Aqueous coating composition

Abstract

A waterborne coating composition comprising: a hydroxyl-containing acrylic resin emulsion (A), a core-shell acrylic resin dispersion (B), and a hydrophobic melamine resin (C), wherein the core-shell acrylic resin dispersion (B) has a branched hydrocarbon group with 4 to 24 carbon atoms in the core and a hydrophilic resin in the shell, and the mass ratio of the solid components of the hydroxyl-containing acrylic resin emulsion (A) to the core-shell acrylic resin dispersion (B) (A:B) is 30:70 to 90:10.

Description

Technical Field

[0001] This invention relates to water-based coating compositions.

[0002] In recent years, environmental pollution has become increasingly serious, and internationally, the emission control of organic solvents is being strengthened. In the coatings industry, there is also a shift from traditional organic solvent-based coatings to water-based coatings. Patent Document 1 discloses a water-based coating composition containing a hydrophobic melamine resin.

[0003] Existing technical documents Patent documents Patent document 1: Japanese Patent Application Publication No. 2020-002244. Summary of the Invention

[0004] The problem that the invention aims to solve In waterborne coating compositions, the dispersibility of hydrophobic melamine resins is easily reduced. If the dispersibility of the resin component decreases, the storage stability of the coating composition and the appearance of the resulting coating film will be compromised.

[0005] The object of the present invention is to provide a waterborne coating composition that can produce a coating film that achieves excellent appearance and suppresses yellowing while ensuring water resistance.

[0006] means for solving problems To address the aforementioned issues, the present invention provides the following approach.

[0007] [1] An aqueous coating composition comprising: a hydroxyl-containing acrylic resin emulsion (A), a core-shell acrylic resin dispersion (B), and a hydrophobic melamine resin (C). The aforementioned core-shell type acrylic resin dispersion (B) has a branched hydrocarbon group with 4 to 24 carbon atoms in the core and a hydrophilic resin in the shell. The solid content mass ratio (A:B) of the aforementioned hydroxyl-containing acrylic resin emulsion (A) and the aforementioned core-shell acrylic resin dispersion (B) is 30:70 to 90:10.

[0008] [2] According to the above-mentioned [1] waterborne coating composition, wherein the mass ratio of the solid components (B:C) of the aforementioned core-shell type acrylic resin dispersion (B) to the aforementioned hydrophobic melamine resin (C) is 10:90 to 50:50.

[0009] [3] According to the above-mentioned [1] or [2] waterborne coating composition, wherein the acid value of the aforementioned core-shell type acrylic resin dispersion (B) is 25 mg KOH / g or more and 50 mg KOH / g or less.

[0010] [4] The waterborne coating composition according to any one of [1] to [3] above, wherein the weight average molecular weight of the aforementioned core-shell type acrylic resin dispersion (B) is 7,600 or more and 80,000 or less.

[0011] Invention Effects According to the present invention, a waterborne coating composition is provided that can produce a coating film that achieves excellent appearance and inhibits yellowing while ensuring water resistance. Detailed Implementation

[0012] In waterborne coating compositions, the water resistance of the resulting coating film is improved by using a hydrophobic melamine resin as a curing component. However, hydrophobic melamine resins exhibit poor dispersibility in waterborne coating compositions. This application improves the dispersibility of hydrophobic melamine resin relative to waterborne solvents, thereby enhancing the storage stability and improving the coating film appearance of the waterborne coating composition.

[0013] This application uses at least two different types of waterborne acrylic resins. Waterborne resins are generally classified into water-soluble and water-dispersible types. Water-dispersible resins are further divided into dispersion types (commonly referred to as colloidal dispersion types) and emulsion types. Typically, colloidal dispersion type waterborne resins are obtained by using a neutralizing agent to partially dissolve the resin synthesized in an organic solvent in water. Typically, emulsion type waterborne resins are manufactured by emulsion polymerization or by mechanically forced emulsification.

[0014] The aqueous acrylic resins used in this application are: a hydroxyl-containing acrylic resin emulsion (A) that is an emulsion type and manufactured by emulsion polymerization; and a core-shell acrylic resin dispersion (B) that is a colloidal dispersion type. The hydroxyl-containing acrylic resin emulsion (A) is used to ensure the film properties (e.g., strength). The core-shell acrylic resin dispersion (B) improves the dispersibility of the hydrophobic melamine resin.

[0015] The core-shell acrylic resin dispersion (B) has branched hydrocarbon groups with 4 to 24 carbon atoms in the core and a hydrophilic resin in the shell. These hydrocarbon groups enhance the hydrophobicity of the core and increase its affinity for the hydrophobic melamine resin. The hydrophilic resin in the shell enhances the water dispersibility of the core-shell acrylic resin dispersion (B). In other words, the core-shell acrylic resin dispersion (B) is micro-dispersed in an aqueous solvent while capturing the hydrophobic melamine resin; as a result, the aggregation of the hydrophobic melamine resin is suppressed, maintaining the dispersed state.

[0016] Hydroxyl-containing acrylic resin emulsions (A) and core-shell acrylic resin dispersions (B) can be fractionated, for example, by centrifugation of an aqueous coating composition. The weight-average molecular weight of each fractionated acrylic resin is determined; substances exceeding 100,000 can be considered emulsions, and those below 100,000 can be considered colloidal dispersions. If the weight-average molecular weight exceeds 1 million, it becomes difficult to determine. Acrylic resins whose weight-average molecular weight cannot be determined can also be considered emulsions.

[0017] Acrylic resins are not prone to yellowing caused by ultraviolet light. By using a core-shell acrylic resin dispersion (B) as the dispersion component of hydrophobic melamine resin, it is possible to obtain a coating film that ensures water resistance while exhibiting excellent appearance and suppressing yellowing.

[0018] The concentration of solid components is calculated based on the amount remaining when the object is heated at 150°C.

[0019] The average particle size is the 50% average particle size (D50) in the volumetric particle size distribution obtained using a particle size distribution measuring device based on laser diffraction / scattering.

[0020] Acid value and hydroxyl value can be calculated from the composition of the raw material monomers according to JIS specifications, and can be determined by neutralization titration using an aqueous potassium hydroxide solution according to JIS K 0070. Acid value and hydroxyl value are based on solid components.

[0021] Weight-average molecular weight and number-average molecular weight were determined according to the polystyrene standard based on GPC (gel permeation chromatography).

[0022] (Meth)acrylates refer to both acrylates and methacrylates. (Meth)acrylic acid refers to both acrylic acid and methacrylic acid.

