A polyurethane which has a number average molecular weight M.sub.n of from
3,000 to 50,000 and an acid number of from 10 to 35 and is preparable by
reacting
(a1) at least one polyesterpolyol having a number average molecular weight
M.sub.n of from 1,000 to 4,000, an acid number of from 0 to 15, and an OH
number of from 35 to 150, based on acyclic aliphatic and cycloaliphatic
dicarboxylic acids,
(a2) a mixture of at least one diol and one triol,
(a3) at least one compound containing at least two isocyanate-reactive
functional groups and at least one functional group capable of forming
anions, and
(a4) a mixture of at least one, acyclic aliphatic and at least one
cycloaliphatic diisocyanate,
to give an isocyanato-containing prepolymer that is then chain extended
with a polyfunctional amine or amino alcohol and, if desired, is
neutralized.
