Aqueous first base coating composition and method for producing coated article

Abstract

This aqueous first base coating composition contains (A) an aqueous polyurethane resin having a glass transition temperature of -20°C or less, a weight average molecular weight of 20,000 or more, and a hydroxyl value of 0-20 mg KOH / g, (B) a melamine resin, and (C) a hydroxyl group-containing acrylic resin emulsion having a weight average molecular weight of 100,000 or more. Per 100 mass parts of the resin solids fraction of the aqueous first base coating composition, the solids fraction content of the (A) aqueous polyurethane resin is 35-60 mass parts, the solids fraction content of the (B) melamine resin is 20-30 mass parts, and the solids fraction content of the (C) hydroxyl group-containing acrylic resin emulsion is 8-20 mass parts. A first base coating film obtained by heating the aqueous first base coating composition at 140°C for 20 minutes has a loss tangent of 0.080 or less at -20° C, a dynamic glass transition temperature of 60°C or more, and a breaking stress of 20 N / mm2 or more.

Description

Aqueous first base coating composition and method for producing coated articles

[0001] The present invention relates to a water-based first base coating composition and a method for making a coated article.

[0002] On the surface of a substrate such as an automobile body, multiple coating films with various functions are formed. For example, on a steel plate, a primer coating such as an electrodeposition coating, a first base coating, a second 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] The first base coating film is required to have high chipping resistance. For example, Patent Document 1 discloses a first base coat containing a water-soluble or water-dispersible polyurethane resin having a glass transition temperature of −20° C. or lower and a weight-average molecular weight of 30,000 to 500,000, and a crosslinkable resin having a weight-average molecular weight of 600 or higher, and a breaking stress of 2050 N / cm ２ The present invention also discloses a first base coating film having a loss tangent (tan δ) at −20° C. of 0.075 or more.

[0004] Japanese Patent Application Laid-Open No. 2018-183722

[0005] The first base coating film described in Patent Document 1 does not have sufficient strength and is poor in adhesion.

[0006] The present invention has been made to solve the above-mentioned problems of the prior art, and has as its object to provide an aqueous first base coating composition which can give a multi-layer coating film having excellent chipping resistance and adhesion.

[0007] In order to solve the above problems, the present invention provides the following aspects. [1] A first aqueous base coating composition comprising: (A) an aqueous polyurethane resin having a glass transition temperature of -20°C or lower, a weight average molecular weight of 20,000 or higher, and a hydroxyl value of 0 to 20 mgKOH / g; (B) a melamine resin; and (C) a hydroxyl-containing acrylic resin emulsion having a weight average molecular weight of 100,000 or higher, wherein, relative to 100 parts by mass of resin solids in the first aqueous base coating composition, the solids content of the (A) aqueous polyurethane resin is 35 to 60 parts by mass, the solids content of the (B) melamine resin is 20 to 30 parts by mass, and the solids content of the (C) hydroxyl-containing acrylic resin emulsion is 8 to 20 parts by mass; and a first base coating film obtained by heating the first aqueous base coating composition at 140°C for 20 minutes has a loss tangent of 0.080 or lower at -20°C, a dynamic glass transition temperature of 60°C or higher, and a viscosity of 20 N / mm ２[2] The (A) aqueous polyurethane resin comprises: (A1) an aqueous polyurethane resin having a glass transition temperature of -20°C or lower, a weight-average molecular weight of 20,000 or higher, and a hydroxyl value of 0 mgKOH / g, and (A2) an aqueous polyurethane resin having a glass transition temperature of -20°C or lower, a weight-average molecular weight of 20,000 or higher, and a hydroxyl value of more than 0 mgKOH / g and not more than 20 mgKOH / g, wherein the content of the (A2) aqueous polyurethane resin is more than 0 part by mass and not more than 32 parts by mass per 100 parts by mass of the resin solids content of the first aqueous base coating composition. [3] The (B) melamine resin has a weight-average molecular weight of 400 to 2,000. [4] A method for producing a coated article, comprising: applying an aqueous first base coating composition according to any one of [1] to [3] above onto an object to be coated to form an uncured first base coating film; applying an aqueous second base coating composition onto the uncured first base coating film to form an uncured second base coating film; applying a clear coating composition onto the uncured second base coating film to form an uncured clear coating film; and heating and curing the uncured first base coating film, the uncured second base coating film, and the uncured clear coating film.

[0008] According to the present invention, there is provided an aqueous first base coating composition that can give a multi-layer coating film that has excellent chipping resistance and adhesion.

[0009] The adhesion of a coating film is affected by the strength of the coating film. The stronger the coating film, the better the adhesion. In the present disclosure, a large amount of aqueous polyurethane resin with a large molecular weight, a low glass transition temperature, and no significant contribution to the curing system is blended. This increases the strength of the first base coating film and improves the adhesion of the multi-layer coating film. Because the polyurethane resin can absorb impacts applied to the coating film, the chipping resistance of the multi-layer coating film is also improved.

[0010] Adhesion refers to the adhesion between the first base coating film and the coating film adjacent to and below the first base coating film (typically an electrocoating film or a primer; hereinafter, these may be collectively referred to as "undercoat coating film").

[0011] One method for evaluating adhesion is the Wind Direct Bond (WDB) adhesion test. The WDB is a bond placed between the windshield glass and a multi-layer paint film during automobile manufacturing. The multi-layer paint film has a first base coat, a second base coat, and a clear coat formed on a substrate with a primer coat.

[0012] In the WDB adhesion test, a strip of WDB is applied to a multi-layer coating film, cured, and then pulled up to cause cohesive failure. The length and area of ​​the peeled portion of the coating film at the point where the WDB has cohesive failure are then measured. Peeling (or cohesive failure) of the coating film occurs between the primer coating film and the multi-layer coating film, which have low strength or low interfacial adhesion. The WDB adhesion test can evaluate the adhesion between the primer coating film and the first base coating film, as well as the cohesive strength (strength) of each coating film.

[0013] The first base coating film obtained using the aqueous first base coating composition of the present disclosure has high strength and adhesion. The first base coating film does not undergo cohesive failure in the WDB adhesion test, and is also less likely to undergo interfacial peeling between the first base coating film and the primer coating.

[0014] The strength of the first base coating film can be expressed by the loss tangent (tan δ), the dynamic glass transition temperature, and the breaking stress.

[0015] Loss tangent (tan δ) The loss tangent (tan δ) is calculated by dividing the loss modulus (viscous component) by the storage modulus (elastic component). Tan δ is an index of how easily a material undergoes viscous deformation when subjected to an external force. The smaller the tan δ, the more difficult it is to undergo viscous deformation, and the higher the strength.

[0016] The tan δ of the first base coating film at -20°C is 0.080 or less. The tan δ of the first base coating film may be 0.070 or less, 0.060 or less, or 0.059 or less. The lower limit of the tan δ of the first base coating film is not particularly limited. The tan δ of the first base coating film is, for example, 0.040 or more.

[0017] Tan δ is measured in accordance with the tensile vibration-non-resonance method of JIS-K7244-4:1999. Specifically, a forced stretching vibration viscoelasticity measuring device is used to detect the stress and vibration strain that occur during temperature rise at a temperature rise rate of 2°C / min and a frequency of 8 Hz, and tan δ at -20°C is determined from the phase difference between these. An example of a measuring device is Orientec's "Vibron."

[0018] Dynamic glass transition temperature (dynamic Tg): In a graph in which tan δ measured in the same manner as above is plotted on the vertical axis and temperature on the horizontal axis, the temperature at which tan δ reaches its peak is the dynamic Tg. The higher the dynamic Tg, the more molecules that behave elastically, and the higher the strength.

[0019] The dynamic Tg of the first base coating film is 60° C. or higher. The dynamic Tg of the first base coating film may be 62° C. or higher, or may be 70° C. or higher. The upper limit of the dynamic Tg of the first base coating film is not particularly limited. The dynamic Tg of the first base coating film is, for example, 100° C. or lower.

[0020] Breaking stress In a tensile test, the coating film usually undergoes elastic deformation and then breaks. The higher the breaking stress, the greater the strength of the coating film.

[0021] The breaking stress was measured by pulling the coating film at a rate of 50 mm / min at 25°C and multiplying the stress (N) at the time of breaking by the cross-sectional area (mm ２ ) The stress is measured multiple times (typically three times) using different samples. The breaking stress is calculated from the average value of these stresses. An example of a measuring device is the Autograph AG-IS manufactured by Shimadzu Corporation.

[0022] The breaking stress of the first base coating is 20 N / mm ２ The breaking stress of the first base coating film is 25 N / mm２ or more, and ２ or more, and ２ The upper limit of the breaking stress of the first base coating film is not particularly limited. The breaking stress of the first base coating film may be, for example, 50 N / mm ２ The following is the result.

