Castable polyurethanes form a part of the overall polyurethane industry. They are
normally prepared by the mixing of two to five ingredients and introduced into a mold. The materials are
cured by the application of moderate heat, approximately 100°C, for 6 to 18 hours. The finished parts can be
post-machined to obtain the right size and shape.
Polyurethanes are made by extending chains of a prepolymer made from a macro diol and a
diisocyanate. The prepolymer is further extended with a diol or an amine curative. The long chains form a solid
which is relatively weak. When the part is given a longer heat treatment, the molecules align themselves and
intermolecular bonds (hydrogen bonding) are formed. At this point the full mechanical properties are established
and the material, if suitably formulated, has excellent mechanical and chemical properties.
Castable polyurethanes can be formulated to contain conventional crosslinks, as in rubbers and
epoxies. They may be either in the form of the liquid system or in the form that requires conventional
rubber-processing equipment. The introduction of conventional cross-links will break the formation of the hydrogen
bonds and normally reduce the hardness of the finished product.
In very hard grades, they are used to increase the compression resistance.