CHAPTER 1
Introduction 1:1 General A plastic material
is
any
of
a
wide
range
of
synthetic
or
semi-
synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs. Monomers of plastic are either natural or synthetic organic compounds. The word plastic is derived from the Greek πλαστικός Greek πλαστικός (plastikos) meaning capable of being shaped or molded, from πλαστός (plastos) meaning molded. It refers to their malleability, or plasticity during manufacture, that allows them to be cast, pressed, or extruded into a variety of shapes — shapes — such such as films, fibers, plates, tubes, bottles, boxes, and much more. Plastic materials trace their origin in The United States back to 1868, when a young printer named John Wesley Hyatt came up with Celluloid, the first American plastic. He mixed pyroxylin, made from cotton (one of nature's polymerics), and nitric acid, with camphor to create an entirely different and new product. Celluloid quickly moved into many markets, including the first photographic film used by George Eastman to produce the first motion picture film in 1882. The material is still in use today under its chemical name, cellulose nitrate. In 1909, Dr. Lee Hendrik Baekeland introduced phenoformaldehyde plastics (or "phenolics", as they are more popularly known), the first plastic to achieve worldwide acceptance. More importantly, Baekeland also evolved techniques for controlling and modifying the phenolformaldehyde reaction so that products could be formed under heat and pressure from the material. This characteristic of liquefying the material so that it can be formed into various shapes under heat and pressure is still common to most plastics. The third major thrust in the development of plastics took place in the 1920s with the introduction of cellulose acetate (which is similar in structure to cellulose nitrate, but safer to process and use), ureaformaldehyde (which can be processed like the phenolics, but can also be molded into light colored articles that are more attractive than the blacks
pg. 1
and browns in which phenolics are available), and polyvinyl chloride (PVC, or vinyl, as it is commonly called). Nylon was also developed in the late 1920s through the classic research of W.T. Carothers. Each decade saw the introduction of new and more versatile plastics. In the 1930's, there were acrylic resins for signs and glazing and the commercialization of polystyrene, which became the third largest-selling plastic, literally revolutionizing segments of the house wares, toys, and packaging industries. Melamine resins were also introduced; these later became a critical element (in the form of a binder) in the development of decorative laminate tops, vertical surfacing, and the like. Today‘s most widely used plastic (Polyethylene) evolved out of the need for a superior insulating material that could be used for such applications as radar cable during World War II. The thermoset polyester resins that only a decade or so later was to radically change the boat-building business in the United States were also a wartime development introduced for military use. And acrylonitrile-butadiene-styrene plastics, or ABS, (the plastic most often used today in appliance housings, refrigerator linens, safety helmets, pipe, telephone headsets, and luggage) owes its origins to research work emanating from the crash wartime program aimed at producing large quantities of synthetic rubber. The decade of the 1950s saw the introduction of polypropylene and the development of acetal and polycarbonate, two plastics that, along with nylon, came to form the nucleus of a sub-group in the plastics family known as the "engineering thermoplastics." Their outstanding impact strength and thermal and dimensional stability enabled them to compete directly and favorably with metal in many applications. The 1960s and 1970s also saw their share of new plastic introductions, most notably thermoplastic polyesters with the kind of outstanding resistance to gas permeation that made them applicable for use in packaging. During this period, another sub-group of the plastics family also started to emerge, the so-called "high temperature plastics," which includes the polyimides, polyamide-imides, aromatic polyesters, polyphenylene sulfide, polyether sulfone, and the like. These materials were designed to meet the demanding thermal needs of aerospace and aircraft applications. Today, however, they have moved into the commercial areas that require their ability to operate at continuous temperatures of 400 degrees F, or more. Like any material, plastics have their origins in nature, in such basic chemical elements as carbon, oxygen, hydrogen, nitrogen, chlorine, or sulfur. These materials are extracted from nature's storehouse of air, water, gas, oil, coal, and even plants. From the basic sources come the feedstock's we call "monomers" (from "mono", which means one, and "mer", which means unit - in this case, the specific chemical unit).
pg. 2
The monomer is subjected to a chemical reaction known as polymerization, which causes the small molecules to link together into longer molecules. Chemically, the polymerization turns the monomer into a "polymer" (many mers). Thus, a polymer may be defined as a high-molecular-weight compound which contains comparatively simple recurring units. A monomer can contribute to the manufacture of a variety of different polymers, each with its own distinctive characteristics. The way in which the monomers link together into polymers, and resulting structural arrangement, is one determinant of the properties of the plastic. The length of the molecules in the molecular chain (referred to as "molecular weight") is a second determinant. And the type of monomer is a third determinant. Polymerizing two or more different monomers together (a process known as "copolymerization") is a fourth determinant. Incorporating various chemicals or additives during or after polymerization is a fifth. [1]
1:2 Objectives The objective of this research is to study methods of recycling of Polyethylene Terephthalate (PET) through the provision of general information on plastics, their structures, chemical properties, uses and environmental effects in addition to study their types, availability and methods of PET recycling that could be adopted in Sudan.
pg. 3
CHAPTER 2
Literature Review
2:1 Background Plastics can be classified by chemical structure, namely the molecular units that make up the polymer's backbone and side chains. Some important groups in these classifications
are the acrylics, polyesters, silicones, polyurethanes,
and halogenated
plastics. Plastics can also be classified by the chemical process used in their synthesis, such as condensation, polyaddition, and cross-linking. Other classifications are based on qualities that are relevant for manufacturing or product
design.
Examples
of
such
classes
are
the
thermoplastic
and
thermoset, elastomer, structural, biodegradable, and electrically conductive. Plastics can also be classified by various physical properties, such as density, tensile strength, glass transition temperature, and resistance to various chemical products. Due to their relatively low cost, ease of manufacture, versatility, and imperviousness to water, plastics are used in an enormous and expanding range of products, from paper clips to spaceships. They have already displaced many traditional materials, such as wood, stone, paper, glass and ceramic, in most of their former uses. The use of plastics is constrained chiefly by their organic chemistry, which seriously limits their properties, such as hardness, density, heat resistance, organic solvents, oxidation, and ionizing radiation. In particular, most plastics will melt or decompose when heated to a few hundred degrees celsius.While celsius. While plastics can be made electrically conductive, with the conductivity of up to 80 k.s/cm in stretchoriented polyacetylene, they are still no match for most metals like copper which have conductivities of several hundreds k.s/cm. Plastics are still too expensive to replace wood, concrete and ceramic in bulky items like ordinary buildings, bridges, dams, pavement and railroad ties. [2] The common plastics include Polyester (PES) which can be used in fibers and textiles, Polyethylene terephthalate (PET) which can be used in carbonated drinks
pg. 4
bottles, peanut butter jars, plastic film and microwavable packaging, Polyethylene (PE) which can be used in wide range of inexpensive uses including supermarket bags and plastic bottles, High-density polyethylene (HDPE) which can be used in detergent bottles and milk jugs, Polyvinyl chloride (PVC) which can be used in plumbing pipes and guttering, shower curtains, window frames and flooring, Polyvinylidene chloride (PVDC) (Saran) which can be used in food packaging, Low-density polyethylene (LDPE) which can be used in outdoor furniture, siding, floor tiles, shower curtains and clamshell packaging, Polypropylene (PP) which can be used in bottle caps, drinking straws, yogurt containers, appliances, car fenders (bumpers) and plastic pressure pipe systems, Polystyrene (PS) which can be used in packaging foam/"peanuts", food containers, plastic tableware, disposable cups, plates, cutlery, CD and cassette boxes, High impact polystyrene (HIPS) which can be used in refrigerator liners, food packaging and vending cups, Polyamides (PA) (Nylons) which can be used in fibers, toothbrush bristles, fishing line and under-the-hood car engine moldings, Acrylonitrile butadiene styrene (ABS) which can be used in electronic equipment cases (e.g., computer monitors, printers, keyboards), drainage pipe, discs, eyeglasses,
riot
Polycarbonate (PC) which can be used in compact
shields,
security
windows,
traffic
lights
and
lenses,
Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS) which is a blend of PC and ABS that creates a stronger plastic used in car interior and exterior parts, and mobile phone bodies and they also include Polyurethanes (PU) which can be used in cushioning foams, thermal insulation foams, surface coatings, printing rollers (Currently 6th or 7th most commonly used plastic material, for instance the most commonly used plastic found in cars). [3]
Chemical Structure of Plastics Common thermoplastics range from 20,000 to 500,000 amu, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known asrepeat units, derived from monomers; each polymer chain will have several thousand repeating units. The vast majority of plastics are composed of polymers of carbon of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. (Some of commercial interests are silicon based.) The backbone is that part of the chain on the main "path" linking a large number of repeat units together. To customize the properties of a plastic, different molecular groups "hang" from the backbone (usually they are "hung" as part of the monomers before linking monomers together to form the polymer chain). This fine tuning of the properties of the polymer by
pg. 5
repeating unit's molecular structure has allowed plastics to become an indispensable part of twenty first-century world. Some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point (the temperature at which the attractive intermolecular forces are overcome) and one or more glass transitions (temperatures above which the extent of localized molecular flexibility is substantially increased). The so-called semi-crystalline plastics include polyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some polyurethanes. Many plastics are completely amorphous, such as polystyrene and its copolymers, poly (methyl methacrylate), methacrylate) , and all thermosets. [1]
2:2 Classifications of Plastics Plastics Families
Amorphous
Ultra polymers
High performance polymers
SRP, TPI, PAI, Hightemperature sulfone|HTS
Semi-crystalline
PFSA, PEEK
Fluoropolymers: LCP, Polyarylamide|PARA, HPN, PPS, PPA PPSU, PEI, PESU, PSU Other polyamides
Mid range polymers
PC, PPC, COC, PMMA, ABS, PVC Alloys
PEX, PVDC, PBT, PET, POM, PA 6,6, UHMWPE
Commodity polymers
PS, PVC
PP, HDPE, LDPE
pg. 6
There exists terrific variety among the many hundreds of different types of plastics. In order to simplify the organization of their similarities and differences, it is useful to classify plastics according to certain specific criteria; we can classify plastics to:
Fossil-based plastics.
Biodegradable plastics.
Special purpose plastics.
