Method for forming multilayer coating film

Abstract

A method for forming a multilayer coating film, the method comprising applying a waterborne first base coating composition to an object to be coated in order to form an uncured first base coating film, applying a waterborne second base coating composition on the uncured first base coating film to form an uncured second base coating film, applying a clear coating composition on the uncured second base coating film to form an uncured clear coating film, and heating to cure the uncured first base coating film, the uncured second base coating film, and the uncured clear coating film, wherein: preheating is not performed between the formation of the uncured first base coating film and the formation of the uncured second base coating film; the waterborne first base coating composition contains, per 100 parts by mass of the solid resin components in the waterborne first base coating composition, 5 to 30 parts by mass of an acrylic resin emulsion (A1), 20 to 60 parts by mass of a waterborne polyurethane resin (B1), 12.5 to 40 parts by mass of a waterborne polyester resin (C1), 10 to 40 parts by mass of a melamine resin (D1), and 5 to 30 parts by mass of an organic solvent (E1); the waterborne second base coating composition contains, per 100 parts by mass of the solid resin components in the waterborne second base coating composition, 31 to 60 parts by mass of an acrylic resin emulsion (A2), 10 to 40 parts by mass of a melamine resin (D2), 5 to 30 parts by mass of a waterborne resin (F2) other than the acrylic resin emulsion (A2), 20 to 40 parts by mass of an organic solvent (E2), and 1 to 40 parts by mass of a bright pigment (G2); and the organic solvent (E1 and the organic solvent (E2) are each at least one alcohol solvent (Ea) having a boiling point of 160 to 280°C and a water solubility of 0.1 to 3.0 g / L at 20°C.

Description

Method for forming a multi-layer coating

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

[0002] Automobile bodies typically have multiple coatings (multilayer coatings) laminated onto them. These multilayer coatings protect the body while simultaneously providing the automobile with superior aesthetic appeal. For example, a multilayer coating is formed by sequentially applying a first water-based paint composition, a second water-based paint composition, and a clear coat composition. From an energy-saving perspective, the second water-based paint composition may be applied after the first water-based paint composition without preheating. In this case, the first and second water-based paint compositions may mix, potentially reducing the aesthetic appeal.

[0003] Patent Document 1 discloses a method to prevent mixed layers by setting the viscosity of the aqueous conductive primer coating film that forms the lower layer to 600,000 mPas or more.

[0004] Japanese Patent Publication No. 2016-87569

[0005] In view of the above, this disclosure aims to provide a method for forming a multi-layer coating film with excellent aesthetic properties when a second aqueous base coating composition is applied after the first aqueous base coating composition has been applied, without preheating.

[0006] To solve the above problems, the present invention provides the following embodiments. [1] A method comprising: applying an aqueous first base coating composition to an object to be coated to form an uncured first base coating film; applying an aqueous second base coating composition on the uncured first base coating film to form an uncured second base coating film; applying a clear coating composition on the uncured second base coating film to form an uncured clear coating film; and heating to cure the uncured first base coating film, the uncured second base coating film, and the uncured clear coating film, wherein there is no preheating between the formation of the uncured first base coating film and the formation of the uncured second base coating film, and the aqueous first base coating composition comprises, per 100 parts by mass of the resin solids content of the aqueous first base coating composition, 5 to 30 parts by mass of acrylic resin emulsion (A1), 20 to 60 parts by mass of aqueous polyurethane resin (B1), and 12.5 to 40 parts by mass of aqueous polyester resin (C1), A method for forming a multilayer coating film, comprising: 10 to 40 parts by mass of melamine resin (D1); 5 to 30 parts by mass of organic solvent (E1); wherein the aqueous second base coating composition contains, per 100 parts by mass of the resin solids of the aqueous second base coating composition, 31 to 60 parts by mass of acrylic resin emulsion (A2), 10 to 40 parts by mass of melamine resin (D2), 5 to 30 parts by mass of aqueous resin other than the acrylic resin emulsion (A2) (F2), 20 to 40 parts by mass of organic solvent (E2), and 1 to 40 parts by mass of luminescent pigment (G2); wherein both the organic solvent (E1) and the organic solvent (E2) are at least one alcohol solvent (Ea) having a boiling point of 160 to 280°C and a solubility in water at 20°C of 0.1 to 3.0 g / L. [2] The method for forming a multilayer coating film according to [1], wherein the aqueous first base coating composition further contains 0.05 to 3.0 parts by mass of a viscous agent (H1) per 100 parts by mass of the resin solids content of the aqueous first base coating composition. [3] The method for forming a multilayer coating film according to [1] or [2], wherein the aqueous second base coating composition further contains 2.0 to 4.0 parts by mass of a viscous agent (H2) per 100 parts by mass of the resin solids content of the aqueous second base coating composition.[4] A method for forming a multilayer coating film according to any of [1] to [3] above, wherein the alcohol solvent (Ea) is at least one selected from the group consisting of 2-ethyl-1-hexanol, ethylene glycol 2-ethylhexyl ether, and diethylene glycol 2-ethylhexyl ether.

[0007] According to this disclosure, a method for forming a multi-layer coating film with excellent aesthetic properties is provided when a second aqueous base coating composition is applied after the first aqueous base coating composition has been applied, without preheating.

[0008] The lower layer, the first base coating, typically requires high smoothness. Therefore, it is desirable for the water-based first base coating composition to have low viscosity. The second base coating, which contains lustrous pigments, requires excellent sagging resistance (resistance to sagging) and flip-flop properties (FF properties). Therefore, it is desirable for the water-based second base coating composition to have high viscosity, and in addition, to have thixotropy (TI properties), so as not to disrupt the arrangement of the lustrous pigments.

[0009] This disclosure describes how to improve the aesthetic properties of a multilayer coating by increasing the viscosity and TI properties of a water-based second base coating composition using a specific organic solvent. Here, aesthetic properties refer to the overall characteristics of smoothness, flip-flop properties, and sagging properties. Smoothness also decreases as sagging properties decrease. Here, the viscosity of the water-based second base coating composition refers to the viscosity of the water-based second base coating composition (uncured second base coating film) after application and before heat curing. Similarly, the TI properties of the water-based second base coating composition refer to the TI properties of the water-based second base coating composition (uncured second base coating film) after application and before heat curing.

[0010] The viscosity and TI properties of the aqueous second base coating composition depend, for example, on the particle size of the acrylic resin emulsion (A2). The larger the particle size of the acrylic resin emulsion (A2), the closer the inter-particle distance becomes, and the easier it is for interactions to occur between the particles. As a result, the viscosity and TI properties of the aqueous second base coating composition increase. The particle size increases as the acrylic resin emulsion (A2) swells by incorporating an organic solvent into its interior.

[0011] The aqueous second base coating composition typically contains an organic solvent that readily causes swelling of the acrylic resin emulsion (A2). The alcohol solvent (Ea) used in this disclosure has very low solubility in water at 20°C, ranging from 0.1 to 3.0 g / L. Therefore, in the aqueous second base coating composition, the alcohol solvent (Ea) is easily incorporated into the acrylic resin emulsion (A2), increasing its particle size. This increases the viscosity and TI properties of the aqueous second base coating composition, improving its sagging and flip-flop properties.

[0012] However, if preheating is not performed, the organic solvent contained in the second aqueous base coating composition (or the organic solvent already incorporated into the acrylic resin emulsion (A2)) can easily migrate to the first aqueous base coating composition during the period until heat curing, and may replace other organic solvents contained in the first aqueous base coating composition. As a result, the swelling of the acrylic resin emulsion (A2) becomes insufficient, the viscosity and TI properties of the second aqueous base coating composition decrease, causing sagging or making it difficult to exhibit flip-flop properties.

[0013] In this disclosure, the first aqueous base coating composition is also formulated with an alcohol solvent (Ea) having the same properties as the second aqueous base coating composition. As a result, even if an organic solvent migrates from the second aqueous base coating composition to the first aqueous base coating composition, only an exchange of organic solvents with the same properties occurs, and the swelling state of the acrylic resin emulsion (A2) is maintained.

[0014] In this specification, the glass transition temperature (Tg) of a resin can be calculated from the type and amount of raw material monomers used in the production of the resin in question. Tg may be measured by a differential scanning calorimeter (DSC).

[0015] The number-average molecular weight (Mn) and weight-average molecular weight (Mw) can be measured by gel permeation chromatography (GPC) using a polystyrene standard sample after removing moisture by vacuum drying or other methods.

[0016] The hydroxyl value (OHV) and acid value (AV) are determined based on the solid content mass. The hydroxyl value (OHV) and acid value (AV) can be measured by known methods described in JIS K 0070:1992. The hydroxyl value (OHV) and acid value (AV) may also be calculated from the amount of unsaturated monomers in the raw material monomers of the resin in question.

