EP4789733A1

EP4789733A1 - Golf ball

Info

Publication number: EP4789733A1
Application number: EP26154471.2A
Authority: EP
Inventors: Takahiro Sajima
Original assignee: Sumitomo Rubber Industries Ltd
Current assignee: Sumitomo Rubber Industries Ltd
Priority date: 2025-02-10
Filing date: 2026-01-27
Publication date: 2026-08-12

Abstract

A golf ball 2 has a plurality of dimples 10 on a surface thereof. An occupation ratio X of the dimples is 70% or more. A value Z of the golf ball 2, calculated by the following mathematical formula, is less than 0.0. Z=−535.36*Db+146.46*Lb−238.52*Da+225.22*La+111.21Db: drag coefficient under a first conditionLb: lift coefficient under the first conditionDa: drag coefficient under a second conditionLa: lift coefficient under the second conditionFirst conditionReynolds number: 1.20 * 10<sup>5</sup>Ratio (rω/V): 0.131Second conditionReynolds number: 0.83 * 10<sup>5</sup>Ratio (rω/V): 0.179The ratio (rω/V) is calculated by dividing a product of a radius r (m) and a backspin rate ω (rad/s) of the golf ball 2 by a speed V (m/s) of the golf ball 2.

Description

CROSS REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to Japanese patent application JP 2025-019878, filed on February 10, 2025 , the entire content of which is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

The present specification discloses a golf ball having a large number of dimples on a surface thereof.

Background Art

A golf ball has a large number of dimples on a surface thereof. The dimples optimize the lift and drag generated during the flight of the golf ball. The dimples contribute to a large flight distance of the golf ball. Improved dimples are disclosed in Japanese Laid-Open Patent Publication No. 2016-7369 .
Due to recent technological advancements, a flight distance achieved with a driver has tended to increase. In particular, for the main purpose of reducing a flight distance achieved with a driver by professionals and advanced players with high head speeds, the Royal and Ancient Golf Club of St Andrews (R&A) and the United States Golf Association (USGA) have announced changes to the test conditions for the Overall Distance Standard (ODS) of golf balls. Therefore, it is necessary to develop golf balls that reduce a flight distance achieved with a driver by professionals and advanced players with high head speeds.
It is an intention of the applicant to provide a golf ball that complies with new rules satisfying new test conditions and particularly reduces a flight distance achieved with a driver by professionals and advanced players with high head speeds.

SUMMARY

A golf ball disclosed in the present specification has a plurality of dimples on a surface thereof. A ratio X of a total area of all the dimples to a surface area of a phantom sphere of the golf ball is 70% or more. A value Z calculated by the following mathematical formula is less than 0.0. $Z = - 536.36 * Db + 146.46 * Lb - 238.52 * Da + 225.22 * La + 111.21,$ where Db represents a drag coefficient under a condition where a Reynolds number is 1.20 * 10⁵ and a ratio (rω/V) is 0.131, Lb is a lift coefficient under a condition where the Reynolds number is 1.20 * 10⁵ and the ratio (rω/V) is 0.131, Da represents a drag coefficient under a condition where the Reynolds number is 0.83 * 10⁵ and the ratio (rω/V) is 0.179, La represents a lift coefficient under a condition where the Reynolds number is 0.83 * 10⁵ and the ratio (rω/V) is 0.179, and the ratio (rω/V) is calculated by dividing a product of a radius r (m) and a backspin rate ω (rad/s) of the golf ball by a speed V (m/s) of the golf ball.
The aerodynamic characteristics of this golf ball when flying under new test conditions are inferior to those of a conventional golf ball. This golf ball can comply with new rules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a golf ball according to one embodiment;
FIG. 2 is a plan view showing the golf ball in FIG. 1;
FIG. 3 is a front view showing the golf ball in FIG. 2;
FIG. 4 is a partially enlarged cross-sectional view of the golf ball in FIG. 1;
FIG. 5 is a plan view showing a golf ball according to another embodiment;
FIG. 6 is a front view showing the golf ball in FIG. 5;
FIG. 7 is a plan view showing a golf ball according to still another embodiment;
FIG. 8 is a front view showing the golf ball in FIG. 7;
FIG. 9 is a plan view showing a golf ball according to still another embodiment;
FIG. 10 is a front view showing the golf ball in FIG. 9;
FIG. 11 is a plan view showing a golf ball according to Comparative Example 5; and
FIG. 12 is a front view showing the golf ball in FIG. 11.

