JP7774521B2 - Water-based paint composition and method for producing multi-layer paint film - Google Patents

Description

This disclosure relates to an aqueous coating composition and a method for producing a multilayer coating film.

Multiple coatings (multilayer coatings) with various functions are sequentially formed on the surface of coated objects such as automobile bodies, protecting the object while also imparting a beautiful appearance and excellent design. A common method of forming multilayer coatings, for example, for steel plate, is to form a primer coating such as an electrodeposition coating on the substrate, which has excellent conductivity, and then sequentially form an intermediate coating, a base coating, and a clear coating on top of that.

Furthermore, in recent years, awareness of reducing environmental impact has grown, leading to demand for a shift to environmentally friendly products. In the paint industry, for example, there is a demand to reduce the amount of organic solvents used, and this demand can be met by using aqueous paint compositions that use water as the solvent.

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.

International Publication No. 2021/084802 International Publication No. 2021/024604 International Publication No. 2020/153057

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

The primer and topcoat coatings used in painting automobile bodies have a significant impact on the paint film and the product's appearance and design. Therefore, if a coating defect, such as bumps caused by foreign matter adhesion or dents or craters due to paint defects, is discovered during the application and baking/curing process of the primer and topcoat coatings, the area where the coating defect exists or the entire substrate is repainted with a primer and topcoat coating. This type of repainting is called recoating. In this recoating process, the interlayer adhesion between the multi-layer coating film, including the primer and topcoat coatings already applied to the substrate, and the repainted coating consisting of the primer and topcoat coatings applied during repainting becomes an issue. Poor interlayer adhesion can lead to delamination between the existing multi-layer coating and the repainted coating. The problem of interlayer adhesion during recoating is particularly pronounced on substrates with sealer sections.

If the appearance of the paint film becomes abnormal due to the adhesion of dirt or other particles, the affected area is repaired by sanding it with sandpaper or by repainting the base paint and clear paint on each block of the car's exterior panel. Even when changing the color tone of only a portion of the car's exterior panel, a process known as two-tone, the base paint and clear top coat are repainted on the area where the color tone is to be changed.

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

As such, not sanding down the first coat of clear paint results in poorer adhesion to the second coat of base paint, so it is common to sand down the first coat of clear paint and then reapply the second coat of base paint and top coat of clear paint.

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 costs required for repairs or repainting on an automotive paint line. Therefore, it has been desirable 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).

The present disclosure was made in consideration of the above-mentioned problems, and aims to provide an aqueous paint composition that can form a coating film with good interlayer adhesion during recoating, even on substrates with sealer sections, and that has excellent coating appearance. Another objective of the present disclosure is to provide a method for producing a multi-layer coating film using such an aqueous paint composition.

The present disclosure relates to a coating composition comprising: [1] 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] 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 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] The content of the melamine resin (B1a) is 1 mass% or more and 20 mass% or less in the total amount of the melamine resin (B1). [1] The aqueous coating composition according to any one of [1] to [3].
[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.

The aqueous paint composition disclosed herein can form a coating film that exhibits good interlayer adhesion during recoating, even on substrates with sealer sections, and that has excellent coating appearance. The present disclosure also provides a method for producing a multi-layer coating film using such an aqueous paint composition.

FIG. 1 is a schematic diagram of the Window Direct Bond (WDB) adhesion test.

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.

The aqueous paint composition disclosed herein can form a coating film with excellent interlayer adhesion and excellent appearance during recoating, even on substrates with sealer sections. Without being bound by any particular theory, it is believed that when a paint composition is applied over a sealer section, components contained in the sealer migrate into the coating film, leaving these components behind as the coating is formed. The decrease in interlayer adhesion during recoating is thought to be due to this component migration. This 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 that require baking and curing each coating film, this method offers the advantage of eliminating several heat curing steps, thereby shortening the process and saving energy during painting. For example, one 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.

The aqueous coating composition of the present disclosure contains both melamine resin (B1a) and melamine resin (B1b) as the melamine resin (B1). This inhibits migration of these components, and is believed to result in good 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.

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

(Water dispersion of acrylic resin (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.

