JP7493696B1 - Method for forming multi-layer coating film, multi-layer coating film, and aqueous primer coating composition - Google Patents

Abstract

A method for forming a multi-layer coating film having an undercoat coating film, a colored base coating film, and a clear coating film on a resin substrate using an aqueous coating composition, the aqueous primer coating composition containing an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D), and water (E) and having a solids content of 5% by mass or more and 17% by mass or less, the aqueous polyolefin resin (A) having a Tg of -40°C to -10°C, the mass ratio of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) being 60/40 to 95/5, the organic solvent (D) containing a poorly water-soluble solvent (D1) and at least one of an amphipathic solvent (D2) and a hydrophilic solvent (D3), the proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) being 1 to 60% by mass, and the VOC amount being 30 to 500 g/L.

Description

The present invention relates to a method for forming a multi-layer coating film, a multi-layer coating film, and an aqueous primer paint composition.

In recent years, environmental pollution has become more serious, and regulations on organic solvent emissions have been strengthened internationally. In the field of paints, there has also been a shift from conventional solvent-based paints to water-based paints using water as a medium. Patent Document 1 discloses a water-based primer paint composition.

Patent No. 7134389

Although the aqueous primer coating composition of Patent Document 1 is water-based, it is unable to sufficiently reduce emissions of volatile organic compounds (VOCs).

The present invention aims to provide a multi-layer coating film with excellent appearance, which is formed from an all-water-based paint composition, including a primer paint composition that has sufficiently low VOC emissions and excellent storage stability and adhesion to resin substrates.

In order to solve the above problems, the present invention provides the following aspects.
[1]
A method for forming a first undercoat coating film by applying an aqueous primer coating composition onto a resin substrate;
Applying an aqueous colored base coating composition onto the first undercoat coating to form a colored base coating;
A water-based two-pack clear coating composition is applied onto the colored base coating film to form a clear coating film.
The aqueous primer coating composition contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
A method for forming a multilayer coating film, wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition.
[2]
The method for forming a multilayer coating film according to the above [1], wherein the mass of the organic solvent (D) in the organic component mass is 30 g/L or more and 350 g/L or less.
[3]
The method for forming a multilayer coating film according to the above [1] or [2], wherein the surface tension of the aqueous primer coating composition is 26.0 mN/m or more and 30.0 mN/m or less.
[4]
The method for forming a multilayer coating film according to the above [1] or [2], wherein the aqueous epoxy resin (B) contains a bisphenol A type epoxy resin.
[5]
The method for forming a multilayer coating film according to the above [1] or [2], wherein the thickener (C) contains a urethane association type thickener.
[6]
The method for forming a multilayer coating film according to the above [1] or [2], further comprising an aqueous polyurethane resin (F).
[7]
The method for forming a multi-layer coating film according to the above [1] or [2], wherein the resinous substrate comprises a substrate and a coating film other than the multi-layer coating film that covers at least a portion of the substrate.
[8]
The method for forming a multilayer coating film according to the above [1] or [2], wherein the difference between the solubility parameter of the polymer constituting the surface of the resin substrate and the solubility parameter of the organic solvent (D) contained in the aqueous primer coating composition is 2.5 or less.
[9]
After forming the first undercoat coating film, and before applying the aqueous colored base coating composition ,
The method for forming a multi-layer coating film according to the above [1] or [2], further comprising applying an aqueous surfacer onto the first undercoat coating film to form a second undercoat coating film.
[10]
A first undercoat coating film formed on a resin substrate using an aqueous primer coating composition;
A colored base coating film formed on the first undercoat coating film using an aqueous colored base coating composition ;
A clear coating film formed on the colored base coating film by an aqueous two-pack clear coating composition ,
The aqueous primer coating composition contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
A multilayer coating film, wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition.
[11]
A first undercoat coating film formed on a resin substrate using an aqueous primer coating composition;
A colored base coating film formed on the first undercoat coating film using an aqueous colored base coating composition ;
A water-based primer coating composition used for forming a multi-layer coating film having a clear coating film formed on the colored base coating film by using a water-based two-pack clear coating composition,
The composition comprises an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
An aqueous primer coating composition , wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less relative to the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition.

According to the present invention, there is provided a multi-layer coating film having excellent appearance, which is formed from an all-water-based paint composition including a primer paint composition that has sufficiently low VOC emissions and has excellent storage stability and adhesion to resin substrates.

The method for forming a multi-layer coating film according to the present disclosure includes applying an aqueous primer coating composition onto a resin substrate to form a first undercoat coating film, applying an aqueous colored base coating composition onto the first undercoat coating film to form a colored base coating film, and applying an aqueous two-pack clear coating composition onto the colored base coating film to form a clear coating film. After forming the first undercoat coating film, the method may also include applying an aqueous surfacer onto the first undercoat coating film before applying the aqueous colored base coating composition to form a second undercoat coating film.

The aqueous primer coating composition used in the present disclosure contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less. The aqueous polyolefin resin (A) has a glass transition temperature of -40°C or more and -10°C or less. The mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5. The organic solvent (D) includes a poorly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, an amphipathic solvent (D2) that dissolves 1 g or more and less than 60 g in 100 g of water at 20°C, and at least one of a hydrophilic solvent (D3) that dissolves 60 g or more in 100 g of water at 20°C. The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1% by mass or more and 60% by mass or less. The proportion of the organic solvent (D) in the organic component mass excluding the mass of water (E) from the total mass of the aqueous primer coating composition is 30 g/L or more and 500 g/L or less.

In recent years, resins have been proposed as a material for automotive parts instead of metals. The surface tension of a resin substrate is usually 40 mN/m or less. On the other hand, the surface tension of water is about 72.7 mN/m. Therefore, in general, water-based paint compositions do not wet resin substrates well, making film formation difficult. In the present disclosure, the water-based primer paint composition contains a small amount of poorly water-soluble solvent as an organic solvent, so that low VOC emissions and high adhesion to resin substrates (particularly adhesion after a water immersion test (hereinafter also referred to as water resistance)) are compatible. The water-based primer paint composition is suitable for painting resin substrates.

Aqueous primer paint compositions are particularly prone to penetrating into minute irregularities (scratches) on the surface of resin substrates. Therefore, when an aqueous primer paint composition is applied to a resin substrate having minute irregularities, repelling and smearing are suppressed, and a uniform film is formed. This results in the formation of a smooth undercoat film with high adhesion. As a result, the adhesion and appearance of the entire multi-layer coating film are improved. Hereinafter, the ability to form a uniform film when applied to a resin substrate having minute irregularities is referred to as film-forming ability. Film-forming ability affects the appearance and adhesion of the entire multi-layer coating film.

The fine irregularities are formed, for example, by polishing. The fine irregularities correspond to the irregularities formed by polishing back and forth 20 times with abrasive paper of #1000 to #2000 (typically #1500). The arithmetic mean roughness Ra of the fine irregularities may be 0.01 μm to 0.5 μm, and may be around 0.1 μm (0.07 μm to 0.12 μm).

The aqueous primer coating composition contains a specified amount of aqueous epoxy resin (B), and therefore has excellent adhesion to old coating films. The aqueous primer coating composition is suitable for painting substrates at least partially covered with old coating films. The aqueous primer coating composition is suitable for painting components partially covered with old coating films and where the substrate is exposed in other areas.

The old coating film is a coating film other than the multilayer coating film of the present disclosure that was formed on the substrate before the multilayer coating film of the present disclosure was formed. The old coating film may be, for example, a coating film for automobile exteriors (at least one of an undercoat, a base layer, and a clear layer). During partial repair, a part of the old coating film (e.g., a damaged portion) is removed by grinding, and then the multilayer coating film of the present disclosure is formed. During repair, the multilayer coating film of the present disclosure may be formed on the old coating film without removing the old coating film.

Hereinafter, the term "water-based resin" refers to a water-soluble resin or a water-dispersible resin. Water-dispersible resins are further divided into dispersion type (generally called colloidal dispersion type) and emulsion type. Colloidal dispersion type water-based resins are typically obtained by semi-dissolving a resin synthesized in an organic solvent in water with a neutralizing agent. Emulsion type water-based resins are typically produced by emulsion polymerization or by mechanical forced emulsification. Water-based resins can also be obtained by modifying a hydrophobic resin with a polar compound or by using a polar monomer in the synthesis of the resin. The water-based resin may be a dispersion type. "Water-based" refers to making a resin water-soluble or water-dispersible.

The aqueous coating composition contains water as a solvent. The proportion of water in the total solvent is 50% by mass or more, may be 60% by mass or more, may be 70% by mass or more, or may be 80% by mass or more.

