Patent Application
20250001264
The present invention relates to a rubber composition for golf balls, and to a golf ball in which the composition is used. More particularly, the invention relates to a rubber composition for golf balls which can be suitably used as the core material in golf balls having a core of one or more layer and a cover of one or more layer, and to a golf ball in which such a composition is used.
Golf balls lately are predominantly either two-piece solid golf balls or three-piece solid golf balls. These golf balls generally have a structure in which a cover of one layer or a plurality of layers made of various resin materials encases a core made of a rubber composition. The core accounts for most of the golf ball volume and exerts a large influence on ball properties such as rebound, feel and durability. Recently, a number of disclosures have been made in which the cross-sectional hardness of the core is suitably adjusted so as to achieve a specific core hardness gradient, thereby optimizing the spin properties of the ball on full shots with a driver or an iron and enabling the ball to travel an increased distance. Enlarging the hardness difference between the core surface and center is known have the effect of reducing the spin rate on full shots with a driver. Moreover, it is known from prior findings that reducing the spin rate on full shots leads to an increased distance. Accordingly, in order to improve the distance traveled by a golf ball, there is a desire for art that enlarges the hardness difference at the core interior. One approach that has been proposed for achieving this aim is to give the core a structure made of two rubber layers. However, producing such a core would entail a relatively large number of operations compared with a single-layer rubber core, and so there remains a desire for art that enlarges the hardness difference within a single-layer core.
Methods for adjusting the cross-sectional hardness of the core include, for example, suitably adjusting the compounding ingredients in the core rubber composition and the vulcanization temperature and time. Alternatively, with regard to the compounding ingredients in the core rubber composition, another method is to select the types of co crosslinking agent and organic peroxide used and to adjust their contents. In terms of co crosslinking agents, the use of methacrylic acid, acrylic acid and metal salts thereof is known in the field of golf balls. However, adjustment in the compounding of such co crosslinking agents is intended primarily to modulate the feel of the ball on impact by regulating the core hardness, and is not capable of satisfying the desired spin properties.
JP-A H11-169485 discloses art in which a specific amount of polyethylene glycol is included in a core-forming rubber composition. However, the object of this prior art is to improve the mold releasability of a rubber molding (core) by including polyethylene glycol as an internal mold release agent. It is not aimed at further improving the internal hardness of a rubber molding and the spin-lowering effect on a golf ball by judicious selection of the types of compounding ingredients used in a core-forming rubber composition.
In addition, as to the golf ball using the core which enlarges the hardness difference at the core interior, the durability of the golf ball to impact is likely to decline. In particular, when undispersed foreign matter accidentally has been mixed in the manufacturing process of the core, it may raise that the core after vulcanizing and molding has early collapsed due to the foreign matter mixed into the core. In this case, even small amount of the foreign matter mixed into the rubber composition raised early collapse unexpectedly. Such the foreign matter often includes respective compounding ingredient used in the formulation process of the rubber composition for the core, for example, solid matter agglomerating the substances such as zinc oxide and zinc stearate, or its ground material.
Therefore, there was the limitation on the enlargement of hardness profile the core interior that is conducted to improve the flight performance, in order to concern that the durability of the golf ball to impact become inferior. Accordingly, it is desired that the low spin properties on golf ball shots are sufficiently achieved by enlarging the hardness profile of a core interior and early collapse of the core is prevented to obtain the core having excellent durability to impact.
It is therefore an object of the present invention to provide a rubber composition for golf balls which, by setting the hardness difference in the hardness profile of a golf ball core to a large value, is able to manifest low spin properties on golf ball shots and thus improve the flight performance of the ball, and is able to improve the durability of the ball to impact. A further object of the invention is to provide a golf ball made using this rubber composition.
