All plastics are polymeric and all polymers may be regarded as potential plastics. Certain polymers occur naturally, i.e. casein (milk solids), cellulose derivatives (wood, cotton), etc.; all others are produced synthetically and are divided into two types, thermoplastic and thermosetting. In thermoplastic materials, the necessary degree of polymerization having been achieved, the long chain molecular structure can then be activated by heat to allow freer movement between the molecules; thus the plastic can be softened and resoftened without deformation provided the degree of heat is kept below the point of actual degrade of the material. In thermosetting plastics the polymerization has been arrested at a stage which produces relatively short chain molecules. Later application of certain simple chemicals (hardeners or catalysts) or simple heat carries the polymerization a stage further, producing cross linkages which destroy the mobility of the molecules, and the plastic sets into a hard, infusible resin which cannot then be resoftened. This latter type is usually known as 'resin plastics" (synthetic resin glues, etc.). The major thermoplastics are tough, resilient and can be given controlled flexibility either by arranging the molecular structure accordingly or by added chemical plasticizers, while the thermosets are either brittle solids which can be extended with other materials (chopped paper, wood flour, etc.) to form moulding powders, or viscous syrups for use as surfacing materials, impregnating liquids and glues, although here again the addition of plasticizers or softeners will allow a limited degree of flexibility. It should be emphasized that the chemistry of the various types of plastic is very much more subtle than this brief resume might suggest, and readers are referred to the standard works on the subject for a thorough understanding of the principles involved. A list of the more important plastics with their applications in the furniture industry is given opposite.
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