Handbook of Plastic Films

Handbook of Plastic Films

Editor: Elsayed M. Abdel-Bary

Rapra Technology Limited


Editor: E.M. Abdel-Bary

rapra TECHNOLOGY

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2003 by

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2003, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library. Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 1-85957-338-X

Typeset by Rapra Technology Limited Cover printed by The Printing House, Crewe, UK Printed and bound by Rapra Technology Limited, Shrewsbury, UK

Contents

1. Technology of Polyolefin Film Production ...................................................... 5 1.1

Introduction ........................................................................................... 5

1.2

Structures of the Polyolefins................................................................... 7 1.2.1

Low-Density Polyethylene (LDPE) ............................................. 7

1.2.2

High-Density Polyethylene (HDPE, MDPE, UHMWPE) ........... 8

1.2.3

Linear Low-Density Polyethylene (LLDPE) ................................ 8

1.2.4

Very- and Ultra-Low-Density Polyethylene (VLDPE, ULDPE) ... 8

1.2.5

Polypropylene (PP) ..................................................................... 9

1.2.6

Polypropylene Copolymers ........................................................ 9

1.3

Morphology of Polyolefin Films ............................................................ 9

1.4

Rheological Characterisation of the Polyolefins ................................... 10

1.5

1.6

1.4.1

High-Density Polyethylene ....................................................... 10

1.4.2

Linear Low-Density Polyethylene ............................................ 11

1.4.3

Very- and Ultra-Low-Density Polyethylene .............................. 11

1.4.4

Low-Density Polyethylene, Long Branches .............................. 11

1.4.5

Polypropylene .......................................................................... 12

Blown Film Production (Tubular Extrusion) ........................................ 13 1.5.1

Extruder Characteristics .......................................................... 14

1.5.2

Screw Design ........................................................................... 15

1.5.3

Frost-line and Blow Ratio ........................................................ 15

Cast Film Production ........................................................................... 16 1.6.1

Extrusion Conditions ............................................................... 16

1.6.2

Calendering Finishing .............................................................. 17

1.6.3

Extrusion Coating .................................................................... 17

iii


1.7

1.8

1.9

Orientation of the Film ........................................................................ 18 1.7.1

Orientation During Blowing .................................................... 18

1.7.2

Orientation by Drawing........................................................... 18

1.7.3

Biaxial Orientation (Biaxially Oriented PP, BOPP) .................. 18

Surface Properties ................................................................................ 19 1.8.1

Gloss ........................................................................................ 19

1.8.2

Haze ........................................................................................ 20

1.8.3

Surface Energy ......................................................................... 20

1.8.4

Slip........................................................................................... 21

1.8.5

Blocking ................................................................................... 21

Surface Modification ........................................................................... 21 1.9.1

Corona Discharge .................................................................... 21

1.9.2

Antiblocking ............................................................................ 22

1.9.3

Slip Additives ........................................................................... 23

1.9.4

Lubricants ................................................................................ 24

1.9.5

Antistatic Agents ...................................................................... 24

1.10 Internal Additives ................................................................................ 24 1.10.1 Antioxidants ............................................................................ 24 1.10.2 Ultraviolet Absorbers ............................................................... 24 1.11 Mechanical Properties .......................................................................... 25 1.11.1 Tensile Properties ..................................................................... 26 1.11.2 Impact Properties ..................................................................... 28 1.11.3 Dynamic Mechanical Properties .............................................. 29 1.11.4 Dielectric Properties ................................................................. 30 1.12 Microscopic Examination .................................................................... 31 1.12.1 Optical – Polarised Light Effect with Strain ............................. 31 1.12.2 Scanning Electron Microscopy (SEM) – Etching ...................... 31 1.12.3 Atomic Force Microscopy (AFM) ............................................ 31 1.13 Thermal Analysis ................................................................................. 31 1.13.1 Differential Scanning Calorimetry (DSC) ................................. 31

iv

Contents

1.13.2 Temperature-Modulated DSC (TMDSC) ................................. 32 1.14 Infrared Spectroscopy .......................................................................... 32 1.14.1 Characterisation ...................................................................... 32 1.14.2 Composition Analysis of Blends and Laminates....................... 33 1.14.3 Surface Analysis ....................................................................... 33 1.14.4 Other Properties ...................................................................... 34 1.15 Applications ......................................................................................... 35 1.15.1 Packaging ................................................................................ 35 1.15.2 Laminated Films ...................................................................... 36 1.15.3 Coextruded Films .................................................................... 37 1.15.4 Heat Sealing ............................................................................. 38 1.15.5 Agriculture ............................................................................... 38 1.16 Conclusion ........................................................................................... 38 2. Processing of Polyethylene Films ................................................................... 41 2.1

Introduction ......................................................................................... 41

2.2

Parameters Influencing Resin Basic Properties ..................................... 42

2.3

2.2.1

Molecular Weight (Molar Mass) and Dispersity Index ............ 42

2.2.2

Melt Index (Flow Properties) ................................................... 42

2.2.3

Density .................................................................................... 44

2.2.4

Chain Branching ...................................................................... 45

2.2.5

Intrinsic Viscosity .................................................................... 46

2.2.6

Melting Point and Heat of Fusion ............................................ 47

2.2.7

Melt Properties – Rheology ..................................................... 48

2.2.8

Elongational Viscosity ............................................................. 49

2.2.9

Elasticity .................................................................................. 49

Blown Film Extrusion (Tubular Film) .................................................. 50 2.3.1

Introduction ............................................................................. 50

2.3.2

Description of the Blown Film Process ..................................... 50

2.3.3

Various Ways of Cooling the Film ........................................... 51

v


2.4

2.3.4

Extruder Size ........................................................................... 54

2.3.5

Horsepower ............................................................................. 55

2.3.6

Selection of Extrusion Equipment ............................................ 55

Cast Film Extrusion ............................................................................. 57 2.4.1

Description of the Cast Film Process ........................................ 57

2.4.2

Effects of Extrusion Variables on Film Characteristics ............. 58

2.4.3

Effect of Blow-up Ratio on Film Properties ............................. 61

2.5

Processing Troubleshooting Guidelines ................................................ 62

2.6

Shrink Film .......................................................................................... 62 2.6.1

Shrink Film Types .................................................................... 65

2.6.2

Shrink Film Properties ............................................................. 66

2.6.3

The Manufacture of Shrink Film ............................................. 67

2.6.4

Shrink Tunnels and Ovens ....................................................... 70

3. Processing Conditions and Durability of Polypropylene Films ...................... 73 3.1

Introduction ......................................................................................... 73

3.2

Structures and Synthesis ....................................................................... 78

3.3

Film Processing .................................................................................... 85

3.4

Additives .............................................................................................. 85

3.5

Ultraviolet Degradation of Polypropylene ............................................ 86

3.6

3.7

vi

3.5.1

UV Degradation Mechanisms .................................................. 86

3.5.2

Effect of UV Degradation on Molecular Structure and Properties of PP................................................................. 87

3.5.3

Stabilisation of PP by Additives ............................................... 88

Case Studies ......................................................................................... 90 3.6.1

Materials and Experimental Procedures ................................... 90

3.6.2

Durability-Microstructure Relationship ................................... 91

3.6.3

Durability-Processing Condition Relationship ......................... 94

3.6.4

Durability-Additive Property Relationship ............................... 97

Concluding Remarks ......................................................................... 101

Contents

4. Solubility of Additives in Polymers.............................................................. 109 4.1

Introduction ....................................................................................... 109

4.2

Nonuniform Polymer Structure.......................................................... 109

4.3

Additive Sorption ............................................................................... 110

4.4

Quantitative Data on Additive Solubility in Polymers ....................... 114

4.5

Factors Affecting Additive Solubility ................................................. 118

4.6

4.5.1

Crystallinity and Supermolecular Structure............................ 118

4.5.2

Effect of Polymer Orientation ................................................ 119

4.5.3

Role of Polymer Polar Groups ............................................... 120

4.5.4

Effect of the Second Compound ............................................ 121

4.5.5

Features of Dissolution of High Molecular Weight Additives .. 122

4.5.6

Effect of Polymer Oxidation .................................................. 124

Solubility of Additives and Their Loss ............................................... 125

5. Polyvinyl Chloride: Degradation and Stabilisation ...................................... 131 5.1

Introduction ....................................................................................... 131

5.2

Some Factors Affecting the Low Stability of PVC .............................. 132

5.3

Identification of Carbonylallyl Groups .............................................. 136

5.4

Principal Ways to Stabilise PVC ......................................................... 138

5.5

Light Stabilisation of PVC ................................................................. 144

5.6

Effect of Plasticisers on PVC Degradation in Solution ....................... 145

5.7

‘Echo’ Stabilisation of PVC ................................................................ 151

5.8

Tasks for the Future ........................................................................... 153

6. Ecological Issues of Polymer Flame Retardants ........................................... 159 6.1

Introduction ....................................................................................... 159

6.2

Mechanisms of Action ....................................................................... 160

6.3

Halogenated Diphenyl Ethers – Dioxins ............................................ 162

vii


6.4

Flame Retardant Systems ................................................................... 166

6.5

Intumescent Additives ........................................................................ 168

6.6

Polymer Organic Char-Former ........................................................... 175

6.7

Polymer Nanocomposites .................................................................. 180

7. Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres . 187 7.1

Introduction ....................................................................................... 187

7.2

Interaction of Nitrogen Dioxide with Polymers ................................. 188 7.2.1

Vinyl Polymers: PE, PP, PS, PMMA, PAN, PVC and PVF ...... 188

7.2.2

Non-Saturated Polymers ........................................................ 191

7.2.3

Polyamides, Polyurethanes, Polyamidoimides ........................ 196

7.3

Reaction of Nitric Oxide with Polymers ............................................ 202

7.4

Conclusion ......................................................................................... 209

8. Modifications of Plastic Films ..................................................................... 213 8.1

Introduction ....................................................................................... 213

8.2

Modification of Mechanical Properties .............................................. 213

8.3

8.4

viii

8.2.1

Orientation ............................................................................ 214

8.2.2

Crystallisation ........................................................................ 214

8.2.3

Crosslinking ........................................................................... 214

Chemical Modifications ..................................................................... 215 8.3.1

Fluorination ........................................................................... 215

8.3.2

Chlorination .......................................................................... 217

8.3.3

Bromination ........................................................................... 217

8.3.4

Sulfonation ............................................................................ 218

8.3.5

Chemical Etching ................................................................... 218

8.3.6

Grafting ................................................................................. 220

Physical Methods Used for Surface Modification............................... 222 8.4.1

Plasma Treatment .................................................................. 222

8.4.2

Corona Treatment ................................................................. 223

Contents

8.5

8.6

Characterisation ................................................................................ 224 8.5.1

Gravimetric Method .............................................................. 224

8.5.2

Thermal Analyses .................................................................. 225

8.5.3

Scanning Electron Microscopy ............................................... 225

8.5.4

Swelling Measurements .......................................................... 226

8.5.5

Molecular Weight and Molecular Weight Distribution .......... 226

8.5.6

Dielectric Relaxation ............................................................. 226

8.5.7

Surface Properties .................................................................. 227

8.5.8

Spectroscopic Analysis ........................................................... 227

8.5.9

Electron Spectroscopy for Chemical Analysis (ESCA) or X-Ray Photoelectron Spectroscopy (XPS) ......................... 228

Applications ....................................................................................... 228

9. Applications of Plastic Films in Packaging .................................................. 235 9.1

Introduction ....................................................................................... 235

9.2

Packaging Functions .......................................................................... 235

9.3

Flexible Package Forms ...................................................................... 236 9.3.1

Wraps .................................................................................... 237

9.3.2

Bags, Sacks and Pouches ........................................................ 238

9.3.3

Pouch Production .................................................................. 239

9.3.4

Dispensing and Reclosure Features ........................................ 239

9.4

Heat-Sealing ...................................................................................... 240

9.5

Other Uses of Packaging Films........................................................... 241

9.6

Major Packaging Films ...................................................................... 241 9.6.1

Low-Density Polyethylene (LDPE) and Linear Low-Density Polyethylene (LLDPE) ....................................... 242

9.6.2

High-Density Polyethylene (HDPE) ....................................... 243

9.6.3

Polypropylene (PP) ................................................................. 244

9.6.4

Polyvinyl Chloride (PVC)....................................................... 245

9.6.5

Polyethylene Terephthalate (PET) .......................................... 245

ix


9.6.6

Polyvinylidene Chloride (PVDC) ........................................... 246

9.6.7

Polychlorotrifluoroethylene (PCTFE) ..................................... 247

9.6.8

Polyvinyl Alcohol (PVOH) .................................................... 248

9.6.9

Ethylene-Vinyl Alcohol (EVOH) ............................................ 248

9.6.10 Polyamide (Nylon) ................................................................. 249 9.6.11 Ethylene-Vinyl Acetate (EVA) and Acid Copolymer Films ..... 250 9.6.12 Ionomers ................................................................................ 251 9.6.13 Other Plastics ......................................................................... 251 9.7

Multilayer Plastic Films ..................................................................... 252 9.7.1

Coating .................................................................................. 252

9.7.2

Lamination ............................................................................ 253

9.7.3

Coextrusion ........................................................................... 253

9.7.4

Metallisation .......................................................................... 253

9.7.5

Silicon Oxide Coating ............................................................ 254

9.7.6

Other Inorganic Barrier Coatings .......................................... 255

9.8

Surface Treatment .............................................................................. 255

9.9

Static Discharge ................................................................................. 256

9.10 Printing .............................................................................................. 256 9.11 Barriers and Permeation ..................................................................... 257 9.12 Environmental Issues ......................................................................... 261 10. Applications of Plastic Films in Agriculture ................................................ 263 10.1 Introduction ....................................................................................... 263 10.2 Production of Plastic Films ................................................................ 263 10.3 Characteristics of Plastic Films Used in Agriculture ........................... 264 10.4 Stability of Greenhouse Films to Solar Irradiation ............................. 265 10.4.1 Ultraviolet Stabilisers ............................................................. 265 10.4.2 Requirements for Stabiliser Efficiency .................................... 268 10.4.3 Evaluation of Laboratory and Outdoor Photooxidation ........ 271

x

Contents

10.5

10.6

10.7

Other Factors Affecting the Stability of Greenhouse Films .............. 272 10.5.1

Temperature ..................................................................... 272

10.5.2

Humidity .......................................................................... 273

10.5.3

Wind ................................................................................ 273

10.5.4

Fog Formation ................................................................. 273

10.5.5

Environmental Pollution .................................................. 274

10.5.6

Effects of Pesticides .......................................................... 274

Ageing Resistance of Greenhouse Films .......................................... 275 10.6.1

Measurement of Ageing Factors ....................................... 275

10.6.2

Changes in Chemical Structure......................................... 276

Recycling of Plastic Films in Agriculture ......................................... 277 10.7.1

Introduction ..................................................................... 277

10.7.2

Contamination by the Environment ................................. 277

11. Physicochemical Criteria for Estimating the Efficiency of Burn Dressings ... 285 11.1

Introduction .................................................................................... 285

11.2

Modern Surgical Burn Dressings ..................................................... 286

11.3

11.4

11.2.1

Dressings Based on Materials of Animal Origin ............... 286

11.2.2

Dressings Based on Synthetic Materials ............................ 286

11.2.3

Dressings Based on Materials of Vegetable Origin ........... 290

Selection of the Properties of Tested Burn Dressings ....................... 290 11.3.1

Sorption-Diffusion Properties ........................................... 291

11.3.2

Adhesive Properties .......................................................... 292

11.3.3

Mechanical Properties ...................................................... 292

Methods of Investigation of Physicochemical Properties of Burn Dressings ................................................................................ 292 11.4.1

Determination of Material Porosity ................................. 292

11.4.2

Determination of Size and Number of Pores .................... 293

11.4.3

Estimation of Surface Energy at Material-Medium Interface ........................................................................... 294

11.4.4

Determination of Sorptional Ability of Materials ............. 294 xi


11.5

11.6

11.7

11.8

11.4.5

Determination of Air Penetrability of Burn Dressings ...... 295

11.4.6

Determination of Adhesion of Burn Dressings ................. 296

11.4.7

Determination of Vapour Penetrability of Burn Dressings .. 296

Results and Discussion .................................................................... 297 11.5.1

Determination of Sorption Ability of Burn Dressings ....... 297

11.5.2

Kinetics of the Sorption of Liquid Media by Burn Dressings ................................................................. 303

11.5.3

Determination of Vapour Penetrability of Burn Dressings .. 305

11.5.4

Determination of the Air Penetrability of Burn Dressings .. 308

11.5.5

Determination of Adhesion of Burn Dressings ................... 315

The Model of Action of a Burn Dressing ........................................ 318 11.6.1

Evaporation of Water from the Dressing Surface ............. 318

11.6.2

Sorption of Fluid by Burn Dressing from Bulk Containing a Definite Amount of Fluid ............................ 320

11.6.3

Mass Transfer of Water from Wound to Surroundings ..... 321

Criteria for the Efficiency of First-Aid Burn Dressings .................... 322 11.7.1

Requirements of a First-Aid Burn Dressing ...................... 322

11.7.2

Characteristics of First-Aid Burn Dressings ...................... 322

Conclusion ...................................................................................... 324

12. Testing of Plastic Films ................................................................................ 329 12.1

Introduction .................................................................................... 329

12.2

Requirements for Test Methods ...................................................... 330

12.3

12.4

xii

12.2.1

List of Requirements ........................................................ 330

12.2.2

Interpretation of Test Results ........................................... 330

Some Properties of Plastic Films ...................................................... 332 12.3.1

Dimensions ...................................................................... 332

12.3.2

Conditioning the Samples ................................................. 332

Mechanical Tests ............................................................................. 333 12.4.1

Tensile Testing (Static) ...................................................... 333

12.4.2

Impact Resistance ............................................................. 336

Contents

12.5

12.4.3

Tear Resistance ................................................................. 337

12.4.4

Bending Stiffness (Flexural Modulus) ............................... 339

12.4.5

Dynamic Mechanical Properties ....................................... 339

Some Physical, Chemical and Physicochemical Tests ....................... 340 12.5.1

Density of Plastics ............................................................ 340

12.5.2

Indices of Refraction and Yellowness ............................... 340

12.5.3

Transparency .................................................................... 341

12.5.4

Resistance to Chemicals ................................................... 341

12.5.5

Haze and Luminous Transmittance .................................. 341

12.5.6

Ignition, Rate of Burning Characteristics and Oxygen Index (OI) ........................................................... 342

12.5.7

Static and Kinetic Coefficients of Friction ........................ 342

12.5.8

Specular Gloss of Plastic Films and Solid Plastics ............. 343

12.5.9

Wetting Tension of PE and PP Films ................................. 344

12.5.10 Unrestrained Linear Thermal Shrinkage of Plastic Films .. 345 12.5.11 Shrink Tension and Orientation Release Stress ................. 345 12.5.12 Rigidity ............................................................................ 345 12.5.13 Blocking Load by Parallel-Plate Method .......................... 346 12.5.14 Determination of LLDPE Composition by 13C NMR ..... 346 12.5.15 Creep and Creep Rupture ................................................. 346 12.5.16 Outdoor Weathering/Weatherability ................................ 347 12.5.17 Abrasion Resistance ......................................................... 347 12.5.18 Mar Resistance ................................................................. 348 12.5.19 Environmental Stress Cracking......................................... 348 12.5.20 Water Vapour Permeability .............................................. 348 12.5.21 Oxygen Gas Transmission ................................................ 349 12.6

Standard Specifications for Some Plastic Films ................................ 349 12.6.1

Standard Specification for PET Films ............................... 350

12.6.2

Standard Specification for LDPE Films (for General Use and Packaging Applications) ..................................... 350

12.6.3

Standard Specification for MDPE and General Grade PE Films (for General Use and Packaging Applications) ... 350 xiii


12.6.4

Standard Specification for OPP Films ............................... 351

12.6.5

Standard Specification for Crosslinkable Ethylene Plastics . 351

13. Recycling of Plastic Waste ........................................................................... 357 13.1

Introduction .................................................................................... 357

13.2

Main Approaches to Plastic Recycling ............................................ 358

13.3

Primary Recycling ............................................................ 358

13.2.2

Secondary Recycling ......................................................... 358

13.2.3

Tertiary Recycling ............................................................ 359

13.2.4

Quaternary Recycling ....................................................... 360

13.2.5

Conclusion ....................................................................... 362

Collection and Sorting .................................................................... 362 13.3.1

Resin Identification .......................................................... 362

13.3.2

General Aspects of Resin Separation ................................ 363

13.3.3

Resin Separation Based on Density .................................. 364

13.3.4

Resin Separation Based on Colour ................................... 365

13.3.5

Resin Separation Based on Physicochemical Properties .... 365

13.4

Recycling of Separated PET Waste .................................................. 367

13.5

Recycling of Separated PVC Waste ................................................. 368

13.6

13.7

xiv

13.2.1

13.5.1

Chemical Recycling of Mixed Plastic Waste ..................... 369

13.5.2

Chemical Recycling of PVC-Rich Waste ........................... 370

Recycling of Separated PE Waste .................................................... 371 13.6.1

Contamination of PE Waste by Additives ......................... 372

13.6.2

Contamination of PE Waste by Reprocessing ................... 372

Recycling of HDPE ......................................................................... 373 13.7.1

Applications for Recycled HDPE ..................................... 373

13.7.2

Rubber-Modified Products ............................................... 373

13.8

Recycling Using Radiation Technology ........................................... 373

13.9

Biodegradable Polymers .................................................................. 374

Preface

The plastic industry continues to grow very rapidly and plays an important role in many fields such as engineering, medical, agriculture and domestic. It is now very difficult to find the point at which plastic cannot be considered as an essential component. The understanding of the nature of plastic films, their production techniques, applications and their characterisation is essential for producing new types of plastic films. This handbook has been written to discuss the production and main uses of plastic films. Chapter 1 deals with the various types of polyolefins and their suitability for film manufacture. The rheology, structure and properties of the polymers are discussed in relation to the type of film manufacturing processes that are most applicable to the types of polymer. Post-extrusion modifications of the films such as orientation, surface chemistry and additives are discussed. Characterisation methods used to measure film mechanical properties; structure and additives are described, as well as other more specific properties. Finally some particularly important applications that require special structures or modifications are given. In Chapter 2, the main parameters influencing resin basic properties are described. The methods of processing of polyethylene films such as cast film extrusion, blow extrusion of tubular films are discussed. Effects of extrusion variables on film characteristics and effect of blow ratio on film properties are considered. Chapter 3 details the structure, synthesis and film processing of polypropylene. The effects of some additives and UV stabilisers are discussed. The solubility of additives plays an important role in determining the efficiency and the properties of the films as well. For this reason Chapter 4 deals with different aspects of additives solubility in polymers in relation to the polymer degradation and stabilisation. The topic covered in Chapter 5 is the stability of polyvinyl chloride (PVC) films during procesing and service. The possibility of increasing the intrinsic stability of PVC during processing with the minimal contents or in total absence of stabilisers and other additives is discussed.

1

Handbook of Plastic Films Chapter 6 discusses flame retardants, which as special additives have an important role in saving lives. These flame retardant system basically inhibit or even suppress the combustion process by chemical or physical action in the gas or condensed phase. Conventional flame retardants have a number of negative attributes and the ecological issues surrounding their applications are driving the search for new polymer flame retardant systems forward. Chapter 7 covers thermal and photochemical oxidation of polymers under the influence of the aggressive, polluting atmospheric gases. Among pollutants, sulfur dioxide, ozone, nitrogen oxides stand out as the most deleterious impurities of atmosphere. Thus, this chapter is devoted to consideration of the results obtained in studies of interactions of nitrogen oxides with polymers. Chapter 8 discusses the modifications of plastic films to improve their mechanical or physical properties to meet the requirements of certain applications. This can be achieved by subjecting the films to mechanical or chemical treatments. A number of surface modification techniques such as plasma, corona discharge and chemical treatments have been used. Chapter 9 deals with applications of plastic films in packaging. A description of the properties of the most common films used in packaging such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), polypropylene (PP), PVC, polyvinylidene chloride (PVDC), polyamide (Nylon), and other plastics are given in this chapter. Chapter 10 deals with the application of plastic films in agriculture. The mechanical properties suitable that make these films suitable for use in agriculture are discussed. Stability of these plastic films under the effect of different environmental conditions is reported. Types of UV stabilisers and their compatibility with plastic are given. Also, recycling of plastic films used in agriculture is of great importance and finally, a case study of their reuse as agriculture films is given. Chapter 11 deals with the principal medical treatment of burns using dressings made with a polymeric layer or layers. It is difficult to estimate the effectiveness of the new burn dressings, as their physicochemical properties are not usually presented in literature. Thus, chapter 11 discusses this subject for the first time. The physicochemical criteria for estimating the efficiency of burn dressings and the possibility of using plastic films is given. Chapter 12 covers the most common test methods generally used for plastic films. The requirements necessary for the test methods are summarised.

2

Preface The problem of plastic films recycling is touched on in Chapter 13. The majority of plastic films are made from polyethylene (LDPE, LLDPE or HDPE) which comprise approximately 68% of the total film production. Non-polyethylene resins constitute the remainder of the plastic film. Different types of recycling are given and recycling of some selected types of films are discussed. This handbook represents the efforts of many experts in different aspects of plastic films. Their efforts in preparing contributions to the volume are to be noted and I take the opportunity to express my heartfelt gratitude for their time and effort. My gratitude extends also to many colleagues for their kind comments in many aspects. A special thanks is extended to the staff of Rapra Technology, for the fine production of this Handbook, particularly Claire Griffiths, Editorial Assistant, Steve Barnfield who typeset the book and designed the cover and Frances Powers who commissioned the book and oversaw the whole project. Elsayed M. Abdel-Bary January 2003

3


4

1

Technology of Polyolefin Film Production Robert Shanks

1.1 Introduction A film is a two-dimensional form of a polymer. A film is typified by a large surface area to volume ratio. Films are required to exhibit barrier properties to any contaminating substances that may try to enter, or any desirable substances that may try to leave, across the film. This property is resistance to diffusion. Since a film is very thin, it must have high mechanical properties such as tensile strength, impact resistance and tear strength. The mechanical properties usually depend on molecular structure, molar mass and molar mass distribution. Visibility through a film is often important, so low haze will be required. These are the bulk properties of the film [1]. The film will often be required to improve the appearance of an item contained within it, so surface properties such as gloss and printability are important. The latter property, printability, is related to a relatively high surface energy to achieve wetting and good work of adhesion. Suitable surface energy may be achieved through modification. Protection may also be improved if the friction is low; this property is called slip. When a film is used to enclose and protect items, it may need to provide adhesion to itself or to the contents. The immediate form of adhesion is called tack. Subsequently the polymer must flow to provide complete adhesion. Manufacture of a film will usually be through an extrusion of the melt, so the melt rheology must be suited to the manufacturing process. Rheology is controlled by chemical structure, molar mass and long branches. The way in which the film is extruded, extended and solidified by cooling will control the microstructure and hence many of the properties. A summary of the various polyolefins used in film manufacture is provided in Table 1.1. In this chapter, polyolefin films are reviewed. First, the various types of polyolefins and their suitability for film manufacture are considered. The rheology, structure and properties of the polymers are discussed in relation to the type of film manufacturing processes that are most applicable to the types of polymer. Post-extrusion modifications of the films, such as orientation, surface chemistry and additives, are discussed. Characterisation methods used to measure film mechanical properties, structure and additives are described, as well as other more specific properties. Finally, some important particular applications that require special structures or modifications are described.

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Table 1.1 Structure, properties and description of the polyolefins used for film production Polyolefin

Comonomer

Density

Preparation method

Mechanical properties

Comments

High-density or linear polyethylene (HDPE, LPE)

No branches

0.94-0.96

Zeigler-Natta

High tensile strength, low impact strength

Brittle film, with good gas barrier properties

Low-density polyethylene (LDPE)

Random short and long branches

0.91

Radical, with autoclave or tubular reactor

Non-Newtonian melt rheology, good impact strength

Good blown extrusion characteristics for flexible films High-clarity, glossy film, difficult to extrude

Linear lowdensity polyethylene (LLDPE)

1-Butene, 1-Hexene, 1-Octene

0.91-0.93

Zeigler-Natta

Intermediate strength with elasticity, melt rheology more Newtonian than LDPE

Very-low-density polyethylene (VLDPE)


0.89-0.91

Single-site metallocene

Tough elastic, moderate strength

High-clarity, very glossy film, very thin films possible

VLDPE with long branches


0.89-0.91

Constrained geometry single site

Tough elastic, moderate strength, nonNewtonian melt rheology

Easy to process, improved melt strength

Ultra-low-density polyethylene (ULDPE), plastomers



Elastic, low tensile strength, low modulus

Thermoplastic elastomer, narrow low temperature melting, good for heat seal

Zeigler-Natta

High tensile strength, brittle, temperature resistance

Transparent, high-strength and temperature-resistant glossy films

Zeigler-Natta

Tough with high melting temperature (block) or softer with lower melting temperature (random)

Tough films, with more milky colour


Narrow molar mass distribution, random comonomer distribution and high isotacticity

Flexible, elastic transparent films

Polypropylene

Polypropylene copolymer with ethylene, block or random

Polypropylene and copolymers with ethylene

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No branches

Ethylene

Ethylene

<0.89

0.90

0.90

0.90

Technology of Polyolefin Film Production

1.2 Structures of the Polyolefins 1.2.1 Low-Density Polyethylene (LDPE) Molecular structures of some example polyethylenes are shown in Figure 1.1. LDPE has rheological properties that are suitable for production of film by the blown film process [2]. LDPE has some long branches and many short branches. Typically, there may be three long branches and 30 short branches per molecule. The molar mass is relatively low, and it has a broad molar mass distribution. The melt strength, or zero-shear viscosity, and the shear-thinning nature of LDPE enhance processing. The film has relatively low tensile strength but good impact strength. LDPE films show good clarity (i.e., low haze) and gloss. The good clarity and gloss result from relatively low crystallinity. LDPE is polymerised by the high-pressure radical process. There are two main reactor types, the autoclave and the tubular reactor. The autoclave tends to provide more branching and broader molar mass distribution. LDPE has a broad melting range, with a peak melting temperature of 110 °C. The density may vary from 0.915 to 0.930 g/cm3 for LDPE.

Figure 1.1 Molecular structures for linear and branched polyethylenes (LPE and BPE) with 100 monomers, four or eight short branches, and one long branch of 40 carbons: (a) LPE (100); (b) BPE (4, 100); (c) BPE (8, 100); and (d) BPE (8, L1(40), 100)

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1.2.2 High-Density Polyethylene (HDPE, MDPE, UHMWPE) HDPE has a linear structure, with little or no branching. HDPE is typically formed by the Ziegler-Natta, Phillips or Unipol processes. Each process involves relatively low pressure and is catalysed by an organometallic complex with a transition metal. Polymerisation is usually performed in slurry with a liquid such as heptane, or in the gas phase with the catalyst in a fluidised bed form. Variations of HDPE are ultra-high molar mass polyethylene (ultra-high molecular weight polyethylene, UHMWPE), where the molar mass is of the order of 1,000,000 g/mol, and medium-density polyethylene (MDPE), where some short branches are introduced by copolymerisation with a 1-alkene, such as 1-butene. HDPE has higher crystallinity and therefore shows higher tensile strength than LDPE, though its impact strength is deficient for many applications. UHMWPE provides increased tensile strength due to the longer molecules providing more tie molecules between crystals. MDPE provides better impact strength because of its reduced crystallinity. HDPE shows a more Newtonian rheology than LDPE, and so is less suitable for extrusion processing, by either the blown film or cast film processes [3].

1.2.3 Linear Low-Density Polyethylene (LLDPE) LLDPE is a copolymer of ethylene and a 1-alkene, typically 1-butene, 1-hexene or 1octene, though branched alkenes such as 4-methyl-1-pentene are also used. These polymers have densities in the range 0.915-0.930 g/cm3 and they contain 2-7% (w/w) or about 12% mol/mol of the 1-alkene. They are polymerised using multisite catalysts such as Ziegler-Natta with either a gas-phase or slurry process. Since the boiling temperature of 1-octene is too high for the gas-phase process, the slurry process must be used. The comonomer composition has a broad distribution, so that some molecules, or segments of molecules, have few branches while others have many branches. This distribution is reflected in the broad melting temperature range of the LLDPE. The properties of LLDPE tend to be in between those of LDPE and HDPE. They have short branches but not long branches, so that crystallisation-dependent mechanical properties are improved, but processing rheological properties are inferior to those of LDPE [4].

1.2.4 Very- and Ultra-Low-Density Polyethylene (VLDPE, ULDPE) Very- (density 0.89-0.915 g/cm3) (VLDPE) and ultra- (density <0.89 g/cm3) (ULDPE) lowdensity polyethylenes have higher copolymer content. ULDPE are also called polyolefin elastomers (POE) because of their properties. These polyethylenes have recently been commercialised as a result of the new metallocene catalyst technology that allows higher comonomer levels and provides a narrower distribution of comonomer composition as

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Technology of Polyolefin Film Production well as of molar mass. These polymers have lower melting temperatures, less crystallinity, greater toughness and elasticity, but lower tensile strength than other polyethylenes. They mainly only have short branches, but some varieties also have some long branches [5].

1.2.5 Polypropylene (PP) Isotactic polypropylene (iPP) is useful for temperature-resistant and glossy film production. iPP has greater strength and higher melting temperature than any of the other polyolefins. The crystal size can be made small by rapid cooling and/or nucleating agents, so that highly transparent, high-gloss films can be produced. The rheological properties are not ideal for the blown film process, but such processing is used in a two-stage extrusion and blowing process. Syndiotactic PP (sPP) is now becoming available commercially as a result of metallocene catalyst polymerisations. sPP provides more elastic films than iPP. iPP has many advantages over polyethylenes because of its strength, thermal resistance, gloss and clarity. It is particularly suitable for more durable products [6].

1.2.6 Polypropylene Copolymers Copolymers of propylene with small amounts of ethylene (0-5% w/w) provide increased toughness, at the expense of tensile strength. Random copolymers show the greatest property changes, such as increased elasticity and a decrease in melting temperature. Copolymers with more block-like structure, where the ethylene is distributed in some of the molecules or molecular segments, provide a good compromise in properties between toughness and strength.

1.3 Morphology of Polyolefin Films All the polyolefins are semicrystalline polymers. The crystallinity provides the tensile strength but reduces the transparency. Larger crystals scatter transmitted light, producing an opalescent appearance, known as haze. Crystals on the surface reduce the surface smoothness and cause surface scattering of incident light and reduce the gloss. An example of the morphology of a polypropylene film is provided in the optical microscope picture in Figure 1.2. Processing conditions can modify the natural tendency of each polyolefin to provide these crystallisation-dependent properties. Rapid cooling will give smaller crystals. So the use of cold rollers in the cast film process usually gives smaller crystals and in particular greater surface smoothness. In the blown film process, the use of a refrigerated air stream increases the crystallisation rate. Crystallisation is evident as a fogging of the film a short distance from the extrusion die; this is called the frost-line height. Orientation can be measured using wide-angle X-ray scattering (WAXS) [7].

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Figure 1.2 Polarised optical microscope picture of polypropylene blended with 30% poly(ethylene-co-propylene). The copolymer is a dispersed phase shown by the dark regions mainly at the edges of the polypropylene spherulites

Orientation of crystals will direct the axes of the crystals, and correspondingly the crystaldependent properties, along the orientation or draw direction. Usually films are oriented, or drawn, in two orthogonal directions, called biaxial drawing, first parallel to the extrusion direction, then laterally. Drawing in the extrusion direction involves cooling the melt until crystallisation takes place, then passing the film between rollers with increasing differential speed. The lateral drawing depends on the method of manufacture. When the blown film process is used, orientation is provided during the blowing process. The cast film process requires a lateral drawing frame called a tenter. The edges of the film are grasped and the frame moves apart as the film moves forward. Orientation provides enhanced physical properties in the drawn directions. When the film is biaxially drawn, the properties are greater in the direction that was drawn last [8].

1.4 Rheological Characterisation of the Polyolefins 1.4.1 High-Density Polyethylene HDPE consists of linear molecules. The shear stress versus shear rate curve will be approximately linear except for very high molar mass. The linear relationship is Newtonian. This means that at high shear rates, as experienced in processing, the viscosity

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Technology of Polyolefin Film Production is high and so the force required for extrusion will be high. Another problem is that the viscosity at low shear rates is not increased. This zero-shear viscosity is related to the melt strength of the polymer. If the melt strength is low, the molten film may rupture as it emerges from the extruder as a tube that is then rapidly expanded by a gas pressure. High melt strength is required to resist rupture and create a dimensionally stable bubble. The melt strength is less critical in the cast film process, although the film must remain stable until it reaches the cooling rollers. The force required for extrusion will still be a problem, since more energy will be needed to extrude a particular mass of polymer, and this will require more electricity and a more powerful extruder motor. HDPE has high tensile strength, but low impact and tear strengths, so damage during processing by tearing is a potential problem. Processing of HDPE can be improved by blending with other polyethylenes, in particular LDPE.

1.4.2 Linear Low-Density Polyethylene LLDPE typically have a broad molar mass distribution and a broad distribution of the 1alkene comonomer, or branches. The tensile strength is lower than that of HDPE but higher than for LDPE. They have improved toughness compared with HDPE. Though they have short branching comparable with LDPE, they do not have long branches. The lack of long branches decreases their shear-thinning rheological characteristics compared with LDPE and so processing is not as efficient. They are often blended with LDPE since the long branches enhance processing. They have greater tensile strength than LDPE, but, with their higher crystallinity, they are less transparent.

1.4.3 Very- and Ultra-Low-Density Polyethylene VLDPE and ULDPE have many short branches distributed along the main chains more evenly than for LLDPE where single-site catalysts have been used in the polymerisation. Short branches are not important in the rheology. These polymers will have essentially Newtonian behaviour. Polymers with very high molar mass have more pronounced shear thinning, though entanglements between only the main chains are not as effective as between several long chains such as when long branches are present.

1.4.4 Low-Density Polyethylene, Long Branches LDPE has long branches that are known to provide non-Newtonian rheological response. LDPE is shear thinning, so that the power required for extrusion at typical high shear rates is less than proportional to the shear rate. This makes extrusion of LDPE more

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Figure 1.3 Parallel plate, continuous shear rheology curve for low-density polyethylene at 200 °C

economical than for other polyethylenes, and the extruder motor does not need to be as powerful. At low shear rates, the viscosity rises significantly, so the zero-shear viscosity, or melt strength, is high. A typical rheology curve for LDPE is shown in Figure 1.3. LDPE has better bubble strength in the blown film process, so that resistance to bursting and bubble stability are greater prior to solidification. In the cast film process, the film will be stable in the molten state between the extrusion die and the cold rollers. The long branches provide more intermolecular entanglements when the shear rate is low. As the shear rate increases, the long branches break free of entanglements and the viscosity decreases markedly. These rheological characteristics are of prime importance during processing [9].

1.4.5 Polypropylene Polypropylene has a methyl branch on each monomer unit and so a pseudo-asymmetric carbon is present. This introduces tacticity, and the isotactic form of polypropylene is the only one that is suitable for film formation. Polymerisation is performed using ZieglerNatta and several other newer proprietary catalysts. The catalysts have been developed

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Technology of Polyolefin Film Production to provide maximum isotactic structure (>99%), so that atactic polypropylene does not need to be extracted and the mechanical properties are maximised. High catalytic activity is desired so that residual catalyst does not need to be extracted from the polymer. Polypropylene has a lower density than most of the polyethylenes (0.905 g/cm3) and a higher strength. Its melting temperature at 162 °C is significantly higher than that of HDPE, making it suitable to form retortable and microwave-resistant products. The glass transition temperature is high, (e.g., 10 °C, but this varies with the crystallinity and method of measurement) so impact resistance is poor. Impact resistance is improved by copolymerisation with ethylene. Usually about 5% ethylene is used, and blocky copolymers are formed, so that the ethylene-containing molecules form an immiscible dispersed phase in a matrix of homopolymer polypropylene. The toughness is increased without decreasing the overall melting temperature significantly. Other random copolymers provide increased toughness and elasticity with decrease in tensile strength and melting temperature. Single-site or metallocene-catalysed polypropylenes have narrower molar mass distribution, though the isotacticity may not be greater. The copolymers with ethylene have a more even distribution of ethylene, and so a very small proportion of ethylene will provide a large decrease in melting temperature compared with the traditional polypropylene copolymers.

1.5 Blown Film Production (Tubular Extrusion) Formation of film is by extrusion. The extrusion process involves a series of events that each affect the stability and consistency of the extrudate and hence the film. The processes in the extruder include feed, melting, mixing, metering and filtration. The die is an annular shape that produces a tube of polymer. The tube is inflated by air pressure injected inside at the die. Inflation of the tube makes the film dimensions greater and provides orientation of the polymer. The tube passes through zones of cooled air, which solidifies the polymer and controls the crystallisation [10]. A diagram showing the essential features of the blown film process is shown in Figure 1.4. In the formation of polypropylene, a two-step tubular orientation process is required. This is because of the poor melt strength of polypropylene. The film must first be cooled to enable crystallisation. The film is reheated to be just at the melting temperature and the tube is blown again before passing through a cooling ring. A comparison of film orientations in the transverse direction (TD) and machine direction (MD) shows the properties to be similar if the stretching occurs simultaneously in each direction. In sequential stretching, the last stretching step predominates, so TD is usually stronger,

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Figure 1.4 Diagram showing the extrusion blown film process

except for tear strength. Oriented PP (OPP) stretch ratios of 6 x 6 are common. Shrink films can be prepared from LLDPE and copolymers of ethylene and propylene, but radiation modification is necessary to partially crosslink the polymer.

1.5.1 Extruder Characteristics The extruder must be able to process a wide variety of polyethylenes with varying molar mass, molar mass distribution, comonomer content and comonomer distribution. Less powerful extruders may only be suitable for LDPE production, since its shearthinning characteristics assist high throughput at lower power. Additives such as antioxidants, ultraviolet stabilisers, lubricants, slip agents and tackifiers may need to be included at the extrusion stage, so a facility for separate injection or dry blending of these additives may be required. The extruder must provide the means to melt and convey the molten polymer through a die that will produce the film. Typically, a singlescrew extruder will be suitable. There are many types of single-screw extruders, but, generally, they are best suited to distributive mixing. Distributive mixing is where the components only need sufficient mixing to provide a uniform melt. Twin-screw extruders provide more intensive mixing, and so are used when dispersive mixing is required. Dispersive mixing is where high shear is needed to subdivide a dispersed phase into smaller particles, where the dispersed phase may be another polymer or a filler with aggregated particles [11].

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1.5.2 Screw Design The extruder screw is usually divided into three zones: feed zone, compression zone and metering zone. The feed zone conveys the polymer pellets, filler and additives from the hopper into the main part of the extruder. In the compression zone, the polymer is melted, mixed with any other components and compressed into a continuous stream of molten polymer compound. The metering zone provides a uniform flow rate to convey the polymer to the die. Polyethylenes are semicrystalline polymers with a broad melting range, particularly if they are copolymers or have random branching, such as LDPE or LLDPE. The melting or compression zones of the screw must be broad. This is the region where the depth of flight is decreased to provide the compression. Polyethylenes have a higher molar mass than other polymers used for extrusion, so the melt viscosity is reasonably high. Polyolefins have weak intermolecular forces, so the mechanical properties are derived from a high molar mass and regularity of the chains for close packing. In addition to the force required for extrusion, the strength of the molten films is important in successful film formation. Of the polyolefins, polypropylene is the most difficult for film production because it has relatively low melt strength. Very high molar mass will improve the film formation, but make the extrusion part of the process more energy-consuming [10].

1.5.3 Frost-line and Blow Ratio The molten film exits from the extruder through an annular die so that a tube of polymer is formed. The tube is sealed at the top as it passes between pinch rollers. The tube is expanded using air pressure. The tube will only expand significantly when the polymer is molten. The rate at which the polymer exits from the die, the air pressure and the impingement of external chilled air determine the blow ratio. The blow ratio is the ratio of the final tube diameter to the diameter of the annulus in the die. This ratio, together with the width of the slot in the die, determines the film thickness and the transverse orientation of the film. The film is also oriented in the direction parallel to the die by a differential between the speed of the polymer exiting the die and the speed of the pinch rollers pressing the tube flat and feeding it to the auxiliary equipment. The transverse orientation occurs up until the polymer solidifies and is often the dominant orientation for properties. The blow ratio able to be used is limited by the melt strength of the polymer. Linear polymers are more likely to exhibit film rupture in the melted region of the tube. Polymers with long branches have higher melt strength and so are much better for production of blown film. The rheology of the polymer is important for other aspects, since an unstable bubble may be formed [12]. The bubble should be symmetrical about the centre-line of the die to the pinch rollers. If the film is uneven in thickness or in solidification, then symmetry will be difficult to control. A thicker portion of the film will be stronger and will resist blowing and so remain thicker. A thinner portion of the

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Handbook of Plastic Films film will expand easier and will become thinner, so this part of the film will bulge outwards even more. The thinner part of the bubble may even rupture. Differential heater bands can be placed around the film near the exit from the extruder to provide fine adjustment of the film temperature as the film is expanded. The frost-line is the point at which the polymer solidifies by crystallisation. The transparency of the polymer is decreased on crystallisation, and this is observed as a sharp transition in the film not very far above the die. The frost-line depends on the extrusion speed and the temperature, as well as on the cooling air that is directed on to the polymer tube from the outside. The cooling air is usually refrigerated, and its temperature, velocity and angle of impingement on to the film may all be varied. Rapid crystallisation will provide smaller crystals, and so the film will be clearer, apparent in a low haze, and have a smoother surface, apparent in a high gloss [13].

1.6 Cast Film Production

1.6.1 Extrusion Conditions Cast film is extruded through a very thin horizontal slit die. The film is drawn from the extruder by calender rolls. This process does not expand the width nor decrease the thickness of the film, though the calendering occurs immediately after extrusion. The extrusion process is the same as for other extrusions. The melted polymer must be distributed evenly along a slit die, usually using channels in the die. The die is referred to as a coat hanger or fish tail die. The calender rolls are chilled so that they provide a melt quenching, giving smaller crystals than the blown film process. The film has a very smooth surface due to the calendering process [8]. The smooth surface can cause self-adhesion of the film, called blocking. An antiblocking agent may be added to reduce the blocking. Cast films will usually have superior gloss and low haze compared with blown films. Orientation of polypropylene flat film uses a tenter frame (chain) with clamps in the transverse direction, a quench roll, then reheating rolls followed by tenter and wind-up roll for the machine direction. The tenter frame is enclosed in an oven that is used to heat and relax the film [6]. Figure 1.5 provides a schematic illustration of the cast film process. Coextrusion is used to make multilayer films by extruding several polymers at the one time through a single complex die. Each individual polymer will have its own extruder feeding into a central die. An individual polymer may be included into more than one layer, yet it only need come from one extruder. Multilayer films are common despite the complexity of the equipment required for their manufacture. Each layer has a special

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Figure 1.5 Diagram showing the extrusion cast film process with calendering and extrusion coating

purpose in the film. The requirements for mechanical protection, diffusion barrier properties, substrate and interlayer adhesion, and heat shrinkage cannot all be met through a single polymer. The most suitable polymer for each purpose can be chosen and assembled into the multilayer structure.

1.6.2 Calendering Finishing The melted film is usually cooled and pressed to provide a high-quality surface by passing it through a set of chilled calender rollers immediately after extrusion. The rollers will be of highly polished steel to provide a smooth glossy surface to the film. Rapid cooling also assists formation of a glossy surface on the film, since the crystals will be kept small and crystallisation may be minimised. A series of rollers may be used to provide orientation by stretching the film in a longitudinal direction.

1.6.3 Extrusion Coating Extrusion coating is when the melted polymer film is extruded on to an existing film before passing through the calender rollers. The existing film will be another polymer, metallic foil or paper. Multiple layers may be formed by extrusion coating both sides of the primary film or building a multilayer structure by introducing several extrusioncoated layers. Coextrusion can only be used for polymers with similar processing conditions. Where the processing conditions are different, particularly in the case of substrates that cannot be melted with the polymer, such as metallic foils and paper, then extrusion coating is the only choice [6].

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1.7 Orientation of the Film 1.7.1 Orientation During Blowing Extrusion blow moulded film usually receives biaxial orientation. During blowing, the diameter of the extruded tube is increased, and this causes the structure of the film to be oriented perpendicular to the extrusion direction. Orientation should take place below the melting temperature when the polymer is crystalline so that the crystals are oriented. The expansion of the extruded tube will take place while the polymer is entirely melted, so that the effect of blowing will not provide a level of orientation equivalent to the diameter expansion [14]. At the same time as the film is being expanded by blowing, it is being drawn by pulling along the axis of extrusion. This provides a parallel orientation. Again, most of the parallel drawing occurs on the polymer melt between when it leaves the extruder and when it crystallises. The crystallisation region is called the frost-line. At the frost-line the film will have its maximum diameter and resist further expansion or drawing compared with the region immediately before the frost-line. At the frost-line the completely transparent melt becomes foggy due to crystallisation. The change in opacity depends on the crystallinity of the particular polymer. Sometimes additional orientation is imparted on the film after the blowing process. This is the case if the film is to be a shrinkable film. Shrinkable films will contract upon heating. This is useful for providing tightly fitting wrapping.

1.7.2 Orientation by Drawing Orientation of a polymer must be performed at a temperature between the glass transition and melting temperatures. Polyolefins, particularly polypropylene, are normally moderately heated. The enhancement of tensile properties is directly related to the draw ratio. After drawing, the film should be further heated to relax or set the structure. This will provide dimensional stability. If the film is to be heat-shrinkable, then the relaxation is not performed. Some crosslinking from radiation treatment prior to drawing may be used to increase the plastic memory effect in the film. Excessive drawing can cause strain hardening because of the introduction of extended chain crystals at the expense of chain-folded crystals. The strain-hardened film will have a stiffer or more leathery feel; it will lose elasticity and may have a rougher surface.

1.7.3 Biaxial Orientation (Biaxially Oriented PP, BOPP) It is desirable to orient films in planar directions, parallel and perpendicular to the flow. The parallel orientation can be provided by a draw-off faster than the extrusion speed.

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Figure 1.6 Schematic diagram for biaxial orientation of cast film

The perpendicular direction has been described for blown film production. In extruded sheet, a frame that attaches to the edges of the film and moves apart must provide the perpendicular orientation. This is called a tenter frame. The film is often drawn to about three times its original width. As well as adding strength to the film, a thinner gauge of film can be produced. The drawing process overcomes the effect of die swell that occurs as the film leaves the die. Orientation of cast film is illustrated in Figure 1.6.

1.8 Surface Properties 1.8.1 Gloss Gloss is the reflection of light from a surface. The nature and origin of gloss and haze are illustrated in Figure 1.7. A high gloss requires a smooth surface. Surface imperfections may be introduced by the processing. Excessive drawing into the strain-hardening region will usually reduce the gloss. Blown film usually has a lower gloss, since crystallisation of the film at the frost-line introduces surface roughness due to the crystals. Rapid crystallisation of the film by the use of chilled air impinging on the bubble reduces the size of crystals and improves the gloss. Extrusion cast film passes through chilled rollers after leaving the

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Figure 1.7 Diagram showing the origin of gloss and haze in polymer films

extruder. The rapid cooling and the polished surface of the rollers provide a high-gloss surface. Extrusion cast films have the higher gloss, but the extrusion blown process produces film at a lower cost. The rheology of the polymer will contribute to the surface of the film. Shark skin is the term applied to a rheological problem in the processing [15].

1.8.2 Haze Haze is an internal bulk property, but, because of the importance of appearance, it has been considered along with surface properties due to its relation to gloss (see Figure 1.7). Crystallinity, optical defects, ‘fish eyes’, phase separation of blends, contaminants, gel particles and dispersion of pigments (carbon black) are structures that increase haze. Haze is the internal scattering of light. Haze makes it difficult to clearly see an object through a film as a result of the interference from randomly scattered light reaching the viewer in addition to light coming straight from the object. Smaller crystals provided by a nucleating agent will decrease haze. The other phenomena described above can also be reduced by nucleating agents, better formulation and processing.

1.8.3 Surface Energy The surface energy of polyolefin films is very low. It is difficult to find other substances that will adhere to polyolefins. Suitable adhesion can be obtained by melt adhesion of polyolefins to each other, but only when the polyolefins are very similar. For instance, polyethylenes have good mutual adhesion. The branched polyethylenes with lower melting temperature are most used because they can be melted more rapidly and they have suitable

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Technology of Polyolefin Film Production rheology to flow on to the adherend. Melt adhesion of films will require higher-melting layers other than the surface layer, since if more than the surface layer melts then the structural integrity of the film will be destroyed. Copolymers of ethylene with vinyl acetate, methyl acrylate, acrylic acid, maleic acid and many other polar monomers are used to increase the surface energy of polyethylenes to make them more readily wettable. Similarly, polypropylene can be grafted with maleic anhydride to increase the adhesion of other substances to it. The surface energy of polyolefins is also increased through corona discharge treatment.

1.8.4 Slip Polyolefin films generally have a smooth surface, with the exception of defects and surface crystal structures. They have a low surface energy and so frictional forces are low. Relative to their strength, the frictional forces can cause damage to the films. Slip additives can decrease the frictional forces. The factors that cause poor slip are often desired for other attributes such as adhesion of printing and adhesion to other surfaces in packaging. Additives that increase self-adhesion of packaging films will decrease the slip, so that desirable properties are not universal – they depend on the intended application of the film.

1.8.5 Blocking The low surface energy and softness of polyethylenes make them self-adhere if pressed together under a load for a considerable time. This self-adhesion is called blocking. The polyolefins can flow, or creep, under load, and so mutual adhesion can occur if the pressure is sufficient or the time of contact is long. This is a significant problem in large film rolls or when film is stacked in large quantity. Blocking is reduced when the surface is less smooth, such as when crystallisation or processing conditions cause microtopographies. A smooth surface with high gloss and clarity is generally preferred, so that blocking will be a serious problem.

1.9 Surface Modification 1.9.1 Corona Discharge Processes for modifying the surface properties of plastic films are important. Surface adhesion can be increased by oxidative treatments such as corona, flame, priming or subcoating. Corona discharge is most widely used, and surface oxidation of the film

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Figure 1.8 Corona discharge surface treatment system

occurs, giving polar functional groups [16]. A corona discharge treatment facility is shown in Figure 1.8. Increased polarity will increase the surface energy and enable wetting by inks or adhesives. Printing is carried out by the flexographic technique, whereby a rubber roller with raised imprints imparts the ink, or by the rotogravure process, whereby an engraved steel roller imparts the ink. Film coating, film wetting, dispersion coating (by an emulsion), solvent coating, barrier and heat seal coatings are all improved after oxidative surface treatment. Extrusion coating is the process whereby a film is extruded on to an existing film. Film lamination occurs when existing films are bonded together with an adhesion layer applied by any of the previously mentioned methods. Typical are the use of poly(vinylidene dichloride), aluminium foil and ionomer films, and metallisation by vapour deposition of aluminium. Coextrusion is the process in which two or more films are extruded and brought together in the die. Each layer will contribute to specific properties, such as barrier and adhesive layers.

1.9.2 Antiblocking Blocking can be reduced by decreasing the surface contact area of the films. Small particles at the surface can decrease the contact. Particles such as silica, diatomaceous earth and talc are useful antiblocking agents [17]. To provide the protection efficiently, they must be included in the polymer during processing. Many of the particles will be in the bulk of the film, so they will not contribute to antiblocking characteristics. Those particles that exist at the surface will be active. The particles must be small enough not to introduce haze when in the interior or decrease gloss when at the surface. Films that have a rougher surface due to surface crystallites or other imperfections will have a natural protection against blocking. The mechanism of antiblock additives is shown in Figure 1.9.

22


Figure 1.9 Mineral particles (silica or talc) prevent blocking, since some particles become located at the surface and so limit contact of the films

1.9.3 Slip Additives The coefficient of friction can be reduced by adding slip agents. The requirement for a slip agent is that it is miscible with the polymer melt, but separates as the polymer is crystallising. Sometimes the separation will take several hours or days to become complete. The slip agent should form a very thin layer on the surface. The layer is more effective if it is randomly structured. The mutual miscibility and separation are important, and long-chain amides have been found to be best when the chain length is about 22 carbon atoms, whereas 18 carbons is less effective. An amide with a cis double bond is better than the fully saturated analogue because the cis double bond prevents surface crystallisation of the amide compared with the saturated amide. Erucamide has thus been found to be the most effective slip agent for polyolefins [18]. Other fatty amides such as ethylene bis-stearamide, oleamide and stearamide have been used. The function of a typical slip additive is shown in Figure 1.10.

Figure 1.10 Structure of erucamide (C21H41CONH2, cis-13-docosenamide) slip additive, and its mechanism for increasing slip. The diagram shows an array of erucamide molecules along the surface of a film, with the bent cis structure ensuring that they stay irregular

23


1.9.4 Lubricants Lubricants are added to assist with processing. The polymer can be extruded more readily when there is a layer of lubricant between the polymer and the surfaces within the extruder. This is particularly important at the die lips, where the lubricant can decrease surface irregularities such as ‘shark skin’ effects. The melt can maintain laminar flow at higher extrusion rates with lubricants present. Common lubricants are stearic acid, stearate salts, paraffin wax and chlorinated paraffins [19]. A fluoropolymer has been reported as a processing additive for polypropylene [20].

1.9.5 Antistatic Agents Polyolefins do not absorb water. The dry surface causes a build-up of static electricity. Antistatic agents are polar substances that are mixed with the polymer and migrate to the surface after the polymer emerges from the extruder. Polyoxyethylenes are one type of antistatic agent. When on the surface, they can absorb sufficient water to prevent static electricity build-up. A problem is that they can attract dust and other materials that would not normally adhere to the polyolefins. Another problem is that they can be easily removed by friction or extraction with polar liquids [19].

1.10 Internal Additives 1.10.1 Antioxidants Immediately the polyolefin is made, it requires protection from oxidation. This is particularly the case during the processing stages when high temperatures are used. Hindered phenols and triaryl phosphites are typically used. During the extrusion process, the polyolefin is then protected from the heat and oxygen. During the lifetime of the film, other thermal processes may be encountered during printing, packaging of food and heat sealing. The antioxidants act by removal of radicals from the polymer – by hydrogen donation in the case of hindered phenols, and by removal of oxygen from peroxy groups in the case of phosphites [18]. Figure 1.11 shows a typical hindered phenol antioxidant and a triphenyl phosphite secondary antioxidant.

1.10.2 Ultraviolet Absorbers Films that are to be used in the sunlight require protection. While the polyolefins do not absorb ultraviolet light, they do contain many other anomalous functional groups, additives and impurities that do absorb. Hydroxybenzophenones, hydroxybenzotriazoles

24


Figure 1.11 Structures of common heat and light stabilisers: (a) hindered phenol antioxidant: (b) tris(2,4-di-tert-butylphenyl) phosphite secondary antioxidant; (c) hydroxybenzophenone ultraviolet absorber: (d) hindered amine light stabiliser

and tetramethylpiperidines (hindered amine light stabilisers, HALS) are typical compounds used for ultraviolet stabilisation. Factors such as compatibility with the polyolefin, low volatility and absence of any colour are just some of the many stringent requirements of these additives. Hydroxybenzophenones and hydroxybenzotriazoles absorb ultraviolet light and form an excited electron state that can undergo a radiationless transfer of the energy to heat, to regenerate the ground state. HALS act by a cyclic mechanism in which they form a nitroxide that can couple with a radical and then transfer the radical to terminate another radical and release the nitroxide. HALS are the most effective stabilisers for polypropylene, but the type of HALS used depends on the application of the polypropylene and the other components in the composition [17]. Figure 1.11 shows a typical hindered amine light stabiliser and a hydroxybenzophenone ultraviolet absorber.

1.11 Mechanical Properties There are many standard test methods to define the performance requirements of polyolefin films. These are specified in the relevant ASTM, DIN and ISO standards. The material mechanical properties are used for polymer specification for a particular purpose. There are many tests available, and each measures a narrowly defined property. Often it is difficult to predict performance by a material property, and so a product-specific test is designed.

25

Handbook of Plastic Films The mechanical properties can be separated into tensile and impact tests, though there are many other tests, such as tear testing, abrasion resistance and adhesion tests. The morphology of the film is of major importance in controlling the mechanical properties [21]. The morphology of a film is strongly connected with the key processing variables [8].

1.11.1 Tensile Properties 1.11.1.1 Strain Rate and Tensile Properties The strain rate is important in measuring the tensile properties of films. During packaging operations, films are often subjected to extremely high strain rates in the machinery. These processes must be duplicated in the testing procedures where possible. Slow strain rates may be preferred when material properties are measured to distinguish between various polyolefin structures. Typical parameters that are obtained from a tensile test are the modulus, yield stress, break stress and elongation at break. The area under the stressstrain curve is used as a measure of the energy to break.

1.11.1.2 Strain Hardening When a polyolefin film is considerably extended, the crystal structure orientation will be significant and the polymer will become harder. Often a strain-hardened film is described as leathery. The morphology may change from a chain-folded to an extended-chain configuration. The tie molecules become fully extended so that further strain is limited before film breakage occurs. Strain hardening is a limit to the useful elongational performance of a film because the elasticity is lost. A stress-strain curve for linear lowdensity polyethylene showing the yield and strain-hardening regions is shown in Figure 1.12.

1.11.1.3 Stress Relaxation Stress relaxation is defined as a change in stress when the material is under constant strain. When a packaging film is stretched around an article, or a large number of articles, the elasticity of the film will keep the article(s) under constant tension to provide protection and ease of transport and handling. When stress relaxation occurs, the tension of the packaging is lost, and the contents are no longer held together. Stress relaxation can be measured with a standard tensile testing instrument, where the stress is measured over time while the specimen is held under a constant strain. Polyolefins with high molar mass and high crystallinity will be the most resistant to stress relaxation. An illustration of the processes involved in stress relaxation is shown in Figure 1.13; first the amorphous molecules are elongated, then on relaxation they slide past each other to return to a random-coil conformation.

26


Figure 1.12 Stress-strain curve for linear low-density polyethylene showing the yield stress after the initial linear elastic region, then the strain-hardening region after 350% strain

Figure 1.13 Schematic for the mechanism of creep and stress relaxation

1.11.1.4 Creep Creep is the change in strain when the specimen is subjected to a constant stress. Polyolefin films that are exposed to pressure for extended periods will gradually elongate. Pressures

27

Handbook of Plastic Films are generated inside many packages, and polymer will creep, causing the package to become larger. Creep is a complementary property to stress relaxation, and so the same molecular characteristics resist creep. Creep is illustrated in Figure 1.13; under constant load the molecules gradually slide past each other, resulting in elongation.

1.11.1.5 Burst Strength When a film experiences a high pressure over a short time, it may burst. This is similar to elongation at break, except that the film will be under a biaxial tension. A test of burst strength may be more suitable for a film under pressure than a tensile test. The shortterm nature of the test is in contrast to the long time for creep.

1.11.2 Impact Properties 1.11.2.1 Dart – Puncture Resistance An impact test is a short-term test; the stress is applied very quickly. The dart test can involve either a dart falling through a constant distance or a dart propelled by gas pressure. The shape of the end of the dart is an important factor in the test. A rounded end is generally used, but the test can be modified to measure the resistance of a film to any puncture by impact with any particular object. The falling dart test will result in a pass/ fail type result compared with a specification for the load. The instrument can be designed to measure the deceleration of the dart as it passes through the film. The energy required to break the film is then calculated from the energy lost by the dart. In this latter situation, the dart is always required to break the film.

1.11.2.2 Tensile – Tear Strength Films often break in shear instead of tension. A film can be tested in a tensile instrument so that the strain is applied under shearing conditions. A cut may be made in the film to direct the tear. The geometry and conditions of the test are defined so that standard conditions are used. The Elmendorf tear strength test is used to measure the performance of films, and it has been related to processing conditions and dart impact strength [22].

1.11.2.3 Tensile Impact This is a short-term impact test where the specimen is mounted in a pendulum test instrument so that it receives a rapid tensile force as the pendulum strikes the specimen

28

Technology of Polyolefin Film Production holder. The tensile impact test is applicable to films, whereas other forms of pendulum impact tests, such as Izod and Charpy tests, require more rigid specimens. The tensile impact test can apply a greater strain rate than a typical tensile test instrument.

1.11.3 Dynamic Mechanical Properties Dynamic mechanical analysis (DMA) measures the properties when an oscillating stress is applied to the material. The stress and strain are often out of phase, and this situation can be used to obtain the viscoelastic properties. The viscous or timedependent properties are out of phase with the stress, while the elastic or instantaneous properties are in phase with the stress. The in-phase property is called the storage modulus, in that the elastic energy is stored and can be subsequently released when the stress is removed. The out-of-phase property is called the loss modulus, in that energy is lost to heat during viscous flow. The properties are usually measured with temperature and/or frequency. Temperature and frequency can be combined to provide a time-temperature transposition, so that long-term or very short-term properties can be measured within real-time limitations [23]. Figure 1.14 shows DMA curves for polypropylene.

Figure 1.14 DMA curve for polypropylene, showing the storage modulus, E′, loss modulus, E″, and damping factor, tan δ

29


1.11.4 Dielectric Properties Polar olefin copolymer films, such as poly(ethylene-co-vinyl acetate), poly(ethyleneco-methyl acrylate) and poly(ethylene-co-butyl acrylate), and blends or laminates can be characterised using dielectric analysis. The dielectric properties of all types of polyolefins are important because of the electrical applications of these polymers. However, dielectric analysis can also be performed in an analogous manner to DMA, in that the mechanical force is replaced by a voltage and the permittivity is measured while the temperature and/or frequency is varied. The viscoelastic properties of the polymer can be measured by the dielectric response over a wider range of frequencies than the mechanical tests. The dielectric test is more sensitive than the mechanical test when the polymer has more polar groups. Figure 1.15 provides a schematic of the processes involved in a typical dielectric analysis.

Capacitance (C) I = current V = voltage f = frequency δ = phase angle

Conductance (R-1)

C=

R−1 =

I sin δ V 2πf

I cos δ V

Permittivity (ε′) ε0 = permittivity of a vacuum, A = area of electrode D = distance between electrodes

⎛ C ⎞⎛ 1 ⎞ ε′ = ⎜ ⎟ ⎜ ⎟ ⎝ ε0 ⎠ ⎝ A / D⎠

Loss factor (ε′′) ω = angular frequency σ = conductivity

ε ′′ = ε dipole + ′′

σ ωε 0

Figure 1.15 Dielectric analysis of polar ethylene copolymers involves measurement of the storage (permittivity) and loss (loss factor) components with temperature

30


1.12 Microscopic Examination 1.12.1 Optical – Polarised Light Effect with Strain The crystallinity of a polyolefin in a film can be viewed with an optical microscope using polarised light. The film must be very thin, although reflected light can be used for thicker opaque films. The microscope can be combined with a hot stage to view crystallisation and melting events.

1.12.2 Scanning Electron Microscopy (SEM) – Etching Scanning electron microscopy (SEM) can reveal morphology, though usually the sample must be etched so that amorphous regions are modified more than crystalline, or a component in a blend is eroded more rapidly. The etching creates a new surface topography that can be viewed with the SEM. Care must be taken so that the electron beam does not damage or create artefacts on the surface.

1.12.3 Atomic Force Microscopy (AFM) The surface of polyolefin films can reveal information about the bulk. Crystal growth at the surface and other irregularities that may arise from processing or treatments such as corona discharge can be studied using atomic force microscopy. Variations in the hardness or friction across the surface as found in blends can be studied to reveal the distribution of components across the surface. Figure 1.16 shows an atomic force microscope picture of the surface of linear low-density polyethylene formed by the extrusion blow moulding process.

1.13 Thermal Analysis 1.13.1 Differential Scanning Calorimetry (DSC) DSC is used to measure the crystallisation and melting temperatures of polyolefins as well as the enthalpies of crystallisation and melting. The results shown in Figure 1.17 are used to identify, characterise and measure the crystallinity [24]. The thermal history and mechanical stresses of a film can be investigated through the melting and crystallisation response measured by DSC [25]. The crystallisation temperature increases with nucleation, so that the efficiency of added nucleation agents can be measured. The crystal structure varies with processing conditions and other treatments such as orientation, and these can be measured by analysis of the melting of the polyolefin on heating.

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Figure 1.16 Atomic force microscope picture of the surface of linear low-density polyethylene film prepared by the extrusion blown film process

1.13.2 Temperature-Modulated DSC (TMDSC) TMDSC is analogous to DMA in that an oscillating force, in this case a temperature programme, is applied to the sample. The response can be resolved into reversing and nonreversing specific heat capacities. Recrystallisation, rearrangement of the crystals and melting can be studied simultaneously. This is important for an understanding of the equilibration of the morphology of the polyolefins after various processing or other thermal treatments [26].

1.14 Infrared Spectroscopy 1.14.1 Characterisation Infrared spectroscopy is a convenient method to identify polyolefin films. The main classes of polyolefins can be easily identified. A detailed interpretation of the infrared spectrum

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Figure 1.17 DSC curves for melting of linear low-density polyethylene and low-density polyethylene (for clarity, this curve has been shifted upwards by five units)

will enable even very similar structures to be distinguished. The extent of branching can be measured and crystallinity can be indirectly measured [27].

1.14.2 Composition Analysis of Blends and Laminates Blends of polyolefins can be identified, and, when the components are known, quantitative analysis can be performed. The component layers of laminates can be identified after separation of the layers, or by analysis of the edge of the film using an infrared microscope.

1.14.3 Surface Analysis Surface additives, such as glyceryl monooleate, polyisobutylene and slip agents, and corona treatments can be measured using surface infrared spectroscopic analysis. Multiple internal reflection is the common method, but specular reflectance and grazing angle reflectance are other useful techniques by which the infrared spectrometer can be used to study surface chemistry. Figure 1.18 shows a schematic for the surface analysis of a polymer

33


Figure 1.18 Apparatus for multiple internal reflectance (attenuated total reflectance) infrared spectroscopy for surface analysis of polymer films

film by multiple internal reflection. The angle of the infrared beam to the surface of the internal reflectance element (typically zinc selenide or germanium) along with the wavelength of the infrared beam determine the depth of penetration into the surface of the film. A low depth of penetration will provide spectra more sensitive to the surface additives or modifications.

1.14.4 Other Properties 1.14.4.1 Thickness Thickness is known as gauge. Uniformity must be achieved by the manufacturing process, Generally films are considered to be of ≤250 μm; greater thicknesses are called sheet. Some films may be 10-20 μm in thickness, and individual layers in multilayer films are often only 5 μm thick.

1.14.4.2 Moisture Resistance Polyolefins are nonpolar, so they are particularly efficient at resisting moisture. Their resistance to liquid water is not necessarily carried on to their resistance to water vapour or humidity. High-density polyethylene film is the most resistant to water vapour because gas molecules have difficulty diffusing through the crystalline structure.

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1.14.4.3 Gas Permeation Films are used to encase and protect other items. An important property is to resist gas permeation, in particular, the permeation of oxygen, carbon dioxide and water vapour. Polyolefins provide very poor barrier layers. Usually a multilayer film that includes another polymer or other material is required to provide suitable barrier properties. The barrier properties are increased when higher-crystallinity polymers are used. The density of the amorphous phase of the polymer has been found to be a guide as to the permeation resistance [28].

1.14.4.4 Orientation Orientation of a film may be measured by annealing the film at temperatures below the melting temperature. Oriented films will shrink more than others. The shrinkage is often controlled by partial crosslinking of the film. There are many applications in the food industry for shrink-wrapping of produce. Shrink-wrapping of polyethylene films can be applied over other packaging materials to group containers into specific quantities.

1.14.4.5 Dimensional Stability Dimensional stability is usually a consequence of orientation. Under heat treatments such as may be experienced during packaging of hot foods, sterilisation processes for foods and heat sealing, the dimensional stability of the film must be maintained. This is the opposite requirement to shrink-wrap films. Humidity can contribute to changes in dimension, due to absorption of moisture by other components of a multilayer film or by the contents of the film package.

1.15 Applications 1.15.1 Packaging The main application of films is for packaging (Figure 1.19). The functions of a package can be summarised as: containment, dispensing, preservation, protection, communication and display. Packaging machinery is diverse, (e.g., vertical fill, shrink-wrap, sleeve-wrap, stretch-wrap and blister packaging machines), and thus requires many properties of the film, (e.g., stiffness, stretchability, heat sealability), which in addition must be suitable for various applications, (e.g., in side weld bags, in bottom seal bags and to hold liquid

35


Figure 1.19 Examples of typical packaging applications for polyolefin films

products). Many of the films are required to be multilayer films to achieve all of the desired properties [14]. Polyethylenes with new improved properties due to high molar mass are being used in heavy-duty applications [29].

1.15.2 Laminated Films The performance requirements for packaging are increasing, with new demands for protection of the product, speed and ease of packaging and sealing, together with printing, handling and storage requirements. An individual polymer cannot meet all these requirements, so multilayer films are necessary. These can be prepared by extrusion of further layers on to existing films or adhering existing films together. This process is called lamination. Most of the layers are polymer, but a metal foil (usually aluminium) may be used. Paper or paperboard is frequently used as substrates for the lamination. A textile layer may be used, but these are mainly classified separately from laminated films.

36

Technology of Polyolefin Film Production A basic requirement for a laminated film is good adhesion between the layers. The materials in the layer will often be chemically different, to provide the diversity of properties, and so adhesion may not be suitable. In such cases, a separate adhesive layer must be included between the functional layers. The adhesive layer functions in the same way as a compatibiliser in polymer blends. Often a copolymer will be used, where each of the component monomers will contribute to the adhesion with one of the adjacent layers. Some multilayer films are produced with the adhesion component included as a blend with a functional layer. In this way the number of individual layers is reduced, simplifying the lamination process. The separate layers of the film may consist of the same polymer but in different forms. A layer could be mineral-filled, pigmented, foamed, oriented, radiation or chemically crosslinked, include antioxidant or ultraviolet stabilisers, be printed or otherwise modified. A further protective layer may then be placed over the modified layer, or the modified layer may be the protective layer. Lamination of films is often preferred to application of a polymer layer as a lacquer since the latter includes solvent or an emulsion is used, so that drying and removal of volatiles is required. The laminated film will often have superior gloss such as when polypropylene is laminated on to printed paper substrates. Surface spreading by the coating layer and film thickness will be easier to control by laminating than by solution coating. Ovens and solvent removal systems can be replaced by calender cooling rollers.

1.15.3 Coextruded Films Coextruded films contain most of the characteristics of laminated films, except that all of the layers are formed by extrusion at the same time. This precludes aluminium foil, paper and textile layers, which must be included by lamination. The coextrusion requires several extruders with a common die. The die has a complex construction and may be of either a tubular film type or a cast film linear slit type. The polymer for each layer is fed into the die by a separate extruder. One extruder per layer may be used or, if more than one layer contains the same polymer, then its extruder can feed into more than one layer. The latter connection uses the minimum number of extruders, but is less flexible than the ‘one extruder per layer’ configuration. After coextrusion, all of the layers must receive any treatments together. For instance, if a layer is to be biaxially oriented, then all layers must be oriented. If a layer is to be radiation-crosslinked, then all of the layers must receive the radiation treatment. Individual layers cannot be printed, then covered with another protective layer. An advantage is that the complete multilayer film is produced in one process. The process is not suitable for polymers with significantly different processing temperatures. Shrinkage stresses may occur on cooling and so layers may separate.

37


1.15.4 Heat Sealing Polyolefin films are joined by heat sealing, a process whereby the film is partially melted while pressing against the other component for a well-defined short time. The strength of adhesion is characterised by the time-temperature-pressure relationship and the proportion of the polymer melted. Melting will destroy the original crystal structure and any orientation in the film, so the mechanical properties will be changed. A multilayer film is preferred so that only the surface adhesive layer is partially melted, while the structural layer is unaffected. This requires that the surface layer have a lower melting temperature than the structural layer [30]. Typically, if the structural layer is polypropylene, then the surface layer can be an ethylenepropylene random copolymer, with a small proportion of ethylene. The adhesive layer may also be grafted with maleic anhydride to provide better adhesion to polar substrates. If the structural layer is a polyethylene, such as LLDPE, then the adhesive layer can be an ethylene copolymer, such as poly(ethylene-co-vinyl acetate). Recently VLDPE have been used as adhesive layers where low surface energy for better wetting is important.

1.15.5 Agriculture Films for agriculture have become important to protect crops and to protect and bind products. The very thin films can provide sufficient tensile strength to contain the product, and with self-adhesion of the film the wrapping process is very efficient. The films may be pigmented black to protect the contents from sunlight. Pigmented film may be spread on the ground to control weeds by removal of sunlight and reduce evaporation of water [31].

1.16 Conclusion Polyolefin films are complex in their manufacture by the blown film extrusion and extrusion cast film processes. The films are typically strengthened by biaxial orientation and can be made very thin. The inert polyolefin surface is often modified by oxidation to provide polar groups for adhesion, and by the use of additives to provide slip or to prevent blocking. Specialised properties such as gas barrier, printability, heat sealing and shrinkability are achieved by coextrusion or lamination of films with different chemical structures. The diversity of films available may seem simple, but when the details are considered even a common packaging film involves a complex range of technologies. Further advances will enable films to possess even more specialised functionality while being thinner and stronger.

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References 1.

K.J. McKenzie in Kirk-Othmer, Concise Encyclopedia of Chemical Technology, Wiley-Interscience, New York, NY, USA, 1999, 845.

2.

J.A. Brydson, Plastics Materials, 4th Edition, Butterworths, London, UK, 1982, 187.

3.

H. Saechtling, International Plastics Handbook, Carl Hanser, Munich, Germany, 1983, 347.

4.

D.Y. Chiu, G.E. Ealer, F.H. Moy and J.O. Buhler-Vidal, Journal of Plastic Film and Sheeting, 1999, 15, 2, 153.

5.

A.K.C. Mehta, M.C. Chen and C.Y. Lin in Metallocene-Based Polyolefins – Preparation, Properties and Technology, Eds., J.K. Scheirs and W. Kaminsky, John Wiley, Chichester, UK, 2000, 463.

6.

E.P. Moore, Polypropylene Handbook, Carl Hanser, Munich, Germany, 1996, 334.

7.

H. Fruitwala, P. Shirodkar, P.J. Nelson and S.D. Schregenberger, Journal of Plastic Film and Sheeting, 1995, 11, 4, 298.

8.

J.A. Degroot, A.T. Doughty, K.B. Stewart and R.M. Patel, Journal of Applied Polymer Science, 1994, 52, 3, 365.

9.

F.J. Velisek, Journal of Plastic Film and Sheeting, 1991, 7, 4, 332.

10. F. Rodriguez, Principles of Polymer Systems, 4th Edition, Taylor and Francis, London, UK, 1996, 451. 11. S.W. Shang and R.D. Kamla, Journal of Plastic Film and Sheeting, 1995, 11, 1, 21. 12. P.J. Carreau, A. Ghaneh-Fard and P.G. Lafleur, Proceedings of ANTECH ‘98, Atlanta, GA, USA, 1998, Volume 3, 3598. 13. A. Ghaneh-Fard, Journal of Plastic Film and Sheeting, 1999, 15, 3, 194. 14. K.R.J. Osborn and W.A. Jenkins, Plastic Films: Technology and Packaging Applications, Technomic, Lancaster, PA, USA, 1992, 141. 15. V. Firdaus and P.P. Tong, Journal of Plastic Film and Sheeting, 1992, 8, 4, 333. 16. E. Foldes, A. Toth, E. Kalman, E. Fekete and A. Tomasovszky-Bobak, Journal of Applied Polymer Science, 2000, 76, 10, 1529.

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Handbook of Plastic Films 17. R. Gachter and H. Muller, Plastics Additives Handbook, Carl Hanser, Munich, Germany, 1985, 97. 18. G. Pritchard, Plastics Additives: An A-Z Reference, Chapman and Hall, London, UK, 1998, 633. 19. J.D. Stepek and H. Daoust, Additives for Plastics, Springer-Verlag, Berlin, Germany, 1983, 243. 20. S. Amos, Modern Plastics, 2000, 77, 10, 131. 21. D. Ferrer-Balas, M.L. Maspoch, A.B. Martinez and O.O. Santana, Polymer, 2000, 42, 4, 1697. 22. W.D. Harris, C.A.A. Van Kerckhoven and L.K. Cantu, Proceedings of ANTEC ‘91, Montreal, Canada, 1991, 178. 23. K.P. Menard, Dynamic Mechanical Analysis, CRC Press, Boca Raton, FL, USA, 1998, 61. 24. M.M. Jaffe, J.D. Menczel and W.E. Bessey, in Thermal Characterisation of Polymeric Materials, Ed., E.I. Turi, Academic Press, New York, NY, USA, 1997, 1956. 25. V.B.F. Mathot in Calorimetry and Thermal Analysis of Polymers, Ed., V.B.F. Mathot, Carl Hanser, Munich, Germany, 1993, 231. 26. J.J. Janimak and G.C. Stevens, Thermochimica Acta, 1999, 332, 2, 125. 27. W. Klopffer, Introduction to Polymer Spectroscopy, Springer-Verlag, Berlin, Germany, 1984, 53. 28. G. Loeber, Kunststoffe Plaste Europe, 1999, 89, 12, 33. 29. J.J. Wooster and B.A. Cobler, Tappi Journal, 1994, 77, 12, 155. 30. F. Martinez and N. Barrera, Tappi Journal, 1991, 74, 10, 165. 31. O.J. Sweeting, The Science and Technology of Polymer Films, Wiley-Interscience, New York, NY, USA, 1971.

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2

Processing of Polyethylene Films Amin Al-Robaidi

2.1 Introduction A plastic is solid in its unprocessed and processed states. It is softened enough through the application of a combination of heat, pressure and mechanical working to be formed into a variety of shapes such as car bumpers, containers and plastic films. Most thermoplastic polymers are linear polymers. However, these chains twist and turn around to form a tangled structure [1]. Commodity polymers are low in cost and high in volume. They are used to a great extent in our day-to-day lives [2]. Worldwide usage is enormous and is increasing year by year. Rates of increase up to 5% per year are not unusual. Of these commodity polymers, polyolefins play the largest role, and therefore their production is of great importance. Polyolefins represent about 45% of total plastic usage: linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) make up 51% of total polyolefin use, the rest being 23% polypropylene (PP) and 26% high-density polyethylene (HDPE). In this chapter, some of the important properties, production techniques, and chemical and physical phenomena that are of interest for polyolefins will be explained. Polyethylenes are made via free-radical and Ziegler-Natta (ZN) processes. The freeradical method, usually at high pressures (up to 3500 atm) and high temperatures (up to 300 °C), is the older of the two methods. The ZN processes usually have much milder conditions. Recent developments use metallocene catalysts to produce tailormade polyolefins. The common means of distinction between the various types of polyethylene is by density. LDPE and LLDPE have a low density below 0.94 g/cm3, whereas that of HDPE is above 0.94 g/cm3. The density is influenced by polymer structure. The properties of each type are given in Chapter 1. LLDPE for film application is characterised by the different processes used to produce the film and also by end-use application. The processes used to produce the film are determined by the molecular weight (molar mass) of the resin. For example, typical melt index ranges of 0.5-2 g/10 min are suitable for blown film, 2-6 g/10 min for slot cast film, and 5-12 g/10 min for extrusion coating.

41

Handbook of Plastic Films A disadvantage of LLDPE resins is the need to modify existing and available extruders at the processor site to convert from LDPE to LLDPE products, because of the higher shear viscosity of LLDPE with narrow molecular weight (molar mass) distribution (MWD). Owing to its different shear and extensional rheology, LLDPE extrusion and processing conditions differ from those for LDPE. To optimise its processing, different screw designs have been developed and air rings have been modified according to the different extensional rheology.

2.2 Parameters Influencing Resin Basic Properties Polyethylene resins are generally characterised by three parameters: (1) Molecular weight distribution (measure of processing ease and product properties), (2) Melt index (measure of molecular weight) and (3) Density (measure of branching or rigidity). These three parameters are considered to be intrinsic properties of polyethylene. The following section discusses each of the basic parameters influencing resin properties.

2.2.1 Molecular Weight (Molar Mass) and Dispersity Index In any polymer, there is a distribution of chain lengths or molecular weights. Accordingly, the molecular weight of polyethylene is not a uniquely defined quantity, but instead depends on what averaging formula is used. The reader is asked to consult any textbook on polymer science regarding this item [3, 4].

