GOLF BALLS HAVING INCREASED IMPACT DURABILITY
Golf balls having cores with increased hardness gradients and improved impact durability are provided. The cores have an increased “positive” hardness gradient (or a “hard-to-soft” hardness) where the outer surface of the core is harder than the center. The cores are formed from rubber formulations including combinations of a terpene-based resin, ethylene-propylene-diene (EPDM) rubber, and/or a water releasing agent. The resulting golf balls have increased impact durability and resilience.
1 . A golf ball, comprising:
a core having an outer surface and a geometric center, wherein the core is formed from a rubber composition comprising:
a base rubber,
a free radical initiator,
a terpene-based resin,
a water releasing agent comprising at least one water molecule available for release during curing of the rubber formulation,
wherein the geometric center and the outer surface each has a hardness and the hardness of the outer surface is greater than the hardness of the geometric center to define a positive Shore C hardness gradient of at least about 40; and
a cover layer surrounding the core.
2 . The golf ball of claim 1 , wherein the terpene-based resin comprises a terpene polymer, a terpene-phenol copolymer, a terpene-styrene copolymer, a terpene-phenol-styrene copolymer, a hydrogenated terpene-phenol copolymer, a hydrogenated terpene-styrene copolymer, a hydrogenated terpene-phenol-styrene copolymer, or combinations thereof.
3 . The golf ball of claim 2 , wherein the terpene-based resin is selected from the group consisting of α-pinene polymer, β-pinene polymer, limonene polymer, dipentene polymer, β-pinene/limonene polymer, α-pinene-phenol copolymer, α-pinene-α-methylstyrene copolymer, α-pinene-α-methylstyrene-phenol copolymer, β-pinene-phenol copolymer, β-pinene-α-methylstyrene copolymer, β-pinene-α-methylstyrene-phenol copolymer, and combinations thereof.
4 . The golf ball of claim 1 , wherein the base rubber comprises ethylene-propylene-diene (EPDM) rubber in an amount of about 1 phr to about 30 phr, and polybutadiene rubber in an amount of about 70 phr to about 99 phr.
5 . The golf ball of claim 1 , wherein the water releasing agent is present in the rubber composition in an amount of about 1 phr to about 3.9 phr.
6 . The golf ball of claim 1 , wherein the hardness of the geometric center ranges from about 45 Shore C to about 65 Shore C and the hardness of the outer surface ranges from about 70 Shore C to about 100 Shore C.
7 . A golf ball, comprising:
a dual core comprising an outer core layer disposed over an inner core layer,
wherein at least one of the outer core layer or the inner core layer is formed of a rubber composition comprising:
a base rubber comprising ethylene-propylene-diene (EPDM) rubber in an amount of about 1 phr to about 30 phr and polybutadiene rubber in an amount of about 70 phr to about 99 phr,
a free radical initiator,
a terpene-based resin,
a water releasing agent comprising at least one water molecule available for release during curing of the rubber formulation,
wherein the layer formed of the rubber composition has an outer surface and a geometric center, each having a hardness, and the hardness of the outer surface is greater than the hardness of the geometric center to define a positive Shore C hardness gradient; and
a cover layer surrounding the dual core.
8 . The golf ball of claim 7 , wherein the terpene-based resin is selected from the group consisting of α-pinene polymer, β-pinene polymer, limonene polymer, dipentene polymer, β-pinene/limonene polymer, α-pinene-phenol copolymer, α-pinene-α-methylstyrene copolymer, α-pinene-α-methylstyrene-phenol copolymer, β-pinene-phenol copolymer, β-pinene-α-methylstyrene copolymer, β-pinene-α-methylstyrene-phenol copolymer, and combinations thereof.
9 . The golf ball of claim 7 , wherein the terpene-based resin is present in the rubber composition in an amount of about 1 phr to about 15 phr.
10 . The golf ball of claim 7 , wherein the free radical initiator comprises an organic peroxide selected from the group consisting of dimethyl terbutyl peroxide, dicumyl peroxide, and combinations thereof.
11 . The golf ball of claim 7 , wherein the water releasing agent is selected from calcium sulfate dihydrate, zinc sulfate dihydrate, or combinations thereof.
12 . The golf ball of claim 7 , wherein the water releasing agent is present in the rubber composition in an amount of about 1 phr to about 3.9 phr.
13 . The golf ball of claim 7 , wherein the positive Shore C hardness gradient is at least about 40.
14 . The golf ball of claim 7 , further comprising a cross-linking agent, wherein the cross-linking agent comprises zinc diacrylate in an amount of about 15 phr to about 40 phr.
15 . A golf ball, comprising:
a multi-layered core comprising a center, an inner core layer disposed about the center, and an outer core layer disposed about the inner core layer, wherein at least one of the center, inner core layer, or outer core layer is formed of a rubber composition comprising:
a base rubber,
a free radical initiator,
a terpene-based resin,
a water releasing agent comprising a metal sulfate hydrate having at least one water of hydration available for release during curing of the rubber formulation,
wherein the layer formed of the rubber composition has an outer surface and a geometric center, each having a hardness, and the hardness of the outer surface is greater than the hardness of the geometric center to define a positive Shore C hardness gradient of at least about 40; and
a cover layer surrounding the multi-layered core.
16 . The golf ball of claim 15 , wherein the terpene-based resin is present in the rubber composition in an amount of about 5 phr to about 15 phr,
17 . The golf ball of claim 15 , wherein the base rubber comprises polybutadiene, ethylene-propylene-diene (EPDM) rubber, or a combination thereof.
18 . The golf ball of claim 15 , wherein the free radical initiator agent comprises an organic peroxide selected from the group consisting of dimethyl terbutyl peroxide, dicumyl peroxide, or combinations thereof.
19 . The golf ball of claim 15 , wherein the water releasing agent has a moisture content of at least about 5 percent by mass.
20 . The golf ball of claim 15 , wherein the water releasing agent is selected from calcium sulfate dihydrate, zinc sulfate dihydrate, or combinations thereof.