Method for forming multi-layer coating films

Abstract

To provide a bilayer coating film formation method that can form a bilayer coating film with excellent smoothness and sealer concealability.SOLUTION: A bilayer coating film formation method includes: a step of coating a sealer (S) to form an uncured sealer coating film; a step of coating a specific aqueous intermediate coating paint (X) to form an uncured intermediate coating film without performing pre-heating and heat curing; a step of coating an aqueous base coat paint (Y) to form an uncured basecoat coating film without performing pre-heating and heat curing; a pre-heating step of pre-heating the uncured sealer coating film, the uncured intermediate coating film, and the uncured basecoat coating film; a step of coating a clear paint (Z) to form an uncured clear coating film; and a printing step of heating and curing a bilayer coating film comprising the pre-heated sealer coating film, intermediate coating film, and basecoat coating film, and the uncured clear coating film to form a cured bilayer coating film.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, from the perspective of protecting the global environment, there has been a demand to reduce volatile organic compounds (VOCs) emitted from paints. As a result, there has been a shift from organic solvent-based paints to water-based paints in various fields of industrial product painting, including industrial machinery, construction machinery, steel furniture and sheds, automobile body panels, and automobile parts. In the automotive painting industry, large quantities of organic solvent-based paints were previously used, and reducing the VOCs emitted from these paints was a pressing need. However, the replacement of organic solvent-based paints with water-based paints has progressed in the various paints used in the undercoat, intermediate coat, and basecoat painting processes of automobiles, and currently, water-based paints are the mainstream.

[0003] Furthermore, to reduce environmental impact, development is underway to shorten the painting process by partially omitting the heat curing process (mainly the heat curing process after the intermediate coat and the preheating process) that was previously performed for each layer of the paint film, such as the intermediate coat. As a result, simplified automotive painting is becoming mainstream.

[0004] However, in the aforementioned process-saving automotive painting using water-based paints, a decrease in the smoothness of the formed coating film is likely to occur due to the mixing of layers between the water-based intermediate coat paint and the water-based base coat paint.

[0005] Furthermore, in the painting of automobile bodies with hemmed sections (parts with a folded-over structure at the edges), generally, a sealer is applied to the hemmed sections such as doors, followed by a preheating process and / or a heat curing process, after which the intermediate coat paint and base coat paint are applied sequentially. From the viewpoint of reducing the number of steps, the heat curing process and preheating process after sealer application have been considered, but the viscosity of the intermediate coat film may decrease due to the influence of plasticizers and other substances contained in the sealer, resulting in a thinner intermediate coat film thickness and inability to completely conceal the sealer.

[0006] Patent Document 1 describes a method for forming a multilayer coating on an automobile body having a hemmed portion, comprising the following steps (1) to (5) in sequence: step (1): a step of applying a sealer (S) to form a sealer coating; step (2): a step of applying an intermediate coating paint (X) on the sealer coating formed in step (1) to form an intermediate coating; step (3): a step of applying an aqueous base coating paint (Y) on the intermediate coating formed in step (2) to form a base coating; step (4): a step of applying a clear coating paint (Z) containing a hydroxyl group-containing acrylic resin and a polyisocyanate compound on the base coating formed in step (3) to form a clear coating; step (5): the same as step (1) The invention describes a method for forming a multilayer coating film, comprising the steps of heating and curing a sealer coating film, an intermediate coating film, a base coating film, and a clear coating film formed in steps ) to (4), wherein the intermediate coating paint (X) contains at least one of (A) a hydroxyl group-containing polyester resin, (B) a melamine resin, and a blocked polyisocyanate compound, (C) a curing catalyst, (D) a pigment, and (E) an organic solvent, wherein the concentration (PWC) of the (D) pigment contained in the intermediate coating paint (X) is 40 to 60%, and the content of the (C) curing catalyst is 0.1 to 0.5% by mass, and the water contact angle of the coated surface after curing the intermediate coating film formed on the uncured sealer coating film is in the range of 70° to 75° 1 minute after water droplets are applied. Patent Document 1 describes that by the above method, defects such as peeling of the coating film due to the influence of plasticizers, etc., can be suppressed, and a multilayer coating film with an excellent finished appearance can be formed. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2016-221469 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] In the multilayer coating method described in Patent Document 1, when a water-based intermediate coating paint is used as the intermediate coating paint, the smoothness and sealer opacity of the multilayer coating film were sometimes insufficient, especially when the heat curing step and preheating step after sealer application were omitted.

[0009] The object of the present invention is to provide a method for forming a multilayer coating film that can form a multilayer coating film with excellent smoothness and sealer opacity. [Means for solving the problem]

[0010] As a result of diligent research to achieve the above objectives, the present inventors have developed a method for forming a multilayer coating on an automobile body having a hemmed portion, wherein the following steps (1) to (6) are performed sequentially on the automobile body: Step (1): A step in which a sealer (S) is applied to form an uncured sealer film. Step (2): A step in which a water-based intermediate coating paint (X) is applied to the uncured sealer coating film formed in step (1) without preheating or heat curing to form an uncured intermediate coating film. Step (3): A step in which an uncured base coat film is formed on the uncured intermediate coating film formed in step (2) by applying a water-based base coat paint (Y) without preheating or heat curing. Step (4): Preheating step in which the uncured sealer film, uncured intermediate coat film, and uncured base coat film are preheated. Step (5): A step of applying a clear coating (Z) onto the base coat coating that has been preheated in step (4) to form an uncured clear coating. Step (6): A baking process in which a multi-layer coating consisting of a preheated sealer coating, an intermediate coating, a base coat, and an uncured clear coating is heated and cured to form a cured multi-layer coating. The aqueous intermediate coating paint (X) contains a hydroxyl group-containing resin (x1), a curing agent (x2), a pigment (x3), and a viscosity modifier (x4), wherein the viscosity modifier (x4) includes an aggregate viscosity modifier (x41) and an alkali-swelling viscosity modifier (x42) different from the aggregate viscosity modifier (x41), wherein the aggregate viscosity modifier (x41) is a (meth)acrylic acid copolymer-based thickener having a hydrophobic group with 8 to 36 carbon atoms, the solid content mass ratio of the aggregate viscosity modifier (x41) to the alkali-swelling viscosity modifier (x42) is in the range of 97 / 3 to 70 / 30, and the dry film thickness of the intermediate coating is less than 20 μm. We found that the above objective can be achieved by a multi-layer coating film formation method. [Effects of the Invention]

[0011] According to the present invention, a method for forming a multilayer coating film that has excellent smoothness and sealer-hiding properties can be provided. [Modes for carrying out the invention]

[0012] The multi-layer coating film formation method of the present invention will be described in detail below. The present invention provides a method for forming a multilayer coating film (hereinafter also simply referred to as "the method of the present invention"), which involves forming a multilayer coating film on an automobile body having a hemmed portion, and performing the following steps (1) to (6) in order. Step (1): A step in which a sealer (S) is applied to form an uncured sealer film. Step (2): A step in which a specific water-based intermediate coating paint (X) is applied to the uncured sealer coating film formed in step (1) without preheating or heat curing to form an uncured intermediate coating film. Step (3): A step in which an uncured base coat film is formed on the uncured intermediate coating film formed in step (2) by applying a water-based base coat paint (Y) without preheating or heat curing. Step (4): Preheating step in which the uncured sealer film, uncured intermediate coat film, and uncured base coat film are preheated. Step (5): A step of forming an uncured clear coating film by applying a clear paint (Z) onto the base coat film preheated in the step (4). Step (6): A baking step of heat-curing a multi-layer coating film composed of the preheated sealer coating film, intermediate coating film, and base coat film, and the uncured clear coating film to form a cured multi-layer coating film.

[0013] Steps (1) to (6) will be described in detail below.

[0014] <Object to be coated> The object to be coated to which the method of the present invention is applied is not particularly limited as long as it is an automobile body having a hemming portion. The hemming portion refers to a portion where the inner panel and outer panel of the steel plate forming the vehicle body are overlapped, and the edge of the outer panel is folded back to the inner panel side so as to sandwich the end of the inner panel, that is, a portion where so-called hemming is performed. Since the hemming portion has such a narrow structure, it is likely to accumulate. Further, in the portion of the corner of the steel plate (or a gently curved shape) where hemming is difficult, the hemming is insufficient, and a gap in the hemming portion is likely to occur. Such a gap portion between the hemming portions is a portion where accumulation is more likely to occur. The method of the present invention is a useful method when forming a multi-layer coating film on an automobile body having such a hemming portion.

[0015] The material of the object to be coated forming the automobile body is not particularly limited. For example, metal materials such as iron, aluminum, aluminum alloy, brass, copper, tinplate, stainless steel, zinc-plated steel, zinc alloy (Zn-Al, Zn-Ni, Zn-Fe, etc.) plated steel, tin-plated steel, etc. can be mentioned.

[0016] Further, the automobile body having a hemming portion may be one in which a surface treatment such as phosphate treatment, chromate treatment, or composite oxide treatment is applied to the metal surface.

[0017] Automobile bodies having the above-mentioned hemmed portion are preferably coated with electrodeposition paint as a base coat, and among these, bodies in which a base coat film has been formed with cationic electrodeposition paint are more preferable.

[0018] <Process (1)> According to the method of the present invention, first, as step (1), a sealer (S) is applied to an automobile body having a hemmed portion, and an uncured sealer film is formed.

[0019] In general, when painting automobile bodies, a sealant, commonly known as a sealer, is often applied to hemmed areas, welded areas, etc., prior to the application of the intermediate coat, in order to provide waterproofing and improve the appearance quality.

[0020] Generally, vinyl resin-based sealants or vinyl chloride resin-based sol paints are often used as the sealer (S). Examples of vinyl resin-based sealants include "Seal Ace 390A" manufactured by Aisin Chemical Co., Ltd. and "Sandine 2690A-2" manufactured by Asahi Rubber Co., Ltd. Examples of vinyl chloride resin-based sol coatings include "615-2" manufactured by Asahi Rubber Co., Ltd., "PV-129" manufactured by Aisin Chemical Co., Ltd., and "PT187" manufactured by Cemedyne Henkel Co., Ltd.

[0021] The above-mentioned sealer (S) often contains plasticizers as a by-component, and the effect of these plasticizers (the plasticizers bleed (migrate) to the intermediate coating film that will be applied in the next step) reduces the viscosity of the intermediate coating film that is formed on the uncured sealer film. This can cause painting defects such as reduced sealer opacity in vertical parts of an automobile body, for example.

[0022] The sealer (S) can be applied by known methods such as air spraying, airless spraying, brush application, or spatula application, and it is preferable that the film thickness be 1 mm or less based on the cured coating.

[0023] <Process (2)> According to the method of the present invention, next, a water-based intermediate coating paint (X) is applied to the sealer coating film formed in step (1) without preheating or heat curing, and an uncured intermediate coating film is formed.

[0024] <Water-based intermediate coat paint (X)> In the method of the present invention, the water-based intermediate coating (X) is a coating composition containing a hydroxyl group-containing resin (x1), a curing agent (x2), a pigment (x3), and a viscosity modifier (x4). The water-based intermediate coating (X) used in the method of the present invention includes an aggregate-type viscosity modifier (x41) and an alkali-swelling-type viscosity modifier (x42) different from the aggregate-type viscosity modifier (x41). By forming an uncured intermediate coating film on a sealer film using such a water-based intermediate coating (X), the intermediate coating film becomes less susceptible to the influence of plasticizers contained in the sealer, and a decrease in the sealer's opacity can be suppressed.

