Method for forming multilayer coating film and transparent coating layer for producing multilayer coating film

Abstract

Problem: To provide a method for forming a multilayer coating film having excellent coating film performance even when cured at low temperature, and a clear coating composition used in the method. Means for solving the problem: a method for forming a multi-layer coating film comprising a step (1) of applying an aqueous first colored coating composition (BC1) to an object to be coated, a step (2) of applying an aqueous one-part second colored coating composition (BC2) to the coating film, and further, a step (3) of forming a multi-layer coating film on the object to be coated, a step (3) of applying a clear coating composition (CC) onto the coating film, in which the clear coating composition (CC) contains a hydroxyl group-containing acrylic resin (A1), an isocyanate curing agent (A2), and an unsaturated fatty acid ester polyol (D) having at least two hydroxyl groups in one molecule; and the distance Ra between the solubility parameter of the hydroxyl group-containing acrylic resin (A1) in the Hansen space and the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space is 4.0 or less; and a step (4) of simultaneously curing the uncured first colored coating film, the uncured second colored coating film, and the uncured clear coating film formed in steps (1) to (3) by heating at 70 DEG C to 100 DEG C; and the clear coating composition (CC).

Description

Technical Field

[0001] The present invention relates to a method for forming a multi-layer coating film and a clear coating composition used in the method. Background Art

[0002] On object to be coated such as automobile body, form multilayer coating film, so that protect object to be coated and attractive outward appearance is provided simultaneously.For this type of automobile body coating, being used to form three coatings and one baking method of multilayer coating is just becoming mainstream, wherein sequentially apply the first aqueous base coating composition, the second aqueous base coating composition and clear coating composition, and then bake and solidify these three layers simultaneously.In recent years, for energy-saving and environmental compliance, need to reduce the temperature that bakes after applying these three layers.Yet, when using conventional coating composition to solidify by low-temperature baking, be difficult to obtain fully cured film.

[0003] Furthermore, at automotive production facilities, as part of ensuring uniform production of high-quality products, the condition of already applied multilayer coatings is inspected, and polishing and / or recoating processes are performed depending on the presence of defects. In the recoating process, repairs are performed by laminating subsequent multilayer coatings on the first multilayer coating layer already applied to the object to be coated using a three-coat-one-bake method to form a laminate of multiple multilayer coatings. For this reason, it is essential that the clear coating of the first multilayer coating layer and the first aqueous base coating layer of the subsequent multilayer coating layer have excellent adhesion, that is, excellent recoat adhesion.

[0004] Under these circumstances, a method has been disclosed for improving the properties of a clear coating film and lowering the baking temperature of a multilayer coating film including the clear coating film by adding an unsaturated fatty acid ester polyol to a clear coating composition (Patent Document 1). In addition, a method has been disclosed for improving the adhesion between multilayer coating films in low-temperature baking by using an aliphatic triisocyanate having a molecular weight in the range of 200 to 350 as part of the curing agent of the clear coating composition and allowing the aliphatic triisocyanate to penetrate into the base coating during baking (Patent Document 2).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: JP 2016-188387 A

[0008] Patent Document 2: JP 7043621 B Summary of the Invention

[0009] Problems to be solved by the present invention

[0010] However, even if an unsaturated fatty acid ester polyol is added to a clear coating composition by the method of Patent Document 1 and cured at a low temperature, sufficient recoat adhesion may not be obtained.

[0011] Furthermore, even when a low-molecular-weight aliphatic triisocyanate is used as part of the curing agent in the clear coating composition and the multilayer coating film is cured at a low temperature as in Patent Document 2, it cannot necessarily be said that the recoat adhesion is sufficient. Furthermore, it is not possible to satisfy coating film properties such as gasoline resistance, chipping resistance, and water resistance in a well-balanced manner.

[0012] Meanwhile, in view of the conventional techniques described above, an object of the present invention is to provide a method for forming a multilayer coating film which has excellent recoat adhesion even when cured at low temperatures and maintains coating film properties such as gasoline resistance, chipping resistance and water resistance.

[0013] Furthermore, an object of the present invention is to provide a clear coating composition that can be used to form a multilayer coating film formed by a three-coat-one-bake method and that can form a multilayer coating film that has excellent recoat adhesion and maintains coating film properties such as gasoline resistance, chipping resistance, and water resistance even when cured at low temperatures.

[0014] Means used to solve problems

[0015] As a result of intensive research by the inventors of the present invention, it has been found that the above-mentioned object is achieved by: a method for forming a multilayer coating film, the method comprising:

[0016] A step (1) of applying an aqueous first colored coating composition (BC1) to an object to be coated to form an uncured first colored coating film;

[0017] a step (2) of applying an aqueous one-component type second colored coating composition (BC2) on the uncured first colored coating film obtained in step (1) to form an uncured second colored coating film; and

[0018] A step (3) of applying a clear coating composition (CC) onto the uncured second colored coating film obtained in step (2) to form an uncured clear coating film, wherein

[0019] The clear coating composition (CC) comprises

[0020] Hydroxyl-containing acrylic resin (A1),

[0021] Isocyanate curing agent (A2), and

[0022] an unsaturated fatty acid ester polyol (D) having at least two hydroxyl groups in one molecule,

[0023] and a distance Ra between a solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space and a solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space is 4.0 or less;

[0024] and a step (4) of simultaneously curing the uncured first colored coating film, the uncured second colored coating film, and the uncured clear coating film formed in steps (1) to (3) by heating at 70°C to 100°C.

[0025] In the method of the present invention, the hydroxyl value of the hydroxyl-containing acrylic resin (A1) is preferably within a range of 80 to 200 mg KOH / g.

[0026] In the method of the present invention, the hydroxyl-containing acrylic resin (A1) preferably has a mass average molecular weight of 3,000 to 30,000.

[0027] In the process of the present invention, the clearcoat composition (CC) preferably comprises a urethane curing catalyst (E).

[0028] In the method of the present invention, the dry film thickness of the first colored coating film is preferably 4 to 30 μm.

[0029] Furthermore, the present invention achieves the above-mentioned object by a clear coating composition (CC) for forming a clear coating film, which is used to form a multilayer coating film by simultaneously curing:

[0030] an uncured first colored coating film formed from the aqueous first colored coating composition (BC1),

[0031] an uncured second colored coating film formed of an aqueous one-component second colored coating composition (BC2) provided on the uncured first colored coating film,

[0032] and an uncured clear coating film disposed on an uncured second colored coating film, wherein:

[0033] The clear coating composition (CC) comprises

[0034] Hydroxyl-containing acrylic resin (A1),

[0035] Isocyanate curing agent (A2), and

[0036] an unsaturated fatty acid ester polyol (D) having at least two hydroxyl groups in one molecule;

[0037] The distance Ra between the solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space and the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space is 4.0 or less; and

[0038] The uncured first colored coating film, the uncured second colored coating film, and the uncured clear coating film are cured by heating at 70°C to 100°C.

[0039] In the present invention, the clear coating composition is preferably such that the amount (solid content) of the blended unsaturated fatty acid ester polyol (D) is 3 to 20% by mass relative to the total mass of the solid contents of all resin components contained in the clear coating composition (CC).

[0040] In the present invention, the aqueous first colored coating composition (BC1) preferably contains a carboxyl group-containing resin (B1) and a carbodiimide group-containing curing agent (B2),

[0041] And the clear coating composition (CC) preferably contains the isocyanate group in the isocyanate curing agent (A2) at a ratio of 0.8 to 1.5 equivalents per 1 equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin (A1).

[0042] Furthermore, in the present invention, it is preferred that the clear coating composition (CC) contains isocyanate groups in the isocyanate curing agent (A2) at a ratio of 1.5 to 3.0 equivalents per 1 equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin (A1), and the isocyanate curing agent (A2) contains the diisocyanate dimer (A2-1) and the diisocyanate trimer or higher compound (A2-2) at a solid content mass ratio in the range of 10 / 90 to 40 / 60.

[0043] In the present invention, the clear coating composition (CC) preferably contains the 3-isocyanatepropyltrialkoxysilane compound (A3), and the amount (solid content) of the 3-isocyanatepropyltrialkoxysilane compound blended is 2% by mass to 50% by mass relative to the total mass of the solid contents of all resin components contained in the clear coating composition (CC).

[0044] Effects of the Invention

[0045] The present invention enables a method for forming a multilayer coating film that can be cured at low temperatures of 70°C to 100°C and has excellent recoat adhesion. Furthermore, the clear coating composition of the present invention is suitable for forming a multilayer coating film by curing at low temperatures using a three-coat, one-bake method, and the resulting clear coating film has excellent recoat adhesion. Furthermore, the method for forming a multilayer coating film of the present invention and the multilayer coating film obtained from the clear coating composition of the present invention exhibit coating film properties, such as gasoline resistance, chipping resistance, and water resistance, that are at least equivalent to or better than those of conventional multilayer coating films. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] [ Figure 1 ] Figure 1(a) and (b) are views illustrating a method for producing a multi-layer coating film according to a first embodiment of the present invention and a clear coating composition used in the method.

[0047] [ Figure 2 ] Figure 2 (a) and (b) are views illustrating a method for producing a multi-layer coating film according to a second embodiment of the present invention and a clear coating composition used in the method.

[0048] [ Figure 3 ] Figure 3 (a) and (b) are views illustrating a method for producing a multi-layer coating film according to a third embodiment of the present invention and a clear coating composition used in the method.

[0049] [ Figure 4 ] Figure 4 (a) and (b) are views illustrating a method for producing a multi-layer coating film according to a fourth embodiment of the present invention and a clear coating composition used in the method. DETAILED DESCRIPTION

[0050] The method for forming a multilayer coating film of the present invention comprises:

[0051] A step (1) of applying an aqueous first colored coating composition (BC1) to an object to be coated to form an uncured first colored coating film;

[0052] a step (2) of applying an aqueous one-component type second colored coating composition (BC2) on the uncured first colored coating film obtained in step (1) to form an uncured second colored coating film;

[0053] a step (3) of applying a clear coating composition (CC) on the uncured second colored coating film obtained in step (2) to form an uncured clear coating film; and

[0054] A step (4) of simultaneously curing the uncured first colored coating film, the uncured second colored coating film, and the uncured clear coating film formed in steps (1) to (3) by heating at 70°C to 100°C.

[0055] In addition, the clear coating composition (CC) used in the present invention contains a hydroxyl group-containing acrylic resin (A1), an isocyanate curing agent (A2), and an unsaturated fatty acid ester polyol (D) having at least two hydroxyl groups in one molecule. In addition, in the clear coating composition, the distance Ra between the solubility parameter in the Hansen space of the hydroxyl group-containing acrylic resin (A1) and the solubility parameter in the Hansen space of the unsaturated fatty acid ester polyol (D) is 4.0 or less.

