Polyurethane coating composition

1. A two-component polyurethane coating composition comprising:

at least one multifunctional isocyanate component having a plurality of reactive isocyanate groups, wherein the isocyanate index is greater than about 1.5;

at least one multifunctional polyester polyol component having a plurality of reactive hydroxyl groups, wherein the polyester polyol has Tg of about 25° C. to −70° C.; and

at least one catalyst capable of catalyzing formation of allophanate or biuret by reaction of the isocyanate component with urethane or urea present on the surface of a coated article.

2. The composition of claim 1 , wherein the at least one isocyanate component has isocyanate index of about 1.5 to 5.0.

3. The composition of claim 1 , wherein the at least one isocyanate component has isocyanate index of about 2.0 to 4.0.

4. The composition of claim 1 , wherein the at least one polyester polyol component has a Tg of about 0° C. to −70° C.

5. The composition of claim 1 , wherein the at least one polyester polyol component with Tg of about −10° C. to −70° C.

6. The composition of claim 1 , wherein the at least one polyester polyol component has hydroxyl equivalent weight of about 300 g/mol.

7. The composition of claim 1 , wherein the at least one polyester polyol component has a functionality of at least 2.

8. The composition of claim 1 , wherein the at least one isocyanate component is selected from 2,4-toluenediisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, polymethylene-polyphenyl diisocyanate, methylenediphenyldiisocyanate, cyclic trimers, cocyclic trimers, or mixtures thereof.

9. The composition of claim 1 , wherein the at least one polyester polyol is the reaction product of ε-caprolactone with one or more hydroxyl-functional compounds.

10. The composition of claim 9 , wherein the one or more hydroxyl-functional compounds are selected from glycerol, trimethylolpropane, neopentyl glycol, diethylene glycol, pentaerythritol, caprolactone-derived triols, and combinations thereof.

11. The composition of claim 1 , wherein the at least one catalyst is selected from metal salts, metal carboxylates, alkyl-substituted metal carboxylates, and combinations thereof.

12. The composition of claim 1 , wherein the at least one catalyst is selected from amine catalysts, zinc salts, cobalt salts, lead salts, tin salts, zinc alkoxylate, cobalt alkoxylate, tin alkoxylate, zinc acetylacetonate, cobalt acetylacetonate, lead acetylacetonate, tin acetylacetonate, zinc naphthenate, cobalt naphthenate, lead naphthenate, tin naphthenate, butyl stannoic acid, dibutyl tin dilaurate, and combinations thereof.

13. The composition of claim 1 , wherein the composition includes less than 250 g/L volatile organic compounds (VOC).

14. The composition of claim 1 , wherein a cured coating derived from the composition has cross-hatch adhesion grade of at least 4B as measured by ASTM D3359-95.

15. A multilayer coating system, comprising:

a first coating wherein the first coating is an aged polyurethane, epoxy, alkyd, or powder coating;

a second coating wherein the second coating is a polyurethane topcoat; and

a tie coat disposed between the first and second coating, the tie coat comprising a coating composition including:

at least one multifunctional isocyanate component having reactive isocyanate groups wherein the isocyanate index is 2.0 to 4.0;

at least one multifunctional polyester polyol component having reactive hydroxyl groups wherein the polyester polyol has Tg of −10° C. to −70° C.; and

at least one catalyst, wherein the tie coat has a dry film thickness of about 2.5 to 25 microns (0.1 to 1.0 mils).

16. A method of forming a two-component polyurethane tie coat composition, comprising:

providing at least one multifunctional isocyanate component having reactive isocyanate groups wherein the isocyanate index is 2.0 to 4.0;

providing at least one multifunctional polyester polyol component having reactive hydroxyl groups wherein the polyester polyol has Tg of −10° C. to −70° C.; and

reacting the isocyanate and polyol components in the presence of a catalyst to form the polyurethane tie coat composition.

17. A method of improving adhesion between a first coating and a second coating applied to a substrate, comprising:

applying the first coating to the substrate;

applying a tie coat composition over the first coating wherein the tie coating comprises at least one multifunctional isocyanate component having reactive isocyanate groups with an isocyanate index of 2.0 to 4.0 and at least one multifunctional polyester polyol component having reactive hydroxyl groups wherein the polyester polyol has Tg of −10° C. to −70° C.;

curing the tie coat composition; and

applying the second coating over the tie coat,

wherein the adhesion between the first and second coating is improved relative to adhesion in the absence of the tie coat.

18. The system of claim 17 , wherein the first coating is an aged coating.

19. The system or method of claim 17 , wherein the first coating is an aged polyurethane, epoxy, alkyd, or powder coating.

20. The method of claim 17 , wherein the second coating is a polyurethane topcoat.

21. The method of claim 17 , wherein the tie coat has a dry film thickness of about 2.5 to 25 microns (0.1 to 1.0 mils).