golf ball

Abstract

The objective is to provide a golf ball that offers excellent printability, such as mark durability, while ensuring the colorfastness and durability of the paint layer. [Solution] The present invention provides a golf ball having at least one coating layer on the surface of a golf ball having a large number of dimples, wherein at least one of the coating layers is formed from a two-component curable urethane coating composition consisting of a polyol component and a polyisocyanate component, the molar ratio (NCO group / OH group) of the hydroxyl groups (OH groups) of the polyol component to the isocyanate groups (NCO groups) of the polyisocyanate component is 0.50 to 0.78, the polyisocyanate component consists of hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI), and the mixing ratio of these is 90 / 10 to 40 / 60 in terms of HDI / TDI (mass ratio).

Description

Technical Field

[0001] The present invention relates to a golf ball having a paint layer made of a two-component curable urethane paint on the ball surface, and more specifically, to a golf ball having excellent printability when a mark is printed on the paint layer by inkjet or the like.

Background Art

[0002] In recent years, the shape and color of the above marks have tended to diversify, and the appearance of golf balls including marks has come to have a great influence on the commercial value of the golf balls themselves. Therefore, even if there is no problem with the performance of the golf ball itself, if the mark peels off even partially due to the use of the golf ball, etc., the commercial value will be significantly reduced or it will become the subject of a claim.

[0003] Particularly, when printing a mark on a coating film, there has been a problem that the mark is easily peeled off. In recent years, it has become common to apply an own name (including alignment marks) on the coating film. Regarding the formation of the own name, in many cases, a desired mark is printed after painting the ball surface. When printing marks such as own names by inkjet printing and existing on the ball surface, since the printing marks are exposed, the printing marks are frequently peeled off or chipped due to the long-term use of the ball, and there have been problems with printability such as mark durability.

[0004] Previous patent documents have included studies on surface treatment and protective film formation to improve the durability of marks, and studies on the molar ratio (NCO / OH) of the paint composition and the elastic modulus of the paint to improve the feel of the golf ball and its stain resistance. For example, Patent Document 1 proposes a method for manufacturing a golf ball in which a base layer is formed in the area corresponding to the mark, and the mark is formed on the base layer by inkjet printing. Patent Document 2 proposes a method in which the area to be marked is heated to 40-70°C, and then the mark is formed by inkjet printing using water-based resin ink while maintaining this heated state. Patent Document 3 proposes forming a protective coating of water-based clear paint or oil-based clear paint on the inkjet-marked area. Patent Document 4 proposes that a golf ball in which the base resin constituting the coating film is polyurethane, the molar ratio (NCO / OH) of hydroxyl groups (OH groups) of the polyol component to the isocyanate groups (NCO groups) of the polyisocyanate component is 0.6 or more, and the elastic modulus of the coating film is 300 MPa or less has excellent feel when struck and stain resistance.

[0005] However, even with these proposed golf balls, the markings such as personalized names on the coating still sometimes peel off, and it was difficult to say that the printability, such as the durability of the markings, was sufficient. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2024-055321 [Patent Document 2] Japanese Patent Publication No. 2007-260356 [Patent Document 3] Japanese Patent Application Publication No. 02-128774 [Patent Document 4] Japanese Patent Publication No. 2016-123632 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] This invention has been made in view of the above circumstances, and aims to provide a golf ball that has excellent printability, such as mark durability, while ensuring the colorfastness and durability of the paint layer. [Means for solving the problem]

[0008] The inventors of the present invention have conducted diligent studies to achieve the above objectives and have found that, for a golf ball having at least one coating layer on the surface of a golf ball having numerous dimples, at least one of the coating layers is formed with a two-component curable urethane coating composition consisting of a polyol component and a polyisocyanate component, the molar ratio (NCO group / OH group) of the hydroxyl groups (OH groups) of the polyol component to the isocyanate groups (NCO groups) of the polyisocyanate component is set to within the range of 0.50 to 0.78, and hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI) are used in combination as the polyisocyanate component, and the mixing ratio of these is adjusted to be within the range of 90 / 10 to 40 / 60 in terms of HDI / TDI (mass ratio), thereby ensuring the colorfastness and durability of the coating layer while also achieving excellent printability such as mark durability, and thus the present invention has been made.

