WÄSSRIGE DISPERSION VON POLYURETHANHARZEN, VERFAHREN ZU DEREN HERSTELLUNG, DIESE ENTHALTENDE ÜBERZUGSMITTEL UND DEREN VERWENDUNG

An aqueous dispersion, suitable for coating compositions, in particular for multicoat finishes, of polyurethane resins containing ionic groups, but no urea groups, and having a number average molecular weight (Mn) of from 10000 to 300000 and an acid number of from 5 to 50, and the process for the preparation by preparing an OH-containing polyurethane prepolymer having an OH number of from 5 to 75 and a number average molecular weight (Mn) of from 5000 to 50000, by reacting, in organic medium, A) one or more isocyanate-reactive compounds having an acid number of from 0 to 10 in the form of a) from 70 to 100% by weight of one or more polyesterdiols and/or polyetherdiols having a molecular weight of from 500 to 6000, together with b) from 30 to 0% by weight of one or more compounds, other than those in a), containing at least two isocyanate-reactive groups and having a molecular weight of from 60 to 400, with B) one or more products, containing free isocyanate groups, of the reaction of c) at least one compound containing two isocyanate-reactive groups and at least one anionic group or group which is capable of forming anions, and d) one or more organic diisocyanates, in such a mixing ratio that the molar ratio between the isocyanate-reactive groups in c) and the isocyanate groups in d) is from 1 : 1.0 to 1 : 4.0, where the reaction product B) can be prepared in the presence of component A), and where component B) is employed in such an amount that the finished polyurethane resins have the desired acid number, and with C) one or more polyisocyanates containing at least two free isocyanate groups per molecule, in such an amount that the molar ratio between the OH groups of component A) and the NCO groups of components B) and C) is from 1.01 : 1 to 3 : 1 the prepolymer obtained from A), B) and C) is then subjected to chain extension, before or after neutralisation of all or some of any groups present which can be converted into ionic groups in the organic medium or after transfer into the water phase, by reaction with D) one or more polyfunctional isocyanates having a functionality of free NCO groups of at least 1.8 in such amounts that the finished polyurethane resins have the desired number average molecular weight.