Aqueous primer coating composition and method for forming multilayer coating layer

Abstract

To provide a coating composition capable of forming a coating layer that exhibits superior adhesion to metal surfaces as well as to organic resin-covered surfaces and also demonstrates superior water resistance and corrosion resistance.SOLUTION: An aqueous primer coating composition contains water, a resin, and a pigment. The mass ratio of the pigment to the resin is 0.30-0.90, where the resin contains at least an acryl-modified epoxy resin (A) and a styrene acrylic resin (B).SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an aqueous undercoat paint composition and a method for forming a multilayer coating film using the aqueous undercoat paint composition, and in particular to a paint composition that has excellent adhesion to both metal surfaces and surfaces coated with an organic resin, and is capable of forming a coating film that is excellent in water resistance and corrosion resistance. [Background technology]

[0002] Undercoat paints for metal substrates used on vehicles, mainly railway cars, construction machinery, such as excavators, industrial machinery, and steel fittings (collectively referred to as vehicle-industrial machinery), are required to form coating films with excellent water resistance and corrosion resistance. In recent years, consideration has also been given to reducing the environmental load and reducing the number of painting steps, and there is a demand for water-based one-component paints capable of forming the above coating films.

[0003] JP 2014-196426 A (Patent Document 1) describes a water-based undercoat paint composition that is characterized by containing an epoxy resin dispersion, an acrylic resin emulsion, and an anti-rust pigment, and describes that by using such a water-based undercoat paint composition, it is possible to impart excellent rust prevention and adhesion to metal sheets such as zinc-plated steel sheets without employing a chromate treatment.

[0004] Japanese Patent Laid-Open Publication No. 2012-224771 (Patent Document 2) describes an aqueous coating composition that contains an aqueous dispersion of a specific aqueous fatty acid modified acrylic resin (A) and a specific maleic fatty acid epoxy ester resin (B) in a solid content mass ratio of 35 / 65 to 90 / 10, and further contains 1 to 60 parts by mass of a metal phosphite (C) based on 100 parts by mass of the total solid content of both components. The document describes that the use of such an aqueous coating composition makes it possible to form a coating film that is excellent in low-temperature curing properties, finishability, coating film hardness, water resistance and corrosion resistance, particularly in corrosion resistance of untreated steel sheets. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2014-196426 A [Patent Document 2] JP 2012-224771 A Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Documents 1 and 2 disclose water-based one-pack epoxy resin coating compositions capable of forming coating films with excellent adhesion, water resistance, and corrosion resistance to metal substrates. On the other hand, the surface to be coated of the vehicle manufacturing machinery may have unevenness or pitting, and in such cases, putty or the like is used to smooth the unevenness or fill the pitting. Here, since organic resins are usually used for the putty, the undercoat coating composition for metal substrates used in the vehicle manufacturing machinery is required to have not only adhesion to metal substrates but also adhesion to surfaces coated with organic resins. However, the water-based one-pack epoxy resin coating compositions described in Patent Documents 1 and 2 have problems with adhesion to surfaces coated with organic resins.

[0007] Therefore, an object of the present invention is to provide a coating composition that has excellent adhesion to both metal surfaces and surfaces coated with organic resins, and is capable of forming a coating film that is excellent in water resistance and corrosion resistance. Another object of the present invention is to provide a method for forming a multi-layer coating film using such a coating composition. [Means for solving the problem]

[0008] As a result of intensive research into achieving the above-mentioned object, the present inventors have found that by adjusting the pigment to resin mass ratio to 0.30 to 0.90 in an aqueous primer coating composition and using an acrylic-modified epoxy resin and a styrene acrylic resin as the resin, it is possible to form a coating film that has excellent adhesion to both metal surfaces and surfaces coated with an organic resin, and that also has excellent water resistance and corrosion resistance, and have thus completed the present invention.

[0009] Accordingly, the coating composition of the present invention is a water-based undercoat coating composition comprising water, a resin, and a pigment, a mass ratio of the pigment to the resin is 0.30 to 0.90; The water-based undercoat paint composition is characterized in that the resin contains at least an acrylic-modified epoxy resin (A) and a styrene-acrylic resin (B).

[0010] In a preferred embodiment of the coating composition of the present invention, the mass ratio (A):(B) of the acrylic modified epoxy resin (A) to the styrene acrylic resin (B) is 40:60 to 80:20.

[0011] In another preferable embodiment of the coating composition of the present invention, the glass transition point (Tg) of the acrylic-modified epoxy resin (A) is 60 to 90°C.

[0012] The method for forming a multilayer coating film of the present invention comprises the steps of applying the above-mentioned aqueous undercoat paint composition of the present invention on at least a substrate to form an undercoat layer (I); and applying a topcoat paint composition on the undercoat layer (I) to form a topcoat layer (II).

[0013] In a preferred embodiment of the method for forming a multilayer coating film of the present invention, the topcoat paint composition contains an acrylic-modified epoxy resin. Effect of the Invention

[0014] According to the coating composition of the present invention, it is possible to provide a coating composition capable of forming a coating film having excellent adhesion to both metal surfaces and surfaces coated with an organic resin, and excellent water resistance and corrosion resistance. Also, according to the method for forming a multilayer coating film of the present invention, it is possible to provide a method for forming a multilayer coating film using such a coating composition. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention relates to a water-based undercoat paint composition and a method for forming a multi-layer paint film using the water-based undercoat paint composition.

[0016] The coating composition of the present invention is a water-based undercoat coating composition that contains water, a resin, and a pigment.

[0017] Here, the water-based coating composition is a coating composition containing water as a main solvent. The water used in the coating composition of the present invention is not particularly limited, but preferred examples include tap water, ion-exchanged water, distilled water, and other pure water. When the coating composition is to be stored for a long period of time, water sterilized by ultraviolet irradiation or the like may be used to prevent the growth of mold and bacteria. In the coating composition of the present invention, the amount of water is preferably 20 to 80% by mass, more preferably 30 to 70% by mass.

[0018] In addition, in the coating composition of the present invention, the amount of solid content is preferably 20 to 70 mass%, more preferably 35 to 55 mass%. In the present invention, the solid content refers to the components excluding volatile components such as solvents, and is the component that will ultimately form a coating film. In the present invention, the components remaining when the coating composition is dried in an oven at 110°C for 3 hours are treated as solid content. Note that the solid content may also be referred to as non-volatile content or coating film forming components.

[0019] The undercoat paint composition means a paint used as an undercoat when painting is performed by applying multiple coats of paint. The undercoat paint may also be called a primer paint, a base paint, or an undercoat.

[0020] The coating composition of the present invention has a pigment to resin mass ratio of 0.30 to 0.90, preferably 0.40 to 0.85, and more preferably 0.45 to 0.70. According to the coating composition of the present invention, by using an acrylic modified epoxy resin and a styrene acrylic resin as the resin, it is possible to form a coating film that is excellent in adhesion to metal surfaces in addition to water resistance and corrosion resistance, and further by adjusting the pigment to resin mass ratio (i.e., pigment / resin mass ratio) to 0.90 or less, a coating film having good adhesion not only to metal surfaces but also to surfaces coated with organic resins can be formed. In addition, according to the coating composition of the present invention, by adjusting the pigment to resin mass ratio (pigment / resin mass ratio) to 0.30 or more, the coating film strength is improved and a coating film having excellent water resistance and corrosion resistance can be formed.

[0021] In the coating composition of the present invention, the resin contains at least an acrylic-modified epoxy resin and a styrene-acrylic resin. The acrylic-modified epoxy resin can improve the adhesion, corrosion resistance, and hardness of the coating film. The styrene-acrylic resin can improve the quick-drying property and water resistance of the coating film.

