Plastics

Plastics are organic substances. They are made synthetically by polymerization, and capable of being formed into an almost endless variety of products, e.g. threads, sheets, tubes and moulded objects.

The ancestor of modern synthetic plastics is celluloid. Celluloid has certain disadvantages – its flammability and the fact that it is not readily moulded. Thus it was not until the discovery of bakelite in 1907 that the real foundation of the synthetic plastics industry laid.

Plastics that consist of long-chain molecules can be softened by heat and moulded into a desire shape. These plastics are called thermoplastics. Plastics in which the polymer chains are cross-linked have much greater rigidity and cannot be softened. They are called thermosettings. The term thermoplastic and thermosetting are also applied to the resins from which plastics are made.

The principal agent incorporated in a plastic is the resin. The resin may be natural, like cellulose, but it is more generally synthetic.

The resin is also known as a binder. Substances added to the plastic to enhance certain properties, e.g. hardness, resistance to shock, or resistance to abrasion, are called litters.

Examples are asbestos, glass fibres, and wood flour. Plasticizers are also included in the formulation. Antioxidants may be added to promote chemical stability and thus prolong life.

Catalysts are added to assist the final cure (final formation of the product), and stabilizers to protect against sunlight, heat and other destructive factors.

The procedure used to shape a plastic into its final form depends on the properties of the plastic. Some plastics can be injection moulded. Other plastics must be compression moulded – after they are filled into the mould they are subjected to pressure. Some plastics are simply cast intomtheir final shape.