US20110306443A1 - Golf ball - Google Patents

Golf ball Download PDF

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Publication number
US20110306443A1
US20110306443A1 US13/089,403 US201113089403A US2011306443A1 US 20110306443 A1 US20110306443 A1 US 20110306443A1 US 201113089403 A US201113089403 A US 201113089403A US 2011306443 A1 US2011306443 A1 US 2011306443A1
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US
United States
Prior art keywords
hardness
equal
golf ball
less
jis
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Abandoned
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US13/089,403
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English (en)
Inventor
Yoshiko Matsuyama
Keiji Ohama
Hirotaka Nakamura
Takahiro Sajima
Toshiyuki Tarao
Kazuyoshi Shiga
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Dunlop Sports Co Ltd
Original Assignee
SRI Sports Ltd
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Publication date
Application filed by SRI Sports Ltd filed Critical SRI Sports Ltd
Assigned to SRI SPORTS LIMITED reassignment SRI SPORTS LIMITED ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: MATSUYAMA, YOSHIKO, NAKAMURA, HIROTAKA, OHAMA, KEIJI, SAJIMA, TAKAHIRO, SHIGA, KAZUYOSHI, TARAO, TOSHIYUKI
Publication of US20110306443A1 publication Critical patent/US20110306443A1/en
Abandoned legal-status Critical Current

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    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63B—APPARATUS FOR PHYSICAL TRAINING, GYMNASTICS, SWIMMING, CLIMBING, OR FENCING; BALL GAMES; TRAINING EQUIPMENT
    • A63B37/00—Solid balls; Rigid hollow balls; Marbles
    • A63B37/0003—Golf balls
    • A63B37/005—Cores
    • A63B37/006—Physical properties
    • A63B37/0062—Hardness
    • A63B37/0063—Hardness gradient
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63B—APPARATUS FOR PHYSICAL TRAINING, GYMNASTICS, SWIMMING, CLIMBING, OR FENCING; BALL GAMES; TRAINING EQUIPMENT
    • A63B37/00—Solid balls; Rigid hollow balls; Marbles
    • A63B37/0003—Golf balls
    • A63B37/005—Cores
    • A63B37/006—Physical properties
    • A63B37/0062—Hardness
    • A63B37/00621—Centre hardness
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63B—APPARATUS FOR PHYSICAL TRAINING, GYMNASTICS, SWIMMING, CLIMBING, OR FENCING; BALL GAMES; TRAINING EQUIPMENT
    • A63B37/00—Solid balls; Rigid hollow balls; Marbles
    • A63B37/0003—Golf balls
    • A63B37/005—Cores
    • A63B37/006—Physical properties
    • A63B37/0062—Hardness
    • A63B37/00622—Surface hardness
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63B—APPARATUS FOR PHYSICAL TRAINING, GYMNASTICS, SWIMMING, CLIMBING, OR FENCING; BALL GAMES; TRAINING EQUIPMENT
    • A63B37/00—Solid balls; Rigid hollow balls; Marbles
    • A63B37/0003—Golf balls
    • A63B37/0023—Covers
    • A63B37/0029—Physical properties
    • A63B37/0031—Hardness
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63B—APPARATUS FOR PHYSICAL TRAINING, GYMNASTICS, SWIMMING, CLIMBING, OR FENCING; BALL GAMES; TRAINING EQUIPMENT
    • A63B37/00—Solid balls; Rigid hollow balls; Marbles
    • A63B37/0003—Golf balls
    • A63B37/0023—Covers
    • A63B37/0029—Physical properties
    • A63B37/0033—Thickness
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63B—APPARATUS FOR PHYSICAL TRAINING, GYMNASTICS, SWIMMING, CLIMBING, OR FENCING; BALL GAMES; TRAINING EQUIPMENT
    • A63B37/00—Solid balls; Rigid hollow balls; Marbles
    • A63B37/0003—Golf balls
    • A63B37/0038—Intermediate layers, e.g. inner cover, outer core, mantle
    • A63B37/004—Physical properties
    • A63B37/0043—Hardness
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63B—APPARATUS FOR PHYSICAL TRAINING, GYMNASTICS, SWIMMING, CLIMBING, OR FENCING; BALL GAMES; TRAINING EQUIPMENT
    • A63B37/00—Solid balls; Rigid hollow balls; Marbles
    • A63B37/0003—Golf balls
    • A63B37/0038—Intermediate layers, e.g. inner cover, outer core, mantle
    • A63B37/004—Physical properties
    • A63B37/0045—Thickness
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63B—APPARATUS FOR PHYSICAL TRAINING, GYMNASTICS, SWIMMING, CLIMBING, OR FENCING; BALL GAMES; TRAINING EQUIPMENT
    • A63B37/00—Solid balls; Rigid hollow balls; Marbles
    • A63B37/0003—Golf balls
    • A63B37/005—Cores
    • A63B37/006—Physical properties
    • A63B37/0064—Diameter

Definitions

  • the present invention relates to golf balls. Specifically, the present invention relates to multi-piece golf balls including a center, an envelope layer, a mid layer and a cover.
  • Flight performance Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver, a long iron and a middle iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball with excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.
  • An appropriate trajectory height is required in order to achieve a large flight distance.
  • a trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained.
  • Use of a core having an outer-hard/inner-soft structure can achieve a low spin rate and a high launch angle.
  • golf players also place importance on spin performance of golf balls.
  • a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point.
  • a sidespin rate is high, the golf ball tends to curve. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve.
  • a golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.
  • U.S. Pat. No. 6,468,169 JP10-328326 discloses a golf ball including a core, an envelope layer, an inner cover and an outer cover.
  • U.S. Pat. No. 6,271,296 JP2001-17575 discloses a golf ball including a core, an envelope layer, a mid layer and a cover.
  • U.S. Pat. No. 6,913,547 JP2002-272880 discloses a golf ball including a core and a cover.
  • the core consists of a center and an outer core layer.
  • the cover consists of an inner cover layer and an outer cover layer.
  • US2003/130066 discloses a golf ball including a core and a cover.
  • the core consists of a center and a mid layer.
  • US2003/166422 discloses a golf ball including a center, a mid layer and a cover.
  • US2004/029648 discloses a golf ball including a core and a cover.
  • the core has a three-layer structure.
  • An object of the present invention is to provide a golf ball that provides a large flight distance when being hit with a driver and that has excellent controllability when being hit with a short iron.
