Method for producing multilayer coating film

Abstract

The purpose of the present invention is to provide a method for producing a coating film, said method being capable of achieving a smooth coating film having a high granular sensation and suppressed unevenness in metal flash colors. A method for producing a multilayer coating film according to the present disclosure comprises: a first base coating film formation step for forming a first base coating film by applying an aqueous first base coating composition on an object to be coated; a second undercoat coating film formation step for forming a second undercoat coating film by applying an aqueous second undercoat coating composition on the first undercoat coating film; and a heating and curing step for simultaneously heating and curing at least the first base coating film and the second base coating film to form a multilayer coating film including the first base coating film and the second base coating film. Wherein the first aqueous base coating composition contains a coating film-forming resin (A1) and a curing agent (B1), the second aqueous base coating composition contains a coating film-forming resin (A2), a curing agent (B2) and a glitter pigment (C2), and the solid content concentration of the first aqueous base coating composition is 15-40% by mass (inclusive). The solid content concentration of the second aqueous base coating composition is from 5% by mass to 20% by mass (inclusive), and the ratio of the solid content concentration in the first aqueous base coating composition to the solid content concentration in the second aqueous base coating composition is from 1.1 to 8 (inclusive) by mass. The average particle diameter (D50) of the brilliant pigment (C2) is 17-30 [mu] m (inclusive), the average thickness of the brilliant pigment (C2) is 0.3-6.0 [mu] m (inclusive), the water surface diffusion area (WCA) of the brilliant pigment (C2) is 1.4 m2 / g or less, and the pigment mass concentration of the brilliant pigment (C2) in the aqueous second base coating composition is 2-40 mass% (inclusive).

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a multi-layer coating film. Background Art

[0002] On the surface of an object to be coated such as an automobile body, a plurality of coating films (multi-layer coating films) having various functions are successively formed to protect the object to be coated and at the same time impart a beautiful appearance and excellent design. As a method for forming such a multi-layer coating film, a method is generally used in which an electrodeposition coating film or the like as a primer coating film is formed on an object to be coated having excellent conductivity, and a middle coat film and a top coat film are successively formed thereon as needed. Among these coating films, in particular, the top coat film composed of a base coat film and a clear coat film largely determines the appearance and design of the coating film. Especially in automobiles, the appearance and design of the top coat film composed of a base coat film and a clear coat film formed on the vehicle body are extremely important.

[0003] The base coat film can be roughly classified into: a coating film containing no scaly pigment called so-called solid color, and a coating film containing scaly pigment called so-called metallic color having a shiny feeling. As a representative example of the scaly pigment contained in the coating film having a shiny feeling, scaly aluminum pigment can be cited. By including scaly aluminum pigment in the base coat film, a metallic-style gloss (metallic feeling) can be exhibited.

[0004] The appearance of the coating film formed on an automobile is closely related to the appearance value such as the premium feeling of the automobile. In addition, customers (consumers) who purchase automobiles tend to pursue automobiles having a coating film with excellent design. Due to the diversification and uniqueness of such preferences of consumers, more unique designs are required.

[0005] Regarding a method for manufacturing a coating film having a metallic-style gloss (metallic feeling), Patent Document 1 describes a method for forming a bright coating film, which includes the following steps: a step of coating a water-based first base bright coating to form a first base coating film; a step of coating a water-based second base bright coating on the first base coating film to form a second base coating film; a step of coating a clear coating on the second base coating film to form a clear coating film; and a step of heating and curing all the uncured coating films at once, wherein it is described that the coating solid content in the water-based first base bright coating is 1 to 45% by mass, the coating solid content in the water-based second base bright coating is 1 to 40% by mass; and the ratio of the coating solid content of the water-based first base bright coating to the water-based second base bright coating is (1.1 / 4) to (4 / 1), etc.

[0006] Patent Document 2 discloses that in the method of applying an aqueous primer coating containing flaky brightening pigments, an aqueous primer coating in which the solid content in the coating has been adjusted to 20 to 40% by mass is applied to the object to be coated in a dry film thickness of 1 to 15 μm, and then an aqueous primer coating in which the solid content in the coating has been adjusted to 2 to 15% by mass is applied to the uncured coating film in a dry film thickness of 0.1 to 5 μm.

[0007] Patent Document 3 discloses a method for forming a multilayer coating film, which includes the following steps: a step of applying an aqueous first colored coating containing a coloring pigment and a brightening pigment to an object to be coated to form a first colored coating film; a step of applying an aqueous second colored coating containing a tackiness regulator, a coloring pigment, and a brightening pigment to the first colored coating film to form a second colored coating film; a step of applying a transparent coating to the second colored coating film to form a transparent coating film; and a step of curing the first colored coating film, the second colored coating film, and the transparent coating film by heating them simultaneously. It is disclosed that the solid content content of the second colored coating is 0.1 to 6% by mass, the dry film thickness of the second colored coating film is 0.2 to 3.0 μm, the average value of the light transmittance of the second colored coating film at a wavelength of 400 to 700 nm is 1% or less, and the difference between the average value of the specular reflectance (110°) of the first colored coating film at a wavelength of 400 to 700 nm and the average value of the specular reflectance (110°) of the multilayer coating film at a wavelength of 400 to 700 nm is 5% or less, etc.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2004-351389

[0011] Patent Document 2: Japanese Patent No. 2006-95522

[0012] Patent Document 3: Japanese Patent No. 7005823 Summary of the Invention

[0013] Problems to be Solved by the Invention

[0014] In recent years, there has sometimes been a demand for a coating film that has an appearance with a high degree of granularity (sparkling) in addition to the metallic luster (metallic feeling). According to the research of the present inventors, a granular luster can be achieved by using a brightening pigment with a large particle size and a thick layer. However, if such a brightening pigment is used, the resulting coating film has poor smoothness, and there is a tendency for metallic flash color unevenness (metallic unevenness: the brightening pigments are not uniformly oriented, forming a mottled appearance) to occur easily. That is, a high degree of granular appearance and the smoothness of the coating film and the suppression of metallic flash color unevenness are mutually exclusive phenomena. In the coating film forming methods described in the previously known Patent Documents 1 to 3, the granularity is insufficient, and even if granularity is obtained, the smoothness and uniformity (suppression of the occurrence of metallic flash color unevenness) of the coating film cannot be sufficiently satisfied.

[0015] In view of the above situation, the present disclosure aims to provide a method for manufacturing a coating film that can achieve a coating film with a high degree of granularity, suppression of metallic flash color unevenness, and smoothness.

[0016] Means for Solving the Problem

[0017] The present disclosure includes the following aspects.

[0018] [1] A method for manufacturing a multilayer coating film, comprising the following steps:

[0019] A first base coating film forming step of coating a water-based first base coating composition on an object to be coated to form a first base coating film;

[0020] A second base coating film forming step of coating a water-based second base coating composition on the first base coating film to form a second base coating film; and

[0021] A heat curing step of simultaneously heat curing at least the first base coating film and the second base coating film to form a multilayer coating film including a first base coating film and a second base coating film;

[0022] wherein the water-based first base coating composition contains a film-forming resin (A1) and a curing agent (B1),

[0023] the water-based second base coating composition contains a film-forming resin (A2), a curing agent (B2), and a brightening pigment (C2),

[0024] the solid content concentration of the water-based first base coating composition is 15% by mass or more and 40% by mass or less,

[0025] the solid content concentration of the water-based second base coating composition is 5% by mass or more and 20% by mass or less,

[0026] The ratio of the solid content concentration in the above-mentioned aqueous first base coating composition to the solid content concentration in the aqueous second base coating composition is 1.1 or more and 8 or less on a mass basis.

[0027] The average particle diameter (D50) of the above-mentioned bright pigment (C2) is 17 μm or more and 30 μm or less.

[0028] The average thickness of the above-mentioned bright pigment (C2) is 0.3 μm or more and 6.0 μm or less.

[0029] The water surface spreading area (WCA) of the above-mentioned bright pigment (C2) is 1.4 m 2 / g or less.

[0030] The pigment mass concentration of the above-mentioned bright pigment (C2) in the above-mentioned aqueous second base coating composition is 2% by mass or more and 40% by mass or less.

[0031] [2][1] The method for manufacturing a multilayer coating film according to claim 1, wherein the coating of the above-mentioned aqueous first base coating composition and the coating of the above-mentioned aqueous second base coating composition are carried out in such a manner that the ratio t1 / t2 of the dry film thickness t1 of the above-mentioned first base coating film to the dry film thickness t2 of the above-mentioned second base coating film is 1.6 or more and 7 or less.

[0032] [3][1] or [2] The method for manufacturing a multilayer coating film according to claim 2, wherein the granularity value (G value) of the above-mentioned multilayer coating film is 9 or more and 18 or less.

[0033] [4][1] to [3] The method for manufacturing a multilayer coating film according to any one of claims 3, wherein the above-mentioned aqueous second base coating composition further contains a viscosity regulator (D).

[0034] The above-mentioned viscosity regulator (D) contains an inorganic layered compound (D-1).

[0035] [5][1] to [4] The method for manufacturing a multilayer coating film according to any one of claims 4, wherein the above-mentioned film-forming resin (A2) contains an acrylic resin aqueous dispersion (A11).

[0036] [6][1] to [5] The method for manufacturing a multilayer coating film according to any one of claims 5, wherein the above-mentioned curing agent (B2) contains an amino resin (B11).

[0037] [7][1] to [6] The method for manufacturing a multilayer coating film according to any one of claims 6, further comprising: a transparent coating film forming step of coating a transparent coating on the above-mentioned second base coating film to form a transparent coating film.

[0038] In the above heating and curing step, the above first base coating film, the above second base coating film, and the above transparent coating film are simultaneously heated and cured to form a multilayer coating film.

[0039] Advantages of the Invention

[0040] According to the manufacturing method of the present disclosure, a coating film with high graininess, suppression of uneven metallic luster, and smoothness can be achieved. Detailed Description of the Invention

[0041] The manufacturing method of the multilayer coating film of the present disclosure is a manufacturing method of a multilayer coating film including the following steps:

[0042] A first base coating film forming step of coating a water-based first base coating composition on an object to be coated to form a first base coating film;

[0043] A second base coating film forming step of coating a water-based second base coating composition on the above first base coating film to form a second base coating film; and

[0044] A heating and curing step of simultaneously heating and curing at least the above first base coating film and the second base coating film to form a multilayer coating film including a first base coating film and a second base coating film;

[0045] Among them, the above water-based first base coating composition contains a film-forming resin (A1) and a curing agent (B1),

[0046] The above water-based second base coating composition contains a film-forming resin (A2), a curing agent (B2), and a brightening pigment (C2),

[0047] The solid content concentration of the above water-based first base coating composition is 15% by mass or more and 40% by mass or less,

[0048] The solid content concentration of the above water-based second base coating composition is 5% by mass or more and 20% by mass or less,

[0049] The ratio of the solid content concentration in the above water-based first base coating composition to the solid content concentration in the water-based second base coating composition is 1.1 or more and 8 or less,

[0050] The average particle diameter (D50) of the above brightening pigment (C2) is 17 μm or more and 30 μm or less,

[0051] The average thickness of the above brightening pigment (C2) is 0.3 μm or more and 6.0 μm or less,

[0052] The water surface spreading area (WCA) of the above brightening pigment (C2) is 1.4 m 2 / g or less,

[0053] The pigment mass concentration of the brightening pigment (C2) in the above aqueous second base coating composition is 2% by mass or more and 40% by mass or less.

[0054] According to the method for manufacturing a multilayer coating film of the present disclosure, a coating film with high graininess, suppression of uneven metallic luster, and smoothness can be achieved. The present disclosure should not be limited to being explained by a specific theory, but the reasons why the manufacturing method of the present disclosure can achieve the above effects are as follows.

