Page 164 - Engineering Plastics Handbook

Polybutylene Terephthalate (PBT) 137

THF by-product formation is largely diminished to approximately 13 mol
%, compared to 6 mol % in the DMT process. The TPAprocess has economic
benefits of lower raw material cost of TPA compared to DMT. The process
is commercially feasible because THF by-product is easier to separate
from water (TPAprocess) than from methanol (DMT process), and energy
consumption and investment costs are lower for the TPA process. Some
manufacturers make a variety of different molecular weight resin grades
by melt polymerization. But other suppliers, especially those with con-
tinuous plant, make a couple of low-molecular-weight and high-molecular-
weight grades, and during the following compounding, a range of molecular
weight grades are made by combining different ratios of these low- and
high-molecular-weight PBT grades. In the commercial continuous process,
combination of low- and high-viscosity resins for the intermediate-viscos-
ity grades has more practical benefits compared to making all viscosity
range grades in the polymerization reactor.

Properties of PBT and PBT Blends

By adding reinforcing agents and impact modifiers, the inherent low creep,
dimensional stability, and good electrical properties of PBT polyester can
be enhanced to provide a range of grades with high strength, stiffness, and
toughness [6]. Alloying with other polymers and adoption of the proper
reinforcement technology can also improve the dimensional stability and
surface appearance of PBT. By making technical modifications, a wide
range of PBT grades are available that can be adapted to a variety of
industrial applications.

Reinforcement of PBT
The properties of PBT resins can be improved by reinforcements, result-
ing in higher modulus, strength, and heat deflection temperature under
load. The properties of reinforced PBT grades are affected by factors such
as the types and content of reinforcement and the average size and dis-
tribution of fillers. The reinforcing fillers are selected based on desired per-
formance and part cost. The most important reinforcing fillers added to
PBT resins are surface-treated short glass fibers. Reinforcing with short
glass fiber gives PBT with improved stiffness and higher heat deflection
temperature. Figure 7.5 illustrates the temperature dependence of mod-
ulus of PBT reinforcement and heat deflection temperature. Besides glass
fibers, glass flake, milled glass, and other mineral fillers are used for rein-
forcement.
The flexural modulus and strength of PBT can be significantly
increased through reinforcement with short glass fibers. Surface-treated
glass fibers with a diameter of 9 to 14 µm and length of 3 to 6 mm are
generally introduced to the PBT during melt mixing. During the melt