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Polyethylene terephthalate (aka. PET, PETE, PETP) is a thermoplastic resin of the polyester family that is used to make beverage, food and other liquid containers, synthetic fibers, as well as for some other thermoforming applications. It is also one of the most important raw materials used in man-made fibers. Depending on its processing and thermal history, it may exist both as an amorphous (transparent) and as a semi-crystalline (opaque and white) material. It can be synthesized by a transesterification reaction between ethylene glycol and dimethyl terephthalate.

High purity is required of all raw materials. In either case, the first step of the reaction is the formation of a prepolymer, bis-hydroxyethyl terephthalate (bis-HET). Subsequent polymerization of this material (with the removal of ethylene glycol) forms the polymeric polyethylene terephthalate. The extent of polymerization (apparent from the molecular weight of the polymer) is a function of the polymerization conditions and significantly affects the properties of the resin that is produced. As the polymer grows in length, both molecular weight and viscosity of the reacting mass increase; thus intrinsic viscosity (IV) is frequently used as a measure of polymer molecular weight. When very high molecular weights are desired, as is the case for bottle-grade PET resins, the polymerization may be carried out in stages, with different reaction conditions being utilized in each stage.

Until the mid-1960s, DMT had been the preferred feedstock for PET manufacture, partly because the ester could generally be made in purer form than the acid. With the development of high-purity TPA processes, notably by Amoco, the free acid gained acceptance and is now the preferred feedstock. The use of high purity TPA (PTA) eliminates the need to recover or recycle methanol and has the added advantage that esterification to the prepolymer is considerably more rapid than the transesterification reaction, which is the first step when starting from DMT.

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