  • a golf ball according to the present invention comprises a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer.
  • the core comprises a center and an envelope layer positioned outside the center.
  • a ratio of a volume of the core to a volume of a phantom sphere of the golf ball is equal to or greater than 76%.
  • a JIS-C hardness Hc of the cover is less than JIS-C hardness Ho at a central point of the core.
  • Ha 1 indicates a JIS-C hardness at a point Pa1 that is located radially inward of each point Pa at a distance of 1 mm from the point Pa
  • Ha 2 indicates a JIS-C hardness at a point Pa 2 that is located radially outward of the point Pa at a distance of 1 mm from the point Pa.
  • Hb 1 indicates a JIS-C hardness at a point Pb 1 that is located radially inward of the point Pb at a distance of 1 mm from the point Pb
  • Hb 2 indicates a JIS-C hardness at a point Pb 2 that is located radially outward of the point Pb at a distance of 1 mm from the point Pb.
  • a hardness distribution of the core is appropriate.
  • the core has a low energy loss when being hit with a driver.
  • a large flight distance is obtained.
  • the golf ball has excellent controllability when being hit with a short iron.
  • the JIS-C hardness Hc of the cover is equal to or less than 65.
  • a thickness of the cover is equal to or less than 0.8 mm.
  • a JIS-C hardness Hm of the mid layer is equal to or greater than 90.
  • a thickness of the mid layer is equal to or less than 1.5 mm.
  • the cover is formed from a resin composition.
  • a principal component of a base material of the resin composition is a thermoplastic polyurethane.
  • a shear loss elastic modulus G′′ of the resin composition which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., is equal to or less than 1.95 ⁇ 10 7 Pa, and a ratio (E′′/G′′) of a tensile loss elastic modulus E′′ of the resin composition, which is measured under the same conditions, to the shear loss elastic modulus G′′ is equal to or greater than 1.76.
  • the tensile loss elastic modulus E′′ is equal to or greater 2.00 ⁇ 10 7 Pa.
  • a polyol component of the thermoplastic polyurethane is polytetramethylene ether glycol having a number average molecular weight of 1500 or less.
  • a difference between a JIS-C hardness He at a surface of the core and the hardness Hb 2 is equal to or greater than 10.
  • a difference between a JIS-C hardness He at a surface of the core and the hardness Ho is equal to or less than 40.
  • the hardness Ho is equal to or greater than 40 but equal to or less than 80.
  • a JIS-C hardness He at a surface of the core is equal to or greater than 75 but equal to or less than 95.
  • a difference between a JIS-C hardness He at a surface of the core and a JIS-C hardness Hi at an innermost portion of the envelope layer is equal to or greater than 10 but equal to or less than 25.
  • a thickness of the envelope layer is equal to or greater than 8 mm but equal to or less than 18 mm.
  • a diameter of the center is equal to or greater than 10 mm but equal to or less than 20 mm.
  • a JIS-C hardness He at a surface of the core is greater than a JIS-C hardness at a surface of the center, and the hardness Hm of the mid layer is greater than the hardness He.
  • a difference between the hardness Ho and the hardness Hc is equal to or greater than 3.
  • FIG. 1 is a partially cutaway cross-sectional view of a golf ball according to one embodiment of the present invention
  • FIG. 2 is a graph showing a hardness distribution of each core of golf balls according to Examples 1 and 4 to 6 of the present invention
  • FIG. 3 is a graph showing a hardness distribution of a core of a golf ball according to Example 2 of the present invention.
  • FIG. 4 is a graph showing a hardness distribution of a core of a golf ball according to Example 3 of the present invention.
  • FIG. 5 is a graph showing a hardness distribution of a core of a golf ball according to Example 7 of the present invention.
  • FIG. 6 is a graph showing a hardness distribution of a core of a golf ball according to Example 8 of the present invention.
  • FIG. 7 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 1;
  • FIG. 8 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 2;
  • FIG. 9 is a graph showing a hardness distribution of each core of golf balls according to Comparative Examples 3 and 4;
  • FIG. 10 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 5.
  • a golf ball 2 shown in FIG. 1 includes a spherical core 4 , a mid layer 6 positioned outside the core 4 , and a cover 8 positioned outside the mid layer 6 .
  • the core 4 includes a spherical center 10 and an envelope layer 12 positioned outside the center 10 .
  • On the surface of the cover 8 a large number of dimples 14 are formed.
  • a part other than the dimples 14 is a land 16 .
  • the golf ball 2 includes a paint layer and a mark layer on the external side of the cover 8 although these layers are not shown in the drawing.
  • the golf ball 2 has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is preferably equal to or less than 44 mm and more preferably equal to or less than 42.80 mm.
  • the golf ball 2 has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is preferably equal to or greater than 44 g and more preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is preferably equal to or less than 45.93 g.
  • the center 10 is obtained by crosslinking a rubber composition.
  • base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers and natural rubbers. In light of resilience performance, polybutadienes are preferred.
  • the polybutadiene is included as a principal component.
  • the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight.
  • the proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% and more preferably equal to or greater than 80%.
  • the rubber composition of the center 10 includes a co-crosslinking agent.
  • the co-crosslinking agent achieves high resilience of the center 10 .
  • Examples of preferable co-crosslinking agents in light of resilience performance include monovalent or bivalent metal salts of an ⁇ , ⁇ -unsaturated carboxylic acid having 2 to 8 carbon atoms.
  • Specific examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate and magnesium methacrylate. In light of resilience performance, zinc acrylate and zinc methacrylate are particularly preferred.
  • the amount of the co-crosslinking agent is preferably equal to or greater than 5 parts by weight, and more preferably equal to or greater than 10 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the co-crosslinking agent is preferably equal to or less than 30 parts by weight, more preferably equal to or less than 25 parts by weight, and particularly preferably equal to or less than 20 parts by weight, per 100 parts by weight of the base rubber.
  • the rubber composition of the center 10 includes an organic peroxide together with a co-crosslinking agent.
  • the organic peroxide serves as a crosslinking initiator.
  • the organic peroxide contributes to the resilience performance of the golf ball 2 .
  • suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.
  • the amount of the organic peroxide is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.2 parts by weight, and particularly preferably equal to or greater than 0.3 parts by weight, per 100 parts by weight of the base rubber.
  • the amount of the organic peroxide is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.
  • the rubber composition of the center 10 includes an organic sulfur compound.
