WO1998008575A1

WO1998008575A1 - Multilayer golf ball and composition

Info

Publication number: WO1998008575A1
Application number: PCT/US1997/015120
Authority: WO
Inventors: Christopher Cavallaro; Murali Rajagopalan; Samuel A. Pasqua, Jr.; Herbert C. Boehm; Kevin M. Harris
Original assignee: Acushnet Company
Priority date: 1996-08-30
Filing date: 1997-08-28
Publication date: 1998-03-05
Also published as: GB9903981D0; AU4165697A; GB2331709B; JP2000502274A; GB2331709A; US5813923A; JP3217379B2

Abstract

The present invention is directed towards a multi-layer golf ball (1) which comprises a core (2) with at least one cover layer (3); and one or more mantle layers (4) disposed between the core (2) and cover layer (3), wherein the mantle layer (4) comprises thermoplastic co-polyether ester block copolymer, thermoplastic co-polyester ester block copolymer, dynamically vulcanized thermoplastic elastomer, functionalized styrene-butadiene elastomer, thermoplastic polyurethane, metallocene polymer or blends thereof, and thermoset materials. Further, the present invention is directed to a ball (1) having a core (2) having a PGA compression less than 60, mantle layer (4) having a flexural modulus of less than 10,000 psi, and the cover (3) having a flexuralmodulus greater than 75,000 psi.

Description

MULTILAYER GOLF BALL AND COMPOSITION This application is a continuation-in-part of co- pending U.S. patent application Serial Nos. 08/482,522, 08/603,057 and 08/606,373 filed June 7, 1995, February 16, 1996 and February 23, 1996, respectively.

FIELD OF THE INVENTION This invention relates generally to golf balls, and more specifically, to a multilayer golf ball. In particular, this invention relates to a golf ball having a core, a cover and one or more mantle layers disposed between the core and cover. This invention is also directed to a composition used for the manufacture of golf ball cores for the multilayer golf ball. The multilayer golf balls of the present invention have been found to provide the distance and durability characteristics approaching that of a conventional two piece ball while also providing good "click and feel" .

BACKGROUND OF THE INVENTION Conventional golf balls can be divided into two general types or groups: two piece balls or wound balls (also know as three piece balls) . The difference in play characteristics resulting from these different types of constructions can be quite significant. Balls having a two piece construction are generally most popular with the average recreational golfer because they provide a very durable ball while also providing maximum distance. Two piece balls are made with a single solid core, usually made of a crosslinked rubber, which is encased by a cover material. Typically the solid core is made of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents and is covered by a tough, cut-proof blended cover. The cover is generally a material such as SURLYN®, which is a trademark for an ionomer resin produced by DuPont. The combination of the core and cover materials provide a "hard" ball that is virtually indestructible by golfers. Further, such a

- l - combination imparts a high initial velocity to the ball which results in improved distance. Because these materials are very rigid, two piece balls have a hard "feel" when struck with a club. Likewise, due to their hardness, these balls have a relatively low spin rate which provides greater distance.

At the present time, the wound ball remains the preferred ball of the more advanced players due to its spin and feel characteristics. Wound balls typically have either a solid rubber or liquid center core around which many yards of a stretched elastic thread or yarn are wound. The wound core is then covered with a durable cover material such as a SURLYN® or similar material or a softer cover such as Balata or polyurethane. Wound balls are generally softer and provide more spin, which enables a skilled golfer to have more control over the ball's flight and final position. Particularly, with approach shots into the green, the high spin rate of soft, wound balls enables the golfer to stop the ball very near its landing position. Golf ball design and technology have now advanced to the point that the United States Golf Association (USGA) , the organization that sets the rules of golf in the United States, has instituted a rule that prohibits the competitive use in any USGA sanctioned event of a golf ball that can achieve an initial velocity of 76.2 meters per second (m/s), or 250 ft/s, when struck by a driver with a velocity of 39.6 m/s, i.e., 130 ft/s (referred to hereinafter as "the USGA test") . However, an allowed tolerance of two percent permits manufacturers to produce golf balls that achieve an initial velocity of 77.7 m/s (255 ft/s).

Manufacturers place a great deal of emphasis on producing golf balls that consistently achieve the highest possible velocity in the USGA test without exceeding the 77.7 m/s (255 ft/s) limit, which are available with a range of different properties and characteristics, such as spin, compression, "click," and "feel." Thus, a variety of different balls is available to meet the needs and desires of a wide range of golfers.

Regardless of the form of the ball, players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to produce golf balls with initial velocities in the USGA test that approximate the USGA maximum of 77.7 m/s or 255 ft/s as closely as possible. To meet the needs of golfers having varying levels of skill, golf ball manufacturers are also concerned with varying the level of the PGA compression of the ball, which is a measurement of the deformation of a golf ball or core in inches under a fixed load. Higher velocity on impact, and, hence, greater distance, can often be achieved by increasing compression, which influences the distance the ball travels or rolls, and may also generate a harder "feel" to the ball. However, because a golf ball must be fully compressed on impact to achieve maximum velocity and distance, amateur golfers, who may not be able to generate the required club head speed, cannot obtain the maximum distance from a high compression ball.

Therefore, golf ball manufacturers are continually searching for new ways in which to provide golf balls that deliver the maximum performance for golfers of all skill levels, and seek to discover compositions that provide the performance of a high compression ball in balls with the lower compression required by amateur golfers.

A number of polymers, such as polybutadiene, natural rubber, styrene butadiene, and isoprene, are commonly used in fabricating golf ball cores. Today, golf ball cores are predominantly made of polybutadiene. Moreover, in order to obtain the desired physical properties for golf balls, manufacturers have added cross-linking agents, such as metallic salts of an unsaturated carboxylic acid. The amount of cross-linking agent added is typically about 20 to 50 parts per hundred parts of polybutadiene. Most commonly, zinc diacrylate or zinc dimethacrylate are used for this purpose. Of these two cross-linkers, zinc diacrylate has been found to produce golf balls with greater initial velocity than zinc dimethacrylate. Typically, about 5 to 50 pph (parts per hundred) of zinc oxide (ZnO) is also added to the composition. This material serves as both a filler and an activation agent for the zinc diacrylate/peroxide cure system. The zinc diacrylate/peroxide cure system, which is well known to those of ordinary skill in this art, cross-links the polybutadiene during the core molding process. The high specific gravity of zinc oxide (5.57) can serve the dual purposes of adjusting the weight of the golf ball, in addition to acting as an activation agent. As zinc oxide is known to be an environmentally unfriendly material, it would be advantageous to eliminate or at least substantially reduce the amount of this material from the manufacturing process. However, when the zinc oxide is eliminated from the composition described above, there is a reduction in cure enhancement, which results in less cross-linking and a corresponding reduction in compression and velocity. This result provides a ball with a softer feel, and allows less skilled golfers to compress the ball fully, but the resulting ball has less than the maximum velocity allowed by the USGA standard.

