For many years the most popular golf balls were formed of three pieces comprising a liquid center, a surrounding thread winding, and a cover made of balata or balata blends.

Such balls were preferred by relatively skilled players because they had a very soft feel and a subdued sound. Furthermore, the relative softness of the cover material allowed the player to apply a spin to the ball to control the ball in flight and on the green.

Such balata covered balls were disadvantageous, however, in that they had low tear resistance and were easily cut in play. Balata covered balls, therefore, had a relatively short life span.

To overcome these deficiencies, golf balls manufactured in more recent years have been primarily of a two-piece type comprising polybutadiene cores with covers selected from a family of ionomers. The most popular ionomers are those sold by E.I. duPont de Nemours & Company under the trademark SURLYN.

The performance of a golf ball is affected by both the cover and core composition.

Sullivan U.S. Patent No. 5,018,740 discusses this in terms of the coefficient of restitution ("COR") of a golf ball. This measurement is achieved by propelling a ball at a given speed against a hard massive surface. The COR is the ratio of the outgoing velocity to the incoming velocity expressed in decimal form.

Sullivan proceeds to emphasize the desire to produce golf ball cores with high COR measurements. This minimizes the need to produce covers with high COR measurements whereby the cover material can take on characteristics enhancing performance in other aspects. For example, "soft" Surlyn monomers can be used to enhance spin rate.

Sullivan also emphasizes that the core material must have satisfactory compressibility. It is pointed out, in particular, that achieving this characteristic often requires a "tradeoff" since core components which increase COR often do so at a sacrifice of compressibility. Sullivan goes on to suggest use of up to five parts per hundred of zinc di-2-ethylexoate in the core resin. Polybutadiene is suggested as the preferred resin.

U.S. Patent No. 5,387,637 issued to Sullivan also discloses a core composition focusing on COR and compressibility. This patent suggests the addition of naturally occurring crystalline silicas to a polybutadiene resin.

Although prior art approaches to improving core compositions have met with a degree of success, problem are still encountered. In particular, consistency in molding of cores is still necessary so that balls produced in mass production environments will have predictable properties. Polybutadiene polymers are known to have low green strength and poor knitting characteristics in the mixing and processing stages making the material difficult to extrude and pre-form prior to the core vulcinization process. Pre-cut slugs tend to form concave ends which trap air in the molding cavities resulting in blemished products. A typical composition of the type suggested by the prior art will include the polybutadiene in combination with other ingredients in the following proportions: Example I Parts bv Weight POLYBUTADIENE 100.00 ZINC ACRYLATE 30.00 DI-CUMYL PEROXIDE 0.75 ZINC OXIDE 5.00 SUMMARY OF THE INVENTION This invention relates to an improved core composition for golf balls and to an improved method for producing the golf balls. The golf ball core composition of this invention is suitable for use in connection with a variety of golf ball designs such as the two-piece balls of the type disclosed, for example, in the Sullivan patents and in Saito Patent No. 4,337,947. The invention is also applicable to three-piece balls such as disclosed in Egashire Patent No. 5,439,227 and Dennis Patent No. 4,431,193.

This invention more specifically involves the use of particular polybutadiene polymer blended together to form the core composition. Specifically, the cores are formed from material selected from the group consisting of high CIS 1.4 polybutadiene of the cobalt, nickel or neodymium type, and combinations thereof.

Cores made in accordance with the invention exhibit dramatically improved mixing, extrusion and pre-form properties. Slugs produced with these compositions demonstrate a very stable profile with no distortion resulting in a virtual elimination of defects. In addition, the cores are characterized by most acceptable COR and compressibility values.

DETAILED DESCRIPTION OF THE INVENTION The core compositions of this invention comprise certain polybutadiene compositions blended together to form the basic core ingredient. Specifically, the invention contemplates the use of: High CIS 1.4 Polybutadiene (Cobalt type) High CIS 1.4 Polybutadiene (Nickel type) High CIS 1.4 Polybutadiene (Neodymium type) The polybutadiene is used in combination with standard additives used in core manufacture. Zinc acrylate, a peroxide initiator and a filler such as zinc oxide are typically used in this regard.

The polybutadienes utilized are preferably included in substantially equal amounts, for example, 30-35 parts by weight of each. Such blends are typically formulated as follows: Example II Parts bv Weight HIGH CIS 1.4 POLYBUTADIENE (COBALT TYPE) 33.33 HIGH CIS 1.4 POLYBUTADIENE (NEODYMIUM TYPE) 33.33 HIGH CIS 1.4 POLYBUTADIENE (NICKEL TYPE) 33.33 ZINC ACRYLATE 30.00 DI-CUMYL PEROXIDE 0.75 ZINC OXIDE 5.00 Two or three of the polymers may be used in the blends in amounts from up to 60 parts of each. The total polybutadiene amount will vary between 80 and 120 parts and the other ingredients in amounts as follows: Zinc acrylate or equivalent: 25-35 parts Peroxide or equivalent: 0.25-1.5 parts Zinc oxide or equivalent: 2-10 parts In place of the zinc acrylate any ingredient selected from the group consisting of acrylic acid, methacrylic acid, iatonic acid, crotonic acid, sorbic acid and mixtures thereof may be used. The initiator may be selected from the group consisting of peroxides, persulfates, hydrazine and azo compounds.

Core formulations made in accordance with this invention are highly satisfactory from the standpoint of the absence of extrusion and pre-form problems. Pre-cut slugs do not form concave ends and blemished product as well as other rejects are essentially eliminated.

More specifically, products made in accordance with prior art Example I exhibit low green strength, poor knitting, irregular extrusion properties and distortion of the individual slugs. This results in rejects during the core molding process. The coefficient of restitution of a molded core using this formula is 0.65025.

The cores of Example II exhibit excellent green strength and complete integrity during the mill roll processing. The batch extrudes very smoothly and the individual pre- forms exhibit excellent shape retention. This results in an almost complete yield from the core molding process. The coefficient of restitution of a molded core using this formula is 0.66132.

The utilization of the blended forms of polybutadiene, as opposed to a singular type, results in dramatically improved processing and elimination of molding rejects, an improvement in coefficient of restitution is also recognized and this improvement is achieved without loss of compressibility to the extent that full performance characteristics are diminished.

It will be understood that various changes and additions may be made to the concepts of this invention without departing from the spirit of this invention particularly as defined in the following claims.