A plastic is a 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 man-made, mainly from petrochemicals, and are available with a vast range of useful properties: hardness, elasticity, transparency, toughness, low density, insulating ability, inertness, and corrosion resistance, etc. They are invariably high polymers with carbon skeletons, each molecule being made up of thousands or even millions of atoms.
Plastics fall into two classes: thermoplastic and thermosetting. Thermoplastics soften or melt reversibly on heating; they include celluloid and other cellulose plastics, lucite, nylon, polythene, styrene polymers, vinyl polymers, polyformaldehyde, polycarbonates, and acrylic. An acrylic is any one of a group of synthetic, short-chain unsaturated carboxylic acids derivatives. Variation in the reagents and the method of formation yields either hard and transparent, soft and resilient, or liquid products. Their transparency, toughness, and dimensional stability make acrylics useful for molded structural parts, lenses, adhesives, and paints.
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, urea-formaldehyde, and melanine-formaldehyde resins, and some polyurethanes.
Most plastics are mixed with stabilizers, fillers, dyes, or pigments and plasticizers if needed. There are several fabrication processes: making films by calendering (squeezing between rollers), casting or extrusion, and making objects by compression molding, injection molding (melting and forcing into a cooled mold) and casting.
The plastics industry began in 1872 with the manufacture of celluloid by the US inventor John W. Hyatt.