Water-based paint composition and method for producing coated object

Abstract

To provide a one-pack aqueous coating composition which is excellent in storage stability and water resistance, and can be cured at low temperature.SOLUTION: An aqueous coating composition is provided, including an aqueous resin (A) having a hydroxyl group and a carboy group, a hydrophilized modified carbodiimide compound (B), and a basic compound (C), wherein the glass transition point of the aqueous resin (A) is 0°C or higher and 20°C or lower, the hydrophilized modified carbodiimide compound (B) has at least a carbodiimide group in the molecule, and structure obtained by removing a hydroxyl group from polyalkylene glycol monoalkyl ether, a pH of the aqueous coating composition is 8.5 or more and less than 10, the aqueous coating composition includes no polyisocyanate curing agent, or includes less than 1 pts.mass of the polyisocyanate curing agent with respect to 100 pts.mass of the solid content of the aqueous coating composition.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] Conventionally, blocked isocyanates have been used as curing agents for one-component water-based paints from the viewpoint of the physical properties of the resulting coating film. However, paints containing blocked isocyanates may not have sufficient storage stability.

[0003] Patent Documents 1 and 2 propose using a carbodiimide compound as a curing agent for a one-liquid type water-based paint or water-based resin composition. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2021-143320 A [Patent Document 2] JP 2018-104605 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, the aqueous coating material of Patent Document 1 has poor storage stability. The aqueous resin composition of Patent Document 2 may have poor water resistance. Furthermore, the aqueous coating material of Patent Document 2 is used for disposable articles, and is not intended for long-term outdoor use. The present invention has been made in view of the above, and aims to provide a one-liquid type aqueous coating material composition that has excellent storage stability and can be cured at low temperature, and a method for producing a coated article that has excellent water resistance. [Means for solving the problem]

[0006] In order to solve the above problems, the present invention provides the following aspects. [1] A one-component aqueous coating composition, (A) an aqueous resin having a hydroxyl group and a carboxyl group; (B) a hydrophilically modified carbodiimide compound; (C) a basic compound, The (A) aqueous resin has a glass transition point of 0° C. or higher and 20° C. or lower, The (B) hydrophilically modified carbodiimide compound has at least one carbodiimide group and a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether in the molecule, The pH of the aqueous coating composition is 8.5 or more and less than 10, The aqueous coating composition does not contain a polyisocyanate curing agent or contains less than 1 part by mass of a polyisocyanate curing agent per 100 parts by mass of the solid content of the aqueous coating composition. [2] The aqueous coating composition according to the above [1], wherein the content of the (C) basic compound is 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. [3] The aqueous coating composition according to the above [1] or [2], wherein the boiling point of the basic compound (C) is 180° C. or lower. [4] The aqueous coating composition according to any one of the above [1] to [3], wherein the (C) basic compound comprises at least one selected from the group consisting of ammonia and amine compounds. [5] The aqueous coating composition according to any one of the above [1] to [4], wherein the (A) aqueous resin has a hydroxyl value of 20 mgKOH / g or more and 100 mgKOH / g or less, and an acid value of 5 mgKOH / g or more and 80 mgKOH / g or less, calculated on a resin solid content basis. [6] The aqueous coating composition according to any one of the above [1] to [5], wherein the ratio of the equivalent weight Ec of the carbodiimide group of the hydrophilically modified carbodiimide compound (B) to the equivalent weight Ea of the acid group of the aqueous resin (A) is 0.1 or more and 1.5 or less: Ec / Ea. [7] The aqueous coating composition according to any one of the above [1] to [6], wherein the content of the (A) aqueous resin is 30 parts by mass or more and 75 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. [8] The aqueous coating composition according to any one of the above [1] to [7], wherein the content of the (B) hydrophilically modified carbodiimide compound is 5 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. [9] The (B) hydrophilically modified carbodiimide compound is represented by the following general formula (α): -OCONH-XQY (α) (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Q is -NHCOO- or NHCONH-.) The aqueous coating composition according to any one of the above [1] to [8], which has one or more structural units represented by the following formula:

[10] The (B) hydrophilically modified carbodiimide compound is The following general formula (I): [ka] (In the formula, X's are each independently a bifunctional organic group containing at least one carbodiimide group, Y's are each independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed from the polyalkylene glycol monoalkyl ether, and Z's are each independently a bifunctional polyol having a number average molecular weight of 200 or more and 5,000 or less having a hydroxyl group removed from the polyol.) The following general formula (II): [ka] (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed, and R 0 is hydrogen, a methyl group, or an ethyl group, and R1 are each independently an alkylene group having 4 or less carbon atoms, n is 0 or 1, and m is 0 to 60; and The following general formula (III): [ka] (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, and each Y independently has a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether.) The aqueous coating composition according to any one of the above [1] to [9], which comprises at least one compound selected from the group consisting of compounds represented by the following formula:

[11] A step of applying the aqueous coating composition according to any one of the above [1] to

[10] to a substrate to form an uncured coating film; and heating the uncured coating film at a temperature of 75°C or higher and 100°C or lower to obtain a cured coating film. Effect of the Invention

[0007] According to the present invention, there are provided a one-liquid type water-based coating composition which has excellent storage stability and can be cured at low temperature, and a method for producing a coated article which has excellent water resistance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] [Water-based paint composition] In this embodiment, a carbodiimide compound is used as a curing agent for the aqueous resin, and a basic compound is used in combination with the carbodiimide compound. The basic compound neutralizes the carboxyl groups of the aqueous resin, thereby suppressing the reaction with the carbodiimide compound.

[0009] Furthermore, the pH of the aqueous coating composition is adjusted to 8.5 or more and less than 10. By keeping the pH at 8.5 or more, the reaction of the carboxyl group of the aqueous resin in the aqueous coating composition is suppressed, and the aqueous coating composition becomes stable for a long period of time (for example, one month or more).

[0010] By making the pH of the aqueous coating composition less than 10, the effect on the components contained in the aqueous coating composition is reduced, and the desired coating film can be obtained. For example, by making the pH less than 10, blackening of aluminum pieces that may be contained as a luster pigment is suppressed.

[0011] In addition, the glass transition point (Tg) of the aqueous resin is set to 0° C. or more and 20° C. or less. This makes it possible to improve the physical properties of the resulting coating film while ensuring low-temperature curing properties in a system using a carbodiimide compound as a curing agent. For example, the aqueous coating composition according to this embodiment can be cured at low temperatures (e.g., 100° C. or less), and the resulting coating film has excellent water resistance.

[0012] That is, the aqueous coating composition according to this embodiment is a one-liquid type, and contains (A) an aqueous resin having a hydroxyl group and a carboxyl group, (B) a hydrophilically modified carbodiimide compound, and (C) a basic compound. The glass transition point of (A) the aqueous resin is 0° C. or higher and 20° C. or lower. The pH of the aqueous coating composition is 8.5 or higher and lower than 10.

[0013] (B) The hydrophilically modified carbodiimide compound has, in the molecule, at least one carbodiimide group and a structure in which a hydroxyl group has been removed from a polyalkylene glycol monoalkyl ether.

[0014] The aqueous coating composition does not contain a polyisocyanate curing agent or contains less than 1 part by mass per 100 parts by mass of the solid content of the aqueous coating composition. The polyisocyanate curing agent can react with the hydroxyl group of the aqueous resin. Since such a curing agent is hardly contained, the storage stability is further improved.

[0015] The aqueous coating composition according to this embodiment can be used even after preparation, even if it is stored at room temperature (between 20°C and 25°C) for one month or more and about two months, and can form a coating film with excellent water resistance.

