Multi-layer coating

Abstract

A multi-layer coating film with little difference in hue from the first base coating film and high flip-flop properties. The present invention provides a coating composition comprising a first base coating film, a second base coating film, and a clear coating film, the first base coating film containing a luster pigment and two or more color pigments, and a solid content concentration NV1 of 20 to 35 mass %. G1 1-25% of PWC, color pigment C1 The second base paint contains a bright pigment and two or more color pigments having the same color index name as the first base paint composition, and the solid content concentration NV2 is 7 to 30 mass%, and the PWC of the bright pigment is G2 5-35% of PWC, color pigment C2 is 0.5 to 20.0%, and the ratio (T1 / T2) of the thickness T1 of the first base coating to the thickness T2 of the second base coating is 1.5 / 1 to 5 / 1, (NV1 / NV2) is 1.3 / 1 to 3 / 1, (PWC G1 / PWC G2 ) is 1 / 10~1 / 1, (PWC C1 / PWC C2 ) is 1 / 5 to 1 / 1.

Description

[Technical Field]

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

[0002] Automobile bodies are laminated with multiple coatings (multi-layer coatings). Multi-layer coatings protect the vehicle body while also providing the vehicle with excellent design. In a multi-layer coating, for example, a first base coating, a second base coating, and a clear coating are laminated in this order. Traditionally, automobile exteriors have required a luxurious feel, and designs with a strong metallic finish and high flip-flop properties are preferred. For this reason, the second base coating usually contains a luster pigment.

[0003] Multilayer paint films are mainly formed on the exterior panels of a vehicle body. Multilayer paint films are often not formed on the interior panels. For example, only a first base paint film used for painting the exterior panels is formed on the interior panels. Because the final paint films formed on the interior and exterior panels are different, a color difference can occur between the interior and exterior panels. This color difference can result in, for example, streaky paint unevenness on the interior panels. Streaky paint unevenness is formed when, after painting the interior panels, the first base paint composition for the first base paint film and the second base paint composition for the second base paint film are applied to the exterior panels, resulting in the second base paint composition splashing onto the interior panels. Conventionally, this problem has been solved by using a paint composition specifically for the interior panels, but this leads to poor productivity and increased costs. Forming the first and second base paint films using the same paint composition does not achieve the desired flip-flop properties.

[0004] In this regard, Patent Document 1 discloses a method for controlling the amount of second base coating composition dispensed and the moving speed of the spray gun. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-126528 Summary of the Invention [Problem to be solved by the invention]

[0006] The present disclosure aims to provide a multilayer coating film (corresponding to the outer panel portion) that has little difference in hue from a single film of the first base coating film (corresponding to the inner panel portion) and has high flip-flop properties when the same first base paint composition is used to form a first base coating film on both the outer panel portion and the inner panel portion. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides the following aspects. [1] A first base coating film formed on an object to be coated from an aqueous first base coating composition; A second base coating film formed on the first base coating film using an aqueous second base coating composition; A clear coating film formed on the second base coating film using a clear coating composition, The first aqueous base coating composition contains a luster pigment and two or more color pigments, The solid content concentration NV1 is 20 to 35 mass%; PWC of the above-mentioned bright pigment G1 is 1-25% PWC of the color pigment C1 is 0.1 to 5.0%, The second aqueous base coating composition contains a brilliant pigment and two or more color pigments having the same color index name as the color pigment contained in the first aqueous base coating composition, The solid content concentration NV2 is 7 to 30 mass%; PWC of the above-mentioned bright pigment G2 is 5-35% PWC of the color pigment C2 is 0.5 to 20.0%, the ratio (T1 / T2) of the thickness T1 of the first base coating film to the thickness T2 of the second base coating film is 1.5 / 1 to 5 / 1; the ratio (NV1 / NV2) of the solid content concentration NV1 to the solid content concentration NV2 is 1.3 / 1 to 3 / 1; PWC of the above-mentioned bright pigment G1 and the PWC of the bright pigment G2 Ratio to (PWC G1 / PWC G2 ) is 1 / 10~1 / 1, PWC of the color pigment C1 and the color pigment PWC C2 Ratio to (PWC C1 / PWC C2 ) is 1 / 5 to 1 / 1. [2] Light I irradiated at a 45 degree angle to the coating surface 45 The brightness L is based on the spectral reflectance of light received at an angle of 25 degrees to the specular reflected light. * 25 value, and the lightness L * 25 values ​​and a * 25 values ​​and b * The color difference E calculated from 25 values * At 25 values, Lightness Lm of the multi-layer coating film * 25 values ​​to the lightness Lbc of the first base coating * ΔL minus 25 * 25 is 0 to 10, The color difference Em of the multi-layer coating film * 25 value and the color difference Ebc of the first base coating * Difference from 25 ΔE * 25 is less than or equal to 10, Light I irradiated at a 45 degree angle to the coating surface 45 The brightness L is based on the spectral reflectance of light received at a 45-degree angle relative to the specular reflection. * 45 value, and the lightness L * 45 values ​​and a * 45 values ​​and b * The color difference E calculated from the 45 values * At 45 values, Lightness Lm of the multi-layer coating film * 45 value to the lightness Lbc of the first base coating * ΔL minus 45 * 45 is -10 to 0, The color difference Em of the multi-layer coating film * 45 value and the color difference Ebc of the first base coating * Difference from 45 ΔE * The multi-layer coating film according to [1] above, wherein 45 is 10 or less. [3] The multilayer coating film according to [1] or [2] above, wherein the difference between the average particle size of the luster pigment contained in the first aqueous base coating composition and the average particle size of the luster pigment contained in the second aqueous base coating composition is 5 μm or less. [Effects of the Invention]

[0008] According to the present disclosure, a multi-layer coating film is provided which has a small difference in hue from the first base coating film and has high flip-flop properties. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view schematically illustrating a coating method for a test plate for evaluating dust streaks in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] In order to reduce the difference in hue between the first base coating film and the multi-layer coating film comprising the first base coating film, the second base coating film, and the clear coating film, two or more color pigments having the same color index names are blended into both the first water-based base coating composition and the second water-based base coating composition. This reduces the difference in hue between the outer and inner panels when the same first base coating composition is used for the inner panel.

[0011] The outer panel parts are typically parts that can be seen when the automobile is viewed from the outside. Specific examples of the outer panel parts include outer doors, the roof, the front fenders, outer side panels, and the hood. The inner panel parts are typically parts that cannot be seen when the automobile is viewed from the outside. Specific examples of the inner panel parts include inner doors, pillars, and the floor.

[0012] As described above, the difference in hue is manifested as streak-like paint unevenness (hereinafter also referred to as dust streaks) formed on the interior panel parts of the vehicle body. In the painting process of an automobile body, a first base paint composition is first applied to the interior panel parts. Next, the first base paint composition and the second base paint composition are applied to the exterior panel parts. The second base paint composition is applied when the door is not completely closed. As a result, the second base paint composition splashes onto the pillars, forming dust streaks. Because the difference in hue between the first base paint film and the multi-layer paint film of the present disclosure is small, the occurrence of dust streaks is suppressed, resulting in a natural finish.

[0013] The Color Index is a database that organizes and classifies industrially manufactured and sold synthetic dyes and pigments based on their species, hue, and chemical structure. The Color Index includes Color Index Generic Names, which classify each chemical structure based on species and hue, and Color Index Constitution Numbers, which classify each chemical structure. This disclosure uses Color Index Names.

[0014] The Color Index is published by the Society of Dyers and Colourists (SDC) and the American Association of Textile Chemists and Colourists (AATCC). Many manufacturers also list the Color Index name along with the product name on their products or in their catalogs. From this information, the Color Index name of a pigment can be obtained.

[0015] According to the "Colour Index™ Online" (https: / / colour-index.com / colour-index-generic-name) published by the SDC and AATCC, pigments with the same chemical structure but different crystal forms are distinguished by a colon (:) followed by a number in the Color Index names. For example, copper phthalocyanine blue has multiple Color Index names: "PB 15:1," "PB 15:3," and "PB 15:6." These copper phthalocyanine blues have the α, β, and ε crystal forms, respectively.

[0016] In this disclosure, "color pigments having the same Color Index name" refers to color pigments that have the same chemical structure (the same symbol before the colon), regardless of their crystal form (i.e., even if the number after the colon is different). In the case of the above-mentioned copper phthalocyanine blue, multiple pigments with the symbol before the colon "PB 15" are considered to be "color pigments having the same Color Index name," even if the number after the colon is different. Hereinafter, "color pigments having the same Color Index name" may be referred to as "similar colors" for convenience.

[0017] In order to achieve high flip-flop properties (FF properties) while minimizing the difference in hue, the thickness ratio of each coating film is further controlled, and the PWC ratio and solid mass ratio of the color pigment and the luster pigment of each coating composition are controlled.

