Aqueous coating composition and method for manufacturing coated article

Abstract

An aqueous coating composition containing 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), the aqueous polyester resin (A) having a number average molecular weight of 1000-6000 inclusive, the aqueous polyurethane resin (B) having a number average molecular weight of 500-600 inclusive, and the melamine resin (C) having a number average molecular weight of 500-600 inclusive. The water-based polyester resin (A) has a glass transition temperature of-30 DEG C to 20 DEG 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, and the solid content of the water-based polyester resin (A) is 12.5 parts by mass to 45 parts by mass per 100 parts by mass of the resin solid content of the water-based coating composition. The solid content of the aqueous polyurethane resin (B) is from 15 parts by mass to 60 parts by mass (inclusive) per 100 parts by mass of the resin solid content of the aqueous coating composition.

Description

Technical Field

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

[0002] As a method for forming a multi-layer coating film on an automobile body, so-called aqueous 3WET coating (3C1B) is generally carried out by sequentially performing coating of an aqueous intermediate coating composition, pre-drying, coating of an aqueous primer coating composition, pre-drying, coating of a clear coating composition, and baking.

[0003] In recent years, from the viewpoint of reducing CO2 emissions, omission of pre-drying after coating the aqueous intermediate coating composition has been required in 3C1B coating. Pre-drying is mainly carried out to remove the solvent contained in the coating composition to increase the solid content concentration. Therefore, if the above pre-drying is omitted, the aqueous intermediate coating composition and the aqueous primer coating composition are likely to be mixed with each other, and a mixed layer may be formed between the uncured intermediate coating film and the uncured primer coating film. The mixed layer deteriorates 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, which uses a coating containing a specific aqueous polyester resin, a specific aqueous acrylic resin, an aqueous urethane resin, and a melamine resin.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: International Publication No. 2013 / 129136. Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] In the coating composition of Patent Document 1, the effect of suppressing the formation of the above mixed layer is insufficient, and the appearance of the resulting coating film is likely to deteriorate. Moreover, the chipping resistance of the coating film obtained from the coating composition of Patent Document 1 is insufficient.

[0010] An object of the present invention is to provide an aqueous coating composition that can obtain a multi-layer coating film having excellent appearance and chipping resistance even when pre-drying after coating the aqueous coating composition is omitted.

[0011] Means for Solving the Problems

[0012] To solve the above problems, the present invention provides the following solutions.

[0013] [1] An aqueous coating composition containing:

[0014] Water-based polyester resin (A),

[0015] Water-based polyurethane resin (B),

[0016] Melamine resin (C), and

[0017] A hydroxyl group-containing resin (D) other than the above water-based polyester resin (A), the above water-based polyurethane resin (B), and the above melamine resin (C),

[0018] Among them, the number-average molecular weight of the above water-based polyester resin (A) is 1000 or more and 6000 or less, the glass transition temperature is -30°C or more and 20°C or less, the acid value is 10 mgKOH / g or more and 25 mgKOH / g or less, the hydroxyl value is more than 60 mgKOH / g and 100 mgKOH / g or less, and relative to 100 parts by mass of the resin solid content of the above water-based coating composition, the solid content of the above water-based polyester resin (A) is 12.5 parts by mass or more and 45 parts by mass or less.

[0019] Relative to 100 parts by mass of the resin solid content of the above water-based coating composition, the solid content of the above water-based polyurethane resin (B) is 15 parts by mass or more and 60 parts by mass or less.

[0020] [2] The water-based coating composition according to [1] above, wherein, relative to 100 parts by mass of the resin solid content of the above water-based coating composition, the solid content of the above hydroxyl group-containing resin (D) is 10 parts by mass or more and 30 parts by mass or less.

[0021] [3] The water-based coating composition according to [1] or [2] above, wherein, relative to 100 parts by mass of the resin solid content of the above water-based coating composition, the solid content of the above melamine resin (C) is 10 parts by mass or more and 40 parts by mass or less.

[0022] [4] The water-based coating composition according to any one of [1] to [3] above, wherein the glass transition temperature of the above water-based polyurethane resin (B) is -90°C or more and -10°C or less.

[0023] [5] The water-based coating composition according to any one of [1] to [4] above, wherein the above hydroxyl group-containing resin (D) includes at least one of a hydroxyl group-containing acrylic resin (D1) and a polyether polyol (D2).

[0024] [6] A method for manufacturing a coated article, which includes the following steps:

[0025] A step of coating the object to be coated with the water-based coating composition according to any one of [1] to [5] above to form an uncured intermediate coating film; and

[0026] A step of coating an aqueous primer coating composition on the uncured intermediate coating film described above to form an uncured primer coating film.

[0027] Among them, there is no pre-drying step between the step of forming the uncured intermediate coating film described above and the step of forming the uncured primer coating film described above.

[0028] [7] The manufacturing method of the coated article according to [6] above, wherein, in the step of forming the uncured intermediate coating film described above, the aqueous coating composition is coated in such a manner that the thickness of the cured intermediate coating film is 6 μm or more and 35 μm or less.

[0029] [8] The manufacturing method of the coated article according to [6] or [7] above, wherein, in the step of forming the uncured primer coating film described above, the aqueous primer coating composition is coated in such a manner that the thickness of the cured primer coating film is 6 μm or more and 35 μm or less.

[0030] [9] The manufacturing method of the coated article according to any one of [6] to [8] above, which further comprises the following steps:

[0031] After the step of forming the uncured primer coating film described above, a step of coating a transparent coating composition on the uncured primer coating film described above to form an uncured transparent coating film; and

[0032] A step of heating the uncured intermediate coating film, the uncured primer coating film, and the uncured transparent coating film described above to form a multilayer coating film.

[0033]

[10] The manufacturing method of the coated article according to [9] above, wherein, in the step of forming the uncured transparent coating film described above, the transparent coating composition is coated in such a manner that the thickness of the cured transparent coating film is 20 μm or more and 50 μm or less.

