Aqueous coating composition, coating film, and method for producing coating film

Abstract

A challenge of the present disclosure is to provide an aqueous coating composition which is superior in storage stability even in a one-pack type and can form a coating film having good processability such as bending and good crack resistance and good scratch resistance during processing. The aqueous coating composition of the present disclosure contains a coating film-forming resin (A), a crosslinking agent (B), a sulfonic acid compound (C), and an amine compound (D), wherein the coating film-forming resin (A) comprises an acrylic resin (A1), the coating film-forming resin (A) has a hydroxyl value of 5 mg KOH / g or more and 35 mg KOH / g or less; the crosslinking agent (B) comprises a fully-alkylated melamine resin (B1); and a molar neutralization ratio of an acid group of the sulfonic acid compound (C) by the amine compound (D) is 100% or more and 1,300% or less.

Description

Water-based coating composition, coating film, and method for producing coating film

[0001] The present disclosure relates to an aqueous coating composition, a coating film, and a method for producing the coating film.

[0002] Coated steel sheets, which are prepared by applying a coating to a metal substrate such as a cold-rolled steel sheet or a plated steel sheet and then subjecting it to forming processing, are also called precoated steel sheets (hereinafter also referred to as "PCMs"), and are used for applications such as architectural components such as shutters, ceiling doors, doors, roofs, and siding; exterior materials for electrical equipment such as outdoor air conditioner units; and interior materials. The precoated steel sheets are typically produced by applying a coating composition to the surface of the metal substrate and then heating (baking) the coated steel sheet at 200 to 270°C for 30 to 60 seconds to form a coating film, which is then subjected to forming processing. For this reason, the coating film of the precoated steel sheet is required to have sufficient workability to prevent cracking or peeling during processing, and sufficient hardness to prevent scratches or dents.

[0003] Coating compositions include one-component coating compositions in which a base agent containing a film-forming resin and a curing agent containing a crosslinking agent coexist in the same system, and two-component coating compositions in which the base agent and curing agent are stored separately and mixed at the time of use. Of these, two-component coating compositions have superior storage stability compared to one-component coating compositions because the base agent and curing agent are not mixed until just before use. However, two-component coating compositions have problems with handling and painting workability, such as the need to mix the base agent and curing agent in a specified ratio and stir until homogeneous when used, and there are limitations on the time they can be used, so one-component coating compositions are in demand.

[0004] In recent years, awareness of reducing environmental impact has increased, leading to a demand for replacement with environmentally friendly products. In the field of paints, for example, there is a demand for reducing the amount of volatile organic compounds (VOCs) used, and the use of aqueous paint compositions can meet this demand. In other words, there is a growing need in the market for one-component aqueous paint compositions.

[0005] Various one-component aqueous coating compositions have been proposed, including, for example, a proposal to neutralize the carboxyl groups in an aqueous coating composition comprising a hydroxyl- and carboxyl-containing acrylic copolymer, an aqueous amino resin, an amine compound, and a hydrophilic organic solvent with an amine compound (Patent Document 1). Also proposed is a proposal to neutralize the carboxyl groups in an acrylic water-soluble coating composition comprising a copolymer of a hydroxyl- and carboxyl-containing (meth)acrylic ester, a carboxyl-containing vinyl monomer, a long-chain alkyl group-containing (meth)acrylic ester, and a vinyl monomer, a water-soluble amino resin, and an aqueous medium with an amine compound (Patent Document 2). Furthermore, it has been proposed to neutralize the carboxyl groups of the hydroxyl- and carboxyl-containing acrylic resin with a basic compound in an aqueous coating composition for metal coating, which contains a hydroxyl- and carboxyl-containing acrylic resin having a glass transition point in the range of -10°C to 80°C, a hydroxyl- and carboxyl-containing acrylic resin having a glass transition point in the range of -50 to 20°C, an aqueous amino resin, a basic compound, and an aqueous medium (Patent Document 3). Also, Patent Document 4 proposes an aqueous coating composition containing an aqueous resin, a melamine resin, and a phosphate ester catalyst as a weak acid catalyst, and proposes neutralizing the aqueous resin with a basic compound (Patent Document 4).

[0006] JP 2001-323207 A JP 2001-240624 A JP 2000-17225 A JP 2015-174958 A

[0007] However, the coating films formed using the coating compositions described in Patent Documents 1 to 4 did not provide fully satisfactory workability (adhesion, crack resistance) or scratch resistance of the resulting precoated steel sheets.

[0008] Furthermore, in one-component coating compositions, the base agent and curing agent coexist in the same system, so there is a trade-off relationship in which increasing the reactivity of the base agent and curing agent with the aim of improving the coating film physical properties reduces the storage stability of the coating composition, and conversely, decreasing the reactivity with the aim of improving the coating composition storage stability reduces the coating film physical properties. For this reason, it has been very difficult to achieve both storage stability and coating film physical properties in one-component coating compositions.

[0009] The present inventors have conducted extensive research to solve these problems and have found that by using a full-alkyl melamine resin as a crosslinking agent, and further using a sulfonic acid compound and an amine compound so as to achieve a specific neutralization rate, it is possible to achieve high storage stability even in a one-component composition, and further, to exhibit good coating film properties (particularly processability (adhesion, crack resistance), and scratch resistance) even when coating is performed under the high temperature and short time conditions typical of pre-coated steel sheets, and have completed the aqueous coating composition and coating film manufacturing method according to the present disclosure.

[0010] An object of the present disclosure is to provide an aqueous coating composition that is a one-component type and yet has excellent storage stability, and is capable of forming a coating film that has good workability, such as bending, and good resistance to cracking and scratches during processing.

[0011] The present disclosure provides the following aspects. [1] An aqueous coating composition comprising a film-forming resin (A), a crosslinking agent (B), a sulfonic acid compound (C), and an amine compound (D), wherein the film-forming resin (A) comprises an acrylic resin (A1), the film-forming resin (A) has a hydroxyl value of 5 mgKOH / g or more and 35 mgKOH / g or less, the crosslinking agent (B) comprises a full-alkyl melamine resin (B1), and the molar neutralization rate of the acid groups of the sulfonic acid compound (C) by the amine compound (D) is 100% or more and 1,300% or less. [2] The aqueous coating composition according to [1], wherein the film-forming resin (A) has a weight-average molecular weight of 100,000 or more. [3] The aqueous coating composition according to [1], wherein the film-forming resin (A) has a weight-average molecular weight of 100,000 or more. [4] The aqueous coating composition according to [1], wherein the film-forming resin (A) has a weight-average molecular weight of 100,000 or more. [5] The aqueous coating composition according to [1], wherein the film-forming resin (A) has a weight-average molecular weight of 100,000 or more. [6] The aqueous coating composition according to [1], wherein the film-forming resin (A) has a weight-average molecular weight of 100,000 or more. [7] The aqueous coating composition according to [1], wherein the film-forming resin (A) has a weight-average molecular weight of 100,000 or more. [8] The aqueous coating composition according to [1], wherein the film-forming resin (A) has a weight-average molecular weight of 100,000 or more. [9] The aqueous coating composition according to [1], wherein the film-forming resin (A) has a weight-average molecular weight of 100,000 or more.

[10] The －１ The shear viscosity measured at a shear rate of 10 s is 30,000 mPa·s or less. －１The shear viscosity measured at a shear rate of 1,000 s is 800 mPa·s or less. －１

[0014] The aqueous coating composition according to [1] or [2], which has a shear viscosity of 150 mPa s or more as measured by a shear viscosity test. [4] The aqueous coating composition according to any one of [1] to [3], further comprising an organic solvent (E1). [5] The aqueous coating composition according to any one of [1] to [4], which is for use in coil coating. [6] A method for producing a coating film, comprising the steps of: applying the aqueous coating composition according to any one of [1] to [5] to a substrate to form a coating film; and drying and / or curing the coating film under conditions where the maximum temperature reached is 180°C or more and the drying and / or curing time is 120 seconds or less.

[0012] According to the present disclosure, there is provided an aqueous coating composition that is a one-component type and has excellent storage stability, and is capable of forming a coating film that has good workability such as bending, and good resistance to cracking and scratches during processing.

[0013] The aqueous coating composition of the present disclosure comprises a film-forming resin (A), a crosslinking agent (B), a sulfonic acid compound (C), and an amine compound (D).

[0014] <Film-forming resin (A)> The film-forming resin (A) includes an acrylic resin (A1). The acrylic resin (A1) represents a polymer having units derived from a monomer having a (meth)acryloyl group, and can be prepared by polymerizing a monomer mixture containing a monomer having an ethylenically unsaturated bond. In this specification, (meth)acrylic acid represents acrylic acid or methacrylic acid.