The resin solids content of an aqueous paint composition is the total solids content of the resin components. The resin components are components other than thickeners, various pigments, organic solvents, water, and other inorganic components. For example, the resin components of an aqueous primer paint composition are aqueous polyolefin resin (A), aqueous epoxy resin (B), and other resins.

Weight average molecular weight and number average molecular weight are measured using polystyrene standards by the GPC (gel permeation chromatography) method.

The glass transition temperature Tg may be calculated from the type and amount of raw material monomers, or may be measured by differential scanning calorimetry (DSC).

The acid value and hydroxyl value can be measured by the known method described in JIS K 0070:1992. The acid value and hydroxyl value may be calculated from the amount of unsaturated monomer in the raw material monomer of the target resin. The acid value and hydroxyl value are determined based on the solid content.

The average particle size is the 50% average particle size (D50) in the volume-based particle size distribution measured using a laser diffraction/scattering particle size distribution measuring device.

Surface tension is measured by the platinum ring method. For example, a Dynometer (manufactured by Byck Gardner, Germany) is used to measure surface tension.

The solubility parameter (SP value) may be a known value. The SP value may be measured by the following method. (Reference: SUH, CLARKE, J.P.S.A-1, 5, 1671-1681 (1967)).

0.5 g of the object to be measured is weighed into a 100 ml beaker and dissolved in 10 ml of a good solvent (acetone) to be used as a sample. A poor solvent is dripped onto this sample using a 50 ml burette at a measurement temperature of 20°C, and the point at which turbidity occurs is regarded as the amount dripped. As poor solvents, ion-exchanged water is used as a high SP poor solvent, and n-hexane is used as a low SP poor solvent, and the turbidity point of each is measured. The SP value δ of the object to be measured is given by the following formula:

Ｖｉ：溶媒の分子容（ｍｌ／ｍｏｌ）
φｉ：濁点における溶媒ｉの体積分率
δｉ：溶媒のＳＰ値
ｍｌ：低ＳＰ貧溶媒混合系
ｍｈ：高ＳＰ貧溶媒混合系

Vi: Molar volume of the solvent (ml/mol)
φi: Volume fraction of solvent i at the clouding point δi: SP value of the solvent ml: Low SP poor solvent mixture mh: High SP poor solvent mixture

First, the coating composition used to form the multi-layer coating film will be described.
(Water-based primer coating composition)
The aqueous primer coating composition according to the present disclosure contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E).

The solid content of the aqueous primer coating composition is 5% by mass or more and 17% by mass or less. This balances film-forming properties, water resistance, and coating workability. The solid content of the aqueous primer coating composition may be 7% by mass or more, or 9% by mass or more. The solid content of the aqueous primer coating composition may be 14% by mass or less, or 12% by mass or less.

The solids content of the aqueous primer coating composition is determined as follows. The aqueous primer coating composition is placed in an aluminum foil cup with a diameter of approximately 5 cm and weighed. This aluminum foil cup is heated at 110°C for 1 hour, and the mass (g) of the residue is weighed. The mass of the residue is divided by the mass before heating to obtain the solids content (mass%), expressed as a percentage.

The proportion of organic solvent (D) in the organic component mass excluding the mass of water (E) from the total mass of the aqueous primer coating composition (hereinafter referred to as the VOC amount) is 30 g/L or more and 500 g/L or less. If the amount of VOC contained in the coating is small, the amount of VOC emitted during coating film formation is also reduced.

The amount of VOCs can be calculated from the following formula. The concentration of water (E) is obtained by dividing the mass of all water contained in the aqueous primer coating composition by the mass of the aqueous primer coating composition. The concentration of organic solvent (D) is obtained by dividing the total mass of organic solvent (D) contained in the aqueous primer coating composition by the mass of the aqueous primer coating composition.

To calculate the amount of VOC, please refer to Mitsuhiko Asakura, "Advances in Water-Based Paints for Environmental Compliance," Surface Technology, Surface Technology Association of Japan, 1997, Vol. 48, No. 8, pp. 760-764.

The lower the VOC amount, the more desirable it is, but from the viewpoint of adhesion to the resin substrate, it may be 40 g/L or more, or 100 g/L or more. The VOC amount may be 350 g/L or less, 300 g/L, or 250 g/L.

The surface tension of the aqueous primer coating composition may be, for example, 26.0 mN/m or more and 30.0 mN/m or less. This further improves the wettability of the aqueous primer coating composition to the resin substrate, making it easier to form a uniform coating film. The surface tension of the aqueous primer coating composition may be 26.6 mN/m or more, or 27.0 mN/m or more. The surface tension of the aqueous primer coating composition may be 29.5 mN/m or less, or 29.0 mN/m or less. The surface tension of the aqueous primer coating composition can be adjusted by the type and amount of the organic solvent (D).

・Water-based polyolefin resin (A)
The aqueous polyolefin resin (A) is not particularly limited and can be produced by a conventionally known method. The aqueous polyolefin resin (A) can be obtained, for example, by adding toluene to a polyolefin resin modified with an acid anhydride, heating the mixture to obtain a resin solution, adding a surfactant to the resin solution, and then dropping ion-exchanged water while forcibly stirring at 50 to 60 ° C. to cause phase inversion emulsification. The aqueous polyolefin resin (A) can also be obtained by heating an acid anhydride-modified polyolefin resin to dissolve it in a solvent such as tetrahydrofuran, neutralizing the carboxyl group of the resin with an excess of amine, and then dropping ion-exchanged water while forcibly stirring at 50 to 60 ° C. to cause phase inversion emulsification. Alternatively, the aqueous polyolefin resin (A) can be obtained by mixing an emulsifier and an amine with the above-mentioned solution, and then dropping ion-exchanged water at about 60 ° C. while forcibly stirring to cause emulsification. The aqueous polyolefin resin (A) can also be obtained by dropping a molten or dissolved acid anhydride-modified polyolefin into warm water in which a neutralizing agent such as an amine and/or a surfactant is dissolved, while forcibly stirring, to cause emulsification.

Examples of aqueous polyolefin resins (A) include homopolymers of olefin monomers (e.g., ethylene, propylene, butene, 3-methyl-1-butene, 3-methyl-1-heptene), ethylene-propylene copolymers, ethylene-propylene-diene copolymers, styrene-butadiene-isoprene copolymers, and copolymers of olefin monomers and other unsaturated monomers (e.g., vinyl acetate, butadiene, acrylic acid esters, methacrylic acid esters). These may be used alone or in combination of two or more.

The aqueous polyolefin resin (A) may be a homopolymer of propylene, a copolymer of propylene and other olefin monomers, a copolymer of propylene and other unsaturated monomers, or a mixture of these with other polymers. These may be used alone or in combination of two or more. 90% by mass or more of the monomers constituting the aqueous polyolefin resin (A) may be propylene.

The aqueous polyolefin resin (A) may be a modified product. Examples of the modified product include an unsaturated carboxylic acid anhydride adduct of an aqueous polyolefin resin. Examples of the unsaturated carboxylic acid anhydride include maleic anhydride, fumaric acid, itaconic anhydride, (meth)acrylic acid, etc. The modified product is usually obtained by reacting polyolefins with an unsaturated carboxylic acid anhydride in the presence of an organic peroxide or the like.

The glass transition temperature Tg of the aqueous polyolefin resin (A) is -40°C or higher and -10°C or lower. The low Tg of the aqueous polyolefin resin (A) improves film-forming properties and facilitates adhesion to the substrate. The Tg of the aqueous polyolefin resin (A) may be -30°C or higher. The Tg of the aqueous polyolefin resin (A) may be -10°C or lower, or -20°C or lower.

The weight average molecular weight Mw of the aqueous polyolefin resin (A) is, for example, 2,000 or more and 300,000 or less. The Mw of the aqueous polyolefin resin (A) may be 10,000 or more, or 30,000 or more. The Mw of the aqueous polyolefin resin (A) may be 200,000 or less, 120,000 or less, or 80,000 or less.

The solid content mass of the aqueous polyolefin resin (A) is, for example, 50 parts by mass or more and 98 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous primer coating composition. When the solid content mass of the aqueous polyolefin resin (A) is 50 parts by mass or more, the adhesion to the resin substrate can be further improved. When the solid content mass of the aqueous polyolefin resin (A) is 98 parts by mass or less, the adhesion to the aqueous colored base coating film can be further improved. The solid content mass of the aqueous polyolefin resin (A) may be 55 parts by mass or more, or may be 60 parts by mass or more. The solid content mass of the aqueous polyolefin resin (A) may be 95 parts by mass or less, may be 90 parts by mass or less, or may be 85 parts by mass or less.

