Multi-piece solid golf ball
In a multi-piece solid golf ball having a core, an envelope layer, an intermediate layer and a cover, the core is formed primarily of a base rubber and has a diameter set in a specific range, the envelope layer, intermediate layer and cover are each formed of resin materials, and the envelope layer is formed into two layers—an inner layer and an outer layer. The center and surface hardnesses of the core, the surface hardness of the inner envelope layer-encased sphere, the surface hardness of the outer envelope layer-encased sphere, the surface hardness of the intermediate layer-encased sphere and the surface hardness of the ball together satisfy a specific relationship. This ball achieves a satisfactory distance on full shots both with a driver and with irons, in addition to which it is superior in the short game and has a good feel at impact and an excellent scuff resistance.
1. A multi-piece solid golf ball comprising a core, an envelope layer, an intermediate layer and a cover, wherein the core is formed primarily of a base rubber and has a diameter of at least 30 mm; the envelope layer and the intermediate layer are each formed of a resin material and the cover is formed primarily of a urethane resin; the envelope layer is formed into two layers—an inner layer and an outer layer; the core has a center hardness and a surface hardness, the sphere obtained by encasing the core with the inner envelope layer (inner envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the inner envelope layer-encased sphere with the outer envelope layer (outer envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the outer envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness and the ball has a surface hardness which together satisfy the following relationship in which the hardnesses are Shore C hardness values:
core center hardness<core surface hardness<surface hardness of inner envelope layer-encased sphere<surface hardness of outer envelope layer-encased sphere<surface hardness of intermediate layer-encased sphere>ball surface hardness;
and the Shore C hardness value obtained by subtracting the core center hardness from the surface hardness of the outer envelope layer-encased sphere is 36 or more, and
wherein the core is formed of a rubber composition comprising:
(a) a base rubber,
(b) a co-crosslinking agent that is an α,β-unsaturated carboxylic acid or a metal salt thereof or both,
(c) a crosslinking initiator, and
(d) a lower alcohol having a molecular weight of less than 200.
2. The golf ball of claim 1 , wherein the hardnesses (Shore C hardness) of the respective encased spheres and the thicknesses (mm) of the respective layers satisfy the following relationship:
(surface hardness of ball×cover thickness)+(surface hardness of intermediate layer-encased sphere×intermediate layer thickness)+(surface hardness of outer envelope layer-encased sphere×thickness of outer envelope layer)+(surface hardness of inner envelope layer-encased sphere×thickness of inner envelope layer)≤[(surface hardness of core+center hardness of core)/2]×core radius.
3. The golf ball of claim 2 , wherein the sum expressed as (surface hardness of ball×cover thickness)+(surface hardness of intermediate layer-encased sphere×intermediate layer thickness)+(surface hardness of outer envelope layer-encased sphere×thickness of outer envelope layer)+(surface hardness of inner envelope layer-encased sphere×thickness of inner envelope layer) has a value of from 300 to 600.
4. The golf ball of claim 1 , wherein the surface hardnesses of the core and the inner envelope layer-encased sphere satisfy the following condition:
1≤surface hardness of inner envelope layer-encased sphere−surface hardness of core≤20.
5. The golf ball of claim 1 , wherein the surface hardnesses of the inner envelope layer-encased sphere and the outer envelope layer-encased sphere satisfy the following condition:
1≤surface hardness of outer envelope layer-encased sphere−surface hardness of inner envelope layer-encased sphere≤20.
6. The golf ball of claim 1 , wherein the surface hardnesses of the outer envelope layer-encased sphere and the intermediate layer-encased sphere satisfy the following condition:
1≤surface hardness of intermediate layer-encased sphere−surface hardness of outer envelope layer-encased sphere≤20.
7. The golf ball of claim 1 , wherein the core has an interior hardness which, letting Cc be the Shore C hardness at the core center, C5 be the Shore C hardness at a position 5 mm from the core center, Cs be the Shore C hardness at the core surface and Cs-2 be the Shore C hardness at a position 2 mm inside the core surface, satisfies the following condition:
Cs−Cc≥ 13.
8. The golf ball of claim 7 , wherein the interior hardness of the core further satisfies the following condition:
Cs−Cs -2≤4.
9. The golf ball of claim 7 , wherein the interior hardness of the core further satisfies the following condition:
C 5 −Cc≥Cs−Cs -2.
10. The golf ball of claim 1 , wherein the content of component (d) is from 0.5 to 5 parts by weight per 100 parts by weight of the base rubber (a).
11. The golf ball of claim 1 , wherein component (d) is a monohydric, dihydric or trihydric alcohol.
12. The golf ball of claim 1 , wherein component (d) is butanol, glycerol, ethylene glycol or propylene glycol.