Method for forming a multi-layer coating

Abstract

This invention provides a method for forming a multi-layer coating film with excellent appearance. [Solution] A method for forming a multilayer coating film, comprising sequentially applying a water-based intermediate coating composition, a water-based base coating composition, and a clear coating composition in a wet-on-wet application and curing them, wherein the water-based intermediate coating composition comprises a core-shell type acrylic resin dispersion (A), a hydrophobic melamine resin (B), a polyester resin dispersion (C), a urethane resin dispersion (D), a glossy pigment (E), a white pigment (F), and a viscosity modifier (G), and has a thixotropic index of 3.5 or more and 10 or less, the core-shell type acrylic resin dispersion (A) has a hydrophilic resin in the shell portion, and the water contact angle of each dried resin film obtained by drying the coating film formed by the aqueous dispersions of each resin dispersion (A), polyester resin dispersion (C), and urethane resin dispersion (D) at 23°C for 45 minutes is 40° or more and 120° or less.

Description

[Technical Field]

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

[0002] Typically, a multi-layer coating comprising an intermediate coating, a base coating, and a clear coating is formed on the surface of an automobile's exterior panel. Patent Document 1 discloses a wet-on-wet coating method using an intermediate coating paint composition containing a glossy pigment. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2015 / 99151 [Overview of the project] [Problems that the invention aims to solve]

[0004] Wet-on-wet painting is prone to causing mixed layers. Furthermore, the glossy pigments contained in the intermediate coating composition can cause irregularities on the surface of the intermediate coating film.

[0005] The present invention aims to provide a method for forming a multi-layer coating film with excellent appearance. [Means for solving the problem]

[0006] To solve the above problems, the present invention provides the following embodiments. [1] A method for forming a multi-layer coating film, comprising sequentially applying a water-based intermediate coating composition, a water-based base coating composition, and a clear coating composition in a wet-on-wet application and curing them, The aforementioned aqueous intermediate coating composition comprises a core-shell type acrylic resin dispersion (A), a hydrophobic melamine resin (B), a polyester resin dispersion (C), a urethane resin dispersion (D), a glossy pigment (E), a white pigment (F), and a viscosity modifier (G), and has a thixotropic index of 3.5 or more and 10 or less. The core-shell type acrylic resin dispersion (A) has a shell portion made of hydrophilic resin. A method for forming a multilayer coating film, wherein the core-shell type acrylic resin dispersion (A), the polyester resin dispersion (C), and the urethane resin dispersion (D) are used, and the coating film formed by the aqueous dispersion of each resin dispersion is dried at 23°C for 45 minutes, resulting in each dried resin film having a water contact angle of 40° or more and 120° or less. [2] The core-shell type acrylic resin dispersion (A) is the aqueous intermediate coating composition of [1] above, having branched hydrocarbon groups having 4 to 24 carbon atoms in the core portion. [3] A method for forming a multilayer coating film according to [1] or [2] above, wherein the content ratio (E:F) of the luminous pigment (E) to the white pigment (F) is 1:20 to 1:260. [4] The method for forming a multilayer coating film according to [1] or [2] above, wherein the viscosity modifier (G) includes a hydrophobic association type viscosity modifier. [5] A method for forming a multilayer coating film according to [1] or [2] above, wherein the average particle size of the luminous pigment (E) is 5 μm or more and 12 μm or less. [6] The method for forming a multilayer coating film according to [1] or [2] above, wherein the luminous pigment (E) contains aluminum particles. [7] Furthermore, a method for forming a multilayer coating film according to [1] or [2] above, comprising a hydrophobic organic solvent (H) having a hydrocarbon group with 5 or more carbon atoms. [8] A method for forming a multilayer coating film according to [1] or [2] above, wherein the PWC of the white pigment (F) is 40% by mass or more and 65% by mass or less. [Effects of the Invention]

[0007] According to the present invention, there is provided a method for forming a multilayer coating film having excellent appearance.

Embodiment for Carrying Out the Invention

[0008] Wet-on-wet coating is a method of applying a plurality of different paint compositions without passing through a curing process. Wet-on-wet coating can improve productivity and the adhesion between coating films. On the other hand, in wet-on-wet coating, the previously applied paint composition (which can be said to be an uncured coating film) and the subsequently applied paint composition can mix and cause a mixed layer. If both have the same properties (for example, both are aqueous), the mixed layer is more likely to occur. The mixed layer is one of the causes of the deterioration of the appearance of the coating film. In the present disclosure, by setting the water contact angle of the resin component contained in the aqueous intermediate coating composition within a predetermined range, the mixed layer between the aqueous intermediate coating composition (uncured intermediate coating film) and its upper layer is suppressed, and the coating film appearance is improved.

[0009] ·Water contact angle The core-shell type acrylic resin dispersion (A), polyester resin dispersion (C), and urethane resin dispersion (D) contained in the aqueous intermediate coating composition used in the present disclosure, the water contact angle of each dry resin film obtained by drying the coating film formed by the aqueous dispersion of each resin dispersion at 23°C for 45 minutes is 40° or more and 120° or less. Therefore, the water contact angle of the aqueous intermediate coating composition (uncured intermediate coating film) also becomes high, and the mixing of the uncured intermediate coating film and the aqueous base coating composition wet-on-wet coated on the uncured intermediate coating film is suppressed.

[0010] The aqueous intermediate coating composition used in the present disclosure contains, as main resin components, a core-shell type acrylic resin dispersion (A), a hydrophobic melamine resin (B), a polyester resin dispersion (C), and a urethane resin dispersion (D). Since the hydrophobic melamine resin (B) has a high water contact angle, the water contact angles of the other resin components are evaluated.

[0011] Three types of dried resin films are prepared, using three different resin dispersions (A, C, and D). A core-shell type acrylic resin dispersion (A) and deionized water are mixed to prepare an aqueous dispersion A with a solid content of 30% by mass. The obtained aqueous dispersion A is coated onto a test plate with an electrodeposited coating using a doctor blade to achieve a dry film thickness of 30 μm, and dried at 23°C for 45 minutes to obtain dried resin film A. Similarly, dried resin film C is prepared from polyester resin dispersion (C), and dried resin film D is prepared from urethane resin dispersion (D). The water contact angles of these three types of dried resin films (A, C, and D) are measured. It is important that the water contact angles of all three types of dried resin films are between 40° and 120°. The dried resin films contain almost no moisture. The moisture content of the dried resin films may be 5% by mass or less.

[0012] The water contact angle is determined by the θ / 2 method (half-angle method). The measurement procedure is as follows: Using a contact angle meter, approximately 1 μL of distilled water is dropped onto the surface of dry resin films A, C, or D in an atmosphere of 23°C and 50% relative humidity, and the angle between the droplet and the surface of each dry resin film after 60 seconds is calculated using the θ / 2 method. An example of a contact angle meter is the automatic contact angle meter DM500-M (manufactured by Kyowa Interface Science Co., Ltd.).

[0013] The water contact angles of each dried resin film (A, C, D) may be 50° or more, or 70° or more. The water contact angles of each dried resin film (A, C, D) may be 115° or less, or 110° or less.

[0014] In this disclosure, wet-on-wet coating comprises the steps of: applying an aqueous intermediate coating composition onto a workpiece to form an uncured intermediate coating film; applying an aqueous base coating composition onto the uncured intermediate coating film to form an uncured base coating film; and applying a clear coating composition onto the uncured base coating film to form an uncured clear coating film. Subsequently, the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film are cured at once.

[0015] The lustrous pigments contained in the water-based intermediate coating composition reduce the light transmittance of the intermediate coating film, improving the weather resistance of the multi-layer coating. On the other hand, the orientation of lustrous pigments is easily disrupted, which is one of the factors that degrades the appearance (surface smoothness) of the intermediate coating film and the multi-layer coating. In this disclosure, the orientation of the lustrous pigments in the water-based intermediate coating composition and the uncured coating film is enhanced by setting the thixotropic index (TI value) of the water-based intermediate coating composition to a specific range, thereby improving the appearance of the coating film.

[0016] • Thixotropic Index (TI value) The thixotropic index (TI value) of the water-based intermediate coating composition is between 3.5 and 10. This TI value is higher than that of conventional water-based intermediate coating compositions.

[0017] The TI value is obtained by dividing the viscosity ηL (Pa·s) at a rotational speed of 6 rpm, measured using a rotational viscometer at 20°C, by the viscosity ηH (Pa·s) at a rotational speed of 60 rpm (TI value = ηL / ηH).

[0018] A high TI value means that the viscosity ηL is high at low shear speeds (6 rpm) and / or the viscosity ηH is low at high shear speeds (60 rpm). In other words, the aqueous intermediate coating composition according to this disclosure has low viscosity during application and high viscosity after being applied to the substrate. Low viscosity during application means that the lustrous pigment is atomized without agglomeration. Therefore, the lustrous pigment can move freely in the aqueous intermediate coating composition during application, increasing its orientation. High viscosity after application allows the lustrous pigment to remain in the coating film with high orientation. In short, a relatively high TI value makes it easier for the lustrous pigment to orient in the aqueous intermediate coating composition, and maintains its high orientation even in the uncured coating film.

[0019] The TI value may be 3.8 or higher, 4.0 or higher, or 4.3 or higher. The TI value may be 8.0 or lower, 7.0 or lower, or 6.5 or lower.

[0020] The TI value can be adjusted, for example, by the solid content concentration of the aqueous intermediate coating composition, the type and amount of resin components (for example, core-shell type acrylic resin dispersion (A), hydrophobic melamine resin (B), polyester resin dispersion (C), urethane resin dispersion (D), and other resin components in the aqueous intermediate coating composition), the type and amount of viscosity modifier (G), and the type and amount of hydrophobic organic solvent (H).

