Aqueous coating composition

Abstract

To provide an aqueous coating composition capable of giving a coating film which has an excellent appearance and suppresses yellowing while securing water resistance.SOLUTION: The aqueous coating composition contains a hydroxyl group-containing acrylic resin emulsion (A), a core-shell type acrylic resin dispersion (B), and a hydrophobic melamine resin (C). The core-shell type acrylic resin dispersion (B) has a C4-24 branched hydrocarbon group in a core part and has a hydrophilic resin in a shell part. The solid content mass ratio (A:B) of the hydroxyl group-containing acrylic resin emulsion (A) to the core-shell type acrylic resin dispersion (B) is 30:70-90:10.SELECTED DRAWING: None

Description

[Technical field]

[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 been a shift from conventional organic solvent-based paints to water-based paints using water as a medium. Patent Document 1 discloses a water-based paint composition containing a hydrophobic melamine resin. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-002244 A Summary of the Invention [Problem to be solved by the invention]

[0004] In aqueous coating compositions, the dispersibility of hydrophobic melamine resins is easily reduced, and when the dispersibility of the resin component is reduced, the storage stability of the coating composition and the appearance of the resulting coating film are deteriorated.

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

[0006] In order to solve the above problems, the present invention provides the following aspects. [1] 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 core portion having a branched hydrocarbon group having 4 to 24 carbon atoms, and a shell portion having a hydrophilic resin, An aqueous coating composition, wherein the solid content mass ratio (A:B) of the hydroxyl group-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 the above [1], wherein the solid content 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 the above [1] or [2], wherein the acid value of the core-shell type acrylic resin dispersion (B) is from 25 mgKOH / g to 50 mgKOH / g. [4] The aqueous coating composition according to the above [1] or [2], wherein the weight average molecular weight of the core-shell acrylic resin dispersion (B) is 7,600 or more and 80,000 or less. Effect of the Invention

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

[0008] In the aqueous coating composition, the use of a hydrophobic melamine resin as a curing component improves the water resistance of the resulting coating film. On the other hand, in the aqueous coating composition, the hydrophobic melamine resin has poor dispersibility. The present disclosure improves the dispersibility of the hydrophobic melamine resin in an aqueous solvent, thereby improving the storage stability of the aqueous coating composition and the appearance of the excellent coating film.

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

[0010] The aqueous acrylic resin used in the present disclosure is 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 the 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 above-mentioned 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 the hydrophobic melamine resin, so that the hydrophobic melamine resin is inhibited from coagulating and maintained in a dispersed state.

[0012] The hydroxyl-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 regarded as emulsion type, and those below 100,000 can be regarded as colloidal dispersion type. If 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 regarded as emulsion type.

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

[0014] The solids concentration is calculated from the residue when the object is heated to 150°C.

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

[0016] The acid value and hydroxyl value may be calculated from the composition of the raw material monomers based on the JIS regulations, and 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 with polystyrene standards by the GPC (gel permeation chromatography) method.

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

[0019] [Water-based paint composition] The aqueous coating composition according to the present disclosure comprises a hydroxyl-containing acrylic resin emulsion (A), a core-shell type acrylic resin dispersion (B), and a hydrophobic melamine resin (C). The core part of the core-shell type acrylic resin dispersion (B) has a branched hydrocarbon group having 4 to 24 carbon atoms, and the shell part has a hydrophilic resin. The solid content mass ratio (A:B) of the hydroxyl-containing acrylic resin emulsion (A) to the core-shell type acrylic resin dispersion (B) is 50:50 to 85:15.

[0020] Hydroxyl-containing acrylic resin emulsion (A) The 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 the form of particles 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.The acrylic resin emulsion (A) may have 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 30 parts by mass or more. The content of the acrylic resin emulsion (A) may be 60 parts by mass or less, or 50 parts by mass or less.

[0024] (Manufacturing 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, such as a (meth)acrylic acid ester or 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 method of emulsion polymerization is not particularly limited. For example, an emulsifier is dissolved in an aqueous medium containing water or, if necessary, an organic solvent such as alcohol, ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and an α,β-ethylenically unsaturated monomer and a polymerization initiator are dropped under heating and stirring. The α,β-ethylenically unsaturated monomer may be emulsified in advance by an emulsifier.