[0015] Examples of the monomer having an ethylenically unsaturated bond include unsaturated carboxylic acids such as (meth)acrylic acid, crotonic acid, isocrotonic acid, 2-propenoic acid, ethacrylic acid, propylacrylic acid, and isopropylacrylic acid; unsaturated polycarboxylic acids such as maleic acid, fumaric acid, and itaconic acid (including their anhydrides); monoalkyl esters of unsaturated polycarboxylic acids such as ethyl maleate, butyl maleate, ethyl fumarate, butyl fumarate, ethyl itaconate, and butyl itaconate; methyl (meth)acrylate, ethyl (meth)acrylate, and p) Propyl acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, n-pentyl (meth)acrylate, neopentyl (meth)acrylate, isopentyl (meth)acrylate, sec-pentyl (meth)acrylate, 3-pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate (meth)acrylic acid alkyl esters such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, tricyclodecyl (meth)acrylate, adamantyl (meth)acrylate, etc.; (meth)acrylic acid esters having an alicyclic hydrocarbon group such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, etc. (meth)acrylic acid esters having a hydroxyl group, such as lactone adducts thereof (the lactone being ε-caprolactone, etc.); monomers having an organosilyl group, such as γ-(meth)acryloxypropyltrimethoxysilane, γ-(meth)acryloxypropylmethyldimethoxysilane, γ-(meth)acryloxypropyltriethoxysilane, γ-(meth)acryloxypropylmethyldiethoxysilane, vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyltriethoxysilane, and vinylmethyldithoxysilane;Monomers having a sulfonic acid group such as α-vinylbenzenesulfonic acid, p-(meth)acrylamidopropanesulfonic acid, t-butyl(meth)acrylamidosulfonic acid, etc.; monomers having a phosphate group such as the above-mentioned (meth)acrylic acid ester phosphoric acid monoesters having a hydroxyl group; (meth)acrylamide monomers such as (meth)acrylamide, N-methylol(meth)acrylamide, methoxybutyl(meth)acrylamide, diacetone(meth)acrylamide, etc.; aminoethyl(meth)acrylamide, dimethylaminoethyl(meth)acrylamide, methylaminopropyl(meth)acrylamide, etc. Examples of such monomers include (meth)acrylamide monomers having an amino group such as acrylamide; (meth)acrylic acid esters having an epoxy group (oxiranyl group) such as glycidyl (meth)acrylate; (meth)acrylonitrile monomers such as (meth)acrylonitrile and α-chloro(meth)acrylonitrile; vinyl carboxylate esters such as vinyl acetate and vinyl propionate; styrene-based monomers such as styrene, α-methylstyrene, α-methylstyrene dimer, vinyltoluene, and divinylbenzene; carbonyl group monomers; and crosslinkable monomers such as polyfunctional vinyl monomers other than those mentioned above. The monomers having an ethylenically unsaturated bond may be used alone or in combination of two or more.

[0016] The acrylic resin (A1) has a hydroxyl group. The hydroxyl group in the acrylic resin (A1) allows a crosslinking reaction to occur between the hydroxyl group and a reactive group of a crosslinking agent, thereby curing a coating film. In order for the acrylic resin (A1) to have a hydroxyl group, a (meth)acrylic acid ester having a hydroxyl group may be used as the monomer having an ethylenically unsaturated bond when polymerizing the acrylic resin (A1).

[0017] The hydroxyl value of the hydroxyl-containing acrylic resin (A1) is preferably 5 mgKOH / g or more, more preferably 7 mgKOH / g or more, even more preferably 10 mgKOH / g or more, and is preferably 50 mgKOH / g or less, more preferably 35 mgKOH / g or less, even more preferably 30 mgKOH / g or less, and even more preferably 25 mgKOH / g or less. Having a hydroxyl value within the above range has the advantage of providing a coating film with good processability (adhesion and crack resistance).

[0018] The (meth)acrylic acid ester having a hydroxyl group preferably has 1 to 3 carbon atoms in the group bonded to the (meth)acryloyl group, and more preferably 2. The inclusion of a (meth)acrylic acid ester having a hydroxyl group and having 1 to 3 carbon atoms in the group bonded to the (meth)acryloyl group has the advantage of providing a coating film with excellent scratch resistance. The content of the (meth)acrylic acid ester having a hydroxyl group and having 1 to 3 carbon atoms in the group bonded to the (meth)acryloyl group is preferably 70% by mass or more, more preferably 80% by mass or more, of the (meth)acrylic acid ester having a hydroxyl group, with the upper limit being 100% by mass.

[0019] The weight average molecular weight of the acrylic resin (A1) is, for example, 50,000 or more, preferably 100,000 or more, and more preferably 150,000 or more, and is, for example, 10,000,000 or less, and preferably 2,000,000 or less. The larger the weight average molecular weight of the acrylic resin (A1), the better the scratch resistance and the more advantageously a coating film with excellent processability can be obtained.

[0020] In this specification, the weight average molecular weight is a value determined by gel permeation chromatography (GPC) and converted into polystyrene.

[0021] The acrylic resin (A1) preferably has an acid group. The acrylic resin (A1) having an acid group can impart dispersibility in the aqueous medium (E) described below. In order for the acrylic resin (A1) to have an acid group, when polymerizing the acrylic resin (A1), a monomer having an acid group, such as an unsaturated monocarboxylic acid, an unsaturated polycarboxylic acid, a monoalkyl ester of an unsaturated polycarboxylic acid, a monomer having a sulfonic acid group, or a monomer having a phosphoric acid group, may be used as the monomer having an ethylenically unsaturated bond.

[0022] The monomer having an acid group is preferably an unsaturated monocarboxylic acid, an unsaturated polycarboxylic acid, or a monoalkyl ester of an unsaturated polycarboxylic acid, more preferably an unsaturated monocarboxylic acid or an unsaturated polycarboxylic acid, still more preferably an unsaturated monocarboxylic acid, and particularly preferably (meth)acrylic acid.

[0023] The acid value of the acrylic resin (A1) is preferably 5 mgKOH / g or more, preferably 50 mgKOH / g or less, more preferably 30 mgKOH / g or less. When the acid value is within the above range, there is an advantage that the acrylic resin (A1) can be stably dispersed in the aqueous medium (E).

[0024] In this specification, the acid value and hydroxyl value of the acrylic resin (A1) represent the acid value and hydroxyl value of the solid content, respectively, and can be measured in accordance with JIS K 0070:1999.

[0025] The glass transition temperature (Tg) of the acrylic resin (A1) is preferably −70° C. or higher, more preferably 0° C. or higher, even more preferably 10° C. or higher, and still more preferably 15° C. or higher, and is preferably 95° C. or lower, more preferably 90° C. or lower, even more preferably 85° C. or lower, and still more preferably 80° C. or lower. Having a Tg within the above range has the advantage of providing a coating film that is excellent in coating processability and scratch resistance.

[0026] The glass transition temperature can be calculated as the reciprocal of the sum of the quotients obtained by dividing the mass fraction of each monomer constituting the acrylic resin (A1) by the Tg (K: Kelvin) value of a homopolymer derived from each monomer. More specifically, in this specification, the glass transition temperature (Tg) can be calculated using Fox's formula (T. G. Fox; Bull. Am. Phys. Soc., 1(3), 123 (1956)). For example, when the resin is a polymer of multiple monomers (monomer A, monomer B, ..., monomer N), the Tg of the resin is defined as the Tg represented by the following general formula: 1 / Tg = wa / Tga + wb / Tgb + ... + wn / Tgn. where Tga is the glass transition temperature (K) of a homopolymer of monomer A, wa is the mass fraction of monomer A, Tgb is the glass transition temperature (K) of a homopolymer of monomer B, wb is the mass fraction of monomer B, Tgn is the glass transition temperature (K) of a homopolymer of monomer N, wn is the mass fraction of monomer N, and wa + wb + ... + wn = 1.

[0027] Among the monomers forming (A1) in the acrylic resin, the monomer having an ethylenically unsaturated bond preferably includes a (meth)acrylic acid alkyl ester, and more preferably includes a (meth)acrylic acid alkyl ester having 1 to 6 carbon atoms in the alkyl group, more preferably 1 to 4 carbon atoms. Using a monomer within the above range has the advantage of providing a coating film with excellent scratch resistance. The content of the (meth)acrylic acid alkyl ester having 1 to 6 carbon atoms in the alkyl group is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and preferably 95% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less, in the (meth)acrylic acid alkyl ester.

[0028] From the viewpoint of weather resistance, the content of styrene-based monomers among the monomers contained in the acrylic resin (A1) is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, with the lower limit being 0% by mass.

[0029] When the acrylic resin (A1) has an acid group, the aqueous coating composition may contain a basic compound. By including a basic compound in the aqueous coating composition, some or all of the acid groups are neutralized, thereby imparting good water dispersibility to the acrylic resin. Examples of the basic compound that can be used include ammonia, amine compounds, alkali metals, etc. Furthermore, a portion of the amine compound (D) described below can also be used as the basic compound. Furthermore, known anionic and / or nonionic surfactants can be used to impart water dispersibility to the acrylic resin.

[0030] The content of the acrylic resin (A1) in the coating film-forming resin (A) is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, with the upper limit being 100% by mass.

