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Polymer Science and Engineering: The Shifting Research Frontiers
raw materials. In part, this observation may reflect our imperfect accounting systems, in which the producer bears no responsibility for the cost of disposal. Manufacturing firms are now moving to design and engineer new products with an eye to the need for recycling at the end of product life. The European automobile industry is a leader in this area, and U.S. automobile manufacturers have formed the Polymer Recycling Consortium to coordinate similiar work.
Recycling often involves the production of lower-grade products from recycled feedstocks. This degradation in quality has several sources: the recycled material may have been degraded to some extent in the first use, it can be further degraded by reprocessing, and the mixtures of polymers that are frequently formed during recycling often have seriously diminished physical properties and appearance. Success in recycling depends very much on the chemistry of the polymer involved and on the purity of the scrap. One outstanding example is polyester soda bottles, whose recycling has been aided by "bottle bills" that encourage their return in relatively uncontaminated form. Labeling of plastics at the time of manufacture has been initiated to facilitate separation, and methods of automated recognition and separation are now being sought. Factory scrap is often fed back into the feed supply to achieve an economic advantage with no sacrifice in quality. Despite considerable effort and progress, however, recycling of plastics generally poses a major unsolved problem.
Incineration is another option for disposal that can be attractive in certain circumstances. Polyolefins, for example, have high fuel value and can be burned cleanly and be disposed of with carbon dioxide and water as the only products. But other polymers pose more difficult problems for incineration. Combustion of poly(vinyl chloride), for example, produces corrosive and toxic smoke that requires complex and expensive scrubbing, followed by subsequent disposal of the scrubbings. Also, the fuel content of this polymer is lower, and net energy consumption is required. Other polymers present different scrubbing problems, and mixtures of polymers complicate incinerator operating strategies. Some progress is being made, but the advances are evolutionary. Nevertheless, until improved strategies for recycling have been implemented, incineration offers many advantages for disposal of plastics.
In recent years, there has been an active effort to invent new polymers that will have the needed properties during use but will decompose by some specific process, such as biodegradation or photodegradation, when disposed of. Some progress has been reported, but the requirements for conflicting properties means that this approach will not be applicable to every situation. Is it reasonable, for example, to hope for a high-strength fishing net that falls apart only after it has been lost or discarded? On the other hand, there may be special situations in which degradable polymers will be valuable, a good example being the use of plastic "six-pack rings" for packaging soft drink and beer cans. A plastic that degrades when exposed to sunlight for just a few weeks presents a minimal hazard to the environment. Another possible example is that of polymers based