Method for forming a multilayer coating film

Abstract

[Problem] To provide a method for forming a multilayer coating film with which a coating film capable of improving adhesion between a coating film surface and an exterior member or the like and showing excellent gasoline resistance, water resistance, and appearance can be obtained. [Solution] A method for forming a multilayer coating film by sequentially applying an intermediate coating composition, a base coating composition, and a clear coating composition on a metal or resin material for a motor vehicle body and then heat-curing the compositions, wherein the clear coating composition is obtained by mixing a main agent (A) and a curing agent (B), the main agent (A) contains a hydroxyl group-containing resin (A-1) and a surface modifier (A-2) in an amount of 0.01 to 3.0 mass% relative to the mass of the solid content of the hydroxyl group-containing acrylic resin, the hydroxyl group-containing acrylic resin (A-1) has a hydroxyl value of 80 to 250 mg KOH / g, a mass average molecular weight of 1,000 to 30,000, and an SP value of 9.0 to 12.0, and the surface modifier (A-2) is a polymer having a mass average molecular weight of 3,000 to 100,000 obtained by grafting an acrylic resin (A-2-i) and a resin (A-2-ii), the acrylic resin (A-2-i) has an SP value of 8.5 to 10.0, and the resin (A-2-ii) has an SP value at least 2.0 higher than the SP value of the acrylic resin (A-2-i).

Description

[Technical Field]

[0001] This invention relates to a method for forming a multilayer coating film by using a curable resin composition in a coating. More specifically, this invention relates to a method for forming a multilayer coating film for automotive applications by sequentially applying an intermediate coating, a base coating, and a clear coating, followed by thermal curing. [Background Technology]

[0002] The method for forming a multi-layer coating on a motor vehicle body typically involves forming an electrodeposited coating on the object to be coated, curing the electrodeposited coating by heating, and subsequently forming a multi-layer coating comprising an intermediate coating, a base coating, and a clear coating. Currently, water-based coatings are beginning to be used as intermediate and base coatings to reduce the use of volatile organic solvents (VOCs).

[0003] Furthermore, to reduce energy consumption, many methods for forming multilayer coatings have emerged in recent years involving the so-called three-coat-one-bake (3C1B) process. This involves forming a base coat and a clear coat on a preheated intermediate coat formed on an electrodeposited coating—without heating and curing the intermediate coat formed on the electrodeposited coating—and subsequently heating and curing all three coatings simultaneously. However, in methods using the 3C1B process to form multilayer coatings, layer mixing occurs between the intermediate and base coats, and a good coating appearance cannot be obtained.

[0004] It is known that aqueous base coating compositions using emulsion resins containing specific core / shell types as base resins are used to prevent layer mixing between intermediate and base coatings and to form multilayer coatings with excellent surface smoothness in methods of forming multilayer coatings using such 3C1B processes, wherein the core portion comprises an acrylic resin, the shell portion comprises a urethane resin, and the urethane resin contains a specific proportion of constituent units derived from dibasic acids and / or diols having 10-60 carbon atoms (see Patent Document 1).

[0005] The method for forming a multilayer coating in Patent Document 1 prevents layer mixing between the intermediate and base coats and can form a multilayer coating with excellent surface smoothness. However, when using double-sided tape to bond external components to the surface of the cured multilayer coating, satisfactory adhesion cannot be achieved between the coating surface and the double-sided tape. Specifically, when applying an isocyanate-curing type transparent coating and the thermosetting temperature of the multilayer coating is 100°C or lower, there is room for improvement in attaching external components using double-sided tape with an acrylic adhesive.

[0006] Patent Document 2 discloses a leveling agent for coatings, characterized by comprising a copolymer with a number average molecular weight of 1,000-60,000, obtained by copolymerizing 2-50% by mass of silicone oil having acrylate or methacrylate groups (monomer component A) and 50-98% by mass of hydroxy acrylate or hydroxymethacrylate having primary hydroxyl groups at their ends (monomer component B). This leveling agent, by adding a small amount to the coating, prevents the formation of a smooth coating surface due to shrinkage from different types of dust and does not impair interlayer adhesion during recoating. However, such leveling agents for coatings prevent a decrease in interlayer adhesion during recoating and do not improve adhesion to double-sided tape.

[0007] [List of citations]

[0008] [Patent Literature]

[0009] [Patent Document 1] Japanese Patent 5995948

[0010] [Patent Document 2] Japanese Patent Application Publication 2011-116880 [Invention Overview]

[0012] [Problems to be solved by this invention]

[0013] The present invention provides a method for forming a multilayer coating film, wherein even if the baking temperature during the thermosetting process is relatively low, good adhesion between the surface of the coating film and external components, etc., is ensured, and a coating film exhibiting good gasoline and water resistance and having an excellent appearance is obtained.

[0014] [Problem-solving methods]

[0015] As a result of diligent research to solve the aforementioned problems, the inventors of this invention discovered that by using a specific two-component transparent coating, good adhesion between external components and the resulting surface can be ensured, even when the baking temperature for heat-curing the multilayer coating is low, such as 100°C or lower. In particular, the inventors of this invention discovered that by using a specific water-based two-component coating as an intermediate coating and a specific water-based base coating as a base coating, a coating film with good water resistance and excellent appearance can be obtained, even when the baking temperature for heat-curing the multilayer coating is 100°C or lower.

[0016] In other words, the present invention provides a method for forming a multilayer coating film by sequentially applying an intermediate coating, a base coating, and a clear coating to a metal for a motor vehicle body—on which an electrodeposited coating has been applied and the formed electrodeposited coating film has been fully baked—and / or a resin material for a motor vehicle exterior, and then thermally curing the coating. The clear coating composition is obtained by mixing a main agent (A) and a curing agent (B). The main agent (A) contains a hydroxyl-containing acrylic resin (A-1) as the main resin and a surface conditioner (A-2) in a mass fraction of 0.01-3.0% by weight relative to the solid content of the hydroxyl-containing acrylic resin (A-1).

[0017] Hydroxyl-containing acrylic resin (A-1) has a hydroxyl value of 80-250 mg KOH / g, a mass average molecular weight of 1,000-30,000, and an SP value of 9.0-12.0.

[0018] The surface conditioner (A-2) is a polymer with a mass average molecular weight of 3,000-100,000, obtained by grafting acrylic resin (A-2-i) and resin (A-2-ii).

[0019] The SP value of acrylic resin (A-2-i) is 8.5-10.0, and the SP value of resin (A-2-ii) is at least 2.0 higher than that of acrylic resin (A-2-i).

[0020] The hydroxyl-containing acrylic resin (A-1) preferably comprises a copolymer of hydroxyl-containing mono(meth)acrylate and other vinyl monomers.

[0021] Acrylic resins (A-2-i) are preferably copolymers containing multiple types of vinyl monomers.

[0022] The resin (A-2-ii) preferably comprises one or more of acrylic resins, polyurethane resins, and monovalent polyoxyethylenes containing ethylene oxide units.

