US20080146377A1 - Multi-piece solid golf ball

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Abstract

Description

Claims

US20080146377A1

Publication number: US20080146377A1
Application number: US11/637,839
Authority: US
Inventors: Hideo Watanabe
Original assignee: Bridgestone Sports Co Ltd
Current assignee: Bridgestone Sports Co Ltd
Priority date: 2006-12-13
Filing date: 2006-12-13
Publication date: 2008-06-19
Also published as: JP2008149130A; JP5286764B2

The invention provides a multi-piece solid golf ball having a core, an envelope layer encasing the core, an intermediate layer encasing the envelope layer, and a cover which encases the intermediate layer and has formed on a surface thereof a plurality of dimples. The core is formed primarily of a rubber material and has a diameter of at least 31 mm, the envelope layer and the intermediate layer are each formed primarily of the same or different resin materials, and the cover is formed primarily of polyurethane. The intermediate layer and the cover have thicknesses which satisfy the relationship: cover thickness<intermediate layer thickness. The core has a surface hardness (Durometer D hardness) and the envelope layer, intermediate layer and cover have material hardnesses (Durometer D hardness) which together satisfy the relationship: cover material hardness<intermediate layer material hardness<envelope layer material hardness>core surface hardness. The golf ball of the invention, as a ball for professionals and skilled amateur golfers, has both an excellent controllability and flight as well as an excellent feel on impact, in addition to which it has a good scuff resistance, enabling it to endure harsh conditions of use.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball composed of a core, an envelope layer, an intermediate layer and a cover that have been formed as successive layers. More specifically, the invention relates to a multi-piece solid golf ball which has a flight performance and controllability that satisfy the needs of professionals and other skilled golfers, and which also has a good feel on impact and an excellent scuff resistance.
A variety of golf balls have hitherto been developed for professionals and other skilled golfers. Of these, multi-piece solid golf balls having an optimized hardness relationship between an intermediate layer encasing the core and the cover layer are in wide use because they achieve both a superior distance in the high head speed range and good controllability on shots taken with an iron and on approach shots. Another important concern is the proper selection of thicknesses and hardnesses for the respective layers of the golf ball so as to optimize not only flight performance, but also both the feel of the ball when played as well as its spin rate after being struck with the club, particularly given the large influence these latter factors have on ball control. A further key concern in ball development, arising from the desire that golf balls also have durability under repeated impact and scuff resistance against burr formation on the surface of the ball when repeatedly played with different types of clubs, is how best to protect the ball from external factors.
The three-piece solid golf balls having an outer layer cover formed primarily of a thermoplastic polyurethane which are disclosed in, for example, JP-A 2003-190330, JP-A 2004-049913, JP-A 2004-97802 and JP-A 2005-319287 were intended to meet such needs. However, because these prior-art golf balls fail to achieve a sufficiently lower spin rate when hit with a driver, professionals and other skilled golfers have desired a ball which delivers an even longer distance.
Meanwhile, efforts to improve the flight and other performance characteristics of golf balls have led to the development of balls having a four-layer construction—i.e., a core enclosed by three intermediate or cover layers—that allows the ball construction to be varied among the several layers at the interior. Such golf balls have been disclosed in, for example, JP-A 2004-180822, JP-A 10-127818, JP-A 10-127819, JP-A 10-295852, U.S. Pat. No. 5,816,937, U.S. Pat. No. 6,152,834, U.S. Pat. No. 6,123,630, U.S. Pat. No. 6,468,169, U.S. Pat. No. 6,045,460, U.S. Pat. No. 6,248,027, U.S. Pat. No. 6,117,026 and U.S. Pat. No. 6,277,036.
Yet, as golf balls for the skilled golfer, such balls provide a poor balance of distance and controllability or they fall short in terms of achieving a lower spin rate on shots with a driver, thus limiting the extent to which the total distance can be increased.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a multi-piece solid golf ball which has a flight performance and controllability that are fully acceptable to professionals and other skilled golfers, while also having an excellent durability to cracking on repeated impact and an excellent scuff resistance.
In the present invention, the golf ball design consists basically of an outermost layer made of polyurethane and a multilayer structure of three or more outer layers (envelope layer/intermediate layer/cover) encasing the core. By making the cover, or outermost layer, of polyurethane, which is relatively soft, a spin performance on approach shots that is acceptable to professionals and other skilled golfers and a high scuff resistance are obtained. By forming the envelope layer of a material which is harder than the core surface and the intermediate layer, the spin rate of the ball on shots with a driver (W#1) can be lowered. In addition, by imparting to the respective layers in the core/envelope layer/intermediate layer/cover construction the following hardness relationship: cover material hardness<intermediate layer material hardness<envelope layer material hardness>core surface hardness, and by optimizing the core diameter and the relationship between the intermediate layer and the cover layer thicknesses, it was possible through the synergistic effects of these hardness relationships and layer thickness relationships to resolve the above-described problems encountered in the prior art. That is, the golf ball of the invention, when used by professionals and other skilled golfers, provides a fully acceptable flight performance and controllability, in addition to which it exhibits an excellent durability to cracking on repeated impact and excellent scuff resistance, effects which were entirely unanticipated. Having thus found that the technical challenges recited above can be overcome by the foregoing arrangement, the inventors ultimately arrived at the present invention.
Accordingly, the invention provides the following multi-piece solid golf balls.