[0034] Advantages of the Invention

[0035] According to the aqueous coating composition of the present invention, even when the pre-drying after coating the aqueous coating composition is omitted, a multilayer coating film having excellent appearance and chipping resistance can be obtained. Detailed Embodiments

[0036] [Aqueous Coating Composition]

[0037] 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 number average molecular weight of the aqueous polyester resin (A) is 1,000 or more and 6,000 or less, the glass transition temperature is -30°C or more and 20°C or less, the acid value is 10 mgKOH / g or more and 25 mgKOH / g or less, and the hydroxyl value is more than 60 mgKOH / g and 100 mgKOH / g or less. The solid content of the aqueous polyester resin (A) is 12.5 parts by mass or more and 45 parts by mass or less with respect to 100 parts by mass of the resin solid content of the aqueous coating composition. The solid content of the aqueous polyurethane resin (B) is 15 parts by mass or more and 60 parts by mass or less with respect to 100 parts by mass of the resin solid content of the aqueous coating composition.

[0038] Generally, if the same aqueous coating composition is applied on an uncured coating film formed from the aqueous coating composition, the solvents are mixed with each other. At this time, in particular, the resin components, pigments, etc. contained in the later-applied aqueous coating composition move to the lower layer together with the solvent, and the boundary between the two coating films becomes blurred. This is the mixed layer. The mixed layer reduces the smoothness of the multi-layer coating film. As a result, the appearance of the multi-layer coating film deteriorates.

[0039] 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. Since the molecular weight of the aqueous polyester resin (A) is low, the viscosity decreases when heated. Therefore, the appearance of the resulting coating film is improved. Moreover, since the hydroxyl value of the aqueous polyester resin (A) is high, the crosslinking density becomes high, and the chipping resistance of the resulting coating film is improved. Moreover, since a specific amount of the aqueous polyurethane resin (B) is contained, the elasticity of the resulting coating film is improved, and the chipping resistance is further improved.

[0040] (aqueous polyester resin (A))

[0041] The number average molecular weight of the aqueous polyester resin (A) is 1,000 or more and 6,000 or less, the glass transition temperature (Tg) is -30°C or more and 20°C or less, the acid value is 10 mgKOH / g or more and 25 mgKOH / g or less, and the hydroxyl value is more than 60 mgKOH / g and 100 mgKOH / g or less. By making the aqueous polyester resin (A) have the above-mentioned glass transition temperature, the viscosity of the aqueous polyester resin (A) decreases during heat curing, and the smoothness of the resulting coating film is improved. Moreover, the softness of the resulting coating film is improved, so the chipping resistance is improved.

[0042] The number-average molecular weight of the aqueous polyester resin (A) may be 1,200 or more, may be 1,500 or more, and may also be 2,000 or more. The number-average molecular weight of the aqueous polyester resin (A) may be 5,000 or less, may be 4,000 or less, and may also be 3,000 or less.

[0043] The number-average molecular weight and the weight-average molecular weight are determined by the GPC method using polystyrene as the standard.

[0044] The Tg of the aqueous polyester resin (A) may be -28°C or higher, may be -25°C or higher, and may also be -23°C or higher. The Tg of the aqueous polyester resin (A) may be 18°C or lower, may be 15°C or lower, and may also be 10°C or lower.

[0045] The hydroxyl value of the aqueous polyester resin (A) may be 95 mgKOH / g or less, and may also be 90 mgKOH / g or less. The hydroxyl value of the aqueous polyester resin (A) may be 70 mgKOH / g or more, and may also be 80 mgKOH / g or more.

[0046] The acid value of the aqueous polyester resin (A) may be 20 mgKOH / g or less, may be 18 mgKOH / g or less, and may also be 15 mgKOH / g or less. The acid value of the aqueous polyester resin (A) may be 11 mgKOH / g or more, and may also be 11.5 mgKOH / g or more.

[0047] The hydroxyl value and the acid value are determined based on the solid content mass. The hydroxyl value and the acid value can be measured by known methods described in JIS K 0070:1992. The hydroxyl value and the acid value can be calculated from the blending amount of the unsaturated monomer in the raw material monomers of the resin (for example, the aqueous polyester resin (A)).

[0048] The aqueous coating composition may contain one or more (especially two or more) aqueous polyester resins (A).

[0049] 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 form of the aqueous polyester resin (A) may be a dispersion.

[0050] The water-soluble polyester resin and the aqueous polyester resin dispersion are obtained, for example, by a method of phase inversion emulsification by neutralizing the polyester resin with an alkali and slowly adding water with stirring, or a method of forcibly emulsifying the polyester resin in water using a high-speed homogenizer.

[0051] The aqueous polyester resin (A) may have hydroxyl groups and carboxyl groups. In one molecule of the aqueous polyester resin (A), the number of hydroxyl groups may be two or more, or three or more. In one molecule of the aqueous polyester resin (A), the number of carboxyl groups may be two or more, or two.

[0052] The polyester resin is obtained, for example, by condensation of a polyol component and a polycarboxylic acid component. The polyol has two or more hydroxyl groups in one molecule. Examples of the polyol include: diols such as propylene glycol, neopentyl glycol, butanediol, hexanediol, octanediol, 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 bis(hydroxyethyl)taurine; triols such as trimethylolpropane and hexanetriol; and tetrols such as pentaerythritol. These can be used alone or in combination of two or more.

[0053] Examples of the polycarboxylic acid component include polycarboxylic acids. The polycarboxylic acid has two or more carboxyl groups in one molecule. Examples of the polycarboxylic acid 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 can be used alone or in combination of two or more. As needed, for example, saturated fatty acids such as stearic acid, lauric acid, and myristic acid; unsaturated fatty acids such as oleic acid, linoleic acid, and linolenic acid; and long - chain fatty acids of natural oils such as castor oil, palm oil, and soybean oil and their modified products can be used in combination.

[0054] As raw materials of the polyester resin, at least one of a monohydric alcohol, a monoepoxide, a lactone, and a hydroxycarboxylic acid can be further used in combination.

[0055] The lactone has a cyclic ester bond. Examples of the lactone include: dimethylpropiolactone, ε - caprolactone, γ - caprolactone, δ - valerolactone, γ - butyrolactone. These can be used alone or in combination of two or more.

