Water-based coating composition

Abstract

To provide a water-based coating composition which suppresses occurrence of coating streaks, and enables formation of a coated film and drawing that is excellent in finishing property.SOLUTION: A water-based coating composition, which is used for a method for approaching and discharging a coating material to a coated surface by pressing a valve body that is movably formed toward a discharge surface having a discharge port for discharging the coating material, contains (A) a hydroxyl group-containing resin, (B) a pigment and (C) a melamine resin, wherein the melamine resin (C) is hydrophilic.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aqueous coating composition. [Background technology]

[0002] Inkjet and dispenser coating methods involve directly ejecting droplets from extremely fine nozzles and depositing them onto the surface to form letters and images. In recent years, these methods have come to be used not only as output devices for offices and homes, but also for industrial purposes.

[0003] For example, Patent Document 1 discloses that inkjet coating and / or topcoat coating of actinic radiation-curable ink is performed in the manufacture of building panels for the exterior and interior of houses, etc. Patent Document 2 also discloses a method for expressing a design inside a coating film using an inkjet method or a dispenser method in which a viscous material is sprayed in a design pattern onto a painted surface such as a car body. When an aqueous coating composition is used in this type of coating in which droplets of ink or paint are ejected in close proximity, the volatile components, such as water and solvent, in the droplets are applied with almost no volatilization. However, after drying, streaks often appear on the coated surface along the scanning direction of the coating head, and problems such as reduced finish quality, including smoothness and gloss, can occur. When the coated surface is a smooth substrate such as metal or plastic, these phenomena are noticeable as paint streaks. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5797314 [Patent Document 2] Japanese Patent Application Publication No. 7-108216 Summary of the Invention [Problem to be solved by the invention]

[0005] The object of the present invention is to provide an aqueous coating composition for use in a method of discharging coating material close to a surface to be coated by pressing a valve body formed to be movable toward a discharge surface having a discharge port from which the coating material is discharged, which aqueous coating composition suppresses the occurrence of paint streaks and enables the formation of coating films and drawings with excellent finish. [Means for solving the problem]

[0006] That is, the present invention provides the following <1> and <2> It is related to. <1> An aqueous coating composition used in a method for discharging a coating material in proximity to a substrate by pressing a valve body formed to be movable toward a discharge surface having a discharge port through which the coating material is discharged, the aqueous coating composition comprising: (A) a hydroxyl group-containing resin; (B) a pigment; and (C) a melamine resin; An aqueous coating composition, wherein the melamine resin (C) is hydrophilic. <2> The melamine resin (C) is an alkyl-etherified melamine resin, and the content ratio of methyl groups to butyl groups is 10 / 90 to 100 / 0 in terms of a molar ratio of (methyl groups) / (butyl groups). <1> The aqueous coating composition according to claim 1. [Effects of the Invention]

[0007] The aqueous coating composition of the present invention can suppress the occurrence of coating streaks when used with an inkjet system or a dispenser system, and can form coating films and patterns with excellent finishes. DETAILED DESCRIPTION OF THE INVENTION

[0008] The following describes aspects of the present invention.

[0009] The aqueous coating composition of the present invention is an aqueous coating composition used in a method for discharging the coating material close to a surface to be coated by pressing a valve body formed to be movable toward a discharge surface having a discharge port from which the coating material is discharged, and is particularly suitable for use in a piezoelectric jet dispenser.

[0010] Generally, a jet dispenser is a device that continuously dispenses liquid material in droplet form at high speed. A plunger, which acts as a valve body, is rapidly advanced toward the outlet in a liquid chamber having an outlet, and then suddenly stopped, thereby discharging the liquid material in droplet form from the outlet. The plunger drive method for these devices includes mechanical and piezo-jet types that use the reciprocating motion of the plunger or rod by air pressure, spring force, or displacement of a piezoelectric element.

[0011] Here, the term "piezo type" refers to an electromechanical conversion method or piezo element method in which a piezo element is driven to eject ink or paint from the inside of the element as minute droplets from a nozzle.

[0012] An example of a jet dispenser is "X JET" (trade name, manufactured by SSI JAPAN, high-viscosity micro-piezo jet dispenser), but is not limited to this.

[0013] The frequency at which droplets are ejected from the dispenser is preferably within the range of 10 to 10,000 Hz, more preferably within the range of 30 to 5,000 Hz, and even more preferably within the range of 50 to 3,000 Hz, from the viewpoints of preventing the occurrence of paint streaks and improving coating efficiency.

[0014] The distance between the nozzle when droplets are ejected from the dispenser and the object to be coated is preferably within the range of 0.1 to 50 mm, more preferably within the range of 0.5 to 30 mm, and even more preferably within the range of 1 to 10 mm, from the viewpoints of preventing paint streaks and improving the finish quality.

[0015] From the viewpoint of stable paint supply, the supply pressure is preferably within the range of 0.001 to 10 MPa, more preferably within the range of 0.005 to 5.0 MPa, and even more preferably within the range of 0.01 to 1.0 MPa.

[0016] From the viewpoints of preventing paint streaks and improving coating efficiency, the scanning speed of the dispenser is preferably within the range of 10 to 1500 mm / s, more preferably within the range of 50 to 1000 mm / s, and even more preferably within the range of 100 to 800 mm / s.

[0017] From the viewpoint of coating efficiency and the like, the pitch when scanning the dispenser is preferably within the range of 0.001 to 1.0 mm, more preferably within the range of 0.005 to 1.0 mm, and even more preferably within the range of 0.01 to 1.0 mm.

[0018] The aqueous coating composition of the present invention contains a hydroxyl group-containing resin (A), a pigment (B), and a melamine resin (C).

[0019] [Hydroxyl group-containing resin (A)] As the hydroxyl group-containing resin (A), a resin having good weather resistance and water resistance is suitable, particularly for exterior applications. Specific examples include hydroxyl group-containing acrylic resin (A1), hydroxyl group-containing polyester resin (A2), and hydroxyl group-containing urethane resin (A3).

