PACKAGING OF DAIRY PRODUCTS

Module 2. Packaging materials

Lesson 10
FORMS OF DIFFERENT PLASTIC MATERIAL - 2

10.1 Introduction

In this lesson the topics related to different plastic materials like polystyrene, PVC, PVDC, EVA, EVOH etc are discussed in detail.

10.2 Polystyrene (PS)

Polystyrene (IUPAC Polyphenylethene) is an aromatic polymer made from the aromatic monomer styrene, a liquid hydrocarbon via the double bond in the ethylene group attached to the benzene ring of the monomer unit that is commercially manufactured from petroleum. Polystyrene foam (Thermocole) or expanded polystyrene (EPS) is made by adding hexane during polymerization.

10.2.1 Characteristics

• Polystyrene is a thermoplastic substance, normally existing in solid state at room temperature, but melting if heated (for molding or extrusion), and becoming solid again when cooling off. It can be cast into molds with fine detail.

• Polystyrene is a rigid, brittle, hard, inexpensive, clear, transparent and sparkling low strength with limited flexibility plastic film with a low melting point and poor impact resistance and can be made to take on various colours.

• PS has a fairly high tensile strength. It softens at about 90-95°C. It is relatively cheap. It has high WVTR & GTR and hence called as breathing film. It deteriorates on exposure to sunlight.

• Polystyrene production methods include sheet stamping (PS) and injection molding.

• Polystyrene may be oriented to improve its gas barrier properties & improve strength.

• It can be used as Expanded Polystyrene (EPS) or Extruded polystyrene (XPS).

• Expanded polystyrene is produced from a mixture of about 90-95% polystyrene and 5-10% gaseous blowing agent, most commonly pentane or carbon dioxide. The solid plastic is expanded into foam through the use of heat, usually steam.

• Extruded polystyrene (XPS) is commonly known by the trade name Styrofoam. The voids filled with trapped air give it low thermal conductivity.

• The density of expanded polystyrene varies greatly from around 25 kg/m³ to 200 kg/m³ depending on how much gas was admixed to create the foam.

10.2.2 Copolymers

• High-impact polystyrene or HIPS: Pure polystyrene is brittle, but hard enough that a fairly high-performance product can be made by giving it some of the properties of a stretchier material, such as polybutadiene rubber. The two such materials can never normally be mixed because of the amplified effect of intermolecular forces on polymer insolubility, but if polybutadiene is added during polymerization it can become chemically bonded to the polystyrene, forming a graft copolymer which helps to incorporate normal polybutadiene into the final mix, resulting in high-impact polystyrene or HIPS, often called "high-impact plastic". HIPS are usually injection molded in production. Autoclaving polystyrene can compress and harden the material.

• Acrylonitrile butadiene styrene or ABS plastic is similar to HIPS: a copolymer of acrylonitrile and styrene, toughened with polybutadiene. Most electronics cases are made of this form of polystyrene, as are many sewer pipes. ABS pipes may become brittle over time. ABS is used for manufacture of Margarine tubs.

• Styrene can be copolymerized with other monomers; for example, divinylbenzene for cross-linking the polystyrene chains.

• It has got shining surface, good printing property and easy thermo-formability.

10.2.3 Uses of polystyrene

• PS films are used as windows in paperboard boxes due to their excellent clarity.

• Polystyrene's most common use is as expanded polystyrene (EPS).

a. EPS is a rigid, low-density material that is widely used to make meat trays, egg cartons, cups, and containers.

b. However, EPS is a poor gas barrier to oxygen and moisture vapor.

c. EPS trays are commonly used with PE or PP over wraps that provide the necessary barrier properties to moisture.

• High Impact styrene is not brittle. It finds much current use as the substance of toy figurines and novelties.

• The voids in Extruded polystyrene (XPS) are filled with trapped air which give it low thermal conductivity. This makes it ideal as a construction material and it is therefore sometimes used in structural insulated panel building systems. It is also used as insulation in building structures, as molded packing material for cushioning fragile equipment inside boxes, as packing "peanuts", as non-weight-bearing architectural structures (such as pillars), and also in crafts and model building, particularly architectural models. Foamed between two sheets of paper, it makes a more-uniform substitute for corrugated cardboard, trade named Foamcore.

• Polystyrene is economical and is used for producing plastic model assembly kits, license plate frames, plastic cutlery, CD "jewel" cases, and many other objects where a fairly rigid, economical plastic is desired.

• Polystyrene can be dish washed at 70°C without deformation since it has a glass transition temperature of 95°C.