[0016] The pH of the aqueous paint composition may be 9.9 or less, or may be 9.8 or less. The pH of a typical aqueous paint is about 8.0. The pH is adjusted by using a basic compound.

[0017] The solid content concentration of the aqueous coating composition is not particularly limited, and may be, for example, 15% by mass or more and 60% by mass or less.

[0018] The aqueous coating composition according to this embodiment is suitable for forming a colored coating film. The colored coating film may be a so-called intermediate coating film having one or more layers, a so-called base coating film having one or more layers, or a combination thereof.

[0019] (A) Water-based resin having hydroxyl and carboxyl groups The aqueous resin is a resin that can be dissolved or dispersed in water. The resin used in the present embodiment has a carboxyl group, and therefore has water-solubility or water-dispersibility. (A) The aqueous resin may be water-dispersible.

[0020] The volume average particle diameter of the (A) aqueous resin dispersed in the aqueous medium is, for example, 0.01 μm or more and 1 μm or less. Within this range, the resin can be stably dispersed in water, and the appearance of the resulting coating film can be improved. The volume average particle diameter of the (A) aqueous resin is adjusted by the composition of the raw material monomers and / or the polymerization conditions.

[0021] The carboxyl group of the water-based resin (A) reacts with the hydrophilic modified carbodiimide compound (B) to form a crosslinked structure, which improves the coating film performance (especially water resistance).

[0022] The Tg of the (A) water-based resin is 0°C or higher and 20°C or lower. If the Tg of the (A) water-based resin is too low, components with small molecular weights such as water will easily penetrate the coating film, and water resistance will decrease. Furthermore, the strength of the resulting coating film will decrease, and it will be difficult to mold the coating film. If the (A) water-based resin has an excessively high Tg, the volume shrinkage during the cooling process after curing will increase, and internal stress will occur. The internal stress will reduce the adhesion of the coating film. In particular, if the (A) water-based resin has a Tg higher than the curing temperature, the stress will be difficult to relieve, which may cause the coating film to crack. Therefore, in a low-temperature curing aqueous coating composition, it is important to set the Tg of the coating film-forming resin. If the (A) water-based resin has a Tg of 0°C or higher and 20°C or lower, a coating film that can be cured at low temperature and has excellent water resistance can be obtained.

[0023] The Tg of the (A) aqueous resin may be 3° C. or higher, or may be 5° C. or higher. The Tg of the (A) aqueous resin may be 18° C. or lower, or may be 15° C. or lower.

[0024] The Tg of the (A) aqueous resin can be calculated from the types and amounts of the raw material monomers. The Tg of the (A) aqueous resin can also be measured by a differential scanning calorimeter (DSC).

[0025] The (A) aqueous resin has, for example, a hydroxyl value of 20 mgKOH / g or more and 100 mgKOH / g or less in terms of resin solid content. This is to provide adhesion to the substrate. The hydroxyl value may be 30 mgKOH / g or more and 80 mgKOH / g or less.

[0026] The (A) aqueous resin has, for example, an acid value of 5 mgKOH / g or more and 80 mgKOH / g or less in terms of resin solid content. This can improve the reaction efficiency with the (B) hydrophilic modified carbodiimide compound, and also makes it easier to improve storage stability. The acid value may be 10 mgKOH / g or more and 60 mgKOH / g or less.

[0027] The number average molecular weight of the (A) aqueous resin is, for example, 50,000 or more. The number average molecular weight of the (A) aqueous resin may be 200,000 or more, or may be 300,000 or more.

[0028] Number average molecular weight is measured by gel permeation chromatography (GPC) using polystyrene standard sample standards.

[0029] The content of the (A) aqueous resin is, for example, 30 parts by mass or more and 75 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. Since the aqueous resin with a relatively large molecular weight accounts for a large proportion of the solid content, the migration of the constituent components during the formation of the coating film is easily suppressed. As a result, the formation of unevenness is suppressed, and the design can be improved.

[0030] The content of the (A) aqueous resin may be 40 parts by mass or more, or 50 parts by mass or more, per 100 parts by mass of the solid content of the aqueous coating composition. The content of the (A) aqueous resin may be 95 parts by mass or less, or 90 parts by mass or less, per 100 parts by mass of the solid content of the aqueous coating composition.

[0031] (A) The total content of components having a number average molecular weight of 100,000 or more, such as an aqueous resin (hereinafter referred to as high molecular weight components), may be 60 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

[0032] From the viewpoint of design, it is desirable to have a small content of components with low molecular weights. For example, the content of components having a number average molecular weight of 1,000 or less (hereinafter referred to as low molecular weight components) may be 30 parts by mass or less, 20 parts by mass or less, or 10 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

[0033] The type of (A) aqueous resin is not particularly limited. (A) aqueous resin may be a single resin or a mixture of different resins. (A) aqueous resin may be at least one of an acrylic resin and a polyester resin having a hydroxyl group and a carboxyl group, in terms of ease of production or availability. (A) aqueous resin may contain at least the acrylic resin, in terms of ease of adjustment of coating film properties.

[0034] When the aqueous coating composition is for undercoating, the (A) aqueous resin is, for example, a mixture of the above-mentioned acrylic resin and polyester resin. The ratio of the acrylic resin and the polyester resin may be acrylic resin / polyester resin=5 / 1 to 1 / 1. When the aqueous coating composition is for base coating, the (A) aqueous resin is, for example, the above-mentioned acrylic resin.

[0035] (Acrylic resin having hydroxyl and carboxyl groups) An acrylic resin having a hydroxyl group and a carboxyl group can be obtained, for example, by acrylic copolymerization of an α,β-ethylenically unsaturated monomer having a hydroxyl group (hereinafter sometimes referred to as a first monomer) and an α,β-ethylenically unsaturated monomer having a carboxyl group (hereinafter sometimes referred to as a second monomer).

[0036] Examples of the first monomer include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, allyl alcohol, methacrylic alcohol, and an adduct of hydroxyethyl (meth)acrylate and ε-caprolactone. The first monomer may be at least one selected from the group consisting of 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and an adduct of 2-hydroxyethyl (meth)acrylate and ε-caprolactone.

[0037] "(Meth)acrylic" means both acrylic and methacrylic.

[0038] Examples of the second monomer include acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, maleic acid, fumaric acid, and itaconic acid. The second monomer may be acrylic acid and / or methacrylic acid.

[0039] In the acrylic copolymerization, other α,β-ethylenically unsaturated monomers (hereinafter, sometimes referred to as third monomers) may be used as necessary. Examples of the third monomer include (meth)acrylic acid esters (e.g., methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl methacrylate, phenyl acrylate, isobornyl (meth)acrylate, cyclohexyl methacrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, dihydrodicyclopentadienyl (meth)acrylate, etc.), and polymerizable amide compounds (e.g., (meth)acrylamide, N-methylol (meth)acrylamide, N-butoxymethyl (meth)acrylamide, etc.).

[0040] The method for preparing the (A) aqueous resin is not particularly limited. For example, the (A) aqueous resin can be prepared by a method in which a polymer is obtained by solution polymerization and then made aqueous, or by a method in which an emulsion is obtained by emulsion polymerization in an aqueous medium.

[0041] <Solution polymerization> The solution polymerization is carried out, for example, by dropping a mixture of the first and second monomers together with a polymerization initiator into a stirred solvent under heating conditions. The solution polymerization is carried out, for example, under conditions of a temperature of 60° C. to 160° C. and a dropping time of 0.5 to 10 hours. The first and second monomers may be polymerized in two stages.

