Coating material composition and method for forming multilayer coating film

Abstract

The purpose of the present disclosure is to provide a coating composition that makes it possible to form a coating film with an excellent appearance in terms of smoothness and sharpness. A coating composition according to the present disclosure has the configuration described below.　Provided is a coating composition containing a film-forming resin (A) and a curing agent (B), wherein in dynamic viscoelasticity measurement under conditions of a temperature of 23°C, a frequency of 0.50 Hz, and a stress measurement range of 0.1-100 Pa, the stress σ2 when the storage elastic modulus decreases by 10% for the first time with reference to the storage elastic modulus G'1 at a stress of 0.1 Pa is in the range of 3.5-100 Pa.

Description

Coating composition and method for forming multi-layer coating film

[0001] The present disclosure relates to a coating composition and a method for forming a multi-layer coating film.

[0002] In recent years, environmental issues have been attracting great attention on a global scale, and the automobile industry is also actively working to improve the environment in its production processes. Problems such as global warming and industrial waste have arisen from the automobile manufacturing process, and measures to address these issues are urgently needed.

[0003] A widely adopted method for forming paint films on automobile bodies is to apply electrodeposition coating to the substrate, then form a multi-layer paint film using the 3-coat, 2-bake (3C2B) method, in which "an intermediate paint is applied on the electrodeposition coating to form an intermediate paint film, the intermediate paint film is baked and cured, a base coat paint is applied on the intermediate paint film and the base coat film is preheated, a clear paint is applied on the base coat film, and the base coat film and clear coat film are baked and cured." However, in recent years, from the perspective of energy conservation, etc., attempts have been made to omit the bake-hardening process after applying the primer paint, and instead, after applying electrodeposition coating to the substrate, a 3-coat, 1-bake (3C1B) method has been attempted in which "the primer paint is applied on the electrodeposition coating, the primer paint is preheated (preheated), the base coat paint is applied on the primer paint, the base coat paint is preheated (preheated), the clear paint is applied on the base coat, and the primer paint, base coat paint, and clear coat paint are baked and hardened."

[0004] However, in the above-mentioned three-coat one-bake method, a mixed layer of the coating film layers occurs between the intermediate coat layer and the base coat layer, and there are cases where a multi-layer coating film with an excellent finished appearance (smoothness, vividness) cannot be obtained, which has been a problem.

[0005] Patent Document 1 describes a method for forming a multilayer coating film, which includes: (1) a step of providing a substrate on which an electrodeposition coating film has been formed; (2) a step of applying a water-based intermediate coating paint on the electrodeposition coating film to form an intermediate coating film; (3) a step of applying a water-based base coating paint and a clear coating paint successively wet-on-wet on the intermediate coating film without curing the intermediate coating film to form a base coating film and a clear coating film; and (4) a step of simultaneously baking and curing the intermediate coating film, the base coating film, and the clear coating film. The present disclosure discloses a method for forming a multilayer coating film having a coating film water absorption rate of 10% or less and a coating film water elution rate of 5% or less, characterized in that the water-based primer coating contains an acrylic resin emulsion having a glass transition temperature of -50 to 20°C, an acid value of 2 to 60 mgKOH / g and a hydroxyl value of 10 to 120 mgKOH / g, a urethane resin emulsion having an acid value of 5 to 50 mgKOH / g, and a curing agent, which effectively prevents intermixing of the primer coating film and the base coating film, thereby forming a multilayer coating film with excellent surface smoothness.

[0006] Furthermore, recently, from the viewpoint of further energy saving, it has been desired to omit the preheating step after application of the intermediate coating in the above-mentioned 3 coat 1 bake (3C1B) system.

[0007] Japanese Patent Application Laid-Open No. 2004-358462

[0008] In the technology described in Patent Document 1, when preheating after application of the intermediate coat is omitted, there is room for improvement in smoothness and image clarity. An object of the present invention is to provide a coating composition capable of forming a coating film having an appearance excellent in smoothness and image clarity.

[0009] As a result of extensive research into solving the above problems, the present inventors have found that the above problems can be solved by using a coating composition comprising a film-forming resin (A) and a curing agent (B), wherein, in dynamic viscoelasticity measurement under conditions of a temperature of 23°C, a frequency of 0.50 Hz, and a stress measurement range of 0.1 to 100 Pa, the stress σ2 at which the storage modulus decreases by 10% for the first time is within the range of 3.5 to 100 Pa, based on the storage modulus G'1 at a stress of 0.1 Pa.

[0010] That is, the present invention relates to <1> to <10>. <1> A coating composition comprising a film-forming resin (A) and a curing agent (B), wherein, in dynamic viscoelasticity measurement under conditions of a temperature of 23°C, a frequency of 0.50 Hz, and a stress measurement range of 0.1 to 100 Pa, the stress σ2 at which the storage modulus first decreases by 10% is within the range of 3.5 to 100 Pa, based on the storage modulus G'1 at a stress of 0.1 Pa. <2> The coating composition according to <1>, wherein the film-forming resin (A) comprises a polyurethane resin (A1). <3> The coating composition according to <2>, wherein the content of the polyurethane resin (A1) is within the range of 1.0 to 30 mass% based on the total resin solids content in the coating composition.

[0011] <4> The coating composition according to any one of <1> to <3>, wherein, in dynamic viscoelasticity measurement under conditions of a temperature of 23°C, a frequency of 0.50 Hz, and a stress measurement range of 0.1 to 100 Pa, the value of tan δ1 (loss modulus / storage modulus) at a stress of 0.1 Pa is less than 1, and the stress σ3 at which the value of tan δ first becomes 1 in a stress range exceeding 0.1 Pa is within the range of 10 to 100 Pa. <5> The coating composition according to any one of <1> to <4>, wherein the storage modulus G'1 is within the range of 5.0 to 150 Pa. <6> The coating composition according to any one of <1> to <5>, wherein the solids concentration is within the range of 40 to 60 mass %. <7> The coating composition according to any one of <1> to <6>, further comprising a viscosity modifier (C).

[0012] <8> The coating composition according to <7>, wherein the content of the viscosity modifier (C) is within the range of 0.05 to 1.00 parts by mass based on 100 parts by mass of the total resin solids in the coating composition. <9> The coating composition according to any one of <1> to <8>, which is used for forming a first coating film when a multi-layer coating film consisting of a first coating film, a second coating film, and a clear coat coating film is formed by a 3-coat 1-bake method. <10> A method for forming a multilayer coating film, comprising: step (I-1): applying the coating composition described in any one of <1> to <8> onto a substrate to form an uncured first coating film; step (I-2): applying a second coating composition onto the uncured first coating film formed in step (I-1) to form an uncured second coating film; step (I-3): applying a clear coat coating composition onto the uncured second coating film formed in step (I-2) to form an uncured clear coat film; and step (I-4): simultaneously heat-curing the uncured first coating film formed in step (I-1), the uncured second coating film formed in step (I-2), and the uncured clear coat film formed in step (I-3).

[0013] According to the present invention, it is possible to provide a coating composition that can form a coating film having an appearance excellent in smoothness and clarity.

[0014] The present invention will be described in detail below, but these are merely examples of preferred embodiments, and the present invention is not limited to these. The coating composition of the present invention is a coating composition containing a film-forming resin (A) and a curing agent (B), and in which, in dynamic viscoelasticity measurement under conditions of a temperature of 23°C, a frequency of 0.50 Hz, and a stress measurement range of 0.1 to 100 Pa, the stress σ2 at which the storage modulus first decreases by 10% is within the range of 3.5 to 100 Pa, based on the storage modulus G'1 at a stress of 0.1 Pa.

[0015] [Paint Composition] In this specification, the dynamic viscoelasticity of a paint composition can be measured, for example, using a rheometer with a temperature control mechanism, according to the following measurement procedures (1) and (2). (1) The paint composition is placed on a cone and plate connected to the measurement device. (2) The stress dependency of the dynamic viscoelasticity is measured while increasing the stress at a logarithmic gradient at a temperature of 23°C and a frequency of 0.50 Hz. Specifically, the paint composition is placed on the measurement platform of the rheometer, and the dynamic viscoelasticity versus stress is measured every 10 seconds while changing the stress at a logarithmic gradient from 0.1 Pa to 100 Pa under the measurement conditions described below, to obtain the storage modulus (G') and loss modulus (G''). For the measurement, a cone and plate viscometer "HAAKE RHEO STRESS 3000" (trade name, manufactured by THERMO SCIENTIFIC, 50 mm diameter, cone and plate tilted at 0.5°) is used.

[0016] [Measurement conditions] Apparatus: Rheometer (HAAKE RHEO STRESS 3000, manufactured by THERMO SCIENTIFIC) Measurement: Stress dependency Measurement range: 0.1 Pa to 100 Pa Stress change: Logarithmic slope Measurement time: 285 s Measurement temperature: 23°C Frequency: 0.50 Hz Measurement gap: 0.100 mm

[0017] If the stress σ2 of the coating composition according to the present invention is less than 3.5 Pa, the resulting coating film will have insufficient image clarity, while if the stress σ2 exceeds 100 Pa, the resulting coating film will have insufficient smoothness, which is undesirable. As used herein, "image clarity of a coating film" refers to a state in which the coating film appears clear and free of turbidity. The image clarity of a coating film can be evaluated by how clearly the outline of the light from a fluorescent lamp reflected on the coating film is defined, and is measured herein by the Wa value obtained by measurement using a "WAVE SCAN" coating surface texture measuring instrument. On the other hand, "smoothness of a coating film" refers to a state in which the surface is physically flat, and is measured herein by the LW value obtained by measurement using a "WAVE SCAN" coating surface texture measuring instrument.

[0018] The reason why the coating composition of the present invention is capable of forming a coating film with an appearance excellent in smoothness and clarity of reflection is presumed to be as follows: In the coating film formation process using the 3-coat-1-bake (3C1B) method, when the second coating composition is applied onto the uncured first coating film using a coating machine or the like, the coating liquid of the second coating composition becomes particulate and collides with the uncured first coating film, causing stress in the uncured first coating film. In this case, if the stress σ2 at the first 10% decrease in storage modulus, based on the storage modulus G'1 at a stress of 0.1 Pa, is 3.5 Pa or more, it is presumed that even if the stress generated in the uncured first coating film due to collisions of particles of the second coating composition is large, the shape of the uncured first coating film will be easily maintained and a coating film with excellent clarity of reflection will be formed.

[0019] On the other hand, when the stress σ2 is 100 Pa or less, it is presumed that the uncured first coating film will have the fluidity necessary to conceal the irregularities of the coated object, and a coating film with excellent smoothness will be formed.

[0020] From the viewpoint of the image sharpness and smoothness of the coating film formed, the coating composition according to the present invention has a stress σ of preferably 5.0 Pa or more, more preferably 10 Pa or more, and even more preferably 15 Pa or more, and a stress σ of preferably 90 Pa or less, more preferably 80 Pa or less, and even more preferably 70 Pa or less.

[0021] Furthermore, from the viewpoint of the image clarity and smoothness of the coating film formed, the coating composition according to the present invention is such that, in the above-mentioned dynamic viscoelasticity measurement under conditions of a stress measurement range of 0.1 to 100 Pa, the value of tan δ1 (loss modulus / storage modulus) at a stress of 0.1 Pa is less than 1, and the stress σ3 at which the value of tan δ first becomes 1 in the stress range exceeding 0.1 Pa is preferably 10 Pa or more, more preferably 12 Pa or more, even more preferably 15 Pa or more, and the stress σ3 is preferably 100 Pa or less, more preferably 90 Pa or less, even more preferably 80 Pa or less.

[0022] Furthermore, from the viewpoint of the image clarity and smoothness of the coating film formed, the coating composition according to the present invention preferably has a storage modulus G'1 at a stress of 0.1 Pa in the dynamic viscoelasticity measurement under the stress measurement range of 0.1 to 100 Pa of 5.0 Pa or more, more preferably 10 Pa or more, and even more preferably 15 Pa or more, and the storage modulus G'1 is preferably 150 Pa or less, more preferably 90 Pa or less, and even more preferably 80 Pa or less.

[0023] [Film-forming resin (A)] The film-forming resin (A) used in the coating composition of the present invention can be any of various film-forming resins known for use in coatings. Examples of the film-forming resin (A) include polyurethane resin (A1), acrylic resin (A2), polyester resin (A3), acrylic-modified polyester resin, polyether resin, polycarbonate resin, epoxy resin, and alkyd resin. These can be used alone or in combination of two or more. From the viewpoint of the image clarity of the coating film formed, it is preferable that the film-forming resin (A) contains the polyurethane resin (A1).

[0024] From the viewpoint of the image clarity and smoothness of the coating film to be formed, the content of the film-forming resin (A) in the coating composition according to the present invention is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, based on the total resin solid content in the coating composition, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

[0025] In this specification, the term "solid content" refers to non-volatile components such as film-forming resins, curing agents, and pigments that remain after drying for 1 hour at 110° C. The solid content can be determined, for example, by weighing a sample into a heat-resistant container such as an aluminum foil cup, spreading the sample on the bottom of the container, drying it for 1 hour at 110° C., and weighing the mass of the components remaining after drying.

