Golf Ball

Abstract

A golf ball capable of reducing the moment of inertial while reducing spin in a driver shot will be provided. To that end, a golf ball of the disclosure is a golf ball having a cover and, provided that I.sub.b (gcm.sup.2) represents the moment of inertia of the golf ball, .mu. (mm) represents deflection hardness corresponding to a deformation amount (mm) of the golf ball in a load direction from when an initial load of 10 kgf is applied to the golf ball to when a final load of 130 kgf is applied to the golf ball, and D represents Shore D hardness of the cover, a spin change amount predictive index .DELTA.S' represented by the following formula: .DELTA. S ' = ( .mu. D ) 2 82 - I b 82 2 10 6 ##EQU00001## is at least 2.0.

Description

TECHNICAL FIELD

[0001] This disclosure concerns a golf ball.

BACKGROUND

[0002] In General, desirable performance of a golf ball (hereinafter, sometimes simply referred to as a "ball") is such that the ball easily flies farther in a driver shot, while the ball easily stops in an approach shot. It has been known that, in order to obtain a ball that easily flies farther in a driver shot, the spin of the ball in a driver shot should be reduced, whereas, in order to obtain a ball that easily stops in an approach shot, the spin of the ball in an approach shot should be increased.

[0003] Conventionally, it had been believed that, in order to reduce the spin in a driver shot, it is effective to increase the moment of inertia of the ball, whereas, in order to increase the spin in an approach shot, it is effective to reduce the moment of inertia of the ball (e.g., PLT 1).

CITATION LIST

Patent Literature

[0004] PLT 1: JP-A-2014-110940

SUMMARY

[0005] We have found, however, that, by adjusting the structure of the ball appropriately, it is possible, even when the moment of inertia of the ball is small, to not only increase the spin in an approach shot, but also to reduce the spin in a driver shot.

[0006] It could be helpful to provide a golf ball capable of reducing the moment of inertia, while reducing the spin in a driver shot.

[0007] A golf ball of the disclosure is a golf ball provided with a cover, wherein, [0008] provided that [0009] I.sub.b (gcm.sup.2) represents a moment of inertia of the golf ball, [0010] .mu. (mm) represents deflection hardness corresponding to a deformation amount (mm) of the golf ball in a load direction from when an initial load of 10 kgf is applied to the golf ball to when a final load of 130 kgf is applied to the golf ball, and [0011] D represents Shore D hardness of the cover, [0012] a spin change amount predictive index .DELTA.S' represented by the following formula:

[0012] .DELTA. S ' = ( .mu. D ) 2 82 - I b 82 2 10 6 ##EQU00002##

is at least 2.0.

[0013] The golf ball of the disclosure can reduce the moment of inertia, while reducing the spin in a driver shot.

[0014] The golf ball of the disclosure may be configured such that the cover is made of urethane.

[0015] According to this configuration, the spin in a driver shot can be further reduced.

[0016] The golf ball of the disclosure may be configured such that the spin change amount predictive index .DELTA.S' is at least 2.5.

[0017] According to this configuration, it is further possible to reduce the moment of inertia, while reducing the spin in a driver shot.

[0018] The golf ball of the disclosure may be configured such that the spin change amount predictive index .DELTA.S' is at least 3.0.

[0019] According to this configuration, it is further possible to reduce the moment of inertia, while reducing the spin in a driver shot.

[0020] The golf ball of the disclosure may be configured such that [0021] the cover is coated with a top coat, and [0022] an elastic work recovery rate of the top coat is 30 to 98%.

[0023] According to this configuration, the spin in a driver shot can be further reduced.

[0024] The golf ball of the disclosure may be configured such that [0025] an outer surface of the cover has a plurality of dimples, and [0026] provided that [0027] PS7 represents an area of the golf ball in contact with a flat surface upon application of a load of 700 kgf to the golf ball against the flat surface, and [0028] VS represents, assuming that the golf ball has no dimples on its surface, an area of a circle of a cross-section of the golf ball taken along a diameter of the golf ball, [0029] the following formula:

[0029] (PS7/VS/.mu.)100.gtoreq.6.70(mm.sup.-1)

is satisfied.

[0030] According to this configuration, the spin in a driver shot can be further reduced.

[0031] According to the disclosure, a golf ball capable of reducing the moment of inertia, while reducing the spin in a driver shot, can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In the accompanying drawings:

[0033] FIG. 1 is a cross-sectional diagram illustrating an example of an internal structure of a golf ball according to one embodiment of the disclosure;

[0034] FIGS. 2A and 2B are diagrams illustrating spins of the golf ball put in a driver shot: FIG. 2A is a schematic diagram illustrating a state of a driver shot, and FIG. 2B is a graph illustrating force acting between the golf ball and a golf club in a driver shot;

[0035] FIG. 3 is a diagram illustrating effects of the golf balls of the disclosure;

[0036] FIG. 4 is a diagram illustrating the effects of the golf balls of the disclosure;

[0037] FIGS. 5A to 5F are diagrams illustrating the effects of the golf balls of the disclosure;

[0038] FIGS. 6A to 6F are diagrams illustrating the effects of the golf balls of the disclosure;

[0039] FIGS. 7A and 7B are diagrams illustrating an example of dimples applicable to the golf ball of the disclosure: FIG. 7A is a side view of an example of the golf ball, and FIG. 7B is a cross-sectional view of a portion of the golf ball illustrated in FIG. 7A;

[0040] FIGS. 8A and 8B are diagrams illustrating another example of the dimples applicable to the golf ball of the disclosure: FIG. 8A is a side view of another example of the golf ball, and FIG. 8B is a cross-sectional view of a portion of the golf ball illustrated in FIG. 8A; and

[0041] FIGS. 9A and 9B are diagrams illustrating states of the same golf ball upon applications of respective loads of 6864 N and 1961 N thereto.

DETAILED DESCRIPTION

[0042] Hereinafter, embodiments of the disclosure will be described by way of example with reference to the drawings.

