GOLF BALL

Abstract

An object of the present disclosure is to provide a golf ball that bends in a great amplitude on short iron shots and rolls in a short distance after landing. The present disclosure provides a golf ball comprising a spherical core, one or more intermediate layer positioned outside the spherical core, and an outermost cover positioned outside the intermediate layer and having a plurality of dimples formed thereon, wherein the spherical core is formed from a rubber composition containing a rubber component, a co-crosslinking agent, and a crosslinking initiator, the rubber component contains a natural rubber, the co-crosslinking agent contains methacrylic acid and/or a metal salt thereof, a total lower volume Vi (mm.sup.3) of the plurality of dimples is less than 365 mm.sup.3, and an occupation ratio of the dimples is 75% or more.

Claims

1. A golf ball comprising a spherical core, at least one intermediate layer positioned outside the spherical core, and an outermost cover positioned outside the intermediate layer and having a plurality of dimples formed thereon, wherein the spherical core is formed from a rubber composition containing a rubber component, a co-crosslinking agent, and a crosslinking initiator, the rubber component contains a natural rubber, the co-crosslinking agent contains methacrylic acid and/or a metal salt thereof, a total lower volume Vi (mm.sup.3) of the plurality of dimples is less than 365 mm.sup.3, and an occupation ratio of the dimples defined by the following formula is 75% or more, Occupation ratio of dimples (%)=100total area of all dimples/surface area of a virtual sphere that is assumed to have no dimples on a golf ball surface.

2. The golf ball according to claim 1, wherein a center hardness (H0) of the spherical core, a hardness (H.sub.2.5) at a point having a radial distance of 2.5 mm from a center of the spherical core, a hardness (H.sub.5) at a point having a radial distance of 5 mm from the center of the spherical core, a hardness (H.sub.7.5) at a point having a radial distance of 7.5 mm from the center of the spherical core, a hardness (H.sub.10) at a point having a radial distance of 10 mm from the center of the spherical core, a hardness (H.sub.12.5) at a point having a radial distance of 12.5 mm from the center of the spherical core, a hardness (H.sub.15) at a point having a radial distance of 15 mm from the center of the spherical core, and a surface hardness (Hs) of the spherical core satisfy a relationship of H0<H.sub.2.5<H.sub.5<H.sub.7.5<H.sub.10<H.sub.12.5<H.sub.15<Hs.

3. The golf ball according to claim 2, wherein the center hardness (H0), the hardness (H.sub.2.5), the hardness (H.sub.5), the hardness (H.sub.7.5), the hardness (H.sub.10), the hardness (H.sub.12.5), the hardness (H.sub.15) and the surface hardness (Hs) satisfy relationships of H.sub.2.5H0<4, H.sub.5H.sub.2.5<4, H.sub.7.5H.sub.5<4, H.sub.10H.sub.7.5<4, H.sub.12.5H.sub.10<4, H.sub.15H.sub.12.5<4, and HsH.sub.15<4 in Shore C hardness.

4. The golf ball according to claim 2, wherein a hardness difference (HsH0) between the surface hardness (Hs) of the spherical core and the center hardness (H0) of the spherical core is 10 or less in Shore C hardness.

5. The golf ball according to claim 1, wherein the total lower volume Vi (mm.sup.3) is 350 mm.sup.3 or less.

6. The golf ball according to claim 1, wherein an amount of the natural rubber is 10 mass % or more and 80 mass % or less in the rubber component.

7. The golf ball according to claim 1, wherein a material hardness Hm of the intermediate layer is greater than a material hardness Hc of the outermost cover.

8. The golf ball according to claim 1, wherein the intermediate layer is formed from an intermediate layer composition having a slab hardness of 50 or more in Shore D hardness and containing an ionomer resin.

9. The golf ball according to claim 1, wherein the outermost cover is formed from a cover composition having a slab hardness of 40 or less in Shore D hardness and containing a polyurethane.

10. The golf ball according to claim 2, wherein the center hardness Ho of the spherical core ranges from 55 to 77 in Shore C hardness.

11. The golf ball according to claim 2, wherein the surface hardness Hs of the spherical core ranges from 60 to 87 in Shore C hardness.

12. The golf ball according to claim 4, wherein the hardness difference (HsH0) ranges from 2 to 10 in Shore C hardness.

13. The golf ball according to claim 3, wherein the center hardness (H0), the hardness (H.sub.2.5), the hardness (H.sub.5), the hardness (H.sub.7.5), the hardness (H.sub.10), the hardness (H.sub.12.5), the hardness (H.sub.15) and the surface hardness (Hs) satisfy relationships of 0.3H.sub.2.5H0<4, 0.5H.sub.5H.sub.2.5<4, 0.5H.sub.7.5H.sub.5<4, 0.5H.sub.10H.sub.7.5<4, 0.5H.sub.12.5H.sub.10<4, 0.1H.sub.15H.sub.12.5<4, and 0<HsH.sub.15<4 in Shore C hardness.

14. The golf ball according to claim 7, wherein a difference (HmHc) between the material hardness Hm of the intermediate layer and the material hardness Hc of the outermost cover ranges from 20 to 40 in Shore D hardness.

15. A golf ball comprising a spherical core, at least one intermediate layer positioned outside the spherical core, and an outermost cover positioned outside the intermediate layer and having a plurality of dimples formed thereon, wherein the spherical core is formed from a rubber composition containing a rubber component, a co-crosslinking agent, and a crosslinking initiator, the rubber component contains a natural rubber, the co-crosslinking agent contains methacrylic acid and/or a metal salt thereof, a total lower volume Vi (mm.sup.3) of the plurality of dimples is less than 365 mm.sup.3, and an occupation ratio of the dimples defined by the following formula is 75% or more, Occupation ratio of dimples (%)=100total area of all dimples/surface area of a virtual sphere that is assumed to have no dimples on a golf ball surface, wherein a center hardness (H0) of the spherical core, a hardness (H.sub.2.5) at a point having a radial distance of 2.5 mm from a center of the spherical core, a hardness (H.sub.5) at a point having a radial distance of 5 mm from the center of the spherical core, a hardness (H.sub.7.5) at a point having a radial distance of 7.5 mm from the center of the spherical core, a hardness (H.sub.10) at a point having a radial distance of 10 mm from the center of the spherical core, a hardness (H.sub.12.5) at a point having a radial distance of 12.5 mm from the center of the spherical core, a hardness (H.sub.15) at a point having a radial distance of 15 mm from the center of the spherical core, and a surface hardness (Hs) of the spherical core satisfy relationships of H0<H.sub.2.5<H.sub.5<H.sub.7.5<H.sub.10<H.sub.12.5<H.sub.15<Hs, 0.3H.sub.2.5H0<4, 0.5H.sub.5H.sub.2.5<4, 0.5H.sub.7.5H.sub.5<4, 0.5H.sub.10H.sub.7.5<4, 0.5H.sub.12.5H.sub.10<4, 0.1H.sub.15H.sub.12.5<4, and 0<HsH.sub.15<4 in Shore C hardness, and a hardness difference (HsH0) between the surface hardness (Hs) of the spherical core and the center hardness (H0) of the spherical core is 10 or less in Shore C hardness.

16. The golf ball according to claim 15, wherein the total lower volume Vi (mm.sup.3) is 350 mm.sup.3 or less.

17. The golf ball according to claim 15, wherein a material hardness Hm of the intermediate layer is greater than a material hardness Hc of the outermost cover.

18. The golf ball according to claim 15, wherein an amount of the natural rubber is 10 mass % or more and 80 mass % or less in the rubber component.

19. The golf ball according to claim 15, wherein the intermediate layer is formed from an intermediate layer composition having a slab hardness of 50 or more in Shore D hardness and containing an ionomer resin.

20. The golf ball according to claim 15, wherein the outermost cover is formed from a cover composition having a slab hardness of 40 or less in Shore D hardness and containing a polyurethane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a partially cutaway cross-sectional view showing a golf ball according to one embodiment of the present disclosure;

[0017] FIG. 2 is a front view of one example of a dimple pattern formed on an outermost cover;

[0018] FIG. 3 is a plane view of one example of a dimple pattern formed on an outermost cover;

[0019] FIG. 4 is an enlarged sectional view of dimples formed on an outermost cover;

[0020] FIG. 5 is a front view of one example of a dimple pattern formed on an outermost cover;

[0021] FIG. 6 is a plane view of one example of a dimple pattern formed on an outermost cover;

[0022] FIG. 7 is a front view of one example of a dimple pattern formed on an outermost cover

[0023] FIG. 8 is a plane view of one example of a dimple pattern formed on an outermost cover; and

[0024] FIG. 9 is an illustration graph illustrating a bending amplitude test on a shot.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] The present disclosure provides a golf ball comprising a spherical core, at least one intermediate layer positioned outside the spherical core, and an outermost cover positioned outside the intermediate layer and having a plurality of dimples formed thereon, wherein [0026] the spherical core is formed from a rubber composition containing a rubber component, a co-crosslinking agent, and a crosslinking initiator, [0027] the rubber component contains a natural rubber, [0028] the co-crosslinking agent contains methacrylic acid and/or a metal salt thereof, [0029] a total lower volume Vi (mm.sup.3) of the plurality of dimples is less than 365 mm.sup.3, and [0030] an occupation ratio of the dimples defined by the following formula is 75% or more, [0031] Occupation ratio of dimples (%)=100total area of all dimples/surface area of a virtual sphere that is assumed to have no dimples on a golf ball surface.

(Construction of Golf Ball)

[0032] The golf ball according to the present disclosure comprises a spherical core, at least one intermediate layer positioned outside the spherical core, and an outermost cover positioned outside the at least one intermediate layer and having a plurality of dimples formed thereon.

[0033] Examples of the golf ball according to the present disclosure include a three-piece golf ball composed of a spherical core, a single-layered intermediate layer covering the spherical core, and a single-layered cover covering the intermediate layer; and a multi-piece golf ball (including a three-piece golf ball) composed of a spherical core, one or more intermediate layers covering the spherical core, and an outermost cover covering the intermediate layers. The present invention can be suitably applied to any one of the above golf balls.

