Golf ball with soft core

[0002] CROSS REFERENCE TO RELATED APPLICATIONS

[0003] This application is a continuation-in-part application of U.S. application Ser. No. 08/975,799 filed Nov. 21, 1997 now U.S. Pat. No. 5,971,870. This application is also a continuation-in-part of U.S. application Ser. No. 09/226,727 filed Jan. 7, 1999, and of U.S. application Ser. No. 09/226,340 filed Jan. 6, 1999, both of which are continuations-in-part of U.S. application Ser. No. 08/819,945 filed Mar. 18, 1997 now U.S. Pat. No. 5,895,105; which is a continuation of U.S. application Ser. No. 08/370,224 filed Jan. 9, 1995 now abandoned; which is a continuation of U.S. application Ser. No. 07/893,277 filed Jun. 4, 1992 now abandoned; and which is a continuation-in-part of U.S. application Ser. No. 07/874,066 filed on Apr. 24, 1992 now abandoned.

[0004] The present invention is directed to golf balls utilizing improved polybutadiene compositions for use in molded golf ball cores in conjunction with a particular type of cover composition. In one aspect, the improved polybutadiene compositions utilize one or more particular butadiene rubbers synthesized through the use of neodymium and cobalt-containing catalysts. The polybutadiene is preferably an ultra-high Mooney viscosity polybutadiene. In another aspect, the improved polybutadiene compositions utilize a particular solid butadiene rubber that exhibits an ultra-high Mooney viscosity and/or a high molecular weight and a low dispersity. The use of such butadiene rubber and/or blend of butadiene rubbers increases the resiliency of the ball. In addition, significantly improved mixing properties are achieved. In another aspect, the golf balls feature particular cover constructions that result in the balls exhibiting a soft feel and particular mechanical impedance.

[0005] Two of the principal properties involved in the performance of golf balls are resilience and hardness. Resilience is determined by the coefficient of restitution (referred to as “C.O.R.”), also expressed as the constant “e”, which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact, or more generally, the ratio of the outgoing velocity to incoming velocity of a rebounding ball. As a result, the coefficient of restitution (i.e. “e”) can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision. Hardness is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball's diameter. The lower the compression value, the harder the material.

[0006] Resilience (C.O.R.), along with additional factors such as clubhead speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determine the distance a ball will travel when hit. Since clubhead speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.

[0007] In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and electronically measuring the ball's incoming and outgoing velocity. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golfers Association (“U.S.G.A.”). Along this line, the U.S.G.A. standards indicate that a “regulation” ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second (250 feet per second with a 2% tolerance). Since the coefficient of restitution of a ball is related to the ball's initial velocity (i.e. as the C.O.R. of a ball is increased, the ball's initial velocity will also increase), it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e. impact resistance) to produce enhanced durability.

[0008] The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e. balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings.

[0009] Polybutadiene has been utilized in forming golf ball cores. Prior artisans have investigated utilizing various grades of polybutadiene in core compositions. For example, such attempts are described in U.S. Pat. Nos. 5,385,440; 4,931,376; 4,683,257; 4,955,613; and 4,984,803; and in Japanese Patent References JP 58225138 and JP 7268132, all of which are hereby incorporated by reference. Although some of the core compositions described in these disclosures are satisfactory, a need remains for an improved composition for forming golf ball cores.

[0010] For example, U.S. Pat. No. 4,929,678 relates to a golf ball formed from a polybutadiene core composition having a broad Mooney viscosity of 45-90, preferably 50-70, and more preferably 55 to 65. However the dispersity of the core composition is limited to the range of 4.0 to 8.0, and preferably 4.0 to 6.0. According to the '678 patent, a dispersity of less then 4.0 produces deleterious workability.

[0011] Similarly, U.S. Pat. No. 5,082,285 generally discloses the preparation of a solid golf ball from an ultra-high molecular weight polybutadiene having a number average molecular weight of 40×10⁴or more, which has dispersity characteristics as noted. See also U.S. Pat. Nos. 4,974,852 and 5,585,440, wherein Mooney viscosity is discussed without reference to dispersity.

[0012] Accordingly, it is an object of the present invention to provide an improved polybutadiene composition which, when utilized to formulate golf ball cores, produces golf balls exhibiting enhanced C.O.R. without increasing hardness. An additional object of the invention is to produce a golf ball core from a polybutadiene composition having a high Mooney viscosity and/or a high molecular weight and low dispersity.

[0013] Accordingly, it is another object of the present invention to provide an improved core composition which, when utilized to formulate golf ball cores, produces golf balls exhibiting enhanced C.O.R. and improved processing.

[0014] The spin rate and “feel” of a golf ball are particularly important aspects to consider when selecting a golf ball for play. A golf ball with the capacity to obtain a high rate of spin allows a skilled golfer the opportunity to maximize control over the ball. This is particularly beneficial when hitting a shot on an approach to the green.

[0015] Golfers have traditionally judged the softness of a ball by the sound of the ball as it is hit with a club. Soft golf balls tend to have a low frequency sound when struck with a club. This sound is associated with a soft feel and thus is desirable to a skilled golfer.

[0016] Balata covered wound golf balls are known for their soft feel and high spin rate potential. However, balata covered balls suffer from the drawback of low durability. Even in normal use, the balata covering can become cut and scuffed, making the ball unsuitable for further play. Furthermore, the coefficient of restitution of wound balls is reduced by low temperatures.

[0017] The problems associated with balata covered balls have resulted in the widespread use of durable ionomeric resins as golf ball covers. However, balls made with ionomer resin covers typically have PGA compression ratings in the range of 90-100. Those familiar with golf ball technology and manufacture will recognize that golf balls with PGA compression ratings in this range are considered to be somewhat harder than conventional balata covered balls. It would be useful to develop a golf ball having a durable cover which has the sound and feel of a balata covered wound ball.

[0018] These and other objects and features of the invention will be apparent from the following summary and description of the invention and from the claims.

[0019] The present invention achieves all of the foregoing objectives and provides, in a first aspect, a golf ball comprising a core that includes a particular combination of polybutadiene rubbers, and a cover disposed about the core which includes a specific combination of ionomer resins. The polybutadiene rubbers used in the particular combination include a first polybutadiene rubber that is obtained utilizing a cobalt catalyst and which exhibits a Mooney viscosity in the range of from about 70 to about 83. The combination of polybutadiene rubbers also includes a second polybutadiene rubber that is obtained utilizing a neodymium series catalyst and which exhibits a Mooney viscosity of from about 30 to about 70. The cover composition used in this golf ball includes a combination of three ionomers. That combination includes a sodium ionomer, a magnesium ionomer, and a zinc ionomer.

[0020] In a second aspect, the present invention provides a golf ball comprising a core that includes a particular combination of first and second polybutadiene rubbers, in particular weight proportions, and a cover generally enclosing the core which is formed from a certain combination of ionomers. Specifically, the core includes from about 30% by weight of a first polybutadiene rubber obtained from a cobalt or cobalt-based catalyst, and about 30% to about 45% by weight of a second polybutadiene rubber obtained from a neodymium or lanthanide-series catalyst. The cover of the golf ball includes a sodium ionomer, a magnesium ionomer, and a zinc ionomer.

[0021] In yet another aspect, the present invention provides a golf ball comprising a core including a certain combination of polybutadiene rubbers, which is enclosed with a cover formed from a particular set of ionomers. The core includes from about 20% to about 30% by weight of a first polybutadiene rubber obtained from a cobalt catalyst and which exhibits a Mooney viscosity in the range of from about 70 to about 83. The core further includes from about 30% to about 45% of a second polybutadiene rubber obtained from a neodymium catalyst and which exhibits a Mooney viscosity of from about 30 to about 70. The cover which is disposed about the core includes from about 40% to about 50% of a sodium ionomer, from about 40% to about 50% of a magnesium ionomer, and from about 5% to about 15% of a zinc ionomer.

[0022] Further scope of the applicability of the invention will become apparent from the detailed description provided below.

[0023] FIG. 1 is a partial sectional view of a first preferred embodiment golf ball in accordance with the present invention.

[0024] FIG. 2 is a cross sectional view of the first preferred embodiment golf ball.

[0025] FIG. 3 is a partial sectional view of a second preferred embodiment golf ball in accordance with the present invention.

[0026] FIG. 4 is a cross sectional view of the second preferred embodiment golf ball.

[0027] FIG. 5 is a cross-sectional view of a third preferred embodiment golf ball according to the present invention having a unitary, solid core and a single cover layer.

[0028] FIG. 6 is a cross-sectional view of a fourth preferred embodiment of the invention in which the ball has two cover layers.

[0029] FIG. 7 is a cross-sectional view of a fifth embodiment of a golf ball according to the present invention in which the ball has a dual layer solid core.

[0030] FIG. 8 is a cross-sectional view of a sixth embodiment of the present invention in which the ball has a dual layer solid core and a dual layer cover.

[0031] FIG. 9 is a cross-sectional view of an embodiment of the invention in which the ball has a mechanical impedance with a primary minimum value in a particular frequency range.

[0032] FIG. 10 is a cross-sectional view of a solid golf ball according to the invention in which the ball has a particular PGA core compression and a mechanical impedance with a primary minimum value in a particular frequency range.

[0033] FIG. 11 shows a cross-sectional view of a golf ball according to yet another embodiment of the invention.

[0034] FIG. 12 shows a cross-sectional view of a golf ball according to a further embodiment of the invention.

[0035] FIG. 13 schematically shows the equipment used to determine mechanical impedance of the golf balls of the present invention.

[0036] FIGS. 14-21 are graphs showing mechanical impedance for the golf balls tested in Example 4.

[0037] The present invention provides a new, improved class of golf balls. As described herein, these golf balls feature a core comprising a particular combination of butadiene rubbers, and a cover comprising a specific array of ionomer resins.