[0031] The acrylic resin (A1) is preferably an aqueous resin, and may be a water-soluble resin that can be dissolved in the aqueous medium (E), or may be a water-dispersible resin that can be dispersed in the aqueous medium (E), such as a colloidal dispersion type or emulsion type (emulsion polymerization type, forced emulsification type). The acrylic resin (A1) is preferably a water-dispersible resin, more preferably an emulsion-type water-dispersible resin, and particularly preferably an emulsion-type water-dispersible resin obtained by emulsion polymerization. The acrylic resin (A1) can be made into an aqueous resin by having an acid group and / or a hydroxyl group and / or coexisting with an emulsifier.

[0032] When the acrylic resin (A1) is an emulsion-type water-dispersible resin, the average particle size of the emulsion particles is preferably 500 nm or less, more preferably 300 nm or less, and even more preferably 200 nm or less, and may be, for example, 10 nm or more, 30 nm or more, or 50 nm or more. Being within the above range has the advantage of providing good storage stability for the emulsion particles and the coating composition containing the emulsion particles. In this specification, the average particle size is the average particle size determined by dynamic light scattering, and specifically can be measured using an electrophoretic light scattering photometer ELSZ series (manufactured by Otsuka Electronics Co., Ltd.) or the like.

[0033] The minimum film-forming temperature (MFT) of the acrylic resin (A1) is preferably 50°C or higher, more preferably 60°C or higher, and even more preferably 70°C or higher, and may be, for example, 200°C or lower, 150°C or lower, or 120°C or lower. Having the MFT within the above range has the advantage of improving the scratch resistance of the resulting coating film and suppressing blocking between coating films. In this specification, the minimum film-forming temperature means the lowest temperature at which a crack-free, uniform coating film is formed when the emulsion-type water-dispersible resin is dried, and can be measured in accordance with JIS K 6828-2:2003.

[0034] When the acrylic resin (A1) is an emulsion-type water-dispersible resin, the emulsion may be an emulsion in which multilayer structure particles each consisting of a core and a shell are dispersed.

[0035] The multilayer structure particles can be prepared, for example, by the method described in JP-A No. 2002-12816.

[0036] The acrylic resin (A1) can be produced by polymerizing the monomer having an ethylenically unsaturated bond, and the polymerization reaction can be carried out, for example, by heating the monomer having an ethylenically unsaturated bond in part or all of the aqueous medium (E) while stirring. The polymerization reaction is preferably an emulsion polymerization reaction. During the polymerization reaction, it is preferable to allow a polymerization initiator to coexist, and if necessary, it is preferable to allow an emulsifier to coexist. The reaction temperature is preferably, for example, 30 to 100°C, and the reaction time is preferably, for example, 1 to 10 hours.

[0037] The polymerization initiator is preferably a radical polymerization initiator. Examples of water-soluble free radical polymerization initiators that can be used include persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate. Redox initiators that combine an oxidizing agent such as potassium persulfate, sodium persulfate, ammonium persulfate, or hydrogen peroxide with a reducing agent such as sodium bisulfite, sodium thiosulfate, Rongalite, or ascorbic acid can also be used. These radical polymerization initiators may be dissolved in part or all of the aqueous medium (E) and used as an aqueous solution.

[0038] The emulsifier may be an anionic or nonionic emulsifier having a hydrophobic moiety, such as a hydrocarbon group having 6 or more carbon atoms, and a hydrophilic moiety, such as a carboxylate, sulfonate, or sulfate partial ester, in the same molecule. Examples of the anionic emulsifier include alkali metal or ammonium salts of sulfuric acid half esters of alkylphenols or higher alcohols; alkali metal or ammonium salts of alkyl or aryl sulfonates; alkali metal or ammonium salts of sulfuric acid half esters of polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl ethers, or polyoxyethylene allyl ethers; and various anionic reactive emulsifiers having an acrylic, methacrylic, propenyl, allyl, allyl ether, maleic acid, or other group and an ethylenically unsaturated bond. Examples of nonionic emulsifiers include polyoxyalkylene ethers such as polyoxyethylene alkylphenyl ether, polyoxyethylene alkyl ether, and polyoxyethylene allyl ether; and nonionic reactive emulsifiers having an ethylenically unsaturated bond and an acrylic, methacrylic, propenyl, allyl, allyl ether, or maleic acid group.

[0039] Furthermore, during polymerization (preferably emulsion polymerization), the combined use of an auxiliary agent (chain transfer agent) for adjusting molecular weight, such as a mercaptan compound or a lower alcohol, is often preferable from the viewpoint of advancing the polymerization (preferably emulsion polymerization) and promoting smooth and uniform formation of a coating film and improving adhesion to the substrate, and is carried out as appropriate depending on the circumstances.

[0040] When emulsion polymerization is carried out, any emulsion polymerization method can be used, such as a conventional one-stage continuous uniform monomer dropping method, a core-shell polymerization method which is a multi-stage monomer feed method, or a power feed polymerization method in which the monomer composition fed during polymerization is continuously changed.

[0041] The acrylic resin (A1) may be used in the preparation of an aqueous coating composition as an aqueous solution or dispersion containing the acrylic resin (A1) and a portion of the aqueous medium (E) described below. The aqueous solution or dispersion may further contain the emulsifier.

[0042] The acrylic resin (A1) may be a commercially available product, and may be used alone or in combination of two or more types.

[0043] The coating film-forming resin (A) may contain, in addition to the acrylic resin (A1), another resin (A2).

[0044] Examples of the other resin (A2) include hydroxyl group-free acrylic resins, urethane resins, vinyl acetate resins, fluororesins, vinyl chloride resins, etc., each of which is preferably an aqueous resin, more preferably a water-dispersible resin, and even more preferably an emulsion-type water-dispersible resin. The other resin (A2) may be used in the preparation of an aqueous coating composition as an aqueous solution or aqueous dispersion containing the other resin (A2) and a portion of the aqueous medium (E). The aqueous solution or aqueous dispersion may contain an emulsifier.

[0045] The hydroxyl group-free acrylic resin represents a polymer having units derived from a monomer having a (meth)acryloyl group, and can be prepared by polymerizing a mixture of the above-mentioned ethylenically unsaturated bond-containing monomers that do not have a hydroxyl group.

[0046] The weight average molecular weight of the hydroxyl group-free acrylic resin is preferably 50,000 or more, more preferably 100,000 or more, even more preferably 150,000 or more, and is preferably 10,000,000 or less, more preferably 2,000,000 or less, even more preferably 500,000 or less. Having a molecular weight within the above range has the advantage of improving the processability of the resulting coating film.

[0047] The glass transition temperature of the hydroxyl group-free acrylic resin is preferably 80° C. or lower, more preferably 60° C. or lower, and even more preferably 50° C. or lower, and is preferably 20° C. or higher, more preferably 30° C. or higher, and even more preferably 40° C. or higher. Having the glass transition temperature within the above range has the advantage of improving scratch resistance.

[0048] The minimum film-forming temperature (MFT) of the acrylic resin is preferably 50° C. or higher, more preferably 60° C. or higher, and even more preferably 70° C. or higher, and may be, for example, 200° C. or lower, 150° C. or lower, or 120° C. or lower. Having the MFT within the above range has the advantages of improving the scratch resistance of the resulting coating film and suppressing blocking between coating films.

[0049] The hydroxyl group-free acrylic resin preferably has an acid group. The hydroxyl group-free acrylic resin preferably has an acid value of 5 mgKOH / g or more, preferably 50 mgKOH / g or less, more preferably 30 mgKOH / g or less.

[0050] When the acrylic resin not containing a hydroxyl group is contained, the content thereof is preferably 20% by mass or more, more preferably 30% by mass or more, and preferably 90% by mass or less, more preferably 85% by mass or less, of the total of the acrylic resin (A1) and the acrylic resin not containing a hydroxyl group.

[0051] The hydroxyl value of the film-forming resin (A) is at least 5 mgKOH / g, preferably at least 7 mgKOH / g, more preferably at least 10 mgKOH / g, and is at most 35 mgKOH / g, preferably at most 30 mgKOH / g, more preferably at most 25 mgKOH / g. Having a hydroxyl value within this range has the advantage of providing a coating film with good processability and scratch resistance.

[0052] The acid value of the film-forming resin (A) is preferably 5 mgKOH / g or more, preferably 50 mgKOH / g or less, and more preferably 30 mgKOH / g or less, which has the advantage that the film-forming resin (A) can be stably dispersed in the aqueous medium (E).

[0053] The weight average molecular weight of the film-forming resin (A) is, for example, 50,000 or more, preferably 100,000 or more, more preferably 150,000 or more, and is, for example, 10,000,000 or less, preferably 2,000,000 or less. Having a molecular weight within this range has the advantage of providing a coating film with good processability.

[0054] The glass transition temperature (Tg) of the coating film-forming resin (A) is preferably −70° C. or higher, more preferably 20° C. or higher, even more preferably 25° C. or higher, and still more preferably 30° C. or higher, and is preferably 95° C. or lower, more preferably 70° C. or lower, even more preferably 60° C. or lower, and still more preferably 50° C. or lower. Having a Tg within this range has the advantage of providing a coating film that is excellent in coating film processability and scratch resistance.

[0055] The film-forming resin (A) may be used alone or in combination of two or more. When two or more film-forming resins (A) are used, each parameter of the film-forming resin (A), except for the glass transition temperature, may be calculated as a weighted average based on the parameters and content of each resin. The glass transition temperature may also be calculated as the reciprocal of the sum of the values ​​obtained by dividing the mass-based content of each film-forming resin by the glass transition temperature (K: Kelvin).

