High Hardness Gradient Golf Ball Cores and Methods of Making Same

The present disclosure relates generally to golf ball cores having a center and a skin with a hardness greater than the hardness of the center to produce a high hardness gradient across the core. More particularly, the present disclosure provides compositions and golf ball cores made from such compositions that have a high hardness gradient. The present disclosure also provides methods of making cores having a center and a skin with a hardness greater than the hardness of the center to produce a high hardness gradient. In some respects, the present disclosure relates to high gradient golf ball cores that, when used in golf balls, provide the ability to achieve one or more desired performance characteristics including, for example, reduced spin on driver shots.

Solid golf balls are typically made with a solid core encased by a cover, both of which can have multiple layers, such as a dual core having a solid center (or inner core) and an outer core layer, or a multi-layer cover having inner and outer cover layers. Generally, golf ball cores and/or centers are constructed with a thermoset rubber, such as a polybutadiene-based composition.

Thermoset rubbers are heated and crosslinked in a variety of processing steps to create a golf ball core having certain desirable characteristics, such as higher or lower compression or hardness, that can impact the spin rate of the ball and/or provide better “feel.” These and other characteristics can be tailored to the needs of golfers of different abilities. For example, professional and highly skilled amateur golfers can place a back spin more easily on balls having a relatively high spin rate, which helps better control the ball and improves shot accuracy and placement. On the other hand, recreational players who cannot intentionally control the spin of the ball when hitting it with a club are less likely to use high spin balls. Due to the nature of thermoset materials and the heating/curing cycles used to form the materials into cores, manufacturers can achieve varying properties across the core (i.e., from the core surface to the center of the core).

In a conventional, polybutadiene-based core, the physical properties of the molded core are highly dependent on the curing cycle (i.e., the time and temperature that the core is subjected to during molding). This time and temperature history, in turn, is inherently variable throughout the core, with the center of the core being exposed to a different time/temperature (i.e., shorter time at a different temperature) than the surface (because of the time it takes for heat to transfer to the center of the core) allowing a property gradient to exist at points between the center and core surface. This physical property gradient is readily measured as a hardness gradient.

The prior art contains a number of references that discuss “hard-to-soft” hardness gradients across a thermoset golf ball core. The “hard-to-soft” hardness gradients are typically in the range of 5 to 30 Shore C. While these hardness gradients can help reduce the spin rate of the golf ball, it would be advantageous to design a core having a greater hardness gradient between the center and core surface than the gradients currently used in the art such that the spin rate of the golf ball can be further reduced while maintaining sufficient impact durability and resilience.

The problems expounded above, as well as others, are addressed by the following inventions, although it is to be understood that not every embodiment of the inventions described herein will address each of the problems described above.

In some embodiments, the present disclosure provides a golf ball including a core and a cover layer disposed over the core, the core having a center and a skin disposed about the center, wherein the core is formed from a rubber formulation cured under infrared radiation, the rubber formulation including a base rubber, a water releasing agent including a metal salt hydrate. In further embodiments, the skin may have a hardness that is at least 2 Shore C harder than any point in the center.

In one embodiment, the skin has a depth that is between about 0.005 and 0.04 inches. In another embodiment, the skin has a depth that is less than 0.01 inch. In yet another embodiment, the skin hardness is at least 5 Shore C harder than any point in the center. In a further embodiment, the water releasing agent is present in the rubber composition in an amount of about 1 to about 3 parts per hundred rubber.

In still another embodiment, the center further includes a geometric center having a first hardness, the skin includes a surface having a second hardness greater than the first hardness, and the first hardness ranges from about 50 Shore C to about 85 Shore C and the second hardness ranges from about 65 Shore C to about 100 Shore C. In another embodiment, the core possesses a hardness gradient substantially equal to the difference in the first hardness and the second hardness, wherein the hardness gradient is at least 30 Shore C. In still another embodiment, the rubber formulation further includes a compression agent. In a further embodiment, the base rubber comprises polybutadiene rubber, butyl rubber, EPDM rubber, or a blend thereof. In yet another embodiment, the cover layer comprises a material selected from the group consisting of polyurethanes, polyureas, and hybrids, copolymers, and blends thereof.

In other embodiments, the present disclosure provides a golf ball including a multi-layer core and a cover layer disposed about the core, the core including a center and an outer core layer disposed about the center, the outer core layer having a skin forming an exterior of the outer core layer and having a hardness that is at least 2 Shore C harder than any point in the center, wherein the outer core layer is formed from a rubber formulation cured under infrared radiation, the rubber formulation comprising a base rubber and a water releasing agent.

