Thread-wound golf ball

DETAILED DESCRIPTION Therefore, an object of the present invention is to provide a thread-wound golf ball having a balata cover which is fully durable, especially improved in cut and shear resistance of the cover.
The inventors have found that a thread-wound golf ball comprising a center, a thread rubber layer wound on the center, and a cover enclosing the thread-wound core is improved in durability, especially in cut and shear resistance of the cover when the cover is formed by shaping and vulcanizing a rubber composition comprising a base rubber containing at least 30% by weight of transpolyisoprene such that the cover has a vulcanizing sulfur content ratio S1/S2 within the range of from 0.
5 to 4, especially from 0.
8 to 3.
5 wherein S1 is a vulcanizing sulfur content in a radially outer half region and S2 is a vulcanizing sulfur content in a radially inner half region of the cover.
More particularly, we examined balata covers of conventional thread-wound golf balls to find that during vulcanization of a cover in a ball manufacturing process, migration of sulfur occurred from the inside to the outside of the cover, resulting in a cover inside region having a substantially lower vulcanizing sulfur content than near the cover outside.
Insufficient resilience was provided near the cover inside.
Mainly because of this loss of internal resilience, the cover had poor cut resistance and shear resistance.
Making investigations in light of this finding in order to obtain a balata cover having high cut resistance and shear resistance, we have found that when a balata cover in which at least 30% by weight of the base rubber is comprised of transpolyisoprene is formed so as to provide a vulcanizing sulfur content ratio S1/S2 within the range of from 0.
5 to 4, preferably from 0.
8 to 3.
5 wherein S1 is a vulcanizing sulfur content in an outer half region of the cover disposed radially outward of a radial midpoint and S2 is a vulcanizing sulfur content in an inner half region of the cover disposed radially inward of the radial midpoint, a sufficient amount of vulcanizing sulfur is available near the cover inside to provide high resilience