Aqueous intermediate coating composition

Abstract

Disclosed is an aqueous intermediate coating composition which contains a core-shell type acrylic resin dispersion (A), a hydrophobic melamine resin (B), a photoluminescent pigment (C), a coloring pigment (D) other than the photoluminescent pigment (C), and a viscosity regulator (E), wherein: the shell part of the core-shell type acrylic resin dispersion (A) comprises a hydrophilic resin; and the thixotropic index of the aqueous intermediate coating composition is 3.5 to 10 inclusive.

Description

Water-based primer coating composition

[0001] The present invention relates to a water-based intermediate coating composition.

[0002] A multi-layer coating film comprising an intermediate coating film, a base coating film, and a clear coating film is usually formed on the surface of an automobile exterior panel. Patent Document 1 discloses an intermediate coating composition containing a luster pigment.

[0003] International Publication No. 2015 / 99151

[0004] The bright pigment contained in the intermediate coating composition can degrade the appearance of the coating film.

[0005] An object of the present invention is to provide a water-based primer coating composition containing a luster pigment, which can give a coating film with excellent appearance.

[0006] In order to solve the above problems, the present invention provides the following aspects. [1] A water-based intermediate coating composition comprising: a core-shell acrylic resin dispersion (A), a hydrophobic melamine resin (B), a bright pigment (C), a color pigment (D) other than the bright pigment (C), and a viscosity modifier (E), wherein the shell portion of the core-shell acrylic resin dispersion (A) comprises a hydrophilic resin, and the thixotropic index of the water-based intermediate coating composition is 3.5 or more and 10 or less. [2] The water-based intermediate coating composition of [1] above, wherein the core-shell acrylic resin dispersion (A) comprises a branched hydrocarbon group having 4 to 24 carbon atoms in the core portion. [3] The water-based intermediate coating composition of [1] or [2] above, wherein the content ratio (C:D) of the bright pigment (C) to the color pigment (D) is 1:20 to 1:260. [4] The aqueous intermediate coating composition of any one of [1] to [3] above, wherein the viscosity modifier (E) comprises a hydrophobic association-type viscosity modifier. [5] The aqueous intermediate coating composition of any one of [1] to [4] above, wherein the average particle size of the bright pigment (C) is 5 μm or more and 12 μm or less. [6] The aqueous intermediate coating composition of any one of [1] to [5] above, wherein the bright pigment (C) comprises aluminum particles. [7] The aqueous intermediate coating composition of any one of [1] to [6] above, further comprising a polyester resin dispersion (F). [8] The aqueous intermediate coating composition of any one of [1] to [7] above, further comprising a urethane resin dispersion (G). [9] The aqueous intermediate coating composition of any one of [1] to [8] above, further comprising a hydrophobic organic solvent (H) having a hydrocarbon group having 5 or more carbon atoms.

[10] The aqueous intermediate coating composition of any one of [1] to [9] above, wherein the color pigment (D) comprises a white pigment.

[11] The aqueous intermediate coating composition according to

[10] above, wherein the PWC of the white pigment is 40% by mass or more and 65% by mass or less.

[0007] According to the present invention, there is provided a water-based intermediate coating composition containing a luster pigment, which can give a coating film with excellent appearance.

[0008] The bright pigment reduces the light transmittance of the intermediate coating film, improving the weather resistance of the multi-layer coating film. On the other hand, the bright pigment is prone to disordered orientation, which reduces the appearance (surface smoothness) of the intermediate coating film and the multi-layer coating film. In the present disclosure, by adjusting the thixotropic index (TI value) of the aqueous intermediate coating composition to a specific range, the orientation of the bright pigment in the aqueous intermediate coating composition and in the uncured coating film is increased, thereby improving the appearance of the coating film.

[0009] Thixotropic Index (TI Value) The thixotropic index (TI value) of the water-based intermediate coating composition is from 3.5 to 10. This TI value is higher than that of conventional water-based intermediate coating compositions.

[0010] The TI value is obtained by dividing the viscosity ηL (Pa s) at a rotational speed of 6 rpm measured at 20°C using a rotational viscometer by the viscosity ηH (Pa s) at a rotational speed of 60 rpm (TI value = ηL / ηH).

[0011] A high TI value means a high viscosity ηL at a low shear rate (6 rpm) and / or a low viscosity ηH at a high shear rate (60 rpm). In other words, the aqueous intermediate coating composition according to the present disclosure has a low viscosity during application and a high viscosity after application to a substrate. A low viscosity during application means that the luster pigment is microparticulated without agglomeration. Therefore, the luster pigment can move freely within the aqueous intermediate coating composition during application, improving its orientation. A high viscosity after application allows the luster pigment to remain highly oriented in the coating film. In other words, a relatively high TI value facilitates orientation of the luster pigment in the aqueous intermediate coating composition, and this high orientation is maintained even in the uncured coating film, improving the coating film appearance.

[0012] The TI value may be 3.8 or more, 4.0 or more, or 4.3 or more. The TI value may be 8.0 or less, 7.0 or less, or 6.5 or less.

[0013] The TI value can be adjusted, for example, by the solids concentration of the aqueous intermediate coating composition, the type and amount of resin components (e.g., the core-shell acrylic resin dispersion (A), the hydrophobic melamine resin (B), and other resin components in the aqueous intermediate coating composition), the type and amount of viscosity modifier (E), and the type and amount of hydrophobic organic solvent (H).

[0014] The aqueous intermediate coating composition according to the present disclosure improves the appearance of both the intermediate coating film and the multilayer coating film comprising the intermediate coating film.

[0015] The appearance of the intermediate coating film can be evaluated by the arithmetic mean roughness Ra. The intermediate coating film obtained according to the present disclosure has an arithmetic mean roughness Ra (cutoff value 0.25 mm) of less than 0.20 μm.

[0016] The appearance of the multilayer coating film can be evaluated using the Wavescan-T appearance measuring instrument manufactured by BYK-Gardner GmbH. The Wavescan-T irradiates the coating surface with long- to short-wavelength laser light emitted from the measuring instrument, and the intensity of the resulting reflected light is detected by a detector to evaluate the optical unevenness of the coating surface. The unevenness is specifically expressed as the W1 value (measured wavelength 2.4 mm or more), the W2 value (measured wavelength 0.8 mm or more but less than 2.4 mm), the W3 value (measured wavelength 0.32 mm or more but less than 0.8 mm), and the W4 value (measured wavelength less than 0.32 mm). The smaller each value, the smoother the surface of the multilayer coating film (the better the appearance). The W1 value can be used to evaluate the surface waviness (roundness) of the multilayer coating film. The W4 value can be used to evaluate the gloss and minute unevenness of the multilayer coating film. The multilayer coating film obtained according to the present disclosure satisfies at least one of W1 being less than 15 and W4 being less than 15.

[0017] Hereinafter, the solid content concentration is calculated from the residue when the object is heated at 150°C.

[0018] The resin solid content mass of the coating composition is the total solid content mass of the resin components.

[0019] The average particle size is the 50% average particle size (D50) in a volumetric particle size distribution measured using a laser diffraction / scattering particle size distribution measuring device, such as UPA-150 (Microtrack particle size distribution measuring device, manufactured by Nikkiso Co., Ltd.).

[0020] The glass transition temperature can be measured by detecting the thermal change accompanying the glass transition of the resin using, for example, a differential scanning calorimeter, such as the DSC220C manufactured by Seiko Instruments Inc.

[0021] The acid value and hydroxyl value may be calculated from the composition of the raw material monomers in accordance with JIS regulations, or may be determined by neutralization titration using an aqueous potassium hydroxide solution in accordance with JIS K 0070. The acid value and hydroxyl value are values ​​based on the solid content.

[0022] The weight average molecular weight and number average molecular weight are measured using polystyrene standards by the GPC (gel permeation chromatography) method.

[0023] (Meth)acrylic acid esters refer to acrylic acid esters and methacrylic acid esters. (Meth)acrylic acid refers to acrylic acid and methacrylic acid.

[0024] A "hydrocarbon group" refers to a group containing carbon and hydrogen, and is a group obtained by removing one hydrogen atom from a hydrocarbon. Examples of hydrocarbon groups include aliphatic hydrocarbon groups and aromatic hydrocarbons having 4 to 24 carbon atoms. Aliphatic hydrocarbon groups may be linear, branched, or cyclic, and may be saturated or unsaturated. Hydrogen bonded to carbon may be substituted with a halogen atom or the like. The hydrocarbon group may have one or more N, O, S, Si, amide, sulfonyl, siloxane, carbonyl, carbonyloxy, etc. at its terminal or in the molecular chain.

[0025] 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.

[0026] The PWC (mass %) of a pigment is calculated by dividing the mass of the pigment by the total mass of all pigments and resin solids contained in the aqueous primer coating composition (see the formula below).

[0027] Aqueous resins are generally broadly classified into water-soluble and water-dispersed types. Water-dispersed types are further classified into dispersion types (commonly referred to as colloidal dispersion types) and emulsion types. Colloidal dispersion-type aqueous resins are typically obtained by semi-dissolving a resin synthesized in an organic solvent in water with a neutralizer. Emulsion-type aqueous resins are typically produced by emulsion polymerization or mechanically forced emulsification. In the case of acrylic resins, those with a weight-average molecular weight of more than 100,000 are considered emulsion-type, and those with a weight-average molecular weight of 100,000 or less are considered colloidal dispersion-type. When the weight-average molecular weight exceeds 1,000,000, it becomes difficult to measure the weight-average molecular weight. Acrylic resins whose weight-average molecular weight cannot be measured can be considered emulsion-type.

[0028] The aqueous coating composition contains water as a solvent. In the aqueous coating composition, the proportion of water in the total solvent (total of organic solvents and water) may be 50% by mass or more, 70% by mass or more, or 100% by mass.

[0029] [Water-based intermediate coating composition] The water-based intermediate coating composition according to the present disclosure comprises a core-shell acrylic resin dispersion (A), a hydrophobic melamine resin (B), a luster pigment (C), a color pigment (D) other than the luster pigment (C), and a viscosity modifier (E). The core-shell acrylic resin dispersion (A) has a shell portion containing a hydrophilic resin. The TI value of the water-based intermediate coating composition is 3.5 or more and 10 or less.