As a result of extensive investigations, I have discovered that, by having a rubber composition for a golf ball core include as the essential compounding ingredients (a) a base rubber, (b) a co-crosslinking agent that is an α,β-unsaturated carboxylic acid and/or a metal salt thereof, (c) a crosslinking initiator, (d) an alcohol, in which further (e) a hindered phenol-type antioxidant including a substituent having thioether structure is included and the content of component (e) is at least 0.2 part by weight per 100 parts by weight of component (a), the hardness difference in the hardness profile at the interior of the core can be set to a large value while maintaining a desired core hardness (deflection), enabling low spin properties to be fully manifested on golf ball shots, and the durability of golf ball to impact can be improved even when the foreign matter has mixed into the core. Specifically, by including in the core material a hindered phenol-type antioxidant including a substituent having thioether structure, it is seen that the heat resistance against the polymer component that deteriorate due to a thermal history in core-molding can be improved as compared with a conventional hindered phenol-type antioxidant, and thus, it is presumed, that the antioxidant effect can be improved in the compounding system of the present invention.
Accordingly, the present invention provides a rubber composition for a golf ball and a golf ball.
1. A rubber composition for golf balls, comprising:
2. The rubber composition of the above 1, wherein the hindered phenol-type antioxidant is a chemical structure having at least one methyl group at ortho position.
3. The rubber composition of the above 1, wherein the number of the substituent having thioether structure in the hindered phenol-type antioxidant is at least two.
4. The rubber composition of the above 1, wherein the hindered phenol-type antioxidant is represented by the following general formula (I).
5. The rubber composition of the above 4, wherein x is an integer of 8 or more in the above formula.
6. The rubber composition of the above 1, wherein component (d) is a lower alcohol having a molecular weight of less than 500.
7. The rubber composition of the above 1, wherein component (d) is a lower alcohol having a molecular weight of less than 200.
8. The rubber composition of the above 1, wherein component (d) is a monohydric alcohol, a dihydric alcohol or a trihydric alcohol.
9. The rubber composition of the above 1, wherein component (d) is butanol, glycerol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, or tripropylene glycol.
10. The rubber composition of the above 1, further comprising (f) an antioxidant which is a benzoimidazole of the following general formula (II) or a metal salt thereof or both.
11. The rubber composition of the above 10, wherein component (f) is selected from the group consisting of 2-mercaptobenzoimidazole, 2-mercaptomethylbenzoimidazole, and metal salts thereof.
12. The rubber composition of the above 1, further comprising (g) sulfur or an organosulfur which is alkylphenoldisulfide polymers represented by the following general formula (III).
13. The rubber composition of the above 12, wherein R in the general formula (III) is an lower alkyl group of 1 to 6 carbon atoms selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, tert-butyl, n-amyl (pentyl), iso-amyl (pentyl), tert-amyl (pentyl), sec-isoamyl, neopentyl, n-hexyl, iso-hexyl, tert-hexyl groups.
14. The rubber composition of the above 12, wherein the organosulfur of component (g) is amylphenoldisulfide polymers.
15. The rubber composition of the above 12, further comprising (h) an organosulfur differing from component (g).
16. The rubber composition of the above 1, wherein a vulcanized form of the rubber composition is a core for a golf ball.
17. The rubber composition of the above 1, wherein the core has a surface and a center with a hardness difference therebetween of at least 15 on JIS-C hardness scale.
18. The rubber composition of the above 1, wherein the hardness of the region of from 25 to 30% at direction of from the surface to the center in a vulcanized form of the rubber composition includes the part which is at least 5 on JIS-C hardness scale smaller than the hardness of the surface.
According to the rubber composition for golf balls of the present invention, a specific rubber crosslinking structure can be obtained, and then, when the rubber member having this rubber crosslinking structure is applied to various structural elements of a golf ball, especially the core, the present invention is able to manifest low spin properties in the golf ball when the ball is hit and can therefore improve the flight performance of the ball, and also early collapse of the core can be prevented and thus the present invention can improve durability to impact even when the foreign matter has been mixed in the core.
The objects, features and advantages of the invention will become more apparent from the following detailed description.
The rubber composition for golf balls of the invention is characterized by including the following components:
The base rubber serving as component (a) is not particularly limited, although it is especially suitable to use polybutadiene.
It is desirable for the polybutadiene to have, in the polymer chain thereof, a cis-1,4 bond content of at least 60%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95%. When cis-1,4 bonds account for too few of the bonds on the polybutadiene molecule, the resilience may decrease.
The content of 1,2-vinyl bonds on the polybutadiene is generally 2% or less, preferably 1.7% or less, and more preferably 1.5% or less, of the polymer chain. When the content of 1,2-vinyl bonds is too high, the resilience may decrease.