[0056] Furthermore, a method for producing a multilayer coating film according to an embodiment of the present invention will be described.

[0057] [Step (1)]

[0058] In the method for forming a multilayer coating film of the present invention, in step (1), an aqueous first colored coating composition (BC1) containing color pigments is applied to an object to be coated to form an uncured first colored coating film layer.

[0059] Object to be coated

[0060] There are no particular limitations on the objects to be coated to which the method for forming a multilayer coating film of the present invention can be applied, and examples include parts made of metals such as iron, zinc, aluminum, and magnesium, parts made of alloys of these metals, parts on which these metals have been plated or vapor-deposited, parts made of glass, plastics, and foams of various materials, among which steel materials and plastic materials constituting automobile bodies are preferred. These parts may be subjected to appropriate treatments such as degreasing and surface treatment as needed.

[0061] Furthermore, in the present invention, a product having a primer film formed on the above-mentioned component can also be used as an object to be coated. The primer film is applied to the surface of the component to conceal the surface of the component or to provide corrosion resistance, rust resistance, adhesion, etc. to the component, and can be formed by coating with a primer paint and curing or drying. These primer paints are not particularly limited, and known primer paints such as electrodeposition paints, solvent-based primers, and water-based primers can be used.

[0062] In the present invention, the object to be coated may be a product that has been subjected to at least one multilayer coating process on the aforementioned component, wherein treatment has optionally been performed on metal, glass, or plastic. Recoating refers to a process of producing another multilayer coating film by performing steps (1) to (4) on the object to be coated including such a multilayer coating film for the purpose of repair.

[0063] Aqueous first colored coating composition (BC1)

[0064] The aqueous first colored coating composition (BC1) used in step (1) of the method for forming a multilayer coating film of the present invention contains a resin component and a color pigment.

[0065] The resin component of the first aqueous colored coating composition (BC1) used in the present invention is not particularly limited as long as it is a component that can form an aqueous colored coating composition, and may be a thermosetting resin that forms a coating film as a result of a cross-linking reaction that proceeds under heating after coating, or may be a thermoplastic resin that forms a coating film by volatilization of a solvent.

[0066] In the present invention, "aqueous" means that the base resin, crosslinking agent, etc. can be stably dissolved or dispersed in water or a mixture of water and an organic solvent.

[0067] For example, a thermosetting resin composition comprising a base resin and a crosslinking agent can be used as the thermosetting resin composition. The content of the crosslinking agent in the resin composition (base resin + crosslinking agent) is not particularly limited, but is preferably 0 to 50 parts by mass, more preferably 5 to 45 parts by mass, and particularly preferably 10 to 40 parts by mass relative to 100 parts by mass of the total amount of non-volatile components in the resin composition.

[0068] The base resin of thermosetting resin composition is not particularly limited, as long as it is water-soluble or water-dispersible, but example includes acrylic resin, polyester resin, polyurethane resin, polyurea resin, acrylic urethane resin, polyurethane-polyurea resin etc.In addition, base resin can be made up of partially crosslinked particles, or particle can be the core / shell particle comprising inside (core part) and outside (shell part).The example of particulate base resin includes polyurethane-polyurea particles, urethane core / acrylic shell particles and acrylic core / urethane shell particles.It should be noted that if base resin particles are partially crosslinked, then this part will serve as the part (gel part) that is insoluble in organic solvent, and therefore gel fraction can be measured, and this gel fraction is the value of the ratio of indication gel part in the solid content of base resin particles.In addition, it is preferred that base resin has hydroxyl and / or carboxyl as functional group.These base resins can be used alone or in combination of two or more.

[0069] Examples of curing agents for thermosetting resin compositions include amino resins, polyisocyanate compounds, blocked polyisocyanate compounds, and polycarbodiimide compounds. Among them, amino resins and polycarbodiimide compounds are particularly preferred. These curing agents can be used alone or in combination of two or more.

[0070] Amino resin is a general term for resins produced by adding formaldehyde to a compound containing an amino group and condensing. Examples of amino resins include melamine resins, urea resins, and guanamine resins, among which melamine resins are particularly preferred.

[0071] Melamine resins include partially or completely methylolated melamine resins obtained by reacting melamine with formaldehyde, partially or completely alkyl ether melamine resins obtained by partially or completely etherifying the methylol groups of methylolated melamine resins with an alcohol component, imino group-containing melamine resins, and mixed melamine resins in which two or more melamine resins are mixed. In addition, examples of the alkyl ether melamine resin include methylated melamine resins, butylated melamine resins, methyl / butyl mixed alkyl ether melamine resins, and the like.

[0072] Examples of the polyisocyanate compound include: aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate; aromatic diisocyanates such as xylylene diisocyanate (XDI), tolylene diisocyanate (TDI) and 4,4-diphenylmethane diisocyanate (MDI); alicyclic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI and hydrogenated MDI; and diisocyanate dimers and trimers such as uretdiones, allophanates, adducts, biuret and isocyanurate, or compounds containing a large amount of diisocyanates.

[0073] Blocked polyisocyanate compounds include those in which the isocyanate groups of the above-mentioned polyisocyanate compounds are blocked by alcohols such as butanol, oximes such as methyl ethyl ketoxime, lactams such as ε-caprolactam, active methylenes such as malonate diesters and acetoacetate, imidazoles such as imidazole and 2-ethylimidazole, and phenols such as m-cresol.

[0074] A hydrophilic carbodiimide compound is preferably used as the polycarbodiimide compound. Examples of the hydrophilic carbodiimide compound include a product obtained by reacting a hydrophilic agent having active hydrogen and a hydrophilic portion with a reaction product obtained by reacting a polycarbodiimide compound having at least two isocyanate groups in one molecule with a polyol having a hydroxyl group at a molecular terminal at a ratio such that the NCO / OH molar ratio exceeds 1.

[0075] As the thermoplastic resin, thermoplastic resins having a mass average molecular weight of 30,000 or more, such as acrylic resins, polyester resins, alkyd resins, urethane resins, polyolefin resins (including chlorinated and / or modified resins), and epoxy resins may be used.

[0076] The color pigments in the aqueous first colored coating composition (BC1) used in the present invention include, for example: inorganic pigments such as titanium oxide pigments, iron oxide pigments, and complex oxide pigments such as titanium yellow; organic pigments such as azo-based pigments, quinacridone-based pigments, diketopyrrolopyrrole-based pigments, perylene-based pigments, pyrenone-based pigments, benzimidazolone-based pigments, isoindoline-based pigments, isoindolinone-based pigments, metal chelate azo-based pigments, phthalocyanine-based pigments, anthraquinone-based pigments, dioxazine-based pigments, threne-based pigments, and indigo-based pigments; carbon black pigments; etc. These color pigments can be used alone or in combination of two or more.

[0077] The total content of the color pigments contained in the aqueous first colored coating composition (BC1) used in the present invention is not particularly limited, but is preferably 10 to 200 parts by mass, more preferably 30 to 180 parts by mass, and particularly preferably 50 to 160 parts by mass relative to 100 parts by mass of the total amount of non-volatile components of the resin used as a carrier.

[0078] The aqueous first colored coating composition (BC1) used in the present invention may further contain a gloss pigment. Examples of gloss pigments include colorless or colored aluminum pigments, vapor-deposited metal flake pigments, and interference pigments in which a transparent or translucent base material is coated with a metal oxide. These gloss pigments may be used alone or in combination of two or more.

[0079] The total content of the glossy pigment contained in the first aqueous colored coating composition used in the present invention is not particularly limited, but is preferably 0 to 30.0 parts by mass, more preferably 0 to 25.0 parts by mass, and particularly preferably 0 to 20.0 parts by mass relative to 100 parts by mass of the total amount of non-volatile components of the resin used as a carrier.

[0080] Various additives for coatings, such as organic solvents and additives such as surface conditioners, thickeners, rheology control agents, pigment dispersants, anti-settling agents, curing catalysts, defoaming agents, antioxidants, ultraviolet absorbers, and extender pigments, can be appropriately blended in the aqueous first colored coating composition (BC1) used in the present invention as needed. As the organic solvent, for the production of the aqueous first colored coating composition (BC1), commonly used organic solvents such as aromatic hydrocarbons such as toluene, xylene, and aromatic naphtha; ketones such as acetone, methyl ethyl ketone, and methyl amyl ketone; esters such as ethyl acetate, butyl acetate, 2-butoxyethyl acetate, amyl acetate, and ethyl ethoxypropionate; alcohols such as isopropyl alcohol, butanol, and 2-butoxyethanol; ethers; aliphatic hydrocarbons including chlorinated hydrocarbons; or mixtures thereof can be mentioned.

[0081] The solid content of the aqueous first colored coating composition used in the present invention during coating is not particularly limited, but it is preferably from 10.0% by mass to 50.0% by mass, and more preferably from 20.0% by mass to 30.0% by mass.

[0082] The aqueous first colored coating composition (BC1) used in the present invention can be applied by methods such as electrostatic coating, air spraying, and airless spraying. After application of the aqueous first colored coating composition (BC1), heating (flash evaporation) at 70°C to 80°C for 3 to 5 minutes to evaporate the water contained in the first colored coating film, or instead of heating, the first colored coating film can be simply left at room temperature. If flash evaporation is performed, the temperature and time ranges should be selected using the above ranges as a guide so that the coating film does not cure.

[0083] The cured film thickness of the aqueous first colored coating composition (BC1) is not particularly limited, but the film thickness (dry film thickness) after the heat treatment in the following step (4) is preferably 4 to 30 μm, more preferably, the aqueous first colored coating composition (BC1) is applied to a thickness of 10 to 30 μm.

[0084] [Step (2)]

[0085] The aqueous one-component type second colored coating composition (BC2) is applied to the uncured first colored coating film obtained in step (1), optionally after flash evaporation or after leaving it at room temperature, to form an uncured second colored coating film.

[0086] Water-based one-component second colored coating composition (BC2)

[0087] The aqueous one-component type second colored coating composition (BC2) used in step (2) contains a resin component and a color pigment.

[0088] The details of the resin component and color pigment contained in the aqueous one-component type second colored coating composition (BC2) are the same as those described for the aqueous first colored coating composition (BC1). However, since the aqueous one-component type second colored coating composition (BC2) is used as a one-component type composition, it is undesirable to use a polyisocyanate compound or a polycarbodiimide compound as a curing agent. The aqueous one-component type second colored coating composition (BC2) contains a resin component and may contain a gloss pigment without containing a color pigment. The details of the amount of each component used in the aqueous one-component type second colored coating composition (BC2) are also the same as those described for the aqueous first colored coating composition (BC1). In addition, the resin component, color pigment and gloss pigment of the aqueous one-component type second colored coating composition (BC2) may be the same as each of the components of the corresponding aqueous first colored coating composition (BC1), or may be different.

