Aqueous coating composition

Abstract

This aqueous coating composition comprises: a hydroxyl group-containing acrylic resin emulsion (A); a core-shell acrylic resin dispersion (B); and hydrophobic melamine resin (C). The core-shell acrylic resin dispersion (B) has a branched hydrocarbon group having a carbon number 4-24 in a core part, a shell part has hydrophilic resin, and the solid content mass ratio (A:B) between the hydroxyl group-containing acrylic resin emulsion (A) and the core-shell acrylic resin dispersion (B) is 30:70 to 90:10.

Description

water-based paint composition

[0001] The present invention relates to an aqueous coating composition.

[0002] In recent years, environmental pollution has become more serious, and regulations on organic solvent emissions have been strengthened internationally. In the field of paints, there has also been a shift from conventional organic solvent-based paints to water-based paints using water as a medium. Patent Document 1 discloses an aqueous paint composition containing a hydrophobic melamine resin.

[0003] Japanese Patent Application Laid-Open No. 2020-002244

[0004] In aqueous coating compositions, the dispersibility of hydrophobic melamine resins is prone to decrease, which leads to deterioration in the storage stability of the coating composition and the appearance of the resulting coating film.

[0005] An object of the present invention is to provide an aqueous coating composition that can provide a coating film that has excellent appearance and is inhibited from yellowing while ensuring water resistance.

[0006] In order to solve the above problems, the present invention provides the following aspects. [1] An aqueous coating composition comprising a hydroxyl-containing acrylic resin emulsion (A), a core-shell type acrylic resin dispersion (B), and a hydrophobic melamine resin (C), wherein the core-shell type acrylic resin dispersion (B) has a branched hydrocarbon group having 4 to 24 carbon atoms in its core portion and a hydrophilic resin in its shell portion, and wherein the solids mass ratio (A:B) of the hydroxyl-containing acrylic resin emulsion (A) to the core-shell type acrylic resin dispersion (B) is 30:70 to 90:10. [2] The aqueous coating composition of above [1], wherein the solids mass ratio (B:C) of the core-shell type acrylic resin dispersion (B) to the hydrophobic melamine resin (C) is 10:90 to 50:50. [3] The aqueous coating composition of the above [1] or [2], wherein the acid value of the core-shell acrylic resin dispersion (B) is 25 mgKOH / g or more and 50 mgKOH / g or less. [4] The aqueous coating composition of any of the above [1] to [3], wherein the weight average molecular weight of the core-shell acrylic resin dispersion (B) is 7,600 or more and 80,000 or less.

[0007] According to the present invention, there is provided an aqueous coating composition that can give a coating film that has excellent appearance and is inhibited from yellowing while ensuring water resistance.

[0008] In aqueous coating compositions, the use of a hydrophobic melamine resin as a curing component improves the water resistance of the resulting coating film. However, hydrophobic melamine resins have poor dispersibility in aqueous coating compositions. The present disclosure improves the dispersibility of hydrophobic melamine resins in aqueous solvents, thereby improving the storage stability of aqueous coating compositions and the appearance of excellent coating films.

[0009] In the present disclosure, at least two types of aqueous acrylic resins with different forms are used. Aqueous resins are generally broadly classified into water-soluble and water-dispersed types. Water-dispersed types are further classified into dispersion types (generally referred to as colloidal dispersion types) and emulsion types. Colloidal dispersion-type aqueous resins are typically obtained by semi-dissolving a resin synthesized in an organic solvent in water with a neutralizing agent. Emulsion-type aqueous resins are typically produced by emulsion polymerization or by mechanically forced emulsification.

[0010] The aqueous acrylic resins used in the present disclosure are an emulsion-type hydroxyl-containing acrylic resin emulsion (A) produced by emulsion polymerization, and a colloidal dispersion-type core-shell acrylic resin dispersion (B). The hydroxyl-containing acrylic resin emulsion (A) ensures coating film properties (e.g., strength). The core-shell acrylic resin dispersion (B) enhances the dispersibility of the hydrophobic melamine resin.

[0011] The core-shell acrylic resin dispersion (B) has a branched hydrocarbon group having 4 to 24 carbon atoms in the core portion and a hydrophilic resin in the shell portion. The hydrocarbon group enhances the hydrophobicity of the core portion and improves the affinity between the core portion and the hydrophobic melamine resin. The hydrophilic resin in the shell portion enhances the water dispersibility of the core-shell acrylic resin dispersion (B). That is, the core-shell acrylic resin dispersion (B) finely disperses the hydrophobic melamine resin in an aqueous solvent while capturing it, thereby suppressing aggregation of the hydrophobic melamine resin and maintaining its dispersed state.

[0012] The hydroxyl group-containing acrylic resin emulsion (A) and the core-shell type acrylic resin dispersion (B) can be fractionated, for example, by centrifuging the aqueous coating composition. The weight average molecular weight of each of the fractionated acrylic resins is determined, and those exceeding 100,000 can be considered emulsion type, while those below 100,000 can be considered colloidal dispersion type. When the weight average molecular weight exceeds 1,000,000, it becomes difficult to measure the weight average molecular weight. Acrylic resins whose weight average molecular weight cannot be measured can be considered emulsion type.

[0013] Acrylic resins are less likely to yellow due to ultraviolet rays, etc. By using the core-shell acrylic resin dispersion (B) as a dispersing component for the hydrophobic melamine resin, a coating film can be obtained that has excellent appearance and is less likely to yellow while ensuring water resistance.

[0014] The solid content concentration is calculated from the residue when the object is heated at 150°C.

[0015] The average particle size is the 50% average particle size (D50) in the volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution measuring device.

[0016] The acid value and hydroxyl value may be calculated from the composition of the raw material monomers in accordance with JIS regulations, or may be determined by neutralization titration using an aqueous potassium hydroxide solution in accordance with JIS K 0070. The acid value and hydroxyl value are values ​​based on the solid content.

[0017] The weight average molecular weight and number average molecular weight are measured using polystyrene standards by the GPC (gel permeation chromatography) method.

[0018] (Meth)acrylic acid esters refer to acrylic acid esters and methacrylic acid esters. (Meth)acrylic acid refers to acrylic acid and methacrylic acid.

[0019] [Aqueous Coating Composition] The aqueous coating composition according to the present disclosure comprises a hydroxyl-containing acrylic resin emulsion (A), a core-shell acrylic resin dispersion (B), and a hydrophobic melamine resin (C). The core-shell acrylic resin dispersion (B) has a branched hydrocarbon group having 4 to 24 carbon atoms in its core portion, and a hydrophilic resin in its shell portion. The solids mass ratio (A:B) of the hydroxyl-containing acrylic resin emulsion (A) to the core-shell acrylic resin dispersion (B) is 30:70 to 90:10.

[0020] Hydroxyl-containing acrylic resin emulsion (A) refers to an acrylic resin prepared by emulsion polymerization. The hydroxyl-containing acrylic resin emulsion (A) (hereinafter sometimes simply referred to as acrylic resin emulsion (A)) is water-dispersible and dispersed in particulate form in an aqueous solvent.