2.2.2 Melt Index (Flow Properties) The melt index (MI) is an inverse measure of the length or average size of polyethylene chains. For a given class of polyethylene, MI can be used to estimate the molecular weight. By ASTM definition, the melt index is the weight of molten material (in grams) extruded in 10 min through an orifice at 190 °C under an applied stress of 2.16 kg, which means a stress of 303 kPa. Only under these conditions is the measurement defined as the melt index. Measurement under different loads is possible in connection with international standards, sometimes reported as I2 (flow rate under specific load). Flow rates under defined loads and other conditions are referred to as flow indices. The common flow index measured at 190 °C and 690 kPa or I5 (flow rate under a load of 5 kg) is used

42

Processing of Polyethylene Films for higher molecular weight resin, usually HDPE grades. The common flow index for all grades is the I21 (HLMI = high-load melt index) measured at 190 °C and 3034 MPa. In practical terms, the melt index is a guide to the relative level of properties of a material and the relative ease of flow that can be expected from the resin during fabrication [5, 6]. The melt index is inversely related to the molecular weight. As the molecular weight increases, the melt index decreases, and vice versa. Since the strength characteristics of polymers are related to the molecular weight, then melt index can be used as an indicator of polymer strength. With an increase in melt index, the tensile strength, tear strength, stress cracking resistance, heat resistance, weatherability, impact strength and shrinkage/ warpage decrease. The modulus of rigidity remains relatively unaffected with melt index increase For HDPE, the increase in melt index improves the gloss but has relatively little effect on transparency [4, 7]. With an increase in melt index, the ease of processing also increases if all other parameters (such as molecular weight distribution) are held constant.

2.2.2.1 Melt Flow Blend Relationship It is often of interest to calculate the melt flow (MF) of a blend of resins whose individual MF values are all somewhat different. The literature contains a number of equations for calculating the viscosity of blends. When blending polyethylene resin, the most frequently used equation is the arithmetic average of the logarithms (ln or log) of the melt indices, which is obtained as follows. Given two polymers, A and B, of the same type and with weight-average molecular weights Mw(A) and Mw(B), respectively, the weight-average molecular weight of the mixture is defined as: Mw(mix) = xAMw(A) + xBMw(B)

(2.1)

where x = weight fraction and so xA + xB = 1. Then the melt index equation for a blend of two samples A and B is given by: ln MI = xA ln MIA + xB ln MIB

(2.2)

The melt flow ratio can either be I21/I2 or I21/I5, where the former is typical for injection moulding and low-density film grades and the latter is typical for higher molecular weight HDPE film, blow moulding and pipe grades. The melt flow ratio is a rough indicator of the molecular weight distribution and the shear-thinning characteristic of the polymer. The higher the melt flow ratio, the broader the expected molecular weight distribution, with the accompanying increases in shear-thinning behaviour. The melt flow ratio has a correlation with the molecular weight distribution, but it is not a purely linear relationship over a broad range [7-9].

43


2.2.3 Density Density can be taken as a measure of the crystallinity of polyethylene. Since branching of the macromolecular chain affects the solid-state structure or crystallinity of polyethylene, density is also an indicator of chain branching. In this relation, we have to differentiate between short-chain branching (SCB) and long-chain branching (LCB). Long-chain branches are mainly present in LDPE, whereby short-chain branches predominate in HDPE and LLDPE. Polyethylene (PE) in its solid state is a semicrystalline material. To first approximation, its structure may be represented as a two-phase composite: a mixture of a hard crystalline phase (density ~1 g/cm3) and a soft noncrystalline amorphous phase (density ~0.84 g/cm3). The actual composite density depends on the relative amounts of crystalline and amorphous phases. Because short-chain branching disturbs the regularity of the PE chain, the crystallisation process is hindered and, accordingly, the crystallinity of the solid-state structure is reduced [9]. Thus, with increasing frequency of chain branching, the crystallinity and density decrease. HDPE, with little or no chain branching, has a density of approximately 0.94-0.97 g/cm3 and a crystallinity by weight in the range 60-85%. LDPE and LLDPE, with more and longer-chain branches, have a density of 0.9-0.93 g/cm3 and crystallinity by weight of 40-60%. The density range can by classified as shown below into four categories in accordance with ASTM D-1248 [10]: (1) Type I (low density)

0.910-0.925 g/cm3

(2) Type II (medium density)

0.926-0.940 g/cm3

(3) Type III (high density)

0.941-0.959 g/cm3

(4) Type IV (very high density)

0.960-0.995 g/cm3

Density is typically measured using a density gradient column. The specimen could be a pellet or a piece of a plaque produced under controlled cooling conditions. The rate of cooling affects the crystallinity, and this will affect the density. A plaque quenched in icewater will have a lower density than a slowly cooled plaque. Boiling a slowly cooled plaque will increase the measured density. As a result of the faster cooling employed, a fabricated pellet will typically have a density (if unmodified by inorganic additives) lower than the ASTM density [9, 11]. The density measured is strongly affected by the addition of inorganic additives with higher density, such as antiblock agents for film-grade resins and other fillers. When blending materials, the specific volume should be arithmetically averaged: 1/ρ = x1(1/ρ1) + x2 (1/ρ2)

(2.3)

where x is the weight fraction, ρ is density, and 1 and 2 represent samples 1 and 2.

44

Processing of Polyethylene Films For PE blends, since the densities are to some extent similar, arithmetically averaging the densities themselves is usually a good approximation. Increasing the density, with all other parameters held constant, will increase the shrinkage, modulus of elasticity, yield strength, heat resistance, gloss for HDPE, permeability resistance and hardness. An increase in density will increase the impact strength, stress cracking resistance, transparency and tear resistance. For HDPE resin, an increase in density will increase the tensile strength, and with increase in SCB, the tensile strength will be lower. Weatherability will remain relatively unaffected.

2.2.4 Chain Branching Polyethylene produced with the high-pressure technology contains both long-chain branches (100-200 or more carbon atoms) and short-chain branches. Polyethylene produced with low-pressure technology has only short-chain branches (1-20 or so carbon atoms). The two types of branching have different effects. For example, LCB has a significant effect on melt rheological (flow) properties, whereas short-chain branching (SCB) has no measurable effects. These differences lead to the definitions of ‘branched’ PE, which contain LCB, and ‘linear’ PE, which essentially do not have any LCB. LCB affects the following melt flow properties: (1) Extensional viscosity (‘strain hardening’), (2) Shear viscosity (‘shear thinning’) and (3) Elasticity (first normal stress differences). LCB also affects the solid-state properties due to its influence on the melt flow properties. Owing to its tendency to strain-harden, the presence of LCB can lead to orientation effects, and those remain in the resultant solidified PE. This is seen in the blown film area. SCB, in contrast, has little effect on melt flow properties but plays a major role in the solid-state properties. SCB will affect the density, modulus of elasticity, heat of fusion, optical properties, impact resistance, tear resistance and melting point. While branch frequency (BF) is undoubtedly the most important SCB parameter, branch length and branch distribution also have important effects. It is well known that, at constant SCB frequency, the longer the branch, the lower the density. The density difference decreases as the density of PE approaches the HDPE region. Homogeneous SCB in the situation in which the branch frequency is the same for all PE chains and, moreover, the branches are randomly distributed along the main chain. Heterogeneous branching refers either to branch frequency varying from molecule to molecule or to a non-random distribution of branches along a given PE chain. At constant

45

Handbook of Plastic Films overall branch frequency, the more heterogeneous the branching, the higher the density. In high-pressure PE, SCB is homogeneous while LCB increases as the molecular weight increases [9, 12]. This distribution can be measured by separating the polymer into fractions using chromatographic techniques. Temperature-rising elution fractionation (TREF) is a method widely used nowadays to determine the comonomer and branching distribution in polyolefins. The technique used to measure the distribution is carbon-13 nuclear magnetic resonance (13C NMR). It gives the sequence distribution along the chain, but averages the distribution among chains. SCB can be detected using Fourier transform infrared spectroscopy (FTIR) or 13C NMR. SCB can be measured using three different units: frequency per 1000 backbone carbon atoms, weight per cent (wt%) or mole per cent (mol%). If SCB is reported in frequency per 1000 backbone carbon atoms, it can be converted using the fact that each monomer or comonomer contributes two carbon atoms to the backbone. Thus, there are 500 monomer/comonomer units making up the 1000 backbone carbon atoms.

2.2.5 Intrinsic Viscosity The viscosity of a polymer solution or polymer melt, in general, is a measure of the fluid’s resistance to flow. Intrinsic viscosity is a measure of the hydrodynamic volume of a polymer molecule in solution. It is sensitive to molecular weight conformation as well as molecular size [9, 12-16]. For a given set of polyethylene resins produced with the same process, intrinsic viscosity can be used as an indicator of the average molecular weight of the polymer. Because intrinsic viscosity is affected by molecular conformation, LDPE (with their long-chain branching) will assume a hydrodynamic volume in solution smaller than that of a linear PE of equivalent molecular weight. PE produced by different processes will have different linear relationships between ln(melt index) and intrinsic viscosity. The molecular weight distribution may be a factor as well. The dilute solution viscosity test defines what is known as the viscosity-average molecular weight. Intrinsic viscosity (dl/g) measurements are normally made by using a three-point zeroconcentration extrapolation. Because it is a zero-concentration measurement, it is independent of concentration; the intrinsic viscosity is dependent on the solvent used in the test. Measurements of PE dilute solution viscosity have been made at 140 °C and at concentrations of 0.1, 0.2 and 0.3 g polymer/100 cm3 decalin.

46

Processing of Polyethylene Films More recent measurements have been made at temperatures higher than 140 °C with trichlorobenzene (TCB) at concentrations ranging from 0.025 up to 0.25 g polymer/ 100 ml solvent. The latter set of conditions (TCB, 140 °C) corresponds to the more typical size exclusion chromatography (SEC) conditions used to determine the molecular weight of PE.

2.2.6 Melting Point and Heat of Fusion As the temperature rises from ambient temperature, the properties of a PE resin change from solid to viscous-like material. Some of these changes are due to facilitated plastic deformation of the crystalline phase. The most part, however, is due to melting of the crystalline phase. The melting temperature of PE lamellar crystallites decreases as lamellar thickness decreases, or as crystallite internal perfection decreases (e.g., due to the partial incorporation of short branches). Since lamellae vary in thickness and perfection, the melting of PE occurs over a range of temperatures. The melting point of PE resin is to some extent a matter of definition. The most popular definition is in terms of a specified point on a differential scanning calorimeter (DSC) trace. The apparent melting point will vary with the method of measurement: mechanical response, dilatometer, X-ray scattering, light scattering or calorimetry. DSC is the most frequently used method for PE. The apparent melting point decreases with increasing short-chain branch frequency (decreasing density), increasing short-chain branch homogeneity, decreasing crystallisation temperature (faster cooling) or decreasing short-chain branch length. In the first three cases, the effect is due to decreasing lamellar thickness. In the last case, the effect is due to decreasing internal perfection of the crystallites. In practical terms, the melting point can be used as an indicator of the density for a given PE type. While LDPE may have a melting point of ~120-127 °C, HDPE resins have melting points in the range of 129-135 °C. The heat of fusion, ΔHf, is the amount of energy required to accomplish the solid-tomelt phase transition. The heat of fusion is linearly proportional to the density. In theory, if the heat of fusion of 100% crystalline PE is known, the heat of fusion of the crystalline fraction of the PE sample can be determined by taking a ratio of the sample to 100% crystalline. In calculating the per cent crystallinity of a sample, a value (average of five literature values) of 68.4-69.2 cal/g (286.6-299.0 J/g) is used as the heat of fusion of 100% crystalline PE.

47


2.2.7 Melt Properties – Rheology For convenience, flow can be classified into shear flow or extensional flow. Shear flows are those in which the velocity component varies only in a plane normal to its flow direction. Shear rheology plays an important role in PE extrusion. Extensional flow, in contrast, is characterised by a velocity component that varies only in its own flow direction. Elongational viscosity is important in film blowing, for example. It is generally not possible to predict the extensional rheology of polyethylene from its shear flow results or vice versa [9, 10]. The viscosity is reported versus either the shear rate or the extension rate.

2.2.7.1 Relation of Viscosity/Shear Rheology In the melt of a high molecular weight polymer like PE, under equilibrium conditions, the chain molecules are not stretched out from chain end to chain end, but instead we assume the so-called ‘random-coil’ configuration, i.e., the contour of a chain resembles a three-dimensional random walk. The random-coil molecules are mutually interpenetrating, so that the environment of a given molecule consists mostly of segments of other molecules. These segments of other molecules collectively form a kind of ‘tube’ in which the molecule of interest is ensheathed. Flow of the polymer melt involves both the axial motion of molecules through their respective tubes (‘reptation’) and the lateral translation of the tubes themselves [7-9]. These processes obviously require the cooperative motion of large numbers of chain segments, which helps to explain why the viscosity (or resistance to flow) of polymer melts is orders of magnitude higher than that of small liquid molecules of similar chemistry. Another difference between a polymer melt and a small liquid molecule is that the viscosity of PE melts depends on the flow rate [5, 6]. In particular, the shear viscosity of PE melts decreases with increasing shear rate, a phenomenon known as ‘shear thinning’. Generally speaking, PE with a broad MWD shear-thins to a greater extent than PE with a narrow MWD. Shear thinning is of particular importance in the various extrusion processes (film, sheet, pipe, tubing, profile, blow moulding) as it permits easy flow through the extruder and die together with shape retention or sag resistance outside the die. Two polyethylenes that have the same melt index may be processed differently at the higher shear rates encountered in fabrication steps depending on their level of shear thinning. PE viscosity decreases with increasing temperature or decreasing molecular weight. For linear PE over a wide range of MWD, the zero-shear viscosity (i.e., the shear viscosity at the limit of zero shear rate) is proportional to: exp(E/RT) Mw3.4 where E is the flow activation energy (5-6 kcal/mol), R is the gas constant, T is absolute temperature and Mw is the weight-average molecular weight. 48

Processing of Polyethylene Films At constant MWD, the effect of increasing temperature or decreasing molecular weight is to shift the logarithmic viscosity versus shear rate vertically towards lower viscosities, and also horizontally towards higher shear rates [13-15]. The vertical shift is the same as for the zero-shear viscosities. For a temperature change, the horizontal shift has the same magnitude as the vertical shift. For a molecular weight change, the horizontal shift is usually smaller in magnitude than the vertical shift. The zero-shear viscosity of branched PE is higher than that of linear PE at constant ‘coil volume’ (i.e., the spherical volume ‘pervaded’ by a random-coil molecule). This is easily understood in the light of the discussion of melt structure. In the case of a branched PE molecule, the axial reptation of the main chain through its tube is necessarily coupled to the lateral translation of the side chains and their tubes [12]. The effect of this added constraint is to increase the viscosity over that of linear polymer. The viscosity of branched PE is also more sensitive to temperature than that of linear PE (apparent flow activation energy 10 kcal/mol versus 6-7 kcal/mol).

2.2.8 Elongational Viscosity The rheological properties of PE in an elongational type of flow are very different from those in shear flow. This viscosity is measured under more of a ‘tensile’ mode. At particular elongation rates, the apparent elongational viscosity does not even reach a steady-state value within the time required to impose the desire total strain. Furthermore, while the viscosity of narrow MWD linear PE resin may begin to level off within this time period, the viscosities of branched or very broad MWD linear PE resins often take a sharp upturn with increasing strain, similar to a rubber band, which becomes stiffer the more it is stretched [9, 17]. This phenomenon is known as ‘strain hardening’. The rubber band analogy is a good one, because strain hardening is more an elastic phenomenon than a viscous one. Strain hardening is observed more often for PE resin containing long-chain branching. It plays an important role in processes that involve highly elongational flows. In film extrusion, strain hardening limits the draw-down capability of branched or very broad MWD linear PE; however, it compensates by providing enhanced aerodynamic bubble stability in blown film. In extrusion coating and slot cast extrusion, strain hardening provides resistance to neck-in edge waver and draw resonance.

2.2.9 Elasticity Polymer melts are, in general, viscoelastic. That is, regardless of the mode of deformation (shear or stretching) imposed on them, they exhibit both viscous and elastic properties.

49

Handbook of Plastic Films The degree of viscous or elastic behaviour depends on the rate of imposed deformation and the temperature. The molecules in a polyethylene melt can be considered as ‘springs’ that prefer a random conformation, but which, under an applied stress, can become uncoiled and extended. Thus, in a flowing melt, the molecules are partially uncoiled and extended in the direction of flow, thereby achieving a balance between the external stress driving the flow on the one hand and the retractive spring force on the other. Upon removal of the external stress, the melt will rebound as the molecules relax to their equilibrium random-coil conformation. The magnitude of this rebound or recoverable strain characterises the elasticity of the melt. Melt elasticity increases with increasing flow stress, with increasing elongational component of the flow, with broadening MWD and with increasing long-chain branch content. With regard to the last two factors, elasticity is especially sensitive to the presence of a few large and/or highly branched molecules in the system. The die swell percentage (ratio of diameter of extrudate/capillary die x 100%) is an indicator of the elasticity of the polymer melt after having been extruded through the capillary die. It is not the most accurate measurement, however, since it involves the physical measurement of the diameter of the extrudate. Elasticity determines the parison swell characteristics in blow moulding, and the warping characteristics in injection moulding. In PE film, heat shrinkability, optical clarity and mechanical anisotropy are directly or indirectly affected by elasticity.

2.3 Blown Film Extrusion (Tubular Film) 2.3.1 Introduction The most widely used method for film extrusion is the tubular or blown film technique, which accounts for about 85% of all film production. Cast film extrusion is the other major process and accounts for about 10-12% of all polyethylene film extrusion [17]. These two methods (blown film and slot cast extrusion) are described in this section and the next.

2.3.2 Description of the Blown Film Process To make polyethylene film, solid pellets are first dropped from a feed hopper into the extruder barrel, melted by subjecting them to heat and pressure, and the melt conveyed by

50

Processing of Polyethylene Films a rotating screw to the die. After having travelled all along the screw channel, the melt passes through a screen pack and supporting breaker plate and adapter into the die. The screen pack filters all contamination and foreign matter and removes them, before finally the melt is forced through the narrow slit of a die. The screen pack and breaker plate also help to increase the back-pressure in the barrel to improve mixing of the melt [18, 19]. The die might be straight or ring-shaped. The resulting thin film has the form of a tube or ‘bubble’. Coming out of the extruder, the film is cooled and is finally rolled up on a core. As the variety of polymers and blends has steadily increased, and film requirements have become more demanding, extruder and screw technologies have evolved to the point where screws are tailor-made for a specific system and application [19-22]. Figure 2.1 depicts a typical decreasing-pitch screw for LLDPE.

Figure 2.1 Typical decreasing-pitch polyethylene screw

In standard tubular blown film extrusion, the hopper feeds the resin to the screw, which conveys, compresses, shears, melts and pumps the material to the die. In tubular film extrusion, dies are circular in shape and are bottom or side fed (Figure 2.2).

2.3.3 Various Ways of Cooling the Film The resin enters the die, flows around the core pin and exits as a thick-walled tube. While the resin is still in a molten state, air is introduced into the tube through a port in

51


Figure 2.2 Blown film die in different arrangements

the centre of the die to expand the bubble to the desired diameter or lay-flat width. No additional air is required once the bubble diameter has been reached. By introducing air through the die-torpedo into the tube of film, the tube can be expanded to two or three times the diameter of the die. Thus, within limits, many different widths of film can be obtained from the same die. By varying the speed of the rollers closing the end of the bubble (the nip rollers), the amount of longitudinal stretching can be varied, and this is normally used to adjust the thickness of the film to the required value. This process has several advantages: With only one die, a range of film widths and thicknesses can be produced as well [23]. A bag can be produced by only one heat seal and two cuts, and very wide film can be produced (by slitting the tube) with equipment wide enough to handle only half this width. On the other hand, a drawback of the process is that the rate of cooling of the film is rather low, particularly if air cooling is used [23, 24]. At high output rates there are difficulties of controlling bubble movements sufficiently to keep film thickness between close limits. Further, because the film is nipped between two rollers at one end of the bubble, the film temperature at this point must be sufficiently low to prevent ‘blocking’. The use of additives to prevent blocking can allow greater production speeds to be attained, and improvements in haul-off techniques have given better control over the swaying and shaking of the bubble. The cooling rate is becoming the limiting factor on the whole process [25-27]. In this respect the process compares unfavourably with the slot-die process; also, with air cooling

52

Processing of Polyethylene Films it is not possible to use shock-cooling and high extrusion temperatures to effect a reduction in haze, as is possible with other processes. The bubble might be externally cooled by means of an air ring encircling the base of the bubble. Cooling air is uniformly distributed and solidifies or quenches the tube. The collapsing frame serves to collapse the tube into a lay-flat (Figure 2.3), whereupon it enters the nip rolls for final flattening. The nip rolls seal the air in the bubble and draw the film upward from the die. For given extruder output rate and blow-up ratio, the film gauge is controlled by the nip roll speed. It is desirable to have nip rolls of adjustable height to allow a lower die-to-nip distance than is normally used for LDPE extrusion [27]. The reduced height provides greater bubble support and stability, and allows the film to enter the nips when warm, minimising wrinkles from improper bubble geometry and/or melt temperature and gauge nonuniformity. Idler rolls guide the lay-flat from that point to the wind-up rolls, which wind the film on to cores.

Figure 2.3 Air ring cooling the blown film

53


2.3.4 Extruder Size In size, extruders are available from bench-top models for laboratory work up to 500 mm diameter screws. Screw sections and compression ratio have important functions. Commonly, the length-to-diameter (L:D) ratios of screws range from 15:1 up to 33:1. Recent developments consider a mixer at the screw end in the length range of 3D [27, 28]. A number of different mixers are available. The compression ratio is the ratio of the channel volume of one screw flight in the feed section to that in the metering section. A compression ratio of about 4:1 is recommended for film extrusion. High compression forces result in high internal heating, in good mixing of the melt and in pushing back any traces of air carried forward with the melt. Barrel diameter is determined primarily by desired output. Knowing the drive motor speed and the reducer ratio of a given extruder, maximum rpm can be determined. Therefore, the throughput T for any given size extruder can be calculated, based on the following formulae (where D is the extruder diameter and h is the depth of the metering section): (1) Neutral screw • when D and h are in inches T = 1.15D2h lb/h rpm

(2.4)

• when D and h are in millimetres T = 7 × 10–5D2h kg/h rpm

(2.5)

(2) Controlled screw • when D and h are in inches T = 1.4D2h lb/h rpm

(2.6)

• when D and h are in millimetres T = 4.6 × 10–5D2h kg/h rpm

(2.7)

Then it is easy to match dies and extruder capacities [28, 29]. Maximum extruder rpm can be calculated by dividing gear reduction into rated motor speed. Some estimates of extruder size and die sizes based on throughput and cooling capacity are shown in Table 2.1.

54

Processing of Polyethylene Films

Table 2.1 Some estimates of extruder size and die sizes based on throughput and cooling capacity Extruder size

Die diameter

inch

cm

inch

cm

1.5

3.81

up to 4

up to 10.16

2.5

6.35

6 to 12

15.2 to 30.5

3.5

8.89

12 to 18

30.5 to 45.7

4.5

11.43

18 to 28

45.7 to 71.1

2.3.5 Horsepower The horsepower needed to drive the extruder motor is important. Based on a given pound per hour per horsepower (lb/h hp) relationship, the die, extruder and motor can be matched to give an optimum equipment arrangement. The examples in Section 2.3.6 give the pound per hour per horsepower values generally expected for 0.2 and 2.0 melt index resins extruded with neutral and water-cooled screws. Table 2.2 gives the normally supplied drive motor horsepower ratings for several extruder sizes.

Table 2.2 Extruder size versus horsepower relationships Extruder size

Drive motor rating

inch

cm

hp

kW

1.5

3.81

5 to 7.5

3.78 to 5.59

2.5

6.35

20 to 40

14.91 to 29.88

3.5

8.89

60 to 100

44.71 to 74.57

4.5

11.43

75 to 150

55.93 to 111.86

2.3.6 Selection of Extrusion Equipment Selection of blown film equipment should be made in a logical sequence [28]. Before deciding on specific equipment, the type, size and end-use of the film to be produced should be determined. Knowing these variables, selection of proper equipment for a given application is fairly simple. Blow ratio is the tool by which the proper die size can be selected (Figure 2.4). In general, blow-up ratios of 1.5:1 to 3.5:1 are best suited for film production.

55


Figure 2.4 Schematic drawing of the blow ratio of film and the blown film width after the nip rolls have flattened the bubble to a double layer of film

Die size and cooling capacity are the major considerations in the selection of an extruder. The general ‘rule of thumb’ in the industry is to expect a throughput of 5-7 lb/h inch (0.9-1.2 kg/h cm) of die circumference. Die capacity and drive motor horsepower, which depend on several extruder size factors, can then be calculated. Some typical values are given in Table 2.3 and Table 2.4, respectively.

56


Table 2.3 Calculated die capacity depending on melt index and type of screw Die diameter

Output

inch

cm

lb/h

kg/h

4

10.16

75

34.0

6

15.24

110

49.9

8

20.32

150

68.0

10

25.40

190

86.2

12

30.48

220

99.8

16

40.64

300

136.1

Table 2.4 Drive motor horsepower depending on several extruder sizes Melt index

Screw

2.0

Rate lb/h hp

kg/h kW

Neutral

8 to 10

13.1 to 16.4

2.0

Cooled

5 to 6

8.2 to 9.8

0.2

Neutral

5 to 6

8.2 to 9.8

0.2

Cooled

3 to 4

9.9 to 6.6

2.4 Cast Film Extrusion 2.4.1 Description of the Cast Film Process In cast film extrusion, the melt is forced through a flat or slotted die opening, either directly into a cooling water-bath, or tangentially contacting a highly polished, water-cooled chill roll (cf. Chapter 1). The flat film is cooled by two or more of these rolls and is carried by idlers to conventional treating and winding equipment. The film is stretched longitudinally between the die and the cooling-bath or rollers to the required thickness. Chilled steel rollers are preferred to a water-bath if the film contains hydrophilic materials such as antistatic agents, which cause some wetting of the film, with subsequent drying difficulties. Chilled rolls also allow greater production speeds than a water-bath [22, 25-27]. Flat film dies are often very long and heavy to install or to change [18]. Unfortunately, there is no close relation between die opening and film thickness. Generally, high gauges

57

Handbook of Plastic Films require large openings. To produce a film of 25-27 μm thickness, the opening is normally around 0.5 mm. Usually one of the jaws is adjustable by means of screws so that the die opening can be reset, using a brass feeler gauge of known thickness and a torque wrench. Die temperature and resin temperature at the die lands are usually higher than in blown film dies, ranging up to 300 °C. It is important to keep melt temperature uniform. The die always has a number of heating zones. To minimise temperature variation and fluctuations in film quality, the temperature along the die should be kept within very strict tolerances of 1 °C. To avoid film faults and imperfections, the inside die surface as well as the polishing rolls must be kept well polished. Slightly surface irregularities will result in gauge variations and die lines (lengthwise parallel groves). A regular die cleaning schedule will usually prevent faults in the inner surface. For the production of very thin films, an air knife is very commonly used. When the hot film is drawn down on to the first cooled chill roll, it may ‘neck-in’ (shrink) at the edges. Neckin is the difference between the hot melt width at the die lips and the film width on the chill roll. When film with beading is wound up, the roll will sag in the middle, making it difficult to use it later for packaging or bag-making. Therefore, such a film must be trimmed at both edges. Neck-in will not occur in blown film production, since the bubble has no edges that may shrink. Another cast film defect, which has something to do with cooling, is ‘puckering’ (a slight bulging across the film recurring at regular intervals). Running the first chill roll hot may reduce puckering. If the roll runs cold, the film may later warm up in storage and expand, and the roll may become loose. If the melt flows well, there is little danger of severe puckering. The production of packaging bags from flat film requires special machinery to effect sealing of the sides of the bag as well as the bottom. A considerable reduction in haze can be obtained by shock-cooling. This is not possible in the tubular film process except by friction contact with water-cooled metal, with the consequent marking of the film.

2.4.2 Effects of Extrusion Variables on Film Characteristics 2.4.2.1 Optical Generally, with increased stock temperatures, higher gloss and lower haze will be obtained for low-density polyethylene. As the melt becomes hotter or more fluid, molecules will

58

Processing of Polyethylene Films have more time to align themselves and give a smooth film, which is a prerequisite for good gloss and low haze. Other problems such as bubble stability may result, however. In tubular extrusion of high-density polyethylene, the melt temperature increase has a minimal effect on the optical properties of the inherently hazy film [28-31]. When the blow-up ratio (BUR) is increased from 1.5:1 to 3:1, the gloss increases and the haze decreases in low-density polyethylene film, but the effect is negligible in high-density polyethylene film. In slot casting and as is observed in tubular film extrusion, increasing the melt temperature generally improves optical properties, although the degree varies with various high-density polyethylene resins. Increasing film speed generally results in poorer optical properties – increased haze and lower gloss.

2.4.2.2 Haze Haze may be of two kinds: surface roughness caused by melt flow phenomena, and surface roughness and internal optical irregularities caused by crystallisation. The faster the film cools between the extrusion die and the freezing point, the less is the haze resulting from crystallisation, and the greater is the haze resulting from flow. In high-density polyethylene, in which the haze caused by crystallinity is dominant, shock-cooling can be used to produce almost haze-free film. Haze in film was formerly accepted as an unavoidable property of polyethylene, but it has been shown that commercially available polymers of slightly higher density than the normal 0.918 g/cm3 give film of lower haze. The higher density of these materials results in a greater susceptibility to ‘brittle’ tearing, other things being equal, and in greater gloss, clarity and stiffness. In soft-goods packaging, haze in the film dulls the colour of the packaged article as seen through the film, and much of the sales appeal of using a transparent package is lost. Provided adequate toughness is retained, film of the lowest possible haze is required for this market.

2.4.2.3 Gloss Since it has been found that small increases in the density of polymers could result in better gloss, the packaging market has demanded this property in order to add sparkle to the package. However, it has been suggested that too much gloss may destroy some of the appeal of a package by reflecting too much of the shop’s lighting and of the variously

59

Handbook of Plastic Films coloured goods in the vicinity. A reduction in chill roll temperature generally improves both haze and gloss in high-density polyethylene films.

2.4.2.4 Impact Strength To have good impact strength, a balance of orientation between machine direction (MD) and transverse direction (TD) molecular structure is needed. Therefore, an increase in blow-up ratio tends to balance this orientation in both directions. As the stock temperature is increased, impact strength increases and a better balance of orientation (machine direction versus transverse direction) results. High-density polyethylene has a greater tendency than low-density polyethylene to orient and show large machine direction versus transverse direction differences, i.e., splittiness. The toughness of film is its most important feature in the packaging of agricultural products (potatoes, carrots, etc.) and building applications. For these uses, the haziness of the film is not of great importance, and, as toughness and haze are to a certain extent mutually exclusive, hazy film has been accepted. The tensile strength of the film is, as expected, mainly dependent on melt flow index, but the apparent brittleness depends mainly on the density of the polymer: the higher the density, the more brittle is the film. Brittleness also depends partly on the extrusion conditions of the film. Brittleness of film should not be mistaken for the kind of brittleness encountered in mouldings. Examination of a ‘brittle’ tear does not show a sharp fracture but reveals a narrow edge of stretched film. A ‘brittle’ film requires much less energy to tear than does a ‘non-brittle’ one because a smaller area of film is stretched before the tear occurs. If the stretching is highly localised, a ‘brittle’ tear occurs; if the stretching is distributed over an area, a ‘non-brittle’ tear results. ‘Brittle’ tears are more likely to occur if the tearing stress is applied at high speed, such as in an impact, and film is normally tested under such conditions. As the density of polyethylene increases, so also does its rigidity. Film made from HDPE is stiffer in both handling and appearance. In over-wrapping machines designed to handle stiff ‘Cellophane’ or paper, stiffness exceeding some minimum value may be essential. Increasing the melt temperature in the slot casting process decreases tensile strength in the machine direction, but produces film with more balanced orientation and a resulting increase in impact strength. Impact strength is increased by a reduction in chill roll temperature in the slot casting process.

60


2.4.2.5 Blocking Although blocking is not strictly a property of the film itself, it is one of the most serious limitations on the high-speed production of film by the blown extrusion process. For higherdensity polymers, blocking is a less serious limitation, because the increased rigidity of the film prevents the intimate contact obtained between flexible films. In contrast, in low-density polyethylene extrusion, increased stock temperature can sometimes cause blocking. Excessive film temperature during the collapsing of the tube will cause the inside surfaces to stick together.

2.4.2.6 Bubble Stability As the blow-up ratio increases, the blown tube becomes larger and nonuniformities in polymer flow from the die and cooling air are magnified. With increasing blow-up ratio, the increased surface area that results is more susceptible to draughts. These external forces tend to make the bubble waver and cause wrinkling and poor gauge.

2.4.3 Effect of Blow-up Ratio on Film Properties 2.4.3.1 Optical Up to a certain point as the frost-line is increased, gloss increases and haze decreases. However, beyond this point, the gloss decreases and the haze increases. The increase in gloss and decrease in haze from increased frost-line height occur because the molecules tend to become better oriented and do not allow molecule ‘ends’ to protrude, which would give a ‘rough’ surface. Too long cooling time gives rise to internal haze inherent in polyethylene.

2.4.3.2 Impact Strength Impact strength is associated with molecular orientation. Thus, on increasing frost-line height without increasing the blow-up ratio, the orientation in the machine direction increases. This ‘one-direction’ orientation allows the molecules to line up or orient, thereby producing a film that is ‘splitty’ and poor in impact strength.

2.4.3.3 Bubble Stability Bubble stability can decrease as stock temperature is increased. Instability results when the bubble (hot melt) lacks stiffness to resist the forces exerted by the cooling air and draughts.

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Handbook of Plastic Films Increased frost-line height produces a tube with a longer length of molten or hot material. This soft material is very susceptible to wavering because of lack of strength, draughts or variables such as nonuniform cooling. In order to maintain the stable bubble necessary for good gauge, low frost-lines are desirable.

2.4.3.4 Puckering Puckering is an expression used in the slot casting process. Puckering is caused by a nonuniform frost-line on the first chill roll, which produces density variations in the machine direction. These density differences cause length variations that appear as small bags, or puckers. Increasing the melt temperature decreases the tendency for puckering, while reduction in chill roll temperature increases the puckering tendency [32, 33]. The draw distance (distance between die and chill roll) affects the film properties. Optimum draw distance for good film production varies with equipment size and production rates, and must be found experimentally. In general, optimum draw distances may range from 1-2 inch (2.54-5.08 cm) in laboratory equipment to over 12 inch (30.48 cm) in commercial equipment.

2.5 Processing Troubleshooting Guidelines Economical film-making means the production of high-quality film in long trouble-free runs at the highest possible production rate. Since certain changes in machine conditions may improve quality while decreasing output, or vice versa, it is frequently necessary to find some kind of compromise between the two goals – high output and superior quality. In Table 2.5 some guidelines are given for machine operators to check their machine(s) and product(s) periodically to prevent unnecessary trouble during film production.

2.6 Shrink Film Shrink-wrapping use is growing very fast worldwide and especially in Europe, both for light/small articles and for heavy/huge pallets. Low-cost polyethylene shrink film is produced on conventional equipment, by the blown extrusion process. No additional machinery is needed. Only processing conditions and resin characteristics need to be properly selected, according to the film’s application. The film shrinks because a high degree of molecular orientation or internal stress was introduced into it during its manufacture. These stresses are ‘frozen’ in the film by the air cooling. When the film is

62


Table 2.5 Processing troubleshooting guidelines Problem

Possible cause and/or solution

Poor gauge

1.

Poor die and/or air ring design

2.

Die and/or air ring need adjustment

Poor optics

Wrinkles

Blocking

3.

Air draughts

4.

Dirt in air ring

5.

Temperature or resin change

1.

Improper resin for property desired

2.

Raise extrusion and die temperatures

3.

Raise frost-line height

4.

Increase blow-up ratio (2, 3, 4 can only be accomplished within limits of bubble stability and blocking)

1.

Poor gauge control

2.

Misaligned rolls or collapsing tent

3.

Air draughts

4.

Unsupported film length too great

5.

Too much static build-up in film; use static eliminators

6.

Film slip is too low

7.

Film too stiff

8.

Keep a level frost-line

1.

Lower melt temperature

2.

Reduce cooling air temperature

3.

Increase air flow

4.

Increase distance from die to pinch roll if possible

5.

Use additional cooling rings

6.

Reduce primary nip roll pressure

7.

Reduce take-off speed

8.

Treatment too high

9.

Wind-up tension too high

10. Improve air circulation in area of nip rolls 11. Resin may need more antiblock

Low impact

1.

Is the correct resin being used

2.

Increase blow-up ratio

3.

Lower melt temperature

4.

Lower frost-line height

5.

Reduce die opening

6.

Eliminate die and weld lines

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Problem

Table 2.5 Processing troubleshooting guidelines continued Possible cause and/or solution

Low impact

Film imperfections (holes, tear-offs, tube collapse, apple sauce, fish eyes)

Bouncing

Bubble diameter control

Poor roll flatness

64

7. 8. 9. 10.

Too high nip roll pressure results in weak crease strength Check gauges for low caliper spots Nip rolls may get too hot Eliminate scratches on film surfaces from collapsing frame, gussets and rollers 1. Dirty screens and/or die 2. Excessive gels in resin 3. Melt either too hot or too cold 4. Improper resin for draw-down required 5. Decrease die opening for improved draw-down 6. Improve homogenisation by greater back-pressure to screw 7. Use water cooling of screw for improved resin homogenisation 8. Improve gauge control 9. Keep resin and scrap clean 10. Check die land surfaces for imperfections 1. Frost-line too high or too low 2. Melt temperature too high or too low 3. Take-off too slow 4. Extruder surging 5. Improper adjustment of air flow from air ring 6. Guide bars too tight 1. Nip rolls not completely closed 2. Nip rolls worn unevenly 3. Air pressure to nip not holding 4. Small pinholes in film 5. Frost-line height change due to melts and/or air temperature change 6. Frost-line height change due to change in air flow from air ring 7. Air leakage into bubble because of leaking valve 8. Air leakage in air lines leading into die for bubble diameter control 9. Extruder surging 1. Die and air ring should be rotating or oscillating 2. Cores should not be out-of-round 3. Eliminate stray air currents around bubble 4. Eliminate wrinkles in film at wind-up 5. Poor gauge control 6. Film temperature at wind-up too hot, resulting in crushed cores


Problem

Table 2.5 Processing troubleshooting guidelines continued Possible cause and/or solution

Drop in roll weight

Insufficient treatment

Overtreatment

1.

Screens becoming clogged

2.

Screw water temperature getting lower

3.

Flow of water to screw greater

4.

Feed hopper beginning to bridge

5.

Take-off speed has increased

6.

Pre-matted polymer in feed section of screw

1.

Take-off speed too great for treater setting

2.

Treater setting too low

3.

Gap setting between electrode and treater roll too great

4.

Dielectric roll (Mylar, Hypalon, etc.), has pinholes or is too thick

5.

Tune the treating unit

6.

Treater bar has too sharp edge

7.

Check for proper spark gap

8.

Check ink batch and how it is being used

9.

Resin may have too much slip

1.

Reduce treater setting

2.

Film not laying flat on treater roll

3.

Dielectric is loose (Mylar)

4.

Gap setting between electrode and treating rolls too small

reheated to a temperature above its softening point (such as in a shrink-tunnel or oven), the molecules tend to revert to their entangled unstrained state. Thus, the internal stresses are released, causing the film to shrink.

2.6.1 Shrink Film Types Two shrink film types are mostly used today (Table 2.6). At present, bi-oriented shrink film is more popular than mono-oriented film. In the future, mono-oriented film is expected to make major gains in the total pallet-wrapping and sleeve-wrapping business. Bi-oriented film will remain for full overwrap of small and medium-sized packages. The most widely used film thicknesses are: •

25-50 μm (thin shrink film) and

•

57-150 μm (heavy-duty shrink film).

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Table 2.6 Shrinkage (%) of the two major shrink film types Type

MD

TD

Mono-oriented

60-80

10-20

Bi-oriented

50-60

30-50

2.6.2 Shrink Film Properties The properties that fully describe a shrink film are the following: (1) Thickness and thickness uniformity, (2) Percent shrinkage (MD, TD), (3) Shrink strength (MD, TD), (4) Tear resistance (Elmendorf), (5) Impact strength (dart drop), (6) Puncture resistance, (7) Clarity (haze, gloss), (8) Slip and antiblock, and (9) UV resistance. Normally a given application needs only some of these properties to be checked. Two examples are shown in Table 2.7.

Table 2.7 Shrink film properties to be checked according to application Display packaging of light articles, with sharp corners

Wrapping of heavy pallets, to be stored in the open air

Thickness

Thickness and thickness uniformity

Percent shrinkage

Percent shrinkage

Clarity

Shrink strength

Puncture resistance

Impact strength

Tear resistance

UV resistance

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2.6.3 The Manufacture of Shrink Film To produce film shrinkable in a given direction, molecular chains must be oriented in the same direction by processing. This orientation is obtained by stretching the film in the required direction. The greater the film stretching, the higher the molecular orientation and hence the shrinkage. Every blown polyethylene film is shrinkable in the machine direction (MD), because it is normally stretched much more in this direction. In fact, from Figure 2.5 it can be seen that a film tube having diameter D1, length L1 and thickness T1, in the molten state, is blown into a film tube having diameter D2, length L2 and thickness T2, in the solid state, at room temperature.

Figure 2.5 Frost-line and blow-up ratio (BUR) in shrink film

These two film tubes must have the same mass. This condition may be written as

ρ2πD2T2L2 = ρ1πD1T1L1

(2.8)

where ρ1 and ρ2 are the film densities at the relative points. These densities are different due to the different film temperatures and pressures.

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Handbook of Plastic Films A normal thin blown film can be stretched to about 9:1 in the machine direction and to only 2:1 in the transverse direction. For this reason such a film will shrink very much in MD and almost nothing in TD. This means that this film is a mono-oriented type shrink film. Usually a thin shrink film is required to be bi-oriented, i.e., with more balanced shrinkage. This condition is achieved by properly regulating the processing conditions shown in Table 2.8.

Table 2.8 Influence of the processing conditions on the shrink properties of LDPE

Shrinkage

Shrink force

MD

TD

MD

TD

Stronger orientation in the machine direction

●

❍

●

❍

Larger BUR

❍

●

❍

●

●●●

❍❍❍

●●●

❍❍❍

Higher take-off speed Neck-type bubble shape

●

●● ● ●●●

Higher frost-line

❍❍❍

●

Production speed

●●●

❍❍❍

Larger die gap speed

●●●

●● ●

Larger length of die gap

❍❍❍

❍❍❍

Thicker film

❍❍❍

❍❍❍

Higher melt temperature

❍❍❍

❍❍❍

❍❍❍

❍❍❍

Higher melt index

❍❍❍

❍❍❍

❍❍❍

❍❍❍

●●● ● ❍ ❍❍❍

●●●

❍❍❍

Strong increase Little increase Fair decrease Strong decrease

To obtain a balanced shrink film, the same amount of molecular orientation must be produced in both MD and TD. First, the stretching ratio in the machine direction (MDSR) and the blow-up ratio (BUR), in other words, the stretching ratio in the transverse direction, should be made equal. This corresponds to putting MDSR = BUR. In practice, the stretching in the machine direction (MD) predominates, due to other factors (shearing suffered by the melt during its passage through the die, bubble shape,

68

Processing of Polyethylene Films etc.). Thus, the BUR needed to obtain balanced shrinkage is somewhat higher than the theoretical value. This corresponds to assuming a slightly higher coefficient (1.1 to 1.2) in the practical formula for balanced shrinkage.

2.6.3.1 Bubble Shape and Frost-Line Bubble shape and frost-line are important parameters controlling the shrinkages in TD and MD. Referring to Figure 2.5, two types of bubble shape can be seen: one with a long neck (continuous line), corresponding to a higher frost-line; the other with no neck (dotted line), corresponding to a lower frost-line. The shape with a long neck gives more balanced shrinkages. This is easy to explain. In fact, for bubbles of this shape, transverse stretching predominates just below the frost-line, where the film is frozen, and no further relaxation can take place. Thus, a high level of molecular orientation in TD is retained in the film.

2.6.3.2 Resin Melt Index Melt index has a remarkable effect on shrink strength and a slight effect on per cent TD shrinkage and on BUR for balanced shrinkages. In particular, the lower the MI, the higher the shrink strength. This means that for heavy-duty shrink film a low MI is preferred. Also, a low MI corresponds to lower BUR for balanced shrinkage and to higher TD shrinkage. The MWD has only a slight effect on shrink temperature. In particular, a film obtained from a resin with narrow MWD (or low swell) shrinks at lower tunnel temperatures. The presence of slip and antiblock additives has no effect on film shrinkage. Ethylene-vinyl acetate copolymers (EVA) with low vinyl acetate (VA) content give shrink films having the following properties: (1) Faster shrinkage (higher tunnel production), (2) Lower shrink temperature, (3) Higher impact resistance (at low temperature), (4) Higher puncture resistance, (5) Lower slip and antiblock, and (6) Higher gas/moisture permeability. These films find application both for light shrink-wrapping and for heavy pallet wrapping.

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Handbook of Plastic Films UV stabilisers are necessary for applications in which the wrapped item (usually a pallet) is stored in the open air. Stabilisers have no effect on shrinkage. They only affect shrink film prices.

2.6.4 Shrink Tunnels and Ovens The shrink-wrapping technique consists of wrapping and heat-sealing the article loosely in the film. The loose film perimeter should exceed the article’s perimeter by no more than 7-10%. The package is then conveyed through a shrink tunnel or into an oven. Many types of heating system are used, the best being that using hot circulating air, because it gives more uniform heating. The most important properties of a shrink film from the point of view of an end-user are the following: (1) Percent shrinkage (MD, TD), (2) Shrink strength (MD, TD), (3) Shrink speed and (4) Shrink temperature. The last two depend on film thickness and on the nature of item to be packed. In fact, both these factors affect the amount of heat required to reach the softening point of the film. Finally, it is worth pointing out that the maximum shrink strength is reached when the film cools outside the oven and not inside. In fact, inside the oven the film shrinks with a small shrink force, because it is hot and soft. When it cools rapidly to room temperature outside the oven, the film shrinks tightly around the article, with a much higher shrink force.

References 1.

Kunststoff-Handbuch IV: Polyolefine, Carl Hanser, Munich, Germany, 1969.

2.

Kunststoff-Handbuch II: Polyvinylchloride, Carl Hanser, Munich, Germany, 1963.

3.

Plastics Engineering Handbook of the Society of the Plastic Industry, 5th Edition, Ed., M. L. Berins, Chapman and Hall, London, UK, 1991.

4.

P.J. Lucchesi, S.J. Kurtz and E.H. Roberts, inventors; Union Carbide Corporation, assignee; US Patent 4,486,377, 1984.

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Processing of Polyethylene Films 5.

J.C. Miller, R. Wu and G.S. Cielozyk, Tappi Extrusion Conference, Hilton Head Island, SC, USA, 1985.

6.

J.C. Miller, Tappi Journal, 1984, 67, 6.

7.

A.V. Ramamurthy, Journal of Rheology, 1986, 30, 2, 337.

8.

A.V. Ramamurthy, Proceedings of the 2nd Annual Meeting of the Polymer Processing Society International, Montreal, Canada, 1986.

9.

J.C. Miller and S.J. Kurtz, Proceedings of the IXth International Congress in Rheology, 1984.

10. ASTM D1248, Standard Specification for Polyethylene Plastics Extrusion Materials for Wire and Cable, 2002. 11. W.A. Fraser and G.S. Cieloszyk, inventors; Union Carbide Corporation, assignee; US Patent 4,243,619, 1981. 12. S.J. Kurtz, T.R. Blakeslee, III and L.S. Scarola, inventors; Union Carbide Corporation, assignee; US Patent 4,282,177, 1981. 13. A.V. Ramamurthy, inventor; Union Carbide Corporation, assignee; US Patent 4,552,712, 1985. 14. A.V. Ramamurthy, inventor; Union Carbide Corporation, assignee; US Patent 4,554,120, 1985. 15. A.V. Ramamurthy, inventor; Union Carbide Corporation, assignee; US Patent 4,522,776, 1985. 16. D.N. Jones in Proceedings of the 1984 Polymers, Laminations and Coating Conference, Tappi Press, 1984. 17. W.H. Darnell and E.A.J. Mohl, SPE Journal, 1956, 12, 20. 18. W.D. Mohr, R.L. Saxton and C.H. Jepson, Industrial Engineering and Chemistry, 1957, 49, 1857. 19. S. Eccher and A. Valentinotti, Industrial Engineering and Chemistry, Industrial Engineering and Chemistry, 1958, 50, 829. 20. B.H. Maddock, SPE Journal, 1959, 15, 383.

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Handbook of Plastic Films 21. W.D. Mohr, J.P. Clapp and F.C. Starr, SPE Technical Papers, 1961, VII, January. 22. L.F. Street, International Plastics Engineering, 1961, 1, 289. 23. B.H. Maddock, SPE Journal, 1960, 16, 373. 24. B.H. Maddock, SPE Journal, 1961, 17, 369. 25. B.H. Maddock, Proceedings of Pressure Development in Extruder Screws International Congress, Amsterdam, The Netherlands, 1960, 139. 26. C. Maillefer, Modern Plastics, 1963, 40, 132. 27. B.H. Maddock, SPE Journal, 1964, 20, 1277. 28. W.A. Fraser, L.S. Scarola and M. Concha, Tappi Journal, 1981, 64, 4. 29. J.C. Miller and S.J. Kurtz, Advances in Rheology, 1984, 3, 629. 30. S.J. Kurtz, L.S. Scarola and J.C. Miller, Plastics Engineering, 1982, 38, 6, 45. 31. J.C. Miller, Tappi Journal, 1984, 67, 6. 32. Film Converting Techniques for Linear Low Density Polyethylene, Union Carbide Corporation, 1985. 33. S.J. Kurtz, Advances in Rheology, 1984, 3, 399.

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3

Processing Conditions and Durability of Polypropylene Films H. Aglan and Y.X. Gan

3.1 Introduction The durability of polypropylene (PP) films under tensile loadings and ultraviolet (UV) irradiation is a very important end-use property. In this chapter, an overview of the structures, synthesis, processing and applications of semicrystalline PP films is introduced. The UV degradation mechanisms and the effect of UV degradation on the durability of PP films are then presented. The functions of different additives in PP films are described. Research findings based on a case study of the durability of several groups of PP films with additives such as UV stabilisers, antioxidants and colouring pigments, (e.g. calcium carbonate), are summarised. In the case study, typical PP film specimens taken from different processing stages are tested to establish the effect of composition and processing conditions on the durability of PP films. Microstructural features of the films are identified and correlated with their durability. It has been found that a lack of proper addition of UV stabiliser and antioxidant agent severely degrades the durability of PP products. UV-degraded PP woven fabrics made from stretched PP films totally lost their load-bearing capability and displayed severely damaged structure with extensive microcracks, voids and dispersed secondary particles. It has also been found that unstretched PP materials have very good durability under static tensile loading. The stress-strain behaviour shows several distinct deformation stages: elastic deformation, yielding and cold flow followed by strain strengthening. The stress-strain relationship of stretched PP films reveals an elastic deformation stage followed by limited plastic deformation from which no obvious cold flow was found. However, stretching results in the drastic decrease in durability of these films. The durability of the stretched films is less than 70%, while before stretching it was about 600%. Calcium carbonate pigment causes a decrease in tensile strength of stretched PP tape, while UV stabiliser does not change the strength and durability of PP films appreciably. A study of the surface morphology of these PP film samples revealed a similar smooth surface with unidirectional texture. Defects in the form of crevices, grooves and intruded particles were found on the surface of PP films with calcium carbonate colouring pigment. PP is a thermoplastic polyolefin polymer [1]. The structure of PP is stereoregular [2-5]. Crystalline PP was invented in the early 1950s by independent groups in the USA and

73

Handbook of Plastic Films Europe. It entered the stage of large-scale production in 1957. Prior to 1950, polypropylene polymer was a branched low molecular weight (molar mass) oil, which had no significant use. After the discovery of polypropylene, obtained from the TiCl3based first-generation catalyst, at the Polytechnic of Milan in 1954, nothing revolutionary happened until the discovery of the active MgCl2-supported high-yield Ziegler-Natta catalysts at the Ferrara Giulio Natta Research Center in 1968. That event was the beginning of the revolution that brought about the creation of the thirdand fourth-generation catalysts. The Ziegler-Natta achievements made the stereoregular polymerisation of PP possible. The fourth-generation catalysts are super-active, and are introducing an innovative and revolutionary new dimension to heterogeneous catalysis. Because of the specific tailored architecture of the catalyst, it is possible to give the catalyst the capability of determining the physical shape of the polymer generated and its external and internal morphology. Thus, the type of specific distribution within a single PP granule can be precisely controlled. This represented a real breakthrough for PP synthesis technology. It was possible to design new, versatile, clean and economical processes to create a new family of materials. PP is a very useful material for various applications because of its good properties and processability in large-scale production by extrusion, injection moulding and casting. Various products can be manufactured from several types of PP, including (1) isotactic, crystalline PP homopolymers, (2) random copolymers and (3) impact or heterophasic copolymers. The advantages of PP are that it is lightweight, unaffected by moisture, fireproof, acid-resistant and possesses high stiffness. Structural PP products can maintain excellent impact capability, high strength, high toughness and good dimensional stability under service conditions. In addition, PP is cost-effective. The principal forms of PP in applications include film and sheet, filament and fibres, pipes, profiles and wire coating. PP has been accepted as a versatile piping material for a considerably long time. The advantages in this application include resistance to chemicals and corrosive media, ease and economy of handling and installation, low friction losses, low thermal and electrical conductivity, high temperature resistance, minimum build-up of soil deposits, and good outdoor durability in all weather. PP piping is used in industrial drainage systems, in the chemical processing industry and in the oil industry for handling salt water and crude oil. PP is also used for internal lining of metal pipes and tanks. PP pipes can withstand temperatures up to 105 °C. Even under pressure, the service temperature can be as high as 90 °C. PP rods are used for the fabrication of prototype and production parts such as gears, spools, pulleys, casters, etc. Various structural elements possessing high strength, toughness and surface hardness have been produced from PP for applications requiring a heat-resistant, noncorrosive material. PP wire and cable coatings possess desirable

74

Processing Conditions and Durability of Polypropylene Films properties such as surface hardness, crush resistance, high softening temperature, low dielectric constant and low environmental stress cracking. PP wire coating can be applied in solid or foamed forms. PP has also found applications in the medical and biochemical fields. The majority of moulded PP used in medical applications is for implantations [6, 7], repairs [8-11], membranes [12] and disposable devices such as syringes [13]. Non-woven fabrics are used in items such as surgical masks and gowns. PP can also serve as a matrix polymer for fabrication of fibre-reinforced composite materials [1420]. In addition, PP can form copolymers and polymer blends [21-25]. PP films are widely used in industry and daily life. Food packaging applications are very good examples. From the early 1960s to now, PP has been the dominant film packaging material in the snack food, bakery and candy (sweets) industries. PP films are heat-sealable and they have the advantages of being grease- and oil-resistant for the protection of the contents. Snack food packaging is the largest single use of PP film. It is used because of its excellent moisture barrier properties, stiffness, gloss, printability and crispness. When needed, special coatings provide excellent oxygen barrier properties as well. Snack foods that are potato-based are adversely affected by UV light, a condition that requires an opaque package. Opaque PP films and/or clear films that are metallised address this need. Snack food packaging consists of more than one layer. At the very least, a heat-seal layer is required to seal the package. Other layers include slip films to facilitate processing through the converting line, oxygen barrier layers for content protection, and adhesive layers to hold it all together. The introduction of highly flavoured snack foods adds still another requirement to the package: fragrance retention. The packaging of food products today requires a sophisticated array of specially engineered films designed specifically for the products they are chosen to protect. Opaque PP films are also used to label soft drinks and other beverages. The films produce an attractive package, do not rip, provide some abrasion resistance, and do not come off when the bottle is chilled, for example, in a refrigerator or by ice-water. Cables, wires and capacitors, widely used in the electronics industry, have made use of special PP films to provide the necessary insulation. Moisture absorption negatively affects the ability of a plastic to provide insulation. Since PP offers low moisture absorption and has inherently good insulating properties, it is ideal for this application. Commercial applications of cast films are growing rapidly. The film is mostly used for packaging purposes [26], e.g., bread wrap, bakery products, bag liners, grocery bags, textiles and miscellaneous wrapping and packaging applications. Other applications include electrical cable wrapping and laminations with substrates such as paper, cellophane and aluminium foil. Oriented PP film, with high mechanical strength in the stretching direction, has found some other important applications. The film is slit,

75

Handbook of Plastic Films stretched in the machine direction and knifed into strands or fibrils. Such types of PP film have been used for weaving and manufacturing of heavy-duty bags, and many other industrial and commercial products [27]. Various tapes and pressure-sensitive labels are also made from PP cast films or oriented films. Cast PP films are also often used as the outer protective layer in diaper (nappy) construction. The purpose of using PP films is to keep the moisture inside the diaper. Recently, cast PP films have been used for manufacturing stationary products, including clear overlays, dividers, photo albums, baseball cards and protector pages. Since PP films can be easily captured and recycled, the environmental concerns surrounding polyvinyl chloride (PVC) have accelerated the use of cast PP films in these applications, many of which were previously served by PVC films. Because of the outstanding combination of cost, performance, excellent physical properties, strong and continuous expansion of process versatility, and environmental friendly processes and materials during manufacturing, use and recycling stages, PP saw an explosive growth in the amount of production in the worldwide market in its early stage. The world market for PP has grown from around 1.5 million tons in the 1970s to about 13 million tons in the early 1990s. The production in 1995 was about 22 million tons and up to 30 million tons in 2000. Following its explosive early growth, the PP business has maintained surprising vigour. Its growth rate for US production has remained above 7% for the past two decades [28]. The recent production of all major plastics and their growth rates are shown in Tables 3.1 and 3.2, respectively.

Table 3.1 Plastic production in kilograms per capita in 1993 [29] World region

PP

LDPE

LLDPE

HDPE

PS

PVC

Total

N. America

10.2

9.8

7.5

13.6

7.2

12.6

60.9

W. Europe

9.7

11.2

1.87

7.8

5.3

12.1

47.97

Japan

15.7

7.0

5.03

8.1

10.9

16.7

63.43

Rest of Asia

1.07

0.65

0.62

0.81

0.54

1.7

5.4

Rest of World

0.94

1.95

0.87

1.15

0.92

2.63

8.46

Total

3.25

3.19

1.55

3.08

2.04

4.39

17.5

PP: Polypropylene LDPE: Low-density polyethylene LLDPE: Linear low-density polyethylene HDPE: High-density polypropylene PS: Polystyrene

76

Processing Conditions and Durability of Polypropylene Films

Table 3.2 Worldwide plastic production growth rate (%) between 1993 and 2000 [29] World region

PP

LDPE

LLDPE

HDPE

PS

PVC

N. America

3.8

2.2

4.1

3.4

2.9

2.2

W. Europe

5.2

1.5

5.5

4.6

3.3

2.2

Japan

3. 1

2.8

7.8

3.8

3.3

2.6

Rest of Asia

11.2

5.9

11.2

12.2

9.8

8

Rest of World

16.1

7.3

20.4

10.9

11.8

8.3

Total

6.9

3.5

9.7

6.1

5.6

4.6

From the results shown in Table 3.1, it can be seen that the production in weight per capita for PP is only slightly lower than that for PVC. The three developed regions, i.e., North America, Western Europe and Japan, consume much more PP than elsewhere. The other areas of the world consume very little of any plastic. Table 3.2 shows that linear low-density polyethylene (LLDPE) has the highest growth rate. PP ranks second and it is one of the fastest-growing plastics in the period from 1993 to 2000. The increase in worldwide PP capacity is shown in Table 3.3 by a comparison of 1994 and 1998 figures [28]. It can be seen that the growth in the developing regions was even more dramatic than that in the three developed regions. Capacity in the ‘Rest of Asia’ region excluding Japan is expected to be the largest producing region in the world.

Table 3.3 Worldwide polypropylene capacity [28] 1994 capacity

1998 capacity

kilotons

% of total

kilotons

% of total

Growth rate (% per year)

N. America

5334

26

6313

23

4.3

W. Europe

5568

27

6618

24

4.4

Japan

2649

13

2979

11

3

Rest of Asia

4250

21

7310

27

14.5

Rest of World

2691

13

4321

15

12.6

Total

20492

100

27541

100

7.7

World region

77


3.2 Structures and Synthesis The propylene molecule is the monomer unit of polypropylene. There are a number of different ways to link the monomer together, depending on the stereo arrangement. Three major factors control the stereoregularity of PP [30, 31]: (1) The first factor is the degree of branching. The molecular chain of PP will be straight (or linear) if the next monomer unit always attaches to the chain end as shown in Figure 3.1(a). If the next monomer may add on to the backbone, this results in the formation of branches, as seen in Figure 3.1(b).

Figure 3.1 Branching Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4. Copyright 1996, Hanser Publishers.

(2) The pendant methyl sequence can also change the stereoregularity of PP. The addition of propylene to the growing PP chain can be regiospecific or non-regiospecific as illustrated in Figure 3.2. We can see that the addition of monomer can be in a head-to-tail manner (Figure 3.2(a)) or in other ways such as head-to-head or tail-to-tail (Figure 3.2(b)).

Figure 3.2 Regiospecificity Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4. Copyright 1996, Hanser Publishers.

78

Processing Conditions and Durability of Polypropylene Films (3) Still another way to control the stereoregularity is the position of the tertiary hydrogen. As shown in Figure 3.3, there exist two possibilities for the arrangement of the tertiary hydrogen. If the propylene monomer is always added in the same stereo arrangement, the alignment of the tertiary hydrogen will be in a same hand way, either right-handed or left-handed (Figure 3.3(a)). Any change in the stereo arrangement of the adding monomer can result in an opposite hand distribution of the tertiary hydrogen (Figure 3.3(b)).

Figure 3.3 Chirality Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4. Copyright 1996, Hanser Publishers.

PP as a commercially used material and in its most widely used form is made with catalysts that produce crystallisable polymer chains. These give rise to a product that is a semicrystalline solid with good physical, mechanical and thermal properties. Another form of PP, produced in much lower volumes as a by-product of semicrystalline PP production and having very poor mechanical and thermal properties, is a soft, tacky material used in adhesive, sealants and caulk products. The first product is often referred to as crystallisable or iPP, while the second type is called noncrystallisable or ‘atactic’ polypropylene (aPP). In addition to the two commonly defined PPs, isotactic and atactic polypropylene, there is an intermediate state, which is defined as syndiotactic polypropylene (sPP). The molecular chain of isotactic PP is linear. The pendant methyl sequence is regiospecific. That is, the addition of propylene to the growing chain is head-to-tail. In addition, the same hand arrangement of the tertiary hydrogen can be found in iPP. The regularity of the isotactic polypropylene allows it to crystallise. The arrangement of carbon atoms in the main chain of crystallised isotactic PP is in the shape of a helix, when it is viewed obliquely from one end. Unlike iPP, sPP results from the consistent insertion of the propylene monomer in the opposite hand from the previous monomer unit. This is a different type of stereoregularity from that of isotactic PP. Syndiotactic PP is not commercially significant. Atactic PP can be produced from any one or more of: the inconsistency in the degree of branching, the change in the pendant sequence, and the non-stereospecificity. The three types of structure of PP are shown in Figure 3.4.

79


Figure 3.4 Three PP structures Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4. Copyright 1996, Hanser Publishers.

Isotactic PP can crystallise in several forms, which have different density of the structural (unit) cell. The α form is dominant. Other forms include the β, γ and mesomorphic (smectic) forms [32]. All of these forms are composed of molecular chains in a helical conformation with a common repeat distance of 6.5 Å. They differ in unit cell symmetry, inter-chain packing and structural order [33, 34]. The structure, conditions for formation, melting behaviour and morphological characterisation of these forms are discussed by Philips and Wolkowicz [32]. The α-form of isotactic polypropylene homopolymer is semicrystalline in nature. As with any semicrystalline polymer, the morphology of α-form iPP exhibits a hierarchy of characteristic scales as shown in Figure 3.5. The macromorphology of PP can be seen on a visual scale in the range of millimetres. The morphology of a gross reactor particle in the as-supplied state is shown in Figure 3.6 at a magnification of x40. The spherical shape of this pure granular PP, as reported in earlier work by other researchers [35-38], can be seen from the top part of the micrograph. However, the skin-core structure [39-43] cannot be so easily identified without using any cross-section slicing. Under carefully controlled optical conditions, such as small-angle light scattering, a spherulite texture is revealed in a finer scale on the order of 1-50 μm. The spherulite structure is built up by smaller blocks and lamellae. At a higher magnification of x500, the lamellar structure of pure PP can be seen, as illustrated in Figure 3.7. These lamellae are composed of crystallographically ordered regions. The molecular chains in the crystalline regions are arranged with specific symmetry, which has been described elsewhere [44-47]. The unit cell of isotactic PP is monoclinic with a monoclinic angle of about β = 99°. The lattice dimensions of the cell are a = 6.6 Å and b = 20.8 Å [46,47]. 80


Figure 3.5 PP morphology at various scales Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p114. Copyright 1996, Hanser Publishers.

Figure 3.6 SEM micrograph of PP granule

81


Figure 3.7 Micrograph of PP showing lamellar structure

The synthesis of isotactic PP has been commercialised through several processes, namely the Spheripol [48], Exxon (Sumitomo) [49], Mitsui Hypol [50], Unipol [51] and Amoco [52] processes. Such processes have been summarised by Lieberman and LeNoir [53]. A flowchart for each process is shown in Figures 3.8-3.12. Because of the application of fourth-generation catalysts, the removal of catalyst and atactic polymer is not necessary. The use of hydrocarbon diluent in liquid or gaseous form is prevented. Thus the yield of the PP products is remarkably increased and the cost of the synthesis of iPP homopolymer

Figure 3.8 Flowchart for the Spheripol process Reproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140. Copyright 1995, Gulf Publishing Company.

82


Figure 3.9 Flowchart for the Exxon (Sumitomo) process Reproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140. Copyright 1995, Gulf Publishing Company.

Figure 3.10 Flowchart for the Mitsui Hypol process Reproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140. Copyright 1995, Gulf Publishing Company.

and/or copolymer is reduced. Among the synthesis processes, the Spheripol process for the production of PP homopolymers or copolymers (as shown in Figure 3.8) has found the widest application. In this process, catalyst components and monomer are fed to a loop reactor for homopolymerisation. The high heat removal capability of the loop reactors 83


Figure 3.11 Flowchart for the Unipol process Reproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 142. Copyright 1995, Gulf Publishing Company.

Figure 3.12 Flowchart for the Amoco process [52]

allows very large outputs. The operating pressure in the synthesis reactors does not require excessive wall thickness or special fabrication techniques because of the small diameter of the loop reactors. Therefore, it is possible to build a large-capacity plant economically.

84


3.3 Film Processing Polypropylene has been processed into films since the beginning of its production. Development of new catalysts and innovation in film processing technology have been a tremendous help in the expansion of this area. Depending on the type of film and the process by which it is made, the resulting film products can be used for various purposes and applications [54]. The melt stability for processing film products is especially important as compared with processing moulded bulk products. In film processing, the melt stability is crucial in preventing rheological changes and maintaining film strength. Additives that control film slippage and antiblocking properties are also critical in the final stage of film processing. The most common process used to produce PP films is the chill roll cast method [55]. Another important process is the tenter frame. The chill roll cast method is for nonoriented film production, and the tenter frame method is for production of oriented films. Blown film processes [56] may also be used for both oriented and non-oriented film production, but they are not widely used.

3.4 Additives Stabilisation agents consisting of phenolic and phosphite antioxidants are usually used to obtain processing stability. The PP polymer for film production also contains other functionalising additives and groups besides the stabilisers. The two main functionalising additives are antiblocking and slip agents. These materials are combined to provide the release of one film from another or from take-off equipment. Since films have large surface areas and may be wound under tension and at high speeds, there can be substantial static charges and compressive forces present between film layers. Thus, an antiblock agent, which is an inert inorganic material, is added to solve these problems. Diatomaceous silica, calcium carbonate, talc, or glass spheres are commonly used antiblock agents [57-62]. Particle size and dispersability are two important characteristics for antiblocks to provide the surface separation effect [63, 64]. Depending on the film thickness, antiblock average particle size can range from less than 1 μm up to 15 μm. Particles or agglomerates larger than 25 μm can appear as defects. In addition, the presence of agglomerates indicates that some of the antiblocking agent was not well dispersed, and the normal concentration may be ineffective. Therefore, the presence of these inorganic antiblock materials at the film surface can affect the film-handling characteristics. Slip agents are generally materials that tend to separate from PP and have some inherent lubricating properties. Since slip agents eventually end up on the film surface, they may increase the haze of the film. The most widely used slip agents for PP films are fatty amides, such as erucamide and oleamide.

85


3.5 Ultraviolet Degradation of Polypropylene 3.5.1 UV Degradation Mechanisms Virgin PP obtained directly from a commercial process is very susceptible to UV irradiation and air oxidation. If stored unstabilised at room temperature, the durability, strength and physical properties of the PP product deteriorate rapidly over a period of weeks or months depending on the physical form, temperature, available oxygen, intensity of UV radiation and other conditions. At elevated temperatures, such as during summer storage, the degradation process can be accelerated. This uncontrolled degradation is exothermic, and the released heat and gases can lead to a further increase in the degradation rate [65, 66]. The oxidation processes of PP are considerably complex and depend on a variety of factors, including oxygen availability, impurities, residual catalyst, crystallinity (or the content of crystallised portion), storage temperature, air pollutants, radiation exposure time, chemical exposure, film thickness, loading or stress conditions in the part, comonomer concentration, and additive type and content. Earlier studies have proven that the degradation of PP can be divided into three steps: initiation, propagation and termination [67]. These steps are briefly described in the following three subsections.

3.5.1.1 Initiation of PP degradation If the PP product is exposed to air, the following reaction proposed by Tudos [68] occurs: RH + O2 → R• + HOO•

(3.1)

where RH stands for the polypropylene molecules and R• is alkyl radical. According to Hawkins [69], UV radiation can assist the initiation of the degradation of PP. In most cases, the products resulting from the oxidation as described in equation (3.1) can remain separate under steady UV irradiation and exposed to air. However, they may also recombine to form a hydroperoxide as shown in the following equation: R• + HOO• → ROOH

(3.2)

3.5.1.2 Propagation of PP degradation The propagation of the degradation, as indicated by Becker and co-workers [67], occurs by a series autoxidation scheme as depicted by the following three reactions:

86

Processing Conditions and Durability of Polypropylene Films R• + O2 → ROO•

(3.3)

ROO• + RH → R• + ROOH

(3.4)

R• + R′H → RH + R′•

(3.5)

Since small free-radical fragments are very active, they may contribute to the propagation of degradation by the following two reactions: RH + HOO• → R• + H2O2

(3.6)

RH + HO• → R• + H2O

(3.7)

It can be seen from the above reactions that the propagation steps cause the radical sites to move, but there is no overall increase in the number of radicals.

3.5.1.3 Termination of PP degradation The last step of the degradation process is termination, where quenching of radicals occurs. The number of radicals can be reduced by combining two radical sites to form a non-radical product. Several reactions may lead to the termination, as described in the following equations [70]: R• + •R′ → R–R′

(3.8)

RO• + •R → ROR

(3.9)

2ROO• → ROOR + O2

(3.10)

2RO• → ROOR

(3.11)

3.5.2 Effect of UV Degradation on Molecular Structure and Properties of PP The degradation of PP, initiated either by UV irradiation or through thermal activation, causes change in crystallisation and melting behaviours of PP [71, 72]. Degradation also leads to chain scission or cleavage, leading to a decrease in the durability of the films. Loss in molecular weight (molar mass) also occurs [73]. There are several types of chain scission found in PP [74]. The most common is a unimolecular scission of carbon- and oxygen-centred radicals. This cleavage produces several products. The products from the carbon-centred radicals are an olefin and a new carbon radical. These products can re-enter the oxidation cycle as PP•, which can be depicted as [67]:

87

Handbook of Plastic Films –CH(CH3)–CH2–CH(CH3)–CH2–CH(CH3)– + R• → RH + –CH(CH3)–CH2–C(CH3)–CH2–CH(CH3)–

(3.12)

Accordingly, the chain scission can be expressed as: –CH(CH3)–CH2–•C(CH3)–CH2–CH(CH3)– → –CH(CH3)–CH2–(CH3)C=CH2 + •CH(CH3)–

(3.13)

The resulting olefin is even more susceptible to oxidation than the original PP with a saturated hydrocarbon structure. If the degradation is initiated by an alkoxy radical, a carbonyl-containing molecule in the form of –CH2–C(CH3)=O, and another carboncentred radical, can be formed. All of these processes lead to an appreciable loss in molecular weight of PP. The reduction in the molecular weight of the PP polymer leads to a change in many of its corresponding properties. One of the most detrimental is the loss of durability and ductility, thus a drastic decrease in toughness of the polymer. In addition, the chain scission will produce products that will tend to cause an increase in the colour of the polymer and the generation of oxygenated compounds, which will adversely affect the durability, strength and physical properties of the final PP products. UV light can accelerate the chain scission processes. In addition, the availability of oxygen and heat are also key factors in the determination of the degradation kinetics. At PP processing temperatures, the degradation reaction rate is extremely rapid. The succeeding extrusion or injection moulding procedure can also result in severe degradation of the PP polymer. In the solid form, PP is a semicrystalline polymer with a crystalline content that is normally between 40% and 60%. The crystalline regions are essentially impervious to oxygen, so the oxidation only occurs in the amorphous region. It has been reported by Mita [75] that the diffusion rate of oxygen is much slower than the reaction rate, so that the oxidation process is basically a surface phenomenon [76]. In most cases, the surface can become dull, crazed, or even powdery. Obviously, unstabilised PP is very prone to oxidation and degradation in the presence of air. Therefore, adding appropriate stabilisers is necessary to convert PP into a durable, useful material.

3.5.3 Stabilisation of PP by Additives Stabilised PP can be obtained by using appropriate additives which can control radical products or potential radicals. There are many stabilisers and UV antioxidants available, and they can be classified into two types, i.e., primary and secondary. Some secondary UV stabilisers in fact also have primary characteristics. The detailed stabilisation mechanisms are still unknown due to the complicated oxidation intermediates.

88

Processing Conditions and Durability of Polypropylene Films Primary antioxidants are those additives that interfere with the oxidation cycle by reacting with the formed radicals and interrupting the cycle. Primary antioxidants are also called radical scavengers. Both hindered phenols (HP) and hindered amines (HA) are effective primary antioxidants. An HP can react with the radical species generated in the initiation and propagation stages of degradation. Specifically, the HP is able to transfer its phenolic hydrogen to the generated radical, causing a non-radical product to be formed. In transferring the hydrogen, the HP itself becomes a radical known as a hindered phenoxy. This is a stable radical that will not abstract a hydrogen from the matrix PP polymer. Hindered phenol enables the radicals to be managed in two different ways. On the one hand, the initial radical species are effectively removed from participation in the propagation steps; on the other hand, the abstraction of hydrogen from the HP prevents another initiating event from occurring on the PP polymer backbone. This step immediately results in at least one less reactive radical being formed. Regardless of the radical being quenched, the overall effect of the HP is to delay the oxidation and, eventually, the degradation of the PP. Hindered phenols are able to terminate more than one radical per phenol moiety. The structure of HP allows the oxygen-based radical to be delocalised to the carbon atom bearing a substituent, forming a quinone-like structure [77]. This is why even a very small amount of HP antioxidant addition can achieve the stabilisation of the degradation for PP. The shortcoming of the phenolic stabiliser is the development of colour. Some of the quinone-like structures that are active in the stabilisation process are also intense colour bodies or colour centres, giving a distinct yellow colour to PP. Even at very low concentration, the matrix polymer of PP demonstrates an obvious colour change. In addition, the interaction between HP and catalyst residues can intensify the development of colour. Further reaction with air pollutants such as nitrogen oxides and sulphur oxides at room temperature results in increase of colour centres. Hindered amines have been used as stabilisers against the oxidative degradation initiated by UV light. More recently, high molecular weight compounds have been shown to be effective as thermal stabilisers [78]. This class of compounds plays an important role in the commercial applications and success of PP. Hindered amines act as radical scavengers through the nitroxyl radical from amines [79, 80]. The stabilisation mechanism of HA can be depicted as: R′–NH (oxidisation) → R′–NO•

(3.14)

R′–NO• + •R → R′–NO–R

(3.15)

From equations (3.14) and (3.15), it can be seen that the active species is not the amine functionality. The species that is active is the nitroxyl radical. The oxidation of the amine leads to the production of nitroxyl groups [80]. The nitroxyl group is regenerative. The process can be ended by cyclic regeneration. Some of the regenerated products may be inefficient as radical scavengers; thus, some of the HA stabiliser is lost during exposure [81].

89

Handbook of Plastic Films The secondary stabilisers decompose hydroperoxides and prevent new oxidation cycles from beginning. This class of compounds is called secondary, because their best performance occurs in the presence of primary antioxidants. When used in PP by themselves, secondary antioxidants do not exhibit any appreciable activity. The value of these compounds comes when they are combined with the correct primary antioxidants. When the appropriate combination is made, a strong synergistic effect results. The commonly used secondary antioxidants can be classified into two categories: one is phosphites, the other is thio compounds. Both the phosphites and thio compounds are synergistic with the hindered phenols because they attack a source of free radicals, the hydroperoxides. They can reduce a hydroperoxide to an alcohol. Consequently, the homolytic cleavage of the ROOH into two radicals can be prevented. Combined with hindered amines, secondary antioxidants are effective in suppressing UV degradation [82]. By absorbing the UV radiation before it has a chance to energise a chromophore, the formation of a radical is prevented. Obviously, it is impossible for the antioxidants to absorb all the UV light, and thus some radicals are still formed. These radicals are subsequently neutralised by the primary antioxidant of hindered amine.

3.6 Case Studies In this work, the UV degradation behaviour of PP films was investigated with emphasis on their durability, strength and surface morphology. Both unaged and UV-degraded woven PP fabrics from stretched, knifed and relaxed film tapes were studied to reveal the degradation effects. Pure PP film and several groups of PP film materials with additives such as UV stabiliser, antioxidant and colouring pigment (calcium carbonate) were characterised. Typical PP material specimens taken from different processing stages were tested to identify the effect of processing conditions on the durability of PP films. Scanning electron microscopy (SEM) was used to study the microstructural features of the films and correlate them with the durability.

3.6.1 Materials and Experimental Procedures 3.6.1.1 Materials and Processes All the PP films used in the current study were first formed using a chill roll film process. Then, several procedures including water-bathing, air-knifing and stretching were applied to obtain PP tapes. The fabric was prepared by weaving. Classification of the test samples taken during processing and for two PP woven fabrics after been exposed to UV degradation for two weeks are given in Table 3.4.

90


Table 3.4 PP film materials taken from different processing conditions and after two weeks of UV exposure Number

Name

Processing condition

1

PP1

After extrusion and water-bathing

2

PP3

After air-knifing

3

PP4

Cut from stretching zone

4

PP7

Stretched and relaxed for weaving

5

J

6

A1

Woven fabrics as sack product UV-degraded PP fabrics

3.6.1.2 Static Tensile Tests Static tensile tests were performed using a Materials Testing System (MTS 810) equipped with a 2100 kN load cell. The specimens were gripped between two hydraulic wedge grips (type 647.10A-01). Static tests were carried out under displacement control condition at a crosshead speed of 1.5 mm/min. The gauge length was 15 mm. All the tests were conducted at room temperature of approximately 25 °C. At least three samples were tested for each material, and the stress-strain behaviour was established based on the average values. The stress was calculated based on the initial cross-sectional area before testing.

3.6.1.3 Microscopic Examination The surface morphology of each film material was examined using a Hitachi S-2150 scanning electron microscope operated at an acceleration voltage of 20 kV. The micrographs were recorded on Polaroid 55 instant films, and the images were captured simultaneously with Quartz PCI Version 3.01 image processing software, and stored for further editing and printing.

3.6.2 Durability-Microstructure Relationship In order to examine the effect of UV degradation on both the durability and surface morphology, comparative studies on unaged and UV-degraded woven PP fabrics were made. The typical stress-strain behaviour of the unaged PP woven fabrics is shown in Figure 3.13. It can be seen from this curve that the relationship between stress and

91


Figure 3.13 Stress-strain behaviour for unaged woven PP fabric material

strain displays almost elastic behaviour in the strain range up to 30%. The calculated modulus based on this linear portion is about 700 MPa. In the strain range from 30% to 40%, the stress-strain relationship is nonlinear. This indicates that plastic deformation dominates the behaviour of the PP woven fabric material in this range. The ultimate tensile strength reached about 200 MPa. After this range, the stress dropped and the specimen failed. A multi-step fracture behaviour was observed due to the mechanical interlocking of the woven fabrics. The durability for this woven fabric is about 63%. The typical surface morphology of woven PP fabrics shows a striped texture, as illustrated in Figure 3.14. Parallel lines along the stretching direction can be seen, in addition to surface defects in the form of longitudinal cracks along the stretching lines. These cracks were only localised in the area where severely deformed material exists. The PP materials raised due to the extrusion and stretching processes exhibit surface crazes. Unlike the unaged PP woven fabrics, the UV-degraded PP woven fabric sample, A1, does not possess any durability and load-bearing capability. It was found that, after aging under intense UV irradiation, the woven fabrics disintegrated. The tensile test specimens prepared from A1 were so brittle that they could not be tested.

92


Figure 3.14 Typical SEM morphology of woven PP fabric material

Figure 3.15 Typical SEM morphology of UV-degraded woven PP fabric material

Microscopic examination of the surface of the UV-degraded PP woven fabric specimen, A1, shows fraying edge strips, broken pieces and fine particles. Such morphological features are shown in Figure 3.15. Numerous microcracks can be easily deposited on the surface. Obviously, severe UV degradation resulted in the formation of these throughthickness cracks. The through-thickness nature of UV-degradation-induced cracks can explain why the fabric material loses its load-bearing capability totally when such a condition of degradation is reached.

93


3.6.3 Durability-Processing Condition Relationship The properties of PP films are dependent on the processing conditions. The effect of crystallinity and orientation of PP crystals on the durability and strength of PP fibres have been studied [83]. Generally, intensive drawing and stretching result in the orientation of PP crystals and decrease the crystallinity of PP. Thus the content of the amorphous portion increases. As indicated by Galanti and Mantell [83], well-oriented PP has a higher durability than that of poorly oriented PP, and stretched amorphous PP displays a higher strength than that of regularly crystallised PP. From the view of engineering design and application, comprehensive studies in this field still need to be carried out. The following section will present the results of the current investigation on the effect of processing conditions on the durability of PP film. Two types of PP film, PP1 and PP7, as defined in Table 3.4, were chosen to investigate the effects of processing conditions. As indicated in Table 3.4, PP1 is the starting film product and PP7 is the tape ready for weaving. This means that these two films were obtained from two extreme conditions. Thus, the change in structure and properties from PP1 to PP7 can reasonably reflect the effect of the entire processing procedure on the durability of PP films. The stress-strain curve for the PP1 sample, extruded and water-bathed from the pure PP material, is shown in Figure 3.16(a). The ultimate tensile strength for this sample was about 37 MPa. The durability was about 500%. The stress-strain relationship displayed a high degree of nonlinear behaviour after an elastic region. The deformation and failure of this material could be divided into five stages. The first stage is elastic deformation corresponding to a strain of approximately 10%. The calculated Young’s modulus for the PP1 sample is about 320 MPa. The second stage is the nonlinear deformation in which both plastic and elastic deformation can be observed. This region corresponds to a strain range from 10% to 20% approximately. The third stage represents yielding. The maximum yield strength was about 31 MPa. Considerably large plastic deformation can be observed after the yield point. The strain at the end of this stage reached 50% with a drop in strength to about 27 MPa. The fourth stage was the cold flow of the PP, which corresponds to the strain range from 50% to 200%. The fifth stage is the strain hardening of the PP material. With further increase in strain, the strength of the material increased about 25%. Following the strain-hardening stage, the material failed catastrophically. The PP7 tape material underwent a series of processing procedures such as knifing, stretching and relaxation. From the typical stress-strain behaviour of the PP7, as shown in Figure 3.16(b), it can be seen that the relationship between stress and strain displays nonlinearity in the strain range up to 20%. The calculated modulus based on the first linear portion of the curve is about 900 MPa. In the strain range from 20% to 33%, the stress-strain relationship is more nonlinear. This indicates that plastic deformation dominates the behaviour of the PP7 tape material in this stage. The ultimate tensile strength reached about 220 MPa. After this range, the stress dropped and the specimen broke. The durability is about 33%. 94


Figure 3.16 Stress-strain behaviour of (a) PP1 and (b) PP7

The difference in the tensile behaviour of these two kinds of PP film materials can be seen clearly by comparison of Figures 3.16(a) and (b). The unstretched PP tape material showed much higher durability than the stretched products. However, its strength is much lower than that of the other. This indicates that stretching followed by knifing can increase the load capability of the PP material and considerably decrease its durability, based on the strain to failure on the stress-strain curve. 95

Handbook of Plastic Films A micrograph taken from the PP1 sample (Figure 3.17) shows raised lines along the extrusion direction. A typical knifed and stretched sample cut from the tape (PP7) was examined. The microstructure is shown in Figure 3.18. Comparing Figures 3.17 and 3.18, it can be seen that the strip size changed from the original width of about 1 mm to about 0.3 mm. In some areas in particular, cracking along the strip lines can be seen on the surface of stretched PP materials.

