Multilayer coating film and method for forming a multilayer coating film

Abstract

A method for forming a multi-layer coated film according to the present invention comprises a first step for coating an aqueous base coating material composition onto a coating target so as to form a base coated film (I), a second step for coating an aqueous clear coating material composition onto the base coated film (I) so as to form a clear coated film (II), and a step for simultaneously drying the base coated film and the clear coated film, wherein at least the aqueous base coating material composition contains a metal compound containing at least one metal selected from the group consisting of zinc, tin, zirconium, bismuth, lead, cobalt, manganese, titanium, aluminum, and molybdenum.

Description

[Technical Field]

[0001] The present invention relates to a multi-layer coating film and a method for forming a multi-layer coating film. [Background technology]

[0002] Traditionally, in the automotive refinish coating field, organic solvent-based acrylic lacquer paints and two-component urethane paints have been used for repairs due to their fast drying time, adhesion, water resistance, and sandability. However, in recent years, from an environmental perspective, the automotive refinish coating industry has been shifting from organic solvent-based paints to water-based paints, and various water-based colored base coats and clear coats have been proposed. Furthermore, in painting environments, there is a growing demand for paints that can dry at lower temperatures and in shorter times to improve work efficiency, for example, paints that can be polished after drying in 20 minutes at 60°C. In addition, in multi-layer coating methods such as two-coat / one-bake painting with water-based paints, where a clear coat is applied wet-on-wet over a base coat and dried simultaneously, there is a simultaneous demand for paints to have a high level of finish and coating performance equivalent to that of solvent-based paints.

[0003] As a method for improving drying properties at low temperatures and in a short time, for example, the applicant proposed in Patent Document 1 a two-component aqueous polyurethane coating composition and a coating method thereof, comprising a first component containing a hydroxyl group-containing resin emulsion component, a molybdenum compound, and water, and a second component containing a polyisocyanate component and an organic solvent. This two-component aqueous polyurethane coating composition makes it possible to form a coating film that requires a short drying time to reach the desired coating film hardness.

[0004] However, as mentioned above, there is a growing demand for further improvements in workability and process shortening. In contrast, the paint composition described in Patent Document 1 sometimes exhibited insufficient drying properties, water resistance, and film hardness when applied wet-on-wet to a water-based coating and then coated with a clear coating to form a multi-layer coating. There is a need to achieve good finish, film hardness, and drying properties (especially polishability) in multi-layer coatings even under conditions where high drying temperatures are not possible. One way to improve drying time is to increase the amount of curing catalyst in the clear coating composition. However, this can significantly reduce the usable time (also known as pot life), affecting the finish, and can also worsen the coating properties, particularly the hardness and water resistance of multi-layer coatings, thus hindering the reduction of drying time and process steps. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-142999 [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention has been made in view of the above circumstances, and aims to provide a method for forming a multilayer coating using an aqueous coating composition as the coating for forming the base coating and the clear coating, which provides a multilayer coating with good finish, excellent drying properties, water resistance and coating hardness, and a method for forming such a multilayer coating that is excellent in drying at lower temperatures and in a shorter time. [Means for solving the problem]

[0007] The inventors of the present invention attempted to form a multi-layer coating by intentionally adding a metal catalyst to the base coating. As a result, they found that the presence of a metal compound in the base coating improved the properties of the coating, particularly the hardness and water resistance of the multi-layer coating, leading to the present invention.

[0008] In other words, the present invention provides a method for forming a multilayer coating film, comprising: a first step of applying an aqueous base coating composition to an object to be coated to form a base coating film (I); a second step of applying an aqueous clear coating composition on the base coating film (I) to form a clear coating film (II); and a step of drying the base coating film and the clear coating film simultaneously, wherein at least the aqueous base coating composition contains a metal compound comprising at least one metal selected from the group consisting of zinc, tin, zirconium, bismuth, lead, cobalt, manganese, titanium, aluminum, and molybdenum.

[0009] The concentration of the metal compound in the water-based paint composition is preferably equal to or greater than the concentration of the metal compound in the clear paint composition.

[0010] The metal is derived from a metal catalyst, and the content of the metal catalyst in the aqueous-based paint composition is preferably 0.01 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the total resin solids in the aqueous-based paint composition.

[0011] The water solubility of metal compounds is 1 × 10⁻⁶. -3 mg / 1000g or more 5×10 5 It is preferable that the amount is mg / 1000g or less.

[0012] The water-based paint composition contains a first film-forming resin, and the first film-forming resin is preferably at least one of a water-dispersible acrylic resin (a1), a water-dispersible polyurethane resin (a2), and a water-soluble acrylic resin (a3).

[0013] The aqueous clear coating composition comprises a second film-forming resin, the second film-forming resin comprising a hydroxyl group-containing resin and a polyisocyanate compound (b), and preferably the polyisocyanate compound (b) comprises a polyisocyanate compound (b1) having three or more isocyanate groups and a molecular weight of 350 or less.

[0014] The aqueous clear coating composition contains a second film-forming resin, and the second film-forming resin preferably contains a dispersion-type acrylic resin (a4).

[0015] The object to be coated is preferably an old coating film or a damaged part of a coated body.

[0016] The object to be coated is preferably an industrial machine, a construction machine, a railway vehicle, a large vehicle, a ship hull, a building or a structure, or a part thereof.

[0017] The multi-layer coating film of the present invention is a multi-layer coating film having a base coating film (I) layer of an aqueous base coating composition and a clear coating film (II) layer of an aqueous clear coating composition on the base coating film (I) layer on an object to be coated, and the Martens hardness value is 7 N / mm 2 or more.

[0018] In addition, another multi-layer coating film of the present invention is a multi-layer coating film having a cured base coating film layer formed by coating and curing an aqueous base coating composition and a cured clear coating film layer formed by coating and curing an aqueous clear coating composition on an object to be coated, and the cured clear coating film contains at least one metal compound selected from the group consisting of zinc, tin, zirconium, bismuth, lead, cobalt, manganese, titanium, aluminum and molybdenum, and the metal element concentration in the cured clear coating film is 20 ppm or more.

Effects of the Invention

[0019] According to the method for forming the multi-layer coating film of the present invention, a multi-layer coating film excellent in drying property, finish property, water resistance, and coating film hardness at low temperature and short time can be obtained. In addition, the multi-layer coating film of the present invention is excellent in finish property, drying property, water resistance and coating film hardness.

Modes for Carrying Out the Invention

[0020] Hereinafter, the method for forming the multi-layer coating film of the present invention will be described. In this specification, "a resin contains monomers that are its raw materials" means that the resin is a (co)polymer of raw material monomers containing the above monomers, unless otherwise specified. In this specification, (co)polymer means polymer or copolymer.

[0021] In this specification, "(meth)acrylate" means acrylate and / or methacrylate, and "(meth)acrylic acid" means acrylic acid and / or methacrylic acid. Also, "(meth)acryloyl" means acryloyl and / or methacryloyl. Furthermore, "(meth)acrylamide" means acrylamide and / or methacrylamide.

[0022] [Method for forming multi-layer coatings] The present invention provides a method for forming a multilayer coating film, comprising: a first step of applying an aqueous base coating composition to a workpiece to form a base coating film (I); a second step of applying an aqueous clear coating composition on the base coating film (I) to form a clear coating film (II); and a step of simultaneously drying the base coating film and the clear coating film, wherein at least the aqueous base coating composition contains a metal compound comprising at least one metal selected from the group consisting of zinc, tin, zirconium, bismuth, lead, cobalt, manganese, titanium, aluminum, and molybdenum. In the present invention, a cured coating film is a coating film in the cured and dried state specified in JIS K 5600-1-1, that is, a coating film in which, when the center of the coated surface is firmly pinched between the thumb and index finger, no indentation from fingerprints is left on the coated surface, no movement of the coating film can be felt, and when the center of the coated surface is rapidly and repeatedly rubbed with the fingertip, no scratches are left on the coated surface. On the other hand, an uncured coating film is a coating film in which the coating film has not reached the above cured and dried state, and also includes the touch-dry state and semi-cured state specified in JIS K 5600-1-1. Furthermore, if the water-based paint composition does not contain a hardener, for example, the base coating immediately after application will be in an uncured state because the cross-linking reaction has not occurred. It will become semi-cured or fully cured as the hardener components from the clear coat penetrate into it. While the curing and drying state can be easily confirmed by the method described above, it can also be confirmed by infrared spectroscopy (IR) analysis, etc., to see if the hardness of the multi-layer coating film becomes constant.

[0023] <First step> The first step is to apply a water-based paint composition to the object to be coated to form a base coating film (I).

[0024] (subject to be coated) In the present invention, the substrate to which the aqueous-based coating composition is applied is not particularly limited. Specifically, examples include automobiles, motorcycles and other automobile vehicles or their parts, industrial machinery, construction machinery, railway vehicles, heavy vehicles, ship hulls, buildings or structures, and their parts.

[0025] The substrate to which the water-based coating composition is applied is not particularly limited, but specifically includes, for example, metals such as aluminum, iron, stainless steel, zinc, copper, and tinplate; inorganic materials such as glass, concrete, and slate; organic materials such as plastics and polyvinyl chloride; wood; and coated objects made by applying a coating film to these substrates. The coated object may also be a molded product containing the above-mentioned substrates.

[0026] Examples of coatings applied to a substrate include a primer coating obtained by applying a primer paint composition to a substrate that has been surface-treated as needed, a cured or uncured colored base coating, a surface treatment coating obtained by applying a primer surfacer to damaged areas of the painted body, known coatings such as new car coatings or repair coatings, or known polished coatings.

[0027] The method for forming a multi-layer coating of the present invention is particularly suitable for repair painting of automobile bodies and is effective for coating substrates such as primer-surfaced coatings, known coatings, or polished known coatings.

[0028] (Water-based paint composition) The aqueous base coating composition of the present invention contains a first film-forming resin. Examples of the first film-forming resin include, but are not limited to, acrylic resins, polyester resins, alkyd resins, urethane resins, fluororesins, epoxy resins, silicone resins, and polyether resins. It is also preferable to contain at least one resin having a crosslinkable functional group such as a hydroxyl group, a carbonyl group, or an amino group, and it is particularly preferable to contain a resin containing a hydroxyl group. This is because a portion of the polyisocyanate compound (b) in the aqueous clear coating composition described later penetrates into the base coating film and undergoes a crosslinking reaction, thereby improving the curability and water resistance of the multi-layer coating film. The film-forming resin can be used individually or in combination of two or more types. When two or more film-forming resins are used, it is preferable that the total hydroxyl value of the film-forming resins contained in the aqueous-based paint composition be adjusted to within the range of 1 to 200 mg KOH / g or 5 to 100 mg KOH / g, from the viewpoint of improving drying properties, finish properties, and water resistance. The film-forming resin contained in the aqueous-based coating composition is preferably one of the following: a water-dispersible acrylic resin (a1), a water-dispersible polyurethane resin (a2), and a water-soluble acrylic resin (a3).

[0029] -Water-dispersible acrylic resin (a1)- The water-dispersible acrylic resin (a1) is preferably a hydroxyl group-containing acrylic resin emulsion, which is an aqueous dispersion of a hydroxyl group-containing acrylic resin, due to its excellent hardness of the formed coating film.

[0030] The water-dispersible acrylic resin (a1) is obtained by dispersing a copolymer in water, in which a (meth)acryloyl group-containing polymerizable unsaturated monomer is an essential component, and other polymerizable unsaturated monomers are copolymerized as needed. For example, a suitable example is one obtained by emulsion polymerization in one or multiple steps using the polymerizable unsaturated monomer component and a polymerization initiator in the presence of water and a dispersion stabilizer.

[0031] The average particle size of the water-dispersible acrylic resin (a1) is preferably 0.02 μm or more and 1.0 μm or less, and more preferably 0.05 μm or more and 0.3 μm or less.

[0032] In this specification, the average particle size of a water-dispersible resin is the volume-average particle size measured by the Coulter counter method at a measurement temperature of 20°C. The Coulter counter method can be used, for example, with a "COULTER N4" (manufactured by Beckman Coulter, trade name).

[0033] The water-dispersible acrylic resin (a1) may be a single-layer type or a multi-layer type such as a core-shell type, but a single-layer type is preferred in terms of recoating compatibility and other factors.

