A first golf ball 2 includes a core 4 , a mid-layer 6 , and a cover 8 . The core 4 includes a center 10 and an envelope layer 12 . The mid-layer 6 includes a metal ion source (C) that can neutralize an ionomer resin. The mid-layer 6 has a melt flow rate (at 190° C. and 2.16 kg) of 4 g/10 minute or greater. The difference (H3−H1) between the JIS-C hardness H3 of the cover 8 and the JIS-C central hardness H1 of the center 10 is equal to or greater than 45. A second golf ball 22 includes a core 24 , a mid-layer 28 , and a cover 30 . The mid-layer 28 has a melt flow rate (at 190° C. and 2.16 kg) of 4 g/10 minute or greater. Preferably, the mid-layer 28 includes a high melt viscosity resin (A), a low melt viscosity ionomer resin (B), and a metal ion source (C1).
1. A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein
the cover has a thickness of 0.3 mm or greater and 1.6 mm or less,
the cover has a Shore D hardness Hc of 56 or greater,
the mid layer has a thickness of 0.5 mm or greater and 1.6 mm or less,
the mid layer has a Shore D hardness Hm of 35or greater and 57or less, and
the mid layer has a melt flow rate, measured under the conditions of: a temperature of 190° C. and a load of 2.16 kg, of 4 g/10 min or greater, and
the mid layer includes a composition (M) that is formed from:
a high melt viscosity resin (A) that is an ionomer resin (a-1), a nonionic resin (a-2), or a mixture of the ionomer resin (a-1) and the nonionic resin (a-2), the ionomer resin (a-1) being a high melt viscosity ionomer resin that is at least one or more types selected from two types of: a metal ion neutralized product (a-11) of a binary copolymer formed with ethylene and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and a metal ion neutralized product (a-12) of a ternary copolymer formed with ethylene, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and an α,β-unsaturated carboxylic acid ester, the high melt viscosity ionomer resin having a melt viscosity (190° C.), measured with a flow tester, of 500 Pa·s or greater and 100000 Pa·s or less, the nonionic resin (a-2) being a high melt viscosity nonionic resin that is at least one or more types selected from two types of: a binary copolymer (a-21) formed with ethylene and an α,β-unsaturated carboxylic acid having 3to 8 carbon atoms; and a ternary copolymer (a- 22) formed with ethylene, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and an α,β-unsaturated carboxylic acid ester, the high melt viscosity nonionic resin having a melt viscosity (190° C.), measured with a flow tester, of 5 Pa·s or greater and 3000 Pa·s or less;
a low melt viscosity ionomer resin (B) that is at least one or more types selected from two types of: a metal ion neutralized product (b-1) of a binary copolymer formed with ethylene and an α,β-unsaturated carboxylic acid having 3to 8 carbon atoms; and a metal ion neutralized product (b-2) of a ternary copolymer formed with ethylene, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and an α,β-unsaturated carboxylic acid ester, the low melt viscosity ionomer resin (B) having a melt viscosity (190° C.), measured with a Brookfield viscometer, of 1 Pa· s or greater and 10 Pa· s or less; and
a metal ion source (C1) that can neutralize carboxyl groups in the high melt viscosity resin (A) and the low melt viscosity ionomer resin (B), wherein
the weight ratio A1/B1of the high melt viscosity resin (A) with respect to the low melt viscosity ionomer resin (B) is equal to or greater than 55/45 and equal to or less than 99 /1, and
the amount of the metal ion source (C1) is equal to or greater than 0.1 part by weight and equal to or less than 10 parts by weight, per total 100 parts by weight of the high melt viscosity resin (A) and the low melt viscosity ionomer resin (B).
2. The golf ball according to claim 1 , wherein the mid layer includes a thermoplastic resin (D) in an amount that is equal to or greater than 1 part by weight and equal to or less than 95 parts by weight per total 100 parts by weight of the high melt viscosity resin (A) and the low melt viscosity ionomer resin (B).
3. The golf ball according to claim 1 , wherein the mid layer has a specific gravity G2 of 1.10 or greater and 1.50 or less.
4. The golf ball according to claim 1 , wherein the core has a diameter of 38.0 mm or greater.
5. The golf ball according to claim 1 , wherein the core has a central hardness H1c of 25 or greater and 55 or less.
6. The golf ball according to claim 1 , wherein the core has a surface hardness H1s of 35 or greater and 65 or less.
7. The golf ball according to claim 1 , wherein the core has a specific gravity G1of 1.00or greater and 1.18or less.
8. The golf ball according to claim 1 , wherein the cover has a specific gravity G3of 0.97or greater and 1.20or less.