[0025] <<Hydroxyl group-containing resin (x1)>> The hydroxyl group-containing resin (x1) is not particularly limited as long as it is a resin containing hydroxyl groups. Specifically, examples of resin types include hydroxyl group-containing acrylic resin, hydroxyl group-containing polyester resin, hydroxyl group-containing polyether resin, hydroxyl group-containing polycarbonate resin, and hydroxyl group-containing polyurethane resin. In particular, it is preferable that the hydroxyl group-containing resin (x1) includes one of either a hydroxyl group-containing acrylic resin (x11) or a hydroxyl group-containing polyester resin (x12).

[0026] (Hydroxyl group-containing acrylic resin (x11)) The hydroxyl group-containing acrylic resin (x11) can be produced, for example, by copolymerizing a hydroxyl group-containing polymerizable unsaturated monomer and other polymerizable unsaturated monomers copolymerizable with the hydroxyl group-containing polymerizable unsaturated monomer by methods known to themselves, such as solution polymerization in an organic solvent or emulsion polymerization in water.

[0027] The above-mentioned hydroxyl group-containing polymerizable unsaturated monomer is a compound having one or more hydroxyl groups and polymerizable unsaturated bonds in one molecule. Examples of the hydroxyl group-containing polymerizable unsaturated monomer 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. However, in the present invention, monomers corresponding to the polymerizable unsaturated monomers having an ultraviolet-absorbing functional group (xvii) described later should be defined as other polymerizable unsaturated monomers copolymerizable with the above-mentioned hydroxyl group-containing polymerizable unsaturated monomers, and are excluded from the hydroxyl group-containing polymerizable unsaturated monomers. These can be used individually or in combination of two or more.

[0028] Other polymerizable unsaturated monomers copolymerizable with the above-mentioned hydroxyl group-containing polymerizable unsaturated monomers include, for example, the monomers (i) to (xx) listed below. These polymerizable unsaturated monomers can be used individually or in combination of two or more.

[0029] (i) Alkyl or cycloalkyl (meth)acrylates: For example, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, tridecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclododecyl (meth)acrylate, tricyclodecanyl (meth)acrylate, etc.

[0030] (ii) Polymerizable unsaturated monomers having an isobornyl group: isobornyl (meth)acrylate, etc.

[0031] (iii) Polymerizable unsaturated monomers having an adamantyl group: adamantyl (meth)acrylate, etc.

[0032] (iv) Polymerizable unsaturated monomers having a tricyclodecenyl group: tricyclodecenyl (meth)acrylate, etc.

[0033] (v) Polymerizable unsaturated monomers containing aromatic rings: benzyl (meth)acrylate, styrene, α-methylstyrene, vinyltoluene, etc.

[0034] (vi) Polymerizable unsaturated monomers having an alkoxysilyl group: vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, γ-(meth)acryloyloxypropyltrimethoxysilane, γ-(meth)acryloyloxypropyltriethoxysilane, etc.

[0035] (vii) Polymerizable unsaturated monomers having a fluorinated alkyl group: perfluoroalkyl (meth)acrylates such as perfluorobutylethyl (meth)acrylate and perfluorooctylethyl (meth)acrylate; fluoroolefins, etc.

[0036] (viii) A polymerizable unsaturated monomer having a photopolymerizable functional group such as a maleimide group.

[0037] (ix) Vinyl compounds: N-vinylpyrrolidone, ethylene, butadiene, chloroprene, vinyl propionate, vinyl acetate, etc.

[0038] (x) Carboxyl group-containing polymerizable unsaturated monomers: (meth)acrylic acid, maleic acid, crotonic acid, β-carboxyethyl (meth)acrylate, etc.

[0039] (xi) Nitrogen-containing polymerizable unsaturated monomers: (meth)acrylonitrile, (meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, methylenebis(meth)acrylamide, ethylenebis(meth)acrylamide, adducts of glycidyl (meth)acrylate with amine compounds, etc.

[0040] (xii) Polymerizable unsaturated monomers having two or more polymerizable unsaturated groups in one molecule: allyl (meth)acrylate, ethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, etc.

[0041] (xiii) Epoxy group-containing polymerizable unsaturated monomers: glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, allyl glycidyl ether, etc.

[0042] (xiv) A (meth)acrylate having a polyoxyethylene chain with an alkoxy group at the molecular terminus.

[0043] (xv) Polymerizable unsaturated monomers having a sulfonic acid group: 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl (meth)acrylate, allylsulfonic acid, 4-styrenesulfonic acid, etc.; sodium salts and ammonium salts of these sulfonic acids, etc.

[0044] (xvi) Polymerizable unsaturated monomers having a phosphate group: acid phosphooxyethyl (meth)acrylate, acid phosphooxypropyl (meth)acrylate, acid phosphooxypoly(oxyethylene) glycol (meth)acrylate, acid phosphooxypoly(oxypropylene) glycol (meth)acrylate, etc.

[0045] (xvii) Polymerizable unsaturated monomers having UV-absorbing functional groups: 2-hydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone, 2-hydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, 2,2'-dihydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone, 2,2'-dihydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, 2-[2-hydroxy-5-[2-(methacryloyloxy)ethyl]phenyl]-2H-benzotriazole, etc.

[0046] (xviii) Photostable polymerizable unsaturated monomers: 4-(meth)acryloyloxy-1,2,2,6,6-pentamethylpiperidine, 4-(meth)acryloyloxy-2,2,6,6-tetramethylpiperidine, 4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 1-(meth)acryloyl-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 1-(meth)acryloyl-4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, 4-crotonoylamino-2,2,6,6-tetramethylpiperidine, 1-crotonoyl-4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, etc.

[0047] (xix) Polymerizable unsaturated monomers having a carbonyl group: acrolein, diacetone acrylamide, diacetone methacrylamide, acetoacetoxyethyl methacrylate, formyl styrene, vinyl alkyl ketones having 4 to 7 carbon atoms (e.g., vinyl methyl ketone, vinyl ethyl ketone, vinyl butyl ketone), etc.

[0048] (xx) Polymerizable unsaturated monomers having an acid anhydride group: maleic anhydride, itaconic anhydride, citraconic anhydride, etc.

[0049] In this specification, a polymerizable unsaturated group means an unsaturated group that can undergo radical polymerization. Examples of such polymerizable unsaturated groups include vinyl groups and (meth)acryloyl groups.

[0050] Furthermore, in this specification, "(meth)acrylate" means acrylate or methacrylate. "(meth)acrylic acid" means acrylic acid or methacrylic acid. Also, "(meth)acryloyl" means acryloyl or methacryloyl. Also, "(meth)acrylamide" means acrylamide or methacrylamide.

[0051] When producing the above-mentioned hydroxyl group-containing acrylic resin (x11), the proportion of the hydroxyl group-containing polymerizable unsaturated monomer used is preferably in the range of 1 to 50% by mass, more preferably in the range of 2 to 40% by mass, and even more preferably in the range of 3 to 30% by mass, based on the total amount of monomer components.

[0052] From the viewpoint of the smoothness of the multilayer coating film formed, the hydroxyl group-containing acrylic resin (x11) described above preferably has a hydroxyl value in the range of 1 to 150 mg KOH / g, more preferably in the range of 2 to 120 mg KOH / g, and even more preferably in the range of 5 to 100 mg KOH / g.

[0053] The hydroxyl group-containing acrylic resin (x11) described above preferably has an acid value in the range of 1 to 150 mg KOH / g, more preferably in the range of 5 to 100 mg KOH / g, and even more preferably in the range of 5 to 80 mg KOH / g, from the viewpoint of the smoothness of the multilayer coating film formed and the ability to conceal the sealer.

[0054] When the above-mentioned hydroxyl group-containing acrylic resin (x11) is obtained by the emulsion polymerization method in water, the emulsion polymerization can be carried out by conventionally known methods. For example, it can be carried out by emulsion polymerization of a polymerizable unsaturated monomer mixture using a polymerization initiator in the presence of an emulsifier. Anionic emulsifiers and nonionic emulsifiers can be suitably used as the emulsifier.

[0055] Examples of anionic emulsifiers include sodium salts and ammonium salts of alkyl sulfonic acid, alkylbenzene sulfonic acid, and alkyl phosphoric acid. Examples of nonionic emulsifiers include polyoxyethylene oleyl ether, polyoxyethylene stearyl ether, polyoxyethylene lauryl ether, polyoxyethylene tridecyl ether, polyoxyethylene phenyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene monolaurate, polyoxyethylene monostearate, polyoxyethylene monooleate, sorbitan monolaurate, sorbitan monostearate, sorbitan trioleate, and polyoxyethylene sorbitan monolaurate.

[0056] Furthermore, polyoxyalkylene group-containing anionic emulsifiers having an anionic group and a polyoxyalkylene group such as a polyoxyethylene group or a polyoxypropylene group in one molecule; reactive anionic emulsifiers having an anionic group and a radically polymerizable unsaturated group in one molecule can also be used. Examples of the above-mentioned reactive anionic emulsifiers include sodium salts of sulfonic acid compounds having radically polymerizable unsaturated groups such as an allyl group, a methallyl group, a (meth)acryloyl group, a propenyl group, or a butenyl group, and ammonium salts of the sulfonic acid compounds.

[0057] The amount of emulsifier used is preferably about 0.1 to 15% by mass, more preferably about 0.5 to 10% by mass, and even more preferably about 1 to 5% by mass, based on the total amount of monomers used.

[0058] Examples of polymerization initiators include organic peroxides such as benzoyl peroxide, octanoyl peroxide, lauroyl peroxide, stearoyl peroxide, cumene hydroperoxide, tert-butyl peroxide, tert-butyl peroxylaurate, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxyacetate, and diisopropylbenzene hydroperoxide; azobisisobutyronitrile, azobis(2,4-dimethyl) Examples of polymerization initiators include azo compounds such as rubaleronitrile, azobis(2-methylpropionnitrile), azobis(2-methylbutyronitrile), 4,4'-azobis(4-cyanobutanoic acid), dimethylazobis(2-methylpropionate), azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and azobis{2-methyl-N-[2-(1-hydroxybutyl)]-propionamide}; and persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate. These polymerization initiators can be used alone or in combination of two or more. In addition, a reducing agent such as sugar, sodium formaldehyde sulfoxylate, or an iron complex can be used in combination with the above polymerization initiators as needed to form a redox initiator.

[0059] The amount of polymerization initiator used is generally preferably in the range of 0.1 to 5% by mass, and more preferably in the range of 0.2 to 3% by mass, relative to the total amount of all monomers used. The method of adding the polymerization initiator is not particularly limited and can be appropriately selected depending on its type and amount. For example, it can be included in the monomer mixture or aqueous medium beforehand, added all at once during polymerization, or added dropwise.

[0060] When the aqueous intermediate coating paint (X) of the present invention contains the hydroxyl group-containing acrylic resin (x11), the content of the hydroxyl group-containing acrylic resin (x11) is preferably in the range of 1 to 70% by mass, more preferably in the range of 2 to 60% by mass, and even more preferably in the range of 3 to 50% by mass, based on the total resin solid content in the aqueous intermediate coating paint (X), from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0061] (Hydroxyl group-containing polyester resin (x12)) Hydroxyl group-containing polyester resin (x12) can be synthesized by known methods, following conventional procedures, by esterifying a polybasic acid with a polyhydric alcohol.

[0062] The polybasic acids mentioned above are compounds having two or more carboxyl groups in one molecule, and examples include phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid, fumaric acid, itaconic acid, trimellitic acid, pyromellitic acid, and their anhydrides. The polyhydric alcohols are compounds having two or more hydroxyl groups in one molecule, and examples include ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, neopentyl glycol hydroxypivalate, and 2-butyl-2-ethyl-1,3-propanediol. Examples include diols such as diols, 3-methyl-1,5-pentanediol, 2,2,4-trimethylpentanediol, and hydrogenated bisphenol A; trivalent or higher polyol components such as trimethylolpropane, trimethylolethane, glycerin, and pentaerythritol; and hydroxycarboxylic acids such as 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolpentanoic acid, 2,2-dimethylolhexanoic acid, and 2,2-dimethyloloctanoic acid.