[0043] [Structure of Golf Ball of the Disclosure]

[0044] A golf ball according to one embodiment of the disclosure includes, for example, in addition to a core and an intermediate layer on an outer side of the core, a cover forming an outermost layer.

[0045] FIG. 1 is a cross-sectional view illustrating an example of an internal structure of the golf ball according to one embodiment of the disclosure. A golf ball 1 of an example of FIG. 1 is what is called a five-piece golf ball including an inner core 11, an intermediate core 12 provided on an outer side of the inner core 11, an outer core 13 provided on an outer side of the intermediate core 12, an intermediate layer 14 provided on an outer side of the outer core 13, and a cover 15 having a plurality of dimples 30 formed on an outer surface thereof and provided on an outer side of the intermediate layer 14. The cover 15 is coated with a top coat 16.

[0046] However, the golf ball of the disclosure can have any internal structure other than that of FIG. 1. For example, the core of the golf ball of the disclosure does not need to have a three-layer structure composed of the inner core 11, the intermediate core 12, and the outer core 13 as illustrated in the example of FIG. 1 but can have a structure composed of one layer, two layers, or 4 or more layers. Also, the intermediate layer of the golf ball of the disclosure can be composed of a plurality of layers.

[0047] According to the golf ball of the disclosure, provided that I.sub.b (gcm.sup.2) represents the moment of inertia of the ball, .mu. (mm) represents deflection hardness of the ball, and D represents Shore D hardness of the cover, a spin change amount predictive index .DELTA.S' represented by the following formula:

.DELTA. S ' = ( .mu. D ) 2 82 - I b 82 2 10 6 ( 1 ) ##EQU00003##

is at least 2.0 (.DELTA.S'.gtoreq.2.0).

[0048] Here, the moment of inertia of the ball (I.sub.b) can be obtained by measurement using a moment of inertial measuring apparatus (for example, M01-005 manufactured inertia Dynamics, Inc.). This measuring apparatus calculates the moment of inertial of the golf ball from a difference between a period of vibration when the golf ball is placed on a jig of the measuring apparatus and a period of vibration when the golf ball is not placed.

[0049] The deflection hardness .mu. (mm) of the golf ball corresponds to a deformation amount (mm) of the golf ball in a load direction from when an initial load of 10 kgf (approx. 98 N) is applied to the golf ball to when a final load of 130 kgf (approx. 1275 N) is applied to the golf ball. The higher the value of the deflection hardness of the golf ball, the softer the golf ball.

[0050] The Shore D hardness (D) of the cover is a value obtained by preparing a sheet-like test piece with a thickness of 2 mm from a material of the cover and measuring hardness of the sheet-like test piece by using an ASTM-D2240 standard durometer "Type D". The higher the value of the Shore D hardness of the cover, the harder the cover.

[0051] Note that, as can be seen from the formula (1), in order to have a positive value (larger than zero) of the spin change amount predictive index .DELTA.S', the moment of inertia of the golf ball I.sub.b needs to be smaller than 82 gcm.sup.2. The value 82 in formula (1) is being used based on the fact that the moment of inertia of existing common golf balls is approximately 81 to 82 gcm.sup.2. That is, the moment of inertia of the golf ball of the disclosure is lower than that of the common golf balls.

[0052] Note that, hereinafter, the golf ball having the moment of inertia of 82 gcm.sup.2 is referred to as a "standard ball.

[0053] Also, the golf ball of the disclosure satisfies .DELTA.S'.gtoreq.2.0 by having the following three factors being appropriately adjusted: the moment of inertia of the ball I.sub.b, the deflection hardness of the ball, and the Shore D hardness of the cover.

[0054] The golf ball according to one embodiment of the disclosure satisfies weight (45.93 g or less) and an outer diameter (42.67 mm or more) prescribed by USGA and R&A.

[0055] As can be understood from the descriptions of Examples and Comparative Examples set forth below, the golf ball of the disclosure, as compared with the standard ball, can reduce the moment of inertia, while, not only increasing spin in an approach shot but also reducing spin in a driver shot.

[0056] [How we Obtained Formula for a Spin Change Amount Predictive Index .DELTA.S']

[0057] As described above, we have found that, depending on the structure of the ball, it is possible, even when the moment of inertia is small, to not only increase the spin in an approach shot but also to reduce the spin in a driver shot. We then conceived that the spin change amount predictive index .DELTA.S' defined by the formula (1) allows an evaluation of an actual spin change amount predictive index.

[0058] Here, how we obtained the spin change amount predictive index .DELTA.S' will be described with reference to FIGS. 2A and 2B. FIG. 2A is a schematic diagram illustrating a state of a driver shot, and FIG. 2B is a graph illustrating force acting between the golf ball 1 and a head 2 of a golf club and generated by a driver shot. In the graph of FIG. 2B, the horizontal axis represents time, and the vertical axis represents force exerted to the golf ball 1 from a club face 21 of the head 2 of the golf club. A "contact period" in FIG. 2B refers to a period in which the ball 1 is in contact with the club face 21. As for waveforms in FIG. 2B, a waveform of a solid line is a waveform of the force actually acting on the ball 1, and a sine-like wave, partially drawn with a broken line smoothly continuous from the waveform of the solid line, is provided to obtain a recoil period T described later.

[0059] As illustrated in FIGS. 2A and 2B, in a driver shot, the force (shear force) acting on the ball 1 from the club face 21 is generated first in a direction of putting a backspin on the ball 1 (in a positive direction) and, later, in a direction of putting a topspin, reverse to the backspin, on the ball 1 (in a negative direction). Here, provided that F.sub.b represents a total sum of the force (impulse) acting on the ball 1 in the direction of putting the backspin while the club face 21 and the ball 1 are in contact with each other, and F.sub.top represents a total sum of the force (the impulse) generated in the direction of putting the topspin while the club face 21 and the ball 1 are in contact with each other (taking positive and negative signs into account), as the absolute value of the total thereof (F.sub.back+t.sub.top) becomes smaller, the spin amount of the spin put on the ball 1 by a driver shot decreases, hence it becomes more favorable.