[0030] Examples of emulsifiers include anionic emulsifiers such as soap, alkyl sulfonate, and polyoxyethylene alkyl sulfate; nonionic emulsifiers such as polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polypropylene glycol ethylene oxide adduct, polyethylene glycol fatty acid ester, and polyoxyethylene sorbitan fatty acid ester; reactive emulsifiers such as nonionic surfactants having a polyoxyethylene alkyl phenyl ether as a basic structure and a radically polymerizable propenyl group introduced into the hydrophobic group, cationic surfactants having a quaternary ammonium salt structure, and anionic surfactants having a radically polymerizable carbon-carbon double bond and containing a sulfonic acid group, a sulfonate group, a sulfate ester group, and / or an ethyleneoxy group. These may be used alone or in combination of two or more. For example, the emulsifier may be used in an amount of 0.5 to 10 parts by mass based on the solid content per 100 parts by mass of the 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-based 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-butyl peroxy-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. or more and 90° C. or less, and the polymerization time is, for example, 3 hours or more and 12 hours or less. The raw material monomer concentration during the polymerization reaction is, for example, 30 mass % or more and 70 mass % or less.

[0033] Optionally, chain transfer agents such as mercaptans (eg, lauryl mercaptan) and alpha-methylstyrene dimer are used.

[0034] After the emulsion polymerization, neutralization is performed using a basic compound as necessary. As the basic compound, an inorganic base or an organic base is used. 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 are used alone or in combination of two or more. For example, the basic compound is used in an amount of 0.2 mol or more and 1.0 mol or less (neutralization rate: 20% or more and 100% or less) per mol of carboxyl group contained in the polymer.

[0035] The neutralization rate is equivalent to the amount of a basic compound used relative to an acid group (e.g., a carboxy group) and is calculated by the following formula: TIFF2025085440000001.tif12150

[0036] The acrylic resin emulsion (A) may be a single-layer type, or may be a core-shell type having a core part and a shell part. The core-shell type acrylic resin emulsion (A) can be prepared by a known production method described in, for example, 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 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 mgKOH / g or more and 50 mgKOH / g or less. This allows the average particle size of the acrylic resin dispersion (B) to be small and have a sharp distribution. Therefore, when the aqueous coating composition according to the present disclosure is used to form a base coating film, color reversion can also 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 the clear coating composition penetrate and mix with the coating film (typically, the base coating film) below, thereby reducing the design. For example, in the case of a colored base coating film having a luster pigment as the lower layer, when the components of the clear coating composition penetrate into the lower layer, the arrangement of the luster pigment is disturbed, and the desired FF property (flip-flop property) cannot be obtained. 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 property that can be exhibited by the coating film below 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 range, the clear coating composition is less likely to penetrate into the base coating film, and color reversion can be suppressed.

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

[0046] The acrylic resin dispersion (B) has a branched hydrocarbon group having 4 to 24 carbon atoms (hereinafter, for convenience, referred to as a hydrophobic group) in the core portion, and a hydrophilic resin (Bs) in the shell portion. The resin forming the core portion is conveniently referred to as a hydrophobic resin (Bc). 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 its interior and a hydrophilic resin (Bs) on its exterior, and at least a part of the hydrophobic resin (Bc) is 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 type 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] The term "hydrocarbon group" refers to a group containing carbon and hydrogen, and a group in which one hydrogen atom has been removed from a hydrocarbon. Examples of the hydrocarbon group 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 acrylic resin having an acid group, which is arranged so as to cover the core portion.

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

[0052] The acrylic resin dispersion (B) is produced, for example, by a method including the steps of: a first step of dropping 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 containing the hydrophobic resin (Bc); a second step of dropping a second monomer mixture containing a second acid group-containing α,β-ethylenically unsaturated monomer (a2) into the liquid to synthesize an acid group-containing resin (Bs') and obtain a core-shell type 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 type acrylic resin (B'); and a step of introducing deionized water to cause phase inversion and obtain a varnish containing the acrylic resin dispersion (B) dispersed in deionized water.

[0053] <1st process> 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 the first step, so that the hydrophobic resin (Bc) can contain a hydrophobic group. For example, the temperature of the reaction system is first lowered (for example, 50°C or higher and lower than 130°C) to polymerize the first monomer mixture to obtain a precursor, and then the temperature is raised (for example, 130°C or higher and 180°C or lower) to ring-open and add 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 the production of the acrylic resin emulsion (A).

