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What are plastics?
Plastics are polymers that are either synthetic or semi-synthetic. Plastics derive their name from the fact they are malleable (and also ductile) and hence they have “plasticity”. They come in many forms and the commonly used ones are Polyvinyl chloride, Polyethylene, Polystyrene, Polyethylene terephthalate etc. They are generally classified by their polymer backbone and also include silicones, acrylics and polyurethanes. While all plastics can be polymers, the reverse is not true. Plastics can be divided into two types: 1) Thermoplastics; that can be reused and recycled and 2) Thermosets; once set cannot be reused (Bakelite). Rubber, the elastic material, which we use in our day-to-day lives is an “elastomer” and has all the properties to qualify to be a polymer (while vulcanization deprives rubber of its thermoplastic property and makes it a thermoset). Although everyone can distinguish between rubber and plastic, sometimes the difference becomes vague; since there are a lot of plastics that are also elastic. These groups of plastics differ in their molecular structure and this determines the very nature of thermal behaviour of plastics (Appendix 1). Commonly plastics are named and identified after their acronyms.
Manufacture of plastics
In the olden days, plastics were manufactured from vegetable matter. The most commonly used raw materials were furfural, various starch derivates, vegetable oils and cellulose. Ever since the use of plastics has burgeoned and the supply of natural raw materials has dwindled, people have turned to synthetic manufacture of plastics from petrochemicals. Petrochemicals are now being used widely since they are relatively cheap compared to other raw materials. The only hitch in this is that the supply is dwindling fast and researchers are trying for other raw materials.
The first step in the manufacture of plastics is polymerization which is done by either by addition or condensation. This is done either in the gaseous or liquid phase of the ingredients and very rarely in the solid phase. During the process of manufacture of plastic chemical additives are added to achieve certain characteristics. These additives are used according to the nature of use; for e.g. for the plastic not to disintegrate in the sun, ultraviolet stabilizers are used. Plasticizers are used to increase flexibility, anti-oxidants to protect from the effects of ozone or oxygen etc. Similarly lubricants, coloring pigments are also added. The way plastics are given “shape” or how is it “moulded” is not mentioned here and is beyond the scope of this paper.
How and why do plastics degrade?
Contrary to popular belief all plastics are degradable and they do deteriorate with age. All plastics are organic in nature and are by products of either petroleum or natural gas hence they have to degrade over time. Plastic starts to deteriorate no sooner than the polymer is produced and it is amplified by the “residual stresses” left by the process of moulding. This is further exacerbated by exposure to sunlight, ultraviolet radiation, heat, stress, oxygen and other bacterial agents. They can also get contaminated by other plastic materials. For example, a polyethylene terephthalate (PET) bottle cane be attacked by its own polyethylene cap, by the process of plasticiser (a form of vitamin that is added as an additive; and is also responsible for that typical “plastic” smell when plasticisers evaporate) migration.
Physical and chemical causes are the main causes for plastic deterioration. Plastics are said to have degraded if any of the following signs are visible: 1) changes in flexibility; 2) distortion; 3) discoloration; 4) getting brittle; 5) emitting odours and 6) bloom (mostly solid) due to plasticiser migrating to the surface. Polyvinyl chloride, cellulose acetate and cellulose nitrate easily degrade and become brittle. Other plastics also degrade but the degree varies. There are certain plastics like acrylic (Plexiglas®, Lucite® and Acrylite®) that do not deteriorate with time. They are completely inert when it is solid and also strong and hence it is the choice roofing material.
“THE CHEMICAL giant ICI produces what appears to be the perfect plastic. Bacteria make it from sugar, and once we have finished with it, other bacteria in the soil digest it back to carbon dioxide. This remarkable plastic is called polyhydroxy butyrate (PHB), and is tradenamed Biopol by ICI. It has recently won environmental awards in the US and Germany”so writes an author on the emergence of biodegradable plastic nearly 15 years back (Emsley, 1991). Degradable plastics have come of age and it is the “in thing” these days. He further goes on to explain about degradable plastics “THERE are two important ways of degrading a plastic and thereby helping it to rot away. One is to make it out of a material which microbes digest in a process called biodegradation. The other option is to make the plastic sensitive to sunlight which fractures its chemical bonds and breaks it down by a process known as photodegradation. To make plastics photodegradable, chemists implant a material in the plastic that will absorb sunlight and become sufficiently reactive to attack the molecules from which the plastic is composed” (Emsley, 1991). Degradable plastics are also called as compostable plastics.
