Method for forming multilayer coating film

Abstract

To provide a method for forming a multilayer coating film excellent in chipping resistance while using an intermediate coating composition having high washability.SOLUTION: A water-based two-component intermediate coating composition, a water-based base coating composition, and a clear coating composition are applied to an object to be coated including a metallic portion and a resin portion, and then cured by heating, wherein the water-based two-component intermediate coating composition contains a water-dispersible acrylic resin (A1), a diisocyanate compound (B1), a water-based polyurethane resin (C), and a pigment (D), and the content of the pigment is 30 mass% or more and 50 mass% or less of the resin solids content, the aqueous base coating composition contains a water-dispersible acryl resin (A2) and a hydrophilicized-modified-carbodiimide compound (E), and the clear coating composition contains an acryl resin (A3) and a polyisocyanate compound (B2).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for forming a multi-layer coating film. [Background technology]

[0002] On the surface of a substrate such as an automobile body, multiple coating films with various functions are sequentially formed. For example, on a steel plate, an undercoat such as an electrodeposition coating, an intermediate coating, a base coating, and a clear coating are sequentially formed. The coating films not only protect the substrate but also provide a beautiful appearance and excellent design.

[0003] Intermediate coating films are required to have high chipping resistance. For example, Patent Document 1 discloses a chipping primer coating composition containing (i) a maleic polyolefin resin and (ii) a butylated melamine resin as coating film-forming components. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-249699 Summary of the Invention [Problem to be solved by the invention]

[0005] Chipping resistance is the ability to prevent coating peeling caused by strong impacts such as those caused by pebbles, and to mitigate the impact energy from reaching the substrate. Intermediate coating compositions often contain resins that possess both viscosity and elasticity. Resins with these properties generally have a large molecular weight and poor solubility in solvents. This makes it difficult to clean equipment used to apply intermediate coating compositions.

[0006] The present invention solves the above-mentioned conventional problems, and aims to provide a method for forming a multi-layer coating film that has excellent chipping resistance while using an intermediate coating composition that has high washability. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides the following aspects. [1] Applying an aqueous two-component intermediate coating composition to a substrate including a metal part and a resin part to form an uncured intermediate coating film; A base coating film forming step of applying an aqueous base coating composition on the uncured intermediate coating film to form an uncured base coating film; Applying a clear coating composition on the uncured base coating film to form an uncured clear coating film; and heating and curing the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film, The aqueous two-component intermediate coating composition comprises: a water-dispersible acrylic resin (A1) having hydroxyl groups and carboxyl groups, a hydroxyl value of 80 mgKOH / g or more and 200 mgKOH / g or less, and an acid value of 10 mgKOH / g or more and 40 mgKOH / g or less; Polyisocyanate compound (B1), An aqueous polyurethane resin (C) having a glass transition temperature (Tg) of -50°C or less, and Contains a pigment (D), the breaking elongation of a cured film formed from the aqueous polyurethane resin (C) is 400% or more at −20° C., the content of the aqueous polyurethane resin (C) is more than 20 parts by mass per 100 parts by mass of the resin solid content of the aqueous two-component intermediate coating composition, The content of the pigment is 30% by mass or more and 50% by mass or less of the resin solid content of the aqueous two-component intermediate coating composition, The aqueous base coating composition comprises: A water-dispersible acrylic resin (A2) having a hydroxyl group and a carboxyl group and a glass transition point of 0°C or higher and 20°C or lower, and a hydrophilically modified carbodiimide compound (E) having a carbodiimide group and a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether; The clear coating composition comprises: a hydroxyl group-containing acrylic resin (A3), and A method for forming a multi-layer coating film, comprising: a polyisocyanate compound (B2). [2] The hydrophilically modified carbodiimide compound (E) is The following general formula (I): [ka] (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y independently has a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, and each Z independently has a structure obtained by removing a hydroxyl group from a bifunctional polyol having a number average molecular weight of 200 or more and 5,000 or less.) The following general formula (II): [ka] (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed, and R 0 is hydrogen, a methyl group, or an ethyl group, and R 1 are each independently an alkylene group having 4 or less carbon atoms, n is 0 or 1, and m is 0 to 60), and The following general formula (III): [ka] (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, and each Y independently has a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether.) The method for forming a multilayer coating film according to [1] above, wherein the compound comprises at least one selected from the group consisting of compounds represented by the following formula: [3] The method for forming a multilayer coating film according to [1] above, wherein the ratio of the carbodiimide group equivalent Ec of the hydrophilically modified carbodiimide compound (E) to the acid group equivalent Ea of the water-dispersible acrylic resin (A2), Ec / Ea, is 0.1 or more and 1.5 or less. [4] The clear coating composition comprises: A polyester polyol (F1) represented by the following formula (1): A polyester polyol (F2) represented by the following formula (2): A polyester polyol (F3) represented by the following formula (3): A polyester polyol (F4) represented by the following formula (4), and The method for forming a multilayer coating film according to [1] above, further comprising at least one polyester polyol (F) selected from the group consisting of polyester polyols (F5) which are reaction products of linear or branched polyhydric alcohols having three or more hydroxyl groups, linear or branched alkanediols, and linear or branched alkanedicarboxylic acids. Formula (1): [ka] (In the formula, a is an integer of 1 to 100, R 11 represents a saturated hydrocarbon group having two bonds, independently for each repeating unit; R 12 and R 13 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (2): [ka] (wherein j and k are each an integer of 0 to 100, and j+k≧1 is satisfied; R 21 represents a saturated hydrocarbon group having two bonds, R 22 and R 23each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (3): [ka] (In the formula, p, q, and r each represent an integer of 1 to 100, R 31 represents a saturated hydrocarbon group having three bonds, R 32 , R 33 and R 34 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (4): [ka] (In the formula, w, x, y, and z each represent an integer of 1 to 100, R 41 represents a saturated hydrocarbon group having four bonds, R 42 , R 43 , R 44 and R 45 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. [5] The method for forming a multilayer coating film according to [1] above, wherein the equivalent ratio (NCO / OH) of the isocyanate groups contained in the polyisocyanate compound (B2) to the hydroxyl groups contained in the water-dispersible acrylic resin (A3) is 1.0 or more and 2.0 or less. [6] The polyisocyanate compound (B2) is The method for forming a multilayer coating film according to [1] above, comprising a polyisocyanate (B21) having an isocyanate group with a uretdione structure and a polyisocyanate (B22) which is a trimer or higher of diisocyanate in a solids mass ratio of 10 / 90 to 50 / 50. [Effects of the Invention]

[0008] According to the present invention, a method for forming a multi-layer coating film having excellent chipping resistance is provided, while using an intermediate coating composition having excellent washability. DETAILED DESCRIPTION OF THE INVENTION

[0009] First, the coating compositions used to form each coating film will be described. Hereinafter, the glass transition temperature can be calculated from the types and amounts of raw material monomers used in the production of the target resin. The glass transition temperature may be measured by a differential scanning calorimeter (DSC).

[0010] The number average molecular weight and weight average molecular weight can be measured by gel permeation chromatography (GPC) using a polystyrene standard sample after removing water by drying under reduced pressure or the like.

[0011] The hydroxyl value and acid value are determined based on the mass of the solid content. The hydroxyl value and acid value can be measured by a known method described in JIS K 0070:1992. The hydroxyl value and acid value may be calculated from the amount of unsaturated monomer in the raw material monomers of the target resin.

[0012] The resin solid content of the coating composition is a film-forming component as described below.

[0013] The thickness of the coating film can be measured using an electromagnetic film thickness meter (for example, SDM-miniR manufactured by SANKO Co., Ltd.) The thickness of the coating film is the average value of the thickness of the coating film at any five points.

[0014] The neutralization rate is the equivalent amount of the basic compound used relative to the carboxyl group, and is calculated using the following formula: TIFF2026021926000008.tif12150

[0015] Water-based primer coating composition The aqueous two-component intermediate coating composition (hereinafter sometimes referred to as the aqueous intermediate coating composition) contains a water-dispersible acrylic resin (A1), a polyisocyanate compound (B1), an aqueous polyurethane resin (C), and a pigment (D).

[0016] The water-dispersible acrylic resin (A1), the polyisocyanate compound (B1), and the aqueous polyurethane resin (C) are film-forming components. The water-dispersible acrylic resin (A1) and the aqueous polyurethane resin (C) are aqueous resins.

[0017] Aqueous resins are generally broadly classified as water-soluble and water-dispersible. Water-dispersible resins are further classified as dispersion type (commonly referred to as colloidal dispersion type) and emulsion type. Colloidal dispersion type aqueous resins are typically obtained by semi-dissolving a resin synthesized in an organic solvent (a solution-polymerized resin) in water with a neutralizer. Emulsion type aqueous resins are typically produced by emulsion polymerization or mechanically forced emulsification. In the case of acrylic resins, those with a weight-average molecular weight of more than 100,000 are considered emulsion type, and those with a weight-average molecular weight of 100,000 or less are considered colloidal dispersion type. When the weight-average molecular weight exceeds 1,000,000, it becomes difficult to measure the weight-average molecular weight. Acrylic resins whose weight-average molecular weight cannot be measured can be considered emulsion type.

[0018] Water-dispersible acrylic resin (A1) The water-dispersible acrylic resin (A1) may be an emulsion type. The water-dispersible acrylic resin (A1) has a hydroxyl group and a carboxyl group. The hydroxyl group of the water-dispersible acrylic resin (A1) reacts with the polyisocyanate compound (B1) to form a crosslinked structure.

[0019] The water-dispersible acrylic resin (A1) has a hydroxyl value of 80 mgKOH / g or more and 200 mgKOH / g or less and an acid value of 10 mgKOH / g or more and 40 mgKOH / g or less. The water-dispersible acrylic resin (A1) has a relatively high hydroxyl value, which increases the crosslink density and improves the chipping resistance of the intermediate coating film.

[0020] The hydroxyl value may be 90 mgKOH / g or more, or 100 mgKOH / g or more. The hydroxyl value may be 180 mgKOH / g or less, or 160 mgKOH / g or less.

[0021] The acid value may be 15 mgKOH / g or more, or 18 mgKOH / g or more. The acid value may be 35 mgKOH / g or less, or 30 mgKOH / g or less.

[0022] The content of the water-dispersible acrylic resin (A1) is, for example, 10 parts by mass or more and 50 parts by mass or less, relative to 100 parts by mass of the resin solids content of the aqueous intermediate coating composition. The content of the water-dispersible acrylic resin (A1) may be 15 parts by mass or more, or may be 20 parts by mass or more. The content of the water-dispersible acrylic resin (A1) may be 40 parts by mass or less, or may be 35 parts by mass or less.

[0023] The water-dispersible acrylic resin (A1) may be used alone or in combination of two or more.

[0024] The water-dispersible acrylic resin (A1) can be obtained, for example, by copolymerizing monomers including an α,β-ethylenically unsaturated monomer having a hydroxyl group and an α,β-ethylenically unsaturated monomer having a carboxyl group in amounts that satisfy the requirements for the hydroxyl value and acid value for the hydroxyl group and carboxyl group.

[0025] Examples of α,β-ethylenically unsaturated monomers having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, allyl alcohol, methacrylic alcohol, and an adduct of hydroxyethyl (meth)acrylate with ε-caprolactone. 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and an adduct of hydroxyethyl (meth)acrylate with ε-caprolactone may also be used.

[0026] "(Meth)acrylic acid" includes both acrylic acid and methacrylic acid.

[0027] Examples of the α,β-ethylenically unsaturated monomer having a carboxy group include acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, maleic acid, fumaric acid, and itaconic acid. The monomer may be acrylic acid or methacrylic acid.

[0028] Other α,β-ethylenically unsaturated monomers may be used as copolymerization components. Examples of other α,β-ethylenically unsaturated monomers include (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl methacrylate, phenyl acrylate, isobornyl (meth)acrylate, cyclohexyl methacrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate; and polymerizable amide compounds such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide.

[0029] The other α,β-ethylenically unsaturated monomer may be a crosslinking monomer. The crosslinking monomer has two or more radically polymerizable ethylenically unsaturated groups in the molecule. Examples of the crosslinking monomer include divinylbenzene, allyl (meth)acrylate, and ethylene glycol di(meth)acrylate.

[0030] The water-dispersible acrylic resin (A1) is synthesized, for example, by emulsion polymerization. The emulsion polymerization method is not particularly limited. For example, an emulsifier is dissolved in an aqueous medium containing water or, if necessary, an organic solvent such as an alcohol or an ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and the raw material monomers and polymerization initiator are added dropwise under heating and stirring. The raw material monomers may be emulsified in advance with an emulsifier.

[0031] Examples of the polymerization initiator include azo-based oily compounds such as azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), and 2,2'-azobis(2,4-dimethylvaleronitrile); aqueous compounds such as anionic 4,4'-azobis(4-cyanovaleric acid), 2,2-azobis(N-(2-carboxyethyl)-2-methylpropionamidine, and cationic 2,2'-azobis(2-methylpropionamidine); redox-based oily peroxides such as benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide, and t-butyl perbenzoate; and aqueous peroxides such as potassium persulfate and ammonium persulfate.

[0032] The emulsifier is not particularly limited. Examples of the emulsifier include reactive emulsifiers. Examples of the reactive emulsifier include Antox MS-60 (manufactured by Nippon Nyukazai Co., Ltd.), Eleminol JS-2 (manufactured by Sanyo Chemical Industries, Ltd.), Adeka Reasoap NE-20 (manufactured by ADEKA Corporation), Aqualon HS-10 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and Latemul PD-104 (manufactured by Kao Corporation). Chain transfer agents such as mercaptans (e.g., lauryl mercaptan) and α-methylstyrene dimers may be used to adjust the molecular weight.

