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Degradable plastics - fact of myth?

Degradable Plastics -- Fact or Myth?

Several factors have given rise to recent growing concerns about environmental pollution and the need to be more careful about what we use, how we use it and what we do when we are finished with it. These include alarming statistics about the rate at which we are filling up garbage dumps (known in polite circles as sanitary landfill sites) and the threat of neighbourhood incinerators. While there is no doubt that we are generating discardable stuff at an excessive and unprecedented rate, the scientifically illiterate population in virtually all countries is not being presented with much in the way of factual information about the real solid waste problem and what to do about it. Taxpayers would like to think that somebody else should take the blame, and pay for it.

Perhaps because they are synthetic and therefore not natural, plastics are sometimes singled out by the uninformed as being a (if not the) major cause of the solid waste mismanagement problem. This fallacious reasoning is pursued further leading to the outright banning of plastics packaging in some jurisdictions or to the requirement that plastic bags should be degradable; commonly the real intent is biodegradability. Hence the title of this article is in the form of a question. Authentic Canadians will appreciate that the correct answer is maybe.

The nature of plastics

All plastics, fibres and rubbers--both natural and synthetic--consist of molecules having very high molecular weights, from tens of thousands to a million or more. Indeed the uniquely useful properties of polymeric materials derive directly from these high-molecular weights and, when a relatively small fraction of the bonds in a macromolecule are broken (usually much less than 0.01%), the useful mechanical properties are lost. Commonly, plastics (and fibres and rubbers) are stabilized to prolong their useful life. Heat, sunlight and mechanical stress, singly or in combination, will degrade unprotected plastics, sometimes in a matter of months depending on the plastic.

Research over many years in numerous laboratories in several countries has contributed to a detailed, if not complete, understanding of the chemistry controlling the service life of plastics. There are special cases such as dehydrochlorination leading to the discoloration and embrittlement of poly(vinyl chloride) and photo-Fries reactions contributing to the degradation of polycarbonates. Nevertheless, oxidative degradation is a major factor in the degradation of numerous commodity plastics of both the addition and the condensation types. Such degradation may be initiated by heat, unfiltered sunlight, ionizing radiation and mechanical stress or combinations of these. Among the many possible reactions that could be involved, a handful of sequential, free-radical reactions has been generally accepted as the key processes that must be prevented if plastics are to be useful for prolonged periods. Conversely, these are the same reactions that must be encouraged in order to enhance degradability.

Photodegradable plastics

Taking advantage of the fact the window glass filters out solar wavelengths shorter than about 315 nm, scientists have been able to devise plastics formulations that are unaffected by indoor light for long periods but that will photodegrade outdoors in a short (a few months) controlled exposure owing to their susceptibility to wavelengths in the 300 to 315 nm (erythemal) region. These formulations include light-sensitive additives or co-monmer units which contain light-absorbing ketone groups. In the latter category, ethylene/carbon monoxide copolymers represent one product type; the other copolymer approach involves the copolymerization of a vinyl ketone with an appropriate monomer, not restricted to ethylene. There is no question that such products are stable indoors but become brittle and disintegrate as a result of outdoor exposure. They are useful as agricultural mulch which retains moisture and discourages weeds during the growing season but is brittle by the end of the summer and can be ploughed under as an inert soil conditioner. Other uses for photodegradable plastics are containers/holders that are likely to end up as widely dispersed litter. There is some evidence that these products would not, as has been feared, encourage littering. Apparently the hard-core slobs that scatter stuff widely around the countryside do so indiscriminantly.

Biodegradable plastics

Such materials are available and have been for some time. Medical sutures are one example. None of the common commodity plastics are biodegradable, however, and does not become so by admixture with something else that is biodegradable. There are tens of thousands of different kinds of fungus which, under warm and moist conditions in air will convert a wide variety of substrates to carbon dioxide and water. Such conditions rarely occur in garbage dumps and there is archaeological evidence (seriously) that newspapers are readable and food wastes are readily identifiable for more than 10 years after burial. It isn't just the inherently bioresistant materials such as metal, glass and plastics in a garbage dump that persist; virtually all solids do. This is a major problem in the case of discarded paper since it represents about 40% by weight of municipal waste. Rigid plastic containers represent about 7% by weight with about 1% thin film packaging. To eliminate plastic bags completely would not add to the useful lifetime of a garbage dump. Replacement of such bags would require a greater weight (and higher volume) of something else. If you want to pick on an item that's big and heavy and really difficult to get rid of, try the telephone directory.

Where do we go from here?

Even though the quantities of paper being used and discarded amount to many times more than the total of all plastics, the quantities of the latter add up to several billions of pounds per annum, and this number is predicted to increase significantly. Factories which fabricate plastic articles already recycle all their own scrap. It is now feasible to collect rigid plastic containers in a municipal blue box (or similar) programme. It can be difficult to wash used plastic containers properly (depending on what's been in them) and reprocessing of waste plastics is not uniformly economically viable yet. Of course, recycling old newspapers must also be subsidized some of the time. The ideal plastics utilization programme would involve the initial use followed by several consecutive recycling modes and ending with incineration to recover the energy invested in the material itself. Many plastics are excellent fuels, and modern incinerators are highly efficient and safe. Appropriate technology is either here or being developed.

Perspective

This is what seems to be missing in most discussions of plastics in the environment. The argument that discarded plastics are killing wildlife is not helped by the fact that everyone seems to use the same photo of the entrapped seagull, and recognition is required that photodegradable plastics are available and do work. There are accusations that foamed polystyrene contributes to ozone layer depletion by people who continue to use refrigerators and air conditioners but who don't realize that the use of CFCs for foaming plastics was never very common and is now not really significant. Apparently, however, there is a municipality in California in which a by-law was recently enacted making it illegal to possess foamed polystyrene. The penalty for conviction of such possession is $250; the penalty for possession of marijuana in this same enlighted town is $150.

The defense rests.
COPYRIGHT 1990 Chemical Institute of Canada
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Wiles, David M.
Publication:Canadian Chemical News
Date:Jan 1, 1990
Words:1204
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