Golf Balls Having Cores with Increased Hardness Gradient

This application is a continuation of U.S. patent application Ser. No. 18/615,578, filed Mar. 25, 2024, which is a continuation of U.S. patent application Ser. No. 17/839,654, filed Jun. 14, 2022, the entire disclosures of which are incorporated by reference herein.

The present disclosure relates generally to golf balls. More particularly, the present disclosure relates to golf balls having cores with an increased hardness gradient. The resulting golf balls have reduced spin and sufficient impact durability.

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 to get heat 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, a golf ball is provided, the golf ball including a solid core having an outer surface and a geometric center, wherein the solid core is formed from a rubber composition cured under heat, the rubber composition including a base rubber consisting of polybutadiene rubber, an organic peroxide, a cross-linking co-agent including a zinc salt of an acrylate, diacrylate, methacrylate, or dimethacrylate, a water releasing agent including a metal sulfate hydrate having one to four waters of hydration, wherein the water releasing agent is present in the rubber composition in an amount of about 1 phr to about 3.9 phr, wherein the geometric center and the outer surface each has a hardness and 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 80 Shore C to about 100 Shore C, and wherein the hardness of the outer surface is greater than the hardness of the geometric center to define a positive hardness gradient of at least 30 Shore C units; and a cover layer surrounding the solid core, wherein the cover layer includes an ionomer, polyurethanes, polyureas, polyurethane-urea hybrids, or copolymers and blends thereof.

The metal may be an alkaline earth metal. For example, the water releasing agent may be calcium sulfate dihydrate. In further embodiments, the solid core has a diameter ranging from about 1.39 inches to about 1.62 inches. In still further embodiments, the cover layer is formed from an ionomer, a thermoplastic polyurethane, or a castable polyurethane. In yet further embodiments, the solid core has a compression of about 10 to about 95. In further embodiments, the organic peroxide is dimethyl terbutyl peroxide, dicumyl peroxide, or combinations thereof, and the organic peroxide is present in the rubber composition in an amount of about 0.25 phr to about 2.5 phr.

In other embodiments, a golf ball is provided, the golf ball including a solid core having an outer surface and a geometric center, wherein the solid core is formed from a rubber composition cured under heat, the rubber composition including a base rubber consisting of polybutadiene rubber having a 1,4 cis-bond content of at least 90 percent, an organic peroxide, zinc diacrylate, a water releasing agent including a metal sulfate hydrate having one to four waters of hydration, wherein the water releasing agent is present in the rubber composition in an amount of about 1 phr to about 3 phr, an additive selected from zinc pentachlorothiophenol, zinc oxide, barium sulfate, or combinations thereof, 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 hardness gradient of 30 Shore C units to 50 Shore C units, wherein the amount of water releasing agent and the number of waters of hydration of the water releasing agent present in the rubber composition are related to the positive hardness gradient, according to equation (I):

where WRArepresents the amount of water releasing agent in parts per hundred; Hrepresents the positive hardness gradient (Shore C); and WRArepresents the number of waters of hydration in the water releasing agent; an inner cover layer surrounding the core, wherein the inner cover layer is formed of an ionomer; and an outer cover layer disposed about the inner cover layer, wherein the outer cover layer is formed of a polymer selected from the group consisting of polyurethanes, polyureas, polyurethane-urea hybrids, and copolymers and blends thereof.

The inner cover layer may be formed of a partially neutralized ionomer, a highly neutralized ionomer, or a combination thereof. In further embodiments, the outer cover is formed of a thermoplastic polyurethane. In still further embodiments, the solid core has a diameter of about 1.39 inches to about 1.62 inches. In yet further embodiments, the hardness of the outer surface is greater than the hardness of the geometric center to define a positive hardness gradient of at least 36 Shore C units. In still other embodiments, the inner cover layer has a thickness of about 0.010 inches to about 0.120 inches and the outer cover layer has a thickness of about 0.004 inches to about 0.080 inches. In further embodiments, the organic peroxide is present in the rubber composition in an amount of about 0.1 phr to about 2.5 phr, the zinc diacrylate is present in the rubber composition in an amount of about 10 phr to 45 phr, and the water releasing agent is present in the rubber composition in an amount of about 3 phr.