[0033] The reaction temperature is determined depending on the polymerization initiator. For example, when an azo-based initiator or peroxide is used, the reaction temperature is 60 to 90°C. When a redox-based initiator is used, the reaction temperature is 30 to 70°C. The reaction time is 1 to 8 hours. The amount of polymerization initiator per 100 parts by mass of the monomer mixture is 0.1 to 5% by mass. Emulsion polymerization can be carried out in multiple stages, for example, in two stages. In two-stage polymerization, a portion of the raw material monomers is emulsion-polymerized, and then the remaining raw material monomers are polymerized.

[0034] The acrylic resin obtained by emulsion polymerization (acrylic resin emulsion) is neutralized with a basic compound from the viewpoint of storage stability. The pH of the acrylic resin emulsion is 5 to 10. The basic compound will be described later. Neutralization is carried out before or after emulsion polymerization.

[0035] The water-dispersible acrylic resin (A1) can also be obtained by solution polymerization and neutralization. The solution polymerization is carried out by a known method.

[0036] The number average molecular weight of the acrylic resin emulsion is, for example, 10,000 to 80,000, which allows the crosslink density to be increased even when cured at a low temperature.

[0037] Polyisocyanate compound (B1) The polyisocyanate compound (B1) may be water-dispersible or hydrophobic. A water-dispersible and a hydrophobic polyisocyanate compound (B1) may be used in combination. This facilitates water dispersion in the aqueous intermediate coating composition.

[0038] Examples of the hydrophobic polyisocyanate compound (B1) include aromatic diisocyanates such as tolylene diisocyanate (TDI), 4,4'-diphenylmethane diisocyanate (MDI), xylylene diisocyanate (XDI), and metaxylylene diisocyanate (MXDI); aliphatic diisocyanates such as hexamethylene diisocyanate (HDI); alicyclic diisocyanates such as isophorone diisocyanate (IPDI) and hydrogenated MDI; compounds obtained by rendering these diisocyanate compounds non-volatile; and derivatives thereof. These may be used alone or in combination of two or more.

[0039] Examples of derivatives of polyisocyanate compounds include adducts such as biuret, uretdione, isocyanurate, and allophanate of diisocyanate compounds; and urethane prepolymers thereof. These may be used alone or in combination of two or more.

[0040] Examples of the water-dispersible polyisocyanate compound (B1) include those obtained by introducing a hydrophilic group into the above-mentioned hydrophobic polyisocyanate compound, and those obtained by self-emulsifying a hydrophobic polyisocyanate compound with a surfactant.

[0041] Examples of hydrophilic groups include anionic groups such as carboxyl groups and sulfonic acid groups, cationic groups such as tertiary amino groups, and nonionic groups such as polyoxyalkylene groups. From the viewpoint of the water resistance of the resulting coating film, nonionic groups are preferred. Specifically, polyoxyethylene groups are preferred.

[0042] Examples of surfactants include anionic surfactants having anionic groups such as carboxyl groups and sulfonic acid groups, cationic surfactants having cationic groups such as tertiary amino groups, and nonionic surfactants having nonionic groups such as polyoxyalkylene groups. From the viewpoint of the water resistance of the resulting coating film, nonionic surfactants are preferred.

[0043] Commercially available water-dispersible polyisocyanate compounds (B1) include, for example, Aquanate 100, Aquanate 110, Aquanate 200, and Aquanate 210 (manufactured by Tosoh Corporation), Bayhydur TPLS-2032, SBU-Isocyanate L801, Bayhydur VPLS-2319, Bayhydur 3100, VPLS-2336, and VPLS-2150 / 1, Bayhydur 305, Bayhydur XP-2655 (manufactured by Sumika Covestro Urethane Co., Ltd.), Takenate WD-720, Takenate WD-725, and Takenate WD-220 (manufactured by Mitsui Chemicals, Inc.), and Resamine D-56 (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.).

[0044] The polyisocyanate compound (B1) may contain at least one selected from the group consisting of aliphatic diisocyanates, alicyclic diisocyanates, and derivatives thereof, which can further improve chipping resistance.

[0045] The content of the polyisocyanate compound (B1) may be 5 parts by mass or more and 55 parts by mass or less, based on 100 parts by mass of the resin solid content of the aqueous intermediate coating composition. The content of the polyisocyanate compound (B1) may be 10 parts by mass or more, 20 parts by mass or more, or 25 parts by mass or more, but may be 45 parts by mass or less.

[0046] The ratio (NCO / OH) of the equivalent weight of the isocyanate groups in the polyisocyanate compound (B1) to the equivalent weight of the hydroxyl groups in the water-dispersible acrylic resin (A1) is, for example, 0.6 to 1.5. When the equivalent ratio is within the above range, the curing reaction is accelerated even at low temperatures, and the crosslink density is increased.

[0047] Water-based polyurethane resin (C) The aqueous polyurethane resin (C) may be water-soluble or water-dispersible. The aqueous polyurethane resin (C) fuses with itself and other components to toughen the coating film and increase its elasticity. The aqueous polyurethane resin (C) further improves chipping resistance.

[0048] The glass transition temperature (Tg) of the aqueous polyurethane resin (C) is −50° C. or lower. When the Tg is −50° C. or lower, chipping resistance is further improved. The Tg of the aqueous polyurethane resin (C) may be −55° C. or lower, or −58° C. or lower.

[0049] The content of the aqueous polyurethane resin (C) is more than 20 parts by mass per 100 parts by mass of the resin solids content of the aqueous intermediate coating composition. This makes it easier for the aqueous polyurethane resin (C) to exhibit its functions. The content of the aqueous polyurethane resin (C) may be 25 parts by mass or more, or may be 30 parts by mass or more. The content of the aqueous polyurethane resin (C) may be 60 parts by mass or less, 50 parts by mass or less, or 40 parts by mass or less.

[0050] The cured film formed from the aqueous polyurethane resin (C) has a breaking elongation of 400% or more at −20° C. This provides excellent chipping resistance. The breaking elongation may be 500% or more.

[0051] The breaking elongation of the cured film of the aqueous polyurethane resin (C) is determined in accordance with JIS K 7127. A tensile performance test is carried out at a test temperature of -20°C, and the elongation at break is measured.

[0052] A cured film of the aqueous polyurethane resin (C) is prepared as follows. First, 95 parts by mass (resin solid content) of the aqueous polyurethane resin (C) and 5 parts by mass (resin solid content) of the hydrophilic modified carbodiimide compound (E) are mixed. The resulting mixture is uniformly coated with a doctor blade to a dry film thickness of 20 μm. After leaving the coating at 20°C for 10 minutes, it is preheated at 80°C for 3 minutes and then heated at 120°C for 30 minutes.

[0053] The aqueous polyurethane resin (C) is produced, for example, by dissolving or dispersing in water a polymer obtained by using a polyol compound (D-1), a compound (D-2) having an active hydrogen group and a hydrophilic group in the molecule, an organic polyisocyanate (D-3), and, if necessary, a chain extender and a polymerization terminator.

[0054] The polyol compound (D-1) contains two or more hydroxyl groups in the molecule. Examples of the polyol compound (D-1) include polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and glycerin; polyether polyols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol; polyester polyols obtained from dicarboxylic acids such as adipic acid, sebacic acid, itaconic acid, maleic anhydride, phthalic acid, and isophthalic acid and glycols such as ethylene glycol, triethylene glycol, propylene glycol, butylene glycol, tripropylene glycol, and neopentyl glycol; polycaprolactone polyols; polybutadiene polyols; polycarbonate polyols; and polythioether polyols. These may be used alone or in combination of two or more.

[0055] Examples of the compound (D-2) having an active hydrogen group and a hydrophilic group in the molecule include compounds containing an active hydrogen and an anionic group, a cationic group, or a nonionic hydrophilic group. The anionic group includes an anionic group and an anion-forming group. The anion-forming group is a group that can form an anionic group by reacting with a base. Specifically, the anionic group is formed by neutralizing with a base before, during, or after the urethanization reaction.

[0056] Compounds containing active hydrogen and anionic groups are described, for example, in JP-B-42-24192 and JP-B-55-41607, and specific examples thereof include α,α-dimethylolpropionic acid and α,α-dimethylolbutyric acid. Compounds having active hydrogen and cationic groups are described, for example, in JP-B-43-9076. Compounds having active hydrogen and nonionic hydrophilic groups are described, for example, in JP-B-43-41718, and specific examples thereof include polyethylene glycol and alkyl alcohol alkylene oxide adducts.

[0057] The organic polyisocyanate (D-3) contains two or more isocyanate groups in the molecule. Examples of the organic polyisocyanate (D-3) include aliphatic diisocyanates such as 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexyl-2,4-diisocyanate, methylcyclohexyl-2,6-diisocyanate, xylylene diisocyanate, 1,3-bis(isocyanato)methylcyclohexane, tetramethylxylylene diisocyanate, transcyclohexane-1,4-diisocyanate, and lysine diisocyanate; 2,4-toluylene diisocyanate; and 2,6-toluylene diisocyanate. Examples of the diisocyanates include aromatic diisocyanates such as methyl 4,4'-diphenylmethane diisocyanate, 1,5'-naphthene diisocyanate, tolidine diisocyanate, diphenylmethylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, 4,4'-dibenzyl diisocyanate, and 1,3-phenylene diisocyanate; triisocyanates such as lysine ester triisocyanate, triphenylmethane triisocyanate, 1,6,11-undecane triisocyanate, 1,8-diisocyanate-4,4-isocyanatomethyloctane, 1,3,6-hexamethylene triisocyanate, and bicycloheptane triisocyanate; their dimers, trimers (isocyanurate bond), and biuret forms; and reaction products of these with polyols. These may be used alone or in combination of two or more.

[0058] The organic polyisocyanate (D-3) may be an aliphatic diisocyanate. The aliphatic diisocyanate suppresses the water permeability of the resulting coating film and improves the initial water resistance at low temperatures.

[0059] The chain extender contains two or more active hydrogen groups in the molecule. Examples of the chain extender include low molecular weight polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, 3-methylpentanediol, 2-ethyl-1,3-hexanediol, and trimethylolpropane; polyamines such as ethylenediamine, hexamethylenediamine, diethylenetriamine, hydrazine, xylylenediamine, and isophoronediamine; and water. These may be used alone or in combination of two or more.

[0060] Examples of polymerization terminators include compounds having one active hydrogen atom in the molecule (e.g., monoalcohols, monoamines, etc.) and monoisocyanate compounds. Examples of monoalcohols include alkyl alcohols such as methanol, butanol, and octanol; and alkyl alcohol alkylene oxide adducts. Examples of monoamines include alkylamines such as butylamine and dibutylamine. Examples of monoisocyanate compounds include methyl isocyanate, ethyl isocyanate, propyl isocyanate, butyl isocyanate, lauryl isocyanate, cyclohexyl isocyanate, phenyl isocyanate, and tolylene isocyanate.

[0061] The polyurethane resin may be synthesized by a one-shot method in which all components are reacted at once, or by a multi-stage method in which the components are reacted in stages. In the multi-stage method, a part of the active hydrogen-containing compound (e.g., a high molecular weight polyol) is reacted with a polyisocyanate to form an NCO-terminated prepolymer, and then the prepolymer is reacted with the remaining active hydrogen-containing compound.

[0062] The reaction temperature is, for example, 40 to 140°C, and may be 60 to 120°C. A catalyst may be used. Examples of the catalyst include tin-based catalysts such as dibutyltin laurate and tin octoate; and amine-based catalysts such as triethylenediamine. The synthesis reaction may be carried out in an organic solvent inert to isocyanates (e.g., acetone, toluene, dimethylformamide, etc.), and a solvent may be added during or after the reaction. The organic solvent is subsequently removed.

[0063] The polyurethane resin is treated by a known method (a method of forming an anionic group by neutralizing with a base in the case of an anion-forming group, a method of forming a cationic group by using a quaternizing agent in the case of a cation-forming group, or a method of forming a cationic group by neutralizing with an acid), and then dispersed in water.

[0064] The polyurethane resin may be dispersed in water after the synthesis reaction or during the multi-stage process. For example, the dispersion treatment is carried out during the formation of an NCO-terminated prepolymer while the prepolymer is chain-extended with water and / or a polyamine.

[0065] Commercially available aqueous polyurethane resins (C) include, for example, the NeoRez series, which are aqueous polyurethane resins sold by Kusumoto Chemicals Co., Ltd.; the HUX series, which are aqueous polyurethane resins sold by ADEKA Corporation; and the U-coat series, Permarin series, and Euprene series, which are aqueous polyurethane resins sold by Sanyo Chemical Industries, Ltd.

[0066] Pigment (D) The pigment (D) reduces the relative content of the aqueous polyurethane resin (C) in the aqueous intermediate coating composition, improving washability. The pigment (D) also improves the hiding power of the intermediate coating film and the weather resistance of the multi-layer coating film.

[0067] The pigment content is 30% by mass or more and 50% by mass or less of the resin solids content of the aqueous intermediate coating composition. When the pigment content is 30% by mass or more, the relative content of the aqueous polyurethane resin (C) is effectively reduced, improving the washability of the aqueous intermediate coating composition. When the pigment content is 50% by mass or less, the effect of the aqueous polyurethane resin (C) is exerted, improving chipping resistance.

[0068] The pigment content may be 32% by mass or more, or 35% by mass or more.The pigment content may be 48% by mass or less, or 45% by mass or less.

[0069] The type of pigment (D) is not particularly limited. Examples of pigment (D) include color pigments, luster pigments, and extender pigments. These may be used alone or in combination of two or more.