[0060] In the graph of FIG. 2B, T representing the period of the waveform partially drawn with a broken line smoothly continuous from the waveform of the solid line (also referred to as the "recoil period") is expressed by the following formula:

T = 2 .pi. / K x m + K t I ( 2 ) ##EQU00004##

Here, K.sub.x represents transverse rigidity of the ball, K.sub.t represents rotational rigidity of the ball, m represents mass of the ball, and I represents the moment of inertia of the ball.

[0061] As a result of various experiments and analyses, we have found that:

[0062] (i) as the recoil period T becomes shorter, the total sum (F.sub.back+F.sub.top) of the impulse of the force exerted on the ball 1 from the club face 21 becomes smaller, thus the spin amount decreases,

[0063] (ii) as the deflection hardness (pi) of the ball 1 becomes higher (i.e., as the ball becomes softer), the contact period of the club face 2 and the ball 1 becomes longer, and the total sum of the force (the impulse) F.sub.top generated in the direction of putting the topspin on the ball increases, and hence the spin amount decreases, and

[0064] (iii) as the Shore D hardness (D) of the cover of the ball 1 becomes smaller (i.e., as the cover becomes softer), the friction between the club face 2 and the ball 1 becomes higher, and the shear force exerted on the ball is generated earlier,

and also found the relationships between the points (i) to (iii). The points (ii) and (iii) can increase or decrease the effect of point (i). Based on these findings, we defined an index S for predicting the effect of the spin amounts in a driver shot and an approach shot from the structure of the golf ball as

S = .mu. D 2 .pi. T = .mu. D K x m + K t I ( 3 ) ##EQU00005##

The meaning of this spin amount predictive index S is such that, when ball structures of the same deflection hardness .mu. and the same Shore D hardness D are compared, where the moment of inertia I is a variable value, the larger the spin amount predictive index S, the less the spin amount in a driver shot.

[0065] In formula (3), when the moment of inertia of the ball is reduced as

I.fwdarw.I-.DELTA.I,

a change amount .DELTA.S of the spin predictive index S is

.DELTA. S = .differential. S .differential. I .DELTA. I = .mu. K t 2 D K x m + K t I .DELTA. I I 2 = K t 2 ( .mu. D ) 2 1 S .DELTA. I I 2 ( 4 ) ##EQU00006##

[0066] In formula (4), suppose

.DELTA.I=I.sub.a-I.sub.b

is satisfied,

.DELTA. S = K t 2 ( .mu. D ) 2 1 S I a - I b I a 2 ( 5 ) ##EQU00007##

is obtained. Here, I.sub.a represents the moment of inertia of the standard ball, and I.sub.b represents the moment of inertia of the ball subject to evaluation.

[0067] In formula (5), K.sub.t and S are values associated with the standard ball and thus can be regarded as constant coefficients. For convenience, the constant coefficients in formula (5) are manipulated as shown in formula (6), whereby a spin change amount predictive index .DELTA.S' is defined by

.DELTA. S ' = 2 K t S .DELTA. S 10 6 = ( .mu. D ) 2 1 S I a - I b I a 2 10 6 ( 6 ) ##EQU00008##

[0068] In formula (6), if the moment of inertia of the standard ball I.sub.a is substituted by 82, the formula (1) set forth above can be obtained.

EXAMPLES AND COMPARATIVE EXAMPLES

[0069] As described above, the golf ball of the disclosure is configured such that .DELTA.S'.gtoreq.2.0 is satisfied by having the following three factors being appropriately adjusted: the moment of inertia of the ball (I.sub.b), the deflection hardness (.mu.) of the ball, and the Shore D hardness (D) of the cover. With this configuration, as compared with the standard ball, the moment of inertia is reduced, while, not only increasing the spin in an approach shot, but also reducing the spin in a driver shot.

[0070] The golf balls of the disclosure according to Examples 1 to 13 and Comparative Examples 1 to 14 were prepared and evaluated. Results of the evaluation will be described with reference to Tables 1 to 5 and FIGS. 3 to 4. Details of Examples 1 to 13 are shown in Table 1, and details of Comparative Examples 1 to 14 are shown in Table 2.

[0071] In Table 1 and Table 2, lower case letters a to u shown in columns of "Composition" of the inner core 11, the intermediate core 12, and the outer core 13 correspond to compositions a to u in Table 3, respectively. In Table 1 and Table 2, also, upper case letters A to H shown in columns of "Composition" of the intermediate core 12, the outer core 13, the intermediate layer 14, and the cover 15 correspond to compositions A to H in Table 4, respectively. The numbers of compositions in Tables 3 and 4 are in unit of parts by weight.

[0072] In Tables 1 and 2, ".mu.: Deflection hardness (mm)" is the deformation amount (mm) of the respective balls in the load direction from when an initial load of 10 kgf (approx. 98 N) is applied to the ball to when a final load of 130 kgf (approx. 1275 N) is applied to the ball.

[0073] In Tables 1 and 2. "I.sub.b: Moment of inertia (gcm.sup.2)" is a value obtained by measuring the respective balls using a moment of inertia measuring apparatus (M01-005 manufactured inertia Dynamics, Inc.).

[0074] In Tables 1 and 2, "Shore D hardness" of the intermediate layer 14 and "D: Shore D hardness" of the cover 15 were obtained by preparing a sheet-like test piece with the thickness of 2 mm from respective materials and measuring the hardness of the test piece using an ASTM-D2240 standard durometer "Type D".

[0075] In Tables 1 and 2, ".DELTA.S': Spin change amount predictive index" is a value calculated from the formula (1) set forth above by using values p, D, and I.sub.b of the respective balls.

[0076] In Tables 1 and 2. "Driver spin (rpm)" and "Approach spin (rpm)" refer to results of experiments of the spin amounts that were obtained by a driver shot and an approach shot, respectively, using respective balls.

