Patent Application
20160354644
This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2015-113941 filed in Japan on Jun. 4, 2015, the entire contents of which are hereby incorporated by reference.
The present invention relates to a multi-piece solid golf ball of three or more pieces having a core, an intermediate layer and a cover.
In the art relating to golf balls of two or more pieces having a core and a cover and multi-piece solid golf balls of three or more pieces having a core, an intermediate layer and a cover, a number of disclosures have hitherto been made which focus on the hardness profile in the core or on the hardness relationship between the intermediate layer and the cover, the intermediate layer material and the like. Such golf balls are described in, for example, US 2014-0187351 A1, JP-A 2011-120898, JP-A 2010-214105, JP-A 2010-172702, JP-A 2008-194474 and JP-A 2008-194473.
However, there is room for further improvement in the core hardness profile of such golf balls. In particular, there exists a desire to provide golf balls which, by optimizing the core hardness profile and the overall hardness and thickness parameters of the ball, are able to achieve the intended spin properties and thus an increased distance. Moreover, in these golf balls, there is a desire not only for increased distance, but also, to increase the enjoyability of the game, for the ability to maintain the spin performance on approach shots at a high level.
It is therefore an object of this invention to provide a multi-piece solid golf ball which retains a good distance on shots with a driver (W#1) yet has a soft feel at impact and which, moreover, is able to maintain the spin performance on approach shots at a high level.
As a result of extensive investigations, we have discovered that, in a multi-piece solid golf ball having a core, a cover and an intermediate layer therebetween, by adjusting the design of the core hardness profile and hardness gradient such that the hardness gradient out to a position 10 mm from the core center is not very steep but the hardness gradient further out from the core interior is steeper, with the hardness difference between a position 10 mm from the core center and the core surface, expressed in terms of JIS-C hardness, being greater than 15, and moreover by constructing the ball such that the intermediate layer is thicker than the cover and the surface hardness of an intermediate layer-encased sphere is higher than the surface hardness of the ball, the spin rate on full shots with a driver (W#1) can be held lower than in conventional golf balls, enabling an increased distance to be achieved, in addition to which a soft feel at impact can be obtained.
Hence, we have succeeded in developing a superior golf ball which, particularly for the mid- or high-level amateur golfer whose head speed is not as high as that of a professional, retains the spin performance on approach shots at a high level while maintaining a good distance on shots with a driver (W#1), and thus provides good enjoyability in the game of golf. In addition, the golf ball of the invention also has an excellent resistance to damage of the cover surface (scuff resistance) when struck with a fully grooved wedge. As used herein, “mid- and high-level amateur” refers to golfers having head speeds (HS) of generally 40 to 50 m/s, with a mid-level amateur golfer having a HS of about 40 to 48 m/s and a high-level amateur golfer having a HS of about 42 to 50 m/s.
Accordingly, the invention provides a multi-piece solid golf having a core, a cover and an intermediate layer therebetween, wherein a sphere made up of the core and the intermediate layer which peripherally encases the core (intermediate layer-encased sphere) and the ball have respective surface hardnesses, expressed in terms of Shore D hardness, which satisfy the relationship:
20≦Cs−Cc, (1)
0<C10−Cc≦10, (2)
C10−Cc<Cs−C10, (3)
15<Cs−C10, (4)
Cs≧80, (5)
and
Cc≧52. (6)
In a preferred embodiment, the golf ball further satisfies condition (3′) below:
(Cs−C10)/(C10−Cc)≧3. (3′)
In another preferred embodiment, the golf ball further satisfies condition (1′) below:
26≦Cs−Cc. (1′)
In yet another preferred embodiment, the golf ball further satisfies condition (7) below:
(C10−C5)≦(C5−C0)≦(Cs−C15)≦(C15−C10). (7)
The core of the golf ball is preferably formed of a material molded under heat from a rubber composition containing: (A) a base rubber, (B) an organic peroxide, and (C) water and/or a metal monocarboxylate.
In the golf ball of the invention, letting tan δ1 be the loss tangent at a dynamic strain of 1% and tan δ10 be the loss tangent at a dynamic strain of 10% when the loss tangents of the core center and the core surface are measured at a temperature of −12° C. and a frequency of 15 Hz, and defining the tan δ slope as (tan δ10−tan δ1)/(10%−1%), the difference between the tan δ slope at the core surface and the tan δ slope at the core center is preferably larger than 0.002.
The golf ball of the invention preferably satisfies the condition V0−V60<0.7, where V0 is the initial velocity of the core in the golf ball after the intermediate layer and cover, collectively referred to herein as “the core-covering layers,” have been molded, as measured after peeling away the core-covering layers, and V60 is the core initial velocity measured 60 days after measuring V0.
In the inventive golf ball, the intermediate layer is preferably formed of a resin composition containing: a combined amount of 100 parts by weight of the following two base resins (I) and (II):
In a further embodiment, when the core surface is photographed with a camera and image data collected by the camera is image processed in such manner as to identify and digitize scratches appearing on the core surface, the number of digitized scratches is 100 or more.
The multi-piece solid golf ball of the invention, when played by mid- and high-level amateur golfers, achieves a good distance on shots with a driver (W#1), has a soft feel at impact and maintains the spin performance at a high level on approach shots, providing good enjoyability in the game of golf. In addition, this golf ball has an excellent resistance to damage of the cover surface (scuff resistance) when struck with a fully grooved wedge.
The objects, features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the foregoing diagrams.
The multi-piece solid golf ball of the invention has, in order from the inside: a solid core, an intermediate layer, and a cover. Referring to
The core diameter, although not particularly limited, is generally from 34.9 to 40.3 mm, preferably from 36.1 to 39.4 mm, and more preferably from 37.3 to 38.5 mm. When the core diameter is too small, the spin rate on shots with a driver (W#1) may rise, as a result of which the intended distance may not be achieved. When the core diameter is too large, the durability to cracking on repeated impact may worsen, or the feel of the ball at impact may worsen.
The core deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), although not particularly limited, is preferably from 3.6 to 4.8 mm, more preferably from 3.7 to 4.4 mm, and even more preferably from 3.8 to 4.2 mm. When the core is too hard, the spin rate may rise excessively, resulting in a poor distance, and the feel of the ball may become too hard. On the other hand, when the core is too soft, the rebound may be too low, resulting in a poor distance, or the feel may become too soft and the durability to cracking on repeated impact may worsen.
The core surface hardness (Cs), expressed in terms of JIS-C hardness, is at least 80, preferably from 80 to 90, more preferably from 81 to 88, and even more preferably from 82 to 86. When the JIS-C hardness value for the core surface hardness is too high, the feel of the ball may be too hard or the durability to cracking on repeated impact may worsen. On the other hand, when this value is too low, the spin rate rises excessively and the rebound is low, resulting in a poor distance.
The core center hardness (Cc), expressed in terms of JIS-C hardness, is at least 52, preferably from 52 to 63, more preferably from 53 to 61, and even more preferably from 55 to 59. When the JIS-C hardness value for the core center hardness is too high, the spin rate may rise excessively, resulting in a poor distance, or the feel at impact may be too hard. On the other hand, when this value is too low, the durability to cracking on repeated impact worsens and the feel at impact becomes too soft.
