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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.