Therefore, it would be advantageous to provide a golf ball core composition with an activation agent other than zinc oxide, i.e., wherein all or at least some of the zinc oxide commonly present was eliminated, which would, as noted above, provide a ball with a lower compression, but would maintain the velocity and distance of a high compression ball. The present invention provides such a golf ball core.

DESCRIPTION OF THE PRIOR ART

Several patents have been issued which are directed towards modifying the properties of a conventional two piece ball by altering the typical single layer core and single cover layer construction to provide a multi-layer core. The inventions disclosed in the prior art patents are directed towards improving a variety of golf ball characteristics. Several patents are directed towards improving the carry distance of the ball. For example, U.S. Patent No. 4,863,167 relates to a three piece solid golf ball having improved rebound characteristics in order to increase its flight distance. This golf ball has a center portion and the outer layer formed from a rubber composition, preferably having a base rubber of polybutadiene, wherein the outer layer further contains a gravity filler such as tungsten or tungsten carbide so as to impart a higher specific gravity to the outer layer than that of the inner layer. The difference in specific gravity of the layers should be 0.15-0.8 for small balls and 0.15-0.45 for large balls. Preferably, the outer layer is harder than the center portion.

U.S. Patent No. 5,184,828 relates to a solid three- piece golf ball having improved rebound characteristics and carry distance while maintaining an adequate spin rate.

These characteristics are obtained by controlling the size of the inner core and outer layer as well as the specific gravity and hardness. The core and mantle layers are made from a rubber compound such as polybutadiene, and have a Shore D hardness of 30-62 and 30-56 respectively. The key to obtaining the desired rebound characteristics is that the maximum hardness (42-62) must be located at the interface between the core and the mantle and the hardness must then decrease both inwardly and outwardly. U.S. Patent No. 4,714,253 is also directed towards a three-piece golf ball having an excellent rebound coefficient. This golf ball has a core with a Shore C hardness of 57-80 in its center, but not more than 83 at a distance between 5-10mm from its center and an outer layer with a Shore C hardness of 70-83.

Additionally, there are a number of patents also directed towards improving the spin, click and feel of solid balls while maintaining the distance provided by the solid construction. A variety of approaches to manipulating the core construction are described in the art. For example, U.S. Patent No. 5,072,944 discloses a three-piece solid golf ball having a center and outer layer which are prepared from a rubber composition, preferably having a base rubber of polybutadiene. It is desirable that the center core is softer than the outer layer, each having a hardness (Shore C) of 25-50 and 70-90 respectively. U.S. Patent No. 4,625,964 relates to a solid golf ball having a polybutadiene rubber core of a diameter not more than 32mm, and a polybutadiene rubber intermediate layer having a specific gravity of lower than that of the core material, and a cover. U.S. Patent No. 4,650,193 is directed towards a solid golf ball having a core comprising of a central portion and an integral outer layer. Preferably the core is a curable elastomer such as polybutadiene which is treated with a cure altering agent to soften an outer layer of the core, thereby producing a central layer with a hardness (Shore C) of greater than 75 and an outer layer with a hardness (Shore A) of less than 80.

U.S. Patent No. 4,848,770 discloses a non-wound three-piece golf ball which includes a core of a highly filled synthetic rubber or polymeric material, an intermediate mantle of an unfilled synthetic rubber and a cover. The core and intermediate mantle have a hardness of between 50-95.

U.S. Patent No. 5,002,281 is directed towards a three-piece solid golf ball which has an inner core having a hardness of 25-70 (Shore C) , an outer shell having a hardness of 80-95 (Shore C) and a cover. Further, the specific gravity of the inner core must be greater than 1.0, but less than or equal to that of the outer shell, which must be less than 1.3.

U.S. Patent No. 5,253,871 concerns a golf ball having a three piece structure comprising an elastomer core, an intermediate layer of a thermoplastic material containing at least 10% of ether block copolymer, preferably blended with an ionomer and a thermoplastic cover.

Further, there are also several patents which are directed to golf balls having multiple cover layers. For example U.S. Patent No. 4,431,193 relates to a golf ball having a multilayer cover wherein the inner layer is a hard, high flexural modulus ionomer resin and the outer layer is a soft, low flexural modulus ionomer resin, and wherein either or both layers may comprise a foamed ionomer resin.

U.S. Patent No. 5,314,187 also relates to golf balls having a multiple layer cover, wherein the outer layer is molded over the inner layer and comprises a blend of balata and an elastomer and the inner layer is an ionomer resin.

U.S. Patent No. 4,919,434 is directed towards a golf ball having a cover which comprises an inner layer and an outer layer each of which comprise a thermoplastic resin, preferably the layers comprise of materials that are capable of fusion bonding with each other.

However, none of these patents disclose a multilayer ball having the materials and material property requirements as disclosed herein to provide the improved golf balls of the present invention.

SUMMARY OF THE INVENTION The present invention is directed towards a multilayer golf ball which provides a softer feel similar to a conventional wound ball, while also providing the distance, durability and relative ease of manufacturing of a conventional two piece ball. Generally, the prior art has been directed at making golf balls perform like a three piece ball by providing a softer cover. The present invention is directed to a golf ball that has durability, a low spin rate and substantial distance more like a conventional two piece ball, but has a soft feel by using a soft mantle layer and core. The present invention is further directed towards a multi-layer golf ball which comprises a core with one or more layers; at least one cover layer; and one or more mantle layers disposed between the core and cover layer, wherein the core is comprised of a polybutadiene rubber having an activation agent of calcium oxide and wherein the mantle layer comprises thermoplastic copolyetherester block copolymer, thermoplastic copolyesterester block copolymer, dynamically vulcanized thermoplastic elastomer, styrene- butadiene, styrene-isoprene and styrene-ixoprene elastomers with a functional group such as maleic anhydride or sulfonic acid, thermoplastic polyurethane, polymer made using a metallocene catalyst, or blends thereof and/or thermoset materials. The present invention is still further directed to a multi-layer golf ball which comprises a core, at least one cover layer and at least one mantle layer disposed between the core and cover layer wherein properties such as the thickness, hardness, flexural modulus, tensile modulus, PGA compression or Bayshore resilience of the various layers is such as to provide a ball with distance performance characteristics and a soft feel.