[0056] The hydroxycarboxylic acid has a hydroxyl group and a carboxyl group in one molecule. Examples of the hydroxycarboxylic acid include: hydroxycarboxylic acids such as dimethylolpropionic acid. These can be used alone or in combination of two or more.

[0057] By modifying a part or all of the hydroxyl groups of the polyester resin with acid anhydrides such as phthalic anhydride, succinic anhydride, hexahydrophthalic anhydride, and trimellitic anhydride, carboxyl groups can be introduced.

[0058] Based on 100 parts by mass of the resin solid content of the aqueous coating composition, the solid content of the aqueous polyester resin (A) is 12.5 parts by mass or more and 45 parts by mass or less. Thus, the mixed layer inhibition effect is exerted. The above-mentioned solid content of the aqueous polyester resin (A) can be 20 parts by mass or more, or can be 25 parts by mass or more. The above-mentioned solid content of the aqueous polyester resin (A) can be 40 parts by mass or less, or can be 38 parts by mass or less, or can also be 35 parts by mass or less.

[0059] The mass of the resin solid content of the aqueous coating composition is the total solid content mass of the aqueous polyester resin (A), the aqueous polyurethane resin (B), the melamine resin (C), the hydroxyl group-containing resin (D), and other resin components.

[0060] (Other aqueous polyester resin (a))

[0061] The aqueous coating composition may contain an aqueous polyester resin (a) other than the aqueous polyester resin (A). The other aqueous polyester resin (a) can be water-soluble. In the aqueous coating composition, the form of the other aqueous polyester resin (a) can be a dispersion.

[0062] As the other aqueous polyester resin (a), for example, a low molecular weight polyester resin (a1) with a number average molecular weight of less than 1000 and a hydroxyl value of 60 mg KOH / g or less can be cited. By using such a low molecular weight aqueous polyester resin with a relatively low hydroxyl value in combination, the dispersibility of the melamine resin (C) can be improved.

[0063] Based on 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) is, for example, 1 part by mass or more and 10 parts by mass or less. The above-mentioned solid content of the low molecular weight polyester resin (a1) can be 2 parts by mass or more, or can be 4 parts by mass or more. The above-mentioned solid content of the low molecular weight polyester resin (a1) can be 8 parts by mass or less, or can be 7 parts by mass or less.

[0064] (Aqueous polyurethane resin (B))

[0065] By means of the aqueous polyurethane resin (B), the elasticity of the coating film is improved and the chipping resistance is improved.

[0066] In the aqueous coating composition, the aqueous polyurethane resin (B) can be dissolved. That is, the aqueous polyurethane resin (B) can be a water-soluble polyurethane resin. In the aqueous coating composition, the form of the aqueous polyurethane resin (B) can be a dispersion.

[0067] Water-soluble polyurethane resins and aqueous polyurethane resin dispersions are obtained, for example, by a method of forcibly emulsifying a polyurethane resin using a surfactant, or a method of neutralizing a urethane resin with an alkali or an acid.

[0068] Polyurethane resins are obtained, for example, by the reaction of a polyol compound, a compound having an active hydrogen group and a hydrophilic group in the molecule, an organic polyisocyanate, and, as required, a chain extender and a polymerization terminator.

[0069] The polyol compound contains two or more hydroxyl groups in the molecule. Examples of the polyol compound include polyols 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 diols such as ethylene glycol, triethylene glycol, propylene glycol, butanediol, tripropylene glycol, and neopentyl glycol; polycaprolactone polyols; polybutadiene polyols; polycarbonate polyols; and polysulfide polyols. These are used alone or in combination of two or more.

[0070] Examples of the compound having an active hydrogen group and a hydrophilic group in the molecule include a compound 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 react with an alkali to form an anionic group. Specifically, it forms an anionic group by neutralization with an alkali before, during, or after the urethanization reaction.

[0071] Examples of the compound containing an active hydrogen and an anionic group are described in the specifications of Japanese Patent Publication No. 42-24192 and Japanese Patent Publication No. 55-41607. Specifically, α,α-dimethylolpropionic acid and α,α-dimethylolbutyric acid can be exemplified. Examples of the compound having an active hydrogen and a cationic group are described in the specification of Japanese Patent Publication No. 43-9076. Examples of the compound having an active hydrogen and a nonionic hydrophilic group are described in Japanese Patent Publication No. 48-41718. Specifically, polyethylene glycol and alkyl alcohol alkylene oxide adducts can be exemplified.

[0072] Organic polyisocyanates contain two or more isocyanate groups in the molecule. Examples of organic polyisocyanates 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(isocyanatomethyl)cyclohexane, tetramethylxylylene diisocyanate, trans-cyclohexane-1,4-diisocyanate, lysine diisocyanate; aromatic diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, diphenylmethane-4,4'-diisocyanate, 1,5'-naphthalene diisocyanate, tolidine diisocyanate, diphenylmethylmethane diisocyanate, tetraalkyl diphenylmethane diisocyanate, 4,4'-dibenzyl diisocyanate, 1,3-phenylene diisocyanate; triisocyanates such as lysine ester triisocyanate, triphenylmethane triisocyanate, 1,6,11-undecane triisocyanate, 1,8-diisocyanato-4,4-isocyanatomethyloctane, 1,3,6-hexamethylene triisocyanate, bicycloheptane triisocyanate; their dimers, trimers (isocyanurate bonds), biuret bodies; and their reaction products with polyols. These can be used alone or in combination of two or more.

[0073] Chain extenders contain two or more active hydrogen groups in the molecule. Examples of chain extenders 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, isophorone diamine; and water. These are used alone or in combination of two or more.

[0074] Examples of polymerization terminators include: compounds having one active hydrogen in the molecule (such as monoalcohols, monoamines, etc.) and monoisocyanate compounds.

[0075] The synthesis method of urethane resins can be a one-shot method in which all components react at once, or a multi-step method with stepwise reactions. In the multi-step method, a part of the compound containing active hydrogen (for example, high molecular weight polyol) is reacted with polyisocyanate to form an NCO-terminated prepolymer, and then the prepolymer is reacted with the remaining part of the compound containing active hydrogen.