[0018] The flip-flop property refers to the property that the brightness of the coating surface changes depending on the viewing angle. In this disclosure, the flip-flop property refers to the property that the brightness of the coating surface changes depending on the viewing angle. 45 L, calculated from the spectral reflectance of light received at an angle of 15 degrees or 75 degrees to the specular reflection light * a * b * Color space (CIE1976L * a * b * Lightness L in color space * (L * 15,L *75) ratio (L * 15 / L * 75) is used as an index of FF. * 15 / L * 75), the larger the value, the better the FF.

[0019] Coating thickness ratio The ratio (T1 / T2) of the thickness T1 of the first base coating to the thickness T2 of the second base coating is 1.5 / 1 to 5 / 1. That is, the first base coating is thicker than the second base coating. When T1 / T2 is in this range, the orientation of the luster pigment is improved, and the flip-flop properties are improved. T1 / T2 may be 1.5 / 1 to 3 / 1.

[0020] PWC ratio of color pigments PWC of two or more color pigments (hereinafter sometimes collectively referred to as "first color pigments") contained in the first aqueous base coating composition C1 and the PWC of the color pigment contained in the second aqueous base coating composition (hereinafter sometimes collectively referred to as the second color pigment). C2 Ratio to (PWC C1 / PWC C2 ) is 1 / 5 to 1 / 1. PWC C1 / PWC C2 When the PWC content is within this range, the difference in hue between the first base coating film and the multi-layer coating film is small, which means that the occurrence of dust streaks is suppressed. C1 / PWC C2 may be 1 / 3 to 1 / 1.1, or may be 1 / 2.5 to 1 / 1.25.

[0021] PWC ratio of photoluminescent pigment PWC of the bright pigment contained in the first aqueous base coating composition (hereinafter sometimes referred to as the first bright pigment) G1 and the PWC of the bright pigment contained in the second aqueous base coating composition (hereinafter sometimes referred to as the second bright pigment). G2 Ratio to (PWC G1 / PWC G2 ) is 1 / 10 to 1 / 1. PWC G1 / PWC G2When the PWC is in this range, the orientation of the bright pigment is improved, and the flip-flop property is improved. G1 / PWC G2 may be 1 / 5 to 1 / 1.1, or may be 1 / 3 to 1 / 1.25.

[0022] Solid mass ratio The ratio (NV1 / NV2) of the solids concentration NV1 of the first aqueous base coating composition to the solids concentration NV2 of the second aqueous base coating composition is 1.3 / 1 to 3 / 1. That is, the solids content of the first aqueous base coating composition is greater than the solids content of the second aqueous base coating composition. When NV1 / NV2 is in this range, the orientation of the luster pigment is improved, resulting in improved flip-flop properties. NV1 / NV2 may be 1.3 / 1 to 2.5 / 1.

[0023] · Difference in brightness and color between the first base coating and the multi-layer coating When both the brightness difference and color difference between the first base coating film and the multi-layer coating film are within a predetermined range, the difference in hue between the two can be said to be small.

[0024] The brightness and color difference were measured by irradiating the surface of the coating with light I at a 45-degree angle. 45 is calculated from the spectral reflectance of light received at a specified angle relative to the specular reflection light, L * a * b * Color space (CIE1976L * a * b * Lightness L in color space * and color difference E * Color difference E * The value is the lightness L * value and a * value and b * It is calculated from the value using the following formula. Color difference E * =√{(L * ) 2 +(a * ) 2 +(b * ) 2}

[0025] Light irradiated onto the surface of a multi-layer coating I 45 Lm based on the spectral reflectance of light received at an angle of 25 degrees to the specular reflected light * From the 25 values, the light I irradiated on the surface of the first base coating 45 Lbc is based on the spectral reflectance of light received at an angle of 25 degrees relative to the specular reflection. * ΔL minus 25 * 25 is 0 to 10. ΔL * 25 may be 0 to 6.

[0026] Light irradiated onto the surface of a multi-layer coating I 45 Lm based on the spectral reflectance of light received at a 45-degree angle to the specular reflected light * From the 45 values, the light I irradiated on the surface of the first base coating 45 Lbc is based on the spectral reflectance of light received at a 45-degree angle relative to the specular reflection. * ΔL minus 45 * 45 is -10 to 0. ΔL * 45 may be from -6 to 0.

[0027] Lm * The color difference Em of the multi-layer coating calculated from the 25 values ​​using the above formula * 25 value and Lbc * The color difference Ebc of the first base coating calculated from the 25 values ​​using the above formula * Difference from 25 values ​​ΔE * 25 is less than 10.0. ΔE * 25 may be 6.0 or less.

[0028] Lm * The color difference Em of the multi-layer coating calculated from the 45 value using the above formula * 45 value and Lbc * The color difference Ebc of the first base coating calculated from the 45 value using the above formula * Difference from 45 values ​​ΔE * 45 is less than 10.0. ΔE * 45 may be 6.0 or less.

[0029] ΔL * 25 is 0 to 10, and ΔL *45 is -10 to 0, which means that the difference in brightness is very small. * 25 is 10 or less and ΔE * A 45 of 10 or less means that the difference in color difference is very small. The small difference in brightness and color difference between the multi-layer coating and the first base coating makes it seem like there is a small difference in hue.

[0030] Lightness L * and color difference E * The lightness L can be obtained using a multi-angle colorimeter (for example, a product name "BYK-mac i" manufactured by BYK). * and color difference E * is the average value of five different samples or five random points of the same sample.

[0031] Lightness L of the first water-based base coating * and color difference E * is measured using a coating film formed on the same substrate as the substrate on which the multi-layer coating film is formed, under the same conditions as for forming the first base coating film and clear coating film that make up the multi-layer coating film.

[0032] Lightness L of water-based first base coating and multi-layer coating * and color difference E * correspond to the lightness and color difference of the inner and outer panels of the car body, respectively. The lightness and color difference of the inner panel (typically the pillar) and outer panel (typically the door outer) of the car body are calculated based on the lightness L of the water-based first base coating and the multi-layer coating. * and color difference E * It is safe to assume this.

[0033] ΔL in the brightness and color difference of the inner and outer panels of the car body * 25 is 0 to 10, ΔL * 45 is -10 to 0, ΔE * 25 is 10.0 or less and ΔE *If 45 is 10.0 or less, it is highly likely that the interior and exterior panels are formed from the first and second water-based base paint compositions used in the present disclosure. The lightness of the interior and exterior panels of a vehicle body is the average value of any five points on the interior panel and any five points on the exterior panel of the same vehicle body.

[0034] In this specification, the glass transition temperature (Tg) of a resin can be calculated from the types and amounts of raw material monomers used in the production of the target resin. Tg may be measured by differential scanning calorimetry (DSC).

[0035] The number average molecular weight (Mn) and weight average molecular weight (Mw) can be measured by gel permeation chromatography (GPC) using polystyrene standard samples after removing water by drying under reduced pressure or the like.

[0036] The hydroxyl value (OHV) and acid value (AV) are determined based on the mass of solids. The hydroxyl value (OHV) and acid value (AV) can be measured by the known method described in JIS K 0070:1992. The hydroxyl value (OHV) and acid value (AV) may be calculated from the amount of unsaturated monomer in the raw material monomers of the target resin.

[0037] The solid content is also referred to as the non-volatile content. Specifically, the solid content of a coating composition is all components excluding the solvent from the coating composition. The solid content concentration is determined by dividing the total mass of the solids excluding the solvent from the coating composition by the mass of the entire coating composition. The solid content concentration can also be calculated from the residue when the coating composition is heated at 140°C in accordance with JIS K 5601-1-2 Heat Residue Measurement Method. Specifically, the solid content of an aqueous coating composition is the aqueous resin, curing agent, luster pigment, color pigment, and other solid components added as needed, as described below.

[0038] The resin solid content is the resin component of the above solid content. Specifically, the solid content of the aqueous coating composition is the aqueous resin, curing agent, and other resins described below.

[0039] The PWC (mass %) of a pigment (e.g., a luster pigment) is calculated by dividing the mass of the pigment by the total mass of all pigments and resin solids contained in the coating composition (see the formula below). The pigment may contain multiple types. For example, the PWC of the first color pigment is C1 is calculated from the total mass of the multiple color pigments contained in the first aqueous base coating composition. TIFF0007808170000002.tif15117

[0040] The thickness of the coating film can be measured using an electromagnetic film thickness meter (for example, SDM-miniR manufactured by SANKO Co., Ltd.) The thickness of the coating film is the average value of the thickness of the coating film at any five points.