The aqueous polyolefin resin (A) may be chlorinated or may not be chlorinated. From the viewpoint of environmental conservation, the aqueous polyolefin resin (A) may not be chlorinated. The chlorination rate may be, for example, 5% or more and 50% or less. Chlorination is carried out, for example, by heating and dissolving the polyolefin resin in a chlorine-based solvent such as carbon tetrachloride, and blowing in chlorine gas at a temperature of 50 to 120°C.

Commercially available water-based polyolefin resins (A) include, for example, the Superclone series and Auroren series (both manufactured by Nippon Paper Industries Co., Ltd.), the Hardlen series (manufactured by Toyobo Co., Ltd.), and the Aptlock series (manufactured by Mitsubishi Chemical Corporation).

Commercially available non-chlorinated water-based polyolefin resins (A) include, for example, Auroren AE-301, Superclone E-415, and E-480T (all manufactured by Nippon Paper Chemicals Co., Ltd.). Commercially available chlorinated water-based polyolefin resins (A) include, for example, Arrowbase DA-1010 (manufactured by Unitika Ltd.), Hardlen EW-5515, Hardlen EW-5303, and Hardlen EW-5250 (all manufactured by Toyobo Co., Ltd.), and Aptlock (manufactured by Mitsubishi Chemical Corporation).

・Water-based epoxy resin (B)
The aqueous epoxy resin (B) contributes to improving the water resistance and moisture resistance of the multi-layer coating film. During repair painting, the aqueous epoxy resin (B) contributes to improving the adhesion between the new coating film and the old coating film. The aqueous primer coating composition is particularly suitable for repairs.

The aqueous epoxy resin (B) is obtained as an aqueous dispersion, for example, by graft polymerizing an epoxy resin with a fatty acid (e.g., a carboxyl group-containing vinyl monomer) and then neutralizing the resulting mixture.

The solid content mass of the aqueous epoxy resin (B) is, for example, 2 parts by mass or more and 50 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous primer coating composition. The solid content mass of the aqueous epoxy resin (B) may be 5 parts by mass or more, 10 parts by mass or more, or 15 parts by mass or more. The solid content mass of the aqueous epoxy resin (B) may be 45 parts by mass or less, or 40 parts by mass or less.

The mass ratio (A/B) of the water-based polyolefin resin (A) and the water-based epoxy resin (B) is 60/40 to 95/5. This allows the multi-layer coating film formed from the all-water-based paint to exhibit excellent water resistance. The mass ratio (A/B) may be 70:30 to 90:10.

Examples of the water-based epoxy resin (B) include bisphenol-type epoxy resins such as bisphenol A-type epoxy resins, bisphenol F-type epoxy resins, and bisphenol AD-type epoxy resins; epoxy ester resins obtained by modifying the above-mentioned bisphenol-type epoxy resins with dibasic acids; novolac-type epoxy resins such as cresol novolac-type epoxy resins and phenol novolac-type epoxy resins; alicyclic epoxy resins; polyglycol-type epoxy resins; hydrogenated bisphenol A-type epoxy resins; ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, Examples of such epoxy resins include aliphatic epoxy resins such as glycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, hexahydrophthalic acid diglycidyl ester, glycerin polyglycidyl ether, diglycerin polyglycidyl ether, and polyglycerin polyglycidyl ether, and epoxy group-containing acrylic resins containing an epoxy group-containing acrylic monomer such as glycidyl (meth)acrylate as a constituent component. These may be used alone or in combination of two or more.

Among these, from the viewpoint of adhesion, it may be an aromatic epoxy resin, a bisphenol type epoxy resin, or a bisphenol A type epoxy resin.

Water-based epoxy resin (B) can be obtained, for example, by adding epichlorohydrin to phenol novolac resin. Commercially available products of water-based epoxy resin (B) include Denacol EM150 (manufactured by Nagase Chemtech Co., Ltd.), Epilez 6006W70, 5003W55 (manufactured by Japan Epoxy Resins Co., Ltd.), Watersol series (manufactured by DIC Corporation), and WEX-5100 (manufactured by Tohto Kasei Co., Ltd.).

The aqueous epoxy resin (B) may be obtained, for example, by forcibly emulsifying a bisphenol-type epoxy resin with an emulsifier. Commercially available products of such aqueous epoxy resin (B) include, for example, Denacol EM101 and EM103 (manufactured by Nagase Chemtech Co., Ltd.), Epilez 3510W60, 3515W6, 3522W60, and 3540WY55 (all manufactured by Japan Epoxy Resins Co., Ltd.).

The aqueous epoxy resin (B) may be, for example, an alkyl type in which epichlorohydrin is added to a polyol such as sorbitol, pentaerythritol, or glycerin. Commercially available products of such aqueous epoxy resin (B) include, for example, Denacol EX-611, EX-614, EX-411, and EX-313 (all manufactured by Nagase Chemtech Co., Ltd.).

The aqueous epoxy resin (B) may be modified. The aqueous epoxy resin (B) may be, for example, acrylic-modified, urethane-modified, amine-modified, or ester-modified. An example of a commercially available acrylic-modified bisphenol A-type aqueous epoxy resin (B) is Modepics 304 (manufactured by Arakawa Chemical Industries Co., Ltd.).

Thickener (C)
The thickener (C) increases the viscosity of the aqueous primer coating composition. Examples of the thickener (C) include inorganic thickeners, cellulose derivatives, urethane association type thickeners, polyamide type thickeners, and alkali swelling type thickeners. These are used alone or in combination of two or more. Among them, from the viewpoint of flowability, the urethane association type thickener may be used.

Examples of urethane association type thickeners include polyurethane-based thickeners that have hydrophobic chains in their molecules, and urethane-urea-based thickeners in which at least a portion of the main chain is a hydrophobic urethane chain.

Commercially available examples of urethane association type thickeners include ADEKA NOL UH-756VF (manufactured by ADEKA Corporation) and BYK-420 (manufactured by BYK Corporation).

The solid content mass of the thickener (C) is, for example, 0.5 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous primer coating composition. The solid content mass of the thickener (C) may be 1 part by mass or more, or may be 2 parts by mass or more. The solid content mass of the thickener (C) may be 4.5 parts by mass or less, or may be 4 parts by mass or less.

Organic solvent (D)
The organic solvent (D) reduces the surface tension of the aqueous primer coating composition and improves the wettability of the composition to the resin substrate. On the other hand, from the viewpoint of the stability of the aqueous coating composition and the reduction of VOC emissions, it is desirable to use a small amount of the organic solvent (D). By using a poorly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, the wettability of the composition to the resin substrate can be improved with a small amount of the organic solvent (D).

The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1% by mass or more and 60% by mass or less. The poorly water-soluble solvent (D1) effectively reduces the surface tension of the aqueous primer coating composition. The above proportion of the poorly water-soluble solvent (D1) may be 5% by mass or more, or 10% by mass or more. The above proportion of the poorly water-soluble solvent (D1) may be 50% by mass or less, 40% by mass or less, or 35% by mass or less.

The organic solvent (D) contains at least one of an amphipathic solvent (D2) and a hydrophilic solvent (D3) together with a poorly water-soluble solvent (D1). This improves the compatibility of the organic solvent (D) with water (E) and improves the storage stability of the aqueous primer coating composition.

Examples of poorly water-soluble solvents (D1) include 2-ethyl-1-hexanol (0.07 g/100 g) and normal butyl propionate (also known as normal butyl propionate, 0.2 g/100 g). The solubility in water at 20°C is shown in parentheses. These may be used alone or in combination of two or more. Commercially available products of poorly water-soluble solvents (D1) include A Solvent (manufactured by JX Nippon Oil & Energy Corporation, 0 g/100 g) and Ipsol TP (manufactured by Idemitsu Kosan Co., Ltd., 0 g/100 g).

Examples of amphipathic solvents (D2) include isoamyl acetate (1.7 g/100 g), methoxypropyl acetate (also known as propylene glycol monomethyl ether acetate, 19.8 g/100 g), ethylene glycol monomethyl ether propionate (18.5 g/100 g), ethoxypropyl acetate (9.5 g/100 g), 3-methoxybutyl acetate (also known as methoxybutyl acetate, 6.5 g/100 g), 3-methyl-3- Examples of such esters include methoxybutyl acetate (also known as 3-methoxy-3-methylbutyl acetate, 6.8 g/100 g), ethyl 3-ethoxypropionate (1.3 g/100 g), dipropylene glycol dimethyl ether (also known as dimethylpropylene diglycol, 37 g/100 g), cyclohexyl acetate (1.4 g/100 g), propylene glycol mono-t-butyl ether (14.5 g/100 g), and propylene glycol monobutyl ether (6.4 g/100 g). These may be used alone or in combination of two or more.

