Process for the production of polyurethanes dissolved in a solvent and also their use for the production of fabricated shapes, especially of microporous structures

Claim 1

A process for making partially cross-linked polyurethanes dissolved in an organic highly polar solvent comprising reacting by continuously adding and mixing at a temperature between 20° C. and 40° C. (1) a linear pre-adduct having a molecular weight of 5000 to 10,000 and having from 1.5 to 5% by weight free isocyanate groups that has been prepared by reacting an excess of an organic diisocyanate with an organic compound having two reactive hydrogen-containing groups dissolved in an organic highly polar solvent, the solids content of the resulting solution being 60 to 80% by weight pre-adduct, into (2) a cross-linking agent selected from the group consisting of hydrazine hydrate and carbodihydrazide in an organic highly polar solvent for the reactants and the resulting partially cross-linked polyurethanes in a concentration of 0.02 to 0.05 mole per cent with continuous measuring of the viscosity of the solution during the addition until the viscosity has reached a range which is indicated by a sudden marked rise in the viscosity to a value from 6,000 to 10,000 cP measured at the reaction conditions and after reaching this range the addition of the linear pre-adduct is terminated, the resulting solids content of the solution of partially cross-linked polyurethanes being between 25 and 35% by weight.