Silver color coating material composition, silver color coating film formation method, and method for improving abrasion resistance of silver color coating film

Abstract

Silver color coating films formed using a silver color coating material composition that includes an aluminum flake pigment have weak coating film strength and need better abrasion resistance.　The present invention provides an abrasion-resistant silver color coating material composition that contains a hydroxyl group–containing acrylic resin, a curing agent, a phosphoric acid group–containing acrylic resin, and an aluminum flake pigment, the silver color coating material composition being characterized in that an aluminum flake pigment that has undergone a molybdenum surface treatment is at least 30 mass% of the entire aluminum flake pigment. The present invention also provides a silver color coating film formation method that involves using an electrostatic coating machine to sequentially apply a silver color coating material composition and a clear coating material to a substrate by wet-on-wet and then simultaneously curing both of the resulting coating films, the silver color coating film formation method being characterized in that the silver color coating material composition is the silver color coating material composition previously described.

Description

Silver-colored paint composition, method for forming a silver-colored coating film, and method for improving the abrasion resistance of a silver-colored coating film.

[0001] This invention relates to a silver paint composition for use on automobile bodies and the like, and a method for improving the abrasion resistance of a silver paint film using the same.

[0002] The laminated coatings of automobiles are formed from various types of paints. For example, in weatherstrips (sealing materials installed around the door openings of automobiles) that seal the space between the opening (door opening) and the opening opening / closing member (door), the weatherstrip comes into contact with the painted interior panel of the automobile, is continuously abraded, and the coating may wear off over time. Therefore, the coating on the painted interior panel requires a highly abrasion-resistant coating that is less likely to wear off even when the weatherstrip comes into contact with the painted interior panel.

[0003] Generally, silver paint, which is widely used in automobiles, uses aluminum pigment to produce a metallic luster. However, it is known that paint films using aluminum pigment have low film strength and problems with abrasion resistance.

[0004] Japanese Patent No. 4612177 (Patent Document 1) describes a method for forming a coating film in which an aqueous base coating and a clear coating are sequentially formed on a substrate using an electrostatic coating machine in a wet-on-wet manner, and then both coating films are cured at once, wherein the appearance is improved by blending a specific organic solvent into the aqueous base coating. This aqueous base coating contains an aluminum pigment.

[0005] Patent No. 4612177

[0006] Silver paint films formed using silver paint compositions containing flaky aluminum pigments have low film strength, and improvement in abrasion resistance is desired.

[0007] In this invention, we have found that using a specific amount of molybdenum-treated flaky aluminum pigment relative to the total amount of flaky aluminum pigment improves the abrasion resistance and coating strength of a silver-colored coating.

[0008] That is, the present invention provides the following aspects: [1] A silver paint composition containing a hydroxyl group-containing acrylic resin, a curing agent, a phosphate group-containing acrylic resin, and a flake-shaped aluminum pigment, wherein the flake-shaped aluminum pigment is a combination of a flake-shaped aluminum pigment whose surface is molybdenum-treated and a flake-shaped aluminum pigment whose surface is not molybdenum-treated, and the flake-shaped aluminum pigment contains 30% by mass or more of the flake-shaped aluminum pigment whose surface is molybdenum-treated relative to the total flake-shaped aluminum pigment. [2] A method for forming a silver-colored coating film, comprising sequentially forming a silver-colored coating composition and a clear coating on a substrate using an electrostatic coating machine in a wet-on-wet manner, and then curing both coating films at once, wherein the silver-colored coating composition contains a hydroxyl group-containing acrylic resin, a curing agent, a phosphate group-containing acrylic resin, and a flake-shaped aluminum pigment, the flake-shaped aluminum pigment is a combination of a flake-shaped aluminum pigment whose surface is molybdenum-treated and a flake-shaped aluminum pigment whose surface is not molybdenum-treated, and the flake-shaped aluminum pigment contains 30% by mass or more of the flake-shaped aluminum pigment whose surface is molybdenum-treated relative to the total flake-shaped aluminum pigment. [3] A method for forming a silver-colored coating film by sequentially forming a silver-colored coating composition and a clear coating on a substrate using an electrostatic coating machine in a wet-on-wet manner, and then curing both coating films at once, wherein the silver-colored coating composition contains a hydroxyl group-containing acrylic resin, a curing agent, a phosphate group-containing acrylic resin, and a flake-shaped aluminum pigment, the flake-shaped aluminum pigment is a combination of a flake-shaped aluminum pigment with a molybdenum-treated surface and a flake-shaped aluminum pigment without a molybdenum-treated surface, and the flake-shaped aluminum pigment is controlled to contain 30% by mass or more of the flake-shaped aluminum pigment with a molybdenum-treated surface relative to the total flake-shaped aluminum pigment.[4] Preferably, the flake aluminum pigment contains a flake aluminum pigment whose surface has been treated with molybdenum in an amount of 30 to 95% by mass relative to the total flake aluminum pigment.

[0009] The present invention focuses on the fact that the abrasion resistance of a silver coating film is improved by using molybdenum-treated flake aluminum pigment as at least a portion of the flake aluminum pigment in a silver coating paint composition containing flake aluminum pigment. The invention provides a silver coating paint composition that improves the abrasion resistance of a silver coating film, as well as a method for forming a coating film that improves the abrasion resistance of a silver coating film and a method for improving the abrasion resistance of a silver coating film.

[0010] This diagram schematically represents the test stand (load arm) of the JSPS-type friction fastness tester used in the abrasion test method. This diagram schematically shows the soft urethane foam and water-resistant abrasive paper used in the abrasion test method.

[0011] The present invention provides a silver coating composition containing a hydroxyl group-containing acrylic resin, a curing agent, a phosphate group-containing acrylic resin, and a flake-shaped aluminum pigment, wherein the flake-shaped aluminum pigment is a combination of a flake-shaped aluminum pigment with a molybdenum-treated surface and a flake-shaped aluminum pigment without a molybdenum-treated surface, and the flake-shaped aluminum pigment contains 30% by mass or more of the molybdenum-treated flake-shaped aluminum pigment relative to the total flake-shaped aluminum pigment. The present invention also provides a coating film forming method in which the above silver coating composition and a clear coating are sequentially formed wet-on-wet using an electrostatic coating machine, and then both coating films are cured at once. Furthermore, the present invention also provides a method for improving the abrasion resistance of a silver coating film.

[0012] While not limited to any specific theory, it is believed that the adhesion between the molybdenum-treated flaky aluminum pigment and the phosphate-containing acrylic resin is enhanced, improving the possibility of delamination starting from the flaky aluminum pigment, which can occur with conventional methods using untreated flaky aluminum pigment and acrylic resin without phosphate groups. This is thought to improve abrasion resistance and coating strength.

[0013] [Silver-colored paint composition] The silver-colored paint composition of the present invention must contain a hydroxyl group-containing acrylic resin, a hardening agent, a phosphate group-containing acrylic resin, and a flake-like aluminum pigment. Each component will be described below.

[0014] The hydroxyl group-containing acrylic resin may be an acrylic resin emulsion obtained by emulsion polymerization of a monomer mixture containing, for example, an alkyl (meth)acrylate (a), an acid group-containing ethylenically unsaturated monomer (b), and a hydroxyl group-containing ethylenically unsaturated monomer (c). The acrylic resin may also be an acrylic resin dispersion obtained by solution polymerization of the above monomer mixture and dispersion of the resulting polymer in an aqueous medium.

[0015] The monomer mixture has the advantage of having a good main skeleton of the acrylic resin emulsion when it contains alkyl (meth)acrylate (a). Specific examples of alkyl (meth)acrylate (a) include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, and stearyl (meth)acrylate. In this specification, (meth)acrylic means acrylic and methacrylic.

[0016] The acid group of the above acid group-containing ethylenically unsaturated monomer (b) is preferably selected from carboxyl groups, sulfonic acid groups, and phosphoric acid groups. A particularly preferred acid group is the carboxyl group, from the viewpoint of improving dispersion stability and promoting the curing reaction. The above monomer mixture has the advantage of improving various stability of the resulting acrylic resin emulsion, such as storage stability, mechanical stability, and freeze resistance, and promoting the curing reaction with a curing agent such as melamine resin during film formation.

[0017] Examples of carboxyl group-containing ethylenically unsaturated monomers, which are one example of acid group-containing ethylenically unsaturated monomers (b), include acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, ethacrylic acid, propylacrylic acid, isopropylacrylic acid, itaconic acid, maleic anhydride, and fumaric acid. Examples of sulfonic acid group-containing ethylenically unsaturated monomers, which are one example of acid group-containing ethylenically unsaturated monomers (b), include p-vinylbenzenesulfonic acid, p-acrylamidopropanesulfonic acid, and t-butylacrylamidosulfonic acid. Examples of phosphate group-containing ethylenically unsaturated monomers, which are one example of acid group-containing ethylenically unsaturated monomers (b), include light ester PM (manufactured by Kyoeisha Chemical Co., Ltd.), such as phosphate monoester of 2-hydroxyethyl acrylate and phosphate monoester of 2-hydroxypropyl methacrylate.

[0018] Examples of the hydroxyl group-containing ethylenically unsaturated monomer (c) mentioned above include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and ε-caprolactone-modified acrylic monomer.

[0019] Specific examples of the above-mentioned ε-caprolactone-modified acrylic monomers include "Praxel FA-1", "Praxel FA-2", "Praxel FA-3", "Praxel FA-4", "Praxel FA-5", "Praxel FM-1", "Praxel FM-2", "Praxel FM-3", "Praxel FM-4", and "Praxel FM-5" manufactured by Daicel Chemical Industries, Ltd.

[0020] The above monomer mixture, by containing a hydroxyl group-containing ethylenically unsaturated monomer (c), imparts hydrophilicity based on the hydroxyl group to the acrylic resin emulsion, improving the workability and freeze-resistant stability of the paint composition, and also providing advantages such as curing reactivity with curing agents such as melamine resin.

[0021] The monomer mixture may contain other components in addition to the above monomers. Examples of other components include at least one monomer selected from the group consisting of styrene monomers, (meth)acrylonitrile, and (meth)acrylamide. Examples of styrene monomers include styrene and α-methylstyrene.

[0022] The monomer mixture may also contain crosslinkable monomers such as carbonyl group-containing ethylenically unsaturated monomers, hydrolyzable silyl group-containing monomers, and various polyfunctional vinyl monomers. For example, including crosslinkable monomers in the monomer mixture has the advantage of imparting self-crosslinking properties to the resulting acrylic resin emulsion.

[0023] In the preparation of acrylic resin emulsions, emulsion polymerization can be carried out by heating the above monomer mixture in an aqueous medium under stirring conditions in the presence of a radical polymerization initiator and an emulsifier. The reaction temperature may be, for example, 30 to 100°C. The reaction time can be appropriately selected depending on the reaction scale and reaction temperature, and may be, for example, 1 to 10 hours. In emulsion polymerization, for example, the monomer mixture or monomer pre-emulsifier may be added all at once to a reaction vessel containing water and an emulsifier, or it may be added dropwise. The reaction temperature can be adjusted by appropriately selecting such a procedure.

[0024] As the radical polymerization initiator mentioned above, known initiators used in emulsion polymerization of acrylic resins can be used. Specifically, as water-soluble free radical polymerization initiators, persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate can be used in aqueous solution. In addition, so-called redox initiators, which are combinations of oxidizing agents such as potassium persulfate, sodium persulfate, ammonium persulfate, and hydrogen peroxide, and reducing agents such as sodium bisulfite, sodium thiosulfate, rongalit, and ascorbic acid, can also be used in aqueous solution.

[0025] As the emulsifiers mentioned above, for example, anionic or nonionic emulsifiers selected from micellar compounds having a hydrocarbon group with six or more carbon atoms and a hydrophilic portion such as a carboxylate, sulfonate, or sulfate partial ester in the same molecule can be used. Among these, anionic emulsifiers include alkali metal salts or ammonium salts of sulfate semi-esters of alkylphenols or higher alcohols; alkali metal salts or ammonium salts of alkyl or allyl sulfonates; alkali metal salts or ammonium salts of sulfate semi-esters of polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl ethers, or polyoxyethylene allyl ethers. Nonionic emulsifiers include polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl ethers, or polyoxyethylene allyl ethers. In addition to these general-purpose anionic and nonionic emulsifiers, various anionic and nonionic reactive emulsifiers having radically polymerizable unsaturated double bonds in their molecules, such as acrylic, methacrylic, propenyl, allyl, allyl ether, and maleic acid groups, can also be used as appropriate.

[0026] In emulsion polymerization, additives for molecular weight adjustment (chain transfer agents), such as mercaptan compounds and lower alcohols, can be used as needed. Using these additives has the advantage of allowing emulsion polymerization to proceed smoothly, and also promotes the smooth and uniform formation of the coating film, thereby improving adhesion to the substrate.

[0027] For emulsion polymerization, any polymerization method can be appropriately selected, including the single-stage continuous monomer uniform dropping method, the core-shell polymerization method which is a multi-stage monomer feed method, and the power-feed polymerization method which continuously changes the monomer composition fed during polymerization.

[0028] The resulting hydroxyl group-containing acrylic resin emulsion may be neutralized partially or completely by adding a basic compound. Neutralization has the advantage of improving the stability of the hydroxyl group-containing acrylic resin emulsion. Examples of basic compounds that can be used include ammonia, various amines, and alkali metals.

[0029] In this way, a hydroxyl group-containing acrylic resin emulsion can be prepared. The weight-average molecular weight of the obtained hydroxyl group-containing acrylic resin emulsion is not particularly limited, but is preferably in the range of 50,000 to 5,000,000, and more preferably in the range of 50,000 to 200,000. The solid content acid value of the hydroxyl group-containing acrylic resin emulsion is preferably in the range of 1 to 80 mg KOH / g, more preferably in the range of 2 to 70 mg KOH / g, and even more preferably in the range of 3 to 60 mg KOH / g. The solid content hydroxyl value of the hydroxyl group-containing acrylic resin emulsion is preferably in the range of 50 to 120 mg KOH / g, and more preferably in the range of 50 to 100 mg KOH / g. By having the weight-average molecular weight, solid content acid value, and solid content hydroxyl value of the hydroxyl group-containing acrylic resin emulsion within the above ranges, it is possible to ensure good paint stability, paint workability, and the physical properties of the resulting coating film.

[0030] A hydroxyl group-containing acrylic resin dispersion can be prepared, for example, by carrying out a polymerization reaction of a monomer mixture containing the above-mentioned alkyl (meth)acrylate (a), an acid group-containing ethylenically unsaturated monomer (b), and a hydroxyl group-containing ethylenically unsaturated monomer (c) in the absence of a solvent or in the presence of a suitable organic solvent, adding it dropwise to water, mixing, and removing excess solvent as necessary.

[0031] Polymerization initiators can be used in polymerization reactions. For example, initiators used in the art as radical polymerization initiators can be used. Specific examples of polymerization initiators include organic peroxides such as benzoyl peroxide, t-butyl peroxide, and cumene hydroperoxide, and organic azo compounds such as azobiscyanovaleric acid and azoisobutyronitrile.

[0032] Polymerization reactions can be carried out at temperatures of, for example, 80 to 140°C. The polymerization reaction time can be appropriately selected depending on the polymerization temperature and reaction scale, and can be carried out for, for example, 1 to 8 hours. Polymerization reactions can be carried out by procedures that are commonly performed by those skilled in the art. For example, polymerization can be carried out by dropping a monomer mixture containing ethylenically unsaturated monomers and a polymerization initiator into a heated organic solvent. The organic solvent that can be used for polymerization is not particularly limited, but those with a boiling point of about 60 to 250°C are preferred. Suitable organic solvents include, for example, water-insoluble organic solvents such as butyl acetate, xylene, toluene, methyl isobutyl ketone, propylene glycol, dipropylene glycol dimethyl ether, and methyl ether acetate; and water-soluble organic solvents such as tetrahydrofuran, ethanol, methanol, propanol, isopropanol, 2-butanol, t-butyl alcohol, dioxane, methyl ethyl ketone, ethylene glycol, ethylene glycol monobutyl ether, 2-methoxypropanol, 2-butoxypropanol, diethylene glycol monobutyl ether, butyl diglycol, N-methylpyrrolidone, ethylene carbonate, and propylene carbonate.

[0033] A neutralizing agent may be added to the hydroxyl group-containing acrylic resin obtained by polymerization to neutralize at least some of the acid groups contained in the hydroxyl group-containing acrylic resin. This step can impart good water dispersibility to the hydroxyl group-containing acrylic resin. The neutralizing agent is not particularly limited as long as it is one that is commonly used to neutralize acid groups contained in water-dispersible resin compositions. Examples include organic amines such as monomethylamine, dimethylamine, trimethylamine, triethylamine, diisopropylamine, monoethanolamine, diethanolamine, and dimethylethanolamine, and inorganic bases such as sodium hydroxide, potassium hydroxide, and lithium hydroxide. These neutralizing agents may be used alone, or two or more may be used in combination.

[0034] A hydroxyl group-containing acrylic resin dispersion can be prepared by mixing water with the neutralized acrylic resin as needed, or by mixing the acrylic resin in water. In preparing the hydroxyl group-containing acrylic resin dispersion, excess organic solvent may be removed before adding the neutralizing agent or after the water dispersion, if necessary.

[0035] In this way, a hydroxyl group-containing acrylic resin dispersion can be prepared. The obtained hydroxyl group-containing acrylic resin dispersion is not particularly limited, but preferably has a solid content hydroxyl value in the range of 5 to 200 mg KOH / g, a solid content acid value in the range of 5 to 100 mg KOH / g, and a weight-average molecular weight in the range of 20,000 to 5,000,000. Having the solid content hydroxyl value, solid content acid value, and weight-average molecular weight of the acrylic resin dispersion within the above ranges has the advantage of ensuring good paint stability, paint workability, and the physical properties of the resulting coating film.

[0036] Curing Agent The silver coating composition of the present invention contains a curing agent. Preferred curing agents include amino resins, blocked isocyanate resins, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, etc. From the viewpoint of the performance of the obtained coating film and cost, amino resins (melamine resins) and / or blocked isocyanate resins are generally used. The content of the above curing agent is 20 to 100% by weight relative to the solid content of the coating film forming resin. If the content is less than 20% by weight, the curing performance will be insufficient, and if it exceeds 100% by weight, the cured film will become too hard and brittle. Note that the coating film forming resin refers only to the resin portion that does not contain the curing agent.

[0037] The phosphate-containing acrylic resin used in this invention is an acrylic resin obtained by copolymerizing a monomer represented by the following general formula (I) with a monomer other than the hydroxyl-containing ethylenically unsaturated monomer (c) already described in the above-mentioned hydroxyl-containing acrylic resin.

[0038] CH ２ =CXCO(OY) ｈ - OPO (OH) ２ ... (I) (In the formula, X is a hydrogen atom or a methyl group, Y is an alkylene group having 2 to 4 carbon atoms, and h is an integer from 3 to 30.) The above phosphate group-containing acrylic resin is used to disperse the above flake-shaped aluminum pigment well. The above phosphate group-containing acrylic resin preferably has a number average molecular weight of 1,000 to 50,000. If the above number average molecular weight is less than 1,000, the flake-shaped aluminum pigment may not be sufficiently dispersed, and if the number average molecular weight exceeds 50,000, the appearance of the resulting coating film may deteriorate.

[0039] Further, the phosphoric acid group-containing acrylic resin preferably has a solid content acid value of 15 to 200 mgKOH / g, and further preferably, the acid value due to the phosphoric acid group in the acid value is 10 to 150 mgKOH / g. When the acid value is less than 15 mgKOH / g, it may not be possible to sufficiently disperse the scaly aluminum pigment, and when the acid value exceeds 200 mgKOH / g, the storage stability of the silver-colored paint composition may decrease. Further, the phosphoric acid group-containing acrylic resin may have a hydroxyl group for ensuring curability, and its hydroxyl value is preferably 20 to 200 mgKOH / g.

[0040] The phosphoric acid group-containing acrylic resin is preferably contained in an amount of 0.01 to 5 parts by weight, more preferably 0.1 to 4 parts by weight, and particularly preferably 0.2 to 3 parts by weight, based on 100 parts by weight of the solid content of the paint resin. If the content of the phosphoric acid group-containing acrylic resin is too small, various properties of the coating film may decrease. If the content of the phosphoric acid group-containing acrylic resin is too large, the storage stability of the paint will deteriorate.

[0041] Specific examples of the monomer represented by the general formula (I) include, for example, acid phosphoxyhexa(oxypropylene) monomethacrylate, acid phosphoxide dodeca(oxypropylene) monomethacrylate, and the like. The monomer copolymerized with the monomer represented by the general formula (I) is the monomer exemplified by other monomers than the hydroxyl group-containing ethylenically unsaturated monomer (c) already described in the description of the hydroxyl group-containing acrylic resin. The aforementioned hydroxyl group-containing ethylenically unsaturated monomer (c) can also be used.

[0042] The phosphoric acid group-containing acrylic resin is an acrylic resin emulsion obtained by emulsion polymerization of a monomer mixture containing the phosphoric acid group-containing monomer of the above formula (1) and other ethylenic monomers (specifically, the alkyl (meth)acrylate (a), acid group-containing ethylenically unsaturated monomer (b), and hydroxyl group-containing ethylenically unsaturated monomer (c) described in the above hydroxyl group-containing acrylic resin); an acrylic resin dispersion obtained by solution polymerization of the above monomer mixture and dispersing the obtained polymer in an aqueous medium; and a water-soluble acrylic resin obtained by solution polymerization of the above monomer mixture. In the preparation of the acrylic resin emulsion, acrylic resin dispersion or water-soluble acrylic resin, the above monomer mixture may not contain either the acid group-containing ethylenically unsaturated monomer (b) or the hydroxyl group-containing ethylenically unsaturated monomer (c).

[0043] As the phosphoric acid group-containing acrylic resin contained in the silver coloring paint composition used in the coating film forming method of the present invention, the number average molecular weight is preferably 5000 to 30000, more preferably 7000 to 25000. If it is less than 5000, the workability and curability are not sufficient, and if it exceeds 30000, the nonvolatile content during painting becomes too low, and conversely, the workability deteriorates. In this specification, the molecular weight is determined by the GPC method using a styrene polymer as a standard.

[0044] Further, the phosphoric acid group-containing acrylic resin preferably has a hydroxyl value of 20 to 200 mgKOH / g, preferably 20 to 180 mgKOH / g. If it exceeds the upper limit, the water resistance of the coating film decreases, and if it is below the lower limit, the curability of the coating film decreases. Also, it preferably has an acid value of 15 to 200 mgKOH / g, more preferably 15 to 180 mgKOH / g. If it exceeds the upper limit, the water resistance of the coating film decreases, and if it is below the lower limit, the curability of the coating film decreases.

[0045] As described above, the silver coating composition of the present invention must contain a molybdenum-treated molybdenum-coated Molybdenum-treated flaky aluminum pigments are those in which the surface of flaky aluminum is coated with a molybdenum compound (e.g., molybdic acid) for purposes such as suppressing hydrogen gas generation in paints. For example, Toyo Aluminum Co., Ltd. offers commercially available products in the WL series.

[0046] The method for treating flaky aluminum pigment with molybdenum is not limited to a specific treatment, but examples include the methods described in Japanese Patent No. 3200473 and Japanese Patent Publication No. 2003-230405. More specifically, for example, the flaky aluminum pigment, particularly the untreated flaky aluminum pigment, is treated with a molybdenum compound (e.g., ammonium paramolybdate (NH4)). ４ ) ６ Mo ７ O ２４ 4H ２ O) Examples include those treated with an aqueous solution.

[0047] The silver coating composition of the present invention may also use other flake aluminum pigments besides the molybdenum-treated flake aluminum pigment described above. It is preferable that the molybdenum-treated flake aluminum pigment be used in an amount that is part of the total flake aluminum pigment, particularly 30% by mass or more, preferably 30 to 95% by mass, more preferably 30 to 90% by mass, and more preferably 40 to 75% by mass. Using only the molybdenum-treated flake aluminum pigment (100% by mass) tends to result in poor hue (flip-flop properties). Conversely, if the amount of molybdenum-treated flake aluminum pigment is less than 30% by mass, a large amount of untreated flake aluminum pigment will be used, making it impossible to maintain adhesion with the phosphate-containing acrylic resin, and reducing abrasion resistance.

[0048] Scaly aluminum pigments other than molybdenum-treated scaly aluminum pigments (untreated scaly aluminum pigments) usually have fatty acids, which are grinding aids, added thereto, or organic solvents added thereto in many cases to avoid the risk of dust explosion and the difficulty of handling. As fatty acids used as grinding aids, higher fatty acids such as oleic acid, stearic acid, isostearic acid, lauric acid, palmitic acid, and myristic acid can be used, and aliphatic amines, aliphatic amides, aliphatic alcohols, etc. can also be used, but usually unsaturated higher fatty acids are used. Examples of unsaturated higher fatty acids include oleic acid, linoleic acid, linolenic acid, ricinoleic acid, elaidic acid, zoomic acid, gadleic acid, and erucic acid. Examples of organic solvents include mineral spirit, petroleum benzine, solvent naphtha, isoparaffin, normal paraffin, benzene, toluene, xylene, cyclohexane, hexane, heptane, octane, chlorobenzene, trichlorobenzene, perchloroethylene, and trichloroethylene. Specific examples of such untreated scaly aluminum pigments include commercially available products such as the 71XX / 81XX grades manufactured by Toyo Aluminium K.K. Alternatively, commercially available products such as the WXM series in which the surface of a scaly aluminum pigment treated by a method other than molybdic acid is coated with a phosphorus-based compound, and the EMR series (both manufactured by Toyo Aluminium K.K.) in which the surface of a scaly aluminum pigment is coated with high-density silica can be mentioned. As the phosphorus-based compound, treatment by phosphoric acid treatment (such as phosphate ester and phosphoric ether) can be considered. The silica coating can be one in which fine silica particles are coated.

[0049] The shape of the above scaly aluminum pigment is not particularly limited, and it may be further colored. For example, it is preferable that the average particle diameter (D ５０ ) is 2 to 50 μm and the thickness is 0.1 to 5 μm. Also, those having an average particle diameter in the range of 10 to 35 μm are excellent in brilliance and are more preferably used.

[0050] The pigment concentration (PWC) of the whole flake aluminum pigment in the silver coating composition of the present invention is generally 20.0% or less. Exceeding this upper limit will degrade the appearance of the coating film. Preferably, it is 0.01% to 18.0%, and more preferably, 0.1% to 15.0%. If the PWC of the flake aluminum pigment exceeds 20.0%, the appearance of the coating film will degrade.

[0051] Solvent The silver coating composition of the present invention contains a solvent. The solvent may be an organic solvent or water. The silver coating composition of the present invention is preferably aqueous, and when water is the main solvent, other organic solvents may be included as needed. The organic solvents that may be included in the silver coating composition are not particularly limited, and examples include ester-based organic solvents such as ethyl acetate, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and diethylene glycol monoethyl ether acetate; ether-based organic solvents; alcohol-based organic solvents; or ketone-based organic solvents.

[0052] Examples of ether-based organic solvents include propylene glycol methyl ether, propylene glycol ethyl ether, ethylene glycol monoethyl ether, methyl methoxybutanol, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, and diethylene glycol monobutyl ether. Examples of alcohol-based organic solvents include methanol, ethanol, and propanol. Examples of ketone-based organic solvents include acetone and methyl ethyl ketone.

[0053] The total weight of volatile matter in the silver paint composition used in the present invention is 50 to 95% by weight. Outside this range, the paintability and color return properties decrease. Preferably, the total weight of volatile matter is 55 to 80% by weight.

[0054] Other Film-Forming Resins: The silver coating composition of the present invention may optionally contain other film-forming resins. Preferred film-forming resins include polyester resins, alkyd resins, polyether resins, polyolefin resins, and urethane resins, and one or more of these can be used in combination. Acrylic resins and polyester resins are preferred in terms of coating performance such as weather resistance and water resistance.

[0055] Other Coloring Pigments: In addition to the above-mentioned flake-like aluminum pigment, the silver coating composition of the present invention may also contain coloring pigments. Examples of coloring pigments include organic azochelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolon pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments. Inorganic pigments include lead yellow, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. Extender pigments such as calcium carbonate, barium sulfate, clay, and talc can be used. Furthermore, luminous pigments may also be added.

[0056] Other Components Furthermore, viscosity regulators may be added to the above-mentioned silver paint composition to prevent adhesion with the clear coating, which is often formed on the silver paint film, and to ensure paintability. Viscosity regulators that generally exhibit thixotropy can be used, and examples of viscosity regulators include polyamide-based substances such as swollen dispersions of fatty acid amides, amide fatty acids, and phosphates of long-chain polyaminoamides; polyethylene-based substances such as colloidal swollen dispersions of oxidized polyethylene; organic acid smectite clay and organic bentonite-based substances such as montmorillonite; inorganic pigments such as aluminum silicate and barium sulfate; flattened pigments whose viscosity is expressed depending on the shape of the pigment; and crosslinked or non-crosslinked resin particles.

[0057] In addition to the above-mentioned components, the silver coating paint composition used in the present invention may also contain additives commonly added to paints, such as surface modifiers, antioxidants, and defoamers. The amounts of these additives are within the range known to those skilled in the art.

[0058] The method for producing the silver coating paint composition used in the present invention is not particularly limited, including those described later, and all methods well known to those skilled in the art can be used, such as kneading and dispersing a compound of aluminum pigment and the like using a kneader or roll.

[0059] The silver coating composition of the present invention may be applied to an object to be coated to form a single layer of silver coating film, but it is more common to apply a clear coating to form a clear coating film. Forming a clear coating film improves the abrasion resistance of the silver coating film. When forming a clear coating film, it is preferable to use a method in which the silver coating composition and the clear coating are sequentially applied to a substrate using an electrostatic coating machine in a wet-on-wet manner, and then both coating films are cured at the same time.

[0060] As a preferred example of a clear coating, from the viewpoint of transparency or acid etching resistance, a combination of acrylic resin and / or polyester resin and amino resin and / or isocyanate, or an acrylic resin and / or polyester resin having a carboxylic acid / epoxy curing system, etc., is included as a film-forming resin. Furthermore, a two-component clear coating with isocyanate as a crosslinking agent is more preferred. In particular, the isocyanate of the clear coating penetrates and hardens into the coating layer formed by the above-mentioned silver coating composition, forming a multi-layer coating with excellent water resistance. The clear coating may be solvent-based or water-based.

[0061] The clear coating composition may contain a viscosity control agent. Examples of viscosity control agents include crosslinked or non-crosslinked resin particles, swollen dispersions of fatty acid amides, polyamide-based substances such as amide fatty acids and phosphates of long-chain polyaminoamides, polyethylene-based substances such as colloidal swollen dispersions of oxidized polyethylene, organic acid smectite clay, and organic bentonite-based substances such as montmorillonite.

[0062] The clear coating described above preferably contains a viscosity control agent to ensure ease of application. Generally, a viscosity control agent exhibiting thixotropy can be used. For example, those listed in the description of the silver coating composition above can be used. Additionally, a curing catalyst, surface modifier, etc., may be included as needed.

[0063] The coating film formation method of the present invention can be advantageously used on various substrates, such as metals, plastics, and foams, particularly on metal surfaces and castings, but is especially suitable for use on metal products that can be coated with cationic electrodeposition.

[0064] Examples of the above-mentioned metal products include iron, copper, aluminum, tin, zinc, and alloys containing these metals. Specifically, examples include automobile bodies and parts for passenger cars, trucks, motorcycles, buses, etc. These metals are particularly preferably those that have been chemically treated with phosphates, chromates, etc.

[0065] Furthermore, the substrate used in the coating film formation method of the present invention may have an electrodeposited coating film and an intermediate coating film already formed on a chemically treated steel plate. Cationic and anionic electrodeposited coatings can be used to form the electrodeposited coating film, but a cationic electrodeposited coating composition is preferred because it provides a laminated coating film with excellent corrosion resistance. The intermediate coating paint may be solvent-based or water-based.

[0066] As the intermediate coating paint that forms the intermediate coating film, gray-colored melamine-curing or isocyanate-curing paints with carbon black and titanium dioxide as the main pigments are used. Furthermore, paints that match the hue of the topcoat or combinations of various coloring pigments can also be used.

[0067] Coating Film Formation Method In the coating film formation method of the present invention, a base coating film formed by applying the above-mentioned silver coating composition to a substrate (for example, an automobile body) and a clear coating film formed by applying a clear coating are applied using an electrostatic coating machine, and the base coating film and the clear coating film are formed by wet-on-wet application.

[0068] In the present invention, when applying a silver-colored paint composition to a substrate, an electrostatic coating machine is used to improve workability and appearance. Examples of such electrostatic coating machines include air electrostatic spray coating machines, such as those called "React Guns," or rotary atomizing electrostatic coating machines, commonly known as "μμ (micro-micro) Bells," "μ (micro) Bells," or "Meta Bells." Preferably, two to three-stage coating is used, and a coating film can also be formed by a coating method that combines air electrostatic spray coating with a rotary atomizing electrostatic coating machine, etc.

[0069] In the present invention, the film thickness of the dried coating film formed by the silver coating composition varies depending on the desired application, but the dried film thickness can be set to 0.5 to 35 μm, preferably 7 to 25 μm. If the film thickness of the base coating film formed from the silver coating composition exceeds 35 μm, the clarity may decrease, or unevenness, blotches, or runs may occur in the coating film. If it is less than 0.5 μm, the substrate concealment will be insufficient, and film breakage (a state in which the coating film is discontinuous) may occur, which is undesirable.

[0070] In the coating film forming method of the present invention, a clear coating film is further formed by applying a clear coating paint wet-on-wet onto an uncured base coating film.

[0071] In the coating film forming method of the present invention, the clear coating film applied after the base coating film is formed is formed to smooth out and protect against irregularities caused by the base coating film, as well as glitter that occurs when a glossy pigment is included. Specifically, as a coating method, it is preferable to form the coating film using a rotary atomizing electrostatic coating machine such as the μμBell or μBell mentioned above.

[0072] The dry film thickness of the clear coating formed by the above clear paint is generally preferably about 10 to 80 μm, and more preferably about 20 to 60 μm. If it exceeds the upper limit, defects such as blotches or sagging may occur during painting, and if it falls below the lower limit, the unevenness of the substrate cannot be concealed.

[0073] The laminated coating obtained as described above is cured simultaneously by a process known as two-coat, one-bake coating, but it is preferable to include a step after the base coat in which moisture is evaporated at 40 to 100°C for 1 to 10 minutes.

[0074] By setting the curing temperature for the above-mentioned laminated coating film to 80 to 180°C, preferably 120 to 160°C, a cured coating film with a high degree of crosslinking can be obtained. If the temperature exceeds the upper limit, the coating film becomes hard and brittle, and if it is below the lower limit, curing is insufficient. The curing time varies depending on the curing temperature, but 10 to 30 minutes at 120 to 160°C is appropriate.

[0075] The film thickness of the laminated coating formed by the present invention is often 30 to 300 μm, preferably 50 to 250 μm. If the upper limit is exceeded, the film properties such as thermal cycling will decrease, and if it falls below the lower limit, the strength of the film itself will decrease.

[0076] The present invention also provides a method for improving the abrasion resistance of a base coating film formed with a silver-colored paint composition. Specifically, the present invention provides a method for forming a silver-colored coating film by sequentially forming a silver-colored paint composition and a clear coating on a substrate using an electrostatic coating machine in a wet-on-wet manner, and then curing both coating films at once. In this method, the abrasion resistance of the silver-colored coating film can be improved by controlling the silver-colored paint composition to contain flaky aluminum pigment, and by controlling the content of flaky aluminum pigment with a molybdenum-treated surface to 30% by mass or more relative to the total flaky aluminum pigment. In the present invention, the amount of abrasion measured by the abrasion test method described in the following examples will be 45 mg or less, more preferably 40 mg or less. If the amount of abrasion exceeds 45 mg, it will result in the exposure of the substrate or steel plate, leading to defects such as reduced aesthetic appeal and rust formation. Although the amount of abrasion is measured only on the silver-colored coating film in the examples, the amount of abrasion will naturally be reduced even in multi-layer coating films with a clear layer.

[0077] The present invention will be described in detail below with reference to specific examples, but the present invention is not limited to the following examples. In the following, "parts" means "parts by weight" or "parts by mass".

[0078] Manufacturing Example 1 330 g of deionized water was added to a reaction vessel for the manufacture of hydroxyl group-containing acrylic resin emulsion A, and the temperature was raised to 80°C while mixing and stirring in a nitrogen atmosphere. Then, 3% of a monomer emulsion consisting of 11.25 parts acrylic acid, 139 parts n-butyl acrylate, 75 parts methyl methacrylate, 187 parts n-butyl methacrylate, 75 parts 2-ethylhexyl methacrylate, 150 parts 2-hydroxyethyl methacrylate, 112 parts styrene, 11.2 parts thiocalcol 20 (n-dodecyl mercaptan, manufactured by Kao Corporation, 100% active ingredient), 74.3 parts latemul PD-104 (emulsifier, manufactured by Kao Corporation, 20% active ingredient), and 300 parts deionized water, along with 30% of an initiator solution consisting of 2.63 parts ammonium persulfate and 90 parts deionized water, were added dropwise to the reaction vessel in parallel over 15 minutes. After the dropwise addition was complete, the mixture was aged at the same temperature for 15 minutes. Furthermore, the remaining monomer emulsion and initiator solution were added dropwise to the reaction vessel in parallel over a period of 180 minutes. After the dropwise addition was complete, the mixture was aged at the same temperature for 1 hour. Next, the mixture was cooled to 40°C and filtered through a 200-mesh filter to obtain a hydroxyl group-containing acrylic resin emulsion A with an average particle size of 200 nm, a non-volatile content of 49%, a solids acid value of 15 mg KOH / g, and a hydroxyl value of 85 mg KOH / g.

[0079] Production Example 2 In a reaction vessel for the production of water-soluble acrylic resin B, 23.89 parts by mass of tripropylene glycol methyl ether and 16.11 parts by mass of propylene glycol methyl ether were added, and the mixture was heated to 105°C while being mixed and stirred under a nitrogen atmosphere. Next, a monomer mixture was prepared containing 13.1 parts by mass of methyl methacrylate, 68.4 parts by mass of ethyl acrylate, 11.6 parts by mass of 2-hydroxyethyl methacrylate, and 6.9 parts by mass of methacrylic acid. An initiator solution consisting of 100 parts by mass of this monomer mixture, 10.0 parts by mass of tripropylene glycol methyl ether, and 1 part by mass of tert-butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel in parallel over a period of 3 hours. After the dropwise addition was completed, the mixture was aged at the same temperature for 0.5 hours.

[0080] Furthermore, an initiator solution consisting of 5.0 parts by mass of tripropylene glycol methyl ether and 0.3 parts by mass of tert-butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel over 0.5 hours. After the addition was complete, the mixture was aged at the same temperature for 2 hours.

[0081] Using a solvent removal apparatus, 16.1 parts by mass of the solvent were removed by distillation under reduced pressure (70 torr) at 110°C. Then, 204 parts by mass of deionized water and 7.1 parts by mass of dimethylaminoethanol were added to obtain a solution of water-soluble acrylic resin B. The non-volatile content of the obtained water-soluble acrylic resin B solution was 30% by mass, the number average molecular weight was 10,000, the solid content acid value was 40 mg KOH / g, and the hydroxyl value was 50 mg KOH / g.

[0082] Production Example 2 In a reactor for the production of hydroxyl group-containing polyester resin C, 25.6 parts isophthalic acid, 22.8 parts phthalic anhydride, 5.6 parts adipic acid, 19.3 parts trimethylolpropane, 26.7 parts neopentyl glycol, 17.5 parts ε-caprolactone, and 0.1 parts dibutyltin oxide were added, and the mixture was heated to 170°C while being mixed and stirred. The reaction mixture was then heated to 220°C over 3 hours, while removing the water produced by the condensation reaction until the acid value reached 8. Next, 7.9 parts trimellitic anhydride was added to the reactor, and the mixture was reacted at 150°C for 1 hour to obtain a polyester resin with an acid value of 40. Furthermore, the obtained polyester resin was cooled to 100°C, and then 11.2 parts butyl cellosolve was added and stirred until homogeneous. Subsequently, the polyester resin was cooled to 60°C, and then 98.8 parts ion-exchanged water and 5.9 parts dimethylethanolamine were added. This yielded a hydroxyl group-containing polyester resin C with a solid content of 50% by mass. The solid content acid value of the hydroxyl group-containing polyester resin C was 40 mg KOH / g, the solid content hydroxyl value was 110 mg KOH / g, and the number average molecular weight was 2,000.

[0083] Manufacturing Example 3: 23 parts of methoxypropanol were added to a reaction vessel for the production of phosphate-containing acrylic resin D, and the temperature was raised to 120°C while mixing and stirring under a nitrogen atmosphere. Next, 22 parts of a solution prepared by dissolving 15 parts of Phosmer PP (acid phosphooxyhexa(oxypropylene) monomethacrylate, manufactured by Unichemical Co., Ltd.) in 7 parts of methoxypropanol, a mixed solution of 12.3 parts of 2-ethylhexyl acrylate, 7.0 parts of 2-hydroxyethyl methacrylate, 7.5 parts of lauryl methacrylate, 4.4 parts of styrene, and 3.8 parts of methacrylic acid, and an initiator solution consisting of 4.5 parts of methoxypropanol and 0.9 parts of azobisisobutyronitrile were added dropwise to the reaction vessel in parallel over a period of 3 hours. After the dropwise addition was complete, the mixture was aged at the same temperature for 0.5 hours.

[0084] Furthermore, an initiator solution consisting of 0.5 parts methoxypropanol and 0.5 parts azobisisobutyronitrile was added dropwise to the reaction vessel over 0.5 hours. After the dropwise addition was complete, the mixture was aged at the same temperature for 1.5 hours, and then 13.7 parts methoxypropanol was added to obtain a phosphate-containing acrylic resin D with a solid content of 50% by weight, a solid content acid value of 61, a hydroxyl value of 60, and a number average molecular weight of 5000.

[0085] Example 1 Hydroxyl group-containing acrylic resin emulsion A: 123.7 parts, water-soluble acrylic resin B: 16.1 parts, hydroxyl group-containing polyester resin C: 21.2 parts, phosphate group-containing acrylic resin D: 20 parts, polyurethane resin (Neoretz R9603, manufactured by Covestro Japan Co., Ltd.): 31.5 parts, polyol resin (Primepol PX-1000, manufactured by Sanyo Chemical Industries, Ltd.): 20 parts, melamine resin (Cymel 327; manufactured by Allnex): 42.8 parts, coloring pigment (Raven 5000 Ultra III, commercially available from Birla Carbon: 3.98 parts, CHROMOFINE BLUE 5000P, commercially available from Dainichi Seika Kogyo Co., Ltd.: 1.67 parts, Hostaperm Red, commercially available from Clariant Chemicals Co., Ltd.) A silver paint composition was prepared by mixing 1.25 parts of P2GL-WD10, 17.5 parts of untreated aluminum pigment (aluminum paste A: 4690NS, commercially available from Toyo Aluminum Co., Ltd.; the surface of the aluminum pigment was untreated), 7.5 parts of molybdenum-treated flaky aluminum pigment (aluminum paste B: WL-4690, commercially available from Toyo Aluminum Co., Ltd.), and 45.0 parts of the organic solvent EHG (2-ethylhexyl glycol: manufactured by Shoei Chemical Co., Ltd.). The viscosity of the silver paint composition measured with a B-type viscometer (manufactured by Toki Sangyo Co., Ltd., model TVB10, single-cylinder rotary viscometer) at a paint temperature of 20°C and 60 rpm was 850 mPa·s, and the solid content was 25% by mass.

[0086] Examples 2-5 and Comparative Examples 1-9: Silver paint compositions were prepared in the same manner as in Example 1, except that the components and mixing ratios shown in Table 2 were used.

[0087] The silver coating compositions obtained in the examples and comparative examples were applied to the substrate using Metabel. After applying the silver coating compositions, the substrate was heated at 140°C for 20 minutes to obtain a coated plate. The film thickness of the silver coating was 15 μm.

[0088] A zinc phosphate treated steel sheet with a cured electrodeposited coating was prepared as the substrate. The cured electrodeposited coating was formed by electrodepositing "Powernics," a cationic electrodeposition coating composition manufactured by Nippon Paint Automotive Coatings, Inc., onto the zinc phosphate treated steel sheet to a dry film thickness of 20 μm, and then heating it at 160°C for 30 minutes.

[0089] <Wear Amount> The amount of wear was measured on the obtained painted objects using the following wear test method. The measured wear amounts were classified as follows and are listed in Table 2. Evaluation ○: Meets the standard value: Wear amount of 45 mg or less △: Slightly below the standard value: Wear amount of more than 45 mg but less than 65 mg ×: Significantly below the standard value: Wear amount of 65 mg or more

[0090] Abrasion Test Method (1) Measure the weight of the test coated plate. (2) Attach the test coated plate to the test stand (load arm) of the JSPS-type friction fastness tester via an abrasive element as shown in Figure 1. (3) Attach a 15 mm x 15 mm soft polyurethane foam (PU102525, manufactured by Inoac Co., Ltd.) and waterproof abrasive paper (DCCS#180, manufactured by Sankyo Rika Kagaku Co., Ltd.) to the abrasive element using double-sided adhesive tape, as shown in Figure 2, and bring the test coated plate and waterproof abrasive paper into contact. (4) Perform friction 200 times back and forth under the test conditions described in Table 1 below. (5) Remove the friction-treated test coated plate from the test stand and remove any shavings or abrasive residue with a brush. (6) Measure the weight of the test coated plate. (7) The amount of weight change before and after the abrasion test, expressed by the following formula, is defined as the amount of abrasion. Abrasion (mg) = Weight of coated plate before test (mg) - Weight of coated plate after test (mg)

[0091]

[0092] <Hue> Using a multi-angle spectrophotometer MA-68 (manufactured by X-Rite Co., Ltd.), the spectral reflectance of light irradiated onto the multi-layer coating at a 45-degree angle was measured relative to the specularly reflected light at a receiving angle of 15 degrees. Next, the lightness L* (L value) in the L*a*b* color system (CIE 1976 L*a*b* color space) was calculated from the spectral reflectance. A higher L value indicates better metallic tone. The following criteria were used for evaluation, and the results are listed in Table 2.

[0093] ○: L value is 110 or higher △: L value is 105 or higher but less than 110 ×: L value is less than 105

[0094]

[0095] In Examples 1 to 5, the content of molybdenum-treated flaky aluminum pigment was 30% by mass or more, and the evaluation of wear amount and hue was excellent (rated ○). In Comparative Example 1, since it did not contain phosphate-containing acrylic resin or molybdenum-treated flaky aluminum pigment, both the wear amount and hue were poorly evaluated. In Comparative Example 2, since it did not contain phosphate-containing acrylic resin and the content of molybdenum-treated flaky aluminum pigment was low (15.1% by mass), both the wear amount and hue were poorly evaluated, similar to Comparative Example 1. In Comparative Example 3, the content of molybdenum-treated flaky aluminum pigment was a predetermined amount, but it did not contain phosphate-containing acrylic resin, so both the wear amount and hue were poorly evaluated. In Comparative Examples 4 to 6, the content of molybdenum-treated flaky aluminum pigment was a predetermined amount, but it did not contain phosphate-containing acrylic resin, so both the wear amount and hue were poorly evaluated. Comparative Example 7 contains a phosphate-containing acrylic resin but does not contain a molybdenum-treated flaky aluminum pigment, so while the hue is good, the amount of wear is reduced. Comparative Example 8 contains a phosphate-containing acrylic resin but contains a small amount of molybdenum-treated flaky aluminum pigment, similar to Comparative Example 2, so while the hue is good, the amount of wear is reduced. Comparative Example 9 contains only a molybdenum-treated flaky aluminum pigment (100% by mass), so while the amount of wear is good, the hue tends to be inferior.

[0096] Examples 6-10 Preparation of Intermediate Coat Paint Compositions Intermediate coat paint compositions were prepared by mixing a film-forming resin, a hardener, and a pigment (108.45 parts of commercially available Typeque CR-97 from Ishihara Sangyo Co., Ltd. and 12.53 parts of commercially available CHROMOFINE BLUE 5000P from Dainichi Seika Kogyo Co., Ltd.) according to the components and mixing ratios shown in Table 3. The viscosity of the intermediate coat composition measured with a B-type viscometer (manufactured by Toki Sangyo Co., Ltd., model TVB10, single-cylinder rotary viscometer) at a paint temperature of 20°C and 60 rpm was 850 mPa·s, and the solid content was 40% by mass.

[0097] Preparation of Clear Coating: PU Excel O-2100 (manufactured by Nippon Paint Automotive Coatings Co., Ltd., a two-component clear coating) was prepared as the clear coating. The clear coating contains a hydroxyl group-containing acrylic resin as the main component and a polyisocyanate compound as the hardener. The viscosity of the clear coating, measured at 20°C using a No. 4 Ford cup, was 25 seconds.

[0098] The intermediate coating compositions obtained in Examples 6 to 10 were each applied to the substrate using Metabel. The silver coating compositions obtained in Examples 1 to 5 were applied to the uncured intermediate coating compositions using Metabel, and then dried at 80°C for 5 minutes. The clear coating was applied to the uncured multi-layer coating using Metabel, and then heated at 140°C for 20 minutes to obtain a substrate with a multi-layer coating. The thickness of the intermediate coating was 6 μm, the thickness of the silver coating was 6 μm, and the thickness of the clear coating was 30 μm. The intermediate coating composition in Example 6 was coated with the silver coating composition of Example 1, the silver coating composition of Example 2 in Example 7, and subsequently the coating of Example 3 in Example 8, the coating of Example 4 in Example 9, and the coating of Example 5 in Example 10.

[0099]

[0100] In Examples 6 to 10, even when the silver paint compositions of Examples 1 to 5 were used as 3WET-type base paints, good hue results were obtained.

Claims

1. A paint composition for silver coating containing a hydroxyl group-containing acrylic resin, a curing agent, a phosphate group-containing acrylic resin, and a flake-shaped aluminum pigment, wherein the flake-shaped aluminum pigment is a combination of a flake-shaped aluminum pigment whose surface is molybdenum-treated and a flake-shaped aluminum pigment whose surface is not molybdenum-treated, and the flake-shaped aluminum pigment contains 30% by mass or more of the flake-shaped aluminum pigment whose surface is molybdenum-treated relative to the total flake-shaped aluminum pigment.

2. The abrasion-resistant silver paint composition according to claim 1, wherein the flake aluminum pigment contains a flake aluminum pigment whose surface has been treated with molybdenum in an amount of 30 to 95% by mass relative to the total flake aluminum pigment.

3. A method for forming a silver-colored coating film, comprising sequentially forming a silver-colored coating composition and a clear coating on a substrate using an electrostatic coating machine in a wet-on-wet manner, and then curing both coating films at once, wherein the silver-colored coating composition contains a hydroxyl group-containing acrylic resin, a curing agent, a phosphate group-containing acrylic resin, and a flake-shaped aluminum pigment, the flake-shaped aluminum pigment is a combination of a flake-shaped aluminum pigment with a molybdenum-treated surface and a flake-shaped aluminum pigment without a molybdenum-treated surface, and the flake-shaped aluminum pigment contains 30% by mass or more of the molybdenum-treated flake-shaped aluminum pigment relative to the total flake-shaped aluminum pigment.

4. The method for forming a highly wear-resistant silver coating film according to claim 3, wherein the flake aluminum pigment contains a flake aluminum pigment whose surface has been treated with molybdenum in an amount of 30 to 95% by mass relative to the total flake aluminum pigment.

5. A method for forming a silver-colored coating film by sequentially forming a silver-colored coating composition and a clear coating on a substrate using an electrostatic coating machine in a wet-on-wet manner, and then curing both coating films at once, characterized in that the silver-colored coating composition contains a hydroxyl group-containing acrylic resin, a curing agent, a phosphate group-containing acrylic resin, and a flake-shaped aluminum pigment, the flake-shaped aluminum pigment is a combination of a flake-shaped aluminum pigment with a molybdenum-treated surface and a flake-shaped aluminum pigment without a molybdenum-treated surface, and the flake-shaped aluminum pigment is controlled to contain 30% by mass or more of the flake-shaped aluminum pigment with a molybdenum-treated surface relative to the total flake-shaped aluminum pigment.

6. A method for improving the abrasion resistance of a silver-colored coating film according to claim 5, wherein the flake-shaped aluminum pigment contains a molybdenum-treated flake-shaped aluminum pigment in an amount of 30 to 95% by mass relative to the total flake-shaped aluminum pigment.

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