[0023] [Water-based coating composition] The waterborne coating composition described in this application 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 branched hydrocarbon groups with 4 to 24 carbon atoms in its core and a hydrophilic resin in its shell. The mass ratio of the solid components of the hydroxyl-containing acrylic resin emulsion (A) to the core-shell acrylic resin dispersion (B) (A:B) is 30:70 to 90:10.

[0024] • Hydroxyl-containing acrylic resin emulsion (A) Hydroxyl-containing acrylic resin emulsion (A) refers to acrylic resin prepared by emulsion polymerization. Hydroxyl-containing acrylic resin emulsion (A) (hereinafter sometimes simply referred to as acrylic resin emulsion (A)) is water-dispersible and disperses in aqueous solvents in particulate form.

[0025] 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) can be 30 nm or more, or 50 nm or more. The average particle size of the acrylic resin emulsion (A) can be 180 nm or less, or 140 nm or less.

[0026] The hydroxyl value of acrylic resin emulsion (A) can be above 20 mg KOH / g and below 180 mg KOH / g. The acid value of acrylic resin emulsion (A) can be above 1 mg KOH / g and below 80 mg KOH / g.

[0027] The solid content of the acrylic resin emulsion (A) is, for example, 20 parts by weight or more and 70 parts by weight or less relative to 100 parts by weight of the resin solid content of the water-based coating composition. The aforementioned content of the acrylic resin emulsion (A) may be 25 parts by weight or more, or 30 parts by weight or more. The aforementioned content of the acrylic resin emulsion (A) may be 60 parts by weight or less, or 50 parts by weight or less.

[0028] (Manufacturing method) Acrylic resin emulsions (A) can be manufactured by emulsion polymerization of α,β-ene unsaturated monomers with hydroxyl groups with other α,β-ene unsaturated monomers. Examples of other α,β-ene unsaturated monomers include (meth)acrylates and α,β-ene unsaturated monomers with acid groups.

[0029] Examples of α,β-ene unsaturated monomers with hydroxyl groups include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methacryl alcohol, and their adducts with ε-caprolactone. They can be used alone or in combination of two or more.

[0030] Examples of (meth)acrylates include methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, phenyl methacrylate, isobornyl methacrylate, cyclohexyl methacrylate, tert-butylcyclohexyl methacrylate, dicyclopentadienyl methacrylate, and dihydrodicyclopentadienyl methacrylate. They can be used alone or in combination of two or more.

[0031] Examples of α,β-olefinic unsaturated monomers with acidic groups 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. They can be used alone or in combination of two or more.

[0032] Other α,β-ene unsaturated monomers can be used in combination. Examples of other α,β-ene unsaturated monomers include, for example, polymerizable amide compounds, polymerizable aromatic compounds, polymerizable nitriles, polymerizable epoxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, α-olefins, dienes, polymerizable carbonyl compounds, polymerizable alkoxysilyl compounds, and other polymerizable compounds. They can be used alone or in combination of two or more.

[0033] There are no particular limitations on the emulsion polymerization method. For example, an emulsifier can be dissolved in an aqueous medium containing water or, if necessary, an organic solvent such as an alcohol or ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and then α,β-ene unsaturated monomers and a polymerization initiator can be added dropwise under heating and stirring. The α,β-ene unsaturated monomers can be pre-emulsified using the emulsifier.

[0034] Examples of emulsifiers include anionic emulsifiers such as soaps, alkyl sulfonates, and polyoxyethylene alkyl sulfates; nonionic emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polypropylene glycol ethylene oxide adducts, polyethylene glycol fatty acid esters, and polyoxyethylene dehydrated sorbitan fatty acid esters; nonionic surfactants with polyoxyethylene alkylphenyl ethers as the basic structure and free radical polymerizable propylene groups introduced into the hydrophobic groups; cationic surfactants with quaternary ammonium salt structures; and reactive emulsifiers such as anionic surfactants containing sulfonic acid groups, sulfonate groups, sulfate groups, and / or ethylene oxide groups, and having free radical polymerizable carbon-carbon double bonds. These can be used alone or in combination of two or more. For example, an emulsifier may be used at a rate of 0.5 to 10 parts by weight relative to 100 parts by weight of the raw material monomer, based on the solids content.

[0035] There are no particular limitations on polymerization initiators; both water-soluble and oil-soluble polymerization initiators can be listed. 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, tert-butyl peroxide, tert-butyl hydroperoxide, tert-butyl peroxide (2-ethylhexanoate), tert-butyl peroxide-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide; and azobis(isobutyronitrile), 2,2'-azobis-2,4-dimethylpentanitrile, 2,2'-azobis(4-methoxy-2,4-dimethylpentanitrile), and 1,1'-azobis-cyclohexane-1-carboxynitrile. They can be used alone or in combination of two or more. The polymerization initiator can be, for example, more than 0.01% by mass and less than 10% by mass of the raw material monomer.

[0036] There are no particular limitations on the polymerization conditions. The polymerization temperature is, for example, above 30°C and below 90°C, and the polymerization time is, for example, above 3 hours and below 12 hours. The concentration of the starting monomers during the polymerization reaction is, for example, above 30% by mass and below 70% by mass.

[0037] Chain transfer agents such as thiols (e.g., lauryl thiols) and α-methylstyrene dimers can be used as needed.

[0038] After emulsion polymerization, neutralization is performed using a basic compound as needed. This basic compound can be either inorganic or organic. Specific examples of basic compounds include organic bases such as ammonia, triethylamine, propylamine, dibutylamine, pentylamine, 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. One or more of these compounds can be used alone or in combination. For example, the basic compound is used in amounts of 0.2 moles to 1.0 mole relative to 1 mole of carboxyl groups contained in the polymer (neutralization rate: 20% to 100%).

[0039] The neutralization rate has the same meaning as the equivalent of a basic compound relative to an acid group (such as a carboxyl group). The neutralization rate is calculated using the following formula.

[0040] The acrylic resin emulsion (A) can be a single-layer type or a core-shell type having a core and a shell. The core-shell type acrylic resin emulsion (A) can be prepared by, for example, a known manufacturing method described in Japanese Patent Application Publication No. 2002-12816.

[0041] Core-shell type acrylic resin dispersions (B) Core-shell acrylic resin dispersion (B) is a core-shell acrylic resin prepared by polymerization without the use of emulsifiers (typically solution polymerization). Additionally, core-shell acrylic resin dispersion (B) (hereinafter sometimes simply referred to as acrylic resin dispersion (B)) also exhibits water dispersibility and is dispersed in particulate form in waterborne coating compositions.

