JPH0470034B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a two-piece solid golf ball and a method for manufacturing the same. [Prior Art] In recent years, the development of two-piece solid golf balls has progressed, and their performance has improved accordingly. For example, regarding the composition of this inner core,
Special Publication No. 55-19615, Special Publication No. 56-9347,
A large number of patents have been filed, including JP-A-51-92225, JP-A-53-83834, JP-A-52-21933, and JP-A-52-21932, and metal salts of acrylic acid have been filed. It has also been revealed that a solid golf ball with good flight characteristics can be obtained by using a metal salt of methacrylic acid as a co-crosslinking agent. Regarding the manufacturing method of these co-crosslinking agents, each company is competing in the development process, as mentioned in, for example, Japanese Patent Publication No. 14416/1982, Japanese Patent Publication No. 17542/1982, and Japanese Patent Application Laid-open No. 141808/1983. It's on. [Problems to be Solved by the Invention] However, although the above-mentioned solid golf balls, especially two-piece solid golf balls, have good flight characteristics, they are disliked by some users because they give a hard feel. ing. In particular, it tends to be avoided by professionals who place emphasis on feeling. By the way, most two-piece solid golf balls currently on the market have ball hardness (PGA ball compression) between 95 and 110, which is compared to 80 to 100 for thread-wound golf balls. It is hard and contributes to the hard feel when hitting the ball. However, solid golf balls with the same hardness as thread-wound golf balls are rarely seen, but because they are soft, they have low rebound and flight characteristics. It is desired that a two-piece solid golf ball with good characteristics be developed. [Means for Solving the Problems] In view of the above-mentioned circumstances, the present inventors have conducted various studies and found that when producing the inner core, 25 to 50% of zinc acrylate was added as a co-crosslinking agent to 100 parts by weight of rubber. 45 parts by weight of higher fatty acids such as palmitic acid, stearic acid, myristic acid as a co-crosslinking aid to zinc acrylate, and zinc oxide as a co-crosslinking aid to 1 mole of zinc acrylate. for
A rubber composition containing 0.5 mol or more of a reaction rate retardant consisting of 2,5-di-tert-butylhydroquinone in an amount of 1 to 3% by weight based on zinc acrylate is prepared, and an inner core made from this rubber composition is When covering a cover made of a resin composition containing 4 to 12 parts by weight of a filler such as titanium oxide (TiO ₂ ) or barium sulfate to 100 parts by weight of a thermoplastic resin whose main material is an ionomer, during impact. The present inventors have discovered that it is possible to obtain a two-piece solid golf ball with good feel and excellent flight characteristics, and have completed the present invention. That is, it is believed that the zinc acrylate causes a polymerization and grafting reaction with a peroxide-based initiator such as dicumyl peroxide, and functions as a co-crosslinking agent as well as a reactive filler. The amount of zinc acrylate is determined so as to obtain the desired hardness and weight, but in order to obtain higher impact resilience,
It is desirable to reduce the blending amount as much as possible. The present invention is based on such circumstances,
When forming the inner core, as shown in Experimental Examples 1 to 3 below, we found a factor that increases the ball hardness, and even with a small amount of zinc acrylate, the desired ball hardness can be obtained and the conventional two-piece solid Compared to golf balls, it provides less resistance when hitting,
In addition, as shown in Experimental Examples 4 and 5, 2,5-di-tert
- A reaction rate retardant made of butylhydroquinone is added to suppress the runaway reaction of zinc acrylate, reduce the width of the hardness distribution of the inner core, eliminate unevenness in the degree of vulcanization of the inner core, and fill the cover. By limiting the amount of the agent within a specific range, it is possible to obtain a two-piece solid golf ball with good feeling and excellent flight characteristics. In the present invention, zinc acrylate is blended in an amount of 25 to 45 parts by weight per 100 parts by weight of rubber, but if the amount of zinc acrylate is less than 25 parts by weight, the ball hardness and coefficient of restitution will be low, and if the amount exceeds 45 parts by weight, the ball hardness and coefficient of restitution will decrease. This is because the hardness of the ball becomes too high. Furthermore, higher fatty acids such as palmitic acid, stearic acid, and myristic acid are blended in an amount of 5 to 25% by weight, preferably 10 to 20% by weight, based on zinc acrylate; If it is less than 5% by weight, it will be difficult to achieve the desired ball hardness and the durability will be low, and if it exceeds 25% by weight, the volume fraction of rubber such as polybutadiene will become small and impact resilience will be inhibited. . Zinc oxide is 0.5 per mole of zinc acrylate
The amount of zinc oxide is more than 0.5 mole per mole of zinc acrylate, since it becomes difficult to achieve the desired hardness of the ball.
In addition to acting as a co-crosslinking aid, zinc oxide also functions as a weight regulator for the inner core, so the upper limit of its amount is not particularly limited, and may vary depending on other ingredients, the size of the golf ball, etc. Although it can be varied in various ways, it is usually blended in an amount of about 60 parts by weight per 100 parts by weight of rubber. As a reaction rate retardant, 2,5-di-tert-
Butylhydroquinone is used, and this reaction rate retarder is 1% relative to zinc acrylate as described above.