[0022] In this specification, the “acrylic-modified epoxy resin” contained in the coating composition of the present invention may be referred to as the “acrylic-modified epoxy resin (A)” or “component (A)”, and the “styrene-acrylic resin” may be referred to as the “styrene-acrylic resin (B)” or “component (B)”.

[0023] In the coating composition of the present invention, the mass ratio (A):(B) of the acrylic-modified epoxy resin (A) to the styrene-acrylic resin (B) is preferably 40:60 to 80:20, and more preferably 50:50 to 70:30, from the viewpoints of adhesion to surfaces coated with organic resins, water resistance, and corrosion resistance.

[0024] The acrylic-modified epoxy resin is an epoxy resin modified with an acrylate-based monomer, and can be obtained, for example, by polymerizing unsaturated monomers including an acrylate-based monomer and a carboxyl-containing polymerizable unsaturated monomer in the presence of a modified epoxy resin obtained by reacting an epoxy resin, a glycidyl-group-containing polymerizable unsaturated monomer, and an amine.

[0025] Examples of epoxy resins used in the production of acrylic modified epoxy resins include bisphenol type epoxy resins such as bisphenol A type epoxy resins and bisphenol F type epoxy resins, naphthalene type epoxy resins, biphenyl type epoxy resins, novolac type epoxy resins, cyclic aliphatic epoxy resins, glycidylamine type resins, heterocyclic epoxy resins, polyfunctional epoxy resins, etc. Among these, bisphenol type epoxy resins are preferred. The epoxy resins used in the production of acrylic modified epoxy resins can be used alone or in combination. Specifically, epoxy resins synthesized from bisphenols and epihalohydrin (epichlorohydrin, etc.), epoxy resins obtained by elongation reaction of bisphenols and epoxy resins derived from bisphenols, dihydric phenols, and epihalohydrin, and bisphenols are preferred.

[0026] Examples of bisphenols used in the production of epoxy resins include bis(4-hydroxyphenyl)methane [bisphenol F], 1,1-bis(4-hydroxyphenyl)ethane, 2,2-bis(4-hydroxyphenyl)propane [bisphenol A], 2,2-bis(4-hydroxyphenyl)butane [bisphenol B], bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-tert-butyl-phenyl)-2,2-propane, p-(4-hydroxyphenyl)phenol, oxybis(4-hydroxyphenyl), sulfonylbis(4-hydroxyphenyl), 4,4'-dihydroxybenzophenone, and bis(2-hydroxynaphthyl)methane, among which bisphenol A is preferably used. Bisphenols can be used alone or in combination.

[0027] The number average molecular weight of the epoxy resin used in producing the acrylic modified epoxy resin is preferably 2,000 to 35,000, more preferably 4,000 to 30,000, and the epoxy equivalent is preferably 1,000 to 12,000, more preferably 3,000 to 10,000.

[0028] In the present invention, the number average molecular weight and the weight average molecular weight are values ​​measured by size exclusion chromatography (SEC), and polystyrene is used as the standard substance.

[0029] The epoxy equivalent of the epoxy resin can be determined in accordance with JIS K 7236:2001 "Determination of epoxy equivalent of epoxy resin".

[0030] Examples of glycidyl group-containing polymerizable unsaturated monomers used in producing the acrylic-modified epoxy resin include glycidyl (meth)acrylate, 2-methylglycidyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate glycidyl ether, 3,4-epoxycyclohexylmethyl (meth)acrylate, and 1,2-epoxy-4-vinylcyclohexane.

[0031] Examples of amines used in the production of acrylic-modified epoxy resins include mono- or di-alkylamines such as monomethylamine, dimethylamine, monoethylamine, diethylamine, monoisopropylamine, diisopropylamine, monobutylamine, and dibutylamine; alkanolamines such as monoethanolamine, diethanolamine, mono(2-hydroxypropyl)amine, di(2-hydroxypropyl)amine, monomethylaminoethanol, and monoethylaminoethanol; alkylene polyamines such as ethylenediamine, propylenediamine, butylenediamine, hexamethylenediamine, tetraethylenepentamine, pentaethylenehexamine, diethylaminopropylamine, diethylenetriamine, and triethylenetetramine; alkylene imines such as ethyleneimine and propyleneimine; and cyclic amines such as piperazine, morpholine, and pyrazine.

[0032] Preferred amines include dialkanolamines and chain aliphatic monoamines, and it is more preferred to use these in combination.

[0033] Dialkanolamine is an amine having two alkanol groups. Its type is not particularly limited, but dialkanolamine (secondary dialkanolamine) having an NH bond is preferred. Dialkanolamine (secondary dialkanolamine) having an NH bond includes, for example, diethanolamine, di-n-propanolamine, diisopropanolamine, bis(2-hydroxybutyl)amine, bis(2-hydroxypentyl)amine, bis(2-hydroxyhexyl)amine, etc., and diethanolamine, di-n-propanolamine, and diisopropanolamine are preferred. Dialkanolamine may be used alone or in combination of two or more kinds.

[0034] The amount of dialkanolamine used is preferably 5 to 40% by weight, and more preferably 7 to 30% by weight, based on the total amount of amines used in producing the acrylic modified epoxy resin.

[0035] The chain aliphatic monoamine is a monoamine having one or more acyclic hydrocarbon groups (alkyl groups, alkenyl groups, alkynyl groups) in the molecule, and is preferably a chain aliphatic amine having one or two NH bonds, that is, a monoalkylamine, a monoalkenylamine, a dialkylamine, or a dialkenylamine. The chain aliphatic monoamine may be used alone or in combination of two or more kinds.

[0036] Examples of monoalkylamines include methylamine, ethylamine, n-propylamine, isopropylamine, n-butylamine, isobutylamine, s-butylamine, t-butylamine, n-pentylamine, isopentylamine, n-hexylamine, n-heptylamine, n-octylamine, isooctylamine, 2-ethylhexylamine, n-nonylamine, n-decylamine, isodecylamine, n-undecylamine, n-dodecylamine (n-laurylamine), n-tridecylamine, n-tetradecylamine (n-myristylamine), n-pentadecylamine, n-hexadecylamine (n-palmitylamine), n-heptadecylamine, n-octadecylamine (n-stearylamine), and isooctadecylamine (isostearylamine).

[0037] Examples of monoalkenylamines include vinylamine, allylamine, butenylamine, pentenylamine, hexenylamine, heptenylamine, octenylamine, nonenylamine, decenylamine, undecenylamine, dodecenylamine, tridecenylamine, tetradecenylamine, pentadecenylamine, hexadecenylamine, heptadecenylamine, octadecenylamine (oleylamine, etc.), nonadecenylamine, icosenylamine, dodecenylamine, etc. The position of the carbon-carbon double bond of these monoalkenylamines is not particularly limited, and their structural isomers can also be used freely.

[0038] Dialkylamines include dimethylamine, N-ethylmethylamine, diethylamine, N-ethyl-N-(n-propyl)amine, di-n-propylamine, n-propylisopropylamine, diisopropylamine, di-n-butylamine, diisobutylamine, di-s-butylamine, di-t-butylamine, Nn-butylethylamine, Ns-butylethylamine, Nt-butylethylamine, Nn-butylpropylamine, Ns-butylpropylamine, Nt-butylpropylamine, and Nt-butylisopropylamine. , dipentylamine (diamylamine), diisopentylamine (diisoamylamine), dihexylamine, diheptylamine, dioctylamine, bis(2-ethylhexyl)amine, dinonylamine, didecylamine, diundecylamine, didodecylamine (dilaurylamine), ditridecylamine, ditetradecylamine (dimyristylamine), dipentadecylamine, dihexadecylamine (dipalmitylamine), diheptadecylamine, dioctadecylamine (distearylamine), and the like.

