Water-based paint composition for coil coating, method for forming a coating film, method for forming a multi-layer coating film, and painted metal sheet

Abstract

To provide an aqueous coating composition for coil coating that can form a coating film with excellent storage stability, processability, and corrosion resistance. [Solution] A water-based paint composition for coil coating, It contains acrylic-modified polyester resin (A), a hardening agent (B), a rust-preventive pigment (C), and water. The mass ratio (a1) / (a2) of the polyester portion (a1) and the acrylic portion (a2) of the acrylic-modified polyester resin (A) is within the range of 60 / 40 to 95 / 5. Water-based paint composition for coil coating.

Description

Technical Field

[0001] The present invention relates to an aqueous coating composition for coil coating, a method for forming a coating film, a method for forming a multilayer coating film, and a coated metal plate.

Background Art

[0002] Pre-coated steel sheets are widely used as coated steel sheets for various members such as building members such as shutters, rain doors, doors, roofs, and siding, exterior materials for electric equipment such as outdoor units of air conditioners, and interior materials.

[0003] A pre-coated steel sheet is a steel sheet used for applications in which a coating is applied to the surface of the steel sheet to form a coating film and then processed into a desired product.

[0004] <​​​​​​​​​​​​​​ [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2022-189126 [Overview of the project] [Problems that the invention aims to solve]

[0008] The technology described in Patent Document 1 had room for improvement in terms of the storage stability of the resulting paint composition, the processability of the formed coating film, and its corrosion resistance.

[0009] The object of the present invention is to provide an aqueous coating composition for coil coating that can form a coating film with excellent storage stability, processability, and corrosion resistance. [Means for solving the problem]

[0010] The inventors of the present invention have conducted extensive research to solve the above problems and have found that the above problems can be solved by using an aqueous coating composition for coil coating which contains an acrylic-modified polyester resin (A), a curing agent (B), a rust-preventive pigment (C), and water, wherein the mass ratio (a1) / (a2) of the polyester portion (a1) to the acrylic portion (a2) of the acrylic-modified polyester resin (A) is in the range of 60 / 40 to 95 / 5.

[0011] In other words, the present invention <1> ~ <12> This concerns... <1> A water-based coating composition for coil coating, It contains acrylic-modified polyester resin (A), a hardening agent (B), a rust-preventive pigment (C), and water. The mass ratio (a1) / (a2) of the polyester portion (a1) and the acrylic portion (a2) of the acrylic-modified polyester resin (A) is within the range of 60 / 40 to 95 / 5. Water-based paint composition for coil coating. <2> The monomer component constituting the acrylic portion (a2) of the acrylic-modified polyester resin (A) contains a polyoxyalkylene group-containing polymerizable unsaturated monomer with a weight-average molecular weight of 400 or more. <1> The aqueous coating composition for coil coating described in [the relevant document]. <3> The acid value of the acrylic portion (a2) of the acrylic-modified polyester resin (A) is in the range of 70 to 500 mgKOH / g. <1> or <2> The aqueous coating composition for coil coating described in [the relevant document]. <4> The hydroxyl value of the acrylic-modified polyester resin (A) is in the range of 20 mg KOH / g to 200 mg KOH / g. <1> or <2> The aqueous coating composition for coil coating described in [the relevant document]. <5> The acid value of the polyester portion (a1) of the acrylic-modified polyester resin (A) is in the range of 0 to 20 mg KOH / g. <1> or <2> The aqueous coating composition for coil coating described in [the relevant document]. <6> The curing agent (B) includes an amino resin (B1), <1> or <2> The aqueous coating composition for coil coating described in [the relevant document]. <7> The rust-preventive pigment (C) comprises at least one rust-preventive pigment selected from metal ion exchange silica fine particles (C1) and metal silicate (C2). <1> or <2> The aqueous coating composition for coil coating described in [the relevant document]. <8> The content of the acrylic-modified polyester resin (A) is within the range of 30 to 95% by mass, based on the total resin solid content in the aqueous coating composition for coil coating. <1> or <2> The aqueous coating composition for coil coating described in [the relevant document]. <9> Used as a primer coat, <1> or <2> The aqueous coating composition for coil coating described in [the relevant document]. <10> On a metal substrate, <1> A method for forming a coating film, comprising a painting step of applying the aqueous coating composition for coil coating described in [the relevant document] to form an uncured coating film, and a heating step of heating the uncured coating film formed in the painting step to dry and / or cure it. <11> Process (I-1): On the metal substrate, <1> A step of applying the water-based paint composition for coil coating described above to form an uncured primer coating film, Step (I-2): A step of heating the uncured primer coating film to dry and / or cure it. Step (I-3): A step of coating a topcoat paint composition on the primer coating film to form an uncured topcoat coating film. Step (I-4): A step of heating the uncured topcoat coating film to dry and / or cure it. A method for forming a multilayer coating film including the above steps. A coated metal sheet coated by the coating film forming method according to <12><10> or <11>.

Effect of the Invention

[0012] According to the present invention, an aqueous coil coating composition capable of forming a coating film excellent in storage stability, workability, and corrosion resistance can be provided.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, the present invention will be described in detail, but these show an example of a desirable embodiment, and the present invention is not limited to these contents.

[0014] [Aqueous Coil Coating Composition] The aqueous coil coating composition of the present invention contains an acrylic-modified polyester resin (A), a curing agent (B), a rust preventive pigment (C), and water, and the mass ratio (a1) / (a2) of the polyester part (a1) to the acrylic part (a2) of the acrylic-modified polyester resin (A) is within the range of 60 / 40 to 95 / 5.

[0015] 〔Acrylic-Modified Polyester Resin (A)〕 The above acrylic-modified polyester resin is one in which a polyester part (a1) composed of a polyester resin is the main chain and is modified with an acrylic part (a2) composed of an acrylic (co)polymer. When the modification is a graft modification, the polyester part (a1) is the trunk polymer and the acrylic part (a2) is the branch polymer, and the acrylic part (a2) is bonded to the polyester part (a1) through a graft point.

[0016] The mass ratio (a1) / (a2) of the polyester part (a1) to the acrylic part (a2) of the acrylic-modified polyester resin (A) of the present invention is within the range of 60 / 40 to 95 / 5.

[0017] When the mass ratio (a1) / (a2) of the polyester part (a1) to the acrylic part (a2) of the above acrylic-modified polyester resin (A) is less than 60 / 40, that is, when the proportion of the polyester part is less than the规定 of the present invention, the processability of the formed coating film is insufficient, which is not preferable.

[0018] Further, when the mass ratio (a1) / (a2) of the polyester part (a1) to the acrylic part (a2) of the above acrylic-modified polyester resin (A) exceeds 95 / 5, that is, when the proportion of the polyester part is more than the规定 of the present invention, the storage stability of the obtained aqueous coil coating composition is insufficient, which is not preferable.

[0019] From the viewpoints of the storage stability of the obtained aqueous coil coating composition, the processability and adhesion of the formed coating film, etc., the mass ratio (a1) / (a2) of the polyester part (a1) to the acrylic part (a2) of the acrylic-modified polyester resin (A) of the present embodiment is preferably within the range of 70 / 30 to 90 / 10, and more preferably within the range of 75 / 25 to 90 / 10.

[0020] The production method of the above acrylic-modified polyester resin (A) is not particularly limited and can be synthesized by a conventional method. Specifically, for example, a method of polymerizing a mixture of an unsaturated group-containing polyester resin and an unsaturated monomer such as an acrylic monomer, a method by an esterification reaction of a polyester resin and an acrylic resin, etc. can be mentioned.

[0021] The method for obtaining the above-mentioned acrylic-modified polyester resin (A) by polymerizing a mixture of an unsaturated group-containing polyester resin and an unsaturated monomer is a method of acrylic-modifying the polyester resin by polymerizing the unsaturated monomer using the unsaturated groups in the polyester resin as graft sites.

[0022] The method for obtaining the above-mentioned unsaturated group-containing polyester resin is not particularly limited, but for example, a polyester resin can be synthesized by a conventional method, and graft sites can be imparted to the polyester resin by reacting the hydroxyl groups of the polyester resin with an acid anhydride group-containing unsaturated monomer. Methods for producing the polyester resin include esterification or transesterification reactions between an acid component and an alcohol component.

[0023] Alternatively, unsaturated group-containing polyester resins can also be synthesized by an esterification or transesterification reaction between an acid component containing a polybasic acid having an unsaturated group and an alcohol component. From the viewpoint of ease of synthesis, it is preferable to synthesize them by an esterification or transesterification reaction between an acid component containing a polybasic acid having an unsaturated group and an alcohol component. In particular, from the viewpoint of storage stability, it is preferable to use an acid component containing an acid anhydride group-containing unsaturated monomer as the polybasic acid having an unsaturated group.

[0024] Here, the above-mentioned acid anhydride group-containing unsaturated monomer is a compound that has one acid anhydride group and one unsaturated group in one molecule. Specifically, compounds with fewer than 6 carbon atoms include maleic anhydride, itaconic anhydride, fumaric anhydride, citraconic anhydride, mesaconic anhydride, 2-pentenioic anhydride, methylene succinic anhydride, and acetylenedicarboxylic anhydride. Compounds with 6 or more carbon atoms include tetrahydrophthalic anhydride, allylmalonic anhydride, isopropylidene succinic anhydride, 2,4-hexadiene dihydroxyanhydride, and unsaturated dicarboxylic acid anhydrides such as 4-cyclohexene-1,2-dicarboxylic acid anhydride. Among these, maleic anhydride is preferred from the viewpoint of reactivity, cost, and availability.

[0025] The amount of the acid anhydride group-containing unsaturated monomer used is preferably in the range of 0.6 to 2.0%, more preferably 0.8 to 1.2%, of the acid component, from the viewpoint of storage stability of the paint composition.

[0026] Examples of acidic components other than unsaturated monomers containing acid anhydride groups include aliphatic polybasic acids, alicyclic polybasic acids, and aromatic polybasic acids.

