WO2025142010A1 - Aqueous multi-component coating composition and method for producing coated article - Google Patents

Abstract

An aqueous multi-component coating composition including: a first liquid including a hydroxyl group-containing acrylic resin (A) and a polyurethane resin (B); and a second liquid including a polyisocyanate compound (C), wherein the polyisocyanate compound (C) includes a hydrophilic group-free polyisocyanate compound (C1) having a number-average molecular weight of 150 to 2,500, and a nonionic hydrophilic group-containing polyisocyanate compound (C2).

Description

Water-based multi-component coating composition and method for producing coated article

The present invention relates to a water-based multi-component paint composition and a method for producing a coated article.

In recent years, consideration for the natural environment has been required in technical fields such as automobiles. Therefore, methods have been developed to lower the heating temperature and shorten the heating time during painting in order to save energy. For example, Patent Document 1 proposes an aqueous multi-liquid paint composition that contains a base agent (I) containing a hydroxyl-containing acrylic resin and a curing agent (II) containing an anionic hydrophilic group-containing polyisocyanate compound and/or a nonionic hydrophilic group-containing polyisocyanate compound. In Patent Document 1, the coating is dried at a low temperature of 60°C.

JP 2021-130812 A

According to Patent Document 1, the above-mentioned aqueous multi-component paint composition is excellent in drying properties, paint handling properties (pot life, ease of manual mixing, etc.), storage properties, weather resistance, and finish properties such as gloss. However, with the above-mentioned aqueous multi-component paint composition, it is difficult to obtain a coating film that is excellent in chipping resistance, water resistance, and appearance.

The present invention has been made in consideration of the above, and aims to provide an aqueous multi-component paint composition that can be cured at low temperatures and yet can form a coating film that is excellent in chipping resistance, water resistance and appearance, and a method for manufacturing a coated article using this aqueous multi-component paint composition.

In order to solve the above problems, the present invention provides the following aspects.
[1]
A first liquid containing a hydroxyl group-containing acrylic resin (A) and a polyurethane resin (B);
A second liquid containing a polyisocyanate compound (C),
The polyisocyanate compound (C) is
A hydrophilic group-free polyisocyanate compound (C1) having a number average molecular weight of 150 to 2,500;
A nonionic hydrophilic group-containing polyisocyanate compound (C2),
Aqueous multi-component paint composition.
[2]
The aqueous multi-component coating composition according to the above [1], wherein the polyurethane resin (B) has a hydroxyl value of 30 mg KOH/g or less.
[3]
The content of the polyurethane resin (B) is 10 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the solid content of the hydroxyl group-containing acrylic resin (A). The aqueous multi-component coating composition according to [1] or [2] above.
[4]
The aqueous multi-component coating composition according to any one of the above [1] to [3], wherein the hydroxyl-containing acrylic resin (A) has an acid value of 5 mg KOH/g or more and 70 mg KOH/g or less.
[5]
The aqueous multi-component coating composition according to any one of the above [1] to [4], wherein the hydrophilic group-free polyisocyanate compound (C1) comprises at least one selected from the group consisting of aliphatic diisocyanates, aliphatic triisocyanates, alicyclic diisocyanates, alicyclic triisocyanates, aromatic diisocyanates, aromatic triisocyanates, and derivatives thereof.
[6]
The water-based multi-component coating composition according to any one of the above [1] to [5], wherein the mass ratio (WC2/WC1) of the content WC2 of the nonionic hydrophilic group-containing polyisocyanate compound (C2) to the content WC1 of the hydrophilic group-free polyisocyanate compound (C1) is 0.1 or more and 4 or less.
[7]
Further, a color pigment (D) is contained,
The content of the color pigment (D) is 1 part by mass or more and 150 parts by mass or less per 100 parts by mass of the total solid content of the hydroxyl group-containing acrylic resin (A), the polyurethane resin (B) and the polyisocyanate compound (C). [1] - [6] The aqueous multi-component coating composition according to any one of the above.
[8]
The aqueous multi-component coating composition according to any one of the above [1] to [7], wherein the nonionic hydrophilic group-containing polyisocyanate compound (C2) has three or more isocyanate groups.
[9]
The aqueous multi-component coating composition according to any one of the above [1] to [8], wherein the nonionic hydrophilic group-containing polyisocyanate compound (C2) further has an allophanate group.
[10]
The polyurethane resin (B) is obtained by chain-extending a terminal NCO group-containing urethane prepolymer, which is a reaction product of a polyisocyanate compound (b1) and a polyol (b2), with a polyamine compound (b3),
The polyisocyanate compound (b1) comprises an aromatic polyisocyanate and at least one selected from the group consisting of an aliphatic polyisocyanate, an alicyclic polyisocyanate, and an araliphatic polyisocyanate. The aqueous multi-component coating composition according to any one of [1] to [9] above.
[11]
A step of applying an aqueous multi-component coating composition according to any one of the above [1] to [10] on a substrate to form an uncured first coating film;
A step of applying a second aqueous coating composition onto the uncured first coating film to form an uncured second coating film;
A step of applying a clear coating composition onto the uncured second coating film to form an uncured clear coating film;
and heating the uncured first coating film, the uncured second coating film and the uncured clear coating film at a temperature of 70°C or higher and 100°C or lower to cure them.
[12]
The method for producing a coated article according to the above-mentioned [11], wherein the substrate includes a metal part and a resin part.

The present invention provides an aqueous multi-component paint composition that can be cured at low temperatures and yet can form a coating film that is excellent in chipping resistance, water resistance, and appearance, and a method for producing a coated article using this aqueous multi-component paint composition.

[Coating composition]
The aqueous multi-liquid coating composition according to the present disclosure includes a first liquid containing a hydroxyl group-containing acrylic resin (A) and a polyurethane resin (B), and a second liquid containing a polyisocyanate compound (C). The polyisocyanate compound (C) includes a hydrophilic group-free polyisocyanate compound (C1) having a number average molecular weight of 150 to 2500, and a nonionic hydrophilic group-containing polyisocyanate compound (C2).

The aqueous multi-liquid paint composition according to the present disclosure (hereinafter sometimes simply referred to as the aqueous paint composition) contains a polyurethane resin. It is generally said that polyurethane resin improves chipping resistance. However, it has been found that when an anionic hydrophilic group-containing polyisocyanate compound is used as a curing agent, even when polyurethane resin is added, a sufficient improvement in chipping resistance cannot be obtained. The reason for this is unclear, but it is thought that it is because the reaction product between a hydroxyl group-containing acrylic resin and an anionic hydrophilic group-containing polyisocyanate compound has a high hardness, and even when polyurethane resin is added, it is difficult to sufficiently lower the elastic modulus of the resulting coating film.

In this disclosure, a non-hydrophilic group-containing polyisocyanate compound (C1) and a nonionic hydrophilic group-containing polyisocyanate compound (C2) are used in combination as a curing agent. This improves water resistance and appearance while also providing the chipping resistance improvement effect of blending polyurethane resin.

The nonionic hydrophilic group-containing polyisocyanate compound (C2) is obtained by modifying a polyisocyanate compound, typically with a hydrophilic polyol and/or a hydrophilic polyether. These modified portions (nonionic hydroxyl group portions) are also known as soft segments, and provide flexibility to the coating film. In addition, the soft segments are highly compatible with polyurethane resins, and can partially dissolve the polyurethane resin in the aqueous coating composition. This is thought to further increase the flexibility of the coating film, further improving chipping resistance.

