Method for forming multilayered coating film

Abstract

Provided is a method for forming a multilayered coating film, the method comprising: applying a thermosetting intermediate coating composition (A) on an object to be coated; after applying the intermediate coating composition (A), applying a first clear coating composition (C1) that is thermosetting and colored, by using a dispenser; heating and curing the intermediate coating composition (A) to obtain an intermediate coating film; and heating and curing the first clear coating composition (C1) to obtain a colored first clear coating film. The first clear coating composition (C1) is solvent-based, and the low shear viscosity LSV measured under conditions of a temperature of 23°C and the shear rate of 0.1 sec-1 is 1 Pa·s or more and 40 Pa·s or less.

Description

Method for forming multi-layer coating film

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

[0002] Traditionally, automobile bodies and parts have been spray painted. In spray painting, the paint is atomized and flows with the air, adhering to the object to be painted. Therefore, the coating efficiency of spray painting is low, generally said to be between 50% and 70%.

[0003] In view of recent environmental issues, efforts have been made to improve coating efficiency. For example, Patent Documents 1 and 2 disclose devices capable of applying droplets of paint to automobile parts.

[0004] JP 2016-144805 A JP 2020-22965 A

[0005] When a paint is applied in droplet form, sagging tends to occur. An object of the present invention is to provide a method for forming a multi-layer coating film using dispense coating that can suppress the occurrence of sagging.

[0006] In order to solve the above problems, the present invention provides the following aspects. The present disclosure includes the following aspects: [1] A method for forming a multi-layer coating film, comprising: applying a thermosetting intermediate coating composition (A) to a substrate; after applying the intermediate coating composition (A), applying a thermosetting, colored first clear coating composition (C1) using a dispenser; heat-curing the intermediate coating composition (A) to obtain an intermediate coating film; and heat-curing the first clear coating composition (C1) to obtain a colored first clear coating film, wherein the first clear coating composition (C1) is solvent-based and has a low shear viscosity LSV of 1 Pa s or more and 40 Pa s or less, measured under conditions of a temperature of 23°C and a shear rate of 0.1 sec-1. [2] The method for forming a multilayer coating film of [1] above, wherein the first clear coating composition (C1) is applied before the intermediate coating composition (A) is heat-cured, and the intermediate coating composition (A) and the first clear coating composition (C1) are heat-cured simultaneously. [3] The method for forming a multilayer coating film of [1] above, wherein the intermediate coating composition (A) is heat-cured to obtain an intermediate coating film, and then the first clear coating composition (C1) is applied. [4] The method for forming a multilayer coating film of any of [1] to [3] above, further comprising applying an aqueous base coating composition (B) after applying the intermediate coating composition (A) and before applying the first clear coating composition (C1), and heat-curing the aqueous base coating composition (B) to obtain a base coating film. [5] The method for forming a multilayer coating film according to any one of the above [1] to [4], further comprising the steps of: applying an aqueous base coating composition (B) after applying the intermediate coating composition (A) and before applying the first clear coating composition (C1); applying a second clear coating composition (C2) after applying the aqueous base coating composition (B) and before applying the first clear coating composition (C1); and heat-curing the second clear coating composition (C2) to obtain a second clear coating film.[6] The method for forming a multilayer coating film according to any one of [1] to [4] above, further comprising the steps of applying a second clear coating composition (C2) after applying the first clear coating composition (C1), and heat-curing the second clear coating composition (C2) to obtain a second clear coating film. [7] The temperature of the first clear coating composition (C1) is 23°C and the shear rate is 1000 sec. －１ [8] The method for forming a multilayer coating film according to any of [1] to [6] above, wherein the high shear viscosity HSV measured under the conditions is 0.009 Pa·s or more and 0.40 Pa·s or less. [8] The method for forming a multilayer coating film according to any of [1] to [7] above, wherein the solids concentration of the first clear coating composition (C1) is 20 mass% or more and 60 mass% or less. [9] The method for forming a multilayer coating film according to any of [1] to [8] above, wherein the rate of decrease in the solids concentration 60 seconds after application of the first clear coating composition (C1) is 10 mass% or less relative to the solids concentration of the first clear coating composition (C1).

[10] The method for forming a multilayer coating film according to any of [1] to [9] above, wherein the diameter of the nozzle of the dispenser is 50 μm or more and 400 μm or less.

[11] The method for forming a multilayer coating film according to any one of [1] to

[10] above, wherein the first clear coating composition (C1) contains at least one selected from the group consisting of color pigments, luster pigments, and dyes.

[12] The method for forming a multilayer coating film according to any one of [1] to

[11] above, wherein the first clear coating composition (C1) contains at least one selected from the group consisting of polyamide-based viscosifying agents, cellulose-based viscosifying agents, polyolefin-based viscosifying agents, polyurea-based viscosifying agents, and organic resin particulate viscosifying agents.

[0007] According to the present invention, a method for forming a multi-layer coating film that can suppress the occurrence of sagging is provided using a dispenser with high coating efficiency.

[0008] FIG. 1 is a flowchart showing a method for forming a multi-layer coating film according to a first embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view showing a multi-layer coating film according to a second embodiment of the present disclosure. FIG. 3 is a schematic cross-sectional view showing a multi-layer coating film according to a third embodiment of the present disclosure. FIG. 4 is a schematic cross-sectional view showing a multi-layer coating film according to a third embodiment of the present disclosure. FIG. 5 is a flowchart showing a method for forming a multi-layer coating film according to a fourth embodiment of the present disclosure. FIG. 6 is a schematic cross-sectional view showing a multi-layer coating film according to a fourth embodiment of the present disclosure. FIG. 7 is a schematic cross-sectional view showing a multi-layer coating film according to a fifth embodiment of the present disclosure. FIG. 8 is a schematic cross-sectional view showing a multi-layer coating film according to a fifth embodiment of the present disclosure. FIG. 9 is a flowchart showing a method for forming a multi-layer coating film according to a sixth embodiment of the present disclosure. FIG. 10 is a flowchart showing a method for forming a multi-layer coating film according to a seventh embodiment of the present disclosure. FIG. 11 is a flowchart showing a method for forming a multi-layer coating film according to an eighth embodiment of the present disclosure.

[0009] Coating using a dispenser (hereinafter referred to as "dispense coating") is a method in which a liquid coating composition is discharged from a nozzle at a fixed amount toward the substrate. In dispense coating, the coating composition is discharged in the liquid form, such as droplets or a liquid column, rather than as a mist. This results in high coating efficiency and reduced dust compared to conventional spray coating.

[0010] In spray coating, when the coating composition becomes atomized, the solvent contained therein evaporates, increasing the solids concentration and increasing the viscosity of the coating composition after dispensing. Therefore, sagging is less likely to occur. On the other hand, in dispense coating, the coating composition is dispensed in liquid form, so sagging is more likely to occur. If the coating composition is prone to sagging, multiple applications are required, which reduces coating workability and causes uneven film thickness, resulting in a poor appearance.

[0011] In dispense coating, the coating composition is required to have a low viscosity before being discharged from a nozzle, but to prevent sagging, it is required to have a high viscosity after being discharged. However, in dispense coating, the coating composition is discharged in a liquid state, so the solids concentration of the coating composition hardly changes before and after being discharged. In other words, if the coating composition is adjusted to a solids concentration suitable for discharging, the viscosity is low and sagging is likely to occur.

[0012] Paints are generally non-Newtonian fluids. The viscosity of non-Newtonian fluids such as paints (especially thixotropic fluids) changes depending on the shear rate applied during dispensing. In the case of dispense coating, where the solid content of the paint composition changes little before and after dispensing, it has been found that controlling the shear viscosity of the paint composition before dispensing is important. The shear viscosity corresponding to the shear viscosity is calculated as follows: －１ It has been found that by setting the low shear viscosity LSV measured under the above conditions to 1 Pa·s or more and 40 Pa·s or less, it is possible to achieve both dischargeability suitable for dispense coating and suppression of sagging.

[0013] In dispense coating, the coating composition is usually applied at a low shear rate of 0.1 sec －１ Therefore, even if the low shear viscosity LSV is increased as described above, it is unlikely to affect the ejection properties.

[0014] The multilayer coating film, which is one aspect of the present disclosure, will be described in detail below with reference to the illustrated embodiments. Note that the drawings include some schematic views and may not reflect actual dimensions or proportions. The present disclosure is not limited to these embodiments.