[0020] The above hydroxyl group-containing resins (A) can be used alone or in combination of two or more depending on the desired coating film performance.

[0021] The content of the hydroxyl group-containing resin (A) in the aqueous coating composition of the present invention is preferably within the range of 40 to 90 mass %, more preferably within the range of 50 to 85 mass %, based on the resin solid content in the aqueous coating composition.

[0022] Hydroxyl group-containing acrylic resin (A1) As the acid group-containing acrylic resin (A1), any known water-soluble or water-dispersible acrylic resin that has been conventionally used in aqueous coating compositions can be used.

[0023] The hydroxyl-containing acrylic resin (A1) can be produced, for example, by copolymerizing a hydroxyl-containing polymerizable unsaturated monomer and another polymerizable unsaturated monomer copolymerizable with the hydroxyl-containing polymerizable unsaturated monomer by a method known per se, such as a solution polymerization method in an organic solvent or an emulsion polymerization method in water.

[0024] The hydroxyl group-containing polymerizable unsaturated monomer is a compound having one or more hydroxyl groups and one or more polymerizable unsaturated bonds per molecule. Examples of the hydroxyl group-containing polymerizable unsaturated monomer 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; N-hydroxymethyl (meth)acrylamide; allyl alcohol; and (meth)acrylates having a polyoxyethylene chain with a hydroxyl group at the molecular terminal. However, in the present invention, a monomer corresponding to the later-described (xvii) polymerizable unsaturated monomer having an ultraviolet-absorbing functional group should be defined as another polymerizable unsaturated monomer copolymerizable with the above-mentioned hydroxyl group-containing polymerizable unsaturated monomer, and is excluded from the hydroxyl group-containing polymerizable unsaturated monomer. These can be used alone or in combination of two or more kinds.

[0025] As other polymerizable unsaturated monomers copolymerizable with the above-mentioned hydroxyl group-containing polymerizable unsaturated monomers, for example, the following monomers (i) to (xx) can be used. These polymerizable unsaturated monomers 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, tridecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclododecyl (meth)acrylate, tricyclodecanyl (meth)acrylate, and the like. (ii) Polymerizable unsaturated monomers having an isobornyl group: isobornyl (meth)acrylate, etc. (iii) Polymerizable unsaturated monomers having an adamantyl group: adamantyl (meth)acrylate, etc. (iv) Polymerizable unsaturated monomers having a tricyclodecenyl group: tricyclodecenyl (meth)acrylate, etc. (v) Aromatic ring-containing polymerizable unsaturated monomers: benzyl (meth)acrylate, styrene, α-methylstyrene, vinyltoluene, and the like. (vi) Polymerizable unsaturated monomers having an alkoxysilyl group: vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, γ-(meth)acryloyloxypropyltrimethoxysilane, γ-(meth)acryloyloxypropyltriethoxysilane, etc. (vii) Polymerizable unsaturated monomers having a fluorinated alkyl group: perfluoroalkyl (meth)acrylates such as perfluorobutylethyl (meth)acrylate and perfluorooctylethyl (meth)acrylate; fluoroolefins, and the like. (viii) Polymerizable unsaturated monomers having a photopolymerizable functional group such as a maleimide group. (ix) Vinyl compounds: N-vinylpyrrolidone, ethylene, butadiene, chloroprene, vinyl propionate, vinyl acetate, etc. (x) Carboxyl group-containing polymerizable unsaturated monomers: (meth)acrylic acid, maleic acid, crotonic acid, β-carboxyethyl (meth)acrylate, etc. (xi) Nitrogen-containing polymerizable unsaturated monomers: (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, adducts of glycidyl (meth)acrylate with amine compounds, and the like. (xii) Polymerizable unsaturated monomers having two or more polymerizable unsaturated groups in one molecule: 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: 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. (xiv) (meth)acrylates having a polyoxyethylene chain with an alkoxy group at the molecular terminal. (xv) Polymerizable unsaturated monomers having a sulfonic acid group: 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl (meth)acrylate, allylsulfonic acid, 4-styrenesulfonic acid, etc.; sodium salts and ammonium salts of these sulfonic acids, etc. (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. (xvii) Polymerizable unsaturated monomers having an ultraviolet-absorbing functional group: 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-[2-(methacryloyloxy)ethyl]phenyl]-2H-benzotriazole, and the like. (xviii) Light-stable polymerizable unsaturated monomers: 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: 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. (xx) Polymerizable unsaturated monomers having an acid anhydride group: maleic anhydride, itaconic anhydride, citraconic anhydride, etc.

[0026] In this specification, the term "polymerizable unsaturated group" refers to an unsaturated group that can undergo radical polymerization. Examples of such polymerizable unsaturated groups include vinyl groups and (meth)acryloyl groups.

[0027] In addition, in this specification, "(meth)acrylate" means acrylate or methacrylate. "(meth)acrylic acid" means acrylic acid or methacrylic acid. "(meth)acryloyl" means acryloyl or methacryloyl. "(meth)acrylamide" means acrylamide or methacrylamide.

[0028] From the viewpoint of improving the curability and finish of the resulting coating film, the hydroxyl value of the hydroxyl-containing acrylic resin (A1) is preferably in the range of 1 to 200 mgKOH / g, more preferably in the range of 2 to 180 mgKOH / g, and even more preferably in the range of 5 to 150 mgKOH / g.

[0029] From the viewpoints of improving the storage stability of the coating material, the water-resistant adhesion of the resulting coating film, and the finish quality, the hydroxyl group-containing acrylic resin (A1) preferably has an acid value in the range of 1 to 150 mgKOH / g, more preferably in the range of 5 to 100 mgKOH / g, and even more preferably in the range of 5 to 80 mgKOH / g.