Figure 3.17 Microstructure of PP1 sample

Figure 3.18 Microstructure of PP7 sample

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3.6.4 Durability-Additive Property Relationship In order to examine the effect of additive on the durability and load-carrying ability of PP films, four different materials were manufactured using four different processing conditions. These PP film materials contain either no additives, a UV stabiliser, a white colouring pigment (calcium carbonate) or a mixed additive (Amoco 100/03). The four processing conditions were designated PP1, PP3, PP4 and PP7 are described in Table 3.4. The stress-strain curves for the four PP1 samples, extruded and water-bathed pure PP material, are shown in Figure 3.19(a). The PP film with Amoco 100/03 mixed additive has the highest tensile strength of 60 MPa, while the specimen of PP1 film with UV stabiliser has the lowest strength of about 35 MPa. The durability for all four materials exceeds 450%. For the Amoco product, this value reached nearly 700%. The stressstrain relationship displayed a high degree of nonlinear behaviour after an elastic region. The deformation and failure of these materials could be divided into five stages. The first stage is elastic deformation corresponding to a strain of approximately 10%. The second stage is the nonlinear deformation in which both plastic and elastic deformation can be observed. This region corresponds to a strain range from 10% to 20% approximately. After this stage, the third stage can be seen. The typical feature in the third stage is yielding. The yield strength for these materials is in the range from 28 to 35 MPa. Considerably large plastic deformation can be observed after the yield point. The strain at the end of this stage reached 50%, with a drop in the strength to about 25 MPa. The fourth stage was the cold flow of the PP, which corresponds to the strain range from 50% to 200%. Again a considerable amount of deformation occurred in this stage. The total strain due to the plastic deformation in the form of cold flow is larger than 150%. The fifth stage is the strain strengthening of the PP1 materials. With further increase in strain, the strength of the materials increased about 25-40%. Following the strain-hardening stage, the material failed catastrophically, and the stressstrain curves displayed a sharp drop. The additive-loaded or unloaded PP3 showed the same stress-strain behaviour as found for the PP1 materials. As shown in Figure 3.19(b), the four PP3 materials also demonstrated several distinct stages of deformation, including elastic deformation, yielding, cold flow followed by strain strengthening. However, both the yield strength and ultimate tensile strength of PP3 materials are a little bit larger than those for the PP1 materials. The typical specimen of PP3 without any additive has a higher yield strength of about 38 MPa, and its ultimate tensile strength is about 62 MPa. The Young’s modulus is approximately 350 MPa. Differences in the durability were found from the stressstrain curves for these PP3 materials with different additives. The durability for all these materials exceeded 400%. The material supplied by Amoco displayed the maximum durability of 700%. The PP3 with white filler also displayed the same durability as that

97


(a)

(b) Figure 3.19 Stress-strain behaviour of (a) PP1, (b) PP3, (c) PP4 and (d) PP7 film materials with various additives

for the Amoco product. The specimens with UV stabiliser kept the same durability. It has the same strain to failure as that for the PP3 material without additive. However, the ultimate tensile strength of the PP3 with added UV stabiliser showed a considerable decrease as compared with the samples without any additives.

98


(c)

(d) Figure 3.19 Continued

The PP4 and PP7 materials underwent a series of processing procedures such as knifing, stretching and relaxation. The four PP4 films were from the stretched zone, while the PP7 films were in the form of tape, and were ready for weaving. From the typical stressstrain behaviour of the PP4 film materials, as shown in Figure 3.19(c), it can be seen that

99

Handbook of Plastic Films three materials – the PP4 without any additive, with UV stabiliser and with white filler – have a similar relationship between stress and strain, while the Amoco product displayed a decrease in both strength and modulus. However, the durability of this film material reached 68%, which is larger than that of the other three films. All the stress-strain curves of the four PP4 materials display considerable nonlinearity in the strain range up to 30%. Beyond this strain range, the stress-strain relationship is more nonlinear. This indicates that plastic deformation dominates the behaviour of the PP4 films. The ultimate tensile strength for these PP4 films is in the range from 275 to 375 MPa. The sharp drop on the stress-strain curves indicated the final rupture of these specimens. The stretched tape materials, PP7, showed a similar nonlinear deformation behaviour under static overloading similar to the PP4 film materials. Nevertheless, both the ultimate tensile strength and the durability of the PP7 materials are smaller than those of the PP4 materials. The stress-strain curves for the four PP7 materials are shown in Figure 3.19(d). The ultimate tensile strength is in the range from about 150 to 275 MPa. The durability was in the range from 20% to about 33%. Moreover, the PP7 samples with calcium carbonate filler show a tendency in decrease in tensile strength. The ultimate tensile strength for this kind of material is about 150 MPa, which is only 60% of that of the other three PP7 materials. The surface morphology of the four groups of PP films with and without additives was examined. Samples from the PP1 group have a unidirectional texture. Parallel lines along the extrusion direction can be seen. These lines are raised materials. The surface morphology for the four PP3 materials was found to be very similar to that of the PP1 materials. Comparing the morphology of PP1 and PP3 with that of PP4 and PP7, it was

Figure 3.20 Micrograph of PP1 containing calcium carbonate pigment

100

Processing Conditions and Durability of Polypropylene Films found that the strip size changed from the original width of about 1 mm for PP1 and PP3 to less than 0.4 mm for PP4 and PP7. The PP films containing colouring pigment show crevices and scratches due to the movement of calcium carbonate particles along the extrusion direction. This is clearly shown in Figure 3.20, a micrograph taken from the surface of the PP1 film with white colouring pigment, calcium carbonate particles.

3.7 Concluding Remarks The durability of PP is highly sensitive to ageing under ultraviolet irradiation. Unaged PP woven fabrics have unidirectional structure and possess good durability and loadbearing capability. The durability is about 60%. The ultimate tensile strength is about 200 MPa. The Young’s modulus reaches 700 MPa. UV degradation causes severe damage to the structure and drastic decrease in durability and strength of PP film materials. The load-bearing capability for typical aged PP woven fabrics was totally lost after two weeks of UV exposure. The surface morphology of the aged PP fabrics was dominated by numerous microcracks. The durability of PP film materials is also sensitive to processing conditions. Without stretching, the films with striped structure demonstrated cold flow followed by strain strengthening. The durability for this material exceeds 490%. The typical specimen of PP1 has a yield strength of 31 MPa, and its ultimate tensile strength is about 37 MPa. The Young’s modulus is approximately 320 MPa. After extensive stretching, knifing and relaxation, the distance between the parallel raised lines decreased, the durability is reduced to about 33%, while the load-carrying capability increased remarkably. Limited plastic deformation without cold flow was found. The sample of the PP7 has a tensile strength of about 220 MPa, and the Young’s modulus is about 900 MPa. The durability of the PP films is not sensitive to the addition of additives. UV stabilisers and antioxidants do not change the durability and morphology of the PP films appreciably. All the PP film materials have a very smooth surface and possess a similar structure with unidirectional raised lines. Calcium carbonate can produce crevices during extrusion, while the durability of the films containing calcium carbonate is almost the same as those without any whitening additives.

Acknowledgements This work was supported by the Egyptian Foreign Relations Coordination Unit (FRCU) of the Supreme Council of Universities/USAID under University Linkage Project II Grant #93-02-16.

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Handbook of Plastic Films 64. F. Stricker, Y. Thomann and R. Mulhaupt, Journal of Applied Polymer Science, 1998, 68, 12, 1891. 65. M.R. Thompson, C. Tzoganakis and G.L. Rempel, Journal of Polymer Science A: Polymer Chemistry, 1997, 35, 14, 3083. 66. T.C. Uzomah and S.C.O. Ugbolue, Journal of Applied Polymer Science, 1997, 66, 7, 1217. 67. R.F. Becker, P.J. Burton and S.E. Amos, in Polypropylene Handbook, Ed., E.P. Moore Jr., Hanser Publishers, Munich, Germany, 1996, 178. 68. E. Tudos in Advances in the Stabilisation and Controlled Degradation of Polymers, Vol. 1, Ed., A.V. Patisis, Technomic Publishing, Lancaster, PA, USA, 1989, 86. 69. Polymer Stabilisation, Ed., W.H. Hawkins, Wiley-Interscience, New York, NY, USA, 1972, 37. 70. E.S. Huyser, Free-Radical Chain Reactions, Wiley-Interscience, New York, NY, USA, 1970, 13. 71. M.S. Rabello and J.R. White, Polymer, 1997, 38, 26, 6379. 72. M.S. Rabello and J.R. White, Polymer, 1997, 38, 26, 6389. 73. M.A. Nesterov, Y.P. Baidarovtsev, G.N. Savenkov and A.N. Ponomarev, High Energy Chemistry, 1998, 32, 1, 42. 74. C. Tzoganakis, Polymer Process Engineering, 1988, 6, 1, 29. 75. I. Mita in Degradation and Stabilisation of Polymers, Volume 1, Eds., H. Jellinek and H. Kachi, Elsevier Applied Science, Amsterdam, The Netherlands, 1983, 277. 76. T. Hirotsu and P. Nugroho, Journal of Applied Polymer Science, 1997, 66, 6, 1049. 77. J. Pospisil, Polymer Degradation and Stability, 1988, 20, 181. 78. P. Gijsman, Polymer Degradation and Stability, 1994, 43, 171. 79. E.T. Denisov, Polymer Degradation and Stability, 1991, 34, 325. 80. J.F. Rabek, Photo-Stabilisation of Polymers: Principles and Applications, Elsevier Applied Science, Amsterdam, The Netherlands, 1990, 318.

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108

4

Solubility of Additives in Polymers Alexander Mar’in

4.1 Introduction Polymer-based materials usually contain, besides the polymer, various low molecular weight (low molar mass) compounds such as stabilisers, plasticisers, dyes, dissolved gases, accidental and technological impurities. During exploitation, polymer materials can come into contact with water, organic liquids, solid substances and foodstuffs, which could result in the transfer of additives and impurities dissolved in the polymer to the surroundings, polluting them and decreasing the lifetime of the polymer. On the other hand, low molecular weight compounds from the surroundings can pass into the polymer. The distribution of additives between polymers and surroundings is controlled by processes based on sorption (dissolution) and diffusion. This topic covers different aspects of additive solubility in polymers in light of polymer degradation and stabilisation.

4.2 Nonuniform Polymer Structure A polymeric substance is nonregular. This irregularity may display itself at the molecular, topological and morphological levels. The molecular irregularity is due to the chain-like structure of the polymer molecule and the existence of non-equivalency (anisotropy) along and across the polymer chain. Topological irregularity is due to the existence of polymer chain ends and various polymer chain entanglements surrounded by relatively ordered substance in which short-range order is obeyed. Morphological irregularity is based on the existence of relatively large zones markedly differing in the character of the arrangement of segments of macromolecules forming these zones and in their physical properties. In crystalline polymers, this irregularity gives rise to the formation of crystalline and amorphous regions, fibrils and spherulites. Gases and additives dissolved in a polymer are mainly present in the amorphous regions in zones around knots, folds and various chain entanglements where there is a free volume large enough to hold the molecule [1-8]. The degree of topological irregularity (disorder) depends on the conditions of polymer synthesis as well as on the conditions of polymer sample preparation, (i.e., on crystallisation) [9-12].

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4.3 Additive Sorption The most important characteristic of the sorption (that is, dissolution) of an additive in a polymer is its sorption isotherm, i.e., the relationship between the concentration or vapour pressure of the compound around the polymer and its concentration in the polymer. The simplest isotherm corresponds to the case of an ideal solution. This isotherm is described by Henry’s law: the concentration of compound A in the polymer ([A]p) is directly proportional to its concentration in the surrounding medium ([A]m) or to its pressure Pa: [A]p = γ*[A]m or

[A]p = (γ*/RT)Pa = γ*pPa

(4.1) (4.1a)

where γ* and γ*p are solubility coefficients. In the case of an ideal solution, d2[A]p/d[A]m2 = 0. That is, dissolution of any compound A does not change the properties of the polymer medium. In practice, linear isotherms (4.1) and (4.1a) are observed only at low concentration of a dissolved compound. Positive and negative deviations from the law (4.1) are possible. In the first case, d2[A]p/ d[A]m2 > 0; and in the second, d2[A]p/d[A]m2 < 0. A positive deviation means that the sorption of any molecule of A facilitates the sorption of the next one. Two mechanisms for the positive deviations are possible: (i) an increase in the mobility of the macromolecules caused by a dissolved compound (for example, plasticising the polymer) and (ii) the formation of aggregates (clusters) of several A molecules dissolved in the polymer. Positive deviations may be described by the following equation obtained from the theory of regular solutions:

μ/RT = ln(P/P0) = ln φ1 + φ2 + χφ22

(4.2)

where P is the vapour pressure of the additive in the system, P0 is the pressure of its saturated vapour, and χ is the Flory-Huggins parameter (solvent-solute interaction parameter). Equation (4.2) connects the chemical potential of the solvent (μ) with the volume fractions of the additive (φ1) and the polymer (φ2). The value of χ can be determined in terms of the solubility parameters of the additive and of the polymer, δ1 and δ2, respectively:

χ = V1(δ2 – δ2)2/RT

(4.3)

where V1 is the molar volume of sorbate. Equations (4.2) and (4.3) are widely used in practice and allow one to predict the additive sorption. The values δ1 and δ2 can be obtained from independent experiments or by simulation. In crystalline polymers, it is necessary to take into account the volume accessible for the molecules of additive, which does not always coincidence with the total volume of amorphous phase of the polymer.

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Solubility of Additives in Polymers A negative deviation from equation (4.1) corresponds to the case when the polymer possesses a limited number (concentration) of centres that can sorb one A molecule each. In this case, with increasing A concentration in the polymer, the number of non-occupied centres decreases. In many cases the negative deviations can be described by a law analogous to the Langmuir equation (4.4) or its combination with Henry’s law (4.5) [2-7, 11]: a[ A] [A]p = 1 + b[Am]

[A]

p

[ ]

=γ A

m

(4.4) m

+

[ ] 1+ b[ A ] aA

m

(4.5) m

where a and b are constants; the ratio a/b corresponds to limit of the A concentration in polymer. The nature of sorption centres may be different. Polymer polar groups interacting with an additive (for example, due to the formation of hydrogen bonds), as well regions with a lower density of a polymer substance (the elements of free volume) in the polymer, may be regarded as such centres. The latter have either a relaxation or a topological nature. Some authors [3-5] consider sorption centres as microvoids and unrelaxed volume in the polymer below the glass transition temperature that disappear at high temperature. In contrast, the centres arising around knots and other chain entanglements are more stable and can also exist in the polymer melt [7, 8, 12]. Suppose that in a polymer a certain concentration of the same centres Zi is present that can interact with compound A. Let us also suppose that the sorption of additive proceeds in two steps. First, the additive forms a true solution, the concentration of A in this solution being related to its concentration around the polymer by Henry’s law, that is, [A] = γ[A]m. Then this truly dissolved additive is reversibly sorbed by centres Zi: K

a A + Zi ←⎯ ⎯ → AZi

(4.6)

The total concentration of A in the polymer is [A]p = [A] + ∑[AZi] = [A] + [AZa], where [A] and [AZa] are the concentrations of true dissolved (mobile) and immobile molecules, respectively. If the additive concentration outside the centres is neglected ([A] << [AZa]), the sorption isotherm of A will be: [A]p = [AZa] = γKa[Za][A]m(1 + γKa[A]m)–1 or

1/[A]p = 1/γKa[Za][A]m + 1/[Za]

(4.7) (4.7a)

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Handbook of Plastic Films where γ is the coefficient of true (outside the centres) solubility of A ([A] = γ[A]m). Formula (4.7) is equivalent to the Langmuir type isotherm (4.4) assuming a = γKa[Za] and b = γKa. Formula (4.7) is observed in the case of the sorption of different additives including antioxidants by polyolefins. The concentration of sorption centres ([Za]) depends on the type of additive and polymer used, and in most cases remains constant over a wide temperature range from solid polymer to polymer melt [6, 7, 9, 11-13], indicating the existence of stable disorder. The precipitation of polymer from different solvents has been used as a method to change the concentration of chain entanglements [12]. During precipitation, macromolecules have to overcome the interaction with molecules of the solvent, which is more difficult in the case of a ‘good’ solvent, and a polymer sample obtained after precipitation from a ‘good’ solvent has to possess a less perfect structure. Figure 4.1 shows that the sorption isotherms of 2,6-di-tert-butyl-4-methylphenol (BMP) by polyethylene (PE) obey equation (4.7a); PE samples precipitated from decane have a higher concentration of sorption centres ([Za ]) than samples precipitated from chlorobenzene [12]. In some cases, at high concentration of additive, the sorption isotherms change their shape and a strong increase in sorption is observed. This dependence can be explained by means of polymer swelling resulting in changes in the polymer properties and mechanism of sorption [7-13]. Polymers usually contain different additives present together. The existence in the polymer of centres capable of sorbing two additives, (i.e. A and B), should result in a decrease in the concentration of one compound in the presence of another one due to competition for sorption centres. Formally it may be presented as: B + AZi ←⎯→ BZi

(4.8)

If ∑[BZi] ≈ [B]p, there should be a linear dependence between the concentrations of the two compounds: the dissolution of B results in an equivalent decrease of the concentration of A in the polymer. However, it is not possible to have the complete replacement of one additive by another one. Substitution is observed only in a limited range of concentration of both additives, which shows that there are some centres that do not take part in the substitution process (Figure 4.2) [7, 9, 13].

112

Solubility of Additives in Polymers

Figure 4.1 Sorption isotherms of BMP by PE samples precipitated from chlorobenzene (1) and decane (2) solutions containing of 1.0% of PE. T = 180 °C

Figure 4.2 Replacement of dibenzoylmethane (DBM) by phenyl benzoate (PB) in polypropylene from ethanol solution; [DBM]0 = 0.2 mol/l, T = 40 °C

113


4.4 Quantitative Data on Additive Solubility in Polymers The solubility of an additive corresponds to the concentration at which the additive in the polymer is present in equilibrium with the additive outside the polymer or with its saturated vapour. Formally, the solubility of A (SA) corresponds to the point on the sorption isotherm ([A]p = SA) where the concentration of A in the surroundings is equal to the concentration of A in its saturated vapour. According to equation (4.7), SA is less than [Za] and reaches [Za] with increasing temperature [8]. Various methods of measuring additive solubility in polymers have been proposed. The direct method includes the study of the kinetics of additive dissolution in a polymer when the additive is present in equilibrium with its saturated vapour or with an additive introduced on the surface of polymer film [7, 14-17]. For this purpose polymer film with an additive is kept in a closed vacuum tube or in an inert medium for different periods of time. Usually, the solubility value corresponds to some plateau on the curve of the concentration of additive in the polymer versus time. At high temperatures, dissolution can be accompanied by change in the polymer structure and the solubility will change with time [8, 16, 17]. To measure additive solubility, the ‘sandwich’ method is also used [18-21]. Polymer film is placed between films oversaturated with additive. The solubility measured in this way may be higher than in the case of the free additive method. This is probably due to the fact that the additive concentration in the oversaturated film does not correspond to the true equilibrium. There are some indirect methods of measuring additive solubility. One of them is based on the measurement of the concentration profile of the additive inside the film [18-23]. This method makes possible the simultaneous determination of the additive diffusion coefficient. Another method includes the determination of the temperature dependence of the vapour pressure of the additive above the pure additive and above the polymer containing a definite concentration of the additive [24]. The intersection point of the two curves (in the coordinates lg Pa versus 1/T) corresponds to the temperature at which the additive concentration in the sample is equal to its solubility. The temperature dependence of the transparency of polymer films with various additive concentrations [25] allows measurement of additive solubility. If the additive concentration in the polymer exceeds its solubility at the given temperature, the excess additive emerges to form crystals or drops, which sharply decrease the sample transparency. This method is not precise. Billingham and coworkers [15] considered the solubility of additives in polymers based on regular solution theory: solubility is defined by the condition that the (negative) free energy of mixing of the liquid additive with the polymer is equal to the (positive) free

114

Solubility of Additives in Polymers energy required to convert the crystalline additive into a liquid at the same temperature. In this case the solubility of the additive in the polymer is represented by: –ln SA = (ΔHf/RT)(1 – T/Tf) + (1 – V1/V2) + χ

(4.9)

where ΔHf is the heat of fusion of the additive, Tf is its melting temperature, V1 and V2 are the molar volumes of the additive and of the solvent, and χ is the interaction parameter. According to equation (4.9) a crystal with a higher heat of fusion is expected to be less soluble in a polymer than one with a lower heat of fusion. The additive solubility in the polymer can be predicted from data on its solubility in a homologous set of solvents by extrapolation of the solubility data in the coordinates ln SA versus 1/V2 to the point 1/V2 = 0. This approach does not take into account the features of the polymer structure. For the description of the temperature dependence of additive solubility in a polymer, the van’t Hoff equation: Ss = Ss0 exp(–ΔH/RT)

(4.10)

is used, where ΔH is the heat of solution. Equation (4.10) is correct in only narrow temperature ranges. Among the reasons for the violation of this dependence are phase transitions in the polymer, (i.e., near its melting point), and the dissolved additive. Another reason is the existence of stable sorption centres whose concentration in the polymer does not depend on temperature. The data published on antioxidant solubility in polymers refer mainly to polyolefins, and markedly differ from one another. These differences are apparently due to differences in the methods of measuring their solubility and to differences in the structures of the samples studied. Table 4.1 shows data on the solubility of different stabilisers in polymers. The solubility of stabilisers decreases with their molecular weight, but there is no simple dependence between these characteristics. The solubility of phenolic-type stabilisers in polyolefins and in rubbers is greater than that of aromatic amines with the same molecular weight. Sulfides are highly soluble in polyolefins, probably due to the presence of aliphatic groups in their molecules [21]. There is a difference in the solubility in PE of two sterically hindered amines with close molecular weight (396-423 and 481) [22]; nitroxides are less soluble than the corresponding amines [23]. As seen from Table 4.1, the solubility of many stabilisers at room temperature is markedly lower than the concentrations at which the additives are usually added to polymers (0.1– 0.5% by weight). Thus, an excess of a stabiliser added to a polymer often emerges (sweats out or blooms) from it.

115


Table 4.1 Solubility of stabilisers in polymers Stabiliser

MW

Polymer*

Temp. range (°C)

SA, at 25 °C (%)

lg SA0

ΔH (kJ/mol)

Ref.

2,6-Di-tert-butyl-4-methylphenol

220

LDPE iPP Butadiene rubber Chloroprene rubber

30-72 30-70

1.9 1.75 18†

7.83 6.11 -

43.0 33.5 -

26 26 27

11.3†

-

-

27

2,4,6-Tri-tert-butylphenol

262

LDPE iPP

30-80 30-50

0.83 0.55

15.78 7.28

90.6 43.2

26 26

2,6-Di-tert-butyl-4-phenylphenol

282

iPP PMP

40-100 30-50

0.45 0.39

6.11 13.82

36.9 81.3

26 26

3,5-Di-tert-butyl-4-hydroxyphenylpropionic methylate (Fenosan-1)

292

LDPE iPP PMP PVB (26%) plasticised

30-90 30-60 30-60 30-60

0.37 0.61 0.05 7.2

8.38 7.34 15.78 10.48

50.3 43.1 97.6 52.6

26 26 26 28

2,2′-Methylenebis(4-methyl6-tert-butyl-phenol)

340

LDPE LDPE iPP iPP PMP Butadiene rubber

30-80 23-90 30-80 50-100 30-90 22-80

0.08 3.5 0.063 1.17 0.012 2.0

3.74 3.34 7.17 4.71 7.75 5.38

27.6 15.9 47.8 26.8 55.3 29

21 21 26 26 26 27

Chloroprene rubber

-

2.3†

-

-

27

4,4′-Thiobis(6-tert-butylm-cresol) (Santonox)

358

LDPE

23-90

9×10-4

11.11

80.9

21

2,2′-Methylenebis(4-chloro6-tert-butylphenol)

360

iPP

40-100

0.38

4.54

28.5

26

2,2′-Methylenebis(4-ethyl6-tert-butylphenol)

368

LDPE

23-90

0.18

3.95

26.8

21

2,2′-Methylenebis(4-methyl6-α-methylcyclohexyl-phenol) (Nonox WSP)

420

LDPE iPP Butadiene rubber Butadieneacrylonitrile (18%) rubber

23-90 40-90 70-100

0.14 0.17 2.0

4.91 4.06 4.34

32.8 27.6 23.0

21 21 29

70-100

2.2

5.11

27.2

29

LDPE

23-90

0.028

5.20

38.5

21

4,4′-Methylenebis(2,6-di-tertbutylphenol) (Ionox 220)

116

424


Table 4.1 Solubility of stabilisers in polymers continued Stabiliser

MW

Polymer*

Temp. range (°C)

SA, at 25 °C (%)

lg SA0

ΔH (kJ/mol)

Ref.

Octadecyl ester of 3,5-di-tertbutyl-4-hydroxyphenylpropionic acid (Irganox 1076)

530

LDPE LDPE

52-90 23-52

0.016

7.60 10.97

45.5 72.8

21 21

1,1,3-Tris(5-tert-butyl-4′-hydroxy-2′-methylphenyl)-butane (Topanol CA)

544

LDPE LDPE

23-90 50-100

0.105 0.009

6.3 6.66

47.3 49.7

21 21

Bis(3,5-di-tert-butyl-4hydroxy-phenyl) ethoxycarbonyl-ethyl sulphide (Irganox 1035)

643

LDPE LDPE

23-74 74-90

0.017 -

9.00 6.78

61.6 46.9

21 21

2,4,6-Tris(3,5-tert-butyl-4hydroxybenzyl)mesitylene (Ionox 330)

775

LDPE iPP

23-90 50-100

0.013 0.004

3.04 8.59

28.1 62.9

21 15

Tetramethylene-3-(3′,5′-di-tert-butyl-4′-hydroxy-phenyl)propionate methane (Irganox 1010)

1178

LDPE LDPE iPP

23-90 50-100 50-100

0.005 0.02 0.15

8.60 5.30 4.45

62.4 39.0 31.8

21 21 15

Phenyl-β-naphthylamine

220

LDPE iPP PMP Butadiene rubber Butadieneacrylonitrile (18%) rubber

30-60 60-100 30-60 28-80

0.06 0.048 0.009 1.4

3.62 2.64 4.26 5.91

27.7 22.6 36.1 33.0

26 26 26 24

-

12.1†

-

-

24

Ester of 2,2,6,6-tetramethyl4-piperidinol and stearic acid (technical grade)

396423

LDPE

-

2-2.2†

-

-

22

Bis(2,2,6,6-tetramethyl-4piperidinyl) sebacate (Tinuvin 770)

481

LDPE

-

0.1†

-

-

22

Bis(2,2,6,6-tetramethyl-4piperidinyl-1-oxyl) sebacate

511

iPP iPP HDPE LDPE LLDPE

25-90 100-114 25-90 25-80 25-80

0.008 0.018 0.02 0.041

11.07 2.76 7.88 8.68 7.65

75.0 16.9 55.2 59.5 52.0

23 23 23 23

Dilauryl thiodipropionate

514

LDPE LDPE

23-40 40-90

0.79 -

9.20 2.15

53.2 10.8

21 21

Distearyl thiodipropionate

682

LDPE LDPE

23-66 66-90

0.75 -

5.00 0.64

29.3 7.7

21 21

*LDPE: low-density polyethylene; iPP: isotactic polypropylene; PMP: poly(4-methyl-1-pentene); PVB: poly(vinyl butyral); HDPE: high-density polyethylene; LLDPE: linear low-density polyethylene. † Solubility at 22-23 °C.

117


4.5 Factors Affecting Additive Solubility 4.5.1 Crystallinity and Supermolecular Structure Additive solubility in nonpolar rubber is greater than that in crystalline polyolefins (see Table 4.1) because the crystalline regions of polyolefins are not available for additives, and crystals decrease the plasticising action of dissolved compounds. There is no simple correlation between polymer crystallinity and additive solubility. The solubility of additives depends not only on the volume of amorphous fraction but also on its structure. It was shown [30] that the solubility of diphenylamine and phenyl-β-naphthylamine in solid polyethylene with different crystallinity is practically constant and only slightly decreases at high crystallinity of the polymer. The authors attribute this to the irregularity of the amorphous regions of the polymer, the density of which decreases with increasing polymer crystallinity. Moisan [31] showed that the solubility of Irganox 1076 in polyethylene at 60 °C only changes weakly with polymer crystallinity in the range from 43 to 57% (density range 0.92–0.94 g/cm3), but at higher temperatures (70 and 80 °C) the solubility decreases with polymer crystallinity. It should be noted that crystallinity measured at room temperature can change considerably with temperature, especially in the polymer premelting region. The role of the polymer supermolecular structure and polymer prehistory on antioxidant solubility has been studied [32-37]. It has been shown that the solubilities of diphenylamine, of the methyl ester of 3,5-di-tert-butyl-β-hydroxypropionic acid and of 2,2′-methylenebis(4-methyl-6-tert-butylphenol) in polyolefins prepared by rapidly cooling the polymer melt (structure with small spherulites) are higher than in the samples prepared by slow crystallisation near the polymer melting temperature (the structure with large spherulites) [32, 33]. The difference in solubility can reach a factor of 2, while the crystallinity measured by the IR method is practically the same [33]. The precipitation of polymer from different solvents has been used as a method to change the polymer structure and antioxidant efficiency [34-36]. It was shown that additive solubility in polypropylene (PP) precipitated from decane (a ‘good’ solvent for PP) was higher than that in chlorobenzene. The solubility of phenyl-β-naphthylamine in PP/PE blends and ethylene-propylene copolymers was studied in solid film and in the melt [37]. It was shown that the solubility of the antioxidant at 60 °C is practically independent of the composition of the polymer mixture, whilst the solubility in copolymers has a minimum at a propylene content in the range near 2% and a wide maximum at 40%. The solubility of different stabilisers in LDPE, in LDPE/LLDPE blends and in ethylene-vinyl acetate copolymer has been studied [18]. The additive solubilities in LDPE and in the blend are close, while in ethylene-vinyl acetate copolymer it is higher, especially in the case of 2,6-di-tert-butyl-4-methylphenol.

118


4.5.2 Effect of Polymer Orientation Orientation drawing of polyolefins results in considerable change in the polymer structure and additive behaviour: spherulites transform into fibrils; in amorphous zones, the amount of regular conformers increases and that of irregular conformers decreases [38-42]. The solubility and diffusion coefficient of additives usually decrease with drawing, but sometimes these relationships are more complicated [40, 43, 44]. Figure 4.3 shows the effect of elongation of PE on the solubility of various antioxidants at 60 °C. The crystallinity of PE determined by differential thermal analysis does not change with drawing, while the crystallinity determined by IR spectroscopy increases from 36 to 48% for (λ = 0-5.5), showing the change in the conformation set of macromolecules [44]. Because the orientation drawing can result in deformation and the disappearance of chain entanglements due to pull-out of macromolecules from knots and other topological irregularities, one may expect that after further melting the additive solubility of oriented samples should tend to decrease compared with that of non–oriented ones. The chain entanglements cannot recover quickly, otherwise a memory of the change in polymer structure should remain after polymer melting. Figure 4.4 shows the effect of orientation drawing on the solubility of phenyl benzoate [45]: curve 1 corresponds to

Figure 4.3 Dependence of the solubility of the methyl ether of 3,5-di-tert-butyl4-hydroxyphenylpropionic acid (1), of 2,2′-methylenebis[4-methyl-6(1-methylcyclohexyl)phenol] (2) and the ester of 3,5-di-tert-butyl-4hydroxyphenylpropionic acid and ethylene glycol (3) in polyethylene on the degree of drawing (l) at 60 °C

119


Figure 4.4 Solubility of phenyl benzoate in polyethylene at 60 °C as a function of degree of drawing: (1) samples after additional heating, (2) samples without additional heating

samples which after drawing were heated in vacuum to 140 °C followed by fast cooling; curve 2 corresponds to samples without additional treatment. As can been seen from Figure 4.4, orientation of the polymer affects the solubility of the additive even if samples were heated above the polymer melting point.

4.5.3 Role of Polymer Polar Groups In a polymer containing polar groups, the mechanism of sorption may also include the interaction of polar groups of the polymer (X) with polar groups of the dissolved additive (A), as represented [8, 46, 47] by: A + X ↔ AX

(4.11)

In going from nonpolar to polar polymers, e.g., from polyolefins to aliphatic polyamides containing –CONH– groups, the polymer density increases as a result of the formation of hydrogen bonds between these groups. It was shown [8] that, for polymers such as polyamide-12 (PA-12), polyamide-6,10 and polyamide-6,6 (or polyamide-6; PA-6), the solubility of phenyl-β-naphthylamine increases in the order polyethylene to polyamide– 12 and decreases with higher concentrations of these groups, whereas the solubilities of

120


Figure 4.5 Solubility of phenyl-β-naphthylamine (1) and of phenyl benzoate (2) in polyethylene and polyamides at 40 °C as a function of the concentration of amide groups

the less polar additives phenyl benzoate and 2,6-di-tert-butyl-4-methylphenol decrease over the whole range studied (Figure 4.5).

4.5.4 Effect of the Second Compound The decrease in the solubility of one compound in the presence of another compound due to competition for sorption centres was mentioned above. In some cases a more complicated situation is observed. The effect of octamethylcyclotetrasiloxane (OMTS) introduced into a PE melt on the solubility of phenyl-β-naphthylamine (PNA) in solid PE is shown in Figure 4.6: the solubility decreases and then passes through a maximum with OMTS concentration. The content of trans and gauche conformations in the polymer is also changed, which can be attributed to rearrangement of the sorption centres. OMTS also affects the polymer melt: the vapour pressure of PNA (the concentration in gas phase over the polymer) depends on the OMTS concentration in the melt [14]. Plasticisers present in polymers change the polymer structure owing to the increase in mobility of polymer chains, which affects the solubility and diffusion of additives in the polymer. The solubility of antioxidants in nonplasticised PVB is low compared with that in pure plasticiser, i.e., for the ester of 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid

121


Figure 4.6 Solubility of phenyl-β-naphthylamine in polyethylene at 60 °C as a function of OMTS concentration

and ethyleneglycol: 0.014 and 0.84 mol/kg (0.9% and 53% by weight) respectively [28]. If the solubility in the polymer (SPVB) and in the plasticiser (SPLA), i.e., of antioxidant in plasticised PVB (Sadd), is considered as a simple sum of the solubilities Sadd = SPVB + x(SPLA – SPVB)

(4.12)

where x is the percentage of plasticiser in the polymer, one can expect a linear growth of antioxidant solubility in the polymer with plasticiser concentration (Figure 4.7, dotted line). Experiment shows that, at low concentrations of the plasticiser (1–5% by weight), the solubility is higher than what it should be according to equation (4.12); but at higher concentrations of plasticiser (10–40%), it is less than Sadd (Figure 4.7, solid line) [28]. The effect observed is due to the fact that, at small concentrations, the plasticiser strongly affects the mobility of macromolecules and, as a result, increases the antioxidant solubility; at high concentrations, there is solution of the polymer in the plasticiser with strong polymer–plasticiser interactions, which disturb the antioxidant dissolution.

4.5.5 Features of Dissolution of High Molecular Weight Additives For additive dissolution, sorption centres should contain an excess volume large enough to locate the additive molecule. If this volume is less than that necessary for sorption,

122


Figure 4.7 Solubility of the methyl ester of 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid as a function of dihexyl adipate content in PVB at 60 °C

dissolution of A can occur only when the rearrangement of this centre results in a change to the polymer structure. The process of centre rearrangement may be represented by equation (4.13) followed by polymer swelling: *

A + Zi → AZi ↔ A + Zi

*

(4.13)

This could be important when large additive molecules are considered. The solubility of sterically hindered amines with molecular weights from 1364 to 2758 in polypropylene has been studied [16, 17]. It was shown that the solubility of the stabilisers in the polymer at 100 °C passes through a maximum with time and depends on the molecular weight of the stabiliser: the higher the molecular weight of the stabiliser, the higher its maximum concentration in PP. It was assumed that, at high temperatures, molecules of larger size are able to change the polymer structure to a greater extent than those of smaller size, so the apparent solubility may increase with the molecular weight of the additive as observed experimentally. Thus, the process of dissolution of high molecular weight additives gives rise to a certain ‘destruction’ of the initial polymer structure. The decrease in additive solubility with time is probably due to annealing of the polymer in the presence of additives. Additive dissolution is accompanied by a change in the polymer crystallinity and in the concentration of irregular conformations in the amorphous zones of the polymer [16, 17].

123


4.5.6 Effect of Polymer Oxidation The oxidation reaction first involves the zones with lower polymer packing density containing polymer chain entanglements and can result in the disappearance of some of them. This process may be represented as: Z1 + nO2 → εZ2

(4.14)

where Z1 and Z2 are different types of sorption centres, and ε < 1. Aliphatic chain scission proceeds in the oxidation reaction according to: O

O

(4.15) CH2

CH

CH2

CH2

C

H

+ CH2

As seen from equation (4.15), polar aldehyde groups are formed in this process at the ends of broken chains. Thus, the newly formed centre Z1 may contain polar groups. In this case the solubility of polar additives may increase with oxidation. The solubilities of diphenylamine and phenyl benzoate were studied in polyethylene and in several aliphatic polyamides after oxidation [47]. Figure 4.8 shows the solubility of phenyl benzoate as a function of oxidation degree. At low oxidation degrees, up to 0.2–0.3 mol/kg of oxygen,

Figure 4.8 Solubility of phenyl benzoate at 60 °C as a function of amount of oxygen absorbed during oxidation of polymers: PE, PA-12 and PA-6 124

Solubility of Additives in Polymers the solubilities of both additives in all polymers studied decrease; at deeper stages, the solubility in polyamides still decreases, whilst in PE it increases. To explain the experimental data, we should assume that oxidation results in the decomposition of one type of sorption centre and the simultaneous formation of other ones. In polyamides the concentration of polar amide groups is higher compared to those of new ones formed in oxidation. For this reason we only observe the decrease in additive solubility caused by polymer oxidation. In nonpolar polyethylene, the effects of both processes are comparable, and we observe a more pronounced and complicated variation of additive solubilities.

4.6 Solubility of Additives and Their Loss An additive dissolved in a polymer can transfer from the polymer into the surrounding medium. This process includes the stages of diffusion of the additive to the surface and its removal from the surface, (i.e., by sweating out, evaporation or washing out). The solubility of an additive in the polymer can affect all these stages [48-57]. Any excess of an additive in a polymer (above its solubility) may exude on the polymer surface, forming either drops or powder; it blooms or sweats out. A quantitative description of the process of sweating out is difficult, because at high additive concentrations exceeding its solubility the diffusion coefficient is not constant and the residual additive concentration after sweating out is greater, the greater its initial concentration in the polymer. The evaporation of an additive, A, from a polymer depends on its solubility in the polymer. In some cases, the rate of additive evaporation, We, is connected with its surface concentration, [A]sf, and its solubility in the polymer, [A]s, by: We = Wa[A]sf/[A]s

(4.16)

where Wa is the rate of evaporation of the individual additive. A detailed description of additive loss due to evaporation may be found elsewhere [51]. High molecular weight additives are not volatile and their diffusion in a polymer at elevated temperature is very slow, which is why washing out is the principal cause of undesired loss of stabilisers and other additives from polymeric material used outdoors or for the flow of liquids in tubes and containers. There are some factors that may influence the washing out of stabiliser from a polymer, i.e., the additive solubility and the solvent solubility in the polymer [55]. Owing to low additive solubility, part of the additive may be present in a polymer in a metastable state or form a separate phase and it can be lost quickly. On the other hand, the solvent facilitates the migration of stabilisers passing into the polymer and increasing the segmental mobility of

125

Handbook of Plastic Films macromolecules. The ability of the solvent to escape the additive is connected with the solvent solubility in the polymer: the higher the solvent solubility, the higher the washing-out effect. The diffusion coefficient of an additive in many cases increases with the additive concentration in the polymer. The dependence of the diffusion coefficient on diffusant concentration may be due either to plasticisation of the polymer or to the presence of sorption centres that bind a part of the diffusing molecules. Assuming that only mobile molecules of A, outside the sorption centres, participate in the diffusion, their concentration is represented [9] by:

[ A] =

[ A ]p Ka ⎛ [ Za ] – [ A]p ⎞⎠ ⎝

(4.17)

It is possible to find the theoretical dependence of the diffusion coefficient (D) on the total concentration of A in the polymer:

Φ = –Dt(d[A]/dx) D=

[ ] 2 Ka ⎛⎝ [ Za ] – [ A]p ⎞⎠ Dt Z

(4.18)

where Dt is the diffusion coefficient of truly dissolved molecules. Figure 4.9 shows that D of phenyl-β-naphthylamine in PP depends on the antioxidant concentration and on the concentration of sorption centres according to equation (4.18). The results obtained show that features of additive dissolution and diffusion could be explained by taking into account non-unique additive distributions in the polymer and the existence of sorption centres around polymer chain entanglements which are able to hold the additive molecule. The concentration of these entanglements may be changed either during polymer synthesis or during polymer treatment (orientation, crystallisation, etc.). The influence of polymer disorder on additive behaviour and polymer properties should be considered as an important factor in the physical chemistry and technology of polymeric materials.

126


Figure 4.9 Dependence of the diffusion coefficient of phenyl-b-naphthylamine on its concentration in PP at 60 °C

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D.R. Paul and W.J. Koros, Journal of Polymer Science, Polymer Physics Edition, 1976, 14, 675.

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Handbook of Plastic Films 9.

P-G. de Gennes, Macromolecules, 1984, 17, 703.

10. A.P. Mar’in, Y.A. Shlyapnikov, A.Z. Makhamov and A.T. Dzhalilov, Oxidation Communications, 1997, 20, 57. 11. P. Bruni, C. Conti, A. Mar’in, Y.A. Shlyapnikov and G. Tosi, European Polymer Journal, 1997, 33, 1665. 12. Y.A. Shlyapnikov, S.G. Kiryushkin and A.P. Mar’in, Antioxidative Stabilisation of Polymers, Taylor and Francis, London, 1996. 13. Y.A. Shlyapnikov and A.P. Mar’in, Acta Chimica Hungarica, 1987, 124, 531. 14. A.P. Mar’in, E.A Sviridova and Y.A. Shlyapnikov, Polymer Degradation and Stability, 1995, 47, 349. 15. N.C. Billingham, P.D. Calvert and A.S. Manke, Journal of Applied Polymer Science, 1981, 26, 3453. 16. A.P. Mar’in, V. Borzatta, M. Bonora and L. Greci, Journal of Macromolecular Science A, 1998, 35, 1299. 17. A.P. Mar’in, V. Borzatta, M. Bonora and L. Greci, Journal of Applied Polymer Science, 2000, 75, 7, 883. 18. E. Foldes, Polymer Degradation and Stability, 1995, 49, 57. 19. E. Foldes and B. Turscanyi, Journal of Applied Polymer Science, 1992, 46, 507. 20. E. Foldes, Journal of Applied Polymer Science, 1993, 48, 1905. 21. J.Y. Moisan, European Polymer Journal, 1980, 16, 979. 22. J. Malik, A. Hrivik and E. Tomova, Polymer Degradation and Stability, 1992, 35, 61. 23. V. Dudler, Polymer Degradation and Stability, 1993, 42, 205. 24. L.S. Feldshtein and A.S. Kuzminsky, Vysokomolekulyarnye Soedineriya Series B, 1971, 13A, 2618. 25. N.P. Frank and R. Frenzel, European Polymer Journal, 1980, 16, 647. 26. I.A. Shlyapnikova, A.P. Mar’in, G.E. Zaikov and Y.A. Shlyapnikov, Vysokomolekulyarnye Soedineriya Series B, 1985, 27A, 1737.

128

Solubility of Additives in Polymers 27. L.S. Feldshtein and A.S. Kuzminsky, Kautchuk i Rezina, 1970, 10, 16. 28. A. Mar’in, L.A. Tatarenko and Y.A. Shlyapnikov, Polymer Degradation and Stability, 1998, 62, 507. 29. B.S. Grishin, I.A. Tutorsky and I.S. Yuroslavskaya, Vysokomolekulyarnye Soedineriya Series B, 1978, 20A, 1967. 30. B.A. Gromov, N.E. Korduner, V.B. Miller and Yu.A. Shlyapnikov, Doklady Akademii Nauk SSSR, Chemistry, 1970, 190, 1381. 31. J.Y. Moisan, European Polymer Journal, 1980, 16, 989. 32. N.Ya. Rapoport, Y.A. Shlyapnikov, B.A. Gromov and V.Z. Dubinsky, Vysokomolekulyarnye Soedineriya Series B, 1972, 14A, 1540. 33. T.V. Monakhova, T.A. Bogaevskaya and Y.A. Shlyapnikov, Vysokomolekulyarnye Soedineriya Series B, 1975, 17, 1243. 34. Y.A. Shlyapnikov, T.V. Monakhova and T.A. Bogaevskaya, Polymer Degradation and Stability, 1994, 46, 247. 35. P. Bruni, A.P. Mar’in, E. Maurelli and G. Tosi, Polymer Degradation and Stability, 1994, 46, 151. 36. P. Bruni, C. Conti, A.P. Mar’in, Y.A. Shlyapnikov and G. Tosi, American Chemical Society, Polymer Preprints, 1996, 37, 1, 101. 37. I.G. Kalinina, I..I. Barashkova, G.P. Belov, K.Z. Gumargalieva, A.P. Mar’in and Y.A. Shlyapnikov, Vysokomolekulyarnye Soedineriya Series B, 1994, 36B, 1028. 38. I.M. Ward and D.W. Hadley, An Introduction to the Mechanical Properties of Solid Polymers, John Wiley, Chichester, UK, 1993. 39. V.A. Marikchin and L.P. Myasnikova, Supermolecular Structure of Polymers, Nauka, Leningrad, Russia, 1977, 86. 40. L.S. Shibryaeva, S.G. Kiryushkin and G.E. Zaikov, Polymer Degradation and Stability, 1992, 36, 17. 41. L.S. Shibryaeva, A.P. Mar’in and Y.A. Shlyapnikov, Vysokomolekulyarnye Soedineriya Series B, 1995, 37B, 696. 42. L.S. Shibryaeva, S.G. Kiryushkin and A.P. Mar’in, Vysokomolekulyarnye Soedineriya Series B, 1987, 29, 113.

129

Handbook of Plastic Films 43. A. Peterlin, Journal of Macromolecular Science B, 1975, 11, 57. 44. J.Y. Moisan, European Polymer Journal, 1980, 16, 997. 45. L.S. Shibryaeva, A.P. Mar’in and Y.A. Shlyapnikov, Polymer Degradation and Stability, 1995, 50, 305. 46. A.P. Mar’in, I.A. Shlyapnikova, G.E. Zaikov and Y.A. Shlyapnikov, Polymer Degradation and Stability, 1991, 31, 61. 47. T.V. Monakhova, A.P. Mar’in and Y.A. Shlyapnikov, Polymer Degradation and Stability, 1993, 40, 365. 48. A.P. Mar’in and Y.A. Shlyapnikov, Polymer Degradation and Stability, 1991, 31, 181. 49. S. Al-Malaika in Free Radicals and Food Additives, Eds., O.I. Aruoma and B. Halliwell, Taylor and Francis, London, UK, 1991, 151. 50. L. Luston in Developments in Polymer Stabilisation, Volume 5, Ed., G. Scott, Applied Science, London, UK, 1980, 185. 51. N.C. Billingham in Oxidation Inhibition in Organic Materials, Volume 2, Eds., J. Pospisil and P.P. Klemchuk, CRC Press, Boca Raton, FL, USA, 1990, 249. 52. N. Haider and S. Karlsson, Polymer Degradation and Stability, 1999, 64, 321. 53. S. Al-Malaika and S. Issenhuth in Polymer Durability, Advances in Chemistry Series No.249, American Chemical Society, Washington, DC, USA, 1996, 425. 54. A.P. Mar’in, V. Borzatta, M. Bonora and L. Greci, Journal of Applied Polymer Science, 2000, 75, 890. 55. A.P. Mar’in, V. Borzatta, M. Bonora and L. Greci, Journal of Applied Polymer Science, 2000, 75, 897. 56. R. Spatafore and L.T. Pearson, Polymer Engineering and Science, 1991, 31, 22, 1610. 57. R. Goydan, A.D. Schwope, R.C. Reid and G. Cramer, Food Additives and Contaminents, 1990, 7, 3, 323.

130

5

Polyvinyl Chloride: Degradation and Stabilisation K.S. Minsker, G.E. Zaikov and V.G. Zaikov

5.1 Introduction Some aspects of the manufacture of polyvinyl chloride (PVC) that does not contain labile groups in the backbone are considered. This will provide a drastic increase in the intrinsic stability of polymeric products, and the possibility of PVC processing with a minimal content or total absence of stabilisers and other additives. The data presented allow the creation of rigid, semi-rigid (semi-flexible) and flexible (plasticised) materials and products with minimal content of chemical additives and increased service lifetime for exploitation in natural and special conditions. PVC is one of the most well-known multi-tonnage and practically important polymeric products. Thousands of rigid, semi-flexible and flexible (plasticised) materials and products based on PVC are widely used in all spheres of national economies and everyday life. PVC was first synthesised by E. Baumann in 1872, but its industrial manufacture began much later – in 1935 in Germany according to the literature data, and in 1930 in the USA according to the data of the DuPont Company. Global PVC production is impressive: 220 thousand tons in 1950, about 1.5 million tons in 1960, more than 3 million tons in 1965, more than 5 million tons in 1970, and its current production (mid-2002) is estimated to be more than 23 million tons. A basic problem with PVC is its low stability. Under the action of heat, ultraviolet (UV) light, oxygen, radiation, etc., it easily disintegrates according to the law of transformation of adjacent groups with the elimination of hydrogen chloride and the formation of sequential carbon-carbon double bonds in the macromolecules and the appearance of undesirable coloration (from yellow to black). Therefore, it is necessary to apply a set of methods that will lead to the increased stability of PVC itself, and of materials and products based on it, when exposed to the various factors that occur during synthesis, storage, processing and use. It is logical to assume that, among the many aspects causing the low stability of PVC and the rather short lifetime of materials and products based on it, the most important point is to understand the reasons for its abnormally high rates of disintegration compared to low molecular weight models. Researchers in the fields of synthesis and processing of

131

Handbook of Plastic Films PVC appear to have found this problem rather complex, for, in essence, it is still under discussion. So far, such workers in industrial research centres in various countries have not been able to agree upon the identification of the weak site in the structure of the PVC macromolecule responsible for causing its abnormally low stability.

5.2 Some Factors Affecting the Low Stability of PVC It used to be thought that the low stability of PVC was connected to the possible presence of labile groups in the macromolecular structure, which activate polymer disintegration. These labile groups are distinct from sequences of regular vinyl chloride repeat units: ~CH2CHCl–CH2–CHCl–CH2–CHCl~ The overwhelming majority of researchers believe that such groupings are [1-8]: (1) Chlorine atoms bonded to tertiary carbon atoms C–Cl (At); (2) Vicinal chlorine atoms in the macromolecular structure: ~CH2–CHCl–CHCl–CH2~ (Av); (3) Unsaturated end-groups such as ~CH=CH2 and/or ~CCl=CH2; (4) β-Chloroallyl groups ~CH2–CH=CH–CHCl~ (Ac); (5) Oxygen-containing hydroxy and peroxy groups (A0). However, even after just a brief consideration of the process of PVC disintegration, it is obvious that there are far fewer labile groups (which can be considered to be the cause of low PVC stability) in the macromolecules. This is because, on PVC dehydrochlorination, tertiary chlorine (At) and vicinal (Av) groups turn into β-chloroallyl groups and the hydroperoxide groups transform into carbonyl groups, as shown in Scheme 5.1. In addition, PVC research throughout the world has shown that the initial (freshly synthesised) PVC macromolecules (which are processed in materials and products) do not contain di- (A2), tri- (A3) and/or polyene (Ap) groups [2, 3, 9-14]. Internal peroxide groups ~CH2–CHCl–O–O–CH2–CHCl~ are not found either, since if they were formed during PVC synthesis they would quickly disappear as a result of hydrolysis and/or homolytic cleavage of the O–O bond. There are reliable experimental results, including those obtained during the study of the thermal

132

Achievements for Polyvinyl Chloride: Degradation and Stabilisation Cl ~H2C

C

At

CH2~

CH3

~H2C

~HC

CH

CH

Cl

Cl

Av

CH2~

O ~H2C

CH~

CH

C

A0

CH2~

– H 2O OOH

~H2C

Cl

Cl

C H

C H

CHCl~

~H2C – HCl

C H

C H

C H

C H

CHCl~

Ap

Ax Scheme 5.1

destruction of fractionated PVC, showing that, although unsaturated end-groups are present in the structure of polymeric molecules, they do not affect the disintegration rate of PVC [10, 13-15]. Thus, the process of gross dehydrochlorination of PVC (overall rate constant VHCl) can be described with sufficient accuracy by Scheme 5.2, where: α0 represents the regular vinyl chloride ~CH2–CHCl~ groups; KCl, Kt, Kv, Kc and Kp are the rate constants for the appropriate dehydrochlorination reactions of PVC; and Ktr is the rate constant for the reaction that terminates polyene growth.

Av α0

Kv

KCl

At

Kp Ac

Kc

Ap

A2 Ktr

Kt

A*

Scheme 5.2

133

Handbook of Plastic Films Following from Scheme 5.2, we have: VHCl = KCl[a0] + Kc[Ac] + Kp[Ap] with real values of: KCl = 10–8 to 10–7 s–1 and [α0] = 1 mol/mol PVC; Kt = 10–4 s–1 and [At] = 10–3 mol/mol PVC; Kc = 10–4 to 10–5 s–1 and [Ac] = 10–4 mol/mol PVC; Kv = 10–3 to 10–4 s–1 and [Av] = 10–5 mol/mol PVC; and Kp = 10–2 s–1 (448 K). It is obvious that Scheme 5.2 assumes the concept of β-chloroallyl-activated disintegration of PVC accepted by the majority of researchers, but without real proof [1-5]. However, this postulate contradicts many experimental facts [16, 17], in particular the following: (1) Calculated values of VHCl differ greatly from the experimental ones. (2) The β-chloroallyl activation of PVC disintegration assumes an autoacceleration of the PVC gross dehydrochlorination process with time [16-18], whereas a linear dependence is observed experimentally (Figure 5.1). The gross rate constant of PVC disintegration, according to experimental data and shown in Figure 5.1 at Kc = 10–4 to 10–5 s–1, should contain the term Kp ≅ 10–2 s–1 (at 448 K) from the very beginning

Figure 5.1 Kinetic curves for PVC dehydrochlorination: (1) calculated data for β-chloroallyl activation; (2) experimental data (448 K, 10–2 Pa) 134

Achievements for Polyvinyl Chloride: Degradation and Stabilisation

Table 5.1 Dehydrochlorination rate constant for thermal destruction of low molecular weight model compounds No.

Compound

Temperature range (K) where compounds start to degrade at noticeable rate

Group index

Decomposition rate constant (K s–1)

1

2,4-Dichloropentane

563-593

α0

2.6 × 10–9

2

meso-2,4-Dichloropentane

563-593

α0

1.9 × 10–9

3

3-Ethyl-3-chloropentane

488-553

At

7.9 × 10–6

4

4-Chlorohexene-2

433-463

Ac

5.1 × 10–4

5

4-Chlorodecene-2

438-468

Ac

5.0 × 10–5

6

7-Chlorononadiene-3,5

343-369

Ap

3.4 × 10–2

7

6-Chlorooctadiene-2,4

360-386

Ap

2.6 × 10–2

of PVC thermal destruction. However, according to data obtained on the thermal disintegration of low molecular weight model compounds [19-21], this is observed only as a result of the destruction of model compounds containing a chlorine atom in a β-position to conjugated (C=C)n bonds (at n ≥ 2), i.e., due to the effect of the adjacent group of the long-range order (Table 5.1). Thus, the concept of β-chloroallyl activation of PVC dehydrochlorination does not satisfy even a preliminary analysis of the experimental results, is therefore erroneous and should no longer be considered a viable theory. On the basis of theoretical considerations of PVC thermal degradation, and in view of all the available experimental data, it can be concluded that: even if internal β-chloroallyl groups (as well as tertiary and vicinal chlorides) are present in the macromolecular structure, they do not contribute to the process of PVC gross dehydrochlorination as a result of their sufficient relative stability. It was assumed, and then proved, that the group that is responsible for the low stability of PVC is an oxovinylene (carbonylallyl) conjugated dienophile group: –C(O)–CH=CHCl–CH2– the double bond of which is activated by the adjacent electrophilic C=O group. Apparently, this group is present in PVC macromolecules in rather small amounts, γ ≅ 10–4 mol/mol PVC, but disintegrates at a rather high rate (Kp ≅ 10–2 s–1) with HCl elimination [14, 17, 22-24].

135

Handbook of Plastic Films It is extremely important to emphasise that the concept of oxovinylene activation of PVC disintegration does not contradict any currently known experimental facts. In addition, new proofs (including original ones) of the existence of basic groups in the structure of PVC macromolecules have been obtained recently. In particular, oxovinylene groups in the PVC macromolecule are easily split (under mild conditions) with alkaline hydrolysis (5% aqueous KOH solution, and 5% solution of PVC in cyclohexanone) [13, 14], which is a characteristic reaction for α,β-unsaturated ketones, as shown in the following reaction [25]: O O

H2O ~CH

CH

C(O)~

+

C

KOH

H3C

C~

(5.1)

H

Using this reaction, it is easy to estimate the content of labile oxovinylene groups (γ0) in the macromolecular structure by the decrease of viscosity-average molecular weight of PVC [13-17].

5.3 Identification of Carbonylallyl Groups It is important to remark that both β-chloroallyl and polyene groups are inert to alkaline hydrolysis, but easily decomposed on oxidation (in the presence of hydrogen peroxide) and by ozonolysis [13]. The ozonolysis method allows estimation of the total amount of internal unsaturated (β-chloroallyl, chloropolyenyl and oxovinylene) groups in the PVC structure by the decrease of PVC molecular weight. Thus, it is experimentally shown that practically all the internal unsaturated groups in PVC macromolecules are oxovinylene ones, and that the PVC dehydrochlorination rate is linearly connected to the content of internal labile oxovinylene groups in polymeric molecules [14, 26], determined by using alkaline hydrolysis (Figure 5.2). It is known that PVC synthesised in the absence of oxygen is always more stable than PVC manufactured industrially. This is due to the presence of sufficiently stable internal β-chloroallyl (not oxovinylene) groups (oxidative ozonolysis) in the first type of PVC structure. As a whole, the real process of HCl elimination during PVC disintegration in the transformation reaction of adjacent groups is complex, since generally this or that contribution is brought in by all abnormal groups contained in the PVC structure. However, apparently, the contribution of different reactions to this process varies and in a number of cases some of them can be neglected. The kinetic analysis takes into account the real contents of characteristic (including abnormal) groups in PVC. Also, the rate constants of their disintegration (Table 5.2)

136


Figure 5.2 Dependence of PVC dehydrochlorination rate on the content of carbonylallyl groups in the polymer molecules (448 K, 10–2 Pa)

have precisely shown [14, 17, 24, 27, 28] that the ratio of the appropriate reaction rate constants is: KCl : Kc : Kt : Kp ≅ 1 : 100 : 100 : 100,000 and, for this reason, the thermal stability of PVC is determined by the effect of the adjacent group of the long-range order (conjugation effect). So the total elimination rate of HCl from PVC is described with sufficient accuracy by the simple equation: VHCl =

[

d HCl dt

] = K [α ] + K [ γ ] = V + V Cl 0 p 0 Cl p

(5.2)

Even taking into account the participation of tertiary chloride (At) and β-chloroallyl (Ac) groups in PVC disintegration, the contribution of the expression Vp = Kp[γ0] comprises about 90% or more of the total gross rate of PVC dehydrochlorination. This confirms oxovinylene (not β-chloroallyl) activation as the major process in PVC thermal disintegration. The development of the concept of oxovinylene activation of PVC thermal destruction appears to be an important point in the theory and practice of PVC chemistry and

137


Table 5.2 Rate constants of dehydrochlorination of characteristic groupings and their contents in the initial PVC structure Contents in PVC Group

Index

Amount (mol/mol PVC)

Rate constant of degradation at 448 K Index

Value (s-1)

K. Minsker 1978 E. Sorvik 1984 G. Zimmerman 1984

Kp

10-1-10-2

K. Minsker 1977 W. Starnes 1985

0

K. Minsker 1978 G. Zimmerman 1984

KCl

10-5-10-4

Z. Meyer 1971 B. Troitsky 1973 W. Starnes 1983

~10-3

E. Sorvik 1984 A. Caraculaku 1981 V. Zegelman 1985

Kt

10-4

W. Starnes 1983 Z. Meyer 1971

K. Minsker 1976

Kp

~10-2

Z. Meyer 1971 K. Minsker 1984

-

KCl

10-7-10-8

Z. Meyer 1971 K. Minsker 1972

Authors

Authors

~CO–CH=CH–CHCl~ γ0

~10-4

(~CH2)CCl–CH2–CH2Cl~ ACl0 ~CCl–CH2CH2Cl At0

~CH2–(CH=CH)n>1–CHCl~ Ap

0

~CH2–CHCl–CH2–CHCl~ α0

1

objectively defines the necessity for a new specific approach for studying various aspects of the destruction and stabilisation of PVC. In particular, studies are needed of the new characteristic reactions with unsaturated ketones, confirming the presence of oxovinylene groups in the PVC structure, or the interaction of ~C(O)–CH=CH–CHCl~ groups with organic phosphites P(OR)3 [29-33] and dienes [34, 35].

5.4 Principal Ways to Stabilise PVC Organic phosphites react easily in mild conditions (290-330 K) with oxovinylene groups in the presence of proton donors to yield the stable ketophosphonates:

138


~C

CHCl~ O

~C O

C H

C H

CH~

+

P-(OR)3

P(OR)3 O

Cl C

CH2 O

CH

CHCl~

P(OR)3

(5.3)

The reaction kinetics for the interaction of organic phosphites with oxovinylene groups are shown in Figure 5.3. The formation of ketophosphonate structures according to reaction (5.3) results in the disappearance of internal C=C bonds in the PVC structure. As a result, neither ozonolysis of a polymeric product nor especially alkaline hydrolysis leads to degradation of macromolecules and, consequently, decrease of PVC molecular weight.

Figure 5.3 The changes in the ~C(O)–CH=CH~ group content in PVC during interaction with tri(2-ethylhexyl) phosphite (C0 = 10–2 mol/mol PVC): (1) 289 K; (2) 298 K; (3) 448 K 139

Handbook of Plastic Films It is important to note that organic phosphites do not react with β-chloroallyl groups, as has been confirmed by the method of competing reactions of organic phosphites (trialkyl-, arylalkyl- and triarylphosphites) with a mixture (1:1 mol/mol) of methyl vinyl ketone (model of an oxovinylene group) and 4-chloropentene-2 (model of a βchloroallyl group) at 353 K. Practically, the organic phosphite selectively reacts quantitatively (with regarding to proton donor) with methyl vinyl ketone, while 4chloropentene-2 is quantitatively allocated after realisation of the reaction, excluding the small amount (less than 7 wt%) of products of its dehydrochlorination. The main reaction product (up to 75 wt%) is: CH3–C(O)–CH2–CH2–P(OR)2 In this reaction, trialkyl- and alkylarylphosphites are more active than triarylphosphites. Dienophilic oxovinylene groups react with conjugated dienes according to the following Diels-Alder reaction: O O RHC ~C

C H

C H

CHCl~

+

CHR– C H

C H

~C

CH

CH

RHC

CHCl~ CHR–

C H

C H

(5.4)

The reactions of PVC with cyclopentadiene, piperylene, isoprene, 5-methylheptatriene1,3,6, etc., proceed in mild conditions (353 K) and result in destruction of internal unsaturated C=C groups in PVC chains. These reactions are new and have not been reported before, and are similar to the reaction of PVC with organic phosphites, as shown in reaction (5.4). The collection of methods used to increase PVC stability to the action of various factors (such as heat, light, oxygen, etc.), in terms of storage, processing and use is closely connected to the level of theoretical development of PVC degradation. Therefore, it is clear that the significant advances in theoretical developments of the reasons for the thermal instability of PVC (the presence of oxovinylene groups in the backbone), the mechanism of the process (the fundamental influence of adjacent groups of the longrange order) and the kinetics of their disintegration were necessary, and have enabled a new look at the determination of effective methods of PVC stabilisation under thermal and other influences.

140

Achievements for Polyvinyl Chloride: Degradation and Stabilisation According to Scheme 5.2 it is impossible (and unnecessary) to increase the stability of PVC by the reduction of rate VHCl, since this process is rather slow. According to the experimental data, the rate of PVC statistical dehydrochlorination, VHCl, is constant (law of randomness) and does not depend on how the polymer was synthesised or its molecular weight. Hence, it is a fundamental characteristic of PVC, showing that all parts in clusters ~BXBXBX~ participate similarly in the process of HCl elimination under the law of randomness. On the other hand, the rate of formation of the conjugated systems, Vp, differs markedly, since it increases linearly with the content of oxovinylene groups in the initial PVC macromolecules (γ0) (Figure 5.2). Thus, the basis of effective PVC stabilisation, which determines processing properties and the durability of rigid materials and products, is due mainly to the increased selfstability of PVC [17, 36-39]. This can be achieved by chemical stabilisation of the labile oxovinylene groups present in the initial PVC macromolecules, first of all by studying specific polymer analogous reactions with either of the reaction centres 1-3: 2 ~C

1

C H

C H

O

CH~

(5.5)

3 Cl

The conjugation ~C(O)CH=CH~ has to be destroyed and/or the labile chlorine atom has to be replaced with a more stable adjacent group by interaction with the appropriate additives (stabilisers). This principle underlies the stabilisation of PVC in real formulations during manufacture of rigid materials and products, which is called ‘chemical stabilisation’ of PVC [17, 36, 37]. The reactions on centres 1-3 mentioned above are as follows: C O fragments of oxovinylene chloride groups:

(1) Polymer analogous reactions on

~ CH

R3SiH R3GeH O ~C

R3

CH

CH

CHCl~

[5]

Si

O CH

CH

CHCl~

R'

CH

CH

OH

OH

CH

CH

R"

R' R'

R"

CH

~C O

O

CH

CH

C

CHCl~

[40]

(5.6)

R"

141

Handbook of Plastic Films C C

(2) Polymer analogous reactions on

~C

CH2

fragments of oxovinylene groups:

CH

~C

CHCl~

P(OR)3 O

and/or

P(OR)3

CH~ CH

O

CHCl~

P

O

O

OR

[31-35] O

R'

CH

CH

CH

CH

~C

R"

CHCl~ CH

CH

CH

R'

[41] CH

HC

R"

CH

O C HC

O

CH

C

CH C O

O

[21]

CH

O

C O

O

CH

~HC

CHCl~

(5.7)

C H

(3) Polymer analogous reactions on labile

~C(O)

R2Sn(COOR)2

CH

C Cl groups:

CH

CH

CH2–

[16, 17]

Cd(COOR)2

OC(O)R

Zn(COOR)2, etc. O R'

CH

CH

(ZnCl2)

142

R"

~C(O)

CH

CH

CH

[43] O—CHR'—CHClR"

(5.8)

Achievements for Polyvinyl Chloride: Degradation and Stabilisation The concept of oxovinylene activation of the disintegration of PVC has allowed revealing new unexpected possibilities for effective stabilisation – not only thermal, but also light stabilisation – of this polymer. This also allows previously unknown classes of chemical compounds to be used for its stabilisation, in particular, conjugated diene hydrocarbons, Diels-Alder reaction adducts, protonic acids, α,β-dicarbonic compounds, etc. [34, 35, 4046]. It has also enabled new real reactions for PVC stabilisation to be revealed, including the application of known additives that have been used for a long time for PVC stabilisation (for instance, organic phosphites, epoxy compounds, proton-donating compounds, etc.). So, on this basis it is possible to manage the PVC ageing process more effectively (Scheme 5.3). The relation between the chemical structure of additives and their efficiency as stabilisers for

Scheme 5.3

143

Handbook of Plastic Films PVC gives an opportunity for the scientifically based and economically expedient selection of the appropriate stabilisers and their synergistic combinations for producing rigid materials based on PVC.

5.5 Light Stabilisation of PVC Polymer analogous transformations of the oxovinylene groups in PVC macromolecules on chemical stabilisation with the appropriate chemical additives lead not only to increased self-stability of PVC and inhibition of macromolecular crosslinking, but also to a noticeable increase in the colour stability of PVC. The transformation of oxovinylene groups as a result of polymer analogous transformations with chemical additives in the ketophosphonate, cyclohexane, dioxolane, dihydropyran, etc., structural groups in PVC and the ‘curing’ of labile oxovinylene chloride groups result in an increase in the optical density of PVC in the UV region of the spectrum. As a result, these groups act as internal light stabilisers and result in the phenomenon of self-photostabilisation of PVC [47] (Figure 5.4).

Figure 5.4 Dependence of whiteness retention coefficient Kw in PVC films on exposure time: (1) unstabilised PVC; and polymer treated with: (2) 2-tris(2-ethylhexyl) phosphite; (3) 2-ethylhexyl-9,10-epoxy stearate with ZnCl2; (4) piperylene; (5) cyclopentadiene (295 K, λ = 254 nm, 1 to 1.5 × 1015 photon/s cm2) 144

Achievements for Polyvinyl Chloride: Degradation and Stabilisation Thus, the determining factor causing the high rate of PVC disintegration and its need for stabilisation is the presence of abnormal groups, mainly oxovinylene ones, in the structure of its macromolecules.

5.6 Effect of Plasticisers on PVC Degradation in Solution In both plasticised (semi-rigid and flexible) PVC materials as well as PVC in solution, the rates of thermal destruction and effective stabilisation are caused by essentially different fundamental phenomena in comparison to those involved in the ageing of PVC in the absence of solvent. The following aspects of both structure and macromolecular dynamics have a significant influence on the stability of PVC: the chemical nature of the solvent, its basicity, specific and nonspecific solvation, the concentration of PVC in the solution, the segmental mobility of macromolecules, the thermodynamic properties of the solvent, the formation of associates, aggregates, etc. The chemical stabilisation of PVC plays a less significant role. As regards PVC destruction in solution, one of the basic reasons for a change in the kinetic parameters is the nucleophilic activation of the PVC dehydrochlorination reaction. The process is described by an E2 mechanism. Thus, there is a linear dependence between PVC thermal dehydrochlorination rate and the relative basicity of the solvent, B cm–1 (Figure 5.5) [48-50]. The value B cm–1 is evaluated by measuring the shift of a characteristic band (phenolic OH) at λ = 3600 cm–1 in the IR spectrum due to interaction with the solvent [51]. It is very important that, in solvents with relative basicity B > 50 cm–1, the rate of PVC dehydrochlorination is always above the rate of PVC dehydrochlorination without the solvent; while, when B < 50 cm–1, PVC disintegration rate is always less than that without the solvent. The revealed dependence VHCl = f(B) is described by the equation: * VHCl = VHCl + k(B − 50)

(5.9)

Inhibition of PVC disintegration in solvents with basicity B < 50 cm–1 is a very interesting and practically important phenomenon. It has been given the name ‘solvational stabilisation’ of PVC. However, ignoring the fact that, even at low concentration (2 wt%), PVC solutions should be represented not as solutions with isolated macromolecules, but rather as structured systems, results in a number of cases of deviation from a linear dependence of PVC dehydrochlorination rate on the solvent basicity B cm–1. In particular, an abnormal destruction behaviour of PVC is observed in certain ester-type solvents (plasticisers) (Figure 5.5, points 25-28), apparently caused by structural changes of the macromolecules. This has never before been taken into account when working with PVC solutions.

145


Figure 5.5 Influence of the solvent’s basicity on the rate of thermal dehydrochlorination in solution: (1) n-dichlorobenzene, (2) o-dichlorobenzene, (3) naphthalene, (4) nitrobenzene, (5) acetophenone, (6) benzonitrile, (7) di-(n-chlorophenylchloropropyl) phosphate, (8) triphenyl phosphite, (9) phenyl-bis(β-chloroethyl) phosphate, (10) tri-(n-chlorophenyl) phosphate, (11) 2-ethylhexylphenyl phosphate, (12) tricresyl phosphate, (13) cyclohexanone, (14) phenyl-bis(β-chloropropyl) phosphate, (15) tri-β-chloroethyl phosphate, (16) tri-β-chloropropyl phosphate, (17) di-(2-ethylhexyl) phosphate, (18) 2-ethylhexylnonyl phosphate, (19) tri-(2ethylhexyl) phosphate, (20) tributyl phosphate, (21,25) dibutyl phthalate, (22,26) di-(2-ethylhexyl) adipate, (23,27) dioctyl phthalate, (24,28) dibutyl sebacate. Concentration of PVC in solution: (1-24) 0.2 wt%, (25-28) 2 wt%; 423 K, under nitrogen

It was revealed quite unexpectedly that not only ‘polymer-solvent’ interactions, but also ‘polymer-polymer’ interactions, have a significant influence on the rate of PVC disintegration in solution. It is known that the structure and properties of the appropriate structural levels depend on the conformational and configurational nature of the macromolecules, including the supermolecular structure of the polymer, which in turn determines all the basic (both physical and chemical) characteristics of the polymer. ‘Polymer-polymer’ interaction leads to the formation of structures on the supermolecular level. In particular, on going to a more concentrated solution, the PVC-solvent system consistently passes through a number of stages, from isolated PVC macromolecules in

146

Achievements for Polyvinyl Chloride: Degradation and Stabilisation solution (infinitely dilute solution) to associates and aggregates of macromolecules in solution. On further increase of PVC concentration, the formation of a spatial fluctuational net with a structure similar to that of the bulk polymer occurs. When the polymer concentration in solution increases, the rate of the PVC dehydrochlorination reaction changes as well, and various types of effect of the solvent on the PVC disintegration rate in solution are observed depending on the numerical value of the basicity B cm–1 [52-57]. If the relative basicity of the solvents used is B > 50 cm–1, the polymer degradation rate decreases when its concentration increases. If the basicity of the employed solvents is B < 50 cm–1, the polymer degradation rate increases with increasing polymer concentration. In all cases the rate of HCl elimination from the polymer tends to approach the values of PVC dehydrochlorination rate usually observed PVC = 5 x 10–8 (mol HCl/mol PVC)/s (Figure 5.6). in the absence of solvent VHCl

Figure 5.6 The change in PVC dehydrochlorination rate as a function of its concentration in solution: (1) cyclohexanol, (2) cyclohexanone, (3) benzyl alcohol, (4) 1,2,3-trichloropropane, (5) o-dichlorobenzene, (6) no solvent; 423 K, under nitrogen

147

Handbook of Plastic Films Equation (5.9) turns into equation (5.10) if one takes into account that the PVC degradation rate is determined not only by the relative basicity of the solvent, B, but also by its concentration in solution, C (mol PVC/litre). Also, the degree of ‘polymer-polymer’ interaction (degree of macromolecule structurisation in a solution is given by ΔC = C – C0, where C0 is the concentration at the beginning of PVC macromolecule association in solution) is considered: 0

VHCl = VHCl +

A1 (B − 50) ΔC + d1

(5.10)

where A1 = (0.8 ± 0.2) × 10–9 (mol HCl/mol PVC)/s; and d1 is a dimensionless factor reflecting the ‘polymer-solvent’ interaction (d1 = 0.5 ± 0.25). The deviation from the onset of macromolecule association in a solution is taken as an absolute value, since it can be changed in both directions to more concentrated or more dilute polymer solution. Equation (5.10) well describes the change of PVC thermal dehydrochlorination rate as a function of its concentration in a solution of relative solvent basicity B, irrespective of the chosen solvent (Figure 5.7). The observable fundamental effect has significant importance in the production of plasticised materials and products made from PVC, in particular when esters are used. Despite the very high basicity of ester-type plasticisers (B = 150 cm–1) in the range of PVC concentration in solutions above 2%, a noticeable reduction in the degradation rate of PVC is observed (Figure 5.5, points 25-28), and stabilisation of PVC occurs. This effect is caused by the formation of dense globules, associates, etc., in the PVC-plasticiser system. Practically, this allows economic formulations of plasticised materials to be created from PVC with very low content of metal-containing stabilisers, used as HCl acceptors, or without their use at all. Temperature is very important in the formation of heterophase systems. Even at low concentrations of PVC in ester-type plasticisers (for example, in dioctyl phthalate at C > 0.1 mol/l), true solutions are formed only at temperatures above 400 K. The globular structure of PVC suspension and the formation of associates are retained at temperatures up to 430-445 K. In other words, plasticised PVC is able to keep its structural individuality on a supermolecular level, which is formed during polymer synthesis. Specifically, under these conditions an ester-type plasticiser behaves not as a highly basic solvent, but as a stabiliser during PVC thermal degradation due to formation of associates, etc. This leads to a reduction of the amount of stabiliser, extension of useful lifetime of materials and products, etc.

148


Figure 5.7 The change in PVC dehydrochlorination rate as a function of its concentration in solution: (1,2) 1,2,3-trichloropropane, (3,4) cyclohexanol; (1,3) experimental data, (2,4) data calculated using equation (5.10) at A1 = 10–9 and d1 = 0.8 and 0.7, respectively, at 423 K, under nitrogen

It is necessary to note that the change in the degradation rate of PVC brought about by association of macromolecules is a general phenomenon and does not depend on how it was achieved. In particular, a change of character of the dehydrochlorination rate of PVC in solution is observed, similar to concentrated PVC solutions (Figures 5.6 and 5.7), if a change of PVC structure in solution occurs upon addition of even chemically inert nonsolvents such as hexane, decane, undecane, polyolefins, polyethylene wax, etc. [53, 56-59] (Figure 5.8). It is interesting to observe that the degree of relative change of PVC disintegration rate under the action of a second inert nonsolvent is much higher than for concentrated PVC solution. This is especially true in the case of using low-basicity solvents (trichloropropane and dichlorobenzene); it is the result of more dense formations on the supermolecular level, corresponding associates and aggregates, and, accordingly, a significant change of PVC destruction rate. The higher the content of nonsolvent (including inert polymer) in a blend and the lower the thermodynamic compatibility of the components in a solution, the more structural

149


Figure 5.8 The change in PVC thermodegradation rate on the content of the second inert polymer in solution of trichloropropane (1,3), dichlorobenzene (2) and cyclohexanol (4-6) for blends of PVC and polyethylene (1,4), polypropylene (2,5) and polyisobutylene (3,6); 423 K, under nitrogen

formation takes place in a solution, including that in the presence of polymer blends (associates, aggregates). Formation of a fluctuational net with participation of macromolecules is the probable explanation. The reason for the change in PVC thermal dehydrochlorination rate in the case of its blends with chemically inert, thermodynamically incompatible polymers is the same. It is due to the fact that in concentrated PVC solution (structural chemical changes of polymer in solution), the parameters determining the rate of PVC disintegration will obviously be similar. Therefore, for PVC thermal destruction, the concentration of the second polymer blended with PVC and its degree of thermodynamic affinity to PVC, in addition to the influence of polymer concentration in the solution, the basicity of the solvent, B cm–1, and ‘polymer-solvent’ interaction forces, have to be taken into account. In view of these factors equation (5.10) turns into: 0 + VHCl = VHCl

150

A1 (B − 50)

ΔC + d1 + C + α n

+

A1α 2α n BC

(5.11)

Achievements for Polyvinyl Chloride: Degradation and Stabilisation where α is the fraction of the second polymer, varying from 0 to 0.99; n is a dimensionless parameter describing the degree of thermodynamic affinity of PVC to the second polymer and varying from 0 (for complete thermodynamic compatibility of the components) up to a value of ~10 (for complete thermodynamic incompatibility of the polymers); and d2 is a dimensionless coefficient reflecting the interaction of the second polymer with the solvent, which equals 2.5 ± 0.1 for the destruction of PVC blended with polyethylene in dichlorobenzene, trichloropropane and cyclohexanol. Observable changes in PVC thermal disintegration rate under the action of a solvent that is thermodynamically incompatible with PVC or for a concentrated solution of PVC are caused by the transformation of the solvent from macromolecular globules of PVC to the structure that existed in the absence of the solvent. This evokes the unexpected effect of ‘solvent action’, either retardation or acceleration of PVC thermal disintegration depending on the solvent basicity, B cm–1. A solvent transformation that accelerates PVC disintegration (B > 50 cm–1) results in a decrease of its interaction with PVC and leads to a delay in the HCl elimination process, i.e., to stabilisation. This occurs in the case of both concentrated PVC solutions as well as the addition of another polymer that is thermodynamically incompatible with PVC. In solvents that slow down PVC disintegration (B < 50 cm–1) by virtue of low nucleophilicity, the effect of solvent transformation and the weakening of its interaction with PVC has the opposite result. In this case an increase of HCl elimination rate from PVC upon increase of its concentration in solution or by using a chemically inert nonsolvent occurs. It is obvious that, irrespective of how the changes to the PVC structure in solution are made, either by increase of its concentration in solution or by addition of another thermodynamically incompatible inert nonsolvent, the varying structuralphysical condition of the polymer results in a noticeable change of its thermal dehydrochlorination rate in solution. These effects are caused by structural-physical changes in the polymer-solvent system, and the previously unknown phenomena can be classified as ‘structural-physical stabilisation’ (in the case of a reduction in the gross rate of PVC disintegration in highly basic solvents at B > 50 cm–1) or ‘structuralphysical antistabilisation’ (in the case of an increase in the gross rate of PVC disintegration in low-basicity solvents with B < 50 cm–1), respectively.

5.7 ‘Echo’ Stabilisation of PVC Finally, it is necessary to describe one more appreciable achievement in the field of ageing and stabilisation of PVC in solution. In real conditions the basic reason for the sharp accelerated ageing of plasticised materials and products is oxidation of the solvent by the oxygen of the air (Figure 5.9, curve 3).

151


Figure 5.9 ‘Echo’ stabilisation of PVC. Elimination of HCl during thermal (argon) (1,2) and thermo-oxidative (air) (3-5) destruction of PVC in solution of dioctyl sebacate: (1-4) unstabilised PVC, (5) PVC stabilised with diphenylpropane (0.02 wt%) – ‘echo’ stabilisation; (2,4) PVC with no solvent; 448 K

•

2 RO2 + RH ⎯ ⎯ ⎯ → ROOH + R •

K

3 ROOH ⎯ ⎯ ⎯ → RO• + HO•

K

•

•

(5.12)

K6

RO2 + RO2 ⎯ ⎯ ⎯→ inactive products

Peroxides, formed by oxidation of ester-type plasticisers, initiate the disintegration of macromolecules. In these conditions the rate of PVC destruction increases by two or more orders of magnitude and is determined by the oxidation stability parameter of the solvent to oxygen Kef = K2 K03.5 K6−0.5 . Thus, a higher oxidation stability of the solvent (in particular, an ester-type plasticiser) lowers the degradation rate of semi-rigid and flexible PVC materials and increases its useful lifetime [60-63]. Inhibition of the oxidation process of the solvents (including plasticisers) due to the incorporation of stabilisers, antioxidants or their synergistic compositions slows down the thermo-oxidative disintegration of PVC in solution (Figure 5.9, curve 5). Effective inhibition of the oxidation of ester-type plasticisers by oxygen of the air causes the rate of PVC thermo-oxidative destruction in concentrated solutions to become closer

152

Achievements for Polyvinyl Chloride: Degradation and Stabilisation to the rate of polymer disintegration. This behaviour is characteristic of the thermal destruction of PVC in the presence of plasticisers acting as solvent. In other words, it becomes slower than PVC disintegration in the absence of solvent. This occurs due to a structural-physical stabilisation. In these cases, inhibition of the solvent oxidation reaction by using ‘echo’-type antioxidant stabilisers improves PVC stabilisation (Figure 5.9, curve 5). This fundamental phenomenon of PVC stabilisation in solution and its thermooxidative destruction has been called ‘echo stabilisation’ of PVC [49, 62, 63].

5.8 Tasks for the Future The creation of high-quality and economic semi-rigid and flexible materials and products made from PVC, including those where solvents are employed, requires specific approaches that are essentially different from the principles of manufacture of rigid PVC materials and products. In particular, consideration and use of the following fundamental phenomena should be considered: solvational, structural-physical and ‘echo’ stabilisation of the polymer in solution. As far as paramount tasks of fundamental and applied research in the field of PVC manufacture and processing at the beginning of the 21st century are concerned, they are obviously the following: (1) The manufacture of industrial PVC that does not contain labile groups in its backbone. This will provide a drastic increase in the intrinsic stability of polymeric PVC products, the possibility of processing with the minimal content or total absence of stabilisers and other chemical additives, and the opportunity to create PVC-based materials and products with essentially increased useful service lifetime. (2) Wide use of the latest achievements in the field of destruction and stabilisation of PVC, in both the presence and the absence of solvents. The phenomena of chemical, solvational, structural-physical, self- and ‘echo’ stabilisation of PVC will allow the creation of rigid, semi-rigid and flexible (plasticised) materials and products with minimal content of chemical additives, and will lead to increased useful service lifetime under natural and special conditions. (3) The use of nontoxic and nonflammable products that do not emit toxic and other poisonous gaseous and liquid products at elevated temperature during the manufacture and processing of PVC materials and their products. (4) Complete elimination of all toxic and even low-toxicity (particularly compounds based on barium, cadmium and lead, etc.), chemical additives from all formulations.