Examples of hydrophilic solvents (D3) include ethylene glycol monoethyl ether (also known as cellosolve, 100g or more/100g), isopropyl glycol (also known as ethylene glycol monoisopropyl ether, 100g or more/100g), ethylene glycol monoisobutyl ether acetate (100g or more/100g), ethylene glycol monobutyl ether (also known as butyl cellosolve, 100g or more/100g), 3-methoxy-3-methylbutanol (100g or more/100g), ethylene glycol mono t-butyl ether (100g or more/100g), propylene glycol monopropyl ether (100g or more/100g), diethylene glycol dimethyl ether (100g or more/100g), diethylene glycol diethyl ether (100g or more/100g), and ethylene glycol monoisoamyl ether (100g or more/100g). These may be used alone or in combination of two or more.

The SP value of the organic solvent (D) may be, for example, 7.90 or more and 9.80 or less. This further improves the wettability of the aqueous primer coating composition to the resin substrate, making it easier to form a uniform coating film. The SP value of the organic solvent (D) may be 8.00 or more, or 8.30 or more. The SP value of the organic solvent (D) may be 9.70 or less, or 9.30 or less.

The SP value of organic solvent (D) is calculated from the volumetric mixing ratio (volume %) of each organic solvent and the SP value of each organic solvent. Specifically, the SP value of organic solvent (D) is the sum of (mixture ratio X1 (volume %) of poorly water-soluble solvent (D1) x its SP value Y1), (mixture ratio X2 (volume %) of amphipathic solvent (D2) x its SP value Y2), and (mixture ratio X3 (volume %) of hydrophilic solvent (D3) x its SP value Y3).

The difference ΔSP between the SP value of the organic solvent (D) and the SP value of the polymer constituting the surface of the resin substrate is, for example, 2.50 or less. This further improves the wettability of the aqueous primer coating composition to the resin substrate, making it easier to form a uniform coating film. ΔSP may be 0.00 or more, or 0.40 or more. ΔSP may be 2.50 or less, or 2.00 or less.

The polymer that constitutes the surface of a resinous substrate is a polymer that occupies 50% or more by mass of the surface of the substrate. For example, if a component labeled as being made of polypropylene resin is used as the substrate, the SP value of the known polypropylene resin may be regarded as the SP value of the polymer that constitutes the surface of the substrate. (Reference: Toshikatsu Kobayashi, "Solubility parameters and surface tension: Basic knowledge of dissolving, mixing, wetting, and adhering," Journal of the Japan Color Materials Association, Vol. 90, No. 9, 2017, p. 324-332)

The SP value of the polymer that constitutes the surface of the coated object can be substituted by the SP value of polypropylene resin. Polypropylene resin is known to be a particularly poorly adhesive resin, and it is difficult to make a coating film adhere to polypropylene resin. Therefore, it is safe to say that an organic solvent (D) whose SP value difference ΔSP with respect to the SP value of polypropylene resin is 2.5 or less also has excellent wettability with other polymers. The SP value of polypropylene resin is known and is 7.90 (Source: Masaki Haneda, "Surface Modification of Plastic Films", Journal of Printing Science and Technology of Japan, Vol. 47, No. 2 (2010) Review, p. 78-83). In other words, the "difference ΔSP between the SP value of the organic solvent (D) and the SP value of the polymer that constitutes the surface of the resin coated object" can be rephrased as the "difference ΔSP between the SP value of the organic solvent (D) and the SP value of polypropylene resin (= 7.90)".

Water (E)
Examples of the water (E) include various types of water such as pure water, ion-exchanged water, tap water, and industrial water.

・Water-based polyurethane resin (F)
The aqueous primer coating composition may contain an aqueous polyurethane resin (F). During repair coating, the aqueous polyurethane resin (F) contributes to improving the adhesion between the new coating film and the old coating film. The aqueous primer coating composition containing the aqueous polyurethane resin (F) is particularly suitable for repair coating.

The aqueous polyurethane resin (F) is prepared, for example, by neutralizing an NCO-terminated prepolymer obtained by the method described below, dispersing it in water, and then subjecting it to a chain extension reaction, i.e., by the so-called prepolymer mixing method.

The solid content mass of the aqueous polyurethane resin (F) is, for example, 0 parts by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous primer coating composition. The solid content mass of the aqueous polyurethane resin (F) may be 2 parts by mass or more, or may be 3 parts by mass or more. The solid content mass of the aqueous polyurethane resin (F) may be 7 parts by mass or less, or may be 6 parts by mass or less.

From the viewpoint of water resistance, the acid value of the aqueous polyurethane resin (F) may be, for example, 0 mgKOH/g or more and 40 mgKOH/g or less. The acid value of the aqueous polyurethane resin (F) may be 5 mgKOH/g or more, or 10 mgKOH/g or more. The acid value of the aqueous polyurethane resin (F) may be 30 mgKOH/g or less, or 20 mgKOH/g or less.

The Tg of the aqueous polyurethane resin (F) may be, for example, -50°C or higher and 5°C or lower. This improves film-forming properties and facilitates adhesion to the substrate. The Tg of the aqueous polyurethane resin (F) may be -40°C or higher, or -35°C or higher. The Tg of the aqueous polyurethane resin (F) may be 0°C or lower, or -10°C or lower.

Polyurethane resins are prepared, for example, by reacting a polyhydric alcohol, a polyisocyanate compound, and a compound having an active hydrogen group in the molecule.

The polyhydric alcohol has two or more hydroxyl groups. Examples of the polyhydric alcohol include the same compounds as those listed as raw materials for the polyester resin dispersion (F). The number average molecular weight of the polyhydric alcohol may be 500 or more and 5,000 or less.

A polyisocyanate compound has two or more isocyanate groups in the molecule. Examples of polyisocyanate compounds include aliphatic diisocyanates having 2 to 12 carbon atoms, such as hexamethylene diisocyanate, 2,2,4-trimethylhexane diisocyanate, and lysine diisocyanate; alicyclic diisocyanates having 4 to 18 carbon atoms, such as 1,4-cyclohexane diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, methylcyclohexylene diisocyanate, and isopropylidenecyclohexyl-4,4'-diisocyanate; 2,4-toluylene diisocyanate, 2,6-toluylene diisocyanate, diphenylmethane-4,4'-diisocyanate, and 1,5'-naphthene diisocyanate. aromatic diisocyanates such as phenyl diisocyanate, tolidine diisocyanate, diphenylmethylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, xylylene diisocyanate, tetramethylxylylene diisocyanate, 4,4'-dibenzyl diisocyanate, and 1,3-phenylene diisocyanate; and triisocyanates such as lysine ester triisocyanate, triphenylmethane triisocyanate, 1,6,11-undecane triisocyanate, 1,8-diisocyanate-4,4-isocyanate methyloctane, 1,3,6-hexamethylene triisocyanate, and bicycloheptane triisocyanate.

The polyisocyanate compound may be a dimer or trimer (isocyanurate bond) of the above polyisocyanate compound, or a biuret obtained by reacting the above polyisocyanate compound with an amine. A polyisocyanate having a urethane bond obtained by reacting the above polyisocyanate compound with a polyhydric alcohol may also be used.

Among these, alicyclic diisocyanates may be used. This introduces a cyclic structure into the molecules of polyurethane resin (F), improving weather resistance.

Compounds having an active hydrogen group in the molecule are used to block the isocyanate group in a polyisocyanate compound. Examples of compounds having an active hydrogen group in the molecule include monohydric alcohols such as methanol, ethanol, and diethylene glycol monobutyl ether; monovalent carboxylic acids such as acetic acid and propionic acid; monovalent thiols such as ethyl mercaptan; primary amines such as diethylenetriamine and monoethanolamine; secondary amines such as diethylamine; and oximes such as methyl ethyl ketoxime.

Chain extenders, monoisocyanate compounds, catalysts, etc. are used as necessary to prepare polyurethane resins.

Polyurethane resins are synthesized, for example, by a one-shot method in which each component is reacted at once, or by a multi-stage method in which the components are reacted in stages (a part of an active hydrogen-containing compound (e.g., a polymer polyol) is reacted with a polyisocyanate to form an NCO-terminated prepolymer, and then the remainder of the active hydrogen-containing compound is reacted). The synthesis reaction is carried out, for example, at a temperature of 40°C or higher and 140°C or lower.

Commercially available water-based polyurethane resins (F) include, for example, the Superflex series, such as Superflex 150, Superflex 420, and Superflex 460 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), the Neosticker series, and Evaphanol series (all manufactured by Nicca Chemical Co., Ltd.), the Bayhydrol series, such as Bayhydrol VP LS2952 (manufactured by Sumika Covestro Urethanes Co., Ltd.), VONDIC 2260, VONDIC 2220, Hydran WLS210, and Hydran WLS213 (all manufactured by Dainippon Ink and Chemicals Inc.), and NeoRez R9603 (manufactured by Avecia).