Typically, the acrylic resin aqueous 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.

The (meth)acrylic acid alkyl ester is a (meth)acrylic acid alkyl ester that does not have an acid group or a hydroxyl group. By including the (meth)acrylic acid alkyl ester in the monomer mixture, the main skeleton of the acrylic resin can be favorably constructed.

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, and stearyl (meth)acrylate. Only one type of (meth)acrylic acid alkyl ester may be used, or two or more types may be used in combination.

Hydroxyl group-containing monomers include (meth)acrylic monomers having a hydroxyl group, specifically hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxybutyl (meth)acrylate; ε-caprolactone-modified (meth)acrylic monomers; and the like.

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. Only one type of hydroxyl group-containing monomer may be used, or two or more types may be used in combination.

By including a hydroxyl group-containing monomer in the monomer mixture, the resulting acrylic resin is made hydrophilic, improving the application workability and stability against freezing of the aqueous coating composition, and also increasing the curing reactivity of the acrylic resin with the curing agent (B), which will be described later.

The acid group-containing monomer may be a (meth)acrylic monomer having an acid group. The acid group may be a carboxyl group, sulfonic acid group, or phosphate group. From the viewpoints of improving dispersion stability and accelerating the curing reaction, a carboxyl group is preferred. By including an acid group-containing monomer in the monomer mixture, the resulting acrylic resin 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 curing agent (B) during coating film formation can be accelerated.

Examples of acid group-containing monomers 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.

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 styrene-based monomers include styrene and α-methylstyrene. Only one type of other monomer may be used, or two or more types may be used in combination.

The monomer mixture may contain crosslinkable monomers such as carbonyl group-containing monomers, hydrolyzable silyl group-containing monomers, and 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.

In preparing the acrylic resin aqueous 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 can be selected appropriately 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.

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.

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.

In emulsion polymerization, molecular weight adjusting agents (chain transfer agents), such as mercaptan compounds and lower alcohols, can be used as needed. The use of these agents allows the emulsion polymerization to proceed smoothly, promotes the smooth and uniform formation of a coating film, and improves the adhesion of the coating film to the substrate.

As 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.

A neutralizing agent may be added to the resulting acrylic resin aqueous 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 aqueous dispersion (A1). Examples of neutralizing agents 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.

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.

The acrylic resin contained in the acrylic resin aqueous dispersion (A1) may also be in the form of core-shell particles.

The weight-average molecular weight of the acrylic resin contained in the acrylic resin aqueous 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.

The acid value of the acrylic resin contained in the acrylic resin aqueous 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.

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 equivalent values and are measured by a method in accordance with JIS K 0070.

The solid content of the acrylic resin aqueous 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, and the resulting coating film has the advantage of good hardness.
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, which 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.

(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 using the urethane resin (A2), 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, it is possible to form a tough coating film, and a multi-layer coating film with excellent inter-coat adhesion, water-resistant adhesion, etc. can be obtained. In addition, the inclusion of the urethane resin (A2) can exhibit high inter-layer adhesion during recoating and high adhesion between the topcoat paint film and Wind Direct Bond (WDB).

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.

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 with hydrophilic groups such as polyols with acid groups (e.g., 2,2-dimethylolpropionic acid) and polyols with amino groups (e.g., N-methyldiethanolamine). Note that the polymer polyols and low molecular weight polyols are different from polyols with hydrophilic groups.

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.

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

Examples of the end-stopper include monools such as methanol, butanol, and octanol; alkylene oxide adducts of the above 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.

Each of the above components may be used alone or in combination of two or more.

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 the components are reacted in stages. 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.

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

Commercially available water-based urethane resins may be used. Examples of commercially available products include the NeoRez series (manufactured by Kusumoto Chemicals), the HUX series (manufactured by ADEKA Corporation), 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).

As the urethane resin (A2), one type may be used alone, or two or more types may be used in combination.

The solid content of the urethane resin (A2) in the solid content of the film-forming resin (A) is 0% by mass or more, and from the viewpoints of high interlayer adhesion during recoating, suppression of cohesive failure in the coating film, and 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.