~3% by weight is blended. This is because if the reaction rate retardant is less than 1% by weight, it will have little effect in suppressing the crosslinking reaction in the center of the inner core, and it will be difficult to obtain an inner core with a narrow hardness distribution. This is because if the amount exceeds % by weight, the crosslinking reaction becomes too slow, resulting in poor production efficiency. In order to obtain the inner core of the two-piece solid golf ball of the present invention, a rubber serving as a base material, an initiator, and the like are required in addition to the above-mentioned components. As the rubber, for example, butadiene rubber such as cis-1,4-polybutadiene, or a blend of butadiene rubber and natural rubber or styrene-butadiene rubber can be used, but in particular, 90% by weight or more of high-cis-1,4-polybutadiene can be used. It is preferable to use a material containing such a material because it is easy to obtain high rebound resilience. As the initiator, peroxides such as dicumyl peroxide and 1,1-bis-(tert-butylperoxy)3,3,5-trimethylcyclohexane are preferably used. The amount of these initiators used is the same as before, per 100 parts by weight of rubber.
The amount is 0.5 to 5.0 parts by weight. In addition, the rubber composition for the inner core may optionally contain additives such as anti-aging agents, fillers such as barium sulfate, calcium carbonate, silica, etc. in addition to the above-mentioned components. In producing the inner core from the above rubber composition, as shown in Experimental Example 6 below, the press pressure was 1 to 20 tons per object to be heated and compression molded.
It is particularly preferable to perform the molding at a press temperature of 130 to 170°C and a time of 20 to 40 minutes. Pressing temperature, time, etc. depend on generally adopted conditions, but press pressure is specially selected and affects the hardness distribution of the resulting inner core, and within the above press pressure range, the center of the inner core A uniform degree of vulcanization with little difference in hardness between the hardness and the surface can be obtained. Even if the press pressure is lower or higher than the above-mentioned press pressure, the uniformity of the hardness distribution will be lost. The cover that covers the inner core contains 4 to 12 parts by weight of a filler such as titanium oxide or barium sulfate based on 100 parts by weight of a thermoplastic resin whose main material is an ionomer or an ionomer blended with other polymers such as nylon. A resin composition containing parts by weight is used. Additionally, known cover additives such as anti-aging agents can be added to the resin composition, if desired. As mentioned above, the resin for the cover is an ionomer or a mixture of an ionomer as a main material and a mixture of the ionomer and other polymers. Examples of ionomers include Surlyn 1605, Surlyn 1706, and Surlyn 1707 manufactured by DuPont, Himilan 1605, and Himilan manufactured by Mitsui Polychemicals.
1706, Himilan 1707 (all trade names), etc. are preferably used. Mixtures of these ionomers are also preferably used. As the polymer to be blended with the ionomer, for example, nylon, linear low-density polyethylene, medium-low density polyethylene, etc. are used, and nylon 12 is particularly preferably used. In the present invention, a blend of nylon 12 and an ionomer is more preferably used than the ionomer alone because it has particularly good impact resilience. The amount of nylon 12 blended with the ionomer is preferably 2 to 8 parts by weight based on 100 parts by weight of the ionomer. This is because if the content of nylon 12 is less than 2 parts by weight, the effect of further increasing the repellency cannot be sufficiently exhibited, and if it is more than 8 parts by weight, the durability will decrease. Blending of the ionomer and nylon 12 can be carried out using a single-screw extruder, but a machine with good dispersibility such as a twin-screw extruder is preferred from the standpoint of improving productivity. The amount of filler in the composition as cover material is
As mentioned above, the amount of filler is 4 to 12 parts by weight per 100 parts by weight of resin, but this is because if the amount of filler is less than 4 parts by weight, the specific gravity distribution of the ball will not change much and the spin performance will not be improved. This is because if the amount exceeds 12 parts by weight, the rebound resilience decreases. As fillers such as barium sulfate and titanium oxide, fillers having a larger particle diameter than those normally used are preferable because they have better impact resilience and durability and less residual strain. As a method for coating the inner core of the cover, either a compression molding method or an injection molding method can be adopted. The thickness of the cover is usually preferably 1.0 to 2.5 mm. [Example] Next, the present invention will be explained in further detail with reference to experimental examples and examples. In addition, all the blending parts in the experimental examples and examples are based on parts by weight. Experimental Example 1 Using zinc acrylate as a co-crosslinking agent, the relationship between the amount of zinc acrylate blended and ball hardness (ball compression measured by PGA) was investigated, and the results are shown in FIG. In addition, the amount of zinc acrylate on the horizontal axis in Figure 1 is based on rubber (cis-
The number of parts is based on 100 parts by weight (1,4-polybutadiene). The rubber composition obtained by kneading with a roll based on the formulations A and B shown in Table 1, a rubber composition with a diameter of about 30 mm,
A plug with a length of approximately 50 mm was prepared and crosslinked by heat compression molding at 165°C for 30 minutes (hereinafter referred to as vulcanization) to obtain a solid inner core with a diameter of 38.1 mm. The cis-1,4-polybutadiene used was manufactured by Nippon Synthetic Rubber.
Co., Ltd.'s BR01 (trade name), and the same applies to subsequent formulations.