[0038] The present invention is directed to improved compositions which, when utilized in formulating golf ball cores, produce cores that exhibit a relatively high degree of resilience. The invention is also directed to improving the processability of polybutadiene, particularly in forming golf ball cores. In these regards, it has been found that the use of a blend of particular polybutadiene resins in a golf ball core composition has the effect of increasing the resiliency of the resultant cores and greatly facilitates core formation.

[0039] The compositions of the present invention comprise one or more rubber or elastomeric components and an array of non-rubber or non-elastomeric components. The rubber components of the core compositions of the invention comprise a particular polybutadiene synthesized with cobalt and having an ultra-high Mooney viscosity and certain molecular weight characteristics described in detail below, one or more particular polybutadienes synthesized with neodymium, and one or more other optional polybutadienes. In some applications, polybutadienes synthesized with nickel catalysts may be used in combination with or instead of polybutadienes synthesized with cobalt catalysts. And, polybutadienes synthesized with lanthanide series catalysts may be used in combination with or instead of polybutadienes synthesized with neodymium catalysts. The non-rubber components of the core compositions of the invention comprise one or more crosslinking agents which preferably include an unsaturated carboxylic acid component, a free radical initiator to promote cross linking, one or more optional modifying agents, fillers, moldability additives, processing additives, and dispersing agents, all of which are described in greater detail below.

[0040] The first preferred polybutadiene resin for use in the present invention composition has a relatively ultra high Mooney viscosity. A “Mooney unit” is an arbitrary unit used to measure the plasticity of raw, or unvulcanized rubber. The plasticity in Mooney units is equal to the torque, measured on an arbitrary scale, on a disk in a vessel that contains rubber at a temperature of 212° F. (100° C.) and that rotates at two revolutions per minute.

[0041] The measurement of Mooney viscosity, i.e. Mooney viscosity [ML₁₊₄(100° C.], is defined according to the standard ASTM D-1646, herein incorporated by reference. In ASTM D-1646, it is stated that the Mooney viscosity is not a true viscosity, but a measure of shearing torque over a range of shearing stresses. Measurement of Mooney viscosity is also described in the Vanderbilt Rubber Handbook, 13th Ed., (1990), pages 565-566, also herein incorporated by reference. Generally, polybutadiene rubbers have Mooney viscosities, measured at 212° F., of from about 25 to about 65. Instruments for measuring Mooney viscosities are commercially available such as a Monsanto Mooney Viscometer, Model MV 2000. Another commercially available device is a Mooney viscometer made by Shimadzu Seisakusho Ltd.

[0042] As will be understood by those skilled in the art, polymers may be characterized according to various definitions of molecular weight. The “number average molecular weight,” M_n, is defined as: $M_n=\frac{\Sigma W_i}{\Sigma W_i/M_i}$

[0043] where W_iis the molecular weight of a fraction or sample of the polymer and M_iis the total number of fractions or samples.

[0044] “Weight average molecular weight,” M_Wis defined as: $M_w=\frac{\Sigma W_i{M}_i}{\Sigma W_i}$

[0045] where W_iand M_ihave the same meanings as noted above.

[0046] The “Z-average molecular weight,” M_Z, is defined as: $M_z=\frac{\Sigma W_i{M}_i^2}{\Sigma W_i{M}_i}$

[0047] where W_iand M_ihave the same meanings as noted above.

[0048] “M_peak” is the molecular weight of the most common fraction or sample, i.e. having the greatest population.

[0049] Considering these various measures of molecular weight, provides an indication of the distribution or rather the “spread” of molecular weights of the polymer under review.

[0050] A common indicator of the degree of molecular weight distribution of a polymer is its “polydispersity”, P: $P=\frac{M_w}{M_n}$

[0051] Polydispersity, also referred to as “dispersity”, also provides an indication of the extent to which the polymer chains share the same degree of polymerization. If the polydispersity is 1.0, then all polymer chains must have the same degree of polymerization. Since weight average molecular weight is always equal to or greater than the number average molecular weight, polydispersity, by definition, is equal to or greater than 1.0:

[0052] P>1.0

[0053] The first particular polybutadiene for use in the preferred embodiment compositions of the present invention exhibits a Mooney viscosity of from about 65 to about 85, and preferably from about 70 to about 83. The first particular polybutadiene has a number average molecular weight M_nof from about 90,000 to about 130,000; and preferably from about 100,000 to about 120,000. The first particular polybutadiene has a weight average molecular weight M_Wof from about 250,000 to about 350,000; and preferably from about 290,000 to about 310,000. The first particular polybutadiene has a Z-average molecular weight M_Zof about 600,000 to about 750,000; and preferably from about 660,000 to about 700,000. The first particular polybutadiene has a peak molecular weight M_peakof about 150,000 to about 200,000; and preferably from about 170,000 to about 180,0006.

[0054] The polydispersity of the first particular polybutadiene for use in the preferred embodiment compositions typically ranges from about 1.9 to about 3.9; and preferably from about 2.4 to about 3.1. Most preferably, the polydispersity is about 2.7.

[0055] The first particular polybutadiene for use in the preferred embodiment compositions preferably contains a majority fraction of polymer chains containing a cis-1,4 bond, more preferably, having a cis-1,4 polybutadiene content of about 90%, and most preferably, having a cis-1,4 polybutadiene content of at least about 95%. Another characteristic of the first preferred polybutadiene is that it is obtained or synthesized by utilizing a cobalt or cobalt-based catalyst. As noted herein, in some applications, a polybutadiene synthesized by using a nickel catalyst may be employed with, or in place of, the polybutadiene synthesized with a cobalt catalyst.

[0056] A commercially available polybutadiene corresponding to the noted first preferred ultra high viscosity polybutadiene, and which is suitable for use in the preferred embodiment compositions in accordance with the present invention is available under the designation Cariflex BCP 820, from Shell Chimie of France. Although this polybutadiene produces cores exhibiting higher C.O.R. values, it is somewhat difficult to process using conventional equipment. The properties and characteristics of this preferred polybutadiene are set forth below in Table 1.

[0057] The second polybutadiene for use in the preferred embodiment golf ball core compositions is a polybutadiene that is obtained or synthesized by utilizing a neodymium or lanthanide series catalyst, and that exhibits a Mooney viscosity of from about 30 to about 70, preferably from about 35 to about 70, more preferably from about 40 to about 65, and most preferably from about 45 to about 60. While the second polybutadiene provides covers exhibiting higher C.O.R. values, it exhibits very poor cold flow properties and very high dry swell characteristics.

[0058] Examples of such second polybutadienes obtained by using a neodymium-based catalyst include NEOCIS 40, NEOCIS 60 from Enichem and CB-22, CB-23, and CB-24 from Bayer. The properties of these polybutadienes are given below.

[0059] It has been found that when the first and second polybutadienes are blended together within certain ranges, golf ball cores can be produced without the individual processing difficulties associated with each polybutadiene. In essence, a synergistic effect is produced allowing the blends to produce golf ball cores using conventional equipment exhibiting enhanced resilience.

[0060] The compositions of the present invention may also utilize other polybutadiene resins in addition to the noted first and second particular polybutadienes. For example, Cariflex BR-1220 polybutadiene available from Shell Chemical (see Table 4 below); and Taktene 220 polybutadiene available from Bayer Corp. of Orange, Tex. (see Tables 5A and 5B below) may be utilized as other polybutadienes in combination with the particular ultra-high Mooney viscosity polybutadiene components described herein. It is also contemplated that these polybutadienes could be used by themselves and without the particular first and second polybutadienes. Generally, these other polybutadienes have Mooney viscosities in the range of about 25 to 65. It is also contemplated that a similar polybutadiene resin, BCP 819, commercially available from Shell Chimie, may be used in conjunction with BCP 820.

[0061] Concerning the elastomeric or rubber portion of the preferred embodiment compositions, it is preferred to utilize the previously described first and second polybutadienes in particular proportions to one another. Generally, it is preferred to utilize the first polybutadiene in a proportion of less than 50 parts per hundred parts of the total amount of the first and second polybutadienes. Unless indicated otherwise, all parts expressed herein are parts by weight. More preferably, the first polybutadiene is utilized in a proportion of about 45 parts or less (most preferably 40 parts or less) per hundred parts of the total amount of the first and second polybutadienes. With respect to the second polybutadiene, it is generally preferred to utilize the second polybutadiene in a proportion of more than 50 parts per hundred parts of the total amount of the first and second polybutadienes. More preferably, the second polybutadiene is utilized in a proportion of about 55 parts or more (most preferably 60 parts or more) per hundred parts of the total amount of the first and second polybutadienes.

[0062] The preferred embodiment core compositions of the present invention generally comprise from about 80 parts to about 120 parts by weight of elastomeric or rubber components, i.e. the first and second polybutadienes, and from about 60 to about 80, or more, parts by weight of non-rubber or non-elastomeric components. Preferably, the core compositions comprise about 100 parts of rubber components and from about 60 to about 80, or more, parts by weight of non-rubber components. It will be understood that depending upon the types and respective function of components added to the non-rubber portion of the preferred embodiment core compositions, that the non-rubber portion may constitute a significant proportion of the rubber component. The rubber components include the previously described first and second polybutadienes. The non-rubber components are as follows.

[0063] Preferably, the crosslinking agent of the core composition is an unsaturated carboxylic acid component which is the reaction product of a carboxylic acid or acids and an oxide or carbonate of a metal such as zinc, magnesium, barium, calcium, lithium, sodium, potassium, cadmium, lead, tin, and the like. Preferably, the oxides of polyvalent metals such as zinc, magnesium and cadmium are used, and most preferably, the oxide is zinc oxide.