[0056] The content of the film-forming resin (A) is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, even more preferably 80 parts by mass or more, and is preferably 100 parts by mass or less, more preferably 95 parts by mass or less, even more preferably 90 parts by mass or less, per 100 parts by mass of the solid content of the aqueous coating composition. In this specification, the content of the film-forming resin (A) refers to the content of the solid content only.

[0057] In this specification, the solid content of the aqueous coating composition refers to the portion excluding the aqueous medium (E) from the entire aqueous coating composition.

[0058] <Crosslinking Agent (B)> The crosslinking agent (B) is a compound having two or more groups per molecule that can react with the hydroxyl groups contained in the coating film-forming resin (A), and is capable of crosslinking with the coating film-forming resin (A) to form a coating film. The crosslinking agent (B) contains an amino resin, and examples of the amino resin include melamine resin, urea resin, and benzoguanamine. From the viewpoint of the storage stability of the resulting coating composition and the physical properties of the resulting coating film (processability, scratch resistance), it is preferable that the amino resin contains a melamine resin.

[0059] The melamine resin is a thermosetting resin synthesized from melamine and aldehyde, and is preferably a compound having three reactive functional groups represented by the following formula as reactive functional groups in one triazine nucleus molecule, or a polycondensate thereof: -NX １ X ２ [X １ , X ２ are each independently a hydrogen atom, a methylol group, or —CH ２ -OR １ Represents R １ represents an alkyl group having 1 to 8 carbon atoms, preferably a linear or branched alkyl group having 1 to 8 carbon atoms. ２ -OR １ If it contains multiple R １ may be the same or different.

[0060] The melamine resin has a reactive functional group of -N(CH ２ OR １ ) ２ a full alkyl type containing only -N(CH ２ OR １ ) (CH ２ methylol group type containing -N(CH ２ OR １ ) (H) as a reactive functional group; ２ OR １ ) (CH ２ -OH) and -N(CH ２ OR １ ) (H), or —N(CH ２ Four types of methylol / imino group types containing R １ is preferably an alkyl group having 1 to 4 carbon atoms, and is preferably a methyl group, an n-butyl group, or an isobutyl group.

[0061] In the present disclosure, among the melamine resins, X １ and X ２ All of the above are -CH ２ -OR １or a polycondensate thereof, and examples of such resins include methylated melamine resins, butylated melamine resins, isobutylated melamine resins, etc. The inclusion of a full-alkyl melamine resin has the advantages of improving the storage stability of the resulting coating composition and improving the reactivity with the acrylic resin (A1) at high temperatures and in the presence of a catalyst.

[0062] The degree of polymerization of the full alkyl melamine resin (B1) is 1 or more, preferably 1.2 or more, more preferably 1.5 or more, and is preferably 10 or less, more preferably 5 or less, and even more preferably 3 or less.

[0063] The number average molecular weight of the full alkyl melamine resin (B1) is preferably 300 or more, and preferably 2,000 or less, more preferably 1,300 or less, even more preferably 1,000 or less, and particularly preferably 800 or less. In this specification, the number average molecular weight is a value measured by gel permeation chromatography (GPC) in terms of polystyrene.

[0064] The full alkyl melamine resin (B1) may be a commercially available product, and examples thereof include Cymel 303, Cymel 325, Cymel 350, Cymel 370, and Mycoat 715 (all methylated melamine resins, manufactured by Allnex Japan Co., Ltd.), Cymel 202, Cymel 235, Cymel 254, Cymel 1123, Cymel 1128, Cymel 1170, and Mycoat 212 (all methyl-butylated mixed melamine resins, manufactured by Allnex Japan Co., Ltd.), Sumimal M-40S (methylated melamine resin, manufactured by Sumitomo Chemical Co., Ltd.), Amidair J-820-60, and Amidair L-127-60 (all butylated melamine resins, manufactured by DIC Corporation). These may be used alone or in combination of two or more.

[0065] The content of the full alkyl melamine resin (B1) in the crosslinking agent (B) is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, with the upper limit being 100% by mass.

[0066] The crosslinking agent (B) may contain, in addition to the full-alkyl melamine resin (B1), another crosslinking agent (B2). Examples of the other crosslinking agent (B2) include melamine resins other than the full-alkyl melamine resin (B1), urea resins, benzoguanamine resins, and other amino resins. The amino resins have high reactivity with the coating film-forming resin (A), and the resulting coating film has good appearance and moisture resistance.

[0067] The crosslinking agent (B) may be used alone or in combination of two or more kinds.

[0068] The ratio of the content of the crosslinking agent (B) to the content of the coating film-forming resin (A) ((B) / (A)) is, by mass, preferably 5 / 95, more preferably 10 / 90 or more, and preferably 30 / 70 or less, more preferably 20 / 80 or less. Having the ratio within this range has the advantage of improving the processability and scratch resistance of the resulting coating film.

[0069] <Sulfonic Acid Compound (C)> The sulfonic acid compound (C) can act as a catalyst to promote the reaction between the coating film-forming resin (A) and the crosslinking agent (B), thereby imparting high reactivity to the resulting coating composition.

[0070] The sulfonic acid compound (C) may be a monosulfonic acid compound or a polysulfonic acid compound. Examples of the sulfonic acid compound include aliphatic sulfonic acids such as methanesulfonic acid; aromatic sulfonic acids such as paratoluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and dinonylnaphthalenedisulfonic acid; and the like. The sulfonic acid compound (C) may be used alone or in combination of two or more.

[0071] The content of the sulfonic acid compound (C) is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and preferably 5 parts by mass or less, more preferably 3 parts by mass or less, relative to 100 parts by mass of the coating film-forming resin (A). When the content of the sulfonic acid compound (C) is within the above range, a coating film having good workability (adhesion, crack resistance) and scratch resistance can be formed on the precoated steel sheet.

[0072] <Amine compound (D)> The amine compound (D) has the effect of neutralizing the sulfonic acid compound (C), and by coexisting it with the sulfonic acid compound (C) to achieve a specific neutralization rate, there is an advantage in that the aqueous coating composition can be made stable during storage (for example, at 15 to 50°C) and highly reactive during heat drying and curing after application. A portion of the amine compound (D) may exist as a salt with the sulfonic acid compound (C).

[0073] The amine compound (D) is a compound having one or more amino groups, and is preferably a secondary or tertiary amine compound.

[0074] The substituent on the nitrogen atom of the amine compound is preferably a saturated or unsaturated aliphatic hydrocarbon group, and the hydrogen atoms contained in the saturated or unsaturated aliphatic hydrocarbon group may each independently be substituted with —COOH, —OH, etc., and the —CH ２ The - may be replaced by -O-. Furthermore, the substituents on the nitrogen atom of the amine compound may be bonded to each other to form a ring containing the nitrogen atom.

[0075] Examples of the amine compound (D) include secondary aliphatic amine compounds such as diethylamine, di-n-propylamine, diisopropylamine, diisobutylamine, di-n-butylamine, di-sec-butylamine, diamylamine, N-ethyl-1,2-dimethylpropylamine, N-methylhexylamine, di-n-octylamine, and diallylamine; tertiary aliphatic amine compounds such as triethylamine, tributylamine, triallylamine, N,N-dimethylethanolamine, N-methyldiallylamine, and N,N-dimethylallylamine; secondary cyclic amine compounds such as piperidine, 2-pipecoline, 3-pipecoline, 4-pipecoline, 2,4-lupetidine, 2,6-lupetidine, 3,5-lupetidine, and 3-piperidinemethanol; tertiary cyclic amine compounds such as N-methylpiperidine, N-methylpiperazine, and N-methylmorpholine; and aromatic amine compounds such as pyridine and 4-ethylpyridine.

[0076] The boiling point of the amine compound (D) is preferably 50° C. or higher, more preferably 70° C. or higher, and even more preferably 100° C. or higher, and is preferably 250° C. or lower, and more preferably 220° C. or lower. Having the boiling point within this range has the advantage of further improving the storage stability of the aqueous coating composition.

[0077] The content of the amine compound (D) is such that the neutralization rate of the sulfonic acid compound (C) by the amine compound (D), i.e., the neutralization rate calculated by the following formula, is in the range of 100% to 1,300%: Neutralization rate (%) = [(base number of amine compound (D) × number of moles of amine compound (D)) / (acid number of sulfonic acid compound (C) × number of moles of sulfonic acid compound (C)] × 100

[0078] The neutralization rate is preferably 200% or more, more preferably 300% or more, and may be, for example, 1,300% or less, 1,100% or less, 1,000% or less, or 900% or less, 800% or less. Although not intended to be limited to a particular theory, it is believed that when the neutralization rate is within the above range, the amine compound (D) blocks the sulfonic acid group of the sulfonic acid compound (C) during storage (e.g., 15 to 30°C) and suppresses catalytic action, thereby improving storage stability, and that during heat drying and curing after application (e.g., 180°C or higher), the block is removed, allowing the sulfonic acid compound (C) to function as a catalyst.

