Page 107 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.47

ductile blend, as in the case of rubber-modified high-impact grades of PS (HIPS), a far
wider range of applications becomes available. HIPS is preferred for durable, molded
items including radio, television, and stereo cabinets as well as compact disc jewel cases.
Copolymerization is also used to produce engineering-grade plastics of higher perfor-
mance as well as higher price, with acrylonitrile-butadiene-styrene (ABS) and styrene-
acrylonitrile (SAN) plastics being of greatest industrial importance.
2.2.26.1 Acrylonitrile Butadiene Styrene (ABS) Terpolymer. As with any copolymers,
there is tremendous flexibility in tailoring the properties of ABS by varying the ratios of
the three monomers: acrylonitrile, butadiene, and styrene. The acrylonitrile component
contributes heat resistance, strength, and chemical resistance. The elastomeric contribu-
tion of butadiene imparts higher impact strength, toughness, low-temperature property re-
tention, and flexibility, while the styrene contributes rigidity, glossy finish, and ease of
processability. As such, worldwide usage of ABS is surpassed only by that of the “big
four” commodity thermoplastics (polyethylene, polypropylene, polystyrene, and polyvi-
nyl chloride). Primary drawbacks to ABS include opacity, poor weather resistance, and
poor flame resistance. Flame retardance can be improved by the addition of fire-retardant
additives or by blending ABS with PVC, with some reduction in ease of processability. 325
As its use is widely prevalent as equipment housings (such as telephones, televisions, and
computers), these disadvantages are tolerated. Figure 2.37 shows the repeat structure of
ABS.

FIGURE 2.37 Repeat structure of ABS.

Most common methods of manufacturing ABS include graft polymerization of styrene
and acrylonitrile onto a polybutadiene latex, blending with a styrene-acrylonitrile latex,
and then coagulating and drying the resultant blend. Alternatively, the graft polymer of
styrene, acrylonitrile, and polybutadiene can be manufactured separately from the styrene
acrylonitrile latex and the two grafts blended and granulated after drying. 326
Its ease of processing by a variety of common methods (including injection molding,
extrusion, thermoforming, compression molding, and blow molding), combined with a
good economic value for the mechanical properties achieved, results in widespread use of
ABS. It is commonly found in under-the-hood automotive applications, refrigerator lin-
ings, radios, computer housings, telephones, business machine housings, and television
housings.
2.2.26.2 Acrylonitrile-Chlorinated Polyethylene-Styrene (ACS) Terpolymer. While ABS
itself can be readily tailored by modifying the ratios of the three monomers and by modi-
fying the lengths of each grafted segment, several companies are pursuing the addition of a
fourth monomer, such as alpha-methylstyrene for enhanced heat resistance and methyl-

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.