(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. By further including the polyester resin (A3), the coating stability, coating workability, and physical properties of the resulting coating film can be improved.

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

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; trivalent 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.

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.

Additional reaction components may be used in combination, such as 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.).

Furthermore, as a further reactive component, fatty acids such as castor oil and dehydrated castor oil, and oil components that are one or a mixture of two or more of these fatty acids may be used. Furthermore, acrylic resins, vinyl resins, etc. may be grafted onto the polyester resin, or a polyisocyanate compound may be reacted therewith.

Each of the above components may be used alone or in combination of two or more.

A neutralizing agent may be added to the resulting 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 neutralizing agents 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.

From the viewpoints of storage stability and coating workability, the weight-average molecular weight of polyester resin (A3) is preferably 500 to 20,000, more preferably 1,500 to 10,000.

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 content acid value and solid content 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.

From the viewpoint of coating film hardness and base hiding power, the glass transition temperature Tg of the polyester resin (A3) is preferably -20°C to 80°C, more preferably -10°C to 60°C. The Tg of the polyester resin can be calculated from the types and amounts of monomers used in preparing the polyester resin. Alternatively, the Tg of the polyester resin may be measured using a differential scanning calorimeter (DSC).

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

The solid content of polyester resin (A3) in the solid content of 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.

The total content of the acrylic resin water dispersion (A1), urethane resin (A2), and polyester resin (A3) in the solids content of the film-forming resin (A) is preferably 70% by mass or more and 100% by mass or less, more preferably 80% by mass or more and 100% by mass or less, and even more preferably 88% by mass or more and 100% by mass or less.

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, and may be, for example, from 1,000 to 30,000.

The solid content of the film-forming resin (A) in the solids 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.

[Curing 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).

(Melamine resin (B1))
The melamine resin (B1) can be obtained by modifying a condensate 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 per molecule of a triazine nucleus, or a polycondensate thereof.
-NX ¹ X ²
[X ¹ and X ² each independently represent a hydrogen atom, a methylol group, or —CH ₂ —OR ^1. ]
^R1 represents an alkyl group having 1 to 8 carbon atoms, preferably a linear or branched alkyl group having 1 to 8 carbon atoms.
When multiple —CH ₂ —OR ¹ groups are contained in the same molecule, the multiple R1 groups may be the same or different.]

Examples of melamine resins include four types: a full alkyl type containing only -N(CH ₂ OR ¹ ) ₂ as the reactive functional group; a methylol group type containing -N(CH ₂ OR ¹ )(CH ₂ OH) as the reactive functional group; an imino group type containing -N(CH ₂ OR ¹ )(H) as the reactive functional group; and a methylol/imino group type containing -N(CH ₂ OR ¹ )(CH ₂ OH) and -N(CH ₂ OR ¹ )(H) or containing -N(CH ₂ OH)(H) as the reactive functional group. In the above-mentioned 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, R 1 may be a mixture of methyl groups and butyl groups, in another embodiment, it may be only methyl groups, or in yet another embodiment, it may be only butyl groups.

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 the present disclosure, the unit of the SP value is (cal/cm ³ ) ^1/2 .

The SP value can be measured using 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. Dissolve 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 point at which the diluted solution becomes cloudy is considered to be the amount of low SP poor solvent added.

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 cloudy is taken as the amount of high SP poor solvent added. The SP value can be calculated from the amount added until the cloudiness point of each poor solvent is reached using known calculation methods described in the references mentioned above.

Commercially available melamine resins may be used. Examples of commercially available melamine resins include methylol group-imino type methyl/butyl mixed etherified melamine resins such as Cymel 202 (SP value: 11.36); and 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). Resins: fully alkylated 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), Cymel 712 (SP value: 14.45); Cymel 247-10 ( Examples of 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.).

The melamine resin (B1) contains 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. This prevents migration of components from the sealer, even on substrates with a sealer, and improves interlayer adhesion during recoating.

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.

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 even more preferably 9.2 or more and less than 10.5.

Commercially available products may be used as the melamine resin (B1a). Examples of commercially available products include methylol group-type methylated melamine resins such as Cymel 247-10 (weight average molecular weight: 11,600, SP value: 9.20) and Cymel 651 (weight average molecular weight: 15,400, SP value: 9.20) (both manufactured by Allnex Japan Co., Ltd.).

