Laminated coating film formation method and laminated coating film structure

Abstract

To suppress appearance deterioration such as sagging and underarms, while maintaining color development, concealment, and weather resistance, in any color tone, in two tone color with black color.SOLUTION: A method for forming a laminated coating film on a coated object W, comprises: forming an electrodeposition coating film 10 on the coated object; forming an intermediate coating film 11 on the electrodeposition coating film; forming a first color-developing coating film 12 on the intermediate coating film; forming a second color, black high-concentration color-developing coating film 13 on a part of the first color-developing coating film; and forming a clear coating film 15 on the first color-developing coating film and the black high-concentration color-developing coating film, wherein in the step of forming the black high-concentration color-developing coating film, the concentration of dispersed black pigment contained in the black high-concentration color-developing coating film is 7% or more and 25% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for forming a multilayer coating film and a multilayer coating film structure. [Background technology]

[0002] As shown in the schematic cross-sectional view of Figure 4, automotive paint films are generally formed by sequentially layering an electrodeposited film 51, which is mainly formed by electrodeposition coating for rust prevention, an intermediate coating film 52, and a top coating film 60 on an object to be coated 50. The topcoat 60 is composed of a color-developing coating 53 containing a pigment for color development and a clear coating 54 on top of that. The color-developing coating 53 has the purpose of improving adhesion between the intermediate coating 52 and the clear coating 54, as well as improving smoothness.

[0003] The intermediate coating film 52 has the purpose of improving adhesion between the electrodeposited film 51 and the topcoat coating film 60, improving smoothness, and also enhancing light degradation resistance, chipping resistance, and color development. In particular, when the electrodeposited film 51 formed from an epoxy-based cationic electrodeposition paint is exposed to a large amount of ultraviolet light, its surface layer deteriorates and the coating film above it peels off. Therefore, the intermediate coating film 52 protects the electrodeposited film 51 from ultraviolet light and improves light degradation resistance.

[0004] When different colors are used in each predetermined area, such as in a two-tone color, as shown in FIG. 5, a first color paint is applied to a portion of an area on an intermediate coating film 52 applied to an electrodeposited film 51 to form a color coating film 53a, and then, after masking is appropriately applied on top of that, a second color paint is applied to an area adjacent to the color coating film 53a to form a color coating film 53b (see Patent Document 1).

[0005] In recent years, deterioration by acid rain has become a problem for topcoat paint films on automobile bodies exposed to wind and rain. In topcoat paint films made with melamine resin-curing paints, the melamine resin and hydroxyl-containing acrylic resin are crosslinked to form ether bonds. Therefore, if the topcoat paint film is exposed to acid rain for a long period of time, the ether bonds can hydrolyze, causing stains, discoloration, etc., and may even lead to the destruction of the paint film.

[0006] In response to these issues, in Patent Document 2, the applicant of the present application proposes a coating method in which a colored top coat layer is formed using a paint in which a pigment is dispersed in a multifunctional clear resin consisting of a resin having a carboxy group and a resin having an epoxy group using a dispersant having a carboxy group and an epoxy group.

[0007] Specifically, as shown in Figure 4, an electrodeposited film 51 is formed on an object 50, and an intermediate coating film 52 is formed on the electrodeposited film 51. A color-developing coating film 53 is then formed, and a clear coating film 54 is then formed on top of that. This clear coating film 54 is a multifunctional clear resin consisting of a resin with a carboxy group and a resin with an epoxy group. In the case of two-tone colors, as shown in Figure 6, a first color-developing coating film 53a is formed on an intermediate coating film 52. This color-developing coating film 53a is formed from a paint (colored top coat layer) in which a pigment is dispersed in a multifunctional clear resin consisting of a resin with a carboxy group and a resin with an epoxy group using a dispersant with a carboxy group and an epoxy group.

[0008] When such a coating material is heated and dried, the carboxyl and epoxy groups in the polyfunctional clear resin crosslink with the carboxyl and epoxy groups in the dispersant to form acid-epoxy bonds, allowing the formation of an outer coating film in which the pigment is uniformly dispersed. Furthermore, the acid-epoxy crosslinking bond between the carboxyl group and the epoxy group is less susceptible to hydrolysis by acid, and a coating film having excellent resistance to acid rain and the like can be formed. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent Publication No. 2021-79338 [Patent Document 2] Patent No. 7241001 Summary of the Invention [Problem to be solved by the invention]