[0076] Based on 100 parts by mass of the total amount of the resin solid components of the aqueous coating composition, the solid content of the aqueous polyurethane resin (B) is 15 parts by mass or more and 60 parts by mass or less. Thus, the effect of suppressing the mixed layer is not impaired, and the chipping resistance is improved. The above-mentioned solid content of the aqueous polyurethane resin (B) can exceed 15 parts by mass, can be 20 parts by mass or more, can also be 22 parts by mass or more, and can be 25 parts by mass or more. The above-mentioned solid content of the aqueous polyurethane resin (B) can be 50 parts by mass or less, can be 40 parts by mass or less, and can also be 30 parts by mass or less.

[0077] The Tg of the aqueous polyurethane resin (B) is, for example, -90°C or higher and -10°C or lower. Thus, the flexibility of the coating film is further improved, and the chipping resistance can be further improved. The Tg of the aqueous polyurethane resin (B) can be -40°C or higher, and can also be -35°C or higher. The Tg of the aqueous polyurethane resin (B) can be -15°C or lower, and can also be -18°C or lower. The Tg of the aqueous polyurethane resin (B) can be measured by a differential scanning calorimeter.

[0078] (Melamine resin (C))

[0079] The melamine resin (C) is a curing agent and mainly reacts with the hydroxyl group-containing resin (D) to form a crosslinked structure. Thus, a coating film with high hardness is obtained.

[0080] The melamine resin (C) can be water-soluble or water-insoluble. The melamine resin (C) contains: around the triazine ring (triazine nucleus), 6 substituents R are bonded via 3 nitrogen atoms N 1 ~N 3 (bonded) to form a structure (-N 1 ~R 6 ), -N 1 (R 1 )(R 2 ), -N 2 (R 3 )(R 4 ), -N 3 (R 5 )(R 6 ). Through the planar and rigid triazine ring, the hardness of the obtained coating film is further improved. Thus, the excellent chipping resistance and excellent hardness generated by the aqueous polyester resin (A) and the aqueous polyurethane resin (B) can be balanced.

[0081] The substituents R 1 ~R 6 are each independently a hydrogen atom, an alkyl ether group (-CH2-OR 7 ), and a hydroxymethyl group (-CH2OH). The substituents R 1 ~R 6 and R7 It may be an alkyl group having 1 to 8 carbon atoms independently, 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, a n-butyl group or an isobutyl group.

[0082] Melamine resin (C) is usually composed of a polynuclear body formed by multiple triazine rings bonded to each other. Melamine resin (C) may be a mononuclear body composed of 1 triazine ring.

[0083] Examples of the melamine resin (C) include those having only an alkyl ether group as the substituent R 1 ~R 6 of the all-alkyl type; those having -N(-CH2-OR 7 )(-CH2OH) of the hydroxymethyl type; those having -N(-CH2-OR 7 )(H) of the imino type; those having -N(-CH2-OR 7 )(-CH2OH) and -N(-CH2-OR 7 )(H) of the hydroxymethyl / imino type.

[0084] Melamine resin (C) may be hydrophobic or hydrophilic. Melamine resin (C) may particularly be hydrophobic. Thus, melamine resin (C) can react with the hydroxyl groups of other resins. On the other hand, it can self-aggregate to form a cluster structure. Through the cluster structure, the hardness of the coating film can be further improved. Moreover, the hydrophobicity of the entire uncured coating film is further improved, and the suppression effect of the mixed layer can be further improved. Moreover, the water resistance of the coating film is improved. A coating film having excellent hardness and water resistance is, for example, suitable for direct bonding of glass. The glass is, for example, a vehicle window. Direct bonding of glass is a method of bonding glass and a vehicle via an adhesive. If the aqueous coating composition according to the present disclosure is used in the coating of a vehicle, the direct bonding of glass becomes easy.

[0085] Regarding the hydrophobic melamine resin (C), for example, in the substituents R 1 ~R 6 the total average number of hydrogen atoms and hydroxymethyl groups is 1 or less. The hydrophobic melamine resin (C) may be of the all-alkyl type. Specific examples of the all-alkyl type melamine resin (C) include methylated melamine resin, butylated melamine resin, and isobutylated melamine resin.

[0086] The weight average molecular weight of the melamine resin (C) may be 1000 or more and 7000 or less. Thus, the hardness of the resulting coating film can be further improved. The weight average molecular weight of the melamine resin (C) may be 1500 or more, or may be 2000 or more. The weight average molecular weight of the melamine resin (C) may be 5000 or less, or may be 4000 or less.

[0087] As the commercially available melamine resin (C), for example, the Cymel series (trade name) manufactured by Allnex Co., Ltd. can be cited. 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, Cymel 1141; the U-VAN (trade name) series manufactured by Mitsui Chemicals, Inc. These are used alone or in combination of two or more.

[0088] With respect to the resin solid content of 100 parts by mass of the aqueous coating composition, the solid content of the melamine resin (C) is, for example, 10 parts by mass or more and 40 parts by mass or less. Thereby, the curing reaction easily proceeds, and a coating film having high hardness can be easily obtained. The above solid content of the melamine resin (C) can be 15 parts by mass or more, or can be 17 parts by mass or more. The above solid content of the melamine resin (C) can be 35 parts by mass or less, or can be 30 parts by mass or less.

[0089] (Other curing agents)

[0090] The aqueous coating composition may contain a curing agent other than the melamine resin (C). As other curing agents, for example, blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, metal ions can be cited. These are used alone or in combination of two or more.

[0091] (Hydroxyl group-containing resin (D))

[0092] As described above, the hydroxyl group-containing resin (D) reacts with the melamine resin (C) to form a crosslinked structure. The hydroxyl group-containing resin (D) is a resin containing a hydroxyl group other than the aqueous polyester resin (A), the aqueous polyurethane resin (B), and the melamine resin (C).

