Aqueous coating composition and method for producing coated article

Abstract

This aqueous coating composition comprises an aqueous polyester resin (A), an aqueous polyurethane resin (B), a melamine resin (C), and a hydroxy group-containing resin (D) other than the aqueous polyester resin (A), the aqueous polyurethane resin (B), and the melamine resin (C), wherein the aqueous polyester resin (A) has a number-average molecular weight of 1,000-6,000, a glass transition temperature of -30°C to 20°C, an acid value of 10-25 mg KOH / g, and a hydroxyl value exceeding 60 mg KOH / g and equal to or less than 100 mg KOH / g; the solid content of the aqueous polyester resin (A) is 12.5-45 parts by mass with respect to 100 parts by mass of the resin solid content of the aqueous coating composition; and the solid content of the aqueous polyurethane resin (B) is 15-60 parts by mass with respect to 100 parts by mass of the resin solid content of the aqueous coating composition.

Description

Water-based coating composition and method for producing coated articles

[0001] The present invention relates to an aqueous coating composition and a method for producing a coated article.

[0002] A common method for forming a multi-layer coating on an automobile body is the so-called aqueous 3WET coating (3C1B) process, in which an aqueous primer coating composition is applied, pre-drying is performed, an aqueous base coating composition is applied, pre-drying is performed, an aqueous clear coating composition is applied, and baking is performed in this order.

[0003] In recent years, CO ２ From the viewpoint of reducing emissions, there is a demand for omitting the pre-drying step that is usually performed after applying a water-based intermediate coating composition in 3C1B coating. Pre-drying is mainly performed to remove the solvent contained in the coating composition and increase the solids concentration. Therefore, if the pre-drying step is omitted, the water-based intermediate coating composition and the water-based base coating composition tend to mix easily, which can lead to the formation of a mixed layer between the uncured intermediate coating film and the uncured base coating film. This mixed layer degrades the appearance of the resulting multi-layer coating film.

[0004] Regarding the omission of pre-drying, Patent Document 1 discloses a method for forming a multi-layer coating film using a paint containing a specific aqueous polyester resin, a specific aqueous acrylic resin, an aqueous urethane resin, and a melamine resin.

[0005] International Publication No. 2013 / 129136

[0006] The coating composition of Patent Document 1 does not have a sufficient effect of suppressing the formation of the mixed layer, and the appearance of the resulting coating film is likely to be poor. In addition, the coating film obtained from the coating composition of Patent Document 1 does not have sufficient chipping resistance.

[0007] The object of the present invention is to provide an aqueous coating composition that can provide a multilayer coating film having excellent appearance and chipping resistance even when the pre-drying step performed after applying the aqueous coating composition is omitted.

[0008] In order to solve the above problems, the present invention provides the following aspects. [1] An aqueous coating composition comprising an aqueous polyester resin (A), an aqueous polyurethane resin (B), a melamine resin (C), and a hydroxyl group-containing resin (D) other than the aqueous polyester resin (A), the aqueous polyurethane resin (B), and the melamine resin (C), wherein the aqueous polyester resin (A) has a number average molecular weight of 1,000 to 6,000, a glass transition temperature of -30°C to 20°C, an acid value of 10 mgKOH / g to 25 mgKOH / g, and a hydroxyl group value of more than 60 mgKOH / g to 100 mgKOH / g, the solids content of the aqueous polyester resin (A) being 12.5 parts by mass to 45 parts by mass per 100 parts by mass of the resin solids of the aqueous coating composition, and the solids content of the aqueous polyurethane resin (B) being 15 parts by mass to 60 parts by mass per 100 parts by mass of the resin solids of the aqueous coating composition. [2] The aqueous coating composition of [1] above, wherein the solid content of the hydroxyl-containing resin (D) is 10 to 30 parts by mass per 100 parts by mass of the resin solids of the aqueous coating composition. [3] The aqueous coating composition of [1] or [2] above, wherein the solid content of the melamine resin (C) is 10 to 40 parts by mass per 100 parts by mass of the resin solids of the aqueous coating composition. [4] The aqueous coating composition of any of [1] to [3] above, wherein the aqueous polyurethane resin (B) has a glass transition temperature of -90°C or higher and -10°C or lower. [5] The aqueous coating composition of any of [1] to [4] above, wherein the hydroxyl-containing resin (D) comprises at least one of a hydroxyl-containing acrylic resin (D1) and a polyether polyol (D2). [6] A method for producing a coated article, comprising the steps of: applying an aqueous coating composition according to any one of [1] to [5] above onto an article to be coated to form an uncured intermediate coating film; and applying an aqueous base coating composition onto the uncured intermediate coating film to form an uncured base coating film, wherein no preliminary drying step is provided between the step of forming the uncured intermediate coating film and the step of forming the uncured base coating film.[7] The method for producing a coated article of [6] above, wherein, in the step of forming the uncured intermediate coating film, the aqueous paint composition is applied so that the thickness of the intermediate coating film after curing is 6 μm or more and 35 μm or less. [8] The method for producing a coated article of [6] or [7] above, wherein, in the step of forming the uncured base coating film, the aqueous base paint composition is applied so that the thickness of the base coating film after curing is 6 μm or more and 35 μm or less. [9] The method for producing a coated article of any of [6] to [8] above, further comprising, after the step of forming the uncured base coating film, the steps of applying a clear paint composition on the uncured base coating film to form an uncured clear coating film, and heating the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film to form a multi-layer coating film.

[10] In the step of forming the uncured clear coating film, the clear coating composition is applied so that the thickness of the clear coating film after curing is 20 μm or more and 50 μm or less, The method for producing a coated article according to [9] above.

[0009] According to the aqueous coating composition of the present invention, a multi-layer coating film having excellent appearance and chipping resistance can be obtained even when the pre-drying step that is usually performed after applying the aqueous coating composition is omitted.