[0027] The above-mentioned aliphatic polybasic acids are generally aliphatic compounds having two or more carboxyl groups in one molecule, acid anhydrides of said aliphatic compounds, and esters of said aliphatic compounds. Examples of aliphatic polybasic acids include aliphatic polycarboxylic acids; anhydrides of said aliphatic polycarboxylic acids; and esters of said aliphatic polycarboxylic acids with a lower alkyl group having about 1 to 4 carbon atoms. Aliphatic polycarboxylic acids include compounds with fewer than 6 carbon atoms, such as succinic acid, succinic anhydride, and glutaric acid; and compounds with 6 or more carbon atoms, such as adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanediic acid, dodecanediic acid, brassic acid, octadecanediic acid, citric acid, and butanetetracarboxylic acid. The above-mentioned aliphatic polybasic acids can be used alone or in combination of two or more types.

[0028] As the above-mentioned aliphatic polybasic acid, it is particularly preferable to use adipic acid and / or adipic anhydride from the viewpoint of the smoothness of the resulting coating film.

[0029] The aforementioned alicyclic polybasic acid is generally a compound having one or more alicyclic structures and two or more carboxyl groups in one molecule, an acid anhydride of the compound, and an ester of the compound. The alicyclic structure is mainly a 4- to 6-membered ring structure. Examples of alicyclic polybasic acids include alicyclic polycarboxylic acids having 6 or more carbon atoms, such as 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4-cyclohexene-1,2-dicarboxylic acid, 3-methyl-1,2-cyclohexanedicarboxylic acid, 4-methyl-1,2-cyclohexanedicarboxylic acid, 1,2,4-cyclohexanetricarboxylic acid, and 1,3,5-cyclohexanetricarboxylic acid; anhydrides of the alicyclic polycarboxylic acids; and esters of lower alkyl groups having about 1 to 4 carbon atoms of the alicyclic polycarboxylic acids. The above alicyclic polybasic acids can be used alone or in combination of two or more types.

[0030] As the alicyclic polybasic acid mentioned above, it is preferable to use 1,2-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid anhydride, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, or 4-cyclohexene-1,2-dicarboxylic acid, from the viewpoint of the smoothness of the resulting coating film, and among these, it is more preferable to use 1,2-cyclohexanedicarboxylic acid and / or 1,2-cyclohexanedicarboxylic acid anhydride.

[0031] The aforementioned aromatic polybasic acid is generally an aromatic compound having two or more carboxyl groups in one molecule, an acid anhydride of the aromatic compound, and an ester of the aromatic compound, and examples include aromatic polycarboxylic acids having 6 or more carbon atoms such as phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, trimellitic acid, and pyromellitic acid; and esters of lower alkyl groups having about 1 to 4 carbon atoms of the aromatic polycarboxylic acid. The above aromatic polybasic acid can be used alone or in combination of two or more types.

[0032] As the aromatic polybasic acid mentioned above, phthalic acid, isophthalic acid, and trimellitic acid are preferred.

[0033] Furthermore, acid components other than the above-mentioned aliphatic polybasic acids, alicyclic polybasic acids, and aromatic polybasic acids can also be used. Such acid components are not particularly limited and include, for example, fatty acids with 6 or more carbon atoms such as coconut oil fatty acid, cottonseed oil fatty acid, hemp seed oil fatty acid, rice bran oil fatty acid, fish oil fatty acid, tall oil fatty acid, soybean oil fatty acid, linseed oil fatty acid, tung oil fatty acid, rapeseed oil fatty acid, castor oil fatty acid, dehydrated castor oil fatty acid, and safflower oil fatty acid; monocarboxylic acids with 6 or more carbon atoms such as lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, benzoic acid, p-tert-butylbenzoic acid, cyclohexanoic acid, and 10-phenyloctadecanoic acid; and hydroxycarboxylic acids with 6 or more carbon atoms such as 3-hydroxy-4-ethoxybenzoic acid. These acid components can be used individually or in combination of two or more.

[0034] Another method for obtaining the aforementioned unsaturated group-containing polyester resin is to use unsaturated fatty acids such as oleic acid and myristic acid (both compounds with 6 or more carbon atoms) as part of the acid component. In this method, the unsaturated groups of the unsaturated fatty acids are used as graft sites.

[0035] As the alcohol component, a polyhydric alcohol having two or more hydroxyl groups in one molecule can be used. Examples of such polyhydric alcohols include ethylene glycol, propylene glycol, diethylene glycol, trimethylene glycol, 1,4-butanediol, 1,3-butanediol, 2,3-butanediol, 1,2-butanediol, 2-methyl-1,3-propanediol, 3-methyl-1,2-butanediol, 1,2-pentanediol, 1,5-pentanediol, 1,4-pentanediol, 2,4-pentanediol, 2,3-dimethyltrimethylene glycol, and neopentyl Polyhydric alcohols with fewer than 6 carbon atoms, such as glycols, dimethylolpropionic acid, glycerin, trimethylolethane, and pentaerythritol; tetraethylene glycol, dipropylene glycol, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl-4,3-pentanediol, 3-methyl-1,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, and 2,5-hexanediol. Examples include diols, 1,4-cyclohexanedimethanol, tricyclodecanedimethanol, neopentyl glycol hydroxypivalate, hydrogenated bisphenol A, and hydrogenated bisphenol F; polylactone diols obtained by adding lactone compounds such as ε-caprolactone to these diols; ester diol compounds such as bis(hydroxyethyl) terephthalate; polyether diol compounds such as alkylene oxide adducts of bisphenol A, polyethylene glycol, polypropylene glycol, and polybutylene glycol; trihydric or higher alcohols such as trimethylolpropane, diglycerin, triglycerin, 1,2,6-hexanetriol, dipentaerythritol, tris(2-hydroxyethyl)isocyanuric acid, sorbitol, and mannitol; polylactone polyol compounds obtained by adding lactone compounds such as ε-caprolactone to these trihydric or higher alcohols; and polyhydric alcohols with 6 or more carbon atoms, such as fatty acid esters of glycerin.

[0036] Furthermore, alcohol components other than the polyhydric alcohols mentioned above can also be used. Such alcohol components are not particularly limited and include, for example, monohydric alcohols with fewer than 6 carbon atoms such as methanol, ethanol, propyl alcohol, and butyl alcohol; monoalcohols such as stearyl alcohol and 2-phenoxyethanol; and monohydric alcohols with 6 or more carbon atoms such as propylene oxide, butylene oxide, and alcohol compounds obtained by reacting monoepoxy compounds such as "Cardura E10" (trade name, manufactured by HEXION Specialty Chemicals, a glycidyl ester of synthetic highly branched saturated fatty acid) with an acid.

[0037] From the viewpoint of processability and weather resistance of the formed coating film, the acrylic-modified polyester resin (A) preferably has a composition of 6 or more carbon atoms, where the proportion of compounds with 6 or more carbon atoms among the components constituting the polyester portion (a1) is 10% to 80% by mass, and more preferably 15% to 75% by mass, of the total amount of components constituting the polyester portion (a1).

[0038] In this specification, "components constituting the polyester portion (a1)" refers to the acidic and alcoholic components used to manufacture the polyester portion.

[0039] Among the compounds having 6 or more carbon atoms, compounds having 4 or more carbon atoms, and especially 6 or more alkylene groups, can be used particularly suitably.

[0040] Examples of such compounds include acidic components such as adipic acid, azelaic acid, sebacic acid, nonanediic acid, and dodecanediic acid, and alcoholic components such as 1,6-hexanediol, 1,7-heptanediol, 1,9-nonanediol, and 2-butyl-2-ethyl-1,3-propanediol.

[0041] The method for producing the unsaturated group-containing polyester resin is not particularly limited and can be carried out according to conventional methods. For example, the unsaturated group-containing polyester resin can be produced by heating the acid component and the alcohol component in a nitrogen stream at 150 to 250°C for 5 to 10 hours to carry out an esterification reaction or transesterification reaction between the acid component and the alcohol component.

[0042] When esterifying or transesterifying the above-mentioned acid component and alcohol component, they may be added to the reaction vessel all at once, or one or both may be added in several portions or continuously. Alternatively, an unsaturated group-containing polyester resin may be synthesized first, and then the resulting unsaturated group-containing polyester resin may be half-esterified to obtain an unsaturated group, carboxyl group, and hydroxyl group-containing polyester resin. Alternatively, an unsaturated group and carboxyl group-containing polyester resin may be synthesized first, and then the above-mentioned alcohol component may be added to obtain an unsaturated group, carboxyl group, and hydroxyl group-containing polyester resin.

[0043] In the aforementioned esterification or transesterification reaction, known catalysts such as dibutyltin oxide, antimony trioxide, zinc acetate, manganese acetate, cobalt acetate, calcium acetate, lead acetate, tetrabutyl titanate, and tetraisopropyl titanate can be used to accelerate the reaction.

[0044] Furthermore, the unsaturated group-containing polyester resin can be modified with a polyisocyanate compound or the like.

[0045] Examples of polyisocyanate compounds include aliphatic diisocyanate compounds such as lysine diisocyanate, hexamethylene diisocyanate, and trimethylhexane diisocyanate; and alicyclic diisocyanate compounds such as hydrogenated xylylene diisocyanate, isophorone diisocyanate, methylcyclohexane-2,4-diisocyanate, methylcyclohexane-2,6-diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), and 1,3-(isocyanatomethyl)cyclohexane. Examples include socyanate compounds; aromatic diisocyanate compounds such as tolylene diisocyanate, xylylene diisocyanate, and diphenylmethane diisocyanate; organic polyisocyanates themselves, such as trivalent or higher polyisocyanates like lysine triisocyanate; adducts of these organic polyisocyanates with polyhydric alcohols, low molecular weight polyester resins, water, etc.; and cyclized polymers (e.g., isocyanurates) and biuret-type adducts of these organic polyisocyanates. These polyisocyanate compounds can be used individually or in combination of two or more.