The present invention is also directed to an improved multilayer golf ball having a softer golf ball core. The composition of the golf ball core comprises a base mixture of polybutadiene, a metal salt diacrylate or dimethacrylate, preferably, zinc diacrylate in an amount of about 20 to 50 parts per hundred parts of polybutadiene, and a free radical initiator, to which calcium oxide (CaO) is added instead of zinc oxide as an activation agent in an amount sufficient to produce a golf ball core with the advantageous properties discussed below.

It has been found that when calcium oxide is used in a golf ball core composition as an activating agent, a lower compression golf ball core is obtained, which, when incorporated into a finished golf ball, provides a ball with an initial velocity in the USGA test that is comparable in velocity and distance to a standard, high compression ball that incorporates a core using zinc oxide. The calcium oxide can be added in an amount that reduces the compression of the golf ball, while maintaining the initial velocity of the ball in the USGA test. Alternatively, calcium oxide can be used in conjunction with an increased amount of zinc diacrylate to maintain a consistent PGA compression and increased initial velocity. Typically, the amount of calcium oxide incorporated into the core composition of the invention is between about 0.1 and 15 parts per 100 parts of polybutadiene. The amount of calcium oxide used is preferably less than about 15 pph because when more than 15 pph is used there appears to be a large decrease in the golf ball core compression that results in a significant reduction in the initial velocity of balls incorporating such cores. The invention thus provides a novel golf ball composition that offers the environmental benefit of eliminating or at least reducing the use of zinc oxide, while providing a golf ball with enhanced performance properties.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a golf ball according to the present invention.

Fig. 2 is a perspective view of a golf ball according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following terms that are used in this application are defined in terms of the enumerated ASTM tests:

Specific gravity ASTM D - 297¹

'Tests were carried out in lab conditions where temperature was controlled to 20-23 °C. Humidity was not controlled. Flexural (Flex) Modulus ASTM D - 790² Shore D Hardness ASTM D - 2240 Tensile Modulus ASTM D - 638³

Bashore resiliency ASTM D - 2632⁴ 5

For purposes of the present invention, the term "reaction conditions" can refer to any reaction condition that can affect the ability of the inventive core compositions to form free radicals. Reaction conditions 0 include, for example, temperature, time and pressure.

As used herein, the terms "points" or "compression points" refer to the PGA compression scale. This scale, which is well known to those working in this field, ranges from 1 to 160 points and is used in determining the relative compression of a core or ball. Some artisans use the Reihle compression scale instead of the PGA compression scale. Based on disclosure in U.S. Patent No. 5,368,304, column 20, lines 55-53 it appears that Reihle compression values can be converted to PGA compression values through the use of the 0 following equation:

PGA compression value = 160 - Reihle compression value.

Referring to Figs. 1 and 2, this invention is 5 particularly directed towards a multi-layer golf ball 1 which

²Tests were carried out using a 0.2 in/min cross-head speed _Q and a 2 inch span length in the three point bending mode. Test samples were annealed at 140°F for 24 hours and the tests were performed no earlier than 24 hours after annealing.

³Tests were carried out on type IV specimens using a 2 in/min cross-head speed without an extensio eter. Test samples were annealed at 140°F for 24 hours and the tests were performed ⁵ no earlier than 24 hours after annealing.

⁴Four (4) 1/8 inch samples were plied together to achieve the 1/2 inch test thickness. comprises a core 2, at least one cover layer 3 and at least one mantle layer 4 disposed therebetween.

A representative base composition for forming the golf ball core 2, prepared in accordance with the present invention, comprises polybutadiene and, in parts by weight based on 100 parts polybutadiene, 20-50 parts of a metal salt diacrylate, dimethacrylate, or monomethacrylate, preferably zinc diacrylate. The polybutadiene preferably has a cis 1,4 content of above about 90% and more preferably above about 96%. Commercial sources of polybutadiene include Shell 1220 manufactured by Shell Chemical, Neocis BR40 manufactured by Enichem Elastomers, and Ubepol BR150 manufactured by Ube Industries, Ltd. If desired, the polybutadiene can also be mixed with other elastomers known in the art, such as natural rubber, styrene butadiene, and/or isoprene in order to further modify the properties of the core. When a mixture of elastomers is used, the amounts of other constituents in the core composition are based on 100 parts by weight of the total elastomer mixture. Metal salt diacrylates, dimethacrylates, and monomethacrylates suitable for use in this invention include those wherein the metal is magnesium, calcium, zinc, aluminum, sodium, lithium or nickel. Zinc diacrylate is preferred, because it provides golf balls with a high initial velocity in the USGA test. The zinc diacrylate can be of various grades of purity. For the purposes of this invention, the lower the quantity of zinc stearate present in the zinc diacrylate the higher the zinc diacrylate purity. Zinc diacrylate containing less than about 10% zinc stearate is preferable. More preferable is zinc diacrylate containing about 4-8% zinc stearate. Suitable, commercially available zinc diacrylates include those from Rockland React-Rite and Sartomer. The preferred concentrations of zinc diacrylate that can be used are 20-50 pph based upon 100 pph of polybutadiene or alternately, polybutadiene with a mixture of other elastomers that equal 100 pph. Free radical initiators are used to promote cross-linking of the metal salt diacrylate, dimethacrylate, or monomethacrylate and the polybutadiene. Suitable free radical initiators for use in the invention include, but are not limited to peroxide compounds, such as dicumyl peroxide, 1,1-di (t-butylperoxy) 3,3,5-trimethyl cyclohexane, a-a bis (t-butylperoxy) diisopropylbenzene, 2 , 5-dimethyl-2 , 5 di (t-butylperoxy) hexane, or di-t-butyl peroxide, and mixtures thereof. Other useful initiators would be readily apparent to one of ordinary skill in the art without any need for experimentation. The initiator(s) at 100% activity are preferably added in an amount ranging between about 0.05 and 2.5 pph based upon 100 parts of butadiene, or butadiene mixed with one or more other elastomers. More preferably, the amount of initiator added ranges between about 0.15 and 2 pph and most preferably between about 0.25 and 1.5 pph.