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. Having a content within this range has the advantage that the curing reaction with the film-forming resin proceeds sufficiently and the resulting coating film has excellent interlayer adhesion.

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.

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 even more preferably 10.0 or more and less than 11.0.

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 groups 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); fully alkylated methylated melamine resins such as 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 711 (weight average molecular weight: 560, SP value: 14.21), Cymel 721 (weight average molecular weight: 560, SP value: 14.21), Cymel 731 (weight average molecular weight: 560, SP value: 14.21), Cymel 741 (weight average molecular weight: 560, SP value: 14.21), Cymel 751 (weight average molecular weight: 560, SP value: 14.21), Cymel 761 (weight average molecular weight: 560, SP value: 14.21), Cymel 771 (weight average molecular weight: 560, SP value: 14.21), Cymel 781 (weight average molecular weight: 560, SP value: 14.21), Cymel 791 (weight average molecular weight: 560, SP value: 14.21), Cymel 801 (weight average molecular weight: 560, SP value: 14.21), Cymel 811 (weight average molecular weight: 560, SP value: 14.21), Cymel 821 (weight average molecular weight: 560, SP value: 14.21), Cymel 831 (weight average molecular weight: 560, SP value: 14.21 Examples of suitable methylated melamine resins include imino-type methylated melamine resins such as Cymel 370 (weight average molecular weight: 985, SP value: 12.76); methylol-type methylated melamine resins such as Cymel 370 (weight average molecular weight: 985, SP value: 12.76) (all manufactured by Allnex Japan Co., Ltd.); imino-type methyl/butyl mixed etherified melamine resins such as Mycoat 212 (weight average molecular weight: 650, SP value: 10.10); imino-type butylated melamine resins such as Mycoat 508 (weight average molecular weight: 1,800, SP value: 10.70); imino-type methylated melamine resins such as Mycoat 723 (weight average molecular weight: 460, SP value: 11.11); and methylol-type methyl/isobutyl mixed etherified melamine resins such as Mycoat 2677 (weight average molecular weight: 1,530, SP value: 10.80) (all manufactured by Allnex Japan Co., Ltd.).

The solid content of the melamine resin (B1b) having a weight-average molecular weight of less than 6,000 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, of the solid content of the melamine resin (B1).

The total content of the solids of melamine resin (B1a) and melamine resin (B1b) in the solids 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.

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.

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.

The content of curing agent (B) in the aqueous coating composition (solids content equivalent) is preferably 20% to 70% by mass, and more preferably 20% to 50% by mass, of the total 100% by mass of the film-forming resin (solids content equivalent) and curing agent (solids content equivalent). Having the curing agent content within this range can improve interlayer adhesion during recoating and adhesion between the topcoat coating and the WDB.

[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.

[Other ingredients]
The aqueous coating composition of the present disclosure may contain various additives known for use in coating compositions other than those described above, as necessary. 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.

The pigments are 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; lead yellow, yellow iron oxide, chromium oxide, molybdate orange, red iron oxide, titanium yellow, zinc oxide, carmine, and the like. Examples include inorganic color pigments such as 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, metal-plated mica); graphite pigments, alumina flake pigments, titanium metal flakes, stainless steel flakes, plate-like iron oxide, phthalocyanine flakes, metal-plated glass flakes, and other colored flat pigments; and extender pigments such as titanium oxide, calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silica, and calcined kaolin.

The aqueous coating composition of the present disclosure can serve as a color base coating composition in addition to serving as a primer 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.

The pigment dispersion paste is obtained by pre-dispersing a pigment, a pigment dispersant, and, if necessary, a portion of the film-forming resin (A) 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 such functional groups in one molecule.

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.