[0034] The construction of the spherical core may be a single-layered construction, or a multiple-layered construction, and the single-layered construction is preferable. The intermediate layer has at least one layer, and may be single-layered or have at least two layers. It is noted that the intermediate layer is sometimes referred to as an outer core or inner cover depending on the construction of the golf ball.

[0035] FIG. 1 is a partially cutaway cross-sectional view showing a golf ball according to one embodiment of the present disclosure. A golf ball 2 shown in FIG. 1 comprises a spherical core 4, an intermediate layer 6 covering the spherical core 4, and an outermost cover 8 positioned outside the intermediate layer 6. The golf ball 2 has a plurality of dimples 10 on the surface. Other portions than the dimples 10 on the surface of the golf ball 2 are lands 12. The golf ball 2 is provided with a paint layer and a mark layer on an outer side of the outermost cover 8, but these layers are not depicted.

(Spherical Core)

[0036] First, the spherical core of the golf ball according to the present disclosure will be explained. The diameter of the spherical core is preferably 34.8 mm or more, more preferably 36.8 mm or more, and even more preferably 38.0 mm or more, and is preferably 42.2 mm or less, more preferably 41.8 mm or less, even more preferably 41.2 mm or less, and most preferably 40.8 mm or less. If the diameter of the spherical core is 34.8 mm or more, the thickness of the cover is not excessively thick and thus the shot feeling is better. On the other hand, if the diameter of the spherical core is 42.2 mm or less, the thickness of the cover is not excessively thin and thus the cover functions better.

[0037] A center hardness (H0) of the spherical core, a hardness (H.sub.2.5) at a point having a radial distance of 2.5 mm from a center of the spherical core, a hardness (H.sub.5) at a point having a radial distance of 5 mm from the center of the spherical core, a hardness (H.sub.7.5) at a point having a radial distance of 7.5 mm from the center of the spherical core, a hardness (H.sub.10) at a point having a radial distance of 10 mm from the center of the spherical core, a hardness (H.sub.12.5) at a point having a radial distance of 12.5 mm from the center of the spherical core, a hardness (H.sub.15) at a point having a radial distance of 15 mm from the center of the spherical core, and a surface hardness (Hs) of the spherical core satisfy a relationship of H0<H.sub.2.5<H.sub.5<H.sub.7.5<H.sub.10<H.sub.12.5<H.sub.15<Hs.

[0038] If the hardness distribution of the spherical core satisfies the above requirements, the spherical core smoothly deforms when the golf ball is hit, and thus the golf ball has better shot feeling on iron shots.

[0039] The center hardness (H0), the hardness (H.sub.2.5), the hardness (H.sub.5), the hardness (H.sub.7.5), the hardness (H.sub.10), the hardness (H.sub.12.5), the hardness (H.sub.15) and the surface hardness (Hs) of the spherical core preferably satisfy the relationships of H.sub.2.5H0<4, H.sub.5H.sub.2.5<4, H.sub.7.5H.sub.5<4, H.sub.10H.sub.7.5<4, H.sub.12.5H.sub.10<4, H.sub.15H.sub.12.5<4 and HsH.sub.15<4 in Shore C hardness.

[0040] If the hardness difference (H.sub.2.5H.sub.0), the hardness difference (H.sub.5H.sub.2.5), the hardness difference (H.sub.7.5H.sub.5), the hardness difference (H.sub.10H.sub.7.5), the hardness difference (H.sub.12.5H.sub.10), the hardness difference (H.sub.15H.sub.12.5) and the hardness difference (HsH.sub.15) fall within the above range, the spherical core smoothly deforms on iron shots, and thus the shot feeling is better.

[0041] The hardness difference (H.sub.2.5H.sub.0) between the hardness (H.sub.2.5) at 2.5 mm point from the center of the spherical core and the center hardness (H0) of the spherical core is preferably 0.3 or more, more preferably 0.4 or more, and even more preferably 0.5 or more, and is preferably less than 4, more preferably 3.8 or less, and even more preferably 3.5 or less in Shore C hardness.

[0042] The hardness difference (H.sub.5H.sub.2.5) between the hardness (H.sub.5) at 5.0 mm point from the center of the spherical core and the hardness (H.sub.2.5) at 2.5 mm point from the center of the spherical core is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more, and is preferably less than 4, more preferably 3.8 or less, and even more preferably 3.5 or less in Shore C hardness.

[0043] The hardness difference (H.sub.7.5H.sub.5) between the hardness (H.sub.7.5) at 7.5 mm point from the center of the spherical core and the hardness (H.sub.5) at 5.0 mm point from the center of the spherical core is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more, and is preferably less than 4, more preferably 3.8 or less, and even more preferably 3.5 or less in Shore C hardness.

[0044] The hardness difference (H.sub.10H.sub.7.5) between the hardness (H.sub.10) at 10 mm point from the center of the spherical core and the hardness (H.sub.7.5) at 7.5 mm point from the center of the spherical core is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more, and is preferably less than 4, more preferably 3.8 or less, and even more preferably 3.5 or less in Shore C hardness.

[0045] The hardness difference (H.sub.12.5H.sub.10) between the hardness (H.sub.12.5) at 12.5 mm point from the center of the spherical core and the hardness (H.sub.10) at 10.0 mm point from the center of the spherical core is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more, and is preferably less than 4, more preferably 3.8 or less, and even more preferably 3.5 or less in Shore C hardness.

[0046] The hardness difference (H.sub.15H.sub.12.5) between the hardness (H.sub.15) at 15 mm point from the center of the spherical core and the hardness (H.sub.12.5) at 12.5 mm point from the center of the spherical core is preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.3 or more, and is preferably less than 4, more preferably 3.8 or less, and even more preferably 3.5 or less in Shore C hardness.

[0047] The hardness difference (HsH.sub.15) between the surface hardness (Hs) of the spherical core and the hardness (H.sub.15) at 15 mm point from the center of the spherical core is preferably more than 0, more preferably 0.1 or more, and even more preferably 0.2 or more, and is preferably less than 4, more preferably 3.8 or less, and even more preferably 3.5 or less in Shore C hardness.

[0048] The hardness difference (HsH0) between the surface hardness (Hs) of the spherical core and the center hardness (H0) of the spherical core is preferably 10 or less, more preferably 9.7 or less, and even more preferably 9.5 or less in Shore C hardness. If the hardness difference (HsH0) is 10 or less, the spin performance on approach shots is better. The hardness difference (HsH0) is preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more.

[0049] The center hardness (H0) of the spherical core is preferably 55 or more, more preferably 58 or more, and even more preferably 60 or more, and is preferably 77 or less, more preferably 75 or less, and even more preferably 73 or less in Shore C hardness.

[0050] The surface hardness (Hs) of the spherical core is preferably 60 or more, more preferably 63 or more, and even more preferably 65 or more, and is preferably 87 or less, more preferably 85 or less, and even more preferably 83 or less in Shore C hardness.

[0051] When the spherical core has a diameter in the range from 34.8 mm to 42.2 mm, the compression deformation amount of the spherical core (shrinking amount of the spherical core along the compression direction) when applying a load from an initial load of 98 N to a final load of 1275 N to the spherical core is preferably 2.0 mm or more, more preferably 2.5 mm or more, and even more preferably 3.0 mm or more, and is preferably 5.0 mm or less, more preferably 4.5 mm or less, and even more preferably 4.0 mm or less. If the compression deformation amount is 2.0 mm or more, the shot feeling on driver shots is better, and if the compression deformation amount is 5.0 mm or less, the durability is better.

(Intermediate Layer)

[0052] The golf ball according to the present disclosure comprises at least one intermediate layer positioned outside the spherical core. The intermediate layer may be single-layered or have at least two layers. It is noted that the intermediate layer is sometimes referred to as an outer core or inner cover depending on the construction of the golf ball.

[0053] The material hardness Hm of the intermediate layer is preferably 50 or more, more preferably 55 or more, and even more preferably 60 or more, and is preferably 75 or less, more preferably 73 or less, and even more preferably 71 or less in Shore D hardness. If the material hardness Hm is 50 or more, the flight distance is better due to the low spin rate on driver shots, and if the material hardness Hm is 75 or less, the shot feeling on driver shots is better. It is noted that the material hardness Hm of the intermediate layer is the slab hardness of the intermediate layer composition forming the intermediate layer. In the case that the intermediate layer has a plurality of layers, the slab hardness of the intermediate layer composition forming the outermost intermediate layer is deemed as the material hardness Hm.

[0054] The thickness Tm of the intermediate layer is preferably 0.8 mm or more, more preferably 0.9 mm or more, and even more preferably 1.0 mm or more, and is preferably 3.0 mm or less, more preferably 2.6 mm or less, and even more preferably 2.2 mm or less. If the thickness Tm is 0.8 mm or more, the durability is better, and if the thickness Tm is 3.0 mm or less, better shot feeling on driver shots is obtained. It is noted that in the case that the intermediate layer has a plurality of layers, the total thickness of all the intermediate layers is deemed as the thickness Tm of the intermediate layer.

(Outermost Cover)

[0055] The golf ball according to the present disclosure comprises an outermost cover positioned outside the intermediate layer. A plurality of dimples are formed on the surface of the outermost cover. The dimples are concaves formed on the outermost cover.

[0056] As shown in FIG. 2 and FIG. 3, the outermost cover of the golf ball 2 has a plurality of dimples 10 formed on the surface. Each of the dimples 10 has a circle contour.

[0057] 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. The top-to-bottom direction in FIG. 4 is the depth direction of the dimple 10. In FIG. 4, a chain double-dashed line 14 indicates a virtual sphere. The surface of the virtual sphere 14 is the surface of the golf ball 2 when it is virtualized that no dimple 10 exists. The diameter of the virtual sphere 14 is equal to the diameter of the golf ball 2. The dimple 10 is recessed from the surface of the virtual sphere 14. The land 12 coincides with the surface of the virtual sphere 14. In the present embodiment, the cross-sectional shape of the dimple 10 is substantially a circular arc. The curvature radius of this circular arc is shown by a reference sign CR in FIG. 4.

[0058] In FIG. 4, an arrow Dm indicates the diameter of the dimple 10. The diameter Dm is a longest distance between one tangent point Ed and another tangent point Ed when a tangent line Tg traversing two sides of the dimple 10 is drawn. The tangent point Ed is also the edge of the dimple 10. The edge Ed defines the contour of the dimple 10.

