Water-based paint composition and method for producing multi-layer paint film

Abstract

To provide a water-based coating composition capable of forming a coating film excellent in a coating film appearance while good in interlayer adhesion at recoating even in a coating object having a sealer part.SOLUTION: A water-based coating composition includes a film-forming resin (A) and a curative (B). The film-forming resin (A) includes an acrylic resin water dispersion (A1). An acrylic resin in the acrylic resin water dispersion (A1) includes a hydroxyl group. The curative (B) includes a melamine resin (B1). The melamine resin (B1) includes a melamine resin (B1a) with a weight-average molecular weight of 6,000 or more and a melamine resin (B1b) with a weight-average molecular weight of less than 6,000.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an aqueous coating composition and a method for producing a multilayer coating film. [Background technology]

[0002] On the surface of a substrate such as an automobile body, multiple coating films (multi-layer coating films) with various functions are sequentially formed to protect the substrate while at the same time imparting a beautiful appearance and excellent design. A common method of forming multi-layer coating films, for example, for steel plate, is to form a primer coating film such as an electrodeposition coating film on the substrate, which has excellent conductivity, and then sequentially form an intermediate coating film, a base coating film, and a clear coating film on top of that.

[0003] In recent years, there has been growing awareness of the need to reduce environmental impact, and there is a demand for products that are more environmentally friendly. In the field of paints, for example, there is a demand for a reduction in the amount of organic solvents used, and this demand can be met by using aqueous paint compositions that use water as the solvent.

[0004] Patent Document 1 describes an aqueous base coating composition that contains a hydroxyl group- and carboxyl group-containing resin, a blocked polyisocyanate compound, a phosphate group-containing compound, and a basic compound, in which the acid dissociation constant of the basic compound is 7.0 to 8.5, the boiling point is 100 to 200°C, and the pH of the entire coating composition is 7.0 to 8.2. Patent Document 2 describes an aqueous coating composition that contains a hydroxyl group-containing acrylic resin and / or a hydroxyl group-containing polyester resin, an oligomer, and an alkyl-etherified melamine resin, wherein the oligomer has a number average molecular weight of 200 to 800 and a solubility parameter of 10.0 to 13.5, and the molar ratio of methyl groups to butyl groups (methyl groups / butyl groups) in the alkyl-etherified melamine resin is 50 / 50 to 0 / 100. Patent Document 3 describes an aqueous coating material containing a hydroxyl group-containing polyester resin, a hydroxyl group-containing acrylic resin, a hydroxyl group-containing polyurethane resin, and a melamine resin. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2021 / 084802 [Patent Document 2] International Publication No. 2021 / 024604 [Patent Document 3] International Publication No. 2020 / 153057 Summary of the Invention [Problem to be solved by the invention]

[0006] On the other hand, coating objects such as automobile bodies have extremely complex structures. For example, in the doors, hoods, roofs, and other parts of an automobile body, there are joints or seams where two steel plates are overlapped. The joints or seams have gaps at their boundaries. These gaps are difficult to cover with conventional coating compositions such as an intermediate coating composition, a base topcoat coating composition, and a clear topcoat coating composition. Furthermore, foreign matter such as water and dust easily penetrates the gaps at the boundaries of the joints or seams, which can cause rust. For this reason, sealer areas are generally provided at the joints or seams by coating and filling with a sealant. The coating and filling of a sealant are generally performed on steel plates on which an electrodeposition coating film has been formed. Then, an intermediate coating film and a topcoat coating film are formed on the substrate with the sealer area.

[0007] The intermediate and topcoat coatings used in the painting of automobile bodies have a significant impact on the appearance and design of the coating and the finished product. Therefore, if coating defects, such as bumps due to foreign matter adhesion or dents or craters due to coating defects, are discovered during the application and baking / curing process of the intermediate and topcoat coatings, the areas where the coating defects exist or the entire substrate are repainted with intermediate and topcoat coatings. This type of repainting is called recoating. In this recoating process, the interlayer adhesion between the multilayer coating film, including the intermediate and topcoat coatings already applied to the substrate, and the repainted coating film consisting of the intermediate and topcoat coatings applied by the repainting process becomes an issue. Poor interlayer adhesion can lead to delamination between the existing multilayer coating film and the repainted coating. The problem of interlayer adhesion during recoating is particularly pronounced on substrates with sealer sections.

[0008] When the adhesion of dirt or other foreign matter causes abnormalities in the appearance of the paint film, the affected area is repaired by sanding it with sandpaper or the like, or the base paint and clear paint are repainted on the block of the automobile exterior panel. Also, when changing the color tone of only a portion of an automobile exterior panel, known as a two-tone finish, the base paint and clear top coat are repainted on the area where the color tone is to be changed.

[0009] In such cases, when comparing the case where the applied (first coat) clear coating is sanded off before repainting and a new (second coat) base paint and top coat clear coating are applied, with the case where the applied (first coat) clear coating is not sanded off and a new (second coat) base paint and top coat clear coating are applied, it is known that the adhesion between the first coat clear coating and the second coat base coating is generally inferior when the first coat clear coating is not sanded off.

[0010] As such, not polishing the first coat of clear paint results in poorer adhesion to the second coat of base paint, so it is common to polish the first coat of clear paint and then reapply the second coat of base paint and top coat of clear paint.

[0011] If the adhesion of the second coat of base paint and top clear paint is not impaired when the second coat of base paint and top clear paint are reapplied without sanding off the first coat of clear paint, the effort of sanding off the first coat of clear paint can be eliminated, shortening the time and reducing the cost required for repairs or repainting on an automobile painting line. Therefore, it has been desired to improve the adhesion of the second coat of base paint (also referred to as "NSR properties" in this disclosure) when the second coat of base paint and top clear paint are reapplied without sanding off the first coat of clear paint (also referred to as "non-sand recoat (NSR)" in this disclosure).

[0012] The present disclosure has been made in view of the above-mentioned problems, and aims to provide an aqueous coating composition that can form a coating film that has good interlayer adhesion during recoating and excellent appearance, even on substrates that have a sealer section. Another object of the present disclosure is to provide a method for producing a multilayer coating film using such an aqueous coating composition. [Means for solving the problem]

[0013] The present disclosure includes [1] A coating composition comprising a film-forming resin (A) and a curing agent (B), the film-forming resin (A) comprises an aqueous acrylic resin dispersion (A1); the acrylic resin in the acrylic resin water dispersion (A1) has a hydroxyl group, the curing agent (B) contains a melamine resin (B1), The melamine resin (B1) is an aqueous coating composition comprising a melamine resin (B1a) having a weight-average molecular weight of 6,000 or more and a melamine resin (B1b) having a weight-average molecular weight of less than 6,000. [2] The aqueous coating composition according to [1], wherein the hydroxyl value of the acrylic resin water dispersion (A1) is 1 mgKOH / g or more and 150 mgKOH / g or less. [3] The aqueous coating composition according to [1] or [2], wherein the content of the melamine resin (B) is 10 to 50 parts by mass per 100 parts by mass of the total of the solid content of the coating film-forming resin (A) and the solid content of the curing agent (B). [4] The aqueous coating composition according to any one of [1] to [3], wherein the content of the melamine resin (B1a) is 1% by mass or more and 20% by mass or less of the total amount of the melamine resin (B1). [5] The aqueous coating composition according to any one of [1] to [4], wherein the coating film-forming resin (A) further comprises a urethane resin (A2) and / or a polyester resin (A3). [6] A step of applying the aqueous coating composition according to any one of [1] to [5] onto a substrate to form a coating film; A step of applying a clear coating wet-on-wet onto the coating film to form a clear coating film; and a step of simultaneously heating and curing the paint film and the clear paint film to form a multi-layer paint film. [Effects of the Invention]

[0014] The aqueous coating composition of the present disclosure can form a coating film that exhibits good interlayer adhesion during recoating and excellent appearance, even on substrates having a sealer section. The present disclosure also provides a method for producing a multilayer coating film using such an aqueous coating composition. [Brief explanation of the drawings]

[0015] [Figure 1] Figure 1 is a schematic diagram of the window direct bond (WDB) adhesion test. DETAILED DESCRIPTION OF THE INVENTION

[0016] The aqueous coating composition of the present disclosure comprises: A coating composition comprising a film-forming resin (A) and a curing agent (B), the film-forming resin (A) comprises an aqueous acrylic resin dispersion (A1); the acrylic resin in the acrylic resin water dispersion (A1) has a hydroxyl group, the curing agent (B) contains a melamine resin (B1), The melamine resin (B1) includes a melamine resin (B1a) having a weight-average molecular weight of 6,000 or more and a melamine resin (B1b) having a weight-average molecular weight of less than 6,000.

[0017] The aqueous coating composition of the present disclosure can form a coating film with excellent interlayer adhesion and excellent coating appearance during recoating, even on substrates with a sealer. Without being bound by any particular theory, it is believed that when a coating composition is applied to a sealer, components contained in the sealer migrate into the coating film, and the coating film is formed with these components remaining. The decrease in interlayer adhesion during recoating is believed to be due to the migration of such components. Such component migration is particularly pronounced in so-called wet-on-wet coating. Wet-on-wet coating is a method of simultaneously curing multiple stacked (uncured) coating films. Compared to methods of baking and curing each coating film, this method has the advantage of eliminating several heat curing steps, thereby shortening the process and saving energy during painting. For example, a wet-on-wet coating method involves applying a base coating composition, then applying a clear coating composition without curing the base coating, and then simultaneously drying the two coating films.

