golf balls

Abstract

To provide a golf ball capable of improving a carry in a sure manner with a configuration in which bottom surfaces of dimples are curved convexly toward an outside direction of the golf ball.SOLUTION: A golf ball of the present invention includes a core and a cover positioned outside the core having a plurality of dimples 10 on a surface. A bottom surface 12 of the dimple includes a first convex part 13A curved convexly toward a ball outside direction and a second convex part 13B further curved convexly toward the ball outside direction in a planar region of the first convex region. A diameter of a circular border 16 of the first convex part and the second convex part is half or less of a diameter of the circular planar region 17 of the first convex part. Relationships between Shore D hardness H of a cover material and a depth d (unit; mm) in the central convex part of the bottom surface of the dimple satisfy a following formula 1; (H-78) / (-300)>d.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a golf ball. [Background technology]

[0002] It is well known that a golf ball is subjected to backspin when hit. If too much backspin is applied on a driver shot, the ball tends to fly up, so in order to increase the flight distance, it is generally necessary to reduce the amount of backspin. To reduce the amount of backspin, golf balls that can increase the friction with the face on driver shots have been proposed.

[0003] For example, Japanese Patent Application Laid-Open No. 2017-006555 describes a golf ball in which the shape of the multiple dimples formed on the surface of the golf ball is such that the bottom surfaces of the dimples are curved convexly outward from the golf ball, and in which the relationship between the amount of deflection H of the golf ball when a predetermined load is applied to the golf ball, the virtual plane area S when there are no dimples on the golf ball surface, and the pressurized area PS, which is the area of ​​the golf ball that comes into contact with the plane when a predetermined load is applied, satisfies a predetermined formula.

[0004] Furthermore, JP 2017-079905 A describes a golf ball having a core, a cover, and at least one intermediate layer therebetween, in which the value obtained by subtracting the surface hardness of the intermediate layer-coated sphere from the surface hardness of the ball and the value obtained by subtracting the core surface hardness from the surface hardness of the intermediate layer-coated sphere are each within a predetermined range, and in the core hardness distribution, the hardness at the core center, the hardness at a position 5 mm from the core center, the hardness at a position 10 mm from the core center, the hardness at a position 15 mm from the core center, and the hardness of the core surface are each within a predetermined range, the value obtained by subtracting the core center hardness from the core surface hardness is within a predetermined range, and the relationship V / H between the ball initial velocity V and the amount of deflection H of the golf ball when a predetermined load is applied to the golf ball is within a predetermined range.

[0005] Furthermore, JP 2017-086579 A describes a golf ball that includes a two-layer core consisting of an inner layer and an outer layer, a cover, at least one intermediate layer between them, and a coating layer on the surface of the cover, in which the core hardness distribution, i.e., the hardness Cc at the center of the inner core, the hardness C10 at a position 10 mm from the center of the inner core, the hardness Cs of the surface of the inner core, and the hardness Css of the surface of the outer core, satisfy two specified formulas, and further, the surface hardness of a sphere formed by covering the core with the intermediate layer is higher than the surface hardness of the ball. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-006555 [Patent Document 2] Japanese Patent Application Publication No. 2017-079905 [Patent Document 3] Japanese Patent Application Publication No. 2017-086579 Summary of the Invention [Problem to be solved by the invention]

[0007] While the above-mentioned Japanese Patent Publications 2017-006555, 2017-079905, and 2017-086579 all describe dimples having a convex curved bottom surface toward the outside of the golf ball, the present inventors have discovered that, because the convex shape of such dimples is unfavorable in terms of aerodynamic characteristics, even if the convex shape increases friction and reduces backspin, it may not necessarily contribute to improving actual flight distance. Furthermore, the frictional force of such convex-bottom dimples against a golf club is affected by the hardness of the cover material, but the above-mentioned publications make no specific mention of this.

[0008] Therefore, an object of the present invention is to provide a golf ball that can reliably improve flight distance by having the bottom surfaces of the dimples have a shape that is convexly curved toward the outside of the golf ball. [Means for solving the problem]

[0009] In order to achieve the above object, the present invention provides a golf ball including a core and a cover located outside the core and having a plurality of dimples on its surface, wherein the bottom surface of each dimple has a first convex portion that is curved convexly outward from the golf ball, and a flat area of ​​the first convex portion has a second convex portion that is further curved convexly outward from the golf ball, the diameter of the circular boundary between the first convex portion and the second convex portion is equal to or less than half the diameter of the flat area of ​​the first convex portion, and the depth d of the second convex portion of the bottom surface of each dimple is the vertical distance from a line S connecting both ends of the outer periphery of the dimple to the highest point of the second convex portion, and the relationship between the Shore D hardness H of the material of the cover and the depth d (unit: mm) is expressed by the following formula 1: (H-78) / (-300)>d (Formula 1) It satisfies the following.

[0010] The volume ratio VR of the dimples on the golf ball may be less than 0.75.

[0011] The cover material may have a Shore D hardness of 50-60.

[0012] The golf club may further include an intermediate layer between the core and the cover, and the material of the intermediate layer may have a Shore D hardness of 55 or greater.

[0013] The material of the mid layer may have a higher Shore D hardness than the material of the cover.

[0014] The cover may further include a coating layer located on the outside thereof, the coating layer may contain matte particles, the average surface roughness Ra of the coating layer may be 0.5 to 1.0, and the volume fraction VR of the dimples may be less than 0.70.

[0015] The dimples having the first and second convex portions may account for 50% or more of the total number of dimples on the surface of the cover.

[0016] The second convex portion may have a spherical crown shape with a radius of curvature R of 5 to 20 mm.