So then how are degradable plastics made? More than this question we are more interested in knowing what happens to degradable plastic? These plastics are made of alternative methods that make them into harmless elements of water and carbon dioxide, when they eventually degrade. Plastic technologists have come about with a process that makes plastic degrade through a process called oxo-degradation. “This technology is based on a small amount of pro-degradent additive (typically 3%) being introduced into the manufacturing process, thereby changing the behaviour of the plastic. This outcome is a plastic that immediately after manufacture starts to degrade without microbes being present. The degradation will accelerate when exposed to heat, light or stress. A process which is irrevocable and continues until the plastic has reduced to nothing more than CO2 and water”.
ASTM International (formerly known as American Society for Testing and Materials) and ISO (International Organization for Standardization) have the following standards on various plastics that can be called degradable. To put it succinctly,
- “Degradable plastic , a plastic designed to undergo a significant change in its chemical structure under specific environmental conditions resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the application in a period of time that determines its classification;
- Biodegradable plastic , a degradable plastic in which the degradation results from the action of naturally-occurring micro-organisms such as bacteria, fungi and algae;
- Compostable plastic, a plastic that undergoes degradation by biological processes during composting to carbon dioxide, water, inorganic compounds, and biomass at a rate consistent with other known, compostable materials and leaves no visually distinguishable or toxic residue;
- Photo degradable plastic , a degradable plastic in which the degradation results from the action of natural daylight and
- Oxidative degradable plastic , a degradable plastic in which the degradation results from oxidation”.
The advantages of degradable plastics are many. First and foremost, they do not leave any traces of “petro-polymers” in the soil when they disintegrate, secondly they release carbon dioxide slowly so that the soil microorganisms absorb instead of getting transmitted into the atmosphere. Finally, they do not release methane into the atmosphere, “if no oxygen is present the carbon and hydrogen remains locked in the remnants of the plastic”.
Plastics are ubiquitous right from the credit cards and computers we use. We are in a world that is filled with this much-maligned material called “plastic” and still we do not know much about them. The only thing that we know and is of great concern is that they take ages to decompose or degrade. We do not know if plastic wastes will outlive us. Considering the pros and cons that are in favour of plastics, we are certain that the future of plastics is not from petroleum products but from plant resources (Stevens, 2002).
There is a lot of debate as to whether plastics should be degradable or recycled. The opinion is divided on this. To seal this article I would like to quote from a leading environmental organization (Friends of the Earth – FOE) that says “Degradable plastic does nothing to promote lasting solutions to plastic waste: cutting the amount of plastic produced and the amount of plastic waste, or recycling existing plastic. Instead of using degradable plastics to make throw-away bags and bottles, the packaging industry could help the environment a lot more by cutting out unnecessary packaging and by making containers re-usable.”
So are we for degradable plastics OR recycling plastics? The debate will go on, although the pros and cons far outweigh the “degradable plastic side”.
|Type of Plastic||Molecular Structure||Characteristics and applications|
|Thermosets||Thermosets are hard and have a very tight-meshed, branched molecular structure. Curing proceeds during shaping, after which it is no longer possible to shape the material by heating. Further shaping may then only be performed by machining. Thermosets are used, for example, to make light switches.|
|Elastomers||While elastomers also have a crosslinked structure, they have a looser mesh than thermosets, giving rise to a degree of elasticity. Once shaped, elastomers also cannot be reshaped by heating. Elastomers are used, for example, to produce automobile tires.|
|Thermoplastics||Thermoplastics have a linear or branched molecular structure which determines their strength and thermal behavior; they are flexible at ordinary temperatures. At approx. 120 – 180°C, thermoplastics become a pasty/liquid mass. The service temperature range for thermoplastics is considerably lower than that for thermosets. The thermoplastics polyethylene (PE), polyvinyl chloride (PVC) and polystyrene (PS) are used, for example, in packaging applications.|
Emsley, J. (1991). Degradable plastics. New Scientist Online.
FOE. (2007). Fact Sheet: Plastics.
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Sanderson, K. (2007). Plastics for posterity: How do you prevent valuable collectors’ items from degrading? Nature News Online, Web.
Stevens, E. S. (2002). Green Plastics: An Introduction to the New Science of Biodegradable Plastics. New Jersey, Princeton University Press.
For a more exhaustive list on the nomenclature of plastics and other elastomers.
Polymer Nomenclature. Web.