[0039] Examples of dialkenylamines include divinylamine, diallylamine, dibutenylamine, dipentenylamine, dihexenylamine, diheptenylamine, dioctenylamine, dinonenylamine, didecenylamine, diundecenylamine, didodecenylamine, ditridecenylamine, ditetradecenylamine, dipentadecenylamine, dihexadecenylamine, diheptadecenylamine, dioctadecenylamine, dinonadecenylamine, diicosenylamine, didodecenylamine, etc. The position of the carbon-carbon double bond of these dialkenylamines is not particularly limited, and structural isomers thereof can also be freely used.

[0040] Among these chain aliphatic monoamines, monoalkylamines, monoalkenylamines and dialkylamines are more preferred.

[0041] The amount of the chain aliphatic monoamine used is preferably 50 to 95% by weight, more preferably 60 to 92% by weight, based on the total amount of amines used in the production of the acrylic modified epoxy resin.

[0042] When producing a modified epoxy resin by reaction of an epoxy resin, a glycidyl group-containing polymerizable unsaturated monomer, and an amine, a monovalent to trivalent organic acid, a monovalent to tetravalent alcohol, an isocyanate compound, or the like can be used as necessary for the purpose of improving water dispersibility and corrosion resistance.

[0043] As the monovalent to trivalent organic acid, various known aliphatic, alicyclic or aromatic carboxylic acids can be used, such as dimer acid and trimellitic acid. As the monovalent to tetravalent alcohol, various known aliphatic, alicyclic or aromatic alcohols can be used, such as neopentyl glycol, trimethylolpropane, pentaerythritol, etc. As the isocyanate compound, various known aromatic, aliphatic or alicyclic polyisocyanates can be used, such as tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, 4,4'-diphenylmethane diisocyanate, etc. These monovalent to trivalent organic acids, monovalent to tetravalent alcohols, isocyanate compounds, etc. can be used as necessary within the range that does not impair the effects of the present invention.

[0044] The modified epoxy resin can be easily produced by heating the above-mentioned components such as the epoxy resin, the glycidyl group-containing polymerizable unsaturated monomer, and the amines in the presence of an organic solvent. The reaction temperature is usually 60 to 200°C, preferably 90 to 150°C, and the reaction time is usually 1 to 10 hours, preferably 1 to 5 hours.

[0045] In producing the modified epoxy resin, the ratio of the epoxy resin, the glycidyl group-containing polymerizable unsaturated monomer, and the amines used is, in terms of the number of epoxy groups / the number of active hydrogen atoms in the amino groups, about 100 / 120 to 100 / 80, preferably about 100 / 110 to 100 / 90, and more preferably about 100 / 105 to 100 / 95.

[0046] Examples of the organic solvent include alcohol-based solvents such as methyl alcohol, ethyl alcohol, n-butyl alcohol, isopropyl alcohol, 2-ethylhexanol, benzyl alcohol, ethylene glycol, and propylene glycol; ether-based solvents such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol mono 2-ethylhexyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monophenyl ether, 3-methyl-3-methoxybutanol, diethylene glycol monoethyl ether, and diethylene glycol monobutyl ether; ketone-based solvents such as acetone, methyl isobutyl ketone, cyclohexanone, isophorone, and acetylacetone; ester-based solvents such as ethylene glycol monoethyl ether acetate and ethylene glycol monobutyl ether acetate; and mixtures thereof.

[0047] An acrylic-modified epoxy resin can be produced by polymerizing an acrylate monomer and a carboxyl group-containing polymerizable unsaturated monomer, and, if necessary, other polymerizable unsaturated monomers, in the presence of the modified epoxy resin obtained as described above.

[0048] The acrylate monomer is an ester monomer having one or more acryloyloxy groups (CH2=CHCOO-) or methacryloyloxy groups (CH2=C(CH3)COO-). Examples of the acrylate monomer used in the production of the acrylic modified epoxy resin include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-, i- or t-butyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, decyl acrylate, lauryl acrylate, cyclohexyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl meth ... Examples of the acrylate monomer include alkyl esters or cycloalkyl esters of acrylic acid or methacrylic acid having 1 to 18 carbon atoms, such as t-butyl, hexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, decyl methacrylate, lauryl methacrylate, and cyclohexyl methacrylate; and C2 to C8 hydroxyalkyl esters of acrylic acid or methacrylic acid, such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, and hydroxybutyl methacrylate. The acrylate monomers can be used alone or in combination of two or more kinds.

[0049] The amount of the acrylate monomer used is preferably about 0.1 to 40 parts by weight, and more preferably about 1 to 20 parts by weight, based on 100 parts by weight of the modified epoxy resin.

[0050] Carboxyl group-containing polymerizable unsaturated monomers are used to make acrylic-modified epoxy resins aqueous (disperse or dissolve in water), and examples thereof include α,β-unsaturated monocarboxylic acids such as (meth)acrylic acid and crotonic acid, α,β-unsaturated dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid, muconic acid and citraconic acid, anhydrides of these carboxylic acids, and alkali metal salts such as sodium salts and potassium salts of these carboxylic acids. The carboxyl group-containing polymerizable unsaturated monomers can be used alone or in combination of two or more kinds.

[0051] The amount of the carboxyl group-containing polymerizable unsaturated monomer used is preferably about 0.1 to 20 parts by weight, and more preferably about 1 to 10 parts by weight, based on 100 parts by weight of the modified epoxy resin.

[0052] The carboxyl group-containing polymerizable unsaturated monomer may be used in combination with a carbodiimide, if necessary.

[0053] Examples of carbodiimides include poly(4,4'-diphenylmethanecarbodiimide), poly(tolylcarbodiimide), poly(p-phenylenecarbodiimide), poly(m-phenylenecarbodiimide), poly(3,3'-dimethyl-4,4'-diphenylmethanecarbodiimide), poly(naphthylenecarbodiimide), poly(1,6-hexamethylenecarbodiimide), poly(1,4-tetramethylenecarbodiimide), poly(1,3-cyclohexylenecarbodiimide), poly(1,4-cyclohexylenecarbodiimide), poly(1,3 ,5-triethylphenylenecarbodiimide), poly(4,4'-methylenebiscyclohexylcarbodiimide), poly(1,3-diisopropylphenylenecarbodiimide), poly(1-methyl-3,5-diisopropylphenylenecarbodiimide), poly(isopropylphenylenecarbodiimide), N,N'-dicyclohexylcarbodiimide, N,N'-diisopropylcarbodiimide, N,N'-diisopropylphenylcarbodiimide, N-ethyl-N'-(3-dimethylaminopropyl)-carbodiimide hydrochloride, etc. The carbodiimide may be used alone or in combination of two or more kinds.

[0054] For the production of acrylic modified epoxy resin, polymerizable unsaturated monomers other than the above-mentioned acrylate monomers and carboxyl group-containing polymerizable unsaturated monomers can also be used. Examples of other polymerizable unsaturated monomers include aromatic vinyl monomers such as styrene, vinyl toluene, 2-methylstyrene, t-butylstyrene, and chlorostyrene; N-substituted acrylamide or N-substituted methacrylamide monomers such as N-methylolacrylamide, N-butoxymethylacrylamide, N-methoxymethylacrylamide, N-methylolmethacrylamide, and N-butoxymethylmethacrylamide, and these can be used alone or in combination of two or more kinds.

[0055] For the copolymerization reaction of the modified epoxy resin with the acrylate monomer and the carboxyl group-containing polymerizable unsaturated monomer, as well as other polymerizable unsaturated monomers used as necessary, various known organic peroxides and azo compounds, such as benzoyl peroxide, tert-butyl peroctoate, 2,2-azobisisobutyronitrile, and 2,2-azobis(2,4-dimethylvaleronitrile), can be used.

[0056] The polymerization method of the copolymerization reaction is not particularly limited, but is preferably a solution polymerization method. For example, the polymerization can be performed in the presence of the above-mentioned polymerization initiator at a reaction temperature of 60 to 150° C. As for the organic solvent, the same one as explained in the production of the modified epoxy resin can be used.