[0021] According to the aqueous intermediate coating composition of this disclosure, the appearance of both the intermediate coating film and the multi-layer coating film comprising it is improved.

[0022] The appearance of the intermediate coating can be evaluated by its arithmetic mean roughness Ra. The intermediate coating obtained by this disclosure has an arithmetic mean roughness Ra (cutoff value 0.25 mm) of less than 0.20 μm.

[0023] The appearance of a multilayer coating can be evaluated using the WaveScan-T appearance measuring instrument manufactured by BYK-Gardner GmbH. The WaveScan-T evaluates the optical surface irregularities of the coating by shining long-wavelength to short-wavelength laser light emitted from the measuring instrument onto the coating surface and detecting the intensity of the resulting reflected light with a detector. Specifically, the irregularities are expressed as W1 value (measurement wavelength 2.4 mm or more), W2 value (measurement wavelength 0.8 mm or more and less than 2.4 mm), W3 value (measurement wavelength 0.32 mm or more and less than 0.8 mm), and W4 value (measurement wavelength less than 0.32 mm). The smaller each value, the smoother (better appearance) the surface of the multilayer coating. Based on the W1 value, the waviness (roundness) of the surface of the multilayer coating can be evaluated. Based on the W4 value, the gloss and minute irregularities of the multilayer coating can be evaluated. The multilayer coating obtained by this disclosure satisfies at least one of the following conditions: W1 is less than 15, or W4 is less than 15.

[0024] The solid content concentration is calculated from the residue after heating the object to 150°C.

[0025] The resin solids content of a paint composition refers to the total solids content of the resin components contained in the paint composition.

[0026] The average particle size is the 50% average particle size (D50) in the volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. An example of a particle size distribution analyzer is the UPA-150 (Nikkiso Co., Ltd., Microtrac particle size distribution analyzer).

[0027] The glass transition temperature can be measured, for example, by detecting the thermal change associated with the glass transition of a resin using a differential scanning calorimeter. An example of a differential scanning calorimeter is the DSC220C manufactured by Seiko Electronics Industries, Ltd.

[0028] The acid value and hydroxyl value may be calculated from the composition of the raw material monomers in accordance with JIS standards, or they may be determined by a neutralization titration method 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.

[0029] The weight-average molecular weight and number-average molecular weight are measured using a polystyrene standard by GPC (gel permeation chromatography).

[0030] (Meth)acrylic acid ester refers to acrylic acid ester and methacrylic acid ester. (Meth)acrylic acid refers to acrylic acid and methacrylic acid.

[0031] A "hydrocarbon group" is a group containing carbon and hydrogen, and refers to 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. The hydrogen bonded to the carbon may be substituted with a halogen atom or the like. A hydrocarbon group may have one or more N, O, S, Si, amide, sulfonyl, siloxane, carbonyl, carbonyloxy, etc. at its terminal or in its molecular chain.

[0032] The thickness of the coating can be measured using an electromagnetic film thickness gauge (for example, SANKO's SDM-miniR). The coating thickness is the average value of the coating thickness at any five points.

[0033] The PWC (mass%) of a pigment is calculated by dividing the mass of the pigment in question by the total mass of all pigments and resin components contained in the water-based intermediate coating paint composition (see formula below). TIFF2026049457000001.tif16168

[0034] Water-based resins are generally classified into two main types: water-soluble and water-dispersible. Water-dispersible resins are further divided into dispersion type (generally referred to as colloidal dispersion type) and emulsion type. Colloidal dispersion type water-based resins are typically obtained by synthesizing a resin in an organic solvent and then partially dissolving it in water using a neutralizing agent. Emulsion type water-based resins are typically produced by emulsion polymerization or by mechanical forced emulsification. In the case of acrylic resins, those with a weight-average molecular weight exceeding 100,000 can be considered emulsion type, while those with a weight-average molecular weight of 100,000 or less can be considered colloidal dispersion type. Measuring the weight-average molecular weight becomes difficult when it exceeds 1 million. Acrylic resins whose weight-average molecular weight cannot be measured can be considered emulsion type.

[0035] Water-based paint compositions contain water as a solvent. In a water-based paint composition, the proportion of water in the total solvent (sum of organic solvent and water) may be 50% by mass or more, 70% by mass or more, or 100% by mass.

[0036] [Water-based intermediate coating paint composition] The aqueous intermediate coating composition used in this disclosure comprises a core-shell type acrylic resin dispersion (A), a hydrophobic melamine resin (B), a polyester resin dispersion (C), a urethane resin dispersion (D), a glossy pigment (E), a white pigment (F), and a viscosity modifier (G). The core-shell type acrylic resin dispersion (A) has a hydrophilic resin in its shell portion. The TI value of the aqueous intermediate coating composition is 3.5 or more and 10 or less.

[0037] • Core-shell type acrylic resin dispersion (A) Core-shell type acrylic resin dispersion (A) (hereinafter sometimes simply referred to as acrylic resin dispersion (A)) is a core-shell type aqueous acrylic resin prepared by a polymerization method without emulsifiers (typically, solution polymerization). Acrylic resin dispersion (A) can improve chipping resistance. Chipping resistance is one of the performance characteristics required for intermediate coating films.

[0038] The resin that forms the shell (shell-forming resin (As)) contains a hydrophilic resin. As a result, the acrylic resin dispersion (A) is dispersed in particulate form in the aqueous intermediate coating composition. The shell-forming resin (As) is, for example, a neutralized acrylic resin having acidic groups.

[0039] In acrylic resin dispersion (A), the "core portion" and the "shell portion" may or may not be chemically crosslinked. In acrylic resin dispersion (A), at least a portion of the "core portion" is covered by the "shell portion".

[0040] 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.

[0041] The weight-average molecular weight of the acrylic resin dispersion (A) is, for example, 13,000 to 80,000. This helps to suppress color return when the aqueous intermediate coating composition according to this 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.

[0042] The acid value of acrylic resin dispersion (A) may be 15 mg KOH / g or more and 50 mg KOH / g or less. The acid value of acrylic resin dispersion (A) may be 25 mg KOH / g or more, 30 mg KOH / g or more, or 35 mg KOH / g or more. From the viewpoint of water resistance, the acid value of acrylic resin dispersion (A) may be 50 mg KOH / g or less, or 45 mg KOH / g or less.

[0043] The content of acrylic resin dispersion (A) is, for example, 3 parts by mass or more and 50 parts by mass or less per 100 parts by mass of resin solids in the aqueous intermediate coating composition. The above content of acrylic resin dispersion (A) may be 5 parts by mass or more, or 10 parts by mass or more. The above content of acrylic resin dispersion (A) may be 40 parts by mass or less, or 30 parts by mass or less, or 25 parts by mass or less.

[0044] The mass ratio (Ac:As) of the resin forming the core (core-forming resin (Ac)) to the resin forming the shell (As) is, for example, 95:5 to 40:60. If the mass ratio of the shell-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 water-based intermediate coating composition is improved. If the mass ratio of the shell-forming resin (As) is 40% or less, the water resistance and appearance of the coating film may be improved. The ratio of core-forming resin (Ac):shell-forming resin (As) may be 90:10 to 45:55, or 85:15 to 45:55.

[0045] The core-forming resin (Ac) may contain a hydrophilic resin or a hydrophobic resin. The core-forming resin (Ac) may have branched hydrocarbon groups having 4 to 24 carbon atoms (hereinafter referred to as hydrophobic groups for convenience) in order to improve the dispersibility of the hydrophobic melamine resin (B). The hydrophobic groups increase the hydrophobicity of the core and improve the affinity between the core and the hydrophobic melamine resin (B). The hydrophilic resin of the shell improves the water dispersibility of the acrylic resin dispersion (A). As a result, the acrylic resin dispersion (A) can be finely dispersed in the aqueous solvent while capturing the hydrophobic melamine resin (B), and consequently, aggregation of the hydrophobic melamine resin (B) is suppressed.

[0046] 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 with 4 to 24 carbon atoms.

[0047] (Manufacturing method) Acrylic resin dispersion (A) can be produced, for example, by multi-step solution polymerization.

[0048] Specifically, the acrylic resin dispersion (A) is produced by a method comprising: a first step of synthesizing a core-forming resin (Ac) by dropwise adding a first monomer mixture containing a first acid group-containing α,β-ethylenically unsaturated monomer (a1) to obtain a liquid containing the core-forming resin (Ac); a second step of synthesizing an acid group-containing resin (As') by dropwise adding a second monomer mixture containing a second acid group-containing α,β-ethylenically unsaturated monomer (a2) to the above liquid to obtain a core-shell type acrylic resin (A') having the core-forming resin (Ac) and the acid group-containing resin (As'); a step of adding a basic compound to neutralize the acid groups remaining in the core-shell type acrylic resin (A'); and a step of adding deionized water to invert the phase and obtain a varnish containing the acrylic resin dispersion (A) dispersed in the deionized water.

[0049] An acrylic resin dispersion (A) having a hydrophobic group in the core can be produced, for example, by performing the first step described above using a reactive solvent (x) having one glycidyl group and a hydrophobic group. The following explanation will use the case where reactive solvent (x) is used as an example, but the method for producing acrylic resin dispersion (A) is not limited to this. An acrylic resin dispersion (A) not having a hydrophobic group in the core can be produced in the same manner as described below, except that reactive solvent (x) is not used.

[0050] <1st process> 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 the acid group-containing monomer (a) proceed. In the first step, so-called solution polymerization takes place.