DETAILED DESCRIPTION

Hereinafter, preferred embodiments will be described in detail with appropriate reference to the drawings.
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 golf ball 2 has a large number of dimples 10 on the surface thereof. Of the surface of the golf ball 2, a part other than the dimples 10 is a land 12. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 8, but these layers are not shown in the drawing.
The golf ball 2 preferably has a diameter of 40 mm or more and 45 mm or less. From the viewpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably 42.67 mm or more. From the viewpoint of suppression of air resistance, the diameter is more preferably 44 mm or less and particularly preferably 42.80 mm or less.
The golf ball 2 preferably has a mass of 40 g or more and 50 g or less. From the viewpoint of attainment of great inertia, the mass is more preferably 44 g or more and particularly preferably 45.00 g or more. From the viewpoint of conformity to the rules established by the USGA, the mass is particularly preferably 45.93 g or less.
The core 4 is formed by crosslinking a rubber composition. Examples of the base rubber of the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. Two or more rubbers may be used in combination. From the viewpoint of resilience performance, polybutadienes are preferable, and high-cis polybutadienes are particularly preferable.
The rubber composition of the core 4 includes a co-crosslinking agent. Preferable co-crosslinking agents from the viewpoint of resilience performance are zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. The rubber composition preferably includes an organic peroxide together with a co-crosslinking agent. Examples of preferable 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.
The rubber composition of the core 4 may include additives such as a filler, sulfur, a vulcanization accelerator, a sulfur compound, an anti-aging agent, a coloring agent, a plasticizer, and a dispersant. The rubber composition may include a carboxylic acid or a carboxylate. The rubber composition may include synthetic resin powder or crosslinked rubber powder.
The core 4 has a diameter of preferably 30.0 mm or more and particularly preferably 38.0 mm or more. The diameter of the core 4 is preferably 42.0 mm or less and particularly preferably 41.5 mm or less. The core 4 may have two or more layers. The core 4 may have a rib on the surface thereof. The core 4 may be hollow.
Typically, the mid layer 6 is formed from a resin composition. A preferable base polymer for the resin composition is an ionomer resin. Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. For the binary copolymers and the ternary copolymers, preferable α-olefins are ethylene and propylene, and preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. In the binary copolymers and the ternary copolymers, 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.
The resin composition of the mid layer 6 may include another polymer instead of an ionomer resin. Examples of the other polymer include polystyrenes, polyamides, polyesters, polyolefins, and polyurethanes. The resin composition may include two or more polymers.
The resin composition of the mid layer 6 may include 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, etc. For the purpose of specific gravity adjustment, the resin composition may include powder of a metal with a high specific gravity such as tungsten and molybdenum.
The mid layer 6 has a thickness of preferably 0.2 mm or more and particularly preferably 0.3 mm or more. The thickness of the mid layer 6 is preferably 2.5 mm or less and particularly preferably 2.2 mm or less. The mid layer 6 has a specific gravity of preferably 0.90 or more and particularly preferably 0.95 or more. The specific gravity of the mid layer 6 is preferably 1.10 or less and particularly preferably 1.05 or less. The mid layer 6 may have two or more layers.
Typically, the cover 8 is formed from a resin composition. It is preferable to contain a polyurethane obtained by reacting a polyol with a polyisocyanate, as a resin component.
The above polyol is a compound having two or more hydroxyl groups in its molecule. As the polyol, one type may be used alone, or two or more types may be used in combination. As the polyol constituting the above polyurethane, it is preferable to use a polyol having a number average molecular weight of 200 or more and 6,000 or less. A polyol having a number average molecular weight of 200 or more and 6,000 or less forms a soft segment and imparts flexibility to the polyurethane.
The number average molecular weight of the polyol constituting the above polyurethane is preferably 200 or more, more preferably 300 or more, and further preferably 1,000 or more, and is preferably 6,000 or less, more preferably 4,000 or less, and further preferably 3,000 or less. When the number average molecular weight is 200 or more and 6,000 or less, the feel at impact of the golf ball is improved.
As the polyol constituting the above polyurethane, at least one polymeric polyol selected from the group consisting of polyether polyols, condensed polyester polyols, lactone polyester polyols, polycarbonate polyols, and acrylic polyols is preferred. The polymeric polyol refers to a polymer obtained by polymerizing a low-molecular-weight compound and having a plurality of hydroxyl groups. The above polymeric polyol may be derived from petroleum resources or from biomass resources.
Examples of the above polyether polyols include polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), polytrimethylene ether glycol (PO3G), and polytetramethylene ether glycol (PTMG). Examples of the above condensed polyester polyols include polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA). Examples of the above lactone polyester polyols include poly-ε-caprolactone (PCL). Examples of the above polycarbonate polyols include polyhexamethylene carbonate.
