Patent Application
20250153009
This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2023-194203 filed in Japan on Nov. 15, 2023, the entire contents of which are hereby incorporated by reference.
The present invention relates to a golf ball having a core and a cover composed primarily of a polyurethane resin having a relatively low crosslink density, and to a method of manufacture thereof.
The property most desired in a golf ball is increased distance, but other desirable properties include the ability for the ball to stop well on approach shots and a good scuff resistance. Many golf balls have hitherto been developed that exhibit a good flight performance on shots with a driver and are suitably receptive to backspin on approach shots. Recently, in golf balls for professional golfers and skilled amateurs, polyurethane resin materials are often used in place of ionomer resin materials as the golf ball cover (outermost layer).
Polyurethane resins are composed of soft segments made up of a polymeric polyol that is a long-chain polyol, and also a chain extender and a polyisocyanate which together make up hard segments. A thermoplastic polyurethane elastomer (TPU) is often used as the polyurethane resin employed in golf balls, and polytetramethylene glycol (PTMG) is often used as the polyol component. Although a number of cover materials that are polymer blends obtained by mixing a polyurethane resin as the base resin with other resin materials have been described, there is scant literature on the chemical structure and physical properties of urethane resins themselves.
One prior-art document that focuses on the crosslink density of polyurethane resin in order to enhance the physical properties of golf balls is JP-A 2012-75911. However, this disclosure is aimed at increasing the crosslink density of polyurethane resin, and thereby enhancing durability, by way of electron beam irradiation. It has nothing to do with enhancing the controllability of the golf ball on approach shots and the scuff resistance of the ball.
It is therefore an object of the present invention to provide a golf ball which, compared with prior-art golf balls having a polyurethane resin cover, has an excellent controllability on approach shots and an excellent scuff resistance. Another object of the invention is to provide a method of manufacturing such a golf ball.
As a result of intensive investigations, I have discovered that when polyurethane is used as the cover material in a golf ball having a core and a cover, by adjusting the crosslink density of the polyurethane within a specific range, the spin rate on approach shots rises and a golf ball having an excellent scuff resistance can be obtained. I have also found that the above advantageous effects of the invention can be fully manifested by specifying the relationship between the crosslink density, the Shore D hardness of the cover and the number-average molecular weight of the polyol component of the polyurethane.
Accordingly, in a first aspect, the invention provides a golf ball having a core and a cover, wherein the cover is formed of a resin composition made chiefly of a polyurethane resin having a crosslink density of at least 1.4×102 mol/m3 and not more than 35×102 mol/m3.
In a preferred embodiment of the golf ball of the invention, the polyurethane resin includes a polyol component which is polytetramethylene glycol (PTMG) and, letting T×102 mol/m3 be the crosslink density of the polyurethane resin, D be the Shore D hardness of the golf ball cover and Mn be the number-average molecular weight of the polytetramethylene glycol (PTMG), the value of (T/D)×Mn is 1,000 or less. In this preferred embodiment, the value of T/D may be 0.6 or less. Also, the value of Mn may be 1,000 or more, may be 2,000 or more or may be 3,000 or more.
In another preferred embodiment of the inventive golf ball, the cover has a thickness of 1.5 mm or less.
In yet another preferred embodiment, the polyurethane resin has a Shore D hardness of from 30 to 60.
In a second aspect, the invention provides a method of manufacturing golf balls having a core and a cover, which method includes the step of forming the cover from a resin composition made chiefly of a polyurethane resin having a crosslink density, as measured by the toluene swelling test, of at least 1.4×102 mol/m3 and not more than 35×102 mol/m3.
The golf ball and golf ball method of manufacture of this invention enable golf balls having an increased spin rate on approach shots and an excellent scuff resistance to be obtained by adjusting the crosslink density of the polyurethane cover within a specific range.
The FIGURE is a graph showing the relationship between the crosslink density and Shore D hardness of the polyurethane in a cover material made of polyurethane resin.
The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagram.
The golf ball of the invention has a core of at least one layer and a cover of at least one layer—that is, a single-layer or multilayer cover—that encases the core.
