Page 108 - Handbook of Plastics Technologies

THERMOPLASTICS

2.48 CHAPTER 2

methacrylate to produce a transparent ABS. One such modification involves using chlori-
nated polyethylene in place of the butadiene segments. This terpolymer, ACS, has very
similar properties to the engineering terpolymer ABS, but the addition of chlorinated poly-
ethylene imparts improved flame retardance, weatherability, and resistance to electrostatic
deposition of dust without the addition of antistatic agents. The addition of the chlorinated
olefin requires more care when injection molding to ensure that the chlorine does not de-
hydrohalogenate. Mold temperatures are recommended to be kept at between 190 and
210°C and not to exceed 220°C. As with other chlorinated polymers, such as polyvinyl
chloride, residence times should be kept relatively short in the molding machine. 327
Applications for ACS include housings and parts for office machines such as desktop
calculators, copying machines, and electronic cash registers, as well as housings for televi-
sion sets and video cassette recorders. 328

2.2.26.3 Acrylic Styrene Acrylonitrile (ASA) Terpolymer. Like ACS, ASA is a spe-
cialty product with similar mechanical properties to ABS, but which offers improved out-
door weathering properties. This is due to the grafting of an acrylic ester elastomer onto
the styrene-acrylonitrile backbone. Sunlight usually combines with atmospheric oxygen to
result in embrittlement and yellowing of thermoplastics, and this process takes a much
longer time in the case of ASA. Therefore, ASA finds applications in gutters, drain pipe
fittings, signs, mail boxes, shutters, window trims, and outdoor furniture. 329
2.2.26.4 General-Purpose Polystyrene (PS). PS is one of the four plastics whose com-
bined usage accounts for 75 percent of the worldwide usage of plastics. 330 These four
commodity thermoplastics are PE, PP, PVC, and PS. Although it can be polymerized via
free-radical, anionic, cationic, and Ziegler mechanisms, commercially available PS is pro-
duced via free-radical addition polymerization. PS’s popularity is due to its transparency,
low density, relatively high modulus, excellent electrical properties, low cost, and ease of
processing. The steric hindrance caused by the presence of the bulky benzene side groups
results in brittle mechanical properties, with ultimate elongations only around 2 to 3 per-
cent, depending on molecular weight and additive levels. Most commercially available PS
grades are atatic, and in combination with the large benzene groups, result in an amor-
phous polymer. The amorphous morphology provides not only transparency but, in addi-
tion, the lack of crystalline regions means that there is no clearly defined temperature at
which the plastic melts. PS is a glassy solid until its T of ~100°C is reached, whereupon
g
further heating softens the plastic gradually from a glass to a liquid. Advantage is taken of
this gradual transition by molders who can eject parts that have cooled to beneath the rela-
tively high Vicat temperature. Also, the lack of a heat of crystallization means that high
heating and cooling rates can be achieved, which reduces cycle time and also promotes an
economical process. Lastly, upon cooling, PS does not crystallize the way PE and PP do.
This gives PS low shrinkage values (0.004 to 0.005 mm/mm) and high dimensional stabil-
ity during molding and forming operations.
Commercial PS is segmented into easy-flow, medium-flow, and high-heat-resistance
grades. Comparison of these three grades is made in Table 2.8. The easy-flow grades are
the lowest molecular weight, to which 3 to 4 percent mineral oil has been added. The min-
eral oil reduces melt viscosity, which is well suited for increased injection speeds while
molding inexpensive thin-walled parts such as disposable dinnerware, toys, and packag-
ing. The reduction in processing time comes at the cost of a reduced softening temperature
and a more brittle polymer. The medium-flow grades are of slightly higher molecular
weight and contain only 1 to 2 percent mineral oil. Applications include injection molded
tumblers, medical ware, toys, injection-blow-molded bottles, and extruded food packag-
ing. The high-heat-resistance plastics are of the highest molecular weight and have the

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