Water-based paint composition and method for forming a multi-layer coating film

Abstract

The purpose of the present invention is to provide an aqueous coating material composition which has excellent storage stability, and demonstrates strong glass adhesion properties and high coating film performance even when cured at a comparatively low temperature.　An aqueous coating material composition which contains (A) a hydroxyl group- and carboxyl group-containing resin, (B) a blocked polyisocyanate compound, (C) a polycarbodiimide compound and (D) a basic compound, wherein: the basic compound (D) contains (D1) a basic compound which has an acid dissociation constant (PKa) in the range of 7.0-8.5 and a boiling point in the range of 100-200°; the content ratio of the basic compound (D1) is within the range of 30-100 mass% of the mass of the basic compound (D); and the pH of the aqueous coating material composition is within the range of 8.0-11.5.

Description

Technical Field

[0001] The present invention relates to an aqueous paint composition and a method for forming a multilayer coating film.

Background Art

[0002] In automotive painting, after applying an electrocoating paint to an object to be painted, a multilayer coating film is formed by a three-coat two-bake (3C2B) method in which painting of an intermediate coat paint → baking and curing → painting of an aqueous base paint → preheating → painting of a clear paint → baking and curing are performed. However, in recent years, from the perspective of energy conservation, the baking and curing process after painting the intermediate coat paint is omitted, and after applying an electrocoating paint to the object to be painted, painting of an aqueous intermediate coat paint → preheating as necessary → painting of an aqueous base paint → preheating → painting of a clear paint → baking and curing are performed, and a three-coat one-bake (3C1B) method is becoming widespread.

[0003] More recently, in order to further reduce the energy used, it has been required to lower the heating temperature in the above baking and curing process.

[0004] However, a paint composition having high curability even when the heating temperature is relatively low generally has high reactivity, and for this reason, there are cases where both the coating film properties such as hardness, chip resistance, and water resistance and the storage stability are insufficient.

[0005] In addition, in the manufacturing specifications of automobiles, members such as glass members such as the front glass and the rear glass generally have an adhesive layer formed on the multilayer coating film and are fixed on the multilayer coating film through the adhesive layer. However, if an attempt is made to improve the chip resistance, for example, by softening the intermediate coating film, the multilayer coating film under the adhesive layer may cause cohesive failure or peeling may occur between the electrocoating film and the interface of the intermediate coating film, resulting in poor adhesion to the member.

[0006] Patent Document 1 describes a method for forming a multilayer coating film on an electrodeposited hot-dip galvanized steel sheet, comprising sequentially performing the following steps (1) to (4): Step (1): A step of applying a water-based intermediate coating paint (A) onto the electrodeposited coating to form an intermediate coating film. Step (2): After preheating, apply water-based paint (B) to the intermediate coating to form a base coating. Step (3): After preheating, apply clear paint (C) to the base coating to form a clear coating. Step (4): A step of heat-curing the intermediate coating film, base coating film and clear coating film formed in steps (1) to (3) above. The aforementioned water-based intermediate coating paint (A) contains a hydroxyl group-containing acrylic resin (a1) having a glass transition temperature (Tg) in the range of 5 to 15°C and a weight-average molecular weight in the range of 30,000 to 40,000, a polyurethane resin (a2) having a glass transition temperature (Tg) of -50°C or lower and having a polyether skeleton, a hydroxyl group-containing polyester resin (a3), a melamine resin (a4), and an activated methylene block polyisocyanate compound (a5), wherein the hydroxyl group-containing acrylic resin (a1) and the polyurethane resin (a A method for forming a multilayer coating film is described, characterized in that the ratio of the two components used is in the range of 20 / 10 to 30 / 10 in terms of solid content, the elongation at break of the coating film formed after heat curing with the water-based intermediate coating (A) at 20°C is in the range of 20 to 30%, the Young's modulus is in the range of 5,000 to 6,000 kgf / cm2, the two-con hardness is in the range of 4 to 6, and the clear coating (C) contains a hydroxyl group-containing acrylic resin (c1) and an allophanate group-containing polyisocyanate compound (c2).

[0007] The above multi-layer coating method offers good glass adhesion, coating performance, and storage stability; however, in recent years, from the perspective of energy conservation, lower heating temperatures have become desirable. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2019-198861 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The present invention aims to provide an aqueous coating composition that exhibits excellent storage stability and high coating performance and glass adhesion even when cured at relatively low temperatures. [Means for solving the problem]

[0010] As a result of diligent research to achieve the above objective, the present inventors have found that the above objective can be achieved by an aqueous coating composition containing (A) a hydroxyl group and carboxyl group-containing resin, (B) a blocked polyisocyanate compound, (C) a polycarbodiimide compound, and (D) a basic compound, wherein the basic compound (D) contains a basic compound having (D1) an acid dissociation constant (PKa) in the range of 7.0 to 8.5 and a boiling point in the range of 100 to 200°C, the content ratio of the basic compound (D1) is in the range of 30 to 100% by mass based on the mass of the basic compound (D), and the pH of the aqueous coating composition is in the range of 8.0 to 11.5.

[0011] According to the present invention, an aqueous coating composition and a method for forming a multi-layer coating film are provided, comprising the following embodiments.

[0012] Item 1. (A) Resins containing hydroxyl groups and carboxyl groups, (B) Blocked polyisocyanate compounds, (C) Polycarbodiimide compounds, and (D) Basic compounds an aqueous paint composition containing, The basic compound (D) is (D1) Basic compounds having an acid dissociation constant (PKa) in the range of 7.0 to 8.5 and a boiling point in the range of 100 to 200°C. It contains and the content of the basic compound (D1) is in the range of 30 to 100% by mass, based on the mass of the basic compound (D). Furthermore, the pH of the aqueous paint composition is within the range of 8.0 to 11.5. Water-based paint composition. Item 2. The aqueous paint composition according to Item 1, wherein at least a portion of the blocking agent of the blocked polyisocyanate compound (B) is an active methylene-based blocking agent. Item 3. The aqueous paint composition according to item 1 or 2, wherein the basic compound (D1) having an acid dissociation constant (PKa) in the range of 7.0 to 8.5 and a boiling point in the range of 100 to 200°C is N-methylmorpholine and / or N-ethylmorpholine. Item 4. The aqueous paint composition according to any one of items 1 to 3, further comprising melamine resin (E). Item 5. Step (1): A step of applying an aqueous paint composition described in any one of Items 1 to 4 onto the object to be coated to form an intermediate coating film. Step (2): A step of applying an aqueous base coat paint composition onto the intermediate coating film formed in step (1) to form a base coat film. Step (3): A step of applying a clear coating composition onto the base coat coating formed in step (2) to form a clear coating, and Step (4): A step of heating and curing the intermediate coating film, base coat film, and clear coating film formed in steps (1) to (3) at a temperature within the range of 60 to 110°C at the same time. A method for forming a multi-layer coating by sequentially performing the following steps. [Effects of the Invention]

[0013] The aqueous coating composition of the present invention has excellent storage stability and, even when cured at relatively low temperatures, can form a coating film that exhibits excellent coating performance such as high hardness and water resistance, as well as good glass adhesion. [Modes for carrying out the invention]

[0014] Hereinafter, the aqueous coating composition of the present invention (hereinafter, may be abbreviated as "this coating") will be described in more detail.

[0015] water-based paint composition The aqueous coating composition of the present invention is an aqueous coating composition containing (A) a resin containing a hydroxyl group and a carboxyl group, (B) a blocked polyisocyanate compound, (C) a polycarbodiimide compound, and (D) a basic compound, wherein the basic compound (D) contains (D1) a basic compound having an acid dissociation constant (PKa) in the range of 7.0 to 8.5 and a boiling point in the range of 100 to 200 °C, and the content ratio of the basic compound (D1) is in the range of 30 to 100% by mass based on the mass of the basic compound (D), and the pH of the aqueous coating composition is in the range of 8.0 to 11.5.

[0016] In this specification, the aqueous coating is a term contrasted with the organic solvent-based coating, Generally, it means a coating in which a film-forming resin, a pigment, etc. are dispersed and / or dissolved in water or a medium mainly composed of water (aqueous medium). Further, the above-mentioned organic solvent-based coating is a coating in which its solvent does not substantially contain water, or all or almost all of its solvent is an organic solvent.

[0017] The pH of the aqueous coating composition of the present invention is in the range of 8.0 to 11.5.

[0018] When the above pH is 8.0 or more, an aqueous coating composition excellent in storage stability can be obtained, and when it is 11.5 or less, an aqueous coating composition excellent in the hardness and water resistance of the formed coating film and glass adhesiveness can be obtained.

[0019] Among them, the above pH is preferably in the range of 8.5 to 11.0, and more preferably in the range of 9.0 to 10.5.

[0020] Hydroxyl group and carboxyl group-containing resin (A) The hydroxyl group and carboxyl group-containing resin (A) is not particularly limited as long as it is a resin containing hydroxyl groups and carboxyl groups. Specifically, examples of resin types include acrylic resins, polyester resins, polyether resins, polycarbonate resins, and polyurethane resins. In particular, acrylic resins, polyester resins, and polyurethane resins containing hydroxyl groups and carboxyl groups can be suitably used as the hydroxyl group and carboxyl group-containing resin (A).

[0021] Acrylic resin containing hydroxyl groups and carboxyl groups (A1) Acrylic resin (A1) containing hydroxyl groups and carboxyl groups can be synthesized by copolymerizing a hydroxyl group-containing polymerizable unsaturated monomer (M-1), a carboxyl group-containing polymerizable unsaturated monomer (M-2), and other copolymerizable polymerizable unsaturated monomers (M-3) by conventional methods.

[0022] The hydroxyl group-containing polymerizable unsaturated monomer (M-1) is a compound having one hydroxyl group and one polymerizable unsaturated bond in one molecule, and this hydroxyl group mainly acts as a functional group that reacts with the crosslinking agent. Specifically, the monomer is preferably a monoester of acrylic acid or methacrylic acid with a dihydric alcohol having 2 to 10 carbon atoms, for example, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, etc.

[0023] In addition to those listed above, other examples of monoesterified acrylic acid or methacrylic acid with polyhydric alcohols include compounds obtained by adding a ring-opening polymer such as ε-caprolactone to a monoesterified 2-hydroxyethyl (meth)acrylate, such as "Praxel FA-1", "Praxel FA-2", "Praxel FA-3", "Praxel FA-4", "Praxel FA-5", "Praxel FM-1", "Praxel FM-2", "Praxel FM-3", "Praxel FM-4", and "Praxel FM-5" (all of which are trade names manufactured by Daicel Chemical Co., Ltd.); polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate; 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-hydroxy-3-butoxypropyl (meth)acrylate; monohydroxyethyl (meth)acrylate phthalate, etc. These can be used individually or in combination of two or more.

[0024] However, in the present invention, monomers that fall under "(xvi) polymerizable unsaturated monomers having an ultraviolet-absorbing functional group" described later should be defined as "other copolymerizable polymerizable unsaturated monomers (M-3)" above, even if they are monomers having a hydroxyl group, and should be excluded from "hydroxyl group-containing polymerizable unsaturated monomers (M-1)".

[0025] Carboxyl group-containing polymerizable unsaturated monomers (M-2) are compounds having one or more carboxyl groups and one polymerizable unsaturated bond in one molecule. Examples include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, and maleic anhydride. These can be used individually or in combination of two or more.

[0026] Other copolymerizable polymerizable unsaturated monomers (M-3) are compounds other than the monomers (M-1) and (M-2) that have one or more polymerizable unsaturated bonds in a single molecule, and specific examples are listed below.

[0027] (i) Alkyl or cycloalkyl (meth)acrylates: For example, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, tridecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclododecyl (meth)acrylate, tricyclodecanyl (meth)acrylate, etc. (ii) Polymerizable unsaturated monomers having an isobornyl group: isobornyl (meth)acrylate, etc. (iii) Polymerizable unsaturated monomers having an adamantyl group: adamantyl (meth)acrylate, etc. (iv) Polymerizable unsaturated monomers having a tricyclodecenyl group: tricyclodecenyl (meth)acrylate, etc. (v) Polymerizable unsaturated monomers containing aromatic rings: benzyl (meth)acrylate, styrene, α-methylstyrene, vinyltoluene, etc. (vi) Polymerizable unsaturated monomers having an alkoxysilyl group: vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, γ-(meth)acryloyloxypropyltrimethoxysilane, γ-(meth)acryloyloxypropyltriethoxysilane, etc. (vii) Polymerizable unsaturated monomers having a fluorinated alkyl group: Perfluoroalkyl (meth)acrylates such as perfluorobutylethyl (meth)acrylate and perfluorooctylethyl (meth)acrylate; fluoroolefins, etc. (viii) A polymerizable unsaturated monomer having a photopolymerizable functional group such as a maleimide group. (ix) Vinyl compounds: N-vinylpyrrolidone, ethylene, butadiene, chloroprene, vinyl propionate, vinyl acetate, etc. (x) Nitrogen-containing polymerizable unsaturated monomers: (meth)acrylonitrile, (meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, methylenebis(meth)acrylamide, ethylenebis(meth)acrylamide, adducts of glycidyl (meth)acrylate with amine compounds, etc.

