Chemistry Reference
In-Depth Information
Unsaturated prepolymers cross-link with peroxides or sulfur. This unsaturation can be present in
the backbone or in the pendant groups. Vulcanization or cross-linking of elastomers with sulfur or
peroxides is discussed in Chap.
8
.
Thermoplastic elastomers exhibit physical properties that are similar to those of cast and millable
elastomers at ambient temperatures. These materials, however, are not cross-linked and flow at
elevated temperatures. They are fabricated like other thermoplastic polymers, are high in molecular
weight, and are hydroxyl-terminated. Such polymers form from linear hydroxyl-terminated polyester
or polyethers that are condensed with diisocyanates and glycols. Strict stoichiometry must be
maintained to achieve high molecular weights.
A structure study was carried out on a model compound of one elastomer [
130
] prepared with 4,4
0
-
diphenylmethane diisocyanate and butanediol hard segments. It was shown that the chains are
probably linked together in stacks through C
ΒΌ
O
H-N hydrogen bonds between the urethane groups.
This bonding stabilizes the overall structure in both directions, perpendicular to the chain axis. Such
an arrangement of the molecules was also proposed earlier [
131
].
7.10.7 Polyurethane Foams
These foams are chemically very similar to other polyurethane materials, except that gas evolutions
during the reactions take place simultaneously with chain lengthening and cross-linking. This results
in formation of cellular structures. The degree of cross-linking determines to a great extent the
rigidity of the foam. In addition, linear or only slightly branched polymers produce flexible foams,
while more highly branched polymers form rigid ones. The foaming is caused by liberation of CO
2
from reactions between added water and isocyanate groups:
H
N
O
H
N
O
+
H
2
O
N
+
CO
2
OH
H
H
H
H
H
N
O
N
N
N
+
2
+
NO
H
2
O
CO
2
O
O
O
In addition, in many industrial practices, additional carbon dioxide or Freon gas (a major source of
environmental pollution) may be introduced into the system as it cures. For rigid foams, a low boiling
liquid may be added to form additional bubbles. Appropriate catalysts and foam stabilizers or
surfactants are added to control foam formation, cell size, and cure. The catalysts are either tin
compounds or tertiary amines. The surfactants that are necessary to control the cell size are usually
based on siloxanes.
7.11 Epoxy Resins
These resins comprise a general class of cross-linkable, low molecular weight materials with epirane
rings as the main functional group [
134
,
135
]. It does not include polyethers formed through ring
opening polymerizations of ethylene and propylene oxides.