[0023] The surface conditioner (A-2) is preferably a graft polymer in which the mass ratio of acrylic resin (A-2-i) to resin (A-2-ii) is 90 / 10-50 / 50.

[0024] Furthermore, the intermediate coating composition used in this invention is preferably an aqueous two-component intermediate coating composition, which contains an acrylic urethane resin with a hydroxyl value of 20-40 mg KOH / g and an acid value of 20-40 mg KOH / g in an amount of 20-60% by mass of the solid resin content in the main agent, and contains a carbodiimide compound having 2 moles or more carbodiimide groups per molecule as a curing agent, wherein the equivalent of the carbodiimide groups is in a ratio of 0.8-1.2 to the equivalent of the carboxylic acid groups contained in the main agent resin.

[0025] Furthermore, the base coating composition used in this invention is preferably an aqueous base coating composition containing an acrylic urethane resin with a hydroxyl value of 20-40 mg KOH / g and an acid value of 20-40 mg KOH / g in an amount of 20-60% by weight of solid resin.

[0026] In addition, the baking temperature during the thermosetting process is preferably 75-100℃.

[0027] [Advantages of the Invention]

[0028] By using the method of forming a multilayer coating according to the present invention, a coating that ensures good adhesion to acrylic double-sided tape, exhibits good gasoline and water resistance, has an excellent appearance, and even shows excellent smoothness and gloss on a highly rough electrodeposited plate, can be obtained, even when the baking temperature during the thermosetting process is 75-100°C after sequentially applying the intermediate coating, base coating and clear coating in the method of forming a multilayer coating using the 3C1B process.

[0029] [Description of the implementation plan]

[0030] The method for forming a multilayer coating according to the present invention will now be explained in detail.

[0031] The transparent coating composition used in this invention is obtained by mixing a main agent (A) and a curing agent (B). The main agent (A) contains a hydroxyl-containing acrylic resin (A-1) with a hydroxyl value of 80-250 mg KOH / g, a mass average molecular weight of 1,000-30,000 g / mol, and an SP value of 9.0-12.0 as the main resin.

[0032] Hydroxyl-containing acrylic resins (A-1) can be copolymers of hydroxyl-containing mono(meth)acrylates and other vinyl monomers.

[0033] Examples of hydroxyl-containing mono(meth)acrylates include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. Other examples include polycaprolactone-modified polyol mono(meth)acrylates, which are commercially available, for example, as Placcel FA-1 (product name, manufactured by Daicel, a monomer obtained by ring-opening addition of 1 mole of ε-caprolactone to 1 mole of 2-hydroxyethyl acrylate), Placcel FM-1D, Placcel FM-2D, Placcel FM-3, and Placcel FM-4 (product names, monomers obtained by ring-opening addition of 1 mole, 2 mole, 3 mole, or 4 mole of ε-caprolactone to 1 mole of 2-hydroxyethyl methacrylate).

[0034] Specific examples of other vinyl monomers capable of copolymerizing with hydroxyl-containing mono(meth)acrylates include (meth)acrylic acid, alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, and stearyl (meth)acrylate, esters of dicarboxylic acids such as itaconic acid, maleic acid, and fumaric acid, styrene, nucleosubstituted styrene compounds such as vinyltoluene, dimethylstyrene, and ethylstyrene, cyclohexyl (meth)acrylate, glycidyl (meth)acrylate, (meth)acrylonitrile, (meth)acrylamide, vinyl acetate, and vinyl chloride. These other copolymerizable vinyl monomers can be used alone or in combination of two or more types.

[0035] The hydroxyl-containing acrylic resin (A-1) has a hydroxyl value of 80-250 mg KOH / g, preferably 100-230 mg KOH / g. When the hydroxyl value is less than 80 mg KOH / g, the crosslinking density of the cured coating is insufficient, and the gasoline resistance tends to deteriorate. Furthermore, when the hydroxyl value exceeds 250 mg KOH / g, the polarity of the main resin becomes too high, resulting in insufficient compatibility with the curing agent and potential turbidity in the coating. The hydroxyl value can be adjusted by copolymerizing hydroxyl-containing mono(meth)acrylates.

[0036] Hydroxyl-containing acrylic resin (A-1) can be obtained by copolymerizing other vinyl monomers capable of copolymerizing with hydroxyl-containing mono(meth)acrylates, but its mass average molecular weight is 1,000-30,000 g / mol, preferably 3,000-18,000 g / mol, more preferably 4,000-16,000 g / mol. When the mass average molecular weight is less than 1,000 g / mol, gasoline resistance may deteriorate. Meanwhile, if the mass average molecular weight exceeds 30,000 g / mol, compatibility with the curing agent deteriorates and defects such as cloudiness in the coating may occur. Furthermore, if the mass average molecular weight exceeds 30,000 g / mol, smoothness deteriorates and an orange-peel appearance may occur when spraying.

[0037] In this specification, the mass-average molecular weight is the molecular weight determined by gel permeation chromatography (GPC) and calculated based on the molecular weight of polystyrene.

[0038] More specifically, molecular weight can be measured, for example, using a commercially available "HLC8120GPC" (product name, manufactured by Tosoh Corporation) as a gel permeation chromatograph and four columns, namely the commercially available "TSKgel G2000HXL", "TSKgel G3000HXL", "TSKgel G4000HXL" and "TSKgel G5000HXL" (product names, all manufactured by Tosoh Corporation), at a measurement temperature of 40°C and a flow rate of 1 mL / min, using tetrahydrofuran as the eluent and a differential refractive index detector (RI detector) as the detector.

[0039] The hydroxyl-containing acrylic resin (A-1) has a solubility parameter (SP value) of 9.0-12.0, preferably 9.5-11.5, measured using a turbidity method. When the SP value is less than 9.0, the gloss of the coating film may decrease, while when the SP value exceeds 12.0, the compatibility with the curing agent decreases and turbidity may occur in the coating film.

[0040] In this article, SP value is an abbreviation for "Solubility Parameter" and is used as an indicator of solubility. Polarity increases with increasing SP value and decreases with decreasing SP value. Specifically, SP value can be measured using the turbidity method described below. At a measurement temperature of 20°C, 0.5 g of sample resin is weighed into a 100 mL beaker, and 10 mL of acetone is added using a pipette while stirring with a magnetic stirrer to dissolve the sample. Next, a low-SP solvent (n-hexane) is slowly added dropwise to the dilute solution using a 50 mL burette, and the amount of low-SP solvent added is recorded when the resin solution becomes turbid. Similarly, a high-SP solvent (ion-exchanged water) is slowly added dropwise to the dilute solution, and the amount of high-SP solvent added is recorded when the resin solution becomes turbid.