• Petri dishes and other containers such as test tubes, made of polystyrene are used in biomedical research and science. For these uses, articles are almost always made by injection molding, and often sterilized post molding, either by irradiation or treatment with ethylene oxide.

• Polystyrene is also used in preparing breath-in boxes type packaging for fruits, which keeps fruits fresh for longer time.

• HIPS is used for thermoformed trays, tubs, cups for packing yogurt, dahi, ice-cream and meats.

10.2.4 Dangers and fire hazard

• The health effects caused by consuming polystyrene when it migrates from food containers into food are under serious investigation.

• Benzene, a material used in the production of polystyrene, is a known human carcinogen. Moreover, butadiene and styrene (in ABS), when combined, become benzene-like in both form and function.

• The acute exposure to styrene in humans results in mucous membrane and eye irritation, and gastrointestinal effects, effects on the central nervous system such as headache, fatigue, weakness, and depression, its dysfunction, hearing loss, and peripheral neuropathy, an increased risk of leukemia and lymphoma. However, the evidence is inconclusive.

• Polystyrene, though is an efficient insulator at low temperatures, it is prohibited from being used in any exposed installations as long the material is not flame retarded e.g. with hexabromocyclododecane.

• Polystyrene is used in some polymer-bonded explosives such as RDX and is also a component of Napalm and a component of most designs of hydrogen bombs.

10.2.5 Environmental concerns and bans

• Expanded polystyrene is not easily recyclable because of its light weight and low scrap value.

• Expanded polystyrene foam takes a very long time to decompose in the environment.

10.3 Polyvinyl Chloride (PVC)

Polyvinyl chloride, (as per IUPAC called as Polychloroethene) commonly abbreviated PVC, is a widely used thermoplastic polymer. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Polyvinyl chloride (PVC) is made by the low-pressure polymerization of vinyl chloride. PVC has side chains incorporating chlorine atoms, which form strong bonds

10.3.1 Characteristics of PVC

About 57% of its mass is chlorine, creating a given mass of PVC requiring less petroleum than many other polymers

• In 1926, Waldo Semon and the B.F. Goodrich Company developed a method to plasticize PVC by blending it with various additives which resulted into a more flexible and more easily-processed material that soon achieved widespread commercial use.

• Plasticized PVC films are tough, clear, and glossy with excellent moisture resistance and low gas permeability, and they can be processed to give films with good shrink properties.

• PVC is cheap, durable, and easy to assemble.

• PVC in its normal form is stiff, strong, heat and weather resistant.

• It is an extremely brittle film, which requires large amounts of plasticizers to soften the film (0 to 150%).

• PVC has a density of 1.35 – 1.4 gm/cm3.

• Chemically it is resistant to weak or strong acids & alkalis.

• PVC has excellent oil and grease resistance.

• Heat sealing is good but gives off corrosive HCl vapours.

• PVC is unsuitable for sterilization. It decomposes at about 60°C

10.3.2 Health and safety

• Plasticizers used to make soft PVC for toys can leach out into the mouths of the children chewing on the toys. In 2006, the EU placed a ban on six types of phthalate softeners, including DEHP (diethylhexyl phthalate), used in toys. An alternative plasticizer, DINP (diisononyl phthalate) is also found to be risky.

• PVC plastic has been used safely for more than 70 years in a variety of medical and commercial applications and humans. No reports of adverse human health effects have been reported from intravenous (IV) bags and medical tubing made with PVC.

• Vinyl chloride monomer: The carcinogenicity of vinyl chloride monomer to humans who were exposed to very high VCM levels, routinely, for many years have been linked. Vinyl chloride is a known human carcinogen that causes a rare cancer of the liver.

• Dioxins: The dioxin is produced as a byproduct of vinyl chloride manufacture and from incineration of waste PVC in domestic garbage.

• Dioxins are a global health threat because they persist in the environment and can travel long distances.

• At very low levels, dioxins have been linked to immune system suppression, reproductive disorders, a variety of cancers, and endometriosis.

10.3.3 Recycling

• Post-consumer PVC is not typically recycled due to the prohibitive cost of regrinding and recompounding the resin compared to the cost of virgin (unrecycled) resin.

• The thermal depolymerization process can safely and efficiently convert PVC into fuel and minerals, according to the company that developed it. It is not yet in widespread use.

• A new process of PVC Recycling is being developed in Europe and Japan called Texiloop®. This process consists of recovering PVC plastic from composite materials through dissolution and precipitation.

10.3.4 Uses of PVC

• Despite the fact that PVC production negatively affects the natural environment and human health, it is still widely used.

• In recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas.

