Aqueous dispersion of polyurethane resins, a method of manufacturing them, coating agents containing them and use thereof

A method for applying an aqueous coating agent to a substrate and curing it at temperatures of up to 150.degree. C., wherein the aqueous coating agent is free of cross-linking agents and comprises a binder consisting of:

• 10-70 wt % of a polyurethane dispersion, and
• 90-30 wt % of a water-dilutable binder, and
• wherein said polyurethane dispersion is an aqueous dispersion of polyurethane resins free from urea groups and containing ionic groups and with a number average molecular weight (Mn) of 10,000 to 300,000 and an acid number of 5 to 50, obtained by preparing a polyurethane prepolymer containing OH groups and with an OH number of 5 to 75 and a number average molecular weight (Mn) of 5,000 to 50,000, by reacting the following in an organic medium:
• A) One or more compounds which react with isocyanate and have an acid number of 0 to 10, in the form of
• a) 70 to 100 wt. % of one or more polyester diols and/or polyether diols with a molecular weight of 500 to 6,000, together with
• b) 30 to 0 wt. % of one or more compounds different from a) and with at least two groups which react with isocyanate and with a molecular weight of 60 to 400, with
• B) One or more substances containing free isocyanate groups and obtained by a reaction between
• c) At least one compound with two groups which react with isocyanate and with at least one anionic or anion-forming group and
• d) One or more organic diisocyanates in a proportion such that the molar ration of the groups reacting with isocyanate in c) to the isocyanate groups in d) is 1:1.5 to 1:2.5 the component B) being used in a proportion such that the final polyurethane resins have the desired acid number, and where component B) could be produced in the presence of component A),
• and with
• C) One or more polyisocyanates with at least two free isocyanate groups per molecule, in a proportion such that the molar ratio of the OH groups in component A) to the NCO groups in components B) and C) is 1.01:1 to 3:1,
• after which the prepolymer obtained from A), B), and C), before or after partial or complete neutralization of any groups present which are convertible into ionic groups, is subjected to a chain-lengthening process in the organic medium, by reaction with
• D) One or more polyfunctional isocyanates with a functionality of free NCO groups of at least 1.8, in proportions such that the final polyurethane resin has the desired number average molecular weight, and the resulting resin is converted in to the aqueous phase.