Method for forming multilayer coating film, multilayer coating film, and aqueous primer coating composition

Abstract

A method for forming a multilayer coating film comprising an undercoating film, a colored base coating film and a clear coating film formed by using aqueous coating compositions on an article to be coated made of a resin, wherein: an 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 mass% to 17 mass% inclusive; the aqueous polyolefin resin (A) has a Tg of -40°C to -10° C; the mass ratio of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60 / 40 to 95 / 5; the organic solvent (D) contains a poorly water-soluble solvent (D1) and at least one of an amphiphilic solvent (D2) and a hydrophilic solvent (D3); the proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1-60 mass %; and the aqueous primer coating composition has a VOC content of 30-500 g / L.

Description

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

[0001] The present invention relates to a method for forming a multi-layer coating film, a multi-layer coating film, and an aqueous primer coating composition.

[0002] 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. Patent Document 1 discloses an aqueous primer paint composition.

[0003] Patent No. 7134389

[0004] Although the aqueous primer coating composition of Patent Document 1 is aqueous, it is unable to sufficiently reduce the amount of volatile organic compounds (VOCs) emitted.

[0005] The present invention aims to provide a multi-layer coating film with excellent appearance, 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.

[0006] In order to solve the above problems, the present invention provides the following aspects: [1] A method for coating a resin substrate with an aqueous primer coating composition 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, wherein 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 solids 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, and 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) comprises a poorly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves 1 g to less than 60 g in 100 g of water at 20° C. and 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 mass % to 60 mass %, and the mass of the organic solvent (D) relative to the mass of the organic components obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer paint is 30 g / L to 500 g / L. [2] The method for forming a multilayer coating film according to [1] above, wherein the mass of the organic solvent (D) relative to the mass of the organic components is 30 g / L to 350 g / L. [3] The method for forming a multilayer coating film according to [1] or [2] above, wherein the surface tension of the aqueous primer coating 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 [1] or [2] above, wherein the aqueous epoxy resin (B) contains a bisphenol A epoxy resin. [5] The method for forming a multilayer coating film according to [1] or [2] above, wherein the thickener (C) contains a urethane associative thickener. [6] The method for forming a multilayer coating film according to [1] or [2] above, further containing an aqueous polyurethane resin (F).[7] The method for forming a multi-layer coating film according to [1] or [2] above, wherein the resin 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 multi-layer coating film according to [1] or [2] above, wherein the difference between the solubility parameter of the polymer that constitutes 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] The method for forming a multi-layer coating film according to [1] or [2] above, comprising, after forming the first undercoat coating film and before applying the water-based colored base paint, applying an aqueous surfacer on the first undercoat coating film to form a second undercoat coating film.

[10] A coating composition comprising: a first undercoat film formed on a resin substrate from an aqueous primer coating composition; a colored base coating film formed on the first undercoat film from an aqueous colored base coating; and a clear coating film formed on the colored base coating film from an aqueous two-component clear coating; wherein 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 solids 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; and the mass ratio (A / B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60 / 40 to 95 / 5; a multilayer coating film, wherein the organic solvent (D) comprises a poorly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, and at least one of an amphipathic solvent (D2) that dissolves 1 g or more but less than 60 g in 100 g of water at 20°C, and a hydrophilic solvent (D3) that dissolves 60 g or more in 100 g of water at 20°C, wherein the proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass % or more and 60 mass % or less, and the mass of the organic solvent (D) in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating is 30 g / L or more and 500 g / L or less.

[11] An aqueous primer coating composition used for forming a multilayer coating film having a first undercoat coating film formed on a resin substrate from an aqueous primer coating composition, a colored base coating film formed on the first undercoat coating film from an aqueous colored base paint, and a clear coating film formed on the colored base coating film from an aqueous two-component clear paint, 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 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) being 60 / 40 to 95 / 5, The organic solvent (D) comprises a slightly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, and at least one of an amphipathic solvent (D2) that dissolves 1 g or more but less than 60 g in 100 g of water at 20°C and a hydrophilic solvent (D3) that dissolves 60 g or more in 100 g of water at 20°C, wherein the proportion of the slightly water-soluble solvent (D1) in the organic solvent (D) is 1 mass % or more and 60 mass % or less, and the mass of the organic solvent (D) in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer paint is 30 g / L or more and 500 g / L or less.

[0007] According to the present invention, there is provided a multi-layer coating film having excellent appearance, 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.

[0008] The method for forming a multi-layer coating film according to the present disclosure comprises applying an aqueous primer coating composition to a resin substrate to form a first undercoat coating film, applying an aqueous colored base coating composition to the first undercoat coating film to form a colored base coating film, and applying an aqueous two-component clear coating composition to the colored base coating film to form a clear coating film. After forming the first undercoat coating film, and before applying the aqueous colored base coating film, the method may also comprise applying an aqueous surfacer to the first undercoat coating film to form a second undercoat coating film.

[0009] 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 solids 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, and at least one of an amphipathic solvent (D2) that dissolves 1 g to less than 60 g in 100 g of water at 20°C, and 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 obtained by subtracting the mass of water (E) from the total mass of the aqueous primer coating is 30 g / L or more and 500 g / L or less.

[0010] In recent years, resins have been proposed as a material for automotive components in place of metals. The surface tension of resin substrates is typically 40 mN / m or less. On the other hand, the surface tension of water is approximately 72.7 mN / m. Therefore, aqueous coating compositions generally have difficulty wetting resin substrates, making film formation difficult. In the present disclosure, the aqueous primer coating composition contains a small amount of a poorly water-soluble organic solvent, thereby achieving both low VOC emissions and high adhesion to resin substrates (particularly adhesion after a water immersion test (hereinafter also referred to as water resistance)). The aqueous primer coating composition is suitable for coating resin substrates.