[0028] (xi) Polymerizable unsaturated monomers having two or more polymerizable unsaturated groups in one molecule: allyl (meth)acrylate, ethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, etc. (xii) Polymerizable unsaturated monomers containing epoxy groups: glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, allyl glycidyl ether, etc. (xiii) A (meth)acrylate having a polyoxyethylene chain with an alkoxy group at the molecular terminus. (xiv) Polymerizable unsaturated monomers having a sulfonic acid group: 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl (meth)acrylate, allylsulfonic acid, 4-styrenesulfonic acid, etc.; sodium salts and ammonium salts of these sulfonic acids, etc. (xv) Polymerizable unsaturated monomers having a phosphate group: acid phosphooxyethyl (meth)acrylate, acid phosphooxypropyl (meth)acrylate, acid phosphooxypoly(oxyethylene) glycol (meth)acrylate, acid phosphooxypoly(oxypropylene) glycol (meth)acrylate, etc. (xvi) Polymerizable unsaturated monomers having UV-absorbing functional groups: 2-hydroxy-4(3-methacryloyloxy-2-hydroxypropoxy)benzophenone, 2-hydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, 2,2'-dihydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone, 2,2'-dihydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, 2-[2-hydroxy-5-[2-(methacryloyloxy)ethyl]phenyl]-2H-benzotriazole, etc. (xvii) Photostable polymerizable unsaturated monomers: 4-(meth)acryloyloxy-1,2,2,6,6-pentamethylpiperidine, 4-(meth)acryloyloxy-2,2,6,6-tetramethylpiperidine, 4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 1-(meth)acryloyl-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 1-(meth)acryloyl-4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, 4-crotonoylamino-2,2,6,6-tetramethylpiperidine, 1-crotonoyl-4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, etc. (xviii) Polymerizable unsaturated monomers having a carbonyl group: acrolein, diacetone acrylamide, diacetone methacrylamide, acetoacetoxyethyl methacrylate, formyl styrene, vinyl alkyl ketones having 4 to 7 carbon atoms (e.g., vinyl methyl ketone, vinyl ethyl ketone, vinyl butyl ketone), etc. (xix) Polymerizable unsaturated monomers having an acid anhydride group: maleic anhydride, itaconic anhydride, citraconic anhydride, etc.

[0029] In this specification, a polymerizable unsaturated group means an unsaturated group that can undergo radical polymerization. Examples of such polymerizable unsaturated groups include vinyl groups and (meth)acryloyl groups.

[0030] Furthermore, in this specification, "(meth)acrylate" means acrylate or methacrylate. "(meth)acrylic acid" means acrylic acid or methacrylic acid. Also, "(meth)acryloyl" means acryloyl or methacryloyl. Also, "(meth)acrylamide" means acrylamide or methacrylamide.

[0031] When producing the hydroxyl group and carboxyl group-containing acrylic resin (A1), the proportion of the hydroxyl group-containing polymerizable unsaturated monomer (M-1) used is preferably 1 to 50% by mass, more preferably 2 to 40% by mass, and even more preferably 3 to 30% by mass, based on the total amount of monomer components.

[0032] The acrylic resin (A1) containing hydroxyl groups and carboxyl groups described above preferably has a hydroxyl value of 1 to 150 mg KOH / g, more preferably 2 to 120 mg KOH / g, and even more preferably 5 to 100 mg KOH / g, from the viewpoint of the curability, chipping resistance, adhesion, hardness, and finished appearance of the resulting coating film, as well as glass adhesion.

[0033] When producing the above-mentioned hydroxyl group and carboxyl group-containing acrylic resin (A1), the proportion of the carboxyl group-containing polymerizable unsaturated monomer (M-2) used is preferably 1 to 50% by mass, more preferably 2 to 40% by mass, and even more preferably 3 to 30% by mass, based on the total amount of monomer components.

[0034] Furthermore, the acrylic resin (A1) containing hydroxyl groups and carboxyl groups is preferably 1 to 80 mg KOH / g, more preferably 5 to 50 mg KOH / g, and even more preferably 5 to 30 mg KOH / g, from the viewpoint of storage stability of the paint and water resistance of the resulting coating film.

[0035] When the aqueous coating composition of the present invention contains the above-mentioned hydroxyl group and carboxyl group-containing acrylic resin (A1), the content of the hydroxyl group and carboxyl group-containing acrylic resin (A1) is preferably 2 to 70% by mass, more preferably 5 to 50% by mass, and even more preferably 10 to 40% by mass, based on the amount of resin solids in the aqueous coating composition.

[0036] Furthermore, the hydroxyl group and carboxyl group-containing acrylic resin (A1) is preferably a hydroxyl group and carboxyl group-containing acrylic resin particle (A1') having a core-shell structure.

[0037] Acrylic resin particles (A1') containing hydroxyl and carboxyl groups having a core-shell structure In the present invention, the "shell portion" of "hydroxyl group and carboxyl group-containing acrylic resin particles having a core-shell structure (A1')" refers to the polymer layer present in the outermost layer of the resin particle, the "core portion" refers to the polymer layer in the inner layer of the resin particle excluding the shell portion, and "having a core-shell structure" refers to a structure having the core portion and the shell portion.

[0038] In the following text, "acrylic resin particles containing hydroxyl groups and carboxyl groups having a core-shell structure (A1')" may simply be referred to as "acrylic resin particles containing hydroxyl groups and carboxyl groups (A1')".

[0039] The core-shell structure described above typically has a layered structure in which the core is completely covered by the shell. However, depending on the mass ratio of the core to the shell, the amount of monomer in the shell may be insufficient to form a layered structure. In such cases, it is not necessary to have a complete layered structure as described above, and a structure in which the shell covers only a portion of the core may also be used.

[0040] Acrylic resin particles containing hydroxyl groups and carboxyl groups (A1') are typically acrylic resin particles consisting of a core portion which is a copolymer (I) with polymerizable unsaturated monomers as copolymer components, and a shell portion which is a copolymer (II) with polymerizable unsaturated monomers as copolymer components. The polymerizable unsaturated monomers can be used in appropriate combinations of the polymerizable unsaturated monomers (M-1) to (M-3).

[0041] By using a polymerizable unsaturated monomer having two or more of the aforementioned polymerizable unsaturated groups in one molecule, a copolymer crosslinking structure can be imparted. When using such a polymerizable unsaturated monomer having two or more polymerizable unsaturated groups in one molecule, the proportion used can be appropriately determined according to the degree of crosslinking of the copolymer, but it is generally preferable that the proportion is in the range of approximately 0.1 to 30% by mass, particularly approximately 0.5 to 10% by mass, and even more preferably approximately 1 to 7% by mass, relative to the total amount of the polymerizable unsaturated monomers (M-1) to (M-3).

[0042] Acrylic resin particles containing hydroxyl groups and carboxyl groups (A1') can be obtained by emulsion polymerization of a polymerizable unsaturated monomer mixture to obtain an emulsion of a core copolymer (I), then adding the polymerizable unsaturated monomer mixture to this emulsion and further emulsion polymerization to prepare a shell copolymer (II).

[0043] Emulsion polymerization to prepare the core copolymer (I) emulsion can be carried out by conventionally known methods. For example, it can be carried out by emulsion polymerization of a polymerizable unsaturated monomer mixture using a polymerization initiator in the presence of an emulsifier.

[0044] Anionic emulsifiers and nonionic emulsifiers can be suitably used as the emulsifiers mentioned above.

[0045] Examples of anionic emulsifiers include sodium salts and ammonium salts of alkyl sulfonic acid, alkylbenzene sulfonic acid, and alkyl phosphoric acid. Examples of nonionic emulsifiers include polyoxyethylene oleyl ether, polyoxyethylene stearyl ether, polyoxyethylene lauryl ether, polyoxyethylene tridecyl ether, polyoxyethylene phenyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene monolaurate, polyoxyethylene monostearate, polyoxyethylene monooleate, sorbitan monolaurate, sorbitan monostearate, sorbitan trioleate, and polyoxyethylene sorbitan monolaurate.

[0046] Furthermore, polyoxyalkylene group-containing anionic emulsifiers having an anionic group and a polyoxyalkylene group such as a polyoxyethylene group or a polyoxypropylene group in one molecule; reactive anionic emulsifiers having an anionic group and a radically polymerizable unsaturated group in one molecule, etc., can also be used.

[0047] Examples of the above-mentioned reactive anionic emulsifiers include sodium salts of sulfonic acid compounds having radically polymerizable unsaturated groups such as allyl groups, methallyl groups, (meth)acryloyl groups, propenyl groups, and butenyl groups, and ammonium salts of the sulfonic acid compounds.

[0048] The amount of emulsifier used is preferably about 0.1 to 15% by mass, particularly preferably about 0.5 to 10% by mass, and even more preferably within the range of about 1 to 5% by mass, relative to the total amount of all monomers used.

[0049] Examples of polymerization initiators include organic peroxides such as benzoyl peroxide, octanoyl peroxide, lauroyl peroxide, stearoyl peroxide, cumene hydroperoxide, tert-butyl peroxide, tert-butyl peroxylaurate, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxyacetate, and diisopropylbenzene hydroperoxide; azobisisobutyronitrile, azobis(2,4-dimethyl) Examples of polymerization initiators include azo compounds such as rubaleronitrile, azobis(2-methylpropionnitrile), azobis(2-methylbutyronitrile), 4,4'-azobis(4-cyanobutanoic acid), dimethylazobis(2-methylpropionate), azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and azobis{2-methyl-N-[2-(1-hydroxybutyl)]-propionamide}; and persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate. These polymerization initiators can be used alone or in combination of two or more. In addition, a reducing agent such as sugar, sodium formaldehyde sulfoxylate, or an iron complex can be used in combination with the above polymerization initiators as needed to form a redox initiator.

[0050] The amount of polymerization initiator used is generally preferably about 0.1 to 5% by mass, and particularly preferably within the range of about 0.2 to 3% by mass, relative to the total amount of monomers used. The method of adding the polymerization initiator is not particularly limited and can be appropriately selected depending on its type and amount. For example, it can be included in the monomer mixture or aqueous medium beforehand, added all at once during polymerization, or added dropwise.

[0051] Acrylic resin particles containing hydroxyl groups and carboxyl groups (A1') can be obtained by adding a polymerizable unsaturated monomer mixture to an emulsion of the core copolymer (I) obtained above, and further polymerizing it to form a shell copolymer (II).

[0052] The monomer mixture forming the shell copolymer (II) may optionally contain components such as polymerization initiators, chain transfer agents, reducing agents, and emulsifiers. While the monomer mixture can be added dropwise as is, it is preferable to add it as a monomer emulsion obtained by dispersing the monomer mixture in an aqueous medium. In this case, the particle size of the monomer emulsion is not particularly limited.

[0053] A polymerization method for the monomer mixture that forms the shell copolymer (II) can be, for example, a method in which the monomer mixture or its emulsion is added to the emulsion of the core copolymer (I) all at once or gradually dropwise, and heated to an appropriate temperature while stirring.

[0054] The resulting hydroxyl group and carboxyl group-containing acrylic resin particles (A1') have a multilayer structure in which copolymer (I) is the core and copolymer (II) is the shell.

[0055] When the aqueous coating composition of the present invention contains the above-mentioned hydroxyl group and carboxyl group-containing acrylic resin particles (A1'), the content of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1') is preferably 2 to 70% by mass, more preferably 5 to 50% by mass, and even more preferably 10 to 40% by mass, based on the amount of resin solids in the aqueous coating composition.

[0056] Furthermore, in preparing the hydroxyl and carboxyl group-containing acrylic resin particles (A1'), a step of supplying polymerizable unsaturated monomers (one or a mixture of two or more) to form another resin layer and performing emulsion polymerization can be added between the step of obtaining the core copolymer (I) and the step of obtaining the shell copolymer (II). In particular, it is preferable to form a gradient polymer layer as an additional layer, resulting in hydroxyl and carboxyl group-containing acrylic resin particles (A1'') having a core-shell structure including the gradient polymer layer.

[0057] Acrylic resin particles (A1'') containing hydroxyl groups and carboxyl groups, having a core-shell structure including a gradient polymer layer.

[0058] In the present invention, the gradient polymer layer of "hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') having a core-shell structure including a gradient polymer layer" means a polymer layer having a layer structure in which the composition changes continuously (has a composition gradient).

[0059] More specifically, it refers to a polymer layer having a compositional gradient, where the monomer (monomer mixture) composition changes continuously, for example, from monomer A (monomer mixture A) to monomer B (monomer mixture B).

[0060] In the following, "acrylic resin particles containing hydroxyl groups and carboxyl groups having a core-shell structure including a gradient polymer layer (A1'')" may simply be referred to as "acrylic resin particles containing hydroxyl groups and carboxyl groups (A1'')".

[0061] The above gradient polymer layer can generally be obtained by a known polymerization method called power feed polymerization. Specifically, for example, when polymerizing two types of monomer A (monomer mixture A) and monomer B (monomer mixture B), a gradient polymer layer can be obtained by introducing monomer A (monomer mixture A) into a reaction vessel while dropping monomer B (monomer mixture B) into a container containing monomer A (monomer mixture A), and then carrying out the polymerization reaction.

[0062] In the power feed polymerization described above, a gradient polymer layer having a desired composition gradient can be obtained by adjusting the synthesis conditions (such as the timing of the start of mixing between monomer A (monomer mixture A) and monomer B (monomer mixture B), the rate at which monomer B (monomer mixture B) is dropped into the container containing monomer A (monomer mixture A), and the rate at which monomer A (monomer mixture A) is introduced into the reaction vessel).

[0063] From the viewpoint of the water resistance of the resulting coating film, the ratio of the gradient polymer layer in the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') is preferably about 20 to 80% by mass, particularly preferably about 25 to 75% by mass, and even more preferably about 30 to 70% by mass, relative to the total amount of the total copolymer components of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'').

[0064] The hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') are preferably 1 to 150 mg KOH / g, more preferably 2 to 120 mg KOH / g, and even more preferably 5 to 100 mg KOH / g, from the viewpoint of the curability, chipping resistance, adhesion, and finished appearance of the resulting coating film, as well as glass adhesion.

[0065] The core portion of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') preferably has a hydroxyl value of 0 to 150 mg KOH / g, more preferably 5 to 120 mg KOH / g, and even more preferably 10 to 100 mg KOH / g, from the viewpoint of the water resistance and chipping resistance of the resulting coating film.

[0066] The shell portion of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') preferably has a hydroxyl value of 0 to 150 mg KOH / g, more preferably 2 to 120 mg KOH / g, and even more preferably 5 to 100 mg KOH / g, from the viewpoint of the water resistance, chipping resistance, and glass adhesion of the resulting coating film.

[0067] The hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') preferably have an acid value of 1 to 80 mgKOH / g, more preferably 5 to 50 mgKOH / g, and even more preferably 5 to 30 mgKOH / g, from the viewpoint of storage stability of the paint and water resistance of the resulting coating film.

