Degradable Plastics

Discarded plastics are hard to eliminate from the environment because they do not degrade and have been designed to last a long time. It is possible to design polymers containing monomer species that may be attacked by chemical, biological, or photochemical action so that degradation by such means will occur over a predetermined period of time. Such polymers can be made by chemical synthesis (as with polylactic acid) or through bacterial or agricultural processes (as with the polyalkonates). Although such processes are often more expensive than conventional ones, cost would undoubtedly drop with increased production volume. One success story was the introduction of carbonyl groups into polyethylene by mixing carbon monoxide with ethylene during synthesis. These carbonyl groups are chomophores that lead to chain breaking upon the absorption of ultraviolet light. The polymer is then broken down into small enough units that are subject to bacterial attack. This approach has been successful, for example, in promoting the disappearance of rings from beverage cans, which are potentially harmful to wildlife.

A problem with the degradation of plastics is that it is probably undesirable in landfills because of the leachants produced that may contaminate water supplies. It is better in these instances to ship the plastics to composting facilities. This requires the separation of degradable plastics from other materials and the availability of such facilities. In most cases, the infrastructure needed for such an approach is not in place. This has discouraged its use for disposable diapers that are said to constitute 1 to 2 percent of landfill volume.

Degradable polymers may have limited use in the reduction of litter and production of flushable plastics, for example, feminine hygiene products, but it seems unlikely that the use of such materials will be a viable means of disposal for large amounts of plastic products. Degradation leads to the loss of most of the potential energy content of plastics that might be recovered by trash-to-energy procedures.

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