[0017] Solids are also called non-volatile components. In specific cases, the solids of an aqueous paint composition are all the components of the aqueous paint composition excluding the solvent. The solids concentration is determined by dividing the total mass of the solids (excluding the solvent) by the total mass of the aqueous paint composition. The solids concentration can also be calculated from the residue when the aqueous paint composition is heated to 140°C, in accordance with JIS K 5601-1-2, the method for measuring residuals after heating. Specifically, the solids of an aqueous paint composition are the aqueous resin, curing agent, pigment, and other solid components added as needed, as described later.

[0018] The resin solids are the resin components among the solids mentioned above. Specifically, the solids of the aqueous paint composition are the aqueous resin and melamine resin described later.

[0019] The thickness of the coating can be measured using an electromagnetic film thickness gauge (for example, SANKO's "SDM-miniR" product). The coating thickness is the average value of the coating thickness at five arbitrary points.

[0020] The average particle size is the 50% average particle size (D50) in the volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. An example of a particle size distribution analyzer is the UPA-150 (Nikkiso Co., Ltd., Microtrac particle size distribution analyzer).

[0021] Water-based resins are generally classified into water-soluble and water-dispersible types. Water-dispersible resins are further divided into colloidal dispersion type and emulsion type. Colloidal dispersion type (hereinafter simply referred to as dispersion type) water-based resins are typically obtained by partially dissolving a resin synthesized in an organic solvent (solution-polymerized resin) in water using a neutralizing agent. Emulsion type water-based resins are typically produced by emulsion polymerization or by mechanical forced emulsification.

[0022] In the case of water-based acrylic resins, those with an Mw (Metal Weight) exceeding 100,000 can be considered emulsion type, while those with an Mw of 100,000 or less can be considered colloidal dispersion type. Measuring Mw becomes difficult when it exceeds 1,000,000. Water-based acrylic resins for which Mw cannot be measured can be considered emulsion type. Water-based resins include water-soluble resins, dispersion-type resins, and emulsion-type resins.

[0023] Acrylic resin emulsions (A1, A2) are emulsion-type acrylic resins.

[0024] First, each paint composition will be described. (Aqueous first base paint composition) The aqueous first base paint composition contains, per 100 parts by mass of the resin solids content of the aqueous first base paint composition, 10 to 30 parts by mass of acrylic resin emulsion (A1), 20 to 60 parts by mass of aqueous polyurethane resin (B1), 12.5 to 40 parts by mass of aqueous polyester resin (C1), 10 to 40 parts by mass of melamine resin (D1), and 5 to 30 parts by mass of organic solvent (E1).

[0025] The organic solvent (E1) comprises at least one alcohol solvent (Ea) having a boiling point of 160 to 280°C and a solubility of 0.1 to 3.0 g / L in water at 20°C.

[0026] The solid content concentration of the first aqueous base coating composition is, for example, 20 to 60% by mass. When the solid content concentration is 20% by mass or higher, the coating efficiency is improved. When the solid content concentration is 60% by mass or lower, the smoothness can be further improved. The lower limit of the solid content concentration may be 25% by mass. The upper limit of the solid content concentration may be 55% by mass.

[0027] - Acrylic resin emulsion (A1) The acrylic resin emulsion (A1) has a hydroxyl group and a carboxyl group. The hydroxyl group of the acrylic resin emulsion (A1) reacts with the melamine resin (D1) to form a crosslinked structure.

[0028] The Mw of the acrylic resin emulsion (A1) is, for example, 100,000 or more. The Mw of the acrylic resin emulsion (A1) may be 200,000 or more. The upper limit value of the Mw of the acrylic resin emulsion (A1) is not limited.

[0029] The OHV of the acrylic resin emulsion (A1) is, for example, 10 to 100 mgKOH / g. Thereby, the crosslinking density becomes high and the chipping resistance can be improved. The above OHV may be 20 to 90 mgKOH / g, and may be 30 to 80 mgKOH / g. The lower limit of the above OHV may be 20 mgKOH / g, and may be 30 mgKOH / g. The upper limit of the above OHV may be 90 mgKOH / g, and may be 80 mgKOH / g.

[0030] The acid value (AV) of the acrylic resin emulsion (A1) is, for example, 5 to 50 mgKOH / g. The above AV may be 10 to 40 mgKOH / g, and may be 15 to 30 mgKOH / g. The lower limit of the above AV may be 10 mgKOH / g, and may be 15 mgKOH / g. The upper limit of the above AV may be 40 mgKOH / g, and may be 30 mgKOH / g.

[0031] The Tg of the acrylic resin emulsion (A1) is, for example, -15 to 60°C. The above Tg may be -10 to 55°C, and may be 25 to 50°C. The lower limit of the above Tg may be -10°C, and may be 25°C. The upper limit of the above Tg may be 55°C, and may be 50°C.

[0032] The average particle diameter of the acrylic resin emulsion (A1) is, for example, 20 to 200 nm. The above average particle diameter may be 25 to 180 nm, or may be 30 to 160 nm. The lower limit of the above average particle diameter may be 25 nm, or may be 30 nm. The upper limit of the above average particle diameter may be 180 nm, or may be 160 nm. The above average particle diameter is the average particle diameter of the acrylic resin emulsion (A1) in the first base paint composition before coating. It is desirable that the average particle diameter of the acrylic resin emulsion (A1) in the first base paint composition after coating and before heat curing is the same as that in the first base paint composition before coating.

[0033] The solid content of the acrylic resin emulsion (A1) is 5 to 30 parts by mass with respect to 100 parts by mass of the resin solid content of the aqueous first base paint composition. Thereby, the gloss, water resistance, weather resistance, and chemical resistance of the coating film are improved. The above content may be 7 to 25 parts by mass, or may be 10 to 20 parts by mass. The lower limit of the above content may be 7 parts by mass, or may be 10 parts by mass. The upper limit of the above content may be 25 parts by mass, or may be 20 parts by mass.

[0034] The acrylic resin emulsion (A1) is used alone or in combination of two or more. When a plurality of types of acrylic resin emulsions are included, the OHV, AV, and Tg of the acrylic resin emulsion (A1) are average values calculated based on the OHV, AV, and Tg of each acrylic resin emulsion and the mass ratio.

[0035] The acrylic resin emulsion (A1) is obtained, for example, by copolymerizing a monomer containing an α,β-ethylenically unsaturated monomer having a hydroxyl group and an α,β-ethylenically unsaturated monomer having a carboxyl group.

[0036] Examples of α,β-ethylenically unsaturated monomers having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, allyl alcohol, methacrylic alcohol, and adducts of hydroxyethyl (meth)acrylate with ε-caprolactone. Alternatively, 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, or adducts of hydroxyethyl (meth)acrylate with ε-caprolactone may be used.

[0037] "(Meth)acrylic acid" includes both acrylic acid and methacrylic acid.

[0038] Examples of α,β-ethylenically unsaturated monomers having a carboxyl group include acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, maleic acid, fumaric acid, and itaconic acid. Acrylic acid or methacrylic acid may also be used.

[0039] Other α,β-ethylenically unsaturated monomers may be used as copolymerization components. Examples of other α,β-ethylenically unsaturated monomers include (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl methacrylate, phenyl acrylate, isobornyl (meth)acrylate, cyclohexyl methacrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate; and polymerizable amide compounds such as (meth)acrylamide, N-methylol(meth)acrylamide, and N-butoxymethyl(meth)acrylamide.

[0040] Other α,β-ethylenically unsaturated monomers may be crosslinkable monomers. Crosslinkable monomers have two or more radically polymerizable ethylenically unsaturated groups in their molecule. Examples of crosslinkable monomers include divinylbenzene, allyl (meth)acrylate, and ethylene glycol di(meth)acrylate.

[0041] Acrylic resin emulsion (A1) is typically synthesized by emulsion polymerization. The emulsion polymerization method is not particularly limited. For example, an emulsifier is dissolved in an aqueous medium containing water, or optionally an organic solvent such as alcohol or ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and the raw material monomers and polymerization initiators are added dropwise under heating and stirring. The raw material monomers may be emulsified beforehand with an emulsifier.

[0042] Examples of polymerization initiators include azo-based oily compounds such as azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), and 2,2'-azobis(2,4-dimethylvaleronitrile); aqueous compounds such as anionic 4,4'-azobis(4-cyanovaleric acid), 2,2-azobis(N-(2-carboxyethyl)-2-methylpropionamidine), and cationic 2,2'-azobis(2-methylpropionamidine); redox-based oily peroxides such as benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide, and t-butyl perbenzoate; and aqueous peroxides such as potassium persulfate and ammonium persulfate.

