Clear coating material composition and method for forming multilayer coating film

Abstract

This clear coating material composition contains a hydroxy group-containing resin (A), a curing agent (B), a defoaming agent (C), and at least one organic solvent (D) selected from the group consisting of an aliphatic hydrocarbon solvent (D1) and an alicyclic hydrocarbon solvent (D2). The defoaming agent (C) contains a silicone defoaming agent (C1), and the solid content at the time of coating is 55 mass% or more.

Description

Clear coating composition and method for forming multi-layer coating film

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

[0002] In recent years, growing awareness of environmental protection has led to calls for the reduction of volatile organic compounds (VOC) emissions from factories and other facilities for environmental conservation. One method for reducing VOC emissions is to increase the solids concentration of the coating composition. High-solids coatings, which contain less solvent and have a reduced environmental impact, have become widely used.

[0003] For example, Patent Document 1 discloses a method for coating plastic materials for automobiles, in which a water-based primer paint, a water-based metallic base paint used for metal materials for automobiles, and a high-solid clear paint are sequentially applied to the surface of a plastic material for automobiles to form a three-layer paint film, and then the three-layer paint film formed is simultaneously baked and cured at a temperature of 70 to 100°C.

[0004] Patent Document 2 describes a coating composition that contains (A) a hydroxyl group-containing acrylic resin having an acid value of 0 to 30 mgKOH / g, (B) a hydroxyl group-containing acrylic resin having an acid value of 60 to 120 mgKOH / g, and (C) a polyisocyanate compound, and that has a solid content of 50 mass% or more when applied.

[0005] However, high-solid clear paints have a problem in that, due to their high solid content, they are prone to develop cracks (small holes or protrusions on the surface of the coating film) on the coating surface.

[0006] Patent Document 3 relates to an aqueous coating composition, and describes that it is preferable for the aqueous coating composition to further contain a hydrophobic solvent in order to improve the water-borne resistance of the aqueous coating composition. This is an aqueous coating composition for forming an aqueous base coat film, not the high-solid clear coating of Patent Document 3. Furthermore, in the examples, 2-ethyl-1-hexanol is used as an example of the hydrophobic solvent.

[0007] JP 2008-200587 A, WO 2022 / 107847 A, WO 2013 / 151143 A

[0008] To improve resistance to popping, it is conceivable to add an organic solvent to the high solid clear, but the basic design is not to increase the viscosity of the high solid clear.

[0009] The problem to be solved by the present invention is to provide a clear coating composition that has a low content of volatile organic compounds and can produce a coating film with excellent resistance to blisters, and a method for producing a multilayer coating film using such a clear coating composition.

[0010] The present invention includes the following embodiments: Item 1. A clear coating composition comprising a hydroxyl group-containing resin (A), a curing agent (B), an antifoaming agent (C), and at least one organic solvent (D) selected from the group consisting of an aliphatic hydrocarbon solvent (D1) and an alicyclic hydrocarbon solvent (D2), wherein the antifoaming agent (C) comprises a silicone antifoaming agent (C1), and the solids concentration at the time of application is 55 mass% or more. Item 2. Item 2. A clear coating composition comprising a first part and a second part, wherein the first part contains a hydroxyl group-containing resin (A), the second part contains a curing agent (B), at least one of the first part and the second part contains an antifoaming agent (C) including a silicone-based antifoaming agent (C1), and at least one of the first part and the second part contains at least one organic solvent (D) selected from the group consisting of an aliphatic hydrocarbon solvent (D1) and an alicyclic hydrocarbon solvent (D2), and the solids concentration at the time of application is 55 mass% or more. Item 3. The clear coating composition according to Item 1 or 2, wherein the hydroxyl group-containing resin (A) contains a hydroxyl group-containing acrylic resin (A1). Item 4. The clear coating composition according to any one of Items 1 to 3, wherein the curing agent (B) contains a polyisocyanate compound (B1). Item 5. Item 6. The clear coating composition according to any one of Items 1 to 4, wherein the content of the antifoaming agent (C) is within the range of 0.01 to 1 part by mass, based on 100 parts by mass of the resin solids content in the clear coating composition. Item 7. The clear coating composition according to any one of Items 1 to 5, wherein the at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) includes an aliphatic hydrocarbon solvent (D11) having 10 or more carbon atoms. Item 8. The clear coating composition according to any one of Items 1 to 6, wherein the content of the at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) is within the range of 3 to 20 parts by mass, based on 100 parts by mass of the resin solids content in the clear coating composition.Item 8. The clear coating composition according to any one of Items 1 to 7, wherein the mass ratio of the antifoaming agent (C) to the at least one organic solvent (D) selected from the group consisting of the aliphatic hydrocarbon solvent (D1) and the alicyclic hydrocarbon solvent (D2), expressed as (C) / (D), is within the range of 0.01 to 0.5. Item 9. A coating film forming method comprising applying the clear coating composition according to any one of Items 1 to 8 to an object to be coated, to form a clear coating film. Item 10. Item 11. A method for forming a multilayer coating film, comprising: step (I-1): applying a first coating composition to a substrate to form an uncured first coating film; step (I-2): applying a second coating composition to the uncured first coating film formed in step (I-1) to form an uncured second coating film; step (I-3): applying the clear coating composition described in any one of items 1 to 8 to the uncured second coating film formed in step (I-2) to form an uncured clear coating film; and step (I-4): heat-curing the uncured first coating film formed in step (I-1), the uncured second coating film formed in step (I-2), and the uncured clear coating film formed in step (I-3) all at once. A method for forming a multilayer coating film, comprising: step (I-1): applying a first coating composition onto a substrate and then curing it to form a cured first coating film; step (I-2): applying a second coating composition onto the cured first coating film formed in step (I-1) to form an uncured second coating film; step (I-3): applying the clear coating composition described in any one of items 1 to 8 onto the uncured second coating film formed in step (I-2) to form an uncured clear coating film; and step (I-4): simultaneously curing the uncured second coating film and clear coating film formed in step (I-3).

[0011] According to the present invention, it is possible to provide a clear coating composition that has a low content of volatile organic compounds and that can produce a coating film with excellent blisters resistance, and further to provide a method for forming a multi-layer coating film using the clear coating composition.

[0012] As used herein, the singular forms "a," "an," and "the" are intended to include both the singular and the plural unless otherwise expressly stated herein or otherwise clearly contradicted by context.

[0013] In this specification, the term "comprise" is a concept that encompasses "consist essentially of" and "consist only of."

[0014] In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. Furthermore, in the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example. Furthermore, in this specification, a numerical value connected with "to" means a numerical range that includes the numbers before and after "to" as the upper and lower limits.

[0015] As used herein, (meth)acrylate refers to acrylate, methacrylate, or both.

[0016] Clear Coating Composition The clear coating composition of the present disclosure will now be described in further detail.

[0017] The clear coating composition of the present disclosure comprises a hydroxyl group-containing resin (A), a curing agent (B), an antifoaming agent (C), and at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2), wherein the antifoaming agent (C) comprises a silicone-based antifoaming agent (C1), and the clear coating composition has a solids concentration at the time of application of 55 mass% or more.

[0018] <Hydroxyl-containing resin (A)> The hydroxyl-containing resin (A) is a resin having at least one hydroxyl group per molecule. As the hydroxyl-containing resin (B1), a wide variety of known resins can be used, including, for example, acrylic resins having hydroxyl groups, polyester resins having hydroxyl groups, acrylic-modified polyester resins having hydroxyl groups, polyether resins having hydroxyl groups, polycarbonate resins having hydroxyl groups, polyurethane resins having hydroxyl groups, epoxy resins having hydroxyl groups, and alkyd resins having hydroxyl groups. These can be used alone or in combination of two or more. Among these, the hydroxyl-containing resin (A) is preferably a hydroxyl-containing acrylic resin (A1) from the viewpoint of the water resistance of the coating film formed.