[0077] In experiments of driver shot, a driver (W#1) was attached to a Golf Swing Robot (manufactured by Miyamae Co., Ltd.), and the spin amount at the time when the robot hit the ball, with a head speed (HS) of 45 m/s, was measured. The golf club that was used for the experiments was "TourStage X-Drive 705 TYPE415 (2011 model)" (loft: 9.5.degree.) manufactured by Bridgestone Sports Co., Ltd.

[0078] In experiments of approach shot, a sand wedge (SW) was attached to a Golf Swing Robot (manufactured by Miyamae Co., Ltd.), and the spin amount at the time when the robot hit the ball, with a head speed (HS) of 20 m/s, was measured. The golf club that was used for the experiments was "TourStage X-WEDGE" (loft: 56.degree.) manufactured by Bridgestone Corporation.

[0079] The columns "Dimples", "PS7: Pressured area", "PS2: Pressured area". "VS: Virtual area", "(PS7/VS/.mu.)100 (mm.sup.-1)", "(PS2/VS/.mu.)100 (mm.sup.-1)", and "Top coat" in Tables 1 and 2 will be described later.

TABLE-US-00001 TABLE 1 Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- ample ample ample ample ample ample ample ample ample ample ample ample ample 1 2 3 4 5 6 7 8 9 10 11 12 13 Inner Diameter (mm) 18.1 18.1 18.1 18.1 18.1 18.1 18.1 18.1 18.1 18.1 18.1 18.1 18.1 core Specific gravity 2.85 2.85 2.85 1.78 1.78 2.85 2.85 2.85 2.85 2.85 2.85 2.85 2.85 Composition d e f h i e f d e f d e f Inter- Diameter (mm) 29 29 29 -- -- 29 29 29 29 29 29 29 29 mediate Specific gravity 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 core Composition E F G F G E F G E F G Outer Diamter (mm) 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 core Specific gravity 0.96 0.96 0.96 1.1 1.1 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 Composition D E H k l E H D E H D E H Inter- Diameter (mm) 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 mediate Specific gravity 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 layer Shore D 62 62 62 62 62 62 62 62 62 62 62 62 62 hardness Composition CI CI CI CI CI CI CI CI CI CI CI CI CI Cover Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Specific gravity 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 D: Shore D 47 47 47 47 47 61 61 47 47 47 47 47 47 hardness Composition A A A A A B B A A A A A A .mu.: Deflection 2.5 3.0 3.5 3.0 3.5 2.8 3.3 2.5 3.0 3.5 2.5 3.0 3.5 hardness (mm) I.sub.b: Moment of 74.5 74.5 74.5 78.7 78.7 74.5 74.5 74.5 74.5 74.5 74.5 74.5 74.5 inertia (g cm.sup.2) Dimples FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. 7A 7A 7A 7A 7A 7A 7A 7A 7A 7A 8A 8A 8A and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 8B and 8B and 8B PS7: pressured area 225 265 297 268 301 252 284 222 269 290 282 329 365 PS2: pressured area 64 72 89 74 92 70 84 62 76 90 75 91 100 VS: virtual area 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 (PS7/VS/.mu.) 100 (mm.sup.-1) 6.28 6.17 5.93 6.24 6.01 6.28 6.01 6.20 6.26 5.79 7.88 7.66 7.28 (PS7/VS/.mu.) 100 (mm.sup.-1) 1.79 1.68 1.78 1.72 1.84 1.75 1.78 1.73 1.77 1.80 20.9 2.12 2.00 Top Composition I I I I I I I J J J I I I Coat Film thickness 15 15 15 15 15 15 15 15 15 15 15 15 15 (.mu.m) Elastic work 16.3 16.3 16.3 16.3 16.3 16.3 16.3 80.1 80.1 80.1 16.3 16.3 16.3 recovery rate (%) .DELTA.S : Spin change 3.2 4.5 6.2 2.0 2.7 2.4 3.3 3.2 4.5 6.2 3.2 4.5 6.2 amount predictive index Driver spin (rpm) 2821 2665 2515 2858 2674 2639 2445 2805 2654 2527 2811 2651 2518 Approach spin (rpm) 6319 6169 6019 6063 5856 5819 5669 6332 6194 6034 6341 6197 6043 indicates data missing or illegible when filed

TABLE-US-00002 TABLE 2 Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative ative ative ative ative ative ative ative ative ative Example Example Example Example Example Example Example Example Example Example Example Example Example Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Inner Diameter (mm) 18.1 18.1 18.1 -- -- -- 18.1 18.1 18.1 18.1 -- -- -- 18.1 core Specific gravity 1.4 1.4 1.4 1.78 1.4 1.4 1.4 2.85 Composition p q r g p q r d Inter- Diameter (mm) -- -- -- -- -- -- -- -- -- -- -- -- -- -- mediate Specific gravity core Composition Outer Diamter (mm) 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 core Specific gravity 1.14 1.14 1.14 1.17 1.17 1.17 1.1 1.14 1.14 1.14 1.17 1.17 1.17 0.96 Composition s t u a b c j s t u m n o D Inter- Diameter (mm) 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 41.05 mediate Specific gravity 0.96 0.96 0.96 1.0 1.0 1.0 0.96 0.96 0.96 0.96 1.0 1.0 1.0 0.96 layer Shore D 62 62 62 62 62 62 62 62 62 62 62 62 62 62 hardness Composition C1 C1 C1 C2 C2 C2 C1 C1 C1 C1 C2 C2 C2 C2 Cover Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Specific gravity 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.16 D: Shore D 47 47 47 47 47 47 47 61 61 61 61 61 61 61 hardness Composition A A A A A A A B B B B B B B .mu.: Deflection 2.5 3.0 3.5 2.5 3.0 3.5 2.5 2.3 2.8 3.3 2.3 2.8 3.3 2.3 hardness (mm) I.sub.b: Moment of 80 80 80 82 82 82 78.7 80 80 80 82 82 82 74.5 inertia (g cm.sup.2) Dimples FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. FIGS. 7A 7A 7A 7A 7A 7A 7A 7A 7A 7A 7A 7A 7A 7A and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B and 7B PS7: pressured area 224 267 301 222 260 295 221 211 250 285 223 261 292 220 PS2: pressured area 63 75 93 63 70 86 62 56 72 83 64 71 85 63 VS: virtual area 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 1432 (PS7/VS/.mu.) 100 (mm.sup.-1) 6.26 6.22 6.01 6.20 6.05 5.89 6.17 6.41 6.24 6.03 6.77 6.51 6.18 6.68 (PS7/VS/.mu.) 100 (mm.sup.-1) 1.76 1.75 1.86 1.76 1.63 1.72 1.73 1.70 1.80 1.76 1.94 1.77 1.80 1.91 Top Composition I I I I I I I I I I I I I I Coat Film thickness 15 15 15 15 15 15 15 15 15 15 15 15 15 15 (.mu.m) Elastic work 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 recovery rate (%) .DELTA.S : Spin change amount 0.8 1.2 1.6 0 0 0 1.4 0.4 0.6 0.9 0 0 0 1.6 predictive index Driver spin (rpm) 3102 2941 2779 3079 2919 2759 3142 2898 2740 2579 2879 2719 2559 2913 Approach spin (rpm) 6201 6023 5836 6155 5986 5816 6251 5883 5692 5486 5805 5636 5466 5969 indicates data missing or illegible when filed