In one embodiment, the skin has a depth that is between about 0.005 and 0.04 inches. In another embodiment, the skin has a depth that is less than 0.01 inch. In a further embodiment, the center further comprises a geometric center having a first hardness, the skin comprises a surface having a second hardness greater than the first hardness, the core possesses a hardness gradient substantially equal to the difference in the first hardness and the second hardness, and the hardness gradient is at least 30 Shore C In still other embodiments, the present disclosure provides a golf ball including a core and a cover layer disposed about the core, the core including: a center having a geometric center hardness H, a skin disposed about the center and including a surface having a surface hardness H, and a hardness gradient Hequal to the difference between Hand H, wherein the core is formed from a rubber formulation comprising a base rubber and a metal salt hydrate having a water releasing agent concentration, WRA, and waters of hydration, WRA, wherein His equal to or greater than 30 Shore C, wherein His at least 2 Shore C harder than any point in the center, and wherein

In another embodiment,

In yet another embodiment,

In still another embodiment,

In a further embodiment,

In yet another embodiment, the skin has a depth that is between about 0.005 and 0.04 inches.

In further embodiments, the present disclosure provides a method of forming a golf ball core including the steps of: providing a core comprising a rubber formulation comprising a base rubber and a water releasing agent, wherein the water releasing agent is a metal salt hydrate, and heating the core using infrared radiation, wherein after the step of heating the core using infrared radiation, the golf ball core comprises a skin having a hardness that is at least 2 Shore C harder than the rest of the core. In one embodiment, the step of heating the golf ball core using infrared radiation further comprises heating the core at a temperature between about 150° F. to about 560° F. for about 0.5 to about 5 minutes. In another embodiment, the step of forming the rubber formulation into the core further comprises compression molding the rubber formulation into the core at a temperature of about 300° F. to about 350° F. for about 4 to about 9 minutes. In still another embodiment, the skin has a depth that is between about 0.005 and 0.04 inches thick. In a further embodiment, the depth of the skin is less than 0.01 inch. In yet another embodiment, the skin hardness is at least 5 Shore D harder than the rest of the core.

The present disclosure relates to compositions and methods that may be used to produce a core with a high hardness gradient, cores including such compositions and/or made using such methods that possess a high hardness gradient, and golf balls including such cores. In some respects, the high hardness gradient cores made in accordance with the present disclosure provides the ability to reduce driver spin when compared to a conventional golf ball hit under the same conditions. In addition, the high hardness gradient cores made in accordance with the present disclosure may be used in golf balls to provide reduced shot dispersion on long shots as well as greater control on approach shots and greenside play.

While the golf ball core is functionally different from the other layers of the golf ball and operates somewhat independently, cores formed in accordance with the present disclosure greatly influence the overall performance of the finished golf ball including such a core. Without being bound by any particular theory, since a core typically represents about 90 percent of the golf ball weight, performance characteristics of a finished golf ball that contains the core of the present disclosure may be tailored by changing the core formulation and/or the curing process. For example, altering the core formulation and/or curing process and, thus, the hardness gradient, may have a significant effect on long shots, e.g., shots off of a driver, and approach shots, e.g., shots made with irons and wedges. In fact, adjusting the hardness gradient of cores made in accordance with this present disclosure, even in relatively small amounts, can significantly affect how a golf ball performs on long and short distance shots. Similarly, adjusting the hardness gradient of the core may allow for tailoring of other properties of the finished golf ball. The core formulations, cores, golf balls, and resulting performance characteristics are discussed in greater detail below.

The present disclosure provides golf balls having single- or multi-layered cores made from core formulations that result in high hardness gradients across the core. In some embodiments, the core formulations of the present disclosure include a base rubber, a water releasing agent, a cross-linking agent, and a free radical initiator. The core formulations may also include a hardening agent and/or a compression agent. The core formulations may also optionally include additives, such as one or more of a metal oxide, metal fatty acid or fatty acid, antioxidant, soft and fast agent, or fillers. Concentrations of components are in parts per hundred (phr) unless otherwise indicated. As used herein, the term, “parts per hundred,” also known as “phr” or “pph” is defined as the number of parts by weight of a particular component present in a mixture, relative to 100 parts by weight of the polymer component. Mathematically, this can be expressed as the weight of an ingredient divided by the total weight of the polymer, multiplied by a factor of 100.