[0030] Core-shell type acrylic resin dispersion (A) The core-shell type acrylic resin dispersion (A) (hereinafter sometimes simply referred to as acrylic resin dispersion (A)) is one of the resin components. The acrylic resin dispersion (A) is a dispersion type prepared by a polymerization method (typically, a solution polymerization method) that does not use an emulsifier, and is a core-shell type aqueous acrylic resin. The acrylic resin dispersion (A) can improve chipping resistance. Chip resistance is one of the properties required for an intermediate coating film.

[0031] The resin forming the shell (shell portion-forming resin (As)) contains a hydrophilic resin. This allows the acrylic resin dispersion (A) to be dispersed in the aqueous intermediate coating composition in a particulate form. The shell portion-forming resin (As) is, for example, a neutralized product of an acrylic resin having an acid group.

[0032] In the acrylic resin dispersion (A), the "core portion" and the "shell portion" may or may not be chemically crosslinked. In the acrylic resin dispersion (A), at least a part of the "core portion" is covered with the "shell portion."

[0033] The average particle size of the acrylic resin dispersion (A) is, for example, 20 nm or more and 200 nm or less. The average particle size of the acrylic resin dispersion (A) may be 180 nm or less, 160 nm or less, 150 nm or less, or 100 nm or less. The average particle size of the acrylic resin dispersion (A) may be 25 nm or more, or 30 nm or more.

[0034] The weight-average molecular weight of the acrylic resin dispersion (A) is, for example, 13,000 or more and 80,000 or less. This allows color reversion to be suppressed when the aqueous intermediate coating composition according to the present disclosure is used to form a base coating film. The weight-average molecular weight of the acrylic resin dispersion (A) may be 15,000 or more, 16,000 or more, 20,000 or more, or 30,000 or more. The weight-average molecular weight of the acrylic resin dispersion (A) may be 60,000 or less, or 45,000 or less.

[0035] The acid value of the acrylic resin dispersion (A) may be 15 mgKOH / g or more and 50 mgKOH / g or less. The acid value of the acrylic resin dispersion (A) may be 25 mgKOH / g or more, 30 mgKOH / g or more, or 35 mgKOH / g or more. From the viewpoint of water resistance, the acid value of the acrylic resin dispersion (A) may be 50 mgKOH / g or less, or 45 mgKOH / g or less.

[0036] The content of the acrylic resin dispersion (A) is, for example, 3 parts by mass or more and 50 parts by mass or less, relative to 100 parts by mass of the resin solids content of the aqueous intermediate coating composition. The content of the acrylic resin dispersion (A) may be 5 parts by mass or more, or may be 10 parts by mass or more. The content of the acrylic resin dispersion (A) may be 40 parts by mass or less, 30 parts by mass or less, or 25 parts by mass or less.

[0037] The mass ratio (Ac:As) of the resin forming the core portion (core portion-forming resin (Ac)) to the shell portion-forming resin (As) is, for example, 95:5 to 40:60. When the mass ratio of the shell portion-forming resin (As) is 5% or more, the dispersibility of the acrylic resin dispersion (A) in water is further improved, and the storage stability of the aqueous intermediate coating composition is improved. When the mass ratio of the shell portion-forming resin (As) is 60% or less, the water resistance and appearance of the coating film can be improved. The core portion-forming resin (Ac):shell portion-forming resin (As) ratio may be 90:10 to 45:55, or may be 85:15 to 45:55.

[0038] The core-forming resin (Ac) may contain a hydrophilic resin or a hydrophobic resin. To enhance the dispersibility of the hydrophobic melamine resin (B), the core-forming resin (Ac) may have a branched hydrocarbon group having 4 to 24 carbon atoms (hereinafter referred to as a hydrophobic group for convenience). The hydrophobic group enhances the hydrophobicity of the core and improves the affinity between the core and the hydrophobic melamine resin (B). The hydrophilic resin in the shell enhances the water dispersibility of the acrylic resin dispersion (A). This allows the acrylic resin dispersion (A) to be finely dispersed in an aqueous solvent while capturing the hydrophobic melamine resin (B), thereby suppressing aggregation of the hydrophobic melamine resin (B).

[0039] The number of carbon atoms in the hydrophobic group may be 7 to 18, or 8 to 15. The hydrophobic group may be saturated. The hydrophobic group may be a branched alkyl group having 4 to 24 carbon atoms.

[0040] (Production Method) The acrylic resin dispersion (A) can be produced, for example, by multi-stage solution polymerization.

[0041] Specifically, the acrylic resin dispersion (A) is produced by a method comprising: a first step of dropping a first monomer mixture containing a first acid group-containing α,β-ethylenically unsaturated monomer (a1) to synthesize a core portion-forming resin (Ac) and obtain a liquid material containing the core portion-forming resin (Ac); a second step of dropping a second monomer mixture containing a second acid group-containing α,β-ethylenically unsaturated monomer (a2) into the liquid material to synthesize an acid group-containing resin (As') and obtain a core-shell acrylic resin (A') having the core portion-forming resin (Ac) and the acid group-containing resin (As'); a step of adding a basic compound to neutralize acid groups remaining in the core-shell acrylic resin (A'); and a step of adding deionized water to cause phase inversion and obtain a varnish containing the acrylic resin dispersion (A) dispersed in deionized water.

[0042] The acrylic resin dispersion (A) having a hydrophobic group in the core portion can be produced, for example, by carrying out the above-mentioned first step using a reactive solvent (x) having one glycidyl group and a hydrophobic group. Hereinafter, the case where the reactive solvent (x) is used will be described as an example, but the production method of the acrylic resin dispersion (A) is not limited thereto. The acrylic resin dispersion (A) having no hydrophobic group in the core portion can be produced in the same manner as the following method, except that the reactive solvent (x) is not used.

[0043] <First Step> In the first step, radical polymerization of the first monomer mixture and a ring-opening addition reaction between the glycidyl group of the reactive solvent (x) and an acid group-containing monomer (for example, the first acid group-containing α,β-ethylenically unsaturated monomer (a1) described below) proceed. In the first step, so-called solution polymerization is carried out.

[0044] Taking advantage of the fact that the ring-opening reaction of the epoxy ring is unlikely to occur at low temperatures, the polymerization reaction of the first monomer mixture and the ring-opening reaction of the epoxy ring can be carried out stepwise in Step 1 to allow the core portion-forming resin (Ac) to contain a hydrophobic group. For example, the temperature of the reaction system is first lowered (e.g., 50°C or higher but lower than 130°C) to polymerize the first monomer mixture to obtain a precursor, and then the temperature is raised (e.g., 130°C or higher but lower than 180°C) to cause ring-opening addition of the reactive solvent (x) to the precursor.

[0045] Examples of the first acid group-containing α,β-ethylenically unsaturated monomer (a1) include acrylic acid, methacrylic acid, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, isocrotonic acid, α-hydro-ω-((1-oxo-2-propenyl)oxy)poly(oxy(1-oxo-1,6-hexanediyl)), maleic acid, fumaric acid, itaconic acid, 3-vinylsalicylic acid, 3-vinylacetylsalicylic acid, 2-acrylamido-2-methylpropanesulfonic acid, p-hydroxystyrene, and 2,4-dihydroxy-4'-vinylbenzophenone. These may be used alone or in combination of two or more.

[0046] The first monomer mixture may contain the first acid group-containing α,β-ethylenically unsaturated monomer (a1) as well as other ethylenically unsaturated monomers.

[0047] Examples of other ethylenically unsaturated monomers include hydroxyl group-containing ethylenically unsaturated monomers such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts of these with ε-caprolactone; esters of the above carboxyl group-containing monomers; styrenes such as styrene and α-methylstyrene; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-, i-, and t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, ... methyl (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, methyl (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, methyl (meth)acrylate, methyl (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, methyl (meth)acrylate, methyl (meth)acrylate, methyl (meth)acrylate, ) Linear aliphatic acrylates such as allyl acrylate and lauryl (meth)acrylate; alicyclic acrylates such as cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate and dihydrodicyclopentadienyl (meth)acrylate; polymerizable amide compounds such as acrylamide and methacrylamide; polymerizable aromatic compounds, polymerizable nitriles, polymerizable alkylene oxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, α-olefins, dienes, polymerizable carbonyl compounds and polymerizable alkoxysilyl compounds. These may be used alone or in combination of two or more.

[0048] In terms of improving the physical properties of the resulting coating film, the mass of the first acid group-containing α,β-ethylenically unsaturated monomer (a1) in the first monomer mixture may be 5 parts by mass or more and 30 parts by mass or less, relative to 100 parts by mass of the first monomer mixture. The mass of the first acid group-containing α,β-ethylenically unsaturated monomer (a1) may be 10 parts by mass or more. The mass of the first acid group-containing α,β-ethylenically unsaturated monomer (a1) may be 25 parts by mass or less.

[0049] The reactive solvent (x) has one glycidyl group and a hydrophobic group. The reactive solvent (x) may be a monocarboxylic acid glycidyl ester. The monocarboxylic acid glycidyl ester is, for example, a compound represented by the following general formula (1): (wherein R is a monovalent organic group, including the above-mentioned hydrophobic group).

[0050] The reactive solvent (x) is used, for example, in an amount such that the mass ratio of the core portion-forming resin (Ac) to the shell portion-forming resin (As) (core portion-forming resin (Ac):shell portion-forming resin (As), mass %) is 95:5 to 40:60.

[0051] <Second Step> In the second step, polymerization of the second monomer mixture mainly proceeds to synthesize an acid group-containing resin (As'). When a basic compound is added in a subsequent step, the acid groups are neutralized, and the acid group-containing resin (As') is hydrophilized to form a shell portion-forming resin (As). The shell portion-forming resin (As) acts as a dispersing component for dispersing the acrylic resin dispersion (A) in water, reducing the particle size of the resin particles and improving their dispersion stability.

[0052] Following the second step, a polymerization initiator may be added, and aging may be performed by stirring and heating. The aging may be performed, for example, at the same temperature as in the second step for 0.5 hours to 3 hours.

[0053] The types and blending ratios of the raw material monomers contained in the second monomer mixture may be the same as or different from those contained in the first monomer mixture. The first acid group-containing α,β-ethylenically unsaturated monomer (a1) and the second acid group-containing α,β-ethylenically unsaturated monomer (a2) may be the same as or different from each other.