[0043] The emulsifier is not particularly limited. Examples of emulsifiers include reactive emulsifiers. Examples of reactive emulsifiers include Antox MS-60 (manufactured by Nippon Emulsifier Co., Ltd.), Eleminor JS-2 (manufactured by Sanyo Chemical Industries, Ltd.), Adekarya Soap NE-20 (manufactured by ADEKA Corporation), Aqualon HS-10 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and Latemul PD-104 (manufactured by Kao Corporation). To adjust the molecular weight, chain transfer agents such as mercaptans (e.g., lauryl mercaptan) and α-methylstyrene dimers may be used.

[0044] The reaction temperature is determined according to the polymerization initiator. For example, when using an azo initiator or peroxide, the reaction temperature is 60 to 90°C. When using a redox initiator, the reaction temperature is 30 to 70°C. The reaction time is 1 to 8 hours. The amount of polymerization initiator per 100 parts by mass of the monomer mixture is 0.1 to 5% by mass. Emulsion polymerization can be carried out in multiple steps, for example, in two steps. In two-step polymerization, a portion of the raw material monomers are emulsion-polymerized, and then the remaining raw material monomers are polymerized.

[0045] Acrylic resin emulsion (A1) may be neutralized with a basic compound from the viewpoint of storage stability. Acrylic resin emulsion (A1) has a pH of 5 to 10. Neutralization is performed before or after emulsion polymerization.

[0046] The basic compound is not particularly limited and, for example, at least one selected from the group consisting of ammonia and amine compounds. Examples of amine compounds include dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, n-propylamine, isopropylamine, triallylamine, triethylenediamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, aminoethanolamine, N-methyl-N,N-diethanolamine, iminobispropylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, methylaminopropylamine, methyliminobispropylamine, 3-methoxypropylamine, monoethanolamine, diethanolamine, triethanolamine, morpholine, allylmorpholine, N-methylmorpholine, and N-ethylmorpholine. These can be used individually or in combination of two or more.

[0047] Acrylic resin emulsion (A1) can also be obtained by solution polymerization and neutralization. Solution polymerization is carried out by known methods.

[0048] • Water-based polyurethane resin (B1) The water-based polyurethane resin (B1) may be water-soluble and may be water-dispersible. The water-based polyurethane resin (B1) may be of the dispersion type. The water-based polyurethane resin (B1) may be used alone or in combination of two or more types.

[0049] Water-based polyurethane resin (B1) fuses with itself and other components to strengthen the coating film and enhance its shock absorption. Water-based polyurethane resin (B1) also improves the chipping resistance and adhesion of multi-layer coatings.

[0050] The OHV of the aqueous polyurethane resin (B1) is, for example, 0 mg KOH / g or more and 20 mg KOH / g or less. This suppresses the reaction between the (B1) aqueous polyurethane resin and the (D1) melamine resin, making it easier for the (B1) aqueous polyurethane resin to exhibit its properties (typically, shock absorption) in the coating film. The OHV of the (B1) aqueous polyurethane resin may be 17 mg KOH / g or less, or 15 mg KOH / g or less.

[0051] The AV of the aqueous polyurethane resin (B1) is, for example, 0 to 20 mg KOH / g. This improves water resistance. The AV may be 5 to 17 mg KOH / g or 10 to 15 mg KOH / g. The lower limit of the AV may be 5 mg KOH / g or 10 mg KOH / g. The upper limit of the AV may be 17 mg KOH / g or 15 mg KOH / g.

[0052] The Tg of the aqueous polyurethane resin (B1) is, for example, -20°C or lower. When Tg is -20°C or lower, chipping resistance is further improved. The above Tg may be -90°C to -20°C, -90°C to -30°C, or -90°C to -50°C. The upper limit of the above Tg may be -30°C or -50°C. The lower limit of the above Tg is not particularly limited. The lower limit of the above Tg may be, for example, -90°C.

[0053] The weight-average molecular weight (Mw) of the aqueous polyurethane resin (B1) is, for example, 20,000 or more. This increases the strength of the coating film and improves adhesion. The above Mw may be 20,000 to 1,000,000, 150,000 to 1,000,000, or 250,000 to 1,000,000. The lower limit of the above Mw may be 150,000 or 250,000. The upper limit of the above Mw is not particularly limited. The upper limit of the above Mw may be, for example, 1,000,000. If the above Mw cannot be measured, it may be considered to be 20,000 or more.

[0054] The solid content of the aqueous polyurethane resin (B1) is 20 parts by mass or more and 60 parts by mass or less per 100 parts by mass of the resin solid content of the first aqueous base coating composition. This improves the flexibility of the coating film and improves chipping resistance. The above content may be 30 to 55 parts by mass, or 35 to 50 parts by mass. The lower limit of the above content may be 30 parts by mass, or 35 parts by mass. The upper limit of the above content may be 55 parts by mass, or 50 parts by mass.

[0055] Aqueous polyurethane resin (B1) can be obtained, for example, by forcibly emulsifying the polyurethane resin using a surfactant, or by neutralizing the polyurethane resin with a base or acid.

[0056] Polyurethane resins can be obtained, for example, by the reaction of polyols, compounds having active hydrogen groups and hydrophilic groups in their molecules, polyisocyanate compounds described later, and, if necessary, chain extenders and polymerization inhibitors. Chain extenders and polymerization inhibitors may be used as needed.

[0057] Examples of polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyacrylate polyols, and polyhydric alcohols. These can be used individually or in combination of two or more. In particular, the polyol may be a polyether polyol.

[0058] Examples of compounds having an active hydrogen group and a hydrophilic group within the molecule include compounds containing an active hydrogen group and an anionic group, a cationic group, or a nonionic hydrophilic group. Anionic groups include anionic groups and anion-forming groups. Anion-forming groups are groups that can react with a base to form an anionic group, specifically by neutralization with a base before, during, or after the urethane reaction to form an anionic group.

[0059] Examples of compounds containing active hydrogen and an anionic group are described in the specifications of Japanese Patent Publication No. 42-24192 and Japanese Patent Publication No. 55-41607, specifically α,α-dimethylolpropionic acid and α,α-dimethylolbutyric acid. Examples of compounds having active hydrogen and a cationic group are described in the specifications of Japanese Patent Publication No. 43-9076. Examples of compounds having active hydrogen and a nonionic hydrophilic group are described in Japanese Patent Publication No. 48-41718, specifically polyethylene glycol and alkyl alcohol alkylene oxide adducts.

[0060] • Aqueous polyester resin (C1) The aqueous polyester resin (C1) may be of the dispersion type. The aqueous polyester resin (C1) may have hydroxyl groups and carboxyl groups.

[0061] The OHV of the aqueous polyester resin (C1) is, for example, 50 to 150 mg KOH / g. The above OHV may be 70 to 120 mg KOH / g or 70 to 100 mg KOH / g. The lower limit of the above OHV may be 70 mg KOH / g. The upper limit of the above OHV may be 120 mg KOH / g or 100 mg KOH / g.

[0062] The AV of the aqueous polyester resin (C1) is, for example, 0 to 60 mg KOH / g. The AV may be 5 to 50 mg KOH / g or 5 to 40 mg KOH / g. The lower limit of the AV may be 5 mg KOH / g. The upper limit of the AV may be 50 mg KOH / g or 40 mg KOH / g.

[0063] The manganese (Mn) of the aqueous polyester resin (C1) is, for example, 500 to 20,000. When the Mn is 500 or higher, storage stability is improved. When the Mn is 20,000 or lower, viscosity increase is suppressed and painting workability is improved. The Mn may be 1,500 to 10,000. The lower limit of the Mn may be 1,500. The upper limit of the Mn may be 10,000.

[0064] The Tg of the water-based polyester resin (C1) is, for example, -20 to 80°C. When the Tg is -20°C or higher, the hardness of the resulting coating film is increased. When the Tg is 80°C or lower, the ability to conceal the substrate (opacity) is improved. The Tg may be 0 to 60°C. The lower limit of the Tg may be 0°C. The upper limit of the Tg may be 60°C.

[0065] The solid content of the aqueous polyester resin (C1) is 12.5 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the resin solid content of the first aqueous base coating composition. This reduces the melt viscosity during flow and improves the smoothness of the coating film. The above content may be 15 to 35 parts by mass, or 18 to 30 parts by mass. The lower limit of the above content may be 15 parts by mass, or 18 parts by mass. The upper limit of the above content may be 35 parts by mass, or 30 parts by mass.

[0066] Aqueous polyester resin (C1) is obtained by neutralizing a polyester resin with a basic compound. Polyester resins are produced, for example, by the condensation of a polyhydric alcohol component and a polybasic acid component.