As briefly discussed above, the core formulations of the present disclosure include a base rubber. In some embodiments, the base rubber may include natural and synthetic rubbers and combinations of two or more thereof. Examples of natural and synthetic rubbers suitable for use as the base rubber include, but are not limited to, polybutadiene, polyisoprene, ethylene propylene rubber (EPR), ethylene-propylene-diene (EPDM) rubber, grafted EPDM rubber, styrene-butadiene rubber, styrenic block copolymer rubbers (such as “SI”, “SIS”, “SB”, “SBS”, “SIBS”, and the like, where “S” is styrene, “I” is isobutylene, and “B” is butadiene), polyalkenamers such as, for example, polyoctenamer, butyl rubber, halobutyl rubber, polystyrene elastomers, polyethylene elastomers, polyurethane elastomers, polyurea elastomers, metallocene-catalyzed elastomers and plastomers, copolymers of isobutylene and p-alkylstyrene, halogenated copolymers of isobutylene and p-alkylstyrene, copolymers of butadiene with acrylonitrile, polychloroprene, alkyl acrylate rubber, chlorinated isoprene rubber, acrylonitrile chlorinated isoprene rubber, and combinations of two or more thereof.

For example, the core may be formed from a core formulation that includes polybutadiene as the base rubber. Polybutadiene is a homopolymer of 1,3-butadiene. The double bonds in the 1,3-butadiene monomer are attacked by catalysts to grow the polymer chain and form a polybutadiene polymer having a desired molecular weight. Any suitable catalyst may be used to synthesize the polybutadiene rubber depending upon the desired properties. In one embodiment, a transition metal complex (for example, neodymium, nickel, or cobalt) or an alkyl metal such as alkyl lithium is used as a catalyst. Other catalysts include, but are not limited to, aluminum, boron, lithium, titanium, and combinations thereof. The catalysts produce polybutadiene rubbers having different chemical structures. In a cis-bond configuration, the main internal polymer chain of the polybutadiene appears on the same side of the carbon-carbon double bond contained in the polybutadiene. In a trans-bond configuration, the main internal polymer chain is on opposite sides of the internal carbon-carbon double bond in the polybutadiene. The polybutadiene rubber can have various combinations of cis- and trans-bond structures. For example, the polybutadiene rubber may have a 1,4 cis-bond content of at least 40 percent. In another embodiment, the polybutadiene rubber has a 1,4 cis-bond content of greater than 80 percent. In still another embodiment, the polybutadiene rubber has a 1,4 cis-bond content of greater than 90 percent. In general, polybutadiene rubbers having a high 1,4 cis-bond content have high tensile strength and rebound.

The polybutadiene rubber may have a relatively high or low Mooney viscosity. Generally, polybutadiene rubbers of higher molecular weight and higher Mooney viscosity have better resiliency than polybutadiene rubbers of lower molecular weight and lower Mooney viscosity. However, as the Mooney viscosity increases, the milling and processing of the polybutadiene rubber generally becomes more difficult. Blends of high and low Mooney viscosity polybutadiene rubbers may be prepared as is described in U.S. Pat. Nos. 6,982,301 and 6,774,187, the disclosures of which are hereby incorporated by reference, and used in accordance with the present disclosure. In general, the lower limit of Mooney viscosity may be about 30 or 35 or 40 or 45 or 50 or 55 or 60 or 70 or 75 and the upper limit may be about 80 or 85 or 90 or 95 or 100 or 105 or 110 or 115 or 120 or 125 or 130. For example, the polybutadiene used in the core formulation may have a Mooney viscosity of about 30 to about 80 or about 40 to about 60.