[0041] The SP value δ of this resin is determined using the following formula:

[0042] δ=(V ml 1 / 2 δ ml +V mh 1 / 2 δmh) / (V ml 1 / 2 +V mh 1 / 2 )

[0043]

[0044]

[0045]

[0046]

[0047] V i : Molar volume of solvent (mL / mol) [V1, V2, and V3 are the molar volumes of solvents 1, 2, and 3 (in this specification, solvents 1, 2, and 3 are acetone, hexane, and ion-exchanged water, respectively. This also applies to δ below)]

[0048] Volume percentage of each solvent at the turbidity point

[0049] δ i : SP values ​​of solvents [δ1, δ2, and δ3 are the SP values ​​of solvents 1, 2, and 3, respectively]

[0050] ml: Low SP poor solvent mixture

[0051] mh: High SP poor solvent mixture

[0052] The method for polymerizing hydroxyl-containing acrylic resins (A-1) is not particularly limited and conventional methods described in publicly available literature, such as solution radical polymerization, can be used. An example is a method involving the dropwise addition of a suitable radical polymerization initiator and monomer mixture solution to a suitable solvent while stirring at a polymerization temperature of 60-160°C for 2-10 hours. The radical polymerization initiator used herein is not particularly limited, as long as it is commonly used in polymerization, and examples include organic peroxide-based polymerization initiators and azo-based polymerization initiators. The amount of polymerization initiator is not particularly limited, but is typically 0.1-15% by mass relative to the total monomer amount, preferably 0.5-12% by mass. Furthermore, the solvent used herein is not particularly limited, as long as it does not affect the reaction, and aromatic solvents such as toluene and xylene, aliphatic solvents such as solvent oils, ester solvents such as ethyl acetate and butyl acetate, ketone solvents such as methyl ethyl ketone, and mixtures thereof can be used.

[0053] The surface conditioner (A-2) in this invention is a graft polymer of an acrylic resin (A-2-i) with an SP value of 8.5-10.0 and a resin (A-2-ii) with an SP value at least 2.0 higher than that of the acrylic resin (A-2-i).

[0054] The acrylic resin (A-2-i) used in the surface conditioner (A-2) of the present invention has an SP value of 8.5-10.0, preferably 8.7-9.8. The acrylic resin (A-2-i) preferably has an SP value at least 0.5 lower than that of the hydroxyl-containing acrylic resin (A-1) which is the main resin. If the SP value of the acrylic resin (A-2-i) is less than that of the hydroxyl-containing acrylic resin (A-1) which is the main resin, the transparent coating of the present invention orients on the surface of the coating fog and reduces the surface tension of the fog, thereby contributing to the wettability of the surface to be coated. When the SP value of the acrylic resin (A-2-i) is less than 8.5, its compatibility with the hydroxyl-containing acrylic resin (A-1) which is the main resin is poor, and defects such as shrinkage may occur. When the SP value exceeds 10.0, the beneficial effect of reducing the surface tension of the wet film decreases, the orientation on the surface of the wet film decreases, and when external components are bonded to the outer surface of the multilayer coating of the present invention, the beneficial effect of achieving good adhesion by means of double-sided tape, especially made of acrylic adhesive, may decrease.

[0055] Acrylic resins (A-2-i) can be synthesized by combining a variety of vinyl monomers that are capable of copolymerizing with each other. Specific examples of vinyl monomers include (meth)acrylic acid, alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, and stearyl (meth)acrylate, esters of dicarboxylic acids such as itaconic acid, maleic acid, and fumaric acid, styrene, nucleosubstituted styrene compounds such as vinyltoluene, dimethylstyrene, and ethylstyrene, cyclohexyl (meth)acrylate, glycidyl (meth)acrylate, (meth)acrylonitrile, (meth)acrylamide, vinyl acetate, and vinyl chloride. These copolymerizable vinyl monomers can be used alone or in combination of two or more types. Acrylic resins (A-2-i) can be obtained by copolymerizing the hydroxyl-containing mono(meth)acrylates mentioned above.

[0056] The resin (A-2-ii) in this invention is a resin with an SP value at least 2.0 higher than that of the acrylic resin (A-2-i), preferably 2.2. Examples of the resin (A-2-ii) include acrylic resins, polyurethane resins, and monovalent polyoxyethylene containing ethylene oxide units.

[0057] The surface conditioner (A-2) in this invention is a polymer obtained by grafting resin (A-2-ii) onto an acrylic resin (A-2-i). This grafted polymer can be produced by copolymerizing the acrylic resin (A-2-i) with resin (A-2-ii) as a free radical polymerizable macromonomer.

[0058] For example, when the resin (A-2-ii) is an acrylic resin, the graft polymer can be obtained as follows: First, a macromonomer is produced using a well-known method, such as the method disclosed in Japanese Patent Application Publication S60-133007, in which free radical polymerizable groups such as (meth)acryloyl and styrene groups are bonded to one end of the polymer backbone. Then, the obtained macromonomer is copolymerized with the monomer of the acrylic resin (A-2-i).

[0059] In this invention, the SP value of the grafted resin (A-2-ii) is considered as the SP value of the produced macromonomer. Furthermore, the SP value of the acrylic resin (A-2-i) to be grafted is determined by producing a separate polymer having the monomer composition of the acrylic resin (A-2-i)—excluding the macromonomer—and subsequently measuring the resulting polymer to determine its SP value.

[0060] The surface conditioner (A-2) in this invention is obtained by grafting and polymerizing acrylic resin (A-2-i) and resin (A-2-ii) in a mass ratio of 90 / 10 to 50 / 50. When the mass ratio of resin (A-2-ii) is less than 10 parts, the beneficial effect of achieving good adhesion between the multilayer coating surface and the acrylic adhesive double-sided tape may decrease. Simultaneously, when the mass ratio of resin (A-2-ii) exceeds 50 parts, the polarity of the surface conditioner (A-2) becomes excessively high, which means that the orientation on the wet coating surface during application is reduced, and the beneficial effect of achieving good adhesion with the acrylic adhesive double-sided tape may decrease.

[0061] In this invention, the surface conditioner (A-2) has a mass-average molecular weight of 3,000-100,000, preferably 5,000-80,000, and more preferably 10,000-50,000. When this mass-average molecular weight is less than 3,000, the beneficial effect of achieving good adhesion between the multilayer coating surface and the acrylic double-sided adhesive tape may decrease. Simultaneously, when this mass-average molecular weight exceeds 100,000, the compatibility with the hydroxyl-containing acrylic resin (A-1) as the main resin deteriorates, the beneficial effect of achieving good adhesion with the double-sided adhesive tape decreases, and defects such as shrinkage may occur.

[0062] In this invention, the mass percentage of solids in the surface conditioner (A-2) is 0.01-3.0% by mass relative to the mass percentage of solids in the main resin (A-1). When the mass percentage of solids in the surface conditioner (A-2) is less than 0.01% by mass, the beneficial effect of achieving good adhesion between the surface of the multilayer coating and the acrylic adhesive double-sided tape may decrease. Simultaneously, when the mass percentage of solids in the surface conditioner (A-2) exceeds 3.0% by mass, turbidity may occur on the cured coating surface.