[0059] In the present disclosure, the lower volume of the dimple is the volume of the dimple lower part surrounded by the plane that is tangent to the golf ball at the edge Ed of the dimple and the surface of the dimple 10. The total lower volume Vi of the dimples is the sum of the lower volume of all the dimples.

[0060] The total lower volume Vi of the plurality of dimples of the golf ball according to the present disclosure is preferably less than 365 mm.sup.3, more preferably 350 mm.sup.3 or less, and even more preferably 335 mm.sup.3 or less. If the total lower volume Vi is less than 365 mm.sup.3, the lift force that acts upon the golf ball on a shot is greater, and the trajectory becomes higher. The total lower volume Vi is preferably 220 mm.sup.3 or more, more preferably 230 mm.sup.3 or more, and even more preferably 240 mm.sup.3 or more. If the total lower volume Vi is 220 mm.sup.3 or more, the trajectory of the golf ball is that the golf does not lift up excessively.

[0061] The diameter Dm of the dimple 10 is preferably 2.0 mm or more, more preferably 2.5 mm or more, and even more preferably 2.8 mm or more, and is preferably 6.0 mm or less, more preferably 5.5 mm or less, and even more preferably 5.0 mm or less. If the diameter Dm is 2.0 mm or more, the dimples easily contribute to the turbulence, and if the diameter Dm is 6.0 mm or less, the nature of the golf ball that is substantially a spherical body can be kept.

[0062] The plurality of dimples may be a plurality of dimples with a single diameter, or a combination of dimples with various types of diameters. The golf ball 2 shown in FIG. 2 and FIG. 3 has five types of dimples, i.e. a dimple A with a diameter of 4.400 mm, a dimple B with a diameter of 4.285 mm, a dimple C with a diameter of 4.150 mm, a dimple D with a diameter of 3.875 mm, and a dimple E with a diameter of 3.000 mm.

[0063] In FIG. 4, a double ended arrow Dp1 indicates a first depth of the dimple 10. The first depth Dp1 is the distance between the deepest part of the dimple 10 and the surface of the virtual sphere 14.

[0064] The first depth Dp1 is preferably 0.15 mm or more, more preferably 0.17 mm or more, and even more preferably 0.20 mm or more, and is preferably 0.45 mm or less, more preferably 0.43 mm or less, and even more preferably 0.40 mm or less. If the first depth Dp1 is 0.15 mm or more, the lift force obtained by the dimples fully generates, and if the first depth Dp1 is 0.45 mm or less, the nature of the golf ball that is substantially a spherical body can be kept.

[0065] In FIG. 4, a double ended arrow Dp2 indicates a second depth of the dimple 10. The second depth Dp2 is the distance between the deepest part of the dimple 10 and the tangent line Tg.

[0066] The second depth Dp2 is preferably 0.08 mm or more, more preferably 0.10 mm or more, and even more preferably 0.12 mm or more, and is preferably 0.30 mm or less, more preferably 0.28 mm or less, and even more preferably 0.26 mm or less. If the second depth Dp2 is 0.08 mm or more, the dimples easily contribute to the turbulence, and if the second depth Dp2 is 0.30 mm or less, the lift force obtained by the dimples is not excessively great, and the flight distance performance on driver shots is better.

[0067] The area A of the dimple 10 is the area of a region surrounded by the contour of the dimple 10 when the central point of the golf ball 2 is viewed at infinity. In the case that the dimple 10 has a circular shape, the area A is calculated by the following mathematical formula.

[00001]$A={\left(\mathrm{Dm}/2\right)}^2$

[0068] In the golf ball 2 shown in FIG. 2 and FIG. 3, the area of the dimple A is 15.21 mm.sup.2, the area of the dimple B is 14.42 mm.sup.2, the area of the dimple C is 13.53 mm.sup.2, the area of the dimple D is 11.79 mm.sup.2, and the area of the dimple E is 7.07 mm.sup.2.

[0069] The occupation ratio of the dimples of the golf ball according to the present disclosure is preferably 75% or more, more preferably 78% or more, and even more preferably 81% or more, and is preferably 95% or less, more preferably 92% or less, and even more preferably 90% or less. If the occupation ratio falls within the above range, the effect of the turbulence by the dimples is greater. As a result, the fly duration of the golf ball is long on a shot, and curves to a large extent. In addition, if the flight duration increases, the height of the highest point of the golf ball becomes great and the falling angle becomes acute. If the falling angle of the golf ball becomes acute, the rolling distance of the golf ball after landing decreases.

[0070] The occupation ratio of the dimples is defined by the following formula.

[0071] Occupation ratio of dimples (%)=100total area of all dimples/surface area of a virtual sphere that is assumed to have no dimples on a golf ball surface

[0072] The number of the dimples can be appropriately adjusted depending on the diameter or occupation ratio of the dimples. It is noted that from the viewpoint of the occupation ratio or the function of the respective dimple, the total number of the dimples 10 is preferably 250 or more, more preferably 280 or more, and even more preferably 300 or more, and is preferably 450 or less, more preferably 410 or less, and even more preferably 390 or less.

[0073] The material hardness Hc of the outermost cover is preferably 20 or more, more preferably 22 or more, and even more preferably 24 or more, and is preferably 40 or less, more preferably 38 or less, and even more preferably 36 or less in Shore D hardness. If the material hardness Hc is 20 or more, the spin rate on driver shots is not excessively great, and thus the flight distance performance is better, and if the material hardness Hc is 40 or less, the spin performance on approach shots is better. It is noted that the material hardness Hc of the outermost cover is the slab hardness of the cover composition forming the outermost cover.

[0074] The thickness Tc of the outermost cover is preferably 0.4 mm or more, more preferably 0.5 mm or more, and even more preferably 0.6 mm or more, and is preferably 1.0 mm or less, more preferably 0.9 mm or less, and even more preferably 0.8 mm or less. If the thickness Tc is 0.4 mm or more, the spin performance on approach shots is better, and if the thickness Tc is 1.0 mm or less, the spin rate on driver shots is not excessively great, and thus the flight distance performance is better.

[0075] The material hardness Hm of the intermediate layer is preferably greater than the material hardness Hc of the cover. The hardness difference (HmHc) between the material hardness Hm of the intermediate layer and the material hardness Hc of the cover is preferably 20 or more, more preferably 22 or more, and even more preferably 24 or more, and is preferably 40 or less, more preferably 39 or less, and even more preferably 38 or less in Shore D hardness. If the hardness difference (HmHc) falls within the above range, the shot feeling on driver shots is better.

[0076] The golf ball according to the present disclosure preferably has a diameter ranging from 40 mm to 45 mm. In light of satisfying the regulation of US Golf Association (USGA), the diameter is particularly preferably 42.67 mm or more. In light of prevention of air resistance, the diameter is more preferably 44 mm or less, and particularly preferably 42.80 mm or less. In addition, the golf ball according to the present disclosure preferably has a mass of 40 g or more and 50 g or less. In light of obtaining greater inertia, the mass is more preferably 44 g or more, and particularly preferably 45.00 g or more. In light of satisfying the regulation of USGA, the mass is particularly preferably 45.93 g or less.

[0077] When the golf ball according to the present disclosure has a diameter in the range of from 40 mm to 45 mm, the compression deformation amount (shrinking amount along the compression direction) of the golf ball when applying a load from an initial load of 98 N to a final load of 1275 N to the golf ball is preferably 2.0 mm or more, more preferably 2.2 mm or more, and even more preferably 2.4 mm or more, and is preferably 3.5 mm or less, more preferably 3.3 mm or less, even more preferably 3.1 mm or less, and most preferably 3.0 mm or less. If the compression deformation amount is 2.0 mm or more, the golf ball is not excessively hard and thus has better shot feeling on driver shots. On the other hand, if the compression deformation amount is 3.5 mm or less, the durability is higher.

[0078] Next, the materials constituting the golf ball according to the present disclosure will be explained.

(Spherical Core)

[0079] The spherical core of the golf ball according to the present disclosure is preferably formed from a rubber composition (hereinafter sometimes referred to as core rubber composition) containing (a) a rubber component, (b) a co-crosslinking agent, and (c) a crosslinking initiator.

[0080] The rubber component contains a natural rubber (NR). If the natural rubber is contained, the velocity of the golf ball when landing is lowered, and the rolling distance decreases. The natural rubber is prepared by slicing plants that produce natural rubber latex, collecting the latex, and coagulating the rubber component contained in the latex. The natural rubber may be used solely, or at least two of them may be used in combination.

[0081] Examples of the plants that produce the natural rubber latex include Para rubber tree and Ceara rubber tree which belong to the Euphorbiaceae family; Indian rubber tree, Panama rubber tree and Lagos rubber tree which belong to the Moraceae family; Arabia rubber tree and Tragacanth rubber tree which belong to the Fabaceae family; Jelutong tree, Zanzibar rubber tree, Funtumia elastica and Urceola which belong to the Apocynaceae family; Guayule rubber tree and Rubber dandelion which belong to the Composite family; Gutta-percha tree, Balata rubber tree and Sapodilla which belong to the Sapotaceae family; Ipomoea nil which belongs to the Asclepiadaceae family; and Eucommia which belongs to the Eucommiaceae family.

[0082] Examples of the natural rubber include a CV grade where a rubber viscosity is stabilized by adding a viscosity modifier or the like to the raw latex, and a non-CV grade where a rubber viscosity is not stabilized. These natural rubbers may be used solely, or at least two of them may be used in combination. Among them, the CV grade having the stabilized viscosity is particularly preferable. It is noted that the natural rubber may be one of SMR (Standard Malaysian Rubber) and SVR (Standard Vietnam Rubber).

[0083] The natural rubber is a cis-1,4-polyisoprene, and each of a sheet rubber and a block rubber can be used. In addition, the natural rubber also includes a modified natural rubber obtained by modifying the natural rubber, such as an epoxidized natural rubber, a methacrylic acid-modified natural rubber, a halogen-modified natural rubber, a deproteinized natural rubber, a maleic acid-modified natural rubber, a sulfonic acid-modified natural rubber, and a styrene-modified natural rubber. Among them, the natural rubber preferably does not contain the epoxidized natural rubber.