[0042] The polymerization initiator is not particularly limited as long as it is used in normal polymerization. For example, azo compounds and peroxides can be used. The amount of the polymerization initiator relative to 100 parts by mass of the monomer mixture is, for example, 0.1 parts by mass or more and 18 parts by mass or less, and may be 0.3 parts by mass or more and 12 parts by mass or less.

[0043] The solvent is not particularly limited as long as it does not adversely affect the reaction. Examples of the solvent include alcohols, ketones, ethers, and hydrocarbon solvents. Furthermore, a chain transfer agent may be added to adjust the molecular weight. Examples of the chain transfer agent include mercaptans such as lauryl mercaptan and α-methylstyrene dimer.

[0044] The acrylic resin obtained by solution polymerization is neutralized by a basic compound. This results in a water-dispersible acrylic resin. As the basic compound, those described below can be used. Among them, the basic compound for water dispersion may be ammonia, methylamine, ethylamine, dimethylamine, diethylamine, trimethylamine, triethylamine, dimethylethanolamine, diethanolamine, diethylaminoethanol, or triethanolamine. The basic compound is added in an amount such that the neutralization rate of the carboxyl group of the acrylic resin obtained by solution polymerization is 60% or more and 100% or less. When the neutralization rate is within this range, the water dispersibility is improved.

[0045] The neutralization rate is equivalent to the amount of the basic compound used relative to the carboxyl group. The amount of the basic compound used is calculated according to the following formula: TIFF0007628072000004.tif12150

[0046] The neutralization of the carboxyl groups is a treatment that is usually carried out to make a carboxyl group-containing resin water-soluble. However, the water-based resin neutralized in this manner is used in an aqueous paint composition having a pH of about 8.0. When the pH is about 8.0, the carboxyl groups are neutralized (e.g., -COO - NH 4 +) and it is not expected to improve storage stability.

[0047] <Emulsion polymerization> The emulsion polymerization is carried out, for example, by dissolving an emulsifier in an aqueous medium containing water or, if necessary, an organic solvent such as alcohol under heating conditions, and dropping a mixture of the first and second monomers together with a polymerization initiator while heating and stirring the mixture. The monomer mixture may be emulsified in advance using an emulsifier and water.

[0048] When emulsion polymerization is performed, a crosslinkable monomer can be used as the third monomer. The crosslinkable monomer has two or more radically polymerizable ethylenically unsaturated groups in the molecule. Examples of the crosslinkable monomer include divinylbenzene, allyl (meth)acrylate, and ethylene glycol di(meth)acrylate.

[0049] Examples of the polymerization initiator include azo-based oily compounds such as azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), and 2,2'-azobis(2,4-dimethylvaleronitrile); aqueous compounds such as anionic 4,4'-azobis(4-cyanovaleric acid), 2,2-azobis(N-(2-carboxyethyl)-2-methylpropionamidine, and cationic 2,2'-azobis(2-methylpropionamidine); redox-based oily peroxides such as benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide, and t-butyl perbenzoate; and aqueous peroxides such as potassium persulfate and ammonium persulfate.

[0050] The emulsifier may be a general emulsifier that is commonly used by those skilled in the art. Examples of the emulsifier include reactive emulsifiers such as Antox MS-60 (manufactured by Nippon Nyukazai Co., Ltd.), Eleminol JS-2 (manufactured by Sanyo Chemical Industries Co., Ltd.), Adeka Reasoap NE-20 (manufactured by Asahi Denka Co., Ltd.), Aqualon HS-10 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and Latemul PD-104 (manufactured by Kao Corporation). In addition, the chain transfer agent may be added to adjust the molecular weight.

[0051] The reaction temperature is set according to the polymerization initiator. For example, when an azo-based initiator or a peroxide is used, the reaction temperature may be 60°C to 90°C, and when a redox-based initiator is used, the reaction temperature may be 30°C to 70°C. The reaction time is, for example, 1 hour to 8 hours. The amount of the polymerization initiator may be 0.1 parts by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the monomer mixture. The emulsion polymerization may be carried out in multiple stages or in two stages. For example, a part of the monomer mixture may be emulsion-polymerized, and the remainder of the monomer mixture may be added thereto, followed by further emulsion polymerization.

[0052] (Polyester resin having hydroxyl and carboxyl groups) The polyester resin having hydroxyl groups and carboxyl groups can be obtained, for example, by a condensation reaction between a polyhydric alcohol component and a polybasic acid component. The obtained polyester resin is made aqueous by neutralizing it with the above-mentioned basic compound.

[0053] Examples of polyhydric alcohol components include ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, hydroxypivalic acid neopentyl glycol ester, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethylpentanediol.

[0054] Examples of the polybasic acid component include aromatic polycarboxylic acids such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrachlorophthalic anhydride, and pyromellitic anhydride; acid anhydrides thereof; alicyclic polycarboxylic acids such as hexahydrophthalic anhydride, tetrahydrophthalic anhydride, and 1,4- and 1,3-cyclohexanedicarboxylic acid; anhydrides thereof; aliphatic polycarboxylic acids such as maleic anhydride, fumaric acid, succinic anhydride, adipic acid, and sebacic acid; and anhydrides thereof.

[0055] If necessary, a monobasic acid such as benzoic acid or t-butylbenzoic acid may be used in combination. As reaction components, monohydric alcohols, monoepoxide compounds such as Cardura E (trade name: manufactured by Shell Chemical Industry Co., Ltd.), and lactones such as β-propiolactone, dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone, and γ-caprolactone may be used in combination.

[0056] Furthermore, fatty acids such as castor oil, dehydrated castor oil, and mixtures of one or more of these fatty acids may be added to the reaction system.

[0057] The polyester resin may be grafted with an acrylic resin and / or a vinyl resin, and may be reacted with a polyisocyanate compound.

[0058] (Urethane resin containing hydroxyl and carboxyl groups) As the (A) aqueous resin, an aqueous polyurethane resin may be used together with the acrylic resin and / or polyester resin. The aqueous polyurethane resin also has a hydroxyl group and a carboxyl group.

[0059] The content of the aqueous polyurethane resin is appropriately set according to the type of coating film. When the aqueous coating composition is for undercoat, the content of the aqueous polyurethane resin may be 30 parts by mass or more and 40 parts by mass or less with respect to 100 parts by mass of the solid content of the aqueous coating composition. When the aqueous coating composition is for base, the content of the aqueous polyurethane resin may be 15 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the solid content of the aqueous coating composition.

[0060] (Other resins) The aqueous coating composition may contain other resins that do not have either or both of a hydroxyl group and a carboxyl group, such as polyether diol, polycarbonate diol, or other resins that have a hydroxyl group, a melamine resin that has a methylol group, or a resin that has a phosphoric acid group.

[0061] The content of the other resin is appropriately set according to the type of coating film. When the aqueous coating composition is for a base coating, the content of the other resin may be 15 parts by mass or more and 45 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

[0062] However, it is desirable to have a small content of resins having only hydroxyl groups, since such resins do not contribute to curing and tend to deteriorate the physical properties of the coating film.

[0063] (B) Hydrophilic modified carbodiimide compound (B) The hydrophilically modified carbodiimide compound has, in the molecule, at least one carbodiimide group and a structure in which a hydroxyl group has been removed from a polyalkylene glycol monoalkyl ether.

[0064] (B) The hydrophilically modified carbodiimide compound may, for example, have the following formula (α) in the molecule: -OCONH-XQY (α) (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Q is -NHCOO- or NHCONH-.) The structural unit has a structure represented by the following formula (hereinafter, referred to as structural unit α).