[0042] 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) can be less than 180 nm, less than 160 nm, less than 150 nm, or less than 100 nm. The average particle size of the acrylic resin dispersion (B) can be greater than 25 nm or greater than 30 nm.

[0043] The weight-average molecular weight of the acrylic resin dispersion (B) is, for example, 7,600 or more and 80,000 or less. Therefore, when the waterborne coating composition described in this application is used to form a base coating film, color reversion can also be suppressed.

[0044] The weight-average molecular weight of acrylic resin dispersion (B) can be above 15,000, above 16,000, or above 20,000. The weight-average molecular weight of acrylic resin dispersion (B) can be below 60,000 or below 48,000.

[0045] The acid value of the acrylic resin dispersion (B) can be above 25 mg KOH / g and below 50 mg KOH / g. Consequently, the average particle size of the acrylic resin dispersion (B) becomes smaller and exhibits a concentrated distribution. Therefore, when the waterborne coating composition described in this application is used to form a base coating film, color reversion can also be suppressed.

[0046] The acid value of acrylic resin dispersion (B) can be above 30 mg KOH / g or above 35 mg KOH / g. From the viewpoint of water resistance, the acid value of acrylic resin dispersion (B) can be below 50 mg KOH / g or below 45 mg KOH / g.

[0047] Color reversion refers to the phenomenon where components of a clear coating composition penetrate and mix into the underlying coating film (typically the base coating film), resulting in a reduction in aesthetic design. For example, in the case of a base coating film with glossy pigments as the underlying layer, if components of the clear coating composition penetrate into the lower layer, the arrangement of the glossy pigments becomes disordered, and the desired angle-dependent color (FF) is not achieved. This reduction in FF is an example of color reversion. Color reversion resistance refers to the property that the aesthetic design achieved by utilizing the coating film beneath the clear coating is not compromised by the clear coating composition. The reason is not yet clear, but when the acid value and / or weight-average molecular weight of the acrylic resin dispersion (B) are within the aforementioned range, penetration of the clear coating composition into the base coating film is less likely, thus suppressing color reversion.

[0048] The content of the acrylic resin dispersion (B) relative to 100 parts by weight of the resin solids component of the water-based coating composition is, for example, 3 parts by weight or more and 50 parts by weight or less. The aforementioned content of the acrylic resin dispersion (B) may be 5 parts by weight or more, or 10 parts by weight or more. The aforementioned content of the acrylic resin dispersion (B) may be 40 parts by weight or less, 30 parts by weight or less, or 20 parts by weight or less.

[0049] The solid content mass ratio (A:B) of the acrylic resin emulsion (A) to the acrylic resin dispersion (B) is 30:70 to 90:10. This allows for the production of a coating that, while ensuring the physical properties of the coating film, exhibits improved storage stability and an excellent appearance. The solid content mass ratio (A:B) can also be 50:50 to 85:15, or 55:45 to 85:15.

[0050] An acrylic resin dispersion (B) has a core containing branched hydrocarbon groups with 4 to 24 carbon atoms (hereinafter referred to as hydrophobic groups) and a shell containing a hydrophilic resin (Bs). The resin forming the core is simply referred to as the hydrophobic resin (Bc). The core and shell may or may not be chemically cross-linked. The acrylic resin dispersion (B) has a hydrophobic resin (Bc) inside and a hydrophilic resin (Bs) on the outside, with at least a portion of the hydrophobic resin (Bc) being covered by the hydrophilic resin (Bs).

[0051] The mass ratio of hydrophobic resin (Bc) to hydrophilic resin (Bs) (Bc:Bs) is, for example, 95:5 to 60:40. If the mass ratio of 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 waterborne coating composition is enhanced. If the mass ratio of hydrophilic resin (Bs) is 40% or less, the water resistance and appearance of the coating film can be improved. The hydrophobic resin (Bc):hydrophilic resin (Bs) ratio can be 90:10 to 70:30, or 85:15 to 75:25.

[0052] A "hydrocarbon group" is a group containing both carbon and hydrogen, obtained by removing one hydrogen atom from a hydrocarbon. Examples of hydrocarbon groups include aliphatic hydrocarbon groups with 4 to 24 carbon atoms and aromatic hydrocarbons. Aliphatic hydrocarbon groups can be linear, branched, or cyclic, and can be saturated or unsaturated. The hydrogen atom bonded to the carbon can be replaced by a halogen atom or the like.

[0053] The hydrophobic group can have 7 to 18 carbon atoms, or 8 to 15. The hydrophobic group can be saturated. The hydrophobic group can be a branched alkyl group with 4 to 24 carbon atoms.

[0054] Hydrophilic resins (Bs) are neutralized acrylic resins with acid groups configured to cover the core.

[0055] (Manufacturing method) The acrylic resin dispersion (B) can be manufactured, for example, by multi-stage polymerization using a reactive solvent (x) having hydrophobic groups as described above.

[0056] An acrylic resin dispersion (B) is manufactured by, for example, a method comprising the following steps: a first step, adding dropwise a mixture of first monomers containing an α,β-olefinic unsaturated monomer (a1) containing a first acid group to a reactive solvent (x) having a glycidyl group and a hydrophobic group to synthesize a hydrophobic resin (Bc) and obtain a liquid containing the hydrophobic resin (Bc); a second step, adding dropwise a mixture of second monomers containing an α,β-olefinic unsaturated monomer (a2) containing a second acid group to the above liquid to synthesize an acid-containing resin (Bs') to obtain a core-shell acrylic resin (B') having a hydrophobic resin (Bc) and an acid-containing resin (Bs'); a step of adding an alkaline 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, obtaining a varnish containing the acrylic resin dispersion (B) dispersed in deionized water.

[0057] <First Process> In the first step, free radical polymerization of the first monomer mixture and ring-opening addition reaction of the glycidyl group of the reactive solvent (x) with the acid-containing monomer (a) are carried out. In the first step, so-called solution polymerization is carried out.

[0058] Taking advantage of the fact that the ring-opening reaction of the epoxy ring is difficult to occur at low temperatures, in the first step, the hydrophobic resin (Bc) can be made to contain hydrophobic groups by carrying out the polymerization reaction of the first monomer mixture and the ring-opening reaction of the epoxy ring in stages. For example, the temperature of the reaction system is first lowered (e.g., above 50°C and below 130°C) to polymerize the first monomer mixture to obtain a precursor, and then the temperature is raised (e.g., above 130°C and below 180°C) to cause the reactive solvent (x) to ring-open and add to the precursor.

[0059] Among the raw material monomers included in the first monomer mixture are those identical to the α,β-olefinic unsaturated monomers used to manufacture acrylic resin emulsions (A).