[0015] [First embodiment] The method for forming a multi-layer coating film of this embodiment comprises applying a thermosetting intermediate coating composition (A) to a substrate, applying a thermosetting and colored first clear coating composition (C1) using a dispenser after applying the intermediate coating composition (A), heat-curing the intermediate coating composition (A) to obtain an intermediate coating film, and heat-curing the first clear coating composition (C1) to obtain a colored first clear coating film. The first clear coating composition (C1) is applied before heat-curing the intermediate coating composition (A), and heat-curing of the intermediate coating composition (A) and the first clear coating composition (C1) is carried out simultaneously (in the same step).

[0016] 1 is a flowchart showing an example of a method for forming a multi-layer coating film according to the first embodiment. First, a thermosetting, solvent-based intermediate coating composition (A) is applied to a substrate (S11), followed by preliminary drying (S12). Next, a thermosetting, colored first clear coating composition (C1) is applied using a dispenser (S13). After that, heating for curing ("heat curing", hereinafter also referred to as "baking") is performed (S14). By a single baking process, both the intermediate coating composition (A) (i.e., the uncured intermediate coating film) and the first clear coating composition (C1) are cured.

[0017] The method of the first embodiment forms a multilayer coating film comprising an intermediate coating film disposed on an object to be coated and a colored first clear coating film (hereinafter also referred to as "colored clear coating film") disposed on the intermediate coating film. Figure 2 is a schematic cross-sectional view showing the multilayer coating film 10 in the first embodiment. The multilayer coating film 10 comprises an intermediate coating film 12 disposed on an object to be coated 11 and a colored clear coating film 13 disposed on the intermediate coating film 12.

[0018] First, the coating compositions used in each step will be described.

[0019] Hereinafter, the weight average molecular weight Mw and the number average molecular weight Mn are measured using polystyrene standards by GPC (gel permeation chromatography) method.

[0020] The hydroxyl value and acid value are determined based on the mass of the solid content. The hydroxyl value and acid value can be measured by a known method described in JIS K 0070:1992. The hydroxyl value and acid value may be calculated from the blending amount of unsaturated monomers in the raw material monomers of the target resin.

[0021] The epoxy equivalent can be measured by a known method described in JIS K 7236:2001.

[0022] The solid content of a coating composition is the total content of the coating composition excluding volatile components (typically, solvents). The solid content concentration of a coating composition can 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.

[0023] 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.

[0024] (First clear coating composition (C1)) The first clear coating composition (C1) is a solvent-based coating composition. A solvent-based coating composition contains an organic solvent as a solvent.

[0025] The first clear coating composition (C1) may be a one-component coating, or may be a multi-component coating such as a two-component coating containing a base agent and a curing agent. The base agent and curing agent constituting the two-component coating may be mixed and then supplied to a nozzle, or may be supplied to the same nozzle and mixed while being discharged.

[0026] The low shear viscosity LSV of the first clear coating composition (C1) is 1 Pa·s or more and 40 Pa·s or less, thereby suppressing sagging of the first clear coating composition (C1) after discharge (after application to the substrate).

[0027] The low shear viscosity LSV of the first clear coating composition (C1) may be 3 Pa s or more, or may be 6 Pa s or more. The low shear viscosity LSV of the first clear coating composition (C1) may be 30 Pa s or less, or may be 10 Pa s or less.

[0028] The low shear viscosity LSV of the first clear coating composition (C1) can affect the leveling properties as well as the sagging suppression effect (hereinafter sometimes referred to as "sagging properties"). When the low shear viscosity LSV is in the above range, the leveling properties of the first clear coating composition (C1) are easily exhibited.

[0029] First clear coating composition (C1) temperature 23°C and shear rate 1000 sec －１ The high shear viscosity HSV measured under the conditions of a shear rate of 1000 sec may be 0.005 Pa·s or more and 0.40 Pa·s or less. －１ is close to the shear rate applied during discharge. By setting the high shear viscosity HSV within the above range, dischargeability is improved and it becomes easier to apply the coating evenly without unevenness. Shear rate 1000 sec －１ Furthermore, this speed allows stable measurement values ​​to be obtained, and is therefore suitable as a condition for evaluating the ejection properties.

[0030] The high shear viscosity HSV of the first clear coating composition (C1) may be 0.008 Pa s or more, 0.05 Pa s or more, or 0.08 Pa s or more. The high shear viscosity HSV of the first clear coating composition (C1) may be 0.40 Pa s or less, 0.20 Pa s or less, or 0.17 Pa s or less.

[0031] In order to easily achieve both sagging suppression and ejection properties, the low shear viscosity LSV and the high shear viscosity HSV may satisfy the following relationship: (a) when 0.005 Pa·s≦HSV<0.11 Pa·s, 1.0 Pa·s≦LSV≦32.0 Pa·s (b) when 0.11 Pa·s<HSV≦0.5 Pa·s, 4.0 Pa·s<LSV≦40 Pa·s, and particularly when 0.005 Pa·s≦HSV<0.11 Pa·s, (c) 1.0 Pa·s≦LSV≦4.0 Pa·s (particularly, 1.0 Pa·s≦LSV<3.0 Pa·s).

[0032] In order to easily achieve both sagging suppression and ejection properties, the index (LSV / HSV) obtained by dividing the low shear viscosity LSV by the high shear viscosity HSV may be 5 or more and 1200 or less. LSV / HSV may be 8 or more, or 10 or more. LSV / HSV may be 500 or less, 400 or less, or 350 or less.

[0033] The solid content concentration of the first clear coating composition (C1) may be 20% by mass or more and 60% by mass or less. When the solid content concentration of the first clear coating composition (C1) is 20% by mass or more, a colored clear coating film with sufficient thickness can be formed even with a small amount of coating, thereby improving coating workability. In addition, the low shear viscosity LSV is easily adjusted to the above range. When the solid content concentration of the first clear coating composition (C1) is 60% by mass or less, the high shear viscosity HSV is easily adjusted to the above range. The solid content concentration of the first clear coating composition (C1) may be 30% by mass or more, or may be 40% by mass or more. The solid content concentration of the first clear coating composition (C1) may be 57% by mass or less, or may be 50% by mass or less.

[0034] The change in solid content concentration before and after discharging the first clear coating composition (C1) is small. For example, the rate of decrease in solid content concentration 60 seconds after applying (discharging) the first clear coating composition (C1) may be 10% by mass or less relative to the solid content concentration of the first clear coating composition (C1) before discharging. Thus, even when the change in solid content concentration before and after discharging is small, sagging of the first clear coating composition (C1) is suppressed. The rate of decrease in solid content concentration of the first clear coating composition (C1) may be 7% by mass or less, or 4% by mass or less. The rate of decrease in solid content concentration of the first clear coating composition (C1) may be 0.1% by mass or more.

[0035] The solids concentration 60 seconds after application of the first clear coating composition (C1) (hereinafter, for convenience, sometimes referred to as "immediately after application") can be calculated as follows. The first clear coating composition (C1) is applied to aluminum foil whose weight has been measured in advance using a dispenser (e.g., Vermes MDS 3200, nozzle diameter 100 μm) so that the dry film thickness is 15 μm. 60 seconds after application, the aluminum foil is folded, its mass is measured while preventing it from drying, and then heated at 140 ° C. The mass W of the first clear coating composition (C1) before heating and the mass Wh of the heating residue are calculated by subtracting the mass of the aluminum foil alone from the samples before and after heating. Finally, the solids concentration of the first clear coating composition (C1) immediately after application is calculated using the formula 100 × Wh / W (mass%).

[0036] Dischargeability can be evaluated, for example, by the degree of voids in the colored clear coating film or scattering of the first clear coating composition (C1). Voids in the coating film refer to unpainted areas. In dispense coating, voids appear as streaks. Voids in dispense coating occur when the first clear coating composition (C1) is not applied to the desired location, indicating poor straightness of the first clear coating composition (C1) after dispensing. The straightness of the dispensed first clear coating composition (C1) is affected by the shear viscosity (corresponding to the high shear viscosity HSV) of the first clear coating composition (C1) during dispensing. If the high shear viscosity HSV is excessively high, straightness decreases. Here, when the low shear viscosity LSV of the first clear coating composition (C1) is 1 Pa·s or more and 40 Pa·s or less, slight voids can be eliminated by leveling the coating composition.

