Golf ball and method of manufacture

Abstract

In a golf ball having a core composed of one or more layer, the outermost layer of the core is formed of a rubber composition containing a base rubber and an α,β-unsaturated metal carboxylate, an envelope layer directly encasing the core is formed of a resin composition containing a thermoplastic resin having a structure that includes α,β-ethylenically unsaturated carboxylic acid copolymerization units, and a surface of the core outermost layer and the envelope layer adjoin each other through an intervening oxazoline group-containing substance. The golf ball has improved adhesion between the rubber-based core and the cover layer that directly encases the core and is formed of an ionomer resin or other α,β-ethylenically unsaturated carboxylic acid copolymer-containing resin material.

Claims

1. A method for producing a golf ball having a core composed of one or more layer, which method comprises the steps of: forming an outermost layer of the core with a rubber composition containing a base rubber and an α,β-unsaturated metal carboxylate; surface-treating the core outermost layer by bringing a solution that contains an oxazoline group-containing substance into contact with a surface of the outermost layer; forming an envelope layer by molding, over the surface-treated core outermost layer, a resin composition containing a thermoplastic resin having a structure that includes α,β-ethylenically unsaturated carboxylic acid copolymerization units; and prior to surface-treating the core outermost layer with a solution that contains an oxazoline group-containing substance, the step of surface-treating the outermost layer of the core by bringing an acid-containing solution into contact with the surface of the outermost layer.

2. The method of claim 1, wherein the acid-containing solution is a hydrochloric acid-containing solution.

3. The method of claim 1, wherein the acid-containing solution is an alcohol-containing solution.

4. The method of claim 1, wherein the acid-containing solution, when brought into contact with the surface of the core outermost layer, has an acid concentration of 0.05 mol/L or more.

5. The method of claim 1, wherein the acid-containing solution is brought into contact with the surface of the core outermost layer by dipping the core in the acid-containing solution.

6. The method of claim 1, wherein the solution containing an oxazoline group-containing substance is an alcoholic solution.

Description

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) The objects, features and advantages of the invention will become more apparent from the following detailed description.

(2) The golf ball of the invention has a core composed of one or more layer. That is, the core in this invention may be a single-layer core or may be a multilayer core such as two-layer core having an inner layer and an outer layer.

(3) The outermost layer of the core is formed of a rubber composition which includes a base rubber and an α,β-unsaturated metal carboxylate. Preferred examples of this rubber composition include rubber compositions formulated as described below.

(4) The base rubber is not particularly limited, although the use of polybutadiene is especially preferred.

(5) It is desirable for the polybutadiene to have a cis-1,4-bond content on the polymer chain of at least 60 wt %, preferably at least 80%, more preferably at least 90 wt %, and most preferably at least 95 wt %. At a cis-1,4-bond content among the bonds on the polybutadiene molecule that is too low, the rebound may decrease.

(6) The polybutadiene has a content of 1,2-vinyl bonds on the polymer chain which is generally not more than 2 wt %, preferably not more than 1.7 wt %, and more preferably not more than 1.5 wt %. At a 1,2-vinyl bond content which is too high, the rebound may decrease.

(7) The polybutadiene has a Mooney viscosity (ML.sub.1+4 (100° C.)) of preferably at least 20, and more preferably at least 30. The upper limit is preferably not more than 120, more preferably not more than 100, and even more preferably not more than 80.

(8) The term “Mooney viscosity” used herein refers to an industrial indicator of viscosity (JIS K 6300) measured with a Mooney viscometer, which is a type of rotary plastometer. This value is represented by the unit symbol ML.sub.1+4 (100° C.), wherein “M” stands for Mooney viscosity, “L” stands for large rotor (L-type) and “1+4” stands for a pre-heating time of 1 minute and a rotor rotation time of 4 minutes. The “100° C.” indicates that measurement is carried out at a temperature of 100° C.

(9) The polybutadiene used may be one synthesized with a rare-earth catalyst or a group VIII metal compound catalyst.