[0019] The hydroxyl-containing acrylic resin (A1) preferably has an acid value in the range of 0 to 40 mgKOH / g. An acid value in the range of 0 to 40 mgKOH / g improves the blisters resistance of the coating film formed. In particular, the acid value of the hydroxyl-containing acrylic resin (A1) is more preferably in the range of 0 to 30 mgKOH / g, more preferably in the range of 1 to 25 mgKOH / g, and even more preferably in the range of 3 to 15 mgKOH / g, from the viewpoints of the storage stability of the resulting coating composition and the blisters resistance of the coating film formed.

[0020] The hydroxyl group-containing acrylic resin (A1) can be obtained, for example, by copolymerizing a hydroxyl group-containing polymerizable unsaturated monomer, an acid group-containing polymerizable unsaturated monomer, and another polymerizable unsaturated monomer (a polymerizable unsaturated monomer other than the hydroxyl group-containing polymerizable unsaturated monomer and the acid group-containing polymerizable unsaturated monomer).

[0021] The hydroxyl group-containing polymerizable unsaturated monomer is a compound having one or more hydroxyl groups and one or more polymerizable unsaturated bonds in one molecule. Examples of the hydroxyl group-containing polymerizable unsaturated monomer include monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; ε-caprolactone-modified monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms; adducts of (meth)acrylic acid with epoxy group-containing compounds (for example, "Cardura E10P" (trade name), manufactured by Momentive Specialty Chemicals, neodecanoic acid glycidyl ester); N-hydroxymethyl (meth)acrylamide; allyl alcohol; and (meth)acrylates having a polyoxyethylene chain whose molecular terminal is a hydroxyl group.

[0022] As the hydroxyl group-containing polymerizable unsaturated monomer, it is preferable to use a secondary hydroxyl group-containing polymerizable unsaturated monomer from the viewpoint of the blisters resistance of the coating film formed from the clear coating composition of the present disclosure.

[0023] Examples of the secondary hydroxyl group-containing polymerizable unsaturated monomer include polymerizable unsaturated monomers having a secondary hydroxyl group in which the alkyl group in the ester moiety has 2 to 8 carbon atoms, preferably 3 to 6 carbon atoms, and more preferably 3 or 4 carbon atoms, such as 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, and 3-hydroxybutyl (meth)acrylate; and adducts of (meth)acrylic acid and epoxy group-containing compounds (e.g., "Cardura E10P" (trade name), manufactured by Momentive Specialty Chemicals, neodecanoic acid glycidyl ester). These can be used alone or in combination of two or more. Of these, 2-hydroxypropyl (meth)acrylate is preferred from the viewpoint of the blisters resistance of the coating film formed by the clear coating composition of the present disclosure.

[0024] The acid group-containing polymerizable unsaturated monomer is a compound having one or more acid groups and one or more polymerizable unsaturated bonds per molecule. Examples of such monomers include carboxyl group-containing monomers such as (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, and maleic anhydride; sulfonic acid group-containing monomers such as vinyl sulfonic acid and 2-sulfoethyl (meth)acrylate; and acidic phosphate ester monomers such as 2-(meth)acryloyloxyethyl acid phosphate, 2-(meth)acryloyloxypropyl acid phosphate, 2-(meth)acryloyloxy-3-chloropropyl acid phosphate, and 2-methacryloyloxyethylphenyl phosphoric acid. These monomers may be used alone or in combination.

[0025] Examples of other polymerizable unsaturated monomers copolymerizable with the hydroxyl group-containing polymerizable unsaturated monomer and the acid group-containing polymerizable unsaturated monomer include the following monomers (1) to (5). These polymerizable unsaturated monomers can be used alone or in combination of two or more.

[0026] (1) Esterification products of acrylic acid or methacrylic acid with monohydric alcohols having 1 to 20 carbon atoms. Specific examples thereof include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, isomyristyl (meth)acrylate, stearyl (meth)acrylate, isostearyl acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, cyclohexyl (meth)acrylate, and isobornyl (meth)acrylate.

[0027] (2) Aromatic vinyl monomers Specific examples include styrene, α-methylstyrene, vinyltoluene, and the like.

[0028] By using an aromatic vinyl monomer as a constituent component, the glass transition temperature of the resulting resin is increased, and a coating film with a high refractive index and hydrophobicity can be obtained, which can improve the gloss of the coating film and thereby the finished appearance.

[0029] When an aromatic vinyl monomer is used as a constituent component, the blending ratio thereof is preferably within the range of 3 to 50% by mass, more preferably 5 to 40% by mass, based on the total amount of the monomer components.

[0030] (3) Glycidyl Group-Containing Polymerizable Unsaturated Monomer The glycidyl group-containing polymerizable unsaturated monomer is a compound having one or more glycidyl groups and one or more polymerizable unsaturated bonds in one molecule, and specific examples thereof include glycidyl acrylate and glycidyl methacrylate.

[0031] (4) Polymerizable Unsaturated Bond-Containing Nitrogen Atom-Containing Compounds Examples thereof include (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-[3-(dimethylamino)propyl](meth)acrylamide, N-butoxymethyl(meth)acrylamide, diacetone(meth)acrylamide, N,N-dimethylaminoethyl(meth)acrylate, vinylpyridine, vinylimidazole, acrylonitrile, and methacrylonitrile.

[0032] (5) Other vinyl compounds Examples include vinyl acetate, vinyl propionate, vinyl chloride, and versatic acid vinyl ester. Examples of versatic acid vinyl ester include commercially available products such as "Veova 9" and "Veova 10" (both trade names, manufactured by Japan Epoxy Resin Co., Ltd.).

[0033] As the other polymerizable unsaturated monomer, the monomers shown in (1) to (5) above can be used alone or in combination of two or more.

[0034] In the present invention, the polymerizable unsaturated monomer refers to a monomer having one or more (e.g., 1 to 4) polymerizable unsaturated groups. The polymerizable unsaturated group refers to an unsaturated group that can undergo radical polymerization. Examples of such polymerizable unsaturated groups include vinyl groups, (meth)acryloyl groups, (meth)acrylamide groups, vinyl ether groups, allyl groups, propenyl groups, isopropenyl groups, and maleimide groups.

[0035] In addition, in this specification, "(meth)acrylate" means acrylate or methacrylate. "(meth)acrylic acid" means acrylic acid or methacrylic acid. "(meth)acryloyl" means acryloyl or methacryloyl. "(meth)acrylamide" means acrylamide or methacrylamide.

[0036] In producing the hydroxyl-containing acrylic resin (A1), the amount of the hydroxyl-containing polymerizable unsaturated monomer used is preferably within the range of 15 to 55% by mass, more preferably 20 to 45% by mass, based on the total amount of copolymerizable monomer components, from the viewpoints of the water resistance and storage stability of the coating film to be formed.

[0037] The hydroxyl value of the hydroxyl-containing acrylic resin (A1) is preferably within the range of 50 to 250 mgKOH / g, more preferably 50 to 210 mgKOH / g, even more preferably 80 to 200 mgKOH / g, and particularly preferably 100 to 190 mgKOH / g, from the viewpoints of the water resistance and finished appearance of the coating film to be formed.

[0038] In the production of the hydroxyl group-containing acrylic resin (A1), the amount of the acid group-containing polymerizable unsaturated monomer used is preferably within the range of 0 to 5.0 mass %, more preferably 0.5 to 3.0 mass %, based on the total amount of copolymerizable monomer components, from the viewpoints of the water resistance and popping resistance of the coating film to be formed.

[0039] Furthermore, from the viewpoints of the water resistance and popping resistance of the coating film formed, and reducing the VOC content of the resulting coating composition, the weight-average molecular weight of the hydroxyl group-containing acrylic resin (A1) is preferably in the range of 3,000 to 10,000, more preferably in the range of 3,500 to 9,000, and even more preferably in the range of 4,000 to 8,000.

[0040] In this specification, the average molecular weight is a value calculated from a chromatogram measured by gel permeation chromatography using the molecular weight of standard polystyrene as a reference. The gel permeation chromatograph used was "HLC8120GPC" (manufactured by Tosoh Corporation). Four columns, "TSKgel G-4000HXL," "TSKgel G-3000HXL," "TSKgel G-2500HXL," and "TSKgel G-2000HXL" (all manufactured by Tosoh Corporation, trade names), were used, and the measurement was performed under the following conditions: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 1 cc / min, and detector: RI.

