CN120265717A

CN120265717A - Water-based coating composition and method for producing coated article

Info

Publication number: CN120265717A
Application number: CN202380082069.0A
Authority: CN
Inventors: 配岛优生; 井阪悠太; 大山阳祐; 柳泽友辉; 山口翔平
Original assignee: Nippon Paint Automotive Coatings Co Ltd
Current assignee: Nippon Paint Automotive Coatings Co Ltd
Priority date: 2022-11-30
Filing date: 2023-11-28
Publication date: 2025-07-04
Also published as: JPWO2024117137A1; WO2024117137A1; JP7721822B2

Abstract

水性涂料组合物，含有：水性聚酯树脂(A)；水性聚氨酯树脂(B)；三聚氰胺树脂(C)；以及除所述水性聚酯树脂(A)、所述水性聚氨酯树脂(B)和所述三聚氰胺树脂(C)以外的含羟基树脂(D)，其中，所述水性聚酯树脂(A)的数均分子量为1000以上且12000以下，玻璃化转变温度为‑30℃以上且20℃以下，酸值为10mgKOH/g以上且45mgKOH/g以下，羟值为10mgKOH/g以上且60mgKOH/g以下，相对于所述水性涂料组合物的树脂固体成分100质量份，所述水性聚酯树脂(A)的含量为17质量份以上且45质量份以下，相对于所述水性涂料组合物的树脂固体成分100质量份，所述水性聚氨酯树脂(B)的含量为超过15质量份且40质量份以下。A water-based coating composition, comprising: a water-based polyester resin (A); a water-based polyurethane resin (B); a melamine resin (C); and a hydroxyl-containing resin (D) other than the water-based polyester resin (A), the water-based polyurethane resin (B) and the melamine resin (C), wherein the water-based polyester resin (A) has a number average molecular weight of 1000 or more and 12000 or less, a glass transition temperature of -30°C or more and 20°C or less, an acid value of 10 mgKOH/g or more and 45 mgKOH/g or less, a hydroxyl value of 10 mgKOH/g or more and 60 mgKOH/g or less, and an amount of the water-based polyester resin (A) of 17 parts by mass or more and 45 parts by mass or less relative to 100 parts by mass of the resin solid content of the water-based coating composition, and an amount of the water-based polyurethane resin (B) of more than 15 parts by mass and less than 40 parts by mass relative to 100 parts by mass of the resin solid content of the water-based coating composition.

Description

Aqueous coating composition and method for producing coated article

Technical Field

The present invention relates to an aqueous coating composition and a method for producing a coated article.

Background

As a method for forming a multilayer coating film on an automobile body, so-called aqueous 3WET coating (3C 1B) is generally carried out by sequentially coating an aqueous intercoat coating composition, pre-drying, coating an aqueous base coating composition, pre-drying, coating a clear coating composition, and sintering.

In recent years, from the viewpoint of reducing CO ₂ emissions, in 3C1B coating, it has been demanded to omit preliminary drying performed after application of an aqueous intercoat coating composition. The preliminary drying is mainly performed to remove the solvent contained in the coating composition and to increase the solid content concentration. Therefore, if the preliminary drying is omitted, the aqueous intercoat coating composition and the aqueous base coating composition are liable to be mixed together, and a mixed layer can be generated between the uncured intercoat coating film and the uncured base coating film. The mixed layer may reduce the appearance of the resulting multilayer coating film.

As for omitting the preliminary drying, patent document 1 discloses a method for forming a multilayer coating film using a coating material containing a specific aqueous polyester resin, a specific aqueous acrylic resin, an aqueous urethane resin and a melamine resin.

Prior art literature

Patent literature

Patent document 1 International publication No. 2013/129136

Disclosure of Invention

Problems to be solved by the invention

In the coating composition of patent document 1, the effect of suppressing the formation of the above mixed layer is insufficient, and the appearance of the obtained coating film is liable to be degraded.

The purpose of the present invention is to provide an aqueous coating composition which can give a multilayer coating film having excellent appearance even when preliminary drying performed after coating the aqueous coating composition is omitted.

Means for solving the problems

In order to solve the above problems, the present invention provides the following means.

[1] A kind of water-based paint composition, the aqueous coating composition contains:

an aqueous polyester resin (A);

an aqueous polyurethane resin (B);

melamine resin (C), and

A hydroxyl-containing resin (D) other than the aqueous polyester resin (A), the aqueous polyurethane resin (B) and the melamine resin (C),

Wherein the aqueous polyester resin (A) has a number average molecular weight of 1000 to 12000, a glass transition temperature of-30 to 20 ℃, an acid value of 10 to 45mgKOH/g, a hydroxyl value of 10 to 60mgKOH/g,

The content of the aqueous polyester resin (A) is 17 to 45 parts by mass based on 100 parts by mass of the resin solid content of the aqueous coating composition,

The content of the aqueous polyurethane resin (B) is more than 15 parts by mass and 40 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous coating composition.

[2] The aqueous coating composition according to the above [1], wherein the content of the hydroxyl group-containing resin (D) is 10 parts by mass or more and 30 parts by mass or less based on 100 parts by mass of the resin solid content of the aqueous coating composition.

[3] The aqueous coating composition according to the above [1] or [2], wherein the melamine resin (C) is contained in an amount of 10 parts by mass or more and 40 parts by mass or less based on 100 parts by mass of the resin solid content of the aqueous coating composition.

[4] The aqueous coating composition according to any one of the above [1] to [3], wherein the glass transition temperature of the aqueous polyurethane resin (B) is from-90 ℃ to-10 ℃.

[5] The aqueous coating composition according to any one of the above [1] to [4], wherein the hydroxyl-containing resin (D) comprises at least one of a hydroxyl-containing acrylic resin (D1) and a polyether polyol (D2).

[6] A method for producing a coated article, comprising:

A step of forming an uncured intercoat coating film by applying the aqueous coating composition according to any one of the above items [1] to [5] to a substrate, and

A step of applying an aqueous base coating composition to the uncured intermediate coating film to form an uncured base coating film,

Wherein a preliminary drying step is not provided between the step of forming the uncured intermediate coating film and the step of forming the uncured base coating film.

[7] The method for producing a coated article according to the above [6], wherein in the step of forming the uncured intercoat film, the aqueous coating composition is coated so that the thickness of the intercoat film after curing is 6 μm or more and 35 μm or less.

[8] The method for producing a coated article according to the above [6] or [7], wherein in the step of forming the uncured base coating film, the aqueous base coating composition is coated so that the thickness of the cured base coating film is 6 μm or more and 35 μm or less.

