Foam-core golf balls

1. A golf ball comprising a core and a cover, wherein the ball has a moment of inertia greater than 84 g·cm 2 ; the core has its specific gravity reduced to less than 1.05, the reduction in specific gravity being less than 15% to minimize the reduction in the coefficient of restitution of the core; and the core comprises a highly neutralized thermoplastic polymer having a specific gravity of less than 0.95.

2. The golf ball of claim 1 , wherein the highly neutralized thermoplastic polymer comprises (a) an ethylene, C 3-8 alpha, beta-ethylenically unsaturated carboxylic acid copolymer, (b) a high molecular weight, monomeric organic acid or salt thereof, (c) a cation source and (d) a thermoplastic elastomer polymer selected from copolyetheresters, copolyetheramides, block styrene polydiene thermoplastic elastomers, elastomeric polyolefins, and thermoplastic polyurethanes.

3. The golf ball of claim 1 , wherein the highly neutralized polymer comprises a melt processible thermoplastic composition comprising (a) aliphatic, mono-functional organic acid(s) having fewer than 36 atoms and (b) an ethylene, C 3-8 alpha, beta-ethylenically unsaturated carboxylic acid copolymer(s) and ionomer(s) thereof.

4. The golf ball of claim 1 , wherein the highly neutralized polymer comprises (a) a salt of a high molecular weight organic acid and (b) an acid containing copolymer ionomer.

5. The golf ball of claim 4 , wherein the highly neutralized polymer further comprises (c) a thermoplastic polymer selected from co-polyesteresters, copolyetheramides, block styrene polydiene thermoplastic elastomers, elastomeric polyolefins, and thermoplastic polyurethanes.

6. The golf ball of claim 1 , wherein the specific gravity of the highly neutralized polymer is reduced by the incorporating less than 5% of low specific gravity fillers into the polymer.

7. The golf ball of claim 6 , wherein the highly neutralized polymer comprises between 1 and 3% low specific gravity fillers.