Patent ID: 7157519

Claim:
A process for preparing a polyurethane resin having a number-average molar mass Mn of at least 25 kg/mol, a weight-average molar mass Mw of at least 40 kg/mol, and an acid number of from 25 to 50 mg/g, which comprises performing the following steps in succession: synthesizing an isocyanate-functional prepolymer by reacting polyisocyanates A with polyols B having a number-average molar mass Mn of at least 400 g/mol, and compounds D which contain at least two isocyanate-reactive groups and at least one group capable of forming anions, to give a prepolymer which contains free NCO groups and has a Staudinger index J0 of at least 20 cm3/g, at least partly neutralizing the group capable of forming anions in the compound D, to form anionic groups, dispersing this prepolymer ABD in water, and reacting the neutralized prepolymer with compounds F which are monofunctional toward isocyanates, and low molar mass polyols E having at least three hydroxyl groups, which compounds F are selected from the group consisting of mono-secondary amines, amidoamines formed from diprimary amines and monocarboxylic acids, monoketimines, and primary-tertiary amines, and which polyols E are selected from the group consisting of glycerol, hexanetriol, pentaerythritol, dipentaerythritol, diglycerol, trimethylol ethane and trimethylol propane, these compounds being used in such an amount that the number of hydroxyl groups in the amount of component E used exceeds the number of isocyanate groups still present in the prepolymer ABD, wherein the reactants are chosen such that the mass fraction of building blocks derived from A is from 20% to 35%, the mass fraction of building blocks derived from B is from 15% to 60%, the mass fraction of building blocks derived from E is from 2% to 15%, and the mass fraction of building blocks derived from F is from 2% to 20%, each based on the mass of the polyurethane resin.