The core may be formed using a known rubber material as the base material. A known base rubber such as a natural rubber or a synthetic rubber may be used as the base rubber. More specifically, it is recommended that polybutadiene, especially cis-1,4-polybutadiene having a cis structure content of at least 40%, be chiefly used. If desired, natural rubber, polyisoprene rubber, styrene-butadiene rubber or the like may be used together with the foregoing polybutadiene in the base rubber.
The polybutadiene may be synthesized with a metal catalyst, such as a neodymium or other rare-earth catalyst, a cobalt catalyst or a nickel catalyst.
Co-crosslinking agents such as unsaturated carboxylic acids and metal salts thereof, inorganic fillers such as zinc oxide, barium sulfate and calcium carbonate, and organic peroxides such as dicumyl peroxide and 1,1-bis(t-butylperoxy)cyclohexane may be included in the base rubber. If necessary, commercial antioxidants and the like may be suitably added.
The core can be produced by vulcanizing/curing the rubber composition containing the above ingredients. For example, production may be carried out by kneading the composition using a mixer such as a Banbury mixer or a roll mill, compression molding or injection molding the kneaded composition using a core mold, and curing the molded body by suitably heating it at a temperature sufficient for the organic peroxide and the co-crosslinking agent to act, i.e., from 100° C. to 200°° C., preferably from 140 to 180° C., for a period of between 10 and 40 minutes.
In the golf ball of the invention, the core is encased by a single-layer or multilayer cover. Such a golf ball may take the form of, for example, a golf ball having a single-layer cover over a core, or a golf ball having a core, an intermediate layer encasing the core, and an outermost layer encasing the intermediate layer.
In the practice of the invention, at least one layer of the cover is formed of, as the resin material, a polyurethane resin-containing resin composition. The polyurethane resin is one capable of serving as the chief material or base resin of the cover-forming resin composition. The polyurethane resin serving as this component (referred to below as the “polyurethane”) is described here in detail.
The polyurethane has a structure which includes soft segments composed of a polymeric polyol that is a long-chain polyol and hard segments composed of a chain extender and a polyisocyanate. Here, the polymeric polyol serving as a starting material may be any that has hitherto been used in the art relating to polyurethane materials, and is not particularly limited. It is exemplified by 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. Specific examples of polyester polyols that may be used include adipate-type polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol and polyhexamethylene adipate glycol; and lactone-type polyols such as polycaprolactone polyol. Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polytetramethylene glycol and polymethyltetramethylene glycol. These polyols may be used singly, or two or more may be used in combination.
It is preferable to use a polyether polyol, especially polytetramethylene glycol (PTMG), as the polymeric polyol. A large number-average molecular weight for the PTMG as the polyol used in the polyurethane is preferable in terms of the spin performance of the golf ball on approach shots and the scuff resistance of the ball. However, although the ball performance improves when the cover material has a lower hardness, ejector pin marks during molding of the ball and surface damage during trimming readily arise, which tends to worsen the moldability.
The polymeric polyol has a number-average molecular weight that is preferably in the range of from 1,000 to 5,000. By using a polymeric polyol having a number-average molecular weight in this range, golf balls that are made with polyurethane compositions and have excellent properties, including a good rebound and good productivity, can be reliably obtained. The number-average molecular weight of the polymeric polyol is more preferably in the range of from 1,500 to 4,000, and even more preferably in the range of from 1,700 to 3,500.
Here and below, “number-average molecular weight” refers to the number-average molecular weight calculated based on the hydroxyl value measured in accordance with JIS-K 1557.
The chain extender is not particularly limited; any chain extender that has hitherto been employed in the art relating to polyurethanes may be suitably used. In this invention, low-molecular-weight compounds with a molecular weight of 2,000 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups may be used. Of these, preferred use can be made of aliphatic diols having from 2 to 12 carbon atoms. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. The use of 1,4-butylene glycol is especially preferred.