[0018] The aqueous coating composition of the present disclosure contains both melamine resin (B1a) and melamine resin (B1b) as the melamine resin (B1), which is believed to inhibit migration of these components and improve interlayer adhesion during recoating, even on substrates with sealer sections. Furthermore, the aqueous coating composition of the present disclosure also provides good interlayer adhesion in wet-on-wet coating.

[0019] [Coating film forming resin (A)] The coating film-forming resin (A) is a resin that can form a coating film by reacting with the curing agent (B) described below, and includes an acrylic resin water dispersion (A1).

[0020] (acrylic resin water dispersion (A1)) The acrylic resin contained in the acrylic resin aqueous dispersion (A1) has a hydroxyl group. By including the acrylic resin aqueous dispersion (A1) as the film-forming resin (A), the film-forming resin (A) and the curing agent (B) undergo a curing reaction to form a coating film. The acrylic resin aqueous dispersion is an acrylic resin dispersed in an aqueous medium, and may be in the form of an emulsion or dispersion, and the acrylic resin may be in the form of particles in the aqueous medium.

[0021] Typically, the acrylic resin water dispersion (A1) is obtained by emulsion polymerization of a monomer mixture containing a (meth)acrylic acid alkyl ester and a hydroxyl group-containing monomer, and the monomer mixture may further contain an acid group-containing monomer and other monomers. The monomer mixture preferably contains a (meth)acrylic acid alkyl ester, a hydroxyl group-containing monomer, and an acid group-containing monomer, and more preferably contains a (meth)acrylic acid alkyl ester, a hydroxyl group-containing monomer, an acid group-containing monomer, and a styrene-based monomer. In the present disclosure, (meth)acrylic acid refers to acrylic acid and methacrylic acid.

[0022] The (meth)acrylic acid alkyl ester is a (meth)acrylic acid alkyl ester having no acid group or hydroxyl group. When the monomer mixture contains the (meth)acrylic acid alkyl ester, the main skeleton of the acrylic resin can be favorably formed.

[0023] Examples of the (meth)acrylic acid alkyl ester include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, etc. As the (meth)acrylic acid alkyl ester, only one type may be used, or two or more types may be used in combination.

[0024] Examples of the hydroxyl group-containing monomer include (meth)acrylic monomers having a hydroxyl group, and specific examples include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxybutyl (meth)acrylate; and ε-caprolactone-modified (meth)acrylic monomers.

[0025] Examples of ε-caprolactone-modified (meth)acrylic monomers include PLACCEL FA-1, PLACCEL FA-2, PLACCEL FA-3, PLACCEL FA-4, PLACCEL FA-5, PLACCEL FM-1, PLACCEL FM-2, PLACCEL FM-3, PLACCEL FM-4, and PLACCEL FM-5, manufactured by Daicel Chemical Industries, Ltd. As the hydroxyl group-containing monomer, only one type may be used, or two or more types may be used in combination.

[0026] By including a hydroxyl group-containing monomer in the monomer mixture, the resulting acrylic resin is made hydrophilic, which improves the coating workability and stability against freezing of the aqueous coating composition, and also increases the curing reactivity of the acrylic resin with the curing agent (B) described below.

[0027] The acid group-containing monomer may be a (meth)acrylic monomer having an acid group, and the acid group may be a carboxyl group, sulfonic acid group, or phosphoric acid group. From the viewpoints of improving dispersion stability and accelerating the curing reaction, the acid group is preferably a carboxyl group. By including the acid group-containing monomer in the monomer mixture, the acrylic resin obtained can be improved in various stabilities such as storage stability, mechanical stability, and stability against freezing, and the curing reaction between the acrylic resin and the curing agent (B) during coating film formation can be accelerated.

[0028] Examples of the acid group-containing monomer include carboxyl group-containing monomers such as (meth)acrylic acid, crotonic acid, isocrotonic acid, ethacrylic acid, propylacrylic acid, isopropylacrylic acid, itaconic acid, maleic anhydride, and fumaric acid; sulfonic acid group-containing monomers such as p-vinylbenzenesulfonic acid, p-acrylamidopropanesulfonic acid, and t-butylacrylamidosulfonic acid; and phosphate group-containing monomers such as 2-hydroxyethyl (meth)acrylate phosphate monoester and 2-hydroxypropyl (meth)acrylate phosphate monoester. Only one type of acid group-containing monomer may be used, or two or more types may be used in combination.

[0029] Examples of the other monomer include at least one monomer selected from the group consisting of styrene-based monomers, (meth)acrylonitrile, and (meth)acrylamide. Examples of the styrene-based monomer include styrene and α-methylstyrene. Only one type of other monomer may be used, or two or more types may be used in combination.

[0030] The monomer mixture may contain a crosslinkable monomer such as a carbonyl group-containing monomer, a hydrolyzable silyl group-containing monomer, or various polyfunctional vinyl monomers. By including a crosslinkable monomer in the monomer mixture, the resulting acrylic resin can be imparted with self-crosslinking properties. Only one type of crosslinkable monomer may be used, or two or more types may be used in combination.

[0031] In preparing the acrylic resin water dispersion (A1), emulsion polymerization can be carried out by heating the monomer mixture in an aqueous medium in the presence of a radical polymerization initiator and an emulsifier while stirring. The reaction temperature may be, for example, about 30 to 100°C. The reaction time may be appropriately selected depending on the reaction scale and reaction temperature, and may be, for example, about 1 to 10 hours. In emulsion polymerization, for example, the monomer mixture or monomer pre-emulsion may be added all at once or gradually dropwise to a reaction vessel containing water and an emulsifier. By appropriately selecting such a procedure, the reaction temperature can be adjusted. The monomer pre-emulsion can be prepared by emulsifying the monomer mixture with at least a portion of the water and emulsifier.

[0032] The radical polymerization initiator may be a known initiator used in emulsion polymerization of acrylic resins. The radical polymerization initiator is preferably a water-soluble radical polymerization initiator, and for example, a persulfate such as potassium persulfate, sodium persulfate, or ammonium persulfate may be used in the form of an aqueous solution. Alternatively, a so-called redox initiator, which is a combination of an oxidizing agent such as potassium persulfate, sodium persulfate, ammonium persulfate, or hydrogen peroxide with a reducing agent such as sodium hydrogen sulfite, sodium thiosulfate, Rongalite, or ascorbic acid, may also be used in the form of an aqueous solution. As the radical polymerization initiator, one kind may be used alone, or two or more kinds may be used in combination.

[0033] The emulsifier may be, for example, an anionic or nonionic emulsifier selected from micellar compounds having a hydrocarbon group with 6 or more carbon atoms and a hydrophilic moiety such as a carboxylate, sulfonate, or sulfate partial ester in the same molecule. Examples of anionic emulsifiers include alkali metal or ammonium salts of sulfate half esters of alkylphenols or higher alcohols; alkali metal or ammonium salts of alkyl or aryl sulfonates; and alkali metal or ammonium salts of sulfate half esters of polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl ethers, or polyoxyethylene allyl ethers. Examples of nonionic emulsifiers include polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl ethers, and polyoxyethylene allyl ethers. Other examples of emulsifiers include various anionic and nonionic reactive emulsifiers having radically polymerizable unsaturated double bonds in the molecule, such as (meth)acrylic, propenyl, allyl, allyl ether, and maleic acid groups. The emulsifier may be used alone or in combination of two or more kinds.

[0034] In emulsion polymerization, an auxiliary agent (chain transfer agent) for adjusting molecular weight, such as a mercaptan compound or a lower alcohol, can be used as needed. The use of such an auxiliary agent allows the emulsion polymerization to proceed favorably and also promotes the smooth and uniform formation of a coating film, thereby improving the adhesion of the coating film to the substrate.

[0035] As the emulsion polymerization, any polymerization method can be appropriately selected, such as a single-stage continuous uniform monomer dropping method, a core-shell polymerization method which is a multi-stage monomer feed method, or a power feed polymerization method in which the monomer composition fed during polymerization is continuously changed.

[0036] A neutralizing agent may be added to the obtained acrylic resin water dispersion (A1) to neutralize at least a portion of the acid groups that may be contained in the acrylic resin. Neutralization can improve the stability of the acrylic resin water dispersion (A1). Examples of the neutralizing agent include basic compounds. Examples of basic compounds include ammonia; organic amines such as monomethylamine, dimethylamine, trimethylamine, triethylamine, diisopropylamine, monoethanolamine, diethanolamine, and dimethylethanolamine (dimethylaminoethanol); and inorganic bases such as sodium hydroxide, potassium hydroxide, and lithium hydroxide. Only one type of neutralizing agent may be used, or two or more types may be used in combination.

[0037] The acrylic resin contained in the acrylic resin water dispersion (A1) may be in the form of particles, and the average particle size of such particles is preferably 0.01 μm or more and 1.0 μm or less. In the present disclosure, the average particle size is a volume average particle size determined by dynamic light scattering, and specifically, can be measured using an electrophoretic light scattering photometer ELSZ series (manufactured by Otsuka Electronics Co., Ltd.) or the like.

[0038] The acrylic resin contained in the acrylic resin water dispersion (A1) may be in the form of core-shell particles.

[0039] The weight-average molecular weight of the acrylic resin contained in the acrylic resin water dispersion (A1) is preferably from 50,000 to 5,000,000, more preferably from 50,000 to 1,000,000. Having a weight-average molecular weight within this range has the advantage that the hardness, adhesion, water resistance, and other properties of the resulting coating film are improved. In the present disclosure, the weight average molecular weight is a value obtained by converting the measurement results of gel permeation chromatography (GPC) into a polystyrene standard.

[0040] The acid value of the acrylic resin contained in the acrylic resin water dispersion (A1) is preferably 1 mgKOH / g or more and 80 mgKOH / g or less, more preferably 2 mgKOH / g or more and 70 mgKOH / g or less, and even more preferably 3 mgKOH / g or more and 60 mgKOH / g or less.

