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.

Prepolymer Preparation see procedure 

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.  

Elastomer preparation (laboratory procedure)

Prepolymer The prepolymer depending on viscosity is heated to 140-150F, (55-60C) and degassed under vacuum (1-5 mm Hg) for 15 minutes. 
The formulation is catalyzed and the chain extender is added and thoroughly mixed
The mixture poured into a preheated mold (212F, 100C) treated with release agent. After molding the parts are post cured at 212F (100C) for 16 hours. The parts are conditioned for several weeks at room temperature and 50 % relative humidity.

Last edited on:

November 14, 2006

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