[0011] Aqueous primer coating compositions are particularly prone to penetrating fine irregularities (scratches) on the surface of resin substrates. Therefore, when an aqueous primer coating composition is applied to a resin substrate having fine irregularities, repelling and smearing are suppressed, resulting in uniform film formation. Therefore, a smooth primer coating film with high adhesion is formed. 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 fine irregularities is referred to as film-forming ability. Film-forming ability affects the appearance and adhesion of the entire multi-layer coating film.

[0012] 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 about 0.1 μm (0.07 μm to 0.12 μm).

[0013] The aqueous primer coating composition contains a predetermined amount of the 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 with the substrate exposed in other areas.

[0014] The old paint film is a paint film other than the multi-layer paint film of the present disclosure that was formed on the substrate before the multi-layer paint film of the present disclosure was formed. The old paint film may be, for example, a paint film for automobile exteriors (at least one of an intermediate coat, a base layer, and a clear coat). During partial repair, a portion of the old paint film (e.g., a damaged portion) is removed by grinding, and then the multi-layer paint film of the present disclosure is formed. During repair, the multi-layer paint film of the present disclosure may be formed on the old paint film without removing the old paint film.

[0015] Hereinafter, aqueous resin refers to a water-soluble resin or a water-dispersible resin. Water-dispersible resins are further divided into dispersion type (commonly referred to as colloidal dispersion type) and emulsion type. Colloidal dispersion type aqueous resins are typically obtained by semi-dissolving a resin synthesized in an organic solvent in water with a neutralizing agent. Emulsion type aqueous resins are typically produced by emulsion polymerization or mechanically forced emulsification. Aqueous resins can also be obtained by modifying a hydrophobic resin with a polar compound or by using a polar monomer in the resin synthesis. Aqueous resins may be dispersion type. "Water-based" refers to making a resin water-soluble or water-dispersible.

[0016] The aqueous coating composition contains water as a solvent, and the proportion of water in the total solvent is 50% by mass or more, or may be 60% by mass or more, or 70% by mass or more, or may be 80% by mass or more.

[0017] The resin solid content of an aqueous coating composition is the total solid 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 coating composition are an aqueous polyolefin resin (A), an aqueous epoxy resin (B), and other resins.

[0018] The weight average molecular weight and number average molecular weight are measured using polystyrene standards by the GPC (gel permeation chromatography) method.

[0019] The glass transition temperature Tg may be calculated from the types and amounts of raw material monomers, or may be measured by a differential scanning calorimeter (DSC).

[0020] The acid value and hydroxyl value can be measured by a 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 monomers of the target resin. The acid value and hydroxyl value are determined based on the solid content.

[0021] 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.

[0022] The surface tension is measured by the platinum ring method using, for example, a Dynometer (manufactured by Byck-Gardner, Germany).

[0023] The solubility parameter (SP value) may be a known value. The SP value may be measured by the following method (Reference: SUH, CLARKE, JPSA-1, 5, 1671-1681 (1967)).

[0024] The sample used is 0.5 g of the object to be measured, weighed into a 100 ml beaker and dissolved in 10 ml of a good solvent (acetone). A poor solvent is added dropwise to this sample using a 50 ml burette at a measurement temperature of 20°C, and the point at which turbidity occurs is recorded as the amount added. As the 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 cloudiness points of each are measured. The SP value δ of the object to be measured is given by the following formula:

[0025] Vi: Molar volume of solvent (ml / mol) φi: Volume fraction of solvent i at cloudiness point δi: SP value of solvent ml: Low SP poor solvent mixture mh: High SP poor solvent mixture

[0026]

[0027]

[0028] First, the coating composition used to form the multi-layer coating film will be described. (Aqueous 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).

[0029] 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 ability, 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.

[0030] The solids content of an 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%) as a percentage.

[0031] 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) (hereinafter referred to as the VOC amount) is 30 g / L or more and 500 g / L or less. If the VOC amount contained in the coating is low, the amount of VOC emitted during coating film formation will also be reduced.

[0032] The VOC amount can be calculated using 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.

[0033] To calculate the amount of VOC, please refer to Mitsuhiko Asakura, "Advances in Water-Based Paints for Environmental Compliance," Surface Technology, Surface Technology Association, 1997, Vol. 48, No. 8, pp. 760-764.

[0034] Although a lower VOC content is desirable, from the viewpoint of adhesion to a resin substrate, the VOC content may be 40 g / L or more, or 100 g / L or more. The VOC content may be 350 g / L or less, 300 g / L, or 250 g / L.

[0035] 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).

[0036] Aqueous Polyolefin Resin (A) The aqueous polyolefin resin (A) is not particularly limited and can be produced by a conventionally known method. For example, the aqueous polyolefin resin (A) can be obtained by adding toluene to an acid anhydride-modified polyolefin resin and heating the mixture to obtain a resin solution, adding a surfactant to the resin solution, and then adding ion-exchanged water dropwise to the solution while forcibly stirring at 50 to 60°C to effect phase inversion emulsification. Alternatively, the aqueous polyolefin resin (A) can be obtained by heating an acid anhydride-modified polyolefin resin to dissolve it in a solvent such as tetrahydrofuran, neutralizing the carboxyl groups of the resin with an excess amine, and then adding ion-exchanged water dropwise to the solution while forcibly stirring at 50 to 60°C to effect phase inversion emulsification. Alternatively, the aqueous polyolefin resin (A) can be obtained by mixing an emulsifier and an amine with the solution, followed by adding ion-exchanged water at about 60°C dropwise to the solution while forcibly stirring to effect emulsification. The aqueous polyolefin resin (A) can also be obtained by a method in which, inverse to the above procedure, a molten or dissolved acid anhydride-modified polyolefin is added dropwise to warm water in which a neutralizing agent such as an amine and / or a surfactant has been dissolved, while the water is being forcibly stirred, to emulsify the mixture.

[0037] Examples of the aqueous polyolefin resin (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 with other unsaturated monomers (e.g., vinyl acetate, butadiene, acrylic esters, methacrylic esters). These may be used alone or in combination of two or more.