[0055] That is, since the thickness of the brightening pigment (C2) of the present disclosure is thick, visible light is not easily transmitted therethrough and is easily reflected on the surface. Generally, a brightening pigment (C2) with a thick thickness is difficult to be uniformly oriented during the coating film formation process, so there may be a situation where it is difficult to exhibit a brightening effect or uneven metallic luster may occur. In the manufacturing method of the present disclosure, a specific brightening pigment (C2) is used, the solid content concentration of the second base coating composition containing the brightening pigment (C2) is set to a range lower than the solid content concentration of the first base coating composition, the ratio of the amount of the solid content contained in the first base coating composition to the amount of the solid content contained in the second base coating composition is set to a specified range, and the pigment mass concentration of the brightening pigment (C2) is set to a specified range. Therefore, it is considered that during heat curing, in addition to the drying and curing of the second base coating film, the solvent easily moves from the second base coating film to the first base coating film, and it is considered that volume shrinkage effectively occurs, so that the brightening pigment (C2) is easily oriented.

[0056] <Aqueous first base coating composition and aqueous second base coating composition>

[0057] The above aqueous first base coating composition contains a film-forming resin (A1) and a curing agent (B1). In one aspect, the above aqueous first base coating composition further contains a brightening pigment (C1), and in another aspect, the above aqueous first base coating composition does not contain a brightening pigment (C1).

[0058] The above aqueous second base coating composition contains a film-forming resin (A2), a curing agent (B2), and a brightening pigment (C2).

[0059] Hereinafter, the aqueous first base coating composition or the aqueous second base coating composition may sometimes be referred to as the "aqueous base coating composition", the film-forming resin (A1) or the film-forming resin (A2) may sometimes be referred to as the "film-forming resin (A)", the above curing agent (B1) or curing agent (B2) may sometimes be referred to as the "curing agent (B)", and the above brightening pigment (C1) or brightening pigment (C2) may sometimes be referred to as the "brightening pigment (C)".

[0060] (Film-forming resin (A))

[0061] The above-mentioned film-forming resin (A) is a resin that can form a film by reacting with a curing agent (B) described later, and preferably contains one or more selected from acrylic resins, polyol resins, polyester resins, polyurethane resins, epoxy resins, fluororesins, and silicone resins, and more preferably contains one or more selected from acrylic resins, polyol resins, and polyester resins. The above-mentioned film-forming resin (A) preferably has a hydroxyl group.

[0062] The above-mentioned film-forming resin (A) preferably contains an aqueous resin. The aqueous resin is a resin dispersed in an aqueous medium and can be an emulsion or a dispersion. Examples of such an aqueous resin include: acrylic resin aqueous dispersion, polyester resin aqueous dispersion, polyurethane resin aqueous dispersion, and epoxy resin aqueous dispersion.

[0063] The resin composition of the film-forming resin (A1) and the resin composition of the film-forming resin (A2) may be the same or different.

[0064] In one embodiment, the above-mentioned film-forming resin (A2) preferably contains an acrylic resin, and preferably contains an acrylic resin aqueous dispersion (A11). By the film-forming resin (A2) containing an acrylic resin, various physical properties such as the storage stability of the resulting coating composition or the water resistance and weather resistance of the resulting film can be improved. In addition, the above-mentioned film-forming resin (A1) preferably contains an acrylic resin, and preferably contains an acrylic resin aqueous dispersion (A11). By the film-forming resin (A1) and the film-forming resin (A2) containing an acrylic resin, the adhesion between the first base film and the second base film can be improved. The acrylic resin aqueous dispersion (A11) can be either an acrylic resin emulsion or an acrylic resin dispersion, and is preferably an acrylic resin emulsion. The acrylic resin contained in the acrylic resin aqueous dispersion may be in the form of particles in the aqueous medium.

[0065] Typically, the above-mentioned acrylic resin is obtained as a polymer of a monomer mixture containing (meth)acrylic monomers, and the acrylic resin aqueous dispersion (A11) is obtained by emulsion polymerization of the above-mentioned monomer mixture. The above-mentioned monomer mixture preferably contains a hydroxyl group-containing monomer, and may further contain an acid group-containing monomer and other monomers. In one embodiment, the above-mentioned monomer mixture preferably contains an alkyl (meth)acrylate, a hydroxyl group-containing monomer, and an acid group-containing monomer, and more preferably contains an alkyl (meth)acrylate, a hydroxyl group-containing monomer, an acid group-containing monomer, and a styrene-based monomer.

[0066] It should be noted that in the present disclosure, (meth)acrylic acid means acrylic acid and methacrylic acid.

[0067] The above-mentioned (meth)acrylic acid alkyl ester is a (meth)acrylic acid alkyl ester that does not have an acid group and a hydroxyl group. By including the (meth)acrylic acid alkyl ester in the above monomer mixture, the main skeleton of the acrylic resin can be well formed.

[0068] Examples of the above-mentioned (meth)acrylic acid alkyl ester include: methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, etc. As the (meth)acrylic acid alkyl ester, only one kind can be used, or two or more kinds can be used in combination.

[0069] Examples of the monomer containing a hydroxyl group include: (meth)acrylic acid monomers having a hydroxyl group. Specifically, examples include: hydroxyl group-containing alkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate; ε-caprolactone-modified (meth)acrylic acid monomers, etc.

[0070] As the ε-caprolactone-modified (meth)acrylic acid monomer, commercially available products can be used. Examples of such commercially available products include: Placcel FA-1, Placcel FA-2, Placcel FA-3, Placcel FA-4, Placcel FA-5, Placcel FM-1, Placcel FM-2, Placcel FM-3, Placcel FM-4, and Placcel FM-5 (all manufactured by Daicel Chemical Industries, Ltd.), etc. As the monomer containing a hydroxyl group, only one kind can be used, or two or more kinds can be used in combination.

[0071] By including the monomer containing a hydroxyl group in the above monomer mixture, hydrophilicity can be imparted to the resulting acrylic resin, and at the same time, the curing reactivity of the acrylic resin with the curing agent (B) described later can be improved.

[0072] The monomer containing an acid group can be a (meth)acrylic acid monomer having an acid group. Such an acid group can be a carboxyl group, a sulfonic acid group, or a phosphoric acid group. From the viewpoint of improving the dispersion stability and the curing reaction promoting function, a carboxyl group is preferred. By including the monomer containing an acid group in the above monomer mixture, various stabilities such as the storage stability, mechanical stability, and anti-freezing stability of the resulting acrylic resin can be improved, and the curing reaction of the acrylic resin with the curing agent (B) during film formation can be promoted.

[0073] Examples of the acid group-containing monomers include carboxyl group-containing monomers such as (meth)acrylic acid, crotonic acid, isocrotonic acid, ethylacrylic acid, propylacrylic acid, isopropylacrylic acid, itaconic acid, maleic anhydride, and fumaric acid; sulfonic acid group-containing monomers such as p-vinylbenzenesulfonic acid, p-acrylamidopropylsulfonic acid, and tert-butylacrylamidesulfonic acid; and phosphoric acid group-containing monomers such as the phosphate monoester of 2-hydroxyethyl (meth)acrylate and the phosphate monoester of 2-hydroxypropyl (meth)acrylate. As the acid group-containing monomer, only one kind may be used, or two or more kinds may be used in combination.

[0074] Examples of the other monomers include at least one monomer selected from styrene-based monomers, (meth)acrylonitrile, and (meth)acrylamide. Examples of the styrene-based monomers include styrene and α-methylstyrene. As the other monomers, only one kind may be used, or two or more kinds may be used in combination.

[0075] The above monomer mixture may further contain crosslinkable monomers such as carbonyl group-containing monomers, hydrolyzable silyl group-containing monomers, and various polyfunctional vinyl monomers. By including crosslinkable monomers in the monomer mixture, self-crosslinkability can be imparted to the resulting acrylic resin. As the crosslinkable monomers, only one kind may be used, or two or more kinds may be used in combination.

[0076] Examples of the carbonyl group-containing monomers include ketone group-containing monomers such as acrolein, diacetone (meth)acrylamide, acetylacetoxyethyl (meth)acrylate, formylstyrene, and alkyl vinyl ketones having 4 to 7 carbon atoms (such as methyl vinyl ketone, ethyl vinyl ketone, and butyl vinyl ketone).

[0077] Examples of the hydrolyzable silyl group-containing monomers include 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, and 3-(meth)acryloxypropyltriethoxysilane.

[0078] The above polyfunctional vinyl monomers are compounds having two or more radically polymerizable ethylenically unsaturated groups in the molecule. Specific examples of the polyfunctional vinyl monomers include polyfunctional vinyl compounds such as divinylbenzene, ethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, allyl (meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate; and polyfunctional monomers such as triallyl cyanurate, pentaerythritol tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.

[0079] The crosslinkable monomer may be used alone or in combination of two or more. Examples of preferred crosslinkable monomers include allyl (meth)acrylate, ethylene glycol di(meth)acrylate, divinylbenzene, etc. By using these crosslinkable monomers, there is an advantage that the average particle diameter of the acrylic resin contained in the obtained acrylic resin aqueous dispersion (A11) can be appropriately controlled to be 100 nm or less.

[0080] Based on the total amount of the monomer mixture, the amount of the crosslinkable monomer is preferably 0.2 to 20% by mass, more preferably 0.5 to 20% by mass. By including the crosslinkable monomer in the monomer mixture within the above range, there is an advantage that the average particle diameter of the acrylic resin contained in the prepared acrylic resin aqueous dispersion (A11) can be appropriately controlled to be 100 nm or less.

[0081] The acrylic resin contained in the acrylic resin aqueous dispersion (A11) may be in the form of particles, and the average particle diameter of such particles is preferably 0.01 μm or more and 1.0 μm or less.

[0082] It should be noted that in the present disclosure, the average particle diameter is the volume average particle diameter determined by the dynamic light scattering method. Specifically, an electrophoretic light scattering photometer ELSZ series (manufactured by Otsuka Electronics Co., Ltd.) etc. can be used for measurement.

[0083] The acrylic resin contained in the acrylic resin aqueous dispersion (A11) may be core-shell type particles.

[0084] The acid value of the acrylic resin contained in the acrylic resin aqueous dispersion (A11) is preferably 1 mgKOH / g or more and 80 mgKOH / g or less, more preferably 2 mgKOH / g or more and 70 mgKOH / g or less, and still more preferably 3 mgKOH / g or more and 60 mgKOH / g or less.

[0085] The hydroxyl value of the acrylic resin contained in the acrylic resin aqueous dispersion (A11) is preferably 30 mgKOH / g or more and 120 mgKOH / g or less, more preferably 35 mgKOH / g or more and 100 mgKOH / g or less. By being within the above range, the curing reaction proceeds sufficiently, and there is an advantage that the hardness of the obtained coating film is good.

[0086] It should be noted that in the present disclosure, both the acid value and the hydroxyl value are values converted based on the solid content and are measured by the method according to JIS K0070.

[0087] The weight-average molecular weight of the acrylic resin contained in the aqueous acrylic resin dispersion (A11) is preferably 50,000 or more and 5,000,000 or less, more preferably 50,000 or more and 1,000,000 or less. By being within the above range, there are advantages such as good various properties of the resulting coating film, such as hardness, adhesion, and water resistance.

[0088] It should be noted that in the present disclosure, the weight-average molecular weight is a value converted from the measurement result of gel permeation chromatography (GPC) using polystyrene as a standard.

[0089] In the solid content of the above coating film-forming resin (A1), the content rate of the solid content of the aqueous acrylic resin dispersion (A11) is preferably 30% by mass or more and 90% by mass or less, more preferably 40% by mass or more and 85% by mass or less, further preferably 50% by mass or more and 80% by mass or less. By being within the above range, the curing reaction proceeds sufficiently, and there is an advantage of good hardness of the resulting coating film.

