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preface to fifteenth edition

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10.18 SECTION 10<br />

10.3.19 Polyurethane<br />

10.3.19.1 Foams. Polyurethane foams are prepared by the polymerization of polyols with isocyanates.<br />

Commonly used isocyanates are <strong>to</strong>luene diisocyanate, methylene diphenyl isocyanate, and polymeric<br />

isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former<br />

[poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free <strong>to</strong><br />

react with the isocyanate. Most flexible foam is made from 80/20 <strong>to</strong>luene diisocyanate (which refers<br />

<strong>to</strong> the ratio of 2,4-<strong>to</strong>luene diisocyanate <strong>to</strong> 2,6-<strong>to</strong>luene diisocyanate). High-resilience foam contains<br />

about 80% 80/20 <strong>to</strong>luene diisocyanate and 20% poly(methylene diphenyl isocyanate), while semiflexible<br />

foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts<br />

by trimerization <strong>to</strong> form isocyanurate rings.<br />

Flexible foams are used in mattresses, cushions, and safety applications. Rigid and semiflexible<br />

foams are used in structural applications and <strong>to</strong> encapsulate sensitive components <strong>to</strong> protect them<br />

against shock, vibration, and moisture. Foam coatings are <strong>to</strong>ugh, hard, flexible, and chemically<br />

resistant.<br />

10.3.19.2 Elas<strong>to</strong>meric Fiber. Elas<strong>to</strong>meric fibers are prepared by the polymerization of polymeric<br />

polyols with diisocyanates.<br />

The structure of elas<strong>to</strong>meric fibers is similar <strong>to</strong> that illustrated for polyurethane foams.<br />

10.3.20 Silicones<br />

Silicones are formed in the following multistage reaction:

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