Abstract:
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; 
     
       When 0≦l≦5 416-6l≦5H≦440-6l 
     
     
       When 5&lt;l≦r 77≦H≦82 
     
     wherein l is a distance in mm from the surface and r is the radius of the golf ball.

Description:
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; 
     
         When 0≦l≦5 415-6 l≦5 H≦440-6 l 
    
     
         When 5&lt;l≦r 77≦H ≦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 &#34;rubber portion&#34; 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 3  -C 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. 
    
    
     BRIEF EXPLANATION OF THE DRAWING 
     FIG. 1 is a graph in which the claimed relation is plotted with the hardness on ordinates and the distance on abscissas. The solid line in FIG. 1 shows the plot of Example 1. 
     FIG. 2 shows a one piece solid golf ball. 
     FIG. 3 shows a two piece solid golf ball comprising a core 2 and a cover 1 covering the core. 
    
    
     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 COMPARATIVE 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               100from Japan Synthetic RubberCo., Ltd. as BR-01)Zinc acrylate       35Zinc oxide          18Dicumylperoxide     1.2Yoshinox 425 (2,2&#39;-methylene-               0.5bis-(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° C., 35 min.                167° C.,                     160° C.,                          148° C.                               145° C., 35 min.                                       145° C., 35 min.        then    24 min.                     27 min.                          35 min.                               then    then        175° C., 15 min.                               185° C., 20 min.                                       160° C., 10 min.Hardness Surface        84      82   79   67   89      77distribution  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      76Hardness difference        6       20   15   11   9       1Exothermic peak        25      13   17   22   25      25occurrence time (min)Ball compression        102     98   99   95   105     100Crack resistance        100     80   90   90   96      96Initial 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.Golferspeed 1       1        2       3        4       5__________________________________________________________________________Pro A50 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-                               entlyPro B50 m/sec      Soft and              Light and fly                       Light and soft                               Good response                                        Slightly hard                                                Soft and good      fly well              insufficiently   but heavy                                        but goodLesson43 m/sec      Soft and good              Too light                       Good and soft                               Heavy and feel                                        Hard and                                                Soft and goodPro C      response                 crushed  strong responseAmateur49 m/sec      Light but fly              Feel crushed                       Soft    Hardcore Too strong                                                Feel slightlyD          more    and no response           response                                                crushed but goodAmateur45 m/sec      Soft    Too light                       Light and good                               Good response                                        Heavy response                                                GoodE                                            but goodTotal 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 COMPARATIVE 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               100from Japan Synthetic RubberCo., Ltd. as BR-01)Zinc acrylate       36Zinc oxide          18Dicumylperoxide     1.2Yoshinox 425 (2,2&#39;-methylene-               0.5bis-(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° C., 35 min.                167° C.,                     160° C.,                          148° C.                               146° C., 35 min.                                       146° C., 35 min.        then    24 min.                     27 min.                          35 min.                               then    then        175° C., 20 min.                               185° C., 25 min.                                       165° C., 15 min.Hardness Surface        84      82   80   66   90      79distribution  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      81Hardness difference        5       14   10   14   9       2Exothermic peak        26      14   19   25   26      26occurrence time (min)Ball compression        106     101  102  98   108     104Crack resistnace        100     85   92   90   98      98Initial 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.Golferspeed     2       6      7       8       9       10__________________________________________________________________________Pro A50 m/sec     Good rebound             Soft and                    Slightly heavy                            Heavy and hard                                    Hard    Slightly hard             slightly good  core            and good responseLesson46 m/sec     Light and good             Light but                    Good response                            Heavy and poor                                    Hard and poor                                            Good respone butPro B     rebound poor fly       rebound rebound slightly heavyLesson43 m/sec     Slightly hard             Light and                    Slightly hard                            Heavy   Effect on                                            Hard but goodPro C     but good             good                   hands   responseAmateur45 m/sec     Light   Soft   Soft    Hard    Hard    SoftTotal 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.