US20020045498A1

US20020045498A1 - Two-piece golf ball

Info

Publication number: US20020045498A1
Application number: US09/924,763
Authority: US
Inventors: Hideo Watanabe
Original assignee: Bridgestone Sports Co Ltd
Current assignee: Bridgestone Sports Co Ltd
Priority date: 2000-08-24
Filing date: 2001-08-09
Publication date: 2002-04-18
Also published as: JP2002065897A

Abstract

A two-piece golf ball includes a solid core and a cover. A difference between the maximum JIS-C hardness and the minimum JIS-C hardness of the solid core is in a range of less than 5. The cover is made from a material mainly containing a base resin composed of a Zn ion neutralized type ionomer resin and an Na ion neutralized type ionomer resin. The shore D hardness of the cover is in a range of less than 62. The thickness of the cover is in a range of 2.1 mm to 2.7 mm. The two-piece golf ball has a very high durability and is suitably used particularly as a training golf ball.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a two-piece golf ball including a core made from a rubber-like elastic body and a cover formed to cover the core, and particularly to a two-piece golf ball having a very high durability and being suitably used particularly as a training golf ball.

In general, two-piece golf balls have been widely used for golfers because of its advantage that they are excellent in durability.

With respect to these two-piece golf balls, the feeling of hitting the ball has been also significantly improved as compared with conventional ones, and at present, a large number of two-piece golf balls acceptable to players from this viewpoint have been proposed.

By the way, since many players properly use golf balls each for game or training purpose, a requirement for golf balls for game purpose and a requirement for golf balls for training purpose are slightly different from each other.

For example, in the case of golf balls for game purpose, the flying performance of the ball is generally most important.

On the contrary, in the case of golf balls for training purpose, the durability against repeated hitting of the ball is most important. Accordingly, as training golf balls, there have been generally used one-piece golf balls.

Recently, however, player's requirements for golf balls have come to be variously broadened, and even training golf balls have been required to give players a feeling of hitting the ball similar to a feeling of hitting a golf ball for game purpose, that is, required to allow players to practice playing golf with a feeling of game.

On the other hand, unless training golf balls not only meet the above-described player's requirement but also have a long exchange cycle, that is, a good durability of the ball, such golf balls are difficult to be adopted by owners of training clubs from the economical viewpoint.

SUMMARY OF THE INVENTION

In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a two-piece golf ball having a very high durability and being suitably used particularly as a training golf ball.

To achieve the above object, the present inventor has earnestly studied and found that a two-piece golf ball including a solid core and a cover, which is characterized at the following points (1) to (4), is effective to optimize respective characteristics of the core and the cover, and hence to prolong the durability and enhance a player's feeling, such as a feeling of hitting the ball comparable to that obtained by a golf ball for game purpose by the synergistic effect of the optimized core and cover.

(1) A difference between the maximum JIS-C hardness and the minimum JIS-C hardness of the solid core is in a range of less than 5;

(2) the cover is made from a material mainly containing a base resin composed of a Zn ion neutralized type ionomer resin and an Na ion neutralized type ionomer resin;

(3) the shore D hardness of the cover is in a range of less than 62; and

(4) the thickness of the cover is in a range of 2.1 mm to 2.7 mm.

The present inventor has also found that such a two-piece golf ball allows players to practice playing golf with a feeling of game while prolonging the ball exchange cycle, and therefore, the golf ball is suitably used particularly in training clubs from the economical viewpoint.

The present inventor has further found that the above two-piece golf ball, characterized at the following point (5), is capable of further improve the durability of the ball.

(5) The cover is made from a material mainly containing the base resin to which an inorganic granular filler is added as a reinforcing material in an amount of 11 parts by mass or more on the basis of 100 parts by mass of the base resin.

The present inventor has accomplished the present invention on the basis of the above-described knowledge.

Accordingly, the present inventor provides a two-piece golf ball including a solid core and a cover; wherein a difference between the maximum JIS-C hardness and the minimum JIS-C hardness of the solid core is in a range of less than 5; the cover is made from a material mainly containing a base resin composed of a Zn ion neutralized type ionomer resin and an Na ion neutralized type ionomer resin; the shore D hardness of the cover is in a range of less than 62; and the thickness of the cover is in a range of 2.1 mm to 2.7 mm.