[0030] When the aqueous coating composition contains the hydroxyl group-containing acrylic resin (A1), the content of the hydroxyl group-containing acrylic resin (A1) is preferably in the range of 2 to 70 mass %, more preferably in the range of 5 to 60 mass %, and even more preferably in the range of 10 to 50 mass %, based on the resin solid content in the aqueous coating composition.

[0031] Hydroxyl group-containing polyester resin (A2) As the hydroxyl group-containing polyester resin (A2), any known water-soluble or water-dispersible polyester resin that has been conventionally used in aqueous coating compositions can be used.

[0032] The hydroxyl group-containing polyester resin (A2) can usually be produced by an esterification reaction or transesterification reaction between an acid component and an alcohol component.

[0033] The acid component may be any compound that is commonly used as an acid component in the production of polyester resins, such as an aliphatic polybasic acid, an alicyclic polybasic acid, or an aromatic polybasic acid.

[0034] The aliphatic polybasic acids are generally aliphatic compounds having two or more carboxyl groups per molecule, acid anhydrides of the aliphatic compounds, and esters of the aliphatic compounds. Examples of the aliphatic polybasic acids 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 polycarboxylic acids; and esters of the aliphatic polycarboxylic acids with lower alkyls having about 1 to 4 carbon atoms. The aliphatic polybasic acids can be used alone or in combination of two or more.

[0035] As the aliphatic polybasic acid, it is preferable to use adipic acid and / or adipic anhydride from the viewpoint of smoothness of the resulting coating film.

[0036] The alicyclic polybasic acids are generally compounds having one or more alicyclic structures and two or more carboxyl groups per molecule, acid anhydrides of the compounds, and esters of the compounds. The alicyclic structures are mainly 4- to 6-membered ring structures. Examples of the 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 the alicyclic polycarboxylic acids; and esters of the alicyclic polycarboxylic acids with lower alkyls having about 1 to 4 carbon atoms. The alicyclic polybasic acids can be used alone or in combination of two or more.

[0037] From the viewpoint of the smoothness of the resulting coating film, it is preferable to use 1,2-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic anhydride, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4-cyclohexene-1,2-dicarboxylic acid, or 4-cyclohexene-1,2-dicarboxylic anhydride as the alicyclic polybasic acid, and it is particularly preferable to use 1,2-cyclohexanedicarboxylic acid and / or 1,2-cyclohexanedicarboxylic anhydride.

[0038] The aromatic polybasic acids are generally aromatic compounds having two or more carboxyl groups per molecule, acid anhydrides of the aromatic compounds, and esters of the aromatic compounds, and examples thereof 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 esters of the aromatic polycarboxylic acids with lower alkyls having about 1 to 4 carbon atoms. The aromatic polybasic acids can be used alone or in combination of two or more.

[0039] As the aromatic polybasic acid, it is preferable to use phthalic acid, phthalic anhydride, isophthalic acid, trimellitic acid, or trimellitic anhydride.

[0040] Acid components other than the aliphatic polybasic acids, alicyclic polybasic acids, and aromatic polybasic acids may 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 may be used alone or in combination.

[0041] As the alcohol component, a polyhydric alcohol having two or more hydroxyl groups in one molecule can be suitably used. Examples of the polyhydric alcohol include 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 suitable polyols 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.

[0042] Furthermore, alcohol components other than the above polyhydric alcohols can also be used. 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 E10P" (trade name, manufactured by HEXION, a glycidyl ester of synthetic highly branched saturated fatty acid) with an acid.

[0043] The method for producing the hydroxyl group-containing polyester resin (A2) is not particularly limited and can be carried out according to a conventional method. For example, the polyester resin can be produced by a method in which the acid component and the alcohol component are heated in a nitrogen stream at about 150 to 250°C for about 5 to 10 hours to cause an esterification reaction or a transesterification reaction between the acid component and the alcohol component.

[0044] When the acid component and alcohol component are subjected to an esterification reaction or transesterification reaction, they 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 be half-esterified to form 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 form a hydroxyl-containing polyester resin.

[0045] In the 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.

[0046] The hydroxyl group-containing polyester resin (A2) can be modified with a fatty acid, a monoepoxy compound, a polyisocyanate compound, an acrylic resin, or the like during or after the preparation of the resin.

[0047] Examples of the fatty acids include 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. An example of the monoepoxy compound that can be suitably used is "Cardura E10P" (trade name, manufactured by HEXION, a glycidyl ester of synthetic highly branched saturated fatty acid).

[0048] Examples of the polyisocyanate compound include aliphatic diisocyanate compounds such as lysine diisocyanate, hexamethylene diisocyanate, and trimethylhexane diisocyanate; and aliphatic diisocyanate compounds such as hydrogenated xylylene diisocyanate, isophorone diisocyanate, methylcyclohexane-2,4-diisocyanate, methylcyclohexane-2,6-diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), and 1,3-(isocyanatomethyl)cyclohexane. Examples of suitable polyisocyanates include cyclic diisocyanate compounds, aromatic diisocyanate compounds such as tolylene diisocyanate, xylylene diisocyanate, and diphenylmethane diisocyanate, organic polyisocyanates themselves such as trivalent or higher polyisocyanates such as lysine triisocyanate, adducts of these organic polyisocyanates with polyhydric alcohols, low-molecular-weight polyester resins, water, and the like, and cyclized polymers (for example, isocyanurates) and biuret-type adducts of these organic polyisocyanates. These polyisocyanate compounds can be used alone or in combination of two or more.

[0049] Furthermore, known methods can be used to modify the polyester resin with the acrylic resin, and examples thereof include a method of polymerizing a mixture of a polymerizable unsaturated group-containing polyester resin and a polymerizable unsaturated monomer, and a method of reacting a polyester resin with an acrylic resin.

[0050] From the viewpoint of improving the curability and finish of the resulting coating film, the hydroxyl group-containing polyester resin (A2) preferably has a hydroxyl value in the range of 1 to 250 mgKOH / g, more preferably in the range of 2 to 200 mgKOH / g, and even more preferably in the range of 5 to 200 mgKOH / g.

