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Metals

Polymers that can be shaped by pressure or heat to the form of a cavity or mold are termed plastics (G. plastikos, fit for molding). Two forms of plastics are generally recognized: thermoplastics and thermosetting plastics. Thermoplastics, which comprise over 80% of all plastics, can be remelted and remolded (e.g., polyethylene, polypropylene, polyvinylchloride, polystyrene), and as such are of interest to numerous recycling efforts. Plastics that once molded cannot be remelted and remolded are termed thermosetting plastics. Concern about plastics as environmental toxicants is twofold. First, many plasticsthermo-plastics in particular-are nondegradable. They resist biological degradation (nonbiodegradable) and degradation from ultraviolet radiation (nonphotodegradable). The possibility is that plastics could persist in landfills or as roadside pollutants for hundreds of years. Although more than a fourth of all aluminum and paper are recycled in the United States each year, only about 1% of plastics are recycled. Second, attempts to incinerate some plastics (e.g., polyvinylchloride or PVC), to reduce their contribution to landfills, result in the production of toxic chemicals. One such toxic chemical, dioxane, may be absorbed via the respiratory system. In animal studies dioxanes are demonstrated carcinogens, most probably involving an epigenetic mechanism.