[0041] The hydroxyl value of the acrylic resin contained in the acrylic resin aqueous dispersion (A1) is preferably 30 mgKOH / g or more and 120 mgKOH / g or less, more preferably 35 mgKOH / g or more and 100 mgKOH / g or less. When the hydroxyl value is within this range, the curing reaction proceeds sufficiently, and the resulting coating film has an advantage of having good hardness. In the present disclosure, the acid value and the hydroxyl value are both solid content converted values, and are measured by a method in accordance with JIS K 0070.

[0042] The solid content of the acrylic resin water dispersion (A1) in the solid content of the coating film-forming resin (A) is preferably 20% by mass or more and 90% by mass or less, more preferably 40% by mass or more and 85% by mass or less, and even more preferably 50% by mass or more and 80% by mass or less. By being within this range, the curing reaction proceeds sufficiently, which is advantageous in that the hardness of the resulting coating film is good. In this disclosure, the solid content of the film-forming resin (A) and the aqueous coating composition described below refers to the heating residue as defined in JIS K 5601-1-2:2008, and is calculated by measuring the percentage of the mass of the residue after heating at 105°C for 60 minutes relative to the original mass.

[0043] (Urethane resin (A2)) The urethane resin (A2) is a resin having a urethane bond in the main chain, typically an aqueous urethane resin dispersed in an aqueous medium. Without being bound by any particular theory, it is believed that the urethane resin (A2) can fuse with itself and other components during curing. Therefore, when the urethane resin (A2) is used, even when the aqueous primer paint and the aqueous paint composition (aqueous base paint) of the present disclosure are applied sequentially and baked and cured under low-temperature curing conditions, a tough coating film can be formed, and a multi-layer coating film with excellent inter-coat adhesion, water-resistant adhesion, etc. can be obtained. Furthermore, the inclusion of the urethane resin (A2) can exhibit high inter-coat adhesion during recoating and high adhesion between the topcoat paint film and Wind Direct Bond (WDB).

[0044] The urethane resin (A2) may be a reaction product of a polyol and a polyisocyanate, or may be a reaction product obtained by further reacting such a reaction product with a chain extender and a terminal terminator, which are used as needed.

[0045] Examples of the polyol include polymer polyols such as polyester polyols, polyether polyols, polycarbonate polyols, and acrylic polyols (e.g., polyols with a number average molecular weight of 500 or more); low molecular weight polyols such as ethylene glycol, propylene glycol, butanediol, 3-methylpentane-1,5-diol, 2-ethylhexane-1,3-diol, and trimethylolpropane (e.g., polyols with a number average molecular weight of less than 500); and polyols having hydrophilic groups such as polyols having acid groups (e.g., 2,2-dimethylolpropionic acid) and polyols having amino groups (e.g., N-methyldiethanolamine). Note that the polymer polyols and low molecular weight polyols are different from polyols having hydrophilic groups.

[0046] Examples of the polyisocyanate include aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates such as cyclohexane diisocyanate, isophorone diisocyanate, and hydrogenated diphenylmethane diisocyanate; and aromatic polyisocyanates such as tolylene diisocyanate, xylene diisocyanate, naphthalene diisocyanate, and diphenylmethane diisocyanate.

[0047] Examples of the chain extender include the low molecular weight polyols mentioned above; and polyamines such as ethylenediamine, hexamethylenediamine, diethylenetriamine, hydrazine, xylylenediamine, and isophoronediamine.

[0048] Examples of the terminal terminator include monools such as methanol, butanol, and octanol; alkylene oxide adducts of the monools; alkylamines such as butylamine and dibutylamine; and monoisocyanates such as methyl isocyanate, ethyl isocyanate, propyl isocyanate, butyl isocyanate, lauryl isocyanate, cyclohexyl isocyanate, phenyl isocyanate, and tolylene isocyanate.

[0049] The above components may be used alone or in combination of two or more.

[0050] The urethane resin (A2) can be produced by a one-shot method in which each component is reacted at once, or a multi-stage method (e.g., a prepolymer method) in which each component is reacted stepwise. The reaction temperature may be, for example, 40 to 140°C, and a tin-based catalyst such as dibutyltin laurate or tin octoate, or an amine-based catalyst such as triethylenediamine may be used. The reaction may be carried out without a solvent or in the presence of a reaction solvent such as acetone, toluene, dimethylformamide, or n-methylpyrrolidone.

[0051] Aqueous urethane resins can be typically produced by using a polyol having a hydrophilic group as the polyol, neutralizing or quaternizing the hydrophilic group after a urethane reaction, dissolving or dispersing the resulting resin in an aqueous medium, and removing the reaction solvent as necessary.

[0052] Commercially available aqueous urethane resins may be used, such as the NeoRez series (manufactured by Kusumoto Chemicals), the HUX series (manufactured by ADEKA), the U-coat series, the Permarin series, and the Euplen series (all manufactured by Sanyo Chemical Industries, Ltd.), and the Bayhydrol series (manufactured by Covestro).

[0053] The urethane resin (A2) may be used alone or in combination of two or more.

[0054] The solid content of the urethane resin (A2) in the solid content of the coating film-forming resin (A) is 0% by mass or more, and from the viewpoints of high interlayer adhesion in recoating, suppression of cohesive failure in the coating film, and further adhesion between the topcoat coating film and the WDB, it is preferably 3% by mass or more and 50% by mass or less, more preferably 5% by mass or more and 30% by mass or less.

[0055] (Polyester resin (A3)) The polyester resin (A3) is a resin having an ester bond in the main chain, and is typically an aqueous polyester resin dispersed in an aqueous medium. The inclusion of the polyester resin (A3) can improve the coating stability, coating workability, and physical properties of the resulting coating film.

[0056] The polyester resin (A3) may be a reaction product of a polyhydric alcohol and a polybasic acid.

[0057] Examples of the polyhydric alcohol include ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, hydroxypivalic acid neopentyl glycol ester, and 2-butyl-2-ethyl-1,3-propanediol. diol components such as 3-methyl-1,5-pentanediol, 2,2,4-trimethylpentanediol, and hydrogenated bisphenol A; tri- or higher polyol components such as trimethylolpropane, trimethylolethane, glycerin, and pentaerythritol; and dihydroxycarboxylic acids such as 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolpentanoic acid, 2,2-dimethylolhexanoic acid, and 2,2-dimethyloloctanoic acid.

[0058] Examples of the polybasic acid include aromatic polycarboxylic acids and acid anhydrides such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrabromophthalic anhydride, tetrachlorophthalic anhydride, and pyromellitic anhydride; alicyclic polycarboxylic acids and anhydrides such as hexahydrophthalic anhydride, tetrahydrophthalic anhydride, and 1,4- or 1,3-cyclohexanedicarboxylic acid; and aliphatic polycarboxylic acids and anhydrides such as maleic anhydride, fumaric acid, succinic anhydride, adipic acid, sebacic acid, and azelaic acid, and their anhydrides. If necessary, monobasic acids such as benzoic acid and t-butylbenzoic acid may be used in combination.

[0059] As additional reaction components, monohydric alcohols, monoepoxide compounds such as Cardura E (trade name: manufactured by Shell Chemical), and lactones (β-propiolactone, dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone, γ-caprolactone, etc.) may be used in combination.

[0060] Further, as a further reactive component, an oil component which is a fatty acid such as castor oil or dehydrated castor oil, or a mixture of two or more of these fatty acids may be used. Furthermore, an acrylic resin, a vinyl resin, or the like may be grafted onto the polyester resin, or a polyisocyanate compound may be reacted therewith.

[0061] The above components may be used alone or in combination of two or more.

[0062] A neutralizing agent may be added to the obtained polyester resin to neutralize at least a portion of the acid groups contained in the polyester resin. Neutralization can improve the water dispersibility or solubility of the polyester resin in water. Examples of the neutralizing agent include basic compounds. Examples of basic compounds are as described above. Only one type of neutralizing agent may be used, or two or more types may be used in combination.

[0063] The weight average molecular weight of the polyester resin (A3) is preferably from 500 to 20,000, more preferably from 1,500 to 10,000, from the viewpoints of storage stability and coating workability.

[0064] The polyester resin (A3) has an acid value of the solid content of preferably 15 mgKOH / g or more and 100 mgKOH / g or less, more preferably 20 mgKOH / g or more and 80 mgKOH / g or less. The polyester resin (A3) has a solid content hydroxyl value of preferably 35 mgKOH / g or more and 170 mgKOH / g or less, more preferably 50 mgKOH / g or more and 150 mgKOH / g or less. When the weight average molecular weight, solid acid value and solid hydroxyl value of the polyester resin (A3) are within the above ranges, there are advantages in that the paint stability, coating workability and the physical properties of the resulting paint film are improved.

[0065] The glass transition temperature Tg of the polyester resin (A3) is preferably −20° C. to 80° C., more preferably −10° C. to 60° C., from the viewpoint of the hardness of the coating film and the ability to conceal the base. The Tg of the polyester resin can be calculated from the type and amount of the monomers used in preparing the polyester resin. Alternatively, the Tg of the polyester resin may be measured by a differential scanning calorimeter (DSC).

[0066] As the polyester resin (A3), one type may be used alone, or two or more types may be used in combination.

[0067] The solid content of the polyester resin (A3) in the solid content of the coating film-forming resin (A) is 0% by mass or more, preferably 5% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 40% by mass or less, and even more preferably 10% by mass or more and 30% by mass or less.