The polybutadiene has a Mooney viscosity (ML1+4 (100° C.)) of preferably at least 20, and more preferably at least 30. The upper limit is preferably not more than 120, more preferably not more than 100, and even more preferably not more than 80.
The term “Mooney viscosity” used herein refers to an industrial indicator of viscosity (JIS K 6300) measured with a Mooney viscometer, which is a type of rotary plastometer. This value is represented by the unit symbol ML1+4 (100° C.), wherein “M” stands for Mooney viscosity, “L” stands for large rotor (L-type) and “1+4” stands for a pre-heating time of 1 minute and a rotor rotation time of 4 minutes. The “100° C.” indicates that measurement was carried out at a temperature of 100° C.
The polybutadiene used may be one synthesized with a rare-earth catalyst or a group VIII metal compound catalyst.
A polybutadiene rubber synthesized with a catalyst differing from the above lanthanum rare-earth compound may be included in the base rubber. In addition, styrene butadiene rubber (SBR), natural rubber, polyisoprene rubber, ethylene propylene diene rubber (EPDM) or the like may also be included. These may be used singly or two or more may be used in combination.
The polybutadiene accounts for a proportion of the overall rubber that is preferably at least 60 wt %, more preferably at least 70 wt %, and most preferably at least 90 wt %. The above polybutadiene may account for 100 wt % of the base rubber; that is, it may account for all of the base rubber.
Next, component (b) is a co-crosslinking agent, this being an α,β-unsaturated carboxylic acid and/or a metal salt thereof. The number of carbon atoms on this unsaturated carboxylic acid is preferably from 3 to 8. Specific examples include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid and fumaric acid. Specific examples of the metal in the metal salts of these unsaturated carboxylic acids include zinc, sodium, magnesium, calcium and aluminum, with zinc being especially preferred. The co-crosslinking agent is most preferably zinc acrylate.
Component (c) is a crosslinking initiator. It is preferable to use an organic peroxide as this crosslinking initiator, and especially preferable to use an organic peroxide having a one minute half-life temperature of between 110 and 185° C. Examples of such organic peroxides include dicumyl peroxide (Percumyl D, from NOF Corporation), 2,5 dimethyl 2,5-di(t-butylperoxy)hexane (Perhexa 25B, from NOF Corporation) and di (2 t butylperoxyisopropyl)benzene (Perbutyl P, from NOF Corporation). The use of dicumyl peroxide is preferred. Other commercial products include Perhexa C-40, Niper BW and Peroyl L (all from NOF Corporation), and Luperco 231XL (from AtoChem Co.). These may be used singly, or two or more may be used together.
Next, component (d) is an alcohol. By including an alcohol as serving component (d) in the rubber composition, a cured rubber material (core) having the desired core hardness profile can be obtained during vulcanization (curing) of the rubber composition, as a result of which the ball fully achieves a lower spin rate when hit, enabling the ball to be endowed with an excellent flight performance. As used herein, an “alcohol” refers to a substance having at least one alcoholic hydroxyl group; substances obtained by polycondensing polyhydric alcohols having two or more hydroxyl groups are also included herein as alcohols.
It is especially preferable for the alcohol to be a hexahydric or lower alcohol (an alcohol having up to six alcoholic hydroxyl groups), particularly, a monohydric alcohol, a dihydric alcohol or a trihydric alcohol is preferably used. Specific examples include, but are not limited to, methanol, ethanol, propanol, butanol, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, glycerol, butanetriol, trimethylolethane, trimethylolpropane, di (trimethylolpropane), pentaerythritol and sorbitol. These have molecular weights which, although not particularly limited, are preferably below 500, more preferably below 300, even more preferably below 250 and most preferably below 200. When the molecular weight is too large, i.e., when the number of carbons is too high, the desired core hardness profile may not be obtained or a reduced ball spin rate on impact may not be fully achieved. The component (d) is not particularly limited, although it is preferably to use an alcohol which is defined as a substance having a value obtained by dividing the molecular weight by the number of hydroxyl groups thereon which is 100 or less, more preferably 70 or less, even more preferably 50 or less. When this numerical value is too large, the sufficient flight performance may not be obtained.
