Lesson- 8 Plastic
8. 0 Introduction
Plastic is an organic macromolecular compounds obtained by polymerisation, polycondensation, polyaddition or any similar process from molecules with a lower molecular weight or by chemical alteration of natural macromolecular compounds.
Plastics are used in the packaging of food because they offer a wide range of appearance and performance properties which are derived from the inherent features of the individual plastic material and how it is processed and used. Plastics are resistant to many types of compound – they are not very reactive with inorganic chemicals, including acids, alkalis and organic solvents, thus making them suitable, i.e. inert, for food packaging. Plastics do not support the growth of microorganisms. Some plastics may absorb some food constituents, such as oils and fats, and hence it is important that a thorough testing is conducted to check all food applications for absorption and migration. Gases such as oxygen, carbon dioxide and nitrogen together with water vapor and organic solvents permeate through plastics. The rate of permeation depends on:
type of plastic
thickness and surface area
method of processing
concentration or partial pressure of the permeant molecule
Plastics have properties of strength and toughness. Polyethylene terephthalate (PET) film has a mechanical strength similar to that of iron, but under load the PET film will stretch considerably more than iron before breaking.
8.1. Application of Plastic in food processing
Plastics are used as containers, container components and flexible packaging. In usage, by weight, they are the second most widely used type of packaging and first in terms of value. Applications of plastic are
rigid plastic containers such as bottles, jars, pots, tubs and trays
flexible plastic films in the form of bags, sachets, pouches and heat-sealable flexible lidding materials
plastics combined with paperboard in liquid packaging cartons
expanded or foamed plastic for uses where some form of insulation, rigidity and the ability to withstand compression is required
plastic lids and caps and the wadding used in such closures
diaphragms on plastic and glass jars to provide product protection and tamper evidence plastic bands to provide external tamper evidence
pouring and dispensing devices to collate and group individual packs in multipacks, e.g. Hi-cone rings for cans of beer, trays for jars of sugar preserves etc.
plastic films used in cling, stretch and shrink wrapping
- films used as labels for bottles and jars, as flat glued labels or heat shrinkable sleeves
- components of coatings, adhesives and inks.
8.2. Types of plastic used in packaging
PE is structurally the simplest plastic and is made by addition polymerization of ethylene gas in a high temperature and pressure reactor. A range of low, medium and high density resins are produced, depending on the conditions (temperature, pressure and catalyst) of polymerization. Polyethylenes are readily heat sealable. They can be made into strong, tough films, with a good barrier to moisture and water vapor. They are not a particularly high barrier to oils and fats or gases such as carbon dioxide and oxygen compared with other plastics, although barrier properties increase with density. The heat resistance is lower than that of other plastics used in packaging, with a melting point of around 120°C, which increases as the density increases.
LDPE and LLDPE can be used in blends with EVA to improve strength and heat sealing. There is a degree of overlap in application between LDPE and LLDPE, due to the fact that there are differences in both, as a result of the conditions of polymer manufacture and on-going product development. The thickness used for specific applications can vary, and this can also have commercial implications.
MDPE or medium-density PE film is mechanically stronger than LDPE and therefore used in more demanding situations. LDPE is coextruded with MDPE to combine the good sealability of LDPE with the toughness and puncture resistance of MDPE, e.g. for the inner extrusion coating of sachets for dehydrated soup mixes.
HDPE or high-density PE is the toughest grade and is extruded in the thinnest gauges. This film is used for boil-in-the-bag applications. To improve heat sealability, HDPE can be coextruded with LDPE to achieve peelable seals where the polymer layers can be made to separate easily at the interface of the co- extrusion.
8.2.2 Polypropylene (PP)
PP is an addition polymer of propylene formed under heat and pressure using Zieger-Natta type catalysts to produce a linear polymer with protruding methyl (CH2) groups. The resultant polymer is a harder and denser resin than PE and more transparent in its natural form.
The high melting point of PP (160°C) makes it suitable for applications where thermal resistance is needed. The surfaces of PP films are smooth and have good melting characteristics. PP films are relatively stiff. When cast, the film is glass clear and heat sealable. It is used for presentation applications to enhance the appearance of the packed product. PP is chemically inert and resistant to most commonly found chemicals, both organic and inorganic. It is a barrier to water vapor and has oil and fat resistance. Aromatic and aliphatic hydrocarbons are, however, able to be dissolved in films and cause swelling and distortion. Many of the PP films are used in the form of laminations with other PP and PE films. This allows for the reverse-side printing of one surface, which is then buried inside the subsequent laminate.