[0021] The average particle size of the acrylic resin emulsion (A) is, for example, 20 nm or more and 200 nm or less. The average particle size of the acrylic resin emulsion (A) may be 30 nm or more, or 50 nm or more. The average particle size of the acrylic resin emulsion (A) may be 180 nm or less, or 140 nm or less.

[0022] The acrylic resin emulsion (A) may have a hydroxyl value of 20 mgKOH / g or more and 180 mgKOH / g or less, and an acid value of 1 mgKOH / g or more and 80 mgKOH / g or less.

[0023] The solid content of the acrylic resin emulsion (A) is, for example, 20 parts by mass or more and 70 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous coating composition. The content of the acrylic resin emulsion (A) may be 25 parts by mass or more, or may be 30 parts by mass or more. The content of the acrylic resin emulsion (A) may be 60 parts by mass or less, or may be 50 parts by mass or less.

[0024] (Production Method) The acrylic resin emulsion (A) can be produced by emulsion polymerization of an α,β-ethylenically unsaturated monomer having a hydroxyl group and another α,β-ethylenically unsaturated monomer. Examples of the other α,β-ethylenically unsaturated monomer include a (meth)acrylic acid ester and an α,β-ethylenically unsaturated monomer having an acid group.

[0025] Examples of α,β-ethylenically unsaturated monomers having a hydroxyl group include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts of these with ε-caprolactone. These may be used alone or in combination of two or more.

[0026] Examples of (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, phenyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate. These may be used alone or in combination of two or more.

[0027] Examples of α,β-ethylenically unsaturated monomers having an acid group include acrylic acid, methacrylic acid, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, isocrotonic acid, α-hydro-ω-((1-oxo-2-propenyl)oxy)poly(oxy(1-oxo-1,6-hexanediyl)), maleic acid, fumaric acid, itaconic acid, 3-vinylsalicylic acid, 3-vinylacetylsalicylic acid, 2-acrylamido-2-methylpropanesulfonic acid, p-hydroxystyrene, and 2,4-dihydroxy-4′-vinylbenzophenone. These may be used alone or in combination of two or more.

[0028] Other α,β-ethylenically unsaturated monomers may be used in combination. Examples of other α,β-ethylenically unsaturated monomers include polymerizable amide compounds, polymerizable aromatic compounds, polymerizable nitriles, polymerizable alkylene oxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, α-olefins, dienes, polymerizable carbonyl compounds, polymerizable alkoxysilyl compounds, and other polymerizable compounds. These may be used alone or in combination of two or more.

[0029] The emulsion polymerization method is not particularly limited. For example, an emulsifier is dissolved in an aqueous medium containing water or, if necessary, an organic solvent such as an alcohol or an ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and an α,β-ethylenically unsaturated monomer and a polymerization initiator are added dropwise with heating and stirring. The α,β-ethylenically unsaturated monomer may be previously emulsified with an emulsifier.

[0030] Examples of emulsifiers include anionic emulsifiers such as soap, alkyl sulfonates, and polyoxyethylene alkyl sulfates; nonionic emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polypropylene glycol ethylene oxide adducts, polyethylene glycol fatty acid esters, and polyoxyethylene sorbitan fatty acid esters; and reactive emulsifiers such as nonionic surfactants having a polyoxyethylene alkyl phenyl ether basic structure and radically polymerizable propenyl groups introduced into the hydrophobic group, cationic surfactants having a quaternary ammonium salt structure, and anionic surfactants containing a sulfonic acid group, a sulfonate group, a sulfate ester group, and / or an ethyleneoxy group and having a radically polymerizable carbon-carbon double bond. These may be used alone or in combination of two or more. The emulsifier may be used in an amount of, for example, 0.5 to 10 parts by mass, based on the solids content, per 100 parts by mass of raw material monomer.

[0031] The polymerization initiator is not particularly limited, and examples thereof include water-soluble polymerization initiators and oil-soluble polymerization initiators. Examples of water-soluble polymerization initiators include persulfate initiators such as ammonium persulfate, sodium persulfate, and potassium persulfate; and inorganic initiators such as hydrogen peroxide. Examples of oil-soluble polymerization initiators include organic peroxides such as benzoyl peroxide, t-butyl peroxybenzoate, t-butyl hydroperoxide, t-butyl peroxy(2-ethylhexanoate), t-butylperoxy-3,5,5-trimethylhexanoate, and di-t-butyl peroxide; and azobis compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 1,1'-azobis-cyclohexane-1-carbonitrile. These may be used alone or in combination of two or more. The polymerization initiator is used in an amount of, for example, 0.01% by mass to 10% by mass of the raw material monomer.

[0032] The polymerization conditions are not particularly limited. The polymerization temperature is, for example, 30° C. to 90° C., and the polymerization time is, for example, 3 hours to 12 hours. The raw material monomer concentration during the polymerization reaction is, for example, 30% by mass to 70% by mass.

[0033] If necessary, chain transfer agents such as mercaptans (eg, lauryl mercaptan) and α-methylstyrene dimers are used.

[0034] After emulsion polymerization, neutralization is carried out using a basic compound, if necessary. The basic compound may be an inorganic or organic base. Specific examples of the basic compound include organic bases such as ammonia, triethylamine, propylamine, dibutylamine, amylamine, 1-aminooctane, 2-dimethylaminoethanol, ethylaminoethanol, 2-diethylaminoethanol, 1-amino-2-propanol, 2-amino-1-propanol, 2-amino-2-methyl-1-propanol, 3-amino-1-propanol, 1-dimethylamino-2-propanol, 3-dimethylamino-1-propanol, 2-propylaminoethanol, ethoxypropylamine, aminobenzyl alcohol, and morpholine; and inorganic bases such as sodium hydroxide and potassium hydroxide. These may be used alone or in combination of two or more. The basic compound is used, for example, in an amount of 0.2 mol to 1.0 mol per mol of carboxyl groups contained in the polymer (neutralization rate: 20% to 100%).

[0035] The neutralization rate is equivalent to the amount of the basic compound used relative to the acid group (e.g., carboxy group). The neutralization rate is calculated by the following formula:

[0036] The acrylic resin emulsion (A) may be a single-layer type or a core-shell type having a core portion and a shell portion. The core-shell type acrylic resin emulsion (A) can be prepared by a known production method, for example, as described in JP-A-2002-12816.

[0037] Core-shell type acrylic resin dispersion (B) The core-shell type acrylic resin dispersion (B) is a core-shell type acrylic resin prepared by a polymerization method (typically, a solution polymerization method) that does not use an emulsifier. The core-shell type acrylic resin dispersion (B) (hereinafter, sometimes simply referred to as the acrylic resin dispersion (B)) is also water-dispersible and is dispersed in the aqueous coating composition in the form of particles.