[0054] In order to improve the dispersibility of the resulting shell-forming resin (As), the mass of the acid group-containing monomer in the second monomer mixture may be 5 parts by mass or more and 30 parts by mass or less, relative to 100 parts by mass of the second monomer mixture. The mass of the acid group-containing monomer may be 10 parts by mass or more. The mass of the acid group-containing monomer may be 25 parts by mass or less.

[0055] A polymerization initiator may be used in the first step, the second step, and the aging step. The total amount of the polymerization initiator used is appropriately set, for example, depending on the type and amount of the raw material monomers. The total amount of the polymerization initiator used may be, for example, 0.2 parts by mass or more and 2.0 parts by mass or less, relative to 100 parts by mass of the total of the first monomer mixture and the second monomer mixture. The amount of the polymerization initiator used may be 0.2 parts by mass or more. The amount of the polymerization initiator used may be 1.5 parts by mass or less.

[0056] Examples of the polymerization initiator include 2,2'-azobisisobutyronitrile, benzoyl peroxide, 2,2-di(t-amylperoxy)butane, di-t-butyl peroxide, di-t-amyl peroxide (DTA), t-butyl peroctoate, and 2,2'-azobis(2-methylbutyronitrile). These may be used alone or in combination of two or more.

[0057] <Neutralization and Phase Inversion Step> The acid groups (typically carboxy groups) remaining in the core-shell acrylic resin (A') are neutralized with a basic compound. This makes the core-shell acrylic resin (A') water-dispersible. Thereafter, deionized water is added to cause phase inversion. This disperses the core-shell acrylic resin (A') in water, and a varnish containing the acrylic resin dispersion (A) is obtained.

[0058] The phase inversion and water dispersion are carried out, for example, by stirring a mixture of a varnish containing the neutralized core-shell acrylic resin (A') and deionized water. Stirring can reduce the average particle size of the acrylic resin dispersion (A).

[0059] Examples of basic compounds include secondary aliphatic amine compounds such as diethylamine, di-n-propylamine, diisopropylamine, diisobutylamine, di-n-butylamine, di-sec-butylamine, diamylamine, N-ethyl-1,2-dimethylpropylamine, N-methylhexylamine, di-n-octylamine, and diallylamine; tertiary aliphatic amine compounds such as triethylamine, tributylamine, triallylamine, N,N-dimethylethanolamine, N-methyldiallylamine, and N,N-dimethylallylamine; secondary cyclic amine compounds such as piperidine, 2-pipecoline, 3-pipecoline, 4-pipecoline, 2,4-lupetidine, 2,6-lupetidine, 3,5-lupetidine, and 3-piperidinemethanol; tertiary cyclic amine compounds such as N-methylpiperidine, N-methylpiperazine, and N-methylmorpholine; and aromatic amine compounds such as pyridine and 4-ethylpyridine. These may be used alone or in combination of two or more.

[0060] The basic compound is added, for example, in an amount such that the neutralization rate of the acid groups contained in the acrylic resin dispersion (A) is 70% or more and 100% or less. When the neutralization rate is within this range, the water dispersibility of the acrylic resin dispersion (A) is improved, and the average particle size can be made smaller. The neutralization rate may be 75% or more, 80% or more, 85% or more, or 90% or more.

[0061] The neutralization rate is equivalent to the amount of the basic compound used relative to the acid group (e.g., carboxy group). The neutralization rate is calculated by the following formula:

[0062] Hydrophobic melamine resin (B) The hydrophobic melamine resin (B) is one of the resin components that acts as a curing agent. The hydrophobic melamine resin can improve the water resistance of the resulting coating film.

[0063] The hydrophobic melamine resin (B) has a melamine nucleus (triazine nucleus) surrounded by R 1 ~R 6The hydrophobic melamine resin (B) may generally be a polynuclear compound in which a plurality of melamine nuclei are bonded to one another, or a mononuclear compound consisting of one melamine nucleus.

[0064] The structure of the melamine nucleus is represented, for example, by the following general formula (2). (In the formula, substituent R １ ~R ６ each independently represents a hydrogen atom, an alkyl ether group, a methylol group, or a bonding moiety to another triazine ring.

[0065] Substituent R １ ~R ６ are each independently a hydrogen atom, an alkyl ether group (—CH ２ -OR ７ ) or a methylol group (-CH ２ The substituent R １ ~R ６ , R ７ may each independently be an alkyl group having 1 to 8 carbon atoms or an alkyl group having 1 to 4 carbon atoms. The alkyl group may be linear or branched. The alkyl group may be a methyl group, an n-butyl group, or an isobutyl group.

[0066] Melamine resins are generally classified into water-soluble melamine resins and hydrophobic melamine resins. Water-soluble melamine resins satisfy all of the following conditions (i) to (iii): (i) the number-average molecular weight of the melamine resin is 1,000 or less; (ii) the R １ ~R ６ In the formula, at least one is a hydrogen atom (imino group) or CH ２ OH (methylol group). That is, the total amount of the average imino group and the average methylol group is 1.0 or more. (iii) R in the above general formula (1) １ ~R ６ In this case, R １ ~R ６ is CH ２ OR ７ If ７ is a methyl group.

[0067] The hydrophobic melamine resin is a melamine resin other than the above-mentioned water-soluble melamine resin. That is, it satisfies any one of the following conditions (iv) to (vi): (iv) The number average molecular weight of the melamine resin is greater than 1,000. (v) The total amount of the average imino group content and the average methylol group content is less than 1.0. (vi) R in the above formula (1) １ ~R ６ In this case, R １ ~R ６ Two or more of them are CH ２ OR ７ and R ７ is an alkyl group having 1 to 4 carbon atoms, １ ~R ６ R that constitutes ７ At least one of the groups is an alkyl group having 2 to 4 carbon atoms.

[0068] Commercially available hydrophobic melamine resins (B) include, for example, the Cymel series (all trade names) manufactured by Allnex, such as Cymel 202, Cymel 204, Cymel 211, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 250, Cymel 251, Cymel 254, Cymel 266, Cymel 267, and Cymel 285 (melamine resins having both methoxy and butoxy groups); Mycoat 506 (a melamine resin having only butoxy groups, manufactured by Mitsui Cytec); and U-Van 20N60 and U-Van 20SE (the U-Van (trade name) series manufactured by Mitsui Chemicals). These may be used alone or in combination of two or more.

[0069] The solid content of the hydrophobic melamine resin (B) is, for example, 10 parts by mass or more and 40 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous intermediate coating composition. This facilitates the curing reaction, making it easier to obtain a coating film with high hardness. The solid content of the hydrophobic melamine resin (B) may be 15 parts by mass or more, 17 parts by mass or more, or 25 parts by mass or more. The solid content of the hydrophobic melamine resin (B) may be 38 parts by mass or less, or 36 parts by mass or less.

[0070] The aqueous intermediate coating composition may contain a curing agent other than the hydrophobic melamine resin (B). Examples of other curing agents include hydrophilic melamine resins, blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more.

[0071] The solid content of the other curing agent may be, for example, less than 10 parts by mass relative to 100 parts by mass of the resin solid content of the aqueous intermediate coating composition. The solid content of the other curing agent may be 5 parts by mass or less, 2 parts by mass or less, or 0 part by mass.

[0072] Luster pigment (C) Luster pigment (C) has light reflectivity and reduces the light transmittance of the intermediate coating film. The luster pigment (C) improves the weather resistance of the multi-layer coating film.

[0073] Examples of the bright pigment (C) include metal particles such as aluminum, copper, zinc, iron, nickel, tin, and aluminum oxide; alloy particles thereof; interference mica; white mica; graphite; and glass flakes. These may be used alone or in combination of two or more. Among these, aluminum particles or interference mica may be used.

[0074] The bright pigment (C) may be uncolored or colored. The bright pigment (C) may be surface-treated or not. Examples of compounds used for the surface treatment include metal oxide compounds, phosphorus compounds, amine compounds, and silane compounds.

[0075] The average particle size (primary particles) of the bright pigment (C) is, for example, 5 μm or more and 12 μm or less. This can further reduce light transmittance and make orientation easier. The average particle size of the bright pigment (C) may be 6 μm or more. The average particle size of the bright pigment (C) may be 11 μm or less, or 10 μm or less.

[0076] The bright pigment (C) may be aluminum particles having an average particle size (primary particles) of 5 μm or more and 12 μm or less.

[0077] The bright pigment (C) may be in the form of a thin film (scale-like). A scale-like shape refers to a shape in which the aspect ratio (average major axis of the pigment / average thickness of the pigment) exceeds 1.0. The aspect ratio of the scaly bright pigment (C) is, for example, 20 or more and 300 or less. The aspect ratio of the scaly bright pigment (C) may be 30 or more. The aspect ratio of the scaly bright pigment (C) may be 200 or less.

[0078] The average thickness of the scaly bright pigment (C) is, for example, 0.01 μm or more and 0.1 μm or less. The average thickness of the scaly bright pigment (C) may be 0.04 μm or more. The average thickness of the scaly bright pigment (C) may be 0.08 μm or less.

[0079] The average thickness of the scaly pigments is obtained by observing the cross section of a coating film containing the scaly pigments using a transmission electron microscope (TEM) and averaging the thicknesses of 100 arbitrarily selected scaly pigments.

[0080] The average major axis of the scaly pigment is obtained by observing the scaly pigment using a shape analysis laser microscope (for example, VK-X 250 manufactured by Keyence Corporation) and averaging the major axes (maximum lengths) of 100 arbitrarily selected scaly pigment particles.

[0081] The PWC (mass%) of the bright pigment (C) is, for example, 0.5% by mass or more and 5% by mass or less. This makes it easier to reduce light transmittance and further improves surface smoothness. The PWC (mass%) of the bright pigment (C) may be 0.7% by mass or more, or 0.9% by mass or more. The PWC (mass%) of the bright pigment (C) may be 4% by mass or less, or 3% by mass or less.

[0082] Coloring Pigment (D) The coloring pigment (D) is an opaque pigment other than the luster pigment (C), and enhances the hiding power of the intermediate coating film.

