Golf ball

#This invention relates to a golf ball having improved resilience.

#In the related art, ionomer resins are widely used as the cover material of golf balls. Ionomer resins are ionic copolymers between olefins (such as ethylene) and unsaturated carboxylic acids (such as acrylic acid, methacrylic acid and maleic acid), wherein some of the acidic groups are neutralized with metal ions such as sodium, lithium, zinc and magnesium. On account of excellent properties including durability, resilience, and scuff resistance, ionomer resins are advantageously used as the base resin of cover materials, and actually become the mainstream of the current cover resins. Although the rebound of ionomer resins is considerably high, the customers are eager to play with golf balls of higher resilience and better flight performance.

#One method for improving the resilience of golf balls using ionomer resins as the cover material is disclosed in JP-A 4-156865. A golf ball is improved in resilience by covering a core with an ionomer resin and annealing the ionomer resin under suitable conditions to modify the crystallographic state thereof. The annealed ionomer resin exhibits such a crystal melting behavior that a first melting peak appears at 85 to 95° C. and a second melting peak appears at 70 to 80° C. when measured by a differential scanning calorimeter. More particularly, the ionomer resin as applied onto the core is annealed at 45 to 63° C. for about 3 to 240 hours. Then the crystallographic state of the ionomer resin is changed whereby the second melting peak temperature is raised to the range of 70 to 80° C., thereby improving resilience.

#The only figure, FIG. 1 schematically illustrates the structure of an ionomer resin.

#The golf ball of the invention includes a core and a cover of one or more layers enclosing the core. At least one layer of the cover is formed of a cover material comprising a heated mixture of an ionomer resin and a metal salt.

#Ionomer resins are ionic copolymers between olefins such as ethylene and unsaturated carboxylic acids such as acrylic acid, methacrylic acid and maleic acid, wherein some acidic groups are neutralized with metal ions such as sodium, lithium, zinc and magnesium. The most widely used ionomer resins are ethylene-(meth)acrylic acid copolymers neutralized with metal ions. They have the structure that the molecular chain of polyethylene has carboxylic groups as side chains, some of which are crosslinked with metal ions between molecular chains.

#The acid content of the ionomer resin is preferably 8 to 22% by weight and more preferably 10 to 20% by weight. An ionomer resin with an acid content of less than 8% is short of rigidity, with a possibility of increased spin and reduced distance upon driver shots. An ionomer resin with an acid content of more than 20% has a too high rigidity, probably exacerbating the feel of a ball when hit.