There are no particular restrictions on pigment dispersants, as long as they do not contain volatile basic substances in their solid content or contain 3% or less by mass. However, dispersants that can efficiently disperse pigments with small amounts are preferred. For example, commercially available dispersants (all trade names below) can be used. Specific examples include anionic and nonionic dispersants manufactured by BYK-Chemie, such as Disperbyk 190, Disperbyk 181, Disperbyk 182 (polymer copolymer), and Disperbyk 184 (polymer copolymer); anionic and nonionic dispersant EFKAPOLYMER 4550 manufactured by BASF; and nonionic dispersant Solsperse 27000, and anionic dispersants Solsperse 41000 and Solsperse 53095, both manufactured by Avecia.

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.

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 known methods. The proportion of 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 solids content of the pigment dispersion paste. By keeping the proportion of pigment dispersant within this range, the pigment dispersion stability and the resulting coating properties can be maintained within better ranges. The proportion of pigment dispersant is more preferably 3% by mass or more, and even more preferably 5% by mass or less.

The pigment may be any of the pigments exemplified above for use in the aqueous paint composition, but colored pigments are preferred 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 paint may be a standard gray paint composition containing carbon black and titanium dioxide as the main pigments. Other examples include paint compositions that match the brightness or hue of the second paint composition, and paint compositions that combine various colored pigments.

The ratio of the pigment mass to the total mass of the resin solids and pigment in the aqueous coating composition (PWC) is preferably 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, ensuring flowability and making it easier to obtain a good coating appearance.

Examples of thickeners include associative nonionic urethane thickeners, alkali swelling thickeners, and inorganic intercalation compounds such as bentonite.

In the aqueous coating composition of the present disclosure, the total solids 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.

<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.

<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.

<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.

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.

The substrate is not particularly limited, and examples include metal substrates, plastic substrates, and foamed versions thereof.

Examples of the material 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 such metal substrates have an electrodeposition coating film formed thereon in advance. Furthermore, prior to 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 standpoint of corrosion resistance, cationic electrodeposition coating compositions are preferred.

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 plastic substrates include automotive parts such as spoilers, bumpers, mirror covers, grilles, and doorknobs. These plastic substrates are preferably washed with pure water and/or a neutral detergent. They may also be coated with a primer to enable electrostatic painting.

If necessary, an intermediate coating film or primer coating film may be formed on the substrate. 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. There are no particular restrictions on the film-forming resin and curing agent contained in the intermediate coating composition, and the film-forming resins and curing agents listed for the water-based coating composition above can be used. The color pigments and extender pigments listed for the water-based coating composition above can also be used.

A primer coating composition is used to form the primer coating. There are no particular limitations on the primer coating composition, and a primer coating containing a film-forming resin and, if necessary, a curing agent, pigment, additives, etc. can be used. Primer coatings can be in the form of water-based or solvent-based. The film-forming resin and curing agent contained in the primer coating are not particularly limited. Examples of such film-forming resins include polyester resins, acrylic resins, and urethane resins, and examples of such curing agents include polyisocyanate compounds and melamine resins. The pigments and additives may be those exemplified for the water-based coating composition, and titanium dioxide, etc., may also be used as the pigment.

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

The vinyl chloride resin contained in vinyl chloride plastisol sealants includes 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 types.

Plasticizers contained in vinyl chloride plastisol sealants include, for example, phthalate esters such as dioctyl phthalate, di-2-ethylhexyl phthalate, diisononyl phthalate, and octylbutyl phthalate; di- or tricarboxylic acid esters such as sebacate esters and azelaate esters; phosphate esters such as triphenyl phosphate and tricresyl phosphate; and epoxy-based plasticizers such as epoxidized soybean oil. High-boiling organic solvents can also be used as plasticizers.

Vinyl chloride plastisol sealants can contain, as needed, fillers such as alkaline earth metal carbonates and sulfates, mica, silica, talc, diatomaceous earth, and kaolin; common adhesion promoters such as epoxy, acrylic, polyamide, and isocyanate; stabilizers such as salts of metals such as zinc, lead, barium, tin, and calcium; and thixotropy promoters such as surface-treated calcium carbonate and ultrafine silica powder.

After applying such a vinyl chloride plastisol sealant, a sealer portion is formed on the substrate by curing under commonly used heat curing conditions. In one embodiment, after applying the sealant, the aqueous paint composition of the present disclosure is applied onto the uncured sealer coating film to form an uncured coating film, and then a clear coating is applied onto the uncured coating film to form an uncured clear coating film, and the sealer coating film, coating film, and clear coating film are simultaneously heat cured.