[Table] Next, the solid inner core was covered with a cover by injection molding to obtain a two-piece solid golf ball with a diameter of 42.8 mm. The cover used is Surlyn
1707 (trade name, ionomer manufactured by DuPont). The composition that forms the inner core of a two-piece solid golf ball depends on the hardness, weight, rebound, and
The material is determined with consideration to crack resistance, but the first consideration is weight and hardness. Note that the hardness here refers to the hardness of the ball after being covered with a cover, and hereinafter it will be expressed as the ball hardness according to PGA. As shown in Figure 1, the ball hardness is increased with formulation A.
To achieve a PGA rating of 80 to 95, 35 to 45 parts by weight of zinc acrylate is required, but in Formulation B, in which the filler is changed from barium sulfate to zinc oxide, zinc acrylate is 30 to 40 parts by weight. The ball hardness is 80 to 95, and there is less zinc acrylate than in the case of formulation A. From this result, increasing the amount of zinc acrylate (with the amount of initiator constant) increases the ball hardness.
It was found that the ball hardness was even higher when zinc acrylate and zinc oxide were combined. That is, graft crosslinking to polybutadiene,
The more zinc acrylate that has a three-dimensional structure, the higher the crosslinking density, and by adding 0.5 mol or more of zinc oxide to 1 mol of zinc acrylate, a condensed ionic bond with the grafted zinc acrylate is formed. formed and the network structure becomes denser,
It is estimated that the ball hardness will further increase. If you visualize this, it will look like this: Note that this phenomenon was unique to zinc acrylate and was not observed in any other zinc acrylate. Experimental Example 2 The effects of adding various fillers containing zinc oxide were investigated, and the results are shown in FIG. The formulation used was formulation C shown in Table 2, and the ball hardness was measured on a two-piece solid golf ball obtained by preparing a solid inner core in the same manner as in Experimental Example 1 and then covering it. Ta.