[0046] The acid value of the polyester portion (a1) is preferably in the range of 0 to 20 mg KOH / g, more preferably in the range of 0.1 to 15 mg KOH / g, and even more preferably in the range of 0.5 to 10 mg KOH / g, from the viewpoint of the storage stability of the resulting aqueous coating composition for coil coating.

[0047] The hydroxyl value of the polyester portion (a1) is preferably in the range of 20 to 250 mg KOH / g, more preferably in the range of 30 to 220 mg KOH / g, and even more preferably in the range of 30 to 200 mg KOH / g, from the viewpoint of the processability and curability of the formed coating film.

[0048] In this specification, the acid value (mgKOH / g) is expressed as the amount of potassium hydroxide equivalent in milligrams when the amount of acid groups contained in 1 g of sample (solid content in the case of resin) is converted to potassium hydroxide. The molecular weight of potassium hydroxide is assumed to be 56.1.

[0049] The acid value of the polyester part can be determined by a titration method.

[0050] The measurement of the acid value is carried out in accordance with JIS K-5601-2-1 (1999). The sample is dissolved in a mixed solvent of toluene / ethanol = 2 / 1 volume ratio, titrated with a potassium hydroxide solution using phenolphthalein as an indicator, and calculated by the following formula.

[0051] Acid value (mgKOH / g) = 56.1×V×C / m V: Titration volume (ml), C: Concentration of the titrant (mol / l), m: Solid content weight of the sample (g) In this specification, the hydroxyl value (mgKOH / g) is represented by the number of mg of potassium hydroxide when the amount of hydroxyl groups contained in 1 g of the sample (in the case of resin, the solid content) is converted to potassium hydroxide. The molecular weight of potassium hydroxide is assumed to be 56.1.

[0052] <00,00207>The hydroxyl value of the polyester part can be calculated from the formulation when synthesizing the polyester part. Specifically, it can be calculated by the following formula.

[0053] Hydroxyl value (mgKOH / g) = {(Number of hydroxy groups in the alcohol component - Number of carboxyl groups in the acid component) × 56.1 × 1000} / Resin solid content weight From the viewpoints of the storage stability of the resulting aqueous coil coating composition and the boiling water resistance of the formed coating film, etc., the acrylic-modified polyester resin (A) preferably contains a polyoxyalkylene group-containing unsaturated monomer having a weight average molecular weight of 400 or more as a monomer component constituting the acrylic part (a2), and more preferably contains a polyoxyalkylene group-containing unsaturated monomer within the range of 400 to 1000 in weight average molecular weight.

[0054] In this specification, the "monomer component constituting the acrylic part (a2)" refers to the monomer component used for producing the acrylic part (a2). The monomer component is an unsaturated monomer.

[0055] As the monomer components constituting the acrylic portion (a2), polyoxyalkylene group-containing unsaturated monomers and other unsaturated monomers can be used. These unsaturated monomers can be used individually or in combination of two or more.

[0056] The above polyoxyalkylene group-containing monomer is an unsaturated monomer having both an unsaturated group and a polyoxyalkylene group, and is given by the following general formula (I) CH2=CR 1 COO(AO) P R 2 (I) [In the formula, AO represents an oxyalkylene unit having 2 to 4 carbon atoms. However, the p oxyalkylene units may be identical or different. If the oxyalkylene units are different, block addition, random addition, or alternating addition may be performed. R 1 is a hydrogen atom or a methyl group, p is an integer from 1 to 50, R 2 This represents a phenyl group which may be substituted with a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, or an alkyl group having 1 to 9 carbon atoms. Unsaturated monomers represented by can be suitably used. Preferred oxyalkylene units are monomers having oxyethylene units, oxypropylene units, and oxytetramethylene units as constituent units.

[0057] In the above general formula (I), p is preferably in the range of 2 to 50. In the above general formula (I), R 2 The group is preferably an alkyl group having 1 to 20 carbon atoms, and more preferably a methyl group.

[0058] Specific examples of the above polyoxyalkylene group-containing monomers include, for example, methoxypolyethylene glycol mono(meth)acrylate, polyethylene glycol mono(meth)acrylate, methoxypolypropylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, ethylene glycol / propylene glycol (meth)acrylate, poly(ethylene glycol / propylene glycol) mono(meth)acrylate, octoxypolyethylene glycol / polypropylene glycol mono(meth)acrylate, octoxypoly(ethylene glycol / propylene glycol) mono(meth)acrylate, and stearoxypolyethylene glycol / polypropylene Examples include pyrene glycol mono(meth)acrylate, stearoxy poly(ethylene glycol / propylene glycol) mono(meth)acrylate, nonylphenoxy polyethylene glycol / polypropylene glycol mono(meth)acrylate, nonylphenoxy poly(ethylene glycol / propylene glycol) mono(meth)acrylate, propylene glycol / tetramethylene glycol mono(meth)acrylate, poly(propylene glycol / tetramethylene glycol) mono(meth)acrylate, propylene glycol / polybutylene glycol mono(meth)acrylate, and poly(propylene glycol / butylene glycol) mono(meth)acrylate. These can be used individually or in combination of two or more types.

[0059] If the acrylic portion (a2) of the acrylic-modified polyester resin (A) contains the polyoxyalkylene group-containing monomer, the content of the polyoxyalkylene group-containing monomer is preferably in the range of 1 to 30% by mass, more preferably in the range of 5 to 25% by mass, and even more preferably in the range of 10 to 20% by mass, relative to the total amount of monomer components constituting the acrylic portion (a2), from the viewpoint of the storage stability of the resulting aqueous coating composition for coil coating and the boiling water resistance of the formed coating film.

[0060] Other unsaturated monomers that can be used include, for example, the monomers (i) to (xix) listed below. (i) Alkyl or cycloalkyl (meth)acrylates: For example, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, tridecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclododecyl (meth)acrylate, tricyclodecanyl (meth)acrylate, etc. (ii) Unsaturated monomers having an isobornyl group: isobornyl (meth)acrylate, etc. (iii) Unsaturated monomers having an adamantyl group: adamantyl (meth)acrylate, etc. (iv) Unsaturated monomers having a tricyclodecenyl group: tricyclodecenyl (meth)acrylate, etc. (v) Aromatic ring-containing unsaturated monomers: benzyl (meth)acrylate, styrene, α-methylstyrene, vinyltoluene, etc. (vi) Unsaturated monomers having an alkoxysilyl group: vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, γ-(meth)acryloyloxypropyltrimethoxysilane, γ-(meth)acryloyloxypropyltriethoxysilane, etc. (vii) Unsaturated monomers having a fluorinated alkyl group: Perfluoroalkyl (meth)acrylates such as perfluorobutylethyl (meth)acrylate and perfluorooctylethyl (meth)acrylate; fluoroolefins, etc. (viii) Unsaturated monomers having photopolymerizable functional groups such as maleimide groups. (ix) Vinyl compounds: N-vinylpyrrolidone, ethylene, butadiene, chloroprene, vinyl propionate, vinyl acetate, etc. (x) Carboxylate-containing unsaturated monomers: (meth)acrylic acid, maleic acid, crotonic acid, β-carboxyethyl acrylate, etc. (xi) Hydroxyl group-containing unsaturated monomers: 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 forms of monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms; N-hydroxymethyl (meth)acrylamide; allyl alcohol, etc. (xii) Nitrogen-containing unsaturated monomers: (meth)acrylonitrile, (meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, methylenebis(meth)acrylamide, ethylenebis(meth)acrylamide, adducts of glycidyl (meth)acrylate with amine compounds, etc. (xiii) Unsaturated monomers having two or more unsaturated groups in one molecule: allyl (meth)acrylate, 1,6-hexanediol di(meth)acrylate, etc. (xiv) Epoxy group-containing unsaturated monomers: Glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, allyl glycidyl ether, etc. (xv) Unsaturated monomers having a sulfonic acid group: 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl (meth)acrylate, allylsulfonic acid, 4-styrenesulfonic acid, etc.; sodium salts and ammonium salts of these sulfonic acids, etc. (xvi) Unsaturated monomers having a phosphate group: acid phosphooxyethyl (meth)acrylate, acid phosphooxypropyl (meth)acrylate, acid phosphooxypoly(oxyethylene) glycol (meth)acrylate, acid phosphooxypoly(oxypropylene) glycol (meth)acrylate, etc. (xvii) Unsaturated monomers having UV-absorbing functional groups: 2-hydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone, 2-hydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, 2,2'-dihydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone, 2,2'-dihydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, 2-(2'-hydroxy-5'-methacryloyloxyethylphenyl)-2H-benzotriazole, etc. (xviii) UV-stable unsaturated monomers: 4-(meth)acryloyloxy-1,2,2,6,6-pentamethylpiperidine, 4-(meth)acryloyloxy-2,2,6,6-tetramethylpiperidine, 4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 1-(meth)acryloyl-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 1-(meth)acryloyl-4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, 4-crotonoylamino-2,2,6,6-tetramethylpiperidine, 1-crotonoyl-4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, etc. (xix) Unsaturated monomers having a carbonyl group: acrolein, diacetone acrylamide, diacetone methacrylamide, acetoacetoxyethyl methacrylate, formyl styrene, vinyl alkyl ketones having 4 to 7 carbon atoms (e.g., vinyl methyl ketone, vinyl ethyl ketone, vinyl butyl ketone), etc.

[0061] In this specification, an unsaturated group means an unsaturated group that can undergo radical polymerization. Examples of such unsaturated groups include vinyl groups and (meth)acryloyl groups.

[0062] 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.

[0063] Of the above-mentioned other unsaturated monomers, it is preferable to use (x) carboxyl group-containing unsaturated monomers and (v) aromatic ring-containing unsaturated monomers in at least a portion thereof, from the viewpoint of graft reactivity with polyester resin and storage stability of the resulting acrylic-modified polyester resin (A).

[0064] If the acrylic portion (a2) of the acrylic-modified polyester resin (A) contains the (x) carboxyl group-containing unsaturated monomer, the content of the (x) carboxyl group-containing unsaturated monomer is preferably in the range of 1 to 70% by mass, more preferably in the range of 9 to 50% by mass, and even more preferably in the range of 10 to 30% by mass, relative to the total amount of monomer components constituting the acrylic portion (a2), from the viewpoint of the storage stability of the acrylic-modified polyester resin (A).