[0093] The hydroxyl group-containing resin (D) has one or more hydroxyl groups. Examples of the hydroxyl group-containing resin include a hydroxyl group-containing acrylic resin (D1) and a polyether polyol (D2). These may be used alone or in combination of two. The aqueous coating composition may contain one or more (especially two or more) hydroxyl group-containing acrylic resins (D1), or one or more hydroxyl group-containing acrylic resins (D1) and one or more polyether polyols (D2).

[0094] With respect to the resin solid content of 100 parts by mass of the aqueous coating composition, the solid content of the hydroxyl group-containing resin (D) is, for example, 10 parts by mass or more and 30 parts by mass or less. Thereby, the curing reaction easily proceeds, and a coating film having high hardness can be easily obtained. The above solid content of the hydroxyl group-containing resin (D) may be 15 parts by mass or more, or may be 16 parts by mass or more. The above solid content of the hydroxyl group-containing resin (D) may be 28 parts by mass or less, or may be 25 parts by mass or less.

[0095] <Hydroxyl group-containing acrylic resin (D1)>

[0096] In the aqueous coating composition, the form of the hydroxyl group-containing acrylic resin (D1) may be an emulsion. In the aqueous coating composition, the hydroxyl group-containing acrylic resin (D1) may be dissolved. That is, the hydroxyl group-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.

[0097] 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)acrylates, α,β-ethylenically unsaturated monomers having an acid group, and α,β-ethylenically unsaturated monomers having a hydroxyl group. The monomers may be used alone or in combination of two or more.

[0098] Examples of the (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, phenyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, tert-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate. (Meth)acrylate means acrylate and methacrylate.

[0099] Examples of the α,β-unsaturated monomer 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, methacrylic acid, α-hydrogen-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-acrylamide-2-methylpropanesulfonic acid, p-hydroxystyrene, 2,4-dihydroxy-4'-vinyldibenzophenone.

[0100] Examples of the α,β-unsaturated monomer having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and their adducts with ε-caprolactone.

[0101] Other α,β-unsaturated monomers can be used in combination. Examples of other α,β-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.

[0102] The method of emulsion polymerization is not particularly limited. For example, an emulsifier is dissolved in water or an aqueous medium containing an organic solvent such as an alcohol, an ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.) as needed, and the α,β-unsaturated monomer and a polymerization initiator are added dropwise under heating and stirring. The α,β-unsaturated monomer can be emulsified with an emulsifier in advance.

[0103] As the polymerization initiator and the emulsifier, substances commonly used by those skilled in the art can be used. As needed, a chain transfer agent such as a mercaptan (e.g., lauryl mercaptan) and α-methylstyrene dimer can be used to adjust the molecular weight. The reaction temperature, reaction time, etc. can be appropriately selected within the range commonly adopted by those skilled in the art. The obtained acrylic resin emulsion is neutralized with an alkali as needed.

[0104] The number average molecular weight of the hydroxyl group-containing acrylic resin (D1) (emulsion of acrylic resin) obtained by emulsion polymerization can be 3,000 or more.

[0105] The hydroxyl value (solid component hydroxyl value) of the hydroxyl group-containing acrylic resin (D1) can be 20 mgKOH / g or more and 180 mgKOH / g or less. The acid value (solid component acid value) of the hydroxyl group-containing acrylic resin (D1) can be 1 mgKOH / g or more and 80 mgKOH / g or less.

[0106] The water-soluble hydroxy group-containing acrylic resin (D1) can be prepared, for example, by solution-polymerizing the above-mentioned α,β-ethylenically unsaturated monomers and solubilizing them using a basic compound.

[0107] <Polyether polyol (D2)>

[0108] In the aqueous coating composition, the polyether polyol (D2) can be in the form of an emulsion. In the coating composition, the polyether polyol (D2) can be dissolved. That is, the polyether polyol (D2) can be a water-soluble polyether polyol.

[0109] The polyether polyol (D2) is obtained, for example, by adding polymerizable alkylene oxides such as ethylene oxide, propylene oxide, and tetrahydrofuran to a polyol. The polyether polyol (D2) can be a polyether diol having two hydroxy groups in one molecule.

[0110] Examples of the polyether diol include polyalkylene diols such as polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol.

[0111] (Solvent)

[0112] The aqueous coating composition contains water as a solvent. The aqueous coating composition can contain both an aqueous solvent and an organic solvent at the same time. In the aqueous coating composition, the proportion of water in the solvent is, for example, 50% by mass or more, 70% by mass or more, 90% by mass or more.

[0113] Examples of the organic solvent 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;

[0114] ether solvents such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, methyl methoxybutanol, ethoxypropanol, ethylene glycol isopropyl ether, ethylene glycol-tert-butyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, methoxybutanol, and propylene glycol monobutyl ether; alcohol solvents such as methanol, ethanol, butanol, and propanol; and ketone solvents such as acetone, butanone, and methyl isobutyl ketone. These are used alone or in combination of two or more.

[0115] (Others)

[0116] The aqueous coating composition can further contain other components. Examples of the other components include pigments, film-forming aids, surface modifiers, preservatives, fungicides, defoamers, light stabilizers, ultraviolet absorbers, antioxidants, and pH regulators.

[0117] (First embodiment)

[0118] In the first embodiment, the aqueous coating composition contains: an aqueous polyester resin (A) in the form of a dispersion, a low molecular weight polyester resin (a1), an aqueous polyurethane resin (B) in the form of a dispersion, a melamine resin (C), and a hydroxy group-containing acrylic resin (D1) in each of the water-soluble and emulsion forms.

[0119] In the first embodiment, with respect to 100 parts by mass of the resin solid content of the aqueous coating composition, the solid content of each resin is as follows.

[0120] The aqueous polyester resin (A) in the form of a dispersion: 20 parts by mass or more and 30 parts by mass or less;

[0121] The low molecular weight polyester resin (a1): 2 parts by mass or more and 10 parts by mass or less;

[0122] The aqueous polyurethane resin (B) in the form of a dispersion: 25 parts by mass or more and 35 parts by mass or less;

[0123] The melamine resin (C): 15 parts by mass or more and 25 parts by mass or less;

[0124] The acrylic resin (D1) in the water-soluble and emulsion forms: 5 parts by mass or more and 15 parts by mass or less, respectively.