[0090] In the solid content of the above coating film-forming resin (A2), the content rate of the solid content of the aqueous acrylic resin dispersion (A11) is preferably 20% by mass or more and 90% by mass or less, more preferably 25% by mass or more and 85% by mass or less, further preferably 30% by mass or more and 80% by mass or less. By being within the above range, the curing reaction proceeds sufficiently, and there is an advantage of good hardness of the resulting coating film.

[0091] It should be noted that in the present disclosure, the solid content contained in each component and composition means the heating residue defined in JIS K 5601-1-2:2008, and is obtained by measuring the percentage of the residual mass after heating at 105 °C for 60 minutes with respect to the original mass.

[0092] The aqueous acrylic resin dispersion (A11) can be produced by subjecting the above monomer mixture to emulsion polymerization with stirring and heating in an aqueous medium in the presence of a radical polymerization initiator and an emulsifier. The reaction temperature can be, for example, about 30 to 100 °C. The reaction time can be appropriately selected according to the reaction scale and reaction temperature, and can be, for example, about 1 to 10 hours. In the emulsion polymerization, for example, the monomer mixture or the monomer pre-emulsion can be added to the reaction vessel containing water and the emulsifier all at once, or can be added dropwise temporarily. By appropriately selecting such steps, the reaction temperature can be adjusted. The above monomer pre-emulsion can be prepared by emulsifying at least a part of the monomer mixture with water and the emulsifier.

[0093] As the above-described radical polymerization initiator, known initiators used in the emulsion polymerization of acrylic resins can be used. The radical polymerization initiator is preferably a water-soluble radical polymerization initiator. For example, persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate can be used in the form of an aqueous solution. In addition, so-called redox initiators composed of an oxidizing agent such as potassium persulfate, sodium persulfate, ammonium persulfate, or hydrogen peroxide and a reducing agent such as sodium bisulfite, sodium thiosulfate, Rongalite, or ascorbic acid can also be used in the form of an aqueous solution.

[0094] As the radical polymerization initiator, only one kind can be used, or two or more kinds can be used in combination.

[0095] As the above-described emulsifier, for example, an anionic or nonionic emulsifier selected from micelle compounds having a hydrocarbon group with 6 or more carbon atoms and a hydrophilic moiety such as carboxylate, sulfonate, or sulfate partial ester in the same molecule can be used. Examples of the anionic emulsifier include: alkali metal salts or ammonium salts of sulfuric acid half esters of alkylphenols or higher alcohols; alkali metal salts or ammonium salts of alkyl sulfonates or allyl sulfonates; alkali metal salts or ammonium salts of sulfuric acid half esters of polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl ethers, or polyoxyethylene allyl ethers, etc. Examples of the nonionic emulsifier include: polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl ethers, or polyoxyethylene allyl ethers, etc. In addition, as other examples of the emulsifier, various anionic and nonionic reactive emulsifiers having groups such as (meth)acrylic, propenyl, allyl, allyl ether, and maleic acid groups having a radically polymerizable unsaturated double bond in the molecule can be cited.

[0096] Commercially available products can be used as the emulsifier. Examples of commercially available products include: Antox MS-60 (manufactured by Nippon Emulsifier Co., Ltd.), Eleminol JS-2 (manufactured by Sanyo Chemical Industries, Ltd.), Adeka Reasoap SR-10 (manufactured by ADEKA Corporation), Aqualon HS-10 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), etc.

[0097] Only one kind of emulsifier can be used, or two or more kinds can be used in combination.

[0098] In the emulsion polymerization, if necessary, a mercaptan compound or a lower alcohol or other additives (chain transfer agents) for adjusting the molecular weight can also be used. By using these additives, the emulsion polymerization can be appropriately carried out, and in addition, the smooth and uniform formation of the coating film can be promoted, and the adhesion of the coating film to the object to be coated can be improved.

[0099] As the emulsion polymerization, any polymerization method such as a single-stage continuous monomer uniform dropping method, a multi-stage monomer feeding method (i.e., core-shell polymerization method), or a power feeding polymerization method in which the monomer composition fed during the polymerization is continuously changed can be appropriately selected.

[0100] A neutralizing agent can be added to the obtained acrylic resin aqueous dispersion (A11) to neutralize at least a part of the acid groups that may be contained in the acrylic resin. By neutralization, the stability of the acrylic resin aqueous dispersion (A11) can be improved. Examples of the neutralizing agent include: basic compounds. Examples of the basic compounds include: ammonia; organic amines such as monomethylamine, dimethylamine, trimethylamine, triethylamine, diisopropylamine, monoethanolamine, diethanolamine, and dimethylethanolamine (dimethylaminoethanol); and inorganic bases such as sodium hydroxide, potassium hydroxide, and lithium hydroxide. As the neutralizing agent, only one kind can be used, or two or more kinds can be used in combination.

[0101] The above polyol resin is a resin having two or more hydroxyl groups in the molecule, and examples thereof include: polyether polyols, polycarbonate polyols, etc. As the above polyol resin, polyether polyols such as polypropylene glycol are preferred.

[0102] In the above first base coating composition, with respect to a total of 100 parts by mass of the solid content of the acrylic resin aqueous dispersion (A11), the content of the solid content of the polyol resin is preferably 1 part by mass or more and 70 parts by mass or less, more preferably 5 parts by mass or more and 50 parts by mass or less, and still more preferably 10 parts by mass or more and 30 parts by mass or less.

[0103] In the above second base coating composition, with respect to a total of 100 parts by mass of the solid content of the acrylic resin aqueous dispersion (A11), the content of the solid content of the polyol resin is preferably 1 part by mass or more and 90 parts by mass or less, more preferably 10 parts by mass or more and 70 parts by mass or less, and still more preferably 30 parts by mass or more and 50 parts by mass or less.

[0104] The polyester resin aqueous dispersion can be prepared, for example, by condensing a polyol component and a polyacid component and dispersing them in water using a basic compound. The polyurethane resin aqueous dispersion can be prepared, for example, by polymerizing a polyol compound, a compound having an active hydrogen group and a hydrophilic group in the molecule, and an organic polyisocyanate as needed with a chain extender and a polymerization terminator, and dispersing the obtained polymer in water.

[0105] The hydroxyl value of the film-forming resin (A) is preferably 30 mgKOH / g or more and 120 mgKOH / g or less, more preferably 35 mgKOH / g or more and 100 mgKOH / g or less. By being within the above range, the curing reaction proceeds sufficiently, and the advantage is that the hardness of the obtained coating film is good.

[0106] In the above-described first base coating composition, the content of the solid component of the polyester resin aqueous dispersion is preferably 10 parts by mass or more and 90 parts by mass or less, more preferably 20 parts by mass or more and 80 parts by mass or less, and still more preferably 30 parts by mass or more and 70 parts by mass or less, based on 100 parts by mass in total of the solid components of the acrylic resin aqueous dispersion (A11).

[0107] In the above-described second base coating composition, the content of the solid component of the polyester resin aqueous dispersion is preferably 20 parts by mass or more and 80 parts by mass or less, more preferably 30 parts by mass or more and 70 parts by mass or less, and still more preferably 40 parts by mass or more and 65 parts by mass or less, based on 100 parts by mass in total of the solid components of the acrylic resin aqueous dispersion (A11).

[0108] In 100% by mass of the solid components of the aqueous first base coating composition, the content of the solid component of the film-forming resin (A1) is preferably 10% by mass or more and 90% by mass or less, more preferably 15% by mass or more and 85% by mass or less, and still more preferably 20% by mass or more and 80% by mass or less.

[0109] In 100% by mass of the solid components of the aqueous second base coating composition, the content of the solid component of the film-forming resin (A2) is preferably 10% by mass or more and 90% by mass or less, more preferably 15% by mass or more and 85% by mass or less, and still more preferably 20% by mass or more and 80% by mass or less.

[0110] (Curing agent (B))

[0111] The above-described curing agent (B) may be a compound having two or more groups capable of reacting with the film-forming resin (A) in one molecule, and together with the film-forming resin (A), belongs to the film-forming components. As the above-described curing agent (B), one or more selected from amino resins (melamine resins, urea resins, benzoguanamine resins, etc.), blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions can be used.

[0112] The curing agent (B1) and the curing agent (B2) may be the same or different.

[0113] In one embodiment, the above-described curing agent (B) preferably contains an amino resin, and more preferably contains a melamine resin (B11). By containing the melamine resin (B11), a coating film can be formed by the self-polymerization of the melamine resin (B11) and the reaction of the amino group contained in the melamine resin (B11) with the hydroxyl group that may be contained in the film-forming resin (A).

[0114] Melamine resin (B11) can be obtained by modifying the condensate of amino compounds such as melamine and aldehyde compounds such as formaldehyde and acetaldehyde with lower alcohols such as methanol, ethanol, propanol, and butanol. Melamine resin (B11) is preferably a compound or its condensate having three reactive functional groups represented by the following formula in one molecule of the triazine nucleus as reactive functional groups.

[0115] -NX 1 X 2

[0116] [X 1 、X 2 each independently represents a hydrogen atom, a hydroxymethyl group, or -CH2-OR 1 .

[0117] R 1 represents an alkyl group having 1 to 8 carbon atoms, preferably a linear or branched alkyl group having 1 to 8 carbon atoms.

[0118] When a molecule contains multiple -CH2-OR 1 , multiple R 1 can be the same or different. ]

[0119] As melamine resins, the following 4 types can be exemplified: a fully alkylated type containing only -N(CH2OR 1 )2 as a reactive functional group; a hydroxymethyl type containing -N(CH2OR 1 )(CH2OH) as a reactive functional group; an imino type containing -N(CH2OR 1 )(H) as a reactive functional group; a hydroxymethyl / imino type containing -N(CH2OR 1 )(CH2OH) and -N(CH2OR 1 )(H) or containing -N(CH2OH)(H) as a reactive functional group. In the above fully alkylated type, hydroxymethyl type, imino type, or hydroxymethyl / imino type melamine resins, R 1 is preferably an alkyl group having 1 to 4 carbon atoms, more preferably methyl, n-butyl, or isobutyl. In one embodiment, methyl and butyl can be mixed. In another embodiment, it can be only methyl. In yet another embodiment, it can be only butyl.

[0120] As the melamine resin, commercially available products can be used. Examples of commercially available products include: hydroxymethyl-imino type methyl / butyl mixed etherified melamine resins such as Cymel 202; imino type methyl / butyl mixed etherified melamine resins such as Cymel 204, Cymel 211, Cymel 250, Cymel 254, Mycoat 212, Mycoat 518, Mycoat 525; fully alkyl type methylated melamine resins such as Cymel 350; imino type methylated melamine resins such as Cymel 325, Cymel 327, Cymel 385, Cymel 701, Cymel 712, Mycoat 723, Mycoat 776; imino type butylated melamine resins such as Mycoat 508, hydroxymethyl type methylated melamine resins such as Cymel 370, hydroxymethyl type methyl / isobutyl mixed etherified melamine resins such as Mycoat 2677 (above, manufactured by Allnex Japan Co., Ltd.), etc.

[0121] In the total 100% by mass of the solid content of the above curing agent (B1), the content of the solid content of the above melamine resin (B11) can be 0% by mass, preferably 80% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less, and still more preferably 95% by mass or more and 100% by mass or less.

[0122] In the total 100% by mass of the solid content of the above curing agent (B2), the content of the solid content of the above melamine resin (B11) can be 0% by mass, preferably 80% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less, and still more preferably 95% by mass or more and 100% by mass or less.