[0065] If the acrylic portion (a2) of the acrylic-modified polyester resin (A) contains the (v) aromatic ring-containing unsaturated monomer, the content of the (v) aromatic ring-containing unsaturated monomer is preferably in the range of 1 to 30% by mass, more preferably in the range of 5 to 30% by mass, and even more preferably in the range of 10 to 30% by mass, relative to the total amount of monomer components constituting the acrylic portion (a2), from the viewpoint of the storage stability of the resulting aqueous coating composition for coil coating.

[0066] The acrylic-modified polyester resin (A) can be synthesized, for example, by copolymerizing the unsaturated group-containing polyester resin and the unsaturated monomer using a known method.

[0067] Specifically, for example, the synthesis can be carried out by adding the above-mentioned unsaturated group-containing polyester resin, unsaturated monomer, polymerization initiator, and, if necessary, a chain transfer agent to a reaction vessel and heating at 90-160°C for 1-5 hours. Alternatively, from the standpoint of controlling the reaction temperature, the unsaturated group-containing polyester resin can be charged into the reaction vessel first, and the other raw materials can be added over time.

[0068] As the polymerization initiators mentioned above, compounds such as organic peroxides and azo compounds can be used. For example, organic peroxide polymerization initiators such as benzoyl peroxide, t-butyl peroxy-2-ethylhexanoate, di-t-butyl peroxide, t-butyl peroxybenzoate, and t-amyl peroxy-2-ethylhexanoate, and azo polymerization initiators such as azobisisobutyronitrile and azobisdimethylvaleronitrile can be used. Furthermore, as the chain transfer agents mentioned above, α-methylstyrene dimers and mercaptans can be used.

[0069] On the other hand, the method for obtaining acrylic-modified polyester resin (A) by an esterification reaction between polyester resin and acrylic resin is a method of grafting a portion of the polyester resin by transesterifying it with acrylic resin.

[0070] The acid value of the acrylic portion (a2) is preferably in the range of 70 to 500 mg KOH / g, more preferably in the range of 80 to 300 mg KOH / g, and even more preferably in the range of 100 to 200 mg KOH / g, from the viewpoint of the storage stability of the resulting aqueous coating composition for coil coating and the corrosion resistance of the formed coating film.

[0071] The acrylic portion (a2) described above either does not have hydroxyl groups, or if it does, its hydroxyl value is preferably in the range of 0 to 70 mg KOH / g, more preferably in the range of 0 to 50 mg KOH / g, and even more preferably in the range of 0 to 10 mg KOH / g, from the viewpoint of the storage stability of the resulting aqueous coating composition for coil coating.

[0072] The acid value of the acrylic portion can be calculated from the formulation used when synthesizing the acrylic portion. Specifically, it can be calculated using the following formula.

[0073] Acid value (mgKOH / g) = (Total number of acid groups derived from acid-containing monomers × 56.1 × 1000) / Weight of resin solids The hydroxyl value of the acrylic portion can be calculated from the formulation used when synthesizing the acrylic portion. Specifically, it can be calculated using the following formula.

[0074] Hydroxyl value (mgKOH / g) = (Total number of hydroxyl groups derived from hydroxyl group-containing monomer × 56.1 × 1000) / Weight of resin solids The acid value of the acrylic-modified polyester resin (A) is preferably in the range of 10 to 100 mg KOH / g, more preferably in the range of 15 to 80 mg KOH / g, and even more preferably in the range of 15 to 60 mg KOH / g, from the viewpoint of the storage stability of the resulting aqueous coating composition for coil coating and the processability of the formed coating film.

[0075] The hydroxyl value of the above-mentioned acrylic-modified polyester resin (A) is preferably in the range of 20 to 200 mg KOH / g, more preferably in the range of 50 to 180 mg KOH / g, and even more preferably in the range of 100 to 170 mg KOH / g, from the viewpoint of the processability and curability of the formed coating film.

[0076] The acid value of the acrylic-modified polyester can be determined by titration, similar to the acid value of the polyester portion.

[0077] The hydroxyl group value of the acrylic-modified polyester can be calculated from the mass ratio (a1) / (a2) of the polyester portion (a1) and the acrylic portion (a2), the acid value of the polyester portion, and the acid value of the acrylic portion. Specifically, it can be calculated using the following formula.

[0078] Hydroxyl value (mgKOH / g) = (Hydroxyl value of polyester portion × {((a1) / ((a1)+(a2))) + (Hydroxyl value of acrylic portion × {((a2) / ((a1)+(a2)))) The number average molecular weight of the acrylic-modified polyester resin (A) is preferably in the range of 1,000 to 15,000, more preferably in the range of 2,000 to 13,000, and even more preferably in the range of 3,000 to 10,000, from the viewpoint of the processability and curability of the formed coating film.

[0079] In this specification, the average molecular weight is the value obtained by converting the retention time (retention volume) measured using gel permeation chromatography (GPC) to the molecular weight of polystyrene using the retention time (retention volume) of standard polystyrene with a known molecular weight measured under the same conditions. Specifically, the measurement can be performed using the "HLC-8120GPC" (product name, manufactured by Tosoh Corporation) as the gel permeation chromatography apparatus, a total of four columns: "TSKgel G4000HXL", "TSKgel G3000HXL", "TSKgel G2500HXL", and "TSKgel G2000HXL" (product names, all manufactured by Tosoh Corporation), a differential refractometer as the detector, and under the conditions of mobile phase: tetrahydrofuran, measurement temperature: 40°C, and flow rate: 1 mL / min.

[0080] The acrylic-modified polyester resin (A) can be neutralized and dispersed in water to obtain an aqueous dispersion. Amine compounds and ammonia can be used as neutralizing agents. Examples of amine compounds include triethylamine, triethanolamine, dimethylethanolamine, diethylethanolamine, and morpholine. Among these, triethylamine and dimethylethanolamine are particularly suitable. The degree of neutralization is not particularly limited, but it is generally preferred to be within the range of 0.3 to 1.0 equivalents relative to the acid groups in the resin.

[0081] The aqueous medium of the aqueous dispersion of the acrylic-modified polyester resin (A) described above may be water alone, or it may be a mixture of water and an organic solvent.

[0082] Preferred organic solvents include alcohol-based solvents and ether-based solvents. Specifically, examples include alcohol-based solvents such as n-butanol; ether-based solvents such as ethylene glycol monobutyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monomethyl ether, propylene glycol monomethyl ether, and diethylene glycol monoethyl ether. Ester-based solvents such as ethyl acetate and butyl acetate, ketone-based solvents such as methyl ethyl ketone and cyclohexanone, and aromatic hydrocarbon-based solvents such as toluene and xylene can also be used.

[0083] To neutralize and disperse the acrylic-modified polyester resin (A) in an aqueous medium, conventional methods can be used, such as gradually adding the acrylic-modified polyester resin to an aqueous medium containing a neutralizing agent while stirring; neutralizing the acrylic-modified polyester resin with a neutralizing agent and then adding the aqueous medium while stirring; or adding the neutralized product to the aqueous medium.

[0084] The content of the acrylic-modified polyester resin (A) is preferably in the range of 30 to 95% by mass, more preferably in the range of 40 to 92% by mass, and even more preferably in the range of 60 to 90% by mass, based on the total resin solids content in the aqueous coating composition for coil coating, from the viewpoint of storage stability of the obtained aqueous coating composition for coil coating, corrosion resistance and curability of the formed coating film, etc.

[0085] In this specification, "solid content" refers to non-volatile components such as resins, curing agents, and pigments that remain after drying at 110°C for 1 hour. The solid content can be determined, for example, by weighing a sample into a heat-resistant container such as an aluminum foil cup, spreading the sample on the bottom surface of the container, drying it at 110°C for 1 hour, and then weighing the mass of the components remaining after drying.

[0086] Furthermore, in this specification, "solid content concentration" refers to the mass percentage of the solid content in the composition. Therefore, the solid content concentration of a composition can be calculated, for example, by weighing 1.0 g of the composition into a heat-resistant container such as an aluminum foil cup, spreading the composition on the bottom surface of the container, drying it at 110°C for 1 hour, weighing the mass of the components remaining in the composition after drying, and determining the ratio of the mass of the components remaining after drying to the total mass of the composition before drying.

[0087] [Hardening agent (B)] The curing agent (B) is a component that contributes to the formation of a cured coating film by reacting with the acrylic-modified polyester resin (A) and the like.

[0088] Examples of the curing agent (B) mentioned above include amino resin (B1); polyisocyanate compounds; blocked polyisocyanate compounds; phenolic resins, etc.

[0089] The above-mentioned hardening agent (B) can be used alone or in combination of two or more types.

[0090] The curing agent (B) above preferably contains an amino resin (B1) from the viewpoint of storage stability of the resulting aqueous coating composition for coil coating and processability of the formed coating film.

[0091] If the aqueous coating composition for coil coating of this embodiment contains the above-mentioned amino resin (B1), the content of the amino resin (B1) is preferably in the range of 10 to 100% by mass, more preferably in the range of 20 to 100% by mass, and even more preferably in the range of 30 to 100% by mass, based on the solid content of the above-mentioned curing agent (B), from the viewpoint of the processability of the formed coating film.

[0092] Examples of the above amino resin (B1) include melamine resin and urea resin, and among these, it is preferable that it contains melamine resin.

[0093] The melamine resin described above is generally a thermosetting resin synthesized from melamine and aldehyde, and has a triazine core and three reactive functional groups (-NX) per triazine core. 1 X 2 The melamine resin has the following characteristics: 1. A fully alkylated type containing -N(CH2OR)2 [where R is preferably an alkyl group having 1 to 8 carbon atoms, the same applies hereinafter] as a reactive functional group; 2. A methylol group type containing -N(CH2OR)(CH2OH) as a reactive functional group; 3. An imino group type containing -N(CH2OR)(H) as a reactive functional group; and 4. A methylol / imino group type containing both -N(CH2OR)(CH2OH) and -N(CH2OR)(H), or containing -N(CH2OH)(H) as reactive functional groups.

