Golf ball

Abstract

A golf ball 2 includes a core 4 , an inner cover 6 , and an outer cover 8 . The core 4 includes a center 10 and an envelope layer 12 . The envelope layer 12 is formed by a rubber composition being crosslinked. The rubber composition 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 Hi of the inner cover 6 is greater than a hardness Hs at a surface of the core 4 . A hardness Ho of the outer cover 8 is less than the hardness Hi.

What is claimed is: 
     
         1 . A golf ball comprising a core, an inner cover positioned outside the core, and an outer cover positioned outside the inner cover, wherein
 the core comprises 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 Hi of the inner cover is greater than a JIS-C hardness Hs at a surface of the core, and   a JIS-C hardness Ho of the outer cover is less than the hardness Hi.   
     
     
         2 . The golf ball according to  claim 1 , wherein a difference (Hi−Hs) between the hardness Hi and the hardness Hs is equal to or greater than 2. 
     
     
         3 . The golf ball according to  claim 1 , wherein a difference (Hi−Ho) between the hardness Hi and the hardness Ho is equal to or greater than 1. 
     
     
         4 . The golf ball according to  claim 1 , wherein an amount of the acid and/or the salt (d) is equal to or greater than 1.0 parts by weight but less than 40 parts by weight, per 100 parts by weight of the base rubber (a). 
     
     
         5 . 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). 
     
     
         6 . A golf ball comprising a core, an inner cover positioned outside the core, and an outer cover positioned outside the inner cover, wherein
 the core comprises 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 Ho of the outer cover is greater than a JIS-C hardness Hs at a surface of the core, and   the hardness Ho is greater than a JIS-C hardness Hi of the inner cover.   
     
     
         7 . The golf ball according to  claim 6 , wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1). 
     
     
         8 . The golf ball according to  claim 6 , wherein a difference (Ho−Hs) between the hardness Ho and the hardness Hs is equal to or greater than 2. 
     
     
         9 . The golf ball according to  claim 6 , wherein a difference (Ho−Hi) between the hardness Ho and the hardness Hi is equal to or greater than 2. 
     
     
         10 . The golf ball according to  claim 6 , wherein the rubber composition includes 1.0 parts by weight or greater but 40 parts by weight or less of the acid and/or the salt (d) per 100 parts by weight of the base rubber (a). 
     
     
         11 . A golf ball comprising a core, an inner cover positioned outside the core, a mid cover positioned outside the inner cover, and an outer cover positioned outside the mid cover, wherein
 the core comprises 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 Hi of the inner cover is greater than a JIS-C hardness Hs at a surface of the core,   a difference (Hi−Hs) between the hardness Hi and the hardness Hs is equal to or greater than 1, and   a JIS-C hardness Ho of the outer cover is greater than the hardness Hi.   
     
     
         12 . The golf ball according to  claim 11 , wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1). 
     
     
         13 . The golf ball according to  claim 11 , wherein the rubber composition includes 1.0 parts by weight or greater but 40 parts by weight or less of the acid and/or the salt (d) per 100 parts by weight of the base rubber (a). 
     
     
         14 . The golf ball according to  claim 11 , wherein a difference (Ho−Hi) between the hardness Ho and the hardness Hi is equal to or greater than 2. 
     
     
         15 . The golf ball according to  claim 11 , wherein
 a JIS-C hardness Hm of the mid cover is greater than the hardness Hi, and   the hardness Ho is greater than the hardness Hm.   
     
     
         16 . A golf ball comprising a core, an inner cover positioned outside the core, a mid cover positioned outside the inner cover, and an outer cover positioned outside the mid cover, wherein
 the core comprises 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 difference (Hi−Hs) between a JIS-C hardness Hi of the inner cover and a JIS-C hardness Hs at a surface of the core is less than 1, and   a JIS-C hardness Ho of the outer cover is greater than the hardness Hi.   
     
     
         17 . The golf ball according to  claim 16 , wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1). 
     
     
         18 . The golf ball according to  claim 16 , 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). 
     
     
         19 . The golf ball according to  claim 16 , wherein a difference (Ho−Hi) between the hardness Ho and the hardness Hi is equal to or greater than 5. 
     
     
         20 . The golf ball according to  claim 16 , wherein
 a JIS-C hardness Hm of the mid cover is greater than the hardness Hi, and   the hardness Ho is greater than the hardness Hm.