Silane-based gradient-initiating solution for thermoplastic core having a positive hardness gradient

1. A golf ball comprising:

a core consisting essentially of a thermoplastic material, the core having an outer diameter of 1.51 inches to 1.59 inches and having an outer surface and a geometric center, each having a hardness;

an outer cover layer; and

an inner cover layer disposed between the core and the outer cover layer;

wherein the thermoplastic core has been exposed to a gradient-initiating solution comprising a silane having the formula:

where R′ is a non-hydrolysable organofunctional group, X is a hydrolysable group, and n is 0-24, such that the hardness of the outer surface is greater than the hardness of the geometric center to define a positive hardness gradient of 5 Shore C or greater;

wherein the thermoplastic material comprises an ionomer, a highly-neutralized ionomer, a thermoplastic polyurethane, a thermoplastic polyurea, a polyester amide, or a polyester ether.

2. The golf ball of claim 1 , wherein the thermoplastic material comprises a highly-neutralized ionomer.

3. The golf ball of claim 1 , wherein the solution penetrates the core to a depth of 0.125 inches or less.

4. The golf ball of claim 1 , wherein R′ is a vinyl group and X is alkoxy, acyloxy, halogen, amino, hydrogen, ketoximate group, amido group, aminooxy, mercapto, alkenyloxy group.

5. The golf ball of claim 1 , wherein X is represented by the formula RO—; where R is selected from the group consisting of a linear or branched C 1 -C 8 alkyl group, and a C 6 -C 12 aromatic group, and R 3 C(O)—.

6. The golf ball of claim 1 , wherein the silane is present in an amount of 1 weight percent to 20 weight percent of the thermoplastic material.

7. The golf ball of claim 6 , wherein the silane is present in an amount of 3 weight percent to about 5 weight percent of the thermoplastic material.

8. The golf ball of claim 1 , wherein the solvent comprises tetrahydrofuran, alcohol, toluene, acetone, ketones, methylene chloride, ethlyacetate, or carbon tetrachloride.

9. The golf ball of claim 1 , wherein the hardness gradient is 15 Shore C or greater.

10. The golf ball of claim 1 , wherein the inner cover comprises an ionomer or a partially- or fully-neutralized ionomer and the outer cover comprises a polyurethane or a polyurea material.

11. The golf ball of claim 1 , wherein the thermoplastic core has been exposed to the solution for a time of 5 minutes to 60 minutes.

12. The golf ball of claim 11 , wherein the thermoplastic core has been exposed to the solution for a time of 15 minutes to 30 minutes.

13. The golf ball of claim 1 , wherein the inner cover layer has a hardness of 60 Shore D or greater.

14. The golf ball of claim 13 , wherein the inner cover layer hardness is 65 Shore D or greater.

15. The golf ball of claim 1 , wherein the inner cover layer has a thickness of 0.015 inches to 0.060 inches.

16. The golf ball of claim 15 , wherein the inner cover layer thickness is 0.02 inches to 0.045 inches.

17. The golf ball of claim 1 , wherein the outer cover layer has a hardness of 60 Shore D or less and is softer than the hardness of the inner cover layer.

18. The golf ball of claim 1 , wherein the outer cover layer has a thickness of 0.015 inches to 0.040 inches.

19. The golf ball of claim 18 , wherein the outer cover layer has a thickness of 0.020 inches to 0.030 inches.

20. A method of forming a golf ball comprising the steps of:

providing a thermoplastic material comprising an ionomer, a highly-neutralized ionomer, a thermoplastic polyurethane, a thermoplastic polyurea, a polyester amide, or a polyester ether;

forming the thermoplastic material into a core having a surface, a geometric center, and an outer diameter of 1.51 inches to 1.59 inches;

combining a silane and a solvent to form a gradient-initiating solution, the silane having the formula:

wherein R′ is a non-hydrolysable organofunctional group, X is a hydrolysable group, and n is 0-24;

soaking the thermoplastic core in the solution for a time of 5 minutes to 60 minutes to create a positive hardness gradient of at least 5 Shore C between the surface and the geometric center;

forming an inner cover layer about the thermoplastic core, the inner cover layer comprising an ionomer; and

forming an outer cover layer about the inner cover layer, the outer cover layer comprising a polyurea or a polyurethane.