[0094] Among the melamine resins mentioned above, it is preferable to use a fully alkylated melamine resin from the viewpoint of storage stability of the resulting aqueous coating composition for coil coating.

[0095] Examples of the fully alkylated melamine resins mentioned above include methylated melamine resins, butylated melamine resins, methyl / butyl mixed melamine resins, and isobutylated melamine resins. Among these, methylated melamine resins, butylated melamine resins, and methyl / butyl mixed melamine resins are preferred.

[0096] Commercially available melamine resins can be used. Examples of commercially available products include Cymel 303 LF, Cymel 254, Cymel 1170, Cymel 235, Cymel 238, Cymel 1123, Mycoat 715 (all manufactured by Daicel Ornex Co., Ltd.), Sumimaru M-40S (manufactured by Sumitomo Chemical Co., Ltd.), Super Beccamine J-820-60, Super Beccamine L-121-60 (both manufactured by DIC Corporation), and Yuban 20SE-60 (manufactured by Mitsui Chemicals, Inc.).

[0097] The above-mentioned melamine resins can be used individually or in combination of two or more types.

[0098] The polyisocyanate compound is a compound having two or more free isocyanate groups in one molecule, and includes, for example, aliphatic diisocyanate compounds such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate; alicyclic diisocyanate compounds such as hydrogenated xylylene diisocyanate, cyclohexylene diisocyanate, methylenebis(cyclohexyl isocyanate), and isophorone diisocyanate; and tolylene diisocyanate and phenylene diisocyanate. Examples include aromatic diisocyanate compounds such as 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate, tetramethyl xylylene diisocyanate, and naphthalene diisocyanate; and trivalent or higher organic polyisocyanate compounds such as 2-isocyanatoethyl-2,6-diisocyanatocaproate, 3-isocyanatomethyl-1,6-hexamethylene diisocyanate, and 4-isocyanatomethyl-1,8-octamethylene diisocyanate (commonly known as triaminononane triisocyanate). Furthermore, examples include cyclized polymers or burettes of these polyisocyanate compounds; or combinations thereof.

[0099] Among the polyisocyanate compounds mentioned above, aliphatic polyisocyanate compounds are preferred from the viewpoint of the processability of the coating film formed, and among these, the nurate form of aliphatic polyisocyanate compounds, the adduct form of aliphatic polyisocyanate compounds, and the burette form of aliphatic polyisocyanate compounds are preferred.

[0100] In particular, the nurate form of hexamethylene diisocyanate, the adduct form of hexamethylene diisocyanate compounds, and the burette form of hexamethylene diisocyanate are preferred.

[0101] The aforementioned blocked polyisocyanate compound is a compound obtained by blocking the isocyanate group of the above polyisocyanate compound with an active hydrogen-containing compound.

[0102] Examples of blocking agents as active hydrogen-containing compounds include pyrazoles such as 3,5-dimethylpyrazole, 3-methylpyrazole, 4-nitro-3,5-dimethylpyrazole, and 4-bromo-3,5-dimethylpyrazole; oximes such as methyl ethyl ketoxime, methyl amyl ketoxime, and cyclohexanone oxime; phenols such as phenol, para-t-butylphenol, and cresol; lactams such as ε-caprolactam and γ-butyrolactam; aliphatic alcohols such as n-butanol and 2-ethylhexanol; aromatic alkyl alcohols such as phenylcarbinol and methylphenylcarbinol; ether alcohols such as ethylene glycol monobutyl ether and diethylene glycol monoethyl ether; and active methylenes such as dimethyl malonate, diethyl malonate, ethyl acetoacetate, methyl acetoacetate, and acetylacetone.

[0103] Among these blocking agents, pyrazoles, oximes, phenols, and lactams, particularly oximes, can be suitably used from the viewpoint of curability and storage stability of the resulting aqueous coating composition for coil coating.

[0104] Examples of the phenolic resin include resol-type phenolic resins obtained by introducing methylol groups by heating a phenolic component and formaldehydes in the presence of a catalyst to cause a condensation reaction, and then alkyl ethering some or all of the methylol groups of a methylolated phenolic resin with an alcohol.

[0105] Examples of the above-mentioned phenol compounds include, as difunctional phenol compounds, o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, and 2,5-xylenol; as trifunctional phenol compounds, carbolic acid, m-cresol, m-ethylphenol, 3,5-xylenol, and m-methoxyphenol; and as tetrafunctional phenol compounds, bisphenol A and bisphenol F. Among these, from the viewpoint of improving scratch resistance, it is preferable to use phenol compounds with three or more functions, especially carbolic acid and / or m-cresol. These phenol compounds can be used individually or in combination of two or more.

[0106] Examples of formaldehydes used in the production of the phenolic resin include formaldehyde, paraformaldehyde, or trioxane, and these can be used individually or in combination of two or more.

[0107] As the alcohol used to alkyl etherify a portion of the methylol groups in the methylolated phenol resin, monohydric alcohols having 1 to 8 carbon atoms, preferably 1 to 4 carbon atoms, can be suitably used. Suitable monohydric alcohols include methanol, ethanol, n-propanol, n-butanol, isobutanol, and the like.

[0108] The phenolic resin is suitable in which, from the viewpoint of reactivity with acrylic-modified polyester resin (A), etc., an average of 0.5 or more, preferably 0.6 to 3.0, alkoxymethyl groups are present per benzene ring.

[0109] The content of the curing agent (B) is preferably in the range of 2 to 40% by mass, more preferably in the range of 5 to 30% by mass, and even more preferably in the range of 8 to 25% by mass, based on the total resin solids content in the aqueous coating composition for coil coating, from the viewpoint of storage stability of the obtained aqueous coating composition for coil coating and processability of the formed coating film.

[0110] [Rust-preventive pigment (C)] As the rust-preventive pigment (C), any rust-preventive pigment known in the paint field can be used, and it may be an inorganic compound or an organic compound. There are no restrictions on its form, such as a single compound, a complex compound, or a composition using multiple such compounds.

[0111] The above-mentioned rust-preventive pigment (C) preferably contains at least one rust-preventive pigment selected from metal ion exchange silica fine particles (C1) and metal silicate (C2), and more preferably contains metal ion exchange silica fine particles (C1), from the viewpoint of storage stability of the resulting aqueous coating composition for coil coating and corrosion resistance of the formed coating film.

[0112] The above-mentioned metal ion exchange silica nanoparticles (C1) are silica nanoparticles in which metal ions such as calcium ions and magnesium ions are introduced into a fine porous silica carrier by ion exchange.

[0113] If the aqueous coating composition for coil coating of this embodiment contains the metal ion exchange silica fine particles (C1), the content of the metal ion exchange silica fine particles (C1) is preferably 3 to 100% by mass, more preferably 5 to 100% by mass, and even more preferably 10 to 100% by mass, based on the solid content of the rust-preventive pigment (C), from the viewpoint of the storage stability of the obtained aqueous coating composition for coil coating and the corrosion resistance of the formed coating film.

[0114] The metal silicate (C2) is a salt composed of silicon dioxide and a metal oxide, and may be any of the following: orthosilicate, polysilicate, etc. Examples of the metal silicate (C2) include calcium silicate, magnesium silicate, zinc silicate, aluminum silicate, orthosilicate, aluminum hydrate, aluminum calcium silicate, aluminum sodium silicate, aluminum beryllium silicate, sodium silicate, orthosilicate, calcium metasilicate, calcium sodium silicate, zirconium silicate, magnesium orthosilicate, magnesium metasilicate, magnesium calcium silicate, manganese silicate, barium silicate, strontium silicate, etc. Among these, calcium silicate, orthosilicate, and calcium metasilicate are preferred as metal silicates (C2).

[0115] If the aqueous coating composition for coil coating of this embodiment contains the above-mentioned metal silicate (C2), the content of the metal silicate (C2) is preferably 3 to 100% by mass, more preferably 5 to 100% by mass, and even more preferably 10 to 100% by mass, based on the solid content of the above-mentioned rust-preventive pigment (C), from the viewpoint of the storage stability of the obtained aqueous coating composition for coil coating and the corrosion resistance of the formed coating film.

[0116] The rust-preventive pigment (C) may include rust-preventive pigments other than the metal ion exchange silica fine particles (C1) and metal silicate (C2). Specific examples include single compounds, complex compounds, compositions using multiple such compounds, etc., with no limitations on its form. Specifically, Metal phosphate compounds such as zinc phosphate, magnesium phosphate, magnesium-ammonium phosphate e-deposit, magnesium monohydrogen phosphate, magnesium dihydrogen phosphate, magnesium-calcium phosphate e-deposit, magnesium-cobalt phosphate e-deposit, magnesium-nickel phosphate e-deposit, calcium phosphate, calcium ammonium phosphate, calcium monohydrogen phosphate, calcium dihydrogen phosphate, calcium fluoride phosphate, aluminum phosphate, and aluminum hydrogen phosphate; Phosphite-based metal compounds such as magnesium phosphite, calcium phosphite, magnesium-calcium phosphite e-deposit, basic zinc phosphite, barium phosphite, manganese phosphite, and hypocalcium phosphite; Condensed phosphate metal compounds such as aluminum dihydrogen tripolyphosphate, magnesium tripolyphosphate, aluminum tripolyphosphate, and zinc dihydrogen tripolyphosphate; Vanadium-based metal compounds such as vanadium pentoxide, calcium vanadate, magnesium vanadate, and ammonium metavanadate, calcined products of manganese oxide and vanadium oxide, and calcined products of calcium phosphate and vanadium oxide; Molybdate-based metal compounds such as aluminum molybdate, calcium molybdate, and phosphomolybdate; Zinc, zinc oxide, and other zinc-based compounds; Composite metal oxides such as composite oxides of iron oxide and magnesium oxide, composite oxides of iron oxide and calcium oxide, and composite oxides of iron oxide and zinc oxide; Examples include the following. These may be used individually or in combination of two or more types, or they may be composites of two or more types. Furthermore, modified or treated products of these example compounds, such as silica, calcium silicate, and other silicate compounds, or magnesium oxide, are also included in rust-preventive pigments (C).

