Photodegradable plastic

Plastics are clearly one of the great chemical inventions of all time. It would probably be impossible to list all the ways in which plastics are used in everyday life. Suffice to say that, in the early 1990s, more than 60 billion pounds (27 billion kg) of plastics were produced in the United States alone each year.

For their many advantages, plastics also create some serious environmental problems. About one third, 20 billion pounds (9 billion kg), of all plastics now produced are used for short-term purposes, such as shopping bags and wrapping material. These plastics are used once, briefly, and then discarded.

Current estimates are that plastic materials make up about 7 percent by weight and 18 percent by volume of all municipal wastes. Since most plastics do not degrade naturally, they will continue to be a part of the nation's (and the world's) solid waste problem for decades.

One solution to the accumulation of plastics is to do a better job of recycling them. Technical, economic, and social factors have limited the success of this approach so far, and no more than 1 percent of all discarded plastics are ever recycled. Another solution is to fabricate plastics in such a way that they will degrade. Addition of starch as a filler in plastics, for example, tends to make them more biodegradable , that is, degradable by naturally-occurring organisms.

Yet another approach is photodegradable plastics, plastics that will decompose when exposed to sunlight. The process of photodegradation is well understood by chemists. In general, when light strikes a molecule, it may initiate any number of reactions that result in the destruction of that molecule. For example, light may dislocate one or more electrons that make up a chemical bond in the molecule. When the bond is broken, the molecule is destroyed.

On a practical level, that means that light can cause certain materials to decompose. Among the plastics, aromatic-based polymers (those that have benzene-like ring structures) are particularly susceptible to photodegradation. Those that lack an aromatic structure are less susceptible to the effects of light. Polyethylene, polypropylene, polyvinyl chloride , polymethyl methacrylate, polyamides, and polystyrene--some of the most widely used plastics--fall into that latter category.

In addition to the type of plastic, the kind of light that falls on a material affects the rate of photodegradation. Ultraviolet (UV) light is more effective, in general, in degrading all plastics than are most other forms of light.

The chemist's task, then, is to find a mechanism for converting a substance that is normally non-photodegradable (like polystyrene ) into a form that is decomposed by light. In principle, that challenge is rather easily met. Certain metallic ions and organic groups are known to absorb visible light strongly. If these ions or groups are included in a plastic material, their absorption of light will contribute to the decay of the plastic.

Photodegradable plastics are not a new market item. Webster Industries, of Peabody, Massachusetts, has been making photodegradable plastic bags since 1978. And Mobil Chemical focused some of its advertising in 1990 on Hefty trash bags that were designed to photodegrade.

But critics raise a number of points about photodegradable (as well as biodegradable) plastic materials. For one thing, most plastics are disposed of in sanitary landfills where they are rapidly covered with other materials. The critics question the good of having a photodegradable plastic that is not exposed to sunlight after it is discarded.

Questions have also been raised about the possible byproducts of photodegradation. It is possible that the very additives used to make a plastic photodegradable may have toxic or other harmful environmental effects, critics say.

Finally, photodegradable plastics take so long to decay (typically two months to two years) that they will still pose a hazard to wildlife and contribute to solid waste volume. The "false promise" of photodegradability may, in fact, actually encourage the use of more plastics and add to the present problem, rather than helping to solve it.

[David E. Newton ]