Solid golf ball

Disclosed is a solid golf ball (including a one piece solid golf ball and another type solid golf ball) which has excellent durability and a high impact resilience, as well as good feeling when struck. The solid golf ball of the present invention is characterized in that a hardness (H) of a rubber portion of the solid golf ball, when measured by a JIS-C hardness meter, decreases with distance from the surface to the center and satifies the following equation; EQU When 0.ltoreq.l.ltoreq.5 416-6l.ltoreq.5H.ltoreq.440-6l EQU When 5<l.ltoreq.r 77.ltoreq.H.ltoreq.82 wherein l is a distance in mm from the surface and r is the radius of the golf ball.

FIELD OF THE INVENTION 
The present invention relates to a solid golf ball which is excellent in 
durability, impact resilience and feeling when struck. 
BACKGROUND OF THE INVENTION 
Thread wound golf balls have a high impact resilience and a high initial 
velocity at the time of an impact, and exhibit good feeling when struck. 
They, however, are poor in durability. 
In order to improve durability, one piece golf balls and two piece golf 
balls (i.e. solid golf balls) have been developed. The solid golf balls, 
however, are poor in impact resilience and feeling when struck, in 
comparison with the thread wound golf balls. 
In order to improve the defects of the solid golf balls, Japanese Kokoku 
Publication (examined) 21426/1986 proposes that the hardness distribution 
of a golf ball, when measured by a JIS-C hardness meter, is controlled to 
72 to 78 on the surface, 77 to 83 at 5 mm from the surface, 72 to 80 at a 
point 5 mm further inside, 67 to 75 at a point 5 further mm inside and 
less than 75 at the center. The proposed golf ball has improved impact 
resilience and durability, but is not sufficiently improved with respect 
to feeling when struck. 
Japanese Kokai Publication (unexamined) 199471/1985 discloses that the 
hardness distribution of a golf ball core, when measured by a JIS-C 
hardness meter, is adjusted to 75 to 85 and a hardness difference between 
any two points of less than 5. This invention makes the hardness 
distribution flat to improve impact resilience, durability and feeling 
when struck. Thus, the feeling when struck becomes soft, but still 
requires improvement. If it is struck outside a sweet spot, one feels it 
to be heavy. 
Japanese Kokai Publication (unexamined) 49840/1976 discloses a golf ball 
which comprises a core and a cover wherein the core is made of a plurality 
of layers, and the farther out the layer, the harder the hardness. 
However, it is complicated to produce the golf ball, thus making it 
expensive. The golf ball also is insufficient in durability. 
SUMMARY OF THE INVENTION 
The present invention provides a solid golf ball (including a one piece 
solid golf ball and another type of solid golf ball) which has excellent 
durability and a high impact resilience, as well as good feeling when 
struck. The solid golf ball of the present invention is characterized in 
that the hardness (H) of a rubber portion of the solid golf ball, when 
measured by a JIS-C hardness meter, discreases with distance from the 
surface to the center and meets the following equation; 
EQU When 0.ltoreq.l.ltoreq.5 415-6 l.ltoreq.5 H.ltoreq.440-6 l 
EQU When 5&lt;l.ltoreq.r 77.ltoreq.H .ltoreq.82 
wherein l is a distance (mm) from the surface and r is the radius of the 
golf ball. 
DETAILED DESCRIPTION OF THE INVENTION 
The hardness of the present invention is determined by cutting a golf ball 
in half and measuring on the center line of the half with a JIS-C hardness 
meter according to JIS vulcanized rubber physical test K-6301. The surface 
hardness is measured on the surface of the spherical rubber portion. 
The hardness is simply reduced from the surface to the center. This does 
not mean the complete reduction of the hardness, but allows small 
scattering. Accordingly, it may happen that the minimum hardness is not on 
the center. 