[0063] Alternatively, α-olefin epoxides such as propylene oxide and butylene oxide, and monoepoxy compounds such as Cardura E10 (manufactured by HEXION, trade name, glycidyl ester of synthetic highly branched saturated fatty acid) may be reacted with an acid to introduce these compounds into the polyester resin.

[0064] Furthermore, the hydroxyl group-containing polyester resin (x12) may also be a fatty acid-modified polyester resin modified with (semi)drying oil fatty acids such as linseed oil fatty acid, coconut oil fatty acid, safflower oil fatty acid, soybean oil fatty acid, sesame oil fatty acid, hen oil fatty acid, tall oil fatty acid, and dehydrated castor oil fatty acid. Generally, the amount of modification by these fatty acids is suitable to be 30% by mass or less in terms of oil length. In addition, the hydroxyl group-containing polyester resin (x12) may also be one in which a monobasic acid such as benzoic acid has been partially reacted.

[0065] Furthermore, the hydroxyl group-containing polyester resin (x12) described above can be modified with fatty acids, monoepoxy compounds, polyisocyanate compounds, acrylic resins, etc., during or after the preparation of the resin.

[0066] Examples of the above fatty acids include coconut oil fatty acids, cottonseed oil fatty acids, hemp seed oil fatty acids, rice bran oil fatty acids, fish oil fatty acids, tall oil fatty acids, soybean oil fatty acids, linseed oil fatty acids, tung oil fatty acids, rapeseed oil fatty acids, castor oil fatty acids, dehydrated castor oil fatty acids, and safflower oil fatty acids.

[0067] Furthermore, as the monoepoxy compound mentioned above, for example, "Cardura E10P" (trade name, manufactured by HEXION, a glycidyl ester of a synthetic highly branched saturated fatty acid) can be suitably used.

[0068] Furthermore, examples of the polyisocyanate compounds include aliphatic diisocyanate compounds such as lysine diisocyanate, hexamethylene diisocyanate, and trimethylhexane diisocyanate; and lipid diisocyanate compounds such as hydrogenated xylylene diisocyanate, isophorone diisocyanate, methylcyclohexane-2,4-diisocyanate, methylcyclohexane-2,6-diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), and 1,3-(isocyanatomethyl)cyclohexane. Examples include cyclic diisocyanate compounds; aromatic diisocyanate compounds such as tolylene diisocyanate, xylylene diisocyanate, and diphenylmethane diisocyanate; organic polyisocyanates themselves, such as trivalent or higher polyisocyanates like lysine triisocyanate; adducts of these organic polyisocyanates with polyhydric alcohols, low molecular weight polyester resins, water, etc.; and cyclized polymers (e.g., isocyanurates) and biuret-type adducts of these organic polyisocyanates. These polyisocyanate compounds can be used individually or in combination of two or more.

[0069] Furthermore, known methods can be used to modify the hydroxyl group-containing polyester resin (x12) with an acrylic resin. Examples include polymerizing a mixture of a polymerizable unsaturated group-containing polyester resin and a polymerizable unsaturated monomer, or reacting a hydroxyl group and carboxyl group-containing polyester resin with an acrylic resin.

[0070] From the viewpoint of the smoothness of the multilayer coating film formed, the hydroxyl group-containing polyester resin (x12) described above preferably has a hydroxyl value in the range of 1 to 250 mg KOH / g, more preferably in the range of 2 to 200 mg KOH / g, and even more preferably in the range of 5 to 200 mg KOH / g.

[0071] Furthermore, from the viewpoint of the smoothness and sealer opacity of the multilayer coating film formed, the acid value of the hydroxyl group-containing polyester resin (x12) is preferably in the range of 1 to 150 mg KOH / g, more preferably in the range of 2 to 100 mg KOH / g, and even more preferably in the range of 2 to 50 mg KOH / g.

[0072] Furthermore, the number-average molecular weight of the above-mentioned hydroxyl group-containing polyester resin (x12) is preferably in the range of 800 to 100,000, more preferably in the range of 1,000 to 50,000, and even more preferably in the range of 1,200 to 10,000.

[0073] In this specification, the average molecular weight is calculated from the chromatogram measured by gel permeation chromatography, using the molecular weight of standard polystyrene as a reference. The gel permeation chromatograph used was "HLC8120GPC" (manufactured by Tosoh Corporation). Four columns were used: "TSKgel G-4000HXL", "TSKgel G-3000HXL", "TSKgel G-2500HXL", and "TSKgel G-2000HXL" (all manufactured by Tosoh Corporation, trade names). The measurements were performed under the following conditions: mobile phase; tetrahydrofuran, measurement temperature; 40°C, flow rate; 1 mL / min, detector; radioisotope (RI).

[0074] When the aqueous intermediate coating paint (X) of the present invention contains the hydroxyl group-containing polyester resin (x12), the content of the hydroxyl group-containing polyester resin (x12) is preferably in the range of 1 to 70% by mass, more preferably in the range of 2 to 50% by mass, and even more preferably in the range of 3 to 30% by mass, based on the total resin solid content in the aqueous intermediate coating paint (X), from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0075] <<Hardening agent (x2)>> The curing agent (x2) is a compound that reacts with the crosslinkable functional group in the hydroxyl group-containing resin (x1) to cure the water-based intermediate coating paint (X). The curing agent (x2) can be used alone or in combination of two or more types.

[0076] Examples of the curing agent (x2) include amino resins (x21), polyisocyanate compounds (x22), blocked polyisocyanate compounds (x23), polycarbodimide compounds, epoxy group-containing compounds, carboxyl group-containing compounds, hydrazide group-containing compounds, and semicarbazide group-containing compounds. In particular, from the viewpoint of the smoothness of the multilayer coating film formed, it is preferable to include at least one selected from amino resin (x21), polyisocyanate compound (x22), and blocked polyisocyanate compound (x23), and it is even more preferable to include one of either amino resin (x21) or blocked polyisocyanate compound (x23).

[0077] (Amino resin (x21)) As the above amino resin (x21), a partially methylolated amino resin or a fully methylolated amino resin obtained by the reaction of an amino component and an aldehyde component can be used. Examples of amino components include melamine, urea, benzoguanamine, acetoganamine, steroguanamine, spiloganamine, and dicyandiamide. Examples of aldehyde components include formaldehyde, paraformaldehyde, acetaldehyde, and benzaldehyde.

[0078] Furthermore, a methylolated amino resin can also be used in which the methylol group has been partially or completely etherified with a suitable alcohol. Examples of alcohols that can be used for etherification include methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, 2-ethyl-1-butanol, and 2-ethyl-1-hexanol.

[0079] As the above amino resin, melamine resin is preferred. In particular, it is preferable to include methyl etherified melamine resin, in which the methylol groups of a partially or completely methylolated melamine resin are partially or completely etherified with methyl alcohol; butyl etherified melamine resin, in which the methylol groups of a partially or completely methylolated melamine resin are partially or completely etherified with butyl alcohol; and methyl-butyl mixed etherified melamine resin, in which the methylol groups of a partially or completely methylolated melamine resin are partially or completely etherified with methyl alcohol and butyl alcohol. It is even more preferable to include methyl-butyl mixed etherified melamine resin.

[0080] From the viewpoint of the smoothness of the multilayer coating film formed, the melamine resin described above preferably has a weight-average molecular weight in the range of 400 to 6,000, more preferably in the range of 500 to 4,000, and even more preferably in the range of 600 to 3,000.

[0081] Commercially available melamine resins can be used as described above. Examples of commercially available product names include "Cymel 202", "Cymel 203", "Cymel 238", "Cymel 250", "Cymel 251", "Cymel 303", "Cymel 323", "Cymel 324", "Cymel 325", "Cymel 327", "Cymel 350", "Cymel 385", "Cymel 1156", "Cymel 1158", "Cymel 1116", "Cymel 1130" (all manufactured by Ornex Japan Co., Ltd.), "Uban 120", "Uban 20HS", "Uban 20SE60", "Uban 2021", "Uban 2028", and "Uban 28-60" (all manufactured by Mitsui Chemicals, Inc.).

[0082] If the aqueous intermediate coating paint (X) of the present invention contains the above-mentioned melamine resin, the aqueous intermediate coating paint (X) may contain, as a curing catalyst, a sulfonic acid such as p-toluenesulfonic acid, dodecylbenzenesulfonic acid, or dinonylnaphthalenesulfonic acid; a neutralized salt of the sulfonic acid and an amine; a neutralized salt of a phosphate ester compound and an amine, etc.

[0083] If the curing agent (x2) contains the amino resin (x21), the content of the amino resin (x21) is preferably in the range of 10 to 100% by mass, more preferably in the range of 30 to 90% by mass, and even more preferably in the range of 50 to 80% by mass, based on the total solid content of the curing agent (x2), from the viewpoint of the smoothness of the multilayer coating film formed.

[0084] (Polyisocyanate compound (x22)) The polyisocyanate compound (x22) is a compound having at least two isocyanate groups in one molecule, and examples include aliphatic polyisocyanates, alicyclic polyisocyanates, aromatic aliphatic polyisocyanates, aromatic polyisocyanates, and derivatives of said polyisocyanates.

[0085] Examples of the above aliphatic polyisocyanates include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate, 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, diisocyanate dimer, and methyl 2,6-diisocyanatohexanoate (common name: lysine). Examples include aliphatic diisocyanates such as diisocyanates; and aliphatic triisocyanates such as 2-isocyanatoethyl 2,6-diisocyanatohexanoate, 1,6-diisocyanato-3-isocyanatomethylhexane, 1,4,8-triisocyanatooctane, 1,6,11-triisocyanatoundecane, 1,8-diisocyanato-4-isocyanatomethyloctane, 1,3,6-triisocyanatohexane, and 2,5,7-trimethyl-1,8-diisocyanato-5-isocyanatomethyloctane.

[0086] Examples of the alicyclic polyisocyanates include 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate), 4-methyl-1,3-cyclohexylene diisocyanate (common name: hydrogenated TDI), and 2-methyl-1,3-cyclohexylene diisocyanate. Alicyclic diisocyanates such as nate, 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane (common name: hydrogenated xylylene diisocyanate) or mixtures thereof, methylenebis(4,1-cyclohexanediyl) diisocyanate (common name: hydrogenated MDI), norbornane diisocyanate; 1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane, 2-(3-isocyanatopropyl)- 2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 2-(3-isocyanatopropyl)-2,6-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 3-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, 6-(2-iso Examples include alicyclic triisocyanates such as cyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, and 6-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane.

[0087] Examples of the aforementioned aromatic aliphatic polyisocyanates include aromatic aliphatic diisocyanates such as methylenebis(4,1-phenylene) diisocyanate (common name: MDI), 1,3- or 1,4-xylylene diisocyanate or mixtures thereof, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (common name: tetramethylxylylene diisocyanate) or mixtures thereof; and aromatic aliphatic triisocyanates such as 1,3,5-triisocyanatomethylbenzene.

[0088] Examples of the aromatic polyisocyanates include aromatic diisocyanates such as m-phenylenediisocyanate, p-phenylenediisocyanate, 4,4'-diphenyldiisocyanate, 1,5-naphthalenediisocyanate, 2,4-tolylenediisocyanate (common name: 2,4-TDI) or 2,6-tolylenediisocyanate (common name: 2,6-TDI) or mixtures thereof, 4,4'-toluidinediisocyanate, and 4,4'-diphenyletherdiisocyanate; aromatic triisocyanates such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, and 2,4,6-triisocyanatotoluene; and aromatic tetraisocyanates such as 4,4'-diphenylmethane-2,2',5,5'-tetraisocyanate.