[0038] The aqueous polyolefin resin (A) may be a homopolymer of propylene, a copolymer of propylene with other olefin monomers, a copolymer of propylene with 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.

[0039] The aqueous polyolefin resin (A) may be a modified product. Examples of the modified product include an adduct of an unsaturated carboxylic acid anhydride with an aqueous polyolefin resin. Examples of the unsaturated carboxylic acid anhydride include maleic anhydride, fumaric acid, itaconic anhydride, and (meth)acrylic acid. The modified product is usually obtained by reacting a polyolefin with an unsaturated carboxylic acid anhydride in the presence of an organic peroxide or the like.

[0040] 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.

[0041] 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.

[0042] 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, adhesion to a resin substrate can be further improved. When the solid content mass of the aqueous polyolefin resin (A) is 98 parts by mass or less, adhesion to an 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, 90 parts by mass or less, or 85 parts by mass or less.

[0043] The aqueous polyolefin resin (A) may be chlorinated or unchlorinated. From the viewpoint of environmental conservation, the aqueous polyolefin resin (A) does not have to 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 chlorinated solvent such as carbon tetrachloride, and blowing chlorine gas into the solution at a temperature of 50 to 120°C.

[0044] Commercially available aqueous polyolefin resins (A) include, for example, the Superchlor series and Auroren series (all manufactured by Nippon Paper Industries Co., Ltd.), the Hardlen series (manufactured by Toyobo Co., Ltd.), and the Aptlock series (manufactured by Mitsubishi Chemical Corporation).

[0045] Commercially available non-chlorinated aqueous polyolefin resins (A) include Auroren AE-301, Superchlorine E-415, and E-480T (all manufactured by Nippon Paper Chemicals Co., Ltd.). Commercially available chlorinated aqueous polyolefin resins (A) include 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).

[0046] - Water-based epoxy resin (B) Water-based epoxy resin (B) contributes to improving the water resistance and moisture resistance of the multi-layer coating film. During repair painting, water-based epoxy resin (B) contributes to improving the adhesion between the new coating film and the old coating film. Water-based primer coating compositions are particularly suitable for repairs.

[0047] The aqueous epoxy resin (B) can be obtained as a water dispersion by, for example, graft polymerizing an epoxy resin with a fatty acid (for example, a carboxyl group-containing vinyl monomer) and then neutralizing the resulting mixture.

[0048] 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.

[0049] The mass ratio (A / B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60 / 40 to 95 / 5. This allows the multi-layer coating film formed using the all-aqueous paint to exhibit excellent water resistance. The mass ratio (A / B) may be 70:30 to 90:10.

[0050] Examples of the aqueous 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 bisphenol type epoxy resins with dibasic acids or the like; 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, as a constituent, an epoxy group-containing acrylic monomer such as glycidyl (meth)acrylate. These may be used alone or in combination of two or more.

[0051] Among these, from the viewpoint of adhesion, aromatic epoxy resins, bisphenol type epoxy resins, and bisphenol A type epoxy resins may be used.

[0052] The aqueous epoxy resin (B) can be obtained, for example, by adding epichlorohydrin to a phenol novolac resin. Commercially available products of such aqueous epoxy resin (B) include Denacol EM150 (manufactured by Nagase Chemtech Co., Ltd.), Epilez 6006W70 and 5003W55 (manufactured by Japan Epoxy Resins Co., Ltd.), Watersol series (manufactured by DIC Corporation), and WEX-5100 (manufactured by Tohto Kasei Co., Ltd.).

[0053] The aqueous epoxy resin (B) may be obtained by, for example, forcibly emulsifying a bisphenol-type epoxy resin with an emulsifier. Commercially available aqueous epoxy resins (B) include Denacol EM101 and EM103 (manufactured by Nagase Chemtech Co., Ltd.), and Epilez 3510W60, 3515W6, 3522W60, and 3540WY55 (all manufactured by Japan Epoxy Resins Co., Ltd.).

[0054] The aqueous epoxy resin (B) may be, for example, an alkyl type in which epichlorohydrin is added to a polyol such as sorbitol, pentaerythritol, glycerin, etc. Commercially available products of such aqueous epoxy resin (B) include Denacol EX-611, EX-614, EX-411, and EX-313 (all manufactured by Nagase Chemtech Co., Ltd.).

[0055] 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, Ltd.).

[0056] 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 thickeners, polyamide thickeners, and alkali swelling thickeners. These may be used alone or in combination of two or more. Among these, urethane association thickeners are preferred from the viewpoint of flowability.

[0057] Examples of urethane associative thickeners include polyurethane thickeners having hydrophobic chains in the molecule, and urethane-urea thickeners in which at least a portion of the main chain is a hydrophobic urethane chain.

[0058] Commercially available urethane associative thickeners include, for example, ADEKA NOL UH-756VF (manufactured by ADEKA Corporation) and BYK-420 (manufactured by BYK Corporation).

[0059] 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 2 parts by mass or more. The solid content mass of the thickener (C) may be 4.5 parts by mass or less, or 4 parts by mass or less.

[0060] Organic solvent (D) The organic solvent (D) reduces the surface tension of the aqueous primer coating composition, improving its wettability to resin substrates. From the standpoint of water-based coating stability and reducing VOC emissions, it is desirable to use a small amount of 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 to resin substrates can be improved with a small amount of organic solvent (D).

[0061] 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 proportion of the poorly water-soluble solvent (D1) may be 5% by mass or more, or 10% by mass or more. The 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.

[0062] The organic solvent (D) contains at least one of an amphipathic solvent (D2) and a hydrophilic solvent (D3) together with the poorly water-soluble solvent (D1), which improves the compatibility between the organic solvent (D) and water (E) and improves the storage stability of the aqueous primer coating composition.

[0063] Examples of the poorly water-soluble solvent (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 the poorly water-soluble solvent (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).

[0064] Examples of the amphipathic solvent (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 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.

[0065] Examples of the hydrophilic solvent (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.

[0066] 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 a 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.