[0068] The core portion of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') preferably has an acid value of 0 to 50 mgKOH / g, more preferably 0 to 30 mgKOH / g, and even more preferably 0 to 10 mgKOH / g, from the viewpoint of manufacturing stability and paint storage stability.

[0069] The shell portion of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') preferably has an acid value of 1 to 100 mg KOH / g, more preferably 5 to 80 mg KOH / g, and even more preferably 10 to 50 mg KOH / g, from the viewpoint of storage stability of the paint and water resistance of the resulting coating film.

[0070] The hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') preferably have a glass transition temperature of -20 to 100°C, more preferably 0 to 100°C, and even more preferably 20 to 100°C, from the viewpoint of the water resistance, chipping resistance, and glass adhesion of the resulting coating film.

[0071] The core portion of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') preferably has a glass transition temperature of -50 to 50°C, more preferably -30 to 50°C, and even more preferably 0 to 50°C, from the viewpoint of the water resistance, chipping resistance, and glass adhesion of the resulting coating film.

[0072] From the viewpoint of the water resistance of the resulting coating film, the glass transition temperature of the shell portion of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') is preferably 0 to 120°C, more preferably 20 to 100°C, and even more preferably 30 to 100°C.

[0073] In this specification, the glass transition temperature (Tg) was calculated using the following formula. 1 / Tg(K)=(W1 / T1)+(W2 / T2)+ (1) Tg(°C) = Tg(K) - 273 (2) In each formula, W1, W2, ... represent the mass fractions of the monomers used in copolymerization, and T1, T2, ... represent the Tg(K) of the homopolymer of each monomer. Note that T1, T2, etc. are values ​​from pages III-139~179 of the Polymer Hand Book (Second Edition, edited by J. Brandup and E. Himmergut). Furthermore, when the Tg of the monomer homopolymer is not clearly defined, the glass transition temperature (Tg) is defined as the static glass transition temperature. For example, using a differential scanning calorimeter "DSC-220U" (manufactured by Seiko Instruments Inc.), the sample was placed in a measuring cup, the solvent was completely removed by vacuum suction, and the change in calorific value was measured in the range of -20°C to +200°C at a heating rate of 3°C / min. The point of change at the first baseline on the lower temperature side was defined as the static glass transition temperature.

[0074] When the aqueous coating composition of the present invention contains the above-mentioned hydroxyl group and carboxyl group-containing acrylic resin particles (A1''), the content of the hydroxyl group and carboxyl group-containing acrylic resin particles (A1'') is preferably 2 to 70% by mass, more preferably 5 to 50% by mass, and even more preferably 10 to 40% by mass, based on the amount of resin solids in the aqueous coating composition.

[0075] Polyester resin containing hydroxyl groups and carboxyl groups (A2) A polyester resin (A2) containing hydroxyl groups and carboxyl groups can be synthesized by known methods, following conventional procedures, by esterifying a polybasic acid with a polyhydric alcohol.

[0076] Polybasic acids are compounds having two or more carboxyl groups in one molecule, and examples include phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid, fumaric acid, itaconic acid, trimellitic acid, pyromellitic acid, and their anhydrides. Polyhydric alcohols are compounds having two or more hydroxyl groups in one molecule, and examples include ethylene 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, neopentyl glycol hydroxypivalate, and 2-butyl-2-ethyl-1,3-propane Examples include diols such as 3-methyl-1,5-pentanediol, 2,2,4-trimethylpentanediol, and hydrogenated bisphenol A; and polyol components with a valency of 3 or higher such as trimethylolpropane, trimethylolethane, glycerin, and pentaerythritol; as well as hydroxycarboxylic acids such as 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolpentanoic acid, 2,2-dimethylolhexanoic acid, and 2,2-dimethyloloctanoic acid.

[0077] Alternatively, α-olefin epoxides such as propylene oxide and butylene oxide, and monoepoxy compounds such as Cardura E10 (manufactured by HEXION Specialty Chemicals, trade name, glycidyl ester of synthetic highly branched saturated fatty acid) may be reacted with an acid to introduce these compounds into the polyester resin.

[0078] The introduction of carboxyl groups into polyester resins can also be carried out, for example, by reacting a polybasic acid with a polyhydric alcohol, and then further reacting it with a polybasic acid such as trimellitic acid or trimellitic anhydride and their anhydrides, or by adding an acid anhydride to a hydroxyl group-containing polyester to perform half-esterification.

[0079] Furthermore, the hydroxyl and carboxyl group-containing polyester resin (A2) may also be a fatty acid-modified polyester resin modified with (semi)drying oil fatty acids such as linseed oil fatty acid, coconut oil fatty acid, safflower oil fatty acid, soybean oil fatty acid, sesame oil fatty acid, hen oil fatty acid, tall oil fatty acid, and dehydrated castor oil fatty acid. Generally, the amount of modification by these fatty acids is suitable to be 30% by mass or less in terms of oil length. In addition, the hydroxyl and carboxyl group-containing polyester resin (A2) may also be obtained by partially reacting it with a monobasic acid such as benzoic acid.

[0080] Furthermore, the hydroxyl group and carboxyl group-containing polyester resin (A2) can be modified with fatty acids, monoepoxy compounds, polyisocyanate compounds, acrylic resins, etc., during or after the preparation of the resin.

[0081] Examples of the above fatty acids include coconut oil fatty acids, cottonseed oil fatty acids, hemp seed oil fatty acids, rice bran oil fatty acids, fish oil fatty acids, tall oil fatty acids, soybean oil fatty acids, linseed oil fatty acids, tung oil fatty acids, rapeseed oil fatty acids, castor oil fatty acids, dehydrated castor oil fatty acids, and safflower oil fatty acids. Furthermore, as the above monoepoxy compound, for example, "Cardura E10P" (trade name, manufactured by HEXION, a glycidyl ester of a synthetic highly branched saturated fatty acid) can be suitably used.

[0082] Furthermore, examples of the polyisocyanate compounds include aliphatic diisocyanate compounds such as lysine diisocyanate, hexamethylene diisocyanate, and trimethylhexane diisocyanate; and lipid diisocyanate compounds such as hydrogenated xylylene diisocyanate, isophorone diisocyanate, methylcyclohexane-2,4-diisocyanate, methylcyclohexane-2,6-diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), and 1,3-(isocyanatomethyl)cyclohexane. Examples include cyclic diisocyanate compounds; aromatic diisocyanate compounds such as tolylene diisocyanate, xylylene diisocyanate, and diphenylmethane diisocyanate; organic polyisocyanates themselves, such as trivalent or higher polyisocyanates like lysine triisocyanate; adducts of these organic polyisocyanates with polyhydric alcohols, low molecular weight polyester resins, water, etc.; and cyclized polymers (e.g., isocyanurates) and biuret-type adducts of these organic polyisocyanates. These polyisocyanate compounds can be used individually or in combination of two or more.

[0083] Furthermore, known methods can be used to modify the hydroxyl group and carboxyl group-containing polyester resin (A2) with an acrylic resin. Examples include polymerizing a mixture of a polymerizable unsaturated group-containing polyester resin and a polymerizable unsaturated monomer, or reacting the hydroxyl group and carboxyl group-containing polyester resin with the acrylic resin.

[0084] The hydroxyl group and carboxyl group-containing polyester resin (A2) preferably has a hydroxyl value of 1 to 250 mg KOH / g, more preferably 2 to 200 mg KOH / g, and even more preferably 5 to 200 mg KOH / g.

[0085] Furthermore, the hydroxyl group and carboxyl group-containing polyester resin (A2) preferably has an acid value of 1 to 150 mg KOH / g, more preferably 2 to 100 mg KOH / g, and even more preferably 2 to 50 mg KOH / g.

[0086] Furthermore, the weight-average molecular weight of the hydroxyl group and carboxyl group-containing polyester resin (A2) is preferably 3,000 to 100,000, more preferably 4,000 to 50,000, and even more preferably 5,000 to 30,000.

[0087] In this specification, the average molecular weight is calculated from the chromatogram measured by gel permeation chromatography, using the molecular weight of standard polystyrene as a reference. The gel permeation chromatograph used was "HLC8120GPC" (manufactured by Tosoh Corporation). Four columns were used: "TSKgel G-4000HXL", "TSKgel G-3000HXL", "TSKgel G-2500HXL", and "TSKgel G-2000HXL" (all product names manufactured by Tosoh Corporation). The measurements were performed under the following conditions: mobile phase; tetrahydrofuran, measurement temperature; 40°C, flow rate; 1 mL / min, detector; radioisotope (RI).

[0088] When the aqueous coating composition of the present invention contains the above-mentioned hydroxyl group and carboxyl group-containing polyester resin (A2), the content of the hydroxyl group and carboxyl group-containing polyester resin (A2) is preferably 2 to 70% by mass, more preferably 5 to 50% by mass, and even more preferably 5 to 40% by mass, based on the amount of resin solids in the aqueous coating composition.

[0089] Polyurethane resin containing hydroxyl groups and carboxyl groups (A3) The polyurethane resin (A3) containing hydroxyl and carboxyl groups can be obtained by conventional methods, for example, by reacting a polyol with a polyisocyanate compound. After the reaction, the chain can be extended in the presence of a chain extender, such as a diol or diamine, which is a low molecular weight compound having at least two active hydrogen atoms per molecule. Furthermore, the resin can be modified with an acrylic resin or the like during or after its preparation.

[0090] Examples of polyols that do not contain carboxyl groups include, among others, low molecular weight polyols such as dihydric alcohols like ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, and hexamethylene glycol, and trihydric alcohols such as trimethylolpropane, glycerin, and pentaerythritol. Examples of high molecular weight polyols include polyether polyols, polyester polyols, acrylic polyols, and epoxy polyols. Examples of polyether polyols include polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. Examples of polyester polyols include the aforementioned dihydric alcohols, polycondensates of alcohols such as dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, and neopentyl glycol with dibasic acids such as adipic acid, azelaic acid, and sebatic acid, lactone-based ring-opening polymer polyols such as polycaprolactone, and polycarbonate polyols.

[0091] Examples of carboxyl group-containing polyols include 2,2-dimethylolpropionic acid and 2,2-dimethylolbutanoic acid, with 2,2-dimethylolpropionic acid being particularly preferred. When using these, a small amount of solvent such as N-methylpyrrolidone may be used to expedite the reaction.

[0092] Examples of polyisocyanate compounds to react with the above polyols include aliphatic polyisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, and lysine diisocyanate; and biuret-type adducts and isocyanurate ring adducts of these polyisocyanates; isophorone diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), and methylcyclohexane-2,4-(or-2,6-) diisocyanate. Alicyclic diisocyanates such as 1,3-(or 1,4-)di(isocyanatomethyl)cyclohexane, 1,4-cyclohexanediisocyanate, 1,3-cyclopentanediisocyanate, and 1,2-cyclohexanediisocyanate; and biuret-type adducts and isocyanurate ring adducts of these polyisocyanates; xylylene diisocyanate, metaxylylene diisocyanate, tetramethylxylylene diisocyanate, tolylenediisocyanate, and 4,4'-diphenylmethanediisocyanate. Aromatic diisocyanate compounds such as cyanates, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4-toluidine diisocyanate, 4,4'-diphenyl ether diisocyanate, (m- or p-)phenylene diisocyanate, 4,4'-biphenylene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, bis(4-isocyanatophenyl)sulfone, isopropylidene bis(4-phenylisocyanate), and their polyisocyanates. Examples include biuret-type adducts and isocyanurate ring adducts of annetes; polyisocyanates having three or more isocyanate groups in one molecule, such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene, and 4,4'-dimethyldiphenylmethane-2,2',5,5'-tetraisocyanate; and biuret-type adducts and isocyanurate ring adducts of these polyisocyanate compounds.

[0093] Examples of diols used as chain extenders include ethylene glycol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, and cyclohexanediol. Examples of diamines include ethylenediamine, propylenediamine, xylylenediamine, and N-(2-hydroxyethyl)ethylenediamine.

[0094] The polyurethane resin (A3) containing hydroxyl groups and carboxyl groups preferably has a hydroxyl value of 1 to 250 mg KOH / g, more preferably 2 to 200 mg KOH / g, and even more preferably 5 to 200 mg KOH / g.

[0095] Furthermore, the polyurethane resin (A3) containing hydroxyl groups and carboxyl groups is preferably such that its acid value is 1 to 100 mg KOH / g, more preferably 2 to 50 mg KOH / g, and even more preferably 2 to 30 mg KOH / g, from the viewpoint of manufacturing stability and the water resistance of the resulting coating film.

[0096] Furthermore, the weight-average molecular weight of the polyurethane resin (A3) containing hydroxyl groups and carboxyl groups is preferably 3,000 or more, more preferably 5,000 or more, and even more preferably 10,000 or more.

[0097] When the aqueous coating composition of the present invention contains the above-mentioned hydroxyl group and carboxyl group-containing polyurethane resin (A3), the content of the hydroxyl group and carboxyl group-containing polyurethane resin (A3) is preferably 2 to 70% by mass, more preferably 5 to 50% by mass, and even more preferably 10 to 40% by mass, based on the amount of resin solids in the aqueous coating composition.

[0098] Blocked polyisocyanate compound (B) Blocked polyisocyanate compound (B) is a compound in which the isocyanate group of polyisocyanate compound (b1) is blocked with a blocking agent (b2).

[0099] Polyisocyanate compound (b1) The polyisocyanate compound (b1) is a compound having at least two isocyanate groups in one molecule, and examples include aliphatic polyisocyanates, alicyclic polyisocyanates, aromatic aliphatic polyisocyanates, aromatic polyisocyanates, and their derivatives, as well as any combination thereof.

[0100] Examples of the above aliphatic polyisocyanates include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate, 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, diisocyanate dimer, and methyl 2,6-diisocyanatohexanoate (common name: lysine). Examples include aliphatic diisocyanates such as diisocyanates; and aliphatic triisocyanates such as 2-isocyanatoethyl 2,6-diisocyanatohexanoate, 1,6-diisocyanato-3-isocyanatomethylhexane, 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.