[Table] As shown in Figure 2, substances that reduce ball hardness when added (e.g. Ca(OH) ₂ , MgO),
Balls with little change in hardness (e.g. SiO ₂ ,
BaSO ₄ , Fe ₂ O ₃ , CaCO ₃ , ZrO ₂ , Fe powder, Zn powder, MgCO ₃ ), and something that has the effect of slightly increasing ball hardness (ZnO ₂ ), but zinc oxide (ZnO)
is by far the most effective at increasing the hardness of the ball, and approximately 0.5 mol of zinc oxide per 1 mol of zinc acrylate will increase the hardness of the ball. Experimental Example 3 Next, the effect of adding palmitic acid was investigated, and the results are shown in FIG. The formulations were formulations D and E shown in Table 3, and the ball hardness was measured on a two-piece solid golf ball obtained by preparing a solid inner core and then covering it in the same manner as in Experimental Example 1. .

[Table] As shown in FIG. 3, in formulation D which does not contain zinc oxide, the more palmitic acid is added, the more the vulcanization is inhibited and the ball hardness becomes lower. However, zinc oxide per mole of zinc acrylate
When 0.5 mol or more of palmitic acid is added, the hardness of the ball increases as the amount of palmitic acid added increases.
In this way, only when zinc acrylate, zinc oxide, and palmitic acid are blended, the crosslinked structure becomes stronger and an excellent highly elastic body can be obtained. This is because in a system that does not contain zinc oxide, palmitic acid has almost no ionic bond interaction with zinc acrylate, making the atmosphere in the rubber acidic and suppressing the generation of peroxide radicals. As the amount of palmitic acid added increases, the ball hardness decreases, but the presence of zinc oxide causes an ionic bonding action,
It is thought that the crosslinked structure becomes stronger, resulting in a ball with high hardness and high elasticity. In addition, in order to investigate whether zinc salts of higher fatty acids have the effect of increasing ball hardness even in the absence of zinc oxide, the amount of zinc stearate added was determined using Formulation F containing zinc stearate. Figure 3 shows the relationship between the hardness of the ball and the hardness of the ball. However, when zinc stearate is used alone and zinc oxide is not included, it has no ionic bonding action and is not a plasticizer. It was found that as the amount of zinc stearate added increases, the ball hardness decreases, and that there is no effect in the absence of zinc oxide. Experimental Example 4 Using the formulations G to L shown in Table 4, Experimental Example 1 was conducted.
A solid inner core was prepared in the same manner as described above, and then covered with a cover to obtain a two-piece solid golf ball. The ball hardness and restitution coefficient of these two-piece solid golf balls were measured and the results are shown in Table 4. Repulsion coefficient is 198.4g on the ball
It was calculated from the ball speed when colliding with a cylindrical object at a speed of 45 m/sec.