[0084] As the natural rubber, Technical Specified Rubbers (TSR), and Ribbed Smoked Sheet (RSS) are preferable. In addition, the natural rubber may contain a viscosity stabilizer.

[0085] The Mooney viscosity (ML.sub.1+4 (100 C.)) of the natural rubber is preferably 30 or more, more preferably 35 or more, and even more preferably 40 or more, and is preferably 80 or less, more preferably 75 or less, and even more preferably 70 or less. It is noted that the Mooney viscosity (ML.sub.1+4 (100 C.)) in the present disclosure is a value measured according to JIS K6300 (2013) using an L rotor under the conditions of preheating time: 1 minute, rotor rotation time: 4 minutes, and temperature: 100 C.

[0086] The rubber component may consist of the natural rubber, or contain the natural rubber and a synthesized rubber. When the rubber component contains the natural rubber and the synthesized rubber, the amount of the natural rubber is preferably 10 mass % or more, more preferably 25 mass % or more, and even more preferably 40 mass % or more, and is preferably 100 mass % or less, more preferably 90 mass % or less, and even more preferably 80 mass % or less in 100 mass % of the rubber component. If the amount is 10 mass % or more, the shot feeling on driver shots is better, and if the amount is 100 mass % or less, the rolling when being hit with a putter is better to give a good distance feeling.

[0087] Examples of the synthesized rubber include a diene-based rubber such as a polybutadiene rubber (BR), a polyisoprene rubber (IR), a styrene-polybutadiene rubber (SBR), a chloroprene rubber (CR), a butyl rubber (IIR), and an acrylonitrile-butadiene rubber (NBR); and a non-diene-based rubber such as an ethylene-propylene rubber (EPM), an ethylene-propylene-diene rubber (EPDM), a urethane rubber, a silicone rubber, an acrylic rubber, an epichlorohydrin rubber, a polysulfide rubber, a fluororubber, and a chlorosulfonated polyethylene rubber. These rubbers may be used solely, or at least two of them may be used in combination.

[0088] The synthetic rubber component preferably contains the diene-based rubber, more preferably contains the polybutadiene rubber. In particular, the synthetic rubber component more preferably contains a high-cis polybutadiene having a cis-1,4 bond in an amount of 40 mass % or more, preferably 80 mass % or more, and more preferably 90 mass % or more. The amount of the high-cis polybutadiene in the synthetic rubber is preferably 50 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more. The synthetic rubber also preferably consists of the high-cis polybutadiene rubber.

[0089] The amount of the 1,2-vinyl bond in the high-cis polybutadiene is preferably 2.0 mass % or less, more preferably 1.7 mass % or less, and even more preferably 1.5 mass % or less.

[0090] The high-cis polybutadiene is preferably one synthesized using a rare-earth element catalyst. When a neodymium catalyst employing a neodymium compound which is a lanthanum series rare-earth element compound, is used, a polybutadiene rubber having a high amount of the cis-1,4 bond and a low amount of the 1,2-vinyl bond is obtained with an excellent polymerization activity, and thus such polybutadiene rubber is particularly preferable.

[0091] The molecular weight distribution Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) of the high-cis polybutadiene is preferably 2.0 or more, more preferably 2.2 or more, and even more preferably 2.4 or more, and is preferably 6.0 or less, more preferably 5.0 or less, and even more preferably 4.0 or less. It is noted that the molecular weight distribution is measured by gel permeation chromatography (HLC-8120GPC available from Tosoh Corporation) using a differential refractometer as a detector under the conditions of column: GMHHXL (available from Tosoh Corporation), column temperature: 40 C., and mobile phase: tetrahydrofuran, and calculated by converting based on polystyrene standard.

[0092] The Mooney viscosity (ML.sub.1+4 (100 C.)) of the high-cis polybutadiene is preferably 30 or more, more preferably 32 or more, and even more preferably 35 or more, and is preferably 140 or less, more preferably 120 or less, and even more preferably 100 or less.

[0093] The core rubber composition used in the present disclosure contains (b) a co-crosslinking agent. (b) The co-crosslinking agent has an action of crosslinking a rubber molecule by graft polymerization to the rubber component. (b) The co-crosslinking agent of the core rubber composition used in the present disclosure contains methacrylic acid and/or a metal salt thereof. If methacrylic acid and/or the metal salt thereof is contained as (b) the co-crosslinking agent, a gradual hardness gradient is easily produced, and thus the controllability on approach shots is better.

[0094] Examples of the metal ion constituting the metal salt of methacrylic acid include a monovalent metal ion such as sodium, potassium and lithium; a divalent metal ion such as magnesium, calcium, zinc, barium and cadmium; a trivalent metal ion such as aluminum; and other metal ions such as tin and zirconium. The above metal component may be used solely or as a mixture of at least two of them.

[0095] In the present disclosure, methacrylic acid is preferably used as (b) the co-crosslinking agent.

[0096] The amount of (b) the co-crosslinking agent is preferably 10 parts by mass or more, more preferably 15 parts by mass or more, and even more preferably 20 parts by mass or more, and is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less, with respect to 100 parts by mass of (a) the rubber component. If the amount of (b) the co-crosslinking agent is 10 parts by mass or more, the formed core has an appropriate hardness, and the durability of the golf ball is better. On the other hand, if the amount of (b) the co-crosslinking agent is 50 parts by mass or less, the formed core is not excessively hard.

[0097] (b) The co-crosslinking agent may contain an ,-unsaturated carboxylic acid having 3 to 8 carbon atoms (excluding methacrylic acid) and/or a metal salt thereof, in addition to methacrylic acid and/or the metal salt thereof.

[0098] Examples of the ,-unsaturated carboxylic acid having 3 to 8 carbon atoms (excluding methacrylic acid) include acrylic acid, fumaric acid, maleic acid and crotonic acid.

[0099] Examples of the metal ion constituting the metal salt of the ,-unsaturated carboxylic acid having 3 to 8 carbon atoms (excluding methacrylic acid) include a monovalent metal ion such as sodium, potassium and lithium; a divalent metal ion such as magnesium, calcium, zinc, barium and cadmium; a trivalent metal ion such as aluminum; and other metal ions such as tin and zirconium. The above metal component may be used solely or as a mixture of at least two of them. Among them, the divalent metal ion such as magnesium, calcium, zinc, barium and cadmium is preferably used as the metal component. This is because if the divalent metal salt of the ,-unsaturated carboxylic acid having 3 to 8 carbon atoms is used, a metal crosslinking easily generates between the rubber molecules.

[0100] In the case that (b) the co-crosslinking agent contains the ,-unsaturated carboxylic acid having 3 to 8 carbon atoms (excluding methacrylic acid) and/or the metal salt thereof in addition to methacrylic acid and/or the metal salt thereof, the amount of methacrylic acid and/or the metal salt thereof in (b) the co-crosslinking agent component is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 70 mass % or more, and is preferably 99 mass % or less, more preferably 98 mass % or less, and even more preferably 97 mass % or less. If the amount of methacrylic acid and/or the metal salt thereof falls within the above range, the reactivity in the rubber is uniform to provide stable quality. (b) The co-crosslinking agent preferably consists of methacrylic acid and/or the metal salt thereof, and more preferably consists of methacrylic acid.

[0101] (c) The crosslinking initiator is blended to crosslink (a) the rubber component. As (c) the crosslinking initiator, an organic peroxide is preferable. Specific examples of the organic peroxide 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. These organic peroxides may be used solely or as a mixture of at least two of them. Among them, dicumyl peroxide is preferably used.

[0102] The amount of (c) the crosslinking initiator is preferably 0.2 part by mass or more, more preferably 0.4 part by mass or more, and even more preferably 0.6 part by mass or more, and is preferably 5.0 parts by mass or less, more preferably 2.5 parts by mass or less, and even more preferably 1.0 part by mass or less, with respect to 100 parts by mass of (a) the rubber component. If the amount of (c) the crosslinking initiator falls within the above range, the formed core has a more appropriate hardness and thus the golf ball has better durability.

[0103] The rubber composition preferably further contains (d) a metal compound. (d) The metal compound can be used, for example, to neutralize (b) the co-crosslinking agent in the rubber composition, or to adjust the weight of the rubber composition.

[0104] Examples of (d) the metal compound include a metal hydroxide such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; a metal oxide such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and a metal carbonate such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. As (d) the metal compound, the divalent metal compound is preferable, the zinc compound is more preferable. This is because the divalent metal compound reacts with the ,-unsaturated carboxylic acid having 3 to 8 carbon atoms to form a metal crosslinking. In addition, use of the zinc compound provides a golf ball with better durability. (d) The metal compound may be used solely, or at least two of them may be used in combination.

[0105] The rubber composition preferably may further contain (e) an organic sulfur compound. (e) The organic sulfur compound is not particularly limited, as long as it is an organic compound having a sulfur atom in the molecule thereof. Examples of (e) the organic sulfur compound include an organic compound having a thiol group (SH) or a polysulfide bond having 2 to 4 sulfur atoms (SS, SSS, or SSSS), and a metal salt thereof (SM, S-M-S or the like; M is a metal atom). Examples of (e) the organic sulfur compound include compounds belong to thiophenols, thionaphthols, polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, and thiazoles.

[0106] Examples of the thiophenols include thiophenol; thiophenols substituted with a fluoro group, such as 4-fluorothiophenol, 2,4-difluorothiophenol, 2,5-difluorothiophenol, 2,6-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, and pentafluorothiophenol; thiophenols substituted with a chloro group, such as 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, and pentachlorothiophenol; thiophenols substituted with a bromo group, such as 4-bromothiophenol, 2,4-dibromothiophenol, 2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, and pentabromothiophenol; thiophenols substituted with an iodo group, such as 4-iodothiophenol, 2,4-diiodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; and their metal salts.

[0107] Examples of the thionaphthols (naphthalenethiols) include 2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol, 2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol, 1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol, 1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol, 2-acetyl-1-thionaphthol, and their metal salts.

[0108] The polysulfides are organic sulfur compounds having a polysulfide bond, and examples thereof include disulfides, trisulfides, and tetrasulfides. As the polysulfides, diphenyl polysulfides are preferable.