[0017] In addition to the dimples having a spherical crown-shaped second convex portion with a radius of curvature R of 5 to 20 mm, dimples having a spherical crown-shaped second convex portion with a radius of curvature R of less than 5 mm may be arranged, and the dimples having a spherical crown-shaped second convex portion with a radius of curvature R of less than 5 mm may account for 1 to 10% of the total number of dimples on the surface of the cover. [Effects of the Invention]

[0018] According to the present invention, the bottom surface of the dimple has a portion that is convexly curved in two stages toward the outside of the golf ball, and the diameter of the circular boundary between the first convex portion and the second convex portion is less than half the diameter of the flat area of ​​the first convex portion.By specifying the relationship between the depth d of the central convex portion at the bottom of the dimple and the material hardness of the cover as shown in Equation 1 above, it is possible to increase the contact area with the club face on full shots (shots with a driver to a middle iron), reduce the amount of backspin, and achieve improved distance. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a perspective view showing an embodiment of a golf ball according to the present invention. [Figure 2] FIG. 2 is an enlarged cross-sectional view of one dimple on the golf ball shown in FIG. 1. [Figure 3]FIG. 1 is a perspective view showing another embodiment of a golf ball according to the present invention. [Figure 4] FIG. 4 is an enlarged cross-sectional view of one dimple on the golf ball shown in FIG. 3. [Figure 5] FIG. 10 is a perspective view showing an example of a golf ball of Comparative Example 4. [Figure 6] FIG. 6 is an enlarged cross-sectional view of one dimple on the golf ball shown in FIG. 5. [Figure 7] 1 is a photograph showing dimples on the golf ball of Example 1. [Figure 8] 1 is a photograph showing dimples on the golf ball of Example 4. [Figure 9] 1 is a photograph showing dimples on a golf ball of Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0020] An embodiment of a golf ball according to the present invention will now be described with reference to the accompanying drawings, but the present invention is not limited thereto.

[0021] As shown in Figure 1, one embodiment of a golf ball according to the present invention includes a core (not shown) and a cover located outside the core and having a plurality of dimples 10 on its surface. The portions of the surface of golf ball 1 between the plurality of dimples 10 are generally referred to as land portions 20. Land portions 20 constitute the spherical surface of golf ball 1, and therefore land portions 20 have a curved surface.

[0022] The planar shape of the dimples 10 formed on the surface of the golf ball 1 (i.e., the shape of the outer periphery 11 of the dimple 10 or the boundary between the dimple 10 and the land portion 20 when viewed from directly above the dimple) may be circular, polygonal (e.g., regular hexagonal), non-circular, or other shapes. In this embodiment, the planar shape is polygonal (approximately regular hexagonal). The diameter of a circular dimple, and the diameter of the circumscribing circle of a polygonal dimple, are preferably within the range of 2 to 5 mm. It is not necessary for all dimples formed on the surface of the golf ball to have the same diameter; they may vary as long as they are within the range of 2 to 5 mm. For example, it is preferable to arrange at least three types of dimples of different sizes, which allows the dimples to be arranged evenly and without gaps on the spherical surface of the golf ball.

[0023] In the present embodiment, a portion of the bottom surface of dimple 10 has a shape that is convexly curved in two stages toward the outside of the ball. A cross-sectional view of dimple 10 taken along its diameter is shown in Figure 2. As shown in Figure 2, dimple 10 has a curved bottom surface 12 that connects from one end of its outer periphery 11 to the other. This bottom surface 12 has, in its central region, a portion that is convexly curved in two stages toward the outside of the ball, i.e., central convex portion 13, and, in its outer circumferential annular region, a portion that is concavely curved toward the outside of the ball.

[0024] This central convex portion 13 has a first convex portion 13A that is initially convexly curved from the deepest part 18 of the dimple toward the outside of the golf ball, and a second convex portion 13B that is further convexly curved in the planar region of this first convex portion 13A.

[0025] The planar area of ​​the first convex portion 13A has a circular planar shape, and its diameter (i.e., the distance W1 between both ends 17 of the planar area shown in Figure 2) is preferably in the range of 35 to 65, more preferably in the range of 40 to 60, and even more preferably in the range of 45 to 55, where 100 is the distance from the outer peripheral edge 12 of the dimple to the center point 15.

[0026] The boundary 16 between the first convex portion 13A and the second convex portion 13B has a circular planar shape, and its diameter (i.e., the distance W2 between both ends of the second convex portion 13B shown in FIG. 2) is equal to or less than half the diameter of the flat region of the first convex portion 13A (i.e., the distance W1). There is no particular lower limit to the diameter of this boundary 16 (i.e., the distance W2) as long as it is large enough to form the second convex portion 13B, but for example, if the diameter of the flat region (i.e., the distance W1) is 100, it is preferable that it be 25 or more.

[0027] The depth d of the central convex portion 13 of the dimple 10 is the vertical distance from the line S connecting both ends of the outer periphery 11 of the dimple to the highest point (center point 15) of the central convex portion 13. The relationship between the depth d (unit: mm) of the central convex portion 13 and the Shore D hardness H of the material forming the cover, which will be described later, is expressed by the following formula 1: (H-78) / (-300)>d (Formula 1) The depth d must satisfy the above condition. In other words, if the depth d is greater than the value of (H-78) / (-300), the contact area with the face of the golf club is insufficient, and frictional force cannot be improved. The value of (H-78) / (-300)-d is preferably 0.002 or greater, and more preferably 0.005 or greater. By setting the depth d to such a value, excellent frictional force can be reliably obtained depending on the hardness of the cover material. There is no particular upper limit to the value of (H-78) / (-300)-d, but it is preferably 0.030 or less, and more preferably 0.025 or less.

