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any organic material with the ability to flow into a desired shape when heat and pressure are applied to it and to retain the shape when they are withdrawn.

Composition and Types of Plastic

A plastic is made up principally of a binder together with plasticizers, fillers, pigments, and other additives. The binder gives a plastic its main characteristics and usually its name. Thus, polyvinyl chloride is both the name of a binder and the name of a plastic into which it is made. Binders may be natural materials, e.g., cellulose derivatives, casein, or milk protein, but are more commonly synthetic resins. In either case, the binder materials consist of very long chainlike molecules called polymerspolymer
, chemical compound with high molecular weight consisting of a number of structural units linked together by covalent bonds (see chemical bond). The simple molecules that may become structural units are themselves called monomers; two monomers combine to form a dimer,
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. Cellulose derivatives are made from cellulose, a naturally occurring polymer; casein is also a naturally occurring polymer. Synthetic resins are polymerized, or built up, from small simple molecules called monomers. Plasticizers are added to a binder to increase flexibility and toughness. Fillers are added to improve particular properties, e.g., hardness or resistance to shock. Pigments are used to impart various colors. Virtually any desired color or shape and many combinations of the properties of hardness, durability, elasticity, and resistance to heat, cold, and acid can be obtained in a plastic.

There are two basic types of plastic: thermosetting, which cannot be resoftened after being subjected to heat and pressure; and thermoplastic, which can be repeatedly softened and remolded by heat and pressure. When heat and pressure are applied to a thermoplastic binder, the chainlike polymers slide past each other, giving the material "plasticity." However, when heat and pressure are initially applied to a thermosetting binder, the molecular chains become cross-linked, thus preventing any slippage if heat and pressure are reapplied.

See epoxy resinsepoxy resins,
group of synthetic resins used to make plastics and adhesives. These materials are noted for their versatility, but their relatively high cost has limited their use.
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; polyacrylicspolyacrylics
, group of thermoplastics that are transparent and highly decorative (see plastic). The polyacrylics, or acrylic plastics, are polymers (and copolymers) of derivatives of acrylic acid, H2C=CH-COOH.
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; polycarbonatespolycarbonates,
group of clear, thermoplastic polymers used mainly as molding compounds (see plastic). Polycarbonates are prepared by the reaction of an aromatic difunctional phenol with either phosgene or an aromatic or aliphatic carbonate.
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; polyethylenepolyethylene
, widely used plastic. It is a polymer of ethylene, CH2=CH2, having the formula (-CH2-CH2-)n, and is produced at high pressures and temperatures in the presence of any one of several catalysts, depending
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; polyolefinspolyolefins
, group of plastics that are polymers of various alkenes, or olefins. The most important are polyethylene and polypropylene.
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; polypropylenepolypropylene
, plastic noted for its light weight, being less dense than water; it is a polymer of propylene. It resists moisture, oils, and solvents. Since its melting point is 121°C; (250°F;), it is used in the manufacture of objects that are sterilized in the course of
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; polystyrenepolystyrene
, widely used plastic; it is a polymer of styrene. Polystyrene is a colorless, transparent thermoplastic that softens slightly above 100°C; (212°F;) and becomes a viscous liquid at around 185°C; (365°F;).
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; polyurethanespolyurethanes
, group of plastics that may be either thermosetting or thermoplastic. Polyurethane can be made into both flexible and rigid foams. The flexible foam is often used in furniture and automobile cushions, in mattresses, and for carpet backings.
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; polyvinyl chloridepolyvinyl chloride
(PVC), thermoplastic that is a polymer of vinyl chloride. Resins of polyvinyl chloride are hard, but with the addition of plasticizers a flexible, elastic plastic can be made. This plastic has found extensive use as an electrical insulator for wires and cables.
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; vinyl plasticsvinyl plastics,
group of thermoplastics used in molded products, flexible tubing, material for raincoats, and laminated safety glass. Vinyl plastics are polymers and copolymers of vinyl derivatives (i.e., derivatives of ethylene, H2C=CH2), e.g.
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Molding of Plastic

Plastics are available in the form of bars, tubes, sheets, coils, and blocks, and these can be fabricated to specification. However, plastic articles are commonly manufactured from plastic powders in which desired shapes are fashioned by compression, transfer, injection, or extrusion molding. In compression molding, materials are generally placed immediately in mold cavities, where the application of heat and pressure makes them first plastic, then hard. The transfer method, in which the compound is plasticized by outside heating and then poured into a mold to harden, is used for designs with intricate shapes and great variations in wall thickness. Injection-molding machinery dissolves the plastic powder in a heating chamber and by plunger action forces it into cold molds, where the product sets. The operations take place at rigidly controlled temperatures and intervals. Extrusion molding employs a heating cylinder, pressure, and an extrusion die through which the molten plastic is sent and from which it exits in continuous form to be cut in lengths or coiled.