[0101] Examples of the above alicyclic polyisocyanates include 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate), 4-methyl-1,3-cyclohexylene diisocyanate (common name: hydrogenated TDI), and 2-methyl-1,3-cyclohexylene diisocyanate. Alicyclic diisocyanates such as nate, 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane (common name: hydrogenated xylylene diisocyanate) or mixtures thereof, methylenebis(4,1-cyclohexanediyl) diisocyanate (common name: hydrogenated MDI), norbornane diisocyanate; 1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane, 2-(3-isocyanatopropyl)- 2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 2-(3-isocyanatopropyl)-2,6-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 3-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, 6-(2-iso Examples include alicyclic triisocyanates such as cyanatoethyl)-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.

[0102] Examples of the above-mentioned aromatic aliphatic polyisocyanates include aromatic aliphatic diisocyanates such as methylenebis(4,1-phenylene) diisocyanate (common name: MDI), 1,3- or 1,4-xylylene diisocyanate or mixtures thereof, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (common name: tetramethylxylylene diisocyanate) or mixtures thereof; and aromatic aliphatic triisocyanates such as 1,3,5-triisocyanatomethylbenzene.

[0103] Examples of the above-mentioned aromatic polyisocyanates include aromatic diisocyanates such as m-phenylenediisocyanate, p-phenylenediisocyanate, 4,4'-diphenyldiisocyanate, 1,5-naphthalenediisocyanate, 2,4-tolylenediisocyanate (common name: 2,4-TDI) or 2,6-tolylenediisocyanate (common name: 2,6-TDI) or mixtures thereof, 4,4'-toluidinediisocyanate, 4,4'-diphenyletherdiisocyanate, etc.; aromatic triisocyanates such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene, etc.; and aromatic tetraisocyanates such as 4,4'-diphenylmethane-2,2',5,5'-tetraisocyanate, etc.

[0104] Furthermore, examples of the above derivatives include dimers, trimers, biuret, allophanate, uretodione, uretoimine, isocyanurate, oxadiazinetrione, etc. of the polyisocyanates mentioned above, as well as polymethylene polyphenyl polyisocyanates (crude MDI, polymeric MDI), crude TDI, etc.

[0105] As the polyisocyanate compound (b1), aliphatic diisocyanates, alicyclic diisocyanates, and their derivatives are preferred because the resulting blocked polyisocyanate compound (B) is less prone to yellowing when heated. Among these, aliphatic diisocyanates and their derivatives are more preferred from the viewpoint of improving the flexibility of the formed coating film.

[0106] Furthermore, the polyisocyanate compound (b1) includes a prepolymer produced by reacting aliphatic polyisocyanates, alicyclic polyisocyanates, aromatic aliphatic polyisocyanates, aromatic polyisocyanates, and their derivatives, as well as any combination thereof, with a compound that can react with the above polyisocyanate, under conditions of excess isocyanate groups. Examples of compounds that can react with the above polyisocyanate include compounds having active hydrogen groups such as hydroxyl groups and amino groups, and specifically, examples include polyhydric alcohols, low molecular weight polyester resins, amines, and water.

[0107] Furthermore, the polyisocyanate compound (b1) includes polymers of isocyanate group-containing polymerizable unsaturated monomers, or copolymers of the above-mentioned isocyanate group-containing polymerizable unsaturated monomer and polymerizable unsaturated monomers other than the above-mentioned isocyanate group-containing polymerizable unsaturated monomer.

[0108] The polyisocyanate compound (b1) may have a number average molecular weight in the range of 300 to 20,000, more preferably 400 to 8,000, and even more preferably 500 to 2,000, from the viewpoint of the reactivity of the resulting blocked polyisocyanate compound (B) and the compatibility of the blocked polyisocyanate compound (B) with other paint components.

[0109] Furthermore, the polyisocyanate compound (b1) preferably has an average number of isocyanate functional groups in one molecule within the range of 2 to 100, from the viewpoint of the reactivity of the resulting blocked polyisocyanate compound (B) and the compatibility of the blocked polyisocyanate compound (B) with other paint components. The average number of isocyanate functional groups is more preferably 3 or more, from the viewpoint of increasing the reactivity of the resulting blocked polyisocyanate compound (B). The average number of isocyanate functional groups is more preferably 20 or less, from the viewpoint of preventing gelation during the production of the blocked polyisocyanate compound (B).

[0110] Blocking agent (b2) Examples of blocking agents (b2) that block the isocyanate group in the polyisocyanate compound (b1) include active methylene-based, alcohol-based, phenol-based, oxime-based, amine-based, acid amide-based, imidazole-based, pyridine-based, and mercaptan-based blocking agents, which can be used individually or in combination of two or more.

[0111] From the viewpoint of low-temperature curing properties, it is preferable that at least a portion of the blocking agent (b2) is an active methylene-based blocking agent, and it is even more preferable that the blocking agent is an active methylene-based blocking agent.

[0112] Examples of the above-mentioned active methylene-based blocking agents include malonic acid diesters such as dimethyl malonate, diethyl malonate, di-n-propyl malonate, diisopropyl malonate, di-n-butyl malonate, diisobutyl malonate, disec-butyl malonate, ditert-butyl malonate, di-n-pentyl malonate, di-n-hexyl malonate, di(2-ethylhexyl) malonate, methylisopropyl malonate, ethylisopropyl malonate, methyln-butyl malonate, ethyln-butyl malonate, methylisobutyl malonate, ethylisobutyl malonate, methylsec-butyl malonate, ethylsec-butyl malonate, diphenyl malonate and dibenzyl malonate; methyl acetoacetate, ethyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate, acetose acetate Examples include acetoacetate esters such as n-butyl acetoacetate, isobutyl acetoacetate, sec-butyl acetoacetate, tert-butyl acetoacetate, n-pentyl acetoacetate, n-hexyl acetoacetate, 2-ethylhexyl acetoacetate, phenyl acetoacetate, and benzyl acetoacetate; isobutyryl acetate esters such as methyl isobutyryl acetate, ethyl isobutyryl acetate, n-propyl isobutyryl acetate, isopropyl isobutyryl acetate, n-butyl isobutyryl acetate, isobutyl isobutyryl acetate, sec-butyl isobutyryl acetate, tert-butyl isobutyryl acetate, n-pentyl isobutyryl acetate, n-hexyl isobutyryl acetate, 2-ethylhexyl isobutyryl acetate, phenyl isobutyryl acetate, and benzyl isobutyryl acetate, as well as any combination thereof.

[0113] As the active methylene-based blocking agent, from the viewpoint of the smoothness and clarity of the coating film formed by the aqueous coating composition of the present invention, it is preferable that the active methylene-based blocking agent be at least one compound selected from the group consisting of dimethyl malonate, diethyl malonate, diisopropyl malonate, methyl acetoacetate, ethyl acetoacetate, methyl isobutyrylacetate, and ethyl isobutyrylacetate.

[0114] The blocking reaction of isocyanate groups in polyisocyanate compound (b1) may optionally include a reaction catalyst. Examples of such reaction catalysts include basic compounds such as metal hydroxides, metal alkoxides, metal carboxylates, metal acetyl acetylates, hydroxides of onium salts, onium carboxylates, metal salts of active methylene compounds, onium salts of active methylene compounds, aminosilanes, amines, and phosphines.

[0115] From the viewpoint of low-temperature curability of the aqueous coating composition of the present invention and the water resistance of the formed coating film, it is preferable that the blocked polyisocyanate compound (B) contains a structure derived from a spacer (b3) having at least two isocyanate-reactive functional groups.

[0116] Spacer (b3) Spacer (b3) is a compound having at least two isocyanate-reactive functional groups.

[0117] The above functional group is not particularly limited as long as it is a functional group that is reactive with an isocyanate group. Examples of such isocyanate-reactive functional groups include hydroxyl groups, amino groups, carboxyl groups, and thiol groups, with hydroxyl groups and amino groups being preferred, and hydroxyl groups being particularly preferred.

[0118] Therefore, the spacer (b3) is preferably a compound having at least two hydroxyl groups or at least two amino groups, and more preferably a compound having at least two hydroxyl groups.

[0119] Examples of compounds having at least two hydroxyl groups include, as low molecular weight compounds, dihydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, and hexamethylene glycol, and trihydric alcohols such as trimethylolpropane, glycerin, and pentaerythritol, and as high molecular weight compounds, polyether polyols, polyester polyols, acrylic polyols, and epoxy polyols. Among these, polyether polyols are preferred from the viewpoint of the water resistance and chipping resistance of the resulting coating film.

[0120] The above-mentioned polyether polyols can also be commercially available. Examples of such commercially available products include "PEG#200", "PEG#300", "PEG#400", "PEG#600", "PEG#1000", "PEG#1500", "PEG#1540", "PEG#2000", "PEG#4000", "PEG#6000" (all manufactured by NOF Corporation, polyethylene glycol), "Sannix GP250", "Sannix GP400", "Sannix GP600", "Sannix GP1000", "Sannix GP1500", "Sannix GP3000", "Sannix GP4000" (all manufactured by Sanyo Chemical Industries, polyoxypropylene glycerin). Examples include polyoxypropylene glycol (Sanyo Chemical Industries), "Sanyo PP200", "Sanyo PP400", "Sanyo PP600", "Sanyo PP950", "Sanyo PP1000", "Sanyo PP1200", "Sanyo PP2000", "Sanyo PP3000", "Sanyo PP4000" (all manufactured by Sanyo Chemical Industries, Ltd.), "PTMG250", "PTMG650", "PTMG1000", "PTMG2000", "PTMG3000", "PTMG4000" (all manufactured by Mitsubishi Chemical Corporation, polyoxytetramethylene glycol), etc.

[0121] Furthermore, examples of compounds having at least two amino groups include polyetheramines.

[0122] The above-mentioned polyetheramines can also be commercially available. Examples of such commercially available products include "JEFFAMINE D-400", "JEFFAMINE D-2000", "JEFFAMINE D-4000", "JEFFAMINE ED-600", "JEFFAMINE ED-900", "JEFFAMINE ED-2003", "ELASTAMINE RT-1000", "JEFFAMINE T-403", "JEFFAMINE T-3000", and "JEFFAMINE T-5000" from HUNTSMAN.

[0123] The molecular weight of the above-mentioned spacer (b3) is preferably in the range of 500 to 6,000, more preferably in the range of 800 to 5,000, and even more preferably in the range of 1,000 to 4,000, from the viewpoint of low-temperature curability of the aqueous coating composition of the present invention and the hardness, chipping resistance, and water resistance of the formed coating film.

[0124] The number of functional groups in the above-mentioned spacer (b3) is preferably 2 to 3, and more preferably 2, from the viewpoint of storage stability of the aqueous coating composition of the present invention.

[0125] By reacting some of the isocyanate groups in the polyisocyanate compound (b1) with a spacer (b3), a polyisocyanate compound (b1) containing a structure derived from the spacer (b3) can be formed. In this case, from the viewpoint of the storage stability of the aqueous coating composition of the present invention, the hardness and water resistance of the formed coating film, and the manufacturing stability of the blocked polyisocyanate compound (B), it is preferable that the ratio of the polyisocyanate compound (b1) to the spacer (b3) is in the range of 0.03 to 0.6 moles of active hydrogen in the spacer (b3) based on 1 mole of isocyanate groups in the polyisocyanate compound (b1). By blocking the polyisocyanate compound (b1) containing a structure derived from the spacer (b3), a blocked polyisocyanate compound (B) containing a structure derived from the spacer (b3) can be formed.

[0126] Furthermore, from the viewpoint of storage stability of the aqueous coating composition of the present invention, the blocked polyisocyanate compound (B) may contain a structure derived from a hydrophilic group-containing compound (b4) having one isocyanate-reactive functional group.

[0127] Hydrophilic group-containing compound (b4) The hydrophilic group-containing compound (b4) has one isocyanate-reactive functional group. Examples of the hydrophilic group-containing compound (b4) include nonionic hydrophilic group-containing compounds, anionic hydrophilic group-containing compounds, cationic hydrophilic group-containing compounds, and any combination thereof. The hydrophilic group-containing compound (b4) is preferably a nonionic hydrophilic group-containing compound because the reaction in which the isocyanate group in the polyisocyanate compound (b1) is blocked by the aforementioned blocking agent (b2) is less likely to be inhibited.

[0128] Examples of the nonionic hydrophilic group-containing compounds include compounds having a polyoxyalkylene group. Examples of the polyoxyalkylene group include a polyoxyethylene group, a polyoxypropylene group, and a polyoxyethyleneoxypropylene group. From the viewpoint of storage stability of the aqueous coating composition of the present invention, it is preferable that the nonionic hydrophilic group-containing compound has a polyoxyethylene group.

[0129] From the viewpoint of storage stability of the aqueous coating composition of the present invention and water resistance of the formed coating film, the compound having the above polyoxyethylene group preferably has 3 or more, preferably 5 to 100, and more preferably 8 to 45 consecutive polyoxyethylene groups, i.e., polyoxyethylene blocks.

[0130] Furthermore, the compound having the polyoxyethylene group may also contain oxyalkylene groups other than the oxyethylene group, in addition to the polyoxyethylene block. Examples of oxyalkylene groups other than the oxyethylene group include the oxypropylene group, the oxybutylene group, and the oxystyrene group.

[0131] In the compound having the polyoxyethylene group described above, the molar ratio of the oxyethylene group in the oxyalkylene group is preferably in the range of 20 to 100 mol%, and more preferably in the range of 50 to 100 mol%, from the viewpoint of storage stability of the aqueous coating composition of the present invention. When the molar ratio of the oxyethylene group in the oxyalkylene group is 20 mol% or more, sufficient hydrophilicity is imparted, and the storage stability of the aqueous coating composition of the present invention is good.

[0132] Furthermore, the nonionic hydrophilic group-containing compound described above preferably has a number average molecular weight in the range of 200 to 2,000, from the viewpoint of storage stability of the aqueous coating composition of the present invention and water resistance of the coating film formed. From the viewpoint of storage stability of the aqueous coating composition of the present invention, the number average molecular weight is more preferably 300 or more, and even more preferably 400 or more. From the viewpoint of water resistance of the coating film formed by the aqueous coating composition of the present invention, the number average molecular weight is more preferably 1,500 or less, and even more preferably 1,200 or less.