[0038] The average particle size of the acrylic resin dispersion (B) is, for example, 20 nm or more and 200 nm or less. The average particle size of the acrylic resin dispersion (B) may be 180 nm or less, 160 nm or less, 150 nm or less, or 100 nm or less. The average particle size of the acrylic resin dispersion (B) may be 25 nm or more, or 30 nm or more.

[0039] The weight-average molecular weight of the acrylic resin dispersion (B) is, for example, from 7,600 to 80,000. This can suppress color reversion when the aqueous coating composition according to the present disclosure is used to form a base coating film.

[0040] The weight average molecular weight of the acrylic resin dispersion (B) may be 15,000 or more, 16,000 or more, or 20,000 or more. The weight average molecular weight of the acrylic resin dispersion (B) may be 60,000 or less, or 48,000 or less.

[0041] The acid value of the acrylic resin dispersion (B) may be 25 mg KOH / g or more and 50 mg KOH / g or less. This results in a small average particle size and a sharp particle size distribution of the acrylic resin dispersion (B). Therefore, when the aqueous coating composition according to the present disclosure is used to form a base coating film, color reversion can be suppressed.

[0042] The acid value of the acrylic resin dispersion (B) may be 30 mgKOH / g or more, or 35 mgKOH / g or more. From the viewpoint of water resistance, the acid value of the acrylic resin dispersion (B) may be 50 mgKOH / g or less, or 45 mgKOH / g or less.

[0043] Color reversion refers to a phenomenon in which the components of a clear coating composition penetrate and mix with the underlying coating film (typically a base coating film), resulting in a deterioration in design. For example, in the case of a colored base coating film whose underlying layer contains a luster pigment, if the components of the clear coating composition penetrate into the underlying layer, the arrangement of the luster pigment is disrupted, making it impossible to obtain the desired flip-flop (FF) property. Such a decrease in FF property is an example of color reversion. Having resistance to color reversion can be said to be a performance in which the design exhibited by the coating film underlying the clear coating film is not impaired by the clear coating composition. Although the reason is not clear, when the acid value and / or weight average molecular weight of the acrylic resin dispersion (B) are within the above ranges, the clear coating composition is less likely to penetrate into the base coating film, thereby suppressing color reversion.

[0044] The content of the acrylic resin dispersion (B) is, for example, 3 parts by mass or more and 50 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous coating composition. The content of the acrylic resin dispersion (B) may be 5 parts by mass or more, or 10 parts by mass or more. The content of the acrylic resin dispersion (B) may be 40 parts by mass or less, 30 parts by mass or less, or 20 parts by mass or less.

[0045] The solids mass ratio (A:B) of the acrylic resin emulsion (A) to the acrylic resin dispersion (B) is 30:70 to 90:10. This improves the storage stability of the aqueous coating composition while ensuring coating film properties, resulting in a coating film with excellent appearance. The solids mass ratio (A:B) may be 50:50 to 85:15, or may be 55:45 to 85:15.

[0046] The acrylic resin dispersion (B) has a branched hydrocarbon group having 4 to 24 carbon atoms (hereinafter referred to as a hydrophobic group for convenience) in the core portion, and a hydrophilic resin (Bs) in the shell portion. The resin forming the core portion is referred to as a hydrophobic resin (Bc) for convenience. The "core portion" and the "shell portion" may or may not be chemically crosslinked. The acrylic resin dispersion (B) has a hydrophobic resin (Bc) in the interior and a hydrophilic resin (Bs) on the exterior, with at least a portion of the hydrophobic resin (Bc) covered with the hydrophilic resin (Bs).

[0047] The mass ratio (Bc:Bs) of the hydrophobic resin (Bc) to the hydrophilic resin (Bs) is, for example, 95:5 to 60:40. When the mass ratio of the hydrophilic resin (Bs) is 5% or more, the dispersibility of the core-shell acrylic resin dispersion (B) in water is further improved, and the storage stability of the aqueous coating composition is improved. When the mass ratio of the hydrophilic resin (Bs) is 40% or less, the water resistance and appearance of the coating film can be improved. The ratio of the hydrophobic resin (Bc):hydrophilic resin (Bs) may be 90:10 to 70:30, or 85:15 to 75:25.

[0048] A "hydrocarbon group" refers to a group containing carbon and hydrogen, and is a group obtained by removing one hydrogen atom from a hydrocarbon. Examples of hydrocarbon groups include aliphatic hydrocarbon groups and aromatic hydrocarbons having 4 to 24 carbon atoms. The aliphatic hydrocarbon group may be linear, branched, or cyclic, and may be saturated or unsaturated. The hydrogen bonded to the carbon may be substituted with a halogen atom or the like.

[0049] The number of carbon atoms in the hydrophobic group may be 7 to 18, or 8 to 15. The hydrophobic group may be saturated. The hydrophobic group may be a branched alkyl group having 4 to 24 carbon atoms.

[0050] The hydrophilic resin (Bs) is a neutralized product of an acrylic resin having an acid group, which is disposed so as to cover the core portion.

[0051] (Production Method) The acrylic resin dispersion (B) can be produced, for example, by multi-stage polymerization using the reactive solvent (x) having a hydrophobic group described above.

[0052] The acrylic resin dispersion (B) can be produced, for example, by a method comprising: a first step of adding dropwise a first monomer mixture containing a first acid group-containing α,β-ethylenically unsaturated monomer (a1) into a reactive solvent (x) having one glycidyl group and a hydrophobic group to synthesize a hydrophobic resin (Bc) and obtain a liquid material containing the hydrophobic resin (Bc); a second step of adding dropwise a second monomer mixture containing a second acid group-containing α,β-ethylenically unsaturated monomer (a2) into the liquid material to synthesize an acid group-containing resin (Bs') and obtain a core-shell acrylic resin (B') having the hydrophobic resin (Bc) and the acid group-containing resin (Bs'); a step of adding a basic compound to neutralize the acid groups remaining in the core-shell acrylic resin (B'); and a step of adding deionized water to cause phase inversion and obtain a varnish containing the acrylic resin dispersion (B) dispersed in deionized water.

[0053] <First Step> In the first step, radical polymerization of the first monomer mixture and ring-opening addition reaction between the glycidyl group of the reactive solvent (x) and the acid group-containing monomer (a) proceed. In the first step, so-called solution polymerization is carried out.

[0054] Taking advantage of the fact that the ring-opening reaction of the epoxy ring is difficult to occur at low temperatures, the polymerization reaction of the first monomer mixture and the ring-opening reaction of the epoxy ring are carried out stepwise in Step 1, thereby making it possible to incorporate a hydrophobic group into the hydrophobic resin (Bc). For example, the temperature of the reaction system is first lowered (e.g., 50°C or higher but lower than 130°C) to polymerize the first monomer mixture to obtain a precursor, and then the temperature is raised (e.g., 130°C or higher but lower than 180°C) to cause ring-opening addition of the reactive solvent (x) to the precursor.