The JIS-C hardness at a position 5 mm from the core center (C5) is preferably from 54 to 66, more preferably from 56 to 64, and even more preferably from 58 to 62. The JIS-C hardness at a position 10 mm from the core center (C10) is preferably from 55 to 68, more preferably from 57 to 66, and even more preferably from 59 to 64. When these hardness values are too high, the spin rate may rise excessively, resulting in a poor distance, or the feel at impact may be too hard. On the other hand, when these values are too low, the durability to cracking on repeated impact may worsen and the feel at impact may be too soft.
The above center hardness (Cc) and cross-sectional hardnesses at specific positions refer to the hardnesses measured at the center and specific positions on a cross-section obtained by cutting a golf ball core in half through the center. The surface hardness (Cs) refers to the hardness measured on the spherical surface of the core.
The JIS-C hardness at a position 15 mm from the core center (C15) is preferably from 68 to 82, more preferably from 70 to 80, and even more preferably from 72 to 78. When this hardness value is too high, the feel at impact may become harder and the durability to cracking under repeated impact may worsen. On the other hand, when this value is too low, the spin rate may rise excessively and the rebound may decrease, resulting in a poor distance.
Next, in this invention, the core satisfies conditions (1) to (4) below:
20≦Cs−Cc, (1)
0<C10−Cc≦10, (2)
C10−Cc<Cs−C10, (3)
and
15<Cs−C10 (4)
In condition (1), the value Cs−Cc is preferably from 21 to 32, more preferably from 23 to 30, and even more preferably from 25 to 28. When this value is too high, the durability to cracking on repeated impact worsens. On the other hand, when this value is too low, the spin rate rises excessively and a good distance is not obtained.
In condition (2), it is critical for the value C10−Cc to be higher than 0 and no higher than 10. This means that, in the core hardness profile of the invention, the gradient from the core center to a position 10 mm from the center is not very steep. The C10−Cc value is preferably from 1 to 8, more preferably from 2 to 7, and even more preferably from 3 to 6. At a C10−Cc value outside of this range, the spin rate on full shots rises and a good distance is not obtained, or the durability to cracking on repeated impact worsens.
In condition (3), it is critical for the value Cs−C10 to be higher than the value C10−Cc. This means that, in the core hardness profile of this invention, the gradient is steeper on the outside than at the core interior. In other words, the value (Cs−C10)/(C10−Cc) must be higher than 1, and is preferably from 2 to 8, more preferably from 3 to 7, and even more preferably from 4 to 6. When this value is too high, the durability to cracking on repeated impact worsens. On the other hand, when this value is too low, the spin rate rises excessively and a good distance is not obtained.
In condition (4), it is critical for the value Cs−C10 to be at least 15. This means that, in the core hardness profile of this invention, the gradient from a position 10 mm from the core center (C10) to the core surface (Cs) is steep to a degree that exceeds a JIS-C hardness of 15. The Cs−C10 value is preferably from 16 to 30, more preferably from 18 to 28, and even more preferably from 20 to 26. When this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate on full shots may rise, as a result of which a good distance may not be achieved.
In addition to above conditions (1) to (4), the core hardness profile may be suitably adjusted so as to satisfy the following conditions.
The value Cs−C15, although not particularly limited, is preferably from 3 to 14, more preferably from 5 to 12, and even more preferably from 7 to 10. When this value is too high, the durability to cracking on repeated impact may worsen. On the other hand, when this value is too low, the spin rate may rise excessively, as a result of which a good distance may not be achieved.
The value C15−C10, although not particularly limited, is preferably from 8 to 19, more preferably from 10 to 17, and even more preferably from 12 to 15. When this value is too high, the durability to cracking on repeated impact may worsen. On the other hand, when this value is too low, the spin rate may rise excessively, as a result of which a good distance may not be achieved.
The value C10−C5, although not particularly limited, is preferably from −1 to 7, more preferably from 0 to 5, and even more preferably from 1 to 3. When this value is outside of this range, the spin rate on full shots may rise excessively and a good distance may not be achieved, or the durability to cracking on repeated impact may worsen.
The value C5−Cc, although not particularly limited, is preferably from 0 to 8, more preferably from 1 to 6, and even more preferably from 2 to 4. When this value is too high, the spin rate may rise excessively and a good distance may not be achieved. On the other hand, when this value is too low, the durability to cracking on repeated impact may worsen.
The core having the above hardness profile and deflection is preferably made of a material that is composed primarily of rubber. For example, use may be made of a rubber composition obtained by compounding (A) a base rubber as the chief component, (B) an organic peroxide, and also a co-crosslinking agent, an inert filler and, optionally, an organosulfur compound.
Polybutadiene is preferably used as the base rubber (A). The polybutadiene has a cis-1,4 bond content on the polymer chain of typically at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. When the content of cis-1,4 bonds among the bonds on the polybutadiene molecule is too low, the resilience may decrease.
Rubber components other than this polybutadiene may be included in the base rubber (A) within a range that does not detract from the advantageous effects of the invention. Examples of such rubber components other than the foregoing polybutadiene include other polybutadienes, and diene rubbers other than polybutadiene, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.
The organic peroxide (B) used in the invention is not particularly limited, although the use of an organic peroxide having a one-minute half-life temperature of 110 to 185° C. is preferred. One, two or more organic peroxides may be used. The amount of organic peroxide included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, and even more preferably not more than 3 parts by weight. A commercially available product may be used as the organic peroxide. Specific examples include those available under the trade names Percumyl D, Perhexa C-40, Niper BW and Peroyl L (all from NOF Corporation), and Luperco 231XL (from Atochem Co.).
The co-crosslinking agent is exemplified by unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids. Illustrative examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred. Metal salts of unsaturated carboxylic acids are not particularly limited, and are exemplified by those obtained by neutralizing the foregoing unsaturated carboxylic acids with the desired metal ions. Illustrative examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.
These unsaturated carboxylic acids and/or metal salts thereof are included in an amount per 100 parts by weight of the base rubber which is typically at least 10 parts by weight, preferably at least 15 parts by weight, and more preferably at least 20 parts by weight. The upper limit is typically 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. When too much is included, the feel of the ball may become too hard and unpleasant. When too little is included, the rebound may decrease.
To satisfy the desired hardness profile described above, the core is preferably formed of a material molded under heat from a rubber composition which includes, as the essential ingredients: (A) a base rubber, (B) an organic peroxide, and (C) water and/or a metal monocarboxylate.
Decomposition of the organic peroxide within the core formulation can be promoted by the direct addition of water (or a water-containing material) to the core material. It is known that the decomposition efficiency of the organic peroxide within the core-forming rubber composition changes with temperature and that, starting at a given temperature, the decomposition efficiency rises with increasing temperature. If the temperature is too high, the amount of decomposed radicals rises excessively, leading to recombination between radicals and, ultimately, deactivation. As a result, fewer radicals act effectively in crosslinking. Here, when a heat of decomposition is generated by decomposition of the organic peroxide at the time of core vulcanization, the vicinity of the core surface remains at substantially the same temperature as the temperature of the vulcanization mold, but the temperature near the core center, due to the build-up of heat of decomposition by the organic peroxide which has decomposed from the outside, becomes considerably higher than the mold temperature. In cases where water (or a water-containing material) is added directly to the core, because the water acts to promote decomposition of the organic peroxide, radical reactions like those described above can be made to differ at the core center and at the core surface. That is, decomposition of the organic peroxide is further promoted near the center of the core, bringing about greater radical deactivation, which leads to a further decrease in the amount of active radicals. As a result, it is possible to obtain a core in which the crosslink densities at the core center and the core surface differ markedly. It is also possible to obtain a core having different dynamic viscoelastic properties at the core center. Along with achieving a lower spin rate, golf balls having such a core are also able to exhibit excellent durability and undergo less change over time in rebound. When zinc monoacrylate is used instead of the above water, water is generated from the zinc monoacrylate by heat during kneading of the compounding materials. An effect similar to that obtained by the addition of water can thereby be obtained.