[0109] Examples of the diphenyl polysulfides include diphenyl disulfide; diphenyl disulfides substituted with a halogen group, such as bis(4-fluorophenyl) disulfide, bis(2,5-difluorophenyl) disulfide, bis(2,6-difluorophenyl) disulfide, bis(2,4,5-trifluorophenyl) disulfide, bis(2,4,5,6-tetrafluorophenyl) disulfide, bis(pentafluorophenyl) disulfide, bis(4-chlorophenyl) disulfide, bis(2,5-dichlorophenyl) disulfide, bis(2,6-dichlorophenyl) disulfide, bis(2,4,5-trichlorophenyl) disulfide, bis(2,4,5,6-tetrachlorophenyl) disulfide, bis(pentachlorophenyl) disulfide, bis(4-bromophenyl) disulfide, bis(2,5-dibromophenyl) disulfide, bis(2,6-dibromophenyl) disulfide, bis(2,4,5-tribromophenyl) disulfide, bis(2,4,5,6-tetrabromophenyl) disulfide, bis(pentabromophenyl) disulfide, bis(4-iodophenyl) disulfide, bis(2,5-diiodophenyl) disulfide, bis(2,6-diiodophenyl) disulfide, bis(2,4,5-triiodophenyl) disulfide, bis(2,4,5,6-tetraiodophenyl) disulfide, and bis(pentaiodophenyl) disulfide; and diphenyl disulfides substituted with an alkyl group, such as bis(4-methylphenyl) disulfide, bis(2,4,5-trimethylphenyl) disulfide, bis(pentamethylphenyl) disulfide, bis(4-t-butylphenyl) disulfide, bis(2,4,5-tri-t-butylphenyl) disulfide, and bis(penta-t-butylphenyl) disulfide.

[0110] Examples of the thiurams include thiuram monosulfides such as tetramethylthiuram monosulfide; thiuram disulfides such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, and tetrabutylthiuram disulfide; and thiuram tetrasulfides such as dipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylic acids include a naphthalene thiocarboxylic acid. Examples of the dithiocarboxylic acids include a naphthalene dithiocarboxylic acid. Examples of the sulfenamides include N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide.

[0111] (e) The organic sulfur compound may be used solely or as a mixture of at least two of them.

[0112] The amount of (e) the organic sulfur compound is preferably 0.05 part by mass or more, more preferably 0.1 part by mass or more, and even more preferably 0.2 part by mass or more, and is preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, and even more preferably 2.0 parts by mass or less, with respect to 100 parts by mass of (a) the rubber component. If the amount of (e) the organic sulfur compound falls within the above range, the processability is better and the rubber composition is more uniform.

[0113] The rubber composition may further contain (f) a carboxylic acid and/or a metal salt thereof. As (f) the carboxylic acid and/or the metal salt thereof, a carboxylic acid having 1 to 30 carbon atoms and/or a metal salt thereof is preferable. As the carboxylic acid, an aliphatic carboxylic acid (a saturated fatty acid or an unsaturated fatty acid), or an aromatic carboxylic acid (e.g. benzoic acid) can be used. The amount of (f) the carboxylic acid and/or the metal salt thereof is preferably 1 part by mass or more and 40 parts by mass or less with respect to 100 parts by mass of the rubber component.

[0114] The rubber composition may further contain an additive such as a filler for adjusting weight or the like, an antioxidant, a peptizing agent, and a softener, where necessary.

[0115] The filler blended in the rubber composition is mainly used as a weight adjusting agent for adjusting the weight of the golf ball obtained as a final product, and may be blended where necessary. Examples of the filler include an inorganic filler such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder. The amount of the filler is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, and even more preferably 2 parts by mass or more, and is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less, with respect to 100 parts by mass of the rubber component. If the amount of the filler is 0.5 part by mass or more, it is easier to adjust the weight, and if the amount of the filler is 30 parts by mass or less, the weight proportion of the rubber component increases and thus the durability tends to be better.

[0116] The amount of the antioxidant is preferably 0.1 part by mass or more and 1 part by mass or less with respect to 100 parts by mass of (a) the rubber component. In addition, the amount of the peptizing agent is preferably 0.1 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass of (a) the rubber component.

(Intermediate Layer Composition and Cover Composition)

[0117] The intermediate layer of the golf ball according to the present disclosure is preferably formed from an intermediate layer composition containing a resin component. The outermost cover of the golf ball according to the present disclosure is preferably formed from a cover composition containing a resin component.

[0118] Examples of the resin component used in the resin composition for forming the outermost cover and the intermediate layer include an ionomer resin, a polyurethane (a thermoplastic polyurethane elastomer, and a thermosetting polyurethane elastomer), a thermoplastic styrene elastomer, a thermoplastic polyamide elastomer, and a thermoplastic polyester elastomer.

[0119] Examples of the ionomer resin include a binary ionomer resin prepared by neutralizing at least a part of carboxyl groups in a binary copolymer composed of an olefin and an ,-unsaturated carboxylic acid having 3 to 8 carbon atoms with a metal ion; a ternary ionomer resin prepared by neutralizing at least a part of carboxyl groups in a ternary copolymer composed of an olefin, an ,-unsaturated carboxylic acid having 3 to 8 carbon atoms and an ,-unsaturated carboxylic acid ester with a metal ion; and a mixture of those.

[0120] Examples of the binary ionomer resin include Himilan (registered trademark) 1555 (Na), 1557 (Zn), 1605 (Na), 1706 (Zn), 1707 (Na), AM7311 (Mg), AM7329 (Zn), AM7337 (available from Dow-Mitsui Polychemicals Co., Ltd.); Surlyn (registered trademark) 8945 (Na), 9945 (Zn), 8140 (Na), 8150 (Na), 9120 (Zn), 9150 (Zn), 6910 (Mg), 6120 (Mg), 7930 (Li), 7940 (Li), AD8546 (Li) (available from E.I. du Pont de Nemours and Company); and lotek (registered trademark) 8000 (Na), 8030 (Na), 7010 (Zn), 7030 (Zn) (available from ExxonMobil Chemical Corporation).

[0121] Examples of the ternary ionomer resin include Himilan AM7327 (Zn), 1855 (Zn), 1856 (Na), AM7331 (Na) (available from Dow-Mitsui Polychemicals Co., Ltd.); Surlyn 6320 (Mg), 8120 (Na), 8320 (Na), 9320 (Zn), 9320W (Zn), HPF1000 (Mg), HPF2000 (Mg) (available from E.I. du Pont de Nemours and Company); and lotek 7510 (Zn), 7520 (Zn) (available from ExxonMobil Chemical Corporation). It is noted that Na, Zn, Li, Mg or the like described in the parentheses after the trade names of the ionomer resin indicate metal ion type for neutralizing the ionomer resin.

[0122] The thermoplastic polyurethane elastomer has a urethane bond in the molecule. The urethane bond may be formed by a reaction between a polyol and a polyisocyanate. The polyol is a raw material for the urethane bond and has a plurality of hydroxy groups, and a low molecular weight polyol or a high molecular weight polyol may be used.

[0123] Specific examples of the thermoplastic polyurethane elastomer include Elastollan (registered trademark) NY80A, NY84A, NY88A, NY95A, ET885, and ET890 (available from BASF Japan Ltd.).

[0124] As the thermoplastic styrene based elastomer, a thermoplastic elastomer containing a styrene block can be suitably used. The thermoplastic elastomer containing the styrene block has a polystyrene block that is a hard segment, and a soft segment.

[0125] The thermoplastic elastomer containing the styrene block includes a styrene-butadiene-styrene block copolymer (SBS), a styrene-isoprene-styrene block copolymer (SIS), a styrene-isoprene-butadiene-styrene block copolymer (SIBS), a hydrogenated product of SBS, a hydrogenated product of SIS, and a hydrogenated product of SIBS. Examples of the hydrogenated product of SBS include a styrene-ethylene-butylene-styrene block copolymer (SEBS). Examples of the hydrogenated product of SIS include a styrene-ethylene-propylene-styrene block copolymer (SEPS). Examples of the hydrogenated product of SIBS include a styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS).

[0126] Examples of the thermoplastic styrene based elastomer include TEFABLOC (registered trademark) T3221C, T3339C, SJ4400N, SJ5400N, SJ6400N, SJ7400N, SJ8400N, SJ9400N, and SR04 (available from Mitsubishi Chemical Corporation).

[0127] The cover composition constituting the cover preferably contains the polyurethane and/or the ionomer resin as the resin component, particularly preferably contains the polyurethane. If the cover contains the polyurethane as the resin component, the bite of the outermost cover into the club face on approach shots is greater, and thus the spin rate is easily increased.

[0128] When the cover composition contains the polyurethane as the resin component, the amount of the polyurethane in the resin component is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 70 mass % or more. The resin component of the cover composition may consist of the polyurethane (preferably the thermoplastic polyurethane elastomer).

[0129] When the cover composition contains the ionomer resin as the resin component, the amount of the ionomer resin in the resin component is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 70 mass % or more. When the ionomer resin is contained, the thermoplastic styrene elastomer is also preferably used in combination.

[0130] The intermediate layer composition preferably contains the ionomer resin as the resin component. When the ionomer resin is contained, the thermoplastic styrene elastomer is also preferably used in combination. The amount of the ionomer resin in the base resin of the intermediate layer composition is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 70 mass % or more.

[0131] The cover composition and the intermediate layer composition may contain a pigment component such as a white pigment (e.g. titanium oxide), a blue pigment and a red pigment, a weight adjusting agent such as zinc oxide, calcium carbonate and barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material or fluorescent brightener, or the like, in addition to the above resin component.

[0132] The amount of the white pigment (e.g. titanium oxide) is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, and is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, with respect to 100 parts by mass of the base resin constituting the outermost cover. If the amount of the white pigment is 0.5 part by mass or more, it is possible to impart the opacity to the cover. In addition, if the amount of the white pigment is 10 parts by mass or less, the obtained cover has better durability.

(Production Method of Golf Ball)

[0133] The spherical core of the golf ball according to the present disclosure is formed from the above core rubber composition. The core rubber composition can be obtained by kneading (a) the rubber component, (b) the co-crosslinking agent, (c) the crosslinking initiator, and the other optional components. The kneading method is not particularly limited. For example, the kneading can be conducted with a conventional kneading machine such as a kneading roll, a banbury mixer and a kneader.