[0041] The glass transition temperature of the hydroxyl group-containing acrylic resin (A1) is preferably within the range of 0 to 70°C, more preferably 10 to 60°C, and even more preferably 20 to 60°C, from the viewpoints of the water resistance and flake resistance of the coating film formed and reducing the VOC content of the resulting coating composition.

[0042] In this specification, the glass transition temperature (°C) of the acrylic resin is calculated by the following formula.

[0043] 1 / Tg(K)=(W1 / T1)+(W2 / T2)+... (1) Tg(℃)=Tg(K)-273 (2)

[0044] In each formula, W1, W2, ... represent the mass fractions of the monomers used in the copolymerization, and T1, T2, ... represent the Tg (K) of the homopolymer of each monomer. Note that T1, T2, ... are values ​​according to pages III-139 to 179 of "Polymer Hand Book" (Second Edition, edited by J. Brandup and E.H. Immergut). When the Tg of the homopolymer of a monomer is unclear, the glass transition temperature (°C) is taken as the static glass transition temperature. For example, using a differential scanning calorimeter "DSC-220U" (manufactured by Seiko Instruments Inc.), a sample is placed in a measuring cup, and the solvent is completely removed by vacuum suction. Then, the change in heat quantity is measured in the range of -20°C to +200°C at a heating rate of 3°C / min, and the first change point of the baseline on the low-temperature side is taken as the static glass transition temperature.

[0045] The hydroxyl group-containing acrylic resin (A1) can be obtained by copolymerizing the above-mentioned polymerizable unsaturated monomer mixture. As the copolymerization method, a solution polymerization method in which polymerization is carried out in an organic solvent in the presence of a polymerization initiator can be preferably used.

[0046] The above hydroxyl group-containing acrylic resins (A1) can be used alone or in combination of two or more.

[0047] From the viewpoints of improving the flake resistance and water resistance of the coating film formed, and reducing the VOC content of the resulting coating composition, the content of the hydroxyl group-containing acrylic resin (A1) in the clear coating composition of the present disclosure is preferably within the range of 20 to 70 parts by mass, more preferably 25 to 65 parts by mass, and even more preferably 30 to 60 parts by mass, based on 100 parts by mass of the resin solids content of the clear coating composition.

[0048] <Curing agent (B)> The curing agent (B) is a compound that reacts with the hydroxyl group-containing resin (A) to form a crosslinked structure and cure the clear coating composition. From the viewpoint of reducing the content of volatile organic compounds, the curing agent (B) preferably contains a polyisocyanate (B1).

[0049] The polyisocyanate compound (B1) is a compound having at least two isocyanate groups in one molecule, and examples thereof include aliphatic polyisocyanate compounds, alicyclic polyisocyanate compounds, aliphatic polyisocyanates having aromatic rings in the molecule that are not bonded to isocyanate groups (araliphatic polyisocyanates), aromatic polyisocyanate compounds, and derivatives of these polyisocyanate compounds.

[0050] Furthermore, examples of derivatives of the polyisocyanate compounds include dimers, trimers, biurets, allophanates, uretdione, uretoimine, isocyanurates, oxadiazinetriones, polymethylene polyphenyl polyisocyanates (crude MDI, polymeric MDI), crude TDI, and the like of the above-mentioned polyisocyanate compounds.

[0051] The above polyisocyanate compounds and derivatives thereof may be used alone or in combination of two or more kinds.

[0052] The isocyanate group of the polyisocyanate compound (B1) may be blocked with a blocking agent, such as phenol, lactam, aliphatic alcohol, ether, ester, oxime, mercaptan, acid amide, amine, imidazole, urea, imine, sulfite, or azole.

[0053] From the viewpoints of water resistance of the coating film formed and reduction of the VOC content of the resulting coating composition, the content of polyisocyanate compound (B1) in the clear coating composition of the present disclosure is preferably within the range of 5 to 60 parts by mass, more preferably 15 to 50 parts by mass, and even more preferably 25 to 45 parts by mass, based on 100 parts by mass of the resin solids content of the clear coating composition.

[0054] The equivalent ratio (NCO / OH) of the isocyanate group contained in the polyisocyanate compound (B) to the hydroxyl group contained in the hydroxyl group-containing resin (A), particularly the hydroxyl group-containing acrylic resin (A1), may be 0.7 or more, or 0.8 or more. The equivalent ratio (NCO / OH) may be 2.0 or less, 1.8 or less, or 1.5 or less. In one embodiment, the equivalent ratio (NCO / OH) is 0.7 or more and 2.0 or less. When the equivalent ratio (NCO / OH) is in this range, a clear coating film having excellent hardness and weather resistance is easily formed.

[0055] <Antifoaming agent (C)> The antifoaming agent (C) is hydrophobic and exhibits defoaming properties by orienting and localizing on the coating film surface. This reduces bubbles in the composition, thereby acting to suppress the occurrence of popping, making it easier to perform a uniform and less uneven coating. Among them, the silicone-based antifoaming agent (C1) has a low surface tension, which makes it highly oriented to the coating film surface and easily localized on the coating film surface, so it has excellent defoaming properties and provides excellent popping resistance to the clear coating composition and / or clear coating film.

[0056] The antifoaming agent includes a silicone-based antifoaming agent (C1) and a non-silicone-based antifoaming agent (C2).

[0057] Examples of the silicone-based antifoaming agent (C1) include surface-active polysiloxanes or antifoaming agents partially containing a polysiloxane structure, such as oil-type, compound-type, self-emulsifying-type, and emulsion-type antifoaming agents.

[0058] Examples of the non-silicone antifoaming agent (C2) include higher alcohol-based, higher alcohol derivative-based, fatty acid-based, fatty acid derivative-based, paraffin-based, polymer-based (e.g., (meth)acrylic, vinyl ether-based), and mineral oil-based antifoaming agents.

[0059] As the defoaming agent (C), a commercially available product may be used.

[0060] The amount of antifoaming agent (C) in the clear coating composition is not particularly limited, but from the viewpoint of the anti-blistering properties of the clear coating composition and / or the clear coating film, it is preferably in the range of 0.01 to 1 part by mass based on 100 parts by mass of the resin solids in the clear coating composition.

[0061] The amount of silicone-based antifoaming agent (C1) in the clear coating composition is not particularly limited, but from the viewpoint of the blisters resistance of the clear coating composition and / or the clear coating film, it is preferably in the range of 0.01 to 1 part by mass based on 100 parts by mass of the resin solids in the clear coating composition.

[0062] The antifoaming agent (C) may contain an antifoaming agent other than the silicone-based antifoaming agent (C1), or may consist solely of the silicone-based antifoaming agent (C1). The proportion of the silicone-based antifoaming agent (C1) in the antifoaming agent (C) is preferably 50% by mass or more, for example, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 100% by mass.

[0063] <Organic Solvent (D)> The clear coating composition of the present disclosure contains an organic solvent (D) which is at least one organic solvent selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2).

[0064] Examples of the aliphatic hydrocarbon solvent (D1) include hexane, heptane, octane, nonane, decane, undecane, dodecane, and isododecane. The aliphatic hydrocarbon solvent (D1) may contain either a linear aliphatic hydrocarbon or a branched aliphatic hydrocarbon. The aliphatic hydrocarbon solvent (D1) may be one type or a combination of two or more types.

[0065] Examples of the alicyclic hydrocarbon solvent (D2) include cyclohexane, cycloheptane, methylcyclohexane, dimethylcyclohexane, etc. The alicyclic hydrocarbon solvent (D2) may be one type or a combination of two or more types.

[0066] In terms of the anti-blister properties of the clear coating composition and / or the clear coating film, the at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) preferably contains an aliphatic hydrocarbon solvent (D11) having 10 or more carbon atoms, and more preferably is a solvent containing a branched-chain aliphatic hydrocarbon having 10 or more carbon atoms.