[0051] By taking advantage of the fact that the ring-opening reaction of epoxy rings is less likely to occur at low temperatures, the core-forming resin (Ac) can be made to contain hydrophobic groups by carrying out the polymerization reaction of the first monomer mixture and the ring-opening reaction of the epoxy ring in a stepwise manner in the first step. For example, the temperature of the reaction system is first lowered (e.g., 50°C or higher and less 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 and less than 180°C) to perform ring-opening addition of the reactive solvent (x) to the above precursor.

[0052] 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-vinyl salicylic acid, 3-vinylacetylsalicylic acid, 2-acrylamido-2-methylpropanesulfonic acid, p-hydroxystyrene, and 2,4-dihydroxy-4'-vinylbenzophenone. These can be used individually or in combination of two or more.

[0053] The first monomer mixture may contain other ethylenically unsaturated monomers along with the first acid group-containing α,β-ethylenically unsaturated monomer (a1).

[0054] Other ethylenically unsaturated monomers include, for example, 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, (meth Examples include linear aliphatic esters of acrylic acid such as allyl acrylate and lauryl (meth)acrylate; alicyclic esters of acrylic acid 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 can be used individually or in combination of two or more.

[0055] 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 per 100 parts by mass of the first monomer mixture. The above mass of the first acid group-containing α,β-ethylenically unsaturated monomer (a1) may be 10 parts by mass or more. The above mass of the first acid group-containing α,β-ethylenically unsaturated monomer (a1) may be 25 parts by mass or less.

[0056] The reactive solvent (x) has one glycidyl group and a hydrophobic group. The reactive solvent (x) may be a monocarboxylic acid glycidyl ester. A monocarboxylic acid glycidyl ester is, for example, the following general formula (1): [ka] (In the formula, R is a monovalent organic group, including the hydrophobic group described above.) It is represented as follows.

[0057] The reactive solvent (x) is used in an amount such that the mass ratio of the core-forming resin (Ac) to the shell-forming resin (As) (core-forming resin (Ac):shell-forming resin (As), mass%) is between 95:5 and 60:40.

[0058] <2nd process> In the second step, polymerization of the second monomer mixture proceeds primarily to synthesize an acid group-containing resin (As'). When a basic compound is added in a subsequent step, these acid groups are neutralized, and the acid group-containing resin (As') becomes hydrophilic, forming a shell-forming resin (As). The shell-forming resin (As) acts as a dispersion component for dispersing the acrylic resin dispersion (A) in water, reducing the particle size of the resin particles and improving their dispersion stability.

[0059] Following the second step, a polymerization initiator may be added, followed by aging by stirring and heating. The aging may be carried out, for example, at the same temperature as the second step for 0.5 hours to 3 hours.

[0060] The types and proportions of 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 monomer (a1) and the second acid group-containing monomer (a2) may be the same or different.

[0061] In terms of improving the dispersion function 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 per 100 parts by mass of the second monomer mixture. The above mass of the acid group-containing monomer may be 10 parts by mass or more. The above mass of the acid group-containing monomer may be 25 parts by mass or less.

[0062] Polymerization initiators may be used in the first step, the second step, and aging. The total amount of polymerization initiator used is appropriately set, for example, depending on the type and amount of raw material monomers. The total amount of polymerization initiator used may be, for example, 0.2 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the total of the first monomer mixture and the second monomer mixture. The above amount of polymerization initiator used may be 0.2 parts by mass or more. The above amount of polymerization initiator used may be 1.5 parts by mass or less.

[0063] Examples of polymerization initiators include 2,2'-azobisisobutyronitrile, benzoyl peroxide, 2,2-di(t-amyl peroxy)butane, di-t-butyl peroxide, di-t-amyl peroxide (DTA), t-butyl peroctoate, and 2,2'-azobis(2-methylbutyronitrile). These can be used individually or in combination of two or more.

[0064] <Neutralization and Phase Inversion Process> A basic compound is used to neutralize the acidic groups (typically carboxyl groups) remaining in the core-shell type acrylic resin (A'). This makes the core-shell type acrylic resin (A') water-dispersible. Then, deionized water is added to invert the phase. This causes the core-shell type acrylic resin (A') to disperse in water, yielding a varnish containing acrylic resin dispersion (A).

[0065] Phase inversion and aqueous dispersion are carried out, for example, by stirring a mixture of varnish containing neutralized core-shell type acrylic resin (A') and deionized water. Stirring can reduce the average particle size of the acrylic resin dispersion (A).

[0066] 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-pipecolin, 3-pipecolin, 4-pipecolin, 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 can be used individually or in combination of two or more types.

[0067] The basic compound is added in an amount such that the neutralization rate of the acid groups in the acrylic resin dispersion (A) is between 70% and 100%. When the neutralization rate is within this range, the water dispersibility of the acrylic resin dispersion (A) is improved, and the average particle size may become smaller. The neutralization rate may be 75% or more, 80% or more, 85% or more, or 90% or more.

[0068] The neutralization rate is equivalent to the amount of basic compound used relative to the acidic group (e.g., carboxyl group). The neutralization rate is calculated using the following formula. TIFF2026049457000003.tif12150

[0069] • Hydrophobic melamine resin (B) The hydrophobic melamine resin (B) acts as a curing agent. The hydrophobic melamine resin can improve the water resistance of the resulting coating film.

[0070] Hydrophobic melamine resin (B) has three nitrogen atoms around a melamine core (triazine core) via R1 ~R 6 It contains a structure to which the group of is bonded. The hydrophobic melamine resin (B) may generally be a polynuclear body in which a plurality of melamine nuclei are bonded to each other, or may be a mononuclear body consisting of one melamine nucleus.

[0071] The structure of the melamine nucleus is represented by, for example, the following general formula (2).

Chemical formula

[0072] The substituents R 1 ~R 6 may each independently be a hydrogen atom, an alkyl ether group (-CH2-OR 7 ) or a methylol group (-CH2OH). The substituents R 1 ~R 6 , R 7 may each independently be an alkyl group having 1 to 8 carbon atoms, or may be 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.

[0073] Melamine resins are generally classified into water-soluble melamine resins and hydrophobic melamine resins. The water-soluble melamine resin satisfies all of the following conditions (i) to (iii). (i) The number average molecular weight of the melamine resin is 1,000 or less. (ii) In R 1 ~R 6 in the above general formula (1), at least one is a hydrogen atom (imino group) or CH2OH (methylol group). That is, the total amount of the average imino group amount and the average methylol group amount is 1.0 or more. (iii) In R 1 ~R 6 in the above general formula (1), R1 ~R 6 CH2OR 7 If that is the case, R 7 It is a methyl group.

[0074] Hydrophobic melamine resin is a melamine resin other than the water-soluble melamine resin described above. That is, it satisfies any of the following conditions (iv) to (vi). (iv) The number-average molecular weight of the melamine resin exceeds 1,000. (v) The sum of the average imino group amount and the average methylol group amount is 1.0 or less. (vi) R in equation (1) above 1 ~R 6 In R 1 ~R 6 Two or more of these are CH2OR 7 And R 7 R is an alkyl group having 1 to 4 carbon atoms, however R 1 ~R 6 R that constitutes 7 At least one of them is an alkyl group having 2 to 4 carbon atoms.

[0075] Examples of commercially available hydrophobic melamine resins (B) include the Allnex Cymel series (all trade names), 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 (all melamine resins having both methoxy and butoxy groups); Mycoat 506 (manufactured by Mitsui Cytec, a melamine resin having only butoxy groups); and Yuban 20N60 and Yuban 20SE (Yuban (trade name) series manufactured by Mitsui Chemicals). These can be used individually or in combination of two or more types.

[0076] 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 per 100 parts by mass of the resin solid content of the aqueous intermediate coating composition. This facilitates the curing reaction and makes 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.

[0077] Other hardeners The water-based 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 individually or in combination of two or more.

[0078] The solid content of the other curing agent may be, for example, less than 10 parts by mass per 100 parts by mass of the resin solid content of the water-based 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 parts by mass.

[0079] • Polyester resin dispersion (C) Polyester resin dispersion (C) is one of the resin components. Polyester resin dispersion (C) contributes to improved chipping resistance. Polyester resin dispersion (C) is a dispersion-type polyester resin.

[0080] The glass transition temperature of the polyester resin dispersion (C) is, for example, between -50°C and 20°C. This can improve chipping resistance and adhesion.

[0081] The acid value of polyester resin dispersion (C) is, for example, 1 mg KOH / g or more and 50 mg KOH / g or less. The acid value of polyester resin dispersion (C) may be 5 mg KOH / g or more, and may be 10 mg KOH / g or more. The acid value of polyester resin dispersion (C) may be 30 mg KOH / g or less.

[0082] The hydroxyl value of polyester resin dispersion (C) is, for example, 50 mg KOH / g or more and 200 mg KOH / g or less. The hydroxyl value of polyester resin dispersion (C) may be 150 mg KOH / g or less.

[0083] Polyester resin dispersion (C) is prepared, for example, by an esterification or transesterification reaction between a polybasic acid and a polyhydric alcohol.

[0084] Polybasic acids have two or more carboxyl groups in one molecule. Examples of polybasic acids include aromatic dicarboxylic acids and their anhydrides, such as terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, and diphenylmethane-4,4'-dicarboxylic acid; alicyclic dicarboxylic acids and their anhydrides, such as hexahydroisophthalic acid, hexahydroterephthalic acid, hexahydrophthalic acid, and tetrahydrophthalic acid; aliphatic dicarboxylic acids and their anhydrides, such as adipic acid, sebacic acid, suberic acid, succinic acid, glutaric acid, maleic acid, chloromaleic acid, fumaric acid, dodecanediic 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 polybasic acids with a valency of 3 or higher, such as trimellitic acid, trimellitic anhydride, pyromellitic acid, pyrometic anhydride, trimesic acid, methylcyclohexentricarboxylic acid, tetrachlorohexentricarboxylic acid, and their anhydrides. These can be used individually or in combination of two or more types.