Any polyisocyanate hitherto employed in the art relating to polyurethanes may be suitably used without particular limitation as the polyisocyanate. For example, use can be made of one 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, 1,4-bis (isocyanatomethyl) cyclohexane and dimer acid diisocyanate. However, depending on the type of isocyanate, crosslinking reactions during injection molding may be difficult to control.
The ratio of active hydrogen atoms to isocyanate groups in the polyurethane-forming reaction may be suitably adjusted within a preferred range. Specifically, in preparing a polyurethane by reacting the above long-chain polyol, polyisocyanate and chain extender, it is preferable to use the respective components in proportions such that the amount of isocyanate groups included in the polyisocyanate per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.
The method of preparing the polyurethane is not particularly limited. Preparation using the long-chain polyol, chain extender and polyisocyanate may be carried out by either a prepolymer process or a one-shot process via a known urethane-forming reaction. Of these, melt polymerization in the substantial absence of solvent is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.
It is preferable to use a thermoplastic polyurethane material as the polyurethane, with an ether-based thermoplastic polyurethane material being especially preferred. The thermoplastic polyurethane material may be a commercial product, illustrative examples of which include those available under the trade name PANDEX from DIC Covestro Polymer, Ltd. and those available under the trade name RESAMINE from Dainichiseika Color & Chemicals Mfg. Co., Ltd.
The above polyurethane is the chief material of the cover-forming resin composition. To fully impart the golf ball with scuff resistance, this polyurethane accounts for at least 50 wt %, preferably at least 60 wt %, more preferably at least 70 wt %, even more preferably at least 80 wt %, and most preferably at least 90 wt %, of the resin composition.
In this invention, a polyurethane resin having a crosslink density of at least 1.4×102 mol/m3 and not more than 35×102 mol/m3 is used as the chief material of the cover-forming resin composition. A golf ball having an excellent controllability on approach shots and also excellent scuff resistance can thereby be obtained. The crosslink density/network chain density that acts effectively on the elasticity governing the mechanical properties of a polyurethane elastomer is very difficult to isolate and quantify given that chemical crosslinking and various types of physical crosslinking contribute thereto and also owing to side reactions during the crosslinking reaction and the complexity of the form and nature of the crosslinks. However, it is possible to measure the crosslink density of the polyurethane elastomer by a swelling method. One example of this swelling method is described below.
Polyurethane is cast into a 2 mm thick sheet from which a test piece having a length of 3 mm, a width of 3 mm and a thickness of 2 mm is cut. Using this as the sample, the sample weight is measured with an electronic balance capable of measuring the weight to four decimal places (mg). The sample and 8 mL of toluene are placed in a 10 mL vial and the vial is closed with a stopper and left at rest for at least 72 hours, after which the solution inside is discarded, toluene adhering to the sample surface is wiped off and the sample weight following immersion is measured. The crosslink density of the polyurethane is calculated from the sample weights before and after swelling using the Flory-Rehner equation.
Here, ν is the crosslink density, vr is the volume fraction of polyurethane in the swollen sample, χ is an interaction coefficient, and VS is the molar volume of toluene.
VTPU represents the volume of polyurethane, VT is the volume of toluene in the swollen sample, vf is the weight fraction of filler in the polyurethane, ρ is the density of the polyurethane, wf is the sample weight before immersion, ws is the sample weight after immersion, and ρT is the density of toluene.
Calculation is carried out at a VS value of 0.1063×10−3 m3/mol and a ρT value of 0.8669, and at a value for χ, based on the literature (Macromolecules 2007, 40, 3669-3675), of 0.47.
In cases where the polyol component of the polyurethane is polytetramethylene glycol (PTMG), letting T×102 mol/m3 be the crosslink density and D be the Shore D hardness of the cover material, the value T/D indicating the crosslink density at the hardness of the polyurethane is preferably 0.6 or less, more preferably 0.5 or less, even more preferably 0.4 or less, still more preferably 0.35 or less, and most preferably 0.2 or less. When this value is greater than 0.6, the scuff resistance is sometimes inferior.