[0067] Examples of commercially available aliphatic hydrocarbon solvents (D11) having 10 or more carbon atoms include Merveille 20, 30, and 40, IP Solvent 1620, and IP Clean LX (isoparaffinic hydrocarbons, Idemitsu Kosan Co., Ltd.), and normal paraffins N-10, N-11, and N-12D (ENEOS Corporation).

[0068] The content of the at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) is not particularly limited, but from the viewpoint of the blisters resistance and storage stability of the clear coating composition and / or clear coating film, it is preferably within the range of 3 to 20 parts by mass based on 100 parts by mass of the resin solids in the clear coating composition.

[0069] The mass ratio of the antifoaming agent (C) to the at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) is not particularly limited, but from the viewpoint of the blisters resistance of the clear coating composition and / or clear coating film, it is preferable that the ratio of (C) / (D) be within the range of 0.01 to 0.5.

[0070] The clear coating composition of the present disclosure has a solids content of 55 mass% or more at the time of application, thereby achieving an effective reduction in VOCs.

[0071] In this specification, "solid content" refers to non-volatile components such as resins, curing agents, and pigments contained in a paint composition that remain after the paint composition has been dried for 1 hour at 105° C. Therefore, for example, the total solid content of a paint composition can be calculated by weighing out the paint composition into a heat-resistant container such as an aluminum foil cup, spreading the paint composition on the bottom of the container, drying it for 1 hour at 105° C., weighing the mass of the components in the paint composition that remain after drying, and determining the ratio of the mass of the components that remain after drying to the total mass of the paint composition before drying.

[0072] The reason why the clear coating composition of the present disclosure is able to form a coating film with excellent popping resistance is not necessarily theoretically clear, but the present inventors have discovered that when at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) is used as the organic solvent to be blended in the clear coating composition, and a silicone-based antifoaming agent (C1) is used as the antifoaming agent, a coating film with a low content of volatile organic compounds and excellent popping resistance can be obtained.

[0073] To improve the blisters resistance of a clear coating composition and / or a clear coating film, it is desirable to maintain a low viscosity of the clear coating composition, and viscosity is usually adjusted using a solvent. However, conventional common knowledge has been that at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) is less likely to reduce the viscosity of a coating composition than other organic solvents, and therefore the use of a paraffinic solvent in a high-solid clear coating to improve blisters resistance was unexpected by those skilled in the art. However, by combining a paraffinic solvent with a silicone antifoaming agent (C1), blisters resistance can be improved while suppressing an increase in the viscosity of the clear coating composition.

[0074] <Other Components> The clear coating composition of the present disclosure may further contain, as necessary, a resin other than the hydroxyl group-containing resin (A), a crosslinking agent other than the polyisocyanate (B1), a pigment, an organic solvent other than at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2), a curing catalyst, a dispersant, an anti-settling agent, a thickener, an ultraviolet absorber, a light stabilizer, a surface conditioner, a scratch resistance improver, and the like.

[0075] Examples of the crosslinking agent other than the polyisocyanate compound (B1) include melamine resins.

[0076] The melamine resin preferably has a weight average molecular weight in the range of 400 to 6,000, more preferably in the range of 500 to 5,000, and even more preferably in the range of 800 to 4,000.

[0077] As the melamine resin, commercially available products can be used. Examples of commercially available product names include "Cymel 202," "Cymel 203," "Cymel 238," "Cymel 251," "Cymel 303," "Cymel 323," "Cymel 324," "Cymel 325," "Cymel 327," "Cymel 350," "Cymel 385," "Cymel 1156," "Cymel 1158," "Cymel 1116," and "Cymel 1130" (all manufactured by Allnex Japan Co., Ltd.), "U-Ban 120," "U-Ban 20HS," "U-Ban 20SE60," "U-Ban 2021," "U-Ban 2028," and "U-Ban 28-60" (all manufactured by Mitsui Chemicals, Inc.).

[0078] The above melamine resins may be used alone or in combination of two or more.

[0079] When the clear coating composition of the present disclosure contains a melamine resin, the content thereof is preferably within the range of 1 to 30 parts by mass, more preferably 2 to 25 parts by mass, and even more preferably 3 to 20 parts by mass, based on 100 parts by mass of the resin solids content of the clear coating composition, from the viewpoint of the scratch resistance of the coating film to be formed, etc.

[0080] Examples of the pigment include color pigments, luster pigments, extender pigments, etc. The pigments may be used alone or in combination of two or more.

[0081] Examples of color pigments include titanium oxide, zinc oxide, carbon black, cadmium red, molybdenum red, chrome yellow, chromium oxide, Prussian blue, cobalt blue, azo pigments, phthalocyanine pigments, quinacridone pigments, isoindoline pigments, threne pigments, and perylene pigments.

[0082] Examples of the luster pigment include aluminum powder, mica powder, and mica powder coated with titanium oxide.

[0083] Examples of extender pigments include talc, clay, kaolin, baryta, barium sulfate, barium carbonate, calcium carbonate, and alumina white.

[0084] Each of the above pigments can be used alone or in combination of two or more.

[0085] When the clear coating composition of the present disclosure contains a pigment, the amount of pigment blended is preferably an amount that does not inhibit the transparency of the resulting coating film; for example, the amount is preferably within the range of 0.1 to 20 mass %, more preferably 0.3 to 10 mass %, and even more preferably 0.5 to 5 mass %, relative to the total amount of solids in the clear coating composition.

[0086] Furthermore, when the clear coating composition of the present disclosure is used as a colored coating and contains a pigment, the amount of the pigment is preferably in the range of usually 1 to 200 mass%, particularly 2 to 100 mass%, and even more particularly 5 to 50 mass%, based on the total amount of solids in the clear coating composition.

[0087] Examples of the organic solvent other than the at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) include ethyl acetate, butyl acetate, propyl propionate, butyl propionate, 1-methoxy-2-propyl acetate, 2-ethoxyethyl propionate, ethyl-3-ethoxypropionate, 3-methoxybutyl acetate, ethylene glycol monoethyl ether acetate, and diethylene glycol monoethyl ether. ketone-based solvents such as methyl ethyl ketone, methyl isobutyl ketone, and methyl amyl ketone; alcohol-based solvents such as isopropanol, n-butanol, isobutanol, and 2-ethylhexanol; and aromatic solvents such as toluene, xylene, Swazol 1000 (trade name, high-boiling point petroleum solvent, manufactured by Cosmo Oil Co., Ltd.) and Swazol 1500 (trade name, high-boiling point petroleum solvent, manufactured by Cosmo Oil Co., Ltd.). These can be used alone or in combination of two or more.

[0088] Examples of curing catalysts include tin octoate, dibutyltin diacetate, dibutyltin di(2-ethylhexanoate), dibutyltin dilaurate, dioctyltin diacetate, dioctyltin di(2-ethylhexanoate), dibutyltin oxide, dibutyltin sulfide, dioctyltin oxide, dibutyltin fatty acid salts, lead 2-ethylhexanoate, zinc octoate, zinc naphthenate, zinc fatty acids, bismuth octanoate, bismuth 2-ethylhexanoate, bismuth oleate, bismuth neodecanoate, and bismuth versatate. organic metal compounds such as bismuth naphthenate, cobalt naphthenate, calcium octoate, copper naphthenate, and tetra(2-ethylhexyl)titanate; sulfonic acids such as paratoluenesulfonic acid, dodecylbenzenesulfonic acid, and dinonylnaphthalenesulfonic acid; alkyl phosphates such as monobutyl phosphate, dibutyl phosphate, mono(2-ethylhexyl)phosphate, and di(2-ethylhexyl)phosphate; and salts of these acids with amine compounds, and these can be used either alone or in combination of two or more.

[0089] When the clear coating composition of the present disclosure contains the above-mentioned curing catalyst, the content of the curing catalyst is preferably within the range of 0.005 to 2 mass%, particularly 0.01 to 1 mass%, relative to the total solid content of the clear coating composition of the present disclosure.