[0010] [Aqueous Coating Composition] The aqueous coating composition according to the present disclosure contains an aqueous polyester resin (A), an aqueous polyurethane resin (B), a melamine resin (C), and a hydroxyl group-containing resin (D) other than these. The aqueous polyester resin (A) has a number average molecular weight of 1,000 to 6,000, a glass transition temperature of -30°C to 20°C, an acid value of 10 mgKOH / g to 25 mgKOH / g, and a hydroxyl value of more than 60 mgKOH / g to 100 mgKOH / g. The solid content of the aqueous polyester resin (A) is 12.5 to 45 parts by mass per 100 parts by mass of the resin solids of the aqueous coating composition. The solid content of the aqueous polyurethane resin (B) is 15 to 60 parts by mass per 100 parts by mass of the resin solids of the aqueous coating composition.

[0011] Typically, when a water-based coating composition is applied to an uncured coating film formed with the same water-based coating composition, the solvents mix. In this case, resin components and pigments, especially those contained in the water-based coating composition applied later, migrate to the lower layer along with the solvent, blurring the boundary between the two coating films. This is called a mixed layer. The mixed layer reduces the smoothness of the multilayer coating film. As a result, the appearance of the multilayer coating film deteriorates.

[0012] The aqueous coating composition according to the present disclosure contains an aqueous polyester resin (A) having a low molecular weight and a high hydroxyl value. Because the aqueous polyester resin (A) has a low molecular weight, its viscosity decreases when heated. This improves the appearance of the resulting coating film. Additionally, because the aqueous polyester resin (A) has a high hydroxyl value, the crosslinking density increases, improving the chipping resistance of the resulting coating film. Furthermore, because a specific amount of aqueous polyurethane resin (B) is included, the elasticity of the resulting coating film increases, further improving chipping resistance.

[0013] (Aqueous Polyester Resin (A)) The aqueous polyester resin (A) has a number average molecular weight of 1,000 or more and 6,000 or less, a glass transition temperature (Tg) of -30°C or more and 20°C or less, an acid value of 10 mgKOH / g or more and 25 mgKOH / g or less, and a hydroxyl value of more than 60 mgKOH / g and 100 mgKOH / g or less. When the aqueous polyester resin (A) has the above glass transition temperature, the viscosity of the aqueous polyester resin (A) is reduced during heat curing, improving the smoothness of the resulting coating film. In addition, the flexibility of the resulting coating film is increased, improving chipping resistance.

[0014] The number average molecular weight of the aqueous polyester resin (A) may be 1,200 or more, 1,500 or more, or 2,000 or more. The number average molecular weight of the aqueous polyester resin (A) may be 5,000 or less, 4,000 or less, or 3,000 or less.

[0015] The number average molecular weight and weight average molecular weight are determined by GPC using polystyrene as a standard.

[0016] The Tg of the aqueous polyester resin (A) may be −28° C. or higher, −25° C. or higher, or −23° C. or higher. The Tg of the aqueous polyester resin (A) may be 18° C. or lower, 15° C. or lower, or 10° C. or lower.

[0017] The hydroxyl value of the aqueous polyester resin (A) may be 95 mgKOH / g or less, or 90 mgKOH / g or less, and may be 70 mgKOH / g or more, or 80 mgKOH / g or more.

[0018] The acid value of the aqueous polyester resin (A) may be 20 mgKOH / g or less, 18 mgKOH / g or less, or 15 mgKOH / g or less. The acid value of the aqueous polyester resin (A) may be 11 mgKOH / g or more, or 11.5 mgKOH / g or more.

[0019] The hydroxyl value and acid value are determined based on the mass of the solid content. The hydroxyl value and acid value can be measured by a known method described in JIS K 0070:1992. The hydroxyl value and acid value may be calculated from the blend amount of the unsaturated monomer in the raw material monomer of the resin (e.g., aqueous polyester resin (A)).

[0020] The aqueous coating composition may contain one or more (particularly two or more) aqueous polyester resins (A).

[0021] In the aqueous coating composition, the aqueous polyester resin (A) may be dissolved. That is, the aqueous polyester resin (A) may be a water-soluble polyester resin. In the aqueous coating composition, the aqueous polyester resin (A) may be in the form of a dispersion.

[0022] The water-soluble polyester resin and the aqueous polyester resin dispersion can be obtained, for example, by a method in which a polyester resin is neutralized with a base and then water is gradually added under stirring to cause phase inversion emulsification, or by a method in which a polyester resin is forcibly emulsified in water using a high-speed homogenizer.

[0023] The aqueous polyester resin (A) may have a hydroxyl group and a carboxyl group. The number of hydroxyl groups per molecule of the aqueous polyester resin (A) may be two or more, or may be three or more. The number of carboxyl groups per molecule of the aqueous polyester resin (A) may be two or more, or may be two.

[0024] Polyester resins are obtained, for example, by condensation of a polyhydric alcohol component and a polybasic acid component. The polyhydric alcohol has two or more hydroxyl groups per molecule. Examples of polyhydric alcohols include diols such as propylene glycol, neopentyl glycol, butylene glycol, hexylene glycol, octylene glycol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,12-dodecanediol, 1,2-cyclohexanediol, 1,3-cyclohexanediol, 1,4-cyclohexanediol, hydrogenated bisphenol A, caprolactone diol, and bishydroxyethyl taurine; triols such as trimethylolpropane and hexanetriol; and tetraols such as pentaerythritol. These may be used alone or in combination of two or more.

[0025] Examples of polybasic acid components include polycarboxylic acids. Polycarboxylic acids have two or more carboxy groups in one molecule. Examples of polycarboxylic acids include aromatic dicarboxylic acids such as phthalic acid and isophthalic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, and tetrahydrophthalic acid; and tricarboxylic acids such as trimellitic acid. These may be used alone or in combination of two or more. If necessary, saturated fatty acids such as stearic acid, lauric acid, and myristic acid; unsaturated fatty acids such as oleic acid, linoleic acid, and linolenic acid; natural oils and fats such as castor oil, palm oil, and soybean oil, and long-chain fatty acids of modified products thereof may be used in combination.

[0026] As a raw material for the polyester resin, at least one of a monohydric alcohol, a monoepoxide compound, a lactone, and a hydroxycarboxylic acid may be used in combination.