[0056] In terms of improving 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, based on 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, represented by the following general formula (1): [ka] (In the formula, R is a monovalent organic group and contains the above-mentioned hydrophobic group.) It is expressed as:

[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 core-shell structured resin particles (hydrophobic resin (Bc):hydrophilic resin (Bs), mass%) is 95:5 to 60:40.

[0059] <2nd process> In the second step, the polymerization of the second monomer mixture mainly proceeds, and an acid group-containing resin (Bs') is synthesized. When a basic compound is added in a later step, the acid group is neutralized, and the acid group-containing resin (Bs') is hydrophilized to become 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 is performed, for example, at the same temperature as in the second step for 0.5 hours to 3 hours.

[0061] The types and mixing 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, based on 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. The total amount of the polymerization initiator used is appropriately set, for example, according to the type and amount of the raw material monomer. 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 with respect 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 Steps The acid groups (typically carboxy groups) remaining in the core-shell acrylic resin (B') are neutralized by the basic compound. This makes the core-shell acrylic resin (B') water-dispersible. Thereafter, deionized water is added to cause phase inversion. This causes the core-shell acrylic resin (B') to be dispersed 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. By stirring, the average particle size of the acrylic resin dispersion (B) can become smaller.

[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 that results in a neutralization rate of the acid groups contained in the acrylic resin dispersion (B) of 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 hardener.

[0069] Hydrophobic melamine resin (C) is a melamine resin having a triazine nucleus and three nitrogen atoms connected thereto. 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). [ka] (In the formula, substituent R 1 ~R 6 each independently represents a hydrogen atom, an alkyl ether group, a methylol group, or a bonding portion to another triazine ring.

[0071] Substituent R 1 ~R 6 are each independently a hydrogen atom, an alkyl ether group (-CH2 -OR 7 ) or methylol group (-CH 2 The substituent R may be aryl, aryl, or aryl. 1 ~R 6 , R 7 may each independently be an alkyl group having 1 to 8 carbon atoms or 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) R in the above general formula (1) 1 ~R 6 At least one of the groups is a hydrogen atom (imino group) or CH 2 OH (methylol group). That is, the total amount of the average imino group amount and the average methylol group amount is 1.0 or more. (iii) R in the above general formula (1) 1 ~R 6 In R 1 ~R 6 CH 2 OR 7 If so, then R 7 is a methyl group.

[0073] The hydrophobic melamine resin is a melamine resin other than the above-mentioned water-soluble melamine resins, 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 sum of the average amount of imino groups and the average amount of methylol groups is 1.0 or less. (vi) R in the above formula (1) 1 ~R 6 In R 1 ~R 6 Two or more of CH 2 OR 7 and R7 is an alkyl group having 1 to 4 carbon atoms, where R 1 ~R 6 Configure R 7 At least one of them is an alkyl group having 2 to 4 carbon atoms.

[0074] Examples of commercially available hydrophobic melamine resins (C) include 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 (all melamine resins having both methoxy and butoxy groups); Mycoat 506 (manufactured by Mitsui Cytec, a melamine resin having only butoxy groups); and U-BAN 20N60 and U-BAN 20SE (manufactured by Mitsui Chemicals, U-BAN (trade name) series). 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, based on 100 parts by mass of the resin solid content of the aqueous coating composition. This makes it easier for the curing reaction to proceed, and makes 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 40:50.

[0077] Other resin components The aqueous coating composition may contain other resin components as necessary. Examples of the 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, based on 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 0 parts by mass, relative to 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 solid 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 hardening ingredients The aqueous coating composition may contain a curing component other than the hydrophobic melamine resin (C). Examples of the 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 necessary, such as film-forming aids, surface conditioners, preservatives, antifungal agents, defoamers, light stabilizers, UV absorbers, antioxidants, and pH adjusters.

[0083] Pigments 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 oxide, and carbon. Inorganic color pigments such as black, titanium dioxide, cobalt green, phthalocyanine green, ultramarine, cobalt blue, phthalocyanine blue, and cobalt violet; mica pigments (titanium dioxide-coated mica, colored mica, metal-plated mica); graphite, aluminum flakes, alumina flakes, metallic titanium flakes, stainless steel flakes, plate-like iron oxide, phthalocyanine flakes, metal-plated glass flakes, and 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 water-based paint composition The method for preparing the aqueous coating composition is not particularly limited, and the composition can be prepared by stirring each component with a stirrer, etc. The pigment can be dispersed in advance in a vehicle containing water, a surfactant, a dispersant, etc., using a sand grind mill or the like to form a pigment paste, which can then be mixed with other components.