[0023] The first base coating film, which is the subject of measurement of tan δ, dynamic Tg, and breaking stress, is prepared as follows. First, the first base coating composition is applied to a polypropylene test plate using an air spray so that the dry film thickness is 20 μm. Next, the polypropylene test plate is heated at 140° C. for 20 minutes. This cures the first base coating composition. The resulting first base coating film is peeled off from the test plate and subjected to measurement.

[0024] "Heating at 140°C for 20 minutes" means that the substrate or the coating composition applied to the substrate (hereinafter sometimes referred to as the substrate, etc.) is heated for 20 minutes while maintaining the temperature at 140°C. More specifically, "heating at 140°C for 20 minutes" means that the substrate, etc. is heated until the temperature of the substrate, etc. reaches 140°C, and then the temperature of the substrate, etc. is maintained at 140°C for 20 minutes. The temperature of the substrate, etc. is the lowest temperature of the substrate, etc. during heating.

[0025] The temperature of the substrate may be an actual measured value measured by a contact or non-contact thermometer. The temperature of the substrate is measured at one or more locations on the substrate. When measuring temperatures at multiple locations, the lowest temperature may be 140°C. Alternatively, the temperature of one location expected to be the coldest may be 140°C. The location expected to be the coldest can be determined, for example, from past data or experience. The temperature of the substrate may also be a predicted value predicted from, for example, the material of the substrate, the shape of the substrate, the heating temperature, and the heating time.

[0026] Tan δ, dynamic Tg, and breaking stress are each adjusted by conventionally known methods. Tan δ is adjusted, for example, by crosslink density. Increasing at least one of the properties related to crosslink density, such as the hydroxyl value, degree of internal crosslinking (molecular weight), and reactivity of the resin constituting the coating film, reduces tan δ. Dynamic Tg is adjusted, for example, by the Tg of the resin involved in the crosslinking reaction, crosslink density, and rigidity of the crosslinked structure. A higher Tg of the resin results in a higher dynamic Tg. Breaking stress is adjusted, for example, by the molecular weight of the coating film-forming resin. A higher molecular weight of the coating film-forming resin results in a higher breaking stress.

[0027] Hereinafter, the glass transition temperature (Tg) of a resin can be calculated from the types and amounts of raw material monomers used in the production of the target resin. Tg may be measured by a differential scanning calorimeter (DSC).

[0028] The number average molecular weight (Mn) and weight average molecular weight (Mw) can be measured by gel permeation chromatography (GPC) using a polystyrene standard sample after removing water by drying under reduced pressure or the like.

[0029] The hydroxyl value (OHV) and acid value (AV) are determined based on the mass of solids. The hydroxyl value (OHV) and acid value (AV) can be measured by a known method described in JIS K 0070:1992. The hydroxyl value (OHV) and acid value (AV) may be calculated from the amount of unsaturated monomer in the raw material monomers of the target resin.

[0030] The solid components are the components remaining as solids after the solvent is removed. Specifically, the solid components of the first aqueous base coating composition are (A) the aqueous polyurethane resin, (B) the melamine resin, (C) the hydroxyl group-containing acrylic resin emulsion, and other solid components added as needed.

[0031] The resin solids are the resin components of the above solids. Specifically, the solids of the first aqueous base coating composition are (A) an aqueous polyurethane resin, (B) a melamine resin, (C) a hydroxyl group-containing acrylic resin emulsion, and other resins.

[0032] 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.

[0033] First Water-Based Base Coating Composition The first water-based base coating composition contains (A) an aqueous polyurethane resin, (B) a melamine resin, and (C) a hydroxyl group-containing acrylic resin emulsion.

[0034] The resin solids concentration of the first aqueous base coating composition is not particularly limited and is set appropriately depending on the coating conditions. The resin solids concentration of the first aqueous base coating composition is, for example, 15 to 60 mass %.

[0035] The (A) aqueous polyurethane resin, the (B) melamine resin, and the (C) hydroxyl-containing acrylic resin emulsion are film-forming resins. The (A) aqueous polyurethane resin and the (C) hydroxyl-containing acrylic resin emulsion are aqueous resins. The (B) melamine resin may be aqueous and may be hydrophobic.

[0036] Aqueous resins are generally broadly classified into water-soluble and water-dispersible resins. Water-dispersible resins are further classified into colloidal dispersion type and emulsion type. Colloidal dispersion type (hereinafter simply referred to as 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 by mechanically forced emulsification.

[0037] In the case of acrylic resins, those with an Mw of more than 100,000 are considered to be emulsion type, and those with an Mw of 100,000 or less can be considered to be colloidal dispersion type. When the Mw exceeds 1,000,000, it becomes difficult to measure the Mw. Acrylic resins whose Mw cannot be measured can be considered to be emulsion type.

[0038] Polyurethane resins whose Mw is too large to be measured may be considered to have an Mw of 20,000 or more, and may be considered to be dispersion type.

[0039] (A) Aqueous Polyurethane Resin The (A) aqueous polyurethane resin may be water-soluble or water-dispersible. The (A) aqueous polyurethane resin may be a dispersion type.

[0040] The aqueous polyurethane resin (A) fuses with itself and with other components to toughen the first base coating film and improve the impact absorption of the first base coating film. The aqueous polyurethane resin (A) also improves the chipping resistance of the multi-layer coating film.

[0041] The glass transition temperature (Tg) of the (A) aqueous polyurethane resin is -20°C or lower. When the Tg is -20°C or lower, chipping resistance is further improved. The Tg of the (A) aqueous polyurethane resin may be -30°C or lower, or may be -50°C or lower. The lower limit of the Tg of the (A) aqueous polyurethane resin is not particularly limited. The Tg of the (A) aqueous polyurethane resin may be, for example, -90°C or higher.

[0042] The weight average molecular weight (Mw) of the (A) aqueous polyurethane resin is 20,000 or more. This increases the strength of the first base coating film and improves adhesion. The Mw of the (A) aqueous polyurethane resin may be 150,000 or more, or may be 250,000 or more. The upper limit of the Mw of the (A) aqueous polyurethane resin is not particularly limited. The Mw of the (A) aqueous polyurethane resin may be, for example, 1,000,000 or less.

[0043] The hydroxyl value (OHV) of the (A) aqueous polyurethane resin is 0 mgKOH / g or more and 20 mgKOH / g or less. This suppresses the reaction between the (A) aqueous polyurethane resin and the (B) melamine resin, making it easier for the (A) aqueous polyurethane resin to exhibit its performance (typically, impact absorption) in the coating film. The OHV of the (A) aqueous polyurethane resin may be 17 mgKOH / g or less, or 15 mgKOH / g or less. The OHV of the (A) aqueous polyurethane resin may be 5 mgKOH / g or more, or 10 mgKOH / g or more.

[0044] The solid content of the (A) aqueous polyurethane resin is 35 to 60 parts by mass per 100 parts by mass of the resin solid content of the first aqueous base coating composition. This makes it easier for the (A) aqueous polyurethane resin to exert its functions, balances the strength and elasticity of the first base coating film, and improves both chipping resistance and adhesion. The content of the (A) aqueous polyurethane resin may be 30 parts by mass or more, or 35 parts by mass or more. The content of the (A) aqueous polyurethane resin may be 45 parts by mass or less, or 40 parts by mass or less.

[0045] Multiple types of (A) aqueous polyurethane resins may be used in combination. At least one of the multiple types of (A) aqueous polyurethane resins may have a Tg of -20°C or less, a Mw of 20,000 or more, and an OHV of 0 to 20 mgKOH / g. Multiple types of (A) aqueous polyurethane resins that satisfy all of the following characteristics may be combined: a Tg of -20°C or less, a Mw of 20,000 or more, and an OHV of 0 to 20 mgKOH / g. The Tg, Mw, and OHV of the (A) aqueous polyurethane resin are controlled by the type of raw material monomer, the degree of polymerization, etc.

[0046] The (A) aqueous polyurethane resin may include an (A1) aqueous polyurethane resin having a Tg of -20°C or less, a Mw of 20,000 or more, and an OHV of 0 mgKOH / g, and an (A2) aqueous polyurethane resin having a Tg of -20°C or less, a Mw of 20,000 or more, and an OHV of more than 0 mgKOH / g but not more than 20 mgKOH / g. The (A1) aqueous polyurethane resin further improves adhesion. The (A2) aqueous polyurethane resin further improves chipping resistance.

[0047] The content of the (A2) aqueous polyurethane resin is, for example, more than 0 parts by mass and not more than 32 parts by mass per 100 parts by mass of the resin solids of the first aqueous base coating composition. This makes it easier to balance the strength and elasticity of the first base coating film. The content of the (A1) aqueous polyurethane resin is the remainder. Specifically, the content of the (A1) aqueous polyurethane resin can be 3 parts by mass or more and less than 60 parts by mass per 100 parts by mass of the resin solids of the first aqueous base coating composition.

[0048] The content of the aqueous polyurethane resin (A2) may be 5 parts by mass or more, or 10 parts by mass or more, and may be 30 parts by mass or less, or 20 parts by mass or less.