[0089] Furthermore, the organic solvents, additives, and the like mentioned for the aqueous first colored coating composition (BC1) can also be used in the aqueous one-pack type second colored coating composition (BC2).

[0090] The solid content of the aqueous one-component type second colored coating composition used in the present invention is not particularly limited, but is preferably from 5.0% by mass to 50.0% by mass, and more preferably from 20.0% by mass to 30.0% by mass.

[0091] As in the case of the aqueous first colored coating composition (BC1), the aqueous second one-component colored coating composition (BC1) is also applied by methods such as electrostatic coating, air spraying, airless spraying, etc. In addition, usually, after applying the aqueous second colored coating composition (BC2), the above-mentioned flash evaporation is performed.

[0092] The aqueous one-pack type second colored coating composition (BC2) is preferably applied so that the dry film thickness of the second colored coating film is preferably 3 to 20 μm, and more preferably 5 to 15 μm. Note that the dry film thickness of the second colored coating film is the film thickness after the heat treatment in the following step (4).

[0093] [Step (3)]

[0094] In the method for forming a multilayer coating film of the present invention, as step (3), a clear coating composition (CC) is applied onto the uncured second colored coating film to form an uncured clear coating film.

[0095] Clear coating composition (CC)

[0096] The clear coating composition (CC) used in the method for forming a multilayer coating film of the present invention comprises a hydroxyl-containing acrylic resin (A1), an isocyanate curing agent (A2), and an unsaturated fatty acid ester polyol (D) having at least two hydroxyl groups in one molecule.

[0097] Hydroxyl-containing acrylic resin (A1)

[0098] The hydroxyl-containing acrylic resin (A) used in the present invention is a homopolymer obtained by homopolymerization of a hydroxyl-containing acrylic monomer, or can be obtained in the form of a copolymer by a known method such as radical copolymerization of a monomer mixture containing a hydroxyl-containing acrylic monomer.

[0099] Examples of hydroxyl-containing acrylic monomers include esters such as 2-hydroxyethyl acrylate or methacrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, or 4-hydroxybutyl acrylate, and ring-opening adducts of 2-hydroxyethyl acrylate or methacrylate with ε-caprolactone, propylene oxide, or ethylene oxide. These hydroxyl-containing acrylic monomers can be used alone or in combination of two or more.

[0100] Other monomers that can be copolymerized with the above-mentioned hydroxyl-containing acrylic monomers include acrylic acid or methacrylic acid and their methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, hexyl, cyclohexyl, 2-ethylhexyl, lauryl, and stearyl esters, as well as acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, styrene, α-methylstyrene, maleic acid, vinyl acetate, and the like. These copolymerizable monomers can be used alone or in combination of two or more with the hydroxyl-containing acrylic monomers. The above-mentioned polymerization is usually carried out using a known polymerization initiator.

[0101] The hydroxyl value of the hydroxyl-containing acrylic resin (A) is determined by the content of the hydroxyl-containing acrylic monomer in the monomers to be copolymerized. The hydroxyl value of the hydroxyl-containing acrylic resin (A) used in the present invention is not particularly limited, but is preferably 80 to 200 mg KOH / g, and more preferably 80 to 190 mg KOH / g.

[0102] In addition, the mass average molecular weight of the hydroxyl-containing acrylic resin (A) is determined by the reaction conditions during copolymerization. The mass average molecular weight of the hydroxyl-containing acrylic resin (A) used in the present invention is not particularly limited, but is preferably 2,000 to 30,000, more preferably 3,000 to 25,000, and particularly preferably 4,000 to 20,000. It should be noted that in the present invention, the mass average molecular weight is measured by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as an eluent, and the value is obtained by converting the data measured at a temperature of 40°C and a flow rate of 1m / min based on the mass average molecular weight of polystyrene. Here, a combination of TSKgel G2000HXL, G3000HXL, G4000HXL and G5000HXL (trade name, manufactured by Tosoh Corporation) is used for the column of gel permeation chromatography (GPC).

[0103] One type alone or a combination of two or more types can be used for the hydroxyl-containing acrylic resin (A).

[0104] Isocyanate curing agent (A2)

[0105] The isocyanate curing agent (A2) used in the present invention is not particularly limited as long as it is a product used in coating applications, and various isocyanate compounds such as aromatic, aliphatic, alicyclic, etc. can be used. Examples of such isocyanate curing agents (A2) include:

[0106] Aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate; aromatic diisocyanates such as xylylene diisocyanate (XDI), tolylene diisocyanate (TDI) and 4,4-diphenylmethane diisocyanate (MDI); alicyclic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, hydrogenated MDI; and diisocyanate dimers and trimers such as uretdione, allophanate, adduct, biuret and isocyanurate, or compounds containing a large amount of diisocyanates. In addition, aliphatic triisocyanate compounds such as 2-isocyanatoethyl-2,6-diisocyanatohexanoate (LTI) and 1,8-diisocyanato-4-isocyanatomethyloctane can be used. In addition, some of these isocyanate groups can be modified with amino-containing silane coupling agents and the like. These isocyanate curing agents (A2) may be used alone or in combination of two or more.

[0107] It is preferred that they are mixed so that the isocyanate groups of the isocyanate curing agent (A2) constitute a ratio of 0.8 to 3.0 equivalents, more preferably 0.9 to 3.0 equivalents, and particularly preferably 0.9 to 2.5 equivalents relative to 1 equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin (A) in the clear coating composition (CC) used in the present invention.

[0108] In the present invention, the clear coating composition (CC) comprises a hydroxyl-containing acrylic resin (A) and an isocyanate curing agent (A2) as essential resin components, and may further contain any other resin components. Examples of optional resin components include polyester resins, amino resins, and the like.

[0109] Unsaturated fatty acid ester polyol (D)

[0110] The unsaturated fatty acid ester polyol (D) having at least two hydroxyl groups in one molecule (also referred to as unsaturated fatty acid ester polyol (D)) used in the present invention is obtained, for example, by esterification of an unsaturated fatty acid with a polyol having at least three hydroxyl groups in the molecule.

[0111] The preferred embodiment of unsaturated fatty acid comprises oleic acid, linoleic acid, linolenic acid, ricinoleic acid and its combination, wherein ricinoleic acid is particularly preferred.In addition, as long as the unsaturated fatty acid ester polyol (D) per molecule obtained has at least one unsaturated group, some of the unsaturated fatty acid also can be replaced by saturated fatty acid.The preferred embodiment of saturated fatty acid comprises palmitic acid, stearic acid, 9,10-dihydroxystearic acid, 12-hydroxystearic acid and its combination.

[0112] Preferred examples of the polyol having at least three hydroxyl groups in one molecule include trimethylolpropane, ditrimethylolpropane, triethylolpropane, ditriethylolpropane, pentaerythritol, dipentaerythritol, tetrakis(2-hydroxyethyl)methane, glycerol, diglycerol, xylitol, sorbitol, galactitol, sucrose, combinations thereof, and the like, among which glycerol is particularly preferred.

[0113] Typically, the unsaturated fatty acid ester polyol (D) is an oligomeric molecule having a mass average molecular weight of, for example, 3,000 or less, and particularly 2,000 or less.

[0114] Regarding the hydroxyl-containing acrylic resin (A1) and the unsaturated fatty acid ester polyol (D) used in the production of the clear coating composition (CC) of the present invention, the distance Ra between the solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space and the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space is 4.0 or less, and preferably in the range of 0.5 to 3.5.

[0115] The solubility parameter (HSP) in the Hansen space is a parameter for indicating the solubility of a substance developed by C.M. Hansen (C.M. Hansen, Hansen Solubility Parameters: A User's Handbook, 1999). In the present invention, the solubility parameter (HSP) in the Hansen space is obtained using the group contribution method proposed by D.W. van Krevelen and P.J. Hoftyzer (D.W. van Krevelen and P.J. Hoftyzer, Properties of Polymers, 2nd edition, 1976). In addition, the distance Ra between the solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space and the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space is obtained according to the following formula (1) and is used as an indicator of the compatibility of the two.

[0116] R a = [4(δD2 - δD1) 2 + (δP2 - δP1) 2 + (δH2 - δH1) 2 ] 1 / 2 ... (1)

[0117] Here, δD1, δP1, and δH1 represent the dispersion term, polarization term, and hydrogen bonding term of the solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space, respectively. Furthermore, δD2, δP2, and δH2 represent the dispersion term, polarization term, and hydrogen bonding term of the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space, respectively.

[0118] By selecting and using a hydroxyl-containing acrylic resin (A1) and an unsaturated fatty acid ester polyol (D) so that the distance Ra between the solubility parameters in the Hansen space according to formula (1) is 4.0 or less, the compatibility between the hydroxyl-containing acrylic resin (A1) and the unsaturated fatty acid ester polyol (D) in the clear coating composition (CC) is improved. As a result, the unsaturated fatty acid ester polyol (D) promotes the curing reaction together with the hydroxyl-containing acrylic resin (A1) while being well mixed with the hydroxyl-containing acrylic resin (A1) in the clear coating film, the crosslinking density of the clear layer is increased, and a cured multi-layer coating film that satisfies coating film properties such as recoat adhesion, gasoline resistance, chipping resistance, and water resistance in a well-balanced manner can be obtained.

[0119] The amount (solid content) of the unsaturated fatty acid ester polyol (D) blended is 1% by mass to 20% by mass, and preferably 3% by mass to 20% by mass, relative to the total mass of the solid contents of all resin components contained in the clear coating composition (CC). By setting the amount of the unsaturated fatty acid ester polyol (D) blended within the above range, the resulting cured multi-layer coating film satisfies coating film properties such as recoat adhesion, gasoline resistance, chipping resistance, and water resistance in a well-balanced manner.

[0120] Furthermore, the clear coating composition (CC) preferably comprises a urethane curing catalyst (E).

[0121] Urethane curing catalyst (E)

[0122] By adding a urethane curing catalyst (E) to the clear coating composition (CC), the urethane bond-forming reaction caused by the hydroxyl-containing acrylic resin (A1), the isocyanate curing agent (A2), and the unsaturated fatty acid ester polyol (D) is accelerated. Examples of the urethane curing catalyst (E) include bismuth-based compounds, aluminum-based compounds, tin-based compounds, zinc-based compounds, and the like.