[0026] In addition, in this specification, the "solid content concentration" means the mass ratio of the solid content in the composition. Therefore, for example, the solid content concentration of the composition can be calculated by weighing out the composition into a heat-resistant container such as an aluminum foil cup, spreading the composition on the bottom of the container, drying it at 110°C for 1 hour, weighing the mass of the components in the composition remaining after drying, and determining the ratio of the mass of the components remaining after drying to the total mass of the composition before drying.

[0027] [Polyurethane Resin (A1)] The polyurethane resin (A1) may be a polyurethane resin obtained by reacting a polyol with a polyisocyanate. Examples of the polyol include low-molecular-weight dihydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, and hexamethylene glycol, and trihydric alcohols such as trimethylolpropane, glycerin, and pentaerythritol.

[0028] Examples of the high-molecular-weight polyols include polyether polyols, polyester polyols, polycarbonate polyols, acrylic polyols, and epoxy polyols. Examples of the polyether polyols include polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. Examples of the polyester polyols include polycondensates of alcohols such as the dihydric alcohols mentioned above, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, and neopentyl glycol with dibasic acids such as adipic acid, azelaic acid, and sebacic acid, and lactone-based ring-opening polymer polyols such as polycaprolactone. Examples of the polyols also include carboxyl-containing polyols such as 2,2-dimethylolpropionic acid and 2,2-dimethylolbutanoic acid.

[0029] Examples of polyisocyanates that can be reacted with the polyols include aliphatic polyisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, and lysine diisocyanate; and biuret-type adducts and isocyanurate ring adducts of these polyisocyanates; isophorone diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), methylcyclohexane-2,4-(or -2,6-)diisocyanate, Alicyclic diisocyanates such as 1,3-(or 1,4-)di(isocyanatomethyl)cyclohexane, 1,4-cyclohexane diisocyanate, 1,3-cyclopentane diisocyanate, and 1,2-cyclohexane diisocyanate; and biuret-type adducts and isocyanurate ring adducts of these polyisocyanates; xylylene diisocyanate, metaxylylene diisocyanate, tetramethylxylylene diisocyanate, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, aromatic diisocyanate compounds such as cyanate, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4-toluidine diisocyanate, 4,4'-diphenyl ether diisocyanate, (m- or p-)phenylene diisocyanate, 4,4'-biphenylene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, bis(4-isocyanatophenyl)sulfone, isopropylidenebis(4-phenylisocyanate); and polyisocyanates thereof. Examples of suitable isocyanates include biuret-type adducts and isocyanurate ring adducts of isocyanates; polyisocyanates having three or more isocyanate groups in one molecule, such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene, and 4,4'-dimethyldiphenylmethane-2,2',5,5'-tetraisocyanate; and biuret-type adducts and isocyanurate ring adducts of these polyisocyanates.

[0030] From the viewpoint of the image clarity of the coating film to be formed, the hydroxyl value of the polyurethane resin (A1) is preferably 0 mgKOH / g or more, more preferably more than 0 mgKOH / g, and is preferably 20 mgKOH / g or less, more preferably 15 mgKOH / g or less, and even more preferably 10 mgKOH / g or less.

[0031] From the viewpoint of the image clarity of the coating film to be formed, the acid value of the polyurethane resin (A1) is preferably 3 mgKOH / g or more, more preferably 5 mgKOH / g or more, even more preferably 10 mgKOH / g or more, and is preferably 31 mgKOH / g or less, more preferably 27.5 mgKOH / g or less, even more preferably 25 mgKOH / g or less.

[0032] When the coating composition according to the present invention contains the polyurethane resin (A1), the content of the polyurethane resin (A1) is, from the viewpoint of the image clarity and smoothness of the coating film to be formed, preferably 1 mass % or more, more preferably 5 mass % or more, even more preferably 7 mass % or more, based on the total resin solid content in the coating composition, and is preferably 30 mass % or less, more preferably 26 mass % or less, even more preferably 23 mass % or less.

[0033] [Acrylic Resin (A2)] The acrylic resin (A2) preferably contains a hydroxyl-containing acrylic resin (A21) from the viewpoint of the image clarity of the coating film formed, etc. The hydroxyl-containing acrylic resin (A21) can usually be produced by copolymerizing a hydroxyl-containing polymerizable unsaturated monomer (a) with another polymerizable unsaturated monomer (b) copolymerizable with the hydroxyl-containing polymerizable unsaturated monomer (a) (hereinafter referred to as "another polymerizable unsaturated monomer (b)") by a method known per se, such as solution polymerization in an organic solvent or emulsion polymerization in an aqueous medium.

[0034] The hydroxyl group-containing polymerizable unsaturated monomer (a) is a compound having at least one hydroxyl group and one polymerizable unsaturated group per molecule, and examples thereof include monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; ε-caprolactone-modified products of these monoesters; N-hydroxymethyl (meth)acrylamide; allyl alcohol; and (meth)acrylates having a polyoxyethylene chain having a hydroxyl group at the molecular terminal.

[0035] However, in the present invention, a monomer corresponding to the polymerizable unsaturated monomer having an ultraviolet-absorbing functional group (xvii) described later should be defined as the other polymerizable unsaturated monomer (b) and is excluded from the hydroxyl group-containing polymerizable unsaturated monomer (a). The hydroxyl group-containing polymerizable unsaturated monomer (a) can be used alone or in combination of two or more kinds.

[0036] In this specification, the term "polymerizable unsaturated group" refers to an unsaturated group that can undergo radical polymerization, such as a vinyl group, a (meth)acryloyl group, a (meth)acrylamide group, a vinyl ether group, an allyl group, a propenyl group, an isopropenyl group, or a maleimide group.

[0037] In this specification, "(meth)acrylate" means acrylate and / or methacrylate, "(meth)acrylic acid" means acrylic acid and / or methacrylic acid, "(meth)acryloyl" means acryloyl and / or methacryloyl, and "(meth)acrylamide" means acrylamide and / or methacrylamide.

[0038] The other polymerizable unsaturated monomer (b) can be appropriately selected and used depending on the properties desired for the hydroxyl group-containing acrylic resin (A1). Specific examples of the other polymerizable unsaturated monomer (b) include the following (i) to (xix). These can be used alone or in combination of two or more. (i) Alkyl or cycloalkyl (meth)acrylates: for example, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, tert-butylcyclohexyl (meth)acrylate, cyclododecyl (meth)acrylate, tricyclodecanyl (meth)acrylate, etc.

[0039] (ii) polymerizable unsaturated monomers having an isobornyl group: for example, isobornyl (meth)acrylate, etc. (iii) polymerizable unsaturated monomers having an adamantyl group: for example, adamantyl (meth)acrylate, etc. (iv) polymerizable unsaturated monomers having a tricyclodecenyl group: for example, tricyclodecenyl (meth)acrylate, etc. (v) aromatic ring-containing polymerizable unsaturated monomers: for example, benzyl (meth)acrylate, styrene, α-methylstyrene, vinyltoluene, etc. (vi) polymerizable unsaturated monomers having an alkoxysilyl group: for example, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, γ-(meth)acryloyloxypropyltrimethoxysilane, γ-(meth)acryloyloxypropyltriethoxysilane, etc.

[0040] (vii) polymerizable unsaturated monomers having a fluorinated alkyl group: for example, perfluoroalkyl (meth)acrylates such as perfluorobutylethyl (meth)acrylate and perfluorooctylethyl (meth)acrylate; fluoroolefins, etc. (viii) polymerizable unsaturated monomers having a photopolymerizable functional group such as a maleimide group (ix) vinyl compounds: for example, N-vinylpyrrolidone, ethylene, butadiene, chloroprene, vinyl propionate, vinyl acetate, etc. (x) polymerizable unsaturated monomers containing a carboxyl group: for example, (meth)acrylic acid, maleic acid, crotonic acid, β-carboxyethyl acrylate, etc. (xi) Nitrogen-containing polymerizable unsaturated monomers: for example, (meth)acrylonitrile, (meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, methylenebis(meth)acrylamide, ethylenebis(meth)acrylamide, 2-(methacryloyloxy)ethyltrimethylammonium chloride, and adducts of glycidyl (meth)acrylate and amines.

[0041] (xii) polymerizable unsaturated monomers having two or more polymerizable unsaturated groups in one molecule: for example, allyl (meth)acrylate, ethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, etc. (xiii) epoxy group-containing polymerizable unsaturated monomers: for example, glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, allyl glycidyl ether, etc.

[0042] (xiv) (meth)acrylates having a polyoxyethylene chain having an alkoxy group at the molecular terminal; (xv) polymerizable unsaturated monomers having a sulfonic acid group: for example, 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl (meth)acrylate, allylsulfonic acid, 4-styrenesulfonic acid, etc.; sodium salts and ammonium salts of these sulfonic acids; (xvi) polymerizable unsaturated monomers having a phosphoric acid group: acid phosphooxyethyl (meth)acrylate, acid phosphooxypropyl (meth)acrylate, acid phosphooxypoly(oxyethylene)glycol (meth)acrylate, acid phosphooxypoly(oxypropylene)glycol (meth)acrylate, etc.

[0043] (xvii) Polymerizable unsaturated monomers having an ultraviolet absorbing functional group: for example, 2-hydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone, 2-hydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, 2,2'-dihydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone, 2,2'-dihydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, 2-(2'-hydroxy-5'-methacryloyloxyethylphenyl)-2H-benzotriazole, etc.

[0044] (xviii) Light-stable polymerizable unsaturated monomers: for example, 4-(meth)acryloyloxy-1,2,2,6,6-pentamethylpiperidine, 4-(meth)acryloyloxy-2,2,6,6-tetramethylpiperidine, 4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 1-(meth)acryloyl-4-(meth)acryloylamino-2,2, 6,6-tetramethylpiperidine, 1-(meth)acryloyl-4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, 4-crotonoylamino-2,2,6,6-tetramethylpiperidine, 1-crotonoyl-4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, etc. (xix) Polymerizable unsaturated monomers having a carbonyl group: for example, acrolein, diacetone acrylamide, diacetone methacrylamide, acetoacetoxyethyl methacrylate, formyl styrene, vinyl alkyl ketones having 4 to 7 carbon atoms (for example, vinyl methyl ketone, vinyl ethyl ketone, vinyl butyl ketone), etc.

[0045] The hydroxyl group-containing acrylic resin (A21) can also be used in combination with a so-called urethane-modified acrylic resin, which is obtained by subjecting some of the hydroxyl groups in the resin to a urethane reaction with a polyisocyanate compound to elongate and increase the molecular weight.

[0046] The content of the hydroxyl group-containing polymerizable unsaturated monomer (a) is preferably 1 mass % or more, more preferably 2 mass % or more, and even more preferably 3 mass % or more, based on the total amount of the hydroxyl group-containing polymerizable unsaturated monomer (a) and the other polymerizable unsaturated monomer (b), and is preferably 50 mass % or less, more preferably 40 mass % or less, and even more preferably 30 mass % or less.

[0047] From the viewpoint of the image clarity and smoothness of the coating film to be formed, the hydroxyl value of the hydroxyl-containing acrylic resin (A21) is preferably at least 1 mgKOH / g, more preferably at least 2 mgKOH / g, even more preferably at least 5 mgKOH / g, and is preferably at most 200 mgKOH / g, more preferably at most 150 mgKOH / g, even more preferably at most 100 mgKOH / g.

[0048] From the viewpoint of the image clarity and smoothness of the coating film to be formed, the acid value of the hydroxyl group-containing acrylic resin (A21) is preferably 1 mgKOH / g or more, more preferably 2 mgKOH / g or more, even more preferably 5 mgKOH / g or more, and is preferably 200 mgKOH / g or less, more preferably 150 mgKOH / g or less, and even more preferably 80 mgKOH / g or less.

[0049] The weight-average molecular weight of the hydroxyl group-containing acrylic resin (A21) is preferably 2,000 or more, more preferably 4,000 or more, and even more preferably 8,000 or more, from the viewpoint of the image clarity and smoothness of the coating film to be formed, and is preferably 5,000,000 or less, more preferably 1,000,000 or less, and even more preferably 500,000 or less.

[0050] In this specification, the number average molecular weight and weight average molecular weight are values ​​obtained by converting the retention time (retention volume) measured using gel permeation chromatography (GPC) into the molecular weight of polystyrene based on the retention time (retention volume) of a standard polystyrene of known molecular weight measured under the same conditions. Specifically, the gel permeation chromatography apparatus used was "HLC-8120GPC" (trade name, manufactured by Tosoh Corporation), and four columns, "TSKgel G4000HXL," "TSKgel G3000HXL," "TSKgel G2500HXL," and "TSKgel G2000HXL" (trade names, all manufactured by Tosoh Corporation), and a differential refractometer was used as the detector. The mobile phase was tetrahydrofuran, the measurement temperature was 40°C, and the flow rate was 1 mL / min.