[0010] As disclosed in Patent Document 2, a paint in which a pigment is dispersed in a multifunctional clear resin consisting of a resin with a carboxyl group and a resin with an epoxy group keeps the amount of pigment in the clear resin low to maintain weather resistance, and a thick single layer is required to achieve color development and hiding power. As shown in Figure 6, when a second color coating film 53b, which is not a paint with pigment dispersed in a multifunctional clear resin consisting of a resin with a carboxyl group and a resin with an epoxy group and has poor weather resistance, is applied on top of a first color coating film 53a, a clear coating film 54 must be applied on top of the color coating film 53b to ensure weather resistance. When the application of the color coating film 53b on top of the color coating film 53a and the application of the clear coating film 54 on top of the color coating film 53b are done in a wet-on state, the weight of the triple structure of the color coating film 53a, the color coating film 53b, and the clear coating film 54 on top of it is heavy, which causes problems such as sagging and popping, which deteriorate the appearance.

[0011] The present invention has been made in view of the above circumstances, and aims to provide a method for forming a multilayer coating film and a multilayer coating film structure that can suppress deterioration of appearance such as sagging and popping, while maintaining color development, hiding power, and weather resistance, regardless of the color tone, in two-tone colors including black. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems, the method for forming a multilayer coating film according to the present invention is a method for forming a multilayer coating film on an object to be coated, and comprises the steps of forming an electrodeposition film on the object to be coated, forming an intermediate coating film on the electrodeposition film, forming a first color color-developing coating film on the intermediate coating film, forming a second color high-concentration black color-developing coating film on a portion of the first color color-developing coating film, and forming a clear coating film on the first color color-developing coating film and the high-concentration black color-developing coating film, wherein the method is characterized in that in the step of forming the high-concentration black color-developing coating film, the concentration of dispersed black pigment contained in the high-concentration black color-developing coating film is 7% or more and 25% or less. In the process of forming a clear coating film on the first color coloring coating film and the high-density black coloring coating film, it is desirable that the resins forming the high-density black coloring coating film and the clear coating film are polyfunctional resins consisting of a resin having one or more carboxy groups in one molecule and a resin having one or more epoxy groups in one molecule, or the same resin containing a polyfunctional resin having one or more carboxy groups and one or more epoxy groups in one molecule. In the step of forming a second color high density black color developing film on a portion of the first color developing film, it is desirable to form the thickness of the second color high density black color developing film to be 10 μm or less.

[0013] According to this method, even if the clear coating film formed on the black high-concentration color-developing coating film has the same coating thickness as before (e.g., 30 μm), the thickness of the black high-concentration color-developing coating film can be made thinner, and the gravity due to the weight of the black high-concentration color-developing coating film and the clear coating film can be reduced, thereby suppressing the occurrence of sagging and popping. Furthermore, by forming acid-epoxy crosslinks in the clear coating film after baking, hydrolysis due to rainfall and the like does not occur, and infiltration into the high-density black color-developing coating film can be prevented. Furthermore, light transmitted through the clear coating film can be reflected by a sufficient amount of black pigment in the black high-concentration color-developing coating film, resulting in color development. The remaining light is absorbed by a sufficient amount of black pigment, preventing it from penetrating into the base layer. Furthermore, by using the same paint resin for both the black high-concentration color-developing coating film and the clear coating film, strong adhesion can be ensured by forming an acid-epoxy crosslink between the black high-concentration color-developing coating film and the clear coating film during baking. Furthermore, by forming the black high-concentration color-developing coating film and the clear coating film from the same resin, excellent durability, including weather resistance, can be ensured. That is, according to the present invention, it is possible to suppress deterioration of appearance such as sagging and popping while maintaining color development, hiding power, and weather resistance.

[0014] In addition, in order to solve the above-mentioned problems, the multilayer coating film structure of the present invention is a multilayer coating film structure formed on a substrate, comprising an electrodeposition film formed on the substrate, an intermediate coating film formed on the electrodeposition film, a first color color-developing coating film formed on the intermediate coating film, a second color high-concentration black color-developing coating film formed on a portion of the first color color-developing coating film, and a clear coating film formed on the first color color-developing coating film and the black high-concentration color-developing coating film, and is characterized in that the concentration of dispersed black pigment contained in the black high-concentration color-developing coating film is 7% or more and 25% or less. Furthermore, the resin forming the black high-concentration color-developing coating film and the clear coating film preferably contains a polyfunctional resin consisting of a resin having one or more carboxy groups in one molecule and a resin having one or more epoxy groups in one molecule, or a polyfunctional resin having one or more carboxy groups and one or more epoxy groups in one molecule, a black pigment, and a dispersant having one or more carboxy groups or one or more epoxy groups in one molecule. It is also desirable that the thickness of the black high density coloring coating film be 10 μm or less.