[0070] The color pigment may be organic or inorganic. Examples of organic color pigments include azo chelate pigments, insoluble azo pigments, condensed azo pigments, monoazo pigments, disazo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, thioindigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, naphthol pigments, pyrazolone pigments, anthraquinone pigments, anthrapyrimidine pigments, and metal complex pigments. These may be used alone or in combination of two or more.

[0071] Examples of inorganic color pigments include zinc oxide, titanium dioxide, yellow lead, yellow iron oxide, chromium oxide, molybdate orange, red iron oxide, titanium yellow, carbon black, cobalt green, phthalocyanine green, ultramarine blue, cobalt blue, phthalocyanine blue, and cobalt violet. These may be used alone or in combination of two or more.

[0072] Examples of extender pigments include calcium carbonate, barium sulfate, barium carbonate, magnesium silicate, clay, talc, silica, and calcined kaolin. These may be used alone or in combination of two or more.

[0073] others The aqueous intermediate coating composition may contain other aqueous resins (for example, the water-soluble acrylic resin described below). The aqueous intermediate coating composition may contain various additives. Examples of the additives include curing catalysts, surface conditioners, antifoaming agents, pigment dispersants, plasticizers, film-forming aids, ultraviolet absorbers, antioxidants, and solvents (water, organic solvents).

[0074] The water-based intermediate coating composition has excellent reactivity at low temperatures and is therefore prepared at the painting site by mixing the above-mentioned components by a known method.

[0075] The resin solids concentration of the aqueous intermediate coating composition is not particularly limited and is set appropriately depending on the coating conditions. The resin solids concentration of the aqueous intermediate coating composition is, for example, 15 to 60 mass %.

[0076] Other hardeners The aqueous intermediate coating composition may contain a curing agent other than the polyisocyanate compound (B1), such as a carbodiimide compound (including a hydrophilically modified carbodiimide compound (E)), an amino resin, an epoxy compound, an aziridine compound, or an oxazoline compound.

[0077] However, the content of the carbodiimide compound is less than 1 part by mass, and may be 0 part by mass, per 100 parts by mass of the solids content of the aqueous intermediate coating composition. The carbodiimide compound improves water resistance.

[0078] Water-based polyester resin The aqueous intermediate coating composition may contain an aqueous polyester resin, which is a film-forming component and is a water-based resin.

[0079] The aqueous polyester resin may be a dispersion type. The aqueous polyester resin may have a hydroxyl group and a carboxyl group.

[0080] The hydroxyl value of the aqueous polyester resin is, for example, 50 to 150 mgKOH / g. The hydroxyl value may be 70 mgKOH / g or more. The hydroxyl value may be 120 mgKOH / g or less, or 100 mgKOH / g or less.

[0081] The acid value of the aqueous polyester resin is, for example, 20 to 80 mgKOH / g. The acid value may be 25 mgKOH / g or more. The acid value may be 50 mgKOH / g or less, or 40 mgKOH / g or less.

[0082] The number average molecular weight of the aqueous polyester resin is, for example, 500 to 20,000. When the number average molecular weight is 500 or more, storage stability is improved. When the number average molecular weight is 20,000 or less, viscosity increase is suppressed and coating workability is improved. The number average molecular weight of the aqueous polyester resin may be 1,500 or more. The number average molecular weight of the aqueous polyester resin may be 10,000 or less.

[0083] The glass transition point of the aqueous polyester resin is, for example, -20 to 80°C. If the glass transition point is -20°C or higher, the hardness of the resulting coating film will be high. If the glass transition point is 80°C or lower, the ability to conceal the base (concealment ability) will be improved. The glass transition point of the aqueous polyester resin may be 0°C or higher. The glass transition point of the aqueous polyester resin may be 60°C or lower.

[0084] The mass-based mixing ratio (A1 / polyester) of the water-dispersible acrylic resin (A1) to the aqueous polyester resin may be, for example, 0.5 to 7. A1 / polyester may be 1 or more, or 2 or more. A1 / polyester may be 6 or less, or 5 or less.

[0085] The content of the aqueous polyester resin may be 2 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the resin solids content of the aqueous intermediate coating composition. The content of the aqueous polyester resin may be 5 parts by mass or more, or may be 8 parts by mass or more. The content of the aqueous polyester resin may be 15 parts by mass or less.

[0086] Aqueous polyester resins are obtained by neutralizing polyester resins with basic compounds, for example, by condensation of a polyhydric alcohol component and a polybasic acid component.

[0087] Examples of polyhydric alcohol components include hydroxycarboxylic acid components such as ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, hydroxypivalic acid neopentyl glycol ester, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethylpentanediol. These may be used alone or in combination of two or more.

[0088] Examples of polybasic acid components include aromatic polycarboxylic acids and acid anhydrides such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrachlorophthalic anhydride, and pyromellitic anhydride; alicyclic polycarboxylic acids and anhydrides such as hexahydrophthalic anhydride, tetrahydrophthalic anhydride, and 1,4- and 1,3-cyclohexanedicarboxylic acid; and aliphatic polycarboxylic acids and anhydrides such as maleic anhydride, fumaric acid, succinic anhydride, adipic acid, and sebacic acid. These may be used alone or in combination of two or more.

[0089] If necessary, a monobasic acid such as benzoic acid or t-butylbenzoic acid may be used in combination.

[0090] As reaction components, monohydric alcohols, monoepoxide compounds such as Cardura E (trade name: manufactured by Shell Chemical), and lactones such as β-propiolactone, dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone, and γ-caprolactone may be used in combination. These may be used alone or in combination of two or more.

[0091] Furthermore, fatty acids such as castor oil, dehydrated castor oil, and mixtures of one or more of these fatty acids may be added to the reaction system.

[0092] The polyester resin may be grafted with an acrylic resin and / or a vinyl resin, or may be reacted with a polyisocyanate compound.

[0093] Water-based coating composition The aqueous base coating composition is a one-component type and contains a water-dispersible acrylic resin (A2) and a hydrophilically modified carbodiimide compound (E).

[0094] The water-dispersible acrylic resin (A2) and the hydrophilically modified carbodiimide compound (E) are film-forming components. The water-dispersible acrylic resin (A2) is a water-based resin.

[0095] The solid content concentration of the aqueous base coating composition is not particularly limited and may be, for example, 15 to 60 mass % or less.

[0096] Water-dispersible acrylic resin (A2) The water-dispersible acrylic resin (A2) may be an emulsion type. The water-dispersible acrylic resin (A2) has a hydroxyl group and a carboxyl group. The carboxyl group of the water-dispersible acrylic resin (A2) reacts with the hydrophilic modified carbodiimide compound (E) to form a crosslinked structure. This improves water resistance.

[0097] The water-dispersible acrylic resin (A2) has a Tg of 0°C or higher and 20°C or lower. If the Tg is 0°C or higher, it becomes difficult for low-molecular-weight components such as water to penetrate the base coating film, improving water resistance. In addition, the strength of the base coating film is increased. If the Tg is 20°C or lower, volume shrinkage after heating is reduced, suppressing the generation of internal stress. Internal stress reduces the adhesion of the base coating film. In particular, if the Tg of the water-dispersible acrylic resin (A2) is higher than the curing temperature, stress is difficult to relieve, which can cause the base coating film to crack. Therefore, in low-temperature curing aqueous base coating compositions, it is important to set the Tg of the film-forming resin. If the Tg of the water-dispersible acrylic resin (A2) is 0°C or higher and 20°C or lower, low-temperature curing is possible and a coating film with excellent water resistance can be obtained.

[0098] The Tg of the water-dispersible acrylic resin (A2) may be 3° C. or higher, or 5° C. or higher. The Tg of the water-dispersible acrylic resin (A2) may be 18° C. or lower, or 15° C. or lower.

[0099] The hydroxyl value of the water-dispersible acrylic resin (A2) is, for example, 10 mgKOH / g or more and 100 mgKOH / g or less. This improves adhesion to the substrate. The hydroxyl value may be 15 mgKOH / g or more. The hydroxyl value may be 80 mgKOH / g or less, or 60 mgKOH / g or less.

[0100] The acid value of the water-dispersible acrylic resin (A2) is, for example, 5 mgKOH / g or more and 80 mgKOH / g or less. This can improve the reaction efficiency with the hydrophilic modified carbodiimide compound (E) and also facilitates increased storage stability. The acid value may be 10 mgKOH / g or more. The acid value may be 60 mgKOH / g or less, or may be 40 mgKOH / g or less.

[0101] The number average molecular weight of the water-dispersible acrylic resin (A2) is, for example, 50,000 or more. The number average molecular weight of the water-dispersible acrylic resin (A2) may be 200,000 or more, or 300,000 or more.

[0102] The content of the water-dispersible acrylic resin (A2) is, for example, 30 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solids content of the aqueous base coating composition. The content of the water-dispersible acrylic resin (A2) may be 40 parts by mass or more, or may be 50 parts by mass or more. The content of the water-dispersible acrylic resin (A2) may be 92 parts by mass or less, or may be 90 parts by mass or less.

[0103] The water-dispersible acrylic resin (A2) is produced in the same manner as the water-dispersible acrylic resin (A1).

[0104] Hydrophilic modified carbodiimide compound (E) The hydrophilically modified carbodiimide compound (E) has at least one carbodiimide group and a structure in which a hydroxyl group has been removed from a polyalkylene glycol monoalkyl ether in the molecule, and therefore has excellent water dispersibility and low-temperature curing properties.

[0105] The hydrophilically modified carbodiimide compound (E) may have, for example, the following formula (α) in the molecule: -OCONH-XQY (α) (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed, and Q is -NHCOO- or NHCONH-.) The structural unit has the following structure (hereinafter referred to as structural unit α):

[0106] The structural unit α improves water dispersibility and curability. One or more structural units α may be present in one molecule. The number of structural units α in one molecule may be one, two, or three.

[0107] X is represented by, for example, the following general formula (a). [ka]

[0108] In the formula, R 2 is a hydrocarbon group having 6 to 15 carbon atoms. 2 Examples of R include a phenylene group, a diphenylenemethyl group, a diphenylene(dimethyl)methyl group, a methylphenylene group, a dimethylphenylene group, a tetramethylxylylene group, a hexylene group, a cyclohexylene group, and a dicyclohexylenemethyl group. 2 may be a dicyclohexylenemethyl group.

[0109] p is an integer of 1 to 10. p is the number of carbodiimide groups present in the structural unit. From the viewpoint of curability, p may be 2 or more and 8 or less.

[0110] The number of repeats (eg, p above) is the average value.

[0111] Y is represented, for example, by the following general formula (b) or (c). [ka]

[0112] In the formula, R 3 is an alkyl group having 1 to 20 carbon atoms. 3 Examples of R include a methyl group, an ethyl group, a butyl group, a hexyl group, an octyl group, a decyl group, a dodecyl group, and a stearyl group. 3 may be a methyl group.

[0113] R 4 is a hydrogen atom or a methyl group. From the viewpoint of water dispersibility, R 4 R may be a hydrogen atom. 4 When is hydrogen, general formulas (b) and (c) show the same structure. q is 4 to 40. From the viewpoint of water dispersibility and reactivity after water volatilization, q may be 4 to 20, or may be 6 to 12.

[0114] The ratio Ec / Ea of the carbodiimide group equivalent Ec of the hydrophilically modified carbodiimide compound (E) to the acid group equivalent Ea of the water-dispersible acrylic resin (A2) is, for example, 0.1 or more and 1.5 or less. When the equivalent ratio is in this range, the crosslink density increases and water resistance can be further improved. The ratio Ec / Ea may be 0.5 or more, or 0.8 or more. The ratio Ec / Ea may be 1.4 or less, or 1.2 or less.

[0115] The content of the hydrophilically modified carbodiimide compound (E) is, for example, 5 to 20 parts by mass per 100 parts by mass of the solids content of the aqueous base coating composition. When the content of the hydrophilically modified carbodiimide compound (E) is within this range, the water resistance of the coating film can be further improved, and shrinkage due to water can be easily suppressed. The content may be 7 parts by mass or more, or 8 parts by mass or more. The content may be 18 parts by mass or less, or 15 parts by mass or less.

[0116] The hydrophilically modified carbodiimide compound (E) may contain at least one compound selected from the group consisting of compounds represented by the following general formulas (I) to (III).

[0117] (Hydrophilic modified carbodiimide compound (I)) The hydrophilic modified carbodiimide compound (E) having two structural units α is represented, for example, by the following general formula (I). [ka]

[0118] In the formula, X and Y have the same meanings as in the above formula (α). Z has a structure in which a hydroxyl group has been removed from a bifunctional polyol with a number average molecular weight of 200 to 5000. Z constitutes a polymer having an ether bond, an ester bond, or a carbonate bond, and is difficult to generalize. For details of Z, please refer to the synthesis method described below.

[0119] <Synthesis method> First, a starting carbodiimide compound containing at least two isocyanate groups per molecule is reacted with a bifunctional polyol having hydroxyl groups at the molecular terminals and a number-average molecular weight of 200 to 5,000, in a ratio such that the molar amount of isocyanate groups in the starting carbodiimide compound exceeds the molar amount of hydroxyl groups in the polyol. Next, the resulting reaction product is reacted with a polyalkylene glycol monoalkyl ether. This produces a hydrophilic modified carbodiimide compound (E) represented by formula (I).

[0120] From the viewpoint of reactivity, the starting carbodiimide compound preferably has isocyanate groups at both ends. Methods for producing such starting carbodiimide compounds are well known to those skilled in the art, and can utilize, for example, a condensation reaction of an organic diisocyanate accompanied by decarbonation.

