CN106938134A - Two-piece golf ball - Google Patents

Abstract

An object of the present invention is to provide a two-piece golf ball (2) that is excellent in impact feeling on putt shots. The method is that the golf ball (2) has a core (4) and a cover (6). The value V calculated by the following mathematical formula is 1080 Hz or less: V=NF(2)2-2/3×NF(2)1 In this mathematical formula, NF(2)1 represents the second order of the spherical core (4). The natural frequency, NF(2)2, represents the second-order natural frequency of the golf ball (2). In the core (4), the difference (H1s-H1o) between the Shore C hardness H1s of the surface and the Shore C hardness H1o of the center is 10 or more. The amount of compressive deformation Df1 of the core (4) is 4.1 mm or more.

Description

double golf ball

technical field

This invention relates to golf balls. In particular, the present invention relates to so-called two piece golf balls having a core and a cover.

Background technique

In golf, a golf ball is hit by a wood, an iron, a hybrid club (multi-purpose), a putter, or the like. The feel of the ball at impact is what players care about. In general, players want golf balls that feel soft to the touch.

In the beginner's game, the frequency of miss shots is high. Therefore, beginners feel dull when hitting golf balls with woods, irons, or hybrids.

On the other hand, in the case of putting, even beginners hit the golf ball with the sweet spot (Sweet spot) of the putter in many cases. Even beginners are sensitive to the feel of the ball on light hits. Beginners want a golf ball that provides a soft feel on light shots.

In the past, so-called string balls were the mainstream of golf. These days, twine balls are hardly ever sold openly on the market. In recent golf balls, two-piece balls, three-piece balls, four-piece balls, five-piece balls, and six-piece balls are used.

A double layer ball has a core and a shell. The structure of the double-layer ball is simple. Double layer balls can be manufactured at low cost. Proposals on double-layer balls are disclosed in Japanese Patent Laid-Open No. 11-76461 and US Patent No. 5971870.

prior art literature

patent documents

Patent Document 1: Japanese Patent Application Laid-Open No. 11-76461

Patent Document 2: US Patent No. 5971870

Contents of the invention

The problem to be solved by the invention

In a beginner's shot, the golf ball often flies in an aimless direction. There are many cases where golf balls fall into pools and fly into forests. Beginners frequently lose golf balls. Therefore, beginners do not like high-priced golf balls. Because it can be manufactured at low cost, double-layer balls are suitable for beginners. As mentioned above, beginners want a soft hitting feel on light hits. It is desired to improve the hitting feeling of double-layer balls on putt.

An object of the present invention is to provide a two-piece golf ball that is excellent in hitting feeling on a putt.

[method to solve the problem]

The double golf ball according to the present invention includes a core and a cover located outside the core. In this golf ball, the value V calculated by the following mathematical formula is 1080 Hz or less.

V＝NF(2)2-2/3×NF(2)1

In this formula, NF(2)1 represents the second-order natural frequency of the core, and NF(2)2 represents the second-order natural frequency of the golf ball.

Preferably, the difference (H1s−H1o) between the Shore C hardness H1s of the surface and the Shore C hardness H1o of the center in the core is 10 or more.

Preferably, the value V is below 1040 Hz. More preferably, the value V is 1000 Hz or less.

Preferably, the amount of compressive deformation Df1 of the core is 4.1 mm or more.

Preferably, Df2 of the golf ball is 3.5 mm or more.

Preferably, the thickness T2 of the casing is not less than 0.80 mm and not more than 2.00 mm. Preferably, the Shore D hardness H2 of the casing is not less than 50 and not more than 65.

Invention effect

The two-layer golf ball according to the present invention has a simple structure. The golf ball can be manufactured at low cost. Since the value V is 1080 Hz or less, the golf ball has a soft hitting feeling when the golf ball is lightly hit. This golf ball can have both low price and hitting feeling.

Description of drawings

[ Fig. 1] Fig. 1 is a cross-sectional view showing a golf ball according to an embodiment of the present invention.

[ Fig. 2] Fig. 2 is a conceptual diagram showing an apparatus for measuring a natural frequency of the golf ball shown in Fig. 1 .

[ Fig. 3] Fig. 3 is a graph showing the relationship between the second-order natural frequency NF(2)1 of the core and the second-order natural frequency NF(2)2 of the golf ball.

Symbol Description

2: Golf

4: Core

6: shell

8: Dent

10: hitting surface (land)

12: Vibration machine

14: board

16: The first acceleration sensor

18: Second acceleration sensor

detailed description

Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to appropriate drawings.