[0090] As the thickener, conventionally known thickeners can be used, and examples thereof include clay minerals (e.g., metal silicates, montmorillonite), acrylics (e.g., those containing a structure consisting of an acrylic acid ester or methacrylic acid ester polymer or oligomer in the molecule), polyolefins (e.g., polyethylene, polypropylene, etc.), amides (higher fatty acid amides, polyamides, oligomers, etc.), polycarboxylic acids (including derivatives having at least two carboxyl groups in the molecule), cellulose (including various derivatives such as nitrocellulose, acetyl cellulose, cellulose ether, etc.), urethanes (polymers, oligomers, etc. containing a urethane structure in the molecule), ureas (polymers, oligomers, etc. containing a urea structure in the molecule), and urethane ureas (polymers, oligomers, etc. containing a urethane structure and a urea structure in the molecule).

[0091] As the ultraviolet absorber, conventionally known ones can be used, for example, ultraviolet absorbers such as benzotriazole-based absorbers, triazine-based absorbers, salicylic acid derivative-based absorbers, benzophenone-based absorbers, etc. These can be used alone or in combination of two or more kinds.

[0092] When the clear coating composition of the present disclosure contains an ultraviolet absorber, the amount of the ultraviolet absorber blended is preferably within the range of 0.1 to 10 mass%, more preferably 0.2 to 5 mass%, and even more preferably 0.3 to 2 mass%, relative to the total amount of solids in the clear coating composition.

[0093] As the light stabilizer, a conventionally known light stabilizer can be used, for example, a hindered amine light stabilizer.

[0094] From the viewpoint of pot life, it is preferable to use a hindered amine light stabilizer having low basicity. Examples of such hindered amine light stabilizers include acylated hindered amines and aminoether hindered amines, and specific examples include "HOSTAVIN 3058" (trade name, manufactured by Clariant) and "TINUVIN 123" (trade name, manufactured by BASF).

[0095] The clear coating composition of the present disclosure encompasses a two-component coating composition in which a hydroxyl-containing resin (A) and a curing agent (B) are crosslinked. In some embodiments, the clear coating composition may be a two-component clear coating composition comprising a first part (also referred to as a main part) containing the hydroxyl-containing resin (A) and a second part (also referred to as a curing agent) containing the curing agent (B), with components other than the hydroxyl-containing resin (A) and the curing agent (B) independently contained in at least one of the first part and the second part.

[0096] The clear coating composition may be a clear coating composition consisting of a first agent and a second agent, wherein the first agent contains a hydroxyl group-containing resin (A), the second agent contains a curing agent (B), at least one of the first agent and the second agent contains an antifoaming agent (C) including a silicone-based antifoaming agent (C1), and at least one of the first agent and the second agent contains at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2), and the solids concentration at the time of application is 55 mass% or more.

[0097] The first part may contain a hydroxyl group-containing resin (A) and a solvent, and the second part may contain a curing agent (B) and a solvent. The solvent in the first part and the solvent in the second part may each be an aqueous solvent containing water, an organic solvent, or a combination thereof. The solvent in the first part and the solvent in the second part may be the same or different. Examples of organic solvents include ketone compounds having 6 or less carbon atoms, such as acetone, acetylacetone, methyl ethyl ketone, methyl i-butyl ketone, and cyclohexanone; aromatic hydrocarbons, such as benzene, toluene, ethylbenzene, propylbenzene, t-butylbenzene, o-xylene, m-xylene, p-xylene, tetralin, and decalin; monocarboxylic acid esters, such as methyl acetate, ethyl acetate, n-butyl acetate, amyl acetate, diethylene glycol monobutyl ether acetate, and ethyl 3-ethoxypropionate; dibasic ester compounds; cellosolves, such as methyl cellosolve, ethyl cellosolve, n-propyl cellosolve, i-propyl cellosolve, n-butyl cellosolve, i-butyl cellosolve, i-amyl cellosolve, phenyl cellosolve, and benzyl cellosolve; carbitols such as n-butyl carbitol, i-butyl carbitol, i-amyl carbitol, carbitol acetate, phenyl carbitol, and benzyl carbitol; ethers such as ethylene glycol monoisopropyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-n-butyl ether, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and dioxane; and alcohols such as methanol, ethanol, propanol, isopropyl alcohol, butanol, and isobutyl alcohol. These may be used alone or in combination of two or more.

[0098] At least a portion of the solvents contained in the first and second agents is at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2). The first agent may contain a defoaming agent (C) containing a silicone-based defoaming agent (C1) and at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2). Alternatively, the second agent may contain a defoaming agent (C) containing a silicone-based defoaming agent (C1) and at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2). Alternatively, one of the first agent and the second agent may contain an antifoaming agent (C) including a silicone-based antifoaming agent (C1), and the other of the first agent and the second agent may contain at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2).

[0099] The clear coating composition of the present disclosure can form a coating film with excellent resistance to cracking, and can therefore be suitably used as a top clear coat coating for the uppermost layer.

[0100] Clear coating film, multi-layer coating film, and coated article A clear coating film is formed by applying the clear coating composition of the present disclosure to a substrate. This results in a coated article having a clear coating film on the substrate. Such coated articles have a low content of volatile organic compounds and excellent resistance to cracking.

[0101] The coated article may, for example, comprise a substrate and a multilayer coating film including a base coating film and a clear coating film. The base coating film is disposed between the substrate and the clear coating film. The base coating film may be one type of layer, or may be two or more types of layers. The multilayer coating film may further comprise an intermediate coating film disposed between the substrate and the base coating film. In other words, the coated article may comprise a substrate and a multilayer coating film in which an intermediate coating film, a base coating film, and a clear coating film are laminated in this order.

[0102] <Substrate> The substrate is not particularly limited, and examples thereof include automobile bodies (e.g., outer panels of automobile bodies) such as passenger cars, trucks, motorcycles, and buses; automobile parts such as spoilers, bumpers, mirror covers, grilles, and door handles; and household electrical appliances such as mobile phones and audio equipment. Of these, outer panels of automobile bodies and automobile parts are preferred.

[0103] The material of the coating may be metal, resin, or glass, with synthetic resin being preferred.

[0104] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of metallic substrates include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0105] The metal substrate may be surface-treated, for example, by phosphate treatment, chromate treatment, zirconium conversion treatment, or composite oxide treatment. After the surface treatment, the metal substrate may be further coated with an electrodeposition paint.

[0106] Examples of resins include polypropylene resin, polycarbonate resin, urethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, and polyamide resin. The resin substrate may be degreased.

[0107] The substrate may be subjected to a surface treatment as required, and then a primer coating film may be formed thereon. For example, when the substrate is an automobile body, the primer coating film may be formed using a known primer and / or intermediate coating composition that is commonly used in the painting of automobile bodies.

[0108] The undercoat paint composition for forming the undercoat paint film can be, for example, an electrodeposition paint, preferably a cationic electrodeposition paint. The intermediate paint composition for forming the intermediate paint film can be, for example, a paint prepared by mixing a base resin having a crosslinkable functional group such as a carboxyl group, a hydroxyl group, or the like, such as an acrylic resin, a polyester resin, an alkyd resin, a urethane resin, or an epoxy resin, with a crosslinking agent such as an amino resin such as a melamine resin or a urea resin, or an optionally blocked polyisocyanate compound, together with a pigment, a thickener, and optionally other components.

[0109] <Intermediate Coating Film> The intermediate coating film is formed from an intermediate coating composition, which will be described later.

[0110] The intermediate coating film has a film thickness (dry film thickness) after curing of, for example, 5 μm or more and 80 μm or less. The intermediate coating film may have a dry film thickness of 10 μm or more. The intermediate coating film may have a dry film thickness of 50 μm or less.

[0111] The thickness of the coating film can be measured using an electromagnetic film thickness meter (for example, SDM-miniR manufactured by SANKO Co., Ltd.) The thickness of the coating film is the average value of the thickness of the coating film at any five points.

[0112] <Base Coating Film> The base coating film is formed from a base coating composition, which will be described later.

