A golf ball 2 includes a core 4 , a mid layer 6 , and a cover 8 . A difference DH in hardness between a surface and a central point of the core 4 , a thickness Tm and a hardness Hm of the mid layer 6 , a thickness Tc and a hardness Hc of the cover 8 , and an amount of compressive deformation Sb of the golf ball 2 meet the following mathematical formulas (DH*Hm)/(Hc*Tc)>80 and ((Sb*Tc)/(Hc*Hm*Tm))*1000>0.75. A hardness Hc of the cover 8 is equal to or less than 40. A material of the cover 8 is a resin composition. The resin composition includes a polyurethane (A) and a polyrotaxane (B).
1. A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer,
wherein a material of the cover is a resin composition, and
the resin composition includes:
(A) a polyurethane; and
(B) a polyrotaxane having a cyclodextrin, a linear molecule that is threaded through a ring structure of the cyclodextrin, and a blocking group that is located at each of both ends of the linear molecule and prevents detachment of the cyclodextrin, at least a part of hydroxyl groups of the cyclodextrin being modified with a caprolactone chain via a —O—C 3 H 6 —O— group;
wherein a thickness Tm (mm) and a Shore D hardness Hm of the mid layer, a thickness Tc (mm) and a Shore D hardness Hc of the cover, and an amount of compressive deformation Sb (mm) of the golf ball meet the following mathematical formula (2):
0.81≤(( Sb*Tc )/( Hc*Hm*Tm ))*1000≤1.03 (2);
wherein a loss elastic modulus E″ of the cover is equal to or greater than 6.0*10 6 Pa and equal to or less than 5.0*10 7 Pa; and
wherein a difference DH in Shore C hardness between a surface and a central point of the core, a Shore D hardness Hm of the mid layer, a thickness Tc (mm) and a Shore D hardness Hc of the cover meet the following mathematical formula (1):
( DH*Hm )/( Hc*Tc )≥90 (1).
2. The golf ball according to claim 1 , wherein a Shore D hardness Hc of the cover is equal to or less than 40.
3. The golf ball according to claim 1 , wherein a Shore D hardness Hm of the mid layer is equal to or greater than 55.
4. The golf ball according to claim 1 , wherein the linear molecule is polyethylene glycol, and the blocking group is an adamantyl group.
5. The golf ball according to claim 1 , wherein an amount of the polyrotaxane (B) in the resin composition of the cover per 100 parts by weight of the polyurethane (A) is equal to or greater than 1 parts by weight but equal to or less than 50 parts by weight.
6. The golf ball according to claim 1 , wherein a loss elastic modulus E″ of the polyurethane (A) is equal to or less than 5.0*10 7 Pa.
7. The golf ball according to claim 1 , wherein a hydroxyl value of the polyrotaxane (B) is equal to or greater than 10 mg KOH/g and equal to or less than 400 mg KOH/g.
8. The golf ball according to claim 1 , wherein a degree of polymerization of the caprolactone chain is 1 to 100.