Method for forming multi-layer coating films

Abstract

This invention provides a multi-layer coating that is less prone to unevenness when a clear coat is applied over a plated-like coating, and exhibits excellent interlayer adhesion. [Solution] The present invention relates to a multilayer coating method comprising: an indium coating film forming step of applying an indium paint containing an indium pigment, an inorganic alkoxide, and a first solvent to a workpiece to form an indium coating film; and a clear coating film forming step of applying a clear paint to the indium coating film to form a clear coating film, wherein the first solvent contains at least one of a first alcohol solvent and a first glycol ether solvent, the content of at least one of the first alcohol solvent and the first glycol ether solvent relative to the total amount of solvent in the indium paint is less than 20% by mass, and the solid content of the inorganic alkoxide relative to the total amount of coating film components in the indium paint is 30% by mass or more and 75% by mass or less.

Description

Technical Field

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

Background Art

[0002] In order to impart high design quality to automotive interior parts, household electrical appliances, bicycles, fishing tackle, etc., plated parts are widely used. However, the production of plated parts requires equipment costs from pretreatment to wastewater treatment, and in terms of design, the colors and material shapes that can be expressed are limited. For this reason, various metallic coating methods and methods for forming metallic multilayer coating films have been studied. In a metallic multilayer coating film, a clear coating film is formed as an overcoat for protecting the metallic base layer. The clear coating film is required not only to have protective performance but also to have interlayer adhesion. In addition, when the clear paint is applied, so-called "return unevenness" may occur, in which the arrangement of the metallic pigments in the metallic coating film is disturbed and unevenness appears in the metallic luster. It is also required to prevent this return unevenness and not degrade the metallic appearance.

[0003] Patent Document 1 discloses a paint set including an undercoat paint and a paint containing vapor-deposited indium thin film flakes for the purpose of forming a coated article having a mirror finish design and excellent radio wave permeability and a low coating film forming temperature.

[0004] Patent Document 2 discloses a method for forming a multilayer coating film having a step of applying a bright paint composition (Y) containing indium particles (y1) to form a bright coating film and a step of applying a clear paint composition (Z) containing a hydroxyl group-containing acrylic resin (z1) and a polyisocyanate compound (z2) to form a clear coating film for the purpose of forming a multilayer coating film having excellent metallic luster and capable of exhibiting high coating film performance.

[0005] Patent Document 3 discloses a method for forming a multilayer coating film that has excellent metallic luster and exhibits excellent coating performance, comprising the steps of: applying a glossy coating composition (Y) to a workpiece to form a glossy coating film; and applying a clear coating composition (Z) containing (z1) a hydroxyl group-containing resin and (z2) a polyisocyanate compound to the glossy coating film obtained in step (1) to form a clear coating film.

[0006] Patent Document 4 discloses a glossy paint that, even when using flake-shaped indium as a flake-shaped metal filler, can be sufficiently oriented during the film formation process and form a paint film with excellent glossiness. The glossy paint contains a hydrocarbon solvent (A) having an aniline point of 40°C or higher, a resin (B1) that is incompatible with the hydrocarbon solvent (A), a solvent (C) that is compatible with the hydrocarbon solvent (A) and the resin (B1) and has a lower boiling point than the hydrocarbon solvent (A), a flake-shaped metal filler (D), and a siloxane-based surfactant (E), wherein the flake-shaped metal filler (D) is flake-shaped indium (D1).