Additives The aqueous primer coating composition may contain various additives as necessary, such as a surface conditioner, a preservative, a mildewproofing agent, a defoaming agent, a light stabilizer, an ultraviolet absorber, an antioxidant, and a pH adjuster.

The surface conditioner can reduce the surface tension of the aqueous primer coating composition. On the other hand, it can reduce the adhesion of the first undercoat coating film to the resin substrate. Therefore, the content of the surface conditioner may be 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the resin solids contained in the aqueous primer coating composition. The content of the surface conditioner may be 5 parts by mass or less, or may be 2 parts by mass or less.

(Water-based surfacer)
The aqueous surfacer is, for example, a two-component type, and includes, for example, a first main agent including a hydroxyl-containing resin (a1) and an aqueous solvent (a2), and a first curing agent including a polyisocyanate compound (b1) and an organic solvent not having a hydroxyl group (b2).

Examples of the hydroxyl group-containing resin (a1) include acrylic polyols, polyester polyols, polyether polyols, polyurethane polyols, and polycarbonate polyols. Among these, acrylic polyols are preferred because they tend to improve the physical properties of the resulting coating film, such as its appearance, weather resistance, and chemical resistance.

The hydroxyl-containing resin (a1) is obtained, for example, by emulsion polymerization of one or more raw material monomers. The emulsion polymerization is performed by a method commonly used by those skilled in the art. Specifically, an emulsifier is mixed into an aqueous medium containing an organic solvent such as alcohol as necessary, and the resulting mixture is heated and stirred while the raw material monomer and polymerization initiator are dropped. Alternatively, an emulsion mixture in which the raw material monomer, emulsifier and water are previously emulsified may be dropped into the aqueous medium. The aqueous medium may be the same as or different from the aqueous solvent (a2).

An emulsion of acrylic polyol can be obtained, for example, by emulsion polymerization of a raw material monomer mixture containing an (meth)acrylic acid alkyl ester monomer, an acid group-containing ethylenically unsaturated monomer, a hydroxyl group-containing ethylenically unsaturated monomer, and a styrene-based monomer.

Examples of the aqueous solvent (a2) include the same as water (E). The first main agent may contain an organic solvent together with water. Examples of the organic solvent include organic solvents that do not have a hydroxyl group, among those exemplified as the organic solvent (D).

The first main agent and the first hardener are mixed to prepare an aqueous surfacer. The first main agent and the first hardener are mixed immediately before painting. After being mixed, the first main agent and the first hardener may be supplied to a painting device, or may be mixed within the painting device.

The first base agent and the first curing agent are mixed so that the hydroxyl group equivalent of the hydroxyl group-containing resin (a1) and the isocyanate group equivalent of the polyisocyanate compound (b1) are isocyanate group equivalent/hydroxyl group equivalent = 1/0.5 to 1/2.

The first main agent may further contain a pigment (c). The content of the pigment (c) may be, for example, 100 parts by mass or more and 300 parts by mass or less, and 120 parts by mass or more and 200 parts by mass or less, relative to 100 parts by mass of the resin solid content contained in the first main agent.

Examples of pigments (c) include color pigments such as titanium dioxide, carbon black, iron oxide, and copper phthalocyanine blue; luster pigments such as aluminum flakes and mica flakes; extender pigments such as calcium carbonate, clay, talc, barium oxide, and silica; and rust-preventive pigments such as aluminum tripolyphosphate, aluminum phosphomolybdate, and zinc phosphate.

In particular, from the viewpoint of the abrasiveness and film-forming properties of the resulting coating film, at least a body pigment may be included. The proportion of the body pigment may be, for example, 30% by mass or more and 100% by mass or less, and 40% by mass or more and 70% by mass or less, relative to 100 parts by mass of the total of all pigments.

The water-based surfacer may contain an epoxy group-containing silane coupling agent. The epoxy group-containing silane coupling agent has a reactive silyl group and one or more epoxy groups as an organic functional group. The epoxy group-containing silane coupling agent may have two or more epoxy groups.

The epoxy group is a three-membered ring (oxirane ring) having an ether bond. The epoxy group has high reactivity and reacts with the hydroxyl group-containing resin (a1) and/or the polyisocyanate compound (b1). This increases the crosslink density of the resulting cured coating film.

The epoxy group-containing silane coupling agent may be added to at least one of the first base agent and the first curing agent. The silane coupling agent is added in an amount of 1 part by mass or more and 8 parts by mass or less per 100 parts by mass of the total solid content of the hydroxyl group-containing resin (a1) and the polyisocyanate compound (b1).

(Water-Based Colored Coating Composition)
The aqueous colored base coating composition may be a curing type or a lacquer type. The aqueous colored base coating composition further contains a pigment. Examples of the pigment include the color pigments and/or luster pigments exemplified as pigment (c).

The curable water-based colored base coating composition contains a curable resin (e.g., the first main agent described above) and a curing agent (e.g., the first curing agent described above), and forms a cured coating film by heating. The curable water-based colored base coating composition may further contain a silane coupling agent (e.g., an epoxy group-containing silane coupling agent).

Lacquer-type water-based colored base paint compositions contain water, an organic solvent, and a binder resin dissolved or dispersed therein, and a cured coating film is formed by the evaporation of the water and the organic solvent. There are no particular limitations on the concentration of the binder resin, and it is set appropriately depending on the composition and application.

The binder resin is not particularly limited. Examples of the binder resin include acrylic resin, amino alkyd resin, alkyd resin, amino resin, isocyanate resin, epoxy resin, vinyl chloride resin, cashew resin, silicone resin, styrene resin, styrenated alkyd resin, cellulose resin (e.g., cellulose nitrate, etc.), urea resin, vinyl resin, phenol resin, phthalic acid resin, fluororesin, polyester resin, unsaturated polyester resin, and modified resins thereof (e.g., rosin-modified, phenol-modified, epoxy resin-modified, styrene-modified, acrylic-modified, urethane-modified). These may be used alone or in combination of two or more. Examples of organic solvents include naphtha, xylene, toluene, and acetone.

(Water-based clear coating composition)
The aqueous clear coating composition is, for example, a two-liquid type. The aqueous clear coating composition includes a second main agent containing the same hydroxyl group-containing resin (a1) and aqueous solvent (a2) as above, and a second curing agent containing the same polyisocyanate compound (b1) and organic solvent (b2) not having a hydroxyl group as above. Each component of the aqueous clear coating composition and each component of the aqueous surfacer may be the same or different.

The second base agent and the second curing agent are mixed so that the hydroxyl group equivalent of the hydroxyl group-containing resin (a1) and the isocyanate group equivalent of the polyisocyanate compound (b1) are isocyanate group equivalent/hydroxyl group equivalent = 1/0.5 to 1/1.5.

The aqueous clear coating composition may contain the same epoxy group-containing silane coupling agent as described above. The epoxy group-containing silane coupling agent is added in an amount of 1.2 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the total solid content of the hydroxyl group-containing resin (a1) and the polyisocyanate compound (b1).

At least one of the water-based surfacer and the water-based clear coating composition may contain a silane coupling agent having one or more epoxy groups. This can further improve the water resistance of the entire multi-layer coating film. Both the water-based surfacer and the water-based clear coating composition may contain an epoxy group-containing silane coupling agent. The epoxy group-containing silane coupling agents contained in the water-based clear coating composition and the water-based surfacer may be the same or different.

(Method of preparing aqueous coating composition)
Each aqueous coating composition can be prepared by a known method. Each aqueous coating composition is prepared by mixing the above-mentioned components. For mixing, a commonly used mixing device such as a paint shaker or mixer is used. The base agent and the curing agent are usually mixed immediately before use (for example, they are used within 60 minutes after mixing).

(Resin substrate)
The substrate may have at least a surface made of resin. The entire substrate may be made of resin, or at least a portion of the surface of a resin or metal substrate may be coated with resin. The resin covering the surface of the substrate may be an old coating film. That is, the substrate made of resin may be made of resin, may have a resin substrate and an old coating film covering at least a portion of its surface, or may have a metal substrate and an old coating film covering at least a portion of its surface. The aqueous primer coating composition used in the present disclosure has excellent adhesion to the old coating film as well as to the resin or metal substrate.

The resin constituting the resin-made coated object may be a thermoplastic resin or a thermosetting resin. Examples of resins include polypropylene (PP) resin, acrylonitrile-butadiene-styrene copolymer (ABS resin), polycarbonate (PC)/ABS resin, PC/acrylonitrile-ethylene-propylene-diene-styrene copolymer (AES resin), AES resin, PC/polybutylene terephthalate (PBT) resin, PC/polyethylene terephthalate (PET) resin, PC resin, polymethyl methacrylate (PMMA) resin, GF-PBT resin, GF-polyamide (PA) resin, Noryl GTX resin, polyvinyl chloride (PVC resin), acrylonitrile-styrene-acrylic (ASA) resin, carbon fiber reinforced plastic (CFRP resin), and glass fiber reinforced plastic (GFRP resin).

Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof. Metallic substrates may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. Metallic substrates may be further coated with an electrocoat paint after surface treatment. The electrocoat paint may be of the cationic type or the anionic type.

[Method of manufacturing coated articles]
The coated article is produced by a method comprising the steps of applying an aqueous primer paint composition onto a resin substrate to form a first undercoat film, applying an aqueous colored base paint composition onto the first undercoat film to form a colored base paint film, and applying an aqueous two-component clear paint composition onto the colored base paint film to form a clear paint film.

After forming the first undercoat coating film, and before applying the aqueous colored base paint composition , a water-based surfacer may be applied onto the first undercoat coating film to form a second undercoat coating film.

(I) Step of Forming First Undercoat Film In this step, the above-mentioned aqueous primer paint composition is applied to a resin substrate to form a first undercoat film.

The amount of the aqueous primer coating composition to be applied is not particularly limited and is set appropriately according to the performance required for the coating film. The aqueous primer coating composition is applied, for example, so that the thickness of the first undercoat coating film after curing is 10 μm or more and 40 μm or less.

The method of applying the aqueous primer coating composition is not particularly limited. Examples of the application method include air spray coating, airless spray coating, rotary atomization coating, and curtain coat coating. These methods may be combined with electrostatic coating. In the case of repairs, air spray coating is preferred because it is easy to apply localized coating. Coating may be performed multiple times. In the coating methods commonly used for automobile painting (e.g., air spray coating), the aqueous primer coating composition exhibits high film-forming properties even on substrates having fine irregularities.

After the aqueous primer coating composition is applied, it is heated to harden the coating (which can also be said to be dried and solidified). The curing conditions are set appropriately depending on the amount applied and the composition. Curing is carried out, for example, at a temperature of 40°C or higher and 70°C or lower for about 30 to 60 minutes.

The coating may be allowed to dry naturally. Natural drying may be performed at room temperature (23°C ± 3°C) for at least 2 hours, at least 24 hours, or even at least 1 week.

When repairing, before applying the water-based primer coating composition, the target area and its surroundings are polished as necessary, and any irregularities are filled in with putty. The area is then polished again.

(II) Step of forming a second undercoat film In this step, a water-based surfacer is applied to the substrate to form a second undercoat film. The second undercoat film is formed as necessary.

The method of applying the water-based surfacer is not particularly limited, and may be the same as that of the water-based primer paint composition. The amount of water-based surfacer applied is not particularly limited, and is set appropriately according to the performance required of the coating film. The water-based surfacer is applied, for example, so that the thickness of the second undercoat coating film after curing is 20 μm or more and 100 μm or less. The water-based surfacer is cured, for example, by the same method as that of the water-based primer paint composition.

After curing, the first and second undercoats are usually polished and degreased. Polishing is done using sandpaper or similar.

(III) Step of forming a colored base coating film In this step, an aqueous colored base coating composition is applied onto the first or second undercoat coating film to form a colored base coating film. The application of the aqueous colored base coating composition may be carried out multiple times.

The method of applying the aqueous colored base coating composition is not particularly limited, and may be the same as that of the aqueous primer coating composition. The amount of the aqueous colored base coating composition applied is not particularly limited, and it is applied so that the thickness of the colored base coating film after curing is, for example, 10 μm or more and 100 μm or less. After application, the coating film is cured. The curing conditions are appropriately set depending on the composition of the aqueous colored base coating composition.

(IV) Step of Forming Clear Coating Film In this step, an aqueous clear coating composition is applied onto the colored base coating film to form a clear coating film.

The prepared aqueous clear coating composition is applied onto the aqueous colored base coating film. The method of applying the aqueous clear coating composition is not particularly limited, and may be the same as that of the aqueous primer coating composition. The amount of the aqueous clear coating composition applied is not particularly limited, and is appropriately set according to the performance required for the coating film. The aqueous clear coating composition is applied, for example, so that the thickness of the clear coating film after curing is 20 μm or more and 100 μm or less. The aqueous clear coating composition is cured, for example, by the same method as that of the aqueous primer coating composition.

In this way, a painted article is obtained having a multilayer coating film in which a first undercoat coating film, an optional second undercoat coating film, a colored base coating film and a clear coating film are laminated in this order.

[Multi-layer coating film]
The multilayer coating film of the present disclosure is disposed on a resin substrate. The multilayer coating film comprises a first undercoat coating film formed from the above-mentioned aqueous primer coating composition, a colored base coating film formed on the first undercoat coating film from the above-mentioned aqueous colored base coating composition , and a clear coating film formed on the colored base coating film from the above-mentioned aqueous two-pack clear coating composition . The multilayer coating film may further comprise a second undercoat coating film formed from the above-mentioned aqueous surfacer between the first undercoat coating film and the colored base coating film.

The first undercoat has excellent adhesion to resin substrates and old coatings. The multi-layer coating is suitable for repairing resin substrates with old coatings.

The thickness of the first undercoat coating film after curing is, for example, 10 μm or more and 40 μm or less. The thickness of the second undercoat coating film after curing is, for example, 20 μm or more and 100 μm or less. The thickness of the colored base coating film after curing is, for example, 10 μm or more and 100 μm or less. The thickness of the clear coating film after curing is, for example, 20 μm or more and 100 μm or less.

The present invention will be described in more detail below using examples, but the present invention is not limited to these examples. In the examples, "parts" and "%" are by weight unless otherwise specified.

[Example 1]
An aqueous primer coating composition, an aqueous surfacer, an aqueous colored base coating composition, and an aqueous clear coating composition were prepared as described below, and multi-layer coating films were formed.

(1) Preparation of the coating composition (1-1) Preparation of the aqueous primer coating composition 200 parts of aqueous polyolefin resin (A) (60 parts of resin solids) and 111 parts of aqueous epoxy resin (B) (40 parts of resin solids) were mixed. Furthermore, 1 part of surface conditioner and 4 parts of thickener (C) were added, and the mixture was stirred for 1 hour. Next, the pH was adjusted to 9.0 with dimethylethanolamine, and 6 parts of poorly water-soluble solvent (D1), 9 parts of amphipathic solvent (D2-1), 18 parts of hydrophilic solvent (D3), and 513 parts of pure water (E) were added and stirred to obtain an aqueous primer coating composition with a solid content of 11.9%. The coating specific gravity of the aqueous primer coating composition was 0.99 g/cm ³ .

(1-2) Preparation of Water-Based Surfacer As the first main agent, nax E-CUBE WB Plassa Vita Gray (manufactured by Nippon Paint Co., Ltd., solid content approximately 56%) was prepared. The first main agent contains acrylic polyol in the form of an emulsion as the hydroxyl group-containing resin (a1), contains pure water as the aqueous solvent (a2), and further contains a pigment. The solid content concentration of the hydroxyl group-containing resin (a1) was 24.4% of the total solid content contained in the first main agent.

The first curing agent was prepared by mixing 60 parts of polyisocyanate compound (b1) (Burnoc DNW-5500, manufactured by DIC Corporation), 19 parts of ethyl ethoxypropionate, 19 parts of ethylene glycol dimethyl ether, and 2 parts of an epoxy group-containing silane coupling agent (3-glycidoxypropyltriethoxysilane).

100 parts of the first base agent, 12.5 parts of the first curing agent, and an appropriate amount of pure water were mixed using a disper to prepare an aqueous surfacer. The content of the silane coupling agent was 1.18 parts by mass per 100 parts by mass of the total solid content of the hydroxyl group-containing resin (a1) and the polyisocyanate compound (b1).

(1-3) Preparation of aqueous colored base coating composition 100 parts of nax E-CUBE WB 412 Silent Black (manufactured by Nippon Paint Co., Ltd., solid content about 22%), 50 parts of nax E-CUBE WB 911 S-binder (auxiliary agent, manufactured by Nippon Paint Co., Ltd., solid content about 26%), and 45 parts of nax E-CUBE WB R20 standard diluent (dilution component, manufactured by Nippon Paint Co., Ltd.) were mixed to prepare a lacquer type aqueous colored base coating composition.

(1-4) Preparation of aqueous clear coating composition As the second main agent, a mixture of nax E-CUBE WB (2:1) NN clear (manufactured by Nippon Paint Co., Ltd., solid content approximately 38%) and pure water (aqueous solvent (a2)) was prepared. The second main agent contains acrylic polyol in the form of an emulsion as the hydroxyl group-containing resin (a1). The solid content concentration of the hydroxyl group-containing resin (a1) was 91.5% of the total solid content contained in the second main agent.