[0063] In addition to hydroxyl-containing resin (A-1) and surface modifier (A-2), the main component (A) of the transparent coating of the present invention may contain organic solvents and various additives, such as ultraviolet absorbers, light stabilizers, antioxidants, surface modifiers other than surface modifier (A-2), electrostatic additives, and rheology modifiers such as polyethylene wax, polyamide wax, and fine particles of internally crosslinked resin.

[0064] The main agent (A) of the transparent coating of the present invention can be used as the main agent of uncolored transparent coatings or as the main agent of colored transparent coatings containing colorants such as dyes or pigments.

[0065] The transparent coating of the present invention is a two-component transparent coating in which the main agent (A) and the curing agent (B) are mixed immediately before application and then the mixture is applied.

[0066] Aliphatic or alicyclic non-yellowing polyisocyanate compounds can be advantageously used as curing agents in the transparent coatings of the present invention. Typical examples include isocyanurates of polymers of hexamethylene diisocyanate and / or isophorone diisocyanate with polyols and / or low molecular weight polyester polyols, and biuret compounds obtained by further reacting the urethane bonds. One of these isocyanate compounds can be used alone or in combination of two or more types. Isocyanurates of hexamethylene diisocyanate are particularly preferred as curing agents in the transparent coatings of the present invention.

[0067] The curing agent content used in the transparent coating of the present invention should be such that the molar ratio (NCO / OH) of the number of hydroxyl functional groups in the hydroxyl-containing acrylic resin (A-1) to the number of isocyanate functional groups (NCO groups) in the curing agent is preferably 0.5-2.0, more preferably 0.8-1.5.

[0068] The intermediate coating composition of the present invention is an aqueous two-component intermediate coating composition, wherein the latter contains an acrylic urethane resin with a hydroxyl value of 20-40 mg KOH / g and an acid value of 20-40 mg KOH / g in an amount of 20-60% by mass of the solid resin content in the main agent, and contains a carbodiimide compound and / or isocyanate compound as a curing agent. The curing agent is preferably a carbodiimide compound having 2 moles or more carbodiimide groups per molecule. Furthermore, the equivalent ratio of the carbodiimide groups in the carbodiimide compound to the equivalent ratio of the carboxylic acid groups contained in the main agent resin is preferably 0.8-1.2.

[0069] A wide variety of pigments, such as coloring pigments, glitter pigments, and extender pigments, can be incorporated into the intermediate coating compositions of the present invention. Examples of coloring pigments include inorganic pigments such as chrome yellow, iron oxide yellow, iron oxide, carbon black, and titanium dioxide; and organic pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, pyrenone pigments, perylene pigments, and dioxin pigments. Alkane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments. Additionally, examples of glitter pigments include aluminum flake pigments, alumina flake pigments, mica pigments, silica flake pigments, and glass flake pigments. Furthermore, examples of extender pigments include calcium carbonate, barite, precipitated barium sulfate, clay, and talc. One of these pigments can be used alone or in combination of two or more types.

[0070] When pigments are added to the intermediate coating composition of the present invention, the mass ratio of pigments is preferably 3-200% by mass, more preferably 30-170% by mass, and even more preferably 50-150% by mass, relative to the total amount of resin solids in the base resin.

[0071] One or more types of additives, such as surface modifiers, defoamers, surfactants, film-forming aids, preservatives, UV absorbers, light stabilizers and antioxidants, rheology control agents and organic solvents, can be incorporated into the intermediate coating composition of the present invention.

[0072] Specific examples of rheology control agents include gum arabic, arabinogalactan, alginate and its salts, gelling polysaccharides, gum arabic, carrageenan, ebony gum, agar, xanthan gum, guar gum, enzymatically degraded guar gum, quince seed gum, gellan gum, gelatin, tamarind gum, indigestible dextrin, tragacanth gum, red algae gum, amylopectin, pectin, polydextrose, galactomannan, locust bean gum, water-soluble polymers such as water-soluble soybean sugars, carboxymethyl cellulose, metal salts of carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and sodium polyacrylate, as well as chlorophyll-based clay minerals. These substances can be used alone or in mixtures of two or more types.

[0073] The intermediate coating composition of the present invention contains water as a medium, but if necessary, the intermediate coating composition can be applied after being diluted to a suitable viscosity by using water or, as appropriate, a small amount of organic solvent or amine.

[0074] By using the aqueous two-component intermediate coating composition of the present invention, even in methods of forming multilayer coatings using such a 3C1B process by forming a base coat and a clear coat on a preheated intermediate coat—without heating and curing the intermediate coat—and subsequently heating and curing the three coats simultaneously, layer mixing between the intermediate coat and the base coat can be prevented and a multilayer coating with excellent surface smoothness can be formed.

[0075] The base coating composition used in this invention may be an aqueous base coating composition, which contains an acrylic urethane resin with a hydroxyl value of 20-40 mg KOH / g and an acid value of 20-40 mg KOH / g in an amount of 20-60% by weight of solid resin.

[0076] A wide variety of pigments, such as coloring pigments, glitter pigments, and extender pigments, can be incorporated into the base coating composition of the present invention. Examples of coloring pigments include inorganic pigments such as chrome yellow, iron oxide yellow, iron oxide, carbon black, and titanium dioxide; and organic pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, pyrenone pigments, perylene pigments, and dioxin pigments. Alkane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments. Additionally, examples of glitter pigments include aluminum flake pigments, alumina flake pigments, mica pigments, silica flake pigments, and glass flake pigments. Furthermore, examples of extender pigments include calcium carbonate, barite, precipitated barium sulfate, clay, and talc. One of these pigments can be used alone or in combination of two or more types.

[0077] When pigments are added to the base coating composition of the present invention, the mass ratio of pigments is preferably 3-200% by mass, more preferably 30-170% by mass, and even more preferably 50-150% by mass, relative to the total amount of resin solids in the base resin.

[0078] One or more types of additives, such as surface modifiers, defoamers, surfactants, film-forming aids, preservatives, UV absorbers, light stabilizers and antioxidants, rheology control agents and organic solvents, can be incorporated into the base coating composition of the present invention.

[0079] Specific examples of rheology control agents include gum arabic, arabinogalactan, alginate and its salts, gelling polysaccharides, gum arabic, carrageenan, ebony gum, agar, xanthan gum, guar gum, enzymatically degraded guar gum, quince seed gum, gellan gum, gelatin, tamarind gum, indigestible dextrin, tragacanth gum, red algae gum, amylopectin, pectin, polydextrose, galactomannan, locust bean gum, water-soluble polymers such as water-soluble soybean sugars, carboxymethyl cellulose, metal salts of carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and sodium polyacrylate, as well as chlorophyll-based clay minerals. These substances can be used alone or in mixtures of two or more types.

[0080] The base coating composition of the present invention contains water as a medium, but the base coating composition may be applied after being diluted to a suitable viscosity by using water or, as appropriate, a small amount of organic solvent or amine.