In still other embodiments, a golf ball is provided, the golf ball including a solid core having an outer surface and a geometric center, wherein the solid core is formed from a rubber composition cured under heat, the rubber composition consisting of: polybutadiene rubber, dicumyl peroxide, zinc oxide, zinc diacrylate, calcium sulfate dihydrate, zinc pentachlorothiophenol, and barium sulfate, wherein the calcium sulfate dihydrate is present in the rubber composition in an amount of about 1 phr to about 3 phr, 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 hardness gradient of at least 34 Shore C units; an inner cover layer surrounding the core; and an outer cover layer disposed about the inner cover layer.

The calcium sulfate dihydrate may be present in the rubber composition in an amount of about 3 phr. In further embodiments, the inner cover layer is formed of an ionomer and the outer cover layer is formed of a polymer selected from the group consisting of polyurethanes, polyureas, polyurethane-urea hybrids, and copolymers and blends thereof. In still further embodiments, the zinc pentachlorothiophenol is present in the rubber composition in an amount of about 0.1 phr to about 3 phr, the zinc oxide is present in the rubber composition in an amount of about 3 phr to about 10 phr, and the barium sulfate is present in the rubber composition in an amount of about 10 phr to about 30 phr. In yet further embodiments, the hardness of the outer surface is greater than the hardness of the geometric center to define a positive hardness gradient of about 34 Shore C units to about 40 Shore C units. In other embodiments, the polybutadiene rubber has a 1,4 cis-bond content of at least 90 percent.

The present disclosure provides golf balls having cores with increased hardness gradients. In some embodiments, the present disclosure provides golf balls having cores with a “positive” hardness gradient (or a “hard-to-soft” hardness) where the outer surface of the core is harder than the center. Without being bound by any particular theory, the inventors of the present disclosure have discovered that an increased “positive” hardness gradient can be achieved by introducing a water-releasing agent into the core rubber formulation during the curing process. It is believed that the water released during the curing process promotes the decomposition of the free radical initiator, which further promotes radical deactivation and reduces the number of radicals at the center of the core. This, in turn, leads to an increased hardness gradient between the surface and the center of the core. By increasing the hardness gradient from the outer surface of the core to the center, it is possible to reduce the spin rate of the golf ball and maintain sufficient durability.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art of this disclosure. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well known functions or constructions may not be described in detail for brevity or clarity.

The terms “about” and “approximately” shall generally mean an acceptable degree of error or variation for the quantity measured given the nature or precision of the measurements. Numerical quantities given in this description are approximate unless stated otherwise, meaning that the term “about” or “approximately” can be inferred when not expressly stated.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well (i.e., at least one of whatever the article modifies), unless the context clearly indicates otherwise.

The terms “first,” “second,” “third,” and the like are used herein to describe various features or elements, but these features or elements should not be limited by these terms. These terms are only used to distinguish one feature or element from another feature or element. Thus, a first feature or element discussed below could be termed a second feature or element, and similarly, a second feature or element discussed below could be termed a first feature or element without departing from the teachings of the present disclosure.

The term “positive hardness gradient” refers to the result of subtracting the hardness value at the innermost portion of the component being measured (for example, the center of a solid core or an inner core in a dual core construction) from the hardness value at the outer surface of the component being measured (for example, the outer surface of a solid core or the outer surface of an inner core in a dual core). For instance, if the outer surface of a solid core has a greater hardness value than the center, the hardness gradient will be deemed a “positive” gradient.

The term “parts per hundred,” also known as “phr,” 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 base rubber component.

The present disclosure provides golf balls having core formulations that result in increased “positive” hardness gradients across the core. In some embodiments, the core formulations of the present disclosure include a base rubber, a cross-linking agent, a free radical initiator, and a water releasing agent that is capable of releasing water into the rubber formulation during the curing process. 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.

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.

