Solid golf ball
The present invention provides a solid golf ball comprising a hot-molded product of a rubber composition comprising (a) 100 parts by weight of a polybutadiene synthesized using a rare-earth catalyst and 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 Mooney viscosity (ML 1+4 (100° C.)) of more than 30, (b) 0.01 to 0.5 parts by weight of a sulfur, (c) 10 to 60 parts by weight of an unsaturated carboxylic acid and/or a metal salt thereof, (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, wherein the hot-molded product has a difference in JIS-C hardness between the center and surface thereof of more than 20 and up to 40. The solid golf balls of the invention have a soft feel upon impact and excellent rebound characteristics. Moreover, the solid golf balls are improved in flight performance by making its spin rate reduced when shot with a driver.
1. A method for manufacturing a solid golf ball comprising a hot-molded product of a rubber composition, said method comprising the steps of:
vulcanizing and curing the rubber composition, wherein the rubber composition comprises
(a) 100 parts by weight of a polybutadiene synthesized using a rare-earth catalyst and 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 Mooney viscosity (ML 1+4 (100° C.)) of more than 30,
(b) 0.01 to 0.5 parts by weight of a sulfur,
(c) 10 to 60 parts by weight of an unsaturated carboxylic acid and/or a metal salt thereof,
(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,
wherein the organic peroxide is 1,1-bis(tert-butylperoxy)cyclohexane, the 1,1-bis(tert-butylperoxy)cyclohexane is the only organic peroxide of the rubber composition, the vulcanization is carried out at a temperature of 100 to 155° C., and the hot-molded product has a difference in JIS-C hardness between the center and surface thereof of more than 20 and up to 40.
2. The method of claim 1 , wherein the amount of (f) the organic peroxide is 1.2 to 3.0 parts by weight per 100 parts by weight of the polybutadiene.