A typical prior art golf ball core incorporates 5 to 50 pph of zinc oxide in a zinc diacrylate-peroxide cure system that cross-links polybutadiene during the core molding process.

In the present invention, the zinc oxide (ZnO) is eliminated in favor of calcium oxide (CaO) in the golf ball core composition. The cores and balls produced from such an admixture typically exhibit enhanced performance properties. The initial velocity of the standard ball is maintained at or near the maximum allowed by the USGA, but the compression of the ball is reduced by at least about 2 compression points on the PGA scale, and may be reduced as much as 14 points. On the other hand, the combination of the use of calcium oxide and a higher percentage of zinc diacrylate can be used to maintain the same PGA compression as with the zinc oxide, but the initial velocity is significantly increased. Where the amount of zinc oxide incorporated in prior art cores is, as noted above, typically about 5 to 50 pph, the amount of calcium oxide added to the core-forming composition of the invention as an activator is typically in the range of about 0.1 to 15, preferably 1 to 10, most preferably 1.25 to 5, parts calcium oxide per hundred parts (pph) of polybutadiene. The compositions of the present invention may also include fillers, added to the elasto eric composition to adjust the density and/or specific gravity of the core. As used herein, the term "fillers" includes any compound or composition that can be used to vary the density and other properties of the subject golf ball core. Fillers useful in the golf ball core according to the present invention include, for example, zinc oxide (in an amount significantly less than that which would be necessary without the addition of the calcium oxide) , barium sulfate, and regrind (which is recycled core molding matrix ground to 30 mesh particle size) . The amount and type of filler utilized is governed by the amount and weight of other ingredients in the composition, since a maximum golf ball weight of 1.620 ounces (45.92 gm) has been established by the USGA. Appropriate fillers generally used range in specific gravity from about 2.0 to 5.6. The golf ball core 2, as shown in Fig. 1, made according to the present invention can be of any specific gravity which can be used in a golf ball. The preferred range of specific gravities of the present invention is from about 0.9 to about 1.5, more preferably in the range of about 1 to about 1.25, depending upon the size of the core, cover, mantle layer and finished ball, as well as the specific gravity of the cover and mantle layer.

Antioxidants may also be included in the elastomer cores produced according to the present invention. Antioxidants are compounds which prevent the breakdown of the elastomer. Antioxidants useful in the present invention include, but are not limited to, quinoline type antioxidants, amine type antioxidants, and phenolic type antioxidants.

Other ingredients such as accelerators, e.g. tetra methylthiuram, processing aids, processing oils, plasticizers, dyes and pigments, as well as other additives well known to the skilled artisan may also be used in the present invention in amounts sufficient to achieve the purpose for which they are typically used.

The core compositions of the invention are produced by forming a mixture comprising at least polybutadiene, zinc diacrylate, and calcium oxide. Either the calcium oxide is used to reduce the compression by at least about 2 points on the PGA compression scale, compared to a core composition having zinc oxide or the amount of zinc diacrylate is increased such that the PGA compression is the same, but the initial velocity is increased. In other words, by using calcium oxide, either the ball compression can be lowered and the initial velocity maintained or the amount of zinc diacrylate can be increased so that the ball compression is the same and the initial velocity is increased. The Examples below set forth these principals and show the advantage of using calcium oxide.

When a set of predetermined conditions is met, i.e., time and temperature of mixing, the free radical initiator is added in an amount dependent upon the amounts and relative ratios of the starting components, as would be well understood by one of ordinary skill in the art. In particular, as the components are mixed, the resultant shear causes the temperature of the mixture to rise. Peroxide (s) free radical initiator (s) are blended into the mixture for crosslinking purposes in the molding process.

After completion of the mixing, the golf ball core composition is milled and hand prepped or extruded into pieces ("preps") suitable for molding. The milled preps are then compression molded into cores at an elevated temperature. Typically, 160°C (320°F) for 15 minutes is suitable for this purpose. These cores can then be used to make finished golf balls by surrounding the cores with mantle and cover materials.

EXAMPLES

These and other aspects of the present invention may be more fully understood with reference to the following non-limiting examples, which are merely illustrative of the preferred embodiment of the present invention golf ball core, and are not to be construed as limiting the invention, the scope of which is defined by the appended claims. The results obtained with golf ball cores and balls prepared according to the examples are representative of the improved performance characteristics of golf ball cores made from the compositions of this invention.

Table I sets forth the contents of the golf ball cores that were made to illustrate the effect of using calcium oxide instead of zinc oxide. The control core and Examples 1-3 all contain zinc oxide. Example 4 was made without zinc oxide or calcium oxide and Examples 5-9 were made with different amounts of calcium oxide. The compositions used to prepare the golf ball cores of these examples are all in parts per hundred (pph) , based on 100 parts of polybutadiene. The fillers used in the compositions of these examples are regrind and barium sulfate (BaS0₄) . Vulcup 40KE® and Varox 231XL® are free radical initiators, and are a-a bis (t-butylperoxy) diisopropylbenzene and 1,1-di (t-butylperoxy) 3,3,5-trimethyl cyclohexane, respectively. Yel MB is a yellow pigment in a styrene butadiene binder, which is used to color the composition for identification purposes. The zinc diacrylate contained no more than about 4-8% zinc stearate.

All the ingredients except the peroxides were mixed in a Process Lab Brabender mixer to 82.2-93.3°C (180-200°F) . The peroxides were added in the second stage to the initial mixture, and the resulting mixture was removed from the Brabender and blended on a lab mill to insure homogeneity. After mixing, the admixture was then hand rolled using a laboratory mill and cut into pieces or "preps". These preps were then compression molded at 160°C (320°F) for 15 minutes to form the cores. To fabricate the finished golf balls, the cores were inserted into two cover hemispheres of a lithium- sodium blend of SURLYN®, which were molded to encase the core. The cores and balls prepared according to the above-described method were tested for their PGA compression and initial velocity. The compression ratings were obtained using a commercial PGA compression tester. The initial velocity results were obtained from a standard technique, whereby the cores or balls are struck at 39.6 m/s (130 ft/s), and pass through light gates, which measure their speed. Both of these standard measurement techniques are well-known to those of ordinary skill in the art of making golf ball cores and balls.