[0028] The depth is greatest at deepest points 18 located on both sides of central convex portion 13, and the bottom surface from those points to land portion 20 is curved convexly outward from the ball. The position of deepest point 18 on a plane is preferably in the range of 20 to 45, more preferably 25 to 40, and even more preferably 30 to 35, where 100 is the distance from outer circumferential edge 11 of the dimple to center point 15.

[0029] The depth D of the dimple 10 varies depending on the depth d of the central convex portion 13, and is preferably at least 0.025 mm deeper, and more preferably at least 0.030 mm deeper, than the depth d of the central convex portion 13. There is no particular upper limit to the depth D of the dimple 10, but it is preferably at most 0.200 mm deeper, and more preferably at most 0.150 mm deeper, than the depth d of the central convex portion 13.

[0030] In this embodiment, the second convex portion 13B has a spherical crown shape (the surface shape of a portion obtained by cutting a sphere with a single plane). The spherical crown shape preferably has a radius of curvature R of 5 to 20 mm. By making the spherical crown shape have a radius of curvature R in this range, the entire central convex portion 13 can be brought into more sufficient contact with the club face during a full shot, further reducing the amount of backspin on the golf ball and further improving the flight distance.

[0031] It is not necessary for all dimples formed on the surface of a golf ball to have the central convex portion 13 curved in a two-stage convex shape described above. It is preferable that 50% or more of the total number of dimples have this central convex portion, more preferably 70% or more, even more preferably 80% or more, and most preferably 90% or more. Of course, it is also possible for all dimples to have this central convex portion. It is preferable that dimples with such a two-stage convexly curved central convex portion be evenly distributed over the entire golf ball in order to achieve excellent aerodynamic isotropy and air resistance.

[0032] It is not necessary for all dimples with a central convex portion having a spherical crown shape to have the same radius of curvature R; they may have different radii of curvature R within a range of 5 to 20 mm. For example, the golf ball may have a configuration in which, of the total number of dimples formed on its surface, 1 to 10% of dimples have a radius of curvature R of 5 mm or more but less than 10 mm, 20 to 40% of dimples have a radius of curvature R of 10 mm or more but less than 15 mm, and 30 to 50% of dimples have a radius of curvature R of 15 mm or more but less than 20 mm. Furthermore, dimples with a central convex portion having a smaller radius of curvature R than the above, i.e., less than 5 mm, may be arranged together with the above-mentioned spherical crown-shaped dimples having a radius of curvature R of 5 to 20 mm. For example, the golf ball may have a configuration in which 1 to 10% of the total number of dimples formed on its surface have a radius of curvature R of less than 5 mm. The lower limit of this radius of curvature R is not particularly limited, but may be, for example, 1 mm or more.

[0033] The upper limit of the total number of dimples is not limited to, but is preferably not more than 500, and more preferably not more than 450. The lower limit of the total number of dimples is not limited to, but is preferably not less than 250, and more preferably not less than 300.

[0034] The dimple volume fraction VR (i.e., the ratio of the total volume of dimples formed below the plane surrounded by the dimple edges to the volume of a virtual sphere of a golf ball hypothetical of a golf ball without dimples) is preferably less than 0.75%. Since dimples with a central convex shape, which is curved convexly in two stages as described above, are disadvantageous in terms of aerodynamic characteristics, keeping the dimple volume fraction VR less than 0.75% improves the trajectory of the golf ball, which, together with the reduction in backspin described above, can further improve the flight distance. The dimple volume fraction VR is preferably 0.73% or less, and more preferably 0.70% or less. There is no particular lower limit to the dimple volume fraction VR, but it is preferably 0.65% or more, and more preferably 0.68% or more.

[0035] The dimple surface coverage SR (i.e., the ratio of the total area of ​​the dimples to the total surface area of ​​the hypothetical spherical surface of the golf ball assuming no dimples) is preferably 70% or more, more preferably 75% or more, and even more preferably 80% or more. There is no particular upper limit to the dimple surface coverage SR, but it is preferably 99% or less.

[0036] The cover may be made of, but is not limited to, an ionomer resin, a polyurethane-based thermoplastic elastomer, a thermosetting polyurethane, or a mixture thereof. In addition to the above-mentioned main components, the cover may also contain other thermoplastic elastomers, polyisocyanate compounds, fatty acids or their derivatives, basic inorganic metal compounds, fillers, and the like.

[0037] As described above, the Shore D hardness H of the material forming the cover satisfies Equation 1. Therefore, although it depends on the depth d of the central convex portion of the dimple, the Shore D hardness H of the material forming the cover is preferably 50 or greater, and more preferably 53 or greater. The Shore D hardness H of the material forming the cover is preferably 65 or less, more preferably 62 or less, and even more preferably 60 or less. By keeping the Shore D hardness H within these numerical ranges, the appropriate amount of spin can be achieved on shots from driver to middle iron.

[0038] Although not limited to a lower limit, the cover thickness is preferably 0.2 mm or more, and more preferably 0.4 mm or more, and the cover thickness is preferably 4 mm or less, more preferably 3 mm or less, and even more preferably 2 mm or less.