In addition, the use of a hydrophilic group-free polyisocyanate compound (C1) as part of the curing agent improves the water resistance of the resulting coating film. Furthermore, the use of a hydrophilic group-free polyisocyanate compound (C1) suppresses the increase in viscosity of the aqueous coating composition, thereby improving the smoothness of the coating film. The hydrophilic group-free polyisocyanate compound (C1) can be dispersed in the aqueous coating composition by the nonionic hydrophilic group-containing polyisocyanate compound (C2). This suppresses local reactions between the hydrophilic group-free polyisocyanate compound (C1) and the hydroxyl group-containing acrylic resin (A), reducing the number of bumps (small protrusions). These are believed to improve the appearance of the coating film.

The aqueous coating composition is a multi-liquid type containing a first liquid and a second liquid. The aqueous coating composition may further contain a third liquid containing other components. The aqueous coating composition is prepared using a method commonly used by those skilled in the art. The aqueous coating composition can be prepared by mixing the first liquid, the second liquid, and further the third liquid. Examples of mixing methods include a kneading and mixing method using a kneader or roll, and a dispersion and mixing method using a sand grind mill or disperser.

Aqueous paint compositions contain water as a solvent. In aqueous paint compositions, the proportion of water in the solvent may be 50% by mass or more, 70% by mass or more, or 100% by mass.

The aqueous coating composition can be cured at low temperatures. The aqueous coating composition can be cured at a temperature of, for example, 70°C or higher and 100°C or lower. The curing temperature may be 75°C or higher, or 80°C or higher. The curing temperature may be 95°C or lower, or 90°C or lower.

(1st liquid)
The first liquid contains a hydroxyl group-containing acrylic resin (A) and a polyurethane resin (B).

(A) Hydroxyl-containing acrylic resin The hydroxyl-containing acrylic resin (A) is a resin (film-forming component) that is the base of the coating film. The hydroxyl-containing acrylic resin (A) reacts with the polyisocyanate compound (C) to form a crosslinked structure. The hydroxyl-containing acrylic resin (A) provides a coating film with sufficient hardness. The hardness of the coating film can be evaluated by its breaking strength.

The hydroxyl-containing acrylic resin (A) has multiple acryloyl groups and one or more (typically two or more) hydroxyl groups in one molecule.

The hydroxyl value (OHV) of the hydroxyl-containing acrylic resin (A) is, for example, 20 mgKOH/g or more and 180 mgKOH/g or less. When the hydroxyl value of the hydroxyl-containing acrylic resin (A) is 20 mgKOH/g or more, the breaking strength of the coating film is likely to be high. When the hydroxyl value of the hydroxyl-containing acrylic resin (A) is 180 mgKOH/g or less, the hydrophilization of the coating film is suppressed, and the water resistance is likely to be improved. The hydroxyl value of the hydroxyl-containing acrylic resin (A) may be 30 mgKOH/g or more, or may be 50 mgKOH/g or more. The hydroxyl value of the hydroxyl-containing acrylic resin (A) may be 150 mgKOH/g or less, or may be 140 mgKOH/g or less.

The hydroxyl value and acid value are determined based on the mass of solids. The hydroxyl value and acid value can be measured by the known method described in JIS K 0070:1992. The hydroxyl value and acid value may be calculated from the amount of unsaturated monomer in the raw material monomer of the resin (e.g., hydroxyl-containing acrylic resin (A)).

The glass transition temperature (Tg) of the hydroxyl-containing acrylic resin (A) is, for example, 15°C or higher and 100°C or lower. If the Tg of the hydroxyl-containing acrylic resin (A) is 15°C or higher, the breaking strength and hardness of the resulting coating film are likely to be improved. If the Tg of the hydroxyl-containing acrylic resin (A) is 100°C or lower, the quick-drying property of the aqueous coating composition is likely to be improved. The Tg of the hydroxyl-containing acrylic resin (A) may be 18°C or higher, or 20°C or higher. The Tg of the hydroxyl-containing acrylic resin (A) may be 90°C or lower, 80°C or lower, or 70°C or lower.

Tg may be calculated from the type and amount of the raw material monomers of the resin. Tg may also be measured by a differential scanning calorimeter (DSC).

From the viewpoint of hardness, the hydroxyl group-containing acrylic resin (A) may have a hydroxyl value of 20 mgKOH/g or more and 180 mgKOH/g or less, and a Tg of 15°C or more and 100°C or less.

The hydroxyl-containing acrylic resin (A) may have an acid value (AV) of 0 mgKOH/g or more and 70 mgKOH/g or less, or 5 mgKOH/g or more and 70 mgKOH/g or less. This makes it easier to improve the hardness of the resulting coating film. The acid value of the hydroxyl-containing acrylic resin (A) may be 8 mgKOH/g or more, or 10 mgKOH/g or more. The acid value of the hydroxyl-containing acrylic resin (A) may be 60 mgKOH/g or less, 50 mgKOH/g or less, or 40 mgKOH/g or less.

The solid acid value and solid hydroxyl value of the hydroxyl-containing acrylic resin (A) are calculated based on the solid acid value and solid hydroxyl value of the monomer mixture used.

The hydroxyl-containing acrylic resin (A) may have a solubility parameter (SP) of 8.5 or more and 12 or less. This makes the hydroxyl-containing acrylic resin (A) more compatible with the aqueous paint composition, and the occurrence of bumps can be further suppressed. The SP value of the hydroxyl-containing acrylic resin (A) may be 9.0 or more, or may be 9.5 or more. The SP value of the hydroxyl-containing acrylic resin (A) may be 11.5 or less, or may be 11.0 or less.

The SP value is one of the measures of a compound's solubility. A higher SP value indicates a higher polarity of the compound, and a lower SP value indicates a lower polarity of the compound.

The SP value of the hydroxyl-containing acrylic resin (A) can be regarded as a weighted average value obtained from the SP values of multiple raw material monomers, taking into account their solid content mass ratios. When the hydroxyl-containing acrylic resin (A) contains multiple types of acrylic resins, the weighted average value of all acrylic resins obtained by further taking into account the solid content mass ratios of each acrylic resin can be regarded as the SP value of the hydroxyl-containing acrylic resin (A).

[Method of measuring solubility parameter (SP)]
The SP value of a monomer is measured, for example, by the following method [Reference: SUH, CLARKE, J. P. S. A-1, 5, 1671-1681 (1967)].

As a sample, 0.5 g of monomer is weighed into a 100 ml beaker and dissolved in 10 ml of acetone. A poor solvent is dripped onto this sample using a 50 ml burette at a measurement temperature of 20°C, and the point at which turbidity occurs is recorded as the amount dripped. As poor solvents, ion-exchanged water is used as a high SP poor solvent, and n-hexane is used as a low SP poor solvent, and the turbidity point of each is measured. The SP value δ of the monomer is given by the following formula.