[0051] When the coating composition according to the present invention is an aqueous coating, from the viewpoint of the smoothness of the coating film to be formed, the hydroxyl-containing acrylic resin (A21) preferably comprises a water-dispersible hydroxyl-containing acrylic resin (A21') (hereinafter sometimes abbreviated as "water-dispersible hydroxyl-containing acrylic resin (A21')") having a core / shell multilayer structure comprising a core of copolymer (I) obtained by copolymerizing a polymerizable unsaturated monomer (c) having at least two polymerizable unsaturated groups per molecule and a polymerizable unsaturated monomer (d) having one polymerizable unsaturated group per molecule, and a shell of copolymer (II) obtained by copolymerizing the hydroxyl-containing polymerizable unsaturated monomer (a) and the other polymerizable unsaturated monomer (b).

[0052] Examples of the polymerizable unsaturated monomer (c) having at least two polymerizable unsaturated groups per molecule that constitute the core include allyl (meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, ) acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, glycerol di(meth)acrylate, 1,1,1-trishydroxymethylethane di(meth)acrylate, 1,1,1-trishydroxymethylethane tri(meth)acrylate, 1,1,1-trishydroxymethylpropane tri(meth)acrylate, triallyl isocyanurate, diallyl terephthalate, divinylbenzene, and the like, which can be used alone or in combination of two or more.

[0053] The content of the polymerizable unsaturated monomer (c) having at least two polymerizable unsaturated groups in one molecule is preferably 0.1% by mass or more, and preferably 30% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, based on the total mass of the monomers (c) and (d).

[0054] The polymerizable unsaturated monomer (d) having one polymerizable unsaturated group per molecule that constitutes the core is a polymerizable unsaturated monomer copolymerizable with the polymerizable unsaturated monomer (c) having at least two polymerizable unsaturated groups per molecule, and includes compounds containing one polymerizable unsaturated group per molecule, such as a vinyl group, a (meth)acryloyl group, or an allyl group.

[0055] Specific examples of the polymerizable unsaturated monomer (d) having one polymerizable unsaturated group per molecule include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, and tridecyl (meth)acrylate. alkyl or cycloalkyl (meth)acrylates such as lauryl (meth)acrylate, stearyl (meth)acrylate, "isostearyl acrylate" (trade name, manufactured by Osaka Organic Chemical Industry Co., Ltd.), cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, tert-butylcyclohexyl (meth)acrylate, cyclododecyl (meth)acrylate; polymerizable unsaturated monomers having an isobornyl group such as isobornyl (meth)acrylate; polymerizable unsaturated monomers having an adamantyl group such as adamantyl (meth)acrylate unsaturated monomers; vinyl aromatic compounds such as styrene, α-methylstyrene, and vinyltoluene; monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; ε-caprolactone-modified products of the monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms; allyl alcohol; and polyoxyethylene having a hydroxyl group at the molecular terminal.

[0033] Examples of the polymerizable unsaturated monomers include hydroxyl group-containing polymerizable unsaturated monomers such as (meth)acrylates having a chain; carboxyl group-containing polymerizable unsaturated monomers such as (meth)acrylic acid, maleic acid, crotonic acid, and β-carboxyethyl acrylate; and nitrogen-containing polymerizable unsaturated monomers such as (meth)acrylonitrile, (meth)acrylamide, dimethylaminopropyl (meth)acrylamide, dimethylaminoethyl (meth)acrylate, and adducts of glycidyl (meth)acrylate and amines, and these can be used either alone or in combination of two or more.

[0056] On the other hand, as described above, examples of the hydroxyl group-containing polymerizable unsaturated monomer (a) that constitutes the shell include monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; ε-caprolactone-modified monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms; allyl alcohol; and (meth)acrylates having a polyoxyethylene chain whose molecular terminal is a hydroxyl group, and these can be used either alone or in combination of two or more.

[0057] The content of the hydroxyl group-containing polymerizable unsaturated monomer (a) is 1 mass % or more, preferably 2 mass % or more, more preferably 3 mass % or more, based on the total mass of the hydroxyl group-containing polymerizable unsaturated monomer (a) and the other polymerizable unsaturated monomer (b), and is preferably 35 mass % or less, more preferably 25 mass % or less, and even more preferably 20 mass % or less.

[0058] The other polymerizable unsaturated monomer (b) constituting the shell is as described above. These can be used alone or in combination of two or more. The other polymerizable unsaturated monomer (b) preferably contains a carboxyl group-containing polymerizable unsaturated monomer (e) as at least a part of its components, in order to ensure the smoothness of the coating film to be formed.

[0059] Examples of the carboxyl group-containing polymerizable unsaturated monomer (e) include (meth)acrylic acid, maleic acid, crotonic acid, and β-carboxyethyl acrylate, with (meth)acrylic acid being particularly preferred.

[0060] From the viewpoint of the stability of the water-dispersible hydroxyl-containing acrylic resin (A21') in an aqueous medium, the content of the carboxyl-containing polymerizable unsaturated monomer (e) is preferably 1 mass % or more, and preferably 40 mass % or less, more preferably 25 mass % or less, and even more preferably 19 mass % or less, based on the total mass of the hydroxyl-containing polymerizable unsaturated monomer (a) and the other polymerizable unsaturated monomer (b).

[0061] The hydroxyl value of the water-dispersible hydroxyl-containing acrylic resin (A21′) is preferably 1 mgKOH / g or more, more preferably 2 mgKOH / g or more, even more preferably 5 mgKOH / g or more, from the viewpoint of the image clarity and smoothness of the coating film formed, and is preferably 100 mgKOH / g or less, more preferably 90 mgKOH / g or less, even more preferably 85 mgKOH / g or less.

[0062] In addition, the acid value of the water-dispersible hydroxyl group-containing acrylic resin (A21') is preferably 3 mgKOH / g or more, more preferably 4 mgKOH / g or more, even more preferably 5 mgKOH / g or more, from the viewpoint of the image clarity and smoothness of the coating film formed, and is preferably 90 mgKOH / g or less, more preferably 70 mgKOH / g or less, even more preferably 50 mgKOH / g or less.

[0063] The water-dispersible hydroxyl-containing acrylic resin (A21′) can be obtained, for example, by emulsion polymerizing a monomer mixture (I) containing 0.1 to 30% by mass of the polymerizable unsaturated monomer (c) having at least two polymerizable unsaturated groups per molecule and 70 to 99.9% by mass of the polymerizable unsaturated monomer (d) having one polymerizable unsaturated group per molecule, adding to the resulting emulsion a monomer mixture (II) containing 1 to 35% by mass of the hydroxyl-containing polymerizable unsaturated monomer (a) and 65 to 99% by mass of the other polymerizable unsaturated monomer (b), and further polymerizing the resulting emulsion.

[0064] The emulsion polymerization of the monomer mixture can be carried out by a known method, for example, using a polymerization initiator in the presence of an emulsifier. Suitable emulsifiers include anionic emulsifiers or nonionic emulsifiers. Examples of the anionic emulsifier include sodium salts and ammonium salts of organic acids such as alkylsulfonic acid, alkylbenzenesulfonic acid, and alkylphosphoric acid. Examples of the nonionic emulsifier include polyoxyethylene oleyl ether, polyoxyethylene stearyl ether, polyoxyethylene lauryl ether, polyoxyethylene tridecyl ether, polyoxyethylene phenyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene monolaurate, polyoxyethylene monostearate, polyoxyethylene monooleate, sorbitan monolaurate, sorbitan monostearate, sorbitan trioleate, and polyoxyethylene sorbitan monolaurate.

[0065] The emulsifier may be a polyoxyalkylene group-containing anionic emulsifier having an anionic group and a polyoxyalkylene group such as a polyoxyethylene group or a polyoxypropylene group in one molecule, or a reactive anionic emulsifier having the anionic group and a radically polymerizable unsaturated group in one molecule, and among these, it is preferable to use a reactive anionic emulsifier.

[0066] Examples of the reactive anionic emulsifier include sodium salts and ammonium salts of sulfonic acid compounds having a radically polymerizable unsaturated group such as a (meth)allyl group, a (meth)acryloyl group, a propenyl group, or a butenyl group. Among these, ammonium salts of sulfonic acid compounds having a radically polymerizable unsaturated group are preferred because they provide excellent water resistance to the formed coating film. Examples of the ammonium salts of sulfonic acid compounds include commercially available products such as "Latemul S-180A" (trade name, manufactured by Kao Corporation).

[0067] Among the ammonium salts of sulfonic acid compounds having a radically polymerizable unsaturated group, ammonium salts of sulfonic acid compounds having a radically polymerizable unsaturated group and a polyoxyalkylene group are more preferred. Examples of the ammonium salts of sulfonic acid compounds having a radically polymerizable unsaturated group and a polyoxyalkylene group include commercially available products such as "Aqualon KH-10" (trade name, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) and "SR-1025A" (trade name, manufactured by Asahi Denka Kogyo Co., Ltd.).

[0068] The amount of the emulsifier is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, and preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, based on the total amount of all monomers used.

[0069] The polymerization initiator may be either oil-soluble or water-soluble, and examples thereof include organic peroxides such as benzoyl peroxide, octanoyl peroxide, lauroyl peroxide, stearoyl peroxide, cumene hydroperoxide, tert-butyl peroxide, tert-butyl peroxylaurate, tert-butylperoxyisopropyl carbonate, tert-butyl peroxyacetate, and diisopropylbenzene hydroperoxide; azobisisobutyronite; Examples of suitable polymerization initiators include azo compounds such as azobis(2,4-dimethylvaleronitrile), azobis(2-methylpropionitrile), azobis(2-methylbutyronitrile), 4,4'-azobis(4-cyanobutanoic acid), dimethylazobis(2-methylpropionate), azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and azobis{2-methyl-N-[2-(1-hydroxybutyl)]-propionamide}; and persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate. These can be used alone or in combination of two or more. Furthermore, if necessary, a reducing agent such as a sugar, sodium formaldehyde sulfoxylate, or an iron complex can be used in combination with the polymerization initiator to form a redox polymerization system.

[0070] The content of the polymerization initiator is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and preferably 5% by mass or less, more preferably 3% by mass or less, based on the total mass of all monomers used. The method of adding the polymerization initiator is not particularly limited and can be appropriately selected depending on the type, amount, etc. For example, the polymerization initiator may be previously contained in the monomer mixture or aqueous medium, or may be added all at once or dropwise during polymerization.

[0071] The water-dispersible hydroxyl-containing acrylic resin (A21') can be obtained by adding a monomer mixture (II) containing the hydroxyl-containing polymerizable unsaturated monomer (a) and the other polymerizable unsaturated monomer (b) to the emulsion obtained as described above, and further polymerizing the mixture.

[0072] The monomer mixture (II) may contain, as necessary, the above-mentioned polymerization initiator, chain transfer agent, reducing agent, emulsifier, and other components. The monomer mixture (II) may be added dropwise as is, but it is preferable to disperse the monomer mixture (II) in an aqueous medium and add it dropwise as a monomer emulsion. The particle size of the monomer emulsion is not particularly limited.

[0073] The polymerization of the monomer mixture (II) can be carried out, for example, by adding the optionally emulsified monomer mixture (II) all at once or dropwise in small amounts to the emulsion, and heating the mixture to an appropriate temperature while stirring.

[0074] The water-dispersible hydroxyl-containing acrylic resin (A21') obtained as described above can have a core / shell multilayer structure in which the core is a copolymer (I) formed from the monomer mixture (I) containing the polymerizable unsaturated monomer (c) having at least two polymerizable unsaturated groups per molecule and the polymerizable unsaturated monomer (d) having one polymerizable unsaturated group per molecule, and the shell is a copolymer (II) formed from the monomer mixture (II) containing the hydroxyl-containing polymerizable unsaturated monomer (a) and the other polymerizable unsaturated monomer (b).

[0075] The water-dispersible hydroxyl group-containing acrylic resin (A21′) may be made into resin particles consisting of three or more layers by adding a step of supplying a polymerizable unsaturated monomer (one or a mixture of two or more) that forms another resin layer and carrying out emulsion polymerization between the step of obtaining the copolymer (I) and the step of obtaining the copolymer (II).

[0076] In the present invention, the "shell" of the water-dispersible hydroxyl-containing acrylic resin (A21') refers to the polymer layer present in the outermost layer of the resin particle, the "core" refers to the polymer layer present in the inner layer of the resin particle excluding the shell portion, and the "core / shell structure" refers to a structure having the core and shell. The core / shell structure is generally a layer structure in which the core is completely covered by the shell, but depending on the mass ratio of the core to the shell, the amount of shell monomer may be insufficient to form a layer structure. In such cases, the complete layer structure described above is not necessary, and the core may be partially covered by the shell, or a polymerizable unsaturated monomer, a component of the shell, may be graft-polymerized onto a portion of the core. The concept of a multilayer structure in the core / shell structure also applies when a multilayer structure is formed on the core of the water-dispersible hydroxyl-containing acrylic resin (A21').

[0077] The ratio of the copolymer (I) to the copolymer (II) in the water-dispersible hydroxyl-containing acrylic resin (A21') having a core / shell multilayer structure is, from the viewpoint of image sharpness and smoothness of the coating film to be formed, preferably in the range of 10 / 90 to 90 / 10, more preferably in the range of 50 / 50 to 85 / 15, and even more preferably in the range of 65 / 35 to 80 / 20, in terms of the solids mass ratio of copolymer (I) / copolymer (II) based on 100 parts by mass of the total of copolymer (I) and copolymer (II).

