Solvent free polyurethane dispersions for hard surface coatings
The invention relates to polyurethane dispersions which develop high hardness and form coating films at room temperature. The dispersions are prepared from prepolymers containing prepolymers containing a polyol or polyol blend having specified equivalent weight and polyisocyanate to polyol molar ratio.
1. An aqueous polyurethane dispersion containing polyurethane solids and a non-aqueous, organic solvent content of less than 5 weight percent wherein the polyurethane solids are obtained from a polyurethane prepolymer prepared by reacting
(a) a polyol or polyol blend having a mean average equivalent weight of 240 to 480, and
(b) at least one polyisocyanate
(c) optionally additional stabilizers; and
(d) optionally chain extenders
wherein the prepolymer has an isocyanate (NCO) content of from 8 to 13 weight percent and the polyisocyanate to polyol molar ratio is from 2.0:1 to 3.0:1; with the proviso when the equivalent weight of the polyol or polyol blend is less than 260, a chain extender is present.
2. The dispersion of claim 1 wherein the solids content is from 40 to 60 weight percent.
3. The dispersion of claims 1 wherein the polyisocyanate is an aliphatic isocyanate.
4. The dispersion of claim 3 wherein the isocyanate is 1,6-hexamethylene-diisocyanate; isophorone diisocyanate, 1,3-, 1,4-(bisisocyanatomethyl)cyclohexane or a mixture thereof.
5. The dispersion of claim 1 wherein the polyisocyanate is an aromatic isocyanate.
6. The dispersion of claim 5 wherein the isocyanate is 2,4- or 2,-6-toluenediisocyanate, 4,4′, 2,4′ or 2,2′-diphenylmethanediisocyanate or a mixture of such isomers, polyphenyl polymethylene polyisocyanate or a mixture thereof.
7. The dispersion of claim 6 wherein the isocyanate diphenylmethanediisocyanate.
8. The dispersion of claim 1 wherein the prepolymer and dispersion are prepared in the absence of an organic solvent.
9. The dispersion of claim 1 wherein the polyol or polyol blend has an average mean equivalent weight of 240 to 450.
10. This dispersion of claim 9 wherein the polyol or polyol blend has an average mean equivalent weight of 240 to 400.
11. The dispersion of claim 1 wherein (a) is a blend of two or more polyols where at least a first polyol has an equivalent weight of from 50 to 300 and at least one second polyol having an equivalent weight of from 250 to 1500.
12. The dispersion of claim 1 wherein the first and second polyols are diols.
13. The dispersion of claim 12 wherein the first polyol is a polyether polyol.
14. The dispersion of claim 11 wherein the second polyol is a polyether.
15. The dispersion of claim 14 wherein the polyol is a poly(propylene oxide) or a poly(propylene oxide) terminated with 5 to 25 weight percent ethylene oxide.
16. The dispersion of claim 1 wherein the prepolymer is made in the presence of a solvent and the solvent is substantially removed prior to dispersion of the prepolymer in water.
17. The dispersion of claim 1 wherein the dispersion is made in a continuous process.
18. The use of the polyurethane dispersion of claim 1 for a coating on a hard surface.
19. The use of claim 18 wherein the hard surface is wood.
20. The dispersion of claim 1 wherein the polyurethane dispersion forms a film at ambient temperature and produces a coating on a hard surface with a Persoz hardness of at least 95 seconds.
21. An aqueous polyurethane dispersion containing polyurethane solids and a non-aqueous, organic solvent content of less than 5 weight percent wherein the polyurethane solids are obtained from a polyurethane prepolymer prepared by reacting
(a) a polyol or polyol blend having a mean average equivalent weight of 240 to 480, and
(b) at least one polyisocyanate selected from 4,4′, 2,4′ or 2,2′-diphenylmethanediisocyanate or a mixture of such isomers;
(c) optionally additional stabilizers; and
(d) optionally chain extenders
wherein the prepolymer has an isocyanate (NCO) content of from 7 to 13 weight percent and the polyisocyanate to polyol molar ratio is from 2.0:1 to 3.0:1; with the proviso when the equivalent weight of the polyol or polyol blend is less than 260, a chain extender is present.