[0067] The SP value of the 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 the organic solvent (D) is the sum of (the mixing ratio X1 (volume %) of the poorly water-soluble solvent (D1) × its SP value Y1), (the mixing ratio X2 (volume %) of the amphipathic solvent (D2) × its SP value Y2), and (the mixing ratio X3 (volume %) of the hydrophilic solvent (D3) × its SP value Y3).

[0068] 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.

[0069] The polymer that makes up the surface of a resinous object is a polymer that accounts for 50% or more by mass of the surface of the object. For example, if a component labeled as made of polypropylene resin is used as the object, the SP value of the known polypropylene resin can be considered the SP value of the polymer that makes up the surface of the object. (Reference: Toshikatsu Kobayashi, "Solubility Parameters and Surface Tension: Fundamentals of Dissolving, Mixing, Wetting, and Adhesion," Journal of the Japan Society of Color Materials, Vol. 90, No. 9, 2017, pp. 324-332)

[0070] The SP value of the polymer that constitutes the surface of the coated substrate can be substituted for the SP value of polypropylene resin. Polypropylene resin is known to be particularly difficult to adhere to, making it difficult to adhere a coating to it. Therefore, organic solvents (D) with an SP difference ΔSP of 2.5 or less from the SP value of polypropylene resin can be said to have excellent wettability with other polymers. The SP value of polypropylene resin is known to be 7.90 (Source: Masaki Haneda, "Surface Modification of Plastic Films," Journal of the Japanese Society of Printing Science, Vol. 47, No. 2 (2010), Review, pp. 78-83). In other words, the "difference ΔSP between the SP value of organic solvent (D) and the SP value of the polymer that constitutes the surface of the resin substrate" can be rephrased as the "difference ΔSP between the SP value of organic solvent (D) and the SP value of polypropylene resin (= 7.90)."

[0071] Water (E) Examples of water (E) include various types of water such as pure water, ion-exchanged water, tap water, and industrial water.

[0072] Aqueous 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. An aqueous primer coating composition containing the aqueous polyurethane resin (F) is particularly suitable for repairs.

[0073] The aqueous polyurethane resin (F) is prepared, for example, by a method in which an NCO-terminated prepolymer obtained by the method described below is neutralized, dispersed in water, and then subjected to a chain extension reaction, that is, a so-called prepolymer mixing method.

[0074] 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 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 6 parts by mass or less.

[0075] 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.

[0076] 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.

[0077] The polyurethane resin is prepared, for example, by reacting a polyhydric alcohol, a polyisocyanate compound, and a compound having an active hydrogen group in the molecule.

[0078] 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.

[0079] The polyisocyanate compound has two or more isocyanate groups in the molecule. Examples of the polyisocyanate compound 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 methyl methyl cyanate, 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.

[0080] 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. Alternatively, a polyisocyanate having a urethane bond obtained by reacting the above polyisocyanate compound with a polyhydric alcohol may be used.

[0081] Among these, alicyclic diisocyanates are preferred, as they introduce a cyclic structure into the molecules of the polyurethane resin (F), thereby improving weather resistance.

[0082] The compound having an active hydrogen group in the molecule is used for blocking the isocyanate group in the polyisocyanate compound. Examples of the compound 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; monohydric thiols such as ethyl mercaptan; primary amines such as diethylenetriamine and monoethanolamine; secondary amines such as diethylamine; and oximes such as methyl ethyl ketoxime.

[0083] In preparing the polyurethane resin, a chain extender, a monoisocyanate compound, a catalyst, etc. may be used as needed.

[0084] The polyurethane resin is synthesized, for example, by a one-shot method in which each component is reacted at once, or a multi-stage method in which the components are reacted in stages (a method in which a part of an active hydrogen-containing compound (e.g., a high molecular weight 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.

[0085]

[0043] Examples of commercially available aqueous polyurethane resins (F) include the Superflex series, such as Superflex 150, Superflex 420, and Superflex 460 (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), the Neosticker series, and the Evaphanol series (all manufactured by Nicca Chemical Co., Ltd.), the Bayhydrol series, such as Bayhydrol VP LS2952 (manufactured by Sumika Covestro Urethane 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).

[0086] Additives The aqueous primer coating composition may contain various additives as needed, such as surface conditioners, preservatives, mildew inhibitors, antifoaming agents, light stabilizers, UV absorbers, antioxidants, and pH adjusters.

[0087] 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 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.

[0088] (Aqueous Surfacer) The aqueous surfacer is, for example, a two-component type, and includes, for example, a first main component containing a hydroxyl group-containing resin (a1) and an aqueous solvent (a2), and a first curing agent containing a polyisocyanate compound (b1) and an organic solvent (b2) that does not have a hydroxyl group.

[0089] 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 provide coating films with good physical properties such as appearance, weather resistance, and chemical resistance.

[0090] The hydroxyl group-containing resin (a1) can be obtained, for example, by emulsion polymerization of one or more raw material monomers. The emulsion polymerization is carried out by a method commonly used by those skilled in the art. Specifically, an emulsifier is mixed with an aqueous medium containing an organic solvent such as alcohol as necessary, and the resulting mixture is heated and stirred while the raw material monomers and polymerization initiator are added dropwise. Alternatively, an emulsion mixture in which the raw material monomers, emulsifier, and water are pre-emulsified may be added dropwise to the aqueous medium. The aqueous medium may be the same as or different from the aqueous solvent (a2).

[0091] The acrylic polyol emulsion can be obtained, for example, by emulsion polymerization of a raw material monomer mixture containing a (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.

[0092] Examples of the aqueous solvent (a2) include those similar to water (E). The first main component 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).

[0093] The water-based surfacer is prepared by mixing the first main component and the first curing agent. The mixing of the first main component and the first curing agent is carried out immediately before painting. The first main component and the first curing agent may be mixed and then supplied to a painting device, or may be mixed within the painting device.

[0094] The first main 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 in the range of isocyanate group equivalent / hydroxyl group equivalent = 1 / 0.5 to 1 / 2.