153

Handbook of Plastic Films (5) The search for nontoxic and highly effective inorganic chemical additives, primarily, stabilisers of zeolite type, modified clays, etc. At the same time, new ‘surprises’ will undoubtedly be presented to us by this outstanding polymer puzzle. Certainly, as we look for a plastic for use as a ‘work-horse’ for many decades, studies on PVC will lead to new stimuli in the development of scientific ideas and practical development, and the opening-up of new pathways. These will result from the essential need to delay PVC ageing in natural and special conditions, and to reduce the amounts of the appropriate chemical additives, down to their complete elimination.

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Achievements for Polyvinyl Chloride: Degradation and Stabilisation 45. S.V. Kolesov, K.S. Minsker, V.M. Yanborisov, G.E. Zaikov, K. De-Jong and R.M. Akhmetkhanov, Plasticheskie Massy, 1983, 12, 39. 46. S.V. Kolesov, A.M. Steklova, G.E. Zaikov and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series A, 1986, 28, 9, 1885. 47. K.S. Minsker, G.T. Fedoseeva, L.D. Strelkova, V.V. Petrov and S.V. Kolesov, Vysokomolekulyarnye Soedineniya, Series B, 1983, 25, 3, 165. 48. K.S. Minsker, M.I. Abdullin, V.I. Manushin, L.N. Malyshev and S.A. Arzhakov, Doklady Akademii Nauk SSSR, 1978, 242, 2, 366. 49. K.S. Minskerin in Polymer Yearbook, Volume 11, Ed., R.A. Pethrick, Harwood Academic, Chur, Switzerland, 1994, 229. 50. K.S. Minsker, E.I. Kulish and G.E. Zaikov, Vysokomolekulyarnye Soedineniya, Series B, 1993, 35, 6, 316. 51. V.A. Palm, Osnovy Kolichestvennoi Teorii Organicheskikh Reaktsii (Foundation of Quantitative Theory of Organic Reactions), Khimiya Publishing House, Leningrad, 1977, 114. 52. S.V. Kolesov, E.I. Kulish and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series B, 1994, 36, 8, 1383. 53. S.V. Kolesov, E.I. Kulish, G.E. Zaikov and K.S. Minsker, Russian Polymer News, 1997, 2, 4, 6. 54. E.I. Kulish, S.V. Kolesov and K.S. Minsker, Bashkirskii Khimicheskii Zhurnal, 1998, 5, 2, 35. 55. E.I. Kulish, S.V. Kolesov, K.S. Minsker and G.E. Zaikov, Vysokomolekulyarnye Soedineniya, Series A, 1998, 40, 8, 1309. 56. S.V. Kolesov, E.I. Kulish, G.E. Zaikov and K.S. Minsker, Journal of Applied Polymer Science, 1999, 73, 1, 85. 57. E.I. Kulish, S.V. Kolesov, K.S. Minsker and G.E. Zaikov, Chemical Physics Reports, 1999, 18, 4, 705. 58. E.I. Kulish, S.V. Kolesov, R.M. Akhmetkhanov and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series B, 1993, 35, 4, 205. 59. E.I. Kulish, S.V. Kolesov, K.S. Minsker and G.E. Zaikov, International Journal of Polymeric Materials, 1994, 24, 1-4, 123.

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6

Ecological Issues of Polymer Flame Retardants G.E. Zaikov and S.M. Lomakin

6.1 Introduction The use of polymer flame retardants has an important role in saving lives. The main flame retardant systems for polymers currently in use are based on halogenated, phosphorus, nitrogen and inorganic compounds. All of these flame retardant systems basically inhibit or even suppress the combustion process by chemical or physical action in the gas or condensed phase. Conventional flame retardants, such as halogenated, phosphorus or metallic additives, have a number of negative attributes. The ecological issue of their application demands the search for new polymer flame retardant systems. Among the new trends in flame retardancy, the following should be pointed out: intumescent systems, polymer nanocomposites, preceramic additives, low-melting glasses, different types of char-formers and polymer morphology modification processing. Brief explanations of the three major types of flame retardant systems (intumescent systems, polymer nanocomposites and polymer organic char-formers) are the subject of this overview. Our environment has a mostly polymeric nature, and all polymers, whether natural or synthetic, will burn, so the use of polymer flame retardants has an important role in saving lives. There are four main families of flame retardant chemicals: (1) Inorganic flame retardants including aluminium trioxide, magnesium hydroxide, ammonium polyphosphate and red phosphorus. This group represents about 50% by volume of global flame retardant production [1]. (2) Halogenated flame retardants, primarily based on chlorine and bromine. The brominated flame retardants (BFR) are included in this group. This group represents about 25% by volume of global production [1]. (3) Organophosphorus flame retardants are primarily phosphate esters and represent about 20% by volume of global production [1]. Organophosphorus flame retardants may contain bromine or chlorine. (4) Nitrogen-based organic flame retardants are used for a limited number of polymers.

159


6.2 Mechanisms of Action Depending on their nature, flame retardants can act chemically and/or physically in the solid, liquid or gas phase. They interfere with combustion during a particular stage of this process, e.g., during heating, decomposition, ignition or flame spread. Substitution of one type of flame retardant by another consequently means a change in the mechanism(s) of flame retardancy. Halogen-containing flame retardants act primarily by a chemical interfering with the radical chain mechanism that takes place in the gas phase during combustion. High-energy OH and H radicals formed during combustion are removed by bromine released from the flame retardant. Although brominated flame retardants are a highly diverse group of compounds, the flame retardancy mechanism is basically the same for all compounds. However, there are differences in the flame retardancy performance of brominated compounds, as the presence of such compounds in the polymer will influence the physical properties of the polymer. In general, aliphatic bromine compounds are easier to break down and hence more effective at lower temperatures, but are also less temperature-resistant than aromatic retardants. Aluminium hydroxide and other hydroxides act in a combination of various processes. When heated, the hydroxides release water vapour, which cools the substrate to a temperature below that required to sustain the combustion processes. The water vapour liberated also has a diluting effect in the gas phase and forms an oxygen-displacing protective layer. Additionally, together with the charring products, the oxide forms an insulating protective layer. Phosphorus compounds mainly influence the reactions taking place in the solid phase. By thermal decomposition, flame retardants are converted to phosphorous acid, which in the condensed phase extracts water from the pyrolysing substrate, causing it to char. However, some phosphorus compounds may, similarly to halogens, act in the gas phase as well by a radical trapping mechanism. Interest in flame retarding polymers goes back to the 19th century with the discovery of highly flammable cellulose nitrate and celluloid. In more recent times a large volume of conventional plastics such as phenolics, rigid polyvinyl chloride (PVC) and melamine resins possess adequate flame retardancy. By the 1970s the major flame retardant polymers were the thermosets, namely, unsaturated polyesters and epoxy resins that utilised reactive halogen compounds and alumina hydrate as an additive. There was also a large market for phosphate esters in plasticised PVC, cellulose acetate film, unsaturated polyesters and modified polyphenylene oxide. Alumina trihydrate (ATH) was the largest-volume flame retardant in unsaturated plastics.

160

Ecological Issues of Polymer Flame Retardancy Consumption of halogen-containing flame retardant additives in the 1970s was much less than that of other additives. The term ‘halogenated flame retardants’ covers a large number of different organic substances, all with chlorine or bromine in their molecular structure. Bromine and chlorine have an inhibitory effect on the formation of fire in organic materials. Flame retardants are added to plastics and textiles in order to comply with fire safety requirements. The halogenated flame retardant additives include: (1) Dechlorane Plus, (2) a chlorinated acyclic (for polyolefins), (3) tris(dibromopropyl) phosphate, (4) brominated aromatics, (5) pentabromochlorocyclohexane and (6) hexabromocyclododecane (for polystyrene). A number of chlorinated flame retardant products were produced under the Dechlorane trade name. The products include: (1) two moles of hexachlorocyclopentadiene and contained 78% chlorine, (2) Dechlorane Plus, (3) a Diels-Alder reaction product of cyclooctadiene and hexachlorocyclopentadiene with 65% chlorine, (4) a Diels-Alder product with furan and (5) a product containing both bromine and chlorine with 77% halogen developed for polystyrene and acrylonitrile-butadiene-styrene (ABS) materials [1]. In 1985-1986 a German study detected brominated dioxins and furans from pyrolysis of a brominated diphenyl oxide in the laboratory at 510-630 °C [2]. The relevance of these pyrolysis studies to the real hazard presented by these flame retardants under actual conditions of use has been questioned. Germany and Holland have considered a ban or curtailed the use of brominated diphenyl oxide flame retardants because of the potential formation of highly toxic and potentially carcinogenic brominated furans and dioxins during combustion [1, 2]. The issue has spread to other parts of Europe, where regulations have been proposed to restrict their use.

161

Handbook of Plastic Films The chemical stability of the substances – particularly in the cases of polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) – is also the reason why brominated flame retardants have been the focus of international environmental debate for many years. PBDE and PBB, which are the most stable of the BFR described, are widespread in the environment, are bioaccumulated and accumulate in sediments, where they are degraded only very slowly.

6.3 Halogenated Diphenyl Ethers – Dioxins Chlorinated dibenzo-p-dioxins and related compounds (commonly known simply as dioxins) are contaminants present in a variety of environmental media. This class of compounds has caused great concern to the general public as well as intense interest in the scientific community. Laboratory studies suggest the probability that exposure to dioxin-like compounds may be associated with other serious health effects, including cancer. Conventional laboratory studies have provided new insights into the mechanisms involved in the impact of dioxins on various cells and tissues and, ultimately, on toxicity [1]. Dioxins have been demonstrated to be potent modulators of cellular growth and differentiation, particularly in epithelial tissues. These data, together with the collective body of information from animal and human studies, when coupled with assumptions and inferences regarding extrapolation from experimental animals to humans, and from high doses to low doses, allow a characterisation of dioxin hazards. Polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDF) and polychlorinated biphenyls (PCB) are chemically classified as halogenated aromatic hydrocarbons. The chlorinated and brominated dibenzodioxins and dibenzofurans are tricyclic aromatic compounds with similar physical and chemical properties, and the two classes are structurally similar. Certain of the PCB (the so-called coplanar or mono-ortho coplanar congeners) are also structurally and conformationally similar. The most widely studied of these compounds is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This compound, often called simply dioxin, represents the reference compound for this class of compounds. The structures of TCDD and several related compounds are shown in Figure 6.1 [3]. These compounds are assigned individual toxicity equivalence factor (TEF) values as defined by the international convention ‘Interim Procedures for Estimating Risks Associated with Exposures to Mixtures of Chlorinated Dibenzo-p-Dioxins and Dibenzofurans’ (US Environmental Protection Agency, USEPA, March 1989). Results of in vitro and in vivo laboratory studies have contributed to the assignment of a relative toxicity value. TEF are estimates of the toxicity of dioxin-like compounds relative to the toxicity of TCDD, which is assigned a TEF of 1.0. All chlorinated dibenzodioxins (CDD) and chlorinated dibenzofurans (CDF) with chlorines substituted in the 2, 3, 7 and 8

162

Ecological Issues of Polymer Flame Retardancy

2,3,7,8-Tetrachlorodibenzo-p-dioxin

1,2,3,7,8-Pentachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzofuran

2,3,4,7,8-Pentachlorodibenzofuran

3,3′,4,4′,5,5′-Hexachlorobiphenyl

3,3′,4,4′,5′-Pentachlorobiphenyl

Figure 6.1 The structures of dioxin and similar compounds

positions are assigned TEF values [1]. Additionally, the analogous brominated dibenzodioxins (BDD) and brominated dibenzofurans (BDF) and certain polychlorinated biphenyls have recently been identified as having dioxin-like toxicity and thus are also included in the definition of dioxin-like compounds. Generally accepted TEF values for chlorinated dibenzodioxins and dibenzofurans are shown in Table 6.1 [4].

163


Table 6.1 Toxicity equivalence factors (TEF) for CDD and CDF [4] Compound*

Toxicity equivalence factors, TEF

Mono-, di- and tri-CDD

0

2,3,7,8-TCDD

1

Other TCDD

0

2,3,7,8-PeCDD

0.5

Other PeCDD

0

2,3,7,8-HxCDD

0.1

Other HxCDD

0

2,3,7,8-HpCDD

0.01

Other HPCDD

0

Mono-, di-, and tri-CDF

0

2,3,7,8-TCDF

0.1

Other TCDF

0

1,2,3,7,8-PeCDF

0.05

2,3,4,7,8-PeCDF

0.5

Other PeCDF

0

2,3,7,8-HxCDF

0.1

Other HxCDF

0

2,3,7,8-HpCDF

0.01

Other HPCDF

0

OCDF

0.001

*CDD, chlorinated dibenzodioxin; CDF, chlorinated dibenzofuran. Prefixes: tetra T, penta Pe, hexa Hx, hepta Hp, octa O.

A World Health Organization/International Program on Chemical Safety meeting held in the Netherlands in December 1993 considered the need to derive internationally acceptable interim TEF for the dioxin-like PCB. Recommendations arising from that meeting of experts suggest that in general only a few of the dioxin-like PCB are likely to be significant contributors to general population exposures to dioxin-like compounds [5]. Dioxin-like PCB may be responsible for approximately one-quarter to one-half of the total toxicity equivalence associated with general population environmental exposures to this class of related compounds.

164

Ecological Issues of Polymer Flame Retardancy There are 75 individual compounds comprising the CDD, depending on the positioning of the chlorine(s), and 135 different CDF. These are called individual congeners. Likewise, there are 75 different positional congeners of the BDD and 135 different congeners of the BDF. Only seven of the 75 congeners of the CDD or the BDD are thought to have dioxin-like toxicity; these are ones with chlorine/bromine substitutions in, at least, the 2, 3, 7 and 8 positions. Only 10 of the 135 possible congeners of the CDF or the BDF are thought to have dioxin-like toxicity; these also are ones with substitutions in the 2, 3, 7 and 8 positions. While this suggests 34 individual CDD, CDF, BDD or BDF with dioxin-like toxicity, inclusion of the mixed chloro/bromo congeners substantially increases the number of possible congeners with dioxin-like activity. There are 209 PCB congeners. Only 13 of these 209 congeners are thought to have dioxin-like toxicity; these are PCB with four or more chlorines with just one or no substitution in the ortho position. These compounds are sometimes referred to as coplanar, meaning that they can assume a flat configuration with rings in the same plane. Similarly configured polybrominated biphenyls are likely to have similar properties; however, the database on these compounds with regard to dioxin-like activity has been less extensively evaluated. Mixed chlorinated and brominated congeners also exist, increasing the number of compounds considered dioxin-like. The physical/chemical properties of each congener vary according to the degree and position of chlorine and/or bromine substitution. Very little is known about the occurrence and toxicity of the mixed (chlorinated and brominated) dioxin, furan and biphenyl congeners. In general, these compounds have very low water solubility, high octanol-water partition coefficients and low vapour pressure, and they tend to bioaccumulate. Although these compounds are released from a variety of sources, the congener profiles of CDD and CDF found in sediments have been linked to combustion sources [1]. The Hazards Substance Ordinance in Germany specifies the maximum level of chlorinated dibenzodioxins and furans that can be present in materials marketed in Germany. This has been extended to the brominated compounds. The two largest-volume flame retardants, decabromodiphenyl oxide and tetrabromo-bisphenol A, are said to meet these requirements [2]. The International Program for Chemical Safety (IPCS) of the World Health Organization has made several recommendations. Polybrominated diphenyls production (in France) and use should be limited because of the concern over high persistency, bioaccumulation and potential adverse effects at low levels. There are limited toxicity data on deca- and octabromodiphenyls. Commercial use should cease unless safety is demonstrated. For the polybrominated diphenyl oxides, a Task Group felt that polybrominated dibenzofurans, and to a lesser extent the dioxins, may be formed. For decabromodiphenyl oxide, appropriate industrial hygiene measures need to be taken, and environmental exposure minimised by effluent and emission control. Controlled incineration procedures should be instituted. For octabromodiphenyl oxide, the hexa- and lower isomers should

165

Handbook of Plastic Films be minimised. There is considerable concern over persistence in the environment and accumulation in organisms, especially for pentabromodiphenyl oxide. There are no regulations proposed or in effect anywhere around the world banning the use of brominated flame retardants. The proposed EU Directive on the brominated diphenyl oxides has been withdrawn. Deca- and tetrabromo-bisphenol A as well as other brominated flame retardants meet the requirements of the German Ordinance regulating the dioxin and furan content of products sold in Germany [6]. The European search for a replacement for decabromodiphenyl oxide in high-impact polystyrene (HIPS) has led to consideration of other bromo-aromatics, such as Saytex 8010 from Albemarle, and a heat-stable chlorinated paraffin from Atochem. The former product is more costly, and the latter, if sufficiently heat-stable, lowers the heat distortion under load (HDUL) significantly. Neither approach has been fully accepted. In September 1994, the USEPA released a final draft of exposure and risk assessment of dioxins and dioxin-like compounds [5]. This reassessment finds the risks greater than previously thought. Based on this reassessment, a picture emerges that tetrachlorodiphenyl dioxins and related compounds are potent toxicants in animals, with the potential to produce a spectrum of effects. Some of these effects may occur in humans at very low levels, and some may result in adverse impacts on human health. The USEPA also concluded that dioxin should remain classified as a probable human carcinogen [5]. Polymer producers have been seeking non-halogen flame retardants, and the search has been successful in several polymer systems. Non-halogen flame retardant polycarbonate/ABS blends are now commercial. They contain triphenyl phosfate or resorcinol diphosfate (RDP) as the flame retardant. Modified polyphenylene oxide (GE’s Noryl) has used phosfate esters as the flame retardant for the past 15-20 years, and the industry recently switched from the alkylated triphenyl phosphate to RDP. Red phosphorus is used with glass-reinforced Polyamide-6,6 (PA-6,6) in Europe, and melamine cyanurate is used in unfilled PA. Magnesium hydroxide is being used commercially in polyethylene wire and cable. The non-halogen solutions present other problems, such as poor properties (plasticisers lower the heat distortion temperature), difficult processing (high loadings of ATH and magnesium hydroxide), corrosion (red phosphorus) and handling problems (red phosphorus). In this chapter, we have tried to present the basic trends in the flame retardants hierarchy.

6.4 Flame Retardant Systems The main flame retardant systems for polymers currently in use are based on halogenated, phosphorus, nitrogen and inorganic compounds (Figure 6.2). Basically, all these flame retardant systems inhibit or even suppress the combustion process by chemical or physical

166

Ecological Issues of Polymer Flame Retardancy action in the gas or condensed phase. To be effective, the flame retardants must be stable at processing temperatures yet decompose near the decomposition temperature of the polymer in order for the appropriate chemistry to take place as the polymer decomposes. Conventional flame retardants, such as halogenated, phosphorus or metallic additives, have a number of negative attributes. The ecological issue of their application requires that new polymer flame retardant systems are sought. Among the new trends in flame retardancy, the use of intumescent systems, polymer nanocomposites, preceramic additives, low-melting glasses, different types of char-formers and polymer morphology modification should be noted [1]. However, the close interactions between the different flame retardant types should be considered in order to achieve synergistic behaviour. A block scheme of polymer flame retardant systems is given in Figure 6.2.

FLAME RETARDANTS (FR)

HALOGENATED FR

PHOSPHORUS FR

Nitrogen-containing FR

ANTIMONY OXIDE Mg HYDROXIDE, ALUMINA TRIHYDRATE, BORON FR

Ecologically friendly flame retardant systems

INTUMESCENT SYSTEMS

POLYMER NANOCOMPOSITES

POLYMER ORGANIC CHARH FORMERS

Preceramics, Low-melting Glass

POLYMER MORPHOLOGY MODIFICATION

Figure 6.2 A block diagram of polymer flame retardant systems

167

Handbook of Plastic Films Brief discussions of the three major types of flame retardant systems (intumescent systems, polymer nanocomposites and polymer organic char-formers) are presented next.

6.5 Intumescent Additives Intumescent behaviour, resulting from the combination of charring and foaming of the surface of burning polymers, is being widely developed for fire retardancy because it is characterised by a low environmental impact. Among alternative candidates, considerable attention has been paid to intumescent materials because they provide fire protection with the minimum of overall fire hazard [7]. Since the first intumescent coating material was patented in 1938 [8], the mechanism of intumescent flame retardancy has referred to the formation of a foam that acts as an insulating barrier between the fire and the substrate. In particular, such intumescence depends significantly on the ratio of carbon, nitrogen and phosphorus atoms in the compound [7, 9]. Although intumescent coatings are capable of exhibiting good fire protection for the substrate, they have several disadvantages, such as water solubility, brushing problems and relatively high cost [10]. The fire retardation of plastic materials is generally achieved by incorporating fire retardant additives into the plastic during processing [11, 12]. Since the processing requires that additives can withstand temperatures up to about 200 °C or more, intumescent systems with insufficient thermal stability cannot be incorporated into various plastics. The phosphate-pentaerythritol system has been investigated and developed as an intumescent material [7]. For example, a systematic study on a mixture of ammonium polyphosphate and pentaerythritol has shown that intumescence occurs on flaming [13, 14]. Thus, new intumescent materials with appropriate thermal stability have been synthesised for better fire retardancy [15]. The most important inorganic nitrogen-phosphorus compound used as an intumescent flame retardant is ammonium polyphosphate, which is applied in intumescent coatings and in rigid polyurethane foams. The most important organic nitrogen compounds used as flame retardants are melamine and its derivatives, which are added to intumescent varnishes or paints. Melamine is incorporated into flexible polyurethane cellular plastics, and melamine cyanurate is applied to unreinforced PA. Guanidine sulfamate is used as a flame retardant for PVC wall coverings in Japan. Guanidine phosphate is added as a flame retardant to textile fibres, and mixtures based on melamine phosphate are used as flame retardants for polyolefins or glass-reinforced PA. All the above-mentioned compounds – ammonium polyphosphate, melamine, guanidine and their salts – are characterised by an apparently acceptable environmental impact.

168

Ecological Issues of Polymer Flame Retardancy Mechanistic studies in PA-6 with added ammonium polyphosphate (APP), ammonium pentaborate (NH4B5O8; APB), melamine and its salts have been carried out using combustion and thermal decomposition approaches [16, 17]. It was shown that APP interacts with PA6 to produce alkylpolyphosphoric ester, which is a precursor of the intumescent char. On the surface of a burning polymer, APB forms an inorganic glassy layer that protects the char from oxidation and hinders the diffusion of combustible gases. Melamine and its salts induce scission of the H–C–C(O) bonds in PA-6, which leads to increased crosslinking and charring of the polymer [17]. APP added at 10-30 wt% to PA-6 is ineffective in the low molecular weight (low molar mass) polymer since the limiting oxygen index (LOI) remains at the level of 23-24 [18] corresponding to non-fire-retarded PA-6. However, APP becomes very effective at loadings of 40 and 50%, where the LOI increases to 41 and 50, respectively. A condensed-phase fire retardant mechanism is proposed for APP in PA-6 [18]. In fact, an intumescent layer is formed on the surface of burning PA-6/APP formulations, which tends to increase the content of APP. Thermal analysis has shown that APP destabilises PA-6, since thermal decomposition is observed at a temperature 70 °C lower than that of pure PA-6 [18]. However, the intumescent layer effectively protects the underlying polymer from the heat flux. Therefore, in the conditions of the linear pyrolysis experiments, the formulation PA-6/APP (40%) decomposes more slowly than pure polymer [18]. These experiments prove the fire retardant action of the intumescent char. Mechanistic studies of thermal decomposition in the PA-6/APP system show that APP catalyses the degradation of the polymer and interacts with it, forming essentially 5-amidopentyl polyphosphate (Scheme 6.1). On further heating, 5-amidopentyl polyphosphate again liberates polyphosphoric acid and produces the char. The intumescent shielding layer on the surface of the polymer is composed of foamed polyphosphoric acid, which is reinforced with the char [18].

Scheme 6.1 Reaction of APP with PA-6

169

Handbook of Plastic Films The effectiveness of APB in high molecular weight PA-6 (Mn = 35,000) is similar to that of APP as measured by oxygen index [19]. In contrast to APP, APB does not give an intumescent layer. Instead, a brown-black glassy-like compact layer is formed. As thermal analysis has shown, APB destabilises PA-6 since the latter decomposes at 50 °C lower. It is likely that freed boric acid catalyses the thermolysis of the Nylon. In contrast to APP, no other chemical interaction of PA-6 and APB was found. In fact, the residue obtained in thermogravimetry in a nitrogen atmosphere for PA-6/APB formulations corresponds to that calculated on the basis of the individual contributions of PA-6 and APB to the residue [19]. It is likely that a molten glassy layer of boric acid/boric anhydride accumulates on the surface of burning polymer, which protects the char from oxidation. This layer reinforced by the char creates a barrier against diffusion of the volatile fuel from the polymer to the flame, which decreases the combustibility of PA-6 [19]. A systematic mechanistic study of halogen-free fire retardant PA-6, via the combustion performance and thermal decomposition behaviour of non-reinforced PA-6 with added melamine, melamine cyanurate, melamine oxalate, melamine phthalate, melamine pyrophosphate or dimelamine phosphate, has been reported [20]. Melamine, melamine cyanurate, melamine oxalate and melamine phthalate promote melt dripping of PA-6, which increases as the additive concentration increases. These formulations self-extinguish very quickly in air, and their LOI increase with increasing concentration (Table 6.2) [20]. The melt dripping effect is very strong in the case of melamine phthalate, where a small amount of the additive (3-10%) leads to large increases in LOI (from 34 to 53). The combustion behaviour of melamine pyrophosphate and dimelamine phosphate is different from that of melamine itself and the other melamine salts (Table 6.2). The former are ineffective at concentrations below 15% and become effective at a loading of 20-30% because an intumescent char is formed on the surface of burning specimens. The mechanism

Table 6.2 Oxygen indices for high molecular weight PA-6 with added melamine or its salts (for pure PA-6, LOI = 24) [20] Additive

Additive concentration (wt%) 3

5

10

15

20

30

Melamine

-

29

31

33

38

39

Dimelamine phosphate

-

23

24

25

26

30

Melamine pyrophosphate

-

24

25

25

30

32

Melamine oxalate

-

28

29

-

33

-

Melamine cyanurate

-

35

37

39

40

40

Melamine phthalate

34

48

53

-

-

-

170

Ecological Issues of Polymer Flame Retardancy of the fire retardant action of both melamine pyrophosphate and dimelamine phosphate is similar to that of APP, since, analogously with ammonia, melamine volatilises, whereas the remaining phosphoric acids produce esters with PA-6, which are precursors of the char [17]. Some part of the freed melamine condenses, probably forming the derivatives melem and melon [21]. Melamine partially evaporates from the composition PA-6/melamine (30%), whereas the other part condenses, giving 8% solid residue at 450 °C. However, similar behaviour with a more thermostable residue is shown by melamine cyanurate. Melamine pyrophosphate, like dimelamine phosphate [17], gives about 15% of thermostable char. As mentioned before, it is likely that a glassy layer of molten boric acid and boric anhydride accumulates on the surface of the burning polymer and protects the char from oxidation. The glass reinforced by the char creates a barrier against diffusion of the volatile fuel from the polymer to the flame, which decreases the combustibility of PA-6 [19]. As infrared characterisation of solid residue and high-boiling products has shown [17], carbodiimide functionalities are formed on thermal decomposition of PA-6 with melamine and its salts. An unusual mechanism of chain scission of PA-6 through CH2–C(O) bonds [22] is likely to become operative in the presence of melamines (Scheme 6.2). The resultant

Scheme 6.2 Mechanism of thermal decomposition of PA-6 in the presence of melamine [22]

171

Handbook of Plastic Films isocyanurate chain ends undergo dimerisation to carbodiimide or trimerisation to Nalkylisocyanurate. Carbodiimide can also trimerise to N-alkylisotriazine. These secondary reactions increase the thermal stability of the solid residue and increase the yield of the char. In order to understand better the chemical reactions that are responsible for the intumescent behaviour of APP/pentaerythritol (PER) mixtures, as model examples, a study of the thermal degradation of pentaerythritol diphosphate (PEDP) was undertaken [23]. PEDP is a model compound for structures identified in APP/PER mixtures heated below 250 °C. Five major degradation steps between room temperature and 950 °C have been identified using thermogravimetric analysis (TGA), and volatile products are evolved in each step. The formation of the foam reaches a maximum at 325 °C, corresponding to the second step of degradation; foam formation decreases at higher temperatures. There are no differences in the TGA or differential scanning calorimetry (DSC) curves in nitrogen or air up to 500 °C. Above this temperature, thermal oxidation leads to almost complete volatilisation in a single step, which is essentially completed at 750 °C. The elucidation of the chemical reactions that occur upon degradation is easier if each step is studied separately. The separation of the steps is accomplished by heating to a temperature at which one step goes to completion, and the following reaction occurs at a negligible rate [23]. The chemical reactions that occur in the first two steps lead to the initial formation of a char-like structure, which will undergo subsequent graphitisation. The first reaction is the elimination of water, with the condensation of OH groups. This overlaps with the elimination of organics when as little as 28% of the possible water has been evolved. This involves essentially complete scission of the phosphate ester bonds and results in a mixture of polyphosphates and a carbonaceous char. Three mechanisms have been proposed in the literature for this reaction [24, 25]: a freeradical mechanism, a carbonium ion mechanism, and a cyclic cis-elimination mechanism. The free-radical mechanism has been ruled out because of the lack of effect of freeradical inhibitors on the rate of pyrolysis [25]. The carbonium ion mechanism is supported by acid catalysis and kinetic behaviour, and may compete with the ciselimination mechanism [24, 25]. The carbonium ion mechanism should occur exclusively if there is no hydrogen atom on the β-carbon atom, as in PEDP, which is necessary for the cyclic transition state of the elimination mechanism. The olefin is generated from the thermodynamically most stable carbonium ion. Hydride migration or skeletal rearrangement may take place to give a more stable carbonium ion of high reactivity. After ring opening in the ionic ester pyrolysis mechanism, a second ester pyrolysis reaction occurs, which could also take place by the cis-elimination mechanism, as shown in Scheme 6.3. 172


Scheme 6.3 Ester pyrolysis mechanism [24]

The formation of char can occur by either free-radical or acid-catalysed polymerisation reactions from the compounds produced in the pyrolysis. For example, the Diels-Alder reaction followed by ester pyrolysis and sigmatropic (1,5) shifts leads to an aromatised structure as shown in Scheme 6.4 [24]. Repetition of these steps can eventually build up the carbonaceous char, which is observed. The reaction pattern shown in Schemes 6.4 and 6.5 should help to provide the structures observed by spectroscopy in the foamed char [24]. These reactions probably occur in an irregular sequence and in competition with other processes; the final products are obtained by some random combination of polymerisation, Diels-Alder condensation, aromatisation, etc. Ester pyrolysis supplies the chemical structures, which build up the charred material through relatively simple reactions [24]. In summary, intumescent behaviour resulting from a combination of charring and foaming of the surface of burning polymers is being widely developed for fire retardancy because it is characterised by a low environmental impact. However, the fire retardant effectiveness of intumescent systems is difficult to predict because the relationship between the occurrence of the intumescence process and the fire protecting properties of the resulting foamed char is not yet understood. 173


Scheme 6.4 Free-radical char formation [24]

174


Scheme 6.5 Acid-catalysed char formation [24]

6.6 Polymer Organic Char-Former There is a strong correlation between char yield and fire resistance. This follows because char is formed at the expense of combustible gases and because the presence of a char inhibits further flame spread by acting as a thermal barrier around the unburned material. Polymeric additives – poly(vinyl alcohol) (PVOH), systems – that promote the formation of char in the PVOH/PA-6,6 system have been studied [26]. These polymeric additives 175

Handbook of Plastic Films usually produce a highly conjugated system – aromatic structures that char during thermal degradation and/or transform into crosslinking agents at high temperatures:

(

CH

CH2)n

OH

CH

CH2

(

CH

CH2)n

CH

CH2

+ H2O

(6.1)

OH

OH

Scission of several carbon-carbon bonds leads to the formation of carbonyl end-groups. For example, aldehyde end-groups arise from the following reaction:

CH

CH2

OH

CH

CH2

n

CH

CH2

OH

OH

(6.2) CH

CH2

CH

CH2

n

OH

CH + CH3

CH

O

OH

The identification of a low concentration of benzene among the volatile products of PVOH has been taken to indicate the onset of a crosslinking reaction proceeding by a Diels-Alder addition mechanism [27]. Clearly, benzenoid structures are ultimately formed in the solid residue, and the IR spectrum of the residue also indicated the development of aromatic structures:

CH2

CH

(a) CH CH

O CH

+ CH2

CH

CH

CH

OH

CH2

CH OH

OH CH2

CH

CH CH CH CH2

(6.3)

CH CH

(b)

CH C O

176

C

CH2

CH OH

Ecological Issues of Polymer Flame Retardancy Acid-catalysed dehydration promotes the formation of conjugated sequences of double bonds (a), and Diels-Alder addition of conjugated and isolated double bonds in different chains may result in intermolecular crosslinking, producing structures that form graphite or carbonisation (b). In contrast to PVOH, PA-6,6 subjected to temperatures above 300 °C in an inert atmosphere is completely decomposed. The wide range of degradation products, which include several simple hydrocarbons, cyclopentanone, water, CO, CO2 and NH3, suggest that the degradation mechanism is highly complex. Further research has led to the generally accepted degradation mechanism for aliphatic polyamides [28]:

O C

O (CH2)x

C

O NH

(CH2)y

NH

H2O

C

n

O (CH2)x

C

OH + NH2

(CH2)y

NH

n

(a) O C

O

(6.4)

(CH2)x + C + *NH (*CH2)y + *NH n

Hydrocarbons, cyclic ketones, esters, nitriles, carbon char

O C

(*CH2)x + CO2 + NH3 + *(CH2)y + *NH

n

The idea of introducing PVOH into PA-6,6 was based on the possibility of hightemperature acid-catalysed dehydration [29]. This reaction can be provided by the acid products of PA-6,6 degradation hydrolysis, which would promote the formation of intermolecular crosslinking and char. Such a system has been called ‘synergetic carbonisation’ because the char yield and flame suppression parameters of the polymer blend of PVOH and PA-6,6 show significant improvement in comparison with those of pure PVOH and PA-6,6 separately [30]. An additional improvement to the flame resistance properties of the PVOH/PA-6,6 system was suggested by means of substitution of pure PVOH by PVOH-ox [poly(vinyl alcohol) oxidised with potassium permanganate (KMnO4)] [30]. Earlier it was reported that the oxidation of PVOH in alkaline solutions occurs through the formation of two intermediate 177

Handbook of Plastic Films complexes. The final step of this process was attributed to the formation of polyvinylketone as a final product of oxidation of the substrate [31]. The fire retardancy approach was made on the basis of the fire behaviour of PVOH-ox samples. Using cone calorimeter tests, a dramatic decrease in the rate of heat release and a significant increase in the ignition time were shown experimentally for the oxidised PVOH in comparison with the original PVOH (see Table 6.3). One reason for this phenomenon may be the ability of PVOH oxidised by KMnO4 (polyvinylketone structures) to act as a neutral and/or monobasic bidentate ligand [32]. Other experimental results (IR and electronic spectra) provide strong evidence of coordination of the ligand (some metal ions Cd2+, Co2+, Cu2+, Hg2+, Ni2+) through the monobasic bidentate mode [33]. Based on the above, the following structure can be proposed for the polymeric complexes (where M = metal):

H C C

C

O

O M

O

O

C

Polymer complex scheme 6A

C C H

n

Table 6.3 Cone calorimeter data for PA-6,6/PVOH [30] Material PVOH

PVOH-ox*

Heat flux (kW/m2)

Char yield (wt%)

Ignition time (s)

Peak RHR (kW/m2)

Total heat release (MJ/m2)

20

8.8

39

255.5

159.6

35

3. 9

52

540.3

111.3

50

2. 4

41

777.9

115.7

20

30.8

1127

127.6

36.9

35

12.7

774

194.0

103.4

50

9. 1

18

305.3

119.8

*Poly(vinyl alcohol) oxidised with potassium permanganate (KMnO4).

178

Ecological Issues of Polymer Flame Retardancy Cone calorimeter combustion tests for PVOH and PVOH oxidised by KMnO4 (Table 6.3) clearly indicate the substantial improvement of fire resistance characteristics for PVOH-ox in comparison with PVOH. PVOH-ox gives about half the peak rate of heat release (peak RHR, kW/m2), when compared with pure PVOH. Even at 50 kW/m2, the yield of char residue for PVOH oxidised by KMnO4 was 9.1% [30]. The result of elemental analysis of PVOH-ox indicates the presence of 1.5% of manganese remaining in this polymeric structure [30]. It has been suggested that the catalytic amount of chelated manganese structure incorporated in the polymer can provide a rapid hightemperature process of carbonisation followed by formation of char [30]. The sample of PVOH-ox displayed even better flame retardant properties due to the catalytic effect of the manganese-chelate fragments on the formation of char (Table 6.3). However, there is a less satisfactory correlation in the determination of total rate of heat release (Table 6.3) [30]. Although, the cone calorimeter measurements indicated a decrease of total heat release for PA-6,6/PVOH and PA-6,6/PVOH-ox in comparison with pure PVOH, the sample of PA-6,6 with PVOH-ox showed a higher value of total heat release than PA6,6 with PVOH (Table 6.3). This fact has been qualitatively explained by the influence of a catalytic amount of chelated manganese structure incorporated in the polymer on the smouldering of the polymer samples. The flame out time for PA-6,6/PVOH-ox is larger than the flame out times of PA-6,6/PVOH and PA-6,6 alone (Table 6.4). The values of average heat of combustion indicate the exothermal process of smouldering provided by chelated manganese structures (Table 6.4). Approximately equal amounts of char yield for PA-6,6/PVOH and PA-6,6/PVOH-ox have been found [30].

Table 6.4 Cone calorimeter data for the heat of combustion and the flame out time for PA-6,6 compositions at a heat flux of 50 kW/m2 Flame out time (s)

Average heat of combustion (MJ/kg)

PA-6,6

512

31.5

PA-6,6/PVOH (80/20, wt%)

429

25.1

PA-6,6/PVOH-ox (80/20, wt%)

747

29.5

Composition

The polymer organic char-former (PVOH system) incorporated in PA-6,6 reduced the peak rate of heat release from 1124.6 kW/m2 (for PA-6,6) and 777.9 kW/m2 (for PVOH) to 476.7 kW/m2 and increased the char yield from 1.4% (for PA-6,6) to 8.7% due to a ‘synergistic’ carbonisation effect. The cone calorimeter was operated at 50 kW/m2 incident flux.

179

Handbook of Plastic Films Cone calorimeter data of PA-6,6 composition with PVOH oxidised by KMnO4 (manganese chelate complexes) show an improvement in the peak rate of heat release from 476.7 kW/m2 (for PA-6,6/PVOH, 80/20 wt%) to 305.3 kW/m2 (for PA-6,6/PVOH-ox, 80/20 wt%) [30]. On the other hand, the exothermal process of smouldering for PA-6,6/PVOHox compositions has been noted [30]. This reaction is evidently provided by chelated manganese structures, which increases the total heat release of PA-6,6/PVOH-ox blend in comparison with PA-6,6/PVOH blend.

6.7 Polymer Nanocomposites Polymer layered silicate (clay) nanocomposites are materials with unique properties when compared with conventional filled polymers. Polymer nanocomposites, especially polymerlayered silicates, represent a radical alternative to conventionally filled polymers. Solventless, melt intercalation of high molecular weight polymers is a new approach to synthesise polymer-layered silicate nanocomposites. This method is quite general and is broadly applicable to a range of commodity polymers from nonpolar polystyrene to strongly polar Nylon. Polymer nanocomposites are thus processable using current technologies and easily scaled to manufacturing quantities. In general, two types of structures are possible: (1) intercalated and (2) disordered or delaminated with random orientation throughout the polymer matrix. Owing to their nanometre size dispersion, the nanocomposites exhibit improved properties compared to the pure polymers or conventional composites. The improved properties include increased modulus, decreased gas permeability, increased solvent resistance and decreased flammability. For example, the mechanical properties of a PA-6 layered-silicate nanocomposite with a silicate mass fraction of only 5% show excellent improvement over those for pure PA-6 [34]. The nanocomposite exhibits 40% higher tensile strength, 68% greater tensile modulus, 60% higher flexural strength and 126% increased flexural modulus [34]. In the polymer industry there is a need for new, more effective and environmentally friendly flame resistant polymers. Recent data on the combustion of polymer nanocomposites indicate that they could be employed for this purpose [35]. There are several proposed mechanisms as to how the layered silicate affects the flame retardant properties of polymers [35]. The first is increased char layer that forms when nanocomposites are exposed to flame. This layer is thought to inhibit oxygen transport to the flame front, as well as gaseous-fuel transport from the polymer, and therefore reduces the heat release rate of the burning surface. At higher temperatures, the inorganic additive has the ability to act as a radical scavenger due to adsorption on to Lewis acid sites. This may interrupt the burning cycle, as radical species are needed to break polymer chains into fuel fragments. The disordered nanocomposites also inhibit the availability of oxygen as a combustible ‘fuel’ species by increasing the path length of these species to

180

Ecological Issues of Polymer Flame Retardancy the flame front. The path length is dramatically increased due to the surface area of the silicates (approximately 700 m2/g for Na+ montmorillonite). There is also a high possibility of alumina-silicate solid-phase catalysis of polymer decomposition, which can dramatically change the overall scheme of the kinetics of the thermal degradation process. Combustibility of some polymer nanocomposite materials was studied using a cone calorimeter [36, 37] under irradiation of 35 kW/m2, which is equivalent to that typical of a small fire [38]. The RHR, which is one of the most important parameters associated with the flammability and combustion of a material, such as those illustrated in Figure 6.3, can be evaluated during this test [36, 37]. Figures 6.3-6.5 compare the results obtained for PA-6,6 as such and for intercalated PA6,6 hybrid produced by using a Carver press to mix PA-6,6 with 5 wt% of Cloisite 15A (montmorillonite modified by ion exchange with dimethyl-ditallow ammonium, a tetraalkylammonium salt from Southern Clay Products Inc.), in an inert nitrogen atmosphere at 260 °C for 30 minutes. It can be seen that the RHR displays a lower maximum peak in the case of the nanocomposite (Figure 6.3), whereas the quantity of heat released (the area under the RHR curve) is about the same for both products, suggesting that their thermal degradation mechanisms are the same [37]. The release of heat by the nanocomposite over a longer period, however, points to its slower degradation. Figures 6.4 and 6.5 on mass loss and specific extinction area illustrate the advantages of nanocomposite over initial PA-6,6 fire behaviour.

Figure 6.3 Rate of heat release versus time for PA-6,6 and PA-6,6 nanocomposite at a heat flux of 35 kW/m2

181


Figure 6.4 Mass loss rate versus time for PA-6,6 and PA-6,6 nanocomposite at a heat flux of 35 kW/m2

Figure 6.5 Specific extinction area (smoke) versus time for PA-6,6 and PA-6,6 nanocomposite at a heat flux of 35 kW/m2

182

Ecological Issues of Polymer Flame Retardancy During the combustion test of the nanocomposite specimen, the carbon layer that formed on its surface from the start grew over time and resisted the heat. The formation of a carbonised layer on the surface of the polymer is a feature of all the nanocomposites studied so far: the pattern illustrated in Figure 6.6 has been reported for other nanocomposites based on polystyrene, polyethylene and polypropylene [37]. Examinations of this residue by X-ray diffraction and transmission electron microscopy (TEM) have revealed an intercalated nanocomposite structure [37]. The TEM image [37] of the carbon residue obtained by combustion of a PA-6,6 nanocomposite in Figure 6.6 shows the intercalation of silicate layers (dark zones) with ‘carbon’ layers (light zones). It should be emphasised that this intercalated structure was derived from the combustion of a delaminated hybrid. It is clear that the disordered structure collapsed during the combustion and was replaced by a self-assembled, ordered structure.

Figure 6.6 TEM image of carbon residue obtained by combustion of PA-6,6 nanocomposite [37] (Reproduced with permission from J.W. Gilman, T. Kashiwagi, C.L. Jackson, E.P. Giannelis, E. Manias, S. Lomakin, J.D. Lichtenhan and P. Jones in Fire Retardancy of Polymers: the Use of Intumescence, Eds., M. Le Bras, G. Camino, S. Bourbigot and R. Delobel, RSC, Cambridge, UK, 1998. Copyright 1998, RSC.)

References 1.

S.M. Lomakin and G.E. Zaikov, Ecological Aspects of Flame Retardancy, VSP International Science Publishers, Utrecht, The Netherlands, 1999, 170.

2.

H. Beck, A. Dross, M. Ende, R. Wolf and P. Trubiroha, Bundesgesundheitsblatt, 1991, 34, 564.

183


R.M.C. Theelen in Biological Basis for Risk Assessment of Dioxin and Related Compounds, Eds., M. Gallo, R. Scheuplein and K. Van der Heijden, Banbury Report No. 35, Cold Spring Harbor Laboratory Press, Plainview, NY, USA, 1991.

4.

U.G. Ahlborg, G.C. Becking, L.S. Birnbaum, A. Brouwer, H.J.G.M. Derks, M. Feeley, G. Golor, A. Hanberg, L.C. Larsen, A.K.D. Liam, S.H. Safe, C. Schlatter, F. Waern, M. Younes and E. Yrjanheikki, Chemosphere, 1994, 28, 6, 1049.

5.

Office of Health and Environmental Assessment Office of Research and Development, Estimating Exposure to Dioxin-Like Compounds, EPA/600/6-88/ 005Ca, Cb, Cc, USEPA, Cinncinnati, OH, USA, 1994.

6.

J. Green, Journal of Fire Sciences, 1996, 14, 426.

7.

C.E. Anderson Jr., J. Dziuk Jr., W.A. Mallow and J. Buckmaster, Journal of Fire Sciences, 1985, 3, 151.

8.

H. Tramm, C. Clar, P. Kuhnel and W. Schuff, inventors; Ruhrchemie AG, assignee, US Patent 2,106,938, 1938.

9.

M. Kay, A.F. Price and I. Lavery, Journal of Fire Retardant Chemistry, 1979, 6, 69.

10. D.E. Cagliostro, S.R. Riccitiello, K.J. Clark and A.B. Shimizu, Journal of Fire and Flammability, 1975, 6, 205. 11. R. Delobel, M. Le Bras, N. Ouassou and F. Alistiqsa, Journal of Fire Sciences, 1990, 8, 85. 12. G. Camino, L. Costa and L. Trossarelli, Polymer Degradation and Stability, 1984, 7, 25. 13. G. Camino, G. Martinasso, L. Costa and R. Gobetto, Polymer Degradation and Stability, 1990, 28, 17. 14. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability, 1992, 36, 31. 15. H. Heinrich, inventor; Chemie Linz (Deutschland) GmbH, assignee, German Patent, DE 4,015,490Al, 1991. 16. S.V. Levchik, G. Camino, L. Costa and G.F. Levchik, Fire and Materials, 1995, 19, 1.

184

Ecological Issues of Polymer Flame Retardancy 17. S.V. Levchik, G.F. Levchik, A.I. Balabanovich, G. Camino and L. Costa, Polymer Degradation and Stability, 1996, 54, 217. 18. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability, 1992, 36, 229. 19. S.V. Levchik, G.F. Levchik, A.F. Selevich and A.I. Leshnikovich, Vesti Akademii Nauk Belarusi, Seryya Khimichnykh, 1995, 3, 34. 20. S.V. Levchik, G.F. Levchik, G. Camino and L. Costa, Journal of Fire Sciences, 1995, 13, 43. 21. L. Costa, G. Camino and M.P. Luda di Cortemiglia in Fire and Polymers: Hazards Identification and Prevention, ACS Symposium Series No.425, Ed., G.L. Nelson, American Chemical Society, Washington, DC, USA, 1990, 211. 22. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability, 1992, 43, 43. 23. G. Camino, G. Martinasso and L. Costa, Polymer Degradation and Stability, 1990, 27, 285. 24. G. Camino and S. Lomakin in Fire Retardant Materials, Eds., A.R. Horrocks and D. Price, CRC Press, Boca Raton, FL, 2001, USA. 25. P. Haake and C.E. Diebert, Journal of the American Chemical Society, 1971, 93, 6931. 26. Y. Tsuchiya and K. Sumi, Journal of Polymer Science, 1969, A17, 3151. 27. Polyvinyl Alcohol. Properties and Applications, Ed., C.A. Finch, John Wiley, London, UK, 1973, 622. 28. B.G. Achhammer, F.W. Reinhard and G.M. Kline, Journal of Applied Chemistry, 1951, 1, 301. 29. S.M. Lomakin and G.E. Zaikov, Khimicheskaia Fizika, 1995, 14, 39. 30. G.E. Zaikov and S.M. Lomakin, Plasticheskie Massy, 1996, 39, 211. 31. R.M. Hassan, Polymer International, 1993, 30, 5. 32. R.M. Hassan, S.A. El-Gaiar and A.M. El-Summan, Polymer International, 1993, 32, 39.

185

Handbook of Plastic Films 33. R.M. Hassan, M.A. El-Gahami and M.A. Abd-Alla, Journal of Materials Chemistry, 1992, 2, 613. 34. Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi and O. Kamigaito, Journal of Materials Research, 1993, 8, 1185. 35. E.P. Giannelis, Advanced Materials, 1996, 8, 1, 29. 36. J.W. Gilman, T. Kashiwagi, M. Nyden, J.E.T. Brown, C.L. Jackson, S. Lomakin, E.P. Giannelis and E. Manias in Chemistry and Technology of Polymer Additives, Eds., S. Al-Malaika, A. Golovoy and C.A. Wilkie, Blackwell Science, Oxford, UK, 1999, 249-265. 37. J.W. Gilman, T. Kashiwagi, C.L. Jackson, E.P. Giannelis, E. Manias, S. Lomakin, J.D. Lichtenhan and P. Jones in Fire Retardancy of Polymers: the Use of

186

7

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres G.B. Pariiskii, I.S. Gaponova and E.Y. Davydov

7.1 Introduction In this chapter, the mechanisms of the reactions of nitrogen oxides with solid polymers are considered. Active participants in reactions with nitrogen oxides are double bonds, the amide groups of macromolecules, alkyl, alkoxy and peroxy radicals, as well as hydroperoxides. The structure of the reaction front during nitration of rubbers has been studied using the electron spin resonance (ESR) imaging technique. The reactions with nitrogen oxides provide a simple way of preparing spin-labelled polymers. The structuralphysical effects on the kinetics and mechanism of reactions of nitrogen dioxide have been demonstrated by the example of filled polyvinylpyrrolidone (PVP). Thermal and photochemical oxidation of polymers have been the subject of detailed and prolonged investigations, because these processes are of major importance for the stabilisation of polymeric materials. However, since the 1960s, the influence of aggressive gases in polluted atmospheres on polymer stability has attracted considerable attention [1]. Among such pollutants in the atmosphere, sulfur dioxide, ozone and the nitrogen oxides stand out as the most deleterious. However, the pursuance of this research has run into a number of problems. The interaction of pollutants with polymers involves the penetration of gases into solids and thus results in a complex kinetic description of the process. Also, as a rule, these reactions are long term for the concentrations of pollutants found in the environment. Consequently, other aging processes occur in the actual conditions of use and storage of polymer materials. To establish the effect of a given aggressive gas on a particular polymer, the reaction is generally studied at pollutant concentrations that are much higher than those actually existing in polluted atmospheres. The results obtained by this means are then linearly extrapolated to the concentrations of reactants found in the atmosphere. This expedient is, a priori, ambiguous in view of the fact that the role of the individual stages of a uniform aging process is changed in conditions of accelerated testing. The problem of non-equivalent kinetics is inherent to polymer reactions in solids [2]. In this case particles existing in different surroundings react with different rate constants. As a result, the most active particles will be removed from the reaction, and the overall rate constant will decrease with time. On the other hand, relaxation processes in polymers restore the initial distribution of particles and so their reactivity. Thus the kinetics will

187

Handbook of Plastic Films depend on the relation between the rate of the chemical reaction and the rate of the relaxation processes [3]. This fact also makes it necessary to reconsider critically the validity of extending the results of accelerated tests for polymer ageing. This chapter is devoted to a consideration of the results obtained in studies of the interactions of nitrogen oxides with polymers. There are eight nitrogen oxides, but only NO, NO2 and N2O4 are actually important as pollutants. Nitric oxide (NO) exists as a free radical, but it is reasonably stable in reactions with organic compounds. The paramagnetic nitrogen dioxide (NO2) is more active compared with NO. This gas is universally present in equilibrium with its dimer molecule: 2NO2

N2O4

with Kp = 0.141 atm at 298 K [4]. Nitrogen dioxide absorbs light in the near-UV and visible spectral range. Excited molecules are generated by light with λ > 400 nm. The dissociation of NO2 into an oxygen atom and NO by light with λ < 365 nm takes place with a quantum yield near to unity [5].

7.2 Interaction of Nitrogen Dioxide with Polymers Detailed investigations of the reactions of NO2 with various polymers have been carried out by Jellinek and co-workers [1, 6]. The degradation of polymer films has been studied at different pressures of NO2, in mixtures of NO2 with air, under the combined action of light (λ > 280 nm), O2 and NO2. Based on the data obtained, Jellinek classified all polymers into three groups: (1) vinyl polymers – polyethylene (PE), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinyl chloride (PVC) and polyvinyl fluoride (PVF); (2) polymers with non-saturation – primarily rubbers; (3) polyamides, polyurethanes and polyamidoimides. The presentation of the results in this section will be carried out according to this classification.

7.2.1 Vinyl Polymers: PE, PP, PS, PMMA, PAN, PVC and PVF The linear extrapolation of the results of accelerated tests to NO2 concentrations likely to be found in the atmosphere (1-5 ppm) predicts that polymer properties will be essentially constant for a long time.

188

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres The first investigations of the interaction of NO2 with PE and PP were performed by Ogihara and co-workers [7, 8] at 298-383 K and NO2 pressure of 20 kPa. It was established that NO2 reacts at room temperature with the >C=C< double bonds originally contained in PE with the formation of dinitro compounds and nitronitrites by the following reactions: >C=C< + NO2 → >C•−C(NO2)<

(= R1•)

(7.1)

R1• + NO2 → >C(NO2)−C(NO2)<

(7.2)

R1• + ONO → >C(ONO)−C(NO2)<

(7.3)

Hydrogen atom abstraction does not take place at room temperature. The nitro, nitrite, nitrate, carbonyl and hydroxyl groups are formed at T > 373 K. The following mechanism was postulated: RH + NO2 → R• + HNO2

(7.4)

R• + NO2 → RNO2

(7.5)

R• + ONO → RONO

(7.6)

RONO → RO• + NO

(7.7)

The reactions of RO• radicals lead to the formation of macromolecular nitrates, alcohols and carbonyl compounds. The activation energy of the NO2 addition to the double bonds of PE is 8-16 kJ/mol. The activation energies of H atom abstraction are 56-68 kJ/mol in PE and 60 kJ/mol in PP. PE, PP, PAN and PMMA change their characteristics slightly at high concentrations of NO2 (1.3-13 kPa) even under the joint action of pollutant, O2 and UV light [6]. Nitrogen dioxide is capable of abstracting tertiary hydrogen atoms in PS with a low rate (P = 20-80 kPa), with the formation of nitro and nitrite side groups [reactions (7.5) and (7.6)]. This process is accompanied by main-chain scission [9, 10]. The combined action of 0.3 kPa NO2 and light (λ > 280 nm) on PS does not lead to mainchain decomposition in the early stage (10 h), after which the degradation process is developed with a constant rate. PVC and PVF show a minor loss of chlorine and fluorine atoms on exposure to NO2 [1, 6]. An attempt to investigate quantitatively the ageing of PS and poly-tert-butyl methacrylate (P-t-BuMA) has been taken by Huber [11]. The research was performed in a flow system of air containing 60-900 ppm of NO2 and/or 60-900 ppm SO2 at 300 K under the simultaneous action of light with λ > 290 nm. The degradation of P-t-BuMA films was

189

Handbook of Plastic Films expressed in terms of the quantity of ruptures per 10,000 monomer units, α. The kinetic dependence is represented by the equation:

α = (P/Q)[exp(Qt) – 1]

(7.8)

where P and Q are constants. This equation describes an autoaccelerated process. As Q → 0, so α → Pt, that is, the degradation proceeds with a constant rate. The P and Q values decrease as the film thickness increases, and yet the P value diminishes more strongly than Q. Therefore, the accelerated character of the degradation appears more clearly for thin films. PS degradation in the same conditions proceeds much more slowly and has a more pronounced autoacceleration (Table 7.1).

Table 7.1 The P and Q values for P-t-BuMA and PS film degradation under the action of 100 ppm NO2 and light in air Film thickness (mg/cm2)

P × 104 (h–1)

Q × 104 (h–1)

P-t-BuMA

1.4

0.071

0.026

P-t-BuMA

2.6

0.050

-

P-t-BuMA

2.8

0.041

0.017

PS

1.4

0.034

0.036

Polymer type

The autoaccelerated character of P-t-BuMA degradation was linked to the ester group decomposition, with isobutylene formation, which gives free radicals in the reaction with NO2 and thus promotes the degradation process. The IR spectrum of PS shows peaks corresponding to carbonyl (1686 cm–1) and hydroxyl (3400 cm–1) groups after exposure to a mixture of NO2 (100 ppm) and air. No bands connected with the insertion of NO2 into the P-t-BuMA and PS macromolecules were observed. It is believed that the following sequence of reactions occurs in PS [11]: RH + NO2 → R• + HNO2

190

(7.9)

R• + O2 → RO2•

(7.10)

RO2• + RH → ROOH + R•

(7.11)

R• + NO2 → RNO2

(7.12)

R• + NO2 → RONO

(7.13)

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres RONO → RO• + NO

(7.14)

ROOH + NO → RO• + •OH + NO

(7.15)

ROOH + hν → RO• + •OH

(7.16)

RO• → degradation + R•

(7.17)

Hydroperoxide decomposition under the action of NO and light gives rise to accelerated PS degradation.

7.2.2 Non-Saturated Polymers Among these are primarily rubbers. These polymers are far more sensitive to NO2 action that the polyolefins. Appreciable degradation of macromolecules as well as moderate crosslinking were observed for rubbers. Comprehensive kinetic investigations of butyl rubber (a copolymer of isobutylene with 1.75% isoprene) in an NO2 atmosphere (NO2 pressure 1.33-133 kPa), in a mixture of NO2 and air, and under the combined action of NO2, O2 and UV light (λ > 280 nm) have been performed by Jellinek and co-workers [12, 13]. According to the proposed mechanism, the total number of chain ruptures is made up of three parts: (1) ruptures that are due to the NO2 interaction only, (2) ruptures that result only from the action of O2, and (3) ruptures that are caused by the combined action of NO2 and O2. The kinetic dependence of the degree of degradation, α = (1/DPt − 1/DP0), is described by the following equation:

α = kef′ t2 + kef″ [NO2][1 − exp(−k3t)]

(7.18)

where DP0 and DPt are the number-average degrees of polymerisation in the original and degraded macromolecule (at time t). The first term in equation (7.18) is connected with ruptures of macromolecules due to photolysis of the reaction products (hydroperoxides, nitro and nitrite groups). The second term describes the degradation for the (NO2 + O2) system in the absence of light. It should be noted that the assumed mechanism [12, 13] is very complex, involving a wealth of elementary reactions, the rate constants of which are unknown in the solid phase. It is well known that the reaction products can be more active relative to the nitrogen oxides than the original polymer. In connection with this, the application of various physical-chemical techniques is extremely important to investigate the

191

Handbook of Plastic Films degradation process. The development of methods to study the movement of the reaction front across the polymer sample is also required. The use of the ESR technique permits one to draw additional conclusions on the mechanisms of the interaction of polymers with nitrogen oxides from the structure of the resulting free radicals and the kinetics of their formation. The interaction of polyisoprene (PI) with NO2 gives rise to di-tert-alkylnitroxyl radicals [14]. The ESR spectra of these radicals show a characteristic anisotropic triplet signal with a width of 2A||N = 6.2 mT and g|| = 2.0028 ± 0.0005 in the solid polymer, and a triplet with aN = 1.53 ± 0.03 mT and g = 2.0057 ± 0.0005 in dilute solutions. These macroradicals are stable in the absence of NO2 during storage for many months in both inert atmosphere and air. The proposed scheme to explain the formation of these radicals involves three main stages: (1) generation of N-containing alkyl radicals, (2) synthesis of tertiary macromolecular nitroso compounds, and (3) spin-trapping of the tertiary alkyl or allyl radicals: ~CH2-C(CH3)=CH-CH2~ + NO2 ~C•(CH3)-CH(ONO)-CH2~ + RH

~C•(CH3)-CH(ONO)-CH2~ ~C(CH3)(NO)-CH(OH)-CH2~

~C(CH3)(NO)−CH(OH)−CH2∼ + •Rtert → Rtert−N(O•)−Rtert

(7.19) (7.20) (7.21)

The reactions of NO2 with double bonds provide a very simple and rapid method for the synthesis of spin-labelled macromolecules of rubbers. The temperature variation of the rotational mobility of macromolecules in block PI has been studied using spin-labelled samples [14]. The temperature dependence of the rotational correlation time τ is described by τc = τ0 exp(E/RT). The τc values within the fast motion region (τc < 10–9 s) are well described by the parameters E = 34.7 kJ/mol and log τ0 = −14.2. The spatial distribution of these macromolecular nitroxyl radicals allows the estimation of the spatial distribution of the nitration reaction in bulk PI. The possibilities of the ESR imaging technique to determine the form of the reaction front of PI nitration has been considered [15]. The ESR imaging spectra were registered in an inhomogeneous magnetic field on cylindrical samples of 0.4 cm diameter and 1 cm height at NO2 and O2 concentrations of 1 x 10–4 to 2 x10–3 mol/l and 2 x 10–3 to 1.4 x 10–2 mol/l, respectively. The spatial distributions of R2NO• radicals at various reaction times are shown in Figure 7.1. The width of the distribution varies over 2030% for 740 h. The maximum concentration of nitroxyl radicals is observed in the superficial layer, and it progressively decreases towards the centre. The width of this layer is ~1 mm, and radicals are unavailable in the sample centre. The nitroxyl radical yield with respect to absorbed NO2 molecules is 0.01. The shape and variation of the distribution in the presence of O2 are the same as in pure NO2, but the reaction front is narrower. The rate of R2NO• formation in the presence of O2 is much lower than 192

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres

Figure 7.1 ESR-imagination of nitroxyl radicals distribution in cylindrical sample (l = 10 mm, d = 4 mm) of PI in course of interaction with nitrogen dioxide ([NO2]) = 8.8 × 10-4 mole/l; 30 min; 20 °C). The contour lines correspond to vertical sections with equal [R2NO*]. The concentrations are given in arbitrary units ([R2NO*]max = 0.125 au).

in pure NO2 at the cost of a decay of alkyl radicals in the reactions with O2: W(NO2)/ W(NO2+O2) = 102. The distribution at a fixed distance from the surface is likely determined by macrodefects in the sample volume, namely, the availability of cracks and porosity. The front form is determined by the ‘membranous’ regime of the nitration process rather than by structural changes. PMMA, which in itself is stable on exposure to NO2, enters into reactions after previous irradiation by UV light at 293 K [16]. The photolysis of PMMA induces the formation of double bonds as a result of ester group decomposition. The ESR spectrum observed after exposure of samples to NO2 is shown in Figure 7.2. The spectrum represents the superposition of the signals of two nitroxyl radicals at low frequencies of rotational mobility (10–9 s < τc < 10–7 s): •

Dialkylnitroxyl radicals ~C(CH3)(COOCH3)–N(O•)–C(CH3)(CHO)–CH2~ give an anisotropic triplet signal with hyperfine interaction (HFI) constant A||N = 3.2 ± 0.1 mT and g|| = 2.0026 ± 0.0005;

193


Figure 7.2 ESR spectrum of nitroxyl radicals generated by NO2 in PMMA pre-irradiated by UV light at 298 K.

•

Acylalkylnitroxyl radicals ~C(CH3)(COOCH3)−CH(OH)–C(CH3)[N(O•)COOCH3]–CH2~ give a triplet signal that is characterised by A||N = 2.1 ± 0.1 mT and g|| = 2.0027 ± 0.0005.

The free-radical process of NO2 interaction with PMMA containing double bonds is represented by the scheme opposite. The formation of nitroxyl radicals testifies to the fact that main-chain decomposition by reaction (7.24) and side-chain ester group cleavage by reaction (7.26) take place in the polymer. Thus, the interaction of NO2 with double bonds is able to initiate free-radical reactions of polymer degradation when hydrogen atom abstraction reactions from C–H bonds are inefficient.

194


(7.22)

(7.23)

(7.24)

(7.25)

(7.26)

(7.27)

195


7.2.3 Polyamides, Polyurethanes, Polyamidoimides Polymers with amide and urethane groups in the macromolecules represent a special class of materials that are sensitive to NO2. The action of NO2 at pressures of 0.5-2 mm Hg on polyamide-6,6 films with various morphologies has been studied by Jellinek and co-workers [17, 18]. It was shown that a degradation process takes place. The degradation of polyamide is a diffusion-controlled reaction and depends on the degree of crystallinity and the sizes of the crystallites. The process is inhibited by small quantities of benzaldehyde or benzoic acid. Increase of the degradation rate was observed during the combined action of NO2, air and UV light. The assumed mechanism of the process as follows: ~CO–NH~ + NO2 → ~CO–N•~ + HNO2

(7.28)

~CO–N•~ + NO2 → ~CO–N(NO2)~

(7.29)

~CO–N•–CH2~

[~CO–N=CH2 + •CH2~] → chain rupture

(7.30)

There is reason to believe that only a small quantity of amide groups, not linked by the hydrogen bonds, enter into the reaction. These groups can be interlocked by benzoic acid with the formation of the following structure:

HO

O

••••••

NH

••••••

CO

C Ph

Research into the effect of NO2 on polyamide textiles has been described [19]. The exposure of samples in an NO2 atmosphere of low concentration at room temperature for 100 h does not lead to a decrease in the whiteness and tensile strength. However, these characteristics are decreased at higher temperatures. The availability of nitrogen oxides in the air under the action of UV light results in the additional degradation of textiles. The conversion of N−H bonds by nitrogen dioxide is also inherent to polycaproamide (PCA). The UV spectra of PCA films display features of absorption at 390-435 nm during exposure to NO2 at concentrations of 10–4 to 10–3 mol/l [20]. The absorption bands were assigned to nitrosamide groups resulting from N−H group conversion. This

196

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres conclusion is confirmed by IR spectroscopy. The intensity of the band with ν = 3293 cm–1, which is associated with stretching vibrations of the hydrogen-linked N−H groups, decreases sharply. The intensities of the amide I (ν = 1642 cm–1) and amide II (ν = 1563 cm–1) bands, which are characteristic of PCA, also decrease. Instead of these bands, absorptions at ν = 1730 cm–1, which corresponds to the absorption of C=O groups, and at ν = 1504 and 1387 cm–1, which correspond to stretching vibrations of N=O groups of nitrosamides, appear in PCA. Thus, nitrosation through the amide group is the main process of PCA transformation in an NO2 atmosphere, which leads to disintegration of the system of hydrogen bonds. Taking into account the equilibrium: NO+NO3−

N2 O 4

(7.31)

2NO2

the formation of nitrosamides can be represented as follows: ∼CONHCH2∼ + N2O4 → ∼CON(NO)CH2∼ + HNO3

(7.32)

It was found that the initial rate of nitrosamide group accumulation is proportional to [NO2]n, where n ≈ 2. As was shown by ESR, the reaction of NO 2 with N−H bonds also produces acylalkylnitroxyl macroradicals: ∼CONHCH2∼ + NO2

~CON(ONO)CH2~

ΝΟ2

→ ∼CON•CH2∼ → ~CON(ONO)CH2~

(7.33)

~CON(O•)CH2~ + NO

(7.34)

–HNO2

As well as in PCA, the interaction of NO2 with PVP leads to UV bands characteristic of the nitrosamide group [20]. The formation of these groups in PVP is associated with splitting of the side-chain cyclic fragments from the main chain:

PVP + NO2 –HNO2

(R1)

CH2 CHCH2 N

O

(7.35)

197


Types of Hot Runner Systems N

R1

CH2CHCH2

O

+

(7.36)

Thereafter, the reaction of NO 2 with the cyclic double bond gives rise to the nitrosamide product: NO2 N

O

O2 N

N

O R1

+ 2NO2

NO

O NOR O2N

N

O2 N

O OR

N

O

(7.37)

+

The ESR spectra observed when NO2 (10–4 to 10–3 mol/l) reacts with PVP represent the superposition of the signals of acylalkylnitroxyl radicals (A||N = 1.94 mT, g|| = 2.003) and iminoxyl radicals (A||N = 4.33 mT, A⊥N = 2.44 mT, g|| = 2.0029, g⊥ = 2.0053). The formation of these iminoxyl radicals is initiated by the hydrogen atom abstraction reaction from C−H bonds that are in the α-position with respect to the amide group by reaction (7.35) and the following reaction: PVP + NO2 –HNO2

(R2)

CH2 CHCH2 N

O

(7.38)

Nitric acid is thought to be the source of nitrogen oxide in the given system: 2HNO2 → H2O + NO2 + NO

198

(7.39)

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres The recombination of NO and R1• initiates the formation of iminoxyl radicals:

R1 + NO

CH2 CHCH2 N

O

–HNO2 CH2 CHCH2

CH2 CHCH2

(7.40)

+NO2 N

N

O

O

ON

The formation of NO explains the production of acylalkylnitroxyl radicals as follows:

R2

CH2 C CH2 N

CH2 C CH2 O

N

NO O

+ R1 (R2)

CH2 C CH2

(7.41)

R N

N O O

An approach based on the analysis of the composition of nitrogen-containing radicals in PVP depending on the content of filler aerosil has been put forward to elucidate the

199

Handbook of Plastic Films effect of polymer structural-physical organisation [21]. The influence of structural organisation may be manifest in the rates of iminoxyl and acylalkylnitroxyl radical formation. Filling gives the possibility of changing the physical structure of the polymer in interface layers. The decrease in the molecular packing density as a result of filling can accelerate the rate of reaction (7.41) involving breakage of the pyrrolidone cycle. The packing density decrease enhances the reaction rate through the promotion of mutual diffusion of R• macroradicals and nitroso compounds. It is well established that the quantitative relation between iminoxyl and acylalkylnitroxyl radicals is changed with the degree of filling. Formation of a gel fraction has been detected on exposure of polyurethane films to NO2 [21]. Degradation of macromolecules simultaneously takes place in the sol fraction of the samples. The changes in the destruction degree and the gel-fraction yield with time are complex to analyse. The gel fraction at 333 K and P(NO2) = 20 mm Hg initially increases up to 20% and thereafter reduces to nearly zero. The number of scissions in the sol fraction increases at the beginning, subsequently reduces, and then grows again. The exposure of films to NO2 is accompanied by the release of CO2 at all temperatures. The IR spectra in this case show N−H bond (3300 cm–1) consumption. The proposed mechanism includes the reaction of NO2 with the N−H groups of both the main chain and the side branches: ~OCO–NH–CH2~ + NO2 → ~OCO–N•–CH2~ + HNO2

(7.42)

~OCO–N(RH)–CH2~ + NO2 → ~OCO–N(R•)–CH2~ + HNO2

(7.43)

The recombination of ~OCO–N•–CH2~ (R1•) and ~OCO–N(R•)–CH2~ (R2•) results in polymer crosslinking. The conversion of R1• causes macromolecule decomposition and CO2 release. The exposure of polyurethane films to an NO2 atmosphere or a mixture of NO2 with air leads to the progressive reduction of the tensile strength limit [22]. The influence of NO2 on the mechanical properties of polyamidoimide films has been considered at 323 K and P(NO2) = 13 kPa [23]. The temperature dependences of the storage modulus E′ and loss modulus E″ have been obtained for various times of NO2 exposure. A nonmonotonic decrease of E′ was observed at 473 K, but the maximum of the E′ temperature dependence appears at approximately the same temperature. Samples exposed to NO2 for eight days show an increase in E′ at the glass transition temperature (563 K). The phenomenon is associated with chain breakage and the recombination of macroradicals giving rise to crosslinking. Chain breakage is supported by results obtained by the present authors. The ESR spectra of polyamidoimide exposed to an NO2 atmosphere show the formation of iminoxyl radicals with spectral parameters that are close to those of PVP iminoxyl radicals. The possible mechanism of their formation includes the main-chain decomposition step as follows: 200


NH

O

CO

CO

NO2

N

–HNO2

CO

O

N

CO

CO N CO

(7.44) R + O

N

CO

CO N CO

O

N

CO

NO2

O

O

N

CO

N

CO

ON

NO

O

NO

N

CO

RH

O

O

HO

O

N HO

(7.45)

NO

NOH

CO

NO2 –HNO2

O

N

CO

HO

201


7.3 Reaction of Nitric Oxide with Polymers Nitric oxide is a low-activity free radical and can be used as a ‘counter’ of radicals in gas and liquid phases. The reactions of alkyl radicals with NO lead to the formation of nitroso compounds, which are spin traps. Thus, the initiation of free-radical reactions in solid polymers in the presence of nitric oxide provides further information on their mechanism. It is well established that at room temperature NO is not able to remove allylic and tertiary hydrogen atoms and add to isolated double bonds [24-26]. There are discrepant opinions on the capability of NO to react with low molecular weight (low molar mass) dienes and polyenes. Some authors believe that NO is able to add to dienes and polyenes, for example, to substituted o-quinonedimethane, phorone and β-carotene, with the formation of free radicals [27-29]. Another way of looking at these reactions lies in the fact that they can be initiated by NO2 impurities [25, 26]. This section of the review is concerned with radical reactions in polymers, induced by photo- and γ-irradiation, in the presence of nitric oxide. Irradiation of powdered PMMA in an NO atmosphere by the light of a mercury lamp results in the formation of three typesof macromolecular nitroxyl radicals [30]. The radical composition depends on temperature and the wavelength of the light. If the photolysis of PMMA is performed at room temperature using unfiltered light from a high-pressure mercury lamp, acylalkylnitroxyl radicals R1N(O•)C(=O)R2 are formed. The irradiation of samples at 383 K produces, in addition to acylalkylnitroxyl radicals, dialkylnitroxyl macroradicals R1N(O•)R2. Finally, if PMMA irradiation is carried out at room temperature using UV light with 260 nm < λ < 400 nm, the signal of iminoxyl radicals R1C(=NO•)R2 is also observed in the ESR spectrum. Acetyl cellulose (AC) under action of light at room temperature gives rise to dialkyl- and acylalkylnitroxyl radicals [30]. The removal of NO from the samples leads to increasing of components of acylalkylnitroxyl radicals in the ESR spectrum. This phenomenon is probably connected with the formation of diamagnetic complexes of NO with acylalkylnitroxyl radicals. Dialkylnitroxyl radicals do not form complexes of this type at 298 K. The γ-irradiation of PMMA at room temperature, as a photolysis, brings about the formation of acylalkylnitroxyl radicals [30]. Iminoxyl radicals also arise, but their quantity is essentially smaller than in AC under γ-irradiation. The formation of nitroxyl radicals during photolysis as well as in the course of radiolysis of PMMA and AC in the presence of NO is explained by the following scheme: polymer

202

γ, hν

→

R1• (R•2)

(7.46)

R1• + NO → R1NO

(7.47)

R2• + R1NO → R1N(•O)R2

(7.48)

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres The structure of nitroxyl radicals is determined by the nature of the free radicals that are generated by γ- and photo-irradiation of PMMA and AC. Photo-irradiation of PMMA and AC leads to the formation of •C(O)OCH3 radicals, which give in turn acylalkylnitroxyl radicals by reactions (7.46)-(7.48). Dialkylnitroxyl radicals arise when two macroradicals are involved in the reactions with NO. The free-radical reactions in solid polymers in the presence of NO are of particular significance for the preparation of spin-labelled polymers. This method has become particularly important for chemically inert, rigid and insoluble polymers, for instance, polytetrafluoroethylene (PTFE), because of the difficult problem of introducing spin labels by chemical reactions of nitroso compounds, nitrons or nitroxyl biradicals [31]. Oriented PTFE films γ-irradiated at room temperature in air after prolonged NO exposure contain nitroxyl radicals whose ESR spectra are displayed in Figure 7.3 [32].

Figure 7.3 ESR spectra of perfluoronitroxyl radicals in PTFE films stretched to fourfold increase in its length at parallel (a) and perpendicular (b) orientation of magnetic field directions.

203

Handbook of Plastic Films The rotation of the samples leads to changes in angle α between the magnetic field and stretching directions. At 298 K and α = 0°, the ESR spectrum is a triplet consisting of quintets with splitting of AN = 0.46 mT and AF = 1.11 mT, and g|| = 2.0060. At α = 90°, the splittings increase to AN = 1.12 mT and AF = 1.61 mT, and g⊥ = 2.0071. The radicals observed are nitroxyl radicals with the following structure: ~CF2–N(O•)–CF2~. A possible mechanism for nitroxyl macroradical synthesis has been suggested [32]. In an oxygen-containing atmosphere, some of the middle alkyl radicals formed in the course of γ-irradiation are capable of decomposing with rupture of the main chain as a result of the high energy transfer to these radicals: ~CF2–CF2–C•F–CF2~ → ~CF2• + CF2=CF–CF2~

(7.49)

In the presence of oxygen, the terminal alkyl macroradicals can be oxidised to form terminal peroxy radicals: ~C•F2 + O2 → ~CF2OO•

(7.50)

Under the action of NO on samples containing neighbouring terminal double bonds and peroxy radicals, the latter are converted into macromolecular nitrates and nitrites:

~CF2

CF2OO• + NO

~CF2OONO …

(7.51) ~CF2ONO2

~CF2O• + NO2

(7.52) ~CF2O• + NO2

~CF2O• + NO

~CF2ONO

(7.53)

Decomposition of alkoxy radicals in an NO atmosphere causes the synthesis of terminal nitroso compounds: NO

~CF2–CF20• → ~C•F2 + CF2O → ~CF2NO

(7.54)

The adjacent terminal double bonds and terminal nitroso compounds formed can enter into a reaction to synthesise nitroxyl radicals: ~CF2N=O + CF2=CF–CF2~ + NO → ~CF2–N(O•)CF2–CF(NO)–CF2~

204

(7.55)

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres The advantage of the suggested method for the preparation of spin-labelled polymer is that the nitroxyl free-radical fragment is incorporated in the basic macromolecular chain without disturbing its orientation. An analogous investigation of the action of NO on γ-irradiated tetrafluoroethylenehexafluoropropylene copolymer (TFE-HFP) containing 13 mol% of HFP units has been performed [33]. After exposure of powders and films of TFE-HFP to a dose of 105 Gy in air, there are three types of stable peroxide macroradicals: (1) End radicals ~CF2–CF2O2• (denoted ReO2•); (2) Secondary mid-chain radicals ~CF2–CF(OO•)–CF2~ (denoted RcO2•); (3) Tertiary mid-chain radicals ~CF2–C(CF3)(OO•)–CF2~ (denoted RtO2•). Their total concentration is [RO2•] ≈ 3 x 10–3 mol/kg, of which (25 ± 5)% are tertiary peroxy radicals. Under the action of NO on evacuated samples, the radicals decay to form peroxy radical conversion products and tertiary nitroso compounds: ~CF2–C(CF3)(NO)–CF2~ Heating these samples in vacuum up to 473 K leads to the formation of nitroxyl radicals of the type: ~CF2–N(O•)–CF2~ The nitroxyl radicals appear in the temperature range where the tertiary nitroso compounds decay in vacuum with the generation of tertiary alkyl radicals (Rt•). The first step of Rt• formation is β-scission by the reaction: ~CF2−C•(CF3)−CF2−CF2~ → ~CF2−C(CF3)=CF2 + •CF2−CF2~

(7.56)

In the presence of NO formed upon decomposition of the tertiary nitroso compounds, the terminal alkyl radicals can be converted into terminal nitroso compounds, which react with the adjacent double bonds to form nitroxyl macroradicals: NO + •CF2−CF2~ → ON−CF2−CF2~

(7.57)

~CF2–C(CF3)=CF2 + ON−CF2−CF2~ → ~CF2–C•(CF3)–CF2–N(O•)–CF2~ → +X

→ ~CF2–C(CF3)(X)–CF2–N(O•)–CF2~

(7.58)

where X is NO or NO2. Nitrogen dioxide can be formed by the interaction of NO with RO2• in reaction (7.51). 205

Handbook of Plastic Films One more type of nitroxyl macroradical is observed if a powdered TFE-HFP, γ-irradiated in air and exposed to NO with subsequent evacuation, is subjected to light irradiation at λ > 260 nm at 298 K [34]. In this case, a new type of nitroxyl macroradical with the structure ∼CF2–N(O•)–CF3 was registered. The following scheme provides an explanation for the radical formation in TFE-HFP under the action of light: RcO2• + NO → [RcOONO] → RcONO2

(7.59)

ReO2• + NO → [ReOONO] → ReONO2

(7.60)

Rt• + NO → RtNO

(7.61)

RcONO2 + hν → RcO• + NO2

(7.62)

ReONO2 + hν → ReO• + NO2

(7.63)

RtNO + hν → Rt• + NO

(7.64)

RcO• → ~CF2–CFO + •CF2–CF2~

(7.65)

ReO• → ~CF2–CF2• + CF2O

(7.66)

~CF2–CF2• + NO → ~CF2–CF2–NO

(7.67)

~CF2−C•(CF3)−CF2~ + NO2 → ~CF2–C(CF3)(ONO)–CF2~

(7.68)

~CF2–C(CF3)(ONO)–CF2~ + hν → ~CF2–C(CF3)(O•)–CF2~ + NO

(7.69)

~CF2–C(CF3)(O•)–CF2~ → •CF3 + ~CF2–C(=O)–CF2~

(7.70)

•

CF3 + NO → CF3NO

(7.71)

•

CF3 + ON–CF2–CF2~ → CF3–N(O•)–CF2–CF2~

(7.72)

CF3NO + •CF2–CF2~ → CF3–N(O•)–CF2–CF2~

(7.73)

It is obvious that the simultaneous action of light and NO on TFE-HFP results in macromolecular decomposition. Polymer hydroperoxides are active participants in degradation processes. The reactions of nitrogen oxides with these particles are of interest to understand the mechanism of the influence of pollutants on polymer stability in the course of the oxidation process. The phenomenon of hydroperoxide decomposition under the action of NO was discussed

206

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres long ago using both macromolecular peroxides and their low molecular weight analogues [34]. Some authors assumed that the primary stage of peroxide decomposition can be represented by the reaction [35]: ROOH + NO → RO• + HONO

(7.74)

Another mechanism [36] suggests that the reaction proceeds with the formation of peroxide radicals: ROOH + NO → ROO• + HNO

(7.75)

The kinetics of hydroperoxide decomposition in PP at 298 K and various partial pressures of NO has been studied in detail [34]. The decomposition kinetics are shown in Figure 7.4.

Figure 7.4 Kinetics of PP hydroperoxide decomposition in NO at various concentrations (1-3) and NO + NO2 mixture (4): (1) 1.61 × 10-3, (2) 3.22 × 10-3, (3) 4.13 × 10-3, (4) 3.1 × 10-3 NO and 3.0 × 10-6 NO2 mol/l.

As can be seen, the hydroperoxide consumption rate is initially low and then sharply increases. The observed character of the kinetic curves cannot be explained by reactions (7.74) or (7.75). According to the ESR data, the decomposition of PP hydroperoxide in an

207

Handbook of Plastic Films NO atmosphere gives dialkylnitroxyl radicals. It was shown that the induction periods for the hydroperoxide decomposition and nitroxyl radical accumulation are very sensitive to the presence of trace amounts of higher nitrogen oxides. This leads to the conclusion that the interaction of hydroperoxide with NO is more likely to proceed according to the scheme: ROOH + N2O3 → [ROONO] + HNO2

ROONO

(7.76) RONO2

(7.77)

RO• + NO2

(7.78)

RO• + NO2

Alkoxy radicals may decompose or enter into substitution reactions with macromolecules to form chain Rc• and end Re• alkyl macroradicals, and low molecular weight alkyl radicals r•, which with NO give nitroso compounds: RO• → Rc• (Re•, r•)

(7.79)

Rc• (Re•, r•) + NO → RcNO (ReNO, rNO)

(7.80)

The increase in the rate of hydroperoxide decomposition with time can be related to reactions proceeding with participation of such nitroso compounds: r′OOH + r″NO → r′O• + r′–N(O•)–OH

(7.81)

r″–N(O•)–OH → r″• + HNO2

(7.82)

The alkyl radicals formed in the system may stimulate hydroperoxide decomposition [37]: r• (Rc•, Re•) + ROOH → rH (RcH, ReH) + RO2•

RO2• + NO

(7.83)

RO• + NO2

(7.84)

RONO2

(7.85)

ROONO

Another process that can increase the hydroperoxide decomposition rate is the disproportionation of NO to N2 and NO3• with the participation of nitroso compounds [24]:

208


RNO• + 2NO

R–N=N–ONO2

RONO2 + N2

(7.86)

R• + N2 + NO3

(7.87)

R• + N2 + •ONO2

NO + NO3• → 2NO2 Reactions (7.83)-(7.88) may lead to an increasing NO2 concentration in the system and, consequently, result in the acceleration of reaction (7.76).

