Multi-piece solid golf ball

Abstract

In a multi-piece solid golf ball comprising a solid core, an inner cover layer and an outer cover layer, the solid core is molded from a rubber composition comprising a base rubber composed of (a) a polybutadiene having a high cis-1,4 content, a minimal 1,2 vinyl content and a viscosity η of up to 600 mPa·s at 25° C. as a 5 wt % toluene solution, being synthesized using a rare-earth catalyst, having a specific Mooney viscosity in combination with (b) a diene rubber other than component (a), (c) an unsaturated carboxylic acid, (d) an organosulfur compound, (e) an inorganic filler, and (f) an organic peroxide; the inner cover layer has a specific Shore D hardness and the outer cover layer has a specific Shore D hardness, and the outer cover layer has a lower Shore D hardness than the inner cover layer. This combination of features gives the ball a good, soft feel upon impact and an excellent spin performance that provides increased distance.

1 . A multi-piece solid golf ball comprising a solid core, an inner cover layer and an outer cover layer, wherein the solid core is molded from a rubber composition comprising 
 100 parts by weight of a base rubber composed of (a) 20 to 100 wt % of a polybutadiene having a cis-1,4 content of at least 60% and a 1,2 vinyl content of at most 2%, having a viscosity η at 25° C. as a 5 wt % solution in toluene of up to 600 mPa·s, and having the Mooney viscosity (ML 1+4  (100° C.)) of the polybutadiene of 50 to 80, being synthesized using a rare-earth catalyst, in combination with (b) 0 to 80 wt % of a diene rubber other than component (a),    (c) 10 to 60 parts by weight of an unsaturated carboxylic acid or a metal salt thereof or both,    (d) 0.1 to 5 parts by weight of an organosulfur compound,    (e) 5 to 80 parts by weight of an inorganic filler, and    (f) 0.1 to 5 parts by weight of an organic peroxide; and    the inner cover layer has a Shore D hardness of 50 to 80, the outer cover layer has a Shore D hardness of 35 to 60, and    the outer cover layer has a lower Shore D hardness than the inner cover layer.    
     
     
         2 . The golf ball of  claim 1 , wherein the polybutadiene (a) satisfies relationship: 10B+5≦A≦10B+60, wherein A is the Mooney viscosity (ML 1+4  (100° C.)) of the polybutadiene and B is the ratio Mw/Mn between the weight-average molecular weight Mw and the number-average molecular weight Mn of the polybutadiene.  
     
     
         3 . The golf ball of  claim 1 , wherein the diene rubber (b) includes 30 to 100 wt % of a second polybutadiene which has a cis-1,4 content of at least 60% and a 1,2 vinyl content of at most 5%, has a Mooney viscosity (ML 1+4  (100° C.)) of not more than 55, and satisfies the relationship:  
       η≦20 A− 550,  
       wherein A is the Mooney viscosity (ML 1+4  (100° C.)) of the second polybutadiene and η is the viscosity of the second polybutadiene, in mPa·s, at 25° C. as a 5 wt % solution in toluene.  
     
     
         4 . The golf ball of  claim 3 , wherein the second polybutadiene in component (b) is synthesized using a Group VIII catalyst.  
     
     
         5 . The golf ball of  claim 1 , wherein the inner cover layer has a thickness of 0.2 to 3.0 mm and the outer cover layer has a thickness of 0.2 to 2.0 mm.