[0068] The total content of the acrylic resin water dispersion (A1), the urethane resin (A2), and the polyester resin (A3) in the solid content of the coating film-forming resin (A) is preferably from 70 to 100% by mass, more preferably from 80 to 100% by mass, and even more preferably from 88 to 100% by mass.

[0069] The film-forming resin (A) may contain a resin other than the acrylic resin of the acrylic resin aqueous dispersion (A1), the urethane resin (A2), and the polyester resin (A3). Examples of such resins include water-soluble acrylic resins. Water-soluble acrylic resins have a low molecular weight and are soluble in aqueous media. The weight-average molecular weight of the water-soluble acrylic resin is less than 50,000, for example, from 1,000 to 30,000.

[0070] The solid content of the film-forming resin (A) in the solid content of the aqueous coating composition is preferably 20% by mass or more and 80% by mass or less, more preferably 30% by mass or more and 75% by mass or less, and even more preferably 40% by mass or more and 70% by mass or less.

[0071] [Hardening agent (B)] The curing agent (B) contains a melamine resin (B1). By including the melamine resin (B1), a coating film can be formed by self-polymerization of the melamine resin (B1) and reaction between amino groups contained in the melamine resin (B1) and hydroxyl groups contained in the acrylic resin dispersion (A1).

[0072] (Melamine resin (B1)) The melamine resin (B1) can be obtained by modifying a condensation product of an amino compound such as melamine with an aldehyde compound such as formaldehyde or acetaldehyde using a lower alcohol such as methanol, ethanol, propanol, butanol, etc. The melamine resin (B1) is preferably a compound having three reactive functional groups represented by the following formula as reactive functional groups in one triazine nucleus molecule, or a polycondensate thereof. -NX 1 X 2 [X 1 , X 2 are each independently a hydrogen atom, a methylol group, or -CH-OR 1 Represents. R 1 represents an alkyl group having 1 to 8 carbon atoms, preferably a linear or branched alkyl group having 1 to 8 carbon atoms. Multiple -CH2-OR in the same molecule 1 When R is included, the multiple R may be the same or different.

[0073] Melamine resins have reactive functional groups such as -N(CH2OR 1 )2 only; -N(CH2OR) as a reactive functional group 1 )(CH2OH) containing methylol group type; -N(CH2OR) as reactive functional group 1 )(H) containing imino group type; as a reactive functional group, -N(CH2OR 1 )(CH2OH) and -N(CH2OR 1In the full alkyl type, methylol group type, imino group type, or methylol / imino group type melamine resin, R 1 is preferably an alkyl group having 1 to 4 carbon atoms, and is preferably a methyl group, an n-butyl group, or an isobutyl group. In one embodiment, methyl groups and butyl groups may be mixed, in another embodiment, methyl groups only, and in yet another embodiment, butyl groups only.

[0074] From the viewpoint of improving the appearance of the coating film and the high adhesion between the base coating film and the WDB, the SP value of the melamine resin (B1) is preferably 9.0 or more and less than 12.0, more preferably 9.2 or more and less than 11.0, even more preferably 9.5 or more and less than 11.0, and still more preferably 10.0 or more and less than 11.0. In this disclosure, the unit of the SP value is (cal / cm 3 ) 1 / 2 is.

[0075] The SP value can be measured by the following method. At a measurement temperature of 20°C, 0.5 g of the component whose SP value is to be measured is weighed into a 100 mL beaker, and 10 mL of a good solvent (acetone) is added using a volumetric pipette. The mixture is then dissolved using a magnetic stirrer to prepare a diluted solution. Next, a low SP poor solvent (n-hexane) is slowly added dropwise to this diluted solution using a 50 mL burette. The amount of low SP poor solvent added is determined to be the point at which the diluted solution becomes cloudy.

[0076] Separately, a high SP poor solvent (ion-exchanged water) is gradually added dropwise to the diluted solution, and the point at which the diluted solution becomes turbid is taken as the amount of high SP poor solvent added. The SP value can be calculated from the amount of each poor solvent added until the turbidity point is reached by a known calculation method described in the references mentioned above.

[0077] As the melamine resin, a commercially available product may be used. Commercially available products include methylol group-imino type methyl / butyl mixed etherified melamine resins such as Cymel 202 (SP value: 11.36); imino type methyl / butyl mixed etherified melamine resins such as Cymel 204 (SP value: 10.94), Cymel 211 (SP value: 11.91), Cymel 250 (SP value: 10.40), and Cymel 254 (SP value: 11.39); fully alkyl type methylated melamine resins such as Cymel 350 (SP value: 12.42); imino group type methylated melamine resins such as Cymel 325 (SP value: 13.69), Cymel 327 (SP value: 12.42), Cymel 385 (SP value: 14.21), Cymel 701 (SP value: 12.86), and Cymel 712 (SP value: 14.45); Cymel 247-10 ( Examples of suitable methylol group-type methylated melamine resins include Cymel 370 (SP value: 9.20), Cymel 370 (SP value: 12.76), and Cymel 651 (9.20) (all manufactured by Allnex Japan Co., Ltd.); imino-type methyl / butyl mixed etherified melamine resins such as Mycoat 212 (SP value: 10.10), Mycoat 518 (SP value: 9.80), and Mycoat 525 (SP value: 11.84); imino-type butylated melamine resins such as Mycoat 508 (SP value: 10.70); imino-type methylated melamine resins such as Mycoat 723 (SP value: 11.11) and Mycoat 776 (SP value: 13.44); and methylol group-type methyl / isobutyl mixed etherified melamine resins such as Mycoat 2677 (SP value: 10.80) (all manufactured by Allnex Japan Co., Ltd.).

[0078] The melamine resin (B1) contains a melamine resin (B1a) having a weight-average molecular weight of 6,000 or more and a melamine resin (B1b) having a weight-average molecular weight of less than 6,000. This makes it possible to suppress migration of components from a sealer section even on a substrate having a sealer section, thereby improving interlayer adhesion during recoating.

[0079] The weight average molecular weight of the melamine resin (B1a) is 6,000 or more, for example, 6,000 or more and 18,000 or less, preferably 6,000 or more and 16,000 or less, more preferably 8,000 or more and 16,000 or less, and even more preferably 10,000 or more and 16,000 or less.

[0080] The SP value of the melamine resin (B1a) having a weight average molecular weight of 6,000 or more is preferably 9.0 or more and less than 12.0, more preferably 9.0 or more and less than 11.0, even more preferably 9.0 or more and less than 10.5, and still more preferably 9.2 or more and less than 10.5.

[0081] The melamine resin (B1a) may be a commercially available product, such as a methylol group-type methylated melamine resin, such as Cymel 247-10 (weight average molecular weight: 11,600, SP value: 9.20) or Cymel 651 (weight average molecular weight: 15,400, SP value: 9.20) (both manufactured by Allnex Japan Co., Ltd.).

[0082] The content of the melamine resin (B1a) having a weight-average molecular weight of 6,000 or more in the melamine resin (B1) is preferably 1% by mass or more and 90% by mass or less, more preferably 1% by mass or more and 30% by mass or less, and even more preferably 1% by mass or more and 20% by mass or less. The above range has the advantage that the curing reaction with the coating film-forming resin proceeds sufficiently and the resulting coating film has excellent interlayer adhesion.

[0083] The weight average molecular weight of the melamine resin (B1b) is less than 6,000, preferably 300 or more and 5,000 or less, more preferably 400 or more and 4,000 or less, and even more preferably 500 or more and 3,000 or less.

[0084] The SP value of the melamine resin (B1b) having a weight average molecular weight of less than 6,000 is preferably 9.0 or more and less than 12.0, more preferably 9.2 or more and less than 11.0, even more preferably 9.5 or more and less than 11.0, and still more preferably 10.0 or more and less than 11.0.

[0085] Commercially available products may be used as the melamine resin (B1b). Examples of commercially available products include methylol group-imino type methyl / butyl mixed etherified melamine resins such as Cymel 202 (weight average molecular weight: 1,170, SP value: 11.36); imino melamine resins such as Cymel 211 (weight average molecular weight: 780, SP value: 11.91), Cymel 250 (weight average molecular weight: 2,700, SP value: 10.40), and Cymel 254 (weight average molecular weight: 1,110, SP value: 11.39); fully alkylated methylated melamine resins such as Cymel 350 (weight average molecular weight: 520, SP value: 12.42); Cymel 325 (weight average molecular weight: 750, SP value: 13.69), Cymel 327 (weight average molecular weight: 585, SP value: 12.42), Cymel 385 (weight average molecular weight: 560, SP value: 14.21), Cymel 701 (weight average molecular weight: 560, SP value: 14.21), Cymel 702 (weight average molecular weight: 560, SP value: 14.21), Cymel 701 ...2 (weight average molecular weight: 560, SP value: 14.21 imino-type methylated melamine resins such as Mycoat 508 (weight average molecular weight: 1,800, SP value: 10.70); imino-type butylated melamine resins such as Mycoat 723 (weight average molecular weight: 460, SP value: 11.11); and methylol-type methyl / isobutyl-etherified melamine resins such as Mycoat 2677 (weight average molecular weight: 1,530, SP value: 10.80) (all manufactured by Allnex Japan Co., Ltd.).

[0086] The content of the solid content of the melamine resin (B1b) having a weight average molecular weight of less than 6,000 in the solid content of the melamine resin (B1) is preferably 10% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 99% by mass or less, and even more preferably 80% by mass or more and 99% by mass or less.

[0087] The total content of the solid contents of the melamine resin (B1a) and the melamine resin (B1b) in the solid contents of the melamine resin (B1) is preferably 80% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less.

[0088] The content of the solid content of the melamine resin (B1) in the solid content of the curing agent (B) is preferably 80% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less, and even more preferably 95% by mass or more and 100% by mass or less.