[0049] The Mw of the (A1) aqueous polyurethane resin may be 150,000 or more, or may be 300,000 or more. The upper limit of the Mw of the (A1) aqueous polyurethane resin is not particularly limited. The Mw of the (A1) aqueous polyurethane resin may be, for example, 1,000,000 or less.

[0050] The Tg of the (A1) aqueous polyurethane resin may be −30° C. or lower, or −50° C. or lower. The lower limit of the Tg of the (A1) aqueous polyurethane resin is not particularly limited. The Tg of the (A1) aqueous polyurethane resin may be, for example, −70° C. or higher.

[0051] The Mw of the (A2) aqueous polyurethane resin may be 50,000 or more, or may be 150,000 or more. The upper limit of the Mw of the (A2) aqueous polyurethane resin is not particularly limited. The Mw of the (A2) aqueous polyurethane resin may be, for example, 500,000 or less.

[0052] The Tg of the (A2) aqueous polyurethane resin may be −50° C. or lower, or −70° C. or lower. The lower limit of the Tg of the (A2) aqueous polyurethane resin is not particularly limited. The Tg of the (A2) aqueous polyurethane resin may be, for example, −100° C. or higher.

[0053] The OHV of the (A2) aqueous polyurethane resin may be 17 mgKOH / g or less, or 15 mgKOH / g or less. The OHV of the (A2) aqueous polyurethane resin may be 5 mgKOH / g or more, or 10 mgKOH / g or more.

[0054] The aqueous polyurethane resin (A) can be obtained, for example, by a method of forcibly emulsifying a polyurethane resin using a surfactant, or by a method of neutralizing a polyurethane resin with a base or an acid.

[0055] The polyurethane resin can be obtained, for example, by reacting a polyol (see the polyol (D4) described below), a compound having an active hydrogen group and a hydrophilic group in the molecule, a polyisocyanate compound, and, if necessary, a chain extender and a polymerization terminator. If necessary, the chain extender and the polymerization terminator may be used.

[0056] Examples of compounds 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, and specifically, the anionic group is formed by neutralizing with a base before, during, or after the urethanization reaction.

[0057] Compounds containing active hydrogen and an anionic group 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 a cationic group are described, for example, in JP-B-43-9076. Compounds having active hydrogen and a nonionic hydrophilic group are described, for example, in JP-B-48-41718, and specific examples thereof include polyethylene glycol and alkyl alcohol alkylene oxide adducts.

[0058] Examples of the polyisocyanate compound include aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, and araliphatic polyisocyanates.

[0059] The aromatic polyisocyanate has two or more isocyanate groups bonded to carbon atoms constituting an aromatic ring. Examples of the aromatic polyisocyanate include aromatic diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, 2,4'- or 4,4'-diphenylmethane diisocyanate, or mixtures thereof, 2,4- or 2,6-tolylene diisocyanate, or mixtures thereof, 4,4'-toluidine diisocyanate, and 4,4'-diphenylether diisocyanate; aromatic triisocyanates such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, and 2,4,6-triisocyanatotoluene; and aromatic tetraisocyanates such as 4,4'-diphenylmethane-2,2',5,5'-tetraisocyanate. These may be used alone or in combination of two or more.

[0060] Aliphatic polyisocyanates do not have an aromatic ring and have two or more isocyanate groups bonded to carbon atoms constituting a linear or branched aliphatic hydrocarbon group. Examples of aliphatic polyisocyanates include ethylene diisocyanate, trimethylene diisocyanate, 1,2-propylene diisocyanate, butylene diisocyanate (tetramethylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate), 1,5-pentamethylene diisocyanate (PDI), 1,6-hexamethylene diisocyanate (HDI), 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, and the like. aliphatic diisocyanates such as lysine ester triisocyanate, 1,4,8-triisocyanatooctane, 1,6,11-triisocyanatoundecane, 1,8-diisocyanato-4-isocyanatomethyloctane, 1,3,6-triisocyanatohexane, and 2,5,7-trimethyl-1,8-diisocyanato-5-isocyanatomethyloctane. These may be used alone or in combination of two or more.

[0061] Alicyclic polyisocyanates do not have an aromatic ring, but have two or more isocyanate groups bonded to carbon atoms constituting a cyclic aliphatic hydrocarbon group. Examples of alicyclic polyisocyanates include 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate), methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane (common name: hydrogenated xylylene diisocyanate), or mixtures thereof, and alicyclic diisocyanates such as norbornane diisocyanate; 1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane, 2-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo[2.2.1] ]heptane, 2-(3-isocyanatopropyl)-2,6-di(isocyanatomethyl)-bicyclo[2.2.1]heptane, 3-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo[2.2.1]heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, 6-(2-isocyanatoethyl)-2-isocyanatomethyl Alicyclic triisocyanates such as isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, and 6-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane can be used alone or in combination of two or more.

[0062] Aromatic aliphatic polyisocyanates have an aromatic ring and two or more isocyanate groups bonded to carbon atoms constituting an aliphatic hydrocarbon group. Examples of araliphatic polyisocyanates include araliphatic diisocyanates such as 1,3- or 1,4-xylylene diisocyanate, or mixtures thereof, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (common name: tetramethylxylylene diisocyanate), or mixtures thereof; and araliphatic triisocyanates such as 1,3,5-triisocyanatomethylbenzene. These may be used alone or in combination of two or more.

[0063] The polyisocyanate compound may be a derivative of each of the above polyisocyanates. Examples of the polyisocyanate derivatives include dimers, trimers, biurets, allophanates, uretdiones, uretimines, isocyanurates, oxadiazinetriones, polymethylene polyphenyl polyisocyanates (crude MDI, polymeric MDI), and crude TDI.

[0064] 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.

[0065] Examples of the polymerization terminator include compounds having one active hydrogen in the molecule (such as monoalcohols and monoamines) and monoisocyanate compounds.

[0066] 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 compound to form an NCO-terminated prepolymer, and then the prepolymer is reacted with the remainder of the active hydrogen-containing compound.

[0067] 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.

[0068] The polyurethane resin is treated by a known method (a method of forming an anion-forming group by neutralizing with a base, or forming a cationic group by using a quaternizing agent, or neutralizing with an acid, in the case of a cation-forming group) and then dispersed in water.

[0069] The polyurethane resin may be dispersed in water after the synthesis reaction or during the multi-stage process, for example, during the formation of an NCO-terminated prepolymer, while the prepolymer is chain-extended with water and / or a polyamine.

[0070] (A) Examples of commercially available aqueous polyurethane resins include the NeoRez series sold by Kusumoto Chemicals Co., Ltd., the HUX series sold by ADEKA Corporation, and the U-Coat series, Permarin series, and Euprene series sold by Sanyo Chemical Industries, Ltd.

[0071] (B) Melamine Resin The (B) melamine resin reacts with the (C) hydroxyl group-containing acrylic resin emulsion to form a crosslinked structure. Using the (B) melamine resin as a curing agent improves curing properties at low temperatures (e.g., 140°C or less), reducing environmental impact. The (B) melamine resin forms a rigid crosslinked structure, which can increase the dynamic Tg of the first base coating film.

[0072] (B) Melamine resin has three nitrogen atoms N around a triazine ring (triazine nucleus). １ ~N ３ Six substituents R １ ~R ６ The structure in which -N １ (R １ ) (R ２ ), -N ２ (R ３ ) (R ４ ), -N ３ (R ５ ) (R ６ )) is included.

[0073] The melamine resin (B) is, for example, a melamine resin represented by the following general formula (1):

[0074] (In the formula, substituent R １ ~R ６ each independently represents a hydrogen atom, an alkyl ether group, a methylol group, or a bonding moiety to another triazine ring.

[0075] Alkyl ether (-CH ２ -OR ７ ) constituting the alkyl group (R ７ The number of carbon atoms in R may be 1 to 8, or may be 1 to 4. ７ R may be linear or branched. ７ may be a methyl group, an ethyl group, a propyl group, or a butyl group.

[0076] The melamine resin (B) may be composed of a polynuclear compound in which a plurality of triazine rings are bonded, or may be a mononuclear compound composed of one triazine ring.

[0077] (B) Melamine resins include, for example, —N(—CH２ -OR ７ ) (-CH ２ methylol group type having —N(—OH); ２ -OR ７ ) (H); imino group type having —N(—CH ２ -OR ７ ) (-CH ２ -OH) and -N(-CH ２ -OR ７ ) (H), methylol / imino group type having substituent R １ ~R ６ Examples of the alkyl ether group include a full alkyl type having only alkyl ether groups.

[0078] From the viewpoint of breaking stress (particularly breaking stress during low-temperature curing), the (B) melamine resin may be a methylol group type, an imino group type, or a methylol / imino group type. The average total number of imino groups and methylol groups per triazine ring (hereinafter sometimes simply referred to as the average functionality) may be greater than 1. The average functionality may be 1.2 or more, 1.5 or more, or 2.0 or more. The average functionality may be 4 or less, or 3.5 or less.