Examples of commercially available polybutadiene rubbers that can be used in rubber formulations in accordance with the present disclosure, include, but are not limited to, BR 01 and BR 1220, available from BST Elastomers of Bangkok, Thailand; PR-040G, available from CHIMEI Corporation of Tainan City, Taiwan; SE BR 1220LA and SE BR1203, available from DOW Chemical Co of Midland, Mich.; BUDENE 1207, 1207s, 1208, and 1280 available from Goodyear, Inc of Akron, Ohio; BR 01, 51 and 730, available from Japan Synthetic Rubber (JSR) of Tokyo, Japan; BUNA CB 21, CB 22, CB 23, CB 24, CB 25, CB 29 MES, CB 60, CB Nd 60, CB 55 NF, CB 70 B, CB KA 8967, and CB 1221, available from Lanxess Corp. of Pittsburgh. Pa.; BR1208, available from LG Chemical of Seoul, South Korea; UBEPOL BR130B, BR150, BR150B, BR150L, BR230, BR360L, BR710, and VCR617, available from UBE Industries, Ltd. of Tokyo, Japan; EUROPRENE NEOCIS BR 60, INTENE 60 AF and P30AF, and EUROPRENE BR HV80, available from Polimeri Europa of Rome, Italy; KBR 01, NdBr 40, NdBR-45, NdBr 60, KBR 710S, KBR 710H, and KBR 750, available from Kumho Petrochemical Co., Ltd. Of Seoul, South Korea; DIENE 55NF, 70AC, and 320 AC, available from Firestone Polymers of Akron, Ohio; and PBR-Nd Group II and Group III, available from Nizhnekamskneftekhim, Inc. of Nizhnekamsk, Tartarstan Republic.

In another embodiment, the core is formed from a rubber formulation including butyl rubber. Butyl rubber is an elastomeric copolymer of isobutylene and isoprene. Butyl rubber is an amorphous, non-polar polymer with good oxidative and thermal stability, good permanent flexibility, and high moisture and gas resistance. Generally, butyl rubber includes copolymers of about 70 percent to about 99.5 percent by weight of an isoolefin, which has about 4 to 7 carbon atoms, for example, isobutylene, and about 0.5 percent to about 30 percent by weight of a conjugated multiolefin, which has about 4 to 14 carbon atoms, for example, isoprene. The resulting copolymer contains about 85 percent to about 99.8 percent by weight of combined isoolefin and about 0.2 percent to about 15 percent of combined multiolefin. A commercially available butyl rubber suitable for use in rubber formulations in accordance with the present disclosure includes Bayer Butyl 301 manufactured by Bayer AG.

The rubber formulations may include a combination of two or more of the above-described rubbers as the base rubber. In some embodiments, the rubber formulation of the present disclosure includes a blend of different polybutadiene rubbers. In this embodiment, the rubber formulation may include a blend of a first polybutadiene rubber and a second polybutadiene rubber in a ratio of about 5:95 to about 95:5. For example, the rubber formulation may include a first polybutadiene rubber and a second polybutadiene rubber in a ratio of about 10:90 to about 90:10 or about 15:85 to about 85:15 or about 20:80 to about 80:20 or about 30:70 to about 70:30 or about 40:60 to about 60:40. In other embodiments, the rubber formulation may include a blend of more than two polybutadiene rubbers or a blend of polybutadiene rubber(s) with any of the other elastomers discussed above.

In other embodiments, the rubber formulation used to form the core includes a blend of polybutadiene and butyl rubber. In this embodiment, the rubber formulation may include a blend of polybutadiene and butyl rubber in a ratio of about 10:90 to about 90:10. For example, the rubber formulation may include a blend of polybutadiene and butyl rubber in a ratio of about 10:90 to about 90:10 or about 20:80 to about 80:20 or about 30:70 to about 70:30 or about 40:60 to about 60:40. In other embodiments, the rubber formulation may include polybutadiene and/or butyl rubber in a blend with any of the other elastomers discussed above.

In further embodiments, the rubber formulation used to form the core may include EPDM or grafted EPDM as the base rubber. The EPDM rubber may be any commercially available EPDM rubber, for example, Dow Nordel® IP 5565 EPDM. In still further embodiments, the core formulations may include a combination of EPDM rubber and grafted EPDM rubber as the base rubber. In other embodiments, the rubber formulation may include a blend of polybutadiene rubber or butyl rubber and EPDM rubber and/or grafted EPDM rubber as the base rubber. In still further embodiments, the rubber formulation may include a blend of polybutadiene rubber or butyl rubber and EPDM rubber as the base rubber. In this embodiment, the rubber formulation may include a blend of EPDM rubber and polybutadiene rubber or butyl rubber in a ratio of about 10:90 to about 90:10. For example, the rubber formulation may include a blend of EPDM rubber and polybutadiene rubber or butyl rubber in a ratio of about 10:90 to about 90:10 or about 20:80 to about 80:20 or about 30:70 to about 70:30 or about 40:60 to about 60:40. In still further embodiments, the base rubber may include a blend of EPDM rubber and two or more different other types of rubber, such as a combination of polybutadiene rubber and butyl rubber or two or more different types polybutadiene rubber or butyl rubber.

