AQUEOUS POLYURETHANE DISPERSION

1 . A process for preparing an aqueous polyurethane dispersion, comprising the steps of:

i) forming an NCO-terminated polyurethane prepolymer from a reaction mixture comprising of:

a) at least one polyol,

b) at least one anionic internal surfactant, wherein the at least one anionic internal surfactant comprises at least two NCO-reactive groups and at least one negatively charged functional group, preferably a carboxyl group,

c) dimethylethanolamine or diethylethanolamine and

d) at least one polyisocyanate, preferably at least one aliphatic, cycloaliphatic or aromatic di- or triisocyanate, wherein the at least one polyisocyanate is used in excess with respect to the molar ratio of the isocyanate groups to the NCO-reactive groups and hydroxyl groups of the other components of the reaction mixture,

ii) dispersing the polyurethane prepolymer into a continuous aqueous phase under application of shear forces, preferably by mechanical stirring, to obtain a prepolymer dispersion; and

iii) reacting the prepolymer dispersion with at least one chain extension agent to obtain an aqueous polyurethane dispersion,

wherein steps i), ii) and iii) are carried out in the absence of an organic solvent.

2 . The process according to claim 1 , wherein the at least one polyol is a polyetherpolyol, preferably a polyalkylene glycol homo- or copolymer, more preferably a polypropylene glycol homo- or copolymer, a polyethylene glycol homo- or copolymer, a polytetramethylene ether glycol homo- or copolymer, a polypropylenglycol/polyethyleneglycol block copolymer, or mixtures thereof.

3 . The process according to claim 1 , wherein the at least one anionic internal surfactant comprises 2,2-bis(hydroxymethyl)propionic acid (DMPA) or 2,2-dihydroxymethyl butyric acid.

4 . The process according to claim 1 , wherein the at least one polyisocyanate is selected from the group consisting of dicyclohexylmethane-4,4′-diisocyanate (H12MDI), isophorone diisocyanate (IPDI), 2,4- or 2,6-toluene diisocyanate (TDI) or hexamethylene diisocyanate (HDI), or mixtures thereof and more preferably isophorone diisocyanate.

5 . The process according to claim 1 , wherein step ii) comprises dispersing the polyurethane prepolymer into a continuous aqueous phase.

6 . The process according to claim 1 , wherein the chain extension agent in step iii) comprises at least two NCO-reactive groups and is preferably selected from the group consisting of ethylene diamine (EDA), hydrazine, water, isophoronediamine, adipic dihydrazide, diethylene triamine, diethanolamine, ethanolamine and N-(2-hydroxyethyl)-ethylene diamine.

7 . The process according to claim 1 , wherein the reaction mixture in step i) further comprises a nonionic internal surfactant, preferably linear mono hydroxyl-functional polyethylene glycol monomethyl ether (M-PEG).

8 . Aqueous polyurethane dispersion (PUD) obtainable according to a process of claim 1 .

9 . The aqueous polyurethane dispersion according to claim 8 , wherein the aqueous polyurethane dispersion is free of organic solvents, more preferably free of organic solvents and amines, and more preferably free of volatile organic compounds (VOC).

10 . The aqueous polyurethane dispersion according to claim 8 , comprising no sulfur-containing compound.

11 . A composition comprising an aqueous polyurethane dispersion according to claim 8 .

12 . The composition according to claim 11 , which is characterized that the composition is a coating, an adhesive, a sealant or a printing ink.

13 . A coating comprising an aqueous polyurethane dispersion according to claim 8 .

14 . An article coated with the coating according to claim 13 .

15 . Use of an aqueous polyurethane dispersion according to claim 7 in the manufacture of a coated article, preferably a coated glass fiber.