In some embodiments, the core formulations may include polybutadiene rubber as the base rubber. The polybutadiene rubber can have various combinations of cis- and trans-bond structures. In one embodiment, the polybutadiene rubber has a 1,4 cis-bond content of at least 40 percent. In other embodiments, the polybutadiene rubber has a 1,4 cis-bond content of at least 80 percent. In still other embodiments, the polybutadiene rubber has a 1,4 cis-bond content of at least 90 percent. In general, polybutadiene rubbers having a high 1,4 cis-bond content have high tensile strength.

The polybutadiene rubber may have a relatively high or low Mooney viscosity.

Examples of commercially-available polybutadiene rubbers that can be used in accordance with the present disclosure, include, but are not limited to, BR 01 and BR 1220, available from BST Elastomers of Bangkok, Thailand; 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; AFDENE 50 and NEODENE BR40, BR45, BR50 and BR60, available from Karbochem (PTY) Ltd. of Bruma, South Africa; 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 combinations of two or more thereof.

The core formulations may include a single base rubber or a combination of two or more of the above-described rubbers as the base rubber. In some embodiments, the core formulations may include polybutadiene rubber, such as high cis-1,4 polybutadiene, as the base rubber. In this embodiment, the core formulation may include a combination of two or more types of polybutadiene rubber, such as two or more different types of high cis-1,4 polybutadiene. In further embodiments, the core formulations may include EPDM rubber or grafted EPDM rubber as the base rubber. In still further embodiments, the core formulations may include a combination of polybutadiene rubber and EPDM rubber as the base rubber. For example, the core formulations may combine EPDM rubber and two or more different types of polybutadiene rubber, such as two or more different types of high cis-1,4 polybutadiene, as the base rubber.

In some embodiments, the core formulations include the base rubber in an amount of 100 phr. That is, when more than one rubber component is used in the core formulation as the base rubber, the sum of the amounts of each rubber component should total 100 phr. In some embodiments, the core formulations include polybutadiene rubber as the base rubber in an amount of 100 phr. In other embodiments, the core formulations include polybutadiene rubber and a second rubber component. In this embodiment, the polybutadiene rubber may be used in an amount of about 70 phr to about 99 phr and the second rubber component may be used in an amount of about 1 phr to about 30 phr. In still other embodiments, the polybutadiene rubber may be used in an amount of about 85 phr to about 95 phr and the second rubber component may be used in an amount of about 5 phr to about 15 phr. In some embodiments, the second rubber component is EPDM rubber.

The base rubber may be used in the core formulation in an amount of at least about 5 percent by weight based on total weight of composition. In some embodiments, the base rubber may be used in an amount of about 20 percent to about 95 percent by weight. In further embodiments, the base rubber may be used in an amount of about 45 percent to about 95 percent by weight. In still other embodiments, the base rubber may be used in an amount of at least about 50 percent by weight. In yet further embodiments, the base rubber may be used in an amount of at least about 70 percent by weight.

The core formulations include a reactive cross-linking co-agent. Suitable cross-linking co-agents include, but are not limited to, metal salts of unsaturated carboxylic acids having from 3 to 8 carbon atoms; unsaturated vinyl compounds and polyfunctional monomers (for example, trimethylolpropane trimethacrylate); phenylene bismaleimide; and combinations thereof. Examples of suitable metal salts include, but are not limited to, one or more metal salts of acrylates, diacrylates, methacrylates, and dimethacrylates, wherein the metal is selected from magnesium, calcium, zinc, aluminum, lithium, or nickel. In some embodiments, the cross-linking co-agent is selected from zinc salts of acrylates, diacrylates, methacrylates, or dimethacrylates. For example, in one embodiment, the cross-linking co-agent is zinc diacrylate (ZDA).

The cross-linking co-agent may be present in the core formulation in an amount of about 5 phr to about 50 phr. In some embodiments, the cross-linking co-agent may be present in the core formulation in an amount of about 10 phr to about 45 phr. In further embodiments, the cross-linking co-agent may be present in the core formulation in an amount of about 15 phr to about 40 phr. In still further embodiments, the cross-linking co-agent may be present in the core formulation in an amount of about 20 phr to about 35 phr. For example, in one embodiment, the cross-linking co-agent may be present in the core formulation in an amount of about 30 phr. In other embodiments, the cross-linking co-agent may be present in the core formulation in an amount of about 35.5 phr.