[0039] The core can be formed from a rubber composition containing rubber as a main component. The main component (base rubber) can be a wide variety of synthetic and natural rubbers, including, but not limited to, polybutadiene rubber (BR), styrene-butadiene rubber (SBR), natural rubber (NR), polyisoprene rubber (IR), polyurethane rubber (PU), butyl rubber (IIR), vinyl polybutadiene rubber (VBR), ethylene propylene rubber (EPDM), nitrile rubber (NBR), and silicone rubber. Examples of polybutadiene rubber (BR) that can be used include 1,2-polybutadiene and cis-1,4-polybutadiene.

[0040] In addition to the base rubber, the core may optionally contain, for example, a co-crosslinking agent, a crosslinking initiator, a filler, an antioxidant, an isomerizing agent, a mastication accelerator, sulfur, and an organic sulfur compound. Furthermore, instead of rubber, a resin may be used as the main component, such as a thermoplastic elastomer, an ionomer resin, or a mixture thereof.

[0041] The co-crosslinking agent is preferably, but not limited to, an α,β-unsaturated carboxylic acid or a metal salt thereof. Examples of the α,β-unsaturated carboxylic acid or a metal salt thereof include acrylic acid, methacrylic acid, and their zinc salts, magnesium salts, and calcium salts. The amount of the co-crosslinking agent is preferably, but not limited to, about 5 parts by weight or more, and more preferably about 10 parts by weight or more, per 100 parts by weight of the base rubber. The amount of the co-crosslinking agent is preferably about 70 parts by weight or less, and more preferably about 50 parts by weight or less.

[0042] The crosslinking initiator is preferably, but not limited to, an organic peroxide, such as dicumyl peroxide, t-butyl peroxybenzoate, di-t-butyl peroxide, or 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane. The amount of crosslinking initiator added is, but not limited to, preferably about 0.10 parts by weight or more, more preferably about 0.15 parts by weight or more, and even more preferably about 0.30 parts by weight or more, based on 100 parts by weight of the base rubber. The amount of crosslinking initiator added is preferably about 8 parts by weight or less, and more preferably about 6 parts by weight or less.

[0043] Examples of fillers that can be used include, but are not limited to, silver, gold, cobalt, chromium, copper, iron, germanium, manganese, molybdenum, nickel, lead, platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zinc oxide, and manganese oxide. The filler is preferably in powder form. The filler content is, but is not limited to, preferably about 1 part by weight or more, more preferably about 2 parts by weight or more, and even more preferably about 3 parts by weight or more, per 100 parts by weight of the base rubber. The filler content is preferably about 100 parts by weight or less, more preferably about 80 parts by weight or less, and even more preferably about 70 parts by weight or less.

[0044] The antioxidant is not limited to, but may be a commercially available product such as Nocrac NS-6 (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.). The amount of antioxidant added is not limited to, but is preferably at least about 0.1 parts by weight, more preferably at least about 0.15 parts by weight, per 100 parts by weight of the base rubber. The amount of antioxidant added is preferably no more than about 1.0 part by weight, more preferably no more than about 0.7 parts by weight.

[0045] The addition of an organic sulfur compound (a stimulant) can improve the resilience of the core 40. The organic sulfur compound is selected from thiophenols, thiocarboxylic acids, and their metal salts. Examples of thiophenols and thiocarboxylic acids include pentachlorothiophenol, 4-t-butyl-o-thiophenol, 4-t-butylthiophenol, and 2-benzamidothiophenol, as well as thiocarboxylic acids such as thiobenzoic acid. Preferred metal salts include zinc salts. The amount of the organic sulfur compound is not limited to, but is preferably at least about 0.5 parts by weight, and more preferably at least about 1 part by weight, per 100 parts by weight of the base rubber. The amount of the organic sulfur compound is preferably at most about 3 parts by weight, and more preferably at most about 2 parts by weight.

[0046] The upper limit of the Shore D hardness of the material forming the core is preferably not more than 60, more preferably not more than 50, and even more preferably not more than 40. On the other hand, the lower limit of the Shore D hardness of the material forming the core is not limited to this, but is preferably not less than 20, and more preferably not less than 30. By setting the hardness of the core material within this range, the feel of the golf ball can be improved.

[0047] The lower limit of the core thickness should be 4.5 mm or more, and more preferably 10 mm or more, to impart a predetermined repulsion force to the golf ball. On the other hand, the upper limit of the core thickness is not particularly limited, but is preferably 25 mm or less, and more preferably 20 mm or less. The core is not limited to a single layer, and may be, for example, a multi-layer core. In this case, it is preferable that the hardness of each layer of the core increases from the inside to the outside of the golf ball.

[0048] An intermediate layer (not shown) may be optionally provided between the core and the cover, which allows for an appropriate amount of spin on shots from driver to middle iron.

[0049] The material for the intermediate layer is preferably, but not limited to, the following heated mixture: By using this material for the intermediate layer, it is possible to achieve low spin at the time of impact and obtain a long flight distance. (a) an olefin-unsaturated carboxylic acid binary random copolymer and / or a metal ion-neutralized product of an olefin-unsaturated carboxylic acid binary random copolymer; (b) a metal ion-neutralized product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer and / or an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer; a base resin blended in a weight ratio of 100:0 to 0:100; (e) a non-ionomer thermoplastic elastomer blended with the base resin in a weight ratio of 100:0 to 50:50; For 100 parts by weight of the resin component including the base resin and component (e), (c) 5 to 150 parts by weight of a fatty acid and / or a derivative thereof having a molecular weight of 228 to 1500; (d) 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing unneutralized acid groups in the base resin and component (c).

[0050] The term "main material" means a material that accounts for 50% by weight or more, preferably 60% by weight or more, and more preferably 70% by weight or more of the total weight of the intermediate layer.