  • organic sulfur compounds include monosubstitutions such as 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 and the like; disubstitutions such as bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)d
  • the amount of the organic sulfur compound is preferably equal to or greater than 0.1 parts by weight and more preferably equal to or greater than 0.2 parts by weight, per 100 parts by weight of the base rubber.
  • the amount of the organic sulfur compound is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.
  • a filler may be included in the center 10 .
  • suitable fillers include zinc oxide, barium sulfate, calcium carbonate and magnesium carbonate.
  • the amount of the filler is determined as appropriate so that the intended specific gravity of the center 10 is accomplished.
  • a particularly preferable filler is zinc oxide.
  • Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.
  • an anti-aging agent a coloring agent, a plasticizer, a dispersant, sulfur, an vulcanization accelerator and the like are added to the rubber composition of the center 10 .
  • Crosslinked rubber powder or synthetic resin powder may be also dispersed in the rubber composition.
  • the center 10 has a central hardness Ho of preferably 40 or greater, more preferably 45 or greater, and particularly preferably 50 or greater.
  • the central hardness Ho is preferably equal to or less than 80, more preferably equal to or less than 75, and particularly preferably equal to or less than 70.
  • the central hardness Ho is measured by pressing a JIS-C type hardness scale against the central point of a cut plane of the center 10 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.
  • the hardness of the center 10 gradually increases from its central point toward its surface.
  • the center 10 has a surface hardness greater than the central hardness Ho.
  • the center 10 has a diameter of preferably 10 mm or greater but 20 mm or less.
  • the center 10 having a diameter of 10 mm or greater can achieve excellent feel at impact.
  • the diameter is more preferably equal to or greater than 12 mm and particularly preferably equal to or greater than 13 mm.
  • the envelope layer 12 having a sufficiently large thickness can be formed.
  • the diameter is more preferably equal to or less than 18 mm and particularly preferably equal to or less than 17 mm.
  • the envelope layer 12 is obtained by crosslinking a rubber composition.
  • base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers and natural rubbers. In light of resilience performance, polybutadienes are preferred.
  • polybutadiene and another rubber are used in combination, it is preferred that the polybutadiene is included as a principal component.
  • the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight.
  • the proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% and more preferably equal to or greater than 80%.
  • a co-crosslinking agent is preferably used.
  • preferable co-crosslinking agents in light of resilience performance include monovalent or bivalent metal salts of an ⁇ , ⁇ -unsaturated carboxylic acid having 2 to 8 carbon atoms.
  • preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate and magnesium methacrylate. In light of resilience performance, zinc acrylate and zinc methacrylate are particularly preferred.
  • the amount of the co-crosslinking agent is preferably equal to or greater than 20 parts by weight, more preferably equal to or greater than 25 parts by weight, and particularly preferably equal to or greater than 30 parts by weight, per 100 parts by weight of the base rubber.
  • the amount of the co-crosslinking agent is preferably equal to or less than 60 parts by weight, more preferably equal to or less than 50 parts by weight, and particularly preferably equal to or less than 45 parts by weight, per 100 parts by weight of the base rubber.
  • the rubber composition of the envelope layer 12 includes an organic peroxide together with a co-crosslinking agent.
  • the organic peroxide serves as a crosslinking initiator.
  • the organic peroxide contributes to the resilience performance of the golf ball 2 .
  • suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.
  • the amount of the organic peroxide is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.3 parts by weight, and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber.
  • the amount of the organic peroxide is preferably equal to or less than 2.0 parts by weight, more preferably equal to or less than 1.5 parts by weight, and particularly preferably equal to or less than 1.0 parts by weight, per 100 parts by weight of the base rubber.
  • the rubber composition of the envelope layer 12 includes an organic sulfur compound.
  • the organic sulfur compounds described above for the center 10 can be used for the envelope layer 12 .
  • the amount of the organic sulfur compound is preferably equal to or greater than 0.1 parts by weight and more preferably equal to or greater than 0.2 parts by weight, per 100 parts by weight of the base rubber.
  • the amount of the organic sulfur compound is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.
  • a filler may be included in the envelope layer 12 .
  • suitable fillers include zinc oxide, barium sulfate, calcium carbonate and magnesium carbonate.
  • Powder of a metal with a high specific gravity may be included as a filler.
  • Specific examples of metals with a high specific gravity include tungsten and molybdenum.
  • the amount of the filler is determined as appropriate so that the intended specific gravity of the envelope layer 12 is accomplished.
  • a particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.
  • various additives such as sulfur, an anti-aging agent, a coloring agent, a plasticizer, a dispersant and the like are included in the envelope layer 12 in an adequate amount.
  • Crosslinked rubber powder or synthetic resin powder may be also included in the envelope layer 12 .
  • the center 10 is covered with two uncrosslinked or semi-crosslinked half shells. These half shells are compressed and heated. By this heating, a crosslinking reaction takes place to complete the envelope layer 12 .
  • the crosslinking temperature is generally equal to or higher than 140° C. but equal to or lower than 180° C.
  • the time period for crosslinking the envelope layer 12 is generally equal to or longer than 10 minutes but equal to or shorter than 60 minutes.
  • the hardness of the envelope layer 12 gradually increases from its innermost portion to its surface.
  • a hardness He at the surface of the envelope layer 12 (namely, the surface of the core 4 ) is preferably equal to or greater than 75, more preferably equal to or greater than 80, and particularly preferably equal to or greater than 85.
  • the hardness He is preferably equal to or less than 95, more preferably equal to or less than 93, and particularly preferably equal to or less than 92.
  • the hardness He is measured by pressing a JIS-C type hardness scale against the surface of the core 4 .
  • an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.
  • the difference (He ⁇ Hi) between the surface hardness He of the envelope layer 12 and a hardness Hi at the innermost portion of the envelope layer 12 is preferably equal to or greater than 10, more preferably equal to or greater than 12, and particularly preferably equal to or greater than 15. In light of ease of production and durability, the difference (He ⁇ Hi) is preferably equal to or less than 25.
  • the hardness Hi is measured for a hemisphere obtained by cutting the core 4 .
  • the hardness Hi is measured by pressing a JIS-C type hardness scale against the cut plane of the hemisphere.
  • the hardness scale is pressed against a region surrounded by a first circle and a second circle.
  • the first circle is the boundary between the center 10 and the envelope layer 12 .
  • the second circle is concentric with the first circle and has a radius larger than the radius of the first circle by 1 mm.
  • an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.
  • the envelope layer 12 has a thickness of preferably 8 mm or greater but 18 mm or less.