[0015] With this configuration, even if the clear coating film formed on top of the black high-concentration color-developing coating film has the same coating thickness as before (e.g., 30 μm), the thickness of the black high-concentration color-developing coating film can be made thinner, and the gravity due to the weight of the black high-concentration color-developing coating film and the clear coating film can be reduced, thereby suppressing the occurrence of sagging and popping. Furthermore, by forming acid-epoxy crosslinks in the clear coating film after baking, hydrolysis due to rainfall and the like does not occur, and infiltration into the high-density black color-developing coating film can be prevented. Furthermore, light transmitted through the clear coating is reflected by a sufficient amount of black pigment in the high-density black color-developing coating, causing it to develop color, while the remaining light is absorbed by a sufficient amount of black pigment, preventing it from penetrating into the underlying layer. Furthermore, by using the same paint resin for the black high-concentration coloring coating film and the clear coating film, strong adhesion is ensured by forming an acid-epoxy crosslinking bond between the black high-concentration coloring coating film and the clear coating film during baking, and by forming the black high-concentration coloring coating film and the clear coating film with the same resin, excellent durability including weather resistance can be ensured.Furthermore, the black high-concentration coloring coating film and the clear coating film can be overcoated without drying. That is, according to the present invention, it is possible to suppress deterioration of appearance such as sagging and popping while maintaining color development, hiding power, and weather resistance. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a multilayer coating film forming method and a multilayer coating film structure that can suppress deterioration of appearance such as sagging and popping while maintaining color development, hiding power, weather resistance, and excellent durability regardless of the color tone in two-tone colors including black. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view schematically showing a multilayer coating film structure according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view illustrating a method for forming the multilayer coating film structure of FIG. [Figure 3] FIG. 3 is a cross-sectional view illustrating a method for forming the multilayer coating film structure of FIG. [Figure 4] FIG. 4 is a cross-sectional view showing a typical structure of a conventional multilayer coating film. [Figure 5] FIG. 5 is a cross-sectional view showing a typical structure of a conventional two-tone multilayer coating film. [Figure 6] FIG. 6 is a cross-sectional view schematically showing another structure of a conventional two-tone multilayer coating film. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the method for forming a multilayer coating film and the multilayer coating film structure according to the present invention will be described with reference to the drawings. Note that the following description of the preferred embodiments is essentially merely illustrative and is not intended to limit the present invention, its applications, or its uses.

[0019] In the following description, as an example, a case will be described in which the first color-developing coating film is white and the second color-developing coating film is black. FIG. 1 is a cross-sectional view schematically showing the multilayer coating film structure of the present invention.

[0020] An electrodeposited film 10 is formed on the entire surface of the substrate W shown in Figure 1. The substrate W may be an iron plate, steel plate, aluminum, stainless steel, or the like, such as the iron plate that forms the body of an automobile. The electrodeposited film 10 is formed by immersing the substrate W in a cationic electrodeposition paint, and depositing it on the substrate W by passing a direct current between the substrate W as the cathode and the electrode plate in the electrodeposition tank as the anode. The thickness of this electrodeposited film 10 is, for example, 20 μm.

[0021] An intermediate coating film 11 is formed on the electrodeposition film 10. A known water-based paint containing a resin component and a curing agent is used as the paint for the intermediate coating film 11. Various additives, such as viscosity modifiers, flow control agents, and pigment dispersants, can be blended into the intermediate coating film as needed. Examples of the resin component include acrylic resin, polyester resin, alkyd resin, epoxy resin, and polyurethane resin. Specifically, acrylic resin or polyester resin having a carboxyl group is used. Examples of the curing agent include melamine resin and blocked polyisocyanate. Various pigments may also be included depending on the paint color. The intermediate coating film 11 is formed to a thickness of 10 μm or more and 50 μm or less.

[0022] Furthermore, a first color developing coating film 12 (referred to as the first color developing coating film 12) is formed on the intermediate coating film 11. The first color developing coating film 12 is formed by depositing an aqueous base paint to which, for example, a white pigment or other color pigment is added as a colorant. The aqueous base paint is, for example, a polyisocyanate-curing aqueous paint containing an acrylic resin, polyisocyanate, and a color pigment. The first color developing coating film 12 is formed to a thickness of, for example, 10 to 20 μm.