The content of component (d) per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 10 parts by weight, more preferably not more than 6 parts by weight, and even more preferably not more than 3 parts by weight. At a component (d) content that is too high, the hardness decreases and the desired feel at impact, durability and rebound may not be obtained. At a content that is too low, the desired core hardness profile may not be obtained and a reduced spin rate may not be fully achieved when the ball is hit.
Component (e) is a hindered phenol-type antioxidant including a substituent having thioether structure. It is preferable that the hindered phenol-type antioxidant is a chemical structure having at least one methyl group at ortho position. Also, it is preferable that the number of the substituent having thioether structure in the hindered phenol-type antioxidant is at least two.
In the present invention, by compounding component (e) into the rubber composition, normal durability to impact can be enhanced. Even when contaminant is incorporated into the rubber material, the decline of durability to impact can maintain at a constant level or more.
It is preferable that the hindered phenol-type antioxidant serving as component (e) is represented by the following general formula (I).
wherein x is an integer of 1 or more, preferably 8 or more in the above formula. Commercial products that may be used as component (e) include “Antage HP-500”, “Antage HP-400” (from Kawaguchi Chemical Industry Co., Ltd.) and “Irganox 1520L” (from BASF Corporation) and the like.
The amount of component (e) included per 100 parts by weight of the base rubber is at least 0.2 part by weight, preferably at least 0.3 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 3.0 parts by weight, more preferably not more than 2.0 parts by weight, and further preferably not more than 1.5 parts by weight. When too much component (e) is included, the hardness decreases and it may not be possible to obtain the desired feel on impact, durability and rebound. When too little component (e) is included, it may not be possible to obtain the desired effect of durability to impact.
In the compounding method of component (e), the component (e) may be added to component (a) at the time of the manufacture of the rubber composition, or previously the component (e) may be added at the time of the manufacture of component (a), and the combination thereof may be adopted.
Also, a benzoimidazole of the following general formula and/or a metal salt thereof is used as (f) an antioxidant.
In formula (II), R is a hydrogen atom or a hydrocarbon group of 1 to 20 carbon atoms and m is an integer from 1 to 4, with each R being the same or different when m is 2 or more. Specific examples of the benzoimidazole of the above formula include 2-mercaptobenzoimidazole, 2-mercaptomethylbenzoimidazole, and metal salts of these. The metal salts are preferably zinc salts.
Further, an antioxidant aside from above components (e) and (f) may be included in the invention. Specific examples of such the antioxidant include hindered phenol-type antioxidants such as 2,6 di tert butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, pentaerythritol tetrakis[3(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 1,3,5-tris(3′,5′-di-t-butyl-4-hydroxybenzyl)isocyanuric acid, 2,2′-methylenebis(4-methyl-6-tert-butylphenol), and 4,4′,4″-(1-methylpropnanyl-3-iridene)tris(6-tert-butyl-m-cresol). Commercial products that can be used include Nocrac 200, Nocrac M-17, Nocrac NS-6 (both from Ouchi Shinko Chemical Industry Co., Ltd.), Irganox 1010 (from BASF) and ADK Stab AO-20, AO-30 (from Adeka). These may be used singly, or two or more may be used together. The amount of this antioxidant included per 100 parts by weight of the base rubber, although not particularly limited, is preferably not more than 1.0 part by weight, more preferably not more than 0.7 part by weight, and further preferably not more than 0.5 part by weight. When too much is included, it may not be possible to achieve the improved effect of the durability due to component (e).
Further, sulfur or organosulfur being specific alkylphenoldisulfide polymers may be included as component (g) in the invention.
As to sulfur, commercial products may be used and include “SULFAX 5” from Turumi Chemical Industry Co., Ltd., “SANMIX IS-60N”, “SANMIX S-80N” from Sansin Chemical Industry Co., Ltd., “AKROFORM S-80/EPR/P” from Akrochem Corporation and the like.