[0083] The term "opaque pigment" refers to a pigment that, when the pigment concentration (PWC) in the solids of the paint composition is 25% by mass, results in a coating film with a black-and-white hiding power of 50 μm or less. The black-and-white hiding power is measured using a black-and-white checkerboard pattern hiding power test paper as specified in JIS K5600-4-1, 4.1.2. Specifically, the hiding power test paper is attached to a steel plate, and the paint is applied at an angle so that the film thickness changes continuously. After the paint has dried or cured, the painted surface is visually observed under diffused daylight. The black-and-white hiding power is the minimum film thickness at which the black-and-white border of the checkerboard pattern on the hiding power test paper becomes invisible.

[0084] The color pigment (D) may be organic or inorganic. Examples of organic color pigments include azo chelate pigments, insoluble azo pigments, condensed azo pigments, monoazo pigments, disazo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, thioindigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, naphthol pigments, pyrazolone pigments, anthraquinone pigments, anthrapyrimidine pigments, and metal complex pigments. These may be used alone or in combination of two or more.

[0085] Examples of inorganic color pigments include white pigments such as zinc oxide and titanium dioxide, yellow lead, yellow iron oxide, chromium oxide, molybdate orange, red iron oxide, titanium yellow, carbon black, cobalt green, phthalocyanine green, ultramarine blue, cobalt blue, phthalocyanine blue, and cobalt violet. These may be used alone or in combination of two or more.

[0086] The color pigment (D) may contain a white pigment. Titanium dioxide may be contained because it has a high refractive index. The titanium dioxide may be of the rutile type or the anatase type. In particular, rutile type titanium dioxide is preferred from the viewpoint of weather resistance. The surface of the titanium dioxide may be treated with an inorganic compound such as silica, zirconium, or aluminum.

[0087] The average particle size (primary particles) of the color pigment (D) is not particularly limited. From the viewpoint of hiding power, the primary particle size of the color pigment (D) may be 100 nm or more and 500 nm or less, or 200 nm or more and 400 nm or less. The primary particle size can be measured from an electron microscope image of the cross section of the multilayer coating film using image processing software.

[0088] The PWC (mass%) of the color pigment (D) is, for example, 40% by mass or more and 65% by mass or less. This can further improve hiding power without impairing chipping resistance. The PWC (mass%) of the color pigment (D) may be 45% by mass or more, or 50% by mass or more. The PWC (mass%) of the color pigment (D) may be 62% by mass or less, or 60% by mass or less.

[0089] The PWC (mass%) of the white pigment may be, for example, 40% by mass or more and 65% by mass or less, 45% by mass or more, or 50% by mass or more, or 62% by mass or less, or 60% by mass or less.

[0090] The content ratio (C:D) of the luster pigment (C) to the color pigment (D) is, for example, 1:20 to 1:260. This allows the light transmittance of the intermediate coating film to be further reduced. This makes the lower layer of the intermediate coating film even less susceptible to the effects of ultraviolet rays, further improving the weather resistance of the coating film. The content ratio (C:D) may be 1:25 to 1:200, or 1:30 to 1:150.

[0091] Other Pigments The aqueous primer coating composition may contain pigments other than those mentioned above. Examples of other pigments include extender pigments such as calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silica, and calcined kaolin.

[0092] Viscosity modifier (E) The viscosity modifier (E) makes it easy to adjust the TI value of the aqueous intermediate coating composition to the above range. It is generally difficult to increase the TI of aqueous coating compositions containing the acrylic resin dispersion (A). The viscosity modifier (E) makes it possible to adjust the TI value higher without reducing the storage stability of the aqueous intermediate coating composition.

[0093] Examples of the viscosity modifier (E) include inorganic viscosity modifiers such as silica-based fine powder, mineral-based viscosity modifiers, and barium sulfate fine powder, and organic viscosity modifiers such as polyamide-based viscosity modifiers, organic resin fine particles, hydrophobic association-type viscosity modifiers, urethane association-type viscosity modifiers, cellulose derivatives, and alkali-swelling thickeners. These may be used alone or in combination of two or more.

[0094] From the viewpoint of improving the appearance, the viscosity modifier (E) may contain at least one of a hydrophobic association type viscosity modifier and a polyamide-based viscosity modifier, and may contain a hydrophobic association type viscosity modifier.

[0095] Examples of hydrophobic association viscosity modifiers include polyacrylic acid-based viscosity modifiers copolymerized with hydrophobic monomers, polyurethane-based viscosity modifiers having hydrophobic chains in the molecule, urethane-urea-based viscosity modifiers in which at least a portion of the main chain is a hydrophobic urethane chain, and amide-urea-based viscosity modifiers in which at least a portion of the main chain is a hydrophobic amide chain. These may be used alone or in combination of two or more.

[0096] The acid value of the solid content of the polyacrylic acid-based viscosity modifier may be, for example, 30 mgKOH / g or more and 300 mgKOH / g or less. The acid value of the solid content of the polyacrylic acid-based viscosity modifier may be 80 mgKOH / g or more. The acid value of the solid content of the polyacrylic acid-based viscosity modifier may be 280 mgKOH / g or less.

[0097] Commercially available polyacrylic acid viscosity modifiers include, for example, Primal ASE-60, Primal TT615, and Primal RM5 (all manufactured by The Dow Chemical Company), SN Thickener 613, SN Thickener 618, SN Thickener 630, SN Thickener 634, and SN Thickener 636 (all manufactured by San Nopco Ltd.), and Disparlon AQ-021 (manufactured by Kusumoto Chemicals Co., Ltd.).

[0098] Commercially available polyurethane viscosity modifiers include, for example, PU 1250 (manufactured by BASF), SN Thickener-660T, SN Thickener-665T (all manufactured by San Nopco), Primal RM-12W, and Primal RM-895 (all manufactured by The Dow Chemical Company).

[0099] Commercially available urethane-urea viscosity modifiers include, for example, BYK-425 and BYK-420 (both manufactured by BYK Chemie) and RHEOLATE 216 (manufactured by ELEMENTIS).

[0100] An example of a commercially available amide-urea viscosity modifier is BYK-430 (manufactured by BYK Chemie).

[0101] Examples of polyamide viscosity modifiers include fatty acid amides, polyamides, acrylamides, long-chain polyaminoamides, aminoamides, and salts thereof (for example, phosphates).

[0102] Commercially available polyamide viscosity modifiers include, for example, Disparlon AQ-600, AQ-607, AQ-620, AQ-630, and AQH-800 (all manufactured by Kusumoto Chemicals Co., Ltd.), Anti-Terra-U, Disperbyk-101, and Disperbyk-130 (all manufactured by BYK Chemie).

[0103] The content of the viscosity modifier (E) may be, for example, 0.05 parts by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the resin solids content contained in the aqueous intermediate coating composition. The content of the viscosity modifier (E) may be 0.08 parts by mass or more, or 0.09 parts by mass or more. The content of the viscosity modifier (E) may be 3 parts by mass or less, or 2 parts by mass or less.

[0104] The proportion of the hydrophobic association type viscosity modifier in the viscosity modifier (E) may be 30% by mass or more, 50% by mass or more, or 100% by mass.

[0105] Other Resin Components: The aqueous intermediate coating composition may contain other resin components as needed. Examples of other resin components include polyester resin dispersions and urethane resin dispersions. These may be used alone or in combination of two or more.

[0106] (Polyester Resin Dispersion (F)) The polyester resin dispersion (F) contributes to improving chipping resistance. The polyester resin dispersion (F) is a dispersion-type polyester resin.

[0107] The glass transition temperature of the polyester resin dispersion (F) is, for example, −50° C. or higher and 20° C. or lower, which can improve chipping resistance and adhesion.

[0108] The acid value of the polyester resin dispersion (F) is, for example, 1 mgKOH / g or more and 50 mgKOH / g or less. The acid value of the polyester resin dispersion (F) may be 5 mgKOH / g or more, or 10 mgKOH / g or more. The acid value of the polyester resin dispersion (F) may be 30 mgKOH / g or less.

[0109] The hydroxyl value of the polyester resin dispersion (F) is, for example, 50 mgKOH / g or more and 200 mgKOH / g or less. The hydroxyl value of the polyester resin dispersion (F) may be 150 mgKOH / g or less.

[0110] The polyester resin dispersion (F) is prepared, for example, by an esterification reaction or transesterification reaction between a polybasic acid and a polyhydric alcohol.

[0111] A polybasic acid has two or more carboxy groups in one molecule. Examples of polybasic acids include aromatic dicarboxylic acids and anhydrides thereof, such as terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, and diphenylmethane-4,4'-dicarboxylic acid; alicyclic dicarboxylic acids and anhydrides thereof, such as hexahydroisophthalic acid, hexahydroterephthalic acid, hexahydrophthalic acid, and tetrahydrophthalic acid; aliphatic dicarboxylic acids and anhydrides thereof, such as adipic acid, sebacic acid, suberic acid, succinic acid, glutaric acid, maleic acid, chloromaleic acid, fumaric acid, dodecanedioic acid, pimelic acid, azelaic acid, itaconic acid, citraconic acid, and dimer acid; lower alkyl esters such as methyl esters and ethyl esters of these dicarboxylic acids; and tri- or higher polybasic acids such as trimellitic acid, trimellitic anhydride, pyromellitic acid, pyromelic anhydride, trimesic acid, methylcyclohexene tricarboxylic acid, and tetrachlorohexene tricarboxylic acid and anhydrides thereof. These may be used alone or in combination of two or more.

[0112] A polyhydric alcohol has two or more hydroxyl groups in one molecule. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, diethylene glycol, trimethylene glycol, tetraethylene glycol, triethylene glycol, dipropylene glycol, 1,4-butanediol, 1,3-butanediol, 2,3-butanediol, 1,2-butanediol, 3-methyl-1,2-butanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,2-pentanediol, 1,5-pentanediol, 1,4-pentanediol, 2,4-pentanediol, 2,3-dimethyltrimethylene glycol, tetramethylene glycol, 3-methyl-4,3-pentanediol, 3-methyl-4,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 2,5-hexanediol, neopentyl glycol, and hydroxypivalic acid. Examples of suitable alcohols include dihydric alcohols such as neopentyl glycol esters; polylactone diols obtained by adding lactones such as ε-caprolactone to these dihydric alcohols; ester diols such as bis(hydroxyethyl) terephthalate; polyether diols such as alkylene oxide adducts of bisphenol A, polyethylene glycol, polypropylene glycol, and polybutylene glycol; trihydric or higher alcohols such as glycerin, trimethylolpropane, trimethylolethane, diglycerin, triglycerin, 1,2,6-hexanetriol, pentaerythritol, dipentaerythritol, sorbitol, and mannite; polylactone polyols obtained by adding lactones such as ε-caprolactone to these trihydric or higher alcohols; and alicyclic polyhydric alcohols such as 1,4-cyclohexanedimethanol, tricyclodecanedimethanol, hydrogenated bisphenol A, and hydrogenated bisphenol F. These may be used alone or in combination of two or more.