As shown below in TABLE II, reduction in the zinc oxide concentrations in Examples 1-3 results in a slight decrease in ball compression and a slight drop in initial velocity compared to the control ball. Similar results are obtained with the core. The complete removal of zinc oxide in Example 4 reduces the compression of both the core and the ball by a significant amount, greater than 10 points, but also reduces the initial velocity of the core and ball significantly, approximately one mile per hour. When the zinc oxide is eliminated from the core composition, and calcium oxide is added in Examples 5-8, both the cores and finished balls containing calcium oxide have a lower compression, but the initial velocity of the low compression balls and cores is comparable to that of the high compression control ball.

TABLE I

Example No. Control 1 2 3 4 5 6 7 8 9

Polybutadiene 100 100 100 100 100 100 100 100 100 100

5

Regrind 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3

Vulcup 40KE® 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23

Varox 231XL® 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43

10 BaS0₄ 20.5 22.0 23.1 24.2 25.8 21.5 22.5 23.5 24.7 12.0

I Yel.MB 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

-vl

Zinc Diacrylate 26.9 26.9 26.9 26.9 26.9 26.9 26.9 26.9 26.9 26.9

Zinc Oxide 5.0 3.75 2.50 1.25

15

Calcium Oxide 5.0 3.75 2.50 1.25 15.0

20

Table III illustrates the effect of varying amounts of calcium oxide, zinc oxide and zinc diacrylate in golf ball cores on PGA compression ratings and initial velocity. Examples A-D below are multilayer balls having a Hytrel® 3078 mantle layer and SURLYN® covers made of 50% high acid SURLYN® and 50% standard SURLYN®. Each of the balls has a 1.39 inch diameter polybutadiene core made with the compositions listed. The results show that the PGA compression can be decreased for a ball made with zinc oxide by decreasing the percentage of zinc diacrylate as shown by the comparison of Examples A and B. However, by decreasing the zinc diacrylate, the ball initial velocity was decreased. When comparing Examples B and C, the effect of substituting calcium oxide for zinc oxide is shown. Here the ball having calcium oxide. Example C, has a lower PGA compression and higher initial velocity than the ball with zinc oxide. Example B. Moreover, Examples D and B show that by using calcium oxide with an increased amount of zinc diacrylate the PGA compression can be decreased and the initial velocity substantially increased. As shown by Table III, both balls having calcium oxide. Examples C and D, exhibit a very low PGA compression, below 80, and high initial velocity.

TABLE III

A B C D

Polybutadiene 100.0 100.0 100.0 100.0

Regrind 17.07 17.07 17.07 17.07

Vulcup 40KE® 0.23 0.23 0.23 0.23

Varox 231XL® 0.42 0.42 0.42 0.42

Cimbar 325 31.70 32.5 32.93 32.56

Zinc diacrylate 23.47 21.33 21.33 22.33

Zinc Oxide 2.13 2.13

Calcium Oxide 2.13 2.13

Ball Initial Velocity 251.06 250.86 251.26 252.08 (ft/s)

Core PGA Compression 67 53 36 47

Ball PGA Compression 92 85 76 79

Turning now to the mantle layer 4, it preferably comprises thermoplastic copolyesterester block copolymer, dynamically vulcanized thermoplastic elastomer, styrene- butadiene elastomer with functional groups such as maleic anhydride or sulfonic acid attached, thermoplastic polyurethane or polymers made using a metallocene catalyst or blends thereof. In a preferred embodiment of the present invention, the mantle layer 4 is a thermoplastic copolyetherester block copolymer. Suitable thermoplastic copolyetheresters include Hytrel® 3078, Hytrel® G3548W and Hytrel® G4078W which are commercially available from DuPont. Hytrel® 3078 being the most preferred. The mantle layer preferably has a specific gravity greater than 1 and less than 1.2.

Suitable dynamically vulcanized thermoplastic elastomers include Santoprene®, Sarlink®, Vyram®, Dytron® and Vistaflex®. Santoprene® is the trademark for a dynamically vulcanized PP/EPDM. Santoprene® 203-40 is an example of a preferred Santoprene® and is commercially available from Advanced Elastomer Systems. Examples of suitable functionalized styrene-butadiene elastomers, i.e., styrene- butadiene elastomers with functional groups such as maleic anhydride or sulfonic acid, include Kraton FG-1901X and FG- 1921x, which are available from the Shell Corporation. Examples of suitable thermoplastic polyurethanes include Estane® 58133, Estane® 58134 and Estane® 58144, which are commercially available from the B.F. Goodrich Company. Suitable metallocene polymers, i.e., polymers made with a metallocene catalyst, whose melting points are higher than the cover materials can also be employed in the mantle layer of the present invention. Further, the materials for the mantle layer described above may be in the form of a foamed polymeric material. For example, suitable metallocene polymers include foams of thermoplastic elastomers based on metallocene single-site catalyst-based foams. Such metallocene-based foam resins are commercially available from Sentinel Products of Hyannis, Massachusetts.

In another preferred embodiment of the present invention, the mantle layer is a blend of a first and a second thermoplastic, wherein the first thermoplastic is a thermoplastic copolyetherester or copolyesterester block copolymer, a dynamically vulcanized thermoplastic elastomer, a functionalized styrene-butadiene elastomer, a thermoplastic polyurethane or a metallocene polymer and the second thermoplastic is a material such as a thermoplastic polyurethane, a thermoplastic polyetherester or polyetheramide, a thermoplastic ionomer resin, a thermoplastic polyester, another dynamically vulcanized elastomer, another functionalized styrene-butadiene elastomer, another metallocene polymer or blends thereof. Suitable thermoplastic polyetheresters include Hytrel® 3078, Hytrel® G3548W and Hytrel® G4078W which are commercially available from DuPont. Suitable thermoplastic polyetheramides include Pebax® 2533, Pebax® 1205 and Pebax® 4033 which are available from Elf-Atochem. Suitable thermoplastic ionomer resins include any number of olefinic based ionomers including SURLYN® and Iotek®, which are commercially available from DuPont and Exxon, respectively. The flexural moduli for these ionomers is about 75 kpsi to about 129 kpsi. Suitable thermoplastic polyesters include polybutylene terephthalate. Likewise, the dynamically vulcanized thermoplastic elastomers, functionalized styrene- butadiene elastomers, thermoplastic polyurethane or metallocene polymers identified above are also useful as the second thermoplastic in such blends. Further, the materials of the second thermoplastic described above may be in the form of a foamed polymeric material.