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, a sealer portion, or an undercoat coating. From the perspective of improving appearance, application methods include multi-stage painting using air electrostatic spray painting, preferably two-stage painting, or a painting method that combines air electrostatic spray painting with a rotary atomizing electrostatic paint applicator known as a metallic bell.

It is typically preferable to apply the water-based base paint 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 adequately conceals the base and prevents 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 multi-layer paint film with a better appearance, it is preferable to heat the resulting uncured base paint film at 40 to 100°C for 2 to 10 minutes to dry it before applying the clear paint.

Next, a clear coating is applied wet-on-wet on top of the uncured base coating without heating it to harden, forming an uncured clear coating. The clear coating smooths and protects any irregularities caused by the base coating, and also adds a beautiful appearance.

The clear paint used to form the clear coating film is not particularly limited, and any clear paint containing a film-forming resin and a curing agent can be used. It can also contain coloring components, as long as the amount does not interfere with the design of the base. Clear paints can be in solvent-based, water-based, or powder form.

Preferred examples of solvent-based clear coatings, from the standpoint of transparency or acid etching resistance, include combinations of acrylic resin and/or polyester resin with amino resin and/or isocyanate, or acrylic resin and/or polyester resin with a carboxylic acid/epoxy curing system.

Examples of water-based clear paints include those containing resins that have been made water-based by neutralizing the film-forming resins contained in the solvent-based clear paints listed above with a base. This neutralization can be carried out by adding a tertiary amine such as dimethylethanolamine or triethylamine before or after polymerization.

Conventional powder coatings, such as thermoplastic and thermosetting powder coatings, can be used as powder clear coatings. Thermosetting powder coatings are preferred because they produce coatings 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 good weather resistance.

It is preferable that a viscosity control agent be added to the clear coating to ensure ease of application. Viscosity control agents that generally exhibit thixotropy can be used. For example, conventionally known agents can be used. The clear coating can also contain curing catalysts, surface conditioners, etc., as needed.

One method for applying clear paint to a base coating is to use a rotary atomizing electrostatic sprayer called a Micro Micro Bell or Micro Bell.

The clear coating is preferably applied so that the dry film thickness is, for example, 10 μm to 80 μm, and preferably 20 μm to 60 μm. A dry film thickness of 10 μm or more can adequately conceal the unevenness of the base, while a dry film thickness of 80 μm or less can suppress defects such as popping or dripping during application.

The uncured clear coat film thus formed and the previously formed uncured base coat film are simultaneously heated and cured to form a multi-layer coating film. From the standpoint of curability and the physical properties of the resulting multi-layer coating film, the heat-curing temperature is preferably 80 to 180°C, and more preferably 120 to 160°C. The heat-curing time can be set as desired depending on the temperature, with a heat-curing temperature of 80 to 160°C and a time of 10 to 30 minutes being appropriate.

The disclosed method for forming a multilayer coating film also relates to a three-coat, one-bake method for forming a multilayer coating film, in which the intermediate coating is applied to the substrate to form an uncured intermediate coating film, and then, without curing it by heat, the aqueous coating composition (aqueous base coating) disclosed herein 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 coatings are simultaneously cured by heat. Examples of various coatings used in this method include those similar to those described above.

The multilayer coating film, including a base coating film and a clear coating film, obtained by the multilayer coating film formation method described above has a good coating appearance, excellent hardness and adhesion, and good water resistance, even when the heat curing temperature is low.

The present disclosure will be further explained in detail using the following examples, but the present disclosure is not limited to these.

<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 with stirring in a stainless steel vessel, 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.

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 inside 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 inside the reaction vessel was maintained at 80°C for 2 hours, then cooled to 40°C and filtered through a 400-mesh filter. 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, yielding 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).

<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 with stirring in a stainless steel container, 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 emulsification mixture.

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 the second monomer, 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. The mixture was stirred at room temperature for 15 minutes using a homomixer to obtain 152.0 parts by mass of a second monomer emulsion mixture.

