Page 170 - Engineering Plastics Handbook

Polybutylene Terephthalate (PBT) 143

In cold water below the glass transition temperature of PBT, little reac-
tion is observed and the molded part will not be affected by water expo-
sure. Long-term exposure of molded solid PBT parts to hot water will give
some degree of hydrolytic decomposition. The resistance to hot water can
be extended through the use of various technologies to suppress hydroly-
sis. The hydrolysis resistant modified PBT grades will give some degree
of extended life to molding parts exposed to high-temperature and high-
humidity atmospheres compared to a standard PBT. For some applications
it might be necessary to use those grades of PBT that have been specially
stabilized against hydrolysis. Those grades normally have only a small
amount of carboxylic end groups and are technically modified to make them
resistant to hydrolysis.

Flammability
PBT has a limiting oxygen index of about 20 to 23% and is fairly com-
bustible. It is classified as horizontal burning (HB) according to UL-94.
Flame resistance is a primary requirement for electrical and electronics
applications. For applications that require flame resistance, flammability
degree is usually achieved by compounding a flame-retardant material that
will generate flame-retarding ingredients on burning. Usually flame-
retardant PBT grades consist of a halogen-containing organic compound
and some inorganic synergists. It is well known that addition of antimony
compounds, such as antimony trioxide, gives a strong synergy when used
with halogen-containing compounds [16, 17]. Bromine is widely used as
the primary flame-retardant ingredient and antimony trioxide as a syn-
ergist. Most flame-retardant PBT grades contain from 10 to 25% of flame-
retardant ingredient mixtures and have a ULV-0 rating at 0.8-mm or
1.6-mm thickness. Small molecule bromine ingredients such as poly-
brominated diphenyl (PBD), polybrominated diphenyl ether (PBDE), and
ethylene bis-tetrabromophthalimide are efficient sources of halogen but
may result in blooming during molding and high-temperature use condi-
tions. Polymeric flame retardants such as brominated epoxy, brominated
polycarbonates, and brominated poystyrenes are widely used for PBT as
non blooming or “non-bleed-out” flame retardants.
There is also concern about the environmental regulations of halo-
genated flame-retardant (FR) additives with respect to recycling and the
possibility of release of toxic ingredients on burning. These issues resulted
in a growing interest in halogen-free flame-retardant PBT grades [18].
However, halogen-free flame-retardant systems for thermoplastics have
their disadvantages. Inorganic flame retardants must be used at high
loading levels, which influence the product’s physical properties. Nitrogen-
based systems, such as melamine cyanurate, have limited effectiveness in
thermoplastics; melamine cyanurate is not effective by itself in PBT.