Components A and B have already been described above.
The water serving as component C is not particularly limited, and may be distilled water or tap water. The use of distilled water which is free of impurities is especially preferred. The amount of water included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5 parts by weight, and more preferably not more than 4 parts by weight.
By including a suitable amount of such water, the moisture content in the rubber composition prior to vulcanization becomes preferably at least 1,000 ppm, and more preferably at least 1,500 ppm. The upper limit is preferably not more than 8,500 ppm, and more preferably not more than 8,000 ppm. When the moisture content of the rubber composition is too low, it may be difficult to obtain a suitable crosslink density and tan δ, which may make it difficult to mold a golf ball having little energy loss and a reduced spin rate. On the other hand, when the moisture content of the rubber composition is too high, the core may end up too soft, which may make it difficult to obtain a suitable core initial velocity.
It is also possible to add water directly to the rubber composition. The following methods (i) to (iii) may be employed to include water:
As used herein, “high-humidity environment” is not particularly limited, so long as it is an environment capable of moistening the rubber composition, although a humidity of from 40 to 100% is preferred.
Alternatively, the water may be worked into a jelly state and added to the above rubber composition. Or a material obtained by first supporting water on a filler, unvulcanized rubber, rubber powder or the like may be added to the rubber composition. In such a form, the workability is better than when water is added directly to the composition, enabling the efficiency of golf ball production to be increased. The type of material in which a given amount of water has been included, although not particularly limited, is exemplified by fillers, unvulcanized rubbers and rubber powders in which sufficient water has been included. The use of a material which causes no loss of durability or resilience is especially preferred. The moisture content of the above material is preferably at least 3 wt %, more preferably at least 5 wt %, and even more preferably at least 10 wt %. The upper limit is preferably not more than 99 wt %, and even more preferably not more than 95 wt %.
In this invention, a metal monocarboxylate may be used instead of the above-described water. Metal monocarboxylates, in which the carboxylic acid is presumably coordination-bonded to the metal, are distinct from metal dicarboxylates such as zinc diacrylate of the formula (CH2═CHCOO)2Zn. A metal monocarboxylate introduces water into the rubber composition by way of a dehydration/condensation reaction, and thus provides an effect similar to that of water. Moreover, because a metal monocarboxylate can be added to the rubber composition as a powder, the operations can be simplified and uniform dispersion within the rubber composition is easy. A monosalt is required in order to carry out the above reaction effectively. The amount of metal monocarboxylate included per 100 parts by weight of the base rubber is preferably at least 1 part by weight, and more preferably at least 3 parts by weight. The upper limit in the amount of metal monocarboxylate included is preferably not more than 60 parts by weight, and more preferably not more than 50 parts by weight. When the amount of metal monocarboxylate included is too small, it may be difficult to obtain a suitable crosslink density and tan δ, as a result of which a sufficient golf ball spin rate-lowering effect may not be achievable. On the other hand, when too much is included, the core may become too hard, as a result of which it may be difficult for the ball to maintain a suitable feel at impact.
The carboxylic acid used may be, for example, acrylic acid, methacrylic acid, maleic acid, fumaric acid or stearic acid. Examples of the substituting metal include sodium, potassium, lithium, zinc, copper, magnesium, calcium, cobalt, nickel and lead, although the use of zinc is preferred. Illustrative examples of the metal monocarboxylate include zinc monoacrylate and zinc monomethacrylate, with the use of zinc monoacrylate being especially preferred.
Core production may be carried out in the usual manner by molding a spherical molded article (core) using heat and compression under vulcanization conditions of at least 140° C. and not more than 180° C. and at least 10 minutes and not more than 60 minutes.
The vulcanized core preferably has a higher moisture content at the core center than at the core surface. The moisture content of the molded core can be suitably controlled by adjusting such conditions as the amount of water included in the rubber composition, the molding temperature and the molding time.
The moisture content at the core center, although not particularly limited, is preferably at least 1,000 ppm, more preferably at least 1,200 ppm, and even more preferably at least 1,500 ppm. The upper limit is preferably not more than 7,000 ppm, more preferably not more than 6,000 ppm, and even more preferably not more than 5,000 ppm. The moisture content at the core surface, although not particularly limited, is preferably at least 800 ppm, more preferably at least 1,000 ppm, and even more preferably at least 1,200 ppm. The upper limit is preferably not more than 5,000 ppm, more preferably not more than 4,000 ppm, and even more preferably not more than 3,000 ppm. The (moisture content at core surface)−(moisture content at core center) value is preferably 0 ppm or below, more preferably −100 ppm or below, and even more preferably −200 ppm or below. The lower limit value is preferably −1,000 ppm or above, more preferably −700 ppm or above, and even more preferably −600 ppm or above.
Measurement of the above moisture content may be carried out with ordinary instruments. For example, the moisture content can be measured using the AQ-2100 coulometric Karl Fischer titrator and the EV-2000 evaporator (both available from Hiranuma Sangyo Co. Ltd.) at a measurement temperature of 130° C., a preheating time of 3 minutes and a background measurement time of 30 seconds.
Letting V0 be the initial velocity of the core measured after removing the intermediate layer and cover (which layers are referred to herein collectively as the “core-covering layers”) from a ball obtained by molding these core-covering layers over the core and V60 be the initial velocity of the core measured 60 days after the day on which V0 was measured, V0 is preferably at least 77.0 m/s, more preferably at least 77.1 m/s, and even more preferably at least 77.2 m/s, but is preferably not more than 78.5 m/s, more preferably not more than 78.3 m/s, and even more preferably not more than 78.0 m/s. V60 is preferably at least 77.0 m/s, and more preferably at least 77.1 m/s, but is preferably not more than 77.8 m/s, more preferably not more than 77.7 m/s, and even more preferably not more than 77.6 m/s. When core initial velocities V0 and V60 within the above ranges cannot be obtained, achieving a satisfactory distance is difficult. Also, if the core initial velocity is too high, the golf ball may not conform to the Rules of Golf. Because the core-covering layer materials are not readily permeable to moisture in the atmosphere, there are cases where the change in core initial velocity over time cannot be measured using the ball as is or where it takes a long time for such change to occur. Therefore, by removing the core-covering layers and exposing the core itself to the atmosphere, it is possible to reliably measure the change in core initial velocity over time.
The value V0−V60 preferably satisfies the relationship V0−V60<0.7, more preferably satisfies the relationship V0−V60<0.6, and still more preferably satisfies the relationship V0−V60<0.5. In this invention, when moisture has been included in a good balance within the core, even if the core comes into direct contact with the atmosphere, it is not readily influenced by the atmospheric humidity, enabling changes in the core initial velocity to be suppressed.
In this invention, the core initial velocity may be measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The core may be tested for this purpose in a chamber at a room temperature of 23±2° C. after being held isothermally in a 23±1° C. environment for at least 3 hours.