[0065] The structural unit α improves water dispersibility and curability. One or more structural units α may be present in one molecule. The number of structural units may be one, two, or three in one molecule.

[0066] X is represented, for example, by the following general formula (a). [ka]

[0067] In the formula, R 2 R is a hydrocarbon group having 6 to 15 carbon atoms. 2 Examples of R include a phenylene group, a diphenylenemethyl group, a diphenylene(dimethyl)methyl group, a methylphenylene group, a dimethylphenylene group, a tetramethylxylylene group, a hexylene group, a cyclohexylene group, and a dicyclohexylenemethyl group. 2 may be a dicyclohexylenemethyl group.

[0068] p is an integer of 1 to 10. p is the number of carbodiimide groups present in the structural unit. From the viewpoint of curability, p may be 2 or more and 8 or less.

[0069] The number of repetitions (eg, p above) is an average value.

[0070] Y is represented, for example, by the following general formula (b) or (c). [ka]

[0071] In the formula, R 3 R is an alkyl group having 1 to 20 carbon atoms. 3 Examples of R include a methyl group, an ethyl group, a butyl group, a hexyl group, an octyl group, a decyl group, a dodecyl group, and a stearyl group. 3 may be a methyl group.

[0072] R 4 is a hydrogen atom or a methyl group. From the viewpoint of water dispersibility, R 4 is a hydrogen atom

[0073] It's okay. R 4 When is hydrogen, general formulas (b) and (c) show the same structure. q is 4 to 40. From the viewpoints of water dispersibility and reactivity after water volatilization, q may be 4 to 20, or may be 6 to 12.

[0074] The ratio Ec / Ea of the equivalent weight Ec of the carbodiimide group of the hydrophilic modified carbodiimide compound (B) to the equivalent weight Ea of the acid group of the aqueous resin (A) is, for example, 0.1 or more and 1.5 or less. When the equivalent ratio is in this range, the crosslinking density is high and the water resistance can be further improved. The ratio Ec / Ea may be 0.5 or more, or 0.8 or more. The ratio Ec / Ea may be 1.4 or less, or 1.2 or less.

[0075] The content of the hydrophilic modified carbodiimide compound (B) is, for example, 5 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. When the content of the hydrophilic modified carbodiimide compound (B) is in this range, the water resistance of the coating film can be further improved, and shrinkage due to water can be easily suppressed. The content may be 7 parts by mass or more, or 8 parts by mass or more, per 100 parts by mass of the solid content of the aqueous coating composition. The content may be 14 parts by mass or less, or 13 parts by mass or less, per 100 parts by mass of the solid content of the aqueous coating composition.

[0076] (Hydrophilic modified carbodiimide compound (I)) The hydrophilically modified carbodiimide compound (B) having two structural units α is represented, for example, by the following general formula (I). [ka]

[0077] In the formula, X and Y are defined as above in formula (α). Z has a structure in which a hydroxyl group is removed from a bifunctional polyol having a number average molecular weight of 200 to 5000. Z constitutes a polymer having an ether bond, an ester bond, or a carbonate bond, and is difficult to generalize. For details of Z, see the synthesis method described below.

[0078] <Synthesis method> First, a raw material carbodiimide compound containing at least two isocyanate groups per molecule is reacted with a bifunctional polyol having hydroxyl groups at the molecular end and a number average molecular weight of 200 to 5,000 inclusive, in such a ratio that the molar amount of the isocyanate groups in the raw material carbodiimide compound exceeds the molar amount of the hydroxyl groups in the polyol. Next, the resulting reaction product is reacted with a polyalkylene glycol monoalkyl ether. This produces a hydrophilic modified carbodiimide compound (B) represented by formula (I).

[0079] From the viewpoint of reactivity, the raw material carbodiimide compound preferably has isocyanate groups at both ends. Methods for producing such raw material carbodiimide compounds are well known to those skilled in the art, and can utilize, for example, a condensation reaction of an organic diisocyanate accompanied by decarbonation.

[0080] Examples of organic diisocyanates include aromatic diisocyanates, aliphatic diisocyanates, alicyclic diisocyanates, and mixtures thereof. Specific examples include 1,5-naphthylene diisocyanate, 4,4-diphenylmethane diisocyanate, 4,4-diphenyldimethylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, hexamethylene diisocyanate, cyclohexane-1,4-diisocyanate, xylylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4-diisocyanate, methylcyclohexane diisocyanate, and tetramethylxylylene diisocyanate. From the viewpoint of reactivity, it may be dicyclohexylmethane-4,4-diisocyanate.

[0081] A carbodiimidization catalyst is usually used for the condensation reaction. Examples of the carbodiimidization catalyst include phospholene oxides such as 1-phenyl-2-phospholene-1-oxide, 3-methyl-2-phospholene-1-oxide, 1-ethyl-2-phospholene-1-oxide, 3-methyl-1-phenyl-2-phospholene-1-oxide, and 3-phospholene isomers thereof. From the viewpoint of reactivity, 3-methyl-1-phenyl-2-phospholene-1-oxide may be used.

[0082] Examples of the bifunctional polyol include polyether diol, polyester diol, and polycarbonate diol.Specific examples include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol, polyester diols such as polyethylene adipate, polybutylene adipate, polyhexamethylene adipate, polyneopentyl adipate, poly-3-methylpentyl adipate, polyethylene / butylene adipate, and polyneopentyl / hexyl adipate, polycaprolactone diol, polylactone diol such as poly-3-methylvalerolactone diol, polycarbonate diol such as polyhexamethylene carbonate diol, and mixtures thereof.

[0083] The molar ratio of the isocyanate group of the raw material carbodiimide compound to the hydroxyl group of the bifunctional polyol may be 1.0:1.1 to 1.0:2.0 from the viewpoints of reaction efficiency and economic efficiency. The polymerization degree of the raw material carbodiimide compound and the bifunctional polyol may be 1 to 10 from the viewpoints of reaction efficiency.

[0084] The reaction product thus obtained is further reacted with polyalkylene glycol monoalkyl ether (hereinafter referred to as PAGAE), thereby obtaining a hydrophilic modified carbodiimide compound (B) having two structural units α.

[0085] PAGAE is, for example, represented by the following general formula (b') or (c'). [ka] (In the formula, R 3 , R 4 , and q are as defined above.)

[0086] The number average molecular weight of PAGAE may be 200 or more and 5,000 or less. The number of carbon atoms in the alkyl group of PAGAE may be 1 to 20. Examples of PAGAE include polyethylene glycol, polypropylene glycol, and mixtures thereof, each of which is end-blocked with an alkyl group having 1 to 20 carbon atoms. Specific examples of PAGAE include polyethylene glycol monomethyl ether, polyethylene glycol mono-2-ethylhexyl ether, polyethylene glycol monolauryl ether, polypropylene glycol monomethyl ether, polypropylene glycol mono-2-ethylhexyl ether, and polypropylene glycol monolauryl ether, each of which has a number average molecular weight of 200 to 5,000.

[0087] The reaction of the reaction product with PAGAE is carried out in such a ratio that the molar amount of isocyanate groups in the reaction product is equal to or exceeds the molar amount of hydroxyl groups in PAGAE. The molar amount of isocyanate groups is determined by direct measurement. The molar amount of isocyanate groups can also be calculated from the amount of charge.

[0088] A catalyst may be used during the reaction. The reaction temperature is not particularly limited, but may be 60° C. or higher and 120° C. or lower from the viewpoint of reaction system control and reaction efficiency. The solvent may be an organic solvent that does not contain active hydrogen.