  • the envelope layer 12 having a thickness of 8 mm or greater can suppress spin.
  • the thickness is more preferably equal to or greater than 9 mm and particularly preferably equal to or greater than 10 mm.
  • the center 10 having a large diameter can be formed.
  • the center 10 having a large diameter can suppress spin.
  • the thickness is more preferably equal to or less than 16 mm and particularly preferably equal to or less than 15 mm.
  • the difference (He ⁇ Ho) between the surface hardness He of the core 4 and the central hardness Ho of the center 10 is preferably equal to or greater than 20 and particularly preferably equal to or greater than 25.
  • the difference (He ⁇ Ho) is preferably equal to or less than 40 and particularly preferably equal to or less than 35.
  • zone A a zone that extends over a distance range from equal to or greater than 1 mm to less than 5 mm from the central point of the core 4
  • zone B a zone that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core 4
  • Ha1 indicates the JIS-C hardness at a point Pa 1 .
  • the point Pa 1 is located radially inward of each point Pa.
  • the distance from the point Pa to the point Pa 1 is 1 mm.
  • Ha 2 indicates the JIS-C hardness at a point Pa 2 .
  • the point Pa 2 is located radially outward of each point Pa.
  • the distance from the point Pa to the point Pa 2 is 1 mm.
  • the hardnesses Ha 1 and Ha 2 are measured by pressing a JIS-C type hardness scale against a cut plane of the center 10 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.
  • the core 4 that satisfies the mathematical formula (I) has a low energy loss when being hit with a golf club.
  • the core 4 can achieve high resilience of the golf ball 2 .
  • the golf ball 2 having the core 4 has excellent flight performance.
  • the difference (Ha 2 ⁇ Ha 1 ) is preferably equal to or less than 4 and particularly preferably equal to or less than 3.
  • the difference (Ha 2 ⁇ Ha 1 ) may be zero.
  • Hb 1 indicates the JIS-C hardness at a point Pb 1 .
  • the point Pb 1 is located radially inward of the point Pb.
  • the distance from the point Pb to the point Pb 1 is 1 mm.
  • Hb 2 indicates the JIS-C hardness at a point Pb 2 .
  • the point Pb 2 is located radially outward of the point Pb.
  • the distance from the point Pb to the point Pb 2 is 1 mm.
  • the hardnesses Hb 1 and Hb 2 are measured by pressing a JIS-C type hardness scale against a cut plane of the center 10 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.
  • the core 4 that satisfies the mathematical formula (II) suppresses spin of the golf ball 2 .
  • the difference (Hb 2 ⁇ Hb 1 ) is particularly preferably equal to or greater than 7.
  • the difference (Hb 2 ⁇ Hb 1 ) is preferably equal to or less than 20 and particularly preferably equal to or less than 15.
  • the ratio of the volume of the core 4 to the volume of a phantom sphere of the golf ball 2 is equal to or greater than 76%. In other words, the core 4 is large.
  • the core 4 can achieve excellent resilience performance of the golf ball 2 .
  • the core 4 can suppress spin of the golf ball 2 . In this respect, the ratio is more preferably equal to or greater than 78% and particularly preferably equal to or greater than 80%.
  • the surface of the phantom sphere is the surface of the golf ball 2 when it is postulated that no dimple 14 exists.
  • the difference (He ⁇ Hb 2 ) between the surface hardness He of the core 4 and the hardness Hb2 is preferably equal to or greater than 10, more preferably equal to or greater than 12, and particularly preferably equal to or greater than 15.
  • a resin composition is suitably used for the mid layer 6 .
  • the base polymer of the resin composition include ionomer resins, styrene block-containing thermoplastic elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers and thermoplastic polyolefin elastomers.
  • Particularly preferable base polymers are ionomer resins.
  • Ionomer resins are highly elastic. As described later, the cover 8 of the golf ball 2 is thin and flexible. Thus, when the golf ball 2 is hit with a driver, the mid layer 6 significantly deforms.
  • the mid layer 6 including an ionomer resin contributes to resilience performance upon a shot with a driver.
  • An ionomer resin and another resin may be used in combination.
  • the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.
  • preferable ionomer resins include binary copolymers formed with an ⁇ -olefin and an ⁇ , ⁇ -unsaturated carboxylic acid having 3 to 8 carbon atoms.
  • a preferable binary copolymer includes 80% by weight or more but 90% by weight or less of an ⁇ -olefin, and 10% by weight or more but 20% by weight or less of an ⁇ , ⁇ -unsaturated carboxylic acid.
  • the binary copolymer has excellent resilience performance.
  • Examples of other preferable ionomer resins include ternary copolymers formed with: an ⁇ -olefin; an ⁇ , ⁇ -unsaturated carboxylic acid having 3 to 8 carbon atoms; and an ⁇ , ⁇ -unsaturated carboxylate ester having 2 to 22 carbon atoms.
  • a preferable ternary copolymer includes 70% by weight or more but 85% by weight or less of an ⁇ -olefin, 5% by weight or more but 30% by weight or less of an ⁇ , ⁇ -unsaturated carboxylic acid, and 1% by weight or more but 25% by weight or less of an ⁇ , ⁇ -unsaturated carboxylate ester.
  • the ternary copolymer has excellent resilience performance.
  • ⁇ -olefins are ethylene and propylene, while preferable ⁇ , ⁇ -unsaturated carboxylic acids are acrylic acid and methacrylic acid.
  • a particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid or methacrylic acid.
  • some of the carboxyl groups are neutralized with metal ions.
  • metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion and neodymium ion.
  • the neutralization may be carried out with two or more types of metal ions.
  • Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion and magnesium ion.
  • ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan MK7320” and “Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF
  • IOTEK 7010 du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000” and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation.
  • Two or more ionomer resins may be used in combination for the mid layer 6 .
  • An ionomer resin neutralized with a monovalent metal ion, and an ionomer resin neutralized with a bivalent metal ion may be used in combination.
  • the mid layer 6 may include a highly elastic resin.
  • highly elastic resins include polybutylene terephthalate, polyphenylene ether, polyethylene terephthalate, polysulfone, polyethersulfone, polyphenylene sulfide, polyarylate, polyamide imide, polyetherimide, polyether ether ketone, polyimide, polytetrafluoroethylene, polyamino bismaleimide, polybisamide triazole, polyphenylene oxide, polyacetal, polycarbonate, acrylonitrile-butadiene-styrene copolymers and acrylonitrile-styrene copolymers.