7.4 Conclusion Nitrogen oxides are capable of influencing the free-radical stages of polymeric material aging in polluted atmospheres. Nitric oxide is a comparatively low-activity free radical, and it cannot abstract even labile hydrogen atoms at ordinary temperatures to initiate the radical degradation process. On the other hand, NO effectively recombines with free radicals. This reaction is apparently controlled in solid polymers by the gas diffusion rate, and NO is capable of terminating the oxidation chain by reaction with peroxy and alkyl macroradicals. The reaction of NO with alkyl radicals gives nitroso compounds, which are spin traps accepting free radicals. This process can slow down polymer degradation in the presence of nitrogen oxides in subsequent conversions, which can break down into alkoxy radicals, effecting the degradation of macromolecules. In addition, nitric oxide initiates the decomposition of hydroperoxides resulting from oxidation of polymers. Nitrogen dioxide is a more active free radical as compared with NO, and is able to break off the labile hydrogen atoms at room temperature as well as to add to the C=C bonds of macromolecules, inducing free-radical degradation of polymers. At the same time, the NO2 radical can inhibit the free-radical reactions giving nitrogen-containing molecules by the reactions with alkyl, alkoxy and peroxy radicals. The thermal and photochemical conversions of these products also affect the aging process of polymeric materials. Nitrogen dioxide is an initiator of the free-radical degradation of polyolefins at elevated temperatures. The low stability of polyamides to the action of NO2 is quite surprising, because the N–H bond of the amide group is rather strong. Therefore, the mechanism of polyamide degradation connected with hydrogen atom abstraction by NO2 from N–H bonds is not fully elucidated.

209


References 1.

H.H.G. Jellinek in Aspects of Degradation and Stabilisation of Polymers, Ed., H.H.G. Jellinek, Elsevier, Amsterdam, The Netherlands, 1978, Chapter 9.

2.

N.M. Emanuel and A.L. Buchachenko, Chemical Physics of Polymer Degradation and Stabilisation, VNU Science Press, Utrecht, The Netherlands, 1987.

3.

O.N. Karpukhin, Usppekhi Khimii, 1978, 47, 6, 1119.

4.

T.C. Hall and F.E. Blacet, Journal of Chemical Physics, 1952, 20, 11, 1745.

5.

J.G. Calvert and J.N. Pitts Jr., Photochemistry, John Wiley, New York, NY, USA, 1966.

6.

H.H.G. Jellinek, F. Flajsman and F.J. Kryman, Journal of Applied Polymer Science, 1969, 13, 1, 107.

7.

T. Ogihara, Bulletin of the Chemical Society of Japan, 1963, 36, 1, 58.

8.

T. Ogihara, S. Tsuchiya and K. Kuratani, Bulletin of the Chemical Society of Japan, 1965, 38, 6, 978.

9.

H.H.G. Jellinek and Y. Toyoshima, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1967, 5, 12, 3214.

10. H.H.G. Jellinek and F. Flajsman, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1969, 7, 4, 1153. 11. A. Huber, Einfluß von Schwefeldioxid und Stickstoffdioxid auf Polymere in Luft unter Belichtung, University of Stuttgart, Germany, 1988, 187. [Ph.D Thesis]. 12. H.H.G. Jellinek and F. Flajsman, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1970, 8, 3, 711. 13. H.H.G. Jellinek and P. Hrdlovic, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1971, 9, 5, 1219. 14. T.V. Pokholok and G.B. Pariiskii, Polymer Science, Series A, 1997, 39, 7, 765. 15. E.N. Degtyarev, T.V. Pokholok, G.B. Pariiskii and O.E. Yakimchenko, Zhurnal Fizicheskoi Khimii, 1994, 68, 3, 461.

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Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres 16. T.V. Pokholok, G.B. Pariiskii and G.O. Bragina, Vysokomolekulyarnye Soedineniya, Seriya A, 1989, 31, 10, 2049. 17. H.H.G. Jellinek and A. Chaudhuri, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1972, 10, 6, 1773. 18. H.H.G. Jellinek, R. Yokota and Y. Itoh, Polymer Journal, 1973, 4, 6, 601. 19. H. Herzlinger, B. Kuster and H. Essig, Textile Praxis International, 1989, 44, 6, 574, 655, 661. 20. I.S. Gaponova, E.Y. Davydov, G.G. Makarov, G.B. Pariiskii and V.P. Pustoshnyi, Polymer Science, Series A, 1998, 40, 4, 309. 21. H.H.G. Jellinek and T.J.Y. Wang, Journal of Polymer Science, Polymer Chemistry Edition, 1973, 11, 12, 3227. 22. H.H.G. Jellinek, F. Martin and J. Wegener, Journal of Applied Polymer Science, 1974, 18, 6, 1773. 23. H. Kambe and R. Yokota, Proceedings of the 2nd International Symposium on Degradation and Stabilisation of Polymers, Dubrovnik, Yugoslavia, 1978, Paper No.39. 24. J.F. Brown, Jr., Journal of the American Chemical Society, 1957, 79, 10, 2480. 25. A. Rockenbauer and L. Korecz, Chemical Communications, 1994, 145. 26. J.S.B. Park and J.C. Walton, Perkin Transactions 2, 1997, 12, 2579. 27. H.-G. Korth, R. Sustmann, P. Lommes, T. Paul, A. Ernst, H. de Groot, L. Hughes and K.U. Ingold, Journal of the American Chemical Society, 1994, 116, 7, 2767. 28. I. Gabr and M.C.R. Symons, Faraday Transactions, 1996, 92, 10, 1767. 29. I. Gabr, R.P. Patel, M.C.R. Symons and M.T. Wilson, Chemical Communications, 1995, 9, 915. 30. I.S. Gaponova, G.B. Pariiskii and D.Ya. Toptygin, Vysokomolekulyarnye Soedineniya, Seriya A, 1988, 30, 2, 262. 31. A.M. Wasserman and A.L. Kovarskii, Spinovye Metki i Zondy v Fizikokhimii Polimerov (Spin Labels and Probes in Physical Chemistry of Polymers), Nauka, Moscow, Russia, 1986.

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Handbook of Plastic Films 32. I.S. Gaponova and G.B. Pariiskii, Chemical Physics Reports, 1997, 16, 10, 1795 33. I.S. Gaponova and G.B. Pariiskii, Polymer Science, Series B, 1998, 40, 11-12, 394. 34. I.S. Gaponova and G.B. Pariiskii, Polymer Science, Series A, 1995, 37, 11, 1133. 35. J.R. Shelton and R.F. Kopczewski, Journal of Organic Chemistry, 1967, 32, 9, 2908. 36. D.J. Carlsson, R. Brousseau, C. Zhang and D.M. Wiles, Polymer Degradation and Stability, 1987, 17, 4, 303. 37. K. Ingold and B. Roberts, Free-Radical Substitution Reactions, John Wiley, New York, NY, USA, 1972.

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8

Modifications of Plastic Films E.M. Abdel-Bary

8.1 Introduction Modifications of plastic films are generally used to improve mechanical or physical properties so that the films are suitable for certain applications. This can be achieved by subjecting the films to mechanical or chemical treatments. Thus, surface treatments modify the crystalline morphology and surface topography, increase the surface energy and remove contaminants. Removal of contaminants is necessary for good adhesion of the surface to other substrates. Other applications, such as printing, decorating, wetting and lamination, are improved by incorporation of a surfactant to change the surface tension of the adherents. Also, the presence of polar nitrogen-containing monomers on a polymer film surface allows one to obtain ionomers for versatile applications. Thus, such films can be used as anion-exchange membranes in electrodialysis processes, in water desalination [1], as a carrier for immobilisation of medical products [2], as a separator in alkaline batteries [3] and in fuel cells, etc. A number of surface modification techniques, such as plasma, corona discharge and chemical treatments, have been used to modify polymer surfaces, and the chemical methods are of particular interest. In this case, adsorption on and adhesion to polymer surfaces have been modified using many different methods, e.g., oxidation and other chemical reactions, high-energy irradiation and plasma treatment. In the following sections, we shall discuss some of the parameters affecting the mechanical and/or physical or physicochemical characteristics of such films.

8.2 Modification of Mechanical Properties Improved mechanical properties of plastic films can be realised by changing the following parameters: orientation, crystallisation and crosslinking. Regarding the orientation process, the properties of some polymer films can be improved by stretching a film above its glass transition temperature (Tg). Orientation may be in one direction only (uniaxial orientation) or in two directions, i.e., in both machine direction and transverse direction (biaxial orientation). Biaxially oriented film can be further categorised as balanced film, where orientation is roughly equal in both directions, or as unbalanced. This orientation of the molecules in thermoplastics is essentially a stretching process, which tends to align

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Handbook of Plastic Films the molecules in the direction of the stretching force. Once the molecules have been aligned, the ordered arrangement is frozen in, giving rise to a strained condition.

8.2.1 Orientation Orientation of plastic films improves some of the physical properties such as tensile strength, impact strength, clarity and stiffness. In many instances there is also improvement in gas and water vapour barrier properties. Particularly in the case of polypropylene (PP), the barrier properties are also improved. Films that benefit appreciably from orientation include PP, polyethylene terephthalate (PET) and polyamide (Nylon). Polystyrene (PS) film is brittle material and becomes tough when biaxially oriented. Another aspect of film orientation is that of shrink-wrapping, where films such as low-density polyethylene (LDPE) and polyvinyl chloride (PVC) are stretched at a temperature above their softening points and then cooled to ‘freeze in’ the consequent orientation of the molecules. When these films are reheated, the molecules tend to return to their unstretched dimensions. In contrast, heat-setting ‘annealing’ is used to prevent shrinkage when heating stretched films. If oriented polypropylene, for example, is heated to about 100 °C immediately after being drawn, it shrinks considerably unless it is restrained in some way. This can be prevented by heat-setting. The film is heated, under controlled conditions, and while held under restraint after cooling, the film will not shrink if heated to below the annealing temperature; the film is said to be heat-set. The physical and optical properties of the film remain unchanged.

8.2.2 Crystallisation Crystallisation of polymers occurs as a result of the close approach of molecular chains in ordered, crystalline areas, which leads to the formation of much stronger intermolecular forces than in the amorphous areas. The rate of cooling has a significant effect on the degree of crystallinity and size of crystallites. Thus, rapid quenching of Nylon films during the casting process produces an amorphous film, whereas slow cooling allows the formation of crystals. The properties of the final film are highly dependent on the crystalline state of the polymer. Rapid quenching and consequent inhibition of crystal growth give a transparent film, which is more easily thermoformed.

8.2.3 Crosslinking Crosslinking is used to improve the mechanical properties and to obtain an infusible film. Crosslinked polyethylene film can be achieved by subjecting the film to high-energy

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Modifications of Plastic Films radiation. When polyethylene film is irradiated, hydrogen and smaller amounts of methane, ethane and propane gases are liberated, and the polymer becomes increasingly insoluble as a result of crosslinking of the molecules via C–C bonds. This process slightly improves the gas and water vapour barrier properties, and the film has good clarity. The tear strength of the film becomes good, and the resistance to tear initiation and to tear propagation becomes high.

8.3 Chemical Modifications Chemical modification of the surface of polymers is an attractive method of improving the barrier characteristics of polymers that are otherwise considered ideal materials for packaging. With the exception of low gas barrier properties, polyolefins are extremely attractive because of their low cost, toughness, processability and excellent water barrier properties. Surface treatment is ideal for such polymers, because they are easily processed and made into better barriers by surface modification, either during processing or afterwards [4, 5]. Chemical reactions of the surface with gases are used to modify the surfaces of existing polymers without changing bulk properties. This modification can be achieved by reacting the polymer surface with gases. Modifications of the surface using fluorine, hydrogen fluoride, sulfur tetrafluoride, chlorine and bromine have been examined.

8.3.1 Fluorination The manufacture of fluoromonomers and their subsequent polymerisations are hazardous and difficult. The fabrication of fluoropolymers is also difficult and expensive. For example, the processing of polytetrafluoroethylene (PTFE) involves costly compaction and a high-temperature (375 °C) sintering process [6]. Hence, the widespread use of fluoropolymers is hindered by these considerations. Fluorination of polymers has been shown to be a successful new route to fluoropolymers. Polymers are fluorinated either directly or indirectly [7]. In direct fluorination, highly active fluorinating agents such as fluorine, hydrogen fluoride, or sulfur tetrafluoride convert the polymeric material completely to a fluorocarbon polymer.

8.3.1.1 Direct Fluorination Fluorine is a highly active fluorinating agent because of its low dissociation energy. It forms extremely stable bonds with carbon [8]. Fluorination of polymers by fluorine may

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Handbook of Plastic Films be divided into two types: bulk fluorination and surface fluorination [9]. The surface fluorination of polyethylene (PE) film by 10% F2 (diluted with N2) results in a depth of fluorination ranging between 0 and 50 Å [10]. The extent and depth of fluorination during surface fluorination of polycarbonate (PC), PS and polymethyl methacrylate (PMMA) films with F2 diluted with He or N2 increase with reaction time, temperature and F2 gas pressure [11]. The extents of fluorination for PS, PC and PMMA are as high as 64.3%, 55.3% and 20% respectively. The relation between depth of fluorination and reaction time is represented by: d = Kt1/2

(8.1)

where d is the depth of fluorination and t is the reaction time. The proportionality constant K depends on the nature of the polymer; for example, the values for K are 13.2 and 5.6 for PS and PC, respectively. Fluorination may be conducted using hydrogen fluoride [7] and sulfur tetrafluoride [12, 13].

8.3.1.2 Indirect Fluorination In an effort to overcome the disadvantages of conventional fluorinating agents such as F2, HF, or SF4, nontoxic fluorocarbons, chlorofluorocarbons and sulfur hexafluoride are used. These gases cannot be used directly as fluorinating agents. However, when exposed to high-energy environments such as plasma, glow discharge, or γ-radiation, they generate active fluorinating agents [14]. Another approach of considerable interest is to modify the surface of existing polymers without changing the bulk properties. Fluorine attaches to the polymer near the surface and, because of its bulkiness and polar nature, improves gas and nonpolar liquid barrier properties [15]. Bulk fluorinated polymers (by F2 under controlled conditions to reduce crosslinking) can be used for the same purpose in place of fluoropolymers with similar structures prepared from respective monomers [8]. However, to make it cost-effective, surface fluorination rather than complete bulk fluorination of fabricated plastic items may be preferred. Such surface treatments could avoid problems encountered in moulding of fluoropolymers. Large fabricated plastic items can be given a surface coating of fluorinated polymer (0.1 mm thickness) [16]. Fluorinated plastic surfaces are impervious to most solvents and have good chemical, solvent and water resistance [16, 17]. Various plastic bottles, containers and tanks are fluorinated to handle chemicals and solvents safely. Hence, fluorinated plastic containers are found in use as containers for gasoline (petrol), paint, turpentine, motor oil and

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Modifications of Plastic Films varnish [18, 19]. However, fluorinated materials may not maintain their barrier properties after repeated flexing. Surface fluorination is used to improve the barrier properties of the inner surface of polyethylene during the blow moulding process for formation of bottles. Polyethylene is nonpolar and therefore a poor barrier to nonpolar hydrocarbons. Such treatments with highly polar fluorine significantly improve its barrier properties. Surface-fluorinated containers are commonly used for gasoline (petrol), herbicides, pesticides and other products that normally penetrate polyethylene [20]. Mild surface treatment of polyethylene with low concentrations of fluorine can reduce the permeability of liquid penetrants such as pentane and hexane depending on the solubility and size of the penetrant [21].

8.3.2 Chlorination The chlorination reaction is too slow and not practicable, but it results in good barrier properties with more resistance to flexing. The gas-phase chlorination of the surface of LDPE has been studied under ambient light [22, 23] as well as in the presence of ultraviolet (UV) radiation [24]. The resultant surface was reported to consist of C–Cl and C–Cl2 moieties [22, 23]. However, chlorination of the surface of PE leads also to the formation of allyl chloride and vinyl chloride moieties [24].

8.3.3 Bromination The introduction of Br moieties on the polyolefin surface opens up a synthetic pathway to introduce a wide range of specific functional groups on the surface under mild conditions via nucleophilic substitution of Br moieties by different nucleophiles [25]. The gas-phase bromination of PE, PP and PS film surfaces by a free-radical photochemical pathway occurs with high regioselectivity. The surface bromination was accompanied by simultaneous dehydrobromination resulting in the formation of long sequences of conjugated double bonds. Thus, the brominated polyolefin surface contains bromide (Br) moieties in different chemical environments. As an example, we consider the free-radical mechanism for the bromination of the surface of PE film. The first step in this reaction is the homolytic bond cleavage of the bromine molecule into two bromine radicals upon exposure to radiation [26]: UV

Br2 → 2Br•

(8.2)

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Handbook of Plastic Films In the second step, the bromine radical abstracts a hydrogen atom from the methylene unit of LDPE, which results in the formation of a radical centre on the LDPE chain: –CH2–CH2– + Br• → –•CH–CH2– + HBr

(8.3)

This radical centre further reacts with a bromine molecule to form the C–Br moiety and a bromine radical: –•CH–CH2– + Br2 → –CHBr–CH2– + Br•

(8.4)

This bromine radical then reacts with another –CH2– unit [equation (8.3)] and this chain reaction continues: –CHBr–CH2– + Br• → –CHBr–•CH– + HBr

(8.5)

The effects of the structure of the polymer on the mechanism of the bromination have been studied. Since the PS backbone contains 50% benzyl carbon atoms and 50% secondary carbon atoms, an increased rate of bromination compared to that of PE is expected.

8.3.4 Sulfonation Sulfonation involves exposure of the polymer surface to SO3/air followed by neutralisation with NH4OH, NaOH, or LiOH. Chemical reduction of copper, tin, or silver counterions present from the neutralisation process following sulfonation is called ‘reductive metallisation’. When combined with a thin protective overcoating of compatible barrier copolymer, dramatic permeation flux reductions of nearly 200-fold have been reported [5]. Sulfonation of polystyrene and aromatic polymers can be used to obtain protonconducting polymer electrolytes for use in fuel cells [27]. The aromatic polymers are easily sulfonated by concentrated sulfuric acid, by chlorosulfonic acid, by pure or complexed sulfur trioxide, or by acetyl sulfate. Sulfonation with chlorosulfonic acid or fuming sulfuric acid sometimes causes chemical degradation in these polymers [1]. Surface sulfonation yields excellent gas barrier properties under dry conditions, is relatively simple and does not affect the mechanical stability of the polymer [5].

8.3.5 Chemical Etching Chemical treatment is usually used for irregular and, in particular, large articles when other treatment methods are not applicable. It involves immersion of the article [LDPE and high-density PE (HDPE)] in an etchant solution such as chromic acid [28],

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Modifications of Plastic Films permanganate, sulfuric acid [29,30] or chlorosulfonic acid. Reflection infrared (IR) studies reveal extensive chemical changes on the surface in the case of LDPE but not HDPE or PP. New bands corresponding to the introduction of –OH, >C=O and SO3H groups were detected. Oxidation of PE by sulfuric acid and potassium chlorate [29, 30] has been carried out. In this case, the free energy of adhesion of the polymer is found to increase linearly with the surface density of the hydrophilic sites created by oxidation. The surface tension, polarity, wettability and bondability of fluoropolymer are improved by sodium etching [31, 32]. The etching solution is the equimolar complex of sodium and naphthalene dissolved in tetrahydrofuran. X-ray photoelectron spectroscopy (XPS) shows the complete disappearance of the fluorine peak, the appearance of an intense oxygen peak, and broadening and shifting of the C 1s peak to lower binding energy. A significant number of functional groups, such as carbonyl, carboxyl and C=C unsaturation, are introduced. The oxidation methods described up to now are heterogeneous in nature, since they involve chemical reactions between substances located partly in an organic phase and partly in an aqueous phase. Recently, a technique that is commonly referred to as phase transfer has come into prominence. This technique involves the use of phasetransferred permanganate (purple hydrocarbon) as an oxidant in a polar medium. Konar and co-workers [33, 34] have oxidised several polyolefins with the help of tetrabutylammonium permanganate in a hydrocarbon medium. Characterisation of the oxidised polyolefins confirmed the introduction of polar functional groups on the polar surface [35, 36]. Other phase-transfer catalysts, such as tetrabutylammonium bromide, tetrapentylammonium iodide, dicyclohexyl-18-crown-6 (DC-18-C6) and benzyl triphenyl phosphonium chloride (BTPC), have been investigated [37]. The results obtained show that LDPE oxidised using DC-18-C6 and BTPC catalysts has a relatively greater polar contribution to the total surface free energies than when using other catalysts. The carboxyl percentage attains 15.0% and 20.0%, respectively [38] while hydroperoxide attains 22.2% and 15.2%, respectively [36]. When a polymer is soaked in a heavily oxidative chemical liquid, such as chromic anhydride/ tetrachloroethane, chromic acid/acetic acid or chromic acid/sulfuric acid, and treated under suitable conditions, polar groups are introduced on the polymer surface [39, 40]. The surface of the polymer is heavily oxidised by nascent oxygen generated during the reaction as follows: K2Cr2O7 + 4H2SO4 → Cr2(SO4)3 + K2SO4 + 4H2O + 3[O]

(8.6)

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8.3.6 Grafting Graft copolymerisation of vinyl monomers on to polymeric materials has been the subject of extensive studies for about four decades. In spite of the huge number of published papers and patents, and the interesting results obtained, there has been comparatively little commercialisation of the grafting process. The reasons for the lack of industrialisation on a large scale have been partly economic. Among the technical problems, which still remain to a considerable extent, are the concurrent formation of homopolymer in most cases and the lack of reproducibility in these largely heterogeneous reactions. In addition, there is the difficulty of controlling the grafted side chains in the molecular weight (molar mass) distribution. There are now a considerable number of methods available for effecting graft copolymerisation on to preformed polymers, each with its own particular advantages and disadvantages. Graft copolymerisation is effected, generally, through an initiation reaction involving attack by a macroradical on the monomer to be grafted. The generation of the macroradical is accomplished by different means such as: (1) Decomposition of a weak bond or liberation of an unstable group present in side groups in the chemical structure of the polymer; (2) Chain transfer reactions; (3) Redox reaction; (4) Photochemical initiation; and (5) Gamma-radiation-induced copolymerisation. Grafting using γ-radiation is concentrated on polyolefins and some vinyl polymers and elastomers, which are usually difficult to graft by chemical means without prior chemical modification of the substrate.

8.3.6.1 Grafting Using High-Energy Radiation The surface properties of commercial polymer thin films can be tailored under appropriate experimental conditions of radiation-induced grafting. The growth in popularity of radiation as the initiating system for grafting arises from the availability and cost of ionising radiation. This is due to the introduction of more powerful nuclear reactors. Apart from its cheapness, radiation is a very convenient method for graft initiation because it allows a considerable degree of control to be exercised over such structural factors as

220

Modifications of Plastic Films the number and length of the grafted chains by careful selection of the dose and dose rate. Thus, the advantages of radiation-chemical methods are: (1) Ease of preparation as compared to the conventional chemical method; (2) General applicability to a wide range of polymer combinations (due to the relatively nonselective absorption of radiation in matter); and (3) More efficient (and thus more economical) energy transfer provided by radiation compared to chemical methods requiring heat. The theory of radiation-induced grafting has received extensive treatment. The direct effect of ionising radiation in material is to produce active radical sites. The typical steps involved in free-radical polymerisation are also applicable to graft copolymerisation, including initiation, propagation and chain transfer. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rate. Among the various methods of radiation grafting, four have received special attention: (1) Direct radiation grafting of a vinyl monomer on to a polymer; (2) Grafting on radiation-peroxidised polymer; (3) Grafting initiated by trapped radicals; and (4) The intercrosslinking of two different polymers. Acrylic acid (AA) has been grafted on to PE films using γ-radiation [41, 42]. Gammaradiation grafting of styrene on to PE films has been carried out [43]. The styrene-grafted films were then sulfonated to form cationic exchange membranes. Rieke and co-workers described the properties obtained from grafting AA on to HDPE [44]. Their study pursued the concept of producing thermally sensitive crosslinks that could improve the properties of PE, (i.e., increase chemical reactivity). In 1977, Toi and co-workers determined the thermal properties for styrene-grafted HDPE by using γ-radiation [45]. No effect was observed on the crystallite size and the glass transition temperature after grafting. Ishigaki and co-workers reported the graft polymerisation of AA on to PE film by the preirradiation method [46, 47]. LDPE and HDPE were irradiated by electron beams of 2-50 Mrad and then immersed in an AA aqueous solution. These products were tested as semipermeable membranes for water desalination under reverse osmosis conditions [48]. Hydrophilic monomers such as AA or vinylpyridine were grafted on to PE via 60Co γradiation. The hydrophilic monomer-grafted PE could be treated further for functionalisation, leading to the investigation of a few applications such as separation membranes, polymeric catalysts and biosensors [49-53].

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8.3.6.2 Photografting The surface photografting process is based on surface grafting reactions initiated by ultraviolet radiation. These reactions are efficient and fast. They are limited to the surface of the polymer without affecting bulk properties, and they give very thin layers (less than 10 nm) of grafted polymer [54]. The grafting of sheets of low-density and high-density polyethylene with acrylic acid by UV irradiation from a high-pressure mercury lamp using a batch process has been reported [55]. The surface of HDPE is more difficult to graft than that of LDPE because of the linear chain structure of HDPE, and consequently its higher degree of crystallinity, which gives it a rougher surface structure than LDPE. Surface grafting with acrylic acid, as expected, decreases the contact angle of water, approaching complete wetting for LDPE. The molecular mechanism of bulk surface photografting has been given [56]. The primary grafting given in this mechanism using benzophenone involves initiation and propagation of short linear chains which is terminated by the addition of ketyl radical. Benzophenone acts as both initiator and terminator. The main effects that are important for applications are increased wettability, as mentioned before, increased adhesion of inks and other substrates, and increased adsorption of dyes. By grafting of reactive monomers like glycidyl acrylate, the polymer surface is made reactive to stabilisers, hydrophilic polymers, heparin and other bioactive agents, which gives functional properties of great interest [57-59]. Biomedical applications are of particular interest [60]. Examples of other recent publications on surface photografting include the preparation of polymeric catalyst [61, 62], polyethylene films for studying electrostatic interactions [63], and films for immobilisation of enzymes [64].

8.4 Physical Methods Used for Surface Modification Modification techniques using physical methods have been designed to achieve increased hydrophilicity, chemical modification and attachment of pharmacologically active agents. These physical methods include plasma treatment, corona treatment, ultraviolet and gamma radiation.

8.4.1 Plasma Treatment The implantation process that occurs in plasma treatment is one of the most effective methods of surface modification of polymeric materials. The plasma activates gas molecules, such as oxygen and nitrogen. The activated species interact with the polymer’s surfaces, and then special functional groups, such as hydroxyl, carbonyl, carboxyl, amino

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Modifications of Plastic Films and amido groups, are formed at the surface of the polymers. As a result, the implantation reactions lead to large changes in the surface properties of the polymer; for example, the polymers change from hydrophobic to hydrophilic. ‘Plasma treatment’ is frequently used for the improvement of the adhesion and wettability of polymeric materials. A polyethylene film was treated with a nitrogen plasma, and its surface was inspected by XPS (C 1s and N 1s core levels) [65]. The original polyethylene film provides a sharp and symmetrical C 1s core-level spectrum whose peak appears at 285 eV with no N 1s core-level spectrum. However, the plasma-treated film gives an asymmetrical C 1s spectrum with a tail at more than 285 eV, and a strong N 1s core-level spectrum. This comparison indicates that some nitrogen functionalities were generated at the polyethylene film surface through nitrogen plasma treatment. Similarly, oxygen plasma treatment leads to the formation of some oxygen functionalities at the surface of polyethylene film [66]. It is clear that plasma treatment implants atomic residues at the surface of polymeric materials. Carbon monoxide, carbon dioxide, nitrogen monoxide, nitrogen dioxide and ammonia are used as plasma gases for hydrophilic surface modification. Polypropylene, polyester, polystyrene, rubber and polytetrafluoroethylene, among others, but not polyethylene, have been successfully modified by plasma treatment. The details of the implantation process are reviewed in the literature [67].

8.4.2 Corona Treatment In this technique, a sufficiently high-voltage electrical discharge is applied to the surface of a moving substrate (sheet or film). Pretreatment of films is usually carried out at the same time as film extrusion, which is an advantage where antistatic and other additives are present in the film. When film was extruded and stored prior to treatment, it was found that the additives had bloomed to the surface, and this made it difficult to achieve an even treatment. In one method, the film is passed between two electrodes, one of which is a metal blade connected to a high-voltage, high-frequency generator. The other is an earthed roller, which is separated from the high-voltage electrode by a narrow gap. The metal electrode should be slightly narrower in width than the film that is to be treated in order to prevent direct discharges to the roller. The electrical discharge is accompanied by the formation of ozone. This oxidises the film surface, rendering it polar. The level of treatment is governed by the generator output and the speed of throughput. Both under- and overtreatment should be avoided – the latter causing surface powdering, brittleness and sealing difficulties. The effect of treatment diminishes with time, and the treated surface is sensitive to handling and dust pickup. The corona treatment functions at atmospheric pressure and relatively high temperature. In this case, very significant surface oxidation occurs [6].

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Handbook of Plastic Films One simple test for determining whether or not a film has been treated is to run water over the surface. If the film is untreated, the water will be repelled, whereas a treated film will retain the liquid for several minutes. Between these two extremes, a partially treated film will tend to show areas of both good and bad adhesion, so that the test is only satisfactory for seeing whether or not the surface of the film has been treated but not for detecting overtreatment. An improvement on this primitive test method is the peel adhesion test. This is carried out by applying a specified pressure-sensitive tape to the film surface, using a roller. The peel strength is then measured with the aid of a tensiometer. The higher the level of treatment, the higher the peel strength. The chemical changes occurring on the surface can be detected by using the XPS technique. This technique enables one to identify the presence of hydroxyl, ether, ester, hydroperoxide, aldehyde, carbonyl or carboxylic groups in corona dischargetreated polyolefins.

8.5 Characterisation Characterisation of modified films depends on the method of modification. For instance, the change in mechanical properties due to stretching can be evaluated by measuring the changes in mechanical properties using tensile testing machines according to standard methods. Characterisation of grafted films also differs somewhat from that of physically treated films. However, the selection of one or other measuring technique depends generally on the extent of modification.

8.5.1 Gravimetric Method Graft products are usually characterised by different methods. The first method is the calculation of graft parameters known as the grafting percentage (GP), grafting efficiency (GE) and weight conversion percentage (WC). These parameters can be calculated according to the following equations: grafting percentage (GP) =

grafting efficiency (GE) =

224

A−B × 100 B A−B × 100 C

(8.7)

(8.8)

Modifications of Plastic Films

weight conversion percentage (WC) =

A × 100 B

(8.9)

where A, B and C are the weights of the extracted graft product, substrate and monomer, respectively. This gravimetric method gives direct and rapid information about the graft reaction. Other characterisation methods are usually used to detect the changes in physical properties, which usually result from the changes in the morphology and structures of the substrates due to grafting.

8.5.2 Thermal Analyses In polymers having a certain degree of crystallinity, differential scanning calorimetry (DSC) is used to determine the heat of fusion and, consequently, the changes in the degree of crystallinity in grafted and ungrafted samples. The changes in the crystallinity of PE found after grafting include a small 2.5 °C drop in the location of the maximum in the melting curve and a significant decrease in the area under the melting peak [69]. Similar results were observed in the case of grafting PP and PE/ethylene-vinyl acetate (EVA) blends [70]. While the decrease in the melting temperature, represented by the shift in the melting curve, indicates that there is some change in the crystallinity caused by grafting, comparison of the areas before and after grafting indicates that this may be a small effect. By assuming that the difference in areas is due only to a difference in the amount of PE or PP present (in other words, no difference in the degree of crystallinity), the per cent graft can be calculated from: %G =

A1 − A2 ρPAN × × 100 A2 ρPE

(8.10)

where A1 is the area before grafting, A2 is the area after grafting, and ρ is the density.

8.5.3 Scanning Electron Microscopy Scanning electron microscopy (SEM) is generally used to detect the topography of a grafted surface, which usually changes due to monomers grafted on to the surface. In addition, this method can also be used to detect the depth of grafting into the matrix. If a binary monomer mixture was used for grafting, scanning electron micrographs help to detect the grafted monomer distribution by comparison with micrographs of each grafted monomer separately.

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8.5.4 Swelling Measurements Equilibrium swelling of grafted samples in a proper solvent helps to detect the presence of grafted monomer. For instance, polyethylene does not swell to any noticeable extent in water. However, if polyethylene is grafted with water-soluble polymers such as polyacrylic acid or polyacrylamide, the equilibrium swelling of the product obtained increases markedly. Accordingly, the increase in swelling is evidence of grafting. In contrast, the swellability of natural rubber or styrene-butadiene-rubber vulcanisates in gasoline or benzene decreases markedly due to grafting with polyacrylonitrile (PAN). This decrease in swelling, again, is evidence of grafting.

8.5.5 Molecular Weight and Molecular Weight Distribution It is essential to know the molecular weight (molar mass) distribution of a graft in order to design functional polymeric membranes precisely by application of radiation-induced graft polymerisation and to control the grafting process. For example, the length and density of the polymer chains grafted on to a cellulose triacetate microfiltration membrane will determine the permeability of liquid through and the adsorptivity of molecules on the functionalised microfiltration membrane. Thus, the molecular weight distribution of methyl methacrylate grafted on to cellulose triacetate has been determined by acid hydrolysis of the substrate. From the gel-permeation chromatogram, the molecular weight distribution was determined [71]. This method is valid only when it is possible to degrade the substrate. In the case of grafted natural rubber, for example, ozonolysis is a very convenient process to use to destroy the natural rubber segments, leaving the plastomer chains intact [72]. Alternatively, oxidation with perbenzoic acid can be used [73]. Osmometry or solution viscosity may then be used to determine the molecular weight of the isolated non-rubber fraction.

8.5.6 Dielectric Relaxation Dielectric relaxation measurements of polyethylene grafted with AA, 2-hydroxyethyl methacrylate (HEMA) and their binary mixture were carried out in a trial to explore the molecular dynamics of the grafted samples [74]. Such measurements enable information to be obtained about their molecular packing and interaction. It was possible to predict that the binary mixture used yields a random copolymer PE-g-P(AA/HEMA) that is greatly enriched with HEMA. This method of characterisation is very interesting and is likely to be developed in different polymer/monomer systems.

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8.5.7 Surface Properties The surface properties of modified plastic films are very important in industrial applications. A number of techniques are available for determining the composition of a solid surface. This is very important in many processes, such as oxidation discoloration, wear and adhesion. The technique used depends upon such important considerations as sampling depth, surface information, analysis environment and surface suitability. The most widely used techniques for surface analysis are Auger electron spectroscopy (AES), XPS, secondary ion - mass spectroscopy (SIMS), Raman and IR spectroscopy, and contact angle measurements.

8.5.8 Spectroscopic Analysis 8.5.8.1 Infrared (IR) Spectroscopy Proof of chemical modification or changes in chemical structure due to physical treatments such as corona discharge can be followed up by spectroscopic analysis using IR. Thus, the amount of acrylonitrile grafting on to PE using an electron beam was determined from the absorbance of the nitrile group at 2240 cm–1 after extraction of homopolymer [69]. In order to minimise the effects of weighing error, an internal reference method utilising the methylene absorbance of PE at 730 cm–1 was adopted. Thus, the mass of PAN in a sample was correlated to the ratio of the absorbance A2240/A730, and the weight per cent graft defined before was computed from the mass of PAN.

8.5.8.2 X-Ray Fluorescence Spectroscopy (XFS) This method can be used to detect and characterise the first several hundred nanometres of depth of a solid. It can be attached to a scanning electron microscope. The main principle is that energetic electrons bombard the sample, where ionisation takes place. Ions with an electron vacancy in their atomic core rearrange to a lower energy state, resulting in the release of electromagnetic energy of a specific wavelength. Analysis of the wavelengths of the X-radiation emitted identifies the atomic species present.

8.5.8.3 Auger Electron Spectroscopy (AES) This technique is used to characterise the chemical bonding state of the elements on the surface. The maximal depth from which Auger electrons can escape is only about 0.30.6 μm. For most materials, AES uses a low-energy, 1-5 keV, electron beam gun for surface bombardment to minimise surface heating.

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8.5.9 Electron Spectroscopy for Chemical Analysis (ESCA) or X-Ray Photoelectron Spectroscopy (XPS) In this method the surface is bombarded with low-energy X-rays, which is less disruptive than an electron beam. The energy is absorbed by ionisation, resulting in the direct ejection of a core-level electron, i.e., a photoelectron. Hence ESCA is also known as Xray photoelectron spectroscopy. These electrons have an escape depth of less than a nanometre. Although XPS is less sensitive than AES, it provides a direct measure of the binding energy of core-level electrons through the relation: binding energy of ionised core-level electron = energy of emitted photoelectron – incident X-ray energy and it gives simpler spectral line shapes than AES. This technique can be used to distinguish between different elements and different chemical bonding configurations. It is the most popular surface analytical technique for providing structural, chemical bonding and composition data for polymeric systems. All elements, except hydrogen, are readily identified by XPS, since the different core-level binding energies are highly characteristic. By measuring the relative peak intensities and dividing them by the appropriate sensitivity factors, one may find the concentration of different elements on a surface. Moreover, small shifts in the binding energy of a core level are corroborated by considering the presence of different functional groups. For example, when a carbon atom is bonded to different groups of atoms of increasing electronegativity, a systematic shift in the binding energy of the C 1s peak is observed. The higher the electronegativity of the group, the higher the binding energy of the C 1s peak.

8.6 Applications Since bringing about changes in physical properties is often the impetus for grafting, it is necessary to touch upon this briefly in this section. A number of general reviews on grafting have also included some discussion on the changes in physical properties that usually determine the field of applications. Grafting has often been employed to change the moisture absorption and transport properties of plastic films when hydrophilic monomers such as acrylamide, acrylic acid and methacrylic acid are grafted. Radiation grafting of anionic and cationic monomers to impart ion-exchange properties to polymer films and other structures is rather promising. Thus, grafting of acrylamide and acrylic acid on to polyethylene and polyethylene/ethylene-vinyl acetate copolymer blend [70] allows a new product to be obtained with reasonable ion-exchange capacity.

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Modifications of Plastic Films A number of possible uses of radiation grafting are being explored for microlithography, diazo printing, and various copying and printing processes. Radiation grafting for various biomedical applications remains an extremely active field of development. The grafted side chains can contain functional groups to which bioactive materials can be attached. These include amine, carboxylic and hydroxyl groups, which can be considered as centres for further modifications. Photodegradation of polyethylene waste can be markedly accelerated via its grafting with acrylamide [70]. In contrast, photostabilisation of polyethylene and polypropylene can be achieved as a result of the grafting of 2-hydroxy-4-(3-methacryloxy-2hydroxypropoxy)benzophenone using γ-radiation [75]. In this case, the grafted compound, acting as a UV stabiliser, is chemically bound to the backbone chain of the polymer, and its evaporation from the surface can be avoided.

References 1.

S. Munari in Advances in Radiation Research: Physics and Chemistry, Eds., J.F. Duplan and A. Chapiro, 1973, Gordon and Breach, New York, NY, USA, 2, 299.

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A. Kabanov and I. Astafieva, Biopolymers, 1991, 32, 1473.

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I. Ishigaki, T. Sugo and J. Okamoto, inventors; Japan Atomic Energy Research Institute, assignee; US Patent 5,075,342, 1991.

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M. Salame in The Wiley Encyclopedia of Packaging Technology, Ed., M. Bakker, John Wiley, New York, NY, USA, 1986.

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W.E. Walles in Barrier Polymers and Structures, Ed., W.J. Koros, ACS Symposium Series No.423, American Chemical Society, Washington, DC, USA, 1990, Chapter 14.

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S. Maiti, P.S. Das and B. Adhikari in The Polymeric Materials Encyclopedia, Ed., J.C. Salamone, CRC Press, Boca Raton, FL, USA, 1996.

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W.A. Sheppard and C.M. Sharts, Organic Fluorine Chemistry, W.A. Benjamin, New York, NY, USA, 1969, pp. 10-14, 53-66.

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R.J. Lagow and J.L. Margrave, Journal of Polymer Science, Polymer Letters Edition, 1974, 12, 177.

10. D.T. Clark, W.J. Feast, W.K.R. Musgrave and I. Ritchie, Journal of Polymer Science, Polymer Chemistry Edition, 1975, 13, 857. 11. J. Shimade and M. Hoshino, Journal of Polymer Science, Polymer Chemistry Edition, 1979, 18, 157. 12. V.P. Bezsolitsen, B.N. Gorbunov, A.A. Nazarov and A.P. Khardin, Vysokomolekuliaryne Soedineriia, Seriya A, 1972, 14, 950. 13. V.P Bezsolitsen, B.N. Gorbunov and A.P. Khardin, Khimiia i Tekhnologiia, 1988, 3. 14. M. Strobel, S. Corn, C.S. Lyons and G.A. Korba, Journal of Polymer Science, Part A: Polymer Chemistry, 1987, 25, 1295. 15. J.P. Hobbs et al., Barrier Polymers and Structures, Ed., W.J. Koros, ACS Symposium Series No.423, American Chemical Society, Washington, DC, USA, 1990, Chapter 15. 16. R.J. Lagow and J.L. Margrave, Chemical and Engineering News, 1970, 48, 40. 17. R. Milker and B. Möller, Kunststoffe, 1992, 82, 10, 978. 18. J.F. Gentilcore, M.A. Trialo and A. Waytek, Journal of Plastics Engineers, 1978, 34, 9, 40. 19. A.G. Frankfurt, Gas Aktuell, 1986, 32, 17. 20. Modern Plastics, 1985, 62, 8, 41. 21. W.J. Koros, V.T. Stannett and H.B. Hopfenberg, Polymer Engineering Science, 1982, 22, 12, 738. 22. E.M. Cross and T.J. McCarthy, Macromolecules, 1992, 25, 10, 2603. 23. J.F. Elman, L.J. Gerenser, K.E. Goppert-Berarducci and J.M. Pochan, Macromolecules, 1990, 23, 17, 3922. 24. S. Balamurugan, A.B. Mandale, S. Badrinarayanan and S.P. Vernekar, Polymer, 2001, 42, 6, 2501. 25. J. March, Advanced Organic Chemistry, 3rd Edition, Wiley Eastern, New Delhi, India, 1986, Chapter 10.

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Modifications of Plastic Films 26. J.G. Calvert and J.N. Pitts, Photochemistry, 1960, John Wiley & Sons, New York, NY, USA. 27. M. Rikukawa, Progress in Polymer Science, 2000, 25, 10, 1463. 28. P. Blais, D.J. Carlsson, G.W. Csullog and D.M. Wiles, Journal of Colloid and Interface Science, 1974, 47, 636. 29. A. Baszkin, L. Ter-Minassian-Saraga and C.R. Lisbeth, Comptes Rendus Academie des Sciences, Paris, Series C, 1969, 268, 315. 30. C. Fonseca, J.M. Perena, J.G. Fatou and A. Bello, Journal of Materials Science, 1985, 20, 3283. 31. D.W. Dwight and W.M. Riggs, Journal of Colloid and Interface Science, 1974, 47, 650. 32. E.H. Andrews and A.J. Kinloch, Proceedings of the Royal Society of London, 1973, A332, 385. 33. J. Konar and P. Maity, Journal of Materials Science Letters, 1994, 13, 197. 34. J. Konar, G. Samanta, B.N. Avasthi and A.K. Sen, Polymer Degradation and Science, 1994, 43, 209. 35. J. Konar and R. Ghosh, Journal of Applied Polymer Science, 1990, 40, 719. 36. J. Konar and R. Ghosh, Polymer Degradation and Science, 1988, 21, 263. 37. J. Konar and R. Ghosh, Journal of Adhesion Science and Technology, 1989, 3, 609. 38. J. Konar, S. Ghosh and A.K. Banthia, Polymer Communications, 1988, 29, 36. 39. D. Briggs, D.M. Brewis and M.B. Konieczo, Journal of Materials Science, 1976, 11, 1270. 40. K. Nakao and M. Nishiuchi, Journal of the Adhesion Society of Japan, 1966, 2, 239. 41. A. Chapiro, M. Magat and J. Sebban, inventors; French Patent 1,125,537, 1956. 42. A. Chapiro, M. Magat and J. Sebban, inventors; British Patent 809,838, 1959. 43. W.K.W. Chen and R.B. Mesrobian, Journal of Polymer Science, 1957, 18, 903. 44. J.E. Rieke and G.M. Hart, Journal of Polymer Science, 1963, C1, 117.

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Handbook of Plastic Films 45. K. Toi, M. Kikuchi and T. Tokuda, Journal of Applied Polymer Science, 1977, 21, 535. 46. I. Ishigaki, T. Sugo and T. Senoo, Radiation Physics and Chemistry, 1981, 18, 899. 47. I. Ishigaki, T. Sugo K. Senco, T. Okada, J. Okamoto and S. Machi, Journal of Applied Polymer Science, 1982, 27, 1033. 48. M.I. Aly, K. Singer, N.A. Ghanem and M.A. El-Azmirly, European Polymer Journal, 1978, 14, 545. 49. G-H. Hsiue and W-K. Huang, Journal of Applied Polymer Science, 1985, 30, 1023. 50. G-H. Hsiue and W-K. Huang, Journal of the Chinese Institute of Chemical Engineers, 1987, 16, 257. 51. G-H. Hsiue and J-S. Yang, Journal of Membrane Science, 1993, 82, 117. 52. G-H. Hsiue, W-K. Huang and H-L. Chu, Journal of Polymer Science, Part A: Polymer Chemistry, 1989, 27, 4397. 53. G-H. Hsiue, T.L Perng and J.M. Yang, Journal of Applied Polymer Science, 1991, 42, 1899. 54. B. Ranby, Die Makromolekulare Chemie - Macromolecular Symposia, 1992, 63, 55. 55. K. Allmer, A. Hult and B. Ranby, Journal of Polymer Science: Polymer Chemistry, 1988, 26, 2099. 56. B. Ranby, International Journal of Adhesion and Adhesives, 1999, 19, 5, 337. 57. B. Zhang and B. Ranby, Journal of Applied Polymer Science, 1991, 43, 621. 58. K. Allmer, A. Hult and B. Ranby, Journal of Polymer Science, Part A: Polymer Chemistry, 1989, 27, 1641. 59. K. Allmer, A. Hult and B. Ranby, Journal of Polymer Science, Part A: Polymer Chemistry, 1989, 27, 3405. 60. K. Allmer, J. Hilborn, P.H. Larsson, A. Hult and B. Ranby, Journal of Polymer Science, Part A: Polymer Chemistry, 1990, 28, 173. 61. H. Kubota, European Polymer Journal, 1992, 28, 3, 267.

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Modifications of Plastic Films 62. H. Kubota, European Polymer Journal, 1993, 29, 4, 551. 63. J. Zhang, E. Uchida, Y. Uyama and Y. Ikada, Journal of Colloid and Interface Science, 1997, 188, 431. 64. J.R. Bellobono, E. Selli, A. Polissi and F. Mufatto, Biotechnology and Bioengineering, 1990, 35, 6, 646. 65. H. Yasuda, H.C. Marsh, S. Brandt and C.N. Reilly, Journal of Polymer Science, Polymer Chemistry Edition, 1977, 15, 991. 66. H. Yasuda, Journal of Polymer Science: Macromolecular Reviews, 1980, 16, 199. 67. N. Inagaki, Plasma Surface Modification and Plasma Polymerisation, Technomic, Lancaster, PA, USA, 1996. 68. C.Y. Kim and D.A.I. Goring, Journal of Applied Polymer Science, 1971, 15, 1357. 69. P.W. Morgan and J.C. Corelli, Journal of Applied Polymer Science, 1983, 28, 1879. 70. E.M. Abdel Bary and E.M. El-Nesr, Radiation Physics and Chemistry, 1996, 48, 5, 689. 71. H. Yamagishi, K. Saito, S. Furusaki, T. Sugo, F. Hoson and J. Okamoto, Journal of Membrane Science, 1993, 85, 71. 72. P.W. Allen, G. Ayrey, C.G. Moore and J. Scanlan, Journal of Polymer Science, 1959, 36, 55. 73. J.A. Blanchette and L.E. Nielson, Journal of Polymer Science, 1956, 22, 317. 74. A.A. Mansour, E.M. Abdel-Bary and E.M. El-Nesr, Journal of Elastomers and Plastics, 1994, 26, 355. 75. F. Ranogajec, M. Mlinac and I. Dvornik, Radiation Physics and Chemistry, 1981, 18, 511.

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9

Applications of Plastic Films in Packaging Susan E. Selke

9.1 Introduction Nearly all products are packaged at some point in their life-cycle. Plastic films are widely used in packaging, and continue to grow in use as more and more applications switch from rigid to flexible packages. Flexible packages generally take up much less space than the rigid structures they replace, especially before they are filled with product. They commonly require less material, as well. Therefore, switching from rigid to flexible packaging can provide significant economic savings in warehouse space and transportation, as well as in package cost. On the other hand, because flexible packaging does not usually have as much strength as rigid packaging, stronger distribution packaging may be required. Opening and reclosing of flexible packaging may also be less userfriendly, and consumers may perceive some types of products in flexible packaging as being lower in quality than equivalent products in rigid or semi-rigid packages. Common flexible packaging forms include wraps, bags and pouches. In these packages, plastic films may be used alone or combined with paper and/or metal to serve the basic packaging functions of containment, protection, communication and utility in the delivery of quality products to the consumer. While plastic films are most often found in flexible package structures, they may also be used as a component in rigid or semi-rigid package structures, for example, as a liner inside a carton, or as lidding on a cup or tray. The most common film used in packaging is low-density polyethylene (LDPE), defined broadly to include linear low-density polyethylene (LLDPE). Appreciable amounts of high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyamide (Nylon) and other plastics are also used

9.2 Packaging Functions Before examining applications of plastic films in packaging, it is useful to take a moment to consider why we use packages at all, since that will help in evaluation of the advantages and disadvantages of plastic films as packaging materials. The functions of packaging can be described in many ways. One simple way of organising them is to consider the basic packaging functions as containment, protection, communication and utility.

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Handbook of Plastic Films The containment function of packaging is the most basic. Many types of goods cannot be readily moved from one place to another unless they are contained in some manner. This is obvious for liquids and gases, but is also true for many small solid items, for example, marbles, nails, laundry detergent powder and potato chips (crisps). The package confines these items in a way that makes it feasible to transport them. Sometimes the containment function is considered part of the more inclusive protection function of packaging. An important attribute of many forms of packaging is the ability of the package to protect the product from some type of damage associated with its interaction with the environment. In the example above, the marbles and nails must be protected against exposure to dust and dirt to remain in a condition that will permit their sale. The laundry detergent needs to be protected from exposure to excess moisture that could cause caking. The potato chips must be protected against light and oxygen, which can cause rancidity. In some cases, protection of the environment from the product is provided by the package. A water-soluble pouch for agricultural chemicals has, as its prime function, protection of the user from exposure to the hazardous undiluted chemical. Packages also serve as a vehicle for communication. In most cases, the package must in some way communicate what it contains. Sometimes this is as simple as being transparent, so that the user can see what is inside. Since the package is often the primary sales tool, however, communication needs are usually much more extensive. The package must not only communicate what is inside, but also act to convince the potential consumer to purchase the product. Often, there are a number of legally required communications, such as the amount of product, where and by whom the product is made, required warnings, etc. Packages also provide utility, either for the end-user or for others who interact with the package along the supply chain. Utility includes attributes such as a tear strip for opening and tamper evidence, a zipper closure for resealing, and a hole for use in hanging the packages on a display. Individual packages or package elements often provide more than one function, simultaneously. For example, a stand-up pouch for snack foods provides: containment; protection of the product against oxygen and moisture; communication of identification, legally required, and sales messages; opening and reclosure features for consumer utility; and the ability to stand conveniently on the retailer’s shelf and present a reasonably flat front panel to catch the consumer’s eye.

9.3 Flexible Package Forms Flexible packages come in two basic forms: wraps, and bags or pouches. A wrap consists of plastic film that has not been formed into a package shape. The film is simply wound around

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Applications of Plastic Films in Packaging the product or products to be contained, and held in place in some fashion. In a bag or pouch, some shaping of the plastic is done, either before or at the same time as the product is added. Most often, this shaping is done by heat-sealing the edges of the plastic together.

9.3.1 Wraps 9.3.1.1 Stretch-Wrap One of the largest uses of plastic films in packaging is in stretch-wrap used for bundling pallet-loads of products together, in order to unitise them for distribution. The plastic film, most often linear low-density polyethylene, is stretched as it is wound around the products and pallet, usually in a spiral fashion. When enough has been applied, the film is cut, and the tail of the film is adhered to the load, usually by self-cling. When the stretching force is released, the film’s tendency to return to its unstretched dimensions causes a restraining force to be exerted on the load, thus unitising it and keeping it from shifting when the load is moved during distribution. In addition to its unitising function, stretch-wrap also protects the load against moisture, dust and abrasion. Stretch-wrap can also be used to provide this protection to single items, or to unitise smaller than pallet-load quantities of goods. While stretch-wrap is simple in conception, it may have a fairly complex structure. It is desirable for each layer of stretch-wrap to stick to the layers below, but it is undesirable for adjacent shrink-wrapped loads to stick to each other, or to other things with which they come in contact. Therefore, the stretch-wrap may have a multilayer structure, with tackifying agents added to the inside layer to enhance cling. Low-density polyethylene, polyvinyl chloride, ethylene-vinyl acetate and other polymers are used as stretch films, in addition to LLDPE.

9.3.1.2 Shrink-Wrap Shrink-wrap is an alternative to stretch-wrap for unitisation. When shrink-wrap is exposed to a source of heat, the previously aligned (oriented) molecules try to return to the lowerenergy, unoriented, random-coil conformation. The product prevents the film from returning to its unstretched dimensions, and the force exerted by the material on the product acts to unitise the load. For unitising pallet-loads of goods, stretch-wrap is much more common than shrinkwrap, since it generally requires less energy and is more economical. Shrink-wrap is

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Handbook of Plastic Films more commonly used as a bundling wrap, unitising two or several products (either the same or different), rather than for full pallet-loads of goods. Often, shrink-wrap is used for product protection rather than unitisation, in applications ranging from meat to toys. It can be designed to form a tight enclosure around the entire product, providing excellent protection against dirt, moisture and abrasion. Usually, the wrap is formed into a loose pouch before it is shrunk tightly around the product in a shrink tunnel, where the packaged product is exposed to hot air. LDPE and LLDPE are common materials for shrink films. PVC and PP are used in lesser quantities, as are some specialty films.

9.3.2 Bags, Sacks and Pouches To make a bag, sack or pouch, two or more edges of a plastic film are sealed together, forming a cavity in which the product can be placed. In most applications, the opening is then closed so that the product is completely enclosed by the package. In some cases, such as merchandise sacks, one side remains open. The terms ‘bag’, ‘sack’ and ‘pouch’ can be confusing. According to some authorities, sacks are larger than bags, and both refer to packages in which the top is open, while pouches are smaller, and refer to packages that are totally sealed. However, these definitions do not conform to common use of these terms, which, in practice, are often used interchangeably. Common styles of pouches include pillow pouches, three-side-seal pouches and fourside-seal pouches. Pillow pouches are produced by forming the plastic film into a cylinder and sealing the edges together in what will become the back seam in the finished package. The bottom of the cylinder is collapsed and sealed, the product introduced, and then the top seam added. The shape of the filled package resembles a pillow – hence its name. Three-side-seal pouches are formed, as the name indicates, by folding the film into a rectangle and sealing the three non-fold sides. In some cases, the fourth side is sealed as well, for additional strength. Four-side-seal pouches are formed from two pieces of material that are sealed together on all sides. Therefore, four-side-seal pouches need not be rectangular in shape. In contrast to pillow and three-side-seal pouches, the front and back of a four-side-seal pouch may be made from different types of plastic film. In any of these pouch styles, gussets may be added to expand the capacity of the pouch without increasing its width or height. Pouches may be used alone, or may be combined with another package for product distribution and/or sale. One very common package structure is bag-in-box packages, which consist of a pouch inside a folding carton or corrugated box.

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Applications of Plastic Films in Packaging The pouch material may be plastic film alone, or a multilayer material containing paper and/or aluminium foil. Paper may be used to add strength, rigidity, printability or bulk to the flexible package. Foil may be incorporated to improve the barrier to permeants such as oxygen, water vapour, odours or flavours. In the past several years, stand-up pouches have increasingly been used as substitutes for cartons or bottles. Stand-up pouches are designed to stand upright on the retail shelf. Their design involves gussets and special shaping of the bottom panel.

9.3.3 Pouch Production There are two primary ways of using bags, sacks and pouches for packaging: as preformed pouches, or in form-fill-seal operations. In a form-fill-seal (FFS) operation, the web stock (usually preprinted, if applicable) is fed into either a horizontal or vertical FFS machine, in which it is formed into a pouch, the product added and the final seal formed. If preformed pouches are used, the packages are formed and an opening left for product introduction. The product is added to the package in a separate operation, and then the package is sealed. Form-fill-seal operations are usually economically advantageous for large-scale production. Buying of preformed pouches is generally more economical if production quantities are small, or in cases where the material is difficult to seal and poses quality control problems.

9.3.4 Dispensing and Reclosure Features One of the long-standing drawbacks of flexible packaging has been the difficulty of providing easy-to-use and effective dispensing and reclosure. In the past few years, several innovations have provided significant improvements in these package attributes. The most common way to dispense products from flexible packages is to cut or tear the package open, or to peel open one of the seams. For some products, such as breakfast cereal in bag-in-box packages, this is a significant source of consumer complaints. The seals often do not peel easily, and all too often the result is a bag with a split down the side, spilling cereal into the carton and making it nearly impossible to reclose the pouch to protect product freshness. Some flexible packages now incorporate zipper closures, often accompanied by a tear strip for initial opening. Other packages have resealable flaps, usually located along a seam. For liquid products, some packages incorporate a threaded spout with a standard threaded cap. This may be located on the top of the pouch, or on the bottom, depending on the

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Handbook of Plastic Films product and the package size. In bag-in-box packages, the outer carton may include a flap through which the spout can be extended for dispensing. For single-serve drinks, it is common to provide an attached straw (protected from dirt in its own pouch, which is glued to the side of the drink pouch), along with a designated spot on the package that has been modified for easy puncturing.

9.4 Heat-Sealing Heat-sealing is the usual method for producing seals and seams in flexible packaging. Occasionally, adhesive systems are used. There are a wide variety of types of heat-sealing systems, but the most common, especially for films, are thermal or bar sealing, and impulse sealing. Thermal or bar sealing uses two heated bars that exert pressure on the materials to be sealed and at the same time conduct heat to the interface, melting the materials. The pressure ensures good contact between the materials, and assists in interpenetration of the melted viscous materials at the interface. When sufficient time has elapsed to produce an initial seal, the materials are released. Therefore, the hot tack of the material is crucial in forming an adequate seal. The full strength of the seal forms as the material cools, but the initial strength must be sufficient to maintain the seal integrity while cooling proceeds. The sealing bars usually have rounded edges to avoid puncturing the material, and often one bar is fitted with a resilient surface to aid in achieving uniform pressure during sealing. Usually, the heat-seal jaws are serrated rather than flat, and produce a patterned seal. In variants of thermal sealing, only one bar is heated and the other is not. Especially for sealing lidding on containers, the bars may be shaped rather than rectangular, producing shaped seals. Another variant uses heated rollers rather than bars; the pouch is sealed as it travels through the rollers. Impulse sealing also uses two jaws to produce the seal, but heat is generated by flow of an impulse of electric current through a nichrome wire. The jaws do not remain hot, but cool down after each electrical impulse. The material being sealed is captured between the jaws, the current flows to produce heating, and the material remains between the jaws for a cool-down period before it is released. Cooling may be aided by circulation of cooling water through the jaws. With impulse sealing, materials do not require as good hot tack as with thermal sealing. The seal will increase in strength during the cooling phase, before it is released from the heat-sealer, so it is not as subject to immediate failure or distortion. On the other hand, the impulse seal is typically much narrower than the bar seal, and therefore is often not as strong. Impulse sealing is particularly advantageous for oriented materials, which have a tendency to wrinkle during sealing. As with bar sealing, the jaws can be shaped to produce shaped seals.

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Applications of Plastic Films in Packaging Hot wire or knife sealing uses a heated wire or knife to cut and seal films simultaneously. It is often used to produce thin polyethylene bags for applications such as produce packaging. The seals are very narrow, often nearly invisible, and relatively weak. In band sealing, often used for the final seal on filled preformed pouches, the materials to be sealed are moved through heated bands. Both heating and cooling phases can be provided. Other types of heat-sealing are used less frequently in production of flexible packaging.

9.5 Other Uses of Packaging Films Plastics films are sometimes used as components in rigid or semi-rigid packaging structures. They can serve as liners inside closures for bottles and jars, as lidding on trays or cups, or can be laminated on paperboard or other materials. While the plastic resins used as coatings are not produced as stand-alone films, they are deposited on or in packages as films. Two common packaging applications of plastic films outside the flexible packaging category are skin packaging and bubble-wrap. In skin packaging, a product is held tightly to a backing material by a plastic film. Usually, the backing material consists of a heat-seal coated paperboard. The product is placed on the board, and then the heated plastic film lowered on to it. A vacuum is drawn through the backing material, causing the film to form tightly around the product and seal to the board. Usually, the product is displayed in the retail environment by hanging the backing from a peg. Obviously, the product must be able to withstand momentary contact with the hot plastic without damage, and the plastic must not adhere to the product. The coated backing often requires perforations to permit adequate evacuation of trapped air. In some cases, films are used that permit sealing to uncoated board. Heavy-duty films sealed to corrugated board can be used to provide protection to products during distribution, by physically isolating the skin-packaged product from impacts to the outer container. Bubble-wrap is a cushioning material produced by forming bubbles of air, of a defined size, between two plastic films. The bubbles can be various sizes, depending on the enduse of the material. Generally, smaller bubbles are used to protect lighter-weight products, and larger bubbles are used for heavier products. Bubble-wrap does not provide suitable protection for products that are very heavy, however.

9.6 Major Packaging Films A variety of plastic resins are used to make packaging films. Sometimes they are used alone, and often they are used in combinations that provide the benefits of multiple materials. The most commonly used packaging resins will be described in this section.

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9.6.1 Low-Density Polyethylene (LDPE) and Linear Low-Density Polyethylene (LLDPE) Low-density and linear low-density polyethylene are the most commonly used packaging films. Low-density polyethylene is produced by a high-temperature, high-pressure process that results in considerable short- and long-chain branching of the molecules. Linear low-density polyethylene is produced at temperatures similar to those used for highdensity polyethylene, resulting in linear molecules. The reduction of density comes about through the use of comonomers that put side groups on the main chain that act like branching in decreasing crystallinity. In traditional Ziegler-Natta catalyst polymerisations, these comonomers are butene, hexene or octene. Some of the new family of polyethylenes using metallocene catalysts incorporate higher alpha-olefins into the polymer structure, producing longer side groups, which act much like the long-chain branching in highpressure LDPE. LDPE and LLDPE are soft, flexible materials, with a hazy appearance. At equal density and thickness, LLDPE has higher impact strength, tensile strength, puncture resistance and elongation than LDPE. LLDPE based on octene generally has the highest strength, followed by hexene- and butene-based polymers, in that order. The cost per unit mass of the materials generally also follows the order octene > hexene > butene. LDPE has better heat-seal properties than LLDPE. It seals at lower temperatures, seals over a wider temperature range, and has better hot tack, all of which result, to a great extent, from its long-chain branching. Metallocene LLDPE containing higher alpha-olefins was designed, in part, to remedy this disadvantage of LLDPE. Another approach that has commonly been taken to producing the best mix of properties for a given application is to blend LLDPE and LDPE. LDPE and LLDPE are good barriers to water vapour, but are poor barriers to oxygen, carbon dioxide and many odour and flavour compounds. They have good grease resistance, and are quite inert. Low-temperature performance is good, as these materials retain their flexibility at very low temperatures. They soften and melt at moderately elevated temperatures, so they are not suitable for applications involving significant exposure to heat. Some characteristic LDPE and LLDPE properties are presented in Table 9.1. LDPE is generally the cheapest plastic film, on a per-unit-mass basis. Since LLDPE often permits considerable down-gauging, it can be the lowest cost alternative on a per-use basis. Very low-density polyethylene (VLDPE) is LLDPE with a higher concentration of comonomer, which reduces crystallinity, and consequently density, below the traditional range for LLDPE, to 0.905-0.915 g/cm3. These materials are very soft films, with excellent cling but reduced strength.

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Applications of Plastic Films in Packaging

Table 9.1 Typical properties of polyethylene films [1, 2] Property

Polymer LDPE

LLDPE

HDPE

–120

–120

–120

105-115

122-124

128-138

Glass transition temperature (Tg; °C) Melting temperature (Tm; °C) Heat distortion temperature, at 455 kPa (°C) Density (g/cm3)

40-44 0.915-0.940

62-91 0.915-0.935

0.94-0.97

Tensile modulus (GPa)

0.2-0.5

0.6-1.1

Tensile strength (MPa)

8-31

20-45

17-45

Elongation (%)

100-965

350-850

10-1200

WVTR* at 37.8 °C and 90% RH (g μm/m2 d)

375-500

125

O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)

160-210

40-73

*WVTR: Water vapour transmission rate (d = day, 24 h) RH: relative humidity

9.6.2 High-Density Polyethylene (HDPE) High-density polyethylene is a linear addition polymer of ethylene, produced at temperatures and pressures similar to those used for LLDPE, and with only very slight branching. HDPE films are stiffer than LDPE films, though still flexible, and have poorer transparency. Their water vapour barrier is better, as is their gas barrier. However, permeability to oxygen and carbon dioxide is still much too high for HDPE to be suitable as a barrier for these permeants. As is the case for LDPE, HDPE is very inert, and has good oil and grease resistance. Using high molecular weight (high molar mass) resin, HMW-HDPE, which permits considerable down-gauging, can reduce the cost of HDPE films on a per-use basis. This material is higher in cost per unit mass, and is also somewhat more difficult to process than lower molecular weight materials, due to its high viscosity. Another alternative for reducing the cost of HDPE film is the use of recycled material, often originating in milk cartons. Because of the distinctly cloudy appearance of HDPE film, a small amount of white pigment is commonly added to provide an attractive opaque white film. Typical HDPE properties are shown in Table 9.1.

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9.6.3 Polypropylene (PP) Polypropylene is a linear addition polymer of propylene; resins used in packaging are predominantly isotactic. PP has the lowest density of the commodity plastics, 0.890.91 g/cm3. PP films are suitable for high-speed packaging applications that demand a relatively stiff material, since they are considerably stiffer than HDPE, and also have much improved clarity. Clarity can be further improved by using copolymer resins containing some ethylene units, to reduce crystallinity. Another approach to improving transparency is the use of nucleating agents to reduce average crystallite size. Barrier properties of PP are comparable to those of HDPE. Unoriented PP film tends to be somewhat brittle, especially at low temperatures. In many applications, biaxially oriented film (BOPP) is preferred. Orientation also increases the stiffness of the film. PP, especially BOPP, does not heat-seal well. Therefore, it is commonly coated or coextruded with sealants to make heat-sealable films. Typical PP properties are shown in Table 9.2.

Table 9.2 Typical properties of polypropylene (PP), biaxially oriented polypropylene (BOPP) and polyvinyl chloride (PVC) films [1-4] Property

Polymer PP

BOPP

PVC

Tg (°C)

–10

–10

75-105

Tm (°C)

160-175

160-175

212

Heat distortion temperature, at 455 kPa (°C)

107-121

Density (g/cm3)

0.89-0.91

0.89-0.91

1.35-1.41


1.1-1.5

1.7-2.4

to 4.1


31-43

120-240

10-55

Elongation (%)

500-650

30-150

14-450

WVTR, at 37.8 °C and 90% RH (g μm/m2 d)

100-300

100-125

750-15,700

50-94

37-58

3.7-240


244

57-82


9.6.4 Polyvinyl Chloride (PVC) Polyvinyl chloride films are formed by combining PVC resin, produced by addition polymerisation of vinyl chloride, with plasticisers and other additives to produce a flexible film. Unmodified PVC is quite brittle and difficult to process because of its heat sensitivity. However, because of its polar nature, PVC has a high affinity for plasticisers, and hence can be substantially modified. Plasticisers generally consist of high-boiling-point organic liquids, which serve a lubricating function in the resin. Some soft and flexible PVC films are approximately 50% plasticiser by weight. For food packaging uses, plasticisers and other ingredients must be suitable for direct food contact. The major plasticisers used in such applications are adipates. Often, epoxidised soybean oil is added as a secondary plasticiser. For non-food use, a wider range of plasticisers is available. Adipates and phthalates are most common. In addition to plasticisers, PVC films contain stabilisers, as the resin is heat-sensitive. Oil epoxides have some stabilising functionality, and in food packaging uses supplement the activity of calcium, magnesium or zinc stearates. Phosphites may also be used. In non-food applications, organometallic salts of barium and zinc are commonly used. The properties of PVC films are strongly influenced by the type and level of modifying ingredients, especially plasticisers, that have been added. In general, the films are quite soft and flexible, easy to heat-seal, and have excellent self-cling, toughness, resilience and clarity. Permeability is relatively high. Both oriented and unoriented films are available. Properties of PVC film are listed in Table 9.2. Heavier gauge PVC, sheet rather than film, is often used in thermoformed packaging, such as in blister packaging.

9.6.5 Polyethylene Terephthalate (PET) Polyethylene terephthalate is formed by condensation polymerisation of ethylene glycol and either terephthalic acid or dimethyl terephthalate. It is commonly used in biaxially oriented form, and has excellent transparency and mechanical properties. Heat-setting enables the film to be used for extended periods at temperatures ranging from –70 to +150 °C. It can tolerate considerably higher temperatures for short periods, such as in dual ovenable packaging for frozen foods. PET has good barrier properties, especially for odours and flavours. The barrier properties can be enhanced by coating with PVDC, or by metallising, as will be discussed in subsequent sections. Coating or coextrusion is often used to provide good heat-seal properties. Typical PET properties are listed in Table 9.3.

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Table 9.3 Typical properties of polyethylene terephthalate (PET) films [1, 2, 5] Property

PET polymer Unoriented

Oriented

Tg (°C)

73-80

73-80

Tm (°C)

245-265

245-265

Heat distortion temperature, at 455 kPa (°C) 3

Density (g/cm )

38-129 1.29-1.40

1.40


2.8-4.1


48-72

220-270

Elongation (%)

30-3,000

70-110

WVTR, at 37.8 °C and 90% RH (g μm/m2 d)

390-510

440


1.2-2.4

1.1

9.6.6 Polyvinylidene Chloride (PVDC) Polyvinylidene chloride is an addition polymer of vinylidene chloride. It is an excellent barrier to oxygen, water vapour, odours and flavours. However, its high crystallinity and sensitivity to heat-induced degradation make it extremely difficult to process. Therefore, homopolymer PVDC is not used commercially. Copolymerisation of vinylidene chloride with various amounts and types of comonomers, usually vinyl chloride, acrylonitrile, methacrylonitrile, methacrylates or alkyl acrylates, produces a family of PVDC copolymer resins with improved processability, while maintaining desired barrier properties. Vinylidene chloride content typically ranges from 72 to 94 wt%; molecular weights range from about 65,000 to 150,000 [6]. In general, the highest barrier resins are not melt-processable, but instead are applied by solvent or latex coating. Extrudable resins have undergone more modification, so consequently have somewhat decreased barrier properties. PVDC films produced for household use are plasticised copolymers, and have even poorer barrier performance. However, they remain much better barriers than competitive polyethylene films. Representative properties are shown in Table 9.4. PVDC copolymer films can be heat-sealed. Therefore, in PVDC copolymer coatings or coextrusions, the PVDC can serve as a combination barrier and heat-seal layer. However, the best barrier films generally do not provide the best heat-seal capability, and vice versa, so when both heat-sealability and barrier are desired, sometimes two differently formulated PVDC copolymer coatings are applied.

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Table 9.4 Typical properties of polyvinylidene chloride (PVDC) films [1, 2, 6] PVDC polymer Property

Generalpurpose

Highbarrier

Tg (°C)

–15 to +2

–15 to +2

Tm (°C)

160-172

160-172

1.60-1.71

1.73


0.3-0.7

0.9-1.1


48-100

83-148

Elongation (%)

40-100

50-100

79

20

0.31-0.43

0.031

Heat distortion temperature, at 455 kPa (°C) Density (g/cm3)

WVTR, at 37.8 °C and 90% RH (g μm/m2 d) O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)

Nearly all cellophane produced in North America is solvent-coated with PVDC copolymers. Solvent and latex coatings are also used on plastic sheet for thermoformed containers, and on blow-moulded plastic bottles. Common substrates include polyolefins, polyesters, polyamides and styrenics. Coextrusions of PVDC copolymer with polyethylene or polypropylene are used in shrinkable films for meat, cheese and other moisture- or oxygen-sensitive foods. Latex coatings of PVDC copolymers are used to provide moisture resistance, grease resistance and barrier to paper and paperboard packages.

9.6.7 Polychlorotrifluoroethylene (PCTFE) Polychlorotrifluoroethylene (PCTFE) is another polymer with good barrier characteristics, especially for water vapour. The homopolymer is very difficult to process because of its extremely high melt viscosity. A small amount of modification by copolymerisation yields AlliedSignal Corporation’s trademarked Aclar films, which contain greater than 95% chlorotrifluoroethylene by weight. These films are considered the best available transparent moisture barriers for flexible packaging; however, they are rather expensive. Aclar films can be used alone, or can be laminated to paper, polyethylene, aluminium foil or other substrates. The film is heat-sealable, and can be thermoformed. Aclar blister packages are often used for unit packages for highly moisture-sensitive pharmaceuticals.

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9.6.8 Polyvinyl Alcohol (PVOH) Polyvinyl alcohol films are unique in several respects. Polyvinyl alcohol polymers are produced by hydrolysis (more correctly, alcoholysis) of polyvinyl acetate. If fully hydrolysed, the polymer is readily soluble in water. Controlling the degree of hydrolysis can produce films that are soluble in hot water but not in cold water. Because PVOH degrades at temperatures well below melt, it cannot be processed by extrusion. Therefore, casting from a water solution is used to make film. As produced, the film is amorphous, but orientation induces some crystallinity. The water-solubility of PVOH is the major reason for its use in niche markets where this is a desired attribute. One application is as an inner pouch in packaging of agricultural or other chemicals, to limit human exposure. The pouch with its contents can be placed into the dilution and dispensing apparatus, without direct contact between the user and the chemical. In the water, the pouch dissolves, releasing the chemical. The dissolved polymer does not clog spray nozzles, and is biodegradable. Another application is in hospital laundry bags. Here, the hot-water-soluble variety is used. Soiled laundry is placed in the bags, and then bag and all can be placed into the washer, so that no contact between the launderer and the potentially infectious linen is required. Since the polymer does not dissolve in cold water, it will not be affected by residual liquid in the linens, but will dissolve readily in the hot wash water.

9.6.9 Ethylene-Vinyl Alcohol (EVOH) Ethylene-vinyl alcohol resins are produced by hydrolysis (alcoholysis) of ethylene-vinyl acetate random copolymer, analogous to the route for production of polyvinyl alcohol from polyvinyl acetate. Commercially available materials contain a substantial percentage of ethylene, typically 27 to 48 mol%. The presence of ethylene renders the resins melt-processable. The presence of –OH groups in the structure results in strong intermolecular hydrogen bonding. While EVOH is a random copolymer, CH2 and CHOH groups are isomorphous; they fit into the same crystalline structure. Therefore, the polymer crystallises readily. The combination of strong intermolecular forces and crystallinity makes it an excellent barrier to gases, odours and flavours. However, the hydrogen bonds also make it a moisture-sensitive material, and high humidity decreases its barrier capability. EVOH is most often used as an oxygen barrier. Since, in most applications, it is likely to be exposed to moisture either from the environment or in the product, it is usually used as a buried inner layer in a coextruded structure, where a good moisture barrier, often a

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Table 9.5 Typical properties of ethylene-vinyl alcohol (EVOH) films [1, 2, 7] EVOH polymer Property

32 mol% ethylene

44 mol% ethylene

Tg (°C)

69

55

Tm (°C)

181

164

Density (g/cm3)

1.19

1.14


2.6

2.1


77

59

Elongation (%)

230

380

WVTR, at 40 °C and 90% RH (g μm/m2 d)

1535

724

0.0078

0.030



polyolefin, protects it. Monolayer EVOH films, oriented or unoriented, are also available, which can be used alone but are usually combined with other materials by laminating, or coating. Typical EVOH properties are listed in Table 9.5.

9.6.10 Polyamide (Nylon) Polyamides, or Nylons, are a family of plastics containing characteristic amide functionality. They are commonly formed by condensation polymerisation of amino acids, or of carboxylic acids and amines. Nylon films are used for specialty applications in packaging, where performance requirements justify their relatively high cost. Nylons have excellent high-temperature performance, so can be used, for example, in boil-in-bag packages. Nylons also provide excellent odour and flavour barrier, and reasonably good oxygen barrier. They are very poor water vapour barriers, and generally have a tendency to lose some barrier performance when exposed to large amounts of moisture. However, their performance is not as water-sensitive as EVOH. Most Nylons used in packaging have some crystallinity; the amount is heavily dependent on processing conditions, since Nylons have a narrow window for crystallisation. Films generally retain good flexibility at low temperatures, and have excellent strength properties. Owing to their relatively high cost, they are often coextruded with other plastics.

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Table 9.6 Typical properties of polyamide (Nylon) films [1, 2] Polymer Property

Nylon-6

Nylon-11

Nylon MXD6

Tg (°C)

60

64

Tm (°C)

210-220

180-190

243

Density (g/cm3)

1.13-1.16

1.03-1.05

1.20-1.25


0.69-1.7

1.3

3.8-4.1


41-165

55-65

220-230

300

300-400

72-76

3,900-4,300

1,000-2,000

630

0.47-1.02

12.5

0.06-0.26


Elongation (%) WVTR, at 40 °C and 90% RH (g μm/m2 d) O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)

Nylon MXD-6: Mitsubishi Gas Chemicals America, Inc., New York, NY, USA

Polyamides manufactured from straight-chain amines and carboxylic acids are typically named with numbers representing the number of carbons in each of the starting monomers. For example, Nylon-6,10 is made from a six-carbon amine and a ten-carbon carboxylic acid. Similarly, polyamides made from amino acids have a number designating the number of carbons in the acid. When the carbons are not in a straight chain, more complex names are necessary. Typical properties of some Nylon films are given in Table 9.6. Nylon-6 tends to be the most-used Nylon packaging film in the USA, and Nylon-11 in Europe.

9.6.11 Ethylene-Vinyl Acetate (EVA) and Acid Copolymer Films Ethylene-vinyl acetate is produced by addition copolymerisation of ethylene and vinyl acetate. The acetate groups provide polar functionality that increases intermolecular forces in the film, and, because of the structural irregularity thus introduced, interfere with crystallisation. These films have excellent transparency, and provide very good heatseal and adhesive properties, with excellent toughness at low temperatures. Typical filmgrade EVA resins contain between 5 and 18% vinyl acetate. Resins designed for use as an adhesive layer in a multilayer structure are typically at the higher end, and standalone films at the lower end, of this concentration range.

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Applications of Plastic Films in Packaging Common markets for EVA are poultry and meat wrap, stretch film and ice bags. The films tend to be sticky, so may require the use of slip and antiblock additives. Copolymers of ethylene with acrylic acid and with methacrylic acid are also available, and are commonly called acid copolymer resins. They are characterised by good clarity, strong adhesion to polar substances such as paper, and also to foil, and low melt and heat-seal temperatures.

9.6.12 Ionomers Ionomers are formed by neutralisation of ethylene-acrylic acid or ethylene-methacrylic acid copolymers containing 7 to 30 wt% acid, to yield sodium or zinc salts. The resulting ionic bonds function as reversible crosslinks in the polymer, readily disrupted by heat, but reforming on cooling. Therefore, these materials provide very strong bonding to numerous substrates. Ionomers can be used for skin packaging to uncoated corrugated board, for example. The heat-seal performance of ionomers is outstanding, even permitting sealing through grease contamination, which makes them ideal for packaging of processed meat. They have superior hot tack, and excellent melt strength. Ionomer films have excellent clarity, flexibility, strength and toughness. They can be used to package sharp objects, which break through many alternative materials when subject to vibration during distribution. Ionomers have relatively poor gas barrier, and tend to absorb water readily. They also are relatively high cost compared to films such as ethylene-vinyl acetate.

9.6.13 Other Plastics Several other types of plastics are used in packaging films to some extent. Polycarbonate films have excellent transparency, toughness and heat resistance, but high cost. They have some use in skin packaging, food packaging where exposure to high temperatures for in-bag preparation is required, and medical packaging. Polystyrene is another film with excellent transparency, often used in window envelopes and window cartons. It has low gas barrier, so can be used for produce where a ‘breathable’ film is required. In heavier gauges, polystyrene is widely used for transparent thermoformed trays. Expanded polystyrene is used for trays, egg cartons and other applications where its cushioning properties are desired. In general, these materials are

251

Handbook of Plastic Films classified as sheet, rather than film. Polystyrene film is generally biaxially oriented to improve its properties, since the unmodified material is too brittle for most applications. Impact-modified polystyrene sheet that incorporates polybutadiene is often used where transparency can be sacrificed for impact resistance. Cellulose-based plastics such as cellulose acetate, cellulose butyrate, cellulose propionate and copolymers are also used to a relatively small extent, most often as sheet rather than film. Their high price and water sensitivity limits their usefulness. A wide variety of copolymers are available. Some of these have been discussed already. It is quite common to modify the chemical structure of a polymer to obtain a more desirable mix of properties. Another way to combine properties is to use blends of polymers. High-impact polystyrene (HIPS) is actually partially a copolymer and partially a blend of polybutadiene and polystyrene.

9.7 Multilayer Plastic Films In many cases, the best combination of packaging attributes at the lowest cost is achieved by using a combination of materials. Therefore, plastic packaging films are often combined with one another or with other materials such as paper, aluminium or even glass, through processes such as coating, lamination, coextrusion and metallisation.

9.7.1 Coating Coating is commonly used to add a thin layer of a plastic on the surface of another plastic film or sheet, or, more commonly, on a non-plastic substrate such as paper, cellophane or foil. The coating may be applied as a solution, a suspension, or a melt. Common reasons for using coating in flexible packaging are: to impart heat-sealability for paper, cellophane, foil or plastics that are not themselves easily heat-sealed; to provide moisture protection for paper or cellophane; to improve barrier properties; and to provide protection from direct contact of the base material with the product. Coating with low-density polyethylene is often used on paper to give heat-sealability and moisture protection, as well as to protect printing from abrasion. It is often used on aluminium foil to provide heat-sealability and abrasion resistance, and to prevent interaction between the foil and the product. PVDC copolymer coatings are often used to improve barrier and heat-sealability.

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9.7.2 Lamination Lamination is the process of combining two webs of film together. In flexible packaging applications, lamination is often used to combine a plastic film with paper or foil, or to join paper and foil together. A variety of lamination methods are used. When plastic films are involved, either as a substrate or as an element in the finished structure, the laminating adhesive is often low-density polyethylene, applied by extrusion, and the process is known as extrusion laminating. When paper is contained in a flexible package, it is most often being used for its excellent printability, along with its ability to impart substance and strength. When aluminium is used, it is most often employed for its excellent barrier to light and to permeation. Occasionally, it is used primarily for its desirable appearance. Another significant use of lamination is to produce a web with buried printing. In these materials, one web is reverse-printed, and is then laminated to a second web, either made from the same or a different polymer. The printing can be seen through the transparent plastic, and is protected against abrasion so it maintains a fresh attractive appearance much better than surface-printed materials.

9.7.3 Coextrusion Coextrusion results in the production of a multilayer web without requiring initial production of individual webs and a separate combining step. The melted polymers are fed together carefully to produce a layered melt, which is then processed in conventional ways to produce a plastic film or sheet. When only plastics are being used in a flexible packaging structure, coextrusion is generally preferred to lamination, unless buried printing is involved. Obviously, coextrusion cannot be used to incorporate nonthermoplastic materials. A major advantage of coextrusion over lamination is its ability to incorporate very thin layers of a material, much thinner than those which can be produced as a single web. This is particularly important for expensive substrates, such as those often used to impart barrier properties. The amount of the expensive barrier resin used need only be enough to provide the desired performance. The thinness of the layer is not limited by the need to produce an unsupported film and handle it in a subsequent lamination step.