[0079] The full-alkyl melamine resin (B1) is known to have lower reactivity than melamine resins commonly used as crosslinkers, such as imino group-type melamine resins and methylol group-type melamine resins. However, as a result of studies by the present inventors, it was found that the low reactivity of the full-alkyl melamine resin (B1) occurs only at low temperatures (e.g., 60 to 80°C), and that when the full-alkyl melamine resin (B1) is used with the sulfonic acid compound (C) and the amine compound (D) at the above-mentioned neutralization ratios, the reactivity at high temperatures is increased. By combining the full-alkyl melamine resin (B1), the sulfonic acid compound (D), the amine compound (D), and the above-mentioned neutralization ratio, an aqueous coating composition having good storage stability and particularly suitable for application at high temperatures and in short periods of time can be obtained. Furthermore, the crosslink density can be increased, resulting in a coating film with excellent coating processability (adhesion and crack resistance).

[0080] The sulfonic acid compound (C) and the amine compound (D) may be used directly in the preparation of an aqueous coating composition, or they may be mixed in advance and used as a mixture in the preparation of an aqueous coating composition. In this case, in the mixture, part or all of the sulfonic acid compound (C) and the amine compound (D) may form a salt (e.g., a salt in which the sulfonic acid group contained in the sulfonic acid compound (C) is blocked by the amino group contained in the amine compound (D)). Alternatively, the salt of part or all of the sulfonic acid compound (C) and the amine compound (D) may be formed and then incorporated into the coating composition. Examples of the salt of part or all of the sulfonic acid compound (C) and the amine compound (D) include aliphatic sulfonic acids such as methanesulfonic acid; aromatic sulfonic acids such as dinonylnaphthalenedisulfonic acid and dinonylnaphthalenesulfonic acid, and amine-blocked versions thereof. Commercially available products may also be used as the salt of part or all of the sulfonic acid compound (C) and the amine compound (D).

[0081] In one embodiment, the content of the sulfonic acid compound (C) is preferably 1 part by mass or more and 5 parts by mass or less per 100 parts by mass of the film-forming resin (A), and the neutralization rate is preferably 100% or more and 1,300% or less; more preferably 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the film-forming resin (A), and the neutralization rate is preferably 200% or more and 1,000% or less; and even more preferably 2 parts by mass or more and 9 parts by mass or less per 100 parts by mass of the film-forming resin (A), and the neutralization rate is preferably 300% or more and 900% or less. When the coating composition has the above-mentioned amounts and neutralization ratios of the sulfonic acid compound (C) and the amine compound (D), the coating composition has high storage stability at low temperatures (storage temperature, for example, 15 to 30°C) and high reactivity at high temperatures, and the coating film obtained has better processability (adhesion, crack resistance) and scratch resistance.

[0082] The total content of the film-forming resin (A), crosslinking agent (B), sulfonic acid compound (C) and amine compound (D) in the solid content of the aqueous coating composition is, for example, preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more, with the upper limit being 100% by mass or less.

[0083] <Aqueous Medium (E)> The aqueous coating composition contains an aqueous medium (E). The aqueous medium (E) is preferably water, an organic solvent (E1), or a mixture of water and an organic solvent (E1).

[0084] The organic solvent (E1) is preferably a hydrophilic organic solvent, for example, a solvent having a solubility in water at 25° C. of 0.1 g / 100 gH ２Examples of such organic solvents include glycol-based solvents such as ethylene glycol, propylene glycol, butanediol, pentanediol, diethylene glycol, dipropylene glycol, and triethylene glycol; glycol ether-based solvents such as ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, triethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, and propylene glycol monomethyl ether acetate; alcohol-based solvents such as methanol, ethanol, isopropyl alcohol, and benzyl alcohol; cyclic ether-based solvents such as dioxane and tetrahydrofuran; alcohol ester-based solvents such as 2,2,4-trimethylpentane-1,3-diol monoisobutyrate; ketone-based solvents such as acetone; and N-methyl-2-pyrrolidone. The use of such organic solvents has the advantage that the resulting coating composition has good wettability with the substrate.

[0085] In one embodiment, the boiling point of the organic solvent (E1) is preferably 150°C or higher, more preferably 180°C or higher, and preferably 300°C or lower, more preferably 250°C or lower. Examples of such organic solvents include glycol-based solvents such as propylene glycol (1,2-propanediol), 1,4-butanediol, 1,5-pentanediol, diethylene glycol, and dipropylene glycol, with diethylene glycol being particularly preferred. These may be used alone or in combination of two or more.

[0086] The solubility of the organic solvent (E1) in water at 25° C. is preferably 0.1 g / 100 gH ２ 0 or more, more preferably 1 g / 100 gH ２ 0 or more, more preferably 5 g / 100 gH ２The organic solvent (E1) may be optionally miscible with water.

[0087] The content of the organic solvent (E1) in the aqueous medium (E) is 3% by mass or more, preferably 4% by mass or more, more preferably 5% by mass or more, and preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less. By having the content in this range, the burden on the environment can be reduced, and there are advantages in that the storage stability of the coating composition, the wettability to the substrate, and the appearance of the resulting coating film are good.

[0088] The content of the aqueous medium (E) is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 55% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 85% by mass or less.

[0089] The aqueous coating composition may contain an organic solvent other than the aqueous medium (E) as needed. Examples of organic solvents other than (E) include diethylene glycol dibutyl ether and 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol). The boiling point of the organic solvent other than (E) is preferably 150°C or higher, more preferably 180°C or higher, and preferably 300°C or lower, more preferably 250°C or lower.

[0090] <Others> The aqueous coating composition may further contain other additives as needed. Examples of the other additives include extender pigments; coloring agents such as coloring pigments and dyes; heat-shielding pigments; luster pigments; aggregates (resin particles, silica particles, etc.); waxes; solvents other than those mentioned above; ultraviolet absorbers (benzophenone-based ultraviolet absorbers, etc.); antioxidants (phenolic, sulfoid, hindered amine antioxidants, etc.); plasticizers; coupling agents (silane-based, titanium-based, zirconium-based coupling agents, etc.); anti-sagging agents; viscosity adjusters; pigment dispersants; pigment wetting agents; surface conditioners (silicone-based, organic polymer-based, etc.); leveling agents; color separation inhibitors; precipitation inhibitors; anti-settling agents; antifoaming agents; surfactants; antifreeze agents; emulsifiers; rust inhibitors; preservatives; mildew inhibitors; antibacterial agents; stabilizers, etc. These additives may be used alone or in combination of two or more.

[0091] Examples of the viscosity modifier (F) include associative viscosity modifiers that utilize the bonding strength (interaction) of hydrophilic groups (parts) or hydrophobic groups (parts); and thickening viscosity modifiers that utilize the solubilizing and thickening action of polymers. Examples of the associative viscosity modifiers include hydrophilic association viscosity modifiers that form hydrogen bonds between viscosity modifiers or between the viscosity modifiers and the base resin and utilize the bonding strength (interaction) thereof, and hydrophobic association viscosity modifiers that utilize the interaction between hydrophobic groups (parts) within the molecules. Examples of the associative viscosity modifiers include alkali thickening viscosity modifiers that utilize the solubilizing and thickening action of polymers by alkali.

[0092] Examples of the hydrophilic association type viscosity modifier include polyamide type viscosity modifiers. Commercially available polyamide type viscosity modifiers may be used, and examples thereof include (hereinafter, all products are trade names): BYK-430, BYK-431 (manufactured by BYK-Chemie), Disparlon AQ-580, Disparlon AQ-600, Disparlon AQ-607 (manufactured by Kusumoto Chemicals Co., Ltd.), Thixol W-300, Thixol W-400LP (manufactured by Kyoeisha Chemical Co., Ltd.).

[0093] As the hydrophobic association type viscosity modifier, commercially available products may be used, and examples thereof include ADEKA NOL UH-420, ADEKA NOL UH-462, ADEKA NOL UH-472, ADEKA NOL UH-526, UH-540, and ADEKA NOL UH-814N (manufactured by ADEKA Corporation), PRIMAL RH-1020, and PRIMAL RM-2020 (manufactured by The Dow Chemical Company), SN Thickener 612, SN Thickener 621, and NOPAL 700N (manufactured by San Nopco Ltd.).

[0094] Examples of the alkali-thickening viscosity modifier include viscose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, sodium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, etc. Commercially available products may also be used, including cellulose-based viscosity modifiers such as Tylose MH and Tylose H (manufactured by Merck & Co.); Primal ASE-60, Primal TT-615, Primal RM-5 (manufactured by The Dow Chemical Company), and Euker Polyphobe (manufactured by Union Carbide). These may be used alone or in combination of two or more.

[0095] The viscosity modifier (F) is preferably an associative viscosity modifier. The inclusion of an associative viscosity modifier has the advantage of improving coating workability with a roll coater (roll coater applicability). Specifically, it can make the viscosity of the coating composition Newtonian at high shear rates. Furthermore, more preferably, the combined use of an associative viscosity modifier with a hydrophobic associative viscosity modifier has the advantage of improving the physical properties, such as the water resistance, of the resulting coating film.