(10) A polybutadiene rubber synthesized with a catalyst other than the above rare-earth catalysts may be included in the base rubber. Other rubber ingredients such as styrene-butadiene rubber (SBR), natural rubber, polyisoprene rubber and ethylene-propylene-diene rubber (EPDM) may also be included. These rubber ingredients may be used alone or two or more may be used in combination.

(11) The polybutadiene accounts for a proportion of all the base rubber in the rubber composition which is preferably at least 60 wt %, more preferably at least 70 wt %, and most preferably at least 90 wt %. It is also possible for 100 wt % of the base rubber, i.e., all of the base rubber, to be polybutadiene.

(12) The α,β-unsaturated metal carboxylate is generally used as a co-crosslinking agent. The number of carbon atoms on this unsaturated carboxylic acid is preferably from 3 to 8. Specific examples include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid and fumaric acid. Specific examples of the metal in the unsaturated metal carboxylate include zinc, sodium, magnesium, calcium and aluminum, with zinc being especially preferred. The co-crosslinking agent is most preferably zinc acrylate.

(13) The α,β-unsaturated metal carboxylate can be mixed as a metal salt with the base rubber, or may be obtained by chemically reacting, within the base rubber, an α,β-unsaturated carboxylic acid with a metal source such as a metal oxide. In cases where the α,β-unsaturated metal carboxylate is obtained from this chemical reaction, it is preferable to react an amount of the metal constituent sufficient to convert acid groups on the α,β-unsaturated carboxylic acid into a metal salt. When the amount of the metal constituent is insufficient, the hardness of the core obtained may decrease or the rebound may decline.

(14) The content of the α,β-unsaturated metal carboxylate per 100 parts by weight of the base rubber may be set to preferably at least 5 parts by weight, more preferably at least parts by weight, and even more preferably at least 15 parts by weight. The upper limit in the content may be set to preferably not more than 60 parts by weight, more preferably not more than 50 parts by weight, and even more preferably not more than 45 parts by weight. When the content is too high, the ball may become too hard, resulting in an unpleasant feel at impact. When the content is too low, the rebound may decrease.

(15) In addition to the above essential ingredients, the rubber composition may also include a co-crosslinking agent other than the above, an organic peroxide, an inert filler, sulfur, an antioxidant, an organosulfur compound and the like.

(16) In cases where the core is composed of a single layer, the single-layer core can be produced from the above-described rubber composition. In cases where the core is composed of a plurality of layers, the material making up the center core may be a rubber material of the same type as that in the outermost layer described above, or the center core may instead be made of a rubber composition in which the types and contents of the compounding ingredients differ from the above or may be made of a known resin material.

(17) A vulcanizate (core) can be produced by vulcanizing/curing the above rubber composition. This vulcanizate may be used as part or all of the single-layer or multilayer core. For example, the core which is a vulcanizate can be produced by using a mixing apparatus such as a Banbury mixer or a roll mill to knead the rubber composition, compression-molding or injection-molding the kneaded composition using a core mold, and suitably heating the molded body at a temperature sufficient for the organic peroxide and co-crosslinking agent to act, such as between about 100° C. and 200° C., for to 40 minutes so as to cure the molded body.

(18) In order to be able to increase the durability of the golf ball while maintaining good spin properties, it is preferable for the core to have a hardness profile in which the hardness difference between the surface and the center is large.

(19) The core center hardness on the JIS-C hardness scale, although not particularly limited, is preferably at least 30, more preferably at least 40, and even more preferably at least 50. The upper limit is preferably not more than 80, more preferably not more than 70, and even more preferably not more than 60. At a core center hardness outside of this range, the feel at impact may worsen or the durability may decrease, and a spin rate-lowering effect may not be obtainable.

(20) The core surface hardness on the JIS-C hardness scale, although not particularly limited, is preferably at least 50, more preferably at least 60, and even more preferably at least 70. The upper limit is preferably not more than 98, more preferably not more than 96, and even more preferably not more than 94. At a core surface hardness that is lower than this range, the ball rebound may decrease and a sufficient distance may not be obtained. On the other hand, at a core surface hardness that is higher than this range, the feel at impact may become too hard or the durability to cracking under repeated impact may worsen.