The above polymeric polyol is preferably a polymeric diol having two hydroxyl groups. By using such a polymeric diol, a linear thermoplastic polyurethane can be obtained, which facilitates molding into a component constituting the golf ball.
As the polyol constituting the above polyurethane, a polyether polyol is preferred. In particular, the content of the polyether polyol in 100% by mass of the polyol constituting the above polyurethane is preferably 50% by mass or more, more preferably 55% by mass or more, and further preferably 60% by mass or more. When the content of the polyether polyol is 50% by mass or more, mechanical strength can be maintained over a long period. In the case where the above golf ball cover material contains two or more polyurethanes, the content of the polyether polyol in 100% by mass of the polyol constituting each polyurethane is preferably within the above range.
The polyisocyanate constituting the above polyurethane is not particularly limited as long as the polyisocyanate has two or more isocyanate groups. As the polyisocyanate, one type may be used alone, or two or more types may be used in combination.
Examples of the above polyisocyanate include aromatic polyisocyanates, alicyclic polyisocyanates, and aliphatic polyisocyanates.
Examples of aromatic polyisocyanates include 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), tetramethylxylylene diisocyanate (TMXDI), and paraphenylene diisocyanate (PPDI).
Examples of alicyclic polyisocyanates or aliphatic polyisocyanates include 4,4'-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenated xylylene diisocyanate (H₆XDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and norbornene diisocyanate (NBDI).
From the viewpoint of improving scratch resistance, it is preferable to use an aromatic polyisocyanate as the polyisocyanate constituting the polyurethane. By using an aromatic polyisocyanate, the mechanical properties of the obtained polyurethane are improved, and a cover having excellent scratch resistance can be obtained.
From the viewpoint of improving weather resistance, it is preferable to use a non-yellowing polyisocyanate (such as TMXDI, XDI, HDI, H₆XDI, IPDI, H₁₂MDI, NBDI, etc.) as the polyisocyanate constituting the polyurethane, and more preferable to use 4,4'-dicyclohexylmethane diisocyanate (H₁₂MDI). 4,4'-dicyclohexylmethane diisocyanate (H₁₂MDI) has a rigid structure, so that the mechanical properties of the obtained polyurethane are improved, and a cover also having excellent scratch resistance can be obtained.
The above polyurethane may also include a chain extender as a component, as long as the effects of the present disclosure are not impaired. As the chain extender component, low-molecular-weight polyols, low-molecular-weight polyamines, etc., can be used.
Examples of the low-molecular-weight polyols include: diols such as ethylene glycol, diethylene glycol, triethylene glycol, propanediols (e.g., 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol), dipropylene glycol, butanediols (e.g., 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol), neopentyl glycol, pentanediol, hexanediol, heptanediol, octanediol, and 1,4-cyclohexanedimethanol; triols such as glycerin, trimethylolpropane, and hexanetriol; and tetraols or hexols such as pentaerythritol and sorbitol.
The low-molecular-weight polyamines that can be used as the chain extender component are not particularly limited as long as the polyamines each have at least two amino groups. Examples of such polyamines include: aliphatic polyamines such as ethylenediamine, propylenediamine, butylenediamine, and hexamethylenediamine; alicyclic polyamines such as isophoronediamine and piperazine; and aromatic polyamines.
The above aromatic polyamines are not particularly limited as long as the polyamines each have at least two amino groups directly or indirectly bonded to an aromatic ring. Here, "indirectly bonded" means that the amino group is bonded to the aromatic ring via, for example, a lower alkylene group. Examples of the aromatic polyamines include monocyclic aromatic polyamines in which two or more amino groups are bonded to a single aromatic ring, and polycyclic aromatic polyamines containing two or more aminophenyl groups, each having at least one amino group bonded to an aromatic ring.
Examples of the above monocyclic aromatic polyamines include: types in which amino groups are directly bonded to an aromatic ring, such as phenylenediamine, toluenediamine, diethyltoluenediamine, and dimethylthiotoluenediamine; and types in which amino groups are bonded to an aromatic ring via a lower alkylene group, such as xylylenediamine.
The above polycyclic aromatic polyamines may each be poly(aminobenzene) in which at least two aminophenyl groups are directly bonded, or may each be a polyamine in which at least two aminophenyl groups are bonded via a lower alkylene group or an alkylene oxide group. Among these, diamino-diphenylalkanes in which two aminophenyl groups are bonded via a lower alkylene group are preferred, and 4,4'-diaminodiphenylmethane and derivatives thereof are particularly preferred.
The molecular weight of the above chain extender is preferably 400 or less, more preferably 350 or less, and further preferably less than 200, and is preferably 30 or more, more preferably 40 or more, and further preferably 45 or more. This is because, if the molecular weight becomes excessively large, it becomes difficult to distinguish the chain extender from the polyol constituting the soft segment of the polyurethane.
As the above polyurethane, either a thermoplastic polyurethane or a thermosetting polyurethane can be used. The thermoplastic polyurethane refers to a polyurethane exhibiting plasticity upon heating and generally means a polyurethane having a linear structure and polymerized to a relatively high molecular weight. The thermosetting polyurethane is a polyurethane obtained by curing a relatively low-molecular-weight prepolymer with a curing agent when used. In the thermosetting polyurethane, a three-dimensional crosslinked structure is formed by controlling the number of functional groups of the prepolymer or curing agent to be used. The above polyurethane is preferably a thermoplastic polyurethane. The thermoplastic polyurethane facilitates molding of the cover.