[0123] In the total 100 parts by mass of the solid content of the above film-forming resin (A1) and the solid content of the curing agent (B1), the content of the above melamine resin (B11) can be 0 parts by mass, preferably 10 parts by mass or more and 90 parts by mass or less, and more preferably 30 parts by mass or more and 80 parts by mass or less. Thus, it has the advantages that various physical properties such as the hardness, adhesion, and water resistance of the obtained coating film are good.

[0124] In the total 100 parts by mass of the solid content of the above film-forming resin (A2) and the solid content of the curing agent (B2), the content of the above melamine resin (B11) can be 0 parts by mass, preferably 10 parts by mass or more and 80 parts by mass or less, and more preferably 30 parts by mass or more and 70 parts by mass or less. Thus, it has the advantages that various physical properties such as the hardness, adhesion, and water resistance of the obtained coating film are good.

[0125] The blocked isocyanate compound can be prepared by adding a blocking agent having active hydrogen to a polyisocyanate composed of trimethylene diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, isophorone diisocyanate, etc. This blocked isocyanate resin dissociates the blocking agent by heating, generates isocyanate groups, and reacts with the functional groups in the above resin components to cure.

[0126] In a total of 100% by mass of the solid content of the film-forming resin (A1) and the solid content of the curing agent (B1), the content of the solid content of the curing agent (B1) in the aqueous first base coating composition is preferably 20% by mass or more and 50% by mass or less, more preferably 20% by mass or more and 40% by mass or less.

[0127] In a total of 100% by mass of the solid content of the film-forming resin (A2) and the solid content of the curing agent (B2), the content of the solid content of the curing agent (B2) in the aqueous second base coating composition is preferably 20% by mass or more and 50% by mass or less, more preferably 20% by mass or more and 40% by mass or less.

[0128] (Brightness pigment (C))

[0129] Examples of the brightness pigment include metallic brightness pigments such as aluminum, copper, zinc, iron, nickel, tin, aluminum oxide, and their alloys, and interference mica pigments, muscovite pigments, graphite pigments, glass flake pigments, etc. In the aqueous base coating composition of the present disclosure, from the viewpoint of the design of the obtained multilayer coating film, the above brightness pigment preferably contains an aluminum pigment.

[0130] The brightness pigment (C1) contained in the aqueous first base coating composition and the brightness pigment (C2) contained in the aqueous second base coating composition may be the same or different.

[0131] The average particle size of the above brightness pigment (C1) can be preferably 1 μm or more and 30 μm or less, more preferably 5 μm or more and 25 μm or less. The average thickness of the above brightness pigment (C1) can be preferably 0.01 μm or more and 6 μm or less, more preferably 0.1 μm or more and 5 μm or less.

[0132] By satisfying the above conditions for the average particle size and average thickness of the brightness pigment (C1), brightness can be imparted to the first base coating film.

[0133] The average particle size of the bright pigment (C2) may be 17 μm to 30 μm, more preferably 19 μm to 25 μm. The average thickness of the bright pigment (C2) is 0.3 μm to 6 μm, preferably 0.35 μm to 6 μm, more preferably 0.5 μm to 6 μm, further preferably 0.5 μm to 5 μm, and even more preferably 0.5 μm to 3 μm.

[0134] When the average particle size and average thickness of the bright pigment (C2) satisfy the above conditions, a highly grainy appearance can be imparted to the second base coating film and further to the multilayer coating film.

[0135] In this disclosure, the average particle size of the bright pigment (C) refers to the average major diameter. The average particle size can be measured by observing the bright pigment using a shape analysis laser microscope (e.g., VK-X250 manufactured by Keyence Corporation) and calculating the number average of the maximum length (major diameter) of 100 randomly selected particles of the pigment.

[0136] In the present disclosure, the average thickness t of the bright pigment can be calculated based on the water diffusion area (WCA: m 2 / g), calculated by the following formula.

[0137] t(μm)=0.4 / [WCA(m 2 / g)]

[0138] In the present disclosure, the water surface spread area can be measured in accordance with JIS K 5906.

[0139] The water diffusion area (WCA) of the bright pigment (C1) is preferably 0.06 m 2 / g or above and 40m 2 / g or less, more preferably 0.08m 2 / g or more and 4m 2 / g or less.

[0140] The water diffusion area (WCA) of the bright pigment (C2) is 1.4m 2 / g or less, preferably 0.06m 2 / g or above and 1.1m 2 / g or less, more preferably 0.08m 2 / g or more and 1m 2 / g or less, more preferably 0.08m 2 / g or above and 0.8m 2 When the water surface diffusion area of the bright pigment (C2) is within the above range, the resulting coating film can have a more grainy appearance.

[0141] When the above-mentioned brightening pigment (C) is an aluminum pigment, surface treatment may be carried out as required. Examples of the surface treatment that can be carried out on the aluminum pigment include: surface treatment using a metal oxide-based compound, surface treatment using a phosphorus compound, surface treatment using an amine compound, surface treatment using a silane compound, etc.

[0142] Examples of the above-mentioned metal oxide-based compound include: metal oxides containing at least a transition metal element as a constituent metal, and their alkali metal salts and ammonium salts. Specifically, examples include: molybdenum trioxide, molybdic acid, alkali metal salts of molybdic acid, ammonium salts of molybdic acid, vanadic acid, alkali metal salts of vanadic acid, ammonium salts of vanadic acid, etc. It should be noted that there is no particular limitation on the above-mentioned alkali metal, and examples include: sodium, potassium, etc. Among them, as the above-mentioned metal oxide-based compound, it is preferable to use one or more metal oxide-based compounds selected from molybdic acid, alkali metal salts of molybdic acid, ammonium salts of molybdic acid, vanadic acid, alkali metal salts of vanadic acid, and ammonium salts of vanadic acid.

[0143] Examples of the above-mentioned phosphorus compound include: organic phosphates, organic phosphites, organic phosphonic acids, and amine salts of these compounds. These phosphorus compounds can be used alone or in combination of two or more.

[0144] Examples of the above-mentioned amine compound include: linear or branched primary amines, linear or branched secondary amines, linear or branched tertiary amines, alicyclic primary amines, alicyclic secondary amines, alicyclic tertiary amines, primary amines containing an aromatic group, secondary amines containing an aromatic group, tertiary amines containing an aromatic group, etc. These amine compounds may also have substituents (such as hydroxyl groups, etc.) as required. These amine compounds can be used alone or in combination of two or more.

[0145] Examples of the above-mentioned silane compound include: alkoxysilane compounds, vinyl-containing silane coupling agents, epoxy-containing silane coupling agents, styryl-containing silane coupling agents, (meth)acryloyl-containing silane coupling agents, amino-containing silane coupling agents, isocyanate group-containing silane coupling agents, urea group-containing silane coupling agents, mercapto-containing silane coupling agents, acid anhydride group-containing silane coupling agents, and their partial condensates, etc. These silane compounds can be used alone or in combination of two or more.

[0146] The above-mentioned surface treatment can be carried out under the treatment conditions generally adopted by those skilled in the art. The above-mentioned surface treatment can be carried out only one kind, or two or more kinds can be used in combination.

[0147] In a preferred embodiment, the brightening pigment (C1) that can be included in the above aqueous first base coating composition is an aluminum pigment, having an average particle size of 1 to 30 μm, particularly 5 to 25 μm. When the above surface treatment is carried out, it will not damage the film appearance of the formed multi-layer coating film, and has advantages such as better stability of the aqueous first base coating composition.

[0148] In a preferred embodiment, the brightening pigment (C2) that can be included in the above aqueous second base coating composition is an aluminum pigment, having an average particle size of 17 to 30 μm, particularly 19 to 25 μm. When the above surface treatment is carried out, it will not damage the film appearance of the formed multi-layer coating film, and has advantages such as better stability of the aqueous first base coating composition.

[0149] As the above brightening pigment, commercially available products can be used. As commercially available products, for example, ALPASTE 93-0647, 06-0672, TCR-2020, MG1000, etc. (all manufactured by Toyo Aluminium Co., Ltd.), Aluminium Paste MH-9901, etc. (manufactured by Asahi Kasei Corporation), etc. can be cited.

[0150] Relative to 100 parts by mass of the resin solid content of the film-forming resin (A1), the above brightening pigment (C1) is preferably 1 part by mass or more and 45 parts by mass or less, more preferably 2 parts by mass or more and 35 parts by mass or less. The pigment mass concentration of the above brightening pigment (C1) in the aqueous first base coating composition is preferably 2% by mass or more and 40% by mass or less, more preferably 2.5% by mass or more and 35% by mass, and further preferably 3% by mass or more and 30% by mass or less.

[0151] It should be noted that the above aqueous first base coating composition may not contain the brightening pigment (C1). In one embodiment, the pigment mass concentration of the brightening pigment (C1) in the above aqueous first base coating composition can be 0% by mass.

[0152] Relative to 100 parts by mass of the resin solid content of the film-forming resin (A2), the above brightening pigment (C2) is preferably 3 parts by mass or more and 50 parts by mass or less, more preferably 5 parts by mass or more and 45 parts by mass or less. The pigment mass concentration of the above brightening pigment (C2) in the aqueous second base coating composition is 2% by mass or more and 40% by mass or less, preferably 3% by mass or more and 35% by mass or less, more preferably 4% by mass or more and 30% by mass, and further preferably 4% by mass or more and 25% by mass or less. By being within the above range, it has the advantage of being able to obtain a coating film with high particle feeling and uniformity.

[0153] The aqueous base coating composition of the present disclosure contains an aqueous medium. The aqueous medium in the present disclosure may be: water; a hydrophilic solvent; a mixture of water and a hydrophilic solvent. As the above-mentioned hydrophilic solvent, examples include: glycol solvents such as ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, and triethylene glycol; glycol ether solvents such as ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monobutyl ether, triethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and dipropylene glycol monoethyl ether; alcohol solvents such as methanol, ethanol, isopropyl alcohol, and benzyl alcohol; cyclic ether solvents such as dioxane and tetrahydrofuran; ketone solvents such as acetone; N-methyl-2-pyrrolidone, etc. alkanes, cyclic ether solvents such as tetrahydrofuran; ketone solvents such as acetone; N-methyl-2-pyrrolidone, etc.

[0154] (Other components)

[0155] In addition to the above components, the aqueous base coating composition may contain additives commonly used by those skilled in the art within the range that does not affect the film properties and film appearance. As the above additives, for example, pigments such as extender pigments, coloring pigments, and anti-rust pigments; anti-sag / anti-settling agents; curing catalysts (organometallic catalysts); anti-separation agents; dispersants; defoaming / anti-wrinkle agents; surface modifiers; viscosity modifiers; thickeners; leveling agents; matting agents; antioxidants; ultraviolet light inhibitors; plasticizers; film-forming aids; defoamers; organophosphorus-containing organic compounds, etc.

[0156] (Viscosity modifier (D)

[0157] The aqueous base coating composition of the present disclosure preferably contains a viscosity modifier (D). By containing a viscosity modifier, thixotropy can be imparted to the aqueous base coating composition, the coating workability is good, and the appearance of the obtained multi-layer film is good.

[0158] As the viscosity modifier, organic viscosity modifiers and inorganic viscosity modifiers can be cited. As the organic viscosity modifiers, for example, crosslinked or uncrosslinked resin particles; swelling dispersions of fatty acid amides, polyamide-based viscosity modifiers such as amide-based fatty acids and phosphates of long-chain polyaminoamides; polyethylene-based viscosity modifiers such as colloidal swelling dispersions of polyethylene oxide. As the inorganic viscosity modifiers, organic bentonite viscosity modifiers such as organic acid montmorillonite clay and montmorillonite can be cited.