(21) With regard to the core hardness profile, from the standpoint of the ball spin performance, it is preferable for the hardness difference between the core surface and the core center to be large. Specifically, the hardness difference between the core surface and core center, expressed on the JIS-C hardness scale, is preferably at least 13, more preferably at least 20, and even more preferably at least 25. The upper limit is preferably not more than 60, more preferably not more than 50, and even more preferably not more than 40. When the hardness difference is too small, the spin rate-lowering effect on shots with a driver (W #1) may be inadequate and a good distance may not be obtained. On the other hand, when the hardness difference is too large, the initial velocity of the ball on actual shots may decrease and a good distance may not be obtained, or the durability to cracking on repeated impact may worsen. As used herein. “center hardness” refers to the hardness measured at the center of the cross-section obtained by cutting the core in half through the center, and “surface hardness” refers to the hardness measured at the spherical surface of the core. Also, “JIS-C hardness” refers to the hardness measured with the spring-type durometer (JIS-C model) specified in JIS K 6301-1975.

(22) The core diameter is not particularly limited and depends also on the layer structure of the golf ball to be produced, but is preferably at least 30 mm, and more preferably at least 35. The upper limit is preferably not more than 41 mm, and more preferably not more than 40 mm. When the core diameter falls outside of this range, the initial velocity of the ball may decrease or a suitable spin performance may not be obtained.

(23) It is preferable to carry out surface treatment of the outermost layer of the core by bringing an acid-containing solution into contact with the surface of the core outermost layer. Generally, following hot molding of the core-forming rubber composition, unsaturated carboxylic acid is neutralized with metal ions owing to the inclusion of a co-crosslinking agent such as zinc acrylate, and so the level of free unsaturated carboxylic acid is not high. It is thought that by bringing an acid-containing solution into contact with the surface of the core outermost layer, a sufficient amount of demetallized carboxyl groups form only at the surface portion and new chemical bonds mediated by an oxazoline group-containing substance form with acid groups in the adjoining envelope layer, so that adhesion improves compared with an untreated core surface.

(24) The acid used in acid treatment is not particularly limited, so long as it is an acid which can remove metal ions from an ionomer resin and protonize a carboxylic acid. Illustrative examples include hydrochloric acid, sulfuric acid and nitric acid. In particular, from the standpoint of the ease of carrying out the operation, the use of hydrochloric acid is preferred. The acid concentration when used, although not particularly limited, is preferably at least 0.05 mol/L, and more preferably at least 0.1 mol/L. The upper limit is preferably not more than 10 mol/L, more preferably not more than 5 mol/L, even more preferably not more than 3 mol/L, and most preferably not more than 1 mol/L. When the acid concentration is too low, an adhesion-improving effect between the core surface and the envelope layer may not be observed. When the acid concentration is too high, this may adversely affect the ball properties rather than enhancing the core surface properties.

(25) For the acid to better penetrate to the interior of the rubber or resin serving as the core material and for increased affinity, it is preferable for the acid-containing solution to include also an alcohol. As a result, demetallization treatment of the metal salt at the core surface due to acid treatment can be made to proceed rapidly. The alcohol used is preferably a lower alcohol having up to four carbon atoms, such as ethanol or 2-propanol.

(26) The acid treatment method used is preferably one that involves subjecting surface regions of the core outermost layer to dipping, painting (spraying), infiltration under applied heat and pressure, dropwise application or the like. The use of a dipping method is especially preferred. For example, when the core is dipped in an acid-containing solution, dipping may be carried out for a period of from 1 to 60 minutes, especially 1 to 10 minutes.

(27) The acid treatment temperature may be set to between 10° C. and 30° C., and especially between 20° C. and 25° C.; room temperature or atmospheric temperature generally suffices. Following acid treatment (acid washing), washing may be carried out in which the core is thoroughly rinsed with water so that acid does not remain on the surface. The water rinsing method is not particularly limited. For example, use can be made of a method such as washing with a large amount of water.