Ｖｉ：溶媒ｉの分子容（ｍｌ／ｍｏｌ）
φｉ：濁点における溶媒ｉの体積分率
δｉ：溶媒ｉのＳＰ値
ｍｌ：低ＳＰ貧溶媒混合系
ｍｈ：高ＳＰ貧溶媒混合系

Vi: Molar volume of solvent i (ml/mol)
φi: Volume fraction of solvent i at the clouding point δi: SP value of solvent i ml: Low SP poor solvent mixture mh: High SP poor solvent mixture

The hydroxyl-containing acrylic resin (A) can be produced by polymerizing a hydroxyl-containing α,β-ethylenically unsaturated monomer with another α,β-ethylenically unsaturated monomer by a known method. The hydroxyl-containing acrylic resin (A) is produced, for example, by solution polymerization. Commercially available hydroxyl-containing acrylic resins may also be used.

Examples of hydroxyl group-containing α,β-ethylenically unsaturated monomers include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts of these with ε-caprolactone.

(Meth)acrylic acid includes both methacrylic acid and acrylic acid.

Examples of α,β-ethylenically unsaturated monomers other than those mentioned above include carboxylic acids such as acrylic acid, methacrylic acid, ethacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, and the like, or dicarboxylic acid monoesters thereof; styrenes such as styrene and α-methylstyrene; acrylic acid esters such as methyl acrylate, ethyl acrylate, propyl acrylate, n-, i-, and t-butyl acrylate, 2-ethylhexyl acrylate, allyl acrylate, and lauryl acrylate; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-, i-, and t-butyl methacrylate, 2-ethylhexyl methacrylate, allyl methacrylate, and lauryl methacrylate; polymerizable amide compounds such as acrylamide and methacrylamide; and other polymerizable compounds such as polymerizable aromatic compounds, polymerizable nitriles, polymerizable alkylene oxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, α-olefins, dienes, polymerizable carbonyl compounds, and polymerizable alkoxysilyl compounds.

(B) Polyurethane resin Polyurethane resin (B) is also a coating film forming component. Polyurethane resin (B) increases the elasticity of the coating film and improves chipping resistance. The elasticity of the coating film can be evaluated not only by chipping resistance but also by elastic modulus and breaking elongation.

The polyurethane resin (B) may have a hydroxyl value of 40 mgKOH/g or less, or 30 mgKOH/g or less. This inhibits the reaction between the polyisocyanate compound (C) and the polyurethane resin (B), and facilitates the reaction between the polyisocyanate compound (C) and the hydroxyl-containing acrylic resin (A). The reaction between the polyisocyanate compound (C) and the hydroxyl-containing acrylic resin (A) increases the hardness of the resulting coating film. That is, the hydroxyl value of 30 mgKOH/g or less makes it easier to ensure the hardness of the resulting coating film. The hydroxyl value of the polyurethane resin (B) may be 20 mgKOH/g or less, 10 mgKOH/g or less, or 0 mgKOH/g.

The content of polyurethane resin (B) may be 10 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the solid content of the hydroxyl group-containing acrylic resin (A). This provides a good balance between the hardness and elasticity of the resulting coating film, making it easier to obtain a coating film that has a moderate hardness and excellent chipping resistance. The content of polyurethane resin (B) may be 15 parts by mass or more, or may be 20 parts by mass or more. The content of polyurethane resin (B) may be 80 parts by mass or less, or may be 70 parts by mass or less.

The above content of polyurethane resin (B) having a hydroxyl value of 30 mgKOH/g or less may be 30 parts by mass or more and 100 parts by mass or less. The above content of polyurethane resin (B) having a hydroxyl value of 30 mgKOH/g or less may be 40 parts by mass or more, and may be 45 parts by mass or more.

Solids are also called non-volatile content. For example, the solids of an aqueous paint composition are all the components of the aqueous paint composition excluding the solvent. The solids concentration is calculated by dividing the total mass of the solids excluding the solvent from the object by the total mass of the object.

In the first liquid, the polyurethane resin (B) may be dissolved. That is, the polyurethane resin (B) may be a water-soluble polyurethane resin. In the first liquid, the polyurethane resin (B) may be in the form of a dispersion.

Water-soluble polyurethane resin and polyurethane resin dispersion can be obtained, for example, by a method of forcibly emulsifying polyurethane resin using a surfactant, or by a method of neutralizing polyurethane resin with a base or acid.

The polyurethane resin (B) is obtained, for example, by chain-extending a urethane prepolymer containing terminal NCO groups, which is a reaction product of a polyisocyanate compound (b1) and a polyol (b2), with a polyamine compound (b3). This polyurethane resin (B) has the advantage that it can have a high molecular weight.

(b1) Polyisocyanate Compound The polyisocyanate compound (b1) has two or more isocyanate groups in the molecule. In this specification, the term "isocyanate group" refers to an unblocked free isocyanate group.

Examples of the polyisocyanate compound (b1) include aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, and araliphatic polyisocyanates. In particular, the polyisocyanate compound (b1) may contain at least one selected from the group consisting of aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, and araliphatic polyisocyanates. This tends to increase the flexibility of the resulting polyurethane resin (B).

Aromatic polyisocyanates have two or more isocyanate groups bonded to carbon atoms that make up an aromatic ring. Examples of aromatic polyisocyanates include aromatic diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, 2,4'- or 4,4'-diphenylmethane diisocyanate, or mixtures thereof, 2,4- or 2,6-tolylene diisocyanate, or mixtures thereof, 4,4'-toluidine diisocyanate, and 4,4'-diphenylether diisocyanate; aromatic triisocyanates such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, and 2,4,6-triisocyanatotoluene; and aromatic tetraisocyanates such as 4,4'-diphenylmethane-2,2',5,5'-tetraisocyanate. These may be used alone or in combination of two or more.

Aliphatic polyisocyanates do not have aromatic rings and have two or more isocyanate groups bonded to carbon atoms constituting a linear or branched aliphatic hydrocarbon group. Examples of aliphatic polyisocyanates include ethylene diisocyanate, trimethylene diisocyanate, 1,2-propylene diisocyanate, butylene diisocyanate (tetramethylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate), 1,5-pentamethylene diisocyanate (PDI), 1,6-hexamethylene diisocyanate (HDI), 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, 1,2-dimethylphenyl diisocyanate, 1,3-dimethylphenyl diisocyanate, 1,5-dimethylphenyl diisocyanate, 1,6 ...5-dimethylphenyl diisocyanate, 1,6-dimethylphenyl diisocyanate, 1,5-dimethylphenyl diisocyanate aliphatic diisocyanates such as ethyl ester triisocyanate, heptamethylene diisocyanate, octamethylene diisocyanate, and dodecamethylene diisocyanate; and aliphatic triisocyanates such as lysine ester triisocyanate, 1,4,8-triisocyanatooctane, 1,6,11-triisocyanatoundecane, 1,8-diisocyanato-4-isocyanatomethyloctane, 1,3,6-triisocyanatohexane, and 2,5,7-trimethyl-1,8-diisocyanato-5-isocyanatomethyloctane. These may be used alone or in combination of two or more.

Alicyclic polyisocyanates do not have aromatic rings and have two or more isocyanate groups bonded to carbon atoms that constitute a cyclic aliphatic hydrocarbon group. Examples of alicyclic polyisocyanates include alicyclic diisocyanates such as 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate), methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane (common name: hydrogenated xylylene diisocyanate), or mixtures thereof, and norbornane diisocyanate; 1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane, 2-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo[2.2.1 ]heptane, 2-(3-isocyanatopropyl)-2,6-di(isocyanatomethyl)-bicyclo[2.2.1]heptane, 3-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo[2.2.1]heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, 6-(2-isocyanatoethyl)-2-isocyanatoethyl Alicyclic triisocyanates such as isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, and 6-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane are included. These may be used alone or in combination of two or more.