[0095] 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.

[0096] Examples of the pigment (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.

[0097] In particular, from the viewpoint of the polishability 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.

[0098] The aqueous 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 organic functional groups. The epoxy group-containing silane coupling agent may have two or more epoxy groups.

[0099] 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.

[0100] The epoxy group-containing silane coupling agent may be added to at least one of the first main agent and the first curing agent in an amount of 1 to 8 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).

[0101] (Aqueous Colored Base 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).

[0102] The curable aqueous colored base coating composition contains a curable resin (e.g., the first main component described above) and a curing agent (e.g., the first curing agent described above), and forms a cured coating film by heating. The curable aqueous colored base coating composition may further contain a silane coupling agent (e.g., an epoxy group-containing silane coupling agent).

[0103] Lacquer-type aqueous 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. The concentration of the binder resin is not particularly limited and is set appropriately depending on the composition and application.

[0104] The binder resin is not particularly limited. Examples of binder resins include acrylic resins, aminoalkyd resins, alkyd resins, amino resins, isocyanate resins, epoxy resins, vinyl chloride resins, cashew resins, silicone resins, styrene resins, styrenated alkyd resins, cellulose-based resins (e.g., cellulose nitrate), urea resins, vinyl resins, phenolic resins, phthalic acid resins, fluororesins, polyester resins, unsaturated polyester resins, and modified resins thereof (e.g., rosin-modified, phenol-modified, epoxy resin-modified, styrene-modified, acrylic-modified, and urethane-modified). These may be used alone or in combination of two or more. Examples of organic solvents include naphtha, xylene, toluene, and acetone.

[0105] (Aqueous clear coating composition) The aqueous clear coating composition is, for example, a two-component type. The aqueous clear coating composition comprises a second main component 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 hydroxyl group-free organic solvent (b2) as above. The components of the aqueous clear coating composition and the components of the aqueous surfacer may be the same or different.

[0106] The second main 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 in the range of isocyanate group equivalent / hydroxyl group equivalent = 1 / 0.5 to 1 / 1.5.

[0107] The aqueous clear coating composition may contain the same epoxy group-containing silane coupling agent as above, which is added in an amount of 1.2 parts by mass to 10 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).

[0108] At least one of the aqueous surfacer and the aqueous 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 aqueous surfacer and the aqueous clear coating composition may contain an epoxy group-containing silane coupling agent. The epoxy group-containing silane coupling agents contained in the aqueous clear coating composition and the aqueous surfacer may be the same or different.

[0109] (Method of preparing aqueous coating compositions) 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 a mixer is used. The base agent and the curing agent are usually mixed immediately before use (for example, used within 60 minutes after mixing).

[0110] (Resin Substrate) The substrate may have at least a surface made of resin. The entire substrate may be formed 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 paint film. That is, the resin substrate may be formed of resin, or may have a resin substrate and an old paint film covering at least a portion of its surface, or may have a metal substrate and an old paint film covering at least a portion of its surface. The aqueous primer coating composition used in the present disclosure has excellent adhesion to old paint films as well as to resin or metal substrates.

[0111] The resin constituting the resin substrate may be a thermoplastic resin or a thermosetting resin, such as 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).

[0112] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof. The metal substrate may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After the surface treatment, the metal substrate may be further coated with an electrodeposition paint. The electrodeposition paint may be either a cationic type or an anionic type.

[0113] [Method for manufacturing a coated article] A coated article is manufactured by a method comprising 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-component clear coating composition onto the colored base coating film to form a clear coating film.

[0114] After forming the first undercoat film, and before applying the water-based colored base paint, a water-based surfacer may be applied on the first undercoat film to form a second undercoat film.

[0115] (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.

[0116] The amount of the aqueous primer coating composition to be applied is not particularly limited and is appropriately determined depending on the performance required of the coating film. For example, the aqueous primer coating composition is applied so that the thickness of the first undercoat coating film after curing is 10 μm or more and 40 μm or less.

[0117] The application method of the aqueous primer coating composition is not particularly limited. Examples of application methods include air spray coating, airless spray coating, rotary atomization coating, and curtain 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 coating methods commonly used in automotive painting (e.g., air spray coating), the aqueous primer coating composition exhibits high film-forming properties even on substrates with fine irregularities.

[0118] After the aqueous primer coating composition is applied, it is heated to cure the coating (which can also be said to be dried and solidified). The curing conditions are appropriately set 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.

[0119] The coating film may be allowed to dry naturally at room temperature (23°C ± 3°C) for 2 hours or more, 24 hours or more, or even one week or more.

[0120] When repairing, before applying the aqueous primer coating composition, the target area and its surrounding area are polished as necessary, and any irregularities are filled in with putty. Subsequently, the area is polished again.

[0121] (II) Step of forming a second undercoat film In this step, an aqueous surfacer is applied to the substrate to form a second undercoat film. The second undercoat film is formed as needed.

[0122] The method for applying the aqueous surfacer is not particularly limited, and examples thereof include methods similar to those used for the aqueous primer coating composition. The amount of the aqueous surfacer to be applied is not particularly limited, and is appropriately determined depending on the performance required for the coating film. The aqueous 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 curing of the aqueous surfacer is carried out, for example, by the same method as that used for the aqueous primer coating composition.

[0123] After curing, the first and second primer coatings are usually polished and degreased using sandpaper or the like.

[0124] (III) Step of forming a colored base coating film In this step, an aqueous colored base coating composition is applied to 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.

[0125] The method for applying the aqueous colored base coating composition is not particularly limited, and may be the same as that for applying the aqueous primer coating composition. The amount of the aqueous colored base coating composition to be applied is not particularly limited, and for example, the coating is applied so that the thickness of the colored base coating film after curing is 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.

[0126] (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.

[0127] The prepared aqueous clear coating composition is applied onto the aqueous colored base coating film. The application method of the aqueous clear coating composition is not particularly limited, and the same method as that for the aqueous primer coating composition can be used. The amount of application of the aqueous clear coating composition is not particularly limited, and is appropriately set depending on 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 curing of the aqueous clear coating composition is carried out, for example, by the same method as that for the aqueous primer coating composition.