[0034] Examples of polymerizable unsaturated monomers that can serve as copolymer components for water-dispersible acrylic resin (a1) include linear or branched alkyl(meth)acrylates such as methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl(meth)acrylate, pentyl(meth)acrylate, hexyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, lauryl(meth)acrylate, and stearyl(meth)acrylate; and cyclohexyl(meth)acrylate. Alicyclic alkyl (meth)acrylates such as isobornyl (meth)acrylate; aralkyl (meth)acrylates such as benzyl (meth)acrylate; alkoxyalkyl (meth)acrylates such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate; perfluoroalkyl (meth)acrylate; N,N-dialkylaminoalkyl (meth)acrylates such as N,N-diethylaminoethyl (meth)acrylate; (meth)acrylamide; (meth)acrylonitrile; vinyl acetate, pro Vinyl ester compounds such as vinyl pionic acid; vinyl aromatic compounds such as styrene and α-methylstyrene; allyl (meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate Polyvinyl compounds having at least two polymerizable unsaturated groups in one molecule, such as acrylates, pentaerythritol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, glycerol di(meth)acrylate, 1,1,1-trishydroxymethylethane di(meth)acrylate, 1,1,1-trishydroxymethylethane tri(meth)acrylate, 1,1,1-trishydroxymethylpropane tri(meth)acrylate, triallyl isocyanurate, diallyl terephthalate, and divinylbenzene;Hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate; allyl alcohol; ε-caprolactone modified forms of the above hydroxyalkyl (meth)acrylates; hydroxyl group-containing polymerizable unsaturated monomers such as polyoxyethylene chain-containing (meth)acrylates with hydroxyl groups at the molecular ends; (meth)acrylic acid, maleic acid, crotonic acid, β-carboxyethyl Carboxyl group-containing polymerizable unsaturated monomers such as acrylate; (meth)acrolein, formyl styrene, C4-C7 vinyl alkyl ketones (e.g., vinyl methyl ketone, vinyl ethyl ketone, vinyl butyl ketone, etc.), acetoacetoxyethyl (meth)acrylate, acetoacetoxyaryl ester, diacetone (meth)acrylamide, and other carbonyl group-containing polymerizable unsaturated monomers; glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexyl Examples include epoxy group-containing polymerizable unsaturated monomers such as silmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, and allyl glycidyl ether; isocyanato group-containing polymerizable unsaturated monomers such as isocyanatoethyl (meth)acrylate and m-isopropenyl-α,α-dimethylbenzyl isocyanate; alkoxysilyl group-containing polymerizable unsaturated monomers such as vinyltrimethoxysilane, vinyltriethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, and γ-methacryloyloxypropyltriethoxysilane; reaction products of epoxy group-containing polymerizable unsaturated monomers or hydroxyl group-containing polymerizable unsaturated monomers with unsaturated fatty acids; and oxidative curable group-containing polymerizable unsaturated monomers such as dicyclopentenyloxyethyl (meth)acrylate, dicyclopentenyloxypropyl (meth)acrylate, and dicyclopentenyl (meth)acrylate. These can be used individually or in combination of two or more. ;

[0035] Furthermore, while there are no particular limitations on the dispersion stabilizers used when emulsion polymerization of water-dispersible acrylic resin (a1), examples include anionic emulsifiers such as sodium dialkyl sulfosuccinate, sodium dodecylbenzenesulfonate, sodium lauryl sulfate, sodium polyoxyethylene alkylphenyl ether sulfate, and sodium alkyldiphenyl ether disulfonate; nonionic emulsifiers such as polyoxyethylene higher alcohol ethers and polyoxyethylene alkylphenyl ethers; and anionic or cationic reactive emulsifiers having radically polymerizable double bonds.

[0036] A reactive emulsifier is an emulsifier that has one or more nonionic, anionic, and cationic groups, along with a polymerizable unsaturated group, in its molecule. Specific examples of polymerizable unsaturated groups include (meth)allyl, (meth)acryloyl, propenyl, and butenyl groups. Commercially available reactive emulsifiers include "Latemul" (trade name, manufactured by Kao Corporation), "Eleminol" (trade name, manufactured by Sanyo Chemical Industries, Ltd.), "Aqualon" (trade name, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), "Adekaria Soap" (trade name, manufactured by Asahi Denka Co., Ltd.), and "ANTOX" (trade name, manufactured by Nippon Emulsifier Co., Ltd.).

[0037] Conventional polymerization initiators can be used without limitation, including peroxide-based polymerization initiators and azo-based polymerization initiators.

[0038] In the present invention, the water-dispersible acrylic resin (a1) is suitable from the viewpoint of water resistance, sandability, and hardness of the colored base coating film if it contains a polyvinyl compound having at least two polymerizable unsaturated groups in one molecule as a copolymer component. Examples of polyvinyl compounds having at least two polymerizable unsaturated groups in one molecule include the compounds described above, which can be used alone or in combination of two or more. The copolymerization amount is preferably 0.1% to 10% by mass, and more preferably 0.5% to 5% by mass, based on the mass of the total polymerizable unsaturated monomer used in the production of the water-dispersible acrylic resin (a1).

[0039] The water-dispersible acrylic resin (a1) is preferably such that the solid content acid value is 5 mg KOH / g or less, and more preferably 4 mg KOH / g or less, from the viewpoint of water resistance and recoating compatibility of the water-based coating film.

[0040] Methods for dispersing resins in water include neutralizing some or all of the anionic groups, such as carboxyl groups, contained in the acrylic resin with a basic compound and dispersing them in water, or adding the acrylic resin to an aqueous medium containing a basic compound and dispersing it therein. The basic compound used as a neutralizing agent for hydroxyl group-containing acrylic resins is not particularly limited, but specifically, examples include inorganic bases such as sodium hydroxide and potassium hydroxide, and organic amines such as trimethylamine, dimethylaminoethanol, 2-methyl-2-amino-1-propanol, triethylamine, and ammonia. Among these, organic amine compounds are preferred, and it is particularly preferable to use tertiary amines such as triethylamine, tributylamine, dimethylethanolamine, and diethylenetriamine.

[0041] In this specification, the resin solids content is calculated by taking approximately 2.0 g of the sample in an aluminum foil cup with a diameter of approximately 5 cm, heating it at 110°C for 1 hour, and measuring the residue (g) as non-volatile content.

[0042] -Water-dispersible polyurethane resin (a2)- As the water-dispersible polyurethane resin (a2), any known in the art can be used without limitation. For example, a polyurethane resin emulsion or polyurethane resin dispersion obtained by dispersing a urethane prepolymer, which is formed by reacting a polyisocyanate, a polyol, and a carboxyl group-containing diol, in water can be used.

[0043] The average particle size of the water-dispersible polyurethane resin (a2) may be 0.01 μm or more and 1.0 μm or less, and in particular, it may be 0.1 μm or more and 0.5 μm or less.

[0044] The constituent polyisocyanate compounds include aliphatic diisocyanate compounds such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, and lysine diisocyanate; biuret-type adducts and isocyanurate ring adducts of these diisocyanate compounds; isophorone diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), methylcyclohexane-2,4-(or-2,6-) diisocyanate, and 1,3 Alicyclic diisocyanate compounds such as -(or 1,4-)di(isocyanatomethyl)cyclohexane, 1,4-cyclohexanediisocyanate, 1,3-cyclopentanediisocyanate, and 1,2-cyclohexanediisocyanate; biuret-type adducts and isocyanurate ring adducts of these diisocyanates; xylylene diisocyanate, metaxylylene diisocyanate, tetramethylxylylene diisocyanate, tolylenediisocyanate, and 4,4'-diphenylmethanediisocyanate. Aromatic diisocyanate compounds such as cyanates, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4'-toluidine diisocyanate, 4,4'-diphenyl ether isocyanate, (m- or p-)phenylene diisocyanate, 4,4'-biphenylene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, bis(4-isocyanatophenyl)sulfone, isopropylidene bis(4-phenylisocyanate); these Diisocyanate compounds such as biuret-type adducts and isocyanurate ring adducts; polyisocyanate compounds having three or more isocyanate groups in one molecule, such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene, and 4,4'-dimethyldiphenylmethane-2,2',5,5'-tetraisocyanate; biuret-type adducts and isocyanurate ring adducts of these polyisocyanate compounds;Urethane adducts are obtained by reacting polyisocyanate compounds with the hydroxyl groups of polyols such as ethylene glycol, propylene glycol, 1,4-butylene glycol, dimethylolpropionic acid, polyalkylene glycol, trimethylolpropane, and hexanetriol in a ratio that results in an excess of isocyanate groups; examples include biuret-type adducts and isocyanurate ring adducts of these urethane adducts.

[0045] Examples of the polyols mentioned above include polyether polyols such as polyethylene glycol, polypropylene glycol, polyethylene-propylene (block or random) glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol; and polyols obtained by condensation polymerization of dicarboxylic acids (adipic acid, succinic acid, sebacic acid, glutaric acid, maleic acid, fumaric acid, phthalic acid, etc.) and glycols (ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol, bishydroxymethylcyclohexane, etc.), such as polyethylene adipate, polybutylene adipate, and polyhexamethylene Examples include polyester polyols such as adipates, polyneopentyl adipates, poly-3-methylpentyl adipates, polyethylene / butylene adipates, and polyneopentyl / hexyl adipates; polycaprolactone polyols and poly-3-methylvalerolactone polyols; polycarbonate polyols; and low molecular weight glycols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,4-butanediol, tetramethylene glycol, hexamethylene glycol, decamethylene glycol, octanediol, tricyclodecanedimethylol, hydrogenated bisphenol A, cyclohexanedimethanol, and 1,6-hexanediol. These can be used individually or in combination of two or more.

[0046] Examples of the carboxyl group-containing diols mentioned above include dimethylolacetic acid, dimethylolpropionic acid, and dimethylolbutyric acid.

[0047] The above-mentioned urethane prepolymer can be manufactured based on conventionally known methods.

[0048] The water-dispersible polyurethane resin (a2) may be neutralized with a neutralizing agent. The neutralizing agent is not particularly limited as long as it can neutralize the carboxyl group; those listed in the section on basic compounds for water-dispersible acrylic resin (a1) can be used.

[0049] The water-dispersible polyurethane resin (a2) is preferable to have a cyclic structure in its molecule because it exhibits good sandability of the colored base coating film. More preferably, the polyisocyanate constituting the water-dispersible polyurethane resin (a2) contains a compound derived from an alicyclic diisocyanate compound as part of its components. Furthermore, the water-dispersible polyurethane resin (a2) is preferably such that the solid content acid value is 20 mg KOH / g or less, and more preferably 5 g KOH / g or more and 18 mg KOH / g or less, from the viewpoint of the water resistance and recoating compatibility of the colored base coating film.

[0050] -Water-soluble acrylic resin (a3)- Water-soluble acrylic resins are resins obtained by polymerizing polymerizable unsaturated monomers having hydrophilic functional groups with other polymerizable unsaturated monomers in the presence of a hydrophilic organic solvent using a polymerization initiator. Since they can exist in a dissolved state (transparent) in an aqueous medium, they are clearly distinguished from dispersion-type acrylic resins, which are dispersed in an aqueous medium, as described later.

[0051] A polymerizable unsaturated monomer containing a hydrophilic functional group is a compound having one or more hydrophilic functional groups and polymerizable unsaturated bonds in one molecule. Examples of hydrophilic functional groups include phosphate groups, sulfonic acid groups, carboxyl groups, amino groups, amide groups, and polyoxyalkylene chains.

[0052] Specifically, examples include N-substituted (meth)acrylamides, polyoxyalkylene chain-containing (meth)acrylates, dialkylaminoalkyl (meth)acrylates, and acid group-containing (meth)acrylates, which can be used individually or in combination of two or more.

[0053] In particular, from the viewpoint of drying properties, water resistance, and weather resistance, polymerizable unsaturated monomers containing acid groups are preferred, and (meth)acrylic acid, maleic acid, polymerizable unsaturated monomers containing phosphate groups, and polymerizable unsaturated monomers containing sulfonic acid are more preferred.

[0054] Examples of polymerizable unsaturated monomers containing phosphate groups include 2-acryloyloxyethyl acid phosphate, 2-methacryloyloxyethyl acid phosphate, 2-acryloyloxypropyl acid phosphate, and 2-methacryloyloxypropyl acid phosphate.

[0055] Examples of polymerizable unsaturated monomers containing sulfonic acid groups include 2-acrylamido-2-methylpropanesulfonic acid, allylsulfonic acid, sodium styrenesulfonate, sulfoethyl methacrylate, and their sodium and ammonium salts, among other sulfonic acid group-containing polymerizable unsaturated monomers.

[0056] In particular, a water-soluble acrylic resin using a combination of a carboxyl group-containing polymerizable unsaturated monomer and a phosphate group-containing polymerizable unsaturated monomer as the acid group-containing polymerizable unsaturated monomer is preferred because it has the effect of improving the recoating properties when the composition of the present invention is applied in multiple layers.

[0057] The above-mentioned hydroxyl group-containing polymerizable unsaturated monomers and other polymerizable unsaturated monomers that can copolymerize with the above-mentioned hydrophilic functional group-containing polymerizable unsaturated monomers can be suitably used.