[0079] The Mw of the (B) melamine resin may be 400 or more and 2000 or less. This keeps the viscosity of the aqueous first base coating composition low, improving the appearance of the coating film. The Mw of the (B) melamine resin may be 1500 or less, or 1300 or less. The Mw of the (B) melamine resin may be 500 or more, or 600 or more.

[0080] The solid content of the (B) melamine resin is 20 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the resin solid content of the first aqueous base coating composition. When the content of the (B) melamine resin is 20 parts by mass or more, the crosslinking reaction with the (C) acrylic resin emulsion proceeds efficiently, improving adhesion. When the content of the (B) melamine resin is 30 parts by mass or less, the amount of melamine resin that does not contribute to the crosslinking reaction with the (C) acrylic resin emulsion (i.e., plasticizing component) is reduced, improving chipping resistance and adhesion. The content of the (B) melamine resin may be 22 parts by mass or more, or may be 23 parts by mass or more. The content of the (B) melamine resin may be 28 parts by mass or less, or may be 26 parts by mass or less.

[0081] Other Curing Agents The first aqueous base coating composition may contain a curing agent other than the melamine resin (B). Examples of other curing agents include blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more.

[0082] Unreacted components of the other curing agents (especially blocked isocyanate compounds) tend to act as plasticizers in the first base coating film. Therefore, it is desirable that the content of the other curing agents be low. The content of the other curing agents may be 10% by mass or less, 5% by mass or less, 1% by mass or less, or 0% by mass, based on the total mass of all curing agents (the sum of the (B) melamine resin and the other curing agents).

[0083] (C) Hydroxyl-containing acrylic resin emulsion (hereinafter simply referred to as (C) acrylic resin emulsion) refers to an acrylic resin prepared, for example, by emulsion polymerization. The (C) acrylic resin emulsion is dispersed in particulate form in the first base coating composition. The (C) acrylic resin emulsion contributes to improving adhesion.

[0084] The acrylic resin emulsion (C) has hydroxyl groups and carboxyl groups, and the hydroxyl groups of the acrylic resin emulsion (C) react with the melamine resin (B) to form a crosslinked structure.

[0085] The Mw of the (C) acrylic resin emulsion is 100,000 or more. The Mw of the (C) acrylic resin emulsion may be 100,000 or more, or may be 200,000 or more. There is no upper limit for the Mw of the (C) acrylic resin emulsion.

[0086] The OHV of the (C) acrylic resin emulsion is, for example, 10 mgKOH / g or more and 100 mgKOH / g or less. This increases the crosslink density and further improves chipping resistance. The OHV of the (C) acrylic resin emulsion may be 20 mgKOH / g or more, or 30 mgKOH / g or more. The OHV of the (C) acrylic resin emulsion may be 90 mgKOH / g or less, or 80 mgKOH / g or less.

[0087] The acid value (AV) of the (C) acrylic resin emulsion is, for example, 5 mgKOH / g or more and 50 mgKOH / g or less. The AV of the (C) acrylic resin emulsion may be 10 mgKOH / g or more, or 15 mgKOH / g or more. The AV of the (C) acrylic resin emulsion may be 40 mgKOH / g or less, or 30 mgKOH / g or less.

[0088] The Tg of the (C) acrylic resin emulsion is, for example, -15°C or higher and 60°C or lower. This makes it easier for the dynamic Tg of the first base coating film to be 60°C or higher. The Tg of the (C) acrylic resin emulsion may be -10°C or higher, or may be 25°C or higher. The Tg of the (C) acrylic resin emulsion may be 55°C or lower, or may be 50°C or lower.

[0089] The average particle size of the (C) acrylic resin emulsion is, for example, 20 nm or more and 200 nm or less. The average particle size of the (C) acrylic resin emulsion may be 180 nm or less, 160 nm or less, 150 nm or less, or 100 nm or less. The average particle size of the (C) acrylic resin emulsion may be 25 nm or more, or 30 nm or more.

[0090] The solid content of the (C) acrylic resin emulsion is 8 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the resin solid content of the first aqueous base coating composition. The content of the (C) acrylic resin emulsion may be 10 parts by mass or more. The content of the (C) acrylic resin emulsion may be 15 parts by mass or less, or 12 parts by mass or less.

[0091] The acrylic resin emulsion (C) may be used alone or in combination of two or more.

[0092] The acrylic resin emulsion (C) 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.

[0093] Examples of the α,β-ethylenically unsaturated monomer 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.

[0094] "(Meth)acrylic acid" includes both acrylic acid and methacrylic acid.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] The (C) acrylic resin emulsion is typically synthesized 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.

[0099] 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.

[0100] 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 dimer may be used to adjust the molecular weight.

[0101] 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.

[0102] 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. Neutralization is carried out before or after emulsion polymerization.

[0103] The basic compound is not particularly limited, and examples thereof include at least one selected from the group consisting of ammonia and amine compounds. Examples of amine compounds include dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, n-propylamine, isopropylamine, triallylamine, triethylenediamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, aminoethanolamine, N-methyl-N,N-diethanolamine, iminobispropylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, methylaminopropylamine, methyliminobispropylamine, 3-methoxypropylamine, monoethanolamine, diethanolamine, triethanolamine, morpholine, allylmorpholine, N-methylmorpholine, and N-ethylmorpholine. These compounds may be used alone or in combination of two or more.

[0104] The acrylic resin emulsion (C) can also be obtained by solution polymerization and neutralization. The solution polymerization is carried out by a known method.

[0105] (D) Other aqueous resins The first aqueous base coating composition may contain (D) other aqueous resins. Examples of (D) other aqueous resins include (D1) water-soluble acrylic resins, (D2) acrylic resin dispersions, (D3) aqueous polyester resins, and (D4) polyols other than those mentioned above. These may be used alone or in combination.

[0106] The solid content of the (D) other aqueous resin is, for example, 40 parts by mass or less relative to 100 parts by mass of the resin solid content of the first aqueous base coating composition. The content of the (D) other aqueous resin may be 5 parts by mass or more, 10 parts by mass or more, or 15 parts by mass or more. The content of the (D) other aqueous resin may be 38 parts by mass or less, 35 parts by mass or less, 30 parts by mass or less, or 25 parts by mass or less.

[0107] (D1) Water-soluble acrylic resin (D1) is produced by solution polymerizing a raw material monomer mixture containing an α,β-ethylenically unsaturated monomer having an acid group, which is exemplified in the raw material monomer mixture used in the production of (C) acrylic resin emulsion, followed by neutralization with a basic compound. Examples of the raw material monomer and basic compound are the same as those exemplified in the production of (C) acrylic resin emulsion.

[0108] The OHV of the (D1) water-soluble acrylic resin is, for example, 10 mgKOH / g or more and 100 mgKOH / g or less. This further improves adhesion. The OHV of the (D1) water-soluble acrylic resin may be 20 mgKOH / g or more, or 30 mgKOH / g or more. The OHV of the (D1) water-soluble acrylic resin may be 80 mgKOH / g or less, or 70 mgKOH / g or less.

[0109] The AV of the (D1) 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 (B) melamine resin and also facilitate increased storage stability. The AV of the (D1) water-soluble acrylic resin may be 10 mgKOH / g or more, or may be 20 mgKOH / g or more. The AV of the (D1) water-soluble acrylic resin may be 60 mgKOH / g or less, or may be 50 mgKOH / g or less.

[0110] The Mw of the (D1) 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 (D1) water-soluble acrylic resin may be 5,000 or more, 8,000 or more, or 15,000 or more. The Mw of the (D1) water-soluble acrylic resin may be 40,000 or less, or 35,000 or less.

[0111] The mass-based mixing ratio (D1 / C) of the water-soluble acrylic resin (D1) to the acrylic resin emulsion (C) may be, for example, 0 to 3. D1 / C may be 1 or more, or 1.5 or more. D1 / C may be 2 or less.

[0112] The solid content of the water-soluble acrylic resin (D1) is, for example, 25 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 (D1) may be 20 parts by mass or less, or 17 parts by mass or less. The content of the water-soluble acrylic resin (D1) may be 3 parts by mass or more, or 5 parts by mass or more.

[0113] (D2) Acrylic Resin Dispersion The (D2) acrylic resin dispersion is prepared by a polymerization method (typically, a solution polymerization method) that does not use an emulsifier. The (D2) acrylic resin dispersion may be a core-shell type. The (D2) acrylic resin dispersion is dispersed in the aqueous first base coating composition in the form of particles.

[0114] The average particle size of the (D2) acrylic resin dispersion is, for example, 20 nm or more and 200 nm or less. The average particle size of the (D2) acrylic resin dispersion may be 180 nm or less, 160 nm or less, 150 nm or less, or 100 nm or less. The average particle size of the (D2) acrylic resin dispersion may be 25 nm or more, or 30 nm or more.