The golf ball 2 shown in FIG. 1 includes a spherical core 4 and a cover 6 positioned outside the core 4 . In this embodiment, the shell 6 is directly bonded to the core 4 . This golf ball 2 is a so-called double ball. This golf ball 2 has a plurality of dimples 8 on its surface. Of the surface of the golf ball 2 , a portion other than the dimple 8 is a land 10 . Although this golf ball 2 includes a paint layer and a mark layer on the outside of the cover 6, illustration of these layers is omitted.

Preferably, the golf ball 2 has a diameter of not less than 40 mm and not more than 45 mm. From the viewpoint of meeting the standards of the United States Golf Association (USGA), the diameter is particularly preferably 42.67 mm or more. From the viewpoint of controlling air resistance, the diameter is more preferably 44 mm or less, particularly preferably 42.80 mm or less. Preferably, the golf ball 2 has a mass of not less than 40 g and not more than 50 g. From the viewpoint of obtaining a large inertia, the mass is more preferably 44 g or more, particularly preferably 45.00 g or more. From the viewpoint of satisfying the standards of USGA, the mass is particularly preferably 45.93 g or less.

The core 4 is formed by cross-linking the rubber composition. Preferred base rubbers include polybutadiene, polyisoprene, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubber. From the standpoint of rebound performance, polybutadiene is preferred. When polybutadiene is used in combination with other rubbers, polybutadiene is preferably used as the main component. Specifically, the ratio of polybutadiene to the entire base rubber is preferably 50% by mass or more, particularly preferably 80% by mass or more. Particularly preferred is polybutadiene having a cis-1,4 bond ratio of 80% or more.

Preferably, the rubber composition of core 4 contains a co-crosslinking agent. From the viewpoint of the rebound performance of the golf ball 2, a preferred co-crosslinking agent is a monovalent or divalent metal salt of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. As a preferable co-crosslinking agent, zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate are mentioned. From the viewpoint of the rebound performance of the golf ball 2, zinc acrylate and zinc methacrylate are particularly preferable.

The rubber composition may contain an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms and a metal oxide. The two react in the rubber composition to obtain the salt. This salt functions as a co-crosslinking agent. Preferable α,β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Zinc oxide and magnesium oxide are exemplified as preferable metal oxides.

Preferably, the amount of the co-crosslinking agent is 10 parts by mass or more relative to 100 parts by mass of the base rubber. The golf ball 2 having the core 4 in an amount of 10 parts by mass or more is excellent in rebound performance. From this viewpoint, the amount is more preferably 15 parts by mass or more, particularly preferably 20 parts by mass or more.

Preferably, the amount of the co-crosslinking agent is 40 parts by mass or less relative to 100 parts by mass of the base rubber. In the golf ball 2 having the core 4 in this amount of 40 parts by mass or more, the impact feeling at the time of putting is soft. From this viewpoint, the amount is more preferably 35 parts by mass or less, particularly preferably 30 parts by mass or less.

Preferably, the rubber composition of core 4 contains an organic peroxide. Organic peroxides function as crosslinking initiators. The organic peroxide contributes to the rebound performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-di Methyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. In particular, a highly versatile organic peroxide is dicumyl peroxide.

Preferably, the amount of the organic peroxide is 0.1 parts by mass or more relative to 100 parts by mass of the base rubber. The golf ball 2 having the core 4 in an amount of 0.1 parts by mass or more is excellent in rebound performance. From this viewpoint, the amount is more preferably 0.3 parts by mass or more, particularly preferably 0.5 parts by mass or more.

Preferably, the amount of the organic peroxide is 3.0 parts by mass or less relative to 100 parts by mass of the base rubber. In the golf ball 2 having the core 4 in an amount of 3.0 parts by mass or more, the impact feeling at the time of putting is soft. From this viewpoint, the amount is more preferably 2.5 parts by mass or less, particularly preferably 2.0 parts by mass or less.

The rubber composition of the core 4 contains an organic sulfur compound. The organic sulfur compounds include naphthalenethiol-based compounds, benzenethiol-based compounds, and disulfide-based compounds.

Examples of naphthalenethiol-based compounds include: 1-naphthalenethiol, 2-naphthalenethiol, 4-chloro-1-naphthalenethiol, 4-bromo-1-naphthalenethiol, 1-chloro-2-naphthalene Thiol, 1-bromo-2-naphthalenethiol, 1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, and 1-acetyl-2-naphthalenethiol.