[0051] Furthermore, from the viewpoint of improving the water-resistant adhesion and finish of the resulting coating film, the hydroxyl group-containing polyester resin (A2) preferably has an acid value in the range of 1 to 150 mgKOH / g, more preferably in the range of 2 to 100 mgKOH / g, and even more preferably in the range of 3 to 80 mgKOH / g.

[0052] The weight average molecular weight of the hydroxyl group-containing polyester resin (A2) is preferably in the range of 1,000 to 100,000, more preferably in the range of 2,000 to 50,000, and even more preferably in the range of 3,000 to 30,000, from the viewpoint of improving the water-resistant adhesion and finish of the resulting coating film.

[0053] In this specification, the average molecular weight is a value calculated from a chromatogram measured by gel permeation chromatography using the molecular weight of standard polystyrene as a reference. The gel permeation chromatograph used was an "HLC8120GPC" (manufactured by Tosoh Corporation). Four columns were used: "TSKgel G-4000HXL," "TSKgel G-3000HXL," "TSKgel G-2500HXL," and "TSKgel G-2000HXL" (all manufactured by Tosoh Corporation, trade names). The analysis was performed under the following conditions: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 1 mL / min, and detector: RI.

[0054] When the aqueous coating composition of the present invention contains the hydroxyl group-containing polyester resin (A2), the content of the hydroxyl group-containing polyester resin (A2) is preferably in the range of 2 to 70 mass %, more preferably in the range of 5 to 50 mass %, and even more preferably in the range of 10 to 40 mass %, based on the resin solid content in the aqueous coating composition.

[0055] Hydroxyl group-containing polyurethane resin (A3) The hydroxyl group-containing polyurethane resin (A3) may be a polyurethane resin obtained by reacting a polyol with a polyisocyanate.

[0056] 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. Examples of high-molecular-weight polyols include polyether polyols, polyester polyols, acrylic polyols, and epoxy polyols. Examples of polyether polyols include polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. Examples of polyester polyols include polycondensates of the above-mentioned dihydric alcohols, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, and neopentyl glycol with dibasic acids such as adipic acid, azelaic acid, and sebacic acid, lactone-based ring-opening polymer polyols such as polycaprolactone, and polycarbonate diols. Carboxyl-containing polyols such as 2,2-dimethylolpropionic acid and 2,2-dimethylolbutanoic acid can also be used.

[0057] 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, and 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 isocyanate adducts 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.

[0058] The hydroxyl value of the hydroxyl-containing polyurethane resin (A3) is preferably within the range of 1 to 150 mgKOH / g, more preferably within the range of 1 to 100 mgKOH / g, and even more preferably within the range of 1 to 50 mgKOH / g, from the viewpoint of improving the curability and finish of the resulting coating film.

[0059] The acid value of the hydroxyl group-containing polyurethane resin (A3) is preferably within the range of 3 to 90 mgKOH / g, more preferably within the range of 4 to 70 mgKOH / g, and even more preferably within the range of 5 to 50 mgKOH / g, from the viewpoint of improving the water-resistant adhesion and finish of the resulting coating film.

[0060] The number average molecular weight of the hydroxyl group-containing polyurethane resin (A3) is preferably 10,000 or more, more preferably 50,000 or more, and even more preferably 100,000 or more, from the viewpoint of improving the water-resistant adhesion and finish of the resulting coating film.

[0061] When the coating composition of the present invention contains the above-mentioned hydroxyl group-containing polyurethane resin (A3), the content of the hydroxyl group-containing polyurethane resin (A3) is preferably in the range of 3 to 60 mass %, more preferably in the range of 5 to 40 mass %, and even more preferably in the range of 7 to 30 mass %, based on the resin solid content in the aqueous coating composition.

[0062] [Pigment (B)] Examples of the pigment (B) include color pigments, extender pigments, and luster pigments. Examples of color pigments include titanium oxide, carbon black, azo pigments, quinacridone pigments, diketopyrrolopyrrole pigments, perylene pigments, perinone pigments, benzimidazolone pigments, isoindoline pigments, isoindolinone pigments, bismuth vanadate pigments, metal chelate azo pigments, phthalocyanine pigments, indanthrone pigments, dioxazine pigments, threne pigments, indigo pigments, and iron oxide. Examples of extender pigments include talc, clay, kaolin, barium sulfate, barium carbonate, calcium carbonate, silica, and alumina white. Examples of luster pigments include aluminum (including vapor-deposited aluminum), copper, zinc, brass, nickel, aluminum oxide, mica, aluminum oxide coated with titanium oxide or iron oxide, mica coated with titanium oxide or iron oxide, glass flakes, and holographic pigments, among which colored pigments are preferred. These pigments can be used alone or in combination of two or more.

[0063] The content of the pigment (B) is preferably in the range of 1 to 100 parts by mass, more preferably in the range of 2 to 80 parts by mass, and even more preferably in the range of 2 to 50 parts by mass, based on 100 parts by mass of the resin solid content in the aqueous coating composition.

[0064] [Melamine resin (C)] In the present invention, it is necessary for the melamine resin (C) to be hydrophilic in order to prevent the occurrence of paint streaks and to improve the finish of the resulting paint film.

[0065] As the melamine resin (C), a methylolated melamine resin having an average of three or more methylol groups per triazine nucleus or an alkyl-etherified melamine resin in which some of the methylol groups have been replaced with alkoxy groups can be used, and it may further contain imino groups.

[0066] In this specification, the hydrophilic melamine resin refers to a methylolated melamine resin and an alkyl-etherified melamine resin having an alkyl group with a small number of carbon atoms.