Instead of the polyurethane, the resin composition of the cover 8 may include another polymer. Examples of the other polymer include ionomer resins, polystyrenes, polyamides, polyesters, and polyolefins. The resin composition may include two or more polymers.
The resin composition of the cover 8 may include 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, etc.
The cover 8 has a thickness of preferably 0.2 mm or more and particularly preferably 0.3 mm or more. The thickness of the cover 8 is preferably 2.5 mm or less and particularly preferably 2.2 mm or less. The cover 8 has a specific gravity of preferably 0.90 or more and particularly preferably 0.95 or more. The specific gravity of the cover 8 is preferably 1.10 or less and particularly preferably 1.05 or less. The cover 8 may have two or more layers.
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 reinforcing layer is formed from a polymer composition. Examples of the base polymer of the reinforcing layer include two-component curing type epoxy resins and two-component curing type urethane resins.
As shown in FIGS. 2 and 3, the contour of each dimple 10 is circular. The golf ball 2 has dimples A each having a diameter of 4.35 mm; dimples B each having a diameter of 4.25 mm; dimples C each having a diameter of 4.10 mm; dimples D each having a diameter of 3.85 mm; and dimples E each having a diameter of 2.95 mm. The number of types of the dimples 10 is five. The golf ball 2 may have dimples 10 each having a non-circular contour.
The number of the dimples A is 60; the number of the dimples B is 158; the number of the dimples C is 72; the number of the dimples D is 36; and the number of the dimples E is 12. The total number N of the dimples 10 is 338. A dimple pattern is formed by these dimples 10 and the land 12.
From the viewpoint of the appearance of the golf ball 2 and the flight performance of the golf ball 2 upon a shot with an iron, the total number N of the dimples 10 is preferably 250 or more and 450 or less. The total number N is more preferably 270 or more and particularly preferably 280 or more. The total number N is more preferably 410 or less and particularly preferably 380 or less.
FIG. 4 shows a cross-section of the golf ball 2 along a plane passing through the central point of the dimple 10 and the central point of the golf ball 2. In FIG. 4, the up-down direction is the depth direction of the dimple 10. In FIG. 4, an alternate long and two short dashes line 14 indicates a phantom sphere. The surface of the phantom sphere 14 is the surface of the golf ball 2 when it is postulated that no dimple 10 exists. The diameter of the phantom sphere 14 is the same as the diameter of the golf ball 2. The dimple 10 is recessed from the surface of the phantom sphere 14. The land 12 coincides with the surface of the phantom sphere 14. In the present embodiment, the cross-sectional shape of each dimple 10 is substantially an arc. The curvature radius of the arc is indicated by reference character CR in FIG. 4.
In FIG. 4, an arrow Dm indicates the diameter of the dimple 10. The diameter Dm is the distance between two tangent points Ed appearing on a tangent line Tg that is drawn tangent to the opposite ends of the dimple 10. Each tangent point Ed is also the edge of the dimple 10. The edge Ed defines the contour of the dimple 10. The diameter Dm is preferably 2.0 mm or more and 6.0 mm or less.
In FIG. 4, a double headed arrow Dp indicates the depth of the dimple 10. The depth Dp is the distance between the deepest part of the dimple 10 and the plane including the contour of the dimple 10. The depth Dp is preferably 0.05 mm or more and preferably 0.25 mm or less.
The area S of the dimple 10 is the area of a region surrounded by the contour line of the dimple 10 when the central point of the golf ball 2 is viewed at infinity. In the case of the dimple 10 having a circular contour, the area S is calculated by the following mathematical formula. $S = {(Dm / 2)}^{2} * π$
In the golf ball 2 shown in FIGS. 2 and 3, the area of each dimple A is 14.86 mm², the area of each dimple B is 14.19 mm², the area of each dimple C is 13.20 mm², the area of each dimple D is 11.64 mm², and the area of each dimple E is 6.83 mm².
In the present specification, the ratio X of the sum of the areas S of all the dimples 10 relative to the surface area of the phantom sphere 14 is referred to as "occupation ratio". From the viewpoint of the appearance of the golf ball 2 and excellent flight performance of the golf ball 2 upon a shot with an iron, the occupation ratio X is preferably 70% or more, more preferably 75% or more, and particularly preferably 80% or more. The occupation ratio X is preferably 95% or less. In the golf ball 2 shown in FIGS. 2 and 3, the total area of the dimples 10 is 4584.8 mm². The surface area of the phantom sphere 14 of the golf ball 2 is 5728.0 mm², and thus the occupation ratio X is 80%.
In the present specification, the "volume of the dimple" means the volume of the portion surrounded by the plane including the contour of the dimple 10 and the surface of the dimple 10. The total of the volumes of all the dimples 10 is referred to as "total volume Y".
In the golf ball 2 shown in FIGS. 2 and 3, the volume of each dimple A is 0.826 mm³, the volume of each dimple B is 0.789 mm³, the volume of each dimple C is 0.734 mm³, the volume of each dimple D is 0.647 mm³, and the volume of each dimple E is 0.380 mm³. Accordingly, the total volume Y of the dimples 10 is 255 mm³.
In the present embodiment, a first drag coefficient Db, a first lift coefficient Lb, a second drag coefficient Da, and a second lift coefficient La are measured. The first drag coefficient Db and the first lift coefficient Lb are measured under the following first condition. The second drag coefficient Da and the second lift coefficient La are measured under the following second condition.