[0009] Therefore, the present invention provides the following golf ball. 1. A golf ball having at least one coating layer on a surface having numerous dimples, wherein at least one of the coating layers is formed from a two-component curable urethane coating composition consisting of a polyol component and a polyisocyanate component, the molar ratio (NCO group / OH group) of the hydroxyl groups (OH groups) of the polyol component to the isocyanate groups (NCO groups) of the polyisocyanate component is 0.50 to 0.78, the polyisocyanate component consists of hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI), and the mixing ratio of these is 90 / 10 to 40 / 60 in terms of HDI / TDI (mass ratio). 2. The golf ball according to claim 1, wherein the paint composition contains a cellulose resin, and the amount of the resin is 0.1 to 5.0 parts by mass per 100 parts by mass of paint solids. 3. The golf ball according to item 2 above, wherein the cellulose resin is cellulose acetate butyrate (CAB) or nitrocellulose. 4. The golf ball according to claim 1 or 2 above, wherein the polyol component is an acrylic polyol or a polyester polyol. 5. In the above two-component curable urethane coating composition, the relationship between the above molar ratio (NCO group / OH group) and the ratio of the above TDI to the above TDI in the polyisocyanate component is given by the following formula (1) (Molar ratio of NCO group / OH group) - {0.29 × (Ratio of TDI)} ≤ 0.75 ···(1) A golf ball as described in item 1 or 2 above that satisfies the requirements. 6. A golf ball according to item 1 or 2 above, wherein the 10% modulus of the above-mentioned paint layer, based on measurements according to JIS K7161 (2014), is 15.0 MPa or less. 7. The golf ball according to claim 1 or 2 above, wherein the peak temperature of the loss tangent (tanδ) obtained by measuring the dynamic viscoelasticity of the above-mentioned paint layer is 70°C or less. 8. The golf ball according to item 1 or 2 above, wherein the peak height of the loss tangent (Tanδ) of the above-mentioned paint layer is 0.35 to 0.80. 9. The golf ball according to claim 1 or 2 above, wherein the above-mentioned paint layer is formed by two layers, an inner layer and an outer layer, with the outer layer having a thickness of 10 to 20 μm and the inner layer having a thickness of 3 to 7 μm. [Effects of the Invention]

[0010] According to the present invention, the golf ball offers excellent printability, such as mark durability, while ensuring the colorfastness and durability of the paint layer. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view of a golf ball, which is one embodiment of the present invention. [Modes for carrying out the invention]

[0012] The present invention will be described in more detail below. The golf ball of the present invention has a paint layer applied to the ball surface, which has numerous dimples, by coating it with a paint composition. The role of this paint layer is to protect the entire ball and to impart gloss and aesthetic appeal to the ball surface. Specifically, a two-component curable urethane paint composition consisting of a polyol component and a polyisocyanate component is used.

[0013] Examples of the polyol components mentioned above include acrylic polyols and polyester polyols. These polyols may also contain modified polyols, and other polyols may be added to further improve workability.

[0014] Examples of acrylic polyols include homopolymers or copolymers of monomers having functional groups that react with isocyanates. Examples of such monomers include alkyl (meth)acrylates, specifically methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. These can be used individually or in combination of two or more.

[0015] In addition, as a modified product of acrylic polyol, for example, polyester-modified acrylic polyol can be used. As other polyols, polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polyoxytetramethylene glycol (PTMG), condensation polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PH2A), lactone polyester polyols such as poly-ε-caprolactone (PCL), polycarbonate polyols such as polyhexamethylene carbonate, etc. can be mentioned. These can be used alone or in combination of two or more. Also, the ratio of these polyols to the total amount of acrylic polyol is preferably 50% by mass or less, more preferably 40% by mass or less.

