Golf balls cover compositions

The Invention provides golf balls formed from the cover compositions comprising a modified ionomer resin which is obtained by adding and reacting with a polyester oligomer in a zirconium complex and zirconia filled state to a typical ionomer resin, said ionomer resin being prepared by neutralizing a part of the unsaturated carboxylic acid with metals including zirconium, being addition polymerization with another ester by a radical reaction. A golf ball having a cover prepared from the compositions, the carry distance has increased in hitting the golf ball with a driver and excellency in back-spin property in hitting with a short iron, and the golf ball has a soft feeling with a durability, so that being got rid of a defect of a golf ball which has a cover composition comprising a conventional soft type ionomer resin, satisfying the golfers.

FIELD OF THE INVENTION
 The present invention relates to a cover composition with a excellent
 repulsive elasticity and back-spin property for golf balls.
 BACKGROUND OF THE INVENTION
 According to the structure, golf balls are classified into many types. For
 example, one-piece golf balls have a single structure made from a
 homogeneous mass of material, two-piece golf balls of a double structure
 consisting of solid matter in which a core is surrounded by a cover, solid
 type three-piece golf balls of a triple structure with a dual core
 consisting of an inner core and an outer layer covering the said inner
 core and a cover covering the said dual core, solid type three-piece golf
 balls of a triple strucure comprising a core and double cover in which an
 inner cover covering the said core and an outer cover covering the said
 inner cover, rubber thread wound three-piece golf balls of a triple
 structure comprising a rubber thread wound liquid or solid core is
 surrounded by a cover, four-piece golf balls of a quadruple structure
 comprising a triple core which is surrounded by a cover, four-piece golf
 balls of a quadruple structure comprising a dual core and double cover in
 which an inner cover covering the said dual core and an outer cover
 covering the said inner cover, four-piece golf balls of a quadruple
 structure comprising a core and triple cover which covered the said core,
 etc., golf balls have got more and more developed in many structures and
 variety of types in actuality. The reason why many types of golf balls are
 prepared as mentioned above, is said to be attributed to the fact that
 there have not yet been developed golf balls with which all golfers can be
 satisfied. In general, the golf ball which can give a satisfaction to
 golfers, that is the golf ball fly a great distance in hitting with a
 driver and excellency in back-spin property in hitting with a short iron,
 and soft enough to give a good impact feeling to the golfers in addition
 to being superior in durability.
 When the golfer hit a golf ball with a golf club, a strong repulsive
 elasticity is generated on the ball by the impact force applied from the
 head of a golf club, bounding the ball at a high speed and thus making a
 carry distance with the aid of the back-spin due to the loft angle of the
 club head and the aerodynamic lifting force due to the dimples on the
 surface of the golf ball.
 The important factors which determine the repulsive elasticity of golf ball
 are materials, their compositions, and the preparation methods for making
 the golf ball. The repulsive elasticity is much more affected by cores,
 such as single core, dual core, triple core, etc., than by covers in the
 solid type golf balls.
 Likewise, rubber thread wound type golf ball's repulsive elasticity is more
 affected by a liquid center or solid core and the rubber threads wound
 around them than by covers. However, the cover or double cover of a golf
 ball plays a critical role in obtaining the back-spin needed when hitting
 a golf ball with a short iron club, in particular, the materials of the
 covers may determine the ball's function. Hereinafter, covers, double
 covers, triple covers are represented simply by covers. The materials of
 the covers of golf balls on the market are usually made from polymeric
 resins, such as ionomer resins, Balata rubber, polyurethane,
 polyetherester, polystyrene-polybutadiene block copolymer, other
 thermoplastics, or the mixtures thereof, or in combination with silicone
 resins, fluorine resins, etc. Among them, ionomer resins are the most
 frequently used. The ionomer resin covers show strong physical properties,
 but it is difficult to make a good golf ball by the reason of too hard to
 give a good impact feeling and the poor back-spin property in short iron
 shot.
 In order to solve the problems, modified soft ionomer resins were developed
 and applied to many golf balls, however the covers made from the soft
 ionomer resins have so many problems, such as the surface of cover is
 easily damaged in hitting or significantly deteriorated the repulsive
 elasticity relative to the surface hardness. On the other hand, balata
 rubber, conventionally used as a cover material for rubber thread wound
 golf balls, which is superior in repusive elasticity and back-spin
 property, however the balata cover is poor in durability, the cover is
 torn or collapsed easily in hitting owing to the weak physical property.