[0051] The Shore D hardness of the material forming the intermediate layer is preferably 55 or higher, and more preferably 57 or higher. The Shore D hardness of the material forming the intermediate layer is preferably higher than the Shore D hardness of the material forming the cover. This allows for an appropriate amount of spin on shots from driver to middle iron. There is no particular upper limit for the Shore D hardness of the material forming the intermediate layer, but it is preferably 65 or lower, and more preferably 63 or lower.

[0052] The thickness of the intermediate layer 20 is preferably 0.5 mm or more, more preferably 1 mm or more, but is not limited thereto. The thickness of the intermediate layer 20 is preferably 10 mm or less, more preferably 5 mm or less, and even more preferably 3 mm or less.

[0053] A coating layer (also called a painted layer) (not shown) may be optionally provided on the surface of the cover. The coating layer is formed from a paint composition. The paint composition may also contain matte particles. The paint composition is not particularly limited, but it is preferable to use, for example, a urethane-based paint. Because it is necessary to be able to withstand the harsh usage environment of the golf ball, it is more preferable to use a two-component curing urethane paint, in particular a non-yellowing urethane paint.

[0054] In the case of two-component curing urethane paints, it is preferable to use various polyols such as saturated polyester polyols, acrylic polyols, polycarbonate polyols, etc. as the base resin. As the isocyanate curing agent, it is preferable to use non-yellowing polyisocyanates such as adducts, biuret forms, isocyanurates, etc. of hexamethylene diisocyanate, isophorone diisocyanate, hydrogenated xylylene diisocyanate, or mixtures thereof.

[0055] Examples of matte particles include silica-based, melamine-based, and acrylic-based particles. Specific examples include silica, polymethyl methacrylate, polybutyl methacrylate, polystyrene, and polybutyl acrylate. While either organic or inorganic particles may be used, silica is particularly preferred.

[0056] If such matte particles were included in the coating layer, aerodynamic performance would be adversely affected, but in this embodiment, the dimple volume fraction VR is less than 0.75, which improves the trajectory and maintains the flight distance. When matte particles are included in the coating layer, the dimple volume fraction VR may be less than 0.70. This further improves the trajectory and flight distance.

[0057] The specific surface area of ​​the matte particles should be 200 to 400 m in BET specific surface area from the viewpoint of extinction and coating properties. 2 / g, and 250 to 350m 2 / g is more preferable. From the viewpoints of spin performance and extinction, the average primary particle diameter of the matte particles is preferably 1.0 to 3.0 μm, and more preferably 2.0 to 2.8 μm. If the average primary particle diameter exceeds 3.0 μm, the ball surface becomes rough, which may adversely affect the spin performance of the golf ball and result in a decrease in spin performance. On the other hand, if the average primary particle diameter is too small, the extinction effect may be reduced.

[0058] The amount of matte particles to be blended is preferably 5 to 10 parts by mass per 100 parts by mass of the base material (total amount of resin component and solvent) of the coating composition for the coating layer. If this amount is too high, the viscosity of the coating composition tends to increase, making application workability difficult, while if it is too low, the quenching effect may be reduced. The average surface roughness Ra of the coating layer is preferably 0.5 to 1.0 from the viewpoint of achieving both the amount of spin of the ball on approach and quenching performance. The surface roughness Ra of this coating film refers to the arithmetic mean roughness according to JIS B0601 (1994).

[0059] Although the embodiment in which the second convex portion 13B of the central convex portion 13 of the dimple 10 has a spherical crown shape as shown in Figure 2 has been described, the present invention is not limited to this embodiment, and for example, dimples having shapes shown in another embodiment shown in Figures 3 and 4 may also be used. This another embodiment will be described below. Note that the same components as those in Figures 1 and 2 are designated by the same reference numerals, and detailed description thereof will be omitted.

[0060] As shown in Figure 3, multiple dimples 10A are formed on the surface of golf ball 1A, and as shown in Figure 4, this dimple 10A has a central convex portion 14 in the center of its bottom surface that is curved in two stages convexly toward the outside of the ball, but the apex is not spherical-crown shaped as in Figure 2, but is flat.

[0061] More specifically, the dimple has a first convex portion 14A that initially curves convexly from the deepest part 18 of the dimple toward the outside of the golf ball, and a second convex portion 14B that curves further convexly in the flat area of ​​the first convex portion 14A, but the apex of the central area of ​​the second convex portion is flat.

[0062] The boundary 16 between the first convex portion 14A and the flat-surfaced second convex portion 14B has a circular planar shape, and its diameter (i.e., the distance W2 between both ends of the second convex portion 14B shown in FIG. 4) is equal to or less than half the diameter of the flat region of the first convex portion 14A (i.e., the distance W1), as in the embodiment of FIGS. 1 and 2. The lower limit of the diameter of this boundary 16 (i.e., the distance W2) is not particularly limited as long as it is large enough to form the second convex portion 14B with a flat apex, but for example, if the diameter of the flat region (i.e., the distance W1) is 100, it is preferable that the diameter be 25 or more.

[0063] The depth d of the flat region of the second convex portion 14B is constant. As described above, the depth d of the second convex portion 14B is measured based on the line S connecting both ends of the outer periphery 11 of the dimple. As in the embodiment shown in FIGS. 1 and 2, the relationship between the depth d of the second convex portion 14B and the Shore D hardness H of the cover material satisfies the above-mentioned formula 1. The value of (H-78) / (-300)-d is preferably 0.002 or greater, more preferably 0.005 or greater. By setting the depth d to such a value, excellent frictional force can be reliably obtained according to the hardness of the cover material. There is no particular upper limit to the value of (H-78) / (-300)-d, but it is preferably 0.010 or less, more preferably 0.008 or less.