　Aromatic aliphatic polyisocyanates have an aromatic ring and two or more isocyanate groups bonded to carbon atoms constituting an aliphatic hydrocarbon group. Examples of aromatic aliphatic polyisocyanates include aromatic aliphatic diisocyanates such as 1,3- or 1,4-xylylene diisocyanate, or mixtures thereof, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (common name: tetramethylxylylene diisocyanate), or mixtures thereof; and aromatic aliphatic triisocyanates such as 1,3,5-triisocyanatomethylbenzene. These may be used alone or in combination of two or more.

The polyisocyanate compound (b1) may be a derivative of each of the above polyisocyanates. Examples of polyisocyanate derivatives include dimers, trimers, biurets, allophanates, uretdione, uretoimine, isocyanurates, oxadiazinetriones, polymethylene polyphenyl polyisocyanates (crude MDI, polymeric MDI), and crude TDI.

(b2) Polyol Polyol (b2) has two or more hydroxyl groups in the molecule. Examples of polyol compounds include polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and glycerin, polyether polyols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol; polyester polyols obtained from dicarboxylic acids such as adipic acid, sebacic acid, itaconic acid, maleic anhydride, phthalic acid, and isophthalic acid and glycols such as ethylene glycol, triethylene glycol, propylene glycol, butylene glycol, tripropylene glycol, and neopentyl glycol; polycaprolactone polyols; polybutadiene polyols; polycarbonate polyols; and polythioether polyols. These may be used alone or in combination of two or more.

(b3) Polyamine Compound The polyamine compound (b3) has two or more amino groups in the molecule. The polyamine compound (b3) functions as a chain extender. Examples of the polyamine compound (b3) include ethylenediamine, tetramethylenediamine, hexamethylenediamine, diethylenetriamine, triethylenetetramine, piperazine, hydrazine, tolylenediamine, xylylenediamine, and isophoronediamine. These may be used alone or in combination of two or more.

(b4) Other components In the preparation of the polyurethane resin (B), other components (b4) may be used as necessary. Representative examples include monoisocyanate compounds. Examples of monoisocyanate compounds include methyl isocyanate, ethyl isocyanate, propyl isocyanate, butyl isocyanate, lauryl isocyanate, cyclohexyl isocyanate, phenyl isocyanate, and tolylene isocyanate. These may be used alone or in combination of two or more.

<Preparation method>
The polyurethane resin (B) can be obtained, for example, by reacting a polyisocyanate compound (b1) with a part of a polyol (b2) to obtain a terminal NCO group-containing urethane prepolymer, and then reacting this prepolymer with a polyamine compound (b3) and the remaining part of the polyol (b2). The other component (b4) may be added at any stage.

The reaction temperature may be from 40°C to 140°C, or from 60°C to 120°C. Tin-based catalysts such as dibutyltin laurate and tin octoate, or amine-based catalysts such as triethylenediamine, which are used in normal urethane reactions, may be used. The reaction may be carried out in an organic solvent inert to isocyanates (e.g., acetone, toluene, ethyl acetate, dimethylformamide, methyl ethyl ketone), and an organic solvent may be added during or after the reaction.

The polyisocyanate compound (b1) and the polyol (b2) are each used in an amount such that the ratio of isocyanate groups to hydroxyl groups (NCO/OH) is, for example, 1.03 to 1.4.

<Other hydroxyl-containing components>
The first liquid may further contain, as another hydroxyl group-containing component, for example, at least one selected from the group consisting of polyester polyol resin, polycarbonate polyol resin, polyether polyol resin, and polycaprolactone polyol resin.

The solid content ratio of the hydroxyl group-containing components other than the hydroxyl group-containing acrylic resin (A) relative to the total solid content of the hydroxyl group-containing components (100% by mass) is, for example, 20% by mass or less, may be 15% by mass or less, or may be 10% by mass or less.

<Solvent>
The first liquid contains water as a solvent and may further contain a water-soluble or water-miscible organic solvent, if necessary.

≪Preparation method≫
The first liquid can be prepared by mixing the above-mentioned components by a method known to those skilled in the art. Examples of the mixing method include the same methods as those used for preparing the aqueous coating composition.

(Second liquid)
The second liquid contains a polyisocyanate compound (C).

(C) Polyisocyanate Compound The polyisocyanate compound (C) is a curing agent that reacts with a hydroxyl-containing resin (typically, a hydroxyl-containing acrylic resin (A)) to form a crosslinked structure, thereby curing the aqueous coating composition.

The polyisocyanate compound (C) includes a hydrophilic group-free polyisocyanate compound (C1) having a number average molecular weight of 150 to 2500, and a nonionic hydrophilic group-containing polyisocyanate compound (C2). By using the above two types of polyisocyanate compounds in combination with a polyurethane resin, a coating film with excellent chipping resistance, water resistance, and appearance can be obtained.

The use of anionic hydrophilic group-containing polyisocyanate compounds is not excluded. However, from the viewpoint of chipping resistance, it is desirable to use a small amount of the compound. For example, the amount of anionic hydrophilic group-containing polyisocyanate compounds used relative to 100% by mass of the total solid content of the polyisocyanate compounds contained in the second liquid may be 20% by mass or less, 10% by mass or less, or 0% by mass.

(C1) Hydrophilic group-free polyisocyanate compound The hydrophilic group-free polyisocyanate compound (C1) has two or more isocyanate groups in the molecule, but does not have a hydrophilic group. Therefore, the water resistance of the resulting coating film is improved. In addition, the hydrophilic group-free polyisocyanate compound (C1) has an unblocked free isocyanate group, which enables low-temperature curing.

Hydrophilic groups are broadly classified into anionic and nonionic. Anionic hydrophilic groups are derived from, for example, carboxylic acid, sulfonic acid, phosphoric acid, silicic acid, sulfate ester, phosphate ester, or metal or organic salts thereof. The hydrophilic group-free polyisocyanate compound (C1) does not have hydrophilic groups derived from these compounds. The hydrophilic group-free polyisocyanate compound (C1) also does not have nonionic hydrophilic groups. Nonionic hydrophilic groups will be described later.

The hydrophilic group-free polyisocyanate compound (C1) includes, for example, at least one selected from the group consisting of aliphatic diisocyanates, aliphatic triisocyanates, alicyclic diisocyanates, alicyclic triisocyanates, aromatic diisocyanates, aromatic triisocyanates, and derivatives thereof. Specific examples of these diisocyanates and triisocyanates include the same compounds as those exemplified as the polyisocyanate compound (b1).

The number average molecular weight of the hydrophilic group-free polyisocyanate compound (C1) is 150 or more and 2500 or less. This allows the coating film to have flexibility and further improves chipping resistance. The number average molecular weight of the hydrophilic group-free polyisocyanate compound (C1) may be 250 or more, 360 or more, or 400 or more. The number average molecular weight of the hydrophilic group-free polyisocyanate compound (C1) may be 2000 or less, 1800 or less, or 1600 or less.