[0067] Examples of polyhydric alcohol components include hydroxycarboxylic acid components such as ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, neopentyl glycol hydroxypivalate ester, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethylpentanediol. These can be used individually or in combination of two or more.

[0068] Examples of polybasic acid components include aromatic polycarboxylic acids and acid anhydrides such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrachlorophthalic anhydride, and pyromellitic anhydride; alicyclic polycarboxylic acids and anhydrides such as hexahydrophthalic anhydride, tetrahydrophthalic anhydride, and 1,4- and 1,3-cyclohexanedicarboxylic acid; and aliphatic polycarboxylic acids and anhydrides such as maleic anhydride, fumaric acid, succinic anhydride, adipic acid, and sebacic acid. These can be used individually or in combination of two or more.

[0069] If necessary, monobasic acids such as benzoic acid and t-butylbenzoic acid may be used in combination.

[0070] Monohydric alcohols, monoepoxide compounds such as Cardura E (trade name: manufactured by Oxalis Chemicals), and lactones such as β-propiolactone, dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone, and γ-caprolactone may be used in combination as reaction components. These may be used individually or in combination of two or more.

[0071] Furthermore, fatty acids such as castor oil, dehydrated castor oil, and one or more of these fatty acids may be added to the reaction system.

[0072] The polyester resin may be grafted with an acrylic resin and / or a vinyl resin, and may be reacted with a polyisocyanate compound.

[0073] ・Melamine resin (D1)The melamine resin (D1) reacts with the hydroxyl groups contained in the aqueous resin to form a cured coating film. The melamine resin (D1) has, around the triazine ring (triazine nucleus), six substituents R １ ~N ３ bonded through N １ ~R ６ (the structure of -N １ (R １ )(R ２ ), -N ２ (R ３ [[ID=2l]])(R ４ ), -N ３ (R ５ )(R ６ )).

[0074] The melamine resin (D1) is represented, for example, by the following general formula (1):

[0075] (In the formula, the substituents R １ ~R ６ [[ID=so]] each independently represent a hydrogen atom, an alkyl ether group, a methylol group, or a bonding portion to another triazine ring.)

[0076] The carbon number of the alkyl group (R ２ -OR ７ ) constituting the alkyl ether (-CH ７ ) may be 1 to 8, and may be 1 to 4. R ７ may be linear or branched. R[[ID=sz]] ７ may be a methyl group, an ethyl group, a propyl group or a butyl group. [[ID=ss]]

[0077] The melamine resin (D1) may be composed of a polynuclear body in which a plurality of triazine rings are bonded, or may be a mononuclear body consisting of one triazine ring.

[0078] Examples of the melamine resin (D1) include -N(-CH ２ -OR ７ )(-CH ２Methylol group having OH; -N(-CH ２ -OR ７ Imino group having (H); -N(-CH ２ -OR ７ ) (-CH ２ OH) and -N(-CH) ２ -OR ７ )(H) methylol / imino group type, substituent R １ ~R ６ One example is the fully alkyl type, which has only alkyl ether groups.

[0079] The Mw of the melamine resin (D1) may be 400 to 3,000. This can lower the viscosity of the first aqueous base coating composition and further improve its smoothness. The above Mw may be 500 to 2,800 or 600 to 2,500. The lower limit of the above Mw may be 500 or 600. The upper limit of the above Mw may be 2,800 or 2,500.

[0080] The solid content of melamine resin (D1) is 10 parts by mass or more and 40 parts by mass or less per 100 parts by mass of resin solid content of the aqueous first base coating composition. This improves the hardness, window bond resistance, and water resistance of the coating film. The above content may be 15 to 35 parts by mass, or 20 to 30 parts by mass. The lower limit of the above content may be 15 parts by mass, or 20 parts by mass. The upper limit of the above content may be 35 parts by mass, or 30 parts by mass.

[0081] • Organic solvent (E1) The organic solvent (E1) is at least one alcohol solvent (Ea) having a boiling point of 160 to 280°C and a solubility of 0.1 to 3.0 g / L in water at 20°C. The organic solvent (E1) may contain multiple types of alcohol solvents (Ea). Details of the alcohol solvents (Ea) will be described later.

[0082] The content of the organic solvent (E1) is 5 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the resin solids of the aqueous first base coating composition. This maintains the swollen state of the acrylic resin emulsion (A2). The above content may be 8 to 25 parts by mass, or 10 to 20 parts by mass. The lower limit of the above content may be 8 parts by mass, or 10 parts by mass. The upper limit of the above content may be 25 parts by mass, or 20 parts by mass.

[0083] The inclusion of an alcohol solvent (Ea) in the first water-based coating composition ensures that even when an exchange of organic solvents occurs, the swelling of the acrylic resin emulsion (A2) within the uncured second base coating film is maintained, improving sagging and flip-flop properties.

[0084] • Hydrophilic solvent (Ee) The first water-based paint composition may further contain an organic solvent other than the alcohol solvent (Ea). Examples of other organic solvents include at least one of an ether solvent (Ee1) with a boiling point of 120 to 240°C and an ester solvent (Ee2) with a boiling point of 140 to 260°C, and a hydrophilic solvent (Ee) with a solubility of 10 g / L or more in water at 20°C. The hydrophilic solvent (Ee) suppresses viscosity changes of the paint composition during and after application, improving sagging and smoothness. Details of the hydrophilic solvent (Ee) will be described later.

[0085] The hydrophilic solvent (Ee) is preferably less than the alcohol solvent (Ea). The proportion of the hydrophilic solvent (Ee) may be 0 to 90% by mass, 10 to 80% by mass, or 30 to 70% by mass of the alcohol solvent (Ea).

[0086] The other organic solvent-based water-based first base coating composition may contain organic solvents other than the alcohol solvent (Ea) and the hydrophilic solvent (Ee). The proportion of the other organic solvent may be 50% by mass or less, 40% by mass or less, or 30% by mass of the alcohol solvent (Ea).

[0087] • Viscosity agent (H1) The aqueous first base coating composition may contain a viscosity agent (H1). The viscosity agent (H1) adjusts the viscosity of the aqueous first base coating composition to a desired value.

[0088] Examples of viscous agents (H1) include inorganic viscous agents, urethane-associated viscous agents, acrylic viscous agents, mineral viscous agents, polyamide viscous agents, cellulose viscous agents, polyolefin viscous agents, polyurea viscous agents, and organic resin fine particle viscous agents. These can be used individually or in combination of two or more.

[0089] Examples of inorganic viscous agents include layered silicates (silicate minerals), halide minerals, oxide minerals, carbonate minerals, borate minerals, sulfate minerals, molybdate minerals, tungstate minerals, phosphate minerals, arsenate minerals, and vanadate minerals.

[0090] Examples of urethane-associated viscous agents include polyurethane-based viscous agents having hydrophobic chains in their molecules, and urethane-urea-based viscous agents in which at least a portion of the main chain is a hydrophobic urethane chain.

[0091] Examples of acrylic viscous agents include poly(meth)acrylic acid-based viscous agents. Specifically, these include poly(meth)acrylic acid, poly(meth)acrylic acid-poly(meth)acrylic acid alkyl ester copolymers, poly(meth)acrylic acid salts, and hydrophobic group-modified polyacrylic acid. Examples of poly(meth)acrylic acid salts include sodium poly(meth)acrylate and potassium poly(meth)acrylate. In hydrophobic group-modified polyacrylic acid, some of the carboxyl groups are modified by hydrophobic groups such as styrene groups and alkyl groups.

[0092] Examples of mineral-based viscous agents include swelling layered silicates having a 2:1 crystal structure. Specifically, these include natural or synthetic smectite group clay minerals such as montmorillonite, saponite, hectorite, stivunsite, bydelite, nontronite, bentonite, and laponite; swelling mica group clay minerals such as Na-type tetrasilicic fluorite, Li-type tetrasilicic fluorite, Na-salt type fluorite teniolite, and Li-type fluorite teniolite; vermiculite; and their substitutions and derivatives. Among these, bentonite is particularly suitable.

[0093] Examples of polyamide-based viscous agents include fatty acid amides, polyamides, acrylamides, long-chain polyaminoamides, aminoamides, and salts thereof (e.g., phosphates).

[0094] Examples of cellulose-based viscous agents include cellulose acetate butyrate (CAB), carboxymethylcellulose, methylcellulose, hydroxyethylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose, methylcellulose, and cellulose nanofiber gel. These can be used individually or in combination of two or more. CAB is particularly suitable.

[0095] Examples of polyolefin-based viscous agents include polyethylene, polypropylene, ethylene-propylene copolymer, polyethylene oxide, polypropylene oxide, ethylene-propylene copolymer, ethylene-(meth)acrylic acid copolymer, ethylene-maleic anhydride copolymer, and propylene-maleic anhydride copolymer.

