Plastics are organic materials made by man which can be shaped by heat or pressure, or both. Plastics are used for brush handles and bristles, toys, transparent wrappings, fountain pens, insulators, costume jewelry, dishes, and many other items. Some day they may be used in place of metals in the building of houses, automobiles, and airplanes.
Plastics are either thermoplastic or thermosetting. Celluloid, which was discovered about 1869 by John W. Hyatt, is thermoplastic—it can be softened again and again by high temperatures and remolded. Bakelite, discovered by Leo H. Baekeland about 1910, is thermosetting—once formed, it becomes insoluble and cannot be remelted. These two properties are caused by the fact that the molding operation does not change the atomic order or chemical structure of thermoplastic materials, whereas the heating operation does change the molecular weight and chemical structure of thermosetting resins. (The vulcanization of rubber is similar to the thermosetting of a plastic.)
Plasticizers, high-boiling-point liquids, are used in paints and in thermoplastics. These materials make the plastic pliable at lower temperatures and improve such properties as water resistance, firmness, and flexibility in the final product.