[0060] From the viewpoint of improving the physical properties of the resulting coating film, in the first monomer mixture, the mass of the acid-containing monomer relative to 100 parts by mass of the first monomer mixture can be 5 parts by mass or more and 30 parts by mass or less. The aforementioned mass of the acid-containing monomer can be 10 parts by mass or more. The aforementioned mass of the acid-containing monomer can be 25 parts by mass or less.

[0061] The reactive solvent (x) has one glycidyl group and a hydrophobic group. The reactive solvent (x) can be a monocarboxylic acid glycidyl ester. A monocarboxylic acid glycidyl ester is represented, for example, by the following general formula (1).

[0062] [Chemistry 1] (In the formula, R is a monovalent organic group, and includes the above-mentioned hydrophobic group.) The reactive solvent (x) is used, for example, in an amount where the mass ratio of hydrophobic resin (Bc) to hydrophilic resin (Bs) in the core-shell structured resin particles (hydrophobic resin (Bc): hydrophilic resin (Bs), mass %) is 95:5 to 60:40.

[0063] <Second Process> In the second step, the polymerization of the second monomer mixture is mainly carried out to synthesize an acid-containing resin (Bs'). If an alkaline compound is added in a subsequent step, the acid group is neutralized, and the acid-containing resin (Bs') becomes hydrophilic, becoming a hydrophilic resin (Bs). The hydrophilic resin (Bs) functions as a dispersion component for dispersing acrylic resin dispersions (B) in water, reducing the particle size of the resin particles while improving its dispersion stability.

[0064] Following the second step, an additional polymerization initiator can be added, followed by stirring and heating for maturation. Maturation is carried out, for example, at the same temperature as the second step, for 0.5 hours to 3 hours.

[0065] The types and proportions of raw material monomers contained in the second monomer mixture can be the same as or different from those contained in the first monomer mixture. The first acid-containing monomer (a1) and the second acid-containing monomer (a2) can be the same or different.

[0066] From the viewpoint of improving the dispersing function of the resulting hydrophilic resin (Bs), in the second monomer mixture, the mass of the acid-containing monomer relative to 100 parts by mass of the second monomer mixture can be 5 parts by mass or more and 30 parts by mass or less. The aforementioned mass of the acid-containing monomer can be 10 parts by mass or more. The aforementioned mass of the acid-containing monomer can be 25 parts by mass or less.

[0067] In the first, second, and maturation processes, a polymerization initiator may be used. The total amount of polymerization initiator used is appropriately set according to, for example, the type and amount of the raw material monomers. The total amount of polymerization initiator used relative to, for example, the total of 100 parts by mass of the first monomer mixture and the second monomer mixture may be 0.2 parts by mass or more and 2.0 parts by mass or less. The above-mentioned amount of polymerization initiator may be 0.2 parts by mass or more. The above-mentioned amount of polymerization initiator may be 1.5 parts by mass or less.

[0068] Examples of polymerization initiators include 2,2'-azobisisobutyronitrile, benzoyl peroxide, 2,2-bis(tert-amyl peroxide)butane, di-tert-butyl peroxide, di-tert-amyl peroxide (DTA), tert-butyl peroctanoate, and 2,2'-azobis(2-methylbutyronitrile).

[0069] <Neutralization and Phase Transformation Process> A basic compound is used to neutralize the acid groups (typically carboxyl groups) remaining in the core-shell acrylic resin (B'). This allows the core-shell acrylic resin (B') to be dispersed in water. Subsequently, deionized water is added to induce phase inversion. Thus, the core-shell acrylic resin (B') is dispersed in water, yielding a varnish containing an acrylic resin dispersion (B).

[0070] Regarding phase inversion and aqueous dispersion, for example, this is carried out while stirring a mixture of varnish containing neutralized core-shell acrylic resin (B') and deionized water. Through stirring, the average particle size of the acrylic resin dispersion (B) becomes smaller.

[0071] As a basic compound, the same compound exemplified as that used to neutralize acrylic resin emulsion (A) can be listed. The basic compound is added in an amount such that the neutralization rate of the acid groups contained in, for example, acrylic resin dispersion (B) is 70% or more and less than 100%. If the neutralization rate is within this range, the water dispersibility of acrylic resin dispersion (B) is improved, and the average particle size becomes smaller. The neutralization rate can be 75% or more, 80% or more, 85% or more, or 90% or more.

[0072] • Hydrophobic melamine resin (C) Hydrophobic melamine resin (C) plays a role as a curing agent.

[0073] Hydrophobic melamine resin (C) contains R atoms bonded around a melamine core (triazine core) via three nitrogen atoms. 1 ~R 6 Structure of the group. Hydrophobic melamine resin (C) can usually be a polynuclear body composed of multiple melamine nuclei bonded together, or a mononuclear body containing one melamine nucleus.

[0074] The structure of the melamine core is represented by, for example, the following general formula (2).

[0075] [Chemistry 2] (In the formula, substituent R) 1 ~R 6 Each group independently represents a hydrogen atom, an alkyl ether group, a hydroxymethyl group, or a portion bonded to another triazine ring. Substituent R 1 ~R 6 Each can be independently a hydrogen atom, an alkyl ether group (-CH2-OR) 7 ) or hydroxymethyl (-CH2OH). Substituent R 1 ~R 6 R 7 Each alkyl group can be an alkyl group with 1 to 8 carbon atoms, or an alkyl group with 1 to 4 carbon atoms. The alkyl group can be straight-chain or branched. The alkyl group can be methyl, n-butyl, or isobutyl.

[0076] Melamine resins are generally classified into water-soluble melamine resins and hydrophobic melamine resins. Water-soluble melamine resins meet all the conditions in (i) to (iii) below.

[0077] (i) The number average molecular weight of the melamine resin is below 1,000.

[0078] (ii) R in the above general formula (1) 1 ~R6 In this composition, at least one is a hydrogen atom (imino) or CH2OH (hydroxymethyl). That is, the total average amount of imino and hydroxymethyl is 1.0 or more.

[0079] (iii) R in the above general formula (1) 1 ~R 6 In the middle, R 1 ~R 6 CH2OR 7 At that time, R 7 It is a methyl group.

[0080] The hydrophobic melamine resin is any melamine resin other than the water-soluble melamine resin mentioned above. That is, it satisfies any of the conditions in (iv) to (vi) below.

[0081] (iv) The number average molecular weight of the melamine resin exceeds 1,000.

[0082] (v) The total amount of average imino content and average hydroxymethyl content is less than 1.0.