[0027] The lactone has a cyclic ester bond. Examples of lactones include dimethylpropiolactone, ε-caprolactone, γ-caprolactone, δ-valerolactone, and γ-butyrolactone. These may be used alone or in combination of two or more.

[0028] Hydroxycarboxylic acids have a hydroxyl group and a carboxyl group in one molecule. Examples of hydroxycarboxylic acids include dimethylolpropionic acid. These may be used alone or in combination of two or more.

[0029] Carboxy groups can be introduced by modifying some or all of the hydroxyl groups of the polyester resin with an acid anhydride such as phthalic anhydride, succinic anhydride, hexahydrophthalic anhydride, or trimellitic anhydride.

[0030] The solid content of the aqueous polyester resin (A) is 12.5 parts by mass or more and 45 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous coating composition. This provides an effect of inhibiting interlayer mixing. The solid content of the aqueous polyester resin (A) may be 20 parts by mass or more, or 25 parts by mass or more. The solid content of the aqueous polyester resin (A) may be 40 parts by mass or less, 38 parts by mass or less, or 35 parts by mass or less.

[0031] The resin solids mass of the aqueous coating composition is the total solids mass of the aqueous polyester resin (A), aqueous polyurethane resin (B), melamine resin (C), hydroxyl group-containing resin (D) and other resin components.

[0032] (Other aqueous polyester resin (a) The aqueous coating composition may contain an aqueous polyester resin (a) other than the aqueous polyester resin (A). The other aqueous polyester resin (a) may be water-soluble. In the aqueous coating composition, the other aqueous polyester resin (a) may be in the form of a dispersion.

[0033] Other examples of the aqueous polyester resin (a) include low-molecular-weight polyester resins (a1) having a number-average molecular weight of less than 1000 and a hydroxyl value of 60 mgKOH / g or less. By using such an aqueous polyester resin having a low molecular weight and a relatively low hydroxyl value in combination, the dispersibility of the melamine resin (C) can be improved.

[0034] The solid content of the low-molecular-weight polyester resin (a1) is, for example, 1 part by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous coating composition. The solid content of the low-molecular-weight polyester resin (a1) may be 2 parts by mass or more, or 4 parts by mass or more. The solid content of the low-molecular-weight polyester resin (a1) may be 8 parts by mass or less, or 7 parts by mass or less.

[0035] (Aqueous Polyurethane Resin (B)) The aqueous polyurethane resin (B) increases the elasticity of the coating film and improves chipping resistance.

[0036] In the aqueous coating composition, the aqueous polyurethane resin (B) may be dissolved. That is, the aqueous polyurethane resin (B) may be a water-soluble polyurethane resin. In the aqueous coating composition, the aqueous polyurethane resin (B) may be in the form of a dispersion.

[0037] The water-soluble polyurethane resin and the aqueous polyurethane resin dispersion can be obtained, for example, by a method of forcibly emulsifying the polyurethane resin using a surfactant, or by a method of neutralizing the polyurethane resin with a base or an acid.

[0038] The polyurethane resin can be obtained, for example, by reacting a polyol compound, a compound having an active hydrogen group and a hydrophilic group in the molecule, an organic polyisocyanate, and, if necessary, a chain extender and a polymerization terminator.

[0039] The polyol compound contains two or more hydroxyl groups in the molecule. Examples of the polyol compound include polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and glycerin; polyether polyols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol; polyester polyols obtained from dicarboxylic acids such as adipic acid, sebacic acid, itaconic acid, maleic anhydride, phthalic acid, and isophthalic acid and glycols such as ethylene glycol, triethylene glycol, propylene glycol, butylene glycol, tripropylene glycol, and neopentyl glycol; polycaprolactone polyols; polybutadiene polyols; polycarbonate polyols; and polythioether polyols. These may be used alone or in combination of two or more.

[0040] Examples of compounds having an active hydrogen group and a hydrophilic group in the molecule include compounds containing an active hydrogen and an anionic group, a cationic group, or a nonionic hydrophilic group. The anionic group includes an anionic group and an anion-forming group. The anion-forming group is a group that can form an anionic group by reacting with a base, and specifically, the anionic group is formed by neutralizing with a base before, during, or after the urethanization reaction.

[0041] Compounds containing active hydrogen and an anionic group are described, for example, in JP-B-42-24192 and JP-B-55-41607, and specific examples thereof include α,α-dimethylolpropionic acid and α,α-dimethylolbutyric acid. Compounds having active hydrogen and a cationic group are described, for example, in JP-B-43-9076. Compounds having active hydrogen and a nonionic hydrophilic group are described, for example, in JP-B-48-41718, and specific examples thereof include polyethylene glycol and alkyl alcohol alkylene oxide adducts.

[0042] The organic polyisocyanate contains two or more isocyanate groups in the molecule. Examples of the organic polyisocyanate include aliphatic diisocyanates such as 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexyl-2,4-diisocyanate, methylcyclohexyl-2,6-diisocyanate, xylylene diisocyanate, 1,3-bis(isocyanato)methylcyclohexane, tetramethylxylylene diisocyanate, transcyclohexane-1,4-diisocyanate, and lysine diisocyanate; and 2,4-toluylene diisocyanate. aromatic diisocyanates such as methyl methyl octane, 2,6-toluylene diisocyanate, diphenylmethane-4,4'-diisocyanate, 1,5'-naphthene diisocyanate, tolidine diisocyanate, diphenylmethylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, 4,4'-dibenzyl diisocyanate, and 1,3-phenylene diisocyanate; triisocyanates such as lysine ester triisocyanate, triphenylmethane triisocyanate, 1,6,11-undecane triisocyanate, 1,8-diisocyanate-4,4-isocyanatomethyloctane, 1,3,6-hexamethylene triisocyanate, and bicycloheptane triisocyanate; dimers, trimers (isocyanurate bond), and biuret forms thereof; and reaction products of these with polyols. These may be used alone or in combination of two or more.