[0096] Polyurea-based viscous agents are reaction products of isocyanates and amines, and each molecule contains multiple urea bonds.

[0097] The organic resin microparticle viscous agent is an organic resin particle having a three-dimensional cross-linked structure inside. The organic resin microparticle viscous agent swells when exposed to an organic solvent (E1), but the amount of swelling is controlled by the three-dimensional cross-linked structure.

[0098] In particular, the viscous agent may be at least one selected from the group consisting of acrylic viscous agents, urethane-associated viscous agents, and polyamide viscous agents.

[0099] The solid content of the viscous agent (H1) is, for example, 0.05 to 3.0 parts by mass per 100 parts by mass of the resin solid content of the aqueous first base coating composition. When the content of the viscous agent (H1) is within this range, it is possible to achieve an appropriate viscosity that does not cause sagging and has excellent smoothness. The above content may be 0.1 to 2.5 parts by mass or 0.1 to 2.0 parts by mass. The lower limit of the above content may be 0.1 parts by mass or 0.2 parts by mass. The upper limit of the above content may be 3.5 parts by mass or 3.0 parts by mass.

[0100] The pigment-based water-based first base coating composition may contain pigments. The pigments enhance the opacity of the first base coating and the weather resistance of the multi-layer coating.

[0101] The pigment content is not particularly limited. In terms of opacity, the pigment content may be 30 to 60% by mass of the resin solids content of the aqueous first base coating composition. The above pigment content may be 35 to 55% by mass, or 40 to 50% by mass.

[0102] The type of pigment is not particularly limited. Examples of pigments include coloring pigments, luminosity pigments, and extender pigments. These can be used individually or in combination of two or more.

[0103] The coloring pigments may be organic or inorganic. Examples of organic coloring pigments include azo chelate pigments, insoluble azo pigments, condensed azo pigments, monoazo pigments, disazo pigments, diketopyrrolopyrrole pigments, benzimidazolon pigments, phthalocyanine pigments, indigo pigments, thioindigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, naphthol pigments, pyrazolone pigments, anthraquinone pigments, antholapyrimidine pigments, and metal complex pigments. These may be used individually or in combination of two or more.

[0104] Examples of inorganic coloring pigments include zinc oxide, titanium dioxide, lead yellow, iron yellow oxide, chromium oxide, molybdate orange, red iron oxide, titanium yellow, carbon black, cobalt green, phthalocyanine green, ultramarine, cobalt blue, phthalocyanine blue, and cobalt violet. These can be used individually or in combination of two or more.

[0105] Examples of luminous pigments include aluminum flakes, alumina flakes, mica, silica flakes, and glass flakes.

[0106] Examples of extender pigments include calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silica, and calcined kaolin. These can be used individually or in combination of two or more.

[0107] The pigment may be incorporated into the first aqueous base coating composition as a pigment dispersion paste. The pigment dispersion paste is obtained by mixing the pigment, a pigment dispersant, and an aqueous medium. The pigment dispersant is a resin having a structure that includes a pigment-affinity portion and a hydrophilic portion. Examples of the pigment-affinity portion and hydrophilic portion include nonionic, cationic, or anionic functional groups. The pigment dispersant may have two or more of the above functional groups in one molecule. The pigment dispersant may be used alone or in combination of two or more types.

[0108] The pigment dispersant is not particularly limited. Examples of commercially available pigment dispersants include Disperbyk 190, Disperbyk 181, Disperbyk 182, Disperbyk 184 (all anionic / nonionic dispersants manufactured by Bic Chemie), EFKA APOLYMER 4550 (anionic / nonionic dispersant manufactured by EFKA), Solspers 27000 (nonionic dispersant manufactured by Abyssia), Solspers 41000, and Solspers 53095 (anionic dispersants manufactured by Abyssia).

[0109] Other water-based first base coating compositions may contain additives. Examples of additives include anti-settlement agents, curing catalysts, UV absorbers, antioxidants, leveling agents, surface modifiers such as silicones and organic polymers, anti-sagging agents, defoaming agents, pigment dispersants, and lubricants.

[0110] The first water-based paint composition contains water as the main solvent. 50% or more by mass of the total mass of the organic solvent (E1) and water may be water. The proportion of water may be 70% or more by mass, and may be 90% or more by mass, of the total mass of the organic solvent (E1) and water.

[0111] The first aqueous base coating composition may contain a curing agent other than the melamine resin (D1). Examples of other curing agents include polyisocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, and oxazoline compounds. The content of the other curing agents is not particularly limited and is appropriately set depending on the aqueous resin.

[0112] The first aqueous base coating composition may contain aqueous resins other than acrylic resin emulsion (A1), aqueous polyurethane resin (B1), and aqueous polyester resin (C1). The content of the other aqueous resins is not particularly limited and is set as appropriate depending on the purpose. Examples of the other aqueous resins include polyols such as polyester polyols and polyether polyols, and water-soluble and / or dispersion-type acrylic resins.

[0113] • Preparation Method: The first aqueous base coating composition is prepared by mixing the above components with water in a known manner. The first aqueous base coating composition is further diluted with a solvent such as water as needed and then used for coating.

[0114] The aqueous second base coating composition contains, per 100 parts by mass of the resin solids content of the aqueous second base coating composition, 31 to 60 parts by mass of acrylic resin emulsion (A2), 10 to 40 parts by mass of melamine resin (D2), 5 to 30 parts by mass of aqueous resin other than acrylic resin emulsion (A2) (F2), 20 to 40 parts by mass of organic solvent (E2), and 1 to 40 parts by mass of luminous pigment (G2).

[0115] The organic solvent (E2) comprises at least one alcohol solvent (Ea) having a boiling point of 160 to 280°C and a solubility of 0.1 to 3.0 g / L in water at 20°C.

[0116] The solid content concentration of the aqueous second base coating composition is, for example, 7 to 30% by mass. When the solid content concentration is 7% by mass or higher, the coating efficiency is improved. When the solid content concentration is 30% by mass or lower, the sagging and flip-flop properties are further improved. The solid content concentration may be 10 to 25% by mass, or 15 to 20% by mass.

[0117] • Acrylic resin emulsion (A2) The acrylic resin emulsion (A2) has hydroxyl groups and carboxyl groups. The hydroxyl groups of the acrylic resin emulsion (A2) react with the melamine resin (D2) to form a crosslinked structure.

[0118] The physical properties of acrylic resin emulsion (A2) may be the same as those of acrylic resin emulsion (A1).

[0119] The solid content of the acrylic resin emulsion (A2) is 31 parts by mass or more and 60 parts by mass or less per 100 parts by mass of the resin solid content of the aqueous second base coating composition. This causes the acrylic resin emulsion (A2) to swell in the solvent, resulting in viscosity and TI properties, and improving sagging and flip-flop properties. As a result of suppressing sagging, smoothness is improved. The above content may be 35 to 60 parts by mass, 40 to 60 parts by mass, or 45 to 60 parts by mass. The lower limit of the above content may be 35 parts by mass, 40 parts by mass, or 45 parts by mass.

[0120] Acrylic resin emulsion (A2) is used either alone or in combination of two or more types. When multiple types of acrylic resin emulsion are included, the OHV, AV, and Tg of acrylic resin emulsion (A2) are average values ​​calculated based on the OHV, AV, Tg, and mass ratio of each acrylic resin emulsion.

[0121] - Melamine resin (D2) The physical properties of melamine resin (D2) may be the same as those of melamine resin (D1).

[0122] The solid content of melamine resin (D2) is 10 parts by mass or more and 40 parts by mass or less per 100 parts by mass of resin solid content of the aqueous second base coating composition. This improves the hardness, window bond resistance, and water resistance of the coating film. The above content may be 15 to 35 parts by mass, or 20 to 30 parts by mass. The lower limit of the above content may be 15 parts by mass, or 20 parts by mass. The upper limit of the above content may be 35 parts by mass, or 30 parts by mass.

[0123] - Aqueous resin (F2) other than acrylic resin emulsion (A2) The aqueous second base coating composition may contain aqueous resin (F2) other than acrylic resin emulsion (A2) and melamine resin (D2).

[0124] Other aqueous resins (F2) include, for example, polyols such as polyester polyols and polyether polyols, aqueous polyurethane resins similar to aqueous polyurethane resin (B1), aqueous polyester resins similar to aqueous polyester resin (C1), and water-soluble and / or dispersion-type acrylic resins.

[0125] Water-soluble acrylic resins can be prepared, for example, by solution polymerization of the above-mentioned α,β-ethylenically unsaturated monomers and then making them water-soluble using a basic compound.