The content of sulfur per 100 parts by weight of the base rubber is, not particularly limited, preferably at least 0.01 part by weight, more preferably at least 0.03 part by weight, most preferably at least 0.05 part by weight. The upper limit is preferably not more than 5.0 parts by weight, more preferably not more than 2.0 parts by weight, most preferably not more than 1.0 parts by weight. At a sulfur content that is too high, the crosslinking reaction with peroxide is prevented by the influence of sulfur such that the entire hardness of the molded article largely soften. At a sulfur content that is too low, the hardness difference in the hardness profile at the interior of the core may not be set to a large value.
It is preferable that sulfur is used in the form of a masterbatch in order to enhance the dispersibility of a small amount of sulfur. The illustrative Examples of such the sulfur includes the above trade names of “SANMIX IS-60N”, “SANMIX S-80N” and “AKROFORM S-80/EPR/P”, each of which is a sulfur masterbatch.
Component (g) may be alkylphenoldisulfide polymers represented by the following chemical formula:
In the above chemical formula (2), R is an alkyl group and n is degree of polymerization in a range of 2 to 20. The alkyl group of R in the chemical formula is preferably an lower alkyl group of 1 to 6 carbon atoms which include methyl, ethyl, n-propyl, iso-propyl, n-butyl, tert-butyl, n-amyl (pentyl), iso-amyl (pentyl), tert-amyl (pentyl), sec isoamyl, neopentyl, n-hexyl, iso-hexyl, tert-hexyl groups. More preferably, the organosulfur is amylphenoldisulfide polymers. Commercial products that may be used include “Sanceler AP” from Sanshin Chemical Industry Co., Ltd., “Vultac 5” from Arkema S.A. and the like.
The content of alkylphenoldisulfide polymers per 100 parts by weight of the base rubber is preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, most preferably at least 0.3 part by weight. The upper limit is preferably not more than 5.0 parts by weight, more preferably not more than 3.0 parts by weight, most preferably not more than 2.0 parts by weight. At its content that is too high, the crosslinking reaction with peroxide is prevented by the influence of sulfur such that the entire hardness of the molded article largely soften.
In the invention, further an organosulfur differing from component (g) may be used as component (h).
The organosulfur compound serving as component (h) is not particularly limited. Examples include thiophenols, thionaphthols, diphenylpolysulfides, halogenated thiophenols, and metal salts of these. Specific examples include the zinc salts of pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol and p-chlorothiophenol, and any of the following having 2 to 4 sulfur atoms: diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides. These may be used singly, or two or more may be used together. Of these, preferred use can be made of the zinc salt of pentachlorothiophenol and/or diphenyldisulfide.
It is recommended that the amount of the above organosulfur compound included per 100 parts by weight of the base rubber be preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and even more preferably at least 0.2 part by weight, and that the upper limit be preferably not more than 3 parts by weight, more preferably not more than 2 parts by weight, and even more preferably not more than 1 part by weight. Including too much organosulfur compound may result in a hot-molded rubber composition that has too low a hardness. On the other hand, including too little may make a rebound-improving effect unlikely.
Aside from above components (a) to (h), various additives such as fillers and processing aids may be included, provided that doing so is not detrimental to the objects of the invention.
Examples of fillers that may be suitably used include zinc oxide, barium sulfate and calcium carbonate. These may be used singly, or two or more may be used together. The filler content per 100 parts by weight of the base rubber may be set to preferably at least 1 part by weight, more preferably at least 3 parts by weight, and even more preferably at least 5 parts by weight. The upper limit in the filler content per 100 parts by weight of the base rubber may be set to preferably not more than 100 parts by weight, more preferably not more than 60 parts by weight, and even more preferably not more than 40 parts by weight. At a filler content that is too high or too low, it may not be possible to obtain a proper weight and a suitable rebound.