[0057] From the viewpoint of the balance between adhesion and water resistance, the acrylic-modified epoxy resin preferably has a glass transition point (Tg) of 60 to 90° C., more preferably 65 to 85° C. On the other hand, if the glass transition point of the acrylic-modified epoxy resin is too low, the water resistance may decrease. The glass transition point may also be called the glass transition temperature.

[0058] In the present invention, the glass transition point can be measured by a differential scanning calorimeter (DSC) in accordance with JIS K 7121-1987. For example, about 10 mg of a sample (e.g., a resin piece or a coating film piece) is placed in an aluminum pan for measurement, and DSC measurement is performed under the following measurement conditions. (Measurement temperature conditions) 1st temperature rise: -80℃~220℃(20℃ / min) Temperature drop condition: 220℃~-80℃(50℃ / min) 2nd temperature rise: -80℃~220℃(10℃ / min)

[0059] Commercially available acrylic modified epoxy resins include, for example, MODEPIX 301, MODEPIX 302, MODEPIX 303, MODEPIX 304, MODEPIX 305, and MODEPIX 307 (all manufactured by Arakawa Chemical Industries, Ltd.).

[0060] In the coating composition of the present invention, the amount of the acrylic-modified epoxy resin in the solid content is preferably 20 to 50 mass%, more preferably 30 to 40 mass%. The acrylic-modified epoxy resin may be used alone or in combination of two or more. If the amount of the acrylic-modified epoxy resin is less than 20 mass%, the adhesion, corrosion resistance, and hardness of the coating film may decrease, and if the amount of the acrylic-modified epoxy resin is more than 50 mass%, the water resistance and corrosion resistance may decrease.

[0061] The styrene-acrylic resin is a resin containing a structural unit derived from a styrene-based monomer and a structural unit derived from an acrylic monomer. The proportion of the structural unit derived from a styrene-based monomer in the styrene-acrylic resin is preferably 0.1 to 50 mass%, more preferably 10 to 40 mass%. The proportion of the structural unit derived from an acrylic monomer in the styrene-acrylic resin is preferably 50 to 99.9 mass%, more preferably 60 to 90 mass%.

[0062] The styrene-acrylic resin can be obtained by copolymerizing a styrene-based monomer and an acrylic monomer. In addition to the styrene-based monomer and the acrylic monomer, a polymerizable unsaturated monomer other than these can be polymerized as necessary. The styrene-acrylic resin thus obtained is a resin containing a structural unit derived from a styrene-based monomer, a structural unit derived from an acrylic monomer, and a structural unit derived from a further polymerizable unsaturated monomer. The proportion of the structural unit derived from the further polymerizable unsaturated monomer in the styrene-acrylic resin is preferably 10% by mass or less, more preferably 5.0% by mass or less.

[0063] Examples of the styrene-based monomer include styrene, α-methylstyrene, etc. The styrene-based monomer may be used alone or in combination of two or more kinds.

[0064] The acrylic monomer is an ester monomer having one or more acryloyloxy groups (CH2=CHCOO-) or methacryloyloxy groups (CH2=C(CH3)COO-). The acrylic monomers can be used alone or in combination of two or more.

[0065] The acrylic monomer used in the production of the styrene-acrylic resin is preferably an alkyl ester of acrylic acid or methacrylic acid having 1 to 18 carbon atoms. Examples of alkyl esters of acrylic acid or methacrylic acid having 1 to 18 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, toluyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, ethylcyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, n-heptyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, cetyl (meth)acrylate, and stearyl (meth)acrylate. The proportion of structural units derived from an alkyl ester of acrylic acid or methacrylic acid having 1 to 18 carbon atoms in the styrene-acrylic resin is preferably 50 to 99.9 mass %, and more preferably 60 to 90 mass %.

[0066] Examples of polymerizable unsaturated monomers (further polymerizable unsaturated monomers) that do not fall under the category of styrene-based monomers or acrylic-based monomers include vinyl acetate, vinyl chloride, vinylidene chloride, butadiene, olefins (propylene, etc.), etc. The further polymerizable unsaturated monomers can be used alone or in combination of two or more.

[0067] In the production of styrene-acrylic resin, a hydroxyl group-containing polymerizable unsaturated monomer can be used as a monomer corresponding to any of acrylic monomers, styrene monomers, or further polymerizable unsaturated monomers. Examples of hydroxyl group-containing polymerizable unsaturated monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, lactone adduct to 2-hydroxyethyl (meth)acrylate, ring-opening adduct of ethylene oxide or ring-opening adduct of propylene oxide to 2-hydroxyethyl (meth)acrylate or 2-hydroxypropyl (meth)acrylate, dimer or trimer of 2-hydroxyethyl (meth)acrylate or 2-hydroxypropyl (meth)acrylate, etc. Hydroxy group-containing polymerizable unsaturated monomers can be used alone or in combination of two or more.

[0068] The styrene-acrylic resin preferably has a glass transition point (Tg) of -5 to 80°C, more preferably 0 to 50°C.

[0069] Commercially available styrene acrylic resins include, for example, Polysol AP-1330, AP-3150, AP-5085N, and AP-6750 (all manufactured by Resonac), Acronal YJ-1070, YJ-1100, YJ-2720, YJ-2716, and 295DN (all manufactured by BASF), Boncoat 4001, 5454, and EC-740EF (all manufactured by DIC), Aron NW-7060 (manufactured by Toa Gosei), Mowinyl DM60, 749E, and LDM6740 (all manufactured by Japan Coating Resins), and VIACRYL VSC 6254W / 40WA, 6265W / 40WA, 6279W / 45WA, 6286W / 45WA, 6295W / 45WA, and SC 6828W / 45WA (manufactured by Daicel-Allnex Co., Ltd.) and the like.

[0070] In the coating composition of the present invention, the amount of styrene-acrylic resin in the solid content is preferably 10 to 40 mass%, more preferably 20 to 30 mass%. The styrene-acrylic resin may be used alone or in combination of two or more. If the amount of styrene-acrylic resin is less than 10 mass%, the quick-drying property and water resistance of the coating film may decrease, and if the amount of styrene-acrylic resin is more than 40 mass%, the adhesion, corrosion resistance and water resistance of the coating film may decrease.

[0071] The coating composition of the present invention may contain additional resins in addition to the acrylic modified epoxy resin and the styrene acrylic resin. Examples of resins that can be used in the coating composition of the present invention include resins that are commonly used in the coating industry. For example, acrylic resins, silicone resins, acrylic silicone resins, polyester resins, fluororesins, rosin resins, petroleum resins, coumarone resins, phenolic resins, urethane resins, melamine resins, urea resins, epoxy resins, cellulose resins, xylene resins, alkyd resins, urethane alkyd resins, aliphatic hydrocarbon resins, butyral resins, maleic acid resins, fumaric acid resins, vinyl resins, amine resins, ketimine resins, etc. The additional resins may be used alone or in combination of two or more.

[0072] In the coating composition of the present invention, the total amount of resin in the solid content is preferably 45 to 70 mass %, more preferably 55 to 65 mass %. If the total amount of resin is less than 45 mass %, the adhesion of the coating film may decrease, and if the total amount of resin is more than 70 mass %, the corrosion resistance may decrease.

[0073] The pigment used in the coating composition of the present invention is not particularly limited, and pigments commonly used in the coating industry, such as extender pigments, coloring pigments, anti-rust pigments, and luster pigments, can be used. The pigments may be used alone or in combination of two or more. In this specification, the "pigment" contained in the coating composition of the present invention may be referred to as "pigment (C)" or "component (C)".

