Elastomers
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Polyurethane elastomers are predominately linear polyurethane
polymers which contain hard and and soft segments. To prepare such
an elastomer a diol or a blend of diols are reacted with a
diisocyanate. At a molar ratio of 1/1 NCO/OH. Mixing of the
components can be in one step or by at first preparing an isocyanate
terminated prepolymer and extending this prepolymer with a diol or a
diamine. Often slight excess of isocyanate is used to compensate for
any side reactions. The combination of hard and soft segments in a
polyurethane elastomer leads to phase separation and is responsible
for the elastomeric characteristics of an elastomer. The formation
of hard segments and phase separation is encouraged by the use of short
diols and also by amines. Because of the high reactivity of
aliphatic amines often aromatic amines and also sterically hindered
amines is used to achieve a lower reaction rate. Elastomers are
often prepared by a prepolymer procedure, this permits a closer
control of functionality and molar ratios and it leads to more
controlled phase separation.
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For many applications of isocyanates it is an advantage to prepare a
prepolymer by reacting a polyol with a diisocyanate. The preparation
of prepolymers raises the cost of polyurethane manufacture, but
reduces the risk of handling volatile isocyanates. The resulting
prepolymers still contain free diisocyanates. The amount of free
diisocyanate will depend on the ratio of hydroxyl/isocyanate and the
relative reactivity of the first and second isocyanate on a
diisocyanate. There are also commercially prepared prepolymers
available which have most of the excess of free diisocyanate removed by thin
film evaporation. Although these prepolymers are higher in cost, they
are saver to handle. If polyols and isocyanates with an average
functionality of >2 are reacted the prepolymer can gel.
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Elastomer preparation (laboratory procedure)
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Prepolymer
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The prepolymer depending on viscosity is heated to 140-150°F,
(55-60°C) and degassed under vacuum
(1-5 mm Hg) for 15 minutes.
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The formulation is catalyzed and the chain extender is added and
thoroughly mixed
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The mixture poured into a preheated mold (212°F,
100°C) treated with release agent. After molding the parts are post
cured at 212°F (100°C) for 16
hours. The parts are conditioned for several weeks at room temperature
and 50 % relative humidity.
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