[0041] The average particle size is the 50% average particle size (D50) in the volumetric particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer, such as the UPA-150 (Microtrac particle size distribution analyzer, manufactured by Nikkiso Co., Ltd.).

[0042] Aqueous resins are generally broadly classified into water-soluble and water-dispersible resins. Water-dispersible resins are further classified into colloidal dispersion type and emulsion type. Colloidal dispersion type (hereinafter simply referred to as dispersion type) aqueous resins are typically obtained by semi-dissolving a resin synthesized in an organic solvent (a solution-polymerized resin) in water with a neutralizer. Emulsion type aqueous resins are typically produced by emulsion polymerization or by mechanically forced emulsification.

[0043] In the case of aqueous acrylic resins, those with an Mw of over 100,000 are considered emulsion type, and those with an Mw of 100,000 or less are considered colloidal dispersion type. When the Mw exceeds 1 million, it becomes difficult to measure the Mw. Aqueous acrylic resins whose Mw cannot be measured can be considered emulsion type. Aqueous resins include water-soluble resins, dispersion type resins, and emulsion type resins.

[0044] [Multi-layer coating] The multilayer coating film comprises a first base coating film formed on the substrate using a first aqueous base coating composition, a second base coating film formed on the first base coating film using a second aqueous base coating composition, and a clear coating film formed on the second base coating film using a clear coating composition.

[0045] Lightness of multi-layer coating film Lm * 25 is, for example, 35 or more and 110 or less. * 25 may be 40 or more, 45 or more, or 55 or more. * 25 may be 100 or less, may be 95 or less, or may be 90 or less.

[0046] Lightness of multi-layer coating film Lm * 45 is, for example, 10 or more and 60 or less. * 45 may be 15 or more, 20 or more, 25 or more, or 30 or more. * 45 may be 50 or less, may be 45 or less, or may be 40 or less.

[0047] Water-based first base coat The dry thickness of the first water-based base coating is, for example, 5 to 15 μm, the dry thickness of the second water-based base coating is, for example, 2 to 8 μm, and the dry thickness of the clear coating is, for example, 10 to 60 μm.

[0048] Lightness Lbc of water-based first base coating * 25 is, for example, between 30 and 100. * 25 may be 40 or more, 45 or more, or 50 or more. * 25 may be 90 or less, 80 or less, or 65 or less.

[0049] Lightness Lbc of water-based first base coating * 45 is, for example, between 10 and 60. *45 may be 20 or more, or 30 or more. * 45 may be 55 or less, may be 50 or less, or may be 40 or less.

[0050] Water-based first base coating composition The water-based first base coating composition contains, for example, a water-based resin, a curing agent, a luster pigment, and a color pigment.

[0051] The solid content concentration NV1 of the first aqueous base coating composition is 20 to 35% by mass. When the solid content concentration NV1 is 20% by mass or more, coating efficiency is improved. When the solid content concentration NV1 is 35% by mass or less, the appearance is improved. The solid content concentration NV1 may be 25% by mass or more. The solid content concentration NV1 may be 30% by mass or less.

[0052] (water-based resin) Examples of aqueous resins include aqueous acrylic resins, aqueous polyester resins, polyether polyols, and aqueous polyurethane resins, which may be used alone or in combination of two or more.

[0053] The ratio of the aqueous resin to the resin solids mass of the first aqueous base coating composition is, for example, 50 to 90 mass%. This makes it easier to achieve both storage stability and curability of the first aqueous base coating composition. The ratio of the aqueous resin may be 60 mass% or more. The ratio of the aqueous resin may be 85 mass% or less.

[0054] Water-based acrylic resin The aqueous acrylic resin may be an emulsion-type acrylic resin. The emulsion-type acrylic resin (acrylic resin emulsion) has hydroxyl groups and carboxyl groups. The hydroxyl groups of the acrylic resin emulsion react with a curing agent to form a crosslinked structure.

[0055] The Mw of the acrylic resin emulsion is 100,000 or more. The Mw of the acrylic resin emulsion may be 100,000 or more, or may be 200,000 or more. There is no upper limit for the Mw of the acrylic resin emulsion.

[0056] The OHV of the acrylic resin emulsion is, for example, 10 mgKOH / g or more and 100 mgKOH / g or less. This increases the crosslink density and further improves chipping resistance. The OHV of the acrylic resin emulsion may be 20 mgKOH / g or more, or 30 mgKOH / g or more. The OHV of the acrylic resin emulsion may be 90 mgKOH / g or less, or 80 mgKOH / g or less.

[0057] The acid value (AV) of the acrylic resin emulsion is, for example, 5 mgKOH / g or more and 50 mgKOH / g or less. The AV of the acrylic resin emulsion may be 10 mgKOH / g or more, or 15 mgKOH / g or more. The AV of the acrylic resin emulsion may be 40 mgKOH / g or less, or 30 mgKOH / g or less.

[0058] The Tg of the acrylic resin emulsion is, for example, -15°C or higher and 60°C or lower. This makes it easier for the Tg of the first base coating film to be 60°C or higher. The Tg of the acrylic resin emulsion may be -10°C or higher, or may be 25°C or higher. The Tg of the acrylic resin emulsion may be 55°C or lower, or may be 50°C or lower.

[0059] The average particle size of the acrylic resin emulsion is, for example, 20 nm or more and 200 nm or less. The average particle size of the acrylic resin emulsion 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 emulsion may be 25 nm or more, or 30 nm or more.

[0060] The solid content of the acrylic resin emulsion is 8 parts by mass or more and 20 parts by mass or less, based on 100 parts by mass of the resin solid content of the first aqueous base coating composition. The content of the acrylic resin emulsion may be 10 parts by mass or more. The content of the acrylic resin emulsion may be 15 parts by mass or less, or may be 12 parts by mass or less.

[0061] The acrylic resin emulsion may be used alone or in combination of two or more. When multiple aqueous acrylic resins are used, the OHV, AV, and Tg of the aqueous acrylic resins are average values ​​calculated based on the OHV, AV, Tg, and mass proportion of each aqueous acrylic resin.

[0062] The aqueous acrylic resin can be obtained, for example, by copolymerizing monomers including an α,β-ethylenically unsaturated monomer having a hydroxyl group and an α,β-ethylenically unsaturated monomer having a carboxyl group.

[0063] Examples of α,β-ethylenically unsaturated monomers having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, allyl alcohol, methacrylic alcohol, and an adduct of hydroxyethyl (meth)acrylate with ε-caprolactone. 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and an adduct of hydroxyethyl (meth)acrylate with ε-caprolactone may also be used.

[0064] "(Meth)acrylic acid" includes both acrylic acid and methacrylic acid.

[0065] Examples of the α,β-ethylenically unsaturated monomer having a carboxy group include acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, maleic acid, fumaric acid, and itaconic acid. The monomer may be acrylic acid or methacrylic acid.

[0066] Other α,β-ethylenically unsaturated monomers may be used as copolymerization components. Examples of other α,β-ethylenically unsaturated monomers include (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl methacrylate, phenyl acrylate, isobornyl (meth)acrylate, cyclohexyl methacrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate; and polymerizable amide compounds such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide.

[0067] The other α,β-ethylenically unsaturated monomer may be a crosslinking monomer. The crosslinking monomer has two or more radically polymerizable ethylenically unsaturated groups in the molecule. Examples of the crosslinking monomer include divinylbenzene, allyl (meth)acrylate, and ethylene glycol di(meth)acrylate.

[0068] Acrylic resin emulsions are typically synthesized by emulsion polymerization. The emulsion polymerization method is not particularly limited. For example, an emulsifier is dissolved in an aqueous medium containing water or, if necessary, an organic solvent such as an alcohol or an ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and the raw material monomers and polymerization initiator are added dropwise with heating and stirring. The raw material monomers may be emulsified in advance with an emulsifier.

[0069] Examples of the polymerization initiator include azo-based oily compounds such as azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), and 2,2'-azobis(2,4-dimethylvaleronitrile); aqueous compounds such as anionic 4,4'-azobis(4-cyanovaleric acid), 2,2-azobis(N-(2-carboxyethyl)-2-methylpropionamidine, and cationic 2,2'-azobis(2-methylpropionamidine); redox-based oily peroxides such as benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide, and t-butyl perbenzoate; and aqueous peroxides such as potassium persulfate and ammonium persulfate.

[0070] The emulsifier is not particularly limited. Examples of the emulsifier include reactive emulsifiers. Examples of the reactive emulsifier include Antox MS-60 (manufactured by Nippon Nyukazai Co., Ltd.), Eleminol JS-2 (manufactured by Sanyo Chemical Industries, Ltd.), Adeka Reasoap NE-20 (manufactured by ADEKA Corporation), Aqualon HS-10 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and Latemul PD-104 (manufactured by Kao Corporation). Chain transfer agents such as mercaptans (e.g., lauryl mercaptan) and α-methylstyrene dimers may be used to adjust the molecular weight.