[0055] Examples of the raw material monomer contained in the first monomer mixture include the same α,β-ethylenically unsaturated monomers as those used in producing the acrylic resin emulsion (A).

[0056] In order to improve the physical properties of the resulting coating film, the mass of the acid group-containing monomer in the first monomer mixture may be 5 parts by mass or more and 30 parts by mass or less, relative to 100 parts by mass of the first monomer mixture. The mass of the acid group-containing monomer may be 10 parts by mass or more. The mass of the acid group-containing monomer may be 25 parts by mass or less.

[0057] The reactive solvent (x) has one glycidyl group and a hydrophobic group. The reactive solvent (x) may be a monocarboxylic acid glycidyl ester. The monocarboxylic acid glycidyl ester is, for example, a compound represented by the following general formula (1): (wherein R is a monovalent organic group, including the above-mentioned hydrophobic group).

[0058] The reactive solvent (x) is used, for example, in an amount such that the mass ratio of the hydrophobic resin (Bc) to the hydrophilic resin (Bs) in the resin particles having a core-shell structure (hydrophobic resin (Bc):hydrophilic resin (Bs), mass %) is 95:5 to 60:40.

[0059] <Second Step> In the second step, polymerization of the second monomer mixture mainly proceeds to synthesize an acid group-containing resin (Bs'). When a basic compound is added in a subsequent step, the acid groups are neutralized, and the acid group-containing resin (Bs') is hydrophilized to form a hydrophilic resin (Bs). The hydrophilic resin (Bs) acts as a dispersing component for dispersing the acrylic resin dispersion (B) in water, reducing the particle size of the resin particles and improving their dispersion stability.

[0060] Following the second step, a polymerization initiator may be added, and aging may be performed by stirring and heating. The aging may be performed, for example, at the same temperature as in the second step for 0.5 hours to 3 hours.

[0061] The types and blending ratios of the raw material monomers contained in the second monomer mixture may be the same as or different from those contained in the first monomer mixture. The first acid group-containing monomer (a1) and the second acid group-containing monomer (a2) may be the same as or different from each other.

[0062] In order to improve the dispersibility of the resulting hydrophilic resin (Bs), the mass of the acid group-containing monomer in the second monomer mixture may be 5 parts by mass or more and 30 parts by mass or less, relative to 100 parts by mass of the second monomer mixture. The mass of the acid group-containing monomer may be 10 parts by mass or more. The mass of the acid group-containing monomer may be 25 parts by mass or less.

[0063] A polymerization initiator may be used in the first step, the second step, and the aging step. The total amount of the polymerization initiator used is appropriately set, for example, depending on the type and amount of the raw material monomers. The total amount of the polymerization initiator used may be, for example, 0.2 parts by mass or more and 2.0 parts by mass or less, relative to 100 parts by mass of the total of the first monomer mixture and the second monomer mixture. The amount of the polymerization initiator used may be 0.2 parts by mass or more. The amount of the polymerization initiator used may be 1.5 parts by mass or less.

[0064] Examples of the polymerization initiator include 2,2'-azobisisobutyronitrile, benzoyl peroxide, 2,2-di(t-amylperoxy)butane, di-t-butyl peroxide, di-t-amyl peroxide (DTA), t-butyl peroctoate, and 2,2'-azobis(2-methylbutyronitrile).

[0065] <Neutralization and Phase Inversion Step> The acid groups (typically carboxy groups) remaining in the core-shell acrylic resin (B') are neutralized with a basic compound. This makes the core-shell acrylic resin (B') water-dispersible. Thereafter, deionized water is added to cause phase inversion. This disperses the core-shell acrylic resin (B') in water, and a varnish containing the acrylic resin dispersion (B) is obtained.

[0066] The phase inversion and water dispersion are carried out, for example, by stirring a mixture of a varnish containing the neutralized core-shell acrylic resin (B') and deionized water. Stirring can reduce the average particle size of the acrylic resin dispersion (B).

[0067] Examples of the basic compound include the same compounds as those exemplified as those used for neutralizing the acrylic resin emulsion (A). The basic compound is added, for example, in an amount such that the neutralization rate of the acid groups contained in the acrylic resin dispersion (B) is 70% or more and 100% or less. When the neutralization rate is within this range, the water dispersibility of the acrylic resin dispersion (B) is improved, and the average particle size can be made smaller. The neutralization rate may be 75% or more, 80% or more, 85% or more, or 90% or more.

[0068] Hydrophobic Melamine Resin (C) The hydrophobic melamine resin (C) acts as a curing agent.

[0069] The hydrophobic melamine resin (C) has a melamine nucleus (triazine nucleus) and three nitrogen atoms surrounding the melamine nucleus. 1 ~R 6 The hydrophobic melamine resin (C) may generally be a polynuclear compound in which a plurality of melamine nuclei are bonded to one another, or a mononuclear compound consisting of one melamine nucleus.

[0070] The structure of the melamine nucleus is represented, for example, by the following general formula (2). (In the formula, substituent R １ ~R ６ each independently represents a hydrogen atom, an alkyl ether group, a methylol group, or a bonding moiety to another triazine ring.

[0071] Substituent R １ ~R ６ are each independently a hydrogen atom, an alkyl ether group (—CH ２ -OR ７ ) or a methylol group (-CH ２ The substituent R １ ~R ６ , R ７ may each independently be an alkyl group having 1 to 8 carbon atoms, or may be an alkyl group having 1 to 4 carbon atoms. The alkyl group may be linear or branched. The alkyl group may be a methyl group, an n-butyl group, or an isobutyl group.

[0072] Melamine resins are generally classified into water-soluble melamine resins and hydrophobic melamine resins. Water-soluble melamine resins satisfy all of the following conditions (i) to (iii): (i) the number-average molecular weight of the melamine resin is 1,000 or less; (ii) the R １ ~R ６ In the formula, at least one is a hydrogen atom (imino group) or CH ２ OH (methylol group). That is, the total amount of the average imino group and the average methylol group is 1.0 or more. (iii) R in the above general formula (1) １ ~R ６ In this case, R １ ~R ６ is CH ２ OR ７ If ７ is a methyl group.

[0073] The hydrophobic melamine resin is a melamine resin other than the above-mentioned water-soluble melamine resin. That is, it satisfies any one of the following conditions (iv) to (vi): (iv) The number average molecular weight of the melamine resin exceeds 1,000. (v) The total amount of the average imino group content and the average methylol group content is 1.0 or less. (vi) R in the above formula (1) １ ~R ６ In this case, R １ ~R ６ Two or more of them are CH ２ OR ７ and R ７ is an alkyl group having 1 to 4 carbon atoms, １ ~R ６ R that constitutes ７ At least one of the groups is an alkyl group having 2 to 4 carbon atoms.