2:2:1 Fossil-based plastics a) Bakelite
The first so called plastic based on a synthetic polymer was made from phenol and formaldehyde, with the first viable and cheap synthesis methods invented in 1907, by Leo Hendrik Baekeland, a Belgian living in New York state. Baekeland was looking for an insulating shellac to coat wires in electric motors and generators. He found that mixtures of phenol (C 6H5OH) and formaldehyde (HCOH) formed a sticky mass when mixed together and heated, and the mass became extremely hard if allowed to cool. He continued his investigations and found that the material could be mixed with wood flour, asbestos, or slate dust to create "composite" materials with different properties. Most of these compositions were strong and fire resistant. The only problem was that the material tended to foam during synthesis, and the resulting product was of unacceptable quality. Baekeland built pressure vessels to force out the bubbles and provide a smooth, uniform product. He publicly announced his discovery in 1912, naming it Bakelite. It was originally used for electrical and mechanical parts, finally coming into widespread use in consumer goods in the 1920s. When the Bakelite patent expired in 1930, the Catalin Corporation acquired the patent and began manufacturing Catalin plastic using a different process that allowed a wider range of coloring. Bakelite was the first true plastic. It was a purely synthetic material, not based on any material or even molecule found in nature. It was also the first thermosetting plastic.
pg. 7
b) Polystyrene and Polyvinyl Chloride
After the First World War, improvements in chemical technology led to an explosion in new forms of plastics. Among the earliest examples in the wave of new plastics were polystyrene (PS) and polyvinyl chloride (PVC). Polystyrene is a rigid, brittle, inexpensive plastic that has been used to make plastic model kits and similar knick-knacks. It would also be the basis for one of the most popular "foamed" plastics, under the name styrene foam or Styrofoam. Foam plastics can be synthesized in an "open cell" form, in which the foam bubbles are interconnected, as in an absorbent sponge, and "closed cell", in which all the bubbles are distinct, like tiny balloons, as in gas-filled foam insulation and flotation devices. In the late 1950s, high impact styrene was introduced, which was not brittle. It finds much current use as the substance of toy figurines and novelties.
.
Polyvinyl Chloride (PVC, commonly called "vinyl") has side chains incorporating chlorine atoms, which form strong bonds. PVC in its normal form is stiff, strong, heat and weather resistant, and is now used for making plumbing, gutters, house siding, enclosures for computers and other electronics gear. PVC can also be softened with chemical processing, and in this form it is now used for shrink-wrap, food packaging, and rain gear.
All PVC polymers are degraded by heat and light. When this happens, hydrogen chloride is released into the atmosphere and oxidation of the compound occurs. Because hydrogen chloride readily combines with water vapor in the air to form hydrochloric acid, polyvinyl chloride is not recommended for long-term archival storage of silver, photographic film or paper (Mylar is preferable).
pg. 8
c) Nylon
The real star of the plastics industry in the 1930s was polyamide (PA), far better known by its trade name nylon. Nylon was the t he first purely synthetic fiber, introduced by DuPont Corporation at the 1939 World's Fair in New York City. In 1927, DuPont had begun a secret development project designated Fiber66, under the direction of Harvard chemist Wallace Carothers and chemistry department director Elmer Keiser Bolton. Carothers had been hired to perform pure research, and he worked to understand the new materials' molecular structure and physical properties. He took some of the first steps in the molecular design of the materials. His work led to the discovery of synthetic nylon fiber, which was very strong but also very flexible. The first application was for bristles for toothbrushes. However, Du Pont's real target was silk, particularly silk stockings. silk stockings. Carothers and his team synthesized a number of different polyamides including polyamide 6.6 and 4.6, as well as polyesters. d) Rubber
Natural rubber is an elastomer (an elastic hydrocarbon polymer) that was originally derived from latex, a milky colloidal suspension found in the sap of some plants. It is useful directly in this form (indeed, the first appearance of rubber in Europe is cloth waterproofed with unvulcanized latex from Brazil) but, later, in 1839, Charles Goodyear invented vulcanized rubber; this a form of natural rubber heated
with,
mostly,
sulfur
forming
cross-links
between
polymer
chains
(vulcanization) , improving elasticity and durability. e) Synthetic rubber
The first fully synthetic rubber was synthesized by Sergei Lebedev in 1910. In World War II, supply blockades of natural rubber from South East Asia caused a boom in development of synthetic rubber, notably styrene-butadiene rubber. In 1941, annual production of synthetic rubber in the U.S. was only 231 tonnes which increased to 840,000 tonnes in 1945. In the space race and nuclear arms race,Caltech race,Caltech researchers experimented with using synthetic rubbers for solid fuel for rockets. Ultimately, all large military rockets and missiles would use synthetic rubber based solid fuels, and they would also play a significant part in the civilian space effort. [4]
pg. 9
2:2:2 Biodegradable (compostable) plastics Research has been done on biodegradable plastics that break down with exposure to sunlight (e.g., ultra-violet radiation), radiation) , water or dampness, bacteria, enzymes, wind abrasion and some instances rodent pest or insect attack are also included as forms of biodegradation of biodegradation or environmental degradation. It is clear some of these modes of degradation will only work if the plastic is exposed at the surface, while other modes will only be effective if certain conditions exist in landfill or composting systems. Starch powder has been mixed with plastic as a filler to allow it to degrade more easily, but it still does not lead to complete breakdown of the plastic. Some researchers have actually genetically engineered bacteria that synthesize a completely biodegradable plastic, but this material, such as Biopol, is expensive at present. The German chemical company BASF makes Ecoflex, a fully biodegradable polyester for food packaging applications.
2:2:3 Special purpose plastics Special purpose plastics include Melamine formaldehyde (MF) – One of the aminoplasts, and used as a multi-colorable alternative to phenolics, for instance in moldings (e.g., break-resistance alternatives to ceramic cups, plates and bowls for children) and the decorated top surface layer of the paper laminates (e.g., Formica), Plastarch
material – material – Biodegradable
and
heat
resistant,
thermoplastic
composed
of modified of modified corn starch., starch. , Phenolics (PF) or (phenol formaldehydes) – High modulus, relatively heat resistant, and excellent fire resistant polymer. Used for insulating parts in electrical fixtures, paper laminated products (e.g., Formica), thermally insulation foams. It is a thermosetting plastic, with the familiar trade name Bakelite that can be molded by heat and pressure when mixed with filler-like wood flour or can be cast in its unfilled liquid form or cast as foam (e.g., Oasis). Problems include the probability of moldings naturally being dark colors (red, green, brown), and as thermoset it is difficult to recycle, Polyetheretherketone (PEEK) – Strong, chemical- and heat-resistant thermoplastic, biocompatibility allows for use in medical implant applications, aerospace moldings. One of the most expensive commercial polymers, Polyetherimide (PEI) (Ultem) – A high temperature, chemically stable polymer that does not crystallize, Polylactic acid (PLA) – (PLA) – A biodegradable, thermoplastic found converted into a variety of aliphatic polyesters derived from lactic acid which in turn can be made by fermentation of various
pg. 10
agricultural products such as corn starch, once made from dairy products, Polymethyl methacrylate (PMMA) – (PMMA) – Contact Contact lenses, glazing (best known in this form by its various trade names around the world; e.g., Perspex, Oroglas, Plexiglas), aglets, fluorescent light diffusers, rear light covers for vehicles. It forms the basis of artistic and commercial acrylic
paints when
suspended
in
water
with
the
use
of
other
agents,
Polytetrafluoroethylene (PTFE) – (PTFE) – Heat-resistant, Heat-resistant, low-friction coatings, used in things like non-stick surfaces for frying pans, plumber's tape and water slides. It is more commonly known as Teflon, also special purpose plastics include Urea-formaldehyde (UF) – (UF) – One One of the aminoplasts and used as a multi-colorable alternative to phenolics. Used as a wood adhesive (for plywood, chipboard, hardboard) and electrical switch housings. [5]
2:3 Plastics Environmental Issues Plastics are durable and degrade very slowly; the molecular bonds that make plastic so durable make it equally resistant to natural processes of degradation. Since the 1950s, one billion tons of plastic have been discarded and may persist for hundreds or even thousands of years. In some cases, burning plastic can release toxic fumes. Burning the plastic polyvinyl chloride (PVC) may create dioxin. Also, the manufacturing of plastics often creates large quantities of chemical pollutants. Prior to the ban on the use of CFCs of CFCs in extrusion of polystyrene (and general use, except in life-critical fire suppression systems), the production of polystyrene contributed to the depletion of the ozone layer; however, non-CFCs are currently used in the extrusion process. [6] By 1995, plastic recycling programs were common in the United States and elsewhere. Thermoplastics can be remelted and reused, and thermoset plastics can be ground up and used as filler, though the purity of the material tends to degrade with each reuse cycle. There are methods by which plastics can be broken back down to a feedstock state. Plastic can be converted as a fuel. Plastic is made from crude oil, so it can be broken down to liquid hydrocarbon. One kilogram of waste plastic produces a liter of hydrocarbon. Plastic wastes are used in cement plants as a fuel. To assist recycling of disposable items, the Plastic Bottle Institute of the Society of the Plastics Industry devised a now-familiar scheme to mark plastic bottles by plastic
pg. 11
type. A plastic container using this scheme is marked with a triangle of three "chasing arrows", arrows", which encloses a number giving the plastic type:
1. PET (PETE), polyethylene terephthalate . 2. HDPE, high-density polyethylene . 3. PVC, polyvinyl chloride . 4. LDPE, low-density polyethylene. 5. PP, polypropylene . 6. PS, polystyrene . 7. Other types of plastics. [7]
2:4 Polyethylene Terephthalate (PET) Polyethylene terephthalate (sometimes
written
poly(ethylene terephthalate)),
commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of terephthalic acid and ethylene glycole (A thermoplastic, also known as thermo-softening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently) PET is a strong, rigid and light material. PET physical properties make it an ideal substance for using in various fields, such as production of packing (bottles, cortexes, etc.), film, constructions elements PET is also ,
used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber. The term 'polyethylene terephthalate' is a source of confusion because this substance, PET, does ‗poly(ethylene terephthalate)' is often not contain polyethylene. Thus, the alternate form, ‗poly(ethylene used in scholarly journals for the sake of accuracy and clarity. Depending on its processing and thermal history, polyethylene terephthalate may exist both as an amorphous (transparent) and as a semi-crystalline polymer. The semi-
pg. 12
crystalline material mi might ght appear transparent (particle size < 500 nm) or opaque and white (particle size up to a few microns) depending on its crystal structure and particle size.
Its
β-hydroxyterephthalate) (bis-β (bis-
monomer
can
be
synthesized
by
the
esterificationr eaction between terephthalic acid and ethylene glycol with water as a byproduct, or by transesterification reaction between ethylene glycol and dimethyl terephthalate
with methanol as
polycondensation
reaction
a
of
byproduct. the
Polymerization
monomers
(done
is
through
immediately
a
after
esterification/transesterification) with water as the byproduct. The majority of the world's PET production is for synthetic fibers (in excess of 60%) with bottle production accounting for around 30% of global demand. In discussing textile applications, PET is generally referred to as simply "polyester" while "PET" is used most often to refer to packaging applications. The polyester industry makes up about 18%
of
world
polymer
production
and
is
third
after polyethylene (PE)
and polypropylene (PP). PET consists of polymerized of polymerized units of the monomer ethylene terephthalate, with repeating C 10H8O4 units. PET is commonly recycled, and has the number "1" as its recycling symbol. The first PET was patented in 1941 by John Rex Whinfield, James Tennant Dickson and their employer the Calico Printers' Association of Manchester. The PET bottle was patented in 1973 by Nathaniel Wyeth. PET is used as substrate in thin film and solar cell. Because PET is an excellent barrier material, plastic bottles made from PET are widely used for soft drinks. For certain specialty bottles, PET sandwiches an additional polyvinyl alcohol to further reduce its oxygen permeability. Biaxially oriented PET film (often known by one of its trade names, "Mylar") can be aluminized by evaporating a thin film of metal onto it to reduce its permeability, and to make it reflective and opaque (MPET). (MPET) . These properties are useful in many applications, including flexible food packaging and thermal insulation, such as "space blankets". blankets" . Because of its high mechanical strength, PET film is often used in tape applications, such as the carrier for magnetic tape or backing for pressure sensitive adhesive tapes. Non-oriented
PET
sheet
can
be thermoformed to
make
packaging
trays
and blisters. If crystallizable PET is used, the trays can be used for frozen dinners, since they
withstand
both
freezing
and
oven
baking
temperatures.