[9] The method for producing a coated article according to any one of the above [6] to [8], further comprising:

A step of applying a clear coating composition onto the uncured base coating film after the step of forming the uncured base coating film, to form an uncured clear coating film, and

And heating the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film to form a multilayer coating film.

[10] The method for producing a coated article according to the above [9], wherein in the step of forming the uncured transparent coating film, the transparent coating composition is coated so that the thickness of the cured transparent coating film is 20 μm or more and 50 μm or less.

Effects of the invention

According to the aqueous coating composition of the present invention, a multilayer coating film having excellent appearance can be obtained even in the case where preliminary drying performed after coating the aqueous coating composition is omitted.

Detailed Description

[ Water-based coating composition ]

The aqueous coating composition comprises an aqueous polyester resin (A), an aqueous polyurethane resin (B), a melamine resin (C) and a hydroxyl-containing resin (D) other than these. The aqueous polyester resin (A) has a number average molecular weight of 1000 to 12000, a glass transition temperature of-30 to 20 ℃, an acid value of 10 to 45mgKOH/g, and a hydroxyl value of 10 to 60 mgKOH/g. The content of the aqueous polyester resin (A) is 17 to 45 parts by mass based on 100 parts by mass of the resin solid content of the aqueous coating composition. The content of the aqueous urethane resin (B) is more than 15 parts by mass and 40 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous coating composition.

In general, when an uncured coating film formed from an aqueous coating composition is coated with the same aqueous coating composition, solvents are mixed with each other. In this case, particularly, the resin component, pigment, and the like contained in the aqueous coating composition for post-coating move downward together with the solvent, and the boundary between the two coating films becomes blurred. This is a hybrid layer. The mixed layer may reduce the smoothness of the multilayer coating film. As a result, the appearance of the multilayer coating film is reduced.

The aqueous coating composition according to the present disclosure comprises a low hydroxyl value aqueous polyester resin (a). The aqueous polyester resin (a) is finely dispersed or dissolved in the aqueous coating composition to improve the hydrophobicity of the whole uncured coating film. Therefore, even if other aqueous coating compositions are applied to the uncured coating film, the solvents are prevented from being mixed with each other. As a result, formation of a mixed layer is suppressed. Furthermore, polyester resins generally improve the smoothness of the coating film. Due to these effects of the aqueous polyester resin (a), the appearance of a multilayer coating film including a coating film formed from the aqueous coating composition of the present disclosure is improved. The appearance referred to herein means smoothness. The smoother the surface of the coating film, the more excellent the appearance.

(Aqueous polyester resin (A))

The aqueous polyester resin (A) has a number average molecular weight of 1000 to 12000, a glass transition temperature (Tg) of-30 to 20 ℃, an acid value of 10 to 45mgKOH/g, and a hydroxyl value of 10 to 60 mgKOH/g. In this way, by providing the aqueous polyester resin (a) with a low hydroxyl value, the hydrophobicity of the aqueous coating composition is improved, and the formation of the above-mentioned mixed layer is suppressed. Further, by providing the aqueous polyester resin (a) with the above-mentioned glass transition temperature, the viscosity of the aqueous polyester resin (a) is reduced upon heat curing, and the smoothness of the obtained coating film is improved. In addition, since the flexibility of the obtained coating film is improved, chipping resistance is improved.

The number average molecular weight of the aqueous polyester resin (a) may be 1200 or more, 1500 or more, or 3000 or more. The number average molecular weight of the aqueous polyester resin (a) may be 10000 or less, 9500 or less, or 9000 or less.

The number average molecular weight and the weight average molecular weight can be determined in a GPC method based on polystyrene.

The Tg of the aqueous polyester resin (A) may be at least-28℃or at least-25℃or at least-23 ℃. The Tg of the aqueous polyester resin (a) may be 18 ℃ or less, 15 ℃ or less, or 10 ℃ or less.

The hydroxyl value of the aqueous polyester resin (A) may be 50mgKOH/g or less, 40mgKOH/g or less, or 38mgKOH/g or less. The hydroxyl value of the aqueous polyester resin (A) may be 11mgKOH/g or more, or may be 11.5mgKOH/g or more.

The acid value of the aqueous polyester resin (A) may be 42mgKOH/g or less, 30mgKOH/g or less, 18mgKOH/g or less, or 15mgKOH/g or less. The acid value of the aqueous polyester resin (A) may be 11mgKOH/g or more, or may be 11.5mgKOH/g or more.

The hydroxyl value and the acid value were obtained based on the mass of the solid content. The hydroxyl value and the acid value can be measured by a known method described in JIS K0070:1992. The hydroxyl value and the acid value can be calculated from the blending amount of the unsaturated monomer in the raw material monomer of the resin (for example, the aqueous polyester resin (a)).

The aqueous coating composition may contain 1 or more (particularly 2 or more) aqueous polyester resins (a). The aqueous coating composition may contain, as the aqueous polyester resin (A), a high molecular weight aqueous polyester resin (A-1) having a number average molecular weight of 3000 or more and 12000 or less, a glass transition temperature of-30 or more and 20 or less, an acid value of 10mgKOH/g or more and 25mgKOH/g or less, a hydroxyl value of 10mgKOH/g or more and 50mgKOH/g or less, and a low molecular weight aqueous polyester resin (A-2) having a number average molecular weight of 1000 or more and less than 3000, a glass transition temperature of-30 or more and 20 or less, an acid value of more than 25mgKOH/g and 45mgKOH/g or less, and a hydroxyl value of 50mgKOH/g or more and 60mgKOH/g or less.

The mixing ratio (A-1:A-2) of the high molecular weight aqueous polyester resin (A-1) to the low molecular weight aqueous polyester resin (A-2) may be, for example, 40:60 to 95:5. The mixing ratio (A-1:A-2) may be 50:50 to 90:10, or may be 60:40 to 90:10, or may be 70:30 to 88:12.

In the aqueous coating composition, the aqueous polyester resin (a) is soluble. That is, the aqueous polyester resin (a) may be a water-soluble polyester resin. In the aqueous coating composition, the aqueous polyester resin (a) may be in the form of a dispersion.

The water-soluble polyester resin and the aqueous polyester resin dispersion can be obtained, for example, by neutralizing the polyester resin with an alkali, gradually adding water with stirring to effect phase inversion emulsification, and forcibly emulsifying the polyester resin in water using a high-speed homogenizer.

The aqueous polyester resin (a) may have hydroxyl groups and carboxyl groups. In the 1-molecule aqueous polyester resin (a), the number of hydroxyl groups may be 2 or more, or 3 or more. In the 1-molecule aqueous polyester resin (A), the number of carboxyl groups may be 2 or more, or may be 2.