[0159] The tackifier (D) more preferably contains an inorganic tackifier, and more preferably contains an inorganic layered compound (D-1). The above-mentioned inorganic layered compound (D-1) is preferably a layered material having a stacked structure formed by stacking a plurality of inorganic crystal layers. Although it should not be limited to a specific theory for explanation, it is considered that the inorganic layered compound (D-1) having such a layered structure swells in the coating composition and forms a card-house structure in the coating composition. Therefore, it is considered that not only can it impart an appropriate viscosity to the coating composition and excellent film strength, but also when forming the first base film or the second base film, it can inhibit the sedimentation of the bright pigment (C) and assist in orientation, and can impart a higher granular appearance to the film.

[0160] Examples of the shape of the primary particles of the inorganic layered compound (D-1) include: disk shape, plate shape, spherical shape, granular shape, cubic shape, needle shape, rod shape, and amorphous shape, etc., and a disk shape or a plate shape is preferred.

[0161] The inorganic layered compound (D-1) in the present disclosure includes layered silicates (silicate minerals), halogenated minerals, oxide minerals, carbonate minerals, borate minerals, sulfate minerals, molybdate minerals, tungstate minerals, phosphate minerals, arsenate minerals, vanadate minerals, etc. From the perspective of being able to impart an appropriate viscosity and excellent film strength to the coating composition, layered silicates are preferred.

[0162] Specific examples of the layered silicate include: natural or synthetic hectorite, saponite, stevensite, hydrite, montmorillonite, nontronite, bentonite and other smectite clay minerals or swelling mica clay minerals such as Na-type tetrasilicic fluoromica, Li-type tetrasilicic fluoromica, Na-type fluorophlogopite, Li-type fluorophlogopite, and vermiculite or kaolin, or a mixture thereof.

[0163] Examples of commercially available products of the inorganic layered compound (D-1) include: Laponite XLG (synthetic lithium montmorillonite analogue, manufactured by BYK), Laponite RD (synthetic lithium montmorillonite analogue, manufactured by BYK), Laponite EP (synthetic lithium montmorillonite analogue, manufactured by BYK), Optigel WX (Na-substituted bentonite, manufactured by BYK), Thermabis (synthetic lithium montmorillonite analogue, manufactured by Henkel), Sumecton SA-1 (talc analogue, manufactured by KUNIMINE INDUSTRIES), BEN-GEL (natural bentonite, manufactured by Hojun), Kunipia-F (natural montmorillonite, manufactured by KUNIMINE INDUSTRIES), Veegum (natural lithium montmorillonite, manufactured by Vanderbilt), Dimnite (synthetic swelling mica, manufactured by Topy Industries), Somasif (synthetic swelling mica, manufactured by Co-op Chemical), SWN (synthetic smectite, manufactured by Co-op Chemical), SWF (synthetic smectite, manufactured by Co-op Chemical), etc.

[0164] With respect to 100 parts by mass of the total of the solid components of the film-forming resin (A1) and the solid components of the curing agent (B1) (hereinafter, the total of the solid components of the film-forming resin (A1) and the solid components of the curing agent (B1) is also referred to as "the resin solid components of the aqueous first base coating composition"), the content of the inorganic layered compound (D-1) in the aqueous first base coating composition of the present disclosure is preferably 1 part by mass or more and 7 parts by mass or less, more preferably 2 parts by mass or more and 7 parts by mass or less, and further preferably 2 parts by mass or more and 5 parts by mass or less. By the content of the inorganic layered compound (D1) being within the above range, it is considered that the viscous behavior of the coating composition is more appropriate, and in the resulting film, it is easier to obtain a higher uniformity, that is, an appearance in which the metallic flake color unevenness is suppressed.

[0165] Based on 100 parts by mass of the total of the solid components of the film-forming resin (A2) and the curing agent (B2) (hereinafter, the total of the solid components of the film-forming resin (A2) and the curing agent (B2) is also referred to as "the resin solid components of the aqueous second base coating composition"), the content of the inorganic layered compound (D-1) in the aqueous second base coating composition of the present disclosure is preferably 1 part by mass or more and 10 parts by mass or less, more preferably 2 parts by mass or more and 8 parts by mass or less, and further preferably 2 parts by mass or more and 6 parts by mass or less. It is considered that when the content of the inorganic layered compound (D-1) is within the above range, the viscosity of the coating composition is better, and in the resulting coating film, it is easier to obtain a higher uniformity, that is, an appearance in which the metal flake color unevenness is suppressed.

[0166] There is no particular limitation on the above coloring pigments, and examples thereof include: azo chelate pigments, azomethine azo pigments, azo lake pigments, insoluble azo pigments, condensed azo pigments, monoazo pigments, indanthrone pigments, bisazo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, thioindigo pigments, pyrene ketone pigments, perylene pigments, di alkane pigments, quinacridone pigments, quinophthalone pigments, isoindolinone pigments, naphthol pigments, pyrazolone pigments, anthrapyrimidine pigments, anthraquinone pigments, anthrapyrimidine pigments, metal complex pigments and other organic coloring pigments; chrome yellow, iron oxide, yellow iron oxide, transparent iron oxide, iron black, iron chromium, bismuth manganese, bismuth vanadate, chromium oxide, molybdenum orange, iron oxide red, titanium yellow, zinc white, zinc yellow, ocher, carbon black, titanium dioxide, cobalt green, phthalocyanine green, ultramarine blue, cobalt blue, phthalocyanine blue, cobalt violet and other inorganic coloring pigments; graphite pigments, other colored, colored flat pigments, etc.

[0167] The above coloring pigments can be colored or achromatic, and can be colors such as red, blue, yellow, green, purple, brown, white, black, gray, etc.

[0168] Examples of the above extender pigments include: titanium oxide, calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silicon dioxide, calcined kaolin, etc.

[0169] As the above-mentioned pigment, from the viewpoints of improving weather resistance and ensuring hiding power, a coloring pigment is preferred. In particular, from the viewpoints of excellent coloring hiding power and low cost, titanium dioxide is more preferred. In addition, as the pigment, a standard gray coating composition mainly composed of carbon black and titanium dioxide may be used. Further, it may also be a coating composition formed by combining a coating composition having brightness or hue compatible with the aqueous second base coating composition and various coloring pigments. The aqueous first base coating composition of the present disclosure can function as a primer when coating the second base coating composition, and can also function as a base color by containing a coloring pigment.

[0170] The above-mentioned brightening pigment (C) and the above-mentioned pigment can be made into a pigment dispersion paste and then used for preparing the aqueous base coating composition. The pigment dispersion paste is obtained by previously dispersing a pigment, a pigment dispersant, and a part of the film-forming resin (A) used as needed in a small amount of an aqueous medium. The pigment dispersant may be a resin having a structure including a pigment-affinitive part and a hydrophilic part. As the pigment-affinitive part and the hydrophilic part, for example, nonionic, cationic, and anionic functional groups can be mentioned. The pigment dispersant may have two or more of the above-mentioned functional groups in one molecule.

[0171] As the nonionic functional group, for example, hydroxyl group, amide group, polyalkylene oxide, etc. can be mentioned. As the cationic functional group, for example, amino group, imino group, hydrazino group, etc. can be mentioned. In addition, as the anion functional group, for example, carboxyl group, sulfo group, phosphate group, etc. can be mentioned. Such a pigment dispersant can be manufactured by methods well-known to those skilled in the art.

[0172] Regarding the pigment dispersant, as long as it does not contain a volatile basic substance in its solid content or the content is 3% by mass or less, there is no particular limitation, and it is preferably possible to effectively disperse the pigment with a small amount of the pigment dispersant. Commercially available products can be used as the pigment dispersant. Examples of commercially available products include, for example, Disperbyk-180 and Disperbyk-190 (both manufactured by BYK-Chemie) as anionic / nonionic dispersants, EFKA POLYMER 4550 (manufactured by BASF), EFKAPOLYMER 4585 (manufactured by BASF), Solsperse 27000 (manufactured by Avecia) as a nonionic dispersant, Solsperse 41000 and Solsperse 53095 (both manufactured by Avecia) as anionic dispersants, Disperbyk-2015 (manufactured by BYK-Chemie) as a polymer copolymer, etc.

[0173] The weight-average molecular weight of the pigment dispersant is preferably 1,000 or more and 100,000 or less, more preferably 2,000 or more and 100,000 or less, and further preferably 4,000 or more and 50,000 or less.

[0174] The pigment dispersion paste can be prepared by mixing and dispersing a pigment dispersant, a pigment, and a part of the film-forming resin (A1) used as needed by a known method. When manufacturing the pigment dispersion paste, the proportion of the pigment dispersant is preferably 1% by mass or more and 50% by mass or less based on the solid content of the pigment dispersion paste. By setting the proportion of the pigment dispersant within the above range, the pigment dispersion stability and the physical properties of the resulting coating film can be maintained in a better range. The above proportion of the pigment dispersant is more preferably 3% by mass or more and more preferably 5% by mass or less.

[0175] In the aqueous base coating composition, the pigment mass concentration of the above pigment (the ratio (PWC) of the pigment mass to the total mass of the resin solid content and the pigment in the aqueous first base coating composition or the aqueous second base coating composition) is preferably 10% by mass or more and 60% by mass or less. By setting the PWC to 10% by mass, it is easier to improve the hiding power. By setting the PWC to 60% by mass or less, it is possible to more easily suppress the increase in viscosity during curing, and thus it is easier to ensure fluidity and obtain a good coating film appearance.

[0176] Examples of the above thickener include: associative nonionic urethane thickeners, alkali-swellable thickeners, bentonite as an inorganic intercalation compound, and the like.

[0177] In the aqueous first base coating composition, the solid content concentration (total content rate of the solid content) is 15% by mass or more and 40% by mass or less, more preferably 18% by mass or more and 35% by mass or less, and further preferably 20% by mass or more and 30% by mass or less.

[0178] In the aqueous second base coating composition, the solid content concentration is 5% by mass or more and 20% by mass or less, preferably 8% by mass or more and 18% by mass or less, and more preferably 10% by mass or more and 15% by mass or less. The solid concentration of the aqueous second base coating composition is preferably less than the total content rate of the solid content of the aqueous first coating composition.

[0179] In addition, the ratio of the solid content concentration in the above aqueous first base coating composition to the solid content concentration in the aqueous second base coating composition (solid content ratio of the aqueous first base coating composition / solid content ratio of the aqueous second base coating composition) exceeds 1 on a mass basis, preferably is 1.1 or more and 8 or less, more preferably is 1.2 or more and 6 or less, further preferably is 1.5 or more and 5 or less, and still more preferably is 2 or more and 4 or less. By having the ratio of the solid content concentrations contained in the aqueous first base coating composition and the aqueous second base coating composition within the above range, the orientation of the brightening pigment (C2) can be made good, and a multilayer coating film that takes into account both a high-granularity appearance and smoothness can be obtained.

[0180] <Method for manufacturing the aqueous first base coating composition or the aqueous second base coating composition>

[0181] The above aqueous first base coating composition can be manufactured by a method commonly used by those skilled in the art, such as kneading / dispersing the film-forming resin (A1), the curing agent (B1), and, if necessary, the brightening pigment (C1), other components, and additives using a disperser, a homogenizer, a kneader, etc.

[0182] In addition, the above aqueous second base coating composition can be manufactured by a method commonly used by those skilled in the art, such as kneading / dispersing the film-forming resin (A1), the curing agent (B1), the brightening pigment (C1), and, if necessary, other components and additives using a disperser, a homogenizer, a kneader, etc.

[0183] In the above manufacturing method, for example, it is preferable to prepare a paste containing the brightening pigment (C) and, if necessary, a pigment dispersant in advance and then mix them. A commercially available pigment dispersant as described above can be used as the pigment dispersant.

