Process for making polyurethane prepolymers

摘要

Polyurethane prepolymer which can be converted into a final product by reaction with a curative or chain extending agent is made by reacting polyisocyanate with polyol in the presence of an amount of an aliphatic hydrocarbon which is a solvent for the polyisocyanate but a non-solvent for the prepolymer. In order to minimize the formation of unwanted oligomers the polyisocyanate is present in stoichiometric excess with respect to the polyol and the amount of hydrocarbon solvent present in the reaction mixture is limited so that a single liquid phase is present in the mixture during most of the synthesis. Solvent containing dissolved isocyanate is then removed from the prepolymer by liquid phase separation. A portion of the solvent can be distilled from this isocyanate solution for use in liquid-liquid extraction of residual isocyanate from the prepolymer, and the remaining solution of isocyanate in hydrocarbon solvent can be used in prepolymer synthesis. A preferred method of liquid-liquid extraction of unreacted polyisocyanate from prepolymer is by counter-current flow of the prepolymer and hydrocarbon solvent through a vertically elongated contact column. This technique can be used independently or in combination with the use of hydrocarbon solvent in the prepolymer synthesis.