[0117] The above-mentioned rust-preventive pigment (C) can be used individually or in combination of two or more types.

[0118] The above-mentioned rust-preventive pigment (C) can be a commercially available product, either alone or in combination with other pigments. Examples of such commercially available products include "EXPERT NP-1000", "EXPERT NP-1020C", "EXPERT NP-1100", "EXPERT NP-1102", "EXPERT NP-3000" (all manufactured by Toho Pigment Industry Co., Ltd., product names), "LF Bowsei CP-Z", "LF Bowsei MZP-500", "LF Bowsei CRFC-1", "LF Bowsei M-PSN", "LF Bowsei MC-400WR", "LF Bowsei PM-300", "LF Bowsei PM-303W", "LF Bowsei PM-308" (all manufactured by Kikuchi Color Co., Ltd., product names), "K-WHITE140", "K-WHITE Ca650", "K-WHITE450H", "K-WHITE G-105", "K WHITE #105", "K-WHITE #82", "K-WHITE G102S" (all manufactured by Teika Co., Ltd., product names), and "SHIELDEX Examples include "C303", "SHIELDEX AC-3", "SHIELDEX C-5" (all manufactured by WRGrace&Co.), "Silomask 52", "Silomask 52M", "Silomask 22MR-H" (manufactured by Fuji Silicia), "ZINC PHOSPHATE PZ20", "Nobinox XCA02", and "Nobinox ACE-110" (manufactured by SNCZ, France).

[0119] The content of the above-mentioned rust-preventive pigment (C) is preferably in the range of 1 to 100 parts by mass, more preferably in the range of 1 to 80 parts by mass, and even more preferably in the range of 5 to 80 parts by mass, based on 100 parts by mass of the total resin solids content in the aqueous coating composition for coil coating, from the viewpoint of the storage stability of the obtained aqueous coating composition for coil coating and the corrosion resistance of the formed coating film.

[0120] [Other ingredients] The aqueous coating composition for coil coating of this embodiment may further contain, if necessary, resins other than the acrylic-modified polyester resin (A), pigments other than the rust-preventive pigment (C), a curing catalyst, a matting agent, aggregate, an organic solvent, an anti-settling agent, an anti-foaming agent, an ultraviolet absorber, a light stabilizer, a surface modifier, and the like.

[0121] Examples of resins other than the acrylic-modified polyester resin (A) mentioned above include acrylic resin, polyester resin, polyurethane resin, urethane-modified polyester resin, acrylic-modified urethane resin, epoxy resin, etc. Among these, polyester resin and acrylic resin can be preferably used.

[0122] If the aqueous coating composition for coil coating of this embodiment contains a resin other than the acrylic-modified polyester resin (A), the content of the resin other than the acrylic-modified polyester resin (A) is preferably in the range of 5 to 70% by mass, more preferably in the range of 8 to 55% by mass, and even more preferably in the range of 10 to 55% by mass, based on the total resin solids content in the aqueous coating composition for coil coating, from the viewpoint of the storage stability of the obtained aqueous coating composition for coil coating.

[0123] In this specification, "total resin solids" includes the acrylic-modified polyester resin (A) and curing agent (B) as essential components, and may include other resins as needed, as exemplified in the other components. Various additives are not included in the resin.

[0124] Examples of pigments other than the aforementioned rust-preventive pigment (C) include coloring pigments, extender pigments, and the like. These pigments can be used individually or in combination of two or more types.

[0125] Examples of the above-mentioned coloring pigments include titanium dioxide, zinc oxide, carbon black, molybdenum red, Prussian blue, cobalt blue, azo pigments, phthalocyanine pigments, quinacridone pigments, isoindoline pigments, surene pigments, perylene pigments, dioxazine pigments, and diketopyrrolopyrrole pigments. Among these, titanium dioxide and carbon black can be preferably used.

[0126] If the aqueous coating composition for coil coating of this embodiment contains the above-mentioned coloring pigment, the amount of the coloring pigment is preferably in the range of 1 to 120 parts by mass, more preferably in the range of 5 to 120 parts by mass, and even more preferably in the range of 15 to 120 parts by mass, based on 100 parts by mass of the total resin solids in the aqueous coating composition for coil coating.

[0127] Examples of extender pigments include barium sulfate, talc, clay, kaolin, barium carbonate, calcium carbonate, and alumina white. Barium sulfate and talc can be suitably used as extender pigments.

[0128] When the aqueous coating composition for coil coating of this embodiment contains the above-mentioned extender pigment, the content of the extender pigment is preferably in the range of 1 to 120 parts by mass, more preferably in the range of 5 to 120 parts by mass, and even more preferably in the range of 10 to 120 parts by mass, based on 100 parts by mass of the total resin solids in the aqueous coating composition for coil coating.

[0129] As the curing catalyst, for example, a sulfonic acid compound or an amine neutralized product of a sulfonic acid compound is preferably used. Representative examples of sulfonic acid compounds include p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and dinonylnaphthalenedisulfonic acid. The amine in the amine neutralized product of the sulfonic acid compound may be a primary amine, a secondary amine, or a tertiary amine. Of these, from the viewpoint of corrosion resistance of the unprocessed portion of the formed coating film, the amine neutralized product of p-toluenesulfonic acid and / or the amine neutralized product of dodecylbenzenesulfonic acid are preferred.

[0130] If the aqueous coating composition for coil coating of this embodiment contains the curing catalyst, the amount of the curing catalyst is preferably in the range of 0.01 to 5.0 parts by mass, more preferably in the range of 0.03 to 5.0 parts by mass, and even more preferably in the range of 0.05 to 4.0 parts by mass, based on 100 parts by mass of the total resin solids in the aqueous coating composition for coil coating.

[0131] As the matting agent, a matting agent that has been conventionally used in paints and is already known can be used. Examples of types of matting agents include inorganic particles and resin particles.

[0132] Examples of the inorganic particles mentioned above include silica particles, alumina particles, titania particles, zirconia particles, zircon particles, tin oxide particles, magnesia particles, or mixtures thereof.

[0133] In particular, from the viewpoint of gloss uniformity of the formed coating film, it is preferable that the inorganic particles include the silica particles. There are no particular limitations on the shape of the silica particles, and spherical, hollow, porous, rod-shaped, plate-shaped, fibrous, or irregularly shaped particles can be used.

[0134] Examples of commercially available silica particles that can be used as described above include the SYLYSIA series ("SYLYSIA350", "SYLYSIA430", "SYLYSIA440", "SYLYSIA435", "SYLYSIA436", "SYLYSIA450", etc.) from Fuji Silycia Co., Ltd., the Silohobic series ("Silohobic 100", "Silohobic 200", "Silohobic 702", "Silohobic 4004", etc.), the Silosphere series ("Silosphere 1504", "Silosphere 1510", etc.) from Evonik Degussa Japan Co., Ltd., and the ACEMATT series ("ACEMATT HK460", "ACEMATT HK400", "ACEMATT OK412", "ACEMATT TS100", "ACEMATT 3200", "ACEMATT 3300", "ACEMATT 3600 etc.), Mizusawa Chemical's Mizukasil series ("Mizukasil P-73", "Mizukasil P-526", etc.), Evonik's Carplex series ("Carplex CS-8", etc.), Nippon Aerosil's AEROSIL series ("AEROSIL 200", "AEROSIL R805", etc.), Showa Chemical Industry's Radiolite series ("Radiolite 100", "Radiolite 200", "Radiolite 500", "Radiolite 500R", "Radiolite 500RS", etc.), Nippon Shokubai's Seahostar series ("Seahostar KE-E150", "Seahostar KE-P100", "Seahostar KE-P150", "Seahostar KE-P250", "Seahostar KE-S100", "Seahostar KE-S150", "Seahostar Examples include the "KE-S250" model.

[0135] The inorganic particles mentioned above may be untreated inorganic particles, or inorganic particles that have been surface-treated with an organic compound or an inorganic compound. In particular, from the viewpoint of gloss uniformity of the formed coating film, it is preferable that the inorganic particles include organically treated inorganic particles, and more preferably that they include organically treated silica particles.

[0136] There are no particular restrictions on the treatment with the above-mentioned organic compounds. Examples include polyethylene treatment, polyethylene wax treatment, and organosilane compound treatment.

[0137] Another embodiment of the matting agent described above is resin particles. Examples of such resin particles include PMMA (polymethyl methacrylate) resin particles, MMA-EGDM (ethylene glycol dimethacrylate) copolymer resin particles, nylon resin particles, urea resin, and polytetrafluoroethylene resin particles.

[0138] Examples of commercially available resin particles that can be used include the "Techpolymer Series" (product name) from Sekisui Chemical Co., Ltd., the "Matsumoto Microsphere Series" (product name) from Matsumoto Oil & Fat Co., Ltd., the "Gantz Pearl Series" (product name) from Aica Industrial Co., Ltd., the "Art Pearl Series" (product name) from Nemoto Kogyo Co., Ltd., the "ORGASOL Series" (product name) from Arkema, and the "PERGOPAK Series" (product name) from JMHuber Corporation.

[0139] If the aqueous coating composition for coil coating of this embodiment contains the above-mentioned matting agent, the amount of the matting agent can be any amount necessary to obtain the desired gloss.

[0140] The water-based coating composition for coil coating according to this embodiment can be applied by diluting it with water, or water and an organic solvent, etc., before use.

[0141] Examples of the above-mentioned organic solvents include ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ester solvents such as ethyl acetate, butyl acetate, methyl benzoate, ethyl ethoxypropionate, ethyl propionate, and methyl propionate; alcohol solvents such as isopropanol, n-butanol, isobutanol, and 2-ethylhexanol; ether solvents such as tetrahydrofuran, dioxane, and dimethoxyethane; glycol ether solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, and 3-methoxybutyl acetate; aromatic hydrocarbon solvents, aliphatic hydrocarbon solvents, and the like.