[0089] The curing agent (B) may contain, in addition to the melamine resin (B1), an amino resin such as a urea resin or a benzoguanamine resin.

[0090] The content of the curing agent (B) in the aqueous coating composition (based on solid content) is preferably 20% by mass or more and 70% by mass or less, more preferably 20% by mass or more and 50% by mass or less, based on 100% by mass of the total of the film-forming resin (based on solid content) and the curing agent (based on solid content). By keeping the curing agent content within this range, high interlayer adhesion in recoating and high adhesion between the topcoat coating film and the WDB can be improved.

[0091] [Aqueous medium] The aqueous coating composition of the present disclosure contains an aqueous medium. The aqueous medium in the present disclosure may be water; a hydrophilic solvent; or a mixture of water and a hydrophilic solvent. Examples of the hydrophilic solvent include glycol-based solvents such as ethylene glycol, propylene glycol, butanediol, diethylene glycol, dipropylene glycol, and triethylene glycol; glycol ether-based solvents such as ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monobutyl ether, triethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and dipropylene glycol monoethyl ether; alcohol-based solvents such as methanol, ethanol, isopropyl alcohol, and benzyl alcohol; cyclic ether-based solvents such as dioxane and tetrahydrofuran; ketone-based solvents such as acetone; and N-methyl-2-pyrrolidone.

[0092] [Other ingredients] The aqueous coating composition of the present disclosure may optionally contain various additives known for use in coating compositions other than those described above. Examples of such additives include pigments such as extender pigments, coloring pigments, and anti-rust pigments, anti-sagging and anti-settling agents, curing catalysts (organometallic catalysts), color-separation inhibitors, dispersants, anti-foaming and anti-popping agents, thickeners, viscosity adjusters, leveling agents, matting agents, UV absorbers, antioxidants, plasticizers, film-forming aids, and organic solvents. The amounts of these components may be adjusted as appropriate within a range that does not impair the effects of the present disclosure.

[0093] The pigment is not particularly limited, and examples thereof include organic color pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, monoazo pigments, disazo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, thioindigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, naphthol pigments, pyrazolone pigments, anthraquinone pigments, anthrapyrimidine pigments, and metal complex pigments; yellow lead, yellow iron oxide, chromium oxide, molybdate orange, red iron oxide, titanium yellow, zinc oxide, carboplatin, and the like. Examples of inorganic color pigments include bomb black, titanium dioxide, cobalt green, phthalocyanine green, ultramarine blue, cobalt blue, phthalocyanine blue, and cobalt violet; mica pigments (titanium dioxide-coated mica, colored mica, and metal-plated mica); graphite pigments, alumina flake pigments, metallic titanium flakes, stainless steel flakes, plate-like iron oxide, phthalocyanine flakes, and metal-plated glass flakes, as well as other colored and colored flat pigments; and extender pigments such as titanium oxide, calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silica, and calcined kaolin.

[0094] The aqueous coating composition of the present disclosure can serve as a primer coating composition and also as a color base coating composition. In one embodiment, the aqueous coating composition of the present disclosure contains a pigment. The pigment can be used in the form of a pigment dispersion paste.

[0095] The pigment dispersion paste is obtained by pre-dispersing a pigment, a pigment dispersant, and a portion of the film-forming resin (A) used as needed in a small amount of aqueous medium. The pigment dispersant may be a resin having a structure containing a pigment affinity moiety and a hydrophilic moiety. Examples of the pigment affinity moiety and the hydrophilic moiety include nonionic, cationic, and anionic functional groups. The pigment dispersant may have two or more types of the functional groups in one molecule.

[0096] Examples of nonionic functional groups include hydroxyl groups, amide groups, and polyoxyalkylene groups. Examples of cationic functional groups include amino groups, imino groups, and hydrazino groups. Examples of anionic functional groups include carboxyl groups, sulfo groups, and phosphate groups. Such pigment dispersants can be produced by methods well known to those skilled in the art.

[0097] The pigment dispersant is not particularly limited as long as it does not contain a volatile basic substance in its solid content or the content is 3 mass% or less, but a pigment dispersant that can efficiently disperse pigments with a small amount of pigment dispersant is preferred. For example, commercially available pigment dispersants (all trade names below) can be used, and specific examples include anionic / nonionic dispersants Disperbyk 190, Disperbyk 181, Disperbyk 182 (polymer copolymer), and Disperbyk 184 (polymer copolymer) manufactured by BYK-Chemie, anionic / nonionic dispersant EFKAPOLYMER 4550 manufactured by BASF, nonionic dispersant Solsperse 27000, anionic dispersant Solsperse 41000, and anionic dispersant Solsperse 53095 manufactured by Avecia.

[0098] The weight average molecular weight of the pigment dispersant is preferably 1,000 or more and 100,000 or less, more preferably 2,000 or more and 100,000 or less, and even more preferably 4,000 or more and 50,000 or less.

[0099] The pigment dispersion paste can be prepared by mixing and dispersing the pigment dispersant, pigment, and a portion of the film-forming resin (A) used as needed according to a known method. The proportion of the pigment dispersant used in producing the pigment dispersion paste is preferably 1% by mass or more and 50% by mass or less based on the solid content of the pigment dispersion paste. By keeping the proportion of the pigment dispersant within this range, the pigment dispersion stability and the physical properties of the resulting coating film can be maintained within better ranges. The proportion of the pigment dispersant is more preferably 3% by mass or more and more preferably 5% by mass or less.

[0100] The pigment may be any of the pigments exemplified above as pigments for use in the aqueous coating composition, but a colored pigment is preferred in order to improve weather resistance and ensure hiding power. Titanium dioxide is particularly preferred because it has excellent color hiding power and is inexpensive. Alternatively, the coating composition may be a standard gray coating composition containing carbon black and titanium dioxide as the main pigments. Alternatively, the coating composition may be a coating composition that matches the brightness or hue of the second coating composition, or a coating composition that combines various colored pigments.

[0101] The pigment preferably has a ratio of the pigment mass to the total mass of the resin solids and pigment in the aqueous coating composition (PWC) of 10% by mass or more and 60% by mass or less. By setting the PWC to 10% by mass, it becomes easier to improve hiding power. By setting the PWC to 60% by mass or less, it becomes easier to suppress an increase in viscosity during curing, so it becomes easier to ensure flowability and obtain a good coating appearance.

[0102] Examples of thickeners include associative nonionic urethane thickeners, alkali swelling thickeners, and bentonite, which is an inorganic intercalation compound.

[0103] In the aqueous coating composition of the present disclosure, the total solid content is preferably 5% by mass or more and 70% by mass or less, more preferably 10% by mass or more and 60% by mass or less, and even more preferably 15% by mass or more and 55% by mass or less.

[0104] <Method for preparing aqueous coating composition> The aqueous coating composition can be prepared by mixing the above-mentioned components by a commonly used means. The aqueous coating composition can be suitably used as an aqueous base coating, particularly an automotive aqueous base coating. Therefore, the aqueous coating composition can be applied to a method for forming a multilayer coating film applied to automobile bodies, parts, etc.

[0105] <Coating film> The coating film according to the present disclosure is formed from the aqueous coating composition according to the present disclosure. Such a coating film is, for example, a cured coating film obtained by applying the aqueous coating composition and then heating and curing the uncured coating film. The coating film according to the present disclosure is suitable as a base coating film in, for example, automotive painting. The coating film according to the present disclosure may be combined with other coating films to form a multi-layer coating film.

[0106] <Method of manufacturing multi-layer coating film> The method for producing a multilayer coating film according to the present disclosure includes: A step of applying the aqueous coating composition of the present disclosure onto a substrate to form an uncured coating film; A step of applying a clear coating wet-on-wet onto the uncured coating film to form an uncured clear coating film; and a step of simultaneously heating and curing the paint film and the clear paint film to form a multi-layer paint film.

[0107] In the method for producing a multilayer coating film disclosed herein, the aqueous coating composition is used, so that a good coating film appearance can be obtained even under low temperature curing conditions or at low heat curing temperatures, and a coating film with excellent hardness, adhesion, and water resistance can be formed. In the present disclosure, the film formed after application of the coating composition and before drying or curing is also referred to as the coating film, and the film formed after drying or curing is also referred to as the coating film.

[0108] The substrate to be coated is not particularly limited, and examples thereof include metal substrates, plastic substrates, and foamed products thereof.

[0109] Examples of materials for the metal substrate include metals and alloys such as iron, steel, copper, aluminum, tin, and zinc. Specific examples of metal substrates include automobile bodies such as passenger cars, trucks, motorcycles, and buses, and parts for automobile bodies. It is preferable that an electrodeposition coating film is formed on such metal substrates in advance. Before forming the electrodeposition coating film, chemical conversion treatments (e.g., zinc phosphate chemical conversion treatment, zirconium chemical conversion treatment, etc.) may be performed as needed. Known cationic electrodeposition coating compositions or anionic electrodeposition coating compositions can be used as the electrodeposition coating composition. From the viewpoint of corrosion resistance, a cationic electrodeposition coating composition is preferable.

[0110] Examples of the plastic substrate include substrates made of polypropylene resin, polycarbonate resin, urethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, polyamide resin, etc. Specific examples of the plastic substrate include automobile parts such as spoilers, bumpers, mirror covers, grilles, and doorknobs. These plastic substrates are preferably washed with pure water and / or a neutral detergent. Furthermore, they may be coated with a primer to enable electrostatic coating.