Processing aids that may be suitably used include higher fatty acids and metal salts thereof. Examples of higher fatty acids include stearic acid, palmitic acid, oleic acid, linoleic acid, linolenic acid and myristic acid. Stearic acid is especially preferred. Examples of higher fatty acid metal salts include lithium salts, sodium salts, potassium salts, copper salts, magnesium salts, calcium salts, strontium salts, barium salts, tin salts, cobalt salts, nickel salts, zinc salts and aluminum salts. The use of zinc stearate is especially preferred. The amount of processing aid included per 100 parts by weight of the base rubber may be set to preferably at least 1 part by weight, more preferably at least 3 parts by weight, and even more preferably at least 5 parts by weight. The upper limit in this amount of addition per 100 parts by weight of the base rubber may be set to preferably not more than 20 parts by weight, more preferably not more than 15 parts by weight, and even more preferably not more than 10 parts by weight. When too much is added, a sufficient hardness and rebound may not be obtained; when too little is added, the chemicals that are added may not fully disperse and it may not be possible to obtain the expected properties. Examples of methods that may be used to add the processing aid include, but are not particularly limited to: charging the processing aid into a mixer at the same time as other chemicals, adding the processing aid after first mixing it together with other chemicals such as component (b), adding the processing aid after coating it onto the surface of other chemicals such as component (b), and adding the processing aid after first preparing a masterbatch of it together with component (a).
A vulcanizate can be produced by vulcanizing/curing the rubber composition for golf balls of the invention. For example, a core which is a vulcanizate can be produced by using a mixing apparatus such as a Banbury mixer or a roll mill to knead the rubber composition, then using a core mold to compression mold or injection mold the kneaded composition and suitably heating the molded body at a temperature suitable for the organic peroxide and co-crosslinking agent to act, such as at between about 100° C. and about 200° C. for a period of 10 to 40 minutes, so as to cure the molded body.
Here, by compounding the ingredients as described above, the vulcanized/cured rubber molding for a golf ball can be conferred with a hardness gradient in which the difference in hardness between the surface and the center thereof is large. By employing this rubber molding for a golf ball as a golf ball core, the durability of the golf ball can be increased while maintaining the good spin properties of the ball.
The core has a center hardness on the JIS-C hardness scale which, although not particularly limited, is preferably at least 40, more preferably at least 45, and even more preferably at least 50. The upper limit is preferably not more than 75, more preferably not more than 70, and even more preferably not more than 65. At a core center hardness outside of this range, the feel on impact may be poor, the durability may decline and it may not be possible to obtain a spin rate-lowering effect.
The core has a surface hardness on the JIS-C hardness scale which, although not particularly limited, is preferably at least 65, more preferably at least 70, and even more preferably at least 72. The upper limit is preferably not more than 95, more preferably not more than 90, and even more preferably not more than 88. When the surface hardness of the core is lower than this range, the ball rebound may decrease, as a result of which a sufficient distance may not be achieved. On the other hand, when the surface hardness of the core is higher than the above range, the feel at impact may be too hard and the durability to cracking on repeated impact may worsen.
The core has a hardness profile such that the hardness difference between the surface and center of the core is sufficiently large. Specifically, the difference in hardness on the JIS-C scale between the surface A and center B of the core, expressed as A-B, is preferably at least 16, more preferably at least 19, and even more preferably at least 20. The upper limit is preferably not more than 50, more preferably not more than 45, and even more preferably not more than 40. When this hardness difference value is too small, the spin rate-lowering effect on shots with a W #1 may be inadequate and a good distance may not be achieved. On the other hand, when this hardness difference is too large, the initial velocity of the ball when struck may become lower, resulting in a shorter distance, or the durability of the ball to cracking on repeated impact may worsen. Here, “center hardness” refers to the hardness measured at the center of the cross-section obtained by cutting the core through the center, and “surface hardness” refers to the hardness measured at the spherical surface of the core. “JIS-C hardness” refers to the hardness measured with a spring-type durometer (JIS-C model) as specified in JIS K 6301-1975.
The core hardness gradient used in this invention is preferably one in which the hardness remains the same or increases, but does not decrease, from the center toward the surface of the core.
In the core being a vulcanized form of the rubber composition, it is preferable that the hardness of the region of from 25 to 30% at direction of from the surface to the center includes the part which is at least 5 on JIS-C hardness scale smaller than the hardness of the surface.
It is recommended that the core (vulcanizate) have a compression hardness (deformation) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably not more than 3.2 mm, and more preferably not more than 3.0 mm. The lower limit is not particularly limited, but, is preferably at least 2.0 mm, more preferably at least 2.3 mm, and even more preferably at least 2.5 mm. When this value is too large, the core becomes too soft, as a result of which the resilience may decrease. When this value is too small, a spin rate lowering effect may not be obtained and the feel of the ball on impact may become hard.
