Dual core golf ball having negative-hardness-gradient thermoplastic inner core and shallow positive-hardness-gradient thermoset outer core layer

Abstract

A golf ball includes an inner core layer formed from a thermoplastic material. The inner core has a hardness at the center that is greater than the hardness at the surface to define a negative hardness gradient. An outer core layer is disposed about the inner core. The outer core has a surface hardness that is greater than the surface hardness of the inner core to define a positive hardness gradient. An inner cover layer and an outer core layer are formed over the outer core layer. The negative hardness gradient of the inner core is about −1 to −5 Shore C, the positive hardness gradient of the outer core layer is less than 25 Shore C, the surface hardness of the outer core layer is about 45 Shore D to about 70 Shore D, and the surface hardness of the inner core is about 20 Shore D to about 60 Shore D.

1 . A golf ball comprising:
 an inner core layer consisting essentially of a thermoplastic material and having a geometric center hardness greater than a first surface hardness to define a negative hardness gradient;   an outer core layer disposed about the inner core, the outer core having a second surface hardness that is greater than the first surface hardness to define a positive hardness gradient;   an inner cover layer disposed outer core layer; and   an outer cover layer disposed about the inner cover layer,   wherein the negative hardness gradient is from −1 to −5 Shore C, the positive hardness gradient is less than 25 Shore C, and the second surface hardness is about 45 Shore D to about 70 Shore D and the first surface hardness is about 20 Shore D to about 60 Shore D.   
     
     
         2 . 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. 
     
     
         3 . The golf ball of  claim 1 , wherein geometric center hardness is about 90 Shore C to about 100 Shore C. 
     
     
         4 . The golf ball of  claim 1 , wherein the first surface hardness is about 85 Shore C to about 95 Shore C. 
     
     
         5 . The golf ball of  claim 1 , wherein the second hardness is about 80 Shore C to about 90 Shore C. 
     
     
         6 . The golf ball of  claim 1 , wherein the outer core layer comprises diene rubber and a metal salt of a carboxylic acid in an amount of about 25 phr to about 40 phr. 
     
     
         7 . The golf ball of  claim 1 , wherein the second hardness is less than the first hardness. 
     
     
         8 . The golf ball of  claim 1 , wherein the outer core layer comprises a soft and fast agent.