8.2.3 Polyethylene terephthalate (PET)
PET can be made into film by blowing or casting. It can be blow moulded, injection moulded, foamed, extrusion coated on paperboard and extruded as sheet for thermoforming. PET can be made into a biaxially oriented range of clear polyester films produced on essentially the same type of extrusion and Stenter-orienting equipment as OPP. PET melts at a much higher temperature than PP, typically 260°C, and due to the manufacturing conditions does not shrink below 180°C. This means that PET is ideal for high-temperature applications using steam sterilization, boiling-the-bag and for cooking or reheating in microwave or conventional radiant heat ovens. The film is also flexible in extremes of cold, down to −100°C. PET is a medium oxygen barrier on its own but becomes a high barrier to oxygen and water vapor when metalized with aluminum. This is used for vacuumised coffee and bag-in-box liquids, where it is laminated with EVA on both sides to produce highly effective seals. It is also used in snack food flexible packaging for products with high fat content requiring barriers to oxygen and ultra violet (UV) light.
PET film is also used as the outer reverse-printed ply in retort pouches, providing strength and puncture resistance, where it is laminated with aluminium foil and either PP or HDPE. PET can be oxide coated with SiO2 to improve the barrier, whilst remaining transparent, retortable and microwaveable. PET is the fastest growing plastic for food packaging applications as a result of its use in all sizes of carbonated soft drinks and mineral water bottles which are produced by injection stretch blow moulding. PET bottles are also used for edible oils, as an alternative to PVC.
8.2.4 Ethylene vinyl acetate (EVA)
EVA is a copolymer of ethylene with vinyl acetate. It is similar to PE in many respects, and it is used, blended with PE, in several ways. The properties of the blend depend on the proportion of the vinyl acetate component. Generally, as the VA component increases, sealing temperature decreases and impact strength, low temperature flexibility, stress resistance and clarity increase. EVA is also a major component of hot melt adhesives, frequently used in packaging machinery to erect and close packs, e.g. folding cartons and corrugated packaging.
Modified EVAs are available for use as peelable coatings on lidding materials such as aluminum foil, OPP, OPET and paper. They enable heat sealing, resulting in controllable heat seal strength for easy, clean peeling. These coatings will seal to both flexible and rigid PE, PP, PET, PS and PVC containers.
8.2.5 Polyamide (PA)
Polyamides (PA) are commonly known as nylon. However, nylon is not a generic name; it is the brand name for a range of nylon products made by Dupont. They were initially used in textiles, but subsequently other important applications were developed including uses in packaging and engineering. Polyamide plastics are formed by a condensation reaction between a diamine and a diacid or a compound containing each functional group (amine). The different types of polyamide plastics are characterized by a number which relates to the number of carbon atoms in the originating monomer.
PA resins can be used to make blown film, and they can be coextruded. PA can be blended with PE, PET, EVA and EVOH. It can be blow moulded to make bottles and jars which are glass clear, low in weight and have a good resistance to impact. PA film is used in retortable packaging in structures such as PA/aluminum foil/PP. The film is non-whitening in retort processing. PA is relatively expensive compared with, for example, PE, but as it has superior properties, it is effective in low thicknesses.
8.2.6 Polyvinyl chloride (PVC)
PVC has excellent resistance to fat and oil. It is used in the form of blowmoulded bottles for vegetable oil and fruit drinks. It has good clarity. As a film, it is tough, with high elongation, though with relatively low tensile and tear strength. The moisture vapor transmission rate is relatively high, though adequate for the packaging of mineral water, fruit juice and fruit drinks in bottles. PVC softens, depending on its composition, at relatively low temperatures (80–95°C). PVC easily seals to itself with heat, but heat sealing with a hot wire has the disadvantage of producing Hcl gas. Most PVC films are produced by extrusion, using the bubble process. It can be oriented to produce film with a high degree of shrinkability. Up to 50% shrinkage is possible at quite low temperatures. The film releases the lowest energy of the commonly used plastic films when it is heat shrunk around products. It is plasticized, and the high stretch and cling make it suitable for overwrapping fresh produce, e.g. apples and meat in rigid trays using semi-automatic and manual methods.
Unplasticised PVC (UPVC) has useful properties but is a hard, brittle material, and modification is necessary for it to be used successfully. Flexibility can be achieved by the inclusion of plasticizers, reduced surface friction with slip agents, various colors by the addition of pigments and improved thermal processing by the addition of stabilizing agents.
8.2.7 Polystyrene (PS)
It is less well known as an oriented plastic film, though the film has interesting properties. It has high transparency (clarity). It is stiff, with a characteristic crinkle, suggesting freshness, and has a dead fold property. It has a low barrier to moisture vapor and common gases, making it suitable for packaging products, such as fresh produce, which need to breathe. PS is easily processed by foaming to produce a rigid lightweight material which has good impact protection and thermal insulation properties.