When
filled
pg. 13
with glass particles or fibers, it becomes significantly stiffer and more durable. PET is also used as substrate in thin film and solar cell.
2:4:1 Chemical Properties of PET Polyethylene terephthalate (PET), is a condensation polymer produced from the monomers ethylene glycol, HOCH 2CH2OH, a dialcohol, and dimethyl terephthalate, CH3O2C – C6H4 – CO CO2CH3, a diester. By the process of transesterification, these monomers form ester linkages between them, yielding polyester. PET chemical resistance is good for concentrated acids, dilute acids, Alcohols, greases and oils, halogens, and ketones, and it has a poor chemical resistance for Alkalis and a fair one for aromatic hydrocarbons.
(Polyethylene (Polyethylene Terephthalate)
Antimony
Antimony (Sb) trioxide (Sb2O3)
or
is
a catalyst that
antimony
triacetate
in
is
often
used
the
production
of
as antimony PET.
After
manufacturing a detectable amount of antimony can be found on the surface of the product. This residue can be removed with washing. Antimony also remains in the material itself and can thus migrate out into food and drinks. Exposing PET to boiling or microwaving can increase the levels of antimony significantly,
possibly
above
USEPA
maximum
contamination
levels.
The
drinking water limit li mit in the t he USA for antimony is 6 parts par ts per billion. bill ion. Although antimony trioxide is of low l ow toxicity toxicit y when taken orally, oral ly, its presence is still stil l of concern. The Swiss Federal Office of Public Health investigated the amount of antimony migration, comparing waters bottled in PET and glass: the antimony concentrations of the water in PET bottles were higher, but still well below the allowed maximum concentration. The Swiss Federal Office
pg. 14
of Public Health concluded that small amounts of antimony migrate from the PET
into
bottled
water,
but
that
the
health
risk
of
the
resulting
low
concentrations is negligible (1% of the "tolerable daily intake" intake " determined by the WHO). WHO). A later (2006) but more widely publicized study found similar amounts of antimony in water in PET P ET bottles. The WHO has published a risk assessment for antimony in drinking water. Fruit juice concentrates (for which no guidelines are established), however, that were produced and bottled in PET in the UK were found to contain up to 44.7 µ g/L of antimony, ant imony, well above the EU limits limi ts for tap water of 5 µg/L.
2:4:2 Physical properties of PET: PET can be semi-rigid to rigid, depending on its thickness, and it is very lightweight. It makes a good gas and fair moisture barrier, as well as a good barrier to alcohol (requires additional "barrier" treatment) and solvents. It is strong and impactresistant. It is naturally colorless with a high transparency. Some of PET physical properties:
Molecular formula: (C 10H8O4)n Density amorphous: 1.370 g/cm Density crystalline: 1.455 g/cm
3
3
Young‘s modulus: 2800-3100 2800-3100 MPa Tensile strength (σt): 55-75 MPa Elastic limit: 50-150% Notch test: 3.6 KJ/m
2
Glass temperature: 75°C 75 °C Melting Point: 260°C 260 °C Vicat B: 170°C 170°C Thermal conductivity: 0.24 W/(m.K) -5
Linear expansion coefficient (α): 7×10 /K
pg. 15
Specific heat (C): (C): 1.0 KJ/(Kg.K) Water absorption (ASTM): 0.16 Refractive Index: 1.5750
Intrinsic viscosity One of the most important characteristics of PET is referred to as intrinsic viscosity (IV). The intrinsic viscosity of the material, measured in deciliters per gram (dℓ/g) is dependent upon the the length of its polymer chains. The longer the polymer chains, the more entanglements between chains and therefore the higher the viscosity. The average
chain
length
of
a
particular
batch
of
resin
can
be
controlled
during polycondensation. The intrinsic viscosity range of PET Fiber grade
0.70 dℓ/g Textile 0.40 – 0.40 – 0.70 0.72 – 0.72 – 0.98 0.98 dℓ/g Technical, tire cord Film grade
0.60 – 0.60 – 0.70 0.70 dℓ/g BoPET (Biaxially oriented PET film) 1.00 dℓ/g Sheet grade for thermoforming 0.70 – 0.70 – 1.00 Bottle grade
0.78 dℓ/g Water bottles (flat) 0.70 – 0.70 – 0.78 0.85 dℓ/g Carbonated soft drink grade 0.78 – 0.78 – 0.85 drink grade 2.00 dℓ/g Monofilament, engineering plastic 1.00 – 1.00 – 2.00
I.
Drying PET is hygroscopic, meaning that it absorbs water from its surroundings.
However, when this 'damp' PET is then heated, the water hydrolyzes the PET, decreasing its resilience. Thus, before the resin can be processed in a molding machine, it must be dried. Drying is achieved through the use of a desiccant or dryers before the PET is fed into the processing equipment.
pg. 16
Inside the dryer, hot dry air is pumped into the bottom of the hopper containing the resin so that it flows up through the pellets, removing moisture on its way. The hot wet air leaves the top of the hopper and is first run through an after-cooler, because it is easier to remove moisture from cold air than hot air. The resulting cool wet air is then passed through a desiccant bed. Finally the cool dry air leaving the desiccant bed is re-heated in a process heater and sent back through the same processes in a closed loop. Typically, residual moisture levels in the resin must be less than 5 parts per million (parts of water per million parts of resin, by weight) before processing. Dryer residence time should not be shorter than about four hours. This is because drying the material in less than 4 hours would require a temperature above 160 °C, at which level hydrolysis would begin inside the pellets before they could be dried out. PET can also be dried in compressed air resin dryers. Compressed air dryers do not reuse drying air. Dry, heated compressed air is circulated through the PET pellets as in the desiccant dryer, then released to the atmosphere.
II.
Copolymers In addition to pure (homopolymer) PET, PET modified by copolymerization is
also available. In some cases, the modified properties of copolymer are more desirable for a particular application. For example, cyclohexane dimethanol (CHDM) can be added to the polymer backbone in place of ethylene. Since this building block is much larger (6 additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighboring chains the way an ethylene glycol unit would. This interferes with crystallization and lowers the polymer's melting temperature. Such PET is generally known as PETG (Eastman Chemical and SK Chemicals are the only two manufacturers). PETG is a clear amorphous thermoplastic that can be injection molded or sheet extruded. It can be colored during processing.
Replacing terephthalic acid (right) with isophthalic acid (center) creates a kink in the PET chain, interfering with crystallization and lowering the polymer's melting point.
pg. 17
Another common modifier is isophthalic acid, replacing some of the 1,4-(para-) linked terephthalate units. The 1,2-(ortho-) or 1,3-(meta1,3- (meta-)) linkage produces an angle in the chain, which also disturbs crystallinity. Such copolymers are advantageous for certain molding applications, such as thermoforming, which is used for example to make tray or blister packaging from coPET film, or amorphous PET sheet (A-PET) or PETG sheet. On the other hand, crystallization is important in other applications where mechanical and dimensional stability are important, such as seat belts. For PET bottles, the use of small amounts of isophthalic acid, CHDM, DEG or other comonomers can be useful: if only small amounts of comonomers are used, crystallization is slowed but not prevented entirely. As a result, bottles are obtainable via stretch blow molding ("SBM"), which are both clear and crystalline enough to be an adequate barrier to aromas and even gases, such as carbon dioxide in carbonated beverages.
III.
Crystals Crystallization of polymers occurs when polymer chains fold up on themselves in a repeating, symmetrical pattern. Long polymer chains tend to become entangled on themselves, which prevents full crystallization in all but the most carefully controlled circumstances. PET is no exception to this rule; About 60% crystallization is the upper limit for commercial products, with the exception of polyester fibers. Besides, about 60% crystalline polymer about 40 % of the polymer chains remaining amorphous. Therefore PET is commonly called a semi-crystalline polymer. PET in its natural state is a semi-crystalline resin. Clear products can be produced by rapidly cooling molten polymer below T g glass transition temperature to form
an amorphous solid. Like glass, amorphous PET forms when its molecules are not given enough time to arrange themselves in an orderly, crystalline fashion as the melt is cooled. At room temperature the molecules are frozen in place, but if enough heat energy is put back into them by heating above T g, they begin to move again, allowing crystals to nucleate and grow. This procedure is known as solid-state crystallization. Like most materials, PET tends to produce spherulites containing many small crystallites when crystallized from an amorphous solid, rather than forming one large single crystal. Light tends to scatter as it crosses the boundaries between crystallites and the amorphous regions between them. This scattering means that crystalline PET is
pg. 18
opaque and white in most cases. Fiber drawing is among the few industrial processes that produce a nearly single-crystal product.
IV.
Degradation PET is subject to various types of degradations during processing. The main
degradations that can occur are hydrolytic, thermal and, probably most important, thermal oxidation. When PET degrades, several things happen: discoloration, chain scissions resulting in reduced molecular weight, formation of acetaldehyde and cross-links ("gel" or "fish-eye" formation). Discoloration is due to the formation of various chromophoric systems following prolonged thermal treatment at elevated temperatures. This becomes a problem when the optical requirements of the polymer are very high, such as in packaging applications. The thermal and thermooxidative degradation results in poor processibility characteristics and performance of the material. One way to alleviate this is to use a copolymer. Comonomers such as CHDM or isophthalic acid lower the melting temperature and reduce the degree of crystallinity of PET (especially important when the material is used for bottle manufacturing). Thus the resin can be plastically formed at lower temperatures and/or with lower force. This helps to prevent degradation, reducing the acetaldehyde content of the finished product to an acceptable (that is, unnoticeable) level. See copolymers, above. Other ways to improve the stability of the polymer is by using stabilizers, mainly antioxidants such as phosphites. Recently, molecular level stabilization of the material using nanostructured chemicals has also been considered. Degradation may cause acetaldehyde:
Acetaldehyde is a colorless, volatile substance with a fruity smell. Although it forms naturally in some fruit, it can cause an off-taste in bottled water. Acetaldehyde forms by degradation of PET through the mishandling of the material. At high temperatures, (PET decomposes above 300 °C or 570 °F), high pressures, extruder speeds (excessive shear flow raises temperature) and long barrel residence times all contribute to the production of acetaldehyde. When acetaldehyde is produced, some of it remains dissolved in the walls of a container and then diffuses into the product stored inside, altering the taste and aroma. This is not such a problem for non-consumables (such as shampoo), for fruit juices (which already contain acetaldehyde), or for strong-tasting drinks like soft drinks. For bottled water, however, low acetaldehyde content is quite
pg. 19
– important, because if nothing masks the aroma, even extremely low concentrations (10 (10 – 20 parts per billion in the water) of acetaldehyde can produce an off-taste.