[0184] <Multilayer coating film>

[0185] The multilayer coating film of the present disclosure has a first base coating film and a second base coating film formed in sequence on an object to be coated, and may further have a transparent coating film formed on the second base coating film.

[0186] It should be noted that in the present disclosure, an uncured film is also referred to as a coating film, and a cured film is also referred to as a coating film. That is, by curing the uncured first or second base coating film, the first or second base coating film can be obtained.

[0187] The granularity value (G value) of the multilayer coating film of the present disclosure is preferably 9 or more and 15 or less, more preferably 9.5 or more and 14 or less, and still more preferably 10 or more and 13 or less. In the multilayer coating film of the present disclosure, when the granularity value (G value) is 9 or more and 15 or less, in the case of forming a cured coating film using a coating composition containing a film-forming resin and a flaky pigment, a coating film with a higher granular feeling of brightness and a unique design compared to conventional coating films is formed. In addition, when the granularity value (G value) of the multilayer coating film is high, metal flake color unevenness is likely to occur, and the smoothness of the surface of the multilayer coating film tends to decrease. However, according to the manufacturing method of the present disclosure, a multilayer coating film with a high granularity value (G value) and good smoothness can be obtained.

[0188] The granularity value (G value) is a value obtained by numerically quantifying granularity through diffuse illumination measurement. The granularity value (G value) is an image acquired using a CCD chip under diffuse illumination inside a white-painted hemisphere, and the acquired image is analyzed using a histogram of brightness levels. The uniformity of the bright and dark regions is represented as a value indicating one granularity, and this value is the granularity value (G value). The granularity value (G value) is represented in the range of 0 to 30. The smaller the value, the finer it indicates, and the larger the value, the more granular it indicates.

[0189] The above granularity value (G value) can be measured using a glossiness meter. Examples of glossiness meters that can measure these include BYK-mac i (manufactured by BYK Gardner).

[0190] For the Me value of the multilayer coating film, for a mottle size of 11 to 24 mm, it is preferably 7 or less, more preferably 6 or less, and can be 1 or more.

[0191] The Me value is an index for evaluating the phenomenon (color mottling, mottling) of a mottled pattern with uneven hue or contrast caused by insufficient uniformity of the orientation of the bright pigment (C) or uneven coating film thickness of the primer coat, etc. The evaluation of color mottling is carried out as follows: The surface of the sample is scanned at a distance between 10 cm and 100 cm, and white LED light is irradiated at an angle of 15° with respect to the direction perpendicular to the sample surface at each location. The brightness is detected at observation angles of 15°, 45°, and 60° with respect to the specularly reflected light. The brightness change curve at each observation angle is wavelength-divided, and for six different mottle sizes (Md: 6 to 13 mm, Me: 11 to 24 mm, Mf: 19 to 42 mm, Mg: 33 to 72 mm, Mh: 57 to 126 mm, Mi: 100 to 200 mm), the Me value of 0 to 10 is calculated. The larger the Me value, the more likely the mottled pattern in that mottle size range is to be prominent. The mottle size with a high correlation with metal flake color unevenness is 11 to 24 mm, and in the present disclosure, the Me value is evaluated for such a mottle size.

[0192] <Method for manufacturing multi-layer coating film>

[0193] The method for manufacturing a multi-layer coating film of the present disclosure includes the following steps:

[0194] A first base coating film forming step of coating an aqueous first base coating composition on an object to be coated to form a first base coating film;

[0195] A second base coating film forming step of coating an aqueous second base coating composition on the first base coating film to form a second base coating film; and

[0196] A heating and curing step of heating and curing at least the first base coating film and the second base coating film simultaneously to form a multi-layer coating film.

[0197] In the method for manufacturing a multi-layer coating film of the present disclosure, an aqueous first base coating composition is coated, and an aqueous second base coating composition is coated wet-on-wet in a state where the first base coating film is not cured to form a second base coating film. Then, the uncured first base coating film and the second base coating film are heated and cured simultaneously. It is considered that by performing wet-on-wet coating, a part of the aqueous medium contained in the second base coating film moves into the first base coating film, thereby promoting volume shrinkage and making the orientation of the brightening pigment (C2) good, and obtaining a coating film with a high granular appearance and smoothness. In the above wet-on-wet coating, during the coating period, that is, between the first base coating film forming step and the second base coating film forming step, or between the second base coating film forming step and the heating and curing step, preheating and / or drying at normal temperature can be performed as needed.

[0198] (Object to be coated)

[0199] As the object to be coated used in the formation of the multi-layer coating film, various substrates can be used, such as metal molded products, plastic molded products, foams, etc. The aqueous base coating composition of the present disclosure can be appropriately used in the coating of automotive outer panels such as automotive bodies and automotive parts. As the metal molded product, for example, plates and molded products made of iron, copper, aluminum, tin, zinc, etc. and alloys containing these metals can be cited. Specifically, automotive bodies and parts such as sedans, trucks, motorcycles, and buses can be cited.

[0200] The above metal molded product can be pre-treated with phosphates, zirconium salts, chromates, etc., and then an electrodeposition coating film is formed. As the electrodeposition coating composition that can be used to form the electrodeposition coating film, cationic electrodeposition coating compositions and anionic electrodeposition coating compositions can be cited.

[0201] Examples of the above-mentioned plastic molded articles include plates and molded articles made of polypropylene resin, polycarbonate resin, polyurethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, polyamide resin, etc. Specific examples of plastic molded articles include automotive parts such as spoilers, bumpers, rearview mirror covers, grilles, door handles, etc. These plastic molded articles can be subjected to primer coating to enable electrostatic coating.

[0202] On the above-mentioned object to be coated, an intermediate coat film may also be further formed as needed. An intermediate coat paint composition is used in the formation of the intermediate coat film. As the intermediate coat paint composition, for example, a paint composition containing a film-forming resin, a curing agent, various coloring components of organic and / or inorganic systems, extender pigments, etc. can be used. There are no particular limitations on the film-forming resin and the curing agent. Specifically, the film-forming resin and the curing agent listed in the above-mentioned aqueous paint composition can be used. As the film-forming resin of the intermediate coat paint composition, from the viewpoint of various properties of the obtained intermediate coat film, etc., a combination of an acrylic resin and / or a polyester resin with an amino resin and / or an isocyanate is preferably used.

[0203] The above-mentioned first paint composition and second paint composition can be applied to the object to be coated by methods commonly used in the paint field. As the coating method, for example, there can be mentioned: multi-stage coating (preferably two-stage coating) based on air spraying, airless spraying, electrostatic spraying, air electrostatic spraying, coating by combining air electrostatic spraying with a rotary atomization type electrostatic coater, etc.

[0204] When the above-mentioned first paint composition and second paint composition are applied and heat-cured, the heating temperature and time can be appropriately selected according to the composition of the paint composition (aqueous or solvent-based) and the type of the object to be coated. The heating temperature is appropriately selected, for example, in the range of 80 to 180 °C, preferably in the range of 100 to 160 °C. The heating time can be appropriately selected, for example, in the range of 5 minutes to 60 minutes, preferably in the range of 10 minutes to 30 minutes.

[0205] The temperature when preheating the first base coat film or the second base coat film can be, for example, 30 to 80 °C, and the preheating time can be, for example, 1 minute to 60 minutes.

[0206] The above-mentioned first paint composition is preferably applied in such a manner that the dry film thickness t1 of the cured first base coat film reaches within the range of 8 to 20 μm, more preferably in such a manner that it reaches 10 to 14 μm. In addition, the second paint composition is preferably applied in such a manner that the dry film thickness of the cured second base coat film t2 reaches within the range of 0.1 to 8 μm.

[0207] In addition, the coating of the above aqueous first base coating composition and the coating of the above aqueous second base coating composition are preferably carried out in such a manner that the ratio t1 / t2 of the dry film thickness t1 of the above first base coating film to the dry film thickness t2 of the above second base coating film is 1.6 or more and 7 or less, and more preferably in such a manner that it is 1.8 or more and 6 or less. By the ratio t1 / t2 being within the above range, the orientation of the brightening pigment (C2) can be made good, and it is easy to obtain a multilayer coating film with higher graininess and smoother surface.

[0208] The method for manufacturing a multilayer coating film of the present disclosure may further include a transparent coating film forming step between the second base coating film forming step and the heat curing step: that is, a transparent coating is applied on the second base coating film to form a transparent coating film. That is, instead of being heat cured, the uncured second base coating film is wet-on-wet coated with a transparent coating to form an uncured transparent coating film. In the case of including the transparent coating film forming step, in the heat curing step, the uncured first base coating film, the second base coating film, and the transparent coating film are simultaneously heat cured to form a multilayer coating film. By forming a transparent coating film, the unevenness and the like caused by the base coating film are smoothed and protected, and beauty can be further imparted.

[0209] There is no particular limitation on the transparent coating used for forming the transparent coating film, and a transparent coating containing a film-forming resin, a curing agent, etc. can be used. A coloring component may also be further contained to the extent that it does not interfere with the underlying design. In addition, a matting agent may be contained to obtain a coating film with a desired gloss (gloss value, for example, from full gloss to full matte). As the form of the transparent coating, solvent type, water type, and powder type can be cited. In the method for manufacturing a multilayer coating film of the present disclosure, the orientation of the brightening pigment (C) is good, and even when a matte (including a matting agent) transparent coating is used, a multilayer coating film with a high-grainy appearance can be obtained.

[0210] As a preferable example of the solvent type transparent coating, from the viewpoints of transparency or acid etching resistance, etc., a combination of an acrylic resin and / or a polyester resin with an amino resin and / or an isocyanate, or an acrylic resin and / or a polyester resin having a carboxylic acid / epoxy curing system, etc. can be cited.

[0211] As an example of the water type transparent coating, a water type transparent coating containing a resin obtained by neutralizing the film-forming resin contained in the substances cited as examples of the above solvent type transparent coating with an alkali and making it water-based can be cited. This neutralization can be carried out by adding a tertiary amine such as dimethylethanolamine and triethylamine before or after polymerization.

[0212] As the powdery transparent coating, ordinary powdery coatings such as thermoplastic and thermosetting powdery coatings can be used. In order to obtain a coating film with good physical properties, thermosetting powdery coatings are preferred. Specific examples of the thermosetting powdery coatings include epoxy-based, acrylic-based, and polyester-based powdery transparent coatings, etc. Acrylic-based powdery transparent coatings with good weather resistance are particularly preferred.

[0213] In the transparent coating, in order to ensure the coating workability, it is preferred to add a viscosity control agent. As the viscosity control agent, substances showing thixotropy can usually be used. As such substances, for example, conventionally known viscosity control agents can be used. The transparent coating may contain a curing catalyst, a surface conditioner, etc. as required.

[0214] As a method of coating the transparent coating on the second base coating film, coating methods using rotary atomizing electrostatic coating machines called Micro MicroBell and Micro Bell can be cited.

[0215] The above-mentioned transparent coating is preferably applied in such a manner that the dry film thickness is, for example, 10 μm or more and 80 μm or less, preferably 20 μm or more and 60 μm. If the above dry film thickness is 10 μm or more, the unevenness of the underlying layer can be sufficiently concealed, and if it is 80 μm or less, defects such as foaming or sagging during coating can be suppressed.

[0216] In the case of including the transparent coating film forming step, from the viewpoints of curability and physical properties of the obtained multilayer coating film, the heating curing temperature in the heating curing step is preferably 80 to 180 °C, more preferably 120 to 160 °C. The heating curing time can be arbitrarily set according to the above temperature, and it is appropriate that the time is 10 to 30 minutes at the heating curing temperature of 80 °C to 160 °C.