[0007] Patent Document 5 discloses a method for forming a coating film, which aims to obtain a coating film that has excellent plating properties while simultaneously satisfying adhesion, comprising the steps of (1) forming a primer coating film and (2) forming a plating-like coating film from a plating-like metallic paint, wherein the evaporation rate of the plating-like metallic paint is 0.3 to 2.0 g / h, and after the gel fraction of the primer coating film is set to 80% or more in step (1), the method proceeds to step (2). [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 7146311 [Patent Document 2] International Publication No. 2024 / 043248 [Patent Document 3] International Publication No. 2022 / 244483 [Patent Document 4] Patent No. 7406176 [Patent Document 5] Japanese Patent Publication No. 2023-147822 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, Patent Document 1 does not describe the improvement of interlayer adhesion. Furthermore, when multi-layer coating is performed using an ester-based + ketone-based or ester-based + ketone-based + aromatic-based topcoat solvent composition, unevenness may occur. However, Patent Document 1 does not focus on this phenomenon and does not disclose any solution. Furthermore, Patent Document 2 does not address the unevenness that occurs when an aromatic + glycol ether-based PMA + ester-based iBA is used as the topcoat solvent composition. Similarly, while the technologies described in Patent Documents 3 and 5 exhibit good interlayer adhesion, they do not address unevenness in the return process due to the composition of the topcoat solvent. Furthermore, since Patent Document 4 does not describe the topcoat film, it does not address issues of interlayer adhesion or unevenness in the return process, and therefore does not describe any solutions to these problems. The present invention has been made in view of the above circumstances, and aims to provide a method for forming a multilayer coating film that is less prone to unevenness when a clear coating is applied to a plated coating film, and that provides a multilayer coating film with a mirror-like appearance and excellent interlayer adhesion. [Means for solving the problem]

[0010] The present inventors have discovered that the above problem can be solved by forming a multilayer coating film consisting of a plating-like coating film and a clear coating film thereon in a plating-like paint by using a specific amount of inorganic alkoxide and a specific amount of a predetermined solvent to form the multilayer coating film, and have arrived at the present invention. In other words, the present invention is as follows: [1] A multilayer coating method comprising: an indium coating film forming step of applying an indium paint containing an indium pigment, an inorganic alkoxide, and a first solvent to a workpiece to form an indium coating film; and a clear coating film forming step of applying a clear paint to the indium coating film to form a clear coating film, wherein the first solvent contains at least one of a first alcohol solvent and a first glycol ether solvent, the content of at least one of the first alcohol solvent and the first glycol ether solvent relative to the total amount of solvent in the indium paint is less than 20% by mass, and the solid content of the inorganic alkoxide relative to the total amount of coating film components in the indium paint is 30% by mass or more and 75% by mass or less. [2] The method for forming a multilayer coating film according to [1] above, wherein the clear coating contains a resin component and a second solvent, the second solvent contains at least one of a second alcohol solvent and a second glycol ether solvent, a ketone solvent and an aliphatic hydrocarbon solvent, the content of at least one of the second alcohol solvent and the second glycol ether solvent relative to the total amount of solvent in the clear coating is 0.1% by mass or more and 15% by mass or less, and the total content of the ketone solvent and aliphatic hydrocarbon solvent relative to the total amount of solvent in the clear coating is 35% by mass or more and 70% by mass or less. [Effects of the Invention]

[0011] According to the multi-layer coating method of the present invention, even when a clear coating is applied to a plated coating, unevenness in the coating is less likely to occur, and a multi-layer coating with a mirror-like appearance and excellent interlayer adhesion can be obtained. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described below.

[0013] [Method for forming multi-layer coatings] The multi-layer coating film forming method of the present embodiment includes an indium coating film forming step of coating an indium coating material containing an indium pigment, an inorganic alkoxide, and a first solvent on an object to be coated to form an indium coating film, and a clear coating film forming step of coating a clear coating material on the indium coating film to form a clear coating film. The first solvent contains at least one of a first alcohol solvent and a first glycol ether solvent, and the content of at least one of the first alcohol solvent and the first glycol ether solvent relative to the total solvent amount of the indium coating material is less than 20% by mass. The solid content of the inorganic alkoxide relative to the total coating film component amount in the indium coating material is 30% by mass or more and 75% by mass or less. This is a multi-layer coating film forming method. Hereinafter, the details of each step will be described.

[0014] <Indium coating film forming step> In the indium coating film forming step, an indium coating material containing an indium pigment, an inorganic alkoxide, and a first solvent is coated on an object to be coated to form an indium coating film.

[0015] (Indium pigment) The indium pigment is a flaky, lustrous pigment having a planar shape such as a rectangular shape, a circular shape, a polygonal shape, or a random shape. Such an indium pigment can make the indium coating film into an excellent mirror surface with a plating finish. The indium pigment has an average particle diameter of 0.1 μm or more and 10 μm or less, and preferably 0.1 μm or more and 1.0 μm or less. Also, from the viewpoint of achieving good orientation and a mirror surface, the aspect ratio of the indium pigment is preferably 3 or more.

[0016] As the indium pigment in the present invention, commercially available leaf powder (registered trademark) (manufactured by Oike Kogyo Co., Ltd.) or the like can be used. The content of the indium pigment is 25% by mass or more and 70% by mass or less, and preferably 30% by mass or more and 60% by mass or less, relative to the total coating film component amount in the indium coating material.

[0017] (Inorganic alkoxide) As the inorganic alkoxide, a known silane coupling agent can be used. As the silane coupling agent, those having reactive groups such as vinyl group, epoxy group, acrylic group, isocyanate group are preferred, and those having an epoxy group are particularly preferred. Specifically, commercially available silane coupling agents such as those manufactured by Shin-Etsu Silicone Co., Ltd., Toray Dow Corning Co., Ltd. and the like can be mentioned. The indium coating film formed by the indium paint contains an inorganic alkoxide and / or its reactant.

[0018] The solid content of the inorganic alkoxide with respect to all the coating film components in the indium paint is 30% by mass or more and 75% by mass or less. The "coating film components" in the indium paint shall include indium pigment, inorganic alkoxide, and resin components. Since the indium paint contains the inorganic alkoxide in the above content, the adhesion to the primer coating film is improved, and in addition, the orientation of the indium pigment is also good, and a coating film having an excellent plating appearance can be obtained.

[0019] (First solvent) The first solvent contains at least one of the first alcohol solvent and the first glycol ether solvent, and the content of at least one of the first alcohol solvent and the first glycol ether solvent with respect to the total solvent amount of the indium paint is less than 20% by mass. When the indium paint contains at least one of the first alcohol solvent and the first glycol ether solvent within the above range, the solubility can be decreased and the drying property of the indium coating film can be increased. Thereby, the orientation of the indium pigment can be made good, and the unevenness in return when a clear coating film is formed on the indium coating film can be prevented.

[0020] - First alcohol solvent - Examples of the first alcohol solvent include methanol, ethanol, propanol, butanol and the like.

[0021] - First glycol ether solvent - Examples of the first glycol ether solvent include ethylene glycol monobutyl ether, ethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, diethylene glycol monobutyl ether, and ethylene glycol monomethyl ether acetate.

[0022] -Other solvents- In addition to the first alcohol solvent and the first glycol ether solvent mentioned above, other solvents that can be used in indium paints include aromatic hydrocarbon solvents, ketone solvents, and ester solvents. Examples of ester solvents include ethyl acetate, butyl acetate, and propyl acetate.

[0023] (Lacquer resin) Indium paint may contain lacquer resin. The lacquer resin is not particularly limited and can be acrylic resin, cellulose resin, polyester resin, urethane resin, polyvinyl chloride resin, etc., but acrylic resin and cellulose resin are preferred.

[0024] The acrylic resin is preferably a polymer or copolymer mainly composed of acrylic acid ester or methacrylic acid ester. Here, "main component" means that it is present in an amount of 50 mol% or more. In the present invention, it is preferable that 90% or more by mass of the acrylic resin has a hydroxyl value of less than 5 mg / KOHg, and that it has a low number of hydroxyl groups. Furthermore, it is preferable that the Tg is 50°C or higher.

[0025] Examples of cellulose resins include cellulose ethers, cellulose esters, nitrocellulose (NC), acrylic-modified cellulose derivatives, polyester-modified cellulose derivatives, and polyurethane-modified cellulose derivatives, with nitrocellulose being particularly preferred. Nitrocellulose preferably has a nitrogen content of 10% or more. Specifically, commercially available industrial nitrated cotton (manufactured by TNCINDUSTRIAL) can be used. The lacquer resin content is preferably 50% by mass or less of the solid content of the indium paint, more preferably 10% by mass or less, and may even be 0% by mass.

[0026] The thickness of the indium coating is preferably between 50 nm and 1000 nm.

[0027] (Method for forming an indium coating) Methods for forming an indium coating include known coating methods such as brush coating, roller coating, trowel coating, spatula coating, flow coater coating, and atomization coating (e.g., air spray coating, airless spray coating, rotary atomization coating, etc.).

[0028] An indium coating can be formed by applying indium paint to an object to be coated and then allowing it to dry. Since indium paint is a lacquer paint, the paint film is obtained by leaving the paint at a temperature of 5°C or higher and below 130°C after application. Furthermore, it is preferable that the evaporation rate at 23°C and 50% RH be 0.3 to 2.0 g / h. This rate is slower than the drying rate of ordinary lacquers. By setting the evaporation rate to 2.0 g / h or less, the volume shrinkage rate during film formation increases, and the indium pigment is arranged densely and parallel, making it possible to form a coating film with a highly vivid and excellent plated appearance.

[0029] -Subject to be coated- Examples of materials to be coated include metals such as iron, aluminum, magnesium, zinc, copper, silver, gold, stainless steel, brass, and galvanized steel, as well as alloys and metal composites; resins such as acrylic resins, polyester resins, polycarbonate resins, polyolefin resins, acrylonitrile-styrene (AS) resins, acrylonitrile-butadiene-styrene (ABS) resins, acrylonitrile-styrene-acrylate (ASA) resins, polyamide resins, vinylidene halogenated resins, polyphenylene ether resins, polyoxymethylene resins, polyurethane resins, epoxy resins, phenolic resins, vinyl halogenated resins, fatty acid vinyl resins, silicone resins, polystyrene resins, vinyl ether resins, glass, ceramics, wood, paper, and textiles.

[0030] (Primer coating) From the viewpoint of improving the adhesion between the object to be coated and the indium paint, it is preferable to form a primer film on the object to be coated before applying the indium paint. As the primer paint, primer paints used for automotive interior and exterior parts, home appliances, etc., can be used. Examples of resins that can be used in primer coatings include acrylic-urethane resins, acrylic resins, fluororesins, alkyd resins, urethane resins, epoxy resins, organically modified silicone resins (e.g., acrylic silicone resins), chlorinated rubber resins, phenolic resins, polyester resins, melamine resins, and nylon resins. In particular, it is preferable to use acrylic-urethane resins or acrylic-polyester resins.

[0031] <Clear coating formation process> In the clear coating formation process, a clear coating is applied to the indium coating to form a clear coating. The clear coating plays a role in improving the scratch resistance and chemical resistance of the entire coating. The clear coating contains a resin component and a second solvent.

[0032] (Resin components) Examples of resin components used in clear coatings include acrylic-urethane resins, acrylic-melamine resins, acrylic resins, fluororesins, urethane resins, epoxy resins, organically modified silicone resins (e.g., acrylic silicone resins), polyester resins, and melamine resins. In particular, the use of acrylic-urethane resins and acrylic-melamine resins is preferred.

[0033] (Second solvent) -Second alcohol solvent and second glycol ether solvent- The second solvent comprises at least one of the second alcohol solvent and the second glycol ether solvent, a ketone solvent, and an aliphatic hydrocarbon solvent. The second alcohol solvent and the second glycol ether solvent can be the same as the first alcohol solvent and the second glycol ether solvent listed in the indium paint described above. Preferably, the content of at least one of the second alcohol solvent and the second glycol ether solvent relative to the total amount of solvent in the clear coating is 0.1% by mass or more and 15% by mass or less. By having the content of at least one of the second alcohol solvent and the second glycol ether solvent relative to the total amount of solvent in the clear coating within the above range, the amount of these highly soluble solvents is small, making them less likely to damage the indium coating and effectively preventing unevenness in the coating.

[0034] -Ketone solvents and aliphatic hydrocarbon solvents- Examples of ketone solvents include acetone, methyl ethyl ketone, methyl isobutyl ketone, and diisobutyl ketone. Examples of aliphatic hydrocarbon solvents include hexane, cyclohexane, and octane. The combined content of ketone solvents and aliphatic hydrocarbon solvents relative to the total amount of solvent in the clear coating is preferably 35% by mass or more and 70% by mass or less. By keeping these solvents, which have low dissolving power and high volatility, within the above range, the solvents evaporate quickly, preventing them from returning to the coating film and causing unevenness, thus enabling the production of an excellent plated, mirror-like coating film.

[0035] The thickness of the clear coating is preferably 15 μm or more and 50 μm or less, and more preferably 20 μm or more and 40 μm or less. The clear coating is preferably a clear coating that does not contain coloring agents or a colored clear coating that contains a small amount of coloring agent, so as not to impair the plated appearance of the indium coating.

[0036] (Method for forming a clear coating) The application method for the clear coating can be the same as that for the indium coating described above. Furthermore, the formation conditions can be the same temperature and time as for the indium coating.

[0037] (Other additives) The indium coatings and clear coatings of the present invention may contain, as appropriate, additives commonly used in the coating industry, such as surface modifiers, wetting agents, dispersants, emulsifiers, thickeners, anti-settling agents, anti-skinning agents, anti-sagging agents, defoaming agents, anti-color separation agents, leveling agents, drying agents, plasticizers, insecticides, light stabilizers, UV absorbers, antistatic agents, and conductivity imparters, depending on the purpose. [Examples]

[0038] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to these examples and comparative examples. In the examples and comparative examples, "parts" and "%" refer to mass unless otherwise specified.

[0039] [Preparation of indium paint] The raw materials were mixed according to the formulations shown in Tables 1 and 2, and dispersed using known methods to prepare indium coatings 1 to 11 (hereinafter abbreviated as Indium 1, Indium 2, etc.). The materials used are as follows:

[0040] (1) Indium pigment: Leaf Powder (trademark registered) In (product name "Indium 49CJ-1120", solid content 20% by mass, average particle size 1 μm or less, manufactured by Oike Kogyo Co., Ltd.) (2) Inorganic alkoxide (silane coupling agent): Product name "KBM-403", manufactured by Shin-Etsu Chemical Co., Ltd. (3) Lacquer resin: 5% nitrated cotton, 95% ester solvent (4) First solvent Glycol ether solvent: PMA (Propylene glycol monomethyl ether acetate) Alcohol solvent: Isopropanol Ester solvent: Ethyl acetate Ketone solvents: Diisobutyl ketone (DIBK), Methyl isobutyl ketone (MIBK)

[0041] [Preparation of clear paint] Clear coatings 1 and 2 (hereinafter referred to as Clear 1 and Clear 2) were prepared by mixing the raw materials according to the formulation shown in Table 3. The materials used are as follows: (1) Resin components Acrylic resin A: 50% resin + 50% butyl acetate Isocyanate resin B: 80% resin + 20% butyl acetate (2) Second solvent Alcohol solvent: Isopropanol Glycol ether solvent: PMA (Propylene glycol monomethyl ether acetate) Ester solvent: Butyl acetate Ketone solvents: Diisobutyl ketone (DIBK), Methyl isobutyl ketone (MIBK) Aliphatic hydrocarbon solvent: cyclohexane

[0042] [Table 1]

[0043] [Table 2]

[0044] [Table 3]

[0045] [Examples 1 to 9 and Comparative Examples 1 to 4] (Test panel preparation) A primer-coated board was prepared by spray-painting an ABS substrate with an amount of Acrytan TG-5 black (manufactured by Dainippon Paint Co., Ltd.) that resulted in a dry film thickness of 25 μm. The board was left to stand for 7 minutes, and then dried at 80°C for 30 minutes. Next, in the combinations shown in Tables 4 and 5, indium paint (Indi 1 to Indi 11) was spray-coated onto primer-coated boards in an amount that resulted in a dry film thickness of 0.3 μm. The boards were left to stand for 20 minutes, and then dried at 80°C for 30 minutes to produce indium-based coated boards. Furthermore, indium-based coated plates were spray-painted with clear paints (Clear 1 and Clear 2) in an amount that resulted in a dry film thickness of 25 μm. After being left for 20 minutes, they were dried at 80°C for 30 minutes to prepare test coated plates for Examples 1 to 9 and Comparative Examples 1 to 4.

[0046] [evaluation] The test coated panels prepared as described above were evaluated as follows. The evaluation results are shown in Tables 4 and 5.

[0047] (exterior) The test painted panels were visually inspected and evaluated according to the following evaluation criteria. ○: Has a mirror-like finish, similar to plating, and the mirror finish is even. △: Inferior to plating, but has a better mirror-like finish and is more even than regular metallic paint, or has a mirror-like finish similar to plating, but with some slight unevenness in the finish. ×: It's inferior to plating, but has more of a mirror-like finish than regular metallic paint, although it is also uneven.

[0048] (Adhesive) After a 240-hour immersion test in 40°C hot water, the samples were allowed to stand for 1 hour, and then an adhesion test was conducted according to JIS K 5600-5-6:1999 (Adhesion (Cross-cut method)). The cuts were spaced 2 mm apart, and the film's appearance was visually inspected using a tape peel test across 100 grid squares, and evaluated according to the following evaluation criteria. ○: Adhesion 100 / 100 △: Adhesion 99 / 100~95 / 100 ×: Less than 95 / 100

[0049] [Table 4]

[0050] [Table 5]

[0051] As shown in Table 4, Examples 1 to 9 using indium paints Indi 1 to 7, in which the total content of the first alcohol solvent and the first glycol ether solvent relative to the total amount of solvent in the indium paint was less than 20% by mass, and the solid content of inorganic alkoxide relative to the total amount of coating film components in the indium paint was 30% by mass or more and 75% by mass or less, showed excellent performance in terms of appearance and adhesion. On the other hand, Comparative Example 1, which used an indium paint (Indi 8) with a high total content of the first alcohol solvent and the first glycol ether solvent relative to the total amount of solvent in the indium paint and without inorganic alkoxide, exhibited inferior appearance and adhesion. Furthermore, Comparative Example 2, which used an indium paint (Indi 9) containing inorganic alkoxides within the above range but with a high total content of the first alcohol solvent and the first glycol ether solvent relative to the total amount of solvent in the indium paint, showed good adhesion but poor appearance due to unevenness in the return process. Comparative Example 3, which used an indium paint (Indi 10) with a high inorganic alkoxide solid content of 89.3% by mass relative to the total amount of coating components in the indium paint due to a low amount of indium pigment, failed to achieve a mirror finish and had an inferior appearance. Comparative Example 4, which used an indium paint (Indi-11) that did not contain inorganic alkoxides, showed poor adhesion.

Claims

1. An indium coating film forming step involves applying an indium paint containing an indium pigment, an inorganic alkoxide, and a first solvent to a workpiece to form an indium coating film, The process includes a clear coating film formation step, in which a clear coating is applied to the indium coating film to form a clear coating film. The first solvent comprises at least one of a first alcohol solvent and a first glycol ether solvent, and the content of at least one of the first alcohol solvent and the first glycol ether solvent relative to the total amount of solvent in the indium paint is less than 20% by mass. A method for forming a multilayer coating film, wherein the solid content of the inorganic alkoxide relative to the total amount of coating film components in the indium paint is 30% by mass or more and 75% by mass or less.

2. The clear coating comprises a resin component and a second solvent. The second solvent comprises at least one of a second alcohol solvent and a second glycol ether solvent, a ketone solvent, and an aliphatic hydrocarbon solvent. The content of at least one of the second alcohol solvent and the second glycol ether solvent relative to the total amount of solvent in the clear coating is 0.1% by mass or more and 15% by mass or less. The method for forming a multilayer coating film according to claim 1, wherein the total content of the ketone solvent and the aliphatic hydrocarbon solvent relative to the total amount of solvent in the clear coating is 35% by mass or more and 70% by mass or less.

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