[1] A multi-piece solid golf ball comprising a core, an envelope layer encasing the core, an intermediate layer encasing the envelope layer, and a cover which encases the intermediate layer and has formed on a surface thereof a plurality of dimples, wherein the core is formed primarily of a rubber material and has a diameter of at least 31 mm, the envelope layer and the intermediate layer are each formed primarily of the same or different resin materials, and the cover is formed primarily of polyurethane; the intermediate layer and the cover have thicknesses which satisfy the relationship: cover thickness<intermediate layer thickness; and the core has a surface hardness (Durometer D hardness) and the envelope layer, intermediate layer and cover have material hardnesses (Durometer D hardness) which together satisfy the relationship: cover material hardness<intermediate layer material hardness<envelope layer material hardness>core surface hardness.
[2] The multi-piece solid golf ball of [1], wherein the thicknesses of the envelope layer, the intermediate layer and the cover satisfy the relationship: cover thickness<intermediate layer thickness<envelope layer thickness.
[3] The multi-piece solid golf ball of [1], wherein the resin material which forms the envelope layer comprises an ionomer resin having an acid content of at least 16 wt %.
[4] The multi-piece solid golf ball of [1], wherein the resin material which forms the cover is composed primarily of (A) a thermoplastic polyurethane material, and (B) an isocyanate mixture obtained by dispersing (b-1) a compound having two or more isocyanate groups as functional groups per molecule in (b-2) a thermoplastic resin which is substantially non-reactive with isocyanate.
[5] The multi-piece solid golf ball of [1], wherein the envelope layer, intermediate layer and cover have material hardnesses (Durometer D hardnesses) such that: 60≦envelope layer material hardness≦70, 55≦intermediate layer material hardness≦70, and 30≦cover material hardness≦55.
[6] The multi-piece solid golf ball of [1], wherein the ball and the core have deflections (mm), when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), which satisfy the following respective conditions: 2.0 mm≦ball deflection≦3.0 mm, and 3.5 mm≦core deflection≦6.0 mm.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional view showing a multi-piece solid golf ball (4-layer construction) according to the invention.