First condition $1.20 * 10^{5}$
Ratio (rω/V): 0.131
Second condition $0.83 * 10^{5}$
Ratio (rω/V): 0.179

The first condition generally corresponds to a condition at a point 70 yards before the peak of a trajectory upon a shot with a driver at a head speed of 50 m/s. The second condition generally corresponds to a condition at the peak of a trajectory upon a shot with driver at a head speed of 50 m/s. According to the findings obtained by the present inventor, the first drag coefficient Db, the first lift coefficient Lb, the second drag coefficient Da, and the second lift coefficient La have a significant influence on the flight distance of the golf ball 2. From this viewpoint, in the present embodiment, the drag and lift coefficients are measured under the first and second conditions.
In the present specification, a value Z is calculated by the following mathematical formula. $Z = - 536.36 * Db + 146.46 * Lb - 238.52 * Da + 225.22 * La + 111.21$ According to the findings obtained by the present inventor, when the golf ball 2 having a value Z of less than 0.0 flies under new test conditions, the flight distance thereof is smaller than that of a conventional golf ball. This golf ball 2 can comply with these new rules. From the viewpoint of compliance with the new rules, the value Z is more preferably -1.0 or less and particularly preferably -3.0 or less. From the viewpoint of flight performance upon a shot at a low head speed (in other words, a shot under a condition different from the new test conditions), the value Z is preferably -20.0 or more.
The golf ball 2 having a small value Z can comply with the new rule even if the golf ball 2 has excellent resilience performance. The golf ball 2 having excellent resilience performance can have excellent flight performance upon a shot at a low head speed. The golf ball 2 may have both a small value Z and inferior resilience performance.
The golf ball 2 having a small value Z can comply with the new rule even if the ball diameter thereof is not excessively large. The golf ball 2 having an appropriate ball diameter has low air resistance during flight. The trajectory of this golf ball 2 is less likely to be affected by wind. The golf ball 2 may have both a small value Z and a large ball diameter.
The golf ball 2 having a small value Z can comply with the new rule even if the mass thereof is not excessively small. The golf ball 2 having an appropriate mass has high inertia. The trajectory of this golf ball 2 is less likely to be affected by wind. The golf ball 2 may have both a small value Z and a small mass.
A value Z of less than 0.0 can be achieved by adopting a small total number N for the golf ball 2. A value Z of less than 0.0 can be achieved by adopting a small occupation ratio X for the golf ball 2. A value Z of less than 0.0 can be achieved by adopting an extremely small total volume Y for the golf ball 2. A value Z of less than 0.0 can be achieved by adopting an extremely large total volume Y for the golf ball 2.
The ratio (Lb/Db) of the first lift coefficient Lb to the first drag coefficient Db is preferably 0.79 or less. When the golf ball 2 having a ratio (Lb/Db) of 0.79 or less flies under the new test condition, the trajectory thereof is low. This trajectory can achieve a short flight distance under the new test condition. From this viewpoint, the ratio (Lb/Db) is more preferably 0.77 or less and more preferably 0.75 or less. From the viewpoint of the controllability of the golf ball 2, the ratio (Lb/Db) is preferably 0.65 or more.
The total volume Y of the golf ball 2 having a ratio (Lb/Db) of 0.79 or less is preferably 330 mm³ or more. The dimples 10 having a total volume Y of 330 mm³ or more can achieve a low trajectory when the golf ball 2 flies under the condition of the new rule. From this viewpoint, the total volume Y is more preferably 390 mm³ or more and particularly preferably 405 mm³ or more. From the viewpoint of the controllability of the golf ball 2, the total volume Y is preferably 550 mm³ or less.
The golf ball 2 having a sufficiently large total volume Y can comply with the new rule even if the number N of the dimples 10 is appropriate. The golf ball 2 having a sufficiently large total volume Y can comply with the new rule even if the occupation ratio X is appropriate. The golf ball 2 having an appropriate number N and occupation ratio X has excellent appearance.
In the present specification, a value T1 is calculated by the following mathematical formula. $T 1 = - 0.55 * X + 0.115 * Y + 0.3$ The value T1 for the golf ball 2 having a ratio (Lb/Db) of 0.79 or less is preferably 0.0 or more. The dimples 10 having a value T1 of 0.0 or more can achieve a low trajectory when the golf ball 2 flies under the condition of the new rule. From this viewpoint, the value T1 is more preferably 1.0 or more and particularly preferably 1.5 or more. From the viewpoint of the controllability of the golf ball 2, the value T1 is preferably 15.0 or less.
The ratio (Lb/Db) of the first lift coefficient Lb to the first drag coefficient Db may be 0.90 or more. When the golf ball 2 having a ratio (Lb/Db) of 0.90 or more flies under the new test condition, the trajectory thereof is high. This trajectory can achieve a short flight distance under the new test condition. From this viewpoint, the ratio (Lb/Db) is more preferably 0.91 or more. From the viewpoint of flight performance upon a shot at a low head speed, the ratio (Lb/Db) is preferably 1.00 or less.
The total volume Y of the golf ball 2 having a ratio (Lb/Db) of 0.90 or more is preferably less than 330 mm³. The dimples 10 having a total volume Y of less than 330 mm³ can achieve a high trajectory when the golf ball 2 flies under the condition of the new rule. From this viewpoint, the total volume Y is more preferably 270 mm³ or less and particularly preferably 260 mm³ or less. From the viewpoint of flight performance upon a shot at a low head speed, the total volume Y is preferably 200 mm³ or more.
The golf ball 2 having a sufficiently small total volume Y can comply with the new rule even if the number N of the dimples 10 is appropriate. The golf ball 2 having a sufficiently small total volume Y can comply with the new rule even if the occupation ratio X is appropriate. The golf ball 2 having an appropriate number N and occupation ratio X has excellent appearance.
In the present specification, a value T2 is calculated by the following mathematical formula. $T 2 = - 0.55 * X + 0.125 * Y + 79.5$ The value T2 for the golf ball 2 having a ratio (Lb/Db) of 0.90 or more is preferably 0.0 or more. The dimples 10 having a value T2 of 0.0 or more can achieve a high trajectory when the golf ball 2 flies under the new test condition. From this viewpoint, the value T2 is more preferably 1.8 or more and particularly preferably 3.6 or more. From the viewpoint of flight performance upon a shot at a low head speed, the value T2 is preferably 5.0 or less.

EXAMPLES

Hereinafter, the advantageous effects of golf balls according to Examples will be shown, but the scope disclosed in the present specification should not be construed in a limited manner on the basis of the description of these Examples.

[Example 1]