[0081] The base coating composition of the present invention may be free of curing agents, but when it does contain curing agents, examples include amino resins, polyisocyanate compounds, blocked polyisocyanate compounds, and polycarbodiimide compounds. Polyisocyanate compounds and polycarbodiimide compounds are preferred from the perspective of coating appearance. Furthermore, one of these curing agents can be used alone or in combination of two or more types.

[0082] By using the above-mentioned water-based coatings, a coating film with particularly excellent water resistance can be formed even at a thermosetting temperature of 100°C or lower.

[0083] Methods commonly used in the automotive industry, such as air spraying, air atomizing electrostatic coating, or clockwork atomizing electrostatic coating, can be used as methods for applying coatings in the method of forming a multilayer coating film of the present invention.

[0084] In the method for forming a multilayer coating of the present invention, the application conditions of the water-containing intermediate coating and the water-containing base coating are preferably a temperature of 10-40°C and a relative humidity of 65-85%.

[0085] In the method for forming a multilayer coating of the present invention, preheating can be performed after applying the aqueous intermediate coating or after applying the aqueous base coating. However, when the aqueous base coating composition of the present invention is used, an excellent coating appearance can be achieved even without preheating after applying the aqueous intermediate coating. Furthermore, when preheating is performed, the preheating temperature is preferably 30-100°C and the preheating time is preferably 3-10 minutes.

[0086] In the method for forming a multilayer coating according to the present invention, the thermosetting temperature is 100°C or lower, preferably 75-90°C. The heating time is preferably 20-120 minutes.

[0087] In the method for forming a multilayer coating of the present invention, the object to be coated is a metal and / or resin material commonly used in motor vehicle bodies on which an electrodeposited coating has already been formed, but may include steps such as applying a primer before applying the aqueous intermediate coating.

[0088] [Working Example]

[0089] The invention will now be explained in more detail through the use of working examples, but the invention is by no means limited to these working examples. Unless otherwise expressly stated, parts, percentages, and proportions in the examples refer to parts by mass, % by mass, and mass ratio.

[0090] <Production Example 1: Production of Polyester Resin Varnish>

[0091] 54.0 parts of dimer acid (product name "PRIPOL 1017", manufactured by CRODA, carbon number: 36), 8.0 parts of neopentyl glycol, 17.76 parts of isophthalic acid, 19.41 parts of 1,6-hexanediol, and 0.81 parts of trimethylolpropane were placed in a reaction vessel equipped with a reflux condenser with a reaction water separator, a nitrogen introduction device, a thermometer, and a stirrer. The temperature was raised to 120°C to dissolve the raw materials, and then the temperature was raised to 160°C while stirring the contents of the reaction vessel. This temperature was maintained at 160°C for 1 hour, and then gradually increased to 230°C over 5 hours. By maintaining the reaction at 230°C and cooling it to 80°C or lower when the resin acid value reaches 4 mg KOH / g, followed by the addition of 31.6 parts of methyl ethyl ketone, a polyester resin varnish with a solid resin content of 74.6%, a resin hydroxyl value of 62 mg KOH / g, a resin acid value of 4 mg KOH / g, and a mass average molecular weight of 3,200 is obtained.

[0092] <Production Example 2: Production of Polyurethane Resin>

[0093] 78.9 parts of the polyester resin varnish obtained in Production Example 1, 7.8 parts of dimethylolpropionic acid, 1.5 parts of neopentyl glycol, and 40.0 parts of methyl ethyl ketone were placed in a reaction vessel equipped with a nitrogen inlet device, a thermometer, and a stirrer. While stirring the contents of the reaction vessel, the temperature was raised to 80°C. 27.8 parts of isophorone diisocyanate were added, and the reaction was continued at 80°C. When the isocyanate value reached 0.43 meq / g, 4.0 parts of neopentyl glycol were added, and the reaction was continued at 80°C. Furthermore, when the isocyanate value reached 0.01 meq / g, 33.3 parts of butyl cellosolve were added, and the reaction was terminated. Next, the temperature was raised to 100°C, and methyl ethyl ketone was removed under reduced pressure. By cooling to 50°C, adding 4.0 parts of dimethylethanolamine to neutralize the acid groups, and adding 147.9 parts of deionized water, a polyurethane resin with a solid resin content of 35.0%, a resin hydroxyl value of 21 mg KOH / g, a resin acid value of 35 mg KOH / g, and a mass average molecular weight of 7,800 was obtained.

[0094] <Production Example 3: Production of Acrylic Urea Resin>

[0095] 46.4 parts of the polyurethane resin obtained in Production Example 2 and 33.1 parts of deionized water were placed in a reaction vessel equipped with a nitrogen inlet device, a thermometer, a dropping funnel, and a stirrer. While stirring the contents of the reaction vessel, the temperature was raised to 85°C, and a homogeneous mixture containing 4.92 parts of styrene, 5.5 parts of methyl methacrylate, 4.02 parts of n-butyl acrylate, 1.62 parts of 2-hydroxyethyl methacrylate, 0.21 parts of acrylic acid, 3.9 parts of propylene glycol monomethyl ether, and 0.2 parts of the polymerization initiator tert-butyl peroxy-2-ethylhexanoate was added dropwise over a period of 3.5 hours. After the dropwise addition was completed, the reaction was terminated by maintaining the temperature at 85°C for 1 hour, adding a polymerization initiator solution obtained by dissolving 0.03 parts of polymerization initiator tert-butyl peroxy-2-ethylhexanoate in 0.11 parts of propylene glycol monomethyl ether as an additional catalyst, and maintaining the temperature at 85°C for another hour to obtain a core / shell type acrylic urethane resin with an acrylic resin / urethane resin ratio of 50 / 50, a solid resin content of 32.5%, a resin hydroxyl value of 32 mg KOH / g, and a resin acid value of 23 mg KOH / g.

[0096] <Production Example 4: Production of Aqueous Intermediate Coating Composition>

[0097] Pigment paste was prepared by adding 29.29 parts of titanium dioxide (product name "Ti-Pure R706", manufactured by DuPont) and 0.3 parts of carbon black (product name "MA-100", manufactured by Mitsubishi Chemical Corp.) to 36.12 parts of polyurethane resin obtained in Production Example 2 and then dispersing it in an electric mill.

[0098] Next, a resin matrix was prepared by mixing 19.46 parts of the core / shell acrylic urethane resin obtained in Production Example 3 and 4.49 parts of an aqueous acrylic resin (product name "SETAQUA 6511", manufactured by Nuplex Resins, acid value 8 mg KOH / g, hydroxyl value 138 mg KOH / g, solids content 47%) in a solvent. This resin matrix was then added to the previously prepared pigment paste and mixed. Immediately before coating, 10.34 parts of a carbodiimide compound (product name "Carbodiimide V02-L2", manufactured by Nisshinbo Chemical Inc., solids content: 40%, carbodiimide group equivalent: 385) was mixed as a curing agent. This aqueous intermediate coating composition contains acrylic urethane resin in an amount of 25% by weight of solids resin and the ratio of carbodiimide group equivalent to the equivalent of carboxylic acid groups in the main resin is 1.0.