[0128] In this way, a painted article is obtained that has a multi-layer 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.

[0129] [Multilayer 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 aqueous primer coating composition described above, a colored base coating film formed on the first undercoat coating film from the aqueous colored base paint described above, and a clear coating film formed on the colored base coating film from the aqueous two-component clear paint described above. The multilayer coating film may further comprise a second undercoat coating film formed from the aqueous surfacer described above between the first undercoat coating film and the colored base coating film.

[0130] The first primer coating has excellent adhesion to the resin substrate and to the old coating film, and the multi-layer coating is suitable for repairing a resin substrate having an old coating film.

[0131] 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.

[0132] 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 mass unless otherwise specified.

[0133] 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 follows, and multi-layer coating films were formed.

[0134] (1) Preparation of Coating Composition (1-1) Preparation of 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 stirring was continued 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 amphiphilic 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 solids content of 11.9%. The coating specific gravity of the aqueous primer coating composition was 0.99 g / cm ３ It was.

[0135] (1-2) Preparation of Water-Based Surfacer Nax E-CUBE WB Plassa Vita Gray (manufactured by Nippon Paint Co., Ltd., solids content approximately 56%) was prepared as the first main component. The first main component contained an acrylic polyol in emulsion form as the hydroxyl group-containing resin (a1), pure water as the aqueous solvent (a2), and a pigment. The solids concentration of the hydroxyl group-containing resin (a1) was 24.4% of the total solids contained in the first main component.

[0136] A first curing agent was prepared by mixing 60 parts of a 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).

[0137] An aqueous surfacer was prepared by mixing 100 parts of the first main agent, 12.5 parts of the first curing agent, and an appropriate amount of pure water using a disper. 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).

[0138] (1-3) Preparation of aqueous colored base coating composition A lacquer-type aqueous colored base coating composition was prepared by mixing 100 parts of nax E-CUBE WB 412 Silent Black (manufactured by Nippon Paint Co., Ltd., solids content: approximately 22%), 50 parts of nax E-CUBE WB 911 S-binder (auxiliary agent, manufactured by Nippon Paint Co., Ltd., solids content: approximately 26%), and 45 parts of nax E-CUBE WB R20 standard diluent (diluting component, manufactured by Nippon Paint Co., Ltd.).

[0139] (1-4) Preparation of aqueous clear coating composition As the second main component, a mixture of Nax E-CUBE WB (2:1) NN Clear (manufactured by Nippon Paint Co., Ltd., solids content approximately 38%) and pure water (aqueous solvent (a2)) was prepared. The second main component contains an acrylic polyol in the form of an emulsion as the hydroxyl group-containing resin (a1). The solids concentration of the hydroxyl group-containing resin (a1) was 91.5% of the total solids contained in the second main component.

[0140] A second curing agent was prepared in the same manner as the first curing agent.

[0141] An aqueous clear coating composition was prepared by mixing 100 parts of the second main agent, 50 parts of the second curing agent, and an appropriate amount of pure water using a disper. 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).

[0142] (2) Formation of multilayer coating film As the substrate, 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 multilayer 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.

[0143] (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 cut to 150 mm x 450 mm x 0.8 mm thick (surface tension: 25.7 mN / m, SP value: 7.90) was prepared as the substrate. The aqueous primer coating composition was applied undiluted to this PP plate using an air spray gun, and air-dried until the coating film lost its luster, forming a first undercoat coating film (cured film thickness 5 μm).

[0144] (2-1-2) Formation of a second primer coating film An aqueous surfacer was applied to the first primer 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 process was repeated two more times. Subsequently, the coated plate was heated in an oven set to 60°C for 30 minutes to form a second primer coating film (cured film thickness: 45 μm). Next, the surface of the second primer coating film was sanded and degreased.

[0145] (2-1-3) Formation of Colored Base Coating Film An aqueous colored base coating composition was applied onto the second undercoat coating film using an air spray gun, and air blow drying was carried out until the paint no longer adhered to the surface when touched with a finger. This application and drying process was repeated two more times to form a colored base coating film (cured film thickness: 20 μm).

[0146] (2-1-4) Formation of clear coating film The aqueous clear coating composition was applied three times onto the colored base coating film using a spray gun. Each application was performed with a 1-minute interval between applications. 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 to 70°C and heated for 40 minutes to cure the clear coating film. The humidity inside the oven was 10 RH%. In this way, a multi-layer coating film P1 was formed on the PP plate with a smooth surface.

[0147] (2-2) Formation of multilayer coating film P2 on PP plate with 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 fabric polishing sheet (equivalent to #1500) to form fine irregularities. In this way, a PP plate with fine irregularities was prepared.

[0148] A 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, except that a PP plate having fine irregularities was used as the substrate.

[0149] (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 "(1-4) Preparation of aqueous clear coating composition" above, an uncured clear coating film was formed on a 70 mm x 150 mm steel plate in the same manner as in "(2-1-4) Formation of clear coating film" above. 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 to 70 ° C (humidity 10 RH%) and heated for 40 minutes to cure the clear coating film. Subsequently, the steel plate was aged at 23 ° C for 1 week to obtain a steel plate with an old coating film with a smooth surface.

[0150] 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 having a smooth surface" above, except that a steel plate having an old coating film with a smooth surface was used as the substrate.

[0151] (2-4) Formation of a multilayer 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 multilayer 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.

[0152] A 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, except that a steel plate with an old coating film having irregularities was used as the substrate.

[0153] 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.

