Process for the manufacture of polymer solutions

Claim 1

A process for the manufacture of a solution of high molecular essentially linear polyurethane elastomer which comprises reacting together, in a first portion of a liquid which is a solvent for the polyurethane, an organic diisocyanate, an essentially linear hydroxyl-terminated polyester, polyesteramide or polyether and a low molecular weight difunctional isocyanate-reactive compound the amount of said liquid in said first portion being less than sufficient for a solution in that amount of solvent of all of the polyurethane to be formed from the initial reactants to have a viscosity which does not exceed the viscosity desired for the final product, initially carrying out the reaction in said first portion of solvent until formation of polyurethane increases the viscosity to substantially the value desired for the final product and thereafter continuing the reaction while adding the remaining solvent for the polyurethane in a continuous manner and at such a rate that the viscosity of the solution remains substantially constant at the value desired for the final product, whereby the viscosity-reducing effect of the solvent addition counteracts the viscosity-increasing effect of the continuing reaction, and finally terminating the reaction when the desired concentration of polyurethane in the solution has been achieved.