[0071] The reaction temperature is determined depending on the polymerization initiator. For example, when an azo-based initiator or peroxide is used, the reaction temperature is 60 to 90°C. When a redox-based initiator is used, the reaction temperature is 30 to 70°C. The reaction time is 1 to 8 hours. The amount of polymerization initiator per 100 parts by mass of the monomer mixture is 0.1 to 5% by mass. Emulsion polymerization can be carried out in multiple stages, for example, in two stages. In two-stage polymerization, a portion of the raw material monomers is emulsion-polymerized, and then the remaining raw material monomers are polymerized.

[0072] From the viewpoint of storage stability, the acrylic resin emulsion may be neutralized with a basic compound. The acrylic resin emulsion has a pH of 5 to 10. Neutralization is carried out before or after emulsion polymerization.

[0073] The basic compound is not particularly limited, and may be, for example, at least one selected from the group consisting of ammonia and amine compounds. Examples of amine compounds include dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, n-propylamine, isopropylamine, triallylamine, triethylenediamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, aminoethanolamine, N-methyl-N,N-diethanolamine, iminobispropylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, methylaminopropylamine, methyliminobispropylamine, 3-methoxypropylamine, monoethanolamine, diethanolamine, triethanolamine, morpholine, allylmorpholine, N-methylmorpholine, and N-ethylmorpholine. These may be used alone or in combination of two or more.

[0074] The acrylic resin emulsion can also be obtained by solution polymerization and neutralization. The solution polymerization is carried out by a known method.

[0075] The water-soluble acrylic resin can be prepared, for example, by solution polymerizing the above-mentioned α,β-ethylenically unsaturated monomer and then solubilizing it in water using a basic compound.

[0076] Water-based polyester resin The aqueous polyester resin may be a dispersion type. The aqueous polyester resin may have a hydroxyl group and a carboxyl group.

[0077] The OHV of the aqueous polyester resin is, for example, 50 to 150 mgKOH / g. The OHV may be 70 mgKOH / g or more. The OHV may be 120 mgKOH / g or less, or 100 mgKOH / g or less.

[0078] The AV of the aqueous polyester resin is, for example, 20 to 80 mgKOH / g. The AV may be 25 mgKOH / g or more. The AV may be 50 mgKOH / g or less, or may be 40 mgKOH / g or less.

[0079] The Mn of the aqueous polyester resin is, for example, 500 to 20,000. When the Mn of the aqueous polyester resin is 500 or more, storage stability is improved. When the Mn of the aqueous polyester resin is 20,000 or less, an increase in viscosity is suppressed, and coating workability is improved. The Mn of the aqueous polyester resin may be 1,500 or more. The Mn of the aqueous polyester resin may be 10,000 or less.

[0080] The Tg of the aqueous polyester resin is, for example, -20 to 80°C. When the Tg of the aqueous polyester resin is -20°C or higher, the hardness of the resulting coating film increases. When the Tg of the aqueous polyester resin is 80°C or lower, the performance of hiding the base (hiding ability) improves. The Tg of the aqueous polyester resin may be 0°C or higher. The Tg of the aqueous polyester resin may be 60°C or lower.

[0081] The aqueous polyester resin is obtained by neutralizing a polyester resin with a basic compound, for example, by condensation of a polyhydric alcohol component and a polybasic acid component.

[0082] Examples of polyhydric alcohol components include hydroxycarboxylic acid components such as ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, hydroxypivalic acid neopentyl glycol ester, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethylpentanediol. These may be used alone or in combination of two or more.

[0083] Examples of polybasic acid components include aromatic polycarboxylic acids and acid anhydrides such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrachlorophthalic anhydride, and pyromellitic anhydride; alicyclic polycarboxylic acids and anhydrides such as hexahydrophthalic anhydride, tetrahydrophthalic anhydride, and 1,4- and 1,3-cyclohexanedicarboxylic acid; and aliphatic polycarboxylic acids and anhydrides such as maleic anhydride, fumaric acid, succinic anhydride, adipic acid, and sebacic acid. These may be used alone or in combination of two or more.

[0084] If necessary, a monobasic acid such as benzoic acid or t-butylbenzoic acid may be used in combination.

[0085] As reaction components, monohydric alcohols, monoepoxide compounds such as Cardura E (trade name: manufactured by Oxalis Chemicals), and lactones such as β-propiolactone, dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone, and γ-caprolactone may be used in combination. These may be used alone or in combination of two or more.

[0086] Furthermore, fatty acids such as castor oil, dehydrated castor oil, and mixtures of one or more of these fatty acids may be added to the reaction system.

[0087] The polyester resin may be grafted with a water-based acrylic resin and / or vinyl resin, or may be reacted with a polyisocyanate compound.

[0088] Polyether polyol Polyether polyol is a resin other than those mentioned above, and has two or more hydroxyl groups in one molecule. The polyether polyol may have an average of three or more hydroxyl groups per molecule. This can further increase the coating strength. The polyether polyol may be an emulsion type or water-soluble.

[0089] The Mn of the polyether polyol is, for example, 300 to 2000. When the Mn of the polyether polyol is 300 or more, the coating film performance is improved. When the Mn of the polyether polyol is 2000 or less, the coating film appearance is improved. The Mn of the polyether polyol may be 400 or more, or may be 500 or more. The Mn of the polyether polyol may be 1800 or less, or may be 1500 or less.

[0090] Polyether polyols can be obtained, for example, by addition polymerization of alkylene oxides such as ethylene oxide, propylene oxide, tetrahydrofuran, etc., to polyhydric alcohols. The polyether polyols may be polyether diols having two hydroxyl groups in one molecule.

[0091] Examples of polyether diols include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol.

[0092] Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and block products thereof. Polyether polyols can be obtained, for example, by adding ethylene oxide and / or propylene oxide to a polyhydric alcohol compound.

[0093] Commercially available polyether polyols include, for example, the Sannix series manufactured by Sanyo Chemical Industries, Ltd. Specific examples include Sannix GP-250, Sannix GP-400, Sannix PP-200, and Sannix GP-600.

[0094] Water-based polyurethane resin The aqueous polyurethane resin may be water-soluble or water-dispersible. The aqueous polyurethane resin may be a dispersion type. The aqueous polyurethane resin may be used alone or in combination of two or more.

[0095] The aqueous polyurethane resin fuses with itself and other components to make the coating film stronger and improve the impact absorption of the coating film. The aqueous polyurethane resin also improves the chipping resistance and adhesion of the multi-layer coating film.

[0096] The OHV of the aqueous polyurethane resin is, for example, 0 mgKOH / g or more and 50 mgKOH / g or less. This improves the coating strength. The OHV of the aqueous polyurethane resin may be 5 mgKOH / g or more, or 10 mgKOH / g or more. The OHV of the aqueous polyurethane resin may be 40 mgKOH / g or less, or 30 mgKOH / g or less.

[0097] The AV of the aqueous polyurethane resin is, for example, 0 mgKOH / g or more and 20 mgKOH / g or less. This improves water resistance. The OHV of the aqueous polyurethane resin may be 5 mgKOH / g or more, or 10 mgKOH / g or more. The AV of the aqueous polyurethane resin may be 17 mgKOH / g or less, or 15 mgKOH / g or less.

[0098] The Tg of the aqueous polyurethane resin is, for example, -20°C or lower. When the Tg is -20°C or lower, chipping resistance is further improved. The Tg of the aqueous polyurethane resin may be -30°C or lower, or may be -50°C or lower. The lower limit of the Tg of the aqueous polyurethane resin is not particularly limited. The Tg of the aqueous polyurethane resin may be, for example, -90°C or higher.

[0099] The weight average molecular weight (Mw) of the aqueous polyurethane resin is, for example, 20,000 or more. This increases the strength of the coating film and improves adhesion. The Mw of the aqueous polyurethane resin may be 150,000 or more, or may be 250,000 or more. The upper limit of the Mw of the aqueous polyurethane resin is not particularly limited. The Mw of the aqueous polyurethane resin may be, for example, 1,000,000 or less.

[0100] The aqueous polyurethane resin can be obtained, for example, by a method of forcibly emulsifying the polyurethane resin using a surfactant, or by a method of neutralizing the polyurethane resin with a base or an acid.

[0101] The polyurethane resin can be obtained, for example, by reacting a polyol, a compound having an active hydrogen group and a hydrophilic group in the molecule, a polyisocyanate compound described below, and, if necessary, a chain extender and a polymerization terminator. If necessary, the chain extender and the polymerization terminator may be used.

[0102] Examples of polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyacrylate polyols, and polyhydric alcohols. These may be used alone or in combination of two or more. Among these, the polyol may be a polyether polyol.