[0113] The polyester resin dispersion (F) may be a condensation reaction product of an aromatic dicarboxylic acid and a polyhydric alcohol, which can further improve chipping resistance.

[0114] The polyester resin dispersion (F) may contain an alcohol compound obtained by reacting an α-olefin epoxide such as propylene oxide or butylene oxide, or a monoepoxy compound such as "Cardura E10" (trade name, manufactured by Japan Epoxy Resins Co., Ltd., glycidyl ester of synthetic highly branched saturated fatty acid) with an acid.

[0115] Commercially available polyester resin dispersions (F) include, for example, Beckolite 46-118 (manufactured by Dainippon Ink and Chemicals, Inc., solids concentration 60%), Beckolite 46-119 (manufactured by Dainippon Ink and Chemicals, Inc., solids concentration 60%), Beckolite M6205-50 (manufactured by Dainippon Ink and Chemicals, Inc., solids concentration 50%), Beckolite M6401-52 (manufactured by Dainippon Ink and Chemicals, Inc., solids concentration 52%), and PRS-1015 (manufactured by Nippon Paint Automotive Coatings, solids concentration 70%).

[0116] The content of the polyester resin dispersion (F) is, for example, 5 parts by mass or more and 35 parts by mass or less per 100 parts by mass of the resin solids content of the aqueous intermediate coating composition. The content of the polyester resin dispersion (F) may be 10 parts by mass or more, or may be 15 parts by mass or more. The content of the polyester resin dispersion (F) may be 30 parts by mass or less, or may be 28 parts by mass or less.

[0117] (Urethane Resin Dispersion (G)) The urethane resin dispersion (G) contributes to improving chipping resistance. The urethane resin dispersion (G) is a dispersion-type urethane resin.

[0118] The average particle size of the urethane resin dispersion (G) is, for example, 10 nm or more and 5,000 nm or less. The average particle size of the urethane resin dispersion (G) may be 20 nm or more, or 50 nm or more. The average particle size of the urethane resin dispersion (G) may be 1,000 nm or less, 500 nm or less, or 140 nm or less.

[0119] From the viewpoint of storage stability, the acid value of the urethane resin dispersion (G) may be, for example, 3 mgKOH / g or more and 75 mgKOH / g or less. The acid value of the urethane resin dispersion (G) may be 5 mgKOH / g or more, or 10 mgKOH / g or more. The acid value of the urethane resin dispersion (G) may be 60 mgKOH / g or less, or 40 mgKOH / g or less.

[0120] From the viewpoint of chipping resistance, the hydroxyl value of the urethane resin dispersion (G) may be, for example, 0 mgKOH / g or more and 45 mgKOH / g or less, and may be 30 mgKOH / g or less or 25 mgKOH / g or less.

[0121] The urethane resin dispersion (G) is prepared, for example, by reacting a polyhydric alcohol, a polyisocyanate compound, and a compound having an active hydrogen group in the molecule.

[0122] The polyhydric alcohol has two or more hydroxyl groups. Examples of the polyhydric alcohol include the same compounds as those listed as raw materials for the polyester resin dispersion (F). The number average molecular weight of the polyhydric alcohol may be 500 or more and 5,000 or less.

[0123] The polyisocyanate compound has two or more isocyanate groups in the molecule. Examples of the polyisocyanate compound include aliphatic diisocyanates having 2 to 12 carbon atoms, such as hexamethylene diisocyanate, 2,2,4-trimethylhexane diisocyanate, and lysine diisocyanate; alicyclic diisocyanates having 4 to 18 carbon atoms, such as 1,4-cyclohexane diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, methylcyclohexylene diisocyanate, and isopropylidenecyclohexyl-4,4'-diisocyanate; 2,4-toluylene diisocyanate, 2,6-toluylene diisocyanate, diphenylmethane-4,4'-diisocyanate, and 1,5'-naphthene diisocyanate. aromatic diisocyanates such as methyl methyl cyanate, tolidine diisocyanate, diphenylmethylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, xylylene diisocyanate, tetramethylxylylene diisocyanate, 4,4'-dibenzyl diisocyanate, and 1,3-phenylene diisocyanate; and triisocyanates such as lysine ester triisocyanate, triphenylmethane triisocyanate, 1,6,11-undecane triisocyanate, 1,8-diisocyanate-4,4-isocyanate methyloctane, 1,3,6-hexamethylene triisocyanate, and bicycloheptane triisocyanate.

[0124] The polyisocyanate compound may be a dimer or trimer (isocyanurate bond) of the above polyisocyanate compound, or a biuret obtained by reacting the above polyisocyanate compound with an amine. Alternatively, a polyisocyanate having a urethane bond obtained by reacting the above polyisocyanate compound with a polyhydric alcohol may be used.

[0125] The polyisocyanate compound may be an araliphatic diisocyanate compound, such as isophorone diisocyanate, 1,4-cyclohexane diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, methylcyclohexylene diisocyanate, or isopropylidenecyclohexyl-4,4'-diisocyanate.

[0126] The compound having an active hydrogen group in the molecule is used for blocking the isocyanate group in the polyisocyanate compound. Examples of the compound having an active hydrogen group in the molecule include monohydric alcohols such as methanol, ethanol, and diethylene glycol monobutyl ether; monovalent carboxylic acids such as acetic acid and propionic acid; monohydric thiols such as ethyl mercaptan; primary amines such as diethylenetriamine and monoethanolamine; secondary amines such as diethylamine; and oximes such as methyl ethyl ketoxime.

[0127] In preparing the urethane resin dispersion (G), a chain extender, a monoisocyanate compound, a catalyst, etc. may be used as needed.

[0128] The urethane resin dispersion (G) can be synthesized, for example, by a one-shot method in which the components are reacted at once, or by a multi-stage method in which the components are reacted in stages (a method in which a part of the active hydrogen-containing compound (e.g., a high molecular weight polyol) is reacted with a polyisocyanate to form an NCO-terminated prepolymer, and then the remainder of the active hydrogen-containing compound is reacted). The synthesis reaction is carried out, for example, at a temperature of 40°C or higher and 140°C or lower.

[0129] Commercially available urethane resin dispersions (G) include, for example, NAC300 (manufactured by Sanyo Chemical Industries, Ltd., solid content concentration 30%), NAC100 (manufactured by Sanyo Chemical Industries, Ltd., solid content concentration 40%), IB-465 (manufactured by Sanyo Chemical Industries, Ltd., solid content concentration 30%), IB-940 (manufactured by Sanyo Chemical Industries, Ltd., solid content concentration 40%), and IB-700 (manufactured by Sanyo Chemical Industries, Ltd., solid content concentration 30%).

[0130] The content of the urethane resin dispersion (G) is, for example, 1 part by mass or more and 60 parts by mass or less, relative to 100 parts by mass of the resin solids content of the aqueous intermediate coating composition. The content of the urethane resin dispersion (G) may be 2 parts by mass or more, or may be 5 parts by mass or more. The content of the urethane resin dispersion (G) may be 50 parts by mass or less, 30 parts by mass or less, or 10 parts by mass or less.

[0131] Hydrophobic Organic Solvent (H) The aqueous primer coating composition may contain a hydrophobic organic solvent (H) having a hydrocarbon group with 5 or more carbon atoms. The hydrophobic organic solvent (H) exhibits hydrophobic properties and is either immiscible or poorly miscible with water. Hydrophobicity can be rephrased as non-polarity (including non-polarity). The hydrophobic organic solvent (H) makes it easy to adjust the TI value to the above range.

[0132] Examples of the hydrophobic organic solvent (H) include hydrocarbon solvents such as benzene, toluene, xylene, cyclohexane, hexane, heptane, octane, chlorobenzene, trichlorobenzene, perchloroethylene, trichloroethylene, mineral spirits, solvent naphtha, petroleum benzine, isoparaffin, normal paraffin, Swazol, Shellsol, and Merveille; 1-hexanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, 1-decanol, benzyl alcohol, and ethylene glycol mono-2-ethoxysilane; Examples of suitable solvents include alcohol-based solvents such as ethylene glycol mono-n-butyl ether, propylene glycol mono-n-butyl ether, dipropylene glycol mono-n-butyl ether, tripropylene glycol mono-n-butyl ether, propylene glycol mono-2-ethylhexyl ether, and propylene glycol monophenyl ether; ester-based solvents such as isoamyl acetate, methyl amyl acetate, and ethylene glycol monobutyl ether; and ketone-based solvents such as methyl isobutyl ketone, cyclohexanone, ethyl n-amyl ketone, and diisobutyl ketone. These may be used alone or in combination of two or more. Among these, hydrocarbon-based solvents are preferred, and aliphatic hydrocarbon-based solvents such as mineral spirits, isoparaffin, normal paraffin, shell sol, and merveilles may be particularly suitable.

[0133] The content of the hydrophobic organic solvent (H) is, for example, 1% by mass or more and 15% by mass or less, based on 100% by mass of the aqueous intermediate coating composition. This makes it easier to adjust the TI value. The content of the hydrophobic organic solvent (H) may be 2% by mass or more, or 3% by mass or more. The content of the hydrophobic organic solvent (H) may be 10% by mass or less, or 8% by mass or less.

[0134] The content of the hydrophobic organic solvent (H) is, for example, 5% by mass or more and 30% by mass or less, based on 100 parts by mass of the resin solids content of the aqueous intermediate coating composition. This makes it possible to more easily adjust the TI value. The content of the hydrophobic organic solvent (H) may be 10% by mass or more, or 12% by mass or more. The content of the hydrophobic organic solvent (H) may be 25% by mass or less, or 20% by mass or less.

