Aqueous multi-pack coating composition and method for producing coated article

Abstract

Provided is an aqueous coating composition which can be cured at a low temperature, but which can form a coating film having high hardness, excellent gloss, and suppressed fogging dust. The aqueous multi-pack coating composition includes a first liquid containing a hydroxyl group-containing resin (A) and a melamine resin (B), and a second liquid containing a hydrophilic polyisocyanate compound (D), the hydroxyl group-containing resin (A) being at least one of a hydroxyl group-containing acrylic resin (A1) and a hydroxyl group-containing polyester resin (A2), the melamine resin (B) is dissolved in the first liquid or has an average particle diameter of less than 1 [mu] m, and the average value of the total number of imino groups and hydroxymethyl groups per triazine ring is greater than 1, and the hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a non-ion-modified polyisocyanate compound (D2).

Description

Technical Field

[0001] The present invention relates to a method for producing an aqueous multi-liquid coating composition and a coated article. Background Art

[0002] In recent years, consideration for the natural environment has been required in technical fields such as automobiles. Therefore, methods have been developed to reduce the heating temperature or shorten the heating time during coating for the purpose of energy conservation. For example, Patent Document 1 proposes an aqueous multi-liquid coating composition comprising: a main agent (I) containing a hydroxyl-containing acrylic resin and a curing agent (II) containing a polyisocyanate compound containing an anionic hydrophilic group and / or a polyisocyanate compound containing a nonionic hydrophilic group. In Patent Document 1, the coating film is cured at a low temperature of 60°C.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2021-130812 Summary of the Invention

[0006] Problems to be solved by the invention

[0007] According to Patent Document 1, the above-mentioned aqueous multi-component coating composition has excellent drying properties, coating workability, storage properties, and weather resistance. However, the coating film obtained from the aqueous multi-component coating composition has low hardness, poor gloss, and may generate mist.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an aqueous multi-component coating composition that can be cured at low temperatures and can form a coating film having high hardness, excellent gloss, and suppressed misting.

[0009] Means for solving problems

[0010] In order to solve the above-mentioned problems, the present invention provides the following solutions.

[0011] [1] An aqueous multi-liquid coating composition comprising: a first liquid containing a hydroxyl-containing resin (A) and a melamine resin (B), and a second liquid containing a hydrophilic polyisocyanate compound (D),

[0012] The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2),

[0013] The melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average value of the total number of imino groups and hydroxymethyl groups per triazine ring is greater than 1,

[0014] The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a non-ion-modified polyisocyanate compound (D2).

[0015] [2] An aqueous multi-liquid coating composition comprising: a first liquid containing a hydroxyl-containing resin (A), a melamine resin (B) and a polyurethane resin (C); and a second liquid containing a hydrophilic polyisocyanate compound (D).

[0016] The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2),

[0017] The melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average value of the total number of imino groups and hydroxymethyl groups per triazine ring is greater than 1,

[0018] The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a non-ion-modified polyisocyanate compound (D2).

[0019] [3] The aqueous multi-component coating composition of [1] or [2] above, wherein the ion-modified polyisocyanate compound (D1) contains at least an anion-modified polyisocyanate compound.

[0020] [4] The aqueous multi-component coating composition of [2] above, wherein the polyurethane resin (C) has a hydroxyl value of 30 mgKOH / g or less.

[0021] [5] The aqueous multi-liquid coating composition according to any one of [1] to [4] above, wherein the acid value of the hydroxyl-containing resin (A) is 5 mgKOH / g or more and 70 mgKOH / g or less.

[0022] [6] The aqueous multi-liquid coating composition according to any one of [1] to [5], wherein the solubility parameter of the melamine resin (B) is 9 or more and 15 or less.

[0023] [7] The aqueous multi-liquid coating composition according to any one of [1] to [6], wherein the solid content of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less relative to 100 parts by mass of the solid content of the hydroxyl-containing resin (A).

[0024] [8] The aqueous multi-liquid coating composition described in [2] above, wherein the solid content of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the total solid content of the hydroxyl-containing resin (A) and the polyurethane resin (C).

[0025] [9] The aqueous multi-liquid coating composition according to any one of [1] to [8] above, wherein the equivalent ratio of the imino group and hydroxymethyl group of the melamine resin (B) to the isocyanate group of the hydrophilic polyisocyanate compound (D) (imino group and hydroxymethyl group / isocyanate group) is 0.2 or more and 1.1 or less.

[0026]

[10] The aqueous multi-liquid coating composition according to any one of [1] to [9] above, wherein the number average molecular weight of the melamine resin (B) is 300 or more and 3000 or less.

[0027]

[11] A method for manufacturing a coated article, comprising:

[0028] A process of applying the aqueous multi-component coating composition according to any one of [1] to

[10] above to an object to be coated to form an uncured first coating film,

[0029] a step of applying a clear coating composition on the uncured first coating film to form an uncured clear coating film, and

[0030] a step of heating the uncured first coating film and the uncured clear coating film at 70° C. or higher and 110° C. or lower to cure them.

[0031]

[12] A method for manufacturing a coated article, comprising:

[0032] A process of applying the aqueous multi-component coating composition according to any one of [1] to

[10] above to an object to be coated to form an uncured first coating film,

[0033] a step of applying a second aqueous coating composition on the uncured first coating film to form an uncured second coating film,

[0034] a step of applying a clear coating composition on the uncured second coating film to form an uncured clear coating film, and

[0035] a step of heating and curing the uncured first coating film, the uncured second coating film, and the uncured clear coating film at 70° C. or higher and 110° C. or lower.

[0036]

[13] The method for manufacturing a coated article according to

[11] or

[12] , wherein the object to be coated includes a metal portion and a resin portion.

[0037] Effects of the Invention

[0038] The present invention provides an aqueous coating composition that can be cured at low temperatures and can form a coating film having high hardness, excellent gloss, and suppressed mist, and a method for producing a coated article using the aqueous coating composition. DETAILED DESCRIPTION

[0039] 1. First Implementation

[0040] [Aqueous multi-liquid coating composition]

[0041] The aqueous multi-liquid coating composition according to the first embodiment (hereinafter sometimes referred to as the aqueous coating composition) comprises a first liquid containing a hydroxyl-containing resin (A) and a melamine resin (B), and a second liquid containing a hydrophilic polyisocyanate compound (D). The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2). In the melamine resin (B), the average value of the total number of imino groups and the number of hydroxymethyl groups per triazine ring is greater than 1. The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound and a non-ion-modified polyisocyanate compound.

[0042] Melamine resins are generally used as curing agents for hydroxyl-containing resins. However, the reaction between melamine resins and hydroxyl-containing resins occurs at high temperatures (e.g., 140°C), making them unsuitable as curing agents for low-temperature curing coating compositions. Therefore, highly reactive polyisocyanate compounds are typically used as curing agents in low-temperature curing coating compositions. However, coating films produced using such coating compositions tend to have low hardness.

[0043] In the present disclosure, melamine resin is not a curing agent for directly forming a cross-linked structure with a hydroxyl-containing resin (corresponding to the second liquid of the present disclosure), but is used as a part of a main agent (corresponding to the first liquid of the present disclosure). That is, melamine resin is used together with a hydroxyl-containing resin as a substance that reacts with a polyisocyanate compound. Melamine resin and polyisocyanate compound can react even at low temperatures. Therefore, when the water-based coating composition involved in the present disclosure is heated at low temperatures, a reaction of the hydroxyl-containing resin with the polyisocyanate compound and a reaction of the melamine resin with the polyisocyanate compound occur. In the melamine resin, each triazine ring has an average of more than one imino group and / or methylol group that reacts with the isocyanate group of the polyisocyanate compound. Therefore, a reaction of a plurality of polyisocyanate compounds combined with the hydroxyl-containing resin and the melamine resin can also be produced. Specifically, the melamine resin is incorporated into the crosslinked structure of the hydroxyl-containing resin and the polyisocyanate compound, forming, for example, a crosslinked structure of hydroxyl-containing resin-polyisocyanate compound-melamine resin-polyisocyanate compound-hydroxyl-containing resin. This incorporation of the planar and rigid triazine rings of the melamine resin into the crosslinked structure increases the hardness of the resulting coating film.

[0044] It has also been found that by including melamine resin as part of the main agent, mist can be suppressed. Mist refers to the convex portion of the coating film formed on the surface of the coating film formed by the coating composition not applied to the coating object flying into the air and forming droplets during spraying. Mist is typically formed during the evaporation of the solvent contained in the droplets and is caused by the non-fusion of the coating film and the droplets. Therefore, the higher the viscosity of the coating composition (droplets), the easier it is to form mist. Melamine resins are generally low molecular weight. Therefore, the viscosity of the coating composition containing melamine resin is suppressed to a low level, and as a result, the formation of mist can be suppressed.

[0045] In the present disclosure, as described above, a hydrophilic polyisocyanate compound (D) is used as a curing agent. The hydrophilic polyisocyanate compound (D) has a hydrophilic group and two or more isocyanate groups in its molecule.

[0046] Since the coating film is formed by the reaction of the curing agent and the main agent, the dispersion state of the curing agent in the coating composition is reflected in the smoothness of the coating film. The hydrophilic polyisocyanate compound (D) used in the present disclosure exists in the aqueous coating composition in the form of micelles surrounded by hydrophilic groups, resulting in high dispersion. Therefore, the resulting coating film has excellent smoothness. The smooth coating film has an excellent gloss.

[0047] In this specification, "isocyanate group" refers to an unblocked (terminated) free isocyanate group. Since the isocyanate group is unblocked, low-temperature curing is possible. The hydrophilic polyisocyanate compound (D) is specifically a water-dispersible polyisocyanate compound.

[0048] The water-based coating composition is a multi-liquid type comprising a first liquid and a second liquid. The water-based coating composition may also comprise a third liquid containing other components. The water-based coating composition is modulated using a method commonly used by those skilled in the art. The water-based coating composition can be modulated by mixing the first liquid and the second liquid. In another embodiment, the water-based coating composition can be modulated by mixing the first liquid and the second liquid and then the third liquid. As a mixing method, a kneading mixing method using a kneader or a roller, a dispersion mixing method using a sand mill or a disperser, etc. can be enumerated.

[0049] The aqueous coating composition contains water as a solvent. In the aqueous coating composition, the proportion of water in the solvent may be 50% by mass or more, 70% by mass or more, or 100% by mass.