[0078] The water-dispersible hydroxyl-containing acrylic resin (A21') obtained as described above preferably has an average particle size of 10 nm or more, more preferably 20 nm or more, and preferably 1,000 nm or less, more preferably 500 nm or less.

[0079] In this specification, the average particle size of the hydroxyl group-containing acrylic resin is a value measured at 20°C using a particle size distribution analyzer based on the dynamic light scattering method after dilution with deionized water in a conventional manner. As a particle size distribution analyzer based on the dynamic light scattering method, for example, "ELSZ-2000" (trade name, manufactured by Otsuka Electronics Co., Ltd.) can be used.

[0080] In order to improve the mechanical stability of the resulting aqueous dispersion particles of the water-dispersible hydroxyl-containing acrylic resin (A21'), it is desirable to neutralize the acidic groups, such as carboxyl groups, of the water-dispersible hydroxyl-containing acrylic resin (A21') with a neutralizing agent. Any neutralizing agent can be used as long as it is capable of neutralizing the acidic groups, and examples of such neutralizing agents include sodium hydroxide, potassium hydroxide, trimethylamine, 2-(dimethylamino)ethanol, 2-amino-2-methyl-1-propanol, triethylamine, and aqueous ammonia. These neutralizing agents are desirably used in an amount such that the pH of the aqueous dispersion of the dispersible hydroxyl-containing acrylic resin (A21') after neutralization is about 6.5 to about 9.0.

[0081] When the coating composition according to the present invention contains the above-mentioned acrylic resin (A2), the content of the acrylic resin (A2) is, from the viewpoint of the image clarity and smoothness of the coating film to be formed, preferably 1 mass % or more, more preferably 10 mass % or more, even more preferably 15 mass % or more, based on the total resin solid content in the coating composition, and is preferably 60 mass % or less, more preferably 55 mass % or less, even more preferably 50 mass % or less.

[0082] [Polyester Resin (A3)] The polyester resin (A3) preferably contains a hydroxyl group-containing polyester resin (A31) from the viewpoint of the image clarity of the coating film formed, etc. The hydroxyl group-containing polyester resin (A31) can be obtained, for example, by using a polyol having two or more hydroxyl groups as the alcohol component and a polycarboxylic acid having two or more carboxyl groups as the acid component, and condensing the alcohol component and the acid component.

[0083] The polyol may be a polyhydric alcohol having two or more hydroxyl groups in one molecule, such as ethylene glycol, propylene glycol, diethylene glycol, trimethylene glycol, tetraethylene glycol, triethylene glycol, dipropylene glycol, 1,4-butanediol, 1,3-butanediol, 2,3-butanediol, 1,2-butanediol, 2-methyl-1,3-propanediol, 3-methyl-1,2-butanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,2-pentanediol, 1,5-pentanediol, 1,4-pentanediol, 2,4-pentanediol, 2,3-dimethyltrimethylene glycol, tetramethylene glycol, 3-methyl-4,3-pentanediol, 3-methyl-1,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 2,5-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, tricyclohexane Examples of the polyol include dihydric alcohols such as rodecanedimethanol, hydroxypivalic acid neopentyl glycol ester, hydrogenated bisphenol A, hydrogenated bisphenol F, and dimethylolpropionic acid; polylactone diols obtained by adding a lactone compound such as ε-caprolactone to these dihydric alcohols; ester diol compounds such as bis(hydroxyethyl)terephthalate; polyether diol compounds such as alkylene oxide adducts of bisphenol A, polyethylene glycol, polypropylene glycol, and polybutylene glycol; trihydric or higher alcohols such as glycerin, trimethylolethane, trimethylolpropane, diglycerin, triglycerin, 1,2,6-hexanetriol, pentaerythritol, dipentaerythritol, tris(2-hydroxyethyl)isocyanuric acid, sorbitol, and mannite; polylactone polyol compounds obtained by adding a lactone compound such as ε-caprolactone to these trihydric or higher alcohols; and fatty acid esters of glycerin.

[0084] Alcohol components other than the above polyols can also be used, and examples of such alcohol components include, but are not limited to, monoalcohols such as methanol, ethanol, propyl alcohol, butyl alcohol, stearyl alcohol, and 2-phenoxyethanol; and alcohol compounds obtained by reacting a monoepoxy compound such as propylene oxide, butylene oxide, or "Cardura E10" (trade name, manufactured by HEXION, a glycidyl ester of synthetic highly branched saturated fatty acid) with an acid.

[0085] The polycarboxylic acid may be a compound that is commonly used in the production of polyester resins, such as an aliphatic polybasic acid, an alicyclic polybasic acid, or an aromatic polybasic acid.

[0086] The aliphatic polybasic acid is generally an aliphatic compound having two or more carboxyl groups per molecule, an acid anhydride of the aliphatic compound, or an ester of the aliphatic compound. Examples of the aliphatic polybasic acid include aliphatic polycarboxylic acids such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, brassylic acid, octadecanedioic acid, citric acid, and butanetetracarboxylic acid; anhydrides of the aliphatic polybasic carboxylic acids; and lower alkyl esters of the aliphatic polybasic carboxylic acids having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms. The aliphatic polybasic acids can be used alone or in combination of two or more.

[0087] The alicyclic polybasic acids are generally compounds having one or more alicyclic structures and two or more carboxyl groups per molecule, acid anhydrides of such compounds, and esterified products of such compounds. The alicyclic structures can be primarily 4- to 6-membered ring structures. Examples of alicyclic polybasic acids include alicyclic polycarboxylic acids such as 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4-cyclohexene-1,2-dicarboxylic acid, 3-methyl-1,2-cyclohexanedicarboxylic acid, 4-methyl-1,2-cyclohexanedicarboxylic acid, 1,2,4-cyclohexanetricarboxylic acid, and 1,3,5-cyclohexanetricarboxylic acid; anhydrides of such alicyclic polycarboxylic acids; and lower alkyl esters of such alicyclic polycarboxylic acids having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms. The alicyclic polybasic acids can be used alone or in combination of two or more.

[0088] The aromatic polybasic acid is generally an aromatic compound having two or more carboxyl groups per molecule, an acid anhydride of the aromatic compound, or an ester of the aromatic compound. Examples of the aromatic polybasic acid include aromatic polycarboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, trimellitic acid, and pyromellitic acid; anhydrides of the aromatic polycarboxylic acids; and lower alkyl esters of the aromatic polycarboxylic acids having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms. The aromatic polybasic acids can be used alone or in combination of two or more. Phthalic acid, phthalic anhydride, isophthalic acid, trimellitic acid, and trimellitic anhydride are preferred as the aromatic polybasic acid, with trimellitic anhydride being particularly preferred.

[0089] Acid components other than the above-mentioned aliphatic polybasic acids, alicyclic polybasic acids, and aromatic polybasic acids can also be used. Examples of such acid components include, but are not limited to, fatty acids such as coconut oil fatty acids, cottonseed oil fatty acids, hempseed oil fatty acids, rice bran oil fatty acids, fish oil fatty acids, tall oil fatty acids, soybean oil fatty acids, linseed oil fatty acids, tung oil fatty acids, rapeseed oil fatty acids, castor oil fatty acids, dehydrated castor oil fatty acids, and safflower oil fatty acids; monocarboxylic acids such as lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, benzoic acid, p-tert-butylbenzoic acid, cyclohexanoic acid, and 10-phenyloctadecanoic acid; and hydroxycarboxylic acids such as lactic acid, 3-hydroxybutanoic acid, and 3-hydroxy-4-ethoxybenzoic acid. These acid components can be used alone or in combination of two or more.

[0090] The method for producing the hydroxyl-containing polyester resin (A31) is not particularly limited and can be carried out according to a conventional method. For example, the hydroxyl-containing polyester resin (A31) can be produced by heating the alcohol component and the acid component in a nitrogen stream at about 150 to 250°C for about 5 to 10 hours to cause an esterification reaction or transesterification reaction between the alcohol component and the acid component.

[0091] When the alcohol component and the acid component are subjected to an esterification reaction or transesterification reaction, these components may be added to a reaction vessel all at once, or one or both may be added in several portions. Alternatively, a hydroxyl-containing polyester resin may be first synthesized, and then the resulting hydroxyl-containing polyester resin may be reacted with an acid anhydride to half-esterify it into a carboxyl- and hydroxyl-containing polyester resin. Alternatively, a carboxyl-containing polyester resin may be first synthesized, and then the alcohol component may be added to produce the hydroxyl-containing polyester resin.

[0092] In the above-mentioned esterification or transesterification reaction, a catalyst known per se, such as dibutyltin oxide, antimony trioxide, zinc acetate, manganese acetate, cobalt acetate, calcium acetate, lead acetate, tetrabutyl titanate, or tetraisopropyl titanate, can be used as a catalyst for promoting the reaction.

[0093] The hydroxyl-containing polyester resin (A31) can be modified with a fatty acid, a monoepoxy compound, a polyisocyanate compound, or the like during or after production of the resin. Examples of the fatty acid include coconut oil fatty acid, cottonseed oil fatty acid, hempseed oil fatty acid, rice bran oil fatty acid, fish oil fatty acid, tall oil fatty acid, soybean oil fatty acid, linseed oil fatty acid, tung oil fatty acid, rapeseed oil fatty acid, castor oil fatty acid, dehydrated castor oil fatty acid, and safflower oil fatty acid. A suitable example of the monoepoxy compound is "Cardura E10P" (trade name, manufactured by HEXION, a glycidyl ester of a synthetic highly branched saturated fatty acid). The polyisocyanate compound can be one of the polyisocyanate compounds exemplified below as the polyisocyanate compound (B2). These compounds can be used alone or in combination of two or more.

[0094] From the viewpoint of the image clarity and smoothness of the coating film to be formed, the hydroxyl value of the hydroxyl-containing polyester resin (A31) is preferably at least 1 mgKOH / g, more preferably at least 2 mgKOH / g, even more preferably at least 5 mgKOH / g, and is preferably at most 200 mgKOH / g, more preferably at most 180 mgKOH / g, even more preferably at most 170 mgKOH / g.

[0095] From the viewpoint of the image sharpness and smoothness of the coating film to be formed, the weight-average molecular weight of the hydroxyl group-containing polyester resin (A31) is preferably at least 500, more preferably at least 1,000, even more preferably at least 1,200, and is preferably at most 50,000, more preferably at most 30,000, even more preferably at most 20,000.

[0096] From the viewpoint of the smoothness of the coating film to be formed, the glass transition temperature (Tg) of the hydroxyl-containing polyester resin (A31) is preferably −20° C. or higher, more preferably −10° C. or higher, even more preferably −5° C. or higher, and is preferably 50° C. or lower, more preferably 40° C. or lower, and even more preferably 35° C. or lower. In this specification, the glass transition temperature (Tg) of the hydroxyl-containing polyester resin is measured by differential thermal analysis (DSC) using a differential scanning calorimeter.

[0097] The hydroxyl group-containing polyester resin (A31) preferably contains a carboxyl group from the viewpoint of the image clarity and smoothness of the coating film to be formed. When the polyester resin (A31) contains a carboxyl group, the acid value of the hydroxyl group-containing polyester resin (A31) is preferably more than 0 mgKOH / g, more preferably 5 mgKOH / g or more, even more preferably 10 mgKOH / g or more, still more preferably 15 mgKOH / g or more, and is preferably 150 mgKOH / g or less, more preferably 140 mgKOH / g or less, and even more preferably 120 mgKOH / g or less, from the viewpoint of the image clarity and smoothness of the coating film to be formed.

[0098] When the coating composition according to the present invention contains the polyester resin (A3), the content of the polyester resin (A3) is, from the viewpoint of the image clarity and smoothness of the coating film to be formed, preferably 1 mass % or more, more preferably 5 mass % or more, even more preferably 10 mass % or more, based on the total resin solid content in the coating composition, and is preferably 45 mass % or less, more preferably 40 mass % or less, even more preferably 35 mass % or less.

[0099] [Curing Agent (B)] The curing agent (B) is not particularly limited. Examples of the curing agent (B) include curing agents that are reactive with the crosslinkable functional groups of the film-forming resin (A). Examples of the curing agent (B) include known curing agents, specifically, amino resins (B1), polyisocyanate compounds (B2), blocked polyisocyanate compounds (B3), polyhydrazide compounds, polysemicarbazide compounds, carbodiimide compounds, oxazoline group-containing compounds, epoxy compounds, polycarboxylic acids, etc. The curing agent (B) may be used alone or in combination of two or more.

[0100] From the viewpoint of smoothness of the coating film to be formed, the curing agent (B) preferably contains at least one crosslinking agent selected from an amino resin (B1), a polyisocyanate compound (B2), and a blocked polyisocyanate compound (B3), more preferably contains an amino resin (B1) and / or a blocked polyisocyanate compound (B3), and even more preferably contains an amino resin (B1) and a blocked polyisocyanate compound (B3).