[0121] Examples of organic diisocyanates include aromatic diisocyanates, aliphatic diisocyanates, alicyclic diisocyanates, and mixtures thereof. Specific examples include 1,5-naphthylene diisocyanate, 4,4-diphenylmethane diisocyanate, 4,4-diphenyldimethylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, hexamethylene diisocyanate, cyclohexane-1,4-diisocyanate, xylylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4-diisocyanate, methylcyclohexane diisocyanate, and tetramethylxylylene diisocyanate. From the viewpoint of reactivity, it may be dicyclohexylmethane-4,4-diisocyanate.

[0122] A carbodiimidization catalyst is usually used for the condensation reaction. Examples of the carbodiimidization catalyst include phospholene oxides such as 1-phenyl-2-phospholene-1-oxide, 3-methyl-2-phospholene-1-oxide, 1-ethyl-2-phospholene-1-oxide, 3-methyl-1-phenyl-2-phospholene-1-oxide, and 3-phospholene isomers thereof. From the viewpoint of reactivity, 3-methyl-1-phenyl-2-phospholene-1-oxide may be used.

[0123] Examples of the bifunctional polyol include polyether diols, polyester diols, and polycarbonate diols. Specific examples include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol, polyester diols such as polyethylene adipate, polybutylene adipate, polyhexamethylene adipate, polyneopentyl adipate, poly-3-methylpentyl adipate, polyethylene / butylene adipate, and polyneopentyl / hexyl adipate, polylactone diols such as polycaprolactone diol and poly-3-methylvalerolactone diol, and polycarbonate diols such as polyhexamethylene carbonate diol, and mixtures thereof.

[0124] The molar ratio of the isocyanate groups of the starting carbodiimide compound to the hydroxyl groups of the bifunctional polyol may be 1.0:1.1 to 1.0:2.0 from the viewpoints of reaction efficiency and economic efficiency. The degree of polymerization between the starting carbodiimide compound and the bifunctional polyol may be 1 to 10 from the viewpoints of reaction efficiency.

[0125] The reaction product thus obtained is further reacted with polyalkylene glycol monoalkyl ether (hereinafter referred to as PAGAE), thereby obtaining a hydrophilic modified carbodiimide compound (E) having two structural units α.

[0126] PAGAE is, for example, represented by the following general formula (b') or (c'). [ka] (In the formula, R 3 , R 4 , and q have the same meanings as above.)

[0127] The number-average molecular weight of the PAGAE may be 200 or more and 5,000 or less. The alkyl group of the PAGAE may have 1 to 20 carbon atoms. Examples of the PAGAE include polyethylene glycol, polypropylene glycol, or a mixture thereof, each of which is end-capped with an alkyl group having 1 to 20 carbon atoms. Specific examples of the PAGAE include polyethylene glycol monomethyl ether, polyethylene glycol mono-2-ethylhexyl ether, polyethylene glycol monolauryl ether, polypropylene glycol monomethyl ether, polypropylene glycol mono-2-ethylhexyl ether, and polypropylene glycol monolauryl ether, all of which have a number-average molecular weight of 200 to 5,000.

[0128] The reaction of the reaction product with PAGAE is carried out in such a manner that the molar amount of isocyanate groups in the reaction product is equal to or exceeds the molar amount of hydroxyl groups in PAGAE. The molar amount of isocyanate groups can be determined by direct measurement. The molar amount of isocyanate groups can also be calculated from the amount of the starting materials.

[0129] A catalyst may be used during the reaction. The reaction temperature is not particularly limited, but may be 60°C or higher and 120°C or lower from the viewpoint of reaction system control and reaction efficiency. The solvent may be an organic solvent containing no active hydrogen.

[0130] The hydrophilically modified carbodiimide compound (E) produced through such a two-step reaction may be a mixture of the compound represented by general formula (I) and other reaction products, but such a mixture may also be considered to be the hydrophilically modified carbodiimide compound (E) having the structure of general formula (I).

[0131] (Hydrophilic modified carbodiimide compound (II)) The hydrophilic modified carbodiimide compound (E) having three structural units α is represented, for example, by the following general formula (II). [ka]

[0132] In the formula, X and Y have the same meanings as in the above formula (α). R 0 is hydrogen, a methyl group, or an ethyl group. 1 are each independently an alkylene group having 4 or less carbon atoms. Specific examples include a methylene group, an ethylene group, a propylene group, and a butylene group. n is 0 or 1. Each m is independently an integer of 0 to 60.

[0133] R 0 , R 1 , n and m are determined depending on the trifunctional polyol used in producing the hydrophilically modified carbodiimide compound (E).

[0134] When m is 11 or more, the ratio of the hydrophilic portion to the hydrophobic portion is preferably 2.0 to 6.3. The ratio is determined by dividing the molecular weight of the oxymethylene group or oxyethylene group portion present in the carbodiimide compound by the molecular weight of the carbodiimide compound.

[0135] <Synthesis method> First, the raw carbodiimide compound is reacted with the PAGAE in such a ratio that the equivalent weight of the isocyanate group in the raw carbodiimide compound exceeds the equivalent weight of the hydroxyl group in the PAGAE. The resulting reaction product is then reacted with a trifunctional polyol. This produces a hydrophilic modified carbodiimide compound (E) represented by general formula (II). The equivalent ratio of the isocyanate group to the hydroxyl group may be 2 / 1.

[0136] The amount of trifunctional polyol may be an amount that results in a hydroxyl equivalent greater than or equal to the isocyanate equivalent of the reaction product, or the isocyanate equivalent and the hydroxyl equivalent may be equal. The isocyanate equivalent of the reaction product is determined by direct measurement. The molar amount of isocyanate groups can also be calculated from the amount charged.

[0137] The reaction can be carried out under conditions well known to those skilled in the art, and a tin catalyst can be used as needed. The reaction is carried out in the same manner as in the case of the hydrophilic modified carbodiimide compound (E) represented by general formula (I).

[0138] The trifunctional polyol may be trimethylolpropane, glycerin, or an alkylene oxide adduct thereof, because they are readily available. Examples of alkylene oxides include ethylene oxide and propylene oxide. Alkylene oxide adducts of glycerin are commercially available from Sanyo Chemical Industries, Ltd. as the GP series. Considering the curing reactivity of the resulting hydrophilic modified carbodiimide compound, the trifunctional polyol may have a structure in which an alkylene oxide is added to each hydroxyl group. Among the GP series, for example, GP-250 and GP-3000 have such a structure.

[0139] The hydrophilic modified carbodiimide compound (E) produced through such a two-step reaction may be a mixture of the compound represented by general formula (II) and other reaction products, but such a mixture may also be considered as the hydrophilic modified carbodiimide compound (E) having the structure of general formula (II).

[0140] (Hydrophilic modified carbodiimide compound (III)) The hydrophilic modified carbodiimide compound (E) having one structural unit α is represented, for example, by the following general formula (III). [ka] (wherein X and Y have the same meanings as in formula (α) above.)

[0141] Each Y may independently have a structure selected from the following (i) or (ii): This can improve water dispersibility and stability, and also makes it easier to increase the crosslink density.

[0142] (i) A structure in which a hydroxyl group is removed from a polyethylene glycol monoalkyl ether in which an alkyl group having 1 to 3 carbon atoms is ether-bonded to the end of a polyethylene oxide unit having a repeating number of 6 to 20. (ii) A structure in which a hydroxyl group is removed from a polypropylene glycol monoalkyl ether in which an alkyl group having 1 to 8 carbon atoms is ether-bonded to the end of a polypropylene oxide unit having a repeating number of 4 to 60.

[0143] Among these, Y may have the structure (ii), and the number of repeating polypropylene oxide units may be 15 to 60.

[0144] One Y may have the structure (i) and the other Y may have the structure (ii). In this case, the ratio of the number of Ys having the structure (i) to the number of Ys having the structure (ii) may be (i):(ii)=1:0.7 to 1:8.

[0145] From the viewpoints of preventing deactivation of the carbodiimide by water and improving the water resistance of the coating film, it is desirable that the area around the carbodiimide group is hydrophobic to some extent and has low contact with water molecules. On the other hand, the carbodiimide compound itself is required to have hydrophilicity. When the ratio (i):(ii) is within the above range, the hydrophobicity and hydrophilicity are easily balanced. The ratio (i):(ii) may be 1:0.7 to 1:1.5.

[0146] <Synthesis method> The hydrophilically modified carbodiimide compound (E) represented by the general formula (III) can be obtained by reacting the above starting carbodiimide compound with the same or a different PAGAE.

[0147] The PAGAE may be at least one selected from the group consisting of the following (i') and (ii'): (i') Polyethylene glycol monoalkyl ether in which an alkyl group having 1 to 3 carbon atoms is ether-bonded to the end of a polyethylene oxide unit having a repeating number of 6 to 20. (ii') Polypropylene glycol monoalkyl ether in which an alkyl group having 1 to 8 carbon atoms is ether-bonded to the end of a polypropylene oxide unit having a repeating number of 4 to 60.

[0148] Specific examples of (i') include polyethylene glycol monomethyl ether, polyethylene glycol monoethyl ether, and polyethylene glycol monopropyl ether. In particular, polyethylene glycol monomethyl ether is preferred. Specific examples of (ii') include polypropylene glycol monomethyl ether, polypropylene glycol monoethyl ether, polypropylene glycol monobutyl ether, and polypropylene glycol 2-ethylhexyl ether. In particular, polypropylene glycol monobutyl ether may be used.

[0149] In terms of crosslink density, the hydrophilically modified carbodiimide compound (E) may be represented by general formula (III).

[0150] Water-soluble acrylic resin The aqueous base coating composition may contain a water-soluble acrylic resin, which is a film-forming component and is a water-based resin.

[0151] The hydroxyl value of the water-soluble acrylic resin is, for example, 10 mgKOH / g or more and 100 mgKOH / g or less. This improves adhesion to the substrate. The hydroxyl value may be 20 mgKOH / g or more, or 30 mgKOH / g or more. The hydroxyl value may be 80 mgKOH / g or less, or 70 mgKOH / g or less.

[0152] The acid value of the water-soluble acrylic resin is, for example, 5 mgKOH / g or more and 80 mgKOH / g or less. This can improve the reaction efficiency with the hydrophilic modified carbodiimide compound (E) and also facilitates increased storage stability. The acid value may be 10 mgKOH / g or more, or 20 mgKOH / g or more. The acid value may be 60 mgKOH / g or less, or 50 mgKOH / g or less.

[0153] The weight average molecular weight (Mw) of the water-soluble acrylic resin is, for example, 4,000 or more and 50,000 or less. This tends to improve the hardness and weather resistance of the resulting coating film. The Mw of the water-soluble acrylic resin may be 5,000 or more, 8,000 or more, or 15,000 or more. The Mw of the water-soluble acrylic resin may be 40,000 or less, or 35,000 or less.

[0154] The mass-based mixing ratio (A2 / water-soluble acrylic resin) of the water-dispersible acrylic resin (A2) to the water-soluble acrylic resin may be, for example, 2 to 80. A2 / water-soluble acrylic resin may be 5 or more, 6.5 or more, or 10 or more. A2 / water-soluble acrylic resin may be 50 or less, or 30 or less.

[0155] The content of the water-soluble acrylic resin is, for example, 5 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the resin solid content of the aqueous base coating composition. The content of the water-soluble acrylic resin may be 8 parts by mass or more, or 10 parts by mass or more. The content of the water-soluble acrylic resin may be 17 parts by mass or less, or 15 parts by mass or less.

[0156] The water-soluble acrylic resin is produced by solution polymerizing a raw material monomer mixture containing an α,β-ethylenically unsaturated monomer having an acid group, which is exemplified as the raw material monomer mixture used in the production of the water-dispersible acrylic resin (A1), and then neutralizing the resulting mixture with a basic compound, which will be described later. Examples of the raw material monomer include those exemplified for the water-dispersible acrylic resin (A1).

[0157] Basic compounds The aqueous base coating composition may contain a basic compound. A basic compound is a compound that acts as a base according to the Bronsted-Lowry definition. A basic compound is a compound that acts as a base by reacting with a proton (H + )

[0158] The basic compound neutralizes the carboxyl groups of the water-dispersible acrylic resin (A2) in the aqueous base coating composition, thereby suppressing the reaction between the water-dispersible acrylic resin (A2) and the hydrophilically modified carbodiimide compound (E). The basic compound improves the storage stability and pot life of the aqueous base coating composition.

[0159] The pH of the aqueous base coating composition is adjusted to 8.5 or more and less than 10 by the basic compound. This further suppresses the reaction between the carbodiimide compound and the aqueous resin. In addition, for example, blackening of aluminum flakes that may be contained as a luster pigment is suppressed. The pH of the aqueous base coating composition may be 9.9 or less, or 9.8 or less.

[0160] The content of the basic compound is appropriately set in consideration of the pH of the aqueous base coating composition. The content of the basic compound is, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the solid content of the aqueous base coating composition. The content of the basic compound may be 1.2 parts by mass or more, or 1.4 parts by mass or more. The content of the basic compound may be 7.0 parts by mass or less, or 5.0 parts by mass or less.

[0161] It is desirable that the basic compound volatilizes after the aqueous base coating composition is applied to the substrate. The volatilization of the basic compound allows the carboxyl group of the aqueous resin to react with the carbodiimide compound. The boiling point of the basic compound may be such that it can be volatilized by heat treatment performed after application. The boiling point of the basic compound may be, for example, 140°C or lower, 120°C or lower, or 100°C or lower. The boiling point of the basic compound may be such that it does not volatilize during storage of the aqueous base coating composition. The boiling point of the basic compound may be, for example, 60°C or higher, 70°C or higher, or 80°C or higher.

