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Material hierarchy: Polymer - Thermoset
The polyesters form a family of polyers made up of the characteristic ester linkages (CO–O). They can be either thermoplastic or thermosetting, depending on whether cross-linking occurs. Of the thermoplastic polyesters, a representative example is polyethylene terephthalate (PET). It can be either amorphous or partially crystallized (up to about 30%), depending on how it is cooled after shaping. Fast cooling favors the amorphous state, which is highly transparent. Significant applications include blow-molded beverage containers, photographic films, and magnetic recording tape. In addition, PET is widely used as fibers in apparel. Polyester fibers have low moisture absorption and good deformation recovery, both of which make them ideal for "wash and wear" garments that resist wrinkling. The PET fibers are almost always blended with cotton or wool. Familiar trade names for polyester fibers include Dacron (DuPont), Fortrel (Celanese), and Kodel (Eastman Kodak).
The unsaturated polyesters are extremely versatile in terms of the forms in which they are used. Polyester resins can be formulated to be brittle and hard, tough and resilient, or soft and flexible. In combination with reinforcements such as glass fibers, they offer outstanding strength, a high strength-to-weight ratio, chemical resistance, and other excellent mechanical properties.
Among the members of the unsaturated polyester family are the orthophthalic (lowest cost) and isophthalic polyesters, vinyl esters, and blends of the various types. The crosslinking of unsaturated polyesters is initiated by a peroxide catalyst, selected by the requirements for curing temperature and rate.
Polyester resins generally are dissolved in a crosslinking monomer, with styrene the common solvent. Styrene has come under fire as a potential carcinogen, and its emissions are being strictly regulated at lower and lower levels. Resin suppliers are helping processors to reduce emissions by incorporating additive that keep the styrene from evaporating. It also is possible to replace styrene with paramethyl styrene, which has a much lower vapor pressure than styrene.
The prime outlet for unsaturated polyesters is in combination with glass fibers in high-strength reinforced plastics, or composites. Using various lay-up, spray-up, filament winding, pultrusion, injection, compression, and resin transfer molding techniques, polyester-glass is being used for such diverse products as boat hulls, automotive body parts, building panels, housings, bathroom components, tote boxes, pipes and pressure vessels, appliances, and electronic and electrical applications.
Where polyesters are used in building applications, they normally are filled with gypsum or alumina trihydrate (ATH) to make them flame-retardant. In sheet molding compounds (SMC) for automotive body panels, they are heavily filled with talc and reinforced with glass fibers; thermoplastic additives are used to provide a smooth Class A surface. Another major application for polyesters is in "cultured marble," highly filled materials that are made to resemble natural materials for sinks and vanities.
Polymer concretes based on polyesters are used to patch highways and bridges; their rapid cure minimizes the disruption of traffic. Polyester patching compounds also are used to repair auto body damage.
Polyester is any of a certain group of widely used plastics materials. The most important polyester, polyethylene terephthalate (PET), can be spun into fiber, extruded (pushed through an opening) to make film, or combined with other materials and molded into plastic parts. PET is a thermoplastic--that is, it softens and melts at high temperatures.
polyester is a semi-crystalline thermoplastic, with a continuous use temperature range of up to 230°F (110°C). It has a low difficulty to machine. It has a high dimensional stability and high chemical resistance. It has low moisture absorption. It has good wear resistance. It has a low coefficient of friction.
Polyesters are synthetic polymers. A polymer is a long, chainlike molecule. The "links" are repeating patterns of simple groups of atoms called monomers. The key feature of the PET repeating unit is two ester groups, each made up of one carbon atom and two oxygen atoms.
PET fiber is one of the most widely produced synthetic fibers. It appears in such products as tire cords and clothing. It is strong and flexible, and it resists wrinkling and mildew. Much PET fiber is sold under the name Dacron, a registered trademark of the DuPont Company.
Uses of PET film include magentic tapes and shrink wrap. PET is also used to make plastic soft drink bottles, which, like PET film, are clear and tough, and resist water and chemicals.
Other important polyesters are the unsaturated polyesters. The monomers in these materials contain unsaturated acids, which are hydrocarbon acids with two carbon atoms joined by double bond. Manufacturers use these polymers to make thermosetting plastics, which do not soften when heated. Polyester "thermosets" can be combined with fiberglass (glass threads) to make a strong rigid material used for automobile body parts, boats, and bowling balls.
Polyester thermoplastics are easy to recycle. The thermosetting materials are difficult to recycle, but can be ground into powders and used as fillers.
Researchers discovered the chemistry of polyesters in the 1930's. In the 1940's, the aircraft industry began to use unsaturated polyesters. In the 1950's, PET fiber became a major product.