[0101] [Amino Resin (B1)] The amino resin (B1) may be a partially methylolated amino resin or a fully methylolated amino resin obtained by reacting an amino component with an aldehyde component. Examples of the amino component include melamine, urea, benzoguanamine, acetoguanamine, steroguanamine, spiroguanamine, and dicyandiamide. Examples of the aldehyde component include formaldehyde, paraformaldehyde, acetaldehyde, and benzaldehyde.

[0102] The amino resin (B1) may be the above-mentioned methylolated amino resin in which the methylol groups have been partially or completely etherified with an appropriate alcohol, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, 2-ethyl-1-butanol, or 2-ethyl-1-hexanol.

[0103] The amino resin (B1) is preferably a melamine resin. In particular, a methyl-etherified melamine resin in which the methylol groups of a partially or fully methylolated melamine resin are partially or completely etherified with methyl alcohol, a butyl-etherified melamine resin in which the methylol groups of a partially or fully methylolated melamine resin are partially or completely etherified with butyl alcohol, or a methyl-butyl mixed etherified melamine resin in which the methylol groups of a partially or fully methylolated melamine resin are partially or completely etherified with methyl alcohol and butyl alcohol, and a methyl-butyl mixed etherified melamine resin is more preferred.

[0104] The weight average molecular weight of the melamine resin is preferably 450 or more, more preferably 500 or more, and even more preferably 550 or more, and is preferably 6,000 or less, more preferably 4,000 or less, and even more preferably 3,000 or less.

[0105] As the melamine resin, commercially available products can be used, for example, under the trade names of "Cymel 202," "Cymel 203," "Cymel 211," "Cymel 238," "Cymel 251," "Cymel 254," "Cymel 303," "Cymel 325," "Cymel 327," "Cymel 350," "Cymel 370," "Cymel 385," "Cymel 1156," "Cymel 1158," and "Cymel 1130" (all manufactured by Allnex Japan Co., Ltd.); "U-Van 20SE60" and "U-Van 28-60" (all manufactured by Mitsui Chemicals, Inc.); and the like.

[0106] When the coating composition contains the melamine resin as the curing agent (B), the coating composition may contain, as necessary, as a curing catalyst, a sulfonic acid such as paratoluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, etc.; a neutral salt of such a sulfonic acid with an amine; a neutral salt of a phosphoric acid ester compound with an amine, etc. The amino resin (B1) can be used, for example, as a crosslinking agent for a resin containing a hydroxyl group.

[0107] [Polyisocyanate Compound (B2)] The polyisocyanate compound (B2) is a compound having two or more isocyanate groups in one molecule, and includes, for example, aliphatic polyisocyanates, alicyclic polyisocyanates, araliphatic polyisocyanates, aromatic polyisocyanates, and derivatives of the polyisocyanates.

[0108] Examples of the aliphatic polyisocyanate include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate, 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, dimer acid diisocyanate, methyl 2,6-diisocyanatohexanoate (common name: lysine diisocyanate), aliphatic diisocyanates such as 2-isocyanatoethyl 2,6-diisocyanatohexanoate, 1,6-diisocyanato-3-isocyanatomethylhexane, 1,4,8-triisocyanatooctane, 1,6,11-triisocyanatoundecane, 1,8-diisocyanato-4-isocyanatomethyloctane, 1,3,6-triisocyanatohexane, and 2,5,7-trimethyl-1,8-diisocyanato-5-isocyanatomethyloctane.

[0109] Examples of the alicyclic polyisocyanate include alicyclic diisocyanates such as 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate), methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane (common name: hydrogenated xylylene diisocyanate) or a mixture thereof, and norbornane diisocyanate; 1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane, 2-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo(2.2.1)hepta 2-(3-isocyanatopropyl)-2,6-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 3-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, 6-(2-isocyanatoethyl)-2-isocyanato alicyclic triisocyanates such as 2-isocyanatoethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, and 6-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane.

[0110] Examples of the araliphatic polyisocyanate include araliphatic diisocyanates such as 1,3- or 1,4-xylylene diisocyanate or a mixture thereof, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (common name: tetramethylxylylene diisocyanate) or a mixture thereof; and araliphatic triisocyanates such as 1,3,5-triisocyanatomethylbenzene.

[0111] Examples of the aromatic polyisocyanate include aromatic diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, 2,4'- or 4,4'-diphenylmethane diisocyanate or mixtures thereof, 2,4- or 2,6-tolylene diisocyanate or mixtures thereof, 4,4'-toluidine diisocyanate, and 4,4'-diphenyl ether diisocyanate; aromatic triisocyanates such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, and 2,4,6-triisocyanatotoluene; and aromatic tetraisocyanates such as 4,4'-diphenylmethane-2,2',5,5'-tetraisocyanate.

[0112] Examples of the derivatives of the polyisocyanates include dimers, trimers, biurets, allophanates, uretdione, uretoimine, isocyanurates, oxadiazinetrione, polymethylene polyphenyl polyisocyanates (crude MDI, polymeric MDI), crude TDI, and the like of the polyisocyanate compounds.

[0113] The polyisocyanates and derivatives thereof may be used alone or in combination of two or more thereof. Among these polyisocyanates, it is preferable to use aliphatic diisocyanates, alicyclic diisocyanates, and derivatives thereof alone or in combination of two or more thereof.

[0114] Furthermore, as the polyisocyanate compound (B2), a prepolymer obtained by subjecting the above polyisocyanate or a derivative thereof to a urethanization reaction with a polyhydric alcohol, a low-molecular-weight polyester resin or water under conditions of an excess of isocyanate groups can also be used.

[0115] When the coating composition contains a polyisocyanate compound (B2) as the curing agent (B), the coating composition may contain, as necessary, a curing catalyst such as an organometallic compound, an acid compound, a base compound, etc. The polyisocyanate compound (B2) can be used, for example, as a crosslinking agent for resins containing hydroxyl groups or amino groups.

[0116] Blocked Polyisocyanate Compound (B3) The blocked polyisocyanate compound (B3) is a compound in which the isocyanate groups of the polyisocyanate compound (B2) are blocked with a blocking agent.

[0117] Examples of the blocking agent include phenol-based agents such as phenol, cresol, xylenol, nitrophenol, ethylphenol, hydroxydiphenyl, butylphenol, isopropylphenol, nonylphenol, octylphenol, and methyl hydroxybenzoate; lactam-based agents such as ε-caprolactam, δ-valerolactam, γ-butyrolactam, and β-propiolactam; aliphatic alcohol-based agents such as methanol, ethanol, propyl alcohol, butyl alcohol, amyl alcohol, and lauryl alcohol; ether-based agents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether, and methoxymethanol; benzyl alcohol, glycolic acid, methyl glycolate, ethyl glycolate, butyl glycolate, lactic acid, methyl lactate, ethyl lactate, butyl lactate, methylol urea, methylol melamine, diacetone alcohol, 2-hydroxyethyl acrylate ... alcohol-based compounds such as diethyl methacrylate; oxime-based compounds such as formamide oxime, acetamide oxime, acetoxime, methyl ethyl ketoxime, diacetyl monooxime, benzophenone oxime, and cyclohexane oxime; active methylene-based compounds such as dimethyl malonate, diethyl malonate, ethyl acetoacetate, methyl acetoacetate, and acetylacetone; butyl mercaptan, tert-butyl mercaptan, hexyl mercaptan, tert-dodecyl mercaptan, 2-mercaptobenzothiazole, thiophenol, and methylthiophenone mercaptans such as acetanilide, acetanisidide, acetotoluide, acrylamide, methacrylamide, acetic amide, stearic acid amide, benzamide, and other acid amides; imides such as succinimide, phthalic acid imide, and maleic acid imide; amines such as diphenylamine, phenylnaphthylamine, xylidine, N-phenylxylidine, carbazole, aniline, naphthylamine, butylamine, dibutylamine, and butylphenylamine; imidazoles such as imidazole and 2-ethylimidazole;Examples of the azole compounds include urea-based compounds such as urea, thiourea, ethyleneurea, ethylenethiourea, and diphenylurea; carbamate ester-based compounds such as N-phenylphenylcarbamate; imine-based compounds such as ethyleneimine and propyleneimine; sulfite-based compounds such as sodium bisulfite and potassium bisulfite; and azole-based compounds. Examples of the azole-based compounds include pyrazoles or pyrazole derivatives such as pyrazole, 3,5-dimethylpyrazole, 3-methylpyrazole, 4-benzyl-3,5-dimethylpyrazole, 4-nitro-3,5-dimethylpyrazole, 4-bromo-3,5-dimethylpyrazole, and 3-methyl-5-phenylpyrazole; imidazoles or imidazole derivatives such as imidazole, benzimidazole, 2-methylimidazole, 2-ethylimidazole, and 2-phenylimidazole; and imidazoline derivatives such as 2-methylimidazoline and 2-phenylimidazoline. As the blocking agent, an oxime-based blocking agent, an active methylene-based blocking agent, pyrazole or a pyrazole derivative is preferred.

[0118] Furthermore, as the blocking agent, a hydroxycarboxylic acid having one or more hydroxyl groups and one or more carboxyl groups, such as hydroxypivalic acid and dimethylolpropionic acid, can also be used.

[0119] When the coating composition according to the present invention contains the above-mentioned blocked polyisocyanate, the coating composition can suitably contain a blocked polyisocyanate compound obtained by blocking the isocyanate groups with the above-mentioned hydroxycarboxylic acid and then neutralizing the carboxyl groups of the hydroxycarboxylic acid to impart water dispersibility.

[0120] The weight average molecular weight of the blocked polyisocyanate compound is preferably 20,000 or more and preferably 200,000 or less from the viewpoint of the image clarity and smoothness of the coating film to be formed.

[0121] When the coating composition contains a blocked polyisocyanate compound (B3) as the curing agent (B), the coating composition can contain, as necessary, an organometallic compound, an acid compound, a base compound, or the like as a curing catalyst.

[0122] In the coating composition according to the present invention, the content of the curing agent (B) is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, based on the total resin solid content in the coating composition, from the viewpoint of the image clarity and smoothness of the coating film to be formed, and is preferably 55% by mass or less, more preferably 50% by mass or less, even more preferably 45% by mass or less.

[0123] [Viscosity modifier (C)] The coating composition according to the present invention preferably contains a viscosity modifier (C) from the viewpoint of the image clarity of the coating film formed. Specific examples of the viscosity modifier (C) include silica-based fine powders, mineral-based viscosity modifiers, barium sulfate fine powder, polyamide (polyamide)-based viscosity modifiers such as fatty acid amides, polyamides, acrylamides, long-chain polyaminoamides, aminoamides and salts thereof (e.g., phosphates), aminoplast-based viscosity modifiers such as hydrophobically modified ethoxylate aminoplasts, organic resin fine particle viscosity modifiers, diurea-based viscosity modifiers, urethane association-type viscosity modifiers, polyacrylic acid-based viscosity modifiers (also called alkali-swelling viscosity modifiers), and cellulose-based viscosity modifiers.

[0124] From the viewpoint of the image clarity of the coating film to be formed, the viscosity modifier (C) preferably includes a mineral-based viscosity modifier, a polyacrylic acid-based viscosity modifier, a cellulose-based viscosity modifier, or a urethane associative viscosity modifier, more preferably includes at least one selected from a cellulose-based viscosity modifier, a polyacrylic acid-based viscosity modifier, and a urethane associative viscosity modifier, and even more preferably includes a urethane associative viscosity modifier. These viscosity modifiers can be used either alone or in appropriate combination of two or more.

[0125] The mineral viscosity modifier may be an inorganic layered compound viscosity modifier such as a swelling layered silicate having a 2:1 crystal structure.Specific examples include natural or synthetic smectite clay minerals such as montmorillonite, saponite, hectorite, stevensite, beidellite, nontronite, bentonite, and laponite; swelling mica clay minerals such as Na-type tetrasilicic fluorine mica, Li-type tetrasilicic fluorine mica, Na-salt-type fluorine taeniolite, and Li-type fluorine taeniolite; vermiculite; substitution products or derivatives thereof; and mixtures thereof.

[0126] Examples of the urethane associative viscosity modifier include polyether polyol-based urethane prepolymers, urethane-modified polyether viscosity modifiers, etc. Commercially available urethane associative viscosity modifiers include the ADEKA NOL series such as "ADEKA NOL UH-814N," "UH-752," "UH-756VF," "UH-420," and "UH-462" (all manufactured by ADEKA Corporation, "ADEKA NOL" is a registered trademark), "SN Thickener 621N" and "SN Thickener 623N" (all manufactured by San Nopco Corporation), "RHEOLATE 244," and "RHEOLATE 278" (all manufactured by Elementis Japan).

[0127] Examples of the polyacrylic acid viscosity modifier include sodium polyacrylate, polyacrylic acid-(meth)acrylic acid ester copolymer, etc. Commercially available polyacrylic acid viscosity modifiers include "Primal ASE-60," "Primal TT615," and "Primal RM5" (all manufactured by The Dow Chemical Company, "Primal" is a registered trademark), "SN Thickener 613," "SN Thickener 618," "SN Thickener 630," "SN Thickener 634," and "SN Thickener 636" (all manufactured by San Nopco).

