Page 132 - Handbook of Plastics Technologies

THERMOSETS

3.2 CHAPTER 3

versatile, finding use in rigid plastics, flexible plastics, elastomers, rigid foams, flexible
foams, fibers, coatings, and adhesives. They offer unique qualities in processability,
strength, abrasion resistance, energy absorption, adhesion, recyclability, and resistance to
oxygen, ozone, gasoline, and motor oil. Thus, they find major use in appliances, autos,
building, furniture, industrial equipment, packaging, textiles, and many other fields.
Their versatility comes from the range of liquid monomers and oligomers that can be
mixed, poured, polymerized, and cured in a minute or so at room temperature. Thus, we
start with a look at their basic chemistry.
3.1.1.1 Polyurethane Chemistry (Figure 3.1)

FIGURE 3.1 Polyurethane chemistry.

Isocyanates and alcohols react readily to form urethanes. When the alcohols and isocyan-
ates are multifunctional,
Polyols R(OH) n
Polyisocyanates R(NCO) n

they form polyurethane polymers. If they are difunctional, they form linear thermoplastic
polyurethanes, which are useful in spandex fibers and thermoplastic elastomers. More of-
ten, they have higher functionality and form cross-linked thermoset polyurethanes. Most
often, the polyols are trifunctional or higher, typically 3-6 OH groups. Less often, the poly-
isocyanates may be trifunctional or higher, typically 3-7 NCO groups. The liquid mono-
mers are easy to mix, and the polymerization/cure reactions take a few minutes or less at
room temperature. The combination of polarity, hydrogen bonding, and cross-linking in
thermoset polyurethanes gives them high strength, adhesion, and chemical resistance.
Isocyanates react even more readily with amines to form ureas. So when the amines
and isocyanates are multifunctional,

Polyamines R(NH )
2 n
Polyisocyanates R(NCO) n

they form polyurea polymers. The urea groups give even stronger hydrogen bonding than
the urethane groups, so they make the polymers even stronger. Many polyurethane proces-
sors use polyamines to speed the polymerization/cure reactions and to build greater
strength into the finished polymer. Thus, many “polyurethanes” are actually urethane/urea
copolymers, even though the manufacturers rarely mention the fact.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.