Golf ball

Abstract

A golf ball 2 includes a spherical core 4 , an inner mid layer 6 , an outer mid layer 8 , a reinforcing layer 10 , and a cover 12 . When distances (%) from a central point of the core 4 to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core 4 to a surface of the core 4 at intervals of 12.5% of a radius of the core 4 , are plotted in a graph, R 2 of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95. Hm1 is greater than Hm2. Hc is less than Hm1. T2 is equal to or greater than 0.5 mm but equal to or less than 1.6 mm.

What is claimed is: 
     
         1 . A golf ball comprising a spherical core, an inner mid layer positioned outside the core, an outer mid layer positioned outside the inner mid layer, and a cover positioned outside the outer mid layer, wherein
 if JIS-C hardness values are measured at nine points in the core obtained by dividing the core radius into intervals of 12.5% and including the core center and core surface, and the hardness values are plotted versus distance in a graph, then R 2  of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95,   a Shore D hardness Hm1 of the inner mid layer is greater than a Shore D hardness Hm2 of the outer mid layer,   a Shore D hardness Hc of the cover is less than the hardness Hm1, and   a thickness T2 of the outer mid layer is equal to or greater than 0.5 mm but equal to or less than 1.6 mm;   the core is formed from a crosslinked rubber composition comprising:
 (a) a base rubber; 
 (b) a co-crosslinking agent; 
 (c) a crosslinking initiator; and 
 (d) a carboxylic acid and/or a salt thereof wherein the carboxylic acid component of the carboxylic acid has a carbon number that is equal to or greater than one but equal to or less than 14, and 
 wherein the co-crosslinking agent (b) is: 
 (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and/or 
 (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, 
   wherein the carboxylic acid is selected from the group consisting of an aromatic carboxylic acid and a fatty acid, and/or the salt thereof is the salt of an aromatic carboxylic acid and does not include the co-crosslinking agent (b), and   wherein the rubber composition does not include a salt of a fatty acid.   
     
     
         2 . The golf ball according to  claim 1 , wherein
 a JIS-C hardness H(0.0) at the central point of the core is equal to or greater than 40 but equal to or less than 70, and   a surface hardness Hs of the core is equal to or greater than 78 but equal to or less than 96.   
     
     
         3 . The golf ball according to  claim 1 , wherein a difference (Hs−H(0.0)) between a surface hardness Hs of the core and a JIS-C hardness H(0.0) at the central point of the core is equal to or greater than 15. 
     
     
         4 . The golf ball according to  claim 1 , wherein a difference (Hm1−Hm2) between the hardness Hm1 and the hardness Hm2 is equal to or greater than 8. 
     
     
         5 . The golf ball according to  claim 1 , wherein a difference (Hm1−Hc) between the hardness Hm1 and the hardness He is equal to or greater than 30. 
     
     
         6 . The golf ball according to  claim 1 , wherein
 a sum (T1+T2) of a thickness T1 of the inner mid layer and the thickness T2 is equal to or greater than 1.0 mm but equal to or less than 3.0 mm, and   a thickness Tc of the cover is equal to or less than 0.8 mm.   
     
     
         7 . The golf ball according to  claim 1 , wherein the carboxylic acid (d) is a zinc salt of a carboxylic acid. 
     
     
         8 . The golf ball according to  claim 1 , wherein the rubber composition includes 100 parts by weight of the base rubber (a) and 1.0 parts by weight or greater but 40 parts by weight or less of the carboxylic acid (d). 
     
     
         9 . The golf ball according to  claim 1 , wherein the co-crosslinking agent is an α,β-unsaturated carboxylic acid (b1), and the rubber composition further includes a metal compound (e). 
     
     
         10 . The golf ball according to  claim 1 , wherein the rubber composition includes 100 parts by weight of the base rubber (a) and 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b). 
     
     
         11 . The golf ball according to  claim 1 , wherein the rubber composition includes 100 parts by weight of the base rubber (a) and 0.2 parts by weight or greater but 5 parts by weight or less of the crosslinking initiator (c). 
     
     
         12 . The golf ball according to  claim 1 , wherein the co-crosslinking agent is a metal salt of an α,β-unsaturated carboxylic acid (b2). 
     
     
         13 . The golf ball according to  claim 1 , wherein
 the outer mid layer is formed from a resin composition,   the cover is formed from another resin composition,   a principal component of a base resin of the resin composition of the outer mid layer is different from a principal component of a base resin of the resin composition of the cover, and   the golf ball further comprises a reinforcing layer between the outer mid layer and the cover.   
     
     
         14 . The golf ball according to  claim 1 , wherein the rubber composition further includes an organic sulfur compound (f). 
     
     
         15 . The golf ball according to  claim 14 , wherein the organic sulfur compound (f) is at least one member selected from the group consisting of thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof. 
     
     
         16 . The golf ball according to  claim 14 , wherein the rubber composition includes 100 parts by weight of the base rubber (a) and 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (f). 
     
     
         17 . The golf ball according to  claim 1 , wherein the hardness Hm1 is equal to or greater than 55 Shore D but equal to or less than 70 Shore D. 
     
     
         18 . The golf ball according to  claim 1 , wherein the hardness Hm2 is equal to or greater than 25 Shore D but equal to or less than 60 Shore D.