[0111] An intermediate coating film or a primer coating film may be formed on the substrate as needed. An intermediate coating composition is used to form the intermediate coating film. This intermediate coating composition is water-based and contains a film-forming resin, a curing agent, organic or inorganic color pigments and extender pigments, etc. The film-forming resin and curing agent contained in the intermediate coating composition are not particularly limited, and the film-forming resins and curing agents listed for the aqueous coating composition above can be used. The color pigments and extender pigments that can be used are also those listed for the aqueous coating composition above.

[0112] A primer coating composition is used to form the primer coating film. This primer coating composition is not particularly limited, and a primer coating containing a film-forming resin and, if necessary, a curing agent, pigment, additives, etc. can be used. The form of the primer coating can be water-based or solvent-based. The film-forming resin and curing agent contained in the primer coating are not particularly limited. For example, polyester resin, acrylic resin, urethane resin, etc. can be used as the film-forming resin, and polyisocyanate compound, melamine resin, etc. can be used as the curing agent. As the pigment and the additive, those exemplified for the water-based coating composition can be used, and titanium dioxide, etc. can also be used as the pigment.

[0113] The above-mentioned sealer section may be formed on the electrodeposition coating of the substrate, and then an intermediate coating film (or a base coating film if no intermediate coating film is formed) may be formed on top of the sealer section. The sealer section can be formed by applying a sealant. For example, a vinyl chloride plastisol sealant containing a vinyl chloride resin and a plasticizer can be used as the sealant.

[0114] Examples of vinyl chloride resins contained in vinyl chloride plastisol sealants include vinyl chloride homopolymers and copolymers of vinyl chloride with other monomers such as vinyl acetate. These vinyl chloride homopolymers and copolymers may be used alone or in combination of two or more.

[0115] Examples of plasticizers contained in vinyl chloride plastisol sealants include phthalates such as dioctyl phthalate, di-2-ethylhexyl phthalate, diisononyl phthalate, and octylbutyl phthalate, di- or tricarboxylic acid esters such as sebacate and azelaate, phosphate esters such as triphenyl phosphate and tricresyl phosphate, and epoxy plasticizers such as epoxidized soybean oil. High-boiling organic solvents can also be used as plasticizers.

[0116] The vinyl chloride plastisol sealant may contain, as required, fillers such as alkaline earth metal carbonates and sulfates, mica, silica, talc, diatomaceous earth, kaolin, etc.; common adhesion-imparting agents such as epoxy-based, acrylic-based, polyamide-based, and isocyanate-based agents; stabilizers such as salts of metals such as zinc, lead, barium, tin, and calcium; and thixotropy-imparting agents such as surface-treated calcium carbonate and ultrafine silica powder.

[0117] After applying such a vinyl chloride plastisol sealant, a sealer portion is formed on the object to be coated by curing it under commonly used heat curing conditions. In one embodiment, after applying the sealant, the aqueous coating composition of the present disclosure is applied on the uncured sealer coating film to form an uncured coating film, and then a clear coating is applied on the uncured coating film to form an uncured clear coating film, and the sealer coating film, the coating film, and the clear coating film may be simultaneously heat cured.

[0118] The aqueous coating composition of the present disclosure is applied as an aqueous base coating to a substrate to form an uncured base coating film. The surface of the substrate to which the aqueous base coating is applied is the surface of an electrodeposition coating film, a sealer portion, or an intermediate coating film. From the viewpoint of improving appearance, the application method may be a multi-stage application by air electrostatic spray coating, preferably a two-stage application, or a combination of air electrostatic spray coating and a rotary atomization electrostatic coater known as a metallic bell.

[0119] Typically, the aqueous base paint is preferably applied so that the dry film thickness is 10 μm or more and 30 μm or less. A dry film thickness of 10 μm or more can adequately conceal the substrate and prevent film breakage, while a dry film thickness of 30 μm or less maintains sharpness and prevents problems such as unevenness or sagging during application. To obtain a multilayer coating with a better appearance, it is preferable to heat the resulting uncured base coating film at 40 to 100°C for 2 to 10 minutes to dry it before applying the clear coating.

[0120] Next, a clear coating is applied wet-on-wet onto the uncured base coating without heat curing to form an uncured clear coating film, which smooths and protects any irregularities caused by the base coating and also adds aesthetic appeal.

[0121] The clear paint for forming the clear coating film is not particularly limited, and a clear paint containing a film-forming resin and a curing agent can be used. A coloring component can also be added as long as it does not interfere with the design of the base. The form of the clear paint can be solvent-based, water-based, or powder-based.

[0122] Preferred examples of solvent-based clear coatings include, from the viewpoint of transparency or acid etching resistance, a combination of an acrylic resin and / or a polyester resin with an amino resin and / or an isocyanate, or an acrylic resin and / or a polyester resin having a carboxylic acid / epoxy curing system.

[0123] Examples of water-based clear coatings include those containing resins obtained by neutralizing the film-forming resins contained in the solvent-based clear coatings listed above with a base to make them water-based. This neutralization can be carried out by adding a tertiary amine such as dimethylethanolamine or triethylamine before or after polymerization.

[0124] As the powder-type clear coating, conventional powder coatings such as thermoplastic and thermosetting powder coatings can be used. Thermosetting powder coatings are preferred because they provide coating films with good physical properties. Specific examples of thermosetting powder coatings include epoxy-based, acrylic-based, and polyester-based powder clear coatings, with acrylic-based powder clear coatings being particularly preferred due to their excellent weather resistance.

[0125] It is preferable that a viscosity control agent be added to the clear coating to ensure coating workability. Viscosity control agents that generally exhibit thixotropy can be used. For example, conventionally known agents can be used. The clear coating can contain a curing catalyst, a surface conditioner, etc., as needed.

[0126] As a method for applying a clear coating to a base coating film, a coating method using a rotary atomizing electrostatic coating machine called a Micro Micro Bell or Micro Bell can be mentioned.

[0127] The clear coating is preferably applied so that the dry film thickness is, for example, 10 μm to 80 μm, preferably 20 μm to 60 μm. When the dry film thickness is 10 μm or more, the unevenness of the base can be sufficiently concealed, and when it is 80 μm or less, defects such as popping or dripping during application can be suppressed.

[0128] The uncured clear coating film thus formed and the previously formed uncured base coating film are simultaneously heat-cured to form a multi-layer coating film, which is a cured coating film. From the viewpoint of curability and the physical properties of the resulting multi-layer coating film, the heat-curing temperature is preferably 80 to 180°C, more preferably 120 to 160°C. The heat-curing time can be set arbitrarily depending on the temperature, and a heat-curing temperature of 80 to 160°C and a time of 10 to 30 minutes are suitable.

[0129] The method for forming a multilayer coating film of the present disclosure also relates to a method for forming a multilayer coating film by a 3-coat, 1-bake process in which the intermediate coating is applied to an object to form an uncured intermediate coating film, and without curing it by heat, the aqueous coating composition (aqueous base coating) of the present disclosure is applied thereon to form an uncured base coating film, and then the clear coating is applied to form an uncured clear coating film, and these coating films are simultaneously cured by heat. Examples of various coating materials used in this method include those similar to those described above.

[0130] The multilayer coating film including the base coating film and the clear coating film obtained by the multilayer coating film forming method has a good coating film appearance, and is excellent in hardness, adhesion, and water resistance, even when the heat curing temperature is low. [Example]

[0131] The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited thereto.

[0132] <Production Example 1: Preparation of acrylic resin water dispersion (A1)> 24.5 parts by mass of methyl acrylate, 48.9 parts by mass of ethyl acrylate, 4.8 parts by mass of styrene, 11.6 parts by mass of 2-hydroxyethyl methacrylate, 1.5 parts by mass of methacrylic acid, and 8.7 parts by mass of allyl methacrylate were mixed and stirred in a stainless steel container, and then 0.7 parts by mass of Aqualon HS-10 (polyoxyethylene alkylpropenylphenyl ether sulfate, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) and 0.5 parts by mass of Adeka Reasoap NE-20 (α-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]-ω-hydroxyoxyethylene, manufactured by Asahi Denka Co., Ltd.) were added as emulsifiers, and 80.0 parts by mass of deionized water were added, and the mixture was stirred at room temperature for 15 minutes using a homomixer to obtain 181.2 parts by mass of a monomer emulsion mixture.

[0133] A reaction vessel equipped with a stirrer, reflux condenser, dropping funnel, and thermometer was charged with 126.5 parts by weight of deionized water, and the temperature in the reaction vessel was raised to 80°C while mixing and stirring in a nitrogen stream. Next, 181.2 parts by weight of the monomer emulsion mixture and an initiator solution consisting of 0.3 parts by weight of ammonium persulfate as a reaction initiator and 10.0 parts by weight of deionized water were simultaneously added dropwise from separate dropping funnels over a period of 2 hours. After the addition was completed, the temperature in the reaction vessel was maintained at 80°C for 2 hours, then cooled to 40°C, filtered through a 400-mesh filter, and the pH was adjusted to 6.5 by adding 70.0 parts by weight of deionized water and 0.3 parts by weight of dimethylaminoethanol to obtain an acrylic resin aqueous dispersion (A1-1) (single-layer type, average particle size: 120 nm, solid acid value: 10 mgKOH / g, solid hydroxyl value: 50 mgKOH / g, solid concentration: 25% by weight).

[0134] <Production Example 2: Preparation of acrylic resin water dispersion (A1-2)> 35.8 parts by mass of methyl methacrylate, 15.1 parts by mass of styrene, 15.2 parts by mass of n-butyl acrylate, 12.9 parts by mass of 2-ethylhexyl acrylate, and 21.0 parts by mass of 2-hydroxyethyl methacrylate were mixed and stirred in a stainless steel container as first monomers, and then 25.00 parts by mass of Aqualon HS-10 and 5.0 parts by mass of Adeka Reasoap NE-20 as emulsifiers and 80.0 parts by mass of deionized water were added, and the mixture was stirred at room temperature for 15 minutes using a homomixer to obtain 190.0 parts by mass of a first monomer emulsified mixture.