The polyester resin can be obtained, for example, by condensing a polyhydric alcohol component with a polybasic acid component. The polyol has 2 or more hydroxyl groups in 1 molecule. Examples of the polyhydric alcohol include diols such as propylene glycol, neopentyl glycol, butanediol, hexanediol, octanediol, 1, 6-hexanediol, 1, 8-octanediol, 1, 9-nonanediol, 1, 10-decanediol, 1, 12-dodecanediol, 1, 2-cyclohexanediol, 1, 3-cyclohexanediol, 1, 4-cyclohexanediol, hydrogenated bisphenol A, caprolactone diol and bis-hydroxyethyl taurine, triols such as trimethylolpropane and hexanetriol, and tetrols such as pentaerythritol. They may be used alone or in combination of 2 or more.

Examples of the polybasic acid component include polycarboxylic acids. The polycarboxylic acid has 2 or more carboxyl groups in 1 molecule. Examples of the polycarboxylic acid include aromatic dicarboxylic acids such as phthalic acid and isophthalic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid and tetrahydrophthalic acid, and tricarboxylic acids such as trimellitic acid. They may be used alone or in combination of 2 or more. If necessary, for example, a saturated fatty acid such as stearic acid, lauric acid and myristic acid, an unsaturated fatty acid such as oleic acid, linoleic acid and linolenic acid, a long-chain fatty acid of a natural oil or fat such as castor oil, palm oil and soybean oil, and a modified product thereof may be used in combination.

As the raw material of the polyester resin, at least 1 of monohydric alcohol, monoepoxy compound, lactone and hydroxycarboxylic acid may also be used in combination.

The lactone has a cyclic ester bond. Examples of the lactone include dimethylpropiolactone, ε -caprolactone, γ -caprolactone, δ -valerolactone and γ -butyrolactone. They may be used alone or in combination of 2 or more.

Hydroxycarboxylic acids have a hydroxyl group and a carboxyl group in 1 molecule. Examples of the hydroxycarboxylic acid include hydroxycarboxylic acids such as dimethylolpropionic acid. They may be used alone or in combination of 2 or more.

The carboxyl groups may be introduced into a part or all of the hydroxyl groups of the polyester resin by modification with an acid anhydride such as phthalic anhydride, succinic anhydride, hexahydrophthalic anhydride, or trimellitic anhydride.

The solid content of the aqueous polyester resin (A) is 17 to 45 parts by mass based on 100 parts by mass of the resin solid content of the aqueous coating composition. This can exert the effect of suppressing the mixed layer. The solid content of the aqueous polyester resin (a) may be 20 parts by mass or more or 25 parts by mass or more. The solid content of the aqueous polyester resin (a) may be 40 parts by mass or less or 35 parts by mass or less.

The mass of the resin solid component of the aqueous coating composition is the total mass of the solid components of the aqueous polyester resin (A), the aqueous polyurethane resin (B), the melamine resin (C), the hydroxyl-containing resin (D) and other resin components.

(Other aqueous polyester resin (a))

The aqueous coating composition may contain an aqueous polyester resin (a) other than the aqueous polyester resin (a). The other aqueous polyester resin (a) may be water-soluble. In the aqueous coating composition, the other aqueous polyester resin (a) may be in the form of a dispersion.

Examples of the other aqueous polyester resin (a) include a low molecular weight polyester resin (a 1) having a number average molecular weight of less than 1000 and a hydroxyl value of more than 60mgKOH/g and 80mgKOH/g or less. By using such an aqueous polyester resin having a low molecular weight and a relatively high hydroxyl value in combination, the dispersibility of the melamine resin (C) can be improved.

The solid content of the low molecular weight polyester resin (a 1) is, for example, 1 to 10 parts by mass based on 100 parts by mass of the resin solid content of the aqueous coating composition. The solid content of the low molecular weight polyester resin (a 1) may be 2 parts by mass or more or 4 parts by mass or more. The solid content of the low molecular weight polyester resin (a 1) may be 8 parts by mass or less or 7 parts by mass or less.

Examples of the other aqueous polyester resin (a) include a high hydroxyl value polyester resin (a 2) having a hydroxyl value of more than 60mgKOH/g and 120mgKOH/g or less. By the high hydroxyl value polyester resin (a 2), the crosslinking density becomes high, and improvement of the direct adhesion and chipping resistance can be expected.

The solid content of the high hydroxyl value polyester resin (a 2) is, for example, 1 to 15 parts by mass based on 100 parts by mass of the resin solid content of the aqueous coating composition. The solid content of the high hydroxyl value polyester resin (a 2) may be 2 parts by mass or more or 4 parts by mass or more. The solid content of the high hydroxyl value polyester resin (a 2) may be 12 parts by mass or less.

(Water-based polyurethane resin (B))

By the aqueous polyurethane resin (B), the elasticity of the coating film is improved and chipping resistance is improved.

In the aqueous coating composition, the aqueous polyurethane resin (B) is soluble. That is, the aqueous urethane resin (B) may be a water-soluble urethane resin. In the aqueous coating composition, the aqueous polyurethane resin (B) may be in the form of a dispersion.

The water-soluble polyurethane resin and the aqueous polyurethane resin dispersion can be obtained, for example, by a method of forcibly emulsifying the polyurethane resin with a surfactant, and a method of neutralizing the polyurethane resin with an alkali or an acid.

The polyurethane resin can be obtained, for example, by reacting a polyol compound, a compound having an active hydrogen group and a hydrophilic group in the molecule, an organic polyisocyanate, and if necessary, a chain extender with a polymerization terminator.

The polyol compound contains 2 or more hydroxyl groups in the molecule. Examples of the polyol compound include polyols 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 diols such as ethylene glycol, triethylene glycol, propylene glycol, butanediol, tripropylene glycol, and neopentyl glycol, polycaprolactone polyols, polybutadiene polyols, polycarbonate polyols, and polythioether polyols. They may be used alone or in combination of 2 or more.

Examples of the compound having an active hydrogen group and a hydrophilic group in the molecule include compounds containing an active hydrogen and an anionic group, a cationic group or a nonionic hydrophilic group. Anionic groups include anionic groups and anion-forming groups. The anion-forming group is a group that can react with a base to form an anion group, specifically, an anion group is formed by neutralization with a base before, during, or after the urethanization reaction.