[0115] The Mw of the (D2) acrylic resin dispersion is, for example, 7,600 or more and 80,000 or less. The Mw of the (D2) acrylic resin dispersion may be 15,000 or more, 16,000 or more, or 20,000 or more. The Mw of the (D2) acrylic resin dispersion may be 60,000 or less, or 48,000 or less.

[0116] The AV of the (D2) acrylic resin dispersion is, for example, 25 mgKOH / g or more and 50 mgKOH / g or less. The AV of the (D2) acrylic resin dispersion may be 30 mgKOH / g or more, or 35 mgKOH / g or more. The AV of the (D2) acrylic resin dispersion may be 50 mgKOH / g or less and 45 mgKOH / g or less.

[0117] The mass-based mixing ratio (D2 / C) of the acrylic resin dispersion (D2) to the acrylic resin emulsion (C) may be, for example, 0 to 3. D2 / C may be 1 or more, or 1.5 or more. D2 / C may be 2 or less.

[0118] The solid content of the (D2) acrylic resin dispersion is, for example, 25 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 (D2) acrylic resin dispersion may be 20 parts by mass or less, or 17 parts by mass or less. The content of the (D2) acrylic resin dispersion may be 3 parts by mass or more, or 5 parts by mass or more.

[0119] (D3) Aqueous Polyester Resin The aqueous polyester resin (D3) may be a dispersion type. The aqueous polyester resin (D3) may have a hydroxyl group and a carboxyl group.

[0120] The OHV of the (D3) aqueous polyester resin is, for example, 50 to 150 mgKOH / g. The OHV may be 70 mgKOH / g or more. The OHV may be 120 mgKOH / g or less, or 100 mgKOH / g or less.

[0121] The AV of the aqueous polyester resin (D3) is, for example, 20 to 80 mgKOH / g. The AV may be 25 mgKOH / g or more. The AV may be 50 mgKOH / g or less, or 40 mgKOH / g or less.

[0122] The Mn of the (D3) aqueous polyester resin is, for example, 500 to 20,000. When the (D3) aqueous polyester resin has an Mn of 500 or more, storage stability is improved. When the (D3) aqueous polyester resin has an Mn of 20,000 or less, viscosity increase is suppressed and coating workability is improved. The (D3) aqueous polyester resin may have an Mn of 1,500 or more. The (D3) aqueous polyester resin may have an Mn of 10,000 or less.

[0123] The Tg of the (D3) aqueous polyester resin is, for example, -20 to 80°C. When the (D3) aqueous polyester resin has a Tg of -20°C or higher, the hardness of the resulting coating film increases. When the (D3) aqueous polyester resin has a Tg of 80°C or lower, the performance of hiding the base (hiding ability) improves. The (D3) aqueous polyester resin may have a Tg of 0°C or higher. The (D3) aqueous polyester resin may have a Tg of 60°C or lower.

[0124] The mass-based mixing ratio (D3 / C) of the aqueous polyester resin (D3) to the acrylic resin emulsion (C) may be, for example, 0 to 5. D3 / C may be 1 or more, 1.5 or more, or 2 or more. D3 / C may be 3 or less.

[0125] The solid content of the (D3) aqueous polyester resin may be, for example, 30 parts by mass or less relative to 100 parts by mass of the resin solid content of the first aqueous base coating composition. The content of the (D3) aqueous polyester resin may be 27 parts by mass or less, or 25 parts by mass or less. The content of the (D3) aqueous polyester resin may be 5 parts by mass or more, 10 parts by mass or more, or 12 parts by mass or more.

[0126] The aqueous polyester resin (D3) is obtained by neutralizing a polyester resin with a basic compound, for example, by condensation of a polyhydric alcohol component and a polybasic acid component.

[0127] 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.

[0128] 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.

[0129] If necessary, a monobasic acid such as benzoic acid or t-butylbenzoic acid may be used in combination.

[0130] As reaction components, monohydric alcohols, monoepoxide compounds such as Cardura E (trade name: manufactured by Oxalis Chemicals), 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.

[0131] 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.

[0132] The polyester resin may be grafted with an acrylic resin and / or a vinyl resin, or may be reacted with a polyisocyanate compound.

[0133] (D4) Polyol The (D4) polyol is a resin other than those described above, and has two or more hydroxyl groups per molecule. The (D4) polyol may have an average of three or more hydroxyl groups per molecule. This can further increase the coating strength.

[0134] The polyol (D4) is aqueous. The polyol (D4) may be an emulsion type or may be water-soluble.

[0135] Examples of the polyol (D4) include polyester polyols, polyether polyols, polycarbonate polyols, polyacrylate polyols, and polyhydric alcohols. These may be used alone or in combination of two or more. Among these, the polyol (D4) may be a polyether polyol.

[0136] Polyether polyols can be obtained, for example, by addition polymerization of a polyhydric alcohol with an alkylene oxide such as ethylene oxide, propylene oxide, or tetrahydrofuran. The polyether polyol may be a polyether diol having two hydroxyl groups in one molecule.

[0137] Examples of polyether diols include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol.

[0138] Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and block products thereof. Polyether polyols can be obtained, for example, by adding ethylene oxide and / or propylene oxide to a polyhydric alcohol compound.

[0139] Commercially available polyether polyols include, for example, the Sannix series manufactured by Sanyo Chemical Industries, Ltd. Specific examples include Sannix GP-250, Sannix GP-400, Sannix PP-200, and Sannix GP-600.

[0140] The polyester polyol can be obtained, for example, by an esterification reaction or transesterification reaction between a dicarboxylic acid and a diol, or by a ring-opening polymerization reaction of a lactone compound.

[0141] Commercially available polyester polyols include, for example, Desmophen VPLS2249 / 1 (manufactured by Sumika Covestro Urethane Co., Ltd.), Desmophen 800 (manufactured by Sumika Covestro Urethane Co., Ltd.), Desmophen XP2488 (manufactured by Sumika Covestro Urethane Co., Ltd.), and Kuraray Polyol P-510 and F-510 (both manufactured by Kuraray Co., Ltd.).

[0142] Polycarbonate polyol can be obtained, for example, by reacting a polyhydric polyol with dimethyl carbonate.

[0143] Commercially available polycarbonate polyols include, for example, Duranol T5650E (manufactured by Asahi Kasei Corporation), C-590 (manufactured by Kuraray Co., Ltd.), and ETERNACOLL PH-50 (manufactured by Ube Industries, Ltd.).

[0144] Examples of polyhydric alcohols include ethylene glycol, glycerin, trimethylolpropane, propylene glycol, tetramethylene glycol, and pentaerythritol.

[0145] The Mw of the polyol (D4) is not particularly limited and may be appropriately set depending on the OHV, etc. The Mw of the polyether polyol may be, for example, 300 or more and 3,000 or less.

[0146] The OHV of the polyol (D4) is, for example, 30 mgKOH / g or more and 700 mgKOH / g or less. The OHV of the polyol (D4) may be 50 mgKOH / g or more. The OHV of the polyol (D4) may be 500 mgKOH / g or less.

[0147] The mixing ratio (D4 / C) of the (D4) polyol to the (C) acrylic resin emulsion on a mass basis may be, for example, 0 to 1. D4 / C may be 0.2 or more. D4 / C may be 0.8 or less.

[0148] The solid content of the polyol (D4) may be, for example, 10 parts by mass or less relative to 100 parts by mass of the resin solid content of the first aqueous base coating composition. The content of the polyol (D4) may be 8 parts by mass or less, or 5 parts by mass or less. The content of the polyol (D4) may be 1 part by mass or more.

[0149] Pigment The aqueous first base paint composition may contain a pigment, which improves the hiding power of the first base paint film and the weather resistance of the multi-layer paint film.

[0150] The pigment content is not particularly limited. In terms of hiding power, the pigment content may be 30% by mass or more and 60% by mass or less of the resin solid content of the aqueous first base coating composition. The pigment content may be 35% by mass or more, or 40% by mass or more. The pigment content may be 55% by mass or less, or 50% by mass or less.

[0151] The type of pigment is not particularly limited. Examples of pigments include color pigments, luster pigments, and extender pigments. These may be used alone or in combination of two or more.

[0152] 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.

[0153] 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.

[0154] Examples of the luster pigment include aluminum flakes, alumina flakes, mica materials, silica flakes, and glass flakes.

[0155] 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.

[0156] The pigment may be blended into the aqueous first base coating composition as a pigment dispersion paste. The pigment dispersion paste is obtained by mixing the pigment, a pigment dispersant, and an aqueous medium. The pigment dispersant is a resin having a structure containing a pigment affinity moiety and a hydrophilic moiety. Examples of the pigment affinity moiety and the hydrophilic moiety include nonionic, cationic, or anionic functional groups. The pigment dispersant may have two or more types of the above functional groups in one molecule. The pigment dispersant may be used alone or in combination of two or more types.