[0135] Other organic solvents The water-based intermediate coating composition may contain organic solvents other than the hydrophobic organic solvent (H). The hydrophobic organic solvent (H) may be used in combination with other organic solvents.

[0136] Examples of other organic solvents include alcoholic solvents such as ethylene glycol, butyl cellosolve, isopropanol, n-butanol, 2-butanol, ethanol, methanol, denatured ethanol, 2-methoxy-1-propanol, 1-methoxy-2-propanol, diacetone alcohol, and glycerin; ethers such as tetrahydrofuran and dioxane; amides such as dimethylacetamide and dimethylformamide; and halogen compounds such as chloroform, methylene chloride, and carbon tetrachloride.

[0137] Additives The aqueous intermediate coating composition may contain various additives as needed, such as film-forming aids, surface conditioners, preservatives, mildew inhibitors, antifoaming agents, light stabilizers, UV absorbers, antioxidants, and pH adjusters.

[0138] Preparation of Water-Based Intermediate Coating Composition There are no particular limitations on the method for preparing the water-based intermediate coating composition, and it can be prepared by stirring the components with a stirrer, etc. The pigment can be pre-dispersed in a vehicle containing water, a surfactant, a dispersant, etc. using a sand grind mill, etc. to form a pigment paste, which can then be mixed with the other components.

[0139] [Coated Article] A coated article can be obtained using the aqueous intermediate coating composition according to the present disclosure. The aqueous intermediate coating composition according to the present disclosure is suitably used to form a coating film (intermediate coating film) that is disposed between the substrate and the base coating film.

[0140] The coated article comprises, for example, a substrate and a multilayer coating film formed by laminating an intermediate coating film, a base coating film, and a clear coating film in this order. The intermediate coating film is formed from the aqueous intermediate coating composition according to the present disclosure. The aqueous intermediate coating composition according to the present disclosure can provide a coated article with excellent weather resistance and appearance (smoothness).

[0141] (Substrate) Examples of the substrate material include metal, resin, and glass. Specific examples of the substrate include automobile bodies and automobile body parts such as passenger cars, trucks, motorcycles, and buses, and automobile parts such as spoilers, bumpers, mirror covers, grilles, and door knobs.

[0142] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of metal substrates include steel sheets such as 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.

[0143] 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.

[0144] 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 (AAS) resin, acrylonitrile styrene (AS) resin, polyamide resin, acetal resin, phenolic resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). Resin substrates may be degreased.

[0145] (Intermediate Coating Film) The intermediate coating film is formed from the aqueous intermediate coating composition according to the present disclosure. The intermediate coating film is formed adjacent to the substrate.

[0146] The intermediate coating film may have a light transmittance of 1.5% or less at wavelengths of 300 to 550 nm. The light transmittance is measured for a single coating film formed using the aqueous intermediate coating composition using a spectrophotometer (Hitachi U-3310) at 10 nm intervals in the wavelength range of 300 to 550 nm. The light transmittance of the single coating film may be 1.5% or less at all wavelengths in the wavelength range of 300 to 550 nm.

[0147] The single coating film is obtained by applying the aqueous intermediate coating composition to a polypropylene plate and heating it at 140°C for 30 minutes. The light transmittance is measured for the coating film (film thickness 25 µm) peeled from the polypropylene plate.

[0148] The thickness of the intermediate coating film is not particularly limited and may be appropriately set depending on the purpose. The thickness of the intermediate coating film after curing may be 5 μm or more and 35 μm or less.

[0149] (Base Coating Film) The base coating film is formed adjacent to the intermediate coating film. The base coating film imparts design properties to the multilayer coating film. The base coating film may be one or more layers, or may be two or more layers. The thickness of each layer of the base coating film after curing may be, for example, 5 μm or more and 60 μm or less.

[0150] The base coating film is formed from a base paint composition. The base paint composition may be aqueous or solvent-based. The base paint composition may be aqueous. An aqueous base paint composition contains, for example, an acrylic resin emulsion, a water-soluble acrylic resin, a curing agent (typically, a melamine resin), and a polyether polyol resin. The base paint composition may further contain the above-mentioned luster pigment, coloring pigment, and various additives. A solvent-based base paint composition contains an organic solvent as the main solvent. In a solvent-based base paint composition, the proportion of the organic solvent in the solvent is 50% by mass or more, may be 70% by mass or more, or may be 100% by mass.

[0151] (Clear Coating Film) The clear coating film is formed on the base coating film. The clear coating film improves the gloss of the coated article and prevents the pigment blended in the lower layer from falling off or popping out. The thickness of the clear coating film after curing may be 15 μm or more and 50 μm or less from the viewpoint of scratch resistance and smoothness.

[0152] The clear coating film is formed by a clear coating composition. The clear coating composition may be solvent-based, water-based, or powder-type. The clear coating composition may be solvent-based. From the viewpoints of transparency or acid etching resistance, the solvent-based clear coating composition may contain an acrylic resin and / or polyester resin as a film-forming resin, and an amino resin and / or isocyanate as a curing agent. The solvent-based clear coating composition may also contain an acrylic resin and / or polyester resin having a carboxylic acid and / or an epoxy group. The clear coating composition may contain the various pigments described above to the extent that transparency is not impaired. The clear coating composition may contain various additives as needed.

[0153] [Method for producing multi-layer coating film] A multi-layer coating film is produced, for example, by a method comprising the steps of applying an aqueous intermediate coating composition according to the present disclosure onto an object to be coated to form an uncured intermediate coating film, curing the uncured intermediate coating film, applying a base coating composition to form an uncured base coating film, curing the uncured base coating film, applying a clear coating composition onto the intermediate coating film to form an uncured clear coating film, and curing the uncured clear coating film.

[0154] When the clear coating film is formed, the intermediate coating film and the base coating film may be cured or uncured. From the viewpoints of productivity, adhesion, and water resistance, each coating film may be laminated without curing (so-called wet-on-wet coating), and then these multiple uncured coating films may be cured simultaneously.

[0155] Wet-on-wet painting comprises the steps of applying an aqueous intermediate coating composition according to the present disclosure onto an object to be coated to form an uncured intermediate coating film, applying a base coating composition onto the uncured intermediate coating film to form an uncured base coating film, applying a clear coating composition onto the uncured base coating film to form an uncured clear coating film, and simultaneously curing the uncured intermediate coating film, uncured base coating film, and uncured clear coating film.

[0156] After application of the aqueous intermediate coating composition, preheating may be performed before application of the base coating composition. After application of the base coating composition, preheating may be performed before application of the clear coating composition. Preheating is performed, for example, by leaving the coating at a temperature of 20°C to 25°C for 30 seconds to 15 minutes, or by heating at a temperature of 50°C to 80°C for 30 seconds to 10 minutes.

[0157] Examples of the coating method include air spray coating, airless spray coating, electrostatic spray coating, multi-stage coating using air electrostatic spray coating (typically, two-stage coating), and coating that combines air electrostatic spray coating with a rotary atomizer-type electrostatic coater.

[0158] The curing of each coating composition is carried out, for example, under conditions of a heating temperature of 80° C. to 180° C. and a heating time of 5 to 60 minutes.

[0159] The present invention will be described in more detail with reference to the following examples, but is not limited thereto. In the examples, "parts" and "%" are by weight unless otherwise specified.

[0160] (Weight-Average Molecular Weight) The weight-average molecular weight was measured using a GPC apparatus "HLC8220GPC" (trade name, manufactured by Tosoh Corporation) and four columns, "Shodex KF-606M" and "Shodex KF-603" (both trade names, manufactured by Showa Denko K.K.), under the following conditions: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 0.6 cc / min, and detector: RI.

[0161] (Average Particle Diameter) The average particle diameter (volume average particle diameter D50) was measured using a laser Doppler particle size analyzer (manufactured by Nikkiso Co., Ltd., "Microtrac UPA-150") by diluting the dispersion with ion-exchanged water so as to obtain an appropriate signal level.

[0162] [Production Example A-1] Production of Core-Shell Acrylic Resin Dispersion (A-1) (1) Synthesis of Hydrophobic Acrylic Resin (Ac) A reaction vessel equipped with a stirrer, a temperature controller, a condenser, and a dropping device was charged with 30 parts of a reactive solvent (CAE, glycidyl ester of monocarboxylic acid having a branched alkyl group having 9 carbon atoms, trade name: Cardura E10P, manufactured by Hexion Corporation, boiling point 251 to 278°C), and the temperature was raised to 165°C with stirring to reflux. Separately, a mixture of 9.47 parts of acrylic acid (AA), 5.8 parts of 2-hydroxyethyl methacrylate (HEMA), 11.6 parts of cyclohexyl methacrylate (CHMA), 7.5 parts of n-butyl acrylate (NBA), 16.9 parts of styrene (ST), 0.28 parts of a polymerization initiator (DTA, trade name: Luperox DTA, manufactured by Arkema Yoshitomi Co., Ltd.), and 6.5 parts of a high-boiling point solvent (dipropylene glycol monomethyl ether (DPM)) was prepared. This mixture was added dropwise to the reaction vessel at 165°C over 3.5 hours to carry out a polymerization reaction and a ring-opening addition reaction.

[0163] (2) Synthesis of carboxyl group-containing acrylic resin (As') Separately, a mixture of 3.43 parts of AA, 3.8 parts of HEMA, 2.7 parts of CHMA, 2.9 parts of NBA, 5.8 parts of ST, 0.11 parts of polymerization initiator (Luperox DTA), and 2.4 parts of high-boiling solvent (DPM) was prepared. This mixture was added dropwise to the reaction vessel over 1 hour at 165 ° C. with stirring, and allowed to polymerize for 1 hour. Furthermore, a mixture of 0.1 parts of polymerization initiator (Luperox DTA) and 0.1 parts of high-boiling solvent (DPM) was added to the reaction vessel, and the mixture was allowed to polymerize for 1 hour at 165 ° C. with stirring.

[0164] In this way, a varnish containing a core-shell type acrylic resin having a core-forming resin (Ac) and an acid group-containing resin (As') and having a solid content concentration of 91% by mass was obtained.