[Table] As shown in formulations G to J in Table 4, the restitution coefficient is approximately proportional to the ball hardness, and the restitution coefficient decreases as the ball hardness is lowered. Therefore, if the ball is made soft, that is, the hardness of the ball is made low with emphasis on feel, the rebound resiliency decreases. However, Nokrac NS-7 (trade name, Ouchi Shinko Chemical Industry Co., Ltd.), which has the effect of retarding the reaction rate as an anti-aging agent,
Formulas K and L containing 2,5-di-tert-butylhydroquinone (2,5-di-tert-butylhydroquinone) exhibited high restitution coefficients despite having lower ball hardness than Formulas H and I. Based on the above findings, various studies were conducted on hindered phenol anti-aging agents and the like to determine whether they have the effect of increasing the repulsion coefficient.
2,4-di-tert-butyl-4-methylphenol, 4,4-thiobis-(6-tert-butyl-3
-methylphenol), mono-(α-methylbenzyl)phenol, mercaptobenzimidazole, dihydroxybisphenol, etc., but the above-mentioned 2,5-di-tert-butylhydroquinone controls the grafting reaction and polymerization reaction better. There was nothing I could do. Experimental Example 5 In order to examine the vulcanization characteristics of individual materials using the formulations shown in Table 5, a test was first conducted using a cuelastrometer, and the results are shown in FIG. The test is
The test was carried out at 160°C and Arc ±1°.

[Table] As shown in Figure 4, blend P vulcanizes more slowly and has a lower degree of vulcanization than blends M and N. Usually, such a formulation has a low degree of vulcanization, making it difficult to obtain ball hardness, and is often not suitable for golf balls. However, in reality, golf balls with a high coefficient of repulsion can be obtained even though the ball hardness is low, as shown below. That is, using the above formulations M, N, and P, vulcanization was performed at 160°C for 30 minutes in the same manner as in Experimental Example 1 to obtain a solid inner core, and the resulting two-piece solid golf ball was evaluated for ball hardness,
The repulsion coefficient was measured and the results are shown in Table 6.
We also investigated the hardness distribution of the inner core of the golf ball mentioned above.
The results are shown in Table 6. Furthermore, the hardness of the inner core is
It is based on a JIS-C type hardness meter.

【table】

[Table] As shown in Table 6 above, in the case of formulation P, the restitution coefficient is higher than in the cases of formulations M and N, although the ball hardness is lower. This means that the hardness distribution of their inner core is
In N, the surface part is hard and the center part is soft, and there is a considerable difference in the degree of vulcanization, whereas in compound P, the hardness is low overall, the hardness distribution is narrow, and the entire inner core is cured without distortion. This is thought to be due to uniform cross-linking and vulcanization. In other words, in the case of the inner core of a golf ball, there is a distance from the surface to the center, so under normal conditions the vulcanization rates between the surface and the center are quite different.
The surface part is close to the mold, which is the heat source, and the reaction is controlled by the mold temperature, that is, the press temperature. However, in the center of the inner core, the press temperature is reached once due to the amount of heat transmitted from the outside, and the peroxidation occurs. The substance undergoes radical decomposition, and the polymerization and grafting reaction of zinc acrylate progresses, and once the reaction begins, the reaction progresses rapidly due to the heat of polymerization. Therefore, the reaction rate is faster at the center than at the surface, and when the surface reaches the desired degree of vulcanization, the center becomes over-vulcanized and an inner core with a large degree of vulcanization distortion is obtained.
By adding 2,5-di-tert-butylhydroquinone, the runaway reaction of zinc acrylate in the center is suppressed, resulting in a uniform vulcanizate with less distortion and a narrow hardness distribution from the surface to the center. will be obtained. Experimental Example 6 Next, the influence of the difference in press pressure on the vulcanization rate will be explained. At the stage of Experimental Example 5, an unexpected fact was discovered while investigating the reaction rate. In other words, when producing the inner core using the composition P shown in Experimental Example 5, a test is normally performed using a 40 ton press in a mold with 20 molds, but the vulcanization speed is different when this is molded using a 10 ton press. It was found that vulcanization was faster at lower pressures. Therefore, the press pressure per rubber compound was changed and heat compression molding was performed at 160°C for 30 minutes, and the hardness of the center and surface of the resulting inner core was measured. The results are shown in FIG. In Fig. 5, the solid line X shows the change in hardness of the surface area as the press pressure changes, and the broken line Y shows the hardness change in the center part as the press pressure changes. . As shown in Figure 5, an inner core with a small difference in hardness between the surface and center, that is, no distortion in vulcanization, can only be obtained by high-pressure pressing with a press pressure of 1 ton or more per piece. . This is because the polymerization and grafting reactions of zinc acrylate involve many side reactions in the low-pressure method.
It is assumed that this is because it is difficult to obtain vulcanized rubber with a high elastic modulus. Examples 1 to 3 and Comparative Examples 1 to 2 Rubber compositions for the inner core having the formulations shown in Table 7 were heated at 160°C.
A solid inner core was prepared by vulcanization for 30 minutes, and the solid inner core was then covered with a cover having the composition shown in Table 7 by injection molding to obtain a two-piece solid golf ball having a diameter of 42.8 mm. The resulting two-piece solid golf ball was measured for ball weight, ball hardness (ball compression by PGA), and restitution coefficient, and was subjected to an actual shooting test using a swing robot. The results are shown in Table 7. Tests using the swing robot were conducted by hitting the ball with a No. 1 wood club at a head speed of 45 m/sec, and the amount of spin was measured by photographic measurement.