[0043] The chain extender contains two or more active hydrogen groups in the molecule. Examples of the chain extender include low molecular weight polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, 3-methylpentanediol, 2-ethyl-1,3-hexanediol, and trimethylolpropane; polyamines such as ethylenediamine, hexamethylenediamine, diethylenetriamine, hydrazine, xylylenediamine, and isophoronediamine; and water. These may be used alone or in combination of two or more.

[0044] Examples of the polymerization terminator include compounds having one active hydrogen in the molecule (such as monoalcohols and monoamines) and monoisocyanate compounds.

[0045] The synthesis method of the urethane resin may be a one-shot method in which each component is reacted at once, or a multi-stage method in which the components are reacted in stages. In the multi-stage method, a part of the active hydrogen-containing compound (e.g., a high molecular weight polyol) is reacted with a polyisocyanate to form an NCO-terminated prepolymer, and then this prepolymer is reacted with the remainder of the active hydrogen-containing compound.

[0046] The solid content of the aqueous polyurethane resin (B) is 15 parts by mass or more and 60 parts by mass or less relative to 100 parts by mass of the total resin solid content of the aqueous coating composition. This improves chipping resistance without impairing the interlayer mixing suppression effect. The solid content of the aqueous polyurethane resin (B) may be more than 15 parts by mass, may be 20 parts by mass or more, may be 22 parts by mass or more, or may be 25 parts by mass or more. The solid content of the aqueous polyurethane resin (B) may be 50 parts by mass or less, may be 40 parts by mass or less, or may be 30 parts by mass or less.

[0047] The Tg of the aqueous polyurethane resin (B) is, for example, -90°C or higher and -10°C or lower. This further increases the flexibility of the coating film, and can further improve chipping resistance. The Tg of the aqueous polyurethane resin (B) may be -40°C or higher, or -35°C or higher. The Tg of the aqueous polyurethane resin (B) may be -15°C or lower, or -18°C or lower. The Tg of the aqueous polyurethane resin (B) can be measured using a differential scanning calorimeter.

[0048] (Melamine Resin (C)) The melamine resin (C) is a curing agent that reacts mainly with the hydroxyl group-containing resin (D) to form a crosslinked structure, thereby providing a coating film with high hardness.

[0049] The melamine resin (C) may be water-soluble or water-insoluble. The melamine resin (C) has three nitrogen atoms N around a triazine ring (triazine nucleus). １ ~N ３ Six substituents R １ ~R ６ A structure in which -N １ (R １ ) (R ２ ), -N ２ (R ３ ) (R ４ ), -N ３ (R ５ ) (R ６ The planar structure and the rigid triazine ring further improve the hardness of the resulting coating film. This allows the aqueous polyester resin (A) and the aqueous polyurethane resin (B) to achieve both excellent chipping resistance and excellent hardness.

[0050] substituent R １ ~R ６ are each independently a hydrogen atom, an alkyl ether group (—CH ２ -OR ７ ) and methylol groups (-CH ２ OH). The substituent R １ ~R ６ , R ７may each independently be an alkyl group having 1 to 8 carbon atoms, or may be an alkyl group having 1 to 4 carbon atoms. The alkyl group may be linear or branched. The alkyl group may be a methyl group, an n-butyl group, or an isobutyl group.

[0051] The melamine resin (C) is generally composed of a polynuclear compound in which a plurality of triazine rings are bonded to each other, but the melamine resin (C) may also be a mononuclear compound composed of one triazine ring.

[0052] The melamine resin (C) may be, for example, a melamine resin having a substituent R １ ~R ６ a full alkyl type having only an alkyl ether group as the alkyl group; ２ -OR ７ ) (-CH ２ methylol group type having —N(—OH); ２ -OR ７ ) (H); imino group type having —N(—CH ２ -OR ７ ) (-CH ２ -OH) and -N(-CH ２ -OR ７ ) (H) and methylol / imino group type.

[0053] The melamine resin (C) may be hydrophobic or hydrophilic. The melamine resin (C) may be particularly hydrophobic. This allows the melamine resin (C) to react with hydroxyl groups of other resins while self-aggregating to form a cluster structure. The cluster structure can further improve the hardness of the coating film. Furthermore, the hydrophobicity of the entire uncured coating film can be further increased, thereby further improving the effect of inhibiting inter-layer mixing. In addition, the water resistance of the coating film is improved. A coating film with excellent hardness and water resistance is suitable, for example, for direct bonding of glass. The glass is, for example, a vehicle window. Direct bonding of glass is a technique of bonding glass to a vehicle using an adhesive. Using the aqueous coating composition according to the present disclosure for vehicle painting facilitates direct bonding of glass.

[0054] The hydrophobic melamine resin (C) may be, for example, a melamine resin having a substituent R １~R ６ Among these, the average total number of hydrogen atoms and methylol groups is 1 or less. The hydrophobic melamine resin (C) may be a full-alkyl type. Specific examples of the full-alkyl type melamine resin (C) include methylated melamine resin, butylated melamine resin, and isobutylated melamine resin.

[0055] The weight average molecular weight of the melamine resin (C) may be 1,000 or more and 7,000 or less. This can further improve the hardness of the resulting coating film. The weight average molecular weight of the melamine resin (C) may be 1,500 or more, or 2,000 or more. The weight average molecular weight of the melamine resin (C) may be 5,000 or less, or 4,000 or less.

[0056] Examples of commercially available melamine resins (C) include the Cymel series (trade name) manufactured by Allnex, specifically Cymel 202, Cymel 204, Cymel 211, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 250, Cymel 251, Cymel 254, Cymel 266, Cymel 267, Cymel 270, Cymel 272, Cymel 285, Cymel 301, Cymel 303, Cymel 325, Cymel 327, Cymel 350, Cymel 370, Cymel 701, Cymel 703, and Cymel 1141; and the U-Ban (trade name) series manufactured by Mitsui Chemicals, Inc. These may be used alone or in combination of two or more.