[0050] The water-based coating composition of the present disclosure is low-temperature curable. For example, the water-based coating composition of the present disclosure can be cured at a temperature of 70°C or higher and 110°C or lower. The curing temperature can be 75°C or higher, or 80°C or higher. The curing temperature can be 105°C or lower, 100°C or lower, 95°C or lower, or 90°C or lower.

[0051] Hereinafter, the solid content is also referred to as non-volatile content. In a specific example, the solid content of the aqueous coating composition is all the components after removing the solvent from the aqueous coating composition.

[0052] (First Liquid)

[0053] The first liquid contains a hydroxyl group-containing resin (A) and a melamine resin (B).

[0054] (A) Hydroxyl-containing resin

[0055] The hydroxyl-containing resin (A) is a resin (film-forming component) that forms the basis of the coating film. The film-forming component chemically reacts with other components contained in the aqueous coating composition to form a coating film, i.e., a resin film. The hydroxyl-containing resin (A) reacts with the hydrophilic polyisocyanate compound (D) to form a crosslinked structure. The hydroxyl-containing resin (A) can provide a coating film with sufficient hardness.

[0056] The hydroxyl group-containing resin (A) has one or more (typically two or more) hydroxyl groups in one molecule. The hydroxyl group-containing resin (A) is at least one of a hydroxyl group-containing acrylic resin (A1) and a hydroxyl group-containing polyester resin (A2).

[0057] The hydroxyl value (OHV) of the hydroxyl-containing resin (A) is, for example, 20 mgKOH / g or more and 180 mgKOH / g or less. If the hydroxyl value of the hydroxyl-containing resin (A) is 20 mgKOH / g or more, the fracture strength of the coating film tends to become higher. If the hydroxyl value of the hydroxyl-containing resin (A) is 180 mgKOH / g or less, the hydrophilization of the coating film is suppressed and the water resistance tends to improve. The hydroxyl value of the hydroxyl-containing resin (A) can be 30 mgKOH / g or more, or 40 mgKOH / g or more. The hydroxyl value of the hydroxyl-containing resin (A) can be 150 mgKOH / g or less, or 140 mgKOH / g or less, or 100 mgKOH / g or less, or 80 mgKOH / g or less.

[0058] The glass transition temperature (Tg) of the hydroxyl-containing resin (A) is, for example, not less than -20°C and not more than 100°C. If the Tg of the hydroxyl-containing resin (A) is not less than -20°C, it is easy to improve the breaking strength and hardness of the resulting coating film. If the Tg of the hydroxyl-containing resin (A) is not more than 100°C, it is easy to improve the quick-drying property of the aqueous coating composition. The Tg of the hydroxyl-containing resin (A) may be not less than -15°C. The Tg of the hydroxyl-containing resin (A) may be not more than 90°C, may be not more than 80°C, may be not more than 70°C, or may be not more than 50°C.

[0059] Tg can be calculated based on the type and amount of the raw material monomers of the resin. Tg can also be measured using a differential scanning calorimeter (DSC).

[0060] The acid value (AV) of the hydroxyl-containing resin (A) may be greater than or equal to 5 mgKOH / g and less than or equal to 70 mgKOH / g. If the acid value of the hydroxyl-containing resin (A) is greater than or equal to 5 mgKOH / g, the hardness of the resulting coating film may be easily improved. If the acid value of the hydroxyl-containing resin (A) is less than or equal to 70 mgKOH / g, the water resistance of the resulting coating film may be improved. The acid value of the hydroxyl-containing resin (A) may be greater than or equal to 7 mgKOH / g, or may be greater than or equal to 10 mgKOH / g. The acid value of the hydroxyl-containing resin (A) may be less than or equal to 60 mgKOH / g, or may be less than or equal to 50 mgKOH / g, or may be less than or equal to 40 mgKOH / g.

[0061] The hydroxyl value and acid value are determined based on the solid content mass. 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 can also be calculated based on the amount of unsaturated monomer in the raw material monomers of the target resin.

[0062] From the viewpoint of hardness, the hydroxyl-containing resin (A) may have a hydroxyl value of 20 mgKOH / g to 180 mgKOH / g, a Tg of -20°C to 100°C, and an acid value of 5 mgKOH / g to 70 mgKOH / g.

[0063] The weight average molecular weight (Mw) of the hydroxyl group-containing resin (A) can be, for example, 2000 to 50000. Mw is determined by a GPC method using polystyrene as a standard.

[0064] When the hydroxyl-containing resin (A) is dispersed in the first liquid, its average particle size can be, for example, 50 nm or more and 500 nm or less. When the average particle size of the hydroxyl-containing resin (A) is within the above range, the coating stability is improved, and the smoothness of the resulting coating film is also improved. The average particle size of the hydroxyl-containing resin (A) can be 80 nm or more. The average particle size of the hydroxyl-containing resin (A) can be 300 nm or less.

[0065] The average particle size of the hydroxyl-containing resin (A) is measured by a laser diffraction / scattering method based on JIS Z 8825. For example, a laser Doppler particle size analyzer can be used for this measurement. As a specific example, a Microtrac UPA 150 (manufactured by Nikkiso Co., Ltd.) can be cited. The average particle size of the hydroxyl-containing resin (A) can be measured using a sample diluted with ion-exchanged water to a resin solid content concentration of 0.01% by mass.

[0066] (A1) Hydroxyl-containing acrylic resin

[0067] The hydroxyl group-containing acrylic resin (A1) can be produced by, for example, polymerizing a hydroxyl group-containing α,β-ethylenically unsaturated monomer and another α,β-ethylenically unsaturated monomer by a known method. Commercially available hydroxyl group-containing acrylic resins can also be used.

[0068] In one embodiment, the hydroxyl-containing acrylic resin (A1) may be an emulsion. The hydroxyl-containing acrylic resin in emulsion form (hydroxyl-containing acrylic resin emulsion) may have a multilayer structure including a core and a shell covering at least a portion of the core surface, or a single layer structure.

[0069] The hydroxyl-containing acrylic resin emulsion is prepared by emulsion polymerization of the above-mentioned monomer mixture. Emulsion polymerization can be carried out by a polymerization method generally performed by those skilled in the art. Specifically, emulsion polymerization is carried out by mixing an emulsifier in an aqueous medium containing water and an organic solvent such as alcohol as needed, and adding the above-mentioned monomer mixture and polymerization initiator dropwise under heating and stirring. Emulsion polymerization can also be carried out by pre-mixing the monomer mixture, emulsifier and aqueous medium and emulsifying them, and then adding the resulting emulsified mixture dropwise. The polymerization initiator, emulsifier, etc. can be used without particular limitation, and substances known to those skilled in the art can be used.

[0070] Examples of the hydroxyl group-containing α,β-ethylenically unsaturated monomer include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts thereof with ε-caprolactone.

[0071] (Meth)acrylic acid includes both methacrylic acid and acrylic acid.

[0072] 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, and itaconic acid, or dicarboxylic acid monoesters thereof; styrenes such as styrene and α-methylstyrene; acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, allyl acrylate, and lauryl acrylate; methacrylates such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl ester, tert-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, multifunctional vinyl compounds, polymerizable amine compounds, α-olefins, dienes, polymerizable carbonyl compounds, and polymerizable alkoxysilyl compounds.

[0073] The average particle size of the hydroxyl-containing acrylic resin emulsion can be, for example, 50 nm to 500 nm. When the average particle size of the hydroxyl-containing acrylic resin emulsion is within this range, the coating stability is improved, and the smoothness of the resulting coating film is also improved. The average particle size of the hydroxyl-containing acrylic resin emulsion can be 80 nm or more. The average particle size of the hydroxyl-containing acrylic resin emulsion can be 300 nm or less.

[0074] In one embodiment, the hydroxyl group-containing acrylic resin (A1) may be water-soluble. The water-soluble hydroxyl group-containing acrylic resin (hydroxyl group-containing water-soluble acrylic resin) can be prepared, for example, by solution polymerization of a monomer mixture containing the above-mentioned monomers.

[0075] The weight average molecular weight (Mw) of the hydroxyl-containing water-soluble acrylic resin is, for example, 4000 or more and 50000 or less. If the Mw of the hydroxyl-containing water-soluble acrylic resin is 4000 or more, it is easy to improve the hardness and weather resistance of the resulting coating film. If the Mw of the hydroxyl-containing water-soluble acrylic resin is 50000 or less, it is easy to suppress the excessive rise in the viscosity of the aqueous coating composition. The Mw of the hydroxyl-containing water-soluble acrylic resin can be 5000 or more, or 8000 or more. The Mw of the hydroxyl-containing water-soluble acrylic resin can be 40000 or less, or 30000 or less.

[0076] In one embodiment, the hydroxyl-containing resin (A) may include a hydroxyl-containing acrylic resin emulsion and a hydroxyl-containing water-soluble acrylic resin. In this case, the ratio of the solid content mass We of the hydroxyl-containing acrylic resin emulsion to the solid content mass Ws of the hydroxyl-containing water-soluble acrylic resin may be, for example, We / Ws = 8 / 1 to 1 / 4, or 7 / 1 to 1 / 3.

[0077] (A2) Hydroxyl-containing polyester resin

[0078] The hydroxyl-containing polyester resin can be obtained, for example, by polycondensing a polyol with a polybasic acid or its anhydride (ester reaction). Commercially available hydroxyl-containing polyester resins can also be used.

[0079] The polyol is not particularly limited, and examples thereof include ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, neopentyl glycol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, hydrogenated bisphenol A, hydroxyalkylated bisphenol A, 1,4-cyclohexanedimethanol, 2,2-dimethyl-3-hydroxypropyl 2,2-dimethyl-3-hydroxypropionate, 2,2,4-trimethyl-1,3-pentanediol, N,N-bis-(2-hydroxyethyl)dimethylhydantoin, polytetramethylene ether glycol, polycaprolactone polyol, glycerol, sorbitol, trimethylolethane, trimethylolpropane, trimethylolbutane, hexanetriol, pentaerythritol, dipentaerythritol, and tris(hydroxyethyl)isocyanate. These can be used alone or in combination of two or more.

[0080] The polybasic acid or its anhydride is not particularly limited, and examples thereof include phthalic acid, phthalic anhydride, tetrahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalic anhydride, methyltetrahydrophthalic acid, methyltetrahydrophthalic anhydride, nadic anhydride, trimellitic acid, trimellitic anhydride, pyromellitic acid, pyromellitic anhydride, isophthalic acid, terephthalic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, adipic acid, azelaic acid, sebacic acid, succinic acid, succinic anhydride, lactic acid, dodecenylsuccinic acid, dodecenylsuccinic anhydride, cyclohexane-1,4-dicarboxylic acid, and endic anhydride. These may be used alone or in combination of two or more.