Examples of the compounds containing active hydrogen and an anionic group include those described in Japanese patent publication No. 42-24192 and Japanese patent publication No. 55-41607, and specifically include α, α -dimethylolpropionic acid and α, α -dimethylolbutyric acid. As a compound having active hydrogen and a cationic group, for example, japanese patent publication No. 43-9076 is described. Examples of the compound having an active hydrogen and a nonionic hydrophilic group include polyethylene glycol and an alkylene oxide adduct of an alkyl alcohol, as described in Japanese patent publication No. 48-41718.

The organic polyisocyanate contains 2 or more isocyanate groups in the molecule. Examples of the organic polyisocyanate include aliphatic diisocyanates such as 1, 4-tetramethylene diisocyanate, 1, 6-hexamethylene diisocyanate, 2, 4-trimethylhexamethylene diisocyanate, 3-isocyanatomethyl-3, 5-trimethylcyclohexyl isocyanate, dicyclohexylmethane-4, 4 '-diisocyanate, methylcyclohexyl-2, 4-diisocyanate, methylcyclohexyl-2, 6-diisocyanate, xylene diisocyanate, 1, 3-bis (isocyanato) methylcyclohexane, tetramethylxylene diisocyanate, trans-cyclohexane-1, 4-diisocyanate, and lysine diisocyanate; aromatic diisocyanates such as 2, 4-toluene diisocyanate, 2, 6-toluene diisocyanate, diphenylmethane-4, 4' -diisocyanate, 1,5 '-cycloalkane diisocyanate (naphthalene diisocyanate), triazine diisocyanate, diphenylmethane diisocyanate, tetraalkyl diphenylmethane diisocyanate, 4' -benzhydryl diisocyanate, 1, 3-phenylene diisocyanate, etc., triisocyanates such as lysine ester triisocyanate, triphenylmethane triisocyanate, 1,6, 11-undecene triisocyanate, 1, 8-diisocyanato-4, 4-isocyanatomethyl octane, 1,3, 6-hexamethylene triisocyanate, bicycloheptane triisocyanate, etc., dimers, trimers (isocyanurate linkages), biurets thereof, and their reactants with polyols. These may be used singly or in combination of 1 or more than 2.

The chain extender contains more than 2 active hydrogen groups in the molecule. Examples of the chain extender include low molecular weight polyols such as ethylene glycol, propylene glycol, 1, 4-butanediol, 3-methylpentanediol, 2-ethyl-1, 3-hexanediol and trimethylolpropane, polyamines such as ethylenediamine, hexamethylenediamine, diethylenetriamine, hydrazine, xylylenediamine and isophoronediamine, and water. They may be used alone or in combination of 2 or more.

Examples of the polymerization terminator include a compound having 1 active hydrogen in the molecule (for example, a monoalcohol, a monoamine, etc.) and a monoisocyanate compound.

The polyurethane resin may be synthesized by a one-step method in which the respective components are reacted at one time, or by a multi-step method in which the respective components are reacted stepwise. In the multi-step process, a portion of the active hydrogen-containing compound (e.g., a polymeric polyol) is reacted with a polyisocyanate to form an NCO-terminated prepolymer, which is then reacted with the remainder of the active hydrogen-containing compound.

The solid content of the aqueous polyurethane resin (B) is more than 15 parts by mass and 40 parts by mass or less relative to 100 parts by mass of the total resin solid content of the aqueous coating composition. Thus, chipping resistance is improved without impairing the effect of suppressing the mixed layer. The solid content of the aqueous urethane resin (B) may be 22 parts by mass or more or 25 parts by mass or more. The solid content of the aqueous urethane resin (B) may be 45 parts by mass or less or 40 parts by mass or less.

The Tg of the aqueous polyurethane resin (B) is, for example, from-90 ℃ to-10 ℃. This can further improve the flexibility of the coating film and further improve chipping resistance. The Tg of the aqueous polyurethane resin (B) may be at least-40℃or at least-35 ℃. The Tg of the aqueous polyurethane resin (B) may be-15℃or less, or-18℃or less. Tg of the aqueous polyurethane resin (B) can be measured by a differential scanning calorimeter.

(Melamine resin (C))

The melamine resin (C) is used as a curing agent and mainly reacts with the hydroxyl-containing resin (D) to form a crosslinked structure. Thus, a coating film having high hardness can be obtained.

The melamine resin (C) may be water-soluble or water-insoluble. In the melamine resin (C), the structure (-N¹(R¹)(R²)、-N²(R³)(R⁴)、-N³(R⁵)(R⁶)). including 6 substituents R ¹～R⁶ bonded via 3 nitrogen atoms N ¹～N³ around the triazine ring (triazine core) further improves the hardness of the obtained coating film by the triazine ring having a planar structure and rigidity. Thus, both excellent chipping resistance and excellent hardness can be achieved based on the aqueous polyester resin (a) and the aqueous polyurethane resin (B).

The substituents R ¹～R⁶ are each independently a hydrogen atom, an alkyl ether group (-CH ₂-OR⁷) and a hydroxymethyl group (-CH ₂ OH). The substituents R ¹～R⁶、R⁷ may be independently an alkyl group having 1 to 8 carbon atoms or an alkyl group having 1 to 4 carbon atoms. The alkyl group may be linear or branched. The alkyl group may be methyl, n-butyl or isobutyl.

The melamine resin (C) is generally composed of a polynuclear body in which a plurality of triazine rings are bonded to each other. The melamine resin (C) may be a mononuclear body composed of 1 triazine ring.

Examples of the melamine resin (C) include a perfluoroalkyl type having only an alkyl ether group as a substituent R ¹～R⁶; hydroxymethyl form with-N (-CH ₂-OR⁷)(-CH₂ OH), imino form with-N (-CH ₂-OR⁷) (H), hydroxymethyl/imino form with-N (-CH ₂-OR⁷)(-CH₂ OH) and-N (-CH ₂-OR⁷) (H).

The melamine resin (C) may be hydrophobic or hydrophilic. The melamine resin (C) may be particularly hydrophobic. Thus, the melamine resin (C) reacts with hydroxyl groups of other resins, and can self-agglomerate to form a cluster structure. The hardness of the coating film can be further improved by the cluster structure. Further, the hydrophobicity of the whole uncured coating film is further improved, and the effect of suppressing the mixed layer can be further improved. The water resistance of the coating film is also improved. The coating film having excellent hardness and water resistance is suitable for direct adhesion of glass, for example. The glass is, for example, a window of a vehicle. Direct bonding of glass is a method in which glass and a vehicle are bonded via an adhesive. When the aqueous coating composition according to the present disclosure is used for coating a vehicle, direct adhesion of glass becomes easy.