The rubber formulations include the base rubber in an amount of 100 phr. That is, when more than one rubber component is used in the rubber formulation as the base rubber, the sum of the amounts of each rubber component should total 100 phr. In some embodiments, the rubber formulations include polybutadiene rubber as the base rubber in an amount of 100 phr. In other embodiments, the rubber formulations include polybutadiene rubber and a second rubber component. In one such embodiment, the polybutadiene rubber may be used in an amount of about 40 to about 99 parts by weight per 100 parts of the total rubber and the second rubber component may be used in an amount of about 1 to about 60 parts by weight per 100 parts of the total rubber. In another embodiment, the polybutadiene rubber may be used in an amount of about 80 to about 98 parts by weight per 100 parts of the total rubber and the second rubber component may be used in an amount of about 2 to about 20 parts by weight per 100 parts of the total rubber. In still another embodiment, the polybutadiene rubber may be used in an amount of about 85 to about 97 parts by weight per 100 parts of the total rubber and the second rubber component may be used in an amount of about 3 to about 15 parts by weight per 100 parts of the total rubber. In yet another embodiment, the polybutadiene rubber may be used in an amount of about 90 to about 99 parts by weight per 100 parts of the total rubber and the second rubber component may be used in an amount of about 1 to about 10 parts by weight per 100 parts of the total rubber. In a further embodiment, the polybutadiene rubber may be used in an amount of about 94 to about 96 parts by weight per 100 parts of the total rubber and the second rubber component may be used in an amount of about 4 to about 6 parts by weight per 100 parts of the total rubber. In some embodiments, the second rubber component is EPDM rubber.

The base rubber may be used in the rubber formulation in an amount of at least about 5 percent by weight based on total weight of the rubber formulation. In some embodiments, the base rubber is included in the rubber formulation in an amount within a range having a lower limit of about 10 percent or 20 percent or 30 percent or 40 percent or 50 percent or 55 percent and an upper limit of about 60 percent or 70 percent or 80 percent or 90 percent or 95 percent or 100 percent. For example, the base rubber may be present in the rubber formulation in an amount of about 30 percent to about 80 percent by weight based on the total weight of the rubber formulation. In another example, the rubber formulation includes about 40 percent to about 70 percent base rubber based on the total weight of the rubber formulation.

The rubber formulations of the present disclosure include a hardening agent. Without being bound to any particular theory, the hardening agent may affect the hardness of the core and the hardness gradient across the core. Suitable hardening agents include, but are not limited to, benzoic compounds comprising a nitro functional group and one of a hydroxyl, amino, or sulfhydryl functional group. Nonlimiting examples of hardening agents include nitrophenol, nitroaniline, and nitrothiophenol. Different isomers of the hardening agent may be used such as, for example, 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitroaniline, 3-nitroaniline, 4-nitroaniline, 2-nitrothiophenol, 3-nitrothiophenol, 4-nitrothiophenol, and combinations thereof. Without being bound by any particular theory, different isomers of the hardening agent may affect the hardness of the core differently and produce different hardness gradients across the core. Some hardening agents, such as nitrophenol, may be advantageous because they are safe and/or easy to handle during manufacturing.

The hardening agent may be included in the rubber formulation in varying amounts depending on the desired characteristics of the golf ball core. For example, the hardening agent may be used in an amount of 0.01 to about 3 parts by weight per 100 parts of the total rubber. In one embodiment, the rubber formulation of the core includes about 0.05 to about 1.5 or about 0.1 to about 1 or about 0.1 to 0.5 parts by weight hardening agent per 100 parts of the total rubber. In another embodiment, the hardening agent is included in the rubber formulation in an amount of about 0.2 to about 0.7 parts by weight per 100 parts of the total rubber. In still another embodiment, the rubber formulation includes about 0.05 to about 0.3 or 0.2 to about 0.4 or about 0.3 to about 0.5 or about 0.4 to about 0.6 parts by weight hardening agent per 100 parts of the total rubber.