[0081] The hydroxyl-containing polyester resin can also be modified using lactones, oils or fatty acids, melamine resins, urethane resins, etc. The oils or fatty acids are not particularly limited, and examples thereof include castor oil, dehydrated castor oil, coconut oil, corn oil, cottonseed oil, linseed oil, perilla oil, poppy oil, safflower oil, soybean oil, tung oil, and the like, or fatty acids extracted from these oils.

[0082] The weight average molecular weight (Mw) of the hydroxyl-containing polyester resin (A2) is, for example, 2000 or more and 20000 or less. If the Mw of the hydroxyl-containing polyester resin (A2) is 2000 or more, it is easy to improve the hardness and weather resistance of the resulting coating film. If the Mw of the hydroxyl-containing polyester resin (A2) is 20000 or less, it is easy to suppress the excessive increase in the viscosity of the aqueous coating composition. The Mw of the hydroxyl-containing polyester resin (A2) may be 2300 or more, or 2500 or more. The Mw of the hydroxyl-containing polyester resin (A2) may be 10000 or less, 9000 or less, or 8000 or less.

[0083] <<Other hydroxyl-containing ingredients>>

[0084] The first liquid may contain, for example, a polycarbonate polyol resin, a polyether polyol resin, or a polycaprolactone polyol resin as other hydroxyl group-containing components.

[0085] The solid content ratio of the hydroxyl-containing components other than the hydroxyl-containing resin (A) in the total solid content of 100% by mass of the hydroxyl-containing components may be, for example, 20% by mass or less, 15% by mass or less, or 10% by mass or less.

[0086] (B) Melamine resin

[0087] Melamine resin (B) is a coating film-forming component. As described above, melamine resin (B) reacts with hydrophilic polyisocyanate compound (D) and is incorporated into the crosslinked structure of hydroxyl-containing resin (A) and hydrophilic polyisocyanate compound (D), thereby forming a coating film with high hardness.

[0088] The melamine resin (B) can be water-soluble or water-insoluble. The water-soluble melamine resin (B) dissolves in the first liquid. This allows the melamine resin (B) to react effectively with the hydrophilic polyisocyanate compound (D). Furthermore, the resulting coating film's smoothness is not easily compromised, resulting in an excellent gloss.

[0089] The dispersion diameter of the melamine resin (B) in the first liquid and the aqueous coating composition can be considered as the average particle size in a liquid having a melamine resin (B) concentration of 70% (the liquid component being n-butanol or isobutanol). The melamine resin (B) is dissolved or finely dispersed in the liquid.

[0090] The average particle size of the melamine resin (B) may be 0.1 μm or less, 0.05 μm or less, or 0.01 μm or less. The average particle size of the melamine resin (B) may be smaller than the effective measurement range of the measuring device.

[0091] The average particle size of the melamine resin (B) is the 50% average particle size (D50) in the volume-based particle size distribution of the liquid material, measured using a laser diffraction / scattering particle size distribution analyzer. An example of a particle size distribution analyzer is the Microtrac UPA 150 (manufactured by Nikkiso Co., Ltd.). The effective measurement range of this analyzer is 0.003 μm to 6.5 μm.

[0092] The first liquid and the aqueous coating composition may contain the melamine resin having an average particle size of 1 μm or greater within a range that does not impair the effects of the present embodiment. For example, the content of the large-particle melamine resin is 1 part by mass or less, 0.5 parts by mass or less, or 0 parts by mass relative to 100 parts by mass of the solid content of the hydroxyl-containing resin (A).

[0093] Since the melamine resin (B) is dissolved or finely dispersed, its solubility parameter (SP value) can be measured by the turbidity point method. Acetone is used as a good solvent (based on the HSP value δ measured by Hansen). g 9.77), hexane was used as the poor solvent (SP value δ pl 7.24) and deionized water (SP value δ phThe SP value of the melamine resin (B) measured by the cloud point method using these good solvents and poor solvents may be 9 or more and 15 or less. The SP value of the melamine resin (B) may be 9.5 or more, or 10.0 or more. The SP value of the melamine resin (B) may be 14.8 or less, or 14.5 or less.

[0094] The SP value of the melamine resin (B) can be determined by dissolving the melamine resin (B) in a good solvent having a known SP value and performing turbidity titration with a poor solvent having a higher SP value than the good solvent and a poor solvent having a lower SP value. For the method of determining the SP value, reference can be made to Reference 1: CM Hansen, J. Paint. Tech., 39

[505] , 104(1967) and Reference 2: Kobayashi Toshikatsu, Color Materials, 77[4], 188-192(2004).

[0095] The SP value of the melamine resin (B) is specifically determined as follows.

[0096] Measurement temperature: 20°C

[0097] Good solvent: acetone (HSP value δ g =9.77)

[0098] Poor solvent: Hexane (SP value δ pl =7.24), deionized water (SP value δ ph =23.50)

[0099] Sample: 0.5 g of melamine resin (B) was weighed into a 100 ml beaker, 10 ml of a good solvent was added using a whole pipette, and the mixture was dissolved using a magnetic stirrer.

[0100] (Turbidity measurement)

[0101] Add hexane dropwise to the sample and calculate the volume fraction of hexane at the point where turbidity occurs. pl Next, the SP value δ of the melamine resin (B) when hexane is used as the poor solvent is determined according to the following formula: ml .

[0102] [Mathematical formula 1]

[0103] δ ml =φ p1 δ p1 +(1-φ p1 )δ g

[0104] In addition, deionized water was added dropwise to the sample prepared in the same manner, and the volume fraction of deionized water at the point where turbidity occurred was calculated. phNext, the SP value δ of the melamine resin (B) when deionized water is used as a poor solvent is determined according to the following formula: mh .

[0105] [Mathematical formula 2]

[0106] δ mh =φ ph δ ph +(1-φ ph )δ g

[0107] The SP value (δ poly ) is δ ml and δ mh The median value is calculated by the following formula:

[0108] [Mathematical formula 3]

[0109] δ poly =(δ m1 +δ mh ) / 2

[0110] Since the melamine resin (B) is dissolved or finely dispersed, it has almost no light diffusivity or light refraction. The Gardner color number of the liquid melamine resin (B) (70% concentration, containing n-butanol or isobutanol as a solvent) obtained by comparison with a standard color glass can be 2 or less, or 1.

[0111] The Gardner color number is an objective method for evaluating the color tone and transparency of chemical products. Gardner color numbers are evaluated in accordance with JIS K 0071-2:1998 (Chemical Color Testing Methods - Part 2: Gardner Color Number). A higher Gardner color number indicates a darker and less transparent color. The liquid product of melamine resin (B) can be evaluated as colorless and transparent.

[0112] Table 1 shows the "Chromaticity Coordinates of Gardner Standard Color Glass" as described in JIS K 0071-2:1998. The standard color glass number for a sample with the same color is the Gardner color number of that sample. A Gardner color number of 2 or less indicates a sample with an x ​​color coordinate of 0.3177 or less, a y color coordinate of 0.3303 or less, and a visual transmittance of 80% or greater.

[0113] [Table 1]

[0114] Table 1 Chromaticity coordinates of Gardner standard color glass

[0115]

[0116] The melamine resin (B) has a low melting point. The melting point of the melamine resin (B) can be below -20°C. The low melting point of the melamine resin (B) lowers the viscosity of the aqueous coating composition, further suppresses fogging, and improves gloss. The melamine resin (B) can be liquid at a temperature of -20°C to 70°C.

[0117] The melting point of the melamine resin (B) can be conveniently evaluated, for example, by the following method. First, a mixture of a solvent and melamine resin (B) is thinly spread on a plastic container. After heating at 70°C for one hour to remove the solvent, the fluidity of the mixture is visually confirmed at temperatures ranging from 20°C to -20°C. For convenience, the temperature at which the mixture loses fluidity is considered the melting point of the melamine resin (B).

[0118] The melamine resin (B) contains three nitrogen atoms N around the triazine ring (triazine core). 1 ~N 3 Bonded to 6 substituents R 1 ~R 6 The structure of (-N 1 (R 1 )(R 2 ),-N 2 (R 3 )(R 4 ),-N 3 (R 5 )(R 6 )).

[0119] The melamine resin (B) is represented by the following general formula (1), for example:

[0120] [Chemical Formula 1]

[0121]

[0122] (wherein, the substituent R 1 ~R 6 Each independently represents a hydrogen atom, an alkyl ether group, a hydroxymethyl group, or a portion bonded to another triazine ring.

[0123] Alkyl ether (-CH2-OR 7 ) of an alkyl group (R 7 ) may have 1 to 8 carbon atoms, or 1 to 4 carbon atoms. 7 It can be straight chain or branched. 7 It can be methyl, ethyl, propyl or butyl.

[0124] The melamine resin (B) is generally composed of a multinuclear structure in which a plurality of triazine rings are bonded. However, the melamine resin (B) may be a mononuclear structure composed of a single triazine ring.

[0125] Examples of the melamine resin (B) include melamine resins having -N(-CH2-OR 7 )(-CH2OH) hydroxymethyl type; with -N(-CH2-OR 7 )(H) imino type; with -N(-CH2-OR 7 )(-CH2OH) and -N(-CH2-OR 7 )(H) of the hydroxymethyl / imino type.

[0126] The average value of the total number of imino groups and methylol groups per triazine ring (hereinafter sometimes referred to as the average functional group number) is greater than 1. This facilitates incorporation of the melamine resin (B) into the cross-linked structure, increasing the hardness of the coating film. The average functional group number can be 1.2 or greater, 1.5 or greater, or 2.0 or greater. The average functional group number can be 4 or less, or 3.5 or less. This facilitates achieving desired physical properties such as hardness and flexibility with respect to metal thermal expansion.

[0127] It is not excluded that only alkyl ether groups are used as substituents R 1 ~R 6 The use of a fully alkylated melamine resin is preferred. However, from the perspective of coating film properties such as flexibility, it is preferred that the amount used be small. For example, the amount of the fully alkylated melamine resin used may be 10% by mass or less, 5% by mass or less, or 0% by mass, based on 100% by mass of the total solid content of the melamine resin contained in the first liquid.