The cover layers of the present invention comprises at least one layer of a thermoplastic or thermosetting material. Any number of a wide variety of cover materials may be used in the present invention. Among the preferred conventional cover materials are ionomer resins. More particularly, ionomers, such as acid-containing ethylene copolymer ionomers, include E/X/Y copolymers where E is ethylene, X is a softening comonomer such as acrylate or methacrylate present in 0-50 (preferably 0-25, most preferably 0-2) , weight percent of the polymer, and Y is acrylic or methacrylic acid present in 5-35 (preferably 10- 35, most preferably 15-20) weight percent of the polymer, wherein the acid moiety is neutralized 1-90% (preferably at least 40%, most preferably at least about 60%) to form an ionomer by a cation such as lithium, sodium, potassium, magnesium, calcium, barium, lead, tin, zinc or aluminum, or a combination of such cations, lithium, sodium and zinc being the most preferred. Specific acid-containing ethylene copolymers include ethylene/acrylic acid, ethylene/methacrylic acid, ethylene/acrylic acid/n-butyl acrylate, ethylene/methacrylic acid/n-butyl acrylate, ethylene/methacrylic acid/iso-butyl acrylate, ethylene/acrylic acid/iso-butyl acrylate, ethylene/methacrylic acid/n-butyl methacrylate, ethylene/acrylic acid/methyl methacrylate, ethylene/acrylic acid/methyl acrylate, ethylene/methacrylic acid/methyl acrylate, ethylene/methacrylic acid/methyl methacrylate, and ethylene/acrylic acid/n-butyl methacrylate. Preferred acid- containing ethylene copolymers include ethylene/methacrylic acid, ethylene/acrylic acid, ethylene/methacrylic acid/n- butyl acrylate, ethylene/acrylic acid/n-butyl acrylate, ethylene/methacrylic acid/methyl acrylate and ethylene/acrylic acid/methyl acrylate copolymers. The most preferred acid-containing ethylene copolymers are ethylene/methacrylic acid, ethylene/acrylic acid, ethylene/ (meth) acrylic acid/n-butyl acrylate, ethylene/ (meth) acrylic acid/ethyl acrylate, and ethylene/ (meth) acrylic acid/methyl acrylate copolymers.

The manner in which these ionomers are made is well known in the art as described in e.g. U.S. Patent No. 3,262,272. Such ionomer resins are commercially available from DuPont Co. under the tradename SURLYN®. A presently preferred cover material is a 50/50 blend of SURLYN® 8140 and SURLYN® 7940, which are a sodium ionomer made with 19% ethacrylic acid and a lithium ionomer made with 15% methacrylic acid, respectively. This cover material has a flexural modulus of about 85 ksi and a specific gravity of about 0.95. Another preferred cover is comprised of a 50/50 blend of SURLYN® 8140 and SURLYN® AD 8546 (SEP671) , which is a lithium SURLYN® having about 19% methacrylic acid. This material has a flexural modulus of about 105 ksi. Preferably, the flexural modulus of the cover is greater than 75 ksi. The most preferred cover is comprised of a 50/50 blend of SURLYN® AD 8546, which has about 19% methacrylic acid, and SURLYN® 9910 which is a standard zinc SURLYN®. Still further, the preferred cover has a hardness of about 70 Shore D. The high flexural modulus of the cover provides increased initial velocity for extra distance. These 50/50 blends of high acid SURLYN® and standard SURLYN® are preferred because they provide high initial velocity without undue hardness. In another preferred embodiment of the present invention, the cover layer comprises an inner layer and an outer layer. The inner layer of the cover is either a thermoplastic material such as a thermoplastic elastomer or a thermoplastic rubber, or a thermoset rubber or thermoset elastomer material. Some examples of materials suitable for use as the inner cover layer include polyether or polyester thermoplastic urethanes as well as thermoset polyurethanes. A preferable thermoplastic material is a thermoplastic urethane with a melting point of about 178° to about 205°F. A preferable thermoset material is a rubber based, castable urethane. The outer layer of the cover is either a thermoplastic plastic material such as an elastomer or a thermoplastic rubber, or a thermosetting material. Suitable materials for the outer layer include Urethanes, ionomers with a low modulus and other "dead" but durable materials such as EPDM and butyl rubber. Additionally, the present invention also contemplates the use of a polymeric foam material, such as the metallocene-based foamed resin described above, as the material for either the outer cover layer or the inner cover layer, but preferably not both layers. In one embodiment of the present invention, it is preferable that the thermoplastic or thermosetting materials of the outer layer have a melting point or reaction (cure) temperature less than the melting point or reaction (cure) temperature of the materials of the inner layer. The present invention also contemplates the use of a variety of non-conventional cover materials. In particular, the covers of the present invention may comprise thermoplastic or engineering plastics such as ethylene or propylene based homopolymers and copolymers including functional monomers such as acrylic and methacrylic acid and fully or partially neutralized ionomers and their blends, methyl acrylate, methyl methacrylate homopolymers and copolymers, imidized, amino group containing polymers, polycarbonate, reinforced polyamides, polyphenylene oxide, high impact polystyrene, polyether ketone, polysulfone, poly (phenylene sulfide) , reinforced engineering plastics, acrylonitrile-butadiene, acrylic-styrene-acrylonitrile, poly (ethylene terephthalate) , poly (butylene terephthalate) , poly (ethylene-vinyl alcohol) , poly (tetrafluoroethylene) and their copolymers including functional comonomers and blends thereof. These polymers or copolymers can be further reinforced by blending with a wide range of fillers and glass fibers or spheres or wood pulp.

The properties such as PGA compression, hardness, Bashore resilience, modulus, core diameter and mantle layer thickness of the golf balls of the present invention have been found to effect play characteristics such as spin, initial velocity and feel of the present golf balls.