[0058] Furthermore, it is desirable to neutralize the acidic groups in the above-mentioned phosphate group-containing water-soluble acrylic resin with a neutralizing agent. Such a neutralizing agent is not particularly limited as long as it can neutralize carboxyl groups, and examples include inorganic bases such as sodium hydroxide and potassium hydroxide, and organic amines such as trimethylamine, dimethylaminoethanol, 2-methyl-2-amino-1-propanol, triethylamine, and ammonia.

[0059] While not strictly distinguished, hydrophilic organic solvents used in polymerization can be those that dissolve in 100g of water at 20°C at a concentration of at least 20g. Examples include alcohol-based organic solvents, ether-based organic solvents, ethylene glycol ether-based organic solvents, diethylene glycol ether-based organic solvents, propylene glycol ether-based organic solvents, dipropylene glycol ether-based organic solvents, and ester-based organic solvents. These can be used individually or in combination of two or more.

[0060] The hydroxyl value of the water-soluble acrylic resin (a3) ​​is not particularly limited, but from the viewpoint of drying properties and finish, it is preferably 1 mg KOH / g or more and 200 mg KOH / g or less, and particularly preferably 5 mg KOH / g or more and 100 mg KOH / g or less.

[0061] The acid value of the water-soluble acrylic resin (a3) ​​is preferably 1 mg KOH / g or more and less than 100 mg KOH / g, and particularly preferably 5 mg KOH / g or more and 60 mg KOH / g or less, from the viewpoint of achieving both storage stability and drying properties of the paint and the water resistance of the coating film.

[0062] The weight-average molecular weight of the water-soluble acrylic resin (a3) ​​is preferably 1,000 to 500,000, more preferably 1,500 to 150,000, and particularly preferably 2,000 to 70,000.

[0063] The amount of water-soluble acrylic resin (a3) ​​included in the aqueous base coating composition is preferably 0.5% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 15% by mass or less, relative to the total resin solids content in the aqueous base coating composition, from the viewpoint of weather resistance and drying properties. <Resin components> In the aqueous base coating composition used in the present invention, it is preferable that at least one of the aforementioned water-dispersible acrylic resin (a1), water-dispersible polyurethane resin (a2), and water-soluble acrylic resin (a3) ​​is included as the main component, and from the viewpoint of sandability, it is more preferable to include water-dispersible polyurethane resin (a2). In particular, from the viewpoint of water resistance and hardness of the multi-layer coating film, it is suitable that the total solid content of resin (a1) and resin (a2), based on the total solid content of resin (a1), resin (a2), and resin (a3), be 80% by mass or more, preferably 90% by mass or more. When the proportion of water-dispersible resin is within the above range, it is particularly preferable that the recoating properties when applying water-based paint compositions are good, and that a multi-layer coating film with excellent coating hardness and a high-quality finish can be obtained.

[0064] -Other resins- Furthermore, other resins include acrylic resins other than resins (a1) and (a3), silicone resins, urethane resins other than resin (a2), fluororesins, epoxy resins, polyester resins, alkyd resins, and mixtures or modified resins thereof, such as acrylic-modified polyester resins, acrylic-silicone resins, acrylic-modified epoxy resins, and epoxy ester resins. These may be used individually or in combination of two or more.

[0065] Other resins may be used as long as they do not affect the paint's storability, film performance, or weather resistance. However, from the viewpoint of weather resistance and paint storability, their amount is preferably 20% by mass or less, and more preferably less than 5% by mass, relative to the total resin solids in the water-based paint composition.

[0066] Furthermore, from the viewpoint of substrate adhesion and water resistance of the colored base coating film, the ratio of water-dispersible acrylic resin (a1) to water-dispersible polyurethane resin (a2) by mass ratio of resin solids is preferably in the range of 99 / 1 to 1 / 99, and more preferably 80 / 20 to 20 / 80.

[0067] -Metal compounds- Water-based paint compositions contain a metal compound. The metal in the metal compound is preferably derived from a metal catalyst. Examples of metal compounds include those containing at least one metal selected from the group consisting of zinc, tin, zirconium, bismuth, lead, cobalt, manganese, titanium, aluminum, and molybdenum. The metal compound is preferably water-soluble or water-dispersible. Such metal compounds can be, for example, carbonates, phosphates, nitrates, sulfates, acetates, fluoroacids and their salts, organic acid salts such as carboxylic acids, oxides, alkali salts, etc. These metal compounds may be anhydrous or hydrated.

[0068] As metal compounds, carboxylate metal salts such as zinc octate, manganese octate, tin octate, cobalt octate, titanium octate, aluminum octate, zirconium octate, bismuth octate, or lead octate can be suitably used due to their excellent catalytic activity and ease of industrial availability; alkali salts of molybdate such as sodium molybdate, sodium phosphomolybdate, calcium molybdate, and ammonium molybdate; and molybdenum oxide such as molybdenum trioxide can be suitably used.

[0069] Particularly preferred are water-dispersible or water-soluble metal compounds, from the viewpoint of migration to the clear coating and improvement of the coating hardness of the multi-layer coating, and water-soluble compounds are especially preferred. Specific examples are shown below (chemical formula, water solubility: unit mg / 1000g [measurement temperature]). A specific example is molybdenum trioxide (MoO3, 1.34 × 10⁻⁶). 3 [20℃]), sodium molybdate (Na2MoO4, 3.94×10⁻⁶) 5 (at 25°C), calcium molybdate (CaMoO4, 50 [at 25°C]), molybdic acid (H2MoO4, 1.33 × 10⁻¹⁰ 3 The temperature is [18℃]. Diammonium molybdate and phosphomolybdic acid are soluble in water. Commercially available metal compounds can be used as metal catalysts. The above water solubility figures are based on Sidney L. Phillips (1997): Properties of Inorganic Compounds: Version 2.0, Boca Raton, CRC Press.

[0070] From the standpoint of water resistance, metal compounds either do not dissolve in water, or their water solubility is 1 × 10⁻⁶. -3 The concentration is preferably 0.05 mg / 1000g or more, more preferably 0.1 mg / 1000g or more. Furthermore, the water solubility of the metal compound is 5 × 10⁻¹⁰. 5 It is preferable that the amount is mg / 1000g or less.

[0071] Here, the metal compound of this application is preferably a colorless (white) or light-colored powder, and preferably does not significantly affect the coloring when applied as a coating film. Therefore, the metal compound of this application is different from the coloring pigments described later.

[0072] The metal compound is particularly preferred to have a small particle size from the viewpoint of water dispersibility and ease of migration from the base coating film. Specifically, the average primary particle size is preferably 1 nm to 1000 nm, more preferably 2 nm to 200 nm, and even more preferably 3 nm to 150 nm. It is convenient and desirable to mix the metal compound into the paint by stirring using a shear-type dispersion method, such as a disperser.

[0073] The concentration of the metal compound in the water-based paint composition is preferably equal to or greater than the concentration of the metal compound in the water-based clear paint composition described later. By having a concentration of the metal compound in the water-based paint composition that is equal to or greater than the concentration of the metal compound in the water-based clear paint composition, sufficient curability can be obtained even if the water-based clear paint composition does not contain a metal catalyst, and the hardness of the coating film can be improved as a multi-layer coating. This is because, when a water-based clear coat is applied to a water-based coating using a wet-on-wet method, both coatings are water-based, causing metal compounds in the base coating to migrate to the clear coating during the drying process. As a result, even when drying under low-temperature, short-duration conditions, a sufficient amount of metal catalyst can be obtained when drying both coatings simultaneously. Therefore, this method has the effect of improving the hardness, water resistance, and drying properties of the multi-layer coating film without compromising the finish quality. Furthermore, when an aqueous clear coating composition containing a metal catalyst is used in this method, the finish quality and coating hardness can also be improved. This is thought to be because, when applied wet-on-wet on an aqueous base coating, the metal catalyst is prevented to some extent from migrating from the clear coating to the base coating during the drying process.

[0074] Furthermore, in the method of the present invention, it is not necessary to add large amounts of metal catalysts or the like, which affect the pot life, to the aqueous clear coating composition as in the conventional method. If a metal catalyst is to be added, it is sufficient to adjust the concentration of the metal compound in the aqueous base coating film to be equal to or greater than the concentration of the metal compound in the clear coating film. The multi-layer coating film obtained by the method of the present invention exhibits excellent workability, such as drying properties, and excellent water resistance.

[0075] The content of metal compounds in the aqueous-based coating composition is preferably in the range of 0.01 parts by mass to 5 parts by mass, more preferably in the range of 0.03 parts by mass to 3.0 parts by mass, and even more preferably in the range of 0.05 parts by mass to 1.5 parts by mass, based on 100 parts by mass of total resin solids in the aqueous-based coating composition. By having a content within the above range, the hardness and workability of the multi-layer coating film after curing are particularly improved. In this context, 100 parts by mass of total resin solids used as a standard for the content of metal compounds in the aqueous-based paint composition refers to 100 parts by mass of the total resin solids of the water-dispersible acrylic resin (a1), the water-dispersible polyurethane resin (a2), and the water-soluble acrylic resin (a3) ​​blended in the aqueous-based paint composition. Furthermore, the solid content concentration of the metal compound in the aqueous-based paint composition is preferably 0.01% by mass or more and 4.0% by mass or less, more preferably 0.02% by mass or more and 2.0% by mass or less, and even more preferably 0.03% by mass or more and 1.0% by mass or less, based on the total solid content in the aqueous-based paint composition. In this case, total solids refer to the sum of the solids of the resins (a1), (a2), and (a3) ​​mentioned above, as well as any other resins, coloring pigments, and other additive components that may be added as needed.

[0076] -Coloring pigments- Water-based paint compositions may contain coloring pigments. Conventionally known coloring pigments can be used without particular limitations. Specific examples include metal oxide pigments such as titanium dioxide and iron oxide, composite metal oxide pigments such as titanium yellow, carbon black, azo pigments, quinacridone pigments, diketopyrrolopyrrole pigments, perylene pigments, perinone pigments, benzimidazolon pigments, isoindoline pigments, isoindolinone pigments, metal chelate azo pigments, phthalocyanine pigments, indanthrone pigments, dioxane pigments, indigo pigments, and other coloring pigments; and luminous pigments such as aluminum (including vapor-deposited aluminum), copper, zinc, brass, nickel, aluminum oxide, mica, aluminum oxide coated with titanium dioxide or iron oxide, mica coated with titanium dioxide or iron oxide, glass flakes, and holographic pigments. These can be used individually or in combination of two or more depending on the desired color and coating performance.

[0077] When an aluminum pigment is included as a coloring pigment, it is preferable to use a surface-treated aluminum pigment. This can be a pigment treated with inorganic phosphate, resin coating, further treatment with a phosphate compound on top of the resin coating, or treatment with a titanium-based treatment agent. In particular, from the viewpoint of improving the hardness and water resistance of the multi-layer coating, it is preferable to use at least one selected from silica-treated aluminum pigment, aluminum pigment treated with a titanium-based treatment agent, and molybdic acid-treated aluminum pigment, and it is especially preferable to use a molybdic acid-treated aluminum pigment.

[0078] The amount of coloring pigment varies depending on the type of coloring pigment, but it is preferably 0.1 parts by mass or more and 300 parts by mass or less, and more preferably 0.5 parts by mass or more and 200 parts by mass or less, per 100 parts by mass of resin solids contained in the aqueous-based paint composition.

[0079] When a surface-treated aluminum pigment is used as a coloring pigment in the aqueous-based paint composition used in this method, the blending ratio is preferably 1 to 100 parts by mass, and more preferably 5 to 50 parts by mass, per 100 parts by mass of resin solids contained in the aqueous-based paint composition, from the viewpoint of paint stability, film hardness, and weather resistance. Furthermore, when a water-based paint composition contains a coloring pigment, it may be referred to as a water-based colored paint composition.

[0080] -Other ingredients- The aqueous-based paint composition according to the present invention may optionally contain paint additives commonly used in the preparation of aqueous paints, such as resin emulsions or water-soluble resins other than water-dispersible acrylic resins (a1), water-dispersible urethane resins (a2), and water-soluble acrylic resins (a3), neutralizing agents, rheology control agents (viscosity modifiers), polymer fine particles, surface modifiers, defoaming agents, hard ultraviolet absorbers, light stabilizers, dispersion aids, preservatives, silane coupling agents, defoaming agents, curing catalysts other than the above-mentioned metal compounds and / or rust-preventive pigments, neutralizing agents, organic solvents, etc.