2:4:3 The Advantages and Disadvantages of PET a) The advantages of PET: PET provides very good alcohol and essential oil barrier properties, generally good chemical resistance (although acetones and ketones will attack PET) and a high degree of impact resistance and tensile strength. The orienting process serves to improve gas and moisture barrier properties and impact strength. This material does not provide resistance to high temperature applications — applications — maximum maximum temperature 160 °F (71 °C). Generally PET offers numerous advantages because it is easily moldable and thus allows production of individual bottle shapes, specifically designed as non-returnable items. Further advantages for the consumer are its stability, lightweight, inexpensive and shatter-resistant. In addition bottles can be made completely from PET, which simplifies recycling considerably. Also recycled polyethylene terephthalate (RPET) can be used to make many new products, including fiber for polyester carpet; fabric for T-shirts, athletic shoes, luggage, upholstery and sweaters; fiberfill for sleeping bags and winter coats; industrial strapping, sheet and film; automotive parts such as luggage racks, fuse boxes, bumpers, grills and door panels; and new PET containers for both food and non-food products.
b) The disadvantages of PET: I.
Its influence on beverages:
Gas permeability is a major difficulty here, as it can lead to problems within the beverage. Because these processes occur as diffusion, independent of pressure, even a carbonated beverage takes up oxygen and at the same time releases carbon dioxide. The intruding oxygen can damage beverage ingredients, in particular vitamins and flavors. In addition, PET can absorb flavor components of the beverage. This is a result of the structure of the plastic. The long polymer molecules are tangled within each other like a sponge. In this "sponge", flavor
pg. 20
components are stored and later released again. When the crystallization level increases, this sponge structure becomes "untangled" and less foreign material may be absorbed. This has made the development of returnable PET bottles possible, despite the possibility of flavor absorption. At the same time, crystallization of the material also improves its resistance to heat, so that hot-fill PET bottles can also be produced using the same technology. However, these disadvantages mean that the shelf life of a beverage in PET is usually shorter than in glass bottles. But through appropriate recipe design, the manufacturer of flavor systems can offset many of the disadvantages of PET.
II.
Toxicity of PET:
Commentary published in Environmental Health Perspectives in April 2010 suggested that PET might yield endocrine disruptors under conditions of common use and recommended research on this thi s topic. Proposed mechanisms include leaching of phthalates as well as leaching of antimony.
2:4:4 Bottle Processing Equipment There are two basic molding methods for PET bottles, one-step and two-step. In two-step molding, two separate machines are used. The first machine injection molds the preform, which resembles a test tube, with the bottle-cap threads already molded into place. The body of the tube is significantly thicker, as it will be inflated into its final shape in the second step using stretch blow molding. In the second step, the preforms are heated rapidly and then inflated against a two-part mold to form them into the final shape of the bottle. Preforms (uninflated bottles) are now also used as containers for candy, and by some Red Cross chapters to distribute to homeowners to store medical history for emergency responders. In one-step machines, the entire process from raw material to finished container is conducted within one machine, making it especially suitable for molding non-standard shapes (custom molding), including jars, flat oval, flask shapes etc. Its greatest merit is
pg. 21
the reduction in space, product handling and energy, and far higher visual quality than can be achieved by the two-step system . [8][9]
2:5 Thermoplastic Products Manufacture Thermoplastic resins are available to the processing industry as pellets of resin. Converting the raw material into useful products can involve separate segments of the plastics industry. As Figure 2.1 suggests, the resin might be compounded by a custom compounder and formed into the final product by a processor or a fabricator. The compounding can also be carried out by the processor in an in-house facility.
Figure 2.1 Flow diagram illustrating components of plastics industry
.
The resin raw material needs to be mixed intimately with a variety of chemical additives to impart specific properties to the end product. Additives are used widely in the plastics industry, in nearly all types of plastic products. The use of common plastics in consumer products would not be possible without the use of additives. For instance, vinyl plastics (particularly PVC) undergo easy thermal and photodegradation; no useful
pg. 22
products can be made with it if stabilizer additives designed to protect the resin during thermal processing and use were not available. Selecting the appropriate set of additives called for by a given product and mixing these in correct proportion with the resin is referred to as compounding. . To ensure adequate mixing or dispersion of the additive, the mixing is accomplished by passing the resin and additive mixture at a temperature high enough to melt the thermoplastic, through a mixing screw in an extruder (a compounding extruder). Care is taken not to overheat or overshear the mix to an extent to cause chemical breakdown of the plastic itself or the additive materials. The now ―compounded‖ resin with the additives evenly distributed within its bulk is repelletized, cooled, dried (where the pelletization is carried out undercooling water), and stored for subsequent processing. Processing is the final step that converts the compounded material into a useful plastic product. Basically, the compounded resin needs to be melted into a liquid and heated to a temperature that allows easy handling of the fluidized plastic or the ―melt.‖ This melt is fed into molds or dies to force the material into required shapes and quickly cooled to obtain the product. Usually, some minor finishing is needed before the product is made available to the consumer. The basic principals involved in common processing methods associated with high- volume products will be discussed briefly below. [10]
2:5:1 Extrusion Processing Extrusion is the process where a solid plastic (also called a resin), usually in the form of beads or pellets, is continuously fed to a heated chamber and carried along by a feedscrew within. The feedscrew is driven via drive/motor and tight speed and torque control is critical to product quality. As it is conveyed it is compressed, melted, and forced out of the chamber at a steady rate through a die. The immediate cooling of the melt results in resolidification of that plastic into a continually drawn piece whose cross section matches the die pattern. This die has been engineered and machined to ensure that the melt flows in a precise desired shape.
pg. 23
Examples of extruders products are blown film, pipe, coated paper, plastic filaments for brush bristles, carpet fibers, vinyl siding, just about any lineal shape, plus many, many more. There is almost always downstream processing equipment that is fed by the extruder. Depending on the end product, the extrusion may be blown into film, wound, spun, folded, and rolled, plus a number of other possibilities. This article limits any equipment discussion to the extruder itself. Plastics are very common substances for extrusion. Rubber and foodstuffs are also quite often processed via extrusion. Occasionally, metals such as aluminum are extruded plus trends and new technologies are allowing an ever-widening variety of materials and composites to be extruded at continually increasing throughput rates. This article will focus only on the extrusion of plastics. Figure 2.2 shows a basic extruder machine. Plastic pellets or beads (also referred
to as resin) are fed from the hopper along a feed screw through a barrel chamber. As the resin travels along the barrel, it is subject to friction, compression, and heated zones. The result is that the resin melts and further travel at the exit end of the screw serves to mix the melt homogeneously. The melt enters a chamber designed to ensure an evenly distributed flow to the die. In many machines, a melt pump is used to prevent any pressure surges. Also, breaker plates serve to prevent any solid particles or foreign objects from passing through the die. The die is a precisely machined part with a patterned opening such that the extruded plastic takes that die pattern for its cross sectional area. With products such as extruded sheet, there are adjustments to the die to allow for a variety of sheet thicknesses with one die. Shapes are varied, and typically are holes for filament, annular rings for pipe and tube, or geometric patterned shapes for items such as vinyl siding and window frame stock. All die surfaces must be free from defects otherwise unwanted patterns will appear on the extruded product. Product from the die solidifies quickly. Depending on the end product, this may be achieved by immersion in cooling water, air cooling, or contact with chill rolls. As mentioned above, overheating the melt is to be avoided at all costs, or the product will not form properly on solidification. Once solid, the product material can be wound, spun, or cut in defined lengths depending upon its intended end-use. [11]
pg. 24
Figure 2.2 Basic extruder machine.
2:5:2 Injection Molding Injection molding is one of the most popular processing operations in the plastics industry. In recent years, more than half the processing machinery manufactured was injection-molding machines. The equipment is basically designed to achieve the melting of the resin, injecting the melt into a cavity mold, packing the material into the mold under high pressure, cooling to obtain solid product, and ejecting the product for subsequent finishing. It is different from extruders in that a mold is used instead of a die, requiring a large force to pack the melt into the mold. A machine is typically classified by the clamping force (which can vary from 1 to 10,000 tons!) and the shot size determined by the size of the article to be manufactured. Other parameters include injection rate, injection pressure, screw design, and the distance between tie bars. The machine is generally made of a hydraulic system, plasticating and injection system, mold system, and a clamping system. The hydraulic system delivers the power for the operation of the equipment, particularly to open and clamp down the heavy mold halves. The injection system consists of a reciprocating screw in a heated barrel assembly and an injection nozzle.
pg. 25
The system is designed to get resin from the hopper, melt and heat to correct temperature, and deliver it into the mold through the nozzle. Electrical heater bands placed at various points about the barrel of the equipment allow close control of the melt temperature. The mold system consists of platens and molding (cavity) plates typically made of tool-grade steel. The mold shapes the plastic melt injected into the cavity (or several cavities). Of the platens, the one attached to the barrel side of the machine is connected to the other platen by the tie bars. A hydraulic knock-out system using ejector pins is built into one of the platens to conveniently remove the molded piece. The machine operates in an injection-molding cycle. The typical cycle sequence is, first, the empty mold closes, and then the screw movement delivers an amount of melt through the nozzle into it. Once the mold is full, the pressure is held to ―pack‖ the melt well into the mold. The mold is then cooled rapidly by a cooling medium (typically water, steam, or oil) flowing through its walls, and finally the mold opens to eject the product. It is common for this cycle to be closely monitored and to be mostly automated by the use of sophisticated control systems. Figure 2.3 shows a diagram of a simple injection molding machine indicating the hydraulic, injection, and mold systems. The mold filling (a), compaction (b), cooling (c), and ejection (d) steps are also illustrated in Figure 2.3
Figure 2.3 Diagram of a simple injection molding machine indicating the hydraulic, -
injection, and mold systems
pg. 26
When a multicavity mold designed for several ―parts‖ is used, the ejected product is complex, consisting of runners, a spruce, and flashing that needs to
be removed (and
recycled) to obtain the plastic product. Figure 2.4 shows a molding with one of the product ―parts‖ removed from it. [10]
Figure 2:4 Injection molded piece -
.
2:5:3 Blow Molding Blow Molding is a highly developed molding technology developed back in the late 1800's to produce celluloid baby rattles. It is best suited for basically hollow parts (such as plastic bottles) with uniform wall thicknesses, where the outside shape is a major consideration. The first polyethylene bottle was manufactured using the blow molding process in December of 1942. This was the real beginning of a huge industry which currently produces 30 to 40 billion plastic bottles per year in the U.S. alone. The basic process in blow molding: a thermoplastic resin is heated to a molten
state, and then it is extruded through a die head to form a hollow tube called a parison, the parison is dropped between two mold halves, which close around it and then the parison is inflated. The plastic solidifies as it is cooled inside the mold and last the mold opens and the finished component is removed (a parison is a plastic tubular form produced by extrusion or injection molding.) . Figure 2.5 illustrates the steps involved in extrusion blow molding.
pg. 27
Figure 2.5 Blow molding of plastic bottle
.