[0085] [Painted items] A coated article is obtained by using the aqueous coating composition according to the present disclosure. The aqueous coating composition according to the present disclosure is preferably used to form 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 by using the aqueous coating composition according to the present disclosure. In this case, the aqueous coating composition according to the present disclosure contains the above-mentioned mica pigment and / or flat pigment. In the coated article, intermixing between the metallic base coating film and the clear coating film is suppressed, and the coated article has an excellent appearance.

[0086] (subject to be coated) Examples of the material of the substrate include metal, resin, and glass. Specific examples of the substrate include automobile bodies such as passenger cars, trucks, motorcycles, and buses, and automobile body parts, 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 metallic substrates include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0088] The metal substrate may be surface-treated. Examples of the surface treatment include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. The metal substrate may be further coated with an electrodeposition paint after the surface treatment. The electrodeposition paint may be of the cationic type or the 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, phenol resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). The resin substrate may be degreased.

[0090] (Colored base coating) 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 from 5 μm to 60 μm, from the viewpoint of smoothness and chipping resistance of the coated article.

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

[0093] (Metallic base coating) 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 film improves the gloss of the coated article and prevents the pigment blended in the lower layer from falling off or popping out.

[0096] The thickness of the cured clear coating film may be 15 μm or more and 50 μm or less from the viewpoints 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 viewpoint 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 a polyester resin having a carboxylic acid and / or an epoxy group. The clear coating composition may contain the above-mentioned various pigments to the extent that the transparency is not impaired. The clear coating composition may contain various additives as necessary.

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

[0099] The isocyanate compound has at least two isocyanate groups in one molecule. Examples of the isocyanate compound include aliphatic polyisocyanates, alicyclic polyisocyanates, aliphatic polyisocyanates (araliphatic polyisocyanates) having an aromatic ring not bonded to an isocyanate group in the molecule, 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] [Manufacturing method for multi-layer coating film] The multilayer coating film is produced, for example, by a method comprising the steps of applying a colored base coating composition onto a substrate 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 curing the uncured colored base coating film, the uncured metallic base coating film, and the uncured clear coating film all at once.

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

[0104] Examples of coating methods include air spray coating, airless spray coating, electrostatic spray coating, multi-stage coating using air electrostatic spray coating (typically, two-stage coating), and coating using a combination of air electrostatic spray coating and a rotary atomizer-type electrostatic coater.

[0105] Each coating composition is cured, for example, at a heating temperature of 80° C. to 180° C. for a heating time of 5 minutes to 60 minutes. EXAMPLES

[0106] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto. In the examples, "parts" and "%" are by weight 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] [Production Example A] Production of Hydroxyl-Containing Acrylic Resin Emulsion (A) 126.5 parts of deionized water was added to the 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 dropped into the reaction vessel in parallel over a period of 2 hours. After the dropwise addition was completed, the mixture was aged at the same temperature for 2 hours.

[0109] 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 to adjust the pH to 6.5. This resulted in a hydroxyl-containing acrylic resin emulsion (A) with an average particle size of 90 nm, Tg-9.5°C, non-volatile content of 30%, acid value of 20 mgKOH / g, and hydroxyl value of 40 mgKOH / g.

[0110] [Production Example B-1] Production of Core-Shell Type Acrylic Resin Dispersion (B-1) (1) Synthesis of hydrophobic acrylic resin (Bc) A reaction vessel equipped with a stirrer, a temperature controller, a cooling tube and a dropping device was charged with 30 parts of a reactive solvent (CAE, glycidyl ester of monocarboxylic acid having a branched alkyl group with 9 carbon atoms, trade name: Cardurer E10P, manufactured by Hexion, boiling point 251-278 ° C), and the temperature was raised to 165 ° C while stirring, and refluxed. 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), 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 polymerization and ring-opening addition reactions.

