The present invention relates to a solid golf ball having a soft xe2x80x9cfeelxe2x80x9d and high-pitched xe2x80x9cclickxe2x80x9d when hit with a golf club.
The current mainstream is soft type golf balls. Attempts to make the core and cover soft are successful in giving the ball a soft feel when hit with a club. As the ball itself becomes soft, the clicking sound made when the ball is hit becomes so dull and weak that golfers cannot make sure that their strike conveys the ball far. This problem is contradictorily solved by making the core or cover hard.
This is also true for golf balls of the multilayer construction which now become predominant. Under the trend toward softening, major efforts are made on golf balls to improve the flight and spin performance thereof, with little attention being paid to the click.
An object of the invention is to provide an improved solid golf ball having a soft feel and an agreeable click.
The invention provides a solid golf ball comprising a solid core and a cover that encloses the solid core, the solid core having a multilayer construction which includes a center core and an outer core of at least one layer that encloses the center core. The center core is formed primarily of a resin and has a diameter from 3 mm to less than 15 mm. At least one layer of the outer core is formed of a rubber composition based on polybutadiene. The center core has a Shore D hardness at its surface which is higher than the Shore D hardness of an innermost layer of the outer core.
Preferably, the center core has a specific gravity of 1.0 to 1.5 which is greater than the specific gravity of the at least one layer of the outer core formed of a rubber composition based on polybutadiene. The center core typically has a natural frequency of at least 1,000 Hz. Also preferably, the solid core has a rebound of at least 90 cm when dropped under gravity from a height of 120 cm.
In an effort to improve the dull click of a soft solid core ball, the inventor investigated the use of hard material in the center core. When a rubber composition as used in forming the center core of conventional solid cores was used as the hard material and molded and vulcanized under such conditions as to achieve a high hardness, there arose several problems including a limit on the achievable hardness and a low productivity resulting from kneading, extruding and grinding steps. It was then attempted to form the center core from a resin base material. Since the use of resinous materials which are less resilient than rubber naturally compromises resilience, it is generally believed undesirable to use a less resilient material in the center core. Even when a resinous material is used in the center core, an improvement in click can be made by suitable steps. That is, by forming the center core to a small diameter of 3 mm to less than 15 mm and using a polybutadiene-base rubber composition in the outer core around the center core, the ball can be given a clicking sound without detracting from the resilience of the solid core. By optimizing the hardness, specific gravity and other physical properties of the respective layers, the clicking sound of the solid golf ball is further improved while maintaining the performance and feel inherent to soft solid cores.