In particular, the thickness of the mantle layer of the present invention is about 0.025 inches to about 0.125 inches. Preferably, the thickness of the mantle layer is about 0.04 inches to about 0.10 inches. Most preferably, the thickness of the mantle layer is about 0.06 inches. Similarly, the diameter of the core of the present invention is about 1.25 inches to about 1.51 inches. Preferably the diameter of the core is about 1.30 inches to about 1.48 inches. Most preferably, the diameter of the core is about 1.39 inches. The overall diameter of the core and mantle layer is about 84% to about 97% of the overall diameter of the finished ball, and is preferably about 1.51 inches. The present multi-layer golf ball can have an overall diameter of any size. Although the United States

Golf Association (USGA) specifications limit the minimum size of a competition golf ball to 1.680 inches in diameter or more, there is no specification as to the maximum diameter. Moreover, golf balls of any size can be used for recreational play. The preferred diameter of the present golf balls is from about 1.680 inches to about 1.800 inches. The more preferred diameter is from about 1.680 inches to about 1.760 inches. The most preferred diameter is about 1.680 inches to about 1.740 inches. The flexural and/or tensile moduli of the mantle layer are believed to have an effect on the "feel" of the golf balls of the present invention. Accordingly, it is preferable that the golf balls of the present invention have a mantle layer with a flexural modulus of about 500 psi to about 50,000 psi in order to impart a softer "feel" to the golf balls of the present invention. Likewise, it is preferred that the mantle layer have a tensile modulus of about 500 psi to about 50,000 psi in order to impart a softer "feel" to the golf balls of the present invention. More preferably, the flexural modulus and tensile modulus of the mantle layer are both less than about 10,000 psi. Most preferably, the flexural modulus of the mantle layer is less than 5000 psi.

Further, the core of the present invention has a Bashore resilience of about 30 to about 80. Preferably the core has a Bashore resilience of about 40 to about 70. The mantle layer of the present invention has a Bashore resilience of about 35 to about 75. Preferably the mantle layer has a Bashore resilience of about 60-70.

The golf balls of the present invention have a mantle layer with a Shore D hardness of less than about 60. Preferably, the Shore D hardness of the mantle layer is about 20 to about 60. Most preferably, the mantle layer has a Shore D hardness of about 30 to provide the soft "feel" desired. The core has a Shore D hardness of about 30 to about 65. Preferably, the core has a Shore D hardness of about 35 to about 60 and a PGA compression less than 60 such that the ball PGA compression is less than about 90. Most preferably, the core compression is low enough to make the ball PGA compression less than or equal to about 80.

The golf balls of the present invention can be made by any conventional process employed in the golf ball art. For example, the solid cores can be either injection or compression molded. Similarly, the undersized wound cores of the present invention are produced through conventional means. The mantle layer is subsequently injection or compression molded about the core. It is important that the mantle material be able to sustain the temperatures applied during the application of the cover layer. The cover layer or layers are then injection or compression molded or cast about the mantle layer.

In the manufacturing of the ball, the core is preferably compression molded as stated above. The mantle layer is molded over the core using a retractable-pin mold or a fixed-pin mold. In a fixed-pin, a plurality of apertures are formed in the mantle layer to form a mechanical interlock with the cover. The cover is molded over the mantle layer. Illustrated in Table IV are a number of batch formulations for several mantle layer compositions of the present invention, as well as properties such a hardness (Shore D) , Bashore resilience, flexural modulus, % strain at break and tensile modulus for each composition.

The following examples of batch compositions are given to illustrate the novel multi-layer golf balls of the present invention. However, it is to be understood that the examples are only for illustrative purposes and in no manner is the present invention limited to the specific disclosures therein.

TABLE IV

Mantle Layer Compositions and Properties

Sample Hardness Resilience Flex Modulus Tensile Modulus % Strain at (Shore D) (psi) (psi) Break

1A 0% Hytrel 5556 30 62 4000 2,078 810 no break 100% Hytrel 3078

IB 25% Hytrel 5556 37 59 8,170 5,122 685 75% Hytrel 3078

IC 50% Hytrel 5556 44 55 15,320 10,879 590 50% Hytrel 3078

10

ID 75% Hytrel 5556 53 50 19,870 16,612 580 25% Hytrel 3078 ro oo IE 100% Hytrel 5556 58 50 24,840 17,531 575 0% Hytrel 3078

2A 0% Estane 58091 28 54 1,720 756 563

15 100% Estane 58861

2B 25% Estane 58091 34 41 2,610 2,438 626 75% Estane 58861

2C 50% Estane 58091 44 31 10,360 10,824 339 50% Estane 58861

2D 75% Estane 58091 61 34 43,030 69,918 149

20 25% Estane 58861

2E 100% Estane 58091 78 46 147,240 211,288 10 0% Estane 58861

3A 0% Hytrel 5556 40 51 9,250 7,071 527 100% Hytrel 4078

3B 25% Hytrel 5556 43 51 10,020 9,726 441

75% Hytrel 4078

3C 50% Hytrel 5556 45 47 12,280 10,741 399 50% Hytrel 4078

3D 75% Hytrel 5556 48 53 13,680 13,164 374 25% Hytrel 4078

3E 100% Hytrel 5556 48 52 12,110 15,231 347 0% Hytrel 4078

10

4A 0% Hytrel 4078 46 51 11,150 8,061 597 100% Pebax 4033

4B 25% Hytrel 4078 46 53 10,630 7,769 644 75% Pebax 4033

4C 50% Hytrel 4078 45 52 9,780 8,117 564

15 50% Pebax 4033

4D 75% Hytrel 4078 42 53 9,310 7,996 660 25% Pebax 4033

4E 100% Hytrel 4078 40 51 9,250 7,071 527 0% Pebax 4033

5A 0% Hytrel 3078 77 50 156,070 182,869 9

20 100 Estane 5809

5B 25% Hytrel 3078 65 48 87,680 96,543 33 75% Estane 5809

5C 50% Hytrel 3078 52 49 53,940 48,941 102 50% Estane 5809

5D 75% Hytrel 3078 35 54 12,040 6,071 852

25% Estane 5809

5E 100% Hytrel 3078 30 62 4000 2,078 810 no break 0% Estane 5809

6A 100% Kraton 1921 29 59 24,300 29,331 515 0% Estane 58091 0% Surlyn 7940

6B 50% Kraton 1921 57 49 56,580 — 145

10 50% Estane 58091 0% Surlyn 7940 eo 6C 50% Kraton 1921 56 55 28,290 28,760 295 o 0% Estane 58091 50% Surlyn 7940

