Page 105 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.45

formed packaging. Fibers are another important application for polypropylene, particu-
larly in carpeting, because of its low cost and wear resistance. Fibers prepared from
polypropylene are used in both woven and nonwoven fabrics.

2.2.25 Polyurethane (PUR)
Polyurethanes are very versatile polymers. They are used as flexible and rigid foams, elas-
tomers, and coatings. Polyurethanes are available as both thermosets and thermoplastics.
In addition, their hardnesses span the range from rigid material to elastomer. Thermoplas-
tic polyurethanes will be the focus of this section. The term polyurethane is used to cover
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materials formed from the reaction of isocyanates and polyols. The general reaction for
a polyurethane produced through the reaction of a diisocyanate with a diol is shown in
Fig. 2.35.

FIGURE 2.35 Polyurethane reaction.

Polyurethanes are phase separated block copoly-
mers as depicted in Fig. 2.36, where the A and B por-
FIGURE 2.36 Block structure of tions represent different polymer segments. One
polyurethanes. segment, called the hard segment, is rigid, while the
other, the soft segment, is elastomeric. In polyure-
thanes, the soft segment is prepared from an elastomeric long-chain polyol, generally a
polyester or polyether, but other rubbery polymers end-capped with a hydroxyl group
could be used. The hard segment is composed of the diisocyanate and a short-chain diol
called a chain extender. The hard segments have high interchain attraction due to hydrogen
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bonding between the urethane groups. In addition, they may be capable of crystallizing.
The soft elastomeric segments are held together by the hard phases, which are rigid at
room temperature and act as physical cross-links. The hard segments hold the material to-
gether at room temperature but, at processing temperatures, the hard segments can flow
and be processed.
The properties of polyurethanes can be varied by changing the type or amount of the
three basic building blocks of the polyurethane: diisocyanate, short-chain diol, or long-
chain diol. Given the same starting materials, the polymer can be varied simply by chang-
ing the ratio of the hard and soft segments. This allows the manufacturer a great deal of
flexibility in compound development for specific applications. The materials are typically
manufactured by reacting a linear polyol with an excess of diisocyanate. The polyol is
end-capped with isocyanate groups. The end-capped polyol and free isocyanate are then
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reacted with a chain extender, usually a short--chain diol to form the polyurethane.

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