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preface to fifteenth edition

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10.8 SECTION 10<br />

10.3 FORMULAS AND KEY PROPERTIES OF<br />

PLASTIC MATERIALS<br />

10.3.1 Acetals<br />

10.3.1.1 Homopolymer. Acetal homopolymers are prepared from formaldehyde and consist of<br />

high-molecular-weight linear polymers of formaldehyde.<br />

The good mechanical properties of this homopolymer result from the ability of the oxymethylene<br />

chains <strong>to</strong> pack <strong>to</strong>gether in<strong>to</strong> a highly ordered crystalline configuration as the polymers change from<br />

the molten <strong>to</strong> the solid state.<br />

Key properties include high melt point, strength and rigidity, good frictional properties, and<br />

resistance <strong>to</strong> fatigue. Higher molecular weight increases <strong>to</strong>ughness but reduces melt flow.<br />

10.3.1.2 Copolymer. Acetal copolymers are prepared by copolymerization of 1,3,5-trioxane with<br />

small amounts of a comonomer. Carbon-carbon bonds are distributed randomly in the polymer chain.<br />

These carbon-carbon bonds help <strong>to</strong> stabilize the polymer against thermal, oxidative, and acidic attack.<br />

10.3.2 Acrylics<br />

10.3.2.1 Poly(methyl Methacrylate). The monomer used for poly(methyl methacrylate), 2-hydroxy-2-methylpropanenitrile,<br />

is prepared by the following reaction:<br />

2-Hydroxy-2-methylpropanenitrile is then reacted with methanol (or other alcohol) <strong>to</strong> yield methacrylate<br />

ester. Free-radical polymerization is initiated by peroxide or azo catalysts and produce<br />

poly(methyl methacrylate) resins having the following formula:<br />

Key properties are improved resistance <strong>to</strong> heat, light, and weathering. This polymer is unaffected<br />

by most detergents, cleaning agents, and solutions of inorganic acids, alkalies, and aliphatic hydrocarbons.<br />

Poly(methyl methacrylate) has light transmittance of 92% with a haze of 1 <strong>to</strong> 3% and its<br />

clarity is equal <strong>to</strong> glass.

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