  • a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener and the like are included in the mid layer 6 in an adequate amount.
  • a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener and the like are included in the mid layer 6 in an adequate amount.
  • known methods such as injection molding, compression molding and the like can be used.
  • the mid layer 6 has a hardness Hm of preferably 90 or greater.
  • the mid layer 6 having a hardness Hm of 90 or greater achieves excellent resilience performance of the golf ball 2 .
  • the mid layer 6 having a hardness Hm of 90 or greater can achieve an outer-hard/inner-soft structure of the sphere consisting of the core 4 and the mid layer 6 .
  • the sphere having the outer-hard/inner-soft structure suppress spin of the golf ball 2 .
  • the hardness Hm is particularly preferably equal to or greater than 92.
  • the hardness Hm is equal to or less than 98 and particularly preferably equal to or less than 97.
  • the hardness Hm of the mid layer 6 is greater than the surface hardness He of the core 4
  • the surface hardness He of the core 4 is greater than the surface hardness of the center 10 .
  • the hardness Hm is measured with an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.) to which a JIS-C type spring hardness scale is mounted.
  • P1 automated rubber hardness measurement machine
  • a slab formed by hot press and having a thickness of about 2 mm is used.
  • a slab kept at 23° C. for two weeks is used for the measurement.
  • three slabs are stacked.
  • a slab formed from a resin composition that is the same as the resin composition of the mid layer 6 is used for the measurement.
  • the mid layer 6 has a thickness of preferably 0.3 mm or greater, more preferably 0.5 mm or greater, and particularly preferably 0.6 mm or greater. In light of feel at impact, the thickness is preferably equal to or less than 1.5 mm, more preferably equal to or less than 1.2 mm, and particularly preferably equal to or less than 1.0 mm.
  • the cover 8 is formed from a resin composition.
  • the base polymer of the resin composition include polyurethanes, polyesters, polyamides, polyolefins, polystyrenes and ionomer resins. Particularly, polyurethanes are preferred. Polyurethanes are flexible. When the golf ball 2 with the cover 8 including a polyurethane is hit with a short iron, the spin rate is high.
  • the cover 8 formed from a polyurethane contributes to the controllability upon a shot with a short iron.
  • the polyurethane also contributes to the scuff resistance of the cover 8 .
  • the golf ball 2 When the golf ball 2 is hit with a driver, a long iron or a middle iron, the sphere consisting of the core 4 and the mid layer 6 becomes significantly distorted since the head speed is high. Since this sphere has an outer-hard/inner-soft structure, the spin rate is suppressed. The suppression of the spin rate achieves a large flight distance. When the golf ball 2 is hit with a short iron, this sphere becomes less distorted since the head speed is low. When the golf ball 2 is hit with a short iron, the behavior of the golf ball 2 mainly depends on the cover 8 . Since the cover 8 including the polyurethane is flexible, a high spin rate is obtained. The high spin rate achieves excellent controllability. In the golf ball 2 , both desired flight performance upon shots with a driver, a long iron, and a middle iron and desired controllability upon a shot with a short iron are achieved.
  • the cover 8 including the polyurethane absorbs the shock. This absorption achieves soft feel at impact. Particularly, when the golf ball 2 is hit with a short iron or a putter, the cover 8 achieves excellent feel at impact.
  • the cover 8 has a shear loss elastic modulus G′′ of preferably 1.95 ⁇ 10 7 Pa or less.
  • the spin rate obtained when being hit with a short iron correlates with the shear loss elastic modulus G′′.
  • the shear loss elastic modulus G′′ is particularly equal to or less than 1.83 ⁇ 10 7 Pa.
  • the shear loss elastic modulus G′′ is preferably equal to or greater than 1.00 ⁇ 10 6 Pa and particularly equal to or greater than 1.10 ⁇ 10 6 Pa.
  • the ratio (E′′/ G′′) of a tensile loss elastic modulus E′′ of the cover 8 to the shear loss elastic modulus G′′ is preferably equal to or greater than 1.76.
  • the spin rate obtained when being hit with a driver correlates with the tensile loss elastic modulus E′′.
  • the spin rate is low, and when the golf ball 2 is hit with a short iron, the spin rate is high.
  • the ratio (E′′/ G′′) is more preferably equal to or greater than 1.86 and particularly preferably equal to or greater than 1.90.
  • the ratio (E′′/ G′′) is preferably equal to or less than 6.0 and particularly preferably equal to or less than 5.5.
  • the tensile loss elastic modulus E′′ is preferably equal to or greater than 2.00 ⁇ 10 7 Pa, more preferably equal to or greater than 2.20 ⁇ 10 7 Pa, and particularly preferably equal to or greater than 2.40 ⁇ 10 7 Pa.
  • the tensile loss elastic modulus E′′ is preferably equal to or less than 1.00 ⁇ 10 8 Pa.
  • the shear loss elastic modulus G′′ and the tensile loss elastic modulus E′′ can be controlled by adjusting the molecular weight of a polyol, the molecular weight of a polyisocyanate, a ratio (NCO/OH), and the like.
  • a sheet having a thickness of 2 mm is obtained by press molding from a resin composition that is the same as the resin composition of the cover 8 .
  • a test piece having a width of 10 mm and an inter-clamp distance of 10 mm is punched out from the sheet.
  • the shear loss elastic modulus G′′ is measured for the test piece.
  • the measurement conditions are as follows.
  • Vibration frequency 10 Hz
  • a sheet having a thickness of 2 mm is obtained by press molding from a resin composition that is the same as the resin composition of the cover 8 .
  • a test piece having a width of 4 mm and an inter-clamp distance of 20 mm is punched out from the sheet.
  • the tensile loss elastic modulus E′′ is measured for the test piece.
  • the measurement conditions are as follows.
  • Vibration frequency 10 Hz
  • a time for which a golf ball and a club contact each other is several hundred microseconds.
  • the frequency of deformation of the golf ball 2 when being hit is several thousand Hz.
  • the golf ball 2 is hit at substantially normal temperature (25° C.).
  • a deformation having a frequency of several thousand Hz in the environment having a temperature of 25° C. corresponds to a deformation having a frequency of 10 Hz in the environment having a temperature of 0° C.
  • the shear loss elastic modulus G′′ and the tensile loss elastic modulus E′′ are measured under the conditions of a vibration frequency of 10 Hz and a temperature of 0° C.