[0125] (Second embodiment)

[0126] In the second embodiment, the aqueous coating composition contains: an aqueous polyester resin (A) in the form of a dispersion, an aqueous polyurethane resin (B) in the form of a dispersion, a melamine resin (C), a hydroxy group-containing acrylic resin (D1) in each of the water-soluble and emulsion forms, and a water-soluble polyether polyol (D2).

[0127] In the second embodiment, with respect to 100 parts by mass of the resin solid content of the aqueous coating composition, the solid content of each resin is as follows.

[0128] The aqueous polyester resin (A) in the form of a dispersion: 12.5 parts by mass or more and 45 parts by mass or less;

[0129] The aqueous polyurethane resin (B) in the form of a dispersion: 15 parts by mass or more and 25 parts by mass or less;

[0130] The melamine resin (C): 15 parts by mass or more and 25 parts by mass or less;

[0131] The acrylic resin (D1) in the water-soluble and emulsion forms: 1 part by mass or more and 10 parts by mass or less, respectively;

[0132] The water-soluble polyether polyol (D2): 5 parts by mass or more and 15 parts by mass or less.

[0133] (Preparation of aqueous coating composition)

[0134] The aqueous coating composition can be prepared, for example, by stirring the respective components with a stirrer or the like. The pigment can be dispersed in a medium containing water, a surfactant or a dispersant, etc. using a sand mill or the like and then mixed with other components.

[0135] (Method for manufacturing a coated article)

[0136] The aqueous coating composition according to the present disclosure can suppress the formation of a mixed layer with the overlapping aqueous coating even when pre-drying is omitted. Therefore, the aqueous coating composition according to the present disclosure is suitable for manufacturing a coated article having a multi-layer coating film formed of a plurality of aqueous coatings. In particular, the aqueous coating composition according to the present disclosure is suitable for forming an intermediate coating film.

[0137] The method for manufacturing a coated article according to the present disclosure includes the following steps: a step of coating the aqueous coating composition according to the present disclosure on an object to be coated to form an uncured intermediate coating film; and a step of coating an aqueous primer coating composition on the uncured intermediate coating film to form an uncured primer coating film. There is no pre-drying step between the step of forming the uncured intermediate coating film and the step of forming the uncured primer coating film. By omitting the pre-drying step, the environmental load and production cost are reduced.

[0138] In the pre-drying step, at least a part of the solvent is intentionally removed. Omitting the pre-drying step means that, before the step of forming the uncured primer coating film, the operation of intentionally removing the solvent from the uncured intermediate coating film is not performed. As a method for intentionally removing the solvent (i.e., the pre-drying method), for example, natural drying and heat drying can be cited. In natural drying, for example, the uncured intermediate coating film is left at a temperature of 20 °C or higher and 25 °C or lower for 5 minutes or more and 15 minutes or less. Heat drying is performed under the condition that the curing reaction of the coating film forming components has not occurred or at least the curing reaction has not been completed. In heat drying, for example, the uncured intermediate coating film is heated at a temperature of 50 °C or higher and 80 °C or lower for 30 seconds or more and 10 minutes or less.

[0139] (I) Step of forming an uncured intermediate coating film

[0140] The aqueous coating composition according to the present disclosure is coated on an object to be coated to form an uncured intermediate coating film. The intermediate coating film is interposed between the object to be coated and the primer coating film. Through the intermediate coating film, the adhesion between the primer coating film and the object to be coated is improved. Moreover, the intermediate coating film in the present disclosure suppresses the formation of a mixed layer. In addition, since the aqueous coating composition according to the present disclosure contains a polyester resin, the coating surface becomes smoother by intermediate coating, and unevenness of the primer coating film is easily suppressed.

[0141] From the viewpoints of the smoothness and chipping resistance of the painted article, the thickness of the cured intermediate coat film can be 6 μm or more and 35 μm or less. The thickness of the cured intermediate coat film can be 8 μm or more, or can be 10 μm or more. The thickness of the cured intermediate coat film can be 25 μm or less, or can be 22 μm or less.

[0142] The thickness of the film can be measured using an electromagnetic film thickness gauge (for example, SDM-miniR manufactured by SANKO Co., Ltd.). The thickness of the film is the average of the film thicknesses at any five points.

[0143] As the painting method, for example, air spraying, airless spraying, rotary atomization painting can be mentioned. These methods can be combined with electrostatic painting. Among them, from the viewpoint of coating efficiency, rotary atomization electrostatic painting is preferred. In rotary atomization electrostatic painting, for example, rotary atomization electrostatic coating machines generally called "Micro·Micro Bell (μμBell)", "Micro Bell (μBell)", "Metallic Bell (MetaBell)" etc. are used.

[0144] (Object to be painted)

[0145] As the material of the object to be painted, for example, metal, resin, glass can be mentioned. According to the aqueous coating composition related to the present disclosure, a film with excellent chipping resistance can be obtained, and thus it is particularly suitable for painting objects to be painted containing metal.

[0146] There is no particular limitation on the shape of the object to be painted. Specifically, examples of the object to be painted include automobile bodies such as sedans, trucks, motorcycles, buses, and automobile body parts, spoilers, bumpers, mirror covers, grilles, door handles, and other automobile parts.

[0147] As the metal, for example, aluminum, tin, zinc or their alloys (for example, steel) can be mentioned. As the object to be painted made of metal, typically, cold-rolled steel sheets, hot-rolled steel sheets, stainless steels, electrogalvanized steel sheets, hot-dip galvanized steel sheets, galvanized-aluminum alloy steel sheets, galvanized-iron alloy steel sheets, galvanized-magnesium alloy steel sheets, galvanized-aluminum-magnesium alloy steel sheets, aluminized steel sheets, aluminized-silicon alloy steel sheets, tin-plated steel sheets, etc. can be mentioned.

[0148] The object to be painted made of metal can be surface-treated. As the surface treatment, for example, phosphate treatment, chromate treatment, zirconium conversion treatment, composite oxide treatment can be mentioned. After the surface treatment, the object to be painted made of metal can be further painted with an electrodeposition coating. The electrodeposition coating can be cationic or anionic.