[0064] Exemplary of the unsaturated carboxylic acids which find utility in the preferred core compositions are acrylic acid, methacrylic acid, itaconic acid, crotonic acid, sorbic acid, and the like, and mixtures thereof. Preferably, the acid component is either acrylic or methacrylic acid. Usually, from about 15 to about 50, and preferably from about 20 to about 35 parts by weight of the carboxylic acid salt, such as zinc diacrylate, is included per 100 parts of the rubber components in the core composition. The unsaturated carboxylic acids and metal salts thereof are generally soluble in the elastomeric base, or are readily dispersible.

[0065] The free radical initiator included in the core composition is any known polymerization initiator (a co-crosslinking agent) which decomposes during the cure cycle. The term “free radical initiator” as used herein refers to a chemical which, when added to a mixture of the elastomeric blend and a metal salt of an unsaturated, carboxylic acid, promotes crosslinking of the elastomers by the metal salt of the unsaturated carboxylic acid. The amount of the selected initiator present is dictated only by the requirements of catalytic activity as a polymerization initiator. Suitable initiators include peroxides, persulfates, azo compounds and hydrazides. Peroxides which are readily commercially available are conveniently used in the present invention, generally in amounts of from about 0.1 to about 10.0 and preferably in amounts of from about 0.3 to about 3.0 parts by weight per each 100 parts of elastomer

[0066] Exemplary of suitable peroxides for the purposes of the present invention are dicumyl peroxide, n-butyl 4,4′-bix(buylperoxy)valerate, 1,1-bis (t-butylperoxy)-3,3,5-trimethyl cyclohexane, di-t-butyl peroxide and 2,5-di-(t-butylperoxy)-2,5 dimethyl hexane and the like, as well as mixtures thereof. It will be understood that the total amount of initiators used will vary depending on the specific end product desired and the particular initiators employed.

[0067] Examples of such commercial available peroxides are Luperco 230 or 231 XL, a peroxyketal manufactured and sold by Atochem, Lucidol Division, Buffalo, N.Y., and Trigonox 17/40 or 29/40, a peroxyketal manufactured and sold by Akzo Chemie America, Chicago, Ill. The one hour half life of Luperco 231 XL and Trigonox 29/40 is about 112° C., and the one hour half life of Luperco 230 XL and Trigonox 17/40 is about 129° C. Luperco 230 XL and Trigonox 17/40 are n-butyl4,4-bis(t-butylperoxy) valerate and Luperco 231 XL and Trigonox 29/40 are 1,1-di(t-butylperoxy) 3,3,5-trimethyl cyclohexane. Most preferably, and as noted in Table 6 herein, Trigonox 42-40B from Akzo Nobel of Chicago, Ill. is used. Most preferably, a solid form of this peroxide is used. Trigonox 42-40B is tert-Butyl peroxy-3,5,5-trimethylhexanoate. The liquid form of this agent is available from Akzo under the designation Trigonox 42S.

[0068] The core compositions of the present invention may additionally contain any other suitable and compatible modifying ingredients including, but not limited to, metal oxides, fatty acids, and diisocyanates. For example, Papi 94, a polymeric diisocyanate, commonly available from Dow Chemical Co., Midland, Mich., is an optional component in the rubber compositions. It can range from about 0 to 5 parts by weight per 100 parts by weight rubber (phr) component, and acts as a moisture scavenger.

[0069] Various activators may also be included in the compositions of the present invention. For example, zinc oxide and/or magnesium oxide are activators for the polybutadiene. The activator can range from about 2 to about 10 parts by weight per 100 parts by weight of the rubbers (phr) component.

[0070] Moreover, filler-reinforcement agents may be added to the composition of the present invention. One such example is polypropylene powder. Since the specific gravity of polypropylene powder is very low, and when compounded, the polypropylene powder produces a lighter molded core, large amounts of higher gravity fillers may be added. Additional benefits may be obtained by the incorporation of relatively large amounts of higher specific gravity, inexpensive mineral fillers such as calcium carbonate. Such fillers as are incorporated into the core compositions should be in finely divided form, as for example, in a size generally less than about 30 mesh and preferably less than about 100 mesh U.S. standard size. The amount of additional filler included in the core composition is primarily dictated by weight restrictions and preferably is included in amounts of from about 10 to about 100 parts by weight per 100 parts rubber.

[0071] The preferred fillers are relatively inexpensive and heavy and serve to lower the cost of the ball and to increase the weight of the ball to closely approach the U.S.G.A. weight limit of 1.620 ounces. Exemplary fillers include mineral fillers such as limestone, zinc oxide, silica, mica, barytes, calcium carbonate, or clays. Limestone is ground calcium/magnesium carbonate and is used because it is an inexpensive, heavy filler. Other heavy filler include metal particles, such as powdered tungsten, bismuth, or molybdenum. Other filler materials are noted herein.

[0072] As indicated, ground flash filler may be incorporated and is preferably 20 mesh ground up center stock from the excess flash from compression molding. It lowers the cost and may increase the hardness of the ball.

[0073] Fatty acids or metallic salts of fatty acids may also be included in the compositions, functioning to improve moldability and processing. Generally, free fatty acids having from about 10 to about 40 carbon atoms, and preferably having from about 15 to about 20 carbon atoms, are used. Exemplary of suitable fatty acids are stearic acid, palmitic, oleic and linoleic acids, as well as mixtures thereof. Exemplary of suitable metallic salts of fatty acids include zinc stearate. When included in the core compositions, the fatty acid component is present in amounts of from about 1 to about 25, preferably in amounts from about 20 to about 15 parts by weight based on 100 parts rubber (elastomer).

[0074] It is preferred that the core compositions include stearic acid as the fatty acid adjunct in an amount of from about 2 to about 5 parts by weight per 100 parts of rubber.

[0075] Diisocyanates may also be optionally included in the core compositions when utilized, the diisocyanates are included in amounts of from about 0.2 to about 5.0 parts by weight based on 100 parts rubber. Exemplary of suitable diisocyanates is 4,4′-diphenylmethane diisocyanate and other polyfunctional isocyanates known to the art.

[0076] Furthermore, the dialkyl tin difatty acids set forth in U.S. Pat. No. 4,844,471, the dispersing agents disclosed in U.S. Pat. No. 4,838,556, and the dithiocarbonates set forth in U.S. Pat. No. 4,852,884 may also be incorporated into the polybutadiene compositions of the present invention. The specific types and amounts of such additives are set forth in the above-identified patents, which are incorporated herein by reference.

[0077] The golf ball core compositions of the invention may also comprise from about 1 to about 100 parts by weight of particulate polypropylene resin, and preferably from about 10 to about 100 parts by weight polypropylene powder resin, per 100 parts by weight of a base elastomer (or rubber) selected from polybutadiene and mixtures of polybutadiene with other elastomers. More preferably, the particulate polypropylene resin, if utilized in the core compositions of the present invention, comprises from about 20 to about 40 parts by weight of a polypropylene powder resin such as that trademarked and sold by Amoco Chemical Co. under the designation “6400 P”, “7000P” and “7200 P”. The ratios of the ingredients may vary and are best optimized empirically.

[0078] As indicated above, additional suitable and compatible modifying agents such as fatty acids, and secondary additives such as Pecan shell flour, ground flash (i.e. grindings from previously manufactured cores of substantially identical construction), barium sulfate, zinc oxide, etc. may be added to the core compositions to increase the weight of the ball as necessary in order to have the ball reach or closely approach the U.S.G.A. weight limit of 1.620 ounces.

[0079] Most preferably, a core formulation in accordance with the present invention is as follows in Table 6:

[0080] In this preferred core formulation, it will be noted that the weight ratio of the polybutadiene formed from the cobalt catalyst (Cariflex BCP-820) to the polybutadiene formed from the neodymium catalyst (NeoCis 60 and NeoCis 40) is about 2:3. The present invention includes a wide range of such ratios, such as from 1:10 to 10:1. Preferably, the amount of the cobalt catalyst polybutadiene ranges from about 20% to about 30% of the core formulation. And, preferably, the amount of the neodymium catalyst polybutadiene ranges from about 30% to about 45% of the core formulation. Most preferably, these polybutadienes are in amounts of 25% and 37%, respectively.

[0081] It will be understood that the present invention golf balls may further include one or more interior or mantle layers. Such layers are usually disposed between the core and the cover components of the ball. It is also contemplated that the preferred blend of first and second polybutadienes described herein could be utilized in one or more of these interior mantle layers.

[0082] The present invention is well suited for forming cores for golf balls as described herein. Referring to FIGS. 1 and 2, a first preferred embodiment golf ball 10 is illustrated. It will be understood that all figures are schematics and not necessarily to scale. The first preferred embodiment golf ball 10 comprises a core 20, most preferably as described herein, and a cover layer 30 disposed about the core 20. The core 30 includes an outer surface 35 that defines a plurality of dimples 40 along the outer surface 35 as is known in the art.

[0083] The present invention core compositions are also well suited for use in multi-layer golf balls such as for example, a second preferred golf ball 50 illustrated in FIGS. 3 and 4. The second preferred embodiment golf ball 50 comprises a core 60, a first inner layer 70 disposed about the core 60, and an outer cover layer 80 disposed about the inner layer 70. The inner layer 70 may include one or more interior layers or mantles. The outer cover layer 80 may include one or more cover layers. The outer layer 80 includes an outer surface 85 that defines a plurality of dimples 90 as known in the art.

[0084] In producing golf ball cores utilizing the present compositions, the ingredients may be intimately mixed using, for example, two roll mills or a Banbury mixer until the composition is uniform, usually over a period of from about 5 to about 20 minutes. The sequence of addition of components is not critical. A preferred blending sequence is as follows.