[0085] Polyhydric alcohols have 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, hydroxypivalic acid, Examples 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 mannitol; 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 can be used individually or in combination of two or more.

[0086] The polyester resin dispersion (C) may be a condensation product of an aromatic dicarboxylic acid and a polyhydric alcohol. This may further improve chipping resistance.

[0087] The polyester resin dispersion (C) may contain an alcohol compound obtained by reacting an α-olefin epoxide such as propylene oxide and butylene oxide, or a monoepoxy compound such as "Cardura E10" (trade name, manufactured by Japan Epoxy Resin Co., Ltd., a glycidyl ester of a synthetic highly branched saturated fatty acid) with an acid.

[0088] Examples of commercially available polyester resin dispersions (C) include Bekkolite 46-118 (manufactured by Dainippon Ink and Chemicals, Inc., solids content 60%), Bekkolite 46-119 (manufactured by Dainippon Ink and Chemicals, Inc., solids content 60%), Bekkolite M6205-50 (manufactured by Dainippon Ink and Chemicals, Inc., solids content 50%), Bekkolite M6401-52 (manufactured by Dainippon Ink and Chemicals, Inc., solids content 52%), and PRS-1015 (manufactured by Nippon Paint Automotive Coatings, Inc., solids content 70%).

[0089] The content of polyester resin dispersion (C) is, for example, 5 parts by mass or more and 35 parts by mass or less per 100 parts by mass of resin solids in the aqueous intermediate coating composition. The above content of polyester resin dispersion (C) may be 10 parts by mass or more, and may be 15 parts by mass or more. The above content of polyester resin dispersion (C) may be 30 parts by mass or less, and may be 28 parts by mass or less.

[0090] • Urethane resin dispersion (D) Urethane resin dispersion (D) is one of the resin components. Urethane resin dispersion (D) contributes to improved chipping resistance. Urethane resin dispersion (D) is a dispersion-type urethane resin.

[0091] The average particle size of the urethane resin dispersion (D) is, for example, 10 nm or more and 5,000 nm or less. The average particle size of the urethane resin dispersion (D) may be 20 nm or more, or 50 nm or more. The average particle size of the urethane resin dispersion (D) may be 1,000 nm or less, 500 nm or less, or 140 nm or less.

[0092] The acid value of the urethane resin dispersion (D) may be, for example, 3 mg KOH / g or more and 75 mg KOH / g or less, from the viewpoint of storage stability. The acid value of the urethane resin dispersion (D) may be 5 mg KOH / g or more, and may be 10 mg KOH / g or more. The acid value of the urethane resin dispersion (D) may be 60 mg KOH / g or less, and may be 40 mg KOH / g or less.

[0093] The hydroxyl value of the urethane resin dispersion (D) may be, for example, 0 mg KOH / g or more and 45 mg KOH / g or less, from the viewpoint of chipping resistance. The hydroxyl value of the urethane resin dispersion (D) may be 30 mg KOH / g or less, and may be 25 mg KOH / g or less.

[0094] The urethane resin dispersion (D) is prepared, for example, by reacting a polyhydric alcohol, a polyisocyanate compound, and a compound having an active hydrogen group in its molecule.

[0095] Polyhydric alcohols have two or more hydroxyl groups. Examples of polyhydric alcohols include compounds similar to those listed as raw materials for polyester resin dispersion (C). Polyhydric alcohols may have a number-average molecular weight of 500 to 5000.

[0096] Polyisocyanate compounds have two or more isocyanate groups in their molecule. Examples of polyisocyanate compounds include aliphatic diisocyanates with 2 to 12 carbon atoms, such as hexamethylene diisocyanate, 2,2,4-trimethylhexane diisocyanate, and lysine diisocyanate; alicyclic diisocyanates with 4 to 18 carbon atoms, such as 1,4-cyclohexane diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, methylcyclohexylene diisocyanate, and isopropylidenecyclohexyl-4,4'-diisocyanate; and 2,4-toluylene diisocyanate, 2,6-toluylene diisocyanate, diphenylmethane-4,4'-diisocyanate, and 1,5'-naphthene diisocyanate. Examples include aromatic diisocyanates such as cyanates, toridine diisocyanate, diphenylmethylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, xylylene diisocyanate, tetramethylxylylene diisocyanate, 4,4'-dibenzyle 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.

[0097] 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. A polyisocyanate having a urethane bond, obtained by reacting the above polyisocyanate compound with a polyhydric alcohol, may also be used.

[0098] The polyisocyanate compound may be an aromatic aliphatic diisocyanate compound, such as isophorone diisocyanate, 1,4-cyclohexane diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, methylcyclohexylene diisocyanate, or isopropylidenecyclohexyl-4,4'-diisocyanate.

[0099] Compounds containing active hydrogen groups within their molecules are used to block isocyanate groups in polyisocyanate compounds. Examples of compounds containing active hydrogen groups within their molecules include monohydric alcohols such as methanol, ethanol, and diethylene glycol monobutyl ether; monohydric 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.

[0100] In preparing the urethane resin dispersion (D), chain extenders, monoisocyanate compounds, catalysts, etc., may be used as needed.

[0101] The urethane resin dispersion (D) is synthesized, for example, by a one-shot method in which each component is reacted at once, or by a multi-stage method in which the reactions are carried out in steps (a method in which a portion 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 between 40°C and 140°C.

[0102] Examples of commercially available urethane resin dispersions (D) include NAC300 (manufactured by Sanyo Chemical Industries, solid content concentration 30%), NAC100 (manufactured by Sanyo Chemical Industries, solid content concentration 40%), IB-465 (manufactured by Sanyo Chemical Industries, solid content concentration 30%), IB-940 (manufactured by Sanyo Chemical Industries, solid content concentration 40%), and IB-700 (manufactured by Sanyo Chemical Industries, solid content concentration 30%).

[0103] The content of urethane resin dispersion (D) is, for example, 1 part by mass or more and 60 parts by mass or less per 100 parts by mass of resin solids in the water-based intermediate coating composition. The above content of urethane resin dispersion (D) may be 2 parts by mass or more, and may be 5 parts by mass or more. The above content of urethane resin dispersion (D) may be 50 parts by mass or less, 30 parts by mass or less, and may be 10 parts by mass or less.

[0104] • Other resin components The water-based intermediate coating composition may contain other resin components as needed. If other resin components are included, one may be included alone, or two or more may be used in combination.

[0105] • Luminous pigment (E) The luminous pigment (E) has light reflectivity and reduces the light transmittance of the intermediate coating. The luminous pigment (E) improves the weather resistance of the multi-layer coating.

[0106] Examples of luminous pigments (E) include metal particles such as aluminum, copper, zinc, iron, nickel, tin, and aluminum oxide; particles of these alloys; interference mica; white mica; graphite; and glass flakes. These can be used individually or in combination of two or more. In particular, aluminum particles or interference mica may be used, or aluminum particles may be used.

[0107] The lustrous pigment (E) may be uncolored or colored. The lustrous pigment (E) may or may not be surface-treated. Examples of compounds used for surface treatment include metal oxide compounds, phosphorus compounds, amine compounds, and silane compounds.

[0108] The average particle size (primary particles) of the luminous pigment (E) is, for example, 5 μm to 12 μm. This can further reduce light transmittance and make orientation easier. The average particle size of the luminous pigment (E) may be 6 μm or more. The average particle size of the luminous pigment (E) may be 11 μm or less, or 10 μm or less.

[0109] The luminous pigment (E) may be aluminum particles with an average particle size (primary particle) of 5 μm or more and 12 μm or less.

[0110] The lustrous pigment (E) may be in the form of a thin film (flaky). "Flaki" refers to a shape where the aspect ratio (average major diameter of the pigment / average thickness of the pigment) is greater than 1.0. The aspect ratio of a flaki lustrous pigment (E) is, for example, between 20 and 300. The aspect ratio of a flaki lustrous pigment (E) may be 30 or greater. The aspect ratio of a flaki lustrous pigment (E) may be 200 or less.

[0111] The average thickness of the flake-like lustrous pigment (E) is, for example, 0.01 μm or more and 0.1 μm or less. The average thickness of the flake-like lustrous pigment (E) may be 0.04 μm or more. The average thickness of the flake-like lustrous pigment (E) may be 0.08 μm or less.

[0112] The average thickness of the flake-like pigment is obtained by observing a cross-section of a coating film containing the flake-like pigment using a transmission electron microscope (TEM) and averaging the thicknesses of 100 arbitrarily selected flake-like pigments.

[0113] The average major axis of the flake-shaped pigment is obtained by observing the flake-shaped pigment using a shape analysis laser microscope (e.g., Keyence VK-X 250) and averaging the major axes (maximum lengths) of 100 arbitrarily selected flake-shaped pigments.

[0114] The PWC (mass%) of the lustrous pigment (E) is, for example, 0.5% by mass or more and 5% by mass or less. This can easily reduce light transmittance and further improve surface smoothness. The PWC (mass%) of the lustrous pigment (E) may be 0.7% by mass or more, and may be 0.9% by mass or more. The PWC (mass%) of the lustrous pigment (E) may be 4% by mass or less, and may be 3% by mass or less.