[0016] The polyester-based polyol is obtained by polycondensation of a polyol and a polybasic acid. Examples of the polyol include diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, hexylene glycol, dimethylolheptane, polyethylene glycol, and polypropylene glycol, triols, tetraols, and polyols having an alicyclic structure. Examples of the polybasic acid include aliphatic dicarboxylic acids such as succinic acid, adipic acid, sebacic acid, azelaic acid, and dimer acid, aliphatic unsaturated dicarboxylic acids such as fumaric acid, maleic acid, itaconic acid, and citraconic acid, aromatic polyvalent carboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, and pyromellitic acid, dicarboxylic acids having an alicyclic structure such as tetrahydrophthalic acid, hexahydrophthalic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, and endomethylenetetrahydrophthalic acid, and tris-2-carboxyethyl isocyanurate.

[0017] As the polyol component, in addition to using only one type of polyester polyol, two types of polyester polyols can also be used in combination. When two types of polyester polyols are used in combination, for example, the following component (A) and component (B) can be mentioned. As the polyester polyol of component (A), a polyester polyol in which a cyclic structure is introduced into the resin skeleton can be adopted. For example, polycondensation of a polyol having an alicyclic structure such as cyclohexanedimethanol and a polybasic acid, or polycondensation of a polyol having an alicyclic structure, diols or triols, and a polybasic acid can be mentioned. On the other hand, as the polyester polyol of component (B), a polyester polyol having a multi-branched structure can be adopted. For example, a polyester polyol having a branched structure such as "NIPPOLAN 800" manufactured by Tosoh Corporation can be mentioned.

[0018] The weight average molecular weight (Mw) of the whole main agent composed of the above two types of polyester polyols is preferably 13,000 to 23,000, more preferably 15,000 to 22,000. Also, the number average molecular weight (Mn) of the whole main agent composed of the above two types of polyester polyols is preferably 1,100 to 2,000, more preferably 1,300 to 1,850. If these average molecular weights (Mw and Mn) deviate from the above range, the abrasion resistance of the paint layer may decrease. The weight average molecular weight (Mw) and the number average molecular weight (Mn) are measured values (polystyrene conversion values) by gel permeation chromatography (hereinafter abbreviated as GPC) measurement using a differential refractive index detector.

[0019] The blending amounts of the above two types of polyester polyols (components (A) and (B)) are not particularly limited, but the blending amount of component (A) is preferably 20 to 30% by mass based on the total amount of the main agent, and the blending amount of component (B) is preferably 2 to 18% by mass based on the total amount of the main agent.

[0020] On the other hand, in this invention, polyisocyanates are a combination of hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI). Generally, when hexamethylene diisocyanate (HDI) is added, the entire paint tends to become softer, while when toluene diisocyanate (TDI) is added, the entire paint tends to become harder. In this invention, by optimizing the HDI / TDI ratio of these curing agents, the printability can be improved without reducing the discoloration resistance and durability of the coating film.

[0021] The mixing ratio of hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI) in the above polyisocyanate component is 90 / 10 to 40 / 60 in terms of HDI / TDI (mass ratio), and preferably 80 / 20 to 50 / 50.

[0022] In the above coating composition, in order to sufficiently achieve both coating film durability and printability, the relationship between the above molar ratio (NCO group / OH group) and the ratio of the above TDI to the above TDI in the polyisocyanate component is given by the following formula (1) (Molar ratio of NCO group / OH group) - {0.29 × (Ratio of TDI)} ≤ 0.75 ···(1) It is desirable that the following conditions be met. That is, the upper limit of the above formula (1) is preferably 0.75 or less, more preferably 0.72 or less, and the lower limit is preferably 0.50 or more, more preferably 0.60 or more.

[0023] Furthermore, in this invention, the molar ratio (NCO group / OH group) of hydroxyl groups (OH groups) of the polyol to isocyanate groups (NCO groups) of the polyisocyanate must be within the range of 0.50 to 0.78, preferably 0.55 or higher, more preferably 0.60 or higher, and the upper limit is preferably 0.75 or lower, more preferably 0.72 or lower. In other words, by making this molar ratio slightly lower than the molar ratio normally used, the mesh of the urethane crosslinked structure becomes slightly looser, making it easier for ink to penetrate into this crosslinked structure, and potentially improving the topcoatability. Also, if this molar ratio is too low, the stain resistance deteriorates, and if it is too high, the paint layer may become brittle.

