POLYESTER

any of a class of organic substances composed of large linear (chainlike) or cross-linked (network) molecules, or polymers, formed from a large number of smaller molecules, or monomers, by establishment of ester linkages between them. Polyesters most commonly are prepared from equivalent amounts of glycols (organic compounds containing two hydroxyl groups) and dibasic acids (containing two carboxyl groups). The long-chain polyester made from ethylene glycol and terephthalic acid is the basis of the fibre called Dacron, Fortrel, or Terylene and the film Mylar. The polycarbonate resin Lexan is also a long-chain polyester. The polyester fibres are generally similar in performance and properties. They recover quickly after extension and absorb very little moisture. They melt at about 260 C (500 F). Prolonged exposure to light reduces their strength but does not affect their colour. Polyesters have good resistance to chemicals. They can be washed or dry-cleaned with most common cleaning solvents; they are resistant to attack by insects and microorganisms. Their low moisture content makes them likely to accumulate static charges unless treated with antistatic agents. Polyesters are made into both woven and knitted fabrics, either alone or blended with other fibres. Industrial applications include ropes, filters, conveyor belts, and tire cords. In surgery, polyester is used to replace or reinforce damaged body tissue. Liquid linear polyesters made from unsaturated dibasic acids, especially maleic and fumaric acids, can be used to impregnate materials such as glass fabrics, then cured, or converted by cross-linking, into durable, resinous polymers; such composite materials have been widely used for hulls of small boats, patches for automobile bodies, and the like. Alkyd resins, used in paints, varnishes, and other kinds of coating materials, are cross-linked polyesters of which the principal components are glycerol and phthalic acid.