[0037] The dischargeability can also be evaluated by the degree of scattering of the first clear coating composition (C1). Scattering of the first clear coating composition (C1) occurs when the first clear coating composition (C1) is scattered in all directions from the nozzle, indicating that the composition has been applied to a location other than the desired location. The scattering of the first clear coating composition (C1) during dispense coating is also affected by the shear viscosity (corresponding to the high shear viscosity HSV) of the first clear coating composition (C1) during discharge. If the high shear viscosity HSV is excessively small, the first clear coating composition (C1) is prone to scattering.

[0038] The first clear coating composition (C1) has excellent discharge properties and is resistant to chipping and scattering, so it can be easily applied to the desired location using a dispenser, eliminating the need for masking.

[0039] The first clear coating composition (C1) contains, for example, a film-forming resin, an organic solvent, and at least one selected from the group consisting of a color pigment, an effective pigment, and a dye. The first clear coating composition (C1) may further contain a viscosity modifier. The first clear coating composition (C1) may further contain a surface conditioner.

[0040] <Film-forming resin> The film-forming resin is a thermosetting resin. Examples of film-forming resins include acrylic resins, polyester resins, polyurethane resins, alkyd resins, fluororesins, silicone resins, epoxy resins, polyether resins, and polycarbonate resins. These resins may be used alone or in combination of two or more. The film-forming resin has a crosslinkable functional group. Examples of the crosslinkable functional group include a carboxy group, a hydroxyl group, an epoxy group, a silanol group, and a (meth)acryloyl group.

[0041] The first clear coating composition (C1) contains, as film-forming resins, for example, a half-esterified acrylic resin (i), a carboxyl-containing polyester resin (ii), and an acrylic resin having hydroxyl groups and epoxy groups (iii). The above three types of polymers react with each other, causing a curing reaction. A resin composition containing the above three types of polymers is called an acid-epoxy curing resin composition.

[0042] Half-esterified acrylic resin (i) The acrylic resin (i) has an acid anhydride group half-esterified with a hydroxyl group. The acrylic resin (i) can be obtained, for example, by copolymerizing an ethylenically unsaturated monomer having an acid anhydride group with another ethylenically unsaturated monomer, and then half-esterifying the acid anhydride group with a low-molecular-weight alcohol compound.

[0043] Examples of ethylenically unsaturated monomers having an acid anhydride group include maleic anhydride, itaconic anhydride, and citraconic anhydride, which may be used alone or in combination of two or more.

[0044] Other ethylenically unsaturated monomers include C monomers of acrylic acid or methacrylic acid such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, and stearyl (meth)acrylate. １ ~C ２４ Examples of suitable vinyl compounds include alkyl esters, vinyl aromatic compounds such as styrene, α-methylstyrene, and vinyltoluene, olefins such as ethylene and propylene, unsaturated nitriles such as acrylonitrile and methacrylonitrile, unsaturated amides such as acrylamide, methacrylamide, and N-methylacrylamide, and vinyl compounds such as vinyl acetate, vinyl chloride, 2-vinylpyridine, and 4-vinylpyridine. These may be used alone or in combination of two or more.

[0045] The acrylic resin (i) may have, for example, an acid value (AV) of 50 mgKOH / g or more and 200 mgKOH / g or less, and a number average molecular weight (Mn) of 1,000 or more and 10,000 or less.

[0046] Carboxy Group-Containing Polyester Resin (ii) The polyester resin (ii) can be obtained, for example, by an addition reaction between a polyester polyol having three or more hydroxyl groups and a compound having an acid anhydride group.

[0047] The polyester resin (ii) may have, for example, an AV of 50 mgKOH / g or more and 200 mgKOH / g or less, an OHV of 5 mgKOH / g or more and 50 mgKOH / g or less, and an Mn of 400 or more and 3500 or less.

[0048] Acrylic resin (iii) having a hydroxyl group and an epoxy group The acrylic resin (iii) can be obtained, for example, by copolymerizing an epoxy group-containing ethylenically unsaturated monomer with a hydroxyl group-containing ethylenically unsaturated monomer. The acrylic resin (iii) may have an average of two or more epoxy groups.

[0049] Examples of epoxy group-containing ethylenically unsaturated monomers include glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, and allyl glycidyl ether. These may be used alone or in combination of two or more.

[0050] Examples of hydroxyl group-containing ethylenically unsaturated monomers include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts of these with ε-caprolactone. These may be used alone or in combination of two or more.

[0051] The acrylic resin (iii) may have, for example, an OHV of 5 mgKOH / g or more and 100 mgKOH / g or less, an Mn of 500 or more and 10,000 or less, and an epoxy equivalent of 200 g / eq or more and 700 g / eq or less.

[0052] Examples of organic solvents include ester-based solvents such as ethyl acetate, butyl acetate, isopropyl acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate; ether-based solvents such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, methyl methoxybutanol, ethoxypropanol, ethylene glycol isopropyl ether, ethylene glycol t-butyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, methoxybutanol, and propylene glycol monobutyl ether; alcohol-based solvents such as methanol, ethanol, butanol, and propyl alcohol; ketone-based solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; aliphatic hydrocarbon-based solvents such as Swazol, Shellsol, and mineral spirits; and aromatic solvents such as xylene, toluene, Solvesso-100 (S-100), and Solvesso-150 (S-150). These may be used alone or in combination of two or more.

[0053] The organic solvent is added, for example, so that the solids concentration of the first clear coating composition (C1) is 20% by mass or more and 60% by mass or less.

[0054] Pigments and Dyes The first clear coating composition (C1) contains at least one selected from the group consisting of color pigments, luster pigments, and dyes. This makes the resulting clear coating film colored. "Colored" means having a color, and may be either an achromatic color or a chromatic color.

[0055] Examples of color pigments include organic color pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, dioxazine pigments, anthraquinone pigments, isoindolinone pigments, and metal complex pigments; and inorganic color pigments such as yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These may be used alone or in combination of two or more.

[0056] Examples of luster pigments include aluminum powder, alumina powder, bronze powder, copper powder, tin powder, zinc powder, iron phosphide, metal-coated mica powder, titanium dioxide-coated mica powder, and titanium dioxide-coated glass powder. These may be used alone or in combination of two or more. The luster pigment may be colored.

[0057] Examples of the dye include 1:2 chromium complex black, 1:2 chromium complex yellow, and 1:2 cobalt complex yellow, which may be used alone or in combination of two or more.

[0058] In particular, the first clear coating composition (C1) may contain a black pigment (typically, carbon black).

[0059] Examples of carbon black include common furnace black and acetylene black. Specific examples include carbon blacks ColorBlack Fw200, ColorBlack Fw200P, and ColorBlack Fw285 manufactured by Degussa, Raven 5750, Raven 5250, Raven 5000, and Raven 3500 manufactured by Columbia Chemical, and Emperor 2000, Monarch 1000, Monarch 1100, Monarch 1300, and Monarch 1400 manufactured by Cabot Corporation. These may be used alone or in combination of two or more.

[0060] The first clear coating composition (C1) may further contain an extender pigment or a matting pigment. Examples of extender pigments include calcium carbonate, barium sulfate, clay, and talc. Examples of matting pigments include silica. These may be used alone or in combination of two or more.

[0061] Total PWC of all pigments and dyes Ｔ In terms of hiding power, PWC may be 0.01 mass % or more and 60 mass % or less. Ｔ may be 1.7% by mass or more, or 14.0% by mass or more. Ｔ may be 60% by mass or less, 40% by mass or less, or 20% by mass or less.

[0062] Total PWC of color pigments, luster pigments and dyes Ｃ In terms of hiding power, PWC may be 0.01 mass % or more and 50 mass % or less. Ｃ may be 1.7% by mass or more, or 4.0% by mass or more. Ｃ may be 50% by mass or less, 30% by mass or less, or 20% by mass or less.

[0063] PWC is the mass ratio of the total of the target pigment and dye to the solid mass of the coating composition.

[0064] From the viewpoint of suppressing nozzle clogging, the average particle size of the pigment may be less than ¼ of the nozzle diameter. The average particle size of the pigment may be, for example, 50 μm or less, or 25 μm or less. Prior to dispense coating, the first clear coating composition (C1) may be filtered to remove particles having an average particle size of ¼ or more of the nozzle diameter.