[0126] The solid content of the other aqueous resin (F2) is 5 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the resin solid content of the second aqueous base coating composition. The above content may be 10 to 25 parts by mass, or 15 to 25 parts by mass. The lower limit of the above content may be 10 parts by mass, or 15 parts by mass. The upper limit of the above content may be 25 parts by mass.

[0127] • Organic solvent (E2) The organic solvent (E2) is at least one of the above alcohol solvents (Ea). The organic solvent (E2) may contain multiple types of alcohol solvents (Ea).

[0128] The content of the organic solvent (E2) is 20 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the resin solids of the aqueous second base coating composition. This maintains the swollen state of the acrylic resin emulsion (A2). The above content may be 25 to 40 parts by mass, or 30 to 40 parts by mass. The lower limit of the above content may be 25 parts by mass, or 30 parts by mass.

[0129] It is desirable that the content of organic solvent (E2) per 100 parts by mass of resin solids in the aqueous second base coating composition is greater than the content of organic solvent (E1) per 100 parts by mass of resin solids in the aqueous first base coating composition.

[0130] The alcohol solvent (Ea) has a boiling point of 160 to 280°C and a solubility of 0.1 to 3.0 g / L in water at 20°C. The alcohol solvent (Ea), with its very low solubility in water at 20°C, maintains the swelling of the acrylic resin emulsion (A2) within the uncured second base coating film, improving its sagging and flip-flop properties.

[0131] If the boiling point of the alcohol solvent (Ea) is 160°C or higher, the evaporation of the alcohol solvent during the application of the aqueous second base coating composition is suppressed, which contributes to the swelling of the acrylic resin emulsion (A2). If the boiling point of the alcohol solvent (Ea) is 280°C or lower, the alcohol solvent can be easily removed by subsequent heat curing. The boiling point may be between 170°C and 250°C. The lower limit of the boiling point may be 170°C. The upper limit of the boiling point may be 250°C.

[0132] If the solubility of the alcohol solvent (Ea) in water at 20°C is 0.1 to 3.0 g / L, it is easier to swell the acrylic resin emulsion (A2). The above solubility may be 0.5 to 2.5 g / L or 0.5 to 2.0 g / L. The lower limit of the above solubility may be 0.5 g / L. The upper limit of the above solubility may be 2.5 g / L or 2.0 g / L.

[0133] The alcohol solvent (Ea) may be a compound having at least one hydroxyl group and a linear or branched aliphatic hydrocarbon group having 6 to 9 carbon atoms. The aliphatic hydrocarbon group may be a 2-ethylhexyl group.

[0134] Examples of alcohol solvents (Ea) include 2-ethyl-1-hexanol, ethylene glycol 2-ethylhexyl ether, and diethylene glycol 2-ethylhexyl ether. These can be used individually or in combination of two or more.

[0135] • Hydrophilic solvent (Ee) The organic solvent (E2) may further contain organic solvents other than the alcohol solvent (Ea). Examples of other organic solvents include at least one of the hydrophilic solvents (Ee) of the ether solvent (Ee1) and ester solvent (Ee2) mentioned above. The hydrophilic solvent (Ee) suppresses changes in the viscosity of the paint composition during and after application, thereby improving the smoothness of the paint film.

[0136] The hydrophilic solvent (Ee) is preferably less than the alcohol solvent (Ea). The proportion of the hydrophilic solvent (Ee) may be 0 to 90% by mass, 10 to 80% by mass, or 30 to 70% by mass of the alcohol solvent (Ea).

[0137] (Ether solvent (Ee1)) The ether solvent (Ee1) has a boiling point of 120 to 240°C and a solubility of 10 g / L or more in water at 20°C. If the boiling point of the ether solvent (Ee1) is 120°C or higher, the volatilization of the ether solvent (Ee1) during the coating of the aqueous second base coating composition is suppressed, which contributes to suppressing viscosity increase. If the boiling point of the ether solvent (Ee1) is 240°C or lower, the ether solvent (Ee1) can be easily removed by subsequent heat curing. The above boiling point may be 120 to 230°C. The lower limit of the above boiling point may be 120°C. The upper limit of the above boiling point may be 230°C.

[0138] The solubility of the ether solvent (Ee1) in water at 20°C may be 20 g / L or more, and may be 40 g / L or more.

[0139] Examples of ether solvents (Ee1) include propylene glycol monomethyl ether (approximately 124°C), di(propylene glycol) methyl ether (approximately 187°C), tri(propylene glycol) monomethyl ether (approximately 220°C), ethylene glycol monomethyl ether (approximately 124°C), ethylene glycol isopropyl ether (approximately 124°C), ethylene glycol-t-butyl ether (approximately 143°C), ethylene glycol monoethyl ether (135°C), ethylene glycol monobutyl ether (approximately 171°C), and propylene glycol monobutyl ether (approximately 171°C) (the boiling point is shown in parentheses). These can be used individually or in combination of two or more.

[0140] The ether solvent (Ee1) may be at least one selected from the group consisting of propylene glycol monomethyl ether (approximately 124°C), di(propylene glycol) methyl ether (approximately 187°C), and tri(propylene glycol) monomethyl ether (approximately 220°C).

[0141] (Ester solvent (Ee2)) The ester solvent (Ee2) has a boiling point of 140 to 260°C and a solubility of 10 g / L or more in water at 20°C. If the boiling point of the ester solvent (Ee2) is 140°C or higher, the volatilization of the ester solvent (Ee2) during the application of the aqueous second base paint composition is suppressed, the viscosity change of the paint composition after application is suppressed, and the smoothness of the paint film is improved. If the boiling point of the ester solvent (Ee2) is 260°C or lower, the ester solvent (Ee2) can be easily removed by subsequent heat curing. The above boiling point may be 140 to 250°C. The upper limit of the above boiling point may be 250°C.

[0142] Examples of ester solvents (Ee2) include propylene glycol monomethyl ether acetate (approximately 145°C), dimethyl glutarate (approximately 214°C), butyl diglycol acetate (approximately 247°C), ethylene glycol monoethyl ether acetate (approximately 156°C), and propylene glycol monoethyl ether acetate (approximately 167°C) (the boiling points are shown in parentheses). These can be used individually or in combination of two or more.

[0143] The solubility of the ester solvent (Ee2) in water at 20°C may be 15 g / L or more, and may be 18 g / L or more.

[0144] The ester solvent (Ee2) may be at least one selected from the group consisting of propylene glycol monomethyl ether acetate (approximately 145°C), dimethyl glutarate (approximately 214°C), dibasic ester (approximately 200°C), and butyl diglycol acetate (approximately 247°C).

[0145] The hydrophilic solvent (Ee) may be at least one selected from the group consisting of propylene glycol monomethyl ether, di(propylene glycol) methyl ether, tri(propylene glycol) monomethyl ether, tri(propylene glycol) monomethyl ether, propylene glycol monomethyl ether acetate, dimethyl glutarate, dibasic ester, and butyl diglycol acetate.

[0146] Other Organic Solvents: Organic solvents (E2) may include organic solvents other than hydrophilic solvents (Ee) and alcohol solvents (Ea). Examples of other organic solvents include other alcohol solvents such as methanol, ethanol, butanol, propyl alcohol, methyl methoxybutanol, methoxybutanol, and ethoxypropanol; and ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. These may be used individually or in combination of two or more.

[0147] The proportion of other organic solvents may be 50% by mass or less, 40% by mass or less, or 30% by mass of the organic solvent (E2).

[0148] The pigment-based aqueous second-base paint composition contains a luminous pigment (G2). The luminous pigment (G2) can produce flip-flop properties. The content of the luminous pigment (G2) is not particularly limited and is set appropriately depending on the purpose, etc. Examples of luminous pigments (G2) are the same as those described above.

[0149] The water-based second base coating composition may contain other pigments (the above-mentioned coloring pigments and extender pigments). The other pigments enhance the opacity of the first base coating and the weather resistance of the multi-layer coating.

[0150] The content of other pigments is not particularly limited. In terms of opacity, the content of other pigments may be 30 to 60% by mass of the resin solids content of the aqueous second base coating composition. The above pigment content may be 35 to 55% by mass, or 40 to 50% by mass.

[0151] • Viscosity agent (H2): The aqueous second base coating composition may contain a viscosity agent (H2). Examples of viscosity agents (H2) include those similar to those used for viscosity agent (H1).

[0152] The solid content of the viscous agent (H2) is, for example, 2.0 to 4.0 parts by mass per 100 parts by mass of the resin solid content of the aqueous second base coating composition. When the content of the viscous agent (H2) is within this range, the viscosity of the aqueous second base coating composition can be easily adjusted to an appropriate range in which sagging does not occur and smoothness is improved. The above content may be 2.5 to 3.5 parts by mass. The lower limit of the above content may be 2.5 parts by mass. The upper limit of the above content may be 3.5 parts by mass.

