PACKAGING OF DAIRY PRODUCTS

Module 2. Packaging materials

Lesson 7
CHARACTERISTICS OF PLASTICS

7.1 Introduction

Plastic is the general term for a wide range of synthetic or semi-synthetic polymerization products derived from ethylene or coal tar products. Plastic/s as per Webster dictionary include any of a large group of materials of a high molecular weight that usually contain as the essential ingredient a synthetic or semi synthetic organic substance made by polymerization or condensation, or derived from a natural material or any chemical treatment, that are molded, cast, extruded, drawn or laminated under various conditions into objects of all sizes and shapes including films and filaments.

They are composed of organic condensation or addition polymers and may contain other substances to improve performance or economics. There are many natural polymers generally considered to be "plastics". Plastics can be formed into objects or films or fibers. Their name is derived from the fact that many are malleable, having the property of plasticity.
Plastics are polymers where long chains of atoms are bonded to one another. A polymer is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. The word is derived from the Greek, πολυ, polu, "many"; and μέρος, meros, "part". Well known examples of polymers include plastics, DNA and proteins.

While the term "polymer" in popular usage suggests "plastic", polymers comprise a large class of natural and synthetic materials with a variety of properties and purposes. Biopolymers such as proteins and nucleic acids play crucial roles in biological processes. A variety of other natural polymers exist, such as cellulose, which is the main constituent of wood and paper. Typical synthetic polymers are Bakelite, neoprene, nylon, PVC (polyvinyl chloride), polystyrene, polyacrylonitrile and PVB (polyvinyl butyral).

7.2 Developments in Polymers

• People experimented with plastics based on natural polymers for centuries.

• The first plastic based on a synthetic polymer was made from phenol and formaldehyde, with the first viable and cheap synthesis methods, the product being known as Bakelite.

• Subsequently poly vinyl chloride, polystyrene, polyethylene (polyethene), polypropylene (polypropene), polyamides (nylons), polyesters, acrylics, silicones, polyurethanes were amongst the many varieties of plastics developed and used commercially.

• The development of plastics has come from the use of natural materials (e.g., chewing gum, shellac) to the use of chemically modified natural materials (e.g., natural rubber, nitrocellulose, collagen) and finally to completely synthetic molecules (e.g., epoxy, polyvinyl chloride, polyethylene).

• Recently, corn has been used to make biodegradable containers. Corn can be used to create non-petroleum plastic, which is often compostable.

7.2.1 Types of plastics

• Thermosets: Thermosets soften and flow when heated but at the same time an irreversible chemical reaction takes place so that once hardening has occurred the material again can not again be softened by heating. If strong heat is applied to thermoset, it will char and decompose bu will not soften and melt. Eg. Urea formaldehyde, Phenol formaldehyde.

• Thermoplastics: Thermoplastics soften on heating and harden again on cooling, a process which can be repeated any number of times. Eg. Polyethylene, NylonCommon thermoplastics range from 20,000 to 500,000 in molecular weight, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as "repeat units", derived from "monomers"; each polymer chain will have several 1000's of repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. (Some of commercial products are silicon based.) The backbone is that part of the chain on the main "path" linking a large number of repeat units together and to vary the properties of plastics, both the repeat unit with different molecular groups "hanging" or "pendant" from the backbone. This customization by repeat unit's molecular structure has allowed plastics to become such an indispensable part of twenty first-century life by fine tuning the properties of the polymer.

7.2.1.1 Urea formaldehyde

It is a thermoset. Urea formaldehyde resins are resistant to solvents but are decomposed by strong acids and attacked by strong alkalis. Their main use in food packaging is for screw cap closures.

7.2.1.2 Polyacetals

The name polyacetals covers polymers and co-polymers of formaldehyde with compounds such as ethylene oxide. They have excellent load bearing properties and have been used in engineering applications. Chemically they are resistant to weak acids and alkalis but are attacked by strong ones. They have excellent solvent resistance.
Polyacetal container is used for hair lacquer aerosol.

7.3 Advantages of Plastics over other Packaging Materials

• Versatility: Glass/paper/board/wood has basic properties. Their modification is possible in narrow limits only. Whereas plastics have wide variety of properties. Their modification is possible by chemical processing. E.g. Rackzine (cloth + PVC+PVDC) Nomex (a fireproof dress withstands 9000C), “Kevlar” (bulletproof jacket -5 x stronger than Iron), PTFE (Kitchen wares), conductor /insulator (PET electric tube light choke)-silicone grease, etc.