[0065] The average particle size of a pigment is the 50% average particle size (D50) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. The average particle size of a pigment may be measured using image processing software from an electron microscope image of the coating film. For example, ten pigments are randomly selected from the image, and the diameter of a circle (equivalent circle) having the same area as the ten pigments is regarded as the diameter of the pigment. The diameters of the ten pigments are calculated, and the average of these can be used as the average particle size of the pigment.

[0066] Generally, larger pigment particle sizes improve the hiding power of the coating film. As mentioned above, the use of large pigments in dispense coatings should be avoided. However, in the present disclosure, the PWC of the pigment and dye can be increased as long as the low-shear viscosity is within a predetermined range. This can improve the hiding power of the colored clear coating film. For example, the black-and-white hiding power film thickness of the colored clear coating film can be 30 μm or less, or 15 μm or less.

[0067] The thinner the black-and-white hiding film thickness, the better the hiding power. In other words, the higher the hiding power, the thinner the film thickness. Therefore, by using the first clear coating composition (C1) that gives a coating film with high hiding power, coating can be performed under conditions that are less likely to cause sagging.

[0068] The black and white hiding film thickness is measured using a hiding rate test paper in accordance with JIS K 5600-4-1(b). A colored clear coating film is dispensed onto a 2 x 2 cm black and white checkered pattern on the hiding rate test paper so as to create a gradient in dry film thickness, and then the film is cured by heating. Next, the limit of the coating film where the black and white checkered pattern cannot be seen through is determined visually, and the film thickness at that point is measured. This measured film thickness is the black and white hiding film thickness.

[0069] Viscosity Agent The first clear coating composition (C1) may contain a viscosity agent, which makes it possible to easily adjust the low shear viscosity of the first clear coating composition (C1) within the above range.

[0070] Examples of viscosity enhancers include silica-based fine powders, mineral-based viscosity enhancers, polyamide-based viscosity enhancers, cellulose-based viscosity enhancers, polyolefin-based viscosity enhancers, polyurea-based viscosity enhancers, barium sulfate fine powder, and organic resin fine particle viscosity enhancers. These may be used alone or in combination of two or more.

[0071] In particular, the viscosity enhancer may be at least one selected from the group consisting of polyamide viscosity enhancers, cellulose viscosity enhancers, polyolefin viscosity enhancers, polyurea viscosity enhancers, and organic resin particulate viscosity enhancers. These viscosity enhancers tend to suppress white haze in the resulting colored clear coating film. White haze can occur due to differences in the optical properties of the pigment and the viscosity enhancer, which reduces the design appeal.

[0072] The silica-based fine powder is SiO ２ Examples of silica-based fine powders include clay, diatomaceous earth, white carbon, and colloidal silica. Among these, colloidal silica is particularly preferred.

[0073] Examples of mineral viscosity enhancers include swellable layered silicates having a 2:1 crystal structure. Specific examples include smectite clay minerals such as natural or synthetic montmorillonite, saponite, hectorite, stevensite, beidellite, nontronite, bentonite, and laponite; swellable mica clay minerals such as Na-type tetrasilicic fluorine mica, Li-type tetrasilicic fluorine mica, Na-salt-type fluorine taeniolite, and Li-type fluorine taeniolite; vermiculite; and substitution products and derivatives thereof. Bentonite is particularly preferred.

[0074] Examples of polyamide viscosity agents include fatty acid amides, polyamides, acrylamides, long-chain polyaminoamides, aminoamides, and salts thereof (for example, phosphates).

[0075] Examples of cellulose-based viscosity modifiers include cellulose acetate butyrate (CAB), carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, and cellulose nanofiber gel. These may be used alone or in combination of two or more. Among these, CAB is particularly preferred.

[0076] Examples of polyolefin viscosity improvers include polyethylene, polypropylene, ethylene-propylene copolymer, polyethylene oxide, polypropylene oxide, ethylene oxide-propylene copolymer, ethylene-(meth)acrylic acid copolymer, ethylene-maleic anhydride copolymer, and propylene-maleic anhydride copolymer.

[0077] The polyurea-based viscosity increasing agent is a reaction product of an isocyanate and an amine, and has a plurality of urea bonds in one molecule.

[0078] The organic resin particulate viscous agent is an organic resin particle having a three-dimensional crosslinked structure inside. The organic resin particulate viscous agent swells in an organic solvent, and the amount of swelling is controlled by the three-dimensional crosslinked structure.

[0079] The solid content of the viscosity agent is, for example, 0.1% by mass or more and 40% by mass or less, based on the solid content mass of the first clear coating composition (C1). When the content of the viscosity agent is in this range, the low shear viscosity can be easily adjusted to the desired range.

[0080] <<Surface Conditioner>> The first clear coating composition (C1) may contain a surface conditioner. The surface conditioner is added to control the surface tension of the first clear coating composition (C1). The surface conditioner improves the wetting ability of the first clear coating composition (C1) onto the underlying layer. The surface conditioner may also improve the recoatability of the multi-layer coating film.

[0081] Examples of surface conditioners include silicone-based, acrylic-based, vinyl-based, and fluorine-based surface conditioners. These may be used alone or in combination of two or more. Among these, at least one of silicone-based and acrylic-based surface conditioners is preferred. Examples of silicone-based surface conditioners include polydimethylsiloxane and modified silicones obtained by modifying the same. Examples of modified silicones include polyether-modified products, acrylic-modified products, and polyester-modified products. Examples of acrylic-based surface conditioners include acrylic copolymers.

[0082] The solid content of the surface conditioner is, for example, 0.01% by mass or more and 10% by mass or less of the solid content mass of the first clear coating composition (C1). The content of the surface conditioner may be 0.02% by mass or more, or 0.04% by mass or more. The content of the surface conditioner may be 8% by mass or less, or 6% by mass or less.

[0083] Others The first clear coating composition (C1) may contain various additives as needed, such as ultraviolet absorbers, antioxidants, antifoaming agents, dispersants, pinhole inhibitors, pigment derivatives, silane coupling agents, and interface control agents.

[0084] For example, a dispersant is added to improve the dispersibility of the pigment. The dispersant is not particularly limited and is selected appropriately depending on the type of solvent and pigment, etc. Examples of dispersants include compounds having an acid or a base, such as acrylic, polyester, and polyurethane compounds. These compounds may be used alone or in combination of two or more. In particular, the dispersant may be a copolymer compound. The solid content of the dispersant may be, for example, 0.01% by mass or more and 8% by mass or less of the solid content mass of the first clear coating composition (C1). The content of the dispersant may be 0.1% by mass or more. The content of the dispersant may be 4.5% by mass or less.

[0085] (Intermediate Coating Composition (A)) The intermediate coating composition (A) is a solvent-based composition. A conventional intermediate coating composition can be used as the intermediate coating composition (A). The intermediate coating composition (A) contains, for example, a film-forming resin, a curing agent, a pigment or dye, and an organic solvent as a solvent.

[0086] <Film-forming resin> Examples of film-forming resins include acrylic resins, polyester resins, polyurethane resins, alkyd resins, fluororesins, epoxy resins, polyether resins, and polycarbonate resins. These may be used alone or in combination of two or more. The film-forming resin has a crosslinkable functional group. Examples of the crosslinkable functional group include a carboxy group, a hydroxyl group, an epoxy group, a silanol group, and a (meth)acryloyl group.

[0087] Examples of the curing agent include melamine resins, blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more.

[0088] Pigments and Dyes Examples of pigments and dyes include the same pigments and dyes as those exemplified in the first clear coating composition (C1).

[0089] <<Organic Solvent>> Examples of the organic solvent include the same ones as those exemplified in the first clear coating composition (C1).

[0090] Others The intermediate coating composition (A) may contain various additives as needed. Examples of additives include the same additives as those exemplified for the first clear coating composition (C1).

[0091] Next, each step will be described.

[0092] - Application of intermediate coating composition (A) The intermediate coating composition is applied to the substrate 11. The application method may be other than dispense coating. Examples of application methods include air spray coating, airless spray coating, and rotary atomization coating. These methods may be combined with electrostatic coating. Of these, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic coating, a rotary atomization electrostatic coater, commonly known as a "micro-microbell (μμbell)", a "microbell (μbell)", or a "metallicbell (metabell)", is used.