[0113] The base coating film may be a single layer or a multi-layer coating film of two or more layers. The dry film thickness of each layer of the base coating film is, for example, 5 μm or more and 40 μm or less. The dry film thickness of each layer of the base coating film may be 7 μm or more. The dry film thickness of each layer of the base coating film may be 30 μm or less.

[0114] <Clear Coating Film> The clear coating film is formed from the clear coating composition of the present disclosure.

[0115] The dry film thickness of the clear coating film is, for example, 10 μm or more and 80 μm or less. The dry film thickness of the clear coating film may be 20 μm or more. The dry film thickness of the clear coating film may be 60 μm or less. In a preferred embodiment, the dry film thickness of the clear coating film is 25 to 50 μm.

[0116] Method for forming a multilayer coating film A multilayer coating film is formed by a method for forming a multilayer coating film, comprising the following steps (I-1) to (I-4): Step (I-1): applying a first coating composition to an object to be coated to form an uncured first coating film; Step (I-2): applying a second coating composition to the uncured first coating film formed in step (I-1) to form an uncured second coating film; Step (I-3): applying any of the clear coating compositions described above to the uncured second coating film formed in step (I-2) to form an uncured clear coating film; and Step (I-4): heat-curing the uncured first coating film formed in step (I-1), the uncured second coating film formed in step (I-2), and the uncured clear coating film formed in step (I-3) all at once.

[0117] The first coating composition and the second coating composition may be an intermediate coating composition and a base coating composition, respectively, or may be two base coating compositions each having a different composition.

[0118] The multilayer coating film is formed, for example, by a method comprising the steps of applying a base coating composition onto an object to be coated to form an uncured base coating film, applying the clear coating composition of the present disclosure to the uncured base coating film to form an uncured clear coating film, and simultaneously curing the uncured base coating film and the uncured clear coating film.

[0119] Before the step of applying the base coating composition, a step of applying an intermediate coating composition to the substrate may be carried out. When the base coating composition is applied, the intermediate coating film may be cured or uncured. When the base coating composition is applied, the intermediate coating film may be cured.

[0120] That is, the method for forming a multi-layer coating film may be a method comprising: step (I-1): applying an intermediate coating composition onto an object to be coated, followed by curing to form a cured intermediate coating film; step (I-2): applying a base coating composition onto the cured intermediate coating film formed in step (I-1), to form an uncured base coating film; step (I-3): applying a clear coating composition onto the uncured base coating film formed in step (I-2), to form an uncured clear coating film; and step (I-4): curing the uncured base coating film and clear coating film formed in step (I-3) at the same time (a 3-coat 2-bake method).

[0121] Alternatively, the method for forming a multi-layer coating film may include: Step (I-1): applying an intermediate coating composition onto an object to be coated to form an uncured intermediate coating film; Step (I-2): applying a base coating composition onto the uncured intermediate coating film formed in Step (I-1) to form an uncured base coating film; Step (I-3): applying a clear coating composition onto the uncured base coating film formed in Step (I-2) to form an uncured clear coating film; and Step (I-4): curing the uncured intermediate coating film formed in Step (I-1), the uncured base coating film formed in Step (I-2), and the uncured clear coating film formed in Step (I-3) all at once (3 coat 1 bake method). From the viewpoint of energy conservation, the 3 coat 1 bake method is preferred.

[0122] Each step will be explained below using as an example a method for forming a multi-layer coating film in which an intermediate coating film, a base coating film, and a clear coating film are laminated in this order by the 3-coat 1-bake method, although the method for forming a multi-layer coating film is not limited to this.

[0123] (I) Step of forming an uncured intermediate coating film First, an intermediate coating composition is applied to a substrate to form an uncured intermediate coating film. The intermediate coating film improves adhesion between the base coating film and the substrate. The intermediate coating also makes the painted surface uniform, making it easier to suppress unevenness in the base coating film.

[0124] Examples of coating methods include roll coating, air spray coating, airless spray coating, and rotary atomization coating. These methods may be combined with electrostatic coating. Among these, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency.

[0125] After applying the intermediate coating composition, pre-drying (also called preheating) may be performed before applying the base coating composition. Pre-drying prevents the uncured intermediate coating film and the base coating composition from mixing, making it difficult for a mixed layer to form. Therefore, the smoothness of the resulting multi-layer coating film can be further improved.

[0126] Examples of pre-drying include a method of leaving the film at a temperature of 20°C or higher and 25°C or lower for 5 to 15 minutes, and a method of heating the film at a temperature of 50°C or higher and 80°C or lower for 30 seconds to 10 minutes.

[0127] <Intermediate Coating Composition> The intermediate coating composition may be water-based or solvent-based. A water-based coating composition contains water as the solvent in an amount of 50 mass% or more of the total solvent. A solvent-based coating composition contains an organic solvent as the solvent in an amount of 50 mass% or more of the total solvent.

[0128] The intermediate coating composition contains, in addition to various solvents, for example, a resin, a pigment, and various additives. Examples of resins include acrylic resins, polyester resins, polyurethane resins, alkyd resins, fluororesins, epoxy resins, and polyether resins. These may be used alone or in combination of two or more. The resin may be a resin having a crosslinkable functional group, and examples of the crosslinkable functional group include a carboxyl group, a hydroxyl group, and an epoxy group. The intermediate coating composition may further contain the above-mentioned curing agent (B). Examples of the curing agent (B) used in the intermediate coating composition include a melamine resin, a polyisocyanate compound, and a blocked polyisocyanate compound.

[0129] As the intermediate coating composition, either an aqueous coating composition or an organic solvent-based coating composition may be used, but from the viewpoint of reducing the environmental load, an aqueous coating composition is preferred.

[0130] For example, when the object to be coated is an automobile body, the coating can be formed using a known intermediate coating composition that is commonly used in painting automobile bodies.

[0131] (II) Step of forming an uncured base coating film A base coating composition is applied onto the cured intermediate coating film to form an uncured base coating film. Two or more layers of uncured base coating films can be formed by applying the same or different base coating compositions two or more times. An interval of several minutes may be provided between the application of the nth base coating composition and the application of the n+1th base coating composition.

[0132] The coating method may be, for example, the same method as that used for coating the intermediate coating composition.

[0133] After applying the base coating composition, pre-drying (also called preheating) may be performed before applying the clear coating composition. This prevents the diluting components contained in the base coating composition from bumping during the curing process, making it easier to prevent popping. Furthermore, pre-drying prevents the uncured base coating film and the clear coating composition from mixing, making it difficult for a mixed layer to form. Therefore, the smoothness of the resulting multi-layer coating film can be further improved.

[0134] Examples of pre-drying include a method of leaving the film at a temperature of 20°C or higher and 25°C or lower for 5 to 15 minutes, and a method of heating the film at a temperature of 50°C or higher and 80°C or lower for 30 seconds to 10 minutes.

[0135] <Base coating composition> The base coating composition may be an aqueous coating composition or an organic solvent-based coating composition. From the viewpoint of reducing the environmental load, the base coating composition is preferably an aqueous coating composition. The aqueous base coating composition contains, for example, an acrylic resin emulsion and a curing agent. The base coating composition may further contain a pigment, a resin other than the acrylic resin emulsion, and various additives.

[0136] Examples of the resin include polyester resin, polyurethane resin, alkyd resin, fluororesin, epoxy resin, and polyether resin. The acrylic resin emulsion and the resin other than the acrylic resin emulsion may be a resin having a crosslinkable functional group, and examples of the crosslinkable functional group include a carboxyl group, a hydroxyl group, and an epoxy group.

[0137] Examples of the curing agent include melamine resin, polyisocyanate compound, and blocked polyisocyanate compound.

[0138] The pigment includes a luster pigment, and examples of the luster pigment include aluminum powder, mica powder, and mica powder coated with titanium oxide.

[0139] (III) Step of forming an uncured clear coating film The clear coating composition of the present disclosure is applied onto an uncured base coating film to form an uncured clear coating film.

[0140] The coating method is not particularly limited. For example, the coating method may be the same as the coating method for the intermediate coating composition. Among them, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. After applying the clear coating composition, preliminary drying may be performed as described above.

