Page 162 - Engineering Plastics Handbook

Polybutylene Terephthalate (PBT) 135

methanol or water during transesterification reaction, with 1,4-butanediol
during second polycondensation stages. The catalyst is usually applied
during reaction; the preferred catalysts are organotitanium or organotin
compounds. The most commonly used catalysts for PBT are tetra alkyl
titanates. The by-product 1,4-butanediol is removed from the melt by
mixing the melt and by using vacuum. Through the elimination of excess
1,4-butanediol, a number-average molecular weight of up to 30,000 g/mol
is attainable. Owing to the high temperatures during the second stage,
oxidative thermal decomposition and yellowing occur. These decomposi-
tion reactions place the upper limit on the molecular weight that can be
realized by the normal melt condensation reaction. High-molecular-weight
PBT grades for extrusion applications are typically produced by further
polycondensation in an additional solid-state process at temperatures 15
to 40°C below the melting temperature of PBT and under vacuum or in
an inert gas atmosphere (see Fig. 7.4). The solid-state reaction is carried
out separately, generally minimizing the degradation and discoloration of
high- molecular-weight PBT [4, 5].

Commercial process
For commercial PBT polymerization, both batch process and continuous
process can be used. When a variety of different grades of PBT are needed,
a batch process is preferred; however, for large commercial production of
PBT of more than 20,000 metric ton capacities, a continuous process is
more economical.
Due to different residence times needed for the reaction steps, the indus-
trial batch processes are typically run in multiple reactors. Usually two
reactors are involved, an ester interchange (EI) reactor designed to extract
methanol or water and a polycondensation (PC) reactor designed to drive
the polymerization and build molecular weight by removing excess
1,4-butanediol as well as volatile by-products. During EI reaction the

Melt polymerization
Direct/Transesterification
O
TPA + 2 Butanediol = BHBT + 2H 2
DMT + 2 Butanediol = BHBT + 2CH OH
3
Polycondensation
+ Butanediol
BHBT + BHBT = PBT 2
+ PBT = PBT + Butanediol
PBT n m n+m
Figure 7.4 Reaction equations for
Solid-state polymerization the polymerization of PBT.
PBT + PBT = PBT n+m + Butanediol
n
m