[0135] In a separate stainless steel container, 35.8 parts by mass of methyl methacrylate, 10.7 parts by mass of styrene, 15.2 parts by mass of n-butyl acrylate, 12.9 parts by mass of 2-ethylhexyl acrylate, 21.0 parts by mass of 2-hydroxyethyl methacrylate, and 4.4 parts by mass of acrylic acid were mixed with stirring as second monomers, and then 2.0 parts by mass of Aqualon HS-10 as an emulsifier and 50.0 parts by mass of deionized water were added, and the mixture was stirred at room temperature for 15 minutes using a homomixer to obtain 152.0 parts by mass of a second monomer emulsified mixture.

[0136] A reaction vessel equipped with a stirrer, reflux condenser, dropping funnel, and thermometer was charged with 126.6 parts by mass of deionized water, and the temperature inside the reaction vessel was raised to 80°C while mixing and stirring in a nitrogen stream. Next, 950.00 parts by mass of the first monomer emulsion mixture and an initiator solution consisting of 1.20 parts by mass of ammonium persulfate as a reaction initiator and 500.00 parts by mass of deionized water were simultaneously added dropwise from separate dropping funnels over 1.5 hours. After the dropping was completed, the temperature inside the reaction vessel was maintained at 80°C for 1 hour. Furthermore, 152.0 parts by mass of the second monomer emulsion mixture and an initiator solution consisting of 0.6 parts by mass of ammonium persulfate as a reaction initiator and 20.0 parts by mass of deionized water were simultaneously added dropwise from separate dropping funnels over 1.5 hours. After the addition was completed, the temperature in the reaction vessel was maintained at 80°C for 2 hours. Thereafter, the mixture was cooled to 40°C and filtered through a 400-mesh filter. Then, 20.0 parts by mass of deionized water and 0.3 parts by mass of dimethylaminoethanol were added to adjust the pH to 6.5, thereby obtaining an acrylic resin water dispersion (A1-2) (core-shell type, average particle size: 150 nm, solid content acid value: 17 mg KOH / g, solid content hydroxyl value: 90 mg KOH / g, solid content concentration: 35% by mass).

[0137] <Production Example 3: Preparation of acrylic resin water dispersion (A1-3)> An acrylic resin water dispersion (A1-3) was obtained in the same manner as in Production Example 1, except that the amount of the monomer blended was changed to the amount shown in Table 1 (single-layer type, average particle size: 120 nm, solid acid value: 20 mg KOH / g, solid hydroxyl value: 40 mg KOH / g, solid concentration: 25 mass%).

[0138] <Production Example 4: Preparation of acrylic resin water dispersion (A1-4)> An acrylic resin water dispersion (A1-4) was obtained in the same manner as in Production Example 2, except that the amounts of the first monomer and the second monomer were changed to those shown in Table 1 (core-shell type, average particle size: 150 nm, solid acid value: 17 mg KOH / g, solid hydroxyl value: 67 mg KOH / g, solid concentration: 35 mass%).

[0139] [Table 1]

[0140] <Production Example 5: Preparation of aqueous polyester resin (A3)> A reaction vessel equipped with a stirrer, reflux condenser, and thermometer was charged with 25.6 parts by weight of isophthalic acid, 22.8 parts by weight of phthalic anhydride, 5.6 parts by weight of adipic acid, 19.3 parts by weight of trimethylolpropane, 26.7 parts by weight of neopentyl glycol, 17.5 parts by weight of ε-caprolactone, and 0.1 parts by weight of dibutyltin oxide. The mixture was heated to 170°C while stirring in a nitrogen stream. The temperature was then raised to 220°C over 3 hours, and the resulting water was distilled off while the condensation reaction was carried out until the solid acid value of the reaction product reached 8 mgKOH / g. Next, 7.9 parts by weight of trimellitic anhydride was added and the reaction was continued for 1 hour at 150°C to obtain a polyester resin (solid acid value: 40 mgKOH / g). After cooling to 100°C, 11.2 parts by weight of butyl cellosolve was added and the mixture was stirred until homogeneous. Thereafter, the mixture was cooled to 60°C, and 98.8 parts by mass of ion-exchanged water and 5.9 parts by mass of dimethylethanolamine were added to obtain an aqueous polyester resin (A3) (acid value of solid content: 40 mgKOH / g, hydroxyl value of solid content: 110 mgKOH / g, number average molecular weight: 2,870, glass transition temperature: -3°C, solid content concentration: 50% by mass).

[0141] <Production Example 6: Preparation of water-soluble acrylic resin> A reaction vessel equipped with a stirrer, reflux condenser, dropping funnel, and thermometer was charged with 23.89 parts by weight of tripropylene glycol methyl ether and 16.11 parts by weight of propylene glycol methyl ether, and the mixture was heated to 105°C while stirring in a nitrogen stream. Next, 100 parts by weight of a monomer mixture containing 13.1 parts by weight of methyl methacrylate, 68.4 parts by weight of ethyl acrylate, 11.6 parts by weight of 2-hydroxyethyl methacrylate, and 6.9 parts by weight of methacrylic acid, and an initiator solution consisting of 10.0 parts by weight of tripropylene glycol methyl ether and 1 part by weight of tert-butyl peroxy 2-ethylhexanoate were added dropwise to the reaction vessel over a period of 3 hours while stirring the reaction vessel. After the addition, stirring was continued at the same temperature for 30 minutes, and then the reaction vessel was cooled to room temperature to obtain a water-soluble acrylic resin (weight average molecular weight: 27,000, solids concentration: 30% by weight). <Production Example 7: Preparation of color pigment paste> 52.5 parts by mass of Shanin Blue G314 (phthalocyanine color pigment, manufactured by Sanyo Dish Co., Ltd.), 83.5 parts by mass of the water-soluble acrylic resin solution obtained in Production Example 4, 50.0 parts by mass of EFKA4550 (nonionic pigment dispersant, manufactured by EFKA Co., Ltd., solids concentration: 50%), and 155.5 parts by mass of ion-exchanged water were pre-mixed using a disper, and then dispersed using an SG mill (dispersion medium: zircon beads) at 3,000 rpm for 5 hours. The zircon beads were then filtered off to obtain a color pigment paste (pigment mass concentration (PWC): 23% by mass, solids concentration: 42% by mass).

[0142] <Preparation of aqueous coating composition> Example 1 40.1 parts by mass of the acrylic resin water dispersion (A1-1) obtained in Production Example 1, 4.3 parts by mass of Cymel 247-10 as the curing agent (B1a-1), 33.9 parts by mass of Mycoat 263 as the curing agent (B1b-1), 13.0 parts by mass of urethane resin (A2-1), 8.7 parts by mass of the aqueous polyester resin (A3-1) obtained in Production Example 5, and 20.0 parts by mass of the color pigment paste obtained in Production Example 5 were mixed with stirring using a disper to obtain aqueous coating composition 1. The blending amounts of each component are expressed as solids content (parts by mass).

[0143] (Examples 2 to 18, Comparative Examples 1 and 2) Aqueous coating compositions were prepared in the same manner as in Example 1, except that the types and amounts of each component were changed as shown in the table below.

[0144] Details of the raw materials used are given below. Paint film forming resin (A2); Film-forming resin (A2-1): U-coat N800T (polyurethane resin, manufactured by Sanyo Chemical Industries, Ltd.), solid content: 37% by mass Film-forming resin (A2-2): UH650 (polyurethane resin, manufactured by Covestro), solid content: 50% by mass Paint film forming resin (A3); Film-forming resin (A3-2): NC3000 (polyester resin, manufactured by Toyobo Co., Ltd.), solid content: 40% by mass hardener (B); Curing agent (B1a-1): Cymel 247-10 (methylol group-type melamine resin, manufactured by Allnex Co., Ltd.), weight average molecular weight: 11,600, SP value: 9.20, solid content: 64% by mass Curing agent (B1a-2): Cymel 651 (methylol group-type melamine resin, manufactured by Allnex Co., Ltd.), weight average molecular weight: 15,400, SP value: 9.20, solid content: 64% by mass Curing agent (B1b-1) Mycoat 263 (imino-type methyl / butyl mixed etherified melamine resin, manufactured by Allnex Corporation), weight average molecular weight: 1,300, SP value: 10.40, solid content: 70% by mass Curing agent (B1b-2) Cymel 202 (imino-type methyl / butyl mixed etherified melamine resin, manufactured by Allnex Corporation), weight average molecular weight: 1,170, SP value: 11.36, solid content: 80% by mass Curing agent (B1b-3) Cymel 211 (imino group type melamine resin, manufactured by Allnex Corporation), weight average molecular weight: 780, SP value: 11.9, solid content: 80% by mass

[0145] <Method for preparing coated panels for evaluating coating appearance> Cationic electrodeposition paint PN-1010 (Nippon Paint Automotive Coatings) was electrodeposited onto an SPC steel plate (Nippon Test Panel, 35 mm x 150 mm x 0.8 mm) to a dry film thickness of 15 μm. The plate was then heated and cured at 170°C for 20 minutes, followed by cooling to prepare a cured electrodeposition coating. The resulting electrodeposition coating surface was then coated with water-based primer paint AR-630 (water-based coating composition, Nippon Paint Automotive Coatings) by air spray coating to a dry film thickness of 15 μm, and the coating was preheated at 80°C for 3 minutes. Next, the water-based coating compositions prepared in the Examples and Comparative Examples were applied by air spray coating to a dry film thickness of 13 μm to form a base coating, which was then preheated at 80°C for 3 minutes. Subsequently, clear paint O-1860 (solvent-based paint composition, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied by air spray coating to a dry film thickness of 30 μm to form a clear coating film, which was then heat-cured at 140°C for 18 minutes using a jet oven to obtain a coated panel for evaluation with a multi-layer coating film.