In addition, letting Mn be the number-average molecular weight of
polytetramethylene glycol (PTMG), the value (T/D)×Mn is preferably 1,000 or less, more preferably 900 or less, even more preferably 800 or less, still more preferably 700 or less, and most preferably 600 or less. This value is obtained by multiplying the number-average molecular weight of PTMG with the crosslink density at the hardness of the polyurethane. When this value is greater than 1,000, the scuff resistance is sometimes inferior.
Increasing the number-average molecular weight (Mn) of PTMG makes it possible to raise the performance of the overall golf ball without altering the low hardness. That is, at the same hardness level, the spin performance on approach shots is better and the rebound and scuff resistance are also better when the number-average molecular weight (Mn) of PTMG is 2,000 as opposed to 1,000, and when it is 3,000 as opposed to 2,000.
In addition to the above polyurethane, other resin materials may be included in the above cover-forming resin composition. The purposes for doing so are, for example, to further improve the flowability of the golf ball resin composition and to enhance such ball properties as the rebound and durability to cracking. A low-rebound resin ingredient may be blended in as a resin ingredient other than polyurethane in order to increase the ball controllability.
Specific examples of resin ingredients other than polyurethane that may be used include polyamide elastomers, ionomer resins, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, polyacetals, polyethylene, nylon resins, styrene resins, polyvinyl chlorides, polycarbonates, polyphenylene ethers, polyarylates, polysulfones, polyethersulfones, polyetherimides and polyamideimides. These may be used singly or two or more may be used together.
With regard to the polyurethane itself, a low hardness is desirable from the standpoint of the spin performance on approach shots and the scuff resistance. The hardness of the polyurethane is preferably in the Shore D hardness range of from 30 to 60.
In addition, an active isocyanate compound may be included in the above cover-forming resin composition. This active isocyanate compound reacts with the polyurethane or polyurea serving as the chief ingredient, enabling the scuff resistance of the overall resin composition to be further enhanced. Moreover, the isocyanate has a plasticizing effect which increases the flowability of the resin composition, enabling the moldability to be improved.
Any isocyanate compound employed in ordinary polyurethanes may be used without particular limitation as the above isocyanate compound. Aromatic isocyanate compounds that may be used include, for example, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and mixtures of both, 4,4-diphenylmethane diisocyanate, m-phenylene diisocyanate and 4,4′-biphenyl diisocyanate. Use can also be made of the hydrogenated forms of these aromatic isocyanate compounds, such as dicyclohexylmethane diisocyanate. Other isocyanate compounds that may be used include aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate (HDI) and octamethylene diisocyanate; and alicyclic diisocyanates such as xylene diisocyanate. Further examples of isocyanate compounds that may be used include blocked isocyanate compounds obtained by reacting the isocyanate groups on a compound having two or more isocyanate groups on the ends with a compound having active hydrogens, and uretdiones obtained by the dimerization of isocyanate.
The amount of the above isocyanate compounds included per 100 parts by weight of polyurethane is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 30 parts by weight, and more preferably not more than 20 parts by weight. When too little is included, a sufficient crosslinking reaction may not be obtained and improvements in the physical properties may not be observable. On the other hand, when too much is included, discoloration over time due to heat and ultraviolet light may increase, or problems such as a loss of thermoplasticity or a decline in resilience may arise.
In addition, optional additives may be suitably included within the above cover-forming resin composition in accordance with the intended use thereof. For example, in cases where the golf ball material of the invention is to be used as a cover material, various additives such as fillers (inorganic fillers), organic staple fibers, reinforcing agents, crosslinking agents, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added to the above ingredients. When including such additives, the amount thereof per 100 parts by weight of the base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight; the upper limit is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.
In order to provide the golf ball with a low rebound and increase the spin rate on approach shots, the above cover-forming resin composition has a rebound resilience, as measured according to JIS-K 6255:2013, which is preferably at least 48%, more preferably at least 50%, and even more preferably at least 52%. The upper limit is preferably not more than 72%, more preferably not more than 70%, and even more preferably not more than 68%.