• Energy requirements for conversion of plastics 4-5 x less energy is required than that of Glass and 30-40 times less as compared to paper.

• Plastic containers are light weight, flexible/rigid, has strength, so less breaking in handling. Attractive shapes, sizes can be formed. Good display and decoration at competitive price and having see-through properties.

• Multilayers used: Multilayers collapsible tube MLCT : LDPE / LDPE / Glassine / EVA foil / EVA / LDPE. Cost of multilayers is at par with aluminum. Collapsible tube has additional benefits like wrinkle resistance, decorative printing, light weight.

• Cost: Cost of plastics is comparatively lower than that of glass and metal containers.

• Consumer’s convenience at lower cost. E.g. Laminates: PET/LDPE or PVC for shampoo pouches, Foil-LDPE for Tablets / Capsules packaging, PET or LDPE / metallized PET/HD or LDPE for Baby foods packaging PET/paper foil/LDPE for coffee packaging and carry home bags made of PP, PE, HDPE.

• Ease of opening compared to metal containers. Grips can be provided, without adding much to cost.

• Retortable pouches also possible (Boil-in-bags) from Nylon-PP; PET-PP thus helps in high temperature processing of food at low cost.

• Intramix packs containing two components which are held separately with rupturable seal which can be broken by squeezing or mixing in pouch and then used. E.g. Self heating pouches containing two chemicals if mixed produce heat by exothermic reaction.

• Ease in Transportation and Distribution

• It is easy to handle and convenient for the manufacturer, retailer, and consumer.

• Plastics have less weight and add little weight to the product so more amount can be loaded and thus less cost. Plastic crates are light in weight than wooden crates so easy to stack.

• Shrink wrapping can act as intermediate of bulk packaging and cause less loss during handling / transportation / distribution.

Plastic packaging fit closely to the shape of the food, thereby wasting little space during storage and distribution.

• Plastics have good barrier properties against moisture and gases.

• Plastics are non-corrosive.

• Heat-sealable to prevent leakage of contents.

• Suitable for high-speed filling.

• Good wet and dry strength.

• Suitable for printing and even sandwiched printing layer can be provided.

• Plastics can be recycled.

Plastics may be made as flexible films or as semi-rigid and rigid containers to meet the varied packaging and processing requirements of food. Plastic films are made with a wide range of mechanical, optical, heat-seal, and barrier properties. Furthermore, they can be coated with another polymer or metallized to give a laminated structure with superior properties. Examples of some of the common flexible films and their properties are discussed here.

Table 7.1 Examples of basic plastics used as packaging material

7.4 Disadvantages of Plastics

7.4.1 Negative health effects

Following plastics have been associated with negative health effects:

PVC (polyvinyl chloride) contains numerous toxic chemicals called adipates and Phthalates ("plasticizers"), which are used to soften brittle PVC into a more flexible form. PVC is commonly used to package foods and liquids, ubiquitous in children's toys and teethers, plumbing and building materials. Traces of these chemicals can leach out of PVC when it comes in contact with food. The World Health Organization's International Agency for Research on Cancer (IARC) has recognized the chemical used to make PVC, vinyl chloride, as a known human carcinogen. The European Union has banned the use of DEHP (di-2-ethylhexyl phthalate), the most widely used plasticizer in PVC, and in children's toys.

PS (polystyrene) is one of the toxin the EPA (Environmental Protection Agency) monitors in America's drinking water. Its production also pollutes the atmosphere, destroying the ozone layer. Some compounds leaching from Styrofoam food containers interfere with hormone functions. It's a possible human carcinogen.

Other (usually polycarbonate - PC) group that consists mainly of polycarbonates, whose primary building block is bisphenol A (BPA), a hormone disrupter that releases into food and liquid and acts like estrogen. Research in Environmental Health Perspectives found that BPA (leached from the lining of tin cans, dental sealants and polycarbonate bottles) can increase body weight of lab animals' offspring, as well as impact hormone levels. A more recent animal study suggests that even low-level exposure to BPA results in insulin resistance, which can lead to inflammation and heart disease.

7.4.2 Negative effects on environment

1. Plastics are durable and degrade very slowly.

2. In some cases, burning plastic can release toxic fumes (Eg. PVC/PVDC).

3. The manufacturing of plastics often creates large quantities of chemical pollutants.

4. Unfortunately, recycling plastics has proven difficult. The biggest problem with plastic recycling is that it is difficult to automate the sorting of plastic waste, and so it is labor intensive.

Recycling certain types of plastics can be unprofitable, as well, e.g. polystyrene is rarely recycled because it is usually not cost effective.