[0089] Furthermore, examples of derivatives of the polyisocyanate include dimers, trimers, biuret, allophanate, uretodione, uretoimine, isocyanurate, oxadiazinetrione, polymethylene polyphenyl polyisocyanate (crude MDI, polymeric MDI), crude TDI, and the like.

[0090] The polyisocyanates and their derivatives described above may be used individually or in combination of two or more. Among these polyisocyanates, aliphatic diisocyanates, alicyclic diisocyanates, and their derivatives are preferred.

[0091] Furthermore, as the polyisocyanate compound, a prepolymer obtained by reacting the above-mentioned polyisocyanate and its derivatives with a compound that can react with the polyisocyanate under conditions of excess isocyanate groups may be used. Examples of compounds that can react with the polyisocyanate include compounds having active hydrogen groups such as hydroxyl groups and amino groups, and specifically, for example, polyhydric alcohols, low molecular weight polyester resins, amines, water, etc. can be used.

[0092] If the curing agent (x2) contains the polyisocyanate compound (x22), the content of the polyisocyanate compound (x22) is preferably in the range of 5 to 80% by mass, more preferably in the range of 8 to 60% by mass, and even more preferably in the range of 10 to 40% by mass, based on the total solid content of the curing agent (x2), from the viewpoint of the smoothness of the multilayer coating film formed.

[0093] Furthermore, as the polyisocyanate compound, a polymer of an isocyanate group-containing polymerizable unsaturated monomer, or a copolymer of the isocyanate group-containing polymerizable unsaturated monomer and a polymerizable unsaturated monomer other than the isocyanate group-containing polymerizable unsaturated monomer may be used.

[0094] (Blocked polyisocyanate compound (x23)) The blocked polyisocyanate compound (x23) is a compound obtained by blocking the isocyanate group of the polyisocyanate compound (x22) with a blocking agent.

[0095] Examples of the above-mentioned blocking agents include phenols such as phenol, cresol, xylenol, nitrophenol, ethylphenol, hydroxydiphenyl, butylphenol, isopropylphenol, nonylphenol, octylphenol, and methyl hydroxybenzoate; lactams such as ε-caprolactam, δ-valerolactam, γ-butyrolactam, and β-propiolactam; aliphatic alcohols such as methanol, ethanol, propyl alcohol, butyl alcohol, amyl alcohol, and lauryl alcohol; ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether, and methoxymethanol; benzyl alcohol, glycolic acid, methyl glycolate, ethyl glycolate, butyl glycolate, lactic acid, methyl lactate, ethyl lactate, butyl lactate, methylolurea, methylolmelamine, diacetone alcohol, 2-hydroxyethyl acrylate, and 2-hydroxyethyl acrylate. Alcohol-based compounds such as droxyethyl methacrylate; oxime-based compounds such as formamide oxime, acetamide oxime, acetoxime, methyl ethyl ketoxime, diacetyl monooxime, benzophenone oxime, and cyclohexane oxime; active methylene-based compounds such as dimethyl malonate, diethyl malonate, ethyl acetoacetate, methyl acetoacetate, and acetylacetone; butyl mercaptan, t-butyl mercaptan, hexyl mercaptan, t-dodecyl mercaptan, 2-mercaptobenzothiazole, thiophenol, and methylthiophenone. Mercaptan-based compounds such as ethylthiophenol; acid amide-based compounds such as acetanilide, acetanisidide, acetotoluid, acrylamide, methacrylamide, acetic acid amide, stearic acid amide, and benzamide; imide-based compounds such as succinimide, phthalimide, and maleimide; amine-based compounds such as diphenylamine, phenylnaphthylamine, xylidine, N-phenylxylidine, carbazole, aniline, naphthylamine, butylamine, dibutylamine, and butylphenylamine; imidazole-based compounds such as imidazole and 2-ethylimidazole;Examples of azole compounds include urea-based compounds such as urea, thiourea, ethyleneurea, ethylenethiourea, and diphenylurea; carbamic acid ester compounds such as phenyl N-phenylcarbamate; imine-based compounds such as ethyleneimine and propyleneimine; sulfite-based compounds such as sodium bisulfite and potassium bisulfite; and azole compounds. Examples of the above-mentioned azole compounds include pyrazoles or pyrazole derivatives such as pyrazole, 3,5-dimethylpyrazole, 3-methylpyrazole, 4-benzyl-3,5-dimethylpyrazole, 4-nitro-3,5-dimethylpyrazole, 4-bromo-3,5-dimethylpyrazole, and 3-methyl-5-phenylpyrazole; imidazoles or imidazole derivatives such as imidazole, benzimidazole, 2-methylimidazole, 2-ethylimidazole, and 2-phenylimidazole; and imidazoline derivatives such as 2-methylimidazoline and 2-phenylimidazoline. Among these, preferred blocking agents include active methylene-based blocking agents, pyrazoles, or pyrazole derivatives.

[0096] When performing the blocking reaction (reacting with the blocking agent), a solvent may be added as needed. Suitable solvents for the blocking reaction are those that are not reactive with isocyanate groups. Examples include acetone, ketones such as methyl ethyl ketone, esters such as ethyl acetate, and solvents such as N-methyl-2-pyrrolidone (NMP).

[0097] Furthermore, as the blocking agent, hydroxycarboxylic acids having one or more hydroxyl groups and one or more carboxyl groups, such as hydroxypivalic acid and dimethylolpropionic acid, can also be used. In particular, a blocked polyisocyanate compound obtained by blocking the isocyanate group with the above-mentioned hydroxycarboxylic acid and then neutralizing the carboxyl group of the hydroxycarboxylic acid to impart water dispersibility can be suitably used.

[0098] If the curing agent (x2) contains a blocked polyisocyanate compound (x23), the content of the blocked polyisocyanate compound (x23) is preferably in the range of 5 to 80% by mass, more preferably in the range of 8 to 60% by mass, and even more preferably in the range of 10 to 40% by mass, based on the total solid content of the curing agent (x2), from the viewpoint of the smoothness of the multilayer coating film formed.

[0099] The content of the above-mentioned curing agent (x2) is preferably in the range of 10 to 80% by mass, more preferably in the range of 15 to 75% by mass, and even more preferably in the range of 20 to 70% by mass, based on the total resin solid content in the water-based intermediate coating paint (X), from the viewpoint of the smoothness of the multilayer coating film that is formed.

[0100] <<Pigment (x3)>> The water-based intermediate coating paint (X) of the present invention contains a pigment (x3). Furthermore, it is preferable that the water-based intermediate coating (X) of the present invention can conceal the color of the underlying coating when applied with a dry film thickness of 15 μm.

[0101] Examples of the aforementioned pigment (x3) include coloring pigments (x31), extender pigments (x32), and luminescence pigments (x33). The pigments (x3) can be used alone or in combination of two or more types.

[0102] (Coloring pigment (x31)) Examples of the above-mentioned coloring pigment (x31) include titanium dioxide, zinc oxide, carbon black, molybdenum red, Prussian blue, cobalt blue, azo pigments, phthalocyanine pigments, quinacridone pigments, isoindoline pigments, surene pigments, perylene pigments, dioxazine pigments, and diketopyrrolopyrrole pigments. Among these, titanium dioxide and / or carbon black can be preferably used.

[0103] If the aqueous intermediate coating paint (X) of the present invention contains the above-mentioned coloring pigment (x31), the content of the coloring pigment (x31) is preferably in the range of 1 to 180 parts by mass, more preferably in the range of 5 to 140 parts by mass, and even more preferably in the range of 10 to 120 parts by mass, based on 100 parts by mass of the total resin solids in the aqueous intermediate coating paint (X), from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0104] (Extender pigment (x32)) Furthermore, examples of the extender pigment (x32) include clay, kaolin, barium sulfate, barium carbonate, calcium carbonate, talc, silica, alumina white, and among these, barium sulfate and / or talc can be preferably used.

[0105] In particular, it is preferable that the aqueous intermediate coating paint (X) of the present invention contains barium sulfate with an average primary particle diameter of 1 μm or less, and more preferably barium sulfate with an average primary particle diameter in the range of 0.01 to 0.8 μm, as the extender pigment (x32), because this allows for the acquisition of a multi-layer coating film with excellent smoothness.

[0106] In this invention, the average primary particle diameter of barium sulfate is the average of the maximum diameters of 20 barium sulfate particles that lie on a randomly drawn straight line on an electron microscope image, obtained by observing barium sulfate with a scanning electron microscope.

[0107] If the aqueous intermediate coating paint (X) of the present invention contains the extender pigment (x32), the content of the extender pigment (x32) is preferably in the range of 1 to 150 parts by mass, more preferably in the range of 5 to 130 parts by mass, and even more preferably in the range of 10 to 110 parts by mass, based on 100 parts by mass of the total resin solids in the aqueous intermediate coating paint (X), from the viewpoint of the smoothness of the multilayer coating film formed.

[0108] (Luminous pigment (x33)) Furthermore, examples of the luminous pigment (x33) include aluminum (including vapor-deposited aluminum), copper, zinc, brass, nickel, glass flakes, aluminum oxide, mica, aluminum oxide coated with titanium oxide and / or iron oxide, mica coated with titanium oxide and / or iron oxide, etc. Among these, it is preferable to use an aluminum pigment. There are non-leafing aluminum pigments and leafing aluminum pigments, and either can be used. Among these, it is preferable to use a non-leafing aluminum pigment.

[0109] The above-mentioned lustrous pigment (x33) is preferably in the form of flakes. Furthermore, the lustrous pigment (x33) is suitable if its longitudinal dimension is within the range of 1 to 100 μm, particularly 5 to 40 μm, and its thickness is within the range of 0.001 to 5 μm, particularly 0.01 to 2 μm.

[0110] If the aqueous intermediate coating paint (X) of the present invention contains the above-mentioned luminous pigment (x33), the content of the luminous pigment (x33) is preferably in the range of 1 to 100 parts by mass, more preferably in the range of 2 to 60 parts by mass, and even more preferably in the range of 3 to 40 parts by mass, based on 100 parts by mass of the total resin solids in the aqueous intermediate coating paint (X), from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0111] Furthermore, in the present invention, the content of the pigment (x3) is preferably in the range of 1 to 180 parts by mass, more preferably in the range of 5 to 170 parts by mass, and even more preferably in the range of 10 to 160 parts by mass, based on 100 parts by mass of the total resin solids in the water-based intermediate coating paint (X), from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0112] <<Viscosity modifier (x4)>> The viscosity modifier (x4) of the present invention comprises an association-type viscosity modifier (x41) and an alkali-swelling type viscosity modifier (x42) different from the association-type viscosity modifier (x41). The association-type viscosity modifier (x41) exhibits viscosity through network formation by hydrophobic association, which can improve the smoothness of multi-layer coating films. However, the association is easily inhibited by plasticizers contained in the sealer. Through our research, we have found that by using the alkali-swelling type viscosity modifier (x42) in combination with the association-type viscosity modifier (x41), it is possible to maintain sealer opacity while suppressing the influence of plasticizers and providing smoothness.

[0113] (Associative viscosity modifier (x41)) The above-mentioned associated viscosity modifier (x41) is a (meth)acrylic acid copolymer-based thickener having hydrophobic groups with 8 to 36 carbon atoms.

[0114] Examples of the above-mentioned associated viscosity modifier (x41) include polyacrylate salts and polyacrylic acid-(meth)acrylic acid ester copolymers.