The hydrophobic melamine resin (C) has a total average number of hydrogen atoms and methylol groups of 1 or less in the substituent R ¹～R⁶, for example. The hydrophobic melamine resin (C) may be of the holoalkyl type. Specific examples of the fully alkyl melamine resin (C) include methylated melamine resin, butylated melamine resin and isobutylated melamine resin.

The weight average molecular weight of the melamine resin (C) may be 1000 or more and 7000 or less. Thus, the hardness of the obtained coating film can be further improved. The weight average molecular weight of the melamine resin (C) may be 1500 or more, or 2000 or more. The melamine resin (C) may have a weight average molecular weight of 5000 or less, or 4000 or less.

Examples of the commercially available melamine resin (C) include the CYMEL series (trade name) manufactured by Allnex, inc., specifically the U-VAN series (trade name) manufactured by CYMEL 202、CYMEL 204、CYMEL 211、CYMEL 232、CYMEL 235、CYMEL 236、CYMEL 238、CYMEL 250、CYMEL 251、CYMEL 254、CYMEL 266、CYMEL 267、CYMEL 270、CYMEL 272、CYMEL 285、CYMEL 301、CYMEL 303、CYMEL 325、CYMEL 327、CYMEL 350、CYMEL 370、CYMEL 701、CYMEL 703、CYMEL 1141; Sanjing chemical Co., ltd.). They may be used alone or in combination of 2 or more.

The solid content of the melamine resin (C) is, for example, 10 to 40 parts by mass based on 100 parts by mass of the resin solid content of the aqueous coating composition. Thus, the curing reaction is easy to proceed, and a coating film having high hardness is easy to obtain. The solid content of the melamine resin (C) may be 15 parts by mass or more or 17 parts by mass or more. The solid content of the melamine resin (C) may be 35 parts by mass or less or 30 parts by mass or less.

(Other curing agent)

The aqueous coating composition may contain a curing agent other than the melamine resin (C). Examples of the other curing agent include blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds,An oxazoline compound and a metal ion. They may be used alone or in combination of 2 or more.

(Hydroxyl-containing resin (D))

As described above, the hydroxyl group-containing resin (D) reacts with the melamine resin (C) to form a crosslinked structure. The hydroxyl group-containing resin (D) is a hydroxyl group-containing resin other than the water-repellent polyester resin (a), the water-repellent polyurethane resin (B) and the melamine resin (C).

The hydroxyl group-containing resin (D) has 1 or more hydroxyl groups. Examples of the hydroxyl group-containing resin include a hydroxyl group-containing acrylic resin (D1) and a polyether polyol (D2). They may be used alone of 1 kind, or in combination of both. The aqueous coating composition may contain 1 or more (particularly 2 or more) hydroxyl-containing acrylic resins (D1), or 1 or more hydroxyl-containing acrylic resins (D1) and 1 or more polyether polyols (D2).

The solid content of the hydroxyl group-containing resin (D) is, for example, 10 to 30 parts by mass based on 100 parts by mass of the resin solid content of the aqueous coating composition. Thus, the curing reaction is easy to proceed, and a coating film having high hardness is easy to obtain. The solid content of the hydroxyl group-containing resin (D) may be 15 parts by mass or more or 16 parts by mass or more. The solid content of the hydroxyl group-containing resin (D) may be 28 parts by mass or less or 25 parts by mass or less.

< Hydroxyl-containing acrylic resin (D1) >)

In the aqueous coating composition, the hydroxyl-containing acrylic resin (D1) may be in the form of an emulsion. In the aqueous coating composition, the hydroxyl-containing acrylic resin (D1) is soluble. That is, the hydroxyl group-containing acrylic resin (D1) may be a water-soluble acrylic resin. The aqueous coating composition may comprise both a water-soluble acrylic resin and an aqueous acrylic resin in the form of an emulsion.

The acrylic resin emulsion may be prepared by emulsion polymerization of alpha, beta-ethylenically unsaturated monomers. Examples of the α, β -ethylenically unsaturated monomer include (meth) acrylic acid esters, α, β -ethylenically unsaturated monomers having an acid group, and α, β -ethylenically unsaturated monomers having a hydroxyl group. The monomer may be used alone or in combination of 2 or more.

Examples of the (meth) acrylic acid ester include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, phenyl (meth) acrylate, isobornyl (meth) acrylate, cyclohexyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate, dicyclopentadiene (meth) acrylate, and dicyclopentadiene (meth) acrylate. (meth) acrylate means acrylate and methacrylate.

Examples of the α, β -ethylenically unsaturated monomer having an acid group include acrylic acid, methacrylic acid, crotonic acid, 2-acryloxyethyl phthalic acid, 2-acryloxyethyl succinic acid, ω -carboxy-polycaprolactone mono (meth) acrylate, isocrotonic acid, α -hydro- ω - ((1-oxo-2-propenyl) oxy) poly (oxy (1-oxo-1, 6-adipoyl)), maleic acid, fumaric acid, itaconic acid, 3-vinylsalicylic acid, 3-vinylacetylsalicylic acid, 2-acrylamide-2-methylpropanesulfonic acid, p-hydroxystyrene, and 2, 4-dihydroxy-4' -vinylbenzophenone.

Examples of the α, β -ethylenically unsaturated monomer having a hydroxyl group include hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, allyl alcohol, methallyl alcohol, and adducts thereof with epsilon-caprolactone.

Other alpha, beta-ethylenically unsaturated monomers may be used in combination. Examples of the other α, β -ethylenically unsaturated monomer include polymerizable amide compounds, polymerizable aromatic compounds, polymerizable nitriles, polymerizable alkylene oxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, α -olefins, dienes, polymerizable carbonyl compounds, polymerizable alkoxysilyl compounds, and other polymerizable compounds.

The method of emulsion polymerization is not particularly limited. For example, an emulsifier is dissolved in an aqueous medium containing water or an organic solvent such as an alcohol or an ether (for example, dipropylene glycol methyl ether, propylene glycol methyl ether or the like) as required, and an α, β -ethylenically unsaturated monomer and a polymerization initiator are added dropwise under heating and stirring. The α, β -ethylenically unsaturated monomers may be pre-emulsified by an emulsifier.

As the polymerization initiator and the emulsifier, those commonly used by those skilled in the art can be used. Chain transfer agents such as mercaptans (e.g., laurylthiol) and alpha-methylstyrene dimers may be used to adjust the molecular weight as desired. The reaction temperature, reaction time, etc. may be appropriately selected within the range commonly used by those skilled in the art. The resulting acrylic resin emulsion is neutralized with a base as needed.