FIG. 2 is a top view of a golf ball showing the arrangement of dimples used in the examples of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below. The multi-piece solid golf ball of the present invention, as shown in FIG. 1, is a golf ball G having four or more layers, including a core 1, an envelope layer 2 which encases the core, an intermediate layer 3 which encases the envelope layer, and a cover 4 which encases the intermediate layer. The cover 4 typically has a large number of dimples D formed on the surface thereof. The core 1 and the intermediate layer 3 are not limited to single layers, and may each be formed of a plurality of two more layers.
In the invention, the core diameter is set to at least 31 mm, and is generally at least 31 mm but not more than 38 mm, preferably at least 32.5 mm but not more than 37 mm, and more preferably at least 34 mm but not more than 36 mm. A core diameter outside this range will lower the initial velocity of the ball or yield a less than adequate spin rate-lowering effect after the ball is hit, as a result of which an increased distance may not be achieved.
The surface hardness of the core, while not subject to any particular limitation, preferably has a Durometer D hardness (the value measured with a type D durometer based on ASTM D2240; the same applies to the hardnesses described below for the respective layers) of generally at least 35 but not more than 60, more preferably at least 40 but not more than 55, and even more preferably at least 43 but not more than 50. Below the above range, the ball may have an inadequate rebound and thus fail to achieve the desired distance, and the durability to cracking on repeated impact may worsen. Conversely, at a core surface hardness higher than the above range, the ball may have an excessively hard feel on full shots with a driver and the spin rate may be too high, as a result of which the desired distance may not be achieved.
The deflection when the core is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), while not subject to any particular limitation, is preferably set within a range of at least 3.5 mm but not more than 6.0 mm, more preferably at least 3.8 mm but not more than 5.6 mm, and even more preferably at least 4.2 mm but not more than 5.2 mm. If this value is too low, the core may lack sufficient rebound, which may result in a less than adequate distance, and the durability of the ball to cracking on repeated impact may worsen. On the other hand, if this value is too high, the ball may have an excessively hard feel on full shots with a driver, and the spin rate may be too high, as a result of which an increased distance may not be achieved.
The core surface hardness must be lower than the material hardness of the envelope layer. The difference between the hardness of the envelope layer material and the core surface hardness is generally from 5 to 35, preferably from 10 to 30, and more preferably from 15 to 25. Outside of this range, the ball as a whole will have an inappropriate hardness and a poor feel. Moreover, the rebound will be insufficient or the spin rate-lowering effect will be inadequate, which may prevent the desired distance from being achieved.
The solid core may be formed of a rubber composition containing, for example, a co-crosslinking agent, an organic peroxide, an inert filler and an organosulfur compound. It is preferable to use polybutadiene as the base rubber of the rubber composition.
It is desirable for the polybutadiene serving as the rubber component to have a cis-1,4-bond content on the polymer chain of at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. Too low a cis-1,4-bond content among the bonds on the molecule may lead to a lower resilience.
Moreover, the polybutadiene has a 1,2-vinyl bond content on the polymer chain of typically not more than 2%, preferably not more than 1.7%, and even more preferably not more than 1.5%. Too high a 1,2-vinyl bond content may lead to a lower resilience.
To obtain a molded and vulcanized rubber composition of good resilience, the polybutadiene used in the invention is preferably one synthesized with a rare-earth catalyst or a Group VIII metal compound catalyst. Polybutadiene synthesized with a rare-earth catalyst is especially preferred.
Such rare-earth catalysts are not subject to any particular limitation. Exemplary rare-earth catalysts include those made up of a combination of a lanthanide series rare-earth compound with an organoaluminum compound, an alumoxane, a halogen-bearing compound and an optional Lewis base.
Examples of suitable lanthanide series rare-earth compounds include halides, carboxylates, alcoholates, thioalcoholates and amides of atomic number 57 to 71 metals.
In the practice of the invention, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth compound is particularly advantageous because it enables a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content to be obtained at an excellent polymerization activity. Suitable examples of such rare-earth catalysts include those mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.
To enhance the resilience, it is preferable for the polybutadiene synthesized using the lanthanide series rare-earth compound catalyst to account for at least 10 wt %, preferably at least 20 wt %, and more preferably at least 40 wt %, of the rubber components.
Rubber components other than the above-described polybutadiene may be included in the base rubber insofar as the objects of the invention are attainable. Illustrative examples of rubber components other than the above-described polybutadiene include other polybutadienes, and other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.
Examples of co-crosslinking agents include unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids.
Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.
The metal salts of unsaturated carboxylic acids, while not subject to any particular limitation, are exemplified by the above-mentioned unsaturated carboxylic acids neutralized with a desired metal ion. Specific examples include the zinc and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.
The unsaturated carboxylic acid and/or metal salt thereof is included in an amount, per 100 parts by weight of the base rubber, of generally at least 10 parts by weight, preferably at least 15 parts by weight, and more preferably at least 20 parts by weight, but generally not more than 60 parts by weight, preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel on impact, whereas too little may lower the rebound.
The organic peroxide may be a commercially available product, suitable examples of which include Percumyl D (produced by NOF Corporation), Perhexa 3M and Perhexa C-40 (NOF Corporation), and Luperco 231XL (Atochem Co.). These may be used singly or as a combination of two or more thereof.
The amount of organic peroxide included per 100 parts by weight of the base rubber is generally at least 0.1 part by weight, preferably at least 0.3 part by weight, more preferably at least 0.5 part by weight, and most preferably at least 0.7 part by weight, but generally not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much or too little organic peroxide may make it impossible to achieve a ball having a good feel on impact, durability and rebound.
Examples of suitable inert fillers include zinc oxide, barium sulfate and calcium carbonate. These may be used singly or as a combination of two or more thereof.
The amount of inert filler included per 100 parts by weight of the base rubber is generally at least 1 part by weight, and preferably at least 5 parts by weight, but generally not more than 50 parts by weight, preferably not more than 45 parts by weight, more preferably not more than 40 parts by weight, and most preferably not more than 35 parts by weight. Too much or too little inert filler may make it impossible to achieve a proper weight and a good rebound.
In addition, an antioxidant may be included if necessary. Illustrative examples of suitable commercial antioxidants include Nocrac 200, Nocrac NS-6 and Nocrac NS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (available from Yoshitomi Pharmaceutical Industries, Ltd.). These may be used singly or as a combination of two or more thereof.
The amount of antioxidant included per 100 parts by weight of the base rubber is generally 0 or more part by weight, preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight, but generally not more than 3 parts by weight, preferably not more than 2 parts by weight, more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight. Too much or too little antioxidant may make it impossible to achieve a good rebound and durability.
To enhance the rebound of the golf ball and increase its initial velocity, it is preferable to include within the core an organosulfur compound.
No particular limitation is imposed on the organosulfur compound, provided it improves the rebound of the golf ball. Exemplary organosulfur compounds include thiophenols, thionaphthols, halogenated thiophenols, and metal salts thereof. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, the zinc salt of pentafluorothiophenol, the zinc salt of pentabromothiophenol, the zinc salt of p-chlorothiophenol; and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. The zinc salt of pentachlorothiophenol is especially preferred.
It is recommended that the amount of the organosulfur compound included per 100 parts by weight of the base rubber be generally at least 0.05 part by weight, and preferably at least 0.1 part by weight, but generally not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2.5 parts by weight. If too much organosulfur compound is included, the effects of addition may peak so that further addition has no apparent effect, whereas the use of too little organosulfur compound may fail to confer the effects of such addition to a sufficient degree.
Next, the envelope layer is described.
The envelope layer material has a hardness, expressed as the Durometer D hardness, which, while not subject to any particular limitation, is generally at least 50 but not more than 75, more preferably at least 60 but not more than 70, and even more preferably at least 62 but not more than 68. If the envelope layer material is softer than the above range, the ball may have too much spin receptivity on full shots, as a result of which an increased distance may not be achieved. On the other hand, if this material is harder than the above range, the durability of the ball to cracking under repeated impact may worsen and the ball may have too hard a feel when played. The envelope layer has a thickness which, while not subject to any particular limitation, is generally at least 1.0 mm but not more than 4.0 mm, preferably at least 1.2 mm but not more than 3.0 mm, and more preferably at least 1.4 mm but not more than 2.0 mm. Outside of this range, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate, as a result of which an increased distance may not be achieved.
The envelope layer in the present invention is formed primarily of a resin material. The resin material in the envelope layer is preferably an ionomer resin. Zinc-neutralized ionomer resins and sodium-neutralized ionomer resins are especially preferred, and may be used either singly or as combinations of two or more such resins. If both types are used in admixture, the mixing ratio therebetween, expressed as zinc-neutralized resin/sodium-neutralized resin (weight ratio), is generally from 25/75 to 75/25, preferably from 35/65 to 65/35, and more preferably from 45/55 to 55/45. Outside of the above range, the rebound may be too small, preventing the desired distance from being achieved, the durability to cracking on repeated impact at normal temperatures may worsen, and the durability to cracking at low temperatures (below 0° C.) may worsen. Moreover, it is desirable for each ionomer to have an acid content of generally at least 16 wt %, preferably at least 17 wt %, and more preferably at least 18 wt %. The amount of such an ionomer resin included in the envelope layer-forming material is generally at least 20%, preferably at least 50%, and more preferably at least 70%. If the acid content of the ionomer and the ionomer content of the envelope layer-forming material are too low, the spin rate-lowering effect may be too small, as a result of which the desired distance may not be achieved.
It is preferable for the envelope layer material to have a higher hardness than the intermediate layer material. In this way, a sufficient spin rate-lowering effect can be achieved on shots with a driver (W#1). The difference between the hardness of the envelope layer material and the hardness of the intermediate layer material is generally at least 1 but not more than 20, preferably at least 2 but not more than 15, and even more preferably at least 3 but not more than 10. Outside of the above range, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate, as a result of which the desired distance may not be achieved.
Next, the intermediate layer is described.
The intermediate layer material has a hardness, expressed as the Durometer D hardness, which, although not subject to any particular limitation, is generally at least 50 but not more than 70, preferably at least 55 but not more than 66, and more preferably at least 60 but not more than 63. If the intermediate layer material is softer than the above range, the ball may have too much spin receptivity on full shots, as a result of which an increased distance may not be attained. On the other hand, if this material is harder than the above range, the durability of the ball to cracking under repeated impact may worsen and the ball may have too hard a feel when played with a putter or on short approach shots. The intermediate layer has a thickness which, while not subject to any particular limitation, is generally at least 0.7 mm but not more than 2.0 mm, preferably at least 0.9 mm but not more than 1.7 mm, and more preferably at least 1.1 mm but not more than 1.4 mm. Outside of this range, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate, as a result of which an increased distance may not be achieved. Moreover, a thickness below the foregoing range may lower the durability to cracking on repeated impact and the low-temperature durability.
The intermediate layer in the invention may be formed primarily of a resin material which is the same as or different from the above-described material used to form the envelope layer. An ionomer resin is especially preferred. Specific examples include sodium-neutralized ionomer resins available under the trade name designations Himilan 1605, Himilan 1601 and Surlyn 8120, and zinc-neutralized ionomer resins such as Himilan 1557, Himilan 1706 and Himilan 1855. These may be used singly or as a combination of two or more thereof.
An embodiment in which the intermediate layer material is composed primarily of, in admixture, both a zinc-neutralized ionomer resin and a sodium-neutralized ionomer resin is especially preferable for attaining the objects of the invention. The mixing ratio, expressed as zinc-neutralized resin/sodium-neutralized resin (weight ratio), is generally from 25/75 to 75/25, preferably from 35/65 to 65/35, and more preferably from 45/55 to 55/45. Outside of the above ratio, the ball rebound may be too low, as a result of which the desired distance may not be achieved, the durability to repeated impact at normal temperatures may worsen, and the durability to cracking at low temperatures (below 0° C.) may worsen.
To increase adhesion between the intermediate layer material and the polyurethane used in the subsequently described cover, it is desirable to abrade the surface of the intermediate layer. In addition, it is preferable to apply a primer (adhesive) to the surface of the intermediate layer following such abrasion or to add an adhesion reinforcing agent to the intermediate layer material. Examples of adhesion reinforcing agents that may be incorporated in the material include organic compounds such as 1,3-butanediol and trimethylolpropane, and oligomers such as polyethylene glycol and polyhydroxy polyolefin oligomers. The use of trimethylolpropane or a polyhydroxy polyolefin oligomer is especially preferred. Examples of commercially available products include trimethylolpropane produced by Mitsubishi Gas Chemical Co., Ltd. and polyhydroxy polyolefin oligomers produced by Mitsubishi Chemical Corporation (under the trade name designation Polytail H; number of main-chain carbons, 150 to 200; with hydroxyl groups at the ends).
Next, the cover is described. As used herein, the term “cover” denotes the outermost layer of the ball construction, and excludes what is referred to herein as the intermediate layer and the envelope layer.
The cover material has a hardness, expressed as the Durometer D hardness, which, while not subject to any particular limitation, is preferably at least 30 but not more than 55, more preferably at least 40 but nor more than 54, and even more preferably at least 45 but not more than 53. At a hardness below this range, the ball tends to take on too much spin on full shots, as a result of which an increased distance may not be achieved. On the other hand, at a hardness above this range, on approach shots, the ball lacks spin receptivity and thus may have an inadequate controllability even when played by a professional or other skilled golfer.
The thickness of the cover, while not subject to any particular limitation, is preferably at least 0.3 mm but not more than 1.5 mm, more preferably at least 0.5 mm but not more than 1.3 mm, and even more preferably at least 0.7 mm but not more than 1.1 mm. If the cover is thicker than the above range, the ball may have an inadequate rebound on shots with a driver (W#1) or the spin rate may be too high, as a result of which an increased distance may not be achieved. Conversely, if the cover is thinner than the above range, the ball may have a poor scuff resistance and inadequate controllability even when played by a professional or other skilled golfer.
From the standpoint of controllability and scuff resistance, it is preferable for the above cover material to be composed primarily of polyurethane. For amenability to mass production, it is especially preferable to use a thermoplastic urethane cover. In the practice of the invention, the use of a cover-molding material (C) composed primarily of the following components A and B is advantageous:

(A) a thermoplastic polyurethane material, and
(B) an isocyanate mixture obtained by dispersing (b-1) a compound having two or more isocyanate groups as functional groups per molecule in (b-2) a thermoplastic resin which is substantially non-reactive with isocyanate.

Components (A), (B) and (C) are described below.

(A) Thermoplastic Polyurethane Material

The thermoplastic polyurethane material has a morphology which includes soft segments composed of a polymeric polyol (polymeric glycol) and hard segments composed of a chain extender and a diisocyanate. The polymeric polyol used as a starting material may be any that has hitherto been employed in the art relating to thermoplastic polyurethane materials, without particular limitation. Exemplary polymeric polyols include polyester polyols and polyether polyols, although polyether polyols are better than polyester polyols for synthesizing thermoplastic polyurethane materials that provide a high rebound resilience and have excellent low-temperature properties. Suitable polyether polyols include polytetramethylene glycol and polypropylene glycol. Polytetramethylene glycol is especially preferred for achieving a good rebound resilience and good low-temperature properties. The polymeric polyol has an average molecular weight of preferably 1,000 to 5,000. To synthesize a thermoplastic polyurethane material having a high rebound resilience, an average molecular weight of 2,000 to 4,000 is especially preferred.
Preferred chain extenders include those used in the prior art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, and 2,2-dimethyl-1,3-propanediol. These chain extenders have an average molecular weight of preferably 20 to 15,000.
Diisocyanates suitable for use include those employed in the prior art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. Depending on the type of isocyanate used, the crosslinking reaction during injection molding may be difficult to control. In the present invention, to ensure stable reactivity with the subsequently described isocyanate mixture (B), it is most preferable to use an aromatic diisocyanate, and specifically 4,4′-diphenylmethane diisocyanate.
A commercial product may be suitably used as the above-described thermoplastic polyurethane material. Illustrative examples include Pandex T-8290, Pandex T-8295 and Pandex T-8260 (all manufactured by DIC Bayer Polymer, Ltd.), and Resamine 2593 and Resamine 2597 (both manufactured by Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.).

(B) Isocyanate Mixture

The isocyanate mixture (B) is prepared by dispersing (b-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (b-2) a thermoplastic resin which is substantially non-reactive with isocyanate. Above isocyanate compound (b-1) is preferably an isocyanate compound used in the prior art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. From the standpoint of reactivity and work safety, the use of 4,4′-diphenylmethane diisocyanate is most preferred.
The thermoplastic resin (b-2) is preferably a resin having a low water absorption and excellent compatibility with thermoplastic polyurethane materials. Illustrative, non-limiting, examples of such resins include polystyrene resins, polyvinyl chloride resins, ABS resins, polycarbonate resins and polyester elastomers (e.g., polyether-ester block copolymers, polyester-ester block copolymers). From the standpoint of rebound resilience and strength, the use of a polyester elastomer, particularly a polyether-ester block copolymer, is especially preferred.
In the isocyanate mixture (B), it is desirable for the relative proportions of the thermoplastic resin (b-2) and the isocyanate compound (b-1), expressed as the weight ratio (b-2):(b-1), to be from 100:5 to 100:100, and especially from 100:10 to 100:40. If the amount of the isocyanate compound (b-1) relative to the thermoplastic resin (b-2) is too small, a greater amount of the isocyanate mixture (B) will have to be added to achieve an amount of addition sufficient for the crosslinking reaction with the thermoplastic polyurethane material (A). As a result, the thermoplastic resin (b-2) will exert a large influence, compromising the physical properties of the cover-molding material (C). On the other hand, if the amount of the isocyanate compound (b-1) relative to the thermoplastic resin (b-2) is too large, the isocyanate compound (b-1) may cause slippage to occur during mixing, making preparation of the isocyanate mixture (B) difficult.
The isocyanate mixture (B) can be obtained by, for example, adding the isocyanate compound (b-1) to the thermoplastic resin (b-2) and thoroughly working together these components at a temperature of 130 to 250° C. using mixing rolls or a Banbury mixer, then either pelletizing or cooling and subsequently grinding. A commercial product such as Crossnate EM30 (made by Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.) may be suitably used as the isocyanate mixture (B).