[0074] Commercially available hydrophobic melamine resins (C) include, for example, the Cymel series (all trade names) manufactured by Allnex, such as Cymel 202, Cymel 204, Cymel 211, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 250, Cymel 251, Cymel 254, Cymel 266, Cymel 267, and Cymel 285 (melamine resins having both methoxy and butoxy groups); Mycoat 506 (a melamine resin having only butoxy groups, manufactured by Mitsui Cytec); and U-Van 20N60 and U-Van 20SE (the U-Van (trade name) series manufactured by Mitsui Chemicals). These may be used alone or in combination of two or more.

[0075] The solid content of the hydrophobic melamine resin (C) is, for example, 10 parts by mass or more and 55 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous coating composition. This facilitates the curing reaction, making it easier to obtain a coating film with high hardness. The solid content of the hydrophobic melamine resin (C) may be 20 parts by mass or more, or 25 parts by mass or more. The solid content of the hydrophobic melamine resin (C) may be 50 parts by mass or less, or 40 parts by mass or less.

[0076] The solid content mass ratio (B:C) of the acrylic resin dispersion (B) to the hydrophobic melamine resin (C) may be, for example, 10:90 to 50:50. This can further improve the water dispersibility of the hydrophobic melamine resin. The solid content mass ratio (B:C) may be 15:85 to 45:55, or 20:80 to 40:60.

[0077] Other Resin Components The aqueous coating composition may contain other resin components as needed. Examples of other resin components include water-soluble acrylic resins, polyester resin dispersions, and polyurethane resin dispersions. These may be used alone or in combination of two or more.

[0078] The content of the water-soluble acrylic resin is, for example, 1 part by mass or more and 60 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous coating composition. The content of the water-soluble acrylic resin may be 2 parts by mass or more, or 5 parts by mass or more. The content of the water-soluble acrylic resin may be 50 parts by mass or less, 30 parts by mass or less, or 10 parts by mass or less.

[0079] The polyester resin dispersion also has the function of dispersing the hydrophobic melamine resin (C). On the other hand, the polyester resin may induce yellowing. The polyester resin may cause yellowing, particularly by reacting with an isocyanate compound contained in the clear coating film. In consideration of this point, the content of the polyester resin dispersion may be 10 parts by mass or less, 5 parts by mass or less, or even 0 parts by mass, per 100 parts by mass of the resin solid content of the aqueous coating composition.

[0080] The content of the polyurethane resin dispersion is, for example, 1 part by mass or more and 60 parts by mass or less relative to 100 parts by mass of the resin solids content of the aqueous coating composition. The content of the polyurethane resin dispersion may be 2 parts by mass or more, or 5 parts by mass or more. The content of the polyurethane resin dispersion may be 50 parts by mass or less, 30 parts by mass or less, or 10 parts by mass or less.

[0081] Other Curing Components The aqueous coating composition may contain curing components other than the hydrophobic melamine resin (C). Examples of other curing components include blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more.

[0082] Additives The aqueous coating composition may contain various additives as needed, such as film-forming aids, surface conditioners, preservatives, mildew inhibitors, antifoaming agents, light stabilizers, UV absorbers, antioxidants, and pH adjusters.

[0083] Pigment The aqueous coating composition may contain a pigment. The pigment is not particularly limited, and examples thereof include organic color pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, monoazo pigments, disazo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, thioindigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, naphthol pigments, pyrazolone pigments, anthraquinone pigments, anthrapyrimidine pigments, and metal complex pigments; yellow lead, yellow iron oxide, chromium oxide, molybdate orange, red iron oxide, titanium yellow, zinc white, and carbon. Examples of inorganic color pigments include black, titanium dioxide, cobalt green, phthalocyanine green, ultramarine, cobalt blue, phthalocyanine blue, and cobalt violet; mica pigments (titanium dioxide-coated mica, colored mica, and metal-plated mica); graphite, aluminum flakes, alumina flakes, metallic titanium flakes, stainless steel flakes, plate-like iron oxide, phthalocyanine flakes, and metal-plated glass flakes, as well as other colored and colored flat pigments; and extender pigments such as titanium oxide, calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silica, and calcined kaolin.

[0084] Preparation of Aqueous Paint Composition There are no particular limitations on the method for preparing the aqueous paint composition, and it can be prepared by stirring the components with a stirrer, etc. The pigment can be pre-dispersed in a vehicle containing water, a surfactant, a dispersant, etc. using a sand grind mill, etc. to form a pigment paste, which can then be mixed with the other components.

[0085] [Coated Article] A coated article is obtained using the aqueous coating composition according to the present disclosure. The aqueous coating composition according to the present disclosure is suitable for use in forming a coating film adjacent to a clear coating film. The coated article comprises, for example, a substrate and a multilayer coating film in which a colored base coating film, a metallic base coating film, and a clear coating film are laminated in this order. The metallic base coating film is formed using the aqueous coating composition according to the present disclosure. In this case, the aqueous coating composition according to the present disclosure contains the mica pigment and / or flat pigment described above. In the coated article, inter-layer mixing between the metallic base coating film and the clear coating film is suppressed, and the coated article has an excellent appearance.

[0086] (Substrate) Examples of the substrate material include metal, resin, and glass. Specific examples of the substrate include automobile bodies and automobile body parts such as passenger cars, trucks, motorcycles, and buses, and automobile parts such as spoilers, bumpers, mirror covers, grilles, and door knobs.

[0087] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of metal substrates include steel sheets such as cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0088] The metal substrate may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After the surface treatment, the metal substrate may be further coated with an electrodeposition paint. The electrodeposition paint may be either a cationic type or an anionic type.

[0089] Examples of resins include polyethylene resin, EVA resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), vinyl chloride resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), polycarbonate resin, acrylic resin, acrylonitrile butadiene styrene (ABS) resin, acrylonitrile styrene (AS) resin, polyamide resin, acetal resin, phenolic resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). Resin substrates may be degreased.

[0090] (Colored base coating film) The colored base coating film is interposed between the substrate and the metallic base coating film. The colored base coating film makes the painted surface uniform, making it easier to suppress unevenness in the metallic base coating film.

[0091] The thickness of the colored base coating film after curing may be 5 μm or more and 60 μm or less, from the viewpoint of smoothness and chipping resistance of the coated article.

[0092] The colored base coating film is formed from a colored base coating composition. The colored base coating composition may be aqueous or solvent-based. The colored base coating composition may be aqueous. An aqueous colored base coating composition contains, for example, the above-mentioned acrylic resin emulsion and melamine resin. A solvent-based colored base coating composition contains an organic solvent as the main solvent. In a solvent-based colored base coating composition, the proportion of the organic solvent in the solvent is 50% by mass or more, may be 70% by mass or more, or may be 100% by mass. The colored base coating composition may further contain a pigment and various additives.

[0093] (Metallic Base Coating Film) The metallic base coating film is formed from the aqueous coating composition according to the present disclosure.

[0094] The thickness of the base coating film is not particularly limited and may be appropriately set depending on the purpose. The thickness of the base coating film after curing may be 0.1 μm or more and 45 μm or less.

[0095] (Clear Coating) The clear coating improves the gloss of the coated article and prevents the pigments blended in the lower layer from falling off or popping out.