[0089] The hydrophilically modified carbodiimide compound (B) produced through such a two-step reaction may be a mixture of the compound represented by general formula (I) and other reaction products, but these mixtures may also be considered as the hydrophilically modified carbodiimide compound (B) having the structure of general formula (I).

[0090] (Hydrophilic modified carbodiimide compound (II)) The hydrophilic modified carbodiimide compound (B) having three structural units α is represented, for example, by the following general formula (II). [ka]

[0091] In the formula, X and Y are defined as above in formula (α). R 0 is hydrogen, a methyl group, or an ethyl group. 1 are each independently an alkylene group having 4 or less carbon atoms. Specific examples include a methylene group, an ethylene group, a propylene group, and a butylene group. n is 0 or 1. Each m is independently an integer of 0 to 60.

[0092] R 0 , R 1 , n and m are determined depending on the trifunctional polyol used in producing the hydrophilically modified carbodiimide compound (B).

[0093] When m is 11 or more, the ratio of the hydrophilic portion to the hydrophobic portion is preferably 2.0 to 6.3. The above ratio is determined by dividing the molecular weight of the oxymethylene group or oxyethylene group portion present in the carbodiimide compound by the molecular weight of the carbodiimide compound.

[0094] <Synthesis method> First, the raw material carbodiimide compound is reacted with the PAGAE in such a ratio that the equivalent of the isocyanate group of the raw material carbodiimide compound exceeds the equivalent of the hydroxyl group of the PAGAE. Then, the resulting reaction product is reacted with a trifunctional polyol. This results in a hydrophilic modified carbodiimide compound (B) represented by general formula (II). The equivalent ratio of the isocyanate group to the hydroxyl group may be 2 / 1.

[0095] The amount of trifunctional polyol may be an amount that results in a hydroxyl equivalent equal to or greater than the isocyanate equivalent of the reactant, and the isocyanate equivalent and the hydroxyl equivalent may be equal. The isocyanate equivalent of the reaction product is determined by direct measurement. The molar amount of isocyanate groups is also calculated from the amount charged.

[0096] The reaction can be carried out under conditions well known to those skilled in the art, and a tin catalyst can be used as necessary. The reaction is carried out in the same manner as in the case of the hydrophilic modified carbodiimide compound (B) represented by the general formula (I).

[0097] The trifunctional polyol may be trimethylolpropane, glycerin, or an alkylene oxide adduct thereof, because of its easy availability. Examples of the alkylene oxide include ethylene oxide and propylene oxide. The alkylene oxide adduct of glycerin is commercially available as the GP series from Sanyo Chemical Industries, Ltd. Considering the curing reactivity of the resulting hydrophilic modified carbodiimide compound, the trifunctional polyol may have a structure in which an alkylene oxide is added to each hydroxyl group. Among the above GP series, for example, GP-250 and GP-3000 have such a structure.

[0098] The hydrophilically modified carbodiimide compound (B) produced through such a two-step reaction may be a mixture of the compound represented by general formula (II) and other reaction products, but these mixtures may also be considered as the hydrophilically modified carbodiimide compound (B) having the structure of general formula (II).

[0099] (Hydrophilic modified carbodiimide compound (III)) The hydrophilically modified carbodiimide compound (B) having one structural unit α is represented, for example, by the following general formula (III). [ka] (In the formula, X and Y are defined as in the above formula (α).)

[0100] Each Y may independently have a structure selected from the following (i) or (ii), which can improve water dispersibility and stability and also facilitate high crosslink density.

[0101] (i) A structure in which a hydroxyl group is removed from a polyethylene glycol monoalkyl ether in which an alkyl group having 1 to 3 carbon atoms is ether-bonded to the end of a polyethylene oxide unit having a repeat number of 6 to 20. (ii) A structure in which a hydroxyl group is removed from a polypropylene glycol monoalkyl ether in which an alkyl group having 1 to 8 carbon atoms is ether-bonded to the end of a polypropylene oxide unit having a repeat number of 4 to 60.

[0102] In particular, Y may have the structure (ii), and the number of repetitions of the polypropylene oxide unit may be 15 to 60.

[0103] One Y may have the structure (i) and the other Y may have the structure (ii). In this case, the ratio of the number of Y having the structure (i) to the number of Y having the structure (ii) may be (i):(ii)=1:0.7 to 1:8.

[0104] From the viewpoint of suppressing the deactivation of carbodiimide by water and improving the water resistance of the coating film, it is desirable that the periphery of the carbodiimide group is hydrophobic to some extent and has low contact with water molecules. On the other hand, the carbodiimide compound itself is required to have hydrophilicity. When (i):(ii) is within the above range, the hydrophobicity and the hydrophilicity are easily balanced. (i):(ii) may be 1:0.7 to 1:1.5.

[0105] <Synthesis method> The hydrophilically modified carbodiimide compound (B) represented by the general formula (III) can be obtained by reacting the above starting carbodiimide compound with the same or a different type of PAGAE.

[0106] The PAGAE may be at least one selected from the group consisting of the following (i') and (ii'): (i') A polyethylene glycol monoalkyl ether in which an alkyl group having 1 to 3 carbon atoms is ether-bonded to the end of a polyethylene oxide unit having a repeating number of 6 to 20. (ii') Polypropylene glycol monoalkyl ether in which an alkyl group having 1 to 8 carbon atoms is ether-bonded to the end of a polypropylene oxide unit having a repeating number of 4 to 60.

[0107] Specific examples of (i') include polyethylene glycol monomethyl ether, polyethylene glycol monoethyl ether, and polyethylene glycol monopropyl ether. In particular, it may be polyethylene glycol monomethyl ether. Specific examples of (ii') include polypropylene glycol monomethyl ether, polypropylene glycol monoethyl ether, polypropylene glycol monobutyl ether, and polypropylene glycol 2-ethylhexyl ether. In particular, polypropylene glycol monobutyl ether may be used.

[0108] In terms of crosslink density, the (B) hydrophilically modified carbodiimide compound may be represented by general formula (III).

[0109] (Other hardeners) The aqueous coating composition may contain other curing agents in addition to the hydrophilically modified carbodiimide compound (B). Examples of other curing agents include polyisocyanate curing agents (including blocked polyisocyanate curing agents), amino resins, epoxy compounds, aziridine compounds, and oxazoline compounds.

[0110] However, the content of the polyisocyanate curing agent is less than 1 part by mass, and may be 0 parts by mass, per 100 parts by mass of the solid content of the aqueous coating composition. This is because the polyisocyanate curing agent reduces storage stability. As mentioned above, from the viewpoint of design, it is desirable that the content of the curing agent corresponding to the low molecular weight component is small.

[0111] (C) Basic compounds A basic compound is a compound that acts as a base according to the Bronsted-Lowry definition. A basic compound is a compound that releases a proton (H + )

[0112] In the aqueous coating composition, the basic compound neutralizes the carboxyl groups of the aqueous resin (A), thereby suppressing the reaction between the aqueous resin (A) and the carbodiimide compound (B), thereby improving the storage stability and pot life.

[0113] The content of the basic compound is not particularly limited as long as the pH of the aqueous coating composition is in the range of 8.5 or more and less than 10. The content of the basic compound may be, for example, 1.0 parts by mass or more, 1.2 parts by mass or more, or 1.4 parts by mass or more relative to 100 parts by mass of the solid content of the aqueous coating composition. The content of the (C) basic compound may be, for example, 10.0 parts by mass or less, 7.0 parts by mass or less, or 5.0 parts by mass or less relative to 100 parts by mass of the solid content of the aqueous coating composition.