[0123] Examples of the bismuth-based compound include bis(acetylacetonate)bismuth, bismuth 2-ethylhexanoate, bismuth neodecanoate, bismuth salicylate, and the like.

[0124] Examples of the aluminum-based compound include tris(acetylacetonate)aluminum, tris(ethyl acetoacetate)aluminum, and the like.

[0125] Examples of the tin-based compound include dimethyltin dilaurate, dibutyltin dilaurate, dimethyltin chloride, dibutyltin chloride, di-n-octyltin dilaurate, and the like.

[0126] Examples of the zinc-based compound include zinc acetylacetonate, zinc propionate, zinc octanoate, zinc 2-ethylhexanoate, zinc neodecanoate, zinc laurate, zinc stearate, zinc linoleate, zinc naphthenate, zinc benzoate, zinc salicylate, and the like.

[0127] The amount (solid content) of the urethane curing catalyst (E) blended is 2% by mass or less, and preferably 0.001% by mass to 1.5% by mass relative to the total mass of the solid contents of all resin components contained in the clear coating composition (CC).

[0128] In addition, the clear coating composition (CC) used in the present invention may contain a color pigment to the extent that transparency is not impaired. Examples of the color pigment include: inorganic pigments such as titanium oxide pigments, iron oxide pigments, and complex oxide pigments such as titanium yellow; organic pigments such as azo pigments, quinacridone pigments, diketopyrrolopyrrole pigments, perylene pigments, pyrene pigments, benzimidazolone pigments, isoindoline pigments, isoindolinone pigments, metal chelate azo pigments, phthalocyanine pigments, anthraquinone pigments, dioxazine pigments, vat pigments, and indigo pigments; carbon black pigments; and the like. These color pigments can be used alone or in combination of two or more.

[0129] The total content of the color pigments contained in the clear coating composition (CC) used in the present invention is not particularly limited, but is preferably not more than 10% by mass, and more preferably 0% by mass to 5% by mass, relative to the total mass of the solid content of all resin components contained in the clear coating composition (CC).

[0130] Various additives for coatings, such as solvents such as organic solvents, rheology control agents, pigment dispersants, anti-settling agents, defoaming agents, antioxidants, ultraviolet absorbers, and extender pigments, can be further appropriately blended into the clear coating composition (CC) used in the present invention as needed. Examples of organic solvents include organic solvents commonly used in the production of clear coating compositions (CC), such as: aromatic hydrocarbons such as toluene, xylene, and aromatic naphtha; ketones such as acetone, methyl ethyl ketone, and methyl amyl ketone; esters such as ethyl acetate, butyl acetate, 2-butoxyethyl acetate, amyl acetate, and ethyl ethoxypropionate; ethers; aliphatic hydrocarbons (including chlorinated hydrocarbons), or mixtures thereof. However, if it is expected that the alcohol will interfere with the curing reaction, the use of alcohol is not recommended.

[0131] The solid content of the clear coating composition (CC) used in the present invention when applied is not particularly limited, but is preferably 30% by mass to 70% by mass.

[0132] The clear coating composition (CC) used in the present invention can be applied by methods such as electrostatic coating, air spraying and airless spraying. Typically, after applying the clear coating composition (CC), it is allowed to stand at room temperature for 5 to 20 minutes, and then heated and cured.

[0133] [Step (4)]

[0134] In the method for forming a multilayer coating film of the present invention, as step (4), the uncured first colored coating film, the uncured second colored coating film and the uncured clear coating film formed in steps (1) to (3) are simultaneously cured by heating at 70°C to 100°C, and preferably 80°C to 100°C, so as to cure all of the first colored coating film, the second colored coating film and the clear coating film and form a multilayer coating film.

[0135] By setting the heating temperature to not less than 70°C, the curing reaction can be sufficiently carried out, and by setting the heating temperature to not more than 100°C, energy consumption can be limited. The heating time is generally preferably 5 to 60 minutes, more preferably 10 to 50 minutes, and particularly preferably 15 to 40 minutes. In the present invention, heating can be carried out by known means, for example, a drying oven such as a hot air oven, an electric oven, an infrared induction heating oven, etc. can be used.

[0136] It should be noted that when the expression "multilayer coating film" is used alone in the present invention, it means a coating film that has been heated and cured through step (4), and is distinguished from an "uncured multilayer coating film" including an uncured first colored coating film, an uncured second colored coating film, and an uncured clear coating film before step (4).

[0137] Figures 1 to 4 Schematic cross-sectional views of the multilayer coating film of the present invention are shown, and these will be used to describe further embodiments of the method of the present invention for producing a multilayer coating film or the multilayer coating film obtained thereby. Figures 1 to 4 The clear coating film included in the multi-layer coating film also corresponds to an embodiment of the clear coating composition of the present invention.

[0138] Figure 1 The first embodiment is shown. Specifically, Figure 1 (a) shows an uncured multilayer coating, which is obtained by sequentially coating an uncured first color coating film BC1-ncd1, an uncured second color coating film BC2-ncd1 and an uncured clear coating film CC-ncd1 on an object to be coated (not shown) such as a metal sheet of an automobile body starting from the bottom according to steps (1) to (4).

[0139] lie in Figure 1 The top uncured clear coating film CC-ncd1 in (a) is obtained from the clear coating composition (CC) as described above and contains a hydroxyl-containing acrylic resin (A1), an isocyanate curing agent (A2), and an unsaturated fatty acid ester polyol (D). In the clear coating composition (CC), the compatibility of the hydroxyl-containing acrylic resin (A1) and the unsaturated fatty acid ester polyol (D) is improved by using a hydroxyl-containing acrylic resin (A1) and an unsaturated fatty acid ester polyol (D) selected so that the distance Ra between the solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space and the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space is 4.0, and preferably 0.5 to 3.5. In addition, in the clear coating composition (CC) of the present invention, the unsaturated fatty acid ester polyol (D) (indicated by ●) is uniformly present in the uncured clear coating film CC-ncd1.

[0140] In the first embodiment, it is preferred to mix them so that the isocyanate groups of the isocyanate curing agent (A2) constitute a ratio of 0.8 to 3.0 equivalents, more preferably 0.9 to 3.0 equivalents, and particularly preferably 0.9 to 2.5 equivalents relative to 1 equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin (A) in the clear coating composition (CC).

[0141] When to start Figure 1 During the heat treatment h (i.e., baking) of the uncured multilayer coating film shown in (a), the unsaturated fatty acid ester polyol (D) remains well mixed with the hydroxyl-containing acrylic resin (A1) in the clear coating film and, together with the hydroxyl-containing acrylic resin (A1), promotes the curing reaction with the isocyanate curing agent (A2). Thus, the multilayer coating film of the present invention comprising a cured clear coating film CC-cd1 having a high crosslinking density is completed.

[0142] In addition to the contents described in the first embodiment, the second embodiment further specifies the aqueous first colored coating composition (BC1) and the clear coating composition (CC). Specifically, Figure 2 The second embodiment shown in shows an example of a case in which the aqueous first colored coating composition (BC1) contains a carboxyl group-containing resin (B1) and a carbodiimide group-containing curing agent (B2) (also referred to as a polycarbodiimide compound (B2)), and the clear coating composition (CC) contains an isocyanate group in the isocyanate curing agent (A2) at a ratio of 0.8 to 1.5 equivalents, and particularly 0.9 to 1.5 equivalents, relative to 1 equivalent of the hydroxyl group in the hydroxyl group-containing acrylic resin (A1).

[0143] That is, in the multilayer coating film according to the second embodiment, the aqueous first colored coating composition (BC1) contains the carboxyl-containing resin (B1) and the polycarbodiimide compound (B2) as the resin components mentioned in step (1). It is preferred that the composition 1 containing the carboxyl-containing resin (B1) and the composition 2 containing the polycarbodiimide compound (B2), which is different therefrom, are prepared as a two-part coating and mixed immediately before coating. The carboxyl-containing resin (B1) and the polycarbodiimide compound (B2) are mixed and applied in step (1), and further reacted by heating in step (4).

[0144] Preferred examples of the carboxyl-containing resin (B1) contained in the aqueous first colored coating composition (BC1) include carboxyl-containing polyester resins, carboxyl-containing acrylic urethane resins, carboxyl-containing polyurethane-polyurea particles, and carboxyl-containing urethane core / acrylic shell particles.

[0145] Furthermore, preferred examples of the polycarbodiimide compound (B2) include products produced by reacting a hydrophilic agent having active hydrogen and a hydrophilic portion with a reaction product obtained by reacting a polycarbodiimide compound containing at least two isocyanate groups in one molecule with a polyol having a hydroxyl group at a molecular terminal at a ratio such that the NCO / OH molar ratio exceeds 1.

[0146] like Figure 2 As shown in (a), in the second embodiment, the uncured first color coating film BC1-ncd2, the uncured second color coating film BC2-ncd2 and the uncured clear coating film CC-ncd2 are stacked in this order. The uncured clear coating film CC-ncd2 contains an isocyanate curing agent (A2) (indicated by ▲). When starting Figure 2 During the heat treatment h (i.e., baking) of the uncured multilayer coating film in (a), due to the high polarity of the isocyanate curing agent (A2), some of the isocyanate curing agent (A2) migrates from the clear coating film, which has not yet been cured, to the second colored coating film. Then, as the heating time elapses, the isocyanate curing agent (A2) in the clear coating film and the second colored coating film reacts with the hydroxyl-containing resin in each coating film, curing the uncured clear coating film CC-ncd2 and the uncured second colored coating film BC2-ncd2. Simultaneously, the uncured first colored coating film BC1-ncd2 is cured by a reaction between the carboxyl groups and the carbodiimide groups within the coating films. This completes the multilayer coating film of the present invention, comprising the cured first colored coating film BC1-cd2, the cured second colored coating film BC2-cd2, and the cured clear coating film CC-cd2.

[0147] In the third embodiment, in addition to the contents described in the first embodiment, a clear coating composition (CC) is further specified. Specifically, Figure 3 In the third embodiment shown, the ratio of the hydroxyl-containing acrylic resin (A1) to the isocyanate curing agent (A2) in the clear coating composition (CC) is within a predetermined range, and further, by additionally determining the type of the isocyanate curing agent (A2) and the ratio in which it is used, a multi-layer coating film having a particularly good cured state can be obtained.