【table】

〔Effect of the invention〕

As explained above, according to the present invention, the feeling of the two-piece solid golf ball was improved, and a two-piece solid golf ball with good feeling and excellent flight characteristics was obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the blending amount of zinc acrylate and ball hardness, and FIG. 2 is a diagram showing the influence of the addition of various fillers on ball hardness.
FIG. 3 is a diagram showing the relationship between the amount of palmitic acid added and ball hardness, and FIG. 4 is a diagram showing the vulcanization characteristics of blends M, N, and P in Experimental Example 5. Fifth
The figure shows changes in hardness at the surface and center of the inner core due to changes in press pressure. X: Change in hardness of the surface of the inner core due to change in press pressure, Y: Change in hardness at the center of the inner core due to change in press pressure.

Claims (1)

[Claims] 1. 25 to 45 parts by weight of zinc acrylate as a co-crosslinking agent is blended with 100 parts by weight of rubber, and higher fatty acids such as palmitic acid, stearic acid, myristic acid, etc. are added to acrylic acid as a co-crosslinking aid. 5~ for zinc
25% by weight, 0.5 mol or more of zinc oxide per 1 mol of zinc acrylate as a co-crosslinking aid, and 1 to 3 mol of a reaction rate retarder consisting of 2,5-di-tert-butylhydroquinone per mol of zinc acrylate. An inner core is prepared from a rubber composition containing % by weight, and in the inner core,
A two-piece solid golf golf ball, characterized in that it is covered with a cover made of a resin composition containing 4 to 12 parts by weight of a filler such as titanium oxide or barium sulfate to 100 parts by weight of a thermoplastic resin whose main material is an ionomer. ball. 2 Rubber contains high-cis-1,4-polybutadiene
Claim 1 which contains 90% by weight or more
The two-piece solid golf ball described in Section 1. 3 The thermoplastic resin for the cover is ionomer 100
The two-piece solid golf ball according to claim 1 or 2, which contains 2 to 8 parts by weight of nylon 12. 4 Add 25 to 45 parts by weight of zinc acrylate as a co-crosslinking agent to 100 parts by weight of rubber, and add 5 to 45 parts by weight of higher fatty acids such as palmitic acid, stearic acid, myristic acid, etc. to zinc acrylate as a co-crosslinking aid.
25% by weight, 0.5 mol or more of zinc oxide per 1 mol of zinc acrylate as a co-crosslinking aid, and 1 to 3 mol of a reaction rate retarder consisting of 2,5-di-tert-butylhydroquinone per mol of zinc acrylate. Object 1 to be heated and compression molded with a rubber composition containing % by weight
Press pressure per piece is 1~20 tons, press temperature 130~
The inner core was prepared at 170°C for 20 to 40 minutes.
The inner core is coated with a cover made of a resin composition containing 4 to 12 parts by weight of a filler such as titanium oxide or barium sulfate to 100 parts by weight of a thermoplastic resin whose main material is an ionomer. A method for manufacturing a two-piece solid golf ball.