There are basically four types of blow molding used in the production of plastic bottles, jugs and jars. These four types are:
1. Extrusion blow molding 2. Injection blow molding 3. Stretch blow molding 4. Reheat and blow molding. Extrusion blow molding is perhaps the simplest type of blow molding. A hot tube
of plastic material is dropped from an extruder and captured in a water cooled mold. Once the molds are closed, air is injected through the top or the neck of the container; just as if one were blowing up a balloon. When the hot plastic material is blown up and touches the walls of the mold the material "freezes" and the container now maintains its rigid shape. Figure 2.6 illustrates the steps involved in extrusion blow molding.
Figure 2.6 Extrusion Blow Molding, (a) Parison extrusion (b) Mold halves close onto parison (c) Parison inflated against internal mold walls and (d) Mold halves open, blow molded part ready for enjection
pg. 28
Injection blow molding is part injection molding and part blow molding. With
injection blow molding, the hot plastic material is first injected into a cavity where it encircles the blow stem, which is used to create the neck and establish the gram weight. The injected material is then carried to the next station on the machine, where it is blown up into the finished container as in the extrusion blow molding process above. Injection blow molding is generally suitable for smaller containers and absolutely no handleware. Figure 2.7 illustrates the steps involved in injection blow molding.
Figure 2.7 Injection Blow Molding, (a) injection cycle, polymer melt supplied to mold halves from injection molding machine, product formed is preform and (b) heated preform inflated against cold mold wall halves
Extrusion blow molding allows for a wide variety of container shapes, sizes and neck openings, as well as the production of handleware. Extrusion blown containers can also have their gram weights adjusted through an extremely wide range, whereas injection blown containers usually have a set gram weight which cannot be changed unless a whole new set of blow stems are built. Extrusion blow molds are generally much less expensive than injection blow molds and can be produced in a much shorter period of time. Stretch blow molding is perhaps best known for producing PET bottles
commonly used for water, juice and a variety of other products. There are two processes
pg. 29
for stretch blow molded PET containers. In one process, the machinery involved injection molds a preform, which is then transferred within the machine to another station where it is blown and then ejected from the machine. This type of machinery is generally called injection stretch blow molding (ISBM) and usually requires large runs to justify the very large expense for the injection molds to create the preform and then the blow molds to finish the blowing of the container. This process is used for extremely high volume (multi-million) runs of items such as wide mouth peanut butter jars, narrow mouth water bottles, liquor bottles etc. Figure 2.8 illustrates the steps involved in stretch blow molding.
Figure 2.8 Stretch Blow Molding, (A) Stretch-blow pin (B) Air entrance (C) Mold vents (D) Preform (E) Stretch rod extended (F) Cooling channels.
The reheat and blow molding process (RHB) is a type of stretch blow process. In this process, a preform is injection molded by an outside vendor. There are a number of companies who produce these "stock" preforms on a commercial basis. Factories buy the preforms and put them into a relatively simple machine which reheats it so that it can be blown. The value of this process is primarily that the blowing company does not have to purchase the injection molding equipment to blow a particular container, so long as a preform is available from a stock preform manufacturer. This process also allows access to a large catalog of existing preforms. Therefore, the major expense is now for the blow molds, which are much less expensive than the injection molds required for preforms.
pg. 30
There are, however, some drawbacks to this process. If you are unable to find a stock preform which will blow the container you want, you must either purchase injection molds and have your own private mold preforms injection molded, or you will have to forego this process. For either type of stretch blow molding, handleware is not a possibility at this stage of development. The stretch blow molding process does offer the ability to produce fairly lightweight containers with very high impact resistance and, in some cases, superior chemical resistance. Whether using the injection stretch blow molding process or the reheat and blow process, an important part of the process is the mechanical stretching of the preform during the molding process. The preform is stretched with a "stretch rod." This stretching helps to increase the impact resistance of the container and also helps to produce a very thin walled container. The extrusion blow molding process allows for the production of bottles in a wide variety of materials, including but not limited to: HDPE, LDPE, PP, PVC, BAREX®, PET, K Resin, PETG, and Polycarbonate. As noted above, a wide variety of shapes (including handleware), sizes and necks are available. Injection blow molding allows for the production of bottles in a variety of materials, including but not limited to: HDPE, LDPE, PP, PVC, BAREX®, P.E.T., and Polycarbonate. Besides the PET noted above for stretch blow molding, a number of other materials have been stretch blown, including polypropylene. As time goes on and technology moves forward, more materials will lend themselves to stretch blow molding as their molecular structures are altered to suit this process. [12]
2:5:4 Extrusion Blowing of Film Extrusion blowing of common plastics such as polyethylenes into film is one of the oldest processing techniques (dating back to the 1930s in the United States). The basic process is simple and is based on a special annular die that is connected to one or
pg. 31
more extruders. In the simple case with a single extruder, the molten plastic material is extruded vertically upwards through the die into a thin-walled plastic tube. Blowing air into the tube expands the soft molten polymer, deforming it circumferentially into a tube with a wider diameter, while the pickup and winding up of the collapsed tube elongates the tube in the machine direction. The ratio of the pickup or haul-off rate to that of extrusion is called the draw-down ratio. The tubular film can be blown up by air only while it is soft and soon forms a ―freeze line‖ at a maximum diameter (the ratio of the diameter at the freeze line to that of the annular die is the blow-up ratio for the film). To obtain a uniform film, it is crucial to maintain constant extrusion rates and a symmetric stable ―bubble‖ or the inflated cylinder of polymer at all times during processing. – 30 Typically the bubble can be 15 15 – 30 ft tall and up to several feet in diameter. The processing variables as well as the grade of resin used for film blowing determines the quality and uniformity of the film product Figure 2.6 shows a diagram of film blowing equipment.
Figure 3.6 Schematic representation of extrusion blowing of plastic film
.
pg. 32
The same process can also be used to produce a multilayered film using several extruders, one for each type of resin used, and a feed block to direct the resin into different layers. The layers need to be selected carefully for their processing characteristics as well as their performance in the final product. For instance, in coextrusion of a barrier film for packaging applications, different layers of the film might be selected for different functionality needed in the prod. [10]
2:5:5 Continuous Strip Molding Continuous strip molding (or reel-to-reel molding) is used to efficiently manufacture and assemble small electronic components and other parts that combine tiny metal and plastic components. The process uses a feed reel and a take-up reel: a continuous metal "carrier" strip is precisely indexed off the feed reel, passes through an injection mold and onto the take-up reel. The molded parts are delivered to customers still on the take-up reel, and placed directly in automated production equipment for hands-off operations that add more components, install covers, or form electrical leads, etc.
2:5:6 Rotational Molding Rotational Molding begins with the melting of a plastic resin in a closed mold. Unlike most other plastic processes, no pressure is involved. The three-stage process includes loading the resin in the mold, heating and fusion of the resin and cooling and unloading the mold. After the charged mold is moved into an oven, the mold is rotated on two axes at low speed. As heat penetrates the mold, the resin adheres to the mold's inner surface until it is completely fused. The mold is then cooled by air or water spray or a combination of both while still rotating, lowering the temperature in a gradual manner. The mold is opened, finished part removed and mold recharged for the next cycle. Cycle times vary from 7 to 60 minutes, depending on part size, material and wall thickness. [12]
pg. 33
2:6 PET
Bottle
Manufacturing
and
Color
Measurement The plastic bottle industry has many producer, each striving to manufacture high quality products, Properties such as barrier characteristics, dimensional stability, shape and appearance are of significant importance. Appearance attributes can include reflected color, yellowness, and haze. For some applications the presence of proper levels of UV inhibitors is also important. The use of color measurement instrumentation to test and quantitate these attributes will ensure customer satisfaction and aid in reducing cost.
2:6:1 General Principles There are two basic types of color measurement instruments,
tristimulus colorimeters
and
colorimetric
spectrophotometers. These instruments are categorized as
psychophysical measurement instruments, meaning they are designed to give measurements that correlate with the human eye-brain impressions. They have built in characteristics that simulate the operation of the eye and brain in judgements of color appearance. The tristimulus colorimeter uses colored glass filters to simulate the human eye response to light and today finds limited application in the plastics industry. On the other hand, the more versatile colorimetric spectrophotometer is widely used in the plastics industry. It measures the wavelength distribution of light reflected or transmitted by a sample and this data is used to calculate color values. Color is measured in terms of a tristimulus color scale such as the CIE L*a*b* scale. The L* value ranges from 0 to 100 and measures dark to light, the a* value measures red to green with positive values being red and negative values green. The b* value measures yellow to blue with yellow having positive values and blue negative values. Comparisons between a standard and a sample is expressed in terms of L*a*b*.
This is the difference in L*, a* and b* between the standard and the sample.
pg. 34
Using this information, one can interpret both the size of the difference (large or small numbers) and direction of color difference (+ or -). When determining acceptability, these numbers must fall within predetermined tolerance limits. In addition to tristimulus color scales there are single number indices for measuring whiteness and yellowness. MSphere geometry spectrophotometers have proven to be the ideal tool for bottle makers to quantify appearance attributes. Users report that they find significant benefit in the spectrophotometer‘s ability to measure both reflectance and transmittance and to obtain both spectral data (to detect UV inhibitors) and tristimulus color data (for visual color), all with the same instrument. This type of instrument can be used to measure raw materials, preforms, bottles and closures during manufacture. It can also be used to measure fiber, film, sheet, and pellets for PET bottle recycling. Data can be displayed, printed, stored and/or exported to other programs. If desired, tolerances can be entered for Pass/Fail indication.
2:6:2 Pellets The color, whiteness or yellowness of plastic pellets is often measured before the pellets are molded into preforms or final product. Plastic pellets have several nonuniform characteristics that require compensating preparation and presentation techniques in order to ensure repeatable sample measurement.
– not a solid sample – They are in the form of a pellet – not and must be measured through the clear window of a glass cell in order to be effectively made into a ―solid‖.
Pellets are irregular in size and shape, requiring the averaging of several readings with refilling of the glass cell between each reading.
Pellets are translucent – not opaque – and can be sensitive to ambient light and to small differences of the optical configuration of the instrument. Using sufficient sample thickness and an opaque cover will minimize these effects.
pg. 35
In
practice
the
pelletsare
poured
into
a
large
transmission type cell with 50mm path length. This large sample thickness makes the translucent, irregular pellets effectively opaque for reflectance measurement. The filled cell is placed on a sample shelf against the instrument reflectance port so that the pellets will be read through the clear glass window of the cell. A color reading of the pellets is then made. The pellets are then dumped out of the cell and poured back in and another reading made. This process is normally repeated three to five times and the readings averaged for a single color measurement. Averaging multiple readings minimizes measurement variation associated with non-uniform samples.
