Method for producing polyurethane emulsion for aqueous one-component coating agent

A method for producing polyurethane emulsion for an aqueous one-component coating agent, comprising:

• reacting an organic diisocyanate (a1), a polyol (a2) having a number average molecular weight of from 500 to 10,000 and a carbonate skeleton and a glycol containing a carboxyl group (a3) to produce an isocyanate-terminated urethane prepolymer having a carboxyl group (A), wherein the glycol containing a carboxyl group (a3) is selected from the group consisting of 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolbutyric acid and 2,2-dimethylolvaleric acid;
• mixing the urethane prepolymer (A) with a polyisocyanate obtained by a method selected from the group consisting of:
• (1) adding an isocyanuration catalyst to an aliphatic isocyanate and/or alicyclic isocyanate to effect an isocyanuration reaction, wherein the unreacted aliphatic isocyanate and/or alicyclic isocyanate is removed;
• and
• (2) reacting an aliphatic isocyanate and/or an alicyclic isocyanate with a low molecular weight polyol used for obtaining the polyester polyol, wherein an isocyanuration catalyst is added thereto to effect an isocyanuration reaction, followed by removing the unreacted aliphatic isocyanate and/or alicyclic isocyanate;
• and wherein, a part of the isocyanate group of the polyisocyanate obtained in the method (1) or (2) is further subjected to a urethanation reaction with a monofunctional or polyfunctional polyol;
• wherein the polyisocyanate contains a nonionic polar group (B);
• neutralizing the carboxyl group in the system with a neutralizing agent (C);
• and
• subjecting the mixture to emulsification in water and chain extension with water or an amine.