Plastic

Plastic, material that can be molded (at least in production) into desired shapes. A few natural plastics are known, e.g., bitumen, resins, and rubber, but almost all are synthetic, made mainly from petrochemicals. They have a vast range of useful properties, including hardness, elasticity, transparency, toughness, low density, insulating ability, inertness, and corrosion resistance. Plastics are high polymers with carbon skeletons, each molecule being made up of thousands or even millions of atoms. Thermoplastics soften or melt reversibly on heating; they include celluloid or other cellulose plastics, Lucite, nylon, polyethylene, styrene polymers, vinyl polymers, polyformal-dehyde, and polycarbonates. Thermosetting plastics, although moldable when produced as simple polymers, are converted by heat and pressure, and sometimes by an admixed hardener, to a cross-linked, infusible form. These include Bakelite and other phenol resins, epoxy resins, polyesters, silicones, ureaformaldehyde and melamineformaldehyde resins, and some polyurethanes.