[0023] Furthermore, a high-concentration black clear coating film 13 (high-concentration black color coating film) is formed as a second color coating film on a portion (area determined by the design) of the first color coating film 12. This high-concentration black clear coating film 13 is a clear coating film to which a high-concentration black pigment has been added. Specifically, the ink contains an acid-epoxy cured acrylic resin, and the concentration of the dispersed black pigment contained therein is 7% or more and 25% or less. More specifically, the resin that forms the clear coating film contains a polyfunctional resin consisting of a resin having one or more carboxy groups in one molecule and a resin having one or more epoxy groups in one molecule, or a polyfunctional resin having one or more carboxy groups and one or more epoxy groups in one molecule, a black pigment, and a dispersant having one or more carboxy groups or one or more epoxy groups in one molecule.

[0024] The resin having one or more carboxy groups per molecule is specifically an acrylic copolymer having a carboxy group and a hydroxy group per molecule. The acrylic copolymer is obtained by copolymerizing an acrylic monomer as a main component with a monomer having a carboxy group and a monomer having a hydroxy group.

[0025] Furthermore, when such a coating material is used to paint a substrate, the carboxyl groups and epoxy groups in the polyfunctional resin crosslink with the carboxyl groups or epoxy groups in the dispersant during heating and drying to form acid-epoxy bonds, making it possible to form a coating film in which the pigment is uniformly dispersed.

[0026] The high-concentration black clear coating film 13 is formed to a thickness of, for example, 10 μm or less. By increasing the concentration of the black pigment in this way, even if the high-concentration black clear coating film 13 is thin (for example, 10 μm), the underlying first color-developing coating film 12 does not show through. Furthermore, if the concentration of dispersed black pigment is high, there is a risk of discoloration due to ultraviolet light exposure, but since the clear coating film 15 described below is formed on top of the high-concentration black clear coating film 13, discoloration can be prevented.

[0027] Furthermore, a transparent, pigment-free clear coating film 15 is formed as a full-surface topcoat layer on the first color coating film 12 and the high-concentration black clear coating film 13. The clear coating film 15 is formed from the same resin as the high-concentration black clear coating film 13, which contains an acid-epoxy curing acrylic resin. The clear coating film 15 has a thickness of, for example, 30 μm.

[0028] With this laminated coating structure, even if the clear coating film 15 formed on the high-concentration black clear coating film 13 has a conventional coating thickness (e.g., 30 μm), the thickness of the high-concentration black clear coating film 13 is thin, so the gravity due to the weight of the high-concentration black clear coating film 13 and the clear coating film 15 can be reduced, and the occurrence of sagging and popping can be suppressed. Furthermore, since the clear coating film 15 after baking forms an acid-epoxy crosslinking bond, it does not undergo hydrolysis due to acid rain as occurs with ether bonds, and acid rain does not penetrate into the high-concentration black clear coating film 13 . Furthermore, light transmitted through the baked clear coating film 15 is reflected by a sufficient amount of black pigment in the baked high-density black clear coating film 13, causing the black pigment to develop color. The remaining light is absorbed by a sufficient amount of black pigment, preventing it from penetrating into the underlying layer. Furthermore, by using the same paint resin for the black high-concentration coloring coating film and the clear coating film, strong adhesion is ensured by forming an acid-epoxy crosslinking bond between the black high-concentration coloring coating film and the clear coating film during baking, and by forming the black high-concentration coloring coating film and the clear coating film with the same resin, excellent durability including weather resistance can be ensured.Furthermore, the black high-concentration coloring coating film and the clear coating film can be overcoated without drying. That is, the multilayer coating film structure of the present invention can suppress deterioration of appearance such as sagging and popping while maintaining color development, hiding power, and weather resistance.

[0029] Next, the method for forming a multilayer coating film of the present invention will be described with reference to FIGS. (Formation of electrodeposited film 10) As described above, the substrate W may be, for example, an iron plate, steel plate, aluminum, or stainless steel. When the substrate W is an automobile steel plate, an electrodeposited film 10 may be formed on the steel plate in advance. The electrodeposited film 10 is a paint or resin film formed on the surface of the vehicle body by electrodeposition coating, and serves to provide corrosion protection and adhesion to the intermediate coating film. As described above, this electrodeposited film 10 is formed by immersing the substrate W in cationic electrodeposition paint, using the substrate W as the cathode and the electrode plate in the electrodeposition tank as the anode, and passing a direct current between them, as shown in FIG. 2(a). The thickness of this electrodeposited film 10 is, for example, 20 μm.

[0030] The cationic electrodeposition paint contains a cationic epoxy resin, a curing agent, and pigments and additives. The cationic epoxy resin includes an amine-modified epoxy resin, which may be modified with a resin such as polyester polyol, polyether polyol, or alkylphenol, or may have an extended chain length.