[0115] The above-mentioned aggregate viscosity modifier (x41) preferably has an acid value in the range of 30 to 350 mgKOH / g, more preferably in the range of 80 to 320 mgKOH / g, and even more preferably in the range of 80 to 300 mgKOH / g, from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0116] The above-mentioned association-type viscosity modifier (x41) is: (x411)(meth)acrylic acid or a salt thereof, (x412) A polymerizable unsaturated monomer represented by the following general formula (1), (x413) alkyl(meth)acrylates having 1 to 4 carbon atoms in the alkyl group, and (x414) A polymerizable unsaturated monomer having two or more polymerizable unsaturated groups in one molecule. It is preferable that the copolymer is obtained by copolymerizing a polymerizable unsaturated monomer mixture containing the above.

[0117] [ka]

[0118] (In formula (1), R 1 R is a hydrogen atom or a methyl group. 2 and R 3 Each of these independently consists of a hydrogen atom, a methyl group or an ethyl group, and R 4 (where n represents a hydrocarbon group with 8 to 36 carbon atoms, and n is an integer between 3 and 100.)

[0119] [(meth)acrylic acid (salt) (x411)] The above (meth)acrylic acid (salt) (x411) means acrylic acid, methacrylic acid, acrylate salt, or methacrylate salt. Salts include alkali metal salts such as sodium salts, potassium salts, and lithium salts; alkaline earth metal salts such as magnesium salts and calcium salts; ammonium salts; alkanolamine salts such as monoethanolamine salts, diethanolamine salts, and triethanolamine salts; and alkylamine salts with 1 to 4 carbon atoms such as methylamine salts, ethylamine salts, propylamine salts, and butylamine salts.

[0120] The above (meth)acrylic acid (salt) is preferably acrylic acid, methacrylic acid, ammonium methacrylate salt, and tertiary amine salts such as dimethylethanolamine salt and triethanolamine salt, more preferably acrylic acid and methacrylic acid, and even more preferably methacrylic acid.

[0121] [Polymerizable unsaturated monomer (x412)] The polymerizable unsaturated monomer (x412) is a polymerizable unsaturated monomer represented by the following general formula (1).

[0122] [ka]

[0123] (In formula (1), R 1 R is a hydrogen atom or a methyl group. 2 and R3 is each independently a hydrogen atom, a methyl group or an ethyl group, R 4 represents a hydrocarbon group having 8 to 36 carbon atoms, and n represents an integer of 3 to 100. C is a carbon atom, H is a hydrogen atom, and O is an oxygen atom.)

[0124] The above R 1 is a hydrogen atom or a methyl group, and is preferably a methyl group.)

[0125] Also, the above R 2 and R 3 each independently represent a hydrogen atom, a methyl group or an ethyl group, and are each independently preferably a hydrogen atom or a methyl group, and more preferably a hydrogen atom, from the viewpoints of smoothness of the formed multilayer coating film and sealer hiding property, etc.)

[0126] Also, the above R 4 is a hydrocarbon group having 8 to 36 carbon atoms, preferably an alkyl group or an alkenyl group having 12 to 32 carbon atoms, and includes, for example, a linear alkyl group, a branched alkyl group, a linear alkenyl group, a branched alkenyl group, etc.)

[0127] Examples of the above linear alkyl group include n-octyl group, n-nonyl group, n-decyl group, n-undecyl group, n-dodecyl group, n-tridecyl group, n-tetradecyl group, n-pentadecyl group, n-hexadecyl group, n-heptadecyl group, n-octadecyl group, n-nonadecyl group, n-eicosyl group, n-henicosyl group, n-docosyl group, etc. Examples of the branched alkyl group include 2-ethylhexyl group, isodecyl group, isotridecyl group, isostearyl group, 2-butyloctyl group, 2-(3-methylbutyl)-1,6-dimethylhexyl group, 2-pentylnonyl group, 2-hexyldecyl group, 2-heptylundecyl group, 3-(1,3,3-trimethylbutyl)-5,7,7-trimethyloctyl group, 2-octyldodecyl group, 2-nonyltridecyl group, etc.)

[0128] Examples of the linear alkenyl group include n-octenyl group, n-decenyl group, n-undecenyl group, n-dodecenyl group, n-tridecenyl group, n-tetradecenyl group, n-pentadecenyl group, n-hexadecenyl group, n-heptadecenyl group, and n-octadecenyl group. Examples of the branched alkenyl group include isooctenyl group, isodecenyl group, isoundecenyl group, indodecenyl group, isotridecenyl group, isotetradecenyl group, isopentadecenyl group, isohexadecenyl group, isoheptadecenyl group, and isooctadecenyl group.

[0129] Furthermore, n is an integer between 3 and 100, and in particular, from the viewpoint of the smoothness of the formed multilayer coating film and the sealer opacity, it is preferably an integer between 10 and 80, and more preferably an integer between 20 and 60.

[0130] Examples of the polymerizable unsaturated monomers (x412) mentioned above include (meth)acrylates of 3 to 60 moles of n-docosanolethylene oxide adducts, (meth)acrylates of 3 to 60 moles of n-heneisanolethylene oxide adducts, (meth)acrylates of 3 to 60 moles of n-eicosanolethylene oxide adducts, (meth)acrylates of 3 to 60 moles of n-nonadecanolethylene oxide adducts, (meth)acrylates of 3 to 60 moles of n-octadecanolethylene oxide adducts, (meth)acrylates of 3 to 60 moles of n-heptadecanolethylene oxide adducts, and (meth)acrylates of 3 to 60 moles of n-hexadecanolethylene oxide adducts.

[0131] In particular, (meth)acrylates of 3 to 60 moles of n-docosanolethylene oxide adduct, (meth)acrylates of 3 to 60 moles of n-octadecanolethylene oxide adduct, and (meth)acrylates of 3 to 60 moles of n-hexadecanolethylene oxide adduct are suitable for use.

[0132] [Alkyl (meth)acrylate (x413) with alkyl group having 1 to 4 carbon atoms] Examples of alkyl(meth)acrylates (x413) having 1 to 4 carbon atoms in the alkyl group include methyl(meth)acrylate, ethyl(meth)acrylate, n-propyl(meth)acrylate, iso-propyl(meth)acrylate, n-butyl(meth)acrylate, iso-butyl(meth)acrylate, tert-butyl(meth)acrylate, and the like.

[0133] In particular, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, and especially methyl (meth)acrylate, ethyl (meth)acrylate, and n-propyl (meth)acrylate can be preferably used from the viewpoint of the smoothness of the formed multilayer coating film and the ability to conceal the sealer.

[0134] [Polymerizable unsaturated monomers (x414) having two or more polymerizable unsaturated groups in one molecule] The polymerizable unsaturated monomer (x414) is not particularly limited as long as it can copolymerize with the (meth)acrylic acid (salt) (x411), polymerizable unsaturated monomer (x412), and alkyl (meth)acrylate (x413), and has two or more polymerizable unsaturated groups in one molecule. For example, a bifunctional polymerizable unsaturated monomer (x4141) having two polymerizable unsaturated groups in one molecule, a trifunctional polymerizable unsaturated monomer (x4142) having three polymerizable unsaturated groups in one molecule, or a 4-8 functional polymerizable unsaturated monomer (x4143) having four to eight polymerizable unsaturated groups in one molecule can be used.

[0135] 《Bifunctional polymerizable unsaturated monomer (x4141)》 As the above-mentioned bifunctional polymerizable unsaturated monomer (x4141), for example, di(meth)acrylates of polyols, di(meth)acrylates of polyol alkylene oxide adducts, etc., can be used.

[0136] Examples of the above polyols include ethylene glycol, propylene glycol, 1,2-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,9-nonanediol, neopentyl glycol, 3-methyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-diol, tricyclodecanedimethylol, cyclohexanediol, cyclohexanedimethylol, hydroquinone, bisphenol A, bisphenol F, hydrogenated bisphenol A, and hydrogenated bisphenol Examples include F, pentaerythritol, trimethylolpropane, and glycerin, among which 1,4-butanediol, 1,6-hexanediol, 1,9-nonanediol, neopentyl glycol, 3-methyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-diol, tricyclodecanedimethylol, cyclohexanediol, cyclohexanedimethylol, hydrogenated bisphenol A, hydrogenated bisphenol F, pentaerythritol, trimethylolpropane, and glycerin can be preferably used.

[0137] The polyol alkylene oxide adduct is not particularly limited as long as it is an alkylene oxide adduct of the polyol, but it is preferable that the number of moles of alkylene oxide added is in the range of 2 to 100 moles, preferably 4 to 80 moles, and more preferably 6 to 60 moles, per mole of polyol.

[0138] As the alkylene oxide mentioned above, alkylene oxides having 2 to 4 carbon atoms can be suitably used. Examples of such alkylene oxides having 2 to 4 carbon atoms include ethylene oxide, propylene oxide, butylene oxide, and tetrahydrofuran. Among these, ethylene oxide is preferred.

[0139] Furthermore, the alkylene oxides can be used individually or in combination of two or more. When two or more alkylene oxides are used, the arrangement order of the alkylene oxides may be block, random, or a mixture thereof. When two or more alkylene oxides are used, it is preferable that the alkylene oxides contain ethylene oxide, and more preferably the ethylene oxide content is in the range of 30 to 100 mol%, more preferably 50 to 100 mol%, and even more preferably 70 to 100 mol%, based on the total amount of alkylene oxides.

[0140] 《Trifunctional polymerizable unsaturated monomer (x4142)》 As the aforementioned trifunctional polymerizable unsaturated monomer (x4142), for example, tri(meth)acrylates of polyols having three or more hydroxyl groups in one molecule, and tri(meth)acrylates of polyol alkylene oxide adducts can be used.

[0141] Examples of polyols having three or more hydroxyl groups in one molecule include pentaerythritol, trimethylolpropane, glycerin, dipentaerythritol, tripentaerythritol, diglycerin, triglycerin, ditrimethylolpropane, tritrimethylolpropane, and cyclohexanetetraol.

[0142] Furthermore, the number of moles of alkylene oxide added to the tri(meth)acrylate of the polyol alkylene oxide adduct is preferably in the range of 3 to 200 mol, more preferably in the range of 6 to 150 mol, and even more preferably in the range of 9 to 100 mol per mol of polyol.

[0143] As the alkylene oxide mentioned above, alkylene oxides having 2 to 4 carbon atoms can be suitably used. Examples of such alkylene oxides having 2 to 4 carbon atoms include ethylene oxide, propylene oxide, butylene oxide, and tetrahydrofuran. Among these, ethylene oxide is preferred.

[0144] Furthermore, the alkylene oxides can be used individually or in combination of two or more. When two or more alkylene oxides are used, the arrangement order of the alkylene oxides may be block, random, or a mixture thereof. When two or more alkylene oxides are used, it is preferable that the alkylene oxides contain ethylene oxide, and more preferably the ethylene oxide content is in the range of 30 to 100 mol%, more preferably 50 to 100 mol%, and even more preferably 70 to 100 mol%, based on the total amount of alkylene oxides.

[0145] 《4-8 Functionally Polymerizable Unsaturated Monomer (x4143)》 As the 4-8 functionally polymerizable unsaturated monomer (x4143), for example, tetra(meth)acrylate, penta(meth)acrylate, hexa(meth)acrylate, hepta(meth)acrylate, octa(meth)acrylate of polyols having 4 or more hydroxyl groups in one molecule, and tetra(meth)acrylate, penta(meth)acrylate, hexa(meth)acrylate, hepta(meth)acrylate, octa(meth)acrylate of polyol alkylene oxide adducts, etc., can be used.

[0146] Examples of polyols having four or more hydroxyl groups in one molecule include pentaerythritol, dipentaerythritol, tripentaerythritol, tetrapentaerythritol, hexapentaerythritol, diglycerin, triglycerin, ditrimethylolpropane, tritrimethylolpropane, cyclohexanetetraol, sorbitan, and the like.