[0093] The intermediate coating composition (A) is applied, for example, so that the thickness (dry film thickness) of the intermediate coating film 12 after curing is 5 μm or more and 40 μm or less.

[0094] (Substrate) Examples of materials for the substrate 11 include metal, resin, and glass. Specific examples of the substrate 11 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 handles.

[0095] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of the metal substrate 11 include steel sheets such as cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0096] The metal substrate 11 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 11 may be further coated with an electrodeposition paint. The electrodeposition paint may be of the cationic type or the anionic type.

[0097] Examples of resins include polypropylene resin, polycarbonate resin, urethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, and polyamide resin. The resin substrate 11 may be degreased.

[0098] Pre-drying of Intermediate Coating Composition (A) After the intermediate coating composition (A) is applied, pre-drying is performed before the application of the colored first clear coating composition. Pre-drying removes at least a portion of the solvent. Examples of pre-drying methods include natural drying (hereinafter also referred to as "wet treatment") and heat drying (hereinafter also referred to as "pH treatment"). Natural drying involves leaving the substrate 11 coated with the intermediate coating composition for 5 to 15 minutes at a temperature of 20°C to 25°C, for example. Heat drying is performed under conditions that do not allow the curing reaction of the film-forming components to proceed, or at least do not complete the curing reaction. Heat drying involves heating the substrate 11 coated with the intermediate coating composition for 30 seconds to 30 minutes at a temperature of 50°C to 100°C, for example.

[0099] - Application of first clear coating composition (C1) The first clear coating composition (C1) is further applied. The first clear coating composition (C1) is applied using a dispenser. This improves coating efficiency and allows the desired area to be painted without the need for masking. According to the first clear coating composition (C1) used in the present disclosure, the occurrence of sagging is suppressed even when dispense coating is performed.

[0100] The first clear coating composition (C1) is applied, for example, so that the thickness of the colored clear coating film after curing is 5 μm or more and 70 μm or less. The thickness of the colored clear coating film after curing may be 10 μm or more, or may be 15 μm or more. According to the present disclosure, sagging is suppressed even when a large amount of the clear coating composition is applied at once.

[0101] In dispense coating, the first clear coating composition (C1) is dispensed in a fixed amount in liquid form from a nozzle hole toward the substrate 11. The first clear coating composition (C1) may be dispensed in droplet form or in the form of a liquid column. The method of dispensing in droplet form or in the form of a liquid column is called dustless coating (oversray-free coating). In dispense coating, for example, the coating composition is dispensed from a plurality of microholes formed at a predetermined pitch in a nozzle head, and is applied to the substrate in droplet or liquid column form. This forms a coating film of a predetermined width on the substrate 11. The method of dispensing in droplet form is similar to inkjet coating. The method of dispensing in the form of a liquid column is described, for example, in JP 2015-196140 A.

[0102] The workpiece 11 is placed on, for example, a table (not shown). The workpiece 11 may be moved together with the table, the dispenser may be moved, or both the table and the dispenser may be moved.

[0103] The discharge mechanism of the dispenser is not particularly limited, and examples thereof include pneumatic, piezoelectric, plunger, non-contact, and thermal valve types. The open / close state of the discharge nozzle during coating may be cyclical or continuous. Depending on the open / close state of the discharge nozzle, the coating composition is discharged in the form of droplets or a liquid column. The number of nozzles is also not particularly limited, and may be one or more, or two or more.

[0104] The diameter of the nozzle of the dispenser is, for example, 50 μm or more and 400 μm or less. If the nozzle diameter is 400 μm or less, excessive ejection is suppressed, making it easy to control the film thickness. If the nozzle diameter is 50 μm or more, nozzle clogging is easily suppressed.

[0105] Heat Curing Next, the intermediate coating composition (A) and the first clear coating composition (C1) are cured by heating. This results in a multi-layer coating film comprising the intermediate coating film 12 and the colored clear coating film 13. According to this embodiment, multiple coating compositions can be cured by heating at one time, thereby reducing the environmental impact and production costs.

[0106] The heating temperature is, for example, 110°C or higher and 180°C or lower. The heating temperature may be 120°C or higher. The heating temperature may be 160°C or lower. The heating time is set appropriately depending on the heating temperature. When the heating temperature is 120°C or higher and 160°C or lower, the heating time may be 10 minutes or higher and 60 minutes or lower.

[0107] [Second embodiment] This embodiment differs from the first embodiment in that after the first clear coating composition (C1) is applied, the second clear coating composition (C2) is applied, and then heat curing is performed. This difference will be explained below. In this embodiment, the other steps are the same as in the first embodiment, so the explanation will be omitted.

[0108] 3 is a flowchart showing an example of a method for forming a multilayer coating film according to the second embodiment. First, a thermosetting, solvent-based intermediate coating composition (A) is applied to a substrate (S31), followed by pre-drying (S32). Next, a thermosetting, colored first clear coating composition (C1) is applied using a dispenser (S33). After pre-drying again (S34), a thermosetting, solvent-based second clear coating composition (C2) is applied (S35). Finally, heat curing is performed, and the intermediate coating composition (A), the first clear coating composition (C1), and the second clear coating composition (C2) are cured simultaneously (S36).

[0109] The method of the second embodiment forms a multilayer coating film comprising an intermediate coating film disposed on an object to be coated, a colored clear coating film disposed on the intermediate coating film, and a second clear coating film disposed on the colored clear coating film. Figure 4 is a schematic cross-sectional view showing a multilayer coating film 10A in the second embodiment. The multilayer coating film 10A comprises an intermediate coating film 12 disposed on an object to be coated 11, a colored clear coating film 13 disposed on the intermediate coating film 12, and a second clear coating film 14 disposed on the colored clear coating film 13.

[0110] Pre-drying of the first clear coating composition (C1) The pre-drying of the first clear coating composition (C1) may be carried out under the same conditions as those for the pre-drying of the intermediate coating composition (A), or may be carried out under different conditions.

[0111] Application of the second clear coating composition (C2) The application method may be the same as the application method for the intermediate coating composition (A), and may be rotary atomization electrostatic coating.

[0112] The second clear coating composition (C2) is applied, for example, so that the thickness of the second clear coating film 14 after curing is 1 μm or more and 50 μm or less. The second clear coating film 14 may be a single layer or a multilayer coating film of two or more layers.

[0113] (Second clear coating composition (C2)) The second clear coating composition (C2) is solvent-based. A conventionally known clear coating composition can be used as the second clear coating composition (C2). The second clear coating composition (C2) may contain, for example, the same film-forming resin, curing agent, organic solvent, and, if necessary, additives as in the intermediate coating composition (A). The second clear coating composition (C2) may or may not contain a color pigment, a luster pigment, and a dye.

[0114] [Third embodiment] This embodiment differs from the first embodiment in that heat curing is performed after application of the intermediate coating composition (A), and that after application of the intermediate coating composition (A) and before application of the first clear coating composition (C1), an aqueous base coating composition (B) is applied, and then heat curing is performed. This difference will be explained below. In this embodiment, the other steps are the same as in the first embodiment, so their explanation will be omitted.

[0115] 5 is a flowchart showing an example of a method for forming a multi-layer coating film according to the third embodiment. First, a thermosetting, solvent-based intermediate coating composition (A) is applied to the substrate (S41), followed by heat curing (S42). Then, an aqueous base coating composition (B) is applied (S43), and pre-drying is performed (S44). Next, a thermosetting, colored first clear coating composition (C1) is applied using a dispenser (S45). Finally, heat curing is performed again (S46), and the aqueous base coating composition (B) and the first clear coating composition (C1) are cured at the same time.

[0116] The method of the third embodiment forms a multilayer coating film comprising an intermediate coating film disposed on an object to be coated, a base coating film disposed on the intermediate coating film, and a colored clear coating film disposed on the base coating film. Figure 6 is a schematic cross-sectional view showing a multilayer coating film 10B in the third embodiment. The multilayer coating film 10B comprises an intermediate coating film 12 disposed on an object to be coated 11, a base coating film 15 disposed on the intermediate coating film 12, and a colored clear coating film 13 disposed on the base coating film 15.