[0133] Examples of the nonionic hydrophilic group-containing compounds mentioned above include polyethylene glycol monoalkyl ethers (also known as ω-alkoxypolyoxyethylene) such as polyethylene glycol monomethyl ether and polyethylene glycol monoethyl ether; polypropylene glycol monoalkyl ethers (also known as ω-alkoxypolyoxypropylene) such as polypropylene glycol monomethyl ether and polypropylene glycol monoethyl ether; ω-alkoxypolyoxyethylene (oxypropylene) such as ω-methoxypolyoxyethylene (oxypropylene) and ω-ethoxypolyoxyethylene (oxypropylene); polyethylene glycol (propylene glycol) monoalkyl ethers such as polyethylene glycol (propylene glycol) monomethyl ether and polyethylene glycol (propylene glycol) monoethyl ether, and any combination thereof.

[0134] As the nonionic hydrophilic group-containing compound mentioned above, polyethylene glycol monomethyl ether and polyethylene glycol monoethyl ether are preferred, and polyethylene glycol monomethyl ether is even more preferred.

[0135] In this specification, "polyethylene glycol (propylene glycol)" means a copolymer of ethylene glycol and propylene glycol, and includes block copolymers and random copolymers.

[0136] Furthermore, commercially available polyethylene glycol monomethyl ether products include, for example, "Uniox M-400," "Uniox M-550," "Uniox M-1000," and "Uniox M-2000" manufactured by NOF Corporation.

[0137] By reacting some of the isocyanate groups in a polyisocyanate compound (b1) with a hydrophilic group-containing compound (b4), a polyisocyanate compound (b1) containing a structure derived from the hydrophilic group-containing compound (b4) can be formed. In this case, from the viewpoint of storage stability of the aqueous coating composition of the present invention and the adhesion, smoothness, clarity, water resistance, chipping resistance, and glass adhesion of the formed coating film, it is preferable that the ratio of polyisocyanate compound (b1) to hydrophilic group-containing compound (b4) is within the range of 0.03 to 0.6 moles, based on 1 mole of isocyanate groups in the polyisocyanate compound (b1). By blocking the polyisocyanate compound (b1) containing a structure derived from the hydrophilic group-containing compound (b4), a blocked polyisocyanate compound (B) containing a structure derived from the hydrophilic group-containing compound (b4) can be formed.

[0138] When the isocyanate group in the polyisocyanate compound (b1) is reacted with the blocking agent (b2) and the spacer (b3) and / or the hydrophilic group-containing compound (b4), the order of the reactions is not particularly limited.

[0139] Specifically, examples include a method in which a spacer (b3) and / or a hydrophilic group-containing compound (b4) are reacted with a portion of the isocyanate groups in a polyisocyanate compound (b1), and then the remaining isocyanate groups are blocked with a blocking agent (b2); a method in which a portion of the isocyanate groups in a polyisocyanate compound (b1) are blocked with a blocking agent (b2), and then the remaining isocyanate groups are reacted with a spacer (b3) and / or a hydrophilic group-containing compound (b4); and a method in which the isocyanate groups in a polyisocyanate compound (b1) are reacted simultaneously with a blocking agent (b2), a spacer (b3), and / or a hydrophilic group-containing compound (b4).

[0140] The weight-average molecular weight of the blocked polyisocyanate compound (B) is preferably in the range of 3,000 to 200,000, more preferably in the range of 10,000 to 150,000, and even more preferably in the range of 30,000 to 120,000, from the viewpoint of manufacturing stability and the water resistance and hardness of the formed coating film.

[0141] The content of the blocked polyisocyanate compound (B) is preferably 2 to 70% by mass, more preferably 5 to 50% by mass, and even more preferably 10 to 40% by mass, based on the amount of resin solids in the aqueous coating composition.

[0142] Polycarbodiimide compounds (C) The polycarbodiimide compound (C) is a compound having at least two carbodiimide groups in one molecule, and for example, a compound obtained by decarbonizing the isocyanate groups of an isocyanate group-containing compound can be used.

[0143] As the polycarbodiimide compound (C) mentioned above, it is preferable to use a water-soluble or water-dispersible polycarbodiimide compound from the viewpoint of forming a coating film that exhibits excellent storage stability, high hardness, water resistance, and other excellent coating film performance, as well as good glass adhesion. The water-soluble or water-dispersible polycarbodiimide compound can be used without particular limitations as long as it is a polycarbodiimide compound that can be stably dissolved or dispersed in an aqueous medium.

[0144] Specifically, examples of the above-mentioned water-soluble polycarbodiimide compounds include "Carbodilite SV-02," "Carbodilite V-02," "Carbodilite V-02-L2," and "Carbodilite V-04" (all manufactured by Nisshinbo Co., Ltd., trade names). Furthermore, examples of the above-mentioned water-dispersible polycarbodiimide compounds include "Carbodilite E-01," "Carbodilite E-02," and "Carbodilite E-05" (all manufactured by Nisshinbo Co., Ltd., trade names).

[0145] The content of polycarbodiimide compound (C) is preferably 1 to 20% by mass, more preferably 2 to 15% by mass, and even more preferably 3 to 10% by mass, based on the amount of resin solids in the aqueous coating composition.

[0146] Basic compounds (D) Basic compound (D) is a compound used for neutralizing the carboxyl groups in the hydroxyl and carboxyl group-containing resin (A) and for adjusting the pH of the aqueous paint composition. Therefore, compounds that chemically react with other functional groups, such as the diamine chain extender described in the description of the hydroxyl and carboxyl group-containing polyurethane resin (A3), are excluded.

[0147] The amount of the basic compound (D) can be such that the pH of the aqueous coating composition of the present invention is within the range of 8.0 to 11.5. The amount of the basic compound (D) is preferably 0.1 to 15% by mass, more preferably 0.5 to 10% by mass, and even more preferably 1 to 8% by mass, based on the amount of resin solids in the aqueous coating composition.

[0148] In the present invention, the basic compound (D) contains a basic compound having an acid dissociation constant (PKa) in the range of 7.0 to 8.5 and a boiling point in the range of 100 to 200°C, and the content of the basic compound (D1) is in the range of 30 to 100% by mass, based on the mass of the basic compound (D).

[0149] Basic compound (D1) Basic compounds (D1) have an acid dissociation constant (PKa) in the range of 7.0 to 8.5 and a boiling point in the range of 100 to 200°C.

[0150] If the above acid dissociation constant (PKa) is 7.0 or higher, an aqueous coating composition with excellent storage stability can be obtained, and if it is 8.5 or lower, an aqueous coating composition with excellent hardness and water resistance of the formed coating film can be obtained.

[0151] In particular, the acid dissociation constant (PKa) of the basic compound (D1) is preferably in the range of 7.2 to 8.2, and more preferably in the range of 7.3 to 7.8.

[0152] If the boiling point is 100°C or higher, an aqueous coating composition with excellent storage stability can be obtained, and if it is 200°C or lower, an aqueous coating composition with excellent hardness and water resistance of the formed coating film can be obtained.

[0153] In particular, the boiling point of the basic compound (D1) is preferably in the range of 105 to 180°C, and more preferably in the range of 110 to 160°C.

[0154] Examples of the basic compound (D1) mentioned above include 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), N-ethylmorpholine (PKa 7.7, boiling point 139°C), and triallylamine (PKa 8.3, boiling point 156°C). These can be used individually or in combination of two or more.

[0155] As the basic compound (D1) mentioned above, it is preferable to use N-methylmorpholine (PKa 7.4, boiling point 116°C) or N-ethylmorpholine (PKa 7.7, boiling point 139°C) from the viewpoint of storage stability of the aqueous coating composition of the present invention, hardness of the formed coating film, and water resistance.

[0156] The content of the basic compound (D1) in the aqueous coating composition of the present invention is in the range of 30 to 100% by mass, based on the mass of the basic compound (D).

[0157] When the basic compound (D1) content in the aqueous coating composition of the present invention is 30% by mass or more, an aqueous coating composition with excellent storage stability, hardness and water resistance of the formed coating film, and glass adhesion can be obtained.

[0158] In particular, the content of the basic compound (D1) in the aqueous coating composition of the present invention is preferably in the range of 35 to 90% by mass, and more preferably in the range of 35 to 80% by mass, based on the mass of the basic compound (D).

[0159] Examples of basic compounds (D) other than basic compound (D1) include ammonia, triethylamine, triethanolamine, 2-(dimethylamino)ethanol, and diethylethanolamine.

[0160] The reason why the aqueous coating composition of the present invention exhibits excellent storage stability and forms a coating film with high hardness, water resistance, and good glass adhesion even when cured at relatively low temperatures is not clear. However, it is presumed that the acid dissociation constant (PKa) of the basic compound (D1) is 7.0 or higher, which allows it to neutralize the carboxyl groups of the hydroxyl and carboxyl group-containing resin (A), resulting in good storage stability. Furthermore, the acid dissociation constant (PKa) of the basic compound (D1) is 8.5 or lower, which does not inhibit the reaction between the hydroxyl and carboxyl group-containing resin (A) and the blocked polyisocyanate compound (B), and the reaction between the hydroxyl and carboxyl group-containing resin (A) and the polycarbodiimide compound (C), thereby enabling the formation of a coating film with excellent coating performance and good glass adhesion. Furthermore, since the boiling point of the basic compound (D1) is 100°C or higher, the amount of basic compound (D1) volatilizing during storage of the aqueous coating composition of the present invention is small, resulting in good storage stability. Also, since the boiling point of the basic compound (D1) is 200°C or lower, the amount of basic compound (D1) volatilizing during heat curing is large, which does not inhibit the reaction between the hydroxyl group and carboxyl group-containing resin (A) and the blocked polyisocyanate compound (B), and the reaction between the hydroxyl group and carboxyl group-containing resin (A) and the polycarbodiimide compound (C), thus forming a coating film that exhibits excellent coating performance and good glass adhesion. Furthermore, by adjusting the pH of the aqueous coating composition to a relatively high range of 8.0 to 11.5, the carboxyl groups of the hydroxyl group and carboxyl group-containing resin (A) are neutralized, which is presumed to suppress the reaction of the carbodiimide compound (C) and improve storage stability.

[0161] Melamine resin (E) The aqueous coating composition of the present invention may further contain melamine resin (E). As the melamine resin (E), a partially methylolated melamine resin or a fully methylolated melamine resin obtained by the reaction of a melamine component with an aldehyde component can be used. Examples of aldehyde components include formaldehyde, paraformaldehyde, acetaldehyde, and benzaldehyde.

[0162] Furthermore, a methylolated melamine resin can also be used in which the methylol groups have been partially or completely etherified with a suitable alcohol. Examples of alcohols that can be used for etherification include methyl alcohol, ethyl alcohol, n-propyl alcohol, i-propyl alcohol, n-butyl alcohol, i-butyl alcohol, 2-ethyl-1-butanol, and 2-ethyl-1-hexanol.

[0163] The melamine resin (E) is preferably a methyl etherified melamine resin in which the methylol groups of a partially or completely methylolated melamine resin are partially or completely etherified with methyl alcohol, a butyl etherified melamine resin in which the methylol groups of a partially or completely methylolated melamine resin are partially or completely etherified with butyl alcohol, or a methyl-butyl mixed etherified melamine resin in which the methylol groups of a partially or completely methylolated melamine resin are partially or completely etherified with methyl alcohol and butyl alcohol, with the methyl-butyl mixed etherified melamine resin being more preferred.

[0164] The melamine resin (B) preferably has a weight-average molecular weight of 400 to 6,000, more preferably 500 to 4,000, and even more preferably 600 to 3,000.

[0165] Commercially available melamine resin (E) can be used. Examples of commercially available product names include "Cymel 202", "Cymel 203", "Cymel 204", "Cymel 211", "Cymel 212", "Cymel 238", "Cymel 251", "Cymel 253", "Cymel 254", "Cymel 303", "Cymel 323", "Cymel 324", "Cymel 325", "Cymel 327", "Cymel 350", "Cymel 370", "Cymel 380", "Cymel 385", "Cymel 1156", "Cymel 1158", "Cymel 1116", and "Cymel 11 Examples include: "30" (manufactured by Ornex Japan Co., Ltd.); "Regimin 735", "Regimin 740", "Regimin 741", "Regimin 745", "Regimin 746", "Regimin 747" (manufactured by Monsanto); "Uban 120", "Uban 20HS", "Uban 20SE", "Uban 2021", "Uban 2028", "Uban 28-60" (manufactured by Mitsui Chemicals, Inc.); "Sumimar M55", "Sumimar M30W", "Sumimar M50W" (manufactured by Sumitomo Chemical Co., Ltd.); and others.

[0166] When the aqueous coating composition of the present invention contains the above-mentioned melamine resin (E), the content of the melamine resin (E) is preferably 1 to 30% by mass, more preferably 2 to 20% by mass, and even more preferably 3 to 15% by mass, based on the amount of resin solids in the aqueous coating composition.

[0167] Other ingredients The aqueous coating composition of the present invention may further contain, if necessary, resins, pigments, organic solvents, curing catalysts, dispersants, anti-settling agents, defoaming agents, thickeners, ultraviolet absorbers, light stabilizers, surface modifiers, etc.

[0168] Examples of resins other than those mentioned above include acrylic resins that do not contain hydroxyl groups and / or carboxyl groups, polyester resins that do not contain hydroxyl groups and / or carboxyl groups, polyurethane resins that do not contain hydroxyl groups and / or carboxyl groups, polyether resins that do not contain hydroxyl groups and / or carboxyl groups, polycarbonate resins that do not contain hydroxyl groups and / or carboxyl groups, epoxy resins that do not contain hydroxyl groups and / or carboxyl groups, and among these, it is preferable to use polyurethane resins that do not contain hydroxyl groups and contain carboxyl groups.

[0169] The carboxyl group-containing polyurethane resin that does not contain the hydroxyl group described above can be obtained by conventional methods, for example, by reacting a polyol with a polyisocyanate. After this reaction, the chain can be extended in the presence of a chain extender, which is a low molecular weight compound having at least two active hydrogens in one molecule, such as a diol or diamine. Furthermore, the resin can be modified with an acrylic resin or the like during or after its preparation.