[0114] After the aqueous coating composition is applied to the substrate, the basic compound is required to volatilize. The volatilization of the basic compound allows the carboxyl group of the aqueous resin to react with the carbodiimide compound. The boiling point of the basic compound may be such that it can be volatilized by the heat treatment performed after coating. The boiling point of the basic compound may be, for example, 180°C or lower, 170°C or lower, or 160°C or lower. The boiling point of the basic compound may be such that it does not volatilize during storage of the aqueous coating composition. The boiling point of the basic compound may be, for example, 60°C or higher, 70°C or higher, or 80°C or higher.

[0115] The pKa (acid dissociation constant) of the basic compound is, for example, 5.0 or more. This makes it easier to suppress the reaction between the carbodiimide compound and the aqueous resin. The pKa of the basic compound may be 7.0 or more, and may be greater than 8.5. The pKa of the basic compound is, for example, 30.0 or less. This makes it easier to improve the water resistance of the resulting coating film. The pKa of the basic compound may be 20.0 or less, and may be 15.0 or less. In one embodiment, the pKa of the basic compound is 5.0 or more and 30.0 or less. The pKa is a value when water is used as a solvent at 25°C.

[0116] The basic compound is not particularly limited, and includes at least one selected from the group consisting of ammonia (pKa 9.25° C.) and amine compounds. Examples of the amine compounds include dimethylamine (pKa 10.64), trimethylamine (pKa 9.76), ethylamine (pKa 10.63), diethylamine (pKa 10.98), triethylamine (pKa 10.7), n-propylamine (pKa 10.53), isopropylamine (pKa 10.63), triallylamine (pKa 8.3, boiling point 156° C.), triethylenediamine (pKa 8.7, boiling point 174° C.), N,N-dimethylethanolamine (boiling point 135° C.), N,N-diethylethanolamine, aminoethanolamine, N -methyl-N,N-diethanolamine, iminobispropylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, methylaminopropylamine, methyliminobispropylamine, 3-methoxypropylamine, monoethanolamine (pKa 9.5), diethanolamine, triethanolamine, morpholine (pKa 8.4, boiling point 129°C), allylmorpholine (pKa 7.1, boiling point 158°C), N-methylmorpholine (pKa 7.4, boiling point 116°C), and N-ethylmorpholine (pKa 7.7, boiling point 139°C). The basic compound is preferably N,N-dimethylethanolamine.

[0117] (others) The aqueous coating composition may contain, as necessary, for example, a diluent, a pigment, a curing catalyst, a surface conditioner, a defoamer, a pigment dispersant, a plasticizer, a film-forming assistant, an ultraviolet absorber, an antioxidant, and a pH adjuster. The aqueous coating composition of the present invention can be prepared by mixing the components constituting the aqueous coating composition by a commonly used means.

[0118] [Manufacturing method for painted items] The aqueous coating composition according to the present embodiment can be cured at low temperature. The coated article is produced, for example, by a method including a step of applying the aqueous coating composition according to the present embodiment to a substrate to form an uncured coating film, and a step of heating the uncured coating film at a temperature of 75°C to 100°C to obtain a cured coating film. The obtained cured coating film has excellent water resistance.

[0119] When a colored coating film is formed using the aqueous coating composition according to this embodiment, the coated article is produced, for example, by a method comprising the steps of applying the aqueous coating composition according to this embodiment to an article to be coated to form an uncured colored coating film, heating the uncured colored coating film at a temperature of 75°C or higher and 100°C or lower to form a cured colored coating film, applying a clear coating composition onto the colored coating film to form an uncured clear coating film, and heating the uncured clear coating film to obtain a cured clear coating film.

[0120] When the clear coating film is formed, the color coating film may be cured or uncured. In particular, from the viewpoints of productivity, adhesion and water resistance, it is preferable to laminate each coating film without curing it (so-called wet-on-wet coating), and then simultaneously cure these multiple uncured coating films. That is, the coated object may be manufactured by a method including a step of coating the aqueous coating composition according to the present embodiment on the coated object to form an uncured color coating film, a step of coating the clear coating composition on the uncured color coating film to form an uncured clear coating film, and a step of curing the uncured color coating film and the uncured clear coating film.

[0121] Hereinafter, the case where the aqueous coating composition according to this embodiment is used to form a colored coating film and is applied wet-on-wet will be described, but the use of the aqueous coating composition according to this embodiment is not limited thereto.

[0122] (1) Process for forming an uncured colored coating film The aqueous coating composition according to the present embodiment is applied to a substrate. This forms an uncured colored coating film. This process may be repeated multiple times before applying the clear coating composition. This forms multiple uncured colored coating films.

[0123] The color coating film may be one layer or two or more laminated coating films. The color coating film may be one or more so-called intermediate coating films, one or more so-called base coating films, or a combination thereof. The intermediate coating film is typically formed adjacent to the substrate. The base coating film is typically formed on the intermediate coating film.

[0124] (subject to be coated) Examples of the substrate include steel plates made of metals such as iron, steel, stainless steel, aluminum, copper, zinc, and tin, and alloys thereof; polyethylene resin, EVA resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), vinyl chloride resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), polycarbonate resin, acrylic resin, acrylonitrile butadiene styrene (ABS) resin, acrylonitrile styrene (AS) resin, polyamide resin, acetal resin, phenol resin, fluororesin, melamine resin, urethane resin, epoxy resin, polyphenylene oxide (PPO), and other resins; and organic-inorganic hybrid materials. The substrate may be flat or have a three-dimensional shape.

[0125] The metal substrate may be previously subjected to a chemical conversion treatment and an electrodeposition coating. Examples of the chemical conversion treatment include zinc phosphate chemical conversion treatment, zirconium chemical conversion treatment, and chromium oxide conversion treatment. The electrodeposition coating may be cationic electrodeposition coating or anionic electrodeposition coating.

[0126] The resin substrate may be degreased in advance. After the degreasing, the resin substrate may be further coated with a primer paint. The primer paint is not particularly limited and may be appropriately selected depending on the type of paint to be applied thereon.

[0127] (Colored coating) The aqueous coating composition used to form a colored coating film contains a pigment, such as a color pigment, a luster pigment, and an extender pigment.

[0128] Examples of color pigments include organic color pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments; inorganic color pigments such as yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These may be used alone or in combination of two or more.

[0129] Examples of the luster pigment include metal flakes (aluminum, chromium, gold, silver, copper, brass, titanium, nickel, nickel chromium, stainless steel, etc.), metal oxide flakes, pearl pigments, glass flakes coated with metal or metal oxide, silica flakes coated with metal oxide, graphite, hologram pigments, and cholesteric liquid crystal polymers. These may be used alone or in combination of two or more.

[0130] Examples of extender pigments include calcium carbonate, barium sulfate, clay and talc. These may be used alone or in combination of two or more.

[0131] The concentration of all pigments, i.e., the mass ratio (PWC) of all pigments to 100% by mass of the resin solid content of the aqueous coating composition, is preferably 0.1% by mass or more and 50% by mass or less. This improves the smoothness of the resulting coating film. The PWC of each pigment is not particularly limited. The PWC of the luster pigment may be, for example, 1% by mass or more and 40% by mass or less. The PWC of the luster pigment is preferably 5% by mass or more. The PWC of the luster pigment is preferably 30% by mass or less.

[0132] The coating method is not particularly limited. Examples of coating methods include air spray coating, airless spray coating, rotary atomization coating, and curtain coat coating. These methods may be combined with electrostatic coating. Among them, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic coating, for example, a rotary atomization electrostatic coater commonly called "micro-microbell (μμbell)", "microbell (μbell)", "metallicbell (metabell)" or the like is used.