[0064] In this way, even if the shape of the bottom surface of dimple 10A is made to be a two-stage convex curve toward the outside of the ball and the apex shape of the second convex portion 14B of the second stage is made flat, the entire central convex portion 14 will be in sufficient contact with the club face during a full shot, just as if the shape of the second convex portion were a spherical crown, thereby reducing the amount of backspin on the golf ball and achieving improved distance. [Example]

[0065] A golf ball was produced with the configuration shown in Example 1 in Table 1. The core formulation is shown in Table 3, the intermediate layer and cover formulations in Table 4, and the paint layer formulation in Table 5. All units are in parts by weight. The dimples were arranged in the pattern shown in Figure 1. As shown in Figure 2, the shape of the bottom of the dimple was such that, in the central region, it was curved in a two-stage convex shape toward the outside of the ball, with the second convex portion having a spherical crown shape with a radius of curvature R in the range of 5 to 20 mm. More detailed specifications of the dimples are shown in Table 6.

[0066] [Table 1]

[0067] [Table 2]

[0068] [Table 3]

[0069] In Table 3, the polybutadiene is manufactured by JSR Corporation under the trade name "BR01," and the zinc acrylate is manufactured by Nippon Shokubai Co., Ltd. The organic peroxide A is manufactured by NOF Corporation under the trade name "Percumyl D," and the organic peroxide B is manufactured by NOF Corporation under the trade name "Peroxa C-40." The antioxidant is 2,2-methylenebis(4-methyl-6-butylphenol), manufactured by Ouchi Shinko Chemical Industry Co., Ltd. under the trade name "Nocrac NS-6." The zinc oxide is manufactured by Sakai Chemical Industry Co., Ltd. under the trade name "Zinc Oxide Type 3." The pentachlorothiophenol zinc salt is manufactured by Zhejiang Cho & Fu Chemical Co., Ltd.

[0070] [Table 4]

[0071] In Table 4, "HPF1000" is an ionomer resin manufactured by The Dow Chemical Company. "Himilan 1605" is an ionomer resin manufactured by Dow Mitsui Polychemicals. "AM7329" is an ionomer resin manufactured by Dow Mitsui Polychemicals. "Surlyn 9320" is an ionomer resin manufactured by The Dow Chemical Company.

[0072] [Table 5]

[0073] The "polyol" of the main component in Table 5 was a polyester polyol synthesized by the following method. First, 140 parts by mass of trimethylolpropane, 95 parts by mass of ethylene glycol, 157 parts by mass of adipic acid, and 58 parts by mass of 1,4-cyclohexanedimethanol were charged into a reaction apparatus equipped with a reflux condenser, a dropping funnel, a gas inlet tube, and a thermometer, and the temperature was raised to 200-240°C with stirring, and the mixture was heated (reacted) for 5 hours. Thereafter, a polyester polyol with an acid value of 4, a hydroxyl value of 170, and a weight-average molecular weight (Mw) of 28,000 was obtained. The "matte particles" used were "Finesil X-35" manufactured by Maruo Calcium Co., Ltd.

[0074] The curing agent "isocyanate" was a nurate (isocyanurate) of hexamethylene diisocyanate (HMDI), manufactured by Asahi Kasei Corporation under the trade name Duranate TPA-100 (NCO content 23.1%, non-volatile content 100%). Butyl acetate was used as the solvent for both the base agent and the curing agent. The two-component curing urethane paint with the above formulation was applied with an air spray gun to the surface of the cover on which the dimples had been formed, forming a paint layer.

[0075] [Table 6]

[0076] The depth d of the central convex portion of the A to E type dimples in Table 6 is all the same as the depth d value shown in Table 1, but the diameter of the dimple (the diameter of the circumscribed circle of the approximately hexagon) varies depending on the radius of curvature R of the convex portion. The size of the dimples is smallest for Type A, and increases in order from Type B to Type E. Type F dimples are conventional dimples formed using a support pin.

[0077] Tests were conducted to evaluate the spin rate and flight distance of the golf ball of Example 1 having such a configuration. First, a driver club ("TourB XD-5" (W#1) (loft angle 9.5°) manufactured by Bridgestone Sports Co., Ltd.) was attached to a golf hitting robot, and the sample golf ball was hit at a head speed of 45 m / s to measure the backspin rate and flight distance. Also, a middle iron club ("TourB X-CB" (I#6) manufactured by Bridgestone Sports Co., Ltd.) was attached to the golf hitting robot, and the sample golf ball was hit at a head speed of 42 m / s to measure the backspin rate. The results are shown in Table 1.

[0078] For comparison, golf balls were produced in Comparative Examples 2 and 3 shown in Table 2, which had the same configuration as Example 1, except that the cover material hardness, dimple volume fraction VR, and central convex depth d of the dimples were appropriately changed so as not to satisfy the condition of Formula 1. Tests were conducted to evaluate the spin rate and flight distance in the same manner as in Example 1. The results are shown in Table 2. The dimples in Comparative Examples 2 and 3 also had the same specifications as in Table 6, and in each example, the diameter of the A to F types of dimples was unchanged, but the depth d of the central convex depth was changed (this also applies to Examples 2 to 8 and Comparative Example 1, which will be described later).

[0079] As shown in Tables 1 and 2, the golf ball of Example 1, which satisfied the condition (H-78) / (-300)>d in Formula 1, had less backspin on driver shots and improved flight distance compared to Comparative Examples 2 and 3, which did not satisfy the condition of Formula 1. Furthermore, the amount of backspin on middle iron shots was also less in Example 1 than in Comparative Examples 2 and 3. Therefore, it is believed that the flight distance with middle irons can also be improved.