The core diameter, which is not particularly limited and depends also on the layer construction of the golf ball to be produced, is preferably at least 30 mm, and more preferably at least 35 mm, but is preferably not more than 41 mm, and more preferably not more than 40 mm. At a core diameter outside of this range, the initial velocity of the ball may become low or suitable spin properties may not be obtained.
As described above, the foregoing rubber composition is suitably used as a golf ball core. The golf ball of the invention preferably has a structure that includes a core and a cover of one or more layers.
Next, the cover of one or more layers encasing the core is described.
The cover material is not particularly limited, although known materials such as various types of ionomeric resins and thermoplastic polyurethane elastomers used in golf balls may be employed.
To realize an even further spin rate-lowering effect in the ball, it is especially preferable to use a highly neutralized ionomeric material in the layer adjoining the core.
Specifically, it is preferable to use a material obtained by blending components (i) to (iv) below:
In particular, when using a mixed material of components (i) to (iv), it is preferable to utilize one in which at least 70% of the acid groups are neutralized.
The material making up the outermost layer of the cover is preferably one composed primarily of a urethane material, especially a thermoplastic urethane elastomer.
One or more cover layer (intermediate layer) may be formed between the layer adjoining the core and the outermost cover layer. In this case, it is preferable to use a thermoplastic resin such as an ionomer as the intermediate layer material.
To obtain the cover in this invention, use may be made of, for example, a method that involves placing within a mold a single-layer core or a multilayer core of two or more layers that has been prefabricated according to the type of ball, mixing and melting the above mixture under applied heat, and injection-molding the molten mixture over the core so as to encase the core with the desired cover. The cover producing operations in this case can be carried out in a state where excellent thermal stability, flowability and processability are assured. As a result, the golf ball ultimately obtained has a high rebound, and moreover has a good feel on impact and excellent scuff resistance. Alternatively, use may be made of a cover-forming method other than the foregoing, such as one in which, for example, a pair of hemispherical half-cups are molded beforehand from the cover material described above, following which the core is enclosed within the half-cups and molding is carried out under applied pressure at between 120° C. and 170° C. for a period of 1 to 5 minutes.
When the cover has only one layer, the thickness of that layer may be set to from 0.3 to 3 mm. When the cover has two layers, the thickness of the outer cover layer may be set to from 0.3 to 2.0 mm and the thickness of the inner cover layer (intermediate layer) may be set to from 0.3 to 2.0 mm. The Shore D hardnesses of the respective layers making up the cover (cover layers), although not particularly limited, are set to preferably at least 40, and more preferably at least 45. The upper limit is preferably not more than 70, and more preferably not more than 65.
Numerous dimples are formed on the surface of the outermost layer of the cover. In addition, the cover may be subjected to various types of treatment, such as surface preparation, stamping and painting. In cases where such surface treatment is imparted to the cover formed of the above cover material, the good moldability of the cover surface enables the work to be carried out efficiently.
The invention provides a golf ball in which the above rubber composition is used as the core material for at least one core layer. With regard to the type of golf ball, this rubber composition may be used without particular limitation in golf balls having a core and one or more cover layer, including solid golf balls such as two-piece or three-piece solid golf balls in which the solid core is encased by the cover and multi-piece golf balls having at least a three-piece construction, and also wound golf balls in which a wound core is encased by a single-layer cover or a cover with a multilayer construction of two or more layers.
Examples according to the invention and Comparative Examples are given below by way of illustration, although the invention is not limited by these Examples.
Cores having a diameter of about 38.6 mm are produced by using the core materials composed primarily of polybutadiene shown in Table 1 below to prepare core compositions formulated for Working Examples 1 to 5 and Comparative Examples 1 to 6, subsequently vulcanizing the compositions at 158° C. for 20 minutes, and then abrading the core surface.
Details on the above formulations are given below.
The durability to impact is evaluated under the circumstance that the core is easily cracked by mixing an artificial foreign matter in the rubber composition of each of Examples. The evaluation of the durability to impact is described later.