[0149] As the resin, for example, the following can be cited: polypropylene resin, polycarbonate resin, urethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, polyamide resin. The resin-coated object is preferably degreased.

[0150] The pre-drying (also referred to as preheating) process after coating with the aqueous coating composition is omitted. Even in this case, the mutual mixing of the uncured intermediate coating film and the aqueous primer coating composition coated thereon is suppressed, and the formation of the mixed layer is suppressed. Therefore, the appearance of the obtained coated article is improved.

[0151] (II) Process of forming an uncured primer coating film

[0152] An aqueous primer coating composition is coated on the uncured intermediate coating film to form an uncured primer coating film. The primer coating film can be a single layer or a laminated coating film of two or more layers.

[0153] In this way, a multi-layer coating film is obtained by the so-called wet-on-wet coating method of coating on an uncured coating film. Since the wet-on-wet coating can omit the baking and drying furnace, the environmental load and production cost can be reduced. In the manufacturing method of the present disclosure, since the pre-drying process after coating the aqueous coating composition can be further omitted, the effect of reducing the environmental load and production cost is higher.

[0154] The thickness of the cured primer coating film is, for example, 6 μm or more and 35 μm or less. The thickness of the cured primer coating film can be 8 μm or more, or can be 10 μm or more. The thickness of the cured primer coating film can be 25 μm or less, or can be 20 μm or less.

[0155] As the coating method, for example, the same method as the coating method of the intermediate coating composition can be cited.

[0156] After coating the aqueous primer coating composition, the above-mentioned pre-drying process can be carried out.

[0157] (Aqueous primer coating composition)

[0158] The aqueous primer 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 primer coating film formed from this aqueous primer coating composition has excellent appearance. The aqueous primer coating composition can further contain pigments and various additives. As additives, for example, the following can be cited: ultraviolet absorber, antioxidant, defoaming agent, surface conditioner, anti-pinhole agent.

[0159] (III) Process of forming an uncured transparent coating film

[0160] A transparent coating composition can be further coated on the uncured primer coating film to form an uncured transparent coating film.

[0161] From the viewpoints of scratch resistance and smoothness, the thickness of the cured transparent coating film can be 20 μm or more and 50 μm or less. The thickness of the cured transparent coating film can be 25 μm or more, or can be 30 μm or more. The thickness of the cured transparent coating film can be 45 μm or less, or can be 40 μm or less.

[0162] There is no particular limitation on the coating method. As the coating method, for example, the same method as the coating method of the intermediate coating composition can be cited. Among them, from the viewpoint of coating efficiency, rotary atomization electrostatic coating is preferred. After coating the transparent coating composition, pre-drying can be carried out in the same manner as above.

[0163] (Transparent coating composition)

[0164] The transparent coating composition can be solvent-based, water-based, or powder-based. Considering aspects such as transparency or acid etch resistance, the solvent-based transparent coating composition may contain an acrylic resin and / or a polyester resin as a film-forming resin, and an amino resin and / or an isocyanate as a curing agent. In addition, the solvent-based transparent coating composition may contain an acrylic resin and / or a polyester resin having a carboxylic acid and / or an epoxy group. The transparent coating composition can contain various pigments within the range that does not impair the transparency and the effects of the coating composition involved in the present disclosure. The transparent coating composition can contain the above-mentioned additives as needed.

[0165] (IV) Curing process

[0166] The uncured coating films are heated to cure them. In this process, the uncured intermediate coating film, the uncured primer coating film, and the uncured transparent coating film are cured at one time.

[0167] The heating conditions are appropriately set according to the composition of each coating composition or the material of the object to be coated, etc. The heating temperature is, for example, 60°C or more and 170°C or less, and can be 90°C or more and 150°C or less.

[0168] The heating time can be appropriately set according to the heating temperature. When the heating temperature is 60°C or more and 170°C or less, the heating time is, for example, 10 minutes or more and 60 minutes or less, and can be 15 minutes or more and 45 minutes or less. The heating time refers to the time when the target temperature is reached in the heating device and the object to be coated is maintained at the target temperature, and does not consider the time before reaching the target temperature. As the heating device, for example, a drying furnace using a heating source such as hot air, electricity, gas, or infrared rays can be cited.

[0169] Examples

[0170] The present invention will be further specifically described by the following examples, but the present invention is not limited to these examples. In the examples, "parts" and "%" are based on mass standards unless otherwise specified. The details of the components described in Tables 1 and 2 are as follows.

[0171] · Aqueous polyester resin (A): Trade name "PED-7301", manufactured by Nippon Paint Automotive Coating Co., Ltd., number average molecular weight of 2400, Tg of 0 °C, acid value of 21 mg KOH / g, hydroxyl value of 90 mg KOH / g;

[0172] · Low hydroxyl value polyester resin (a1): Trade name "PED-5501", manufactured by Nippon Paint Automotive Coating Co., Ltd., water-soluble, number average molecular weight of 2300, Tg of -5 °C, acid value of 21 mg KOH / g, hydroxyl value of 57 mg KOH / g;

[0173] · Aqueous polyurethane resin (B): Trade name "U-Coat N-800T", manufactured by Sanyo Chemical Industries, Ltd., dispersion, Tg of -20 to -30 °C;

[0174] · Melamine resin (C): Trade name "Cymel 250", manufactured by Allnex, imino type, weight average molecular weight of 2500;

[0175] · Hydroxyl group-containing acrylic resin (D1): Trade name "EMA-1046", manufactured by Nippon Paint Automotive Coatings Co., Ltd., emulsion, hydroxyl value of 30 mg KOH / g.

[0176] [Examples 1 to 7, Comparative Examples 1 to 3]

[0177] (1) Preparation of aqueous coating composition

[0178] As shown in Table 1, the blending amounts of the aqueous polyester resin (A) and the aqueous polyurethane resin (B) were changed, and aqueous coating compositions were prepared respectively.