[0083] (vi) R in the above formula (1) 1 ~R 6 In the middle, R 1 ~R 6 Two or more of them are CH2OR 7 R 7 It is an alkyl group having 1 to 4 carbon atoms, wherein R constitutes 1 ~R 6 R 7 At least one of them is an alkyl group having 2 to 4 carbon atoms.

[0084] Commercially available hydrophobic melamine resins (C) include, for example, the Allnex CYMEL series (all trade names): 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 of which are melamine resins containing both methoxy and butoxy groups); MYCOAT 506 (a melamine resin containing only a butoxy group manufactured by MITSUI SITECH); and U-VAN 20N60 and U-VAN 20SE (the U-VAN series manufactured by Mitsui Chemicals). These can be used alone or in combination of two or more.

[0085] 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 waterborne coating composition. This facilitates the curing reaction and easily yields a coating film with high hardness. The aforementioned solid content of the hydrophobic melamine resin (C) can be 20 parts by mass or more, or 25 parts by mass or more. The aforementioned solid content of the hydrophobic melamine resin (C) can be 50 parts by mass or less, or 40 parts by mass or less.

[0086] The solids mass ratio (B:C) of the acrylic resin dispersion (B) to the hydrophobic melamine resin (C) can be, for example, 10:90 to 50:50. This further improves the water dispersibility of the hydrophobic melamine resin. The solids mass ratio (B:C) can also be 15:85 to 45:55, or 20:80 to 40:60.

[0087] Other resin components Waterborne coating compositions may include other resin components as needed. Examples of other resin components include water-soluble acrylic resins, polyester resin dispersions, and polyurethane resin dispersions. They may be used alone or in combination of two or more.

[0088] The content of water-soluble acrylic resin is, for example, 1 part by weight or more and 60 parts by weight or less relative to 100 parts by weight of the resin solids component of the water-based coating composition. The aforementioned content of water-soluble acrylic resin may be 2 parts by weight or more, or 5 parts by weight or more. The aforementioned content of water-soluble acrylic resin may be 50 parts by weight or less, 30 parts by weight or less, or 10 parts by weight or less.

[0089] In addition, the polyester resin dispersion also functions to disperse hydrophobic melamine resin (C). On the other hand, polyester resin may induce yellowing. Polyester resin may cause yellowing, in particular, due to its reaction with isocyanate compounds contained in the transparent coating film. Taking this into account, the content of the polyester resin dispersion relative to 100 parts by weight of the resin solids component of the waterborne coating composition may be 10 parts by weight or less, 5 parts by weight or less, or 0 parts by weight.

[0090] The content of the polyurethane resin dispersion is, for example, 1 part by mass and 60 parts by mass or less per 100 parts by mass of the resin solids component of the waterborne coating composition. The aforementioned content of the polyurethane resin dispersion may be 2 parts by mass or more, or 5 parts by mass or more. The aforementioned 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.

[0091] Other curing components Waterborne coating compositions may contain curing components other than hydrophobic melamine resin (C). Other curing components include, for example, end-capped isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. They may be used alone or in combination of two or more.

[0092] ·additive Waterborne coating compositions may contain various additives as needed. Examples of additives include film-forming aids, surface conditioners, preservatives, fungicides, defoamers, light stabilizers, UV absorbers, antioxidants, and pH adjusters.

[0093] ·pigment Waterborne coating compositions may contain pigments. There are no particular limitations on the type of pigment, but examples include organic coloring pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, monoazo pigments, diazo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, thioindigo pigments, perylene ketone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolone pigments, naphthol pigments, pyrazolone pigments, anthraquinone pigments, anthraquinone pyrimidine pigments, and metal complex pigments; chrome yellow, yellow oxide... Inorganic coloring pigments such as iron, chromium oxide, molybdenum chromium orange, Indian red, titanium yellow, zinc white, carbon 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, titanium flakes, stainless steel flakes, plate-shaped iron oxide, phthalocyanine flakes, metal-plated glass flakes, and other coloring and colored flat pigments; extender pigments such as titanium oxide, calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silicon dioxide, and calcined kaolin.

[0094] Preparation of water-based coating compositions There are no particular limitations on the preparation method of the water-based coating composition; it can be prepared by stirring the various components using a mixer or the like. The pigment can be mixed with other components in the form of a pigment paste, which is pre-dispersed in a color carrier containing water, surfactant, or dispersant using a sand mill or the like.

[0095] [Painted Items] Using the water-based coating composition described in this application, coated articles can be obtained. The water-based coating composition described in this application is suitable for forming a coating film adjacent to a clear coating film. The coated article comprises, for example, a substrate and a multilayer coating film sequentially stacked with a colored base coating film, a metallic base coating film, and a clear coating film. The metallic base coating film is formed by the water-based coating composition described in this application. In this case, the water-based coating composition described in this application contains the aforementioned mica pigments and / or flat pigments. In the coated article, interlayer mixing between the metallic base coating film and the clear coating film is suppressed, and the coated article has an excellent appearance.

[0096] (Object to be painted) Materials that can be coated include, for example, metal, resin, and glass. More specifically, examples of objects that can be coated include car bodies and body parts such as those for cars, trucks, motorcycles, and buses, as well as spoilers, bumpers, rearview mirror covers, grilles, door handles, and other automotive components.

[0097] Examples of metals include iron, copper, aluminum, tin, zinc, or their alloys (e.g., steel). Examples of metal coatings include, for instance, cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electro-galvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy coated steel sheets, zinc-iron alloy coated steel sheets, zinc-magnesium alloy coated steel sheets, zinc-aluminum-magnesium alloy coated steel sheets, aluminum-based coated steel sheets, aluminum-silicon alloy coated steel sheets, and tin-based coated steel sheets.

[0098] Metallic objects can undergo surface treatment. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium formation treatment, and composite oxide treatment. After surface treatment, the metallic objects can be further coated with electroplating paint. Electroplating paint can be cationic or anionic.

[0099] Examples of resins include polyethylene resin, EVA resin, polyolefin resins (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, fluoropolymer resin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). Resin-coated substrates can undergo degreasing treatment.

[0100] (Colored base coating) The colored base coating is sandwiched between the substrate and the metal base coating. The coated surface becomes more uniform due to the colored base coating, which easily suppresses the unevenness of the metal base coating.

[0101] From the perspective of the smoothness and spill resistance of the coated items, the thickness of the cured base coating can be greater than 5μm and less than 60μm.