Examples of benzenethiol-based compounds include: benzenethiol, 4-chlorobenzenethiol, 3-chlorobenzenethiol, 4-bromobenzenethiol, 3-bromobenzenethiol, 4-fluorobenzenethiol, 4-iodobenzenethiol, 2,5-dichlorobenzenethiol, 3,5-dichlorobenzenethiol, 2,6-dichlorobenzenethiol, 2,5-dibromobenzenethiol, 3,5 -Dibromobenzenethiol, 2-chloro-5-bromobenzenethiol, 2,4,6-trichlorobenzenethiol, 2,3,4,5,6-pentachlorobenzenethiol, 2,3, 4,5,6-pentafluorobenzenethiol, 4-cyanobenzenethiol, 2-cyanobenzenethiol, 4-nitrobenzenethiol, and 2-nitrobenzenethiol.

Examples of disulfide-based compounds include: diphenyl disulfide, bis(4-chlorophenyl) disulfide, bis(3-chlorophenyl) disulfide, bis(4-bromophenyl) Disulfide, bis(3-bromophenyl) disulfide, bis(4-fluorophenyl) disulfide, bis(4-iodophenyl) disulfide, bis(4-cyanophenyl) disulfide Sulfide, bis(2,5-dichlorophenyl) disulfide, bis(3,5-dichlorophenyl) disulfide, bis(2,6-dichlorophenyl) disulfide, bis( 2,5-dibromophenyl) disulfide, bis(3,5-dibromophenyl) disulfide, bis(2-chloro-5-bromophenyl) disulfide, bis(2-cyano -5-bromophenyl) disulfide, bis(2,4,6-trichlorophenyl) disulfide, bis(2-cyano-4-chloro-6-bromophenyl) disulfide, bis (2,3,5,6-tetrachlorophenyl) disulfide, bis(2,3,4,5,6-pentachlorophenyl) disulfide, and bis(2,3,4,5, 6-pentabromophenyl) disulfide.

From the viewpoint of the rebound performance of the golf ball 22, the amount of the organic sulfur compound is preferably 0.1 parts by mass or more, particularly preferably 0.2 parts by mass or more, based on 100 parts by mass of the base rubber. The amount is preferably 1.5 parts by mass or less, more preferably 1.0 parts by mass or less, particularly preferably 0.8 parts by mass or less, from the viewpoint of the feeling at impact on a putt. Two or more organic sulfur compounds may be used in combination.

The rubber composition of the core 4 may contain additives for the purpose of adjusting specific gravity and the like. Examples of suitable additives include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is appropriately determined in order to achieve the target specific gravity of the core 4 .

The rubber composition may contain an appropriate amount of various additives such as sulfur, carboxylic acid, carboxylate, antiaging agent, colorant, plasticizer, and dispersant. The rubber composition may contain crosslinked rubber powder or synthetic resin powder.

Preferably, the diameter of the core 4 is 39.0 mm or more. In the golf ball 2 having the core 4 having a diameter of 39.0 mm or more, the cover 6 is thin. Therefore, in this golf ball 2, the impact feeling at the time of putting is soft. Furthermore, this golf ball 2 is excellent in rebound performance. From these viewpoints, the diameter is more preferably 39.3 mm or more, particularly preferably 39.5 mm or more. From the viewpoint of durability of the golf ball 2, the diameter thereof is preferably 41.0 mm or less, more preferably 40.6 mm or less, particularly preferably 40.2 mm or less.

Preferably, the compressive deformation amount Df1 of the core 4 is 4.1 mm or more. In the core 4 whose compression deformation amount Df1 is 4.1 mm or more, the impact feeling at the time of putting is soft. From this viewpoint, the amount of compressive deformation Df1 is more preferably 4.2 mm or more, particularly preferably 4.4 mm or more. From the viewpoint of the rebound performance of the golf ball 2, the amount of compressive deformation Df1 is preferably 6.5 mm or less, more preferably 6.0 mm or less, particularly preferably 5.5 mm or less.

For the measurement of the amount of compression deformation, a YAMADA-type compression testing machine was used. In this testing machine, a ball (core 4 or golf ball 2)) is placed on a metal hard plate. A metal cylinder is gradually lowered toward the ball. The ball sandwiched between the bottom surface of the cylinder and the hard plate is deformed. The moving distance of the column from the state where the initial load of 98N was applied to the ball to the final load of 1274N was measured. The moving speed of the cylinder until the initial load was applied was 0.83 mm/s. The moving speed of the cylinder from the application of the initial load to the application of the final load was 1.67 mm/s.