[0067] The alkyl etherified melamine resin preferably has a methyl group / butyl group molar ratio of 10 / 90 to 100 / 0, since this increases hydrophilicity, improves compatibility with coating components such as the hydroxyl group-containing resin (A), and improves the water dispersibility of the melamine resin.

[0068] In particular, the molar ratio of (methyl group) / (butyl group) of the alkyl etherified melamine resin is preferably 30 / 70 to 80 / 20, more preferably 35 / 65 to 60 / 40, and even more preferably 37 / 63 to 50 / 50, from the viewpoints of suppressing the occurrence of paint streaks and improving the finish and water-resistant adhesion of the resulting coating film.

[0069] The molar ratio of methyl groups to butyl groups can be adjusted by adjusting the types and amounts of methyl alcohol, n-butyl alcohol, and isobutyl alcohol used for modification.

[0070] The weight average molecular weight of the melamine resin (C) is preferably in the range of 400 to 6,000, more preferably in the range of 500 to 4,000, and even more preferably in the range of 600 to 3,000.

[0071] The content of the melamine resin (C) in the aqueous coating composition of the present invention is preferably within the range of 10 to 60 mass %, more preferably 15 to 45 mass %, based on the resin solid content in the aqueous coating composition, from the viewpoints of suppressing the occurrence of paint streaks, improving the finish quality and water-resistant adhesion of the resulting coating film, etc.

[0072] Furthermore, when the aqueous coating composition of the present invention contains an alkyl etherified melamine resin having a (methyl group) / (butyl group) molar ratio of 10 / 90 to 100 / 0, the content of said alkyl etherified melamine resin having a (methyl group) / (butyl group) molar ratio of 10 / 90 to 100 / 0 is preferably in the range of 50 to 100 mass%, more preferably in the range of 70 to 100 mass%, even more preferably in the range of 80 to 100 mass%, and particularly preferably 100 mass%, based on the total solids content of the melamine resin (C), from the viewpoints of suppressing the occurrence of paint streaks and improving the finish and water-resistant adhesion of the resulting coating film.

[0073] In the present invention, the aqueous coating composition containing the hydroxyl-containing resin (A), pigment (B), and hydrophilic melamine resin (C) can suppress the occurrence of paint streaks when used with the inkjet method or dispenser method, enabling the formation of coating films and drawings with excellent finishes.The reason for this is thought to be that the use of the hydrophilic melamine resin (C), which has high compatibility with the hydroxyl-containing resin (A), suppresses the separation of coating components within droplets with a low coating solids content, and makes it difficult for the melamine components to be unevenly distributed even in areas where the ejected droplets overlap, making it difficult for the surface roughness to increase.As a result, the occurrence of paint streaks, which are variations in color tone derived from the pigment (B), can be suppressed, enabling the formation of coating films and drawings with excellent finishes.

[0074] The aqueous coating composition of the present invention may contain a solvent such as water or an organic solvent, and may further contain various additives as necessary, such as a resin that does not contain hydroxyl groups, a crosslinking agent other than the melamine resin (C), a pigment dispersant, a curing catalyst, an antifoaming agent, an antioxidant, an ultraviolet absorber, a light stabilizer, a thickener, and a surface conditioner.

[0075] Examples of the crosslinking agent other than the melamine resin (C) include polyisocyanate compounds, blocked polyisocyanate compounds, and carbodiimide compounds.

[0076] As the ultraviolet absorber, conventionally known ones can be used, for example, benzotriazole-based absorbers, triazine-based absorbers, salicylic acid derivative-based absorbers, benzophenone-based absorbers, etc. The ultraviolet absorber may also have a polymerizable unsaturated group.

[0077] Specific examples of the benzotriazole-based absorbents include 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-5'-t-butylphenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-t-butylphenyl)benzotriazole, 2-(2'-hydroxy-3'-t-butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-3',5'-di-t-butylphenyl)-5-chlorobenzotriazole, Examples include 2-(2'-hydroxy-3',5'-di-t-amylphenyl)benzotriazole, 2-(2'-hydroxy-4'-octoxyphenyl)benzotriazole, 2-{2'-hydroxy-3'-(3'',4'',5'',6''-tetrahydrophthalimidomethyl)-5'-methylphenyl}benzotriazole, and 2-[2-hydroxy-5-[2-(methacryloyloxy)ethyl]phenyl]-2H-benzotriazole.

[0078] Specific examples of the triazine-based absorbents include 2,4-bis(2,4-dimethylphenyl)-6-(2-hydroxy-4-isooctyloxyphenyl)-1,3,5-triazine, 2-[4((2-hydroxy-3-dodecyloxypropyl)-oxy)-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, 2-[4-((2-hydroxy-3-tridecyloxypropyl)-oxy)-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, and 2-(2,4-dihydroxyphenyl)-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine.

[0079] Specific examples of the salicylic acid derivative-based absorbents include phenyl salicylate, p-octylphenyl salicylate, and 4-tert-butylphenyl salicylate.

[0080] Specific examples of the benzophenone-based absorbent include 4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2-hydroxy-4-methoxy-2'-carboxybenzophenone, 2-hydroxy-4-methoxy-5-sulfobenzophenone trihydrate, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, 2-hydroxy-4-octoxybenzophenone, 2-hydroxy-4-octadecyloxybenzophenone, and naphthalene. Examples include thorium 2,2'-dihydroxy-4,4'-dimethoxy-5-sulfobenzophenone, 2,2',4,4'-tetrahydroxybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone, 5-chloro-2-hydroxybenzophenone, resorcinol monobenzoate, 2,4-dibenzoylresorcinol, 4,6-dibenzoylresorcinol, hydroxydodecylbenzophenone, and 2,2'-dihydroxy-4(3-methacryloxy-2-hydroxypropoxy)benzophenone.

[0081] Commercially available examples of the ultraviolet absorber include "TINUVIN 900," "TINUVIN 928," "TINUVIN 384-2," "TINUVIN 479," "TINUVIN 405," "TINUVIN 400" (product names manufactured by BASF, TINUVIN is a registered trademark), and "RUVA 93" (product name manufactured by Otsuka Chemical Co., Ltd.).