TABLE-US-00003 TABLE 3 Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- sition sition sition sition sition sition sition sition sition sition sition sition a b c d e f g h i j k l Polyb 100 100 100 100 100 100 100 100 100 100 100 100 Zinc 27.0 23.0 19.0 20.0 17.0 14.0 20.0 17.0 14.0 27.0 23.0 19.0 acrylate Peroxide *1 3 3 3 3 3 3 3 3 3 3 3 3 Tungsten 0 0 0 268 266 264 102 101.8 101.6 0 0 0 Anti-aging 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 agent *2 Zinc oxide 21.0 22.5 24.0 5.0 5.0 5.0 5. 5.0 5.0 10.0 11.5 13.5 Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- sition sition sition sition sition sition sition sition sition m n o p q r s t u Polyb 100 100 100 100 100 100 100 100 100 Zinc 25.5 21.5 17.5 20.0 17.0 14.0 27.0 23.0 19.0 acrylate Peroxide *1 3 3 3 3 3 3 3 3 3 Tungsten 0 0 0 48 48.5 49 0 0 0 Anti-aging 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 agent *2 Zinc oxide 22.0 23.5 25.0 5.0 5.0 5.0 8.0 10.0 12.0 *1 Peroxide(2): Mixture of 1,1-Di(t-butylperoxy)cyclohexane and silica, product name PER (product of NOF Corporation) *2 Anti-aging agent: product name No NS-6 (product of Ouchi Shinko Chemical Industrial Co. Ltd) indicates data missing or illegible when filed

TABLE-US-00004 TABLE 4 Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- sition sition sition sition sition sition sition sition sition A B C1 C2 D E F G H T-8290 75 -- -- -- -- -- -- -- -- T-8283 25 -- -- -- -- -- -- -- -- T-8260 -- 100 -- -- -- -- -- -- -- Himilan 1706 -- -- 35 35 -- -- -- -- -- Himilan 1557 -- -- 15 15 -- -- -- -- -- Himilan 1605 -- -- 50 50 -- -- -- -- -- HPF1000 -- -- -- -- 100 -- -- -- -- HPF2000 -- -- -- -- -- 100 -- -- 50 AD1035 -- -- -- -- -- -- 100 -- 50 AD1172 -- -- -- -- -- -- -- 100 -- Hytrel 4001 11 11 -- -- -- -- -- -- -- Titanium oxide 3.9 3 -- 7 -- -- -- -- -- Polyethylene 1.2 1 -- -- -- -- -- -- -- wax Isocyanate 7.5 7.5 -- -- -- -- -- -- -- compound Trimethylol- -- -- 1.1 1.1 -- -- -- -- -- propane

[0080] The following are details of materials in Table 4.

[0081] T-8290: PANDEX.RTM. produced by DIC Bayer Polymer Ltd., MDI-PTMG-type thermoplastic polyurethane

[0082] T-8283: PANDEX.RTM.@ produced by DIC Bayer Polymer Ltd., MDI-PTMG-type thermoplastic polyurethane

[0083] T-8260: PANDEX.RTM. produced by DIC Bayer Polymer Ltd., MDI-PTMG-type thermoplastic polyurethane

[0084] Himilan 1706: ionomer produced by Du Pont-Mitsui Polychemicals Co., Ltd.

[0085] Himilan 1557: ionomer produced by Du Pont-Mitsui Polychemicals Co., Ltd.

[0086] Himilan 1605: ionomer produced by Du Pont-Mitsui Polychemicals Co., Ltd.

[0087] HPF1000: Dupont HPF

[0088] HPF2000: Dupont HPF

[0089] AD1035: Dupont HPF

[0090] AD1172: Dupont HPF

[0091] Hytrel 4001: thermoplastic polyether-ester elastomer produced by Du Pont-Toray Co., Ltd.

[0092] Polyethylene wax: "SANWAX161P" (product of Sanyo Chemical Industries, Ltd.)

[0093] Isocyanate composition: 4,4'-diphenylmethane diisocyanate

*Note that C1 and C2 have equivalent physical property values, and differ only in specific gravity.