A second hardener was prepared in the same manner as the first hardener described above.

100 parts of the second base agent, 50 parts of the second curing agent, and an appropriate amount of pure water were mixed using a disper to prepare an aqueous clear coating composition. The content of the silane coupling agent was 1.35 parts by mass per 100 parts by mass of the total solid content of the hydroxyl group-containing resin (a1) and the polyisocyanate compound (b1).

(2) Formation of multi-layer coating film As the coating object, a polypropylene plate (PP plate) with a smooth surface, a PP plate with fine irregularities, a steel plate with a smooth old coating film, or a steel plate with a finely irregular old coating film was prepared, and a multi-layer coating film was formed on each surface. The smooth PP plate and the old coating film were used for adhesion evaluation. The finely irregular PP plate and the old coating film were used for appearance evaluation. After preparation of each coating composition, painting work was started within 60 minutes.

(2-1) Formation of multilayer coating film A on a PP plate with a smooth surface (2-1-1) Formation of a first undercoat coating film A PP plate (surface tension: 25.7 mN/m, SP value: 7.90) cut to 150 mm x 450 mm x thickness 0.8 mm was prepared as a substrate. The aqueous primer coating composition was applied to this PP plate undiluted with an air spray gun, and the coating was dried by air blowing until the coating film lost its luster, forming a first undercoat coating film (cured film thickness 5 μm).

(2-1-2) Formation of the second undercoat coating film A water-based surfacer was applied onto the first undercoat coating film using an air spray gun, and air-blow drying was performed until the paint no longer adhered to the surface when touched with a finger. This application and drying was repeated two more times. The coated plate was then heated in an oven set at 60°C for 30 minutes to form a second undercoat coating film (cured film thickness 45 μm). The surface of the second undercoat coating film was then polished and degreased.

(2-1-3) Formation of colored base coating film On the second undercoat coating film, the aqueous colored base coating composition was applied with an air spray gun, and the composition was dried by air blowing until the paint did not adhere to the surface when touched with a finger. This application and drying were repeated two more times to form a colored base coating film (cured film thickness 20 μm).

(2-1-4) Formation of clear coating film The aqueous clear coating composition was applied three times on the colored base coating film by a spray gun. Each application was performed with an interval of 1 minute. In this way, an uncured clear coating film was formed. Thereafter, the PP plate with the uncured clear coating film was left to stand for 15 minutes in an environment of a temperature of 23±2°C and a humidity of 50±2 RH%. Next, the PP plate was placed in an oven set at 70°C and heated for 40 minutes to cure the clear coating film. The humidity in the oven was 10 RH%.
In this manner, a multi-layer coating film P1 was formed on the smooth surface of the PP plate.

(2-2) Formation of multi-layer coating film P2 on PP plate having fine irregularities The surface of a PP plate (surface tension: 25.7 mN/m) cut to 150 mm x 450 mm x 0.8 mm thick was polished back and forth 20 times with a nonwoven abrasive sheet (equivalent to #1500) to form fine irregularities. In this way, a PP plate having fine irregularities was prepared.

Except for using a PP plate having fine irregularities as the substrate, multi-layer coating film P2 was formed in the same manner as in “(2-1) Formation of a multi-layer coating film on a PP plate having a smooth surface” above.

(2-3) Formation of a multilayer coating film on an old coating film with a smooth surface Using an aqueous clear coating composition prepared in the same manner as in the above "(1-4) Preparation of an aqueous clear coating composition", an uncured clear coating film was formed on a 70 mm x 150 mm steel plate by the same method as in the above "(2-1-4) Formation of a clear coating film". Thereafter, the steel plate with the uncured clear coating film was left to stand for 15 minutes in an environment of a temperature of 23 ± 2 ° C and a humidity of 50 ± 2 RH%. Next, the steel plate was placed in an oven set at 70 ° C (humidity 10 RH%) and heated for 40 minutes to cure the clear coating film. Subsequently, the plate was aged at 23 ° C for one week to obtain a steel plate with an old coating film with a smooth surface.

A multi-layer coating film M1 was formed in the same manner as in "(2-1) Formation of a multi-layer coating film on a PP plate with a smooth surface" above, except that a steel plate with an old coating film with a smooth surface was used as the substrate.

(2-4) Formation of a multi-layer coating film on an old coating film having fine irregularities The surface of a steel plate having an old coating film with a smooth surface obtained in the same manner as in "(2-3) Formation of a multi-layer coating film on an old coating film with a smooth surface" was polished back and forth 20 times with a nonwoven fabric polishing sheet (equivalent to #1500) to form fine irregularities. In this way, a steel plate having an old coating film with fine irregularities was prepared.

Except for using a steel plate with an old coating film having irregularities as the substrate, multi-layer coating film M2 was formed in the same manner as in “(2-1) Formation of a multi-layer coating film on a PP plate having a smooth surface” above.

[Examples 2 to 18, Comparative Examples 1 to 9]
Except for preparing the aqueous primer coating compositions as shown in Table 1, multi-layer coating films were formed on four types of substrates in the same manner as in Example 1.

Details of each component in the table are as follows:
・Water-based polyolefin resin (A)
Product name: Auroren AE-301, emulsion of non-chlorinated polypropylene resin, manufactured by Nippon Paper Industries Co., Ltd., Tg-28°C, Mw 50,000
・Water-based epoxy resin (B)
Product name: MODEPIX 304, emulsion of acrylic modified bisphenol A type epoxy resin, manufactured by Arakawa Chemical Industries, Ltd.; thickener (C)
Product name: ADEKA NOL UH-756VF, urethane association type, manufactured by ADEKA Corporation; surface conditioner Product name: Tego Wet 260, manufactured by EVONIC Corporation; polyether modified silicone compound; 100% active ingredient
Poorly water-soluble solvent (D1)
2-Ethyl-1-hexanol (SP value: 9.85, specific gravity: 0.83 g/cm ³ )
Amphiphilic solvent (D2-1)
Propylene glycol monobutyl ether (SP value: 9.69, specific gravity: 0.88 g/cm ³ )
Amphiphilic solvent (D2-2)
Dipropylene glycol dimethyl ether (SP value: 7.88, specific gravity: 0.903 g/cm ³ )
Hydrophilic solvent (D3)
Propylene glycol monopropyl ether (SP value: 9.82, specific gravity: 0.885 g/cm ³ )
・Water-based polyurethane resin (F)
Product name: HYDRAN WLS-210, manufactured by DIC Corporation, Tg-32°C, acid value 28 mgKOH/g

[Evaluation of Water-Based Primer Coating Compositions]
The aqueous primer coating composition was evaluated.

Surface tension was measured by the platinum ring method using a Dynometer (manufactured by Bykgardner, Germany).

Storage stability 900 ml of the aqueous primer coating composition was filled into a 1 L inner surface coated can and allowed to stand at 23° C. After one month, the state of the aqueous primer coating composition was visually evaluated. The evaluation criteria are as follows. If the evaluation is B or higher, it can be said that the storage stability is excellent.

(Evaluation criteria)
A: No change before and after storage. B: Separation has occurred, but the mixture returns to the state before storage by stirring. C: The mixture has separated or gelled, and the mixture does not return to the state before storage by stirring.

Film-forming properties The surface of a polypropylene plate cut to 150 mm x 450 mm x 0.8 mm thick was polished back and forth 20 times with a nonwoven abrasive sheet (equivalent to #1500) to form fine irregularities. An aqueous primer coating composition (coating amount 150 g/m ² ) was applied to this PP plate using a spray gun. The appearance from immediately after painting until natural drying was visually evaluated. The evaluation criteria are as follows. If the evaluation is B or higher, it can be said that the film-forming properties are excellent.

(Evaluation criteria)
A: No creasing or smearing was observed immediately after painting, and a smooth coating was formed. B: Smearing was observed immediately after painting, but as painting was completed and the paint filled in the unevenness to form a smooth coating. Unevenness was observed on the coating surface of the wet coating, and unevenness was also observed in the dried coating, presenting problems. C: Crackling and/or smearing was observed on the coating surface of the wet coating, and a uniform coating was not formed after drying.

[Evaluation of multi-layer coating film]
The water resistance of the multi-layer coating film to the above-mentioned substrate (polypropylene plate) or to the old coating film was evaluated. As the water resistance, the adhesion and the appearance after the immersion test were evaluated.

Adhesion evaluation was performed on multi-layer coating films P1 and M1 formed on smooth PP plates or steel plates with old coatings. Since adhesion tends to decrease when the painted surface is smooth, it can be evaluated that if the adhesion of multi-layer coating films P1 and M1 is high, the adhesion of multi-layer coating films P2 and M2 is also high.