• The material is often used in Plastic Pressure Pipe Systems for pipelines in the water and sewer industries because of its inexpensive nature and flexibility.

• PVC is widely used to make clear plastic bottles and as an overwrap with EPS trays for meat and fresh produce.

• PVC can also be softened with chemical processing, and in this form it is now used for shrink-wrap, food packaging, and raingear.

• UPVC (Unplasticized PVC) can sometimes be used as bullet proof glass for a cars window as it is very hard and thick.

• Food grade PVC can be used for packaging, butter, fish, poultry and bakery items.

• PVC bottles are used for edible oils, wines, and non-carbonated beverages.

• PVC jars are used for chocolate drink powders, instant coffee and pickles.

10.4 Polyvinylidene Chloride

Polyvinylidene chloride (PVDC) is made by copolymerizing the monomers vinylidene chloride and vinyl chloride.

• Ralph Wiley, a Dow Chemical lab worker, accidentally discovered polyvinylidene chloride in 1933. Dow researchers made this material into a greasy, dark green film, first called "Eonite" and then "Saran" (mainly made from PVC and PVDC).

• The most well known use of polyvinylidene chloride came in 1953, when Saran Wrap, a plastic food wrap was introduced.

• Saran fiber is manufactured by melt spinning vinylidene chloride copolymer. Saran is pigment dyed before fiber spinning if color is desired

10.4.1 Characteristics of PVDC

• PVDC is a clear, heavy, very strong film with excellent cling properties and is commonly used for packaging cheese (as a layer in a multilayer film).

• It is a remarkable barrier against water, oxygen and aromas, has superior chemical resistance to alkalis and acids, is insoluble in oil and organic solvents, has very low moisture regain and is impervious to mold, bacteria, and insects. Saran fiber has a high elastic recovery and resists wrinkling and creasing.

• It is heat-shrinkable and heat-sealable.

• It is expensive to produce and hence it is commonly used in very thin gauges and laminated to other films for mechanical support and strength

• Because it is pigment dyed before fiber spinning, it has excellent color fastness and high light permeability.

• Saran is also flame-retardant and self extinguishing, it may soften or char in flame, and decomposes in moderate heat.

10.4.2 Uses of PVDC

• Packaging: Polyvinylidene chloride is applied as a water-barrier coating to other plastic films such as biaxially-oriented polypropylene (BOPP) and polyester (PET). This coating increases the barrier properties of the film, reducing the permeability of the film to oxygen and flavours and thus extending the shelf life of the food inside the package. So used when high barrier characteristics are required, e.g., gas packaging.

• Household: Cleaning cloths, filters, screens, tape, shower curtains, garden furniture.

• Industry: Screens, artificial turf, waste-water treatment materials, underground materials and industrial applications

10.4.3 Limitations of PVDC

• While extremely useful as a food packaging material, the major disadvantage of Saran is that it will undergo thermally induced dehydrochlorination at temperatures very near to processing temperatures.

• This degradation easily propagates, leaving polyene sequences long enough to absorb visible light, and change the color of the material from colorless to an undesirable transparent brown (unacceptable as food packaging).

• Therefore, there is a significant amount of product loss in the manufacturing process, which increases production and consumer costs

10.5 Ethylene Vinyl Acetate

Ethylene vinyl acetate (EVA) is comprised of low-density polyethylene copolymerized with vinyl acetate.

10.5.1 Characteristics of EVA

• EVA is a clear, tough film (especially at low storage temperatures), and it has moderate moisture and poor gas barrier properties.

• It is heat-sealable at low temperatures and is commonly used as a heat-sealant layer in many laminated structures.

• EVA is used as an overwrap for fresh meat and poultry and as a tie layer when two films of dissimilar properties are laminated together.

10.6 Ethylene Vinyl Alcohol

Ethylene vinyl alcohol (EVOH) is a hydrolyzed copolymer of vinyl alcohol and ethylene.

10.6.1 Characteristics of EVOH

• EVOH films are strong, have good clarity, are heat-sealable, and have excellent odor, gas, and moisture barrier characteristics.

• The major disadvantage of EVOH films is that they are hydrophilic and hygroscopic.

• When they absorb moisture at high relative humidity, the absorbed moisture acts as a plasticizer and the gas barrier properties of the film decrease. This can be overcome by

a. Increasing the ethylene content of the film,

b. Laminating it between two films that protect it against moisture, or

c. Adding a desiccant to the tie layer.

• EVOH is commonly used in laminated structures where high gas and moisture barrier characteristics are desired, e.g., modified atmosphere packaging applications.