Details
Material hierarchy: Polymer - Engineering Thermoplastic
The two dominant materials in this family are polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). The thermoplastic polyesters are similar in properties to types 6 and 6/6 nylons but have lower water absorption and higher dimensional stability than the nylons.
To develop the maximum properties of PET, the resin has to be processed to raise its level of crystallinity and/or to orient the molecules. Orientation increases the tensile strength by 300 to 500% and reduces permeability. PET is a water-white polymer and is made into fibers, films and sheets, and blow-molded and thermoformed containers for soft drinks and foods. Glass-reinforced PET compounds can be injection-molded into parts for automotive, electrical/electronic, and other industrial and consumer products.
PBT, on the other hand, crystallizes readily, even in chilled molds. Most PBT is sold in the form of filled and reinforced compounds for engineering applications. Its uses include appliances, automotive, electrical/electronic, materials handling, and consumer products.
Some newer members of the polyester family are starting to find application in specific niche markets. PCT (poly-1-4-cyclohexylene-dimethylene terephthalate) has higher heat resistance than either PET or PBT, and is used mainly as a component of blends. PEN (polyethylene naphthalate) has a combination of higher heat resistance, mechanical strength, chemical resistance, and dimensional stability, and can be processed into films, fibers, and containers. PEN has up to five times the oxygen barrier of PET.
Thermoplastic polyesters can be used as a component in alloys and blends. Widely used combinations include those with polycarbonate, polysulfone, and elastomers.
Also in the family of thermoplastic polyesters is the class of materials known as liquid crystal polymers (LCPs), aromatic copolyesters with a tightly ordered structure that is self-reinforcing. LCPs generally flow very well in processing, but they have to be thoroughly dried and molded at high temperatures. They also exhibit very high mechanical properties although molded structures tend to be quite anisotropic. The anisotrophy can be reduced by proper gating and mold design and by incorporating mineral fillers and glass fiber reinforcements. LCPs are resistant to most organic solvents and acids. They are inherently flame-resistant (UL94 V-O) and meet federal standards for aircraft interior use. Some LCPs withstand temperatures over 1000°F before decomposing.
LCPs have found application in aviation, electronics (connectors, sockets, chip carriers), automotive underhood parts, and chemical processing, and are used to mold household cookware for conventional and microwave ovens.