Appearance evaluation was performed on multi-layer coating films P2 and M2 formed on PP plates with fine irregularities or steel plates with old coating films. Since appearance tends to be reduced when the coating surface is irregular, if the appearance of multi-layer coating films P2 and M2 is good, it can be evaluated that the adhesion of multi-layer coating films P1 and M1 is also good.

(Evaluation of polypropylene plate)
Adhesion A constant temperature water bath with a circulation function was set to 40°C and filled with deionized water. A coated article having a multi-layer coating film P1 was immersed in the water. After immersion for 72 hours, the coated article was pulled out and the water droplets were lightly wiped off. After pulling out, the coated article was left at room temperature (23°C) for 24 hours.
After the immersion test, the multilayer coating film P1 was cut with an NT cutter (manufactured by NT Corporation) to make 100 squares (11 vertical lines, 11 horizontal lines, 2 mm intervals) that reached the substrate. Tape (Nichiban Corporation, Cellotape (registered trademark), 24 mm wide) was applied to cover all the squares, and the tape was firmly attached from above with a fingernail or the like. The tape was then peeled off while being pulled so that the angle between the tape and the coating film was about 45°. After the tape was peeled off, the number of squares where the coating film remained was counted. The more squares that remained, the higher the adhesion.

Appearance: In the same manner as described above, an immersion test was performed on the coated article having the multi-layer coating film P2. The appearance of the multi-layer coating film P2 was then evaluated visually. The evaluation criteria are as follows. If the evaluation is B or higher, it can be said that the appearance is excellent.

(Evaluation criteria)
A: No blistering or whitening B: Slight blistering or whitening C: Obvious blistering and/or whitening

(Evaluation of old coating film)
The coated article having the multi-layer coating film M1 was subjected to an adhesion evaluation in the same manner as described above. The coated article having the multi-layer coating film M2 was subjected to an appearance evaluation in the same manner as described above.

The aqueous primer coating compositions of Examples 1 to 18 had excellent storage stability and film-forming properties, and the resulting coating films had good adhesion and appearance.

The aqueous primer coating composition of Comparative Example 1 gelled immediately after preparation and lost fluidity, making it impossible to apply. As a result, it was not possible to form a multi-layer coating film, and it was not possible to evaluate the multi-layer coating film. The aqueous primer coating compositions of Comparative Examples 2 to 4 showed good results in the film-forming evaluation using a large amount of coating, but the results were poor in the adhesion and appearance evaluation using an amount close to that of automobile repair paint. The aqueous primer coating composition of Comparative Example 5 had poor storage stability. The aqueous primer coating compositions of Comparative Examples 6 to 9 showed poor film-forming evaluation, and also showed poor results in the adhesion and appearance evaluation.

The present invention includes the following aspects.
[1]
A method for forming a first undercoat film by applying an aqueous primer coating composition onto a resin substrate;
Applying an aqueous colored base coating composition onto the first undercoat coating to form a colored base coating;
A water-based two-pack clear coating composition is applied onto the colored base coating film to form a clear coating film.
The aqueous primer coating composition contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
A method for forming a multilayer coating film, wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition.
[2]
The method for forming a multilayer coating film according to the above [1], wherein the mass of the organic solvent (D) in the organic component mass is 30 g/L or more and 350 g/L or less.
[3]
The method for forming a multilayer coating film according to the above [1] or [2], wherein the surface tension of the aqueous primer coating composition is 26.0 mN/m or more and 30.0 mN/m or less.
[4]
The method for forming a multilayer coating film according to any one of the above [1] to [3], wherein the aqueous epoxy resin (B) contains a bisphenol A type epoxy resin.
[5]
The method for forming a multilayer coating film according to any one of the above [1] to [4], wherein the thickener (C) contains a urethane association type thickener.
[6]
The method for forming a multilayer coating film according to any one of the above [1] to [5], further comprising an aqueous polyurethane resin (F).
[7]
The method for forming a multilayer coating film according to any one of the above [1] to [6], wherein the resinous substrate comprises a substrate and a coating film other than the multilayer coating film that covers at least a portion of the substrate.
[8]
The method for forming a multilayer coating film according to any one of the above [1] to [7], wherein the difference between the solubility parameter of the polymer constituting the surface of the resin substrate and the solubility parameter of the organic solvent (D) contained in the aqueous primer coating composition is 2.5 or less.
[9]
After forming the first undercoat coating film, and before applying the aqueous colored base coating composition ,
The method for forming a multilayer coating film according to any one of [1] to [8] above, further comprising applying an aqueous surfacer onto the first undercoat coating film to form a second undercoat coating film.
[10]
A first undercoat coating film formed on a resin substrate using an aqueous primer coating composition;
A colored base coating film formed on the first undercoat coating film using an aqueous colored base coating composition ;
A clear coating film formed on the colored base coating film by an aqueous two-pack clear coating composition ,
The aqueous primer coating composition contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
A multilayer coating film, wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition.
[11]
A first undercoat coating film formed on a resin substrate using an aqueous primer coating composition;
A colored base coating film formed on the first undercoat coating film using an aqueous colored base coating composition ;
A water-based primer coating composition used for forming a multi-layer coating film having a clear coating film formed on the colored base coating film by using a water-based two-pack clear coating composition,
The composition comprises an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
An aqueous primer coating composition , wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less relative to the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition.

According to the present invention, a multi-layer coating film with excellent appearance is provided, which is formed from an all-water-based coating composition including a primer coating composition that has sufficiently low VOC emissions and excellent storage stability and adhesion to resin substrates. Therefore, the water-based coating composition is suitable for painting automobiles, particularly for painting automobile repairs.

Claims (10)

A method for forming a first undercoat coating film by applying an aqueous primer coating composition onto a resin substrate;
Applying an aqueous colored base coating composition onto the first undercoat coating to form a colored base coating;
A water-based two-pack clear coating composition is applied onto the colored base coating film to form a clear coating film.
The aqueous primer coating composition contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
The aqueous epoxy resin (B) contains a bisphenol A type epoxy resin,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
A method for forming a multilayer coating film, wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition. The method for forming a multilayer coating film according to claim 1, wherein the mass of the organic solvent (D) in the organic component mass is 30 g/L or more and 350 g/L or less. The method for forming a multi-layer coating film according to claim 1 or 2, wherein the surface tension of the aqueous primer coating composition is 26.0 mN/m or more and 30.0 mN/m or less. The method for forming a multi-layer coating film according to claim 1 or 2 , wherein the thickener (C) comprises a urethane association type thickener. The method for forming a multilayer coating film according to claim 1 or 2 , further comprising an aqueous polyurethane resin (F). 3. The method for forming a multi-layer coating film according to claim 1 or 2 , wherein the resinous substrate comprises a substrate and a coating film other than the multi-layer coating film that covers at least a portion of the substrate. The method for forming a multilayer coating film according to claim 1 or 2, wherein the difference between the solubility parameter of the polymer constituting the surface of the resin-made coating object and the solubility parameter of the organic solvent (D) contained in the aqueous primer coating composition is 2.5 or less. After forming the first undercoat coating film, and before applying the aqueous colored base coating composition ,
The method for forming a multilayer coating film according to any one of claims 1 to 7, further comprising applying an aqueous surfacer onto the first undercoat coating film to form a second undercoat coating film. A first undercoat coating film formed on a resin substrate using an aqueous primer coating composition;
A colored base coating film formed on the first undercoat coating film using an aqueous colored base coating composition ;
A clear coating film formed on the colored base coating film by an aqueous two-pack clear coating composition ,
The aqueous primer coating composition contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
The aqueous epoxy resin (B) contains a bisphenol A type epoxy resin,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
A multilayer coating film, wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition. A first undercoat coating film formed on a resin substrate using an aqueous primer coating composition;
A colored base coating film formed on the first undercoat coating film using an aqueous colored base coating composition ;
A water-based primer coating composition used for forming a multi-layer coating film having a clear coating film formed on the colored base coating film by using a water-based two-pack clear coating composition,
The composition comprises an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D) and water (E), and has a solid content of 5% by mass or more and 17% by mass or less;
The aqueous polyolefin resin (A) has a glass transition temperature of −40° C. or higher and −10° C. or lower,
The aqueous epoxy resin (B) contains a bisphenol A type epoxy resin,
a mass ratio (A/B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60/40 to 95/5;
The organic solvent (D) includes a slightly water-soluble solvent (D1) that dissolves at less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves at least 1 g but less than 60 g in 100 g of water at 20° C., and a hydrophilic solvent (D3) that dissolves at least 60 g in 100 g of water at 20° C.,
The proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass% or more and 60 mass% or less,
An aqueous primer coating composition , wherein the mass of the organic solvent (D) is 30 g/L or more and 500 g/L or less relative to the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating composition.