[0081] The aqueous-based paint composition in the present invention may contain a curing agent and / or a diluent. If a curing agent and / or a diluent is included, it is preferable to mix them immediately before painting. An aqueous-based paint composition that does not contain a curing agent is called a one-component type. An aqueous-based paint composition being a one-component type is particularly preferable from the viewpoint of shortening the process and from the viewpoint of hardness and drying properties of the multi-layer coating film. When a diluent is added to a water-based paint composition, the mass ratio of the main component and the diluent component can be adjusted as appropriate depending on the painting environment. Generally, from the viewpoint of the appearance and properties of the base coating film, it is preferable that the diluent component be 30 parts by mass or more and 300 parts by mass or less, based on 100 parts by mass of the main component, and more preferably 40 parts by mass or more and 250 parts by mass or less.

[0082] Furthermore, when a curing agent is included in the water-based paint composition, examples of curing agents include polyisocyanate curing agents, blocked isocyanate curing agents, melamine curing agents, oxazoline curing agents, and carbodiimide curing agents. The curing agent can be included in either the main component or the diluent component, or it can be used as a separate component and mixed immediately before painting.

[0083] Furthermore, while there are no particular limitations on the viscosity of the above-mentioned aqueous-based paint composition, from the viewpoint of storage stability, the viscosity when no curing agent or diluent is added is preferably 100 mPa·sec to 3000 mPa·sec, and can be 600 mPa·sec to 2000 mPa·sec. In this specification, viscosity is defined as the value measured within 10 minutes after preparing the sample at 25°C using a digital bismetron viscometer VDA type (manufactured by Shibaura Systems Co., Ltd.) at a rotation speed of 60 rpm.

[0084] Furthermore, while there are no particular limitations on the solid content of the above-mentioned aqueous-based paint composition, from the viewpoint of the finish of the formed coating film, it is preferable that the solid content concentration be 5% by mass or more and 60% by mass or less, and particularly preferable that it be 10% by mass or more and 50% by mass or less. In this specification, "solids" (or sometimes referred to as "solids concentration") refers to the non-volatile content, which is the residue remaining after removing volatile components such as water and organic solvents from the sample. It can be calculated by multiplying the mass of the sample by the solids concentration. Specifically, it can be measured by dividing the mass of the residue (non-volatile matter) obtained by drying approximately 3 grams of the sample at 105°C for 3 hours by the mass before drying. It may also be expressed as a percentage. The residue may be solid or liquid at room temperature.

[0085] (Painting with water-based paint compositions) Methods for applying a water-based paint composition include, for example, air spraying, airless spraying, rotary atomization, brushes, rollers, hand guns, multi-purpose guns, dipping, roll coaters, curtain flow coaters, roller curtain coaters, and die coaters, which can be appropriately selected depending on the application of the object to be coated. Furthermore, the water-based paint composition may be applied in multiple coats using these methods.

[0086] It is preferable to perform the second step while the base coating (I) is still wet after applying the water-based paint composition, but the process may also include a step to dry the base coating (I) before performing the second step. Examples of drying steps include room temperature drying or forced drying. In the case of room temperature drying, the coating can be left to stand at room temperature (e.g., 10 to less than 40°C) for 10 minutes or more (preferably 1 hour or more, more preferably 2 hours or more), or in the case of forced drying, it can be heated at 40 to 120°C for 1 to 120 minutes (preferably 5 to 60 minutes). In the case of forced drying, from the viewpoint of finish, the coating can be set (left to stand) at room temperature for 1 to 120 minutes (preferably 5 to 60 minutes) before heat curing. For example, a blower may be used for drying.

[0087] The thickness of the base coating (I) can be adjusted as appropriate depending on the condition of the surface to be coated, but generally, a dry thickness of 5 μm to 60 μm is preferred, and a dry thickness of 10 μm to 40 μm is particularly preferred.

[0088] <Second step> The second step is to apply a water-based clear coating composition onto the base coating (I) to form a clear coating (II).

[0089] (Water-based clear paint composition) The aqueous clear coating composition used in the present invention contains a second film-forming resin, and preferably the second film-forming resin contains a dispersion-type acrylic resin (a4). The water-based clear coating composition is preferably a two-component water-based coating composition comprising a first component containing a dispersion-type acrylic resin (a4) and water, and a second component containing a polyisocyanate compound (b), which is mixed immediately before application and diluted as appropriate. The first component functions as the main component, and the second component functions as a curing agent.

[0090] In an aqueous clear coating composition, the ratio of the first liquid to the second liquid is preferably such that the second liquid is 5 parts by mass or more and 100 parts by mass or less, based on 100 parts by mass of the first liquid, and more preferably 30 parts by mass or more and 70 parts by mass or less.

[0091] The water-based clear coating composition may optionally contain components other than the first and second liquids. Specific examples of components other than the first and second liquids include, for example, a rheology control agent and a diluent component containing water, a siloxane bond-forming component such as an organosilane compound, and its catalyst component. These components can be mixed together when mixing the first and second liquids immediately before application. The details of the first and second solutions are described below.

[0092] (1) 1st liquid The first liquid of the aqueous clear coating composition preferably contains a hydroxyl group-containing resin as the second film-forming resin, and the hydroxyl group-containing resin preferably contains a dispersion-type acrylic resin (a4) and water. Since hydroxyl group-containing resins are applied to water-based paints, they are either water-soluble resins or water-dispersible resins. Here, in this specification, a "water-soluble" resin is one in which the resin can be dissolved or partially dissolved in water when mixed with water, and is a different form from a "water-dispersible" resin. Water-dispersible resins are further divided into emulsion type and colloidal dispersion type. In this specification, the emulsion type refers to a form obtained by emulsion polymerization in the presence of an emulsifier using water as a solvent, or a form obtained by mechanically forcibly dispersing a resin that is completely insoluble in water in water. On the other hand, a method in which the polymerization reaction of the monomer mixture is carried out in the presence of no solvent or a suitable organic solvent, and then dispersed by dropping it into water, mixing, and removing excess organic solvent as necessary, or by removing excess organic solvent as necessary after the polymerization reaction and then adding water to disperse it, is called the dispersion type. The first liquid of the aqueous clear coating composition preferably has a solid content concentration of 20% to 60% by mass, and particularly preferably 30% to 50% by mass, from the viewpoint of pot life and the finish of the formed coating film. The dispersion-type acrylic resin (a4) is described below.

[0093] -Dispersion-type acrylic resin (a4)- The dispersion-type acrylic resin (a4) is preferably contained in an amount of 50% by mass or more relative to the total resin solids contained in the first liquid. From the viewpoint of the drying properties, gloss and other finish properties, weather resistance and water resistance of the resulting multilayer coating film, the content of the dispersion-type acrylic resin (a4) can be 60% by mass or more, exceeding 70% by mass, and 75% by mass or more relative to the total resin solids contained in the first liquid, with an upper limit of 99.5% by mass or less, and more preferably 98% by mass or less.

[0094] Here, the total resin solids contained in the first liquid refer to the total mass of the resin solids of the dispersion-type acrylic resin (a4), the polyether polyol (a5) which is added as needed, and, if any hydroxyl group-containing resins other than the dispersion-type acrylic resin (a4) are included, the total mass of those resin solids.

[0095] The dispersion-type acrylic resin (a4) can be, for example, a copolymer of a hydroxyl group-containing polymerizable unsaturated monomer (a4-1) and other polymerizable unsaturated monomers copolymerizable with the hydroxyl group-containing polymerizable unsaturated monomer.

[0096] Hydroxyl group-containing polymerizable unsaturated monomers (a4-1) are compounds having one or more hydroxyl groups and polymerizable unsaturated bonds in one molecule. Specifically, examples include monoesters of (meth)acrylic acid and dihydric alcohols having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; ε-caprolactone modified forms of the monoesters of (meth)acrylic acid and dihydric alcohols having 2 to 8 carbon atoms; N-hydroxymethyl (meth)acrylamide; allyl alcohol; and (meth)acrylates having polyoxyethylene chains with hydroxyl groups at the molecular ends. Among these, 2-hydroxyethyl methacrylate and / or 2-hydroxypropyl methacrylate are preferred in terms of drying properties and reactivity with curing agents described later.

[0097] Other monomers copolymerizable with hydroxyl group-containing polymerizable unsaturated monomers (a4-1) include, for example, carboxyl group-containing unsaturated monomers such as acrylic acid and methacrylic acid; methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and isooctyl (meth)acrylate. Alkyl ester compounds with acrylic acid or methacrylic acid such as lauryl (meth)acrylate, isomiristyl (meth)acrylate, stearyl (meth)acrylate, and isostearyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., trade name) (linear or branched alkyl (meth)acrylate); alicyclic alkyl (meth)acrylates such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate; polymerizable organic siloxanes such as polysiloxane mono(meth)acrylate; and aralkyl compounds such as benzyl (meth)acrylate. (Meth)acrylate; aromatic ring-containing polymerizable unsaturated monomers such as styrene and vinyltoluene; alkoxyalkyl (meth)acrylates such as 2-methoxyethyl (meth)acrylate; nitrogen-containing polymerizable unsaturated monomers such as (meth)acrylonitrile, (meth)acrylamide, and diacetone (meth)acrylamide; allyl (meth)acrylate, ethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, pentaerythritol di(meth)acrylate, 1,1,1-trishydroxymethylpropane Examples include polymerizable unsaturated monomers having at least two polymerizable unsaturated groups in one molecule, such as tri(meth)acrylate; vinyl monomers such as vinyl acetate; epoxy group-containing polymerizable unsaturated monomers such as glycidyl(meth)acrylate, allyl glycidyl ether, and 3,4-epoxycyclohexylmethyl(meth)acrylate; alkoxysilyl group-containing polymerizable unsaturated monomers such as 3-methacryloyloxypropyltrimethoxysilane; and oxidatively curable group-containing polymerizable unsaturated monomers such as dicyclopentenyloxyethyl(meth)acrylate. These can be used individually or in combination of two or more.

[0098] Methods for preparing dispersion-type acrylic resin (a4) include polymerizing monomers by conventionally known methods such as solution polymerization in the presence of a solvent or an organic solvent, and then dispersing them in water as needed. Conventional methods can be used for water dispersion. For example, methods such as neutralizing some or all of the anionic groups, such as carboxyl groups, contained in the resin with a basic compound such as an amine to ionize them and disperse them in water, or adding the resin to an aqueous medium containing a basic compound and dispersing it, can be used.

[0099] In the present invention, it is more preferable that the dispersion-type acrylic resin (a4) is copolymerized in multiple steps using polymerizable unsaturated monomer components of different compositions, as this provides excellent storage stability for the main component (I).

[0100] Dispersion-type acrylic resin (a4) has superior water resistance and weather resistance compared to emulsion-type acrylic resin because it does not use emulsifiers. Because dispersion-type acrylic resin (a4) is not manufactured from an aqueous medium, it is easy to obtain relatively low molecular weight and small, well-balanced fine particles. When applied as a water-based paint, it has excellent handling properties such as easy manual stirring, as well as excellent drying properties and gloss. Furthermore, dispersion-type acrylic resin (a4) can obtain an extremely excellent coating appearance despite being a water-based paint, and because it has excellent reactivity with polyisocyanate compounds (b), it is possible to obtain a coating with extremely excellent weather resistance.

[0101] The hydroxyl value of the dispersion-type acrylic resin (a4) is not particularly limited, but from the viewpoint of balancing drying properties, finish properties, weather resistance, and handling properties of the paint such as pot life and ease of manual stirring, it is preferably 30 mg KOH / g or more and 200 mg KOH / g or less, and particularly preferably 50 mg KOH / g or more and 170 mg KOH / g or less.

[0102] The acid value of the dispersion-type acrylic resin (a4) is preferably less than 40 mgKOH / g, and more preferably between 5 mgKOH / g and 35 mgKOH / g, from the viewpoint of achieving both storage stability and drying properties of the paint.

[0103] In this specification, both the acid value and hydroxyl value are expressed on a solid content basis and are measured according to the method compliant with JIS K 0070.

[0104] The weight-average molecular weight of the dispersion-type acrylic resin (a4) is preferably 1,000 or more and 100,000 or less, and particularly preferably 2,000 or more and 70,000 or less. In this specification, the weight-average molecular weight is the value on a polystyrene basis, measured by gel permeation chromatography (GPC).

[0105] The glass transition temperature (hereinafter sometimes abbreviated as Tg) of the dispersion-type acrylic resin (a4) is preferably 40°C or higher, more preferably 42°C to 70°C, and particularly preferably 45°C to 65°C, from the viewpoint of improving drying speed in a short time, and especially from the viewpoint of enabling polishing repair in a shorter time. To obtain such a dispersion-type acrylic resin (a4) with a relatively high glass transition temperature, it is preferable that the component includes a monomer whose homopolymer glass transition temperature is 30°C or higher, and more preferably that the copolymer component includes a monomer whose glass transition temperature is 40°C or higher and 130°C or lower.