[0094] FIG. 3 is a diagram illustrating the driver spin (rpm) and the approach spin (rpm) of the balls of the Examples 1 to 5 and Comparative Examples 1 to 7, the covers 15 of which have the Shore D hardness (D) of 47. FIG. 4 is a diagram illustrating the driver spin (rpm) and the approach spin (rpm) of the balls of the Examples 6 to 7 and Comparative Examples 8 to 14, the covers 15 of which have the Shore D hardness (D) of 61. In FIG. 3 and FIG. 4, the horizontal axis represents the driver spin (rpm), and the vertical axis represents the approach spin (rpm). As described above, it is desirable when the spin amount is small in a driver shot and when the spin amount is large in an approach shot. Therefore, in FIG. 3 and FIG. 4, performance of the ball becomes more favorable while moving from the lower right towards the upper left.

[0095] As can be seen in FIG. 3 and FIG. 4, when comparing the balls of the Comparative Examples and Examples which have the covers 15 of the same Shore D hardness (D), the balls of the Examples which satisfied .DELTA.S'.gtoreq.2.0, as compared with the balls of the Comparative Examples, favorably achieved both increase of the spin amount in an approach shot and reduction of the spin amount in a driver shot.

[0096] FIGS. 5A to 5F each illustrate the driver spin (rpm) of the balls of the Comparative Examples and Examples which have the covers 15 of the same Shore D hardness (D) and deflection hardness (.mu.). FIGS. 6A to 6F each illustrate the approach spin (rpm) of the balls of the Comparative Examples and Examples which have the covers 15 of the same Shore D hardness (D) and deflection hardness (p).

[0097] As can be seen in FIGS. 5A to 5F and FIGS. 6A to 6F, when comparing the balls of the Comparative Examples and Examples which have the covers 15 of the same Shore D hardness (D) and deflection hardness (pi), the balls of the Examples which satisfied .DELTA.S'.gtoreq.2.0, as compared with the balls of the Comparative Examples, favorably achieved reduction of the spin amount in a driver shot or increase of the spin amount in an approach shot.

[0098] Note that, from the viewpoint of reducing the moment of inertia of the ball while reducing the spin amount of a driver shot, the ball 1 of the present embodiment has the spin change amount predictive index .DELTA.S' of preferably at least 2.5, more preferably at least 3.0.

[0099] From a similar viewpoint, the ball 1 of the present embodiment preferably satisfies I.sub.b.ltoreq.80 gcm.sup.2, 2.0 mm.ltoreq..mu..ltoreq.4.5 mm, and D.ltoreq.65. More preferably, the ball 1 of the present embodiment satisfies 72 gcm.sup.2.ltoreq.I.sub.b.ltoreq.79 gcm.sup.2, 2.5 mm.ltoreq..mu..ltoreq.3.0 mm, and D.ltoreq.55.

[0100] The cover 15 of the ball 1 of the present embodiment is preferably made of urethane. Therein a driver shot, the frictional force between the ball 1 and the club face 21 of the golf club can be increased, hence the spin amount can be further reduced.

[0101] [Dimples]

[0102] Next, the dimples 30 of the ball 1 of the present embodiment will be described in more detail. The dimples 30 of the ball 1 of the present embodiment can have any shape. FIGS. 7A to 7B and FIGS. 8A to 8B illustrate different examples of the dimples applicable to the ball 1 of the present embodiment.

[0103] In the example illustrated in FIGS. 7A to 7B, each dimple 30 is curved in a convex shape protruding toward the inside of the golf ball.

[0104] In the example illustrated in FIGS. 8A to 8B, the bottom surface of each of the dimples 30, only in the central area thereof, has a shape protruding toward the outside of the golf ball. In this case, without compromising original aerodynamic performance of the dimples 30, the pressured area, which will be described later, can be enhanced. Note that, as illustrated in FIG. 8B, the portion protruding toward the outside of the ball 1 in the central area of the dimples 30 may have a flat shape in the further central region of the central area. In this case, as illustrated in FIG. 8B, the peripheral portion of the flat region may have a chamfered (rounded) corner, whereby the contact area between the ball and the club face 21 at the time when the ball is hit can be increased, and hence the spin amount in a driver shot can be reduced.

[0105] Here, the ball 1 of the present embodiment preferably satisfies:

(PS7/VS/.mu.)100.gtoreq.5.70(mm.sup.-1) (7)

[0106] In formula (7), "PS7" represents an area (referred to as the "pressured area") (mm.sup.2) of the golf ball contacting with a flat surface when a load of 700 kgf (approx. 6864 N) is applied to the golf ball against the flat surface. In formula (7), "VS" represents an area (referred to as a "virtual plane area") (mm.sup.2) of the circle of the cross-section of the golf ball taken along the diameter of the golf ball, when it is assumed that the golf ball has no dimples 30 on its surface. In formula (7). ".mu." represents the deflection hardness (mm) of the ball 1 described above.

[0107] Note that "PS7/VS/.mu." in formula (7) is synonymous with "PS7/(VS.mu.)". That is, ".mu." in formula (7) is a variable of the denominator.

[0108] When the pressured area PS7 of the golf ball upon application of the load in a driver shot by a typical golfer satisfies the above formula (7), the contact area between the ball 1 and the club face 21 of the golf club increases and, simultaneously, the frictional force between the ball 1 and the club face 21 is enhanced. As a result, the backspin amount in a driver shot can be reduced, and hence the fly distance can be improved.

[0109] Note that, from a similar viewpoint, the ball 1 of the present embodiment more preferably satisfies the following formula:

(PS7/VS/.mu.)100.gtoreq.6.70(mm.sup.-1) (8)

[0110] Also, the ball 1 of the present embodiment preferably satisfies the following formula:

(PS2/VS/.mu.)100.gtoreq.1.70(mm.sup.-1) (9)

[0111] In formula (9), "PS2" is the area (referred to as the "pressured area") (mm.sup.2) of the golf ball contacting with a flat surface when a load of 200 kgf (approx. 1961 N) is applied to the golf ball against the flat surface. VS and p are the same as those of the formulas (7) and (8).

[0112] Note that "PS2/VS/.mu." in formula (9) is synonymous with "PS2/(VS.mu.)". That is, ".mu." in formula (9) is a variable of the denominator.