[0153] • Clear coating composition The clear coating composition may be solvent-based, water-based, or in powder form. The clear coating composition may be solvent-based. The clear coating composition may contain the above-mentioned coloring pigments to the extent that transparency is not impaired.

[0154] The clear coating composition may be, for example, an acid epoxy curing type containing a polyepoxide and a polycarboxylic acid, or a urethane curing type containing a hydroxyl group-containing resin and a polyisocyanate compound. The urethane curing type clear coating composition may be a two-component type.

[0155] An acid epoxy curing clear coating composition includes, for example, (a) an acid anhydride group-containing aqueous acrylic resin, (b) a carboxyl group-containing polyester resin, and (c) an aqueous acrylic resin having hydroxyl groups and epoxy groups. From the viewpoint of storage stability, the acid anhydride groups of (a) the acid anhydride group-containing aqueous acrylic resin may be half-esterified with a low molecular weight alcohol or the like. The carboxyl group-containing polyester resin may further have hydroxyl groups.

[0156] The resins (a) to (c) described above are blended such that, for example, the molar ratio of carboxyl groups contained in (a) aqueous acrylic resin and (b) polyester resin to epoxy groups contained in (c) aqueous acrylic resin is 1 / 1.4 to 1 / 0.6 (preferably 1 / 1.2 to 1 / 0.8), and the molar ratio of carboxyl groups derived from acid anhydride groups contained in (a) aqueous acrylic resin to hydroxyl groups contained in (b) polyester resin and (c) aqueous acrylic resin is 1 / 2.0 to 1 / 0.5 (preferably 1 / 1.5 to 1 / 0.7).

[0157] A urethane-curing clear coating composition includes, for example, a hydroxyl group-containing resin and a polyisocyanate compound. Examples of polyisocyanate compounds include those listed above. Examples of hydroxyl group-containing resins include polyester resins, polyurethane resins, aqueous acrylic resins, and polyols containing hydroxyl groups.

[0158] The OHV of the hydroxyl group-containing resin is, for example, 20 to 200 mg KOH / g. The OHV may be 30 mg KOH / g or more. The OHV may be 180 mg KOH / g or less. The Mw of the hydroxyl group-containing resin is, for example, 1000 to 20000. The Mw may be 2000 or more. The Mw may be 15000 or less. The AV of the hydroxyl group-containing resin is, for example, 0 to 30 mg KOH / g. The AV may be 25 mg KOH / g or less.

[0159] The hydroxyl group-containing resin and the polyisocyanate compound are blended, for example, such that the equivalent ratio of isocyanate groups (NCO) to hydroxyl groups (OH) (NCO / OH) is 0.5 to 1.7. The lower limit of the above equivalent ratio may be 0.7. The upper limit of the above equivalent ratio may be 1.5.

[0160] Alternatively, an acrylic melamine-curing type clear coating composition may be used. The clear coating composition may be a commercially available product.

[0161] The solid content concentration of the clear coating composition is not particularly limited and is set appropriately according to the painting conditions. For example, the solid content concentration of the clear coating composition is 40 to 70% by mass.

[0162] [Method for forming a multi-layer coating] The method for forming a multi-layer coating according to this disclosure comprises: applying an aqueous first base coating composition to a workpiece to form an uncured first base coating; applying an aqueous second base coating composition onto the uncured first base coating to form an uncured second base coating; applying a clear coating composition onto the uncured second base coating to form an uncured clear coating; and heating to cure the uncured first base coating, the uncured second base coating, and the uncured clear coating. On the other hand, the method does not include preheating between the formation of the uncured first base coating and the formation of the uncured second base coating.

[0163] In this method, by including the above-mentioned alcohol solvent (Ea) in both the first aqueous base coating composition and the second aqueous base coating composition, a multi-layer coating film with excellent design properties can be obtained.

[0164] (I) Step of forming an uncured first base coating film The uncured first base coating film is formed by applying the above-mentioned aqueous first base coating composition to the object to be coated. The aqueous first base coating composition is applied, for example, so that the thickness of the first base coating film after curing is 10 to 50 μm.

[0165] The painting method is not particularly limited. Examples of painting methods include air spray painting, airless spray painting, and rotary atomization painting. These methods may be combined with electrostatic painting. Among these, rotary atomization electrostatic painting is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic painting, rotary atomization electrostatic painting machines commonly known as "micro-microbell (μμbell)", "microbell (μbell)", or "metallicbell (metabell)" are used.

[0166] The material of the object to be coated is not particularly limited. Examples of materials to be coated include metal, resin, and glass.

[0167] The shape of the object to be painted is not particularly limited. Specifically, the objects to be painted include automobile bodies and parts for automobile bodies such as passenger cars, trucks, motorcycles, and buses, as well as automobile parts such as spoilers, bumpers, mirror covers, grilles, and door handles.

[0168] Examples of metals include iron, copper, aluminum, tin, zinc, or alloys thereof (e.g., steel). Typical examples of metals to be coated include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electro-galvanized 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.

[0169] Metallic workpieces may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After surface treatment, metallic workpieces may be further coated with electrodeposition paint. The electrodeposition paint may be cationic or anionic.

[0170] Examples of resins include polyethylene resin, EVA resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), vinyl chloride resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), polycarbonate resin, acrylic resin, acrylonitrile butadiene styrene (ABS) resin, acrylonitrile styrene (AS) resin, polyamide resin, acetal resin, phenolic resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). The resin-coated object may be degreased. The resin part may be primed.

[0171] (II) Step of forming an uncured second base coating The uncured second base coating is formed by coating the uncured first base coating with the above-described aqueous second base coating composition. The aqueous second base coating composition is coated, for example, so that the thickness of the second base coating after curing is 2 to 30 μm. The coating method may be, for example, the same method as the coating method for the aqueous first base coating composition.

[0172] No preheating is performed between the application of the first water-based paint composition and the application of the second water-based paint composition. Even in this case, apparent migration of organic solvents is less likely to occur between the uncured first base coating and the uncured second base coating, resulting in a multi-layer coating with excellent aesthetic appeal.

[0173] In preheating, at least some of the solvent is intentionally removed. Failure to preheat means that no work is performed to intentionally remove the solvent from the uncured first base coating before applying the water-based second base coating composition. Methods for intentionally removing the solvent (i.e., preheating methods) include, for example, air drying and heat drying. In air drying, for example, the uncured coating is left at a temperature of 15°C to 30°C for 5 to 20 minutes. Heat drying is performed under conditions where the curing reaction by the coating film-forming components does not proceed, or at least the curing reaction is not completed. In heat drying, for example, the uncured coating is heated at a temperature of 50°C to 90°C for 30 seconds to 10 minutes.

[0174] (III) Step to form an uncured clear coating The uncured clear coating is formed by painting the above-mentioned clear coating composition onto an uncured second base coating. The clear coating composition is painted, for example, so that the thickness of the clear coating after curing is 15 μm or more and 60 μm or less.

[0175] Preheating may be performed between the application of the water-based second base coating composition and the application of the clear coating composition. Preheating can be performed, for example, by leaving the mixture at a temperature of 15°C to 30°C for 5 to 20 minutes, or by heating it at a temperature of 50°C to 90°C for 30 seconds to 10 minutes.

[0176] The painting method is not particularly limited. Examples of painting methods include those similar to those used for painting the first water-based paint composition. Among these, rotary atomizing electrostatic painting is preferred from the viewpoint of coating efficiency.

[0177] (IV) Curing process: Each uncured coating is cured. Each coating can be cured by heating. In this process, the first base coating, the second base coating, and the clear coating are cured at the same time.

[0178] The curing of each coating composition is carried out under conditions such as a heating temperature of 80°C to 180°C and a heating time of 5 to 60 minutes (preferably 10 to 30 minutes). The heating time refers to the time during which the heating device is maintained at the target temperature, and does not consider the time it takes to reach the target temperature. Examples of heating devices include drying ovens that utilize heat sources such as hot air, electricity, gas, and infrared radiation.

[0179] [Coated Articles] By the method of the present disclosure, a coated article having a multilayer coating comprising a first base coating formed on an object to be coated, a second base coating formed on the first base coating, and a clear coating formed on the second base coating is obtained.

[0180] The present invention will be further described by the following examples, but the present invention is not limited thereto. In the examples, "parts" and "%" are based on mass unless otherwise specified.