[0085] The elastomer, powder resin, fillers, zinc salt, metal oxide, fatty acid, and any other optional components, if desired, are blended for about 7 minutes in an internal mixer such as a Banbury mixer. As a result of shear during mixing, the temperature rises to about 200° F., whereupon the batch is discharged onto a two roll mill, mixed for about one minute and sheeted out.

[0086] The sheet is then placed in a Barwell preformer and slugs are produced. The slugs are then subjected to compression molding at about 320° F. for about 14 minutes. After molding and cooling, the cooling effected at room temperature for about 4 hours, the molded cores are subjected to a centerless grinding operation whereby a thin layer of the molded core is removed to produce a round core having a diameter of 1.545 inches.

[0087] The mixing is desirably conducted in such a manner that the composition does not reach incipient polymerization temperatures during the blending of the various components.

[0088] Usually the curable component of the composition will be cured by heating the composition at elevated temperatures on the order of from about 275° F. to about 350° F., preferably and usually from about 290° F. to about 325° F., with molding of the composition effected simultaneously with the curing thereof. The composition can be formed into a core structure by any one of a variety of molding techniques, e.g. injection, compression, or transfer molding. When the composition is cured by heating, the time required for heating will normally be short, generally from about 10 to about 20 minutes, depending upon the particular curing agent used. Those of ordinary skill in the art relating to free radical curing agents for polymers are conversant with adjustments of cure times and temperatures required to effect optimum results with any specific free radical agent.

[0089] After molding, the core is removed from the mold and the surface thereof, preferably treated to facilitate adhesion thereof to the covering materials. Surface treatment can be effected by any of the several techniques known in the art, such as corona discharge, ozone treatment, sand blasting, and the like. Preferably, surface treatment is effected by grinding with an abrasive wheel.

[0090] The core is converted into a golf ball by providing at least one layer of covering material thereon, ranging in thickness from about 0.050 to about 0.250 inch and preferably from about 0.060 to about 0.090 inch.

[0091] The cover layers can be formed over the cores by injection molding, compression molding, casting or other conventional molding techniques. Each layer preferably is separately formed. It is preferable to form each layer by either injection molding or compression molding. A more preferred method of making a golf ball of the invention with a multi-layer cover is to successively injection mold each layer in a separate mold. First, the inner cover layer is injection molded over the core in a smooth cavity mold, subsequently any intermediate cover layers are injection molded over the inner cover layer in a smooth cavity mold, and finally the outer cover layer is injection molded over the intermediate cover layers in a dimpled cavity mold.

[0092] The outer cover layer of the golf ball of the present invention is based on a resin material. Non-limiting examples of suitable materials are ionomers, plastomers such as metallocene catalyzed polyolefins, e.g., EXACT, ENGAGE, INSITE or AFFINITY which preferably are cross-linked, polyamides, amide-ester elastomers, graft copolymers of ionomer and polyamide such as CAPRON, ZYTEL, PEBAX, etc., blends containing cross-linked transpolyisoprene, thermoplastic block polyesters such as HYTREL, or thermoplastic or thermosetting polyurethanes, and polyureas such as ESTANE, which is thermoplastic.

[0093] Any inner cover layers which are part of the ball can be made of any of the materials listed in the previous paragraph as being useful for forming an outer cover layer. Furthermore, any inner cover layers can be formed from a number of other non-ionomeric thermoplastics and thermosets. For example, lower cost polyolefins and thermoplastic elastomers can be used. Non-limiting examples of suitable non-ionomeric polyolefin materials include low density polyethylene, linear low density polyethylene, high density polyethylene, polypropylene, rubber-toughened olefin polymers, acid copolymers which do not become part of an ionomeric copolymer when used in the inner cover layer, such as PRIMACOR, NUCREL, ESCOR and ATX, flexomers, thermoplastic elastomers such as styrene/butadiene/styrene (SBS) or styrene/ethylene-butylene/styrene (SEBS) block copolymers, including Kraton (Shell), dynamically vulcanized elastomers such as Santoprene (Monsanto), ethylene vinyl acetates such as Elvax (DuPont), ethylene methyl acrylates such as Optema (Exxon), polyvinyl chloride resins, and other elastomeric materials may be used. Mixtures, blends, or alloys involving the materials described above can be used. It is desirable that the material used for the inner cover layer be a tough, low density material. The non-ionomeric materials can be mixed with ionomers.

[0094] The outer cover layer and any inner cover layers optionally may include processing aids, release agents and/or diluents. Another useful material for any inner cover layer or layers is a natural rubber latex (prevulcanized) which has a tensile strength of 4,000-5,000 psi, high resilience, good scuff resistance, a Shore D hardness of less than 15 and an elongation of 500%.

[0095] When the ball has a single cover layer, it has a thickness of 0.010-0.500 inches, preferably 0.015-0.200 inches, and more preferably 0.025-0.150 inches. When the ball has two or more cover layers, the outer cover layer typically has a thickness of 0.01-0.20 inches, preferably 0.02-0.20 inches, and more preferably 0.025-0.15 inches. The one or more inner cover layers have thicknesses appropriate to result in an overall cover thickness of 0.03-0.50 inches, preferably 0.05-0.30 inches and more preferably 0.10-0.20 inches, with the minimum thickness of any single inner cover layer preferably being 0.01 inches.

[0096] The core and/or cover layers of the golf ball optionally can include fillers to adjust, for example, flex modulus, density, mold release, and/or melt flow index. A description of suitable fillers is provided below in the “Definitions” section.

[0097] The physical characteristics of the cover are such that the ball has a soft feel. When a single cover layer is used, the Shore D hardness of that cover layer is at least 60 in one preferred form of the invention. When the ball has a multi-layer cover, the Shore D hardness of the outer cover layer is at least 60 in another preferred form of the invention. Preferably, the outer cover layer in a single or multi-layer covered ball has a Shore D hardness of at least 63, more preferably at least 65, and most preferably at least 67. The preferred maximum Shore D hardness for the outer cover layer is 90.

[0098] A particularly preferred embodiment of an outer cover layer for use in forming the golf ball of the present invention incorporates high molecular weight ionomer resins, such as EX 1005, 1006, 1007, 1008 and 1009, provided by Exxon Chem. Co., or any combination thereof, as shown in Table 7. These resins are particularly useful in forming the outer cover layer because they have a tensile modulus/hardness ratio that allows for a hard cover over a soft core while maintaining durability.

[0099] Appropriate fillers or additive materials may also be added to produce the cover compositions of the present invention. These additive materials include dyes (for example, Ultramarine Blue sold by Whitaker, Clark and Daniels of South Plainfield, N.J.), and pigments, i.e., white pigments such as titanium dioxide (for example UNITANE 0-110 commercially available from Kemira, Savannah, Ga.) zinc oxide, and zinc sulfate, as well as fluorescent pigments. As indicated in U.S. Pat. No. 4,884,814, the amount of pigment and/or dye used in conjunction with the polymeric cover composition depends on the particular base ionomer mixture utilized and the particular pigment and/or dye utilized. The concentration of the pigment in the polymeric cover composition can be from about 1% to about 10% as based on the weight of the base ionomer mixture. A more preferred range is from about 1% to about 5% as based on the weight of the base ionomer mixture. The most preferred range is from about 1% to about 3% as based on weight of the base ionomer mixture. The most preferred pigment for use in accordance with this invention is titanium dioxide (Anatase).

[0100] Moreover, since there are various hues of white, i.e. blue white, yellow white, etc., trace amounts of blue pigment may be added to the cover stock composition to impart a blue white appearance thereto. However, if different hues of the color white are desired, different pigments can be added to the cover composition at the amounts necessary to produce the color desired.

[0101] In addition, it is within the purview of this invention to add to the cover compositions of this invention compatible materials which do not affect the basic novel characteristics of the composition of this invention. Among such materials are antioxidants (i.e. Santonox R), commercially available from Flexysys, Akron, Ohio, antistatic agents, stabilizers, compatablizers and processing aids. The cover compositions of the present invention may also contain softening agents, such as plasticizers, etc., and reinforcing materials, as long as the desired properties produced by the golf ball covers of the invention are not impaired.

[0102] Furthermore, optical brighteners, such as those disclosed in U.S. Pat. No. 4,679,795 may also be included in the cover composition of the invention. Examples of suitable optical brighteners which can be used in accordance with this invention are Uvitex OB as sold by the Ciba-Geigy Chemical Company, Ardsley, N.Y. Uvitex OB is believed to be 2,5-Bis(5-tert-butyl-2-benzoxazoyl)-thiophene. Examples of other optical brighteners suitable for use in accordance with this invention are as follows: Leucopure EGM as sold by Sandoz, East Hanover, N.J. 07936. Leucopure EGM is thought to be 7-(2n-naphthol(1,2-d)-triazol-2yl(3phenyl-coumarin. Phorwhite K-20G2 is sold by Mobay Chemical Corporation, P.O. Box 385, Union Metro Park, Union, N.J. 07083, and is thought to be a pyrazoline derivative. Eastobrite OB-1 as sold by Eastman Chemical Products, Inc., Kingsport, Tenn. is thought to be 4,4-Bis(-benzoxaczoyl) stilbene. The above-mentioned UVITEX and EASTOBRITE OB-1 are preferred optical brighteners for use in accordance with this invention.

[0103] Moreover, since many optical brighteners are colored, the percentage of optical brighteners utilized must not be excessive in order to prevent the optical brightener from functioning as a pigment or dye in its own right.

[0104] The percentage of optical brighteners which can be used in accordance with this invention is from about 0.01% to about 0.5% as based on the weight of the polymer used as a cover stock. A more preferred range is from about 0.05% to about 0.25% with the most preferred range from about 0.10% to about 0.20% depending on the optical properties of the particular optical brightener used and the polymeric environment in which it is a part.