[0074] The extender pigment is usually a white or colorless pigment. Since the refractive index is low, even if it is mixed with a vehicle, it hardly affects the hiding power, and therefore it is used as an extender to reduce the cost of paints, coating materials, cosmetics, etc., and to adjust the coloring power, gloss, strength, usability, etc. Known materials can be used as the extender pigment, and examples of such materials include precipitated barium sulfate, silica, cristobalite, calcium carbonate, alumina, alum, white clay, magnesium hydroxide, magnesium oxide, etc. In addition, the extender pigment may be a pigment having a thin and flat shape like a foil (scale-like pigment), and specific examples thereof include glass flake, talc, mica, kaolin clay, etc. In the coating composition of the present invention, the amount of the extender pigment in the solid content is, for example, 5.0 to 30 mass %.

[0075] Color pigments are generally classified into inorganic pigments and organic pigments based on their composition. Examples of inorganic pigments include titanium oxide, red iron oxide, yellow iron oxide, and carbon black, while examples of organic pigments include phthalocyanine blue, phthalocyanine green, naphthol red, quinacridone red, benzimidazolone yellow, Hansa yellow, benzimidazolone orange, and dioxazine violet. In the coating composition of the present invention, the amount of color pigment in the solid content is, for example, 0.1 to 20 mass%.

[0076] The rust-preventive pigment may be a known material, such as zinc powder, zinc oxide, barium metaborate, calcium silicate, aluminum phosphate, condensed aluminum phosphate, aluminum tripolyphosphate, zinc phosphate, zinc phosphite, potassium phosphite, calcium phosphite, aluminum phosphite, calcium zinc phosphate, zinc aluminum phosphate, zinc phosphomolybdate, aluminum phosphomolybdate, magnesium phosphate, vanadate / phosphoric acid mixed pigment, etc. In the coating composition of the present invention, the amount of the rust-preventive pigment in the solid content is, for example, 1.0 to 20 mass%.

[0077] Known materials can be used as the luster pigment, and examples thereof include scaly metal pigments made of metals such as zinc, nickel, chromium, tin, copper, silver, platinum, gold, and aluminum, which are thin and flat like foil, and pearl pigments made by surface-treating talc or mica with a metal oxide such as titanium oxide. The scaly metal pigments also include alloy pigments such as stainless steel. In the coating composition of the present invention, the amount of the luster pigment in the solid content is, for example, 0.1 to 10 mass%.

[0078] In the coating composition of the present invention, the total amount of pigments in the solid content is preferably 20 to 45 mass %, more preferably 30 to 40 mass %.

[0079] The coating composition of the present invention may contain other components, such as flexibility imparting agents, surface conditioners, wetting agents, pigment dispersants, emulsifiers, viscosity modifiers, anti-settling agents, anti-skinning agents, anti-sagging agents, defoamers, color separation inhibitors, leveling agents, drying agents, plasticizers, film-forming assistants, neutralizing agents, electrostatic lubricants, antistatic agents, and conductivity imparting agents, as appropriate depending on the purpose.

[0080] The coating composition of the present invention can be prepared by mixing various components appropriately selected as necessary. The coating composition of the present invention is suitable as a one-liquid type coating composition in which various components are premixed and used as is at the time of coating.

[0081] The coating composition of the present invention has a viscosity of 1 to 1000 (Pa s, 23°C) at a shear rate of 0.1 (1 / s), and preferably has a viscosity of 0.05 to 10 (Pa s, 23°C) at a shear rate of 1000 (1 / s). In the present invention, the viscosity is measured using a rheometer (e.g., ARES rheometer manufactured by TA Instruments) after adjusting the liquid temperature to 23°C.

[0082] The means for applying the coating composition of the present invention is not particularly limited, and known coating means such as dipping, spin coating, flow coating, roll coating, spray coating, blade coating and air knife coating can be used.

[0083] The means for drying the coating composition of the present invention is not particularly limited, and may be either natural drying at ambient temperature or forced drying using a dryer or the like.

[0084] The substrate to be coated with the coating composition of the present invention may have various shapes, for example, two-dimensional substrates such as film, sheet, plate, etc., and three-dimensional substrates that are complex three-dimensional objects, etc. The surface of the substrate may be smooth or may have irregularities.

[0085] Examples of substrates to be coated with the coating composition of the present invention include plastic substrates such as epoxy resins, ABS resins, polycarbonates, polyvinyl chloride, polystyrene, acrylic resins, such as polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), polyolefins, such as polypropylene (PP), metal substrates such as steel, galvanized steel, tinned steel, stainless steel, magnesium alloys, aluminum, aluminum alloys, titanium, titanium alloys, and other metal substrates such as cement, mortar, concrete, slate, gypsum, calcium silicate, glass, ceramics, calcium carbonate, marble, and artificial marble, and other inorganic substrates other than metals, wood substrates such as wood, paper substrates, and composite substrates such as a combination of two or more of these substrates. Examples of composite substrates include composite substrates such as wood fiber reinforced cement boards, fiber reinforced cement boards, and fiber reinforced cement-calcium silicate boards, metal substrates that have been subjected to various surface treatments, such as oxidation treatments, and plastic substrates whose surfaces are coated with inorganic substances (for example, plastic substrates coated with glass).

[0086] Since the coating composition of the present invention is capable of forming a coating film that has excellent adhesion and preferably also excellent corrosion resistance, the substrate to be coated is preferably a metal substrate or a composite substrate made of a metal and another material.

[0087] The substrate may have various shapes, for example, a two-dimensional substrate such as a film, sheet, or plate, or a three-dimensional substrate that is a complex three-dimensional object, etc. The surface of the substrate may be smooth or may have irregularities.

[0088] The surface of the substrate may be subjected to pretreatment such as degreasing, chemical conversion treatment, polishing, etc., painting with a sealer, primer, putty, zinc-rich paint, etc., plating, metal spraying, etc.

[0089] The substrate may have an old coating film on its surface. The old coating film may cover a part or the whole of the substrate surface. In the present invention, the old coating film means a coating film that is already present on the substrate when the coating composition of the present invention is applied.

[0090] The substrate to be coated with the coating composition of the present invention is particularly preferably a metal substrate used in vehicle production machinery such as vehicles, mainly railway vehicles, construction machinery, for example, excavators, industrial machinery, and steel building fixtures.

[0091] Next, the method for forming the multi-layer coating film of the present invention will be described.

[0092] The method for forming a multilayer coating film of the present invention comprises the steps of applying the coating composition of the present invention described above on at least a substrate to form an undercoat layer; and applying a topcoat paint composition onto the undercoat layer to form a topcoat layer.

[0093] In this specification, the undercoat layer may be referred to as "undercoat layer (I)" and the topcoat layer may be referred to as "topcoat layer (II)".

[0094] In the method for forming a multilayer coating film of the present invention, the substrate and the coating composition of the present invention are as described in the description of the coating composition of the present invention.

[0095] The undercoat layer may be one layer or two or more layers. The undercoat layer has a thickness of preferably 20 to 150 μm, more preferably 25 to 100 μm. When the undercoat layer is made up of multiple layers, the numerical range described here refers to the total thickness of the undercoat layer.

[0096] In the step of forming the undercoat layer, the coating composition of the present invention is applied as an undercoat coating on at least the substrate. Here, when there are defects such as depressions on the surface of the substrate, putty is often used to fill the defects. Putty is a build-up coating used to improve the smoothness of the coating system, and is usually made of an organic resin such as polyester. In this case, the coating composition of the present invention is applied not only to the substrate but also to the surface coated with the organic resin.