[0106] Examples of monomers with a glass transition temperature of 30°C or higher include styrene, methyl methacrylate, ethyl methacrylate, methacrylates having a branched alkyl group with 3 to 4 carbon atoms (e.g., i-propyl methacrylate, i-butyl methacrylate, tert-butyl methacrylate), cyclohexyl methacrylate, isobornyl acrylate, isobornyl methacrylate, cetyl acrylate (also known as hexadecyl acrylate), benzyl methacrylate, and other (meth)acrylates having a cyclic alkyl structure, as well as isostearyl acrylate, stearyl methacrylate, etc. From the viewpoint of adhesion to the substrate and coating film hardness, it is particularly preferable to use at least one monomer selected from styrene, methyl methacrylate, and methacrylates having a branched alkyl group with 3 to 4 carbon atoms. These monomers can also be used individually or in combination.

[0107] When using monomers with a glass transition temperature of 30°C or higher, the amount used is preferably 50% to 100% by mass, and more preferably 70% to 95% by mass, in addition to other polymerizable unsaturated monomers, from the viewpoint of coating hardness.

[0108] In this specification, the static glass transition temperature of a resin can be measured, for example, by taking a sample in a measuring cup, completely removing the solvent by vacuum suction, and then measuring the change in heat quantity in the range of -100°C to 150°C at a heating rate of 3°C / min using a differential scanning calorimeter "DSC-50Q" (manufactured by Shimadzu Corporation, trade name), and defining the point of change at the first baseline on the low-temperature side as the static glass transition temperature.

[0109] The above-mentioned dispersion-type acrylic resin (a4) preferably has an average particle diameter of 50 nm to 500 nm, and more preferably 80 nm to 300 nm, from the viewpoint of water resistance, weather resistance, and finish properties such as gloss (smoothness). In this specification, the average particle diameter is the value measured at room temperature (approximately 20°C) using a Coulter Counter N4 (product name, manufactured by Beckman Coulter, Inc., particle size distribution analyzer) after diluting the sample with deionized water to a concentration suitable for measurement.

[0110] The solid content concentration of the dispersion-type acrylic resin (a4) after manufacturing is preferably 35% by mass or more and 65% by mass or less, from the viewpoint of dispersion stability of the dispersion-type acrylic resin (a4) in water.

[0111] Commercially available dispersion-type acrylic resins (a4) can also be used. Commercially available products are not particularly limited and include, for example, the MACRYNAL series (manufactured by allnex), such as MACRYNAL SM 6825w / 37WA, MACRYNAL SM 6810w / 42WA, MACRYNAL VSM6299 / 42WA, etc.; the Bihydrol series (manufactured by Sumika Covestro Urethane), such as Bihydrol A145, Bihydrol A2470, Bihydrol A2542, Bihydrol A2469, Bihydrol A2646, etc.; the Barnock series (manufactured by DIC), such as Barnock WD-551, etc.; and the NeoCryl series (manufactured by DSM), such as NeoCryl XK-555, etc.

[0112] The solid content concentration of the dispersion-type acrylic resin (a4) in the aqueous clear coating composition is preferably 20% by mass or more and 70% by mass or less, and more preferably 35% by mass or more and 65% by mass or less, relative to the total amount of solid content in the aqueous clear coating composition.

[0113] The first component of the water-based clear coating composition may optionally contain other components such as a water-dispersible resin or water-soluble resin other than the dispersion-type acrylic resin (a4), pigments, neutralizing agents, rheology control agents, surface modifiers, defoamers, curing catalysts, ultraviolet absorbers, light stabilizers, and organic solvents. When other components are included, the solid content concentration is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, based on the total amount of solid content of the aqueous clear coating composition.

[0114] Examples of rheology control agents include polyamide-based rheology control agents such as fatty acid amides, polyamides, acrylamides, long-chain polyaminoamides, aminoamides and their salts (e.g., phosphates); urethane-based rheology control agents such as polyether polyol urethane prepolymers and urethane-modified polyether-type viscosity modifiers; polycarboxylic acid-based rheology control agents such as high molecular weight polycarboxylic acids, high molecular weight unsaturated acid polycarboxylic acids and their partial amidates; cellulose-based rheology control agents such as hydroxyethylcellulose and hydroxypropylcellulose; inorganic layered compound-based rheology control agents such as montmorillonite, bentonite, and clay; aminoplast-based rheology control agents such as hydrophobic modified ethoxylate aminoplasts; and polyolefin waxes such as polyethylene, polypropylene, oxidized polyethylene, oxidized polypropylene, ethylene vinyl acetate, chloride polyethylene, and chlorinated polypropylene. Only one type may be used, or a mixture of two or more types may be used.

[0115] Commercially available rheology control agents include polyamide-based rheology control agents such as "Disparon AQ-600" and "Disparon AQH-800" (product names, manufactured by Kusumoto Chemical Co., Ltd.); aminoplast-based rheology control agents such as "REOBYK H370," "REOBYK H400," "REOBYK H600," and "RHEOBYK H600VF" (all manufactured by BYK Chemie); and "ACRYSOL Polycarboxylic acid-based rheology control agents such as "ASE60" (Dow Chemical), "Viscarex HV-30" (BASF), "SN Thickener 613", "SN Thickener 617", "SN Thickener 618", "SN Thickener 630", "SN Thickener 634", "SN Thickener 636" (all product names, manufactured by Sunnopco); urethane-based rheology control agents such as "Adekanol UH-814N", "UH-752", "UH-750", "UH-462" (all product names, manufactured by ADEKA), "SN Thickener 621N", "SN Thickener 623N" (all product names, manufactured by Sunnopco), "Leolate 244", "Leolate 278" (both product names, manufactured by Elementis Japan); cellulose-based rheology control agents such as "HEC Daicel SP600N" (product name, manufactured by Daicel Chemical Industries); "BENTONE Inorganic layered compound rheology control agents such as "HD" (product name, manufactured by Elementis Japan); polyolefins from BYK Chemie's AQUACER series, Nippon Paper Group's Superclon series, Unitika's Arrowbase series, Mitsui Chemicals' Chemipearl series, etc., can be used alone or in combination of two or more.

[0116] In this invention, from the viewpoint of the sagging resistance of the formed coating film, the use of a polycarboxylic acid-based rheology control agent and / or a nonionic rheology control agent is suitable as the rheology control agent.

[0117] Examples of nonionic rheology control agents include urethane-based rheology control agents, cellulose-based rheology control agents, layered compound-based rheology control agents, and aminoplast-based rheology control agents from the above examples.

[0118] The above-mentioned rheology control agent preferably contains the active ingredient of the rheology control agent in an amount of 0.01 parts by mass or more and 1.0 parts by mass or less, based on 100 parts by mass of the resin solids content of the dispersion-type acrylic resin (a4), and is particularly preferably contained in an amount of 0.1 parts by mass or more and 0.5 parts by mass or less.

[0119] —Polyether polyol (a5)— In the present invention, from the viewpoint of obtaining a coating film with excellent finished appearance, polyether polyol (a5) may be included as part of the second film-forming resin. It is preferable to include polyether polyol (a5) in the first liquid.

[0120] Examples of polyether polyols (a5) include polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and polyoxyalkylene glyceryl ether, which may be used individually or in combination of two or more. Among these, polyoxyalkylene glyceryl ether is preferred from the viewpoint of improving the finish of the coating film. The polyoxyalkylene structure in polyoxyalkylene glyceryl ether can be selected from polyoxyethylene, polyoxypropylene, and polyoxybutylene, with polyoxypropylene being preferred.

[0121] It is preferable to use a polyether polyol (a5) with a number average molecular weight of 400 to 5000, particularly within the range of 500 to 1500, and a hydroxyl value of 30 mg KOH / g to 400 mg KOH / g, particularly within the range of 100 mg KOH / g to 350 mg KOH / g. Examples of commercially available polyether polyols (a5) include Sannix PP-1000, PP-2000, PP-3000, GP-600, GP-1000, GP-3000, GL-300, FA-103, FA-703 (all manufactured by Sanyo Chemical Industries, Ltd.), Exenol EL-1020, EL-2020, EL-3020, EL-510, EL-540, EL-3030, EL-5030, EL-823, EL-828, EL-830, EL-837, EL-840, EL-850, EL-851B (all manufactured by Asahi Glass Urethane Co., Ltd.), and Preminol PML-3005, PML-3012, PML-4002, PML-5001, PML-7001 (all manufactured by Asahi Glass Urethane Co., Ltd.).

[0122] When the first liquid contains polyether polyol (a5), the content is preferably 0.05% by mass or more, more preferably 1% by mass or more, even more preferably 2% by mass or more, preferably 40% by mass or less, more preferably 25% by mass or less, and even more preferably 15% by mass or less, relative to the total resin solids contained in the first liquid, from the viewpoint of balancing finish, curability and the hardness of the resulting coating film.

[0123] The first liquid component may optionally contain hydroxyl group-containing resins other than resin (a4) and component (a5), other resin emulsions or water-soluble resins, pigments, neutralizing agents, rheology control agents, surface modifiers, defoaming agents, curing catalysts, ultraviolet absorbers, light stabilizers, organic solvents, etc. When a curing catalyst is included in the first liquid component, from the viewpoint of water resistance and maintaining catalytic activity, the amount of the active ingredient of the curing catalyst is preferably 0.001 parts by mass or more, more preferably 0.005 parts by mass or more, preferably 1.0 part by mass or less, more preferably 0.5 parts by mass or less, and even more preferably 0.1 parts by mass or less, based on 100 parts by mass of solid content of the hydroxyl group-containing resin.

[0124] (2)Second liquid The second liquid of the aqueous clear coating composition according to the present invention preferably contains a polyisocyanate compound (b) as a curing agent component.

[0125] -Polyisocyanate compound (b)- The polyisocyanate compound (b) is a compound having two or more free isocyanate groups in one molecule, and those conventionally used in the production of polyurethanes can be used. In particular, it is preferable that the isocyanate group content is within a specific range. Specifically, from the viewpoint of water resistance and adhesion, it is preferable that the isocyanate group content is 10% by mass or more, more preferably 12% by mass or more, particularly 18% by mass or more, and 60% by mass or less, and more preferably 55% by mass or less.

[0126] In this specification, the isocyanate group content is expressed as the mass fraction of isocyanate groups contained in polyisocyanate compound (b). The amount of isocyanate groups can be measured in accordance with JIS K 1603-1 (2007).

[0127] As the polyisocyanate compound (b), it is preferable to use a polyisocyanate compound for water-based coatings, such as a hydrophilic polyisocyanate compound in which a hydrophilic group has been introduced into the polyisocyanate compound, or a water-dispersible polyisocyanate compound that can be dispersed in water using a surfactant. Examples of hydrophilic groups include anionic groups such as acid groups and nonionic groups containing polyoxyalkylene (polyether chain) units. Examples of acid groups include carboxyl groups, phosphate groups, and sulfonic acid groups.

[0128] Furthermore, the curing agent may also contain a hydrophobic polyisocyanate compound in addition to the polyisocyanate compound for water-based paints. Such hydrophobic polyisocyanate compounds can be those commonly used in solvent-based paint compositions.

[0129] The polyisocyanate compounds used in water-based paints or hydrophobic polyisocyanate compounds can be those conventionally used in the production of polyurethanes. Examples include aliphatic diisocyanates such as tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, and lysine diisocyanate; alicyclic diisocyanates such as 4,4'-methylenebis(cyclohexyl isocyanate) and isophorone diisocyanate; aromatic diisocyanates such as xylylene diisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate, and polyphenylmethane diisocyanate (hereinafter referred to as polymeric MDI); and similar compounds such as their isocyanurates and biuret compounds, which can be used individually or in combination of two or more. Of these, it is particularly preferable from the viewpoint of faster drying time to include a polyisocyanate compound (b1) having three or more isocyanate groups and a molecular weight of 350 or less.

[0130] -Polyisocyanate compounds with a molecular weight of 350 or less (b1)- Examples of polyisocyanate compounds (b1) having three or more isocyanate groups and a molecular weight of 350 or less include aliphatic triisocyanate compounds such as 1,8-diisocyanato-4-isocyanatomethyloctane, (2S)-2,6-diisocyanatohexanoate 2-isocyanatoethyl (common name: lysine triisocyanate), 2,6-diisocyanatohexanoate 2-isocyanatoethyl, and 1,6,11-triisocyanatoundecane; alicyclic triisocyanate compounds such as 1,3,5-triisocyanatocyclohexane and 1,3,5-trimethylisocyanatocyclohexane; and aromatic triisocyanates such as 1,3,5-triisocyanatobenzene and 2,4,6-triisocyanatotoluene. These can be used individually or in combination of two or more.