(28) Prior to the above acid treatment, it is preferable to abrade the surface of the core outermost layer. The method of abrasion is exemplified by a process that involves randomly rotating the core in a diamond grinding wheel, a process that uses diamond paper, and a sand blasting process.

(29) Additionally, in the practice of this invention, to construct the golf ball such that the surface of the core outermost layer and the envelope layer adjoin each other through an intervening oxazoline group-containing substance, it is preferable to surface-treat the core outermost layer by bringing a solution containing an oxazoline group-containing substance into contact with the surface of the outermost layer. Owing to this oxazoline group-containing substance, carboxylic acid groups from which metal ions have been removed in the core outermost layer chemically bond with the oxazoline groups, and the oxazoline groups chemically bond with unneutralized carboxyl groups remaining in the ionomer resin or the like present within the subsequently described envelope layer, enabling the core and the envelope layer to firmly adhere through the oxazoline group-containing substance.

(30) No particular limitation is imposed on the oxazoline group-containing substance, so long as it is a chemical substance having oxazoline groups. However, from the standpoint of, for example, the means used to impregnate the oxazoline group-containing substance into the core surface, the use of an oxazoline group-containing water-soluble polymer is preferred. Examples include oxazoline group-containing acrylic polymers and oxazoline group-containing styrene polymers.

(31) To achieve both a good adhesive effect between the core surface and the envelope and also a good ball durability, the oxazoline group content per gram of polymer in the acrylic polymer or styrene polymer is preferably from 0.1×10.sup.−3 to 10×10.sup.−3 mol/g (solids), and especially from 0.2×10.sup.−3 to 8×10.sup.−3 mog (solids).

(32) A commercial product may be used as the oxazoline group-containing polymer.

(33) Illustrative examples include the acrylic polymers Epocros® WS-500, Epocros® WS-300 and Epocros® WS-700 and the styrene polymer Epocros® RPS-100, all from Nippon Shokubai Co., Ltd.

(34) From the standpoint of uniformly treating the surface of the core outermost layer, the solution containing the oxazoline group-containing substance is preferably an alcoholic solution. Preferred examples of the alcohol used in the alcoholic solution include methanol, ethanol, propanol and 1-methoxy-2-propanol.

(35) Following surface treatment at the surface of the core outermost layer with the above solution containing an oxazoline group-containing substance, a step in which drying is carried out at a given temperature and for a given time may be provided. The reason for doing so is to induce partial reaction of the demetallized carboxyl groups in the outermost layer surface with oxazoline groups and thereby further strengthen adhesion with the subsequently described envelope layer. Treatment may be carried out at a treatment temperature of generally 80° C. or less, preferably 60° C. or less, and more preferably 50° C. or less. Even treatment at room temperature is acceptable. The treatment time is preferably a time sufficient for about half of the oxazoline groups to react. For example, as a general rule, at a treatment temperature of 60° C., the treatment time may be set to about 4 hours. If the treatment temperature is too high or the treatment time is too long, all of the oxazoline groups may end up reacting and subsequent reaction with acid groups within the envelope layer may not adequately take place, as a result of which the adhesion may diminish.

(36) The golf ball of the invention has an envelope layer which directly encases the core. This envelope layer makes up part or all of the cover. That is, in cases where the cover consists of a single layer, this envelope layer (cover layer) serves as the outermost layer in the layer structure of the golf ball. In cases where the cover is composed of a plurality of layers, one or more additional cover layer is formed on the outside of this envelope layer.

(37) The envelope layer is formed of a resin composition having a thermoplastic resin with a structure that includes α,β-ethylenically unsaturated carboxylic acid copolymerization units.

(38) The thermoplastic resin having a structure that includes α,β-ethylenically unsaturated carboxylic acid copolymerization units is not particularly limited, although it is preferably one which includes either (a) an ethylene-α,β-unsaturated carboxylic acid copolymer and/or a metal salt thereof or (b) an ethylene-α,β-unsaturated carboxylic acid-α,β-unsaturated carboxylic acid ester copolymer and/or a metal salt thereof.

