Two-piece golf ball

Abstract: The two-piece golf ball of the present invention comprises a core formed from a rubber composition comprising a base rubber, a co-crosslinking agent and an organic peroxide, and a cover covering the core, wherein the core has the following hardness distribution when measured by a JIS-C hardness meter; (1) hardness at the center; 58 to 73 (2) hardness at 5 to 10 mm from the center; 68 to 78 (3) hardness at 15 mm from the center: 76 to 88 (4) surface hardness; 78 to 88 (in the hardness distribution, hardness (2) is substantially constant being within the above range, of which the tolerance is within ± 3 and the other values satisfy the relation of (1) < (2) < (3) ≦ (4) ), the compression deformation of the core is in the range of from 2.8 to 3.5 mm when pressurized by an initial load of 10 kg up to a final load of 130 kg and the cover has a thickness of 1.5 to 2.1 mm. The shot feel of these two-piece golf balls is closer to that of thread-wound golf balls.

Description: The present invention relates to a two-piece golf ball and, in particular, to a two-piece golf ball with an excellent shot feel. Two-piece golf balls are widely used because of their excellent flight performance. However, they have some drawbacks, for example, the shot feel is harder than thread-wound golf balls and, particularly, when a bad shot is made, it gives an even harder feeling and therefore the advent of a two-piece golf ball with a good shot feel has been desired. Various efforts have been made to bring the shot feel of the two-piece golf ball closer to that of the thread-wound golf ball. For example, the core of a two-piece golf ball has been made more flexible to reduce the hardness of the entire ball and thereby improve the shot feel (see Japanese Kokai Publication 63(1988)-220889). However, when the golf ball obtained by this method is hit, it feels heavy although it is more flexible and its shot feel is not necessarily similar to the shot feel of the thread-wound golf ball. When viewed from hardness distribution and compression strength, its impact force is large and the shot feel is not good enough. The present invention provides a two-piece golf ball having a shot feel closer to that of a thread-wound golf ball, which is obtained by controlling the characteristics of the core and the thickness of the cover. We have now discovered that by making the hardness distribution of the core more rigid at the outside and softer at the inside, adjusting the compression strength of the core, reducing the thickness of the cover to 1.5 to 2.1 mm (which is normally 2.1 to 2.4 mm) and optimizing these factors, it is possible to obtain a two-piece golf ball having a cover without a hard shot feel and a core with a light shot feel and appropriate flexibility, the ball as a whole being soft and light, having a better carry than conventional balls, without excessively reducing its hardness (expressed as PGA). Accordingly, the present invention provides a two-piece golf ball comprising a core formed from a rubber composition comprising a base rubber, a co-crosslinking agent and an organic peroxide, and a cover covering the core, wherein the core has the following hardness distribution when measured by a JIS-C hardness meter; (1) hardness at the center; 58 to 73 (2) hardness at 5 to 10 mm from the center; 68 to 78 (3) hardness at 15 mm from the center: 76 to 88 (4) surface hardness; 78 to 88 (in the hardness distribution, hardness (2) is substantially constant being within the above range, of which the tolerance is within ± 3 and the other values satisfy the relation of (1) < (2) < (3) ≦ (4) ), the compression deformation of the core is in the range of from 2.8 to 3.5 mm when pressurized by an initial load of 10 kg up to a final load of 130 kg and the cover has a thickness of 1.5 to 2.1 mm. The base rubber used in the present invention may be a natural and/or synthetic rubber which has been conventionally used for a two-piece core. In particular, cis-1,4-polybutadiene rubber containing at least more than 40% of the cis structure is preferred. If desired, natural rubber, polyisoprene rubber, styrene rubber, EPDM etc. may be blended into the polybutadiene rubber in appropriate amounts. The co-crosslinking agent is not particularly restricted but, for example, a metal salt of an unsaturated carboxylic acid, in particular, a monavalent or divalent metal salt of an unsaturated carboxylic acid having 3 to 8 carbon atoms (for example, acrylic acid, methacrylic acid etc.) may be used. Zinc diacrylate is particularly preferred. The amount of the co-crosslinking agent is 20 to 35 wt parts, preferably 25 to 32 wt parts based on 100 wt parts of the base rubber. If the amount is less than 20 wt parts, the hardness