(C2) Nonionic hydrophilic group-containing polyisocyanate compound The nonionic hydrophilic group-containing polyisocyanate compound (C2) has two or more isocyanate groups in the molecule. The nonionic hydrophilic group-containing polyisocyanate compound (C2) also has an unblocked free isocyanate group, so that low-temperature curing is possible. In addition, the nonionic hydrophilic group-containing polyisocyanate compound (C2) has a nonionic hydrophilic group, so that the resulting coating film becomes flexible and chipping resistance is improved. Furthermore, the nonionic hydrophilic group-containing polyisocyanate compound (C2) disperses the hydrophilic group-free polyisocyanate compound (C1) in the aqueous coating composition, suppressing the local reaction between the hydrophilic group-free polyisocyanate compound (C1) and the hydroxyl group-containing acrylic resin (A). This improves the appearance of the resulting coating film. The appearance of the coating film can be evaluated by the presence or absence of bumps and smoothness.

The nonionic hydrophilic group is derived from a hydrophilic compound. In other words, the nonionic hydrophilic group-containing polyisocyanate compound (C2) can be obtained by modifying, for example, the polyisocyanate compound exemplified as the polyisocyanate compound (b1) with a hydrophilic compound.

Hydrophilic compounds include, for example, hydrophilic polyols and hydrophilic polyethers. Hydrophilic polyols include, for example, ethylene glycol, glycerol, trimethylolpropane, pentaerythritol, and sorbitol.

An example of a hydrophilic polyether is polyalkylene glycol (-(OC _n H _2n ) _a -). The number of carbon atoms n of the alkyl group is, for example, 2 or more and 4 or less, and may be 2 or more and 3 or less. The number of repeating units a is, for example, 2 or more and 12 or less. The number of repeating units a may be 3 or more. The number of repeating units a may be 8 or less.

The hydrophilic polyether may be polyethylene glycol or polypropylene glycol. The hydrophilic polyether may be polyethylene glycol. One of the hydroxyl groups of the polyalkylene glycol may be alkoxylated with an alkyl group such as a methyl group, an ethyl group, a propyl group, or a butyl group.

The modification with a hydrophilic compound is carried out so that two or more isocyanate groups remain in one molecule. The nonionic hydrophilic group-containing polyisocyanate compound (C2) may have three or more isocyanate groups. This improves reactivity and makes low-temperature curing more likely to proceed. The nonionic hydrophilic group-containing polyisocyanate compound (C2) having three or more isocyanate groups can be obtained, for example, by modifying the triisocyanate compound or tetraisocyanate compound exemplified above with a hydrophilic compound (typically, a polyalkylene glycol).

The nonionic hydrophilic group-containing polyisocyanate compound (C2) may further have an allophanate group (-NH-CO-N-CO(=O)-). This improves compatibility with the polyurethane resin (B). As a result, the polyurethane resin (B) can be more easily mixed uniformly in the aqueous coating composition, further improving chipping resistance.

The mass ratio (WC2/WC1) of the content WC2 of the nonionic hydrophilic group-containing polyisocyanate compound (C2) to the content WC1 of the hydrophilic group-free polyisocyanate compound (C1) may be 0.1 or more and 4 or less. This makes it easier to achieve both improved chipping resistance and appearance, and improved water resistance.

The mass ratio (WC2/WC1) may be 0.2 or more, 0.3 or more, 0.5 or more, or 0.7 or more. The mass ratio (WC2/WC1) may be 3 or less, 2 or less, 1.5 or less, or less than 1.0.

The equivalent ratio (NCO/OH) of the total isocyanate groups contained in the polyisocyanate compound (C) to the total hydroxyl groups contained in the hydroxyl group-containing component is, for example, 0.7 or more and 2.0 or less. The above equivalent ratio (NCO/OH) may be 0.8 or more. The above equivalent ratio (NCO/OH) may be 1.8 or less, or may be 1.5 or less.

Other hardeners
The aqueous coating composition may contain other curing agents other than the polyisocyanate compound (C). Examples of other curing agents include amino resins, epoxy compounds, aziridine compounds, carbodiimide compounds and oxazoline compounds. These are used alone or in combination of two or more. The content of other curing agents is appropriately set according to the hydroxyl group-containing resin.

<Solvent>
The second liquid may contain a solvent that does not have a hydroxyl group. Examples of such a solvent include glycol ether-based organic solvents, acetate-based organic solvents, ketone-based organic solvents, and ester-based organic solvents. These may be used alone or in combination of two or more.

≪Preparation method≫
The second liquid can be prepared by mixing the above-mentioned components by a method known to those skilled in the art. Examples of the mixing method include the same method as that for preparing the first liquid.

(D) Coloring Pigment The aqueous coating composition may contain a coloring pigment (D). The coloring pigment (D) enhances the hiding power of the resulting coating film. The aqueous coating composition containing the coloring pigment (D) is particularly suitable for forming an intermediate coating film (first coating film described later). The coloring pigment (D) may be added to any of the first liquid, the second liquid, and the third liquid. The coloring pigment (D) may be added after being dispersed with a pigment dispersant to form a paste.

The content of the color pigment (D) is, for example, 1 part by mass or more and 150 parts by mass or less per 100 parts by mass of the total solid content of the hydroxyl group-containing acrylic resin (A), the polyurethane resin (B), and the polyisocyanate compound (C). This allows the aqueous paint composition to exhibit sufficient hiding power when used to form an undercoat coating film. The content of the color pigment (D) may be 5 parts by mass or more, 10 parts by mass or more, 20 parts by mass or more, or 40 parts by mass or more. The content of the color pigment (D) may be 130 parts by mass or less, or 110 parts by mass or less.

The color pigment (D) may be inorganic or organic. The color pigment (D) may be chromatic or achromatic.

Examples of organic coloring pigments include azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments. Examples of inorganic coloring pigments include yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These may be used alone or in combination of two or more.

(Other pigments)
The aqueous coating composition may contain a pigment other than the color pigment (D) in place of or in addition to the color pigment (D). Examples of the other pigments include extender pigments and anti-rust pigments. Examples of the extender pigments include calcium carbonate, barium sulfate, clay, and talc.

(Other ingredients)
The aqueous coating composition may contain additives that are commonly used in the coating field. The additives may be added to any of the first, second and third liquids. Examples of additives include ultraviolet absorbers, hindered amine light stabilizers, antioxidants, crosslinked resin particles, leveling agents, defoamers, curing accelerators and viscosity adjusters.

[Painted items]
A coated article can be obtained by using the aqueous coating composition according to the present disclosure. The coated article includes, for example, a substrate and a multi-layer coating film in which a first coating film, a second coating film, and a clear coating film are laminated in this order. The first coating film is formed by using the aqueous coating composition according to the present disclosure. Thus, the coated article has excellent chipping resistance, water resistance, and appearance.

(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.

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, electrolytic galvanized 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.

The metallic substrate may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After surface treatment, the metallic substrate may be further coated with an electrocoating paint. The electrocoating paint may be of the cationic type or the anionic type.

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

The aqueous coating composition according to the present disclosure can be cured at low temperatures, making it suitable for application to resins. The coating film obtained by the aqueous coating composition according to the present disclosure has excellent chipping resistance, making it suitable for application to metals. The substrate may include both a metal part (a part formed of metal) and a resin part (a part formed of resin). The metal part may be a steel plate.