Next, the method of measuring the dynamic viscoelasticity of the core is explained. In this invention, letting tan δ1 be the loss tangent at a dynamic strain of 1% and tan δ10 be the loss tangent at a dynamic strain of 10% when the loss tangents are measured at a temperature of −12° C. and a frequency of 15 Hz in a dynamic viscoelasticity test on vulcanized rubber at the core center and core surface, and defining the tan δ slope as (tan δ10−tan δ1)/(10%−1%), a desirable feature of the invention is that the difference between the tan δ slope at the core surface and the tan δ slope at the core center be larger than 0.002. This difference in slopes is preferably larger than 0.003, and more preferably larger than 0.004. At a smaller difference in slope, the energy loss by the core ends up being larger, making a spin rate-lowering effect more difficult to obtain. Various methods may be employed to measure the dynamic viscoelastic properties of the core. For example, a circular disk having a thickness of 2 mm may be cut out of the cover-encased core by passing through the geometric center thereof and then, treating this disk as the sample, using a punching machine to punch out 3 mm diameter specimens at the places of measurement. In addition, by employing a dynamic viscoelasticity measuring apparatus (such as that available under the product name EPLEXOR 500N from GABO) and using a compression test holder, the tan δ values under dynamic strains of 0.01 to 10% can be measured at an initial strain of 35%, a measurement temperature of −12° C. and a frequency of 15 Hz, and the slopes determined based on the results of these measurements.
Regarding the viscoelastic behavior measured in this way, there is known to be a correlation between the viscoelastic behavior in the high-strain region and the spin rate of the golf ball when struck. Thus, when the tan δ in the high-strain region is relatively large, i.e., when the tan δ slope between a dynamic strain of 10% and a dynamic strain of 1% is large, the spin rate rises; conversely, when the tan δ in the high-strain region is relatively small, i.e., when the tan δ slope between a dynamic strain of 10% and a dynamic strain of 1% is small, the spin rate falls. Also, the amount of deformation varies depending on the club used to strike the golf ball, with deformation occurring even at the ball center when the ball is struck with a driver or a middle iron (e.g., a number six iron). Therefore, when striving to reduce the spin rate on shots with a driver or a number six iron, good results can be obtained by making the tan δ slope between a dynamic strain of 10% and a dynamic strain of 1% at the core center small. In cases where the deformation on impact is small, such as on approach shots near the green, the influence of the tan δ at the core surface is large. Hence, to increase or maintain the spin rate on approach shots, good results can be obtained by making the tan δ slope between a dynamic strain of 10% and a dynamic strain of 1% at the core surface large. Accordingly, to obtain a golf ball that travels well on shots with a driver and stops on approach shots, what is desired is for the tan δ slope between a dynamic strain of 10% and a dynamic strain of 1% at the core center to be made small and for the tan δ slope between a dynamic strain of 10% and a dynamic strain of 1% at the core surface to be made large; that is, for the difference between the tan δ slope at the core surface and the tan δ slope at the core center to be made large.
In this invention, when the core surface is photographed with a camera and image data collected by the camera is image processed in such manner as to identify and digitize scratches appearing on the core surface, the number of digitized scratches is preferably 100 or more, and more preferably 200 or more. By expressing the degree of core surface roughness in terms of the number of scratches, an attempt is made here to evaluate adhesion between the core and the intermediate layer encasing the core. This is based on the observation that the number of scratches on the core surface has an influence on adhesion between the core and the intermediate layer. In a core having a small number of such scratches, the adhesive strength between the core and the intermediate layer is weak, likely resulting in an inadequate durability to cracking by the core itself.
The method of measuring scratches on the core surface is based on the technology of capturing a photographic image for the purpose of digitizing the roughness of the core surface as the number of scratches, and processing the captured image. Use may be made of, for example, the image processing technique and equipment described below.
The measuring equipment setup may include, for example, as shown in
Using the image processing software, processing is carried out which, basically, treats regions of the captured image data that are darker than a threshold setting for brightness and where there are at least a set number of connected pixels (connectivity number) in such dark areas (shadows) as scratches, and counts the number of such scratches. The brightness threshold and the connectivity number setting used for counting the number of scratches are made to agree with the depth and size of scratches generally visible on the core surface. The depth and size of generally visible scratches are each about 0.5 mm, although these thresholds and settings may be adjusted as necessary. It is also possible to classify scratches by the degree of darkness, to classify the size of scratches by the size of regions of connected shadows or, from the shape characteristics of connected shadows, to classify the shape of scratches by prioritizing shapes that are linear, for example. When carrying out such shape processing, because the photographed image surface of the core surface is curved rather than flat, it is preferable to use a dynamic threshold method.
Specific commercially available products that may be used as the image processing equipment and image software (image processing means) are discussed in the subsequently described examples. However, because of rapid development recently in image processing equipment, the equipment mentioned in the examples may not necessarily be the most suitable. Therefore, it is advisable to select appropriate equipment in keeping with the core to be photographed and recent technical advances, and to combine and use such equipment together.
In this invention, as mentioned above, by specifying the roughness of the core surface in terms of a desired number of scratches, the adhesive strength between the core and the intermediate layer described below can be increased. Specifically, based on the method of measuring adhesive strength described in the examples, the adhesion may be set to preferably at least 1.00 N/4 mm, and more preferably at least 1.15 N/4 mm. In order to achieve a core surface having such a high adhesion, the core material and intermediate layer material may be suitably selected and a method for abrading the core surface may be employed. Abrasion of the core surface may be carried out using a known method and under known conditions.
Next, the intermediate layer is described. The intermediate layer has a material hardness expressed in terms of Shore D hardness which, although not particularly limited, is preferably from 48 to 68, more preferably from 52 to 62, and even more preferably from 55 to 57. The sphere encased by the intermediate layer (referred to below as the “intermediate layer-encased sphere”) has a surface hardness, expressed in terms of Shore D hardness, which is preferably from 55 to 75, more preferably from 59 to 69, and even more preferably from 62 to 64. When the intermediate layer is too soft, the spin rate on full shots may rise excessively, as a result of which a good distance may not be achieved. On the other hand, when the intermediate layer is too hard, the durability to cracking on repeated impact may worsen and the feel of the ball on shots with a putter or on short approaches may worsen.
The intermediate layer-encased sphere has a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably from 2.8 to 4.0 mm, more preferably from 3.0 to 3.8 mm, and even more preferably from 3.2 to 3.6 mm. When this value is too high, the feel of the ball may be too soft, the durability to repeated impact may be poor, or the initial velocity on full shots may be low, as a result of which the intended distance may not be achieved. On the other hand, when this value is too low, the feel of the ball may be too hard and the spin rate on full shots may rise, as a result of which the intended distance may not be achieved.
The intermediate layer has a thickness of preferably from 0.9 to 2.4 mm, more preferably from 1.2 to 2.1 mm, and even more preferably from 1.5 to 1.8 mm. In this invention, it is preferable for the thickness of the intermediate layer to be higher than that of the subsequently described cover (outermost layer). When the intermediate layer thickness is outside of this range or thinner than the cover, the spin rate-reducing effects on driver (W#1) shots may be inadequate, as a result of which a good distance may not be achieved.
The intermediate layer material is not particularly limited, although preferred use can be made of various thermoplastic resin materials. From the standpoint of fully achieving the desired effects of the invention, it is especially preferable to use a high-resilience resin material as the intermediate layer material. For example, the use of an ionomer resin material or the subsequently described highly neutralized resin material is preferred.
Specifically, a molded material obtained by molding a resin composition of components (I) to (IV) described below under applied heat may be used as the highly neutralized resin material.
Preferred use can be made of the two following components (I) and (II) as the base resins:
The weight-average molecular weight (Mw) of component (I) is at least 140,000, and preferably at least 145,000. The weight-average molecular weight (Mw) of component (II) is at least 140,000, and preferably at least 160,000. By thus making these molecular weights large, the resin material can be assured of having sufficient resilience.