[0103] Examples of compounds having an active hydrogen group and a hydrophilic group in the molecule include compounds containing an active hydrogen and an anionic group, a cationic group, or a nonionic hydrophilic group. The anionic group includes an anionic group and an anion-forming group. The anion-forming group is a group that can form an anionic group by reacting with a base, and specifically, the anionic group is formed by neutralizing with a base before, during, or after the urethanization reaction.

[0104] Compounds containing active hydrogen and anionic groups are described, for example, in JP-B-42-24192 and JP-B-55-41607, and specific examples thereof include α,α-dimethylolpropionic acid and α,α-dimethylolbutyric acid. Compounds having active hydrogen and cationic groups are described, for example, in JP-B-43-9076. Compounds having active hydrogen and nonionic hydrophilic groups are described, for example, in JP-B-43-41718, and specific examples thereof include polyethylene glycol and alkyl alcohol alkylene oxide adducts.

[0105] (hardening agent) Examples of curing agents include blocked isocyanate compounds, melamine resins, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more. The curing agent may be a melamine resin, blocked isocyanate compound, carbodiimide compound, or melamine resin.

[0106] The ratio of the crosslinking agent to the resin solids mass of the first aqueous base coating composition is, for example, 10 to 50 mass%. The ratio of the crosslinking agent may be 15 mass% or more. The ratio of the aqueous resin may be 40 mass% or less.

[0107] Melamine resin Melamine resin has three nitrogen atoms (N) around the triazine ring (triazine nucleus). 1 ~N 3 Six substituents R 1 ~R 6 The structure in which -N 1 (R 1 )(R 2 ), -N 2 (R 3 )(R 4 ), -N 3 (R 5 )(R 6 )) is included.

[0108] The melamine resin is, for example, a compound represented by the following general formula (1):

[0109] [ka] (In the formula, substituent R 1 ~R 6 each independently represents a hydrogen atom, an alkyl ether group, a methylol group, or a bonding moiety to another triazine ring. It is expressed as:

[0110] Alkyl ether (-CH2-OR 7 ) consisting of alkyl groups (R 7 The number of carbon atoms in R may be 1 to 8, or may be 1 to 4. 7 R may be linear or branched. 7 may be a methyl group, an ethyl group, a propyl group, or a butyl group.

[0111] The melamine resin may be composed of a polynuclear compound in which a plurality of triazine rings are bonded together, or may be a mononuclear compound consisting of one triazine ring.

[0112] Examples of melamine resins include -N(-CH2-OR 7 )(-CH2OH) with methylol group; -N(-CH2-OR 7 )(H) with imino group type; -N(-CH2-OR 7 )(-CH2OH) and -N(-CH2-OR 7 ) (H) with methylol / imino group type, substituent R 1 ~R 6 Examples of the alkyl ether group include a full alkyl type having only alkyl ether groups.

[0113] The Mw of the melamine resin may be 400 or more and 2000 or less. This keeps the viscosity of the first aqueous base coating composition low, improving the appearance of the coating film. The Mw of the melamine resin may be 1500 or less, or 1300 or less. The Mw of the melamine resin may be 500 or more, or 600 or more.

[0114] Blocked isocyanate compounds The blocked isocyanate compound can be prepared by blocking a polyisocyanate compound with a blocking agent.

[0115] The polyisocyanate compound has at least two isocyanate groups per molecule. Examples of polyisocyanate compounds include aliphatic polyisocyanates, alicyclic polyisocyanates, aliphatic polyisocyanates having aromatic rings not bonded to isocyanate groups in the molecule (araliphatic polyisocyanates), aromatic polyisocyanates, and derivatives of these polyisocyanates. Specific examples include aromatic polyisocyanates such as tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate, and metaxylylene diisocyanate; aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate; and polymers of these compounds, such as biuret, nurate, and adduct types. These compounds may be used alone or in combination of two or more.

[0116] Examples of blocking agents include monovalent alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenol carbinol, and methylphenyl carbinol; cellosolves such as ethylene glycol monohexyl ether and ethylene glycol mono-2-ethylhexyl ether; polyether-type diols terminated at both ends such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol phenol; polyester-type polyols terminated at both ends obtained from diols such as ethylene glycol, propylene glycol, and 1,4-butanediol and dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, suberic acid, and sebacic acid; phenols such as para-t-butylphenol and cresol; oximes such as dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime, and cyclohexanone oxime; and lactams represented by ε-caprolactam and γ-butyrolactam. As the blocking agent, active hydrogen compounds such as methyl diketone, methyl ketoester and methyl diester compounds, for example, alkyl esters such as acetylacetone, ethyl acetoacetate, diethyl malonate, etc. Also, blocked isocyanates using pyrazole compounds or imidazole compounds as blocking agents may be used.

[0117] The blocking rate of the blocked isocyanate compound is preferably 100%, which improves the storage stability of the first base coating composition.

[0118] Carbodiimide compounds The carbodiimide compound has at least two carbodiimide groups (-N=C=N-) in the molecule.

[0119] Examples of the carbodiimide compound include poly(4,4'-diphenylmethanecarbodiimide), poly(3,3'-dimethyl-4,4'-biphenylmethanecarbodiimide), poly(tolylcarbodiimide), poly(p-phenylenecarbodiimide), poly(m-phenylenecarbodiimide), poly(3,3'-dimethyl-4,4'-diphenylmethanecarbodiimide), poly(naphthylenecarbodiimide), poly(1,6-hexamethylenecarbodiimide), poly(4,4'-methylenebiscyclohexylcarbodiimide), poly(1,4-tetramethylenecarbodiimide), poly(1,3-cyclohexylenecarbodiimide), poly(1, Examples of suitable polyisopropylphenylene carbodiimide include poly(1,3-diisopropylphenylene carbodiimide), poly(1-methyl-3,5-diisopropylphenylene carbodiimide), poly(1,3,5-triethylphenylene carbodiimide), and poly(triisopropylphenylene carbodiimide).

[0120] (First bright pigment) The first bright pigment is not particularly limited and may be at least one selected from the group consisting of aluminum flake pigments, metal oxide-coated alumina flake pigments, metal oxide-coated silica flake pigments, graphite pigments, interference mica pigments, colored mica pigments, titanium metal flake pigments, stainless steel flake pigments, plate-like iron oxide pigments, metal-plated glass flake pigments, metal oxide-coated plated glass flake pigments, hologram pigments, and flake pigments made of cholesteric liquid crystal polymers.

[0121] First bright pigment PWC G1 The PWC is 1-25%. G1 If the PWC is 1% or more, sufficient concealment is achieved. G1 When the PWC content is 25% or less, good coating film properties can be obtained. G1 The PWC may be 2.0% or more, or 5.0% or more. G1 may be 20.0% or less, may be 15.0% or less, or may be 10.0% or less.

[0122] The average particle size of the first bright pigment is not particularly limited, and may be, for example, 8 to 25 μm, in order to easily improve the brightness.

[0123] (First color pigment) The first color pigment contains two or more color pigments of a similar color to the second color pigment. That is, the first aqueous base coating composition contains at least a color pigment X' of a similar color to the color pigment X contained in the second aqueous base coating composition, and a color pigment Y' of a similar color to the color pigment Y (not similar in color to the color pigment X) contained in the second aqueous base coating composition. This reduces the difference in hue between the multilayer coating film (corresponding to the outer panel part) and the first base coating film (corresponding to the inner panel part). Three or more color pigments of a similar color may be contained.

[0124] The first color pigment is not particularly limited. Examples of the first color pigment include organic pigments such as azo lake pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, phthalone pigments, dioxazine pigments, quinacridone pigments, isoindolinone pigments, benzimidazolone pigments, diketopyrrolopyrrole pigments, and metal complex pigments; and inorganic pigments such as yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These pigments may be used in combination of two or more.

[0125] First color pigment PWC C1 The PWC is 0.1 to 5.0%. C1 If the PWC content is 0.1% or more, the hue of the first base coating film can be easily adjusted, and the color difference between the first base coating film and the multi-layer coating film can be easily reduced. C1 When the PWC is 5.0% or less, the flip-flop properties of the first base coating film can be improved. C1 The PWC may be 0.3% or more, or may be 0.5% or more. C1 may be 4.0% or less, or may be 3.0% or less.

[0126] (extender pigment) The first aqueous base coating composition may contain an extender pigment. Examples of the extender pigment include talc, calcium carbonate, precipitated barium sulfate, and silica. The PWC of the extender pigment is not particularly limited.

[0127] (solvent) The first aqueous base coating composition contains water as a solvent. The first aqueous base coating composition may contain an organic solvent in addition to the aqueous solvent. In the first aqueous base coating composition, the proportion of water in the solvent is, for example, 50 mass % or more, 70 mass % or more, or 90 mass % or more.