Preferably, in the core 4, the difference (H1s−H1o) between the Shore C hardness H1s of the surface and the Shore C hardness H1o of the center is 10 or more. The core 4 having a difference (H1s-H1o) of 10 or more has a so-called hard-outside-soft-inside structure. The golf ball 2 having the core 4 can suppress spin when it is hit with a driver. When the golf ball 2 having the core 4 is hit with a driver, a large launch angle can be obtained.

In the case of a driver hit, a moderate trajectory height and moderate air time are required. In the golf ball 2 that realizes the trajectory height and the air time at a high spin speed, the movement (run) after falling is small. In the golf ball 2 that realizes the trajectory height and air time at a large impact angle, the movement after falling is large. From the viewpoint of flight distance, the golf ball 2 that achieves a trajectory height and air time at a large hitting angle is preferable. As described above, the core 4 having a structure that is hard on the outside and soft on the inside can contribute to a large hitting angle and low spin speed. This core 4 can contribute to the flight performance of the golf ball 2 despite the small amount of compressive deformation Df1.

From the standpoint of flight performance, the difference (H1s-H1o) is more preferably 11 or more, particularly preferably 12 or more. From the viewpoint of the durability of the golf ball 2, the difference (H1s-H1o) is preferably 30 or less, more preferably 28 or less, particularly preferably 25 or less.

The center hardness H1o is preferably 40 or more, more preferably 45 or more, and particularly preferably 50 or more from the viewpoint of durability and rebound performance. From the viewpoint of suppressing rotation, the hardness H1o is preferably 70 or less, more preferably 65 or less, particularly preferably 60 or less.

The hardness H1o was measured with a Shore C hardness meter equipped with an automatic hardness meter (trade name "Digitest II" of H.Barleys Co., Ltd.). The durometer is pressed against the center of the cross-section of the hemisphere obtained by cutting the golf ball 2 . The measurement was performed in an environment of 23°C.

From the viewpoint of suppressing rotation, the surface hardness H1s is preferably 64 or higher, more preferably 66 or higher, and particularly preferably 68 or higher. From the viewpoint of the durability of the golf ball 2, the hardness H1s is preferably 85 or less, more preferably 83 or less, particularly preferably 80 or less.

The hardness H1s was measured with a Shore C-type durometer attached to an automatic durometer (trade name "Digitest II" of H.Barleys Co.). This durometer is pressed against the surface of the core 4 . The measurement was performed in an environment of 23°C.

Preferably, the mass of the core 4 is not less than 10 g and not more than 42 g. The crosslinking temperature of the core 4 is not less than 140°C and not more than 180°C. The crosslinking time of the core 4 is not less than 10 minutes and not more than 60 minutes.

The cover 6 is located outside the core 4 . If apart from the marking layer and the paint layer, the outer shell 6 is the outermost layer. The case 6 is formed of a thermoplastic resin composition. Examples of the base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. Ionomer resins are particularly preferred. Ionomer resins are highly elastic. Golf ball 2 having cover 6 containing ionomer resin is excellent in rebound performance. The housing 6 may also be formed of a thermosetting resin composition.

The ionomer resin can also be used in combination with other resins. When used in combination, the ionomer resin is used as the main component of the base polymer from the viewpoint of rebound performance. The ratio of the ionomer resin to the entire base polymer is preferably 50% by mass or more, more preferably 70% by mass or more, particularly preferably 85% by mass or more.

Preferred ionomer resins include binary copolymers of an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferred binary copolymer contains 80% by mass to 90% by mass of α-olefin and 10% by mass to 20% by mass of α,β-unsaturated carboxylic acid. The binary copolymer is excellent in rebound performance. Other preferred ionomer resins include α-olefins, α,β-unsaturated carboxylic acids with 3 to 8 carbon atoms, and α,β-unsaturated carboxylic acids with 2 to 22 carbon atoms. Terpolymers of acid esters. A preferred terpolymer contains 70% by mass to 85% by mass of α-olefin, 5% by mass to 30% by mass α, β-unsaturated carboxylic acid, and 1% by mass to 25% by mass of α, β-unsaturated carboxylate. The terpolymer has excellent rebound performance. Among binary copolymers and terpolymers, preferred α-olefins are ethylene and propylene, and preferred α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferred ionomer resin is a copolymer of ethylene and acrylic acid. A particularly preferable other ionomer resin is a copolymer of ethylene and methacrylic acid.

In binary copolymers and terpolymers, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for neutralization include sodium ions, potassium ions, lithium ions, zinc ions, calcium ions, magnesium ions, aluminum ions, and neodymium ions. For neutralization, two or more metal ions may be used. Particularly suitable metal ions are sodium ions, zinc ions, lithium ions, and magnesium ions from the standpoint of rebound performance and durability of the golf ball 2 .