[0128] The number average molecular weight (Mn) of melamine resin (B) can be 300 or more and 3000 or less. Thus, the viscosity of the aqueous coating composition is suppressed to be relatively low, mist and dust are further suppressed, and glossiness is further improved. The number average molecular weight of melamine resin (B) can be 2500 or less, 2000 or less, 1500 or less, or 1300 or less. The number average molecular weight of melamine resin (B) can be 350 or more, or 400 or more.

[0129] For example, relative to 100 parts by mass of the solid content of hydroxy-containing resin (A), the solid content of melamine resin (B) can be more than 7 parts by mass and less than 50 parts by mass. Thus, curing reaction is easily carried out, and it is easy to obtain a coating film with high hardness. The above-mentioned solid content of melamine resin (B) can be more than 15 parts by mass, can be more than 17 parts by mass, or can be more than 20 parts by mass. The above-mentioned solid content of melamine resin (B) can be less than 45 parts by mass, can be less than 40 parts by mass, or can be less than 35 parts by mass. In one embodiment, relative to 100 parts by mass of the solid content of hydroxy-containing resin (A), the above-mentioned solid content of melamine resin (B) is more than 7 parts by mass and less than 40 parts by mass.

[0130] The equivalent ratio (imino groups, etc. / isocyanate groups) of the imino groups and methylol groups (hereinafter sometimes referred to as imino groups, etc.) of the melamine resin (B) to the isocyanate groups of the hydrophilic polyisocyanate compound (D) can be 0.2 or more and 1.1 or less. This facilitates the reaction between the melamine resin (B) and the hydrophilic polyisocyanate compound (D).

[0131] If the equivalent ratio (imino group, etc. / isocyanate group) is 1.0, the imino group and the methylol group react with the isocyanate group without excess or deficiency. If the equivalent ratio (imino group, etc. / isocyanate group) is 0.2 or more, the reaction between the melamine resin (B) and the hydrophilic polyisocyanate compound (D) is likely to occur. An equivalent ratio (imino group, etc. / isocyanate group) of 1.1 or less means that the number of imino groups and methylol groups is not too large compared to the number that reacts with the isocyanate group without excess or deficiency, and therefore the reaction between the hydrophilic polyisocyanate compound (D) and the hydroxyl-containing resin (A) is likely to occur.

[0132] The equivalent ratio (imino group, etc. / isocyanate group) may be 0.3 or more, or 0.38 or more. The equivalent ratio (imino group, etc. / isocyanate group) may be 1.0 or less, or 0.9 or less.

[0133] Since the imino group or hydroxymethyl group reacts with the isocyanate group in a 1:1 ratio, the equivalent ratio (imino group, etc. / isocyanate group) can be obtained as the number ratio (Tm / Ti) of the total number of imino groups and hydroxymethyl groups possessed by the melamine resin (B) contained in the coating composition and the total number of isocyanate groups Ti possessed by the hydrophilic polyisocyanate compound (D) contained in the coating composition.

[0134] For example, the total number Tm of imino groups and the like contained in 100 g of the coating composition can be calculated based on the following formula from the mass (compounding amount) Wm of the melamine resin (B) contained in 100 g of the coating composition, the molecular weight Mm, and the total number Nm of imino groups and methylol groups per molecule.

[0135] Total number Tm ( / 100g) = total number of imino groups, etc. Nm × (compounding amount Wm / molecular weight Mm)

[0136] The total number Ti is similarly calculated based on the following formula from the mass (compounding amount) Wi of the hydrophilic polyisocyanate compound (D) contained in 100 g of the coating composition, the molecular weight Mi, and the number Ni of isocyanate groups per molecule (or the NCO content Ri (%)).

[0137] Total Ti ( / 100g) = Number of isocyanate groups Nix (compounding amount Wi / molecular weight Mi)

[0138] or

[0139] Total Ti ( / 100g) = blending amount Wi × NCO content Ri / (NCO molecular weight = 42)

[0140] Solvents

[0141] The first liquid contains water as a solvent. If necessary, the first liquid may further contain a water-soluble or water-miscible organic solvent.

[0142] <<Preparation Method>>

[0143] The first liquid can be prepared by mixing the above components by a method known to those skilled in the art. The mixing method may be the same as that for preparing the aqueous coating composition.

[0144] (Second liquid)

[0145] The second liquid contains a hydrophilic polyisocyanate compound (D). The hydrophilic polyisocyanate compound (D) is a curing agent that reacts with a hydroxyl-containing component (representatively a hydroxyl-containing resin (A)) to form a cross-linked structure, thereby curing the aqueous coating composition. The hydrophilic polyisocyanate compound (D) also reacts with the melamine resin (B). As a result, the rigid triazine rings of the melamine resin (B) are incorporated into the cross-linked structure of the hydroxyl-containing resin (A) and the hydrophilic polyisocyanate compound (D), thereby increasing the hardness of the resulting coating film.

[0146] The equivalent ratio (NCO / OH) of the isocyanate groups contained in the hydrophilic polyisocyanate compound (D) to the hydroxyl groups contained in the hydroxyl-containing component is, for example, 0.7 or more and 2.0 or less. The equivalent ratio (NCO / OH) may be 0.7 or more, or 0.8 or more. The equivalent ratio (NCO / OH) may be 2.0 or less, 1.8 or less, or 1.5 or less.

[0147] As the hydrophilic polyisocyanate compound (D), at least one of an ion-modified polyisocyanate compound (D1) and a nonion-modified polyisocyanate compound (D2) having unblocked free isocyanate groups can be used. These are water-dispersible. At least the ion-modified polyisocyanate compound (D1) can be used.

[0148] (D1) Ion-modified polyisocyanate compound

[0149] The ion-modified polyisocyanate compound (D1) has two or more isocyanate groups in its molecule. The ion-modified polyisocyanate compound (D1) has unblocked free isocyanate groups, thus enabling low-temperature curing. The ion-modified polyisocyanate compound (D1) also has an ionic group that is a hydrophilic group. Therefore, when the ion-modified polyisocyanate compound (D1) is mixed with the aqueous first liquid, it is fully dispersed, thereby improving the smoothness of the coating film.

[0150] An anion-modified polyisocyanate compound may be contained as the ion-modified polyisocyanate compound (D1). The anion-modified polyisocyanate compound is obtained, for example, by modifying the polyisocyanate compound exemplified as the organic polyisocyanate compound (b1) with an anionic hydrophilic group. The modification with the anionic hydrophilic group is performed so that two or more isocyanate groups remain in one molecule.

[0151] The anionic hydrophilic group is derived from, for example, carboxylic acid, sulfonic acid, phosphoric acid, silicic acid, sulfate, phosphate, or a metal salt or organic salt thereof, among which an anionic hydrophilic group derived from sulfonic acid may be used.

[0152] (D2) Nonionic modified polyisocyanate compound

[0153] The nonionic modified polyisocyanate compound (D2) has two or more isocyanate groups in the molecule. Since the nonionic modified polyisocyanate compound (D2) also has unblocked free isocyanate groups, low-temperature curing is possible. In addition, the nonionic modified polyisocyanate compound (D2) has a nonionic hydrophilic group. As a result, the nonionic modified polyisocyanate compound (D2) is fully dispersed when mixed with the aqueous first liquid, improving the smoothness of the coating film.

[0154] The nonionic hydrophilic group is derived from a hydrophilic compound. The nonionic modified polyisocyanate compound (D2) is obtained by, for example, modifying the polyisocyanate compound exemplified as the organic polyisocyanate compound (b1) with a hydrophilic compound.

[0155] Examples of the hydrophilic compound include hydrophilic polyols and hydrophilic polyethers. Examples of the hydrophilic polyol include ethylene glycol, glycerin, trimethylolpropane, pentaerythritol, and sorbitol.

[0156] <<Other polyisocyanate compounds>>

[0157] The use of a non-hydrophilic polyisocyanate compound is not excluded. However, from the perspective of viscosity, it is desirable to use a small amount. For example, the amount of the non-hydrophilic polyisocyanate compound used may be 50% by mass or less, 30% by mass or less, or 0% by mass, based on 100% by mass of the total solid content of the polyisocyanate compounds contained in the second liquid.

[0158] The use of blocked polyisocyanate compounds whose isocyanate groups are blocked by a blocking agent is not excluded. However, from the perspective of curability at low temperatures, it is desirable that the amount used is extremely small. The amount used of the blocked polyisocyanate compound can be 1% by mass or less, or it can be 0% by mass.

[0159] <<Other curing agents>>

[0160] The aqueous coating composition may contain other curing agents in addition to the hydrophilic polyisocyanate compound (D). Examples of other curing agents include epoxy compounds, aziridine compounds, carbodiimide compounds, and Oxazoline compounds. These can be used alone or in combination of two or more. The content of other curing agents is appropriately set according to the hydroxyl-containing resin.

[0161] Solvents

[0162] The second liquid may contain a solvent without a hydroxyl group. Examples of such solvents include glycol ether organic solvents, acetate organic solvents, ketone organic solvents, and ester organic solvents. These may be used alone or in combination of two or more.

[0163] <<Preparation Method>>

[0164] The second liquid can be prepared by mixing the above components by a method known to those skilled in the art. The mixing method may be the same as that for preparing the first liquid.

[0165] (Other ingredients)

[0166] The water-based coating composition may contain additives commonly used in the field of pigments and coatings. The additives may be added to any of the first liquid, the second liquid, and the third liquid. Examples of pigments include coloring pigments, extender pigments, and rust-proof pigments. Examples of additives include ultraviolet absorbers, hindered amine light stabilizers, antioxidants, crosslinked resin particles, leveling agents, defoamers, curing accelerators, and viscosity modifiers.

[0167] [Painted Items]

[0168] The aqueous coating composition of the present disclosure provides a coated article having a coating film with high hardness, excellent gloss, and suppressed misting.

[0169] In one embodiment, a coated article includes a coated article and a multilayer coating film in which a first coating film and a clear coating film are sequentially laminated. The first coating film is formed from the aqueous coating composition of the present disclosure.

[0170] In another embodiment, a coated article includes a coated article and a multilayer coating film, wherein a first coating film, a second coating film, and a clear coating film are sequentially laminated. The first coating film is formed from the aqueous coating composition of the present disclosure.

[0171] (Object to be coated)

[0172] Examples of materials for the coating include metals, resins, and glass. Specifically, examples of the coating include automobile bodies of cars, trucks, motorcycles, buses, and the like, as well as parts thereof, and automobile parts such as spoilers, bumpers, mirror covers, grilles, and door handles.