[0096] The content of the viscosity modifier (F) in the aqueous coating composition of the present disclosure is preferably 0.01 to 20 parts by mass, and more preferably 0.1 to 10 parts by mass, per 100 parts by mass of the total solids content of the coating film-forming resin (A) and crosslinking agent (B). Having the amount of viscosity modifier (F) within this range has the advantage of improving coating workability with a roll coater (roll coater applicability) and the appearance and water resistance of the resulting coating film.

[0097] Examples of the extender pigment include calcium carbonate, barium sulfate, clay, talc, mica, glass fiber, etc. These may be used alone or in combination of two or more.

[0098] In one embodiment, the amount of the extender pigment is preferably from 1 to 40 parts by mass, more preferably from 10 to 30 parts by mass, per 100 parts by mass of the total solids content of the film-forming resin (A) and the curing agent (B). By keeping the amount of the extender pigment within this range, the scratch resistance of the coating film is likely to be improved.

[0099] Examples of the color pigment include color inorganic pigments such as titanium dioxide, carbon black, graphite, iron oxide, and coal dust; color organic pigments such as phthalocyanine blue, phthalocyanine green, quinacridone, perylene, anthrapyrimidine, carbazole violet, anthrapyridine, azo orange, flavanthrone yellow, isoindoline yellow, azo yellow, industhrone blue, dibromoanzathrone red, perylene red, azo red, and anthraquinone red; aluminum powder, alumina powder, bronze powder, copper powder, tin powder, zinc powder, iron phosphide, and finely divided titanium. These may be used alone or in combination of two or more.

[0100] The heat-shielding pigment refers to a pigment that does not absorb light in the near-infrared wavelength region (wavelength: 780 nm to 2,500 nm) or has a low absorptivity for light in the near-infrared wavelength region (wavelength: 780 nm to 2,500 nm). The heat-shielding pigment is not particularly limited, and the following inorganic heat-shielding pigments and organic heat-shielding pigments can be used.

[0101] Examples of inorganic heat-shielding pigments include metal oxide pigments such as titanium oxide, magnesium oxide, barium oxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, indium oxide, sodium titanate, silicon oxide, nickel oxide, manganese oxide, chromium oxide, iron oxide, copper oxide, cerium oxide, and aluminum oxide; iron oxide-manganese oxide, iron oxide-chromium oxide (for example, Dipyroxide Color Black #9595 manufactured by Dainichiseika Color & Chemicals Co., Ltd., and Black 6350 manufactured by Asahi Kasei Kogyo Co., Ltd.), iron oxide-cobalt oxide-chromium oxide (for example, Dipyroxide Color Brown manufactured by Dainichiseika Color & Chemicals Co., Ltd.), and Examples of suitable pigments include composite oxide pigments such as copper oxide-magnesium oxide (e.g., Dipyroxide Color Black #9290, Dipyroxide Color Black #9590 manufactured by Dainichiseika Color & Chemicals Co., Ltd.), manganese oxide-bismuth oxide (e.g., Black 6301 manufactured by Asahi Kasei Kogyo Co., Ltd.), and manganese oxide-yttrium oxide (e.g., Black 6303 manufactured by Asahi Kasei Kogyo Co., Ltd.); metal pigments such as silicon, aluminum, iron, magnesium, manganese, nickel, titanium, chromium, and calcium; and alloy pigments such as iron-chromium, bismuth-manganese, iron-manganese, and manganese-yttrium. These pigments may be used alone or in combination of two or more.

[0102] Examples of organic heat-shielding pigments include azo pigments, azomethine pigments, lake pigments, thioindigo pigments, anthraquinone pigments (such as anthranthrone pigments, diaminoanthraquinonyl pigments, indanthrone pigments, flavanthrone pigments, and anthrapyrimidine pigments), perylene pigments, perinone pigments, diketopyrrolopyrrole pigments, dioxazine pigments, phthalocyanine pigments, quiniphthalone pigments, quinacridone pigments, isoindoline pigments, and isoindolinone pigments. These pigments may be used alone or in combination of two or more.

[0103] Examples of the luster pigment include aluminum foil, bronze foil, tin foil, gold foil, silver foil, titanium metal foil, stainless steel foil, alloy foil of nickel, copper, etc., foil pigment such as foil-like phthalocyanine blue, etc. These may be used alone or in combination of two or more.

[0104] As the wax, waxes known to those skilled in the art for use in paints can be used, such as microcrystalline, polyethylene, polypropylene, paraffin, carnauba, and modified products thereof, etc. These may be used alone or in combination of two or more.

[0105] The shear viscosity of the aqueous coating composition is 0.01 s at a temperature of 23°C. －１ When measured at a shear rate of 10 s, the viscosity is preferably 30,000 mPa·s or less, more preferably 20,000 mPa·s or less, even more preferably 10,000 mPa·s or less, and is preferably 3,000 mPa·s or more, more preferably 4,000 mPa·s or more, even more preferably 5,000 mPa·s or more. －１ When measured at a shear rate of 1,000 s, the viscosity is preferably 800 mPa·s or less, more preferably 700 mPa·s or less, even more preferably 600 mPa·s or less, and is preferably 300 mPa·s or more, more preferably 400 mPa·s or more, even more preferably 500 mPa·s or more. －１ When measured by a shear viscosity measuring method, the viscosity is preferably 1,000 mPa·s or less, more preferably 150 mPa·s or more, and preferably 500 mPa·s or less. Being within this range results in a viscosity suitable for the paint's pick-up properties during roll coater application. The shear viscosity can be, for example, a value measured immediately after preparation of the paint composition. The shear viscosity can be measured using a rotational viscometer, for example, a stress-controlled rheometer MCR301 (manufactured by Anton Paar).

[0106] <Method for preparing aqueous coating composition> The method for preparing the aqueous coating composition of the present disclosure is not particularly limited, and the composition can be prepared by mixing the components. For example, the components can be mixed using a mixer / disperser such as a roller mill, ball mill, bead mill, pebble mill, sand grind mill, pot mill, paint shaker, or disperser, or a kneader.

[0107] A coating film formed from the aqueous coating composition and a method for producing the coating film are also included within the technical scope of the present disclosure.

[0108] <Substrate> Examples of substrates (substrates) to be coated with the aqueous coating composition of the present disclosure include zinc-plated steel sheets, zinc-aluminum alloy-plated steel sheets, aluminum alloy-plated steel sheets, hot-dip zinc-aluminum-magnesium alloy-plated steel sheets, stainless steel sheets, cold-rolled steel sheets, etc., all of which are produced by a hot-dip method or an electrolytic method. In addition to these steel sheets or plated steel sheets, metal sheets such as aluminum sheets (including aluminum alloy sheets) can also be coated.

[0109] The substrate is preferably surface-treated. Specifically, the substrate is preferably subjected to a pretreatment such as alkaline degreasing, hot water washing, or water washing, followed by a chemical conversion treatment. The chemical conversion treatment may be performed by a known method, and examples include chromate treatment and non-chromate treatments such as zinc phosphate treatment. The surface treatment can be appropriately selected depending on the steel sheet to be used, but a treatment that does not contain heavy metals is preferred. By applying the coating composition of the present disclosure to a substrate that has been subjected to such a chemical conversion treatment, the adhesion of the coating film to the metal sheet surface and corrosion resistance are improved. Alternatively, a primer coating film can be formed on the metal sheet surface that has been subjected to the chemical conversion treatment, and the coating can be applied on top of that. The thickness of the primer coating film is preferably 3 μm or more, more preferably 5 μm or more, and preferably 15 μm or less, more preferably 10 μm or less.

[0110] <Method for producing a coating film> The method for producing a coating film of the present disclosure includes the steps of: applying the aqueous coating composition of the present disclosure to an object to be coated to form a coating film; and drying and / or curing the coating film under conditions where the maximum temperature reached is 180°C or higher and the drying and / or curing time is 120 seconds or less to form a coating film.

[0111] The method for applying the aqueous coating composition of the present disclosure to a substrate is not particularly limited, but conventionally known methods such as a roll coater method, an airless spray method, an electrostatic spray method, or a curtain flow coater method can be used. Preferred are the roll coater method and the curtain flow coater method, and more preferred is the roll coater method.

[0112] The maximum temperature is preferably reached in 200 seconds or more, and may be, for example, 280 seconds or less, 270 seconds or less, or 250 seconds or less. The drying and / or curing time is 120 seconds or less, and may be 60 seconds or less, 30 seconds or less, 10 seconds or less, or 6 seconds or less, and is preferably 1 second or more.

[0113] The method for drying and / or curing the coating film is not particularly limited, but heating means such as hot air heating, infrared heating, and induction heating can be used.

[0114] The film thickness (dry film thickness) of the coating film after drying and / or curing is preferably 1 μm or more, more preferably 5 μm or more, and preferably 30 μm or less, more preferably 25 μm or less.

[0115] A laminate having the above-described substrate and the above-described coating film formed on the substrate is also included within the technical scope of the present disclosure.

[0116] When the substrate has the coating film on one side, it may also have a coating film formed from a known coating composition, such as a coating composition containing an epoxy resin, on the other side.