The cover-forming resin composition has a material hardness on the Shore D hardness scale which, from the standpoint of the scuff resistance of the golf ball and to impart a suitable spin rate on approach shots, is preferably 50 or less, more preferably 48 or less, and even more preferably 45 or less. In terms of the moldability, the lower limit in the material hardness on the Shore D hardness scale is preferably at least 30, more preferably at least 35, and even more preferably at least 37.
The above cover-forming resin composition may be prepared by mixing together the ingredients using any of various types of mixers, such as a kneading-type single-screw or twin-screw extruder, a Banbury mixer, a kneader or a Labo Plastomill. Alternatively, the ingredients may be mixed together by dry blending when the resin composition is to be injection molded. In addition, when an active isocyanate compound is used, it may be incorporated at the time of resin mixture using various types of mixers, or a resin masterbatch already containing the active isocyanate compound and other ingredients may be separately prepared and the various components mixed together by dry blending when the resin composition is to be injection molded.
The method of molding the cover from the above cover-forming resin composition may involve, for example, feeding the resin composition into an injection molding machine and molding the cover by injecting the molten resin composition over the core. In this case, the molding temperature differs according to the type of polyurethane, but is typically in the range of from 150 to 270° C.
The cover has a thickness which is preferably 0.4 mm or more, more preferably 0.5 mm or more, and even more preferably 0.6 mm or more. The upper limit is preferably not more than 2.0 mm, and more preferably not more than 1.5 mm.
In cases where at least one intermediate layer is interposed between the above core and the above cover, various types of thermoplastic resins used in golf ball cover materials, especially ionomer resins, may be used as the intermediate layer material. A commercial product may be used as the ionomer resin. In such a case, the thickness of the intermediate layer may be set within the same range as the above cover thickness.
In the golf ball of the invention, numerous dimples are provided on the surface of the outermost layer for reasons having to do with the aerodynamic performance. The number of dimples formed on the surface of the outermost layer is not particularly limited. However, to enhance the aerodynamic performance and increase the distance traveled by the ball, this number is preferably at least 250, more preferably at least 270, even more preferably at least 290, and most preferably at least 300. The upper limit is preferably not more than 400, more preferably not more than 380, and even more preferably not more than 360.
In the practice of the invention, a coating layer is formed on the surface of the golf ball cover. A two-part curable urethane coating may be suitably used as the coating that forms this coating layer. Specifically, in this case, the two-part curable urethane coating is one that includes a base resin composed primarily of a polyol resin and a curing agent composed primarily of a polyisocyanate.
A known method may be used without particular limitation as the method for applying this coating onto the cover surface and forming a coating layer. Use can be made of a desired method such as air gun painting or electrostatic painting.
The thickness of the coating layer, although not particularly limited, is typically from 8 to 22 μm, and preferably from 10 to 20 μm.
The golf ball of the invention can be made to conform to the Rules of Golf for play. 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 between 45.0 and 45.93 g.
The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.
A core-forming rubber composition formulated as shown in Table 1 and common to all of the Examples is prepared and then molded/vulcanized to produce a 38.6 mm diameter core.
Details on the above core material are given below.
An intermediate layer-forming resin material is injection-molded over the 38.6 mm diameter core, thereby producing an intermediate layer-encased sphere having a 1.25 mm thick intermediate layer. This intermediate layer-forming resin material, which is a resin blend common to all of the Examples, consists of 50 parts by weight of the sodium neutralization product of an ethylene-unsaturated carboxylic acid copolymer having an acid content of 18 wt % and 50 parts by weight of the zinc neutralization product of an ethylene-unsaturated carboxylic acid copolymer having an acid content of 15 wt %, for a total of 100 parts by weight.
Resin compositions for the covers in Examples 1 to 7 and 14 to 20 are prepared by blending 4,4′-diphenylmethane diisocyanate as the isocyanate, 1,4-butylene glycol as the chain extender and PTMG 1000 or PTMG 2000 as the long-chain polyol in specific ratios so as to achieve the respective Shore D hardnesses indicated in Tables 2 and 3. The polyurethane is synthesized by the one-shot process.