[0131] As the polyisocyanate compound (b1) having three or more isocyanate groups and a molecular weight of 350 or less, it is preferably an aliphatic triisocyanate compound from the viewpoint of pot life and the finish of the formed coating film, and among these, 1,8-diisocyanato-4-isocyanatomethyloctane and (2S)-2,6-diisocyanatohexanoate 2-isocyanatoethyl (common name: lysine triisocyanate) are more preferred, and 1,8-diisocyanato-4-isocyanatomethyloctane is particularly preferred.

[0132] In the present invention, the polyisocyanate compound (b1) having three or more isocyanate groups and a molecular weight of 350 or less is preferably in the range of 200 to 300, and more preferably in the range of 230 to 280, from the viewpoint of paintability and the finish of the formed coating film.

[0133] In the present invention, the polyisocyanate compound (b1) having three or more isocyanate groups and a molecular weight of 350 or less is preferably such that its viscosity at 23°C is 1 mPa·s or more and 50 mPa·s or less, and more preferably 1 mPa·s or more and 30 mPa·s or less, from the viewpoint of paintability and the finish of the formed coating film.

[0134] The amount of polyisocyanate compound (b) in the second liquid is preferably 20 parts by mass or more and 100 parts by mass or less, and more preferably 30 parts by mass or more and 80 parts by mass or less, per 100 parts by mass of the total mass of the second liquid. In the case where the second liquid contains a polyisocyanate compound (b1), from the viewpoint of paintability, finish of the formed coating film, quick drying, and coating film hardness, it is preferable that the total solid content of the polyisocyanate compound (b) be 5% by mass or more and 100% by mass or less, more preferably 10% by mass or more and 90% by mass or less, and even more preferably 10% by mass or more and 60% by mass or less.

[0135] These polyisocyanate compounds (b) can be used individually or in combination of two or more. When two or more polyisocyanate compounds (b) are used in the second liquid, it is more preferable to adjust the blending amount so that the isocyanate group content in the second liquid is, on average, 5% by mass or more, more preferably 8% by mass or more, 55% by mass or less, and more preferably 50% by mass or less, from the viewpoint of storage and weather resistance.

[0136] The amount of polyisocyanate compound (b) in the above aqueous clear coating composition can be appropriately adjusted so that the equivalent ratio (NCO / OH) of isocyanate groups in the polyisocyanate compound to hydroxyl groups in the dispersion-type acrylic resin (a4) is generally between 0.5 and 5.0. However, from the viewpoint of curability and weather resistance, an amount between 1.1 and 3.0 is preferred, and an amount between 1.2 and 2.0 is more preferred. Having an equivalent ratio (NCO / OH) within the above preferred range has the advantage of ensuring good curing reactivity of the aqueous clear coating composition. Note that all of the above equivalent ratios are calculated on a solid content basis.

[0137] -Other ingredients- The second liquid of the aqueous clear coating composition according to the present invention may optionally contain a resin that does not contain crosslinking reactive groups such as hydroxyl groups, a film-forming aid, a pigment, a neutralizing agent, a rheology control agent, a surface modifier, an antifoaming agent, a curing catalyst, an ultraviolet absorber, a light stabilizer, a dehydrating agent, and the like.

[0138] The second component of the aqueous clear coating composition preferably contains an organic solvent, from the viewpoint of ease of application and the finish of the resulting coating film.

[0139] The organic solvent is preferably a compound that does not have a hydroxyl group, and specifically, for example, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol divinyl ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, triethylene glycol dimethyl ether, triethylene glycol divinyl ether, tetraethylene glycol diethyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, propylene glycol di-n-propyl ether, propylene glycol diisopropyl ether, propylene glycol di-n-butyl ether, propylene glycol diisobutyl ether, propylene glycol diallyl ether, propylene glycol diphenyl ether, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, dipropylene glycol di- Glycol ether-based organic solvents such as n-butyl ether, dipropylene glycol diisobutyl ether, dipropylene glycol allyl ether, tripropylene glycol dimethyl ether, tripropylene glycol diethyl ether, tripropylene glycol di-n-butyl ether, tripropylene glycol diisobutyl ether, tripropylene glycol diallyl ether, butylene glycol dimethyl ether, butylene glycol diethyl ether, butylene glycol di-n-butyl ether, 2-butoxyethyl diethoxyethyl ether, 2-butoxyethyl triethoxy ether, 2-butoxyethyl tetraethoxyethyl ether, etc.; acetate-based organic solvents such as ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol mono-n-butyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol mono-n-butyl ether acetate, 3-methoxybutyl acetate, propylene glycol monomethyl ether acetate, etc.;Examples include ketone-based organic solvents such as acetone, methyl ethyl ketone, methyl amyl ketone, and methyl isobutyl ketone; and ester-based organic solvents such as ethyl acetate, butyl acetate, isobutyl acetate, methyl benzoate, ethyl ethoxypropionate, ethyl propionate, and methyl propionate. These can be used individually or in combination of two or more. In particular, from the viewpoint of the finish of the resulting coating film, it is preferable that the organic solvent contains an organic solvent with a boiling point higher than water, especially a compound with a boiling point of 150-250°C, and that it is also hydrophilic.

[0140] Regarding the amount of organic solvent in the second liquid, from the viewpoint of miscibility with polyisocyanate compound (b), i.e., finish quality, it is preferable that the mass of the organic solvent be 10 parts by mass or more and 300 parts by mass or less, based on 100 parts by mass of polyisocyanate compound (b), and particularly preferable that it be 30 parts by mass or more and 250 parts by mass or less.

[0141] The aqueous clear coating composition used in this method is obtained by mixing a first component containing the hydroxyl group-containing resin and a second component containing the polyisocyanate component and an organic solvent, etc., immediately before use, and then diluting the resulting mixture with an appropriate diluent before application. The appropriate ratio of the first and second components is such that the second component is 20 to 100 parts by mass, particularly 30 to 70 parts by mass, based on 100 parts by mass of the first component. Furthermore, paint additives such as film-forming aids, pigments, neutralizing agents, rheology control agents, surface modifiers, defoamers, curing catalysts, UV absorbers, light stabilizers, and dehydrating agents can be added later as a third component as needed.

[0142] In this method, sufficient curability can be obtained even without containing a curing catalyst in the aqueous clear coating composition. However, if a curing catalyst is included, the amount of the active ingredient of the curing catalyst is preferably 0.001 parts by mass or more, more preferably 0.005 parts by mass or more, more preferably 1.0 part by mass or less, more preferably 0.5 parts by mass or less, and even more preferably 0.1 parts by mass or less, based on 100 parts by mass of the solid content of the hydroxyl group-containing resin contained in the first component. As a curing catalyst, for example, conventionally known urethane curing catalysts can be used. Examples of urethane curing catalysts include amine compounds such as triethylamine, organotin compounds such as dibutyltin dilaurate, dibutyltin diacetate, dioctyltin diacetate, and tin dioctoate, and organometallic compounds such as zinc octoate (zinc 2-ethylhexanoate). From the viewpoint of water resistance, a catalyst different from those listed in the section on metal compounds in the base coating composition above can be used.

[0143] (Painting with a water-based clear coating composition) Methods for applying the aqueous clear coating composition applicable to the present invention include, for example, air spraying, airless spraying, rotary atomization, brushes, rollers, hand guns, multi-purpose guns, dipping, roll coaters, curtain flow coaters, roller curtain coaters, die coaters, etc., which can be appropriately selected depending on the application of the object to be coated, and multiple coats may be applied.

[0144] The drying conditions for the base coating and the clear coating in the multilayer coating formation method of the present invention are not particularly limited, and room temperature drying, forced drying, bake drying, etc., can be selected depending on the workpiece and working environment. Forced drying or bake drying is preferred from the viewpoint of abrasiveness, finish and water resistance, and forced drying is particularly preferred from the viewpoint of selectivity of the workpiece and energy reduction. For forced drying, heating can be performed at a mild temperature of 40-120°C, preferably 40-70°C, for 10-120 minutes. A setting time may be provided as needed, during which the solvent is allowed to evaporate by leaving the material at room temperature (5-45°C). Air drying (ventilation) may also be used in combination as needed. Setting can usually be done by leaving the painted object in a dust-free environment at room temperature for 30 seconds to 60 minutes. The relative humidity (hereinafter sometimes abbreviated as RH) during setting is preferably 80% or less, and particularly preferably 70% or less. Drying can be performed using an IR furnace, an electric hot air dryer, or the like.

[0145] As for the dry film thickness of the clear coating, from the standpoint of ensuring a film thickness that can be polished and repaired, and from the standpoint of finish, a thickness of 5 μm to 500 μm is preferable, 10 μm to 100 μm is more preferable, and 15 μm to 60 μm is particularly preferable.

[0146] The method for forming a multi-layer coating of the present invention is particularly suitable for repair painting of automobiles and the like. Since a coating with excellent hardness can be obtained with a short drying time, surface polishing can be performed at an early stage after the coating is formed.

[0147] One polishing method involves using waterproof sandpaper to polish the clear coating, then rinsing it with the waterproof sandpaper, and finally polishing the polished surface sequentially with a coarse polishing compound and a finishing polishing compound. This method yields a clear coating with excellent gloss and shine, and a finish that is virtually indistinguishable from the unrepaired area.

[0148] In addition, since the coating composition used in this method is wet-on-wet coated on an aqueous base coating film, there is no step of baking the aqueous base paint, so energy can be saved and the process can be shortened. Both the base composition and the clear composition are aqueous, so it is an environmentally friendly coating film forming method. Further, in this method, it is not necessary to add a large amount of a metal catalyst to the aqueous clear coating composition, so the pot life of the aqueous clear coating composition is improved, the finishability and water resistance performance are maintained, and an excellent effect is exhibited in that sufficient coating film hardness can be obtained with a multilayer coating film. Further, this method enables wet-on-wet coating and exhibits an effect of excellent workability in terms of improving the pot life.

[0149] According to the method for forming a multilayer coating film of the present invention, by incorporating a metal compound into the aqueous base coating composition, the metal compound derived from the catalyst in the uncured base coating film migrates into the uncured clear coating film, so the reactivity in the clear coating film is improved and low-temperature curing can be completed in a short time.

[0150] [Multilayer coating film] The multilayer coating film of the present invention has a base coating film (I) formed by coating an aqueous base coating composition on an object to be coated, and a clear coating film (II) layer formed by coating an aqueous clear coating composition on the base coating film (I), and the Martens hardness value of the multilayer coating film is 7 N / mm 2 or more. From the viewpoint of abrasion resistance, the Martens hardness value of the multilayer coating film is preferably 7 N / mm 2 or more, and more preferably 10 N / mm 2 or more. The upper limit can be said to be a cured and dried state as long as the Martens hardness reaches a certain value. As a guideline, it is preferably 120 N / mm 2 or less, and more preferably 80 N / mm​​​​​Specifically, although it depends on the film thickness and drying time of the multilayer coating, for example, a multilayer coating obtained with an uncured base coating layer (I) of 5 μm to 60 μm and an uncured clear coating layer (II) of 5 μm to 500 μm (evaluation test coated board: drying time 60°C for 20 minutes) is dried at 60°C for 20 minutes after coating, and after being removed from the drying oven and left for 1 hour in an environment of 50% relative humidity and 20°C, the Martens hardness value of the coated board is within this range. The state of the multilayer coating at this drying time and temperature is semi-cured, but even in a semi-cured state, the Martens hardness value of the multilayer coating of the present invention is above the lower limit within the above range, making polishing repair possible and thus improving workability. Further hardening occurs from this state, and once it reaches a hardened and dry state, the Martens hardness value becomes 120 N / mm². 2 A value exceeding a certain threshold becomes a constant value.

[0152] The present invention provides a multilayer coating comprising a cured base coating obtained by applying and curing an aqueous base coating composition onto a substrate, and a cured clear coating obtained by applying and curing an aqueous clear coating composition, wherein the cured clear coating contains at least one metal compound selected from the group consisting of zinc, tin, zirconium, bismuth, lead, cobalt, manganese, titanium, aluminum, and molybdenum, and the concentration of the metal element in the metal compound is 20 ppm or more. The concentration of metal elements in metal compounds within a coating can be measured by scraping off a predetermined amount of the transparent clear coating layer of a hardened and dried multi-layer coating and performing elemental analysis.

[0153] The multilayer coating of the present invention, by including a metal compound in the aqueous base coating composition, allows the catalyst-derived metal compound in the uncured base coating to migrate into the uncured clear coating. This improves the reactivity in the clear coating, allowing for faster low-temperature curing. Furthermore, compared to cases where the aqueous base coating composition does not contain a metal compound, the hardness and water resistance of the multilayer coating are also improved. [Examples]

[0154] The present invention will be described in more detail below with reference to manufacturing examples, examples, and comparative examples. However, the present invention is not limited thereto. In each example, "parts" and "%" are based on mass unless otherwise specified. The film thickness of the coating is based on the cured film.

