Patent ID: 7691941

Claim:
Process for preparing dispersions of blocked polyurethane prepolymers, comprising I) preparing a blocked polyurethane prepolymer by reacting (a) 100 equivalent % of at least one polyisocyanate having a functionality of >2, with b) 50 to 90 equivalent %, based on the isocyanate-reactive groups, of a thermally eliminable blocking agent, c) 5 to 45 equivalent %, based on the isocyanate-reactive groups, of a monohydroxycarboxylic acid as hydrophilicizing agent d) 0 to 25 equivalent %, based on the isocyanate-reactive groups, of a polyhydroxycarboxylic acid as hydrophilicizing agent and (e) greater than 0 to 15 equivalent %, based on the isocyanate-reactive groups, of a chain extender component which is at least, based on isocyanate-reactive groups, trifunctional or higher and has a molecular weight of 32 to 350 g/mol, wherein i) step I) takes place using (partly) water-miscible organic solvents which are inert towards NCO groups and have boiling points below 120° C. (at 1013 mbar), and ii) components a)-e) and the organic solvents are used in an amount such that the polyurethane prepolymer in the reaction solution is present, following complete reaction, in an amount of 70% to 98% by weight, II) deprotonating, at least partially, the carboxylic acid groups with a base before, during or after step III), III) dispersing the polyurethane prepolymer solution obtained from step (I) in water, and IV) distilling the dispersion to completely remove the organic solvent, the distillation occurring during or after step III).