[0117] Heat Curing of Intermediate Coating Composition (A) The intermediate coating composition (A) may be heat cured under the same conditions as those for the heat curing of the first clear coating composition (C1), or may be heat cured under different conditions.

[0118] Application of the aqueous base coating composition (B) The application method may be the same as the application method for the intermediate coating composition (A), and in particular may be rotary atomization electrostatic coating.

[0119] The aqueous base coating composition (B) is applied, for example, so that the thickness of the base coating film after curing is 2 μm or more and 30 μm or less. The base coating film may be a single layer or a multilayer coating film of two or more layers.

[0120] (Aqueous base coating composition (B)) A conventionally known base coat coating composition can be used as the aqueous base coating composition (B). The aqueous base coating composition (B) may contain, for example, the same film-forming resin, curing agent, pigment or dye, solvent containing water, and additives as necessary as the intermediate coating composition (A). In the aqueous coating, the film-forming resin may be contained as an emulsion, may be contained as a dispersion, or may be contained in a state dissolved in a solvent.

[0121] [Fourth embodiment] This embodiment differs from the third embodiment in that the second clear coating composition (C2) is applied after the application of the aqueous base coating composition (B) and before the application of the first clear coating composition (C1). This difference will be explained below. In this embodiment, the other steps are the same as in the third embodiment, so the explanation will be omitted.

[0122] 7 is a flowchart showing an example of a method for forming a multilayer coating film according to the fourth embodiment. First, a thermosetting, solvent-based intermediate coating composition (A) is applied to the substrate (S51), followed by heat curing (S52). Then, an aqueous base coating composition (B) is applied (S53), followed by pre-drying (S54). Next, a second clear coating composition (C2) is applied (S55), followed by pre-drying again (S56). Next, a thermosetting, colored first clear coating composition (C1) is applied using a dispenser (S57). Finally, heat curing is performed again (S58), and the aqueous base coating composition (B), the second clear coating composition (C2), and the first clear coating composition (C1) are cured at the same time.

[0123] The method of the fourth embodiment forms a multilayer coating film comprising an intermediate coating film disposed on an object to be coated, a base coating film disposed on the intermediate coating film, a second clear coating film disposed on the base coating film, and a colored clear coating film disposed on the second clear coating film. Figure 8 is a schematic cross-sectional view showing a multilayer coating film 10C in the fourth embodiment. The multilayer coating film 10C comprises an intermediate coating film 12 disposed on an object to be coated 11, a base coating film 15 disposed on the intermediate coating film 12, a second clear coating film 14 disposed on the base coating film 15, and a colored clear coating film 13 disposed on the colored clear coating film 13.

[0124] Pre-drying of second clear coating composition (C2) Pre-drying of the second clear coating composition (C2) may be carried out under the same conditions as those for pre-drying the intermediate coating composition (A), or may be carried out under different conditions.

[0125] [Fifth embodiment] This embodiment differs from the fourth embodiment in that the order of application of the first clear coating composition (C1) and the second clear coating composition (C2) is reversed. This difference will be explained below. In this embodiment, the other steps are the same as in the fourth embodiment, so the explanation will be omitted.

[0126] 9 is a flowchart showing an example of a method for forming a multi-layer coating film according to the fifth embodiment. First, a thermosetting, solvent-based intermediate coating composition (A) is applied to the substrate (S61), followed by heat curing (S62). Then, an aqueous base coating composition (B) is applied (S63), followed by pre-drying (S64). Next, a thermosetting, colored first clear coating composition (C1) is applied using a dispenser (S55), followed by pre-drying again (S66). Next, a second clear coating composition (C2) is applied (S67). Finally, heat curing is performed again (S68), and the aqueous base coating composition (B), the first clear coating composition (C1), and the second clear coating composition (C2) are cured at the same time.

[0127] The method of the fifth embodiment forms a multilayer coating film comprising an intermediate coating film disposed on an object to be coated, a base coating film disposed on the intermediate coating film, a colored clear coating film disposed on the base coating film, and a second clear coating film disposed on the colored clear coating film. Figure 10 is a schematic cross-sectional view showing a multilayer coating film 10D in the fifth embodiment. The multilayer coating film 10D comprises an intermediate coating film 12 disposed on an object to be coated 11, a base coating film 15 disposed on the intermediate coating film 12, a colored clear coating film 13 disposed on the base coating film 15, and a second clear coating film 14 disposed on the colored clear coating film 13.

[0128] [Sixth embodiment] This embodiment differs from the third embodiment in that the intermediate coating composition (A) is water-based, and that after application of the intermediate coating composition (A), preliminary drying is carried out instead of heat curing. These differences are explained below. In this embodiment, the other steps are the same as in the third embodiment, so explanations thereof will be omitted.

[0129] 11 is a flowchart showing an example of a method for forming a multi-layer coating film according to the sixth embodiment. First, a thermosetting, aqueous intermediate coating composition (A) is applied to a substrate (S71), followed by pre-drying (S72). Then, an aqueous base coating composition (B) is applied (S73), followed by pre-drying again (S74). Next, a thermosetting, colored first clear coating composition (C1) is applied using a dispenser (S75). Finally, heat curing is performed (S76), and the intermediate coating composition (A), aqueous base coating composition (B), and first clear coating composition (C1) are cured simultaneously.

[0130] The method of the sixth embodiment forms a multilayer coating film comprising an intermediate coating film disposed on an object to be coated, a base coating film disposed on the intermediate coating film, and a colored clear coating film disposed on the base coating film. The multilayer coating film in the sixth embodiment has a configuration similar to that of the third embodiment (Fig. 6).

[0131] The aqueous intermediate coating composition (A) contains water as a solvent and, if necessary, an organic solvent, and may contain, as a film-forming resin, for example, an aqueous polyester resin, an aqueous polyurethane resin, or a hydroxyl group-containing resin other than these (e.g., an aqueous acrylic resin).

[0132] [Seventh embodiment] This embodiment differs from the fourth embodiment in that the intermediate coating composition (A) is aqueous, and that after application of the intermediate coating composition (A), preliminary drying is carried out instead of heat curing. These differences are explained below. In this embodiment, the other steps are the same as in the fourth embodiment, so explanations thereof will be omitted.

[0133] 12 is a flowchart showing an example of a method for forming a multilayer coating film according to the seventh embodiment. First, a thermosetting, aqueous intermediate coating composition (A) is applied to the substrate (S81), followed by pre-drying (S82). Then, an aqueous base coating composition (B) is applied (S83), followed by pre-drying again (S84). Next, a second clear coating composition (C2) is applied (S85), followed by pre-drying a third time (S86). Then, a thermosetting, colored first clear coating composition (C1) is applied using a dispenser (S87). Finally, heat curing is performed (S88), and the intermediate coating composition (A), aqueous base coating composition (B), second clear coating composition (C2), and first clear coating composition (C1) are cured simultaneously.

[0134] The method of the seventh embodiment forms a multilayer coating film comprising an intermediate coating film disposed on an object to be coated, a base coating film disposed on the intermediate coating film, a second clear coating film disposed on the base coating film, and a colored clear coating film disposed on the second clear coating film. The multilayer coating film in the seventh embodiment has a configuration similar to that of the fourth embodiment (Fig. 8).

[0135] [Eighth embodiment] This embodiment differs from the seventh embodiment in that the order of application of the first clear coating composition (C1) and the second clear coating composition (C2) is reversed. This difference will be explained below. In this embodiment, the other steps are the same as in the seventh embodiment, so the explanation will be omitted.

[0136] 13 is a flowchart showing an example of a method for forming a multilayer coating film according to the eighth embodiment. First, a thermosetting, aqueous intermediate coating composition (A) is applied to the substrate (S91), followed by pre-drying (S92). Then, an aqueous base coating composition (B) is applied (S93), followed by pre-drying again (S94). Next, a second clear coating composition (C2) is applied (S95), followed by pre-drying a third time (S96). Next, a thermosetting, colored first clear coating composition (C1) is applied using a dispenser (S97). Finally, heat curing is performed (S98), and the aqueous intermediate coating composition (A), aqueous base coating composition (B), first clear coating composition (C1), and second clear coating composition (C2) are cured simultaneously.