(39) Specific examples of the α,β-unsaturated carboxylic acid in components (a) and (b) include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred. The α,β-unsaturated carboxylic acid ester in component (b) is preferably a lower alkyl ester of the above unsaturated carboxylic acid, specific examples of which include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Butyl acrylate (butyl n-acrylate, butyl i-acrylate) is especially preferred.

(40) Metal ion neutralization products of the copolymers in components (a) and (b) can be obtained by partially neutralizing acid groups in the olefin-unsaturated carboxylic acid copolymer or the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer with metal ions. Illustrative examples of metal ions which neutralize the acid groups include Na.sup.+, K.sup.+, Li.sup.+, Zn.sup.++, Cu.sup.++, Mg.sup.++, Ca.sup.++, Co.sup.++, Ni.sup.++ and Pb.sup.++. Preferred use can be made of Na.sup.+, Li.sup.+, Zn.sup.++, Mg.sup.++ and Ca.sup.++ in particular. Such neutralization products may be obtained by a known method. For example, a neutralization product may be obtained by using, for reaction with the above copolymer, a compound such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide of the above metal ion.

(41) Known substances may be used as components (a) and (b). Illustrative examples include commercial products such as the following acid copolymers: Nucrel® N1560, Nucrel® N1214, Nucrel® N1035, Nucrel® AN4221C, Nucrel® AN4311, Nucrel® AN4318 and Nucrel® AN4319 (all products of Dow-Mitsui Polychemicals Co., Ltd.). Illustrative examples of metal ion neutralization products of acid copolymers include Himilan® 1554, Himilan® 1557, Himilan® 1601, Himilan® 1605, Himilan® 1706, Himilan® AM7311, Himilan® 1855, Himilan® 1856 and Himilan® AM7316 (all products of Dow-Mitsui Polychemicals Co., Ltd.), and Surlyn® 7930. Surlyn® 6320, Surlyn® 8320, Surlyn® 9320 and Surlyn® 8120 (E.I. DuPont de Nemours and Company).

(42) The resin composition in the envelope layer may also suitably include known resins other than the above thermoplastic resin having a structure that includes α,β-ethylenically unsaturated carboxylic acid copolymerization units, and various types of additives.

(43) In cases where the cover has a plurality of layers, one or more other cover layer may additionally be formed outside of this envelope layer. A known thermoplastic resin or thermoplastic elastomer, such as an ionomer or a thermoplastic polyurethane elastomer, may be used as the chief material in this cover layer.

(44) The method used to obtain the cover layers (envelope layer, and cover layers other than the envelope layer) in this invention may be, for example, a method in which, depending on the type of ball being produced, a pre-fabricated single-layer core or multilayer core of two or more layers is placed in a mold and the above mixture is mixed and melted under heating and then injection-molded over the core, thereby encasing the core with the desired cover layer or layers. Another method that may be used to form the cover layers involves molding the cover material into a pair of hemispherical half-cups, enclosing the core with these half-cups, and then molding under applied pressure at between 120° C. and 170° C. for 1 to 5 minutes.

(45) When the cover is a one-layer cover, that is, when the cover is composed of only an envelope layer, the thickness of that layer may be set to from 0.3 to 3 mm. When the cover is composed of two layers, the cover layer outside of the envelope layer (i.e., the outermost layer) may be set to a thickness of from 0.3 to 2.0 mm and the envelope layer (inner cover layer) may be set to a thickness of from 0.3 to 2.0 mm. The cover layers making up the cover each have a Shore D hardness which, although not particularly limited, is preferably at least 40, and more preferably at least 45, but is preferably not more than 70, and more preferably not more than 65.

(46) Numerous dimples may be formed on the surface of the outermost layer of the cover. Also, various types of treatment, such as surface preparation, stamping and painting, may be carried out on the surface of the cover outermost layer.