[0128] The acid value of the solid content of the polyacrylic acid viscosity modifier is preferably 30 mgKOH / g or more, more preferably 80 mgKOH / g or more, and preferably 300 mgKOH / g or less, more preferably 280 mgKOH / g or less.

[0129] Examples of the cellulose-based viscosity adjuster include carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, cellulose nanofiber, etc., and from the viewpoint of the image clarity of the coating film formed, cellulose nanofiber is preferred.

[0130] The cellulose nanofibers are also called cellulose nanofibrils, fibrated cellulose, or nanocellulose crystals. From the viewpoint of the clarity of the coating film formed, the cellulose nanofibers preferably have a number-average fiber diameter of 2 nm or more and preferably 500 nm or less, more preferably 250 nm or less, and even more preferably 150 nm or less. From the viewpoint of the clarity of the coating film formed, the cellulose nanofibers preferably have a number-average fiber length of 0.1 μm or more and preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 10 μm or less.

[0131] The number average fiber diameter and number average fiber length are measured and calculated from an image obtained by, for example, dispersing a sample of cellulose nanofibers diluted with water, casting it on a carbon film-coated grid that has been hydrophilized, and observing the result with a transmission electron microscope (TEM).

[0132] The cellulose nanofibers can be prepared by defibrating a cellulose raw material and stabilizing it in water. Here, the cellulose raw material refers to various forms of materials primarily composed of cellulose, including, for example, pulp (wood pulp, pulp derived from herbs such as jute, Manila hemp, and kenaf); natural cellulose such as cellulose produced by microorganisms; regenerated cellulose obtained by dissolving cellulose in a solvent such as a cuprammonium solution or a morpholine derivative and then spinning it; and fine cellulose obtained by depolymerizing the cellulose raw material by subjecting the cellulose to mechanical treatments such as hydrolysis, alkaline hydrolysis, enzymatic decomposition, explosive treatment, or a vibration ball mill.

[0133] Anion-modified cellulose nanofibers can also be used as the cellulose nanofibers. Examples of anion-modified cellulose nanofibers include carboxylated cellulose nanofibers, carboxylmethylated cellulose nanofibers, sulfonic acid group-containing cellulose nanofibers, and phosphate group-containing cellulose nanofibers. The anion-modified cellulose nanofibers can be obtained, for example, by introducing functional groups such as carboxyl groups and carboxylmethyl groups into a cellulose raw material using a known method, washing the resulting modified cellulose to prepare a dispersion of the modified cellulose, and then defibrating this dispersion. The carboxylated cellulose is also known as oxidized cellulose.

[0134] The oxidized cellulose can be obtained, for example, by oxidizing the cellulose raw material in water using an oxidizing agent in the presence of a compound selected from the group consisting of N-oxyl compounds, bromides, iodides, or mixtures thereof. Commercially available cellulose nanofibers include, for example, Leocrysta (registered trademark) manufactured by Daiichi Kogyo Seiyaku Co., Ltd. and AuroVisco (registered trademark) manufactured by Oji Holdings Co., Ltd.

[0135] When the coating composition of the present invention contains the viscosity modifier (C), the content of the viscosity modifier (C) is, from the viewpoint of the image clarity and smoothness of the coating film to be formed, preferably 0.05 parts by mass or more, more preferably 0.10 parts by mass or more, even more preferably 0.15 parts by mass or more, based on 100 parts by mass of the total resin solids in the coating composition, and is preferably 1.00 parts by mass or less, more preferably 0.90 parts by mass or less, even more preferably 0.80 parts by mass or less.

[0136] [Other Components] The coating composition according to the present invention may further contain, as necessary, pigments, water, organic solvents, curing catalysts, dispersants, anti-settling agents, antifoaming agents, ultraviolet absorbers, light stabilizers, surface conditioners, etc.

[0137] Examples of the pigment include coloring pigments, extender pigments, and luster pigments. These pigments can be used alone or in combination of two or more. When the coating composition of the present invention contains the pigment, the content of the pigment is preferably 0.1 parts by mass or more, more preferably 1 part by mass or more, and even more preferably 3 parts by mass or more, based on 100 parts by mass of the total resin solids in the coating composition, and is preferably 200 parts by mass or less, more preferably 150 parts by mass or less, and even more preferably 120 parts by mass or less.

[0138] Examples of the color pigment include titanium oxide, zinc oxide, carbon black, molybdenum red, Prussian blue, cobalt blue, azo pigments, phthalocyanine pigments, quinacridone pigments, isoindoline pigments, threne pigments, perylene pigments, dioxazine pigments, and diketopyrrolopyrrole pigments, with titanium oxide and carbon black being preferred.

[0139] When the coating composition of the present invention contains the above-mentioned coloring pigment, the content of the coloring pigment is preferably 1 part by mass or more, more preferably 5 parts by mass or more, even more preferably 15 parts by mass or more, based on 100 parts by mass of the total resin solids in the coating composition, and is preferably 180 parts by mass or less, more preferably 150 parts by mass or less, even more preferably 130 parts by mass or less.

[0140] Examples of the extender pigment include barium sulfate, talc, clay, kaolin, barium carbonate, calcium carbonate, silica, alumina white, etc. Of the extender pigments, barium sulfate and talc are preferred.

[0141] When the coating composition of the present invention contains the above-mentioned extender pigment, the content of the extender pigment is preferably 1 part by mass or more, more preferably 5 parts by mass or more, even more preferably 10 parts by mass or more, based on 100 parts by mass of the total resin solids in the coating composition, and is preferably 180 parts by mass or less, more preferably 140 parts by mass or less, even more preferably 120 parts by mass or less.

[0142] Examples of the bright pigment include aluminum (including vapor-deposited aluminum), copper, zinc, brass, nickel, glass flakes, aluminum oxide, mica, aluminum oxide coated with titanium oxide and / or iron oxide, and mica coated with titanium oxide and / or iron oxide, with aluminum pigments being preferred. Examples of the aluminum pigment include non-leafing aluminum pigments and leafing aluminum pigments, and either can be used.

[0143] The bright pigment is preferably scaly. The longitudinal dimension of the bright pigment is preferably 1 μm or more, more preferably 5 μm or more, and preferably 100 μm or less, more preferably 40 μm or less. The thickness of the bright pigment is preferably 0.001 μm or more, more preferably 0.01 μm or more, and preferably 5 μm or less, more preferably 2 μm or less.

[0144] When the coating composition of the present invention contains the above-mentioned luster pigment, the content of the luster pigment is preferably 0.1 parts by mass or more, more preferably 1 part by mass or more, and even more preferably 3 parts by mass or more, based on 100 parts by mass of the total resin solids in the coating composition, and is preferably 100 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 25 parts by mass or less.

[0145] The coating composition of the present invention can be applied by diluting it to an appropriate viscosity by adding water and / or an organic solvent, etc., as necessary. Examples of the organic solvent include ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ester solvents such as ethyl acetate, butyl acetate, methyl benzoate, ethyl ethoxypropionate, ethyl propionate, and methyl propionate; alcohol solvents such as isopropanol, n-butanol, isobutanol, and 2-ethylhexanol; ether solvents such as tetrahydrofuran, dioxane, and dimethoxyethane; glycol ether solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, and 3-methoxybutyl acetate; aromatic hydrocarbon solvents; and aliphatic hydrocarbon solvents.

[0146] The appropriate viscosity varies depending on the coating composition, but for example, the viscosity after 1 minute at 60 rpm measured with a Brookfield viscometer at a temperature of 20°C (sometimes referred to herein as the "B60 value") is, from the viewpoint of the storage stability of the resulting coating composition, preferably 100 mPa·s or more, more preferably 300 mPa·s or more, even more preferably 500 mPa·s or more, and preferably 3,000 mPa·s or less, more preferably 2,000 mPa·s or less, even more preferably 1,500 mPa·s or less. The viscometer used here is "LVDV-I" (trade name, Brookfield, Brookfield viscometer).

[0147] From the viewpoint of the image clarity and smoothness of the coating film to be formed, the solids concentration of the coating composition according to the present invention is preferably 40% by mass or more, more preferably 43% by mass or more, even more preferably 45% by mass or more, and is preferably 60% by mass or less, more preferably 58% by mass or less, even more preferably 55% by mass or less.

[0148] The coating composition according to the present invention may be either a one-component coating or a multi-component coating, but is preferably a one-component coating from the viewpoints of excellent productivity due to the absence of a coating mixing step and simplification of maintenance of coating machines.

[0149] [Method for forming a multi-layer coating film] The coating composition of the present invention can also be suitably used for forming the first coating film when a multi-layer coating film consisting of a first coating film, a second coating film and a clearcoat coating film is formed on a substrate such as an automobile body using a 3-coat 1-bake method. In this case, the coating film formation method can be carried out according to Method I below.

[0150] <Method I> A method for forming a multilayer coating film, comprising: step (I-1): applying a first coating composition to an object to be coated to form an uncured first coating film; step (I-2): applying a second coating composition onto the uncured first coating film formed in step (I-1) to form an uncured second coating film; step (I-3): applying a clear coat coating composition onto the uncured second coating film formed in step (I-2) to form an uncured clear coat film; and step (I-4): heat-curing the uncured first coating film formed in step (I-1), the uncured second coating film formed in step (I-2), and the uncured clear coat film formed in step (I-3) all at once, wherein the first coating composition is the coating composition according to the present invention.

[0151] Examples of the substrate include outer and inner panels of automobile bodies such as passenger cars, trucks, motorcycles, and buses; automobile parts; and outer panels of household electrical appliances such as mobile phones and audio equipment. The substrate is preferably an outer or inner panel of an automobile body or an automobile part.

[0152] The material of the substrate is not particularly limited. Examples of the material include metal materials such as iron, aluminum, brass, copper, tinplate, stainless steel, zinc-plated steel, and zinc alloy (Zn-Al, Zn-Ni, Zn-Fe, etc.)-plated steel; resins such as polyethylene resin, polypropylene resin, acrylonitrile-butadiene-styrene (ABS) resin, polyamide resin, acrylic resin, vinylidene chloride resin, polycarbonate resin, polyurethane resin, and epoxy resin; plastic materials such as various FRPs; inorganic materials such as glass, cement, and concrete; wood; and fibrous materials such as paper and cloth. The material is preferably a metal material or a plastic material.

[0153] Furthermore, among the above-mentioned substrates, the surfaces to which the coating film is applied may be metal surfaces such as outer and inner panels of automobile bodies, automobile parts, household electrical appliances, and metal substrates such as steel plates that make up these products, which have been subjected to surface treatments such as phosphate treatment, chromate treatment, complex oxide treatment, etc.

[0154] A coating film may be further formed on the object, which may or may not have been subjected to the above-mentioned surface treatment. For example, a substrate to be coated may be subjected to a surface treatment as necessary, and a primer coating film may be formed thereon. For example, when the substrate to be coated is an automobile body, the primer coating film may be formed using a known primer paint that is commonly used in painting automobile bodies.

[0155] In particular, the substrate is preferably a metal substrate, which may or may not have been subjected to a surface treatment, on which an undercoat coating film is formed using an electrodeposition paint, preferably a cationic electrodeposition paint, and it is even more preferable that the undercoat coating film formed using the electrodeposition paint, preferably a cationic electrodeposition paint, has been heat-cured.

[0156] When the substrate is coated with a primer paint, an intermediate coating film may be further formed thereon. For example, when the substrate is an automobile body, the intermediate coating film may be formed using a known intermediate coating material that is commonly used in painting automobile bodies. The intermediate coating film may or may not be heat-cured.

[0157] The coating thickness of the first coating composition, as a cured film thickness, is preferably 10 μm or more, more preferably 15 μm or more, even more preferably 20 μm or more, and preferably 60 μm or less, more preferably 50 μm or less, even more preferably 40 μm or less. The coating thickness of the second coating composition, as a cured film thickness, is preferably 0.5 μm or more, more preferably 2 μm or more, even more preferably 5 μm or more, and preferably 50 μm or less, more preferably 40 μm or less, even more preferably 30 μm or less. The coating thickness of the clearcoat coating composition, as a cured film thickness, is preferably 10 μm or more, more preferably 15 μm or more, and preferably 80 μm or less, more preferably 60 μm or less.

[0158] Furthermore, preheating can be carried out after application of the first coating composition. The preheating temperature is usually 40°C or higher and 100°C or lower. The preheating time is usually 30 seconds or longer and 15 minutes or shorter. In the method for forming a multilayer coating film according to the present invention, from the viewpoint of energy saving, etc., it is preferable not to carry out preheating after application of the first coating composition.

[0159] An interval can be provided between the application of the first coating composition and the application of the second coating composition, which interval is preferably 1.5 minutes or more, more preferably 3 minutes or more, and even more preferably 4 minutes or more, from the viewpoints of the image clarity and smoothness of the coating film formed and production efficiency, and is preferably 35 minutes or less, more preferably 25 minutes or less, and even more preferably 20 minutes or less.

[0160] It is also preferable to preheat after application of the second coating composition. The preheating temperature is preferably 40°C or higher, more preferably 50°C or higher, and even more preferably 60°C or higher, and is preferably 100°C or lower, more preferably 90°C or lower, and even more preferably 80°C or lower. The preheating time is preferably 30 seconds or longer, more preferably 1 minute or longer, and even more preferably 2 minutes or longer, and is preferably 15 minutes or shorter, more preferably 10 minutes or shorter, and even more preferably 5 minutes or shorter.