[0154] Details of each component in the table are as follows: Aqueous polyolefin resin (A) Trade name: Auroren AE-301, emulsion of non-chlorinated polypropylene resin, manufactured by Nippon Paper Industries Co., Ltd., Tg -28°C, Mw 50,000 Aqueous epoxy resin (B) Trade name: Modepics 304, emulsion of acrylic-modified bisphenol A-type epoxy resin, manufactured by Arakawa Chemical Industries, Ltd. Thickener (C) Trade name: Adekanol UH-756VF, urethane association type, manufactured by ADEKA Corporation Surface conditioner Trade name: Tego Wet 260, manufactured by EVONIC Corporation, polyether-modified silicone compound, active ingredient 100% 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 Aqueous polyurethane resin (F) Product name: Hydran WLS-210, manufactured by DIC Corporation, Tg -32°C, acid value 28 mg KOH / g

[0155] [Evaluation of Water-Based Primer Coating Composition] The water-based primer coating composition was evaluated.

[0156] Surface tension: Measured by the platinum ring method using a Dynometer (manufactured by Byck-Gardner, Germany).

[0157] Storage Stability 900 ml of the aqueous primer coating composition was filled into a 1 L coated can and allowed to stand at 23°C. After one month, the condition of the aqueous primer coating composition was visually evaluated. The evaluation criteria are as follows. A rating of B or higher can be said to indicate excellent storage stability.

[0158] (Evaluation criteria) A No change before and after storage B Separation occurred, but the product returned to the state before storage by stirring C Separation or gelation occurred, and the product did not return to the state before storage by stirring

[0159] Film-forming properties: The surface of a polypropylene plate cut to a size of 150 mm x 450 mm x 0.8 mm thick was polished back and forth 20 times with a nonwoven fabric polishing sheet (equivalent to #1500) to form fine irregularities. An aqueous primer coating composition (coating amount 150 g / m) was sprayed onto the PP plate using a spray gun. ２ The condition of the surface was visually evaluated from immediately after application until it dried naturally. The evaluation criteria are as follows. A rating of B or higher can be said to be excellent in film-forming ability.

[0160] (Evaluation criteria) A: No crazing or smearing was observed immediately after application, and a smooth coating film was formed. B: Smearing was observed immediately after application, but the paint filled in the unevenness during the application and drying process, resulting in a smooth coating film. Unevenness was observed on the coating surface of the wet coating film, and unevenness was also observed in the dried coating film, presenting a problem. C: Cracking and / or smearing was observed on the coating surface of the wet coating film, and a uniform film was not formed after drying.

[0161] [Evaluation of multi-layer coating film] The multi-layer coating film was evaluated for water resistance to the above-mentioned substrate (polypropylene plate) or to the old coating film. As the water resistance, adhesion and appearance after an immersion test were evaluated.

[0162] 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, if the adhesion of multi-layer coating films P1 and M1 is high, it can be evaluated that the adhesion of multi-layer coating films P2 and M2 is also high.

[0163] 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 deteriorate when the coating surface is irregular, if the appearance of the multi-layer coating films P2 and M2 is good, it can be evaluated that the adhesion of the multi-layer coating films P1 and M1 is also good.

[0164] (Evaluation for polypropylene plate) - Adhesion A thermostatic water bath with a circulation function was set to 40 ° C and filled with deionized water. A coated article having a multilayer 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, slits (11 vertical lines, 11 horizontal lines, 2 mm intervals) reaching the substrate were made in the multilayer coating film P1 using an NT cutter (manufactured by NT Corporation) to create 100 grids. Tape (Nichiban Co., Ltd., Cellotape (registered trademark), 24 mm wide) was applied to cover all the grids and firmly adhered from above with a fingernail or the like. Next, the tape was peeled off while pulling so that the angle between the tape and the coating film was approximately 45 °. After peeling the tape, the number of grids with remaining coating film was counted. The more grids remaining, the higher the adhesion.

[0165] Appearance: In the same manner as above, an immersion test was carried out on a coated article having the multilayer coating film P2. The appearance of the multilayer coating film P2 was then evaluated visually. The evaluation criteria are as follows. A rating of B or higher can be said to indicate excellent appearance.

[0166] (Evaluation criteria) A: No swelling or whitening B: Slight swelling or whitening C: Obvious swelling and / or whitening

[0167] (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 above. The coated article having the multi-layer coating film M2 was subjected to an appearance evaluation in the same manner as above.

[0168] The aqueous primer coating compositions of Examples 1 to 18 were excellent in storage stability and film-forming ability, and the resulting coating films also had good adhesion and appearance.

[0169] The aqueous primer coating composition of Comparative Example 1 gelled immediately after preparation and lost fluidity, making it impossible to apply. Therefore, it was not possible to form a multi-layer coating film, and evaluation of the multi-layer coating film was not possible. The aqueous primer coating compositions of Comparative Examples 2 to 4 showed good results in the film-forming property evaluation using a large application amount, but the results were poor in the adhesion and appearance evaluation using an application amount similar to that of an 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 property evaluation and also poor results in the adhesion and appearance evaluation.

[0170]

[0171]

[0172]

[0173] The present invention encompasses the following aspects: [1] A method for coating a resin substrate with an aqueous primer coating composition 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, wherein 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 solids 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, and 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) comprises a poorly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20° C., and at least one of an amphipathic solvent (D2) that dissolves 1 g to less than 60 g in 100 g of water at 20° C. and 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 mass % to 60 mass %, and the mass of the organic solvent (D) relative to the mass of the organic components obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer paint is 30 g / L to 500 g / L. [2] The method for forming a multilayer coating film according to [1] above, wherein the mass of the organic solvent (D) relative to the mass of the organic components is 30 g / L to 350 g / L. [3] The method for forming a multilayer coating film according to [1] or [2] above, wherein the surface tension of the aqueous primer coating 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 [1] to [3] above, wherein the aqueous epoxy resin (B) contains a bisphenol A epoxy resin. [5] The method for forming a multilayer coating film according to any one of [1] to [4] above, wherein the thickener (C) contains a urethane associative thickener. [6] The method for forming a multilayer coating film according to any one of [1] to [5] above, further containing an aqueous polyurethane resin (F).[7] The method for forming a multilayer coating film according to any one of [1] to [6] above, wherein the resin 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 [1] to [7] above, wherein the difference between the solubility parameter of the polymer that constitutes 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] The method for forming a multilayer coating film according to any one of [1] to [8] above, comprising, after forming the first undercoat coating film and before applying the water-based colored base paint, applying an aqueous surfacer on the first undercoat coating film to form a second undercoat coating film.

