Practice golf ball

Abstract

A practice golf ball has a core made of a rubber composition which includes a base rubber, a co-crosslinking agent, a crosslinking initiator and a metal oxide, and has a cover which encases the core and is made of a resin material. The co-crosslinking agent is methacrylic acid. The resin material has a breaking strength of 20 to 80 MPa and an elongation of 150 to 600%. The ball is endowed with the properties required of practice balls intended for long-term use, including better durability to cracking and durability of appearance than ordinary game balls, and also better durability to ball surface loss than the one-piece golf balls which are commonly used as practice balls.

1 . A practice golf ball comprising:
 a core made of a rubber composition comprising a base rubber and, as compounding ingredients: a co-crosslinking agent, a crosslinking initiator and a metal oxide; and   a cover which encases the core and is made of a resin material,   
       wherein the co-crosslinking agent is methacrylic acid, and the resin material has a breaking strength of from 20 MPa to 80 MPa and an elongation of from 150% to 600%. 
     
     
         2 . The practice golf ball of  claim 1 , wherein the ball has an initial velocity of not more than 76 m/s. 
     
     
         3 . The practice golf ball of  claim 1 , wherein the resin material making up the cover is composed primarily of polyurethane, the cover material has a Shore D hardness of from 30 to 57, and the cover has a thickness of from 0.3 mm to 2.5 mm. 
     
     
         4 . The practice golf ball of  claim 1 , wherein the ball has formed on a surface thereof a plurality of dimples, each dimple having a spatial volume below a flat plane circumscribed by an edge of the dimple, and the sum of the dimple spatial volumes, expressed as a percentage (VR) of the volume of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, being from 0.8% to 1.7%. 
     
     
         5 . The practice golf ball of  claim 1 , wherein the ball has formed on a surface thereof a plurality of dimples which satisfy conditions (1) and (2) below:
 (1) the dimples have a peripheral edge provided with a roundness represented by a radius of curvature R of from 0.5 mm to 2.5 mm; and   (2) the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by a total number of dimples N on the surface of the ball, is from 15% to 95%.   
     
     
         6 . The practice golf ball of  claim 5  which further satisfies condition (3) below:
 (3) the ball has thereon a plurality of dimple types of differing diameter, and the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than said own diameter plus dimples of a type having a largest diameter, divided by the total number of dimples N on the surface of the ball, is at least 80%. 
 
     
     
         7 . The practice golf ball of  claim 6  which further satisfies conditions (4) to (6) below:
 (4) the number of dimple types of differing diameter is 3 or more; 
 (5) the total number of dimples N is not more than 380; and 
 (6) the surface coverage SR of the dimples, which is the sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple, expressed as a percentage of the surface area of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, is from 60% to 74%. 
 
     
     
         8 . The practice golf ball of  claim 1 , wherein the core has a hardness profile in which, letting A be the JIS-C hardness at a surface of the core, B be the JIS-C hardness at a position 2 mm inside the core surface, C be the JIS-C hardness at a position 5 mm inside the core surface, D be the JIS-C hardness at a position 10 mm inside the core surface, E be the JIS-C hardness at a position 15 mm inside the core surface, and F be the JIS-C hardness at a center of the core: A is from 60 to 88, B is from 54 to 83, C is from 56 to 85, D is from 54 to 80, E is from 51 to 75, and F is from 48 to 72; the relative hardness conditions A>B<C≧D>E>F are satisfied; the value A−F is not more than 19; the core is formed in such a way that A has the highest value among A to F; the value A−C is from 0 to 8; the core has a specific gravity of from 1.05 to 1.2; and, when the core and the ball are each compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), letting deflection by the core be CH and deflection by the ball be BH, the core deflection CH is from 2.0 mm to 7.0 mm, the ball deflection BH is from 2.0 mm to 7.0 mm, and the ratio CH/BH is from 0.95 to 1.1. 
     
     
         9 . The practice golf ball of  claim 1  wherein the metal oxide is zinc oxide, and the compounding ingredients are included in respective amounts of from 10 to 40 parts by weight of methacrylic acid, from 0.3 to 5.0 parts by weight of crosslinking initiator, and from 15 to 30 parts by weight of zinc oxide, per 100 parts by weight of the base rubber.