[0148] like Figure 3As shown in (a), in the third embodiment, an uncured first colored coating film BC1-ncd3, an uncured second colored coating film BC2-ncd3, and an uncured clear coating film CC-ncd3 are laminated in this order. The clear coating composition (CC) constituting the uncured clear coating film CC-ncd3 contains a hydroxyl-containing acrylic resin (A1) and an isocyanate curing agent (A2), and the isocyanate curing agent (A2) contains a diisocyanate dimer (A2-1) (also referred to as dimer (A2-1)) (indicated by ◇) and a diisocyanate trimer or higher compound (A2-2) (also referred to as (A2-2) trimer or the like) (indicated by △).

[0149] A diisocyanate compound having a uretdione structure is preferable as an example of the diisocyanate dimer (A2-1), and a triisocyanate compound having an isocyanurate structure is preferable as the diisocyanate trimer or the like (A2-2).

[0150] In addition, the diisocyanates used to produce the dimer (A2-1) and the trimer or the like (A2-2) may be the same or different, and examples include the diisocyanates mentioned with respect to step (1). In the third embodiment, it is preferred that the molecular weight of the dimer (A2-1) is smaller than that of the trimer or the like (A2-2). The isocyanate curing agent (A2) may further contain polyisocyanates other than the dimer (A2-1) and the trimer or the like (A2-2) (also referred to as other polyisocyanates).

[0151] Then, when it starts Figure 3 During the heat treatment h (that is, baking) in step (4) of the uncured multilayer coating film of (a), due to the high polarity of the diisocyanate dimer (A2-1) and trimer or the like (A2-2), some of each of the diisocyanate dimer (A2-1) and trimer or the like (A2-2) migrates from the clear coating film that has not yet completed curing to the second colored coating film BC2-ncd3 and the first colored coating film BC1-ncd3.

[0152] In the third embodiment, the clear coating composition (CC) contains a hydroxyl-containing acrylic resin (A1) and an isocyanate curing agent (A2) in an amount such that there are 1.5 to 3.0 equivalents, and preferably 1.5 to 2.5 equivalents, of isocyanate groups in the isocyanate curing agent (A2) per 1 equivalent of hydroxyl groups in the hydroxyl-containing acrylic resin (A1). That is, by making the isocyanate groups present in the isocyanate curing agent (A2) excessive relative to the hydroxyl groups in the hydroxyl-containing acrylic resin (A1), a configuration is produced in which unreacted isocyanate curing agent (A2) migrates to the second colored coating film BC2-ncd3 and further migrates to the first colored coating film BC1-ncd3. The isocyanate curing agent (A2) containing a diisocyanate dimer (A2-1) and a trimer or the like (A2-2) migrates to the second colored coating film BC2-ncd3 and further migrates to the first colored coating film BC1-ncd3, and forms the second colored coating film BC2-cd3 and the first colored coating film BC1-cd3 which have been cured by reacting with the hydroxyl-containing resin in each coating film.

[0153] By selecting the molecular weight of the dimer (A2-1) lower than that of the trimer or the like (A2-2), the dimer (A2-1) will be able to migrate to the first colored coating film (BC1-ncd3) as the lowermost layer, thereby achieving sufficient curing of the first colored coating film BC1-ncd3.

[0154] Furthermore, by blending the dimer (A2-2) and the trimer or the like (A2-1) at a solid content mass ratio within a range of 10 / 90 to 40 / 60, both the second colored coating film BC2-ncd3 and the first colored coating film BC1-ncd3 can be sufficiently cured.

[0155] Thereby, the multi-layer coating film of the present invention including the cured first colored coating films BC1-cd3, the cured second colored coating films BC2-cd3, and the cured clear coating films CC-cd3 is completed.

[0156] In the fourth embodiment, in addition to the contents described in the first embodiment, a clear coating composition (CC) is further specified. Specifically, Figure 4 The fourth example shown in is an example in which a 3-isocyanatepropyltrialkoxysilane compound (A3) is blended into the clear coating composition (CC).

[0157] Regarding the fourth embodiment, Figure 4(a) shows an uncured multilayer coating in which an uncured first colored coating film BC1-ncd4, an uncured second colored coating film BC2-ncd4, and an uncured clear coating film CC-ncd4 are stacked in this order. Clear coating film CC-ncd4, composed of a clear coating composition (CC), contains an isocyanate curing agent (A2) (indicated by ▲) and a 3-isocyanatepropyltrialkoxysilane compound (A3) (indicated by ■).

[0158] Then, when it starts Figure 4 During the heat treatment of the uncured multilayer coating film in (a) for h, due to the high polarity of the isocyanate curing agent (A2) and the 3-isocyanatepropyltrialkoxysilane compound (A3), some of each of the isocyanate curing agent (A2) and the 3-isocyanatepropyltrialkoxysilane compound (A3) migrates from the clear coating film, which has not yet been completely cured, to the second colored coating films BC1-ncd4 and the first colored coating films BC2-ncd4. Since the 3-isocyanatepropyltrialkoxysilane compound (A3) has a lower molecular weight and lower viscosity than the isocyanate curing agent (A2), it can migrate to the first colored coating film BC1-ncd4, which is the lowermost layer, and the first colored coating film BC1-ncd4 can be fully cured.

[0159] Furthermore, by setting the blending amount (solid content) of the 3-isocyanatepropyltrialkoxysilane compound (A3) to 2% by mass to 50% by mass, and preferably 2% by mass to 20% by mass, relative to the total mass of the solid content of all resin components contained in the clear coating composition (CC), both the second colored coating film BC2-ncd4 and the first colored coating film BC1-ncd4 can be sufficiently cured. Thus, the multilayer coating film of the present invention comprising the cured first colored coating film BC1-cd4, the cured second colored coating film BC2-cd4, and the cured clear coating film CC-cd4 is completed.

[0160] As described above, the multilayer coating of the present invention is Figures 1 to 4 The form of a single multilayer coating film is shown for use as an outer coating on an object to be coated, such as an automobile body.

[0161] Furthermore, if recoating of a multilayer coating film is desired, a laminate of the multilayer coating films can be produced by sequentially applying an uncured first colored coating film, an uncured second colored coating film, and an uncured clear coating film on top of the cured clear coating film CC-cd1, followed by heat treatment. In this case, the clear coating film CC-cd1 exhibits good recoat adhesion because the unsaturated fatty acid ester polyol (D) increases the crosslinking density.

[0162] As described above, using the method for producing a multi-layer coating film of the present invention or the clear coating composition (CC) of the present invention, it is possible to produce a multi-layer coating film that can satisfy coating film properties such as recoat adhesion, gasoline resistance, chipping resistance and water resistance in a well-balanced manner even when heated at low temperatures (70° C. to 100° C.).

[0163] The method for forming a multilayer coating film of the present invention and the multilayer coating film obtained thereby can be applied to automobile bodies, components, and parts, such as those of passenger cars, trucks, motorcycles, and buses. When the object to be coated is metal, it is particularly effective for automobile bodies, and when the object to be coated is plastic, it is particularly effective for automobile interior and exterior parts.

[0164] Furthermore, the clear coating composition (CC) of the present invention is applied to the uncured second colored coating film formed from the aqueous one-component colored coating composition (BC2) on the uncured first colored coating film formed from the aqueous first colored coating composition (BC1) to form an uncured clear coating film, and then heated and cured simultaneously with the first and second colored coating films to form a multilayer coating film. That is, the clear coating composition (CC) of the present invention is applied to the above-mentioned automotive bodies, components, and parts as part of the multilayer coating film formed by the method of the present invention.

[0165] Examples

[0166] Hereinafter, the present invention will be described in more detail by examples, but the present invention is not limited to these examples. In addition, in these examples, unless otherwise indicated, "parts" means "parts by mass", and about blending amount and content, "%" means "mass %".

[0167] Method for measuring resin characteristic values

[0168] In the present invention, the resin characteristic value is measured by the following method.

[0169] 1. Resin solid content

[0170] The resin solids content was determined by measuring the mass of a 1.0 g sample after heating at 130° C. for 60 minutes.

[0171] 2. Gel Fraction

[0172] First, a 1.0g sample was heated at 130°C for 60 minutes and its mass was measured. Next, the heated sample was immersed in an excess of tetrahydrofuran at 25°C for 24 hours to extract the dissolved components. The gel fraction was then determined by drying the remaining insoluble components at 50°C for 4 hours and measuring their mass.

[0173] 3. Hydroxyl value

[0174] The measurement was performed according to JIS-K1557.

[0175] 4. Acid value

[0176] The measurement was performed according to JIS-K5601.

[0177] <Production Example 1: Production of Aqueous Dispersion CS-1 of Urethane Core / Acrylic Shell Particles for the Aqueous First Colored Coating Composition and the Aqueous One-Component Second Colored Coating Composition (BC1 and BC2)>

[0178] 1-(1) Production of polyester polyol solution PE-1

[0179] 54.0 parts of dimer acid PRIPOL 1017 (trade name, manufactured by Croda Japan Co., Ltd.), 16.3 parts of isophthalic acid and 29.7 parts of 1,6-hexanediol, which are produced by dimerization of C36 dicarboxylic acid as a main component, are charged into a flask equipped with a reflux condenser, a thermometer, a stirrer and a nitrogen inlet tube of a separation tube with reaction water, and heated to 160° C. under a nitrogen stream while stirring. After being kept at 160° C. for 1 hour, the temperature is raised to 230° C. within 5 hours. While maintaining the temperature at 230° C., the acid value is periodically measured, and when the acid value reaches 3.5 mg KOH / g, it is cooled to 80° C. or lower. Finally, 31.0 parts of methyl ethyl ketone are added to obtain a polyester polyol solution PE-1. The polyester polyol solution PE-1 was characterized by an acid value of 3.5 mg KOH / g, a hydroxyl value of 73 mg KOH / g and a resin solids content of 75% by mass.

[0180] 1-(2) Production of aqueous dispersion PU-1 of unsaturated polyester urea urethane containing allyl groups

[0181] 586.7 parts of the polyester polyol solution PE-1 obtained in Production Example 1-(1), 71.0 parts of dimethylolpropionic acid and 278.7 parts of methyl ethyl ketone were charged into a flask equipped with a reflux condenser, a thermometer, a stirrer and a nitrogen inlet tube, and the mixture was stirred under a nitrogen stream until uniform.