The core formulations may include a free radical initiator selected from an organic peroxide, a high energy radiation source capable of generating free radicals, or a combination thereof. In some embodiments, the free radical initiator is an organic peroxide. Suitable organic peroxides include, but are not limited to, dicumyl peroxide; n-butyl-4,4-di(t-butylperoxy) valerate; 1,1-di(t-butylperoxy) 3,3,5-trimethylcyclohexane; 2,5-dimethyl-2,5-di(t-butylperoxy) hexane; di-t-butyl peroxide; di-t-amyl peroxide; t-butyl peroxide; t-butyl cumyl peroxide; 2,5-dimethyl-2,5-di(t-butylperoxy) hexyne-3; di(2-t-butyl-peroxyisopropyl)benzene; dilauroyl peroxide; dibenzoyl peroxide; t-butyl hydroperoxide; dimethyl terbutyl peroxide blend; and combinations thereof. In one embodiment, the free radical initiator is dicumyl peroxide, including, but not limited to Perkadox® BC, commercially available from Akzo Nobel. In other embodiments, the free radical initiator is dimethyl terbutyl peroxide, including, but not limited to Trigonox® 101-50D-PD, commercially available from Nouryon.

The free radical initiator may be present in the core formulation in an amount of about 0.05 phr to about 15 phr. In some embodiments, the free radical initiator may be present in the core formulation in an amount of about 0.1 phr to about 10 phr. In other embodiments, the free radical initiator may be present in the core formulation in an amount of about 0.5 phr to about 6 phr. In still other embodiments, the free radical initiator may be present in the core formulation in an amount of about 1 phr to about 5 phr. In further embodiments, the free radical initiator may be present in the core formulation in an amount of about 1.5 phr to about 3 phr. In yet further embodiments, the free radical initiator is present in the core formulation in an amount of about 0.1 phr to about 2.5 phr. In still further embodiments, the free radical initiator is present in the core formulation in an amount of about 0.25 phr to about 1.5 phr. For example, the free radical initiator may be present in the core formulation in an amount of about 0.35 phr. In other embodiments, the free radical initiator may be present in the core formulation in an amount of about 0.6 phr. In still other embodiments, the free radical initiator may be present in the core formulation in an amount of about 1 phr.

The core 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. As briefly described above, 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 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. 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 has 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 one embodiment, the metal of the metal sulfate hydrate is calcium. In 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. In some embodiments, the metal of the metal sulfate hydrate is zinc.

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 dihydrate (CaSO·2HO), magnesium sulfate heptahydrate (MgSO·7HO), zinc sulfate dihydrate (ZnSO·2HO), zinc sulfate heptahydrate (ZnSO·7HO), vanadium oxide sulfate hydrate (VOSO·xHO), neodymium sulfate hydrate (Nd(SO)·xHO), lanthanum oxalate hydrate (La(CO)·xHO), zinc sulfate monohydrate (ZnSO·HO), zirconium sulfate hydrate (Zr(SO)·xHO), nickel sulfate heptahydrate (NiSO·7HO), nickel sulfate hexahydrate (NiSO·6HO), aluminum sulfate hydrate (Al(SO)·xHO), and copper sulfate pentahydrate (CuSO·5HO).

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, the water releasing agent is present in the core formulation in an amount of about 1 phr to about 15 phr. In other embodiments, the water releasing agent is present in the core formulation in an amount of about 2 phr to about 10 phr. In still further embodiments, the water releasing agent is present in the core formulation in an amount of about 3 phr to about 8 phr. In yet further embodiments, the water releasing agent is present in the core formulation in an amount of about 5 phr to about 7 phr. In still further embodiments, the water releasing agent is present in the core formulation in an amount of about 1 phr to about 4 phr. In further embodiments, the water releasing agent is present in the core formulation in an amount of about 1 phr to about 3.9 phr. In still further embodiments, the water releasing agent is present in the core formulation in an amount of about 1 phr to about 3.75 phr. In yet further embodiments, the water releasing agent is present in the core formulation in an amount of about 1 phr to about 3 phr. For example, in some embodiments, the water releasing agent is present in the core formulation in an amount of about 3 phr.