[0128] Examples of the organic solvent include hydrocarbons such as toluene and xylene, ketones such as acetone and methyl ethyl ketone, esters such as ethyl acetate, butyl acetate, cellosolve acetate and butyl cellosolve, and alcohols. Among these, hydrophilic organic solvents such as alcohols may be used.

[0129] Examples of hydrophilic organic solvents include methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, sec-butyl alcohol, tert-butyl alcohol, ethylene glycol, 1,2-propylene glycol, 1,3-butylene glycol, 2,3-butylene glycol, hexylene glycol, 2,5-hexanediol, and dipropylene glycol.

[0130] (others) The first aqueous base coating composition may further contain additives such as anti-settling agents, curing catalysts, ultraviolet absorbers, antioxidants, leveling agents, surface conditioners such as silicones and organic polymers, anti-sagging agents, thickeners, defoamers, pigment dispersants, lubricants, and crosslinkable polymer particles (microgels).

[0131] (Preparation method) The first aqueous base coating composition is prepared by mixing the above components by a known method. If necessary, the first aqueous base coating composition is diluted with a solvent such as water and then used for coating.

[0132] Water-based second base coating composition The second aqueous base coating composition contains, for example, the same aqueous resin, curing agent, second luster pigment, and second color pigment as the first aqueous base coating composition. The second aqueous base coating composition may optionally contain the same extender pigment and additives as the first aqueous base coating composition.

[0133] The second color pigment contains two or more color pigments of similar colors to the first color pigment, and may contain three or more color pigments of similar colors.

[0134] The solid content concentration NV2 of the second aqueous base coating composition is 7.0 to 30.0% by mass. When the solid content concentration NV2 is 7.0% by mass or more, coating efficiency is improved. When the solid content concentration NV2 is 30.0% by mass or less, FF properties are improved. The solid content concentration NV2 may be 15.0% by mass or more. The solid content concentration NV2 may be 20.0% by mass or less.

[0135] The second aqueous base coating composition may contain a wettability improver such as a fluorine-containing compound or a silicone compound. This reduces the surface tension of the second aqueous base coating composition, improving its wettability to the first base coating film. The amount of the wettability improver may be 0.1 to 5.0 parts by mass per 100 parts by mass of the resin solids content of the second aqueous base coating composition.

[0136] Examples of the fluorine-containing compound include fluorinated alkyl carboxylates, fluorinated alkyl alkoxylates, and fluorinated alkyl esters.

[0137] Examples of silicone-based additives include polyether-modified polymethylalkylsiloxane, polyether-modified polydimethylsiloxane, polyester-modified polymethylalkylsiloxane, silicone-modified polyacrylic, and aralkyl-modified polymethylalkylsiloxane.

[0138] (PWC of the second photoluminescent pigment G2 ) Secondary photoluminescent pigment PWC G2 PWC is 5.0 to 35.0%. G2 If the content of PWC is 5.0% or more, sufficient brightness can be obtained. G2 When the PWC content is 35.0% or less, a good appearance can be obtained. G2 The PWC may be 7.0% or more, or 9.0% or more. G2 may be 30% or less, or may be 25.0% or less.

[0139] The average particle size of the second bright pigment is not particularly limited. In terms of facilitating an improvement in brightness, the average particle size of the second bright pigment may be, for example, 8.0 to 25 μm.

[0140] The difference between the average particle size of the first bright pigment and the average particle size of the second bright pigment may be 5.0 μm or less. This can further reduce the difference in hue. The difference in average particle size may be 3.0 μm or less, 1.0 μm or less, or 0 μm.

[0141] (PWC of the second color pigment C2 ) Secondary color pigment PWC C2 PWC is 0.5 to 20.0%. C2 If the PWC content is 0.5% or more, the hue of the second base coating film can be easily adjusted, and the color difference between the second base coating film and the multi-layer coating film can be easily reduced. C2 When the PWC content is 20.0% or less, the flip-flop properties of the multi-layer coating film can be improved. C2 The PWC may be 1.0% or more, or 1.5% or more. C2 may be 10.0% or less, may be 7.0% or less, or may be 5.0% or less.

[0142] Clear paint composition The clear coating composition may be solvent-based, water-based, or powder-type. The clear coating composition may be solvent-based. The clear coating composition may contain the above-mentioned color pigments to the extent that transparency is not impaired.

[0143] The clear coating composition may be, for example, an acid-epoxy curing type containing a polyepoxide and a polycarboxylic acid, or a urethane curing type containing a hydroxyl group-containing resin and a polyisocyanate compound. The urethane curing type clear coating composition may be a two-component type.

[0144] The acid-epoxy curing clear coating composition contains, for example, (a) an acid anhydride group-containing aqueous acrylic resin, (b) a carboxyl group-containing polyester resin, and (c) an aqueous acrylic resin having hydroxyl groups and epoxy groups. From the viewpoint of storage stability, the acid anhydride groups of (a) the acid anhydride group-containing aqueous acrylic resin may be half-esterified with a low-molecular-weight alcohol, etc. The (b) carboxyl group-containing polyester resin may further contain hydroxyl groups.

[0145] The resins (a) to (c) are blended, for example, so that the molar ratio of the carboxyl groups contained in the (a) aqueous acrylic resin and the (b) polyester resin to the epoxy groups contained in the (c) aqueous acrylic resin is 1 / 1.4 to 1 / 0.6 (preferably 1 / 1.2 to 1 / 0.8), and so that the molar ratio of the carboxyl groups derived from the acid anhydride groups contained in the (a) aqueous acrylic resin to the hydroxyl groups contained in the (b) polyester resin and the (c) aqueous acrylic resin is 1 / 2.0 to 1 / 0.5 (preferably 1 / 1.5 to 1 / 0.7).

[0146] The urethane-curing clear coating composition contains, for example, a hydroxyl-containing resin and a polyisocyanate compound. Examples of the polyisocyanate compound include those listed above. Examples of the hydroxyl-containing resin include polyester resins, polyurethane resins, aqueous acrylic resins, and polyols containing hydroxyl groups.

[0147] The OHV of the hydroxyl-containing resin is, for example, 20 mgKOH / g or more and 200 mgKOH / g or less. The OHV may be 30 mgKOH / g or more. The OHV may be 180 mgKOH / g or less. The Mw of the hydroxyl-containing resin is, for example, 1,000 or more and 20,000 or less. The Mw may be 2,000 or more. The Mw may be 15,000 or less. The AV of the hydroxyl-containing resin is, for example, 0 mgKOH / g or more and 30 mgKOH / g or less. The AV may be 25 mgKOH / g or less.

[0148] The hydroxyl group-containing resin and the polyisocyanate compound are blended, for example, so that the equivalent ratio (NCO / OH) of the isocyanate group (NCO) to the hydroxyl group (OH) is 0.5 or more and 1.7 or less. The equivalent ratio may be 0.7 or more. The equivalent ratio may be 1.5 or less.

[0149] Alternatively, an acrylic melamine curing type clear coating composition may be used. The clear coating composition may be a commercially available product.

[0150] The resin solid content concentration of the clear coating composition is not particularly limited and is set appropriately depending on the coating conditions. The resin solid content concentration of the clear coating composition is, for example, 40 to 70 mass %.

[0151] (subject to be coated) The material of the substrate is not particularly limited, and examples of the substrate material include metal, resin, and glass.

[0152] The shape of the substrate is not particularly limited, and specific examples of the substrate include automobile bodies such as passenger cars, trucks, motorcycles, and buses, and automobile body parts, as well as automobile parts such as spoilers, bumpers, mirror covers, grilles, and door knobs.

[0153] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of metal substrates include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0154] The metal substrate may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After the surface treatment, the metal substrate may be further coated with an electrodeposition paint. The electrodeposition paint may be either a cationic type or an anionic type.

[0155] Examples of resins include polyethylene resin, EVA resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), vinyl chloride resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), polycarbonate resin, acrylic resin, acrylonitrile butadiene styrene (ABS) resin, acrylonitrile styrene (AS) resin, polyamide resin, acetal resin, phenolic resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). Resin substrates may be degreased. Resin parts may be coated with a primer.

[0156] [Method for forming multi-layer coating film] The multilayer coating film can be formed by a method comprising applying a first aqueous base coating composition onto an object to be coated to form an uncured first base coating film, applying a second aqueous base coating composition onto the uncured first base coating film to form an uncured second base coating film, applying a clear coating composition onto the uncured second base coating film to form an uncured clear coating film, and heating the uncured first base coating film, the uncured second base coating film, and the uncured clear coating film.

[0157] The first base coating film may be cured or uncured when the second base coating composition is applied. The first base coating film may be uncured when the second base coating composition is applied.