[0080] Golf balls of Examples 2 and 3 shown in Table 1 were produced with the same configuration as Example 1, except that the cover material hardness was the same as that of Comparative Examples 2 and 3, and the dimple volume fraction VR and the depth d of the central convex portion of the dimple were changed to satisfy the condition of Formula 1. Tests to evaluate spin rate and flight distance were conducted in the same manner as in Example 1. The results are shown in Table 1. As shown in Tables 1 and 2, Examples 2 and 3 exhibited less backspin on driver shots and improved flight distance compared to Comparative Examples 2 and 3, which did not satisfy the condition of Formula 1. Furthermore, Examples 2 and 3 also exhibited less backspin on middle iron shots than Comparative Examples 2 and 3. Therefore, it is believed that the flight distance with middle irons can also be improved.

[0081] Table 2 shows a golf ball of Comparative Example 1, which has the same configuration as Example 1, except that the cover material hardness is the same as that of Example 1, and the dimple volume fraction VR and the depth d of the central convex portion of the dimple are changed to result in a configuration that does not satisfy the condition of Formula 1. In a simulation that takes the above test results into consideration, as shown in Table 2, in Comparative Example 1, the depth d of the convex portion is too deep, which reduces the contact area with the club, resulting in more backspin on driver shots than in Example 1 and reduced flight distance. The amount of backspin on middle iron shots in Comparative Example 1 is also greater than in Example 1, which is thought to result in reduced flight distance.

[0082] Furthermore, golf balls of Example 4 were produced with the same configuration as Example 1, except that the depth d of the central convex portion of the dimple was maintained the same as in Example 1, and the shape of the second convex portion of the dimple was made flat rather than spherical-capped, as shown in Figures 3 and 4. Tests to evaluate the spin rate and flight distance were conducted in the same manner as in Example 1. The results are shown in Table 1. Further detailed dimple specifications are shown in Table 7.

[0083] [Table 7]

[0084] The G-K type dimples in Table 7 correspond to the A-E type dimples in Table 6, respectively, and have the same dimple diameter, the same diameter of the flat area of ​​the first convex portion, and the same diameter of the circular boundary of the first and second convex portions. The L type dimple is a conventional dimple formed using a support pin and is identical to the F type dimple in Table 6.

[0085] As shown in Table 1, the golf ball of Example 4, in which the surface of the second-stage convex portion was flat, had a slightly higher amount of backspin on driver shots and a slightly shorter flight distance than Example 1, in which the second-stage convex portion had a spherical crown shape, but was able to improve flight distance compared to Comparative Example 1. Furthermore, the amount of backspin with a middle iron was also slightly higher in Example 4 than in Example 1, but less than in Comparative Example 1, which is thought to enable an improvement in flight distance with a middle iron.

[0086] In contrast, a golf ball of Comparative Example 5 was produced with the same configuration as Example 1, except that the depth d of the central convex portion of the dimple was maintained the same as in Example 1, but the bottom of the dimple was curved convexly in a single step at its center, and the surface of this central convex portion 35 was flat, as shown in Figures 5 and 6. More specifically, the bottom surface 34 connecting one end of the outer periphery 32 of the dimple 30 is formed in its central region with a central convex portion 35 that curves convexly in a single step toward the outside of the ball, and the surface is flat. The distance W between both ends 37 of this flat portion is equal to the distance W1 between both ends 17 of the flat regions of the first convex portions 13A and 14A shown in Figures 2 and 4. Furthermore, the depth d of the flat region of the central convex portion 35 is also measured relative to the line S connecting both ends of the outer periphery 32 of the dimple, as in Figures 2 and 4. The regions on both sides of the central convex portion 35 are curved so that the depth D is greatest at the deepest point 38. More detailed specifications of the dimples are shown in Table 8.

[0087] [Table 8]

[0088] The MQ to Q type dimples in Table 8 correspond to the A to E type dimples in Table 6, respectively, and have similar dimple diameters. The diameters of the flat areas of the convex portions of the MQ to Q type dimples in Table 8 are also the same as the diameters of the flat areas of the first convex portions of the A to E type dimples in Table 6, respectively. The R type dimples are conventional dimples formed using a support pin, and are identical to the F type dimples in Table 6. Tests were then conducted to evaluate the spin rate and flight distance in the same manner as in Example 1. The results are shown in Table 2.

[0089] FIG. 7 shows a photograph of the dimples formed when the golf ball of Example 1 was shot at a speed of 35 m / s and collided with a transparent plate, and FIGS. 8 and 9 show photographs of the dimples formed when the golf balls of Example 4 and Comparative Example 4 were collided with transparent plates under similar conditions.

[0090] As shown in Figures 7 and 8, in Examples 1 and 4, where the convex portion at the center of the dimple is two-tiered, almost the entire convex portion is in contact with the transparent plate, whereas in Comparative Example 4, where the convex portion at the center of the dimple is one-tiered and its surface is flat, as shown in Figure 9, the center of the convex portion does not sufficiently contact the transparent plate. Therefore, as shown in Table 2, although Comparative Example 4 satisfies Equation 1, which represents the relationship between the cover material hardness and the depth d of the convex portion, the contact area with the club face on full shots is smaller than in Examples 1 and 4, resulting in a greater amount of backspin on driver shots and a shorter flight distance than in Examples 1 and 4. Furthermore, the amount of backspin on middle iron shots in Comparative Example 4 is also greater than in Examples 1 and 4, which is thought to be why the flight distance is shorter.