[0099] <Production Example 5: Production of Base Coating Composition>

[0100] Pigment paste was prepared by adding 1.55 parts of carbon black (product name "MA-100", manufactured by Mitsubishi Chemical Corp.) to 50.5 parts of polyurethane resin obtained in Production Example 2 and then dispersing it in an electric mill.

[0101] Next, a resin matrix was prepared by mixing 27.21 parts of the core / shell acrylic urethane resin obtained in Production Example 3 and 6.28 parts of an aqueous acrylic resin (product name "SETAQUA 6511", manufactured by Nuplex Resins, acid value 8 mg KOH / g, hydroxyl value 138 mg KOH / g, solids content 47%) in a solvent. This resin matrix was then added to the previously prepared pigment paste and mixed. Immediately before coating, 14.46 parts of a carbodiimide compound (product name "Carbodiimide V02-L2", manufactured by Nisshinbo Chemical Inc., solids content: 40%, carbodiimide equivalent: 385) was mixed as a curing agent. This aqueous base coating composition contains acrylic urethane resin at a solids content of 25% by mass and the carbodiimide equivalent is 1.0 relative to the carboxylic acid equivalent in the main resin.

[0102] <Production Example 6: Production of Hydroxyl Acrylic Resin Varnish A-1-1>

[0103] 27.0 parts xylene and 9.0 parts propylene glycol monomethyl ether acetate were added to a flask equipped with a thermometer, reflux condenser, stirrer, dropping funnel, and nitrogen introduction device. The mixture was heated to 130°C while stirring and introducing nitrogen. Next, 6.0 parts styrene, 0.8 parts acrylic acid, 16.7 parts 2-hydroxyethyl methacrylate, and 36.6 parts n-butyl methacrylate (as free radical polymerizable monomers), and 1.0 part tert-butyl peroxy-2-ethylhexanoate (as polymerization initiator) were uniformly mixed. This mixture was then added dropwise through the dropping funnel at the same rate over 3 hours at 130°C. After the addition was complete, the temperature was maintained at 130°C for 1 hour, and then the reaction temperature was lowered to 110°C.

[0104] Then, by adding 0.1 parts of tert-butyl peroxy-2-ethylhexanoate as an additional catalyst as a polymerization initiator, maintaining the temperature at 110°C for another 2 hours, adding 2.9 parts of xylene to dilute the mixture, and then cooling, hydroxyl-containing acrylic resin varnish A-1-1 was obtained.

[0105] <Production Example 7: Production of Hydroxyl Acrylic Resin Varnish A-1-2>

[0106] Hydroxyl acrylic resin varnish A-1-2 was obtained using a production method similar to that used for A-1-1, except that the amount of raw materials added was changed as shown in Table 1.

[0107] [Table 1]

[0108]

[0109] <Production Example 8: Production of Surface Conditioner A-2-a>

[0110] The macromonomer (A-2-ii) (mass average molecular weight 4,000) of an acrylonitrile / methyl methacrylate copolymer with a methacryloyl group at the end is produced by the following method from the monomer composition (basic structure) that constitutes part of the resin (A-2-ii) in Table 2, namely 15 parts acrylonitrile and 15 parts methyl methacrylate.

[0111] 50.0 parts of butyl acetate were added to a flask equipped with a thermometer, reflux condenser, stirrer, dropping funnel, and nitrogen introduction device. While stirring and introducing nitrogen, a liquid mixture containing 23.85 parts of acrylonitrile, 23.85 parts of methyl methacrylate, 1.29 parts of 3-mercaptopropionic acid, and 1.0 part of azobisisobutyronitrile was uniformly mixed and added dropwise through the dropping funnel at the same rate over 4 hours at 80°C. After the addition was complete, the temperature was maintained at 80°C for 2 hours, then the reaction temperature was raised to 95°C and maintained for 1 hour. The resulting prepolymer reaction solution had an acid value of 6.8 mg KOH / g. After heating at 130°C for 1 hour, the mass of the prepolymer reaction solution was 50.0% of its initial mass.

[0112] Next, 3.32 parts of glycidyl methacrylate, 0.48 parts of tetrabutylammonium bromide as a catalyst, and 0.02 parts of hydroquinone monomethyl ether as a polymerization inhibitor were added to 96.18 parts of the prepolymer reaction solution, and the resulting mixture was reacted at a reaction temperature of 90°C for 8 hours. After heating at 130°C for 1 hour, the heating residue of the obtained macromonomer was 51.3%. The obtained macromonomer had an acid value of 0.03 mg KOH / g or less and an SP value of 11.5.

[0113] A solution of a surface conditioner (A-2-a) with a solid resin content of 50% and a mass average molecular weight of 30,000 was obtained by copolymerizing 65.3 parts of a macromonomer solution comprising 30 parts of the macromonomer and the monomers of the acrylic resin (A-2-i) shown in Table 2 (35.0 parts by weight of stearyl methacrylate and 35.0 parts by weight of 2-ethylhexyl acrylate) using xylene as a polymerization initiator.

[0114] Separately, an acrylic resin (A-2-i) with a solid resin content of 50% and a mass average molecular weight of 10,000 is produced by copolymerizing the monomers of the acrylic resin (A-2-i) in xylene using tert-butyl peroxy-2-ethylhexanoate as a polymerization initiator (comprising a monomer mixture of 35.0 parts stearyl methacrylate and 35.0 parts 2-ethylhexyl acrylate), and the acrylic resin (A-2-i) has a measured SP value of 9.1.

[0115] [Table 2]

[0116]

[0117] <Production Examples 9-14: Production of Surface Conditioners A-2-b to A-2-g>

[0118] The same method used to produce surface conditioner A-2-a involves first producing a macromonomer (A-2-ii) with a terminal methacryloyl group from the monomer composition of the resin (A-2-ii) shown in Table 2, and then copolymerizing the macromonomer and the monomer of the acrylic resin (A-2-i) shown in Table 2 in xylene using tert-butyl peroxy-2-ethylhexanoate as a polymerization initiator to produce a solution of surface conditioners (A-2-b to A-2-g) having the performance values ​​shown in Table 2.

[0119] Separately, acrylic resins (A-2-i) with a solid resin content of 50% and a mass average molecular weight of 10,000 were produced by copolymerizing a mixture of monomers of the acrylic resins (A-2-i) shown in Table 2 using xylene as a polymerization initiator, and the SP values ​​of these acrylic resins were measured.

[0120] <Working Examples 1-3 and Comparative Examples 1-7>

[0121] The two-component transparent coating composition is produced by sequentially mixing the raw materials of the main agent shown in Table 3, stirring until uniformly mixed, and then combining the main agent with the curing agent shown in the table.

