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

Abstract

A golf ball includes an inner core layer and an outer core layer. The inner core layer is formed from a first thermoplastic material and has a geometric center hardness that is greater than its surface hardness to define a first negative hardness gradient. The outer core layer is formed from a second thermoplastic material and has an inner surface hardness greater than its outer surface hardness to define a second negative hardness gradient. Inner and outer cover layers are disposed over the outer core layer. The negative hardness gradient of the inner core layer is about −1 to −5 Shore C. The negative hardness gradient of the outer core layer is at least −1 Shore C but less than −7 Shore C. A difference between the inner core layer surface hardness and the outer core layer inner surface hardness is at least −3 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 greater than a surface hardness to define a first negative hardness gradient;   an outer core layer disposed about the inner core, the outer core layer comprising a second thermoplastic material and having an inner surface hardness greater than an outer surface hardness to define a second negative hardness gradient;   an inner cover layer disposed outer core layer; and   an outer cover layer disposed about the inner cover layer,   wherein the first negative hardness gradient is from −1 to −5 Shore C, the second negative hardness gradient is at least −1 Shore C but less than −7 Shore C, and a difference between the inner core surface hardness and the outer core inner surface hardness, Δh, is at least −3 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 1 , wherein the difference between the inner core layer surface hardness and the outer core layer inner surface hardness, Δh, is at least −5 Shore C. 
     
     
         4 . The golf ball of  claim 3 , wherein the difference between the inner core layer surface hardness and the outer core layer inner surface hardness, Δh, is at least −7 Shore C. 
     
     
         5 . The golf ball of  claim 1 , wherein the geometric center hardness is about 84 Shore C to about 96 Shore C. 
     
     
         6 . The golf ball of  claim 1 , wherein the inner core layer surface hardness is about 80 Shore C to about 92 Shore C. 
     
     
         7 . The golf ball of  claim 1 , wherein the hardness of the inner surface of the outer core layer is about 65 Shore C to about 77 Shore C. 
     
     
         8 . The golf ball of  claim 1 , wherein the hardness of the outer surface of the outer core layer is about 64 Shore C to about 74 Shore C. 
     
     
         10 . The golf ball of  claim 1 , wherein the second negative hardness gradient is at least −1 Shore C but less than −5 Shore C. 
     
     
         11 . The golf ball of  claim 10 , wherein the second negative hardness gradient is at least −3 Shore C but less than −5 Shore C. 
     
     
         12 . A golf ball comprising:
 an inner core layer consisting of a first thermoplastic material and having a geometric center hardness greater than a surface hardness to define a first negative hardness gradient between −1 Shore C and −5 Shore C;   an outer core layer disposed about the inner core, the outer core being formed from a second thermoplastic material and having an inner surface hardness greater than an outer surface hardness to define a second negative hardness gradient of at least −1 Shore C but less than −7 Shore C;   a cover layer disposed outer core layer, the cover layer comprising an inner cover layer comprising an ionomer and an outer cover layer comprising a castable polyurethane or polyurea material,   wherein a difference between the inner core layer surface hardness and the outer core layer inner surface hardness, Δh, is at least −5 Shore C.