The hardness (H) and the distance (l) from the surface must meet the 
claimed equation. This equation is plotted on a graph with the hardness on 
the ordinate and the distance on the abscissa, thus showing a diagonal 
portion in FIG. 1. If the hardness of the surface is more than 89, impact 
resilience and crack resistance are poor. If the hardness between the 
center and a 5 mm inside from the surface is more than 82, the striking 
feeling becomes poor and one feels a hard feeling. If the hardness of the 
surface is less than 83, one feels the ball heavy at impact and impact 
resilience is also poor. If the hardness of the center is less than 77, 
impact resilience and crack resistance are lowered. 
The term "rubber portion" herein means a ball itself in the case of a one 
piece solid golf ball as shown in FIG. 2, and a rubber core in the case of 
a two piece solid golf ball comprising a core and a cover covering the 
core as shown in FIG. 3. For the sake of simplicity a two piece golf ball 
is exemplified hereinafter. 
The core of the golf ball is generally prepared by heating and molding a 
rubber composition which comprises base rubber, a metal salt of an 
unsaturated aliphatic acid, filler (e.g. zinc oxide) and polymerization 
initiator. Typical examples of the base rubbers are polybutadiene which is 
preferred, styrene-butadiene rubber, natural rubber, high-styrene resin 
and a mixture thereof. More preferred is cis 1,4-polybutadiene rubber. The 
unsaturated aliphatic metal salt includes a metal salt of a C.sub.3 
-C.sub.8 unsaturated aliphatic acid, such as zinc acrylate, zinc 
methacrylate, magnesium acrylate, magnesium methacrylate and the like, but 
preferred are zinc acrylate and zinc methacrylate. Typical examples of the 
polymerization initiators are peroxides, such as dicumylperoxide and the 
like. The polymerization initiator may be present in the rubber 
composition in an amount of 0.5 to 4 parts by weight based on 100 parts by 
weight of the base rubber. Amounts outside the range of the initiator do 
not provide suitable hardness range. The rubber composition may further 
contain filler, such as zinc white, barium sulfate, calcium carbonate, 
silica and the like. The rubber composition is generally prepared by 
mixing the above mentioned components, using a Bunbury mixer or a roll. It 
is pressure-molded or injection-molded into molds and then heated at a 
suitable temperature to form the core for the two piece solid golf ball. 
A process for controlling the hardness within the claimed range is known to 
the art and can be effected by varying rubber composition, heating 
conditions and the like. For example, a rubber composition is heated under 
pressure to give an exothermic peak by the internal exothermic phenomenon, 
but the exothermic peak is controlled by adjusting a heating temperature 
to occur after 20 minutes from heat starting. At that temperature, the 
rubber composition is heated under pressure so as to adjust a hardness 
more than 77 near the center. Subsequently, the heating temperature is 
elevated to such a temperature that the hardness between the surface and a 
5 mm inside from the surface may be adjusted to more than 83 and the 
hardness at other portion is made uniform, at which heating is then 
continued to prepare the golf ball of the present invention. 
The cover covering the core is generally an ionomer resin, such as HI-MILAN 
(available from Dupont-Mitsui Polychemicals Co., Ltd.). The ionomer resin 
may contain inorganic fillers, such as titanium dioxide, zinc oxide and 
the like. 
A process for covering the core with the ionomer resin is also known to the 
art, and not limited. For example, the core is covered with two half 
shells of the ionomer resin and then molded under pressure. Also, the 
ionomer resin may be injection-molded to cover the core. 
The present invention provides a solid golf ball which has excellent 
durability and a high impact resilience, as well as a good striking 
feeling.

EXAMPLES 
The present invention is illustrated by the following examples which, 
however, are not construed as limiting the invention to their details. 
EXAMPLE 1 AND COMATIVE EXAMPLES 1 TO 5 
The following ingredients were mixed and then vulcanized in a mold at 
conditions shown in Table 1 according to a press molding process to form a 
solid core having a diameter of 38.2 mm. 