[0142] The appropriate viscosity varies depending on the paint composition, but it is controlled by measuring Ford Cup No. 4 according to the method specified in JIS K 5600 2-2 (flow cup method). The viscosity measured by Ford Cup No. 4 is preferably in the range of 60 to 200 seconds, and more preferably in the range of 80 to 150 seconds, from the viewpoint of storage stability and handling during painting of the resulting aqueous paint composition for coil coating.

[0143] The solid content concentration of the aqueous coating composition for coil coating in this embodiment is preferably in the range of 30 to 70% by mass, and more preferably in the range of 35 to 65% by mass, from the viewpoint of the storage stability of the resulting aqueous coating composition for coil coating.

[0144] The coating film formed from the above-mentioned aqueous coating composition for coil coating has excellent processability and corrosion resistance, and can therefore be suitably used as a primer coat.

[0145] Furthermore, the above-mentioned water-based coating composition for coil coating can also be used for monocoating.

[0146] [Coating film formation method] The coating film forming method of this embodiment is a coating film forming method that includes a coating step of applying the aqueous coating composition for coil coating to a metal substrate to form an uncured coating film, and a heating step of heating the uncured coating film formed in the coating step to dry and / or cure it.

[0147] Examples of the above-mentioned metal substrates include zinc-plated steel sheets such as electro-galvanized steel sheets, electro-galvanized zinc-nickel steel sheets, hot-dip galvanized steel sheets, and zinc-aluminum hot-dip galvanized steel sheets, as well as hot-rolled steel sheets, cold-rolled steel sheets, stainless steel sheets, copper-plated steel sheets, aluminum sheets, hot-dip tin-zinc (Sn-10%Zn) plated steel sheets, hot-dip aluminum plated steel sheets, and Al-Mg alloys. The surfaces of these metal substrates may be subjected to chemical conversion treatment (pretreatment).

[0148] Furthermore, chemical treatments (pretreatments) that can be performed as needed include, for example, phosphate treatments such as zinc phosphate treatment and iron phosphate treatment, composite oxide film treatment, chromium phosphate treatment, and chromate treatment.

[0149] The coating using the aqueous coating composition for coil coating of this embodiment can be performed on the above-mentioned metal substrate by the coil coating method.

[0150] The dry film thickness of the coating film formed by the coating film forming method of this embodiment varies depending on the application of the object to be coated and is not particularly limited, but from the viewpoint of processability, corrosion resistance, and boiling water resistance of the formed coating film, it is preferably in the range of 0.5 to 20 μm, and more preferably in the range of 2 to 10 μm.

[0151] In this specification, the dry film thickness is determined by measurement using an electromagnetic film thickness gauge.

[0152] The drying and / or curing of the uncured coating can be carried out by known heating means, such as a hot air furnace, electric furnace, or infrared induction heating furnace. From the viewpoint of the curability of the formed coating, the heating temperature is preferably in the range of 160 to 250°C, and more preferably in the range of 180 to 250°C. The heating time is preferably in the range of 15 to 180 seconds, and more preferably in the range of 20 to 120 seconds, from the viewpoint of the curability of the formed coating. This heating allows the uncured coating to be dried and / or cured.

[0153] The coating film forming method of this embodiment makes it possible to obtain a painted metal plate in which the metal substrate is covered with the coating film.

[0154] [Method for forming multi-layer coatings] The aqueous coating composition for coil coating of this embodiment can be suitably used for forming a primer film when forming a multi-layer coating including a primer film and a topcoat film on an object to be coated. In this case, the coating film formation method can be carried out according to Method I below.

[0155] <Method I> Step (I-1): A step of applying the aqueous coating composition for coil coating of this embodiment to a metal substrate to form an uncured primer coating film. Step (I-2): A step of heating the uncured primer coating to dry and / or cure it. Step (I-3): A step of applying a topcoat paint composition onto the primer coating to form an uncured topcoat coating, Step (I-4): A step of heating the uncured topcoat film to dry and / or cure it. A method for forming a multilayer coating film, including the following:

[0156] The process for forming the primer coating described above is as explained in the section on [Coating Formation Method].

[0157] As the topcoat paint, solid color paints, metallic paints, light interference color paints, and clear paints can be used, and one or more selected from these can be used to form a single-layer or multi-layer topcoat film of two or more layers on the primer film. Various water-based or organic solvent-based paints that are known themselves can be used as these topcoats.

[0158] The dry thickness of the topcoat film varies depending on the application of the object to be coated and is not particularly limited, but from the viewpoint of processability, corrosion resistance, and boiling water resistance of the formed coating film, it is preferably in the range of 5 to 30 μm, and more preferably in the range of 10 to 25 μm.

[0159] The drying and / or curing of the uncured topcoat film can be carried out by known heating means, such as a hot air furnace, electric furnace, or infrared induction heating furnace. The heating temperature is preferably in the range of 160 to 250°C, and more preferably in the range of 180 to 250°C, where the maximum temperature the material reaches. The heating time is preferably in the range of 15 to 180 seconds, and more preferably in the range of 20 to 120 seconds. This heating can dry and / or cure the uncured topcoat film.

[0160] The multi-layer coating method of this embodiment makes it possible to obtain a painted metal plate in which the metal substrate is covered with the primer coating and the topcoat coating. [Examples]

[0161] The present invention will be described in more detail below with reference to manufacturing examples, examples, and comparative examples. However, the present invention is not limited thereto. In each example, "parts" and "%" are based on mass unless otherwise specified. Furthermore, the film thickness of the coating is based on the cured coating.

[0162] Manufacturing of acrylic-modified polyester resin (A) Manufacturing Example 1 In a reaction apparatus equipped with a thermometer, thermostat, stirrer, heating device, and rectification column, 14.7 parts of adipic acid, 18.7 parts of succinic anhydride, 1.1 parts of maleic anhydride, 36.0 parts of 2-butyl-2-ethyl-1,3-propanediol, and 20.4 parts of trimethylolpropane were charged, and the temperature was raised to 160°C while stirring. Next, the temperature of the contents was gradually raised from 160°C to 230°C over 3.5 hours, and the condensed water produced was removed by distillation through the rectification column. After continuing the reaction at 230°C for 90 minutes, the rectification column was replaced with a water separator, and approximately 4 parts of toluene were added to the contents. The condensed water was removed by azeotropic distillation of water and toluene. The acid value was measured starting one hour after the addition of toluene. Heating was stopped after confirming that the acid value was less than 6, and after removing the toluene under reduced pressure, 20 parts of dipropylene glycol monomethyl ether were added to dilute the solution, and 2.1 parts of methoxypolyethylene glycol methacrylate (Mw400) were added.

[0163] Next, the reaction mixture was cooled to 130°C, and a mixture of 3.0 parts styrene, 3.0 parts acrylic acid, 6.9 parts 2-ethylhexyl acrylate, and 0.85 parts t-butyl peroxy-2-ethylhexanoate was added dropwise over 30 minutes. After that, the mixture was aged at 130°C for 30 minutes, and then 0.05 parts t-butyl peroxy-2-ethylhexanoate was added as an additional catalyst, and the mixture was aged for a further 1 hour.

[0164] Subsequently, the reaction solution was cooled to 85°C, neutralized with dimethylethanolamine, deionized water was added, and the mixture was dispersed in water to obtain an aqueous dispersion of acrylic-modified polyester resin (A-1) with a solid content of 40%. The obtained acrylic-modified polyester resin (A-1) had an acid value of 26.8 mg KOH / g, a hydroxyl value of 155.3 mg KOH / g, and a number-average molecular weight of 3200.

[0165] Manufacturing Examples 2-21 In Production Example 1, an aqueous dispersion of acrylic-modified polyester resin (A-2) to (A-21) was obtained in the same manner as in Production Example 1, except that the compound composition was as shown in Table 1 below.

[0166] [Table 1]

[0167] [Table 2]

[0168] Preparation of aqueous coating compositions for coil coating Example 1 212.5 parts (85 parts solids) of the acrylic-modified polyester resin (A-1) obtained in Production Example 1, 10 parts (10 parts solids) of "SHIELDEX C303" (product name, manufactured by Grace Co., Ltd., calcium ion exchange silica fine particles, solid content concentration 100%), 15 parts (15 parts solids) of "JR-806" (product name, manufactured by Teika Co., Ltd., rutile-type titanium dioxide, solid content concentration 100%), and 5 parts (5 parts solids) of "MICRO ACE S-3" (product name, manufactured by Nippon Talc Co., Ltd., talc powder, solid content concentration 100%) were mixed, and the pigment was dispersed until the particle size (particle diameter of pigment elementary particles) was 15 microns or less. Next, 15.2 parts (15 parts solids) of "Cymel 303 LF" (trade name, manufactured by Ornex Japan, melamine resin, solid content concentration 99%) and 0.6 parts (0.15 parts solids) of "Nacure 2547" (trade name, manufactured by KING NDUSTRIES, amine salt of p-toluenesulfonic acid, solid content concentration 25%) were added to this dispersion and mixed uniformly. Deionized water was then added to obtain water-based coating composition No. 1 for coil coating with a solid content concentration of 45%.

[0169] Examples 2-32 and Comparative Examples 1-6 In Example 1, aqueous coating compositions No. 2 to 38 for coil coating were obtained in the same manner as in Example 1, except that the formulation was as shown in Table 2 below. The amounts of each material in the table are expressed as solid content.

[0170] Preparation of test painted panels A 55% aluminum-zinc plated steel sheet (0.35 mm thick, AZ150 plating) that had undergone chemical conversion treatment was coated with the water-based coating composition for coil coating obtained in each example and comparative example using a bar coater to achieve a dry film thickness of 5 μm. The sheet was then heated to a maximum temperature of 220°C and baked for 30 seconds. The same process was then repeated on the opposite side of the steel sheet to obtain a primer-coated steel sheet. Next, KP Color 1555 Brown Full Gloss (manufactured by Kansai Paint Co., Ltd., polyester / melamine paint, glass transition temperature 50°C) was applied to the surface that had been initially coated with the water-based coating composition for coil coating to achieve a dry film thickness of 15 μm. The sheet was then heated to a maximum temperature of 220°C and baked for 45 seconds to create a test coated plate.