[0170] The polyol, polyisocyanate, and chain extender mentioned above can be any compound described in the description section for the hydroxyl group and carboxyl group-containing polyurethane resin (A3).

[0171] Examples of the above-mentioned pigments include coloring pigments, extender pigments, and luminosity pigments. These pigments can be used individually or in combination of two or more types.

[0172] When the aqueous coating composition of the present invention contains the above-mentioned pigment, the amount of the pigment can be preferably in the range of 1 to 200 parts by mass, more preferably 5 to 160 parts by mass, and even more preferably 15 to 140 parts by mass, based on 100 parts by mass of the resin solids in the aqueous coating composition of the present invention.

[0173] Examples of the aforementioned coloring pigments include titanium dioxide, zinc oxide, carbon black, molybdenum red, Prussian blue, cobalt blue, azo pigments, phthalocyanine pigments, quinacridone pigments, isoindoline pigments, surene pigments, perylene pigments, dioxazine pigments, and diketopyrrolopyrrole pigments.

[0174] When the aqueous coating composition of the present invention contains the above-mentioned coloring pigment, the amount of the coloring pigment can be preferably in the range of 1 to 180 parts by mass, more preferably 5 to 150 parts by mass, and even more preferably 15 to 130 parts by mass, based on 100 parts by mass of resin solids in the aqueous coating composition of the present invention.

[0175] Examples of the extender pigments include barium sulfate, talc, clay, kaolin, barium carbonate, calcium carbonate, silica, and alumina white. Barium sulfate can be suitably used as the extender pigment from the viewpoint of improving design aesthetics.

[0176] When the aqueous coating composition of the present invention contains the above-mentioned extender pigment, the amount of the extender pigment can be preferably in the range of 1 to 180 parts by mass, more preferably 5 to 140 parts by mass, and even more preferably 10 to 120 parts by mass, based on 100 parts by mass of the resin solids content in the aqueous coating composition of the present invention.

[0177] Examples of the aforementioned luminous pigments include aluminum (including vapor-deposited aluminum), copper, zinc, brass, nickel, glass flakes, aluminum oxide, mica, aluminum oxide coated with titanium oxide and / or iron oxide, and mica coated with titanium oxide and / or iron oxide. Among these, the use of aluminum pigments is preferred. Aluminum pigments include non-leafing aluminum pigments and leafing aluminum pigments, and either can be used.

[0178] The above-mentioned lustrous pigment is preferably in the form of flakes. Furthermore, suitable lustrous pigments have a longitudinal dimension of 1 to 100 μm, particularly 5 to 40 μm, and a thickness of 0.001 to 5 μm, particularly 0.01 to 2 μm.

[0179] If the aqueous coating composition of the present invention contains the above-mentioned luminous pigment, the amount of the luminous pigment can be preferably in the range of 0.1 to 100 parts by mass, more preferably 1 to 50 parts by mass, and even more preferably 3 to 25 parts by mass, based on 100 parts by mass of the resin solids in the aqueous coating composition of the present invention.

[0180] Examples of the aforementioned organic solvents include ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ester solvents such as ethyl acetate, butyl acetate, methyl benzoate, ethyl ethoxypropionate, ethyl propionate, and methyl propionate; alcohol solvents such as isopropanol, n-butanol, isobutanol, and 2-ethylhexanol; ether solvents such as tetrahydrofuran, dioxane, and dimethoxyethane; glycol ether solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, and 3-methoxybutyl acetate; aromatic hydrocarbon solvents, aliphatic hydrocarbon solvents, and the like.

[0181] The curing catalysts include, specifically, tin octoate, dibutyltin diacetate, dibutyltin di(2-ethylhexanoate), dibutyltin dilaurate, dioctyltin diacetate, dioctyltin di(2-ethylhexanoate), dibutyltin oxide, dibutyltin sulfide, dioctyltin oxide, dibutyltin fatty acid salts, lead 2-ethylhexanoate, zinc octoate, zinc naphthenate, zinc fatty acids, bismuth octanoate, bismuth 2-ethylhexanoate, bismuth oleate, bismuth neodecanoate, and Versatile Organometallic compounds such as bismuth citrate, bismuth naphthenate, cobalt naphthenate, calcium octylate, copper naphthenate, and tetra(2-ethylhexyl) titanate; sulfonic acid group-containing compounds such as p-toluenesulfonic acid, dodecylbenzenesulfonic acid, and dinonylnaphthalenesulfonic acid; and phosphate group-containing compounds such as monobutyl phosphate, dibutyl phosphate, mono-2-ethylhexyl phosphate, di-2-ethylhexyl phosphate, alkyl ether phosphate, polyoxyethylene alkyl ether phosphate, and phosphate group-containing resins can be used. In particular, from the viewpoint of storage stability of the aqueous coating composition and water resistance and chipping resistance of the formed coating film, phosphate group-containing compounds are preferred, phosphate group-containing resins are more preferred, and phosphate group-containing acrylic resins are even more preferred.

[0182] The above-mentioned phosphate group-containing acrylic resin can be synthesized by copolymerizing a polymerizable unsaturated monomer having a phosphate group and other polymerizable unsaturated monomers by conventional methods.

[0183] In a phosphate-containing acrylic resin, the polymerizable unsaturated monomer having a phosphate group is preferably 1 to 50% by mass, and particularly 5 to 40% by mass, based on the total amount of each monomer component constituting the phosphate-containing acrylic resin.

[0184] The weight-average molecular weight of the phosphate-containing acrylic resin is preferably in the range of 3,000 to 30,000, more preferably in the range of 5,000 to 25,000, and even more preferably in the range of 10,000 to 20,000, from the viewpoint of the finished appearance and coating film performance.

[0185] When a phosphate group-containing compound is used as a curing catalyst, its content is preferably 0.1 to 20% by mass, more preferably 0.5 to 15% by mass, and even more preferably 1 to 10% by mass, based on the amount of resin solids in the aqueous paint composition.

[0186] The aqueous coating composition of the present invention can be used for painting by diluting it with water and / or an organic solvent as needed and adjusting it to an appropriate viscosity.

[0187] The appropriate viscosity varies depending on the paint composition, but for example, when adjusted using a Ford Cup viscometer No. 4, it can typically be set to a viscosity of about 20 to 100 seconds, preferably 25 to 70 seconds, at 20°C.

[0188] Furthermore, the coating solid content concentration of the aqueous coating composition of the present invention is preferably about 5 to 70% by mass, and more preferably about 10 to 55% by mass.

[0189] The aqueous paint composition of the present invention may be either a one-component paint or a multi-component paint, but it is preferable to be a one-component paint from the viewpoint of having no paint mixing step and having excellent productivity, and simplifying the maintenance of painting machinery.

[0190] The aqueous coating composition of the present invention can be applied to an object by known methods, such as air spray coating, airless spray coating, rotary atomization coating, curtain coating, etc., and electrostatic application may be performed during coating. Of these, air spray coating and rotary atomization coating are preferred. Furthermore, such coating methods can be carried out in one to several stages until the desired film thickness is obtained.

[0191] The amount of aqueous coating composition of the present invention applied can typically be such that the cured film thickness is 5 to 40 μm, preferably 7 to 35 μm, and more preferably 10 to 30 μm.

[0192] Method for forming a multi-layer coating film The aqueous coating composition of the present invention can form a coating film that exhibits excellent coating performance, such as high hardness and water resistance, as well as good glass adhesion, even when cured at relatively low temperatures, making it suitable for use as an intermediate coating composition. This coating is particularly suitable for use as an automotive coating.

[0193] As a method for forming a multi-layer coating film in which this paint is applied as an intermediate coating composition, for example, the following method can be suitably used.

[0194] Step (1): A step of applying the aqueous coating composition of the present invention onto the object to be coated to form an intermediate coating film. Step (2): A step of applying an aqueous base coat paint composition onto the intermediate coating film formed in step (1) to form a base coat film. Step (3): A step of applying a clear coating composition onto the base coat coating formed in step (2) to form a clear coating, and A method for forming a multilayer coating film, comprising the steps of: (4) heating and curing the intermediate coating film, base coat film, and clear coating film formed in steps (1) to (3) at a single temperature within the range of 60 to 110°C; and so on.

[0195] Examples of objects to be coated include the exterior panels of automobile bodies such as passenger cars, trucks, motorcycles, and buses; automobile parts; and the exterior panels of household electrical appliances such as mobile phones and audio equipment. Of these, the exterior panels of automobile bodies and automobile parts are preferred.

[0196] The materials of these objects to be coated are not particularly limited. Examples include metallic materials such as iron, aluminum, brass, copper, tinplate, stainless steel, galvanized steel, and zinc alloy (Zn-Al, Zn-Ni, Zn-Fe, etc.) plated steel; plastic materials such as polyethylene resin, polypropylene resin, acrylonitrile-butadiene-styrene (ABS) resin, polyamide resin, acrylic resin, vinylidene chloride resin, polycarbonate resin, polyurethane resin, epoxy resin, and various types of FRP; inorganic materials such as glass, cement, and concrete; wood; and fibrous materials such as paper and cloth. Of these, metallic and plastic materials are preferred.

[0197] Furthermore, the surface to which the coating film is applied may be a metal surface such as the exterior body panels of an automobile, automobile parts, household electrical appliances, or steel plates or other metal substrates that make up these, which may have been subjected to surface treatments such as phosphate treatment, chromate treatment, or composite oxide treatment.

[0198] A coating film may be formed on an object whether or not it has undergone surface treatment. For example, the substrate (the object to be coated) may be surface-treated as needed, and then a primer coating film may be formed on it. If the object to be coated is, for example, an automobile body, the primer coating film can be formed using a known primer paint that is commonly used in automobile body painting.

[0199] For example, an electrodeposition paint, preferably a cationic electrodeposition paint, can be used as the primer paint for forming the undercoat film.

[0200] As the above-mentioned aqueous base coat paint composition, a base resin such as an acrylic resin, polyester resin, alkyd resin, urethane resin, or epoxy resin having crosslinkable functional groups such as carboxyl groups or hydroxyl groups, and a crosslinking agent such as an amino resin such as a melamine resin or urea resin, or a polyisocyanate compound which may be blocked, can be used in a paint mixture with a pigment, a thickener, and other optional components.

[0201] Any thermosetting clear coating composition known for use in painting automobile bodies and the like can be used as the clear coating composition. Examples of such thermosetting clear coating compositions include organic solvent-type thermosetting coating compositions containing a base resin having a crosslinkable functional group and a curing agent, aqueous thermosetting coating compositions, powder thermosetting coating compositions, and the like.

[0202] Examples of crosslinkable functional groups in the above-mentioned base resin include carboxyl groups, hydroxyl groups, epoxy groups, and silanol groups. Examples of base resin types include acrylic resins, polyester resins, alkyd resins, urethane resins, epoxy resins, and fluororesins. Examples of curing agents include polyisocyanate compounds, blocked polyisocyanate compounds, melamine resins, urea resins, carboxyl group-containing compounds, carboxyl group-containing resins, epoxy group-containing resins, and epoxy group-containing compounds.

[0203] Preferred combinations of base resin / curing agent for the above clear coating composition include hydroxyl group-containing resin / polyisocyanate compound, carboxyl group-containing resin / epoxy group-containing resin, hydroxyl group-containing resin / blocked polyisocyanate compound, and hydroxyl group-containing resin / melamine resin, with hydroxyl group-containing resin / polyisocyanate compound being more preferred.

[0204] Furthermore, the clear coating composition may be a one-component coating or a multi-component coating such as a two-component urethane resin coating.

[0205] Furthermore, the above clear coating composition may contain, if necessary, coloring pigments, luminescence pigments, dyes, etc., to an extent that does not impair transparency, and may also contain, as appropriate, extender pigments, ultraviolet absorbers, light stabilizers, defoamers, thickeners, rust inhibitors, surface modifiers, etc.

[0206] The method of applying the clear coating composition is not particularly limited, but a wet coating can be formed by methods such as air spray coating, airless spray coating, rotary atomization coating, and curtain coat coating. Electrostatic application may be performed as needed in these coating methods. Of these, air spray coating or rotary atomization coating is particularly preferred. The amount of clear coating composition applied is usually such that the cured film thickness is 10 to 70 μm, preferably 20 to 50 μm.

[0207] Furthermore, when performing air spray painting, airless spray painting, or rotary atomization painting, it is preferable to adjust the viscosity of the clear coating composition using an organic solvent or other solvent to a viscosity range suitable for the painting method, typically a viscosity range of about 15 to 60 seconds at 20°C, and particularly about 20 to 50 seconds, using a Ford Cup No. 4 viscometer.

[0208] The heating can be carried out by known means, for example, by drying furnaces such as hot air furnaces, electric furnaces, and infrared induction heating furnaces. The heating temperature is in the range of 60 to 110°C, preferably 70 to 90°C. The heating time is not particularly limited, but is preferably in the range of 10 to 40 minutes, more preferably 20 to 40 minutes. [Examples]

[0209] The present invention will be described in more detail below with reference to manufacturing examples, embodiments, and comparative examples. These manufacturing examples, embodiments, and comparative examples are merely illustrative and are not intended to limit the scope of the present invention. In the manufacturing examples, embodiments, and comparative examples, "parts" and "%" are based on mass unless otherwise specified. Furthermore, the film thickness of the coating is based on the cured coating.