It is thought that because the acid components and ester contents of the respective copolymers serving as the base resins (I) and (II) differ, these two types of base resins interlock in a complex manner, giving rise to molecular synergistic effects that can increase the rebound and durability of the ball. In this invention, by specifying the weight-average molecular weight, acid content and ester content as indicated above in such a way as to select a material that is relatively soft as the terpolymer serving as base resin (I), and by specifying the type of acid, weight-average molecular weight and acid content in such a way as to select a relatively hard material as base resin (II), it is possible with a blend of these polymers to ensure sufficient resilience and durability for use as a golf ball material.
Here, the weight-average molecular weight (Mw) is a value calculated relative to polystyrene in gel permeation chromatography (GPC). A word of explanation is needed here concerning GPC molecular weight measurement. It is not possible to directly take GPC measurements for copolymers and terpolymers because these molecules are adsorbed to the GPC column owing to unsaturated carboxylic acid groups within the molecules. Instead, the unsaturated carboxylic acid groups are generally converted to esters, following which GPC measurement is carried out and the polystyrene-equivalent average molecular weights Mw and Mn are calculated.
The olefins used in component (I) and component (II) preferably have 2 to 6 carbons, with ethylene being especially preferred. The unsaturated carboxylic acid used in component (I) is not particularly limited, although preferred use can be made of acrylic acid or methacrylic acid. To ensure resilience, the unsaturated carboxylic acid used in component (II) is acrylic acid. This is because, when methacrylic acid is used as the unsaturated carboxylic acid in component (II), the methacrylic acid with its pendant methyl group may give rise to a buffering action, lowering the reactivity.
The unsaturated carboxylic acid content (acid content) within each of components (I) and (II), although not particularly limited, is preferably at least 10 wt %, with the upper limit being preferably less than 15 wt %, and more preferably less than 13 wt %. When this acid content is low, moldings of the golf ball material may lack sufficient resilience. On the other hand, when the acid content is high, the hardness may become excessively high, adversely affecting the durability.
The unsaturated carboxylic acid ester used in the terpolymer serving as component (I) is preferably a lower alkyl ester, with butyl acrylate (butyl n-acrylate, butyl i-acrylate) being especially preferred.
The ester content of the unsaturated carboxylic acid ester in component (I), in order to employ a resin that is relatively soft compared with the binary copolymer serving as component (II), is set to at least 15 wt %, preferably at least 18 wt %, and more preferably at least 20 wt %, with the upper limit being preferably not more than 25 wt %. At an ester content higher than this range, moldings of the intermediate layer material may lack sufficient resilience. On the other hand, when the ester content is low, the hardness may increase, adversely affecting the durability.
The hardness of the base resin (I), that is, the hardness when the resin itself is molded alone (material hardness), expressed in terms of Shore D hardness, is preferably at least 30, and more preferably at least 35, with the upper limit being preferably not more than 50, and more preferably not more than 45. The hardness of the base resin (II), that is, the hardness when the resin itself is molded alone (material hardness), expressed in terms of Shore D hardness, is preferably at least 40, and more preferably at least 50, with the upper limit being preferably not more than 60, and more preferably not more than 57. When base resins outside of these respective hardness ranges are used, a material having the desired hardness may not be obtained, or an adequate resilience and durability may not be obtained.
In this invention, it is preferable for component (I) and component (II) to be used together. The mixing proportions of component (I) and component (II), expressed as the weight ratio (I):(II), is set to preferably 90:10 to 10:90, more preferably 85:15 to 30:70, and even more preferably 80:20 to 50:50. When the proportion of component II is higher than this range, the hardness increases, as a result of which the material may be difficult to mold.
When metal neutralization products of resins (i.e., ionomers) are used as component (I) and component (II), the type of metal neutralization product and the degree of neutralization are not particularly limited. Illustrative examples include 60 mol % Zn (degree of neutralization with zinc) ethylene-methacrylic acid copolymers, 40 mol % Mg (degree of neutralization with magnesium) ethylene-methacrylic acid copolymers, and 40 mol % Mg (degree of neutralization with magnesium) ethylene-methacrylic acid-acrylic acid ester terpolymers.
To ensure at least a given degree of flowability during injection molding and provide a good molding processability, it is essential for the melt flow rates of the resins serving as components (I) and (II) to each be from 0.5 to 20 g/10 min. The difference between the melt flow rates of components (I) and (II) is set to not more than 15 g/10 min. When the difference in melt flow rates between these base resins is too large, the components cannot be uniformly mixed together during the compounding of components (I) and (II) in an extruder, and so the mixture becomes non-uniform, which may lead to injection molding defects.
As noted above, copolymers or ionomers with weight-average molecular weights (Mw) set in specific ranges are used as components (I) and (II). Illustrative examples of commercial products that may be used for this purpose include the Nucrel series (DuPont-Mitsui Polychemicals Co., Ltd.), the Escor series (ExxonMobil Chemical), the Surlyn series (E.I. DuPont de Nemours & Co.), and the Himilan series (DuPont-Mitsui Polychemicals Co., Ltd.).
In addition, (III) a basic inorganic metal compound is preferably included as a component for neutralizing acid groups in above components (I) and (II) and subsequently described component (IV). By even more highly neutralizing the resin material in this way, the spin rate of the ball on full shots is even further reduced without adversely affecting the feel of the ball, thus making an increased distance fully achievable even by amateur golfers. Illustrative examples of the metal ions in the basic inorganic metal compound include Na+, K+, Li+, Zn2+, Ca2+, Me, Ce and Co2+. Of these, Na+, Zn2+, Ca2+ and Mg2+ are preferred, and Mg2+ is more preferred. These metal salts may be introduced into the resin using, for example, formates, acetates, nitrates, carbonates, bicarbonates, oxides and hydroxides.
This basic inorganic metal compound (III) is included in the resin composition in an amount equivalent to at least 70 mol %, based on the acid groups in the resin composition. Here, the amount in which the basic inorganic metal compound serving as component (III) is included may be selected as appropriate for obtaining the desired degree of neutralization. Although this amount depends also on the degree of neutralization of the base resins (components (I) and (II)) that are used, in general it is preferably from 1.0 to 2.5 parts by weight, more preferably from 1.1 to 2.3 parts by weight, and even more preferably from 1.2 to 2.0 parts by weight, per 100 parts by weight of the combined amount of the base resins (components (I) and (II)). The degree of neutralization of the acid groups in components (I) to (IV) is preferably at least 70 mol %, more preferably at least 90 mol %, and even more preferably at least 100 mol %.
Next, the anionic surfactant serving as component (IV) is described. The reason for including an anionic surfactant is to improve the durability after resin molding while ensuring good flowability of the overall resin composition. The anionic surfactant is not particularly limited, although the use of one having a molecular weight of from 140 to 1,500 is preferred. Exemplary anionic surfactants include carboxylate surfactants, sulfonate surfactants, sulfate ester surfactants and phosphate ester surfactants. Preferred examples include one, two or more selected from the group consisting of various fatty acids such as stearic acid, behenic acid, oleic acid and maleic acid, derivatives of these fatty acids, and metal salts thereof. Selection from the group consisting of stearic acid, oleic acid and mixtures thereof is especially preferred. Alternatively, exemplary organic acid metal salts that may serve as component (IV) include metal soaps, with the metal salt being one in which a metal ion having a valence of 1 to 3 is used. The metal is preferably selected from the group consisting of lithium, sodium, magnesium, aluminum, potassium, calcium and zinc, with the use of metal salts of stearic acid being especially preferred. Specifically, the use of magnesium stearate, calcium stearate, zinc stearate or sodium stearate is preferred.