Environmental Considerations

Plastics are so durable that they will not rot or decay as do natural products such as those made of wood. As a result great amounts of discarded plastic products accumulate in the environment as waste. It has been suggested that plastics could be made to decompose slowly when exposed to sunlight by adding certain chemicals to them. Plastics present the additional problem of being difficult to burn. When placed in an incinerator, they tend to melt quickly and flow downward, clogging the incinerator's grate. They also emit harmful fumes; e.g., burning polyvinyl chloride gives off hydrogen chloride gas.

Development of Plastics

The first important plastic, celluloidcelluloid
[from cellulose], transparent, colorless synthetic plastic made by treating cellulose nitrate with camphor and alcohol. Celluloid was the first important synthetic plastic and was widely used as a substitute for more expensive substances, such as ivory, amber,
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, was discovered (c.1869) by the American inventor John W. Hyatt and manufactured by him in 1872; it is a mixture of cellulose nitrate, camphor, and alcohol and is thermoplastic. However, plastics did not come into modern industrial use until after the production (1909) of BakeliteBakelite
[for its inventor, L. H. Baekeland], synthetic thermosetting resin. It has been widely used both alone, to form whole objects, and in combination with other materials, as a laminate or a surface coating.
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 by the American chemist L. H. Baekeland. Bakelite, made by the polymerization of phenol and formaldehyde, is thermosetting. New uses for plastics are continually being discovered. Following World War II optical lenses, artificial eyes, and dentures of acrylic plastics, splints that X rays may pierce, nylon fibers, machine gears, fabric coatings, wall surfacing, and plastic lamination were developed. More recently a hydrophilic, or water-attracting, plastic suitable for use in non-irritating contact lenses has been developed. Among the trade names by which many plastic products are widely known are Plexiglas, Lucite, Polaroid, Cellophane, Vinylite, and Koroseal. Plastics reinforced with fiberglass are used for boats, automobile bodies, furniture, and building panels.


See L. K. Arnold, Introduction to Plastics (1968); J. H. DuBois, Plastics History, U.S.A. (1972); H. D. Junge, Dictionary of Plastics Technology (1987); A. W. Birley et al., Plastics Materials: Properties and Applications (1988).


Any of the various synthetic complex organic compounds produced by polymerization; can be molded, extruded, or cast into various shapes or drawn into fibers.

acrylic fiber

A synthetic polymer fiber.


The generic term for a material consisting of extremely fine filaments of glass that are mixed with a resin to give the desired form in a mold. Layers of this combination are laid or sprayed into the mold.


A class of thermoplastics characterized by extreme toughness, strength, and elasticity and capable of being extruded into filaments, fibers and sheets.


A polymeric material (usually organic) of large molecular weight which can be shaped by flow; usually refers to the final product with fillers, plasticizers, pigments, and stabilizers included (versus the resin, the homogeneous polymeric starting material); examples are polyvinyl chloride, polyethylene, and urea-formaldehyde.
Displaying, or associated with, plasticity.


1. A natural or artificially prepared organic polymer of low extensibility, as compared with rubber; can be molded, extruded, cut, or worked into a great variety of objects, rigid or nonrigid, relatively light, which are formed by condensation polymerization and by vinyl polymerization; plastics.


The condition in which material is capable of being shaped. It is normally in a semisolid state.


1. any one of a large number of synthetic usually organic materials that have a polymeric structure and can be moulded when soft and then set, esp such a material in a finished state containing plasticizer, stabilizer, filler, pigments, etc. Plastics are classified as thermosetting (such as Bakelite) or thermoplastic (such as PVC) and are used in the manufacture of many articles and in coatings, artificial fibres, etc.
2. Fine arts
a. of or relating to moulding or modelling
b. produced or apparently produced by moulding
3. Biology of or relating to any formative process; able to change, develop, or grow