[0162] The pKa (acid dissociation constant) of the basic compound is, for example, 5.0 or more. This makes it easier to suppress the reaction between the hydrophilically modified carbodiimide compound (E) and the aqueous resin. The pKa of the basic compound may be 7.0 or more, and may be greater than 8.5. The pKa of the basic compound is, for example, 30.0 or less. This makes it easier to improve the water resistance of the resulting coating film. The pKa of the basic compound may be 20.0 or less, and may be 15.0 or less. In one aspect, the pKa of the basic compound is 5.0 or more and 30.0 or less. The pKa is a value measured at 25°C using water as a solvent.

[0163] The basic compound is not particularly limited and includes, for example, at least one selected from the group consisting of ammonia (pKa 9.25) and amine compounds. Examples of the amine compounds include dimethylamine (pKa 10.64), trimethylamine (pKa 9.76), ethylamine (pKa 10.63), diethylamine (pKa 10.98), triethylamine (pKa 10.7), n-propylamine (pKa 10.53), isopropylamine (pKa 10.63), triallylamine (pKa 8.3, boiling point 156°C), triethylenediamine (pKa 8.7, boiling point 174°C), N,N-dimethylethanolamine (boiling point 135°C), N,N-diethylethanolamine, aminoethanolamine, and N Examples of suitable basic compounds include N,N-methyl-N,N-diethanolamine, iminobispropylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, methylaminopropylamine, methyliminobispropylamine, 3-methoxypropylamine, monoethanolamine (pKa 9.5), diethanolamine, triethanolamine, morpholine (pKa 8.4, boiling point 129°C), allylmorpholine (pKa 7.1, boiling point 158°C), N-methylmorpholine (pKa 7.4, boiling point 116°C), and N-ethylmorpholine (pKa 7.7, boiling point 139°C). These may be used alone or in combination of two or more. The basic compound may be N,N-dimethylethanolamine.

[0164] others The aqueous base coating composition may contain a pigment and various additives, such as a curing catalyst, a surface conditioner, an antifoaming agent, a pigment dispersant, a plasticizer, a film-forming aid, an ultraviolet absorber, an antioxidant, a solvent (water, organic solvent), and a pH adjuster.

[0165] The aqueous base coating composition is prepared by mixing the above components in a known manner.

[0166] The resin solids concentration of the aqueous base coating composition is not particularly limited and is set appropriately depending on the coating conditions. The resin solids concentration of the aqueous base coating composition is, for example, 10 to 60 mass %.

[0167] Other film-forming ingredients The aqueous base coating composition may contain other film-forming components. The aqueous base coating composition may contain other resins that do not have either or both of a hydroxyl group and a carboxyl group. Examples of other resins include resins that have hydroxyl groups, such as polyether diols and polycarbonate diols, melamine resins that have methylol groups, and resins that have phosphate groups.

[0168] The content of other resins is appropriately set depending on the type of coating film. The content of other resins may be 15 to 45 parts by mass per 100 parts by mass of the solid content of the aqueous base coating composition. However, it is desirable that the content of resins having only hydroxyl groups is low. Such resins do not contribute to curing and tend to deteriorate the physical properties of the coating film.

[0169] Other hardeners The aqueous base coating composition may contain other curing agents in addition to the hydrophilically modified carbodiimide compound (E). Examples of other curing agents include polyisocyanate curing agents (including blocked polyisocyanate curing agents), amino resins, epoxy compounds, aziridine compounds, and oxazoline compounds.

[0170] However, the content of the polyisocyanate curing agent is less than 1 part by mass, and may be 0 part by mass, per 100 parts by mass of the solid content of the aqueous base coating composition, because the polyisocyanate curing agent reduces storage stability.

[0171] Clear coating composition The clear coating composition contains an acrylic resin (A3) and a polyisocyanate compound (B2).

[0172] The acrylic resin (A3) and the polyisocyanate compound (B2) are film-forming components. The acrylic resin (A3) is a water-based resin.

[0173] The clear coating composition may be a solvent-based or water-based clear coating composition.The clear coating composition may be a solvent-based clear coating composition.

[0174] The clear coating composition may be a two-component clear coating composition. The two-component clear coating composition comprises, for example, a base agent containing an acrylic resin (A3) and a curing agent containing a polyisocyanate compound (B2).

[0175] Acrylic resin (A3) The acrylic resin (A3) may be water-dispersible and may be hydrophobic. The acrylic resin (A3) has hydroxyl groups.

[0176] The hydroxyl value of the acrylic resin (A3) is, for example, 80 mgKOH / g or more and 200 mgKOH / g or less. This improves adhesion to the substrate. The hydroxyl value may be 15 mgKOH / g or more. The hydroxyl value may be 80 mgKOH / g or less, or 60 mgKOH / g or less.

[0177] The acid value of the acrylic resin (A3) is, for example, 5 mgKOH / g or more and 40 mgKOH / g or less. This tends to further improve storage stability. The acid value may be 8 mgKOH / g or more. The acid value may be 30 mgKOH / g or less, or 20 mgKOH / g or less.

[0178] The number average molecular weight (Mn) of the acrylic resin (A3) may be from 1500 to 6000. When the number average molecular weight is within the above range, coating workability and curing properties can be improved.

[0179] The acrylic resin (A3) can be produced, for example, by solution polymerization of a raw material monomer mixture containing the above-mentioned α,β-ethylenically unsaturated monomer having a hydroxyl group. Examples of the raw material monomer include those exemplified for the water-dispersible acrylic resin (A1).

[0180] Polyisocyanate compound (B2) The polyisocyanate compound (B2) has one to two isocyanate groups per hydroxyl group contained in the acrylic resin (A3), and is water-dispersible.

[0181] The polyisocyanate compound (B2) is not particularly limited as long as it satisfies the above-mentioned relationship. Examples of the polyisocyanate compound (B2) include the same compounds as those exemplified for the polyisocyanate compound (B1).

[0182] The polyisocyanate compound (B2) may contain at least one selected from the group consisting of aliphatic diisocyanates, alicyclic diisocyanates, and derivatives thereof, which can further improve chipping resistance.

[0183] The polyisocyanate compound (B2) may contain a polyisocyanate (B21) having an isocyanate group with a uretdione structure and a polyisocyanate (B22) that is a trimer or higher of a diisocyanate. This increases the reaction rate with the acrylic resin (A3). In addition, the weather resistance of the clear coating film is improved.

[0184] The solid content mass ratio (B21 / B22) of the polyisocyanate (B21) to the polyisocyanate (B22) may be 10 / 90 to 50 / 50. B21 / B22 may be 0.1 to 1.0.

[0185] The equivalent ratio (NCO / OH) of the isocyanate groups contained in the polyisocyanate compound (B2) to the hydroxyl groups contained in the acrylic resin (A3) is, for example, 1.0 or more and 2.0 or less. This further increases the coating film strength. NCO / OH may be 1.5 or less, or 1.2 or less.

[0186] Polyester polyol (F) The clear coating composition may contain a polyester polyol. The polyester polyol (F) generally has a low viscosity and is easy to increase the hydroxyl value. The acrylic resin (A3) is easy to increase the viscosity of the clear coating composition. By using the polyester polyol (F) in combination with the acrylic resin (A3), it is possible to improve the crosslink density while suppressing an increase in viscosity.

[0187] The polyester polyol (F) reacts with the polyisocyanate compound (B2) to form a crosslinked structure. The polyester polyol (F) has two or more ester bonds and two or more hydroxyl groups.

[0188] The amount of polyester polyol (F) is 10 parts by mass or more per 100 parts by mass of the resin solid content of the clear coating composition. This can further improve chipping resistance. The content of polyester polyol (F) may be 20 parts by mass or less, or may be 30 parts by mass or less.

[0189] From the viewpoint of abrasion resistance, all of the hydroxyl groups of the polyester polyol (F) may be located at the terminals of the molecular chain. Examples of the polyester polyol (F) include at least one selected from the group consisting of the following polyester polyols (F1) to (F5). The polyester polyols (F1) to (F4) are represented by formulas (1) to (4), respectively.

[0190] The polyester polyols (F1) to (F5) have hydroxyl groups only at their terminals, and the hydroxyl groups are bonded to linear hydrocarbon groups (hydrocarbon groups that do not have a cyclic structure). Therefore, the reaction between the polyester polyols (F1) to (F5) and the polyisocyanate compound (B2) forms a portion having a linear structure (linear portion) in the clear coating film. This enhances the stress relaxation properties of the entire coating film, further improving its scratch resistance.

[0191] In the formulas (1) to (4), the linear alkylene group is -(C n H 2n)-, a divalent aliphatic hydrocarbon group. The number of carbon atoms, n, is, for example, 1 to 20, and may be 2 to 12. Examples of linear alkylene groups include methylene, ethylene, n-propylene, n-butylene, n-pentylene, n-hexylene, n-heptylene, n-octylene, n-nonylene, and n-decylene.

[0192] In the formulas (1) to (4), the branched alkylene group branched at a position other than the α-position and the β-position is -C α H2-C β H2-RC β H2-C α C is a divalent aliphatic hydrocarbon group represented by H2-. α and C β represent the carbon atoms at the α-position and the β-position, respectively. The carbon atom at the α-position is the carbon atom bonded to a hydroxyl group (-OH). The carbon atom at the β-position is the carbon atom bonded to the carbon atom to which a hydroxyl group (-OH) is bonded. R represents an alkylene group which may be branched. The number of carbon atoms in R is, for example, 1 to 20, and may be 2 to 12.

[0193] Examples of branched alkylene groups include a 3-methyl-n-pentylene group, a 3-ethyl-n-pentylene group, a 3,4-dimethyl-n-hexylene group, and a neopentylene group.

[0194] A hydrocarbon group is a group containing carbon and hydrogen, and is a group obtained by removing one or more hydrogen atoms from a hydrocarbon. A saturated hydrocarbon group is a hydrocarbon group that does not have a carbon-carbon unsaturated bond (e.g., -C=C-, -C≡C-). Specific examples of saturated hydrocarbon groups include aliphatic hydrocarbon groups. The aliphatic hydrocarbon group may be linear, branched, or cyclic, and may be linear or branched. The saturated hydrocarbon group may have one or more N, O, S, Si, amide, sulfonyl, siloxane, carbonyl, carbonyloxy, etc. at its terminal or in the molecular chain. The hydrogen atoms of the saturated hydrocarbon group may be substituted with one or more substituents other than a hydroxyl group.

[0195] Polyester polyol (F1) The polyester polyol (F1) is represented by the following formula (1): [ka] (In the formula, a is an integer of 1 to 100, R 11 represents a saturated hydrocarbon group having two bonds, independently for each repeating unit; R 12 and R 13 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. It is expressed as:

[0196] a may be an integer of 3 to 50, or may be an integer of 5 to 30.

[0197] R 11 R may be a straight chain alkylene group or a branched alkylene group. 11 Specific examples of the alkylene group include linear or branched alkylene groups having 3 to 12 carbon atoms.

[0198] R 12 and R 13 Specific examples of the R include linear or branched alkylene groups having 3 to 12 carbon atoms. 12 may be the same. R 12 and R 13 may be the same.

[0199] R 11 is a linear alkylene group having 3 to 12 carbon atoms, and R 12 is a linear or branched alkylene group having 3 to 12 carbon atoms, and R 13 may be a linear or branched alkylene group having 3 to 12 carbon atoms. 11 and R 12 and R 13 may be the same.

[0200] The polyester polyol (F1) can be obtained, for example, by an esterification reaction or transesterification reaction between a specific dicarboxylic acid and a specific diol. In the esterification reaction or transesterification reaction, the specific dicarboxylic acid and the diol are mixed and heated to 180 to 250°C in the presence of a catalyst.

[0201] Specific examples of the specific dicarboxylic acid include succinic acid, malonic acid, adipic acid, sebacic acid, azelaic acid, or lower alkyl esters thereof (e.g., C1-C4 alkyl esters), or derivatives thereof such as acid anhydrides and acyl halides. These may be used alone or in combination of two or more.

[0202] Examples of specific diols include ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, and 1,9-nonanediol. These may be used alone or in combination of two or more. These may be used alone or in combination of two or more.

[0203] Polyester polyol (F2) The polyester polyol (F2) is represented by the following formula (2): [ka] (wherein j and k are each an integer of 0 to 100, and j+k≧1 is satisfied; R 21 represents a saturated hydrocarbon group having two bonds, R 22 and R 23 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. It is expressed as:

[0204] j and k may each be an integer of 1 to 50, or an integer of 2 to 30. j+k may be 2 or more. j+k may be 50 or less, 30 or less, or 20 or less.

[0205] R 21 R may be a linear, branched, or alicyclic alkylene group. 21 Specific examples of R include linear alkylene groups having 2 to 12 carbon atoms, branched alkylene groups having 3 to 12 carbon atoms, and alicyclic alkylene groups having 5 to 12 carbon atoms. 21 may be a linear, branched, or alicyclic alkylene group having 5 to 10 carbon atoms, or may be an alicyclic alkylene group having 5 to 10 carbon atoms.

[0206] R 22 and R 23 Specific examples of R include linear alkylene groups having 2 to 12 carbon atoms and branched alkylene groups having 3 to 12 carbon atoms. 22 and R 23 In particular, R may be a linear alkylene group having 2 to 12 carbon atoms, a linear alkylene group having 3 to 10 carbon atoms, or a linear alkylene group having 4 to 8 carbon atoms. 22 In two or more repeating units, multiple R 23 may be the same. R 22 and R 23 may be the same.