[0181] Details of each component listed in the table are as follows: • Acrylic resin emulsion (A1) Acrylic resin emulsion, product name "EMA-1046", manufactured by Nippon Paint Automotive Coatings Co., Ltd., Mw: (unmeasurable), AV: 30 mg KOH / g, OHV: 30 mg KOH / g, Tg: 40℃

[0182] • Acrylic resin emulsion (A2) Acrylic resin emulsion, product name "EMA-1015", manufactured by Nippon Paint Automotive Coatings Co., Ltd., Mw: (not measurable), AV: 20 mg KOH / g, OHV: 40 mg KOH / g, Tg 20℃

[0183] • Water-based polyurethane resin (B1) Polyurethane dispersion, product name "ISW-1005", manufactured by Nippon Paint Co., Ltd., Mw: (not measurable), AV: 13 mg KOH / g, OHV: 0 mg KOH / g, Tg: -55℃

[0184] • Water-based polyester resin (C1) Polyester dispersion, product name "PRW-1006", manufactured by Nippon Paint Co., Ltd., Mn: 2,400, AV: 21 mg KOH / g, OHV: 90 mg KOH / g, Tg: 0℃

[0185] - Melamine resin (D1) Product name "C211", manufactured by Ornex Co., Ltd., imino-based, weight-average molecular weight 780 - Melamine resin (D2) Product name "C250", manufactured by Ornex Co., Ltd., imino-based, weight-average molecular weight 2,500

[0186] • Alcohol solvent (Ea) 2EHOH: 2-ethyl-1-hexanol EHG: ethylene glycol 2-ethylhexyl ether

[0187] Other organic solvents: BuOH (butyl alcohol)

[0188] Ether solvents (Ee1) PGM: Propylene glycol monomethyl ether DPM: Di(propylene glycol) methyl ether TPM: Tripropylene glycol monomethyl ether ester solvents (Ee2) GADE: Dimethyl glutarate BDGAc: Butyl diglycol acetate

[0189]

[0190] • Other water-based resins (F2): Water-based polyester resin, product name "PRS-1045", manufactured by Nippon Paint Automotive Coatings Co., Ltd., Mn: 2,500, Tg: 5.6℃, AV: 40.5 mg KOH / g, OHV: 52.6 mg KOH / g

[0191] • Luminous pigment (G2), product name "Aluminum Paste MH-8801", manufactured by Asahi Kasei Corporation.

[0192] • Viscosity agent (H1): Acrylic viscosity agent, product name "Disparon AQ-001", manufactured by Kusumoto Kasei Co., Ltd. • Viscosity agent (H2): Urethane association viscosity agent, product name "Adekanol UH-814", manufactured by ADEKA Corporation

[0193] [Examples 1-33, Comparative Examples 1-22] (1) As shown in the preparation table for the first aqueous base coating composition, the components were mixed to prepare the first aqueous base coating composition. The solid content concentration was adjusted to 50% by mass using deionized water. (2) As shown in the preparation table for the second aqueous base coating composition, the components were mixed to prepare the second aqueous base coating composition. The solid content concentration was adjusted to 25% by mass using deionized water.

[0194] (3) Formation of multi-layer coating A zinc phosphate treated dull steel plate was electrodeposited with cationic electrodeposition paint (product name Powernix 150, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) so that the dry coating film was 20 μm thick. Subsequently, it was heated at 160°C for 30 minutes to form the electrodeposited coating film and obtain the coated object.

[0195] The substrate was coated with a first water-based paint composition using a rotary atomizing electrostatic coating apparatus to a dry film thickness of 20 μm. Subsequently, the second water-based paint composition was coated using the rotary atomizing electrostatic coating apparatus to a dry film thickness of 15 μm. A 6-minute interval was observed between the application of the first and second water-based paint compositions. After the application of the second water-based paint composition, preheating was performed at 80°C for 3 minutes.

[0196] A clear coating composition (product name "PolyureExcel O-4300", manufactured by Nippon Paint Automotive Coatings Co., Ltd., a two-component acrylic urethane-based organic solvent-type clear coating containing a polyisocyanate compound) was applied in one stage to achieve a dry film thickness of 35 μm.

[0197] After being left at room temperature for 10 minutes, the coated board with a multi-layer coating was obtained by heating it at 140°C for 20 minutes.

[0198] [Evaluation] The evaluation was conducted as follows. The evaluation results are shown in the table.

[0199] (Sagging properties) A ​​coated plate having an electrodeposited coating with a 5 mm diameter hole was sequentially coated with the first water-based coating composition and the second water-based coating composition in the same manner as above, and preheated at 80°C for 3 minutes. Subsequently, the clear coating composition was coated in the same manner as above, heated at 140°C for 20 minutes, and the length of the sagging at the bottom of the hole was measured. An evaluation of B or higher indicates excellent sagging properties.

[0200] Evaluation Criteria A: Sauce length 0 mm B: Sauce length greater than 0 mm but less than 2 mm C: Sauce length 2 mm or more

[0201] (Smoothness) The smoothness of the multilayer coating was evaluated by measuring the amount of reflected light (LW0) in the long wavelength region (1200 μm to 12000 μm) using a surface measuring instrument (BYK Wave Scan-dual), according to the following criteria. A smaller LW0 indicates smoother surface. With Wave Scan-dual, laser light is shone onto the test piece at a 60° angle while moving the light source, and the reflected light is measured. A rating of B or higher indicates high smoothness.

[0202] Evaluation Criteria A: LW0 is less than 15 B: LW0 is 15 or more but less than 20 C: LW0 is 20 or more

[0203] (Flip-flop (FF) properties) An electrodeposited coating film and an intermediate coating film were formed on a substrate. Furthermore, a multilayer coating film having a first base coating film, a second base coating film, and a clear coating film was created on top of these. The FI value (flip-flop properties) of the obtained multilayer coating film was measured using "BYK-mac i" (manufactured by BYK Corporation). The higher the FI value, the better the flip-flop properties. An evaluation of B or higher indicates high flip-flop properties.

[0204] Evaluation Criteria A: FI score of 15 or higher B: FI score of 14 or higher but less than 15 C: FI score less than 14

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[0212] According to the method of the present invention, when a second aqueous base coating composition is applied after the first aqueous base coating composition has been applied without preheating, a multi-layer coating film with excellent aesthetic properties is provided. The multi-layer coating film of the present invention is suitable for painting automobile bodies.

[0213] This application claims priority under Japanese Patent Application No. 2024-230967, filed in Japan on 26 December 2024, the entirety of which is incorporated herein by reference.

Claims

1. The method comprises: applying an aqueous first base coating composition to an object to be coated to form an uncured first base coating film; applying an aqueous second base coating composition onto the uncured first base coating film to form an uncured second base coating film; applying a clear coating composition onto the uncured second base coating film to form an uncured clear coating film; and heating to cure the uncured first base coating film, the uncured second base coating film, and the uncured clear coating film, wherein there is no preheating between the formation of the uncured first base coating film and the formation of the uncured second base coating film, and the aqueous first base coating composition contains, per 100 parts by mass of the resin solids content of the aqueous first base coating composition, 5 to 30 parts by mass of acrylic resin emulsion (A1), 20 to 60 parts by mass of aqueous polyurethane resin (B1), and 12.5 to 40 parts by mass of aqueous polyester resin (C1), A method for forming a multilayer coating film, comprising: 10 to 40 parts by mass of melamine resin (D1); 5 to 30 parts by mass of organic solvent (E1); wherein the aqueous second base coating composition contains, per 100 parts by mass of the resin solids of the aqueous second base coating composition, 31 to 60 parts by mass of acrylic resin emulsion (A2), 10 to 40 parts by mass of melamine resin (D2), 5 to 30 parts by mass of aqueous resin other than the acrylic resin emulsion (A2) (F2), 20 to 40 parts by mass of organic solvent (E2), and 1 to 40 parts by mass of luminescent pigment (G2); wherein both the organic solvent (E1) and the organic solvent (E2) are at least one alcohol solvent (Ea) having a boiling point of 160 to 280°C and a solubility in water at 20°C of 0.1 to 3.0 g / L.

2. The method for forming a multilayer coating film according to claim 1, wherein the aqueous first base coating composition further contains 0.05 to 3.0 parts by mass of a viscous agent (H1) per 100 parts by mass of the resin solids content of the aqueous first base coating composition.

3. The method for forming a multilayer coating film according to claim 1 or 2, wherein the aqueous second base coating composition further contains 2.0 to 4.0 parts by mass of a viscous agent (H2) per 100 parts by mass of the resin solids content of the aqueous second base coating composition.

4. The method for forming a multilayer coating film according to any one of claims 1 to 3, wherein the alcohol solvent (Ea) is at least one selected from the group consisting of 2-ethyl-1-hexanol, ethylene glycol 2-ethylhexyl ether, and diethylene glycol 2-ethylhexyl ether.

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