[10] A coating composition comprising: a first undercoat film formed on a resin substrate from an aqueous primer coating composition; a colored base coating film formed on the first undercoat film from an aqueous colored base coating; and a clear coating film formed on the colored base coating film from an aqueous two-component clear coating; wherein 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 solids 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; and the mass ratio (A / B) of the aqueous polyolefin resin (A) to the aqueous epoxy resin (B) is 60 / 40 to 95 / 5; a multilayer coating film, wherein the organic solvent (D) comprises a poorly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, and at least one of an amphipathic solvent (D2) that dissolves 1 g or more but less than 60 g in 100 g of water at 20°C, and a hydrophilic solvent (D3) that dissolves 60 g or more in 100 g of water at 20°C, wherein the proportion of the poorly water-soluble solvent (D1) in the organic solvent (D) is 1 mass % or more and 60 mass % or less, and the mass of the organic solvent (D) in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer coating is 30 g / L or more and 500 g / L or less.

[11] An aqueous primer coating composition used for forming a multilayer coating film having a first undercoat coating film formed on a resin substrate from an aqueous primer coating composition, a colored base coating film formed on the first undercoat coating film from an aqueous colored base paint, and a clear coating film formed on the colored base coating film from an aqueous two-component clear paint, 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 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) being 60 / 40 to 95 / 5, The organic solvent (D) comprises a slightly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, and at least one of an amphipathic solvent (D2) that dissolves 1 g or more but less than 60 g in 100 g of water at 20°C and a hydrophilic solvent (D3) that dissolves 60 g or more in 100 g of water at 20°C, wherein the proportion of the slightly water-soluble solvent (D1) in the organic solvent (D) is 1 mass % or more and 60 mass % or less, and the mass of the organic solvent (D) in the organic component mass obtained by subtracting the mass of the water (E) from the total mass of the aqueous primer paint is 30 g / L or more and 500 g / L or less.

[0174] According to the present invention, there is provided a multi-layer coating film having excellent appearance, 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 automotive painting, particularly for automotive repair painting.

Claims

1. Coating an aqueous primer paint composition on a resin substrate to form a first undercoat film; coating an aqueous colored base paint composition on the first undercoat film to form a colored base film; and coating an aqueous two-component clear paint composition on the colored base film to form a clear film, wherein the aqueous primer paint composition contains an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D), and water (E), 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 at least one of a sparingly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, an amphiphilic solvent (D2) that dissolves 1 g or more and less than 60 g in 100 g of water at 20°C, and a hydrophilic solvent (D3) that dissolves 60 g or more in 100 g of water at 20°C, the proportion of the sparingly water-soluble solvent (D1) in the organic solvent (D) is 1% by mass or more and 60% by mass or less, and the mass of the organic solvent (D) in the organic component mass excluding the mass of water (E) from the total mass of the aqueous primer paint is 30 g / L or more and 500 g / L or less. A method for forming a multilayer coating film.

2. 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.

3. The method for forming a multilayer coating film according to claim 1 or 2, wherein the surface tension of the aqueous primer paint 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 claims 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 claims 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 claims 1 to 5, further containing an aqueous polyurethane resin (F).

7. The method for forming a multilayer coating film according to any one of claims 1 to 6, wherein the resin substrate includes a base material and a coating film other than the multilayer coating film that covers at least a part of the base material.

8. The method for forming a multilayer coating film according to any one of claims 1 to 7, wherein the difference between the solubility parameter of the polymer constituting the surface of the resin object to be coated and the solubility parameter of the organic solvent (D) contained in the aqueous primer coating composition is 2.5 or less.

9. The method for forming a multilayer coating film according to any one of claims 1 to 8, comprising forming a second undercoat film by applying an aqueous surfacer onto the first undercoat film after forming the first undercoat film and before applying the aqueous colored base paint.

10. A multilayer coating film having a first undercoat film formed by an aqueous primer coating composition on a resin object to be coated, a colored base film formed by an aqueous colored base paint on the first undercoat film, and a clear film formed by an aqueous two-component clear paint on the colored base film, wherein 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), 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 at least one of a hardly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, an amphiphilic solvent (D2) that dissolves 1 g or more and less than 60 g in 100 g of water at 20°C, and a hydrophilic solvent (D3) that dissolves 60 g or more in 100 g of water at 20°C, the proportion of the hardly water-soluble solvent (D1) in the organic solvent (D) is 1% by mass or more and 60% by mass or less, and the mass of the organic solvent (D) in the organic component mass excluding the mass of the water (E) from the total mass of the aqueous primer paint is 30 g / L or more and 500 g / L or less.

11. An aqueous primer paint composition used for forming a multilayer coating film having a first undercoat film formed of an aqueous primer paint composition on a resin object to be coated, a colored base film formed of an aqueous colored base paint on the first undercoat film, and a clear coat film formed of an aqueous two-component clear paint on the colored base film, the aqueous primer paint composition containing an aqueous polyolefin resin (A), an aqueous epoxy resin (B), a thickener (C), an organic solvent (D), and water (E), having a solid content of 5% by mass or more and 17% by mass or less, the aqueous polyolefin resin (A) having 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) being 60 / 40 to 95 / 5, the organic solvent (D) including at least one of a poorly water-soluble solvent (D1) that dissolves less than 1 g in 100 g of water at 20°C, an amphiphilic solvent (D2) that dissolves 1 g or more and less than 60 g in 100 g of water at 20°C, and 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) being 1% by mass or more and 60% by mass or less, and the mass of the organic solvent (D) in the organic component mass obtained by removing the mass of the water (E) from the total mass of the aqueous primer paint being 30 g / L or more and 500 g / L or less.

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