[0165] (3) A basic compound (dimethylethanolamine, DMEA) was added to the neutralized and phase-inverted varnish at a ratio of 5.33 parts per 100 parts of acrylic resin at 80°C, followed by stirring for 15 minutes. The neutralization rate of the carboxyl groups was adjusted to 90%. Subsequently, 150 parts of deionized water was added dropwise to the neutralized varnish at 80°C while stirring, yielding a milky white dispersion containing acrylic resin particles.

[0166] The acrylic resin particles contained in the dispersion were predicted to have a core-shell structure, with the core portion containing a hydrocarbon group. The mass proportion of the core portion was calculated to be 81.4%, and the mass proportion of the shell portion was calculated to be 18.6%. The average particle size of the acrylic resin particles was 57 nm, the acid value was 37.2 mg KOH / g, and the weight-average molecular weight was 32,000.

[0167] [Production Example A-2] Production of Core-Shell Acrylic Resin Dispersion (A-2) (1) Synthesis of Hydrophobic Acrylic Resin (Ac) A reaction vessel equipped with a stirring, cooling, and heating device was charged with 210 parts by mass of butyl acetate and heated to 120 ° C. Thereafter, a monomer mixture of 73.65 parts by mass of styrene (ST), 178.96 parts by mass of methyl methacrylate (MMA), 75.94 parts by mass of n-butyl acrylate (BA), 64.45 parts by mass of 2-ethylhexyl acrylate (2-EHA), 105.00 parts by mass of hydroxyethyl methacrylate (HEMA), and 2.0 parts by mass of acrylic acid (AA), and a solution obtained by dissolving 70 parts by mass of tert-butylperoxy-2-ethylhexanate in 45 parts by mass of butyl acetate, was added dropwise in two series at a constant rate over 90 minutes. While maintaining the reaction vessel at 120 ° C., 10 parts by mass of methacrylic anhydride was added and the mixture was maintained for 2 hours. Thereafter, the mixture was cooled to 60° C. to obtain a hydrophobic acrylic resin (Ac).

[0168] (2) Synthesis of carboxy group-containing acrylic resin (As') Separately, 210 parts by mass of butyl acetate was charged into a reaction vessel equipped with a cooling and heating device and heated to 120 ° C. Thereafter, a monomer mixture of 835.00 parts by mass of hydrophobic acrylic resin (Ac), 73.65 parts by mass of styrene (ST), 178.96 parts by mass of methyl methacrylate (MMA), 75.94 parts by mass of n-butyl acrylate (BA), 64.45 parts by mass of 2-ethylhexyl acrylate (2-EHA), 105.00 parts by mass of hydroxyethyl methacrylate (HEMA), and 20.0 parts by mass of acrylic acid (AA), and a solution obtained by dissolving 140 parts by mass of tert-butylperoxy-2-ethylhexanate in 120 parts by mass of butyl acetate, was added dropwise to the reaction vessel in two series at a constant rate over 90 minutes.

[0169] The reaction mixture was maintained at 120°C for 30 minutes, and then a solution of 3 parts by mass of tert-butylperoxy-2-ethylhexanate in 80 parts by mass of butyl acetate was added dropwise in one series at a constant rate over 30 minutes. After stirring at 120°C for 60 minutes, the mixture was cooled to 70°C.

[0170] In this way, a varnish containing a core-shell type acrylic resin having a core-forming resin (Ac) and an acid group-containing resin (As') and having a solid content concentration of 58.5% by mass was obtained.

[0171] (3) The neutralized and phase inversion varnish was diluted with 170 parts by mass of dipropylene glycol monomethyl ether. The butyl acetate was distilled off under reduced pressure until the solid content reached 85% by mass. 27.21 parts by mass of dimethylethanolamine (DMEA) was added thereto, followed by the addition of 1,680 parts by mass of water to obtain a milky white dispersion containing acrylic resin particles.

[0172] The acrylic resin particles contained in the dispersion were expected to have a core-shell structure, and the mass ratio of the core portion was calculated to be 50%, and the mass ratio of the shell portion was calculated to be 50%. The acrylic resin particles had an average particle size of 90 nm, an acid value of 17 mg KOH / g, and a weight-average molecular weight of 25,000.

[0173] [Production Example a-1] Production of Acrylic Resin Emulsion (a-1): 126.5 parts of deionized water was added to a reaction vessel, and the temperature was raised to 80°C while mixing and stirring in a nitrogen stream. Next, a monomer emulsion consisting of 100 parts of a monomer mixture (containing 27.61 parts of methyl acrylate, 53.04 parts of ethyl acrylate, 4.00 parts of styrene, 9.28 parts of 2-hydroxyethyl methacrylate, 3.07 parts of methacrylic acid, and 3.00 parts of allyl methacrylate), 1.1 parts of an emulsifier (trade name: Adeka Reasoap SR-10, manufactured by ADEKA Corporation), and 80 parts of deionized water, and an initiator solution consisting of 0.3 parts of ammonium persulfate and 10 parts of deionized water, were added dropwise to the reaction vessel in parallel over a period of 2 hours. After completion of the dropwise addition, the mixture was aged at the same temperature for 2 hours.

[0174] The mixture was then cooled to 40°C and filtered through a 400-mesh filter. 20 parts of deionized water and 0.32 parts of dimethylaminoethanol were added to the filtrate, and the pH was adjusted to 6.5. This yielded a hydroxyl-containing acrylic resin emulsion (A) with an average particle size of 90 nm, a Tg of -9.5°C, a solids concentration of 30%, an acid value of 20 mgKOH / g, and a hydroxyl value of 40 mgKOH / g.

[0175] [Melamine resin (B)] B-1: Hydrophobic, product name Cymel 211, manufactured by Allnex Corporation B-2: Hydrophobic, product name Cymel 202, manufactured by Allnex Corporation b-1: Hydrophilic, product name Cymel 370, manufactured by Allnex Corporation

[0176] [Brilliant pigment (C) paste] C-1: Product name FD-5090, manufactured by Asahi Kasei Corporation, aluminum pigment, average particle diameter 9 μm C-2: Product name 4690NS, manufactured by Toyo Aluminum K.K., aluminum pigment, average particle diameter 7 μm

[0177] [Preparation Example D-1] Preparation of pigment dispersion paste (D-1) 4.5 parts of a pigment dispersant (trade name: Disperbyk 190, manufactured by BYK-Chemie, anionic / nonionic dispersant), 0.5 parts of an antifoaming agent (BYK-011, manufactured by BYK-Chemie), 29.6 parts of ion-exchanged water, and 65.4 parts of rutile titanium dioxide were premixed. Subsequently, the mixture was dispersed in a paint conditioner using a glass bead medium at room temperature until the particle size became 5 μm or less, thereby obtaining a pigment dispersion paste (D-1) containing a white pigment.

[0178] [Viscosity modifiers (E)] E-1: Hydrophobic association type (polyacrylic acid type), product name Disparlon AQ-021, manufactured by Kusumoto Chemical Industries, Ltd. E-2: Polyamide type, product name Disparlon AQ-600, manufactured by Kusumoto Chemical Industries, Ltd. E-3: Urethane association type, product name Nopal 3303, manufactured by San Nopco Ltd. E-4: Alkali swelling type (carboxylic acid type), product name HV30, manufactured by BASF

[0179] [Production Example F-1] Production of Polyester Resin Dispersion (F-1) 250 parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts of cyclohexanedicarboxylic acid were added to a reaction vessel equipped with a stirrer, nitrogen inlet tube, temperature control device, condenser, and decanter. The temperature was raised to 180°C, and a condensation reaction was carried out until water no longer distilled. After cooling to 60°C, 120 parts of phthalic anhydride was added to the reaction vessel. Next, the temperature was raised to 140°C and maintained at this temperature for 60 minutes to obtain a polyester resin. After cooling to 80°C, 59 parts of dimethylaminoethanol (equivalent to 80% of the acid value of the resin (neutralization rate 80%)) and 1,920 parts of deionized water were added to the reaction vessel and stirred. This resulted in a polyester resin dispersion (F-1) with a solids content of 45% by mass, a hydroxyl value of 110 mg KOH / g, an acid value of 15 mg KOH / g, a Tg of -14°C, and an Mw of 7,000.

[0180] [Production Example G-1] Production of Urethane Resin Dispersion (G-1) In a 5000 ml four-neck flask equipped with a thermometer, a nitrogen inlet tube, and a condenser, 645.2 parts of PTG-2000SN (manufactured by Hodogaya Chemical Co., Ltd., polytetramethylene ether glycol, molecular weight 2000), 114.8 parts of 2,2-dimethylolpropionic acid, 50.2 parts of neopentyl glycol, and 21.5 parts of trimethylolpropane were dissolved in 683.8 parts of acetone. To this solution, 368.3 parts of tolylene diisocyanate were added dropwise over 2 hours. Subsequently, the mixture was allowed to react at 56°C for 7 hours. The prepolymer thus obtained was continuously added dropwise over 2 hours at 50°C to a mixed aqueous solution of 64.7 parts of N,N-dimethylethanolamine and 1914.7 parts of ion-exchanged water, followed by stirring for 1 hour. Next, the acetone was removed under reduced pressure at 50°C to obtain Urethane Resin Dispersion (G-1).

[0181] [Organic Solvent (H)] H-1: Hydrophobic, 2-ethyl-1-hexanol H-2: Hydrophobic, isoparaffin, trade name: Merveille 30, manufactured by Idemitsu Kosan Co., Ltd. h-1: Hydrophilic, 1-methoxy-2-propanol

[0182] [Production Example 1] Production of Phosphate Group-Containing Acrylic Resin 40 parts of ethoxypropanol were charged into a 1-liter reaction vessel equipped with a stirrer, temperature regulator, and condenser. Separately, a monomer solution was prepared consisting of 40 parts of a solution obtained by dissolving 20 parts of Phosmer PP (acid phosphooxyhexa(oxypropylene)monomethacrylate, manufactured by Unichemical Co., Ltd.) in 4 parts of styrene, 35.96 parts of n-butyl acrylate, 18.45 parts of ethylhexyl methacrylate, 13.92 parts of 2-hydroxyethyl methacrylate, 7.67 parts of methacrylic acid, and 20 parts of ethoxypropanol, and 1.7 parts of azobisisobutyronitrile. 121.7 parts of this monomer solution was added dropwise to the reaction vessel at 120°C over 3 hours. Stirring was continued for another hour to obtain a phosphate group-containing acrylic resin (solid concentration 63%) having an acid value of 105 mgKOH / g, of which the acid value due to phosphate groups was 55 mgKOH / g, a hydroxyl value of 60 mgKOH / g, and a number average molecular weight of 6,000.