9.7.4 Metallisation Metallisation is a way of applying a thin metal layer on a plastic film (or on paper), as an alternative to using a lamination with aluminium foil. In commercial packaging practice, the metal being deposited is almost always aluminium. The process, known as vacuum

253

Handbook of Plastic Films metallising, involves evaporation of aluminium inside a vacuum chamber, and deposition of the aluminium vapour on a plastic film. The operation is usually done in a batch mode, with the substrate being metallised and an aluminium wire placed inside the vacuum chamber. The film is rolled past a chill roll, which removes the heat from the condensing aluminium, preventing melting of the film. Very high vacuums are needed, which has retarded the development of continuous metallisation processes, although some are available. Vaporisation of the aluminium is most often achieved by resistance heating. Induction and electron-beam heating are used to a lesser extent. Metallised films have significantly enhanced barrier characteristics, and are usually chosen for this reason. Cost of metallised film is generally less than that of foil-containing laminated materials. In snack packaging, for example, metallised film has almost totally replaced foil laminations. The barrier performance of metallised film, as initially produced, is somewhat inferior to foil, and is dependent on the thickness of the deposited metal layer. However, stress during product distribution can lead to the development of flex cracks in foil, which then provide a route for gas transfer. Metallised foil, since it retains the flexibility and other mechanical characteristics of the film substrate, is not usually subject to flex cracking. Therefore, the barrier characteristics of metallised foil are sometimes superior to those of foil laminations, at later points in distribution. Also, many oxygen-sensitive products require better barrier than can be attained with plastic alone, but can be successfully protected with metallised film. In addition to gas barrier, metallised film provides an essentially total light barrier. Occasionally, metallised foil is used for its appearance, rather than for its barrier characteristics. This is particularly the case when it is used for labels. In many such applications, however, paper, rather than film, is the metallisation substrate.

9.7.5 Silicon Oxide Coating One of the disadvantages of metallised film is that the resultant material is opaque, and is not suitable for use in microwave ovens. The desire for transparent high-barrier coatings led to the development of glass-like coatings based on silicon oxide, SiO2. Silicon oxide coatings on film are usually applied in a manner analogous to vacuum metallising. The silicon oxide is evaporated, using electron-beam heating, and condensed on the film substrate in a vacuum chamber. The film is very thin, 400 to 1000 Å, and does not affect the mechanical properties of the material to any significant degree. The chemical composition of the deposited film depends somewhat on conditions, and is

254

Applications of Plastic Films in Packaging characterised as SiOx, where the value of x is between 1.0 and 2.0. At values close to 1, the layer imparts a distinct yellowish colour to the film. At values close to 2, it is nearly colourless. The yellowing is a significant concern in some applications. The SiOx layer greatly increases barrier of the film, and it is transparent to microwave radiation. Therefore, it can be used in packages that will be heated in microwave ovens. The most common substrate is PET, in thickness of 12.5-25 μm, although polypropylene, polystyrene and polyamides can also be used. An alternative to evaporative deposition is chemical plasma deposition, in which a siliconcontaining gas such as tetramethyldisiloxane or hexamethyldisiloxane is used as the silica source. Little heat is required, and the degree of vacuum needed is lower. Therefore, plasma deposition can be used on heat-sensitive materials such as LDPE and oriented PP. The coating produced is thinner, and less yellow. Plasma deposition is the method of choice for SiOx coating of containers, and can also be used for film.

9.7.6 Other Inorganic Barrier Coatings Processes have also been developed that deposit aluminium oxide coatings on plastic films, to increase barrier properties. Combinations of SiO and MgO have also been used. Another type of inorganic barrier coating uses clay nanocomposites, which are deposited on the film from a solution of PVOH/EVOH copolymer, in a mix of water and isopropyl alcohol, with nanodispersed 7 nm diameter silica and titanium dioxide particles. Microgravure equipment is used to coat the solution on to the film substrate. Barrier is reportedly comparable to that of films metallised with aluminium, but the coatings are transparent. These materials are all still in relatively early phases of development.

9.8 Surface Treatment In many packaging applications, it is necessary for something to stick to a plastic film. This may involve placing a label on a pouch or on a stretch-wrapped pallet, adhering two films together in a lamination, or, as is often the case, printing the film. Adequate adhesion requires that secondary bonding forces between the film and the object, such as the ink, which is to be adhered, be sufficient to retain the material. Historically, this has been a significant problem for plastic films, since the surface energy of the films is often low, causing poor adhesion. Several techniques are commonly used to increase the surface energy of polymers, hence improving adhesion. For films, the most common treatment is corona discharge.

255

Handbook of Plastic Films In corona discharge treatment, the film surface is exposed to a discharge between grounded and powered electrodes, at high voltage. The discharge of the electric current ionises the air in the gap between the electrodes. The ions produced initiate free-radical reactions with the film surface, causing bond cleavage followed by oxidation. Oxidation of the surface increases its ability to adhere to substances such as inks and adhesives. The effectiveness of corona discharge treatment dissipates with time, so ideally it should be applied within a short time of the subsequent printing. In a roll of corona dischargetreated material that has been stored, the effectiveness of the treatment is likely to be significantly higher on the inside layers than on the first few outside layers of film. Other surface treatments that are sometimes applied to film also exist. However, corona discharge is by far the most frequently encountered.

9.9 Static Discharge Plastics, because they are nonconductive, are subject to build-up of electrostatic charges. When such charges build up, the result can range from the attraction of dust and lint to material handling problems, shocks and sparks. Methods for controlling the build-up of static charges include charge neutralisation through ionisation of the surrounding air and incorporation of conductive materials to dissipate the charge. Antistatic agents can be incorporated into the film as additives, or can be used as a surface treatment. The agents commonly used include non-ionic ethoxylated alkylamines, anionic aliphatic sulfonates and phosphates, and cationic quaternary ammonium compounds. In some cases, humidifying the area can control static, so that a thin layer of water is absorbed on the film surface, which conducts the charge to ground. Control of static discharge is especially important for packaging for sensitive electronic devices. Film designed for such applications, usually polyethylene, is generally pigmented pink to denote that it contains antistatic agents.

9.10 Printing In many packaging applications, plastic films are printed to convey information to the user. When printing is desired, it is usually done on roll stock before packages, such as pouches, are formed. Printing on formed flexible packages is usually limited to date or lot coding. Flexography is the printing method used most often for flexible packaging materials. In this process, a subcategory of relief printing, the printing plates are flexible elastomers,

256

Applications of Plastic Films in Packaging with the images, or printing areas, raised above the nonprinting surrounding areas. Thin, highly fluid, rapid-drying inks are used. The ink is transferred by a system of rollers to the top surface of the printing plates, which in turn transfer the ink to the film. In lithography, the printing image and the background are on the same plane of the thin metal printing plate. The plate is treated to attract water and repel ink in the non-image areas, and the reverse in the image areas. A system of rollers is used to transfer both ink and water to the plate. The image on the plate is then transferred (offset) to a rubberblanket-covered cylinder, and then to the film. Rotogravure uses copper-plated printing cylinders, which have the image engraved into the cylinder in the form of tiny cells. The cylinder rotates in an ink bath, filling the cells with ink. Excess ink is wiped off by a doctor blade, and then the image is transferred to the film as it is pressed against the printing cylinder by an elastomer-covered impression cylinder. For printing date and lot codes on formed packages, ink-jet printing is commonly used. In this process, electrically charged drops of ink are sprayed out of jets, and electrostatically directed to the desired printing location. This is an impactless form of printing, and is ideal for printing rapidly changing information such as these codes. Other types of printing, such as screen printing, as well as variations of the basic processes described above, are used less frequently for plastic packaging films.

9.11 Barriers and Permeation As has been discussed, in many packaging applications, protection of the product from gain or loss of gases or vapours is important. The mechanism by which substances travel through an intact plastic film is known as permeation. It involves dissolution of the penetrating substance, the permeant, in the plastic, followed by diffusion of the permeant through the film, and finally by evaporation of the permeant on the other side of the film, all driven by a partial pressure differential for the permeant between the two sides of the film. The barrier performance of the film is generally expressed in terms of its permeability coefficient. For one-dimensional steady-state mass transfer, the permeability coefficient is related to the quantity of permeant, which is transferred through the film as represented by the equation: P=

Ql AtΔp

(9.1)

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Handbook of Plastic Films where P is the permeability coefficient, Q is the mass of permeant passing through the material, l is the thickness of the plastic film, A is the surface area available for mass transfer, t is time, and Δp is the change in permeant partial pressure across the film. It can be shown that the permeability coefficient, as defined by equation (9.1), is equal to the product of the Fick’s law diffusion coefficient, D, and the Henry’s law solubility coefficient, S, in situations where these laws adequately represent mass transfer (ideally dilute solutions, diffusion independent of concentration): P = DS

(9.2)

The permeability coefficient, under these circumstances, is a function of temperature, but is not a function of film thickness or permeant concentration. While this is a very simplified approach to mass transfer, it is adequate for many packaging situations. For example, with oxygen-sensitive products, reaction with oxygen is commonly rapid compared to the rate of transfer, so the oxygen concentration within the package is relatively constant at nearly zero. Oxygen concentration in the surrounding air, measured as partial pressure, is constant at approximately 21 kPa. Regardless of the shape of the flexible package, mass transfer is essentially one-dimensional, through the thickness of the film. If temperature is constant and P is known, the amount of oxygen transported through the film in a given period can be easily calculated using equation (9.1). Conversely, if the sensitivity of the product is known in terms of the maximum amount of oxygen that can be taken up without resulting in unacceptable product quality, the time required for that amount of transfer (the product shelf-life) can be calculated. A similar approach can often be taken for transfer of odour or flavour compounds. While the diffusivity, and hence the permeability coefficient, of such organic substances is likely to be concentration-dependent, at the low levels associated with most packaging situations, the dependence is slight. Calculating shelf-life when water vapour transmission is involved is more problematic. In such cases, the partial pressure difference for water vapour between the inside and the outside of the package is almost never constant. Simplifying assumptions generally used consider the time for moisture in the product itself and in the product headspace to reach equilibrium to be small compared to the time required for permeation, and ignore moisture change in the headspace itself, calculating only moisture gain or loss in the product. The resulting differential equation is: dQ 1 = PA( p2 − p1 ) dt l

258

(9.3)

Applications of Plastic Films in Packaging where p1 is the partial pressure of water vapour outside the package, p2 is the partial pressure of water vapour inside the package, and p2 is a function of Q. Solution of this equation requires knowledge of the moisture sorption isotherm for the product, which relates the moisture content of the product to the equilibrium relative humidity of the air in contact with the product, and thus to p2. In the case where the sorption isotherm at the storage temperature can be approximated as linear over the range of moisture contents of interest, it can be written as: W = a + bM

(9.4)

where W is the water activity of the air in equilibrium with the product with moisture content M (dry weight basis), and a and b are the best-fitting straight-line constants. Rewriting the basic permeability equation [equation (9.3)] in terms of water activity, and substituting: Q = (M – Mi)w

(9.5)

where Mi is the initial moisture content and w is the dry weight of the product gives: w

dM PAps = (W2 − W1 ) dt l

(9.6)

where W1 and W2 are the water activities at times 1 and 2, and ps is the saturation water vapour pressure at the storage temperature. This equation can be integrated, giving the following relationship for moisture gain or loss:

[ [

] ]

1 ⎛ W2 − W1 t ⎞ PApst - ln⎜ ⎟= b ⎝ W2 − W1 ⎠ lw 0

(9.7)

Mass transfer characteristics for plastics are often expressed in terms of water vapour transmission rates (WVTR), rather than permeability coefficients. WVTR reflect the rate of water vapour transfer under specific conditions, and must be translated to permeability coefficients for application at different conditions. The relationship between Pwater and WVTR is the following: Pwater =

WVTR Δp

(9.8)

259

Handbook of Plastic Films where Δp is the difference in water vapour partial pressure under the conditions at which the WVTR was measured. In many cases, this was the ASTM standard condition of 32.2 °C (90 °F) and 90% relative humidity (RH). When permeability coefficients are not available at the temperature of interest, an Arrhenius relationship can be used to determine the required value, from the permeability coefficient at a nearby temperature and the activation energy. The equation used is the following: 1 ⎤ ⎪⎫ ⎪⎧⎛ E ⎞ ⎡ 1 P2 = P1 exp⎨⎜ a ⎟ ⎢ − ⎥ ⎬ ⎪⎩⎝ R ⎠ ⎣ T1 T2 ⎦ ⎪⎭

(9.9)

where T1 is the temperature at which P1 is known, T2 is the temperature at which P2 is to be calculated, Ea is the activation energy, and R is the gas constant. Care must be taken in applying equation (9.9). The permeability coefficient, as indicated, is a product of the diffusion coefficient and the Henry’s law solubility constant. Since these vary in different ways with temperature, equation (9.9) is valid only over reasonably small temperature ranges. A particular concern is that permeation rates are much higher above the Tg than below this temperature, and the rate of change with temperature differs. Therefore, equation (9.9) should never be used to calculate the permeability coefficient across a temperature range that spans Tg of the plastic. Permeability coefficients for multilayer plastic film or sheet, either coextrusions or laminations, can be calculated from the thickness and permeability coefficients of the individual layers, as follows: Pt =

lt

∑ (l / P ) i=n

i=1 i

i

(9.10)

where the subscript t indicates the value for the total structure, i indicates the value for an individual layer, and there are n layers in the structure. Special care must be taken when the barrier characteristics of a polymer are affected by the presence of the permeant or of some other substance that may also be permeating. This situation is most often encountered with water-sensitive plastics, such as ethylenevinyl alcohol, since co-permeation of water vapour and other components of interest, such as oxygen, may well occur during processing and storage. It may also arise in other situations, such as co-permeation of organics involved in odour and taste.

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9.12 Environmental Issues In recent years, consideration of the environmental effects of packaging decisions has become more common. While thorough discussion of such issues is beyond the scope of this chapter, some general observations and conclusions will be made. It is generally agreed that evaluation of the environmental impacts of a product or package requires consideration of the total life-cycle of the object. Such ‘cradle to grave’ analysis is commonly referred to as life-cycle assessment. Usually, when such analyses are carried out, the most influential life-cycle stage is that of production of the raw materials and packages, rather than transportation or disposal. Packages that minimise material use are therefore likely to have reduced environmental impact. Since flexible packaging systems usually (although not always, since distribution packaging must be included) use less overall packaging material, they often have reduced environmental impact, compared to the rigid packaging systems they replace. In examining the impacts of waste disposal, two general conclusions can be drawn. In most cases, flexible packaging is less likely to be recovered for recycling than rigid or semi-rigid packaging. Therefore, a higher proportion of flexible packaging is likely to require disposal. On the other hand, flexible packaging, as discussed above, usually means less total material requires handling. Unless recycling rates for the alternatives to the flexible packages are very high, use of flexible packaging is likely to mean less material requiring disposal. Also, flexible packages containing plastics are sources of recoverable energy in appropriate systems.

References 1.

R.J. Hernandez, S.E.M. Selke and J.D. Culter, Plastics Packaging: Properties, Processing, Applications, and Regulations, Hanser, Munich, Germany, 2000.

2.

S.E.M. Selke, Understanding Plastics Packaging Technology, Hanser, Munich, Germany, 1997.

3.

D. Kong in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S Marsh, Wiley, New York, NY, USA, 1997, 407.

4.

E. Mount and J. Wagner in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 415.

5.

J. Newton in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 408.

261


P. DeLassus, W. Brown and B. Howell in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 958.

7.

R. Foster Newton in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 355.

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10

Applications of Plastic Films in Agriculture E.M. Abdel-Bary, A.A. Yehia and A.A. Mansour

10.1 Introduction The quantity of plastic materials used annually in the world in the agricultural sector amounts to 2 million tons. About 50% of this is used in protected cultivation greenhouses, as mulching, for low tunnels, as temporary coverings of structures for fruit trees, etc. [1]. Thin plastic film produced with low investment is economically and technically feasible, and provides the best cost/benefit ratio for use in greenhouses and low tunnels. The area covered by both greenhouses and tunnels has been experiencing continual growth. This growth is expected to appear in many countries where protected cultivation replaces the traditionally used more expensive glass-clad greenhouses. Low-density polyethylene (LDPE), ethylene-vinyl acetate (EVA) and linear low-density polyethylene (LLDPE) films are the most common greenhouse covering films in agriculture. This chapter looks at the production of polyethylene-based plastic films for protected cultivation. The mechanical properties that make these films suitable for the use in agriculture are discussed. The stability of these plastic films under the effects of different environmental conditions is reported. These include solar irradiation, temperature, humidity, wind, fog formation and pesticides. Types of ultraviolet (UV) stabilisers and a determination of their compatibility are given. Also, the recycling of plastic films used in agriculture is of great importance, and a case study of their recycling as agricultural films is given.

10.2 Production of Plastic Films LDPE films dominate the market for protected cultivation in the countries of both the Mediterranean region and worldwide. Most of these contain special additives, which are used either to enhance the performance of the film in the special conditions met in a greenhouse, or to prolong its lifetime by minimising the effects of the environment on the structure of the film. Advances in the formulation of the LDPE films in use today have led to an expected lifetime of between one and five cultivating seasons [2]. The expected lifetime is, in fact, significantly affected by the environmental conditions that the film will face during its use. The climate of the region, the greenhouse design, the microclimate developing inside

263

Handbook of Plastic Films the greenhouse, the use of agrochemicals and the environmental pollution of the area can all severely affect the lifetime of the material by inducing ageing of the plastic film to various degrees. Thus, a film whose lifetime is estimated to be four seasons in NorthCentral Europe will only last two or three seasons in the Mediterranean [2]. The varying requirements for greenhouse systems between different regions because of different climatic conditions and differences in production methods has led, until very recently, to a significant variation in approaches, standards and practices adopted or implemented by the interested National Research Institutions, Commercial Agencies and relevant industries [3]. Some of the consequences of this differentiation and variability are reflected in the lack of standardisation concerning the testing methods for covering materials of greenhouses. Usually, the testing methods used for plastics in general are also applied to greenhouse covers, despite important functional differences. As a consequence, quality control data provided by the producers of the covering materials of greenhouses are usually limited to only a few properties of these materials. In most cases, it is not possible to reproduce the relevant technical information provided, as this is not obtained systematically and is available in a somewhat confusing way [3]. The manufacturing of plastic films used in agriculture is usually carried out by blown film extrusion (tubular extrusion). Readers are asked to consult Chapters 1 and 2, where the manufacturing process is given in detail.

10.3 Characteristics of Plastic Films Used in Agriculture The film products of interest here have been evaluated for their applied efficiency on the basis of their characteristics and requirements related to mechanical resistance, total percentage light transmittance (T %) to visible solar and long-wavelength ultraviolet (UV-A) radiation, useful lifetime and energy-saving potential (greenhouse effect). Visible solar radiation regulates the nutrition of plants through the ‘chlorophyll function’. Longwavelength ultraviolet radiation favours the formation of pigments and vitamins, which is advantageous for the quality characteristics of the crop involved, with regard to flavour, intensity of colour, perfume or smell and good keeping of fruit or vegetables. With reference to energy saving, values of the total thermal transmittance measured in W/m2 °C for the different covering materials and their possible combinations have been estimated. The total thermal transmittance of a covering material indicates the general heat loss (management, convection, radiation), estimated in watts, through a 1 m2 surface referred to a difference of 1 °C between the internal and external environmental temperatures of the prepared covering. These values enable estimation of the theoretical thermal yield of a manufactured film related to the heating needs for a thermal difference of 1 °C across the film.

Applications of Plastic Films in Agriculture The incident heat calculation enables the agricultural operator to choose the proper covering material with respect to any thermal (°C) and luminous (flux) needs of the species to be established in the agricultural crop rotation desired. This can be done by using the results showing the total thermal transmittance (W/m2 °C) and total light transmittance (T %) of some materials for greenhouse covering [4].

10.4 Stability of Greenhouse Films to Solar Irradiation The performance and lifetime of the plastic films used as covering materials in protected cultivation depend strongly on: (a) the original chemical structure of the materials, (b) the change in the properties of the material brought about by induced ageing, (c) the type of physical structure used, (d) the climatic conditions of the area where the structure is installed, and (e) the use of agrochemicals, among other things. A brief description of the factors affecting the stability of polyethylene (PE) as a greenhouse covering under the effect of different environmental conditions is given below. It is well known that photodegradation of many plastic materials occurs on subjecting these materials to solar radiation with wavelengths of 290-1400 nm [5, 6], the most energetic part of the solar spectrum. UV radiation in the range 290-400 nm can be absorbed by the plastic, and this is followed by bond cleavage and depolymerisation, causing photodegradation. The photodegradation process of the covering materials of a greenhouse is further complicated by various interacting factors. The effect of UV radiation combined with varying temperature, humidity, critical mechanical loads, friction, abrasion, exposure to agrochemicals, etc., accelerate ageing at various rates. Accordingly, it is difficult to predict the lifetime of plastic films by laboratory testing of the photostability of films. For instance, high abrasion of the film by sand or soil particles carried by the wind leads to the formation of high concentrations of active centres giving rise to an increase in photodegradation.

10.4.1 Ultraviolet Stabilisers Theoretically, LDPE should be stable under the effect of UV due to its stable structure and the absence of chromophores. However, during processing, it suffers partial oxidation, in which carbonyl and hydroxyl groups are formed. Also, it contains some impurities (photo-absorbing chromophores). Both impart photosensitivity to LDPE films [7, 8]. Special measures are therefore needed in order to protect greenhouse films against solar radiation and especially its most energetic and therefore harmful portion, namely, UV

265

Handbook of Plastic Films radiation. Inhibition or at least retardation of the reaction responsible for degradation is, of course, a necessity for successful UV stabilisation. Retardation or protection against photodegradation can take place by using additives. These additives may retard the photodegradation of the polymer in three ways, namely, ultraviolet screening, ultraviolet absorption and excited-state quenching. Thus, stabilisers are often included in the polymer to provide stability against photooxidation to protect the material from UV light damage. The effectiveness of a light stabiliser depends on many factors, including its solubility and concentration in the polymer matrix [9].

10.4.1.1 Ultraviolet Screening Ultraviolet screening compounds are based on inorganic or organic additives. In this type of protection, the ultraviolet light is blocked before it can reach the polymer. Screening is provided by pigments or by reflective coatings. Carbon black is also very effective and is used to stabilise many outdoor grades of polymers. In this case of UV screeners, any damage is confined to surface regions because UV penetration is restricted to very short distances. However, many of the pigments, like chalk, talc, short glass fibres and carbon black, impart an unattractive appearance, the grey, brown and black colours generally being unappealing. TiO2 is another common additive, which may act as a screener, but it may occur in different forms, some of which are chemically active and can promote photodegradation. The first class of organic additives for improving the resistance to UV radiation is the UV absorbers. They act by absorbing the harmful UV radiation above 290 nm, and thus do not allow it to reach the chromophores present in the chemical structure of LDPE as a result of processing or as impurities. Many organic compounds absorb light in the desired region but few act as stabilisers. Some have little or no effect when added to polymers and may actually increase the rate of degradation. For a UV absorber to be effective, it must be able to dispose of its excitation energy without interacting with the polymer in harmful ways and without undergoing any photochemical reaction that would destroy its effectiveness. Accordingly, a stabiliser must have a structure that provides a rapid cascade back to the ground state through thermally excited levels with a quantitative efficiency for return to the ground state not less than 0.999%, i.e., less than one molecule can be destroyed for every 100,000 molecules that are excited. Derivatives of o-hydroxybenzophenone or benzotriazole are examples of UV absorbers. However, this class of stabilisers seems to perform better in thicker materials and not well in the thin LDPE greenhouse films [8].

Applications of Plastic Films in Agriculture

10.4.1.2 Excited-State Quenchers The second class of UV stabilisers is the nickel excited-state quenchers. These quenchers act by deactivating the excited states of the chromophoric groups responsible for the photo-initiation by energy transfer, instead of relying on direct absorption of the UV radiation [8]. With proper selection of the Ni quenchers, the results of the UV stabilisation are satisfactory. A typical example of a nickel excited-state quencher is nickel dibutyldithiocarbamate. However, formulations containing such Ni quenchers are prohibited because of the environmental impact of nickel compounds.

10.4.1.3 Hindered-Amine Light Stabilisers (HALS) HALS, based on bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, are the most recent and innovative class of light stabilisers. HALS do not absorb any light above 250 nm and so cannot be regarded as UV absorbers or as excited-state quenchers. Through oxidation of the piperidyl group to the nitrosyl group, a radical becomes available that starts a very efficient cycle of radical scavenging and peroxide decomposition. Thus, HALS are converted to the corresponding nitroxyl radicals, which are the real species responsible for polymer stabilisation. Hindered nitroxyl radicals are effective chain breaking antioxidants that act by trapping alkyl radicals to give hydroxylamines and/or alkylhydroxylamines – the former regenerates nitroxyl. The overall high efficiency of HALS as UV stabilisers in polyolefins is attributed to the regeneration of the nitroxyl radical. The complementary nature of the chain breaking antioxidant mechanisms involved [10] are shown in Scheme 10.1. From these reactions, nitroxyl and alkoxyl radicals are formed according to equations (10.1) and (10.3)-(10.5). These radicals act as scavengers for any radicals formed during UV irradiation [equation (10.2)]. This means that HALS operate as excellent antioxidants. Some of the HALS contain further antioxidant groups; others are polymeric and less extractable. The main difference between UV absorbers and HALS is that the former absorb UV radiation and in turn are destroyed by it, while the latter do not absorb UV radiation and are much more slowly altered by secondary side reactions. Thus HALS act as radical traps for radicals produced from photochemical oxidation [8, 9]. They offer an excellent approach to ultraviolet stabilisation and have replaced nickel quenchers and ultraviolet absorbers in many applications. Highly efficient chemically resistant light stabiliser systems have been developed. Market demands for extended-life greenhouses and thinner mulch films require even more powerful stabilisers. New noninteracting chemistries based on alkoxylamine HALS will offer a new generation of stabilisers for agricultural polyethylene films.

267


hν, [O]

N

H

N

O• + P•

N

O

N

ΔH

O•

N

P + POO•

NH + POOH

N• + POO•

(10.1)

O

P

N

(10.2)

O• + POOP

(10.3)

N• + H2O + PO•

(10.4)

NO• + PO•

(10.5)

Scheme 10.1

10.4.2 Requirements for Stabiliser Efficiency The effectiveness of long-time stabilisation depends not only on the chemical nature but also on the rate of additive loss, which in turn depends on the compatibility of the additive with the polymer and is controlled by its volatility, solubility and diffusion coefficient.

10.4.2.1 Compatibility of the Additive Compatibility is the main problem for light stabilisers, because light stabilisers are generally used in concentrations up to 2%. Ideally, stabiliser molecules should be disposed singly throughout the polymer matrix. This will not generally happen, but compatibility with the polymer should be sufficient to prevent gross phase separation.

Applications of Plastic Films in Agriculture Stabilisers are normally dissolved in the polymer melt at the processing temperature. However, their solubility limit may be exceeded on cooling and this may lead to visually observable blooming. These processes depend on the nature of the chosen substrate as well as on its morphology. Additive diffusion is primarily a consequence of the thermal motion of the polymer chains above the glass transition temperature and of the related formation and disappearance of free volumes. If the chains are flexible and move easily, only small amounts of energy are necessary to move the polymer segments. With increasing orientation of the polymer chains or on crosslinking and with increasing crystallinity, the diffusion constant decreases. For this reason, the diffusion of additives is faster in LDPE than in high-density polyethylene (HDPE). Numerous studies [11-14] confirm that the diffusion behaviour of UV absorbers depends mainly on polymer structure and morphology and to a minor extent on additive structure. The solubility and compatibility of light stabilisers are particularly a problem when highly polar light stabilisers are used for non-polar plastics such as polyolefin. However, even in polyurethane, the compatibility of light stabilisers may become a problem.

10.4.2.2 Determination of Compatibility Stabilisers such as antioxidants, metal deactivators and UV absorbers are added to polymers to reduce degradation during the manufacturing process and throughout the lifetime of the polymer products. In order to study the degradation of polymers or the compatibility between additives and polymers, it is essential to have an analytical method that can provide both identification and a quantitative measure of additives in the polymers. Fourier transform infrared (FTIR) spectroscopy [15, 16], UV spectroscopy [17], near-infrared reflectance gas chromatography, high-performance liquid chromatography (HPLC) and differential scanning calorimetry (DSC) can all be used as analytical tools for identification and determination of the concentration of dissolved stabilisers and their homogeneous distribution. FTIR and UV spectroscopy are the most important techniques used, as they can be applied directly to the sample without disturbing the morphology in the solid state. In addition, it is possible to detect any degradation or changes taking place at earlier stages due to the sensitivity of these tools. Furthermore, the diffusion coefficient of additives can be estimated by using the disc-stacking technique [18], where a disc doped with the additives is placed in the middle of a stack of undoped discs. Diffusion is then allowed to take place at an appropriate temperature for an appropriate time. Then spectroscopic measurements can be done on different discs to evaluate the concentration of the additives in each disc. Accordingly, the diffusion

269

Handbook of Plastic Films coefficient can be determined by knowing both the thickness of the discs and the concentration of the additive by using a characteristic absorption band. The principle behind the use of near-infrared reflectance analysis is the measurement of the light reflected by a sample when exposed to light in this region. The logarithm of the inverse of this reflected light can be related to the concentration of a particular component found in the sample. The obtained concentration value for different discs can also be used for determination of either the diffusion coefficient or the solubility of the additive. The same can be said for gas chromatography or HPLC. On the other hand, differential scanning calorimetry can be used to determine the exothermic peak of the oxidation of antioxidants, which allows direct determination of the concentration of the dissolved antioxidants. However, the commonly used methods for determination of compatibility do not give adequate information about the molecular association of the stabilisers, but only information about the volume concentration of the stabilisers in the polymeric matrix. This means that it makes no difference whether the stabilisers are present as stacks of molecules or as single molecules. Such aggregation will lead to lower efficiency of stabilisers. Of course, FTIR can offer some information from the change in the perturbation potential that results from polymer-stabiliser and stabiliser-stabiliser molecular interactions. However, this needs a very careful study of the sample, and to have references for molecularly dispersed stabilisers as well as sophisticated calculation for the obtained spectra. On the other hand, broad-band dielectric spectroscopy [19-22] can be applied to investigate solubility and compatibility, as it offers an excellent possibility of detecting the molecular reorientation of stabiliser molecules and segments simultaneously at the same temperature. Accordingly, the degree of compatibility with most polymeric segments can be evaluated, where detailed investigation of the molecular dynamics have been carried out [19-22] for various additives having different shapes, sizes and polarities in different polymeric matrices. An empirical relation that determines the dependence of the relaxation frequency differences between the cooperative process of the additive and the glass process of the matrix (macro-Brownian cooperative reorientation of the segment associated with the glass temperature) and the additive length has been given [21]: Δ log fm = 4 log[(L/d) – 1]

(10.6)

where Δ log fm is the difference between log fm of the cooperative process of the additive and log fm of the glass process of the matrix; L is the length of the additive; and d is the

Applications of Plastic Films in Agriculture polymer inter-chain distance. This equation was found to be valid not only for the relaxation process of an additive in a polymer but also for the δ relaxation process of side-chain liquid-crystalline polymers and their additives. This equation implies that, if the molecules are molecularly dispersed, and have a length not longer than 1.8 nm, they must relax cooperatively with the cooperatively reorienting segments of the glass process at the same relaxation frequency. However, in the case of Tinuvin P, a commercially available UV stabiliser (a benzotriazole derivative), the difference in the relaxation frequency maxima of the stabiliser peak and the glass process of the matrix, Δ log fm, is greater than three decades of frequency. These results indicate that the reorientations of the stabiliser are not coupled with the glass process of polystyrene segments, where the stabiliser molecules can relax locally at higher frequencies, (i.e., faster by a factor of 1000). Furthermore, short additives can relax either cooperatively with the polymeric segments at the same relaxation frequency as the segments, or locally at higher frequencies. The ratio of the local contributions to the total relaxation strength (cooperative plus local) of the additives depends on the size of the stabiliser. The biodegradation of representative samples of available commercial photo(bio)degradable polyethylene films was examined with respect to the rate and extent of degradation, oxidation products and changes in molecular weight both during outdoor exposure and in laboratory photo-ageing devices with different accelerating factors [23]. Although the rate of photooxidation was found to depend on the type of degradation system used, all the samples showed a rapid rate of carbonyl formation, with concomitant reduction in molecular weight and mechanical properties on exposure to UV light. The photo-fragmented polymers were shown to be much more hydrophilic in nature compared to the unoxidised analogues, and photo-fragments of all samples were found to contain high levels of low molecular weight (low molar mass) bioassimble carboxylic acids and esters. The recycling behaviour of virgin polyolefins, both as homopolymers and as heterogeneous polymer blends, which contained 10% of non-oxidised and photooxidised photo(bio)degradable plastics, has been examined. It was found that the initial mechanical performance of homogeneous blends was not greatly affected by the presence of nonoxidised degradable materials. However, blends containing degradable films that were initially partially photooxidised had a much more detrimental effect on the properties of the recycled blends during processing and weathering; the effect was minimal for degradable polymers containing the iron-nickel dithiocarbamate system.

10.4.3 Evaluation of Laboratory and Outdoor Photooxidation Laboratory and outdoor photooxidation of plastic films were evaluated using different techniques [24]. Thus melt blown, biaxially oriented, unstabilised and stabilised LDPE films with various thicknesses were exposed in two accelerated artificial weathering devices

271

Handbook of Plastic Films with xenon (Xenotest) and UV-B fluorescent tube (QUV Weatherometer) sources under controlled temperature and humidity conditions. The structural changes during combined photo- and thermal degradation have been studied using tensile tensiometric, IR spectrophotometric and DSC methods. The effects of HALS and film thickness on the time-dependent changes in elongation, carbonyl group concentration, crystallinity and the onset temperature (Ton) of the post-fusion DSC oxidation exotherm have been observed. Photooxidation is accompanied by increased crystallinity, which maximises as mechanical properties start to deteriorate significantly and the rate of carbonyl group formation increases. While there is a poor correlation between the reduction in mechanical properties and increased carbonyl index values, the former correlates well with the DSCderived Ton values for unstabilised and stabilised films. This suggests that thermal analysis may be used to detect the physicochemical changes occurring in exposed films more effectively than other techniques such as IR. However, many problems of premature film failure can occur during their use in greenhouses, due to their interaction with the agrochemicals used. Both sulfur- and chlorine-containing agrochemicals inhibit the functioning of HALS, and can have a very detrimental effect on the life of greenhouse films [25, 26]. The concentration of HALS in LDPE covering films before and after exposure to natural weathering and accelerated photooxidation conditions has been determined [27]. It has been found that photostabiliser disappearance above 0.4% up to 600 days is mostly probably the result of its physical loss during long photooxidation times under both photooxidative conditions. On the contrary, photostabiliser disappearance in the initial stage is due to chain scission and the consequent volatilisation and diffusion of these fragments on the surface.

10.5 Other Factors Affecting the Stability of Greenhouse Films 10.5.1 Temperature Cyclic temperature changes and the high temperatures developed at the metallic parts of greenhouse constructions during hot and sunny days lead to increased degradation. One can observe a lot of damage in the places where the plastic greenhouse films come into contact with the metallic structural elements, especially when they are not painted. The temperature at these contact points may reach up to 70 °C and more depending on climatic conditions. In this case the diffusion of metal ions enhances the degradation process. Metal particles, especially iron, may catalyse the decomposition of hydroperoxides formed as a result of oxidation, leading to unnecessarily high rates of degradation. The degradation

Applications of Plastic Films in Agriculture mechanism of PE films containing additives with metal ions at a simulated composting temperature has been studied. The hydroperoxide concentration [POOH] in the films was traced quantitatively by using iodometric potentiometric titration, and compared with Fourier transform infrared spectrometry (FTIR). The results show that [POOH] increases during the early stage of degradation, followed by a more or less flat maximum, before it starts to decrease. At the same time, similar results are obtained by FTIR analysis. It is also found that the rate laws for the carbonyl index and [POOH] increases seem more complicated than an exponential-type increase in the early stage of oxidation [28]. Moreover, high temperatures lead to an increase in the rate of reaction for both photooxidation and chemical oxidation by agrochemicals, and thus to higher degradation rates. As mentioned before, HALS compounds act as radical traps, and consequently they also act as heat stabilisers and minimise the effects of high temperature [29].

10.5.2 Humidity Lower resistance to oxidation and enhancement of degradation occur as a result of increased humidity as well as rainfall. This is due to the gradual washout of additives that may bloom on to the surface of plastic films. Besides, the degradation of plastic films may occur due to hydroxyl radicals or other reactive species generated as a result of photolysis [30].

10.5.3 Wind It has been suggested that tearing due to high winds can be a major problem in greenhouses. Another problem connected with windy areas is the wind load. This load can impose increased stress on plastic films and lead to premature failure of the film. Abrasion caused by soil and other particles, which are carried by the wind and impinge on the surface of the greenhouse film, may also be another problem.

10.5.4 Fog Formation The term ‘fog’ is used to describe the condensation of water vapour in the form of small discrete droplets on the surface of transparent plastic films. The physical conditions that lead to this formation may be summarised as follows [31]: (1) A fall in temperature of the inside surface of the film to below the dew point of the enclosed air/water vapour mixture;

273

Handbook of Plastic Films (2) Cooling of the air near the film to a temperature at which it can no longer retain all the water vapour, so that excess water condenses upon the film; (3) The difference between the surface tension of condensed water and the critical wetting tension of the film surface, which causes the water to condense as discrete droplets, rather than as a continuous film. A number of undesirable effects may result from fog formation in greenhouse films and leads to the following: (1) Light transmission will be reduced where the total internal reflection of incident light occurs. Consequently, the rate of plant growth will reduce, crop maturity is delayed and the crop yield decreases; (2) Light and heat transmission may be focused on delicate plant tissues owing to water droplets acting as lenses. This causes burning of the plants and crop spoilage. To prevent fog formation, surface-active agents are usually added to PE during film production. These compounds are incompatible with the polymer and subsequently migrate to the film surface where they increase the critical wetting tension. The result is reduction in contact angle between water and polymer surface, permitting the water to spread into a more uniform layer [31].

10.5.5 Environmental Pollution Atmospheric pollution, such as nitrogen oxides, sulfur dioxides, hydrocarbons and particulates, can enhance the degradation of polymers [32] and must also be taken into consideration. For instance, infrared studies have revealed that polyethylene reacts with NO2 at elevated temperature and that chemical attack is observed even at 25 °C. Similarly, SO2 is rather reactive, especially in the presence of UV radiation, which it readily absorbs and forms triplet excited sulfur dioxide. This species is capable of abstracting hydrogen from polymer chains, leading to the formation of macroradicals in the polymer structure, which in turn can undergo further depolymerisation [33].

10.5.6 Effects of Pesticides The use of agrochemicals in greenhouses severely affects polymer films [29]. The pesticides used for the protection of the crop influence the degradation and lifetime of the films. Usually pesticides have complicated formulations, and contain a number of compounds besides the active component. They contain sulfur and halogen in their chemical structure. It is a well-known fact that films are destroyed under the effect of pesticides.

Applications of Plastic Films in Agriculture Pesticides react with the stabilisers present in the film, decreasing its effect or completely destroying it. Experimental results clearly show that pesticides with sulfur-containing active compounds enter into antagonistic interaction with the stabilisers. One simple explanation [34] is that the interaction of the pesticide and the HALS compound prohibits the latter in executing its effect. Some sulfur-containing compounds and organic halogenides initiate the oxidation of PE and bring about rapid deterioration of its mechanical properties. The extent of this negative effect depends on the molecular weight, dispersion, allotropic modification, etc., of the elementary sulfur. The introduction of a UV absorber into film considerably improves the lifetime and light stability of the film. Also, HALS-stabilised greenhouse films were shown to last 33% longer than Ni-stabilised films in testing under real conditions. However, the polymer materials used for greenhouse films are changing, and, in particular, the use of blends is continuously increasing, like the use of additives. These additives are used in relatively large amounts for different aims, like photooxidation resistance, antifogging, etc. Moreover, the films can absorb fertilisers and pesticides, which can compromise the use of secondary materials coming from greenhouse covering films in many applications. UV exposure gives rise to major modifications of the macromolecular chains, with chain breaking, formation of oxygenated groups, possible formation of branching and crosslinking, and so on [35-38]. Finally, the reprocessing operations can induce further degradation due to the thermomechanical treatment in the melt [39-42].

10.6 Ageing Resistance of Greenhouse Films 10.6.1 Measurement of Ageing Factors Evaluation of the stability and durability of greenhouse films is usually carried out using laboratory equipment. There is no standard testing scheme for evaluating the degradation of these properties when the plastic film is used as a greenhouse covering material. This is due to the fact that there are several interconnected factors that can lead to the degradation of the mechanical properties. These factors are usually difficult to realise in the laboratory. Following up the changes that occur in the mechanical properties of plastic films as a result of ageing is very important in order to throw some light on the problem and to identify the conditions of the film. However, other properties of the film, such as physical and chemical properties, are also affected by the degradation, e.g., abrasion directly affects light transmittance and also other mechanical properties. Many research groups have paid attention to this problem and concentrated their efforts on measuring the effects of various ageing factors on the degradation of plastic materials [6, 43, 44]. Some of them are concerned with the very specific problem of ageing of agricultural plastic film [24, 45].

275

Handbook of Plastic Films In principle, all parameters should be covered in order to be able to predict accurately the performance and lifetime of the material under the particular circumstances that would occur when the material is used in a greenhouse. The accelerated ageing process consists of simulating in an intensified manner the most critical parameters that lead to the degradation of plastics. Several accelerated ageing tests for plastic films have become commercially available during the past decade. However, their relationship to real outdoor ageing is extremely questionable. These tests are based on inducing artificial ageing in the material through an intense UV source coupled with a day-night cycle and a water spray cycle. Only a few studies have considered the use of pesticides or stress due to wind loading combined with the effect of UV-induced ageing [25]. An empirical correlation between the lifetime of films under artificial weathering and in the greenhouse situation has been given [46]. This standard defines three climatic zones depending on the level of solar radiation energy that they receive: 70-100, 100-130 or 130-160 kLy/yr, where the kilolangley is given by 1 kLy = 4.184 kJ/cm2. However, no special conditions – such as the contact between the film and the metal parts of the greenhouse structure, and the application of pesticides that can strongly influence the actual weathering of the plastic films – have been considered. Thus, the artificial ageing tests can only provide a rough estimate of the actual behaviour of plastic films when exposed to the real and complicated environmental factors that affect the plastic during its use [47]. For this reason several researchers have studied the degradation of plastic films under natural weathering conditions (outdoor tests) [48]. Only a limited number of tests were performed in greenhouses by this group [48].

10.6.2 Changes in Chemical Structure Changes in chemical structure resulting from plastic film ageing have been followed using spectroscopic methods. FTIR is the most frequently used technique [47, 49]. It provides information about the chemical structure of the macromolecules. For instance, when the chains are oxidised, carbonyl and OH groups are formed. The additive concentrations and their changes can also be detected by the same technique. The presence of parts of the agrochemicals in the film can also be detected spectroscopically. Electron spin resonance (ESR) is also used to detect the creation of free radicals during degradation [49]. Thermal analysis, such as DSC, is used to study the oxidation process as well as the changes in the crystallinity of plastic films due to ageing [50]. Gel permeation chromatography (GPC) may be used to evaluate the changes in the molecular weight and molecular weight distribution of the films [8]. The integrated area between 1770 and 1690 cm–1 of the absorption band at 1734 cm–1 was used to determine the concentration of Tinuvin 622 [poly(N-β-hydroxyethyl-2,2,6,6-tetramethyl-4-

Applications of Plastic Films in Agriculture hydroxypiperidyl succinate)] before and after film exposure. The oxidation degree, i.e., carbonyl index (CI), under different oxidation conditions was obtained by calculating the carbonyl absorption at 1713 cm–1 from the FTIR spectra at various oxidation times using the spectrum of the unoxidised starting material as a reference. All measured absorbances should be normalised by the film thickness using the equation: CI = (A1713/d) x 100

(10.7)

where A1713 is the measured absorbance at 1713 cm–1 at a certain exposure time, and d is the film thickness in micrometres.

10.7 Recycling of Plastic Films in Agriculture 10.7.1 Introduction The amount of plastic materials used in agriculture has been continually increasing. Plastic materials are used for greenhouse covers, mulching, piping, packaging and other applications. Films used for greenhouses can be considered as an easy source of materials for recycling. Indeed, large amounts of film can be easily collected and, because of the homogeneity of the polymers used for this application, the recycling operations can be relatively easy. However, UV exposure gives rise to major modifications of the macromolecular chains, with chain breaking, formation of oxygenated groups, possible formation of branching and crosslinking, and so on [35, 36, 51, 52]. The recycling of post-consumer films for greenhouses is strongly dependent on the initial structure of the plastic materials and on the processing conditions. Such films contain small amounts of low molecular weight compounds probably coming from the photooxidation of the PE molecules and from the absorption of fertiliser and pesticide residues. The amount of these compounds is small, however, and does not prevent the use of the recycled materials in many applications. The properties of the secondary materials deteriorate with the number of extrusion steps, but especially with the increasing extent of photooxidative degradation. However, the mechanical properties of the recycled post-consumer film remain relatively good even after many extrusion passes, and such film is useful for many applications [53].

10.7.2 Contamination by the Environment Dow Chemicals have actively investigated the recycling of mulch film because the normal practice of disposal by burning it on the fields is environmentally undesirable. The

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Handbook of Plastic Films contamination levels in mulch film make its recycling particularly challenging. For instance, soil contamination can be as high as 30-40%. Furthermore, the soil can contain up to 3% iron, which is a polyethylene prodegradant [54]. In addition, it was found that vegetable matter derived from harvested plants could not be removed during the washing operations [54]. Other contaminants are fumigants, (e.g., methyl bromide), and the oxidised fractions of LDPE resulting from photodegradation of the mulch film. Recently effort has been focused on the recycling of LDPE mulch film and greenhouse film, both of which contain pesticide residues. This area poses special problems that are difficult to overcome. It has been discovered that organochlorine and organosulfur pesticide residues can deactivate HALS. This deactivation is believed to occur as a result of hydrolysis of the pesticides to acidic species that then react with the HALS. This has implications with respect to the long-term outdoor stability of the recycled product. During in-service use, PE can become badly degraded and can form low molecular weight oxygenated products, (e.g., aldehydes, acids, ketones, waxes, etc.). These impurities can lead to embrittlement of the recycled polymer because low molecular weight oxidised fractions are segregated from the melt during crystallisation and concentrate at the spherulite boundaries [55]. The resulting zone, rich in oxidised material, has very low fracture toughness. Moreover, oxygenated degradation products of PE, such as carbonyl groups, are active chromophores and can sensitise the reprocessed polymer to photodegradation. Plastic waste management, in general, is a global environmental problem. The management of such waste may be through the famous 4R approach: •

Reduction (of source material);

•

Reuse;

•

Recycling;

•

Recovery.

The recycling and reuse of plastic waste films generated from greenhouses can share in solving the problem. The disposal of municipal solid waste has become an environmental issue of growing concern [56]. It was determined that discarded plastics represent close to 20% of municipal solid waste on a volume basis [57, 58]. This is due to the high volume-to-weight ratio of polymeric materials. The management of plastic waste follows the scheme: •

Source reduction;

•

Recycling;

Applications of Plastic Films in Agriculture •

Thermal reduction by incineration;

•

Land-filling.

The most feasible methods for developing countries are source reduction and recycling. Source reduction is any measure that reduces the volume of plastic waste produced. This is accomplished through material efficiency, i.e., reducing the quantity of plastic material used to produce a particular item. Recycling generally involves the collection of waste plastic materials for reprocessing [59, 60]. Polyolefin blend technology is of critical importance to various applications, including greenhouse films. For instance, the LLDPE/LDPE blend is characterised by reduced haze and better bubble stability. One of the most common blends is LDPE/ ethylene-propylene-diene terpolymer (EPDM) with improved low-temperature flexibility, rubbery properties, weathering resistance and high-temperature mechanical properties. The addition of EVA to LDPE has been commercially utilised to improve environmental stress cracking resistance, toughness, film tearing resistance, flexibility and optical properties. Both blending and coextrusion have been employed to deal with the problem of agricultural plastic film waste. The main goal is to find a solution to the problem of agricultural plastic waste from greenhouses by recycling and converting the waste into products usable in the mulch and greenhouse film applications. The proposed solution is based on the development of multilayer films consisting essentially of a top layer made from virgin resin and a bottom layer consisting of a blend of recycled PE waste film material in combination with virgin resin and other ingredients. Evaluation of greenhouse plastic wastes revealed that it is possible to obtain useful transparent plastic films to be reused with reduced cost [61]. Multilayer films for greenhouses are a current trend in the industry. LDPE films for the top layer, stabilised with different concentrations (0.1, 1.0 and 2.5%) of UV quencher, have been produced in the laboratory by blow extrusion. The effect of natural weathering on the film properties was investigated over a period of 12 months [62]. Significant changes in the mechanical properties were observed in the later stages of degradation. Films stabilised with 0.1% stabiliser crumbled after 12 months of natural weathering, whereas films with higher concentrations retained their mechanical properties. It is believed that the inclusion of a UV stabiliser interferes with the crystallisation process and that the stabiliser particles accumulate in the amorphous matrix. Degradation of the imperfect crystalline region with its low oxygen permeability proceeds via crosslinking, whereas chain scission predominates in the amorphous region with excess of oxygen. Films stabilised with 2.5% UV quencher form a barrier against the transmission of UV radiation and the bottom layers are less affected by UV radiation. Recycled material can, therefore, be incorporated at high concentrations into these layers.

279

Handbook of Plastic Films Optimisation of the top layer was based on a fixed concentration of three thermoplastics, i.e., 80% LDPE, 10% LLDPE and 5% EVA, the remaining 5% being a specially prepared master batch of LDPE containing 25% UV and heat stabilisers. Different types of UV stabilisers were taken in different concentrations. These are: Cyasorb 1084 [n-butylaminenickel-2,2′-thio-bis(4-tert-octyl phenolate)] and Chimassorb 81 [2-hydroxy-4-noctoxybenzophenone], acting as UV light absorbers; Chimassorb 944 LD [poly{6-(1,1,3,3tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl amino-hexamethylene-4-(2,2,2,6tetramethylpiperidyl)imine}] and Tinuvin 622 LD, acting as radical scavengers; an energy transfer agent; and a peroxide decomposer. The data obtained show that the haziness of all plastics films are within the range required for agricultural films. It has also been found in the case study [62] that the utilisation of a single UV stabiliser is less efficient than the utilisation of a two- or three-component UV stabiliser. Thus, films containing three-component UV stabilisers in addition to a thermal stabiliser (Irganox 1076) can retain at least 94% and 81% of tensile strength and elongation at break, respectively, after exposure to UV radiation for 600 h. The good resistance of these plastic films can be attributed to the different mechanisms of action of the utilised stabilisers. In other words, if the UV absorber Chimassorb 81 is added alone to the plastic blend, the films retain about 63% of the original elongation; whereas in combination with Cyasorb 1084, the retained elongation is increased to about 80%. Consequently, these results indicate the necessity of using a combination of UV absorbers and radical scavengers [62]. Furthermore, some plastic films of various compositions were subjected to outdoor weathering tests in two different locations, Cairo and Upper Egypt. The results obtained indicate that the unprotected films deteriorate completely within three months, whereas the protected films can withstand almost one year without a drastic decrease in mechanical properties.

References 1.

J.C. Garnaud in Proceedings of the 13th International Congress of CIPA, Verona, Italy, 1994.

2.

P.A. Dilara and D. Briassoulis, Journal of Agricultural Engineering Research, 2000, 76, 309.

3.

D. Briassoulis, D. Waaijenberg, J. Gratraud and B. von Elsner, Journal of Agricultural Engineering Research, 1997, 67, 1.

4.

L. Pacini, Plasticulture, 1999, 117, 25.

5.

M.B. Amin, S.H. Hamid and J.H. Khan, Journal of Polymer Engineering, 1995, 14, 253.

Applications of Plastic Films in Agriculture 6.

S.H. Hamid, A.G. Maadhah and M.B. Amin in Handbook of Polymer Degradation, Eds., S.H. Hamid, A.G. Maadhah and M.B. Amin, Marcel Dekker, New York, NY, USA, 1992, 219.

7.

J.F. Rabek, Polymer Photodegradation. Mechanisms and Experimental Methods, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1994, 73.

8.

F. Gugumus in Developments in Polymer Stabilisation – I, Ed., G. Scott, Applied Science, London, UK, 1979.

9.

P.P. Klemchuk in Polymer Stabilisation and Degradation, Ed., P.P. Klemchuk, American Chemical Society, Washington, DC, USA, 1985, 1.

10. S. Al-Malaika, E.O. Omikorede and G. Scott, Journal of Applied Polymer Science, 1987, 33, 703. 11. R.G. Hauserman and M. Johnson, Journal of Applied Polymer Science, 1976, 20, 2533. 12. M. Johnson and R.G. Hausermann, Journal of Applied Polymer Science, 1977, 21, 3457. 13. F. Gugumus, Proceedings of the 3rd International Conference on Polypropylene Fibres and Textiles, York, UK, 1983, Paper No.18. 14. F. Gugumus, Kunststoffe, 1987, 77, 1065. 15. K. Moeller, T.O. Gevert and I. Jakubowicz, Proceedings of the International Conference on Environmental Science, Mount Prospect, IL, USA, 1990, 635-640. 16. D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C.P. Peck and R.O. Carter, Polymer Degradation and Stability, 1990, 28, 1, 39. 17. B. Bell, D.E. Beyer, N.L. Maeker, R.R. Papenfus and D.B. Priddy, Journal of Applied Polymer Science, 1994, 54, 1605. 18. J.Y. Moisan, European Polymer Journal, 1980, 16, 979. 19. A.A. Mansour, B. Stoll and W. Pechhold, Colloid & Polymer Science, 1992, 270, 219. 20. A.A. Mansour and B. Stoll, Colloid & Polymer Science, 1994, 272, 25. 21. A.A. Mansour and B. Stoll, Colloid & Polymer Science, 1994, 272, 17.

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Handbook of Plastic Films 22. A.A. Mansour, Ph.D. Thesis, University of Ulm, Germany, 1992. 23. S. Al-Malaika, S. Chohan, M. Coker, G. Scott, R. Arnaud, P. Dabin, A. Fauve and J. LeMarie, Journal of Macromolecular Science A, Applied Chemistry, 1995, 32, 4, 709. 24. M.G. Liu, A.R. Horrocks and M.E. Hall, Polymer Degradation and Stability, 1995, 49, 1, 151. 25. P.C. Powell, Engineering Design Guides, 1979, 19, 1. 26. P. Desriac, Plasticulture, 1991, 89, 1, 9. 27. M. Scoponi, S. Cimmino and M. Kaci, Polymer, 2000, 41, 2, 7969. 28. J.G. Yu, H.S. Li, M.Q. Zhang and M.L. Zhang, Journal of Applied Polymer Science, 2000, 75, 4, 523. 29. F. Henninger in Handbook of Polymer Degradation, Ed., S.H. Hamid, A.G. Maadhah and M.B. Amin, Marcel Dekker, New York, NY, USA, 1992, 411. 30. J.E. Bonekamp and N.L. Maecker, Journal of Applied Polymer Science, 1994, 54, 1593. 31. ICI Europe, Surfactants, Report, Ciba Speciality, Everberg, Belgium, 1998. 32. B. Ranby and J.F. Rabek in The Effects of Hostile Environments on Coatings and Plastics, Eds., D.P. Garner and G.A. Stahl, American Chemical Society, Washington, DC, USA, 1983, 291-307. 33. W. Schnabel in Polymer Degradation: Principles and Practical Applications, Hanser International, New York, NY, USA, 1981. 34. E. Epacher and B. Pukanszky, Proceedings of Antec ’99, New York, NY, USA, 1999, Volume III, 3785. 35. F.P. La Mantia, Radiation Physics and Chemistry, 1984, 23, 699. 36. A. Tidjani, R. Arnaud and A. Dasilva, Journal of Applied Polymer Science, 1993, 47, 211. 37. M. Sebaa, C. Servens and J. Pouyet, Journal of Applied Polymer Science, 1993, 47, 1897.

Applications of Plastic Films in Agriculture 38. J.L. Angùlo-Sanchez, H. Ortega-Ortiz and S. Sànchez-Valdes, Journal of Applied Polymer Science, 1994, 53, 847. 39. M.K. Loultcheva, M. Proietto, N. Jilov and F.P. La Mantia, Polymer Degradation and Stability, 1997, 57, 77. 40. A.T.P. Zahavich, B. Latto, E. Takacs and J. Vlachopoulos, Advances in Polymer Technology, 1997, 16, 11. 41. M. Marrone and F.P. La Mantia, Polymer Recycling, 1996, 2, 17. 42. J.I. Eguiazàbal and J. Nazàbal, Polymer Engineering Science, 1990, 30, 527. 43. F.S. Qureshi, M.B. Amin, A.G. Maadhah and S.H. Hamid, Polymer Plastics Technology and Engineering, 1989, 28, 649. 44. G. Yanai, A. Ram and J. Miltz, Journal of Applied Polymer Science, 1995, 57, 303. 45. A. Ram, T. Meir and J. Miltz, International Journal of Polymeric Materials, 1980, 8, 323. 46. G. Grünwald, Plastics: How Structure Determines Properties, Hanser, Germany, Munich, 1992. 47. M.R. Kamal and B. Huang in Handbook of Polymer Degradation, Eds., S.H. Hamid, M.B. Amin and A.G. Maadhah, Marcel Dekker, New York, NY, USA, 1992, 127. 48. J.H. Khan and S.H. Hamid, Polymer Degradation and Stability, 1995, 48, 137. 49. A.A. Popov, N.N. Blinov, B.E. Krisyuk, S.G. Karpov, L. Privalova and G.E. Zoukov, Journal of Polymer Science, 1983, 21, 1017. 50. L. Peeva and S. Evtimova, European Polymer Journal, 1984, 20, 1049. 51. A. Tidjani, R. Arnaud and A. Dasilva, Journal of Applied Polymer Science, 1993, 47, 211. 52. L. Angùlo, H. Ortega and S. Sànchez, Journal of Applied Polymer Science, 1994, 53, 847. 53. N.T. Dintcheva, F.P. La Mantia, D. Acierno, L. Di Maio, G. Camino, F. Trotta, M.P. Luda and M. Paci, Polymer Degradation and Stability, 2001, 72, 1, 141.

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Handbook of Plastic Films 54. C. Llop and A. Perez, Die Makromolekulare Chemie - Macromolecular Symposia, 1992, 57, 115. 55. D.T. Wark, Proceedings of the ECM International Conference, Advances in High Performance Polymer Blends and Alloys, 1991. 56. D.R. Paul in Multicomponent Polymer Materials, Eds., D.R. Paul and L.R. Sperling, American Chemical Society, Washington, DC, USA, 1986, 2-19. 57. A.P. Plochocki, Polymer Engineering and Science, 1983, 23, 618. 58. Plastic Recycling, Ed., R.J. Ehrig, Hanser, Munich, Germany, 1989. 59. A. Hansen in Plastic Extrusion Technology, 2nd Edition, Ed., F Hensen, Hanser, Munich, Germany, 1997. 60. A. Yehia, E.M. Abdel-Bary, A.A. Abdel-Hakim and M.N. Ismail, Proceedings of the 1st Egyptian-Syrian Conference on Chemical Engineering, Suez, Egypt, 1995. 61. N. Khraishi and A. Al-Robaidi, Polymer Degradation and Stability, 1991, 32, 1, 105. 62. E.M. Abdel-Bary, M.N. Ismail, A.A. Yehia and A.A. Abdel-Hakim, Polymer Degradation and Stability, 1998, 62, 1, 111.

11

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Klara Z. Gumargalieva and Gennady E. Zaikov

11.1 Introduction The principal medical treatment of burns is the use of dressings, which often worsen the effects of the injury. It is difficult to estimate the effectiveness of new burn dressings, as their physicochemical properties are not usually presented in the literature. This chapter is devoted to a discussion of this subject for the first time. The authors address the complexity of physicochemical methods of analysis in order to create criteria for efficient dressings for a burn wound surface. The characteristics typical of burn wounds for which dressings are required are shown in Table 11.1.

Table 11.1 Characteristics of burn wounds [3] Burn degree Image of damage

Physiological process

Burn depth (mm)

I

Redness and oedema (medium oedema)

Aseptic inflammatory process

0

II

Sac formation

Aseptic inflammatory process

0

III

Damage of skin cover, exuding wound surface

Skin necrosis, tissue necrosis

1-2

IV

Exuding wound surface

Full necrosis of tissues, carbonisation of tissues

2-5

Based on theoretical and experimental data, it was found that the maximal sorptional ability of a burn dressing is determined by the free volume of the dressing material calculated from the value of the material density. Kinetic parameters were determined from the sorption curves. These parameters help in predicting the behaviour of burn dressings. Criteria for estimating the efficiency of first-aid burn dressings are then formulated.

285


11.2 Modern Surgical Burn Dressings Dressings for wounds and burns must primarily be protective, sorptional and atraumatic. In currently used dressings, these properties are provided by a multilayer structure or structural modifications. Different classifications of dressings can be found in the literature: by material, by construction or by function [1-3]. The dressings applied in the modern treatment of wounds and burns are subdivided into three groups according to the material of the layer sorbing the wound exudate. The material may be of animal origin, synthetic foamed polyurethane or of vegetable origin (Table 11.2).

11.2.1 Dressings Based on Materials of Animal Origin Typical dressings in this group are collagen sponges. Besides hydrophilic properties, collagen sponges provide higher sorption of liquid (in the range of 40-90 g/g) [1, 4-9]. The patent literature describes in detail the methods of obtaining collagen dressings for wounds and burns in the form of sponges and felt [10-13] based on materials of animal origin. Also, the materials used include that made from biological artificial leathers based on lyophilised bodies and swine cutis, produced as plates 0.5-0.7 mm thick. However, these materials possess lower sorptional capacity than collagen dressings. Dressings called ‘cultivated cutis’ are also obtained from the epithelia of cells of the patient himself [13]. The shortcoming of biological artificial leathers or bio-dressings is their expense and, as a rule, their inability to retain their properties on storage.

11.2.2 Dressings Based on Synthetic Materials The demands for inexpensive raw materials for the production of wound and burn dressings has led to the production of materials based on synthetic polymers, particularly cellular polyurethane [10, 14-18]. Cellular polyurethane intended for medical purposes is synthesised using toluene diisocyanate and polyoxypropyleneglycol [19]. Dressings based on polyurethanes have a pore distribution of about 200-300 pores/cm2, and allow the regulation of the number and size of pores in layers [20]. Dressings from this group are prepared as a double layer; the density of the outside layer is high in order to prevent liquid evaporation and penetration of microorganisms. In rare cases, these dressings are homogeneous through their thickness. The influence of the pore size on the sorption properties of polyurethane sponges has been reported [21], where macroporous sponge with a pore size from 200 up to 2000 μm is completely nourished by exudate under pressure only. In this case, the size of the pores should be of the order of several micrometres.

286

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings

Table 11.2 Characteristics of dressings in brief No. Name

Company

Country Structure

Composition

Group 1 1.

Collagen burn dressing

‘Helitrex’

USA

Dressings uniform by thickness, dense, pores of 0.01 mm size. It has gauze cover

Collagen

2.

Collagen sponge

‘Helitrex’

US A

Similar to No. 1, differs by big radius of pores, formed by fibril weaving of cylindrical form preferably

Collagen

3.

Collagen dressing

‘Bayer’

Germany Friable dressing, possesses rough porous structure with pore/hole sizes from 1.5 to 0.1 mm

Collagen

4.

Burn curative dressing

‘Combutec-2’

USA

Dressing of large-porous structure with pore size from 1 to 0.05 mm. Pores are of cylindrical form preferably formed by fibril weaving of collagen

Collagen

5.

Biological dressing

‘Corretium-2’

US A

Dense, pressed plate. Fibrillar structure is observed in dense layers

Collagen

6.

Biological dressing

‘Corretium-3’

USA

The same as No. 5

Collagen

Group 2 7.

Compositional burn dressing

‘Biobrant’

USA

Double-layered elastic, porous dressing, consists of the upper layer of 0.010.005 mm and flexible fabric Nylon base. It represents combination of hydrophilic components with elastic silicon films

Silicon, the main layer, base made from polyamide

8.

Synthetic dressing

‘Epigard’

USA

Double-layered elastic porous dressing. Upper layer is dense, nonporous 0.2 mm thick

The main layer made from polyurethane, the upper one made from polypropylene

287


Table 11.2 Characteristics of dressings in brief continued No. Name Company Country Structure Composition 9.

Synthetic burn dressing

10.

‘Syncrite’

ChSSR

Single-layered dressing on gauze base with through large pores, medium flexible

Polyurethane

Synthetic ‘Syspurderm’ wound dressing

Germany

Dressings of homogeneous Polyurethane composition, with different pore distribution: upper layer is 0.1 mm thick, possesses small porous structure with pores of 0.01 mm size; lower layer, adjoining wound possesses large pores of 0.05 mm size. The dressing is ‘elastic’, accepts a form badly

11.

Synthetic ‘Farmexplant’ wound dressing

PB R

Antiseptic double-layered dressing. Main polyurethane layer possesses pores of 0.1-1.5 mm. Upper layer is 0.1 mm thick, more dense, non-porous

Polyurethane

12.

Atraumatic caproic dressing

USSR

Large-cellular dressing on basis of woven Nylon

Polyamide

USSR

Wound large-cellular dressing, homogeneous by its composition

Alginic acid salts

USSR

Porous cotton balling dressing with atraumatic layer

Cellulose

Group 3 13.

Cover for wound, burns

14.

Needle-pierced fabric

15.

Wound nonadhering dressing

288

VNII medpolymer ‘Algipor’

‘Bayersdorf’

Germany Three-layered dressing of plaster type with tricot lower layer. Dressing is of the sandwich type: upper layer is crepe paper, main part is cotton balling, lower layer is tricot network. Atraumatic action is provided by the effect of dressing ‘bending’ (tunnelling effect)

Main and upper layers made from cellulose, lower one is a film of Dacron or Nylon


Table 11.2 Characteristics of dressings in brief continued No. Name

Company

Country Structure

Composition

16.

Wound absorbing dressing

‘Johnson-Joh- USA nson’

17.

Haemostatic

18.

Wound dressing

19.

Surgical dressing

20.

Dressing with perforated metallised layer

Germany First aid dressing with hydrophobic layer and lower metallised layer. Internal layers represent non-fabric pressed layer of crepe paper

Main layer made from cellulose, lower layer is aluminium spray-coated

21.

Dressing lower layer is not metallised

Germany Similar to No. 20

Cellulose with spray-coated lower layer

22.

Non-adhesive dressing

ChSSR

Dense cotton balling dressing, lower and upper layers are nonfixed Nylon networks

Cellulose and polyamide

23.

Series of experimental dressings with various quantitative viscose-cotton composition

USSR

Cellulose or viscose dressings with atraumatic layer

Three-layered dressing with perforated lower and upper layers 0.01 mm thick, main part is cotton balling, porous

Main layer made from cellulose, external layers made from polypropylene

Sweden

Double-layered dressing with perforated lower layer, sewn to the main layer

Viscose main layer, atraumatic one made from polyethylene

‘Mesorb’

France

Cotton balling or viscose dressing, crepe paper lower and upper layers

Cellulose

‘Kendall’

US A

Similar to No. 16 with cellulose base and atraumatic synthetic lower layer

Cellulose

‘Switin’

289

Handbook of Plastic Films Apart from polyurethanes, other polymeric materials (polyvinyl chloride, Nylon, etc.) have been used as the sorbing layer [22-25]. This group includes a compositional burn dressing based on a silicon film, polyamide network and hydrophilic admixture, produced by Hall Woodroof Co., (USA) [13]. Polyurethane coverings with an atraumatic lower layer made from polyglycolic acid may be considered as a variety of compositional dressings [26]. It is characteristic of dressings from this group that they preserve their high strength properties even after absorption of wound exudate. A two-component protective dressing ‘Hydron’ was recently applied in the treatment of burns. It is a film formed on the wound, and consists of a powder of poly(2-hydroxyethyl methacrylate) dissolved in polyethyleneglycol 400 [2, 27]. Although they possess good protective properties, ‘Hydron’ dressings have low strength and sorptional capacity.

11.2.3 Dressings Based on Materials of Vegetable Origin A large number of burn dressings, the so-called ‘cotton balling’, are based on cellulose, viscose or a combination of the two [28-32]. These dressings differ from each other by structure and composition of the upper and lower layers. Most often, a sorption layer based on cellulose is used in complex dressings. Such dressings are usually layered, with the separate layers being produced from either the same or different materials; the layers may be fixed mechanically or by using thermoplastic material. To decrease their adhesion to the wound surface, the lower layer is produced from various fabric and non-fabric materials (perforated Dacron, polypropylene, pressed paper, metallised fabric material, etc.). The total sorptional ability of these dressings is defined by the hydrophilicity and porosity of the basic material and is usually equal to 15-25 g/g. Data on the action of wound and burn dressings based on another vegetable material – derivatives of alginic acid – have been reported [1, 33, 34]. Typical ‘Algipor’ specimens used are based on the mixed sodium-calcium salts of alginic acid as spongy plates of about 10 mm thickness with high absorption ability.

11.3 Selection of the Properties of Tested Burn Dressings The data from the literature showed that burn dressings, particularly the first-aid ones, must perform three main functions [1, 2, 35, 36]: (1) Absorb the wound exudate, which contains metabolic products and toxins; (2) Provide optimum water, air and heat exchange between the wound and the atmosphere; (3) Protect the wound from the penetration of microorganisms from the air.

290

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Moreover, the burn dressing must be removable from the wound without further injury to the patient. Therefore, the following properties of burn dressings have been studied to determine their efficiency.

11.3.1 Sorption-Diffusion Properties The sorption-diffusion properties of dressings are extremely important, because they determine the performance of the three main functions of dressings just mentioned.

11.3.1.1 Water absorption Water is the main component of the exudate from wounds. At present, there is no opinion on how fast and to what degree the dressing must absorb the exudate in order to clean the wound from toxins and metabolic products while at the same time keeping the wound wet enough to prevent the removal of water from healthy tissue [1, 2, 35, 36].

11.3.1.2 Air penetrability Sufficient air must be allowed to penetrate the dressing, since an increase of oxygen concentration helps the healing process.

11.3.1.3 Vapour penetrability Vapour penetrability of the skin of a healthy man may reach 0.5 mg cm–2 h–1 [37]. Water loss by evaporation from burns is even higher (Table 11.3). In the absence of technical data, it may be concluded that high vapour penetrability will lead to ‘drying’ of the dressing, with a corresponding change in the surface energy of the dressing-wound

Table 11.3 Water losses by evaporation from different types of burns Surface type

Evaporation (cm3/cm2-h)

Natural skin

1-2

First degree burn

1-2.5

Second degree burn with blisters intact

2.8

Second degree burn with no damage of fermentative layer

37

Third and fourth degree burns

20-31

291

Handbook of Plastic Films interface. This will promote undesirable removal of water from the tissues, and may cause the dressing to come off the wound. Low vapour penetrability of the dressing will lead to the accumulation of liquid under the dressing, which may cause oedema.

11.3.1.4 Microorganism Penetrability Penetration of microorganisms through the dressing must be blocked to prevent infection.

11.3.2 Adhesive Properties The adhesive properties of dressings determine their ability to stay attached to the wound. Thus, the surface energy of the dressing surface facing the wound must always be lower than that of the wound surface.

11.3.3 Mechanical Properties Two mechanical properties are important for dressings: (a) flexural rigidity and (b) strength at break. The former defines the ability of the dressing to mould to the wound profile; the latter is important since it allows the dressing to be removed from the wound completely without breaking.

11.4 Methods of Investigation of Physicochemical Properties of Burn Dressings 11.4.1 Determination of Material Porosity The porosity of materials (the relation of pore space volume to total volume) is determined by the following two methods. (1) By measuring the density, and then using: l pores 1 l = + mat P∑ l ∑ Ppores l ∑ Pmat

(11.1)

where Q is the material porosity, ρ is the observable density, and ρ0 is the density of the material forming the porous medium. The value of ρ is determined by weighing

292

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings a sample of known geometrical size. The value of ρ0 is determined similarly for samples pressed at 500 GPa. (2) From photos obtained by a light microscope (MIN-10) we get: ⎡ S pores ⎤ Q=⎢ ⎥ ⎢⎣ S0 ⎥⎦

3/ 2

(11.2)

where Spores and S0 are total surface area of pores and general surface area of the material in the field of vision of the microscope, respectively.

11.4.2 Determination of Size and Number of Pores The number and size of the pores are determined with the help of the MIN-10 microscope in reflected light. The pore distribution curve (number of pores as a function of radius) is calculated; typical results are given in Figure 11.1.

Figure 11.1 Typical curves of pore size distribution for various burn dressing materials. 1: Farmexplant; 2: Syncrite; 3: Bayer brown collagen dressing; 4: Syspurderm

293


11.4.3 Estimation of Surface Energy at Material-Medium Interface The surface energy of a material-medium interface is estimated using the wetting angle of the material surface by the medium. A drop of liquid is applied to the surface of the material, and the angle is measured between the tangent at the base of the drop and the material surface. The wetting angle is determined using a horizontal microscope. The accuracy of angle measurement does not exceed ±1°.

11.4.4 Determination of Sorptional Ability of Materials The total amount of liquid sorbed by a ‘tiled’ material includes the liquid in macropores with size over 0.1 μm, that is micropores with size smaller than 0.1 μm, and that in the material matrix itself (dissolved liquid). The amount of dissolved liquid, and of liquid filling the micropores, is calculated from the vapour pressure of the sorbed liquid over the sample (sorption isotherms). The sorption isotherm for a material with micropores possesses an S-type form (Figure 11.2). The first part of the curve is connected with the real dissolved liquid, and the second part with the condensed liquid in micropores.

Figure 11.2 A typical sorption isotherm of a low molecular weight liquid by a microporous material: part ➀ of the curve represents real dissolving of the liquid by the material, and ➁ represents condensation of the liquid in the micropores within the material. Here Δm is the (change in) sorbed liquid mass; and P/P0 is the relative pressure of liquid in the thermostated vessel (where P0 is the saturated vapour pressure of the liquid under the conditions used)

294


Figure 11.3 A schematic diagram of the device for determining the absorbtion ability of porous materials. 1: sample; 2: perforated plate; 3: thermostated bath; 4: float

The maximal sorption (the amount of liquid, really dissolved and filling micro- and macropores) is determined using the device shown in Figure 11.3. The device represents a vessel with liquid medium, in which a float of special perforated square construction is placed. The float construction is calculated to prevent its sinking. This requires that the liquid medium does not penetrate through the perforations of the square, but instead forms a meniscus on the side of the square facing the porous interlayer. The change of the mass of the porous material is determined from the immersion of the float with the sample. It is measured using a horizontal microscope.

11.4.5 Determination of Air Penetrability of Burn Dressings Air penetrability (the volume of air that passes through a specific surface area during a specific time) was determined using a device specially designed for this purpose. The device is a cylindrical cell with perforated plate supporting the sample (Figure 11.4). The air was passed through the cell with the help of an air compressor, equipped with a manometer and pressure controller. The time required to fill a polyethylene sack (45 litres in volume) with air was measured. A round form sample was prepared. The sample was then placed on the perforated plate of the cell. The compressed air passed through the cell pressed on the sample. The time taken for the polyethylene sack to fill was measured. The method allows determination of the air penetrability of dry or wet materials.

295


Figure 11.4 A schematic diagram of the device for determining the air penetrability of porous materials. 1: sample; 2: perforated plate; 3: polyethylene sack; 4: manometer; 5: pressure controller

11.4.6 Determination of Adhesion of Burn Dressings Adhesion of burn dressings was investigated on a modified form of a device previously described [38]. Thus, a 1 mm thick fibreglass plate, covered with three layers of medical gauze, was placed into a fibreglass cell having a working surface of 3 × 10 mm2. The cell was filled with 5 ml of whole blood and 1 ml of 2% thrombin. The dressing to be tested was then placed on the plate surface for 1 min. The cell containing the sample was placed into a thermostat at 37 °C for 24 hours. Sample removal was performed at a 90° angle to the surface of the tested material.

11.4.7 Determination of Vapour Penetrability of Burn Dressings Vapour penetrability (the mass of water that passes through a specific surface area during a specific time) was determined using a device described elsewhere [39]. A glass vessel was filled with a known amount of liquid, e.g., water or aqueous solution of sulfuric acid. This amount provided a known relative humidity. The investigated sample was placed on the vessel surface; and a metal ring was set and pressed to the vessel by a special clamp. The vessel with the contents was weighed and placed into desiccator with

296

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings dryer at 37 °C. After measured time periods the vessel was taken out from the desiccator, weighed and then put back in the desiccator. The amount of water that has passed through the sample was determined by the mass loss of the vessel contents. The vessel dimensions used in the experiments were 40 mm diameter and 20 mm height.

11.5 Results and Discussion 11.5.1 Determination of Sorption Ability of Burn Dressings On applying a dressing to a burn wound, first wetting of the surface layer of the material occurs, followed by sorption of the wound exudate into the dressing volume. Thus it is necessary: (1) to know the components of the burn wound exudate, which need to be sorbed by the material, and the way in which sorption occurs; and (2) to determine the maximum sorption of the separate components of the exudate by the dressing material. With respect to the second item, the maximum water sorption of different materials has been determined previously [40]. For this purpose, the sample was immersed in water, dried rapidly using filter paper and then weighed. However, this method did not allow the sorption kinetics to be measured, and the accuracy of the maximum sorption was low. That is why we have developed the device for continuous measurement of sorption. The first item mentioned above has not yet been addressed in the published literature. The exudate from wounds contains water, salts, proteins, damaged cells and various low and high molecular weight (low and high molar mass) substances in relatively lower amounts. Table 11.4 shows the approximate composition of oedema liquid in a burn wound. The composition of oedema liquid changes depending on the burn degree: the worse the burn, the higher the content of protein and the lower the albumin/globulin ratio [3]. Similar data for blood plasma are also shown for comparison in Table 11.4.

Table 11.4 Composition of oedema liquid and blood plasma (g/cm3) Components

Oedema liquid

Blood plasma

Urea

5.1 × 10

5.5 × 10-4

Sugar

5.8 × 10-6

11.0 × 10-6

Protein

3.4 × 10-2

7.2 × 10-2

Salts

1.0 × 10-2

1.0 × 10-2

3. 9

1.5

Albumin/Globulin

-4

297

Handbook of Plastic Films Sorption of wound exudate may proceed via filling of micro- and macropores, or dissolving in the material matrix. Let us consider the sorption of the various components of the wound exudate by the dressing material. (1) Water fills pores and dissolves in the material matrix. Water solubility is defined by the material hydrophilicity. The solubility of water, salts and other low molecular weight substances in polymers is subject to the following rules: • In hydrophilic polymers, solubility is defined by the size and charge of the low molecular weight substance; • In hydrophobic polymers, solubility is defined by vapour pressure (the higher the vapour pressure, the higher the solubility) [41]. (2) Protein fills pores up to 10–2 m in size and may dissolve only in hydrogels of ‘Hydron’ type with water content over 30% by mass. (3) Cells fill only open pores over 0.1-0.2 μm in size.

11.5.1.1 Solubility of water in polymers As mentioned previously, modern burn dressings are heterogeneous materials, usually consisting of several layers. The upper one exposed to the air is usually more hydrophobic and less porous than the others. The solubility of water in this layer will define its evaporation from the dressing surface and the heat exchange between the wound and the surroundings. Information about solubility of water in various polymers is reported in Table 11.5 [42]. The solubility of water was determined by the sorption method. Extreme values of sorption at known water vapour pressures were calculated from the sorption curves, and then the sorption isotherms were constructed using the method described elsewhere [43]. ∞ Extreme values of solution φH at the saturation pressure were determined by extrapolation 2O ∞ of φH 2O to P/Ps = 1. The value φH of equals the solubility of water in the polymer. 2O

11.5.1.2 Maximum sorption ability of burn dressings Modern burn dressings are heterogeneous materials that either have large pores or are fibrillar, and they possess a high free volume. In contact with a wound, the exudate will fill the free volume of the dressing. The degree of filling is defined by the hydrophilicity of the material, and the size and geometry of the free volume fraction.

298


Table 11.5 Solubility of water in various polymers Solubility (102 g/g)

T (K)

Cellophane

40

303

Viscose fibre

46

303

Cotton

23

303

Cellulose diacetate

18

303

Cellulose triacetate

11.5

303

Polycaproamide

8.5

303

Polyethyleneterephthalate

0.3

303

Polydimethylsiloxane

0.07

308

Poly(2-oxyethylmethacrylate)

40*

310

Polypropylene

0.007

298

Polytetrafluoroethylene

0.01

293

Polyethylene (ρ = 0.923)

0.006

298

Polyurethane

1*

298

Polyvinyl chloride

1.5

307

Polymer

*Measured by the authors.

11.5.1.3 Maximum sorption of water by burn dressings Sorption of water by burn dressings is measured using a device developed for this purpose by the present authors. Experiments were performed in the following way. First, different masses were placed on the perforated plate of the device, and the relative immersion of the device into the water was measured, in units of the eyepiece graticule of the horizontal microscope. A calibration curve was then drawn using the coordinates ‘mass’ versus ‘depth of immersion’. The slope coefficient of this calibration curve equals 0.70 ± 0.02 g/ unit. Then, a sample of a dressing was placed into the device, and the depth of immersion during time h was measured. The mass of the medium sorbed by the material was calculated from the correlation: mc – 0.70h

(11.3)

The extreme value of the mass of sorbed medium was determined at t → ∞.

299

Handbook of Plastic Films Table 11.6 shows the experimental and theoretical [calculated from equation (11.3)] values of CH∞ 2O , and values of ρ0 determined experimentally and used for the theoretical calculations. A good correlation was observed between the experimental and theoretical values of CH∞ 2O for the majority of dressings. This shows that practically the entire free volume is filled by liquid medium for the contact of dressings with water.

Table 11.6 Experimental and theoretical data of the maximum sorption of water by burn dressings ∞ CH 2O

Covering name (material)

ρ0 (g/cm3)

Experimental

Theoretical

Helitrex (collagen)

32 ± 2

0.030 ± 0.007

33 ± 2

Helitrex (collagen sponge)

58 ± 3

0.018 ± 0.005

55 ± 3

Collagen dressing

1.8 ± 0.1

0.350 ± 0.07

2.8 ± 0.3

Corretium-2 (collagen)

3.5 ± 0.3

0.300 ± 0.07

3.3 ± 0.3

Corretium-3 (collagen)

2.1 ± 0.2

0.330 ± 0.05

3.0 ± 0.1

Combutec-2 (collagen)

77.0 ± 5.0

0.015 ± 0.005

66.0 ± 3.0

Epigard (foamy polyurethane

10.0 ± 0.3

0.067 ± 0.005

15.0 ± 1.0

Silicon-Nylon composite

7.5 ± 0.2

0.130 ± 0.03

7.7 ± 0.5

Syspurderm (foamy polyurethane)

6.2 ± 0.2

0.140 ± 0.03

7.1 ± 0.5

Syncrite (foamy polyurethane)

20.0 ± 2.0

0.050 ± 0.01

22.0 ± 1.5

Farmexplant (foamy polyurethane)

12.0 ± 0.5

0.064 ± 0.007

15.6 ± 3.0

Johnson & Johnson (cellulose)

11.4 ± 0.5

0.100 ± 0.03

10.0 ± 1.5

Blood-stopping (cellulose)

15.7 ± 0.9

0.080 ± 0.006

12.5 ± 0.7

Tunnelling (cellulose)

4.3 ± 0.2

0.200 ± 0.005

5.0 ± 0.4

Switin (cellulose)

18.0 ± 2.0

0.050 ± 0.005

20.0 ± 1.0

Metallised (cellulose-paper)

12.4 ± 0.7

0.100 ± 0.04

10.0 ± 0.5

Needle-perforated (cellulose-viscose)

28.0 ± 2.5

0.033 ± 0.007

30.0 ± 2.0

100% Viscose

25.0 ± 2.0

0.033 ± 0.007

30.0 ± 2.0

70% cotton + 30% viscose

31.0 ± 3.0

0.030 ± 0.007

33.3 ± 3.0


25.0 ± 2.0

0.035 ± 0.007

28.5 ± 2.0


28.0 ± 2.0

0.036 ± 0.007

27.7 ± 2.0

Algipor (vegetable)

30.0 ± 3.0

0.011 ±0.0002

90.0 ± 5.0

300

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings The exception is the ‘Algipor’ dressing, the large pores of which become denser on filling with water because of the collapse of the pore walls. At the end this leads to the decrease of the total volume of the dressing. The liquid medium may not fill the whole volume of the dressing if the material is sufficiently hydrophobic and poorly wetted with water. To test this assumption, seven collagen materials that differ in production method were investigated for: density, maximal water sorption, wetting angle and heat of sorption of water by the material. The latter was determined using a microcalorimeter (LKB 2107) as follows: A sample of known mass was exposed to vacuum in a thermostated Butch-type cell, and then an excess amount of water was introduced into the cell, causing the forced filling of the material volume. The results obtained are presented in Table 11.7 and Figure 11.5.

