Golf ball

1. A golf ball comprising a core and a cover covering the core,

wherein the cover is made from a cover composition containing, as a resin component,

a thermoplastic polyurethane (A) and

a polyisocyanate mixture (B) in which a urethane prepolymer (b-1) having at least two isocyanate groups is dispersed in a thermoplastic resin (b-2) which does not substantially react with an isocyanate group; and

wherein a number average molecular weight of a polyol component constituting the urethane prepolymer (b-1) is 650 or more, the polyisocyanate mixture (B) has NCO content (NCO %) of 0.1 mass % to 3.0 mass %, the cover composition has a slab hardness of more than 60 and not more than 75 in Shore D hardness, and

the cover has a thickness of 2.5 mm or less.

2. The golf ball according to claim 1 , wherein the urethane prepolymer (b-1) is represented by the following formula (1)

Polyisocyanate-(polyol-polyisocyanate)n  (1)

wherein the connecting number n denotes an integer of not less than 1 and not higher than 10.

3. The golf ball according to claim 1 , wherein the thermoplastic resin (b-2) which does not substantially react with the isocyanate group is at least one kind selected from the group consisting of a polyester elastomer, an acrylic elastomer, a styrene elastomer, an olefin elastomer, and a vinyl chloride elastomer.

4. The golf ball according to claim 1 , wherein the cover composition contains the polyisocyanate mixture (B) in an amount of 1 part to 50 parts by mass based on 100 parts by mass of the thermoplastic polyurethane (A).

5. The golf ball according to claim 1 , wherein the urethane prepolymer (b-1) and the thermoplastic resin (b-2) are blended in the polyisocyanate mixture (B) (a total of 100 mass %) in a ratio of the urethane prepolymer (b-1)/the thermoplastic resin (b-2)=5 mass % to 50 mass %/50 mass % to 95 mass %.

6. The golf ball according to claim 1 , wherein the urethane prepolymer (b-1) has a number average molecular weight of 1000 to 30000.

7. The golf ball according to claim 1 , wherein the urethane prepolymer (b-1) is an isocyanate group terminated urethane prepolymer selected from the group consisting of 4,4′-diphenylmethane diisocyanate based urethane prepolymer, tolylene diisocyanate based urethane prepolymer, and 4,4′-dicyclohexylmethane diisocyanate based urethane prepolymer.

8. The golf ball according to claim 1 , wherein the urethane prepolymer (b-1) has NCO content (NCO %) of 0.5 mass % to 15.0 mass %.

9. The golf ball according to claim 8 , wherein the cover composition contains the polyisocyanate mixture (B) in an amount of 1 part to 50 parts by mass based on 100 parts by mass of the thermoplastic polyurethane (A).

10. The golf ball according to claim 9 , wherein the urethane prepolymer (b-1) and the thermoplastic resin (b-2) are blended in the polyisocyanate mixture (B) (a total of 100 mass %) in a ratio of the urethane prepolymer (b-1)/the thermoplastic resin (b-2)=5 mass % to 50 mass %/50 mass % to 95 mass %.

11. The golf ball according to claim 10 , wherein the thermoplastic resin (b-2) which does not substantially react with the isocyanate group is at least one kind selected from the group consisting of a polyester elastomer, an acrylic elastomer, a styrene elastomer, an olefin elastomer, and a vinyl chloride elastomer.

12. The golf ball according to claim 11 , wherein the urethane prepolymer (b-1) is an isocyanate group terminated urethane prepolymer selected from the group consisting of 4,4′-diphenylmethane diisocyanate based urethane prepolymer, tolylene diisocyanate based urethane prepolymer, and 4,4′-dicyclohexylmethane diisocyanate based urethane prepolymer.

13. The golf ball according to claim 12 , wherein the urethane prepolymer (b-1) has a number average molecular weight of 1000 to 30000.