Component (IV) is included in an amount, per 100 parts by weight of the base resins serving as components (I) and (II), of 1 to 100 parts by weight, preferably 10 to 90 parts by weight, and more preferably 20 to 80 parts by weight. When the component (IV) content is too low, it may be difficult to lower the hardness of the resin material. On the other hand, at a high content, the resin material is difficult to mold and bleeding at the material surface increases, adversely affecting the molded article.
In this invention, the moldability of the material and the productivity can be further increased by suitably adjusting the compounding ratio between components (III) and (IV). When the content of the basic inorganic metal compound serving as component (III) is too high, the amount of gases such as organic acids that evolve during molding decreases, but the flowability of the material diminishes. Conversely, when the content of component (III) is low, the amount of gases generated increases. On the other hand, when the content of the anionic surfactant serving as component (IV) is too high, the amount of gas consisting of fatty acids and other organic acids increases during molding, which has a large impact in terms of molding defects and productivity. Conversely, when the content of component (IV) is low, the amount of gases generated decreases, but the flowability and durability decline. Therefore, achieving a proper compounding balance between components (III) and (IV) is also important. Specifically, it is desirable to set the compounding ratio between components (III) and (IV), expressed as the weight ratio (III):(IV), to from 4.0:96.0 to 1.0:99.0, and especially from 3.0:97.0 to 1.5:98.5.
The resin composition of above components (I) to (IV) accounts for preferably at least 50 wt %, more preferably at least 60 wt %, even more preferably at least 70 wt %, and most preferably at least 90 wt %, of the total amount of the intermediate layer material.
A non-ionomeric thermoplastic elastomer may be included in the intermediate layer material. The non-ionomeric thermoplastic elastomer is preferably included in an amount of from 1 to 50 parts by weight per 100 parts by weight of the combined amount of the base resins.
The non-ionomeric thermoplastic elastomer is exemplified by polyolefin elastomers (including polyolefins and metallocene-catalyzed polyolefins), polystyrene elastomers, diene polymers, polyacrylate polymers, polyamide elastomers, polyurethane elastomers, polyester elastomers and polyacetals.
Optional additives may be suitably included in the intermediate layer material according to the intended use. For example, various additives such as pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When such additives are included, the content thereof per 100 parts by weight of components (I) to (IV) combined is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight, with the upper limit being preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.
It is advantageous to abrade the surface of the intermediate layer in order to increase adhesion with the polyurethane that is preferably used in the subsequently described cover (outermost layer). In addition, it is desirable to apply a primer (adhesive) to the surface of the intermediate layer following such abrasion treatment or to add an adhesion reinforcing agent to the intermediate layer material.
The intermediate layer material has a specific gravity which is typically less than 1.1, preferably from 0.90 to 1.05, and more preferably from 0.93 to 0.99. Outside of this range, the rebound becomes small, as a result of which a good distance may not be obtained, or the durability to cracking on repeated impact may worsen.
Next, the cover, which is the outermost layer of the ball, is described.
The cover (outermost layer) has a material hardness expressed in terms of Shore D hardness which, although not particularly limited, is preferably from 44 to 58, more preferably from 48 to 56, and even more preferably from 52 to 54.
The cover (outermost layer) encased sphere, i.e., the ball, has a surface hardness expressed in terms of Shore D hardness which is preferably from 52 to 67, more preferably from 56 to 65, and even more preferably from 60 to 63. When the cover-encased sphere is too much softer than this range, the spin rate on driver (W#1) shots and iron shots may become too high, as a result of which a good distance may not be obtained. When the cover is too much harder than this range, the spin rate on approach shots may be inadequate or the feel at impact may be too hard.
The cover (outermost layer) encased sphere, that is, the ball, has a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably from 2.4 to 3.7 mm, more preferably from 2.6 to 3.5 mm, and even more preferably from 2.8 to 3.3 mm. When this value is too high, the feel of the ball may be too soft, the durability to repeated impact may worsen, or the initial velocity on full shots may be low, as a result of which the intended distance may not be achieved. On the other hand, when this value is too low, the feel of the ball may be too hard and the spin rate on full shots may rise, as a result of which the intended distance may not be achieved.
The cover (outermost layer) has a thickness which, although not particularly limited, is preferably from 0.3 to 1.5 mm, more preferably from 0.45 to 1.2 mm, and even more preferably from 0.6 to 0.9 mm. When the cover is thicker than this range, the rebound on W#1 shots and iron shots may be inadequate and the spin rate may rise, as a result of which a good distance may not be obtained. On the other hand, when the cover is thinner than this range, the scuff resistance may worsen and the ball may lack spin receptivity on approach shots, resulting in poor controllability.
The cover (outermost layer) material is not particularly limited, although the use of any of various types of thermoplastic resin materials is preferred. For reasons having to do with controllability and scuff resistance, it is preferable to use a urethane resin as the cover material of the invention. In particular, from the standpoint of the mass productivity of manufactured golf balls, it is preferable to use a cover material composed primarily of a thermoplastic polyurethane, with formation more preferably being carried out using a resin blend composed primarily of (O) a thermoplastic polyurethane and (P) a polyisocyanate compound.
In the thermoplastic polyurethane composition containing above components (O) and (P), to improve the ball properties even further, a necessary and sufficient amount of unreacted isocyanate groups should be present in the cover resin material. Specifically, it is recommended that the combined weight of above components (O) and (P) be at least 60%, and more preferably at least 70%, of the weight of the overall cover layer. Components (O) and (P) are described below in detail.
The thermoplastic polyurethane (O) has a structure which includes soft segments composed of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments composed of a chain extender and a polyisocyanate compound. Here, the long-chain polyol serving as a starting material may be any that has hitherto been used in the art relating to thermoplastic polyurethanes, and is not particularly limited. Illustrative examples include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly, or two or more may be used in combination. Of these, in terms of being able to synthesize a thermoplastic polyurethane having a high rebound resilience and excellent low-temperature properties, a polyether polyol is preferred.
Any chain extender that has hitherto been employed in the art relating to thermoplastic polyurethanes may be advantageously used as the chain extender. For example, low-molecular-weight compounds with a molecular weight of 400 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these, an aliphatic diol having 2 to 12 carbons is preferred, and 1,4-butylene glycol is more preferred, as the chain extender.
Any polyisocyanate compound hitherto employed in the art relating to thermoplastic polyurethanes may be advantageously used without particular limitation as the polyisocyanate compound. For example, use may be made of one, two or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use the following aromatic diisocyanate: 4,4′-diphenylmethane diisocyanate.
Commercially available products may be used as the thermoplastic polyurethane serving as component (O). Illustrative examples include Pandex T-8295, T-8290, T-8283 and T-8260 (all from DIC Bayer Polymer, Ltd.).
Although not an essential ingredient, a thermoplastic elastomer other than the above thermoplastic polyurethane may be included as an additional component together with above components (O) and (P). By including this component (Q) in the above resin blend, a further improvement in the flowability of the resin blend can be achieved and the properties required of a golf ball cover material, such as resilience and scuff resistance, can be enhanced.
