2 - Material Basics
Review articleOpen access

Publisher SummaryPolyethylene terephthalate (PET) belongs to the group of materials known as thermoplastic polymers. The application of heat causes the softening and deformation of thermoplastics. There are a few chemical routes to manufacturing PET. A compound with two acids, such as terephthalic acid (TPA), is esterified with a compound with two alcohols, such as ethylene glycol (EG). Since the catalyst residues remain in the PET, they are still present during drying and processing. Therefore, different grades of PET from different manufacturers react differently if not processed at optimum conditions. PET occurs in three different states: amorphous, nonoriented, and clear, such as preforms and melted plastic resin; thermally crystallized, such as resin pellets; strain-induced crystallized, such as bottle side walls. PET is transformed several times as it goes from pellet to preform to bottle. The properties of the PET polymer are largely dependent upon the average molecular weight or the average number of repeat units of the polymer chains. This is usually determined by the measurement of intrinsic viscosity (IV). The relationship between molecular weight and IV is fairly linear. High IV PET has a higher molecular weight than low IV PET. The longer chains give the resin better properties in the final product and also affect the processing in predictable ways.

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