There are several types of melt-processible thermoplastics, including polybutylene tere-phthalate, polyethylene terephthalate, and aromatic copolyesters.
POLYBUTYLENE TEREPHTHALATE (PBT)
This plastic material is made by the transester-ification of dimethyl terephthalate with butane-diol through a catalyzed melt polycondensa-tion. These molding and extrusion resins have good resistance to chemicals, low moisture absorption, relatively high continuous-use temperature, and good electrical properties (track resistance and dielectric strength). PBT resin is sensitive to alkalies, oxidizing acids, aromatics, and strong bases. Various additives, fillers, and fiber reinforcements are used with PBT resins, in particular flame retardants, mineral fillers, and glass fibers. PBT resins and compounds are used extensively in automotive, electrical-electronic, appliance, military, communications, and consumer product applications.
Polyethylene Terephthalate (pet)
This is a widely used thermoplastic packaging material. Beverage bottles and food trays for microwave and convection oven use are the most prominent applications. PET resins are made from ethylene glycol and either terephthalic acid or the dimethyl ester of terephthalic acid. Most uses for PET require the molecular structure of the material to be oriented. Orientation of PET significantly increases tensile strength and reduces gas permeability and water vapor transmission. For packaging uses, PET is processed by blow molding and sheet extrusion. Typical trade names are Cleartuf, Traytuf, Tenite, and Kodapak. Injection-molding grades of PET, with fillers and/or reinforcements, also are available for making industrial products.
Aromatic polyesters (liquid crystal polymers, or LCP) have high mechanical properties and heat resistance. Commercial grades are aromatic polyesters, which have a highly ordered or liquid crystalline structure in solution and molten states. A high degree of molecular orientation develops during processing and, hence, anistropy in properties. Typically, these melt-processible resins can be molded or extruded to form products capable of use at temperatures over 260°C. Tensile strengths up to 240.3 MPa and flexural moduli up to 3586.3 MPa are reported for LCPs. Chemical resistance also is excellent. Trade names for LCPs include Vectra and Xydur Granlar. Applications are in chemical processing, electronic, medical, and automotive components.