[0207] R 21 is an alicyclic alkylene group having 5 to 10 carbon atoms, and R 22 is a linear alkylene group having 4 to 8 carbon atoms, and R 23 may be a linear alkylene group having 4 to 8 carbon atoms.

[0208] The polyester polyol (F2) can be obtained, for example, by the ring-opening polymerization reaction of a lactone compound. In the ring-opening polymerization reaction of a lactone compound, the lactone compound is mixed with a diol as an initiator and heated to 130 to 220°C in the presence of a catalyst.

[0209] Examples of lactone compounds include ε-caprolactone, δ-valerolactone, γ-butyrolactone, β-propiolactone, α-acetolactone, and cyclic esters of these with lactide.

[0210] Examples of diols include ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 1,9-nonanediol, 1,2-propanediol, 1,3-butanediol, 1,2-butanediol, neopentyl glycol, diethylene glycol, triethylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 2-methyl-1,8-octanediol, 2-methyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-ethyl-1,3-hexanediol, and 2,4-diethyl-1,5-pentanediol.

[0211] (B3) Polyester polyol The polyester polyol (F3) is represented by the following formula (3): [ka] (In the formula, p, q, and r each represent an integer of 1 to 100, R 31 represents a saturated hydrocarbon group having three bonds, R 32 , R 33 and R 34 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. It is expressed as:

[0212] p, q, and r may each be an integer of 3 to 50, or may be an integer of 5 to 30. p+q+r is 3 or greater, or may be 5 or greater, or may be 6 or greater. p+q+r may be 50 or less, or may be 30 or less, or may be 20 or less.

[0213] R 31 R may be a linear or branched trivalent aliphatic hydrocarbon group. 31 Specific examples of the alkyl group include branched trivalent aliphatic hydrocarbon groups having 3 to 10 carbon atoms.

[0214] R 32 , R 33 or R 34 Specific examples of R include linear alkylene groups having 2 to 12 carbon atoms and branched alkylene groups having 3 to 12 carbon atoms. 32 , R 33 or R 34 In particular, R may be a linear alkylene group having 2 to 12 carbon atoms, a linear alkylene group having 3 to 10 carbon atoms, or a linear alkylene group having 4 to 8 carbon atoms. 32 In two or more repeating units, multiple R 33 In two or more repeating units, multiple R 34 may be the same. R 32 and R 33 and R 34 may be the same.

[0215] R 31 is a branched trivalent aliphatic hydrocarbon group having 3 to 10 carbon atoms, and R 32 , R 33 and R 34 may each be a linear alkylene group having 4 to 8 carbon atoms.

[0216] The polyester polyol (F3) can be obtained, for example, by the same procedure as the ring-opening polymerization reaction of the lactone compound of the polyester polyol (F2) except that the diol is changed to a trihydric alcohol.

[0217] Examples of trihydric alcohols include glycerin, trioxyisobutane, 1,2,3-butanetriol, 1,2,3-pentanetriol, 2-methyl-1,2,3-propanetriol, 2-methyl-2,3,4-butanetriol, 2-ethyl-1,2,3-butanetriol, 2,3,4-pentanetriol, 2,3,4-hexanetriol, 4-propyl-3,4,5-heptanetriol, 2,4-dimethyl-2,3,4-pentanetriol, pentamethylglycerin, pentaglycerin, 1,2,4-butanetriol, 1,2,4-pentanetriol, trimethylolethane, and trimethylolpropane.

[0218] Polyester polyol (F4) The polyester polyol (F4) is represented by the following formula (4): [ka] (In the formula, w, x, y, and z each represent an integer of 1 to 100, R 41 represents a saturated hydrocarbon group having four bonds, R 42 , R 43 , R 44 and R 45 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. It is expressed as:

[0219] Each of w, x, y, and z may be an integer of 3 to 50, or may be an integer of 5 to 30. w+x+y+z may be 4 or greater, 5 or greater, or 6 or greater. w+x+y+z may be 50 or less, 30 or less, or 20 or less.

[0220] R 41 R may be a linear or branched tetravalent aliphatic hydrocarbon group. 41 Specific examples of the alkyl group include branched tetravalent aliphatic hydrocarbon groups having 3 to 10 carbon atoms.

[0221] R 42 , R 43 , R 44 and R 45 Specific examples of R include linear alkylene groups having 2 to 12 carbon atoms and branched alkylene groups having 3 to 12 carbon atoms. 42 , R 43 , R 44 and R 45 A more preferred embodiment of the repeating unit is a linear alkylene group having 3 to 10 carbon atoms, and an even more preferred embodiment is a linear alkylene group having 4 to 8 carbon atoms. 42 In two or more repeating units, multiple R 43 In two or more repeating units, multiple R 44 In two or more repeating units, multiple R 45 may be the same. R 42 and R 43 and R 44 and R 45 may be the same.

[0222] R 41 is a branched tetravalent aliphatic hydrocarbon group having 3 to 10 carbon atoms, and R 42 is a linear alkylene group having 4 to 8 carbon atoms, and R 43 is a linear alkylene group having 4 to 8 carbon atoms, and R 44 is a linear alkylene group having 4 to 8 carbon atoms, and R 45 may be a linear alkylene group having 4 to 8 carbon atoms.

[0223] The polyester polyol (F4) can be obtained, for example, by the same procedure as the ring-opening polymerization reaction of the lactone compound of the polyester polyol (F2) except that the diol is changed to a tetrahydric alcohol.

[0224] Examples of tetrahydric alcohols include pentaerythritol, 1,2,3,4-pentanetetrol, 2,3,4,5-hexanetetrol, 1,2,4,5-pentanetetrol, 1,3,4,5-hexanetetrol, diglycerin, and sorbitan.

[0225] Polyester polyol (F5) The polyester polyol (F5) is a reaction product of a linear or branched polyhydric alcohol having three or more hydroxyl groups, a linear or branched alkanediol, and a linear or branched alkanedicarboxylic acid. That is, the polyester polyol (F5) does not have a cyclic structure such as an alicyclic hydrocarbon group or an aromatic hydrocarbon group.

[0226] Examples of linear or branched polyhydric alcohols having three or more hydroxyl groups include glycerin, trioxyisobutane, 1,2,3-butanetriol, 1,2,3-pentanetriol, 2-methyl-1,2,3-propanetriol, 2-methyl-2,3,4-butanetriol, 2-ethyl-1,2,3-butanetriol, 2,3,4-pentanetriol, 2,3,4-hexanetriol, 4-propyl-3,4,5-heptanetriol, 2,4-dimethyl-2,3,4-pentanetriol, pentamethylglycerin, pentaglycerin, and 1,2,4-butanetriol. trihydric alcohols such as pentaerythritol, 1,2,4-pentanetriol, trimethylolethane, trimethylolpropane, etc.; tetrahydric alcohols such as pentaerythritol, 1,2,3,4-pentanetetrol, 2,3,4,5-hexanetetrol, 1,2,4,5-pentanetetrol, 1,3,4,5-hexanetetrol, diglycerin, sorbitan, etc.; pentahydric alcohols such as adonitol, arabitol, xylitol, triglycerin, etc.; and hexahydric alcohols such as dipentaerythritol, sorbitol, mannitol, iditol, inositol, dulcitol, talose, allose, etc. These may be used alone or in combination of two or more.

[0227] Examples of linear or branched alkanediols include ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,3-butanediol, 1,2-butanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, and 3-methyl-1,5-pentanediol. These may be used alone or in combination of two or more.

[0228] Examples of linear or branched alkanedicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. These may be used alone or in combination of two or more.

[0229] The polyester polyol (F5) can be obtained by mixing the above components and heating the mixture, for example, at 180° C. or higher and 250° C. or lower in the presence of a catalyst.

[0230] The polyester polyol (F) may contain at least one selected from the group consisting of polyester polyols (F1), (F2) and (F5). The polyester polyol (F) may contain polyester polyol (F2).

[0231] The hydroxyl value of the polyester polyol (F) is, for example, 50 mgKOH / g or more and 600 mgKOH / g or less. When the hydroxyl value of the polyester polyol (F) is 50 mgKOH / g or more, the crosslinking density tends to be high. When the hydroxyl value of the polyester polyol (F) is 600 mgKOH / g or less, hydrophilization of the coating film is suppressed, and the water resistance of the clear coating film tends to be improved. The hydroxyl value of the polyester polyol (F) may be 75 mgKOH / g or more, or may be 500 mgKOH / g or more. The hydroxyl value of the polyester polyol (F) may be 100 mgKOH / g or less, or may be 400 mgKOH / g or less.

[0232] It is sufficient that the hydroxyl value of at least one polyester polyol (F) selected from the group consisting of polyester polyols (F1) to (F5) contained in the clear coating composition is within the above range.

[0233] The Mw (or molecular weight) of the polyester polyol (F) is, for example, 250 or more and 150,000 or less. When the Mw of the polyester polyol (F) is 250 or more, the hardness and weather resistance of the resulting coating film are likely to be improved. The Mw of the polyester polyol (F) may be 300 or more, or 350 or more. The Mw of the polyester polyol (F) may be 120,000 or less, or 100,000 or less.

[0234] It is sufficient that the weight average molecular weight of at least one polyester polyol (F) selected from the group consisting of polyester polyols (F1) to (F5) contained in the clear coating composition is within the above range.

[0235] others The clear coating composition may contain various additives, such as curing catalysts, surface conditioners, antifoaming agents, pigment dispersants, plasticizers, film-forming aids, UV absorbers, antioxidants, solvents (water, organic solvents), pH adjusters, and viscosity control agents.

[0236] The clear coating composition is prepared by mixing the above components in a known manner.

[0237] The resin solid content concentration of the clear coating composition is not particularly limited and is set appropriately depending on the coating conditions. The resin solid content concentration of the clear coating composition is, for example, 40 to 70 mass %.

[0238] Multi-layer coating film formation method Multi-layer coating film Applying the aqueous two-component intermediate coating composition to a substrate including a metal part and a resin part to form an uncured intermediate coating film; Applying the aqueous base coating composition on the uncured intermediate coating film to form an uncured base coating film; Applying the clear coating composition on an uncured base coating film to form an uncured clear coating film; heating and curing the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film; The compound is formed by a method comprising:

[0239] The above-mentioned water-based intermediate coating composition, water-based base coating composition and clear coating composition can provide a coating film with excellent chipping resistance.

[0240] Object to be coated The substrate includes both metal parts (parts formed from metal) and resin parts (parts formed from resin). The coating composition can be cured at low temperatures (e.g., 75 to 100°C), so it is possible to avoid applying heat that would cause thermal deformation to the resin parts. Because the metal parts and the resin parts can be coated with the same coating composition, it is possible to match the color of the coating film on each part to a high degree. The substrate may be an automobile body.

[0241] Examples of metals include iron, steel, stainless steel, aluminum, copper, zinc, tin, and alloys thereof. Representative examples of the metal part include steel sheets such as cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0242] The metal part may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After the surface treatment, the metal part may be further coated with an electrodeposition paint. The electrodeposition paint may be of the cationic type or the anionic type.

[0243] Examples of resins include polyethylene resin, EVA resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), vinyl chloride resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), polycarbonate resin, acrylic resin, acrylonitrile butadiene styrene (ABS) resin, acrylonitrile styrene (AS) resin, polyamide resin, acetal resin, phenolic resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). Resin substrates may be degreased. Resin parts may be coated with a primer.

[0244] coating Each coating composition can be applied by a commonly used coating method. The water-based primer coating composition and the water-based base coating composition are applied, for example, by multi-stage coating using an air electrostatic spray coater (typically, coating in two stages), or by a method that combines an air electrostatic spray coater with a rotary atomization electrostatic coater commonly known as a "μμ (micro-micro) bell," "μ (micro) bell," or "meta-bell."

[0245] The clear coating composition is applied, for example, by a rotary atomizing electrostatic coating machine called a Microbell.

[0246] The application of the clear coating composition may be carried out after applying the aqueous intermediate coating composition and the aqueous base coating composition and drying the resulting uncured intermediate coating film and base coating film. The drying (also called preheating) is carried out, for example, at 60 to 90°C for 1 to 15 minutes.

[0247] hardening The uncured intermediate coating film, uncured base coating film, and uncured clear coating film are heated, which simultaneously cures each of the uncured coating films, forming a multi-layer coating film that includes the intermediate coating film, base coating film, and clear coating film.

[0248] The heating temperature may be 75 to 100° C. The heating temperature may be 80° C. or higher. The heating temperature may be 95° C. or lower, or may be 90° C. or lower. The heating time may be, for example, 10 minutes or more and 40 minutes or less.

[0249] The thickness of the intermediate coating film may be appropriately set depending on the application, etc. The thickness of the intermediate coating film is, for example, 5 μm or more and 40 μm or less. The thickness of the intermediate coating film may be 10 μm or more. The thickness of the intermediate coating film may be 30 μm or less.

[0250] The thickness of the base coating film may be appropriately set depending on the application, etc. The thickness of the base coating film is, for example, 5 μm or more and 30 μm or less. The thickness of the base coating film may be 10 μm or more. The thickness of the base coating film may be 20 μm or less.

[0251] The thickness of the clear coating film may be appropriately set depending on the application, etc. The thickness of the clear coating film is, for example, 10 μm or more and 70 μm or less. The thickness of the clear coating film may be 20 μm or more, or 30 μm or more. The thickness of the clear coating film may be 60 μm or less.

[0252] The thickness of the multi-layer coating film is, for example, 20 μm or more and 140 μm or less. The thickness of the multi-layer coating film may be 30 μm or more. The thickness of the multi-layer coating film may be 120 μm or less, or may be 100 μm or less. [Example]

[0253] The present invention will be described in more detail with reference to the following examples, but is not limited thereto. In the examples, "parts" and "%" are by weight unless otherwise specified.