EXAMPLES

(47) The following Examples of the invention and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 4, Comparative Examples 1 to 4

(48) Using the three types of core materials (X, Y, Z) composed primarily of polybutadiene shown in Table 1 below, core compositions having the rubber formulations shown in Table 1 are prepared. The core compositions are subjected to 20 minutes of vulcanization at 155° C., and the core surface is abraded, thereby producing 38.6 mm-diameter cores.

(49) TABLE-US-00001 TABLE 1 Type of core X Y Z Core Polybutadien 100 100 100 formulation Zinc acrylate 40 26 25 (pbw) Organic peroxide 1 1 1 Zinc oxide 15.4 21.1 21.4 Propylene glycol 1 Water 1 Antioxidant (1) 0.2 0.2 Antioxidant (2) 0.3 Core Diameter (mm) 38.6 38.6 38.6 properties Weight (g) 35.0 35.0 35.0 Compressive hardness (mm) 4.03 4.01 4.05 Surface hardness A (JIS-C) 76.8 80.4 70.5 Center hardness B (JIS-C) 53.1 54.2 56.6 Hardness difference A-B (JIS-C) 23.7 26.2 13.9 Details on the above formulations are given below. Polybutadiene: Available under the trade name “BR 01” from JSR Corporation Zinc acrylate: Available under the trade name “ZN-DA85S” (85% zinc acrylate/15% zinc stearate) from Nippon Shokubai Co., Ltd. Organic peroxide (dicumyl peroxide): Available under the trade name “Percumyl D” from NOF Corporation Zinc oxide: Available as “Zinc Oxide Grade 3” from Sakai Chemical Co., Ltd. Propylene glycol (a dihydric alcohol): molecular weight, 76.1 (from Hayashi Pure Chemical Ind., Inc.) Water: Distilled water Antioxidant (1): Available under the trade name “Nocrac NS-6” from Oucin Shinko Chemical Industry Co., Ltd. Antioxidant (2): Available under the trade name “Nocrac MB” from Ouchi ShinkoChemical Industry Co., Ltd.
Center and Surface Hardnesses of Core

(50) The surface and center hardnesses of the three above types of 38.6 mm-diameter cores are measured by the following methods.

(51) At a temperature of 23±° C., the hardnesses at four random points on the core surface are measured with a JIS-C durometer by perpendicularly setting the durometer indenter against the spherical surface of the core. The average value of these measurements is treated as the measured value for one core, and the average value for three measured cores is determined. Also, the core is cut through the center to obtain a flat cross-section. At a temperature of 23*1° C., the center hardness of the hemispherical core is measured with a JIS-C durometer by perpendicularly setting the durometer indenter against the flat cross-section, thereby obtaining the measurement for one core. The average value for three measured cores is determined. These measurements are presented in Table 1.

(52) Compressive Hardnesses of Core

(53) The compressive hardness (deformation)(mm) of each core when compressed at a rate of 10 mm/s under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) is measured at a temperature of 23±1° C. The average value for ten measured cores is determined.

(54) Surface Treatment of Core

(55) The following four types of surface treatment are carried out on the three above types of cores. Core surface treatment (I): After being abraded, the core is washed with water and then dried at room temperature (20 to 25° C.). Core surface treatment (II): After being abraded, the core is washed with an ethanol solution and then dried at room temperature (20 to 25° C.). Core surface treatment (III): After being abraded, the core is dipped for 1 minute at a liquid temperature of 23° C. in an Epocros® solution (mixed solution of Epocros® WS-500: ethanol=1:20 (weight ratio)) and then dried at room temperature (20 to 25° C.). Core surface treatment (IV): After being abraded, the core is dipped for 1 minute at 23° C. in a 0.1 mol/L hydrochloric acid 2-propanolic treatment solution, following which the dipped core is washed with an ethanol solution and then dried at room temperature (20 to 25° C.). The core is subsequently dipped for 1 minute at a liquid temperature of 23° C. in an Epocros® solution (mixed solution of Epocros® WS-500: ethanol=1:20 (weight ratio)) and then dried at room temperature (20 to 25° C.).

