AQUEOUS BARRIER COAT BASED ON POLYURETHANE DISPERSIONS

The present invention relates to aqueous polyurethane dispersions wherein the polyurethanes are reaction products of A) at least difunctional polyols having a molecular weight of 500 to 6000, B) at least difunctional low molecular weight alcohols, C) di- and/or trifunctional isocyanates and D) compounds with an acid group and one or two hydroxy- and/or primary or secondary amino groups in an amount sufficient to provide an acid number, based on resin solids, of < 25 mg KOH/g of substance, wherein i) the neutralizing agent is added in an amount sufficient to neutralize 40 to 105% of the acid groups and an amount sufficient to neutralize at least 60% of the acid groups is added prior to the chain extension reaction, ii) at least 1 wt. % of components A) and B), based on the total weight of components A) to D), are tri- or higher-functional compounds and iii) the polyurethane contains 1 to 4 wt. % of the urea groups formed by the reaction of water and isocyanate groups. The present invention also relates to a process for preparing these polyurethane dispersions and to coating compositions containing these polyurethane dispersions.