[0096] The thickness of the clear coating film after curing may be 15 μm or more and 50 μm or less from the viewpoint of scratch resistance and smoothness.

[0097] The clear coating film is formed by a clear coating composition. The clear coating composition may be solvent-based, water-based, or powder-type. The clear coating composition may be solvent-based. From the standpoint of transparency or acid etching resistance, the solvent-based clear coating composition may contain a hydroxyl group-containing acrylic resin and / or polyester resin as a coating film-forming resin, and an amino resin and / or an isocyanate compound as a curing agent. The solvent-based clear coating composition may also contain an acrylic resin and / or polyester resin having a carboxylic acid and / or an epoxy group. The clear coating composition may contain the various pigments described above to the extent that transparency is not impaired. The clear coating composition may contain various additives as needed.

[0098] The clear coating composition may contain an isocyanate compound as a curing agent. Metallic base coatings containing an acrylic resin as the main coating film-forming resin are less likely to yellow due to reaction with the isocyanate compound contained in the clear coating.

[0099] The isocyanate compound has at least two isocyanate groups per molecule. Examples of the isocyanate compound include aliphatic polyisocyanates, alicyclic polyisocyanates, aliphatic polyisocyanates having an aromatic ring not bonded to an isocyanate group in the molecule (araliphatic polyisocyanates), aromatic polyisocyanates, and derivatives of these polyisocyanates. Specific examples include aromatic polyisocyanates such as tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate, and metaxylylene diisocyanate; aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate; and polymers of these compounds, such as biuret type, nurate type, and adduct type. These compounds may be used alone or in combination of two or more.

[0100] [Method for producing multilayer coating film] The multilayer coating film is produced, for example, by a method comprising the steps of applying a colored base coating composition onto an object to be coated to form an uncured colored base coating film, curing the uncured colored base coating film, applying an aqueous coating composition according to the present disclosure to form an uncured metallic base coating film, curing the uncured metallic base coating film, applying a clear coating composition onto the metallic base coating film to form an uncured clear coating film, and curing the uncured clear coating film.

[0101] When the clear coating film is formed, the colored base coating film and the metallic base coating film may be cured or uncured. From the viewpoints of productivity, adhesion, and water resistance, each coating film may be laminated without curing (so-called wet-on-wet coating), and then these multiple uncured coating films may be cured simultaneously.

[0102] Wet-on-wet coating comprises the steps of applying a colored base coating composition onto an object to be coated to form an uncured colored base coating film, applying the aqueous coating composition according to the present disclosure onto the uncured colored base coating film to form an uncured metallic base coating film, applying a clear coating composition onto the uncured metallic base coating film to form an uncured clear coating film, and simultaneously curing the uncured colored base coating film, the uncured metallic base coating film, and the uncured clear coating film.

[0103] After application of the colored base coating composition, preheating may be performed before application of the aqueous coating composition according to the present disclosure. After application of the aqueous coating composition according to the present disclosure, preheating may be performed before application of the clear coating composition. Preheating may be performed, for example, by leaving the coating at a temperature of 20°C to 25°C for 5 to 15 minutes, or by heating at a temperature of 50°C to 80°C for 30 seconds to 10 minutes.

[0104] Examples of the coating method include air spray coating, airless spray coating, electrostatic spray coating, multi-stage coating using air electrostatic spray coating (typically, two-stage coating), and coating that combines air electrostatic spray coating with a rotary atomizer-type electrostatic coater.

[0105] The curing of each coating composition is carried out, for example, under conditions of a heating temperature of 80° C. to 180° C. and a heating time of 5 to 60 minutes.

[0106] The present invention will be described in more detail with reference to the following examples, but is not limited thereto. In the examples, "parts" and "%" are based on the mass of the solid content unless otherwise specified.

[0107] (Weight-Average Molecular Weight) The weight-average molecular weight was measured using a GPC apparatus "HLC8220GPC" (trade name, manufactured by Tosoh Corporation) and four columns, "Shodex KF-606M" and "Shodex KF-603" (both trade names, manufactured by Showa Denko K.K.), under the following conditions: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 0.6 cc / min, and detector: RI.

[0108] The solid content concentration was calculated from the residue when the object was heated at 150°C. The average particle size was the 50% average particle size (D50) in the volume-based particle size distribution measured using a Microtrac particle size distribution analyzer with the product name "UPA-150" manufactured by Nikkiso Co., Ltd. The acid value and hydroxyl value were calculated from the composition of the raw material monomers.

[0109] [Production Example A] Production of Hydroxyl-Containing Acrylic Resin Emulsion (A) 126.5 parts of deionized water was added to a reaction vessel, and the temperature was raised to 80°C while mixing and stirring in a nitrogen stream. Next, a monomer emulsion consisting of 100 parts of a monomer mixture (containing 27.61 parts of methyl acrylate, 53.04 parts of ethyl acrylate, 4.00 parts of styrene, 9.28 parts of 2-hydroxyethyl methacrylate, 3.07 parts of methacrylic acid, and 3.00 parts of allyl methacrylate), 1.1 parts of an emulsifier (trade name: Adeka Reasoap SR-10, manufactured by ADEKA Corporation), and 80 parts of deionized water, and an initiator solution consisting of 0.3 parts of ammonium persulfate and 10 parts of deionized water, were added dropwise to the reaction vessel in parallel over a period of 2 hours. After completion of the dropwise addition, the mixture was aged at the same temperature for 2 hours.

[0110] The mixture was then cooled to 40°C and filtered through a 400-mesh filter. 20 parts of deionized water and 0.32 parts of dimethylaminoethanol were added to the filtrate, and the pH was adjusted to 6.5. This yielded a hydroxyl-containing acrylic resin emulsion (A) with an average particle size of 90 nm, a Tg of -9.5°C, a nonvolatile content of 30%, an acid value of 20 mgKOH / g, and a hydroxyl value of 40 mgKOH / g.

[0111] [Production Example B-1] Production of Core-Shell Acrylic Resin Dispersion (B-1) (1) Synthesis of Hydrophobic Acrylic Resin (Bc) A reaction vessel equipped with a stirrer, a temperature controller, a condenser, and a dropping device was charged with 30 parts of a reactive solvent (CAE, glycidyl ester of monocarboxylic acid having a branched alkyl group having 9 carbon atoms, trade name: Cardurer E10P, manufactured by Hexion Corporation, boiling point 251 to 278°C), and the temperature was raised to 165°C with stirring to reflux. Separately, a mixture of 9.47 parts of acrylic acid (AA), 5.8 parts of 2-hydroxyethyl methacrylate (HEMA), 11.6 parts of cyclohexyl methacrylate (CHMA), 7.5 parts of n-butyl acrylate (NBA), 16.9 parts of styrene (ST), 0.28 parts of a polymerization initiator (DTA, trade name: Luperox DTA, manufactured by Arkema Yoshitomi Co., Ltd.), and 6.5 parts of a high-boiling point solvent (dipropylene glycol monomethyl ether (DPM)) was prepared. This mixture was added dropwise to the reaction vessel at 165°C over 3.5 hours to carry out a polymerization reaction and a ring-opening addition reaction.