[0157] The pigment dispersant is not particularly limited. Examples of commercially available pigment dispersants include Disperbyk 190, Disperbyk 181, Disperbyk 182, and Disperbyk 184 (all of which are anionic / nonionic dispersants manufactured by BYK-Chemie), EFKAPOLYMER 4550 (anionic / nonionic dispersant manufactured by EFKA), Solsperse 27000 (nonionic dispersant manufactured by Avecia), Solsperse 41000, and Solsperse 53095 (anionic dispersants manufactured by Avecia).

[0158] The aqueous first base coating composition may contain various additives, such as a curing catalyst, a surface conditioner, an antifoaming agent, a plasticizer, a film-forming aid, an ultraviolet absorber, an antioxidant, and a solvent.

[0159] The first aqueous base coating composition contains water as a solvent. The first aqueous base coating composition may contain an organic solvent together with the aqueous solvent. In the first aqueous base coating composition, the proportion of water in the solvent is, for example, 50% by mass or more, 70% by mass or more, or 90% by mass or more.

[0160] Examples of organic solvents include ester solvents such as ethyl acetate, butyl acetate, isopropyl acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate; ether solvents such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, methyl methoxybutanol, ethoxypropanol, ethylene glycol isopropyl ether, ethylene glycol t-butyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, methoxybutanol, and propylene glycol monobutyl ether; alcohol solvents such as methanol, ethanol, butanol, and propyl alcohol; and ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. These may be used alone or in combination of two or more.

[0161] The first aqueous base coating composition is prepared by mixing the above components by a known method. If necessary, the first aqueous base coating composition is diluted with a solvent such as water and then used for coating.

[0162] The coated article comprises a substrate and a multilayer coating comprising a first base coating film formed on the substrate using the above-mentioned aqueous first base coating composition, a second base coating film formed on the first base coating film using the aqueous second base coating composition, and a clear coating film formed on the second base coating film using a clear coating composition. The multilayer coating also has excellent chipping resistance and adhesion.

[0163] The coated article may further comprise a third base coating film interposed between the second base coating film and the clear coating film. That is, the multi-layer coating film may comprise a first base coating film, a second base coating film, a third base coating film, and a clear coating film in this order.

[0164] The third base coating film is formed from a third aqueous base coating composition containing the same components as the second aqueous base coating composition, and the second aqueous base coating composition and the third aqueous base coating composition may be the same or different.

[0165] Examples of materials for the substrate include metal, resin, and glass. Specific examples of the substrate include automobile bodies and body parts for automobiles such as passenger cars, trucks, motorcycles, and buses, as well as automobile parts such as spoilers, bumpers, mirror covers, grilles, and door knobs.

[0166] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of metal substrates 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.

[0167] The metal substrate 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 substrate may be further coated with an electrodeposition paint. The electrodeposition paint may be either a cationic type or an anionic type.

[0168] 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, polyurethane resin, epoxy resin, and polyphenylene oxide (PPO). Resin substrates may be degreased. Resin substrates may be coated with a primer.

[0169] The thickness of the first base coating film may be appropriately set depending on the application, etc. The thickness of the first base coating film is, for example, 5 μm or more and 40 μm or less. The thickness of the first base coating film may be 10 μm or more. The thickness of the first base coating film may be 30 μm or less.

[0170] The thickness of the second base coating film may be appropriately set depending on the application, etc. The thickness of the second base coating film is, for example, 5 μm or more and 30 μm or less. The thickness of the second base coating film may be 10 μm or more. The thickness of the second base coating film may be 20 μm or less.

[0171] 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.

[0172] 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.

[0173] Second and third aqueous base coating compositions The second and third aqueous base coating compositions contain an aqueous resin and a curing agent. The aqueous resin is not particularly limited. Examples of aqueous resins include acrylic resins, polyester resins, polyurethane resins, epoxy resins, and polyols. These may be used alone or in combination of two or more. The aqueous resins contained in the first, second, and third aqueous base coating compositions may be the same or different.

[0174] In the second and third aqueous base coating compositions, the aqueous resin may be contained as an emulsion, a dispersion, or in a state dissolved in a solvent.

[0175] The second and third aqueous base coating compositions may contain at least an acrylic resin, a polyurethane resin, and a polyol. The acrylic resin may include both an acrylic resin emulsion and a water-soluble acrylic resin. The polyol may include a polyether polyol.

[0176] The acrylic resin emulsion, water-soluble acrylic resin, polyurethane resin and polyether polyol may be the same as those in the first base coating composition.

[0177] Examples of curing agents include melamine resins, blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more. The curing agents contained in the first, second, and third aqueous base coating compositions may be the same or different. The second and third aqueous base coating compositions may contain at least a melamine resin.

[0178] The solid content of the curing agent is, for example, 10 parts by mass or more and 45 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous second or third base coating composition. The content of the curing agent may be 20 parts by mass or more, or may be 30 parts by mass or more.

[0179] The second and third aqueous base coating compositions may contain the same pigments, solvents and additives as the first aqueous base coating composition.

[0180] The water-based second and third base coating compositions may be commercially available products.

[0181] The resin solids concentrations of the second and third aqueous base coating compositions are not particularly limited and may be appropriately set depending on the coating conditions. The resin solids concentrations of the second and third aqueous base coating compositions are, for example, 10 to 60 mass%.

[0182] Clear Coating Composition The clear coating composition may be solvent-based, water-based, or powder-type. The clear coating composition may be solvent-based.

[0183] The clear coating composition may be, for example, an acid epoxy curing type containing a polyepoxide and a polycarboxylic acid, or a urethane curing type containing a hydroxyl group-containing resin and a polyisocyanate curing agent. The urethane curing type clear coating composition may be a two-component type.

[0184] The acid-epoxy curing clear coating composition contains, for example, (a) an acid anhydride group-containing acrylic resin, (b) a carboxyl group-containing polyester resin, and (c) an acrylic resin having a hydroxyl group and an epoxy group. From the viewpoint of storage stability, the acid anhydride group of the (a) acid anhydride group-containing acrylic resin may be half-esterified with a low-molecular-weight alcohol, etc. The (b) carboxyl group-containing polyester resin may further contain a hydroxyl group.

[0185] The resins (a) to (c) are blended, for example, so that the molar ratio of the carboxyl groups contained in the (a) acrylic resin and (b) polyester resin to the epoxy groups contained in the (c) acrylic resin is 1 / 1.4 to 1 / 0.6 (preferably 1 / 1.2 to 1 / 0.8), and so that the molar ratio of the carboxyl groups derived from the acid anhydride groups contained in the (a) acrylic resin to the hydroxyl groups contained in the (b) polyester resin and (c) acrylic resin is 1 / 2.0 to 1 / 0.5 (preferably 1 / 1.5 to 1 / 0.7).

[0186] The urethane curing clear coating composition contains, for example, a hydroxyl group-containing resin and a polyisocyanate compound. Examples of the polyisocyanate compound include those listed above. Examples of the hydroxyl group-containing resin include polyester resins, polyurethane resins, acrylic resins, and polyols containing hydroxyl groups. Examples of these resins include those listed above.

[0187] The OHV of the hydroxyl group-containing resin is, for example, 20 mgKOH / g or more and 200 mgKOH / g or less. The OHV may be 30 mgKOH / g or more. The OHV may be 180 mgKOH / g or less. The Mw of the hydroxyl group-containing resin is, for example, 1,000 or more and 20,000 or less. The Mw may be 2,000 or more. The Mw may be 15,000 or less. The AV of the hydroxyl group-containing resin is, for example, 2 mgKOH / g or more and 30 mgKOH / g or less. The AV may be 3 mgKOH / g or more. The AV may be 25 mgKOH / g or less.

[0188] The hydroxyl group-containing resin and the polyisocyanate curing agent are blended, for example, so that the equivalent ratio (NCO / OH) of the isocyanate group (NCO) to the hydroxyl group (OH) is 0.5 or more and 1.7 or less. The equivalent ratio may be 0.7 or more. The equivalent ratio may be 1.5 or less.

[0189] Alternatively, an acrylic melamine curing type clear coating composition may be used. The clear coating composition may be a commercially available product.

[0190] The resin solids concentration of the clear coating composition is not particularly limited and is set appropriately depending on the coating conditions. The resin solids concentration of the clear coating composition is, for example, 40 to 70 mass %.

[0191] Method for manufacturing a coated article A coated article is manufactured by a method comprising: applying the above-mentioned aqueous first base paint composition onto an object to be coated to form an uncured first base paint film; applying the above-mentioned aqueous second base paint composition onto the uncured first base paint film to form an uncured second base paint film; applying the above-mentioned clear paint composition onto the uncured second base paint film to form an uncured clear paint film; and heating and curing the uncured first base paint film, the uncured base paint film, and the uncured clear paint film.

[0192] The method for producing a coated article may further comprise forming an uncured third base coating film between forming the uncured second base coating film and forming the uncured clear coating film. That is, the method for producing a coated article may comprise forming an uncured second base coating film, and then applying the above-mentioned aqueous third base coating composition on the uncured second base coating film to form an uncured third base coating film, and then applying the above-mentioned clear coating composition.