A reaction vessel equipped with a stirrer, a reflux condenser, a dropping funnel, and a thermometer was charged with 126.6 parts by mass of deionized water, and the temperature in 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 in the reaction vessel was maintained at 80°C for 1 hour.
Further, 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 completion of the addition, 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 mgKOH/g, solid content hydroxyl value: 90 mgKOH/g, solid content concentration: 35% by mass).

<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 amounts of the monomers were changed to those shown in Table 1 (single-layer type, average particle size: 120 nm, acid value of solid content: 20 mgKOH/g, hydroxyl value of solid content: 40 mgKOH/g, solid content concentration: 25% by mass).

<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 blending amounts of the first monomer and the second monomer were changed to the amounts shown in Table 1 (core-shell type, average particle size: 150 nm, solid content acid value: 17 mgKOH/g, solid content hydroxyl value: 67 mgKOH/g, solid content concentration: 35 mass%).

<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 mass of isophthalic acid, 22.8 parts by mass of phthalic anhydride, 5.6 parts by mass of adipic acid, 19.3 parts by mass of trimethylolpropane, 26.7 parts by mass of neopentyl glycol, 17.5 parts by mass of ε-caprolactone, and 0.1 parts by mass 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, while distilling off the water produced, and the reaction product was subjected to a condensation reaction until the solid acid value of the reaction product reached 8 mg KOH / g. Next, 7.9 parts by mass of trimellitic anhydride was added and the mixture was reacted at 150 ° C. for 1 hour to obtain a polyester resin (solid acid value: 40 mg KOH / g). After cooling to 100 ° C., 11.2 parts by mass 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).

<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 tertiary butyl peroxy 2-ethylhexanoate were added dropwise to the reaction vessel in parallel over 3 hours while continuing to stir in the reaction vessel. After completion of the addition, stirring in the reaction vessel was continued at the same temperature for 30 minutes, and then 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 Shannin 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 Corporation, solids concentration: 50%), and 155.5 parts by mass of ion-exchanged water were premixed using a disper, and then the mixture was subjected to a dispersion treatment at 3,000 rpm for 5 hours using an SG mill (dispersion medium: zircon beads), and the zircon beads were filtered off to obtain a color pigment paste (pigment mass concentration (PWC): 23% by mass, solids concentration: 42% by mass).

<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 colored pigment paste obtained in Production Example 5 were mixed with stirring using a disper to obtain an aqueous coating composition 1. The blending amount of each component is the amount (parts by mass) converted to solids content.

(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.

Details of the raw materials used are given below.

Coating 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

Coating film forming resin (A3);
Film-forming resin (A3-2): NC3000 (polyester resin, manufactured by Toyobo Co., Ltd.), solid content: 40% by mass

Curing agent (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

<Method for preparing coated panels for evaluating coating appearance>
An SPC steel plate (manufactured by Nippon Test Panel, 35 mm x 150 mm x 0.8 mm) was electrodeposited with cationic electrodeposition paint PN-1010 (manufactured by Nippon Paint Automotive Coatings) to a dry film thickness of 15 μm, and then heated and cured at 170°C for 20 minutes, followed by cooling to prepare a cured electrodeposition coating. On the surface of the resulting electrodeposition coating, an aqueous primer paint AR-630 (aqueous coating composition, manufactured by Nippon Paint Automotive Coatings) was applied 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 aqueous 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, and the coating was preheated at 80°C for 3 minutes. Thereafter, clear coating O-1860 (solvent-based coating composition, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied by air spray coating so that the dry film thickness was 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 plate for evaluation having a multi-layer coating film.

<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.

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

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), to obtain 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.
The coating film of the test plate obtained above was cut with a cutter, with 11 cuts in each direction at 1 mm intervals, and Cellophane tape (registered trademark) (manufactured by Nichiban Co., Ltd.) was applied on the cuts and 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 a case where the coating film peeled off area was 0%, for example, 10/100 indicates a case where the coating film peeled off area was 10%, and 50/100 indicates a case where the coating film peeled off area was 50%. 0/100 was considered a pass.

