Submitted To
Mr. Istiaq Department of Daity Technology Submitted by
Mohsin Shahzad 2007-va-376 B.S. (Hons.) Dairy Technology The use of polymers in packaging has made tremendous progress in recent years all over the world. A wide variety of polymers can be used as thermoformed; injection moulded or blow moulded containers, such as bottles, cartons, cups, boxes etc. The merits of rigid plastic containers are its low cost
and ease of fabrication. The demerits cited are lack of product compatibility, low barrier properties, plastic deterioration, lack of resistance to high heat and fragility at lower temperatures. Flexible plastic packaging films are used as wrappers or sachets or bags or pouches for packaging milk and dairy products. The flexible polymers can be classified in to two types.
Low polymers They include cellophane (coated with plain or nitrocellulose / saran / polyethylene), treated with cellulose etc. Cellophane
Cellophane is a thin, transparent sheet made of regenerated cellulose. Its low permeability to air, oils, greases, and bacteria makes it useful for food packaging. Production Cellulose is treated with alkali and carbon disulfide to yield viscose. Cellulose from wood, cotton, hemp, or other sources is dissolved in alkali and carbon disulfide to make a solution called viscose, which is then extruded through a slit into a bath of dilute sulfuric acid and sodium sulfate to reconvert the viscose into cellulose. The film is then passed through several more baths, one to remove sulfur, one to bleach the film, and one to add glycerin to prevent the film from becoming brittle. A similar process, using a hole (a spinneret) instead of a slit, is used to make a fibre called rayon. Chemically, cellophane, rayon and cellulose are polymers of glucose and contain the chemical elements carbon, hydrogen, and oxygen.
High polymers Polyethylene, polypropylene, polystyrene, poly vinyl chloride, poly vinyledene chloride (cryovac), rubber hydrochloride (pliofilm), polyester, polyamide (nylon), saran (a mixed polymer), etc. form good packaging materials. The merits cited for flexible packaging films are they can be easily applied and the packaging process can be readily mechanized; loss of moisture from the dairy product is practically nil; it confers protection to dairy products against attack by microorganisms, insects etc. The demerits are: not all technical problems in film packaging have been solved; failure to obtain a perfect seal and removal of all air before packaging may lead to spoilage; the most careful attention to detail is necessary, else faulty production will result; etc. Care has to be exercised in selecting food grade plastics for packaging of milk and dairy products; otherwise toxicity, if any, from the package will be transferred to the products. Polyethylene
Polyethylene or polythene (IUPAC name polyethene or poly(methylene)) is the most widely used plastic, with an annual production of approximately 80 million metric tons. Its primary use is within packaging. Description Polyethylene is a thermoplastic polymer consisting of long chains of the monomer ethylene (IUPAC name ethene). The recommended scientific name polyethene is systematically derived from the scientific name of the monomer. In certain circumstances it is useful to use a structure-based nomenclature; in such cases IUPAC recommends poly(methylene) (poly(methanediyl) is an nonpreferred alternative). The difference in names between the two systems is due to the opening up of the monomer's double bond upon polymerisation. The name is abbreviated to PE in a manner similar to that by which other polymers like polypropylene and polystyrene are shortened to PP and PS respectively. In the United Kingdom the polymer is commonly called polythene , although this is not recognized scientifically. The ethene molecule (known almost universally by its common name ethylene) C2H4 is CH2=CH2, Two CH2 groups connected by a double bond, thus:
Polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging, textiles (e.g. ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes. An addition polymer made from the monomer propylene, it is rugged and unusually resistant to many chemical solvents, bases and acids. In 2007, the global market for polypropylene had a volume of 45.1 million tons which led to a turnover of about US$65 billion (€47.4 billion).
Chemical and physical properties
Most commercial polypropylene is isotactic and has an intermediate level of crystallinity between that of low density polyethylene (LDPE) and high density polyethylene (HDPE); its Young's modulus is also intermediate. Polypropylene is normally tough and flexible, especially when copolymerized with ethylene. This allows polypropylene to be used as an engineering plastic, competing with materials such as ABS. Polypropylene is reasonably economical, and can be made translucent when uncolored but is not as readily made transparent as polystyrene, acrylic or certain other plastics. It is often opaque or colored using pigments. Polypropylene has good resistance to fatigue. The melting of PP occurs as a range, so a melting point is determined by finding the highest temperature of a differential scanning calorimetry chart. Perfectly isotactic PP has a melting point of 171 °C (340 °F). Commercial isotactic PP has a melting point that ranges from 160 to 166 °C (320 to 331 °F), depending on atactic material and crystallinity. Syndiotactic PP with a crystallinity of 30% has a melting point of 130 °C (266 °F). The melt flow rate (MFR) or melt flow index (MFI) is a measure of molecular weight of polypropylene. The measure helps to determine how easily the molten raw material will flow during processing. Polypropylene with higher MFR will fill the plastic mold more easily during the injection or blow molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease. There are three general types of polypropylene: homopolymer, random copolymer and block copolymer. The comonomer used is typically ethylene. Ethylene-propylene rubber or EPDM added to polypropylene homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to polypropylene homopolymer decreases the polymer crystallinity and makes the polymer more transparent.