 As for a polyurethane cover, it is superior in back-spin property, but the
 golf ball flies a significantly reduced distance in hitting with a driver
 owing to the poor repulsive elasticity, and the ball becomes too hard in
 the cold season to give the good feeling for the golfers. In addition, the
 molding process is too difficult to make a cover from polyurethane, the
 productivity is disadvantageous. Likewise, the other resins do not give
 remarkable advantages as cover materials for golf balls, and they have a
 lot of problems in many aspects.
 Technical Assignment to be Achieved in this Invention
 It is an object of the present invention to solve the above-mentioned
 problems of the covers in prior arts and to make a cover composition for
 golf balls which can fly a longer carry distance in hitting with a driver
 and have a superior back-spin property in short iron shot, and to provide
 a cover composition for golf balls which is superior in durability and
 gives the golfers a soft impact feeling.
 DETAILED DESCRIPTION OF THE INVENTION
 This invention provides a cover composition for golf balls which comprises
 a modified ionomer resin by adding and reacting with a polyester oligomer
 in a zirconium complex and zirconia-filled state which are catalysts in
 esterification reaction and dehydration reaction to a typical ionomer
 resin, said ionomer resin being prepared by neutralizing a part of the
 unsaturated carboxylic acid with metals including zirconium, being
 addition polymerization with another ester by a radical reaction, can be
 used as a cover material for golf balls, that is much better in repulsive
 elasticity and back-spin property than conventional cover materials. The
 polyester oligomer is highly viscous and named A-paste throughout the
 specification. In order to prepare the A-paste of the present invention,
 there are needed a polyol which has hydroxyl groups at opposite ends with
 a molecular weight ranging from 1,000 to 2,400, a bi- or tri-functional
 acrylic or methacrylic monomer and a zirconia, and an organic peroxide is
 used as a reaction initiator. The said polyol is selected from materials,
 such as polyester diol which can be derived from the reactants by
 polycondensation or ester exchange reaction of dibasic acid (such as
 adipic acid and sebasic acid) and diol (such as ethylene glycol, propylene
 glycol, butanediol and hexanediol), polyether diol which can be derived
 from the reactants by addition polymerization of epoxide and ethylene
 oxide (or butylene oxide), polytetramethyleneetherglycol which can be
 derived from the reactants by polymerization of polyalkylene oxide and
 tetrahydrofuran, or polydimethylsiloxanepolyoxyethylene diol,
 polycaprolactone diol, and polycarbonate diol, those polyols have hydroxyl
 groups at opposite ends with an average molecular weight of from 1,000 to
 2,400. The said bi- or tri-functional acrylic or methacrylic monomer is
 selected from materials, such as ethylene diacrylate, ethylene
 dimethacrylate, 1,3-butyleneglycoldimethacrylate, acid, trimethylolpropane
 triacrylate and trimethylolpropane trimethacrylate. As for the zirconia,
 it is a zirconium oxide(ZrO.sub.2) powder with an average particle size
 passable through 250 mesh sieve, whose crystal structure is partially
 stabilized by addition of yttrium oxide (Y.sub.2 O.sub.3), calcium
 oxide(CaO) or magnesium oxide(MgO), and comprises hafnium oxide(HfO.sub.2)
 at an amount of 0.01.about.2%. Functioning as a reaction initiator, the
 organic peroxide is selected from materials, such as dicumylperoxide,
 2,5-dimethylhexyl-2,5-di(peroxybenzoate),
 1,1-di-tertiallybutyl-peroxy-3,3,5-trimethylcyclohexane, and
 bis(tertially-butylperoxyisopropyl)benzene.
 The said ionomer resin is meant to be a resin comprising a three component
 type copolymer comprised an .alpha.-olefin, such as ethylene (or
 propylene) and an unsaturated carboxylic acid and at least a part of the
 carboxylic group forms a salt with a metal, or a four component type
 copolymer comprised an .alpha.-olefin and an unsaturated carboxylic acid
 and an ester of unsaturated carboxylic acid and at least a part of the
 carboxylic group forms a salt with a metal. Preferably used unsaturated
 carboxylic acid is mono-basic acid, such as acrylic acid and methacrylic
 acid, sometimes, di-basic acid, such as maleic acid and fumaric acid can
 be used. In ionomer resin, the content of the acid is represented by the
 weight % of the unsaturated carboxylic acid irrespective of whether it is
 neutralized or not, and is usually 6.about.20 weight %.