[0057] The solid content of the melamine resin (C) is, for example, 10 parts by mass or more and 40 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous coating composition. This facilitates the curing reaction, making it easier to obtain a coating film with high hardness. The solid content of the melamine resin (C) may be 15 parts by mass or more, or 17 parts by mass or more. The solid content of the melamine resin (C) may be 35 parts by mass or less, or 30 parts by mass or less.

[0058] (Other Curing Agents) The aqueous coating composition may contain a curing agent other than the melamine resin (C). Examples of other curing agents include blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more.

[0059] (Hydroxyl-containing resin (D)) As described above, the hydroxyl-containing resin (D) reacts with the melamine resin (C) to form a crosslinked structure. The hydroxyl-containing resin (D) is a resin containing hydroxyl groups other than the aqueous polyester resin (A), the aqueous polyurethane resin (B), and the melamine resin (C).

[0060] The hydroxyl-containing resin (D) has one or more hydroxyl groups. Examples of the hydroxyl-containing resin include a hydroxyl-containing acrylic resin (D1) and a polyether polyol (D2). These may be used alone or in combination. The aqueous coating composition may contain one or more (particularly two or more) hydroxyl-containing acrylic resins (D1), or one or more hydroxyl-containing acrylic resins (D1) and one or more polyether polyols (D2).

[0061] The solid content of the hydroxyl-containing resin (D) is, for example, 10 parts by mass or more and 30 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous coating composition. This facilitates the curing reaction, making it easier to obtain a coating film with high hardness. The solid content of the hydroxyl-containing resin (D) may be 15 parts by mass or more, or 16 parts by mass or more. The solid content of the hydroxyl-containing resin (D) may be 28 parts by mass or less, or 25 parts by mass or less.

[0062] <Hydroxyl-containing acrylic resin (D1)> In the aqueous coating composition, the hydroxyl-containing acrylic resin (D1) may be in the form of an emulsion. In the aqueous coating composition, the hydroxyl-containing acrylic resin (D1) may be dissolved. That is, the hydroxyl-containing acrylic resin (D1) may be a water-soluble acrylic resin. The aqueous coating composition may contain both a water-soluble acrylic resin and an aqueous acrylic resin in the form of an emulsion.

[0063] The acrylic resin emulsion can be prepared by emulsion polymerization of α,β-ethylenically unsaturated monomers. Examples of the α,β-ethylenically unsaturated monomers include (meth)acrylic acid esters, α,β-ethylenically unsaturated monomers having an acid group, and α,β-ethylenically unsaturated monomers having a hydroxyl group. The monomers can be used alone or in combination of two or more.

[0064] Examples of (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, phenyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate. (Meth)acrylic acid esters refer to acrylic acid esters and methacrylic acid esters.

[0065] Examples of α,β-ethylenically unsaturated monomers having an acid group include acrylic acid, methacrylic acid, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, isocrotonic acid, α-hydro-ω-((1-oxo-2-propenyl)oxy)poly(oxy(1-oxo-1,6-hexanediyl)), maleic acid, fumaric acid, itaconic acid, 3-vinylsalicylic acid, 3-vinylacetylsalicylic acid, 2-acrylamido-2-methylpropanesulfonic acid, p-hydroxystyrene, and 2,4-dihydroxy-4′-vinylbenzophenone.

[0066] Examples of the α,β-ethylenically unsaturated monomer having a hydroxyl group include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts of these with ε-caprolactone.

[0067] Other α,β-ethylenically unsaturated monomers may be used in combination. Examples of the other α,β-ethylenically unsaturated monomers include polymerizable amide compounds, polymerizable aromatic compounds, polymerizable nitriles, polymerizable alkylene oxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, α-olefins, dienes, polymerizable carbonyl compounds, polymerizable alkoxysilyl compounds, and other polymerizable compounds.

[0068] The emulsion polymerization method is not particularly limited. For example, an emulsifier is dissolved in an aqueous medium containing water or, if necessary, an organic solvent such as an alcohol or an ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and an α,β-ethylenically unsaturated monomer and a polymerization initiator are added dropwise with heating and stirring. The α,β-ethylenically unsaturated monomer may be previously emulsified with an emulsifier.

[0069] Polymerization initiators and emulsifiers commonly used by those skilled in the art can be used. If necessary, a chain transfer agent such as a mercaptan (e.g., lauryl mercaptan) or α-methylstyrene dimer may be used to adjust the molecular weight. The reaction temperature, reaction time, and the like can be appropriately selected within ranges commonly used by those skilled in the art. The resulting acrylic resin emulsion is neutralized with a base, if necessary.

[0070] The hydroxyl group-containing acrylic resin (D1) (acrylic resin emulsion) obtained by emulsion polymerization may have a number average molecular weight of 3,000 or more.

[0071] The hydroxyl group-containing acrylic resin (D1) may have a hydroxyl value (solid content hydroxyl value) of 20 mgKOH / g or more and 180 mgKOH / g or less. The hydroxyl group-containing acrylic resin (D1) may have an acid value (solid content acid value) of 1 mgKOH / g or more and 80 mgKOH / g or less.

[0072] The water-soluble hydroxyl group-containing acrylic resin (D1) can be prepared, for example, by solution polymerizing the above-mentioned α,β-ethylenically unsaturated monomer and then solubilizing it in water with a basic compound.

[0073] <Polyether polyol (D2)> In the aqueous coating composition, the polyether polyol (D2) may be in the form of an emulsion. In the coating composition, the polyether polyol (D2) may be dissolved. That is, the polyether polyol (D2) may be a water-soluble polyether polyol.

[0074] The polyether polyol (D2) can be obtained, for example, by addition polymerization of an alkylene oxide such as ethylene oxide, propylene oxide, or tetrahydrofuran to a polyhydric alcohol. The polyether polyol (D2) may be a polyether diol having two hydroxyl groups in one molecule.

[0075] Examples of polyether diols include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol.

[0076] (Solvent) The aqueous coating composition contains water as a solvent. The aqueous coating composition may contain an organic solvent together with the aqueous solvent. In the aqueous coating composition, the proportion of water in the solvent is, for example, 50 mass % or more, 70 mass % or more, or 90 mass % or more.