[0112] (2) Synthesis of carboxyl group-containing acrylic resin (Bs') Separately, a mixture of 3.43 parts of AA, 3.8 parts of HEMA, 2.7 parts of CHMA, 2.9 parts of NBA, 5.8 parts of ST, 0.11 parts of polymerization initiator (Luperox DTA), and 2.4 parts of high-boiling solvent (DPM) was prepared. This mixture was added dropwise to the reaction vessel over 1 hour at 165 ° C. with stirring, and allowed to polymerize for 1 hour. Furthermore, a mixture of 0.1 parts of polymerization initiator (Luperox DTA) and 0.1 parts of high-boiling solvent (DPM) was added to the reaction vessel, and the mixture was allowed to polymerize for 1 hour at 165 ° C. with stirring.

[0113] In this way, a varnish containing a core-shell type acrylic resin having a hydrophobic resin (Bc) and an acid group-containing resin (Bs') and having a solid content concentration of 91% by mass was obtained.

[0114] (3) Neutralization and Phase Inversion A basic compound (dimethylethanolamine, DMEA) was added to the varnish at a ratio of 5.33 parts per 100 parts of acrylic resin at 80°C, and the mixture was stirred for 15 minutes. The neutralization rate of the carboxyl groups was adjusted to 90%. Subsequently, 150 parts of deionized water was added dropwise to the neutralized varnish at 80°C while stirring, to obtain a milky white dispersion containing acrylic resin particles.

[0115] The acrylic resin particles contained in the dispersion were expected to have a core-shell structure, and the mass ratio of the core was calculated to be 81.4%, and the mass ratio of the shell was calculated to be 18.6%. The acrylic resin particles had an average particle size of 57 nm, an acid value of 37.2 mg KOH / g, and a weight-average molecular weight of 32,000.

[0116] [Production Examples B-2 to B-6, Comparative Production Example b-1] Core-shell type acrylic resin dispersions were obtained in the same manner as in Production Example B-1, except that the types of raw material monomers, neutralization rates, etc. were changed as shown in Table 1.

[0117] [Production Example 1] Production of Water-Soluble Acrylic Resin 23.89 parts of tripropylene glycol methyl ether and 16.11 parts of propylene glycol methyl ether were added to a reaction vessel, and the mixture was heated to 105°C while being mixed and stirred in a nitrogen stream. Separately, a monomer mixture containing 13.1 parts of methyl methacrylate, 68.4 parts of ethyl acrylate, 11.6 parts of 2-hydroxyethyl methacrylate, and 6.9 parts of methacrylic acid was prepared. 100 parts of this monomer mixture and an initiator solution consisting of 10.0 parts of tripropylene glycol methyl ether and 1 part of t-butylperoxy 2-ethylhexanoate were added dropwise to the reaction vessel in parallel over a period of 3 hours. After completion of the dropwise addition, the mixture was aged at the same temperature for 0.5 hours.

[0118] Further, an initiator solution consisting of 5.0 parts of tripropylene glycol methyl ether and 0.3 parts of t-butylperoxy 2-ethylhexanoate was added dropwise to the reaction vessel over 0.5 hours. After the completion of the addition, the mixture was aged at the same temperature for 2 hours.

[0119] Subsequently, 16.1 parts of the solvent were distilled off under reduced pressure (70 torr) at 110°C using a solvent remover, and then 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added, thereby obtaining a water-soluble acrylic resin solution having a nonvolatile content of 30%, an acid value of 40 mgKOH / g, a hydroxyl value of 50 mgKOH / g, a Tg of 10°C, and an Mw of 30,000.

[0120] [Production Example 2] Production of Polyester Resin Dispersion 250 parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts of cyclohexanedicarboxylic acid were added to a reaction vessel equipped with a stirrer, nitrogen inlet tube, temperature control device, condenser, and decanter. The temperature was raised to 180°C, and a condensation reaction was carried out until water no longer distilled. After cooling to 60°C, 120 parts of phthalic anhydride was added to the reaction vessel. The temperature was then raised to 140°C and maintained at this temperature for 60 minutes to obtain a polyester resin. After cooling to 80°C, 59 parts of dimethylaminoethanol (equivalent to 80% of the acid value of the resin (neutralization rate 80%)) and 1,920 parts of deionized water were added to the reaction vessel and stirred. This resulted in a polyester resin dispersion with a solids content of 45% by mass, a hydroxyl value of 110 mgKOH / g, an acid value of 15 mgKOH / g, a Tg of -14°C, and an Mw of 7,000.

[0121] [Production Example 3] Production of Phosphate Group-Containing Acrylic Resin 40 parts of ethoxypropanol were charged into a 1-liter reaction vessel equipped with a stirrer, temperature controller, and condenser. Separately, a monomer solution was prepared consisting of 40 parts of a solution obtained by dissolving 20 parts of Phosmer PP (acid phosphooxyhexa(oxypropylene) monomethacrylate, manufactured by Unichemical Co., Ltd.) in 4 parts of styrene, 35.96 parts of n-butyl acrylate, 18.45 parts of ethylhexyl methacrylate, 13.92 parts of 2-hydroxyethyl methacrylate, 7.67 parts of methacrylic acid, and 20 parts of ethoxypropanol, and 1.7 parts of azobisisobutyronitrile. 121.7 parts of this monomer solution was added dropwise to the reaction vessel at 120°C over 3 hours. Stirring was continued for another hour to obtain a phosphate group-containing acrylic resin (non-volatile content 63%) having an acid value of 105 mgKOH / g, of which the acid value due to phosphate groups was 55 mgKOH / g, a hydroxyl value of 60 mgKOH / g, and a number average molecular weight of 6,000.

[0122]

[0123] Example 1 (i) Preparation of aqueous coating composition 5 parts of the above water-soluble acrylic resin (resin solids content: 30%), 3.6 parts of 10% by mass dimethylaminoethanol, 40 parts of hydroxyl group-containing acrylic resin emulsion (A), 10 parts of core-shell type acrylic resin dispersion (B-1) (resin solids content: 36%), 40 parts of melamine resin (C-1), and 5 parts of urethane resin dispersion (trade name: N-800T, manufactured by Sanyo Chemical Industries, Ltd.) were mixed and dispersed uniformly.

[0124] Next, 27.6 parts of aluminum flakes (average particle size 14 μm, manufactured by Toyo Aluminum Co., Ltd., active ingredient 66%) per 100 parts of resin solids, 5.52 parts of the above phosphate group-containing acrylic resin, 0.5 parts of lauryl acid phosphate, 21.25 parts of 2-ethylhexanol, 8.5 parts of 2-ethylhexyl glycol, and 15 parts of a surfactant (trade name: Surfynol 440, manufactured by Air Products Co., Ltd., polyol product of acetylene dialcohol (solids content 100%)) were uniformly dispersed. Dimethylaminoethanol was added to this dispersion to adjust the pH to 8.1, and the mixture was diluted with deionized water to obtain an aqueous coating composition.