Specific examples of ionomer resins include: Mitsui Dupont Poly Chemical Co., Ltd. (Mitsui DuPont Poly Chemical Co., Ltd.) trade names "Himilan 1555" (Himilan 1555), "Himilan 1557", "Himilan 1605", "Himilan 1706", "Himilan 1707" , "Himilan1856", "Himilan1855", "HimilanAM7311", "Himilan AM7315", "Himilan AM7317", "Himilan AM7329" and "Himilan AM7337"; DuPont's trade names "SURLYN6120" (サーリン6120), "SURLYN6910", "SURLYN7930", "SURLYN7940", "SURLYN8140", "SURLYN8150", "SURLYN8940", "SURLYN8945", "SURLYN9120", "SURLYN9150", "SURLYN9910", "SURLYN9945", "SURLYN AD851000", "HPF HPF2000"; and ExxonMobil Chemical Company trade names "IOTEK7010", "IOTEK7030", "IOTEK7510", "IOTEK7520", "IOTEK8000" and "IOTEK8030". Two or more ionomer resins may be used in combination.

The resin composition of the case 6 may contain a thermoplastic elastomer containing a styrene block. The thermoplastic elastomer containing a styrene block has a polystyrene block as a hard segment, and a soft segment. Typical soft segments are diene blocks. Examples of the diene block compound include butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two or more compounds may be used in combination.

Thermoplastic elastomers containing styrene blocks include styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene block copolymer (SIS), styrene- Isoprene-butadiene-styrene block copolymer (SIBS), hydrides of SBS, hydrides of SIS, and hydrides of SIBS. As a hydrogenated product of SBS, styrene-ethylene-butylene-styrene block copolymer (SEBS) is mentioned. As a hydrogenated product of SIS, styrene-ethylene-propylene-styrene block copolymer (SEPS) is mentioned. Examples of hydrogenated products of SIBS include styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

From the viewpoint of the rebound performance of the golf ball 2, the content of the styrene component in the styrene block-containing thermoplastic elastomer is preferably 10% by mass or more, more preferably 12% by mass or more, particularly preferably 15% by mass or more. From the viewpoint of the impact feeling of the golf ball 2, the content thereof is preferably 50% by mass or less, more preferably 47% by mass or less, particularly preferably 45% by mass or less.

In the present invention, the styrene block-containing thermoplastic elastomer contains an alloy of one or two or more selected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, and SEEPS with an olefin. It is speculated that the olefin component in the mixture helps to improve the compatibility with other base polymers. This mixture can contribute to the rebound performance of the golf ball 2 . An olefin having 2 to 10 carbon atoms is preferable. Examples of suitable olefins include ethylene, propylene, butene and pentene. Ethylene and propylene are particularly preferred.

Specific examples of the polymer mixture include Mitsubishi Chemical Corporation's trade names "RABALONT3221C" (ラバロン T3221C), "RABALON T3339C", "RABALON SJ4400N", "RABALON SJ5400N", "RABALON SJ6400N", "RABALON SJ7400N", "RABALON SJ8400N", "RABALON SJ9400N", and "RABALON SR04". Specific examples other than thermoplastic elastomers containing styrene blocks include Daicel Chemical Industry Co., Ltd.'s trade name "EPOFRIENDA1010" (Epofrend A1010) and Kuraray Co., Ltd. (Kurare Co., Ltd.). ) trade name "SEPTON HG-252" (セプトンHG-252).

From the viewpoint of the impact feeling at the time of putt, the ratio of the thermoplastic elastomer containing a styrene block to the total base polymer is preferably 2% by mass or more, more preferably 4% by mass or more, particularly preferably 6% by mass above. From the viewpoint of suppressing rotation, the ratio is preferably 30% by mass or less, more preferably 25% by mass or less, particularly preferably 20% by mass or less.

The resin composition of the shell 6 may contain an appropriate amount of colorants, fillers, dispersants, antioxidants, ultraviolet absorbers, light stabilizers, fluorescent agents, fluorescent whitening agents, and the like. When the hue of the golf ball 2 is white, a typical colorant is titanium dioxide.

Preferably, the thickness T2 of the casing 6 is 2.00 mm or less. The shell 6 whose thickness T2 is 2.00 mm or less does not hinder the soft hitting feeling at the time of putting. From this viewpoint, the thickness is more preferably 1.85 mm or less, particularly preferably 1.70 mm or less. The thickness T2 is preferably 0.80 mm or more, more preferably 0.95 mm or more, and particularly preferably 1.05 mm or more from the viewpoint of ease of forming the cover 6 and the durability of the golf ball 2 . The thickness T2 is measured directly under the land 10 .