[0102] The coloring base coating film is formed from a coloring base coating composition. The coloring base coating composition can be water-based or solvent-based. The coloring base coating composition can be water-based. Water-based coloring base coating compositions, for example, include the aforementioned acrylic resin emulsion and melamine resin. Solvent-based coloring base coating compositions contain an organic solvent as the main solvent. In solvent-based coloring base coating compositions, the organic solvent accounts for 50% by mass or more, 70% by mass or more, or 100% by mass of the solvent. The coloring base coating composition may also contain pigments and various additives.

[0103] (Metal-based coating) The metal base coating is formed from the water-based coating composition described in this application.

[0104] There is no particular limitation on the thickness of the base coating; it can be set appropriately according to the purpose. The thickness of the cured base coating can be greater than 0.1 μm and less than 45 μm.

[0105] (Transparent coating) Transparent coatings enhance the gloss of coated items and prevent the pigments mixed into the lower layers from peeling off and flying off.

[0106] From the perspective of scratch resistance and smoothness, the thickness of the cured transparent coating can be greater than 15μm and less than 50μm.

[0107] A transparent coating film is formed from a transparent coating composition. The transparent coating composition can be solvent-based, water-based, or powder-based. The transparent coating composition can be solvent-based. From the viewpoint of transparency or acid etching resistance, solvent-based transparent coating compositions may contain hydroxyl-containing acrylic resins and / or polyester resins as film-forming resins and amino resins and / or isocyanate compounds as curing agents. Additionally, solvent-based transparent coating compositions may contain acrylic resins and / or polyester resins having carboxylic acid and / or epoxy groups. The transparent coating composition may contain the aforementioned pigments to a extent that does not impair transparency. The transparent coating composition may contain various additives as needed.

[0108] Transparent coating compositions may include isocyanate compounds as curing agents. Metal-based coatings containing acrylic resins as the main film-forming resin are less prone to yellowing caused by reactions with isocyanate compounds contained in the transparent coating.

[0109] Isocyanate compounds have at least two isocyanate groups in one molecule. Examples of isocyanate compounds include, for example, aliphatic polyisocyanates, alicyclic polyisocyanates, aliphatic polyisocyanates having an aromatic ring in the molecule that is not bonded to the isocyanate group (aromatic aliphatic polyisocyanates), aromatic polyisocyanates, and derivatives of these polyisocyanates. Specifically, examples include aromatic polyisocyanates such as toluene diisocyanate, 4,4′-diphenylmethane diisocyanate, phenylenediamine diisocyanate, and isophthalimethylene diisocyanate; aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate; and their biuret, ureate, adduct, and other polymeric forms. They can be used alone or in combination of two or more.

[0110] [Manufacturing method of multilayer coating] Multilayer coatings can be manufactured, for example, by a method comprising the following steps: applying a coloring base coating composition to a substrate to form an uncured coloring base coating film; curing the uncured coloring base coating film; applying the water-based coating composition described in this application to form an uncured metal base coating film; curing the uncured metal base coating film; applying a clear coating composition to the metal base coating film to form an uncured clear coating film; and curing the uncured clear coating film.

[0111] When forming a transparent coating, the colored base coating and the metallic base coating may or may not be cured. From the point of view of productivity, adhesion and water resistance, it is possible to prevent the individual coatings from curing and instead cure these multiple uncured coatings simultaneously after lamination (so-called wet-on-wet coating).

[0112] The wet-on-wet coating process includes the following steps: applying a coloring base coating composition to the substrate to form an uncured coloring base coating film; applying the water-based coating composition described in this application to the uncured coloring base coating film to form an uncured metal base coating film; applying a clear coating composition to the uncured metal base coating film to form an uncured clear coating film; and simultaneously curing the uncured coloring base coating film, the uncured metal base coating film, and the uncured clear coating film.

[0113] Preheating can be performed after applying the base coloring paint composition and before applying the water-based paint composition described in this application. Preheating can be performed, for example, by placing the product at a temperature of 20°C or higher and 25°C or lower for 5 minutes or more and 15 minutes; or by heating the product at a temperature of 50°C or higher and 80°C or lower for 30 seconds or more and 10 minutes.

[0114] Examples of coating methods include multi-stage coating based on air spray coating, airless spray coating, electrostatic spray coating, and air electrostatic spray coating (typically, two-stage coating), as well as coating that combines air electrostatic spray coating with a rotary atomizing electrostatic coating machine.

[0115] The curing of each coating composition is carried out, for example, at a heating temperature of 80°C to 180°C and a heating time of 5 to 60 minutes. Example

[0116] The invention is illustrated in more detail by means of the following examples, but is not limited thereto. In the examples, "parts" and "%" are based on the mass of solid components unless otherwise specified.

[0117] (weight-average molecular weight) Regarding the weight-average molecular weight, four GPC devices were used: "HLC8220GPC" (trade name, manufactured by Tosoh Corporation) as the GPC apparatus, and "Shodex KF-606M" and "Shodex KF-603" (both manufactured by Showa Denko Corporation, trade names) as columns. The measurements were performed under the following conditions: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 0.6 cc / min, and detector: RI.

[0118] The concentration of solid components is calculated based on the amount remaining when the object is heated at 150°C.

[0119] The average particle size is set as the 50% average particle size (D50) in the volume-based particle size distribution measured using the MICROTRAC particle size distribution measuring device manufactured by Nikkiso Corporation and named "UPA-150".

[0120] Acid value and hydroxyl value are calculated based on the composition of the raw material monomers.

[0121] [Manufacturing Example A] Manufacturing of a hydroxyl-containing acrylic resin emulsion (A) 126.5 parts of deionized water were added to the reaction vessel, and the mixture was stirred and heated to 80°C under a nitrogen atmosphere. Then, over a period of 2 hours, 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), a monomer emulsion containing 1.1 parts of emulsifier (trade name: ADEKA REASOAP SR-10, manufactured by ADEKA), and 80 parts of deionized water, and an initiator solution containing 0.3 parts of ammonium persulfate and 10 parts of deionized water were added dropwise to the reaction vessel. After the addition was complete, the mixture was allowed to mature at the same temperature for 2 hours.

[0122] Subsequently, the solution was cooled to 40°C and filtered through a 400-mesh filter. 20 parts deionized water and 0.32 parts dimethylaminoethanol were added to the filtrate to adjust the pH 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 non-volatile content of 30%, an acid value of 20 mg KOH / g, and a hydroxyl value of 40 mg KOH / g.

