The term plastic can be used as both an adjective and a noun. As an adjective, the term refers to any material that can be shaped or molded, with or without the application of heat. In this respect, objects such as soft waxes, asphalt, and moist clays are said to be plastic.
As a noun, the term describes a natural or synthetic polymer. A polymer is a material whose molecules consist of very long chains of one or two repeating units known as monomers. As an example, the synthetic polymer called polyethylene consists of thousands of ethylene units joined to each other in long chains. If the letter E is taken to represent an ethylene unit (monomer), then the polymer polyethylene can be represented as:
Although the term plastic is strictly defined as either a natural or synthetic material, it is probably understood by most people today to refer
primarily to artificial materials. Substances such as nylon, Styrofoam™, Plexiglass™, Teflon™, and polyvinyl chloride (PVC) are examples of such materials.
Plastics can be subdivided into two large categories: thermoplastic and thermosetting. The former term refers to a material that can be melted and shaped over and over again. Examples of thermoplastics include acetal, acrylic, cellulose acetate, polyethylene, polystyrene, vinyl, and nylon.
A thermosetting plastic, in contrast, can be melted and shaped only once. If it is then heated a second time, it tends to crack or disintegrate. Examples of thermosetting plastics (or just thermosets) include amino, epoxy, and phenolic and unsaturated polyesters.
Composite: A combination of a plastic and one or more additives that has special properties not possessed by the plastic alone.
Monomer: A fundamental unit of which a polymer is composed.
Polymer: A substance composed of very large molecular chains that consist of repeating structural units known as monomers.
Thermoplastic: A polymer that softens when heated and that returns to its original condition when cooled to ordinary temperatures.
Thermosetting plastic (or thermoset): A polymer that solidifies when heated and that cannot be melted a second time.
Very few plastics are used in their pure state. Many different materials known as additives are added to improve their properties. Products consisting of pure plastics and additives are known as composites. For example, the strength of a plastic can be increased by adding glass, carbon, boron, or metal fibers to it. Materials known as plasticizers make the plastics more pliable and easier to work with. Some typical plasticizers include low-melting solids, organic liquids, camphor, and castor oil. Fillers are materials made of small particles that make a plastic more resistant to fire; attack by heat, light, or chemicals; and abrasion. They also can be used to add color to the plastic.
[ See also Polymer ]