[0105] Generally, the additives are admixed with an ionomer to be used in the cover composition to provide a masterbatch (abbreviated herein as MB) of desired concentration and an amount of the masterbatch sufficient to provide the desired amounts of additive is then admixed with the copolymer blends.

[0106] Most preferably, the preferred embodiment golf balls of the present invention utilize a particular cover composition in combination with a core having a particular set of physical properties. The formulation of this most preferred core is set forth in Table 6. This most preferred golf ball is as follows in Tables 8A and 8B set forth below.

[0107] Details of the individual components of this most preferred cover composition are set forth below in Tables 9A-9C.

[0108] It will be noted that this most preferred cover formulation includes the combination of a sodium ionomer, a magnesium ionomer, and a zinc ionomer. The amounts of each of these may be tailored as desired, however the preferred amounts are as followed. The sodium ionomer is preferably employed within the range of from about 40% to about 50% of the cover formulation, and most preferably at about 45%. The magnesium ionomer is preferably used within the range of from about 40% to about 50% of the cover formulation, and most preferably at about 45%. The zinc ionomer is preferably used within the range of from about 5% to about 15%, and most preferably at about 10%.

[0109] The golf ball of the present invention preferably has a mechanical impedance with a primary minimum value in the frequency range of 3100 Hz or less, and preferably 100-3100 Hz. This low mechanical impedance provides the ball with a soft feel. This soft feel in combination with excellent distance provide a golf ball which is particularly well suited for use by intermediate players who like a soft ball but desire a greater distance than can be achieved with a conventional balata ball.

[0110] Mechanical impedance is defined as the ratio of magnitude and force acting at a particular point to a magnitude of a responsive velocity at another point when the force is acted. Stated another way, mechanical impedance Z is given by Z=F/V, where F is an externally applied force and V is a responsive velocity of the object to which the force is applied. The velocity V is the internal velocity of the object.

[0111] Mechanical impedance and natural frequency can be depicted graphically by plotting impedance on the “Y” axis and frequency N (Hz) on the “X” axis. A graph of this type is shown in FIGS. 14-21.

[0112] As shown in FIG. 14, a golf ball of Example 2 which is analyzed in Example 4 has a mechanical impedance with a primary minimum value at a first frequency, a mechanical impedance with a secondary minimum value at a higher frequency, and a third minimum value at an even higher frequency. These are known as the primary, secondary and tertiary minimum frequencies. The first minimum value which appears on the graph is not the primary minimum frequency of the ball but instead represents the forced node resonance of the ball due to the introduction of an artificial node, such as a golf club. The forced node resonance is a frequency which may depend in part upon the nature of the artificial node. The existence of forced node resonance is analogous to the change in frequency which is obtained on a guitar by placing a finger over a fret.

[0113] The mechanical impedance can be measured using an accelerometer. Further details regarding natural frequency determinations are provided below in the Examples.

[0114] Referring to FIG. 5, a third embodiment of a golf ball according to present invention is shown and is designated as 110. The ball includes a central core 112 formed from polybutadiene or another cross-linked rubber. A cover layer 114 surrounds the core. The core has a PGA compression of 55 or less. The cover has a Shore D hardness of at least 60. The ball has a PGA compression of 80 or less.

[0115] Referring now to FIG. 6, a cross-sectional view of a fourth embodiment of the invention is shown, and is designated as 120. The ball 120 has a solid core 122, an inner cover layer 124, and an outer cover layer 126. The core has a PGA compression of 55 or less. The outer cover layer has a Shore D hardness of 60 or more. The inner cover layer can be softer or harder than the outer cover layer, but provides the overall ball with a PGA compression of 80 or less.

[0116] A fifth embodiment of a golf ball according to the present invention is shown in FIG. 7, and is designated as 130. The ball includes a solid core 131 which is formed from two layers, namely, an inner core layer 132 and an outer core layer 133. A cover 134 surrounds the core 131. The inner core layer 132 and outer core layer 133 are selected to provide the overall core 131 with a PGA compression of 55 or less. The inner core layer may be harder or softer than the outer core layer and may also be higher in durability. The cover has a Shore D hardness of at least 60. The ball has a PGA compression of 80 or less.

[0117] FIG. 8 shows a cross-sectional view of a sixth embodiment of a golf ball according to the present invention, which is designated as 140. The ball includes a core 141 having an inner core layer 142 and an outer core layer 143. A dual layer cover 144 surrounds the core 141. The dual layer cover 144 includes an inner cover layer 145 and an outer cover layer 146. The core 141 has a PGA compression of 55 or less. The outer cover layer 146 has a Shore D hardness of 60 or more. The ball has a PGA compression of 80 or less.

[0118] FIG. 9 shows yet another preferred embodiment of the present invention, which is designated as 150. The ball 150 has a core 152 formed from one or more layers and a cover 54 formed from one or more layers. The ball is constructed such that the outer cover layer has a Shore D hardness of at least 60, and the ball has a mechanical impedance with a primary minimum value in the frequency range of 3100 Hz, after the ball has been maintained at 21.1° C., 1 atm. and about 50% relative humidity for at least 15 hours.

[0119] Yet another embodiment of a golf ball according to the invention is shown in FIG. 10 and is designated as 160. The ball has a solid core 162 and a cover 164, each of which can be formed of one or more layers. The core 162 has a PGA compression of 55 or less and the cover has a Shore D hardness of at least 58. The ball has a mechanical impedance with a primary minimum value in the frequency range of 3100 Hz or less after the ball has been maintained at 21.1° C., 1 atm. and about 50% relative humidity for at least 15 hours.

[0120] Yet another embodiment of a golf ball according to the invention is shown in FIG. 11. The ball 170 includes a solid or wound core 172 and a cover 174. Each of the core and cover can have one or more layers. The outer cover layer of the ball has a Shore D hardness of at least 60. The ball has a mechanical impedance with a primary minimum value in the frequency range of 2600 or less after the ball has been maintained at 21.1, 1 atm. and about 50% relative humidity for at least 15 hours.

[0121] Yet another preferred form of the invention is shown in FIG. 12 and is designated as 180. The ball 180 has a core 182 which can be solid or wound, and a cover 184. The ball includes a core 182 which can be solid or wound, and can have one or more layers, and a cover 184 which can have one or more layers. The core has a PGA compression of 55 or less. The ball has a mechanical impedance with a primary minimum value in the frequency range of 2600 Hz or less after the ball has been maintained at 21.1° C., 1 atm. and about 50% relative humidity for at least 15 hours.

[0122] The composition of the cover may vary depending upon the desired properties for the resulting golf ball. A wide array of cover formulations may be utilized such as those disclosed in U.S. Pat. Nos. 4,986,545; 5,098,105; 5,120,791; 5,187,013; 5,306,760; 5,312,857; 5,324,783; 5,328,959; 5,330,837; 5,338,610; 5,542,677; 5,580,057; 5,591,803; and 5,733,206, all of which are hereby incorporated by reference.

[0123] The covered golf ball can be formed in any one of several methods known in the art. For example, the molded core may be placed in the center of a golf ball mold and the ionomeric resin-containing cover composition injected into and retained in the space for a period of time at a mold temperature of from about 40° F. to about 120° F.

[0124] Alternatively, the cover composition may be injection molded at about 300° F. to about 450° F. into smooth-surfaced hemispherical shells, a core and two such shells placed in a dimpled golf ball mold and unified at temperatures on the order of from about 200° F. to about 300° F.

[0125] The golf ball produced is then painted and marked, painting being effected by spraying techniques.

[0126] The following is a series of definitions used in the specification and claims.

[0127] PGA compression is an important property involved in the performance of a golf ball. The compression of the ball can affect the playability of the ball on striking and the sound or “click” produced. Similarly, compression can effect the “feel” of the ball (i.e., hard or soft responsive feel), particularly in chipping and putting.

[0128] Moreover, while compression itself has little bearing on the distance performance of a ball, compression can affect the playability of the ball on striking. The degree of compression of a ball against the club face and the softness of the cover strongly influences the resultant spin rate. Typically, a softer cover will produce a higher spin rate than a harder cover. Additionally, a harder core will produce a higher spin rate than a softer core. This is because at impact a hard core serves to compress the cover of the ball against the face of the club to a much greater degree than a soft core thereby resulting in more “grab” of the ball on the clubface and subsequent higher spin rates. In effect the cover is squeezed between the relatively incompressible core and clubhead. When a softer core is used, the cover is under much less compressive stress than when a harder core is used and therefore does not contact the clubface as intimately. This results in lower spin rates.

[0129] The term “compression” utilized in the golf ball trade generally defines the overall deflection that a golf ball undergoes when subjected to a compressive load. For example, PGA compression indicates the amount of change in golf ball's shape upon striking. The development of solid core technology in two-piece balls has allowed for much more precise control of compression in comparison to thread wound three-piece balls. This is because in the manufacture of solid core balls, the amount of deflection or deformation is precisely controlled by the chemical formula used in making the cores. This differs from wound three-piece balls wherein compression is controlled in part by the winding process of the elastic thread. Thus, two-piece and multi-layer solid core balls exhibit much more consistent compression readings than balls having wound cores such as the thread wound three-piece balls.

[0130] In the past, PGA compression related to a scale of from 0 to 200 given to a golf ball. The lower the PGA compression value, the softer the feel of the ball upon striking. In practice, tournament quality balls have compression ratings around 70-110, preferably around 80 to 100.

[0131] In determining PGA compression using the 0-200 scale, a standard force is applied to the external surface of the ball. A ball which exhibits no deflection (0.0 inches in deflection) is rated 200 and a ball which deflects {fraction (2/10)}th of an inch (0.2 inches) is rated 0. Every change of 0.001 of an inch in deflection represents a 1 point drop in compression. Consequently, a ball which deflects 0.1 inches (100×0.001 inches) has a PGA compression value of 100 (i.e., 200-100) and a ball which deflects 0.110 inches (110×0.001 inches) has a PGA compression of 90 (i.e., 200-110).