[0082] When the coating composition of the present invention contains the above-mentioned ultraviolet absorber, the content of the ultraviolet absorber is preferably within the range of 0.3 to 10 mass %, more preferably within the range of 0.4 to 9 mass %, and even more preferably within the range of 0.5 to 8 mass %, relative to the total amount of the coating composition.

[0083] The solids concentration of the aqueous coating composition at the time of application is preferably within the range of 5 to 50 mass %, more preferably within the range of 10 to 40 mass %, and even more preferably within the range of 15 to 30 mass %, from the viewpoint of preventing the occurrence of paint streaks.

[0084] Furthermore, the viscosity of the aqueous coating composition, measured at 20°C with a Brookfield viscometer at 60 rpm after 1 minute (sometimes referred to herein as the "B60 value"), is preferably in the range of 100 to 3000 mPa·s, more preferably 300 to 2000 mPa·s, and even more preferably 500 to 1500 mPa·s, from the viewpoint of preventing paint streaks. The viscometer used here is the "LVDV-I" (trade name, Brookfield, Brookfield Brookfield viscometer).

[0085] The aqueous coating composition of the present invention can be heated, if necessary, after droplets have been ejected in the vicinity of the substrate as described above. The heating temperature is preferably within the range of 80 to 180°C, and more preferably within the range of 100 to 160°C. The heating time is preferably within the range of 10 to 60 minutes, and more preferably within the range of 15 to 40 minutes. If necessary, before heat curing, the composition may be heated directly or indirectly by preheating, air blowing, or the like, at a temperature within the range of about 50 to about 110°C, preferably within the range of about 60 to about 90°C, for about 1 to 60 minutes. [Example]

[0086] 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 to these Examples. Note that "parts" and "%" are all based on mass. The coating film thickness is based on the cured coating film.

[0087] Preparation of hydroxyl group-containing acrylic resin (A1) Manufacturing Example 1 A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device was charged with 130 parts of deionized water and 0.52 parts of "Aqualon KH-10" (trade name, Daiichi Kogyo Seiyaku Co., Ltd., emulsifier, active ingredient 97%), and the mixture was stirred and mixed in a nitrogen stream and heated to 80°C. Next, 1% of the total amount of the monomer emulsion (1) described below and 5.3 parts of a 6% aqueous ammonium persulfate solution were introduced into the reaction vessel and maintained at 80°C for 15 minutes. Thereafter, the remaining monomer emulsion (1) was added dropwise over 3 hours to the reaction vessel maintained at the same temperature, and the mixture was aged for 1 hour after the completion of the dropwise addition. Then, the following monomer emulsion (2) was added dropwise over 1 hour, and after aging for 1 hour, 40 parts of a 5% aqueous dimethylethanolamine solution was gradually added to the reaction vessel while cooling to 30°C, and the mixture was discharged while filtering through a 100-mesh nylon cloth, yielding a hydroxyl-containing acrylic resin (A1-1) emulsion with a solids concentration of 30%. The resulting hydroxyl-containing acrylic resin (A1-1) emulsion had an acid value of 33 mg KOH / g and a hydroxyl value of 25 mg KOH / g.

[0088] Monomer emulsion (1): 42 parts of deionized water, 0.72 parts of "Aqualon KH-10", 2.1 parts of methylenebisacrylamide, 2.8 parts of styrene, 16.1 parts of methyl methacrylate, 28 parts of ethyl acrylate, and 21 parts of n-butyl acrylate were mixed and stirred to obtain a monomer emulsion (1).

[0089] Monomer emulsion (2): A monomer emulsion (2) was obtained by mixing and stirring 18 parts of deionized water, 0.31 parts of "Aqualon KH-10," 0.03 parts of ammonium persulfate, 5.1 parts of methacrylic acid, 5.1 parts of 2-hydroxyethyl acrylate, 3 parts of styrene, 6 parts of methyl methacrylate, 1.8 parts of ethyl acrylate, and 9 parts of n-butyl acrylate.

[0090] Manufacturing Example 2 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 heated to 85°C. 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 over 4 hours, and the mixture was aged for 1 hour after the completion of the dropwise addition. A mixture of 10 parts of propylene glycol monopropyl ether and 1 part of 2,2'-azobis(2,4-dimethylvaleronitrile) was then added dropwise over 1 hour, and the mixture was aged for 1 hour after the completion of the dropwise addition. Further addition of 7.4 parts of diethanolamine and 13 parts of propylene glycol monopropyl ether gave a hydroxyl group-containing acrylic resin (A1-2) solution with a solids concentration of 55%. The resulting hydroxyl group-containing acrylic resin (A1-2) had an acid value of 47 mgKOH / g, a hydroxyl value of 72 mgKOH / g, and a weight average molecular weight of 58,000.

[0091] Preparation of hydroxyl group-containing polyester resin (A2) Manufacturing Example 3 A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and water separator was charged with 109 parts trimethylolpropane, 141 parts 1,6-hexanediol, 126 parts hexahydrophthalic anhydride, and 120 parts adipic acid. The temperature was raised from 160 to 230°C over 3 hours, and then the mixture was condensed at 230°C for 4 hours. Next, 38.3 parts trimellitic anhydride was added to the resulting condensation reaction product to add carboxyl groups. The mixture was then reacted at 170°C for 30 minutes, and then diluted with 2-ethyl-1-hexanol to obtain a hydroxyl-containing polyester resin (A2-1) solution with a solids concentration of 70%. The resulting hydroxyl-containing polyester resin (A2-1) had an acid value of 46 mg KOH / g, a hydroxyl value of 150 mg KOH / g, a solids concentration of 70%, and a weight-average molecular weight of 6,400.

