1. Field of the Invention
The present invention generally relates to multi-layered golf balls and more particularly to golf balls having multi-layered covers. More particularly, the cover includes an inner cover layer made of an ionomer composition; an intermediate cover layer made of an aromatic polyurethane composition; and an outer cover layer made of an aliphatic polyurethane composition. The resulting ball has many advantageous physical properties including good light-stability, cut/shear-resistance, and durability along with optimum playing performance properties.
2. Brief Review of the Related Art
Manufacturers of golf balls are constantly looking at new materials for developing multi-piece solid golf balls. In general, the materials should be cost-effective, have good processability, and be capable of producing golf balls with desirable physical and playing performance properties. A two-piece solid golf ball basically includes a solid inner core protected by an outer cover. The inner core is made commonly of a rubber material such as natural and synthetic rubbers: styrene butadiene, polybutadiene, or polyisoprene. Highly neutralized ethylene acid copolymer ionomer resins (HNPs) also may be used to form the core. The outer cover is made commonly of thermoplastic or thermoset resins such as ionomers, polyolefins, polyamides, polyesters, polyurethanes, and polyureas. As new materials and manufacturing processes have become more economically feasible, multi-piece solid golf balls such as, for example, three-piece, four-piece, and five-piece balls have been introduced. Different materials are used in these golf ball constructions to impart specific properties and playing features to the ball.
For instance, in recent years, there has been high interest in using thermoset, castable polyurethanes and polyureas to make golf ball covers. Basically, polyurethane compositions contain urethane linkages formed by reacting an isocyanate group (—N═C═O) with a hydroxyl group (OH). Polyurethanes are produced by the reaction of a multi-functional isocyanate with a polyol in the presence of a catalyst and other additives. The chain length of the polyurethane prepolymer is extended by reacting it with a hydroxyl-terminated curing agent. Polyurea compositions, which are distinct from the above-described polyurethanes, also can be formed. In general, polyurea compositions contain urea linkages formed by reacting an isocyanate group (—N═C═O) with an amine group (NH or NH2). The chain length of the polyurea prepolymer is extended by reacting the prepolymer with an amine curing agent. Hybrid compositions containing urethane and urea linkages also may be produced as discussed further below. In general, polyurethane and polyurea covered golf balls are described in the patent literature, for example, U.S. Pat. Nos. 5,334,673; 5,484,870; 6,476,176; 6,506,851; 6,867,279; 6,958,379; 6,960,630; 6,964,621; 7,041,769; 7,105,623; 7,131,915; and 7,186,777.
Particularly, Sullivan, US Patent Application Publication 2002/0151380 discloses a golf ball having a core and cover wherein the cover comprises: a) an inner cover layer having a first thickness and being disposed directly about the core; b) an outer cover layer having a second thickness no greater than about 0.050 inches; and c) an intermediate cover layer having a third thickness and being disposed between the inner and outer cover layers; wherein the outer cover layer comprises a composition formed of a reactive liquid material (thermoset material comprising polyurethane, polyurea, polyurethane ionomer, epoxy, or a mixture thereof) and the combination of the first, second, and third thickness is no greater than about 0.125 inches.
Sullivan et al., US Patent Application Publication 2004/0235587 discloses a golf ball having a core and a cover comprising: an inner cover layer; an outer cover layer having a material hardness of 60 Shore D or less; and an intermediate cover layer disposed between the inner and outer cover layers. At least two of the inner, intermediate, and outer cover layers comprise a non-ionomeric material. Preferably, the outer cover layer comprises a polyurethane, a polyurea, a copolymer of a polyurethane, a copolymer of a polyurea, or an interpenetrating polymer network.
As discussed above, isocyanates with two or more functional groups are used in producing polyurethane and polyurea polymers. Manufacturers often use aromatic isocyanates for several reasons including their high reactivity and costs benefits. It normally is more economically advantageous to use aromatic isocyanates over other isocyanate compounds, because the raw material costs for aromatic isocyanates are generally lower. Furthermore, the aromatic isocyanates are able to react with the hydroxyl or amine compounds and form a durable and tough polymer having a high melting point. The resulting polyurethane or polyurea generally has good mechanical strength and cut/shear-resistance. However, one disadvantage with using aromatic isocyanates is the polymeric reaction product tends to have poor light stability and may discolor upon exposure to light, particularly ultraviolet (UV) light. Because aromatic isocyanates are used as a reactant, some aromatic structures may be found in the reaction product. Such aromatic structures are inherently unstable and the resulting material tends to discolor when exposed over long time periods to UV light rays. Hence, UV light stabilizers are commonly added to the formulation, but the covers may still develop a yellowish appearance over prolonged exposure to sunlight. Thus, golf balls are normally painted with a white paint and then covered with a transparent coating to protect the ball's appearance.
In a second approach, aliphatic isocyanates are used to form the prepolymer. Examples of aliphatic isocyanates include, but are not limited to, isophorone diisocyanate (IPDI), 1,6-hexamethylene diisocyanate (HDI), 4,4′-dicyclohexylmethane diisocyanate (“H12 MDI”), and homopolymers and copolymers thereof. These aliphatic isocyanates can provide polyurethane and polyurea polymers having good light stability but such polymers tend to have reduced mechanical strength and cut/shear-resistance.
As discussed above, golf ball covers having good light stability are needed. At the same time, the golf ball should have high tensile strength, impact durability, and cut/shear-resistance. The present invention provides multi-layered golf balls having such characteristics as well as other advantageous properties and features. In the present invention, the cover of the golf ball is essentially “split” into two separate and distinct layers, an aromatic polyurethane inner cover (which will become the intermediate cover layer in the three-layered cover of the present invention) and an aliphatic polyurethane outer cover layer, wherein each cover layer contributes to provide the optimum combination of physical; playing; cosmetic; and color-stable properties.