In the above two-piece golf ball, preferably, the cover is made from a material mainly containing the base resin to which an inorganic granular filler is added as a reinforcing material in an amount of 11 parts by mass or more on the basis of 100 parts by mass of the base resin.

The two-piece golf ball of the present invention thus configured is very excellent in durability, and is suitably used as a training golf ball.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a two-piece golf ball of the present invention will be described in further detail. The two-piece golf ball of the present invention includes a core made from a rubber composition which mainly contain a base rubber. As the base rubber, there may be used a known natural rubber or synthetic rubber, more specifically, polybutadiene, particularly, 1,4-cis-polybutadiene having at least 40% or more of a cis-structure. The base rubber may further contain a natural rubber, polyisoprene rubber, styrene-butadiene rubber, in addition to the above-described synthetic rubber, that is, polybutadiene, as needed.

According to the present invention, to give a desirable resilience to the core, an organic sulfur compound may be added to the base rubber. As the organic sulfur compound, there may be used thiophenol, thionaphthol, halogenated thiophenol, or a metal salt thereof. Specific examples of the organic sulfur compounds may include zinc salts of pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, parachlorothiophenol, and pentachlorothiophenol; and polysulfides each having the sulfur number of 2 to 4, such as diphenyl polysufide, dibenzil polysulfide, dibenzoyl polysulfide, dibenzothiazoyl polysulfide, and dithiobenzoyl polysulfide. In particular, a zinc salt of pentachlorothiophenol or diphenyldisulfide can be preferably used.

According to the present invention, the added amount of the organic sulfur compound is generally in a range of 0.05 part by mass or more, preferably, 0.1 part by mass or more, more preferably, 0.2 part by mass or more on the basis of 100 parts by mass of the base rubber. If the added amount is excessively small, the effect of improving the resilience cannot be expected. The upper limit of the added amount of the organic sulfur compound is generally in a range of 2.0 parts by mass or less, preferably, 1.2 parts by mass or less, more preferably, 1.0 part by mass or less on the basis of 100 parts by mass of the base rubber. If the added amount of the organic sulfur compound is excessively large, the effect of improving the resilience (particularly, at the time of hitting the golf ball with the driver (or number one wood (W#1)) cannot be expected so much, and the core is excessively softened to degrade the feeling of hitting the ball.

In addition to the above-described base rubber and the organic sulfur compound, known components may be added to the rubber composition of the present invention, as needed. Examples of the known components may include zinc salts of unsaturated carboxylic acids, organic peroxides, and antioxidants. The added amount of the above known component can be suitably adjusted.

The core can be produced from the above-described rubber composition for the core in accordance with a known process. One example of the process of producing the core includes the steps of kneading the rubber composition by a Banbury mixer or kneader into a slug, putting the slug into a specific mold, and heating the slug at a heating temperature, at which the peak of the temperature rise at the central portion caused by internal heat generation appears after an elapse of 20 min or more since starting the heating, for 20 min or more and continuously heating the slug at a temperature higher than the above heating temperature.

The diameter of the core of the present invention may be, while not particularly limited thereto, generally in a range of 37.3 mm to 38.8 mm, preferably, 37.5 mm to 38.5 mm.

With respect to the hardness of the core of the present invention, arbitrary points of the core are measured by a JIS-C type hardness tester, and a difference between the maximum value and the minimum value of the JIS-C hardnesses at the measured points of the core is specified to be less than 5.

From the practical viewpoint, however, it is difficult to strictly measure a JIS-C hardness at each point of the core and determine the maximum value and the minimum value of the measured hardnesses, and therefore, according to the present invention, the difference between the maximum JIS-C hardness and the minimum JIS-C hardness of the core is defined as follows:

The core to be measured is cut into halves, and JIS-C hardnesses at the following points (1) to (5) of one of the halves of the core are measured: (1) the core center, (2) the core surface, (3) a point apart from the core center by 5 mm, (4) a point apart from the core center by 10 mm, and (5) a point apart from the core center by 15 mm. The highest and lowest values of the JIS-C hardnesses at the above measured points are specified as the maximum value and the minimum values, respectively.

With respect to the solid core of the present invention, the difference between the maximum value and the minimum value of the JIS-C hardnesses at the above measured points is specified to be in a range of less than 5. If the differential hardness is out of the above range, the durability against cracking due to repeated hitting of the ball is degraded.