(First coating film)
The first coating film is formed by the aqueous coating composition according to the present disclosure. The film thickness (dry film thickness) of the first coating film after curing is, for example, 5 μm or more and 80 μm or less. The dry film thickness of the first coating film may be 7 μm or more. The dry film thickness of the first coating film may be 50 μm or less.

The thickness of the coating can be measured using an electromagnetic coating thickness gauge (e.g., SANKO's SDM-miniR). The coating thickness is the average value of the coating thickness at any five points.

(Second coating film)
The second coating film is formed by the second coating composition. The second coating composition will be described later. The second coating film may be a single layer, or may be a laminated coating film of two or more layers. The dry film thickness of each layer of the second coating film is, for example, 5 μm or more and 35 μm or less. The dry film thickness of each layer of the second coating film may be 7 μm or more. The dry film thickness of each layer of the second coating film may be 30 μm or less.

(Clear coating)
The clear coating film is formed by a clear coating composition. The clear coating composition will be described later. The dry film thickness of the clear coating film is, for example, 10 μm or more and 80 μm or less. The dry film thickness of the clear coating film may be 20 μm or more. The dry film thickness of the clear coating film may be 60 μm or less.

[Method of manufacturing coated articles]
The above-mentioned coated article is manufactured by a method including, for example, a step of applying the above-mentioned aqueous coating composition on a substrate to form an uncured first coating film, a step of applying a second aqueous coating composition on the uncured first coating film to form an uncured second coating film, a step of applying a clear coating composition on the uncured second coating film to form an uncured clear coating film, and a step of heating and curing the uncured first coating film, the uncured second coating film, and the uncured clear coating film. The heating temperature may be 70°C or higher and 100°C or lower. According to the aqueous coating composition of the present disclosure, even at such a low temperature, a coating film excellent in chipping resistance, water resistance, and appearance is formed.

(I) Step of forming an uncured first coating film The aqueous coating composition according to the present disclosure is applied onto a substrate to form an uncured first coating film. The first coating film improves adhesion between the second coating film and the substrate. The first coating film also makes the coating surface uniform, making it easier to suppress unevenness in the second coating film. As described above, the substrate may include both a metal part and a resin part.

Examples of coating methods include roll coater method, air spray coating, airless spray coating, and rotary atomization coating. These methods may be combined with electrostatic coating. Among them, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic coating, for example, rotary atomization electrostatic coating machines commonly known as "micro-microbell (μμbell)", "microbell (μbell)", "metallicbell (metabell)", etc. may be used.

After applying the aqueous paint composition, pre-drying (also called pre-heating) may be performed before applying the second paint composition. This prevents the solvent contained in the aqueous paint composition from bumping during the curing process, making it easier to prevent popping. Furthermore, pre-drying prevents the uncured first coating film and the second paint composition from mixing, making it difficult for a mixed layer to form. This can further improve the smoothness of the resulting coated article.

Examples of pre-drying include leaving the material at a temperature of 20°C to 25°C for 5 to 15 minutes, or heating the material at a temperature of 50°C to 80°C for 30 seconds to 10 minutes.

(II) Step of forming an uncured second coating film A second coating composition is applied on the uncured first coating film to form an uncured second coating film. Two or more layers of uncured second coating films can be formed by applying the same or different second coating compositions two or more times. An interval of several minutes may be provided between the application of the nth second coating composition and the application of the n+1th second coating composition.

The coating method may be, for example, the same method as the coating method for the aqueous coating composition. After coating the second coating composition, preliminary drying may be performed in the same manner as described above.

(Second coating composition)
The second coating composition may be water-based or solvent-based. The second coating composition may be water-based. The water-based second coating composition contains, for example, an acrylic resin emulsion, a water-soluble acrylic resin, a curing agent (typically, a melamine resin), and a polyether polyol resin. The second coating composition may further contain the above-mentioned various pigments, luster pigments, and various additives.

(III) Step of forming an uncured clear coating film The clear coating composition is applied onto the uncured second coating film to form an uncured clear coating film.

The coating method is not particularly limited. Examples of coating methods include methods similar to those used for coating water-based coating compositions. Among these, rotary atomization electrostatic coating is preferred from the standpoint of coating efficiency. After coating the clear coating composition, preliminary drying may be performed in the same manner as described above.

(Clear Coating Composition)
The clear coating composition may be solvent-based, water-based, or powder-type. The solvent-based clear coating composition may contain an acrylic resin and/or a polyester resin as a coating film-forming resin, and an amino resin and/or an isocyanate as a curing agent, from the viewpoints of transparency or acid etching resistance. 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 and additives to the extent that the transparency is not impaired.

(IV) Curing step: Each uncured coating film is cured. Each coating film can be cured by heating. In this embodiment, the first coating film, the second coating film, and the clear coating film are cured at the same time.

The heating temperature is, for example, 70°C or higher and 100°C or lower. The heating temperature may be 75°C or higher, or 80°C or higher. The heating temperature may be 95°C or lower, or 90°C or lower. The heating time refers to the time during which the temperature inside the heating device reaches the target temperature and the workpiece is maintained at the target temperature, and does not take into account the time until the target temperature is reached. Examples of heating devices include drying furnaces that use heat sources such as hot air, electricity, gas, and infrared rays.

The heating time may be set appropriately depending on the heating temperature. When the heating temperature is 70°C or higher and 100°C or lower, the heating time may be, for example, 10 minutes or longer and 60 minutes or shorter, and may be 15 minutes or longer and 45 minutes or shorter.

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

The acid value and hydroxyl value were calculated based on the amount of each unsaturated monomer used and the solid acid value and solid hydroxyl value of each monomer.

[Production Example 1-1] Production of hydroxyl group-containing acrylic resin (A-1) 127 parts of deionized water was added to a reaction vessel, and the temperature was raised to 80°C while mixing and stirring in a nitrogen stream. Next, a monomer emulsion consisting of 1.9 parts of acrylic acid (AA), 30.2 parts of 2-hydroxyethyl methacrylate (HEMA), 8.1 parts of butyl acrylate (nBA), 47.9 parts of n-butyl methacrylate (nBMA), 10.9 parts of styrene (ST), 1.0 parts of allyl methacrylate (AMA), 4.0 parts of Adeka Reasoap SR-10 (polyoxyethylene-1-alkoxymethyl-2-(2-propenyloxy)ethyl ether sulfate ester ammonium salt, 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. The mixture was then cooled to 40°C and filtered through a 400 mesh filter, after which 20 parts of deionized water and 0.32 parts of dimethylaminoethanol were added to adjust the pH. The non-volatile content, Tg, Sp, hydroxyl value and acid value of the resulting acrylic emulsion (A-1) are shown in Table 1 below.

[Production Examples 1-2 to 1-15] Production of Hydroxyl-Containing Acrylic Resins (A-2) to (A-15) Acrylic resin emulsions (A-2) to (A-15) were obtained in the same manner as in Production Example 1-1, except that the monomer compositions and blending amounts of each copolymerization component in Production Example 1-1 were changed to those shown in Table 1 below.