2:6:3 Preforms The color of preforms is frequently measured prior to being reheat blow molded (RHB) into final bottle form. Ensuring that the preform color is correct greatly increases the probability that the blown bottle will have the proper color. Thus off-color bottles are reduced and money is saved. However, preforms can also be measured earlier in the process. Measuring the preform during the product changeover process helps to get the process on color more quickly saving time and money. Transparent preforms are measured by transmission and near opaque or opaque preforms are measured by reflection. Because of their cylindrical shape, preforms require special sample-handling to precisely position the sample to be measured. This is accomplishe d by use of ―Preform Holders‖. For transmission, a holder having an aperture and an insert that is the proper size for the preform being measured, are mounted in the instrument transmission compartment. The instrument is standardized for total transmission and small
pg. 36
area view. The preform is placed on the preform insert and a reading taken. The preform is then rotated and another reading made. After a few readings are taken the average is recorded as the measurement. For ―colorless‖ preforms it may m ay be desirable to measure yellowness and for some applications confirm that the proper levels of UV inhibitors are present. Yellowness is measured as a single number scale that is calculated and displayed by the instrument‘s software. For UV inhibitors the spectral data between 360nm to 420nm is viewed. Low transmittance values in this range indicate the presence of UV inhibitors. Higher transmittance values indicate less or no inhibitor. For reflection measurement, a modified sample shelf is mounted at the reflectance measurement port of the instrument. After the instrument is standardized for small area reflectance a preform holder attachment having the proper diameter for the preform to be measured is snapped onto the sample shelf. The preform is then inserted and measurements made much like those for transmission.
2:6:4 Bottles The color of bottles can also be measured. For opaque or near opaque bottles, reflected color is measured. Although a whole bottle can be measured by placing a relatively flat and smooth surface of the bottle against the instrument‘s reflectance port, greater precision will be achieved by cutting a section from the bottle. The cut section is pressed flat against the instrument‘s measurement port. For transparent bottles a positioning device is mounted in the instrument transmission compartment. This device is used to reproducibly position the bottle. The bottle should be filled with distilled water to reduce the reflection from interior bottle surfaces. This simulates the optical conditions of the bottle in use. If a smooth section can be cut from a bottle for measurement, greater precision will be achieved and if desired, transmission haze of the bottle material can also be measured. Haze is the scattering of light within a nearly clear specimen and is responsible for cloudy appearance. The cut
pg. 37
section to be measured is held in place against the sphere on the inside of the instrument transmission compartment by a ―Transmission Sample Clamp‖.
2:6:5 Caps The color of bottle closures can be measured. In most cases the small area reflectance mode of the instrument is used and a relatively flat surface, such as the top of the closure, is placed against the instrument port. If the top is not flat, then the side of the closure is measured. A positioning fixture mounted at the reflectance port would be used to ensure that the closures are reproducibly positioned. Collected PET bottles are frequently recycled into fiber, film, sheet, pellets and new bottles. Fiber color, whiteness or yellowness can be measured by using a ―Compression Cell Attachment‖ on the instrument. This device is used for compressing fibers into a compact mass to permit repeatable color analysis. A specified amount of fiber is placed in a container having a glass window in the bottom and air pressure is applied to a piston to compress the fiber. By applying a consistent amount of pressure to the piston and using a consistent amount of fiber, a consistent yarn density is achieved for reproducible measurements. Recycled film and sheet would be measured similar to methods used for measuring bottles. The colorimetric spectrophotometer is a useful tool for measuring color at many phases of the bottle making process. Checking the color of raw material such as pigment, resin and compound helps to ensure that the molded preform will be the proper color. And checking preform color ensures that the blown bottle will meet color requirements. This enables the bottle manufacturer to produce the highest volume of quality product at the lowest cost. [13]
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2:7 PET Bottle Recycling 2:7:1 General Approximately 5.8 million tons of PET were collected in 2009 worldwide. This gave 4.7 million tons of flake. 3.4 million tons were used to produce fiber, 500,000 tons to produce bottles, 500,000 tons to produce APET sheet for thermoforming, 200,000 tons to produce strapping tape and 100,000 tons for miscellaneous applications. Petcore, the European trade association that fosters the collection and recycling of PET, reported that in Europe alone, 1.45 million tonnes of PET bottles were collected in 2010 - more than 48.3% of all bottles. After exported bales were taken into account, 975,000 tons of PET flake were produced. 382,000 tons were used to produce fibers, 244,000 tons to produce more bottles, 221,000 tons to produce APET sheets, 93,000 tons for strapping tape and 33,000 tons for miscellaneous applications. [14] From its beginning, the PET plastic packaging industry has demonstrated its commitment to environmental responsibility through recycling. Prior to the introduction of the PET soft drink bottle on grocery shelves, PET bottle manufacturers and consumer product companies worked with private recycling companies to demonstrate that this new packaging material could be recycled, a major concern for new packaging, given the popularity of recycling with the American public. Reportedly, the first PET bottle recycling process was established by a company called St. Jude Polymers in 1976, that began recycling PET bottles into plastic strapping and paint brush bristles. In 1977, St. Jude became to first to ―repelletize‖ post-consumer post-consumer PET plastic. This was an important step, as many PET remanufacturing companies rely on plastic in pelletized form for their processes, increasing the variety of products that can be made from recycled, postconsumer PET plastic. However, a major push in the development of both the demand and the capacity for postconsumer PET recycling occurred when a major plastic fiber manufacturer named Wellman, Inc., entered the picture. As early as 1978, Wellman began recycling PET bottles into a fiber product that was suitable for both carpet and fiberfill applications. Wellman continued to increase its use of recycled PET and throughout the 1980s and
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early 1990s increased their processing capacity and consequently the market demand for post -consumer PET post-consumer post-consumer PET. The major event in Wellman‘s development of post-consumer processing capacity was the vertical integration of the recycled PET it processed into its own product lines. Another was the development of the first textile fiber manufactured from 100% recycled PET in 1993, called ―Eco Spun,‖ which is now a familiar fabric material particularly in sportswear where it was first used. Today, St. Jude and Wellman are joined by more than a dozen other companies, whose combined PET recycling processing capacity produces over 1/2 billion pounds of recycled PET resin annually. With recent advances in PET recycling technology, it is now possible to ―close the loop,‖ by recycling bottles and containers back into bottles and containers, even in some food-contact packaging applications. The federal Food and Drug Administration (FDA) has issued ―letters of non non--objection‖ for the use of post-consumer PET in a number of food-contact packaging applications. This has greatly increased the demand for recycled PET plastic and the ability to produce new PET packages from 100%, post-consumer recycled PET plastic. [15] Purification and decontamination are the most important processing steps during polyester recycling The success of any recycling concept is hidden in the efficiency of .
purification and decontamination at the right place during processing and to the necessary or desired extent. Generally, the following applies: the sooner foreign substances are removed, in the process, and the more thoroughly this is done, the more efficient the process is
.
The high plasticization temperature of PET in the range of 280°C is the reason why almost all common organic impurities such as PVC, PLA, polyolefin, chemical wood-pulp and paper fibers, polyvinyl acetate, melt adhesive, coloring agents, sugar and proteins residues are transformed into colored degradation products which, in their turn, might release reactive degradation products additionally. Then, the number of defects in the polymer chain increases considerably. Naturally, the particle size distribution of impurities is very wide, the big particles of 60 – 60 – 1000 1000 µm (which are visible by naked eye and easy to filter) representing the lesser evil since their total surface is relatively small and the degradation speed is therefore lower. The influence of the microscopic particles,
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which (because they are many) increase the frequency of defects in the polymer, is comparable bigger
.
The motto "What the eye does not see the heart cannot grieve over" is considered to be very important in many recycling processes. Therefore besides efficient sorting the removal of visible impurity particles by melt filtration processes is playing a particular part in this case
.
In general one can say that the processes to make PET bottle flakes from collected bottles are as versatile as the different waste streams are different in their composition and quality. In view of technology there isn‘t just one way to do it. There are meanwhile many engineering companies which are offering flake production plants and components, and it is difficult to decide for one or other plant design. Nevertheless there are principles which are sharing most of these processes.
Depending on composition and impurity level of input material the general following process steps are applied: 1. Bale opening, briquette opening. 2. Sorting and selection for different colors, foreign polymers especially PVC,
foreign
matter, removal of film, paper, glass, sand, soil, stones and metals.
3. Pre-washing without cutting. 4. Coarse cutting dry or combined to pre-washing. 5. Removal of stones, glass and metals. 6. Air sifting to remove film, paper and labels. 7. Grinding, dry and/or wet. 8. Removal of low-density polymers (cups) by density differences. 9. Hot wash. 10. Caustic wash. 11. Caustic surface etching, maintaining intrinsic viscosity and decontamination. 12. Rinsing. 13. Clean water rinsing. 14. Drying.
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15. Air sifting of flakes. 16. Automatic flake sorting. 17. Water circuit and water treatment technology. 18. Flake quality control.
When recycling polyethylene terephthalate or PET or polyester, two ways generally have to be differentiated: 1- The chemical recycling back to the initial raw materials purified terephthalic acid
(PTA) or dimethyl terephthalate (DMT) and ethylene glycol (EG) where the polymer structure is destroyed completely, or in process intermediates like bis-ßhydroxyterephthalate
.
2- The mechanical recycling where the original polymer properties are being maintained
or reconstituted
.
Chemical recycling of PET will become cost-efficient only applying high capacity recycling lines of more than 50,000 tons/year. Such lines could only be seen, if at all, within the production sites of very large polyester producers. Several attempts of industrial magnitude to establish such chemical recycling plants have been made in the past but without resounding success. Even the promising chemical recycling in Japan has not become an industrial break through so far. The two reasons for this are at first the difficulty of consistent and continuous waste bottles sourcing in such a huge amount at one single site and at second the steadily increased prices and price volatility of collected bottles. The prices of baled bottles increased for instance between the years 2000 and 2008 from about 50 Euro/ton to over 500 Euro/ton in 2008
.
Mechanical recycling or direct circulation of PET in the polymeric state is operated in most diverse variants today. These kinds of processes are typical of small and medium-sized industry. Cost-efficiency can already be achieved with plant capacities within a range of 5,000 – 20,000 tons/year. In this case, nearly all kinds of recycledmaterial feedback into the material circulation are possible today
.
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Besides chemical contaminants and degradation products generated during first processing and usage, mechanical impurities are representing the main part of quality depreciating impurities in the recycling stream. Recycled materials are increasingly introduced into manufacturing processes, which were originally designed for new materials only. Therefore, efficient sorting, separation and cleaning processes become most important for high quality recycled polyester
.
When talking about polyester recycling industry we are concentrating mainly on recycling of PET bottles which are meanwhile used for all kinds of liquid packaging like water, carbonated soft drinks, juices, beer, sauces, detergents, household chemicals and so on. Bottles are easily to distinguish because of shape and consistency and separate from waste plastic streams either by automatic or hand sorting processes.
The established polyester recycling industry exists of three major sections : PET bottle collection and waste separation (waste logistics). Production of clean bottle flakes (flake production). Conversion of PET flakes to final products (flake processing).
Intermediate product from the first section is baled bottle waste with a PET content greater than 90%. Most common trading form is the bale but also bricked or even loose, pre-cut bottles are common in the market. In the second section the collected bottles are converted to clean PET bottle flakes. This step can be more or less complex and complicated depending on required final flake quality. During third step PET bottle flakes are processed to any kind of products like film, bottles, fiber, filament, strapping or intermediates like pellets for further processing and engineering plastics
.