[0132] In order to assist in the determination of compression, several devices have been employed by the industry. For example, PGA compression is determined by an apparatus fashioned in the form of a small press with an upper and lower anvil. The upper anvil is at rest against a 200-pound die spring, and the lower anvil is movable through 0.300 inches by means of a crank mechanism. In its open position the gap between the anvils is 1.780 inches allowing a clearance of 0.100 inches for insertion of the ball. As the lower anvil is raised by the crank, it compresses the ball against the upper anvil, such compression occurring during the last 0.200 inches of stroke of the lower anvil, the ball then loading the upper anvil which in turn loads the spring. The equilibrium point of the upper anvil is measured by a dial micrometer if the anvil is deflected by the ball more than 0.100 inches (less deflection is simply regarded as zero compression) and the reading on the micrometer dial is referred to as the compression of the ball. In practice, tournament quality balls have compression ratings around 80 to 100 which means that the upper anvil was deflected a total of 0.120 to 0.100 inches.

[0133] An example to determine PGA compression can be shown by utilizing a golf ball compression tester produced by Atti Engineering Corporation of Newark, N.J. The value obtained by this tester relates to an arbitrary value expressed by a number which may range from 0 to 100, although a value of 200 can be measured as indicated by two revolutions of the dial indicator on the apparatus. The value obtained defines the deflection that a golf ball undergoes when subjected to compressive loading. The Atti test apparatus consists of a lower movable platform and an upper movable spring-loaded anvil. The dial indicator is mounted such that it measures the upward movement of the springloaded anvil. The golf ball to be tested is placed in the lower platform, which is then raised a fixed distance. The upper portion of the golf ball comes in contact with and exerts a pressure on the springloaded anvil. Depending upon the distance of the golf ball to be compressed, the upper anvil is forced upward against the spring.

[0134] Alternative devices have also been employed to determine compression. For example, Applicant also utilizes a modified Riehle Compression Machine originally produced by Riehle Bros. Testing Machine Company, Phil., Pa. to evaluate compression of the various components (i.e., cores, mantle cover balls, finished balls, etc.) of the golf balls. The Riehle compression device determines deformation in thousandths of an inch under a fixed initialized load of 200 pounds. Using such a device, a Riehle compression of 61 corresponds to a deflection under load of 0.061 inches.

[0135] Additionally, an approximate relationship between Riehle compression and PGA compression exists for balls of the same size. It has been determined by Applicant that Riehle compression corresponds to PGA compression by the general formula PGA compression=160−Riehle compression. Consequently, 80 Riehle compression corresponds to 80 PGA compression, 70 Riehle compression corresponds to 90 PGA compression, and 60 Riehle compression corresponds to 100 PGA compression. For reporting purposes, Applicant's compression values are usually measured as Riehle compression and converted to PGA compression.

[0136] Furthermore, additional compression devices may also be utilized to monitor golf ball compression so long as the correlation to PGA compression is know. These devices have been designed, such as a Whitney Tester, to correlate or correspond to PGA compression through a set relationship or formula.

[0137] The resilience or coefficient of restitution (COR) of a golf ball is the constant “e,” which is the ratio of the relative velocity of an elastic sphere after direct impact to that before impact. As a result, the COR (“e”) can vary from 0 to 1, with 1 being equivalent to a perfectly or completely elastic collision and 0 being equivalent to a perfectly or completely in elastic collision.

[0138] COR, along with additional factors such as club head speed, club head mass, ball weight, ball size and density, spin rate, angle of trajectory and surface configuration (i.e., dimple pattern and area of dimple coverage) as well as environmental conditions (e.g. temperature, moisture, atmospheric pressure, wind, etc.) generally determine the distance a ball will travel when hit. Along this line, the distance a golf ball will travel under controlled environmental conditions is a function of the speed and mass of the club and size, density and resilience (COR) of the ball and other factors. The initial velocity of the club, the mass of the club and the angle of the ball's departure are essentially provided by the golfer upon striking. Since club head, club head mass, the angle of trajectory and environmental conditions are not determinants controllable by golf ball producers and the ball size and weight are set by the U.S.G.A., these are not factors of concern among golf ball manufacturers. The factors or determinants of interest with respect to improved distance are generally the coefficient of restitution (COR) and the surface configuration (dimple pattern, ratio of land area to dimple area, etc.) of the ball.

[0139] The COR in solid core balls is a function of the composition of the molded core and of the cover. The molded core and/or cover may be comprised of one or more layers such as in multi-layered balls. In balls containing a wound core (i.e., balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings. As in the solid core balls, the center and cover of a wound core ball may also consist of one or more layers.

[0140] The coefficient of restitution is the ratio of the outgoing velocity to the incoming velocity. In the examples of this application, the coefficient of restitution of a golf ball was measured by propelling a ball horizontally at a speed of 125+/−5 feet per second (fps) and corrected to 125 fps against a generally vertical, hard, flat steel plate and measuring the ball's incoming and outgoing velocity electronically. Speeds were measured with a pair of Oehler Mark 55 ballistic screens available from Oehler Research, Inc., P.O. Box 9135, Austin, Tex. 78766, which provide a timing pulse when an object passes through them. The screens were separated by 36″ and are located 25.25″ and 61.25″ from the rebound wall. The ball speed was measured by timing the pulses from screen 1 to screen 2 on the way into the rebound wall (as the average speed of the ball over 36″), and then the exit speed was timed from screen 2 to screen 1 over the same distance. The rebound wall was tilted 2 degrees from a vertical plane to allow the ball to rebound slightly downward in order to miss the edge of the cannon that fired it. The rebound wall is solid steel 2.0 inches thick.

[0141] As indicated above, the incoming speed should be 125±5 fps but corrected to 125 fps. The correlation between COR and forward or incoming speed has been studied and a correction has been made over the ±5 fps range so that the COR is reported as if the ball had an incoming speed of exactly 125.0 fps.

[0142] The coefficient of restitution must be carefully controlled in all commercial golf balls if the ball is to be within the specifications regulated by the United States Golf Association (U.S.G.A.). As mentioned to some degree above, the U.S.G.A. standards indicate that a “regulation” ball cannot have an initial velocity exceeding 255 feet per second in an atmosphere of 75° F. when tested on a U.S.G.A. machine. Since the coefficient of restitution of a ball is related to the ball's initial velocity, it is highly desirable to produce a ball having sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of softness (i.e., hardness) to produce enhanced playability (i.e., spin, etc.).

[0143] As used herein, “Shore D hardness” of a cover layer is measured generally in accordance with ASTM D-2240, except the measurements are made on the curved surface of a molded cover layer, rather than on a plaque. Furthermore, the Shore D hardness of the cover layer is measured while the cover layer remains over the core and any underlying cover layers. When a hardness measurement is made on a dimpled cover, Shore D hardness is measured at a land area of the dimpled cover.

[0144] Plastomers are polyolefin copolymers developed using metallocene single-site catalyst technology. Polyethylene plastomers generally have better impact resistance than polyethylenes made with Ziegler-Natta catalysts. Plastomers exhibit both thermoplastic and elastomeric characteristics. In addition to being comprised of a polyolefin such as ethylene, plastomers contain up to about 35 wt % comonomer. Plastomers include but are not limited to ethylene-butene copolymers, ethylene-octene copolymers, ethylene-hexene copolymers, and ethylene-hexene-butene terpolymers, as well as mixtures thereof.

[0145] The plastomers which are useful in the invention preferably are formed by a single site metallocene catalyst such as those disclosed in EP 29368, U.S. Pat. No. 4,752,597; U.S. Pat. No. 4,808,561; and U.S. Pat. No. 4,937,299 the teachings of which are incorporated herein by reference. Blends of plastomers can be used. As is known in the art, plastomers can be produced by solution, slurry and gas phase accesses but the preferred materials are produced by metallocene catalysis using a high pressure process by polymerizing ethylene in combination with other olefin monomers, such as butene-1, hexene-1, octene-1 and 4-methyl-1-pentene in the presence of catalyst system comprising a cyclopentadienyl-transition metal compound and an alumoxane.

[0146] Plastomers found especially useful in the invention are those sold by Exxon Chemical under the trademark “EXACT” and include linear ethylene-butene copolymers such as EXACT 3024 having a density of about 0.905 g/cc (ASTM D-1505) and a melt index of about 4.5 g/10 min. (ASTM D-2839); EXACT 3025 having a density of about 0.910 g/cc (ASTM D-1505) and a melt index of about 1.2 g/10 min. (ASTM D-2839); EXACT 3027 having a density of about 0.900 g/cc (ASTM D-1505) and a melt index of about 3.5 g/10 min. (ASTM D-2839). Other useful plastomers include but are not limited to ethylene-hexene copolymers such as EXACT 3031 having a density of about 0.900 g/cc (ASTM D-1505) and a melt index of about 3.5 g/10 min. (ASTM D-2839), as well as EXACT 4049, which is an ethylene-butene copolymer having a density of about 0.873 g/cc (ASTM D-1505) and a melt index of about 4.5 g/10 min. (ASTM D-2839). All of the above EXACT series plastomers are available from EXXON Chemical Co.

[0147] EXACT plastomers typically have a dispersion index (M_W/M_nwhere M_Wis weight average molecular weight and M_nis number average molecular weight) of about 1.5 to 4.0, preferably 1.5-2.4, a molecular weight of about 5,000 to 50,000, preferably about 20,000 to about 30,000 a density of about 0.86 to about 0.93 g/cc, preferably about 0.87 g/cc to about 0.91 g/cc, a melting point of about 140-220 F, and a melt flow index (MI) above about 0.5 g/10 mins, preferably about 1-10 g/10 mins as determined by ASTM D-1238, condition E. Plastomers which may be employed in the invention include copolymers of ethylene and at least one C₃-C₂₀-olefin, preferably a C₄-C₈-olefin present in an amount of about 5 to about 32 wt %, preferably about 7 to about 22 wt %, more preferably about 9-18 wt %. These plastomers are believed to have a composition distribution breadth index of about 45% or more.