[0137] The method of the eighth embodiment forms a multilayer coating film comprising an intermediate coating film disposed on an object to be coated, a base coating film disposed on the intermediate coating film, a colored clear coating film disposed on the base coating film, and a second clear coating film disposed on the colored clear coating film. The multilayer coating film in the eighth embodiment has a configuration similar to that of the fifth embodiment (Fig. 10).

[0138] The present disclosure is not limited to the above-described embodiments, and design modifications are possible without departing from the spirit and scope of the present disclosure. For example, any two or more of the features of the above-described embodiments may be combined.

[0139] In the above-described embodiment, the intermediate coating composition, the aqueous base coating composition, and the second clear coating composition are applied by a method other than dispense coating, but this is not limiting. The intermediate coating composition, the aqueous base coating composition, and the second clear coating composition may be applied by dispense coating.

[0140] In the above-described embodiment, an aqueous base coating composition is used, but the present invention is not limited to this. The base coating composition may be aqueous or solvent-based.

[0141] In the above-described embodiment, a solvent-based second clear coating composition is used, but the present invention is not limited to this. The second clear coating composition may be water-based or solvent-based.

[0142] In the above-described embodiment, the preliminary drying may be a wet treatment or a pH treatment.

[0143] The present invention will be described in more detail with reference to the following examples, but is not limited thereto. In the examples, "parts" and "%" are by weight unless otherwise specified.

[0144] The solids concentration was calculated from the residue when the coating composition was heated at 140°C in accordance with JIS K 5601-1-2 Heating Residue Measurement Method.

[0145] The LSV and HSV were measured using an "MCR302" (manufactured by Anton Paar) rheometer at a measurement temperature of 23°C, with a cone plate of 25φ or 50φ, and at various shear rates.

[0146] Mn was measured using three GPC apparatuses, namely, "HLC8220GPC" (trade name, manufactured by Tosoh Corporation) and "TSK Gel SuperMultipore HZ-M" (manufactured by Tosoh Biosciences), under the following conditions: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 0.35 mL / min, and detector: RI.

[0147] The AV and OHV were calculated based on the acid value and hydroxyl value of the raw material monomers used.

[0148] [Preparation of Pigment Dispersion Paste] 28.6 parts of a dispersant (Disperbyk 161, manufactured by BYK), 57.1 parts of an organic solvent (Solvesso 150), and 14.3 parts of a black pigment (trade name: RAVEN 5000, carbon black, manufactured by COLUMBIAN) were premixed. Then, using a paint conditioner and glass bead medium, the mixture was mixed at room temperature until the average particle size of the pigment became 5 μm or less. In this way, a solvent-based black pigment dispersion paste with a solids concentration of 22.9% by mass was obtained.

[0149] [Preparation of first clear coating composition (C1-1)] 62.7 parts of the half-ester acid anhydride group-containing acrylic resin (i), 20.0 parts of a carboxyl group-containing polyester resin (ii), 73.7 parts of an acrylic resin having a hydroxyl group and an epoxy group (iii), 1 part of an ultraviolet absorber, 1 part of a light stabilizer, 0.3 parts of an acrylic surface conditioner, 4.9 parts of a viscosity agent a, and 1.0 parts of a surface conditioner were mixed. Subsequently, 30 parts of an organic solvent and 35.7 parts of the above pigment dispersion paste were blended to obtain a first clear coating composition (C1-1).

[0150] [Preparation of first clear coating compositions (C1-2 to 13, c1-1 to 6)] As shown in Table 1, the first clear coating compositions (C1-2 to 13, c1-1 to 6) were prepared in the same manner as the first clear coating composition (C1-1), except that the ingredients and amounts were changed.

[0151] The descriptions in Tables 1 and 2 are as follows.

[0152] (Acrylic resin (i)) Half-ester acid anhydride group-containing acrylic resin, manufactured by Nippon Paint Automotive Coatings, solid content concentration 51.5 mass%, AV: 150 mg KOH / g, Mn 2800 (Polyester resin (ii)) Carboxy group-containing polyester resin, manufactured by Nippon Paint Automotive Coatings, solid content concentration 75.0 mass%, AV: 120 mg KOH / g, Mn 2500 (Acrylic resin (iii)) Acrylic resin having hydroxyl groups and epoxy groups, manufactured by Nippon Paint Automotive Coatings, solid content concentration 71.5 mass%, epoxy equivalent 400 g / eq, OHV: 50 mg KOH / g, Mn 2200

[0153] (Thickening agents) a: Trade name BHEOBYK-410, manufactured by BYK, polyurea-based thickening agent, solid content concentration 52% by mass b: Trade name SETALUX 91756 VS-60YA, manufactured by Allnex, polyurea-based thickening agent c: Trade name BHEOBYK-430, manufactured by BYK, urea-modified polyamide-based thickening agent d: Trade name Disparlon BB102, manufactured by Kusumoto Chemicals, polyamide-based thickening agent e: Trade name Disparlon 6900-20X, manufactured by Kusumoto Chemicals, polyamide-based thickening agent f: Trade name Disparlon 4200-10, manufactured by Kusumoto Chemicals, polyolefin-based (polyethylene oxide) thickening agent g: Trade name CAB-1000 (cellulose-based thickening agent, EASTMAN h: 10% ethyl acetate solution of Pliolite AC4 (organic resin fine particle viscous agent, manufactured by Eriochem) in Solvesso 100

[0154] (UV absorber) Product name: Tinuvin 928, manufactured by BASF, solids concentration 100% (Light stabilizer) Product name: Tinuvin 123, manufactured by BASF, solids concentration 100% (Acrylic surface conditioner) Product name: Resiflow LV, manufactured by Estron Chemical, solids concentration 100% (Anti-popping agent) Product name: Disparlon OX-881, manufactured by Kusumoto Chemicals, solids concentration 30% (Organic solvent) A 1:1 mixture of ethyl 3-ethoxypropionate and Solvesso 150

[0155]

[0156] Example 1 A multi-layer coating film having the same structure as that shown in FIG. 6 was formed by a method similar to that of the third embodiment (flowchart shown in FIG. 5).

[0157] (1) Preparation of substrate: A zinc phosphate-treated dull steel plate was electrodeposited with a cationic electrodeposition paint (product name: Powernics 110, manufactured by Nippon Paint Co., Ltd.) so that the dry coating film was 20 μm thick. The coating was then heat-cured at 160° C. for 30 minutes to form a cured electrodeposition coating film, thereby obtaining a coated substrate.

[0158] (2) Application of intermediate coating composition and heat curing The solvent-based intermediate coating composition (A-2) shown below was applied onto the cured electrodeposition coating film by rotary atomization electrostatic coating so that the cured film thickness was 20 μm, and the coating was heat cured at 140° C. for 30 minutes to obtain an intermediate coating film.

[0159] (3) Application of aqueous base coating composition and preliminary drying Next, the aqueous base coating composition (B) was applied by rotary atomization electrostatic coating so that the cured film thickness was 15 μm. Then, preliminary drying (PH treatment 1) described below was carried out.

[0160] (4) Dispense Coating of First Clear Coating Composition Subsequently, the first clear coating composition (C1-1) was applied using a dispenser (Vermes, MDS 3200, nozzle diameter 100 μm) so that the cured film thickness was 15 μm.

[0161] (5) Heat Curing Finally, heat curing was carried out at 140°C for 30 minutes to form a multi-layer coating film comprising an intermediate coating film (thickness 20 µm), a base coating film (thickness 15 µm) and a colored clear coating film (thickness 15 µm).

[0162] [Examples 2 to 13, Comparative Examples 1 to 6] A multilayer coating film comprising an intermediate coating film (thickness 20 μm), a base coating film (thickness 15 μm) and a colored clear coating film (thickness 15 μm) was formed in the same manner as in Example 1, except that the first clear coating composition (C1-2 to 13, c1-1 to 6) was used instead of the clear coating composition (C-1).

[0163]

[0164] [Examples 14 to 23] In the same manner as in Example 1, each coating composition was applied in order from bottom to top as shown in Table 3 to form a multi-layer coating film. Between applications of each coating composition, pre-drying or baking was carried out as shown in Table 3. After the last coating composition was applied, heat curing was carried out at 140°C for 30 minutes.