[0077] Examples of organic solvents include ester solvents such as ethyl acetate, butyl acetate, isopropyl acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate; ether solvents such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, methyl methoxybutanol, ethoxypropanol, ethylene glycol isopropyl ether, ethylene glycol t-butyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, methoxybutanol, and propylene glycol monobutyl ether; alcohol solvents such as methanol, ethanol, butanol, and propyl alcohol; and ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. These may be used alone or in combination of two or more.

[0078] (Others) The aqueous coating composition may further contain other components, such as pigments, film-forming aids, surface conditioners, preservatives, mildew inhibitors, antifoaming agents, light stabilizers, UV absorbers, antioxidants, and pH adjusters.

[0079] (First Aspect) In the first aspect, the aqueous coating composition comprises an aqueous polyester resin (A) in a dispersion form, a low-molecular-weight polyester resin (a1), an aqueous polyurethane resin (B) in a dispersion form, a melamine resin (C), and each of water-soluble and emulsion-form hydroxyl group-containing acrylic resins (D1).

[0080] In the first embodiment, the solid content of each resin relative to 100 parts by mass of the resin solid content of the aqueous coating composition is as follows: Aqueous polyester resin (A) in dispersion form: 20 parts by mass or more and 30 parts by mass or less Low molecular weight polyester resin (a1): 2 parts by mass or more and 10 parts by mass or less Aqueous polyurethane resin (B) in dispersion form: 25 parts by mass or more and 35 parts by mass or less Melamine resin (C): 15 parts by mass or more and 25 parts by mass or less Water-soluble and emulsion-form acrylic resin (D1): 5 parts by mass or more and 15 parts by mass or less

[0081] (Second Aspect) In the second aspect, the aqueous coating composition comprises an aqueous polyester resin (A) in dispersion form, an aqueous polyurethane resin (B) in dispersion form, a melamine resin (C), water-soluble and emulsion-form hydroxyl group-containing acrylic resins (D1), and a water-soluble polyether polyol (D2).

[0082] In the second embodiment, the solid content of each resin relative to 100 parts by mass of the resin solid content of the aqueous coating composition is as follows: Aqueous polyester resin (A) in dispersion form: 12.5 parts by mass or more and 45 parts by mass or less Aqueous polyurethane resin (B) in dispersion form: 15 parts by mass or more and 25 parts by mass or less Melamine resin (C): 15 parts by mass or more and 25 parts by mass or less Water-soluble and emulsion-form acrylic resin (D1): 1 part by mass or more and 10 parts by mass or less Water-soluble polyether polyol (D2): 5 parts by mass or more and 15 parts by mass or less

[0083] (Preparation of aqueous coating composition) The aqueous coating composition can be prepared, for example, by stirring the components using a stirrer, etc. The pigment may be dispersed in a vehicle containing water, a surfactant, a dispersant, etc. using a sand grind mill, etc., and then mixed with other components.

[0084] [Method for manufacturing coated articles] The aqueous coating composition according to the present disclosure can suppress the formation of mixed layers between aqueous coatings applied in succession, even when pre-drying is omitted. Therefore, the aqueous coating composition according to the present disclosure is suitable for use in manufacturing coated articles having multi-layer coating films formed by multiple aqueous coatings. The aqueous coating composition according to the present disclosure is particularly suitable for forming intermediate coating films.

[0085] The method for producing a coated article according to the present disclosure comprises the steps of applying an aqueous coating composition according to the present disclosure to a substrate to form an uncured intermediate coating film, and applying an aqueous base coating composition to the uncured intermediate coating film to form an uncured base coating film. No pre-drying step is provided between the steps of forming the uncured intermediate coating film and the step of forming the uncured base coating film. Omission of the pre-drying step reduces environmental impact and production costs.

[0086] In the pre-drying step, at least a portion of the solvent is intentionally removed. Omission of the pre-drying step means that no work is performed to intentionally remove the solvent from the uncured intermediate coating film before the step of forming the uncured base coating film. Methods for intentionally removing the solvent (i.e., pre-drying methods) include, for example, natural drying and heat drying. In natural drying, the uncured intermediate coating film is left for 5 to 15 minutes at a temperature of 20°C to 25°C. Heat drying is performed under conditions that do not allow the curing reaction of the coating film-forming components to proceed, or at least do not complete the curing reaction. In heat drying, the uncured intermediate coating film is heated for 30 seconds to 10 minutes at a temperature of 50°C to 80°C.

[0087] (I) Step of forming an uncured intermediate coating film The aqueous coating composition according to the present disclosure is applied to a substrate to form an uncured intermediate coating film. The intermediate coating film is interposed between the substrate and the base coating film. The intermediate coating film improves adhesion between the base coating film and the substrate. Furthermore, the intermediate coating film according to the present disclosure suppresses the formation of a mixed layer. Furthermore, because the aqueous coating composition according to the present disclosure contains a polyester resin, the intermediate coating further smooths the painted surface, making it easier to suppress unevenness in the base coating film.

[0088] From the viewpoint of smoothness and chipping resistance of the coated article, the thickness of the intermediate coating film after curing may be 6 μm or more and 35 μm or less. The thickness of the intermediate coating film after curing may be 8 μm or more, or 10 μm or more. The thickness of the intermediate coating film after curing may be 25 μm or less, or 22 μm or less.

[0089] The thickness of the coating film can be measured using an electromagnetic film thickness meter (for example, SDM-miniR manufactured by SANKO Co., Ltd.) The thickness of the coating film is the average value of the thickness of the coating film at any five points.

[0090] Examples of coating methods include air spray coating, airless spray coating, and rotary atomization coating. These methods may be combined with electrostatic coating. Among these, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic coating, a rotary atomization electrostatic coater, commonly known as a "micro-microbell (μμbell)," a "microbell (μbell)," or a "metallicbell (metabell)," is used.