[0148] Plastomers such as those sold by Dow Chemical Co. under the trade name ENGAGE also may be employed in the invention. These plastomers are believed to be produced in accordance with U.S. Pat. No. 5,272,236, the teachings of which are incorporated herein by reference. These plastomers are substantially linear polymers having a density of about 0.85 g/cc to about 0.93 g/cc measured in accordance with ASTM D-792, a melt index (MI) of less than 30 g/10 minutes, a melt flow ratio (I₁₀/I₂)of about 7 to about 20, where I₁₀is measured in accordance with ASTM D-1238 (190/10) and I₂is measured in accordance with ASTM D-1238 (190/2.16), and a dispersion index M_W/M_nwhich preferably is less than 5, and more preferably is less than about 3.5 and most preferably is from about 1.5 to about 2.5. These plastomers include homopolymers of C₂-C₂₀olefins such as ethylene, propylene, 4-methyl-1-pentene, and the like, or they can be interpolymers of ethylene with at least one C₃-C₂₀-olefin and/or C₂-C₂₀acetylenically unsaturated monomer and/or C₄-C₁₈diolefins. These plastomers have a polymer backbone that is either unsubstituted or substituted with up to 3 long chain branches/1000 carbons. As used herein, long chain branching means a chain length of at least about 6 carbons, above which the length cannot be distinguished using ¹³C nuclear magnetic resonance spectroscopy. The preferred ENGAGE plastomers are characterized by a saturated ethylene-octene backbone and a narrow dispersion index M_W/M_nof about 2. Other commercially available plastomers may be useful in the invention, including those manufactured by Mitsui.

[0149] The dispersion index M_W/M_nof plastomers made in accordance with U.S. Pat. No. 5,272,236 most preferably is about 2.0. Non-limiting examples of these plastomers include ENGAGE CL 8001 having a density of about 0.868 g/cc, a melt index of about 0.5 g/10 mins, and a Shore A hardness of about 75; ENGAGE CL 8002 having a density of about 0.87 g/cc, a melt index of about 1 gms/10 min, Shore A hardness of about 75; ENGAGE CL 8003 having a density of about 0.885 g/cc, melt index of about 1.0 gms/10 min, and a Shore A hardness of about 86; ENGAGE EG 8100 having a density of about 0.87 g/cc, a melt index of about 1 gms/10 min., and a Shore A hardness of about 87; ENGAGE 8150 having a density of about 0.868 g/cc, a melt index of about 0.5 gms/10 min, and a Shore A hardness of about 75; ENGAGE 8200 having a density of about 0.87 g/cc, a melt index of about 5 g/10 min., and a Shore A hardness of about 75; and ENGAGE EP 8500 having a density of about 0.87 gms/cc, a melt index of about 5 g/10 min., and a Shore A hardness of about 75.

[0150] Fillers preferably are used to adjust the density, flex modulus, mold release, and/or melt flow index of the inner cover layer. More preferably, at least when the filler is for adjustment of density or flex modulus, it is present in an amount of at least five parts by weight based upon 100 parts by weight of the resin composition. With some fillers, up to about 200 parts by weight probably can be used. A density adjusting filler according to the invention preferably is a filler which has a specific gravity which is at least 0.05 and more preferably at least 0.1 higher or lower than the specific gravity of the resin composition. Particularly preferred density adjusting fillers have specific gravities which are higher than the specific gravity of the resin composition by 0.2 or more, even more preferably by 2.0 or more. A flex modulus adjusting filler according to the invention is a filler which, when used in an amount of e.g. 1-100 parts by weight based upon 100 parts by weight of resin composition, will raise or lower the flex modulus (ASTM D-790) of the resin composition by at least 1% and preferably at least 5% as compared to the flex modulus of the resin composition without the inclusion of the flex modulus adjusting filler. A mold release adjusting filler is a filler which allows for easier removal of part from mold, and eliminates or reduces the need for external release agents which otherwise could be applied to the mold. A mold release adjusting filler typically is used in an amount of up to about 2 wt % based upon the total weight of the inner cover layer. A melt flow index adjusting filler is a filler which increases or decreases the melt flow, or ease of processing of the composition.

[0151] The cover layers may contain coupling agents that increase adhesion of materials within a particular layer e.g. to couple a filler to a resin composition, or between adjacent layers. Non-limiting examples of coupling agents include titanates, zirconates and silanes. Coupling agents typically are used in amounts of 0.1-2 wt % based upon the total weight of the composition in which the coupling agent is included.

[0152] A density adjusting filler is used to control the moment of inertia, and thus the initial spin rate of the ball and spin decay. The additional a filler with a lower specific gravity than the resin composition results in a decrease in moment of inertia and a higher initial spin rate than would result if no filler were used. The addition of a filler with a higher specific gravity than the resin composition results in an increase in moment of inertia and a lower initial spin rate. High specific gravity fillers are preferred as less volume is used to achieve the desired inner cover total weight. Nonreinforcing fillers are also preferred as they have minimal effect on COR. Preferably, the filler does not chemically react with the resin composition to a substantial degree, although some reaction may occur when, for example, zinc oxide is used in a cover layer which contains some ionomer.

[0153] The density-increasing fillers for use in the invention preferably have a specific gravity in the range of 1.0-20. The density-reducing fillers for use in the invention preferably have a specific gravity of 0.06-1.4, and more preferably 0.06-0.90. The flex modulus increasing fillers have a reinforcing or stiffening effect due to their morphology, their interaction with the resin, or their inherent physical properties. The flex modulus reducing fillers have an opposite effect due to their relatively flexible properties compared to the matrix resin. The melt flow index increasing fillers have a flow enhancing effect due to their relatively high melt flow versus the matrix. The melt flow index decreasing fillers have an opposite effect due to their relatively low melt flow index versus the matrix.

[0154] Fillers may be or are typically in a finely divided form, for example, in a size generally less than about 20 mesh, preferably less than about 100 mesh U.S. standard size, except for fibers and flock, which are generally elongated. Flock and fiber sizes should be small enough to facilitate processing. Filler particle size will depend upon desired effect, cost, ease of addition, and dusting considerations. The filler preferably is selected from the group consisting of precipitated hydrated silica, clay, talc, asbestos, glass fibers, aramid fibers, mica, calcium metasilicate, barium sulfate, zinc sulfide, lithopone, silicates, silicon carbide, diatomaceous earth, polyvinyl chloride, carbonates, metals, metal alloys, tungsten carbide, metal oxides, metal stearates, particulate carbonaceous materials, micro balloons, and combinations thereof. Non-limiting examples of suitable fillers, their densities, and their preferred uses are as follows in Table 10:

[0155] The amount of filler employed is primarily a function of weight requirements and distribution.

[0156] Ionomeric resins include copolymers formed from the reaction of an olefin having 2 to 8 carbon atoms and an acid which includes at least one member selected from the group consisting of alpha, beta-ethylenically unsaturated mono- or dicarboxylic acids with a portion of the acid groups being neutralized with cations. Terpolymer ionomers further include an unsaturated monomer of the acrylate ester class having from 1 to 21 carbon atoms. The olefin preferably is an alpha olefin and more preferably is ethylene. The acid preferably is acrylic acid or methacrylic acid. The ionomers typically have a degree of neutralization of the acid groups in the range of about 10-100%.

[0157] The present invention is further illustrated by the following examples in which the parts of the specific ingredients are by weight. It is to be understood that the present invention is not limited to the examples, and various changes and modifications may be made in the invention without departing from the spirit and scope thereof.

[0158] Example 1

[0159] Using the ingredients tabled below, golf ball cores having a finished diameter of about 1.470 to about 1.475 inches were produced by compression molding and subsequent removal of a surface layer by grinding. Each core was formulated using 100 parts elastomer (rubber). In the formulations, the amounts of remaining ingredients are expressed in parts by weight, and the coefficient of restitution and compression achieved are set forth below.

[0160] Tables 11 and 12 summarize the results of testing of four core compositions.

[0161] The results of Table 12 show that the high Mooney BCP-820 is 9 points higher in C.O.R. vs. the control (low Mooney BR-1220). Blends of the high Mooney BCP-820 with Neodymium catalyzed Neo Cis 60 and CB-22 also show a 5 to 6 point gain in C.O.R.

[0162] The high Mooney BCP-820, while giving high C.O.R. values, is extremely difficult to process using conventional equipment. Blending the high Mooney BCP-820 with Neodymium catalyzed polybutadiene rubber solves the processing problems but maintains the high C.O.R. values.

[0163] Tables 13 and 14 summarize the results of additional testing.

[0164] The results in Table 14 show there is very little difference in the 6 trials when the C.O.R. is corrected for differences in compression, i.e. Nes Factor. The Neodymium rubber when used at 100% or when blended with high Mooney BCP-820 is equal to the properties of the high Mooney BCP-820 when used at 100%. Neodymium rubber when used at 100% is also extremely difficult to process due to high die swell during extrusion of preforms and high cold flow of the rubber causing deformed preforms resulting in very high rejects. Neodymium polybutadiene and high Mooney polybutadiene rubber such as BCP-820 cannot be processed when used alone at 100 parts or conventional equipment such as two roll mills and extruders.

[0165] When the two rubbers above are blended together, processing of the synergistic mixture becomes easy and practical without losing any performance or C.O.R.

[0166] Tables 15 and 16 summarize the results of testing of four additional core compositions.