______________________________________ 
Ingredients Parts by weight 
______________________________________ 
Butadiene rubber (available 
100 
from Japan Synthetic Rubber 
Co., Ltd. as BR-01) 
Zinc acrylate 35 
Zinc oxide 18 
Dicumylperoxide 1.2 
Yoshinox 425 (2,2'-methylene- 
0.5 
bis-(4-ethyl-6-t-butylphenol) 
______________________________________ 
The obtained core was covered with a cover resin composition which contains 
100 parts by weight of HI-MILAN 1702 and 2 parts by weight of titanium 
oxide to obtain a large size two piece solid golf ball. The physical 
properties of the golf ball were evaluated and the results are shown in 
Table 1. 
TABLE 1 
__________________________________________________________________________ 
Example No. 
Comparative Example No. 
1 1 2 3 4 5 
__________________________________________________________________________ 
Vulcanizing conditions 
145.degree. C., 35 min. 
167.degree. C., 
160.degree. C., 
148.degree. C. 
145.degree. C., 35 min. 
145.degree. C., 35 min. 
then 24 min. 
27 min. 
35 min. 
then then 
175.degree. C., 15 min. 
185.degree. C., 20 min. 
160.degree. C., 10 min. 
Hardness 
Surface 
84 82 79 67 89 77 
distribution 
5 mm inside 
80 80 80 76 85 77 
10 mm inside 
78 72 78 78 82 77 
15 mm inside 
78 67 72 78 80 77 
Center 78 62 65 77 80 76 
Hardness difference 
6 20 15 11 9 1 
Exothermic peak 
25 13 17 22 25 25 
occurrence time (min) 
Ball compression 
102 98 99 95 105 100 
Crack resistance 
100 80 90 90 96 96 
Initial velocity (45 m/sec) 
65.5 64.5 64.6 64.5 65.2 65.1 
__________________________________________________________________________ 
Compresion: According to the PGA rule. 
Crack resistance: A golf ball repeatedly collides with a panel at 45 m/sec, 
and the number of collisions at which the ball cracks is expressed with an 
index in which the number of Example 1 is regarded as 100. 
Initial velocity: An initial velocity of a golf ball which is struck at a 
head speed of 45 m/sec. 
Five golfers strike the golf ball and evaluate feeling when struck. The 
results are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Head Example No. 
Comparative Example No. 
Golfer 
speed 1 1 2 3 4 5 
__________________________________________________________________________ 
Pro A 
50 m/sec 
Light and good 
Too light 
Light Hard core, 
Hard but fly 
Soft and good, 
response slightly heavy 
more but slightly heavy 
fly insuffici- 
ently 
Pro B 
50 m/sec 
Soft and 
Light and fly 
Light and soft 
Good response 
Slightly hard 
Soft and good 
fly well 
insufficiently but heavy 
but good 
Lesson 
43 m/sec 
Soft and good 
Too light 
Good and soft 
Heavy and feel 
Hard and 
Soft and good 
Pro C response crushed strong response 
Amateur 
49 m/sec 
Light but fly 
Feel crushed 
Soft Hardcore Too strong 
Feel slightly 
D more and no response response 
crushed but good 
Amateur 
45 m/sec 
Soft Too light 
Light and good 
Good response 
Heavy response 
Good 
E but good 
Total evaluation 
Very good 
Bad Good Bad Bad Ordinary 
__________________________________________________________________________ 
The golf ball of Example 1 is excellent in crack resistance and impact 
resilience (initial velocity). 
Comparative Example 1 provides a golf ball which has a lower hardness than 
the claimed hardness range near center and is poor in crack resistance and 
initial velocity. 
Comparative Example 2 provides a golf ball which also has a lower hardness 
than the claimed hardness range at a center. The golf ball is good in 
striking feeling, but poor in crack resistance and initial velocity. 
Comparative Example 3 provides a golf ball which has a lower hardness than 
the claimed hardness range on a surface. The golf ball is good in initial 
velocity, but poor in striking feeling and crack resistance. 
Comparative Example 4 provides a golf ball which has a higher hardness than 
the claimed hardness range at a surface. The golf ball is good in initial 
velocity, but very poor in striking feeling. 
Comparative Example 5 provides a golf ball which has substantially uniform 
hardness throughout the core which is outside the claimed hardness range. 
The golf ball is quite good properties, but less than that of the present 
invention. 