[0097] In one embodiment of the method for forming a multilayer coating film of the present invention, the step of forming an undercoat layer is a step of applying the coating composition of the present invention on the substrate and the putty layer to form an undercoat layer. For example, putty is filled into defects such as depressions on the substrate surface to form a putty layer, and the coating composition of the present invention is applied onto the substrate and the putty layer to form an undercoat layer, or the coating composition of the present invention is applied onto the substrate to form a first undercoat layer, and then putty is filled into defects such as depressions on the substrate surface to form a putty layer (wherein the first undercoat layer is present between the substrate surface and the putty layer), and then the coating composition of the present invention is applied onto the first undercoat layer and the putty layer to form a second undercoat layer. It is preferable to perform a polishing treatment on the surface to be coated before applying the coating composition of the present invention to form the second undercoat layer. The coating composition of the present invention has excellent adhesion to both metal surfaces and surfaces coated with organic resins, and is a coating composition capable of forming a coating film with excellent water resistance and corrosion resistance, and is therefore suitable as an undercoat coating used in such an embodiment.

[0098] The topcoat paint composition refers to a paint used as a topcoat when painting by applying multiple coats of paint. The topcoat paint is sometimes called a finishing paint or an overcoat.

[0099] The topcoat paint composition used in the method for forming the multilayer coating film of the present invention is preferably a water-based paint composition, similar to the paint composition of the present invention. The topcoat paint composition can be appropriately blended with resins, pigments, solvents, matting agents, flexibility imparting agents, surface conditioners, wetting agents, pigment dispersants, emulsifiers, viscosity modifiers, anti-settling agents, anti-skinning agents, anti-sagging agents, defoamers, color separation inhibitors, leveling agents, drying agents, plasticizers, film-forming assistants, neutralizing agents, antifungal agents, antibacterial agents, insecticides, light stabilizers, ultraviolet absorbers, electrostatic lubricants, antistatic agents and electroconductive agents according to the purpose.

[0100] In the method for forming a multilayer coating film of the present invention, the topcoat paint composition preferably contains an acrylic-modified epoxy resin. This can improve the adhesion between the undercoat layer and the topcoat layer. Here, the acrylic-modified epoxy resin is as described in the description of the paint composition of the present invention.

[0101] The amount of the acrylic-modified epoxy resin in the solid content of the topcoat paint composition is preferably less than the amount of the acrylic-modified epoxy resin in the solid content of the paint composition of the present invention as an undercoat paint. In the topcoat paint composition, the amount of the acrylic-modified epoxy resin in the solid content is preferably 5.0 to 30 mass %, more preferably 10 to 20 mass %.

[0102] The topcoat paint composition preferably contains at least one resin selected from an acrylic silicone resin and a urethane resin, and more preferably contains an acrylic-modified epoxy resin, an acrylic silicone resin, and a urethane resin. By containing an acrylic silicone resin and a urethane resin, a coating film having excellent weather resistance can be formed.

[0103] In the topcoat paint composition, the total amount of resins in the solid content is preferably 60 to 100 mass %, more preferably 70 to 95 mass %.

[0104] The topcoat paint composition preferably does not contain a pigment or the total amount of pigments in the solid content is 20 mass % or less. In particular, the topcoat paint composition generally does not contain an anti-rust pigment.

[0105] The topcoat paint composition preferably has a pigment to resin mass ratio of 0.50 or less, more preferably 0.10 to 0.30. The topcoat paint composition preferably has a pigment to resin mass ratio lower than that of the undercoat paint composition, more preferably 0.1 to 0.5 times that of the undercoat paint composition. If the topcoat paint composition has a pigment to resin mass ratio within the above specified range, a coating film having excellent weather resistance and appearance is likely to be obtained.

[0106] The overcoat layer may be one layer or two or more layers. The overcoat layer preferably has a thickness of 10 to 100 μm, more preferably 20 to 60 μm. When the overcoat layer is made up of multiple layers, the numerical range described here refers to the total thickness of the overcoat layers. EXAMPLES

[0107] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0108] The following materials were used to prepare coating compositions (undercoat coating compositions) according to the formulations shown in Tables 1 and 2. Styrene acrylic resin 1 was also used to prepare the topcoat coating composition. (1) Acrylic modified epoxy resin (A) Acrylic modified epoxy resin 1 (product name: MODEPIX 301, manufactured by Arakawa Chemical Industries, Ltd., this product is an aqueous dispersion of acrylic modified epoxy resin, glass transition temperature: 70°C, non-volatile content: 33% by mass) Acrylic modified epoxy resin 2 (product name: MODEPIX 303, manufactured by Arakawa Chemical Industries, Ltd., this product is an aqueous dispersion of acrylic modified epoxy resin, glass transition temperature: 70°C, non-volatile content: 33% by mass) Acrylic modified epoxy resin 3 (product name: MODEPIX 305, manufactured by Arakawa Chemical Industries, Ltd., this product is an aqueous dispersion of acrylic modified epoxy resin, glass transition temperature: 58°C, non-volatile content: 39% by mass) (2) Styrene acrylic resin (B) Styrene-acrylic resin 1 (a styrene-acrylic resin emulsion, a preparation example of which will be described later, glass transition temperature: 20°C, non-volatile content: 48% by mass) Styrene-acrylic resin 2 (a styrene-acrylic resin emulsion, a preparation example of which will be described later, glass transition temperature: 32°C, non-volatile content: 46% by mass) (3) Other resins Epoxy ester resin 1 (the product is an aqueous dispersion of epoxy ester resin, product name: Watersol EFD-5580, manufactured by DIC Corporation, non-volatile content: 40% by mass) (4) Pigment (C) Color pigment (product name: TITONS R-5N, manufactured by Sakai Chemical Industry Co., Ltd., titanium oxide) · Extender pigment 1 (Product name: Precipitated barium sulfate #100, manufactured by Sakai Chemical Industry Co., Ltd., Precipitated barium sulfate) · Extender pigment 2 (product name: Talc DN-2, manufactured by Fuji Talc Co., Ltd., talc) Anti-rust pigment 1 (Product name: Zinc oxide type 2, Sakai Chemical Industry Co., Ltd., zinc oxide-based anti-rust pigment) Anti-rust pigment 2 (Product name: K-WHITE #140W, manufactured by Sakai Chemical Industry Co., Ltd., zinc phosphate-based anti-rust pigment) (5) Additives Pigment dispersant (polymeric pigment dispersant, non-volatile content: 100% by mass) Anti-sagging agent (acrylic thickener, non-volatile content: 30% by weight) Defoamer (mineral oil-based defoamer, non-volatile content: 100% by mass) Film-forming aid (dipropylene glycol n-butyl ether)

[0109] <Preparation example of styrene acrylic resin 1> Into a reactor equipped with a stirrer, a thermometer, a reflux condenser, a dropping device, and a nitrogen inlet tube, 25.92 parts by mass of ion-exchanged water and 0.3 parts by mass of polyoxyethylene-1-(allyloxymethyl)alkyl ether ammonium sulfate (Aqualon KH-10, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were charged, and while the atmosphere inside the reactor was replaced with nitrogen, the temperature was raised to 80°C, and then 0.13 parts by mass of potassium persulfate (polymerization initiator) was added. Subsequently, a mixture of 0.15 parts by mass of 2,4-diphenyl-4-methyl-1-pentene (NOF Corp.; Nofumer MSD), 14.0 parts by mass of styrene, 14.0 parts by mass of methyl methacrylate, 19.2 parts by mass of n-butyl acrylate, 0.5 parts by mass of methacrylic acid, 25 parts by mass of ion-exchanged water, and 0.5 parts by mass of polyoxyethylene-1-(allyloxymethyl) alkyl ether ammonium sulfate (Dai-ichi Kogyo Seiyaku Co., Ltd.; Aqualon KH-10), which had been previously stirred and mixed in a separate container, was added dropwise over 3 hours. After the dropwise addition was completed, the mixture was kept at 80°C for another 2 hours, then cooled to 40°C, and the non-volatile content was adjusted to 48% by mass and pH 9.0 to obtain a styrene-acrylic resin emulsion (styrene-acrylic resin 1). The glass transition temperature of the resin contained in the styrene-acrylic resin 1 obtained above was 20°C.