The Shore D hardness H2 of the cover 6 is preferably 50 or higher, more preferably 53 or higher, and particularly preferably 55 or higher, from the viewpoint that the golf ball 2 as a whole has a structure that is hard on the outside and soft on the inside. The hardness H2 is preferably 65 or less, more preferably 63 or less, particularly preferably 61 or less, from the viewpoint of the impact feeling at the time of putting.

The hardness H2 of the case 6 is measured based on the regulations of "ASTM-D 2240-68". The hardness H2 is measured by a Shore D-type hardness meter installed on an automatic hardness meter (trade name "Digitest II" of H.Barleys Co., Ltd.). A sheet with a thickness of about 2 mm. Before the measurement, the sheet was stored at a temperature of 23° C. for 2 weeks. During the measurement, three sheets were stacked.

Preferably, the amount of compressive deformation Df2 of the golf ball 2 is 3.5 mm or more. In the golf ball 2 having a compressive deformation amount Df2 of 3.5 mm or more, the ball feeling at the time of putting is soft. From this viewpoint, the amount of compressive deformation Df2 is more preferably 3.6 mm or more, particularly preferably 3.8 mm or more. From the viewpoint of the rebound performance of the golf ball 2, the amount of compressive deformation Df2 is preferably 6.0 mm or less, more preferably 5.5 mm or less, particularly preferably 5.0 mm or less.

An apparatus for measuring the natural frequencies of the core 4 and the golf ball 2 is shown in FIG. 2 . The device has a vibration machine 12 , a plate 14 , a first acceleration pick up 16 and a second acceleration pick up 18 . The plate 14 is mounted on the vibration machine 12 . A ball (the core 4 or the golf ball 2 ) is placed on the plate 14 . A first acceleration sensor is mounted on the board 14 . The second acceleration sensor 18 is mounted on the ball. Vibration is applied to the ball by the vibration machine 12 . A signal of the acceleration applied to the ball is output from the first acceleration sensor 16 . From the second acceleration sensor 18, a signal of the acceleration of the ball is output. Feed these signals into a dynamic signal analyzer. Through the calculation of the analyzer, a curve showing the frequency in the sphere as a function of the mechanical impedance can be obtained. The frequency at the minimum point of this curve is the natural frequency. The frequency in the minimum represented as the first in the curve is the first natural frequency. The frequency in the minimum point indicated as the second in the curve is the second order natural frequency. A typical vibrator 12 is a vibrator under the trade name "PET" of the International Institute of Mechanical Vibration Corporation. A typical dynamic signal analyzer is a dynamic signal analyzer under the trade name "HP-5420A" of Yokogawa Hewlett-Packard Company.

The present inventor conducted in-depth research and found that: when the second-order natural frequency NF(2)1 of the core 4 and the second-order natural frequency NF(2)2 of the golf ball 2 have a prescribed relationship, soft hitting feel.

FIG. 3 is a graph showing the relationship between the second-order natural frequency NF(2)1 of the core 4 and the second-order natural frequency NF(2)2 of the golf ball 2 . In this graph, the horizontal axis represents the second-order natural frequency NF(2)1 of the core, and the vertical axis represents the second-order natural frequency NF(2)2 of the golf ball. The straight line represented by the symbol L1 in this graph is represented by the following mathematical formula.

NF(2)2＝2/3×NF(2)1+1080

In this graph, in the region below the straight line L1, the value V calculated by the following mathematical formula is 1080 Hz or less.

V＝NF(2)2-2/3×NF(2)1

According to the findings obtained by the present inventors, the golf ball 2 having a value V of 1080 Hz or less has an excellent impact feeling on a putt. In this golf ball 2, the impact feeling at the time of putting is soft.

The straight line represented by symbol L2 in the graph of FIG. 3 is represented by the following mathematical formula.

NF(2)2＝2/3×NF(2)1+1040

In the region below the straight line L2 in this graph, the value V is 1040 Hz or less. The golf ball 2 having a value V of 1040 Hz or less has an excellent impact feeling on a putt. In this golf ball 2, the impact feeling at the time of putting is soft.

The straight line represented by symbol L3 in the graph of FIG. 3 is represented by the following mathematical formula.

NF(2)2＝2/3×NF(2)1+1000

In the region below the straight line L3 in this graph, the value V is 1000 Hz or less. The golf ball 2 having a value V of 1000 Hz or less has an excellent impact feeling on a putt. In this golf ball 2, the impact feeling at the time of putting is soft.