Polystyrene
Polystyrene (IUPAC Poly(1-phenylethane-1,2-diyl)), abbreviated following ISO Standard PS, is an aromatic polymer made from the aromatic monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum by the chemical industry. Polystyrene is one of the most widely used kinds of plastic. Polystyrene is a thermoplastic substance, which is in solid (glassy) state at room temperature, but flows if heated above its glass transition temperature (for molding or extrusion), and becomes solid again when it cools off. Pure solid polystyrene is a colorless, hard plastic with limited flexibility. It can be cast into molds with fine detail. Polystyrene can be transparent or can be made to take on various colors. Solid polystyrene is used, for example, in disposable cutlery, plastic models, CD and DVD cases, and smoke detector housings. Products made from foamed polystyrene are nearly ubiquitous, for example packing materials, insulation, and foam drink cups. Polystyrene can be recycled, and has the number "6" as its recycling symbol. Polystyrene does not biodegrade, and is often abundant as a form of pollution in the outdoor environment, particularly along shores and waterways.
A polystyrene yogurt container
Ethylene Polymers in Packaging Ethylene polymers are thermoplastic polymers which are used in the production of various packaging materials. These polymers (especially the ones mentioned below) generally have good resistance to alcohols, oils grease and dilute acids and alkalis. One of the most important features of this type of material in the context of packaging is that it has moderate to good barrier properties to oxygen and carbon dioxide which makes is suitable for food packaging. Below is a description of some of the common ethylene polymers used in various forms, especially for the packaging of perishable consumer items (foodstuff and pharmaceuticals). Ethylene Vinyl Acetate (EVA) This is known as foam rubber. Amongst its properties: Flexible, elastic, transparent, good low temp flexibility and it has a high friction co-efficient. EVA is used primarily as a padding material in packaging cartons e.g. for electronics etc. Ethylene Vinyl Alcohol Copolymer (EVOH) This is a formal copolymer of ethylene and vinyl alcohol. It is used as a high barrier material to gases (O2 and CO2) but is a low moisture vapor barrier material. These properties diminish in a humid environment. . This is expensive packaging material. Widely used as the barrier film in a laminate. EVOH is used primarily for food packaging and usually as a thin layer within cardboard or foil, to improve the shelf life of food. High Density Polyethylene (HDPE) HDPE is a polyethylene thermoplastic made from petroleum. It is Semi-rigid, translucent, very tough, and also has low water absorption. HDPE has moderate barrier properties but is not suitable for use at high temperatures. It is widely blow molded to form soft bottle-containers, e.g. to store milk, detergents, etc. It is a low cost and easily produced material. Low Density Polyethylene / Linear Low Density Polyethylene (LDPE -LLDPE) LDPE is a thermoplastic made from oil. It is extremely tough but flexible, waxy, moisture/ weather-proof, has good low temperature toughness and is easy to process by most methods at low cost and has good overall resistance and moderate barrier properties. It is widely used for its heat sealing properties in a laminated material. It is not suitable for use at temperatures above 100° C and is recyclable. It is also corrosion-resistant. These properties make it appropriate for the production of general purpose containers for foodstuff (jars etc) and laboratory use and is also used to coat/ laminate cartons and make six-packsoda-holder rings. Polyethylene Terephthalate (PET or PETE) PET is a thermoplastic polymer resin of and is used in synthetic fibers. It is rigid, extremely tough and impact-resistant, has good creep and fatigue resistance, wide range temperature resistance (-40 to 220° C) and does not flow on heating. This material is usually oriented to improve its barrier properties to gasses and water vapor. It has excellent clarity and is often used as a glass replacement. CPET is widely used to produce containers for carbonated beverages and solvents
due to its good resistance and barrier properties and is extremely easy to recycle, which makes it very environment friendly. Crystallized PET is also used to contain frozen dinners, and a thin film of PET (Mylar) can be coated with other materials (like aluminum) to cater to a variety of uses, including as flexible food packaging etc.
Laminates They are formed by combining the complete surfaces of 2 or more webs of different films with the primary object of overcoming the defects of single films. Usually laminations are made to strengthen the film material, to improve barrier properties, to improve grease resistance, to provide a surface that will heat seal, etc. Some of the typical laminates available for packaging are paper-polythene, cellophane-polythene, aluminium foil-polythene, paper aluminium foil-polythene, polyester-polythene, etc.
