Chemistry Reference
In-Depth Information
The prepolymers are spun into fibers and subsequently cross-linked, or cross-linked while they are
being spun, to obtain resilient elastomeric fibers.
7.10.6 Polyurethane Elastomers
Solid elastomers can be divided into three categories, namely cast, millable, and thermoplastic ones.
The cast elastomers are formed by casting liquid reaction mixtures of low molecular weight
prepolymers into heated molds where they cross-link and convert to high molecular weight materials.
Slightly branched polyesters are combined first with diisocyanates, like toluene diisocyanate, to form
the prepolymers and degassed in a vacuum at elevated temperature (about 70
C). The reaction
mixtures are then poured into molds and heated for several hours at about 110
C to form solid
elastomers. These elastomers are soft and resilient, but they lack good mechanical strength.
Elastomers with better mechanical strength form from linear, hydroxyl-terminated polyesters or
polyethers. These macro glycols are also prereacted first with the diisocyanates, similarly to the
procedure used for expandable fibers. The products, however, are mixed with low molecular weight
glycols or diamines and then heated in molds at 110
C for 24 h. Slightly less than stoichiometric
amounts of glycols or diamines are used so that the polymers are terminated with isocyanate groups.
These terminal isocyanate groups react in the mold with urethane hydrogens to form allophanate
cross-links. Trimerization of the isocyanate groups might also take place, though this usually requires
a catalyst to form at temperatures below 130
C:
O
N
O
O
N
+
N
N
O
N
O
N
O
The concentrations of the allophanate links vary with the time of cure [
129
]. Also, if the cross-
linking reactions are conducted in inert nitrogen atmospheres, very little scission of cross-links takes
place and a network structure forms during the cure. In open air, however, the scissions of cross-links
are extensive [
129
] and the products have poorer physical properties.
A drawback to the cast elastomers is limited shelf life and a need to store them in the absence of
moisture. As a result, millable elastomers were developed. These are produced by first forming hydroxy-
terminated linear polyurethanes through reactions of linear aliphatic polyesters or polyethers with
diisocyanates. The prepolymers are rubbers or gums that can be compounded on rubber mills with
other ingredients and cross-linked. Cross-linking is accomplished by adding either more diisocyanates,
or sulfur, or peroxides. Diisocyanates dimers that dissociate at about 150
C are often used:
O
Δ
NN
2
NO
O
N
N
N
O
O
O