[0134] The spherical core can be molded, for example, by heat pressing the core rubber composition. The molding conditions for heat pressing the core rubber composition may be determined appropriately depending on the rubber composition. Generally, the heat pressing is preferably carried out at a temperature of 130 C. to 200 C. for 10 to 60 minutes, or carried out in a two-step heating of heating at a temperature of 130 C. to 150 C. for 20 to 40 minutes followed by heating at a temperature of 160 C. to 180 C. for 5 to 15 minutes.

[0135] The method for molding the intermediate layer of the golf ball according to the present disclosure is not particularly limited, and examples thereof include a method which comprises molding the intermediate layer composition into a half shell in advance, covering the spherical core with two of the half shells, and performing compression molding; and a method which comprises injection molding the intermediate layer composition directly onto the spherical core.

[0136] Examples of the method for molding the outermost cover of the golf ball according to the present disclosure include a method which comprises molding the cover composition into a hollow shell, covering the spherical body having the intermediate layer formed with a plurality of the shells, and performing compression molding (preferably a method which comprises molding the cover composition into a hollow half-shell, covering the spherical body with two of the half-shells, and performing compression molding); and a method which comprises injection molding the cover composition directly onto the spherical body having the intermediate layer formed.

[0137] The golf ball body having the cover formed thereon is ejected from the mold, and is preferably subjected to surface treatments such as deburring, cleaning and sandblast where necessary.

[0138] In addition, if desired, a paint film or a mark may be formed. The thickness of the paint film is not particularly limited, and is preferably 5 m or more, more preferably 6 m or more, and even more preferably 7 m or more, and is preferably 50 m or less, more preferably 40 m or less, and even more preferably 30 m or less. If the thickness of the paint film is 5 m or more, the paint film is hard to wear off even if the golf ball is used continuously, and if the thickness of the paint film is 50 m or less, the dimple effect is fully obtained. It is noted that the effect of the present disclosure is not impaired since the paint film is very thin.

EXAMPLES

[0139] Next, the present disclosure will be described in detail by way of examples. However, the present disclosure is not limited to the examples described below. Various changes and modifications without departing from the spirit of the present disclosure are included in the scope of the present disclosure.

[Evaluation Method]

(1) Compression Deformation Amount (mm)

[0140] The compression deformation amount was measured with a YAMADA type compression tester SCH. The golf ball or core was placed on a metal rigid plate of the tester. A metal cylinder slowly fell toward the golf ball or core. The golf ball or core sandwiched between the bottom of the cylinder and the rigid plate deformed. The travelling distance of the cylinder when applying a load from an initial load of 98 N to a final load of 1275 N to the golf ball or core was measured. The compression deformation amount (mm) is the travelling distance. The travelling speed of the cylinder before applying the initial load was 0.83 mm/s. The travelling speed of the cylinder when applying the load from the initial load to the final load was 1.67 mm/s.

(2) Material (Slab) Hardness (Shore D Hardness)

[0141] Sheets with a thickness of about 2 mm were produced by injection molding the intermediate layer composition or the cover composition. The sheets were stored at a temperature of 23 C. for two weeks. At least three of these sheets were stacked on one another so as not to be affected by the measuring substrate on which the sheets were placed, and the hardness of the stack was measured with an automatic hardness tester (Digitest II, available from Bareiss company) using a testing device of Shore D.

(3) Core Hardness Distribution (Shore C Hardness)

[0142] An automatic hardness tester (Digitest II, available from Bareiss company) using a testing device of Shore C was used to measure the hardness. The Shore C hardness measured at the surface portion of the spherical core was adopted as the surface hardness of the spherical core. In addition, the spherical core was cut into two hemispheres along a plane passing through the center of the spherical core to obtain a cut plane, and the hardness at the central point of the cut plane and the hardness at the predetermined distance from the central point in the radius direction were measured. It is noted that each hardness was measured at four points, and the average value thereof was calculated.

(4) Surface Hardness of Golf Ball

[0143] The hardness measured at the land portion of the golf ball surface was adopted as the surface hardness of the ball. The hardness was measured at four points, and the average value thereof was calculated. An automatic hardness tester (Digitest II, available from Bareiss company) using a testing device of Shore D was used to measure the hardness.

(5) Measurement of Curving Width (m) and Rolling Distance (m) on Iron Shot

[0144] A 8-iron (I #7) (SRIXON ZX7, Shaft hardness: X, Loft angle: 36, available from Sumitomo Rubber Industries, Ltd.) was installed on a swing machine available from Golf Laboratories, Inc. The hitting point was set at a 10 mm point on the toe side from the face center. The golf ball was hit at a head speed of 39 m/sec, and the side spin rate right after hitting the golf ball, the rolling distance (a distance from the fall point to the stop point), and the trajectory were measured. The spin rate right after hitting the golf ball was measured by continuously taking a sequence of photographs right after hitting the golf ball. The trajectory measurement was carried out with a launch monitor TRACK MAN 4 available from TRACK MAN Golf. As shown in FIG. 9, the trajectory viewed from the sky was plotted in a coordinate where the line connecting the launch point and the fall point is the Y axis and the perpendicular coordinate axis is the X axis (the right direction from the launch direction is +), and the maximum value in the X axis direction is defined and calculated as the curving width. The measurement was conducted twelve times for each golf ball, and the average value thereof was adopted as the measurement value for that golf ball.

[Production of Golf Ball]

(1) Preparation of Core Composition

[0145] According to the formulations shown in Table 1, the materials were kneaded with a kneading roll to obtain the core compositions.

TABLE-US-00001 TABLE 1 Spherical core No. A1 A2 B1 B2 Formulation Polybutadiene rubber 20 20 40 40 (parts by Natural rubber 80 80 60 60 mass) Isoprene rubber Methacrylic acid 22 22 22.5 22.5 Zinc acrylate Zinc oxide 5 5 5 5 Barium sulfate Appropriate Appropriate Appropriate Appropriate amount*.sup.1) amount*.sup.1) amount*.sup.1) amount*.sup.1) Benzoic acid Pentabromodiphenyl disulfide Diphenyl disulfide Dicumyl peroxide 0.75 0.75 0.9 0.9 Molding Vulcanizing temperature ( C.) 170 170 170 170 condition Vulcanizing time (min) 20 20 20 20 Core diameter (mm) 39.5 39.1 39.5 38.5 Core compression deformation amount (mm) 3.17 3.2 3.21 3.26 Core Center hardness (H0) 63.6 63.2 63.4 62.9 hardness Hardness at 2.5 mm point from 64.5 64.2 64.2 63.7 (Shore C) center (H.sub.2.5) Hardness at 5.0 mm point from 66.4 66.1 66.1 65.7 center (H.sub.5.0) Hardness at 7.5 mm point from 68.4 68.2 68.2 67.7 center (H.sub.7.5) Hardness at 10 mm point from 70.2 69.9 69.9 69.6 center (H.sub.10) Hardness at 12.5 mm point from 71.9 71.5 71.1 70.6 center (H.sub.12.5) Hardness at 15 mm point from 72.2 71.9 71.8 71.4 center (H.sub.15) Surface hardness (Hs) 72.4 72.1 72.0 71.5 Hardness difference (Hs H0) 8.8 8.9 8.6 8.6 Spherical core No. C D E F Formulation Polybutadiene rubber 60 100 100 100 (parts by Natural rubber 40 mass) Isoprene rubber Methacrylic acid 23 Zinc acrylate 34.3 30.6 33.2 Zinc oxide 5 5 5 10 Barium sulfate Appropriate Appropriate Appropriate Appropriate amount*.sup.1) amount*.sup.1) amount*.sup.1) amount*.sup.1) Benzoic acid 2 Pentabromodiphenyl disulfide 0.4 0.4 Diphenyl disulfide 0.4 Dicumyl peroxide 1.4 0.7 0.7 0.7 Molding Vulcanizing temperature ( C.) 170 150 155 170 condition Vulcanizing time (min) 20 20 20 15 Core diameter (mm) 39.5 39.5 39.5 39.5 Core compression deformation amount (mm) 3.22 3.18 3.16 3.2 Core Center hardness (H0) 62.9 70.7 64.8 55.4 hardness Hardness at 2.5 mm point from 63.8 71.9 67.7 65.1 (Shore C) center (H.sub.2.5) Hardness at 5.0 mm point from 65.4 72.5 69.3 69.6 center (H.sub.5.0) Hardness at 7.5 mm point from 67.1 73.0 69.9 70.2 center (H.sub.7.5) Hardness at 10 mm point from 68.6 75.5 72.6 70.6 center (H.sub.10) Hardness at 12.5 mm point from 70.2 77.1 75.1 71.0 center (H.sub.12.5) Hardness at 15 mm point from 70.6 77.7 79.8 76.7 center (H.sub.15) Surface hardness (Hs) 70.8 79.3 82.2 86.2 Hardness difference (Hs H0) 7.9 8.6 17.4 30.8 *.sup.1)The amount of barium sulfate was adjusted such that the golf balls had a mass of 45.3 g.

[0146] The materials used in Table 1 are shown as follows.

[0147] Polybutadiene rubber: BR-730 (high-cis polybutadiene rubber, amount of cis-1,4 bond: 95 mass %, amount of 1,2-vinyl bond: 1.3 mass %, Moony viscosity (ML.sub.1+4 (100 C.): 55, molecular weight distribution (Mw/Mn): 3) available from JSR Corporation

[0148] Natural rubber: CV60 (Moony viscosity (ML.sub.1+4 (100 C.)=60) available from Dau Tieng Rubber Corporation

[0149] Methacrylic acid: available from Mitsubishi Gas Chemical Company, Inc.

[0150] Zinc diacrylate: ZN-DA90S available from Nisshoku Techno Fine Chemical Co., Ltd.

[0151] Zinc oxide: Ginrei R available from Toho Zinc Co., Ltd.

[0152] Barium sulfate: Barium sulfate BD available from Sakai Chemical Industry Co., Ltd.

[0153] Benzoic acid: (purity: at least 98%) available from Tokyo Chemical Industry Co. Ltd.