Regarding an artificial foreign matter, zinc oxide and fine powdered zinc stearate are mixed in weight ratio of 2:1 and the mixture thereof is heated in an oven at 150° C. for 30 minutes and then the temperature lower to room temperature, after that, the heated solid article is ground roughly. And this ground material is screened with 1 mm mesh and 0.5 mm mesh to obtain an aggregation having a diameter of 0.5 to 1.0 mm. Such the aggregation as an article foreign matter is added to a kneaded rubber of a rubber formulation shown in Table 1 with a roll in an amount (1phr) of Table 1.
The cross-sectional hardnesses at various positions, including the surface and center, of the about 38.6 mm diameter core in each of the above Working Examples and Comparative Examples are measured by the following methods.
At a temperature of 23±1° C., the indenter of a durometer is perpendicularly set against a surface portion of the spherical core and the JIS-C hardness is measured at four random points on the core surface. The average value of these measurements is treated as the measured value for one core, and the average value for three measured cores is determined. These results are presented in Table 3.
The core is cut through the center to obtain a flat cross-sectional plane. At a temperature of 23±1° C., the indenter of a durometer is perpendicularly set against the cross sectional plane and the JIS-C hardness is measured at the center of the hemispherical core and at 2 mm intervals from the center toward the surface, thereby collecting the measurements for one core. The average values for three measured cores are determined. These results are presented in Table 3.
The amount of deflection (mm) by each core and ball when compressed at a speed of 10 mm/s under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is measured at a temperature of 23±1° C. In each case, the average value for 10 measured cores or balls is determined.
Using an injection mold, the intermediate layer material (ionomer resin material) shown in Table 2 is then injection-molded over the surface of the above core, thereby forming an intermediate layer having a thickness of 1.3 mm and a Shore D hardness of 64. Next, using a different injection mold, the outermost layer material (urethane resin material) shown in Table 2 is injection-molded over the intermediate layer-encased sphere, thereby forming an outermost layer having a thickness of 0.8 mm and a Shore D hardness of 40.
Details on the compounding ingredients in the table are given below.
The spin rates of the resulting golf balls on shots with a driver and the durability of the resulting golf balls to impact are evaluated by the following method. The results are shown in Table 3.
The durability of the golf ball is evaluated using an ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). This tester fires a golf ball pneumatically and causes it to repeatedly strike two metal plates arranged in parallel. The incident velocity against the metal plates is set to 43 m/s. The number of shots required for the golf ball to crack is measured, and the average value of the measurements taken for ten golf balls is calculated. Durability indices for the balls in the respective Examples are determined relative to a reference value of 1.00 for the average number of shots required for the ball obtained in Comparative Example 1 (none of foreign matter) to crack, and are shown in Table 3. In addition, the durability to impact in each of Examples and Comparative Examples is evaluated by compounding artificial foreign matters into rubber compositions. Preparing method of an artificial foreign matter is described above. In this case, as well as the above, durability indices for the balls in the respective Examples are determined relative to a reference value of 1.00 for the average number of shots required for the ball obtained in Comparative Example 1 to crack, and are shown in Table 3.
As is apparent from the results in Table 3, in each of the golf balls in Examples 1 to 5 of the invention, normal durability to impact is enhanced, as compared with Comparative Examples 1 to 6. Even when contaminant is incorporated into the rubber composition, it is seen that the decline of durability to impact can maintain at a constant level or more.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-111044 | Jun 2018 | JP | national |
This application is a continuation-in-part of copending application Ser. No. 18/100,789 filed on Jan. 24, 2023, which is a continuation-in-part of copending two applications Ser. No. 17/386,656 filed on Jul. 28, 2021 and Ser. No. 17/386,681 filed on Jul. 28, 2021, each of which is a continuation-in-part of copending application Ser. No. 16/406,099 filed on May 8, 2019 (now is U.S. Pat. No. 11,104,781), claiming priority based on Japanese Patent Application No. 2018-111044 filed in Japan on Jun. 11, 2018, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 18100789 | Jan 2023 | US |
| Child | 18764867 | US | |
| Parent | 17386681 | Jul 2021 | US |
| Child | 18100789 | US | |
| Parent | 17386656 | Jul 2021 | US |
| Child | 17386681 | US | |
| Parent | 16406099 | May 2019 | US |
| Child | 17386681 | US | |
| Parent | 16406099 | May 2019 | US |
| Child | 17386656 | US |