[0182] Then, 279.5 parts of isophorone diisocyanate are added to the resulting solution. After the exothermic reaction subsided, while stirring, the reaction mixture was gradually heated to 80°C, and continued to stir at this temperature until the content of isocyanate groups in the solution was 3.3% by mass. After that, 9.4 parts of allyl alcohol, 22.2 parts of trimethylolpropane and 21.3 parts of methyl ethyl ketone were added, and the mixture was stirred at 80°C until the content of isocyanate groups in the solution was reduced to 1.0% by mass. Then, 22.6 parts of diethanolamine were added and the content of isocyanate groups was monitored until free isocyanate groups were no longer detectable. Then, 28.8 parts of methyl ethyl ketone, 142.3 parts of methoxypropanol and 45.5 parts of triethylamine were added to the resulting polyurethane solution. After adding triethylamine for 30 minutes, the temperature of the solution was reduced to 60°C and 1977 parts of deionized water were added over a 30-minute period while stirring. Thereafter, a separation tube was attached to a reflux condenser, and the solvent was removed under reduced pressure at 60° C. until the resin solid content of the dispersion reached 29% by mass to obtain an aqueous dispersion PU-1 of allyl-containing unsaturated polyester urea urethane.

[0183] The resulting aqueous dispersion PU-1 of the allyl-containing unsaturated polyester urea urethane had an acid value of 35 mg KOH / g, a hydroxyl value of 28 mg KOH / g, and a pH of 7.1.

[0184] 1-(3) Production of aqueous dispersion CS-1 of urethane core / acrylic shell particles

[0185] 1,961.2 parts of the aqueous dispersion PU-1 of the allyl-containing unsaturated polyester urea urethane obtained in Production Example 1-(2), 43.3 parts of methoxypropanol, 744.4 parts of deionized water, and 3.6 parts of triethylamine were charged into a flask equipped with two dropping devices, a reflux condenser, a thermometer, a stirrer, and a nitrogen inlet tube, stirred under a nitrogen stream, and heated to 80° C. Thereafter, 0.6 parts of ammonium persulfate was dissolved in 35.7 parts of deionized water and added to the reaction vessel.

[0186] Subsequently, while stirring, a monomer mixture consisting of 538.3 parts of n-butyl methacrylate, 26.3 parts of 2-hydroxyethyl acrylate, 4.2 parts of allyl methacrylate, and 70.0 parts of butanediol was added dropwise at a constant rate over 5 hours using a dropping device. Simultaneously with the dropwise addition using the above dropping device, a polymerization initiator solution consisting of 1.1 parts of ammonium persulfate and 71.3 parts of deionized water was added dropwise at a constant rate over 5 hours using another dropping device.

[0187] After the monomer mixture solution and the polymerization initiator solution were added dropwise, the resulting reaction mixture was further stirred at 80° C. for 1 hour, and then cooled to room temperature to obtain an aqueous dispersion CS-1 of urethane core / acrylic shell particles.

[0188] The resulting aqueous dispersion CS-1 of urethane core / acrylic shell particles had an acid value of 18 mg KOH / g, a hydroxyl value of 25 mg KOH / g, a resin solids content of 33 mass %, a pH of 7.1, and a gel fraction of 83 mass %.

[0189] <Production Example 2: Production of Aqueous Dispersion UU-1 of Polyurethane-Polyurea Particles for the Aqueous First Colored Coating Composition and the Aqueous One-Component Second Colored Coating Composition (BC1 and BC2)>

[0190] 2-(1) Production of diketimine solution DK-1 using diethylenetriamine and methyl isobutyl ketone

[0191] 180.0 parts of diethylenetriamine and 531.9 parts of methyl isobutyl ketone were charged into a flask equipped with a reflux condenser with a separation tube for reaction water, a thermometer, a stirrer, and a nitrogen inlet tube, which was stirred under a nitrogen stream, and the reaction water was removed at 130° C. to 150° C. When the distillation of the reaction water was completed, the mixture was cooled to obtain a diketimine solution DK-1.

[0192] 2-(2) Production of aqueous dispersion UU-1 of polyurethane-polyurea particles

[0193] 746.3 parts of the polyester polyol solution PE-1 obtained in Production Example 1-(1), 27.2 parts of dimethylolpropionic acid, 3.8 parts of dibutyltin dilaurate and 157.9 parts of methyl ethyl ketone were charged into a flask equipped with a reflux condenser, a thermometer, a stirrer and a nitrogen inlet tube, and the mixture was stirred under a nitrogen stream until uniform.

[0194] Then, 201.8 parts of dicyclohexylmethane 4,4'-diisocyanate were added to the resulting solution. After the exothermic reaction subsided, the reaction mixture was gradually heated to 80°C while stirring. Stirring was continued at this temperature until the content of isocyanate groups in the solution reached 1.5% by mass. Thereafter, 626.2 parts of methyl ethyl ketone were added and the reaction mixture was cooled to 40°C. After cooling, 11.8 parts of triethylamine were added within 2 minutes and stirred for 5 minutes. Then, 30.2 parts of the diketimine solution DK-1 obtained in Production Example 2-(1) were added within 1 minute and stirred at 40°C for 30 minutes. In addition, 1206 parts of deionized water were added within 7 minutes while stirring. Thereafter, a separation tube was attached to a reflux condenser, and the solvent was removed under reduced pressure at 45°C until the resin solid content of the dispersion reached 40% by mass to obtain an aqueous dispersion UU-1 of polyurethane-polyurea particles.

[0195] The resulting aqueous dispersion UU-1 of polyurethane-polyurea particles had an acid value of 17 mg KOH / g, a pH of 7.4, and a gel fraction of 87% by mass.

[0196] <Production Example 3: Production of Aqueous Acrylic Urethane Dispersion AU-1 for Aqueous First Colored Coating Composition (BC1)>

[0197] 3-(1) Production of polyester polyol solution PE-2

[0198] 49.9 parts of adipic acid, 18.5 parts of 1,6-hexanediol and 31.6 parts of neopentyl glycol were charged into a flask equipped with a reflux condenser with a separation tube for reaction water, a thermometer, a stirrer and a nitrogen inlet tube, and the temperature was raised to 160°C while stirring under a nitrogen flow. After being maintained at 160°C for 1 hour, the temperature was raised to 230°C over 5 hours. While maintaining the temperature at 230°C, the acid value was periodically measured, and when the acid value reached 3.5 mg KOH / g, the contents were cooled to 80°C or lower. Finally, 21.9 parts of methyl ethyl ketone were added to obtain polyester polyol solution PE-2. The characteristic values ​​of polyester polyol solution PE-2 were an acid value of 3.5 mg KOH / g, a hydroxyl value of 155 mg KOH / g and a resin solid content of 80% by mass.

[0199] 3-(2) Production of Waterborne Acrylic Urethane Dispersion AU-1

[0200] 420.0 parts of the polyester polyol solution PE-2 obtained in Production Example 3-(1), 31.0 parts of neopentyl glycol, 27.8 parts of trimethylolpropane monoallyl ether, 0.5 parts of dibutyltin dilaurate and 195.7 parts of methyl ethyl ketone were charged into a flask equipped with two dropping devices, a reflux condenser, a thermometer, a stirrer and a nitrogen inlet tube, and stirred under a nitrogen stream until uniform.

[0201] Next, 259.9 parts of isophorone diisocyanate were added to the resulting solution. After the exothermic reaction subsided, the reaction mixture was gradually heated to 80°C while stirring, and stirring was continued at this temperature until the isocyanate content of the solution reached 2.2% by mass. 66.7 parts of trimethylolpropane were then added and stirred at 80°C until free isocyanate groups were no longer detectable in the solution. Thereafter, 248.9 parts of methyl ethyl ketone were added to the resulting polyurethane solution.

[0202] Subsequently, the temperature was adjusted to 82° C., and a monomer mixture containing 312.5 parts of n-butyl acrylate, 312.5 parts of methyl methacrylate, 74.7 parts of 2-hydroxypropyl methacrylate, and 58.4 parts of acrylic acid was added dropwise at a constant rate over 3 hours using a dropping device. While the addition was being performed using the above dropping device, a polymerization initiator solution containing 22.8 parts of 2,2′-azobis(methylbutyronitrile) and 152.3 parts of methyl ethyl ketone was added dropwise at a constant rate over 3.5 hours using another dropping device.

[0203] After the monomer mixture and polymerization initiator solution were added dropwise, the resulting reaction mixture was further stirred at 82°C for 2.5 hours, and 56.9 parts of dimethylethanolamine and 2242 parts of deionized water were added. Thereafter, a separation tube was attached to a reflux condenser, and the solvent was removed under reduced pressure at 45°C until the resin solids content of the dispersion reached 40% by mass, to obtain aqueous acrylic urethane dispersion AU-1.

[0204] The resulting aqueous acrylic urethane dispersion AU-1 had an acid value of 32 mg KOH / g, a hydroxyl value of 57 mg KOH / g, and a pH of 8.1.

[0205] <Production Example 4: Production of Aqueous Dispersion PE-3 of Polyester Polyol for Aqueous One-Component Second Colored Coating Composition (BC2)>

[0206] 171.0 parts of dimer acid PRIPOL 1017 (trade name, manufactured by Croda Japan Co., Ltd.) having a C36 dicarboxylic acid produced by dimerization of C18 unsaturated fatty acids as a main component, 72.9 parts of neopentyl glycol, 82.7 parts of 1,6-hexanediol and 46.2 parts of hexahydrophthalic anhydride were charged into a flask equipped with a reflux condenser, a thermometer, a stirrer and a nitrogen inlet tube with a separation tube for reaction water, and heated to 160°C while stirring under a nitrogen stream. After being kept at 160°C for 1 hour, the temperature was raised to 230°C over 5 hours. While maintaining the temperature at 230°C, the acid value was periodically measured, and when the acid value reached 8.5mg KOH / g, the contents were cooled to 140°C. Next, 76.8 parts of trimellitic anhydride were added, and the reaction was continued at 180°C until the acid value reached 30mg KOH / g. Thereafter, the contents were cooled to 120° C., 140.2 parts of butanol were added, and after further cooling to 90° C., 1.6 parts of dimethylethanolamine and 140.2 parts of deionized water were added, thereby obtaining an aqueous dispersion PE-3 of polyester polyol.

[0207] The resulting aqueous dispersion PE-3 of polyester polyol had an acid value of 30 mg KOH / g, a hydroxyl value of 83 mg KOH / g, a resin solid content of 60% by mass, and a pH of 7.8.

[0208] <Production Example 5: Production of Aqueous First Colored Coating Compositions BC1-1 and BC1-2>

[0209] 5-(1) Production of black paste PP-1

[0210] 25.0 parts of the aqueous acrylic urethane dispersion AU-1 obtained in Production Example 3, 10.0 parts of carbon black pigment MA100 (trade name, manufactured by Mitsubishi Chemical Corporation), 0.1 part of methyl isobutyl ketone, 1.4 parts of dimethylethanolamine, 2.0 parts of polyalkylene glycol Pluriol P900 (trade name, manufactured by BASF) and 61.5 parts of deionized water were mixed and dispersed with a motor mill to obtain black paste PP-1.