[0115] • White pigment (F) The white pigment (F) is an opaque pigment that does not have any specific absorption in the visible region. The white pigment (F) enhances the opacity of the intermediate coating film.

[0116] An "opaque pigment" refers to a pigment such that, when its concentration (PWC) in the solid content of the paint composition is 25% by mass, the resulting black-and-white opacity film thickness is 50 μm or less. The black-and-white opacity film thickness is measured using the black-and-white checkerboard opacity test paper specified in JIS K5600-4-1, 4.1.2. Specifically, the opacity test paper is attached to a steel plate, and the paint is applied in a gradient so that the film thickness changes continuously. After the paint dries or hardens, the painted surface is visually observed under diffused daylight. The minimum film thickness at which the black-and-white boundary of the checkerboard pattern on the opacity test paper disappears is the black-and-white opacity film thickness.

[0117] Examples of the white pigment (F) include zinc oxide and titanium dioxide. Titanium dioxide may be used due to its high refractive index. The titanium dioxide may be rutile or anatase. In particular, rutile titanium dioxide may be used 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.

[0118] The average particle size (primary particle size) of the white pigment (F) is not particularly limited. From the viewpoint of opacity, the primary particle size of the white pigment (F) may be between 100 nm and 500 nm, or between 200 nm and 400 nm. 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.

[0119] The PWC (mass%) of the white pigment (F) may be, for example, 40% by mass or more and 65% by mass or less. This can further improve opacity without impairing chipping resistance. The PWC (mass%) of the white pigment (F) may be 45% by mass or more and 50% by mass or more. The PWC (mass%) of the white pigment (F) may be 62% by mass or less and 60% by mass or less.

[0120] The content ratio (E:F) of the luminous pigment (E) to the white pigment (F) is, for example, 1:20 to 1:260. This allows for a lower light transmittance of the intermediate coating. As a result, the lower layer of the intermediate coating becomes less susceptible to the effects of ultraviolet rays, which can further improve the weather resistance of the coating. The content ratio (E:F) may be 1:25 to 1:200, or 1:30 to 1:150.

[0121] Other pigments The water-based intermediate coating paint composition may contain pigments other than those mentioned above. Other pigments include, for example, coloring pigments other than white pigments; extender pigments such as calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silica, and calcined kaolin.

[0122] (Other coloring pigments) Other coloring pigments may be organic or inorganic. Examples of organic coloring pigments include azo chelate pigments, insoluble azo pigments, condensed azo pigments, monoazo pigments, disazo pigments, diketopyrrolopyrrole pigments, benzimidazolon pigments, phthalocyanine pigments, indigo pigments, thioindigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindlinone pigments, naphthol pigments, pyrazolone pigments, anthraquinone pigments, antholapyrimidine pigments, and metal complex pigments. These may be used individually or in combination of two or more.

[0123] Examples of inorganic coloring pigments include lead yellow, iron yellow oxide, chromium oxide, molybdate orange, red iron oxide, titanium yellow, carbon black, cobalt green, phthalocyanine green, ultramarine, cobalt blue, phthalocyanine blue, and cobalt violet. These can be used individually or in combination of two or more.

[0124] The PWC (mass%) of other coloring pigments is, for example, 0.1% by mass or more and 10% by mass or less. The PWC (mass%) of other coloring pigments may be 0.5% by mass or more and may be 1% by mass or more. The PWC (mass%) of other coloring pigments may be 8% by mass or less and may be 6% by mass or less.

[0125] • Viscosity modifier (G) The viscosity modifier (G) makes it easier to adjust the TI value of the water-based intermediate coating composition to the above range. Water-based coating compositions containing acrylic resin dispersion (A) are generally difficult to raise to a high TI. The viscosity modifier (G) allows for a higher TI value without reducing the storage stability of the water-based intermediate coating composition.

[0126] Examples of viscosity modifiers (G) include inorganic viscosity modifiers such as silica-based fine powder, mineral-based viscosity modifiers, and barium sulfate pulverized 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-type thickeners. These can be used individually or in combination of two or more.

[0127] Viscosity modifier (G) 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, from the viewpoint of improving appearance.

[0128] Examples of hydrophobic association-type viscosity modifiers include polyacrylic acid-based viscosity modifiers copolymerized with hydrophobic monomers, polyurethane-based viscosity modifiers containing hydrophobic chains in their molecules, 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 can be used individually or in combination of two or more.

[0129] The solid content acid value of the polyacrylic acid-based viscosity modifier may be, for example, 30 mg KOH / g or more and 300 mg KOH / g or less. The solid content acid value of the polyacrylic acid-based viscosity modifier may be 80 mg KOH / g or more. The solid content acid value of the polyacrylic acid-based viscosity modifier may be 280 mg KOH / g or less.

[0130] Examples of commercially available polyacrylic acid-based viscosity modifiers include Primal ASE-60, Primal TT615, Primal RM5 (all manufactured by Dow Chemical), SN Thickener 613, SN Thickener 618, SN Thickener 630, SN Thickener 634, SN Thickener 636 (all manufactured by Sunopco), and Disparon AQ-021 (manufactured by Kusumoto Chemical Co., Ltd.).

[0131] Examples of commercially available polyurethane viscosity modifiers include PU 1250 (manufactured by BASF), SN Thickener-660T, SN Thickener-665T (all manufactured by Sunnopco), Primal RM-12W, and Primal RM-895 (all manufactured by Dow Chemical).

[0132] Examples of commercially available urethane-urea viscosity modifiers include BYK-425, BYK-420 (both manufactured by BYK Chemie), and RHEOLATE216 (manufactured by ELEMENTIS).

[0133] A commercially available example of an amido-urea viscosity modifier is BYK-430 (manufactured by BYK Chemie).

[0134] Examples of polyamide-based viscosity modifiers include fatty acid amides, polyamides, acrylamides, long-chain polyaminoamides, aminoamides, and salts thereof (e.g., phosphates).

[0135] Examples of commercially available polyamide-based viscosity modifiers include Disparon AQ-600, AQ-607, AQ-620, AQ-630, and AQH-800 (all manufactured by Kusumoto Chemical Co., Ltd.), as well as Anti-Terra-U, Disperbyk-101, and Disperbyk-130 (all manufactured by BYK Chemie).

[0136] The content of viscosity modifier (G) may be, for example, 0.05 parts by mass or more and 5 parts by mass or less per 100 parts by mass of resin solids contained in the water-based intermediate coating paint composition. The above content of viscosity modifier (G) may be 0.08 parts by mass or more, and may be 0.09 parts by mass or more. The above content of viscosity modifier (G) may be 3 parts by mass or less, and may be 2 parts by mass or less.

[0137] The proportion of the hydrophobic association-type viscosity modifier in the viscosity modifier (G) may be 30% by mass or more, 50% by mass or more, or 100% by mass.

[0138] • Hydrophobic organic solvent (H) The water-based intermediate coating composition may contain a hydrophobic organic solvent (H) having hydrocarbon groups with 5 or more carbon atoms. The hydrophobic organic solvent (H) exhibits hydrophobicity and is either immiscible or poorly miscible with water. Hydrophobicity can be rephrased as nonpolarity (including nonpolarity). The hydrophobic organic solvent (H) makes it easier to adjust the TI value to the above range.

[0139] Examples of hydrophobic organic solvents (H) include benzene, toluene, xylene, cyclohexane, hexane, heptane, octane, chlorobenzene, trichlorobenzene, perchloroethylene, trichloroethylene, mineral spirits, solvent naphtha, petroleum benzine, isoparaffin, normal paraffin, Swarzol, Shellzol, Merveille hydrocarbon solvents; 1-hexanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, 1-decanol, benzyl alcohol, ethylene glycol mono-2-ethyl Examples of solvents include alcoholic solvents such as ethylhexyl 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; esteric solvents such as isoamyl acetate, methylamyl acetate, and ethylene glycol monobutyl ether; and ketoneic solvents such as methyl isobutyl ketone, cyclohexanone, ethyl n-amyl ketone, and diisobutyl ketone. These can be used individually or in combination of two or more. Among these, hydrocarbon solvents are preferred, and in particular, aliphatic hydrocarbon solvents such as mineral spirits, isoparaffins, normal paraffins, shellsols, and merveilles may be used.

[0140] 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 water-based intermediate coating composition. This makes it easier to adjust the TI value. The above content of the hydrophobic organic solvent (H) may be 2% by mass or more, or 3% by mass or more. The above content of the hydrophobic organic solvent (H) may be 10% by mass or less, or 8% by mass or less.

[0141] The content of the hydrophobic organic solvent (H) is, for example, 5% by mass or more and 30% by mass or less per 100 parts by mass of the resin solids of the aqueous intermediate coating composition. This makes it easier to adjust the TI value. The above content of the hydrophobic organic solvent (H) may be 10% by mass or more and 12% by mass or more. The above content of the hydrophobic organic solvent (H) may be 25% by mass or less and 20% by mass or less.

[0142] 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.

[0143] Other organic solvents include, for example, 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, and diacetone alcohol glycerin; ethers such as tetrahydrofuran and dioxane; amides such as dimethylacetamide and dimethylformamide; and halogen compounds such as chloroform, methylene chloride, and carbon tetrachloride.

[0144] • Additives Water-based intermediate coating compositions may contain various additives as needed. Examples of additives include film-forming aids, surface modifiers, preservatives, fungicides, defoamers, light stabilizers, UV absorbers, antioxidants, and pH adjusters.