The relative proportions of above components (O), (P) and (Q) are not particularly limited. However, to fully elicit the desirable effects of the invention, the weight ratio (O):(P):(Q) is preferably from 100:2:50 to 100:50:0, and more preferably from 100:2:50 to 100:30:8.
In addition to the ingredients making up the thermoplastic polyurethane, various additives may be optionally included in the above resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and internal mold lubricants may be suitably included.
The manufacture of multi-piece solid golf balls in which the above-described core, intermediate layer and cover (outermost layer) are formed as successive layers may be carried out by a customary method such as a known injection-molding process. For example, a multi-piece golf ball may be obtained by placing a molded and vulcanized product composed primarily of a rubber material as the core in a given injection mold, injecting an intermediate layer material over the core to give an intermediate sphere, and subsequently placing the resulting sphere in another injection mold and injection-molding a cover (outermost layer) material over the sphere. Alternatively, a cover may be formed over the intermediate layer by a method that involves encasing the intermediate sphere with a cover (outermost layer), this being carried out by, for example, enclosing the intermediate sphere within two half-cups that have been pre-molded into hemispherical shapes, and then molding under applied heat and pressure.
The golf ball of the invention preferably satisfies the following conditions.
The relationship between the deflections of the core and the ball under specific loading is optimized within a specific range. That is, letting CH be the deflection of the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and BH be the deflection of the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the value CH−BH is preferably from 0.7 to 1.5, more preferably from 0.9 to 1.3, and even more preferably from 1.0 to 1.2. When this value is too large, the durability to cracking on repeated impact may worsen, or the feel of the ball on full shots may be too soft. On the other hand, when this value is too small, the spin rate on full shots may become too high, as a result of which the intended distance may not be obtained.
The relative thicknesses of the intermediate layer and the cover are set in a specific range. The value obtained by subtracting the cover thickness from the intermediate layer thickness is preferably from 0 to 2.0 mm, more preferably from 0.1 to 1.5 mm, and even more preferably from 0.3 to 1.0 mm. When this value is too large, the feel at impact may become too hard or the core may become too soft, resulting in a poor durability to cracking on repeated impact. On the other hand, when this value is too small, the spin rate on full shots may become too high, as a result of which the intended distance may not be obtained.
In order for the ball to have a structure in which the cover is hard on the inside and soft on the outside and the intermediate layer is hard, it is critical for the surface hardnesses of the ball and the intermediate layer-encased sphere to satisfy the relationship:
The relationship between the surface hardnesses of the core and the ball is optimized within a specific range. That is, the value obtained by subtracting the surface hardness of the ball from the surface hardness of the core, expressed in terms of Shore D hardness, is preferably from −15 to 5, more preferably from −8 to −4, and even more preferably from −7 to −5. When this value is too large, the cover may be too hard, making the ball poorly suited for the short game, or the core may be soft, which may result in a poor durability to cracking on repeated impact. On the other hand, when this value is too small, the spin rate on full shots may rise excessively, as a result of which the intended distance may not be obtained.
Letting CH be the deflection of the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and MH be the deflection of the intermediate layer-encased sphere when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the value CH−MH is preferably from 0.3 to 1.4, more preferably from 0.5 to 1.2, and even more preferably from 0.7 to 1.0. When this value is too large, the durability to cracking on repeated impact may worsen, or the initial velocity of the ball on full shots may decrease, as a result of which the intended distance may not be obtained. On the other hand, when this value is too small, the spin rate on full shots may become too high, as a result of which the intended distance may not be obtained.
The relationship between the surface hardnesses of the intermediate layer-encased sphere and the core is optimized within a specific range. That is, the value obtained by subtracting the surface hardness of the core from the surface hardness of the intermediate layer-encased sphere, expressed in terms of Shore D hardness, is preferably from 3 to 20, more preferably from 5 to 15, and even more preferably from 7 to 10. When this value is too large, the durability to cracking under repeated impact may worsen, or the feel at impact may worsen. On the other hand, when this value is too small, the spin rate on full shots may be too high, as a result of which the intended distance may not be obtained.
Numerous dimples may be formed on the cover (outermost layer). The number of dimples arranged on the cover surface, although not particularly limited, is preferably at least 280, more preferably at least 300, and even more preferably at least 320, with the upper limit being preferably not more than 360, more preferably not more than 350, and even more preferably not more than 340. When the number of dimples is larger than this range, the ball trajectory becomes lower, as a result of which the distance may decrease. On the other hand, when the number of dimples is too small, the ball trajectory becomes higher, as a result of which a good distance may not be achieved.
The dimple shapes that are used may be of one type or a combination of two or more types selected from among circular shapes, various polygonal shapes, dewdrop shapes and oval shapes. When circular dimples are used, the dimple diameter may be set to at least about 2.5 mm and up to about 6.5 mm, and the dimple depth may be set to at least 0.08 mm and up to about 0.30 mm.
In order to fully manifest the aerodynamic properties, it is desirable for the surface coverage ratio of dimples on the spherical surface of the golf ball, i.e., the ratio SR of the sum of the individual dimple surface areas, each defined by the flat plane circumscribed by the edge of a dimple, with respect to the spherical surface area of the ball were it to have no dimples thereon, to be set to at least 60% and up to 90%. Also, to optimize the ball trajectory, it is desirable for the value Vo, defined as the spatial volume of the individual dimples below the flat plane circumscribed by the dimple edge, divided by the volume of the cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the base, to be set to at least 0.35 and up to 0.80. Moreover, it is preferable for the ratio VR of the sum of the spatial volumes of the individual dimples, each formed below the flat plane circumscribed by the edge of a dimple, with respect to the volume of the ball sphere were the ball surface to have no dimples thereon, to be set to at least 0.6% and up to 1.0%. Outside of the above ranges in these respective values, the resulting trajectory may not enable a good distance to be obtained, and so the ball may fail to travel a fully satisfactory distance.
The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for play. Specifically, the inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and is not more than 42.80 mm, and to a weight which is preferably from 45.0 to 45.93 g.
The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.
Solid cores for the respective Examples of the invention and Comparative Examples were produced by preparing the rubber compositions shown in Table 1 below, then molding and vulcanizing the compositions under the vulcanization conditions shown in the same table.
Details on the ingredients shown in Table 1 are given below.
An intermediate layer material formulated as shown in Table 2 was injected-molded over the core obtained above, thereby giving an intermediate layer-encased sphere. Next, using the cover materials formulated as shown in Table 2, a cover (outermost layer) was injection-molded over the resulting intermediate layer-encased sphere, thereby producing a golf ball having an intermediate layer and a cover (outermost layer) over the core. Although not shown in the diagram, a common dimple pattern was formed on the surface of the ball in each of the Examples of the invention and the Comparative Examples.
Details on the materials shown in Table 2 are as follows.
For each of the resulting golf balls, properties such as the thicknesses and material hardnesses of the respective layers and the surface hardnesses of various layer-encased spheres were evaluated by the methods described below. The results are shown in Table 3. In addition, the flight performance, properties on approach shots, feel, and scuff resistance for each golf ball were evaluated as described below. Those results are shown in Table 4.
The indenter of a durometer was set so as to be substantially perpendicular to the spherical surface of the core, and the core surface hardness in terms of JIS-C hardness was measured as specified in JIS K6301-1975.
To obtain the cross-sectional hardnesses at the center and other specific positions of the core, the core was hemispherically cut so as form a planar cross-section, and measurements were carried out by pressing the indenter of a durometer perpendicularly against the cross-section at the measurement positions. These hardnesses are indicated as JIS-C hardness values.