(C) Cover-Molding Material

The cover-molding material (C) is composed primarily of the above-described thermoplastic polyurethane material (A) and isocyanate mixture (B). The relative proportions of the thermoplastic polyurethane material (A) and the isocyanate mixture (B) in the cover-molding material (C), expressed as the weight ratio (A):(B), is preferably from 100:1 to 100:100, more preferably from 100:5 to 100:50, and even more preferably from 100:10 to 100:30. If too little isocyanate mixture (B) is included relative to the thermoplastic polyurethane material (A), a sufficient crosslinking effect will not be achieved. On the other hand, if too much is included, unreacted isocyanate may discolor the molded material.
In addition to the above-described ingredients, other ingredients may be included in the cover-molding material (C). For example, thermoplastic polymeric materials other than the thermoplastic polyurethane material may be included; illustrative examples include polyester elastomers, polyamide elastomers, ionomer resins, styrene block elastomers, polyethylene and nylon resins. Thermoplastic polymeric materials other than the thermoplastic polyurethane material may be included in an amount of 0 to 100 parts by weight, preferably 1 to 75 parts by weight, and more preferably 10 to 50 parts by weight, per 100 parts by weight of the thermoplastic polyurethane material serving as the essential component. The amount of such thermoplastic polymeric materials used is selected as appropriate for such purposes as adjusting the hardness of the cover material, improving the rebound, improving the flow properties, and improving adhesion. If necessary, various additives such as pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and parting agents may also be suitably included in the cover-molding material (C).
Formation of the cover from the cover-molding material (C) can be carried out by adding the isocyanate mixture (B) to the thermoplastic polyurethane material (A) and dry mixing, then using an injection molding machine to mold the mixture into a cover over the core. The molding temperature varies with the type of thermoplastic polyurethane material (A), although molding is generally carried out within a temperature range of 150 to 250° C.
Reactions and crosslinking which take place in the golf ball cover obtained as described above are believed to involve the reaction of isocyanate groups with hydroxyl groups remaining on the thermoplastic polyurethane material to form urethane bonds, or the creation of an allophanate or biuret crosslinked form via a reaction involving the addition of isocyanate groups to urethane groups in the thermoplastic polyurethane material. Although the crosslinking reaction has not yet proceeded to a sufficient degree immediately after injection molding of the cover-molding material (C), the crosslinking reaction can be made to proceed further by carrying out an annealing step after molding, in this way maintaining properties useful for a golf ball cover. “Annealing,” as used herein, refers to heat aging the cover at a constant temperature for a given length of time, or aging the cover for a fixed period at room temperature.
In addition to the above resin components, various optional additives may be included in the above-described resin materials for the envelope layer, the intermediate layer and the cover. Such additives include, for example, pigments, dispersants, antioxidants, ultraviolet absorbers, ultraviolet stabilizers, parting agents, plasticizers, and inorganic fillers (e.g., zinc oxide, barium sulfate, titanium dioxide).

Relationship between Core Surface Hardness and Hardnesses of Envelope Layer, Intermediate Layer and Cover Materials

In the present invention, it is critical that the relationship among the core surface hardness and the hardnesses of the respective materials for the envelope layer, intermediate layer and cover, expressed in terms of the Durometer D hardness, satisfy the conditions: cover material hardness<intermediate layer material hardness<envelope layer material hardness>core surface hardness. The reasons are the same as given above in the description of the envelope layer.

