The present invention provides a golf ball comprising a golf ball body with a spherical core having one or more layers, an intermediate layer disposed outside the spherical core and a cover disposed outside the intermediate layer and a paint film formed on a surface of the golf ball body, and having a friction coefficient calculated using a contact force tester of 0.35 or more and 0.60 or less, wherein the golf ball body has specific plurality of dimples on the surface thereof, at least one layer of the spherical core is a rubber layer having a specific hardness distribution, and the rubber layer has a hardness at the outermost point which is greater than a hardness at the innermost point, and the intermediate layer has a slab hardness (Hm) which is greater than a slab hardness (Hc) of the cover.
1. A golf ball comprising
a golf ball body having
a spherical core composed of at least one layer,
an intermediate layer disposed outside the spherical core,
a cover disposed outside the intermediate layer, and
a paint film formed on a surface of the golf ball body,
wherein
the golf ball has a plurality of dimples on a surface thereof, the dimples satisfying a following mathematical formula (I);
Y≦ 3.8 *X− 2.894 (I)
wherein X represents a ratio of a total area of all the dimples to a surface area of a phantom sphere of the golf ball, and Y represents a standard deviation (mm) of diameters of all the dimples,
at least one layer of the spherical core is a rubber layer formed from a rubber composition, wherein if the JIS-C hardness values of the rubber layer are measured at nine points obtained by dividing a thickness of the rubber layer into equal parts having 12.5% intervals in a radial direction of the spherical core and including innermost and outermost portions of the rubber layer, and the hardness values are plotted against distance (%) from an innermost point of the rubber layer, then R 2 of a linear approximation curve obtained from a least square method is 0.95 or higher;
the rubber layer has a hardness at an outermost point which is greater than a hardness at the innermost point; and
the intermediate layer has a slab hardness (Hm) which is greater than a slab hardness (Hc) of the cover.
2. The golf ball according to claim 1 , wherein the spherical core has a center hardness (Ho) ranging from 40 to 70 in JIS-C hardness and a surface hardness (Hs) ranging from 76 to 96 in JIS-C hardness.
3. The golf ball according to claim 1 , wherein the spherical core surface hardness (Hs) is greater than the center hardness (Ho) by a difference of 15 or more in JIS-C hardness.
4. The golf ball according to claim 1 , wherein the intermediate layer slab hardness (Hm) is greater than the cover slab hardness (Hc) by a difference of 30 or more in JIS-C hardness.
5. The golf ball according to claim 1 , wherein the intermediate layer has a thickness ranging from 0.5 mm to 1.6 mm, and the cover has a thickness of 0.8 mm or less.
6. The golf ball according to claim 1 , wherein the rubber layer is formed from a rubber composition containing
(a) a base rubber,
(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or (b2) a metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as (b) a co-crosslinking agent,
(c) a crosslinking initiator, and
(d) an acid and/or a salt thereof excluding the (b) co-crosslinking agent α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and metal salt thereof.
7. The golf ball according to claim 6 , wherein (d) the acid and/or the salt thereof is a carboxylic acid and/or a salt thereof.
8. The golf ball according to claim 7 , wherein (d) the carboxylic acid and/or the salt thereof is a fatty acid and/or a salt thereof.
9. The golf ball according to claim 8 , wherein (d) the carboxylic acid and/or the salt thereof has a fatty acid component having 1 to 30 carbon atoms.
10. The golf ball according to claim 6 , wherein the rubber composition contains (d) the acid and/or the salt thereof in an amount of 1 part by mass or more and less than 40 parts by mass with respect to 100 parts by mass of (a) the base rubber.
11. The golf ball according to claim 1 , wherein a base resin constituting the paint film is a polyurethane obtained by a reaction between a polyol and two or more polyisocyanates.
12. The golf ball according to claim 11 , wherein the two or more polyisocyanates include a derivative of hexamethylene diisocyanate and a derivative of isophorone diisocyanate.
13. The golf ball according to claim 12 , wherein the derivative of hexamethylene diisocyanate includes a biuret-modified product of hexamethylene diisocyanate and isocyanurate of hexamethylene diisocyanate.
14. The golf ball according to claim 12 , wherein the derivative of isophorone diisocyanate includes isocyanurate of isophorone diisocyanate.
15. The golf ball according to claim 11 , wherein a molar ratio (NCO/OH) of the isocyanate group (NCO) of the two or more polyisocyanates to the hydroxyl group (OH) of the polyol is 0.1 or more and 1.0 or less in the reaction between the two or more polyisocyanates and the polyol.
16. The golf ball according to claim 1 , wherein the ratio X is 0.78 or more.
17. The golf ball according to claim 1 , wherein the standard deviation Y (mm) of diameters of all the dimples is 0.30 mm or less.
18. The golf ball according to claim 1 , wherein a total number of dimples is 300 or more and 400 or less.
19. The golf ball according to claim 1 , wherein each of the dimples has a contour in a circular shape.
20. The golf ball according to claim 1 , further satisfying the following mathematical formula (II):
Y≦ 3.8 *X− 2.944 (II).
21. The golf ball according to claim 1 , further satisfying the following mathematical formula (III):
Y≦ 3.8 *X− 2.994 (III).
22. The golf ball according to claim 6 , wherein (d) the acid and/or the salt thereof is a carboxylic acid having 1 to 13 carbon atoms and /or a salt thereof.
23. The gold ball according to claim 6 , wherein
a base resin constituting the paint film is a polyurethane obtained by a reaction between a polyol and two or more polyisocyanates, and
a molar ratio (NCO/OH) of the isocyanate group (NCO) of the two or more polyisocyanates to the hydroxyl group (OH) of the polyol is 0.1 or more and 1.0 or less in the reaction between the two or more polyisocyanates and the polyol.
24. The gold ball according to claim 6 , wherein
a base resin constituting the paint film is a polyurethane obtained by a reaction between a polyol and two or more polyisocyanates, and
the two or more polyisocyanates include a derivative of hexamethylene diisocyanate including a biuret-modified product of hexamethylene diisocyanate and isocyanurate of hexamethylene diisocyanate and a derivative of isophorone diisocyanate including isocyanurate of isophorone diisocyanate.
25. The golf ball according to claim 6 , wherein
a base resin constituting the paint film is a polyurethane obtained by a reaction between a polyol and two or more polyisocyanates,
the polyol includes a urethane polyol obtained by a reaction between a polyol component and a polyisocyanate component, and
the polyol component constituting the urethane polyol includes a triol component and a diol component, and a mixing ratio (triol component / diol component) of the triol component to the diol component is 0.2 or more and 6.0 or less in a mass ratio.