[0031] Compounds that can introduce cationic groups include acid salts, sulfides, and acid mixtures of primary, secondary, and tertiary amines, such as butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine hydrochloride, N,N-dimethylethanolamine acetate, diethyldisulfide-acetic acid mixture, ketimine of aminoethylethanolamine, and diketimine of diethylenetriamine, among others.

[0032] As the curing agent, a blocked polyisocyanate obtained by blocking a polyisocyanate with a blocking agent can be used. The polyisocyanate may be any of aliphatic, alicyclic, aromatic-aliphatic, etc.

[0033] Among polyisocyanates, examples of aromatic isocyanates include tolylene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI). Examples of aliphatic isocyanates include hexamethylene diisocyanate (HDI) and 2,2,4-trimethylhexane diisocyanate. Examples of alicyclic isocyanates include 1,4-cyclohexyanate (hydrogenated MDI), 1,3-diisocyanatomethylcyclohexane (hydrogenated XDI), hydrogenated TDI, and norbornane diisocyanate. Examples of aromatic-aliphatic isocyanates include xylylene diisocyanate (XDI) and tetramethylxylylene diisocyanate (TMXDI). Furthermore, modified products of these isocyanates, such as urethane compounds, biuret compounds, and isocyanurate-modified compounds, are also included. These include, for example, chlorodiisocyanate (CDI), isophorone diisocyanate (IPDI), and 4,4'-dicyclohexylmethane diisocyanate, which can be used alone or in combination of two or more.

[0034] Examples of the blocking agent include lactam-based blocking agents such as ε-caprolactam, and oxime-based blocking agents such as formaldoxime. The amount of curing agent, expressed as the solids weight ratio of cationic epoxy resin to curing agent, is generally in the range of 80 / 20 to 50 / 50, and the amount of cationic epoxy resin and curing agent is generally in the range of 30 to 80% by weight of the total solids of the electrodeposition coating composition.

[0035] Electrodeposition paints generally contain pigments as colorants. Examples of color pigments include titanium oxide, carbon black, and iron oxide. Examples of extender pigments include kaolin, talc, aluminum silicate, calcium carbonate, mica, and clay. Examples of rust-preventive pigments include zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, and calcium molybdate. The amount of pigment can be in the range of 10 to 30% by weight of the total solids content of the electrodeposition paint composition.

[0036] (Formation of intermediate coating film 11) After the electrodeposition film 10 is baked and hardened, as shown in FIG. 2(b), a coating film is applied on the electrodeposition film 10 by, for example, air spray coating, airless spray coating, rotary atomization coating, curtain coating, etc., to a dry film thickness (when the solvent has completely evaporated) of 10 μm to 50 μm. The intermediate coating film 11 is formed so that the intermediate coating film 11 becomes as follows. Electrostatic charge may be applied during coating. If the dry thickness of the intermediate coating film 11 is thinner than 10 μm, the color tone of the color-developing paint will not be fully expressed, which is not preferable. On the other hand, if the dry thickness of the intermediate coating film 11 is thicker than 50 μm, sagging and peeling will occur, which is not preferable.

[0037] The intermediate coating film 11 can be formed by applying a water-based intermediate coating material, such as an isocyanate-curing oil-based paint containing acrylic resin, polyisocyanate, color pigment, extender pigment, talc, a surface conditioner, a dispersant, etc.

[0038] Pigments are added to the intermediate coating as colorants, and examples of such pigments include color pigments, extender pigments, and luster pigments. Examples of color pigments include organic azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments. Inorganic pigments include yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. Furthermore, calcium carbonate, barium sulfate, clay, talc, and the like may be used in combination as extender pigments.

[0039] (Formation of first color coating film 12) After the intermediate coating film 11 has been baked and cured, a first color coating film 12 is applied onto the intermediate coating film 11 as shown in Figure 2(c). This coating is applied by, for example, air spray coating, airless spray coating, rotary atomization coating, curtain coating, etc., so that the dry film thickness (when the solvent has completely evaporated) is 10 μm to 20 μm. Electrostatic force may be applied during coating.

[0040] The first color coating film 12 can be formed by applying a water-based paint, such as an isocyanate-curing oil-based paint containing acrylic resin, polyisocyanate, color pigment, extender pigment, talc, a surface conditioner, a dispersant, etc.

[0041] Pigments are added to the base coating as colorants, and examples of such pigments include color pigments, extender pigments, and luster pigments. Examples of color pigments include organic azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments. Inorganic pigments include yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. Furthermore, calcium carbonate, barium sulfate, clay, talc, and the like may be used in combination as extender pigments.