[0122]

[0123] <>

[0124] 1) Tinuvin 384-2: Product name, manufactured by BASF Japan, benzotriazole UV absorber (95% by weight solids)

[0125] 2) Tinuvin 292: Product name, manufactured by BASF Japan, light stabilizer.

[0126] 3) BYK-300: Product name, manufactured by BYK Japan, silicone-based surface conditioner (52% by mass solids).

[0127] 4) Solvesso 100: Product name, manufactured by Exxon Mobil, aromatic hydrocarbon-based solvent.

[0128] 5) Sumidur N3300: Product name, manufactured by Sumika Covestro Urethane Co., Ltd., is an isocyanurate type resin of 1,6-hexamethylene diisocyanate (100% by weight solids, 21.8% by weight NCO).

[0129] <<Evaluation plate 1 is produced by applying an electrodeposited coating to a steel plate>>

[0130] A cationic electrodeposited coating (product name "Cathoguard No. 500", manufactured by BASF Coatings) was electrodeposited on a zinc phosphate-treated low-carbon steel plate to obtain a dry film thickness of 20 μm, and then baked at 175°C for 25 minutes to obtain an electrodeposited coated plate (hereinafter referred to as "electrodeposited plate") for evaluation of this invention.

[0131] Next, evaluation panel 1 was produced using a rotary atomizing bell coater (product name "Metallic Bell G1-COPES Bell", manufactured by ABB) under coating conditions of 25°C and 75% relative humidity, following the procedures described below. Furthermore, in the following multilayer coating formation, the intermediate coating and the base coating were applied after dilution with deionized water, resulting in a viscosity of 40 seconds (20°C) measured using a Ford cup (size 4).

[0132] The intermediate coating was applied to the electrodeposition plate to obtain a dry film thickness of 20 μm. The coated plate was then left to stand at room temperature for 5 minutes, followed by the application of the base coating to obtain a dry film thickness of 12 μm. After coating, the plate was left to stand at room temperature for 5 minutes, then preheated at 80°C for 3 minutes. The coated plate was then cooled to room temperature. Next, immediately before coating, a mixture was prepared by mixing the main agents and curing agents of the transparent coatings CC-1 to CC-10 shown in Table 3 and diluted with Solvesso 100 to achieve a viscosity of 25 seconds (20°C) measured using a Ford cup (size 4). This mixture was then applied to the coated plate, which had been adjusted to room temperature, to obtain a dry film thickness of 30 μm. After coating, the coated plate was left to stand at room temperature for 10 minutes, then baked at 80°C for 30 minutes to obtain evaluation plate 1 of the electrodeposition plate.

[0133] <<Production of Evaluation Board 2 by Coating with Resin Material>>

[0134] The resin-coated board (hereinafter referred to as the "resin board") used for evaluation in this invention was obtained by spraying a degreased polypropylene board with dimensions of 70×150×3mm with a primer coating (product name "Plymac No. 1501", manufactured by BASF Coatings) to obtain a dry film thickness of 10μm.

[0135] Next, evaluation panel 2 was produced using a rotary atomizing bell coater (product name "Metallic Bell G1-COPES Bell", manufactured by ABB) under coating conditions of 25°C and 75% relative humidity, following the procedures described below. Furthermore, in the following multilayer coating formation, the intermediate coating and the base coating were applied after dilution with deionized water, resulting in a viscosity of 40 seconds (20°C) measured using a Ford cup (size 4).

[0136] The intermediate coating was applied to the resin board to obtain a dry film thickness of 20 μm. The coated board was then left to stand at room temperature for 5 minutes, followed by the application of the base coating to obtain a dry film thickness of 12 μm. After application, the board was left to stand at room temperature for 5 minutes, then preheated at 80°C for 3 minutes. After the coated board cooled to room temperature, a mixture was prepared immediately before application by mixing the main agents and curing agents of the transparent coatings CC-1 to CC-10 shown in Table 3 and diluting with Solvesso 100 to achieve a viscosity of 25 seconds (20°C) measured using a Ford cup (size 4). This mixture was then applied to the resin board to obtain a dry film thickness of 30 μm. After application, the board was left to stand at room temperature for 10 minutes, then baked at 80°C for 30 minutes to obtain evaluation board 2 of the resin board.

[0137] <<Evaluation Methods for Evaluation Boards>>

[0138] The following coating performance evaluations were performed on the obtained evaluation plates 1 and 2.

[0139] (1) Coating appearance

[0140] The coating appearance of each evaluation panel was evaluated visually according to the following criteria. Furthermore, the evaluation result was the average of the observations from 20 evaluators.

[0141] ◎: When fluorescent light shines on the coating, it reflects the fluorescent light very clearly.

[0142] ○: When fluorescent light shines on the coating, the fluorescent light is not reflected very clearly.

[0143] △: When fluorescent light shines on the coating, the outline of the fluorescent light is blurred.

[0144] ×: When fluorescent light shines on the coating, the outline of the fluorescent light is very blurry.

[0145] (2) Water resistance

[0146] After production, the evaluation panels were left at room temperature for one week, then immersed in pure water at 40°C and remained submerged for 240 hours. Following immersion, the coating was visually evaluated according to the following criteria. Furthermore, the evaluation result was the average of observations from 20 evaluators.

[0147] ○: No turbidity was observed.

[0148] △: Slight turbidity was observed.

[0149] ×: Significant turbidity was observed.

[0150] (3) Gasoline resistance

[0151] The test panels were immersed in ordinary unleaded gasoline (as described in JIS K2202#2) at 20°C for 24 hours, and then the appearance of the test panels was evaluated visually according to the following criteria. Furthermore, the evaluation results were the average of the observations of 20 evaluators.

[0152] ○: No abnormalities were observed.

[0153] △: Minor abnormalities such as yellowing or swelling were observed.

[0154] ×: Abnormalities such as yellowing or swelling were observed.

[0155] (4) Double-sided tape adhesion

[0156] Acrylic foam tape GT5912 manufactured by 3M Japan Ltd. was used as a double-sided tape in which the adhesive is an acrylic adhesive, and this tape was cut into 30mm × 30mm dimensions. The double-sided tape cut as described above was bonded to the upper surface of an L-shaped clamp—formed by bending a stainless steel plate with a thickness of 1mm, a width of 30mm, and a length of 80mm at a 90° angle from the top 30mm—that is, bonded to an area of ​​30mm × 30mm to cover the entire upper surface. Next, the upper surface of the L-shaped clamp was bonded to the center portion of evaluation plates 1 and 2, each measuring 70mm × 150mm, while confirming that there were no gaps between the evaluation plates and the upper surface and that the tape did not shift. The L-shaped clamp was extended downwards from the evaluation plates, supporting both sides of the evaluation plates, and a 2kg or 3kg weight was suspended from the L-shaped clamp and held in this position at room temperature for 7 days. Evaluation was performed according to the following criteria.

[0157] ◎: When a 3kg weight is suspended, the double-sided tape does not come off after 7 days or more.