[0125] Details of the melamine resins used are shown in Table 2.

[0126]

[0127] (ii) Formation of multilayer coating film A zinc phosphate-treated dull steel plate (thickness 0.8 mm, length 30 cm, width 40 cm) was electrodeposited with a cationic electrodeposition paint (product name: Powernics 150, manufactured by Nippon Paint Co., Ltd.) so that the dry coating film would be 20 μm thick. After heat curing at 160° C. for 30 minutes, the plate was cooled to obtain a coated object having a cured electrodeposition coating film.

[0128] Aqualex AR-3100 (trade name, water-based base paint, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied to the substrate using a rotary atomizer electrostatic coating device so that the dry film thickness would be 10 μm. After application, the coating was allowed to set for 4 minutes, yielding an uncured colored base coating film.

[0129] The aqueous coating composition prepared above was then diluted with ion-exchanged water to a solids concentration of 23% by mass. The composition was then air-spray coated onto the uncured colored base coating film at room temperature of 23°C to a dry film thickness of 8 μm. After setting for 4 minutes, the coating was preheated at 80°C for 5 minutes. This resulted in an uncured metallic base coating film.

[0130] The coated plate obtained above was allowed to cool to room temperature, and a clear coating composition (two-component urethane curing clear coating (product name: Polyurexcel O-3100 Clear, manufactured by Nippon Paint Automotive Coatings Co., Ltd.)) was applied by air spray to a dry film thickness of 35 μm, and the resulting coating was allowed to set for 7 minutes. This gave an uncured clear coating film.

[0131] Finally, the coated plate was heated in a dryer at 140° C. for 30 minutes to form a multi-layer coating film comprising a colored base coating film, a metallic base coating film and a clear coating film in this order.

[0132] [Examples 2 to 15, Comparative Examples 1 to 4] Aqueous coating compositions were prepared and multi-layer coating films were formed in the same manner as in Example 1, except that the components and / or their amounts were changed as shown in Table 3.

[0133] [Evaluation] The aqueous coating compositions or multi-layer coating films obtained in the examples and comparative examples were evaluated as follows, and the evaluation results are shown in the table below.

[0134] (1) Coating Film Appearance The Short Wave (SW) value (measurement wavelength: 300 to 1,200 μm) of the multilayer coating film was measured using a Wavescan DOI (manufactured by BYK Gardner). The obtained SW values ​​were evaluated according to the following criteria. The smaller the SW value, the higher the smoothness of the coating film. A rating of B or higher can be evaluated as excellent in coating film appearance.

[0135] (Evaluation criteria) A: SW value≦20 B: 20<SW value≦30 C: 31<SW value

[0136] (2) Yellowing A 1600-hour accelerated weathering test was carried out on the multilayer coating film in accordance with JIS B 7753 using a Sunshine Weatherometer S80 (Sunshine carbon arc accelerated weathering tester, manufactured by Suga Test Instruments Co., Ltd.). The b value was measured before and after the accelerated weathering test using a colorimeter (model: CR-331, manufactured by Minolta Co., Ltd.). The difference between the two values ​​was calculated and evaluated according to the following criteria. The b value indicates the yellowness of the coating film, and the smaller the Δb value, the less yellowing there is. A rating of B or higher can be evaluated as being suitable for practical use.

[0137] (Evaluation Criteria) A: Δb value<0.3 B: 0.3≦Δb value<0.5 C: 0.5≦Δb value

[0138] (3) Storage Stability Using the aqueous coating composition immediately after preparation in the Examples or Comparative Examples and the aqueous coating composition stored at 40°C for one month after preparation, multilayer coating films (X, Y) for evaluation were formed in the same manner as above. The appearance (smoothness) and FF properties of both were visually observed and evaluated according to the following criteria. A rating of B or higher indicates that the aqueous coating composition has excellent storage stability.

[0139] (Evaluation criteria) A: No difference is observed between multilayer coating films X and Y. B: The smoothness of multilayer coating film Y is slightly less than that of multilayer coating film X, or slight sagging and / or a decrease in FF properties is observed at the coating film edge of multilayer coating film Y. C: The smoothness of multilayer coating film Y is significantly less than that of multilayer coating film X, or clear sagging and / or a significant decrease in FF properties is observed at the coating film edge of multilayer coating film Y, or coating defects such as repelling, dents, or bumps have occurred in multilayer coating film Y.

[0140] (4) Color Reversion Using a spectrophotometer (trade name: X-Rite MA68II, manufactured by X-Rite Corporation), the lightness (L ５ ) and 110° (shade) lightness (L １１０ ) were measured and the difference was calculated. The larger the difference, the higher the FF property and the more the color reversion was suppressed. A rating of B or higher can be evaluated as being suitable for practical use.

[0141] (Evaluation criteria) A: 90≦L ５ -L１１０ B: 80≦L ５ -L １１０ <90 C:L ５ -L １１０ <80

[0142] (5) Water Resistance The test plates obtained in the same manner as above were immersed in warm water at 40°C for 240 hours. Then, they were taken out of the water and dried at room temperature for 1 hour. The appearance of the coating film after drying was visually observed, and the presence or absence of white blur was evaluated according to the following criteria. A rating of B or higher can be evaluated as excellent water resistance.

[0143] (Evaluation criteria) A: No white blur is observed B: Slight white blur is observed C: Clear white blur is observed

[0144]

[0145] The aqueous coating composition of the present disclosure provides a coating film that has excellent appearance and suppresses yellowing while ensuring water resistance, and is suitable for coating automobile bodies and automotive components.

[0146] This application claims priority based on Japanese Patent Application No. 2023-199319, filed on November 24, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. An aqueous coating composition comprising: a hydroxyl-containing acrylic resin emulsion (A); a core-shell type acrylic resin dispersion (B); and a hydrophobic melamine resin (C), wherein the core-shell type acrylic resin dispersion (B) has a branched hydrocarbon group having 4 to 24 carbon atoms in its core portion and a hydrophilic resin in its shell portion, and the solids mass ratio (A:B) of the hydroxyl-containing acrylic resin emulsion (A) to the core-shell type acrylic resin dispersion (B) is 30:70 to 90:

10.

2. The aqueous coating composition according to claim 1, wherein the solids mass ratio (B:C) of the core-shell type acrylic resin dispersion (B) to the hydrophobic melamine resin (C) is 10:90 to 50:

50.

3. The aqueous coating composition according to claim 1 or 2, wherein the acid value of the core-shell type acrylic resin dispersion (B) is 25 mg KOH / g or more and 50 mg KOH / g or less.

4. An aqueous coating composition according to any one of claims 1 to 3, wherein the weight average molecular weight of the core-shell type acrylic resin dispersion (B) is 7,600 or more and 80,000 or less.

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