Aqueous polyurethane coating compositions

1. An aqueous coating composition comprising a physical blend of a crosslinkable water-dispersible polyurethane oligomer(s) and 10 to 56 wt % of a dispersed vinyl polymer, 0 to 25% of co-solvent by weight of the composition, said polyurethane oligomer(s) having:

a) a measured weight average molecular weight in the range of from 1,500 to 50,000 Daltons; and

b) a solution viscosity ≦150 Pa·s, as determined from an 80% by weight solids solution of the crosslinkable polyurethane oligomer(s) in at least one of the solvents selected from the group consisting of N-methylpyrrolidone, n-butylglycol and mixtures thereof, using a shear rate of 90±5 s −1 and at 50±2° C.; and

said dispersed vinyl polymer(s) having:

a) a measured weight average molecular weight ≧90,000 Daltons; and

b) an acid value up to 100 mgKOH/g;

said composition when drying to form a coating having the following properties:

i) an open time of at least 20 minutes;

ii) a wet edge time of at least 10 minutes;

iii) a tack-free time of ≦20 hours;

iv) an equilibrium viscosity of ≦5,000 Pa·s, at any solids content when drying in the range of from 20 to 55% by weight of the composition, using any shear rate in the range of from 9±0.5 to 90±5 s −1 and at 23+2° C.; and

wherein the crosslinkable water-dispersible polyurethane oligomer(s) is crosslinkable by autooxidation optionally in combination with silane or Schiff base condensation.

2. An aqueous coating composition according to claim 1 wherein said polyurethane oligomer(s) has a solution viscosity ≦250 Pa·s, as determined from a 70% by weight solids solution of the crosslinkable polyurethane oligomer(s) in a solvent mixture consisting of:

i) at least one of the solvents selected from the group consisting of N-methylpyrrolidone, n-butylglycol and mixtures thereof;

ii) water and

iii) N,N-dimethylethanolamine;

where i), ii) and iii) are in weight ratios of 20/7/3 respectively, using a shear rate of 90±5 s −1 and at 23±2° C.

3. An aqueous coating composition comprising a physical blend of a crosslinkable water-dispersible polyurethane oligomer(s) which is crosslinkable by autooxidation and 10 to 56 wt % of a dispersed polymer(s) and 0 to 25% of co-solvent by weight of the composition, said crosslinkable water-dispersible polyurethane oligomer(s) having:

a) a measured weight average molecular weight in the range of from 1,500 to 50,000 Daltons; and

b) a solution viscosity ≦150 Pa·s, as determined from an 80% by weight solids solution of the crosslinkable water-dispersible polyurethane oligomer(s) in at least one of the solvents selected from the group consisting of N-methylpyrrolidone, n-butylglycol and mixtures thereof, using a shear rate of 90±5 s −1 and at 50±2° C.;

and said dispersed polymer(s) having:

a) a measured weight average molecular weight ≧90,000 Daltons; and

b) an acid value up to 100 mgKOH/g;

wherein said composition when drying to form a coating has the following properties:

i) an open time of at least 20 minutes;

ii) a wet edge time of at least 10 minutes;

iii) a tack-free time of ≦20 hours; and

iv) an equilibrium viscosity of ≦5,000 Pa·s, at any solids content when drying in the range of from 20 to 55% by weight of the composition, using any shear rate in the range of from 9±0.5 to 90±5 s −1 and at 23±2° C.

4. An aqueous composition according to claim 3 wherein the crosslinkable water-dispersible polyurethane oligomer(s) contains autooxidisable groups and carbonyl functional groups.

5. An aqueous coating composition comprising a physical blend of a crosslinkable water-dispersible polyurethane oligomer(s) which is crosslinkable by autooxidation and a dispersed polymer(s) in a ratio by weight of 90:10 to 10:90, and 0 to 25% of co-solvent by weight of the composition, said polyurethane oligomer(s):

a) having a measured weight average molecular weight in the range of from 1,500 to 50,000 Daltons;

b) having a solution viscosity ≦150 Pa·s, as determined from an 80% by weight solids solution of the crosslinkable polyurethane oligomer(s) in at least one of the solvents selected from the group consisting of N-methylpyrrolidone, n-butylglycol and mixtures thereof, using a shear rate of 90±5 s −1 and at 50±2° C.; and

c) being obtained by capping an isocyanate-terminated polyurethane oligomer with a monofunctional isocyanate-reactive compound and/or by using a stoichlometric excess of reactants having isocyanate-reactive groups during the oligomer preparation;

said dispersed polymer(s) having:

a) a measured weight average molecular weight ≧90,000 Daltons; and

b) an acid value up to 100 mgKOH/g;

wherein said composition when drying to form a coating has the following properties:

i) an open time of at least 20 minutes;

ii) a wet edge time of at least 10 minutes;

iii) a tack-free time of ≦20 hours;

iv) an equilibrium viscosity of ≦5,000 Pa·s, at any solids content when drying in the range of from 20 to 55% by weight of the composition, using any shear rate in the range of from 9±0.5 to 90±5 s −1 and at 23±2° C.