Golf ball

Abstract

A golf ball 2 includes a core 4 and a cover 6 . The core 4 includes a center 8 and an envelope layer 10 each of which is formed by a rubber composition being crosslinked. At least one of the rubber compositions of the center 8 and the envelope layer 10 includes a base rubber (a), a co-crosslinking agent (b), a crosslinking initiator (c), and an acid and/or a salt (d). The co-crosslinking agent (b) is: (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A hardness H(Ie) at an innermost portion of the envelope layer 10 is greater than a hardness H(Sc) at an outermost portion of the center 8 . A hardness Ho of the cover 6 is greater than a hardness Hs at a surface of the core 4.

What is claimed is: 
     
         1 . A golf ball comprising a core and a cover positioned outside the core, wherein
 the core includes a center and an envelope layer positioned outside the center,   the center is formed by a rubber composition being crosslinked,   the envelope layer is formed by a rubber composition being crosslinked,   at least one of the rubber composition of the center and the rubber composition of the envelope layer includes:
 (a) a base rubber; 
 (b) a co-crosslinking agent; 
 (c) a crosslinking initiator; and 
 (d) an acid and/or a salt, 
   the co-crosslinking agent (b) is:
 (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or 
 (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, 
   a JIS-C hardness H(Ie) at an innermost portion of the envelope layer is greater than a JIS-C hardness H(Sc) at an outermost portion of the center, and   a JIS-C hardness Ho of the cover is greater than a JIS-C hardness Hs at a surface of the core.   
     
     
         2 . The golf ball according to  claim 1 , wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1). 
     
     
         3 . The golf ball according to  claim 2 , wherein a carbon number of a carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is equal to or greater than 1 but equal to or less than 30. 
     
     
         4 . The golf ball according to  claim 2 , wherein the carboxylic acid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof. 
     
     
         5 . The golf ball according to  claim 2 , wherein the carboxylic acid and/or the salt thereof (d1) is a zinc salt of a carboxylic acid. 
     
     
         6 . The golf ball according to  claim 2 , wherein the carboxylic acid and/or the salt thereof (d1) is one or more members selected from the group consisting of zinc octoate, zinc laurate, zinc myristate, and zinc stearate. 
     
     
         7 . The golf ball according to  claim 1 , wherein the rubber composition includes 1.0 parts by weight or greater but less than 40 parts by weight of the acid and/or the salt (d) per 100 parts by weight of the base rubber (a). 
     
     
         8 . The golf ball according to  claim 1 , wherein the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a). 
     
     
         9 . The golf ball according to  claim 1 , wherein the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a). 
     
     
         10 . The golf ball according to  claim 1 , wherein
 the rubber composition includes the α,β-unsaturated carboxylic acid (b1), and   the rubber composition further includes a metal compound (f).   
     
     
         11 . The golf ball according to  claim 1 , wherein a difference (H(Ie)−H(Sc)) between the hardness H(Ie) and the hardness H(Sc) is equal to or greater than 2. 
     
     
         12 . The golf ball according to  claim 1 , wherein a difference (Ho−Hs) between the hardness Ho and the hardness Hs is equal to or greater than 7. 
     
     
         13 . The golf ball according to  claim 1 , wherein a thickness To of the cover is equal to or greater than 0.8 mm but equal to or less than 2.0 mm. 
     
     
         14 . The golf ball according to  claim 1 , wherein a diameter of the center is equal to or greater than 10 mm but equal to or less than 25 mm. 
     
     
         15 . The golf ball according to  claim 1 , wherein a difference (Hs−H(0.0)) between the hardness Hs and a JIS-C hardness H(0.0) at a central point of the core is equal to or greater than 15. 
     
     
         16 . The golf ball according to  claim 1 , wherein the rubber composition further includes an organic sulfur compound (e). 
     
     
         17 . The golf ball according to  claim 16 , wherein the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof. 
     
     
         18 . The golf ball according to  claim 16 , wherein the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).