Page 163 - Engineering Plastics Handbook

136 Engineering Plastics

temperature is increased to about 210°C, and the pressure is controlled
at about ambient conditions. The esterification by-product—water or
methanol—is removed via a process column during reaction, while 1,4-
butanediol is refluxed. In the PC stage polycondensation is the equilibrium
reaction, to shift the chemical equilibrium to the polymerization side. The
by-product 1,4-butanediol is removed via vacuum. The final molecular
weight of PBT can be controlled by either setting the reaction time to a
predetermined value or by stopping the reaction at a fixed melt viscosity
level. After the PI reaction, the reactor vessel is emptied by inert gas pres-
sure or mechanical gear pump, and the final PBT strands are cooled by
water and cut into granules. During the batch process, by controlling the
reaction time, monomer types and contents, additives, and reaction con-
ditions, a variety of different molecular weights and specific grades of
PBT are easily attainable.
Continuous processes involve a series of continuous reactors followed by
finishing reactors. In most commercial processes, the melt-phase reac-
tions are performed by three to five (or even more) continuous reactors—
the one or two esterification reactors, one or two polycondensation reactors,
and high-viscosity finishing reactors. The series of reactors are used to
gradually extract by-product such as tetrahydrofuran (THF), methanol,
and water. Pressure is also gradually reduced with a series of reactors as
the polycondensation reaction goes further and 1,4-butanediol is removed
from the reactor. In finishing reactors, the volatile by-products and excess
1,4-butanediol are removed under vacuum. The final PBT polymer is dis-
charged from the finisher by pumping, water-cooled, and pelletized to a
size. The melt polymerization process can be used to make a variety of
molecular weight PBT grades. The higher-molecular-weight grades are
generally made by using the solid-state process under inert gas or vacuum,
or are produced by melt polymerization equipped with special mixing
devices designed for high-melt-viscosity agitation during removal of by-
products. In the commercial process, additional solid-state polymeriza-
tion may be carried out in three ways, which are under vacuum, under a
stream of inert gas, and in a fluidized bed under inert gas.
PBT is produced from both terephthalic acid and dimethyl terephtha-
late with 1,4-butanediol. Most commercial PBT grades are initially devel-
oped with DMT because the TPA process generates a larger amount of
tetrahydrofuran by-product and DMT is easier to purify than the TPA.
Also, the higher solubility of the DMT allows a handling convenience and
faster reaction rates in the transesterification stage with lower boiling
point of methanol rather than water. The DMT process produces lower
amounts of THF, a by-product formed by the irreversible acid-catalyzed
dehydration of 1,4-butanediol, compared to the TPA process. However,
in the commercial continuous process, more and more processes have
shifted to TPA as the feedstock. In the newly developed TPA process,