Multi-piece solid golf ball

Details Multi-piece solid golf ball Multi-piece solid golf ball

2002-05-29

2003-08-05

Sewell, Paul T. (Department: 3711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S526000, C524S534000, C473S357000

Reexamination Certificate

active

06602941

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-piece solid golf ball which has been imparted with a good, soft feel upon impact and an excellent flight performance.
2. Prior Art
Various improvements are being made in formulating the polybutadiene used as the base rubber in golf balls so as to confer the balls with outstanding rebound characteristics.
For example, JP-A 62-89750 describes rubber compositions for use as the base rubber in solid golf balls, which compositions are arrived at by blending a polybutadiene having a Mooney viscosity of 70 to 100 and synthesized using a nickel or cobalt catalyst with another polybutadiene having a Mooney viscosity of 30 to 90 and synthesized using a lanthanide catalyst or polybutadiene having a Mooney viscosity of 20 to 50 and synthesized using a nickel or cobalt catalyst.
However, further improvements in the materials are required in the above art to achieve golf balls endowed with an excellent flight performance.
JP-A 2-268778 describes golf balls molded using a blend composed of a polybutadiene having a Mooney viscosity of less than 50 and synthesized using a Group VIII catalyst in combination with a polybutadiene having a Mooney viscosity of less than 50 and synthesized with a lanthanide catalyst. However, golf balls with an excellent flight performance cannot be obtained in this way.
The existing art also teaches multi-piece solid golf balls in which an intermediate layer is molded of a low-Mooney viscosity polybutadiene (JP-A 11-70187), solid golf balls molded from rubber compositions comprising a polybutadiene having a Mooney viscosity of 50 to 69 and synthesized using a nickel or cobalt catalyst in combination with a polybutadiene having a Mooney viscosity of 20 to 90 and synthesized using a lanthanide catalyst (JP-A 11-319148), solid golf balls molded from compositions based on a rubber having a 1,2 vinyl content of at most 2.0% and a weight-average molecular weight to number-average molecular weight ratio Mw/Mn of not more than 3.5 (JP-A 11-164912), golf balls molded from rubber compositions containing a high Mooney viscosity polybutadiene (JP-A 63-275356), and golf balls molded from rubber compositions comprising polybutadiene having a high number-average molecular weight in admixture with polybutadiene having a low number-average molecular weight (JP-A 3-151985). However, none of these prior-art golf balls truly have a good, soft feel upon impact and an excellent flight performance that helps increase the distance traveled by the ball.
Golf balls having a cover composed of a relatively soft inner layer and a relatively hard outer layer have already been disclosed in JP-A 2-228978, JP-A 6-142228, JP-A 7-24084, JP-A 9-10358, JP-A 11-253578, JP-A 11-253579, JP-A 11-319149, JP-A 2000-70408, JP-A 2000-70409, JP-A 2000-70410, and JP-A 2000-70411. However, further improvements in distance are desired for the golf balls described in all of these specifications.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide multi-piece solid golf balls which are endowed with a good, soft feel when hit with a golf club and an excellent flight performance that helps increase the distance traveled by the ball when played.
The inventor has discovered that golf balls having a solid core, an inner cover layer over the cover, and an outer cover layer over the inner cover layer, wherein the solid core is made of a rubber composition formulated from a particular type of base rubber combined in specific proportions with certain other materials, and the outer cover layer is harder than the inner cover layer, exhibit a good synergy from optimization of the solid core materials and an appropriate distribution of hardness between the inner and outer cover layers. Multi-piece solid golf balls thus constituted have a good, soft feel when hit with a golf club and an excellent flight performance that enables the ball to travel further when played.