  • a polyurethane and another resin may be used in combination for the cover 8 .
  • the polyurethane in this case, in light of spin performance and feel at impact, the polyurethane is included as the principal component of the base polymer.
  • the proportion of the polyurethane to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.
  • thermoplastic polyurethanes and thermosetting polyurethanes can be used. In light of productivity, thermoplastic polyurethanes are preferred.
  • a thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment.
  • the polyurethane includes a polyol component.
  • a polymeric polyol is preferred.
  • polymeric polyols include polyetherpolyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG) and polytetramethylene ether glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA) and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly- ⁇ -caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. Two or more polyols may be used in combination.
  • polytetramethylene ether glycol is preferred.
  • the spin rate obtained when the golf ball 2 is hit with a short iron has a high correlation with the content of polytetramethylene ether glycol. Meanwhile, the spin rate obtained when the golf ball 2 is hit with a driver has a low correlation with the content of polytetramethylene ether glycol.
  • the golf ball 2 including a polyurethane that includes polytetramethylene ether glycol in an appropriate amount has both excellent flight performance when being hit with a driver and excellent controllability when being hit with a short iron.
  • the polyol has a number average molecular weight of preferably 200 or greater, more preferably 400 or greater, and particularly preferably 650 or greater.
  • the molecular weight is preferably equal to or less than 1500, more preferably equal to or less than 1200, and particularly preferably equal to or less than 850.
  • the number average molecular weight is measured by gel permeation chromatography.
  • the measurement conditions are as follows.
  • HLC-8120GPC manufactured by Tosoh Corporation
  • the polymeric polyol component has a hydroxyl value of preferably 94 mg KOH/g or greater and particularly preferably 112 mg KOH/g or greater.
  • the hydroxyl value is preferably equal to or less than 561 mg KOH/g and particularly preferably equal to or less than 173 mg KOH/g.
  • an isocyanate component in the polyurethane examples include aromatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, a mixture (TDI) of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylene diisocyanate (TMXDI) and paraphenylene diisocyanate (PPDI); and alicyclic polyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate (H 12 MDI), hydrogenated xylylene diisocyanate (H 6 XDI)
  • the polyurethane may include a chain extender as its component.
  • chain extenders include low-molecular-weight polyols and low-molecular-weight polyamines.
  • low-molecular-weight polyols examples include diols, triols, tetraols and hexaols.
  • diols include ethylene glycol, diethylene glycol, propanediol, dipropylene glycol, butanediol, neopentyl glycol, pentanediol, hexanediol, heptanediol and octanediol.
  • triols examples include glycerin, trimethylolpropane and hexanetriol.
  • tetraols include pentaerythritol and sorbitol. 1,4-butanediol is preferred.
  • low-molecular-weight polyamines examples include aliphatic polyamines, monocyclic aromatic polyamines and polycyclic aromatic polyamines.
  • aliphatic polyamines include ethylenediamine, propylenediamine, butylenediamine and hexamethylenediamine.
  • monocyclic aromatic polyamines include phenylenediamine, toluene diamine, dimethyl toluene diamine, dimethylthio toluene diamine and xylylenediamine.
  • the chain extender has a number average molecular weight of preferably 30 or greater, more preferably 40 or greater, and particularly 45 or greater.
  • the molecular weight is preferably equal to or less than 400, more preferably equal to or less than 350, and particularly preferably equal to or less than 200.
  • Low-molecular-weight polyols and low-molecular-weight polyamines that are used as chain extenders are low-molecular-weight compounds that almost do not have a molecular weight distribution. Thus, the low-molecular-weight polyols and the low-molecular-weight polyamines can be distinguished from the polymeric polyol.
  • the cover 8 may be formed from a composition including a thermoplastic polyurethane and an isocyanate compound. During or after forming the cover 8 , the polyurethane is crosslinked with the isocyanate compound.
  • a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener and the like are included in the cover 8 in an adequate amount.
  • the cover 8 has a JIS-C hardness Hc of 65 or less.
  • the hardness Hc is more preferably equal to or less than 60, even more preferably equal to or less than 55, and particularly preferably equal to or less than 50. If the hardness Hc is excessively low, the flight performance upon a shot with a driver is insufficient.
  • the hardness Hc is preferably equal to or greater than 20, more preferably equal to or greater than 25, and particularly preferably equal to or greater than 35.
  • a slab formed from a resin composition that is the same as the resin composition of the cover 8 is used. The measurement method is the same as the measurement method for the hardness Hm of the mid layer 6 .
  • the hardness Hc of the cover 8 is less than the central hardness Ho of the core 4 .
  • the golf ball 2 has excellent controllability upon a shot with a short iron.
  • the difference (Ho ⁇ Hc) is preferably equal to or greater than 3, more preferably equal to or greater than 5, and particularly preferably equal to or greater than 8.
  • the difference (Ho ⁇ Hc) is preferably equal to or less than 15.
  • the cover 8 In light of flight performance upon a shot with a driver, the cover 8 has a thickness of preferably 0.8 mm or less, more preferably 0.6 mm or less, even more preferably 0.5 mm or less, and particularly preferably 0.4 mm or less. In light of controllability upon a shot with a short iron, the thickness is preferably equal to or greater than 0.10 mm and particularly preferably equal to or greater than 0.15 mm.
  • the cover 8 For forming the cover 8 , known methods such as injection molding, compression molding and the like can be used. When forming the cover 8 , the dimples 14 are formed by pimples formed on the cavity face of a mold.
  • the golf ball 2 has an amount of compressive deformation of preferably 2.0 mm or greater, more preferably 2.1 mm or greater, and particularly preferably 2.2 mm or greater.
  • the amount of compressive deformation is preferably equal to or less than 3.5 mm, more preferably equal to or less than 3.0 mm, and particularly preferably equal to or less than 2.6 mm.
  • the golf ball 2 is placed on a hard plate made of metal.
  • a cylinder made of metal gradually descends toward the golf ball 2 .
  • the golf ball 2 squeezed between the bottom face of the cylinder and the hard plate, becomes deformed.
  • a migration distance of the cylinder starting from the state in which an initial load of 98 N is applied to the golf ball 2 up to the state in which a final load of 1274 N is applied thereto, is measured.
  • the golf ball 2 may include a reinforcing layer between the mid layer 6 and the cover 8 .
  • the reinforcing layer firmly adheres to the mid layer 6 and also to the cover 8 .
  • the reinforcing layer suppresses separation of the cover 8 from the mid layer 6 .