[0141] (IV) Curing Step The uncured base coating film and clear coating film are cured at the same time. Each coating film can be cured by heating. The heating time refers to the time during which the interior of the heating device reaches the target temperature and the substrate is maintained at the target temperature, and does not take into account the time required to reach the target temperature. Examples of heating devices include drying ovens that use heat sources such as hot air, electricity, gas, and infrared rays. The curing (heating) conditions are appropriately set depending on the composition of the intermediate coating composition and the material of the substrate. The heating temperature is, for example, 60°C to 180°C, and may be 70°C to 160°C. The heating time may be appropriately set depending on the heating temperature. When the heating temperature is 100°C to 180°C, the heating time is, for example, 10 minutes to 60 minutes, and may be 10 minutes to 30 minutes.

[0142] When the uncured intermediate coating film is cured by the 3-coat 2-bake method, the intermediate coating composition is cured by heating after the step (I) of forming the uncured intermediate coating film. The curing (heating) conditions may be the same as the heating conditions in the curing step (IV).

[0143] The method of forming a multi-layer coating film on a substrate can be considered a method of producing a coated article.

[0144] The present invention will be explained in more detail below with reference to Production Examples, Examples, and Comparative Examples. However, the present invention is not limited thereto. In each example, "parts" and "%" are by mass unless otherwise specified. Furthermore, the film thickness of the coating film is based on the cured coating film.

[0145] [1] Preparation of coated object A degreased and zinc phosphate-treated steel plate (JIS G3141, size 110 mm × 300 mm × 0.8 mm) was electrodeposited with the cationic electrodeposition paint "Elecron GT-10" (product name: manufactured by Kansai Paint Co., Ltd., which uses an epoxy resin polyamine-based cationic resin and a blocked polyisocyanate compound as a curing agent) so that the film thickness based on the cured coating was 20 μm, and the coating was heated at 170°C for 20 minutes to crosslink and cure, forming an electrodeposition coating.

[0146] Onto the electrodeposition coated surface of the steel plate obtained, "WP-523H" (trade name, acrylic melamine resin-based water-based intermediate coating composition, manufactured by Kansai Paint Co., Ltd.) was electrostatically coated using a rotary atomizing electrostatic coater so as to give a cured film thickness of 20 μm, and the coating was left to stand for 5 minutes to form an uncured intermediate coating film, thereby preparing a coated object.

[0147] [2] Preparation of Paint <Production of Hydroxyl-Containing Acrylic Resin (A1)> Production Example 1 A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device was charged with 27 parts of "Swasol 1000" (trade name, manufactured by Cosmo Oil Co., Ltd., aromatic organic solvent) and 5 parts of propylene glycol monomethyl ether acetate. The reaction vessel was stirred at 150 ° C while blowing nitrogen gas into it, and a monomer mixture consisting of 20 parts of styrene, 32.5 parts of 2-hydroxypropyl acrylate, 46.8 parts of isobutyl methacrylate, 0.7 parts of acrylic acid, and 3.5 parts of di-tertiary amyl peroxide (polymerization initiator) was added dropwise at a uniform rate over 4 hours. The mixture was then aged at 150 ° C for 1 hour, cooled, and further diluted with 21 parts of isobutyl acetate to obtain a hydroxyl-containing acrylic resin (A1-1) solution with a solids concentration of 65%. The resulting hydroxyl-containing acrylic resin (A1-1) had an acid value of 5.5 mgKOH / g, a hydroxyl value of 140 mgKOH / g, a weight average molecular weight of 7,500, and a glass transition temperature of 38°C.

[0148] Production Example 2 A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device was charged with 27 parts of "Swasol 1000" (trade name, aromatic organic solvent, manufactured by Cosmo Oil Co., Ltd.) and 5 parts of propylene glycol monomethyl ether acetate. The reaction vessel was stirred at 150°C while nitrogen gas was blown into it, and a monomer mixture consisting of 20 parts of styrene, 32.5 parts of 2-hydroxypropyl acrylate, 46.8 parts of isobutyl methacrylate, 2 parts of acrylic acid, and 5.5 parts of di-tertiary amyl peroxide (polymerization initiator) was added dropwise at a uniform rate over 4 hours. The mixture was then aged at 150°C for 1 hour, cooled, and further diluted with 21 parts of isobutyl acetate to obtain a hydroxyl group-containing acrylic resin (A1-2) solution with a solids concentration of 65%. The resulting hydroxyl-containing acrylic resin (A1-2) had an acid value of 15.5 mgKOH / g, a hydroxyl value of 140 mgKOH / g, a weight average molecular weight of 4,500, and a glass transition temperature of 39°C.

[0149] Production Example 3: A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device was charged with 27 parts of "Swasol 1000" (trade name, aromatic organic solvent, manufactured by Cosmo Oil Co., Ltd.) and 5 parts of propylene glycol monomethyl ether acetate. The reaction vessel was stirred at 150°C while nitrogen gas was blown into it, and a monomer mixture consisting of 13.7 parts of 2-ethylhexyl methacrylate, 40 parts of 2-hydroxyethyl methacrylate, 38 parts of isobutyl acrylate, 8.3 parts of acrylic acid, and 6.5 parts of di-tertiary amyl peroxide (polymerization initiator) was added dropwise at a uniform rate over 4 hours. The mixture was then aged at 150°C for 1 hour, cooled, and further diluted with 21 parts of isobutyl acetate to obtain a hydroxyl group-containing acrylic resin (A1-3) solution with a solids concentration of 65% by mass. The resulting hydroxyl-containing acrylic resin (A1-3) had an acid value of 64.7 mgKOH / g, a hydroxyl value of 173 mgKOH / g, a weight average molecular weight of 3,500, and a glass transition temperature of 15°C.

[0150] <Preparation of Hydroxyl-Containing Polyester Resin (A2)> Preparation Example 4 A reactor equipped with a stirrer, reflux condenser, and thermometer was charged with 148 parts of dimethylolbutanoic acid, 735 parts of "Cardura E10P" (trade name, manufactured by Momentive Specialty Chemicals, neodecanoic acid monoglycidyl ester), and 308 parts of hexahydrophthalic anhydride, and the mixture was reacted at 190°C for 3 hours. Thereafter, 342 parts of ε-caprolactone was added, and the mixture was reacted at 190°C for 3 hours. The mixture was then diluted with butyl acetate to obtain a hydroxyl-containing polyester resin (A2-1) solution with a solids concentration of 80%. The resulting hydroxyl-containing polyester resin (A2-1) had an acid value of 4 mgKOH / g, a hydroxyl value of 120 mgKOH / g, and a number average molecular weight of 2,100.

[0151] Production Example 5: A reactor equipped with a stirrer, a reflux condenser, a water separator, and a thermometer was charged with 100.1 parts of hexahydrophthalic anhydride and 118 parts of 1,6-hexanediol, and the mixture was reacted at 230°C for 6 hours. The mixture was then diluted with butyl acetate to obtain a solution of hydroxyl-containing polyester resin (A2-2) with a solids concentration of 80%. The obtained hydroxyl-containing polyester resin (A2-2) had an acid value of 2 mgKOH / g, a hydroxyl value of 192 mgKOH / g, and a number average molecular weight of 578.