[0254] The number average molecular weight was measured under the following GPC system measurement conditions. Apparatus: Tosoh HLC-8220 GPC Column: Shodex KF-606M, KF-603 Flow rate: 0.6ml / min Detector: RI, UV254nm Mobile phase: tetrahydrofuran Standard samples: TSK STANDARD POLYSTYRENE (manufactured by Tosoh Corporation), A-500, A-2500, F-1, F-4, F-20, F-80, F-700, 1-phenylhexane (manufactured by Aldrich)

[0255] Manufacturing example of components to be blended into water-based two-component intermediate coating composition [Production Example 1-1] Production of Water-Dispersible Acrylic Resin (A1) A reaction vessel equipped with a stirrer, a nitrogen inlet tube, a temperature controller, a condenser, and a dropping funnel was charged with 2,000 parts of deionized water, and the temperature was raised to 80° C. with stirring under a nitrogen atmosphere. Separately, a pre-emulsion was prepared by emulsifying 103 parts of styrene, 290 parts of n-butyl methacrylate, 280 parts of n-butyl acrylate, 302 parts of hydroxyethyl acrylate, 26 parts of acrylic acid, 3 parts of dodecyl mercaptan, and 100 parts of an emulsifier (trade name Latemul PD-104, manufactured by Kao Corporation, 20% aqueous solution) in 1,000 parts of deionized water.

[0256] The pre-emulsion and an aqueous initiator solution prepared by dissolving 3 parts of ammonium persulfate in 300 parts of deionized water were added dropwise to the reaction vessel over a period of 2 hours. After the dropwise addition was completed, the reaction was continued for 1 hour at 80° C. After cooling, 8.2 parts of N,N-dimethylaminoethanol was added to obtain an emulsion-type water-dispersible acrylic resin (A1) with a resin solid content of 35% by mass.

[0257] The water-dispersible acrylic resin (A1) had a hydroxyl value of 130 mgKOH / g, an acid value of 20 mgKOH / g, and a number average molecular weight of 1,000,000 or more.

[0258] [Production Example 1-2] Production of aqueous polyester resin A reaction vessel equipped with a stirrer, nitrogen inlet tube, temperature controller, condenser, and decanter was charged with 250 parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts of cyclohexanedicarboxylic acid, and the mixture was heated to 180°C and subjected to a condensation reaction until water no longer distilled off. After cooling to 60°C, 120 parts of phthalic anhydride was added. The mixture was then heated to 140°C and maintained at this temperature for 60 minutes. This yielded a polyester resin with a number average molecular weight of 2,000.

[0259] Next, 59 parts of dimethylaminoethanol (equivalent to 80% of the acid value of the resin (neutralization rate 80%)) was added at 80°C, and 1920 parts of deionized water was added and stirred to obtain a colloidal dispersion type aqueous polyester resin with a resin solids content of 45% by mass. The aqueous polyester resin had a hydroxyl value of 90 mgKOH / g and an acid value of 35 mgKOH / g.

[0260] [Production Example 1-3] Production of color pigment paste 9.2 parts of a pigment dispersant (trade name Disperbyk 190, manufactured by BYK-Chemie), 17.8 parts of ion-exchanged water, and 73.0 parts of rutile titanium dioxide were premixed, and then mixed and dispersed together with a bead medium in a paint conditioner at room temperature until the particle size became 5 μm or less. The bead medium was then removed by filtration to obtain a color pigment paste.

[0261] [Production Example 1-4] Production of water-soluble acrylic resin 23.89 parts of tripropylene glycol methyl ether and 16.11 parts of propylene glycol methyl ether were added to a reaction vessel, and the mixture was heated to 105 ° C. while being mixed and stirred in a nitrogen stream. Next, a monomer mixture containing 13.1 parts of methyl methacrylate, 68.4 parts of ethyl acrylate, 11.6 parts of 2-hydroxyethyl methacrylate, and 6.9 parts of methacrylic acid was prepared, and 100 parts of this monomer mixture and an initiator solution consisting of 10.0 parts of tripropylene glycol methyl ether and 1 part of t-butylperoxy 2-ethylhexanoate were added dropwise to the reaction vessel in parallel over 3 hours. After the completion of the dropwise addition, the mixture was aged at the same temperature for 0.5 hours.

[0262] Further, an initiator solution consisting of 5.0 parts of tripropylene glycol methyl ether and 0.3 parts of t-butylperoxy 2-ethylhexanoate was added dropwise to the reaction vessel over 0.5 hours. After the addition was completed, the mixture was aged at the same temperature for 2 hours.

[0263] Subsequently, 16.1 parts of the solvent was distilled off under reduced pressure (70 torr) at 110°C using a solvent remover, and then 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added, thereby obtaining a water-soluble acrylic resin with a resin solid content of 30% by mass.

[0264] The water-soluble acrylic resin had an acid value of 40 mgKOH / g, a hydroxyl value of 50 mgKOH / g, a Tg of 10°C, and an Mw of 30,000.

[0265] Examples of manufacturing components to be incorporated into aqueous base coating compositions [Production Example 2-1] Production of water-dispersible acrylic resin (A2) To a reaction vessel charged with 194.1 parts of ion-exchanged water, 0.2 parts of an emulsifier (trade name Adeka Reasoap NE-20, manufactured by ADEKA Corporation, α-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]-ω-hydroxyoxyethylene, aqueous solution with a solids content of 80% by mass) and 0.2 parts of an emulsifier (trade name Aqualon HS-10, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., polyoxyethylene alkylpropenyl phenyl ether sulfate) were added, and the mixture was heated to 80°C while being stirred and mixed in a nitrogen stream.

[0266] Separately, 18.5 parts of methyl acrylate, 31.7 parts of ethyl acrylate, 5.8 parts of 2-hydroxyethyl acrylate, 10.0 parts of styrene, 4.0 parts of acrylamide, 0.3 parts of Adeka Reasoap NE-20, 0.2 parts of Aqualon HS-10, and 70 parts of ion-exchanged water were mixed to obtain a first-stage raw material monomer mixture.

[0267] Separately, 24.5 parts of ethyl acrylate, 2.5 parts of 2-hydroxyethyl acrylate, 3.1 parts of methacrylic acid, 0.3 parts of Aqualon HS-10, and 30 parts of ion-exchanged water were mixed to obtain a second-stage raw material monomer mixture.

[0268] The first-stage raw material monomer mixture and an initiator solution consisting of 0.2 parts of ammonium persulfate and 7 parts of ion-exchanged water were added dropwise to the reaction vessel over a period of 2 hours. After the addition was completed, the mixture was aged at the same temperature for 1 hour.

[0269] Next, the second-stage raw material monomer mixture and an initiator solution consisting of 0.1 parts of ammonium persulfate and 3 parts of ion-exchanged water were added dropwise to the reaction vessel over 0.5 hours at 80°C. After the addition was completed, the mixture was aged at the same temperature for 2 hours.

[0270] The mixture was cooled to 40°C and filtered through a 400 mesh filter, and then a 10% by mass aqueous solution of dimethylaminoethanol was added to adjust the pH to 7, yielding an emulsion-type water-dispersible acrylic resin (A2) with a resin solid content of 35% by mass.

[0271] The water-dispersible acrylic resin (A2) had an average particle size of 110 nm, an acid value of 20 mgKOH / g, a hydroxyl value of 40 mgKOH / g, and a Tg of 10°C.

[0272] [Production Example 2-2] Production of hydrophilically modified carbodiimide compound (E) 700 parts of 4,4-dicyclohexylmethane diisocyanate and 7 parts of 3-methyl-1-phenyl-2-phospholene-1-oxide were reacted at 170°C for 7 hours to obtain a carbodiimide compound having a structure represented by the above general formula (a), three carbodiimide groups per molecule, and isocyanate groups at both ends.

[0273] Next, 95 parts of polytetramethylene glycol (trade name PTMD-1000, manufactured by Mitsubishi Chemical Corporation, number average molecular weight 1,000, tetramethylene oxide repeating units calculated from the number average molecular weight 13.6) and 0.2 parts of dibutyltin dilaurate were added to 180 parts of the above carbodiimide compound, and the mixture was heated to 85°C and maintained at this temperature for 2 hours.

[0274] Subsequently, 86.4 parts of polyethylene glycol monomethyl ether (trade name: Methyl Polyglycol 130, manufactured by Nippon Nyukazai Co., Ltd., hydroxyl value: 130 mg KOH / g, number of ethylene oxide repeat units calculated from the hydroxyl value: 9) was added, and the mixture was maintained at 85°C for 3 hours. The reaction was terminated when it was confirmed by IR measurement that the NCO peak had disappeared. After cooling to 60°C, deionized water was added to obtain an aqueous dispersion of the hydrophilic modified carbodiimide compound (E) represented by the above general formula (I) with a resin solids content of 40% by mass.

[0275] Manufacturing examples of components to be blended into clear coating compositions [Production Example 3-1] Production of acrylic resin (A3) 30 g of butyl acetate was placed in a vessel equipped with a stirrer, a temperature controller, and a reflux condenser, and the temperature was raised to 120°C. Separately, 20 parts of styrene, 15.8 parts of n-butyl acrylate, 21.8 parts of n-butyl methacrylate, 41.1 parts of 2-hydroxypropyl methacrylate, and 1.3 parts of acrylic acid were mixed to obtain a raw material monomer mixture.

[0276] To the raw material monomer mixture, 12 parts of Kayaester O and 6 parts of butyl acetate were simultaneously added dropwise over 3 hours. After leaving the mixture for 30 minutes, a solution consisting of 0.5 parts of Kayaester O and 4 parts of butyl acetate was added dropwise to the container over 30 minutes. The contents of the container were then stirred for 1 hour to terminate the reaction. This yielded a hydroxyl-containing acrylic resin (A3) with a resin solids content of 70% by mass.

[0277] The acrylic resin (A3) had a number average molecular weight of 3,800, a hydroxyl value of 160 mgKOH / g, and an acid value of 10 mgKOH / g.

[0278] [Example 1] (1) Preparation of water-based primer coating composition 65 parts of water-dispersible acrylic resin (A1) (resin solids content: 35%), 0.6 parts of water-soluble acrylic resin (resin solids content: 30%), and 22 parts of aqueous polyester resin (resin solids content: 45%) were stirred. To this mixture, 113 parts of aqueous polyurethane resin (C) (trade name: Permarine U150, manufactured by Sanyo Chemical Industries, Ltd., resin solids concentration: 30% by weight, Tg: -60°C, elongation at break: 610%), 113 parts of water-soluble acrylic resin (resin solids content: 30%), and 137.7 parts of color pigment paste were added. The pH was adjusted to 8.0 with 0.01 parts of dimethylethanolamine (manufactured by Kishida Chemical Co., Ltd.), and then 1.0 part of a urethane associative viscosity enhancer (trade name: Adekanol UH-814N, solids content: 30% by weight, manufactured by ADEKA Corporation) was added and stirred until uniform.

[0279] Subsequently, 33 parts of a polyisocyanate compound (B1) (trade name Bayhydur 305, manufactured by Sumika Covestro Urethane Co., Ltd., a polyisocyanate compound having an ethylene oxide group, an ethylene oxide content of 20% by mass, an isocyanate group content of 16% by mass) was added and stirred to obtain an aqueous primer coating composition (solids concentration of 45% by mass). The NCO / OH ratio was 1.7.

[0280] (2) Preparation of aqueous base coating composition A container equipped with a stirrer was charged with 217 parts of water-dispersible acrylic resin (A2) (resin solids content: 35%), 37 parts of the above water-soluble acrylic resin (resin solids content: 30%), 34 parts of hydrophilically modified carbodiimide compound (E) (resin solids content: 40%), 10 parts of a glitter pigment (trade name: Aluminum Paste MH8801, manufactured by Asahi Kasei Corporation, aluminum pigment, solids content: 65%) (PWC: 12%), and 0.3 parts of lauryl acid phosphate. Further, 30 parts of 2-ethylhexanol, 2 parts of a thickener (trade name: Adekanol UH-814N, manufactured by ADEKA Corporation, solids content: 30%), 1 part of a basic compound (N,N-dimethylethanolamine, manufactured by Kishida Chemical Co., Ltd.), and 50 parts of ion-exchanged water were added and uniformly dispersed to obtain an aqueous base coating composition (solids concentration: 25% by mass).

[0281] The pH of the aqueous base coating composition was 9.5. The content of the basic compound was 5 parts by mass per 100 parts by mass of the resin solids of the aqueous base coating composition. The ratio of the carbodiimide group equivalent Ec of the hydrophilically modified carbodiimide compound (E) to the acid group equivalent Ea of the water-dispersible acrylic resin (A2), Ec / Ea, was 1.2.

[0282] (3) Preparation of clear coating composition To a 1 L metal container were sequentially added 82 parts of acrylic resin (A3), 6 parts of polyester polyol (F2) (compound represented by formula (2), trade name: PCL205H, manufactured by Daicel Corporation), 1.78 parts of ultraviolet absorber (trade name: Tinuvin 384-2, manufactured by BASF), 0.9 parts of light stabilizer (trade name: Tinuvin 123, manufactured by BASF), 0.05 parts of acrylic surface conditioner, 5 parts of butyl acetate, and 5 parts of butanol, and the mixture was thoroughly stirred using a disper to obtain a base compound.

[0283] Into another metal container, 70 parts of a polyisocyanate compound (B2) (trade name Dismodur N-3300, manufactured by Sumitomo Covestro Urethane Co., Ltd., NCO active ingredient 22%, solid content 100%) and 30 parts of 2-ethylethoxypropanol were added in this order and thoroughly stirred to obtain a curing agent.