[0024] As the curing catalyst (organometallic compound), amine-based catalysts or organometallic catalysts can be used. Suitable organometallic compounds include metal soaps such as aluminum, nickel, zinc, and tin, which have conventionally been used as curing agents for two-component urethane coatings.

[0025] Depending on the painting conditions, various organic solvents can be mixed into the paint composition. Examples of such organic solvents include aromatic solvents such as toluene, xylene, and ethylbenzene; ester solvents such as ethyl acetate, butyl acetate, propylene glycol methyl ether acetate, and propylene glycol methyl ether propionate; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; ether solvents such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, and dipropylene glycol dimethyl ether; alicyclic hydrocarbon solvents such as cyclohexane, methylcyclohexane, and ethylcyclohexane; and petroleum hydrocarbon solvents such as mineral spirits.

[0026] In the present invention, it is preferable to include a cellulose-based resin in the above-mentioned paint composition. Specific examples of cellulose-based resins include cellulose acetate butyrate (CAB) and nitrocellulose. Adding this cellulose-based resin to the paint composition can modify the surface area of ​​the paint layer. That is, this cellulose-based resin helps maintain a smooth state of the paint layer after application to the substrate or during curing, thereby improving the adhesion between the paint and the mark, and potentially improving the durability of the mark.

[0027] The amount of cellulose resin used is 0.1 to 5.0 parts by mass per 100 parts by mass of paint solids, preferably 0.5 parts by mass or more, more preferably 1.0 part by mass or more, with an upper limit of preferably 4.0 parts by mass or less, more preferably 3.0 parts by mass or less. If the amount is too high, the viscosity will increase and workability may deteriorate, while if the amount is too low, the compatibility between the paint and the mark may deteriorate.

[0028] The paint composition may contain known paint components as needed. Specifically, appropriate amounts of thickeners, UV absorbers, fluorescent whitening agents, slippery agents, pigments, etc., can be added.

[0029] There are no particular restrictions on the thickness of the paint layer made from the above paint composition, but it is usually 5 to 40 μm, preferably 10 to 20 μm. The thickness of the paint layer referred to here is not the thickness of the paint layer formed within the dimples, but rather the thickness of the paint layer formed on the ball surface other than the dimples (i.e., also called the land portion or embankment portion).

[0030] Furthermore, the above-mentioned paint layer can be formed by two layers: an inner layer and an outer layer. In this case, the thickness of the inner layer is preferably 3 to 7 μm, more preferably 4 to 6 μm, and the thickness of the outer layer is preferably 10 to 20 μm, more preferably 12 to 18 μm. When forming the paint layer in multiple layers in this manner, it is preferable to use the above-mentioned paint composition as the outermost layer in order to fully ensure the desired effects of the present invention.

[0031] When using the above-mentioned paint composition, the paint composition can be prepared at the time of painting on a golf ball manufactured by a known method, applied to the surface using a normal painting process, and a paint layer can be formed on the ball surface after a drying process. In this case, spray painting, electrostatic painting, dipping, etc., can be suitably used as the painting method, and there are no particular restrictions.

[0032] The drying process described above is the same as that of known two-component curing urethane paints, with a drying temperature of approximately 40°C or higher, particularly 40-60°C, and a drying time of 20-90 minutes, particularly 40-50 minutes.

[0033] By applying corona treatment, plasma treatment, ultraviolet irradiation treatment, or electron beam irradiation treatment to the cover surface, which is the base for forming the above-mentioned paint layer, the adhesion between the cover surface and the paint layer can be improved, and these surface treatments can be suitably adopted.

[0034] The above-mentioned paint composition can be used with any type of golf ball, such as a one-piece golf ball, a two-piece solid golf ball consisting of a core and a cover that covers the core, or a multi-piece solid golf ball consisting of one or more layers of core and multiple layers of cover that covers the core.