[0147] Furthermore, the number of moles of alkylene oxide added to the polyol alkylene oxide adducts tetra(meth)acrylate, penta(meth)acrylate, hexa(meth)acrylate, hepta(meth)acrylate, and octa(meth)acrylate is preferably in the range of 3 to 200 mol, more preferably in the range of 6 to 150 mol, and even more preferably in the range of 9 to 100 mol per mole of polyol.

[0148] As the alkylene oxide mentioned above, alkylene oxides having 2 to 4 carbon atoms can be suitably used. Examples of such alkylene oxides having 2 to 4 carbon atoms include ethylene oxide, propylene oxide, butylene oxide, and tetrahydrofuran. Among these, ethylene oxide is preferred.

[0149] Furthermore, the alkylene oxides can be used individually or in combination of two or more. When two or more alkylene oxides are used, the arrangement order of the alkylene oxides may be block, random, or a mixture thereof. When two or more alkylene oxides are used, it is preferable that the alkylene oxides contain ethylene oxide, and more preferably the ethylene oxide content is in the range of 30 to 100 mol%, more preferably 50 to 100 mol%, and even more preferably 70 to 100 mol%, based on the total amount of alkylene oxides.

[0150] Other polymerizable unsaturated monomers (x415) The aforementioned associated viscosity modifier (x41) also includes a copolymer of polymerizable unsaturated monomer mixtures containing (x411) to (x414) above, as well as other polymerizable unsaturated monomers (x415).

[0151] The above-mentioned other polymerizable unsaturated monomers (x415) are not particularly limited as long as they can copolymerize with the polymerizable unsaturated monomers (x411) to (x414). However, from the viewpoint of the smoothness and sealer opacity of the formed multilayer coating film, ethylenically unsaturated carboxylic acids (salts), polyoxyalkylene (2 to 100 moles of alkylene oxide added) (meth)acrylic acid monoesters, alkoxy polyalkylene glycols (2 to 100 moles of alkylene oxide added) (meth)acrylic acid esters, hydroxyalkyl (meth)acrylates, and amide group-containing polymerizable unsaturated monomers are preferred, preferably crotonic acid, isocrotonic acid, itaconic acid, maleic acid, and maleic anhydride. Fumaric acid, polyoxyethylene (ethylene oxide addition moles 2 to 100) mono(meth)acrylate, (meth)acrylate of aliphatic alcohol ethylene oxide adducts with 1 to 6 carbon atoms (ethylene oxide addition moles 2 to 100), hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, dihydroxyethyl (meth)acrylate, and (meth)acrylamide can be suitably used. More preferably, maleic acid, fumaric acid, polyoxyethylene (ethylene oxide addition moles 2 to 100) mono(meth)acrylate, 2-hydroxyethyl (meth)acrylate, dihydroxyethyl (meth)acrylate, and (meth)acrylamide can be suitably used.

[0152] The polymerizable unsaturated monomers (x411) to (x415) described above can each be used individually or in combination of two or more.

[0153] The associated viscosity modifier (x41) is obtained by copolymerizing a polymerizable unsaturated monomer mixture containing the polymerizable unsaturated monomers (x411) to (x414) and optionally (x415). The proportion of polymerizable unsaturated monomers (x411) to (x415) used in copolymerization can be within the following range, based on the total mass of polymerizable unsaturated monomers, i.e., the total mass of (x411) to (x415). The blending ratio of (meth)acrylic acid (salt) (x411) is preferably in the range of 1 to 50% by mass, more preferably in the range of 1 to 45% by mass, and even more preferably in the range of 7 to 40% by mass. The blending ratio of polymerizable unsaturated monomer (x412) is preferably in the range of 5 to 60% by mass, more preferably in the range of 10 to 55% by mass, and even more preferably in the range of 20 to 50% by mass. The blending ratio of alkyl (meth)acrylate (x413) having 1 to 4 carbon atoms in the alkyl group is preferably in the range of 5 to 80% by mass, more preferably in the range of 7 to 75% by mass, and even more preferably in the range of 10 to 70% by mass. The blending ratio of polymerizable unsaturated monomers (x414) having two or more polymerizable unsaturated groups in one molecule is preferably in the range of 0.05 to 5% by mass, more preferably in the range of 0.07 to 4% by mass, and even more preferably in the range of 0.1 to 3% by mass. The blending ratio of other polymerizable unsaturated monomers (x415) is preferably in the range of 0 to 20% by mass, more preferably in the range of 0 to 15% by mass, and even more preferably in the range of 0 to 10% by mass.

[0154] When the blending ratio of polymerizable unsaturated monomers (x411) to (x415) is within the above range, the smoothness and sealer opacity of the multi-layer coating film are good.

[0155] Copolymerization of the above polymerizable unsaturated monomers (x411) to (x415) can be carried out by conventionally known methods such as emulsion polymerization, solution polymerization, suspension polymerization, and bulk polymerization. Among these, emulsion polymerization and solution polymerization are preferred, and solution polymerization is preferable.

[0156] Furthermore, in the present invention, the amount of the aggregate viscosity modifier (x41) is preferably in the range of 0.05 to 0.7 parts by mass, more preferably in the range of 0.1 to 0.6 parts by mass, and even more preferably in the range of 0.2 to 0.5 parts by mass, based on 100 parts by mass of the total resin solids in the water-based intermediate coating paint (X), from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0157] (Alkali-swelling viscosity modifier (x42)) The aforementioned alkali-swelling viscosity modifier (x42) is generally a vinyl polymer emulsion containing acidic groups, which dissolves or swells in water upon neutralization with an alkaline substance, thereby imparting shear-reducing viscosity and exhibiting thickening properties. The alkali swelling type viscosity modifier (x42) of the present invention is a viscosity modifier different from the association type viscosity modifier (x41) and does not contain hydrophobic groups with 8 to 36 carbon atoms.

[0158] In the water-based intermediate coating paint (X) of the present invention, by including the above-mentioned alkali swelling type viscosity modifier (x42), the influence of the plasticizer contained in the sealer (S) can be reduced.

[0159] The vinyl polymer emulsion is a low-viscosity liquid in an acidic region, and by neutralizing it with ammonia, an amine compound, or an inorganic base, the vinyl polymer emulsion swells and is given thicker properties. The vinyl polymer in the aforementioned vinyl polymer emulsion may be crosslinked, uncrosslinked, or partially crosslinked.

[0160] Examples of the alkali swelling type viscosity modifier (x42) include Primal ASE-60, Primal ASE-75, Primal ASE-95NP (all manufactured by Rodau Chemicals, trade names, acrylic emulsions), Viscarex HV-30, Viscarex HM, Viscarex VS, Viscarex VG2 (all manufactured by Hoechst Synthesizer, trade names, acrylic emulsions), SN Thickener 618, SN Thickener 630, SN Thickener 634, SN Thickener 636, SN Thickener 650 (all manufactured by Sunopco, trade names, acrylic), etc.

[0161] Furthermore, in the present invention, the alkali swelling type viscosity modifier (x42) is preferably in the range of 0.02 to 0.3 parts by mass, more preferably in the range of 0.03 to 0.2 parts by mass, and even more preferably in the range of 0.05 to 0.1 parts by mass, based on 100 parts by mass of the total resin solids in the water-based intermediate coating paint (X), from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0162] The solid content mass ratio ((x41) / (x42)) of the association-type viscosity modifier (x41) and the alkali-swelling-type viscosity modifier (x42) is within the range of 97 / 3 to 70 / 30. The solid content mass ratio of the association-type viscosity modifier (x41) to the alkali-swelling-type viscosity modifier (x42) is preferably in the range of 95 / 5 to 73 / 27, more preferably in the range of 92 / 8 to 77 / 23, and even more preferably in the range of 90 / 10 to 80 / 20, from the viewpoint of the smoothness of the formed multilayer coating film and the ability to conceal the sealer.

[0163] The water-based intermediate coating paint (X) of the present invention may also use known viscosity modifiers other than association-type viscosity modifiers (x41) and alkali-swelling-type viscosity modifiers (x42) (hereinafter also referred to as "other viscosity modifiers (x43)") as necessary.

[0164] Examples of the above-mentioned other viscosity modifiers (x43) include inorganic thickeners such as silicates, metal silicates, montmorillonite, and colloidal alumina; protein-based thickeners such as casein, sodium caseate, and ammonium caseate; alginate-based thickeners such as sodium alginate; polyether-based thickeners such as pluronic polyether, polyether dialkyl ester, polyether dialkyl ether, and polyether epoxy modified products; maleic anhydride copolymer-based thickeners such as partial esters of vinyl methyl ether-maleic anhydride copolymer; polyamide-based thickeners such as polyamide amine salts; and cellulose derivative-based viscosity modifiers such as carboxymethylcellulose, methylcellulose, and hydroxyethylcellulose.

[0165] These other viscosity modifiers (x43) can be used individually or in combination of two or more.

[0166] <<Other ingredients>> The aqueous intermediate coating paint (X) of the present invention may further contain, if necessary, resins other than the hydroxyl group-containing resin (x1) and curing agent (x2), organic solvents, curing catalysts, dispersants, anti-settling agents, defoaming agents, ultraviolet absorbers, light stabilizers, surface modifiers, etc.

[0167] Examples of resins other than the hydroxyl group-containing resin (x1) and curing agent (x2) mentioned above include polyurethane resins that do not contain hydroxyl groups, acrylic resins that do not contain hydroxyl groups, polyester resins that do not contain hydroxyl groups, acrylic-modified polyester resins that do not contain hydroxyl groups, acrylic-modified polyurethane resins that do not contain hydroxyl groups, polyether resins that do not contain hydroxyl groups, polycarbonate resins that do not contain hydroxyl groups, epoxy resins that do not contain hydroxyl groups, alkyd resins that do not contain hydroxyl groups, and polyolefin resins that do not contain hydroxyl groups. In particular, it is preferable that the water-based intermediate coating paint (X) of the present invention contains a polyurethane resin that does not contain hydroxyl groups as at least one of the resins other than the hydroxyl group-containing resin (x1) and curing agent (x2).

[0168] If the water-based intermediate coating paint (X) of the present invention contains a polyurethane resin that does not contain hydroxyl groups, the content of the polyurethane resin that does not contain hydroxyl groups is preferably in the range of 2 to 40% by mass, more preferably in the range of 3 to 20% by mass, and even more preferably in the range of 5 to 15% by mass, based on the total resin solid content in the water-based intermediate coating paint (X), from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0169] Examples of the aforementioned organic solvents include ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ester solvents such as ethyl acetate, butyl acetate, methyl benzoate, ethyl ethoxypropionate, ethyl propionate, and methyl propionate; alcohol solvents such as isopropanol, n-butanol, isobutanol, and 2-ethylhexanol; ether solvents such as tetrahydrofuran, dioxane, and dimethoxyethane; glycol ether solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, and 3-methoxybutyl acetate; aromatic hydrocarbon solvents, aliphatic hydrocarbon solvents, and the like.

[0170] The water-based intermediate coating paint (X) of the present invention can be applied by diluting it with water and / or an organic solvent as needed and adjusting it to an appropriate viscosity.

[0171] The appropriate viscosity varies depending on the paint composition, but for example, the viscosity measured at 1 minute at 6 rpm using a B-type viscometer at a temperature of 20°C (sometimes referred to as the "B6 value" in this specification) is preferably in the range of 100 to 3000 mPa·s, more preferably in the range of 300 to 2000 mPa·s, and even more preferably in the range of 500 to 1500 mPa·s, from the viewpoint of the smoothness of the formed multi-layer coating film and the opacity of the sealer. The viscometer used in this case is the "LVDV-I" (product name, manufactured by Brookfield, a B-type viscometer).