The hydroxyl group-containing acrylic resin (D1) (emulsion of acrylic resin) obtained by emulsion polymerization may have a number average molecular weight of 3,000 or more. The hydroxyl value (solid component hydroxyl value) of the hydroxyl-containing acrylic resin (D1) may be 20mgKOH/g or more and 180mgKOH/g or less. The acid value (solid content acid value) of the hydroxyl group-containing acrylic resin (D1) may be 1mgKOH/g or more and 80mgKOH/g or less.

The water-soluble hydroxyl-containing acrylic resin (D1) can be prepared, for example, by solution-polymerizing the above-mentioned α, β -ethylenically unsaturated monomer and dissolving it in water using an alkaline compound.

< Polyether polyol (D2) >)

In the aqueous coating composition, the polyether polyol (D2) may be in the form of an emulsion. In the coating composition, the polyether polyol (D2) is soluble. That is, the polyether polyol (D2) may be a water-soluble polyether polyol.

The polyether polyol (D2) is obtained by, for example, polyaddition of an alkylene oxide such as ethylene oxide, propylene oxide, tetrahydrofuran or the like with a polyol. The polyether polyol (D2) may be a polyether glycol having 2 hydroxyl groups in 1 molecule.

Examples of the polyether glycol include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polyethylene-polypropylene glycol (polyethylene propylene glycol), polytetramethylene ether glycol, polyhexamethylene ether glycol, and octamethylene ether glycol.

(Solvent)

The aqueous coating composition comprises water as solvent. The aqueous coating composition may also comprise an aqueous solvent and an organic solvent. In the aqueous coating composition, the proportion of water in the solvent is, for example, 50 mass% or more, 70 mass% or more, or 90 mass% or more.

Examples of the organic solvent include ester solvents such as ethyl acetate, butyl acetate, isopropyl acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate;

Ether solvents such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, methyl methoxybutanol, ethoxypropanol, ethylene glycol isopropyl ether, ethylene glycol-t-butyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, methoxybutanol, and propylene glycol monobutyl ether, alcohol solvents such as methanol, ethanol, butanol, and propanol, and ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. They may be used alone or in combination of 2 or more.

(Others)

The aqueous coating composition may also contain other ingredients. Examples of the other components include pigments, film-forming aids, surface regulators, preservatives, mold inhibitors, defoamers, light stabilizers, ultraviolet absorbers, antioxidants, and pH regulators.

(First scheme)

In a first embodiment, the aqueous coating composition comprises an aqueous polyester resin (A) in the form of a dispersion, an aqueous polyurethane resin (B) in the form of a dispersion, a melamine resin (C), and hydroxyl-containing acrylic resins (D1) in the form of water-soluble and emulsion.

In the first embodiment, the solid content of each resin per 100 parts by mass of the solid content of the resin of the aqueous coating composition is as follows.

An aqueous polyester resin (A) in the form of a dispersion, wherein the aqueous polyester resin (A) is 20 to 40 parts by mass inclusive;

An aqueous polyurethane resin (B) in the form of a dispersion, wherein the aqueous polyurethane resin (B) is 25 to 35 parts by mass inclusive;

15 to 25 parts by mass of melamine resin (C);

the water-soluble and emulsion-form acrylic resin (D1) is 5 to 15 parts by mass inclusive.

(Second scheme)

In a second embodiment, the aqueous coating composition comprises an aqueous polyester resin (A) in the form of a dispersion, an aqueous polyurethane resin (B) in the form of a dispersion, a melamine resin (C), and hydroxyl-containing acrylic resins (D1) and water-soluble polyether polyols (D2) in the form of water-soluble and emulsion.

In the second embodiment, the solid content of each resin per 100 parts by mass of the solid content of the resin of the aqueous coating composition is as follows.

17 To 45 parts by mass inclusive of an aqueous polyester resin (A) in the form of a dispersion;

15 to 25 parts by mass of an aqueous polyurethane resin (B) in the form of a dispersion;

15 to 25 parts by mass of melamine resin (C);

1 to 10 parts by mass of water-soluble and emulsion-form acrylic resin (D1);

the water-soluble polyether polyol (D2) is 5 to 15 parts by mass.

(Third scheme)

In a third embodiment, the aqueous coating composition comprises 2 aqueous polyester resins (high molecular weight aqueous polyester resin (A-1) and low molecular weight aqueous polyester resin (A-2)) in the form of a dispersion, aqueous polyurethane resin (B) in the form of a dispersion, melamine resin (C), and hydroxyl-containing acrylic resin (D1) in the form of an emulsion.

In the third embodiment, the solid content of each resin per 100 parts by mass of the solid content of the resin of the aqueous coating composition is as follows.

15 To 38 parts by mass inclusive of a high molecular weight aqueous polyester resin (A-1) in the form of a dispersion;

2 to 7 parts by mass inclusive of a low molecular weight aqueous polyester resin (A-2) in the form of a dispersion;

15 to 25 parts by mass of melamine resin (C);

The emulsion-form acrylic resin (D1) is 15-30 parts by mass.

(Preparation of aqueous coating composition)

The aqueous coating composition can be prepared, for example, by stirring the components with a stirrer or the like. The pigment may be mixed with other ingredients after being dispersed in a vehicle containing water, a surfactant, a dispersant, or the like using a sand mill or the like.

[ Method for producing coated article ]

The aqueous coating composition according to the present disclosure can suppress formation of a mixed layer with the aqueous coating material for the over-coating even when preliminary drying is omitted. Therefore, the aqueous coating composition according to the present disclosure is suitable for use in the production of coated articles having a multilayer coating film formed from a plurality of aqueous coatings. In particular, the aqueous coating composition according to the present disclosure is suitable for the formation of an intercoat coating film.

The method for producing a coated article according to the present disclosure includes a step of applying the aqueous coating composition according to the present disclosure to an object to be coated to form an uncured intercoat coating film, and a step of applying the aqueous base coating composition to the uncured intercoat coating film to form an uncured base coating film. The preliminary drying step is not provided between the step of forming an uncured intermediate coating film and the step of forming an uncured base coating film. By omitting the preliminary drying process, the environmental load and the production cost are reduced.

In the preliminary drying step, at least a part of the solvent is intentionally removed. Omitting the preliminary drying step means that the operation of intentionally removing the solvent from the uncured intercoat coating film is not performed before the step of forming the uncured base coating film. Examples of the method for intentionally removing the solvent (i.e., the preliminary drying method) include natural drying and heat drying. In the natural drying, for example, the uncured intercoat film is left for 5 minutes to 15 minutes under a temperature condition of 20 ℃ to 25 ℃. The heat drying is performed under the condition that the curing reaction based on the coating film forming component does not proceed or at least the curing reaction is not completed. In the heat drying, for example, the uncured intercoat film is heated at a temperature of 50 ℃ to 80 ℃ for 30 seconds to 10 minutes.