Preferably, the value V is 800 Hz or more from the viewpoint of the golf ball's durability and rebound performance.

Example

Hereinafter, the effects of the present invention will be clarified through examples, but the present invention should not be limitedly interpreted based on the description of the examples.

[Example 1]

With 100 mass parts of high-cis polybutadiene (JSR company's trade name "BR-730"), 22.0 mass parts of zinc acrylate, 5 mass parts of zinc oxide, an appropriate amount of barium sulfate, 0.5 mass parts of diphenyl A rubber composition A was obtained by kneading the base disulfide and 0.9 parts by mass of dicumyl peroxide. This rubber composition A was put into a mold composed of an upper mold and a lower mold equipped with a hemispherical cavity, and heated at 160° C. for 20 minutes to obtain a core with a diameter of 39.8 mm. The amount of barium sulfate is adjusted so that the quality of the golf ball is appropriate.

In a twin-screw kneading extruder, 47 parts by mass of ionomer resin ("HIMILAN1555" mentioned above), 46 parts by mass of other ionomer resin ("HIMILAN1557" mentioned above), 7 parts by mass of styrene block-containing The thermoplastic elastomer (the above "RABALON T3221C"), 4 parts by mass of titanium dioxide and 0.2 parts by mass of a light stabilizer (trade name "JF-90" of Johoku Chemical Industry Co., Ltd.) were kneaded to obtain a resin composition a. The core is placed in a final mold consisting of an upper mold and a lower mold each equipped with a hemispherical cavity. The final mold has many pimples on the cavity face. The resin composition melted by injection molding is filled around the core to form the cover. The thickness T2 of the case is 1.45 mm. In the case, a dimple having a shape in which the shape of the small protrusion is reversed is formed.

A varnish based on a two-component curing type polyurethane was applied around the cover to obtain a golf ball of Example 1 having a diameter of about 42.7 mm and a mass of about 45.6 g.

[Examples 2-11 and Comparative Examples 1-3]

The golf balls of Examples 2-11 and Comparative Examples 1-3 were obtained in the same manner as in Example 1 except that the specifications of the core and the cover were shown in Tables 4-6 below. The details of the specifications of the core are shown in Table 1-2 below. The details of the specifications of the case are shown in Table 3 below.

[flight test]

A driver (trade name "XXIO8" of Dunlop Sporting Goods Co., Ltd.; shaft hardness: R; face Inclination (LOFT): 10.5°). The golf ball was hit at a head speed of 40 m/sec, and the initial speed, spin speed, and flight distance of the golf ball were measured. Carry distance is the distance between the point of impact and the point at which the golf ball comes to rest. The average values of the values obtained in the 12 measurements are shown in Tables 4-6 below.

[feel at impact]

20 players were asked to hit a golf ball with a putter and asked about their feeling when playing the ball. According to the number of golfers who answered "feeling good at playing", the following prescribed rankings were performed.

A: 16-20 people

B: 10-15 people

C: 3-9 people

D: 0-2 people

The results are shown in Tables 3-6 below.

[Table 1]

Table 1: Composition of rubber (parts by mass)

A B C D. polybutadiene 100 100 100 100 Zinc oxide 5 5 5 5 Zinc acrylate 22.0 24.8 21.6 21.2 Barium sulfate Appropriate amount Appropriate amount Appropriate amount Appropriate amount diphenyl disulfide 0.5 0.5 0.5 0.5 dicumyl peroxide 0.9 0.9 0.9 0.9 2-Naphthalenethiol — — — — Cross-linking temperature (°C) 160 160 160 160 Cross-linking time (min) 20 20 20 20

[Table 2]

Table 2: Composition of rubber (parts by mass)

E. f G h polybutadiene 100 100 100 100 Zinc oxide 5 5 5 5 Zinc acrylate 20.8 26.0 21.0 27.0 Barium sulfate Appropriate amount Appropriate amount Appropriate amount Appropriate amount diphenyl disulfide 0.5 0.5 0.5 0.5 dicumyl peroxide 0.9 0.9 0.9 0.9 2-Naphthalenethiol — — — — Cross-linking temperature (°C) 160 160 140 160 Cross-linking time (min) 20 20 20 20

[table 3]

Table 3: Composition of the shell (parts by mass)

a b c d Himilan1555 47 — — Himilan AM7329 — 40 40 50 Himilan1557 46 — — — Himilan1605 — 52 57 47 RABALON T3221C 7 8 3 3 Titanium dioxide 4 4 4 4 JF-90 0.2 0.2 0.2 0.2 H2 (Shore D) 57 59 61 63

[Table 4]