[0133] After the aqueous coating composition is applied, preliminary drying (also called preheating) may be performed. At this time, the basic compound may volatilize. In addition, bumping of the dilution components is suppressed, and the occurrence of popping is easily suppressed. Furthermore, mixing of the uncured colored coating film and the coating composition applied thereon is suppressed, and mixed phases are less likely to be formed. Therefore, the smoothness of the resulting coated article is easily improved.

[0134] The conditions for pre-drying are not particularly limited. Examples of pre-drying include a method of leaving the film at a temperature of 20° C. to 25° C. for 15 to 30 minutes, and a method of heating the film at a temperature of 50° C. to 100° C. for 30 seconds to 10 minutes.

[0135] The thickness of the intermediate coating film is not particularly limited. In terms of the smoothness and chipping resistance of the coated article, the thickness of each layer of the intermediate coating film after curing is preferably 5 μm or more and 40 μm or less. The thickness of each layer of the intermediate coating film after curing is more preferably 15 μm or more. The thickness of each layer of the intermediate coating film after curing is more preferably 30 μm or less.

[0136] The thickness of the base coating film is not particularly limited and is appropriately set according to purpose.The thickness of each layer of the base coating film after curing is, for example, 10 μm or more, may be 15 μm or more, or may be 20 μm or more.The thickness of each layer of the base coating film after curing is, for example, 50 μm or less, may be 45 μm or less, or may be 40 μm or less.

[0137] (2) Step to form uncured clear coating A clear coating composition is applied onto the uncured colored coating film, thereby forming an uncured clear coating film.

[0138] The clear coating composition is not particularly limited. The clear coating composition contains, for example, a hydroxyl-containing resin and a polyisocyanate curing agent. The hydroxyl value of the hydroxyl-containing resin may be 20 mgKOH / g or more and 200 mgKOH / g or less. The number average molecular weight of the hydroxyl-containing resin may be 1000 or more and 20000 or less. The polyisocyanate curing agent is not particularly limited, and examples thereof include aliphatic isocyanates, aliphatic cyclic isocyanates, aromatic isocyanates, alicyclic isocyanates, and their biuret and nurate polymers and mixtures.

[0139] As the clear coating composition, commercially available products can be used, such as Polyurexel O-1100 Clear and O-1200 Clear (manufactured by Nippon Paint Automotive Coatings Co., Ltd., isocyanate-cured clear coating composition).

[0140] The coating method is not particularly limited. For example, the coating method may be the same as the coating method of the above-mentioned aqueous coating composition. Among them, from the viewpoint of coating efficiency, rotary atomization electrostatic coating is preferred. After applying the clear coating composition, preliminary drying may be performed. The conditions of preliminary drying are not particularly limited and may be the same as the preliminary drying of the colored coating film.

[0141] (4) Curing process Each uncured coating is heated to a temperature of 75°C or more and 100°C or less. (C) The basic compound volatilizes in at least one step selected from the group consisting of pre-drying for the colored coating film, pre-drying for the clear coating film, and heating in the curing step. That is, the cation derived from the basic compound is removed from the neutralized carboxyl group of the aqueous resin, and the carboxyl group is regenerated. This causes the aqueous resin to react with the hydrophilic modified carbodiimide compound, and the coating composition is cured.

[0142] The curing temperature may be 90° C. or lower. However, the aqueous coating composition according to this embodiment may be cured at a higher temperature. For example, the curing temperature may be higher than 100° C. and lower than 120° C.

[0143] The heating time may be appropriately set according to the heating temperature. The heating time may be, for example, 5 minutes or more and 60 minutes or less, or 10 minutes or more and 45 minutes or less. The heating time means the time during which the inside of the heating device is kept at the target temperature, and does not take into account the time until the target temperature is reached. Examples of the heating device include a drying furnace that uses a heating source such as hot air, electricity, gas, or infrared rays. EXAMPLES

[0144] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto. In the examples, "parts" and "%" are by weight unless otherwise specified.

[0145] (Preparation of acrylic emulsion 1 having hydroxyl and carboxyl groups) A reaction vessel equipped with a stirrer, a nitrogen inlet tube, a temperature control device, a condenser, and a dropping funnel was charged with 607 parts of deionized water and 5 parts of LATEMULL PD-104 (Kao Corporation, 20% aqueous solution) as an emulsifier, and the temperature was raised to 80°C with stirring under a nitrogen atmosphere. Separately, 191 parts of styrene, 61 parts of methyl methacrylate, 179 parts of 2-ethylhexyl acrylate, 46 parts of hydroxyethyl acrylate, 8 parts of methacrylic acid, and 15 parts of allyl methacrylate were added to an emulsifier aqueous solution prepared by mixing 271 parts of deionized water and 45 parts of Latemul PD-104, and emulsified to obtain a pre-emulsion.

[0146] This pre-emulsion was added dropwise to a reaction vessel over a period of 2 hours together with an aqueous initiator solution in which 1.5 parts of ammonium persulfate was dissolved in 204 parts of deionized water. After completion of the addition, the reaction was continued for another 2 hours at 80° C. Then, the mixture was cooled and 0.8 parts of N,N-dimethylamino ether (corresponding to 10% of the acid value of the resin (neutralization rate 10%)) was added to obtain an acrylic emulsion 1 with a resin solid content of 30% by mass.

[0147] The hydroxyl value and acid value of Acrylic Emulsion 1 calculated based on the resin solid content, calculated from the monomer composition, were 40 mgKOH / g and 10 mgKOH / g, respectively.

[0148] (Production of hydrophilically modified carbodiimide compounds) 700 parts of 4,4-dicyclohexylmethane diisocyanate and 7 parts of 3-methyl-1-phenyl-2-phospholene-1-oxide were reacted at 170°C for 7 hours to obtain a carbodiimide compound having the structure represented by the above general formula (a), three carbodiimide groups per molecule, and isocyanate groups at both ends.

[0149] Next, 95 parts of PTMG-1000 (polytetramethylene glycol having a number average molecular weight of 1,000, manufactured by Mitsubishi Chemical Corporation, with 13.6 repeating units of tetramethylene oxide calculated from the number average molecular weight) and 0.2 parts of dibutyltin dilaurate were added to 180 parts of the produced carbodiimide compound, and the mixture was heated to 85° C. and maintained at this temperature for 2 hours.

[0150] Next, 86.4 parts of Methyl Polyglycol 130 (polyethylene glycol monomethyl ether manufactured by Nippon Nyukazai Co., Ltd., ethylene oxide repeat number 9 calculated from hydroxyl value 130 mg KOH / g) was added and maintained at 85°C for 3 hours. The reaction was terminated when it was confirmed by IR measurement that the NCO peak had disappeared, and the mixture was cooled to 60°C, and deionized water was added to obtain an aqueous dispersion of a hydrophilic modified carbodiimide compound with a resin solid content of 40 mass%. The obtained hydrophilic modified carbodiimide compound was a compound represented by the above general formula (I).

[0151] (Adjustment of color pigment paste) 9.2 parts of a commercially available dispersant "Disperbyk 190" (manufactured by BYK-Chemie), 17.8 parts of ion-exchanged water, and 73.0 parts of rutile-type titanium dioxide were premixed. Next, using a paint conditioner, the premix was mixed and dispersed at room temperature with a bead medium until the particle size became 5 μm or less, to obtain a color pigment paste.