[0110] <Preparation example of styrene acrylic resin 2> A flask equipped with a stirrer, a reflux condenser, a thermometer, a dropping device, and a nitrogen inlet tube was charged with 31.0 parts by mass of ion-exchanged water, 0.5 parts by mass of α-sulfonato-ω-(1-(allyloxymethyl)-alkyloxypolyoxyethylene)ammonium salt (Aqualon KH10, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and 0.2 parts by mass of polyoxyalkylene alkyl ether (Noigen ET-170, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.). The flask was heated to 80° C. while replacing the atmosphere with nitrogen, and then ammonium persulfate was added. A pre-emulsion for dropping was prepared by adding 9.0 parts by mass of styrene, 10.5 parts by mass of methyl methacrylate, 2.0 parts by mass of butyl acrylate, 0.5 parts by mass of an 80% aqueous solution of acrylic acid, 11.0 parts by mass of ion-exchanged water, and 0.5 parts by mass of α-sulfonato-ω-(1-(allyloxymethyl)-alkyloxypolyoxyethylene)ammonium salt (Aqualon KH10, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), and this pre-emulsion was added dropwise uniformly into the flask over a period of 120 minutes. After the dropwise addition was completed, the contents of the flask were maintained at 80°C for 60 minutes. Subsequently, while maintaining the temperature in the flask at 80°C, a pre-emulsion for dropping consisting of monomers of 6.0 parts by mass of styrene, 12.5 parts by mass of butyl acrylate, 2.5 parts by mass of methyl methacrylate, 0.5 parts by mass of an 80% by mass aqueous solution of acrylic acid, 1.0 parts by mass of diacetone acrylamide, 11.0 parts by mass of ion-exchanged water, and 0.5 parts by mass of α-sulfonato-ω-(1-(allyloxymethyl)-alkyloxypolyoxyethylene)ammonium salt (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.; Aqualon KH10) was prepared and dropped uniformly into the flask over 120 minutes. After the dropwise addition was completed, the mixture was further aged for 120 minutes while continuing to stir. The mixture was cooled to room temperature, adjusted to 46% by mass of nonvolatile matter and pH 9.0, and a core-shell type styrene acrylic resin emulsion (styrene acrylic resin 2) with an acid value of 7.7 mg / KOH was obtained. The glass transition temperature of the resin contained in the styrene-acrylic resin 2 obtained above was 31.5°C.

[0111] <Measurement and evaluation method> (1) How to prepare a painted board (1-1) How to prepare the test plate (metal surface) The coating composition was spray-coated on a polished mild steel plate (SPCC-SB, 150 x 70 x 0.8 mm) degreased with xylene so that the dry film thickness was 45 μm, and then dried to form a coating film. The coating film was dried at room temperature for one week to obtain the target test plate. Since the test plate had a coating film on the metal surface, it is hereinafter referred to as the test plate (metal surface). The obtained test plate (metal surface) was subjected to an evaluation of adhesion to a metal substrate. In addition, a coating film was formed in the same manner, and the back side of the obtained test plate (metal surface) was sealed with the same paint, and the test plate was dried at room temperature for one week, and then the obtained test plate (metal surface) was subjected to a water resistance test and a corrosion resistance test. (1-2) How to prepare the test panel (putty surface) A polyester putty (product name: Polybest Putty #20AF #1, manufactured by Dai Nippon Toryo Co., Ltd.) was brush-painted on a polished mild steel plate (SPCC-SB, 150 x 70 x 0.8 mm) degreased with xylene, the surface was polished to 30 μm, and dried. The paint composition was then spray-painted to a dry film thickness of 45 μm and dried to form a coating film. The coating film was dried at room temperature for one week to obtain the intended test plate. Since the test plate had a coating film on the putty surface, it is hereinafter referred to as the test plate (putty surface). The obtained test plate (putty surface) was used to evaluate adhesion to the putty surface. (2) Adhesion to metal substrates The surface of the test plate (metal surface) obtained by the above-mentioned coating plate preparation method was cut with a cutter to create 100 1 mm square grids, and adhesive tape (cellophane tape) was then applied on top of the grid, after which a test was conducted in which the tape was forcibly peeled off, and the results were evaluated according to the following criteria. ⊚: No peeling of the coating was observed. ○: Peeling was observed on the edges of the grid, but there was no problem with adhesion. ×: Peeling of the coating film from the substrate was observed. (3) Adhesion to putty surface The surface of the test plate (putty surface) obtained by the above-mentioned coating plate preparation method was cut with a cutter to create 100 1 mm square grids, and adhesive tape (cellophane tape) was then applied on top of the grid, after which a test was conducted in which the tape was forcibly peeled off, and the results were evaluated according to the following criteria. ⊚: No peeling of the coating film was observed, or slight peeling was observed only on the edges of the grid. ○: Peeling was observed on the edges of the grid, but there was no problem with adhesion. ×: Peeling of the coating film from the substrate was observed. (4) Water resistance The test plate (metal surface) obtained by the above-mentioned coating plate preparation method was immersed in water at 23°C for 120 hours, and the appearance and adhesion of the coating film were evaluated immediately after wiping off the water droplets on the surface. The evaluation of adhesion here was performed according to the above-mentioned (2) Adhesion to metal substrates. ⊚: There is no blistering of the coating film, and in the evaluation of adhesion, no peeling of the coating film is observed at all. ◯: The coating film is not blistered, and peeling is observed at the edges of the grid, but there is no problem with adhesion. ×: The coating film is blistered, or peeling of the coating film from the substrate is observed in the adhesion evaluation. (5) Corrosion resistance The test panels (metal surface) obtained by the above-mentioned coating method were exposed to salt spray for 240 hours in accordance with the neutral salt spray resistance of JIS K5600-7-1:1999, and the degree of rust, blisters, etc. that occurred on the test panels was evaluated according to the following criteria. ⊚: No abnormalities on the coating surface, and the coating appearance is excellent. ○: Rust, blisters, and other abnormalities are observed on some parts of the coating surface. ×: Rust, blisters, and other abnormalities are observed over the entire coating surface.

[0112] <Examples and Comparative Examples> Coating compositions were prepared and coating film performance was evaluated according to the formulations shown in Tables 1 and 2. The results are shown in Tables 1 and 2. The values ​​of each component in the formulations shown in the tables are expressed in parts by mass.

[0113] [Table 1]

[0114] [Table 2]

[0115] In Tables 1 and 2, "Amount of solids in paint (mass %)" indicates the amount of solids (mass %) contained in the paint composition. "Mass ratio of component (A) / component (B)" refers to the mass ratio (A / B) of the acrylic-modified epoxy resin (A) to the styrene-acrylic resin (B) contained in the coating composition. "Pigment / resin mass ratio" refers to the mass ratio of pigment to resin contained in the coating composition.

[0116] Next, the coating performance of the multi-layered film was evaluated. The multi-layered film was a multi-layered film having an undercoat coating film formed from Coating Composition 3, 7, or 8, and a topcoat coating film formed from Topcoat Paint Composition 1 or 2 described below on the undercoat coating film.

[0117] <Preparation of topcoat paint composition 1> In a container, 20 parts by mass of acrylic modified epoxy resin 4 (product name: MODEPIX 307, manufactured by Arakawa Chemical Industries, Ltd.; this product is an aqueous dispersion of acrylic modified epoxy resin; non-volatile content: 33% by mass), 38 parts by mass of styrene acrylic resin 1 (non-volatile content: 48% by mass), 25 parts by mass of acrylic silicone resin 1 (product name: POLYDUREX G613, manufactured by Asahi Kasei Chemicals Corporation; this product is an acrylic silicone resin emulsion; non-volatile content: 41.5% by mass), and urethane resin 1 (product name: RACCOAT) were added. Topcoat paint composition 1 (non-volatile content: 42.5% by mass) was prepared by a known manufacturing method using 2.0 parts by mass of WU-412 (manufactured by Seiko Chemical Corporation, this product is a urethane resin emulsion, non-volatile content: 33% by mass), 5.0 parts by mass of titanium oxide, 1.0 part by mass of inorganic yellow pigment, 1.0 part by mass of anti-settling agent, 0.5 parts by mass of defoamer, 0.5 parts by mass of surface conditioner, 2.0 parts by mass of film-forming aid, and 5.0 parts by mass of water.