In this case, the hardness of the core center may be generally in a range of 65 to 80, preferably, 68 to 77, more preferably, 70 to 74. If the hardness of the core center is excessively low, the durability of the ball is degraded, and if the hardness of the core center is excessively high, the feeling of hitting the ball may become hard.

The hardness of the core surface may be generally in a range of 69 to 85; preferably, 72 to 82, more preferably, 74 to 79. If the hardness of the core surface is excessively low, the durability of the ball is degraded, and if the hardness of the core surface is excessively high, the feeling of hitting the ball may become hard.

With respect to the solid core of the present invention, the compressive deformation amount of the entire core, measured by applying, to the core, a load from an initial load of 10 kgf (98.07 N) to a final load of 130 kgf (1274.91 N), may be generally in a range of 2.5 mm to 5.0 mm, preferably, 2.8 mm to 4.0 mm, more preferably, 3.0 mm to 3.8 mm. If the compressive deformation amount is excessively small, the feeling of hitting the ball may become hard, and if the compressive deformation amount is excessively large, the durability against cracking may be degraded and the feeling of hitting the ball may become excessively soft.

According to the golf ball of the present invention, the core configured as described above is covered with a cover made from a material which mainly contains a base resin composed of a Zn ion neutralized type ionomer resin and an Na ion neutralized type ionomer resin for the purpose of improving the resilience and the durability against cracking.

Examples of the above Zn ion neutralized type ionomer resins may include commercial ionomer resins (trade name: “Himilan” 1557, 1706, and 1855, sold by Du Pont-Mitsui Polychemicals Co., Ltd.). Examples of the Na ion neutralized type ionomer resins may include commercial ionomer resins (trade name: “Surlyn” 1605, 1601, and 1707, sold by Du Pont DE NEMOURS & COMPANY).

According to the present invention, the content of each of the Zn ion neutralized type ionomer resin and the Na ion neutralized type ionomer resin in the cover material may be generally in a range of 20 parts by mass or more, preferably, 30 parts by mass or more, more preferably, 40 parts by mass or more. If the content of each ionomer resin is excessively small, the effect of improving the resilience and the durability against cracking may be degraded.

The cover material of the present invention, which mainly contains the base resin composed of the above-described Zn ion neutralized type ionomer resin and Na ion neutralized type ionomer resin, may further contain an inorganic granular filler as a reinforcing material.

To improve the dispersibility of the inorganic granular filler, while not exclusively, the average particle size thereof may be set in a range of 0.01 to 100 μm.

As the inorganic granular filler of a reinforcing material, there may be used a barium sulfate.

The added amount of the inorganic granular filler may be generally in a range of 11 parts by mass or more, preferably, 15 parts by mass or more on the basis of 100 parts by mass of the ionomer resins as the base resin of the cover material. The addition of the inorganic granular filler in the above range is effective to significantly improve the durability without degradation of the flying performance. If the added amount is excessively small, the reinforcing effect may be insufficient. The upper limit of the added amount of the inorganic granular filler may be generally in a range of 45 parts by mass or less, preferably, 35 parts by mass or less. If the added amount is excessively large, the resilience may be degraded.

The two-piece golf ball of the present invention can be produced by covering the above-described core with the above-described cover material in accordance with, for example, a known injection molding process or a known compression molding process. The former process is performed by putting a previously prepared solid core in a mold, and injection-molding a cover material on the surface of the solid core. The latter process is performed by previously preparing a pair of cup halves with a specific cover material, covering a solid core with the cup halves, and pressing the solid core covered with the cup halves. In each process, the molding condition may be suitably changed.

According to the present invention, the thickness of the cover is required to be adjusted so as to obtain a synergistic effect with the improved core of the present invention. To be more specific, the thickness of the cover may be in a range of 2.1 mm or more, preferably, 2.2 mm or more. The upper limit of the thickness of the cover may be 2.7 mm or less, preferably, 2.5 mm or less, more preferably, 2.4 mm or less. If the thickness of the cover is excessively small, the durability against cracking is significantly degraded, while if the thickness of the cover is excessively large, the resilience is reduced to degrade the flying performance of the ball.

The two-piece golf ball thus produced has a very high durability and gives players a feeling of hitting the ball comparable to that obtained by a golf ball for game purpose, thereby allowing players to practice playing golf with a feeling of game while prolonging the ball exchange cycle because of its high durability. The golf ball of the present invention, therefore, is suitably used particularly in training clubs from the economical viewpoint.

