| abstract |
The present invention relates to an aqueous coating composition comprising dispersed polyurethane-vinyl polymer hybrid particles wherein (i) the polyurethane-vinyl polymer hybrid is obtained by free-radical polymerization of at least one vinyl monomer in the presence of a polyurethane, whereby the polyurethane is obtained by the reaction of at least (I) an isocyanate-terminated polyurethane prepolymer and (II) at least one active-hydrogen containing chain extending compound selected from the group consisting of hydrazine, hydrazine derivative and any mixture thereof, wherein the isocyanate-terminated polyurethane prepolymer is obtained by the reaction of at least one polyol with at least one polyisocyanate, whereby at least 30 wt.% of the polyisocyanate is aromatic polyisocyanate, (ii) the polyurethane-vinyl polymer hybrid is ketone functional and hydrazide (- C=ONHNH2) functional whereby the molar ratio of hydrazide groups to ketone groups in the polyurethane-vinyl polymer hybrid is from 0.05:1 to 1.5:1, (iii) the weight ratio of the polyurethane to the vinyl polymer in the polyurethane- vinyl polymer hybrid ranges from 90:10 to 35:65, and (iv) the amount of free hydrazine in the aqueous coating composition is at most 50 ppm. |