Thickness Relationship Between Envelope Layer, Intermediate Layer and Cover

In the present invention, it is critical for the thicknesses of the envelope layer, the intermediate layer and the cover to satisfy the relationship
cover thickness<intermediate layer thickness, and preferably also the relationship
cover thickness<intermediate layer thickness<envelope thickness.
By setting the core diameter to at least 31 mm and also designing the ball construction so that the relationships among the thicknesses of the envelope layer, intermediate layer and cover are as indicated above, there can be obtained a golf ball which exhibits a good flight performance, good controllability and a good feel when played. Should the cover be thicker than the intermediate layer, the ball rebound will decrease or the ball will have excessive spin receptivity on full shots, as a result of which an increased distance will not be attainable. Should the envelope layer be thinner than the intermediate layer, the spin rate-lowering effect will be inadequate, preventing the desired distance from being achieved.
The multi-piece solid golf ball of the invention can be manufactured using an ordinary process such as a known injection molding process to form on top of one another the respective layers described above—the core, envelope layer, intermediate layer, and cover. For example, a molded and vulcanized article composed primarily of the rubber material may be placed as the core within a particular injection-molding mold, following which the envelope layer material and the intermediate layer material may be injection-molded in this order to give an intermediate spherical body. The spherical body may then be placed within another injection-molding mold and the cover material injection-molded over the spherical body to give a multi-piece golf ball. Alternatively, the cover may be formed as a layer over the intermediate spherical body by, for example, placing two half-cups, molded beforehand as hemispherical shells, around the intermediate spherical body so as to encase it, then molding under applied heat and pressure.
Numerous dimples may be formed on the surface of the cover. The dimples arranged on the cover surface, while not subject to any particular limitation, number preferably at least 280 but not more than 360, more preferably at least 300 but not more than 350, and even more preferably at least 320 but not more than 340. If the number of dimples is higher than the above range, the ball will tend to have a low trajectory, which may shorten the distance of travel. On the other hand, if the number of dimples is too small, the ball will tend to have a high trajectory, as a result of which an increased distance may not be achieved.
Any one or combination of two or more dimple shapes, including circular shapes, various polygonal shapes, dewdrop shapes and oval shapes, may be suitably used. If circular dimples are used, the diameter of the dimples may be set to at least about 2.5 mm but not more than about 6.5 mm, and the depth may be set to at least 0.08 mm but not more than 0.30 mm.
To fully manifest the aerodynamic characteristics of the dimples, the dimple coverage on the spherical surface of the golf ball, which is the sum of the individual dimple surface areas, each defined by the border of the flat plane circumscribed by the edge of the dimple, expressed as a ratio (SR) with respect to the spherical surface area of the ball were it to be free of dimples, is preferably at least 60% but not more than 90%. Also, to optimize the trajectory of the ball, the value V₀obtained by dividing the spatial volume of each dimple below the flat plane circumscribed by the edge of that dimple by the volume of a cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the cylinder base is preferably at least 0.35 but not more than 0.80. In addition, the VR value, which is the sum of the volumes of individual dimples formed below flat planes circumscribed by the dimple edges, as a percentage of the volume of the ball sphere were it to have no dimples thereon, is preferably at least 0.6% but not more than 1%. Outside of the above ranges for these values, the ball may assume a trajectory that is not conducive to achieving a good distance, as a result of which the ball may fail to travel a sufficient distance when played.
The golf ball of the invention, which can be manufactured so as to conform with the Rules of Golf for competitive play, may be produced to a ball diameter which is of a size that will not pass through a ring having an inside diameter of 42.672 mm, but is not more than 42.80 mm, and to a weight of generally from 45.0 to 45.93 g.
As explained above, by using primarily a polyurethane material in the cover, by optimizing the respective thicknesses and hardnesses of the envelope layer, intermediate layer and cover as described above, and by setting the core diameter to at least a particular size, the inventive golf ball having a multi-layer construction is highly beneficial for professionals and other skilled golfers because the spin rate of the ball on full shots with a driver is lowered, providing an increased distance of travel and a good controllability, and because the ball has an excellent durability to cracking under repeated impact and an excellent scuff resistance.

EXAMPLES

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 and 2, Comparative Examples 1 to 6

Core Formation

Rubber compositions were formulated as shown in Table 1, then molded and vulcanized under the conditions shown in the table to form cores. Numbers shown for the ingredients in the table indicate parts by weight.

	TABLE 1

	Example	Comparative Example

	1	2	1	2	3	4	5	6

Core	Polybutadiene	100	100	100	100	100	100	100	100
formulation	Zinc acrylate	28.1	24.6	28.1	24.6	24.6	31.2	24.6	31.0
	Peroxide (1)	—	—	—	—	—	—	—	0.3
	Peroxide (2)	1.2	1.2	1.2	1.2	1.2	1.2	1.2	0.3
	Antioxidant	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
	Zinc oxide	30.6	31.9	34.5	64.2	22.5	12.8	28.8	22.4
	Zinc salt of	2	2	2	2	2	2	2	0.2
	pentachlorothiophenol
	Zinc stearate	5	5	5	5	5	5	5	5
Vulcanization	Temperature (° C.)	155	155	155	155	155	155	155	155
conditions	Time (min)	15	15	15	15	15	15	15	15

Trade names for some the materials appearing in the table are given below.

Polybutadiene: Available from JSR Corporation under the trade name BR730. Synthesized with a neodymium catalyst.
Peroxide (1): Dicumyl peroxide, available from NOF Corporation under the trade name Percumyl D.
Peroxide (2): A mixture of 1,1-di(t-butylperoxy)cyclohexane and silica, available from NOF Corporation under the trade name Perhexa C-40.
Antioxidant: 2,2-Methylenebis(4-methyl-6-butylphenol), available from Ouchi Shinko Chemical Industry Co., Ltd. as Nocrac NS-6.
Sulfur: Zinc white-sulfur mixture, available from Tsurumi Chemical Industry Co., Ltd.

Formation of Envelope Layer, Intermediate Layer and Cover

Next, the envelope layer, intermediate layer and cover formulated from the various resin components shown in Table 2 were injection-molded, thereby forming over the core, in order: an envelope layer, an intermediate layer and a cover. Next, the dimples shown in Table 3, which were common to all the examples, were formed on the cover surface, thereby producing multi-piece solid golf balls.

TABLE 2

No. 1	No. 2	No. 3	No. 4	No. 5	No. 6	No. 7

Himilan 1605			50	68.75
Himilan 1557			15		30
Himilan 1706			35
Himilan 1855					20
AM 7317	50
AM 7318	50
AM 7311					50
Dynaron 6100P				31.25
Hytrel 3046		100
Behenic acid				18
Calcium hydroxide				2.3
Calcium stearate				0.15
Zinc stearate				0.15
Trimethylolpropane			1.1
Polytail H				2
T-8295						94.8
T-8260							94.8
Titanium oxide	2				2.2	3.8	3.8
Polyethylene wax						1.4	1.4
Isocyanate compound						18	18

Note:
Numbers in the table indicate parts by weight.

Trade names for the chief materials appearing in Table 2 are given below.

Himilan: Ionomer resins produced by DuPont-Mitsui Polychemicals Co., Ltd.
AM 7317: A high-stiffness zinc ionomer resin having an acid content of 18% produced by DuPont-Mitsui Polychemicals Co., Ltd.
AM 7318: A high-stiffness sodium ionomer resin having an acid content of 18% produced by DuPont-Mitsui Polychemicals Co., Ltd.
AM 7317: An ionomer resin produced by DuPont-Mitsui Polychemicals Co., Ltd.
Dynaron: A hydrogenated polymer produced by JSR Corporation.
Hytrel: A polyester elastomer produced by DuPont-Toray Co., Ltd.
Behenic acid: NAA222-S (beads), produced by NOF Corporation.
Calcium hydroxide: CLS-B, produced by Shiraishi Kogyo.
Trimethylolpropane: Produced by Mitsubishi Gas Chemical Co., Inc.
Polytail H: A low-molecular-weight polyolefin polyol produced by Mitsubishi Chemical Corporation.
T-8260, T-8295: MDI-PTMG type thermoplastic polyurethane produced by DIC Bayer Polymer under the trademark designation Pandex.
Polyethylene wax: Produced by Sanyo Chemical Industries, Ltd. under the trade name Sanwax 161P.
Isocyanate compound: Crossnate EM30 (trade name), an isocyanate masterbatch which is produced by Dainichi Seika Colour & Chemicals Mfg. Co., Ltd., contains 30% of 4,4′-diphenylmethane diisocyanate (measured concentration of amine reverse-titrated isocyanate according to JIS-K1556, 5 to 10%), and in which the masterbatch base resin is a polyester elastomer. The isocyanate compound was mixed with Pandex at the time of injection molding.