[0217] In the present disclosure, an uncured first base coating film, an uncured second base coating film, and an uncured transparent coating film formed as required are formed, and at least the first base coating film and the second base coating film are heated and cured simultaneously. However, the aqueous first base coating composition and the aqueous second base coating composition used in the manufacturing method of the present disclosure can be used not limited to this manufacturing method, and can be used, for example, in the following Scenarios 1 to 2.

[0218] (Scenario 1)

[0219] After coating the aqueous first base coating composition on the object to be coated to form the first base coating film, the above first base coating film is heated and cured to form the first base coating film. Then, the aqueous second base coating composition is coated on the first base coating film to form the second base coating film, and thereafter, the above first base coating film is heated and cured to form the second base coating film. In this scenario, the first base coating film and / or the second base coating film can also be dried at room temperature.

[0220] (Scheme 2)

[0221] Apply an aqueous first base coating composition to the object to be coated to form a first base coating film, and then heat-cure the above first base coating film to form a first base coating film. Then, apply an aqueous second base coating composition to the first base coating film to form a second base coating film, and then heat-cure the above first base coating film to form a second base coating film. Then, apply a transparent coating composition to the second base coating film to form a transparent coating film, and then heat-dry the above transparent coating film to form a transparent coating film. In this scheme, the first base coating film and / or the second base coating film can also be air-dried at room temperature.

[0222] Examples

[0223] The present invention will be described more specifically by the following examples, but the present invention is not limited to these.

[0224] <Production Example 1: Preparation of Acrylic Resin Aqueous Dispersion (A11-1)>

[0225] In a stainless steel container, 20.0 parts by mass of styrene as the first monomer, 30.0 parts by mass of methyl methacrylate, 10.7 parts by mass of ethyl acrylate, 10.0 parts by mass of ethyl methacrylate, 10.0 parts by mass of isobutyl methacrylate, 10.0 parts by mass of n-butyl methacrylate, and 9.3 parts by mass of 2-hydroxyethyl methacrylate were stirred and mixed, and then 25.0 parts by mass of Aqualon HS-10, 5.0 parts by mass of Adeka Reasoap NE-20, and 80.0 parts by mass of deionized water as emulsifiers were added. The mixture was stirred with a homogenizer at room temperature for 15 minutes to obtain 190.0 parts by mass of the first monomer emulsion mixture.

[0226] In another stainless steel container, 12.7 parts by mass of methyl methacrylate, 40.0 parts by mass of ethyl acrylate, 30.0 parts by mass of isobutyl methacrylate, 5.0 parts by mass of 2-ethylhexyl acrylate, 2.1 parts by mass of 2-hydroxyethyl methacrylate, and 10.3 parts by mass of acrylic acid as the second monomer were stirred and mixed, and then 2.0 parts by mass of Aqualon HS-10 as an emulsifier and 50.0 parts by mass of deionized water were added. The mixture was stirred with a homogenizer at room temperature for 15 minutes to obtain 152.0 parts by mass of the second monomer emulsion mixture.

[0227] In a reaction vessel equipped with a stirrer, a reflux condenser, a dropping funnel, and a thermometer, 126.6 parts by mass of deionized water was charged. While mixing and stirring in a nitrogen stream, the temperature in the reaction vessel was raised to 80°C. Then, over 1.5 hours, 950.0 parts by mass of the above-mentioned first monomer emulsion mixture and an initiator solution composed of 1.20 parts by mass of ammonium persulfate as a reaction initiator and 500.0 parts by mass of deionized water were simultaneously dropped from their respective dropping funnels. After the dropping was completed, the temperature in the reaction vessel was maintained at 80°C for 1 hour.

[0228] Further, over 1.5 hours, 152.0 parts by mass of the above-mentioned second monomer emulsion mixture and an initiator solution composed of 0.6 parts by mass of ammonium persulfate as a reaction initiator and 20.0 parts by mass of deionized water were simultaneously dropped from their respective dropping funnels. After the dropping was completed, the temperature in the reaction vessel was maintained at 80°C for 2 hours.

[0229] After that, it was cooled to 40°C, filtered through a 400-mesh filter, and then 20.0 parts by mass of deionized water and 0.3 parts by mass of dimethylaminoethanol were added to adjust the pH to 6.5, obtaining an aqueous dispersion of an acrylic resin (A11-1) (core-shell type, average particle size: 100 nm, solid content acid value: 40 mgKOH / g, solid content hydroxyl value: 25 mgKOH / g, solid content concentration: 30% by mass).

[0230] <Production Example 2: Preparation of Aqueous Dispersion of Acrylic Resin (A11-3)>

[0231] In a stainless-steel container, 24.0 parts by mass of styrene, 40.0 parts by mass of methyl methacrylate, 16.0 parts by mass of n-butyl methacrylate, 70.0 parts by mass of 2-ethylhexyl acrylate, 30.9 parts by mass of 2-ethylhexyl methacrylate, 16.6 parts by mass of 2-hydroxyethyl acrylate, and 2.6 parts by mass of acrylic acid as monomers were stirred and mixed. Then, 2.2 parts by mass of Adeka Reasoap SR-10 (ammonium polyoxyethylene-1-alkoxymethyl-2-(2-propenyloxy)ethyl ether sulfate, manufactured by ADEKA Corporation) as an emulsifier and 160.0 parts by mass of deionized water were added, and the mixture was stirred with a homogenizer at room temperature for 15 minutes to obtain 362.2 parts by mass of a monomer emulsion mixture.

[0232] In a reaction vessel equipped with a stirrer, a reflux condenser, a dropping funnel, and a thermometer, 253.0 parts by mass of deionized water was charged. While mixing and stirring in a nitrogen stream, the temperature in the reaction vessel was raised to 80°C. Then, 362.2 parts by mass of the above monomer emulsion mixture and an initiator solution composed of 0.6 part by mass of ammonium persulfate as a reaction initiator and 20.0 parts by mass of deionized water were simultaneously dropped from their respective dropping funnels over 2 hours. After the dropping was completed, the temperature in the reaction vessel was maintained at 80°C for 2 hours, then cooled to 40°C, filtered through a 400-mesh filter, and 40.0 parts by mass of deionized water and 0.6 part by mass of dimethylaminoethanol were added to adjust the pH to 6.5, obtaining an aqueous dispersion of an acrylic resin (A11-3) (single layer, average particle diameter: 150 nm, acid value of solid content: 10 mgKOH / g, hydroxyl value of solid content: 40 mgKOH / g, solid content concentration: 30% by mass).

[0233] <Production Example 3: Preparation of Aqueous Dispersion of Acrylic Resin (A11-2)>

[0234] Except for changing the blending amounts of the first monomer and the second monomer to the amounts shown in Table 1, the same operations as in Production Example 1 were carried out to obtain an aqueous dispersion of an acrylic resin (A11-2) (core-shell type, average particle diameter: 150 nm, acid value of solid content: 10 mgKOH / g, hydroxyl value of solid content: 40 mgKOH / g, solid content concentration: 30% by mass).

[0235] <Production Example 4: Preparation of Aqueous Dispersion of Acrylic Resin (A11-4)>

[0236] Except for changing the monomer blending amount to the amount shown in Table 1, the same operations as in Production Example 2 were carried out to obtain an aqueous dispersion of an acrylic resin (A11-4) (single-layer type, average particle diameter: 200 nm, acid value of solid content: 40 mgKOH / g, hydroxyl value of solid content: 0 mgKOH / g, solid content concentration: 30% by mass).

[0237] [Table 1]

[0238]

[0239]

[0240] <Production Example: Preparation of Phosphoric Acid Group-Containing Organic Compound>

[0241] In a reaction vessel equipped with a stirrer, a temperature regulator, and a condenser, 40 parts by mass of ethoxypropanol was charged. At 120 °C, a monomer solution composed of 121.7 parts by mass of a solution prepared by dissolving 4 parts by mass of styrene, 35.96 parts by mass of n-butyl acrylate, 18.45 parts by mass of 2-ethylhexyl methacrylate, 13.92 parts by mass of 2-hydroxyethyl methacrylate, 7.67 parts by mass of methacrylic acid, 20 parts by mass of Phosmer PP (acid phosphonyloxy hexa(oxypropylene) monomethacrylate manufactured by Unichemical) in 20 parts by mass of ethoxypropanol and 1.7 parts by mass of azobisisobutyronitrile was added dropwise thereto over 3 hours. Thereafter, stirring was continued for 1 hour. The obtained phosphoric acid group-containing organic compound had an acid value of 105 mgKOH / g, a phosphoric acid group value of 55 mgKOH / g, a hydroxyl value of 60 mgKOH / g, a number average molecular weight of 6,000, and a non-volatile content of 63%.

[0242] In addition, in the present disclosure, the acid value and the phosphoric acid group value of the phosphoric acid group-containing organic compound were calculated according to the definition of the acid value in JIS K5601 2-1 (the number of mg of potassium hydroxide (KOH) required to neutralize the free acid in 1 g of the sample (non-volatile matter)). In addition, the hydroxyl value was calculated according to the definition of the hydroxyl value in JIS K 0070 (the number of mg of potassium hydroxide required to neutralize acetic acid bonded to a hydroxyl group when 1 g of the sample was acetylated).

[0243] <Preparation of aqueous base coating composition>

[0244] <Production Example: Preparation of colored pigment dispersion paste>

[0245] Using a disperser, 16 parts by mass of Cyanine Blue G314 as a colored pigment, 39 parts by mass of Dispex Ultra PA 4550 (manufactured by BASF) as a pigment dispersant, 44 parts by mass of ion-exchanged water, and 0.9 part by mass of BYK-011 as an antifoaming agent were premixed, and then subjected to a dispersion treatment at 3,000 rpm for 5 hours using an SG mill (dispersion medium: zirconium silicate beads). The zirconium silicate beads were filtered off to obtain a colored pigment paste (pigment weight concentration (PWC): 44 mass%, solid content concentration: 36 mass%).

[0246] <Preparation of aqueous first base coating composition>

[0247] 35 parts by mass of the acrylic resin aqueous dispersion (A11-1) of Production Example 1, 5 parts by mass of a polyol resin (A12), 15 parts by mass of a polyester resin (A13), 2 parts by mass of dimethylaminoethanol, 40 parts by mass of a curing agent (B11-1), 5 parts by mass of a curing agent (B11-2), 20 parts by mass of the above-described colored pigment dispersion paste, 3 parts by mass of a brightening pigment (C1-1), 5 parts by mass of the phosphoric acid group-containing organic compound of Production Example 2, 0.4 parts by mass of lauryl acid phosphate, 50 parts by mass of butyl cellosolve, 5.5 parts by mass (3 parts by mass in terms of solid content) of Noigen EA-207D (an amphiphilic compound, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., number average molecular weight 4,200, solid content 55%), 3 parts by mass of linoleic acid (manufactured by KISHIDA CHEMICAL Co., Ltd.) were uniformly dispersed, dimethylaminoethanol was added to adjust the pH to 8.1, and the mixture was diluted with deionized water to prepare an aqueous first base coating composition (solid content concentration: 30.0 mass%).

[0248] <Preparation of aqueous second base coating composition>

[0249] 30 parts by mass of the acrylic resin emulsion (A11-1) of Production Example 1, 10 parts by mass of a polyol resin (A12), 15 parts by mass of a polyester resin (A13), 2 parts by mass of dimethylaminoethanol, 45 parts by mass of a curing agent (B11-1), 5 parts by mass of the colored pigment dispersion paste prepared in the preparation of the above-described aqueous first base coating composition, 30 parts by mass of a brightening pigment (C1-1), 5 parts by mass of the phosphoric acid group-containing organic compound of Production Example 2, 0.4 parts by mass of lauryl acid phosphate, 50 parts by mass of butyl cellosolve, 5.5 parts by mass (3 parts by mass in terms of solid content) of Noigen EA-207D (an amphiphilic compound, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., number average molecular weight 4,200, solid content 55%), 3 parts by mass of linoleic acid (manufactured by KISHIDA CHEMICAL Co., Ltd.) were uniformly dispersed, dimethylaminoethanol was added to adjust the pH to 8.1, and the mixture was diluted with deionized water to prepare an aqueous second base coating composition (solid content concentration: 10 mass%).