[0161] After application of the above-mentioned clear coat paint composition, if necessary, an interval of preferably 1 minute or more and preferably 60 minutes or less can be allowed to pass at room temperature, or the surface can be preheated at a temperature of preferably 40°C or more and preferably 80°C or less for a period of preferably 1 minute or more and preferably 60 minutes or less.

[0162] The three-layer coating film consisting of the uncured first coating film, the uncured second coating film, and the uncured clearcoat coating film can be cured by known heating means, such as a hot air oven, an electric oven, or an infrared induction heating oven. The heating temperature for curing the three-layer coating film is preferably 60°C or higher, more preferably 65°C or higher, and even more preferably 70°C or higher, and is preferably 180°C or lower, more preferably 170°C or lower, and even more preferably 160°C or lower. The heating time for curing the three-layer coating film is preferably 10 minutes or higher, more preferably 20 minutes or higher, and preferably 60 minutes or lower, more preferably 40 minutes or lower. This heating allows the three-layer coating film consisting of the first coating film, the second coating film, and the clearcoat coating film to be cured simultaneously.

[0163] Any known thermosetting coating composition can be used as the second coating composition, for example, a thermosetting coating composition containing a base resin having a crosslinkable functional group, a crosslinking agent, a color pigment, and an extender pigment can be suitably used as the second coating composition.

[0164] Examples of crosslinkable functional groups contained in the base resin include carboxyl groups, hydroxyl groups, and epoxy groups. Examples of types of the base resin include acrylic resins, polyester resins, alkyd resins, and urethane resins. Examples of crosslinking agents include melamine resins, polyisocyanate compounds, and blocked polyisocyanate compounds.

[0165] The second coating composition may be any of an organic solvent-based coating composition, a water-based coating composition, and a powder coating composition, with the use of a water-based coating composition being preferred.

[0166] The clear coat paint composition can be any known thermosetting clear coat paint composition for painting automobile bodies, etc. Examples of the clear coat paint composition include organic solvent-based thermosetting paint compositions containing a base resin having a crosslinkable functional group, a crosslinking agent, etc., aqueous thermosetting paint compositions, and powder thermosetting paint compositions.

[0167] Examples of crosslinkable functional groups possessed by the base resin include carboxyl groups, hydroxyl groups, epoxy groups, silanol groups, etc. Types of the base resin include acrylic resins, polyester resins, alkyd resins, urethane resins, epoxy resins, fluororesins, etc. Examples of the crosslinking agent include polyisocyanate compounds, blocked polyisocyanate compounds, melamine resins, urea resins, carboxyl group-containing compounds, carboxyl group-containing resins, epoxy group-containing resins, epoxy group-containing compounds, etc.

[0168] Preferred combinations of base resin / crosslinking agent for the above clear coat paint composition include carboxyl group-containing resin / epoxy group-containing resin, hydroxyl group-containing resin / polyisocyanate compound, hydroxyl group-containing resin / blocked polyisocyanate compound, hydroxyl group-containing resin / melamine resin, etc. From the viewpoint of the weather resistance of the coating film to be formed, a preferred combination is hydroxyl group-containing resin / polyisocyanate compound.

[0169] The clear coat paint may be a one-component paint or a multi-component paint such as a two-component urethane resin paint. The clear coat paint composition may contain coloring pigments, luster pigments, dyes, etc., as needed, to the extent that transparency is not impaired, and may further contain extender pigments, ultraviolet absorbers, light stabilizers, antifoaming agents, thickeners, rust inhibitors, surface conditioners, etc., as appropriate.

[0170] In the method for forming a multilayer coating film according to the present invention, the first coating composition, the second coating composition and / or the clear coat coating can be applied to the substrate by a known method, such as air spray coating, airless spray coating, rotary atomization coating, inkjet coating, curtain coating, etc., and electrostatic application may be performed during the coating. The coating is preferably performed by air spray coating or rotary atomization coating.

[0171] The present invention will be explained in more detail below with reference to Production Examples, Examples, and Comparative Examples. However, the present invention is not limited thereto. In each example, "parts" and "%" are by mass unless otherwise specified. Furthermore, the film thickness of the coating film is based on the cured coating film.

[0172] [Production of Polyurethane Resin (A1)] [Production Example 1] A reaction vessel equipped with a thermometer, a stirrer, and a reflux condenser was charged with 211.9 parts of "PTMG1000" (trade name, manufactured by Mitsubishi Chemical Corporation, polytetramethylene ether glycol having a number average molecular weight of 1,000), 11.5 parts of 2,2-dimethylolpropionic acid, 6.9 parts of trimethylolpropane, 112.2 parts of isophorone diisocyanate, and 298.5 parts of methyl ethyl ketone. After purging the reaction system with nitrogen gas, the contents of the reaction vessel were reacted with stirring at 80°C to obtain a methyl ethyl ketone solution of an NCO-terminated urethane prepolymer having a free isocyanate group content of 3.2%. The resulting methyl ethyl ketone solution was cooled to 40°C, and 493.2 g of deionized water containing 9.8 parts of N-ethylmorpholine was added to the reaction vessel to emulsify the contents. Thereafter, 159.2 parts of a 5% aqueous ethylenediamine solution was added to the reaction vessel, and the contents were stirred for 60 minutes. The methyl ethyl ketone was then distilled off under reduced pressure and heating, and the concentration of the contents was adjusted with deionized water to obtain a hydroxyl group-free polyurethane resin (A1-1) dispersion having a solids content of 35%, an acid value of 14 mgKOH / g, and an average particle size of 120 nm.

[0173] [Production Example 2] A reaction vessel equipped with a thermometer, a stirrer, and a reflux condenser was charged with 316.0 parts of "PTMG1000" (trade name, manufactured by Mitsubishi Chemical Corporation, polytetramethylene ether glycol having a number average molecular weight of 1,000), 17.0 parts of 2,2-dimethylolpropionic acid, 21.9 parts of trimethylolpropane, 113.0 parts of isophorone diisocyanate, 0.19 parts of "Neostan U-600" (trade name, manufactured by Nitto Kasei Co., Ltd., bismuth catalyst), and 235.0 parts of methyl ethyl ketone. After purging the reaction system with nitrogen gas, the contents of the reaction vessel were reacted with stirring at 80°C to obtain a methyl ethyl ketone solution of an NCO-terminated urethane prepolymer having a free isocyanate group content of 3.2%. The resulting methyl ethyl ketone solution was cooled to 40°C, and 764.5 g of deionized water containing 13.8 parts of N-methylmorpholine was added to the reaction vessel to emulsify the contents. Thereafter, 96.4 parts of a 5% aqueous solution of N-(2-hydroxyethyl)ethylenediamine was added to the reaction vessel, and the contents were stirred for 120 minutes. The methyl ethyl ketone was then distilled off under reduced pressure and heating, and the concentration of the contents was adjusted with deionized water to obtain a hydroxyl-containing polyurethane resin (A1-2) dispersion having a solids concentration of 35%, an acid value of 15 mgKOH / g, a hydroxyl value of 12 mgKOH / g, and an average particle size of 120 nm.

[0174] [Production of Hydroxyl-Containing Acrylic Resin (A21)] [Production Example 3] A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and dropping device was charged with 100 parts of deionized water and 0.5 parts of Aqualon KH-10 (trade name, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., and polyoxyethylene alkyl ether sulfate ester ammonium salt, active ingredient 97%). The contents of the reaction vessel were stirred and mixed in a nitrogen stream, and the temperature of the contents was raised to 80°C. Next, 10.3 parts of a 1% and 3% aqueous ammonium persulfate solution, the total amount of the monomer emulsion 1 described below, was introduced into the reaction vessel, and the contents were maintained at 80°C for 15 minutes. Thereafter, the remaining monomer emulsion 1 was added dropwise to the reaction vessel over 3 hours, and after the completion of the dropwise addition, the mixture was aged for 1 hour. Thereafter, Monomer Emulsion 2 below was added dropwise to the reaction vessel over 2 hours, and the reaction vessel was aged for 1 hour. Then, 42 parts of a 5.0% aqueous dimethylethanolamine solution was gradually added to the reaction vessel while the contents were cooled to 30°C. The contents were then discharged while being filtered through a 100-mesh nylon cloth, yielding a dispersion of a hydroxyl-containing acrylic resin (A21-1) having a core / shell structure, with an average particle size of 100 nm (measured at 20°C after dilution with deionized water using a particle size distribution measuring device "ELSZ-2000" (trade name, manufactured by Otsuka Electronics Co., Ltd.)), an acid value of 32 mgKOH / g, a hydroxyl value of 43 mgKOH / g, and a solids content of 30% by mass.

[0175] Monomer emulsion 1: 70 parts of deionized water, 1 part of Aqualon KH-10 (trade name, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., polyoxyethylene alkyl ether sulfate ester ammonium salt, active ingredient 97%), 3 parts of methylene bisacrylamide, 4 parts of styrene, 13 parts of methyl methacrylate, 30 parts of ethyl acrylate, and 20 parts of n-butyl acrylate were mixed and stirred to obtain monomer emulsion 1.

[0176] Monomer emulsion 2: Monomer emulsion 2 was obtained by mixing and stirring 10 parts of deionized water, 1 part of Aqualon KH-10 (trade name, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., polyoxyethylene alkyl ether sulfate ester ammonium salt, active ingredient 97%), 0.1 parts of ammonium persulfate, 3 parts of styrene, 6 parts of methyl methacrylate, 2 parts of ethyl acrylate, 4 parts of n-butyl acrylate, 10 parts of hydroxyethyl acrylate, and 5 parts of methacrylic acid.

[0177] [Production Example 4] A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device was charged with 35 parts of propylene glycol monopropyl ether, and the contents of the reaction vessel were heated to 85°C. Then, a mixture of 30 parts of methyl methacrylate, 20 parts of 2-ethylhexyl acrylate, 29 parts of n-butyl acrylate, 15 parts of 2-hydroxyethyl acrylate, 6 parts of acrylic acid, 15 parts of propylene glycol monopropyl ether, and 2.3 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the reaction vessel over 4 hours. After completion of the addition, the contents were aged for 1 hour. Thereafter, a mixture of 10 parts of propylene glycol monopropyl ether and 1 part of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the reaction vessel over 1 hour. After completion of the addition, the contents were aged for 1 hour. Further, 7.4 parts of diethanolamine and 13 parts of propylene glycol monopropyl ether were added to the reaction vessel to obtain a solution of hydroxyl-containing acrylic resin (A21-2) with a solids concentration of 55%. The obtained hydroxyl-containing acrylic resin (A21-2) had an acid value of 47 mg KOH / g, a hydroxyl value of 72 mg KOH / g, and a weight-average molecular weight of 58,000.

[0178] [Production of Hydroxyl Group-Containing Polyester Resin (A31)] [Production Example 5] A reaction vessel equipped with a thermometer, a thermostat, a stirrer, a reflux condenser, and a water separator was charged with 174 parts of trimethylolpropane, 327 parts of neopentyl glycol, 352 parts of adipic acid, 109 parts of isophthalic acid, and 101 parts of 1,2-cyclohexanedicarboxylic anhydride, and the content of the reaction vessel was heated from 160°C to 230°C over 3 hours. The temperature was then maintained at 230°C while the generated condensed water was distilled off using the water separator, and the reaction was carried out until the acid value reached 3 mgKOH / g or less. To this reaction product, 59 parts of trimellitic anhydride was added, and an addition reaction was carried out at 170°C for 30 minutes. The contents were then cooled to below 50°C, and an equivalent amount of 2-(dimethylamino)ethanol was added relative to the acid groups to neutralize the mixture. Deionized water was then gradually added to obtain a hydroxyl-containing polyester resin (A31-1) solution with a solids concentration of 45% and a pH of 7.2. The resulting hydroxyl-containing polyester resin had a hydroxyl value of 128 mgKOH / g, an acid value of 35 mgKOH / g, and a number average molecular weight of 1,440.

[0179] [Production Example 6] A reactor equipped with a thermometer, thermostat, stirrer, reflux condenser, and water separator was charged with 236 parts of 1,6-hexanediol, 308 parts of hexahydrophthalic anhydride, and 490 parts of Cardura E10P. The contents of the reactor were reacted at 100 to 230°C for 3 hours, after which 230 parts of trimellitic anhydride was added to the reactor and the contents were subjected to a condensation reaction at 180°C. The contents were then diluted with propylene glycol monomethyl ether to obtain a hydroxyl-containing polyester resin (A31-2) solution with a solids content of 70%. The resulting hydroxyl-containing polyester resin (A31-2) had a hydroxyl value of 69 mgKOH / g, an acid value of 50 mgKOH / g, and a number average molecular weight of 1900.