The two-piece golf ball of the present invention can be produced with its diameter and weight adjusted in accordance with the Rules of Golf. That is to say, the golf ball having a diameter of 42.67 mm or more and a weight of 45.93 g or less can be produced.

EXAMPLES

The present invention will be more clearly understood by way of, while not limited thereto, the following Examples and Comparative Examples. [0046]

Examples 1 to 3 and Comparative Examples 1 to 7

Two-piece golf balls were produced using core materials shown in Table 1 and cover materials shown in Table 2 in accordance with a known process. [0047]
Each of the two-piece golf balls thus produced was then subjected to the following tests. The main materials used for producing the two-piece golf balls and evaluation methods used for the tests are as follows: [0048]
[BR]: polybutadiene, sold by Japan Synthetic Rubber Co., Ltd. [0049]
Peroxide 1: dicumyl peroxide (trade name: “Percumyl D”, sold by NOF CORPORATION [0050]
Peroxide 2: 1,1-bis(t-butylperoxy)3,3,5-trimethyl cyclohexane (trade name: “Perhexa 3M-40”, sold by NOF CORPORATION) [0051]
Antioxidant: “NOCRAC NS-6” (trade name, sold by Ouchi-shinko Chemical Industrial Co., Ltd.) [0052]
Zinc salt of pentachlorothiophenol: “Renacit IV” (trade name, sold by Bayer LTD.) [0053]
[Himilan]: ionomer resin sold by Du Pont-Mitsui Polychemicals Co., Ltd. [0054]
[Surlyn]: ionomer resin sold by Du Pont DE NEMOURS & COMPANY) [0055]
[Nucrel]: ethylene-metacryl acid-acrylic ester three component copolymer sold by Du Pont-Mitsui Polychemicals Co., Ltd. [0056]
Barium sulfate 1: “BARICO #100” (trade name, sold by Hakusui Tec Co., Ltd.) [0057]
Barium Sulfate 2: “Barium Sulfate 300” (trade name, sold by Sakai Chemical Industry Co., Ltd.) [0058]
Flying Performance [0059]
Each golf ball was hit at a specific point by a hitting robot with its head speed set to a low head speed of 45 m/sec. The flying distance of the ball was measured and evaluated on the basis of the following criterion: [0060]
G: total flying distance=220 m or more [0061]
B: total flying distance=215 m or less [0062]
Durability Against Cracking [0063]
Each golf ball was repeatedly hit at random points by the hitting robot with its head speed set to 45 m/s, and the number of initially appeared cracks of the ball was evaluated. The number of initially appeared cracks was expressed by an index based on the number of initially appeared cracks of the golf ball in Example 3, which was taken as 100. In addition, the hitting of each golf ball was repeated by ten times, and the number of earliest appeared cracks was taken as the number of initially appeared cracks to be evaluated. [0064]
G: index=100 or more [0065]

B: index=90 or less

	TABLE 1


	Example	Comparative Example

	1	2	3	1	2	3	4	5	6

Core	BR 01	50	50	50	50	50	50	50	50	50
composition	BR 11	50	50	50	50	50	50	50	50	50
(parts	Zinc acrylate	33.4	31.6	30.7	30.7	31.6	31.6	27.5	30.7	30.7
by mass)	Peroxide 1	0.6	0.6	0.6	0.6	0.6	0.6	0.6	0.6	0.6
	Peroxide 2	0.6	0.6	0.6	0.6	0.6	0.6	0.6	0.6	0.6
	Antioxidant	0.2	0.2	0.2	0.2	0.2	0.2	0	0.2	0.2
	Barium sulfate 1	5.5	6.4	6.5	18.1	26.8	15.0	9.0	18.1	18.1
	Zinc oxide	5	5	5	5	5	5	5	5	5
	Zinc salt of	1	1	1	1	1	1	1	1	1
	pentachlorothiophenol
	Zinc stearate	0	0	0	0	0	0	5	0	0

Vulcanizing	Primary	Vulcanizing	145	145	145	145	145	145	157	145	145
condition	stage	temperature (° C.)
		Vulcanizing time	30	30	30	30	30	30	15	30	30
		(min)
	Secondary	Vulcanizing	170	170	170	170	170	170		170	170
	stage	temperature (° C.)
		Vulcanizing time	10	10	10	10	10	10		10	10
		(min)