  • the cover 8 of the golf ball 2 is thin. When the golf ball 2 is hit by the edge of a clubface, a wrinkle is likely to occur.
  • the reinforcing layer suppresses occurrence of a wrinkle.
  • thermosetting resin As the base polymer of the reinforcing layer, a two-component curing type thermosetting resin is suitably used.
  • two-component curing type thermosetting resins include epoxy resins, urethane resins, acrylic resins, polyester resins, and cellulose resins. In light of strength and durability of the reinforcing layer, two-component curing type epoxy resins and two-component curing type urethane resins are preferred.
  • the reinforcing layer may include additives such as a coloring agent (typically, titanium dioxide), a phosphate-based stabilizer, an antioxidant, a light stabilizer, a fluorescent brightener, an ultraviolet absorber, an anti-blocking agent and the like.
  • a coloring agent typically, titanium dioxide
  • a phosphate-based stabilizer typically, an antioxidant
  • a light stabilizer typically, a fluorescent brightener
  • an ultraviolet absorber typically, titanium dioxide
  • the additives may be added to the base material of the two-component curing type thermosetting resin, or may be added to the curing agent of the two-component curing type thermosetting resin.
  • the reinforcing layer is obtained by applying, to the surface of the mid layer 6 , a liquid that is prepared by dissolving or dispersing the base material and the curing agent in a solvent.
  • a liquid that is prepared by dissolving or dispersing the base material and the curing agent in a solvent In light of workability, application with a spray gun is preferred. After the application, the solvent is volatilized to permit a reaction of the base material with the curing agent, thereby forming the reinforcing layer.
  • the reinforcing layer has a thickness of preferably 3 ⁇ m or greater and more preferably 5 ⁇ m or greater.
  • the thickness is preferably equal to or less than 300 ⁇ m, more preferably equal to or less than 50 ⁇ m, and particularly preferably equal to or less than 20 ⁇ m. The thickness is measured by observing a cross section of the golf ball 2 with a microscope. When the mid layer 6 has concavities and convexities on its surface from surface roughening, the thickness of the reinforcing layer is measured at a convex part.
  • the reinforcing layer has a pencil hardness of preferably 4 B or greater and more preferably B or greater.
  • the pencil hardness of the reinforcing layer is preferably equal to or less than 3 H. The pencil hardness is measured according to the standard of “JIS K5400”.
  • Dicyclohexylmethane diisocyanate polytetramethylene ether glycol (PTMG) having a number average molecular weight of 1500, and 1,4-butanediol (BD) were prepared.
  • H 12 MDI and PTMG were heated to 80° C., and PTMG was put into a container containing H 12 MDI, to obtain a mixed liquid.
  • Dibutyltin dilaurate manufactured by Aldrich, Inc.
  • the mixed liquid was stirred at a temperature of 80° C. for 2 hours under a blanket of nitrogen gas.
  • BD heated to 80° C. was put into the container. The mixed liquid was stirred at a temperature of 80° C.
  • the mixed liquid was cooled to room temperature.
  • the mixed liquid was depressurized for 1 minute. By the depressurization, the mixed liquid was deaerated. After the deaeration, the mixed liquid was spread out in another container, and kept at 110° C. for 6 hours under a blanket of nitrogen gas. By the keeping, a urethane reaction took place, thereby obtaining a polyurethane #1.
  • the polyurethane #1 had a JIS-C hardness of 45. Details of the materials are as follows.
  • H 12 MDI manufactured by Sumika Bayer Urethane Co. Ltd.
  • PTMG trade name “PTMG-1500SN”, manufactured by Hodogaya Chemical Co., Ltd.
  • BD manufactured by Wako Pure Chemical Industries, Ltd.
  • the mole ratio of H 12 MDI, PTMG, and BD was (3.39:1.00:2.39).
  • the amount of dibutyltin dilaurate per 100 parts by weight of the total amount of H 12 MDI, PTMG and BD was 0.005 parts by weight.
  • a polyurethane #2 was synthesized in the same manner as the synthesis of the polyurethane #1, except the mole ratio of H 12 MDI, PTMG and BD was (3.61:1.00:2.61).
  • the polyurethane #2 had a JIS-C hardness of 47.
  • a polyurethane #3 was synthesized in the same manner as the synthesis of the polyurethane #1, except PTMG having a number average molecular weight of 1000 (trade name “PTMG-1000SN”, manufactured by Hodogaya Chemical Co., Ltd.) was used and the mole ratio of H 12 MDI, PTMG and BD was (2.63:1.00:1.63).
  • the polyurethane #3 had a JIS-C hardness of 44.
  • a rubber composition (1) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 20 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.7 parts by weight of dicumyl peroxide.
  • the rubber composition (1) was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 15 minutes to obtain a center with a diameter of 15 mm.
  • a rubber composition (3) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (the aforementioned “BR-730”), 42 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.7 parts by weight of dicumyl peroxide.
  • Half shells were formed from the rubber composition (3). The center was covered with two half shells. The center and the half shells were placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 20 minutes to obtain a core with a diameter of 39.7 mm.
  • An envelope layer was formed from the rubber composition (3). The amount of barium sulfate was adjusted such that the specific gravity of the envelope layer agrees with the specific gravity of the center and the weight of a golf ball is 45.4 g.
  • a resin composition (a) was obtained by kneading 50 parts by weight of an ionomer resin (the aforementioned “Surlyn 8945”) and 50 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”) with a twin-screw kneading extruder.
  • the core was placed into a mold including upper and lower mold halves each having a hemispherical cavity.
  • the core was covered with the resin composition (a) by injection molding to form a mid layer with a thickness of 1.0 mm.
  • a paint composition (trade name “POLIN 750LE”, manufactured by SHINTO PAINT CO., LTD.) including a two-component curing type epoxy resin as a base polymer was prepared.
  • the base material liquid of this paint composition includes 30 parts by weight of a bisphenol A type solid epoxy resin and 70 parts by weight of a solvent.
  • the curing agent liquid of this paint composition includes 40 parts by weight of a modified polyamide amine, 55 parts by weight of a solvent, and 5 parts by weight of titanium oxide.
  • the weight ratio of the base material liquid to the curing agent liquid is 1/1.
  • This paint composition was applied to the surface of the mid layer with a spray gun, and kept at 40° C. for 24 hours to obtain a reinforcing layer with a thickness of 10 ⁇ m.