[0158] That is, the multilayer coating film may be produced by a method (3-coat, 1-bake method) that includes the steps of sequentially applying a first base coating composition, a second base coating composition, and a clear coating composition onto an object to be coated to form an uncured first base coating film, a second base coating film, and a clear coating film in that order, and curing the uncured first base coating film, the second base coating film, and the clear coating film all at once.

[0159] Examples of coating methods include air spray coating, airless spray coating, electrostatic spray coating, multi-stage coating (preferably two-stage coating) using air electrostatic spray coating, and coating that combines air electrostatic spray coating with a rotary atomizer-type electrostatic coater.

[0160] Each coating composition is cured, for example, at a heating temperature of 80°C to 180°C for a heating time of 5 to 60 minutes (preferably 10 to 30 minutes).

[0161] Preheating may be performed between the application of the first base coating composition and the application of the second base coating composition, and between the application of the second base coating composition and the application of the clear coating composition. Preheating is performed, for example, by leaving the coating at a temperature of 20°C to 25°C for 5 to 15 minutes, or by heating at a temperature of 50°C to 80°C for 30 seconds to 10 minutes. Preheating does not need to be performed between the application of the first base coating composition and the application of the second base coating composition.

[0162] During preheating, at least a portion of the solvent is intentionally removed. "No preheating" means that no work is done to intentionally remove the solvent from the uncured first base coating film before applying the second base coating composition. Methods for intentionally removing the solvent (i.e., preheating methods) include, for example, natural drying and heat drying. In natural drying, the uncured first base coating film is left for 5 to 15 minutes at a temperature of 20°C to 25°C. Heat drying is performed under conditions that do not allow the curing reaction of the coating film-forming components to proceed, or at least do not complete the curing reaction. In heat drying, the uncured first base coating film is heated for 30 seconds to 10 minutes at a temperature of 50°C to 80°C. [Example]

[0163] Details of each component of the coating composition used in the examples are as follows: Water-based resin 1 (acrylic resin emulsion) 126.5 parts of deionized water was added to a reaction vessel, and the temperature was raised to 80°C while mixing and stirring in a nitrogen stream. The mixture was heated. Next, a monomer emulsion consisting of 100 parts of a monomer mixture of 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, 0.7 parts Aqualon HS-10 (polyoxyethylene alkylpropenylphenyl ether sulfate, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), 0.5 parts Adeka Reasoap NE-20 (α-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]-ω-hydroxyoxyethylene, manufactured by Asahi Denka Co., Ltd.), and 80 parts deionized water was added dropwise to the reaction vessel over a period of 2 hours. An initiator solution consisting of 0.3 parts ammonium persulfate and 10 parts deionized water was also added dropwise to the reaction vessel over a period of 2 hours. After the addition was completed, the mixture was aged at the same temperature for 2 hours. The mixture was then cooled to 40°C and filtered through a 400 mesh filter, after which 70 parts of deionized water and 0.32 parts of dimethylaminoethanol were added to adjust the pH to 6.5.

[0164] The obtained acrylic resin emulsion was a single-layer type with an average particle size of 150 nm, a nonvolatile content of 20%, an acid value of the solid content of 20 mg KOH / g, and a hydroxyl value of 40 mg KOH / g.

[0165] Water-based resin 2 (water-soluble acrylic resin) 23.89 parts of tripropylene glycol methyl ether and 16.11 parts of propylene glycol methyl ether were added to a reaction vessel, and the mixture was heated to 105°C while stirring under a nitrogen stream. Next, a monomer mixture containing 13.1 parts of methyl methacrylate, 68.4 parts of ethyl acrylate, 11.6 parts of 2-hydroxyethyl methacrylate, and 6.9 parts of methacrylic acid was prepared. An initiator solution consisting of 100 parts of this monomer mixture, 10 parts of tripropylene glycol methyl ether, and 1 part of tertiary butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel over a period of 3 hours. After the addition was completed, the mixture was aged at the same temperature for 0.5 hours. Furthermore, an initiator solution consisting of 5 parts of tripropylene glycol methyl ether and 0.3 parts of tertiary butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel over a period of 0.5 hours. After the addition was completed, the mixture was aged at the same temperature for 2 hours. Using a solvent remover, 16.1 parts of the solvent were distilled off under reduced pressure (70 torr) at 110° C., and then 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added.

[0166] The resulting water-soluble acrylic resin (B) had a nonvolatile content of 30.0%, an acid value of the solid content of 40 mgKOH / g, and a hydroxyl value of 50 mgKOH / g.

[0167] Water-based resin 3 (polyether polyol) Sanyo Chemical Industries, Ltd. product name "Primepol PX-1000", bifunctional polyether polyol, number average molecular weight 400, hydroxyl value 278 mg KOH / g, primary / secondary hydroxyl value ratio = 63 / 37, non-volatile content 100%

[0168] Water-based resin 4 (urethane resin emulsion) Avecia's product name "Neolet's R-9603", polycarbonate-based urethane emulsion resin, non-volatile content 33%

[0169] Hardener Mitsui Chemicals' product name "Cymel 204", mixed alkylated melamine resin, 100% non-volatile content

[0170] Photoluminescent pigment 1 Toyo Aluminum's product name "Alpaste (registered trademark) 06-0672", silver color, average particle size 17 μm Photoluminescent pigment 2 Toyo Aluminum's product name "Alpaste (registered trademark) WM-2025", silver color, average particle size 15 μm Photoluminescent pigment 3 Toyo Aluminum's product name "Alpaste (registered trademark) WL-4690", silver color, average particle size 7 μm

[0171] Color pigment 1 (color index name: P.BK.7) Birla Raven 5000 Black, black Color pigment 2 (color index name: PB15:1) Sanyo Pigment's product name "Cyanine Blue G-314" Color pigment 3 (color index name: PB15:3) "Cyanine Blue 3011" manufactured by Sanyo Pigment Co., Ltd., blue, similar in color to Coloring Pigment 2. Color pigment 4 (color index name: PV19Y) DIC's product name "FASTOGEN SUPER RED 400RG", red Color pigment 5 (color index name: P.BR.7) Lanxess product name "Bayferrox 120NM", red Color pigment 6 (color index name: PW6) Ishihara Sangyo Kaisha's product name "Tipaaque CR-97", white Coloring pigment 7 (color index name: PV23) Clariant's product name "Violet RL-NF", violet

[0172] Tertiary amine (neutralizer) Dimethylethanolamine 10% by mass aqueous solution

[0173] Clear paint Nippon Paint Automotive Coatings' Macflow O-1810 Clear, an acid-epoxy curing acrylic resin paint.

[0174] ·Base material Zinc phosphate treated SPCC-SD steel plate, dimensions 20cm x 30cm x thickness 0.8cm

[0175] ·Multi-angle colorimeter BYK's product name "BYK-mac i"

[0176] [Examples 1 to 6 and Comparative Examples 1 to 5] (1) Preparation of first base coating composition (1bc) In addition to the bright pigment and color pigment in the formulation (PWC) shown in Tables 1 and 2, the following amounts of resin component, curing agent and tertiary amine were blended and uniformly dispersed to obtain a first base coating composition.

[0177] Water-based resin 1: 200 parts by weight (non-volatile content 40 parts by weight) Water-based resin 2: 16.7 parts by weight (non-volatile content 5 parts by weight) Water-based resin 3: 10 parts by weight (non-volatile content 10 parts by weight) Water-based resin 4:5 parts by mass (non-volatile content 1.65 parts by mass) Hardener: 15.2 parts by weight (non-volatile content 15.2 parts by weight) Tertiary amine: 40 parts by mass (concentration 10% by mass) Corrosion inhibitor: 0.2 parts by weight (non-volatile content 0.1 parts by weight)

[0178] (2) Preparation of second base coating composition (2bc) The luster pigment and color pigment were blended as shown in Tables 1 and 2, and the same amounts of resin component, curing agent, and tertiary amine as in the first base coating composition were blended and uniformly dispersed to obtain a second base coating composition. The solid content concentration was adjusted by the amount of solvent.

[0179] (3) Formation of multi-layer coating The substrate (steel plate) was coated with a cationic electrodeposition paint (product name "Power Top V-50", manufactured by Nippon Paint Automotive Coatings) and a primer paint (product name "OP-30", manufactured by Nippon Paint Automotive Coatings) to prepare the coated object. The first base coating composition was applied to the substrate with a spray gun to a dry film thickness of 12 μm, followed by the second base coating composition with a spray gun to a dry film thickness of 4 μm, and the coating was preheated at 80°C for 3 minutes.

[0180] Next, a clear coating composition was applied to the substrate to a dry film thickness of 35 μm, and finally, the substrate was heated at 140° C. for 30 minutes to form a multi-layer coating film having a first base coating film, a second base coating film, and a clear coating film.