[0173] Examples of the metal include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of the metal to be coated include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy steel sheets, zinc-iron alloy steel sheets, zinc-magnesium alloy steel sheets, zinc-aluminum-magnesium alloy steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy steel sheets, and tin-plated steel sheets.

[0174] Metal substrates can also be surface treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium treatment, and composite oxide treatment. After surface treatment, metal substrates can be coated with an electrodeposition coating. Electrodeposition coatings can be either cationic or anionic.

[0175] Examples of the resin include polyethylene resin, EVA resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), vinyl chloride resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), polycarbonate resin, acrylic resin, acrylonitrile butadiene styrene (ABS resin), acrylonitrile styrene (AS resin), polyamide resin, acetal resin, phenolic resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene ether (PPO). The resin-made coating material may be degreased.

[0176] Because the water-based coating composition involved in the present disclosure can be cured at low temperatures, it is suitable for coating resins. The coating film obtained by the water-based coating composition involved in the present disclosure is excellent in resistance to cracking, so it is suitable for coating metals. The coated object can include both a metal portion (a portion formed by a metal) and a resin portion (a portion formed by a resin). The metal portion can be a steel plate.

[0177] (First coating)

[0178] The first coating film is formed by the aqueous coating composition of the present disclosure. The film thickness (dry film thickness) after the first coating film is cured is, for example, 5 μm or more and 80 μm or less. The dry film thickness of the first coating film can be 7 μm or more. The dry film thickness of the first coating film can be 50 μm or less.

[0179] The thickness of the coating film can be measured with an electromagnetic film thickness meter (for example, SDM-miniR manufactured by SANKO Co., Ltd.). The thickness of the coating film is the average value of the coating film thickness at any five points.

[0180] (Second coating)

[0181] The second coating film is formed from a second coating composition. The second coating composition will be described later. The second coating film may be a single layer or 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.

[0182] (Transparent coating)

[0183] The clear coating film is formed from 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 can be 20 μm or more. The dry film thickness of the clear coating film can be 60 μm or less.

[0184] [Method for producing coated articles]

[0185] In one embodiment, a coated article is produced by a method comprising the following steps: applying the aqueous multi-component coating composition onto an article to be coated to form an uncured first coating film; applying a clear coating composition onto the uncured first coating film to form an uncured clear coating film; and heating the uncured first coating film and the uncured clear coating film at a temperature of not less than 70° C. and not more than 110° C. to cure them.

[0186] In another embodiment, a coated article is manufactured by a method comprising the following steps: a step of applying the above-mentioned aqueous coating composition on the coated article 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 the uncured first coating film, the uncured second coating film, and the uncured clear coating film at a temperature of not less than 70° C. and not more than 110° C. to cure them.

[0187] Even at a low temperature of 70° C. to 110° C., a coating film having high hardness, excellent gloss, and suppressed misting is formed.

[0188] Hereinafter, a method for producing a coated article including a multilayer coating film in which a first coating film, a second coating film, and a clear coating film are sequentially laminated will be described as an example.

[0189] (I) Step of forming an uncured first coating film

[0190] The aqueous coating composition involved in the present disclosure is applied to the object to be coated to form an uncured first coating film. Through the first coating film, the adhesion of the second coating film to the object to be coated is improved. In addition, through the first coating film, the coating surface becomes uniform, thereby easily suppressing the unevenness of the second coating film. As mentioned above, the object to be coated can include both a metal portion and a resin portion.

[0191] As a coating method, for example, a roller coater method, air spray coating, airless spray coating, and rotary atomization coating can be cited. These methods can also be combined with electrostatic coating. Among them, from the viewpoint of coating efficiency, rotary atomization electrostatic coating is preferred. In rotary atomization electrostatic coating, for example, a rotary atomization electrostatic coating machine commonly known as "Micro·Micro Bell (μμBell)", "Micro Bell (μBell)", "Metallic Bell (MetaBell)" or the like can be used.

[0192] After the aqueous coating composition is applied and before the second coating composition is applied, preliminary drying (also referred to as preheating) can also be performed. Thus, it is possible to suppress the sudden boiling of the solvent contained in the aqueous coating composition during the curing process, thereby easily suppressing the occurrence of blistering. And then, by preliminary drying, it is suppressed that the uncured first coating film and the second coating composition are mixed together, thereby being difficult to form a mixed layer. Therefore, the smoothness of the obtained coated article can be further improved.

[0193] Examples of pre-drying include a method of leaving the mixture at a temperature of 20°C to 25°C for 5 to 15 minutes, and a method of heating the mixture at a temperature of 50°C to 80°C for 30 seconds to 10 minutes.

[0194] (II) Step of forming an uncured second coating film

[0195] A second coating composition is applied to 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. A few minutes may be allowed between the nth application of the second coating composition and the n+1th application of the second coating composition.

[0196] As a coating method, the same method as the coating method of the water-based coating composition can be mentioned, for example. After the second coating composition is applied, preliminary drying can also be performed in the same manner as above.

[0197] (Second coating composition)

[0198] The second coating composition can be either water-based or solvent-based. The second coating composition can be water-based. The water-based second coating composition, for example, comprises 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 can also comprise the various pigments, bright pigments, and various additives mentioned above.

[0199] (III) Step of forming an uncured clear coating film

[0200] The clear coating composition is applied on the uncured second coating film to form an uncured clear coating film.

[0201] The coating method is not particularly limited. As a coating method, for example, the same method as the coating method of the water-based coating composition can be cited. Among them, from the viewpoint of coating efficiency, rotary atomization electrostatic coating is preferred. After applying the clear coating composition, preliminary drying can also be carried out in the same manner as above.

[0202] (Clear coating composition)

[0203] The clear coating composition may be solvent-based, water-based, or powder-based. Considering transparency and acid etching resistance, the solvent-based clear coating composition may contain an acrylic resin and / or a polyester resin as a film-forming resin, and an amino resin and / or an isocyanate as a curing agent. The solvent-based clear coating composition may also contain an acrylic resin and / or a polyester resin having a carboxylic acid and / or epoxy group. The clear coating composition may contain the various pigments and additives mentioned above, as long as transparency is not impaired.

[0204] (IV) Curing process

[0205] The uncured coatings are cured. Each coating can be cured by heating. In this embodiment, the first coating, the second coating, and the clear coating are cured at once.

[0206] The heating temperature is, for example, 70°C to 110°C. The heating temperature may be 75°C or higher, or 80°C or higher. The heating temperature may be 105°C or lower, 100°C or lower, 95°C or lower, or 90°C or lower. The heating time indicates the time required for the target temperature to be reached in the heating device and for the coated object to be maintained at the target temperature, without taking into account the time required to reach the target temperature. Examples of the heating device include drying ovens utilizing a heating source such as hot air, electricity, gas, or infrared rays.

[0207] The heating time can be appropriately set according to the heating temperature. When the heating temperature is 70° C. or higher and 110° C. or lower, the heating time is, for example, 10 minutes or higher and 60 minutes or lower, or 15 minutes or higher and 45 minutes or lower.

[0208] 2. Second Implementation

[0209] The difference between the second embodiment and the first embodiment is that the first liquid contains a polyurethane resin (C) while containing a hydroxyl-containing resin (A) and a melamine resin (B). This different structure is described below. The other structures of the second embodiment are the same as those of the first embodiment, so their description is omitted. In the second embodiment, since the structure of the coated article and the manufacturing method of the coated article are the same as those of the first embodiment, their description is omitted.

[0210] (C) Polyurethane resin

[0211] The polyurethane resin (C) is also a coating film-forming component. The polyurethane resin (C) increases the elasticity of the coating film and improves chipping resistance.

[0212] The hydroxyl value of the polyurethane resin (C) can be 30mgKOH / g or less. Thus, the viscosity of the aqueous coating composition can be suppressed to be relatively low. Therefore, mist and dust are further suppressed, and the glossiness is further improved. In addition, the reaction of the polyurethane resin (C) and the hydrophilic polyisocyanate compound (D) is suppressed, and the reaction of the hydroxyl-containing resin (A) and the hydrophilic polyisocyanate compound (D) is easily controlled. The hydroxyl value of the polyurethane resin (C) can be 20mgKOH / g or less, can be 10mgKOH / g or less, or can be 0mgKOH / g.

[0213] In one embodiment, the solid content of the polyurethane resin (C) can be 10 parts by mass or more and 110 parts by mass or less relative to 100 parts by mass of the solid content of the hydroxyl-containing resin (A). Thus, the hardness and elasticity of the resulting coating are well balanced, and a coating having crack resistance and good coating film strength can be easily obtained. The solid content of the polyurethane resin (C) can be 15 parts by mass or more, or 20 parts by mass or more. The solid content of the polyurethane resin (C) can be 105 parts by mass or less, or 50 parts by mass or less.

[0214] The polyurethane resin (C) may be soluble in the first liquid. That is, the polyurethane resin (C) may be a water-soluble polyurethane resin. The polyurethane resin (C) may be in the form of a dispersion in the first liquid.

[0215] The water-soluble polyurethane resin and the polyurethane resin dispersion are obtained, for example, by a method of forcibly emulsifying the polyurethane resin using a surfactant or a method of neutralizing the polyurethane resin with an alkali or an acid.

[0216] The polyurethane resin is 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 compound (b1), and, if necessary, a chain extender and a polymerization terminator.

[0217] Polyol compounds contain two or more hydroxyl groups in their molecules. Examples of polyol compounds (polyols) include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and glycerol; polyether polyols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol; polyester polyols derived 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. These can be used alone or in combination.

[0218] Examples of compounds having an active hydrogen group and a hydrophilic group in the molecule include compounds containing active hydrogen and an anionic group, a cationic group, or a nonionic hydrophilic group. Anionic groups include anionic groups and anion-forming groups. Anion-forming groups are groups that can react with a base to form anionic groups. Specifically, anionic groups are formed by neutralization with a base before, during, or after the urethanization reaction.

[0219] Examples of compounds containing active hydrogen and an anionic group include those described in Japanese Patent Publication Nos. 42-24192 and 55-41607, with specific examples including α,α-dimethylolpropionic acid and α,α-dimethylolbutanoic acid. Examples of compounds containing active hydrogen and a cationic group include those described in Japanese Patent Publication No. 43-9076. Examples of compounds containing active hydrogen and a nonionic hydrophilic group include those described in Japanese Patent Publication No. 48-41718, with specific examples including polyethylene glycol and alkyl alcohol alkylene oxide adducts.