(I) A step of forming an uncured intercoat film

The aqueous coating composition according to the present disclosure is applied to a substrate to form an uncured intercoat film. The intercoat coating film is sandwiched between the substrate and the base coating film. The intercoat coating film improves the adhesion between the base coating film and the object to be coated. Furthermore, the intercoat film in the present disclosure suppresses the formation of a mixed layer. Further, since the aqueous coating composition according to the present disclosure contains a polyester resin, the coating surface becomes smoother by the intercoat coating, and unevenness of the base coating film is easily suppressed.

The thickness of the intercoat film after curing may be 6 μm or more and 35 μm or less from the viewpoint of smoothness and chipping resistance of the coated article. The thickness of the cured intercoat film may be 8 μm or more or 10 μm or more. The thickness of the cured intercoat film may be 25 μm or less or 22 μm or less.

The thickness of the coating film can be measured by an electromagnetic film thickness meter (for example, SDM-miniR manufactured by SANKO Co.). The thickness of the coating film is an average value of the thickness of the coating film at any 5 points.

Examples of the coating method include air spray coating, airless spray coating, and rotary atomizing coating. These methods may also be combined with electrostatic painting. Among them, rotary atomizing electrostatic coating is preferable from the viewpoint of coating efficiency. For example, a rotary atomizing type electrostatic coater called "Micro Bell (μbell)", "Micro Bell (μbell)", or "Metallic Bell (MetaBell)", etc. can be used for the rotary atomizing type electrostatic coating.

(Coated article)

Examples of the material of the object to be coated include metal, resin, and glass. According to the aqueous coating composition of the present disclosure, a coating film excellent in chipping resistance can be obtained, and thus is particularly suitable for coating an object to be coated containing a metal.

The shape of the coating is not particularly limited either. Specifically, examples of the coating material include automobile bodies such as cars, trucks, motorcycles, buses, and automobile parts for automobile bodies such as spoilers, bumpers, mirror covers, grilles, and door handles.

Examples of the metal include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Typical examples of the metal coating include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel sheets, electrogalvanized steel sheets, hot-dip galvanized steel sheets, galvanized-aluminum alloy steel sheets, galvanized-iron alloy steel sheets, galvanized-magnesium alloy steel sheets, galvanized-aluminum-magnesium alloy steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy steel sheets, tin-plated steel sheets, and the like.

The metal substrate may be subjected to surface treatment. Examples of the surface treatment include phosphate treatment, chromate treatment, zirconium formation treatment, and composite oxide treatment. The metal substrate may be coated with an electrodeposition paint after the surface treatment. The electrodeposition coating material may be cationic or anionic.

Examples of the resin include polypropylene resin, polycarbonate resin, polyurethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, and polyamide resin. The resin coating material is preferably subjected to degreasing treatment.

The preliminary drying (also referred to as preheating) step after application of the aqueous coating composition is omitted. In this case, it is also possible to suppress the uncured intercoat film from being mixed with the aqueous base coating composition applied thereon, thereby suppressing the formation of a mixed layer. Thus, the appearance of the resulting coated article is improved.

(II) a step of forming an uncured base coating film

The aqueous base coating composition is applied to the uncured intermediate coating film to form an uncured base coating film. The base coating film may be 1 layer or 2 or more layers of laminated coating film.

In this way, the multilayer coating film is obtained by a so-called wet-on-wet coating method in which coating is performed on an uncured coating film. Because the wet-on-wet coating can omit a sintering drying furnace, the environmental load and the production cost are reduced. In the production method of the present disclosure, since the preliminary drying step after the application of the aqueous coating composition is also omitted, the effect of reducing the environmental load and the production cost is higher.

The thickness of the cured base coating film is, for example, 6 μm or more and 35 μm or less. The thickness of the cured base coating film may be 8 μm or more, or 10 μm or more. The thickness of the cured base coating film may be 25 μm or less or 20 μm or less.

Examples of the coating method include the same method as the coating method of the intercoat coating composition.

The preliminary drying step may be performed after the application of the aqueous base coating composition.

(Aqueous base coating composition)

The aqueous base coating composition contains, for example, an acrylic resin emulsion, a water-soluble acrylic resin, a curing agent (typically, melamine resin), and a polyether polyol resin. The base coating film formed from such an aqueous base coating composition is excellent in appearance. The aqueous base coating composition may also contain pigments and various additives. Examples of the additives include ultraviolet absorbers, antioxidants, antifoaming agents, surface regulators, and pinhole inhibitors.

(III) a step of forming an uncured clear coating film

The clear coating composition may be further applied onto the uncured base coating film to form an uncured clear coating film.

The thickness of the transparent coating film after curing may be 20 μm or more and 50 μm or less from the viewpoint of scratch resistance and smoothness. The thickness of the transparent coating film after curing may be 25 μm or more or 30 μm or more. The thickness of the transparent coating film after curing may be 45 μm or less or 40 μm or less.

The coating method is not particularly limited. Examples of the coating method include the same method as the coating method of the intercoat coating composition. Among them, rotary atomizing electrostatic coating is preferable from the viewpoint of coating efficiency. After the application of 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, aqueous, or powder. 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 viewpoint of transparency, acid etching resistance, and the like. The solvent-borne clear coating composition may also comprise an acrylic resin and/or a polyester resin having carboxylic acid and/or epoxy groups. The clear coating composition may contain various pigments within a range that does not impair transparency and effects of the coating composition to which the present disclosure relates. The clear coating composition may contain the additives described above as needed.

(IV) curing step

Each uncured coating film is cured by heating. In this step, the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film are cured at one time.

The heating conditions are appropriately set according to the composition of each coating composition, the material of the object to be coated, and the like. The heating temperature is, for example, 60 ℃ to 170 ℃, and may be 90 ℃ to 150 ℃.

The heating time may be appropriately set according to the heating temperature. When the heating temperature is 60 ℃ to 170 ℃, the heating time is, for example, 10 minutes to 60 minutes, and may be 15 minutes to 45 minutes. The heating time is a time period in which the object to be coated is maintained at the target temperature by reaching the target temperature in the heating device, regardless of a time period before reaching the target temperature. Examples of the heating device include a drying furnace using a heating source such as hot air, electricity, gas, or infrared rays.