[0042] After the formation of the first color-developing coating film 12, a preheat treatment is carried out if necessary, and a high-density black clear coating film 13 is formed on a portion of the first color-developing coating film 12 to a thickness of 10 μm or less, as shown in Figure 3(a).

[0043] The high-concentration black clear coating film 13 contains a black paint, which is prepared by blending a film-forming base resin, a black pigment, a crosslinking agent, a solvent, and other additive components. Various black pigments for paints can be used as the black pigment to be incorporated into the black paint, and they may be either organic or inorganic. Examples of such black pigments include carbon black, acetylene black, lamp black, bone black, graphite, black iron oxide (iron tetroxide), black titanium oxide, copper manganese black, copper chromium black, cobalt black, cyanine black, and aniline black. These can be used alone or in combination of two or more. The concentration of the dispersed black pigment is set to a high concentration of 7% to 25%. If the concentration is below 7%, hiding power may be insufficient at a film thickness of 10 μm, and the black color may not be expressed. If the concentration is above 25%, the stability of the paint may be impaired.

[0044] The base resin of the black paint contains an acid-epoxy cured acrylic resin. Specifically, the resin contains a polyfunctional resin consisting of a resin having one or more carboxy groups per molecule and a resin having one or more epoxy groups per molecule, or a polyfunctional resin having one or more carboxy groups and one or more epoxy groups per molecule, a black pigment, and a dispersant having one or more carboxy groups or one or more epoxy groups per molecule.

[0045] The resin having one or more carboxy groups per molecule is specifically an acrylic copolymer having a carboxy group and a hydroxy group per molecule. The acrylic copolymer is obtained by copolymerizing an acrylic monomer as a main component with a monomer having a carboxy group and a monomer having a hydroxy group.

[0046] Examples of acrylic monomers include C1-24 alkyl esters of acrylic acid, such as methyl acrylate, n-butyl acrylate, isobutyl acrylate, lauryl acrylate, and 2-ethylbutyl acrylate, and C1-24 alkyl esters of methacrylic acid, such as methyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, and stearyl methacrylate. Of these, those with a relatively small number of carbon atoms, such as n-butyl acrylate and n-butyl methacrylate, are preferred.

[0047] The monomer having a carboxy group is generally, for example, acrylic acid or methacrylic acid, but ethylenically unsaturated carboxylic acids such as maleic acid, fumaric acid, crotonic acid, and itaconic acid may also be used.

[0048] Examples of the monomer having a hydroxyl group include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, N-methylol acrylamide, N-methylol methacrylamide, N-hydroxyethyl acrylamide, and N-dihydroxyethyl methacrylamide. Among these, 2-hydroxyethyl methacrylate is preferred.

[0049] In addition to the above-mentioned monomers, styrene, vinyltoluene, acrylonitrile, methacrylonitrile, acrylamide, vinyl acetate, etc. may also be used in appropriate combination.

[0050] Resins having one or more carboxy groups per molecule are obtained by copolymerizing the above-mentioned monomer components, and their number-average molecular weight (Mn) is usually 1,000 to 200,000, preferably 2,000 to 100,000. If the number-average molecular weight (Mn) is less than 1,000, it may be difficult to form a sufficiently cured resin coating film. On the other hand, if the number-average molecular weight (Mn) exceeds 200,000, the solubility of the resin decreases, the viscosity of the coating increases, and preparation may become difficult.

[0051] Next, the resin having one or more epoxy groups in one molecule is specifically an acrylic copolymer having an epoxy group and a hydroxyl group in one molecule, and forms an acid-epoxy curing resin together with the resin having one or more carboxyl groups in one molecule.

[0052] The resin having one or more epoxy groups in one molecule may be obtained, for example, by copolymerizing a monomer having an epoxy group with the acrylic monomer.

[0053] Examples of the monomer having an epoxy group include glycidyl methacrylate, glycidyl acrylate, 3,4-epoxycyclohexyl acrylate, 3,4-epoxycyclohexyl methacrylate, and allyl glycidyl ether.

[0054] The number average molecular weight (Mn) of the resin having one or more epoxy groups in one molecule is usually 1,000 to 100,000, and preferably 1,500 to 100,000. The average molecular weight of one of the resins containing one or more carboxyl groups per molecule and the resin containing one or more epoxy groups per molecule may be slightly larger, which increases the compatibility of the acrylic resin and facilitates the crosslinking reaction between the two resins.