[0113] When the pressured area PS2 of the golf ball upon application of the load in an approach shot by a typical golfer satisfies the above formula (9), the contact area between the ball 1 and the club face 21 of the golf club increases and, simultaneously, the frictional force between the ball 1 and the club face 21 is enhanced. Therefore, the backspin amount in an approach shot can be increased, and hence the ball 1 can stop sooner near its falling point.

[0114] Also, when the above formula (9) is satisfied, the total sum of the impulse (F.sub.back+F.sub.top) of the force exerted on the ball 1 from the club face 21 in a driver shot becomes smaller and, simultaneously, the contact period of the club face 2 and the ball 1 becomes longer. Therefore, the total (the impulse) of the force generated in the direction of putting the top spin on the ball is increased, thereby the spin amount can be further reduced.

[0115] Note that, from a similar viewpoint, the ball 1 of the present embodiment more preferably satisfies the following formula:

(PS2/VS/.mu.)100.gtoreq.1.90(mm.sup.-1) (10)

[0116] In reference to Tables 1 and 2, the balls of the Examples 1 to 10 and Comparative Examples 1 to 14 had the dimples 30 in the shape illustrated in FIGS. 7A and 7B, and the balls of the Examples 11 to 13 had the dimples 30 in the shape illustrated in FIGS. 8A and 8B.

[0117] The balls of the Examples 1 to 10 and Comparative Examples 1 to 14 each had the dimples 30 of six types with different diameters, out of which the dimples 30 with a typical diameter of 4.4 mm, as illustrated in FIG. 7B, had a depth L of 0.150 mm at its deepest point.

[0118] The balls of the Examples 11 to 13 had the dimples 30 of six types with different diameters, out of which the dimples 30 with a typical diameter of 4.4 mm, as illustrated in FIG. 8B, had a depth H of 0.097 mm at its central point C, a depth D of 0.131 mm at its deepest point, and, provided that a distance L1 along a virtual extension plane (a chain double-dashed line in FIG. 8B) of the peripheral surface of the ball 1 from the peripheral edge E to the central point C is 100, a distance L2 along the virtual extension plane of the peripheral surface of the ball 1 from the peripheral edge E to an adjacent deepest position was 39. Further, the dimples 30 with the typical diameter of 4.4 mm had a radius of curvature R of 0.5 mm and an edge angle A2 of 10.5.degree..

[0119] In Tables 1 and 2, the pressured areas PS7 and PS2 of each ball were measured by the following method. First, a pressure-sensitive sheet (a pressure measuring film, PRESCALE for medium pressure produced by Fujifilm Corporation) was placed on a flat surface, and the golf balls of the Examples and Comparative Examples were placed thereon. Then, by using Model 4204 produced instron Corporation, the load of 700 kgf (approx. 6864 N) and the load of 200 kgf (approx. 1961 N) were separately applied to the golf balls, and then the total area where the pressure-sensitive sheet developed color due to contact with the golf ball was measured by using PRESCALE pressure image analysis system FPD-9270 (product of Fujifilm Corporation). The pressured areas PS7 and PS2 in Tables 1 and 2 indicate results of the measurement conducted on a random portion of the golf ball.

[0120] FIG. 9A illustrates an example of an actual pressure-sensitive sheet which developed color upon application of the load of 700 kgf (approx. 6864 N) to the golf ball, and FIG. 9B illustrates an example of an actual pressure-sensitive sheet which developed color upon application of the load of 200 kgf (approx. 1961 N) to the same golf ball as that of FIG. 9A. In these figures, the circle portions are the dimples 30, and the colored area is where the color was developed.

[0121] [Top Coat]

[0122] Next, the top coat 16 coated on the cover 15 of the ball 1 of the present embodiment will be described in more detail. For the ball 1 of the present embodiment, the method of forming the top coat 16 (a coating layer) by coating the outer surface of the cover 15 with a coating material may be any method including, for example, an air gun coating method, an electrostatic coating method, and the like.

[0123] The thickness of the top coat 16 is not particularly limited but is normally 8 to 22 .mu.m, preferably 10 to 20 .mu.m.

[0124] The top coat 16 preferably has an "elastic work recovery rate", which will be described later, of 30 to 98%, more preferably 70 to 90%. When the elastic work recovery rate of the top coat 16 is within the above ranges, the coating film formed on the surface of the golf ball has higher self-repair-and-recovery function while maintaining constant hardness and elasticity, thereby contributing to excellent durability and abrasion resistance of the ball. However, when the elastic work recovery rate of the top coat 16 deviates from the above ranges, there is a risk that sufficient approach spin may not be obtained.

[0125] The elastic work recovery rate of the top coat 16 is one of parameters of a nanoindentation method, which is used for evaluating physical properties of a coating film, and which is an ultra-micro hardness testing method where indentation load is controlled in the order of micronewton (.mu.m) and the depth of an indenter at the time of indentation is tracked with the accuracy of nanometer (nm). Although the conventional method could only measure the size of a deformation mark (a plastic deformation mark) corresponding to the maximum load, the nanoindentation method can obtain a relationship between the indentation load and the indentation depth by automatic and continuous measurement. Therefore, unlike the conventional method, there is no individual differences in visual measurement of the deformation mark using an optical microscope, and hence the nanoindentation method is considered to be able to reliably and highly accurately evaluate the physical properties of a coating film. Accordingly, since the coating film on the surface of the golf ball can be greatly affected by the hitting by the driver or various golf clubs and can have more than little influence on various physical properties of the golf ball, measuring the coating film of the golf ball more accurately than the conventionally by using the ultra-micro hardness testing method enables a very effective evaluation.

[0126] For the balls of the Examples and Comparative Examples shown in Table 1 and 2, on the outer surface of the cover 15 (the outermost layver) having numerous dimples 30 formed thereon, the coating material was painted with an air spray gun so as to form the top coat 16 with a thickness of 15 .mu.m. In Tables 1 and 2, alphabets I and J in columns of the "Composition" of the top coat 16 correspond to Composition I and Composition J in the following Table 5, respectively.