[0146] <Coating film appearance> The appearance of the evaluation coated plates obtained in the Examples and Comparative Examples was evaluated by measuring the W1 value (measurement wavelength: 2.4 mm or more) using a Microwave Scan-T (manufactured by BYK Gardner). Here, the smaller the W1 value measured by this device, the higher the smoothness of the coating surface.

[0147] <Sealer NSR performance test> Cationic electrodeposition paint PN-1010 (Nippon Paint Automotive Co., Ltd.) was electrodeposited onto an SPC steel plate (Nippon Test Panel, 50 mm x 160 mm x 0.8 mm) to a dry film thickness of 15 μm. The plate was then heated and cured at 170°C for 20 minutes, followed by cooling to prepare a cured electrodeposition coating. Sealer SN-2650-2 (Aisin Chemical Co., Ltd.) was applied to the surface of the resulting electrodeposition coating with an applicator to a dry film thickness of 1 mm to form a sealer coating. Next, the aqueous coating compositions prepared in the Examples and Comparative Examples were sprayed onto the surface of the resulting sealer coating to a dry film thickness of 10 μm to form a base coating, and the resulting coating was preheated at 80°C for 3 minutes. Subsequently, clear paint O-1860 (solvent-based paint composition, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied by air spray coating to a dry film thickness of 10 μm to form a clear coating film, which was then heat-cured at 130°C for 12 minutes using a jet oven.The test plate was then left to stand for 24 hours or 240 hours in an environment of 23±2°C and 50±5% RH to obtain a test plate with a multi-layer coating film.

[0148] A base coating film and a clear coating film were formed on the surface of the test plate obtained above in the same manner as described above (recoating process), thereby obtaining a test plate having, in order from the steel plate side, an electrodeposition coating film, a sealer coating film, a base coating film, a clear coating film, a base coating film, and a clear coating film. Eleven vertical and horizontal cuts were made with a cutter at 1 mm intervals in the coating film of the test plate obtained above, and Cellophane Tape (registered trademark) (manufactured by Nichiban Co., Ltd.) was applied to the cuts and then peeled off, and the number of remaining squares out of 100 squares was counted (cross-cut test). The cross-cut test was performed on each test plate that had been left for 24 hours. Note that 0 / 100 indicates that the coating film peeled off 0% of the area; for example, 10 / 100 indicates that the coating film peeled off 10% of the area, and 50 / 100 indicates that the coating film peeled off 50% of the area. 0 / 100 was considered a pass.

[0149] <Window Direct Bond (WDB) Adhesion Test 1> Procedure 1: Cationic electrodeposition paint PN-1010 (Nippon Paint Automotive) was electrodeposited onto an SPC steel plate (Nippon Test Panel, 35 mm x 150 mm x 0.8 mm) to a dry film thickness of 15 μm. The plate was then heated and cured at 170°C for 20 minutes, followed by cooling to prepare a cured electrodeposition coating. The surface of the resulting electrodeposition coating was then coated with water-based primer paint AR-630 (water-based coating composition, Nippon Paint Automotive Coatings) by air spray coating to a dry film thickness of 15 μm, and the coating was preheated at 80°C for 3 minutes. Next, the water-based coating compositions prepared in the Examples and Comparative Examples were applied by air spray coating to a dry film thickness of 13 μm to form a base coating, which was then preheated at 80°C for 3 minutes. Subsequently, clear paint O-1860 (solvent-based paint composition, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied by air spray coating to a dry film thickness of 30 μm to form a clear paint film, which was then heat-cured at 140°C for 18 minutes using a jet oven, drying the intermediate paint film, base paint film, and clear paint film simultaneously to form a multi-layer paint film, and a test specimen was obtained. Step 2: Bond Hamatite WS-373 (urethane sealant, manufactured by Yokohama Rubber Co., Ltd.) was applied in a strip shape to the surface of the clear coating so that the thickness of the bond after drying was 3 mm, the width was approximately 15 mm, and the length was approximately 100 mm. Release paper was then placed on top of the bond, and the bond was cured at room temperature for 72 hours while pressure was applied from above the release paper to obtain a test panel. Procedure 3: The test plate was immersed in 40°C warm water for 240 hours, and then cooled by immersing in room temperature water for 1 to 2 hours. Procedure 4: In the first test, the edge of the bond was pulled up at an angle of 90° or more from the coating surface (horizontal surface) in the longitudinal direction of the test plate, causing the bond to fail cohesively. Step 5: In the areas where the bond had cohesively failed and remained on the surface of the test plate, a cutter knife was used to make a cut at an angle of 60° from the coating surface (horizontal surface) along the length of the test plate to a depth that reached the base material (SPC steel plate). The cutter knife cut ran from one end of the strip of bond in the width direction to the other. Step 6: The length of the peeled part of the coating was measured with a ruler along the cut line made by the cutter knife (a line extending from one end of the strip of bond to the other in the width direction), and the percentage of the area of ​​the peeled part of the coating from the edge of the bond to the cut line was calculated to evaluate the adhesion of the WDB. If the coating is peeled, this indicates that cohesive failure has occurred in the base coating. Step 7: For the nth test, as in Step 4, the edge of the bond was pulled upward at an angle of 90° or more from the coating surface (horizontal plane) along the length of the test panel, causing cohesive failure. As in Step 5, a cutter knife was used to make incisions at 2–3 mm intervals along the length of the test panel from the incision line made in the n-1th test, at an angle of 60° from the coating surface (horizontal plane) along the length of the test panel, reaching deep into the substrate (SPC steel plate). As in Step 6, the length of the peeled area along the newly formed incision line was measured with a ruler, and the percentage of the area of ​​the peeled area from the incision line in the n-1th test to the incision line in the nth test was calculated to evaluate the adhesion of the WDB. This procedure was repeated for at least 10 tests (Figure 1). Step 8: The window direct bond (WDB) adhesion was evaluated based on the average of the evaluation results from the above steps.

[0150] The evaluation criteria are as follows: A: No peeling of the coating is observed. B: Peeling of the coating film of less than 1 mm is observed. C: Peeling of the coating film is observed, with a thickness of 1 mm or more but less than 2 mm. D: Peeling of the coating film of 2 mm or more is observed, but the peeled area is less than 50% of the total area. E: The peeled area is 50% or more and less than 70% of the total area. F: The peeled area is 70% or more and less than 100% of the total area. G: Peeled area is 100% of the total (entire coating).

[0151] <Window Direct Bond (WDB) Adhesion - Test 2> The adhesiveness of the WDB was evaluated in the same manner as described above, except that in step 3 of the Window Direct Bond Adhesion Test 1, instead of immersing the test plate in warm water at 40°C for 240 hours, the test plate was immersed in a mixed solution of Nikkemi Window Washer Fluid JCW-34 (manufactured by Nippon Chemical Industry Co., Ltd.) and deionized water in a mass ratio of 1:1 at room temperature for 240 hours.

[0152] The results are shown in Tables 2A and 2B.

[0153] [Table 2A]

[0154] [Table 2B]

[0155] Examples 1 to 18 are examples of the present disclosure, and even on substrates having a sealer portion, it was possible to form a coating film with good interlayer adhesion during recoating and excellent coating film appearance. Comparative Example 1 is an example that does not contain a melamine resin (B1a) having a weight-average molecular weight of 6,000 or more, and the interlayer adhesion was not fully satisfactory. Comparative Example 2 is an example that does not contain a melamine resin (B1b) having a weight average molecular weight of less than 6,000, and the sealer NSR properties were not fully satisfactory, and the interlayer adhesion during recoating in the sealer area was not fully satisfactory. [Explanation of symbols]

[0156] 1 Clear coating 2 Base coating 3. Intermediate coating 4 Electrodeposition coating 5 SPC steel plate 6. Bond 10 Cutting

Claims

1. A coating composition comprising a film-forming resin (A) and a curing agent (B), the film-forming resin (A) comprises an aqueous acrylic resin dispersion (A1); the acrylic resin in the acrylic resin water dispersion (A1) has a hydroxyl group, The curing agent (B) contains a melamine resin (B1), The melamine resin (B1) in the aqueous coating composition comprises a melamine resin (B1a) having a weight-average molecular weight of 6,000 or more and a melamine resin (B1b) having a weight-average molecular weight of less than 6,000.

2. 2. The aqueous coating composition according to claim 1, wherein the hydroxyl value of the acrylic resin water dispersion (A1) is 1 mgKOH / g or more and 150 mgKOH / g or less.

3. 2. The aqueous coating composition according to claim 1, wherein the content of the melamine resin (B) is 10 parts by mass or more and 50 parts by mass or less, based on 100 parts by mass of the total of the solid content of the coating film-forming resin (A) and the solid content of the curing agent (B).

4. 2. The aqueous coating composition according to claim 1, wherein the content of the melamine resin (B1a) is 1% by mass or more and 20% by mass or less of the total melamine resin (B1).

5. 2. The aqueous coating composition according to claim 1, wherein the film-forming resin (A) further comprises a urethane resin (A2) and / or a polyester resin (A3).

6. A step of applying the aqueous coating composition according to any one of claims 1 to 5 onto a substrate to form a coating film; A step of applying a clear coating wet-on-wet onto the coating film to form a clear coating film; and a step of simultaneously heating and curing the paint film and the clear paint film to form a multi-layer paint film.

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