[0091] Furthermore, golf balls of Example 5 were fabricated with the same configuration as Example 1, except that the dimple volume fraction VR and the depth d of the central convex portion of the dimple were changed, and tests were conducted to evaluate the spin rate and flight distance in the same manner as in Example 1. The results are shown in Table 1.

[0092] Example 5, which had a dimple volume fraction VR of 0.78, had slightly more backspin on driver shots and shorter flight distance than Example 1, which had a dimple volume fraction VR of 0.72. It is also believed that Example 5 had slightly more backspin on driver shots and shorter flight distance than Example 1. In other words, it can be seen that by setting the dimple volume fraction VR to less than 0.75, it is possible to further improve flight distance on both driver shots and middle irons.

[0093] Furthermore, as shown in Table 9, golf balls of Reference Examples 1 and 2 were produced with a dimple configuration with a single-stage convex portion and a flat surface, similar to Comparative Example 4, except that matte particles were blended into the paint layer, the conditions of Formula 1 were not satisfied, and the dimple volume fraction VR was set to 0.68 and 0.75, respectively. Tests were then conducted to evaluate the spin rate and flight distance in the same manner as in Example 1.

[0094] [Table 9]

[0095] Reference Examples 1 and 2, which contained matte particles, both had slightly more backspin on driver shots than Comparative Example 4, but Reference Example 1, which had a low volume occupancy rate VR of 0.68, improved the flight distance, while Reference Example 2, which had a high volume occupancy rate VR of 0.75, reduced the flight distance. Furthermore, the amount of backspin on middle irons was also roughly the same for Reference Examples 1 and 2, but Reference Example 1, which had a low volume occupancy rate VR of 0.68, improved the flight distance, while Reference Example 2, which had a high volume occupancy rate VR of 0.75, reduced the flight distance.

[0096] Table 1 shows golf balls of Examples 4 and 5, which have the same configuration as Example 1, except that matte particles were blended into the paint layer and the volume fraction VR of the dimples and the depth d of the central convex portion of the dimples were appropriately changed. Table 1 also shows golf balls of Examples 6 and 7. In a simulation that took the above test results into account, as shown in Table 1, Examples 6 and 7 had roughly the same amount of backspin on driver shots. However, Example 6, with its volume fraction VR of 0.68, achieved a greater improvement in flight distance than Example 7, which had the same volume fraction VR of 0.72 as Example 1. Furthermore, Examples 6 and 7 also had roughly the same amount of backspin on middle irons. Therefore, it is believed that Example 6 also achieved a better improvement in flight distance on middle irons than Example 7.

[0097] Table 1 also shows a golf ball of Example 8, which has the same configuration as Example 1, except that the material hardness of the mid layer is lower than that of the cover. In a simulation that took the above test results into consideration, as shown in Table 1, Example 8 had the same amount of backspin on driver shots as Example 1, and was able to maintain distance, but the amount of backspin on middle iron shots was higher. Therefore, it is thought that the improvement in distance on middle iron shots was inferior. [Explanation of symbols]

[0098] 1, 3 golf balls 10, 30 dimples 11, 31 outer edge 13, 14 Central convex part 13A, 14A First convex portion 13B, 14B Second convex portion 16 Boundary between the first convex portion and the second convex portion 17 End of the flat area of ​​the first convex portion 20, 40 Rikubu

Claims

1. A golf ball having a core and a cover located outside the core and having a plurality of dimples on a surface thereof, The bottom surface of the dimple has a first convex portion that is curved convexly outward from the golf ball, and the flat area of ​​the first convex portion has a second convex portion that is curved convexly outward from the golf ball, and the diameter of the circular boundary between the first convex portion and the second convex portion is half or less of the diameter of the flat area of ​​the first convex portion, a depth d of the second convex portion of the bottom surface of the dimple is the vertical distance from a line S connecting both ends of the outer periphery of the dimple to the highest point of the second convex portion, The relationship between the Shore D hardness H of the cover material and the depth d (unit: mm) is expressed by the following formula 1: (H-78) / (-300)>d...(Formula 1) Meet the golf ball.

2. 2. The golf ball according to claim 1, wherein the volume ratio VR of the dimples is less than 0.

75.

3. 2. The golf ball according to claim 1, wherein the cover material has a Shore D hardness of 50 to 60.

4. 2. The golf ball according to claim 1, further comprising an intermediate layer between the core and the cover, the intermediate layer being made of a material having a Shore D hardness of 55 or greater.

5. 5. The golf ball according to claim 4, wherein the Shore D hardness of the material of the mid layer is higher than the Shore D hardness of the material of the cover.

6. 2. The golf ball according to claim 1, further comprising a coating layer located on the outside of the cover, the coating layer containing matte particles, the average surface roughness Ra of the coating layer being 0.5 to 1.0, and the volume ratio VR of the dimples being less than 0.

70.

7. 2. The golf ball according to claim 1, wherein the dimples having the first and second convex portions account for 50% or more of the total number of dimples on the surface of the cover.

8. 2. The golf ball according to claim 1, wherein the second convex portion has a spherical crown shape with a radius of curvature R of 5 to 20 mm.

9. 9. The golf ball according to claim 8, wherein, in addition to the dimples having a spherical crown-shaped second convex portion with a radius of curvature R of 5 to 20 mm, dimples having a spherical crown-shaped second convex portion with a radius of curvature R of less than 5 mm are arranged, and the dimples having a spherical crown-shaped second convex portion with a radius of curvature R of less than 5 mm account for 1 to 10% of the total number of dimples on the surface of the cover.

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