[0220] Examples of the organic polyisocyanate compound (b1) include aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, and aromatic aliphatic polyisocyanates.

[0221] The aromatic polyisocyanate has two or more isocyanate groups bonded to the carbon atoms constituting the aromatic ring. Examples of the aromatic polyisocyanate include 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-toluene 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.

[0222] Aliphatic polyisocyanates do not have an aromatic ring, but 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. Aliphatic diisocyanates such as isocyanate, 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.

[0223] Alicyclic polyisocyanates do not have an aromatic ring, but have two or more isocyanate groups bonded to carbon atoms constituting a cyclic aliphatic hydrocarbon group. Examples of the alicyclic polyisocyanates include 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5-trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate), methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, 1,3- or 1,4-bis(isocyanate). alicyclic diisocyanates such as 1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane, 2-(3-isocyanatopropyl)-2,5-bis(isocyanatomethyl)-bicyclo[2.2.1]hept ... propyl)-2,6-bis(isocyanatomethyl)-bicyclo[2.2.1]heptane, 3-(3-isocyanatopropyl)-2,5-bis(isocyanatomethyl)-bicyclo[2.2.1]heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, 6-(2-isocyanatoethyl)-2-isocyanatomethyl Alicyclic triisocyanates such as 2-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 can be used alone or in combination of two or more.

[0224] Aromatic aliphatic polyisocyanates have an aromatic ring and contain two or more isocyanate groups bonded to the carbon atoms that constitute the aliphatic hydrocarbon group. Examples of aromatic aliphatic polyisocyanates include 1,3- or 1,4-xylylenediisocyanate or mixtures thereof, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (commonly known as tetramethylxylylenediisocyanate), or mixtures thereof; and aromatic aliphatic triisocyanates such as 1,3,5-triisocyanatomethylbenzene. These can be used alone or in combination of two or more.

[0225] The organic polyisocyanate compound (b1) may be a derivative of the above-mentioned polyisocyanate. Examples of the polyisocyanate derivatives include dimers, trimers, biuret, allophanate, uretdione, uretonimine, isocyanurate, Diazinetrione, polymethylene polyphenyl polyisocyanate (crude MDI, polymeric MDI) and crude TDI.

[0226] Chain extenders contain two or more active hydrogen groups in their molecules. Examples of chain extenders 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, xylene diamine, and isophorone diamine; and water. These can be used alone or in combination.

[0227] Examples of the polymerization terminator include compounds having one active hydrogen atom in the molecule (for example, monoalcohols, monoamines, etc.) and monoisocyanate compounds.

[0228] The synthesis method of the polyurethane resin can be a one-step method in which all components are reacted at once, or a multi-step method in which the reaction is carried out in stages. In the multi-step method, a portion of the active hydrogen-containing compound (e.g., a high molecular weight polyol) is reacted with the organic polyisocyanate compound (b1) to form an NCO-terminated prepolymer, which is then reacted with the remaining portion of the active hydrogen-containing compound.

[0229] In the present embodiment, relative to the total solid content mass 100 mass parts of hydroxy-containing resin (A) and polyurethane resin (C), the solid content mass of melamine resin (B) can be more than 7 mass parts and less than 50 mass parts. Thus, curing reaction is easily carried out, and the coating properties such as hardness and flexibility to metal thermal expansion are easily become good. The solid content mass of melamine resin (B) in the present embodiment can be more than 15 mass parts, can be more than 17 mass parts, or can be more than 20 mass parts. The solid content mass of melamine resin (B) in the present embodiment can be less than 45 mass parts, can be less than 40 mass parts, or can be less than 35 mass parts. Particularly relative to the total solid content mass 100 mass parts of hydroxy-containing resin (A) and polyurethane resin (C), the solid content mass of melamine resin (B) can be more than 7 mass parts and less than 40 mass parts.

[0230] Example

[0231] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto. In the examples, "parts" and "%" are based on the mass of the solid content unless otherwise specified.

[0232] The acid value and the hydroxyl value are calculated based on the solid content acid value and the solid content hydroxyl value of the monomer mixture used.

[0233] [Manufacturing Example 1-1] Manufacture of Hydroxyl-Containing Acrylic Resin (A1-1)

[0234] 126.5 parts of deionized water were added to a reaction vessel, and the temperature was raised to 80°C while mixing and stirring under a nitrogen stream. Next, a monomer emulsion consisting of 30 parts of a monomer mixture (containing 6.98 parts of ethyl acrylate, 12.42 parts of butyl acrylate, 4.75 parts of styrene, 2.78 parts of 2-hydroxyethyl methacrylate, and 3.07 parts of methacrylic acid), 0.33 parts of ADEKA REASOAP SR-10 (polyoxyethylene-1-alkoxymethyl-2-(2-acryloyloxy)ethyl ether sulfate ammonium salt, manufactured by ADEKA Corporation), and 24 parts of deionized water was added dropwise to the reaction vessel over 1.5 hours, along with an initiator solution consisting of 0.09 parts of ammonium persulfate and 3 parts of deionized water. After the additions were completed, the mixture was aged at the same temperature for 1 hour.

[0235] Next, a monomer emulsion consisting of 70 parts of a monomer mixture (containing 16.28 parts of ethyl acrylate, 28.97 parts of butyl acrylate, 15.25 parts of styrene, 6.5 parts of 2-hydroxyethyl methacrylate, and 3 parts of allyl methacrylate), 0.77 parts of ADEKAREASOAP SR-10, and 56 parts of deionized water was added dropwise to the reaction vessel over 1.5 hours, along with an initiator solution consisting of 0.21 parts of ammonium persulfate and 7 parts of deionized water. After the addition was complete, the mixture was aged at the same temperature for 1 hour.

[0236] The mixture was then cooled to 40°C and filtered through a 400-mesh filter. The filtrate was adjusted to pH 6.5 by adding 20 parts of deionized water and 0.32 parts of dimethylaminoethanol. This yielded an emulsion of a hydroxyl-containing acrylic resin (A1-1) having a core-shell multilayer structure and an average particle size of 100 nm, a Tg of 27°C, a non-volatile content of 30%, an acid value of 20 mgKOH / g, and a hydroxyl value of 40 mgKOH / g.

[0237] [Manufacturing Example 1-2] Manufacture of Hydroxyl-Containing Acrylic Resin (A1-2)

[0238] 126.5 parts of deionized water were added to a reaction vessel, and the temperature was raised to 80°C while mixing and stirring under a nitrogen stream. Next, a monomer emulsion consisting of 100 parts of a monomer mixture (containing 27.61 parts of methyl acrylate, 53.04 parts of ethyl acrylate, 4.00 parts of styrene, 9.28 parts of 2-hydroxyethyl methacrylate, 3.07 parts of methacrylic acid, and 3.00 parts of allyl methacrylate), 1.1 parts of ADEKA REASOAP SR-10, and 80 parts of deionized water was added dropwise to the reaction vessel over 2 hours, along with an initiator solution consisting of 0.3 parts of ammonium persulfate and 10 parts of deionized water. After the additions were completed, the mixture was aged at the same temperature for 2 hours.

[0239] The mixture was then cooled to 40°C and filtered through a 400-mesh filter. 20 parts of deionized water and 0.32 parts of dimethylaminoethanol were added to the filtrate to adjust the pH to 6.5. This yielded a single-layer emulsion of a hydroxyl-containing acrylic resin (A1-2) having an average particle size of 90 nm, a Tg of -9.5°C, a nonvolatile content of 30%, an acid value of 20 mgKOH / g, and a hydroxyl value of 40 mgKOH / g.

[0240] [Manufacturing Example 1-3] Manufacture of Hydroxyl-Containing Acrylic Resin (A1-3)

[0241] 23.89 parts of tripropylene glycol methyl ether and 16.11 parts of propylene glycol methyl ether were added to a reaction vessel and heated to 105°C while mixing and stirring under a nitrogen stream. Next, a monomer mixture containing 13.1 parts of methyl methacrylate, 68.4 parts of ethyl acrylate, 11.6 parts of 2-hydroxyethyl methacrylate, and 6.9 parts of methacrylic acid was prepared. 100 parts of this monomer mixture was added dropwise to the reaction vessel over 3 hours, along with an initiator solution consisting of 10.0 parts of tripropylene glycol methyl ether and 1 part of t-butyl peroxy 2-ethylhexanoate. After the addition was complete, the mixture was aged at the same temperature for 0.5 hours.

[0242] Furthermore, an initiator solution consisting of 5.0 parts of tripropylene glycol methyl ether and 0.3 parts of t-butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel over 0.5 hours. After completion of the dropwise addition, the mixture was aged at the same temperature for 2 hours.

[0243] Next, 16.1 parts of the solvent was distilled off at 110°C under reduced pressure (70 torr) using a desolventizer, and then 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added. This yielded a hydroxyl-containing water-soluble acrylic resin solution (A1-3) having a nonvolatile content of 30%, an acid value of 40 mgKOH / g, a hydroxyl value of 50 mgKOH / g, a Tg of 10°C, and an Mw of 30,000.

[0244] [Manufacturing Example 2] Manufacture of hydroxyl-containing polyester resin (A2)

[0245] In a reaction vessel equipped with a stirrer, nitrogen inlet pipe, temperature control device, condenser, and decanter, 250 parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts of cyclohexanedicarboxylic acid were added. The temperature was raised to 180°C, and a condensation reaction was carried out until no more water was distilled off. After cooling to 60°C, 120 parts of phthalic anhydride were added. The temperature was then raised to 140°C and maintained for 60 minutes to obtain a polyester resin having a number average molecular weight of 2000 as measured by GPC. 59 parts of dimethylaminoethanol (equivalent to 80% of the resin's acid value (neutralization rate 80%)) was added at 80°C, and 1920 parts of deionized water was added and stirred to obtain a polyester aqueous dispersion having a solids content of 45% by mass. The hydroxyl-containing polyester resin (A2) contained in this polyester aqueous dispersion had a hydroxyl value of 110 mgKOH / g, an acid value of 15 mgKOH / g, a Tg of -14°C, and an Mw of 7000.