[0167] Tables 17 and 18 summarize the results of testing of additional core compositions.

[0168] Tables 17 and 18 demonstrate that when the Zinc Diacrylate (ZDA) level is adjusted to obtain the same Riehle compression as the Control, the C.O.R. increased 7 points higher for the BCP-820 and the Nes Factor was also 7 points higher.

[0169] Tables 19 and 20 summarize the results of additional testing of core compositions.

[0170] Tables 19 and 20 demonstrate that, despite adjusting the ZDA level, the Riehle compressions were different. However, the Nes Factor shows that Trial #3 using 100% BCP-820 is 7 points higher than the Control.

[0171] Table 21 summarizes additional testing.

[0172] Tables 22A-22D and 23 summarize the resulting balls and their components.

[0173] Cores were centerless ground to 1.470″ and injection molded with a high modulus clear ionomer mantle. See Table 23 for mantle composition.

[0174] Mantled cores were injection molded with a soft, low modulus ionomer cover into dimpled molded golf balls. See Table 23 for cover composition.

[0175] Molded balls were trimmed, brush tumbled, primed, stamped, and clear coated.

[0176] It is evident from the proceeding tables that the high Mooney cobalt catalyzed polybutadiene BCP-820 produces a higher C.O.R. (3-7 points) vs. the low Mooney cobalt catalyzed polybutadiene. Blending with the low Mooney polybutadiene produces less of a gain in C.O.R.

[0177] A number of golf ball cores were made having the following formulation and characteristics as shown in Table 24.

[0178] The cores had a diameter of 1.560 inches, a PGA compression of about 40 and a COR of about 0.775. To make the cores, the core ingredients were intimately mixed in an internal mixer until the compositions were uniform, usually over a period of from about 5 to about 20 minutes. The sequence of addition of the components was not found to be critical. As a result of shear during mixing, the temperature of the core mixtures rose to about 190° F. whereupon the batch was discharged onto a two roll mill, mixed for about one minute and sheeted out.

[0179] The sheet was rolled into a “pig” and then placed in a Barwell preformer and slugs produced. The slugs were then subjected to compression molding at about 310° F. for about 11½ minutes. After molding, the cores were cooled under ambient conditions for about 4 hours. The molded cores were then subjected to a centerless grinding operation whereby a thin layer of the molded core was removed to produce a round core having a diameter of 1.2 to 1.5 inches. Upon completion, the cores were measured for size and in some instances weighed and tested to determine compression and COR.

[0180] The cores were covered with an injection-molded cover blend of 35 parts by weight EX® 1006 (Exxon Chemical Corp., Houston, Tex.), 55.6 parts by weight EX 1007 (Exxon Chemical Corp., Houston, Tex.) and 9.4 parts by weight of Masterbatch. The Masterbatch contained 100 parts by weight lotek 7030, 31.72 parts by weight titanium dioxide (Unitane 0-110), 0.6 parts by weight pigment (Ultramarine Blue), 0.35 parts by weight optical brightener (Eastobrite OB1) and 0.05 parts by weight stabilizer (Santanox R).

[0181] The cover had a thickness of 0.055 inches and a Shore D hardness of 67. The balls had a PGA compression of 65 and a COR of 0.795.

[0182] The procedure of Example 2 was repeated with the exception that a different cover formulation was used.

[0183] The cores were covered with a cover blend of 54.5 parts by weight Surlyn 9910, 22.0 parts by weight Surlyn 8940, 10.0 parts by weight Surlyn 8320, 4.0 parts by weight Surlyn 8120, and 9.5 parts by weight of Masterbatch. The Masterbatch had the same formulation as that of Example 2.

[0184] The cover had a thickness of 0.55 inches and a Shore D hardness of 63. The balls had a PGA compression of 63 and a COR of 0.792.

[0185] A number of frequency measurements based upon audible sound were made for the sound of contact between a putter and 11 different types of golf balls, including the balls of Example 2. Three balls of each type were tested.

[0186] The putter was a 1997 Titleist Scotty Cameron putter. An accelerometer (Vibra-Metrics, Inc., Hamden, Conn., Model 9001A, Serial No. 1225) was placed on the back cavity of the putter head. The output of the accelerometer was powered by a Vibra-Metrics, Inc., Hamden, Conn., Model P5000 accelerometer power supply, at a gain of ×1. A microphone was positioned proximate to the intended point of contact between the putter and the ball. The microphone stand was placed at the distal end of the putter head such that the microphone itself was positioned 3 centimeters above the sweet spot at a downfacing angle of 30°. A preamplifier (Realistic Model 42-2101A, Radio Shack) was used for the microphone. Signals were collected using a Metrabyte Das-58 A-D board with a SSH-04 simultaneous sample and hold module (Keithley Instruments, Cleveland, Ohio) at a rate of 128 kHz. The microphone was a Radio Shack Model 33-3007 unidirectional condenser microphone with a frequency response of 50-15000 Hz.

[0187] The putter was positioned by a putting pendulum so that when properly balanced the ground clearance was one millimeter. The balls were hit from the sweet spot of the putter. The club was drawn back to the 20° mark on the putting pendulum. Contact with the ball occurred when the putter was at a 90° angle relative to the ground.

[0188] The point of contact between the club and the ball could be determined by viewing the signal from the accelerometer. Pre-trigger and post-trigger data was collected for each shot. Data was collected at 128 kHz for a duration of 64 microseconds, resulting in 8,192 data points per shot. The data was saved in ASCII text files for subsequent analysis. Each ball was struck 10 times in a random sequence, i.e., all 33 balls were struck before any ball was struck a second time and the striking order was randomly changed for each set of hits. Data for the three balls of each particular type was averaged. The results are shown below on Table 25.

[0189] As shown by the results on Table 25, the balls of Example 2 had a lower frequency measurement based upon sound than all of the other balls that were tested.

[0190] Mechanical impedance and natural frequency of the golf balls of the invention were determined, along with the mechanical impedance and natural frequency of commercially available golf balls.

[0191] Impedance was determined using a measurement of acceleration response over sine-sweep based frequencies.

[0192] FIG. 13 schematically shows the equipment used to determine mechanical impedance of golf balls in accordance with the present invention. A power amplifier 210 (IMV Corp. PET-0A) was obtained and connected to a vibrator 212 (IMV Corp. PET-01). A dynamic signal analyzer 214 (Hewlett Packard 35670A) was obtained and connected to the amplifier 210 to provide a sine-sweep source to 10,000 Hz. An input accelerometer 216 (PCB Piezotronics, Inc., New York, A353B17) was physically connected to the vibrator 212 (with Loctite 409 adhesive and electrically connected to the dynamic signal analyzer 214. The dynamic signal analyzer 214 was programmed such that it could calculate the mechanical impedance given two acceleration measurements and could plot this data over a frequency range.

[0193] An output accelerometer 218 (PCB Piezotronics, Inc., New York, A353B17) was obtained and electrically connected to the dynamic signal analyzer 214. A first golf ball sample 220 was obtained and bonded to the vibrator 212 using Loctite 409 adhesive. The output accelerometer 218 also was bonded to the ball using Loctite 409 adhesive. The vibrator 212 was turned on and a sweep was made from 100 to 10,000 Hz. Mechanical impedance was then plotted over this frequency range.

[0194] The natural frequency was determined by observing the frequency at which a second minimum occurred in the impedance curve. The first minimum value was determined to be a result of forced node resonance resulting from contact with the accelerometer 218 or the vibrator 212. This determination about the first minimum value is based upon separate tests which compared the above described mechanical impedance test method, referred to the “sine-sweep method” of determining mechanical impedance, as compared to an “impact method” in which a golf ball is suspended from a string and is contacted with an impact hammer on one side with accelerometer measurements taken opposite the impact hammer.

[0195] The mechanical impedance and natural frequency of the balls of Examples 2 and 3 above were determined using the above-described method. The first set of data was taken with the balls at room temperature. The second set of data was taken after the balls had been maintained at 21.1° C. (70° F.) for a period of time, preferably at least 15 hours. Furthermore, 12 commercially available golf balls also were tested. The results are shown below on Table 26.

[0196] Additionally, a non-commercial, non-wound ball with a liquid (salt/sugar water) core was tested and was found to have a natural frequency of 3961.

[0197] As shown by the results on Table 26, the balls of the present invention have a low natural frequency in combination with a relatively high COR. The low natural frequency provides the balls with a soft sound and feel while maintaining good distance.

[0198] Another series of trials were carried out in which the core hardness gradient was measured for the most preferred core composition according to the present invention. As will be recalled, that most preferred core composition is set forth in Table 6 herein.

[0199] In yet another series of trials, a series of particularly preferred cores and covers were formed to produce particularly preferred golf balls in accordance with the present invention. These preferred cover compositions and core compositions are set forth below in Tables 28A and 28B, respectively.

[0200] Golf balls were produced using various combinations of these cover and core compositions. A summary of these balls is set forth below in Table 28C. The noted barrel test is utilized to determine an indication of durability. This test is described in detail in U.S. Pat. Nos. 5,827,134 and 5,820,489 herein incorporated by reference. Basically, the test involves the use of an air cannon that fires a golf ball at about 135 ft/second into a five-sided steel drum until the ball breaks. One dozen of each ball type is tested. The average is the average number of runs until the ball breaks. Higher averages indicate greater durability.

[0201] In another set of trials, a series of particularly preferred cores and covers were formed to produce particularly preferred golf balls in accordance with the present invention. These preferred cover compositions and core compositions are set forth below in Tables 29A and 29B, respectively.

[0202] These various preferred cover and core formulations were utilized to provide a series of particularly preferred golf balls. A summary of these balls is set forth in Table 29C.

[0203] The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such alternations and modifications insofar as they come within the scope of the claims and the equivalents thereof.