In some respects, the amount of hardening agent in the rubber formulation required to produce the desired hardness gradient may differ based on the compound, and even the particular isomer of the compound, used as the hardening agent. For example, when the rubber formulation includes 2-nitrophenol, which has a nitro functional group ortho to a hydroxyl functional group, the hardening agent may be used in an amount of about 0.1 to about 0.3 parts by weight per 100 parts of the total rubber to achieve the desired hardness gradient. In other embodiments, when the rubber formulation includes 3-nitrophenol, which has a nitro functional group meta to a hydroxyl functional group, the hardening agent may be used in an amount of about 0.2 to about 0.4 parts by weight per 100 parts of the total rubber to achieve the desired hardness gradient. In further embodiments, when the rubber formulation includes 4-nitrophenol, which has a nitro functional group para to a hydroxyl functional group, the hardening agent may be used in an amount of about 0.3 to about 0.5 parts by weight hardening agent per 100 parts of the total rubber to achieve the desired hardness gradient. Without being bound by any particular theory, the relative positions of the functional groups on disubstituted benzoic hardening agents are believed to influence the effectiveness of the compound as a hardening agent. Accordingly, the amount of hardening agent needed to produce a desired hardness gradient may change when different isomers within a class of compounds are used.

The rubber formulations of the present disclosure may include a compression agent. Without being bound to any particular theory, including a compression agent in the rubber formulation of a golf ball core may increase the compression and COR of the golf ball core as well as the hardness gradient of the golf ball core. The compression agent may also be aromatic or aliphatic in nature. As discussed in greater detail below, some compression agents may have increased activity when the rubber formulation also includes a water releasing agent.

In some embodiments, the compression agent is a polyfunctional isocyanate. A polyfunctional isocyanate is an organic compound having two or more isocyanate (—NCO) groups per molecule and may include monomers, polymers, quasi prepolymers, or prepolymers. In some aspects, the compression agent may be a monomeric polyfunctional isocyanate, a polymeric polyfunctional isocyanate, or a blend thereof. The polyfunctional isocyanate may be aromatic or aliphatic in nature. Nonlimiting examples of polyfunctional isocyanates for use as the compression agent with the present invention include methylene diphenyl diisocyanate (“MDI”) and isomers thereof including 4,4′-diphenylmethane diisocyanate (“4,4′-MDI”), polymeric MDI, carbodiimide-modified liquid MDI, 4,4′-dicyclohexylmethane diisocyanate (“H12MDI”), polymethylene polyphenlisocyante containing MDI, p-phenylene diisocyanate (“PPDI”), toluene diisocyanate (“TDI”), 3,3′-dimethyl-4,4′-biphenylene diisocyanate (“TODI”), isophoronediisocyanate (“IPDI”), hexamethylene diisocyanate (“HDI”), naphthalene diisocyanate (“NDI”); xylene diisocyanate (“XDI”); p-methylxylene diisocyanate; m-methylxylene diisocyanate; o-methylxylene diisocyanate; para-tetramethylxylene diisocyanate (“p-TMXDI”); meta-tetramethylxylene diisocyanate (“m-TMXDI”); ethylene diisocyanate; propylene-1,2-diisocyanate; tetramethylene-1,4-diisocyanate; cyclohexyl diisocyanate; 1,6-hexamethylene-diisocyanate (“HDI”); dodecane-1,12-diisocyanate; cyclobutane-1,3-diisocyanate; cyclohexane-1,3-diisocyanate; cyclohexane-1,4-diisocyanate; 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane; methyl cyclohexylene diisocyanate; triisocyanate of HDI; triisocyanate of 2,4,4-trimethyl-1,6-hexane diisocyanate (“TMDI”); tetracene diisocyanate; naphthalene diisocyanate; anthracene diisocyanate; and combinations thereof. In one embodiment, the compression agent is a polymethylene polyphenylisocyanate containing MDI. A non-limiting example of a commercially available polyfunctional isocyanate includes PAPI® 94 manufactured by The Dow Chemical Company.

The compression agent may be included in the rubber formulation in varying amounts depending on the desired characteristics of the golf ball core. For example, the compression agent may be used in an amount of 0.1 to about 10 parts by weight per 100 parts of the total rubber. In one embodiment, the rubber formulation of the core includes about 0.2 to about 7 parts by weight polyfunctional isocyanate per 100 parts of the total rubber. In another embodiment, the polyfunctional isocyanate is included in the rubber formulation in an amount of about 0.5 to about 5 or about 0.5 to about 2 or about 0.7 to about 1.3 parts by weight per 100 parts of the total rubber. In yet another embodiment, the rubber formulation includes 0.6 to about 0.9 or about 0.8 to about 1.2 or about 1.1 to about 1.6 parts by weight polyfunctional isocyanate per 100 parts of the total rubber.