[0246] [Preparation of pigment dispersion paste]

[0247] 9.2 parts of the dispersant "Disperbyk 190" (manufactured by BYK-Chemie GmbH), 17.8 parts of ion-exchanged water, and 73.0 parts of rutile titanium dioxide were premixed. Then, using a beaded media in a paint conditioner, the mixture was mixed and dispersed at room temperature until the particle size reached 5 μm or less. Finally, the beaded media was removed by filtration to obtain a pigment paste.

[0248] The details of the triamino resin (B), the polyurethane resin (C), and the hydrophilic polyisocyanate compound (D) used are as follows.

[0249] [Melamine resin (B)]

[0250] Melamine resins (B-1) to (B-7) and (b-1) to (b-3) having the following average number of functional groups, average particle diameters, and number average molecular weights were used. Details are shown in Table 2.

[0251] The average particle size of the melamine resin (B) was measured using a 70% concentration of a liquid containing n-butanol or isobutanol. The volume-based particle size distribution of the liquid was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac UPA150, manufactured by Nikkiso Co., Ltd.), and the 50% average particle size (D50) in this particle size distribution was recorded as the average particle size of the melamine resin (B). The effective measurement range of the Microtrac UPA 150 is 0.003 μm to 6.5 μm. The fact that D50 is less than 0.003 μm in Table 2 indicates that D50 is less than the effective measurement range of the measuring instrument.

[0252] [Table 2]

[0253]

[0254] [Polyurethane resin (C)]

[0255] (C-1): N800T, manufactured by Sanyo Chemical Co., Ltd., hydroxyl value 0 mgKOH / g

[0256] (C-2): Bayhydrol UH2648 / 1, manufactured by Cobestro, water-based polyurethane resin, hydroxyl value 0 mgKOH / g

[0257] (C-3): Bayhydrol U2787, manufactured by Cobestro, water-based polyurethane resin, hydroxyl value 56 mgKOH / g

[0258] [Ionic (anionic) modified polyisocyanate compound (D1)]

[0259] (D1): Bayhydur 2655, manufactured by Sumika Covestro Urethane Co., Ltd., containing sulfonic acid groups

[0260] [Nonionic modified polyisocyanate compound (D2)]

[0261] (D2): Bayhydur 304, manufactured by Sumika Covestro Urethane Co., Ltd., hydrophilic polyether-modified HDI trimer

[0262] [Blocked polyisocyanate compound]

[0263] (d1): DURANATE WM44-L70G, manufactured by Asahi Kasei Corporation, HDI-based blocked isocyanate

[0264] [Non-hydrophilic polyisocyanate compound]

[0265] (d2): Sumidur N3300, manufactured by Sumika Covestro Urethane Co., Ltd., uretdione of hexamethylene diisocyanate

[0266] Examples 1 to 12 corresponding to the first embodiment were carried out.

[0267] [Example 1]

[0268] (1) Preparation of the first liquid

[0269] In a container equipped with a stirrer, 35 parts of a hydroxyl-containing acrylic resin (A1-1), 10 parts of a hydroxyl-containing polyester resin (A2), 10 parts of a melamine resin (B-1), 122.87 parts of a coloring pigment paste, and 25 parts of ion-exchanged water were placed, and the pH was adjusted to 8.0 with 0.01 part of dimethylethanolamine (manufactured by Kishida Chemical Co., Ltd.). Furthermore, 1.0 part of ADEKA NOL UH-814N (urethane associative adhesive, 30% active ingredient, manufactured by Asahi Denka Kogyo Co., Ltd., trade name) was mixed and stirred. Tipaque CR-97 (manufactured by Ishihara Sangyo Co., Ltd., titanium dioxide, primary average particle size 200 nm) was then added and dispersed to a pH of 53.5%. This yielded a first liquid.

[0270] (2) Preparation of the second liquid

[0271] 45 parts of the anion-modified polyisocyanate compound (D1) and an appropriate amount of a solvent (dipropylene glycol dimethyl ether and / or ethylene glycol monobutyl acetate) were mixed and stirred thoroughly with a disperser to obtain a second liquid.

[0272] (3) Preparation of water-based coating composition

[0273] The first liquid and the second liquid are mixed to obtain an aqueous coating composition.

[0274] (4) Preparation of the coated object

[0275] (Metal coating)

[0276] A pear-grain steel plate (400 mm x 600 mm) was used as the coating material and washed and zinc-phosphated according to conventional methods. Next, electrodeposition coating was performed using a cationic electrodeposition paint (Powertop U-100, manufactured by Nippon Paint Co., Ltd.). The coating was then dried at 170°C for 20 minutes to obtain a 15 μm-thick electrodeposition coating film on the metal substrate.

[0277] (Resin coated article)

[0278] A polypropylene plate was degreased to obtain a resin-coated object.

[0279] (5) Production of coated articles with multi-layer coatings

[0280] The aqueous multi-component coating composition was electrostatically applied to each of a metal substrate and a resin substrate using a rotary atomizing electrostatic coating machine so that the dry film thickness became 15 μm, and the coatings were left to stand for 5 minutes.

[0281] Next, a water-based base coating composition (Nippon Paint Automotive Coatings, water-based AR-3020-1 (gray metallic)) was applied using a rotary atomizing electrostatic coater to a dry film thickness of 15 μm, and preheated at 80° C. for 3 minutes.

[0282] Next, a clear coating (Polyure Exel O-1200 (trade name), a two-component acrylic urethane organic solvent clear coating containing a polyisocyanate compound, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied using a rotary atomizing electrostatic coating machine to a dry film thickness of 35 μm.

[0283] Then, the product was heated at 80° C. for 20 minutes to obtain a coated article having a multilayer coating film.

[0284] [Examples 2 to 12 and Comparative Examples 1 to 4 and 6]

[0285] A water-based coating composition was prepared by the same procedure as in Example 1, except that the types and amounts of the ingredients, the solid content mass during coating, etc. were changed as shown in Tables 3 and 4, to obtain a coated article.

[0286] [Comparative Example 5]

[0287] An aqueous coating composition was prepared by the same procedure as in Example 1, except that the melamine resin (B-1) was added to the second liquid instead of the first liquid.

[0288] The aqueous coating composition was left to stand at 23° C., and after 24 hours, the viscosity of the coating composition became significantly high, making spray coating impossible. Therefore, the following evaluations could not be performed.

[0289] [Comparative Example 7]

[0290] In the preparation of the first liquid, an aqueous coating composition was prepared by the same procedure as in Example 1 except that 10 parts of melamine resin particles having an average particle size of 2 μm (trade name: Epostar MS, manufactured by Nippon Shokubai Co., Ltd.) were added instead of 10 parts of melamine resin (B-1).

[0291] [evaluate]

[0292] The following evaluations were performed using the aqueous coating compositions prepared in the above Examples and Comparative Examples. Comparative Example 7 was evaluated for hardness and glossiness.

[0293] (1) Hardness of single-layer coating

[0294] A water-based multi-component coating composition was electrostatically applied to a glass plate using a rotary atomizer electrostatic coater to a dry film thickness of 15 μm. The plate was then left to stand for 5 minutes. The plate was then heated at 85°C for 20 minutes (hold time) to produce a single-layer coating film, test plate A for evaluation. Test plate A was placed in a pendulum hardness tester (manufactured by BYK) and the number of vibrations required to reduce the Konig oscillator's oscillation angle from 6 degrees to 3 degrees at 23°C was measured. Evaluation was performed according to the following criteria.

[0295] (Evaluation Criteria)

[0296] AThe number of vibrations is more than 80 times

[0297] B The number of vibrations is more than 65 times and less than 80 times

[0298] C The number of vibrations is more than 50 times and less than 65 times

[0299] D The number of vibrations is more than 35 times and less than 50 times

[0300] E vibration number is less than 35 times

[0301] (2) Glossiness of single-layer coating

[0302] A water-based multi-component coating composition was electrostatically applied to a metal substrate prepared in the same manner as above using a rotary atomizing electrostatic coater to a dry film thickness of 15 μm. The coating was then allowed to stand for 5 minutes. Subsequently, the coating was heated at 85°C for 20 minutes (hold time) to obtain a single-layer coating film, test plate B, for evaluation.

[0303] The gloss value of the evaluation test plate B was measured using a "MULTI GLOSS 268 plus" (manufactured by Konica Minolta, Inc.) in accordance with JIS K5600-4-7. Specifically, the gloss value of the evaluation test plate B was measured three times under the geometric condition of a 60° incident light axis (60° gloss). The average of the measured values ​​was calculated, and evaluation was performed according to the following criteria.

[0304] A 60° gloss value is above 70

[0305] B 60° gloss value is above 60 and less than 70

[0306] C 60° gloss value is 50 or more and less than 60

[0307] D 60° gloss value is above 40 and less than 50

[0308] E 60° gloss value is less than 40

[0309] (3) Fog and dust

[0310] The water-based coating composition was applied to a metal coating object prepared in the same manner as above by air spray coating under the following conditions to obtain a test plate C for evaluation.

[0311] (Air spray coating conditions)

[0312] Discharge: From fully closed to 2 turns back

[0313] Air pressure: 0.25MPa

[0314] (Painting method)

[0315] The metal object to be coated is kept in a substantially vertical state and is coated according to the following steps.

[0316] Step 1. Apply the aqueous multi-component coating composition to the substrate so that the dry film thickness reaches 15 μm (spray gun distance 30 cm)

[0317] Step 2. Dry at room temperature for 10 minutes

[0318] Step 3. Stand at a 45° angle to the object being coated and 150 cm from the spray gun, and spray the dust for 10 seconds.

[0319] Step 4. Dry at room temperature for 10 seconds

[0320] Step 5. Perform steps 3 and 4 5 times in total

[0321] Step 9. Dry in a jet oven at 60°C for 1 hour

[0322] The coating film of the obtained evaluation test plate C was visually observed from above, and the number of mist particles attached to the test plate (30 cm×40 cm) was evaluated according to the following criteria: Protrusions with a diameter of 100 μm or more were considered mist particles.

[0323] 5 Less than 10

[0324] 4 More than 10 and less than 30

[0325] 3 More than 30 and less than 100

[0326] 2 More than 100 and less than 200

[0327] 1,200 or more

[0328] [Table 3]

[0329]

[0330] [Table 4]

[0331]

[0332] Examples 13 to 31 corresponding to the second embodiment were carried out.