[0117] The aqueous coating composition of the present disclosure is highly curable, even when applied under conditions of higher temperature and shorter time than those typically employed for coating metal substrates (e.g., drying / curing temperature of 60 to 80°C, drying / curing time of 30 minutes to 1 hour), and can provide a coating film with good coating film properties (adhesion, processability such as crack resistance, scratch resistance).

[0118] The aqueous coating composition of the present disclosure has high storage stability, and the resulting coating film has good adhesion and is resistant to peeling from the substrate even during processing such as bending, and also suppresses cracking, providing good crack resistance and excellent scratch resistance. Therefore, the aqueous coating composition of the present disclosure is suitable for use in coating metals, particularly as a precoat.

[0119] The present disclosure will be described in more detail with reference to the following examples, but the present disclosure is not limited thereto. In the examples, "parts" and "%" are by mass unless otherwise specified.

[0120] <Production Example of Coating Film-Forming Resin (A-1)> 0.6 parts by mass of Pelex SS-H (surfactant, manufactured by Kao Corporation) was dissolved in 60 parts by mass of ion-exchanged water. To this was added a monomer mixture consisting of 53.0 parts by mass of methyl methacrylate, 39.2 parts by mass of n-butyl acrylate, 5.8 parts by mass of 2-hydroxyethyl methacrylate, and 2.0 parts by mass of methacrylic acid, and the mixture was stirred to prepare 150.5 parts by mass of a monomer pre-emulsion. Separately, 1.0 part by mass of ammonium persulfate was dissolved in 20 parts by mass of ion-exchanged water as an initiator to prepare an aqueous initiator solution.

[0121] A reaction vessel equipped with a thermometer, condenser, and stirrer was charged with 40 parts by weight of ion-exchanged water and 0.4 parts by weight of Pelex SS-H, and heated to 80 ° C. under a nitrogen atmosphere. While maintaining the temperature at 80 ° C., the aqueous initiator solution was added dropwise over 180 minutes, and 10 minutes after the start of the dropwise addition, the monomer pre-emulsion was added dropwise from another opening of the reaction vessel over 150 minutes, thereby carrying out emulsion polymerization. After the dropwise addition of the aqueous initiator solution was completed, the mixture was heated and stirred at 80 ° C. for a further 60 minutes, then cooled to room temperature, and 2.10 parts by weight of dimethylethanolamine was added to prepare an acrylic emulsion (solids concentration: 45% by weight) in which the coating film-forming resin (A-1) was dispersed in an aqueous medium.

[0122] Film-forming resins (A-2) to (A-11) were prepared in the same manner as above, except that the type and amount of monomer and the amount of initiator were changed as shown in Table 1. Table 1 shows the specific values ​​of each film-forming resin, such as the hydroxyl value.

[0123] Details of each component shown in the following table and used in the examples and comparative examples are as follows. Film-forming resins (A) (A-12) Vylonal MD2000 (manufactured by Toyobo Co., Ltd., polyester resin emulsion); hydroxyl value: 6 mg KOH / g, acid value: 2 mg KOH / g, weight average molecular weight: 30,000, glass transition temperature: 67°C, minimum film-forming temperature: 48°C, average particle size: 125 nm, solid content: 40 mass% Crosslinking agents (B) (B-1) Cymel 303 (manufactured by Allnex Japan Co., Ltd., full-alkyl methylated melamine resin); solid content: 100 mass%, number average molecular weight: 455 (B-2) Cymel 300 (manufactured by Allnex Japan Co., Ltd., full-alkyl methylated melamine resin); solid content: 100 mass%, number average molecular weight: 390 Other crosslinking agents (b-1) Cymel 327 (manufactured by Allnex Japan Co., Ltd., imino group-type methylated melamine resin); solid content: 90 mass% Number average molecular weight: 470 (b-2) Mycoat 508 (manufactured by Allnex Japan Co., Ltd., imino group-type butylated melamine resin): solid content: 80 mass%, number average molecular weight: 1,500 Sulfonic acid compounds (C) (C-1) AC400S (manufactured by Teika Corporation, dodecylbenzenesulfonic acid); solid content: 25 mass% (C-2) AC700 (manufactured by Teika Corporation, paratoluenesulfonic acid); solid content: 25 mass% (C-3) Nacure-1051 (manufactured by Kusumoto Chemicals Co., Ltd., dinonylnaphthalenesulfonic acid); solid content: 51 mass% Other acid compounds (c-1) Cycat 296 (manufactured by Allnex Japan Co., Ltd., phosphoric acid compound); solid content: 50 mass% Amine compounds (D) (D-1) DMEA (dimethylethanolamine, manufactured by Mitsubishi Gas Chemical Co., Ltd.); boiling point: 134 ° C. (D-2) AMP (2-amino-2-methyl-1-propanol, manufactured by Kokusan Chemical Co., Ltd.); boiling point: 165°C (D-3) TEA (triethylamine, manufactured by Mitsubishi Gas Chemical Co., Ltd.); boiling point: 90°C. Aqueous medium (E) (E1-1) Diethylene glycol (manufactured by Nippon Shokubai Co., Ltd.); boiling point: 244°C, solubility in water: infinite (freely miscible with water) (E1-2) Propylene glycol (manufactured by Sankyo Chemical Co., Ltd.); boiling point: 187°C, solubility in water: infinite (freely miscible with water) (E1-3) Dipropylene glycol (manufactured by Showa Chemical Co., Ltd.); boiling point: 232°C, solubility in water: infinite (freely miscible with water) (E1-4) 1,4-butanediol (manufactured by Sankyo Chemical Co., Ltd.);Boiling point: 228°C, solubility in water: infinite (freely miscible with water) (E1-5) 1,5-pentanediol (manufactured by Ube Industries, Ltd.); boiling point: 242°C, solubility in water: infinite (freely miscible with water) Viscosity modifiers (F) (F-1) SN Thickener 612 (polyether urethane hydrophobic association type viscosity modifier, manufactured by San Nopco); solid content: 40% by mass (F-2) SN Thickener 621 (polyether urethane hydrophobic association type viscosity modifier, manufactured by San Nopco); solid content: 30% by mass (F-3) ADEKA NOL UH-526 (polyether urethane hydrophobic association type viscosity modifier, manufactured by ADEKA); solid content: 30% by mass (F-4) Primal RM-2020NPR (polyether urethane-based hydrophobic association type viscosity modifier, manufactured by The Dow Chemical Company); solid content: 20 mass% (F-5) Primal ASE-60 (polyacrylic acid ester emulsion-based alkali swelling type thickener, manufactured by The Dow Chemical Company); solid content: 28 mass%

[0124] <Production Example of Pigment Dispersion Paste> 1.63 parts by mass of Disperbyk 190 (manufactured by BYK-Chemie) as a dispersant, 0.25 parts by mass of dimethylethanolamine, 0.05 parts by mass of SN-477T (manufactured by San Nopco) as an antifoaming agent, 32.9 parts by mass of ion-exchanged water, and 65.2 parts by mass of titanium dioxide (Ti-Pure R-706, manufactured by DuPont) as a pigment were premixed, and then dispersed at 1,600 rpm using an SG mill (dispersion medium: glass beads) until the maximum particle size of the pigment coarse particles reached 5 μm, thereby obtaining a pigment dispersion paste.

[0125] <Production Example of Aqueous Coating Composition 1> 55.1 parts by weight of the pigment dispersion paste obtained in the above Production Example, 80.0 parts by weight of the film-forming resin (A-1) obtained in the above Production Example, 20.0 parts by weight of the film-forming resin (A-8), and 17.6 parts by weight of Cymel 303 as a crosslinker (B-1) were mixed, and then 5.4 parts by weight of diethylene glycol (E1-1) and 5.4 parts by weight of propylene glycol (E1-2) were mixed and stirred as an aqueous medium. Next, 1.2 parts by weight of dodecylbenzenesulfonic acid as a sulfonic acid compound (C-1) and 1.9 parts by weight of dimethylethanolamine as an amine compound (D-1) were mixed with a disper, and further, 0.2 parts by weight of Thickener SN-612 as a viscosity modifier (F-1) were mixed with stirring to obtain coating composition 1.

[0126] (Coating Compositions 2 to 45, Comparative Examples 1 to 10) Coating compositions were prepared in the same manner as for coating composition 1, except that the type and amount of each component was changed as shown in Tables 2 to 7.

[0127] <Production Example of Coated Steel Sheet> A hot-dip galvanized steel sheet with a thickness of 0.4 mm was alkaline degreased, and then a phosphate treatment agent, Surfcoat EC2310 (manufactured by Nippon Paint Surf Chemicals Co., Ltd.), was applied to the front and back surfaces of the steel sheet to perform a non-chrome chemical conversion treatment, followed by drying. Next, 1 aliquot of the coating composition obtained in Production Example was applied to the surface of the steel sheet using a bar coater so that the dried coating film thickness was 18 μm, and the steel sheet was baked for 30 seconds under conditions that brought the material's maximum temperature to 230°C to form a surface coating film, thereby obtaining a coated steel sheet.