[0118] <Preparation of topcoat paint composition 2> Topcoat paint composition 2 (non-volatile content: 42.5% by mass) was prepared using the same manufacturing method as topcoat paint composition 1, except that acrylic-modified epoxy resin 4 was not used and the styrene-acrylic resin 1 was changed to 51.75 parts by mass and the water to 11.25 parts by mass.

[0119] <Measurement and evaluation methods for multi-layered films> (1) How to prepare a painted board (1-1) How to prepare the test plate (metal surface) A polished mild steel plate (SPCC-SB, 150×70×0.8 mm) degreased with xylene was spray-coated with the undercoat paint composition to a dry film thickness of 45 μm, and dried to form an undercoat paint film. The test plate on which the undercoat paint film was formed was spray-coated with the topcoat paint composition prepared by the above method to a dry film thickness of 35 μm to form a topcoat paint film. The resulting two-layered paint film was dried at room temperature for one week to obtain the intended test plate. Since the test plate had a paint film on the metal surface, it is hereinafter referred to as the test plate (metal surface). The resulting test plate (metal surface) was subjected to an evaluation of adhesion to a metal substrate. In addition, two layers of paint film were formed in the same manner, and the back surface of the obtained test plate (metal surface) was sealed with the same paint as the paint used as the undercoat paint, and after drying at room temperature for one week, the obtained test plate (metal surface) was subjected to a water resistance test, a corrosion resistance test, and a weather resistance test. (1-2) How to prepare the test panel (putty surface) A polyester putty (trade name: Polybest Putty #20AF #1, manufactured by Dai Nippon Toryo Co., Ltd.) was applied by brushing to a polished mild steel plate (SPCC-SB, 150 x 70 x 0.8 mm) degreased with xylene, the surface was polished to 30 μm, and then dried. The undercoat paint composition was spray-applied to a dry film thickness of 45 μm and dried to form an undercoat paint film. The topcoat paint composition prepared by the above method was spray-applied to the test plate on which the undercoat paint film was formed to a dry film thickness of 35 μm to form a topcoat paint film. The obtained two-layer paint film was dried at room temperature for one week to obtain the desired test plate. Since the test plate had a paint film on the putty surface, it is referred to as the test plate (putty surface) below. The obtained test plate (putty surface) was used for evaluation of adhesion to the putty surface. (2) Adhesion to metal substrates The surface of the test plate (metal surface) obtained by the above-mentioned coating plate preparation method was cut with a cutter to create 100 2 mm square grids, and adhesive tape (cellophane tape) was then applied on top of the grid, after which a test was conducted in which the tape was forcibly peeled off, and the results were evaluated according to the following criteria. ⊚: No peeling of the coating was observed. ○: Peeling was observed on the edges of the grid, but there was no problem with adhesion. ×: Peeling of the coating film from the substrate was observed. (3) Adhesion to putty surface The surface of the test plate (putty surface) obtained by the above-mentioned coating plate preparation method was cut with a cutter to create 100 2 mm square grids, and adhesive tape (cellophane tape) was then applied on top of the grid, followed by a forced peel-off test, which was evaluated according to the following criteria. ⊚: No peeling of the coating film was observed, or slight peeling was observed only on the edges of the grid. ○: Peeling was observed on the edges of the grid, but there was no problem with adhesion. ×: Peeling of the coating film from the substrate was observed. (4) Water resistance The test plate (metal surface) obtained by the above-mentioned coating plate preparation method was immersed in water at 23° C. for 120 hours, and the appearance and adhesion of the coating film were evaluated immediately after wiping off the water droplets on the surface. ⊚: There is no blistering of the coating film, and in the evaluation of adhesion, no peeling of the coating film is observed at all. ◯: The coating film is not blistered, and peeling is observed at the edges of the grid, but there is no problem with adhesion. ×: The coating film is blistered or peeled off from the substrate. (5) Corrosion resistance The test panels (metal surface) obtained by the above-mentioned coating method were exposed to salt spray for 240 hours in accordance with the neutral salt spray resistance of JIS K5600-7-1:1999, and the degree of rust, blisters, etc. that occurred on the test panels was evaluated according to the following criteria. ⊚: No abnormalities on the coating surface, and the coating appearance is excellent. ○: Rust, blisters, and other abnormalities are observed on some parts of the coating surface. ×: Rust, blisters, and other abnormalities are observed over the entire coating surface. (6) Weather resistance The test plate (metal surface) obtained by the above-mentioned coating plate preparation method was tested according to cycle A of JIS K 5600-7-7 coating film long-term durability, accelerated weather resistance, and accelerated light resistance (xenon lamp method), and the gloss retention and appearance of the coating film were judged visually. The gloss retention was calculated by measuring the 60° specular gloss of the coating film before and after the test using a gloss meter (BYK Gardner Micro-Gloss) manufactured by BYK Gardner, and using the following formula. *Gloss retention rate (%) = Specular gloss of coating film after test (60°) / Specular gloss of coating film before test (60°) x 100 ⊚: Gloss retention after 2000 hours of testing was 80% or more. ◯: The gloss retention rate after a test time of 500 hours was 80% or more, and the gloss retention rate after a test time of 2000 hours was less than 80%. ×: Gloss retention after 500 hours of testing was less than 80%.

[0120] <Examples and Comparative Examples> A multi-layer film was formed using the undercoat paint composition and the topcoat paint composition according to the paint combination shown in Table 3, and the coating film performance of the multi-layer film was evaluated. The results are shown in Table 3.

[0121] [Table 3]

[0122] In Table 3, for the undercoat paint, the "mass ratio of component (A) / component (B)" indicates the mass ratio (A / B) of the acrylic-modified epoxy resin (A) and the styrene acrylic resin (B) contained in the undercoat paint composition, and the "mass ratio of pigment / resin" indicates the mass ratio of the pigment to the resin contained in the undercoat paint composition. For the topcoat paint, "content of acrylic-modified epoxy resin (mass %)" indicates the amount (mass %) of acrylic-modified epoxy resin contained in the solid matter of the topcoat paint composition, and "mass ratio of pigment / resin" indicates the mass ratio of pigment to resin contained in the topcoat paint composition.

Claims

1. A water-based undercoat paint composition comprising water, a resin, and a pigment, a mass ratio of the pigment to the resin is 0.30 to 0.90; 1. A water-based undercoat paint composition, characterized in that the resin comprises at least an acrylic-modified epoxy resin (A) and a styrene-acrylic resin (B).

2. The water-based undercoat paint composition according to claim 1, characterized in that the mass ratio (A):(B) of the acrylic-modified epoxy resin (A) to the styrene-acrylic resin (B) is 40:60 to 80:

20.

3. 2. The water-based undercoat paint composition according to claim 1, wherein the acrylic-modified epoxy resin (A) has a glass transition point (Tg) of 60 to 90° C.

4. A step of applying the aqueous undercoat paint composition according to any one of claims 1 to 3 onto at least a substrate to form an undercoat layer (I); and applying a topcoat paint composition on the undercoat layer (I) to form a topcoat layer (II).

5. 5. The method for forming a multi-layer coating film according to claim 4, wherein the topcoat paint composition contains an acrylic-modified epoxy resin.

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