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preface to fifteenth edition

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POLYMERS, RUBBERS, FATS, OILS, AND WAXES 10.19<br />

The silanols formed above are unstable and under dehydration. On polycondensation, they give<br />

polysiloxanes (or silicones) which are characterized by their three-dimensional branched-chain structure.<br />

Various organic groups introduced within the polysiloxane chain impart certain characteristics<br />

and properties <strong>to</strong> these resins.<br />

Methyl groups impart water repellency, surface hardness, and noncombustibility.<br />

Phenyl groups impart resistance <strong>to</strong> temperature variations, flexibility under heat, resistance <strong>to</strong><br />

abrasion, and compatibility with organic products.<br />

Vinyl groups strengthen the rigidity of the molecular structure by creating easier cross-linkage<br />

of molecules.<br />

Methoxy and alkoxy groups facilitate cross-linking at low temperatures.<br />

Oils and gums are nonhighly branched- or straight-chain polymers whose viscosity increases with<br />

the degree of polycondensation.<br />

10.3.21 Styrenics<br />

10.3.21.1 Polystyrene. Polystyrene has the following formula:<br />

Polystyrene is rigid with excellent dimensional stability, has good chemical resistance <strong>to</strong> aqueous<br />

solutions, and is an extremely clear material.<br />

Impact polystyrene contains polybutadiene added <strong>to</strong> reduce brittleness. The polybutadiene is<br />

usually dispersed as a discrete phase in a continuous polystyrene matrix. Polystyrene can be grafted<br />

on<strong>to</strong> rubber particles, which assures good adhesion between the phases.<br />

10.3.21.2 Acrylonitrile-Butadiene-Styrene (ABS) Copolymers. This basic three-monomer system<br />

can be tailored <strong>to</strong> yield resins with a variety of properties. Acrylonitrile contributes heat resistance,<br />

high strength, and chemical resistance. Butadiene contributes impact strength, <strong>to</strong>ughness, and<br />

retention of low-temperature properties. Styrene contributes gloss, processibility, and rigidity. ABS<br />

polymers are composed of discrete polybutadiene particles grafted with the styrene-acrylonitrile<br />

copolymer; these are dispersed in the continuous matrix of the copolymer.<br />

10.3.21.3 Styrene-Acrylonitrile (SAN) Copolymers. SAN resins are random, amorphous copolymers<br />

whose properties vary with molecular weight and copolymer composition. An increase in<br />

molecular weight or in acrylonitrile content generally enhances the physical properties of the copolymer<br />

but at some loss in ease of processing and with a slight increase in polymer color.<br />

SAN resins are rigid, hard, transparent thermoplastics which process easily and have good dimensional<br />

stability—a combination of properties unique in transparent polymers.

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