[0211] 5-(2) Production of white paste PP-2

[0212] 43.0 parts of the aqueous acrylic urethane dispersion AU-1 obtained in Production Example 3, 50.0 parts of titanium dioxide pigment Ti-Pure R706 (trade name, manufactured by Chemours Company), 0.3 parts of 1-propoxy-2-propanol, and 6.7 parts of deionized water were mixed and dispersed with a motor mill, thereby obtaining white paste PP-2.

[0213] 5-(3) Production of Water-Based First Colored Coating Compositions BC1-1 and BC1-2

[0214] The components listed in Table 1 were mixed in the order listed and stirred for an additional 30 minutes. Next, dimethylethanolamine was added to adjust the pH to 8.5. Thereafter, deionized water was added to adjust the viscosity to 100 mPa·s, as measured using a rotational viscometer Rheomat RM180 (trade name, manufactured by METTLER TOLEDO) at 23° C. and 1000 s -1 Measured at a shear rate of .

[0215] [Table 1]

[0216]

[0217]

[0218]

[0219] 1) Surfynol 104PA (trade name, manufactured by Nissin Chemical Industry Co., Ltd., active ingredient 50% by mass)

[0220] 2) Rheovis HS1162 (trade name, manufactured by BASF)

[0221] 3) CYMEL 327 (trade name, manufactured by Allnex, resin solid content 90% by mass)

[0222] 4) CARBODILITE V-02-L2 (trade name, manufactured by Nisshinbo Chemical Inc., resin solid content 40% by mass)

[0223] <Production Example 6: Production of Aqueous One-Component Second Colored Coating Composition BC2-1>

[0224] The components listed in the aqueous phase portion of Table 2 were mixed in the order listed and stirred for an additional 60 minutes to obtain an aqueous mixture. Next, in a separate container, the components listed in the organic phase portion were mixed in the order listed to obtain an organic mixture. The resulting organic mixture was then added to the aqueous mixture and stirred for an additional 30 minutes. Subsequently, dimethylethanolamine was added to adjust the pH to 8.0. Thereafter, deionized water was added to adjust the viscosity to 100 mPa·s, as measured using a rotational viscometer Rheomat RM180 (trade name, manufactured by Mettler Toledo) at 23°C and 1000 s -1 Measured at a shear rate of .

[0225] [Table 2]

[0226]

[0227]

[0228]

[0229] 5) A mixture of 3 parts by mass of Laponite RD (trade name, manufactured by BYK), 3 parts by mass of Pluriol P900 (trade name, manufactured by BASF), and 94 parts by mass of deionized water

[0230] 6) Rheovis AS1130 (trade name, manufactured by BASF)

[0231] 7) STAPA IL HYDROLAN 2192 55900 / G (trade name, manufactured by ECKART, pigment content 60% by mass)

[0232] <Production Example 7: Production of Solutions of Hydroxyl-Containing Acrylic Resins A1-1 to A1-10 for Clear Coating Composition (CC)>

[0233] 7-(1) Production of Hydroxyl-Containing Acrylic Resin A1-1 Solution

[0234] 99.0 parts of ethyl 3-ethoxypropionate (serving as a solvent) were charged into a flask equipped with a dropping device, a reflux condenser, a thermometer, a stirrer, and a nitrogen inlet tube, and heated to 75° C. under a nitrogen stream while stirring. While maintaining the temperature at 75° C., a mixture of 10.0 parts of styrene, 25.0 parts of 4-hydroxybutyl acrylate, 10.0 parts of 2-hydroxyethyl methacrylate, 55.0 parts of isobutyl methacrylate (serving as a monomer), and 1.0 part of 2,2'-azobis(isobutyronitrile) (serving as a polymerization initiator) in the dropping device was added dropwise at a constant rate over 3 hours, and then stirring was continued at 75° C. for another 5 hours. After confirming that the conversion rate exceeded 98% by measuring the resin solid content, the contents were cooled to obtain a hydroxyl-containing acrylic resin A1-1 solution.

[0235] 7-(2) Production of Hydroxyl-Containing Acrylic Resins A1-2 to A1-10 Solutions

[0236] Hydroxyl-containing acrylic resin solutions A1-2 to A1-10 were prepared in the same manner as the hydroxyl-containing acrylic resin (A1-1) solution, except that the initial solvent amount, monomer composition, blending amount, and initiator amount were changed as shown in Table 3. Table 3 also lists the solubility parameters (HSP) in Hansen space of the hydroxyl-containing acrylic resins A1-1 to A1-10.

[0237] [Table 3]

[0238]

[0239]

[0240] The abbreviations in Table 3 are indicated as follows.

[0241] ST: Styrene

[0242] 4HBA: 4-Hydroxybutyl acrylate

[0243] HEMA: 2-Hydroxyethyl Methacrylate

[0244] IBMA: Isobutyl Methacrylate

[0245] HPMA: 2-Hydroxypropyl Methacrylate

[0246] MMA: Methyl Methacrylate

[0247] EHA: 2-Ethylhexyl acrylate

[0248] CHMA: Cyclohexyl methacrylate

[0249] <Production Example 8: Production of Clear Coating Compositions CC1 to CC52>

[0250] 8-(1) Unsaturated fatty acid ester polyols D-1 to D-5

[0251] The structure of the unsaturated fatty acid ester polyol D-1 (glyceryl trilinolenate) used is shown below.

[0252] [Chemical Formula 1]

[0253]

[0254] The structure of the unsaturated fatty acid ester polyol D-2 (glyceryl diricinoleate) used is shown below.

[0255] [Chemical Formula 2]

[0256]

[0257] The structure of the unsaturated fatty acid ester polyol D-3 (glycerol monoricinoleate) used is shown below.

[0258] [Chemical Formula 3]

[0259]

[0260] The structure of the unsaturated fatty acid ester polyol D-4 (diglycerol monoricinoleate) used is shown below.

[0261] [Chemical Formula 4]

[0262]

[0263] The structure of the unsaturated fatty acid ester polyol D-5 (dipentaerythritol monoricinoleate) used is shown below.

[0264] [Chemical Formula 5]

[0265]

[0266] Furthermore, the solubility parameters (HSP) of the unsaturated fatty acid ester polyols D-1 to D-5 in Hansen space are shown together in Table 4.

[0267] [Table 4]

[0268]

[0269]

[0270] 8-(2) Production of Clear Coating Composition CC1

[0271] 84.6 parts of a hydroxyl-containing acrylic resin A1-1 solution, 3 parts of an unsaturated fatty acid ester polyol D-1, 22.4 parts of an isocyanate curing agent A2-1 (1.0 equivalent of isocyanate groups in the isocyanate curing agent A2-1 per equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin A1-1), and 0.01 parts of a urethane curing catalyst E-1 were weighed out. Ethyl 3-ethoxypropionate was then used as a solvent, diluted to a Ford #4 cup viscosity of 25 seconds (20°C), and stirred with a paint shaker until completely homogeneous. Thereafter, the mixture was filtered using a membrane filter with an opening of 200 μm to remove coarse particles, and a clear coating composition CC1 was obtained.

[0272] 8-(3) Production of Clear Coating Compositions CC2 to CC52

[0273] Clear coating compositions CC2 to CC52 were obtained in the same manner as clear coating composition CC1, with changes made so as to produce the blending ratios shown in Table 5. Note that in Table 5, the amount of the hydroxyl-containing acrylic resin (A1-1 to A1-10) solution used was 84.6 parts in all cases.

[0274] In addition, for each composition in each Example and Comparative Example, the distance Ra between the solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space and the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space was determined. That is, the values ​​obtained by substituting the values ​​of δD1, δP1, δH1, δD2, δP2, and δH2 listed in Tables 3 and 4 into the following formula (1) are shown as Ra in Table 5.

[0275] R a =[4(δD2-δD1) 2 +(δP2-δP1) 2 +(δH2-δH1) 2 ] 1 / 2 ...(1)

[0276] [Table 5]

[0277]

[0278]

[0279]

[0280]

[0281] The abbreviations in Table 5 are indicated as follows.

[0282] Isocyanate curing agent A2-a: DURANATE TPA-100 (trade name, manufactured by Asahi Kasei Corporation, diisocyanate trimer or higher compound)

[0283] Isocyanate curing agent A2-b: DURANATE TKA-100 (trade name, manufactured by Asahi Kasei Corporation, diisocyanate trimer or higher compound)

[0284] Isocyanate curing agent A2-c: DURANATE TUL-100 (trade name, manufactured by Asahi Kasei Corporation, diisocyanate dimer content 14%)

[0285] Isocyanate curing agent A2-d: DURANATE TLA-100 (trade name, manufactured by Asahi Kasei Corporation, diisocyanate dimer content 12%)

[0286] Isocyanate curing agent A2-e: DESMODUR N3400 (trade name, manufactured by Covestro, diisocyanate dimer content 38%)

[0287] 3-Isocyanatopropyltrialkoxysilane compound A3-1: KBE-9007N (trade name, manufactured by Shin-Etsu Chemical Co., Ltd., (3-isocyanatopropyl)triethoxysilane)

[0288] 3-Isocyanatopropyltrialkoxysilane compound A3-2: (3-isocyanatopropyl)trimethoxysilane

[0289] Urethane curing catalyst E-1: K-KAT XK-614 (trade name, manufactured by Kusumoto Chemicals, Ltd.)

[0290] Urethane curing catalyst E-2: K-KAT 670 (trade name, manufactured by Kusumoto Chemicals Co., Ltd.)

[0291] Urethane curing catalyst E-3: dibutyltin dilaurate

[0292] <Examples 1 to 45 and Comparative Examples 1 to 7>

[0293] Preparation of test plates

[0294] CathoGuard No. 500 (trade name, manufactured by BASF Coatings) cationic electrodeposition paint was applied to a film thickness of 20 μm by electrodeposition on a cold-rolled steel sheet (150 mm x 75 mm x 0.8 mm) that had been treated with zinc phosphate, and cured by heating at 170° C. for 30 minutes.

[0295] The test plate was electrostatically coated with the first aqueous colored coating composition (BC1) shown in Table 6 to give the dry film thickness shown in Table 6, and left at room temperature for 10 minutes, and then the second aqueous one-component colored coating composition BC2-1 was electrostatically coated to give a dry film thickness of 10 μm, left at room temperature for 5 minutes, and then subjected to preliminary drying (flash evaporation) at 80°C for 5 minutes. Next, the clear coating composition (CC) listed in Table 6 was electrostatically coated to give a dry film thickness of 35 μm, left at room temperature for 10 minutes, and then heated at 85°C for 20 minutes to obtain a test coated plate on which a multilayer coating had been formed.