Table 11.7 Density, maximal water sorption, wetting angle and heat of sorption of water by different collagens ∞ CH (g/g) 2O

ρ0 (g/cm3)

Experimental

0.011

Theoretical

ø (deg)

ΔH (cal/g)

74

91

170

34.6

0.016

53

62.5

70

25.4

0.013

49

77

90

30.2

0.013

47

77

110

31.9

0.013

8

77

120

31.2

0.014

4

71.4

110

29.8

0.014

30

71.4

50

27.2

Figure 11.5 The dependence of the maximum sorption of water on the heat of sorption for various collagen materials

301

Handbook of Plastic Films The following conclusions can be derived from the data presented in Table 11.7: (1) The experimental value of CH∞ 2O is lower than the ‘theoretical’ one. This may be explained by two reasons: the decrease of the total volume (as in the case of ‘Algipor’), and the non-filling of a part of the material free volume by water. (2) A satisfactory correlation exists between the theoretical values of CH∞ 2O and ΔH. Thus, the main reason for the difference between the experimental and theoretical values of ∞ CH is evidently the non-filling of a part of the material free volume by water. 2O (3) The absence of a correlation between maximal water sorption and wetting angle, defined on the external surface of the material, shows that the values obtained as mentioned above do not reflect the real interaction of water with the internal surface of the collagen material.

∞ Figure 11.6 The dependence of C H on the free volume of various burn dressing 2O materials: 1, Helitrex collagen dressing; 2, Neutron collagen sponge; 3, Bayer brown collagen dressing; 4 and 5, Corretium-2 and -3 artificial leathers; 6, Combutec-2; 7, Epigard synthetic dressing; 8, Syspurderm foamy polyurethane dressing; 9, Syncrite synthetic dressing; 10, Farmexplant foamy polyurethane dressing; 11, burn face mask; 12, Biobrant compositional dressing; 13, Johnson & Johnson cellulose dressing; 14, Kendall cellulose dressing; 15, Torcatee non-adhesive cellulose dressing; 16, bloodstopping cellulose dressing; 17, Mesorb cotton balling dressing; 18, dressing with tunnelling effect; 19, cellulose dressing with non-adhesive synthetic layer; 20, Switin cotton balling dressing; 21 and 22, metallised dressings; 23 and 24, needle-perforated fabric with atraumatic layer; 25, 100% viscose; 26 to 29, viscose-cotton balling

302

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Thus, it may be concluded that, for the majority of hydrophilic burn dressings, the maximum sorption capacity with reference to water may be predicted satisfactorily. For example, the experimental values of CH∞ 2O correlate well with the free volume part of the materials (Figure 11.6), the correlation coefficient being 0.96.

11.5.1.4 Maximum sorption of plasma by burn dressings Sorption of blood plasma by burn dressings was determined by a similar method. Plasma was obtained by centrifugation of conserved blood. The treatment of the experimental results was carried out similarly to the case of the investigation of the maximum sorption ∞ of water. The value of C plasma differs from CH∞ 2O . The difference is not higher than 10%, ∞ are not presented in Table 11.7. which is why data for C plasma

11.5.2 Kinetics of the Sorption of Liquid Media by Burn Dressings The study of the kinetics of the sorption of wound exudate by burn dressings is of great importance for the estimation of their efficiency. There are difficulties in the mathematical description of the kinetics of the sorption process connected with the absence of a strictly quantitative description of dressing structure.

11.5.2.1 Structure of burn dressings Burn dressings are heterogeneous systems, consisting of several component phases. As general attention in dressings must be paid to the material possessing the maximum penetrability with reference to the liquid medium, it is necessary to classify the types of heterogeneous systems. For example, the penetrable parts of the material are placed under a layer of another weakly penetrable material in such a way that diffusing flow is perpendicular to the surface layer. This is the case for double-layered dressings with a dense external layer. The penetrable parts of the material can be dispersed in a continuous weakly penetrable phase. Dressings based on collagen and cellulose possess fibrillar structure and the fibres are randomly placed. In some cases, spatial orientation of fibres is present. The number of open pores in dressings of this type is large but the open pores possess irregular form and great tortuosity in the direction of mass transfer. Modern burn dressings are multilayer with a denser external layer. Table 11.8 shows the mean radius of macropores and their number per unit area for dressings based on polyurethane.

303


Table 11.8 Mean radius R of macropores and their number N per unit area for dressings based on polyurethane R (10–2 cm)

N (cm–2)

Epigard

2.2 ± 0.2

370 ± 10

Syspurderm

1.8 ± 0.2

266 ± 5

Syncrite

2.8 ± 0.2

275 ± 5

Farmexplant

2.2 ± 0.2

300 ± 10

Dressing name

Detailed analysis of a number of mathematical models and results of experimental investigations of heterogeneous systems has been performed by Zaikov [44]. It is known that the calculation of diffusion coefficients in heterogeneous systems is very difficult. According to ideas accepted at the present time, the penetration of liquid into a porous body is ruled by the laws of capillarity. These ideas have been successfully applied to interpret the penetration of water into paper, leather, fabrics, etc. [45, 46]. An equation that takes into account the real structure of porous bodies was obtained by Deriagin [47].

11.5.2.2 Kinetics of sorption The kinetics of sorption of water and blood plasma was investigated using the device for the maximal sorption of water. Figure 11.7 shows typical kinetic curves of sorption of water and plasma by various dressings. All curves are satisfactorily described by the equation: 1/ 2

⎡ Dt ⎤ mt = 2⎢ 2 ⎥ m∞ ⎣ πl ⎦

(11.4)

The following conclusions can be made from the data obtained: (1) Burn dressings differ significantly in their rates of sorption of liquid media; (2) The rate of sorption is determined by the pore size and the material hydrophilicity.

304


Figure 11.7 Curves for the sorption of water and blood plasma by various burn dressings. 1: water, and 2: plasma, by needle-perforated material; 3: water, and 4: plasma, by Syspurderm polyurethane dressing

11.5.3 Determination of Vapour Penetrability of Burn Dressings With multilayer dressings, the external layer, which regulates the mass transfer of water from the wound into the surroundings, is denser than the inner ones. The process of mass transfer of water through the material layers is often called aqua-, water or vapour penetrability. Penetrability and diffusion of water in polymers has been the subject of numerous investigations. The results given in some reviews and monographs [47, 48] are presented in Table 11.9. The mass transfer of water molecules in polymers possesses a list of features. In hydrophobic matrices, the interaction between water molecules and the material matrix is weak (low solubility). Nevertheless, the interaction of water molecules with each other stipulates a specific transfer mechanism. In hydrophilic materials, the interaction between water molecules and the hydrophilic groups of the material matrix stipulates high solubility of water in the matrix and increased aqua-penetrability. Consequently, high aqua-penetrability may be a property

305


Table 11.9 Penetrability and diffusion of water vapours in polymers [48] Polymer

T (K)

p p0

P × 1015 (mol m/m2 s Pa)

D × 1012 (m2/s)

Cellulose

298

1.0

8500

–

Regenerated cellulose

298

0.2

5700

0.1

Cellulose acetate

303

0.5 - 1.0

2000

1.7

Cellulose diacetate

298

1.0

15.7

–

Cellulose triacetate

298

1.0

5.5

–

Ethylcellulose

298

0.84

7950

18

Polydimethylorganosiloxane

308

0.2

14400

7000

Polyethylene (ρ= 0.922)

298

0 - 0.1

30

23


298

0 - 0.1

58.6

0.39

Polypropylene

298

0 - 0.1

17

24

Polyvinyl chloride

303

–

–

2.3

Polycaproamide

298

0.5

134

0.097

of hydrophobic as well as of hydrophilic materials; however, the causes will be different. For example, in hydrophilic polydimethylorganosiloxane, the high mobility of water molecules is stipulated by the high mobility of the chain units in this polymer. That is why, despite the low solubility of water in polydimethylorganosiloxane, the coefficient of aqua-penetrability is significant. In contrast, in regenerated cellulose, the diffusion coefficient is low because only the dissolved water molecules, which are not connected with the matrix of this polymer, participate in the mass transfer. In this case the high value of aqua-penetrability is stipulated by increasing the dissolved water content in the regenerated cellulose, which increases the fraction of the water molecules participating in mass transfer. This in turn leads to the increase of both the diffusion coefficient and the penetrability coefficient. The mass transfer of water through a porous body is practically equal to that of gases in a polymer, provided there is no interaction between water molecules and the matrix of the polymeric material. Since hydrophilic materials, which actively interact with water molecules, are commonly used for the production of dressings, diffusion should be considered simultaneously with absorption.

306

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings As a rule, the rate of the absorption process is significantly higher than the diffusion rate. Therefore, it can be assumed that the absorption equilibrium is immediately reached, and the concentration of water in the material CH 2O is obtained from the equation: ∂C H 2 O ∂t

= DH 2O

∂2CH 2O ∂x 2

−

a ∂CH 2O

(11.5)

∂t

where DH 2O is the coefficient of water diffusion in the material, x is the diffusion a is the concentration of absorbed water. coordinate, and CH 2O The concentration of absorbed water can be calculated for particular cases. For example, if the concentration of functional groups able to link water molecules irreversibly is limited and equals Cf, we can assume that the bonded water molecules no longer participate in the diffusion process, but form domains in which fast absorption occurs. For the case when the concentration of water at one of the surfaces (x = 0) is constant 0 and equals CH , the reaction zone reaches the second surface of the membrane, which 2O has thickness l, during the time t [49]. Thus, during time t there will be no water flow through the surface x = l on the membrane exterior, and then steady-state flow will be set up immediately. The amount of water passing through the membrane is given by: mH 2O = DH 2O

ΔCH 2O l

(11.6)

St

where S is the area of the membrane and

ΔCH 2O l

is the concentration gradient.

If the solubility of water in the material is ruled by Henry’s law: CH 2O = σP

(11.7)

where P is the water vapour pressure over the material, then substituting equation (11.7) into equation (11.6) gives: mH 2O = DH 2O σ H 2O

ΔP St l

(11.8)

Considering the diffusion coefficient DH2O being given by: PH 2O = DH 2O σ H 2O

(11.9)

307

Handbook of Plastic Films we obtain: PH 2O =

mH 2O

(11.10)

ΔP S t

The aqua-penetrability of burn dressings has been determined on the device described in Section 11.3. The values of the penetrability coefficients were calculated by equation (11.10). Table 11.10 shows the values of the coefficients of aqua-penetrability PH 2O for various burn dressings.

Table 11.10 Values of aqua-penetrability coefficients of burn dressings at 37 °C Dressing name (material)

PH 2 O × 10 9

(mol m/m2 s Pa)

Helitrex (collagen)

1.6 ± 0.1

Helitrex sponge (collagen)

11.0 ± 1.0

Brown dressing (collagen)

6.6 ± 0.6


0.8 ± 0.2


1.2 ± 0.2

Epigard (foamy polyurethane)

4.3 ± 0.4


3.3 ± 0.3

Biobrant (silicon-polyamide)

1.6 ± 0.16


2.0 ± 0.2

Perforated metallised dressing (cellulose)

9.0 ± 0.7

Face mask dressing (cellulose)

2.5 ± 0.2

Burn towel (cellulose)

5.4 ± 0.5


8.0 ± 0.7


8.0 ± 0.7

100% viscose

7.0 ± 0.7

11.5.4 Determination of the Air Penetrability of Burn Dressings As mentioned in Section 11.5.1, active sorption of wound exudate occurs for several minutes after putting a dressing on a burn wound. Later, the evaporation of water from the external

308

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings side of the dressing proceeds. This leads to a change in the state of the exudate in the material mass. On the whole, this changes the penetrability of the dressing with respect to air. In this case, in order for anaerobic conditions not to be created in the wound, it is necessary to provide optimal air penetrability during the entire period of application. Data on the penetrability of dressings to dry air are known in the literature. Thus, for example, it is recommended [50] to determine air penetrability with the help of the industrially produced VPTM-2 device. This device records automatically the amount of air passing through a dressing of known area during time t under pressure oscillations of about 5 mm H2O. However, the application of such a device does not allow investigation of the air penetrability of dense materials such as foamy polyurethane compositions and, most importantly, of dressings in the wet state. The construction and principle of action of a device, developed by the present authors, that allows thse determination of the air penetrability of any material in any state and under any conditions were described in Section 11.3.

11.5.4.1 Penetrability of various materials to oxygen and nitrogen The coefficient of gas penetrability (as well as the coefficient of vapour penetrability) is calculated according to equation (11.10). Literature data on the penetrability of various polymers to oxygen and nitrogen are given in Table 11.11. As the data in this table show,

Table 11.11 Penetrability and separation coefficients of gases in polymers. Polymer

Penetrability coefficient ×1015 (mol m/m2 s Pa)

Separation coefficient

O2

N2

O2 / N2

Polycaproamide

0.013

0.0033

3.8

Polyvinyl chloride

0.022

0.008

2.8

Polyurethane elastomer

0.032

0.10

3.2

Polyethylene (ρ = 0.922)

0.35

0.13

2.7

Polystyrene

3.13

0.73

2.9

Teflon

2.07

0.67

3.1

Ethylcellulose

3.2

0.93

3.4

Polydimethylsiloxane

168

83.0

2.0

Silicon rubber

200

87.0

2.3

309


Table 11.12 Values of penetrability coefficients P (mol m/m2 s Pa), diffusion D ( m 2/ s ) a n d s o l u b i l i t y σ ( m o l / m 3 P a ) o f g a s e s i n t o p o l y d i m e t h y l s i l o x a n e at 20 °C [51]. P × 1015

D × 1010

σ × 106

O2

83

23.3

36

N2

164

30

55.6

CO2

720

–

–

Gases

the penetrability of polymers may differ by four orders of magnitude. Special attention should be paid to the high gas penetrability of polydimethylsiloxane and compositions based on it, which is the result of the increased solubility of gases in them at high rates of diffusion (Table 11.12) [51].

11.5.4.2 Penetrability of porous materials filled by a liquid medium A short list of studies considering the investigation of the gas penetrability of polymeric membranes in contact with a liquid is given elsewhere [52]. It is observed that the sorption of liquid by a polymer leads to a decrease in the gas penetrability coefficient in comparison with that of the liquid-free polymer.

11.5.4.3 Air permeability Let us consider the mass transfer of air through a porous body in two cases: one in which the free volume of all the pores is filled by air, and the other with the free volume filled by a liquid medium. The porous body may be represented as consisting of two phases: the material forming the body’s matrix, and the free space. We also assume that pores have cubic form and are disposed within the volume of the body in such a way that they do not join up with each other. Such a model is sufficient for porous burn dressings. Let us determine the total thickness of the body in the direction of mass transfer, the total thickness of free space occupied by pores, and the total thickness of the layer occupied by the material.

310

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings •

The total thickness of the body in the direction of mass transfer is given by: lΣ = V/S

(11.11)

where V and S are the volume and surface area, respectively. •

The total thickness of the free space occupied by pores is given by: Qpores = lΣQ1/3 = (V/S)Q1/3

(11.12)

where Qpores = Vpores/V is the porosity. •

The total thickness of the layer occupied by the material is given by: lmat = lΣ – lpores = (V/S)(1 – Q1/3)

(11.13)

Thus, air passing through a porous body will overcome the resistance of two layers, each possessing its own penetrability coefficient with respect to air. The total penetrability coefficient PΣ of a porous body is thus given by: l pores 1 l = + mat P∑ l ∑ Ppores l ∑ Pmat

(11.14)

where Ppores and Pmat are the penetrability coefficients of the porous medium and the material forming the body’s matrix, respectively. The following equation can be used to determine the ratio of the penetrability coefficients of air for the porous body when its pores are filled with liquid and air: P∑( liq) P∑(air )

=

( Pmat / Pair ) + 1 ξ( Pmat / Pliq )

(11.15)

where: ξ = Q1/3/(1 – Q1/3)

(11.16)

Values of Pmat are shown in Table 11.10. Values of Pair and Pliq can be estimated from the coefficients of diffusion and solubility of oxygen in air, water, plasma and blood at 37 °C (Table 11.13).

311


Table 11.13 Values of the coefficients of penetrability, diffusion and solubility of oxygen in air, water, plasma and blood at 37 °C (dimensions as in Table 11.9) P

D

σ

Air

2.5 × 10-9

2.7 × 10-5

9.4 × 10-5

Water

7.4 × 10-14

3.0 × 10-9*

2.5 × 10-5*

Plasma

–

2.0 × 10-9*

–

Blood

1.4 × 10-14

1.4 × 10-9*

1.0 × 10-5*

Medium

* Values taken from [53]

Values of the penetrability coefficient of oxygen in various media may be calculated according to the following expression: P = Dσ

(11.17)

For any material Pair >> Pmat, so we obtain the simpler expression: P∑( air ) P∑( liq )

≈ξ

Pmat +1 Pliq

(11.18)

As, for the majority of dressings, ξ >> 1, and Pmat and Pliq are of the same order of magnitude, the decrease in air penetrability of a dressing when the pores fill with liquid must be significant. It has been shown by special experiments that air humidity (from 40 to 100%) does not practically influence the rate of penetration. The experiments were performed according to the following scheme. First we determined the time t0 to fill a polyethylene sack, of 45 litre volume, with air in conditions when the sample was not in the cell. This time t0 (a constant of the device) depended on the pressure in the system (p): log(1/t0) = –2.00 + 0.44 log p

(11.19)

The time for polyethylene sack filling at p = 100 Pa was selected as the standard. At T = 21 ± 1 °C, it is found that t0 = 16.0 ± 0.1 min. Subsequently, the time tx to fill the polyethylene sack when the sample was placed into the cell was similarly determined. It was observed (Figure 11.8) that the dependence of tx on p

312


Figure 11.8 The dependence of 1/log t on the pressure in the system for dry air. 1: needle-perforated material; 2: collagen sponge; 3: Syspurderm; 4: Syncrite; 5: Switin cellulose dressing; 6: Farmexplant; 7: Epigard

has the same slope as in equation (11.19) for all investigated dressings in conditions of dry air penetration: lg

1 = − Ax + 0.44 lg p tx

(11.20)

where Ax is a constant depending on the structure and properties of the dressing material. By bubbling humid air through a dressing saturated by water, the slope increased significantly. That is why it is necessary to perform several experiments for each dressing at different pressures in order to extrapolate tx to the pressure of 100 Pa with the required accuracy (Figure 11.9). The increase in the slope of log(1/tx) versus log p on bubbling air through a dressing saturated with water was attributed to the change in the material structure of the dressing resulting from the changes of form and size of the macropores. This is often accompanied by a decrease of the total volume of the dressing. The coefficient of air penetrability of the dressing (Px) was calculated according to: Px =

ml x S t (t x − t0 )

(11.21)

where m is the polyethylene sack bulk (equal to 2 mol of air at 21 ± 1 °C), and S is the surface area in contact with the bubbling air (equal to 1.8 × 10–3 m2); and p = 100 Pa. Thus,

313


Figure 11.9 The dependence of 1/log t on the pressure in the system for humid air. 1: needle-perforated material; 2: Farmexplant; 3: Combutec-2; 4: Syspurderm; 5: Biobrant (silicon-polyamide) compositional dressing; 6: Epigard

Px = 11

lx

(t x − t0 )

(11.22)

The values of air penetrability coefficients for dry dressings and dressings saturated with water are shown in Table 11.14. From this table it can be seen that a significant decrease of air penetrability takes place on saturation with water for all dressings except for ‘Biobrant’. The penetrability coefficient for dry dressings can be calculated according to the equation:

(

13 Q1 3 1 + Q = + P∑(air ) Pair Pmat

1

)

(11.23)

Pair is obtained from equation (11.17) using D = 2.7 × 10–5 m2/s and solubility at atmospheric pressure equal to 45 mol/m3. The value of Pair is 1.2 × 10–3 mol m/m2 s Pa. The values of PΣ(air) were taken from Table 11.11.Values of Pmat were calculated from equation (11.18). The values of PΣ(H2O) can be obtained from the equation:

(

1 + Q1 3 Q1 3 = + P∑(H 2O) PH 2O Pmat 1

314

)

(11.24)


Table 11.14 Coefficients of air penetrability for dry and water-saturated burn dressings at temperature of 21 ± l °C. Dressing name (material)

Coefficient of air penetrability (mol m/m2 s Pa) Dry

Wet

Helitrex (collagen)

2.7 × 10

Combutec-2 (collagen)

1.1 × 10

0


-4

1.3 × 10

1.3 × 10-5


1.3 × 10-4

1.0 × 10-6


1.1 × 10-3

4.0 × 10-5


4.5 × 10-5

0

Biobrant (polyamide + silicon)

-4

1.8 × 10

7.0 × 10-5


1.6 × 10-4

3.0 × 10-6

Needle-perforated material (cellulose)

1.1 × 10-3

–

-5

0

-3

The calculated values of PΣ(H 2O) fall close to 10–8 mol m/m2 s Pa for the majority of dressings. This result reveals the extremely low air penetrability for the listed dressings. For some dressings, the value of PΣ(H 2O) is significantly higher than 10–8 mol m/m2 s Pa. This can be explained by two effects: (1) The presence of air flow along the surface of pores (surface flow) [49]; (2) The pressure of channels in the materials that are free of water. To test these suppositions, additional investigations are required.

11.5.5 Determination of Adhesion of Burn Dressings Adhesion properties play a key role in dressing performance. The lower layer of a dressing must be easily wetted, providing good adhesion of the dressing to the wound. Besides, the surface energy at the dressing-wound interface must be minimal to provide the smallest trauma on its removal from the wound.

315


11.5.5.1 Adhesive strength: theory Adhesive strength characterises the ability of an adhesive structure to preserve its integrity. Adhesive strength as well as the strength of homogeneous solids is of kinetic nature. That is why the rates of surface tension and temperature increase affect the adhesive strength, and why the scale factors, (i.e., sample dimensions), are also of great importance. Different theories of adhesion of polymers have previously been suggested [53, 54] as follows: (1) Mechanical theory (MacBain), according to which the main role is devoted to mechanical filling of defects and pores of the surface (dressing) by the adhesive (blood); (2) Adsorption theory (Mac-Loren), considering adhesion as a result of the performance of molecular interaction forces between contacting phases – according to this theory, low adhesion, for example, may be reached between a substrate (dressing) with nonpolar groups and polar adhesive (blood); (3) Electrical theory (Deriagin), based on the idea that the main factor controlling the strength of adhesive compounds rests in the double electrical layer that is formed on the adhesive-substrate interface; (4) Diffusion theory (Vojytzky), considering the adhesion to be a result of interweaving of the polymer chains; (5) Molecular-kinetic theory (Lavrentiev), which assumes that a continuous process of restoration and breakage of bonds proceeds in the zone of adhesive-substrate contact – thus, adhesive strength is defined by the difference between the activation energies for breakage and formation of bonds, and also depends on the correlation between the amount of segments participating in the formation of bonds and the average number of molecular bonds per unit contact area. In recent years, the thermodynamic concept has received the most attention. Thus, the main role is devoted to the correlation of the surface energies of adhesive and substrate. The thermodynamic work of adhesion of a liquid to a solid (Wa) is described by the Dupret-Jung equation: Wa = γl(1 – cos θ)

(11.25)

where gl is the surface tension of the liquid, and θ is the wetting angle. Substituting Jung’s equation:

γs-l = γs – γs-l cos θ

316

(11.26)

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings into equation (11.25), we obtain the correlation: Wa = γs + γl – γs-l

(11.27)

where γs and γs-l are the surface tension of the solid and of the solid-liquid interface, respectively. It follows from equation (11.27) that, the higher Wa, the larger are the values of γs and γl while γs-l are smaller. However, according to equation (11.27), the increase of γs must lead to the growth of Wa and to an increase of γs-l. That is why the increase of the surface tension of the substrate is accompanied by the action of two effects. The necessary condition for adhesive strength is γl >> γs. Values of γl and Ws −H 2O for different materials are shown in Table 11.15.

Table 11.15 Values of the surface tension and thermodynamic work of adhesion of various materials [27] Material

γs (mN/m)

Ws −H 2O

(mN/m)


18.5

83

Silicon rubber

21.0

78

Polyethylene

31.0

99

Polystyrene

33.0

105

Polymethylmethacrylate

39.0

103

Polyvinyl chloride

39.0

101


43.0

104

Polycaproamide

46.0

107

Glass

170.0

222

11.5.5.2 Adhesive strength of dressings The adhesive strength of burn dressings was determined according to the method described in Section 11.3. Table 11.16 shows the adhesive strength of various burn dressings and the angle of wetting by water.

317


Table 11.16 Adhesive strength (A) and the angle of wetting by water (θ) of various burn dressings A (mN/m)

θ (deg)

Corretium (collagen)

220 ± 20

75 ± 2


210 ± 20

–


350 ± 50

125 ± 3


200 ± 20

130 ± 2

Bern-pack (cellulose)

170 ± 20

–

Biobrant (silicon-polyamide)

70 ± 10

–


20

–

Blood-stopping non-adhesive dressing (cellulose)

20

–

170 ± 50

–

Dressing name (material)

Dressing with metallised lower layer (cellulose)

11.6 The Model of Action of a Burn Dressing Three main processes proceed after the application of a dressing to a wound: (1) Sorption of the wound exudate by the dressing; (2) Water evaporation from the dressing surface; (3) Mass transfer of gases through the dressing under conditions of ongoing sorption and evaporation. Processes (1) and (3) were analysed in detail in Section 11.4. It was found that sorption of liquid media (water, plasma) proceeds rapidly and reaches a limiting value (maximal sorption ability) after several minutes for most dressings, i.e., a time that is significantly shorter than the time for which the dressing acts (2-3 days). The mass transfer of gases (oxygen and nitrogen) through the dressing is 2-4 orders of magnitude slower with wet samples than with the dry ones in similar conditions. Next, we consider water evaporation from the dressing surface.

11.6.1 Evaporation of Water from the Dressing Surface Suppose that a dressing is saturated with water in air at 20 °C and 50% humidity. The temperature of the dressing surface is 32 °C. These conditions are chosen to take into account the temperature gradient in the matrix of the dressing.

318

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Let us determine the amount of water that evaporates from the surface of the dressing during a given time period under stationery atmospheric pressure, and when the dressing surface is completely saturated with water. The partial pressure of air at 20 °C and 50% relative humidity equals: PH 2O = 1.26 × 10–3 kg/cm2, Pair = 1.02 kg/cm2

For air at 32 °C in the saturated state: PH 2O = 4.85 × 10–2 kg/cm2, Pair = 0.98 kg/cm2

The values of density, viscosity, heat conductivity and heat capacity of air at 26 °C equal: ρ = 1.185 kg/cm3 μ = 1.861 × 10–6 g/m s λ = 6.1 × 10–6 kcal/m s °C Cp = 0.24 kcal/°C After mathematical transformations using the method described elsewhere [55-58], the following equation for the mass transfer of water in a dressing can be obtained: W = am

P ( p1 − p2 ) RT ρ av

(11.28)

where am is the coefficient of heat conductivity, ρav is the average value of the mixture density over and near the surface of the dressing, p1 and p2 are the partial pressure PH 2O at 37 °C and 20 °C, respectively, R is the universal gas constant, and P is the normal pressure. Substituting numerical values for a dressing of 1 m × 1 m size, we obtain: W = 1.2 × 10–1 g/m2 s If the dressing surface is not completely occupied by water, we should apply the equation: W=

Csurf (H 2O) 0 Csurf (H 2 O )

× 1.2 × 10 −1 g/m 2 s

(11.29)

0 where Csurf (H 2O) and Csurf (H 2O) are the surface concentrations of water on the external side of the dressing and on the free water surface, respectively.

319


11.6.2 Sorption of Fluid by Burn Dressing from Bulk Containing a Definite Amount of Fluid Let us consider the case where a burn dressing is applied to a wound containing a definite amount of liquid. Assume that a dressing membrane of given size (thickness and surface area S) is in contact with the solution of restricted bulk volume V, which contains a concentration C0(s-s) of diffusive substance. As the dressing becomes saturated by this substance, the concentration of the latter in the bulk will decrease. The solution of the diffusion equation has the following form [55-58]: m m∞

= 1−

2a(1 − a) 1 + a + a2 q 2

⎡ 4Dq 2t ⎤ exp⎢ 2 ⎥ ⎢⎣ l ⎥⎦

(11.30)

where q is the positive solution of the characteristic equation: tgq = − aq ; a =

V (σSl )

where σ is the distribution coefficient of the substance between the membrane and the solution. When a sufficient part of the substance in solution is sorbed by the membrane, the value of a is small and a simpler expression can be used: ⎡

⎤ ⎥ mt ≈ m ⎢1 − 12⎥ ⎢ ⎥ 4πDt / l 2 ⎣ ⎦ ∞⎢

a

(

)

(11.31)

From equations (11.30) and (11.31), two important correlations can be obtained. The sorption ability of the dressing, i.e., the part of the substance sorbed from the solution under equilibrium conditions, equals: m∞/m0 = 1/(1 + a)

(11.32)

Thus, for the efficient action of the dressing, it is necessary that the concentration C be as high as possible in relation to the products of metabolism and toxins. Relating to water, CH2O ~ 1, it is desirable that the dressing volume (lS) should be close to the volume of wound exudate (V).

320

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings The time to reach the degree 0.85 of maximum sorption of liquid media by the dressing equals: t0.85 = 12

a2l 2 V2 = 12 πD πDσ 2 S 2

(11.33)

It depends on many parameters, each being able to affect the time of completion of the sorption process.

11.6.3 Mass Transfer of Water from Wound to Surroundings Generally, the change in the amount of water under the dressing in the wound ( mH 2O ) is determined from the correlation derived from equations (11.29) and (11.31):

0 mH 2O = V CH 2O

⎡ ⎢ a − mH O dressing ⎢1 − ) 2 ( ⎢ 4πDt / l 2 ⎣

(

⎤ ⎥ Csurf (H 2O) − 0 × 1.2 × 10 −1 S t 12⎥ C surf (H 2 O ) ⎥ ⎦

)

(11.34)

Let us consider the application of the correlation (11.34) for the following case. The wound characteristics are: 0 0 S = 10–2 m–2, CH = 106 g/m3, mH = 50 g 2O 2O

V = 5 × 10–5 m3, l = 10–3 m, σ H 2O = 1 Csurf (H 2O) 0 Csurf (H 2 O )

= 0.5

Under these conditions: t0.85 = 12

m∞ m

0

=

25 × 10 −10 π × 10 −9 × 10 −4

(

1 + 5 × 10

= 9.5 × 102 s# (or ~15 min)

1 −5

/ 10 −2 × 10 −3

)

= 0.17#

(or 8.5 g)

321

Handbook of Plastic Films During the same time the following amount of water will evaporate from the dressing surface: mevap ( H2O) < 0.5 × 1.2 × 10 −1 × 950 × 10 −2 = 0.6 g

i.e., the rate of evaporation is significantly (14 times) lower than that of water sorption by the dressing. All the amount of water from the wound (wound exudate) will evaporate during the time: t=

50 0.5 × 1.2 × 10 −1 × 10 −2

= 8.3 × 10 4 s# (or ~23 h)

11.7 Criteria for the Efficiency of First-Aid Burn Dressings 11.7.1 Requirements of a First-Aid Burn Dressing A first-aid burn dressing must meet the following criteria: (1) Sorption of the wound exudate, containing products of metabolism and toxic substances, during the period of dressing action (24-48 h); (2) Wound isolation from infection of the external medium; (3) Optimum air and water transfer between wound and surroundings; (4) Easy removal from the wound, causing no damage to the wound surface. The characteristics of burn wound dressings, based on the approximate estimations discussed previously, are listed next. Note that no quantitative data have been reported in the literature.

11.7.2 Characteristics of First-Aid Burn Dressings 11.7.2.1 Sorption ability of dressings A second- or third-degree burn wound releases on average 5 × 103 g/m2 of exudate. As may be seen from Table 11.11, the water amount is about 90%. The sorption of different components of the exudate proceeds at different rates. In this case, the free volume of the dressing material will be first filled with water. The diffusion of proteins and cells takes place in space occupied by water.

322

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Modern burn dressings possess a porosity of 0.9 and almost the entire free volume can be filled with water (Figure 11.5). The maximum sorption ability for such dressings equals: CH 2O ≈

ρH 2O ρ

and the amount of the liquid sorbed per unit area is: ∞ mρ ≈ C H ρl = 2O

ρH 2O ρ

ρ l ≈ 106 l g/m 2

because ρH 2O = 106 g/m3. As a first-aid burn dressing must sorb 5 × 103 g/m3, it follows that: 5 × 103 ≈ 106l

(11.35)

and therefore the thickness of a first-aid burn dressing equals: l ≈ 5 × 103/106 ≈ 5 × 10–3 m (or 0.5 cm) Thus, the first criterion for the efficiency of a first-aid burn dressing can be formulated as follows: A first-aid burn dressing must use its entire free volume for sorption. This volume must be 0.9 or more of the total volume of the dressing. Dressing thickness must be 0.5 cm or more. The majority of foreign, (i.e., non-Russian), first-aid dressings fulfil this criterion.

11.7.2.2 Air penetrability of dressings The air penetrability of most of the dry dressings ranges between 10–4 and 10–5 mol m/m2 s Pa (Table 11.14). The air penetrability of the dressings saturated with water is much lower and decreases to values between 10–6 and 10–5 mol m/m2 s Pa, that is, 0.2-2 dm3/ m2 s. Thus, the second criterion for the efficiency of first-aid burn dressings can be formulated as follows: A first-aid burn dressing must possess an air penetrability of 10–5 mol m/m2 s Pa or higher after the sorption of water. For example, the Biobrant burn dressing fulfils this criterion. 323


11.7.2.3 Adhesion of dressing to wound The adhesion strength of dressings with respect to coagulated blood (Table 11.16) varies in a wide range, but it has the minimum value of ~20 N/m. This value should be accepted as the optimal one, because it corresponds to the minimal pain and damage on removal from the surface of natural skin. Thus, the third criterion for the efficiency of first-aid burn dressing can be formulated as follows: A first-aid burn dressing must possess an adhesive strength to the wound of 20 N/m or less after the end of its action. The following burn dressings, for example, fulfil this criterion: Biobrant, blood-stopping remedy, Johnson & Johnson.

11.7.2.4 Isolation of wound from infection from external medium It is known that microorganisms causing wound infection do not penetrate through filters possessing average pores size ~0.5 μm. So the fourth criterion for the efficiency of first-aid burn dressings is as follows: A first-aid burn dressing must possess no open pores with average diameter larger than 5 × 10–7 m (0.5 μm). Moreover, it is implied that first-aid burn dressings possess sufficient mechanical strength and elasticity in both dry and humid conditions.

11.8 Conclusion Experimental methods to estimate the main physicochemical properties of burn dressings were worked out. Based on theoretical and experimental data we found the following: (1) The maximal sorption ability of a burn dressing equals the free volume of the dressing material, calculated from the value of the material density. (2) Water can be used as a model liquid in the study of sorption ability instead of blood plasma. (3) Kinetic parameters were determined from the sorption curves. These parameters showed that first-aid burn dressings markedly differ in the value of the rate of liquid media sorption at stages close to the sorption limits. (4) The air penetrability parameter in the wet state decreases abruptly by 2-3 orders of magnitude for the majority of tested dressings. This is due to the filling of pore space by the liquid medium.

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Physicochemical Criteria for Estimating the Efficiency of Burn Dressings (5) Accordingly, it is recommended that the air penetrability parameter should be determined in the wet state, which represents the common condition of action for first-aid burn dressings. (6) The value of the adhesive strength after the end of its action on the wound should not exceed 20 N/m. From the data obtained in this study, we formulated the following criteria to estimate the efficiency of first-aid burn dressings: •

Maximum sorption ability for water must be at least 10 g/g;

•

Optimal thickness of dressings, fulfilling this value of sorptional capacity, must be about 5 × 10–3 m (0.5 cm);

•

Adhesive strength must not exceed 20 N/m;

•

Average diameter of open (connected) pores must not exceed 5 × 10–7 m.

References 1.

M.I. Fel’dshtein, V.S. Yakubovich, L.V. Raskina and T.T. Daurova, Polymer Coatings for Wound and Burn Treatment, Institute of Information, Moscow, Russia, 1981, 299 (in Russian).

2.

G.B. Park, Biomaterials, Medical Devices and Artificial Organs, 1978, 6, 1.

3.

V. Rudkovsky, V. Nezelovsky, V. Zitkevich and N. Zinkevich, Theory and Practice of Burn Treatment, Meditsina, Moscow, Russia, 1988, 200 (in Russian).

4.

A. Robin and K.H. Stenzel in Biomaterials, Eds., L. Stark and G. Agarwal, Plenum Press, New York, NY, USA, 1969, 157.

5.

A. Robin, R.R. Riggio and R.L. Nachman, Transactions of the American Society of Artificial Internal Organs, 1968, 14, 1669.

6.

H.C. Grillo and I. Gross, Surgical Research, 1962, 2, 69.

7.

J. Oluwasanmi and M. Chapil, Journal of Trauma, 1976, 16, 348.

8.

G.E. Zaikov, International Journal of Polymeric Materials, 1994, 24, 1.

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J.I. Abbendhaus, R.A. McMahon, J.G. Rosenkranz and I.C. McNeil, Surgical Forum, 1965, 16, 477.

10. F.J. Richter and C.T. Riall, inventors; American Cyanamid Company, assignee; US Patent 3,566,871, 1971. 11. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York, NY, USA, 1997. 12. G.E. Zaikov, Degradation and Stabilisation of Polymers, Nova Science Publishers, New York, NY, USA, 1998. 13. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York, NY, USA, 1995, 286. 14. J.H. Gardner and D.T. Rovee, inventors; Johnson & Johnson, assignee; US Patent 3,521,631, 1970. 15. L.M. Wheeler, inventor; Parke Davis and Company, assignee; US Patent 3,648,692, 1972. 16. No inventors; Johnson & Johnson, assignee; UK Patent 1,309,768, 1973. 17. G.L. Wilks and L.L.J. Samuels, Biomedical Materials Research, 1973, 7, 541. 18. I.A. Agureev, Voenno-Meditsinskii Zhurnal, 1963, 6, 74 (in Russian). 19. P. Lock, inventor; no assignee; French Patent 2,156,068A1, 1973. 20. K. Gorkisch, E. Vaubel and K. Hopf, Proceedings of the 2nd International Congress on Plastics in Medicine, Amsterdam, The Netherlands, 1973, Paper No.16. 21. A.L. Iordanskii, G.E. Zaikov and T.E. Rudakova in Transport, Kinetics, Mechanism, VSP Science Press, Utrecht, The Netherlands, 1993, 288. 22. USSR Certificate No. 245,281, 1969, Bulletin of Certificates, No. 19. 23. W.M. Chardack, M.M. Martin, T.C. Jewett and E.M. Pearce, Plastic and Reconstructive Surgery, 1962, 30, 554. 24. C.W. Hall, D. Liotta, J.J. Chidoni, V.M.M. Lobo and A. Valente, Journal of Biomedical Materials Research, 1972, 6, 571. 25. J.J. Guldarian. C. Jelenko, D. Calloway, L. Kalle and M.Lewin, Journal of Trauma, 1973, 13, 32.

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Physicochemical Criteria for Estimating the Efficiency of Burn Dressings 26. E.E. Schmitt and R.A. Polistina, inventors; American Cyanamid Company, assignee; US Patent 3,875,937, 1975. 27. S. Madou, Ed., Polymers for Medicine, Meditsina, Moscow, Russia, 1981, 350 (in Russian). 28. USSR Certificate No. 267,010, Bulletin of Certificates, 1970, No.12. 29. G.E. Zaikov, A.L. Buchachenko and V.B. Ivanov, Ageing of Polymers, Polymer Composites and Polymer Blends, Nova Science Publishers, New York, NY, USA, 2002. 30. F.C. Moore and L.A. Perkinson, inventors; Moore-Perk Corporation, assignee; US Patent 3,678,933, 1972. 31. M.G.M. Nilsson, R.G.A.B. Udden, P.E.C. Udden and B.A. Wennerblom, inventors; Svenska cellulos Aktiebolaget, assignee, US Patent 3,654,929, 1972. 32. H. Kinkel and S. Holzman, Chirurgie, 1965, 36, 535. 33. USSR Certificate No. 6,658,148, Bulletin of Certificates, 1979, No.15. 34. M.I. Kuzin, V.K. Sologub, V.V. Yudenich, Y.B. Monakov, K.S.Minsker and A.A. Berlin, Khirurgiya, 1979, 8, 86 (in Russian). 35. I.V. Yannas and J.F. Burke, Journal of Biomedical Materials Research, 1980, 14, 65. 36. S. Jacobson and U. Rothenaw, Journal of Plastic and Reconstructive Surgery, 1976, 10, 65. 37. D. Spruit and K.E. Malten, Dermatology, 1966, 132, 115. 38. USSR Certificate No. 685,292, Bulletin of Certificates, 1979, No.34. 39. Textbook on Polymer Materials, Ed., N.A. Plate, Khimiya Publishers, Moscow, Russia, 1980, 255 (in Russian). 40. G.E. Zaikov, A.L. Iordanskii and V.S. Markin, Diffusion of Electrolytes in Polymers, VSP Science Press, Utrecht, The Netherlands, 1988, 328. 41. Y.V. Moiseev and G.E. Zaikov, Chemical Resistance of Polymers in Reactive Media, Plenum Press, New York, NY, USA, 1986, 586. 42. I.A. Barrie in Diffusion in Polymers, Eds., J. Crack and G.S. Park, Academic Press, London, UK, 1968, 452.

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Handbook of Plastic Films 43. S.I. Papkov and E.Z. Fainberg, Interaction of Cellulose and Cellulose Materials with Water, Khimiya, Moscow, Russia, 1976, 231 (in Russian). 44. G.E. Zaikov, Chemical and Biochemical Kinetics, Nova Science Publishers, New York, NY, USA, 2002. 45. S.S. Voyutskii, Physico-Chemical Principles of Fiber Materials – Sorption by Polymer Dispersions, Khimiya, Leningrad, 1969, 336 (in Russian). 46. D.A. Fridrikhsberg, Course of Colloid Chemistry, Khimiya, Leningrad, 1974, 351 (in Russian). 47. M.I. Al’tshuller and B.V. Deryagin, in Investigations in the Field of Surface Force, Nauka, Moscow, Russia, 1967, 235 (in Russian). 48. M.M. Mikhailov, Moisture Permeability of Organic Dielectrics, Gosenergoizdat, Moscow, Russia, 1960, 162 (in Russian). 49. N.I. Nikolaev, Diffusion in Membranes, Khimiya, Moscow, Russia, 1980, 232 (in Russian). 50. Textbook on Textile Materials, Legkaya Industriya, Moscow, Russia, 1974, 342 (in Russian). 51. I.M. Raigorodskii and V.A. Savin, Plasticheskie Massy, 1976, 1, 65 (in Russian). 52. V.N. Manin and A.N. Gromov, Physico-Chemical Resistance of Polymer Materials During Exploitation, Khimiya, Moscow, Russia, 1980, 247 (in Russian). 53. E. Laifut, Transfer Phenomena in Living Systems, Mir, Moscow, Russia, 1977, 520 (in Russian). 54. V.E. Basin, Adhesion Durability, Khimiya, Moscow, Russia, 1981, 208 (in Russian). 55. L.M. Batuner and M.E. Pozin, Mathematical Methods in Chemical Technology, Khimiya, Leningrad, 1971, 822 (in Russian). 56. Polymer Analysis and Degradation, Eds., A. Jimenez and G. Zaikov, Nova Science Publishers, Huntington, NY, USA, 2000, 287. 57. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York, NY, USA, 1998, 245. 58. A.Y. Polishchuk and G.E. Zaikov, Multicomponent Transport in Polymer Systems for Controlled Release, Gordon and Breach, New York, NY, USA, 1996, 231.

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12

Testing of Plastic Films E.M. Abdel-Bary and G. Akovali

12.1 Introduction Plastics are a very important group of materials. They differ from most of the ‘natural’ materials – such as metals, papers, ceramics, natural fibres – mainly as a result of their ‘viscoelastic’ behaviour. The word ‘viscoelastic’ is used to describe behaviour that shows both viscous and elastic characteristics even at ambient conditions, when stressed. This behaviour is a direct result of the long-chain nature of the polymeric molecules that constitute the plastic material. Whereas the gross mechanical behaviour of most ‘natural’ materials under stress could be considered as elastic or deformation flow, the response of all plastics to stress is a combination of the two. The ratio of viscous and elastic components, termed ‘damping’, can vary greatly over quite a narrow temperature range for plastics and it also depends markedly on the rate of stressing. One of the most common forms of plastic material is the ‘film’. Test methods for plastic films have evolved not only from the techniques of the preceding technologies. The bigger manufacturers and users have also devised their own laboratory procedures to enable them to control film properties or determine the suitability of a film for a particular process or application. In addition, research scientists have published the methods that they have used to study the theoretically interesting properties of polymers. Standards organisations have attempted to devise standard test methods acceptable to all branches of the industry. This chapter reports briefly on the most common test methods generally used for plastic films, according to the field of applications. Although most countries have their own standards and standards organisation(s), consideration here will be restricted to tests published by the American Society for Testing and Materials (ASTM). Anyone interested in the details of the ASTM tests can find them in ASTM D883 [1], which is one of the many parts of the ASTM Standards and is available in libraries or directly from the ASTM or the US Government Printing Office.

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12.2 Requirements for Test Methods 12.2.1 List of Requirements There are several requirements necessary for a test method to be developed, some of which are summarised below: (1) The test method should be rapid, so that results can be used in quality control on high-output machinery without delaying producing or dispatch. (2) Results must be reproducible and consistent between different testing stations and machines. This means that the test should be insensitive to minor variations in specimen preparation, to wear and to other small differences in test apparatus. (3) The precision of the results should be no more than is required. The cost of extreme accuracy is rarely justified in industry, and often a value that is accurate to within a few per cent will give all the information that is wanted. (4) It is preferable that the results are scientifically significant. It is imperative that they are of technological significance and give a meaningful indication of the real-life performance of the film. The main advantage of a standard method is that results obtained by its use in different laboratories can be compared.

12.2.2 Interpretation of Test Results The main difficulties encountered both in deriving significant tests for polymers and in interpreting the results are the (relatively rapid) changes in properties with rate of deformation and, particularly, with temperature. The mechanical behaviour of conventional materials is fairly insensitive to temperature in the normal range of ambient and packaging-processing temperature for the films used in the packaging industry. However, a polymer, being viscoelastic, may change from a glassy solid through a leathery and then a rubbery stage to a sticky liquid in a temperature range of less than 100 °C. This variation can be of practical importance not only for the manufacturer, who is prevented from using the high temperature sometimes demanded (for example, in print drying), but also for the designer wishing to provide packages that can be used in

330

Testing of Plastic Films environments ranging from cold storage at –30 °C to a window display in hot sunshine, where the temperature can exceed 60 °C. Viscoelasticity is a complex subject, and all polymers exhibit a similar pattern of behaviour, the details of which are determined by the chemical nature of the polymer, its molecular weight (molar mass) and molecular weight distribution, degree of crystallinity and so on. Taking polystyrene (PS) as an example of a simple amorphous polymer, one finds that the elastic modulus is constant in the temperature range up to about 100 °C, which is the glassy region. Increasing the temperature above 100 °C leads to a drastic decrease in the elastic modulus, as it exists in the leathery region. Further increase in temperature has no effect on the elastic modulus as PS falls in the rubbery region. In all these three regions – glassy, leathery and rubbery – the moduli of commercially useful polymers are independent of molecular chain length. In the last region, at temperatures exceeding about 170 °C, the polymer falls in the flow region. The basic molecular phenomena causing these different types of behaviour are reasonably well understood. In the glassy region, the long polymer molecule is frozen, with the atoms vibrating about fixed positions as in any rigid solid. In the leathery (transition) region, where the modulus changes rapidly with temperature, short-range diffusion of segments of the polymer chains takes place, but any movement is restricted to individual atoms of two or three adjacent segments, and the molecule as a whole does not move. In the rubbery region, the modulus is fairly constant; here the shortrange motions of polymer segments are very fast, and the cooperative movement of adjacent segments takes place. Entanglements restrict the length of chain that can move. In the rubbery flow region, the motion of molecules as a whole becomes important as a result of slippage of the entanglements; while in the region of flow, changes in the entire molecule take place quicker than the rate of testing, and there is little elastic recovery at this time-scale. For the last two regions, the modulus depends on the chain length and its distribution. The modulus versus temperature curve is also rate- (of testing or stressing) dependent, since the major changes in the modulus take place when a particular molecular activity is occurring at large magnitudes at rates faster than the test. This behaviour for the modulus holds true for any usual mechanical property such as yield strength, breaking strength, breaking elongation, impact strength (or total breaking energy), etc. Meanwhile, it is important to consider the parameters in the test, which are important for the proposed application, e.g., temperature, rate, humidity and geometry, so that they cover the range met in use. If not, the data of some other standard test should be able to provide the necessary, and probably the most important, information.

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12.3 Some Properties of Plastic Films Several standard methods can be used to determine the properties of plastic films. Properties can be purely physical, physico-chemical, chemical or mechanical. For the first three, usually the form of the sample does not matter, and the same methods are used for film samples as well as samples with bar shape. Most of the mechanical tests are also the same methods that are employed in testing plastics in any form, while some are specific for the plastic films. Following the procedure described in a relevant standard for tensile properties, it was shown that it is very important to define appropriately the whole set of parameters involved in the test. In addition, special adaptation of the equipment used is required. Harmonisation of the testing methods for impact and initial tear resistance proved to be more readily obtained. However, some parameters entering the corresponding measuring procedures had to be adapted. In general, harmonisation has been achieved regarding the measurement of the specific mechanical properties [2]. Some of the characteristics of these films, usually taken into consideration, are first given below, followed by the mechanical tests and then other tests.

12.3.1 Dimensions Measurement of the average thickness of a film is straightforward, and no special problems should be encountered in their measurement. The accurate measurement of film thickness is important because the values of some of the other properties – such as tensile strength, elongation at break, impact resistance, resistance to tear propagation – depend strongly on the thickness of the material. In general, the thicknesses of plastic films are several tens of micrometres, (e.g., low-density polyethylene (LDPE) agricultural films usually range from 50 μm to more than 200 μm, the latter for greenhouse films). The trend is to reduce thickness to avoid the huge amount of waste at the end of their lifetime.

12.3.2 Conditioning the Samples In general, the physical and electrical properties of plastics and electrical insulating materials are strongly influenced by the temperature and stress history of the samples (used during their preparation) as well as the humidity. In order to make reliable comparisons, it is necessary to standardise the temperature and humidity conditions to which plastics are subjected prior to and during testing. Unless otherwise specified for special polymers, the standard procedure recommended for conditioning samples prior to testing is described by ASTM D618-61/90 Procedure A [3]. In this method, for specimens thinner or thicker than 7 mm, condition the specimens for a minimum 40 h immediately prior to testing (or 88 h for the latter, over 7 mm thickness) in the

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Testing of Plastic Films standard laboratory atmosphere at 23 °C and 50% relative humidity (RH), whilst providing adequate air circulation on all sides. This can be achieved by placing the samples in suitable racks, hanging them from metal clips or laying them on widemesh, wire screen frames with at least 25 mm between the screen and the surface of the bench.

12.4 Mechanical Tests 12.4.1 Tensile Testing (Static) Tensile tests are mainly used to determine the tensile strength of a material. Such testing provides data for research, development and engineering design as well as for quality control and specification. In tensile testing there are certain difficulties with thin films. It is essential that the cut edges of the tensile specimen are free from nicks or flaws from which premature failure could start. For thinner films, grip surfaces are a problem. Both slippage in the grip and fracture of the sample at the grips must be avoided. Any technique, such as the use of a thin coating of rubber on the faces or the use of emery cloth, that prevents slipping in the grips, prevents grip fractures and does not interfere with the portion of the sample under test, is acceptable. From tensile tests, some material characteristics – such as the (tensile) modulus, percent elongation at break, yield stress and strain, tensile strength and tensile energy to break values – can be obtained. Tensile properties (static) of plastics are covered in ASTM D638 (general) [4] and ASTM D882 (films) [5].

12.4.1.1 Tensile Strength Tensile strength is calculated by dividing the maximum load by the initial crosssectional area of the specimen, and is expressed as force per unit area (usually in megapascals, MPa).

12.4.1.2 Yield Strength Yield strength is the load at the yield point divided by the initial cross-sectional area, and is expressed as force per unit area (MPa), usually to three significant figures.

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12.4.1.3 Tensile Modulus of Elasticity The tensile modulus of elasticity (or simply the elastic modulus, E) is an index of the stiffness, while the tensile energy to break (TEB, or toughness) is the total energy absorbed per unit volume of the specimen up to the point of rupture. The tensile modulus of elasticity is calculated by drawing a tangent to the initial linear portion of the load versus extension curve, selecting any point on this tangent, and dividing the tensile force by the corresponding strain. The results are expressed in MPa, and are usually reported to three significant figures. Secant modulus (used for cases where no initial linear proportionality exists between stress and strain) is defined at a designated strain. TEB is calculated by integrating the energy per unit volume under the stress-strain curve, or by integrating the total energy absorbed divided by the volume of the original gauge region of the specimen. TEB is expressed as energy per unit volume (in megajoules per cubic metre, MJ/m3), usually to two significant figures.

12.4.1.4 Tensile Strength at Break Tensile strength at break is calculated in the same way as the tensile strength, except that the load at break is used in place of the maximum load. It should be noted that, in most cases, tensile strength and tensile strength at break values are identical.

12.4.1.5 Percent Elongation at Break Percent elongation at break is the extension at the point of rupture divided by the initial gauge length. It is usually reported to two significant figures.

12.4.1.6 Percent Elongation at Yield Percent elongation at yield is the extension at the yield point divided by the initial gauge length of the specimen, usually given to two significant figures.

12.4.1.7 Package Yield of a Plastic Film A specific ASTM test method (ASTM D4321; [6]) exists for the determination of the ‘package yield’ of plastic films, in terms of area per unit mass of the sample. In this test, values such as the nominal yield (the target value of the yield as agreed between the user and supplier), package yield (yield calculated by the standard), nominal thickness (the

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Testing of Plastic Films target value of the film thickness as agreed between the user and supplier), nominal density and measured density are defined and obtained. The value of package yield is important for the manufacturer, because it determines the actual number of units or packages that can be derived from a given mass of film in a particular application.

12.4.1.8 ASTM D882 Test for Thin Films In tensile measurements, discrepancies can and usually do occur in the results, either because of the use of different specimen types with different geometries and/or because different test speeds are employed in the testing procedure. However, the data from such tests cannot be considered appropriate for applications whose load time-scales differ widely from those actually used in the test employed. In fact, the shape of the specimens suggested may be different depending on the film thickness. They are specified in different standards (such as ISO 527 for thick films [7-9], and ISO 1184 [9] and ASTM D882 for films less than 0.25 mm; [5]). A brief description of D882-95a is given next. A load range is selected such that specimen failure occurs within its upper two-thirds, for which a few trial runs are recommended. The cross-sectional area, width (to an accuracy of 0.25 mm) and thickness (to an accuracy of 0.025 mm for thin films with thicknesses less than 0.25 mm, and for thicker films to an accuracy of 1%) of the sample are measured at several points. The grip separation rate is set and the test specimen is placed in the grips and tightened evenly. The machine is started, and load versus extension values are recorded. Some characteristic tensile values of different plastic films are presented in the table given in ASTM D882-95a. LDPE is one of the weakest films used as the covering of greenhouses, in terms of tensile strength (11-37.9 MPa) [10]. As the density of polyethylene (PE) increases from LDPE to high-density polyethylene (HDPE), tensile strength at yield and stiffness values are seen to increase, while elongation and flexibilities decrease [11]. This is because the crystalline regions significantly increase the modulus of elasticity and hence the ability of the plastics to support loads at elevated temperature [12]. Another effect observed from the table in ASTM D882-95a is that of strengthening due to the molecular orientation imparted during film blowing. This is because, on a molecular level, tensile properties are higher in the direction of the covalent C–C bond in the chain than in the transverse direction, which is dominated by the much weaker van der Waals’ bonds. Since the crystals of LDPE films are preferentially oriented parallel to the machine direction, load applied in the machine direction may yield higher values of tensile strength than load

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Handbook of Plastic Films applied perpendicular to that direction. In fact, not only the direction of the film, but also the process parameters – such as melt temperature, die parameters, blow-up ratio, draw ratio, frost-line height and cooling conditions – can lead to different mechanical properties between two films with the same composition [13] (details are given in Chapter 2).

12.4.2 Impact Resistance Impact values represent the total ability of the material to absorb impact energy, which is composed of two parts: (a) the energy required to break the bonds, and (b) the work consumed in deforming a certain volume of the material. The impact resistance of plastics in general is specified by ASTM D256 [14] as the energy extracted from standardised pendulum-type hammers with one pendulum swing done either with milled notched (Izod and Charpy tests) or unnotched samples, for relatively brittle samples. The results are reported in terms of energy absorbed per unit specimen width. For tough plastic films, on the other hand, the free-falling dart method is recommended. There is one specific ASTM standard given for the impact resistance of LDPE measured by the free-falling dart method (ASTM D1709 [15] or ISO 7765-1 [16] and ISO 7765-2 [17]), which is reported in two different cases, for 260 g and 881 g (for 0.20 mm thick) film. LDPE has good toughness, which decreases with the density of the material. ASTM D1790 [17a] and D746 [18] are test methods for the routine determination of the specific ‘brittleness’ temperature at which plastics exhibit brittle failure under specified impact conditions. The first method is given for a thin (0.25 mm or less) plastic film, and the second is for real loading conditions. Thus ways to predict the behaviour of the material at low temperatures can be made, which is important for plastic films that are used in variable temperature conditions. The test applies for similar conditions of deformation, and the brittleness temperature is estimated statistically in the test as that at which 50% of the specimens would fail.

12.4.2.1 Impact Resistance by Free-Falling Dart Method The test method ASTM D1709-91 [15] covers the determination of the energy that causes a plastic film to fail under specified conditions of impact of a free-falling dart. This energy is expressed in terms of the weight (mass of the missile), falling from a specified height, that would result in 50% failure of the specimens tested. The impact resistance of a plastic film, while partly dependent on its thickness, has no simple correlation with sample thickness.

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Testing of Plastic Films The specimen for the test should be large enough to extend outside the specimen clamp gaskets at all points. The specimens will be representative of the film under study, and should be free from pinholes, wrinkles, folds or other obvious imperfections, unless such imperfections constitute variables under study.

12.4.2.2 Pendulum Impact Resistance Like other techniques to measure toughness, this test method (ASTM D256 [14]) provides a means to determine the parameters of a material at strain rates close to those applicable in some enduse applications, and the results are more valid than those provided by low-speed uniaxial tensile tests. The dynamic tensile behaviour of a film is important, particularly when the film is used as a packaging material. The same uncertainties about correlations with thickness that apply to other impact tests (such as ASTM D1709 [15]) also apply to this test. Several impact test methods are used for film samples. It is sometimes desirable to know the relationships among the test results derived by different methods. A study was conducted in which films made from two resins [polypropylene (PP) and linear low-density polyethylene (LLDPE)], with two film thicknesses for each resin, were impacted using ASTM test methods D1709 [15], D3420 [19] and D4272 [20]. Differences in results between test methods D1709 and D4272 may be expected, since test method D1709 represents failure-initiated energy while test method D4272 represents initiation plus completion energy.

12.4.2.3 Hail Resistance Although impact resistance is a valuable property to measure, the complexity and multiplicity of events occurring during impact make the value obtained applicable only under narrow conditions and not suitable for general design purposes. Thus, servicerelated impact tests have been devised for large-volume applications as greenhouse coverings. According to this method, a complete half of a greenhouse roof is built horizontally and is randomly shot with Nylon balls. The impact damage is registered with a camera. Single glass, 4 mm thick, is considered to be the reference material, and all other materials are compared to that.

12.4.3 Tear Resistance The tear resistance of a plastic film is a complex function of its ultimate resistance to rupture. There are different ASTM standards available for the tear resistance of films: ASTM D1004 [21] is designed to measure the force necessary to initiate tearing at very

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Handbook of Plastic Films low rates of loading, while ASTM D1938 [22]covers the force necessary to propagate a tear by a single tear. ASTM D1922 [23] is the determination of the average force to propagate tearing through a specified length of plastic film by use of an Elmendorf-type tearing tester. In ASTM D2582 [24], the puncture-propagation tear resistance of films is of interest. In these tests, two different values are of interest and are measured: (1) the force required to initiate the tear (ASTM D1004 and ISO 344 [25]); (2) the force needed to propagate the tear (ASTM D1938, D1922 and ISO 6383-1; [26]). ISO standards are specific to applications in greenhouses. The second value (the force needed to propagate the tear) can be considered to be of most interest, because, while it might occasionally be impossible to prevent a film from tearing in greenhouse applications, (e.g., when the film is not fastened securely, flaps in the wind, and hits a protruding part of the structure), it is highly beneficial if the tear propagates with great difficulty. Resistance to initiation of tear is also important and cannot be neglected in general. The tear resistance of plastic films is very important with regard to their overall mechanical behaviour and common failure mechanisms, i.e., for agricultural plastic films. The resistance to tear propagation for LDPE film is found to vary significantly. The reported value of resistance to tear propagation is 5-20 N [27]. Possible sources of this variation are the anisotropy, elongation effects and variable thickness of the tested films, as well as the use of different speeds during tearing.

12.4.3.1 Propagation Tear Resistance of Plastic Film and Thin Sheeting by Pendulum Method This test (ASTM D1922-94a; [23]) covers the determination of the average force to propagate tearing through a specified length of plastic film. It is widely used in packaging applications. While it may not always be possible to correlate film tearing data with other mechanical or roughness properties, the apparatus for this test method provides a controlled means for tearing specimens at straining rates approximating some of those found in actual packaging service. Owing to orientation during manufacture, plastic films and sheeting frequently show marked anisotropy in their resistance to tearing. This is further complicated by the fact that some films elongate greatly during tearing, even at the relatively rapid rates of loading encountered in this test method. The degree of this elongation is dependent in turn on film orientation and the inherent mechanical properties of the polymer from which it is made. There is no direct relationship between tearing force and specimen thickness. The tearing force is usually expressed in milli newtons (mN) or gram-force.

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Testing of Plastic Films A comparison of propagation tear resistance (Elmendorf tear) in machine direction and transverse direction of different types of plastic film is given in a table in ASTM D192294a. From the data given in this table,, it can be observed that LLDPE possesses the highest value of tear resistance in both machine and transverse directions. PP, which shows a low value of tear resistance in the machine direction, has a higher value in the transverse direction. The difference between the two directions reflects the degree of orientation and anisotropy of the material. PS orientation during processing is not remarkable, and consequently its tear resistance does not differ in the two directions.

12.4.3.2 Puncture-Propagation Tear Resistance This test method (ASTM D2582-93; [24]) covers the determination of the dynamic tear resistance of plastic film and film sheeting subjected to enduse snagging-type hazards. The puncture-propagation tear test measures the resistance of a material to snagging, or, more precisely, to dynamic puncture and propagation of that puncture resulting in a tear. Failure due to snagging hazard occurs in a variety of enduses, including industrial bags, liners and tarpaulins. The tear resistance measured by the instrument in this test is in newtons (N). Tear resistance can be measured using a standard drop height of 508 ± 2 mm or a nonstandard drop height (or carriage weight).

12.4.4 Bending Stiffness (Flexural Modulus) Test methods ASTM D747 [28] and D790 [29] cover the determination of the bending stiffness of plastic sheets and films. In the test, specimens are subjected to three- or fourpoint bending loads, such as a cantilever beam, and the force and angle of bending are used to determine the apparent flexural modulus (or bending stiffness) and yield strength.

12.4.5 Dynamic Mechanical Properties Tests by dynamic mechanical analysis (DMA) provide the elastic and loss moduli as well as the loss tangent (damping) as functions of temperature, frequency and/or time. These plots are indicative of the viscoelastic characteristics of the plastic. As the modes of molecular motion in the specimen change with temperature (or frequency), a corresponding transition temperature occurs. The most significant transition temperatures are the glass transition temperature (Tg) and the melting temperature (Tm). In addition,

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Handbook of Plastic Films there may be a number of sub-glass transition temperatures, which can be very important in determining the toughness of the material. In the temperature ranges where significant changes are observed in the modes of molecular motion, a number of mechanical properties, e.g., elastic modulus, decrease rapidly with increasing temperature (at constant or near-constant frequency) or increase with increasing frequency (at constant temperature). Hence DMA tests (provided by ASTM D4065; [30]) provide determination of transition temperatures, elastic modulus and loss modulus over a range of temperatures (from –160 °C to degradation), frequencies (0.01 to 1000 Hz) and times, by free vibration and resonant or nonresonant forced vibration techniques. DMA is usually applied for materials with elastic modulus from 0.5 MPa to 100 GPa [31]. DMA tests have been shown to be useful to evaluate a number of properties, for example, (1) degree of phase separation (in multicomponent systems), (2) effects of a certain processing treatment, and (3) filler type and amount, among others. DMA is very useful for quality control in general, for specification acceptance and in research, and it can also be used to determine, e.g., (1) stiffness and its change with temperature, (2) degree of crystallinity, (3) magnitude of triaxial stress state in rubber phase for rubber-modified plastics, etc. DMA tests incorporate laboratory practice for determining the dynamic mechanical properties of plastic films subjected to various oscillatory deformations on a variety of instruments (generally called dynamic mechanical analysers, thermomechanical analysers, mechanical spectrometers or even viscoelastometers).

12.5 Some Physical, Chemical and Physicochemical Tests 12.5.1 Density of Plastics The density of solid plastics is a conveniently measurable property, which is useful to follow the occurrence of physical changes, as well as to indicate uniformity among samples. ASTM D1505 [32] covers the method for density determination through observation of the level to which a test specimen sinks in a liquid column exhibiting a density gradient, in comparison with standards of known density.

12.5.2 Indices of Refraction and Yellowness The refractive index test is useful for controlling the purity and composition of films of transparent plastics for simple identification purposes, and it is done by use of a refractometer (ASTM D542; [33]), usually to four significant figures.

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Testing of Plastic Films For homogeneous, non-fluorescent, nearly colourless transparent and/or nearly white translucent-opaque plastic films, the yellowness index test is recommended to determine the degree of yellowness or degree of its change. Yellowness is defined as the deviation in chroma from whiteness in the dominant wavelength range from 570 to 580 nm relative to magnesium oxide for CIE Source C. In the test, data are collected using a Hardy GE type spectrophotometer or an equivalent system. A change in the yellowness index is taken as a measure of degradation (under exposure to heat, light or other environment) and has proved to be a very useful parameter for plastic films.

12.5.3 Transparency The clarity of a film is measured by its ability to transmit light in the visible region. The regular transmittance of film and sheet materials (defined as the ratio of undiffused transmitted flux to the incident flux) can be obtained by following ASTM D1746 [34].

12.5.4 Resistance to Chemicals Plastic films can be subjected to various chemicals and corrosive conditions, and their resistance to these should be tested. ASTM D543 [35] covers a general test method for all type of plastic materials. The test follows the changes in weight, dimensions, appearance and strength properties. As indicated in the test, the choice of type and concentration of reagent, duration of immersion and temperature are all arbitrary, and this poses the main limitation of the method.

12.5.5 Haze and Luminous Transmittance Light scattered from a film can produce a hazy or smoky field when viewed through the material. Haze is the cloudy or turbid appearance of an otherwise transparent material as a result of light scattered within or from the surface of the specimen. ASTM D1003 [36] provides a test method for the evaluation of specific light-transmitting and lightscattering properties of transparent plastic films. A hazemeter or a spectrometer [37] is used, which can provide very useful diagnostic data for the reason for the haze. In the test, the intensity of the incident light (I1), the total light transmitted by the specimen (I2), the light scattered by the instrument (I3) and the light scattered by the instrument and specimen (I4) are all measured. From these, the total transmittance (Tt) is calculated as Tt = I2/I1; and the diffuse transmittance (Td) is calculated from: Td =

I 4 − I 3( I2 / I1 ) I1

(12.1)

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Handbook of Plastic Films From these, the per cent haze is calculated as: haze =

Td × 100 Tt

(12.2)

Materials having a haze value greater than 30% are considered diffusing and should be tested.

12.5.6 Ignition, Rate of Burning Characteristics and Oxygen Index (OI) Most plastic films are flammable. There are three different ASTM methods available to test the ignition and rate of burning characteristics and to evaluate the per cent oxygen necessary to initiate burning, namely the oxygen index. ASTM D635 [38] covers a smallscale laboratory screening procedure to compare the relative rates of burning of selfsupporting plastic films tested in the horizontal position using a burner. ASTM D1929 [39] is for determination of the self-ignition temperature (the lowest initial temperature of air passing around the specimen at which, in the absence of an ignition source, the selfheating of the specimen leads to ignition) and flash ignition temperature (the lowest initial temperature of air passing around the specimen at which a sufficient amount of combustible gas is evolved to be ignited by a small external flame) of plastics by using a hot-air ignition furnace. The oxygen index test (ASTM D2863; [40]) covers tests that find the minimum oxygen concentration to support candle-like combustion of plastic film.

12.5.7 Static and Kinetic Coefficients of Friction The frictional properties of film surfaces may contribute markedly to film behaviour in packaging machinery and to the stacking properties of sacks. Slip agents are frequently added to film to improve its frictional behaviour. However, films containing additives often take considerable time to develop their full properties while the additives diffuse to the surface, and care must be taken in choosing the time after manufacture to carry out the test. The ASTM D1894-95 test method [41] covers determination of the coefficients of starting and sliding friction of plastic film and sheeting, when relative sliding occurs between the film and other substances under specified test conditions. The procedure permits the use of a stationary sled with a moving plane film, or the use of a moving sled with a stationary plane film. The static or starting coefficient of friction (μs) is related to the force measured to begin movement of the surfaces relative to each other. The kinetic or sliding coefficient of friction (μk) is related to the force required to sustain this movement.

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Testing of Plastic Films Measurements of frictional properties may also be made on a film or sheet specimen when sliding over itself or over another substance. The coefficient of friction is related to the slip properties of plastic films, which are of wide interest in packaging applications. These methods yield empirical data for control purposes in film production. For instance, slip properties are generated by additives in some plastic films, for example, polyethylene. These additives have varying degrees of compatibility with the film matrix. Some of them bloom, or extrude to the surface, lubricating it and making it more slippery. Because this blooming action may not always be uniform in all areas of the film surface, values from these tests may be limited in reproducibility. Besides, this blooming action of many slip additives is time-dependent. For this reason, it is sometimes meaningless to compare the slip and friction properties of films or sheets produced at different times, unless the method is designed to study this effect. Plastic films (not greater than 0.245 mm thick) and sheeting (greater than 0.245 mm thick) may exhibit different frictional properties in their respective principal directions due to anisotropy or extrusion effects. Specimens may be tested with their long dimensions in either the machine direction or transverse direction of the sample, but it is more common to test the specimen with its long direction parallel to the machine direction. The test surface must be kept free of dust, lint, fingerprints, or any foreign matter that might change the surface characteristics of the specimen. The static and kinetic coefficients of friction (μs and μk, respectively), are calculated from:

μs = As/B

(12.3)

μk = Ak/B

(12.4)

where As is the initial scale reading (g) at which motion just begins, Ak is the average scale reading (g) obtained during uniform sliding of the film surface and B = sled weight (g).

12.5.8 Specular Gloss of Plastic Films and Solid Plastics This test (ASTM D2457-90; [42]) covers the measurement of gloss of plastic films, both opaque and transparent. Specular gloss is defined as the relative luminous reflectance factor of a specimen in the mirror direction. Specular gloss is used primarily as a measure of the shiny appearance of film and surfaces. Precise comparisons of gloss values are meaningful only when they refer to the same measurement procedure and the same general type of material. In particular, gloss values for transparent films should not be compared with those of opaque films, and vice versa. Gloss is a complex attribute of a surface, which cannot be completely measured by any single number. Specular gloss usually varies with surface smoothness and flatness. The instrument

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Handbook of Plastic Films used consists of an incandescent light source to produce the incident beam, a means to locate the surface of the specimen, and a receptor located to receive the required pyramid of rays reflected by the specimen. The receptor is a photosensitive device that is responsive to visible radiation. The receptor measurement mechanism should give a numerical indication that is proportional to the light flux passing the receptor field stops within ±1% of full-scale readings. Specimen surfaces should have good planarity, since surface warpage, waviness or curvature may seriously affect test results. The direction of machine marks, or similar texture effects, should be parallel to the plane of the axes of the two beams. Surface test areas must be kept free of soiling and abrasion. Gloss is due chiefly to reflection at the surface; therefore, anything that changes the surface physically or chemically is likely to affect gloss.

12.5.9 Wetting Tension of PE and PP Films In this test method (ASTM D2578-94 [43]) drops of a series of mixtures of formamide and cellosolve (ethyleneglycol monoethyl ether) of gradually increasing surface tension are applied to the surface of the polyethylene or polypropylene film until a mixture is found that just wets the film surface. The wetting tension of the PE or PP film surface will be approximated by the surface tension of this particular mixture. The ability of PE and PP films to retain inks, coating, adhesives, etc., is primarily dependent upon the character of their surfaces, and can be improved by one of several surface-treatment techniques mentioned in Chapter 8. The same treatment techniques have been found to increase the wetting tension of PE or PP film surfaces in contact with a mixture of formamide and ethyl cellosolve in the presence of air. It is therefore possible to relate the wetting tension of a PE or PP film surface to its ability to accept and retain inks, coating, adhesives, etc. The measured wetting tension of a specific film surface can only be related to acceptable ink, coating, or adhesive retention through experience. Wetting tension in itself is not a completely reliable measure of ink or coating retention, or adhesion. A wetting tension of 3.5 × 10-2 N/m or higher has generally been found to reveal a degree of treatment normally regarded as acceptable for tubular film made from PE and intended for commercial flexographic printing. A table showing the measured wetting tension of PE and PP film as a function of the concentration of a mixture of ethyl cellosolve and formamide is given in ASTM D257894 [43]. Note that a solution is considered to wet a test specimen when it remains intact as a continuous film of liquid for at least 2 seconds. The reading of the liquid film behaviour should be made in the centre of the liquid film. Shrinking of the liquid film about its periphery does not indicate lack of wetting. Breaking of the liquid film into droplets

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Testing of Plastic Films within 2 seconds does indicate lack of wetting. Too much liquid being placed upon the film surface may cause severe peripheral shrinkage.

12.5.10 Unrestrained Linear Thermal Shrinkage of Plastic Films Unrestrained linear thermal shrinkage, expressed as the percentage of the original dimension, is defined as ‘the irreversible reduction in linear dimension at elevated temperatures where no restraint to inhibit shrinkage is present’. During the manufacturing processes, internal stresses that occur might be locked into the film, which can be released afterwards by proper heating. The temperature at which shrinkage occurs is mainly related to the processing techniques employed and may also be related to the phase transition in the base resin. The magnitude of the shrinkage varies with the temperature. Shrinkage of a particular material produced by a process may be characterised by the ASTM D2732 test method [44], by making measurements at several temperatures through the shrinkage range of the material. The experiment is usually carried out in a constant-temperature liquid bath accurate to ±0.5 °C. It is a prerequisite that the liquid of the bath should not plasticise or react with the specimen. Polyethyleneglycol, glycerine and water have been found to have wide applicability for this purpose. Immersion of the sample (100 × 100 mm2) for 10 s has been determined to be generally adequate for most thermoplastics of up to 50 μm thickness. Unrestrained linear shrinkage is calculated using: unrestrained linear shrinkage (%) =

L0 − Lf × 100 L0

(12.5)

where L0 is the initial length of side (100 mm) and Lf is the length of side after shrinkage.

12.5.11 Shrink Tension and Orientation Release Stress The ASTM D2838 test [45] measures the maximum force of a totally restrained specimen and the maximum force of a specimen permitted to shrink a predetermined amount prior to restraint in a liquid bath at selected temperatures. The results obtained are especially important and useful for shrink-wrap films and shrink-wrap packaging design.

12.5.12 Rigidity Rigidity affects the machinability of plastics. It depends mainly on the stiffness of the material, on its thickness, as well as on a number of other factors such as static electricity, frictional

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Handbook of Plastic Films properties, etc. The standard test method ASTM D2923 [46] is specific for the rigidity of polyolefin films and sheeting. In the test, the resistance of the sample to flexure is measured (by a strain gauge affixed to the end of the sample) and by use of a microammeter connected to the gauge and calibrated; rigidity is read directly as grams per centimetre of sample width.

12.5.13 Blocking Load by Parallel-Plate Method Blocking (unwanted adhesion) is a problem with plastic films, which develops during processing and/or storage, and happens when touching layers of films are in intimate contact with almost complete exclusion of air between them. Blocking is induced by increase of temperature and/or pressure. The standard test method provided by ASTM D3354 [47] simulates the operation of separating blocked films in some enduse applications. The load (in grams) needed to separate blocked samples [five groups of specimens each cut to 100 × 180 mm2] is measured by a beam-balance system (similar to an analytical balance). The test, in summary, is as follows: One sheet of the blocked specimen is secured to an aluminium block suspended from the end of the balance beam, while the other end is fixed to another aluminium block fastened to the balance base. Weight is then added equivalent to 90 ± 10 g/m to the other side of the beam until the films totally separate (or until they reach 1.905 cm separation). The film-to-film adhesion is expressed as grams, and the test is limited to maximum 200 g of load.

12.5.14 Determination of LLDPE Composition by 13C NMR The performance properties of ethylene copolymer plastic films depend on the number and type of short-chain branches. The ASTM D5017 method [48] allows one to measure them for ethylene copolymers with propylene, 1-butene, 1-octene and 4-methyl-1-pentene. For this, the polymer sample (about 1.2 g) is dispersed in a solvent (1.5 ml) and a deuterated solvent (1.3 ml), put into a 10 mm nuclear magnetic resonance (NMR) tube, and analysed at high temperatures by using 13C NMR spectroscopy, usually a 13C pulsed Fourier transform with a field strength of at least 2.35 T. Spectra are recorded under conditions such that the responses of each chemically different carbon are identical. The integrated responses for carbons originating from different comonomers are used for calculation of the copolymer composition. Results are presented as mole per cent alkene and/or branches per 1000 carbon atoms.

12.5.15 Creep and Creep Rupture Creep is defined as the increasing strain over time in the presence of a constant stress, and is expressed as the per cent extension (creep strain per cent). The practical importance

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Testing of Plastic Films of creep is due to the need (a) to determine the limits of excessive deformation and (b) to understand creep rupture. The two mechanisms are especially important for LDPE greenhouse covering materials, because LDPE, with 8.23% creep, has the second highest creep value of all greenhouse covering films [11]. There is a significant variation in the creep values of LDPE, which is attributed to the fact that the creep resistance of LDPE increases with density and with the content of ethylene-vinyl acetate (EVA) in the material’s composition. Creep is also strongly dependent on the service temperature of the covering material. ASTM D2990 [49] is a general test method for plastics to characterise creep and creep rupture. The test is applicable to different loading conditions, (e.g., tensile, flexural, compressive, etc.), and helps to determine the creep strength and modulus of standard specimens for use in comparing materials and in design.

12.5.16 Outdoor Weathering/Weatherability The ASTM D1435 test [50] is used to evaluate the stability of plastic films when exposed outdoors to the varied influences of the atmosphere and weather. The general climate, the season, the time of day, the presence or absence of industrial pollutants in the atmosphere, and annual variations in the weather are the most important factors, and the results are taken as indicative only. Short-term accelerated exposure tests are also available by use of a special chamber equipped with a carbon-arc light (ASTM G152 [51] and ASTM G153 [52]), which can indicate the relative outdoor performance, but cannot be used to predict the absolute long-term performance.

12.5.17 Abrasion Resistance Abrasion is a surface phenomenon that occurs mechanically, and it is important in the sense that it can significantly degrade certain physical properties (light transmission, thermal effect through loss of thickness, etc.), as well as some mechanical properties, (e.g., impact resistance, tear resistance). As a result it has a direct impact on the functional characteristics of covering materials. Abrasive damage to transparent plastic films is judged by following the change of the optical properties (ASTM D1044; [53]) as well as by volume loss in general (by using abrasion testing machines, ASTM D1242; [54]). The abrasion resistance of plastic films used in greenhouses is of utmost importance. Abrasion, in this case, occurs due to the effect of particles carried by the wind, which can be significant in some areas where greenhouses are built. In this case, abrasion can lead to the loss of transparency and reduction in mechanical properties much earlier than expected. Abrasion in general is affected by the exact formulation of the film, and by the incorporation of (amount and type of) filler, additives and pigments, which can lead to varying results. Another important factor is that rapid chemical oxidation of the surface layer may occur due to the buildup of localised high temperatures during abrasion [55].

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Handbook of Plastic Films It is worth mentioning that abrasion can ultimately lead to increased degradation of the film, since more active centres for photooxidation are in general created by this procedure. There is a direct relationship between the density of a PE film and its abrasion resistance: increase of density increases the abrasion resistance.

12.5.18 Mar Resistance Test ASTM D673 [56] covers the determination of the extent of resistance of plastic film surfaces to surface marring, mainly caused by falling abrasive particles. The test simulates the relatively mild airborne abrasive action that occurs in actual use, and different materials are ranked according to their relative mar resistances.

12.5.19 Environmental Stress Cracking The ability of a polymer surface to withstand an aggressive medium under load is known as environmental stress-cracking resistance (ESCR). Environmental stress cracking is a characteristic that depends on the nature and level of stresses applied as well as on the thermal history of the sample and the environment, and is also called stress corrosion [57]. Under certain conditions of stress and in the presence of certain environments, environmental stress cracking occurs. For example, in the presence of soaps, wetting agents and detergents, ethylene plastics may exhibit mechanical failure by cracking. Typically, increased ESCR is obtained with increased polymer molecular weight. ASTM D1693 [58] is specific for the environmental stress cracking of ethylene plastics. A stress crack is an external or internal rupture in the film caused by tensile stresses less than its short-time mechanical strength. The environment accelerates the development of stress cracks. The appearance of what seem to be cracks on the surfaces of transparent polymers develops under tensile stress, with the plane of the craze being normal to the stress direction. Crazes usually initiate at surfaces but can develop internally under special circumstances as well. They reflect light in a manner similar to cracks, and indeed often precede early fracture of the film. In the test, bent specimens, each having a controlled imperfection on one surface, are exposed to the action of a surface-active agent, and the proportion of the total number of specimens that crack in a given time is reported.

12.5.20 Water Vapour Permeability In the packaging of hygroscopic materials, and particularly in packaging of food, the permeability properties of the film to water vapour and other gases is very important.

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Testing of Plastic Films ASTM D3079 [59] and E96 [60]/F372 [61]/F1249 [62] cover standard test methods for plastic packaging films and plastics for water vapour permeability. These tests are especially important and are used for packaging plastic films. In the first of these, desiccant- or product-filled packages are exposed to a normal atmosphere of 90 ± 2% RH at constant temperature, and weighings are repeated to constant rate of moisture gain. Water vapour permeabilities are reported in grams per 30 days. In the second test method, desiccant- or product-filled packages are again exposed to a normal atmosphere of 90 ± 2% RH at two different temperatures for 24 hours and 6 days, respectively. Hence cycling between cold and hot/moist atmospheres is achieved. In this test, weighings are repeated to constant rate of moisture gain, and water vapour permeability is reported in grams per cycle. In the third method mentioned, desiccant- or product-filled packages are exposed to a normal atmosphere of 90 ± 2% RH at constant temperature for at least 1 month; average rate of water gain is reported. In the last two methods, infrared (F372) and modulated infrared (F1249) detection of water vapour transmitted from a moist atmosphere to a dry air stream is made, which provides a measure of water vapour transmission rates.

12.5.21 Oxygen Gas Transmission ASTM D1434 [63] and D3985 [64] cover standard test methods for packaging plastic films and sheeting materials for their oxygen gas transmission. Basically the methods used can be divided into three types, varying either pressure, volume or concentration. In variablevolume methods, gas is introduced at a high pressure on one side of the film, the chamber on the other side normally being at atmospheric pressure. The change in volume is followed as a function of time. A manometer is used to measure the pressure of oxygen transmitted, from which the rate of transmission at steady state can be calculated. In another method, a coulometric sensor is used, which measures the rate of oxygen transmitted through a specimen exposed on one surface to oxygen and on the other to nitrogen. Considerable experimental difficulties are normally encountered in achieving airtight seals and in the initial calibration of the instrument to allow for the deadspace in the filter paper and discs used to support the film

12.6 Standard Specifications for Some Plastic Films There are several standards available to specify plastic films, such as: •

ASTM D5047 [65] for polyethylene terephthalate (PET) films;

•

ASTM D4635 [66] for LDPE films;

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Handbook of Plastic Films •

ASTM D3981 [67] for medium-density polyethylene (MDPE) films;

•

ASTM D2103 [68] for PE films;

•

ASTM D2673 [69] for oriented polypropylene (OPP) films;

•

ASTM D2647 [70] for crosslinkable ethylene plastics.

12.6.1 Standard Specification for PET Films Specification ASTM D5047 [65] covers biaxially oriented PET films in the range of 1.5-35.5 μm, containing at least 90% PET homopolymer. The thicknesses should be within ±18% to ±14% of nominal for film tested in accordance with ASTM D374 [71]; while the requirements for the width (within ±1.6 mm and ±3.2 mm of nominal for rolls up to 1 m or larger, respectively), and weight (within ±10% and ±5% for orders up to 110 kg or over, respectively), are also given. The film will be tested appropriately to establish conformance to the critical requirements as agreed by the purchaser and seller.