[0092] Preparation of hydroxyl group-containing polyurethane resin (A3) Manufacturing Example 4 A reaction vessel equipped with a thermometer, a stirrer, and a reflux condenser was charged with 211.9 parts of polytetramethylene glycol (number average molecular weight 1000), 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, and after the reaction system was purged with nitrogen gas, the mixture was reacted at 80°C with stirring to obtain an NCO-terminated urethane prepolymer with 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 8.6 parts of triethylamine was added to emulsify the solution. 275.9 parts of a 5% aqueous solution of N-(2-hydroxyethyl)ethylenediamine was then added and stirred for 60 minutes. The methyl ethyl ketone was then distilled off under reduced pressure and heating, and the concentration was adjusted with deionized water to obtain a hydroxyl group-containing polyurethane resin (A3-1) dispersion with a solids content of 35%, an acid value of 14 mg KOH / g, a hydroxyl value of 12 mg KOH / g, and an average particle size of 120 nm.

[0093] Pigment dispersion manufacturing Manufacturing Example 5 A container equipped with a stirrer was charged with 9.1 parts (5 parts solids) of the hydroxyl-containing acrylic resin (A1-2) solution obtained in Production Example 2, 4 parts of "Carbon MA-100" (trade name, carbon black, manufactured by Mitsubishi Chemical Corporation), and 50 parts of deionized water, and mixed uniformly. 2-(dimethylamino)ethanol was added to the mixed solution to adjust the pH to 7.5. The resulting mixed solution was then placed in a wide-mouth glass bottle, and glass beads with a diameter of approximately 1.3 mm were added as a dispersion medium, after which the bottle was sealed. The bottle was then dispersed for 4 hours using a paint shaker to obtain pigment dispersion (P-1).

[0094] Manufacturing Example 6 A container equipped with a stirrer was charged with 9.1 parts (5 parts solids) of the hydroxyl-containing acrylic resin (A1-1) solution obtained in Production Example 2, 7 parts of "Chlorinated Copper Cyanine Blue G-314" (trade name, manufactured by Sanyo Dish Co., Ltd., phthalocyanine blue pigment), and 50 parts of deionized water, and mixed uniformly. 2-(dimethylamino)ethanol was added to the mixed solution, and the pH was adjusted to 7.5. The resulting mixed solution was then placed in a wide-mouth glass bottle, and glass beads with a diameter of approximately 1.3 mm were added as a dispersion medium, after which the bottle was sealed. The bottle was then dispersed for 4 hours using a paint shaker to obtain pigment dispersion (P-2).

[0095] Manufacturing Example 7 A container equipped with a stirrer was charged with 9.1 parts (5 parts solids) of the hydroxyl-containing acrylic resin (A1-1) solution obtained in Production Example 2, 10 parts of "PERRIND MAROON 179 229-6440" (trade name, manufactured by Sun Chemical Co., organic perylene pigment), and 50 parts of deionized water, and mixed uniformly. 2-(dimethylamino)ethanol was added to the mixed solution, and the pH was adjusted to 7.5. The resulting mixed solution was then 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 4 hours using a paint shaker to obtain pigment dispersion (P-3).

[0096] Manufacturing Example 8 A container equipped with a stirrer was charged with 9.1 parts (5 parts solids) of the hydroxyl-containing acrylic resin (A1-1) solution obtained in Production Example 2, 10 parts of "YELLOW 2GLMA" (trade name, manufactured by Dominion Colour Corporation, a bismuth vanadate-based yellow pigment), and 50 parts of deionized water, and mixed uniformly. 2-(dimethylamino)ethanol was added to the mixed solution, and the pH was adjusted to 7.5. The resulting mixed solution was then placed in a wide-mouth glass bottle, and glass beads with a diameter of approximately 1.3 mm were added as a dispersion medium, and the bottle was sealed. The bottle was then dispersed for 4 hours using a paint shaker to obtain pigment dispersion (P-4).

[0097] Preparation of aqueous coating composition Example 1 Into a stirring mixing vessel, 63.1 parts (solid content 9 parts) of the pigment dispersion (P-1) obtained in Production Example 5, 116.7 parts (solid content 35.0 parts) of the hydroxyl-containing acrylic resin (A1-1) emulsion obtained in Production Example 1, 21.4 parts (solid content 15 parts) of the hydroxyl-containing polyester resin (A2-1) solution obtained in Production Example 3, 42.9 parts (solid content 15 parts) of the hydroxyl-containing polyurethane resin (A3-1) dispersion obtained in Production Example 4, 37.5 parts (solid content 30 parts) of "melamine resin (C-1) (alkyl etherified melamine resin, (methyl group / (butyl group) molar ratio 40 / 60, solid content concentration 80%)," and "TINUVIN 1.1 parts (1 part solids content) of "384-2" (trade name, manufactured by BASF, benzotriazole-based ultraviolet absorber, solids concentration 95%) was mixed uniformly, and "UH-752" (trade name, manufactured by ADEKA, thickener, solids concentration 28%), 2-(dimethylamino)ethanol, and deionized water were added to obtain aqueous coating composition No. 1. The aqueous coating composition had a pH of 8.0, a coating solids concentration of 20%, and a viscosity of 800 mPa·s after 1 minute at 60 rpm as measured with a Brookfield viscometer at 20°C.

[0098] Examples 2 to 10, Comparative Examples 1 and 2 Water-based coating compositions Nos. 2 to 12 were obtained in the same manner as in Example 1, except that the blending compositions were as shown in Table 1.

[0099] Preparation of test specimens A degreased and zinc phosphate-treated steel plate (JIS G3141, size 400 mm × 300 mm × 0.8 mm) was electrodeposited with the cationic electrodeposition paint "Elecron GT-10" (product name: manufactured by Kansai Paint Co., Ltd., which uses an epoxy resin polyamine-based cationic resin and a blocked polyisocyanate compound as a curing agent) to a film thickness of 20 μm based on the cured coating, and heated at 170°C for 20 minutes to crosslink and cure. After that, the primer paint "TP-65" (product name, manufactured by Kansai Paint Co., Ltd., polyester resin-based solvent primer paint) was electrostatically applied on top using a rotary atomizing bell-type coater to a cured film thickness of 35 μm, and heated at 140°C for 30 minutes to crosslink and cure to form the coated substrate.

