Thermoplastic dual core having a positive gradient inner core and a negative gradient outer core

Abstract

A golf ball includes an inner core layer formed from a first thermoplastic material and having a geometric center hardness less than a surface hardness to define a positive hardness gradient. An outer core layer is disposed about the inner core and is formed from a second thermoplastic material. The outer core layer has a surface hardness less than a hardness at a point about 0.5 to 2 mm from an interface between the inner core layer and the outer core layer to define a negative hardness gradient. An inner cover layer is disposed outer core layer and an outer cover layer is disposed about the inner cover layer. The positive hardness gradient os the inner core layer is up to about 10 Shore C and the negative hardness gradient of the outer core layer is less than about 10 Shore C.

What is claimed is: 
     
         1 . A golf ball comprising:
 an inner core layer comprising a first thermoplastic material and having a geometric center hardness less than a surface hardness to define a positive hardness gradient;   an outer core layer disposed about the inner core, the outer core layer comprising a second thermoplastic material and having a surface hardness less than a hardness at a point about 0.5 to 2 mm from an interface between the inner core layer and the outer core layer to define a negative hardness gradient;   an inner cover layer disposed outer core layer; and   an outer cover layer disposed about the inner cover layer,   wherein the positive hardness gradient is up to about 10 Shore C and the negative hardness gradient is less than about 10 Shore C.   
     
     
         2 . The golf ball of  claim 1 , wherein the first and second thermoplastic materials comprise 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. 
     
     
         3 . The golf ball of  claim 2 , wherein the thermoplastic material is a highly-neutralized ionomer. 
     
     
         4 . The golf ball of  claim 1 , wherein the geometric center of the inner core has a hardness of about 50 Shore C to 70 Shore C. 
     
     
         5 . The golf ball of  claim 1 , wherein the surface of the inner core has a hardness of about 52 Shore C to 78 Shore C. 
     
     
         6 . The golf ball of  claim 1 , wherein the surface of the outer core layer has a hardness of about 60 Shore C to 80 Shore C. 
     
     
         7 . The golf ball of  claim 1 , wherein the hardness at a point about 0.5 to 2 mm from the interface is about 66 Shore C to 80 Shore C. 
     
     
         8 . The golf ball of  claim 7 , wherein the hardness at a point about 0.5 to 2 mm from the interface is about 70 Shore C to 76 Shore C. 
     
     
         9 . The golf ball of  claim 1 , wherein the positive hardness gradient is about 1 to 7 Shore C. 
     
     
         10 . The golf ball of  claim 9 , wherein the positive hardness gradient is about 3 to 5 Shore C. 
     
     
         11 . The golf ball of  claim 1 , wherein the negative hardness gradient is about 2 to 8 Shore C. 
     
     
         12 . The golf ball of  claim 11 , wherein the negative hardness gradient is about 3 to 6 Shore C.