[0165] The descriptions in Table 3 are as follows: A-1: ​​Water-based intermediate coating composition, trade name Aqualex AR620, manufactured by Nippon Paint Automotive Coating A-2: Solvent-based intermediate coating composition, trade name Orga-30-P, manufactured by Nippon Paint Automotive Coating B: Water-based base coating composition, trade name Aqualex AR2000, water-based, manufactured by Nippon Paint Automotive Coating C2: Solvent-based second clear coating composition, trade name Macflow O-1820KN, manufactured by Nippon Paint Automotive Coating

[0166] WET treatment: Pre-drying, left at 23°C for 7 minutes PH treatment 1: Pre-drying, left at 23°C for 3 minutes, then heated at 80°C for 3 minutes PH treatment 2: Pre-drying, left at 23°C for 7 minutes, then heated at 90°C for 4 minutes Cure: Baking, left at 23°C for 7 minutes, then heated at 140°C for 30 minutes

[0167]

[0168] Example 24 A multi-layer coating film was formed in the same manner as in Example 2, except that the nozzle diameter was changed to 50 μm.

[0169] Example 25 A multi-layer coating film was formed in the same manner as in Example 2, except that the nozzle diameter was changed to 400 μm.

[0170]

[0171] [Evaluation] The multi-layer coating film was evaluated as follows, and the evaluation results are shown in Tables 2 to 4.

[0172] (1) Sagging property A multilayer coating film obtained in the same manner as in Example 1, except that the first clear coating composition was applied vertically, was visually observed and evaluated based on the following criteria. A rating of C or higher can be evaluated as the first clear coating composition being excellent in sagging property (sagging is suppressed).

[0173] (Evaluation criteria) A: No dripping B: Between evaluations A and C, usable C: Accumulation of coating composition observed at the bottom, but usable D: Accumulation of coating composition observed at the bottom, not suitable for practical use

[0174] (2) Leveling Ability The multilayer coating films obtained in the examples were visually observed, and whether or not the irregularities (streaks) extending in the scanning direction of the nozzle were leveled was evaluated based on the following criteria. A rating of C or higher can be evaluated as the first clear coating composition having excellent leveling ability.

[0175] (Evaluation criteria) A: No irregularities can be seen B: Between evaluations A and C, usable C: The irregularities can be seen by looking closely, usable D: The irregularities immediately after painting remain, not suitable for practical use

[0176] (3) Straight-line Adhesion The multilayer coating films obtained in the examples were visually observed, and the presence or absence of unpainted areas (voids) was evaluated based on the following criteria. A rating of B or higher indicates that the first clear coating composition has excellent straight-line adhesivity.

[0177] (Evaluation criteria) A: No gaps were observed. B: Lines thought to be due to gaps were observed, but the product was still usable. C: Clear gaps were observed, and the product was not suitable for practical use.

[0178] (4) Scattering Resistance The multilayer coating films obtained in the examples were visually observed, and whether or not the first clear coating composition had scattered to areas other than the painted areas was evaluated based on the following criteria: A rating of B or higher indicates that the first clear coating composition has excellent scatter resistance (scattering is suppressed).

[0179] (Evaluation criteria) A: No scattering observed B: A few scattered spots observed, but usable C: Many scattered spots observed, not suitable for practical use

[0180] (5) Concealment A concealment test paper (manufactured by Nippon Test Panel Co., Ltd.) conforming to JIS K 5600-4-1(b) was used. The first clear coating composition was dispensed onto the 2x2 cm black and white checkered pattern on the concealment test paper so as to create a gradient in dry film thickness, and then heat cured. Next, the limit of the coating film where the black and white checkered pattern could not be seen through was determined visually, and the film thickness at that point was measured. The measured film thickness was taken as the black and white concealing film thickness. The concealing property of the colored clear coating film was evaluated from the obtained black and white concealing film thickness based on the following evaluation criteria. If the colored clear coating film was rated A, it can be evaluated that the colored clear coating film has excellent concealing property.

[0181] (Evaluation criteria) A: Black and white hiding film thickness is 13 μm or less B: Black and white hiding film thickness is more than 13 μm

[0182] (6) Whitening The multilayer coating films obtained in the examples were visually observed, and the whitening (cloudiness) of the coating film was evaluated. The evaluation was carried out using a standard colored clear coating film made with the first clear coating composition (c1-1) as a comparison. Since the first clear coating composition (c1-1) does not contain a viscosity agent, the resulting colored clear coating film is less likely to develop whitening. A rating of B or higher can be evaluated as being less likely to develop whitening.

[0183] (Evaluation criteria) A: Equivalent to the standard colored clear coating film, and can be evaluated as having no white blurring. B: More white blurring than the standard colored clear coating film can be seen when viewed closely, but it is still usable. C: Clear white blurring can be seen compared to the standard colored clear coating film, and it is not suitable for practical use.

[0184] According to the method of the present invention, a multi-layer coating film with reduced sagging can be obtained using dispense coating. The method of the present disclosure is particularly suitable for painting automobiles.

[0185] This application claims priority based on Japanese Patent Application No. 2023-221531, filed on December 27, 2023, the entire contents of which are incorporated herein by reference.

[0186] 10, 10A to 10D Multilayer coating film 11 Coating object 12 Intermediate coating film 13 Colored first clear coating film (colored clear coating film) 14 Second clear coating film 15 Base coating film

Claims

1. Coating a thermosetting intermediate coating composition (A) on an object to be coated; after coating the intermediate coating composition (A), coating a thermosetting and colored first clear coating composition (C1) using a dispenser; heating and curing the intermediate coating composition (A) to obtain an intermediate coating film; heating and curing the first clear coating composition (C1) to obtain a colored first clear coating film, wherein the first clear coating composition (C1) is a solvent-based composition and has a low-shear viscosity LSV measured under the conditions of a temperature of 23°C and a shear rate of 0.1 sec-1 of 1 Pa·s or more and 40 Pa·s or less. A method for forming a multilayer coating film.

2. The first clear coating composition (C1) is coated before the intermediate coating composition (A) is heated and cured, and the heating and curing of the intermediate coating composition (A) and the first clear coating composition (C1) are carried out at once. The method for forming a multilayer coating film according to claim 1.

3. After heating and curing the intermediate coating composition (A) to obtain an intermediate coating film, the first clear coating composition (C1) is coated. The method for forming a multilayer coating film according to claim 1.

4. After coating the intermediate coating composition (A), before coating the first clear coating composition (C1), coating an aqueous-based coating composition (B); heating and curing the aqueous-based coating composition (B) to obtain a base coating film. The method for forming a multilayer coating film according to any one of claims 1 to 3.

5. After coating the intermediate coating composition (A), before coating the first clear coating composition (C1), coating an aqueous-based coating composition (B); after coating the aqueous-based coating composition (B), before coating the first clear coating composition (C1), coating a second clear coating composition (C2); heating and curing the second clear coating composition (C2) to obtain a second clear coating film. The method for forming a multilayer coating film according to any one of claims 1 to 4.

6. After coating the first clear coating composition (C1), coating the second clear coating composition (C2); heating and curing the second clear coating composition (C2) to obtain a second clear coating film. The method for forming a multilayer coating film according to any one of claims 1 to 4.

7. The high-shear viscosity HSV measured under the conditions of a temperature of 23°C and a shear rate of 1000 sec －１ for the first clear coating composition (C1) is 0.009 Pa·s or more and 0.40 Pa·s or less. The method for forming a multilayer coating film according to any one of claims 1 to 6.

8. The solid content concentration of the first clear coating composition (C1) is 20% by mass or more and 60% by mass or less. The method for forming a multilayer coating film according to any one of claims 1 to 7.

9. The rate of decrease in the solid content concentration after 60 seconds from the application of the first clear coating composition (C1) is 10% by mass or less with respect to the solid content concentration of the first clear coating composition (C1). The method for forming a multilayer coating film according to any one of claims 1 to 8.

10. The diameter of the nozzle of the dispenser is 50 μm or more and 400 μm or less. The method for forming a multilayer coating film according to any one of claims 1 to 9.

11. The first clear coating composition (C1) contains at least one selected from the group consisting of a coloring pigment, a pearlescent pigment, and a dye. The method for forming a multilayer coating film according to any one of claims 1 to 10.

12. The first clear coating composition (C1) contains at least one selected from the group consisting of a polyamide-based binder, a cellulose-based binder, a polyolefin-based binder, a polyurea-based binder, and an organic resin fine particle binder. The method for forming a multilayer coating film according to any one of claims 1 to 11.

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