[0145] [Water-based paint composition] The components of the water-based paint composition are not particularly limited. For example, the water-based paint composition may include an acrylic resin emulsion, a water-soluble acrylic resin, a curing agent (typically a melamine resin), and a polyether polyol resin. The water-based paint composition may further include the above-mentioned glossy pigments, coloring pigments, and various additives.

[0146] [Clear paint composition] The components of the clear coating composition are not particularly limited. The clear coating composition may be solvent-based, water-based, or powder-based. The clear coating composition may be solvent-based. A solvent-based coating composition contains an organic solvent as the main solvent. In a solvent-based coating 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.

[0147] 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, from the viewpoint of transparency or acid etching resistance. The solvent-based clear coating composition may also contain an acrylic resin and / or polyester resin having a carboxylic acid and / or epoxy group. The clear coating composition may contain the above-mentioned various pigments to the extent that transparency is not impaired. The clear coating composition may contain various additives as needed.

[0148] Preparation of each paint composition The method for preparing the paint composition is not particularly limited, and each component can be prepared by stirring with a stirrer or the like. The pigment can be mixed with other components as a pigment paste, which is obtained by pre-dispersing the pigment in a vehicle containing water, a surfactant or a dispersant using a sand grind mill or the like.

[0149] [Method for forming multi-layer coatings] The multi-layer coating is formed by a method in which an uncured intermediate coating, an uncured base coating, and an uncured clear coating, all formed by wet-on-wet painting, are cured in a single process.

[0150] Specifically, the multi-layer coating comprises the steps of: applying the aqueous intermediate coating composition according to the present disclosure onto a workpiece to form an uncured intermediate coating; applying the base coating composition onto the uncured intermediate coating to form an uncured base coating; applying the clear coating composition onto the uncured base coating to form an uncured clear coating; and curing the uncured intermediate coating, the uncured base coating, and the uncured clear coating all at once.

[0151] Preheating may be performed after applying the water-based intermediate coating composition and before applying the base coating composition. Preheating may also be performed after applying the base coating composition and before applying the clear coating composition. Preheating is performed, for example, by leaving the surface at a temperature of 20°C to 25°C for 5 to 15 minutes, or by heating at a temperature of 50°C to 80°C for 30 seconds to 10 minutes.

[0152] Painting methods include, for example, air spray painting, airless spray painting, electrostatic spray painting, multi-stage painting using air electrostatic spray painting (typically two-stage painting), and painting combining air electrostatic spray painting with a rotary atomizing electrostatic painting machine.

[0153] The curing of each paint 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.

[0154] [Painted items] A painted article can be obtained by the method for forming a multilayer coating according to this disclosure. The painted article comprises an object to be coated and a multilayer coating in which an intermediate coating, a base coating, and a clear coating are laminated in that order. According to the water-based intermediate coating composition used in this disclosure, a painted article having excellent weather resistance and appearance (smoothness) can be obtained.

[0155] (subject to be coated) Examples of materials to be coated include metal, resin, and glass. Specifically, examples of materials to be coated include automobile bodies and parts for automobile bodies such as passenger cars, trucks, motorcycles, and buses, as well as automobile parts such as spoilers, bumpers, mirror covers, grilles, and door handles.

[0156] Examples of metals include iron, copper, aluminum, tin, zinc, or alloys thereof (e.g., steel). Typical examples of metals to be coated include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electro-galvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy plated steel sheets, zinc-iron alloy plated steel sheets, zinc-magnesium alloy plated steel sheets, zinc-aluminum-magnesium alloy plated steel sheets, aluminum plated steel sheets, aluminum-silicon alloy plated steel sheets, and tin plated steel sheets.

[0157] Metallic workpieces may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After surface treatment, metallic workpieces may be further coated with electrodeposition paint. The electrodeposition paint may be cationic or anionic.

[0158] 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). The resin-coated object may be degreased.

[0159] (Intermediate coating) The intermediate coating is formed adjacent to the object to be coated. The thickness of the intermediate coating is not particularly limited and is set appropriately according to the purpose. The thickness of the intermediate coating after curing may be 15 μm or more and 35 μm or less.

[0160] The light transmittance of the intermediate coating film at wavelengths of 400 to 700 nm may be 0.25% or less. Light transmittance is measured every 10 nm in the wavelength range of 400 to 700 nm using a spectrophotometer (Hitachi U-3310) for individual coating films formed using the intermediate coating paint. At any wavelength within the range of 400 to 700 nm, the light transmittance of the individual coating film may be 0.25% or less.

[0161] The above-mentioned single coating film is obtained by applying an aqueous intermediate coating composition to a polypropylene board and heating it (140°C for 30 minutes). Light transmittance is measured for the coating film (film thickness 25 μm) peeled off from the polypropylene board.

[0162] (Base coating) The base coating is formed adjacent to the intermediate coating. The base coating imparts aesthetic appeal to the multi-layer coating. The base coating consists of one or more layers, and may consist of two or more layers. The thickness of each layer of the cured base coating may be, for example, 5 μm to 60 μm.

[0163] (Clear coating) The clear coating is formed on the base coating. The clear coating improves the gloss of the painted article and prevents the pigments mixed in the underlying layer from falling off or splashing out. The thickness of the clear coating after curing may be between 15 μm and 50 μm from the viewpoint of scratch resistance and smoothness. [Examples]

[0164] The present invention will be further illustrated by the following examples, but the present invention is not limited thereto. In the examples, "parts" and "%" are based on mass unless otherwise specified.

[0165] (Weight average molecular weight) The weight-average molecular weight was measured using a GPC instrument, "HLC8220GPC" (product name, manufactured by Tosoh Corporation), and four columns, "Shodex KF-606M" and "Shodex KF-603" (both manufactured by Showa Denko K.K., product names). The conditions were: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 0.6 cc / min, detector: radioisotope (RI).

[0166] (Average particle size) The average particle size (volume-average particle size D50) was measured using a laser Doppler particle size analyzer (Nikkiso Co., Ltd., "Microtrac UPA-150") after diluting the dispersion with deionized water to achieve an appropriate signal level.

[0167] [Manufacturing Example A-1] Manufacturing of core-shell type acrylic resin dispersion (A-1) (1) Synthesis of hydrophobic acrylic resin (Ac) In a reaction vessel equipped with a stirrer, temperature controller, condenser, and dropping device, 30 parts of the reactive solvent (CAE, glycidyl ester of a monocarboxylic acid having a branched alkyl group with 9 carbon atoms, trade name: Cardura E10P, manufactured by Hexion, boiling point 251-278°C) were charged, and the temperature was raised to 165°C while stirring, and then refluxed. Separately, a mixture was prepared consisting 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 polymerization initiator (DTA, trade name: Luperox DTA, manufactured by Arkema Yoshitomi), and 6.5 parts of a high-boiling point solvent (dipropylene glycol monomethyl ether (DPM)). This mixture was added dropwise to the reaction vessel at 165°C over 3.5 hours to carry out polymerization and ring-opening addition reactions.

[0168] (2) Synthesis of carboxyl group-containing acrylic resin (As') Separately, a mixture was prepared consisting 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). This mixture was added dropwise to the reaction vessel at 165°C over 1 hour while stirring, and polymerization was carried out 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 polymerization was carried out at 165°C for 1 hour while stirring.

[0169] In this way, a varnish with a solid content of 91% by mass was obtained, containing a core-shell type acrylic resin having a core-forming resin (Ac) and an acid group-containing resin (As').

[0170] (3) Neutralization and phase inversion To the varnish, a basic compound (dimethylethanolamine, DMEA) was added at a ratio of 5.33 parts per 100 parts of acrylic resin at 80°C, and the mixture was stirred for 15 minutes. The neutralization rate of the carboxyl groups was 90%. Subsequently, at 80°C, 150 parts of deionized water was added dropwise to the neutralized varnish while stirring to obtain a milky white dispersion containing acrylic resin particles.

[0171] The acrylic resin particles contained in the dispersion were expected to have a core-shell structure, with the core containing hydrocarbon groups. The mass percentage of the core was calculated to be 81.4%, and the mass percentage of the shell was 18.6%. The average particle size of the acrylic resin particles was 57 nm, the acid value was 37.2 mgKOH / g, and the weight-average molecular weight was 32,000.

[0172] [Manufacturing Example A-2] Manufacturing of core-shell type acrylic resin dispersion (A-2) (1) Synthesis of hydrophobic acrylic resin (Ac) In a reaction vessel equipped with stirring, cooling, and heating devices, 210 parts by mass of butyl acetate was charged and heated to 120°C. Then, a monomer mixture consisting 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) was added dropwise in two series over 90 minutes using a solution prepared by dissolving 70 parts by mass of tert-butylperoxy-2-ethylhexanate in 45 parts by mass of butyl acetate. While maintaining the reaction vessel at 120°C, 10 parts by mass of methacrylic anhydride was added and the mixture was held for 2 hours. The mixture was then cooled to 60°C to obtain a hydrophobic acrylic resin (Ac).

[0173] (2) Synthesis of carboxyl 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. Then, 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), along with a solution of 140 parts by mass of tert-butylperoxy-2-ethylhexanate dissolved in 120 parts by mass of butyl acetate, was added dropwise in two series at a constant rate over 90 minutes to the above reaction vessel.

[0174] The reaction mixture was maintained at 120°C for 30 minutes. Then, a solution of 3 parts by mass of tert-butylperoxy-2-ethylhexanate dissolved in 80 parts by mass of butyl acetate was added dropwise in a single series over 30 minutes. After stirring at 120°C for 60 minutes, the mixture was cooled to 70°C.

[0175] In this way, a varnish with a solid content of 58.5% by mass was obtained, containing a core-shell type acrylic resin having a core-forming resin (Ac) and an acid group-containing resin (As').