[0123] [Manufacturing Example B-1] Manufacturing of Core-Shell Type Acrylic Resin Dispersion (B-1) (1) Synthesis of hydrophobic acrylic resin (Bc) Add 30 parts of a reactive solvent (CAE, glycidyl ester of a branched alkyl monocarboxylic acid with 9 carbon atoms, trade name: Cardura E10P, manufactured by HEXION, boiling point 251~278℃) to a reaction vessel equipped with a stirrer, temperature controller, condenser, and dropping device, and heat to 165℃ while stirring, then reflux. Separately prepare 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 ARKEMAYOSHITOMI), and 6.5 parts of a high-boiling solvent (dipropylene glycol monomethyl ether (DPM)). The mixture was added dropwise to the above reaction vessel at 165°C for 3.5 hours to induce polymerization and ring-opening addition reactions.

[0124] (2) Synthesis of carboxyl-containing acrylic resins (Bs') A mixture of 3.43 parts AA, 3.8 parts HEMA, 2.7 parts CHMA, 2.9 parts NBA, 5.8 parts ST, 0.11 parts polymerization initiator (LUPEROX DTA), and 2.4 parts high-boiling-point solvent (DPM) was prepared separately. This mixture was added dropwise to the above reaction vessel while stirring for 1 hour at 165°C, allowing it to polymerize directly for 1 hour. Then, a mixture of 0.1 parts polymerization initiator (LUPEROX DTA) and 0.1 parts high-boiling-point solvent (DPM) was added to the above reaction vessel, and the polymerization reaction was carried out at 165°C for 1 hour while stirring.

[0125] This process yields a varnish with a solid content of 91% by mass of a core-shell acrylic resin containing a hydrophobic resin (Bc) and an acid-containing resin (Bs').

[0126] (3) Neutralization and phase transition At 80°C, an alkaline compound (dimethylethanolamine, DMEA) was added to the varnish at a ratio of 5.33 parts per 100 parts of acrylic resin, and the mixture was stirred for 15 minutes. The neutralization rate of the carboxyl groups was set to 90%. Then, at 80°C, 150 parts of deionized water were added dropwise to the neutralized varnish while stirring to obtain a milky white dispersion containing acrylic resin particles.

[0127] It is anticipated that the acrylic resin particles in the dispersion will have a core-shell structure, with the calculated mass proportion of the core being 81.4% and the mass proportion of the shell being 18.6%. The average particle size of the acrylic resin particles is 57 nm, the acid value is 37.2 mg KOH / g, and the weight-average molecular weight is 32,000.

[0128] [Manufacturing Examples B-2~B-6, Comparative Manufacturing Example b-1] As shown in Table 1, the types and neutralization rates of the raw material monomers were changed, and the same procedure as in Manufacturing Example B-1 was followed to obtain a core-shell acrylic resin dispersion.

[0129] [Manufacturing Example 1] Manufacturing of water-soluble acrylic resins Add 23.89 parts of tripropylene glycol methyl ether and 16.11 parts of propylene glycol methyl ether to the reaction vessel, and heat to 105°C while mixing and stirring under a nitrogen atmosphere. Separately prepare 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. Over 3 hours, add 100 parts of this monomer mixture and an initiator solution containing 10.0 parts of tripropylene glycol methyl ether and 1 part of tert-butyl peroxide. After the addition is complete, allow to mature at the same temperature for 0.5 hours.

[0130] Then, an initiator solution containing 5.0 parts of tripropylene glycol methyl ether and 0.3 parts of tert-butyl peroxide-2-ethylhexanoate was added dropwise to the reaction vessel over 0.5 hours. After the addition was completed, the mixture was allowed to mature at the same temperature for 2 hours.

[0131] Next, using a solvent removal apparatus, 16.1 parts of solvent were removed by distillation under reduced pressure (70 torr) and at 110°C. Then, 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added. This yielded a water-soluble acrylic resin solution with 30% non-volatile components, an acid value of 40 mg KOH / g, a hydroxyl value of 50 mg KOH / g, a Tg of 10°C, and a Mw of 30,000.

[0132] [Manufacturing Example 2] Manufacturing 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 pipe, temperature control device, condenser, and decanter. The temperature was raised to 180°C to carry out a condensation reaction until water no longer distilled out. After cooling to 60°C, 120 parts of phthalic anhydride were added to the reaction vessel. Then, the temperature was 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. A polyester resin dispersion with a solid content of 45% by mass, a hydroxyl value of 110 mg KOH / g, an acid value of 15 mg KOH / g, a Tg of -14°C, and a Mw of 7,000 was obtained.

[0133] [Manufacturing Example 3] Manufacturing of phosphate-containing acrylic resins 40 parts of ethoxypropanol were added to a 1-liter reaction vessel equipped with a stirrer, temperature controller, and condenser. Separately, a monomer solution was prepared containing 4 parts styrene, 35.96 parts n-butyl acrylate, 18.45 parts ethylhexyl methacrylate, 13.92 parts 2-hydroxyethyl methacrylate, 7.67 parts methacrylic acid, and 20 parts ethoxypropanol, dissolved with 20 parts Phosmer PP (hexa(oxopropyl) monomethacrylate, manufactured by Union Chemical), and 1.7 parts azobisisobutyronitrile. 121.7 parts of this monomer solution were added dropwise to the reaction vessel at 120°C for 3 hours. Stirring was continued for 1 hour to obtain a phosphate-containing acrylic resin (63% non-volatile component) with an acid value of 105 mg KOH / g, of which the phosphate-based acid value was 55 mg KOH / g, the hydroxyl value was 60 mg KOH / g, and the number average molecular weight was 6000.

[0134] [Table 1] .

[0135] [Example 1] (i) Preparation of water-based coating compositions Mix 5 parts of the above-mentioned water-soluble acrylic resin (30% resin solids), 3.6 parts of 10% by mass dimethylaminoethanol, 40 parts of hydroxyl-containing acrylic resin emulsion (A), 10 parts of core-shell acrylic resin dispersion (B-1) (36% resin solids), 40 parts of melamine resin (C-1), and 5 parts of urethane resin dispersion (trade name: N-800T, manufactured by Sanyo Chemical Industry Co., Ltd.) and disperse them evenly.

[0136] Next, aluminum flakes (average particle size 14 μm, manufactured by Toyo Aluminum Co., Ltd., active ingredient 66%), at a ratio of 27.6 parts per 100 parts of resin solids, 5.52 parts of the aforementioned phosphate-containing acrylic resin, 0.5 parts of lauryl phosphate, 21.25 parts of 2-ethylhexanol, 8.5 parts of 2-ethylhexanediol, and 15 parts of surfactant (trade name: SURFYNOL 440, manufactured by Air Products Co., Ltd., a polyol compound of acetylene diol (100% solids)) were uniformly dispersed. Dimethylaminoethanol was added to the dispersion to achieve a pH of 8.1, and the mixture was diluted with deionized water to obtain an aqueous coating composition.