[0193] The above-mentioned first aqueous base coating composition, second aqueous base coating composition (and even third base coating composition) and clear coating composition make it possible to obtain a coating film that is excellent in chipping resistance and adhesion.

[0194] Each coating composition can be applied by a commonly used coating method. The first, second, and third water-based base coating compositions can be applied, for example, by multi-stage coating using an air electrostatic spray coater (typically, coating in two stages), or by a method combining an air electrostatic spray coater with a rotary atomizing electrostatic coater commonly known as a "μμ (micro-micro) bell," "μ (micro) bell," or "meta-bell."

[0195] The clear coating composition is applied, for example, by a rotary atomizing electrostatic coating machine called a Microbell.

[0196] The application of the clear coating composition may be carried out after successively applying the first, second, and third aqueous base coating compositions and pre-drying the resulting uncured first, second, and third base coating films. Pre-drying (also called preheating) is carried out, for example, at 60 to 90°C for 1 to 15 minutes.

[0197] Curing The uncured first base coating film, the uncured second base coating film (and the third base coating film), and the uncured clear coating film are heated, which simultaneously cures each of the uncured coating films to form a multi-layer coating film including the first base coating film, the second base coating film (and the third base coating film), and the clear coating film.

[0198] The heating temperature may be 75°C or higher and 160°C or lower. The heating temperature may be 80°C or higher. The heating temperature may be 150°C or lower, or 140°C or lower. The heating temperature is synonymous with the above-mentioned "temperature of the object to be coated, etc." The object to be coated, etc. is heated so that the lowest temperature is 75°C or higher and 160°C or lower. When measuring temperatures at multiple locations, the lowest temperature may be in the range of 75°C or higher and 160°C or lower. Alternatively, the temperature of one location that is expected to be the lowest may be in the range of 75°C or higher and 160°C or lower.

[0199] The heating time may be, for example, 10 minutes or more and 40 minutes or less. The heating time is the time during which the above-mentioned "temperature of the object to be coated, etc." is maintained at the measurement location.

[0200] 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.

[0201] [Examples 1 to 7, Comparative Examples 1 to 9] (1) Preparation of first base coating composition As shown in Table 1, each component was mixed to prepare aqueous first base coating compositions X1 to X7, x1 to x9. The solid content concentration was adjusted to 35 to 45 mass% using ion-exchanged water. Details of each component listed in Table 1 are as follows.

[0202] (A) Water-based polyurethane resins A1: Polyurethane dispersion, trade name "ISW-1005", manufactured by Nippon Paint Co., Ltd., Tg -55°C, Mw not measurable (20,000 or more), OHV 0 mg KOH / g A2: Polyurethane dispersion, trade name "UH-2800", manufactured by Covestro Urethanes, Tg -83°C, Mw not measurable (20,000 or more), OHV 15 mg KOH / g A-3: Polyurethane dispersion, trade name "ASP-038", manufactured by Nippon Paint Co., Ltd., Tg -55°C, Mw 80,000, OHV 10 mg KOH / g a-1: Polyurethane dispersion, trade name "U-coat D359", manufactured by Sanyo Chemical Industries, Ltd., Tg -80°C, Mw 10,000, OHV 19 mg KOH / g

[0203] a-2: Polyurethane dispersion, Tg 5°C, Mw 19000, OHV 64 mg KOH / g

[0204] The method for producing aqueous polyurethane resin (a-2) is as follows. - Production of polyester polyol A flask equipped with a reflux condenser with a reaction water separation tube attached, a thermometer, a stirrer, and a nitrogen gas inlet tube was charged with 20.0 parts of dimer acid (trade name "PRIPOL 1017", manufactured by CRODA, carbon number 36), 35.0 parts of isophthalic acid, 8.6 parts of adipic acid, 35.6 parts of neopentyl glycol, and 0.8 parts of trimethylolpropane, and the temperature was raised to 160 ° C. with stirring. After holding at 160 ° C. for 1 hour, the temperature was raised to 230 ° C. over 5 hours. The acid value was measured periodically while maintaining the temperature at 230 ° C. After the resin acid value reached 4 mg KOH / g, the temperature was lowered to 80 ° C. or below. Finally, 59.4 parts of methyl ethyl ketone was added to obtain a polyester polyol with an acid value of 4 mg KOH / g, a hydroxyl value of 62 mg KOH / g, and a weight average molecular weight of 7,200.

[0205] - Production of aqueous polyurethane resin (a-2) A flask equipped with a thermometer, a stirrer, and a nitrogen gas inlet tube was charged with 110.0 parts of the above polyester polyol, 4.6 parts of dimethylolpropionic acid, 2.0 parts of neopentyl glycol, and 20.4 parts of methyl ethyl ketone, and the temperature was raised to 80 ° C. while stirring. Next, 24 parts of isophorone diisocyanate was added, and the reaction was continued at 80 ° C. After the isocyanate value reached 0.40 mmol / g, 4.9 parts of trimethylolpropane was added, and the temperature was maintained at 80 ° C. After the isocyanate value reached 0.03 mmol / g, 4.8 parts of butyl cellosolve was added. The temperature was lowered to 50 ° C., 3.3 parts of dimethylethanolamine was added to neutralize the acid groups, and 150.0 parts of deionized water was added. Thereafter, the temperature was raised to 100 ° C., and the methyl ethyl ketone was removed under reduced pressure to obtain a dispersion-type aqueous polyurethane resin (a-2).

[0206] (B) Melamine Resins B-1: Trade name "C211", manufactured by Allnex Corporation, Mw 780, imino group type B-2: Trade name "C370", manufactured by Allnex Corporation, Mw 985, methylol group type B-3: Trade name "C385", manufactured by Allnex Corporation, Mw 550, methylol / imino group type B-4: Trade name "C202", manufactured by Allnex Corporation, Mw 1200, methylol / imino group type

[0207] (b) Other curing agents b-1: Blocked isocyanate, trade name "Bayhydur BL2867", manufactured by Covestro

[0208] (C) Hydroxyl-containing acrylic resin emulsion C-1: Trade name "EMA-1036", manufactured by Nippon Paint Automotive Coatings Co., Ltd., Mw not measurable (1 million or more), OHV 40 mg KOH / g, Tg -10°C C-2: Trade name "EMA-1046", manufactured by Nippon Paint Automotive Coatings Co., Ltd., Mw not measurable (1 million or more), OHV 30 mg KOH / g, Tg 40°C

[0209] (D) Other aqueous resins D1: Water-soluble acrylic resin, trade name "ACW-1011", manufactured by Nippon Paint Automotive Coatings Co., Ltd., OHV 70 mg KOH / g, MW 27,000

[0210] D2: Acrylic resin dispersion, Mw 30,000

[0211] The method for producing acrylic resin dispersion (D2) is as follows. A reaction vessel equipped with a stirrer, a temperature controller, a condenser, and a dropping device was charged with 30 parts of a glycidyl ester (trade name: Cardura E10P, manufactured by Oxalis Chemicals, viscosity 7 mPa s, boiling point 251 to 278°C), and the temperature was raised to 165°C with stirring, followed by reflux. Separately, a mixture was prepared: 9.47 parts of acrylic acid (AA), 5.8 parts of 2-hydroxyethyl methacrylate (HEMA), 11.6 parts of cyclohexyl methacrylate (CHMA), 7.5 parts of n-butyl acrylate (NBA), 16.9 parts of styrene (ST), 0.28 parts of a polymerization initiator (DTA, trade name: Luperox DTA, manufactured by Arkema Yoshitomi Co., Ltd.), and 6.5 parts of a high-boiling solvent (dipropylene glycol monomethyl ether (DPM)). This mixture was added dropwise to the reaction vessel at 165° C. over 3.5 hours to carry out a polymerization reaction and a ring-opening addition reaction, thereby obtaining an acrylic resin dispersion (D2).

[0212] D3: Water-based polyester resin, trade name "NP-6100", manufactured by DIC Corporation. D4: Polyether polyol, trade name "Sannyx GP-1000", manufactured by Sanyo Chemical Industries, Ltd.

[0213] (2) Production of Coated Article 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.) so that the dry coating film would be 20 μm thick. The plate was then heated at 160° C. for 30 minutes to form an electrodeposition coating film, thereby obtaining a coated article.

[0214] The first aqueous base coating compositions X1 to X7 and x1 to x9 were each applied to a substrate using a rotary atomization electrostatic coating device to a dry film thickness of 20 μm. Subsequently, the second aqueous base coating composition (product name "AR-3510", Nippon Paint Automotive Coatings Co., Ltd., aqueous base paint) was applied to a substrate using a rotary atomization electrostatic coating device to a dry film thickness of 15 μm. The substrate was then preheated at 80°C for 3 minutes. A 6-minute interval was allowed between the application of the first aqueous base coating composition and the application of the second aqueous base coating composition.