[0155] [Example 1] (Water-based paint composition) First, the acrylic resin emulsion and pigment to be contained in the water-based paint composition were synthesized using the following procedure.

[0156] -Acrylic resin emulsion (a1)- In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropper, 100 parts of deionized water, 2.5 parts of "Newcol 707SF" (trade name, manufactured by Nippon Emulsifier Co., Ltd., polyoxyethylene polycyclic phenyl ether sulfate), and 1 part of the monomer mixture (9 parts styrene, 40 parts n-butyl acrylate, 40 parts 2-ethylhexyl acrylate, 10 parts 2-hydroxyethyl acrylate, 1 part allyl methacrylate) were added and stirred under a nitrogen stream. At 60°C, 3 parts of 3% ammonium persulfate aqueous solution were added. The temperature was then raised to 80°C, and the remaining 99 parts of the monomer mixture, 2.5 parts of "Newcol 707SF", 4 parts of 3% ammonium persulfate, and 100 parts of deionized water were added to the reaction vessel over 4 hours using a metering pump. After the addition was complete, the mixture was allowed to mature for 1 hour. Subsequently, 33 parts of deionized water were added, and the pH was adjusted to 7.5 with dimethylethanolamine to obtain an aqueous dispersion solution of acrylic resin (a1) with an average particle size of 100 nm and a solid content of 30%.

[0157] -Urethane resin emulsion (a2)- In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropper, 115.5 parts of polybutylene adipate with a number average molecular weight of 2000, 115.5 parts of polycaprolactone diol with a number average molecular weight of 2000, 15.2 parts of dimethylolpropionic acid, 14.5 parts of 1,4-butanediol, and 120.1 parts of isophorone diisocyanate were charged, and the mixture was reacted under a nitrogen atmosphere with stirring at 85°C for 7 hours to obtain a prepolymer with an NCO content of 4.0%. Next, the prepolymer was cooled to 50°C, 165 parts of acetone were added and dissolved uniformly, then 15.7 parts of triethylamine were added under stirring, and 600 parts of deionized water were added while maintaining the temperature below 50°C. The resulting aqueous dispersion was held at 50°C for 2 hours to complete the water extension reaction, and then the acetone was removed by distillation under reduced pressure at below 70°C. The pH was adjusted to 8.0 with triethylamine and deionized water to obtain an aqueous dispersion solution of urethane resin (a2) with a solid content of 30% and an average particle size of 150 nm.

[0158] -Water-soluble acrylic resin (a3)- A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device contained a mixed solvent of 27.5 parts methoxypropanol and 27.5 parts isobutanol, and was heated to 110°C. 121.5 parts of a mixture consisting of 25 parts styrene, 27.5 parts n-butyl methacrylate, 20 parts isostearyl acrylate (manufactured by Osaka Organic Chemical Co., Ltd.), 7.5 parts hydroxybutyl acrylate, 15 parts (solids) of a phosphate-containing polymerizable monomer solution (Note 1), 12.5 parts 2-methacryloyloxyethyl acid phosphate, 10 parts isobutanol, and 4 parts t-butyl peroxyoctanoate were added to the mixed solvent over 4 hours. A mixture consisting of 0.5 parts t-butyl peroxyoctanoate and 20 parts isopropanol was then added dropwise over 1 hour. The mixture was then stirred and aged for 1 hour to obtain a water-soluble acrylic resin (a3) ​​solution with a solids content of 50%. (Note 1) Phosphate group-containing polymerizable monomer solution: 57.5 parts monobutyl phosphate and 41.1 parts isobutanol were placed in a flask, and 42.5 parts glycidyl methacrylate were added dropwise over 2 hours under aeration, followed by stirring and aging for another hour. Then, 5.9 parts isopropanol was added to obtain a phosphate group-containing polymerizable monomer solution with a solid content of 50%.

[0159] -Synthesis of Pigments- Preparation of pigment paste P1 In a stirring and mixing container, the aluminum pigment paste "Hydrolan 2156" (manufactured by Ekart, silica-coated aluminum flakes, pigment content 60%), the above-mentioned water-soluble acrylic resin (a3) ​​solution with a solid content of 50%, and methoxypropanol were stirred and mixed in a mass ratio of 50 / 20 (of which 10 parts by mass of resin solids) / 30 to obtain the aluminum pigment paste (P1).

[0160] Preparation of pigment paste P2 Aluminum pigment paste (P2) was obtained by stirring and mixing aluminum pigment paste "Alpaste WL-7640" (manufactured by Toyo Aluminum Co., Ltd., molybdic acid coated aluminum flakes, pigment content 59%), the above water-soluble acrylic resin (a3), and 35 parts of methoxypropanol in a mass ratio of 50 / 20 (of which resin solids are 10 parts by mass) / 30 in a stirring and mixing container.

[0161] -Preparation of water-based paint composition (X-1)- 150 parts of acrylic resin emulsion (a1) (45 parts resin solids) and 150 parts of urethane resin emulsion (a2) (45 parts resin solids) were mixed, and 17.8 parts of "Primal ASE60" (trade name, manufactured by Rohm & Haas, polyacrylic acid-based thickener, acid value 270 mg KOH / g, active ingredient 28%) and 1 part of "BYK-024" (trade name, manufactured by BYK-CHEMIE, polyether-modified silicone compound, active ingredient 100%) were added. The mixture was then stirred for 1 hour, and 100 parts of the above aluminum pigment paste (P1) were added to the stirring and mixing container. The pH was adjusted with dimethylethanolamine, and then deionized water was added to obtain the first solution with a solid content of 20%. In a separate stirring and mixing container, 995 parts of water and 5 parts of "LaponiteRD" (trade name, manufactured by Rochwood Additives Limited, inorganic thickener, synthetic hectorite) were added and stirred to obtain a diluent component. The two were then mixed in a disperser so that 100 parts of the first liquid and 100 parts of the diluent component were combined to obtain an aqueous base paint composition. To this, K-KAT XK614 (trade name, manufactured by KING INDUSTRIES, zinc compound, zinc compound content 65%) was added as a metal compound in an amount of 0.3 parts by mass per 100 parts by mass of the resin solids of the base coating composition and mixed, then mixed with a disperser to obtain an aqueous base coating composition (X-1).

[0162] (Water-based clear paint composition) -Dispersion-type acrylic resin- 400 parts of propylene glycol monopropyl ether were placed in a four-necked glass flask equipped with a thermometer, stirrer, reflux condenser, and nitrogen inlet, and the temperature was raised to 120°C under a nitrogen stream while stirring. Once 120°C was reached, mixed solution 1, which consisted of the first-stage monomer formulation and a polymerization initiator (12 parts of t-butyl peroxy-2-ethylhexanoate), was added dropwise over 4 hours, and the mixture was then maintained at 120°C for 1 hour after the dropwise addition was complete. While maintaining a temperature of 120°C, a mixed solution 2, which had been pre-mixed with the second-stage monomer formulation and polymerization initiator (t-butyl peroxy-2-ethylhexanoate 2.4 parts), was added dropwise to the flask over 1 hour. After the addition was complete, the mixture was maintained at 120°C for 1.5 hours to obtain the acrylic resin solution. The obtained acrylic resin solution had a resin solids content of 66.7%, a hydroxyl value of 120 mgKOH / g, a glass transition temperature of 53°C, a weight-average molecular weight of 14,000, and an average particle size of 170 nm. Next, propylene glycol monopropyl ether was removed from the obtained acrylic resin solution under reduced pressure until the solid content was 85%. This was cooled to 95°C, the pH was adjusted to 8.0 with dimethylethanolamine, and the mixture was stirred for 30 minutes. Furthermore, while stirring, deionized water was added dropwise over 2 hours until the resin solid content reached 50% to obtain an aqueous dispersion of acrylic resin (a4), a colloidal dispersion.

[0163] (Contains first-stage monomers) Styrene 96 parts Methyl methacrylate 104 parts Isobutyl methacrylate 152 parts n-butyl methacrylate 108 parts n-butyl acrylate 32 parts 2-Hydroxyethyl methacrylate 192 parts Glycidyl methacrylate 8 parts

[0164] (Contains second-stage monomers) Styrene 8 parts Methyl methacrylate 8 parts Isobutyl methacrylate 12 parts n-butyl methacrylate 16 parts n-butyl acrylate 8 parts 2-Hydroxyethyl methacrylate 32 parts Acrylic acid 24 parts

[0165] - Manufacturing of clear coating composition (Y-1) - (1) 1st liquid 65 parts of dispersion-type acrylic resin (a4) with a resin solids content of 50% (32.5 parts resin solids), 2 parts of Sannix GP-600 (Note 2), 0.5 parts of "BYK-348" (Note 3), 1 part of "BYK-015" (Note 4), 1 part of "TINUVIN384-2" (Note 5), 0.5 parts of "TINUVIN292" (Note 6), 0.5 parts of "SN Thickener 621N" (Note 7), and 29.5 parts of deionized water were mixed and stirred at room temperature, and dimethylethanolamine was added dropwise until the pH reached 7.6 to prepare the first solution.

[0166] (Note 2) "Sannix GP-600": Product name, manufactured by Sanyo Chemical Industries, Ltd., polyoxypropylene glyceryl ether, number average molecular weight 600, hydroxyl value 280 mgKOH / g, non-volatile content 100% (Note 3) "BYK-348": Product name, manufactured by BIC Chemie Japan, polyether-modified siloxane, weight-average molecular weight 1,500, non-volatile content 100% (Note 4) "BYK-015": Product name, manufactured by BIC Chemie Japan, polyether-modified siloxane, weight-average molecular weight 2,200, non-volatile content 100% (Note 5) "TINUVIN 384-2": Trade name, manufactured by BASF, benzotriazole-based UV absorber, 95% non-volatile content, 5% 1-methoxy-2-propyl acetate. (Note 6) "TINUVIN 292": Trade name, manufactured by BASF, hindered amine light stabilizer, 100% non-volatile content. (Note 7) "SN Thickener 621N": Product name, manufactured by Sunopco, nonionic rheology control agent, non-volatile content 30%

[0167] (2)Second liquid The first solution was prepared by mixing 25 parts of the polyisocyanate compound "Baihijur XP2655" (Note 8), 15 parts of dipropylene glycol dimethyl ether, 5 parts of ethylene glycol monobutyl ether acetate, and 5 parts of diethylene glycol monoethyl ether acetate in a separate container until homogeneous.

[0168] (Note 8) "Baihijur XP2655": Trade name, manufactured by Sumika Covestro Urethane Co., Ltd., hexamethylene diisocyanate-based polyisocyanate with sulfonic acid groups, NCO content 21%, non-volatile content 100%

[0169] (3) Preparation of clear coating composition (Y-1) Immediately before painting, the proportions of the first and second liquids were adjusted so that the equivalent ratio of hydroxyl value in the main component to isocyanate groups (NCO) in the hardener was NCO / OH = 1.6. After hand-stirring with a spatula, 21.7 parts of deionized water were added to the main component and hand-stirred further to prepare a water-based clear paint composition (Y-1) that did not contain a metal catalyst. The pot life was 2 hours and the workability was good.

[0170] (Preparation of coated objects for evaluation testing) A painted board with an electrodeposited epoxy resin-based electrodeposition paint coating and a primer surfacer base coat was sanded with #240 grit sandpaper. A commercially available water-based primer surfacer, "Retan WB Eco Plasaf" (product name, Kansai Paint Co., Ltd., two-component water-based urethane primer surfacer for automotive repair), was spray-coated onto the partially exposed steel plate to a dry film thickness of 60 μm. After drying at 60°C for 30 minutes, it was wet-sanded with #400 grit sandpaper. This primer surfacer base coat was designated as the substrate S.

[0171] (Formation of multi-layer coating) The object to be coated S was placed horizontally, and the above-mentioned water-based coating composition (X-1) was spray-coated under conditions of 25°C and 40% relative humidity. Air blowing was performed until the gloss of the base coating film was 25 or less. This spray coating and air blowing process was repeated two more times to obtain a colored base coating film with a dry film thickness of 15 μm. Next, the above-mentioned aqueous clear coating composition (Y-1) was air-sprayed onto the uncured colored base coating film using a wet-on-wet (sometimes indicated as W / W in the table) method to achieve a dry film thickness of 40 μm. After that, the coated board was kept horizontally at room temperature for 20 minutes, and then dried at 60°C for 20 minutes using an electric hot air dryer and cooled to room temperature to form a multi-layer coating film (XY-1).

