Page 80 - Handbook of Plastics Technologies

THERMOPLASTICS

2.20 CHAPTER 2

Applications where PC is blended with ABS increase the heat distortion temperature of
the ABS and improve the low-temperature impact strength of PC. The favorable ease of
processing and improved economics make PC/ABS blends well suited for thin-walled
electronic housing applications such as laptop computers. Blends with PBT are useful for
improving the chemical resistance of PC to petroleum products and its low-temperature
impact strength. PC alone is widely used as vacuum cleaner housings, household appli-
ance housings, and power tools. These are arenas where PC’s high impact strength, heat
resistance, durability, and high-quality finish justify its expense. It is also used in safety
helmets, riot shields, aircraft canopies, traffic light lens housings, and automotive battery
cases. Design engineers take care not to design with tight radii where PC’s tendency to
stress crack could be a hindrance. PC cannot withstand constant exposure to hot water and
can absorb 0.2 percent of its weight of water at 33°C and 65 percent relative humidity.
This does not impair its mechanical properties but, at levels greater than 0.01 percent, pro-
cessing results in streaks and blistering.

2.2.12 Polyester Thermoplastics
The broad class of organic chemicals called polyesters are characterized by the fact that
they contain an ester linkage,

and may have either aliphatic or aromatic hydrocarbon units. As an introduction, Table 2.4
offers some selected thermal and mechanical properties as a means of comparing polybu-
tylene terephthalate (PBT), polycyclohexylenedimethylene terephthalate (PCT), and
poly(ethylene terephthalate) (PET).
2.2.12.1 Liquid Crystal Polymers (LCPs). Liquid crystal polyesters, known as liquid
crystal polymers, are aromatic copolyesters. The presence of phenyl rings in the backbone
of the polymer gives the chain rigidity, forming a rod-like chain structure. Generally, the
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phenyl rings are arranged in para linkages to give good rod-like structures. This chain
structure orients itself in an ordered fashion, both in the melt and in the solid state, as
shown in Fig. 2.15. The materials are self-reinforcing with high mechanical properties, but

FIGURE 2.15 Melt configurations.

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