Examples

The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples. In the examples, unless otherwise indicated, "parts" and "%" are based on mass. Details of the components described in tables 1 and 2 are as follows.

VYLONAL NC-3000, manufactured by Toyo-yo, dispersion, number average molecular weight 8400, tg 7 ℃, acid value 12.0mgKOH/g, hydroxyl value 12.0mgKOH/g;

The aqueous polyester resin (A-2) is produced by PRS-1045,NIPPON PAINT AUTOMOTIVE COATING, has a water-soluble, number average molecular weight of 2500, tg5.6deg.C, an acid value of 40.5mgKOH/g, and a hydroxyl value of 52.6mgKOH/g;

Gao Qiang polyester resin NP-6100, manufactured by DIC Co., ltd., water-soluble, having a weight average molecular weight of 7000, tg-3 ℃, an acid value of 15.4mgKOH/g, and a hydroxyl value of 90mgKOH/g;

The aqueous polyurethane resin (B) is UCoat N-800T, manufactured by Sanyo chemical industry Co., ltd., dispersion, tg of-20 to-30 ℃;

Melamine resin (C) CYMEL 250, manufactured by Allnex corporation, imino type, weight average molecular weight 2500;

The hydroxyl-containing acrylic resin (D1) was manufactured by EMA-1046,NIPPON PAINT AUTOMOTIVE COATINGS, inc., and was an emulsion having a hydroxyl value of 30mgKOH/g.

Examples 1 to 9, comparative examples 1 to 3

(1) Preparation of aqueous coating composition

As shown in table 1, the blending amount of the aqueous polyester resin (a) was changed from 0 to 31 parts by mass, and aqueous coating compositions were prepared, respectively.

(2) Manufacture of coated articles

POWERNICS150 (trade name, cationic electrodeposition paint manufactured by NIPPON PAINT AUTOMOTIVE COATINGS company) was electrodeposition coated on the zinc phosphate treated matte steel sheet in such a manner that the dry film thickness was 20 μm. Then, heating was performed at 160 ℃ for 30 minutes. Then, cooling was performed to form a cured electrodeposition coating film.

The aqueous coating composition was applied to the obtained electrodeposition coating film by a rotary atomizing type electrostatic coating apparatus so that the dry film thickness was 12. Mu.m. Then, using a rotary atomizing electrostatic coating device, the coating of the aqueous base coating composition, namely, the aleurous AR-3400 (trade name, NIPPON PAINT AUTOMOTIVE COATINGS company) was preheated at 80 ℃ for 3 minutes.

Next, PU-device O-4300 (a two-component clear paint manufactured by NIPPON PAINT AUTOMOTIVE COATINGS company, trade name) was applied as a clear paint composition so that the dry film thickness was 35 μm by a rotary atomizing electrostatic coating apparatus. Then, heat curing was performed at 140 ℃ for 30 minutes to obtain a coated article.

[ Evaluation ]

The coated article was evaluated by the following method. The results are shown in tables 1 and 2.

(1) Appearance of

Wd (measurement wavelength: 3.0mm to 10.0 mm) was measured using wave-scan DOI (manufactured by BYK Gardner Co.). Wd is a value obtained by digitizing the roughness of the coating film at the measured wavelength. The smaller Wd means that the roughness of the coating film is smaller, and the appearance is good. If Wd is 24 or less, the present invention is suitable for practical use.

TABLE 1

TABLE 2

The present disclosure includes the following aspects.

an aqueous polyester resin (A);

an aqueous polyurethane resin (B);

melamine resin (C), and

[6] A method for producing a coated article, comprising:

Industrial applicability

The aqueous coating composition of the present invention can give a multilayer coating film having excellent appearance even in the case where preliminary drying is omitted. Therefore, the coating composition is suitable for forming various multilayer coating films.

The present application claims priority based on 2022, 11 and 30, japanese patent application No. 2022-191997 to japanese application, the contents of which are incorporated herein by reference in their entirety.

Claims

1. A water-based coating composition, comprising:

Waterborne polyester resin (A);

Waterborne polyurethane resin (B);

Melamine resin (C); and

The aqueous polyester resin (A) has a number average molecular weight of 1000 to 12000, a glass transition temperature of -30°C to 20°C, an acid value of 10 mgKOH/g to 45 mgKOH/g, and a hydroxyl value of 10 mgKOH/g to 60 mgKOH/g.

The content of the aqueous polyester resin (A) is 17 parts by mass or more and 45 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous coating composition,

The content of the aqueous polyurethane resin (B) is more than 15 parts by mass and less than 40 parts by mass based on 100 parts by mass of the resin solid content of the aqueous coating composition.

2 . The aqueous coating composition according to claim 1 , wherein the content of the hydroxyl-containing resin (D) is 10 parts by mass or more and 30 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous coating composition.

3. The aqueous coating composition according to claim 1 or 2, wherein the content of the melamine resin (C) is 10 parts by mass or more and 40 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous coating composition.

4 . The aqueous coating composition according to claim 1 , wherein the aqueous polyurethane resin (B) has a glass transition temperature of -90° C. or higher and -10° C. or lower.

5 . The aqueous coating composition according to claim 1 , wherein the hydroxyl-containing resin (D) comprises at least one of a hydroxyl-containing acrylic resin (D1) and a polyether polyol (D2).

6. A method for producing a coated article, comprising:

A step of coating an object with the aqueous coating composition according to any one of claims 1 to 5 to form an uncured intermediate coating film; and

The step of applying a water-based base coating composition on the uncured intermediate coating film to form an uncured base coating film,

However, no preliminary drying step is provided between the step of forming the uncured intermediate coating film and the step of forming the uncured base coating film.

7 . The method for producing a coated article according to claim 6 , wherein, in the step of forming the uncured intermediate coating film, the aqueous coating composition is applied so that the thickness of the intermediate coating film after curing is 6 μm or more and 35 μm or less.

8 . The method for producing a coated article according to claim 6 , wherein, in the step of forming the uncured base coating film, the aqueous base coating composition is applied so that the base coating film has a thickness of 6 μm or more and 35 μm or less after curing.

9. The method for producing a coated article according to any one of claims 6 to 8, further comprising:

After the step of forming the uncured base coating film, a step of applying a clear coating composition on the uncured base coating film to form an uncured clear coating film; and

A step of heating the uncured intermediate coating film, the uncured base coating film and the uncured clear coating film to form a multilayer coating film.

10 . The method for producing a coated article according to claim 9 , wherein, in the step of forming the uncured clear coating film, the clear coating composition is applied so that the thickness of the clear coating film after curing is 20 μm or more and 50 μm or less.