[0055] (Formation of clear coating film 15) Next, as shown in Figure 3(b), a clear coating is applied over the first color coating film 12 and the high-density black clear coating film 13 using an airless spray, air spray, rotary atomizer, or other sprayer. Electrostatic charge may be applied during application. After application to a dry film thickness of 25 to 40 μm, the coating is heated at 140°C for 20 minutes to cure.

[0056] The resin forming the clear coating film 15 is made of the same components as the resin forming the high-density black clear coating film 13, and contains an acid-epoxy curing acrylic resin. The clear coating may contain, as needed, non-aqueous dispersion resins, polymer microparticles, curing catalysts, ultraviolet absorbers, light stabilizers, coating surface conditioners, antioxidants, flowability adjusters, waxes, etc. Examples of curing catalysts include organotin compounds, triethylamine, diethanolamine, etc. Examples of ultraviolet absorbers include ultraviolet stabilizers such as benzophenone-based, benzotriazole-based, cyanoacrylate-based, salicylate-based, and anilide oxalate-based compounds, and hindered amine-based compounds.

[0057] (Heat curing of first color coating 12, high density black clear coating 13, clear coating 15) After application of the clear coating film 15, the first color-developing coating film 12, the high-density black clear coating film 13, and the clear coating film 15 are all heated and cured at the same time to form a cured multilayer coating film. The baking temperature and baking time for heat curing can be set as appropriate.

[0058] As described above, according to the embodiment of the present invention, even if the clear coating film 15 formed on the high-concentration black clear coating film 13 has a conventional coating thickness (e.g., 30 μm), the thickness of the high-concentration black clear coating film 13 is thin, so the gravity due to the weight of the high-concentration black clear coating film 13 and the clear coating film 15 can be reduced, and the occurrence of sagging and popping can be suppressed. Furthermore, since the clear coating film 15 after baking forms an acid-epoxy crosslink, it does not undergo hydrolysis due to rainfall or the like, and can prevent infiltration into the high-density black clear coating film 13 . Furthermore, light transmitted through the baked clear coating film 15 is reflected by a sufficient amount of black pigment in the baked high-density black clear coating film 13, causing the black pigment to develop its color. The remaining light is absorbed by a sufficient amount of black pigment, preventing it from penetrating into the underlying layer. Furthermore, by using the same paint resin for the black high-concentration coloring coating film and the clear coating film, strong adhesion is ensured by forming an acid-epoxy crosslinking bond between the black high-concentration coloring coating film and the clear coating film during baking, and by forming the black high-concentration coloring coating film and the clear coating film with the same resin, excellent durability including weather resistance can be ensured.Furthermore, the black high-concentration coloring coating film and the clear coating film can be overcoated without drying. That is, the multilayer coating film structure of the present invention can suppress deterioration of appearance such as sagging and popping while maintaining color development, hiding power, and weather resistance.

[0059] In addition, in the above embodiment, the first color-developing coating film 12 is white, but the present invention is not limited to this form and may be any color different from the high-density black clear coating film 13. For example, the first color-forming coating film 12 may be not only pure white, but also a white-based color with saturation. Furthermore, there is no limitation on the brightness. Alternatively, the first color-forming coating film 12 may be a chromatic color, such as red, blue, or yellow. Alternatively, it may be an achromatic color or a metallic color, and any color tone is possible without limiting the brightness or saturation. [Example]

[0060] The method for forming a multilayer coating film and the multilayer coating film structure according to the present invention will be further described with reference to examples.

[0061] In this example, the outer panel of a vehicle body was used as the substrate, and an electrodeposition film and an intermediate coating film (15 μm) were formed on it. Then, a first color-developing coating film (white, 15 μm) was formed on the intermediate coating film, and then a high-density black clear coating film and a clear coating film (30 μm) were formed on top of that.

[0062] Example 1 In Example 1, the concentration of the dispersed black pigment in the high-concentration black clear coating film was set to 7%, and the film thickness of the high-concentration black clear coating film was set to 10 μm. Example 2 In Example 2, the concentration of the dispersed black pigment in the high-concentration black clear coating film was set to 11%, and the film thickness of the high-concentration black clear coating film was set to 10 μm. Example 3 In Example 3, the concentration of the dispersed black pigment in the high-concentration black clear coating film was set to 15%, and the film thickness of the high-concentration black clear coating film was set to 10 μm. Example 4 In Example 4, the concentration of the dispersed black pigment in the high-concentration black clear coating film was set to 20%, and the film thickness of the high-concentration black clear coating film was set to 10 μm. Example 5 In Example 5, the concentration of the dispersed black pigment in the high-concentration black clear coating film was set to 11%, and the film thickness of the high-concentration black clear coating film was set to 6 μm.