[Production Example 2-1] Production of polyurethane resin (B-1) Using 100 parts of polybutylene adipate diol, 2 parts of trimethylolpropane, 5 parts of dimethylolpropionic acid, and 40 parts of isophorone diisocyanate, a prepolymer having an isocyanate terminal was synthesized in methyl ethyl ketone. The reaction temperature was 85°C. Then, 350 parts of a mixed solvent of 3.6 parts of triethylamine and water was added to emulsify the prepolymer. Furthermore, 21.5 parts of a 5% aqueous solution of ethylenediamine was added and stirred at the same temperature for 80 minutes. Then, the mixture was heated under reduced pressure to distill off the methyl ethyl ketone. As a result, a milky white polyurethane resin (B-1) with a hydroxyl value of 0 mgKOH/g was obtained.

[Production Example 2-2] Production of polyurethane resin (B-3) A polyurethane resin having a hydroxyl group terminal was synthesized in a methyl ethyl ketone solvent using 100 parts of polytetramethylene ether glycol having a number average molecular weight of 250, 13 parts of dimethylolpropionic acid, and 45 parts of isophorone diisocyanate. The reaction temperature was 85°C. Then, 350 parts of a mixed solvent of 8.0 parts of triethylamine and water was added and emulsified. Then, the mixture was heated under reduced pressure to distill off the methyl ethyl ketone. As a result, a milky white polyurethane resin (B-3) having a hydroxyl value of 37 mgKOH/g was obtained.

Superflex-460S (manufactured by Daiichi Kogyo Co., Ltd., hydroxyl value 0 mgKOH/g) was used as the polyurethane resin (B-2).

Details of the polyisocyanate compound (C) used are as follows.
[Hydrophilic Group-Non-Containing Polyisocyanate Compound (C1)]
(1) Desmodur N3300: manufactured by Sumika Covestro Urethane Co., Ltd., hydrophilic group-free, HDI trimer type, number average molecular weight 550
(2) Desmodur N3800: manufactured by Sumika Covestro Urethane Co., Ltd., hydrophilic group-free, HDI trimer type, number average molecular weight 900
(3) Desmodur N3400: manufactured by Sumika Covestro Urethane Co., Ltd., hydrophilic group-free, HDI dimer type, number average molecular weight 350

[Nonionic hydrophilic group-containing polyisocyanate compound (C2)]
(1) Bayhydur 304: manufactured by Sumika Covestro Urethane Co., Ltd., hydrophilic polyether modified HDI trimer, containing allophanate group (2) Bayhydur 3100: manufactured by Sumika Covestro Urethane Co., Ltd., hydrophilic polyether modified HDI trimer, not containing allophanate

[Anionic hydrophilic group-containing polyisocyanate compound (C3)]
Bayhydur XP2655: Sumika Covestro Urethane Co., Ltd., hexamethylene diisocyanate type, containing sulfonic acid groups

[Preparation of pigment dispersion paste]
19.2 parts of Typec CR-97 (titanium dioxide, manufactured by Ishihara Sangyo Kaisha), 4.8 parts of MA-100 (carbon black, manufactured by Mitsubishi Carbon Co., Ltd.), and 16 parts of barium sulfate B-34 (barium sulfate, manufactured by Sakai Chemical Industry Co., Ltd.) were charged into a 1 L stainless steel container and mixed and dispersed at room temperature for 45 minutes using a paint conditioner to obtain a pigment dispersion paste (pigment solid content: 70%).

[Example 1]
(1) Preparation of First Liquid To a 1 L metal container, 50 parts of the above pigment dispersion paste, 30 parts of the hydroxyl group-containing acrylic resin (A-1), 20 parts of the polyurethane resin (B-1), and an appropriate amount of water were added in that order, and the mixture was thoroughly stirred with a disper to obtain a first liquid.

(2) Preparation of second liquid In a separate metal container, 12 parts of a hydrophilic group-free polyisocyanate compound (C1) (Desmodur N3300), 10 parts of a nonionic hydrophilic group-containing polyisocyanate compound (C2) (Bayhydur 304), and an appropriate amount of a solvent (dipropylene glycol dimethyl ether and/or ethylene glycol monobutyl acetate) were mixed and thoroughly stirred with a disper to obtain a second liquid.

(3) Preparation of Water-Based Coating Composition The first and second liquids were mixed to obtain a water-based coating composition.

[Examples 2 to 24 and Comparative Examples 1 to 4]
Aqueous coating compositions were prepared in the same manner as in Example 1, except that the types and amounts of the components, the solid content mass at the time of coating, etc. were changed as shown in Tables 2 to 4.

[evaluation]
The following evaluations were carried out using the aqueous coating compositions prepared in the above Examples and Comparative Examples. Test panels having a first coating film formed from the aqueous coating composition were used for the evaluations. The evaluation results are shown in Tables 2 to 4. In the present invention, it is important that all performances are excellent, and if any one of them is rated as failing "D", it is a failure.

(Preparation of steel test panels)
The matte steel plate was washed and zinc phosphate-treated in a conventional manner. Then, the steel plate was electrocoated using a cationic electrocoating paint (Power Top U-100, manufactured by Nippon Paint Co., Ltd.) and heated at 170°C for 20 minutes. This resulted in the formation of an electrocoating film having a dry thickness of 15 μm.

Then, the aqueous coating composition was electrostatically coated onto the electrodeposition coating film using a rotary atomizing electrostatic coating machine. After leaving it for 5 minutes, it was heated at 85°C for 20 minutes. This resulted in a steel test panel with a cured first coating film (dry film thickness 15 μm).

(Creating a resin test plate)
A polypropylene plate was degreased to obtain a resin test plate. Then, in the same manner as above, a resin test plate provided with a cured first coating film (dry film thickness: 15 μm) was obtained.

(1) Chipping resistance A steel test plate was placed on the specimen holder of a flying stone tester (Grabelo Tester KSS-1, manufactured by Suga Testing Machines Co., Ltd.), and 100 g of crushed stone with a grain size of No. 6 was collided with the test plate at an angle of 45 degrees from a distance of 30 cm from the test plate at -30°C using compressed air of 0.5 MPa. The test plate was then washed with water and dried, and a cloth adhesive tape (manufactured by Nichiban Co., Ltd.) was attached to the coated surface. The tape was then peeled off, and the maximum diameters of multiple points where the steel plate was exposed were measured and averaged. The obtained average diameter (average exposed diameter) was evaluated according to the following criteria. In the following evaluation, a rating of C or higher is considered to be a pass.

(Evaluation Criteria)
A: Average exposed diameter less than 0.5 mm B: Average exposed diameter 0.5 mm or more and less than 1.0 mm C: Average exposed diameter 1.0 mm or more and less than 2.0 mm D: Average exposed diameter 2.0 mm or more and less than 2.5 mm E: Average exposed diameter 2.5 mm or more

(2) Water resistance A steel test plate was immersed in 40°C warm water for 10 days. 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 evaluated according to the following criteria. A rating of C or higher in the following evaluations was considered to be a pass.