[0111] (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 point solvent (DPM) was prepared. This mixture was dropped into the reaction vessel over a period of 1 hour at 165°C while stirring, and polymerized for 1 hour. Furthermore, a mixture of 0.1 parts of polymerization initiator (Luperox DTA) and 0.1 parts of high boiling point solvent (DPM) was added to the reaction vessel, and polymerization reaction was carried out for 1 hour at 165°C while stirring.

[0112] In this manner, 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 mass % was obtained.

[0113] (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%. Next, 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.

[0114] The acrylic resin particles contained in the dispersion liquid were expected to have a core-shell structure, with the mass ratio of the core being calculated to be 81.4% and the mass ratio of the shell being 18.6%. The average particle size of the acrylic resin particles was 57 nm, the acid value was 37.2 mg KOH / g, and the weight average molecular weight was 32,000.

[0115] [Manufacturing examples B-2 to B-6, comparative manufacturing example b-1] Except for changing the types of raw material monomers, the neutralization rate, etc. as shown in Table 1, the same procedure as in Production Example B-1 was carried out to obtain core-shell type acrylic resin dispersions.

[0116] [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. 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 separately. 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 dropped into the reaction vessel in parallel over 3 hours. After 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-butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel over 0.5 hours. After the addition was completed, the mixture was aged at the same temperature for 2 hours.

[0119] Next, 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. This resulted in a water-soluble acrylic resin solution with a non-volatile 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 A reaction vessel equipped with a stirrer, a nitrogen inlet tube, a temperature control device, a condenser, and a decanter was charged with 250 parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts of cyclohexanedicarboxylic acid. The temperature was raised to 180°C, and a condensation reaction was carried out until water was no longer distilled. After cooling to 60°C, 120 parts of phthalic anhydride were added to the reaction vessel. The temperature was then raised to 140°C and maintained 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 1920 parts of deionized water were added to the reaction vessel and stirred. This resulted in a polyester resin dispersion with a solid content of 45% by mass, a hydroxyl value of 110 mgKOH / g, an acid value of 15 mgKOH / g, Tg-14°C, and Mw of 7,000.

[0121] [Production Example 3] Production of phosphoric acid group-containing acrylic resin 40 parts of ethoxypropanol were charged into a 1-liter reaction vessel equipped with a stirrer, a temperature controller, and a cooling tube. Separately, a monomer solution consisting of 40 parts of a solution in which 20 parts of Hosmer PP (acid phosphooxyhexa(oxypropylene)monomethacrylate manufactured by Unichemical Co., Ltd.) were dissolved 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 was prepared. 121.7 parts of this monomer solution were dropped into 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%) with an acid value of 105 mgKOH / g, including an acid value of 55 mgKOH / g due to phosphate groups, a hydroxyl value of 60 mgKOH / g, and a number average molecular weight of 6,000.

[0122] [Example 1] (i) Preparation of Water-Based Coating Composition 5 parts of the above water-soluble acrylic resin (resin solid content 30%), 3.6 parts of 10 mass % dimethylaminoethanol, 40 parts of the hydroxyl group-containing acrylic resin emulsion (A), 10 parts of the core-shell type acrylic resin dispersion (B-1) (resin solid content 36%), 40 parts of the melamine resin (C-1), and 5 parts of a urethane resin dispersion (product name: N-800T, manufactured by Sanyo Chemical Industries, Ltd.) were mixed and uniformly dispersed.

[0123] 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 phosphoric acid 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-based dialcohol (solids content 100%)) were uniformly dispersed. Dimethylaminoethanol was added to this dispersion to adjust the pH to 8.1, and the dispersion was diluted with deionized water to obtain an aqueous coating composition.

[0124] Details of the melamine resins used are shown in Table 2.

[0125] [Table 2]

[0126] (ii) Formation of multi-layer coating A zinc phosphate-treated dull steel plate (thickness 0.8 mm, length 30 cm, width 40 cm) was electrocoated 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. The plate was then heated to harden at 160°C for 30 minutes and then cooled to obtain a coated object with a hardened electrodeposition coating film.

[0127] AQUALEX AR-3100 (product name, Nippon Paint Automotive Coatings, water-based base paint) was applied to the substrate using a rotary atomizing electrostatic coating device so that the dry film thickness was 10 μm. After coating, the coating was allowed to set for 4 minutes to obtain an uncured colored base coating film.

[0128] The aqueous coating composition prepared above was then diluted with ion-exchanged water to a solid content of 23% by mass. The uncured colored base coating was then air-spray coated at room temperature of 23°C to a dry 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.