[0172] In the present invention, "water-based paint" is a term used in contrast to "organic solvent-based paint," and generally refers to a paint in which a film-forming resin, pigment, etc., is dispersed and / or dissolved in water or a water-based medium (water-based medium).

[0173] The water content of the aqueous intermediate coating paint (X) of the present invention is preferably in the range of 50 to 90% by mass, more preferably in the range of 55 to 85% by mass, and most preferably in the range of 60 to 80% by mass, based on the total amount of solvent, from the viewpoint of the smoothness of the formed multilayer coating film and the opacity of the sealer.

[0174] Furthermore, the paint solid content concentration of the aqueous intermediate coating paint (X) of the present invention is typically about 5 to 70% by mass, preferably about 10 to 60% by mass.

[0175] The water-based intermediate coating paint (X) of the present invention may be either a one-component paint or a multi-component paint, but it is preferable to be a one-component paint from the viewpoint of having no paint mixing step and having excellent productivity, and simplifying the maintenance of painting machinery.

[0176] The above-mentioned water-based intermediate coating paint (X) can be applied as needed by known methods such as rotary atomization, air spraying, or airless spraying, after being adjusted to a viscosity suitable for painting.

[0177] The thickness of the intermediate coating film formed from the above-mentioned water-based intermediate coating paint (X) is less than 20 μm based on the dry thickness, and is preferably in the range of 7 to 19 μm, more preferably in the range of 10 to 18 μm, and even more preferably in the range of 11 to 17 μm, from the viewpoint of the smoothness of the formed multi-layer coating film and the sealer opacity.

[0178] The intermediate coating film obtained by applying the water-based intermediate coating paint (X) can typically be cured at a temperature of approximately 120°C to 180°C.

[0179] <Process (3)> According to the method of the present invention, next, an aqueous base coat paint (Y) is applied to the uncured intermediate coating film formed in step (2) without preheating and heat curing, thereby forming an uncured base coat film.

[0180] <Water-based base coat paint (Y)> Any known thermosetting water-based basecoat paint (Y) can be used. For example, a paint can be made by dissolving or dispersing a resin component consisting of a base resin such as acrylic resin, polyester resin, alkyd resin, urethane resin, or epoxy resin having crosslinkable functional groups such as carboxyl groups or hydroxyl groups, and a curing agent such as a polyisocyanate compound, melamine resin, or urea resin, which may be blocked, in water together with a pigment and other additives.

[0181] Examples of the above-mentioned pigments include those similar to those of pigment (x3).

[0182] Furthermore, the paint solid content concentration of the aqueous base coat paint (Y) is usually about 5 to 60% by mass, preferably about 10 to 50% by mass.

[0183] The above-mentioned water-based basecoat paint (Y) may be either a one-component paint or a multi-component paint, but a one-component paint is preferred from the viewpoint of having no paint mixing process and having excellent productivity, as well as simplifying the maintenance of painting machinery.

[0184] The above-mentioned water-based base coat paint (Y) can be applied as needed by known methods such as rotary atomization, air spraying, or airless spraying, after being adjusted to a viscosity suitable for painting.

[0185] The thickness of the base coat film formed from the above-mentioned aqueous base coat paint (Y) is preferably in the range of 10 to 25 μm, more preferably in the range of 10 to 20 μm, and even more preferably in the range of 13 to 17 μm, based on the dry thickness, from the viewpoint of the smoothness of the formed multi-layer coating and the sealer opacity.

[0186] The base coat film obtained by applying the above-mentioned water-based base coat paint (Y) can typically be cured at a temperature of approximately 120°C to 180°C.

[0187] <Process (4)> According to the method of the present invention, the uncured sealer film, uncured intermediate coating film, and uncured base coat film formed in steps (1) to (3) are then preheated. Preheating increases the solid content concentration in each film, thereby suppressing the generation of bubbles during the curing of the coating film as described later, and resulting in a good finished appearance.

[0188] The temperature of the preheating process described above is preferably in the range of 40 to 100°C, more preferably in the range of 50 to 90°C, and even more preferably in the range of 60 to 80°C. The duration of the preheating is preferably in the range of 30 seconds to 15 minutes, more preferably in the range of 1 to 10 minutes, and even more preferably in the range of 2 to 5 minutes.

[0189] <Process (5)> According to the method of the present invention, a clear coating (Z) is then applied to the preheated base coat film to form an uncured clear coating film.

[0190] <Clear Coating (Z)> Any thermosetting clear coating composition known for use in painting automobile bodies and the like can be used as the clear coating (Z). Examples of such thermosetting clear coating compositions include organic solvent-type thermosetting coating compositions containing a base resin having crosslinkable functional groups and a curing agent, aqueous thermosetting coating compositions, and powder thermosetting coating compositions. Among these, organic solvent-type thermosetting coating compositions containing a base resin having crosslinkable functional groups and a crosslinking agent are preferred from the viewpoint of the smoothness of the resulting multilayer coating film.

[0191] Examples of crosslinkable functional groups in the above-mentioned base resin include carboxyl groups, hydroxyl groups, epoxy groups, and alkoxysilyl groups. Examples of base resin types include acrylic resins, polyester resins, alkyd resins, urethane resins, epoxy resins, and fluororesins. Examples of curing agents include polyisocyanate compounds, blocked polyisocyanate compounds, melamine resins, urea resins, carboxyl group-containing compounds, carboxyl group-containing resins, epoxy group-containing resins, and epoxy group-containing compounds.

[0192] Furthermore, the paint solid content concentration of the clear coating (Z) is typically around 20-70% by mass, preferably around 30-60% by mass.

[0193] The clear coating (Z) described above may be either a one-component coating or a multi-component coating, but a one-component coating is preferred from the viewpoint of having no coating mixing process, which is superior in terms of productivity, and which simplifies the maintenance of the coating machine.

[0194] The above clear coating (Z) can be applied as needed by known methods such as rotary atomization, air spraying, or airless spraying, after being adjusted to a viscosity suitable for painting.

[0195] The thickness of the clear coating film formed from the above clear coating (Z) is preferably in the range of 10 to 60 μm, more preferably in the range of 15 to 55 μm, and even more preferably in the range of 20 to 50 μm, based on the dry thickness, from the viewpoint of the smoothness of the formed multi-layer coating film.

[0196] The clear coating obtained by applying the above clear paint (Z) can usually be cured at a temperature of approximately 120°C to 180°C.

[0197] <Process (6)> According to the method of the present invention, a multilayer coating consisting of a preheated sealer coating, an intermediate coating, a base coat coating, and an uncured clear coating is then heat-cured to form a cured multilayer coating.

[0198] The heating in this baking process can be carried out by conventional coating heating methods, such as hot air heating, infrared heating, or high-frequency heating. The heating temperature is preferably in the range of 60 to 180°C, more preferably in the range of 110 to 170°C, and even more preferably in the range of 130 to 160°C. The heating time is not particularly limited, but is preferably in the range of 10 to 90 minutes, more preferably in the range of 15 to 60 minutes, and even more preferably in the range of 15 to 30 minutes.

[0199] Preheating may be performed as appropriate before heat curing. The preheating temperature is preferably in the range of 40 to 100°C, and more preferably in the range of 50 to 90°C. The preheating time is preferably in the range of 30 seconds to 15 minutes, and more preferably in the range of 1 to 10 minutes.

[0200] This heating process allows for the curing of a multi-layer coating consisting of a sealer film, an intermediate coating film, a base coat film, and a clear coating film.

[0201] Furthermore, from the viewpoint of the smoothness of the formed multilayer coating, the film thickness of the multilayer coating obtained by the method of the present invention is preferably in the range of 45 to 95 μm, and more preferably in the range of 50 to 90 μm, based on the dry coating film. [Examples]

[0202] The present invention will be described in more detail below with reference to examples and comparative examples. However, the present invention is not limited to these examples. Note that "parts" and "%" are all based on mass. The film thickness of the coating is based on the cured coating.

[0203] <Manufacturing of hydroxyl group-containing acrylic resin (x11)> Manufacturing Example 1 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropper, 30 parts of propylene glycol monopropyl ether were charged and the temperature was raised to 85°C. Then, a mixture of 6 parts styrene, 30 parts methyl methacrylate, 25 parts n-butyl acrylate, 20 parts 2-ethylhexyl acrylate, 13 parts 4-hydroxybutyl acrylate, 6 parts acrylic acid, 10 parts propylene glycol monopropyl ether, and 2 parts 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the flask over 4 hours, and the mixture was aged for 1 hour after the dropwise addition was complete. Subsequently, a mixture of 5 parts propylene glycol monopropyl ether and 1 part 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the flask over 1 hour, and the mixture was aged for 1 hour after the dropwise addition was complete. Furthermore, 7.4 parts of 2-(dimethylamino)ethanol were added to neutralize the mixture, and deionized water was gradually added to obtain an aqueous dispersion of hydroxyl-containing acrylic resin (x11-1) with a solid content of 40%. The obtained hydroxyl-containing acrylic resin (x11-1) had an acid value of 47 mgKOH / g, a hydroxyl value of 51 mgKOH / g, and a weight-average molecular weight of 50,000.

[0204] Manufacturing Example 2 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and dropper, 82 parts of deionized water and 1.0 part of Adeka Soap SR-1025 (trade name, manufactured by Adeka, emulsifier, 25% active ingredient) were charged and stirred under a nitrogen atmosphere, and the temperature was raised to 75°C. Next, 3% of the total amount of the monomer and initiator emulsion (Note 1) and 10 parts of a 0.5% ammonium persulfate aqueous solution were introduced into the reaction vessel and maintained at 75°C for 2 hours. Subsequently, the remaining monomer and initiator emulsion was added dropwise to the reaction vessel over 5 hours, and after the completion of dropwise addition, maturation was carried out for 6 hours. After that, it was cooled to 30°C and adjusted to a solid content of 40% and pH of 6.8 using a 5.0% dimethylethanolamine aqueous solution and deionized water. Then, it was discharged while filtering through a 200-mesh nylon cloth to obtain an aqueous dispersion of hydroxyl group-containing acrylic resin (x11-2) with a solid content of 40%. The obtained hydroxyl group-containing acrylic resin (x11-2) had an acid value of 11 mg KOH / g, a hydroxyl group value of 24 mg KOH / g, and a weight-average molecular weight of 2.9 million. (Note 1) Emulsion of monomer and initiator: 55 parts of deionized water, 4 parts of Latemul E-118B (trade name, manufactured by Kao Corporation, emulsifier, active ingredient 26%), 10 parts of styrene, 53.5 parts of methyl methacrylate, 30 parts of n-butyl acrylate, 5 parts of 2-hydroxyethyl acrylate, 1.5 parts of acrylic acid, and 0.2 parts of 2,2′-azobis[2-(2-imidazolin-2-yl)propane] were mixed and stirred to obtain an emulsion of monomer and initiator.

[0205] <Manufacturing of hydroxyl group-containing polyester resin (x12)> Manufacturing Example 3 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and water separator, 174 parts of trimethylolpropane, 327 parts of neopentyl glycol, 352 parts of adipic acid, 109 parts of isophthalic acid, and 101 parts of 1,2-cyclohexanedicarboxylic acid anhydride were charged. The temperature was raised from 160°C to 230°C over 3 hours, and the mixture was maintained at 230°C while the generated condensed water was removed by distillation using a water separator until the acid value was 3 mg KOH / g or less. To this reaction product, 59 parts of trimellitic anhydride were added, and an addition reaction was carried out at 170°C for 30 minutes. After cooling to below 50°C, 2-(dimethylamino)ethanol was added in an equivalent amount relative to the acid groups to neutralize the mixture, and then deionized water was gradually added to obtain an aqueous dispersion of hydroxyl group-containing polyester resin (x12-1) with a solid content of 45%. The obtained hydroxyl group-containing polyester resin (x12-1) had an acid value of 35 mgKOH / g, a hydroxyl value of 128 mgKOH / g, and a number-average molecular weight of 1500.

