Page 84 - Handbook of Plastics Technologies

THERMOPLASTICS

2.24 CHAPTER 2

FIGURE 2.18 Direct esterification of a diacid (dimethyl terephthalate) with
a diol(ethylene glycol) in the first stages of PET polymerization.

lution polymerization of one mole of dimethyl terephthalate with 2.1 to 2.2 moles of ethyl-
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ene glycol. The excess ethylene glycol increases the rate of formation of bis(2-
hydroxyethyl) terephthalate. Small amounts of trimer, tetramer, and other oligomers are
formed. A metal alkanoate, such as manganese acetate, is often added as a catalyst; this is
later deactiviated by the addition of a phosphorous compound such as phosphoric acid.
The antioxidant phosphate improves the thermal and color stability of the polymer during
194
the higher-temperature second-stage process. The first stage of the reaction is run at
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150 to 200°C with continuous methanol distillation and removal.
The second step of the polymerization, shown in Fig. 2.19, is a melt polymerization as
the reaction temperature is raised to 260 to 290°C. This second stage is carried out under
either partial vacuum (0.13 kPa) 196 to facilitate the removal of ethylene glycol or with an
inert gas being forced through the reaction mixture. Antimony trioxide is often used as a
polymerization catalyst for this stage. 197 It is critical that excess ethylene glycol be com-
pletely removed during this alcoholysis stage of the reaction so as to proceed to high-mo-
lecular-weight products; otherwise, equilibrium is established at an extent of reaction of
less than 0.7. This second stage of the reaction proceeds until a number-average-molecular
weight, M , of about 20,000 g/mol is obtained. The very high temperatures at the end of
n
this reaction cause thermal decomposition of the end groups to yield acetaldehyde. Ther-
mal ester scission also occurs, which competes with the polymer step-growth reactions. It
is this competition, which limits the ultimate M , that can be achieved through this melt
n
condensation reaction. 198 Weight-average molecular weights of oriented films are around
35,000 g/mol.
Other commercial manufacturing methods have evolved to a direct esterification of
acid and glycol in place of the ester-exchange process. In direct esterification, terephthalic
acid and ethylene glycol are reacted rather than esterifying terephthalic acid with metha-
nol to produce the dimethyl terephthalate intermediate. The ester is easier to purify than
the acid, which sublimes at 300°C and is insoluble. However, better catalysts and purer

FIGURE 2.19 Polymerization of bis(2-hydroxyethyl) terephthalate to PET.

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