Radical scavengers, such as a halogenated organosulfur or metal salt thereof, organic disulfide, or inorganic disulfide compound, may be added to the core formulations of the present disclosure. These compounds also may function as “soft and fast agents.” As used herein, a “soft and fast agent” means any compound or a blend thereof that is capable of making a core: 1) softer (having a lower compression) at a constant “coefficient of restitution” (CoR); and/or 2) faster (having a higher COR at equal compression), when compared to a core equivalently prepared without a soft and fast agent. Examples of halogenated organosulfur compounds that may be used with the core formulations include, but are not limited to, pentachlorothiophenol (PCTP) and salts of PCTP such as zinc pentachlorothiophenol (ZnPCTP). Using PCTP and ZnPCTP in golf ball inner cores helps produce softer and faster inner cores. The PCTP and ZnPCTP compounds help increase the resiliency and the coefficient of restitution of the core. In some embodiments, the soft and fast agent is selected from ZnPCTP, PCTP, ditolyl disulfide, diphenyl disulfide, dixylyl disulfide, 2-nitroresorcinol, or combinations thereof. In some embodiments, the soft and fast agent may be used in the core formulation in an amount of about 0.1 phr to about 3 phr. In further embodiments, the soft and fast agent may be used in the core formulation in an amount of about 0.2 phr to about 1 phr. For example, the soft and fast agent may be used in the core formulation in an amount of about 0.3 phr to about 0.35 phr.

The core formulations of the present disclosure also may include “fillers,” which are added to adjust the density and/or specific gravity of the formulation. Suitable fillers include, but are not limited to, polymeric or mineral fillers, metal fillers, metal alloy fillers, metal oxide fillers and carbonaceous fillers. The fillers can be in any suitable form including, but not limited to, flakes, fibers, whiskers, fibrils, plates, particles, and powders. Rubber regrind, which is ground, recycled rubber material obtained from discarded rubber golf ball cores, also can be used as a filler. The amount and type of fillers utilized are governed by the amount and weight of other ingredients in the golf ball, since a maximum golf ball weight of 45.93 g (1.62 ounces) has been established by the United States Golf Association (USGA).

Suitable polymeric or mineral fillers that may be added to the core formulations include, for example, precipitated hydrated silica, clay, talc, asbestos, glass fibers, aramid fibers, mica, calcium metasilicate, barium sulfate, zinc sulfide, lithopone, silicates, silicon carbide, tungsten carbide, diatomaceous earth, polyvinyl chloride, carbonates such as calcium carbonate and magnesium carbonate. Suitable metal fillers include titanium, tungsten, aluminum, bismuth, nickel, molybdenum, iron, lead, copper, boron, cobalt, beryllium, zinc, and tin. Suitable metal alloys include steel, brass, bronze, boron carbide whiskers, and tungsten carbide whiskers. Suitable metal oxide fillers include zinc oxide, iron oxide, aluminum oxide, titanium oxide, magnesium oxide, and zirconium oxide. Suitable particulate carbonaceous fillers include graphite, carbon black, cotton flock, natural bitumen, cellulose flock, and leather fiber. Micro balloon fillers such as glass and ceramic, and fly ash fillers can also be used. In some embodiments, the core formulations of the present disclosure include zinc oxide. The zinc oxide may be used in an amount ranging from about 1 phr to about 15 phr. In some embodiments, the zinc oxide may be used in an amount ranging from about 3 phr to about 10 phr, for example, about 5 phr. In further embodiments, the core formulations of the present disclosure include barium sulfate. The barium sulfate may be used in an amount ranging from about 10 phr to about 30 phr, for example, about 12 phr to about 14 phr.

The core formulations may also include antioxidants to prevent the breakdown of the elastomers. In addition, processing aids, such as high molecular weight organic acids and salts thereof, may be added to the formulations.

