A golf ball 2 includes a core 4, a cover 6, and a paint layer 8. The golf ball 2 has numerous dimples 10 on the surface thereof. The cover 6 is formed of a resin composition including thermoplastic polyurethane (A) and a polyisocyanate mixture (B). The polyisocyanate mixture (B) includes a polyisocyanate compound (B1) having two or more isocyanate groups or a urethane prepolymer (B2) having two or more isocyanate groups. The cover 6 has a hardness of 30 or greater and 55 or less. The cover 6 has a thickness of 1.0 mm or greater and 2.0 mm or less. The ratio of the total volume of all the dimples 10 to the surface area of a phantom sphere of the golf ball is less than 75%. The dimples 10 have a curvature radius at the edge thereof in a range of 0.1 mm to 10 mm.
1. A golf ball comprising a core and a cover positioned outside the core, the golf ball having numerous dimples on a surface of the cover, wherein
the cover is formed of a resin composition including thermoplastic polyurethane (A) and a polyisocyanate mixture (B),
the polyisocyanate mixture (B) includes a urethane prepolymer (B2) having two or more isocyanate groups,
the cover has a hardness, measured with a Shore D type hardness scale, of 30 or greater and 55 or less,
the cover has a thickness of 1.0 mm or greater and 2.0 mm or less,
a difference (Hb−Hc) between a hardness Hb of the golf ball and a hardness Hc of the core which are measured with the Shore D type hardness scale is equal to or greater than −10 and equal to or less than 10,
a ratio of the total area of all the dimples to a surface area of a phantom sphere of the golf ball is less than 75%, and
the numerous dimples include a dimple having a curvature radius at an edge thereof in a range of 0.1 mm to 10 mm.
2. The golf ball according to claim 1 , wherein an amount of the polyisocyanate mixture (B) in the resin composition of the cover is equal to or greater than 1 part by weight and equal to or less than 40 parts by weight, per 100 parts by weight of the thermoplastic polyurethane (A).
3. The golf ball according to claim 1 , wherein an amount of isocyanate groups in the polyisocyanate mixture (B) is equal to or greater than 0.1% by weight and equal to or less than 10% by weight.
4. The golf ball according to claim 1 , wherein an MDI-based thermoplastic polyurethane is used for the cover.
5. The golf ball according to claim 1 , wherein the polyisocyanate mixture (B) includes a thermoplastic resin (B3) which substantially does not react with an isocyanate group, and the urethane prepolymer (B2) is dispersed in the thermoplastic resin (B3).
6. The golf ball according to claim 1 , wherein a ratio of a number of dimples with a curvature radius at an edge thereof in a range of 0.1 mm to 10 mm, to the total number of the dimples, is equal to or greater than 50%.
7. The golf ball according to claim 1 , wherein the numerous dimples include a dimple having a curvature radius at an edge thereof in a range of 0.1 mm to 1.0 mm.
8. The golf ball according to claim 1 , wherein the thermoplastic polyurethane (A) includes a residue obtained by reacting thermoplastic polyurethane with a urethane prepolymer (B2) having two or more isocyanate groups.
9. The golf ball according to claim 6 , wherein 100% of the dimples have a curvature radius at an edge thereof in a range of 0.1 mm to 10 mm.
10. The golf ball according to claim 2 , wherein an amount of isocyanate groups in the polyisocyanate mixture (B) is equal to or greater than 0.1% by weight and equal to or less than 10% by weight.