7A 33.3% Pebax 4033 48 50 41,240 30,032 294

15 33.3% Estane 58091 33.3% Hytrel 3078

7B 50% Pebax 4033 48 50 30,650 14,220 566 40% Estane 58091 10% Hytrel 3078

7C 20% Pebax 4033 41 54 24,020 16,630 512

20 40% Estane 58091 40% Hytrel 3078

Tables V and VI provide test data obtained from embodiments of the present invention. In particular, the spin rate and velocity were measured for golf balls formed according to the present invention. In the first ball, the mantle layer was Hytrel 3078, the cover was a conventional lithium/high acid sodium SURLYN® 50/50 blend (SURLYN 7940/SURLYN 8140) and the core was a conventional cross- linked polybutadiene core. In the second ball, the mantle layer was Hytrel 3078, the cover was high acid. lithium/standard zinc 50/50 blend (SURLYN® SEP 671/SURLYN® 9910) and the core was a polybutadiene core with 2.13 pph calcium oxide. Both cores had a diameter of 1.39 inches and a specific gravity of about 1.25. The mantle layers were 0.06 inches thick and the covers were 0.085 inches thick. Also tested were conventional two piece golf balls (Titleist HVC 90 and Pinnacle Gold) for comparison. Each ball was tested in a True Temper Test Machine. The test machine was configured to strike the balls with a Driver and an Eight Iron.

TABLE V Spin Rate Test Results for Driver

Sample Launch Angle Spin (rpm) Velocity PGA (ft/sec) Compression

Titleist HVC (control) 9.3° 3038 234.9 100

Pinnacle Gold (control) 9.3° 3027 233.6 97

SURLYN® 7940/SURLYN® 8140 cover, 9.1° 3066 234.7 92 Hytrel 3078 mantle layer, and 1.39" polybutadiene core

SURLYN® SEP 671/SURLYN® 9910 cover, 9.1° 2900 233.3 79

10 Hytrel 3078 mantle layer, and 1.39" polybutadiene core with calcium oxide

TABLE VI

As shown by test data above, golf balls having a low modulus mantle layer, like Hytrel 3078, have a high initial velocity and low spin rate for good distance. Moreover, the ball having calcium oxide in the core composition has a very soft "feel", more like a wound ball. Still further, the balls of this invention are easy to manufacture compared to the conventional wound ball. Thus, these balls provide the advantages of two piece balls with low spin rates, high velocity and soft "feel".

Claims

THE CLAIMS We claim:

1. A golf ball comprising:

(a) a core having a PGA compression less than about 70;

(b) a cover layer having a flexural modulus of greater than about 75,000 psi; and

(c) a mantle layer disposed between the core and cover layer, wherein the mantle layer has a flexural modulus of less than about 10,000 psi.

2. The golf ball of claim 1 wherein the mantle layer comprises a polymer selected from the group of thermoplastic copolyetherester block copolymer and thermoplastic copolyesterester block copolymer.

3. The golf ball of claim 1, wherein the mantle layer has a flexural modulus of less than about 5,000 psi.

4. The golf ball of claim 3, wherein the mantle layer has a tensile modulus of less than about 10,000 psi.

5. The golf ball of claim 4, wherein the mantle layer has a Shore D hardness of less than about 60.

6. The golf ball of claim 5 wherein the mantle layer has a Shore D hardness of about 30.

7. The golf ball of claim 1, wherein the core is formed from a mixture comprising polybutadiene, a metal salt of a material selected from the group consisting of diacrylates, dimethacrylates, and monomethacrylates, and calcium oxide.

8. A golf ball comprising:

(a) a core formed from a mixture comprising polybutadiene, a metal salt of a material selected from the group consisting of diacrylates, dimethacrylates, and monomethacrylates, and calcium oxide;

(b) a cover layer having a flexural modulus of greater than about 75,000 psi; and (c) a mantle layer disposed between the core and cover layer,

(d) wherein the amount of the calcium oxide and the hardness of the mantle layer are such to make the PGA compression of the golf ball about or below 90 PGA compression points.

9. The golf ball of claim 9, wherein the PGA compression of the golf ball about or below 80 PGA compression points.

10. The golf ball of claim 9, wherein the amount of calcium oxide in said mixture is from about 0.1 to 15 parts per 100 parts of polybutadiene.

li. The golf ball of claim 11, wherein the amount of calcium oxide in said mixture is from about 1.25 to 5 parts per 100 parts of polybutadiene.

12. The golf ball of claim 12, wherein the polybutadiene has a cis-1, 4-polybutadiene content of at least about 90%.

13. The golf ball of claim 13, wherein the polybutadiene has a cis-1, 4-polybutadiene content of at least about 96%.

14. The golf ball of claim 9, wherein the amount of the metal salt in said mixture is about 20 to 50 parts per 100 parts of polybutadiene.

15. The golf ball of claim 15, wherein the metal salt is zinc diacrylate.

16. The golf ball of claim 16, wherein the core further comprises a filler selected from the group consisting of barium sulfate, regrind, limestone, and mixtures thereof.

17. The golf ball of claim 9, wherein the hardness of the mantle layer is less than about 60.

18. The golf ball of claim 18, wherein the flexural modulus of the mantle layer is less than about 10,000 psi.

AMENDED CLAIMS

[received by the International Bureau on 30 December 1997 (30.12.97); original claims 9-18 amended; remaining claims unchanged (2 pages)] group consisting of diacrylates, dimethacrylates, and monomethacrylates, and calcium oxide;

9. The golf ball of claim 8, wherein the PGA compression of the golf ball is about or below 80 PGA compression points.

10. The golf ball of claim 8, wherein the amount of calcium oxide in said mixture is from about 0.1 to 15 parts per 100 parts of polybutadiene.

11. The golf ball of claim 10, wherein the amount of calcium oxide in said mixture is from about 1.25 to 5 parts per 100 parts of polybutadiene.

12. The golf ball of claim 11, wherein the polybutadiene has a cis-1, 4-polybutadiene content of at least about 90%.

13. The golf ball of claim 12, wherein the polybutadiene has a cis-1, 4-polybutadiene content of at least about 96%.

14. The golf ball of claim 8, wherein the amount of the metal salt in said mixture is about 20 to 50 parts per 100 parts of polybutadiene.

15. The golf ball of claim 14, wherein the metal salt is zinc diacrylate.

16. The golf ball of claim 15, wherein the core further comprises a filler selected from the group consisting of barium sulfate, regrind, limestone, and mixtures thereof.

5 17. The golf ball of claim 8, wherein the hardness of the mantle layer is less than about 60.

18. The golf ball of claim 17, wherein the flexural modulus of the mantle layer is less than about 10 10,000 psi.

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