[0206] <Manufacturing of Associative Viscosity Modifier (x41)> Manufacturing Example 4 350 parts of methyl triglycol were charged into a four-necked flask equipped with a heater, stirrer, thermometer, and reflux condenser, and the temperature was raised to 80-90°C. Next, under stirring, a monomer mixture consisting of 20 parts of methacrylic acid, 19.5 parts of acrylate of a 60-mol adduct of n-octadecyl alcohol ethylene oxide, 60 parts of ethyl acrylate, and 0.5 parts of diacrylate of a 15-mol adduct of ethylene glycol ethylene oxide, and 50 parts of a 1% methyl triglycol solution of 2,2'-asobisisobutyronitrile were added dropwise to the methyl triglycol over 1.5 hours. During this time, the reaction temperature was maintained at 80-90°C. After the dropwise addition was complete, the reaction product was kept at the same temperature for 3 hours and then cooled to room temperature to obtain a 20% solids solution of the associated viscosity modifier (x41-1).

[0207] Manufacturing Example 5 350 parts of methyl triglycol were charged into a four-necked flask equipped with a heater, stirrer, thermometer, and reflux condenser, and the temperature was raised to 80-90°C. Next, under stirring, a monomer mixture consisting of 20 parts of methacrylic acid, 19.5 parts of acrylate of a 30-mol adduct of n-hexyl alcohol ethylene oxide, 60 parts of propyl acrylate, and 0.5 parts of diacrylate of a 15-mol adduct of ethylene glycol ethylene oxide, and 50 parts of a 1% methyl triglycol solution of 2,2'-asobisisobutyronitrile were added dropwise to the methyl triglycol over 1.5 hours. During this time, the reaction temperature was maintained at 80-90°C. After the dropwise addition was complete, the reaction product was kept at the same temperature for 3 hours, and then cooled to room temperature to obtain a 20% solids solution of the associated viscosity modifier (x41-2).

[0208] <Manufacturing of polyurethane resin (U) that does not contain hydroxyl groups> Manufacturing Example 6 In a reaction vessel equipped with a thermometer, a stirrer, and reflux condenser, 211.9 parts of "PTMG1000" (trade name, manufactured by Mitsubishi Chemical Corporation, polytetramethylene ether glycol with a number average molecular weight of 1000), 11.5 parts of 2,2-dimethylolpropionic acid, 6.9 parts of trimethylolpropane, 112.2 parts of isophorone diisocyanate, and 298.5 parts of methyl ethyl ketone were charged. After purging the reaction system with nitrogen gas, the mixture was reacted at 80°C under stirring to obtain an NCO-terminated urethane prepolymer with a free isocyanate group content of 3.2%. The obtained methyl ethyl ketone solution was cooled to 40°C, and 493.2 g of deionized water containing 9.8 parts of triethylamine was added and emulsified. Then, 159.2 parts of a 5% aqueous ethylenediamine solution were added, and after stirring for 60 minutes, the methyl ethyl ketone was removed by distillation under reduced pressure and heating. The concentration was then adjusted with deionized water to obtain an aqueous dispersion of polyurethane resin (U-1) with a solid content of 35% and an acid value of 14 mg KOH / g, which does not contain hydroxyl groups.

[0209] <Manufacturing of pigment dispersion paste> Manufacturing example 7 37.5 parts (15 parts solids) of the aqueous dispersion of hydroxyl group-containing acrylic resin (x11-1) obtained in Production Example 1, 55 parts of "JR-806" (product name, manufactured by Teika Co., Ltd., rutile-type titanium dioxide), 40 parts of "Variace B-35" (product name, manufactured by Sakai Chemical Industry Co., Ltd., barium sulfate powder, average primary particle size 0.5 μm), 2.5 parts of "Carbon MA-100" (product name, manufactured by Chemical Co., Ltd., carbon black), and 5 parts of deionized water were mixed, the pH was adjusted to 8.0 with 2-(dimethylamino)ethanol, and then dispersed in a paint shaker for 30 minutes to obtain a pigment dispersion paste (P-1).

[0210] <Manufacturing of water-based intermediate coating paint (X)> Manufacturing Example 8 140 parts (112.5 parts solids) of pigment dispersion paste (P-1) obtained in Production Example 7, 37.5 parts (15 parts solids) of aqueous dispersion of hydroxyl group-containing acrylic resin (x11-2) obtained in Production Example 2, 11.1 parts (5 parts solids) of aqueous dispersion of hydroxyl group-containing polyester resin (x12-1) obtained in Production Example 3, 50 parts (40 parts solids) of "Cymel 325" (trade name, manufactured by Ornex, melamine resin, 80% solids), 39.5 parts (15 parts solids) of "Baihijur VPLS2310" (trade name, manufactured by Sumika Covestro Urethane, oxime-blocked polyisocyanate compound, 38% solids), 2.125 parts (0.425 parts solids) of aggregate viscosity modifier (x41-1) obtained in Production Example 4, "Primal 0.268 parts (0.075 parts solids) of "ASE-60" (product name, manufactured by Dow Chemical, polyacrylic acid-based thickener, 28% solids) and 28.6 parts (10 parts solids) of the aqueous dispersion of hydroxyl group-free polyurethane resin (U-1) obtained in Production Example 6 were uniformly mixed. Next, 2-(dimethylamino)ethanol and deionized water were added to the resulting mixture to obtain a water-based intermediate coating paint (X-1) with a pH of 8.0, a paint solids content of 51%, a temperature of 20°C, and a paint viscosity of 1200 mPa·s measured by a Brookfield viscometer at a rotor speed of 6 rpm.

[0211] Manufacturing examples 9-20 In Production Example 8, water-based intermediate coatings (X-2) to (X-13) were obtained in the same manner as in Production Example 8, except that the compound composition was as shown in Tables 1 and 2 below.

[0212] [Table 1]

[0213] [Table 2]

[0214] The components listed in the table are as follows: (Note 2) "SN Thickener 634" is a product name, manufactured by Sunopco, and is a polyacrylic acid-based thickener. (Note 3) "UH-752": Product name, manufactured by ADEKA Corporation, urethane association type thickener

[0215] <Preparation of test panels> (Preparation of test substrates) A 10cm x 30cm zinc phosphate-treated cold-rolled steel sheet with a hemmed portion at the top edge was electrodeposited with "Elecron 9910" (product name, manufactured by Kansai Paint Co., Ltd., thermosetting epoxy resin-based cationic electrodeposition coating) to a film thickness of 20μm, and then heated at 170°C for 30 minutes to cure, and this was used as the test substrate.

[0216] Example 1 (Preparation of test painted panels) On the hemming portion of the upper end of the above-mentioned test workpiece, a sealer (product name "Sandine 2690A-2", manufactured by Asahi Rubber Co., Ltd., automotive sealer) was applied to the surface uniformly with a spatula to a width of 5 cm and a dry film thickness of approximately 0.5 mm. After standing at room temperature for 5 minutes, a test workpiece was obtained in which a partially uncured sealer film was formed. Next, the water-based intermediate coating paint (X-1) obtained in Manufacturing Example 8 was applied to the test workpiece with the partially uncured sealer film using a rotary atomizing electrostatic coating machine so that the dry film thickness in the areas not coated with sealer was 15 μm. After standing for 5 minutes, an uncured intermediate coating film was obtained. Next, "WBC-713D No.B82" (product name, manufactured by Kansai Paint Co., Ltd., acrylic melamine resin-based water-based base coat paint, blue color) was electrostatically applied onto the uncured intermediate coat film using a rotary atomizing electrostatic coating machine so that the dry film thickness in the areas where the sealer was not applied was 15 μm. After letting it stand for 5 minutes, it was preheated at 80°C for 3 minutes to obtain an uncured base coat film. Next, "DKC-K12-1" (product name, manufactured by Kansai Paint Co., Ltd., acrylic resin-based solvent-type topcoat clear paint) was electrostatically applied onto the uncured base coat film using a rotary atomizing electrostatic coating machine so that the dry film thickness in the areas where the sealer was not applied was 35 μm. The painted board was then placed almost vertically and left for 7 minutes, and then heated at 140°C for 30 minutes to cure the multi-layer coating consisting of the sealer film, intermediate coat film, base coat film, and clear coat film, thereby producing test painted board No. 1.

[0217] Examples 2-11, Comparative Examples 1-7 In Example 1, test painted panels No. 2 to 18 were prepared in the same manner as in Example 1, except that the dry film thickness of the water-based intermediate coating (X) in the areas where the water-based intermediate coating (X) and sealer were not applied was as shown in Tables 3 to 5.

[0218] <Sealer opacity test> The areas of the above-mentioned test painted boards No. 1 to 18 that were coated with sealer were observed for their concealment, and evaluated according to the following criteria. ◎ and ○ were considered passing grades. The results are shown in Tables 3 to 5. (Evaluation Criteria) ◎: The sealant is concealed. ○: The sealant is slightly visible, but this does not pose a practical problem. △: The sealant is visible through the material, which poses a practical problem. ×: The sealant is significantly visible, which is clearly a practical problem.

[0219] <Smoothness Test> The smoothness of the uncoated areas of the aforementioned test painted panels No. 1 to 18 was evaluated based on the Long Wave (LW) value measured by "Wave Scan" (product name, manufactured by BYK Gardner) according to the following criteria. A smaller LW value indicates higher surface smoothness, and ◎ and ○ were considered passing grades. The results are shown in Tables 3 to 5. (Evaluation Criteria) ◎: LW value is less than 15. ○: LW value is 15 or greater and less than 20. △: LW value is 20 or higher but less than 25. ×: LW value is 25 or higher.

[0220] [Table 3]

[0221] [Table 4]

[0222] [Table 5]

[0223] Tables 3-5 show that the test coated panels No. 1-11 of Examples 1-11 exhibited excellent sealer opacity and smoothness, demonstrating that both properties can be achieved simultaneously.

Claims

[Claim 1] A method for forming a multilayer coating on an automobile body having a hemmed portion, wherein the following steps (1) to (6) are performed sequentially on the automobile body, Step (1): A step of applying sealer (S) to form an uncured sealer film. Step (2): A step in which a water-based intermediate coating paint (X) is applied to the uncured sealer coating film formed in step (1) without preheating or heat curing to form an uncured intermediate coating film. Step (3): A step in which an uncured base coat film is formed on the uncured intermediate coating film formed in step (2) by applying a water-based base coat paint (Y) without preheating or heat curing. Step (4): Preheating step in which the uncured sealer film, uncured intermediate coat film, and uncured base coat film are preheated. Step (5): A step of applying a clear coating (Z) onto the base coat coating that has been preheated in step (4) to form an uncured clear coating. Step (6): Baking process to heat-cur the multi-layer coating consisting of a preheated sealer coating, an intermediate coating, a base coat, and an uncured clear coating to form a cured multi-layer coating. The aforementioned water-based intermediate coating paint (X) It contains a hydroxyl group-containing resin (x1), a curing agent (x2), a pigment (x3), and a viscosity modifier (x4), The viscosity modifier (x4) comprises an aggregate viscosity modifier (x41) and an alkali swelling viscosity modifier (x42) different from the aggregate viscosity modifier (x41). The associated viscosity modifier (x41) is a (meth)acrylic acid copolymer-based thickener having a hydrophobic group with 8 to 36 carbon atoms. The solid content mass ratio of the association-type viscosity modifier (x41) and the alkali-swelling-type viscosity modifier (x42) is in the range of 97 / 3 to 70 / 30. The dry film thickness of the aforementioned intermediate coating is less than 20 μm. Method for forming a multi-layer coating.

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