[0179] (2) Manufacture of coated article

[0180] Powernix 150 (trade name, cationic electrodeposition coating manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was electrodeposited on a dull-finished sheet (phosphate-treated) so that the dry film thickness was 20 μm. Then, it was heated at 160 °C for 30 minutes. After that, it was cooled to form a cured electrodeposition coating film.

[0181] Using a rotary atomization type electrostatic coating apparatus, the above aqueous coating composition was applied to the obtained electrodeposited coating film so that the dry film thickness was 12 μm. Subsequently, Aqurex AR-3400 (trade name, an aqueous metallic primer coating manufactured by Nippon Paint Automotive Coatings Co., Ltd.), which is an aqueous primer coating composition, was applied using a rotary atomization type electrostatic coating apparatus so that the dry film thickness was 15 μm, and preheating was carried out at 80 °C for 3 minutes.

[0182] Subsequently, PU Excel O-4300 (trade name, a two-component transparent coating manufactured by Nippon Paint Automotive Coatings Co., Ltd.), which is a transparent coating composition, was applied using a rotary atomization type electrostatic coating apparatus so that the dry film thickness was 35 μm. Thereafter, heat curing was carried out at 140 °C for 30 minutes to obtain a coated article.

[0183] [Evaluation]

[0184] The evaluation of the coated article was carried out according to the following method. The results are shown in Table 1.

[0185] (1) Appearance

[0186] Using Wave-Scan DOI (manufactured by BYK Gardner), Wd (measurement wavelength: 3.0 mm to 10.0 mm) was measured. Wd is a value obtained by quantifying the roughness of the coating film in the measurement wavelength. The smaller Wd is, the smaller the roughness of the coating film, indicating good appearance. If Wd is 24 or less, it is suitable for practical use.

[0187] (2) Chipping resistance

[0188] Using a gravelometer KSS-1 (manufactured by Suga Test Instruments Co., Ltd.), a gravel impact test was carried out on the test panel under the following conditions.

[0189] <Test conditions>

[0190] Size of stones: 2.36 to 4.75 mm;

[0191] Amount of stones: 100 g;

[0192] Distance: 35 cm;

[0193] Shot pressure: 0.3 kg / cm 2 ;

[0194] Shot angle: 90°;

[0195] Test temperature: -20 °C.

[0196] Evaluate the test panel after the flying stone test visually according to the following criteria. The evaluation is set as the average value of three test panels for each level. If the following criteria are 3.0 points or more, it is suitable for practical use.

[0197] 5: Almost no peeling is found.

[0198] 4: The peeling area is small, and almost no peeling is found at the interface between the electrodeposition coating film and the intermediate coating film.

[0199] 3: The peeling area is slightly larger, and peeling is found at the interface between the electrodeposition coating film and the intermediate coating film.

[0200] 2: The peeling area is large, and peeling is found at the interface between the electrodeposition coating film and the intermediate coating film.

[0201] 1: The peeling area is large and the electrodeposition coating film is damaged.

[0202] [Table 1]

[0203]

[0204] Industrial Applicability

[0205] The aqueous coating composition of the present invention can obtain a multi-layer coating film with excellent appearance and chipping resistance even when pre-drying is omitted. Therefore, it is suitable for forming various multi-layer coating films.

[0206] This application claims priority based on Japanese Patent Application No. 2023-200889 filed on November 28, 2023, and the entire description thereof is incorporated herein by reference.

Claims

1. An aqueous coating composition, which contains: an aqueous polyester resin (A), an aqueous polyurethane resin (B), a melamine resin (C), and a hydroxyl group-containing resin (D) other than the above-mentioned aqueous polyester resin (A), the above-mentioned aqueous polyurethane resin (B), and the above-mentioned melamine resin (C), Among them, the number-average molecular weight of the above-mentioned aqueous polyester resin (A) is 1000 or more and 6000 or less, the glass transition temperature is -30°C or more and 20°C or less, the acid value is 10 mgKOH / g or more and 25 mgKOH / g or less, and the hydroxyl value is more than 60 mgKOH / g and 100 mgKOH / g or less, relative to 100 parts by mass of the resin solid content of the above-mentioned aqueous coating composition, the solid content of the above-mentioned 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 above-mentioned aqueous coating composition, the solid content of the above-mentioned aqueous polyurethane resin (B) is 15 parts by mass or more and 60 parts by mass or less.

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

3. The aqueous coating composition according to claim 1 or 2, wherein, relative to 100 parts by mass of the resin solid content of the above-mentioned aqueous coating composition, the solid content of the above-mentioned melamine resin (C) is 10 parts by mass or more and 40 parts by mass or less.

4. The aqueous coating composition according to any one of claims 1 to 3, wherein the glass transition temperature of the above-mentioned aqueous polyurethane resin (B) is -90°C or more and -10°C or less.

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

6. A method for manufacturing a coated article, which includes the following steps: a step of coating the aqueous coating composition according to any one of claims 1 to 5 on an object to be coated to form an uncured intermediate coating film; and a step of coating an aqueous primer coating composition on the uncured intermediate coating film to form an uncured primer coating film, Among them, there is no pre-drying step between the step of forming the uncured intermediate coating film and the step of forming the uncured primer coating film.

7. The method for manufacturing a coated article according to claim 6, wherein, In the step of forming the uncured intermediate coating film, the aqueous coating composition is coated in such a manner that the thickness of the cured intermediate coating film is 6 μm or more and 35 μm or less.

8. The method for manufacturing a coated article according to claim 6 or 7, wherein, In the step of forming the uncured primer coating film, the aqueous primer coating composition is coated in such a manner that the thickness of the cured primer coating film is 6 μm or more and 35 μm or less.

9. The method for manufacturing a coated article according to any one of claims 6 to 8, which further includes the following steps: a step of coating a transparent coating composition on the uncured primer coating film after the step of forming the uncured primer coating film to form an uncured transparent coating film; and a step of heating the uncured intermediate coating film, the uncured primer coating film, and the uncured transparent coating film to form a multi-layer coating film.

10. The method for manufacturing a coated article according to claim 9, wherein, In the step of forming the uncured transparent coating film, the transparent coating composition is coated in such a manner that the thickness of the cured transparent coating film is 20 μm or more and 50 μm or less.

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