A rubber composition was obtained by kneading 100 parts by mass of a high-cis polybutadiene (trade name "BR-730", manufactured by JSR Corporation), 22.5 parts by mass of zinc acrylate, 5 parts by mass of zinc oxide, 5 parts by mass of barium sulfate, 0.5 parts by mass of diphenyl disulfide, and 0.6 parts by mass of dicumyl peroxide. The rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and was heated at 170°C for 18 minutes to obtain a core having a diameter of 38.5 mm.
A resin composition was obtained by kneading 50 parts by mass of an ionomer resin (trade name "HIMILAN 1605", manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 50 parts by mass of another ionomer resin (trade name "HIMILAN AM7329", manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), and 4 parts by mass of titanium dioxide with a twin-screw kneading extruder. The core was covered with the resin composition by injection molding to form a mid layer. The thickness of the mid layer was 1.6 mm.
A 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 composition includes 30 parts by mass of a bisphenol A type solid epoxy resin and 70 parts by mass of a solvent. The curing agent liquid of this composition includes 40 parts by mass of a modified polyamide amine, 55 parts by mass of a solvent, and 5 parts by mass of titanium dioxide. The weight ratio of the base material liquid to the curing agent liquid is 1/1. This composition was applied to the surface of the mid layer with a spray gun, and kept at 23°C for 6 hours to obtain a reinforcing layer. The thickness of the reinforcing layer was 10 µm.
A resin composition was obtained by kneading 100 parts by mass of a thermoplastic polyurethane elastomer (trade name "Elastollan XNY85A", manufactured by BASF JAPAN Ltd.) and 4 parts by mass of titanium dioxide with a twin-screw kneading extruder. Half shells were obtained from this resin composition by compression molding. The sphere consisting of the core, the mid layer, and the reinforcing layer was covered with two of these half shells. These half shells and the sphere were placed into a final mold including upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face, and a cover was obtained by compression molding. The thickness of the cover was 0.5 mm. Dimples having a shape that is 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 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. The dimple pattern of the golf ball is shown in FIGS. 2 and 3. The specifications of the dimples of the golf ball are shown in Table 1 below.

[Examples 2 to 9 and Comparative Examples 1 to 5]

Golf balls of Examples 2 to 9 and Comparative Examples 1 to 5 were obtained in the same manner as Example 1, except that the final mold was changed. The specifications of the dimples of these golf balls are shown in Tables 1 to 4 below.

[Flight Test]

A driver (trade name "SRIXON ZX7 Mk II", manufactured by Sumitomo Rubber Industries, Ltd., loft angle: 9.5°, shaft hardness: S) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 50.0 m/sec, and the flight distance was measured. The flight distance is the distance from the hitting spot to the spot at which the golf ball stopped. The measurement was conducted 12 times, and the average value of the obtained data was calculated. The results are shown in Tables 5 to 7 below.

[Trajectory]

Twenty players with powerful swings were asked to hit golf balls and rate the trajectory heights according to the following criteria.

A: Significantly Low
B: Low
C: Slightly Low
D: Appropriate
E: Slightly High
F: High
G: Significantly High

[Appearance]

Twenty players were asked to visually observe golf balls and provide their impressions. The rating was conducted according to the following criteria.

A: The number of players who answered "The appearance is poor" is five or less.
B: The number of players who answered "The appearance is poor" is six or more and fourteen or less.
C: The number of players who answered "The appearance is poor" is fifteen or more.

[Table 1]

Table 1 Dimple specifications
	Type	Number	Diameter (mm)	Depth (mm)	Curvature (mm)	Volume (mm³)	Area (mm²)
Example 1	A	60	4.350	0.1111	21.35	0.826	14.86
	B	158	4.250	0.1111	20.38	0.789	14.19
	C	72	4.100	0.1111	18.97	0.734	13.20
	D	36	3.850	0.1111	16.73	0.647	11.64
	E	12	2.950	0.1111	9.85	0.380	6.83
Example 2	A	60	4.350	0.1177	20.15	0.875	14.86
	B	158	4.250	0.1177	19.24	0.836	14.19
	C	72	4.100	0.1177	17.91	0.778	13.20
	D	36	3.850	0.1177	15.80	0.686	11.64
	E	12	2.950	0.1177	9.30	0.403	6.83
Comparative Example 1	A	60	4.350	0.1242	19.11	0.924	14.86
	B	158	4.250	0.1242	18.24	0.882	14.19
	C	72	4.100	0.1242	16.98	0.821	13.20
	D	36	3.850	0.1242	14.98	0.724	11.64
	E	12	2.950	0.1242	8.82	0.425	6.83

[Table 2]

Table 2 Dimple specifications
	Type	Number	Diameter (mm)	Depth (mm)	Curvature (mm)	Volume (mm³)	Area (mm²)
Example 3	A	16	4.592	0.1067	24.76	0.884	16.56
	B	30	4.492	0.1067	23.69	0.846	15.85
	C	30	4.392	0.1067	22.65	0.809	15.15
	D	150	4.292	0.1067	21.63	0.773	14.47
	E	30	4.192	0.1067	20.64	0.737	13.80
	F	66	4.092	0.1067	19.67	0.702	13.15
	G	10	3.792	0.1067	16.90	0.603	11.29
	H	12	3.392	0.1067	13.53	0.483	9.04
Comparative Example 2	A	16	4.592	0.1231	21.47	1.020	16.56
	B	30	4.492	0.1231	20.55	0.976	15.85
	C	30	4.392	0.1231	19.65	0.933	15.15
	D	150	4.292	0.1231	18.77	0.891	14.47
	E	30	4.192	0.1231	17.91	0.850	13.80
	F	66	4.092	0.1231	17.06	0.810	13.15
	G	10	3.792	0.1231	14.66	0.696	11.29
	H	12	3.392	0.1231	11.74	0.557	9.04
Comparative Example 3	A	60	4.400	0.1594	15.26	1.214	15.21
	B	158	4.300	0.1594	14.58	1.160	14.52
	C	72	4.150	0.1594	13.59	1.080	13.53
	D	36	3.900	0.1594	12.01	0.954	11.95
	E	12	3.000	0.1594	7.14	0.565	7.07
Example 4	A	60	4.400	0.1658	14.68	1.263	15.21
	B	158	4.300	0.1658	14.02	1.206	14.52
	C	72	4.150	0.1658	13.07	1.124	13.53
	D	36	3.900	0.1658	11.55	0.993	11.95
	E	12	3.000	0.1658	6.87	0.588	7.07

[Table 3]