Accordingly, the invention provides a multi-piece solid golf ball having a solid core, an inner cover layer enclosing the core, and an outer cover layer enclosing the inner cover layer. The solid core is molded from a rubber composition comprising 100 parts by weight of a base rubber composed of (a) 20 to 100 wt % of a polybutadiene having a cis-1,4 content of at least 60% and a 1,2 vinyl content of at most 2%, having a viscosity &eegr; at 25° C. as a 5 wt % solution in toluene of up to 600 mPa·s, and satisfying the relationship:
10
B+
5
≦A≦
10
B+
60,
wherein A is the Mooney viscosity (ML
1+4
(100° C.)) of the polybutadiene and B is the ratio Mw/Mn between the weight-average molecular weight Mw and the number-average molecular weight Mn of the polybutadiene, in combination with (b) 0 to 80 wt % of a diene rubber other than component (a). The rubber composition includes also (c) 10 to 60 parts by weight of an unsaturated carboxylic acid and/or a metal salt thereof, (d) 0.1 to 5 parts by weight of an organosulfur compound, (e) 5 to 80 parts by weight of an inorganic filler, and (f) 0.1 to 5 parts by weight of an organic peroxide. The outer cover layer has a higher Shore D hardness than the inner cover layer.
The polybutadiene (a) is typically synthesized using a rare-earth catalyst.
Preferably, the diene rubber (b) includes 30 to 100 wt % of a second polybutadiene which has a cis-1,4 content of at least 60% and a 1,2 vinyl content of at most 5%, has a Mooney viscosity (ML
1+4
(100° C.)) of not more than 55, and satisfies the relationship
&eegr;≦20
A−
550,
wherein A is the Mooney viscosity (ML
1+4
(100° C.)) of the second polybutadiene and &eegr; is the viscosity, in mPa·s, of the second polybutadiene at 25° C. as a 5 wt % solution in toluene. The second polybutadiene in component (b) is typically synthesized using a Group VIII catalyst.
In the multi-piece solid golf ball of the invention, it is generally advantageous for the inner cover layer to have a Shore D hardness of 10 to 60 and the outer cover layer to have a Shore D hardness of 50 to 80.
DETAILED DESCRIPTION OF THE INVENTION
The golf ball of the invention includes a solid core made of a rubber composition in which the base rubber is at least partly polybutadiene. It is critical that the base rubber contain as component (a) a specific amount of a polybutadiene in which the cis-1,4 and 1,2 vinyl contents, the viscosity &eegr; at 25° C. as a 5 wt % solution in toluene, and the relationship between the Mooney viscosity and the polydispersity index Mw/Mn have each been optimized.
That is, the polybutadiene (a) has a cis-1,4 content of at least 60%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95%; and has a 1,2 vinyl content of at most 2%, preferably at most 1.7%, more preferably at most 1.5%, and most preferably at most 1.3%. Outside of the above ranges, the resilience declines.
The polybutadiene (a) must also have a viscosity &eegr; at 25° C. as a 5 wt % solution in toluene of not more than 600 mPa·s. “Viscosity &eegr; at 25° C. as a 5 wt % solution in toluene” refers herein to the value in mPa·s units obtained by dissolving 2.28 g of the polybutadiene to be measured in 50 ml of toluene and carrying out measurement with a specified viscometer at 25° C. using a standard solution for the viscometer (JIS Z8809).
The polybutadiene (a) has a viscosity &eegr; at 25° C. as a 5 wt % solution in toluene of not more than 600 mPa·s, preferably not more than 550 mPa·s, more preferably not more than 500 mPa·s, even more preferably not more than 450 mPa·s, and most preferably not more than 400 mPa·s. Too high a viscosity &eegr; lowers the workability of the rubber composition. It is recommended that the viscosity &eegr; be at least 50 mPa·s, preferably at least 100 mPa·s, more preferably at least 150 mPa·s, and most preferably at least 200 mPa·s. Too low a viscosity &eegr; may lower the resilience.
In addition, the polybutadiene (a) must satisfy the relationship:
10
B+
5
≦A≦
10
B+
60,
wherein A is the

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