(Evaluation Criteria)
A: No abnormality B: At least one of the abnormalities of gloss loss, cracks, and blisters (blisters) is slightly observed C: At least one of the abnormalities of gloss loss, cracks, and blisters (blisters) is partially observed D: At least one of the abnormalities of gloss loss, cracks, and blisters (blisters) is significantly observed partially or over the entire surface of the coating film

(3) Surface Smoothness The appearance of the coating film on the steel test panel was visually observed, and the smoothness was evaluated according to the following criteria. A rating of C or higher in the following evaluations was deemed to be acceptable.
A: Very good B: Good C: Slightly poor D: Poor E: Obvious irregularities are observed

(4) The appearance of the coating on the surface-depleted steel test panel was visually observed and evaluated according to the following criteria. A rating of C or higher was deemed to be a pass.
A: No abnormality B: Very small amount of small bumps are observed C: Bumps are partially observed D: Solid matter is observed over the entire surface of the coating film

(5) Coating Elastic Modulus A sample of 10 mm width x 50 mm length was cut out from a resin test plate, and the thickness was measured. The sample was set in an autograph AG-IS manufactured by Shimadzu Corporation, and pulled at a rate of 50 mm/min at 25°C, and the elastic modulus (also called Young's modulus) was measured from the stress when the coating film stretched 0.5 mm, the width of the coating film, and the thickness. The measurement was carried out three times using different samples, and the average value was taken as the coating elastic modulus. In the following evaluation, a rating of C or higher was considered to be a pass.

(Evaluation Criteria)
A: Less than 700N/ ^mm2 B: 700N/mm2 or ^more , less than 1000N/ ^mm2 C: 1000N/mm2 ^{or more} , less than 1200N/ ^mm2 D: 1200N/mm2 ^or more, less than 1500N/ ^mm2 E: 1500N/mm2 or ^more

(6) Coating strength: In the same manner as in (5) above, the coating was pulled until it broke, and the stress at the break was divided by the cross-sectional area of the break to measure the strength of the coating. The measurement was carried out three times with different samples, and the average value was taken as the coating strength. In the following evaluation, a rating of C or higher was considered to be a pass.

(Evaluation Criteria)
A: 25 N/mm2 or ^more B: 22 N/mm2 ^{or more} , less than 25 N/ ^mm2 C: ¹⁸ N/mm2 or more, less than 22 N/ ^mm2 D: 15 N/ ^mm2 or more, less than 18 N/ ^mm2 E: Less than 15 N/ ^mm2

(7) Coating elongation: In the same manner as in (5) above, the coating was pulled until it broke, and the elongation of the coating was measured at the time of breakage. The measurement was carried out three times with different samples, and the average value was taken as the coating elongation. In the following evaluation, a rating of C or higher was considered to be a pass.

(Evaluation Criteria)
A: 35% or more B: 30% or more, less than 35% C: 20% or more, less than 30% D: 15% or more, less than 20% E: Less than 15%

All of the aqueous coating compositions in the examples were able to form coating films with excellent chipping resistance, water resistance and appearance, even though they were cured under low temperature conditions.

Comparative Example 1 is an aqueous coating composition that does not contain the nonionic hydrophilic group-containing polyisocyanate compound (C2). In this example, chipping resistance is poor, and defects such as surface bumps are observed. In addition, it was confirmed that the coating film elastic modulus and coating film elongation are poor.
Comparative Example 2 is an aqueous coating composition that does not contain the hydrophilic group-free polyisocyanate compound (C1). In this example, it was confirmed that the water resistance and surface smoothness were poor.
Comparative Example 3 is an aqueous coating composition that does not contain polyurethane resin (B). In this example, it was confirmed that the chipping resistance was poor, and the coating elastic modulus and elongation were also poor.
Comparative Example 4 is an aqueous multi-liquid coating composition that uses an anionic hydrophilic group-containing polyisocyanate compound instead of the nonionic hydrophilic group-containing polyisocyanate compound (C2). In this example, chipping resistance is poor, and problems such as surface bumps are observed. In addition, it is confirmed that the coating film elasticity modulus and coating film elongation are poor.

The coating composition of the present invention can form a coating film that is curable at low temperatures and has excellent chipping resistance, water resistance, and appearance. Therefore, it can be used particularly effectively on substrates that include both metal and resin parts.

This application claims priority to Japanese Patent Application No. 2023-218559, filed on December 25, 2023, the entire contents of which are incorporated herein by reference.

Claims (12)

A first liquid containing a hydroxyl group-containing acrylic resin (A) and a polyurethane resin (B);
A second liquid containing a polyisocyanate compound (C),
The polyisocyanate compound (C) is
A hydrophilic group-free polyisocyanate compound (C1) having a number average molecular weight of 150 to 2,500;
A nonionic hydrophilic group-containing polyisocyanate compound (C2),
Aqueous multi-component paint composition. The aqueous multi-component coating composition according to claim 1, wherein the polyurethane resin (B) has a hydroxyl value of 30 mg KOH/g or less. The aqueous multi-component coating composition according to claim 1 or 2, wherein the content of the polyurethane resin (B) is 10 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the solid content of the hydroxyl group-containing acrylic resin (A). The aqueous multi-component coating composition according to any one of claims 1 to 3, wherein the hydroxyl-containing acrylic resin (A) has an acid value of 5 mgKOH/g or more and 70 mgKOH/g or less. The aqueous multi-liquid coating composition according to any one of claims 1 to 4, wherein the hydrophilic group-free polyisocyanate compound (C1) includes at least one selected from the group consisting of aliphatic diisocyanates, aliphatic triisocyanates, alicyclic diisocyanates, alicyclic triisocyanates, aromatic diisocyanates, aromatic triisocyanates, and derivatives thereof. The aqueous multi-component coating composition according to any one of claims 1 to 5, wherein the mass ratio (WC2/WC1) of the content WC2 of the nonionic hydrophilic group-containing polyisocyanate compound (C2) to the content WC1 of the hydrophilic group-free polyisocyanate compound (C1) is 0.1 or more and 4 or less. Further, a color pigment (D) is contained,
The content of the color pigment (D) is 1 part by mass or more and 150 parts by mass or less per 100 parts by mass of the total solid content of the hydroxyl group-containing acrylic resin (A), the polyurethane resin (B) and the polyisocyanate compound (C). The aqueous multi-component coating composition according to any one of claims 1 to 6. The aqueous multi-component coating composition according to any one of claims 1 to 7, wherein the nonionic hydrophilic group-containing polyisocyanate compound (C2) has three or more isocyanate groups. The aqueous multi-component coating composition according to any one of claims 1 to 8, wherein the nonionic hydrophilic group-containing polyisocyanate compound (C2) further has an allophanate group. The polyurethane resin (B) is obtained by chain-extending a terminal NCO group-containing urethane prepolymer, which is a reaction product of a polyisocyanate compound (b1) and a polyol (b2), with a polyamine compound (b3),
The polyisocyanate compound (b1) comprises an aromatic polyisocyanate and at least one selected from the group consisting of an aliphatic polyisocyanate, an alicyclic polyisocyanate, and an aromatic aliphatic polyisocyanate. The aqueous multi-component coating composition according to any one of claims 1 to 9. A step of applying the aqueous multi-component coating composition according to any one of claims 1 to 10 onto a substrate to form an uncured first coating film;
A step of applying a second aqueous coating composition onto the uncured first coating film to form an uncured second coating film;
A step of applying a clear coating composition onto the uncured second coating film to form an uncured clear coating film;
and heating the uncured first coating film, the uncured second coating film and the uncured clear coating film at a temperature of 70°C or higher and 100°C or lower to cure them. The method for producing a coated article according to claim 11, wherein the substrate includes a metal part and a resin part.