Aside this external polyester bottle recycling numbers of internal recycling processes exist, where the wasted polymer material does not exit the production site to the free market and where the waste is reused at one and the same production circuit. In this way for instance fiber waste is directly reused to produce fiber, preform waste is directly reused to produce performs and film waste is directly reused to produce film. [16]
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Impurities and Material Defects The number of possible impurities and material defects which accumulate in the polymeric material is increasing permanently (when processing as well as when using polymers) taking into account a growing service life time, growing final applications and repeated recycling.
As far as recycled PET bottles are concerned, the defects mentioned can be sorted in the following groups : a) Reactive polyester OH- or COOH- end groups are transformed into dead / not
reactive end groups, e.g. formation of vinyl ester end groups through dehydration or decarboxylation of terephthalate acid, reaction of the OH- or COOH- end groups with mono-functional degradation products like mono-carbonic acids or alcohols. Results are decreased reactivity during re-polycondensation or re-SSP and broadening the molecular weight distribution
.
b) The end group proportion shifts toward the direction of the COOH end groups built
up through a thermal and oxidative degradation. Results are decrease in reactivity, increase in the acid autocatalytic decomposition during thermal treatment in presence of humidity
.
c) Number of poly-functional macromolecules increases. Accumulation of gels and
long-chain branching defects
.
d) Concentration and variety of non polymer-identical organic and inorganic foreign
substances are increasing. With every new thermal stress, the organic foreign substances will react by decomposition. This is causing the liberation of further degradation-supporting substances and coloring substances
.
e) Hydroxide and peroxide groups build up at the surface of the products made of
polyester in presence of air (oxygen) and humidity. This process is accelerated by ultraviolet light. During an ulterior treatment process, hydro peroxides are a source of oxygen-radicals which are source of oxidative degradation. Destruction of hydro peroxides is to happen before the first thermal treatment or during plasticization and can be supported by suitable additives like antioxidants
.
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Taking in consideration the above mentioned chemical defects and impurities, there is ongoing a modification of the following polymer characteristics during each recycling cycle, which are detectable by chemical and physical laboratory analysis
.
In particular:
Increase of COOH end groups. Increase of color number. Increase of haze (transparent products). Increase of oligomer content. Reduction in filterability. Increase of by-products content such as acetaldehyde, formaldehyde. Increase of extractable foreign contaminants. Decrease in color. Decrease of intrinsic viscosity or dynamic viscosity. Decrease of crystallization temperature and increase of crystallization speed. Decrease of the mechanical properties like tensile strength, elongation at break or elasticity modulus. Broadening of molecular weight distribution. The recycling of PET bottles is meanwhile an industrial standard process which is offered by a wide variety of engineering companies. [10]
2:7:2 Recycling Back to the Initial Raw Materials 2:7:2:1 Glycolysis and partial glycolysis The polyester which has to be recycled is transformed into an oligomer by adding ethylene glycol or other glycols during thermal treatment. The aim and advantage of this way of processing is the possibility of separating the mechanical deposits directly and efficient through a progressive and stepwise filtration. The filtration fineness of the last filtration step has a decisive effect on the quality of the end product. Taking partial recycling with partial glycolysis as an example, it is to be demonstrated how bottle waste
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can successfully be recycled in a continuously operating polyester line which is manufacturing pellets for bottle applications
.
The task consists in feeding 10 – 10 – 25% 25% bottle flakes and maintaining at the same time the quality of the bottle pellets which are manufactured on the line. This aim is solved by degrading the PET bottle flakes — flakes — already already during their first plasticization which can be carried out in a single- or multi-screw extruder — extruder — to to an intrinsic viscosity of about 0.30 dℓ/g by adding small quantities of ethylene glycol and by subjecting the low viscosity melt stream to an efficient filtration directly after plasticization. Furthermore, temperature is brought to the lowest possible limit. In addition, with this way of processing, the possibility of a chemical decomposition of the hydro peroxides is possible by adding a corresponding P-stabilizer directly when plasticizing. The destruction of the hydro peroxide groups is, with other processes, already carried out during the last step of flake treatment for instance by adding H3PO3. The partially glycolyzed and finely filtered recycled material is continuously fed to the esterification or prepolycondensation reactor, the dosing quantities of the raw materials are being adjusted accordingly
.
The treatment of polyester waste through total glycolysis to convert the polyester to bis-beta hydroxy-terephthalate, which is vacuum distilled and can be used, instead of DMT or PTA, as a raw material for polyester manufacture, has been executed on an industrial scale in Japan as experimental production
.
2:7:2:2 Hydrolysis Recycling processes, through hydrolysis of the PET to PTA and MEG, are operating under high pressures under supercritical conditions. In this case, PET-waste will be directly hydrolyzed applying for instance supercritical water steam. Purification of crude terephthalic acid will be carried out by re-crystallization in acetic acid / water mixtures similar to PTA purification. Industrial-scale lines based on this chemistry have not been known to date
.
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2:7:2:3 Methanolysis Methanolysis is the recycling process which has been practiced and tested on a large scale for many years in the past. In this case, polyester waste is transformed with methanol into DMT, under pressure and in presence of catalysts. After this an efficient filtration of the methanolysis product is applied. Finally the crude DMT is purified by vacuum distillation. The methanolysis is only rarely carried out in industry today because polyester production based on DMT shrunk tremendously and with this DMT producers disappeared step by step during the last decade. [17]
2:7:3 Practices in Collection and Recycling of PET 2:7:3:1 Collection There are four basic ways in which communities around the world offer recycling collection services for PET plastic bottles and containers (in addition, to other recyclable materials) to their residents. The first method is not up to individual communities but is created as a result of statewide laws known as Returnable Container Legislation, or “Bottle Bills.”
Many states around the United States have passed such legislation, which
establishes a redemption value on carbonated beverage (and, in some cases, noncarbonated beverage) containers. These containers, when returned by the consumer for the redemption value, facilitate recycling by aggregating large quantities of recyclable materials at beverage retailers and wholesalers to be collected by recyclers, while simultaneously providing the consumer with an economic incentive to return soft drink containers for recycling. The second, and most widely accessible, collection method is curbside collection of recyclables. Curbside recycling programs are generally the most convenient for
community residents to participate in and yield high recovery rates as a result. In the United States, research conducted by the Center for Plastics Recycling Research at Rutgers University estimates that curbside collection gathers 70%-90% of available
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recyclables. In addition, estimates by the National Association for Plastic Container Recovery (NAPCOR) indicate that approximately 55% of all the PET plastic containers collected for recycling are generated through curbside programs. Communities that provide curbside collection generally request residents to separate designated recyclables from their household garbage and to place them into special receptacles or bags, which are then set out at the curb for collection by municipal or municipally-contracted crews. Some communities allow their residents to commingle recyclables, that is, mix recyclable materials of different kinds into the same receptacle. Others require some level of material segregation known as ―source separation.‖ For example, many curbside collection programs require that newspapers and cardboard be bundled separately and placed alongside the receptacle containing their commingled recyclable containers. Some communities will collect recyclables on the same day as normal garbage collection, while others have separate days for trash collection and collection of recyclables. The third collection method is known as drop-off recycling. In this method, containers for designated recyclable materials are placed at central collection locations throughout the community, such as parking lots, churches, or other civic associations. The containers are generally marked as to which recyclable material should be placed in them. Residents are requested to deliver their recyclables to the drop-off location, where recyclables are separated by material type into their respective collection containers. Drop-off centers require much less investment to establish than curbside programs, yet do not offer the convenience of curbside collection. However, drop-off collection centers work well in rural locations where curbside collection is impractical. The last collection method employs the use of buy-back centers. While communities do not provide this service per se, as most buy-back recycling centers are operated by private companies, they often provide incentives, through legislation or grants and loan programs, which can assist in the establishment of buy-back centers for their residents. Buy-back centers pay consumers for recyclable materials that are brought to them. Most buy-back centers have purchasing specifications that require consumers to source separate recyclable materials brought for sale, in addition to other requirements they may have (for example, removal of caps from bottles). These purchase specifications
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can greatly reduce contamination levels and allow the buy-back center to immediately begin processing the recyclables they purchase, while providing consumers with an economic incentive to comply with the specifications. Finally, many communities that offer curbside recycling collection services will augment this service with drop-off and buy-back centers where curbside is not as effective, such as near multi-family housing units. While buy-back centers may not be as convenient as curbside collection, they offer an economic incentive to the public that curbside collection does not.
PET plastic wastes are also collected by the following ways: Private Collection: This type of collection is done in restaurants, hotels, business establishments, supermarkets and fast food chains. Household Consumer: The household consumers segregate and sell their plastic waste to eco-aids. However, some of them dispose their commingled solid waste to garbage bins or containers for pick- up by dump trucks or garbage collectors. Junk Shops: There are many junk shops collecting recyclable items and separate them. They buy from scavengers and household consumers and sell their scrap to the recyclers/ processors. PET bottles are sold after sorting and cleaning (removal of cover and label) from the commingled waste. Middleman: The middleman or consolidators operates in the following ways: a) Collects and grinds PET industrial waste "on- site". b) Collects and grinds PET industrial and post consumer waste in their own
plant. c) Collects PET industrial/ consumer waste and sell them to PET recyclers.
2:7:3:2 Sorting and grinding After PET plastic containers are collected they must be sorted and prepared for sale. Each subsequent step in the recycling process adds value to the post-consumer PET and puts it into marketable form for other processors and end-users that will use them to manufacture new products. The amount and type of sorting and processing required will
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depend upon purchaser specifications and the extent to which consumers separate recyclable materials of different types and remove contaminants. Collected PET plastic containers are delivered to a materials recovery facility (MRF) or a plastics intermediate processing facility (IPC) to begin the recycling process. The value of the post-consumer PET plastic and its ability to be economically remanufactured into new products is dependent on the quality of the material as it passes through the recycling process. MRFs accept commingled curbside collected recyclables and separate them into their respective material categories. PET plastic bottles and containers are separated from other recyclables and baled for sale to IPCs, plastics recycling facilities (PRFs), or reclaimers. Unlike MRFs and IPCs, plastic recycling facilities only accept plastic containers, either commingled or source separated from other plastic containers. PRFs will generally accept plastics in both loose and baled form. Very often, these materials are supplied by drop-off and buy-back centers, which require source separation of recyclable materials that are brought to them. Once again, PET plastic bottles and containers are sorted from other plastic containers at PRFs and, in most cases, further processed by color sorting and granulating PET for shipment to reclaimers as ―dirty‖ regrind. Some PRFs merely separate PET and other plastic containers by resin category and bale them for shipment to reclaimers or end-users. However, IPCs shall generally refer to recycling facilities that take in loose; source separated plastic bottles and densifies them for shipment to PRFs, reclaimers or endusers. And, PRFs will be used to describe sorting, baling, and/or grinding facilities.