12.6.2 Standard Specification for LDPE Films (for General Use and Packaging Applications) Specification ASTM D4635 [66] covers unpigmented, unsupported, tubular LDPE films with densities between 910 and 925 kg/m3 (0.910-0.925 gm/cm3), for general use and for packaging applications. It is also applicable to polyethylene copolymer (low-pressure PE and LLDPE) as well as for blends of homopolymers and copolymers, including ethylenevinyl acetate copolymers. The thicknesses are 100 μm or less and the maximum widths are 3 m. The specification covers dimensional tolerances (including thickness, width, length and yield), intrinsic quality requirements (density, workmanship, tensile strength, heat sealability, odour, impact strength, coefficient of friction, optical properties, surface treatment, etc.), and test methods.

12.6.3 Standard Specification for MDPE and General Grade PE Films (for General Use and Packaging Applications) Specification ASTM D3981 [67] is for unpigmented, unsupported, sheet or tubular MDPE films with densities between 926 and 938 kg/m3 (0.926-0.938 g/cm3), for general use and for packaging applications. It is also applicable to polyethylene copolymer

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Testing of Plastic Films (low-pressure PE and LLDPE) as well as for blends of homopolymers and copolymers, including ethylene-vinyl acetate copolymers. The thicknesses are 25-100 μm and the maximum widths are 3.05 m. The standard excludes heat-shrinkable films. The specification covers dimensional tolerances (including thickness, width, length and yield), intrinsic quality requirements (density, workmanship, tensile strength, heat sealability, odour, impact strength, coefficient of friction, optical properties, surface treatment, etc.) and test methods. Specification ASTM D2103 [68] covers general specifications for polyethylene films.

12.6.4 Standard Specification for OPP Films Specification ASTM D2673 [69] covers OPP films of 10-50 μm thickness with ±10% of the nominal value, composed of Group 1 or 2 propylene (ASTM D4101 [72]), or a blend of such Group 1 and/or Group 2 polypropylene with one or more other types of polymers where the polypropylene fraction is the main component. It must have normal appearance (be free of gel, streaks, pinholes, particulates, etc., as well as undispersed raw materials) and it should not block excessively. The average width will be within –3 to +19 mm of nominal. If the film yields a minimum tensile strength of 103 MPa in at least one principal (machine or transverse) direction, it is termed oriented polypropylene (OPP). If the film is oriented in one (machine or transverse) direction and yields a minimum tensile strength of 103 MPa in the orientation direction, it is called as uniaxially oriented PP film. If the film tensile strengths in both the machine and transverse directions exceed 103 MPa, it is biaxially oriented PP. If the film tensile strengths in both the machine and transverse directions exceed 103 MPa, but do not differ by more than 55 MPa, and the machine and transverse elongations do not differ by more than 60%, it is balanced oriented PP.

12.6.5 Standard Specification for Crosslinkable Ethylene Plastics Specification ASTM D2647 [70] covers crosslinkable ethylene plastics and compounds. There are mainly two different types: mechanical types (type I) and electrical types (type II). In the former, mechanical properties (strength, ultimate elongation, elongation retention after ageing, apparent modulus of rigidity, brittleness temperature) are the most important in applications.

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References 1.

D833, Terminology Relating to Plastics, 2000.

2.

D. Briassoulis and A. Aristopoulou, Polymer Testing, 2001, 20, 615.

3.

ASTM D618-00, Standard Practice for Conditioning Plastics for Testing, 2000.

4.

ASTM D638-02, Standard Test Method for Tensile Properties of Plastics, 2002.

5.

ASTM D882-02, Standard Test Method for Tensile Properties of Thin Plastic Sheeting, 2002.

6.

ASTM D4321-99, Standard Test Method for Package Yield of Plastic Film, 1999.

7.

ISO 527-1, Plastics - Determination of Tensile Properties - General Principles, 1994.

8.

ISO 527-2, Plastics - Determination of Tensile Properties - Test Conditions for Moulding and Extrusion Plastics, 1994.

9.

ISO 527-3, Plastics - Determination of Tensile Properties - Part 3: Test Conditions for Films and Sheets, 2001.

10. D. Briassoulis, D. Waayenberg, J. Gratraud and B.J. Von Elsner, Journal of Agricultural Engineering Research, 1997, 67, 81. 11. P.C. Powell, Engineering Design Guides, 1979, 19, 1. 12. G. Gruenwald, Plastics: How Structure Determines Properties, Hanser, Munich, Germany, 1992. 13. R.M. Patel, Polymer Engineering Science, 1994, 34, 1506. 14. ASTM D256-00e1, Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics, 2000. 15. ASTM D1709-01, Standard Test Methods for Impact Resistance of Plastic Film by the Free-Falling Dart Method, 2001. 16. ISO 7765-1, Plastics Film and Sheeting - Determination of Impact Resistance by the Free-Falling Dart Method - Part 1: Staircase Methods, 1999. 17. ISO 7765-2, Plastics Film and Sheeting — Determination of Impact Resistance by the Free-Falling Dart Method — Part 2: Instrumented Puncture Test, 1999.

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Testing of Plastic Films 17a. ASTM D1790-02, Standard Test Method for Brittleness Temperature of Plastic Sheeting by Impact, 2002. 18. ASTM D746-98e1, Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact, 1998. 19. ASTM D3420-95, Standard Test Method for Pendulum Impact Resistance of Plastic Film, 1995. 20. ASTM D4272-99, Standard Test Method for Total Energy Impact of Plastic Films By Dart Drop, 1999. 21. ASTM D1004-94a, Standard Test Method for Initial Tear Resistance of Plastic Film and Sheeting, 1994. 22. ASTM D1938-02, Standard Test Method for Tear-Propagation Resistance (Trouser Tear) of Plastic Film and Thin Sheeting by a Single-Tear Method, 2002. 23. ASTM D1922-00a, Standard Test Method for Propagation Tear Resistance of Plastic Film and Thin Sheeting by Pendulum Method, 2000. 24. ASTM D2582-00, Standard Test Method for Puncture-Propagation Tear Resistance of Plastic Film and Thin Sheeting, 2000. 25. ISO 344, Textile Machinery and Accessories - Spinning Machines - Flyer Bobbins, 1981. 26. ISO 6383-1, Plastics - Film and Sheeting - Determination of Tear Resistance Trouser Tear Method, 1983. 27. F. Henninger in Handbook of Polymer Degradation, Eds., S.H. Hamid, M.B. Amin and A.G. Maadhah, Marcel Dekker, New York, NY, USA, 1992, 411. 28. ASTM D747-02, Standard Test Method for Apparent Bending Modulus of Plastics by Means of a Cantilever Beam, 2002. 29. ASTM D790-02, Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials, 2002. 30. ASTM D4065-01, Standard Practice for Plastics: Dynamic Mechanical Properties: Determination and Report of Procedures, 2001. 31. J.D. Ferry, Viscoelastic Properties of Polymers, 2nd Edition, Wiley, New York, NY, USA, 1961.

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Handbook of Plastic Films 32. ASTM D1505-98e1, Standard Test Method for Density of Plastics by the Density-Gradient Technique, 1998 33. ASTM D542-00, Standard Test Method for Index of Refraction of Transparent Organic Plastics, 2000. 34. ASTM D1746-97, Standard Test Method for Transparency of Plastic Sheeting, 1997. 35. ASTM D543-95 (2001), Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents, 2001. 36. ASTM D1003-00, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, 2000. 37. F.W. Billmeyer, Jr. and Y. Chen, Color Research and Application, 1985, 10, 219. 38. ASTM D635-98, Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position, 1998. 39. ASTM D1929-96(2001)e1, Standard Test Method for Determining Ignition Temperature of Plastics, 2001. 40. ASTM D2863-00, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), 2000. 41. ASTM D1894-01, Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, 2001. 42. ASTM D2457-97, Standard Test Method for Specular Gloss of Plastic Films and Solid Plastics, 1997. 43. ASTM D2578-99a, Standard Test Method for Wetting Tension of Polyethylene and Polypropylene Films, 1999. 44. ASTM D2732-01, Standard Specification for Polyethylene (PE) Plastic Tubing, 2001. 45. ASTM D2838-02, Standard Test Method for Shrink Tension and Orientation Release Stress of Plastic Film and Thin Sheeting, 2002. 46. ASTM D2923-01, Standard Test Method for Rigidity of Polyolefin Film and Sheeting, 2001. 47. ASTM D3354-96, Standard Test Method for Blocking Load of Plastic Film by the Parallel Plate Method, 1996.

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Testing of Plastic Films 48. ASTM D5017-96, Standard Test Method for Determination of Linear Low Density Polyethylene (LLDPE) Composition by Carbon-13 Nuclear Magnetic Resonance, 1996. 49. ASTM D2990-01, Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics, 2001. 50. ASTM D1435-99, Standard Practice for Outdoor Weathering of Plastics, 1999. 51. ASTM G152-00ae1, Standard Practice for Operating Open Flame Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials, 2000. 52. ASTM G153-00ae1, Standard Practice for Operating Enclosed Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials, 2000. 53. ASTM D1044-99, Standard Test Method for Resistance of Transparent Plastics to Surface Abrasion, 1999. 54. ASTM D1242-95a, Standard Test Methods for Resistance of Plastic Materials to Abrasion, 1995. 55. V. Shah, Handbook of Plastic Testing Technology, Wiley, New York, NY, USA, 1984. 56. ASTM D673 discontinued not replaced 57. R.P. Kambour and A.S. Holik, Journal of Polymer Science, A-2: Polymer Physics, 1969, 7, 1393. 58. ASTM D1693-01, Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics, 2001. 59. ASTM D3079-94 (1999), Standard Test Method for Water Vapor Transmission of Flexible Heat-Sealed Packages for Dry Products, 1999. 60. ASTM E96-00e1, Standard Test Methods for Water Vapor Transmission of Materials, 2000. 61. ASTM F372-99, Standard Test Method for Water Vapor Transmission Rate of Flexible Barrier Materials Using an Infrared Detection Technique, 1999. 62. ASTM F-1249-01, Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor, 2001. 63. ASTM D1434-82(1998), Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting, 1998.

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Handbook of Plastic Films 64. ASTM D3985-02, Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor, 2002. 65. ASTM D5047-95, Standard Specification for Polyethylene Terephthalate Film and Sheeting, 1995. 66. ASTM D4635-01, Standard Specification for Polyethylene Films Made from Low-Density Polyethylene for General Use and Packaging Applications, 2001. 67. ASTM D3981-95, Standard Specification for Polyethylene Films Made from Medium-Density Polyethylene for General Use and Packaging Applications, 1995. 68. ASTM D2103-97 Standard Specification for Polyethylene Film and Sheeting, 1997. 69. ASTM D2673-99, Standard Specification for Oriented Polypropylene Film, 1999. 70. ASTM D2647-94 (2000) e1, Standard Specification for Crosslinkable Ethylene Plastics, 2000. 71. ASTM D374, Standard Test Methods for Thickness of Solid Electrical Insulation, 1999. 72. ASTM D4101, Standard Specification for Polypropylene Injection and Extrusion Materials, 2002.

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13

Recycling of Plastic Waste E.M. Abdel-Bary

13.1 Introduction Polymeric materials (plastics and rubbers) comprise a steadily increasing proportion of the municipal and industrial waste going into landfill. Owing to the huge amount of plastic wastes and environmental pressures, recycling of plastics has become a predominant subject in today’s plastics industry. Development of technologies for reducing polymeric waste, which are acceptable from the environmental standpoint and are cost-effective, has proven to be a difficult challenge, because of the complexities inherent in the reuse of polymers. Establishing optimal processes for the reuse/recycling of polymeric materials thus remains a worldwide challenge in the new century. Compared with other countries, there is a huge amount of plastic waste in the USA (taken as a reference), where five main types of polymers dominate in the plastics waste stream. The highest polymer waste results from low-density polyethylene (LDPE), at 5 million tons per year; high-density polyethylene (HDPE) is second, at 4.1 million tons; then come polypropylene (PP) at 2.6 million tons, followed by polystyrene (PS) at 2 million tons and polyethylene terephthalate (PET) at 1.7 million tons [1]. These five polymer types, together with polyvinyl chloride (PVC), also dominate the plastics waste stream in the European Community [2]. Plastic films find applications in agriculture as well as in plastic packaging, which is a high-volume market owing to the many advantages of plastics over other traditional materials. However, such material is also the most visible in the waste stream, and has received a great deal of public criticism as films have comparatively short life-cycles and usually are non-degradable [3]. The majority of plastic films are made from LDPE or linear low-density polyethylene (LLDPE), comprising approximately 68% of the total film production. In addition, HDPE resins are commonly used in film plastics. Non-polyethylene resins constitute the remainder of the film plastic types found in the market place. PP, PVC and Nylon resins comprise the bulk of these other film types. Increasingly, certain multilayer or coextruded films are used in special applications that seek to combine the performance attributes of two or more resins for such applications.

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Handbook of Plastic Films Businesses can save money by reducing their disposal expenses, in the form of both tonnagebased tipping fees and container hauling fees. This is especially evident with plastic films, where a high volume-to-weight ratio can mean more container pulls per ton hauled.

13.2 Main Approaches to Plastic Recycling There are four main approaches to recycling plastics (excluding, as not acceptable, dumping on land or at sea with or without prior treatment) [4]. These are primary, secondary, tertiary and quaternary recycling.

13.2.1 Primary Recycling This is the recycling of clean, uncontaminated, single-type waste, and it remains the most popular as it ensures simplicity and low cost, especially when done ‘in-plant’ and fed with scrap of controlled history [5]. The recycled scrap or waste is either mixed with virgin material to assure product quality or used as second-grade material [6]. Primary recycling is very simple without any precautions except the proper and clean collection of the waste in the plant.

13.2.2 Secondary Recycling 13.2.2.1 Approaches to Secondary Recycling There are two main approaches to secondary recycling. One approach is to separate the plastics from their contaminants and then segregate the plastics into generic types, one or more of which is then recycled into products produced from virgin or primary recycled material. The other approach is to separate the plastics from their associated contaminants and remelt them as a mixture without segregation. The treatment of the plastics-containing waste streams may include: size reduction by granulators, shredders or crumblers; separation of plastics from other waste materials and from one another; cleaning; drying; and compounding [7, 8]. The actual order and number of operations in a particular treatment system depends on the waste being processed and the desired quality of the final material [9].

13.2.2.2 Mechanical Recycling Mechanical recycling is mainly related to secondary recycling. The main steps include separating, sorting and washing to get rid of contamination, especially for plastic films,

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Recycling of Plastic Waste which possess a large surface area and consequently have a large degree of contamination. In a chemical recycling plant, one should have shredders, metal and mineral separators, prewashing and granulation, second washing stage, mechanical grinding and dirt removal, hydrocyclone separation, dewatering and melt processing. The separation of plastic waste is one of the main factors restricting high performance in plastic recycling. The separation of plastics into desired categories as well as the elimination of contaminants is an ongoing technological development process. The aim is to develop automatic and continuous separation technology to minimise the handling of waste and to achieve a more efficient recycling process. Probably the best alternative for pure plastic streams is not to allow them to mix in the first place, neither among themselves nor with contaminants. If separation starts at the consumer level and at the source point of collection, there will be fewer difficulties during the recycling.

13.2.3 Tertiary Recycling Tertiary recycling includes chemical recycling. The terms ‘chemical recycling’ and ‘feedstock recycling’ of plastics are sometimes collectively referred to as ‘advanced recycling technologies’. In these processes, solid plastic materials are converted into smaller molecules as chemical intermediates through the use of heat. These chemical intermediates, usually liquids or gases, but sometimes solids or waxes, are suitable for use as feedstocks for the production of new petrochemicals and plastics. The technical and economic feasibility and overall commercial viability of advanced recycling methods must be considered in each step of the recycling chain, consisting of collection, processing and marketing. All of them are critical to the success of chemical and feedstock recycling. Today, most of these technologies remain developmental and have not yet proven themselves sustainable in a competitive market. Nevertheless, they remain of considerable interest in their longer-term potential. The term ‘feedstock recycling’ encompasses chemical recycling but is often applied to the thermal depolymerisation of polyolefins and substituted polyolefins into a variety of smaller hydrocarbon intermediates. Fluidised bed pyrolysis investigations of LDPE have provided data on the suitability of the process and on the influence of the process conditions on the compatibility of the feedstock produced with the conventional petroleum feedstock [10]. The gases produced from the pyrolysis of LDPE are mainly hydrogen, methane, ethane, ethylene, propane, propene, butane and butene. Also, it has been reported that the thermolysis products of HDPE consist of 80-90 wt% straight-chain alkanes and 1-alkenes. Subsequent hydrogenation of the PE oil resulted in a diesel fuel with high cetane index and low sulfur and aromatic contents [11].

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Handbook of Plastic Films In some cases, addition polymers such as polystyrene and polymethyl methacrylate can be thermally depolymerised back to their corresponding monomers in reasonably high yield. The term ‘chemical recycling’ is often applied also to the depolymerisation of certain condensation polymers back to monomers. Examples of these types of plastics are polyesters, polyamides and polyurethanes. Chemical recycling thus mainly includes pyrolysis, gasification, hydrogenation, hydrolysis, glycolysis and depolymerisation. A new reactor system was developed for the recovery of fuels from waste plastic mixtures in a steam atmosphere. The degradation mechanisms of two polyolefins (PE and PP), two polyamides (Nylon-6 and Nylon-6,6), polystyrene and three polyesters (polycarbonate, polybutylene terephthalate and polyethylene terephthalate) in both nitrogen and steam as the carrier gas have been investigated [12]. The oil produced from the proposed reactor system was continuously upgraded to produce gasoline and kerosene over a Raney nickel catalyst in a steam atmosphere.

13.2.4 Quaternary Recycling Quaternary recycling includes the recovery of the energy content of plastic wastes. Owing to a lack of other recycling possibilities, incineration (combustion) aimed at the recovery of energy is currently the most effective way to reduce the volume of organic material. This may then be disposed of to landfill. Plastics, either thermoplastics or thermosets, are actually high-yielding energy sources. For example, one litre of heating oil has a net calorific value of 10,200 kcal, whereas 1 kg of plastics releases 11,000 kcal worth of energy; for comparison, it should be added that 1 kg of briquettes (blocks of pressed coal dust) has a net calorific value of 4,800 kcal. It has been estimated that, by burning 1 ton of organic waste, approximately 250 litres of heating oil could be saved [13]. Clean incineration of municipal solid waste (MSW) is widely accepted in countries like Sweden and Germany (50% of total MSW), Denmark (65%), Switzerland (80%) and Japan (70%) [14]. Although there are very stringent emissions regulations, more than 50 refuse incineration units are working in Germany. The energy that can be recovered from the incineration of plastics depends on the type of plastic. It has been estimated (in kcal/kg) as: 18,720 for PE; 18,343 for PP; 16,082 for PS; 13,179 for phenol-formaldehyde; 11,362 for foamed polyurethane (PU); 10,138 for Nylon; 8,565 for polyvinyl acetate (PVAc); 7,516 for PVC; and 7,014 for PU. This energy is on average 10,000 kcal/kg. Each ton will release about 107 kcal. However, plastics emit some objectionable gases and form some hazardous compounds. Thus, recovering energy from plastic waste is not cheap. The main goal must be to avoid the formation of these hazardous compounds by the correct construction of incinerators

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Recycling of Plastic Waste and by considering all proper means to avoid pollution. The costs of operating, maintaining and monitoring an incinerator are quite substantial compared to those for a conventional power plant. Energy from waste (EFW) is a reliable and renewable source of energy, especially if the MSW is rich in organic matter. Furthermore, it reduces the amount of waste to be landfilled at the final stage. Costs involved in developing new landfills can partly offset the high costs of energy recovery from an EFW facility. Incinerators with EFW installations are not considered just as power plants, as their main purpose is to reduce the amount of garbage being landfilled within the purpose of an integrated waste management system. Incineration plants should be designed and operated to produce the least amount of pollution. The use of incineration plants is mandatory for plastic wastes from hospitals and similar institutions, which is considered as a potential source of disease. Incinerators do not emit ethane gas, as this gas is completely combusted into CO2 and water, even at low temperature. However, incinerators have often been associated with dioxin and furan emissions, which are avoided in modern ones by working at temperatures that are high enough to decompose such chemicals and prevent them from reaching the ecosystem. Although dioxin and furan are often perceived as two individual chemical products, there are in fact 75 congeners of polychlorinated dibenzodioxins (PCDD) and 135 congeners of polychlorinated dibenzofurans (PCDF), each differing in its chemical configuration and degree of toxicity. The most toxic of the dioxins is 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD). When assessing health risks, the total congeners are converted to the equivalent TCDD. Dioxins and furans are present in Nature and are generated by many sources, such as forest fires and as a by-product of a certain chemical processes and the burning of wood in stoves and fireplaces, barbecues, diesel engines, power plants, ponds and so on. Scientists can account for about 60% of the dioxins found in Nature (referred to as ‘background level’), while the source of the remainder is still unknown. Dioxins enter the human body through the food chain, inhalation and skin contact. As long as the quantities absorbed are very minute, however, they do not represent a health hazard. The complete combustion of organic matter removes all the dioxins present in the garbage However, during cooling of the flue gases, traces of dioxins are formed. An energy-fromwaste facility acts as a reliable and renewable source of energy. It is a safe method of reducing the volume of waste dumped in landfills. A considerable reduction in the emission of greenhouse gases compared to landfills can be achieved. However, further research is needed to avoid completely this emission. Recovery of energy from solid waste constitutes the fourth ‘R’ after reducing, reusing and recycling. Research as the fifth ‘R’ is the key element. Scientists and environmental scientists have to work together to develop new methods for recycling more products.

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13.2.5 Conclusion In conclusion, primary recycling is ideal for clean, uncontaminated single-type scrap, but degradation during service life or reprocessing should be taken under serious consideration. Secondary recycling by type can be accomplished by various methods, but the cost associated with the separation and decontamination of the wastes undoubtedly poses an inherent obstacle. Dissolution-based techniques seem worth developing, but cannot yet be considered to be the complete answer. Secondary recycling of plastics mixtures by remelting is intended to produce downgraded products as a result of incompatibility problems. Compatibilisation is effective only in specific cases of plastics mixtures. Tertiary or ‘chemical’ recycling processes involve high levels of investment and succeed in recovering the chemical products, but negate the value added during the polymerisation. The latter comment is valid also for the last resort, quaternary recycling (energy recovery of plastics waste), which can substitute other energy sources and solve disposal problems. However, it stands strongly accused of undesirable emissions. The first and most important steps in plastic recycling are collection and sorting, after which the recycling process depends on the type of plastic and the field of application. These issues will now be addressed.

13.3 Collection and Sorting Collection involves gathering lightweight packaging films and other materials. Plastic packaging from separate collection streams is separated from other lightweight packaging material and sorted into fractions comprising film containers, mixed plastics and residues. Identification of the plastic type is one of the most important elements in recycling because most recycling processes prohibit certain types of plastics. For example, severe problems appear during processing of recycled resin of unknown origin. Thus, extrusion and injection moulding require accurate identifications of plastic waste, otherwise a product with bad appearance and impaired quality, especially poor mechanical properties, is obtained.

13.3.1 Resin Identification Identification has become more complicated, not only due to the presence of plastic materials compounded with additives such as plasticisers, stabilisers, flame retardants, fillers and others, but also due to the presence of polymer blends in the waste. Some time ago, the Society of the Plastic Industry (SPI) introduced a labelling system for recyclable

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Recycling of Plastic Waste plastic material. It is now common for manufacturers to use this code system printed or moulded on the product surface for easy plastic identification. The code is a three-sided triangular arrow with a number in the centre and letters underneath, indicating the type of resin: 1 for PET; 2 for HDPE; 3 for vinyl polymers, especially PVC; 4 for LDPE; 5 for PP; 6 for PS; and 7 for others. It is thus easy to develop automated scanning systems that can read the SPI or other codes. This will help to identify the resin used [15]. Furthermore, separation of plastic containers has been proposed by printing bar codes on them [16]. Plastic films are more difficult to identify than plastic containers, because most films do not carry a code, and producers and recyclers need training on how to distinguish between film types. Sorting generally must occur early in the recovery process, near the initial point of generation, to be successful. Optical systems for identification of mixed plastics have been used. A few technologies originally designed and used in the film and packaging industry were considered earlier. Electromagnetic scanning equipment was used to recognise chlorine molecules and so to sort PVC from PET [17]. An X-ray fluorescence (XRF) analyser as a photoelectric sensor was also used to identify transparent PET, green PET, translucent or natural HDPE, pigmented HDPE and PVC. The sensor system is connected to an automatic sorting line. Automation of the process reduces costs and improves the resale value of the separated plastics. Although dirt does not significantly influence the fluorescence intensity from bottles, paper labels do reduce the intensity but do not pose an obstacle in detecting vinyl bottles [17]. Paper labels are virtually blind to X-rays. Infrared and other spectral separation devices have been reported for the continuous examination of waste products [18]. However, a satisfactory process for the identification of plastic products for commercial purposes has yet to be developed.

13.3.2 General Aspects of Resin Separation Resin separation from contaminants or from undesired materials to obtain the desired stream can be achieved by a number of processes. These are: magnetic separation for the removal of ferrous materials; an electrostatic method for nonferrous, mainly aluminium, separation; air separation via cyclones to separate paper; and flotation tanks or hydrocyclones used to separate various resins based upon specific gravity. After that the processed materials are shredded. Automatic separation of shredded plastic waste is very difficult if the resins have similar specific gravity. Fortunately, 85% by volume of world plastic consumption is of four main thermoplastic resins: PE, PP, PVC and PS [19]. In the next sections, some separation techniques based on different properties will be discussed.

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13.3.3 Resin Separation Based on Density Air classification can be used to separate plastics on the basis of their bulk densities; thus film and foamed plastics may be separated from heavier forms of plastic material or paper [20-22]. The densities of the major thermoplastics give the potential to separate them into types by a series of float-sink operations [23, 24]. Water may be used to separate PP, LDPE and HDPE from PS, PVC and PET. A liquid having a density of about 930 kg/m3 may be used to separate PP and LDPE from HDPE; the PP and LDPE may then be separated using a liquid having a density of about 910 kg/m3. The PS and PVC may be separated using a liquid with a density of about 1150 kg/m3 [20]. Blending or filling a plastic may change its density to the point where it could cause difficulties in the float-sink operation processes. Labels, residual adhesives, metals and metallic-plastic composites cause similar difficulties, and therefore processes have been developed to remove these contaminants before the mixed plastic materials enter a separation system [25]. As an example, a solvent washing stage, using either tetrachloroethylene- or hexane-related solvents, was added to the classic water-washing treatment. These solvents were believed to remove not only the glues but also any toxic organic chemicals that have been stored in beverage bottles by consumers or are present as additives in plastics and that inadvertently will be present in the end-products [26]. A different approach for separating mixed plastic wastes by density has been reported [27, 28]. The process uses the properties of a fluid near its critical point to allow fine separations at mild temperatures and pressures. The density of the medium can be varied over a wide range and controlled to a sensitivity of ±0.01 g/cm3. Carbon dioxide is the most commonly used supercritical fluid and can be compressed to densities in the range of 1000 kg/m3. Since the separation of non-olefin thermoplastics will require fluid densities up to approximately 1400 kg/m3, mixtures of carbon dioxide and sulfur hexafluoride, a very dense supercritical fluid, may be used. By effecting small incremental changes of pressure, pure CO2 efficiently separated LDPE, HDPE and PP. Separation of green PET, clear PET and PVC has also been demonstrated, and separation of light- and dark-coloured HDPE is possible. The different densities exhibited by PET in the neck and the body of PET bottles can be separated by CO2/SF6. The possibility of separating various components of wire and cable scrap also exists. The centrifugal field produced in a hydrocyclone has been extensively used for the separation of plastics. In a hydrocyclone, the flow rate referred to the separation area is 100 times higher than in a static float-sink separator. The contamination of plastics is of only minor relevance in this process compared to flotation. For the separation of an n-component mixture, n – 1 separation stages (cyclone plants) are necessary.

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Recycling of Plastic Waste Furthermore, continuously operated plants must be equipped with a feeding device and screens for dehydration purposes [29]. Using hydrocyclones, PS, PET and PVC can be separated from one another or from polyvinyl alcohol; polyolefins from municipal solid wastes; PET from PE, PP and paper; and PP car bumpers from metals and other contaminants [30-32].

13.3.4 Resin Separation Based on Colour Photoelectric sensors are used for the separation of mixed, whole, or baled plastic containers. One of the systems uses mechanical means to reduce the baled plastic into individual bottles and to screen contaminants. After deballing and screening, the containers are manipulated into a single-line presentation to an optical sensor that performs a threeclass identification: Class 1, dairy HDPE and PP; Class 2, PET and PVC; and Class 3, mixed colour HDPE containers. Another optical sensor can be used to discriminate green and amber PET from clear PET containers, PP from dairy HDPE containers, and mixed colour HDPE according to seven colour classifications. However, reliable identification of post-consumer containers requires that measurements from much of the container surface should be ignored. These areas include closures or necks, labels, edges, bottoms and areas with residue of dirt [33, 34].

13.3.5 Resin Separation Based on Physicochemical Properties 13.3.5.1 Electrification The separation of mixed plastic wastes can be achieved using high-voltage drums, taking advantage of their different relative positions in the charging sequence. The process involves first tribo-electrification of the shredded plastic particles of the mixture by fluidisation. Subsequently, the electrified mixture is conveyed through an electrostatic field that separates the individual particles according to the magnitude and the polarity of the electric charges acquired during the tribo-electrification. When fluidising a mixture of two shredded plastics, the particles of the plastic with the lower work function transfer electronic charges to those with the higher work function. For example, the tribo-electrical contact between PVC and PET results in PVC having a negative charge and PET a positive charge. In the case of PET/PS mixtures, PET has a negative charge and PS a positive charge [35]. Mixtures containing more than two plastic species pose a substantial problem with regard to their charging behaviour. Also, owing to the various additives contained in different types of resin, the respective positions of the plastic species are prone to change.

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13.3.5.2 Spectroscopy Spectroscopy in the near-infrared (NIR) region of the spectrum could be a key to the rapid identification of various plastics and their subsequent recovery. By illuminating the sample with light in the near-infrared and measuring the reflected light, a so-called NIR spectrum of the material is obtained, which contains information about the molecular vibrations excited by the light energy. For example, the IR vibrational spectra of plastics show characteristic absorption bands at wavenumbers ν of 1200, 1400, 1700 and 22002500 cm–1 for CH, and at 1300-1500 and 1900-2100 cm–1 for OH. The NH (1500 and 2050 cm–1) and the CO (1730-1740 cm–1) vibrations contain the relevant spectral information for plastics. Plastic objects, such as beverage bottles, can be dropped through a vertical tube, and are identified and separated while falling. This also simulates other transportation possibilities like conveyor belts. From the performance data, it was found that identification can be achieved within 0.2 s, although several measurements were needed to avoid mistakes due to dirt or labels. The problem of transparency for satisfactory measurements could be overcome by reflectance measurements [36]. Other spectroscopic methods are also possible. The identification of several thermoplastics – such as polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PP, PS, high-impact PS (HIPS) and PVC – can be achieved by Fourier transform IR (FTIR), based on similar principles [37]. When exposed to γ-radiation, the high molecular weight (high molar mass) molecules of PVC containing chlorine atoms emit an X-ray return signature easily visible by an XRF analyser. Polyolefins, which have much lower molecular weights, emit a lower backscattering signal that barely shows up on the XRF analyser, and so is easily identified and separated [16, 38]. Bayer have proposed a process for automatic identification and sorting of post-consumer plastics, in which fluorescent dyes were added to resins during compounding, a different one for each resin type. These dyes, having high detection sensitivity, can be added in minute quantities, and so 5 g of dye per ton of polymer were sufficient for identification by a diode device [39].

13.3.5.3 Selective Dissolution of Polymer Mixtures Finally, it must be emphasised that solvent recycling of a single-type plastic scrap serves as a model process providing fundamental knowledge for the development of a selective dissolution process. The principle of the selective dissolution of a single polymer in a polymer mixture can be used to separate the polymers. According to the concept of selective dissolution, one polymer could be dissolved at a time, and thus dissolution-based processes can deal with mixtures of polymers. This has an evident impact on the recycling of plastics in municipal solid

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Recycling of Plastic Waste waste. Mixtures of polyolefins, PS, PVC, thermosetting plastics and natural polymers (rubbers, fibres, paper, etc.) can be separated using certain solvent systems [40]. The mixture is first treated with xylene at 5-50 °C to dissolve PS, and after separation the mixture is further heated at 90-150 °C to dissolve PE, leaving PP insoluble. The three main thermoplastics – polyolefins, PS and PVC – may be separated by dissolving them in a mixed solvent of xylene (85%) and cyclohexane (15%). From the dissolution, three separate phases could be obtained containing 99% each of the pure plastics, indicating that excellent separation can be achieved. [41]. In relevant studies, toluene, xylene and kerosene have been proposed as suitable solvents for the selective dissolution of LDPE, but the information given is very limited [42, 43]. Such selective dissolution is accomplished for each of the polymers of the mixture by heating the waste dispersion at various temperatures.

13.4 Recycling of Separated PET Waste The worldwide production of PET is above 1 × 106 tonnes per year. With such large consumption, the effective utilisation of PET waste is of considerable commercial and technological significance. PET waste may be converted into extruded or moulded articles after repelletising it. Recently, waste PET films or sheets have been cleaned, crushed, dried and mixed with LDPE waste [44]. The obtained mix was pelletised and blowextruded into films. The maximum concentration of PET does not exceed 20%. The films obtained were found to possess very good mechanical properties compared with LDPE only. Also, the films are expected to possess good printability due to the polar nature of PET. PET may be depolymerised to yield raw materials for resin synthesis. Recycling of segregated waste may be possible by blending in small quantities with virgin monomer, bis(hydroxyethyl) terephthalate. However, it often lowers the quality of the final product [45]. It is therefore desirable to break down the polymer into smaller fragments or oligomers [46]. PET can also be fully depolymerised into dimethyl terephthalate (DMT). However, the regenerated DMT exhibits a significantly higher carboxyl content, adversely affecting product quality. It is more economic to convert PET into low molecular weight oligomers by glycolysis in the presence of a transesterification catalyst [47-49]. When glycolysis is carried out using ethyleneglycol, the oligomers may be directly recycled into the polycondensation stage in PET manufacture, but this also lowers the product quality. Glycolysis can be carried out using different glycols, and the oligomers can be used in the synthesis of unsaturated polyester by reaction with an unsaturated anhydride [50] or used to synthesise other polymers [51].

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Handbook of Plastic Films There are two distinct advantages of the process: (1) the PET waste is converted into a commercial value-added product; and (2) unsaturated polyester resins based on terephthalic acid (TPA) are obtained without the processing difficulties encountered with the use of plain TPA. Yang and Tsai [52] degraded PET fabric waste to glycolysed product by treating the PET waste with sodium hydroxide using ethyleneglycol or glycerol as the solvent. Compared with the conventional aqueous alkaline hydrolysis, they found that the degradation rate in ethyleneglycol increases tenfold. They reported that the kinetics of the alkaline ethyleneglycol treatment show that the weight loss is linear with respect to time. They concluded that using ethyleneglycol can greatly shorten the treatment time to achieve results similar to those with the conventional aqueous system. A new chemical recycling process for PET using supercritical water has been developed by Yoshiyuki and co-workers [53]. In this method, the monomers obtained from supercritical water hydrolysis are the raw materials of each polymer. The purity of the terephthalic acid obtained from PET is about 99 wt%. It was confirmed that this process has the advantage of reducing the reaction time and simplicity of the process when compared with conventional methods such as methanolysis and glycolysis.

13.5 Recycling of Separated PVC Waste As mentioned before, most of the technologies for the recycling of plastic wastes include degradative extrusion, pyrolysis, hydrogenation, gasification, glycolysis, hydrolysis, methanolysis, incineration with HCl recovery, or input as a reducing agent into blast furnaces. Most of these technologies are still in the research phase, or simply are not suitable for PVCcontaining waste. The latter is particularly true for technologies such as glycolysis and hydrolysis, which play a role only for well-defined single-waste streams such as PET. Some of these technologies are currently generally regarded as the most feasible ones for realisation on a practical scale. However, one group of these technologies is not designed specifically for PVC waste, but deals with mixed plastic waste (MPW) in general. These technologies mainly concentrate on recovering the organic part of the MPW. They often have restrictions with regard to the maximum permissible chlorine (or PVC) input. Other technologies are designed to deal specifically with PVC waste (chlorine concentrations of well over 10%). They emphasise recovery of the chlorine fraction in a useful form. Hence, together with the competing technologies for chemical recycling, three types of technologies have been discussed [54]: (1) Technologies for chemical recycling of mixed plastic waste; (2) Technologies for chemical recycling of PVC-rich waste; (3) Alternatives to chemical recycling (incineration, mechanical recycling).

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13.5.1 Chemical Recycling of Mixed Plastic Waste Regarding the chemical recycling of MPW with a PVC content of up to several per cent, the process consists of two parts, a liquefaction step and an entrained bed gasifier. In the liquefaction step, the plastic waste is mildly thermally cracked (depolymerised) into synthetic heavy oil and some condensable and non-condensable gas fractions. The non-condensable gases are reused in the liquefaction as fuel together with natural gas. The heavy oil is filtered to remove large inorganic particles. The oil and condensed gas are then injected into the entrained gasifier. Also, chlorine-containing gases from the plastic waste are fed to the gasifier. The gasification is carried out with oxygen and steam at a temperature of 1200-1500 °C [55]. The products of the process are synthesis gas (predominantly H2/CO), pure sulfur and NH4Cl.

13.5.1.1 Polymer Cracking Process In the polymer cracking process, some elementary preparation of the waste plastics feed is required, including size reduction and removal of most nonplastics. The reactor operates at approximately 500 °C in the absence of air. The plastics crack thermally under these conditions to hydrocarbons, which vaporise and leave the bed with the fluidising gas. The gas has a high content of monomers (ethylene and propylene) and other useful hydrocarbons, with only some 15% being methane.

13.5.1.2 Conversion Process The feedstock recycling process was designed to handle the recycling of mixed plastic waste supplied by the collection system. The conversion of the pretreated mixed plastic into petrochemical raw materials takes place in a multistage melting and reduction process. In the first stage the plastic is melted and dehalogenised to preserve the subsequent plant segments from corrosion. The hydrogen chloride separated out in this process is absorbed and processed in the hydrochloric acid production plant. Hence, the major part of the chlorine present in the input, (e.g., from PVC), is converted into saleable HCl. Minor amounts become available as NaCl or CaCl2 effluent [56]. Gaseous organic products are compressed and can be used as feedstock in a cracker. In the subsequent stages the liquefied plastic waste is heated to over 400 °C and cracked into components of different chain lengths. About 20-30% of gases and 60-70% of oils are produced and subsequently separated in a distillation column.

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Handbook of Plastic Films Naphtha produced by the feedstock process is treated in a steam cracker, and the monomers, (e.g., ethylene, propylene), are recovered. These raw materials are used for the production of virgin plastic materials. The process is carried out under atmospheric pressure in a closed system and, therefore, no other residues or emissions are produced. In sum, the products of the process are: (1) HCl, which is neutralised or processed in a hydrochloric acid production plant; (2) Naphtha, to be treated in a steam cracker; (3) Monomers, (e.g., ethylene, propylene), which can be used for the production of virgin plastic materials; (4) High-boiling oils, which can be processed into synthesis gas or conversion coke and then transferred for further use; (5) Residues.

13.5.2 Chemical Recycling of PVC-Rich Waste These processes aim to recover as much as possible of the chlorine present in PVC in a usable form (HCl or a saleable chloride salt). The two processes in question, which are discussed below, are: •

Incineration process;

•

Pyrolysis process.

13.5.2.1 Incineration Process A plant for the processing of chlorine-containing fluid and solid waste streams is used. The goal is to process the waste by thermal treatment and to produce HCl using the energy from the process itself. The plant is based on a rotary kiln and has a capacity of 45 kilotonnes per year, (i.e., not only PVC waste), with a heat production capacity of 25 MW at ca. 7500 production hours per year. The waste is incinerated in the rotary kiln and a post-combustion chamber, directly after the rotary kiln, at temperatures of 900-1200 °C. During this treatment HCl is released and recovered. In this way a continuous production of high-quality HCl can be assured. Also, the formation of dioxins and furans can be diminished in this way, as the goal of the process is to oxidise the waste fully, so that no toxic chemicals (dioxins and furans) are formed.

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13.5.2.2 Pyrolysis Process In this process the chemical and thermal degradation of the PVC waste takes place in a reactor at low pressures (200-300 kPa) and moderate temperatures (maximum 375 °C). Chlorine generated from the PVC reacts with fillers, forming calcium chloride. Simultaneously, the metal stabilisers that may be present in PVC waste (lead, cadmium, zinc and/or barium) are converted to metal chlorides. This product consists of over 60% lead and may be purified and reused. After completion of the reactions, three main intermediate products are formed: a solid-phase product, a liquid product and a gas-phase product. In sum the products of the process are: (1) Calcium chloride (<1 ppm lead), which may be used as thaw salt or for other purposes; (2) Coke (<0.1 wt% lead and <0.1 wt% chlorine), which may be used as fuel in a cement kiln; (3) Metal concentrate (up to 60 wt% lead); (4) Organic condensate, which may be used as fuel for the process. To treat the PVC waste, lime and water are needed to run the process. No dioxins, chlorine, metals or plasticisers are emitted from the process. Also, there are no liquid waste streams in the process, since all streams are recycled within the system. There is a small volume of carbon dioxide gas formed by the reaction between lime/limestone and hydrogen chloride.

13.6 Recycling of Separated PE Waste LDPE recycling is widespread, although not to the same extent as HDPE recycling. The majority of LDPE that is recycled originates from post-industrial waste such as bundle shrink-wrap used to stabilise loads on pallets as well as greenhouse films and mulch films. There is only a limited proportion of recycled LDPE that can be classified as postconsumer recycle (PCR). Contamination in recycled PE can arise from a number of sources: (1) From multicomponent systems that use dissimilar polymers such as PP closures, from adhesive-backed paper labels, and even through the incorporation of additives such as pigments; (2) During use, (e.g., by the contents of the packaging); (3) During collection, (e.g., owing to consumers mixing plastic types); (4) By the environment, (e.g., soil in LDPE mulch film); and (5) By reprocessing, (e.g., gels and black specks).

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13.6.1 Contamination of PE Waste by Additives The pigments used in PE mouldings and films are often based on inexpensive metal oxides. For instance, the common brown, grey and orange pigments are based on various iron oxides and hydrates that act as prooxidants/prodegradants at the high temperatures, (i.e., 220 °C), encountered during reprocessing of HDPE. Moreover, green pigments are usually based on chromium(III) oxide, which can readily catalyse the thermooxidative degradation of HDPE when present even in trace quantities. Virgin PE is usually adequately stabilised so that these catalytic compounds do not cause in-service degradation of the polymer; however, with reprocessing, the antioxidants are usually consumed and these pigments may then be able to exert their prodegradant effects [57]. The recycled polymer can also be contaminated by pigmented components in the feedstock. In the recycling of HDPE bottles by melt processing, a major effort has been directed toward producing a naturally coloured recycle stream [58]. The major barrier to overcome to reach this end is the removal of the coloured bottle caps. LDPE film is extensively used for packaging and for the production of shopping bags. These films often contain a fatty acid amide lubricant (usually cis-docosenamide) that can be oxidised during thermal reprocessing of LDPE film. The lubricant readily undergoes cleavage at the unsaturated site to give a series of aldehydes that have very low odour thresholds. Such contamination imparts to the recycled material a rancid odour that may restrict its application potential.

13.6.2 Contamination of PE Waste by Reprocessing Recycling of HDPE PCR (usually milk bottles) by melt extrusion can lead to crosslinking during the thermal reprocessing stage since the antioxidant added initially (during manufacture of the polymer) is consumed [59]. Loosely crosslinked regions (known as ‘gels’) can act as stress concentrations in film and cause ‘blow-outs’ in bottles made from recycled HDPE. Another common source of contamination in recycled HDPE (as well as virgin HDPE) is ‘black specks’. These are small areas of highly degraded polymer that have been carbonised owing to excessive residence time in an extruder. These ‘black specks’ typically occur in low-flow regions in the extruder where ‘hang-ups’ form. Often, such contamination may also appear yellow, brown, or amber depending on the extent of degradation. Black specks cause a major problem in the blow-moulding of natural or white bottles because they are aesthetically undesirable.

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13.7 Recycling of HDPE 13.7.1 Applications for Recycled HDPE New markets for recycled HDPE will also emerge as a result of advances in fabrication technology allowing recycled HDPE to be used in more demanding applications that are currently the domain of LLDPE and high-modulus HDPE. For instance, there is the potential use of recycled HDPE in oriented laminates for applications such as pond liners and moisture barriers. High-performance sheets are obtained by laminating oriented HDPE webs to equalise cross-web and longitudinal properties [60]. Compared to the same thickness of monolayer HDPE, the advantages are greatly improved tensile, tear and puncture strength, and doubling of the moisture barrier properties. Specialised processing equipment has been developed to handle recycled material [61, 62]. The following are some of the commercial applications of the modified recycled HDPE: (1) Large mouldings; (2) Detergent bottles are another major outlet; (3) Grocery sacks (‘check-out’ bags) are generally produced from high molecular weight (HMW) HDPE film and have a thickness in the range 15-18 μm, their most critical performance parameters being tear strength and dart impact strength.

13.7.2 Rubber-Modified Products Recycled rubber crumb from used car tyres is another class of modifier that is finding widespread potential for blending with HDPE PCR to yield truly 100% recycled polymeric materials [44]. Profiles extruded from recycled HDPE are finding increased use in applications such as decking, fence posts, boundary stakes, road posts, railroad sleepers and similar areas, replacing wood and concrete [63]. Such applications are generally thick sections and are ideal outlets for consuming large volumes of HDPE recycle. Furthermore, they are resistant to rotting, not liable to insect attack, and do not splinter.

13.8 Recycling Using Radiation Technology Owing to the ability of ionising radiation to alter the structure and properties of bulk polymeric materials, and the fact that it is applicable to essentially all polymer types,

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Handbook of Plastic Films irradiation holds promise for impacting the polymer waste problem. The three main possibilities for use of radiation in this application are: (1) Enhancing the mechanical properties and performance of recovered materials or material blends, principally through crosslinking, or through surface modification of different phases being combined; (2) Treatment causing or enhancing the decomposition of polymers, particularly through chain scission, leading to recovery of either low molecular weight mixtures, or powders, for use as chemical feedstocks or additives; (3) Production of advanced polymeric materials designed for environmental compatibility. An overview of the polymer recycling problem describes the major technological obstacles to the implementation of recycling technologies, and outlines some of the approaches being taken [64].

13.9 Biodegradable Polymers The synthetic polymer industry has brought great benefits to modern society. For example, in the packaging and distribution of foodstuffs and other perishable commodities, the commercial thermoplastic polymers are hydrophobic and biologically inert, and this has made them essential to modern retailing [65]. Similarly, in agriculture, plastics have largely replaced glass in greenhouses and cloches, and they have gained a unique position in the growing of soft fruits and vegetables over very thin polymer films (mulching films) [66]. The major group of polymers used both in packaging and in agriculture are the polyolefins, which, due to their resistance to peroxidation, water and microorganisms, are durable during use. In the 1970s, it became evident that the very technical advantages which made polymers so useful were disadvantages when polymer-based products were discarded at the end of their useful life and in particular when they appeared as litter in the environment. Some items of plastics packaging waste were found to have very damaging effects on wildlife [67], and this led to calls from the ‘Green’ movement to return to biologically based (renewable) polymers. Materials made from naturally occurring or biologically produced polymers are the only truly biodegradable ‘plastics’ available. Since living things construct these materials, living things can metabolise them.

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Recycling of Plastic Waste In practice, a relatively small weight proportion of polymeric materials ends up as litter. In most developed societies, domestic organic waste, including plastics packaging, is disposed of in sanitary landfill or by incineration. However, burying waste is no longer an ecologically acceptable way of disposing of consumer wastes [65, 68]. Another approach aimed at the solid waste problem is the development or evaluation of biodegradable polymers [69]. These are based on a variety of natural products (often structurally modified to optimise properties), or are laboratory-made polymers with structures designed to be susceptible to enzymatic attack. A rather large effort has grown up in this area. Polymer types being studied include cellulose derivatives (such as cellulose acetate) [70], polysaccharides such as chitin [71], starch and poly(3-hydroxybutrate) [72]. A related approach to materials that break down under natural environmental conditions is the development of UV-degradable plastics, designed to decompose in sunlight should they become ‘litter’ [73]. Examples include a copolymer of ethylene and carbon monoxide, and modified PET. Two different applications have emerged over the past two decades for degradable polymers. The first is where biodegradability is part of the function of the product. Examples of this are temporary sutures in the body or in controlled release of drugs, where cost is relatively unimportant. Similarly, in agriculture, very thin films of photobiodegradable polyethylene are used to ensure earlier cropping and to reduce weed formation [65, 66]. By increasing soil temperature, they also increase crop yields and ensure earlier harvest. A major ecological benefit of mulching films is the reduction of irrigation water and fertiliser utilisation [74]. No residues must persist in the soil in subsequent seasons to make the land less productive by interfering with root growth. The technology of biodegradable polymers, as a solution to minimising the huge amount of plastic waste, is developing. There is an ever-widening range of polymers satisfying the requirements necessary for the numerous applications for which biodegradability of the materials is essential.

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Handbook of Plastic Films 41. R.J. Sperber and S.L. Rosen, Polymer Engineeering and Science, 1976, 16, 4, 246. 42. R.B. Seymour and G.A. Stahl, Journal of Chemical Education, 1976, 53, 10, 653. 43. M. Date, inventors; Japan Kokai 78:30,675, 1978. 44. E.M. Abdel-Bary, Recycling of Plastic Wastes, Final Report, Project No. 62, Academy of Scientific Research and Technology, Cairo, Egypt, 2001. 45. M. Matsuura, T. Habara and Y. Katagiri, inventors; Japan Kokai 75:71,639, 1975. 46. H. Hemmi, H. Nagashima, Y. Kimura, I. Teresaki and M. Satani, inventors; Japan Kokai 73:62,732, 1973. 47. S.C. Rustagi, D.P.A. Dabholkar, J.K. Niham, M.N. Marathe and K.B. Iyer, inventors; Indian Patent 145,323, 1977. 48. H.S. Ostrowski, inventor; Fiber Industries, Inc., assignee; US Patent 3,884,850, 1975. 49. H. Toshima, inventor; Japan Kokai 75:64,382, 1975. 50. S.N. Tong, D.S. Chen, C.C. Chen and L.Z. Chung, Polymer, 1983, 24, 469. 51. S-C. Lee, V-W. Sze and C.C. Lin, Journal of Applied Polymer Science, 1995, 55, 1271. 52. M.C. Yang and H.Y. Tsai, Textile Research Journal, 1997, 67, 10, 760. 53. N. Yoshiyuki, Y. Masahiro and F. Ryuichi, R&D Kobe Steel Engineering Report, 1997, 47, 3, 43. 54. D.A. Tukker, H. de Groot, L. Simons and S. Wiegersma, Chemical Recycling of Plastic waste (PVC and other Resins), TNO Report STB-99-55 Final, TNO, Delft, The Netherlands, 1999. 55. H. Croezen and H. Sas, Evaluation of the Texaco Gasification Process for Treatment of Mixed Household Waste, Final Report of Phase I and II, Council of Europe, Delft, The Netherlands, 1997. 56. M. Heyde and S. Kremer, LCA Packaging Plastic Waste, LCA Documents 2(5), Ecomed, Landsberg, Germany, 1999. 57. J. Scheirs and S.W. Bigger, Proceedings of the 35th International Symposium Macromolecules (IUPAC), Akron, OH, USA, 1994, 606.

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Recycling of Plastic Waste 58. P.M. Phillips, Proceedings of Antec ’93, New Orleans, LA, USA, 1993, 998. 59. F.A. Sitek, Modern Plastics International, 1993, 23, 10, 74. 60. Uniloy article, ‘Use of post-consumer plastic in bottles’, Modern Plastics International, 1991, 21, 6. 61. P. S. Blatz in Emerging Technologies in Plastics Recycling, Eds., G.D. Andrews and P.M. Subramanian, ACS Symposium Series No.513, American Chemical Society, Washington, DC, USA, 1992, Chapter 20. 62. G.S. Bowes, Proceedings of Antec ’91, Montreal, Canada, 1991, 2556. 63. N. Humber, Proceedings of Recycle ’93, Davos, Switzerland, 1993, p.17/2-1. 64. G. Burillo, R.L. Clough, T. Czvikovszky, O. Guven, A. Le Moel, W. Liu, A. Singh, J. Yang and T. Zaharescu, Radiation Physics and Chemistry, 2002, 64, 1, 41. 65. G. Scott, Polymers and the Environment, Royal Society of Chemistry, Cambridge, UK, 1999. 66. Degradable Polymers: Principles and Applications, Eds., G. Scott and D. Gilead, Kluwer Academic, Dordrecht, The Netherlands, 1995, Chapters 9-11. 67. Proceedings of 2nd International Conference on Marine Debris, Eds., R.S. Showmura and M.L. Godfrey, Honolulu, HI, USA, 1990. 68. G. Scott, Wastes Management, 1999, May, 38. 69. S.J. Huang in Degradable Polymers, Recycling, and Plastics Waste Management, Eds., A-C. Albertsson and S.J. Huang, Marcel Dekker, New York, NY, USA, 1995, pp.1-5. 70. R.A. Gross, J. Gu , D. Eberiel and S.P. McCarthy in Degradable Polymers, Recycling, and Plastics Waste Management, Eds., A-C. Albertsson and S.J. Huang, Marcel Dekker, New York, NY, USA, 1995, pp.21-22. 71. M.G. Peter in Degradable Polymers, Recycling, and Plastics Waste Management, Eds., A-C. Albertsson and S.J. Huang, Marcel Dekker, New York, NY, USA, 1995, pp.37-48. 72. M.K. Cox in Degradable Polymers, Recycling, and Plastics Waste Management, Eds., A-C. Albertsson and S.J. Huang, Marcel Dekker, New York, NY, USA, 1995, pp.15-20.

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Abbreviations and Acronyms

AA

Acrylic acid

ABS

Acrylonitrile-butadiene-styrene

AC

Acetyl cellulose

AES

Auger electron spectroscopy

AFM

Atomic force microscopy

APB

Ammonium pentaborate

APP

Ammonium polyphosphate

aPP

Atactic polypropylene

ASTM

American Society for Testing and Materials

ATH

Alumina trihydrate

au

Arbitrary units

BDD

Brominated dibenzodioxin(s)

BDF

Brominated dibenzofuran(s)

BF

Branch frequency

BFR

Brominated flame retardant

BMP

2,6-di-tert-butyl-4-methylphenol

BOPP

Biaxially oriented polypropylene

BPE

Branched polyethylene

BTPC

Benzyl triphenyl phosphonium chloride

BUR

Blow-up ratio

CDD

Chlorinated dibenzodioxin(s)

CDF

Chlorinated dibenzofuran(s)

CI

Carbonyl index

DBM

Dibenzoylmethane

DIN

Deutsch Institut für Normung

DMA

Dynamic mechanical analysis

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Handbook of Plastic Films DMT

Dimethyl terephthalate

DSC

Differential scanning calorimetry

EFW

Energy from waste

EPDM

Ethylene-propylene-diene terpolymer

ESCA

Electron spectroscopy for chemical analysis

ESCR

Environmental stress cracking

ESR

Electron spin resonance

EU

European Union

EVA

Ethylene vinyl acetate

EVOH

Ethylene-vinyl alcohol

FFS

Form-fill-seal

FTIR

Fourier transform infrared spectroscopy

GE

Grafting efficiency

GP

Grafting percentage

GPC

Gel permeation chromatography

HA

Hindered amines

HALS

Hindered amine light stabilisers

HDPE

High-density polyethylene

HDUL

Heat distortion under load

HEMA

2-Hydroxyethyl methacrylate

HFI

Hyperfine interaction

HIPS

High impact polystyrene

HLMI

High-load melt index

HMW

High molecular weight

HP

Hindered phenols

HPLC

High-performance chromatography

IPCS

International Program for Chemical Safety

iPP

Isotactic polypropylene

iPP

Isotactic polypropylene

IR

Infra red

ISO

International Standards Organisation

382

Abbreviations and Acronyms L:D

Length-to-diameter ratio

LCB

Long chain branching

LDPE

Low-density polyethylene

LLDPE

Linear low-density polyethylene

LOI

Limiting oxygen index

LPE

Linear polyethylene

MD

Machine direction

MDPE

Medium-density polyethylene

MDSR

Machine direction stretching ratio

MF

Melt flow

MI

Melt index

MPW

Mixed plastic waste

MSW

Municipal solid waste

MWD

Molecular weight distribution

NIR

Near infra red spectroscopy

NMR

Nuclear magnetic resonance

NO

Nitric oxide

NO2

Nitrogen dioxide

OMTS

Octamethylcyclotetrasiloxane

OPP

Oriented polypropylene

PA-12

Polyamide-12

PA-6

Polyamide-6

PAN

Polyacrylonitrile

PB

Phenyl benzoate

PBB

Polybrominated biphenyl(s)

PBDE

Polybrominated diphenyl ether(s)

PC

Polycarbonate

PCA

Polycaproamide

PCB

Polychlorinated biphenyl(s)

PCDD

Polychlorinated dibenzodioxin(s)

PCDF

Polychlorinated dibenzofuran(s)

383

Handbook of Plastic Films PCR

Post-consumer recyclable

PCTFE

Polychlorotrifluoroethylene

PE

Polyethylene

PER

Pentaerythritol

PET

Polyethylene terephthalate

PI

Polyisoprene

PMMA

Polymethyl methacrylate

PMP

Poly(4-methyl-1-pentene)

PNA

Phenyl-β-naphthylamine

PP

Polypropylene

PS

Polystyrene

P-t-BuMA

Poly-tert-butyl methacrylate

PTFE


PU

Polyurethane

PVAc

Polyvinyl acetate

PVA-ox

Oxidised polyvinyl alcohol

PVB

Poly(vinyl butyral)

PVC

Polyvinyl chloride

PVDC

Polyvinylidene chloride

PVF

Polyvinyl fluoride

PVOH

Polyvinyl alcohol

PVP

Polyvinylpyrrolidone

RDP

Rescorcinol diphosphate

RH

Relative humidity

RHR

Rate of heat release

SCB

Short chain branching

SEC

Size exclusion chromatography

SEM

Scanning electron microscopy

SIMS

Secondary ion-mass spectroscopy

SPI

Society of the Plastics Industry

sPP

Syndiotactic polypropylene

384

Abbreviations and Acronyms sPP

Syndiotactic polypropylene

TCB

Trichlorobenzene

TCDD

2,3,7,8-Tetrachlorodibenzo-p-dioxin

TD

Transverse direction

TEB

Tensile energy to break

TEF

Toxic equivalence factor

TEM

Transmission electron microscopy

TFE-HFP

Tetrafluoroethylene-hexafluoropropylene copolymer

Tg

Glass transition temperature

TGA

Thermogravimetric analysis

Tm

Melting temperature

TMDSC

Temperature modulated DSC

Ton

Onset temperature

TPA

Terephthalic acid

TREF

Temperature-rising elution fractionation

UHMWPE

Ultra high molecular weight polyethylene

ULDPE

Ultra low-density polyethylene

USEPA

United States Environmental Protection Agency

UV

Ultra violet

VA

Vinyl acetate

VLDPE

Very low-density density polyethylene

WAXS

Wide angle X-ray scattering

WC

Weight conversion percentage

WVTR

Water vapour transmission rate

XFS

X-ray fluorescene spectroscopy

XPS

X-ray photoelectron spectroscopy

XRF

X-ray fluorescence analyser

ZN

Ziegler-Natta

385


386

Contributors

Elsayed M Abdel-Bary Department of Chemistry Faculty of Science Mansoura University Mansoura Egypt Heshmat A Aglan Department of Mechanical Engineering Tuskegee University Tuskegee AL 36088 USA Guneri Akovali Middle-East Technical University Department of Chemistry TR-06531 Ankara Turkey Amin Al-Robaidi Jubeiha 11941 PO Box 1628 Amman Jordan Evgenii Y Davydov NM Emanuel Institute of Biochemical Physics Russian Academy of Sciences ul. Kosygina 4 Moscow 117977 Russia Yong X. Gan Department of Mechanical Engineering

387

Handbook of Plastic Films Tuskegee University Tuskegee AL 36088 USA Irina S. Gaponova, Emanuel Institute of Biochemical Physics Russian Academy of Sciences ul. Kosygina 4 Moscow 117977 Russia Klara Z Gumargalieva N.N. Semenov Institute of Chemical Physics Russian Academy of Sciences Moscow Russia S.M. Lomakin Institute of the Biochemical Physics of Russian Academy of Sciences 119991, Kosygin 4 Russia Ashraf A Mansour Department of Chemistry Faculty of Science Cairo University Cairo Egypt Alexander Mar´in Parallel Solutions, Inc. 763 Concord Avenue Cambridge MA 02138 USA Karl S Minsker, Bashkirian State University 32 Frunze Str. Ufa Bashkiriya 450074 Russia

388

Contributors Georgii B Pariiskii NM Emanuel Institute of Biochemical Physics Russian Academy of Sciences ul. Kosygina 4 Moscow 117977 Russia Susan E Selke School of Packaging Michigan State University East Lansing MI 48824-1223 USA Robert A Shanks Department of Applied Chemistry Faculty of Applied Science RMIT University GPO Box 2476V Melbourne Victoria 3001 Australia Abbas A Yehia Department of Polymers and Pigements National Research Center Dokki Cairo Egypt Gennady E Zaikov N.M. Emanuel Institute of Biochemical Physics Russian Academy of Sciences 4 Kosygin Str. Moscow 119991 Russia Vadim G. Zaikov N. M. Emanuel Institute of Biochemical Physics 4 Kosygin Str. Moscow 117334 Russia

389


390

Main Index

Index

A Aclar films 247 Additive dissolution 122, 123, 126 kinetics of 114 Additive solubility 115 additive loss 125 crystallinity 118 factors 118 high molecular weight additives 122 polymer orientation 119 polymer oxidation 124 polymer polar groups 120 quantitative data 114 sorption 110-112 supermolecular structure 118 Additives antioxidants 14 lubricants 14 slip agents 14 solubility of 109 tackifiers 14 ultraviolet stabilisers 14 Adhesive strength adsorption theory 316 diffusion theory 316 electrical theory 316 mechanical theory 316 molecular-kinetic theory 316 theory 316 Agricultural films 263 blown film extrusion 264 characteristics of plastic films 264 greenhouses 263 light transmittance 264

polyethylene films stabilisers 267 Albemarle 166 Algipor 290, 301, 302 Aliphatic chain scission 124 Allied Signal Corporation 247 Amoco 82 Amoco process 84 Antiblocking agents 22, 85 mineral particles 23 Antioxidants dissolution 122 hindered amines 89 hindered phenols 24, 89 phosphites 90 polyolefins 112 solubility 122 thio compounds 90 triphenyl phosphite 24 Antistatic agents polyoxyethylenes 24 Ammonium polyphosphate/ pentaerythritol mixtures intumescent behaviour 172 Applications agriculture 38, 263 coextruded films 37 heat sealing 38 laminated films 36 packaging 35 Aqua-penetrability 305 Artificial weathering devices 271 Atactic polypropylene 13, 79 Atochem 166

391


B Biaxial Biaxially oriented film 18, 213 drawing 10 polypropylene 351 properties of 244 Biobrant 314, 324 burn dressing 323 Biodegradable polymers 374 Blending agricultural plastic film waste 279 Blow-up ratio 59, 61 Blown film air ring cooling 53 blow ratio 56 die 52 extrusion process 14 extrusion (tubular film) 50 process 50 cooling the film 51 extruder size 54 extrusion equipment 55 horsepower 55 Tenter frame 19 production 13 Branched polyethylenes 7 molecular structures 7 Burn dressings 285-286 adhesion of 296, 315 adhesive properties 292 adhesive strength 317-318 air penetrability 291, 295, 308, 315, 323 air permeability 310 aqua-penetrability coefficients 308 cellulose 303 characteristics 322 collagen 303 compositional 290 cotton balling 290 degree of filling 298 efficiency of 322

392

evaporation of water from 318 hydrophilic 303 kinetics of sorption 304 materials free volume of 303 material porosity 292 mechanical properties 292 microorganism penetrability 292 model of action 318 nitrogen penetrability 309 number of pores 293 oxygen penetrability 309 physicochemical properties 292 pore size distribution 293 properties of 290 size of pores 293 solubility of water in polymers 298 sorption ability 297-298, 322 sorption of fluid 320 sorption of liquid media 303 sorption of plasma 303 sorption of water 299-300 sorption-diffusion properties 291 sorptional ability 285 sorptional ability of materials 294 structure of 303 vapour penetrability 291-292, 296, 305 water absorption 291 Butyl rubber effect of nitrogen oxides on 191

C Carbonium ion mechanism 172 Carbonylallyl groups identification of 136 Cast film biaxial orientation 19 calendering 16 extrusion 57 packaging 75

Index polypropylene films applications 76 process chilled roll 57 production calendering finishing 17 extrusion coating 17 extrusion conditions 16 tenter frame 16 Char formation acid-catalysed 175 free-radical 174 Characterisation dielectric relaxation 226 electron spectroscopy for chemical analysis 228 gravimetric method 224 molecular weight 226 molecular weight distribution 226 scanning electron microscopy 225 spectroscopic analysis Auger electron spectroscopy 227 infrared spectroscopy 227 x-ray fluorescence spectroscopy 227 surface properties 227 swelling measurements 226 thermal analyses 225 x-ray photoelectron spectroscopy 228 Charpy tests 29, 336 Chemical modification 215 bromination 217, 218 chemical etching 218 chlorination 217 fluorination 215 bulk 216 direct 215 indirect 216 surface 216 grafting 220 high-energy radiation 220 radiation-induced 221 photografting 222 bulk surface 222

sulfonation 218 Chemical recycling 359 depolymerisation 360 Chimassorb 280 Chromatographic techniques 46 Coextrusion 16, 17, 22 agricultural plastic film waste 279 Collagens 301 maximum sorption of water 301 Corona discharge surface treatment system 22 Creep mechanism of 26 Crosslinkable ethylene plastics standard specifications 351 Cultivated cutis 286 Cyasorb 1084 280

D Dacron 290 Dehydrochlorination 134 rate constant 135 Density gradient column 44 Die capacity 57 Die size 55-56 Die swell percentage polymer melt 50 Dielectric properties 30 Differential thermal analysis 119 Dimensional stability 35 Dioxin structure of 163 Dow Chemicals 277 Dressings adhesion of 324 air penetrability 313 animal origin 286 characteristics of 287-289 multilayer 305 penetrability 309

393

Handbook of Plastic Films polyurethane 304 synthetic materials 286 vegetable origin 290 DuPont Company 131 Dynamic mechanical analysis 339 Dynamic mechanical properties 29, 339

E Elmendorf tear strength test 28 Energy recovery 361 Erucamide structure of 23 Ester pyrolysis mechanism 173 Ethylene copolymers dielctric analysis 30 Ethylene-vinyl alcohol films oxygen barrier properties 248 typical properties 249 Extruder blow ratio 15 characteristics 14 compression zone 15 dispersive mixing 14 distributive mixing 14 feed zone 15 frost-line 15, 16 metering zone 15 screw 15 design 15 size 55 twin-screw 14 Extrusion 10 cast film 19 process calendering 17 extrusion coating 17 tubular extrusion 13 film 51, 59 Exxon 82 process 83

394

F Feedstock recycling 359 Film 5 brittleness 60 drawing 18-19 extrusion blow moulded 18 extrusion coating 22 extrusion of the melt 5 film lamination 22 gloss 5 lamination 37 manufacture 5 melt adhesion 21 pigmented 38 printing 5, 22 slip agents 23, 342 surface properties 5 tensile strength 60 toughness 60 Film application linear low-density polyethylene 41 Film blowing 48 Film characteristics blocking 61 bubble stability 61 extrusion variables 58 gloss 59 haze 59 impact strength 60-61 optical 58 puckering 62 Film extrusion blown film 50 slot cast extrusion 50 Film orientation shrink-wrapping 214 Film packaging 75 Film properties blow ratio 61 gloss 61 haze 61

Index optical 61 Film samples impact test methods 337 Film shrinkage 69 Fire retardants ammonium pentaborate 169 ammonium polyphosphate 168-169 brominated 166 brominated diphenyl oxide 161 char-formers 167 chlorinated 161 chlorinated dibenzo-p-dioxins 162 diagram of 167 dimelamine phosphate 170 environmental impact 168 guanidine sulfamate 168 halogen-containing 159-161 halogen-free 170 halogenated diphenyl ethers - dioxins 162 inorganic 159 intumescent systems 167-168 low-melting glasses 167 mechanism of action 160 condensed-phase 169 melamine pyrophosphate 170 nitrogen-based organic 159 organophosphorus 159 phosphorus 160 polybrominated biphenyls 162, 165 polybrominated diphenyl ethers 162 polymer morphology modification 167 polymer nanocomposites 167 polymer organic char-former 175 preceramic additives 167 Flat film dies 57 packaging bags 58 Flexible packaging bags, sacks and pouches 238 dispensing 239

forms 236 heat-sealing 240 pouch production 239 reclosure 239 wraps shrink-wrap 237 stretch-wrap 237 Fluidised bed pyrolysis 359 Fluoropolymer sodium etching 219 Free-falling dart method 336 Free-radical processes 41

G Gas permeation 35 General grade polyethylene films standard specifications 350 General Electric 166 Greenhouse films additive 268 ageing factors 275 ageing resistance 275 changes in chemical structure 276 compatibility 269 effects of pesticides 274 environmental pollution 274 excited-state quenchers nickel dibutyldithiocarbamate 267 fog formation 273 hindered-amine light stabilisers 267, 275 humidity 273 light stabilisers 269 recycling 278 solar irradiation 265 solar radiation 265 stabilisation 265, 268-269, 272 temperature 272 ultraviolet screening carbon black 266 chalk 266 short glass fibres 266

395

Handbook of Plastic Films stabilisation 265-266 talc 266 TiO2 266 wind 273

Intrinsic viscosity polyethylene resin 46 Isotactic polypropylene 9, 12, 79-80 Izod tests 29, 366

H

L

Hall Woodroof Co. 290 Heat stabilisers structures of 25 High-density polyethylene 10 melt strength 11 recycled 373 applications 373 detergent bottles 373 grocery sacks 373 large mouldings 373 viscosity 11 Hindered amine light stabilisers hydroxybenzophenones 25 hydroxybenzotriazoles 25 tetramethylpiperidines 25 Hydron 290, 298 Hydroperoxide decomposition 208

Light stabilisers structures of 25 Linear low-density polyethylene 11 differential scanning calorimetry curves 33 film atomic force microscopy of 32 stress-strain curve 27 Linear polyethylene 7 molecular structures 7 Linear low density polyethylene 8 composition by 13C nuclear magnetic resonance 346 properties 8 shrink films 14 Low-density polyethylene 7, 11 continuous shear rheology curve 12 covering films hindered-amine ligt stabilising 272 differential scanning calorimetry curves 33 films standard specifications 350 high-pressure radical process 7 mulch film recycling 278 long branches 11 resistance to tear propagation 338 shrink properties 68 Lubricants chlorinated paraffins 24 paraffin wax 24 stearate salts 24 stearic acid 24

I Impact properties dart-puncture resistance 28 tensile impact 28 tensile–tear strength resistance 28 Infrared spectroscopy characteriastion 32 composition analysis of blends and laminates 33 surface analysis 33 Inorganic barrier coatings aluminium oxide 255 clay nanocomposites 255 Internal additives antioxidants 24 ultraviolet absorbers 24

396

Index

M Mechanical properties abrasion resistance 26 adhesion tests 26 impact tests 26 polyolefin films 25 modification of 213 tear testing 26 tensile 26 Mechanical tests bending stiffness (flexural modulus) 339 free-falling dart method 336 hail resistance 337 impact resistance 336 impact test methods 337 package yield of a plastic film 334 pendulum impact resistance 337 pendulum method 338 percent elongation at break 334 percent elongation at yield 334 propagation tear resistance 338 puncture-propagation tear resistance 339 tear resistance 337 tensile modulus of elasticity 334 tensile strength 333 tensile strength at break 334 tensile testing (static) 333 yield strength 333 Medium-density polyethylene films standard specifications 350 Melt elasticity 50 Microporous material typical sorption isotherm 294 Microscopic examination atomic force microscopy 31 optical–polarised light effect with strain 31 scanning electron microscopy–etching 31

Mitsui Hypol 82-83 Mixed plastic waste chemical recycling 369 conversion process 369 feedstock recycling process 369 gasification 369 identification of electromagnetic scanning 363 optical systems 363 X-ray fluorescence 363 polymer cracking process 369 Modulus 331 Moisture resistance 34 Mulch film 277 recycling 278 Multilayer plastic films 16 coating 252 coextrusion 253 greenhouses 279 lamination 253 metallisation 253 silicon oxide coating 254

N Nitroxyl radicals 194 Electron spin resonance spectrum 194 structure of 203 Noryl 166 Nylon-6 thermal decomposition 171

O Orientation 35 biaxial 18 by drawing 18 during blowing 18 machine direction 60 of film 18 transverse direction 60 Oriented polypropylene 351

397

Handbook of Plastic Films standard specifications 351 Oxygen indices 170

P Polyamide-6,6 cone calorimeter data 179-180 nanocomposite carbon residue 183 Polyamide-6,6/Polyvinyl alcohol cone calorimeter data 178 Packaging 235 Packaging films acid copolymer films 250 biaxially oriented film 244 cellophane 247 ethylene-vinyl acetate 250 ethylene-vinyl alcohol 248 high-density polyethylene 243 linear low-density polyethylene 242 low-density polyethylene 242 polyamide (Nylon) 249 polychlorotrifluoroethylene 247 polyethylene terephthalate 245 polypropylene 244 polyvinyl alcohol 248 polyvinyl chloride 245 polyvinylidene chloride 246 uses of 241 Packaging materials barrier 257 cellulose 252 environmental issues 261 high-impact polystyrene 252 ionomers 251 permeation 257 plastics 251 polystyrene 251 printing flexography 256 ink-jet printing 257 lithography 257

398

rotogravure 257 screen printing 257 static discharge 256 surface treatment corona discharge 255 Polyethylene chlorination of 217 films wetting tension 344 interaction of nitrogen dioxide with 189 long-chain branching 45 melt flow properties 45 melts shear viscosity 48 oxidation of 219 processing troubleshooting 63-65 resin basic properties 42 chain branching 45 density 44, 45 dispersity index 42 elasticity 49 elongational viscosity 49 heat of fusion 47 intrinsic viscosity 46 melt flow blend relationship 43 melt index 42 melt properties 48 melting point 47 molecular weight 42 rheology 48 viscosity/shear rheology 48 short-chain branching 44-45 waste contamination by additives 372 contamination by reprocessing 372 Perfluoronitroxyl radicals electron spin resonance spectra of 203 Physical property modification corona treatment 223 plasma treatment 222

Index Physicochemical tests abrasion resistance 347 blocking load parallel-plate method 346 creep 346 creep rupture 346 density of plastics 340 environmental stress cracking 348 haze transmittance 341 ignition 342 indices of refraction 340 kinetic coefficients of friction 342 luminous transmittance 341 mar resistance 348 orientation release stress 345 outdoor weathering 347 oxygen gas transmission 349 oxygen index 342 rate of burning characteristics 342 resistance to chemicals 341 rigidity 345 shrink tension 345 specular gloss 343 static coefficients of friction 342 transparency 341 water vapour permeability 348 weatherability 347 yellowness 340 Piping material polypropylene 74 Plastic films abrasive damage 347 applications 228 artificial ageing tests 276 contamination by the environment 277 crosslinking 213-214 crystallisation 213-214 dimensions 332 gloss 343 grafting 228 greenhouses 263-264

in packaging 235 mechanical properties 213 modification of 213 orientation 213-214 photooxidation 271, 277 physical properties 213 premature failure 272 production of 263 properties of 332 recycling in agriculture 277 removal of contaminants 213 resistance to tearing 338 slip properties 343 specular gloss 343 stability 263, 347 standard specifications 349 tear resistance 338 testing of 329-356 thicknesses 332 unrestrained linear thermal shrinkage 345 Plastic materials fire retardation 168 photodegradation 265 recycling 279 Plastic production 76 growth rate 77 Plastic recycling collection 362 mechanical recycling 358 primary recycling 358, 362 quaternary recycling 360 secondary recycling 358, 362 sorting 362 tertiary recycling 359, 362 Plastic surfaces fluorinated 216 Plastic waste films recycling 278 reuse 278 management 278

399

Handbook of Plastic Films recycling 357 resin identification 362 Plastics separation of 359 Polymethyl methacrylate exposure to nitrogen dioxide 193 Polluted atmospheres 187 Polyamide films typical properties 250 Polyamides interaction of nitrogen dioxide with 196 Polyamidoimide film influence of nitrogen dioxide on 200 Polydimethylsiloxane diffusion of gases 310 penetrability of gases 310 solubility of gases 310 Polyethylene 7, 8 decreasing-pitch screw 51 density 60 high-density 8 high-pressure technology 45 linear low-density 8 low-density 7 low-pressure technology 45 self-adhesion 21 solubility of phenyl benzoate in 120 solubility of phenyl-b-naphthylamine in 122 surface treatment of 217 ultra-low-density 8 very-low-density 8 Polyethylene films crosslinked 214 irradiated 215 photo(bio)degradable 271 processing 41 properties of 243 shrink-wrapping 35 Polyethylene resin intrinsic viscosity 46

400

Polyethylene terephthalate films properties of 246 standard specifications 350 waste chemical recycling process 368 depolymerised 367 glycolysis 367-368 recycling of 367 Polyisoprene interaction with nitrogen dioxide 192 Polymer films gloss 20 haze 20 surface analysis 34 Polymer mixtures selective dissolution 366 Polymer nanocomposites combustibility 181 disordered 180 intercalated 180 Polymer structure morphological irregularity 109 nonuniform 109 Polymeric materials graft copolymerisation 220 Polymers barrier characteristics 215 diffusion of water vapour 306 flame retardancy 159-160 interaction of nitrogen dioxides with 188 interaction of nitrogen oxides with 187-188 nitric oxide 188 nitrogen oxide 188 non-saturated 191 penetrability 306, 310 penetrability of gases 309 photochemical oxidation 187 reaction of nitric oxide with 202 rheology 15 separation coefficients of gases 309

Index solubility of stabilisers 116-117 solubility of water in 299 thermal oxidation 187 Polyolefin elastomers 8 Polyolefin films 9 crystallisation 9 morphology of 9 packaging applications 36 production 5 Polyolefins 6-7, 10 corona discharge treatment 21 dielectric properties 30 orientation drawing 119 properties 6 rheological characterisation 10 structure of 6-7 virgin recycling behaviour of 271 Polypropylene 9, 12 additives 88 balanced oriented 351 barrier properties 214 branching 78 calcium carbonate pigment 101 chain scission 87-88 chirality 79 degradation 86 durability-additive property 97 durability-processing condition 94 dynamic mechanical analysis curve 29 films durability 73 processing conditions 73 hydroperoxide decomposition 207 interaction of nitrogen dioxide with 189 metallocene-catalysed 13 micrograph 82 microstructure 96 morphology 81 regiospecificity 78 rods 74

stress-strain behaviour 94-95, 98-99 stretched tape materials 100 structures 80 properties of 244 two-step tubular orientation 13 ultraviolet degradation 86 uniaxially oriented 351 wire coating 75 worldwide capacity 77 Poly-tert-butyl methacrylate film degradation 190 Polytetrafluoroethylene processing of 215 Polyurethane cellular 286 films exposure to nitrogen dioxide 200 interaction of nitrogen dioxide with 196 sponges pore size 286 Polyvinyl chloride properties of 244 Polyvinylidene chloride films typical properties 247 Porous materials determining absorbtion ability 295 determining air penetrability of 296 penetrability 310 Post-consumer films recycling 277 Polypropylene degradation 87 Polypropylene fabric scanning electron microscope morphology 93 of ultraviolet light degraded 93 stress-strain behaviour 92 Polypropylene films 75 additives 85 chill roll cast method 85 durability-microstructure relationship 91

401

Handbook of Plastic Films film processing 85 microscopic examination 91 oriented 75 opaque 75 static tensile tests 91 structures 78 surface morphology 100 synthesis 78 ultraviolet degradation behaviour 90 ultraviolet exposure 91 wetting tension 344 Polypropylene granules Scanninmg electron micrograph 80 Polypropylene woven fabrics stress-strain behaviour 91 Polypropylene-polyethylene-copolypropylene 10 micrograph 10 Processing troubleshooting 62 Polystyrene degradation 191 film degradation 190 Polyvinyl chloride 134 degradation effect of plasticisers 145 rate 148 dehydrochlorination 135, 137, 145, 147-148 kinetic curves for 134 rate constants 138 thermal 150 disintegration 132, 134, 136 thermal 151 ‘echo’ stabilisation 151-152, 153 global production 131 light stabilisation 144 low stability 132 stabilisation 138, 140-141 thermodegradation rate 150 thermoformed packaging 245

402

Polyvinyl chloride films plasticisers 245 properties 245 Polyvinyl chloride waste chemical recycling 368, 370 incineration 368, 370 rotary kiln 370 mechanical recycling 368 pyrolysis process chemical 371 thermal degradation 371 recycling 368 Polyvinyl alcohol hospital laundry bags 248 water-solubility 248 Polyvinylpyrrolidone interaction of nitrogen dioxide with 197

R Recycling dioxins 361 energy recovery 360 furans 361 incineration 360 incinerator 361 radiation technology 373 Regenerated cellulose diffusion coefficient 306 Resin separation air separation 363 colour 365 density 364 electrification 365 float-sink operations 364 flotation tanks 363 fluidisation 365 Fourier transform IR 366 high-voltage drums 365 hydrocyclone 364 magnetic separation 363

Index photoelectric sensors 365 physicochemical properties 365 spectroscopy 366 supercritical fluid 364 X-ray fluorescence analyser 366 Rheology 5 Rubber-modified products 373

S Saytex 8010 166 Separated PE waste recycling 371 Shock-cooling 58 Short-term tests dart test 28 impact test 28 Shrink film 18, 62, 65 bi-oriented 65 blow-up ratio 67 bubble shape 69 frost-line 67, 69 manufacture 67 mono-oriented 65 ovens 70 properties 66 resin melt index 69 shrinkage 66 shrink-wrapping 62 tunnels 70 Slip agents erucamide 23, 85 ethylene bis-stearamide 23 oleamide 23, 85 stearamide 23 Slot casting 59 process melt temperature 60 Sorption isotherm 110 Spheripol 82 process 82-83 Stabilisation agents

phenolic antioxidants 85 phosphite antioxidants 85 Stress relaxation mechanism of 26 Surface additives corona treatments 33 glyceryl monooleate 33 polyisobutylene 33 slip agents 33 Surface modification antiblocking 22 antistatic agents 24 chemical treatments 213 chlorinated paraffins 24 corona discharge 21, 213 hydrophilic 223 lubricants 24 paraffin wax 24 physical methods 222 plasma 213 slip additives 23 stearate salts 24 stearic acid 24 Surface properties blocking 21 gloss 19 haze 20 slip 21 surface energy 20 Syndiotactic polypropylene 79

T Tensile properties burst strength 28 creep 27, 28 strain hardening 26 strain rate 26 stress relaxation 26 Testing ASTM D882 335 methods

403

Handbook of Plastic Films requirements 330 sample conditioning 332 results interpretation of 330 thin films 335 Tetrafluroroethylene-hexafluoropropylene gamma-irradiated action of nitric oxide on 205 Thermal analysis differential scanning calorimetry 31 temperature-modulated 32 Thermal dehydrochlorination 146 Thickness 34 Thin films ‘neck-in’ 58 Tinuvin 622 LD 280 Toxicity equivalence factors 162-164 CDD 164 CDF 164

U Ultra-low-density polyethylene 11 Ultraviolet stabilisation 25 Unipol 82 Unipol process 84 Ultraviolet degradation 87 Ultraviolet stabilisers 70

404

V Very-low-density polyethylene 11 Vinyl polymers interaction of nitrogen dioxide with 188 Viscoelasticity 331

W Wound exudate sorption of by dressings 298

Z Ziegler-Natta processes 41

ISBN: 1-85957-338-X

Rapra Technology Limited Rapra Technology is the leading independent international organisation with over 80 years of experience providing technology, information and consultancy on all aspects of rubbers and plastics. The company has extensive processing, analytical and testing laboratory facilities and expertise, and produces a range of engineering and data management software products, and computerised knowledge-based systems. Rapra also publishes books, technical journals, reports, technological and business surveys, conference proceedings and trade directories. These publishing activities are supported by an Information Centre which maintains and develops the world’s most comprehensive database of commercial and technical information on rubbers and plastics.

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Handbook of Plastic Films

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