[0176] (3) Neutralization and phase inversion The varnish was diluted with 170 parts by mass of dipropylene glycol monomethyl ether. Butyl acetate was removed under reduced pressure until the solid content was 85% by mass. Then, 27.21 parts by mass of dimethylethanolamine (DMEA) was added, followed by 1680 parts by mass of water to obtain a milky white dispersion containing acrylic resin particles.

[0177] The acrylic resin particles contained in the dispersion were expected to have a core-shell structure, and the mass ratio of the core portion to the shell portion was calculated to be 50%. The average particle size of the acrylic resin particles was 90 nm, the acid value was 17 mg KOH / g, and the weight-average molecular weight was 25,000.

[0178] [Manufacturing Example a-1] Manufacturing of acrylic resin emulsion (a-1) 126.5 parts of deionized water were added to the reaction vessel, and the temperature was raised to 80°C while mixing and stirring under a nitrogen atmosphere. Next, a monomer emulsion consisting of 100 parts of a monomer mixture (containing 27.61 parts methyl acrylate, 53.04 parts ethyl acrylate, 4.00 parts styrene, 9.28 parts 2-hydroxyethyl methacrylate, 3.07 parts methacrylic acid, and 3.00 parts allyl methacrylate), 1.1 parts of an emulsifier (product name: Adekarya Soap 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 2 hours. After the dropwise addition was complete, the mixture was allowed to mature at the same temperature for 2 hours.

[0179] 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 to adjust the pH to 6.5. This yielded a hydroxyl group-containing acrylic resin emulsion (A) with an average particle size of 90 nm, a Tg of -9.5°C, a solids content of 30%, an acid value of 20 mg KOH / g, and a hydroxyl value of 40 mg KOH / g.

[0180] [Melamine resin (B)] • B-1: Hydrophobic, product name Cymel 211, manufactured by Allnex. • B-2: Hydrophobic, product name Cymel 202, manufactured by Allnex. • b-1: Hydrophilic, product name Cymel 370, manufactured by Allnex.

[0181] [Manufacturing Example C-1] Manufacturing of polyester resin dispersion (C-1) In a reaction vessel equipped with a stirrer, nitrogen inlet tube, temperature control device, condenser, and decanter, 250 parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts of cyclohexanedicarboxylic acid were added. The temperature was raised to 180°C, and the condensation reaction was carried out until no more water distilled out. After cooling to 60°C, 120 parts of phthalic anhydride were added to the reaction vessel. The temperature was then 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 of 80%)) and 1920 parts of deionized water were added to the reaction vessel and stirred. This yielded a polyester resin dispersion (C-1) with a solid content of 45% by mass, a hydroxyl value of 110 mgKOH / g, an acid value of 15 mgKOH / g, a Tg of -14°C, and an Mw of 7,000.

[0182] [Manufacturing Example D-1] Manufacturing of urethane resin dispersion (D-1) In a 5000 ml four-necked flask equipped with a thermometer, nitrogen inlet tube, and 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. The mixture was then reacted at 56°C for 7 hours. The resulting prepolymer was then continuously added dropwise over 2 hours under 50°C conditions to an aqueous solution of 64.7 parts of N,N-dimethylethanolamine and 1914.7 parts of deionized water, followed by stirring for 1 hour. Next, the acetone was removed under reduced pressure under 50°C conditions to obtain a urethane resin dispersion (D-1).

[0183] [Urethane resin dispersion (D)] • D-2: Product name UH-2800, manufactured by Covestro, solids content 40% by mass, average particle size 125 nm, hydroxyl value 15 mg KOH / g, acid value 9 mg KOH / g ·D-3: Product name AP-10, manufactured by DIC, solid content 31% by mass ·D-4: Product name APX-101H, manufactured by DIC, solid content 45% by mass

[0184] [Luminous Pigment (E)] • E-1: Product name FD-5090, manufactured by Asahi Kasei Corporation, aluminum pigment, average particle size 9 μm • E-2: Product name 4690NS, manufactured by Toyo Aluminum Co., Ltd., aluminum pigment, average particle size 7 μm

[0185] [Preparation Example F-1] Preparation of Pigment Dispersion Paste (F-1) 4.5 parts of a pigment dispersant (product name: Disperbyk 190, manufactured by Bic Chemie, an anionic / nonionic dispersant), 0.5 parts of an antifoaming agent (BYK-011, manufactured by Bic Chemie), 29.6 parts of deionized water, and 65.4 parts of rutile-type titanium dioxide were premixed. Subsequently, the mixture was dispersed in paint conditioner using a glass bead medium at room temperature until the particle size was 5 μm or less, to obtain a pigment dispersion paste (F-1) containing a white pigment.

[0186] [Viscosity modifier (G)] • G-1: Hydrophobic association type (polyacrylic acid-based), product name Disparon AQ-021, manufactured by Kusumoto Chemical Industries Co., Ltd. G-2: Polyamide type, product name Disparon AQ-600, manufactured by Kusumoto Kasei Kogyo Co., Ltd. • G-3: Urethane composite type, product name Nopal 3303, manufactured by Sannopco. • G-4: Alkali-swelling type (carboxylic acid type), trade name HV30, manufactured by BASF.

[0187] [Organic solvent (H)] H-1: Hydrophobic, 2-ethylhexanol H-2: Hydrophobic, isoparaffinic hydrocarbon, trade name: Merveille 30, manufactured by Idemitsu Kosan Co., Ltd. • h-1: Hydrophilic, 1-methoxy-2-propanol

[0188] [Manufacturing Example 1] Manufacturing of phosphate-containing acrylic resin Forty parts of ethoxypropanol were charged into a 1-liter reaction vessel equipped with a stirrer, temperature controller, and condenser. Separately, a monomer solution was prepared consisting of 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, 20 parts of ethoxypropanol, 40 parts of a solution containing 20 parts of Phosmer PP (acid phosphooxyhexa(oxypropylene) monomethacrylate, manufactured by Unichemical), and 1.7 parts of azobisisobutyronitrile. 121.7 parts of this monomer solution were added dropwise to the reaction vessel at 120°C over 3 hours. Stirring was continued for another hour to obtain a phosphate-containing acrylic resin (solid content concentration 63%) with an acid value of 105 mg KOH / g, of which 55 mg KOH / g was due to phosphate groups, a hydroxyl value of 60 mg KOH / g, and a number average molecular weight of 6000.

[0189] [Example 1] (i) Preparation of water-based intermediate coating composition A mixture of 22 parts of core-shell type acrylic resin dispersion (A-1) (22% resin solids), 35 parts of hydrophobic melamine resin (B-1), 22 parts of polyester resin dispersion, 20 parts of urethane resin dispersion (D-2), 10 parts of hydrophobic organic solvent (H-1), 5 parts of hydrophobic organic solvent (H-2), 10 parts of hydrophilic organic solvent (h-1), 75 parts of deionized water, and pigment dispersion paste (F-1) was mixed in an amount equal to 150 parts in terms of colored pigment (F) (titanium dioxide). Furthermore, 1 part of phosphate-containing acrylic resin, 2.6 parts of lustrous pigment (E-1) in terms of aluminum pigment content, and 1.1 parts of lustrous pigment (E-2) in terms of aluminum pigment content were mixed and stirred to obtain an aqueous intermediate coating paint composition. The total content of hydrophobic organic solvent (H) was 15% by mass relative to 100% by mass of resin solids contained in the aqueous intermediate coating paint composition. The above proportions are based on solids content. The above formulations are based on solid content.

[0190] (ii) Formation of the intermediate coating film A zinc phosphate-treated dull steel plate (0.8 mm thick, 30 cm long, 40 cm wide) was coated with 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. Subsequently, the electrodeposition paint was heat-cured to prepare the workpiece.

[0191] Next, a water-based intermediate coating composition was applied to the substrate to a dry film thickness of 8 μm. Then, after preheating at 80°C for 3 minutes, the substrate was baked and cured at 140°C for 30 minutes to produce test plate A with an intermediate coating film.

[0192] (iii) Formation of multi-layer coating In the same manner as described above, the water-based intermediate coating composition was applied to the workpiece to a dry film thickness of 8 μm. Then, the base coating composition (product name: Aquarex AR-3020-1 Pearl Mica, manufactured by Nippon Paint Automotive Coatings Co., Ltd., water-based mica base coating) was applied by air spray painting to a dry film thickness of 8 μm, and preheated at 80°C for 3 minutes.

[0193] Next, a clear coating composition (product name: PolyureExcel O-2300 Clear, manufactured by Nippon Paint Automotive Coatings Co., Ltd., a two-component clear coating) was applied by air spray painting to achieve a dry film thickness of 35 μm. Finally, test panel B with a multi-layer coating was prepared by heat curing at 140°C for 30 minutes.

[0194] The water-based intermediate coating composition, base coating composition, and clear coating composition were diluted under the following conditions and used for painting.

[0195] • Water-based intermediate coating paint composition (solids content 49% by mass) Thinner: Ion-exchanged water Viscosity: 4500mPa·s / 6rpm / B type viscometer

[0196] • Base paint composition Thinner: Ion-exchanged water Viscosity: 2200mPa·s / 6rpm / B type viscometer

[0197] • Clear paint composition Thinner: A mixed solvent of EEP (ethoxyethyl propionate) / BDG-AC (butyl diglycol acetate) = 8 / 2 (mass ratio). Viscosity: 25 seconds / No. 4 Ford Cup / 20℃

[0198] [Examples 2-15, Comparative Examples 1-7] Except for changing the components and / or their amounts as shown in Tables 2 and 3, a water-based intermediate coating composition was prepared using the same procedure as in Example 1, and test panels A and B were made.