[0035] The 10% modulus of the paint layer formed by the above paint composition is 15.0 MPa or less, preferably 10.0 MPa or less, more preferably 8.0 MPa or less, with a lower limit of preferably 1.0 MPa or more, more preferably 1.5 MPa or more, and even more preferably 2.0 MPa or more. Specifically, the above "10% modulus" refers to the measurement of the tensile stress (10% modulus) at 10% strain on a test piece prepared by punching a cured product of the paint composition into a test piece type 2 as specified in JIS K7127 (1999), in accordance with JIS K7161 (2014). If the 10% modulus of the paint layer is 3.0 MPa or less, the paint layer becomes flexible and can smoothly deform in accordance with the deformation of the ball when struck with a club, thereby preventing chipping and cracking of marks on the paint layer as much as possible, resulting in good mark durability.

[0036] For the paint layer formed by the above paint composition, the peak height of the loss tangent (tanδ) in the dynamic viscoelasticity test is preferably 0.35 or higher, more preferably 0.40 or higher, with an upper limit of preferably 0.80 or lower, more preferably 0.70 or lower. Furthermore, the temperature at this peak height (peak temperature) is preferably 70°C or lower, more preferably 60°C or lower, and even more preferably 50°C or lower, with a lower limit of preferably 20°C or higher, more preferably 25°C or higher. A paint layer having a loss tangent (tanδ) within the above range has high frictional force and high shear force, and as a result, a high spin amount can be obtained during approach.

[0037] The loss tangent (tanδ) of the paint layer formed by the above-mentioned paint composition is expressed as the loss modulus divided by the storage modulus, and is also called the dynamic viscoelastic modulus. This loss tangent (tanδ) can be measured using a commercially available measuring device, for example, a dynamic viscoelasticity measuring device (DMA Q800) manufactured by T.A. Instruments.

[0038] The golf ball of the present invention has a ball surface with numerous dimples and at least one paint layer, but the internal structure of the ball usually comprises a core and a cover consisting of at least one layer.

[0039] The above core can be formed using a known rubber material as a base material. As the base rubber, a known base rubber such as natural rubber or synthetic rubber can be used, and more specifically, it is recommended to mainly use polybutadiene, in particular cis-1,4-polybutadiene having at least 40% or more of a cis structure. In addition, in the base rubber, natural rubber, polyisoprene rubber, styrene-butadiene rubber, etc. can be used in combination with the above-mentioned polybutadiene as desired. Furthermore, polybutadiene can be synthesized using Ziegler catalysts such as titanium-based, cobalt-based, nickel-based, and neodymium-based catalysts, as well as metal catalysts such as cobalt and nickel.

[0040] The above-mentioned base rubber can be compounded with co-crosslinking agents such as unsaturated carboxylic acids and their metal salts, 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. Furthermore, commercially available antioxidants may be added as needed.

[0041] The core described above is a heat-molded product obtained by heat-curing the rubber material described above. The core may be a single layer or multiple layers, and the heat-molded product can be used for all or part of a single-layer or multiple-layer core. For example, the core can be manufactured by kneading the material using a kneader such as a Banbury mixer or roll, compression molding or injection molding using a core mold, and then appropriately heating the molded body at a temperature of approximately 100 to 200°C for 10 to 40 minutes, which is sufficient for the organic peroxide and co-crosslinking agent to act, thereby curing the molded body.

[0042] The above-mentioned cover is a component that covers the core and has at least one layer, such as a two-layer cover or a three-layer cover. In the case of a two-layer cover, the inner layer may be called the intermediate layer and the outermost layer. In the case of a three-layer cover, the layers may be called the surrounding layer, intermediate layer, and outermost layer, in order from the inside. The outer surface of the outermost layer usually has many dimples formed to improve aerodynamic characteristics.

[0043] There are no particular restrictions on the material of each layer of the cover; for example, they can be formed from ionomer resin, polyester resin, polyamide resin, or even polyurethane resin. For instance, the intermediate layer can be made from ionomer resin or highly neutralized ionomer resin, and the outermost layer from polyurethane resin.

[0044] The above-mentioned cover layers can be formed by conventional methods such as known injection molding. For example, a covered sphere can be obtained by injecting cover material around the core using an injection molding die, or a two-piece golf ball with a cover surrounding the core can be manufactured by wrapping the core with two pre-formed hemispherical half-cups as an intermediate layer material and then heat-pressure molding it.

