Page 77 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.17

FIGURE 2.12 General structure of polybenzimidazoles.

conditions, crystallinity may be obtained. The lack of many single bonds and the high
glass transition temperature give this polymer its superior high-temperature resistance. In
addition to the high-temperature resistance, the polymer exhibits good low-temperature
toughness. PBI polymers show good wear and frictional properties along with excellent
153
compressive strength and high surface hardness. The properties of PBI at elevated tem-
peratures are among the highest of the thermoplastics. In hot, aqueous solutions, the poly-
mer may absorb water, with a resulting loss in mechanical properties. Removal of
moisture will restore the mechanical properties. The heat deflection temperature of PBI is
higher than most thermoplastics, and this is coupled with a low coefficient of thermal ex-
pansion. PBI can withstand temperatures up to 760°C for short durations and exposure to
425°C for longer durations.
The polymer is not available as a resin and is generally not processed by conventional
thermoplastic processing techniques, but rather by a high-temperature and high-pressure
sintering process. 154 The polymer is available in fiber form, certain shaped forms, finished
parts, and solutions for composite impregnation. PBI is often used in fiber form for a vari-
ety of applications such as protective clothing and aircraft furnishings. 155 Parts made from
PBI are used as thermal insulators, electrical connectors, and seals. 156

2.2.10 Polybutylene (PB)
Polybutylene polymers are prepared by the polymerization of 1-butene using Ziegler-
Natta catalysts The molecular weights range from 770,000 to 3,000,000. 157 Copolymers
with ethylene are often prepared as well. The chain structure is mainly isotactic and is
shown in Fig. 2.13. 158

FIGURE 2.13 General structure for polybuty-
lene.

The glass transition temperature for this polymer ranges from –17 to –25°C. Polybuty-
lene resins are linear polymers exhibiting good resistance to creep at elevated tempera-
159
tures and good resistance to environmental stress cracking. They also show high impact
strength, tear resistance, and puncture resistance. As with other polyolefins, polybutylene
shows good resistance to chemicals, good moisture barrier properties, and good electrical

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