<Window Direct Bond (WDB) Adhesion Test 1>
Procedure 1: An SPC steel plate (manufactured by Nippon Test Panel, 35 mm x 150 mm x 0.8 mm) was electrodeposited with cationic electrodeposition paint PN-1010 (manufactured by Nippon Paint Automotive Co., Ltd.) to a dry film thickness of 15 μm, and then heated and cured at 170°C for 20 minutes, followed by cooling to prepare a cured electrodeposition coating. On the surface of the resulting electrodeposition coating, an aqueous primer paint AR-630 (aqueous coating composition, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied 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 aqueous 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, and the coating was preheated at 80°C for 3 minutes. Thereafter, clear paint O-1860 (solvent-based paint composition, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied by air spray coating so that the dry film thickness was 30 μm to form a clear paint film, which was then heat-cured using a jet oven at 140°C for 18 minutes, and the intermediate paint film, base paint film, and clear paint film were simultaneously dried 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 to the surface of the clear coating in a strip shape so that the film thickness after drying of the bond was 3 mm, the width was about 15 mm, and the length was about 100 mm. After that, release paper was placed on the bond, and the bond was cured at room temperature for 72 hours while applying pressure from above the release paper to obtain a test panel.
Procedure 3: The test plate was immersed in warm water at 40° C. for 240 hours, and then cooled by immersing in water at room temperature 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 cohesive failure of the bond.
Step 5: At the point where the bond had cohesively failed and remained on the surface of the test plate, a cutter knife was used to make a cut from the coating surface (horizontal surface) at an angle of 60° 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 end.
Step 6: The length of the peeled part of the coating film along the cut line made by the cutter knife (the line extending from one end of the strip bond to the other in the width direction) was measured with a ruler, and the percentage of the area of the peeled part of the coating film in the region from the end of the bond to the cut line was calculated to evaluate the adhesiveness of the WDB. If the coating film is peeled off, this indicates that cohesive failure has occurred in the base coating film.
Step 7: For the nth test, as in Step 4, the edge of the bond was pulled up at an angle of 90° or more from the coating surface (horizontal surface) along the length of the test plate, causing cohesive failure of the bond. As in Step 5, a cutter knife was used to make a cut at a 60° angle from the coating surface (horizontal surface) along the length of the test plate, leaving a 2-3 mm gap from the cut line made in the n-1th test, to a depth that reached the base material (SPC steel plate). As in Step 6, the length of the peeled coating along the newly formed cut line was measured with a ruler, and the percentage of the area of the peeled coating from the cut line in the n-1th test to the cut line in the nth test was calculated to evaluate the adhesion of the WDB. The above procedure was repeated 10 or more times (Figure 1).
Procedure 8: Window Direct Bond (WDB) adhesion was evaluated based on the average of the evaluation results from the above procedures.

The evaluation criteria are as follows:
A: No peeling of the coating film was observed.
B: Peeling of the coating film of less than 1 mm is observed.
C: Peeling of the coating film of 1 mm or more but less than 2 mm is observed.
D: Peeling of the coating film of 2 mm or more was observed, but the peeled area was 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 film).

<Window Direct Bond (WDB) Adhesion - Test 2>
The adhesiveness of the WDB was evaluated in the same manner as 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.

The results are shown in Tables 2A and 2B.

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 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 property was not fully satisfactory, and the interlayer adhesion during recoating in the sealer part was not fully satisfactory.

1 Clear coating film 2 Base coating film 3 Intermediate coating film 4 Electrodeposition coating film 5 SPC steel plate 6 Bond 10 Cut

Claims (6)

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. The aqueous coating composition according to claim 1, wherein the hydroxyl value of the acrylic resin aqueous dispersion (A1) is 1 mgKOH/g or more and 150 mgKOH/g or less. The aqueous coating composition according to claim 1, wherein the content of the melamine resin (B) is 10 to 50 parts by mass per 100 parts by mass of the total solid content of the film-forming resin (A) and the curing agent (B). 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). The aqueous coating composition according to claim 1, wherein the film-forming resin (A) further contains a urethane resin (A2) and/or a polyester resin (A3). 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.