[0045] A specific layer structure of the golf ball of the present invention is, for example, the ball structure shown in Figure 1. Figure 1 shows a three-layer golf ball G having a core 1, an intermediate layer 2 covering the core 1, and a cover 3 covering the intermediate layer. The intermediate layer is formed as a single layer, but it can also be formed as multiple layers, and it may also be a two-piece golf ball with a core without an intermediate layer and a single-layer cover. The cover 3, excluding the coating layer, is located as the outermost layer in the layer structure of the golf ball. The surface of the cover (outermost layer) 3 usually has many dimples D formed on it to improve aerodynamic characteristics. In addition, a paint layer 4 is formed on the surface of the cover 3, and a mark M such as a personal name is applied to the outside of the paint layer.

[0046] There are no particular restrictions on the method of printing this mark M onto the surface of the paint layer. Various methods can be used, such as the conventional inkjet method, the transfer film method using transfer tape, and the pad method (engraving method). However, in this invention, it is particularly useful when the mark is formed by an inkjet printing method.

[0047] Furthermore, the ball properties of the golf ball of the present invention, such as ball mass and diameter, can be appropriately set in accordance with the rules of golf. Specifically, it can be formed with a diameter of 42.67 mm or more and a mass of 45.93 g or less. [Examples]

[0048] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0049] [Examples 1-7, Comparative Examples 1, 2] A dimpled golf ball was prepared by injection molding a 1.25 mm thick ionomer resin cover onto a 40.2 mm diameter core, and then it was painted. For Examples 1-5 and Comparative Example 1, the coating was applied in two layers. The inner coating layer (hereinafter referred to as "coating film") was applied using an automatic spray gun with a water-based primer to a thickness of 5 μm. The water-based primer composition was a mixture of Cashew's "Acrylic Primer" (main component), "Crosslinker CX-100" (hardener), and water in a mass ratio of 100:1.5:3. This mixture was dried at 55°C for 30 minutes and then subjected to testing. Next, the outer coating (paint layer) was applied using a paint composition consisting of the main agent and hardener shown in Table 1, with an automatic spray gun to achieve a coating thickness of 15 μm. The golf balls were painted and then marked with an inkjet print. For the markings, a Roland VersaUV LEF2-200 printer was used, along with Roland ECO UV ink.

[0050] For Examples 6 and 7 and Comparative Example 2, the inner coating layer was prepared in the same manner as described above, and the outer coating layer was applied using a coating composition consisting of the main agent and hardener shown in Table 1, with an automatic spray gun to achieve a coating layer thickness of 15 μm. Furthermore, in the same manner as described above, a mark was printed on the painted golf ball surface using inkjet printing.

[0051] The sheet properties of the coating films produced in each example are as follows:

[0052] [10% Modulus (MPa)] The tensile properties of the coating film were measured in accordance with JIS K7161 (2014). Specifically, a coating film (thickness 0.05 mm) was prepared by drying and curing the paint at 40°C for 4 hours. This coating film was punched out into test specimens of type 2 (parallel section width 10 mm, standard distance 50 mm) as specified in JIS K7127 (1999) to prepare test specimens. Tensile tests were performed on these test specimens using the A&D testing machine "Tensilon RTG1310". The test conditions were a grip distance of 100 mm, a tensile speed of 50 mm / min, and a test temperature of 23°C. The tensile stress (10% modulus) at 10% strain was recorded.