Resin compositions for the covers in Examples 8 to 13 and 21 to 34 and Comparative Examples 1 and 2 are prepared by blending 4,4′-diphenylmethane diisocyanate as the isocyanate, 1,4-butylene glycol as the chain extender and PTMG 1000, PTMG 2000 or PTMG 3000 as the long-chain polyol in specific ratios so as to achieve the respective Shore D hardnesses indicated in Tables 2 and 4. The polyurethane is synthesized by the one-shot process.
Next, the above cover-forming resin composition is injection-molded over the intermediate layer-encased sphere, thereby producing a 42.7 mm diameter three-piece golf ball having a 0.8 mm thick outermost layer. Dimples common to all of the Examples are formed at this time on the cover surface in each Example and Comparative Example. Also, a coating layer of about 15 μm is formed with the urethane coating composition on the surface of the golf ball in each Example.
Each of the above polyurethane resins is formed into 2 mm thick sheets and left to stand for two weeks at a temperature of 23±2° C. Three sheets are stacked together at the time of measurement. The material hardness of the resin is measured using a Shore D durometer in accordance with ASTM D2240. The P2 Automatic Rubber Hardness Tester (Kobunshi Keiki Co., Ltd.) equipped with a Shore D durometer is used for measuring the hardness.
Each of the above polyurethane resins is cast into a 2 mm thick sheet from which a test piece having a length of 3 mm, a width of 3 mm and a thickness of 2 mm is cut. Using this as the sample, the sample weight is measured with an electronic balance capable of measuring the weight to four decimal places (mg). The sample and 8 mL of toluene are placed in a 10 mL vial and the vial is closed with a stopper and left at rest for at least 72 hours, after which the solution inside is discarded, toluene adhering to the sample surface is wiped off and the sample weight following immersion is measured. The crosslink density of the polyurethane is calculated from the sample weights before and after swelling using the above-described Flory-Rehner equation.
The coefficient of restitution, spin rate on approach shots and scuff resistance of the golf balls produced in each Example are evaluated by the following methods. Examples 8 to 13 and 21 to 34 and Comparative Examples 1 and 2 can be evaluated from the Shore D hardness in each Example and the respective physical property values obtained. Those results are shown in Tables 2, 3 and 4.
A sand wedge (SW) is mounted onto a golf swing robot and the backspin rate of the
ball immediately after being struck at a head speed (HS) of 20 m/s is measured with a launch monitor.
The golf balls are held isothermally at 23° C. and five balls of each type are hit at a head speed of 33 m/s using as the club a pitching wedge (PW) mounted on a golf swing robot. The damage to the ball from the impact is visually rated according to the following criteria.
As shown in Tables 2 to 4, in the golf balls in the Examples according to the invention, the polyurethane resin serving as the cover material has a crosslink density of at least 1.4×102 mol/m3 and not more than 35×102 mol/m3. As a result, a high spin rate on approach shots is obtained and the scuff resistance is excellent. In particular, comparing Comparative Example 1, Example 27 and Example 34, all of which have the same Shore D hardness of 50, Examples 27 and 34 have a better scuff resistance than Comparative Example 1 and the difference in the spin rate on approach shots is large. Also, the spin rate on approach shots in Example 34 in which PTMG having an average molecular weight of 3,000 was used is higher than that in Example 27 in which PTMG having an average molecular weight of 2,000 was used.
The FIGURE presents a graph showing the relationship between the crosslink density and Shore D hardness of polyurethane based on the data shown in Tables 2 to 4. At the same hardness, PTGM having an average molecular weight of 1,000 has a higher crosslink density. That is, the hardness is presumably obtained by having a greater number of crosslink points and thus increasing the number of network chains.
At the same Shore D hardness, PTMG having an average molecular weight of 2,000 has a small crosslink density. It appears here that, because the number-average molecular weight of polyol is high, a given Shore D hardness can be obtained even when the number of network chains is not all that high and flexibility can also be achieved, resulting in a good scuff resistance. A smaller crosslink density value with respect to the Shore D hardness is better in terms of the golf ball properties.
Japanese Patent Application No. 2023-194203 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 |
|---|---|---|---|
| 2023-194203 | Nov 2023 | JP | national |