[0154] Bis(pentabromophenyl) disulfide: available from Kawaguchi Chemical Industry Co., Ltd.

[0155] Diphenyl disulfide: available from Sumitomo Seika Chemicals Co., Ltd.

[0156] Dicumyl peroxide: Percumyl (registered trademark) D available from NOF Corporation

(2) Preparation of Intermediate Layer Composition and Cover Composition

[0157] The materials having the formulations shown in Table 2 were extruded with a twin-screw kneading type extruder to prepare the intermediate layer composition and the cover composition in a pellet form.

TABLE-US-00002 TABLE 2 Intermediate layer/cover composition No. a b c d e f g Formulation Surlyn 8150 50 (parts by mass) Himilan 1605 47 Himilan AM7329 50 50 Himilan 1555 45 41 Himilan 1557 45 41 TEFABLOC T3221C 3 10 18 Elastollan NY80A 100 Elastollan NY84A 100 Elastollan NY88A 50 Elastollan NY95A 50 Tinuvin 770 0.2 0.2 0.2 Titanium dioxide 4 4 4 4 4 4 4 Slab hardness (Shore D) 68 63 55 50 27 31 40

[0158] Surlyn (registered trademark) 8150: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from E.I. du Pont de Nemours and Company.

[0159] Himilan (registered trademark) 1605: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Dow-Mitsui Polychemicals Co., Ltd.

[0160] Himilan (registered trademark) AM7329: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Dow-Mitsui Polychemicals Co., Ltd.

[0161] Himilan (registered trademark) 1555: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Dow-Mitsui Polychemicals Co., Ltd.

[0162] Himilan (registered trademark) 1557: zinc ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Dow-Mitsui Polychemicals Co., Ltd.

[0163] TEFABLOC (registered trademark) T3221C: thermoplastic styrene based elastomer available from Mitsubishi Chemical Corporation

[0164] Elastollan (registered trademark) NY80A: thermoplastic polyurethane elastomer available from BASF Japan Ltd.

[0165] Elastollan (registered trademark) NY84A: thermoplastic polyurethane elastomer available from BASF Japan Ltd.

[0166] Elastollan (registered trademark) NY88A: thermoplastic polyurethane elastomer available from BASF Japan Ltd.

[0167] Elastollan (registered trademark) NY95A: thermoplastic polyurethane elastomer available from BASF Japan Ltd.

[0168] Tinuvin (registered trademark) 770: hindered amine-based light stabilizer available from BASF Japan Ltd.

[0169] Titanium dioxide: A-220 available from Ishihara Sangyo Kaisha, Ltd.

(4) Production of Spherical Core

[0170] The core compositions shown in Table 1 were heat-pressed in upper and lower molds, each having a hemispherical cavity, to obtain the spherical cores. It is noted that barium sulfate was added in an appropriate amount such that the obtained golf balls had a mass of 45.3 g.

(5) Formation of Intermediate Layer and Cover

[0171] The intermediate layer composition was injection molded on the spherical core to obtain the intermediate layer-covering spherical body. The obtained intermediate layer-covering spherical body was charged into a final mold provided with a plurality of pimples on the cavity surface. Half shells were obtained from the cover composition by a compression molding method. The intermediate layer-covering spherical body charged into the final mold was covered with two of the half shells to obtain the golf balls having an outermost cover on which a plurality of dimples with an inverted shape of the pimple shape on the cavity surface were formed. The specifications of the dimples formed on the outermost cover are shown in Tables 3-4 and FIGS. 2 to 3, 5 to 8. The evaluation results regarding the obtained golf balls are shown in Tables 5 to 6.

TABLE-US-00003 TABLE 3 Lower Curvature Diameter Depth depth radius Lower Dm Dp1 Dp2 CR volume Type Number (mm) (mm) (mm) (mm) (mm.sup.3) I A 60 4.400 0.2186 0.1049 23.1 0.80 B 158 4.285 0.2127 0.1049 21.9 0.76 C 72 4.150 0.2060 0.1049 20.6 0.71 D 36 3.875 0.1930 0.1049 17.9 0.62 E 12 3.000 0.1577 0.1049 10.8 0.37 II A 60 4.400 0.2356 0.1219 19.9 0.93 B 158 4.285 0.2297 0.1219 18.9 0.88 C 72 4.150 0.2230 0.1219 17.7 0.83 D 36 3.875 0.2100 0.1219 15.5 0.72 E 12 3.000 0.1747 0.1219 9.3 0.43 III A 60 4.400 0.2526 0.1389 17.5 1.06 B 158 4.285 0.2467 0.1389 16.6 1.00 C 72 4.150 0.2400 0.1389 15.6 0.94 D 36 3.875 0.2270 0.1389 13.6 0.82 E 12 3.000 0.1917 0.1389 8.2 0.49 IV A 60 4.400 0.2696 0.1559 15.6 1.19 B 158 4.285 0.2637 0.1559 14.8 1.13 C 72 4.150 0.2570 0.1559 13.9 1.06 D 36 3.875 0.2440 0.1559 12.1 0.92 E 12 3.000 0.2087 0.1559 7.3 0.55 V A 60 4.400 0.2866 0.1729 14.1 1.32 B 158 4.285 0.2807 0.1729 13.4 1.25 C 72 4.150 0.2740 0.1729 12.5 1.17 D 36 3.875 0.2610 0.1729 10.9 1.02 E 12 3.000 0.2257 0.1729 6.6 0.61 VI A 60 4.400 0.3036 0.1899 12.8 1.45 B 158 4.285 0.2977 0.1899 12.2 1.37 C 72 4.150 0.2910 0.1899 11.4 1.29 D 36 3.875 0.2780 0.1899 10.0 1.12 E 12 3.000 0.2427 0.1899 6.0 0.67 VII A 24 4.600 0.2418 0.1176 22.6 0.98 B 54 4.500 0.2365 0.1176 21.6 0.94 C 210 4.400 0.2313 0.1176 20.6 0.90 D 24 4.000 0.2115 0.1176 17.1 0.74 E 12 3.000 0.1704 0.1176 9.6 0.42 VIII A 12 4.600 0.2404 0.1162 22.8 0.97 B 48 4.500 0.2351 0.1162 21.8 0.93 C 86 4.400 0.2299 0.1162 20.9 0.88 D 60 4.300 0.2247 0.1162 19.9 0.85 E 120 4.200 0.2197 0.1162 19.0 0.81 F 12 3.050 0.1707 0.1162 10.1 0.43

TABLE-US-00004 TABLE 4 Dimple pattern No. I II III IV V VI VII VIII Front view FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 5 FIG. 7 Plane view FIG. 3 FIG. 3 FIG. 3 FIG. 3 FIG. 3 FIG. 3 FIG. 6 FIG. 8 Total number 338 338 338 338 338 338 324 338 Total lower volume Vi (mm.sup.3) 245 285 325 365 405 445 285 285 Occupation ratio (%) 81.6 81.6 81.6 81.6 81.6 81.6 84.4 85.4

TABLE-US-00005 TABLE 5-1 Golf ball No. 1 2 3 4 5 6 Spherical core Spherical core No. A1 B1 A1 A1 A2 B2 Diameter (mm) 39.5 39.5 39.5 39.5 39.1 38.5 Compression deformation amount 3.17 3.2 3.17 3.17 3.2 3.26 Hardness Hardness difference (H.sub.2.5-H0) 0.9 0.8 0.9 0.9 1.0 0.8 difference Hardness difference (H.sub.5.0-H.sub.2.5) 1.9 1.9 1.9 1.9 1.9 2.0 (Shore C) Hardness difference (H.sub.7.5-H.sub.5.0) 2.0 2.1 2.0 2.0 2.1 2.0 Hardness difference (H.sub.10-H.sub.7.5) 1.8 1.7 1.8 1.8 1.7 1.9 Hardness difference (H.sub.12.5-H.sub.10) 1.7 1.2 1.7 1.7 1.6 1.0 Hardness difference (H.sub.15-H.sub.12.5) 0.3 0.7 0.3 0.3 0.4 0.8 Hardness difference (Hs-H.sub.15) 0.2 0.2 0.2 0.2 0.2 0.1 Hardness difference (Hs-H0) 8.8 8.6 8.8 8.8 8.9 8.6 Intermediate Intermediate layer composition No. a a b c a a layer Thickness (mm) 1 1 1 1 1 1.3 Material hardness Hm (Shore D) 68 68 63 55 68 68 Cover Cover composition No. f f g e f f Thickness (mm) 0.6 0.6 0.6 0.6 0.8 0.8 Material hardness Hc (Shore D) 31 31 40 27 31 31 Dimple Dimple pattern No. II II II II II II Total lower volume Vi (mm.sup.3) 285 285 285 285 285 285 Golf ball Surface hardness (Shore D) 59 59 60 52 56 56 Compression deformation amount 2.72 2.76 2.75 2.8 2.78 2.8 (mm) Iron shots Side spin rate (rpm) 650 630 620 700 660 650 (I#8 39 m/s) Bending amplitude (m) 7.9 7.6 7.5 8.6 8 7.9 Rolling distance (m) 7.9 8.1 8.1 7.6 7.8 8 Golf ball No. 7 8 9 10 11 12 Spherical core Spherical core No. C A1 A1 A1 A1 A1 Diameter (mm) 39.5 39.5 39.5 39.5 39.5 39.5 Compression deformation amount 3.22 3.17 3.17 3.17 3.17 3.17 Hardness Hardness difference (H.sub.2.5-H0) 0.9 0.9 0.9 0.9 0.9 0.9 difference Hardness difference (H.sub.5.0-H.sub.2.5) 1.6 1.9 1.9 1.9 1.9 1.9 (Shore C) Hardness difference (H.sub.7.5-H.sub.5.0) 1.7 2.0 2.0 2.0 2.0 2.0 Hardness difference (H.sub.10-H.sub.7.5) 1.5 1.8 1.8 1.8 1.8 1.8 Hardness difference (H.sub.12.5-H.sub.10) 1.6 1.7 1.7 1.7 1.7 1.7 Hardness difference (H.sub.15-H.sub.12.5) 0.4 0.3 0.3 0.3 0.3 0.3 Hardness difference (Hs-H.sub.15) 0.2 0.2 0.2 0.2 0.2 0.2 Hardness difference (Hs-H0) 7.9 8.8 8.8 8.8 8.8 8.8 Intermediate Intermediate layer composition No. a a a a a d layer Thickness (mm) 1 1 1 1 1 1 Material hardness Hm (Shore D) 68 68 68 68 68 50 Cover Cover composition No. f f f f f c Thickness (mm) 0.6 0.6 0.6 0.6 0.6 0.6 Material hardness Hc (Shore D) 31 31 31 31 31 55 Dimple Dimple pattern No. II I III VII VII II Total lower volume Vi (mm.sup.3) 285 245 325 285 285 285 Golf ball Surface hardness (Shore D) 59 59 59 59 59 53 Compression deformation amount 2.77 2.72 2.72 2.72 2.72 2.68 (mm) Iron shots Side spin rate (rpm) 630 650 650 650 650 560 (I#8 39 m/s) Bending amplitude (m) 7.6 8.8 7.1 8 8.1 6.6 Rolling distance (m) 8.1 7.4 8.4 8.1 8.1 8.8