In some embodiments, the total amount of additive(s) and filler(s) present in the core formulation may be about 15 percent by weight or less, based on the total weight of the core formulation. In other embodiments, the total amount of additive(s) and filler(s) present in the core formulation may be about 12 percent by weight or less, based on the total weight of the core formulation. In still other embodiments, the total amount of additive(s) and filler(s) present in the core formulation may be about 10 percent by weight or less, based on the total weight of the core formulation. In further embodiments, the total amount of additive(s) and filler(s) present in the core formulation may be about 8 percent by weight or less, based on the total weight of the core formulation. In yet further embodiments, the total amount of additive(s) and filler(s) present in the core formulation may be about 5 percent by weight or less, based on the total weight of the core formulation.

The base rubber, free radical initiator, cross-linking agent, water releasing agent, fillers, and any other materials used in forming the core, in accordance with the present disclosure, may be combined to form a mixture by any type of mixing known to one of ordinary skill in the art.

Suitable types of mixing include single pass and multi-pass mixing, and the like. A single pass mixing process where ingredients are added sequentially is preferred, as this type of mixing tends to increase efficiency and reduce costs for the process. The formulation may be cured using any technique known in the art for rubber compositions for golf balls.

The core formulations of the present disclosure may be used with golf balls of various constructions. In one version, shown in, a golf ball of the present disclosure is a two-piece ballcomprising a single core layerand a single cover layer. As shown in, in one embodiment, the golf ballcomprises a core layer, an intermediate layer, and a cover layer. In, the intermediate layercan be considered an outer core layer, an inner cover layer, a mantle or casing layer, or any other layer disposed between the coreand the cover layer. Referring to, in another embodiment, a four-piece golf ballcomprises an inner core layer, an outer core layer, an intermediate layer, and an outer cover layer. In, the intermediate layermay be considered a casing or mantle layer, or inner cover layer, or any other layer disposed between the outer core layerand the outer cover of the ball. Referring to, in another version, a five-piece golf ballincludes a three-layered core having an inner core layer, an intermediate core layer, an outer core layer, an inner cover layer, and an outer cover layer. As exemplified herein, a golf ball in accordance with the present disclosure can comprise any combination of any number of core layers, intermediate layers, and cover layers.

The core formulations of the present disclosure may be used with single- or multi-layered cores. The core formulations may be used in one or more layers of the core. In one embodiment, the core formulations described herein may be used in a solid core of a golf ball. In other embodiments, the core formulations described herein may be used in a dual core having an inner core (center) and a surrounding outer core layer. In one embodiment, the inner core layer (center) may be formed of the core formulation of the present disclosure while the outer core layer may be formed of a rubber composition. In another embodiment, the outer core layer may be formed of the core formulation while the inner core layer may be formed of a rubber composition. In other embodiments, both the inner core layer and the outer core layer may be formed of the core formulations of the present disclosure. In still other embodiments, the core formulations described herein may be used in a multi-layered core having three or more layers. For example, the center of the core may be formed of the core formulation of the present disclosure while the other layers of the core may be formed of a rubber composition. In still other embodiments, two or more layers of the core may be formed of the core formulations of the present disclosure.

In some embodiments, when the golf ball core is a dual core or a multi-layered core, the surrounding outer core layer(s) may be formed of a polybutadiene rubber composition. The rubber compositions may contain any of the base rubbers, free radical initiators, cross-linking agents, soft and fast agents, additives, and fillers described above, and the composition may be cured using conventional methods as described above. In some embodiments, the surrounding outer core layer(s) may include a combination of polybutadiene rubber and styrene butadiene rubber (SBR) as the base rubber. In this embodiments, the polybutadiene rubber may be used in an amount of about 80 phr to about 99 phr and the SBR may be used in an amount of about 1 phr to about 20 phr. For example, in some embodiments, the surrounding outer core layer(s) may be formed of a rubber composition including polybutadiene rubber, SBR, dicumyl peroxide, regrind, zinc pentachlorothiophenol, zinc diacrylate, and zinc oxide.