1	12	4.6	0.15	0.47	0.81	0.783
2	234	4.4	0.15	0.47
3	60	3.8	0.14	0.47
4	6	3.5	0.13	0.46
5	6	3.4	0.13	0.46
6	12	2.6	0.10	0.46
Total	330

Dimple Definitions

Diameter: Diameter of flat plane circumscribed by edge of dimple.
Depth: Maximum depth of dimple from flat plane circumscribed by edge of dimple.
V₀: Spatial volume of dimple below flat plane circumscribed by dimple edge, divided by volume of cylinder whose base is the flat plane and whose height is the maximum depth of dimple from the base.
SR: Sum of individual dimple surface areas, each defined by the border of the flat plane circumscribed by the edge of the dimple, as a percentage of surface area of ball sphere were it to have no dimples thereon.
VR: Sum of volumes of individual dimples formed below flat plane circumscribed by the edge of the dimple, as a percentage of volume of ball sphere were it to have no dimples thereon.

The golf balls obtained in Examples 1 and 2 of the invention and Comparative Examples 1 to 6 were tested and evaluated according to the criteria described below with regard to the following: surface hardness and other physical properties of each layer and of the ball, flight performance, spin rate on approach shots (controllability), and scuff resistance. The results are shown in Table 4. All measurements were carried out in a 23° C. atmosphere.

(1) Core Deflection

The core was placed on a hard plate, and the deflection (mm) by the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured.

(2) Surface Hardness of Core

The surface of the core is spherical. The durometer indenter was set substantially perpendicular to this spherical surface, and Durometer D hardness measurements (using a type D durometer in accordance with ASTM-2240) were taken at two randomly selected points on the surface of the core. The average of the two measurements was used as the core surface hardness.

(3) Hardness of Envelope Layer Material

The resin material for the envelope layer was formed into a sheet having a thickness of about 2 mm, and the hardness was measured with a type D durometer in accordance with ASTM D2240.

(4) Hardness of Intermediate Layer Material

The same method of measurement was used as in (3) above.

(5) Hardness of Cover Material

The same method of measurement was used as in (3) above.

(6) Ball Deflection

The ball was placed on a hard plate, and the deflection (mm) by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. Ball deflection was measured only in Examples 1 and 2.

(7) Flight

The carry and total distance of the ball when hit at a head speed (HS) of 45 m/s with a driver (abbreviated below as “W#1”; TourStage X-Drive Type 405, manufactured by Bridgestone Sports Co., Ltd.; loft angle, 9.5°) mounted on a swing robot were measured. The results were rated according to the criteria indicated below. The spin rate, which was measured with an apparatus for measuring initial conditions, was the value obtained for the ball immediately following impact.

- Good: Total distance was 232.0 m or more
- NG: Total distance was less than 232.0 m

(8) Spin Rate on Approach Shots

The spin rate of a ball hit at a head speed of 22 m/s with a sand wedge (abbreviated below as “W”; J's Classical Edition, manufactured by Bridgestone Sports Co., Ltd.) was measured. The results were rated according to the criteria indicated below. The spin rate was measured by the same method as that used above when measuring distance.

- Good: Spin rate was at least 6,600 rpm
- Fair: Spin rate was at least 6,300 rpm but less than 6,600 rpm
- NG: Spin rate was less than 6,300 rpm

(9) Scuff Resistance

A non-plated pitching sand wedge was set in a swing robot, and the ball was hit once at a head speed of 40 m/s, following which the surface state of the ball was visually examined and rated as follows.

- Good: Can be used again
- NG: Cannot be used again

	TABLE 4

	Example	Comparative Example

	1	2	1	2	3	4	5	6

Core	Diameter (mm)	34.9	34.9	34.9	29.0	34.9	34.9	34.9	37.3
	Weight (g)	27.2	27.1	28.0	18.0	26.4	25.5	27.2	31.7
	Deflection (mm)	4.4	4.9	4.4	4.9	4.4	3.3	4.9	3.1
	Surface hardness (D)	48	45	48	45	48	55	45	56
Envelope	Material	No. 1	No. 1	No. 1	No. 1	No. 1	No. 2	No. 1	—
layer	Thickness (mm)	1.7	1.7	1.7	4.7	1.1	1.7	1.7	—
material	Specific gravity	0.96	0.96	0.96	0.96	0.96	1.07	0.96	—
	Material hardness (D)	66	66	66	66	66	30	66	—
Sphere	Outside diameter (mm)	38.4	38.4	38.4	38.4	37.2	38.4	38.4	—
encased by	Weight (g)	34.1	34.1	35.0	34.1	30.8	33.3	34.2	—
envelope
layer
Intermediate	Material	No. 3	No. 3	No. 3	No. 3	No. 3	No. 3	No. 4	No. 3
layer	Thickness (mm)	1.15	1.15	1.15	1.15	0.95	0.35	1.15	1.67
material	Specific gravity	0.95	0.95	0.95	0.95	0.95	0.95	0.93	0.95
	Material hardness (D)	62	62	62	62	62	62	56	62
Sphere	Outside diameter (mm)	40.7	40.7	40.7	40.7	39.1	39.1	40.7	40.6
encased by	Weight (g)	39.4	39.4	40.4	39.5	34.9	34.9	39.4	39.3
intermediate
layer
Cover	Material	No. 6	No. 6	No. 5	No. 6	No. 6	No. 6	No. 7	No. 6
material	Thickness (mm)	1.00	1.00	1.00	1.00	1.80	1.80	1.00	1.03
	Specific gravity	1.13	1.13	0.96	1.13	1.13	1.13	1.13	1.13
	Material hardness (D)	53	53	53	53	53	53	58	53
Ball¹⁾	Diameter (mm)	42.7	42.7	42.7	42.7	42.7	42.7	42.7	42.7
	Weight (g)	45.5	45.5	45.5	45.5	45.5	45.4	45.5	45.5
Flight	Spin rate (rpm)	2894	2749	2724	3177	2816	2985	2705	2856
W#1	Carry (m)	222.0	220.5	220.1	214.6	217.2	216.2	218.5	219.2
HS: 45 m/s	Total distance (m)	236.4	235.3	235.1	227.7	230.3	228.5	234.0	230.5
	Rating	good	good	good	NG	NG	NG	good	NG
SW	Spin rate (rpm)	6977	6955	6554	7008	6912	6950	6037	6962
HS: 22 m/s	Rating	good	good	fair	good	good	good	good	good
Scuff resistance		good	good	NG	good	good	good	NG	good

