Golf ball

This application claims priority on Patent Application No. 2007-274682 filed in JAPAN on Oct. 23, 2007. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

1. Field of the Invention

The present invention relates to golf balls.

2. Description of the Related Art

For golf balls, in addition to flight performances, approach performances and spin stability have been demanded. Golf balls having favorable approach performances are accompanied by a great backspin rate upon approach shots. Golf balls that are superior in the spin stability are less likely to be accompanied by variance of the backspin rate. In the golf balls that are superior in spin stability, for example, the difference between dry spin rate Sd and wet spin rate Sw is small. The ratio (Sw/Sd) is also referred to as spin retention, which has been employed as a marker of the spin stability. In particular, high-level golf players tend to place a high priority on approach performances, and spin stability upon approach shots. The dry spin rate Sd means backspin rate in a dry state, while the wet spin rate Sw means a backspin rate in a wet state. The wet state refers to any state in which ball and/or club face get wet with water.

Specifications of coating films (paint layers) can affect the performances of golf balls. In order to improve characteristics of golf balls, golf balls in which thickness and the like of the coating film have been considered were proposed. Japanese Unexamined Patent Application, First Publication No. 2003-265650 (United States Patent Publication No. 2003/176242A1) discloses a golf ball having a favorable spin retention, in which the thickness of the coating film, the 50% modulus of the coating film, and the ratio of the thickness of the coating film and the thickness of the cover are defined. Japanese Unexamined Patent Application, First Publication No. 2000-176047 (United States Patent No. 6488596) discloses a golf ball that is excellent in scuff resistance in which the coating film is from 5 to 20 micron. Japanese Unexamined Patent Application, First Publication No. Hei 9-276445 (U.S. Pat. No. 5,902,190) discloses that design freedom of golf balls can be increased by providing a coating film having a thickness of 50 to 700 μm. Japanese Unexamined Patent Application, First Publication No. Hei 8-322961 (U.S. Pat. No. 5,695,414) discloses a golf ball in which the proportion of the depth of the dimple and the average thickness of the coated film that covers the dimple is defined.

Specifications of the cover can affect the performances of the golf ball. Suitable covers that provide superior the approach performance and spin stability, and also scuff resistance include a polyurethane resin as a base polymer (urethane cover). However, when the thickness of the urethane cover is too large, the backspin rate is increased upon shots with a driver, whereby the flight distance is likely to be decreased. In light of the flight distance upon shots with a driver, it is preferred to minimize the thickness of the urethane cover.

In general, the surface of the cover is provided with a paint layer. Relationship between a thin urethane cover and a paint layer has not been studied in the aforementioned prior arts.

It was revealed that the spin stability upon approach shots and the flight performance upon shots with a driver can be improved by appropriately defining the relationship between the urethane cover and the paint layer.

An object of the present invention is to provide a golf ball which can achieve both the spin stability upon approach shots, and the flight distance upon shots with a driver.

The golf ball according to the present invention has a core, a cover situated on the external side of the core, and a paint layer provided to cover this cover. The base polymer of the cover is a urethane resin. The base polymer of the paint layer is a urethane resin. The cover has a thickness Tc of 0.1 mm or greater and 0.7 mm or less. The paint layer has a thickness Tp (mm) of 0.015 mm or greater and 0.040 mm or less. The ratio (Tp/Tc) is 0.021 or greater and 0.40 or less.

Preferably, the paint layer is formed with two or more layers.

Preferably, the base polymer of the paint layer is a two-pack type urethane resin or a two-pack type epoxy resin.

Preferably, the base polymer of the cover is a thermoplastic polyurethane elastomer.

Preferably, one or more mid layers are provided between the core and the cover. Preferably, the base polymer of the mid layer that is in contact with the core is an ionomer resin.

Preferably, the cover has a Shore D hardness of 20 or greater and 50 or less.

Accordingly, golf balls that are superior in spin stability upon approach shots and flight distance upon shots with a driver.