  • a resin composition (b) was obtained by kneading 100 parts by weight of a thermoplastic polyurethane elastomer (trade name “Elastollan XNY85A”, manufactured by BASF Japan Ltd.) and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder.
  • Half shells were obtained from the resin composition (b) by compression molding.
  • the sphere consisting of the core, the mid layer and the reinforcing layer was covered with two of these half shells.
  • the sphere and the half shells were placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and that has a large number of pimples on its cavity face.
  • a cover was obtained by compression molding.
  • the cover had a thickness of 0.5 mm. Dimples having a shape that was the inverted shape of the pimples were formed on the cover. A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of 42.7 mm. A hardness distribution of the core of this golf ball is shown in Table 3.
  • Example 2 Golf balls of Examples 2 to 8 and Comparative Examples 1 to 5 were obtained in the same manner as Example 1, except the specifications of the center, the envelope layer, the mid layer and the cover were as shown in Tables 6 to 9 below.
  • the rubber composition of the core is shown in detail in Table 1 below.
  • the resin compositions of the mid layer and the cover are shown in detail in Table 2 below.
  • a hardness distribution of the core is shown in Tables 3 to 5.
  • the golf ball according to Comparative Example 1 does not have an envelope layer.
  • a driver with a titanium head (trade name “SRIXON W505”, manufactured by SRI Sports Limited, shaft hardness: X, loft angle: 8.5°) was attached to a swing machine manufactured by Golf Laboratories, Inc.
  • a golf ball was hit under the condition of a head speed of 50m/sec. The ball speed immediately after the hit and the distance from the launch point to the stop point were measured.
  • the average value of data obtained by 12 measurements is shown in Tables 6 to 9 below.
  • a sand wedge (SW) was attached to a swing machine manufactured by Golf Laboratories, Inc.
  • a golf ball was hit under the condition of a head speed of 21 m/sec, and the spin rate was measured immediately after the hit.
  • the average value of data obtained by 12 measurements is shown in Tables 6 to 9 below.
  • water was applied to a clubface and a golf ball, and the golf ball was hit. The spin rate was measured immediately after the hit.
  • the average value of data obtained by 12 measurements is shown in Tables 6 to 9 below.
  • Elastollan XNY85A, Elastollan XNY90A, Elastollan XNY97A, polyurethane #1, polyurethane #2 and polyurethane #3 are thermoplastic polyurethane elastomers in each of which a polyol component is polytetramethylene ether glycol.
  • the number average molecular weight of each polytetramethylene ether glycol is as follows.
  • Example 1 Example 2
  • Example 3 Example 4 0 60 60 60 1.0 61 60.8 61 61 2.0 62 61.6 62 62 3.0 63 62.4 63 63 4.0 64 63.2 64 64 5.0 65 64 65 65 6.0 65.7 65 65.7 65.7 7.0 66.5 66 66 66.5 8.0 74.5 67 74.5 74.5 9.0 75.7 68.76 75.7 75.7 10.0 77 77 77 77 77 11.0 77.8 77.8 77.8 12.0 78.6 78.6 78.6 78.6 13.0 79.4 79.4 79.4 79.4 14.0 80.2 80.2 80.2 80.2 15.0 81 81 81 81 81 81 81 81 81 81 81 81 81
  • Example 8 0 60 60 54 62 1.0 61 61 56 63 2.0 62 62 58 64 3.0 63 63 60 65 4.0 64 64 62 66 5.0 65 65 64 67 6.0 65.7 65.7 66 67.7 7.0 66.5 66.5 68 68.5 8.0 74.5 74.5 74.5 73.5 9.0 75.7 75.7 75.7 74.7 10.0 77 77 77 76 11.0 77.8 77.8 77.8 76.8 12.0 78.6 78.6 78.6 77.6 13.0 79.4 79.4 79.4 78.4 14.0 80.2 80.2 80.2 79.2 15.0 81 81 81 80
  • Example 3 Center Composition (1) (1) (1) Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 15 15 15 Diameter (mm) 15 18 15 Envelope Composition (3) (3) (3) layer Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 20 20 20 Core Diameter (mm) 39.7 40.1 40.3 Volume ratio (%) 80.4 82.8 84.1 Central hardness Ho 60 60 60 (JIS-C) Surface hardness He 88 88 88 (JIS-C) Hardness distribution FIG. 2 FIG. 3 FIG.
  • Example 6 Center Composition (1) (1) (1) Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 15 15 15 Diameter (mm) 15 15 15 Envelope Composition (3) (3) (3) layer Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 20 20 20 Core Diameter (mm) 39.7 39.7 39.7 Volume ratio (%) 80.4 80.4 80.4 Central hardness Ho 60 60 60 (JIS-C) Surface hardness He 88 88 88 (JIS-C) Hardness distribution FIG. 2 FIG. 2 FIG.
  • Example 7 Example 8 Example 1 Center Composition (8) (6) (2) Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 15 15 20 Diameter (mm) 15 15 39.7 Envelope Composition (3) (7) — layer Crosslinking temperature 170 170 — (° C.) Crosslinking time (min) 20 20 — Core Diameter (mm) 39.7 39.7 39.7 Volume ratio (%) 80.4 80.4 80.4 Central hardness Ho 54 62 70 (JIS-C) Surface hardness He 88 87 86 (JIS-C) Hardness distribution FIG. 5 FIG. 6 FIG.
  • FIG. 9 FIG. 9 FIG. 10 Mid Composition (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) layer Hardness (JIS-C) 94 94 94 94 Thickness (mm) 1.0 1.6 1.0 1.0 Cover Composition (b) (b) (d) (b) Hardness (JIS-C) 47 47 67 47 Thickness (mm) 0.8 0.5 0.5 0.5 Molecular weight of 1800 1800 1800 PTMG E′′ ( ⁇ 10 7 Pa) 3.24 3.24 7.57 3.24 G′′ ( ⁇ 10 7 Pa) 2.77 2.77 4.01 2.77 E′′/G′′ 1.17 1.17 1.89 1.17 Ball Amount of 2.40 2.40 2.40 2.40 compressive deformation (mm) Ha2-Ha1 (maximum value) 1.6 2.0 2.0 7.0 Hb2-Hb1 (maximum value) 11 7 7 9.0 W #1 Ball speed (m/s) 73.5 73.6 74.0 73.4 Spin (rpm) 2360 2420 2270 2400 Flight
  • the golf balls according to Examples are excellent in various performance characteristics. From the results of evaluation, advantages of the present invention are clear.

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