[0053] [Measurement of friction coefficient] The coefficient of friction of the coating film was measured in accordance with JIS K7125 (1999). Specifically, a coating film (thickness of 0.5 mm or less) was prepared by drying and curing the paint at 40°C for 4 hours. This coating film was measured on a surface measuring 80 mm x 200 mm, with a thickness of 0.5 mm or less and a contact area of ​​40 cm². 2 A test specimen was prepared with a side length of 63 mm. This specimen was tested using the A&D Corporation's "Tensilon RTG1310" testing machine. The test conditions were a speed of 100 mm / min, a test temperature of 23°C, and a sliding piece of 200 g ± 2 g. The frictional force was measured under these conditions. Friction is classified into static friction and kinetic friction. • Static friction: The friction that occurs when the "threshold" is exceeded at the start of sliding. • Dynamic friction: Friction during a sliding motion at a given speed. • Static friction coefficient: The force increases linearly, causing friction and reaching a maximum load. This peak represents the static friction force Fs. The static friction coefficient μs is given by the following equation. μs=Fs / N [Fs: Static friction force (N)] • Coefficient of kinetic friction: The frictional force acting during sliding motion often deviates from the constant value that appears under ideal conditions due to secondary effects related to the increase in distance traveled. The kinetic friction force FD is the average value up to the first 6 cm after the start of relative shear motion between the contact surfaces, neglecting the peak static friction force Fs. The coefficient of kinetic friction μD is calculated from the kinetic friction force using the following formula. μD=FD / N [FD: Dynamic friction force (N)]

[0054] [Peak height and peak temperature of loss tangent Tanδ] The loss tangent (tan δ) of the coating film was measured under the following conditions. • Equipment: TA Instruments' "Q800" dynamic viscoelasticity measuring instrument. • Measurement sample: A paint composition containing the main agent and hardener was dried and cured at 40°C for 4 hours to produce a coating film with a thickness of 0.11-0.14 mm. From this coating film, sample pieces were cut out to a width of 4 mm and a clamping distance of 20 mm. • Measurement mode: Tensile ·Measurement temperature: -100℃~150℃ • Heating rate: 4°C / min • Measurement data acquisition interval: 4℃ ·Excitation frequency: 10Hz • Measurement distortion: 0.10%

[0055] For each of the obtained examples and comparative examples, golf balls with marks on the surface of the coating were evaluated for discoloration, coating durability, printability, and spin performance during approach shots using the method described below. The results are shown in Table 1.

[0056] [Discoloration] First, 20g of spinach powder and 1L of water were mixed in a blender for 5 minutes. This mixture was then placed in a 10L plastic container and mixed with the painted golf ball for 3 hours. The degree to which the golf ball's coating changed to green was determined using a colorimeter to calculate the color difference ΔE. The settings for the colorimeter were as follows: [Setting conditions] The color difference is measured using a colorimeter (SC-P spectrometer manufactured by Suga Test Instruments Co., Ltd.) on golf balls before and after the test, in accordance with JIS Z 8722 "Method for measuring reflective objects" (diffuse light illumination, optical system receiving light at 8 degrees: condition c), using the d / 8 method (measurement without including the specular reflection component of the sample: with light trap). A measurement hole diameter of φ30 mm is used. Then, the L, a, and b values, as well as the color difference ΔE, are measured based on the Lab color chart of JIS Z 8701. <Judgment criteria> Then, the color change properties are evaluated based on the color difference ΔE value measured using the above-mentioned colorimeter, according to the following criteria. ◎: ΔE is within 10 ○: ΔE is between 10 and 20. △: ΔE is between 20 and 30. ×: ΔE is 30 or greater

[0057] [Coating film durability] Approximately 1.7 kg of sand (about 5 mm in size) and 1.7 L of water are placed in a porcelain pot with an outer diameter of approximately 210 mm, and 15 balls are added to the pot. The mixture is then stirred in a ball mill at a rotation speed of 60 ± 5 rpm for 120 minutes. After that, the balls are removed from the porcelain pot, the degree of peeling of the surface coating is checked, and evaluated according to the following criteria. <Judgment criteria> ○...There is either no paint peeling at all, or it is hardly noticeable. △ ··· Slight paint peeling can be seen in several places. × ··· Paint peeling was observed in several places.

[0058] [Printability (Mark durability)] The printability (durability of the mark) will be evaluated using the ADC Ball COR Durability Tester manufactured by Automated Design Corporation, USA. This test machine launches a golf ball using air pressure and then continuously impacts it against two parallel metal plates. The incident velocity on the metal plates is set to 43 m / s, and the test is performed 30 times for each ball. The appearance of the mark after the test is then visually observed. <Judgment criteria> ◎: There is absolutely no peeling or chipping of the mark. ○: There are slight peeling or chipping marks. △: There are small areas of peeling or chipping around the mark. Large peeling and chipping are visible in the × mark.