[0171] Evaluation of storage stability For each water-based paint composition for coil coating, the viscosity of the paint was adjusted to 90 to 110 seconds using a Ford Cup No. 4 (manufactured by Ueshima Seisakusho Co., Ltd.), in accordance with the method specified in JIS K 5600 2-2 (flow cup method). Next, 300g of each water-based paint composition for coil coating was placed in a 500ml poly bottle, and a storage test was conducted at 50°C for two weeks. After storage, the bottom of the container was scraped with a spatula, and the settling properties of the pigment and resin were evaluated. A, B, and C are considered acceptable. The results are shown in Table 2.

[0172] A: There were no precipitated components, and the condition was good. B: Slight pigment sedimentation was observed, but it returned to a uniform state after disperser stirring. C: Pigment sedimentation was observed, but it returned to a uniform state after disperser stirring. D: Pigment sedimentation was observed, and the mixture did not return to a uniform state with disperser stirring. E: Aggregation of pigments and resins was observed, and the mixture did not return to a uniform state with disperser stirring.

[0173] Evaluation of machinability At a room temperature of 20°C, each test coated board, cut to a size of 6cm x 12cm, was subjected to a 6T bending process (bending the coated board with the surface side facing outwards, inserting six boards of the same thickness as the coated board inside, and then bending the coated board 180 degrees using a vise) so that the length of the bend was 6cm. The occurrence of cracks was evaluated visually and using a 10x magnifying glass according to the following criteria. A, B, and C are considered acceptable. The results are shown in Table 2.

[0174] A: No cracks were visible even with a 10x magnifying glass or by the naked eye. B: A very small crack is visible under a 10x magnifying glass, but no cracks are visible to the naked eye. C: A few large cracks visible to the naked eye are present. D: A large, visible crack is observed in a portion of the machined area. E: Large, visible cracks are observed across the entire surface of the machined area.

[0175] Evaluation of corrosion resistance A composite corrosion test was conducted in accordance with JASO M609-91 (Automotive Material Corrosion Test, 1991). Each test coated panel was cut to a size of 6 cm x 12 cm so that the burrs on the long edge of each panel faced the surface side of the coating on the right side and the back side on the left side. Step 1: A process in which 5% saline solution is sprayed at 35°C for 2 hours. Step 2: Drying at 60°C and 95% relative humidity for 4 hours. Step 3: A process of wetting at 50°C and 95% relative humidity for 2 hours. The above steps 1-3 constituted one cycle, and the test was conducted for 150 cycles (totaling 1200 hours). After this test, the degree of corrosion progression at both edges of the test painted board was evaluated. A, B, and C are considered acceptable. The results are shown in Table 2.

[0176] A: The total width of corrosion progression at both edges was less than 4 mm. B: The total width of corrosion progression at both edges was 4 mm or more and less than 8 mm. C: The total width of corrosion progression at both edges was 8 mm or more and less than 12 mm. D: The total width of corrosion progression on both edges was 12 mm or more and less than 20 mm. E: The total width of corrosion progression on both edges was 20 mm or more.

[0177] Evaluation of curing properties A 55% aluminum-zinc plated steel sheet (0.35 mm thick, AZ150 plating) that had undergone chemical conversion treatment was coated with each coil coating water-based paint composition using a bar coater to achieve a dry film thickness of 5 μm. The sheet was then heated to a maximum temperature of 220°C and baked for 30 seconds to create a coated sheet for curing evaluation. A 700 g hammer with gauze wrapped around its round head was prepared, and a rubbing test was conducted by impregnating the gauze with 2 g of xylol, and evaluated according to the following criteria. A, B, and C are considered acceptable. The evaluation results are shown in Table 2.

[0178] A: The paint film did not peel off after 100 back-and-forth rubbing cycles. B: The coating peeled off after 80 to 99 round trips. C: The paint film peeled off after 50 to 79 round trips. D: The paint film peeled off after 30 to 49 passes. E: The coating peeled off after 1 to 29 passes.

[0179] Evaluation of adhesion At a room temperature of 20°C, each test coated plate, cut to a size of 6cm x 12cm, was subjected to a 2T bend process (the coated plate was bent with the surface side facing outwards, two plates of the same thickness as the coated plate were placed inside, and the coated plate was bent 180 degrees using a vise) so that the length of the bend was 6cm. To assess the adhesion of the coating film to the bent portion, cellophane adhesive tape was applied to the bend, and the degree of peeling of the coating film in the bent portion was evaluated when the tape was instantly removed. A, B, and C are considered acceptable. The results are shown in Table 2.

[0180] A: No peeling of the coating was observed in the processed area. B: A small number of small areas (5 or fewer) of peeling, with the longest side being less than 1 mm, were observed. C: Small areas of peeling with a long side of less than 1 mm were observed in several locations (6 or more places). D: Large areas of peeling with a long side of 1 mm or more were observed in less than half of the processed area. E: Large areas of peeling with a long side of 1 mm or more were observed in more than half of the processed area.

[0181] Evaluation of boiling water resistance Each test coated panel was immersed in boiling water for 5 hours. Immediately after removal, the coating was observed and visually evaluated according to ASTM D714-56 for its appearance and blister formation. Grades A and B are considered acceptable. The evaluation results are shown in Table 2.

[0182] A: No abnormalities were observed compared to the untested test painted panel. B: A slight loss of gloss was observed compared to the untested test painted panel, but no blistering occurred. C: Significant gloss loss or blistering occurred compared to the untested test painted panel.

[0183] [Table 3]

[0184] [Table 4]

[0185] [Table 5]

[0186] [Table 6]

[0187] The components listed in the table are as follows: (Note 1) "CKS-3898": Product name, manufactured by Aica Kogyo Co., Ltd., phenolic resin, solid content concentration 50%, (Note 2) "WM44-L70G": Product name, manufactured by Asahi Kasei Corporation, blocked polyisocyanate compound, solid content concentration 70%, (Note 3) "Expert NP-3000": Product name, manufactured by Toho Pigment Industry Co., Ltd., calcium silicate-based rust-preventive pigment, solid content concentration 100% (Note 4) "ZINC PHOSPHATE PZ20": Product name, manufactured by SNCZ, zinc phosphate, solid content concentration 100%, (Note 5) "LF Bowsei PM-303W": Product name, manufactured by Kikuchi Color Co., Ltd., aluminum tripolyphosphate, solid content concentration 100%, (Note 6) "Byronaal MD-2000": Product name, manufactured by Toyobo MC Co., Ltd., polyester resin, number average molecular weight 18000, hydroxyl value 6 mg KOH / g, acid value less than 2 mg KOH / g, solid content concentration 40%, (Note 7) "JONCRYL OH 8300": Product name, manufactured by BASF, acrylic resin, hydroxyl value 42 mg KOH / g, solids content concentration 43%, (Note 8) "SYLYSIA 440": Product name, manufactured by Fuji Silysia Chemical Co., Ltd., matting agent (silica particles), volume average particle size (d50) 6.2 μm, solid content concentration 100%.

Claims

1. A water-based coating composition for coil coating, It contains acrylic-modified polyester resin (A), a hardening agent (B), a rust-preventive pigment (C), and water. The mass ratio (a1) / (a2) of the polyester portion (a1) and the acrylic portion (a2) of the acrylic-modified polyester resin (A) is within the range of 60 / 40 to 95 / 5. Water-based paint composition for coil coating.

2. The aqueous coating composition for coil coating according to claim 1, wherein the monomer component constituting the acrylic portion (a2) of the acrylic-modified polyester resin (A) contains a polyoxyalkylene group-containing polymerizable unsaturated monomer having a weight-average molecular weight of 400 or more.

3. The aqueous coating composition for coil coating according to claim 1 or 2, wherein the acid value of the acrylic portion (a2) of the acrylic-modified polyester resin (A) is in the range of 70 to 500 mg KOH / g.

4. The aqueous coating composition for coil coating according to claim 1 or 2, wherein the hydroxyl value of the acrylic-modified polyester resin (A) is in the range of 20 mg KOH / g to 200 mg KOH / g.

5. The aqueous coating composition for coil coating according to claim 1 or 2, wherein the acid value of the polyester portion (a1) of the acrylic-modified polyester resin (A) is in the range of 0 to 20 mg KOH / g.

6. The aqueous coating composition for coil coating according to claim 1 or 2, wherein the curing agent (B) comprises an amino resin (B1).

7. The aqueous coating composition for coil coating according to claim 1 or 2, wherein the rust-preventive pigment (C) comprises at least one rust-preventive pigment selected from metal ion exchange silica fine particles (C1) and metal silicate (C2).

8. The aqueous coating composition for coil coating according to claim 1 or 2, wherein the content of the acrylic-modified polyester resin (A) is in the range of 30 to 95% by mass, based on the total amount of resin solids in the aqueous coating composition for coil coating.

9. An aqueous coating composition for coil coating according to claim 1 or 2, used as a primer coat.

10. A method for forming a coating film, comprising a painting step of applying the aqueous coating composition for coil coating described in claim 1 to a metal substrate to form an uncured coating film, and a heating step of heating the uncured coating film formed in the painting step to dry and / or cure it.

11. Step (I-1): A step of applying the aqueous coating composition for coil coating described in claim 1 to a metal substrate to form an uncured primer coating film. Step (I-2): A step of heating the uncured primer coating to dry and / or cure it. Step (I-3): A step of applying a topcoat paint composition onto the primer coating to form an uncured topcoat coating, Step (I-4): A step of heating the uncured topcoat film to dry and / or cure it. A method for forming a multilayer coating film, including the following:

12. A painted metal plate coated by the coating film forming method described in claim 10 or 11.

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