[0100] Preparation of test painted panels Preparation of painted panels for finish test and paint streak test Water-based coating compositions No. 1 to No. 12 were filled into an "X JET" (trade name, manufactured by SSI JAPAN, high-viscosity, minute-volume piezo-jet dispenser, discharge orifice diameter 100 μm). They were then applied to substrates at a frequency of 350 Hz, a supply pressure of 0.03 MPa, a speed of 230 mm / s, a pitch of 1 mm, a distance of 10 mm between the orifice and the substrate, and conditions of 23°C and 68% relative humidity. The compositions were then heated at 140°C for 30 minutes to crosslink and cure, yielding coated panels for finish testing and paint streak testing.

[0101] Preparation of coated panels for water-resistant adhesion tests Water-based coating compositions No. 1 to No. 12 were filled into an "X JET" (product name, manufactured by SSI JAPAN, high-viscosity micro-piezo jet dispenser, discharge port diameter 100 μm). They were then applied to the substrate under conditions of a frequency of 350 Hz, a supply pressure of 0.03 MPa, a speed of 230 mm / s, a pitch of 1 mm, and a distance of 10 mm between the discharge port and the substrate, and then heated at 140°C for 30 minutes to crosslink and cure.

[0102] Next, the clear paint "Magicron KINO-1210" (product name, manufactured by Kansai Paint Co., Ltd., acrylic resin-based solvent-based topcoat clear paint) was electrostatically applied using a rotary atomizing bell-type coating machine to a cured film thickness of 35 μm, and the coating was heated at 140°C for 30 minutes to crosslink and cure, producing a coated panel for a water-resistant adhesion test.

[0103] Test Method Finishability The coated panels for finish test obtained above were visually observed, and the finish was evaluated according to the following evaluation criteria. ◎ and ◯ are acceptable. The evaluation results are shown in Table 1. -Evaluation criteria- ◎: Smooth and glossy ○: Smooth but lacking in gloss ×: Not smooth and has poor gloss.

[0104] Paint streaks The coated panels for the paint streak test obtained above were visually observed, and the paint streaks were evaluated according to the following evaluation criteria. ◎ and ○ are acceptable. The evaluation results are shown in Table 1. -Evaluation criteria- ◎: No paint streaks are visible. ◯: Slight paint streaks are observed, but this does not pose a practical problem. ×: Paint streaks are clearly visible, and there is a problem in practical use.

[0105] Water-resistant adhesion The resulting coated panels for water adhesion tests were immersed in 40°C water for 240 hours, removed, and dried at 20°C for 12 hours. Following JIS K 5600-5-6 (1990), 100 2mm x 2mm square grids were created on the coating surface. Adhesive tape was applied to the grids and then quickly peeled off. The number of square grids remaining on the coating surface was evaluated. ◎ and ○ indicate passing. The evaluation results are shown in Table 1. -Evaluation criteria- ◎: Remaining number / total number = 100 / 100 with no chipped edges ○: Remaining number / total number = 100 / 100 with chipped edges ×: Remaining number / total number = 99 or less / 100.

[0106] [Table 1]

[0107] [Table 2]

[0108] In addition, (Note 1) to (Note 7) in the table mean the following: (Note 1) Melamine resin (C-2): Alkyl etherified melamine resin, (methyl group / (butyl group) molar ratio is 100 / 0, solid content concentration is 80%, (Note 2) Melamine resin (C-3): Alkyl etherified melamine resin, (methyl group / (butyl group) molar ratio is 60 / 40, solid content concentration is 80%, (Note 3) Melamine resin (C-4): Alkyl etherified melamine resin, (methyl group / (butyl group) molar ratio is 35 / 65, solid content concentration is 80%, (Note 4) Melamine resin (C-5): Alkyl etherified melamine resin, (methyl group / (butyl group) molar ratio is 30 / 70, solid content concentration is 70%, (Note 5) Melamine resin (C-6): Alkyl etherified melamine resin, (methyl group / (butyl group) molar ratio is 10 / 90, solid content concentration is 70%, (Note 6) Melamine resin (C-7): Alkyl etherified melamine resin, (methyl group / (butyl group) molar ratio is 0 / 100, solid content concentration is 60%, (Note 7) "Duranate SBN-70D": Product name, manufactured by Asahi Kasei Chemical Corporation, pyrazole-blocked polyisocyanate compound, solids concentration 70%.

Claims

1. An aqueous coating composition used in a method for discharging a coating material in proximity to a surface to be coated by pressing a valve body formed to be movable toward a discharge surface having a discharge port from which the coating material is discharged, the composition comprising: (A) a hydroxyl group-containing resin; (B) a pigment; and (C) a melamine resin; the hydroxyl-containing resin (A) comprises a hydroxyl-containing acrylic resin (A1), a hydroxyl-containing polyester resin (A2), and a hydroxyl-containing polyurethane resin (A3), An aqueous coating composition, wherein the melamine resin (C) is hydrophilic.

2. 2. The aqueous coating composition according to claim 1, wherein the melamine resin (C) is an alkyl-etherified melamine resin having a methyl group / butyl group content ratio of 10 / 90 to 100 / 0 in terms of a (methyl group) / (butyl group) molar ratio.

3. A method for discharging paint in proximity to a surface to be coated by pressing a valve body formed to be movable toward a discharge surface having a discharge port from which the paint is discharged, comprising: A method of using the aqueous coating composition of claim 1 as said coating material.

4. 4. The method of claim 3, comprising dispensing the aqueous coating composition as droplets adjacent to a surface to be coated.

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