Polyester resins

(organic chemistry) A thermosetting or thermoplastic synthetic resin made by esterification of polybasic organic acids with polyhydric acids; examples are Dacron and Mylar; the resin has high strength and excellent resistance to moisture and chemicals when cured.

Synthetic polymers made by esterification of dicarboxylic acids with diols. The aliphatic polyesters tend to be relatively soft, and the aromatic derivatives are usually hard and brittle, or tough. The properties of either group may be modified by cross linking, crystallization, plasticizers, or fillers. See also Ester.

The commercial products are alkyds which are used in paints, enamels, and molding compounds; unsaturated polyesters or unsaturated alkyds which are used extensively with fiber glass for boat hulls and panels; aliphatic saturated polyesters; aromatic polyesters, such as polyethylene terephthalate which is used in the form of fibers and films; and the aromatic polycarbonates. The polydiallyl esters, while frequently listed with the polyesters, are not true polyesters as defined above. See also Polyvinyl resins.

The alkyds are commonly used as coatings. Combinations of conventional vegetable drying oils and alkyd resins represent the basis of most of the oil-soluble paints. The drying oil–alkyd may be further modified by the inclusion of a vinyl monomer, such as styrene. Some of the styrene polymerizes, probably as a graft polymer, and the remainder polymerizes and copolymerizes in the final drying or curing of the pain. See also Polymerization.

The unsaturated polyesters, in combination with glass fiber, have found applications as panels, roofing, radar domes, boat hulls, and protective armor for soldiers. The compositions are distinguished by ease of fabrication and high impact resistance. See also Polymeric composite.

Saturated aliphatic polyesters have long been frequently used as intermediates in the preparation of prepolymers for making segmented polyurethanes. Lactone rings can also be opened to yield linear polyesters.

The aromatic polyesters which have achieved general importance are the polyethylene terephthalates, which yield very strong and chemically resistant fibers and films. Polyethylene terephthalate is the principal ingredient of polyester fibers. Polyethylene terephthalate may be molded or extruded to yield materials that can replace metals or thermoset resins in some automotive, electrical, and specialty applications, especially when reinforced with glass fibers or mineral fillers.

Aromatic polycarbonates are a strong, tough group of thermoplastic polymers formed most frequently from bisphenol A and phosgene. The products, polycarbonates, are noted for high softening temperatures, and high impact resistance, clarity, and resistance to creep. Polycarbonate is usually available as a molding compound. Because of its high strength, toughness, and softening point, the resin, both by itself and as a glass-reinforced material, has found many electrical domestic and engineering applications. It is often used to replace glass and metals. Examples include bottles, unbreakable windows, applicance parts, electrical housings, marine propellers, and shotgun shells. Flame-retardant grades are of interest because of low toxicity and smoke emission on burning.

Polydiallyl esters are polymers of diallyl esters. Thermosetting molding compounds may be produced by careful limitation of the initial polymerization to yield a product which is fusible. Major applications are in electronic components, sealants, coatings, and glass-fiber composites. See also Plastics processing.

One of a group of synthetic resins which undergo polymerization during curing; advantageous because high pressure is not required for curing; has excellent adhesive properties, high strength, good chemical resistance; esp. used in laminating and impregnating materials.