[0129] The coated plate obtained above was allowed to cool to room temperature, and a clear coating composition (two-liquid urethane curing clear coating (product name: Polyurethane Excel 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 plate was allowed to set for 7 minutes. This resulted in an uncured clear coating film.

[0130] Finally, the coated plate was heated in a dryer at 140° C. for 30 minutes to form a multi-layer coating film having a colored base coating film, a metallic base coating film and a clear coating film in that order.

[0131] [Examples 2 to 15, Comparative Examples 1 to 4] An aqueous paint composition was prepared and a multilayer coating film was formed in the same procedure as in Example 1, except that the components to be blended and / or their amounts were changed as shown in Table 3.

[0132] [Evaluation] Using the aqueous paint compositions or multilayer coating films obtained in the examples and comparative examples, the following evaluations were carried out. The evaluation results are shown in the following table.

[0133] (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 product name: Wave Scan DOI (manufactured by BYK Gardner). The obtained SW value was evaluated according to the following criteria. The smaller the SW value, the higher the smoothness of the coating film. If the evaluation is B or higher, it can be evaluated that the coating film appearance is excellent.

[0134] (Evaluation criteria) A: SW value ≤ 20 B: 20 < SW value ≤ 30 C: 31 < SW value

[0135] (2) Yellowing For the multilayer coating film, an accelerated weather resistance test of 1,600 hours was carried out in accordance with JIS B 7753 using a Sunshine Weather Ometer S80 (Sunshine carbon arc type accelerated weather resistance tester, manufactured by Suga Test Instruments Co., Ltd.). Before and after the accelerated weather resistance test, the b value was measured with a color difference meter (model: CR-331, manufactured by Minolta). The difference between the two 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. If the evaluation is B or higher, it can be evaluated that it is suitable for practical use.

[0136] (Evaluation criteria) A: Δb value < 0.3 B: 0.3 ≤ Δb value < 0.5 C: 0.5 ≤ Δb value

[0137] (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, multi-layer coating films (X, Y) for evaluation were formed in the same manner as above. The appearance (smoothness) and FF property 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.

[0138] (Evaluation Criteria) A: No difference was observed between multi-layer coatings X and Y. B: The smoothness of the multi-layer coating film Y is slightly lower than that of the multi-layer coating film X, or the edge of the coating film Y shows some sagging and / or a decrease in FF property. C: The smoothness of the multi-layer coating film Y is significantly lower than that of the multi-layer coating film X, or the edge of the coating film Y shows clear sagging and / or a significant decrease in FF properties, or the coating film Y has coating defects such as repelling, dents, or bumps.

[0139] (4) Color reversion The lightness (L) of the multi-layer coating film obtained in the same manner as above was measured at 15° (front) using a spectrophotometer (product name: X-Rite MA68II, manufactured by X-Rite Corporation). 5 ) and 110° (shade) lightness (L 110 ) was 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 suitable for practical use.

[0140] (Evaluation Criteria) A:90≦L 5 -L 110 B:80≦L 5 -L 110 <90 C:L 5 -L 110 <80

[0141] (5)Water resistance The test plate obtained in the same manner as above was immersed in warm water at 40°C for 240 hours. It was then removed from 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 indicates excellent water resistance.

[0142] (Evaluation Criteria) A: No white blur is observed B: A slight white blur is visible. C: White blur is clearly visible

[0143] [Table 3]

[0144] The present disclosure includes the following aspects. [1] 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 core portion having a branched hydrocarbon group having 4 to 24 carbon atoms, and a shell portion having a hydrophilic resin, An aqueous coating composition, wherein the solid content mass ratio (A:B) of the hydroxyl group-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 the above [1], wherein the solid content 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 the above [1] or [2], wherein the acid value of the core-shell type acrylic resin dispersion (B) is 25 mgKOH / g or more. [4] The aqueous coating composition according to any one of the above [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. [Industrial Applicability]

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

Claims

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

10.

2. 2. The aqueous coating composition according to claim 1, wherein the solid content 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. 3. The aqueous coating composition according to claim 1, wherein the acid value of the core-shell type acrylic resin dispersion (B) is from 25 mgKOH / g to 50 mgKOH / g.

4. 3. The aqueous coating composition according to claim 1, 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.

Images