[0128] 1) Shear viscosity measurement The shear viscosity of the coating compositions obtained in the examples and comparative examples was measured using a stress-controlled rheometer MCR301 (manufactured by Anton Paar, jig: 50 mm parallel plate, gap: 0.5 mm) at a shear rate of 0.1 s －１ , 10s －１ and 1,000s －１ The shear viscosity was measured at 23°C.

[0129] 2) Storage stability: Evaluation was performed in accordance with the method specified in JIS K 5600 2-2 (Flow Cup Method) using a Ford Cup No. 4 (manufactured by Ueshima Seisakusho Co., Ltd.). Ion-exchanged water was added to the coating compositions obtained in the Examples and Comparative Examples to adjust the viscosity to 60±10 seconds (initial viscosity (seconds)). Specifically, the initial viscosity was determined by diluting the composition with the ion-exchanged water, stirring it at 1,000 rpm for 3 minutes using a disper, and then measuring the viscosity immediately. The coating temperature was 25°C.

[0130] The coating composition adjusted to the initial viscosity (60±10 seconds (25°C)) was filled 80-90% into a 1 / 5 can, sealed, and then left to stand in a thermostatic chamber at 40°C. After 14 days (2 weeks), the can was removed and the viscosity was measured in the same manner as above (viscosity over time (seconds)). The rate of change in viscosity over time relative to the initial viscosity was calculated using the following formula, and storage stability was evaluated according to the following criteria. A score of ○ or higher was rated as passing. Viscosity change rate (%) = viscosity over time (seconds) / initial viscosity (seconds) × 100 ◎: The rate of viscosity change is 0% or more and less than 30%. ○: The rate of viscosity change is 30% or more and less than 50%. △: The rate of viscosity change is 50% or more and less than 100%. ×: The rate of viscosity change is 100% or more.

[0131] 5) Coating Workability (Roll Coater Coatability) The coating compositions obtained in the Examples and Comparative Examples were applied to substrates under the following conditions using a small test coater (manufactured by NK Tech Co., Ltd.) equipped with three rolls (backup roll, application roll, and pickup roll), and the roll coater coatability was evaluated according to the following criteria. A score of ○ or higher was considered to be pass. The test conditions were a room temperature of 23°C and a humidity of 60% RH. Substrate: GL steel plate (manufactured by Nippon Steel & Co., Ltd.) measuring 300 mm x 2,000 mm x 0.35 mm Coating conditions: Line speed: 50 m / min Roll peripheral speed: Application roll: 65 m / min (130% of line speed), pickup roll: 20 m / min (40% of line speed) Backup roll pressure: 60 kgf Reference coating amount: 28 g / m of dried coating film ２Baking conditions: 30 seconds under conditions where the maximum temperature of the material to be coated reaches 230°C. ◎: The entire surface can be coated evenly with the standard coating amount. ○: The entire surface can be coated evenly, but the coating amount is 20 to 28 g / m. ２ △: The entire surface can be coated, but the coating amount is 20 g / m ２ x: Uncoated areas remain and the entire surface cannot be coated.

[0132] In a roll coater, the paint is wound up by a pickup roll, transferred to an application roll, and then transferred to a backup roll, where it is applied to the object to be coated. If the paint is properly wound up by the pickup roll and properly transferred to the backup roll by the pressure of the application roll and backup roll, the paint will be applied evenly to the object to be coated, but if only a small amount of paint is wound up by the pickup roll, unevenness will occur during transfer between the rolls, and the object will not be coated evenly.

[0133] 4) Workability (Adhesion) Each coated steel sheet obtained in the Examples and Comparative Examples was cut into a 5 cm x 3 cm piece and pre-bent using a seam folding machine (Ueshima Seisakusho Co., Ltd.) so that the coating surface was facing outward. Two steel sheets of the same thickness (0.4 mm) were sandwiched between the test pieces and then folded using a press (Kyoritsu Kogyo Co., Ltd.). Cellophane tape (registered trademark) (LP-24, Nichiban Co., Ltd.) was adhered to the processed area of ​​the coated steel sheet and then quickly peeled off, and the adhesion of the coating film in the processed area was evaluated. The appearance of the tape-peeled area was evaluated according to the following criteria. A score of 4 or higher was considered acceptable. 5: No metal base was observed in the tape-peeled area. 4: Metal base was observed in more than 0% but less than 20% of the area of ​​the tape-peeled area. 3: Metal base was observed in 20% or more but less than 50% of the area of ​​the tape-peeled area. 2: Metallic base material is observed in 50% or more but less than 80% of the area of ​​the part where the tape was peeled off. 1: Metallic base material is observed in 80% or more of the area of ​​the part where the tape was peeled off.

[0134] 5) Workability (Crack Resistance) Each coated steel plate obtained in the Examples and Comparative Examples was cut into a 5 cm x 3 cm piece and pre-bent using a seam folding machine (Ueshima Seisakusho Co., Ltd.) so that the coating surface was facing outward. Five steel plates of the same thickness (0.4 mm) were sandwiched between the test pieces and then bent using a press (Kyoritsu Kogyo Co., Ltd.). The condition (cracks) of the coating in the processed area was observed with a 15x magnifying glass, and the workability was evaluated according to the following criteria. A score of 4 or higher was considered pass. The test conditions were a temperature of 23°C and a humidity of 60% RH. 5: No cracks were observed in the processed area. 4: Cracks were observed in less than 20% (more than 0%) of the processed area. 3: Cracks were observed in 20% to less than 50% of the processed area. 2: Cracks were observed in 50% to less than 80% of the processed area. 1: Cracks were observed in 80% or more of the processed area.

[0135] 6) Scratch Resistance Using a continuous load scratch strength tester, Type: 18 / 18L (manufactured by Shinto Scientific Co., Ltd.), a diamond stylus (a conical scratch stylus with a radius of 0.4 mm) that had been rounded to a radius of 0.4 mm was applied to the coating surface of each coated steel sheet obtained in the Examples and Comparative Examples, and the coating surface was rubbed once at a speed of 300 mm / min with a movement width of 10 cm under a load of 300 mm / min. The load at which the coating surface was scratched and the substrate was exposed was evaluated according to the following criteria. A score of ○ or higher was considered to be acceptable. The load was applied in increments of 500 gf, and the test conditions were a temperature of 23°C and a humidity of 60% RH. ⊚: The substrate was not exposed even when the load exceeded 3,000 g. ○: A load of more than 2,000 and up to 3,000 g. △: A load of more than 1,000 g and up to 2,000 g. ×: A load of 1,000 g or less.

[0136]

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143] Examples 1 to 45 are examples of the present disclosure, and had high storage stability, excellent processability, and good scratch resistance.

[0144] Comparative Examples 1 and 2 are examples in which the hydroxyl value of the acrylic resin (A1) was less than 5 mgKOH / g, and the scratch resistance was poor. Comparative Examples 3 and 4 are examples in which the hydroxyl value of the acrylic resin (A1) exceeded 35 mgKOH / g, and the storage stability and processability were poor. Comparative Examples 5 and 6 are examples in which the neutralization rate of the sulfonic acid compound (C) by the amine compound (D) was less than 100%, and the storage stability was poor. Comparative Example 7 is an example in which the neutralization rate of the sulfonic acid compound (D) by the amine compound (D) exceeded 1,300%, and the storage stability was poor. Comparative Examples 8 and 9 are examples in which the crosslinking agent (B) did not contain a full alkyl melamine resin (B1), and the processability was poor. Comparative Example 10 is an example in which a phosphoric acid compound was used without containing a sulfonic acid compound (C), and the storage stability and scratch resistance were poor.

Claims

1. An aqueous coating composition comprising a film-forming resin (A), a crosslinking agent (B), a sulfonic acid compound (C), and an amine compound (D), wherein the film-forming resin (A) contains an acrylic resin (A1) having a hydroxyl group, the hydroxyl value of the film-forming resin (A) is 5 mgKOH / g or more and 35 mgKOH / g or less, the crosslinking agent (B) contains a fully alkylated melamine resin (B1), and the neutralization rate of the acid group of the sulfonic acid compound (C) in terms of moles by the amine compound (D) is 100% or more and 1,300% or less.

2. The aqueous coating composition according to claim 1, wherein the weight average molecular weight of the film-forming resin (A1) is 100,000 or more.

3. At a temperature of 23°C, the shear viscosity measured at a shear rate of 0.01 s －１ is 30,000 mPa·s or less, and the shear viscosity measured at a shear rate of 10 s －１ is 800 mPa·s or less, and the shear viscosity measured at a shear rate of 1,000 s －１ is 150 mPa·s or more. The aqueous coating composition according to claim 1 or 2.

4. The aqueous coating composition according to any one of claims 1 to 3, further comprising an organic solvent (E1).

5. The aqueous coating composition according to any one of claims 1 to 4, which is for coil coating.

6. A method for producing a coating film, comprising a step of applying the aqueous coating composition according to any one of claims 1 to 5 to an object to be coated to form a coating film, and a step of drying and / or curing the coating film under conditions where the maximum reaching temperature is 180°C or more and the drying and / or curing time is 120 seconds or less to obtain a coating film.