[0284] Next, the base resin and curing agent were mixed in a mass ratio of 2.5 parts base resin to 1.0 parts curing agent to obtain a clear coating composition (resin solids concentration 48 mass%). The equivalent ratio (NCO / OH) of the isocyanate groups contained in the polyisocyanate compound (B2) to the hydroxyl groups contained in the acrylic resin (A3) was 1.2.

[0285] (4) Formation of multi-layer coating A zinc phosphate-treated dull steel plate was electrodeposited with a cationic electrodeposition paint (trade name Powernics 150, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) to a dry coating thickness of 20 μm. The plate was then heated at 160°C for 30 minutes to form an electrodeposition coating, yielding a test plate.

[0286] The aqueous intermediate coating composition was applied to the test panel using a rotary atomizer electrostatic coating device to a dry film thickness of 25 μm. Subsequently, the aqueous base coating composition was applied using a rotary atomizer electrostatic coating device to a dry film thickness of 15 μm. The test panel was then preheated at 80°C for 3 minutes. A 6-minute interval was allowed between the application of the aqueous intermediate coating composition and the aqueous base coating composition.

[0287] The clear coating composition was diluted with a dilution solvent consisting of 2-ethylethoxypropanol / xylene = 1 / 1 to adjust the viscosity to 30 seconds (measured at 20°C using a No. 4 Ford cup). After adjusting the viscosity, the clear coating composition was applied in one stage using a Microbell so that the dry film thickness would be 40 μm.

[0288] After leaving it at room temperature for 10 minutes, it was heated at 85°C for 30 minutes to obtain a coated plate having a multi-layer coating film.

[0289] [Examples 2 to 4 and Comparative Examples 1 to 3] Except for changing the amount of each component as shown in the table below, each paint was prepared in the same manner as in Example 1. Using the resulting paint composition, a coated plate having a multi-layer coating film was obtained in the same manner as in Example 1.

[0290] The amount of each component used shown in the table below is the part by mass of the resin solids or active ingredient.

[0291] [evaluation] (1) Chipping resistance The test plates having the multilayer coating film obtained in each of the Examples and Comparative Examples were subjected to a stone chipping test under the following conditions using a Gravello tester KSS-1 (manufactured by Suga Test Instruments Co., Ltd.). <Test conditions> Shot material: M2 nut Amount of shot: 250g Distance: 35cm Shot pressure: 0.49 MPa Shot angle: 45° Test temperature: -20℃

[0292] After the stone chipping test, the test plate was visually evaluated according to the following criteria, where a score of 5 indicates that the plate is suitable for practical use and is judged to be acceptable. 5: Almost no peeling is observed. 4: The peeling area is small, and almost no peeling is observed at the interface between the electrodeposition coating film and the intermediate coating film. 3: The peeled area is somewhat large, and peeling is observed at the interface between the electrodeposition coating and the intermediate coating. 2: The peeling area is large, and peeling is observed at the interface between the electrodeposition coating and the intermediate coating. 1: The peeled area is large and the electrocoating film is destroyed.

[0293] (2) Cleanability The aqueous intermediate coating composition was applied to a tinplate using a bar coder to a film thickness (uncured coating film) of approximately 50 μm, and then dried for 5 minutes in a hot air dryer set at 60°C to obtain a test plate. The test plate was immersed in a cleaning solution at 20°C and left to stand for 3 minutes. The test plate was then pulled out and the removability of the coating film was visually confirmed. Separately, a test plate was immersed in a cleaning solution at 20°C, and ultrasonic waves were immediately generated in the tank. After 2 minutes, the test plate was pulled out and the removability of the coating film was visually confirmed.

[0294] Removability was evaluated based on the area ratio of the remaining coating film. 100% removability means that the entire coating film was removed, indicating that the aqueous intermediate coating composition has excellent cleanability. 0% removability means that the coating film remained without being removed, indicating that the aqueous intermediate coating composition has poor cleanability. A rating of A can be said to have excellent cleanability.

[0295] The cleaning liquid was obtained by adjusting a mixed liquid of 70% by mass of deionized water and 30% by mass of butyl cellosolve to pH 10 with N,N-dimethylethanolamine.

[0296] (Evaluation criteria) A: 100% removability B: Removability: 50% or more but less than 100% C: Removability: 0% to less than 50%

[0297] [Table 1]

[0298] The aqueous primer coating compositions prepared in the examples contained a large amount of aqueous polyurethane resin, but had excellent washability. The resulting multi-layer coating films, which were made with many aqueous polyester resins, had excellent chipping resistance.

[0299] In Comparative Example 1, the water-based primer paint contained an excess amount of pigment, resulting in poor chipping resistance. In Comparative Example 2, the water-based primer paint contained a small amount of pigment, resulting in poor cleanability. In Comparative Example 3, the water-based primer paint contained a small amount of water-based polyurethane resin, resulting in poor chipping resistance.

[0300] The present disclosure includes the following aspects. [1] Applying an aqueous two-component intermediate coating composition to a substrate including a metal part and a resin part to form an uncured intermediate coating film; A base coating film forming step of applying an aqueous base coating composition on the uncured intermediate coating film to form an uncured base coating film; Applying a clear coating composition on the uncured base coating film to form an uncured clear coating film; and heating and curing the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film, The aqueous two-component intermediate coating composition comprises: a water-dispersible acrylic resin (A1) having hydroxyl groups and carboxyl groups, a hydroxyl value of 80 mgKOH / g or more and 200 mgKOH / g or less, and an acid value of 10 mgKOH / g or more and 40 mgKOH / g or less; Polyisocyanate compound (B1), An aqueous polyurethane resin (C) having a glass transition temperature (Tg) of -50°C or less, and Contains a pigment (D), the breaking elongation of a cured film formed from the aqueous polyurethane resin (C) is 400% or more at −20° C., the content of the aqueous polyurethane resin (C) is more than 20 parts by mass per 100 parts by mass of the resin solid content of the aqueous two-component intermediate coating composition, The content of the pigment is 30% by mass or more and 50% by mass or less of the resin solid content of the aqueous two-component intermediate coating composition, The aqueous base coating composition comprises: A water-dispersible acrylic resin (A2) having a hydroxyl group and a carboxyl group and a glass transition point of 0°C or higher and 20°C or lower, and a hydrophilically modified carbodiimide compound (E) having a carbodiimide group and a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether; The clear coating composition comprises: a hydroxyl group-containing acrylic resin (A3), and A method for forming a multi-layer coating film, comprising: a polyisocyanate compound (B2). [2] The hydrophilically modified carbodiimide compound (E) is The following general formula (I): [ka] (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y independently has a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, and each Z independently has a structure obtained by removing a hydroxyl group from a bifunctional polyol having a number average molecular weight of 200 or more and 5,000 or less.) The following general formula (II): [ka] (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed, and R 0 is hydrogen, a methyl group, or an ethyl group, and R 1 are each independently an alkylene group having 4 or less carbon atoms, n is 0 or 1, and m is 0 to 60), and The following general formula (III): [ka] (In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, and each Y independently has a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether.) The method for forming a multilayer coating film according to [1] above, wherein the compound comprises at least one selected from the group consisting of compounds represented by the following formula: [3] The method for forming a multilayer coating film according to [1] or [2] above, wherein the ratio of the carbodiimide group equivalent Ec of the hydrophilically modified carbodiimide compound (E) to the acid group equivalent Ea of the water-dispersible acrylic resin (A2), Ec / Ea, is 0.1 or more and 1.5 or less. [4] The clear coating composition comprises: A polyester polyol (F1) represented by the following formula (1): A polyester polyol (F2) represented by the following formula (2): A polyester polyol (F3) represented by the following formula (3): A polyester polyol (F4) represented by the following formula (4), and The method for forming a multilayer coating film according to any one of the above [1] to [3], which comprises at least one polyester polyol (F) selected from the group consisting of polyester polyols (F5) which are reaction products of a linear or branched polyhydric alcohol having three or more hydroxyl groups, a linear or branched alkanediol, and a linear or branched alkanedicarboxylic acid. Formula (1): [ka] (In the formula, a is an integer of 1 to 100, R 11 represents a saturated hydrocarbon group having two bonds, independently for each repeating unit; R 12 and R 13each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (2): [ka] (wherein j and k are each an integer of 0 to 100, and j+k≧1 is satisfied; R 21 represents a saturated hydrocarbon group having two bonds, R 22 and R 23 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (3): [ka] (In the formula, p, q, and r each represent an integer of 1 to 100, R 31 represents a saturated hydrocarbon group having three bonds, R 32 , R 33 and R 34 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (4): [ka] (In the formula, w, x, y, and z each represent an integer of 1 to 100, R 41 represents a saturated hydrocarbon group having four bonds, R 42 , R 43 , R 44 and R 45 each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. [5] The method for forming a multilayer coating film according to any one of the above [1] to [4], wherein the equivalent ratio (NCO / OH) of the isocyanate groups contained in the polyisocyanate compound (B2) to the hydroxyl groups contained in the water-dispersible acrylic resin (A3) is 1.0 or more and 2.0 or less. [6] The polyisocyanate compound (B2) is The method for forming a multilayer coating film according to any one of [1] to [5] above, comprising a polyisocyanate (B21) having an isocyanate group with a uretdione structure and a polyisocyanate (B22) which is a trimer or higher of diisocyanate in a solids mass ratio of 10 / 90 to 50 / 50. [Industrial Applicability]

[0301] According to the method for forming a multilayer coating film of the present invention, the multilayer coating film obtained has high chipping resistance even when an intermediate coating composition with high washability is used. The method for forming a multilayer coating film of the present invention is suitable for coating an object having a metal part and a resin part.

Claims

1. Applying an aqueous two-component intermediate coating composition to a substrate including a metal part and a resin part to form an uncured intermediate coating film; A base coating film forming step of applying an aqueous base coating composition on the uncured intermediate coating film to form an uncured base coating film; Applying a clear coating composition on the uncured base coating film to form an uncured clear coating film; and heating and curing the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film, The aqueous two-component intermediate coating composition comprises: a water-dispersible acrylic resin (A1) having hydroxyl groups and carboxy groups, a hydroxyl value of 80 mgKOH / g or more and 200 mgKOH / g or less, and an acid value of 10 mgKOH / g or more and 40 mgKOH / g or less; Polyisocyanate compound (B1), An aqueous polyurethane resin (C) having a glass transition temperature (Tg) of −50° C. or lower, and Contains a pigment (D), the breaking elongation of a cured film formed from the aqueous polyurethane resin (C) is 400% or more at −20° C., the content of the aqueous polyurethane resin (C) is more than 20 parts by mass per 100 parts by mass of the resin solids content of the aqueous two-component intermediate coating composition, The content of the pigment is 30% by mass or more and 50% by mass or less of the resin solid content of the aqueous two-component intermediate coating composition, The aqueous base coating composition comprises: a water-dispersible acrylic resin (A2) having a hydroxyl group and a carboxyl group and a glass transition point of 0°C or higher and 20°C or lower; and a hydrophilically modified carbodiimide compound (E) having a carbodiimide group and a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, The clear coating composition comprises: a hydroxyl group-containing acrylic resin (A3), and A method for forming a multi-layer coating film, comprising the step of:

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

3. The method for forming a multilayer coating film according to claim 1, wherein the ratio of the carbodiimide group equivalent Ec of the hydrophilically modified carbodiimide compound (E) to the acid group equivalent Ea of the water-dispersible acrylic resin (A2): Ec / Ea is 0.1 or more and 1.5 or less.

4. The clear coating composition comprises: A polyester polyol (F1) represented by the following formula (1): A polyester polyol (F2) represented by the following formula (2): A polyester polyol (F3) represented by the following formula (3): A polyester polyol (F4) represented by the following formula (4), and 2. The method for forming a multilayer coating film according to claim 1, further comprising at least one polyester polyol (F) selected from the group consisting of polyester polyols (F5) which are reaction products of a linear or branched polyhydric alcohol having three or more hydroxyl groups, a linear or branched alkane diol, and a linear or branched alkane dicarboxylic acid. Formula (1): 【Chemistry 4】 (wherein a is an integer from 1 to 100, R １１ represents, independently for each repeating unit, a saturated hydrocarbon group having two bonds; R １２ and R １３ each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (2): 【Transformation 5】 (wherein j and k are each an integer of 0 to 100, and j+k≧1 is satisfied; R ２１ represents a saturated hydrocarbon group having two bonds, R ２２ and R ２３ each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (3): 【Transformation 6】 (wherein p, q, and r each represent an integer of 1 to 100, R ３１ represents a saturated hydrocarbon group having three bonds, R ３２ , R ３３ and R ３４ each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position. Formula (4): 【Transformation 7】 (wherein w, x, y, and z each represent an integer of 1 to 100, R ４１ represents a saturated hydrocarbon group having four bonds, R ４２ , R ４３ , R ４４ and R ４５ each independently of the other and independently of each other, represents a linear alkylene group or a branched alkylene group branched at a position other than the α-position and the β-position.

5. The method for forming a multilayer coating film according to claim 1, wherein the equivalent ratio (NCO / OH) of the isocyanate groups contained in the polyisocyanate compound (B2) to the hydroxyl groups contained in the water-dispersible acrylic resin (A3) is 1.0 or more and 2.0 or less.

6. The polyisocyanate compound (B2) is 2. The method for forming a multilayer coating film according to claim 1, comprising a polyisocyanate (B21) having an isocyanate group with a uretdione structure and a polyisocyanate (B22) that is a trimer or higher of a diisocyanate in a solids mass ratio of 10 / 90 to 50 / 50.

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