[0137] Details of the melamine resin used are shown in Table 2.

[0138] [Table 2] .

[0139] (ii) Formation of multilayer coatings For a matte steel sheet treated with zinc phosphate (thickness 0.8mm, length 30cm, width 40cm), a cationic electroplating coating (trade name: Powernix 150, manufactured by Nippon Paint Co., Ltd.) was applied to achieve a dry coating film thickness of 20μm. After heating and curing at 160°C for 30 minutes, the coating was cooled to obtain a coated object with a cured electroplated film.

[0140] Using a rotary atomizing electrostatic coating apparatus, Aquarex AR-3100 (trade name, manufactured by Nippon Paint Automotive Coating Co., Ltd., water-based base coating) was applied to the substrate to achieve a dry film thickness of 10 μm. After coating, a setting period of 4 minutes was allowed to obtain an uncured colored base coating film.

[0141] Next, the prepared waterborne coating composition was diluted with deionized water to a solids concentration of 23% by mass. Then, on the uncured colored base coating, air spray coating was performed at room temperature (23°C) to achieve a dry film thickness of 8 μm. After setting for 4 minutes, preheating was performed at 80°C for 5 minutes. This yielded an uncured metallic base coating.

[0142] The coated panel obtained above was naturally cooled to room temperature. The transparent coating composition (two-component urethane-cured transparent coating (trade name: Polyurethane O-3100 Clear, manufactured by Nippon Paint Automotive Coating Co., Ltd.)) was then applied by air spraying to a dry film thickness of 35 μm and allowed to set for 7 minutes. This resulted in an uncured transparent coating film.

[0143] Finally, the coated board was heated at 140 °C for 30 minutes using a dryer to form a multi-layer coating film successively having a colored base coating film, a metal base coating film, and a transparent coating film.

[0144] [Examples 2 to 15, Comparative Examples 1 to 4] As shown in Table 3, the components and / or their amounts were changed, and except for this, an aqueous coating composition was prepared and a multi-layer coating film was formed according to the same procedure as in Example 1.

[0145] [Evaluation] Using the aqueous coating compositions or multi-layer coating films obtained in the examples and comparative examples, the following evaluations were carried out. The evaluation results are shown in the following table.

[0146] (1) Coating film appearance For the short-wave (SW) value (measurement wavelength: 300 to 1,200 μm) of the multi-layer coating film, using the trade name: WAVE SCANDOI (manufactured by BYK Gardner), it was measured. The obtained SW value was evaluated according to the following criteria. The smaller the SW value, the higher the smoothness of the coating film. A coating film appearance excellent can be evaluated as B or above.

[0147] (Evaluation criteria) A: SW value ≤ 20 B: 20 < SW value ≤ 30 C: 31 < SW value.

[0148] (2) Yellowing For the multi-layer coating film, using a sunshine weather resistance test chamber S80 (sunshine carbon arc type accelerated weather resistance test machine, manufactured by SugaTest Instruments), according to JIS B 7753, an accelerated weather resistance test of 1600 hours was carried out.

[0149] Before and after the accelerated weather resistance test, the b value was measured using 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 represents the yellowness of the coating film, and the smaller the Δb value, the less yellowing. A suitability for practical use can be evaluated as B or above.

[0150] (Evaluation criteria) A: Δb value < 0.3 B: 0.3 ≤ Δb value < 0.5 C: 0.5 ≤ Δb value.

[0151] (3) Storage stability Using the waterborne coating compositions freshly prepared in the examples or comparative examples and those stored at 40°C for one month after preparation, multilayer coatings (X, Y) for evaluation were formed, following the same procedures as described above. The appearance (smoothness) and FF properties of both were visually observed and evaluated according to the following criteria. A rating of B or above indicates that the waterborne coating compositions exhibit excellent storage stability.

[0152] (Evaluation Criteria) A: No changes were observed in X and Y of the multilayer coating. B: The smoothness of multilayer coating Y is slightly reduced compared to multilayer coating X, or some flow and / or reduced smoothness are observed at the coating end face of multilayer coating Y. C: The smoothness of the multilayer coating Y is significantly reduced compared to the multilayer coating X, or a significant reduction in flow and / or FF is observed on the coating end face of the multilayer coating Y, or coating defects such as repulsion, pitting or particulate matter occur in the multilayer coating Y.

[0153] (4) Color recovery Using a spectrophotometer (trade name: X-Rite MA68II, manufactured by XRITE Corporation), the lightness (L5) at 15° (front) and the lightness (L) at 110° (shade) of the multilayer coating obtained by the same procedure as described above were measured. 110 Calculate the difference. The larger the difference, the higher the FF property and the more the color return is suppressed. A rating of B or above can be considered suitable for practical use.

[0154] (Evaluation Criteria) A: 90≤L5-L 110 B: 80≤L5-L 110 <90 C: L5-L 110 <80.

[0155] (5) Water resistance The test panels obtained through the same procedure as described above were immersed in warm water at 40°C for 240 hours. Afterward, they were removed from the water and dried at room temperature for 1 hour. The appearance of the dried coating was visually observed, and the presence or absence of white spots was evaluated according to the following criteria. A rating of B or higher indicates excellent water resistance.

[0156] (Evaluation Criteria) A: No white stains can be observed. B: Slight white stains were observed. C: White stains were clearly observed.

[0157] [Table 3] .

[0158] Industrial utilization The waterborne coating composition of this application can achieve a coating film that ensures water resistance while exhibiting excellent appearance and suppressing yellowing. This waterborne coating composition is suitable for coating automotive bodies and automotive components.

[0159] This application claims priority based on Japanese Patent Application No. 2023-199319, filed in Japan on November 24, 2023, the contents of which are incorporated herein by reference in their entirety.

Claims

1. A waterborne coating composition comprising: a hydroxyl-containing acrylic resin emulsion (A), a core-shell acrylic resin dispersion (B), and a hydrophobic melamine resin (C). The core-shell type acrylic resin dispersion (B) has branched hydrocarbon groups with 4 to 24 carbon atoms in the core and a hydrophilic resin in the shell. The solid component 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.

2. The aqueous coating composition according to claim 1, wherein, The solid component 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 core-shell type acrylic resin dispersion (B) has an acid value of 25 mg KOH / g or higher and 50 mg KOH / g or lower.

4. The aqueous coating composition according to any one of claims 1 to 3, wherein, The core-shell type acrylic resin dispersion (B) has a weight-average molecular weight of 7600 or more and 80000 or less.

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