Table 4: Evaluation Results

Example 1 Example 2 Example 3 Example 4 Example 5 nucleus composition A B C D. E. Diameter (mm) 39.8 39.8 39.8 39.8 39.8 Df1(mm) 4.85 4.15 4.95 5.05 5.15 H1o (Shore C) 58 62 57 56 55 H1s (Shore C) 72 76 71 70 69 H1s-H1o 14 14 14 14 14 NF(2)1(Hz) 1355 1631 1315 1275 1235 shell composition a a b c d T2(mm) 1.45 1.45 1.45 1.45 1.45 H2 (Shore D) 57 57 59 61 63 ball NF(2)2(Hz) 1855 2031 1865 1875 1885 Df2(mm) 4.25 3.65 4.25 4.25 4.25 V(Hz) 952 944 988 1025 1062 Feel A B A B C flight test rotation (rpm) 2500 2588 2450 2400 2350 Initial speed (m/s) 57.50 57.70 57.63 57.76 57.89 Flight distance (m) 195.9 196.0 196.9 198.0 199.0

[table 5]

Table 5: Evaluation Results

Example 6 Example 7 Example 8 Example 9 Example 10 nucleus composition A B f C G Diameter (mm) 39.5 39.5 39.5 39.5 40.2 Df1(mm) 4.85 4.15 3.85 4.95 4.75 H1o (Shore C) 58 62 64 57 60 H1s (Shore C) 72 76 78 71 68 H1s-H1o 14 14 14 14 8 NF(2)1(Hz) 1347 1623 1761 1307 1316 shell composition a a a b a T2(mm) 1.60 1.60 1.60 1.60 1.25 H2 (Shore D) 57 57 57 59 57 ball NF(2)2(Hz) 1927 2103 2191 1937 1816 Df2(mm) 4.23 3.63 3.38 4.23 4.27 V(Hz) 1029 1021 1017 1066 939 Feel A B C B A flight test rotation (rpm) 2520 2608 2683 2470 2650 Initial speed (m/s) 57.52 57.72 57.82 57.65 57.48 Flight distance (m) 195.8 195.9 195.7 196.8 194.4

[Table 6]

Table 6: Evaluation Results

Example 11 Comparative example 1 Comparative example 2 Comparative example 3 nucleus composition G D. A h Diameter (mm) 39.5 39.5 39.1 39.5 Df1(mm) 4.75 5.05 4.85 4.00 H1o (Shore C) 60 56 58 58 H1s (Shore C) 68 70 72 78 H1s-H1o 8 14 14 20 NF(2)1(Hz) 1304 1267 1507 1779 shell composition a c a a T2(mm) 1.60 1.60 1.80 1.60 H2 (Shore D) 57 61 57 57 ball NF(2)2(Hz) 1834 1947 2167 2269 Df2(mm) 4.23 4.23 4.21 3.66 V(Hz) 965 1102 1162 1083 Feel A D. D. D. flight test rotation (rpm) 2685 2420 2540 2618 Initial speed (m/s) 57.52 57.78 57.54 58.29 Flight distance (m) 194.3 197.9 195.7 198.4

As shown in Tables 4 to 6, the golf balls of each example are excellent in the impact feeling under putt shots and the flight performance under driver impacts. The superiority of the present invention is evident from the evaluation results.

Industrial availability

The golf ball according to the present invention is suitable for games on a golf course, practice on a driving range, and the like.

Claims (8)

1. A double-layer golf ball comprising a core and a shell positioned outside the core, It is characterized in that the value V calculated by the following mathematical formula is 1080 Hz or less: V＝NF(2)2－2/3×NF(2)1 In the formula, NF(2)1 represents the second-order natural frequency of the core, and NF(2)2 represents the second-order natural frequency of the golf ball. 2. The golf ball according to claim 1, wherein in the core, a difference H1s-H1o between the Shore C hardness H1s of the surface and the Shore C hardness H1o of the center is 10 or more. 3. The golf ball according to claim 1 or 2, wherein the value V is 1040 Hz or less. 4. The golf ball according to claim 3, wherein the value V is 1000 Hz or less. 5. The golf ball according to any one of claims 1 to 4, wherein the compression deformation amount Df1 of the core is 4.1 mm or more. 6. The golf ball according to any one of claims 1 to 5, wherein the amount of compressive deformation Df2 is 3.5 mm or more. 7. The golf ball according to any one of claims 1 to 6, wherein the thickness T2 of the cover is 0.80 mm to 2.00 mm. 8 . The golf ball according to claim 1 , wherein the cover has a Shore D hardness H2 of 50 or more and 65 or less.