Table 3 Dimple specifications
	Type	Number	Diameter (mm)	Depth (mm)	Curvature (mm)	Volume (mm³)	Area (mm²)
Example 5	A	60	4.400	0.1721	14.15	1.311	15.21
	B	158	4.300	0.1721	13.52	1.252	14.52
	C	72	4.150	0.1721	12.60	1.167	13.53
	D	36	3.900	0.1721	11.13	1.031	11.95
	E	12	3.000	0.1721	6.62	0.611	7.07
Example 6	A	60	4.400	0.1912	12.75	1.457	15.21
	B	158	4.300	0.1912	12.18	1.392	14.52
	C	72	4.150	0.1912	11.36	1.297	13.53
	D	36	3.900	0.1912	10.04	1.146	11.95
	E	12	3.000	0.1912	5.98	0.679	7.07
Comparative Example 4	A	16	4.592	0.1395	18.96	1.157	16.56
	B	30	4.492	0.1395	18.15	1.107	15.85
	C	30	4.392	0.1395	17.35	1.058	15.15
	D	150	4.292	0.1395	16.58	1.011	14.47
	E	30	4.192	0.1395	15.82	0.964	13.80
	F	66	4.092	0.1395	15.07	0.919	13.15
	G	10	3.792	0.1395	12.95	0.789	11.29
	H	12	3.392	0.1395	10.38	0.632	9.04
Example 7	A	16	4.592	0.1722	15.39	1.429	16.56
	B	30	4.492	0.1722	14.73	1.367	15.85
	C	30	4.392	0.1722	14.09	1.307	15.15
	D	150	4.292	0.1722	13.46	1.248	14.47
	E	30	4.192	0.1722	12.84	1.191	13.80
	F	66	4.092	0.1722	12.24	1.135	13.15
	G	10	3.792	0.1722	10.52	0.975	11.29
	H	12	3.392	0.1722	8.44	0.781	9.04

[Table 4]

Table 4 Dimple specifications
	Type	Number	Diameter (mm)	Depth (mm)	Curvature (mm)	Volume (mm³)	Area (mm²)
Example 8	A	16	4.592	0.2048	12.97	1.700	16.56
	B	30	4.492	0.2048	12.42	1.627	15.85
	C	30	4.392	0.2048	11.88	1.556	15.15
	D	150	4.292	0.2048	11.35	1.486	14.47
	E	30	4.192	0.2048	10.83	1.418	13.80
	F	66	4.092	0.2048	10.32	1.351	13.15
	G	10	3.792	0.2048	8.88	1.161	11.29
	H	12	3.392	0.2048	7.12	0.930	9.04
Example 9	A	60	4.190	0.1601	13.79	1.106	13.79
	B	158	4.090	0.1601	13.14	1.054	13.14
	C	72	3.940	0.1601	12.20	0.978	12.19
	D	36	3.690	0.1601	10.71	0.858	10.69
	E	12	2.790	0.1601	6.16	0.492	6.11
Comparative Example 5	A	60	4.052	0.1716	12.05	1.109	12.90
	B	158	3.952	0.1716	11.46	1.055	12.27
	C	72	3.802	0.1716	10.62	0.977	11.35
	D	36	3.552	0.1716	9.28	0.853	9.91
	E	12	2.652	0.1716	5.21	0.477	5.52

[Table 5]

Table 5 Evaluation results
	Example 1	Example 2	Comparative Example 1	Example 3	Comparative Example 2
Plan view	FIG. 2	FIG. 2	FIG. 2	FIG. 5	FIG. 5
Front view	FIG. 3	FIG. 3	FIG. 3	FIG. 6	FIG. 6
Number	338	338	338	344	344
Total area [mm²]	4584.8	4584.8	4584.8	4868.5	4868.5
Occupation ratio X [%]	80	80	80	85	85
Total volume Y [mm³]	255	270	285	260	300
T1	-	-	-	-	-
T2	3.6	1.8	-0.1	0.3	-4.8
Db	0.268	0.263	0.243	0.264	0.243
Lb	0.243	0.238	0.217	0.240	0.216
Da	0.256	0.278	0.294	0.279	0.298
La	0.197	0.232	0.263	0.238	0.269
Lb/Db	0.91	0.90	0.89	0.91	0.89
Z	-13.7	-9.1	1.7	-8.3	2.0
Flight distance (yard)	268.8	272.8	282.5	271.5	282.3
Trajectory	G	F	E	G	E
Appearance	A	A	A	A	A

[Table 6]

Table 6 Evaluation results
	Comparative Example 3	Example 4	Example 5	Example 6
Plan view	FIG. 7	FIG. 7	FIG. 7	FIG. 7
Front view	FIG. 8	FIG. 8	FIG. 8	FIG. 8
Number	338	338	338	338
Total area [mm²]	4695.6	4695.6	4695.6	4695.6
Occupation ratio X [%]	82	82	82	82
Total volume Y [mm³]	375	390	405	450
T1	-1.7	0.1	1.8	7.0
T2	-	-	-	-
Db	0.238	0.239	0.239	0.244
Lb	0.192	0.190	0.184	0.178
Da	0.273	0.274	0.269	0.265
La	0.243	0.241	0.233	0.221
Lb/Db	0.81	0.79	0.77	0.73
Z	1.5	-0.5	-1.4	-6.8
Flight distance (yard)	282.7	281.1	279.4	273.2
Trajectory	C	B	B	A
Appearance	A	A	A	A

[Table 7]

