TWO-COMPONENT POLYURETHANE COATING COMPOSITIONS

1 . A two-component coating composition comprising:

(1) an isocyanate-functional component, and

(2) an isocyanate-reactive component comprising a hydroxyl-functional acrylic polymer and/or a hydroxyl-functional polyester,

wherein the isocyanate-functional component comprises:

(a) an aliphatic polyisocyanate containing allophanate structural units and having the structure:

in which: (i) Q 1 and Q 2 independently of one another are the radical of an aliphatic diisocyanate, (ii) R 1 and R 2 independently of one another are hydrogen or a C 1 -C 4 alkyl radical, (iii) Y is the radical of a starter molecule with a functionality of from 2 to 6, (iv) n is a number from 2 to 6, and (v) m corresponds to a number of monomer units such that the number-average molecular weight of the polyether on which the structure is based is 300 to 20,000 g/mol; and

(b) a cycloaliphatic polyisocyanate comprising an allophanate group and an isocyanurate trimer group.

2 . The two-component coating composition of claim 1 , wherein the aliphatic polyisocyanate containing allophanate structural units corresponds to the general formula:

in which Q is the radical of an aliphatic diisocyanate.

3 . The two-component coating composition of claim 2 , in which Q is —(CH 2 ) 6 —.

4 . The two-component coating composition of claim 1 , wherein the aliphatic polyisocyanate has an isocyanate functionality of at least 4, a glass transition temperature less than −40° C., and a % NCO less than 10%.

5 . The two-component coating composition of claim 1 , wherein the cycloaliphatic polyisocyanate is derived from isophorone diisocyanate and has: (i) an NCO content of 10% to 47% by weight, (ii) a viscosity of less than 10,000 mPas, and (iii) isocyanurate and allophanate groups in a molar ratio of monoisocyanurates to monoallophanates of 10:1 to 1:5.

6 . The two-component coating composition of claim 1 , wherein the isocyanate-functional component comprises: (a) 50 to 90 weight percent, based on the total weight of isocyanate-functional materials in the isocyanate-functional component, of the cycloaliphatic polyisocyanate, and (b) 10 to 50 weight percent, based on the total weight of isocyanate-functional materials in the isocyanate-functional component, of the aliphatic polyisocyanate.

7 . The two-component coating composition of claim 1 , wherein the isocyanate-reactive component comprises a hydroxyl-functional acrylic polymer.

8 . The two-component coating composition of claim 7 , wherein the hydroxyl-functional acrylic polymer comprises a reaction product of reactants comprising:

(a) 10 to 40 percent by weight of one or more vinyl aromatic monomers;

(b) 5 to 40 percent by weight of one or more olefinic monomers containing hydroxyl groups;

(c) 10 to 30 percent by weight of one or more (meth)acrylic esters of an alcohol containing 1 to 8 carbon atoms; and

(d) 0.1 to 2 percent by weight of one or more (meth)acrylic acids,

wherein the weight percents are based on the total weight of the reactants used to make the acrylic polymer.

9 . The two-component coating composition of claim 1 , wherein the isocyanate-functional component and the isocyanate-reactive component are present in amounts such that the ratio of isocyanate groups to hydroxyl groups in the composition is 0.5:1 to 5:1.

10 . A method of using the two-component coating composition of claim 1 , comprising applying the coating composition to a substrate such that the cured coating has a dry film thickness of at least 3 mils.

11 . The method of claim 10 , wherein the substrate comprises a storage tank, a process vessel, pipework, a pump, a building structure, or a bridge structure

12 . A method for coating a substrate, comprising:

(a) combining (1) an isocyanate-functional component with (2) an isocyanate-reactive component comprising a hydroxyl-functional polymer in relative amounts to provide a ratio of isocyanate groups to hydroxyl groups in the combined composition of 0.5 to 5.0:1; and

(b) depositing the combined composition over at least a portion of a substrate, wherein the isocyanate-functional component comprises:

(i) an aliphatic polyisocyanate containing allophanate structural units and having the structure:

in which Q 1 and Q 2 independently of one another are the radical of an aliphatic diisocyanate, R 1 and R 2 independently of one another are hydrogen or a C 1 -C 4 alkyl radical, Y is the radical of a starter molecule with a functionality of from 2 to 6, n is a number from 2 to 6, and m corresponds to a number of monomer units such that the number-average molecular weight of the polyether on which the structure is based is 300 to 20,000 g/mol; and

(H) a cycloaliphatic polyisocyanate comprising an allophanate group and an isocyanurate trimer group.

13 . The method of claim 12 , in which Q 1 and Q 2 are —(CH 2 ) 6 —.

14 . The method of claim 12 , wherein the aliphatic polyisocyanate is prepared by a process comprising: (1) reacting a polyisocyanate (a) and a polyether polyol (b) that contains less than or equal to 0.02 milliequivalent of unsaturated end groups per gram of polyol, has a polydispersity of from 1.0 to 1.5 and an OH functionality of at least 1.9 to give an isocyanate-functional polyurethane polymer, and (2) partly or fully allophanatizing the urethane groups of the isocyanate-functional polyurethane polymer by further reaction with a polyisocyanate.

15 . The method of claim 12 , wherein the aliphatic polyisocyanate containing allophanate structural units corresponds to the general formula:

in which Q is —(CH 2 ) 6 .

16 . The method of claim 12 , wherein the cycloaliphatic polyisocyanate is derived from isophorone diisocyanate and has: (i) an NCO content of 10% to 47% by weight, (ii) a viscosity of less than 10,000 mPas, and (iii) isocyanurate and allophanate groups in a molar ratio of monoisocyanurates to monoallophanates of 10:1 to 1:5.

17 . The method of claim 12 , wherein the isocyanate functional component comprises: (a) 50 to 90 percent by weight, based on the total weight of isocyanate-functional materials in the isocyanate-functional component, of the cycloaliphatic polyisocyanate; and (b) 10 to 50 percent by weight, based on the total weight of isocyanate-functional materials in the isocyanate-functional component, of the aliphatic polyisocyanate.

18 . The method of claim 12 , wherein the isocyanate-reactive component comprising a hydroxyl-functional acrylic polymer.

19 . The method of claim 18 , wherein the hydroxyl-functional acrylic polymer comprises a reaction product of reactants comprising:

(a) 10 to 40 percent by weight of one or more vinyl aromatic monomers;

(b) 5 to 40 percent by weight of one or more olefinic monomers containing hydroxyl groups;

(c) 10 to 30 percent by weight of one or more (meth)acrylic esters of an alcohol containing 1 to 8 carbon atoms; and

(d) 0.1 to 2 percent by weight of one or more (meth)acrylic acids, wherein the weight percents are based on the total weight of the reactants used to make the acrylic polymer.