[0183] Example 1 (i) Preparation of an aqueous intermediate coating composition 22 parts of a core-shell acrylic resin dispersion (A-1) (resin solids content 22%), 35 parts of a hydrophobic melamine resin (B-1), 22 parts of a polyester resin dispersion (F-1), 20 parts of a urethane resin dispersion (G-1), 10 parts of a hydrophobic organic solvent (H-1), 5 parts of a hydrophobic organic solvent (H-2), 10 parts of a hydrophilic organic solvent (h-1), 75 parts of deionized water, and a pigment dispersion paste (D-1) were mixed in an amount to give a color pigment (D) (titanium dioxide) content of 150 parts. Furthermore, 1 part of a phosphate group-containing acrylic resin, 2.6 parts of an aluminum pigment luster pigment (C-1), and 1.1 parts of an aluminum pigment luster pigment (C-2) were mixed and stirred to obtain an aqueous intermediate coating composition. The total content of the hydrophobic organic solvent (H) was 15% by mass based on 100% by mass of the resin solids content of the aqueous intermediate coating composition. The above blending parts are based on the solids content.

[0184] (ii) Formation of intermediate coating film A zinc phosphate-treated dull steel plate (thickness 0.8 mm, length 30 cm, width 40 cm) was coated with a cationic electrodeposition paint (product name: Power Top U-50, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) to a dry film thickness of 25 μm. The electrodeposition paint was then heated and cured to prepare a coated object.

[0185] The aqueous intermediate coating composition was then applied to the substrate so that the dry film thickness was 8 μm, and the coating was then preheated at 80° C. for 3 minutes, followed by baking and curing at 140° C. for 30 minutes to produce test panel A having an intermediate coating film.

[0186] (iii) Formation of multi-layer coating film In the same manner as above, the aqueous intermediate coating composition was applied to the substrate so that the dry film thickness was 8 μm, and then the base coating composition (trade name: Aqualex AR-3020-1 Pearl Mica, manufactured by Nippon Paint Automotive Coatings Co., Ltd., aqueous mica base paint) was applied by air spray coating so that the dry film thickness was 8 μm, and the coating was preheated at 80°C for 3 minutes.

[0187] Next, a clear coating composition (trade name: Polyurexcel O-2300 Clear, manufactured by Nippon Paint Automotive Coatings Co., Ltd., a two-component clear coating) was applied by air spray coating so that the dry coating film thickness was 35 μm. Finally, heat curing was carried out at 140°C for 30 minutes to prepare test panel B having a multi-layer coating film.

[0188] The water-based intermediate coating composition, base coating composition and clear coating composition were diluted under the following conditions and used for coating.

[0189] Water-based primer paint composition (solid content 49% by mass) Thinner: ion-exchanged water Viscosity: 4500 mPa·s / 6 rpm / B-type viscometer

[0190] Base paint composition thinner: ion-exchanged water Viscosity: 2200 mPa·s / 6 rpm / B-type viscometer

[0191] Clear coating composition thinner: EEP (ethoxyethyl propionate) / BDG-AC (butyl diglycol acetate) = 8 / 2 (mass ratio) mixed solvent Viscosity: 25 seconds / No. 4 Ford cup / 20°C

[0192] [Examples 2 to 10, Comparative Examples 1 to 6] Aqueous intermediate coating compositions were prepared and test panels A and B were prepared in the same manner as in Example 1, except that the components and / or their amounts were changed as shown in Table 1.

[0193] [Evaluation] The following evaluations were carried out using the aqueous intermediate coating compositions obtained in the Examples and Comparative Examples and the test panels A and B. The evaluation results are shown in the table below.

[0194] (1) TI Value After allowing the aqueous intermediate coating composition to stand under conditions of 20°C and 50% RH, the viscosity ηL when stirred at a low speed (6 rpm) and the viscosity ηH when stirred at a high speed (60 rpm) were measured using a rotational viscometer (B-type viscometer, "TVB-10M", manufactured by Toki Sangyo Co., Ltd., SPINDLER Rotor No. M3). The viscosity ηL was divided by the viscosity ηH to calculate the TI value (ηL / ηH).

[0195] (2) Appearance of intermediate coating film Test panel A having an intermediate coating film was used. In accordance with JIS B 0601, the arithmetic mean roughness Ra value of the intermediate coating film was measured using a surface roughness measuring instrument (Mitutoyo Corporation, SURFTESTSJ-201P). Specifically, the surface roughness was measured 10 times at different locations under the condition of a cutoff value of 0.25 mm (number of sections: 5), and the average Ra value was calculated. The obtained Ra values ​​were evaluated according to the following criteria. A rating of B or higher can be considered to be excellent in coating film appearance.

[0196] (Evaluation criteria) A: Less than 0.18 μm B: 0.18 μm or more and less than 0.20 μm C: 0.20 μm or more

[0197] (3) Appearance of multilayer coating film Test panel B having a multilayer coating film was used. The W1 value and W4 value of the surface of the multilayer coating film were measured using a "Wavescan-T" manufactured by BYK-Gardner GmbH. The smaller each W value, the higher the smoothness of the coating film. A rating of B or higher can be evaluated as excellent in appearance.

[0198] (Evaluation Criteria) A: W1 is less than 12 and W4 is less than 12. B: W1 is 12 or more and less than 15, or W4 is 12 or more and less than 15. C: W1 is 15 or more, or W4 is 15 or more.

[0199] (4) Water resistance test panel B was immersed in warm water at 40°C. After 10 days, test panel B was pulled out and the water droplets were lightly wiped off. After pulling out, the coated article was left at room temperature (23°C) for 1 hour. The cutting blade of an NT Cutter S type (manufactured by NT Corporation) was held at approximately 30 degrees to the coating surface to make 100 squares (11 vertical lines, 11 horizontal lines, 2 mm intervals) that reached the substrate. Adhesive tape (Nichiban Co., Ltd., Cellotape (registered trademark), 24 mm wide) was evenly applied with fingertips to cover all squares, ensuring that no air bubbles remained. The adhesive tape was immediately peeled off while being pulled so that the angle between the tape and the coating was approximately 90°. After peeling the tape, the number of squares where the coating had peeled off was counted and evaluated according to the following criteria. A rating of B or higher indicates excellent water resistance.

[0200] (Evaluation criteria) A: 0 squares / 100 squares B: 1-5 squares / 100 squares C: 5 squares / 100 or more

[0201] (5) Weathering test panel B was subjected to a xenon lamp accelerated weathering test with an integrated light intensity of 1000 MJ / m using a xenon lamp accelerated weathering tester (manufactured by Suga Test Instruments Co., Ltd.). ２ After that, a tape peeling test was carried out in the same manner as above, and the film was evaluated. A rating of B or higher can be evaluated as excellent weather resistance.

[0202] (6) Chipping Resistance: A stone chipping test was carried out on test panel B using a Gravelo tester KSS-1 (manufactured by Suga Test Instruments Co., Ltd.) under the following conditions: <Test conditions> Shot material: No. 6 crushed stone as specified in JIS A5001 (crushed stone for roads) Amount of shot material: 300 g Distance: 35 cm Shot pressure: 0.50 MPa Shot angle: 45° Test temperature: -20°C

[0203] Next, test panel B was washed with water and dried. Subsequently, industrial gummed tape manufactured by Nichiban Co., Ltd. was evenly pressed onto the multilayer coating film with the fingertips so as not to leave any air bubbles. The gummed tape was immediately peeled off while being pulled so that the angle between it and the coating film was approximately 90°. After peeling off the tape, the degree of peeling of the coating film was visually evaluated according to the following criteria. A rating of B or higher can be evaluated as suitable for practical use. (Evaluation criteria) A: Peeling area was small and the frequency was low B: Peeling area was somewhat large and the frequency was somewhat high C: Peeling area was large and the frequency was high

[0204]

[0205] The aqueous intermediate coating composition of the present disclosure provides a multi-layer coating film with excellent appearance, and is suitable for coating automobile bodies and automobile components.

[0206] This application claims priority based on Japanese Patent Application No. 2024-154143, filed on September 6, 2024, the entire contents of which are incorporated herein by reference.

Claims

1. A water-based intermediate coating composition comprising: a core-shell acrylic resin dispersion (A); a hydrophobic melamine resin (B); a photoluminescent pigment (C); a color pigment (D) other than the photoluminescent pigment (C); and a viscosity modifier (E), wherein the shell portion of the core-shell acrylic resin dispersion (A) comprises a hydrophilic resin, and the thixotropic index of the water-based intermediate coating composition is 3.5 or more and 10 or less.

2. The aqueous primer coating composition according to claim 1, wherein the core-shell acrylic resin dispersion (A) has a branched hydrocarbon group having 4 to 24 carbon atoms in the core portion.

3. The aqueous primer coating composition according to claim 1 or 2, wherein the content ratio (C:D) of the luster pigment (C) to the coloring pigment (D) is 1:20 to 1:

260.

4. The aqueous primer coating composition according to any one of claims 1 to 3, wherein the viscosity modifier (E) comprises a hydrophobic association type viscosity modifier.

5. The aqueous intermediate coating composition according to any one of claims 1 to 4, wherein the average particle size of the luster pigment (C) is 5 μm or more and 12 μm or less.

6. The water-based primer coating composition according to any one of claims 1 to 5, wherein the bright pigment (C) contains aluminum particles.

7. The aqueous intermediate coating composition according to any one of claims 1 to 6, further comprising a polyester resin dispersion (F).

8. The aqueous primer coating composition according to any one of claims 1 to 7, further comprising a urethane resin dispersion (G).

9. The water-based primer coating composition according to any one of claims 1 to 8, further comprising a hydrophobic organic solvent (H) having a hydrocarbon group having 5 or more carbon atoms.

10. The water-based primer coating composition according to any one of claims 1 to 9, wherein the color pigment (D) includes a white pigment.

11. The aqueous primer coating composition according to claim 10, wherein the PWC of the white pigment is 40% by mass or more and 65% by mass or less.

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