[0091] (Substrate) Examples of the material of the substrate include metal, resin, and glass. The aqueous coating composition according to the present disclosure can provide a coating film with excellent chipping resistance, and is therefore particularly suitable for coating substrates containing metal.

[0092] The shape of the substrate is not particularly limited. Specific examples of the substrate include automobile bodies such as passenger cars, trucks, motorcycles, and buses, and automobile body parts, as well as automobile parts such as spoilers, bumpers, mirror covers, grilles, and door knobs.

[0093] Examples of metals include tungsten, tin, zinc, and alloys thereof (e.g., steel). Representative examples of metal substrates include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0094] The metal substrate may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After the surface treatment, the metal substrate may be further coated with an electrodeposition paint. The electrodeposition paint may be either a cationic type or an anionic type.

[0095] Examples of resins include polypropylene resin, polycarbonate resin, urethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, and polyamide resin. It is preferable that the resin substrate be degreased.

[0096] The preliminary drying (also called preheating) step after applying the aqueous coating composition is omitted. In this case, too, mixing of the uncured intermediate coating film and the aqueous base coating composition applied thereon is suppressed, thereby suppressing the formation of a mixed layer. As a result, the appearance of the resulting coated article is improved.

[0097] (II) Step of forming an uncured base coating film The aqueous base coating composition is applied onto the uncured intermediate coating film to form an uncured base coating film. The base coating film may be a single layer or a multilayer coating film of two or more layers.

[0098] In this way, the multilayer coating film is obtained by the so-called wet-on-wet coating method, in which coating is performed on an uncured coating film. Wet-on-wet coating can omit the baking drying oven, thereby reducing the environmental impact and production costs. In the manufacturing method disclosed herein, the pre-drying process after coating the aqueous coating composition is also omitted, so the effect of reducing the environmental impact and production costs is even greater.

[0099] The thickness of the base coating film after curing is, for example, 6 μm or more and 35 μm or less. The thickness of the base coating film after curing may be 8 μm or more, or 10 μm or more. The thickness of the base coating film after curing may be 25 μm or less, or 20 μm or less.

[0100] The coating method may be, for example, the same method as that used for coating the intermediate coating composition.

[0101] After the aqueous base coating composition has been applied, the above-mentioned preliminary drying step may be carried out.

[0102] (Aqueous base coating composition) The aqueous base coating composition contains, for example, an acrylic resin emulsion, a water-soluble acrylic resin, a curing agent (typically, a melamine resin), and a polyether polyol resin. The base coating film formed by such an aqueous base coating composition has excellent appearance. The aqueous base coating composition may further contain a pigment and various additives. Examples of additives include an ultraviolet absorber, an antioxidant, an antifoaming agent, a surface conditioner, and a pinhole inhibitor.

[0103] (III) Step of Forming Uncured Clear Coating Film A clear coating composition may be further applied onto the uncured base coating film to form an uncured clear coating film.

[0104] From the viewpoint of scratch resistance and smoothness, the thickness of the cured clear coating film may be 20 μm or more and 50 μm or less. The thickness of the cured clear coating film may be 25 μm or more, or 30 μm or more. The thickness of the cured clear coating film may be 45 μm or less, or 40 μm or less.

[0105] The coating method is not particularly limited. For example, the coating method may be the same as the coating method for the intermediate coating composition. Among them, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. After applying the clear coating composition, preliminary drying may be performed in the same manner as above.

[0106] (Clear Coating Composition) The clear coating composition may be solvent-based, water-based, or powder-type. From the viewpoints of transparency or acid etching resistance, the solvent-based clear coating composition may contain an acrylic resin and / or polyester resin as a film-forming resin, and an amino resin and / or isocyanate as a curing agent. The solvent-based clear coating composition may also contain an acrylic resin and / or polyester resin having a carboxylic acid and / or an epoxy group. The clear coating composition may contain various pigments to the extent that the transparency and the effects of the coating composition according to the present disclosure are not impaired. The clear coating composition may contain the above-mentioned additives as needed.

[0107] (IV) Curing step: Each uncured coating film is heated to cure. In this step, the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film are cured at the same time.

[0108] The heating conditions are appropriately set depending on the composition of each coating composition, the material of the substrate, etc. The heating temperature is, for example, 60°C or higher and 170°C or lower, and may be 90°C or higher and 150°C or lower.

[0109] The heating time may be set appropriately depending on the heating temperature. When the heating temperature is 60°C or higher and 170°C or lower, the heating time may be, for example, 10 minutes or higher and 60 minutes or lower, or 15 minutes or higher and 45 minutes or lower. The heating time means the time during which the inside of the heating device reaches the target temperature and the coated object is maintained at the target temperature, and does not take into account the time until the target temperature is reached. Examples of the heating device include a drying furnace that uses a heat source such as hot air, electricity, gas, or infrared rays.

[0110] The present invention will be described in more detail with reference to the following examples, but is not limited thereto. In the examples, "parts" and "%" are by mass unless otherwise specified. Details of the components listed in Tables 1 and 2 are as follows.

[0111] Water-based polyester resin (A): trade name "PED-7301", manufactured by Nippon Paint Automotive Coating Co., Ltd., number average molecular weight 2400, Tg 0°C, acid value 21 mgKOH / g, hydroxyl value 90 mgKOH / g Low hydroxyl value polyester resin (a1): trade name "PED-5501", manufactured by Nippon Paint Automotive Coating Co., Ltd., water-soluble, number average molecular weight 2300, Tg -5°C, acid value 21 mgKOH / g, hydroxyl value 57 mgKOH / g Water-based polyurethane resin (B): trade name "U-coat N-800T", manufactured by Sanyo Chemical Industries, Ltd., dispersion, Tg -20 to -30°C Melamine resin (C): trade name "Cymel 250", manufactured by Allnex, imino group type, weight average molecular weight 2500 Hydroxyl group-containing acrylic resin (D1): trade name "EMA-1046", manufactured by Nippon Paint Automotive Coatings Co., Ltd., emulsion, hydroxyl value 30 mg KOH / g

[0112] [Examples 1 to 7, Comparative Examples 1 to 3] (1) Preparation of aqueous coating compositions As shown in Table 1, aqueous coating compositions were prepared by varying the blending amounts of the aqueous polyester resin (A) and the aqueous polyurethane resin (B).