[0210] Production of hydroxyl group and carboxyl group-containing resin (A) Production of acrylic resin particles (A1') containing hydroxyl and carboxyl groups having a core-shell structure Manufacturing Example 1 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device, 130 parts of deionized water and 0.52 parts of "Aqualon KH-10" (product name, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., emulsifier, 97% active ingredient) were charged, stirred and mixed in a nitrogen atmosphere, and the temperature was raised to 80°C. Next, 1.72 parts of the monomer emulsion (1) and 5.3 parts of a 6% ammonium persulfate aqueous solution were introduced into the reaction vessel and maintained at 80°C for 15 minutes. Then, the remaining monomer emulsion (1) was added dropwise over 3 hours to the reaction vessel, which was maintained at the same temperature, and the mixture was allowed to mature for 1 hour after the dropwise addition was complete. Subsequently, the monomer emulsion (2) was added dropwise over 1 hour and matured for 1 hour. Then, 20 parts of a 5% N-ethylmorpholine aqueous solution were gradually added to the reaction vessel while cooling to 30°C, and the mixture was drained while filtering through a 100-mesh nylon cloth to obtain hydroxyl and carboxyl group-containing acrylic resin particles (A1'-1) with a solid content of 30%. The obtained hydroxyl and carboxyl group-containing acrylic resin particles (A1'-1) had an acid value of 16 mg KOH / g, a hydroxyl value of 66 mg KOH / g, and a glass transition temperature of 21°C. Monomer emulsion (1): 42 parts of deionized water, 0.72 parts of "Aqualon KH-10", 2 parts of methylenebisacrylamide, 5 parts of styrene, 15 parts of methyl methacrylate, 5 parts of 2-hydroxyethyl methacrylate, and 23 parts of n-butyl acrylate were mixed and stirred to obtain monomer emulsion (1). Monomer emulsion (2): 42 parts of deionized water, 0.72 parts of Aqualon KH-10, 0.05 parts of ammonium persulfate, 2.5 parts of methacrylic acid, 10 parts of 2-hydroxyethyl methacrylate, 5 parts of styrene, 12.5 parts of methyl methacrylate, 10 parts of n-butyl acrylate, and 10 parts of n-butyl methacrylate were mixed and stirred to obtain monomer emulsion (2).

[0211] Manufacturing Example 2 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device, 130 parts of deionized water and 0.52 parts of "Aqualon KH-10" (product name, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., emulsifier, 97% active ingredient) were charged, stirred and mixed in a nitrogen atmosphere, and the temperature was raised to 80°C. Next, 1.72 parts of the monomer emulsion (1) and 5.3 parts of a 6% ammonium persulfate aqueous solution were introduced into the reaction vessel and maintained at 80°C for 15 minutes. Then, the remaining monomer emulsion (1) was added dropwise over 3 hours to the reaction vessel, which was maintained at the same temperature, and the mixture was allowed to mature for 1 hour after the dropwise addition was complete. Subsequently, the monomer emulsion (2) was added dropwise over 1 hour and matured for 1 hour. Then, 20 parts of a 5% 2-(dimethylamino)ethanol aqueous solution were gradually added to the reaction vessel while cooling to 30°C, and the mixture was discharged while filtering through a 100-mesh nylon cloth to obtain hydroxyl and carboxyl group-containing acrylic resin particles (A1'-2) with a solid content of 30%. The obtained hydroxyl and carboxyl group-containing acrylic resin particles (A1'-2) had an acid value of 16 mg KOH / g, a hydroxyl value of 66 mg KOH / g, and a glass transition temperature of 21°C. Monomer emulsion (1): 42 parts of deionized water, 0.72 parts of "Aqualon KH-10", 2 parts of methylenebisacrylamide, 5 parts of styrene, 15 parts of methyl methacrylate, 5 parts of 2-hydroxyethyl methacrylate, and 23 parts of n-butyl acrylate were mixed and stirred to obtain monomer emulsion (1). Monomer emulsion (2): 42 parts of deionized water, 0.72 parts of Aqualon KH-10, 0.05 parts of ammonium persulfate, 2.5 parts of methacrylic acid, 10 parts of 2-hydroxyethyl methacrylate, 5 parts of styrene, 12.5 parts of methyl methacrylate, 10 parts of n-butyl acrylate, and 10 parts of n-butyl methacrylate were mixed and stirred to obtain monomer emulsion (2).

[0212] Production of hydroxyl and carboxyl group-containing acrylic resin particles (A1'') having a core-shell structure including a gradient polymer layer. Manufacturing Example 3 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropping device, 130 parts of deionized water and 0.52 parts of "Aqualon KH-10" (product name, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., emulsifier, 97% active ingredient) were charged, stirred and mixed in a nitrogen atmosphere, and the temperature was raised to 80°C. Next, 1.72 parts of monomer emulsion (1) and 5.3 parts of a 6% ammonium persulfate aqueous solution were introduced into the reaction vessel and maintained at 80°C for 15 minutes. Then, 35.4 parts of monomer emulsion (1) were added dropwise to the reaction vessel, which was maintained at the same temperature, over a period of 1 hour. Immediately after the dropwise addition was complete, the remaining 55.6 parts of monomer emulsion (1) were added dropwise to the reaction vessel. At the same time, 55.6 parts of monomer emulsion (2) were added dropwise to monomer emulsion (1), and the dropwise addition of monomer emulsion (1) and monomer emulsion (2) was completed over a period of 2 hours. Subsequently, 37.17 parts of the remaining monomer emulsion (2) were added dropwise to the reaction vessel over 1 hour, and after aging for 1 hour, 20 parts of a 5% N-ethylmorpholine aqueous solution were gradually added to the reaction vessel while cooling to 30°C. Then, the mixture was filtered through a 100-mesh nylon cloth to obtain hydroxyl and carboxyl group-containing acrylic resin particles (A1''-1) with a solid content of 30%. The obtained hydroxyl and carboxyl group-containing acrylic resin particles (A1''-1) had an acid value of 16 mg KOH / g, a hydroxyl value of 66 mg KOH / g, and a glass transition temperature of 21°C. Monomer emulsion (1): 42 parts of deionized water, 0.72 parts of "Aqualon KH-10", 2 parts of methylenebisacrylamide, 5 parts of styrene, 15 parts of methyl methacrylate, 5 parts of 2-hydroxyethyl methacrylate, and 23 parts of n-butyl acrylate were mixed and stirred to obtain monomer emulsion (1). Monomer emulsion (2): 42 parts of deionized water, 0.72 parts of Aqualon KH-10, 0.05 parts of ammonium persulfate, 2.5 parts of methacrylic acid, 10 parts of 2-hydroxyethyl methacrylate, 5 parts of styrene, 12.5 parts of methyl methacrylate, 10 parts of n-butyl acrylate, and 10 parts of n-butyl methacrylate were mixed and stirred to obtain monomer emulsion (2).

[0213] Production of hydroxyl group and carboxyl group-containing polyester resin (A2) Manufacturing Example 4 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and water separator, 174 parts of trimethylolpropane, 327 parts of neopentyl glycol, 352 parts of adipic acid, 109 parts of isophthalic acid, and 101 parts of 1,2-cyclohexanedicarboxylic acid anhydride were charged. The temperature was raised from 160°C to 230°C over 3 hours, and the mixture was maintained at 230°C while the generated condensed water was removed by distillation using a water separator until the acid value was 3 mg KOH / g or less. To this reaction product, 59 parts of trimellitic anhydride were added, and an addition reaction was carried out at 170°C for 30 minutes. After cooling to below 50°C, 75.8 parts of N-ethylmorpholine were added to neutralize the mixture, and then deionized water was gradually added to obtain a polyester resin solution (A2-1) containing hydroxyl and carboxyl groups with a solid content of 45%. The obtained hydroxyl and carboxyl group-containing polyester resin had a hydroxyl value of 128 mgKOH / g, an acid value of 35 mgKOH / g, and a weight-average molecular weight of 13,000.

[0214] Manufacturing Example 5 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and water separator, 174 parts of trimethylolpropane, 327 parts of neopentyl glycol, 352 parts of adipic acid, 109 parts of isophthalic acid, and 101 parts of 1,2-cyclohexanedicarboxylic acid anhydride were charged. The temperature was raised from 160°C to 230°C over 3 hours, and the mixture was maintained at 230°C while the generated condensed water was removed by distillation using a water separator until the acid value was 3 mg KOH / g or less. To this reaction product, 59 parts of trimellitic anhydride were added, and an addition reaction was carried out at 170°C for 30 minutes. After cooling to below 50°C, 58.7 parts of 2-(dimethylamino)ethanol were added to neutralize the mixture, and then deionized water was gradually added to obtain a polyester resin solution (A2-2) containing hydroxyl and carboxyl groups with a solid content of 45%. The obtained hydroxyl and carboxyl group-containing polyester resin had a hydroxyl value of 128 mgKOH / g, an acid value of 35 mgKOH / g, and a weight-average molecular weight of 13,000.

[0215] Production of polyurethane resin (A3) containing hydroxyl and carboxyl groups. Manufacturing Example 6 In a reaction vessel equipped with a thermometer, stirrer, and reflux condenser, 211.9 parts of polytetramethylene glycol (number average molecular weight 1000), 11.5 parts of 2,2-dimethylolpropionic acid, 6.9 parts of trimethylolpropane, 112.2 parts of isophorone diisocyanate, and 298.5 parts of methyl ethyl ketone were charged. After purging the reaction system with nitrogen gas, the mixture was reacted at 80°C under stirring to obtain an NCO-terminated urethane prepolymer with a free isocyanate group content of 3.2%. The obtained methyl ethyl ketone solution was cooled to 40°C, and 493.2 g of deionized water containing 9.8 parts of N-ethylmorpholine was added and emulsified. Then, 275.9 parts of a 5% N-(2-hydroxyethyl)ethylenediamine aqueous solution were added, and after stirring for 60 minutes, the methyl ethyl ketone was removed by distillation under reduced pressure and heating, and the concentration was adjusted with deionized water to obtain a polyurethane resin dispersion (A3-1) containing hydroxyl and carboxyl groups with a solid content of 35%, an acid value of 14 mg KOH / g, a hydroxyl value of 12 mg KOH / g, and an average particle size of 120 nm.

[0216] Manufacturing example 7 In a reaction vessel equipped with a thermometer, stirrer, and reflux condenser, 211.9 parts of polytetramethylene glycol (number average molecular weight 1000), 11.5 parts of 2,2-dimethylolpropionic acid, 6.9 parts of trimethylolpropane, 112.2 parts of isophorone diisocyanate, and 298.5 parts of methyl ethyl ketone were charged. After purging the reaction system with nitrogen gas, the mixture was reacted at 80°C under stirring to obtain an NCO-terminated urethane prepolymer with a free isocyanate group content of 3.2%. The obtained methyl ethyl ketone solution was cooled to 40°C, and 493.2 g of deionized water containing 8.6 parts of triethylamine was added and emulsified. Then, 275.9 parts of a 5% N-(2-hydroxyethyl)ethylenediamine aqueous solution was added, and after stirring for 60 minutes, the methyl ethyl ketone was removed by distillation under reduced pressure and heating, and the concentration was adjusted with deionized water to obtain a polyurethane resin dispersion (A3-2) containing hydroxyl and carboxyl groups with a solid content of 35%, an acid value of 14 mg KOH / g, a hydroxyl value of 12 mg KOH / g, and an average particle size of 120 nm.

[0217] Production of carboxyl group-containing polyurethane resin (U-1) that does not contain hydroxyl groups Manufacturing Example 8 In a reaction vessel equipped with a thermometer, stirrer, and reflux condenser, 211.9 parts of polytetramethylene glycol (number average molecular weight 1000), 11.5 parts of 2,2-dimethylolpropionic acid, 6.9 parts of trimethylolpropane, 112.2 parts of isophorone diisocyanate, and 298.5 parts of methyl ethyl ketone were charged. After purging the reaction system with nitrogen gas, the mixture was reacted at 80°C under stirring to obtain an NCO-terminated urethane prepolymer with a free isocyanate group content of 3.2%. The obtained methyl ethyl ketone solution was cooled to 40°C, and 493.2 g of deionized water containing 9.8 parts of N-ethylmorpholine was added and emulsified. Then, 159.2 parts of a 5% aqueous ethylenediamine solution were added, and after stirring for 60 minutes, the methyl ethyl ketone was removed by distillation under reduced pressure and heating. The concentration was then adjusted with deionized water to obtain a carboxyl group-containing polyurethane resin dispersion (U-1) with a solid content of 35%, an acid value of 14 mg KOH / g, and an average particle size of 120 nm, which does not contain hydroxyl groups.

[0218] Production of blocked polyisocyanate compound (B) Manufacturing Example 9 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, dropping device, and simple solvent removal trap, 1500 parts of "Sumijoule N-3300" (trade name, manufactured by Sumika Bayer Urethane Co., Ltd., polyisocyanate containing isocyanurate structure derived from hexamethylene diisocyanate, 100% solids, 21.8% isocyanate group content) and 0.9 parts of 2,6-di-t-butyl-4-methylphenol were charged and thoroughly mixed, and heated at 130°C for 3 hours under a nitrogen stream. Next, 1200 parts of ethyl acetate and 1300 parts of diisopropyl malonate were charged, and while stirring under a nitrogen stream, 14 parts of a 28% methanol solution of sodium methoxide were added, and the mixture was stirred at 65°C for 8 hours to obtain a blocked polyisocyanate compound (B-1) with a solids content of 70% and a weight-average molecular weight of 4,000.

[0219] Manufacturing Example 10 In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, and dropper, 1610 parts of "Sumijoule N-3300" (product name, manufactured by Sumika Bayer Urethane Co., Ltd., polyisocyanate containing isocyanurate structure derived from hexamethylene diisocyanate, 100% solids content, 21.8% isocyanate group content), 1200 parts of "PTMG2000" (product name, manufactured by Mitsubishi Chemical Corporation, polytetramethylene ether glycol, average molecular weight 2,000, 100% solids content), and 0.9 parts of 2,6-di-t-butyl-4-methylphenol were charged, mixed thoroughly, and heated at 130°C for 3 hours under a nitrogen atmosphere. Next, 1200 parts of ethyl acetate and 1250 parts of diisopropyl malonate were charged, and while stirring under a nitrogen atmosphere, 14 parts of a 28% methanol solution of sodium methoxide were added, and the mixture was stirred at 65°C for 8 hours. The mixture was then diluted with ethyl acetate to obtain a blocked polyisocyanate compound (B-2) with a solid content of 70% and a weight-average molecular weight of 50,000.