JIS-C	Surface	79	76	74	74	76	76	80	74	74
hardness	Point apart from center by	76	74	72	72	74	74	75	72	72
	15 mm
	Point apart from center by	76	73	71	71	73	73	68	71	71
	10 mm
	Point apart from center by	74	71	70	70	71	71	65	70	70
	5 mm
	Center	74	71	70	70	71	71	63	70	70

Difference in hardness	5	5	4	4	5	5	17	4	4
(maximum value − minimum value)

	TABLE 2


	Example	Comparative Example

	1	2	3	1	2	3	4	5	6	7*

Core	Outer diameter (mm)	38.3	38.3	37.9	38.3	35.7	39.5	38.3	38.3	38.3	—
	Compressive deformation	3.1	3.5	3.7	3.7	3.5	3.5	3.7	3.7	3.7	—
	amount (mm)
Cover com-	Himilan 1706 (Zn)				50				100
position	Himilan 1557 (Zn)	35	35	35			50	35
(parts	Himilan 1855 (Zn)	15	15	15		35		15
by mass)	Himilan 1707 (Na)									100
	Himilan 1605 (Na)	30	30	30	50			30
	Surlyn 8120 (Na)	20	20	20		35		20
	Himilan 1601 (Na)						50
	Nucrel AN4311					30
	Barium sulfate 2	22	22	22				22
	Titanium oxide	5	5	5	5	5	5	5	5	5
Cover	Thickness (mm)	2.2	2.2	2.4	2.2	3.5	1.6	2.2	2.2	2.2
	Shore D hardness	61	61	61	63	43	60	61	60	62
Ball	Structure	2P	2P	2P	2P	2P	2P	2P	2P	2P	1P
	Outer diameter (mm)	42.7	42.7	42.7	42.7	42.7	42.7	42.7	42.7	42.7	42.6
	Weight (g)	45.3	45.3	45.3	45.3	45.3	45.3	45.3	45.3	45.3	45.5

Performance

W#1

Carry (m)

216.0

215.5

215.1

218.2

205.1

214.0

213.9

213.8

213.4

197.2

of Ball

HS45

Total (m)

225.5

225.3

224.1

228.0

214.6

223.8

225.2

222.2

221.2

208.4

Evaluation of

flying distance

G

B

G

B

Random

Durability

hitting

against

G

B

G

B

G

with

cracking

HS45

As is apparent from the results shown in Table 2, each of the golf balls in Examples 1 to 3 exhibits the flying performance comparable to that obtained by a general two-piece golf ball for game purpose, and a high durability against cracking. [0068]
On the contrary, the golf balls in Comparative Examples 1 to 7 are inferior to the golf balls in Examples 1 to 3 in the following points: [0069]
In Comparative Example 1, since the cover is excessively hard, the durability against cracking is poor. [0070]
In Comparative Example 2, since the cover is excessively thick and the resilience of the ball is low, the flying performance is significantly poor. [0071]
In Comparative Example 3, since the cover is excessively thin, the durability against cracking is poor. [0072]
In Comparative Example 4, a difference between the maximum JIS-C hardness and the minimum JIS-C hardness of the core is excessively large, the durability against cracking is poor. [0073]
In Comparative Example 5, since the cover is made from the single Zn ion neutralized type ionomer resin, the durability against cracking is poor. [0074]
In Comparative Example 6, since the cover is made from the single Na ion neutralized type ionomer resin, the durability against cracking is poor. [0075]
Comparative Example 7, which is a commercial one-piece golf ball, is very inferior to the two-piece golf ball of the present invention in terms of flying performance. [0076]
While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purpose only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims. [0077]

Claims

1. A two-piece golf ball comprising a solid core and a cover;

wherein a difference between the maximum JIS-C hardness and the minimum JIS-C hardness of said solid core is in a range of less than 5;

said cover is made from a material mainly containing a base resin composed of a Zn ion neutralized type ionomer resin and an Na ion neutralized type ionomer resin;

the shore D hardness of said cover is in a range of less than 62; and

the thickness of said cover is in a range of 2.1 mm to 2.7 mm.

2. A two-piece golf ball according to claim 1, wherein said cover is made from a material mainly containing said base resin to which an inorganic granular filler is added as a reinforcing material in an amount of 11 parts by mass or more on the basis of 100 parts by mass of said base resin.