(56) Epocros® WS-500 is an acrylic polymer from Nippon Shokubai Co., Ltd. which is a light-yellow liquid (water-soluble) and has an oxazoline group content of 4.5 mmol/g (solids), a glass transition temperature of 50° C., a number-average molecular weight (Mn) of 20,000 and a weight-average molecular weight (Mw) of 70,000.

(57) Formation of Envelope Laver and Outermost Laver

(58) Next, using an injection mold, the envelope layer material (ionomer resin material) shown in Table 2 is injection-molded over the core surface, thereby forming an envelope layer having a thickness of 1.25 mm and a Shore D hardness of 64. Using a different injection mold, the outermost layer material (urethane resin material) shown in Table 2 is then injection-molded over the envelope layer-encased sphere, thereby forming an outermost layer having a thickness of 0.8 mm and a Shore D hardness of 41.

(59) TABLE-US-00002 TABLE 2 Compounding Envelope Outermost ingredients (pbw) layer layer Himilan 1706 33 Himilan 1557 15 Himilan 1605 50 TPU 100 Polyethyline wax 1.0 Isocyanate compound 6.3 Titanium oxide 3.3 Trimethylolpropane 1.1 Details on the compounding ingredients in this table are given below. Himilan 1706, Himilan 1557, Himilan 1605: Ionomer resins available from Dow-Mitsui Polychemicals Co., Ltd. TPU: An ether-type thermoplastic polyurethane available under the trade name “Pandex” from DIC Covestro Polymer, Ltd., Shore D hardness, 41 Polyethylene wax: Available under the trade name “Sanwax 161P” from Sanyo Chemical Industries, Ltd. Isocyanate compound: 4,4′-Diphenylmethane diisocyanate

(60) The compressive hardness and durability to cracking of the resulting golf balls are evaluated by the following methods. The results are presented in Table 3. The compressive hardness and durability to cracking for all of the balls are measured after letting the manufactured balls stand for one month at 23° C. following completion.

(61) Compressive Hardness of Ball

(62) The compressive hardness (deformation)(mm) of the golf ball in each Example when compressed at a speed of 10 mm/s under a final load of 1,275 N (130 kg) from an initial load state of 98 N (10 kgf) is measured at a temperature of 23*1° C. In each case, the average value for ten measured balls is determined.

(63) Durability to Cracking

(64) The durability of the golf ball is evaluated using an ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). This tester fires a golf ball pneumatically and causes it to consecutively strike two metal plates arranged in parallel. The incident velocity against the metal plates is set to 43 m/s. The number of shots required for the golf ball to crack is measured, and the average value of the measurements taken for ten golf balls is calculated. Durability indices for the balls in the respective Examples are calculated relative to an arbitrary index of 100 for the average number of shots required for the ball obtained in Comparative Example 4 to crack and are shown in Table 3. The durability index difference between the ball in an Example of the invention and the ball in a Comparative Example having the same type of core is indicated in Table 3 as the “Degree of improvement.”

(65) TABLE-US-00003 TABLE 3 Example Comparative Example 1 2 3 4 1 2 3 4 Core Type X Y Z Z X Y Z X (before Compressive hardness (mm) 4.03 4.01 4.05 4.05 4.03 4.01 4.05 4.03 surface JIS-C hardness difference 23.7 26.2 13.9 13.9 23.7 26.2 13.9 23.7 treatment) (surface − center) Core surface treatment (IV) (IV) (IV) (III) (I) (I) (I) (II) Ball Compressive hardness (mm) 3.26 3.24 3.30 3.30 3.26 3.24 3.30 3.26 Durability Index 125 128 130 112 100 103 109 100 to cracking Degree of +25 +25 +21 +3 — — — 0 improvement* *“Degree of improvment” refers to the durability index difference relative to Treatment I (non-treatment of core surface) in balls having the same type of core.

(66) Japanese Patent Application No. 2019-233130 is incorporated herein by reference.

(67) Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. Itis therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.