[0059] [Spin ​​performance during approach shots] Using a swing robot machine, the amount of backspin (rpm) when hitting with a sand wedge (SW) at a head speed (HS) of 20 m / s will be measured and evaluated according to the following criteria. The club used will be a Bridgestone Sports "TourStage TW-03" (loft angle 57°). <Judgment criteria> ◎: 5801 rpm or higher ○: 5501 rpm or higher, 5800 rpm or lower ×: 5500rpm or less

[0060] [Table 1]

[0061] The main components to be incorporated into the paint composition are as follows: • Polyol • Polyester polyol synthesized by the following method • Organic solvent: A mixture of ethyl acetate and butyl acetate in a mass ratio of 5.5:1. • Unreactive resin • Cellulose acetate butyrate (CAB)

[0062] Synthesis of "polyester polyols" The main polyol used in Table 1 was a polyester polyol synthesized by the following method. In a reaction apparatus equipped with a reflux condenser, dropping funnel, gas inlet tube, and thermometer, 140 parts by mass of trimethylolpropane, 95 parts by mass of ethylene glycol, 157 parts by mass of adipic acid, and 58 parts by mass of 1,4-cyclohexanedimethanol were charged and heated to 200-240°C while stirring for 5 hours (reaction). Subsequently, a polyester polyol with an acid value of 4, a hydroxyl value of 170, and a weight-average molecular weight (Mw) of 28,000 was obtained.

[0063] As shown in Table 1, Examples 1 to 7 maintain good discoloration resistance and durability of the coating film while exhibiting good printability (mark durability) and achieving a high spin rate during approach. In contrast, Comparative Example 1 has a high molar ratio of NCO / OH in the coating composition of 0.80, resulting in poor coating film durability and printability (mark durability). Comparative Example 2 does not contain HDI in the polyisocyanate component, and therefore does not show excellent results in either coating film discoloration resistance or coating film durability.

Claims

1. A golf ball having at least one coating layer on a surface having numerous dimples, wherein at least one of the coating layers is formed from a two-component curable urethane coating composition comprising a polyol component and a polyisocyanate component, the molar ratio (NCO group / OH group) of hydroxyl groups (OH groups) of the polyol component to isocyanate groups (NCO groups) of the polyisocyanate component is 0.50 to 0.78, the polyisocyanate component consists of hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI), and the mixing ratio of these is 90 / 10 to 40 / 60 in terms of HDI / TDI (mass ratio).

2. The golf ball according to claim 1, wherein the above-mentioned paint composition contains a cellulose resin, and the amount of the resin is 0.1 to 5.0 parts by mass per 100 parts by mass of paint solids.

3. The golf ball according to claim 2, wherein the cellulose-based resin is cellulose acetate butyrate (CAB) or nitrocellulose.

4. The golf ball according to claim 1 or 2, wherein the polyol component is an acrylic polyol or a polyester polyol.

5. In the above two-component curable urethane coating composition, the relationship between the molar ratio (NCO group / OH group) and the ratio of TDI to the TDI in the polyisocyanate component is given by the following formula (1): (Molar ratio of NCO group / OH group) - {0.29 × (Ratio of TDI)} ≤ 0.75 ... (1) A golf ball according to claim 1 or 2 that satisfies the requirements.

6. The golf ball according to claim 1 or 2, wherein the 10% modulus of the coating layer, based on measurements according to JIS K7161 (2014), is 15.0 MPa or less.

7. The golf ball according to claim 1 or 2, wherein the peak temperature of the loss tangent (tanδ) obtained by measuring the dynamic viscoelasticity of the above-mentioned paint layer is 70°C or less.

8. The golf ball according to claim 1 or 2, wherein the peak height of the loss tangent (Tanδ) of the above-mentioned coating layer is 0.35 to 0.

80.

9. The golf ball according to claim 1 or 2, wherein the above-mentioned paint layer is formed of two layers, an inner layer and an outer layer, the thickness of the outer layer being 10 to 20 μm and the thickness of the inner layer being 3 to 7 μm.

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