of the ball is insufficient and the ball has a heavy and inferior shot feel and the durability is also inferior. When the amount exceeds 35 wt parts, the ball is too hard and the shot feel is also inferior. The organic peroxide may be dicumyl peroxide or di-t-butylperoxide, but dicumyl peroxide is particularly preferred. The amount of the organic acid is from 0.5 to 5.0 wt parts, preferably 1.0 to 3.0 wt parts based on 100 wt parts of the base rubber. If it is less than 0.5 wt parts, the hardness of the ball is insufficient and the shot feel of the ball is heavy and inferior, while if it exceeds 5.0 wt parts, the ball is too hard and the shot feel is inferior. The rubber composition used for the golf ball of the present invention may contain such additives as fillers, antioxidants, etc. if necessary. The filler generally used may be zinc oxide, barium sulfate etc. and the amount in the composition depends on the specific weight, size etc. of the cover and the core and is not particuarly restricted, but it is usually from 10 to 40 wt parts based on 100 parts of the base rubber. The core for the two-piece golf ball is formed by kneading the rubber composition sufficiently and curing it in a mold. The kneading conditions and curing conditions are well known to the industry but, usually curing is conducted at a temperature of from 140 to 180°C for 15 to 55 minutes. The core of the golf ball of the present invention has such a hardness distribution when measured by a JIS-C hardness meter that it satisfies the conditions of (1) 58 to 73 at the center, (2) 68 to 78 at 5 - 10 mm from the center, (3) 76 to 88 at 15 mm from the center and (4) 78 to 88 at the surface. Particularly, the hardness (2) should be substantially constant of which the tolerance is less than ± 3. The hardness values also satisfy the relation of (1) < (2) < (3) ≦ (4). The technology to specifically control the hardness distribution as above is described in Japanese Kokai Publication Sho 60(1985)-90575. When the hardness is outside the aforesaid ranges, the durability decreases and the ball is exceedingly flexible and gives a heavy shot feel. When the hardness is higher than the above range, the impact force at the time of hitting is too large and the shot feel is inferior. When the hardness distribution is such that the hardness is higher as it is closer to the core surface, the impact force is larger than that described in the present invention, the softness of the shot feel is inferior, whilst in the case of a hardness distribution having a flat section as in the present invention, the impact force is small, a ball with a soft shot feel is obtained and the durability is also good. Also it is necessary that the compression deformation of the core is in the range of from 2.8 to 3.5 mm when it is loaded with an initial load of 10 kg up to a final load of 130 kg. Such compression deformation may be controlled mainly by the amount of the metal salt of the unsaturated carboxylic acid, but it may also be controlled by the amounts of other chemicals, curing conditions etc. By whichever method it may be controlled, if the deformation of the core exceeds 3.5 mm, the rebound coefficient decreases and the flight performance is inferior. Basically, the ball is too soft and heavy and its shot feel is insecure and its durability is lower. On the contrary, if the deformation is less than 2.8 mm, the ball is too hard and the shot feel is inferior. By covering the core obtained above with a cover having a thickness of from 1.5 to 2.1 mm, a two-piece golf ball is obtained. The cover is generally made from a cover composition which mainly comprises an ionomer resin and, as necessary, a filler or coloring agent (for example, titanium dioxide, barium sulfate, etc.). When the thickness of the cover is less than 1.5 mm, the ball spins easily and the shot feel is closer to that of a one-piece golf ball. When it is thicker than 2.1 mm, the shot feel is inferior. In the present invention, it is preferred to adjust the stiffness of the cover to within the range of 3,000 to 4,500 kg/cm 2 . When it is less than 3,000 kg/cm 2 , the cover is too flexible and the ball spins easily and the shot feel is soft, heavy and insecure. Whereas if it exceeds 4,500 kg/cm 2 , the cover is hard and gains a heavy shot feel. The method to coat the ionomer resin cover on the core is well known and it is generally executed by injection molding. EXAMPLES The present invention is described in further detail according to Examples. However, the present invention is not limited to these Examples. Examples 1 - 3 and Comparative Examples 1 - 4 A rubber composition was obtained by kneadding the rubber composition for the core shown in Table 1. Thus obtained rubber composition