The core rubber formulations of the present disclosure include a water releasing agent. A “water releasing agent,” as used herein, refers to a compound having at least one water molecule available for release during the curing process. When the free radical initiator decomposes to generate decomposition heat at the time of curing of the core, the temperature near the surface of the core is kept substantially the same as the temperature of the mold, while the temperature near the center of the core increases because of the accumulated decomposition heat of the free radical initiator. Without being bound by any particular theory, it is believed that, by adding a water releasing agent to the core rubber formulation that can release water at the desired curing temperature, the water can promote further decomposition of the free radical initiator and deactivation of radicals at the center of the core, which, in turn, results in a difference in crosslinking density and an increased hardness gradient between the center and the surface.

The water releasing agent of the present disclosure has a moisture content capable of releasing a sufficient amount of water to promote decomposition of the free radical initiator and deactivation of radicals during the curing process. The moisture content in the water releasing agent can be calculated as the mass of water in the water releasing agent divided by the total weight of the water releasing agent. In some embodiments, the water releasing agent has a moisture content (in its molecular form) of at least about 5 percent by mass. In further embodiments, the water releasing agent has a moisture content ranging from about 5 percent by mass to about 95 percent by mass. In still further embodiments, the water releasing agent has a moisture content ranging from about 10 percent by mass to about 90 percent by mass. In yet further embodiments, the water releasing agent has a moisture content ranging from about 15 percent by mass to about 85 percent by mass. In further embodiments, the water releasing agent has a moisture content of at least about 50 percent by mass. For example, the water releasing agent may have a moisture content of about 50 percent by mass to about 95 percent by mass.

In some embodiments, the water releasing agent of the present disclosure may be a metal sulfate hydrate having one or more waters of hydration capable of being released during the reactions of the present disclosure. In one embodiment, the metal may be an alkaline earth metal. For example, the metal may be calcium, magnesium, beryllium, strontium, barium, or radium. In another embodiment, the metal of the metal sulfate hydrate is calcium. In yet another embodiment, the metal of the metal sulfate hydrate is magnesium. In further embodiments, the metal may be a transition metal or a post-transition metal. For instance, the metal may be zinc, copper, iron, cobalt, manganese, chromium, nickel, aluminum, zirconium, cadmium, indium, or vanadium. In still further embodiments, the metal may be neodymium or lanthanum.

The metal sulfate hydrate may have any number of waters of hydration. In some embodiments, the metal sulfate hydrate may have from 0.5 to ten waters of hydration. For instance, the metal sulfate hydrate may be a hemihydrate, monohydrate, dihydrate, trihydrate, tetrahydrate, pentahydrate, hexahydrate, heptahydrate, octahydrate, nonahydrate, or decahydrate. In further embodiments, the metal sulfate hydrate may have from one to seven waters of hydration. In still further embodiments, the metal sulfate hydrate may have from one to four waters of hydration. In yet further embodiments, the metal sulfate hydrate may have from one to three waters of hydration. In other embodiments, the metal sulfate hydrate may have two waters of hydration. For example, in one embodiment, the metal sulfate hydrate may be a dihydrate. In still further embodiments, the metal sulfate hydrate may be a heptahydrate (i.e., having seven waters of hydration).

Examples of suitable metal sulfate hydrates contemplated for use as the water releasing agent in accordance with the present disclosure include, but are not limited to, calcium sulfate hemihydrate (CaSO4·0.5H2O) calcium sulfate dihydrate (CaSO4·2H2O), magnesium sulfate heptahydrate (MgSO4·7H2O), zinc sulfate dihydrate (ZnSO4·2H2O), zinc sulfate heptahydrate (ZnSO4·7H2O), vanadium oxide sulfate hydrate (VOSO4·xH2O), neodymium sulfate hydrate (Nd2(SO4)3·xH2O), lanthanum oxalate hydrate (La2(C2O4)3·xH2O), zinc sulfate monohydrate (ZnSO4·H2O), zirconium sulfate hydrate (Zr(SO4)2·xH2O), beryllium sulfate tetrahydrate (BeSO4·4H2O), manganese sulfate hydrate (MnSO4·xH2O), iron sulfate hydrate (FeSO4·xH2O), cobalt sulfate hydrate (CoSO4·xH2O), cadmium sulfate monohydrate (CdSO4·H2O), cadmium sulfate octahydrate (CdSO4·8H2O), indium sulfate nonahydrate (InSO4·9H2O), nickel sulfate heptahydrate (NiSO4·7H2O), nickel sulfate hexahydrate (NiSO4·6H2O), aluminum sulfate hydrate (Al2(SO4)3·xH2O), and copper sulfate pentahydrate (CuSO4·5H2O). In some embodiments, the water releasing agent is a metal sulfate dihydrate such as calcium sulfate dihydrate and zinc sulfate dihydrate.