[0250] <Method for forming multilayer coating film>

[0251] On a phosphated matte steel sheet with a thickness of 0.8 mm, a length of 30 cm, and a width of 40 cm, an cationic electrodeposition coating composition "Power Top U-50)" (manufactured by Nippon Paint Automotive Coatings) was electrodeposition-coated in such a manner that the dry film thickness was 20 μm, sintered at 160 °C for 30 minutes. For the obtained coated sheet, an air spray gun W-101-132G manufactured by ANEST IWATA was used to air-spray an intermediate coating composition "OP-200 dark gray" (manufactured by Nippon Paint Automotive Coatings, polyester / melamine-based coating, pre-diluted to 25 seconds (measured at 20 °C using a No. 4 Ford cup)) in such a manner that the dry film thickness was 35 μm, and then sintered at 140 °C for 30 minutes to cure it, forming a cured intermediate coating film.

[0252] Then, under the conditions of a room temperature of 23 °C and a humidity of 68%, an aqueous first base coating composition was air-sprayed in such a manner that the dry film thickness was 12 μm. After standing for 2.5 minutes, under the conditions of a room temperature of 23 °C and a humidity of 68%, an aqueous second base coating composition was air-sprayed wet-on-wet in such a manner that the dry film thickness was 4 μm. After standing for 1.5 minutes, preheating was carried out at 80 °C for 3 minutes.

[0253] After preheating after coating the aqueous second base coating composition, the coated sheet was allowed to cool to room temperature, and Macflow-O-2100 (a solvent-based clear coating manufactured by Nippon Paint Automotive Coatings) as a clear coating was air-sprayed in such a manner that the dry film thickness was 35 μm, and left standing for 7 minutes. Then, the coated sheet was sintered at 140 °C for 30 minutes using a dryer, thereby obtaining a coated test panel having a multilayer coating film.

[0254] (Examples 2 to 34, Comparative Examples 1 to 7)

[0255] Except for changing the types and amounts of the respective components as described in the following table, the operation was carried out in the same manner as in Example 1 to prepare an aqueous base coating composition. The details of the raw materials used are described below. It should be noted that the amounts of the components in the following table all represent the amounts of solid components.

[0256] Film-forming resin (A)

[0257] Film-forming resin (A12): Primepol PX-1000 (manufactured by Sanyo Chemical Industries, Ltd., polyol resin), solid component concentration: 100% by mass;

[0258] Coating-forming resin (A13): Vylonal NC-6210 (manufactured by Toyobo Co., Ltd., polyester resin, solid content concentration: 33% by mass);

[0259] Curing agent (B):

[0260] Curing agent (B11-1) Cymel 327 (fully alkylated melamine resin, manufactured by Allnex Japan), solid content concentration: 90% by mass;

[0261] Curing agent (B11-2): Cymel 250 (imino-type methyl / butyl mixed etherified melamine resin, manufactured by Allnex Japan), solid content concentration: 70% by mass;

[0262] Curing agent (B12) WM44-L70G (active methylene-blocked polyisocyanate compound, manufactured by Asahi Kasei Corporation), solid content concentration: 70.7% by mass;

[0263] Brightening pigment (C):

[0264] Brightening pigment (C1-1): TCR-2020 (aluminum pigment, manufactured by Toyo Aluminum Co., Ltd.), average particle size: 22 μm, average thickness: 2 μm, water surface spreading area: 0.2 m 2 / g, active ingredient concentration: 78% by mass;

[0265] Brightening pigment (C1-2): 93-0647 (aluminum pigment, manufactured by Toyo Aluminum Co., Ltd.), average particle size: 20 μm, average thickness: 0.36 μm, water surface spreading area: 1.1 m 2 / g, active ingredient concentration: 71% by mass;

[0266] Brightening pigment (C1-3): 06-0672 (aluminum pigment, manufactured by Toyo Aluminum Co., Ltd.), average particle size: 17 μm, average thickness: 0.5 μm, water surface spreading area: 0.8 m 2 / g, active ingredient concentration: 71% by mass;

[0267] Brightening pigment (c2-1): MH-8801 (aluminum pigment, manufactured by Asahi Kasei Corporation), average particle size: 15 μm, average thickness 0.2 μm, water surface spreading area: 2.4 m 2 / g, active ingredient concentration: 64% by mass;

[0268] Other materials

[0269] (D1-1) Laponite EP (manufactured by BYK, synthetic lithium montmorillonite analogue);

[0270] (D1-2) Optigel WX (manufactured by BYK, Na-substituted bentonite);

[0271] (D1-3) Dymolite (manufactured by Topy Industries, synthetic swelling mica).

[0272] In Example 5, except that R2550-3 (SEMIGLOSS) clear (a solvent-based clear coating manufactured by Nippon Paint Automotive Coatings) was used as the clear coating, the same operations as in Example 1 were carried out to obtain a coated test panel with a multilayer coating film.

[0273] <Evaluation Items>

[0274] 1) Evaluation of Coating Film Appearance (Graininess)

[0275] For the multilayer coating films obtained in the above Examples and Comparative Examples, the granularity value (G value) at an incident angle of 45° was measured using a spectrophotometer BYK-mac i (manufactured by BYK Gardner) to evaluate the graininess of the multilayer coating films. The granularity value (G value) is represented by a range of 0 to 30. The smaller the numerical value, the finer it indicates, and the larger the numerical value, the more granular it indicates.

[0276] 2) Evaluation of Coating Film Appearance (Unevenness)

[0277] For the appearance (unevenness) of the multilayer coating films obtained in the above Examples and Comparative Examples, the Me value (spot size: 11 - 24 mm) was measured using Cloud Runner (manufactured by BYK-Gardner) to evaluate the unevenness of the coating films.

[0278] It should be noted that the larger the numerical value of the Me value measured by the above device, the more likely the unevenness (spot pattern) of the coating film is to be prominent. An Me value less than 7 is considered qualified.

[0279] 3) Evaluation of Coating Film Appearance (Clarity)

[0280] For the appearance of the multilayer coating films of the evaluation use panels obtained in the Examples and Comparative Examples, the SW value (measurement wavelength: 300 - 1,200 nm) was measured using micro wavescan-T (manufactured by BYK Gardner) to evaluate the smoothness and clarity of the coating film surface.

[0281] It should be noted that the smaller the numerical value of the SW value measured by the above device, the higher the smoothness and clarity of the coating film surface. An SW value less than 15 is considered qualified.

[0282]

[0283]

[0284]

[0285]

[0286] Examples 1 to 34 are examples of the present disclosure, and a smooth multi-layer coating film with high granularity and suppressed uneven metallic flash color is obtained.

[0287] It should be noted that in Example 7, a transparent coating material capable of obtaining a coating film with low gloss (matte style) is used as the transparent coating material. Therefore, although the clarity of the multi-layer coating film of Example 7 cannot be evaluated, the granularity value (G value) increases. From this, it can also be known that: according to the manufacturing method of the present disclosure, a multi-layer coating film with high granularity can be formed.

[0288] Comparative Example 1 is an example in which the specific numerical value of the bright pigment (C2) does not satisfy the requirements of the present invention, and the obtained multi-layer coating film has a small G value and poor granularity.

[0289] Comparative Example 2 is an example in which the solid content concentration of the aqueous first base coating composition is less than 15% by mass, and significant sagging occurs during the coating of the aqueous first base coating composition, and a multi-layer coating film cannot be formed.

[0290] Comparative Example 3 is an example in which the solid content concentration of the aqueous second base coating composition exceeds 20% by mass, and the SW value and Me value of the obtained multi-layer coating film are large, and the smoothness of the multi-layer coating film is poor. Therefore, the clarity is poor, and the uneven metallic flash color is large, forming a mottled appearance.

[0291] Comparative Example 4 is an example in which the solid content concentration of the aqueous second base coating composition is less than 5% by mass, and significant sagging occurs during the coating of the aqueous second base coating composition, and a multi-layer coating film cannot be formed.

[0292] Comparative Example 5 is an example in which the amount of solid content contained in the aqueous first base coating composition is less than 1.1 with respect to 1 part by mass of the solid content contained in the aqueous second base coating composition. The obtained multi-layer coating film has a large SW value and Me value, and the smoothness of the multi-layer coating film is poor. Therefore, the clarity is poor and the uneven metallic flash color is large, forming a mottled appearance.

[0293] Comparative Example 6 is an example in which the pigment mass concentration of the bright pigment (C2) in the aqueous second base coating composition is less than 2% by mass, and the obtained multi-layer coating film has a small G value and poor granularity.

[0294] Comparative Example 7 is an example in which the pigment mass concentration of the bright pigment (C2) in the aqueous second base coating composition exceeds 40% by mass, and the obtained multi-layer coating film has a large SW value and poor smoothness. Therefore, the clarity is poor.

Claims

1. A method for manufacturing a multilayer coating film, comprising the following steps: A first base coating film forming step of coating a water-based first base coating composition on an object to be coated to form a first base coating film; A second base coating film forming step of coating a water-based second base coating composition on the first base coating film to form a second base coating film; and A heat curing step of simultaneously heat curing at least the first base coating film and the second base coating film to form a multilayer coating film including a first base coating film and a second base coating film; Among them, The water-based first base coating composition contains a film-forming resin (A1) and a curing agent (B1), The water-based second base coating composition contains a film-forming resin (A2), a curing agent (B2), and a brightening pigment (C2), The solid content concentration of the water-based first base coating composition is 15% by mass or more and 40% by mass or less, The solid content concentration of the water-based second base coating composition is 5% by mass or more and 20% by mass or less, The ratio of the solid content concentration in the water-based first base coating composition to the solid content concentration in the water-based second base coating composition is 1.1 or more and 8 or less on a mass basis, The average particle diameter (D50) of the brightening pigment (C2) is 17 μm or more and 30 μm or less, The average thickness of the brightening pigment (C2) is 0.3 μm or more and 6.0 μm or less, The water surface spreading area (WCA) of the above-mentioned bright pigment (C2) is 1.4 m 2 / g or less. The pigment mass concentration of the brightening pigment (C2) in the water-based second base coating composition is 2% by mass or more and 40% by mass or less.

2. The method for manufacturing a multilayer coating film according to claim 1, wherein, The coating of the water-based first base coating composition and the coating of the water-based second base coating composition are carried out in such a manner that the ratio t1 / t2 of the dry film thickness t1 of the first base coating film to the dry film thickness t2 of the second base coating film is 1.6 or more and 7 or less.

3. The manufacturing method of the multilayer coating film according to claim 1, wherein, The granularity value, i.e., the G value, of the multilayer coating film is 9 or more and 18 or less.

4. The manufacturing method of the multilayer coating film according to claim 1, wherein, The water-based second base coating composition further contains a viscosity modifier (D), The viscosity modifier (D) contains an inorganic layered compound (D-1).

5. The manufacturing method of the multilayer coating film according to claim 1, wherein, The film-forming resin (A2) contains an acrylic resin aqueous dispersion (A11).

6. The method for manufacturing a multilayer coating film according to claim 1, wherein, The curing agent (B2) contains an amino resin (B11).

7. The manufacturing method of the multilayer coating film according to any one of claims 1 to 6, further comprising: A transparent coating film forming step of coating a transparent coating on the second base coating film to form a transparent coating film, In the heat curing step, the first base coating film, the second base coating film, and the transparent coating film are simultaneously heat cured to form a multilayer coating film.

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