Health problems in the manufacture and use of plastics.

The increasing variety of plastic materials produced during the last few years is due mainly to the ability of the modern chemist to synthesize the chemical compounds from which plastics are made. The basis of most plastics today is the synthetic polymer and its introduction has made available many substances hitherto unknown in industry. Modern plastics are manufactured to serve specific purposes so that the synthetic polymers can no longer be regarded as substitutes for those found naturally as cellulose, rubber, and proteins. A wide range of materials can be made from them and so designed as to meet the actual properties required for any particular application. In addition to the polymer, synthetic plastic materials contain varying amounts of other chemicals as catalysts, accelerators, stabilizers, hardeners, pigments, and so on, which are added to assist and regulate polymerization or to influence the properties of the final material, such as colour and flexibility. Most polymers are chemically inert and under ordinary circumstances the final plastic material should be comparatively harmless. It is admittedly difficult, however, in practice to effect complete polymerization and traces of free monomer or other chemicals may be present which can sometimes produce pathological conditions such as dermatitis. Polymerized chemicals may be broadly divided into two groups, condensation and addition polymers. They may also be divided into thermoplastic or thermosetting types depending upon whether the polymer is softened or hardened by heating. The change that takes place on heating the thermosetting materials is irreversible.