Table 7 Evaluation results
	Comparative Example 4	Example 7	Example 8	Example 9	Comparative Example 5
Plan view	FIG. 5	FIG. 5	FIG. 5	FIG. 9	FIG. 11
Front view	FIG. 6	FIG. 6	FIG. 6	FIG. 10	FIG. 12
Number	344	344	344	338	338
Total area [mm²]	4868.5	4868.5	4868.5	4239.3	3952.3
Occupation ratio X [%]	85	85	85	74	69
Total volume Y [mm³]	340	420	500	340	340
T1	-7.4	1.9	11.1	-1.3	1.5
T2	-	-	-	-	-
Db	0.237	0.240	0.244	0.246	0.250
Lb	0.199	0.177	0.167	0.206	0.210
Da	0.278	0.266	0.264	0.281	0.283
La	0.248	0.224	0.208	0.246	0.245
Lb/Db	0.84	0.74	0.69	0.84	0.84
Z	3.0	-4.5	-11.5	-2.1	-4.7
Flight distance (yard)	284.3	276.1	269.0	278.9	276.3
Trajectory	D	B	A	D	D
Appearance	A	A	A	B	C

As shown in Tables 5 to 7, the flight distance of the golf ball of each Example is short. The golf ball of each Example can comply with new rules established by the R&A and the USGA. With the golf ball of each Example, the trajectory thereof exhibits no unnatural characteristics. The golf ball of each Example has excellent appearance. From the evaluation results, advantages of the golf ball are clear.

[Disclosure Items]

Each of the following items is a disclosure of a preferred embodiment.

[Item 1]

A golf ball having a plurality of dimples on a surface thereof, wherein

a ratio X of a total area of all the dimples to a surface area of a phantom sphere of the golf ball is 70% or more, and
a value Z calculated by the following mathematical formula is less than 0.0, $Z = - 536.36 * Db + 146.46 * Lb - 238.52 * Da + 225.22 * La + 111.21,$ where Db represents a drag coefficient under a condition where a Reynolds number is 1.20 * 10⁵ and a ratio (rω/V) is 0.131, Lb is a lift coefficient under a condition where the Reynolds number is 1.20 * 10⁵ and the ratio (rω/V) is 0.131, Da represents a drag coefficient under a condition where the Reynolds number is 0.83 * 10⁵ and the ratio (rω/V) is 0.179, La represents a lift coefficient under a condition where the Reynolds number is 0.83 * 10⁵ and the ratio (rω/V) is 0.179, and the ratio (rω/V) is calculated by dividing a product of a radius r (m) and a backspin rate ω (rad/s) of the golf ball by a speed V (m/s) of the golf ball.

[Item 2]

The golf ball according to Item 1, wherein a ratio (Lb/Db) of the lift coefficient Lb to the drag coefficient Db is 0.79 or less.

[Item 3]

The golf ball according to Item 2, wherein a total volume Y of all the dimples is 330 mm³ or more, and a value T1 calculated by the following mathematical formula is 0.0 or more, $T 1 = - 0.55 * X + 0.115 * Y + 0.3 .$

[Item 4]

The golf ball according to Item 1, wherein a ratio (Lb/Db) of the lift coefficient Lb to the drag coefficient Db is 0.90 or more.

[Item 5]

The golf ball according to Item 4, wherein a total volume Y of all the dimples is less than 330 mm³, and a value T2 calculated by the following mathematical formula is 0.0 or more, $T 2 = - 0.55 * X + 0.125 * Y + 79.5 .$
The above-described golf ball is suitable for, for example, playing golf on golf courses and practicing at driving ranges.

Claims

A golf ball comprising a plurality of dimples on a surface thereof, wherein a ratio X of a total area of all the dimples to a surface area of a phantom sphere of the golf ball is 70% or more, and
a value Z calculated by the following mathematical formula is less than 0.0, $Z = - 536.36 * Db + 146.46 * Lb - 238.52 * Da + 225.22 * La + 111.21,$ where Db represents a drag coefficient under a condition where a Reynolds number is 1.20 * 10⁵ and a ratio (rω/V) is 0.131, Lb is a lift coefficient under a condition where the Reynolds number is 1.20 * 10⁵ and the ratio (rω/V) is 0.131, Da represents a drag coefficient under a condition where the Reynolds number is 0.83 * 10⁵ and the ratio (rω/V) is 0.179, La represents a lift coefficient under a condition where the Reynolds number is 0.83 * 10⁵ and the ratio (rω/V) is 0.179, and the ratio (rω/V) is calculated by dividing a product of a radius r (m) and a backspin rate ω (rad/s) of the golf ball by a speed V (m/s) of the golf ball.
The golf ball according to claim 1, wherein a ratio (Lb/Db) of the lift coefficient Lb to the drag coefficient Db is 0.79 or less.
The golf ball according to claim 2, wherein a total volume Y of all the dimples is 330 mm³ or more, and a value T1 calculated by the following mathematical formula is 0.0 or more, $T 1 = - 0.55 * X + 0.115 * Y + 0.3 .$
The golf ball according to claim 1, wherein a ratio (Lb/Db) of the lift coefficient Lb to the drag coefficient Db is 0.90 or more.
The golf ball according to claim 4, wherein a total volume Y of all the dimples is less than 330 mm³, and a value T2 calculated by the following mathematical formula is 0.0 or more, $T 2 = - 0.55 * X - 0.125 * Y + 79.5 .$
The golf ball according to any one of claims 1 to 5, wherein a total number of the dimples is 250 or more and 450 or less.
The golf ball according to any one of claims 1 to 6, wherein a contour of each dimple is circular.