TABLE-US-00005 TABLE 5 Costing composition (parts by mass) Composition I Composition J Main Polyol (1) 100.0 -- agent Polyol (2) -- 100.0 Ethyl acetate 100.0 60.0 Propylene glycol 40.0 40.0 monomethyl ester acetate Curing catalyst 0.03 0.03 Curing Nurate body of 30.5 52.5 agent hexamethylene diisocyanate (1) Modified polyster of 46.8 -- hexamethylene diisocyanate (2) Ethyl acetate 42.7 47.5 Mixing molar ratio (NCO/OH) 1.08 1.08 *Coating composition A (molar ratio of NCO) ***Nurate body of hexamethylene diisocyanate (1): Modified polyester of hexamethylene diisocyanate (2) = 0.79:0.29.

[0127] Here, synthesis examples of acrylic polyol (1) and (2) in Table 5 will be described. Note that, in the following description, "parts" means "parts by mass".

Synthesis Example 1 of Acrylic Polyol

[0128] Into a reactor vessel equipped with a stirrer, a thermometer, a cooling pipe, a nitrogen gas introducing pipe, and a dropping device, 1000 parts of butyl acetate was introduced and, while being stirred, heated to 100.degree. C. Into thus obtained butyl acetate, a mixture of 220 parts of acrylic monomer containing polyester (PLACCEL FM-3 produced by Daicel Chemical Industries, Ltd.), 610 parts of methyl methacrylate, 170 parts of 2-hydroxyethyl methacrylate, and 30 parts of 2,2'-azobisisobutyronitrile was dropped over the period of 4 hours. After the dropping, a mixture thus obtained was left to react at the same temperature for 6 hours. After the reaction, 180 parts of butyl acetate and 150 parts of polycaprolactone diol (PLACCEL L205AL produced by Daicel Chemical Industries, Ltd.) were introduced into a resulting mixture and mixed. Thereby, transparent acrylic polyol resin solution (Polyol (1) in Table 5) with 50% solid content, viscosity of 100 mPas (25.degree. C.), weight average molecular weight of 10,000, and a hydroxyl value of 113 mgKOH/g (solid content) was obtained.

Synthesis Example 2 of Acrylic Polyol

[0129] Into a reactor vessel equipped with a stirrer, a thermometer, a cooling pipe, a nitrogen gas introducing pipe, and a dropping device, 1000 parts of butyl acetate was introduced and, while being stirred, heated to 100.degree. C. Into thus obtained butyl acetate, a mixture of 620 parts of acrylic monomer containing polyester (PLACCEL FM-3 produced by Daicel Chemical Industries, Ltd.), 317 parts of methyl methacrylate, 63 parts of 2-hydroxyethyl methacrylate, and 12 parts of 2.2'-azobisisobutyronitrile was dropped over the period of 4 hours. After the dropping, a mixture thus obtained was left to react at the same temperature for 6 hours. After the reaction, 532 parts of butyl acetate and 520 parts of polycaprolactone diol (PLACCEL L205AL produced by Daicel Chemical Industries, Ltd.) were introduced into a resulting mixture and mixed. Thereby, transparent acrylic polyol resin solution (Polyol (2) in Table 5) with 50% solid content, viscosity of 60 mPas (25.degree. C.), weight average molecular weight of 70,000, and ahydroxyl value of 142 mgKOH/g (solid content) was obtained.

[0130] The elastic work recovery rate of the top coat 16 of the ball 1 of the Examples and Comparative Examples in Tables 1 and 2 was measured as follows. First, from the coating material used for the top coat 16, a coating film sheet with a thickness of 100 .mu.m was prepared. Then, by using an ultra-micro hardness tester "ENT-2100" produced by ELIONIX Inc., the elastic work recovery rate was measured under the following condition. [0131] Indenter: Berkovich indenter (material: diamond, angle .alpha.: 65.03.degree.) [0132] Load F: 0.2 mN [0133] Loading period: 10 seconds [0134] Maintaining period: 1 second [0135] Unloading period: 1 second

[0136] Note that, based on an indention work amount W.sub.elast (Nm) due to a restoring deformation of the coating film and a mechanical indention work amount W.sub.total (Nm), the elastic work recovery rate can be calculated from the following equation.

Elastic work recovery rate=(W.sub.elast/W.sub.total)100(%) (11)

Claims

1. A golf ball having a cover, wherein, provided that I.sub.b (gcm.sup.2) represents a moment of inertia of the golf ball, .mu. (mm) represents deflection hardness corresponding to a deformation amount (mm) of the golf ball in a load direction from when an initial load of 10 kgf is applied to the golf ball to when a final load of 130 kgf is applied to the golf ball, and D represents Shore D hardness of the cover, a spin change amount predictive index .DELTA.S' represented by the following formula: .DELTA. S ' ( .mu. D ) 2 82 - I b 82 2 10 6 ##EQU00009## is at least 2.0.

2. The golf ball according to claim 1, wherein the cover is made of urethane.

3. The golf ball according to claim 1, wherein the spin change amount predictive index .DELTA.S' is at least 2.5.

4. The golf ball according to claim 1, wherein the spin change amount predictive index .DELTA.S' is at least 3.0.

5. The golf ball according to claim 1, wherein the cover is coated with a top coat, and an elastic work recovery rate of the top coat is 30 to 98%.

6. The golf ball according to claim 1, wherein an outer surface of the cover has a plurality of dimples, and provided that PS7 represents an area of the golf ball in contact with a flat surface upon application of a load of 700 kgf to the golf ball against the flat surface, and VS represents, assuming that the golf ball has no dimples on its surface, an area of a circle of a cross-section of the golf ball taken along a diameter of the golf ball, the following formula: (PS7/VS/.mu.)100.gtoreq.6.70(mm.sup.-1) is satisfied.