[0063] (Comparative Example 1) In Comparative Example 1, the concentration of the dispersed black pigment in the high-concentration black clear coating film was set to 5%, and the film thickness was set to 10 μm. (Comparative Example 2) In Comparative Example 2, the concentration of the dispersed black pigment in the high-concentration black clear coating film was set to 11%, and the film thickness of the high-concentration black clear coating film was set to 15 μm. (Comparative Example 3) In Comparative Example 3, the concentration of the dispersed black pigment in the high-concentration black clear coating film was set to 30%, and the film thickness of the high-concentration black clear coating film was set to 10 μm.

[0064] The evaluation items for this example were color tone (whether the base shows through, etc.), appearance (whether there is sagging, etc.), and finish. The results of this example are shown in Table 1. In Table 1, ◯ indicates that there were no problems in any of the evaluation items, and × indicates that there was a problem in any of the evaluation items.

[0065] [Table 1]

[0066] As shown in Table 1, Examples 1 to 5 (dispersed black pigment concentration of 7% to 20%) had no problems with the evaluation items, but Comparative Example 1, where the dispersed black pigment concentration was 5%, showed through to the base due to insufficient black hiding power. Furthermore, Comparative Example 2, where the film thickness was 15 μm, had a high film thickness, resulting in increased interphase with the clear coating, resulting in a poor finish. Furthermore, Comparative Example 3, where the dispersed black pigment concentration was 30%, had a high concentration of black pigment, resulting in excessive solvent carried over by the black pigment, making painting difficult. From the results of the above examples, it was confirmed that the concentration of the dispersed black pigment in the high-concentration black clear coating film is preferably 7% to 20%, and the film thickness is preferably 10 μm or less. [Explanation of symbols]

[0067] W Object to be coated 10 Electrodeposited film 11 Undercoat 12 First color coating 13 High-density black clear coating (high-density black color coating) 15 Clear coating

Claims

1. A method for forming a multilayer coating film on a substrate, comprising: forming an electrodeposited film on the substrate; forming an intermediate coating film on the electrodeposition film; forming a first color coating film on the intermediate coating film; forming a second color black high density color developing coating film on a portion of the first color developing coating film; and forming a clear coating film on the first color coating film and the black high-density color coating film, In the step of forming the black high-density color-developing coating film, A method for forming a multilayer coating film, characterized in that the concentration of the dispersed black pigment contained in the black high-concentration color-developing coating film is 7% or more and 25% or less.

2. In the step of forming a clear coating film on the first color coating film and the black high-density color coating film, The resin forming the black high-density color-developing coating film and the clear coating film is 2. The method for forming a multilayer coating film according to claim 1, characterized in that the multilayer coating film is a polyfunctional resin consisting of a resin having one or more carboxy groups in one molecule and a resin having one or more epoxy groups in one molecule, or a polyfunctional resin having one or more carboxy groups and one or more epoxy groups in one molecule, and the high-concentration black color-developing coating film contains a black pigment and a dispersant having one or more carboxy groups or one or more epoxy groups in one molecule.

3. In the step of forming a second color black high density color developing coating film on a part of the first color developing coating film, 2. The method for forming a multilayer coating film according to claim 1, wherein the thickness of the black high-density color-developing coating film is formed to be 10 μm or less.

4. A multilayer coating film structure formed on a substrate, An electrodeposited film formed on the substrate; an intermediate coating film formed on the electrodeposition film; a first color coating film formed on the intermediate coating film; a second color black high density color developing coating film formed on a portion of the first color developing coating film; a clear coating film formed on the first color coating film and the black high-density color coating film; Equipped with A multilayer coating structure characterized in that the concentration of the dispersed black pigment contained in the black high-concentration color-developing coating film is 7% or more and 25% or less.

5. The resin forming the black high-density color-developing coating film and the clear coating film is a polyfunctional resin consisting of a resin having one or more carboxy groups in one molecule and a resin having one or more epoxy groups in one molecule, or a polyfunctional resin having one or more carboxy groups and one or more epoxy groups in one molecule, and the black high-density color-developing coating film comprises a black pigment, a dispersant having one or more carboxy groups or one or more epoxy groups in one molecule, The multilayer coating structure according to claim 4, characterized in that it contains

6. 5. The multilayer coating structure according to claim 4, wherein the thickness of the black high-density color-developing coating film is 10 [mu]m or less.

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