[0215] A clear coating composition (trade name "PolyureExcel O-1200", manufactured by Nippon Paint Automotive Coatings Co., Ltd., a two-component acrylic urethane organic solvent-based clear coating containing a polyisocyanate compound) was diluted with a 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 viscosity adjustment, the clear coating composition was applied in one stage using a Microbell so that the dry film thickness would be 40 μm.

[0216] After leaving it at room temperature for 10 minutes, it was heated at 140°C for 20 minutes to obtain a coated article having a multi-layer coating film.

[0217] The amount of each component used shown in Table 1 is the part by mass of the resin solids or active ingredient.

[0218] [Physical properties of the first base coating film] Preparation of the first base coating film The first base coating compositions X1 to X7, x1 to x9 were each applied to a polypropylene test plate by air spray so that the dry film thickness was 20 μm. Then, the film was heated at 140 ° C for 20 minutes. After that, the coating film was peeled off from the test plate to obtain a first base coating film.

[0219] Tan δ Using a forced stretching vibration viscoelasticity measuring device "Vibron" manufactured by Orientec Co., Ltd., the stress and vibration strain generated in the first base coating film during temperature rise were detected under conditions of a temperature rise rate of 2°C / min and a frequency of 8 Hz in accordance with the tensile vibration-non-resonance method of JIS-K7244-4:1999, and tan δ at -20°C was determined from the phase difference between these.

[0220] Dynamic Tg In a graph in which the tan δ measured in the same manner as above is plotted on the vertical axis and the temperature on the horizontal axis, the temperature at which tan δ reaches its peak was taken as the dynamic Tg.

[0221] Breaking stress A sample measuring 10 mm wide x 50 mm long was cut out from the first base coating film, and the thickness was measured. The sample was set in an autograph AG-IS manufactured by Shimadzu Corporation, and pulled at 25°C at a rate of 5 mm / min until the coating film broke, and the stress at break was measured. The stress was divided by the cross-sectional area of ​​the coating film to calculate the breaking stress. The measurement was carried out three times using different samples, and the average value was taken as the breaking stress.

[0222] [Evaluation] (1) Chipping resistance Test plates having the multilayer coating film obtained in each Example and Comparative Example were subjected to a stone chipping test using a Gravelo tester KSS-1 (manufactured by Suga Test Instruments Co., Ltd.) under the following conditions: <Test conditions> Shot material: M2 nut Amount of shot material: 250 g Distance: 35 cm Shot pressure: 0.49 MPa Shot angle: 45° Test temperature: -20°C

[0223] After the stone chipping test, the test plate was visually evaluated according to the following criteria: In the following criteria, a rating of A to B indicates excellent chipping resistance.

[0224] (Evaluation criteria) A: The peeled area per peeled portion is small, or the number of peeled portions is small, and the total peeled area relative to the entire coating film is 3% or less. B: The peeled area per peeled portion is small, but the number of peeled portions is somewhat large, and the total peeled area relative to the entire coating film is 10% or less. C: The peeled area per peeled portion is large, and the number of peeled portions is also large, and the total peeled area relative to the entire coating film is 30% or less. D: The peeled area per peeled portion is very large, and the number of peeled portions is also very large, and the total peeled area relative to the entire coating film is more than 30%.

[0225] (2) WDB Adhesion The adhesion of WDB was evaluated according to the following procedure: (Procedure 1) Formation of multi-layer coating film In the same manner as above, an electrodeposition coating film and a multi-layer coating film having a first base coating film, a second base coating film, and a clear coating film were formed on a substrate.

[0226] (Step 2) Preparation of test panel Bond Hamatite WS-272 (urethane sealant, manufactured by Yokohama Rubber Co., Ltd.) was applied in a strip shape to the surface of the clear coating film so that the film thickness after drying of the bond was 3 mm, the width was about 15 mm, and the length was about 100 mm. After that, release paper was placed on the bond, and the bond was cured at room temperature for 72 hours while applying pressure from above the release paper to obtain a test panel.

[0227] (Procedure 3) Cohesive Failure of Bond In the first test, the edge of the bond was pulled up at an angle of 90° or more from the coating surface (horizontal surface) in the longitudinal direction of the test plate, causing cohesive failure of the bond.

[0228] (Step 4) Cutting the coating film At the location where the bond had cohesively failed and remained on the surface of the test plate, a cutter knife was used to make a cut from the coating surface (horizontal plane) at an angle of 60° in the length direction of the test plate to a depth that reached the substrate (SPC steel plate). The cut with the cutter knife extended from one end of the strip-shaped bond in the width direction to the other end.

[0229] (Step 5) Measurement of the size of peeling The length of the peeled part of the coating film on the cut line made by a cutter knife (a line extending from one end of the strip-shaped bond to the other end in the width direction) was measured with a ruler, and the percentage of the area of ​​the peeled part of the coating film in the region from the end of the bond to the cut line was calculated to evaluate the adhesiveness of the WDB. If the coating film is peeled off, this is the part where cohesive failure has occurred in the base coating film.

[0230] (Procedure 6) Subsequently, as the nth test, as in Procedure 3, the end of the bond was pulled up at an angle of 90° or more from the coating surface (horizontal surface) in the longitudinal direction of the test plate, causing cohesive failure of the bond. As in Procedure 4, a cutter knife was used to make incisions at an angle of 60° from the coating surface (horizontal surface) in the longitudinal direction of the test plate, leaving 2 to 3 mm intervals from the incision line made in the n-1th test, to a depth reaching the base material (SPC steel plate). As in Procedure 5, the length of the newly formed incision line where the coating film had peeled was measured with a ruler, and the percentage of the area of ​​the incision line from the incision line in the n-1th test to the incision line in the nth test where the coating film had peeled was calculated. The above procedure was repeated 10 or more times.

[0231] (Procedure 7) The test results obtained by multiple runs of the procedure above were comprehensively evaluated according to the following criteria.

[0232] The evaluation criteria are as follows: A: Neither cohesive failure of the first base coating film nor peeling at the interface between the first base coating film and the electrodeposition coating film was observed. B1: Cohesive failure of the first base coating film was observed. B2: Peeling at the interface between the first base coating film and the electrodeposition coating film was observed. B3: Both cohesive failure of the first base coating film and peeling at the interface between the first base coating film and the electrodeposition coating film were observed.

[0233]

[0234] According to the present invention, a multi-layer coating film having excellent chipping resistance and adhesion can be obtained. The first base coating composition of the present invention is suitable for coating automobile bodies.

[0235] This application claims priority based on Japanese Patent Application No. 2024-109027, filed on July 5, 2024, the entire contents of which are incorporated herein by reference.

Claims

1. A first aqueous base coating composition comprising: (A) an aqueous polyurethane resin having a glass transition temperature of -20°C or lower, a weight average molecular weight of 20,000 or higher, and a hydroxyl value of 0 to 20 mgKOH / g; (B) a melamine resin; and (C) a hydroxyl-containing acrylic resin emulsion having a weight average molecular weight of 100,000 or higher, wherein, relative to 100 parts by mass of resin solids in the first aqueous base coating composition, the solids content of the (A) aqueous polyurethane resin is 35 to 60 parts by mass, the solids content of the (B) melamine resin is 20 to 30 parts by mass, and the solids content of the (C) hydroxyl-containing acrylic resin emulsion is 8 to 20 parts by mass; and a first base coating film obtained by heating the first aqueous base coating composition at 140°C for 20 minutes has a loss tangent at -20°C of 0.080 or lower, a dynamic glass transition temperature of 60°C or higher, and a viscosity of 20 N / mm ２ and a breaking stress of at least 100%.

2. The first aqueous base coating composition according to claim 1, wherein the (A) aqueous polyurethane resin comprises: (A1) an aqueous polyurethane resin having a glass transition temperature of -20°C or lower, a weight average molecular weight of 20,000 or higher, and a hydroxyl value of 0 mgKOH / g; and (A2) an aqueous polyurethane resin having a glass transition temperature of -20°C or lower, a weight average molecular weight of 20,000 or higher, and a hydroxyl value of more than 0 mgKOH / g and not more than 20 mgKOH / g; and wherein the content of the (A2) aqueous polyurethane resin is more than 0 parts by mass and not more than 32 parts by mass per 100 parts by mass of resin solids in the first aqueous base coating composition.

3. The first water-based base coating composition according to claim 1 or 2, wherein the melamine resin (B) has a weight average molecular weight of 400 to 2,000.

4. A method for manufacturing a coated article, comprising: applying an aqueous first base coating composition according to any one of claims 1 to 3 onto an object to be coated to form an uncured first base coating film; applying an aqueous second base coating composition onto said uncured first base coating film to form an uncured second base coating film; applying a clear coating composition onto said uncured second base coating film to form an uncured clear coating film; and heating and curing said uncured first base coating film, said uncured second base coating film and said uncured clear coating film.

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