EXAMPLE 2 AND COMATIVE EXAMPLES 6 TO 10 
The following ingredients were mixed and then vulcanized in a mold at 
conditions shown in Table 3 according to a press molding process to form a 
solid core having a diameter of 38.2 mm. 
______________________________________ 
Ingredients Parts by weight 
______________________________________ 
Butadiene rubber (available 
100 
from Japan Synthetic Rubber 
Co., Ltd. as BR-01) 
Zinc acrylate 36 
Zinc oxide 18 
Dicumylperoxide 1.2 
Yoshinox 425 (2,2'-methylene- 
0.5 
bis-(4-ethyl-6-t-butylphenol) 
______________________________________ 
The obtained core was covered with a cover resin composition which contains 
100 parts by weight of Hi-Miran 1707 and 2 parts by weight of titanium 
oxide to obtain a large size two piece solid golf ball. The physical 
properties of the golf ball were evaluated and the results are shown in 
Table 3. 
TABLE 3 
__________________________________________________________________________ 
Example No. 
Comparative Example No. 
2 6 7 8 9 10 
__________________________________________________________________________ 
Vulcanizing conditions 
146.degree. C., 35 min. 
167.degree. C., 
160.degree. C., 
148.degree. C. 
146.degree. C., 35 min. 
146.degree. C., 35 min. 
then 24 min. 
27 min. 
35 min. 
then then 
175.degree. C., 20 min. 
185.degree. C., 25 min. 
165.degree. C., 15 min. 
Hardness 
Surface 
84 82 80 66 90 79 
distribution 
5 mm inside 
82 79 79 76 86 80 
10 mm inside 
81 74 78 79 83 79 
15 mm inside 
79 72 73 80 81 80 
Center 80 68 70 79 82 81 
Hardness difference 
5 14 10 14 9 2 
Exothermic peak 
26 14 19 25 26 26 
occurrence time (min) 
Ball compression 
106 101 102 98 108 104 
Crack resistnace 
100 85 92 90 98 98 
Initial velocity (45 m/sec) 
65.7 64.7 64.9 64.8 65.3 65.4 
__________________________________________________________________________ 
Four golfers strike the golf ball and evaluate feeling when struck. The 
results are shown in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Head Example No. 
Comparative Example No. 
Golfer 
speed 
2 6 7 8 9 10 
__________________________________________________________________________ 
Pro A 
50 m/sec 
Good rebound 
Soft and 
Slightly heavy 
Heavy and hard 
Hard Slightly hard 
slightly good core and good response 
Lesson 
46 m/sec 
Light and good 
Light but 
Good response 
Heavy and poor 
Hard and poor 
Good respone but 
Pro B rebound poor fly rebound rebound slightly heavy 
Lesson 
43 m/sec 
Slightly hard 
Light and 
Slightly hard 
Heavy Effect on 
Hard but good 
Pro C but good 
good hands response 
Amateur 
45 m/sec 
Light Soft Soft Hard Hard Soft 
Total evaluation 
Good Good Ordinary 
Bad Bad Ordinary 
__________________________________________________________________________ 
Example 2 provides a golf ball which has harder near center than that of 
Example 1. The ball is excellent in crack resistance and impact resilience 
(initial velocity), as well as striking feeling. 
Comparative Example 6 provides a golf ball which has higher hardness 
difference and is good in striking feeling, but poor in crack resistance 
and initial velocity. 
Comparative Example 7 provides a golf ball which has lower hardness 
difference than Comparative Example 6, but outside the claimed range. The 
golf ball is poor in all properties. 
The golf ball of Comparative Example 8 is fairly good in crack resistance, 
but poor in striking feeling and initial velocity. 
Comparative Example 9 provides a golf ball which has a higher hardness than 
that of the present invention at a surface. The golf ball is good in 
initial velocity and crack resistance, but very poor in striking feeling. 
Comparative Example 10 provides a golf ball which has uniform hardness 
throughout a core, but higher hardness than that of Comparative Example 5. 
The golf ball is fairly good properties, but less than that of the present 
invention.