[0333] [Example 13]

[0334] (1) Preparation of the first liquid

[0335] In a container equipped with a stirrer, 40 parts of a hydroxyl-containing acrylic resin (A1-1), 10 parts of a hydroxyl-containing polyester resin (A2), 15 parts of a melamine resin (B-1), 10 parts of a polyurethane resin (C-1), 122.11 parts of a color pigment paste, and 68 parts of ion-exchanged water were placed. The pH was adjusted to 8.0 with 0.01 part of dimethylethanolamine (manufactured by Kishida Chemical Co., Ltd.). Furthermore, 1.0 part of ADEKA NOL UH-814N (urethane associative adhesive, 30% active ingredient, manufactured by Asahi Denka Kogyo Co., Ltd., trade name) was mixed and stirred. Tipaque CR-97 (manufactured by Ishihara Sangyo Co., Ltd., titanium dioxide, primary average particle size 200 nm) was then added and dispersed to a PWC of 53.5%. This yielded a first liquid.

[0336] (2) Preparation of the second liquid

[0337] 25 parts of the anion-modified polyisocyanate compound (D1) and an appropriate amount of a solvent (dipropylene glycol dimethyl ether and / or ethylene glycol monobutyl acetate) were mixed and stirred thoroughly with a disperser to obtain a second liquid.

[0338] A water-based coating composition and a coated article were obtained in the same manner as in Example 1 except that the first and second liquids were used.

[0339] [Examples 14 to 31 and Comparative Examples 8 to 11 and 13]

[0340] Aqueous coating compositions were prepared by the same procedures as in Example 13, except that the types and amounts of the ingredients, the solid content mass during coating, etc. were changed as shown in Tables 5 to 7, to obtain coated articles.

[0341] [Comparative Example 12]

[0342] An aqueous coating composition was prepared by the same procedure as in Example 13, except that the melamine resin (B-1) was added to the second liquid instead of the first liquid.

[0343] The aqueous coating composition was left to stand at 23° C., and after 24 hours, the viscosity of the coating composition became significantly high, making spray coating impossible. Therefore, the following evaluations could not be performed.

[0344] [Table 5]

[0345]

[0346] [Table 6]

[0347]

[0348] [Table 7]

[0349]

[0350] The water-based coating compositions of the examples can form coating films that are curable at low temperatures, have high hardness, excellent gloss, and have suppressed mist and dust.

[0351] Melamine resins having a small average number of functional groups were used in Comparative Examples 1, 2, 8, and 9. It was confirmed that the hardness of coating films obtained from these aqueous coating compositions by low-temperature curing was different.

[0352] Blocked isocyanate compounds were used in Comparative Examples 3 and 10. It was confirmed that the hardness of the coating film obtained from the aqueous coating composition was significantly deteriorated by low-temperature curing.

[0353] In Comparative Examples 4 and 11, non-hydrophilic isocyanate compounds were used. It was confirmed that the coating films obtained from these aqueous coating compositions by low-temperature curing had a significantly reduced gloss and generated a significant amount of mist.

[0354] In Comparative Examples 5 and 12, melamine resin was added to the second liquid to prepare an aqueous coating composition. The viscosity of the aqueous coating composition increased significantly, making spray coating difficult.

[0355] No polyisocyanate compound was used in Comparative Examples 6 and 13. It was confirmed that the hardness of the coating film obtained from the aqueous coating composition was significantly deteriorated by low-temperature curing.

[0356] In Comparative Example 7, a melamine resin with a large average particle size was used. The coating film obtained from this aqueous coating composition by low-temperature curing had poor glossiness. This is believed to be because the melamine resin has light diffusing properties.

[0357] The present disclosure includes the following aspects.

[0358] [1] An aqueous multi-liquid coating composition comprising: a first liquid containing a hydroxyl-containing resin (A) and a melamine resin (B), and a second liquid containing a hydrophilic polyisocyanate compound (D),

[0359] The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2),

[0360] The melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average value of the total number of imino groups and hydroxymethyl groups per triazine ring is greater than 1,

[0361] The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a non-ion-modified polyisocyanate compound (D2).

[0362] [2] An aqueous multi-liquid coating composition comprising: a first liquid containing a hydroxyl-containing resin (A), a melamine resin (B) and a polyurethane resin (C); and a second liquid containing a hydrophilic polyisocyanate compound (D).

[0363] The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2),

[0364] The melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average value of the total number of imino groups and hydroxymethyl groups per triazine ring is greater than 1,

[0365] The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a non-ion-modified polyisocyanate compound (D2).

[0366] [3] The aqueous multi-component coating composition of [1] or [2] above, wherein the ion-modified polyisocyanate compound (D1) contains at least an anion-modified polyisocyanate compound.

[0367] [4] The aqueous multi-component coating composition of [2] above, wherein the polyurethane resin (C) has a hydroxyl value of 30 mgKOH / g or less.

[0368] [5] The aqueous multi-liquid coating composition according to any one of [1] to [4] above, wherein the acid value of the hydroxyl-containing resin (A) is 5 mgKOH / g or more and 70 mgKOH / g or less.

[0369] [6] The aqueous multi-liquid coating composition according to any one of [1] to [5], wherein the solubility parameter of the melamine resin (B) is 9 or more and 15 or less.

[0370] [7] The aqueous multi-liquid coating composition according to any one of [1] to [6], wherein the solid content of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less relative to 100 parts by mass of the solid content of the hydroxyl-containing resin (A).

[0371] [8] The aqueous multi-liquid coating composition described in [2] above, wherein the solid content of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the total solid content of the hydroxyl-containing resin (A) and the polyurethane resin (C).

[0372] [9] The aqueous multi-liquid coating composition according to any one of [1] to [8] above, wherein the equivalent ratio of the imino group and hydroxymethyl group of the melamine resin (B) to the isocyanate group of the hydrophilic polyisocyanate compound (D) (imino group and hydroxymethyl group / isocyanate group) is 0.2 or more and 1.1 or less.

[0373]

[10] The aqueous multi-liquid coating composition according to any one of [1] to [9] above, wherein the number average molecular weight of the melamine resin (B) is 300 or more and 3000 or less.

[0374]

[11] A method for manufacturing a coated article, comprising:

[0375] A process of applying the aqueous multi-component coating composition according to any one of [1] to

[10] above to an object to be coated to form an uncured first coating film,

[0376] a step of applying a clear coating composition on the uncured first coating film to form an uncured clear coating film, and

[0377] a step of heating the uncured first coating film and the uncured clear coating film at 70° C. or higher and 110° C. or lower to cure them.

[0378]

[12] A method for manufacturing a coated article, comprising:

[0379] A process of applying the aqueous multi-component coating composition according to any one of [1] to

[10] above to an object to be coated to form an uncured first coating film,

[0380] a step of applying a second aqueous coating composition on the uncured first coating film to form an uncured second coating film,

[0381] a step of applying a clear coating composition on the uncured second coating film to form an uncured clear coating film, and

[0382] a step of heating and curing the uncured first coating film, the uncured second coating film, and the uncured clear coating film at 70° C. or higher and 110° C. or lower.

[0383]

[13] The method for manufacturing a coated article according to

[11] or

[12] , wherein the object to be coated includes a metal portion and a resin portion.

[0384] Industrial applicability

[0385] The aqueous coating composition of the present invention can form a coating film that is curable at low temperatures, has high hardness, excellent gloss, and suppressed misting. Therefore, it is particularly suitable for coating objects containing both metal and resin components.

[0386] This application claims priority based on Japanese Patent Application No. 2023-013411 filed in Japan on January 31, 2023, and priority based on International Application No. PCT / JP / 2023 / 035702 filed on September 29, 2023, the contents of which are incorporated herein by reference in their entirety.

Claims

1. A water-based multi-liquid coating composition comprising: a first liquid containing a hydroxyl-containing resin (A) and a melamine resin (B), and a second liquid containing a hydrophilic polyisocyanate compound (D). The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2), The melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average value of the total number of imino groups and hydroxymethyl groups per triazine ring is greater than 1, The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a non-ion-modified polyisocyanate compound (D2).

2. A water-based multi-liquid coating composition comprising: a first liquid containing a hydroxyl-containing resin (A), a melamine resin (B) and a polyurethane resin (C); and a second liquid containing a hydrophilic polyisocyanate compound (D). The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2), The melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average value of the total number of imino groups and hydroxymethyl groups per triazine ring is greater than 1, The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a non-ion-modified polyisocyanate compound (D2).

3. The aqueous multi-liquid coating composition according to claim 1 or 2, wherein The ion-modified polyisocyanate compound (D1) contains at least an anion-modified polyisocyanate compound.

4. The aqueous multi-liquid coating composition according to claim 2, wherein The hydroxyl value of the polyurethane resin (C) is 30 mgKOH / g or less.

5. The aqueous multi-liquid coating composition according to any one of claims 1 to 4, wherein The acid value of the hydroxyl-containing resin (A) is 5 mgKOH / g or more and 70 mgKOH / g or less.

6. The aqueous multi-liquid coating composition according to any one of claims 1 to 5, wherein The solubility parameter of the melamine resin (B) is 9 or more and 15 or less.

7. The aqueous multi-liquid coating composition according to any one of claims 1 to 6, wherein The solid content of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less relative to 100 parts by mass of the solid content of the hydroxyl-containing resin (A).

8. The aqueous multi-liquid coating composition according to claim 2, wherein The solid content of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the total solid content of the hydroxyl-containing resin (A) and the polyurethane resin (C).

9. The aqueous multi-liquid coating composition according to any one of claims 1 to 8, wherein The equivalent ratio of the imino group and methylol group of the melamine resin (B) to the isocyanate group of the hydrophilic polyisocyanate compound (D) (imino group and methylol group / isocyanate group) is 0.2 or more and 1.1 or less.

10. The aqueous multi-liquid coating composition according to any one of claims 1 to 9, wherein The number average molecular weight of the melamine resin (B) is 300 or more and 3000 or less.

11. A method for producing a coated article, comprising: A step of applying the aqueous multi-component coating composition according to any one of claims 1 to 10 to a coating object to form an uncured first coating film, a step of applying a clear coating composition on the uncured first coating film to form an uncured clear coating film, and a step of heating the uncured first coating film and the uncured clear coating film at 70° C. or higher and 110° C. or lower to cure them.

12. A method for producing a coated article, comprising: A step of applying the aqueous multi-component coating composition according to any one of claims 1 to 10 to a coating object 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 at 70° C. or higher and 110° C. or lower.

13. The method for producing a coated article according to claim 11 or 12, wherein: The object to be coated includes a metal portion and a resin portion.

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