The present invention relates to a multi-piece solid golf ball having a solid core composed of an inner core layer and an outer core layer, and having one or a plurality of cover layers encasing the core. More particularly, the invention relates to a multi-piece solid golf ball endowed with an excellent flight performance and an excellent spin performance.
To increase the distance traveled by a golf ball and improve the feel of the ball when played, efforts have hitherto been made to design golf balls with a multi-layer structure. Such efforts have led to the disclosure of various multi-piece golf balls in which the core, as well as the cover, has been provided with a two-layer structure. For example, JP-A 11-57070 and the corresponding JP No. 4006550 and U.S. Pat. No. 6,071,201 disclose a multi-piece solid golf ball having an inner core layer made of resin and an outer core layer made of rubber, wherein the inner core layer has a diameter of from 15 to 25 mm and a Shore D hardness of from 55 to 90, and the outer core layer has a JIS-C hardness of from 35 to 75 and a thickness of from 0.5 to 3.0 mm. However, because the inner core layer (center) of this golf ball is hard, the ball has a hard feel on impact and an increased spin rate on full shots.
JP-A 2001-17571 and the corresponding U.S. Pat. No. 6,394,912 describe a golf ball in which the core is composed of a center core made of a thermoplastic resin or a thermoplastic elastomer and having a diameter of from 3 to 18 mm and a Shore D hardness of from 15 to 50, and of an outer core layer having a Shore D hardness near the boundary between the outer core layer and the center core that is from 1 to 15 units harder than the Shore D hardness of the center core. Also, JP-A 2000-229133 and the corresponding JP No. 3656806 and U.S. Pat. No. 6,605,009 disclose a golf ball having a construction composed of an inner core layer, an outer core layer and a cover, wherein the inner core layer is made primarily of resin and has a diameter of from 3 to 15 mm, the outer core layer is formed of a rubber composition, the center core has a Shore D surface hardness which is from 4 to 50 units harder than the innermost side of the outer core layer, and the specific gravities of these layers have been adjusted. However, this golf ball has a small center core and thus lacks a sufficient distance performance.
U.S. Pat. No. 7,241,232 discloses a multi-piece solid golf ball having a multi-layer core in which an inner core layer is formed of a resin such as an ionomer, a polyamide or a polyester elastomer and the outer core layer is formed of a rubber composition, and having an inner cover layer and an outer cover layer composed of specific resins and having certain thicknesses. However, a sufficient distance is not achievable with this golf ball either.
U.S. Pat. No. 7,468,006 discloses a golf ball having an inner core layer and an outer core layer, wherein the outer core layer is formed of a copolymer-based highly saturated ionomer having a Shore D hardness of 45 or more, and the inner core layer is formed of a terpolymer-based highly saturated ionomer having a Shore D hardness of 55 or less. However, in this golf ball, the outer core layer has been set to a higher hardness than the inner core layer and the ball does not have a high initial velocity. As a result, a sufficient distance is not achievable.
JP-A 2008-301985 describes a golf ball having a ball construction of three or more layers in which a center core is made primarily of a thermoplastic resin and has a diameter of from 18 to 35 mm. However, the center core in this golf ball is soft, and so the ball does not have a high initial velocity. As a result, the distance on shots with a driver (W#1) leaves something to be desired.