The water releasing agent may be included in the rubber formulation in varying amounts depending on the desired characteristics of the golf ball core. For example, the water releasing agent may be used in an amount of 0.1 to about 15 or about 0.5 to about 10 or about 1 to about 8 parts by weight water releasing agent per 100 parts of the total rubber. In one embodiment, the rubber formulation of the core includes about 0.2 to about 6 parts by weight water releasing agent per 100 parts of the total rubber. In some embodiments, the water releasing agent is included in the rubber formulation in an amount of about 2 to about 10 or about 3 to about 8 or about 3 to about 6 or about 5 to about 8 parts by weight water releasing agent per 100 parts of the total rubber. In other embodiments, the water releasing agent is included in the rubber formulation in an amount of about 0.5 to about 4 or about 0.7 to about 3.5 or about 1 to about 3 parts by weight water releasing agent per 100 parts of the total rubber. In yet further embodiments, the rubber formulation includes about 0.7 to about 1.4 or about 1.3 to about 1.7 or about 1.6 to about 2.3 or about 2.2 to about 3 parts by weight water releasing agent per 100 parts of the total rubber. For example, in some embodiments, the water releasing agent is present in the core rubber formulation in an amount of about 2 parts by weight water releasing agent per 100 parts of the total rubber.

In some respects, the amount of water releasing agent in the rubber formulation may differ based on the compound used as the water releasing agent. In particular, the amount of water releasing agent included may depend on the moisture content and moisture availability in the water releasing agent. When the water releasing agent has a greater moisture content, less water releasing agent may be needed in the rubber formulation to give the golf ball core the desired performance characteristics. For example, more water releasing agent may be required when the water releasing agent is calcium sulfate dihydrate, which has a moisture content of approximately 21 percent, than if water releasing agent is magnesium sulfate heptahydrate, which has a moisture content of about 51 percent. The moisture availability refers to how readily the water molecules dissociate from the water releasing agent. If the water releasing agent used has a high moisture availability, less water releasing agent may be needed than if the water releasing agent has a low moisture availability. Further, the amount of water releasing agent in the rubber formulation may differ based on the amount, type, or both of the hardening agent or the compression agent.

The core formulations may include two or more of any of the water releasing agents described above. For example, the core formulations may include two or more of any of the metal sulfate hydrates described above. In some embodiments, it may be desirable to formulate the rubber formulation of the golf ball core based on the water content of the rubber formulation instead of or in addition to the concentration of the water releasing agent. In this aspect, the water content of the rubber formulation can be determined by multiplying the concentration of water releasing agent in parts by weight per 100 parts of the total rubber by the moisture content of the water releasing agent. For example, if the water releasing agent is included in the rubber formulation at about 2 parts by weight per 100 parts of the total rubber and the moisture content in the water releasing agent is 50 percent, the water content of the rubber formulation is 1 part by weight per 100 parts of the total rubber. As such, the desired or targeted water content of the rubber formulation may be used to determine the concentration (and/or type) of water releasing agent used.

In some embodiments, the rubber formulation has a water content of about 0.01 to about 10 or about 0.05 to about 7 or about 0.1 to about 5 parts by weight water per 100 parts of the total rubber. In other embodiments, the water content of the rubber formulation is about 0.1 to about 2.5 or about 0.2 to about 2 or about 0.5 to about 1.5 parts by weight per 100 parts of the total rubber. In other embodiments, the water content of the rubber formulation is about 0.05 to about 1 or about 0.1 to about 0.5 or about 0.2 to about 0.6 parts by weight per 100 parts of the total rubber. In further embodiments, the rubber formulation has a water content of about 1 to about 3 or about 1.5 to about 2.5 or about 1 to about 2 parts by weight per 100 parts of the total rubber.