[0152] <Production of Clear Coating Composition> Example 1 84.6 parts (solid content 55 parts) of the hydroxyl group-containing acrylic resin (A1-1) solution obtained in Production Example 1, 6.3 parts (solid content 5 parts) of the hydroxyl group-containing polyester resin (A2-1) solution obtained in Production Example 3, 6 parts (solid content 0.18 parts) of "BYK-088" (trade name, manufactured by BYK-Chemie, silicone-based antifoaming agent, purity 3%), 12 parts of "IP Solvent 1620" (trade name, isoparaffinic hydrocarbon consisting of 97.9% by mass of isoparaffins having 10 or more carbon atoms and 2.1% by mass of nonane, Idemitsu Kosan Co., Ltd.), 8 parts of ethyl 3-ethoxypropionate, 5 parts of butyl acetate, "SETALUX 61767 A base compound consisting of 0.5 parts (0.3 parts solids) of "VX-60" (trade name, manufactured by Allnex Corporation, viscosity modifier, solids content 60%) and 0.4 parts (0.2 parts solids) of "BYK-300" (trade name, manufactured by BYK-Chemie Corporation, surface modifier, active ingredient 52%) was uniformly mixed with 35 parts of the curing agent "Desmodur N3300" (trade name, manufactured by Sumika Covestro Urethane Co., Ltd., isocyanurate of hexamethylene diisocyanate, solids content 100%) and 5 parts of "U-Ban 20SE60" (trade name, manufactured by Mitsui Chemicals, Inc., melamine resin, solids content 3%). Immediately before application, the mixture was uniformly mixed, and 5 parts of butyl acetate was added to adjust the solids content at application to 58%, to obtain Clear Coating Composition No. 1.

[0153] Examples 2 to 26 and Comparative Examples 1 to 6 Clear coating composition Nos. 2 to 32 were obtained in the same manner as clear coating composition No. 1, except that the blending compositions and solid contents at the time of application were as shown in Table 2. The blending compositions shown in Table 2 are based on the solid contents by mass of each component.

[0154] "BYK-065": Trade name, manufactured by BYK Corporation, fluorine-modified silicone-based defoaming agent, solid content concentration 0.7% "BYK-077": Trade name, manufactured by BYK Corporation, silicone-based defoaming agent, solid content concentration 52% "BYK-081": Trade name, manufactured by BYK Corporation, silicone-based defoaming agent, solid content concentration 90% "Disparlon OX-883": Trade name, manufactured by Kusumoto Chemicals Co., Ltd., acrylic-based defoaming agent, solid content concentration 30% "Florene AC-326": Trade name, manufactured by Kyoeisha Scientific Co., Ltd., vinyl ether-based defoaming agent, solid content concentration 100%

[0155] <Preparation of test panel> On the substrate prepared in [1] above, "WBC-713T No. 1F7" (product name, manufactured by Kansai Paint Co., Ltd., acrylic-melamine resin-based water-based base coat paint, silver paint color) was applied in one stage using a rotary atomization bell-type coater "ABB cartridge bell coater" so that the dry film thickness was 12 μm under the conditions of bell diameter 77 mm, bell rotation speed 35,000 rpm, shaping air flow rate 700 NL / min, applied voltage -60 kV, temperature 23°C and humidity 75%, and after leaving it for 3 minutes, it was preheated at 80°C for 5 minutes to form an uncured base coat film.

[0156] Next, clear coating composition No. 1 was applied onto the uncured base coat film using a mini-bell type rotary atomizing electrostatic coating machine at a discharge rate of 200 cc, a rotation speed of 40,000 rpm, and a shaping air pressure of 1 kg / cm so that the dry film thickness was 50 μm. ２ The test panels of Example 1 were prepared by applying the paint in one stage at a gun distance of 30 cm, a temperature of 23°C, and a humidity of 75%, forming a clear coat film, which was then left to stand for 7 minutes. The test panels were then heated at 140°C for 30 minutes to heat-cure the intermediate coat film, base coat film, and clear coat film.

[0157] In preparing the test panels for clear coating composition No. 1, the test panels for Examples 2 to 26 and Comparative Examples 1 to 6 were each prepared in the same manner as in preparing the test panels for clear coating composition No. 1, except that clear coating composition No. 1 was replaced with any of clear coating compositions Nos. 2 to 32.

[0158] Each test panel obtained above was evaluated by the following test methods. The evaluation results are shown in Table 1 together with the coating composition.

[0159] (Test method) Popping resistance: Popping resistance was evaluated according to the following evaluation criteria based on the number of bubbles caused by popping that occurred on each test panel. A, B, and C are acceptable. The evaluation results are shown in Table 1. A: No popping was observed on the coating surface. B: One or two poppings were observed on the coating surface. C: Three or four poppings were observed on the coating surface. D: Five or more but less than 10 poppings were observed on the coating surface. E: Ten or more poppings were observed on the coating surface.

[0160] Storage stability: Each coating composition was placed in a 1 L glass container and stored sealed in a thermostatic chamber at 40°C. After 30 days of storage, storage stability was evaluated according to the following evaluation criteria. A, B, and C are acceptable. The evaluation results are shown in Table 1. A: No varnish separation, no change from pre-storage state; B: Some varnish separation, but recovery to pre-storage state after less than 1 minute of stirring (500 rpm using a 3 cm diameter stirring blade); C: Varnish separation observed, but recovery to pre-storage state after 1 to 60 minutes of stirring (500 rpm using a 3 cm diameter stirring blade); D: Varnish separation observed, varnish separation remaining even after stirring for more than 60 minutes (500 rpm using a 3 cm diameter stirring blade).

[0161]

[0162]

Claims

1. A clear coating composition comprising a hydroxyl group-containing resin (A), a curing agent (B), an antifoaming agent (C), and at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2), wherein the antifoaming agent (C) comprises a silicone-based antifoaming agent (C1), and the solids concentration at the time of application is 55 mass% or more.

2. A clear coating composition comprising a first agent and a second agent, wherein the first agent contains a hydroxyl group-containing resin (A), the second agent contains a curing agent (B), at least one of the first agent and the second agent contains an antifoaming agent (C) including a silicone-based antifoaming agent (C1), and at least one of the first agent and the second agent contains at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2), and the solids concentration at the time of application is 55 mass% or more.

3. A clear coating composition according to claim 1 or 2, wherein the hydroxyl-containing resin (A) comprises a hydroxyl-containing acrylic resin (A1).

4. A clear coating composition according to any one of claims 1 to 3, wherein the curing agent (B) comprises a polyisocyanate compound (B1).

5. A clear coating composition according to any one of claims 1 to 4, wherein the content of the antifoaming agent (C) is within the range of 0.01 to 1 part by mass based on 100 parts by mass of the resin solids in the clear coating composition.

6. A clear coating composition according to any one of claims 1 to 5, wherein the at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) comprises an aliphatic hydrocarbon solvent (D11) having 10 or more carbon atoms.

7. A clear coating composition according to any one of claims 1 to 6, wherein the content of at least one organic solvent (D) selected from the group consisting of aliphatic hydrocarbon solvents (D1) and alicyclic hydrocarbon solvents (D2) is within the range of 3 to 20 parts by mass based on 100 parts by mass of the resin solids in the clear coating composition.

8. A clear coating composition according to any one of claims 1 to 7, wherein the mass ratio of the antifoaming agent (C) to the at least one organic solvent (D) selected from the group consisting of the aliphatic hydrocarbon solvents (D1) and the alicyclic hydrocarbon solvents (D2), (C) / (D), is within the range of 0.01 to 0.

5.

9. A method for forming a coating film, which comprises applying the clear coating composition according to any one of claims 1 to 8 to an object to be coated, to form a clear coating film.

10. A method for forming a multi-layer coating film, comprising: step (I-1): applying a first coating composition onto a substrate to form an uncured first coating film; step (I-2): applying a second coating composition onto the uncured first coating film formed in step (I-1) to form an uncured second coating film; step (I-3): applying the clear coating composition according to any one of claims 1 to 8 onto the uncured second coating film formed in step (I-2) to form an uncured clear coating film; and step (I-4): simultaneously heating and curing the uncured first coating film formed in step (I-1), the uncured second coating film formed in step (I-2), and the uncured clear coating film formed in step (I-3).

11. A method for forming a multi-layer coating film, comprising: step (I-1): applying a first coating composition onto an object to be coated, followed by curing to form a cured first coating film; step (I-2): applying a second coating composition onto the cured first coating film formed in step (I-1) to form an uncured second coating film; step (I-3): applying the clear coating composition according to any one of claims 1 to 8 onto the uncured second coating film formed in step (I-2) to form an uncured clear coating film; and step (I-4): simultaneously curing the uncured second coating film and clear coating film formed in step (I-3).

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