[0172] [Examples 2-23, Comparative Examples 1-4] In Example 1, the base coating composition and the clear coating composition were as described in Tables 1 to 3, except that the multilayer coating films (XY-2) to (XY-26) were formed in the same manner as in Example 1. The base coating compositions and clear coating compositions were manufactured as follows.

[0173] - Manufacturing of water-based paint compositions (X-2) to (X-18) - Except for the types and content of metal compounds in the aqueous-based paint compositions and the combinations of pigment pastes being as described in Tables 1 to 3, aqueous-based paint compositions (X-2) to (X-18) were prepared in the same manner as aqueous-based paint composition (X-1), and each aqueous-based paint composition was prepared.

[0174] -Manufacturing of solvent-based base coating composition (X-19)- Sodium molybdate was added in an amount of 0.30 parts by mass per 100 parts by mass of solids of Retan PGHB Eco Base 202 Sunmetallic (product name, solvent-based metallic base paint composition for automotive repair manufactured by Kansai Paint Co., Ltd.) and mixed with a disperser to prepare a solvent-type base paint composition (X-19).

[0175] - Manufacturing of clear coating composition (Y-2) - In the preparation of clear coating composition (Y-1), the same procedure was followed except that the polyisocyanate compound "Baihijur XP2655" (Note 8) was replaced with 12.5 parts of "Baihijur XP2655" and 12.5 parts of 1,8-diisocyanato-4-isocyanatomethyloctane to obtain an aqueous clear coating composition (Y-2) that does not contain a metal catalyst. The pot life was 2 hours and the workability was good.

[0176] - Manufacturing of clear coating composition (Y-3) - Aqueous clear coating composition (Y-3) was prepared in the same manner as in the production of clear coating composition (Y-1), except that sodium molybdate was added as a metal catalyst in an amount of 0.01 parts by mass per 100 parts by mass of resin solids in the clear coating composition to the first liquid. The pot life was 2 hours and the workability was good.

[0177] - Manufacturing of clear coating composition (Y-4) - Aqueous clear coating composition (Y-4) was prepared in the same manner as in the preparation of clear coating composition (Y-1), except that (1) 0.03 parts by mass of sodium molybdate per 100 parts by mass of resin solids in the clear coating composition was added as a metal catalyst to the first liquid. The pot life was 2 hours and the workability was good.

[0178] - Manufacturing of clear coating composition (Y-5) - A clear coating composition (Y-5) was prepared in the same manner as in the production of clear coating composition (Y-1), except that (1) 0.10 parts by mass of sodium molybdate per 100 parts by mass of resin solids in the clear coating composition was added as a metal catalyst to the first liquid, thereby obtaining an aqueous clear coating composition (Y-5). The pot life was 1 hour and the workability was somewhat poor.

[0179] - Manufacturing of clear coating composition (Y-6) - As a top clear coating composition, "Retan PG Multi HX (Q) Clear" and its hardener (manufactured by Kansai Paint Co., Ltd., an organic solvent-type urethane curing coating composition) were mixed at a weight ratio of 2:1 of the main agent and hardener (sometimes abbreviated as main-hardener ratio) immediately before application to create the clear coating composition (Y-6). The pot life was 1 hour, and the workability was somewhat poor.

[0180] - Manufacturing of clear coating composition (Y-7) - A water-based clear coating composition (Y-7) was prepared in the same manner as in the production of clear coating composition (Y-1), except that (1) 0.30 parts by mass of sodium molybdate per 100 parts by mass of resin solids in the clear coating composition was added as a metal catalyst to the first liquid. The pot life was extremely short, less than 30 minutes, requiring painting in a short time, resulting in poor workability.

[0181] - Manufacturing of clear coating composition (Y-8) - To the clear coating composition (Y-1), 0.05 parts by mass of dioctyl tin diacetate (urethane curing catalyst) was added per 100 parts by mass of hydroxyl group-containing resin solids contained in the clear coating composition, and the mixture was mixed until uniform to obtain clear coating composition (Y-8). The pot life was 2 hours and the workability was good.

[0182] - Manufacturing of clear coating composition (Y-9) - To the clear coating composition (Y-3) mentioned above, 0.05 parts by mass of dioctyl tin diacetate (urethane curing catalyst) was added per 100 parts by mass of hydroxyl group-containing resin solids contained in the clear coating composition and mixed until uniform to obtain an aqueous clear coating composition (Y-9). The pot life was 2 hours and the workability was good.

[0183] [evaluation] The multilayer coatings obtained in the above examples and comparative examples were evaluated as follows. The test results are shown in Tables 1 to 3.

[0184] [Drying properties (polishing properties)] The multi-layer coatings (evaluation test coated panels) of the examples and comparative examples were dried in an electric hot air dryer for 30 minutes. In addition, multi-layer coatings (drying performance evaluation test coated panels) were also prepared with drying times of 20 minutes, 30 minutes, and 40 minutes in an electric hot air dryer. Polishing repair was performed on each of the multi-layer coatings (evaluation test coated panels) with different drying times in an electric hot air dryer using the polishing repair method described below. (Polishing and repair method) Each test coating plate was wet-sanded with #2000 grit waterproof sandpaper, then polished for 60 seconds with a coarse polishing compound using a coarse polishing buff to remove sandpaper scratches, and finally polished for 60 seconds with a finishing compound using a finishing buff to remove buffing scratches.

[0185] From the paints that underwent the polishing and repair described above, those with good paint film condition, free from sanding marks and loss of gloss, were selected. The following evaluation criteria were used. A, B, and C are considered passing grades, while D is considered a failing grade. A: Polishing and repair are possible on test coated panels with a drying time of 20 minutes or more. B: Polishing and repair are possible on test coated panels with a drying time of 30 minutes or more. C: Polishing and repair are possible on a test coated panel with a drying time of 40 minutes. D: Polishing and repair are impossible on the test coated panel after a drying time of 40 minutes.

[0186] [Finishing properties] The multi-layer coating films (evaluation test coated panels) obtained in the examples and comparative examples were visually inspected to evaluate whether there were any problems such as unevenness, blotches, shrinkage, smoothness, and gloss. A, B, and C were considered passing grades, while D was considered a failing grade. A: The appearance of the coating is very good. B: Almost no unevenness, undercoating, or shrinkage is observed, and the smoothness and gloss are excellent. C: Slight reduction in smoothness and glossiness are observed, but unevenness, undercoating, and shrinkage are almost absent, and the level is acceptable for practical use. D: Significant unevenness, undercoating, shrinkage, and loss of shine are present, clearly indicating a problem.

[0187] [Coating film hardness] The multilayer coatings obtained in the examples and comparative examples, each having a 15 μm dry film base coating layer and a 40 μm dry film clear coating layer (evaluation test coated panels: drying times 30 minutes at 60°C and 20 minutes at 60°C), were removed from the drying oven and left for 1 hour in an environment of 50% relative humidity and 20°C before being used as test panels. When the multilayer coatings were measured by IR (infrared spectroscopy), peaks of isocyanate groups were confirmed in all cases, confirming that they were in a semi-cured state. Using a Fischerscope HM2000 (Fischer), the indenter was pressed into the surface of the coating film, and the Martens hardness value obtained from the indentation depth and force was measured. The measurement conditions were 5 mN × 4 μm / 20 seconds. The measured Martens hardness was evaluated according to the following evaluation criteria. A: Martens hardness value is 7 N / mm 2 That's all. B: Martens hardness value is 5 N / mm 2 The above and 7N / mm 2 Less than C: Martens hardness value is 3 N / mm 2 The above and 5N / mm 2 Less than D: Martens hardness value is 3 N / mm 2 Less than

[0188] [water resistance] The multilayer coatings (evaluation test coated panels) obtained in the examples and comparative examples were immersed in a constant temperature water bath at 40°C for 10 days. After removal, the condition of the coatings was visually observed after standing for 1 hour, and evaluated according to the following evaluation criteria to check for any coating abnormalities such as gloss loss, cracking, or blistering. A: No abnormalities B: A very small amount of at least one of the following abnormalities is observed: glossiness, cracking, and blistering. C: Partially, at least one of the following abnormalities is observed: glossiness, cracking, and blistering. D: Significant abnormalities such as gloss loss, cracking, and blistering are observed in part or across the entire paint film.

[0189] [Environmentally friendly] A paint composition containing 420 g / L or less of harmful volatile organic compounds (VOCs) immediately before application, and with a water content of 50% or more in the solvent, is marked with a circle (○). A paint composition in which the main solvent is almost entirely an organic solvent, or which does not meet the above criteria, is marked with a cross (×).

[0190] [Table 1]

[0191] [Table 2]

[0192] [Table 3]

[0193] (metal concentration measurement) As a representative example, the multi-layer coating shown in Table 4 below was completely cured at 80°C for 30 minutes, and only the clear coating (transparent portion) was scraped off with a cutter, with 0.2 g collected. This sample was dissolved in a strong acid solution, and elemental analysis was performed. The measuring instrument used was the ICPS-8100 manufactured by Shimadzu Corporation. The results of calculating the increase in the concentration of metal elements in the clear coating are shown in Table 4.

[0194] [Table 4]

[0195] As shown in Tables 1 to 4, the examples in which at least the water-based coating composition contains a metal compound, compared to Comparative Example 1, which does not contain a metal compound in the water-based coating composition, and Comparative Example 4, which contains a metal compound only in the clear coating composition, show superior drying properties, finish, hardness, and water resistance of the multi-layer coating film. Furthermore, according to the method for forming a multi-layer coating film of the present invention, since the harmful volatile organic compounds (VOCs) contained in the paint composition immediately before painting are 420 g / L or less, and the water content in the solvent is 50% or more, an environmentally friendly multi-layer coating film can be obtained.

Claims

1. A first step involves applying an aqueous base coating composition to the object to be coated to form a base coating film (I), and A second step involves applying an aqueous clear coating composition onto the base coating (I) to form a clear coating (II), and A method for forming a multilayer coating film, comprising the step of simultaneously drying the base coating film and the clear coating film, At least the aqueous-based paint composition contains a metal compound comprising at least one metal selected from the group consisting of zinc, tin, zirconium, bismuth, lead, cobalt, manganese, titanium, aluminum, and molybdenum. The aqueous clear coating composition comprises a second film-forming resin, A method for forming a multilayer coating film, wherein the second coating-forming resin is a dispersion-type acrylic resin (a4) in a form that is dispersed in water.

2. The method for forming a multilayer coating film according to claim 1, wherein the concentration of the metal compound in the aqueous base coating composition is equal to or greater than the concentration of the metal compound in the clear coating composition.

3. The aforementioned metal is derived from a metal catalyst. The method for forming a multilayer coating film according to claim 1 or 2, wherein the content of the metal catalyst in the aqueous base coating composition is 0.01 parts by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the total resin solids in the aqueous base coating composition.

4. The water solubility of the aforementioned metal compound is 1 × 10 －３ mg / 1000g or more 5×10 ５ A method for forming a multilayer coating film according to any one of claims 1 to 3, wherein the amount is mg / 1000g or less.

5. The aqueous base coating composition comprises a first film-forming resin, A method for forming a multilayer coating film according to any one of claims 1 to 4, wherein the first film-forming resin is at least one of a water-dispersible acrylic resin (a1), a water-dispersible polyurethane resin (a2), and a water-soluble acrylic resin (a3).

6. A method for forming a multilayer coating film according to any one of claims 1 to 5, wherein the aqueous clear coating composition contains a urethane curing catalyst.

7. The aqueous clear coating composition comprises a second film-forming resin, A method for forming a multilayer coating according to any one of claims 1 to 6, wherein the second coating-forming resin further comprises a polyisocyanate compound (b), and the polyisocyanate compound (b) comprises a polyisocyanate compound (b1) having three or more isocyanate groups and a molecular weight of 350 or less.

8. The method for forming a multilayer coating according to any one of claims 1 to 7, wherein the object to be coated is an old coating or a damaged part of a coated body.

9. The method for forming a multilayer coating according to any one of claims 1 to 8, wherein the object to be coated is industrial machinery, construction machinery, railway vehicles, large vehicles, ship hulls, buildings or structures, or parts thereof.

10. A multilayer coating having a cured base coating layer obtained by applying and curing an aqueous base coating composition on a substrate, and a cured clear coating layer obtained by applying and curing an aqueous clear coating composition, The hardened clear coating contains at least one metal compound selected from the group consisting of zinc, tin, zirconium, bismuth, lead, cobalt, manganese, titanium, aluminum, and molybdenum. The concentration of metal elements in the cured clear coating is 20 ppm or more. The aqueous clear coating composition comprises a second film-forming resin, A multilayer coating film comprising a dispersion-type acrylic resin (a4) in which the second film-forming resin is dispersed in water.

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