Thermoplastic core having a hardness gradient formed from a gradient-initiating solution

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 solvent, free radical initiator, and a reactive co-agent comprising a metal salt of an acrylic or methacrylic acid, a mono- or multi-functional acrylate or methacrylate, or having the chemical structure

where R=linear, branched, C 1 -C 14 or aromatic and n=1-5, such that the hardness of the outer surface is different than the hardness of the geometric center to define a positive or negative hardness gradient of 5 Shore C or greater.

2. The golf ball of claim 1 , wherein the free radical initiator comprises an azo compound or a peroxide.

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

4. The golf ball of claim 3 , wherein the gradient-initiating solution penetrates the core to a depth of 0.0625 inches or less.

5. The golf ball of claim 1 , wherein the reactive co-agent comprises phenylene bismaleimide, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, 1,6-hexanediol diacrylate, zinc diacrylate, zinc dimethacrylate, or dipentaerythritol pentaacrylate.

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

7. The golf ball of claim 1 , wherein the hardness gradient is 10 Shore C or greater.

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

9. The golf ball of claim 1 , wherein the thermoplastic material comprises an ionomer, a highly-neutralized ionomer, a thermoplastic polyurethane, a thermoplastic polyurea, a styrene block copolymer, a polyester amide, polyester ether, a polyethylene acrylic acid copolymer or terpolymer, or a polyethylene methacrylic acid copolymer or terpolymer.

10. The golf ball of claim 1 , wherein the inner cover layer comprises a partially- or fully-neutralized ionomer.

11. The golf ball of claim 10 , wherein the inner cover layer has a hardness of 60 Shore D or greater.

12. The golf ball of claim 11 , wherein the inner cover layer hardness is 65 Shore D or greater.

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

14. The golf ball of claim 13 , wherein the inner cover layer thickness is 0.02 inches to 0.045 inches.

15. The golf ball of claim 1 , wherein the outer cover layer comprises polyurethane, polyurea, or a blend thereof.

16. The golf ball of claim 15 , 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.

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

18. The golf ball of claim 1 , wherein the outer cover layer has a thickness of 0.020 inches to 0.030 inches.

19. 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 styrene block copolymer, a polyester amide, polyester ether, a polyethylene acrylic acid copolymer or terpolymer, or a polyethylene methacrylic acid copolymer or terpolymer;

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 solvent, a free radical initiator, a reactive co-agent comprising a metal salt of an acrylic or methacrylic acid, a mono- or multi-functional acrylate or methacrylate, or having the chemical structure

where R=linear, branched, or aromatic C 1 -C 14 and n=1-5 to form a gradient-initiating solution;

soaking the thermoplastic core in the gradient-initiating solution for a pre-determined time to create a positive or negative hardness gradient of 5 Shore C or greater between the surface and the geometric center;

forming an inner cover layer about the thermoplastic core, the inner cover layer comprising an ionomer or a highly-neutralized ionomer; and

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