The Shore D hardness at the core surface was measured with a type D durometer in accordance with ASTM D2240-95.
A circular disk having a thickness of 2 mm was cut out by passing through the geometric center of the core and, treating the core center and surface vicinity on this disk as the respective samples, a punching machine was used to punch out 3 mm diameter specimens at the places of measurement. The loss tangents (tan δ) under dynamic strains of from 0.01% to 10% were measured at an initial strain of 35%, a measurement temperature of −12° C. and a frequency of 15 Hz using a dynamic viscoelasticity measuring apparatus (such as that available under the product name EPLEXOR 500N from GABO) and a compression test holder. Measurement results obtained within a radius of 5 mm from the core center were treated as the tan δ at the core center, and measurement results within 5 mm of the core surface were treated as the tan δ at the core surface.
Using the AQ-2100 coulometric Karl Fischer titrator and the EV-2000 evaporator (both available from Hiranuma Sangyo Co., Ltd.), measurement of the moisture content was carried out at a measurement temperature of 130° C., a preheating time of 3 minutes and a background measurement time of 30 seconds. The interval time was set to 99 seconds and the current was set to “Fast.” Measurement results obtained within a radius of 5 mm from the core center were treated as the moisture content for the center of the core, and measurement results obtained within 5 mm of the core surface were treated as the moisture content for the surface of the core.
Initial Velocity of Core after Standing
A core was prepared by peeling the intermediate layer and cover from a golf ball. The core initial velocity measured on the day that the core-covering layers—these being the intermediate layer and cover—were peeled off was treated as the Day 0 result, and the initial core velocity when 60 days had elapsed thereafter was treated as the Day 60 result. During this time, the core was kept in a chamber controlled to a temperature of 24° C. and 40% humidity. The initial velocity was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The core was tested in a chamber at a room temperature of 23±2° C. after being held isothermally in a 23±1° C. environment for at least 3 hours. Twenty cores were each hit twice. The time taken for the core to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity. This cycle was carried out over a period of about 15 minutes.
A grinding wheel was mounted on a centerless grinder commonly used for grinding spheres and the core surface was abraded for 5 seconds at 2,500 rpm, following which the surface roughness of the core was determined by the following method. An electrodeposited diamond wheel (40/50 grit) was used as the grinding wheel in Examples 1 and 2 of the invention and in Comparative Examples 1 to 5, and a common grinding wheel (GC 46) differing in the frequency of use was used in Comparative Examples 6 and 7. Core surface data was collected from an area having a diameter of about 10 mm at each of five places and the data for each subjected to image processing, thereby obtaining the number of scratches on the core surface. Next, using the average value for the five places as the measured value for a single ball, the average value for five balls (N=5) was determined. The measurement apparatus and method were as shown in
Lighting means 60: UV LED lamp (LDR2-60VL385-BTTPTK), from CCS Inc.
Lighting power source 90: PD3-3024-3-PI, also from CCS Inc.
Camera 70: Sony XC-73 CCD camera
Computer 100: A PC using Windows™ 7 as the operating system, and HALCON sold by LinX Corporation as the image processing software
Referring to
The diameters at five random places on the surface were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single core or intermediate layer-encased sphere, the average diameter for five measured cores or intermediate layer-encased spheres was determined.
The diameters at five random dimple-free areas on the surface of a ball were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single ball, the average diameter for five measured balls was determined.
A core, intermediate layer-encased sphere or ball was placed on a hard plate and the amount of deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. The amount of deflection here refers in each case to the measured value obtained after holding the test specimen isothermally at 23.9° C. In the table, letting A be the core deflection, B be the deflection by the intermediate layer-encased sphere and C be the ball deflection, the values A-B and A-C were calculated.
The intermediate layer and cover-forming resin materials were molded into sheets having a thickness of 2 mm and left to stand for at least two weeks, following which the Shore D hardnesses were measured in accordance with ASTM D2240-95.
Measurements were taken by pressing the durometer indenter perpendicularly against the surface of the intermediate layer-encased sphere or ball (i.e., the surface of the cover). The surface hardness of the ball (cover) is the measured value obtained at dimple-free places (lands) on the ball surface. The Shore D hardnesses were measured with a type D durometer in accordance with ASTM D2240-95.
In addition, the flight performance (W#1), spin performance on approach shots, feel, scuff resistance, and durability to cracking of the golf balls obtained in the respective Examples of the invention and the Comparative Examples were evaluated according to the criteria indicated below. The results are shown in Table 4.
Flight Performance on Shots with a Driver
A driver (W#1) was mounted on a golf swing robot, the distance traveled by the ball when struck at a head speed (HS) of 45 m/s was measured, and the flight performance was rated according to the criteria shown below. The club used was a TourStage X-Drive 709 D430 driver (2013 model; loft angle, 9.5°) manufactured by Bridgestone Sports Co., Ltd. The above head speed corresponds to the average head speed of mid- and high-level amateur golfers.
Rating Criteria:
A sand wedge was mounted on a golf swing robot, and the spin rate of the ball when hit at a head speed (HS) of 20 m/s was rated according to the following criteria.
Rating Criteria:
Sensory evaluations were carried out when the balls were hit with a driver (W#1) by amateur golfers having head speeds of 40 to 50 m/s. The feel of the ball was rated according to the following criteria.
Rating Criteria:
Here, a “good feel” refers to a feel at impact that is appropriately soft.
A non-plated pitching sand wedge was set in a swing robot and the ball was hit once at a head speed of 35 m/s, following which the surface state of the ball was visually examined and rated as follows.
Rating Criteria:
The same type of driver (W#1) as in the flight performance evaluation was mounted on a golf swing robot and the ball was repeatedly struck at a head speed of 45 m/s. For the ball in each Example, a loss of durability was judged to have occurred when the initial velocity of the ball fell to or below 97% of the average initial velocity for the first ten shots. The average value for three measured golf balls (N=3) was used as the basis for evaluation in each Example. The durability indexes for the balls in the respective Examples were calculated relative to an arbitrary index of 100 for the number of shots taken with the ball in Example 1, and the durability was rated according to the following criteria.
Rating Criteria:
In Comparative Example 1, the hardness profile of the core fell outside the range in values for the invention. As a result, the spin rate rose on full shots with a driver (W#1) and a good distance was not achieved.
In Comparative Example 2, the hardness profile of the core fell outside the range in values for the invention. As a result, the spin rate rose on full shots with a driver and a good distance was not achieved.
In Comparative Example 3, the hardness profile of the core fell outside the range in values for the invention, making the core soft and holding down the spin rate on W#1 shots. As a result, the initial velocity of the ball when struck was low and a good distance was not achieved.
The ball in Comparative Example 4 had a cover that was harder than the intermediate layer. As a result, the spin rate of approach shots was insufficient, making the ball performance inferior in the short game.
The ball in Comparative Example 5 had a cover (outermost layer) that was thicker than the intermediate layer. As a result, on W#1 shots, the spin rate rose and a good distance was not achieved.
In Comparative Example 6, the hardness profile of the core fell outside the range of values for the invention. As a result, the spin rate on full shots with a driver (W#1) rose and a good distance was not achieved.
In Comparative Example 7, the hardness profile of the core fell outside the range of values for the invention. As a result, the spin rate on full shots with a driver (W#1) rose and a good distance was not achieved. Also, the number of scratches on the core surface was small and the durability to cracking was low.
Japanese Patent Application No. 2015-113941 is incorporated herein by reference.
Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-113941 | Jun 2015 | JP | national |
