PLASTIC Any synthetic organic material that can be moulded under heat and pressure into a shape that is retained after the heat and pressures are removed. There are two basic types of plastic:
Plastics are made up chiefly of a binder consisting of long chainlike molecules called POLYMERS. Binders can be natural materials, e.g., CELLULOSE, or (more commonly) synthetic RESINS, e.g., BAKELITE. The permanence of thermosetting plastics is due to the heat- and pressure-induced cross-linking reactions the polymers undergo. Thermoplastics can be reshaped because their linear or branched polymers can slide past one another when heat and pressure are applied. Adding plasticizers and fillers to the binder improves
a wide range of properties, Adding PIGMENTS imparts color. Plastic products are commonly made from plastic powders.
The first important plastic, CELLULOID, has been largely replaced by a wide variety of plastics known by such trade names as Plexiglas, Lucite, Polaroid, and CELLOPHANE. New uses continue to be found and include contact lenses, machine gears, non-moving engine parts, and artificial body parts. The widespread use of plastics has led to environmental problems. Because plastic products do not decay, large amounts accumulate as waste. Disposal is difficult because they melt when burned, clogging incinerators and often emitting harmful fumes, e.g., the hydrogen chloride gas given off by POLYVINYL CHLORIDE, and because many products into which they are made, such as plastic bottles, do not compact as readily in landfills. However, recycling is increasingly emerging as an alternative to disposal, particularly in communities with limited landfill capacity. See also
POLYPROPYLENE;
POLYURETHANES;
TEFLON.
Chemical compound with high molecular weight consisting of a number of structural units linked together by covalent bonds. The simple molecules that may become structural units are themselves called monomers. A structural unit is a group having two or more bonding sites. In a linear polymer, the monomers are connected in a chain arrangement and thus need only have two bonding sites. When the monomers have three bonding sites, a nonlinear, or branched, polymer results. Naturally occurring polymers include CELLULOSE, PROTEINS, natural RUBBER, and SILK; those synthesized in the laboratory have led to such commercially important products as PLASTICS, synthetic fibers, and synthetic rubber.
Substance that imparts color to other materials. Most paint pigments are metallic compounds, but organic compounds are also used. Some metallic pigments occur naturally, e.g., the oxides that produce the brilliant coloring of rocks and soil in the W U.S. Plants and animals also contain pigments. CHLOROPHYLL (green) and carotene (yellow) produce bright colors in plants. Blood receives its red color from HEMOGLOBIN, and various pigments color human skin.
(PVC), thermoplastic that is a POLYMER of vinyl chloride. By adding plasticizers, hard PVC RESINS can be made into a flexible, elastic PLASTIC, used as an electrical insulator and as a coating for paper and cloth in making fabric for upholstery and raincoats.
Lightweight PLASTIC, a POLYMER of propylene. It is less dense than water and resists moisture, oils, and solvents. It is used to make packaging material, textiles, luggage, ropes that float, and, because of its high melting point (250°F/121°C), objects that must be sterilized.
Large group of PLASTICS that occur in a wide variety of forms. As a flexible foam, it is used for cushions and carpet backings. As a rigid foam, it can be moulded into furniture or used as insulation. Some polyurethanes are highly elastic, e.g., Lycra, a fiber used in stretch clothing; others form hard protective coatings.
Trade name for a solid, chemically inert POLYMER of tetrafluoroethylene, F2C&dbond;CF2. Stable up to temperatures around 572 degrees Fahrenheit (300 degrees centigrade), Teflon is used in electrical insulation, gaskets, and in making low-adhesion surfaces, e.g., for non-stick cookware.
Any of a class of amorphous solids or semisolids. Natural resins occur as plant exudations (e.g., of pines and firs), and are also obtained from certain scale insects. They are typically yellow to brown in color, tasteless, and translucent or transparent. Oleoresins contain ESSENTIAL OILS and are often sticky or plastic; other resins are exceedingly hard, brittle, and resistant to most solvents. Resins are used in VARNISH, SHELLAC, and lacquer and in medicine. Synthetic resins, e.g., BAKELITE, are widely used in making PLASTICS. See also AMBER; TURPENTINE.
Any solid substance, usually elastic, that can be vulcanized to improve its elasticity and add strength; the term includes natural rubber, or caoutchouc, and a wide variety of synthetic rubbers, which have similar properties. Rubbers are composed chiefly of CARBON and HYDROGEN, but some synthetics also have other elements, e.g., chlorine, fluorine, nitrogen, or silicon. All are compounds of high molecular weight; each consists of a series of one kind of molecule (e.g., isoprene in natural rubber) hooked together in a long chain to form a very flexible, larger molecule, the POLYMER. Natural rubber is obtained as latex, a milky suspension of rubber globules found in a large variety of plants, chiefly tropical and subtropical. An important source is the PARá RUBBER TREE. Latex can be shipped for processing either as a liquid or coagulated by acid and rolled into sheets. For most purposes rubber is ground, dissolved in a solvent, and compounded with other ingredients, e.g., fillers, PIGMENTS, and plasticizers. Known in pre-Columbian times by the indigenous peoples of South and Central America, rubber first attracted interest in Europe in the 18th cent. Vulcanization, a process invented (1839) by Charles GOODYEAR, revolutionized the rubber industry. It usually involves heating raw or compounded rubber with SULFUR, causing sulfur bridges to form between molecules. The product is nonsticky, elastic, and resistant to heat and cold. Natural rubber is used chiefly to make tires and inner tubes because it is cheaper than synthetic rubber and has greater resistance to tearing when hot. Natural rubber can be treated to make foam rubber and sponge rubber. The first synthetic rubber was made in Germany in World War I. Today synthetics, e.g., Buna S, neoprene, butyl, and nitrile, account for most of the world's rubber production. Made from COAL, PETROLEUM, NATURAL GAS, and ACETYLENE, synthetic rubbers are resilient over a wider temperature range than natural rubber and are more resistant to aging, weathering, and attack by certain substances, notably, oil, solvents, oxygen, and ozone. SILICONE rubbers are used in insulation. POLYURETHANES are used in tires, in shoes, and as foams. Neoprene is used for making hose and tank linings. Butyl rubber is used in inner tubes and as insulation. The above definitions were obtained from Encyclopedia.com
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