[0199] [evaluation] The following evaluations were performed using the aqueous intermediate coating compositions obtained in the examples and comparative examples, and test panels A or B. The evaluation results are shown in Tables 1 to 3 below.

[0200] (1) Water contact angle (1-1) Preparation of a dried resin film An acrylic resin dispersion (A) used in the preparation of the aqueous intermediate coating composition and deionized water were mixed to prepare an aqueous dispersion A with a solid content of 30% by mass. The obtained aqueous dispersion A was applied to a test plate having an electrodeposited coating using a doctor blade to a dry film thickness of 30 μm, and dried at 23°C for 45 minutes to obtain a dried resin film A.

[0201] Two types of dried resin films C and D were obtained in the same manner as described above, except that a polyester resin dispersion (C) or a urethane resin dispersion (D) was used instead of an acrylic resin dispersion (A).

[0202] (1-2) Measurement of water contact angle Using an automatic contact angle meter DM500-M manufactured by Kyowa Interface Science Co., Ltd., approximately 1 μL of distilled water was dropped onto the surface of dry resin films A, C, or D in an atmosphere of 23°C and 50% relative humidity. The angle between the droplet and the surface of each dry resin film after 60 seconds was calculated using the θ / 2 method.

[0203] (2) TI value After allowing the water-based intermediate coating composition to stand at 20°C and 50% RH, the viscosity ηL at low speed (6 rpm) and viscosity ηH at high speed (60 rpm) were measured using a rotational viscometer (Type B viscometer, "TVB-10M", manufactured by Toki Sangyo Co., Ltd., SPINDLE rotor No. M3). The (ηL / ηH)TI value was calculated by dividing viscosity ηL by viscosity ηH.

[0204] (3) Appearance of the intermediate coating Test plate A, which had 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, under the condition of a cutoff value of 0.25 mm (5 sections), the surface roughness was measured 10 times at different locations, and the average Ra value was calculated. The obtained Ra values ​​were evaluated according to the following criteria. A rating of B or higher indicates excellent coating appearance.

[0205] (Evaluation Criteria) A: Less than 0.18 μm B: 0.18 μm or more and less than 0.20 μm C: 0.20μm or more

[0206] (4) Appearance of multi-layer coating Test plate B, which has a multi-layer coating, was used. The W1 and W4 values ​​of the surface of the multi-layer coating were measured using the "WaveScan-T" manufactured by BYK-Gardner GmbH. A smaller W value indicates higher coating smoothness. A rating of B or higher indicates excellent appearance.

[0207] (Evaluation Criteria) A: W1 is less than 12, and W4 is less than 12. B:W1 is 12 or more but less than 15, or W4 is 12 or more but less than 15 C:W1 is 15 or higher, or W4 is 15 or higher

[0208] (5) Water resistance Test plate B was immersed in 40°C warm water. After 10 days, test plate B was removed and water droplets were lightly wiped off. After removal, the painted object was left at room temperature (23°C) for 24 hours. On the resulting multi-layer coating, the cutting blade of an NT Cutter S-type (manufactured by NT Corporation) was held at approximately 30 degrees to the coating surface, and cuts (11 vertical and 11 horizontal, at 2mm intervals) reaching the object to be coated were made, creating 100 squares. Adhesive tape (manufactured by Nichiban Co., Ltd., Cellotape®, 24mm wide) was evenly pressed onto all squares with fingertips, ensuring no air bubbles remained. The adhesive tape was immediately peeled off while pulling it so that the angle between it and the coating was approximately 90°. After tape removal, 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.

[0209] (Evaluation Criteria) A: 0 squares / 100 squares B: 1-5 squares / 100 squares C: 5 squares / 100 or more

[0210] (6) Weather resistance Test plate B was subjected to an accelerated weathering test using a xenon lamp (manufactured by Suga Test Instruments Co., Ltd.), with an integrated light intensity of 1000 MJ / m². 2 The material was irradiated with ultraviolet light. Afterward, a tape peel test was performed and evaluated in the same manner as described above. A rating of B or higher indicates excellent weather resistance.

[0211] (7) Chipping resistance A stone chip test was conducted on test plate B using a Gravelo testing machine KSS-1 (manufactured by Suga Test Machine Co., Ltd.) under the following conditions. <Test Conditions> Shot material: No. 6 crushed stone as specified in JIS A5001 (Crushed stone for roads) Shot material quantity: 300g Distance: 35cm Shot pressure: 0.50 MPa Shot angle: 45° Test temperature: -20℃

[0212] Next, test plate B was washed with water and dried. Then, industrial gum tape manufactured by Nichiban Co., Ltd. was evenly pressed onto the multi-layer coating using fingertips, ensuring no air bubbles remained. The gum tape was immediately peeled off while pulling it so that the angle between it and the coating was approximately 90°. After tape removal, the degree of coating peeling was visually evaluated according to the following criteria. A rating of B or higher indicates suitability for practical use.

[0213] (Evaluation Criteria) A: The peeling area is small and the frequency is low. B: The peeling area is somewhat large, and the frequency is somewhat high. C: Large peeling area and frequent occurrence.

[0214] [Table 1]

[0215] [Table 2]

[0216] [Table 3]

[0217] This disclosure includes the following aspects: [1] A method for forming a multi-layer coating film, comprising sequentially applying a water-based intermediate coating composition, a water-based base coating composition, and a clear coating composition in a wet-on-wet application and curing them, The aforementioned aqueous intermediate coating composition comprises a core-shell type acrylic resin dispersion (A), a hydrophobic melamine resin (B), a polyester resin dispersion (C), a urethane resin dispersion (D), a glossy pigment (E), a white pigment (F), and a viscosity modifier (G), and has a thixotropic index of 3.5 or more and 10 or less. The core-shell type acrylic resin dispersion (A) has a shell portion made of hydrophilic resin. A method for forming a multilayer coating film, wherein the core-shell type acrylic resin dispersion (A), the polyester resin dispersion (C), and the urethane resin dispersion (D) are used, and the coating film formed by the aqueous dispersion of each resin dispersion is dried at 23°C for 45 minutes, resulting in each dried resin film having a water contact angle of 80° or more and 120° or less. [2] The core-shell type acrylic resin dispersion (A) is the aqueous intermediate coating composition of [1] above, having branched hydrocarbon groups having 4 to 24 carbon atoms in the core portion. [3] A method for forming a multilayer coating film according to [1] or [2] above, wherein the content ratio (E:F) of the luminous pigment (E) to the white pigment (F) is 1:20 to 1:260. [4] The viscosity modifier (G) includes a hydrophobic association type viscosity modifier, and the method for forming a multilayer coating film is one of the above [1] to [3]. [5] A method for forming a multilayer coating film according to any of the above [1] to [4], wherein the average particle size of the luminous pigment (E) is 5 μm or more and 12 μm or less. [6] The method for forming a multilayer coating film according to any of the above [1] to [5], wherein the luminous pigment (E) contains aluminum particles. [7] Furthermore, a method for forming a multilayer coating film according to any of the above [1] to [6], comprising a hydrophobic organic solvent (H) having a hydrocarbon group with 5 or more carbon atoms. [8] A method for forming a multilayer coating film according to any of the above [1] to [7], wherein the PWC of the white pigment (F) is 40% by mass or more and 65% by mass or less. [Industrial applicability]

[0218] The formation method disclosed herein yields a multi-layer coating with excellent appearance. This multi-layer coating formation method is suitable for painting automobile bodies and automobile components.

Claims

1. A method for forming a multi-layer coating film, comprising sequentially applying a water-based intermediate coating composition, a water-based base coating composition, and a clear coating composition in a wet-on-wet application and curing them, The aforementioned aqueous intermediate coating composition comprises a core-shell type acrylic resin dispersion (A), a hydrophobic melamine resin (B), a polyester resin dispersion (C), a urethane resin dispersion (D), a glossy pigment (E), a white pigment (F), and a viscosity modifier (G), and has a thixotropic index of 3.5 or more and 10 or less. The core-shell type acrylic resin dispersion (A) has a shell portion made of hydrophilic resin. A method for forming a multilayer coating film, wherein the core-shell type acrylic resin dispersion (A), the polyester resin dispersion (C), and the urethane resin dispersion (D) are used, and the coating film formed by the aqueous dispersion of each resin dispersion is dried at 23°C for 45 minutes, resulting in each dried resin film having a water contact angle of 40° or more and 120° or less.

2. The method for forming a multilayer coating film according to claim 1, wherein the core-shell type acrylic resin dispersion (A) has branched hydrocarbon groups having 4 to 24 carbon atoms in the core portion.

3. The method for forming a multilayer coating film according to claim 1 or 2, wherein the content ratio (E:F) of the luminous pigment (E) to the white pigment (F) is 1:20 to 1:

260.

4. The method for forming a multilayer coating film according to claim 1 or 2, wherein the viscosity modifier (G) includes a hydrophobic association type viscosity modifier.

5. The method for forming a multilayer coating film according to claim 1 or 2, wherein the average particle size of the luminous pigment (E) is 5 μm or more and 12 μm or less.

6. The method for forming a multilayer coating film according to claim 1 or 2, wherein the luminous pigment (E) contains aluminum particles.

7. Furthermore, the method for forming a multilayer coating film according to claim 1 or 2, comprising a hydrophobic organic solvent (H) having a hydrocarbon group with 5 or more carbon atoms.

8. The method for forming a multilayer coating film according to claim 1 or 2, wherein the PWC of the white pigment (F) is 40% by mass or more and 65% by mass or less.

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