[0158] ○: When a 2kg weight is suspended, the double-sided tape does not come off after 7 days or more.

[0159] △: When a 2kg weight is suspended, the double-sided tape will detach after a period of not less than 2 days but less than 7 days.

[0160] ×: When a 2kg weight is suspended, the double-sided tape will detach in less than 2 days.

[0161] For the above tests (1)-(4), the results for evaluation plate 1 of the electrodeposition plate are shown in Table 4 and the results for evaluation plate 2 of the resin plate are shown in Table 5.

[0162]

[0163]

[0164] As can be clearly seen from the working examples and comparative examples, the working examples of the present invention achieve good results in all evaluation items, while the comparative examples do not achieve good results in all evaluation items.

[0165] The invention has been clearly described above with reference to the embodiments, but the invention is not limited to these embodiments, and it is self-evident that various modifications are possible as long as they do not depart from the spirit of the invention.

Claims

1. A method of forming a multilayer coating film by sequentially applying an intermediate coating composition, a base coating composition and a clear coating composition on a metal for a motor vehicle body on which an electrodeposition coating has been applied and an electrodeposition coating film formed thereby has been completely baked, and then heat-curing, wherein the clear coating composition is obtained by mixing a main agent (A) and a curing agent (B), the main agent (A) containing a hydroxyl group-containing acrylic resin (A-1) as a main resin and a surface modifier (A-2) in an amount of 0.01 to 3.0 mass% relative to the mass of the solid content in the hydroxyl group-containing acrylic resin (A-1), the hydroxyl group-containing acrylic resin (A-1) has a hydroxyl value of 80 to 250 mg KOH / g, a mass average molecular weight of 1,000 to 30,000 and an SP value of 9.0 to 12.0, the surface modifier (A-2) is a polymer having a mass average molecular weight of 3,000 to 100,000 and obtained by grafting an acrylic resin (A-2-i) and a resin (A-2-ii), the acrylic resin (A-2-i) has an SP value of 8.5 to 10.0, and the resin (A-2-ii) has an SP value at least 2.0 higher than the SP value of the acrylic resin (A-2-i).

2. A method of forming a multilayer coating film by sequentially applying an intermediate coating composition, a base coating composition and a clear coating composition on a resin material for the exterior of a motor vehicle and then heat-curing, wherein the clear coating composition is obtained by mixing a main agent (A) and a curing agent (B), the main agent (A) containing a hydroxyl group-containing acrylic resin (A-1) as a main resin and a surface modifier (A-2) in an amount of 0.01 to 3.0 mass% relative to the mass of the solid content in the hydroxyl group-containing acrylic resin (A-1), the hydroxyl group-containing acrylic resin (A-1) has a hydroxyl value of 80 to 250 mg KOH / g, a mass average molecular weight of 1,000 to 30,000 and an SP value of 9.0 to 12.0, the surface modifier (A-2) is a polymer having a mass average molecular weight of 3,000 to 100,000 and obtained by grafting an acrylic resin (A-2-i) and a resin (A-2-ii), the acrylic resin (A-2-i) has an SP value of 8.5 to 10.0, and the resin (A-2-ii) has an SP value at least 2.0 higher than the SP value of the acrylic resin (A-2-i).

3. The method of forming a multilayer coating film according to claim 1, wherein the hydroxyl group-containing acrylic resin (A-1) comprises a copolymer of a hydroxyl group-containing mono(meth)acrylate and another vinyl monomer.

4. The method of forming a multilayer coating film according to claim 2, wherein the hydroxyl group-containing acrylic resin (A-1) comprises a copolymer of a hydroxyl group-containing mono(meth)acrylate and another vinyl monomer.

5. The method of forming a multilayer coating film according to any one of claims 1 to 4, wherein the acrylic resin (A-2-i) comprises a copolymer of a plurality of types of vinyl monomers. ​ ​ 6. The method for forming a multilayer coated film according to any one of claims 1 to 4, wherein the resin (A-2-ii) contains one or more of an acrylic resin, a polyurethane resin, and a monovalent polyoxyalkylene containing ethylene oxide units.

7. The method for forming a multilayer coated film according to claim 5, wherein the resin (A-2-ii) contains one or more of an acrylic resin, a polyurethane resin, and a monovalent polyoxyalkylene containing ethylene oxide units.

8. The method for forming a multilayer coated film according to any one of claims 1 to 4, wherein the surface conditioner (A-2) is a graft polymer in which the mass ratio of the acrylic resin (A-2-i) / the resin (A-2-ii) is from 90 / 10 to 50 / 50.

9. The method for forming a multilayer coated film according to claim 7, wherein the surface conditioner (A-2) is a graft polymer in which the mass ratio of the acrylic resin (A-2-i) / the resin (A-2-ii) is from 90 / 10 to 50 / 50.

10. The method for forming a multilayer coated film according to any one of claims 1 to 4, wherein the intermediate coating composition is an aqueous two-component type intermediate coating composition containing an acrylic urethane resin having a hydroxyl value of from 20 to 40 mg KOH / g and an acid value of from 20 to 40 mg KOH / g in an amount of from 20 to 60 mass% of the solid resin content in the main agent and containing a carbodiimide compound having 2 moles or more of carbodiimide groups per molecule as a curing agent and in which the ratio of the equivalent of the carbodiimide groups to the equivalent of the carboxylic acid groups contained in the resin in the main agent is from 0.8 to 1.

2.

11. The method for forming a multilayer coated film according to claim 9, wherein the intermediate coating composition is an aqueous two-component type intermediate coating composition containing an acrylic urethane resin having a hydroxyl value of from 20 to 40 mg KOH / g and an acid value of from 20 to 40 mg KOH / g in an amount of from 20 to 60 mass% of the solid resin content in the main agent and containing a carbodiimide compound having 2 moles or more of carbodiimide groups per molecule as a curing agent and in which the ratio of the equivalent of the carbodiimide groups to the equivalent of the carboxylic acid groups contained in the resin in the main agent is from 0.8 to 1.

2.

12. The method for forming a multilayer coated film according to any one of claims 1 to 4, wherein the base coating composition is an aqueous base coating composition containing an acrylic urethane resin having a hydroxyl value of from 20 to 40 mg KOH / g and an acid value of from 20 to 40 mg KOH / g in an amount of from 20 to 60 mass% of the solid resin content.

13. The method for forming a multilayer coated film according to claim 11, wherein the base coating composition is an aqueous base coating composition containing an acrylic urethane resin having a hydroxyl value of from 20 to 40 mg KOH / g and an acid value of from 20 to 40 mg KOH / g in an amount of from 20 to 60 mass% of the solid resin content.

14. The method for forming a multilayer coated film according to any one of claims 1 to 4, wherein the baking temperature during heat curing is from 75 to 100°C.

15. The method for forming a multilayer coating film according to claim 13, wherein the baking temperature during the thermal curing is 75 to 100 °C.

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