was cured and molded under the conditions given in Table 1. Hardness distribution and compression strength of thus obtained core are shown in Table 1. The cover obtained by the ordinary method with the composition shown in Table 2 was covered with the aforesaid core. Stiffness and thickness of the cover used are shown in Table 2. Table 3 shows the hardness (indicated by PAG) durability index, rebound coefficient, flight characteristics, such as launch angle, spin, carry etc. and results of hit feeling evaluation. Core Ex. 1 Ex. 2 Ex.3 Comp. Ex.1 Comp. Ex.2 Comp. Ex.3 Comp. Ex. 4 Formulation BR-01 100 100 100 100 100 100 100 Zinc diacrylate 31 29 27 31 25 31 31 Zinc oxide 17.5 18.2 19.0 20.0 20.0 20.4 16.6 Antioxidant 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Dicumyl peroxide 2.2 2.2 2.2 2.2 2.0 1.8 2.3 Condition of curing Temperature 155 °C 155 °C 155 °C 145 °C 160 °C 160 °C 152 °C Period of time 25 min. 25 min. 25 min. 40 min. 25 min. 25 min. 25min. Hardness distribution Center 71 65 60 70 56 62 65 5mm from center 76 73 70 76 67 75 76 10mm from center 76 73 70 81 67 75 76 15mm from cener 86 84 79 83 75 79 86 Surface 87 85 80 78 77 85 86 Compression (mm) 2.9 3.1 3.4 2.6 3.7 3.2 2.7 Cover Formulation HIMILAN 1076 50 50 50 50 50 50 50 HIMILAN 1605 50 50 50 50 50 50 50 Stiffness 23 °C X 2 weeks 3300 3300 3300 3300 3300 3300 3300 Thickness (mm) 1.9 1.9 1.9 1.9 1.9 2.3 1.4 Ball evaluation Ex.1 Ex.2 Ex.3 Comp. Ex.1 Comp. Ex.2 Comp. Ex.3 Comp. Ex.4 Hardness (PGA) 105 100 95 110 88 100 92 Durability index 100 99 97 115 75 105 80 Rebound coefficient (45 m/s) 0.7820 0.7810 0.7800 0.7830 0.7720 0.7790 0.7735 Flying characteristics Launch angle (°) 12 12.2 12.5 11.4 12.8 12 10 Spin (rpm) 2600 2580 2550 2800 2500 2600 3200 Carry (yard) 230 230 230 230.5 227 228 220 Feeling evaluation Light, soft and good Light, soft and good Light, soft and good Hard Too soft and heavy Slightly hard and feel hardness cover Soft, heavy and of dull As it is evident from the results of Tables 1, 2 and 3, the balls of Examples 1 - 3 are light, soft, gives good hit feeling and their characteristics are closer to those of thread - wound golf balls. Durability and flight performance are also good. The golf ball of Comparative Example 1 has the core with small compression strength (2.6 m/m), hardness of the ball is high, and the hit feeling is hard and inferior. The ball of Comparative Example 2 has a large core compression strength (3.7 mm) and low hardness, and it is too soft and heavy and its hit feeling is insecure and inferior. Durability is low. The ball of Comparative Example 3 has thick cover and its hit feeling was hard and inferior. The ball of Comparative Example 4 has thin cover, its hit feeling is closer to that of one piece golf ball and its hit feeling is soft and heavy. The ball is easy to catch spin and has poor flight performance and durability. The two-piece golf ball of the present invention has hit feeling close to that of thread-wound golf ball and its flight performance is the one specific to the two-piece golf ball and it has preferred characteristics as a golf ball in regard to both flight performance and hit feeling.

Claims: A two-piece golf ball comprising a core formed from a rubber composition comprising a base rubber, a co-crosslinking agent and an organic peroxide, and a cover covering the core, wherein the core has the following hardness distribution when measured by a JIS-C hardness meter: (1) hardness at the center 58 to 73 (2) hardness at 5 to 10 mm from the center 68 to 78 (3) hardness at 15 mm from the center 76 to 88 (4) surface hardness 78 to 88 wherein in the hardness distribution, hardness (2) is substantially constant being within the above range, of which the tolerance is within ± 3 and the other values satisfy the relation of (1) < (2) < (3) ≤ (4), and wherein the compression deformation of the core is in the range of from 2.8 to 3.5 mm when pressurized by an initial load of 10 kg up to a final load of 130 kg and the cover has a thickness of 1.5 to 2.1 mm. A golf ball as claimed in claim 1 wherein the core is made from a rubber composition which comprises cis-1,4-butadiene rubber, zinc diacrylate and dicumyl peroxide. A golf ball as claimed in claim 2 wherein the rubber composition further comprises a filler and an antioxidant. A golf ball as claimed in any one of the preceding claims wherein the cover is formed from an ionomer resin and a filler. A golf ball as claimed in any one of the preceding claims wherein the cover has a stiffness in the range of from 3,000 to 4,500 kg/cm 2 .

Inventors: ENDO SEIICHIRO, YABUKI YOSHIKAZU, ENDO, SEIICHIRO, YABUKI, YOSHIKAZU
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1993
Main classification: A61B17
IPC codes: A63B37, C08J3