 The metal, useful for neutralizing the unsaturated carboxylic acid
 comprises monovalent metal, such as sodium, lithium and potassium, and
 divalent metal, such as zinc and magnesium, and the neutralization degree
 of 20.about.80% by the metallic ion is commercially available. Useful
 ester of unsaturated carboxylic acid is an alkylester of unsaturated
 carboxylic acid, and the said alkyl group contains 1.about.10 carbon atoms
 and methyl, ethyl, propyl are proper for use.
 A preparing process of the A-paste of the invention using the enumerated
 chemicals is as follows.
 First, put a polyol having hydroxyl group at opposite ends with an average
 molecular weight of 1,000.about.2,400 into a proper sized vessel equipped
 with a stirrer and the bi- or tri-functional acrylic or methacrylic
 monomer is added at a mol ratio of 1.3.about.1.6 mol per mol of the polyol
 and mixed well 20.about.30 min.
 Then, the zirconia powder is added slowly at an amount of 60.about.100
 weight parts per 100 weight parts of the polyol while stirring at a speed
 of 200.about.300 r.p.m. Meanwhile, on the basis of 100 weight parts of the
 polyol, 2.about.3 weight parts of the organic peroxide is dissolved in
 50.about.60 weight parts of an organic solvent, such as methylethylketone
 or toluene, and this organic peroxide solution is mixed with the above
 mixture little by little while stirring for 20.about.30 min.
 Thereafter, the resulting solution is heated gradually for 1.about.1.2
 hours with stirring, if the temperature reaches 100.about.110.degree. C.,
 the solution is cooled down to 60.degree. C. with stirring. And again, the
 solution is warmed while stirring for 30 min., from the point where the
 temperature is elevated to 105.degree. C., the reaction violently occurs,
 leading to a rapid elevation of temperature to 130.degree. C. or higher.
 By rapidly reducing this high temperature to 60.degree. C. or less, a
 polyester oligomer in a metal complex and metal-filled state is obtained.
 This highly viscous A-paste should be reacted with an ionomer resin in a
 short time in order to produce a cover material suitable for the present
 invention, as follows.
 100 weight parts of the A-paste are well mixed with 350.about.450 weight
 parts of the ionomer resin with the aid of a suitable mixer and then, the
 remaining organic solvent and volatiles are completely removed by
 maintaining the mixture at 60.degree. C. for 2.about.4 hours in a dryer.
 The dried mixture of the A-paste and the ionomer resin is extruded at a
 screw speed of 200.about.300 r.p.m., maintaining the temperature at
 200.about.240.degree. C., from a nozzle of an extruder equipped with a
 pelletizer, and cooled to yield the primary pellets. Seperately,
 150.about.300 weight parts of a fresh ionomer resin are mixed with
 additives, such as white pigment or other coloring pigments, antioxidants,
 organic or inorganic fillers containing metal, etc., and this ionomer
 resin is mixed with 100 weight parts of the primary pellets in a suitable
 mixer. This resulting mixture is again extruded in the same manner as in
 the above to give secondary pellets which can be used as a cover material
 for golf balls, in accordance with the present invention.
 As occasion demands, the additives may be used or not. Optionally, the
 cover material prepared in accordance with the present invention may be
 used in combination with conventional cover materials, such as balata
 rubber, polyurethane, polyetherester, polystyrene-polybutadiene block
 copolymer, polyethylene-polystyrene-polybutadiene block copolymer,
 silicone resin, fluorine resin and other thermoplastic resins. When the
 material according to the present invention is applied for golf ball
 covers, any type of cores, for example, solid cores, dual cores, triple
 cores, liquid-containing solid cores, rubber thread wound liquid cores,
 rubber thread wound solid cores, may be used.
 The golf balls, which have the covers prepared from the material of the
 present invention, fly a farther carry distance than do conventinal ones
 in hitting by a driver, and superior in back-spin property in hitting with
 a short iron, that control the ball with ease, and have a soft feeling
 with a improved durability, satisfying the golfers.