TABLE-US-00006 TABLE 6-1 Golf ball No. 13 14 15 16 17 18 Spherical core Spherical core No. D E F A1 A1 A1 Diameter (mm) 39.5 39.5 39.5 39.5 39.5 39.5 Compression deformation amount 3.18 3.16 3.2 3.17 3.17 3.17 Hardness Hardness difference (H.sub.2.5-H0) 1.2 2.9 9.7 0.9 0.9 0.9 difference Hardness difference (H.sub.5.0-H.sub.2.5) 0.6 1.6 4.5 1.9 1.9 1.9 (Shore C) Hardness difference (H.sub.7.5-H.sub.5.0) 0.5 0.6 0.6 2.0 2.0 2.0 Hardness difference (H.sub.10-H.sub.7.5) 2.5 2.7 0.4 1.8 1.8 1.8 Hardness difference (H.sub.12.5-H.sub.10) 1.6 2.5 0.4 1.7 1.7 1.7 Hardness difference (H.sub.15-H.sub.12.5) 0.6 4.7 5.7 0.3 0.3 0.3 Hardness difference (Hs-H.sub.15) 1.6 2.4 9.5 0.2 0.2 0.2 Hardness difference (Hs-H0) 8.6 17.4 30.8 8.8 8.8 8.8 Intermediate Intermediate layer composition No. a a a a a a layer Thickness (mm) 1 1 1 1 1 1 Material hardness Hm (Shore D) 68 68 68 68 68 68 Cover Cover composition No. f f f f f f Thickness (mm) 0.6 0.6 0.6 0.6 0.6 0.6 Material hardness Hc (Shore D) 31 31 31 31 31 31 Dimple Dimple pattern No. II II II IV V VI Total lower volume Vi (mm.sup.3) 285 285 285 365 405 445 Golf ball Surface hardness (Shore D) 59 59 59 59 59 59 Compression deformation amount (mm) 2.73 2.71 2.75 2.72 2.72 2.72 Iron shots Side spin rate (rpm) 550 530 520 650 650 650 (I#8 39 m/s) Bending amplitude (m) 6.4 6.1 6 6.4 5.8 5.3 Rolling distance (m) 9.2 9.4 9.4 8.9 9.5 10.1 Golf ball No. 19 20 21 22 23 Spherical core Spherical core No. E E E E E Diameter (mm) 39.5 39.5 39.5 39.5 39.5 Compression deformation amount 3.16 3.16 3.16 3.16 3.16 Hardness Hardness difference (H.sub.2.5-H0) 2.9 2.9 2.9 2.9 2.9 difference Hardness difference (H.sub.5.0-H.sub.2.5) 1.6 1.6 1.6 1.6 1.6 (Shore C) Hardness difference (H.sub.7.5-H.sub.5.0) 0.6 0.6 0.6 0.6 0.6 Hardness difference (H.sub.10-H.sub.7.5) 2.7 2.7 2.7 2.7 2.7 Hardness difference (H.sub.12.5-H.sub.10) 2.5 2.5 2.5 2.5 2.5 Hardness difference (H.sub.15-H.sub.12.5) 4.7 4.7 4.7 4.7 4.7 Hardness difference (Hs-H.sub.15) 2.4 2.4 2.4 2.4 2.4 Hardness difference (Hs-H0) 17.4 17.4 17.4 17.4 17.4 Intermediate Intermediate layer composition No. d d a a a layer Thickness (mm) 1 1 1 1 1 Material hardness Hm (Shore D) 50 50 68 68 68 Cover Cover composition No. b c f f f Thickness (mm) 0.6 0.6 0.6 0.6 0.6 Material hardness Hc (Shore D) 63 55 31 31 31 Dimple Dimple pattern No. II II IV V VI Total lower volume Vi (mm.sup.3) 285 285 365 405 445 Golf ball Surface hardness (Shore D) 61 53 59 59 59 Compression deformation amount (mm) 2.62 2.66 2.71 2.71 2.71 Iron shots Side spin rate (rpm) 470 490 530 530 530 (I#8 39 m/s) Bending amplitude (m) 5.2 5.5 5.1 4.7 4.3 Rolling distance (m) 9.8 9.6 10.1 10.7 11.4

[0172] It is apparent from the results shown in Tables 5 to 6 that the golf ball according to the present disclosure that comprises a spherical core, at least one intermediate layer positioned outside the spherical core, and an outermost cover positioned outside the intermediate layer and having a plurality of dimples formed thereon, wherein the spherical core is formed from a rubber composition containing a rubber component, a co-crosslinking agent, and a crosslinking initiator, the rubber component contains a natural rubber, the co-crosslinking agent contains methacrylic acid and/or a metal salt thereof, a total lower volume Vi (mm.sup.3) of the plurality of dimples is less than 365 mm.sup.3, and an occupation ratio of the dimples defined by the following formula is 75% or more, curves to a large extent on short iron shots and rolls a short distance after landing.

[0173] Occupation ratio of dimples (%)=100total area of all dimples/surface area of a virtual sphere that is assumed to have no dimples on golf ball surface

[0174] The golf ball according to the present disclosure curves to a large extent on short iron shots and rolls in a short distance after landing.

[0175] The preferable embodiment (1) according to the present disclosure is a golf ball comprising a spherical core, at least one intermediate layer positioned outside the spherical core, and an outermost cover positioned outside the intermediate layer and having a plurality of dimples formed thereon, wherein [0176] the spherical core is formed from a rubber composition containing a rubber component, a co-crosslinking agent, and a crosslinking initiator, [0177] the rubber component contains a natural rubber, [0178] the co-crosslinking agent contains methacrylic acid and/or a metal salt thereof, [0179] a total lower volume Vi (mm.sup.3) of the plurality of dimples is less than 365 mm.sup.3, and [0180] an occupation ratio of the dimples defined by the following formula is 75% or more.

[0181] Occupation ratio of dimples (%)=100total area of all dimples/surface area of a virtual sphere that is assumed to have no dimples on golf ball surface

[0182] The preferable embodiment (2) according to the present disclosure is the golf ball according to the embodiment (1), wherein a center hardness (H0) of the spherical core, a hardness (H.sub.2.5) at a point having a radial distance of 2.5 mm from a center of the spherical core, a hardness (H.sub.5) at a point having a radial distance of 5 mm from the center of the spherical core, a hardness (H.sub.7.5) at a point having a radial distance of 7.5 mm from the center of the spherical core, a hardness (H.sub.10) at a point having a radial distance of 10 mm from the center of the spherical core, a hardness (H.sub.12.5) at a point having a radial distance of 12.5 mm from the center of the spherical core, a hardness (H.sub.15) at a point having a radial distance of 15 mm from the center of the spherical core, and a surface hardness (Hs) of the spherical core satisfy a relationship of H0<H.sub.2.5<H.sub.5<H.sub.7.5<H.sub.10<H.sub.12.5<H.sub.15<Hs.

[0183] The preferable embodiment (3) according to the present disclosure is the golf ball according to the embodiment (2), wherein the center hardness (H0), the hardness (H.sub.2.5), the hardness (H.sub.5), the hardness (H.sub.7.5), the hardness (H.sub.10), the hardness (H.sub.12.5), the hardness (H.sub.15) and the surface hardness (Hs) satisfy relationships of [0184] H.sub.2.5H0<4, [0185] H.sub.5H.sub.2.5<4, [0186] H.sub.7.5H.sub.5<4, [0187] H.sub.10H.sub.7.5<4, [0188] H.sub.12.5H.sub.10<4, [0189] H.sub.15H.sub.12.5<4, and [0190] HsH.sub.15<4 in Shore C hardness.

[0191] The preferable embodiment (4) according to the present disclosure is the golf ball according to the embodiment (2) or (3), wherein a hardness difference (HsH0) between the surface hardness (Hs) of the spherical core and the center hardness (H0) of the spherical core is 10 or less in Shore C hardness.

[0192] The preferable embodiment (5) according to the present disclosure is the golf ball according to any one of the embodiments (1) to (4), wherein the total lower volume Vi (mm.sup.3) is 350 mm.sup.3 or less.

[0193] The preferable embodiment (6) according to the present disclosure is the golf ball according to any one of the embodiments (1) to (5), wherein an amount of the natural rubber is 10 mass % or more and 80 mass % or less in the rubber component.

[0194] The preferable embodiment (7) according to the present disclosure is the golf ball according to any one of the embodiments (1) to (6), wherein a material hardness Hm of the intermediate layer is greater than a material hardness Hc of the outermost cover.

[0195] The preferable embodiment (8) according to the present disclosure is the golf ball according to any one of the embodiments (1) to (7), wherein the intermediate layer is formed from an intermediate layer composition having a slab hardness of 50 or more in Shore D hardness and containing an ionomer resin.

[0196] The preferable embodiment (9) according to the present disclosure is the golf ball according to any one of the embodiments (1) to (8), wherein the outermost cover is formed from a cover composition having a slab hardness of 40 or less in Shore D hardness and containing a urethane.

[0197] This application is based on Japanese patent application No. 2023-201042 filed on Nov. 28, 2023, the content of which is hereby incorporated by reference.