[0181] Comparative Example 6 A first base coating composition and a second base coating composition were prepared in the same manner as in Example 1, except that the luster pigment and color pigment were blended as shown in Tables 1 and 2.

[0182] A multi-layer coating film was obtained in the same manner as in Example 1, except that the obtained first base coating composition and second base coating composition were both applied so as to give a dry film thickness of 7.5 μm.

[0183] [evaluation] Lightness and color difference A first base coating for evaluation was prepared as follows. First, a substrate (steel plate) was coated with a cationic electrodeposition paint (trade name "Power Top V-50", manufactured by Nippon Paint Automotive Coatings) and an undercoat paint (trade name "OP-30", manufactured by Nippon Paint Automotive Coatings) to prepare a coating object. The first base coating composition was applied to the substrate with a spray gun to a dry film thickness of 10 μm, and the substrate was preheated at 80°C for 3 minutes to form an uncured first base coating film. Next, the above-mentioned clear coating composition was applied to a dry film thickness of 10 μm. Finally, the substrate was heated at 140°C for 30 minutes.

[0184] Using a multi-angle colorimeter, the Lbc of the first base coating for evaluation * 25 value, Lbc * 45 value, Ebc * 25 and Ebc * 45 value and the Lm of the above multi-layer coating film * 25 values, Lm * 45 values, color difference Em * 25 values ​​and color difference Em * 45 values ​​were measured, and ΔL * 25, ΔL * 45, ΔE * 25 and ΔE * 45 was calculated.

[0185] ·FF nature Light I was irradiated from a 45-degree angle onto the surface of the multi-layer coating using a multi-angle colorimeter. 45 The light is received at an angle of 15 degrees or 75 degrees to the specular reflection light, and the brightness Lm * 15,Lm * 75) were measured. From these values, the ratio (Lm * 15 / Lm * 75) was calculated, and the FF property was evaluated according to the following criteria. * 15 / Lm * 75), the larger the value, the better the FF. A: Ratio (Lm * 15 / Lm * 75) is 4 or more B: Ratio (Lm * 15 / Lm * 75) is less than 4

[0186] Dust streaks An inner panel test panel 1 corresponding to an inner panel and an outer panel test panel corresponding to an outer panel (an outer panel test panel 2A intended for a front door and an outer panel test panel 2B intended for a rear door) were prepared as follows.

[0187] (a) Preparation of inner plate test panel 1 Three substrates were prepared by coating a substrate (steel plate) with a cationic electrodeposition paint (trade name "Power Top V-50", manufactured by Nippon Paint Automotive Coatings) and an undercoat paint (trade name "OP-30", manufactured by Nippon Paint Automotive Coatings). The first base paint composition was applied to one of the substrates with a spray gun to a dry film thickness of 10 μm, forming an uncured first base coating film. This was designated inner panel test panel 1, corresponding to the inner panel.

[0188] (b) Preparation of outer panel test panels Next, as shown in Figure 1, another object A to be coated was placed above the inner panel test panel 1 so that the height from the surface of the inner panel test panel 1 to the surface of object A was 25 mm. Another object B to be coated was placed so that the height from the surface of the inner panel test panel 1 to the surface of object B was 10 mm. The distance between object A and object B was 10 mm.

[0189] Next, under standard robotic coating conditions, the first base coating composition and the second base coating composition were applied in sequence from the left end of substrate A to the right end of substrate B in Figure 1, to dry film thicknesses of 12 μm and 4 μm, respectively. The coatings were then preheated at 80°C for 3 minutes. A clear coating composition was then applied to a dry film thickness of 30 μm. Finally, the coatings were baked at 140°C for 30 minutes to form multi-layer coatings comprising the first base coating, the second base coating, and the clear coating on substrate A and substrate B. An exterior panel test panel 2A, simulating a front door, was prepared from substrate A, and an exterior panel test panel 2B, simulating a rear door, was prepared from substrate B.

[0190] The surface of the inner panel test panel 1 was visually inspected and the dust streaks were evaluated according to the following criteria: A rating indicates that the difference in hue between the first base coating film and the multi-layer coating film is reduced. A: Dust streaks can be seen slightly depending on the viewing angle, but there is nothing unusual about them. B: Dust streaks can be seen depending on the observation angle, creating a sense of incongruity C: Dust streaks are easily visible and there is a clear sense of incongruity.

[0191] [Table 1]

[0192] [Table 2]

[0193] In Examples 1 to 6, the difference in hue between the first base coating film and the multi-layer coating film was small, the occurrence of dust streaks was reduced, and the FF properties were also excellent. In Comparative Example 1, the PWC ratio of the bright pigment was outside the specified range, and dust streaks were observed. In Comparative Examples 2 and 3, the PWC ratio of the color pigment was outside the specified range, and dust streaks were observed. In Comparative Examples 4 and 5, dust streaks were observed because only one type of color pigment of a similar color was used or no color pigment was included. In Comparative Example 6, the first base coating composition and the second base coating composition were the same, and therefore the FF properties were poor. [Industrial Applicability]

[0194] The multilayer coating film of the present invention provides a multilayer coating film in which the difference in hue between the first base coating film and the multilayer coating film is small, and in addition, a high flip-flop property is obtained. The multilayer coating film of the present invention is suitable for painting the outer panel parts of an automobile body when the same first base paint composition is used to form the first base coating film on the outer panel parts and the inner panel parts. [Explanation of symbols]

[0195] A,B Coated object 1. Inner plate test panel 2A outer panel test panel 2B outer panel test panel

Claims

1. a first base coating film formed on an object to be coated from an aqueous first base coating composition; A second base coating film formed on the first base coating film using an aqueous second base coating composition; A clear coating film formed on the second base coating film using a clear coating composition, The first aqueous base coating composition contains a luster pigment and two or more color pigments, The solid content concentration NV1 is 20 to 35% by mass, PWC of the bright pigment Ｇ１ is 1 to 25%, PWC of the color pigment Ｃ１ is 0.1 to 5.0%, The second aqueous base coating composition comprises a brilliant pigment and two or more color pigments having the same color index name as the color pigment contained in the first aqueous base coating composition, The solid content concentration NV2 is 7 to 30 mass%; PWC of the bright pigment Ｇ２ is 5 to 35%, PWC of the color pigment Ｃ２ is 0.5 to 20.0%, the ratio (T1 / T2) of the thickness T1 of the first base coating film to the thickness T2 of the second base coating film is 1.5 / 1 to 5 / 1; the ratio (NV1 / NV2) of the solid content concentration NV1 to the solid content concentration NV2 is 1.3 / 1 to 3 / 1; PWC of the bright pigment Ｇ１ and the PWC of the bright pigment Ｇ２ Ratio to (PWC Ｇ１ / PWC Ｇ２ ) is 1 / 10 to 1 / 1, PWC of the color pigment Ｃ１ and the color pigment PWC Ｃ２ Ratio to (PWC Ｃ１ / PWC Ｃ２ ) is 1 / 5 to 1 / 1.

2. Light I irradiated at a 45 degree angle to the coating surface ４５ The brightness L is based on the spectral reflectance of light received at an angle of 25 degrees relative to the specular reflection light. ＊ 25 value, and the lightness L ＊ 25 values ​​and a ＊ 25 value and b ＊ 25 values ​​and the color difference E calculated from ＊ In 25 values, Lightness Lm of the multi-layer coating film ＊ 25 value to the lightness Lbc of the first base coating film ＊ The value ΔL obtained by subtracting 25 ＊ 25 is 0 to 10, The color difference Em of the multi-layer coating film ＊ The color difference Ebc between the .25 value and the first base coating film ＊ Difference from 25 ΔE ＊ 25 is 10 or less, Light I irradiated at a 45 degree angle to the coating surface ４５ The brightness L is based on the spectral reflectance of light received at an angle of 45 degrees to the specular reflected light. ＊ 45 value, and the lightness L ＊ 45 values ​​and a ＊ 45 value and b ＊ 45 values ​​and the color difference E calculated from ＊ At 45 values, Lightness Lm of the multi-layer coating film ＊ 45 value to the lightness Lbc of the first base coating film ＊ The value obtained by subtracting 45 is ΔL ＊ 45 is −10 to 0, The color difference Em of the multi-layer coating film ＊ 45 value and the color difference Ebc of the first base coating film ＊ Difference from 45 ΔE ＊ 45 is 10 or less, The multi-layer coating film according to claim 1.

3. The difference between the average particle diameter of the luster pigment contained in the first aqueous base coating composition and the average particle diameter of the luster pigment contained in the second aqueous base coating composition is 5 μm or less. A multilayer coating film as described in claim 1 or 2.

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