[0220] Manufacturing of phosphate-containing acrylic resins Manufacturing Example 11 A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and dropping device was filled with a mixed solvent of 27.5 parts methoxypropanol and 27.5 parts isobutanol, and heated to 110°C. Next, 121.5 parts of a mixture consisting of 25 parts styrene, 27.5 parts n-butyl methacrylate, 20 parts isostearyl acrylate (trade name, manufactured by Osaka Organic Chemical Industry Co., Ltd., branched higher alkyl acrylate), 7.5 parts 4-hydroxybutyl acrylate, 15 parts of the following phosphate group-containing polymerizable monomer, 12.5 parts 2-methacryloyloxyethyl acid phosphate, 10 parts isobutanol, and 4 parts t-butyl peroxyoctanoate were added to the mixed solvent over 4 hours. A mixture consisting of 0.5 parts t-butyl peroxyoctanoate and 20 parts isopropanol was then added dropwise over 1 hour. After that, the mixture was stirred and aged for 1 hour to obtain a phosphate group-containing acrylic resin solution (C1'-1) with a solid content of 50%. The acid value due to the phosphate group of this resin was 83 mgKOH / g, the hydroxyl value was 29 mgKOH / g, and the weight-average molecular weight was 10,000. Phosphate-containing polymerizable monomer: In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and dropping device, 57.5 parts of monobutyl phosphate and 41 parts of isobutanol were added. After raising the temperature to 90°C, 42.5 parts of glycidyl methacrylate were added dropwise over 2 hours, followed by stirring and aging for another hour. Then, 59 parts of isopropanol were added to obtain a phosphate-containing polymerizable monomer solution with a solid content of 50%. The acid value of the obtained monomer due to the phosphate group was 285 mgKOH / g.

[0221] Manufacturing of pigment dispersions Manufacturing Example 12 44.4 parts (20 parts solids) of the hydroxyl and carboxyl group-containing polyester resin solution (A2-1) obtained in Production Example 4, 100 parts of "JR-806" (trade name, manufactured by Teika Co., Ltd., rutile-type titanium dioxide), 1 part of "Carbon MA-100" (trade name, manufactured by Mitsubishi Chemical Corporation, carbon black), and 10 parts of deionized water were mixed, and 1.2 parts of N-ethylmorpholine were added to adjust the pH to 8.0. Next, the resulting mixture was placed in a wide-mouthed glass bottle, glass beads with a diameter of approximately 1.3 mmφ were added as a dispersion medium, the bottle was sealed, and the mixture was dispersed in a paint shaker for 30 minutes to obtain a pigment dispersion (P-1).

[0222] Manufacturing Example 13 44.4 parts (20 parts solids) of the hydroxyl and carboxyl group-containing polyester resin solution (A2-2) obtained in Production Example 5, 100 parts of "JR-806" (trade name, manufactured by Teika Co., Ltd., rutile-type titanium dioxide), 1 part of "Carbon MA-100" (trade name, manufactured by Mitsubishi Chemical Corporation, carbon black), and 10 parts of deionized water were mixed, and 0.7 parts of 2-(dimethylamino)ethanol were added to adjust the pH to 8.0. Next, the resulting mixture was placed in a wide-mouthed glass bottle, glass beads with a diameter of approximately 1.3 mmφ were added as a dispersion medium, the bottle was sealed, and the mixture was dispersed in a paint shaker for 30 minutes to obtain a pigment dispersion (P-2).

[0223] Manufacturing of water-based paint compositions Example 1 155.9 parts of the pigment dispersion (P-1) obtained in Production Example 12, 83.3 parts (25 parts solids) of hydroxyl and carboxyl group-containing acrylic resin particles (A1'-1) obtained in Production Example 1, 57.1 parts (20 parts solids) of hydroxyl and carboxyl group-containing polyurethane resin dispersion (A3-1) obtained in Production Example 6, 42.9 parts (30 parts solids) of the blocked polyisocyanate compound (B-1) obtained in Production Example 9, 12.5 parts (5 parts solids) of "Carbodilite SV02" (trade name, manufactured by Nisshinbo, carbodiimide compound, 40% solids, carbodiimide equivalent per solids 429) and 4 parts (2 parts solids) of the phosphate group-containing acrylic resin solution obtained in Production Example 11 were uniformly mixed. Then, 1.6 parts of N-ethylmorpholine were added as a basic compound (D) for pH adjustment to make the pH 8.2. Next, "UH-752" (product name, manufactured by ADEKA Corporation, thickener) and deionized water were added to obtain aqueous paint composition No. 1 with a pH of 8.2, a paint solids content of 48%, and a viscosity of 30 seconds according to the Ford Cup No. 4 at 20°C.

[0224] Examples 2-18, Comparative Examples 1-6 Except for the formulations shown in Tables 1-1 to 1-4 below, aqueous paint compositions No. 2 to 24 were obtained in the same manner as in Example 1, with a viscosity of 30 seconds measured by Ford Cup No. 4 at 20°C. The storage stability of water-based paint compositions No. 1 to 24 was evaluated by the rate of viscosity change. Viscosity change rate: Evaluated based on the viscosity measured at 60 rpm for 1 minute using "LVDV-I" (product name, manufactured by Brookfield, type B viscometer), comparing the viscosity immediately after manufacturing with the viscosity after standing at 40°C for 10 days. Viscosity change rate (%) = |(Viscosity after standing at 40°C for 10 days / Viscosity immediately after manufacturing) - 1| × 100 ◎ and ○ indicate a passing grade. ◎: Viscosity change rate is less than 20%. ○: Viscosity change rate is 20% or more and less than 50%. ×: Viscosity change rate is 50% or more. The storage stability results are shown in Tables 1-1 to 1-4.

[0225] [Table 1-1]

[0226] [Table 1-2]

[0227] [Table 1-3]

[0228] [Table 1-4]

[0229] Preparation of test substrates A thermosetting epoxy resin-based cationic electrodeposition coating composition (product name "Elecron GT-10," manufactured by Kansai Paint Co., Ltd.) was electrodeposited onto a zinc phosphate-treated cold-rolled steel sheet to a film thickness of 20 μm, and then cured by heating at 170°C for 30 minutes. Thus, a coated object was prepared by forming an electrodeposited coating film on the steel sheet.

[0230] Preparation of test painted panels Example 19 The aqueous coating composition No. 1 obtained in Example 1 was electrostatically applied to the above-mentioned test substrate using a rotary atomizing electrostatic coating machine to a cured film thickness of 20 μm, and left for 5 minutes to form an uncured intermediate coating film. Next, "WBC-713T No.202" (product name, manufactured by Kansai Paint Co., Ltd., acrylic melamine resin-based water-based base coat paint, black color) was electrostatically applied onto the uncured intermediate coating film using a rotary atomizing electrostatic coating machine to a dry film thickness of 15 μm. After standing for 5 minutes, preheating was performed at 80°C for 3 minutes to form an uncured base coat film. Next, "Softlex #520 Clear" (a two-component acrylic urethane-based organic solvent-type clear paint manufactured by Kansai Paint Co., Ltd., trade name, containing a hydroxyl group-containing acrylic resin and a polyisocyanate compound) was electrostatically applied onto the uncured base coat film to a dry film thickness of 35 μm, left for 7 minutes, and a clear coat film was formed. Next, it was heated at 80°C for 30 minutes to heat-cure the intermediate coat film, the base coat film, and the clear coat film, thereby producing a test painting board.

[0231] Examples 20 to 38, Comparative Examples 7 to 12 In Example 19, test boards were produced in the same manner as in Example 19, except that the types of the aqueous paint compositions and the heating temperature were changed as shown in Table 2 below. For each of the test boards obtained above, evaluation was carried out by the following test methods. The evaluation results are shown in Table 2 below.

[0232] (Test Methods) Low-temperature curability: In accordance with JIS K 5600-5-4 (1999) "Scratch hardness (pencil method)", the pencil hardness of the test boards obtained in the above Examples and Comparative Examples was measured. The pencil hardness is in the order of 3B < 2B < B < HB < F, and if the pencil hardness is HB or higher, it is considered qualified. The evaluation results are shown in Table 2.

[0233] Water adhesion resistance: The test boards obtained in the above Examples and Comparative Examples were immersed in warm water at 40°C for 240 hours, pulled out, and the surface moisture was wiped off. Immediately afterwards, in accordance with JIS K 5600-5-6 (1990), 100 2 mm × 2 mm lattice patterns were made on the paint surface, an adhesive tape was adhered to the surface, and after rapidly peeling it off, the number of lattice paint films remaining on the paint surface was evaluated. ◎ and ○ are considered qualified. The evaluation results are shown in Table 2. ◎: Remaining number / Total number = 100 pieces / 100 pieces with no edge chipping ○: Remaining number / Total number = 100 pieces / 100 pieces with edge chipping △: Remaining number / Total number = 99 pieces to 90 pieces / 100 pieces ×: Remaining number / Total number = 89 pieces or less / 100 pieces.

[0234] Hardness after water resistance: The test plates obtained in the above Examples and Comparative Examples were immersed in warm water at 40°C for 240 hours and dried at 20°C for 12 hours. Then, in accordance with JIS K 5600-5-4 (1999) "Scratch hardness (pencil method)", the pencil hardness of the painted surface of each test plate was measured. If the pencil hardness is B or higher, it is considered qualified. The evaluation results are shown in Table 2.

[0235] Chipping resistance: The test plates obtained in the above Examples and Comparative Examples were placed on the specimen holder of a flying stone tester "JA-400 type" (trade name, manufactured by Suga Test Instruments Co., Ltd., chipping resistance test device). At 0°C, 50 g of granite crushed stones of No. 7 particle size were made to collide with the test plate at an angle of 45 degrees from a position 30 cm away from the test plate by compressed air at 0.39 MPa (4 kgf / cm2). Then, the obtained test plate was washed with water, dried, a cloth adhesive tape (manufactured by Nichiban Co., Ltd.) was adhered to the painted surface, and after peeling it off, the degree of occurrence of scratches on the coating film was visually observed and evaluated according to the following criteria. ◎ and ○ are considered qualified. The evaluation results are shown in Table 2. ◎: The size of the scratches is extremely small, and the electrodeposited surface and the base steel plate are not exposed. ○: The size of the scratches is small, and the electrodeposited surface and the base steel plate are not exposed. △: The size of the scratches is small, but the electrodeposited surface and the base steel plate are exposed. ×: The size of the scratches is quite large, and the base steel plate is also largely exposed.

[0236] Glass adhesion: A urethane-based adhesive (trade name "3740", manufactured by Sunstar Co., Ltd., automotive windshield agent) was further applied to each test coating plate obtained in the above Examples and Comparative Examples in such a way that the coating shape had a width of 20 mm, a thickness of 3 mm, and a length of 100 mm or more. After covering it with a release paper, it was evenly pressed with a flat plate. After removing the flat plate, it was left at a temperature of 23 ± 2°C and a humidity of 50 ± 5% for 72 hours to cure. Then, the release paper was peeled off. Next, each test coating plate was immersed in a constant temperature water bath set at 50°C for 240 hours, and then immersed in water at 23°C for 1 hour for cooling, and then the following peel test was conducted. While pulling the cured adhesive layer by hand in a direction of 90 degrees or more with respect to the coating film at intervals of 2 to 3 mm, make a cut with a cutter knife until it reaches the surface of the coating film at an angle of about 60 degrees with respect to the coating film. The peeling state after peeling off the adhesive layer was evaluated according to the following criteria. ◎, ○+ and ○ are qualified. ◎: No peeling of the adhesive layer is observed, and no exposure of the coating film is observed. ○+: The coating film is not broken, only the adhesive layer undergoes cohesive failure and peels off, but the adhesion between the coating film and the adhesive layer is almost maintained. ○: The coating film undergoes cohesive failure and peels off, and the width is less than 1 mm from the part where the cutter knife was inserted. △: The coating film undergoes cohesive failure and peels off, and the width is 1 mm or more from the part where the cutter knife was inserted. ×: Peeling is observed at the interface between the coating film and the adhesive layer.

[0237]

Table 2

[0238] As described above, the embodiments and examples of the present invention have been specifically described. However, the present invention is not limited to the above-described embodiments, and various modifications based on the technical idea of the present invention are possible. For example, the configurations, methods, processes, shapes, materials, and numerical values mentioned in the above-described embodiments and examples are merely examples, and different configurations, methods, processes, shapes, materials, and numerical values, etc. may be used as necessary. In addition, the configurations, methods, processes, shapes, materials, and numerical values, etc. of the above-described embodiments can be combined with each other as long as they do not deviate from the gist of the present invention.

Claims

1. (A) Resins containing hydroxyl groups and carboxyl groups, (B) Blocked polyisocyanate compounds, (C) Polycarbodiimide compounds, and (D) Basic compounds an aqueous paint composition containing, The basic compound (D) is (D1) Basic compounds having an acid dissociation constant (PKa) in the range of 7.0 to 8.5 and a boiling point in the range of 100 to 200°C. It contains and the content of the basic compound (D1) is in the range of 30 to 100% by mass, based on the mass of the basic compound (D). Furthermore, the pH of the aqueous paint composition is in the range of 8.0 to 11.

5. An aqueous paint composition in which the basic compound (D1), having an acid dissociation constant (PKa) in the range of 7.0 to 8.5 and a boiling point in the range of 100 to 200°C, is N-methylmorpholine and / or N-ethylmorpholine.

2. The aqueous paint composition according to claim 1, wherein at least a portion of the blocking agent of the blocked polyisocyanate compound (B) is an active methylene-based blocking agent.

3. Furthermore, the aqueous paint composition according to claim 1 or 2 further contains melamine resin (E).

4. Step (1): A step of applying the aqueous coating composition described in any one of claims 1 to 3 onto the object to be coated to form an intermediate coating film. Step (2): A step of applying an aqueous base coat paint composition onto the intermediate coating film formed in step (1) to form a base coat film. Step (3): A step of applying a clear coating composition onto the base coat coating formed in step (2) to form a clear coating, and Step (4): A step of heating and curing the intermediate coating film, base coat film, and clear coating film formed in steps (1) to (3) at a temperature within the range of 60 to 110°C at the same time. A method for forming a multi-layer coating by sequentially performing the following steps.

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