¹⁾The measured deflection of the ball was 2.2 mm in Example 1, and 2.4 mm in Example 2.

From the results in Table 4, the golf balls obtained in Comparative Examples 1 to 6 were inferior to the balls obtained according to the invention (Examples 1 and 2) in the following respects.
In Comparative Example 1, because the cover was made of an ionomer resin, the ball had a low scuff resistance and was not receptive to spin on approach shots.
In Comparative Example 2, because the core diameter was less than 31 mm, on shots taken with a driver (W#1), the spin rate rose and an increased distance was not achieved.
In Comparative Example 3, because the cover layer was thicker than the intermediate layer, on shots taken with a driver (W#1), the spin rate rose and an increased distance was not achieved.
In Comparative Example 4, because the envelope layer was softer than the intermediate layer, on shots taken with a driver (W#1), the spin rate rose and an increased distance was not achieved.
In Comparative Example 5, because the cover layer was harder than the intermediate layer, the ball had a poor scuff resistance and lacked a sufficient spin rate on approach shots.
The ball in Comparative Example 6 was a three-piece golf ball which lacked an envelope layer; that is, the core was encased by only two layers. In this ball, the spin rate remained too high, as a result of which an increased distance was not achieved.

Number of dimples

1. A multi-piece solid golf ball comprising a core, an envelope layer encasing the core, an intermediate layer encasing the envelope layer, and a cover which encases the intermediate layer and has formed on a surface thereof a plurality of dimples, wherein the core is formed primarily of a rubber material and has a diameter of at least 31 mm, the envelope layer and the intermediate layer are each formed primarily of the same or different resin materials, and the cover is formed primarily of polyurethane; the envelope layer, the intermediate layer and the cover have thicknesses which satisfy the relationship:

cover thickness<intermediate layer thickness<envelope layer thickness;

and the core has a surface hardness (Durometer D hardness) and the envelope layer, intermediate layer and cover have material hardnesses (Durometer D hardness) which together satisfy the relationship;

cover material hardness<intermediate layer material hardness<envelope layer material hardness<core surface hardness.

2. (canceled)

3. The multi-piece solid golf ball of claim 1, wherein the resin material which forms the envelope layer comprises an ionomer resin having an acid content of at least 16 wt %.

4. The multi-piece solid golf ball of claim 1, wherein the resin material which forms the cover is composed primarily of:

(A) a thermoplastic polyurethane material, and

(B) an isocyanate mixture obtained by dispersing (b-1) a compound having two or more isocyanate groups as functional groups per molecule in (b-2) a thermoplastic resin which is substantially non-reactive with isocyanate.

5. The multi-piece solid golf ball of claim 1, wherein the envelope layer, intermediate layer and cover have material hardnesses (Durometer D hardnesses) such that

60≦envelope layer material hardness≦70,

55≦intermediate layer material hardness≦70, and

30≦cover material hardness≦55.

6. The multi-piece solid golf ball of claim 1, wherein the ball and the core have deflections (mm), when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), which satisfy the following respective conditions:

2.0 mm≦ball deflection≦3.0 mm, and

3.5 mm≦core deflection≦6.0 mm.

7. The multi-piece solid golf ball of claim 1, wherein the core surface hardness is lower than the material hardness of the envelope layer by a difference in Durometer D hardness of 5 to 35.

8. The multi-piece solid golf ball of claim 1, wherein the difference between the hardness of the envelope layer material and the hardness of the intermediate layer material is at least 1 but not more than 20 in Durometer D hardness.

9. The multi-piece solid golf ball of claim 1, wherein the surface of the intermediate layer is abraded.

10. The multi-piece solid golf ball of claim 9, wherein a primer is applied to the surface of the intermediate layer after the abrasion.

11. The multi-piece solid golf ball of claim 9, wherein an adhesion reinforcing agent is added to the intermediate layer material after the abrasion.

12. The multi-piece solid golf ball of claim 11, wherein the adhesion reinforcing agent is selected from the group consisting of 1,3-butanediol, trimethylolpropane, polyethylene glycol oligomer and polyhydroxy polyolefin oligomer.

13. The multi-piece solid golf ball of claim 1, wherein the numbers of the dimples arranged on the cover surface is at least 280 but not more than 360.

14. The multi-piece solid golf ball of claim 1, wherein the diameter of the dimples is set to at least about 2.5 mm but not more than about 6.5 mm.

15. The multi-piece solid golf ball of claim 1, wherein the dimple coverage on the spherical surface of the golf ball, which is the sum of the individual dimple surface areas, each defined by the border of the flat plane circumscribed by the edge of the dimple, expressed as a ratio (SR) with respect to the spherical surface area of the ball were it to be free of dimples, is at least 60% but not more than 90%.

16. The multi-piece solid golf ball of claim 1, wherein the value V₀obtained by dividing the spatial volume of each dimple below the flat plane circumscribed by the edge of that dimple by the volume of a cylinder whose base is the flat plane and whose height from the base to the maximum depth of the dimple is at least 0.35 but not more than 0.80.

17. The multi-piece solid golf ball of claim 1, wherein the VR value, which is the sum of the volumes of individual dimples formed below flat planes circumscribed by the dimple edges, as a percentage of the volume of the ball sphere were it to have no dimples thereon, is at least 0.6% but not more than 1.0%.

18. The multi-piece solid golf ball of claim 1, wherein the ball has deflection of 2.0 to 2.2 mm, when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf).