Method for forming multi-layer coating film

Abstract

A method for forming a multi-layer coating film is provided, which allows a coating film with good appearance and high hardness to be obtained. [Solution] A method for forming a multilayer coating film, wherein a base coating composition (Y) comprises a hydroxyl-containing acrylic resin (y1) having a weight-average molecular weight of 13,000 or more and 35,000 or less and a glass transition temperature of 20°C or more and less than 80°C, and a polyol compound (y2) having a hydroxyl value of more than 200 mgKOH / g and 1,000 mgKOH / g or less and a molecular weight of 100 or more and 1,000 or less, wherein the solid content of the hydroxyl-containing acrylic resin (y1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y), and the clear coating composition (Z) comprises a polyol compound (y2) having a hydroxyl value of 90 mgKOH / g or more and 1,000 mgKOH / g or less. The clear coating composition (Z) comprises a hydroxyl-containing acrylic resin (z1) having a weight-average molecular weight of 4,000 to 6,000 and a glass transition temperature of 15°C to 100°C, a hydroxyl-containing polyester resin (z2), a polyisocyanate compound (z3), a metal catalyst (z4) containing at least one metal selected from the group consisting of zinc, bismuth and tin, and an organic amine catalyst (z5) having an amidine group, wherein the solid content of the hydroxyl-containing acrylic resin (z1) is 50 parts by mass to 95 parts by mass per 100 parts by mass of the resin solids of the clear coating composition (Z).

Description

[Technical Field]

[0001] The present invention relates to a method for forming a multi-layer coating film. [Background technology]

[0002] On the substrates that make up automobiles, a multi-layer paint film is usually applied, consisting of a base paint film and a clear paint film on top of it, for the purpose of imparting design and durability, etc. From the viewpoint of energy conservation, the base paint film and the clear paint film are cured simultaneously (2-coat 1-bake method), eliminating the heating process.

[0003] As a coating composition used in the above method, Patent Document 1 discloses a clear coating composition containing an organometallic catalyst consisting of a metal compound and an amidine compound. According to Patent Document 1, this clear coating composition cures at low temperatures in a short time. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2013 / 047209 Summary of the Invention [Problem to be solved by the invention]

[0005] In the two-coat one-bake method, a clear coating film is typically formed using a two-component clear coating composition containing a polyisocyanate compound as a curing agent. In this case, the polyisocyanate compound is expected to penetrate from the uncured clear coating film into the uncured base coating film, thereby improving the crosslink density of the base coating film.

[0006] When an organometallic catalyst consisting of a metal compound and an amidine compound is blended into a clear coating composition as in Patent Document 1, the curing time of the uncured clear coating film is shortened, making it difficult for the polyisocyanate compound to sufficiently penetrate from the uncured clear coating film into the uncured base coating film, resulting in insufficient crosslink density in the base coating film and reduced coating film properties.

[0007] The present invention has been made to solve the above-mentioned problems of the prior art, and has as its object to provide a method for forming a multi-layer coating film which can provide a coating film with good appearance and high hardness. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention provides the following aspects. [1] Applying at least one layer of base coating composition (Y) to a substrate to form at least one layer of uncured base coating film; Applying a clear coating composition (Z) onto the uncured base coating film to form a clear coating film; A method for forming a multilayer coating film, comprising heating and curing the uncured base coating film and the uncured clear coating film, The base coating composition (Y) A hydroxyl group-containing acrylic resin (y1) having a weight average molecular weight of 13,000 or more and 35,000 or less and a glass transition temperature of 20°C or more and less than 80°C, and a polyol compound (y2) having a hydroxyl value of more than 200 mgKOH / g and not more than 1000 mgKOH / g and a molecular weight of 100 or more and not more than 1000; the solid content of the hydroxyl group-containing acrylic resin (y1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y); The clear coating composition (Z) a hydroxyl group-containing acrylic resin (z1) having a hydroxyl value of 90 mgKOH / g or more and 190 mgKOH / g or less, a weight average molecular weight of 4000 or more and 6000 or less, and a glass transition temperature of 15°C or more and 100°C or less; Hydroxyl group-containing polyester resin (z2), Polyisocyanate compound (z3), a metal catalyst (z4) containing at least one metal selected from the group consisting of zinc, bismuth, and tin; and an organic amine catalyst (z5) having an amidine group, The solid content of the hydroxyl group-containing acrylic resin (z1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the clear coating composition (Z). A method for forming a multi-layer coating film. [2] The method for forming a multi-layer coating film according to the above item [1], wherein the polyol compound (y2) has two hydroxyl groups and an alicyclic hydrocarbon group. [3] The method for forming a multi-layer coating film according to the above [1] or [2], wherein the hydroxyl group-containing acrylic resin (y1) has a hydroxyl value of 30 mgKOH / g or more and 100 mgKOH / g or less. [4] The method for forming a multilayer coating film according to [1] or [2] above, wherein the solid content of the polyol compound (y2) is 2 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y). [5] The method for forming a multi-layer coating film according to [1] or [2] above, wherein the hydroxyl group-containing polyester resin (z2) has a hydroxyl value of 250 mgKOH / g or more and less than 500 mgKOH / g, and a weight average molecular weight of 1500 or more and 2500 or less. [6] The method for forming a multi-layer coating film according to the above [1] or [2], wherein the base coating composition (Y) has a solids concentration of 30% by mass or more and 60% by mass or less. [7] The method for forming a multi-layer coating film according to the above [1] or [2], wherein the clear coating composition (Z) has a solids concentration of 45% by mass or more and 60% by mass or less. [8] The method for forming a multilayer coating film according to [1] or [2] above, wherein the substrate comprises a resin portion containing at least one resin selected from the group consisting of ABS resin, PC-PBT resin, PC-ABS resin, polycarbonate resin, vinyl chloride resin, polyamide resin, and polypropylene resin. [9] The method for forming a multi-layer coating film according to the above [1] or [2], wherein the organic amine catalyst (z5) has a heterocycle containing a nitrogen atom.

[10] The method for forming a multilayer coating film according to [1] or [2] above, wherein the metal catalyst (z4) comprises at least one selected from the group consisting of an acylate compound, an alkoxide compound, a chelate compound, and an oxide compound of zinc, bismuth, or tin. [Effects of the Invention]

[0009] According to the present invention, there is provided a method for forming a multi-layer coating film, which can provide a coating film with good appearance and high hardness. DETAILED DESCRIPTION OF THE INVENTION

[0010] In this embodiment, a polyol compound (y2) with a low molecular weight and a high hydroxyl value is used as a component for forming the base coating film. Because the polyol compound (y2) has a low molecular weight, the base coating composition (Y) (hereinafter also referred to as the uncured base coating film) has a low viscosity. Therefore, the polyisocyanate compound (z3) can easily penetrate from the uncured clear coating film (hereinafter also referred to as the clear coating composition) into the uncured base coating film. As a result, the crosslink density of the base coating film increases. Additionally, because the polyol compound (y2) has a high hydroxyl value, the crosslink density increases, resulting in a multilayer coating film with excellent physical properties. Examples of the physical properties of the coating film include water resistance, hardness, tackiness, and adhesion.

[0011] The clear coating composition contains an organic amine catalyst (z5) having an amidine group, so the curing rate of the clear coating composition is fast. However, because the viscosity of the base coating composition (Y) is sufficiently low, it is thought that the polyisocyanate compound (z3) penetrates into the uncured base coating film before the clear coating composition cures. The effect of the polyol compound (y2) is more pronounced when the curing rate of the clear coating composition is fast.

[0012] First, the base coating composition (Y) and the clear coating composition (Z) will be explained.

[0013] (Y) Base coating composition The base coating composition (Y) according to this embodiment contains a hydroxyl-containing acrylic resin (y1) and a polyol compound (y2) other than the hydroxyl-containing acrylic resin (y1).

[0014] The solids concentration of the base coating composition (Y) is, for example, 30% by mass or more and 60% by mass or less. When the solids concentration of the base coating composition (Y) is within the above range, curing can be performed in a shorter time at a lower temperature, contributing to energy savings. The solids concentration of the base coating composition (Y) may be 35% by mass or more. The solids concentration of the base coating composition (Y) may be 50% by mass or less.

[0015] Solid content is also called nonvolatile content. The solid content of a material is, for example, all components of the material excluding the solvent. The solid content concentration is calculated by dividing the total mass of the solids excluding the solvent by the total mass of the material. The solid content concentration can also be calculated from the residue when the material is heated to 140°C in accordance with JIS K 5601-1-2 Heat Residue Measurement Method.

[0016] The base coating composition (Y) has a high solids concentration within the above range, but its viscosity is low. The viscosity of a base coating composition (Y) with a solids concentration of 35% by mass, measured by the flow cup method at 23°C, is, for example, 20 seconds or less. This allows the polyisocyanate compound (z3) to more easily penetrate from the clear coating composition into the uncured base coating film.

[0017] The above viscosity is that of the base coating composition (Y) immediately after preparation, and is assumed to be the viscosity when actually applied. Hereinafter, this viscosity will be referred to as the application viscosity η0.

[0018] The viscosity of the base coating composition (Y) is measured at 23°C using a No. 4 Ford cup in accordance with "3. Flow cup method" of JIS K5600-2-2:1999. The viscosity is the average value of the viscosities of five different coating compositions with the same composition. The coating viscosity η0 of the clear coating composition (Z) is measured in the same manner.

[0019] (y1) Hydroxyl group-containing acrylic resin The hydroxyl group-containing acrylic resin (y1) is a resin (film-forming component) that serves as the base of the base coating film. The hydroxyl group-containing acrylic resin (y1) reacts with a curing agent such as a polyisocyanate compound and / or a melamine resin to form a crosslinked structure.

[0020] A hydroxyl-containing acrylic resin has multiple acryloyl groups and one or more (typically two or more) hydroxyl groups in one molecule. "Acrylic resin" is obtained by polymerizing at least one monomer selected from the group consisting of acrylic acid and its esters, and methacrylic acid and its esters.

[0021] The weight-average molecular weight of the hydroxyl-containing acrylic resin (y1) is 13,000 or more and 35,000 or less. When the weight-average molecular weight of the hydroxyl-containing acrylic resin (y1) is 13,000 or more, the hardness and weather resistance of the resulting coating film are improved. When the weight-average molecular weight of the hydroxyl-containing acrylic resin (y1) is 35,000 or less, an excessive increase in viscosity of the base coating composition (Y) is suppressed, and penetration of the polyisocyanate compound (z3) into the uncured base coating film is promoted. The weight-average molecular weight of the hydroxyl-containing acrylic resin (y1) may be 15,000 or more, or may be 20,000 or more. The weight-average molecular weight of the hydroxyl-containing acrylic resin (y1) may be 30,000 or less, or may be 25,000 or less.

[0022] The weight-average molecular weight can be calculated from a chromatogram measured by gel permeation chromatography, using the molecular weight of standard polystyrene as a reference. For example, an HLC-8200 (manufactured by Tosoh Corporation) is used as the gel permeation chromatograph. The measurement conditions using this are as follows: Column: TSgel Super Multipore HZ-M (3 columns) Developing solvent: tetrahydrofuran Column inlet oven: 40℃ Flow rate: 0.35ml Detector: Refractive index detector (RI) Standard polystyrene: PS oligomer kit manufactured by Tosoh Corporation

[0023] The glass transition temperature (Tg) of the hydroxyl-containing acrylic resin (y1) is 20°C or higher and lower than 80°C. When the Tg of the hydroxyl-containing acrylic resin (y1) is 20°C or higher, the stain resistance, scratch resistance, and hardness of the resulting coating film are improved. When the Tg of the hydroxyl-containing acrylic resin (y1) is lower than 80°C, the quick-drying properties of the base coating composition (Y) are improved. The Tg of the hydroxyl-containing acrylic resin (y1) may be 40°C or higher. The Tg of the hydroxyl-containing acrylic resin (y1) may be 70°C or lower, or may be 50°C or lower.

[0024] The glass transition temperature (Tg) is determined using a differential scanning calorimeter (DSC) by the following method. A hydroxyl-containing acrylic resin (y1) is subjected to the following steps: (1) a step of increasing the temperature from 20°C to 150°C at a rate of 10°C / min; (2) a step of decreasing the temperature from 150°C to -50°C at a rate of 10°C / min; and (3) a step of increasing the temperature from -50°C to 150°C at a rate of 10°C / min after step 2. The value obtained from the chart during the temperature increase in step 3 is the Tg of the hydroxyl-containing acrylic resin (y1). For example, a thermal analyzer SSC5200 (manufactured by Seiko Electronics) is used as the DSC.

[0025] The hydroxyl value (OHV) of the hydroxyl-containing acrylic resin (y1) is, for example, 30 mgKOH / g or more and 100 mgKOH / g or less. When the hydroxyl value is 30 mgKOH / g or more, the crosslinking density tends to be high. When the hydroxyl value of the hydroxyl-containing acrylic resin (y1) is 100 mgKOH / g or less, hydrophilization of the coating film is suppressed, and the water resistance of the multilayer coating film tends to be improved. The hydroxyl value may be 30 mgKOH / g or more, or 40 mgKOH / g or more. The hydroxyl value may be 100 mgKOH / g or less, 70 mgKOH / g or less, or 60 mgKOH / g or less.

[0026] The hydroxyl value can be determined by the neutralization titration method described in JIS K 0070 using an aqueous potassium hydroxide solution.

[0027] The hydroxyl group-containing acrylic resin (y1) may be used alone or in combination of two or more.

[0028] The solid content of the hydroxyl-containing acrylic resin (y1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y). When the solid content of the hydroxyl-containing acrylic resin (y1) is 50 parts by mass or more, the hardness of the base coating film is improved. When the solid content of the hydroxyl-containing acrylic resin (y1) is 95 parts by mass or less, a sufficient amount of polyol compound (y2) can be blended, allowing the polyisocyanate compound to easily penetrate through the uncured clear coating film. The solid content of the hydroxyl-containing acrylic resin (y1) may be 60 parts by mass or more, or may be 70 parts by mass or more. The solid content of the hydroxyl-containing acrylic resin (y1) may be 90 parts by mass or less, or may be 88 parts by mass or less.

[0029] Examples of raw material monomers for the hydroxyl group-containing acrylic resin (y1) include acrylic acid hydroxy esters such as 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate; and methacrylic acid hydroxy esters such as 2-hydroxyethyl methacrylate and 4-hydroxybutyl methacrylate. Furthermore, if necessary, acrylic acid; acrylic acid esters such as methyl acrylate, butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, and isobornyl acrylate; methacrylic acid; methacrylic acid esters such as methyl methacrylate, butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, and isobornyl methacrylate; and ethylenically unsaturated monomers having an aromatic ring such as styrene. These may be used alone or in combination of two or more. Commercially available hydroxyl group-containing acrylic resins (y1) may also be used.

[0030] (y2) Polyol compound The polyol compound (y2) is also a film-forming component. The polyol compound (y2) reacts with the polyisocyanate compound (z3) to form a crosslinked structure.

[0031] The polyol compound (y2) is a compound other than the hydroxyl-containing acrylic resin (y1) and has two or more hydroxyl groups per molecule. The molecular weight (weight average molecular weight in the case of a polymer) of the polyol compound (y2) is low, ranging from 100 to 1,000. This facilitates penetration of the polyisocyanate compound (z3) from the clear coating composition (Z) into the uncured base coating film. In addition, the hydroxyl value of the polyol compound (y2) is high, ranging from more than 200 mg KOH / g to 1,000 mg KOH / g. Therefore, even a small amount of addition can improve the crosslink density of the base coating composition (Y).

[0032] The molecular weight of the polyol compound (y2) may be 110 or more, or 120 or more. The molecular weight of the polyol compound (y2) may be 800 or less, 500 or less, or 300 or less.

[0033] The hydroxyl value of the polyol compound (y2) may be 300 mgKOH / g or more, or 400 mgKOH / g or more, and may be 950 mgKOH / g or less, or 900 mgKOH / g or less.

[0034] The solid content of the polyol compound (y2) relative to 100 parts by mass of the resin solid content of the base coating composition (Y) is, for example, 2 parts by mass or more and 10 parts by mass or less. When the solid content of the polyol compound (y2) is 2 parts by mass or more, the polyisocyanate compound is more likely to penetrate into the uncured clear coating film. When the solid content of the polyol compound (y2) is 10 parts by mass or less, the drying properties of the base coating composition are likely to be improved. The solid content of the polyol compound (y2) may be 3 parts by mass or more, or may be 5 parts by mass or more. The solid content of the polyol compound (y2) may be 8 parts by mass or less, or may be 7 parts by mass or less.

[0035] The polyol compound (y2) is not particularly limited as long as it is other than the hydroxyl group-containing acrylic resin (y1) and the hydroxyl group-containing polyester resin (z2) and has two or more hydroxyl groups in one molecule. The polyol compound (y2) may have an alicyclic hydrocarbon group, which facilitates the improvement of the physical properties (particularly hardness and weather resistance) of the resulting coating film. From the same viewpoint, the polyol compound (y2) may have two hydroxyl groups.

[0036] At least one hydroxyl group of the polyol compound (y2) may be directly bonded to the alicyclic hydrocarbon group or may be bonded via a hydrocarbon group having 1 or 2 carbon atoms. The alicyclic hydrocarbon group may be a 5-membered ring or a 6-membered ring. The alicyclic hydrocarbon group may be a 6-membered ring.

[0037] The polyol compound (y2) may have two hydroxyl groups and an alicyclic hydrocarbon group. Examples of such polyol compounds (y2) include diol compounds such as cyclohexanediol, cyclohexanedimethanol, cyclohexanediethanol, and cyclopentanedimethanol. The arrangement of the two hydroxyl groups of the diol compound is not particularly limited and may be 1,1-, 1,2-, 1,3-, or 1,4-. From the viewpoint of hardness, 1,4-cyclohexanedimethanol, in which the two hydroxyl groups are bonded to cyclohexane via a methylene group, may be used.

[0038] (Other hydroxyl group-containing components) The base coating composition (Y) according to this embodiment may contain other hydroxyl-containing components, such as hydroxyl-containing acrylic resins other than the hydroxyl-containing acrylic resin (y1), hydroxyl-containing polyester resins, and polyol compounds other than the polyol compound (y2). These may be used alone or in combination of two or more.

[0039] Examples of other hydroxyl group-containing acrylic resins include hydroxyl group-containing acrylic resins that satisfy at least one of the following: (a) a weight average molecular weight of less than 13,000, (b) a weight average molecular weight of more than 35,000, (c) a Tg of less than 20° C., and (d) a Tg of 80° C. or higher. Examples of other polyol compounds include polycarbonate polyol resins, polyether polyol resins, and polycaprolactone polyol resins.

[0040] The amount of other hydroxyl group-containing components is desirably small. The solid content of other hydroxyl group-containing components relative to 100 parts by mass of the total solid content of the hydroxyl group-containing components is, for example, 20 parts by mass or less, or may be 15 parts by mass or less, or 10 parts by mass or less, or 5 parts by mass or less, or 3 parts by mass or less, or may be 0 parts by mass.

[0041] (hardening agent) The base coating composition (Y) may contain a curing agent. The curing agent further enhances adhesion between the substrate and the base coating film, or between the base coating film and an intermediate coating film or primer coating film provided on the substrate. The curing agent also improves the coating film properties of the base coating film.

[0042] The curing agent may include other curing agents such as blocked isocyanate compounds, melamine resins, guanamine resins, urea resins, and other amino resins. These may be used alone or in combination of two or more. The content of the curing agent is, for example, 5 to 30 parts by mass per 100 parts by mass of the resin solids content of the base coating composition.

[0043] The blocked isocyanate compound can be prepared by blocking a polyisocyanate compound with a blocking agent. Examples of the polyisocyanate compound include the same compounds as the polyisocyanate compound (z3) described below.

[0044] Examples of blocking agents include monovalent alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenol carbinol, and methylphenyl carbinol; cellosolves such as ethylene glycol monohexyl ether and ethylene glycol mono-2-ethylhexyl ether; polyether-type diols terminated at both ends such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol phenol; polyester-type polyols terminated at both ends obtained from diols such as ethylene glycol, propylene glycol, and 1,4-butanediol and dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, suberic acid, and sebacic acid; phenols such as para-t-butylphenol and cresol; oximes such as dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime, and cyclohexanone oxime; and lactams represented by ε-caprolactam and γ-butyrolactam. As the blocking agent, active hydrogen compounds such as methyl diketone, methyl ketoester and methyl diester compounds, for example, alkyl esters such as acetylacetone, ethyl acetoacetate, diethyl malonate, etc. Also, blocked isocyanates using pyrazole compounds or imidazole compounds as blocking agents may be used.

[0045] The blocking rate of the blocked isocyanate compound is preferably 100%, which improves the storage stability of the base coating composition.

[0046] (pigment) The base coating composition (Y) may contain a pigment, which imparts design properties to the coating film.

[0047] Examples of pigments include color pigments, luster pigments, and extender pigments. Examples of color pigments include organic color pigments such as 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; and inorganic color pigments such as yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These may be used alone or in combination of two or more.

[0048] Examples of luster pigments include metal flakes (aluminum, chromium, gold, silver, copper, brass, titanium, nickel, nickel chromium, stainless steel, etc.), metal oxide flakes, pearl pigments, glass flakes coated with metal or metal oxide, silica flakes coated with metal oxide, graphite, hologram pigments, and cholesteric liquid crystal polymers. These may be used alone or in combination of two or more.

[0049] Examples of extender pigments include calcium carbonate, barium sulfate, clay, and talc, which may be used alone or in combination of two or more.

[0050] The concentration of the total pigments, i.e., the mass ratio (PWC) of the total pigments to 100% by mass of the resin solids content of the base coating composition (Y), is, for example, 0.1% by mass or more and 60% by mass or less. This makes it difficult for the smoothness of the resulting coating film to be impaired. The PWC of each pigment is not particularly limited. The PWC of the bright pigment may be, for example, 1% by mass or more and 40% by mass or less. The PWC of the bright pigment may be 5% by mass or more. The PWC of the bright pigment may be 30% by mass or less.

[0051] (Dilution ingredients) The base coating composition (Y) may contain a diluent component. The base coating composition (Y) is diluted with a diluent component as appropriate, taking into consideration the coating method and the coating environment, such as temperature and humidity. Examples of the diluent component include water and non-aqueous solvents. The base coating composition (Y) according to this embodiment may also contain the non-aqueous solvents used in the production of each component.

[0052] Examples of non-aqueous solvents include aliphatic or alicyclic hydrocarbon solvents such as cyclohexane, methylcyclohexane, cycloheptane, methylcycloheptane, and mineral split; ketone organic solvents such as acetone, acetylacetone, methyl ethyl ketone, methyl-i-butyl ketone, methyl amyl ketone, and cyclohexanone; aromatic hydrocarbon organic solvents such as benzene, toluene, ethylbenzene, propylbenzene, t-butylbenzene, o-xylene, m-xylene, p-xylene, tetralin, and decalin; ester organic solvents such as methyl acetate, ethyl acetate, n-butyl acetate, and aluminum acetate; cellosolve organic solvents such as methyl cellosolve, ethyl cellosolve, n-propyl cellosolve, i-propyl cellosolve, n-butyl cellosolve, i-butyl cellosolve, i-amyl cellosolve, phenyl cellosolve, and benzyl cellosolve; Examples of suitable organic solvents include carbitol-based organic solvents such as tyl carbitol, n-propyl carbitol, i-propyl carbitol, n-butyl carbitol, i-butyl carbitol, i-amyl carbitol, carbitol acetate, phenyl carbitol, and benzyl carbitol; and ether-based organic solvents such as ethylene glycol monoisopropyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-n-butyl ether, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and dioxane. These may be used alone or in combination of two or more.

[0053] (additives) The base coating composition (Y) according to this embodiment may contain other additives commonly used in the coating field, such as ultraviolet absorbers, hindered amine light stabilizers, antioxidants, crosslinked resin particles, viscosity modifiers, surface conditioners, film-forming aids, and rust inhibitors.

[0054] (Preparation of base coating composition) The method for producing the base coating composition (Y) is not particularly limited, and any method known in the art can be used, such as stirring, kneading, or dispersing the above-mentioned components using a disperser, homogenizer, roll, sand grind mill, kneader, or the like.

[0055] (Z) Clear coating composition The clear coating composition (Z) according to this embodiment contains a hydroxyl group-containing acrylic resin (z1), a hydroxyl group-containing polyester resin (z2), a polyisocyanate compound (z3), a metal catalyst (z4), and an organic amine catalyst (z5).

[0056] The solid content concentration of the clear coating composition (Z) is, for example, 45% by mass or more and 60% by mass or less. The above solid content concentration is that of the clear coating composition (Z) to be used for coating. The clear coating composition (Z) to be used for coating is diluted, for example, with a diluting component to a viscosity suitable for coating.

[0057] The solid content concentration of the clear coating composition (Z) may be 50% by mass or more, or 55% by mass or more, and may be 59% by mass or less, or 58% by mass or less.

[0058] The clear coating composition (Z) has a high solids concentration within the above range, but its viscosity is low. The coating viscosity η0 of a clear coating composition (Z) with a solids concentration of 58% by mass, measured by the flow cup method at 23°C, is, for example, 25 seconds or less. The coating viscosity η0 of the clear coating composition (Z) is measured immediately after preparation (specifically, immediately after the curing agent and other components are mixed).

[0059] (z1) Hydroxyl group-containing acrylic resin The hydroxyl group-containing acrylic resin (z1) is a resin (film-forming component) that serves as the base of the clear coating film. The hydroxyl group-containing acrylic resin (z1) reacts with a curing agent such as a polyisocyanate compound to form a crosslinked structure.

[0060] The weight-average molecular weight of the hydroxyl-containing acrylic resin (z1) is 4,000 or more and 6,000 or less. When the weight-average molecular weight of the hydroxyl-containing acrylic resin (z1) is 4,000 or more, the hardness and weather resistance of the resulting coating film are improved. When the weight-average molecular weight of the hydroxyl-containing acrylic resin (z1) is 6,000 or less, an excessive increase in viscosity of the clear coating composition (Z) is suppressed, and penetration of the polyisocyanate compound (z3) into the uncured clear coating film is promoted. The weight-average molecular weight of the hydroxyl-containing acrylic resin (z1) may be 4,200 or more, or 4,300 or more. The weight-average molecular weight of the hydroxyl-containing acrylic resin (z1) may be 5,800 or less, or 5,500 or less.

[0061] The glass transition temperature (Tg) of the hydroxyl-containing acrylic resin (z1) is 15°C or higher and 100°C or lower. When the Tg of the hydroxyl-containing acrylic resin (z1) is 15°C or higher, the stain resistance, scratch resistance, and hardness of the resulting coating film are improved. When the Tg of the hydroxyl-containing acrylic resin (z1) is 100°C or lower, the quick-drying properties of the clear coating composition (Z) are improved. The Tg of the hydroxyl-containing acrylic resin (z1) may be 18°C ​​or higher, or 20°C or higher. The Tg of the hydroxyl-containing acrylic resin (z1) may be 95°C or lower, or 90°C or lower.

[0062] The hydroxyl value (OHV) of the hydroxyl-containing acrylic resin (z1) is 90 mgKOH / g or more and 190 mgKOH / g or less. When the hydroxyl value is 90 mgKOH / g or more, the crosslinking density is high. When the hydroxyl value of the hydroxyl-containing acrylic resin (z1) is 190 mgKOH / g or less, hydrophilization of the coating film is suppressed, and the water resistance of the multilayer coating film is improved. The hydroxyl value may be 100 mgKOH / g or more, or 110 mgKOH / g or more. The hydroxyl value may be 180 mgKOH / g or less, or 170 mgKOH / g or less.

[0063] The acid value of the hydroxyl-containing acrylic resin (z1) may be, for example, 2 mgKOH / g or more and 30 mgKOH / g or less. This can further improve the smoothness of the resulting coating film. Furthermore, when the coating composition is applied onto another uncured coating film, the occurrence of a mixed layer can be suppressed. The acid value of the hydroxyl-containing acrylic resin (z1) may be 3 mgKOH / g or more. The acid value of the hydroxyl-containing acrylic resin (z1) may be 20 mgKOH / g or less, or may be 15 mgKOH / g or less.

[0064] The hydroxyl group-containing acrylic resin (z1) may be used alone or in combination of two or more.

[0065] The solid content of the hydroxyl-containing acrylic resin (z1) relative to 100 parts by mass of the resin solid content of the clear coating composition (Z) is, for example, 50 parts by mass or more and 95 parts by mass or less. When the solid content of the hydroxyl-containing acrylic resin (z1) is 50 parts by mass or more, the hardness of the base coating film is improved. When the solid content of the hydroxyl-containing acrylic resin (z1) is 95 parts by mass or less, a sufficient amount of polyisocyanate compound (z3) can be blended, and the polyisocyanate compound (z3) can easily penetrate into the uncured base coating film. The solid content of the hydroxyl-containing acrylic resin (z1) may be 55 parts by mass or more, or may be 60 parts by mass or more. The solid content of the hydroxyl-containing acrylic resin (z1) may be 90 parts by mass or less, or may be 88 parts by mass or less.

[0066] Examples of raw material monomers for the hydroxyl group-containing acrylic resin (z1) include the same as those for the hydroxyl group-containing acrylic resin (y1).

[0067] (z2) Hydroxyl group-containing polyester resin The hydroxyl group-containing polyester resin (z2) is also a coating film-forming component. The hydroxyl group-containing polyester resin (z2) reacts with the polyisocyanate compound (z3) to form a crosslinked structure. The hydroxyl group-containing polyester resin (z2) has multiple ester bonds and one or more hydroxyl groups.

[0068] The hydroxyl-containing acrylic resin (z1) tends to increase the viscosity of the clear coating composition (Z). On the other hand, the hydroxyl-containing polyester resin (z2) generally has a low viscosity and tends to increase the hydroxyl value. By using the hydroxyl-containing polyester resin (z2) in combination, it is possible to improve the crosslink density while suppressing the increase in viscosity.

[0069] The hydroxyl value of the hydroxyl-containing polyester resin (z2) is, for example, 250 mgKOH / g or more and less than 500 mgKOH / g. When the hydroxyl value is 250 mgKOH / g or more, the crosslinking density can be further increased. When the hydroxyl value is less than 500 mgKOH / g, hydrophilization of the coating film is suppressed, and the water resistance of the clear coating film can be further improved. The hydroxyl value of the hydroxyl-containing polyester resin (z2) may be 260 mgKOH / g or more, or 270 mgKOH / g or more. The hydroxyl value of the hydroxyl-containing polyester resin (z2) may be 480 mgKOH / g or less, or 450 mgKOH / g or less.

[0070] The weight average molecular weight of the hydroxyl group-containing polyester resin (z2) is, for example, 1,500 or more and 2,500 or less. When the weight average molecular weight is 1,500 or more, the hardness and weather resistance of the resulting coating film can be further improved. When the weight average molecular weight is 2,500 or less, excessive viscosity increase of the coating composition can be further suppressed. The weight average molecular weight of the hydroxyl group-containing polyester resin (z2) may be 1,600 or more, or 1,700 or more. The weight average molecular weight of the hydroxyl group-containing polyester resin (z2) may be 2,400 or less, or 2,300 or less.

[0071] From the viewpoint of viscosity, the hydroxyl group-containing polyester resin (z2) may have a hydroxyl value of 250 mgKOH / g or more and less than 500 mgKOH / g, and a weight average molecular weight of 1,500 or more and 2,500 or less.

[0072] The solid content of the hydroxyl-containing polyester resin (z2) relative to 100 parts by mass of the total solid content of the hydroxyl-containing components is, for example, 4 parts by mass or more. This tends to improve the smoothness of the resulting coating film. The solid content of the hydroxyl-containing polyester resin (z2) may be 7 parts by mass or more, or 10 parts by mass or more. The solid content of the hydroxyl-containing polyester resin (z2) is, for example, 30 parts by mass or less. This tends to improve the drying properties of the clear coating composition (Z). The solid content of the hydroxyl-containing polyester resin (z2) may be 25 parts by mass or less, or 20 parts by mass or less. In one embodiment, the solid content of the hydroxyl-containing polyester resin (z2) relative to 100 parts by mass of the total solid content of the hydroxyl-containing components is 4 parts by mass or more and 30 parts by mass or less.

[0073] The hydroxyl group-containing polyester resin (z2) can be obtained, for example, by polycondensation (ester reaction) of a polyhydric alcohol with a polybasic acid or its anhydride. Commercially available hydroxyl group-containing polyester resin (z2) may also be used.

[0074] The polyhydric alcohol is not particularly limited, and examples thereof include ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, neopentyl glycol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, hydrogenated bisphenol A, hydroxyalkylated bisphenol A, 1,4-cyclohexanedimethanol, 2,2-dimethyl- Examples of such an alkyl acrylate include 3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate, 2,2,4-trimethyl-1,3-pentanediol, N,N-bis-(2-hydroxyethyl)dimethylhydantoin, polytetramethylene ether glycol, polycaprolactone polyol, glycerin, sorbitol, trimethylolethane, trimethylolpropane, trimethylolbutane, hexanetriol, pentaerythritol, dipentaerythritol, and tris-(hydroxyethyl)isocyanate. These may be used alone or in combination of two or more.

[0075] The polybasic acid or anhydride thereof is not particularly limited, and examples thereof include phthalic acid, phthalic anhydride, tetrahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalic anhydride, methyltetrahydrophthalic acid, methyltetrahydrophthalic anhydride, himic anhydride, trimellitic acid, trimellitic anhydride, pyromellitic acid, pyromellitic anhydride, isophthalic acid, terephthalic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, adipic acid, azelaic acid, sebacic acid, succinic acid, succinic anhydride, lactic acid, dodecenylsuccinic acid, dodecenylsuccinic anhydride, cyclohexane-1,4-dicarboxylic acid, and endo acid anhydride. These may be used alone or in combination of two or more.

[0076] The hydroxyl group-containing polyester resin (z2) may be modified with lactone, fats and oils or fatty acids, melamine resin, urethane resin, etc. The fats and oils or fatty acids are not particularly limited, and examples thereof include castor oil, dehydrated castor oil, palm oil, corn oil, cottonseed oil, linseed oil, perilla oil, poppy seed oil, safflower oil, soybean oil, tung oil, and other fats and oils, and fatty acids extracted from these fats and oils.

[0077] (Other hydroxyl group-containing components) The clear coating composition (Z) according to this embodiment may contain other hydroxyl group-containing components, such as hydroxyl group-containing acrylic resins other than the hydroxyl group-containing acrylic resin (z1) and various other polyol compounds, which may be used alone or in combination of two or more.

[0078] Examples of other hydroxyl group-containing acrylic resins include hydroxyl group-containing acrylic resins that satisfy at least one of the following: (a) a hydroxyl value of less than 90 mgKOH / g, (b) a hydroxyl value of more than 190 mgKOH / g, (c) a weight average molecular weight of less than 4,000, (d) a weight average molecular weight of more than 6,000, (e) a Tg of less than 15° C., and (f) a Tg of more than 100° C. Examples of other polyol compounds include polyol compound (y2), polycarbonate polyol resins, polyether polyol resins, and polycaprolactone polyol resins.

[0079] The amount of other hydroxyl group-containing components is desirably small. The solid content of other hydroxyl group-containing components relative to 100 parts by mass of the total solid content of the hydroxyl group-containing components is, for example, 20 parts by mass or less, or may be 15 parts by mass or less, or 10 parts by mass or less, or 5 parts by mass or less, or 3 parts by mass or less, or may be 0 parts by mass.

[0080] (z3) Polyisocyanate compounds The polyisocyanate compound (z3) is a curing agent that reacts with the hydroxyl group-containing component to form a crosslinked structure and cure the coating composition. A portion of the polyisocyanate compound (z3) penetrates into the uncured base coating film (base coating composition) and acts as a curing agent for the base coating composition.

[0081] The polyisocyanate compound (z3) has at least two isocyanate groups per molecule. Examples of the polyisocyanate compound (z3) include aliphatic polyisocyanates, alicyclic polyisocyanates, aliphatic polyisocyanates having an aromatic ring not bonded to an isocyanate group in the molecule (araliphatic polyisocyanates), aromatic polyisocyanates, and derivatives of these polyisocyanates. Specific examples include aromatic polyisocyanates such as tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate, and metaxylylene diisocyanate; aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate; and multimers of these compounds, such as biuret, nurate, and adduct types. These compounds may be used alone or in combination of two or more.

[0082] The equivalent ratio (NCO / OH) of the isocyanate groups contained in the polyisocyanate compound (z3) to the hydroxyl groups contained in the hydroxyl group-containing component may be 0.7 or more, or 0.8 or more. The equivalent ratio (NCO / OH) may be 2.0 or less, 1.8 or less, or 1.5 or less. In one embodiment, the equivalent ratio (NCO / OH) is 0.7 or more and 2.0 or less. When the equivalent ratio (NCO / OH) is in this range, a sufficient amount of the polyisocyanate compound (z3) can penetrate into the uncured base coating film, while the curing reaction of the clear coating film also occurs easily.

[0083] (Other hardeners) The clear coating composition (Z) according to this embodiment may further contain at least one other curing agent selected from the group consisting of, for example, amino resins, epoxy compounds, aziridine compounds, carbodiimide compounds, and oxazoline compounds. The content of the other curing agent is appropriately determined depending on the coating film-forming components.

[0084] (z4) Metal compounds The metal catalyst (z4) is a curing catalyst. The metal catalyst (z4) can form a complex with the organic amine catalyst (z5) in the clear coating composition (Z). Upon heating, the metal catalyst (z4) and the organic amine catalyst (z5) dissociate, and each acts as a curing catalyst. The combined use of the metal catalyst (z4) and the organic amine catalyst (z5) improves low-temperature curing properties.

[0085] The metal catalyst (z4) contains at least one metal M selected from the group consisting of zinc, bismuth, and tin. The metal catalyst (z4) may be an organometallic catalyst containing the metal M. The metal catalyst (z4) may be, for example, an acylate compound, alkoxide compound, chelate compound, or oxide compound of the metal M. These may be used alone or in combination of two or more.

[0086] For example, the zinc acylate compound is represented by the following general formula (C11): Zn-(OC(=O)-R c11 )2(C11) (In the formula, R c11 represents an aliphatic hydrocarbon group having 1 to 10 carbon atoms. It is expressed as:

[0087] R c11 The number of carbon atoms in R may be 3 or more, 4 or more, or 5 or more. c11 The number of carbon atoms in R may be 9 or less, 8 or less, or 7 or less. c11 may be a linear, branched or cyclic aliphatic hydrocarbon group, and may be a linear or branched aliphatic hydrocarbon group.

[0088] For example, a tin acylate compound is represented by the following general formula (C13): Sn-(OC(=O)-R c13 )2(C13) (In the formula, R c13 represents an aliphatic hydrocarbon group having 1 to 12 carbon atoms. It is expressed as:

[0089] R c13 The number of carbon atoms in R may be 3 or more, 4 or more, or 5 or more. c13 The number of carbon atoms in R may be 11 or less, or may be 10 or less. c13 may be a linear, branched or cyclic aliphatic hydrocarbon group, and may be a linear or branched aliphatic hydrocarbon group.

[0090] For example, a bismuth acylate compound is represented by the following general formula (C12): Bi-(OC(=O)-R c12 )3(C12) (In the formula, R c12 represents an aliphatic hydrocarbon group having 1 to 10 carbon atoms. It is expressed as:

[0091] R c12 The number of carbon atoms in R may be 3 or more, 4 or more, or 5 or more. c12 The number of carbon atoms in R may be 9 or less, 8 or less, or 7 or less. c12 may be a linear, branched or cyclic aliphatic hydrocarbon group, and may be a linear or branched aliphatic hydrocarbon group.

[0092] The alkoxide compound of the metal M is, for example, a compound represented by the following general formula (C2): M(OR c2 ) s (C2) (In the formula, R c2 each independently represents an aliphatic hydrocarbon group having 1 to 12 carbon atoms, s represents the valence of the metal M. It is expressed as:

[0093] R c2 The number of carbon atoms in R may be 2 or more, 3 or more, or 4 or more. c2 The number of carbon atoms in R may be 11 or less, 10 or less, or 9 or less. c2 may be a linear, branched or cyclic aliphatic hydrocarbon group, and may be a linear or branched aliphatic hydrocarbon group.

[0094] The chelate compound of metal M is, for example, represented by the following general formula (C3): M(R c3 ) t (C3) (In the formula, R c3 represents an acetylacetonato group, an ethylacetonato group, a propylacetonato group, an isopropylacetonato group, a butylacetonato group, a propionylacetonato group, or a proponylacetonato group, t represents the coordination number of the metal M. It is expressed as:

[0095] When the metal M is tin, the organic oxide compound of tin is, for example, a compound represented by the following general formula (C4): Sn(=O)-(-R c14 )2(C4) (In the formula, R c13 represents an aliphatic hydrocarbon group having 1 to 12 carbon atoms. It is expressed as:

[0096] Specific examples of the metal catalyst (z4) include organic zinc compounds such as zinc naphthenate; organic bismuth compounds such as bismuth oxide, bismuth hydroxide, and bismuth carboxylate; and organic tin compounds such as diacetyltin diacetate, dibutyltin dilaurate, dibutyltin diacetate, dioctyltin dilaurate, diacetyltin dioctoate, tin octoate, dibutyltin diacetate, and dibutyltin dioctoate.

[0097] The content of the metal catalyst (z4) is, for example, 0.005% by mass or more and 0.05% by mass or less based on the resin solid content. This can further improve the smoothness and pot life of the resulting coating film. The content of the metal catalyst (z4) may be 0.008% by mass or more, or 0.01% by mass or more. The content of the metal catalyst (z4) may be 0.05% by mass or less, or 0.03% by mass or less.

[0098] (z5) Organic amine catalyst The organic amine catalyst (z5) has an amidine group. The organic amine catalyst (z5) does not contain a metal atom. The organic amine catalyst (z5) exhibits excellent catalytic activity under heating. In particular, the organic amine catalyst (z5) exhibits high activity even at low temperatures (e.g., 100°C or less), improving the physical properties of the resulting coating film. In addition, the organic amine catalyst (z5) can complete the curing reaction in a short time (e.g., 20 minutes or less) even at low temperatures. Therefore, post-treatments such as polishing can be carried out promptly after the curing treatment.

[0099] By using a combination of a clear coating composition (Z) containing an organic amine catalyst (z5) and a base coating composition containing a polyol compound (y2), low-temperature curing becomes possible in the two-coat, one-bake method, and a coating film with excellent physical properties can be obtained.

[0100] An amidine group has a carbon (C) atom with one nitrogen (N) atom attached via a double bond and one N atom attached via a single bond. The amidine group has the general formula: -C(=NR 1 )-N(R 2 )- It is expressed as:

[0101] In the formula, R 1 and R 2 are each independently a hydrogen atom or a monovalent or divalent hydrocarbon group having 1 to 3 carbon atoms. 1 and R 2 may form a ring structure (heterocycle containing an N atom) via one or more C atoms.

[0102] Examples of the organic amine catalyst (z5) not having a heterocycle include N'-cyclohexyl-N,N-dimethylformamidine, N'-methyl-N,N-di-n-butylacetamidine, N'-octadecyl-N,N-dimethylformamidine, N'-cyclohexyl-N,N-dimethylvaleroamidine, 1-methyl-2-cyclohexyliminopyrrolidine, 3-butyl-3,4,5,6-tetrahydropyrimidine, N-(hexyliminomethyl)morpholine, N-(α-(decyliminoethyl)ethyl)pyrrolidine, N'-decyl-N,N-dimethylformamidine, N'-dodecyl-N,N-dimethylformamidine, and N'-cyclohexyl-N,N-acetamidine.

[0103] Examples of the heterocycle include an imidazoline ring, an imidazole ring, a tetrahydropyrimidine ring, a dihydropyrimidine ring, and a pyrimidine ring. The organic amine catalyst (z5) may have an imidazole ring.

[0104] The organic amine catalyst (z5) may have the above heterocycle, or may be a polycyclic compound having the above heterocycle.

[0105] Examples of the organic amine catalyst (z5) having an imidazole ring include N-(2-hydroxyethyl)imidazole, N-(3-aminopropyl)imidazole, 4-(hydroxymethyl)imidazole, 1-(tert-butoxycarbonyl)imidazole, imidazole-4-propionic acid, 4-carboxyimidazole, 1-butylimidazole, 1-methylimidazole, 2-methyl-4-imidazolecarboxylic acid, 4-formylimidazole, 1-(ethoxycarbonyl)imidazole, reaction products of propylene oxide with imidazole and 2-methylimidazole, 1-trimethylsilylimidazole, 4-(hydroxymethyl)imidazole hydrochloride, 1H-imidazolium, 3-ethyl-1-methyl-, benzo ester, copolymer of 1-chloro-2,3-epoxypropane and imidazole, 1-(p-toluenesulfonyl)imidazole, 1,1-carbonylbisimidazole, 1-(2-cyanoethyl)-2-ethyl-4-methylimidazole, 2-phenyl-2-imidazoline pyromellitate, 4-(hydroxymethyl)imidazole picrate, disodium salts of 2-propenoic acid, 4,5-dihydro-2-nonyl-1H-imidazole-1-ethanol, and 2-heptyl-4,5-dihydro-1H-imidazole-1-ethanol, 1-(cyanoethyl)-2-undecylimidazole trimellitate, formate ester of 1-(2-hydroxypropyl)imidazole, sodium imidazole salt, and silver imidazole salt.

[0106] Examples of the polycyclic organic amine catalyst (z5) include 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo[5.4.0]-7-undecene (DBU), 1,4-diazabicyclo[3.3.0]oct-4-ene, 2-methyl-1,5-diazabicyclo[4.3.0]one-5-ene, 2,7,8-trimethyl-1,5-diazabicyclo[4.3.0]one-5-ene, 2-butyl-1,5-diazabicyclo[4.3.0]one-5-ene, and 1,9-diazabicyclo[6.5.0]tridec-8-ene.

[0107] The organic amine catalyst (z5) may be used alone or in combination of two or more.

[0108] The content of the organic amine catalyst (z5) is, for example, 0.1% by mass or more and 1% by mass or less based on the resin solid content. This can further improve the smoothness and pot life of the resulting coating film. The content of the organic amine catalyst (z5) may be 0.2% by mass or more, or 0.3% by mass or more. The content of the organic amine catalyst (z5) may be 1.5% by mass or less, 1.0% by mass or less, or 0.8% by mass or less.

[0109] (Other catalysts) The clear coating composition (Z) according to this embodiment may contain a catalyst other than the organic amine catalyst (z5). Examples of other catalysts include organometallic compounds other than zinc, bismuth, and tin, amine compounds without amidine groups, and boric acid compounds. These may be used alone or in combination of two or more.

[0110] Other organometallic compounds include, for example, organoaluminum compounds such as aluminum trimethoxide, aluminum tris(acetylacetonate), aluminum tri-n-butoxide, aluminum tris(ethyl acetoacetate), aluminum diisopropoxy(ethyl acetoacetate), and aluminum acetylacetonate; organotitanium compounds such as titanium tetra(monoethylethoxide), titanium tetra(monoethylethoxide), titanium tetra(monobutylethoxide), titanium tetrakis(acetylacetonate), and tetra-n-butyltitanate; and organozirconium compounds such as zirconium tetra(monomethylethoxide), zirconium tetra(monoethylethoxide), zirconium tetra(monobutylethoxide), zirconium n-propylate, zirconium n-butylate, and zirconium tetrakis(acetylacetonate).

[0111] Examples of amine compounds having no amidine group include trimethylamine, triethylamine, 2-(dimethylamino)ethyl methacrylate, 1-methylpiperidine, 1-methylpyrrolidine, pyridine, 4-dimethylaminopyridine, 4-(1-piperidyl)pyridine, N-methylimidazole, and N,N-dimethylaniline.

[0112] Examples of boric acid compounds include boric acid esters such as trimethyl borate, triethyl borate, tripropyl borate, tributyl borate, triphenyl borate, tri(4-chlorophenyl) borate, and trihexafluoroisopropyl borate.

[0113] The content of the other catalyst is, for example, 0.05% by mass or less based on the resin solid content. The content of the other catalyst may be 0.03% by mass or less, or 0.02% by mass or less. The content of the other catalyst may be 0.01% by mass or more. The content of the other catalyst may be 0% by mass or more.

[0114] (Dilution ingredients) The clear coating composition (Z) according to this embodiment may contain a diluent component. The clear coating composition (Z) is diluted with a diluent component as appropriate, taking into consideration the coating method and the coating environment, such as temperature and humidity. Examples of the diluent component include water and the non-aqueous solvents described above. The clear coating composition (Z) according to this embodiment may also contain the non-aqueous solvents used in the production of each component.

[0115] (additives) The clear coating composition (Z) according to this embodiment may contain other additives commonly used in the coating field. For example, it may contain color pigments and / or luster pigments to the extent that transparency is not impaired. It may also contain ultraviolet absorbers, hindered amine light stabilizers, antioxidants, crosslinked resin particles, viscosity modifiers, surface conditioners, film-forming aids, rust inhibitors, etc.

[0116] [Method for forming multi-layer coating film] The multilayer coating film is formed by applying at least one layer of base coating composition (Y) onto an object to be coated to form at least one layer of uncured base coating film, applying a clear coating composition (Z) onto the uncured base coating film to form a clear coating film, and heating the uncured base coating film and the uncured clear coating film to cure them.

[0117] A primer coating film may be formed between the substrate and the base coating film. The primer coating film can be formed in the same manner as the base coating film.

[0118] (I) Step of forming an uncured base coating film The uncured base coating film is formed by applying the above-mentioned base coating composition (Y) to a substrate. The base coating composition (Y) is applied so that the thickness of the base coating film after curing is 10 μm or more and 50 μm or less.

[0119] The coating method is not particularly limited. Examples of coating methods include air spray coating, airless spray coating, and rotary atomization coating. These methods may be combined with electrostatic coating. Among them, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic coating, a rotary atomization electrostatic coater, commonly known as a "micro-microbell (μμbell)," "microbell (μbell)," or "metallicbell (metabell)," is used.

[0120] After applying the base coating composition (Y), pre-drying (also called preheating) may be performed. This prevents the diluting components contained in the base coating composition (Y) from bumping during the curing process, making it easier to prevent popping. Furthermore, pre-drying prevents the uncured base coating film and the clear coating composition (Z) applied thereon from mixing, making it harder for a mixed layer to form. This makes it easier to further improve the smoothness of the resulting coated article.

[0121] The conditions for pre-drying are not particularly limited. Examples of pre-drying include leaving the film at room temperature for 5 to 15 minutes, or heating the film at a temperature of 50°C to 80°C for 30 seconds to 5 minutes.

[0122] (II) Step of forming an uncured clear coating film The uncured clear coating film is formed by applying the above-mentioned clear coating composition (Z) onto the uncured base coating film. The clear coating composition (Z) is applied so that the thickness of the clear coating film after curing is 15 μm or more and 60 μm or less.

[0123] The coating method is not particularly limited. Examples of coating methods include the same methods as those used for coating the base coating composition. Among these, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. After applying the clear coating composition (Z), pre-drying may be performed. The conditions for pre-drying are not particularly limited and may be the same as those for pre-drying the base coating film.

[0124] (III) Curing process Each uncured coating film is cured. Each coating film can be cured by heating. In this process, the base coating film and the clear coating film are cured at the same time.

[0125] The heating conditions are appropriately set depending on the composition of each coating composition, the material of the substrate, etc. The heating temperature is, for example, 60°C or higher and 100°C or lower. With the clear coating composition (Z) according to this embodiment, a clear coating film having high hardness is formed even at such a low temperature. The heating temperature may be, for example, 75°C or lower, or may be 70°C or lower.

[0126] The heating time may be set appropriately depending on the heating temperature. When the heating temperature is 60°C or higher and 100°C or lower, the heating time is, for example, 5 minutes to 20 minutes, or may be 5 minutes to 15 minutes. The heating time means the time during which the interior of the heating device is maintained at the target temperature, and does not take into account the time required to reach the target temperature. Examples of heating devices include drying ovens that use a heat source such as hot air, electricity, gas, or infrared rays.

[0127] (subject to be coated) The material of the substrate is not particularly limited, and examples of the substrate material include metal, resin, and glass.

[0128] The shape of the substrate is not particularly limited, and specific examples of the substrate include automobile bodies such as passenger cars, trucks, motorcycles, and buses, and automobile body parts, as well as automobile parts such as spoilers, bumpers, mirror covers, grilles, and door knobs.

[0129] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of metal substrates include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0130] The metal substrate may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After the surface treatment, the metal substrate may be further coated with an electrodeposition paint. The electrodeposition paint may be either a cationic type or an anionic type.

[0131] Examples of resins include polyethylene resin, EVA resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), acrylic resin, acrylonitrile butadiene styrene (ABS) resin, polycarbonate-polybutylene terephthalate (PC-PBT) resin, polycarbonate-acrylonitrile butadiene styrene (PC-ABS resin), polycarbonate resin, vinyl chloride resin, polyamide resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), acrylonitrile styrene (AS) resin, acetal resin, phenolic resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). Resin substrates may be degreased.

[0132] The clear coating composition (Z) according to this embodiment can be cured at low temperatures, and is therefore suitable for application to resin substrates. The substrate may contain both a resin portion (a portion formed from a resin) and a metal portion (a portion formed from a metal). The substrate may be made of a resin. The substrate may contain a resin portion containing at least one resin selected from the group consisting of ABS resin, PC-PBT resin, PC-ABS resin, polycarbonate resin, vinyl chloride resin, polyamide resin, and polypropylene resin.

[0133] [Painted items] According to this embodiment, a coated article is obtained that comprises a base coating film formed on an object to be coated and a clear coating film formed on the base coating film.

[0134] The thickness of the base coating film is not particularly limited and is appropriately set according to the purpose. The thickness of each layer of the base coating film is, for example, 10 μm or more, may be 15 μm or more, or may be 20 μm or more. The thickness of each layer of the base coating film is, for example, 50 μm or less, may be 45 μm or less, or may be 40 μm or less.

[0135] The thickness of the coating film can be measured using an electromagnetic film thickness meter (for example, SDM-miniR manufactured by SANKO Co., Ltd.) The thickness of the coating film is the average value of the thickness of the coating film at any five points.

[0136] The thickness of the clear coating film is not particularly limited. From the viewpoint of scratch resistance and smoothness, the thickness of the clear coating film after drying is, for example, 15 μm or more, and may be 20 μm or more. The thickness of the clear coating film may be 60 μm or less, and may be 40 μm or less.

[0137] A primer coating may be interposed between the substrate and the base coating. The primer coating further improves adhesion between the base coating and the substrate (especially a resin substrate). Furthermore, if the surface of the substrate is uneven, the primer coating makes the painted surface uniform, making it easier to suppress unevenness in the base coating.

[0138] The primer coating film is formed, for example, from a primer coating composition containing a film-forming component, a substrate adhesion component, a viscosity modifier, a diluent component, a pigment, and, if necessary, a curing agent. The primer coating composition may contain various of the above-mentioned additives as needed. The primer coating composition may be solvent-based or water-based. Examples of the film-forming component, curing agent, viscosity modifier, diluent component, and pigment include the components exemplified as those to be incorporated into the base coating composition.

[0139] The thickness of the primer coating is not particularly limited. In terms of the smoothness and chipping resistance of the coated article, the thickness of the primer coating may be 5 μm or more and 40 μm or less. The thickness of the primer coating may be 7 μm or more. The thickness of the primer coating may be 25 μm or less. [Example]

[0140] The present invention will be described below with reference to examples, but the present invention is not limited to the examples described below.

[0141] [Examples 1 to 7, Comparative Examples 1 to 7] (1) Preparation of base coating composition (Y) and clear coating composition (Z) A base coating composition (Y) and a clear coating composition (Z) were obtained by mixing the components according to the ingredients and amounts shown in Table 1. The solids concentration of the base coating composition (Y) was 35 mass %, and the solids concentration of the clear coating composition (Z) was 58 mass %.

[0142] (2) Formation of multi-layer coating An ABS resin substrate was prepared as the substrate and wiped with isopropyl alcohol. Next, the base coating composition (Y) was applied to the substrate using a spray gun (W-101-134G, manufactured by Anest Iwata Corporation) to a dry film thickness of 15 μm. The coating was then dried for 3 minutes in an environment of 20±5°C and a relative humidity of 78% or less to form an uncured base coating film on the ABS substrate.

[0143] Next, the clear coating composition (Z) immediately after preparation was applied onto the base coating film using a spray gun (W-101-134G, manufactured by Anest Iwata Corporation) so that the dry film thickness was 30 μm. The substrate was then left to stand for 10 minutes in an environment of a temperature of 20±5°C and a relative humidity of 78% or less to form an uncured clear coating film.

[0144] The coating was then heated at 70°C for 10 minutes using a dryer to obtain a coated article having a multi-layer coating film with a cured base coating film and a clear coating film.

[0145] The components used in the examples and comparative examples are as follows: Hydroxyl-containing acrylic resins (y1) and (z1) were produced as follows.

[0146] [Production of hydroxyl group-containing acrylic resin (y1-1)] A reactor equipped with a stirring blade, thermometer, dropping device, temperature control device, nitrogen gas inlet, and cooling tube was charged with 57 parts of butyl acetate, and the temperature was raised to 120 ° C while stirring and introducing nitrogen gas. Next, a mixture consisting of 0.5 parts of methacrylic acid, 56.6 parts of 2-ethylhexyl methacrylate, 16.7 parts of methyl methacrylate, 15.0 parts of styrene, 2.3 parts of 2-hydroxyethyl methacrylate, and 8.9 parts of lactone-modified 2-hydroxyethyl methacrylate and a solution of 2.0 parts of t-butylperoxy-2-ethylhexanate in 5 parts of butyl acetate were added dropwise to the reactor over 3 hours. After the addition was completed, the mixture was aged for 1 hour, and then a solution of 0.2 parts of t-butylperoxy-2-ethylhexanate in 5 parts of butyl acetate was added dropwise to the reactor over 1 hour. The mixture was maintained at 120 ° C and aged for 2 hours to complete the reaction. The resulting hydroxyl-containing resin had a non-volatile content of 60% and a weight-average molecular weight of 13000. The glass transition temperature was 20°C and the hydroxyl value was 30 mgKOH / g.

[0147] [Table 1]

[0148] [Production of Hydroxyl-Containing Acrylic Resins (y1-2), (a1), and (a2)] Hydroxyl-containing acrylic resins (y1-2), (a1), and (a2) were produced in the same manner as for hydroxyl-containing acrylic resin (y1-1), except that the types and amounts of monomers were as shown in Table 1.

[0149] [Production of hydroxyl group-containing acrylic resin (z1-1)] A reactor equipped with a stirring blade, thermometer, dropping device, temperature control device, nitrogen gas inlet, and cooling tube was charged with 57 parts of butyl acetate, and the temperature was raised to 120°C while stirring with nitrogen gas introduced. A mixture consisting of 0.8 parts methacrylic acid, 26.8 parts 2-ethylhexyl acrylate, 33.0 parts methyl methacrylate, and 39.4 parts 2-hydroxyethyl methacrylate and a solution of 10 parts t-butylperoxy-2-ethylhexanate in 5 parts butyl acetate were added dropwise to the reactor over a period of 3 hours. After the addition was completed, the mixture was allowed to age for 1 hour. Then, a solution of 0.2 parts t-butylperoxy-2-ethylhexanate in 5 parts butyl acetate was added dropwise to the reactor over a period of 1 hour. The reactor was maintained at 120°C for 2 hours while the mixture was aged, completing the reaction and obtaining hydroxyl-containing acrylic resin (z1-1). The resulting hydroxyl-containing acrylic resin (z1-1) had a non-volatile content of 60%, a weight average molecular weight of 4,500, and a glass transition temperature of 20° C. The hydroxyl value (OHV) was calculated to be 170 from the monomer blend.

[0150] [Production of hydroxyl group-containing acrylic resins (z1-2) and (a3)] Hydroxyl-containing acrylic resins (z1-2) and (z1-3) were produced in the same manner as for hydroxyl-containing acrylic resin (z1-1), except that the types and amounts of monomers were as shown in Table 2.

[0151] [Table 2]

[0152] Polyol Compounds y2-1: 1,4-cyclohexanedimethanol, hydroxyl value 780 mg KOH / g, molecular weight 144 [ka]

[0153] y2-2: 1,1-cyclohexanediethanol, hydroxyl value 652 mg KOH / g, molecular weight 172 [ka]

[0154] p1: Polypropylene glycol diol type 2000, manufactured by Wako Pure Chemical Industries, Ltd., hydroxyl value 56 mg KOH / g, molecular weight 2,000 p2: Polycarbonate diol, Duranol T-5650J, manufactured by Asahi Kasei Corporation, hydroxyl value 140 mg KOH / g, molecular weight 800

[0155] Blocked isocyanate compound (y3) Product name: Duranate MF-K60B, manufactured by Asahi Kasei Corporation melamine resin Product name: "RESIMENE 747", manufactured by Prefere Resins

[0156] Hydroxyl group-containing polyester resin (z2) Product name: Basonol HPE1170B, manufactured by BASF, hydroxyl value 280 mg KOH / g, weight average molecular weight 1,800

[0157] Polyisocyanate compound (z3) Product name "N3300", manufactured by Covestro, isocyanurate of hexamethylene diisocyanate

[0158] metal compounds Z4-1: Dibutyltin dilaurate z4-2: Bismuth carboxylate

[0159] Organic amine catalyst (z5) DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene [ka]

[0160] KAT-1:1H-Imidazolium,3-ethyl-1-methyl-,benzoate, CAS No. 150999-33-0 [ka]

[0161] (h) Diluent n-Butyl acetate

[0162] [evaluation] The evaluation was carried out as follows, and the evaluation results are shown in Tables 3 and 4.

[0163] (paint viscosity η0) The viscosity of the base coating composition (Y) or clear coating composition (Z) immediately after preparation was measured at 23°C using a No. 4 Ford cup in accordance with "3. Flow cup method" of JIS K5600-2-2:1999. The average viscosity of five different clear coating compositions (Z) with the same composition was taken as the coating viscosity η0. The coating viscosity η0 was evaluated according to the following criteria.

[0164] Base paint composition (Y), solids concentration 35% Good: Coating viscosity η0 is 20 seconds or less Poor: Paint viscosity η0 exceeds 20 seconds

[0165] Clear coating composition (Z), solids concentration 58% Good: Coating viscosity η0 is 25 seconds or less Poor: Paint viscosity η0 exceeds 25 seconds

[0166] (smoothness) The smoothness of the multilayer coating film was evaluated using a surface measuring device (BYK Wave Scan-dual) to measure the amount of reflected light (LW0) in the long wavelength region (1200 μm to 12000 μm) and according to the following criteria: The smaller the LW0, the smoother the surface. With Wave Scan-dual, a laser beam is irradiated onto the test piece at an angle of 60° while the light source is moved, and the reflected light is measured. A rating of B or higher indicates high smoothness and excellent appearance.

[0167] A: LW0 is 20 or less B: LW0 is over 20 and 25 or less C:LW0 is over 25

[0168] (Initial adhesion) A single-blade cutting tool specified in JIS K-5600-5-6 was placed perpendicularly on the multilayer coating film to make incisions (parallel lines 1) that reached all the way to the substrate. Furthermore, 10 incisions parallel to the parallel lines 1 were made at equal intervals. Eleven incisions (parallel lines 2) were made at equal intervals that intersected perpendicularly with the 11 parallel lines 1 and reached all the way to the substrate. The distance between parallel lines 1 and between parallel lines 2 was 2 mm. In this way, a grid pattern having 100 squares surrounded by four straight lines was formed.

[0169] A transparent pressure-sensitive adhesive tape as specified in JIS K-5600-5-6 was adhered to the grid area, taking care not to trap air bubbles between the tape and the coating surface. The tape was then quickly peeled off within 0.5 to 1.0 seconds, and the peeling state of the grid area was visually evaluated. The evaluation criteria are as follows: A: When no peeling of the coating is observed B: When peeling of the coating is observed

[0170] (water resistance) The coated article was immersed in a water tank maintained at 40°C for 240 hours. The coated article was then removed from the water and dried at room temperature for 1 hour. The multilayer coating was then evaluated visually and by the same tape peel test as above. A rating of B or higher was considered to be water resistant. Visual evaluation consisted of observing a loss of gloss and the occurrence of blisters. A: No tape peeling, no loss of gloss, and no blistering observed. B: No tape peeling, but slight loss of gloss and / or slight blistering is observed C: Tape peeling, loss of gloss and blistering observed

[0171] (tackiness) In an atmosphere of 23°C, the surface of the multi-layer coating film immediately after curing was pressed with a finger and the presence or absence of tackiness (adhesion) of the surface was evaluated. The evaluation criteria are as follows. Tack is more likely to occur in coating films cured at low temperatures. Tack is one of the indicators of the degree of curing. If tackiness is felt, it can be said that the curing is insufficient.

[0172] Good: No stickiness felt Poor: Sticky feeling

[0173] (Amount of polyisocyanate compound soaked in) The base coating composition (Y) and the clear coating composition (Z) were applied in that order to a fluororesin sheet in the same manner as above, and cured by heating at 70°C for 10 minutes to obtain a multi-layer coating film for evaluation.

[0174] The multilayer coating film was peeled off from the fluororesin sheet, and ATR-IR measurement was performed on the surface of the multilayer coating film that had been in contact with the sheet under the following conditions. -1 Infrared absorption peak intensity at 1670 cm -1 The infrared absorption peak intensity was measured. The peak area ratio (urethane bond area / ester bond area) was determined and applied to a calibration curve prepared in advance using the procedure described below to calculate the isocyanate amount α (the number of parts of solid content of polyisocyanate compound per 100 parts of solid content of base coating composition (Y)) at the interface side of the base coating film with the substrate. The isocyanate amount α was used as an index of the amount of penetration.

[0175] The greater the isocyanate amount α, the better the polyisocyanate compound penetrates from the clear coating composition into the base coating composition. A rating of fair or better can be evaluated as indicating that the polyisocyanate compound has penetrated sufficiently.

[0176] Good: The above isocyanate amount α is 5 parts or more Acceptable: The above isocyanate amount α is 1 part or more and less than 5 parts Poor: The above isocyanate amount α is less than 1 part

[0177] (ATR-IR measurement conditions) Equipment: Fourier transform infrared spectrophotometer "FT / IR610", manufactured by JASCO Corporation Measurement mode: ATR method (prism: zinc selenide, incident angle: 45°) Resolution: 4cm -1 Accumulation count: 16 times Wavelength range: 400cm -1 ~4000cm -1

[0178] (Calibration curve creation procedure) Three types of reference paints were prepared by adding 2, 5, and 10 mass% of the polyisocyanate compound (trade name "Desmodur N3300, manufactured by Covestro Japan Co., Ltd.") used in the clear paint composition (Z) to the base paint composition (Y). Each reference paint was applied to a fluororesin sheet and cured by heating at 70°C for 10 minutes to obtain three types of reference coating films. A portion of the reference coating film was peeled from the sheet, and the peak area ratio (urethane bond area / ester bond area) was determined in the same manner as above to obtain a calibration curve.

[0179] [Table 3]

[0180] [Table 4]

[0181] The multi-layer coating film of the example had a good appearance and high hardness. The multilayer coating films of Comparative Examples 1, 2, and 4 to 5 had good appearance but poor physical properties. The multilayer coating films of Comparative Examples 3 and 6 had high hardness but poor appearance. The multilayer coating film of Comparative Example 7 was poor in both hardness and appearance.

[0182] The present invention provides the following aspects. [1] Applying at least one layer of base coating composition (Y) to a substrate to form at least one layer of uncured base coating film; Applying a clear coating composition (Z) onto the uncured base coating film to form a clear coating film; A method for forming a multilayer coating film, comprising heating and curing the uncured base coating film and the uncured clear coating film, The base coating composition (Y) A hydroxyl group-containing acrylic resin (y1) having a weight average molecular weight of 13,000 or more and 35,000 or less and a glass transition temperature of 20°C or more and less than 80°C, and a polyol compound (y2) having a hydroxyl value of more than 200 mgKOH / g and not more than 1000 mgKOH / g and a molecular weight of 100 or more and not more than 1000; the solid content of the hydroxyl group-containing acrylic resin (y1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y); The clear coating composition (Z) a hydroxyl group-containing acrylic resin (z1) having a hydroxyl value of 90 mgKOH / g or more and 190 mgKOH / g or less, a weight average molecular weight of 4000 or more and 6000 or less, and a glass transition temperature of 15°C or more and 100°C or less; Hydroxyl group-containing polyester resin (z2), Polyisocyanate compound (z3), a metal catalyst (z4) containing at least one metal selected from the group consisting of zinc, bismuth, and tin; and an organic amine catalyst (z5) having an amidine group, The solid content of the hydroxyl group-containing acrylic resin (z1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the clear coating composition (Z). A method for forming a multi-layer coating film. [2] The method for forming a multi-layer coating film according to the above item [1], wherein the polyol compound (y2) has two hydroxyl groups and an alicyclic hydrocarbon group. [3] The method for forming a multi-layer coating film according to the above [1] or [2], wherein the hydroxyl group-containing acrylic resin (y1) has a hydroxyl value of 30 mgKOH / g or more and 100 mgKOH / g or less. [4] The method for forming a multilayer coating film according to any one of the above [1] to [3], wherein the solid content of the polyol compound (y2) is 2 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y). [5] The method for forming a multilayer coating film according to any one of the above [1] to [4], wherein the hydroxyl group-containing polyester resin (z2) has a hydroxyl value of 250 mgKOH / g or more and less than 500 mgKOH / g, and a weight average molecular weight of 1500 or more and 2500 or less. [6] The method for forming a multi-layer coating film according to any one of the above [1] to [5], wherein the base coating composition (Y) has a solids concentration of 30% by mass or more and 60% by mass or less. [7] The method for forming a multilayer coating film according to any one of the above [1] to [6], wherein the clear coating composition (Z) has a solids concentration of 45% by mass or more and 60% by mass or less. [8] The method for forming a multilayer coating film according to any one of [1] to [7] above, wherein the substrate comprises a resin portion containing at least one resin selected from the group consisting of ABS resin, PC-PBT resin, PC-ABS resin, polycarbonate resin, vinyl chloride resin, polyamide resin, and polypropylene resin. [9] The method for forming a multi-layer coating film according to any one of the above [1] to [8], wherein the organic amine catalyst (z5) has a heterocycle containing a nitrogen atom.

[10] The method for forming a multilayer coating film according to any one of the above [1] to [9], wherein the metal catalyst (z4) comprises at least one selected from the group consisting of an acylate compound, an alkoxide compound, a chelate compound, and an oxide compound of zinc, bismuth, or tin. [Industrial Applicability]

[0183] The multi-layer coating film formed by the present invention is suitable for, for example, automobile vehicles and automobile parts.

Claims

1. Applying at least one layer of base coating composition (Y) to a substrate to form at least one layer of uncured base coating film; Applying a clear coating composition (Z) onto the uncured base coating film to form a clear coating film; A method for forming a multilayer coating film, comprising heating and curing the uncured base coating film and the uncured clear coating film, The base coating composition (Y) a hydroxyl group-containing acrylic resin (y1) having a weight average molecular weight of 13,000 or more and 35,000 or less and a glass transition temperature of 20°C or more and less than 80°C; and a polyol compound (y2) having a hydroxyl value of more than 200 mgKOH / g and not more than 1000 mgKOH / g and a molecular weight of 100 or more and not more than 1000; the solid content of the hydroxyl group-containing acrylic resin (y1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y); The clear coating composition (Z) a hydroxyl group-containing acrylic resin (z1) having a hydroxyl value of 90 mgKOH / g or more and 190 mgKOH / g or less, a weight average molecular weight of 4,000 or more and 6,000 or less, and a glass transition temperature of 15°C or more and 100°C or less; hydroxyl group-containing polyester resin (z2), Polyisocyanate compound (z3), A metal catalyst (z4) containing at least one metal selected from the group consisting of zinc, bismuth, and tin, and an organic amine catalyst (z5) having an amidine group, The solid content of the hydroxyl group-containing acrylic resin (z1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the clear coating composition (Z). A method for forming a multi-layer coating film.

2. 2. The method for forming a multi-layer coating film according to claim 1, wherein the polyol compound (y2) has two hydroxyl groups and an alicyclic hydrocarbon group.

3. 3. The method for forming a multi-layer coating film according to claim 1, wherein the hydroxyl group-containing acrylic resin (y1) has a hydroxyl value of 30 mg KOH / g or more and 100 mg KOH / g or less.

4. The solid content of the polyol compound (y2) is 2 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y). A method for forming a multilayer coating film according to claim 1 or 2.

5. The method for forming a multilayer coating film according to claim 1 or 2, wherein the hydroxyl group-containing polyester resin (z2) has a hydroxyl value of 250 mg KOH / g or more and less than 500 mg KOH / g and a weight average molecular weight of 1500 or more and 2500 or less.

6. 3. The method for forming a multi-layer coating film according to claim 1, wherein the base coating composition (Y) has a solids concentration of 30% by mass or more and 60% by mass or less.

7. 3. The method for forming a multi-layer coating film according to claim 1, wherein the clear coating composition (Z) has a solids concentration of 45% by mass or more and 60% by mass or less.

8. 3. The method for forming a multilayer coating film according to claim 1, wherein the substrate comprises a resin portion containing at least one resin selected from the group consisting of ABS resin, PC-PBT resin, PC-ABS resin, polycarbonate resin, vinyl chloride resin, polyamide resin, and polypropylene resin.

9. 3. The method for forming a multi-layer coating film according to claim 1, wherein the organic amine catalyst (z5) has a heterocycle containing a nitrogen atom.

10. 3. The method for forming a multilayer coating film according to claim 1 or 2, wherein the metal catalyst (z4) comprises at least one selected from the group consisting of an acylate compound, an alkoxide compound, a chelate compound, and an oxide compound of zinc, bismuth, or tin.

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