[0296] [Table 6]

[0297]

[0298]

[0299]

[0300]

[0301]

[0302]

[0303] evaluate

[0304] The following coating film performance tests were performed on the test plates 1 to 52 prepared as described above. The evaluation results are also shown in Table 6. It should be noted that the visual evaluation results are the average of the observation results of 20 evaluators.

[0305] <Disintegration resistance>

[0306] The test plate was mounted on a sample holder of Gravel Test Instrument JA-400 (trade name, manufactured by Suga Test Instruments Co., Ltd.), and 50 g of No. 7 grit stone was struck at an angle of 45 degrees from a distance of 30 cm from the test plate at -20°C by compressed air of 0.39 MPa (4 kgf / cm2). Subsequently, the resulting test plate was washed with water and dried, and a cloth-based tape (manufactured by NICHIBAN Co., Ltd.) was attached to the coated surface, and after peeling the tape, the occurrence of damage on the coating film on the test plate was visually observed and evaluated according to the following criteria.

[0307] Very good: The size of the damage is very small, and the electrodeposition coating surface and the base steel plate are not exposed.

[0308] Good: The size of the damage is small, and the electrodeposition coating film surface and the base steel plate are not exposed.

[0309] General: The damage is small in size, but exposes the electrodeposition coating surface and the base steel plate.

[0310] Poor: The size of the lesion is quite large and also greatly exposes the base steel plate.

[0311] <Gasoline resistance>

[0312] The test plate was immersed in regular unleaded gasoline (No. 2 defined in JIS K2202:2012) at 20° C. for 24 hours, and the appearance was visually observed and evaluated according to the following criteria.

[0313] Good: No issues observed.

[0314] Fair: Minor issues such as yellowing and blistering are seen.

[0315] Poor: Problems such as yellowing and bubbling are seen.

[0316] <Water resistance test: blistering>

[0317] The test plate was immersed in warm water at 40° C. for 10 days, taken out and dried, and then the coated surface was visually observed, and the occurrence of bubbles was evaluated using the following criteria.

[0318] Very good: no bubbles appear.

[0319] Good: The area where bubbles appear is no more than 10% of the total area.

[0320] Normal: The area where bubbles appear is 11% to 30% of the total area.

[0321] Poor: The area where bubbles appear is 31% or more of the total area.

[0322] <Water resistance test: Adhesion>

[0323] The test plate was immersed in warm water at 40°C for 10 days, then removed and dried. Subsequently, a cutter was used to create 100 squares measuring 2 mm x 2 mm on the coated surface of the test plate so as to reach the substrate. Cellophane tape was then attached to the surface and abruptly peeled off at 45°C at 20°C. Evaluation was performed based on the number of coating film pieces remaining in the grid, according to the following criteria.

[0324] Very good: 100 pieces (no peeling)

[0325] Good: 99 pieces (some peeling)

[0326] Average: 51 to 98

[0327] Difference: no more than 50

[0328] <Recoat Adhesion Test>

[0329] The test plate was left at room temperature for 7 days, and the same coating was reapplied on the coated surface under the same conditions and cured. That is, in Example 1, a multilayer coating film was formed by sequentially applying and heating the aqueous first coloring coating composition BC1-1, the aqueous one-pack type second coloring coating composition BC2-1, and the clear coating composition CC1, and then left at room temperature for 7 days; then, on its surface (on the clear coating film obtained from the clear coating composition CC1), the same materials (that is, the aqueous first coloring coating composition BC1-1, the aqueous one-pack type second coloring coating composition BC2-1, and the clear coating composition CC1) were reapplied and heated to form a multilayer coating film.

[0330] The laminate of the multi-layer coating films obtained in this manner was allowed to stand at room temperature for 3 days and then subjected to the same adhesion test as above, and the number of remaining coating film pieces was seen and evaluated according to the following criteria.

[0331] Very good: 100 pieces (no peeling)

[0332] Good: 99 pieces (some peeling)

[0333] Average: 51 to 98

[0334] Difference: no more than 50

[0335] The present invention accomplished by the inventors has been described above in specific terms based on the embodiments, but it goes without saying that the present invention is not limited to the above embodiments and can be variously modified within the scope not departing from the gist thereof.

[0336] Explanation of Reference Numerals

[0337] BC1-ncd1, BC1-ncd2, BC1-ncd3, BC1-ncd4

[0338] Uncured first colored coating film

[0339] BC2-ncd1, BC2-ncd2, BC2-ncd3, BC2-ncd4

[0340] Uncured second colored coating film

[0341] CC-ncd1, CC-ncd2, CC-ncd3, CC-ncd4

[0342] Uncured clear coating film

[0343] BC1-cd1, BC1-cd2, BC1-cd3, BC1-cd4

[0344] Cured first colored coating film

[0345] BC2-cd1, BC2-cd2, BC2-cd3, BC2-cd4

[0346] Cured second colored coating film

[0347] CC-cd1, CC-cd2, CC-cd3, CC-cd4

[0348] Cured clear coating film

[0349] D Unsaturated fatty acid ester polyol.

Claims

1. A method for forming a multilayer coating film, the method comprising: A step (1) of applying an aqueous first colored coating composition (BC1) to an object to be coated to form an uncured first colored coating film; a step (2) of applying an aqueous one-component type second colored coating composition (BC2) onto the uncured first colored coating film obtained in the step (1) to form an uncured second colored coating film; A step (3) of applying a clear coating composition (CC) onto the uncured second colored coating film obtained in step (2) to form an uncured clear coating film, wherein The clear coating composition (CC) comprises Hydroxyl-containing acrylic resin (A1), Isocyanate curing agent (A2), and an unsaturated fatty acid ester polyol (D) having at least two hydroxyl groups in one molecule, and a distance Ra between the solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space and the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space is 4.0 or less; as well as A step (4) of simultaneously curing the uncured first colored coating film, the uncured second colored coating film, and the uncured clear coating film formed in steps (1) to (3) by heating at 70°C to 100°C.

2. The method for forming a multilayer coating film according to claim 1, wherein The amount (solid content) of the unsaturated fatty acid ester polyol (D) blended is 3% to 20% by mass relative to the total mass of the solid contents of all resin components contained in the clear coating composition (CC).

3. The method for forming a multilayer coating film according to claim 1 or 2, wherein: The hydroxyl value of the hydroxyl-containing acrylic resin (A1) is within a range of 80 to 200 mg KOH / g.

4. The method for forming a multilayer coating film according to claim 1 or 2, wherein The hydroxyl-containing acrylic resin (A1) has a mass average molecular weight of 3,000 to 30,000.

5. The method for forming a multilayer coating film according to claim 1 or 2, wherein The clear coating composition (CC) comprises a urethane curing catalyst (E).

6. The method for forming a multilayer coating film according to claim 1 or 2, wherein: The first colored coating film has a dry film thickness of 4 to 30 μm.

7. The method for forming a multilayer coating film according to claim 1 or 2, wherein: The aqueous first colored coating composition (BC1) comprises a carboxyl group-containing resin (B1) and a carbodiimide group-containing curing agent (B2), And the clear coating composition (CC) contains isocyanate groups in the isocyanate curing agent (A2) at a ratio of 0.8 to 1.5 equivalents per 1 equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin (A1).

8. The method for forming a multilayer coating film according to claim 1 or 2, wherein The clear coating composition (CC) contains isocyanate groups in the isocyanate curing agent (A2) at a ratio of 1.5 to 3.0 equivalents per 1 equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin (A1), and the isocyanate curing agent (A2) contains a diisocyanate dimer (A2-1) and a diisocyanate trimer or higher compound (A2-2) at a solid content mass ratio within the range of 10 / 90 to 40 / 60.

9. The method for forming a multilayer coating film according to claim 1 or 2, wherein: The clear coating composition (CC) contains a 3-isocyanatopropyltrialkoxysilane compound (A3).

10. The method for forming a multilayer coating film according to claim 9, wherein The amount (solid content) of the 3-isocyanatepropyltrialkoxysilane compound (A3) blended is 2% by mass to 50% by mass relative to the total mass of the solid contents of all resin components contained in the clear coating composition (CC).

11. A clear coating film-forming clear coating composition (CC) for forming a multilayer coating film by simultaneously curing: an uncured first colored coating film formed of an aqueous first colored coating composition (BC1), an uncured second colored coating film formed from an aqueous one-component second colored coating composition (BC2) provided on the uncured first colored coating film, and an uncured clear coating film disposed on the uncured second colored coating film, wherein: The clear coating composition (CC) comprises Hydroxyl-containing acrylic resin (A1), Isocyanate curing agent (A2), and an unsaturated fatty acid ester polyol (D) having at least two hydroxyl groups in one molecule; The distance Ra between the solubility parameter of the hydroxyl-containing acrylic resin (A1) in the Hansen space and the solubility parameter of the unsaturated fatty acid ester polyol (D) in the Hansen space is 4.0 or less; and The uncured first colored coating film, the uncured second colored coating film, and the uncured clear coating film are cured by heating at 70°C to 100°C.

12. The clear coating composition (CC) according to claim 11, wherein The amount (solid content) of the unsaturated fatty acid ester polyol (D) blended is 3% to 20% by mass relative to the total mass of the solid contents of all resin components contained in the clear coating composition (CC).

13. The clear coating composition according to claim 11 or 12, wherein The aqueous first colored coating composition (BC1) comprises a carboxyl group-containing resin (B1) and a carbodiimide group-containing curing agent (B2), And the clear coating composition (CC) contains isocyanate groups in the isocyanate curing agent (A2) at a ratio of 0.8 to 1.5 equivalents per 1 equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin (A1).

14. The clear coating composition (CC) according to claim 11 or 12, wherein The clear coating composition (CC) contains isocyanate groups in the isocyanate curing agent (A2) at a ratio of 1.5 to 3.0 equivalents per 1 equivalent of the hydroxyl group in the hydroxyl-containing acrylic resin (A1), and the isocyanate curing agent (A2) contains a diisocyanate dimer (A2-1) and a diisocyanate trimer or higher compound (A2-2) at a solid content mass ratio within the range of 10 / 90 to 40 / 60.

15. The clear coating composition (CC) according to claim 11 or 12, wherein The clear coating composition (CC) contains a 3-isocyanatopropyltrialkoxysilane compound (A3).

16. The clear coating composition (CC) according to claim 15, wherein The amount (solid content) of the 3-isocyanatepropyltrialkoxysilane compound (A3) blended is 2% by mass to 50% by mass relative to the total mass of the solid contents of all resin components contained in the clear coating composition (CC).

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