[0180] [Production of Pigment Dispersion] [Production Example 7] 22.2 parts (10 parts solids) of the hydroxyl group-containing polyester resin (A31-1) solution obtained in Production Example 5, 65 parts of "JR-806" (trade name, manufactured by Teika Corporation, rutile-type titanium dioxide), 20 parts of "Variace B-35" (trade name, manufactured by Sakai Chemical Industry Co., Ltd., barium sulfate powder), 0.7 parts of "Carbon MA100" (trade name, manufactured by Mitsubishi Chemical Corporation, carbon black), and 10 parts of deionized water were mixed in a container equipped with a stirrer, and the pH of the contents of the container was adjusted to pH 8.0 with 2-(dimethylamino)ethanol. Next, the resulting mixture was placed in a wide-mouth glass bottle, and glass beads with a diameter of approximately 1.3 mm were added as a dispersion medium, the bottle was sealed, and the mixture was dispersed for 30 minutes using a paint shaker to obtain pigment dispersion (P-1).

[0181] [Production Example 8] In a container equipped with a stirrer, 14.3 parts of the hydroxyl group-containing polyester resin (A31-2) solution obtained in Production Example 6 (10 parts solids), 65 parts of "JR-806" (trade name, manufactured by Teika Corporation, rutile-type titanium dioxide), 20 parts of "Variace B-35" (trade name, manufactured by Sakai Chemical Industry Co., Ltd., barium sulfate powder), 0.7 parts of "Carbon MA100" (trade name, manufactured by Mitsubishi Chemical Corporation, carbon black), and 10 parts of deionized water were mixed, and the pH of the contents of the container was adjusted to pH 8.0 with 2-(dimethylamino)ethanol. Next, the resulting mixture was placed in a wide-mouth glass bottle, and glass beads with a diameter of approximately 1.3 mm were added as a dispersion medium, sealed, and dispersed for 30 minutes using a paint shaker to obtain a pigment dispersion (P-2).

[0182] [Production Example 9] In a container equipped with a stirrer, 18.2 parts of the hydroxyl group-containing acrylic resin (A21-2) solution obtained in Production Example 4 (10 parts solids), 65 parts of "JR-806" (trade name, manufactured by Teika Corporation, rutile-type titanium dioxide), 20 parts of "Variace B-35" (trade name, manufactured by Sakai Chemical Industry Co., Ltd., barium sulfate powder), 0.7 parts of "Carbon MA100" (trade name, manufactured by Mitsubishi Chemical Corporation, carbon black), and 10 parts of deionized water were mixed, and the pH of the contents of the container was adjusted to pH 8.0 with 2-(dimethylamino)ethanol. Next, the resulting mixture was placed in a wide-mouth glass bottle, and glass beads with a diameter of approximately 1.3 mm were added as a dispersion medium, sealed, and dispersed for 30 minutes using a paint shaker to obtain a pigment dispersion (P-3).

[0183] [Preparation of Coating Composition] [Example 1] 117.9 parts (solid content 95.7 parts) of the pigment dispersion (P-1) obtained in Production Example 7, 57.1 parts (solid content 20 parts) of "U-coat UX-8100" (trade name, manufactured by Sanyo Chemical Industries, Ltd., polyurethane resin emulsion, solid content concentration 35%), 33.3 parts (solid content 10 parts) of the hydroxyl group-containing acrylic resin (A21-1) dispersion obtained in Production Example 3, 18.2 parts (solid content 10 parts) of the hydroxyl group-containing acrylic resin (A21-2) solution obtained in Production Example 4, 14.3 parts (solid content 10 parts) of the hydroxyl group-containing polyester resin (A31-2) solution obtained in Production Example 6, and "Cymel 325" (trade name) 37.5 parts (solids content: 30 parts) of "Allnex Japan Co., Ltd., melamine resin, solids content: 80%)" and 26.3 parts (solids content: 10 parts) of "Bayhydur VPLS2310" (trade name, Sumika Bayer Urethane Co., Ltd., blocked polyisocyanate compound, solids content: 38%) were uniformly mixed, and 1.43 parts (solids content: 0.4 parts) of "UH-752" (trade name, urethane associative viscosity modifier, solids content: 28%), 2-(dimethylamino)ethanol, and deionized water were further added to obtain Coating Composition No. 1, which had a pH of 8.0 and a solids content of 50%.

[0184] [Examples 2 to 34 and Comparative Examples 1 to 6] Coating compositions Nos. 2 to 40 were obtained in the same manner as in Example 1, except that the blending compositions in Example 1 were as shown in Tables 1 to 4 below. The blending amounts of each material in the tables are expressed as solid content. In Tables 1 to 3, "polyester resin" is abbreviated as "PE," "acrylic resin" as "Ac," "polyurethane resin" as "PU," and "polyisocyanate compound" as "PNCO."

[0185] [Dynamic Viscoelasticity Measurement at a Temperature of 23°C, a Frequency of 0.50 Hz, and a Stress Measurement Range of 0.1 to 100 Pa] For coating compositions No. 1 to No. 40 obtained in Examples 1 to 34 and Comparative Examples 1 to 6, dynamic viscoelasticity measurements were carried out at a temperature of 23°C, a frequency of 0.50 Hz, and a stress measurement range of 0.1 to 100 Pa according to the method described herein, and stress σ2 (Pa), stress σ3 (Pa), tanδ1 (loss modulus / storage modulus) at a stress of 0.1 Pa, and storage modulus G'1 (Pa) were measured. The measurement results are also shown in Tables 1 to 4.

[0186] [0.1s -1 Viscosity of uncured coating film 5 minutes after application at a temperature of 23°C and a shear rate of 0.1 s -1 The viscosity at 1000 kJ / min was measured according to the following viscosity measurement method. The measurement results are also shown in Tables 1 to 4.

[0187] [Viscosity Measurement Method] The coating composition was electrostatically coated onto a tinplate measuring 45 cm long x 30 cm wide x 0.8 mm thick using a rotary atomizing electrostatic coater so that the dry film thickness was 25 μm. Five minutes after the coating composition was applied to the tinplate, a portion of the coating film was scraped off with a spatula and measured using a viscoelasticity measuring device (HAAKE RHEO STRESS 3000, manufactured by THERMO SCIENTIFIC) at a temperature of 23° C. and a shear rate of 10,000 sec -1 to 0.0001 sec -1 0.1 sec when changed to -1 The viscosity at 1000 kJ / min was measured.

[0188]

[0189]

[0190]

[0191]

[0192] The ingredients listed in the table are as follows: (Note 1) "Primal ASE-60": Trade name, manufactured by The Dow Chemical Company, polyacrylic acid viscosity modifier (Note 2) "Leocrysta I-2SX": Trade name, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., cellulose nanofiber, viscosity modifier.

[0193] [Preparation of coated test panels] [Example 35] A cold-rolled steel panel that had been subjected to a zinc phosphate conversion treatment was electrodeposited with "Elecron GT-10" (product name, cationic electrodeposition paint, manufactured by Kansai Paint Co., Ltd.) to a dry film thickness of 20 μm, and heated at 170°C for 30 minutes to cure the electrodeposition coating, thereby preparing a coated object.

[0194] [Preparation of Test Coating Plate (S1)] The first coating composition, coating composition No. 1 obtained in Example 1, was electrostatically applied onto the above substrate using a rotary atomizer electrostatic coater to a dry film thickness of 25 μm, and then left for 5 minutes to form an uncured first coating film. Next, on the uncured first coating film, "WBC-713T" (trade name, manufactured by Kansai Paint Co., Ltd., acrylic melamine resin-based water-based base coat paint, black paint) was electrostatically applied onto the uncured first coating film using a rotary atomizer electrostatic coater to a dry film thickness of 15 μm, and after leaving for 5 minutes, the coating was preheated at 80 ° C for 3 minutes to form an uncured second coating film. "KINO6510" (trade name, manufactured by Kansai Paint Co., Ltd., hydroxyl group / isocyanate group curing type acrylic resin / urethane resin-based two-component organic solvent-based clear coat paint) was electrostatically applied to the uncured second coating film to a cured film thickness of 35 μm to form an uncured clear coat coating film. The substrate having the uncured clear coat film was left to stand for 7 minutes, and then heated at 80°C for 30 minutes to simultaneously bake the uncured first coat film, the uncured second coat film, and the uncured clear coat film, to produce test coated panel No. S1-1.

[0195] [Preparation of Test Coating Plate (S2)] The first coating composition, coating composition No. 1 obtained in Example 1, was electrostatically applied onto the above substrate using a rotary atomizer electrostatic coater to a dry film thickness of 25 μm, and then allowed to stand for 15 minutes to form an uncured first coating film. Next, the second coating composition, "WBC-713T" (trade name, manufactured by Kansai Paint Co., Ltd., acrylic melamine resin-based water-based base coat paint, black paint color), was electrostatically applied onto the uncured first coating film using a rotary atomizer electrostatic coater to a dry film thickness of 15 μm, and after allowing to stand for 5 minutes, the coating was preheated at 80°C for 3 minutes to form an uncured second coating film. "KINO6510" (trade name, manufactured by Kansai Paint Co., Ltd., hydroxyl group / isocyanate group curing type acrylic resin / urethane resin-based two-component organic solvent-based clear coat paint) was electrostatically applied to the uncured second coating film to a cured film thickness of 35 μm to form an uncured clear coat coating film. The substrate with the uncured clear coat film was left to stand for 7 minutes, and then heated at 80°C for 30 minutes to simultaneously bake the uncured first coat film, the uncured second coat film, and the uncured clear coat film, to produce test coated panel No. S2-1.

[0196] [Examples 36 to 68 and Comparative Examples 7 to 12] Coated test plates No. S1-2 to No. S1-40 and No. S2-2 to No. S2-40 were prepared in the same manner as in Example 35, except that the coating compositions in Example 35 were changed to those shown in Tables 5 to 7 below.

[0197] [Evaluation of image clarity] For coated test panels No. S1-1 to No. S1-40 and No. S2-1 to No. S2-40 obtained in Examples 35 to 68 and Comparative Examples 7 to 12, image clarity was evaluated according to the following criteria based on the Wa value measured using a "Wave Scan" (trade name, manufactured by BYK Gardner). The smaller the Wa value, the higher the image clarity of the coated surface, with A and B indicating a pass. The evaluation results are shown in Tables 5 to 7. A: Wa value less than 8. B: Wa value 8 or more and less than 12. C: Wa value 12 or more.

[0198] [Evaluation of Smoothness] For coated test panels No. S1-1 to No. S1-40 and No. S2-1 to No. S2-40 obtained in Examples 35 to 68 and Comparative Examples 1 to 12, smoothness was evaluated according to the following criteria based on the Long Wave (LW) value measured using a "Wave Scan" (trade name, manufactured by BYK Gardner). The smaller the LW value, the higher the smoothness of the coated surface, with A and B indicating a pass. The evaluation results are shown in Tables 5 to 7. A: LW value less than 12. B: LW value 12 or more and less than 15. C: LW value 15 or more.

[0199]

Claims

1. A coating composition comprising a film-forming resin (A) and a curing agent (B), characterized in that, in dynamic viscoelasticity measurement under conditions of a temperature of 23°C, a frequency of 0.50 Hz, and a stress measurement range of 0.1 to 100 Pa, the stress σ2 at which the storage modulus first decreases by 10% is within the range of 3.5 to 100 Pa, based on the storage modulus G'1 at a stress of 0.1 Pa.

2. The coating composition according to claim 1, wherein the film-forming resin (A) comprises a polyurethane resin (A1).

3. The coating composition according to claim 2, wherein the content of said polyurethane resin (A1) is within the range of 1.0 to 30 mass % based on the total resin solid content in said coating composition.

4. A coating composition according to any one of claims 1 to 3, in which, in dynamic viscoelasticity measurement under conditions of a temperature of 23°C, a frequency of 0.50 Hz and a stress measurement range of 0.1 to 100 Pa, the value of tanδ1 (loss modulus / storage modulus) is less than 1 when the stress is 0.1 Pa, and the stress σ3 at which the value of tanδ first becomes 1 in the stress range exceeding 0.1 Pa is in the range of 10 to 100 Pa.

5. The coating composition according to any one of claims 1 to 4, wherein the storage modulus G'1 is in the range of 5.0 to 150 Pa.

6. A coating composition according to any one of claims 1 to 5, having a solids concentration in the range of 40 to 60 mass %.

7. The coating composition according to any one of claims 1 to 6, further comprising a viscosity modifier (C).

8. The coating composition according to claim 7, wherein the content of the viscosity modifier (C) is within the range of 0.05 to 1.00 parts by mass based on 100 parts by mass of the total resin solids in the coating composition.

9. A coating composition according to any one of claims 1 to 8, used for forming a first coating film when forming a multi-layer coating film consisting of a first coating film, a second coating film and a clear coat coating film using a three-coat, one-bake method.

10. A method for forming a multi-layer coating film, comprising: step (I-1): applying the coating composition according to any one of claims 1 to 8 onto a substrate to form an uncured first coating film; step (I-2): applying a second coating composition onto the uncured first coating film formed in step (I-1) to form an uncured second coating film; step (I-3): applying a clear coat coating composition onto the uncured second coating film formed in step (I-2) to form an uncured clear coat film; and step (I-4): simultaneously heating and curing the uncured first coating film formed in step (I-1), the uncured second coating film formed in step (I-2), and the uncured clear coat film formed in step (I-3).

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