Page 109 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.49

TABLE 2.8 Properties of Commercial Grades of General-Purpose PS *
High-heat-
Property Easy-flow PS Medium-flow PS resistance PS
M w 218,000 225,000 300,000
M n 74,000 92,000 130,000
Melt flow index, g/10 min 16 7.5 1.6
Vicat softening temperature, °C 88 102 108
Tensile modulus, MPa 3,100 2,450 3,340
Ultimate tensile strength, MPa 1.6 2.0 2.4

* Source: Encyclopedia of Polymer Science, 2nd ed., vol. 6, Mark, Bikales, Overberger, Menges,and Kroschwitz,
Eds., Wiley Interscience, 1986, p. 65.
least level of additives such as extrusion aids. These products are used in sheet extrusion
and thermoforming, and extruded film applications for oriented food packaging. 331
2.2.26.5 Styrene-Acrylonitrile Copolymers (SAN). Styrene-acrylonitrile polymers are
copolymers prepared from styrene and acrylonitrile monomers. The polymerization can be
done under emulsion, bulk, or suspension conditions. 332 The polymers generally contain
between 20 to 30 percent acrylonitrile. 333 The acrylonitrile content of the polymer influ-
ences the final properties with tensile strength, elongation, and heat distortion temperature
increasing as the amount of acrylonitrile in the copolymer increases.
SAN copolymers are linear, amorphous materials with improved heat resistance over
pure polystyrene. 334 The polymer is transparent but may have a yellow color as the acry-
lonitrile content increases. The addition of a polar monomer, acrylonitrile, to the backbone
gives these polymers better resistance to oils, greases, and hydrocarbons when compared
to polystyrene. 335 Glass-reinforced grades of SAN are available for applications requiring
higher modulus combined with lower mold shrinkage and lower coefficient of thermal ex-
pansion. 336
As the polymer is polar, it should be dried before processing. It can be processed by in-
jection molding into a variety of parts. SAN can also be processed by blow molding, ex-
337
trusion, casting, and thermoforming.
SAN competes with polystyrene, cellulose acetate, and polymethyl methacrylate. Ap-
plications for SAN include injection-molded parts for medical devices, PVC tubing con-
338
nectors, dishwasher-safe products, and refrigerator shelving. Other applications include
packaging for the pharmaceutical and cosmetics markets, automotive equipment, and in-
dustrial uses.
2.2.26.6 Olefin-Modified SAN. SAN can be modified with olefins, resulting in a poly-
mer that can be extruded and injection molded. The polymer has good weatherability and
is often used as a capstock to provide weatherability to less expensive parts such as swim-
339
ming pools, spas, and boats.
2.2.26.7 Styrene-Butadiene Copolymers. Styrene-butadiene polymers are block copol-
ymers prepared from styrene and butadiene monomers. The polymerization is performed
340
using sequential anionic polymerization. The copolymers are better known as thermo-
plastic elastomers, but copolymers with high styrene contents can be treated as thermo-

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