2:7:3:3 Cleaning and drying Sorting and grinding alone are not sufficient preparation of PET bottles and containers for remanufacturing. There are many items that are physically attached to the PET bottle or containers that require further processing for their removal. These items include the plastic cups on the bottom of many carbonated beverage bottles (known as ―base cups‖), labels and caps.
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Dirty regrind from PRFs is then sent to reclaimers that process post-consumer PET plastic into a form that can be used by converters. Converters process the recycled PET plastic into a commodity-grade form that can then be used by end-users to manufacture new products. At a reclaiming facility, the dirty flake passes through a series of sorting and cleaning stages to separate PET from other materials that may be contained on the bottle or from contaminants that might be present. First, regrind material is passed through an ―air classifier‖ which removes materials lighter than the PET such as plastic or paper labels and ―fines‖ -- very small PET particle fragments that are produced during granulating. The flakes are t hen washed with a special detergent in a ―scrubber.‖ This step removes food residue that might remain on the inside surface of PET bottles and containers, glue that is used to adhere labels to the PET containers, and any dirt that might be present. Next, the flakes pass through what is known as a ―float/sink‖ classifier. During this process, PET flakes, which are heavier than water, sink in the classifier, while base cups made from highdensity polyethylene plastic (HDPE) and caps and rings made from polypropylene plastic (PP), both of which are lighter than water, float to the top. The ability of the float/sink stage to yield pure PET flakes is dependent upon the absence of any other plastics that might also be heavier than water and sink with the PET. It should be noted that some reclaimers use a different device known as a ―hydrocyclone‖ to perform this same step. This device essentially operates like a centrifuge and separates materials based on their weight (density) differences. Following the float/sink stage the flakes are thoroughly dried. After they have dried, the PET flakes pass through what is known as an electrostatic separator, which produces a magnetic field to separate PET flakes from any aluminum that might be present as a result of bottle caps and tennis ball can lids and rings. Some reclaimers use a number of different particle separation technologies where PET flakes are further processed to remove any residual contaminants that may still be present, such as x-ray separation devices for PVC removal, or optical sorting devices to remove other contaminants. The purity level to which PET flakes are processed depends on the end-use applications for which they are intended.
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Once all of these processing steps have been completed, the PET plastic is now in a form known as ―clean flake.‖ In some cases reclaimers will further process clean flake in a ―repelletizing‖ stage, which turns the flake into ―pellet.‖ Clean PET flake or pellet is then processed by reclaimers or converters which transform the flake or pellet into a commodity-grade raw material form such as fiber, sheet, or engineered or compounded pellet, which is finally sold to end-users to manufacture new products.
2:7:3:4 End-use categories recycled PET: There are five major generic end-use categories for recycled PET plastic: 1) Packaging applications (such as new bottles). 2) Sheet and film applications (including some thermoforming applications, such as
laundry scoops). 3) Strapping. 4) Engineered resins applications (such as reinforced components for automobiles). 5) Fiber applications (such as carpets, fabrics and fiberfill).
There are a number of emerging technologies that are generically referred to as depolymerization processes. These processes -- like glycolysis and methanolysis -- break down the PET plastic into its individual chemical components, which can then be recombined back into PET plastic. While not used extensively, these technologies are employed when the economics warrant and offer yet another market opportunity for postconsumer PET plastic containers. One of the highest value end-uses for recycled PET plastic is to manufacture new PET bottles and containers. Recycled PET can be made into numerous other products including: Belts, blankets, boat hulls, business cards, caps, car parts (bumpers, distributor caps, and exterior panels), carpets, egg cartons, furniture, insulation, landfill liners, overhead transparencies, paint brush bristles, pillows, polyester fabric for (upholstery, Tshirts, sweaters, backpacks, athletic wear and shoes), recycling bins, sails, scouring pads,
pg. 52
strapping, stuffing for ski jackets, cushions, mattresses, sleeping bags and quilts, tennis ball cans, tennis ball felt, twine and welcome mats.
2:7:3:5 Designing Community for PET Recycling Collection Program Properly designed PET recycling collection programs greatly increase the quantity and quality of PET collected and can reduce overall recycling system costs. In order to maximize the recovery and value of PET plastic containers our community recycling collection program. Two best practices should be followed when designing program. The first is to establish an effective and ongoing consumer education program. The second best practice is to designate all PET bottles with screw-neck tops as acceptable for recycling. There are seven basic messages that should be included in any consumer education or promotional program aimed at the collection of PET bottles: 1) Only PET bottles with screw-neck tops to be placed for collection or brought to a
collection location. PET can be identified by looking for the #1 code, any non-bottle PET should be excluded. 2) Only PET bottles that are clear or transparent green should be included for recycling,
other colors to be excluded. 3) Consumers should remove lips, caps and other closures from PET bottles placed for
recycling. 4) All PET bottles that are set out for recycling should be completely free of contents
and rinsed clean. 5) Consumers should flatten PET bottles prior to setting out for collection. 6) Consumers should never place any material other than the original content into PET
bottles for recycling. 7) Hypodermic needles are increasing safety concern at recycling facilities. [15]
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2:7:4 Basic Benefits of PET Recycling The greenhouse gas emissions from management of selected materials in municipal solid waste‘s report issued by the EPA, states: ―recycling of PET and other materials positively infects the environment by helping to reduce green house gas emissions and global warming‖. wa rming‖. [18] Recycling of PET plastic waste can help reduce waste disposal costs (since the PET Plastic is removed from the waste stream) and it can generate revenues from the sale of the recycled PET, it can also reduce labor costs associated with the handling of PET during the waste disposal process, further more the recycling of PET can help with streamlining overall waste processing operations, it can help free up space (used for the temporary storage of PET), and also it can help improve workplace safety and neatness.
2:8 Safety Issues at the PET Intermediate Processing Facility: Maintaining a safe workplace environment is essential for reducing the incidence of worker injury, complying with safety regulations at the federal state and local levels, reducing liability costs associated with worker injury, and is corporate best practice in maintaining the health and well-being of its employees. There are many laws and regulations that deal with worker safety at the federal, state and local levels of government. It is every facility operator‘s responsibility to make sure that they are in compliance with all laws and regulations. The most important law in workplace safety is the federal Occupational Safety and Health Act. This law is administered by the Occupational Safety and Health Administration (OSHA), a division of the federal Department of Labor. Unlike other regulatory agencies that may have jurisdiction over the operations of recycling facilities, OSHA does not issue permits for construction or operation, which could help define worker safety requirements for specific types of operations. Given the
pg. 54
number and complexity of safety regulations, many plastics recycling companies have a designated compliance officer who is responsible for identifying and complying with all regulations that might effect a facility‘s operations. OSHA regulations and standards are contained in two volumes and are quite extensive (CFR 29, Parts 1900 to 1910.999, and CFR 29, Part 1910, Secs. 1910.1000 to end). OSHA regulations relate to almost every aspect of a facility‘s oper ation ation and include such generic regulatory categories as processing, receiving, shipping and storage practices; the general condition of the building and grounds; exiting or egress; general inplant housekeeping practices; electrical equipment; lighting; heating and ventilation; machinery, personnel, hand and power tools, chemicals, fire prevention, maintenance, personal protective equipment and transportation, that must be complied with in specific detail. Hazards at plastics recycling facilities can be divided div ided into three general categories: 1) Health and hygiene hazards (noise, dust, climate, EMFs - electromagnetic
frequencies). 2) Safety hazards (vehicle and machine hazards) 3) Ergonomic hazards (fatigue and musculoskeletal).
Compliance with worker safety regulations and proper system design and maintenance are the best practices to be followed to minimize the incidence of workplace hazards. In addition, reducing fatigue through proper ergonomic design can increase worker productivity and improve material quality at PET processing facilities. While a discussion of regulatory compliance for all OSHA regulations is beyond the scope of this document, there are a number of major safety issues and safety best practices at PET recycling facilities that should be discussed. Once again, it is the responsibility of the facility operator to ensure that all safety regulations that apply to their specific operations are complied with at all levels of government. And, the Best Practices presented below are not intended as a comprehensive listing for regulatory compliance.
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General Safety Best Practices:
Provide all employees with adequate personal protection equipment, which may include such items as safety glasses, ear protection, gloves, hard hats, protective footwear, back-support belts, dust masks, etc.
Make sure all conveyors, balers, grinders, and other processing equipment are equipped with emergency power-cut-off switch es (often referred to as ―kill‖ switches) and machine guards. This will allow plant personnel to react to safety hazard or emergency situations or to ensure worker safety during normal equipment operation and when performing equipment repair or maintenance.
Make sure all grinders and regrind evacuation systems (blowers) are insulated or enclosed in a separate room to maintain noise levels below the OSHA regulated noise exposure level for workers.
Make sure that all cyclone discharges from grinders are properly exhausted into baghouse or other dust collection systems, or are otherwise properly filtered in compliance with regulatory requirements, to maintain ambient dust levels within OSHA guidelines.
Ensure that all equipment is properly maintained for safe and efficient operation through the implementation of a regular and preventative maintenance schedule for all equipment within the facility.
Ensure that ergonomic considerations are factored into system design. For example, the width of sorting conveyors must not exceed to ability of the line inspector to comfortably reach the material on the belt, whether single-sided or double-sided sorting stations are used. In addition, proper belt speeds on manual sorting lines can greatly decrease worker fatigue and improve overall material quality.
Provide adequate space for vehicle and worker activities.
Ensure that adequate lighting is provided for general plant visibility and to prevent worker eye fatigue.
Ensure that adequate fire protection equipment is in place based on the nature of the materials being processed and stored, the types of equipment being used, local fire codes and insurance requirements.
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Make sure that proper signage is maintained throughout the facility.
If the facility is equipped with sorting systems that use electromagnetic frequencies (EMFs), like x-rays or ultraviolet light, as the detection signal, make sure that the equipment is properly shielded to eliminate worker exposure to EMFs.
Ensure that only trained personnel operate specific equipment and that designated operators have any required operating certificates or licenses for that type of equipment.
Provide adequate disposal containers that are in regulatory compliance for the disposal of oily, hazardous, or combustible wastes.
Finally, hypodermic needles are an increasing safety concern at plastic recycling facilities. Many recycling programs request community members who require intravenous injections to store used needles in plastic containers that are then collected through special needle collection programs. Unfortunately, many of these containers make their way into plastic recycling facilities, increasing the safety concerns of worker exposure to blood borne pathogens. Every plastics recycling facility should have at least one employee who is trained in the proper handling and disposal of used hypodermic needles and has been inoculated for the hepatitis B virus. If a hypodermic needle is identified by an employee, they should hit the emergency cut-off switch for their conveyor or particular piece of equipment. Without handling the hypodermic needle or the plastic bottle containing it, they should notify their supervisor to summons properly trained and inoculated personnel to remove the hypodermic needle from the system. Removed hypodermic needles should then be placed in approved medical waste ―sharps‖ containers for removal by trained medical or medical waste disposal professionals. In addition, if employees should be stuck with hypodermic needles encountered in the workplace, OSHA guidelines for proper medical attention should be followed for vaccination and post-exposure evaluation and follow-up. [19] [20]
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