[0152] [Example 1] (1) Preparation of Water-Based Coating Composition In a container with a stirrer, add acrylic emulsion. 1 (resin solid content 30%), 10 parts of hydrophilic modified carbodiimide compound (resin solid content 40%), 10 parts of Alpaste MH8801 (Asahi Kasei aluminum pigment, solid content 65%) as a glitter pigment (PWC 12%), 0.3 parts of lauryl acid phosphate were added, and 30 parts of 2-ethylhexanol, 2 parts of Adekanol UH-814N (ADEKA thickener, solid content 30%), 1 part of N,N-dimethylethanolamine (Kishida Chemical Co., Ltd.), and 50 parts of ion-exchanged water were uniformly dispersed to obtain an aqueous base coating composition. The pH of the aqueous base coating composition was 9.5.

[0153] (2) Formation of multi-layer coating A zinc phosphate-treated dull steel plate was electrocoated with Powernics 150 (product name, cationic electrocoating paint manufactured by Nippon Paint Automotive Coatings Co., Ltd.) so that the dry coating film was 20 μm thick. The plate was then heated to harden at 160°C for 30 minutes and then cooled to prepare a steel plate substrate. The above-mentioned water-based base coating material was applied to the obtained substrate by a rotary atomizing electrostatic coating device so that the dry film thickness was 15 μm, and then the substrate was preheated at 80° C. for 3 minutes.

[0154] Furthermore, a clear coating (Polyurexcel O-1200, manufactured by Nippon Paint Automotive Coatings Co., Ltd., a two-liquid acrylic urethane organic solvent-based clear coating containing a polyisocyanate compound) was applied to the coated plate using a rotary atomizing electrostatic coating device so that the dry film thickness was 35 μm. After that, the coating was heated and cured at 80°C for 20 minutes to obtain a coated object with a cured multi-layer coating film.

[0155] [Examples 2 to 4, Comparative Examples 1 to 5] An aqueous base coating composition was prepared and a coated article was obtained in the same manner as in Example 1, except that the composition of the aqueous base coating composition was changed to the components and amounts thereof shown in the table below.

[0156] [evaluation] The following evaluation tests were carried out on the water-based base coating composition and the coated object. The evaluation results are shown in Table 1.

[0157] (1) Storage stability After preparing the coating composition, it was left to stand for 10 days at 40° C. If a non-flowable lump was formed in the coating composition or if the viscosity of the entire coating composition was 0% or less or 150% or more of the initial viscosity, it was rated as "poor", and if not, it was rated as "good".

[0158] (2) Water resistance The coated object was immersed in 40°C warm water for 240 hours. The coated object was then removed and dried at 20°C for 24 hours. The multi-layer coating film of the coated object was then cut into a grid pattern with a cutter until it reached the base material, and divided into 100 grids measuring 2 mm x 2 mm. Next, adhesive cellophane tape was applied to the surface of the grid. (商標) The tape was then rapidly peeled off at 20°C. The number of squares where the multi-layer coating film had peeled off was counted. The lower this number, the better the water resistance. Good: No peeled squares Defective: 1 or more squares peeled off

[0159] [Table 1]

[0160] The aqueous coating compositions of the examples have excellent storage stability, and even when cured at a low temperature of 80°C, they can form coating films having excellent water resistance.

[0161] On the other hand, the aqueous coating composition of Comparative Example 1 does not contain a hydrophilic modified carbodiimide compound, and therefore has excellent storage stability. However, the Tg of the aqueous resin is less than 0°C, and therefore the water resistance of the multi-layer coating film is low. The aqueous coating composition of Comparative Example 2 has a pH of less than 8.5, and therefore has poor storage stability. Furthermore, the Tg of the aqueous resin is less than 0°C, and therefore the water resistance of the multi-layer coating film is also low. The aqueous coating composition of Comparative Example 3 has excellent storage stability, but the pH is high, and therefore the water resistance of the multi-layer coating film is low. The aqueous coating compositions of Comparative Examples 4 and 5 have a pH of 8.5 or more and less than 10, and therefore have excellent storage stability. On the other hand, the Tg of the aqueous resin is not in the range of 0 to 20°C, and therefore the water resistance of the multi-layer coating film is low. [Industrial Applicability]

[0162] The aqueous coating composition of the present invention has excellent storage stability and can be cured at low temperatures, and therefore can be used in a variety of applications.

Claims

1. A one-component aqueous coating composition, (A) an aqueous resin having a hydroxyl group and a carboxyl group; (B) a hydrophilically modified carbodiimide compound; (C) a basic compound, The glass transition point of the (A) aqueous resin is 0° C. or higher and 20° C. or lower, The hydrophilically modified carbodiimide compound (B) has at least one carbodiimide group and a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether in the molecule, The pH of the aqueous coating composition is 8.5 or more and less than 10, The aqueous coating composition does not contain a polyisocyanate curing agent or contains less than 1 part by mass of a polyisocyanate curing agent per 100 parts by mass of the solid content of the aqueous coating composition.

2. 2. The aqueous coating composition according to claim 1, wherein the content of the basic compound (C) is from 1 part by mass to 10 parts by mass per 100 parts by mass of the solid content of the aqueous coating composition.

3. 3. The aqueous coating composition according to claim 1, wherein the boiling point of the basic compound (C) is 180° C. or lower.

4. 4. The aqueous coating composition according to claim 1, wherein the basic compound (C) comprises at least one selected from the group consisting of ammonia and amine compounds.

5. The aqueous resin (A) has a hydroxyl value of 20 mgKOH / g or more and 100 mgKOH / g or less, and an acid value of 5 mgKOH / g or more and 80 mgKOH / g or less, calculated as a resin solid content. The aqueous coating composition according to any one of claims 1 to 4.

6. The aqueous coating composition according to any one of claims 1 to 5, wherein the ratio of the equivalent weight Ec of the carbodiimide group of the hydrophilically modified carbodiimide compound (B) to the equivalent weight Ea of the acid group of the aqueous resin (A): Ec / Ea is 0.1 or more and 1.5 or less.

7. The aqueous coating composition according to any one of claims 1 to 6, wherein the content of the aqueous resin (A) is 30 parts by mass or more and 75 parts by mass or less per 100 parts by mass of solids in the aqueous coating composition.

8. The aqueous coating composition according to any one of claims 1 to 7, wherein the content of the hydrophilically modified carbodiimide compound (B) is 5 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

9. The hydrophilically modified carbodiimide compound (B) is represented by the following general formula (α): -OCONH-X-Q-Y (α) (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Q is -NHCOO- or NHCONH-.) The aqueous coating composition according to any one of claims 1 to 8, which has one or more structural units represented by the following formula:

10. The hydrophilically modified carbodiimide compound (B) is The following general formula (I): 【Chemistry 1】 (In the formula, X's are each independently a bifunctional organic group containing at least one carbodiimide group, Y's are each independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed, and Z's are each independently a bifunctional polyol having a number average molecular weight of 200 or more and 5,000 or less having a hydroxyl group removed.) The following general formula (II): 【Chemistry 2】 (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed, and R ０ is hydrogen, a methyl group, or an ethyl group; R １ are each independently an alkylene group having 4 or less carbon atoms, n is 0 or 1, and m is 0 to 60; and The following general formula (III): 【Chemistry 3】 (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, and each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether.) The aqueous coating composition according to any one of claims 1 to 9, comprising at least one compound selected from the group consisting of compounds represented by the following formula:

11. A step of applying the aqueous coating composition according to any one of claims 1 to 10 to a substrate to form an uncured coating film; and heating the uncured coating film at a temperature of 75° C. or higher and 100° C. or lower to obtain a cured coating film.

Images