[0113] (2) Production of Coated Articles A zinc phosphate-treated dull steel plate was electrodeposited with Powernics 150 (product name, cationic electrodeposition paint manufactured by Nippon Paint Automotive Coatings Co., Ltd.) to a dry coating thickness of 20 μm. The plate was then heated at 160° C. for 30 minutes. After that, the plate was cooled to form a cured electrodeposition coating.

[0114] The above aqueous coating composition was applied onto the obtained electrodeposition coating film using a rotary atomizer electrostatic coating device to a dry film thickness of 12 μm. Subsequently, as an aqueous base coating composition, Aqualex AR-3400 (trade name, aqueous metallic base paint manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied using a rotary atomizer electrostatic coating device to a dry film thickness of 15 μm, and the coating was preheated at 80°C for 3 minutes.

[0115] Next, a clear coating composition, PU Excel O-4300 (trade name, two-component clear coating, manufactured by Nippon Paint Automotive Coatings Co., Ltd.), was applied using a rotary atomization electrostatic coating device so that the dry film thickness was 35 μm. This was then heat-cured at 140° C. for 30 minutes to obtain a coated article.

[0116] [Evaluation] The coated articles were evaluated using the following methods. The results are shown in Table 1. (1) Appearance Wd (measurement wavelength: 3.0 mm to 10.0 mm) was measured using a Wavescan DOI (manufactured by BYK Gardner). Wd is a numerical representation of the roughness of the coating film at the measurement wavelength. The smaller the Wd, the smaller the roughness of the coating film and the better the appearance. A Wd of 24 or less is suitable for practical use.

[0117] (2) Chipping Resistance A stone chipping test was carried out on the test plate using a Grarobe tester KSS-1 (manufactured by Suga Testing Instruments Co., Ltd.) under the following conditions: <Test conditions> Stone size: 2.36 to 4.75 mm, Stone quantity: 100 g, Distance: 35 cm, Shot pressure: 0.3 kg / cm２ Shot angle: 90° Test temperature: -20°C

[0118] The test panels after the stone chipping test were visually evaluated according to the following criteria. The evaluation was the average of three test panels per level. A score of 3.0 or higher on the following criteria was deemed suitable for practical use. 5: Almost no peeling was observed. 4: The peeled area was small, but almost no peeling was observed at the interface between the electrodeposition coating and the intermediate coating. 3: The peeled area was somewhat large, and peeling was observed at the interface between the electrodeposition coating and the intermediate coating. 2: The peeled area was large, and peeling was observed at the interface between the electrodeposition coating and the intermediate coating. 1: The peeled area was large, and the electrodeposition coating was destroyed.

[0119]

[0120] The aqueous coating composition of the present invention can produce a multi-layer coating film with excellent appearance and chipping resistance even when pre-drying is omitted, and is therefore suitable for use in forming a variety of multi-layer coating films.

[0121] This application claims priority based on Japanese Patent Application No. 2023-200889, filed on November 28, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. An aqueous paint composition comprising an aqueous polyester resin (A), an aqueous polyurethane resin (B), a melamine resin (C), and a hydroxyl-containing resin (D) other than the aqueous polyester resin (A), the aqueous polyurethane resin (B), and the melamine resin (C), wherein the aqueous polyester resin (A) has a number average molecular weight of 1,000 or more and 6,000 or less, a glass transition temperature of -30°C or more and 20°C or less, an acid value of 10 mgKOH / g or more and 25 mgKOH / g or less, and a hydroxyl value of more than 60 mgKOH / g and 100 mgKOH / g or less, the solids content of the aqueous polyester resin (A) is 12.5 parts by mass or more and 45 parts by mass or less per 100 parts by mass of resin solids of the aqueous paint composition, and the solids content of the aqueous polyurethane resin (B) is 15 parts by mass or more and 60 parts by mass or less per 100 parts by mass of resin solids of the aqueous paint composition.

2. The aqueous coating composition according to claim 1, wherein the solid content of the hydroxyl-containing resin (D) is 10 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the resin solid content of the aqueous coating composition.

3. An aqueous coating composition as described in claim 1 or 2, wherein the solid content of the melamine resin (C) is 10 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the resin solid content of the aqueous coating composition.

4. An aqueous coating composition according to any one of claims 1 to 3, wherein the aqueous polyurethane resin (B) has a glass transition temperature of -90°C or higher and -10°C or lower.

5. An aqueous coating composition according to any one of claims 1 to 4, wherein the hydroxyl-containing resin (D) comprises at least one of a hydroxyl-containing acrylic resin (D1) and a polyether polyol (D2).

6. A method for producing a coated article, comprising the steps of: applying an aqueous coating composition according to any one of claims 1 to 5 onto an object to be coated, to form an uncured intermediate coating film; and applying an aqueous base coating composition onto the uncured intermediate coating film to form an uncured base coating film, wherein no preliminary drying step is provided between the steps of forming the uncured intermediate coating film and forming the uncured base coating film.

7. A method for producing a coated article as described in claim 6, wherein in the step of forming the uncured intermediate coating film, the aqueous paint composition is applied so that the thickness of the intermediate coating film after curing is 6 μm or more and 35 μm or less.

8. A method for producing a coated article as described in claim 6 or 7, wherein in the step of forming the uncured base coating film, the aqueous base paint composition is applied so that the thickness of the base coating film after curing is 6 μm or more and 35 μm or less.

9. A method for producing a coated article as described in any one of claims 6 to 8, further comprising the steps of: applying a clear paint composition onto the uncured base coating film after the step of forming the uncured base coating film to form an uncured clear coating film; and heating the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film to form a multi-layer coating film.

10. A method for producing a coated article as described in claim 9, wherein in the step of forming the uncured clear coating film, the clear paint composition is applied so that the thickness of the clear coating film after curing is 20 μm or more and 50 μm or less.

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