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

Abstract

To provide a water-based paint composition that has sufficient pot life regardless of seasonal factors, and also exhibits excellent paintability and coating performance, as well as a multi-layer coating film using the same and a method for forming the same. [Solution] An aqueous coating composition comprising an aqueous main component (I) and a water-dispersible curing agent (II), wherein the main component (I) comprises a water-dispersible hydroxyl group-containing acrylic resin (a), and the curing agent (II) comprises a water-dispersible polyisocyanate (b), wherein (b) comprises a polyisocyanate (b-1) with an isocyanate group content of 8-14% and a polyisocyanate (b-2) with an isocyanate group content of 16-20%, the blending mass ratio of (b-1) and (b-2) ((b-1) / (b-2)) is 9.5 / 0.5-1.5 / 8.5, and the equivalent ratio of isocyanate groups in (b) to hydroxyl groups in (a) (isocyanate groups / hydroxyl groups) is 0.5-3.0, characterized in that an aqueous coating composition comprising an aqueous main component (I) and a water-dispersible curing agent (II), wherein (b) comprises a water-dispersible hydroxyl group-containing acrylic resin (a), and (b) comprises a water-dispersible polyisocyanate (b), wherein (b) comprises a polyisocyanate (b-1) with an isocyanate group content of 8-14% and a polyisocyanate (b-2) with an isocyanate group content of 16-20%, the blending mass ratio of (b-1) and (b-2) is 9.5 / 0.5-1.5 / 8.5, and the equivalent ratio of isocyanate groups in (b) to hydroxyl groups in (a) (isocyanate groups / hydroxyl groups) is 0.5-3.0, and a multilayer coating film using this and a method for forming the same.

Description

TECHNICAL FIELD

[0001] The present invention relates to an aqueous coating composition, a method for forming a multilayer coating film using the same, and a multilayer coating film. BACKGROUND ART

[0002] In recent years, replacement of solvent-based paints with water-based paints has been widely studied from the perspective of environmental protection. Although water-based conversion has also been studied for metal base materials such as steel structures, automobiles, heavy machinery, and railway vehicles, it is difficult to design water-based paints with excellent adhesion especially for railway vehicles and heavy vehicles that are large, thick-walled coated objects with no suction into the base material and thus not suitable for bake hardening. Here, various paints suitable for coating railway vehicles have been proposed (Patent Documents 1 to 4).

[0003] Patent Document 1 describes that the railway vehicle outer panel with a laminated coating film obtained by sequentially laminating (A) an undercoat film formed of an epoxy-curable paint and having a gel fraction of 90% or more, (B) a putty coating film having a hydrolysis rate of 0.5% or less under the condition of being immersed in warm water at 40°C for 30 days, (C) a primer-surfacer coating film, and (D) a topcoat film on a metal base material forming the structure of a railway vehicle is excellent in hot water resistance.

[0004] Further, Patent Document 2 is an invention relating to an aqueous coating composition that describes being capable of forming a coating film having the coating film appearance and coating film physical properties required for heavy vehicles such as railway vehicles. Document 2 describes that the use amount of organic solvents can be reduced, and the aqueous coating composition having excellent adhesion even to a coated object having a coating film can be provided.

[0005] Patent Document 3 describes providing a water-based, multi-component paint composition and film-forming method that are optimal for painting large objects such as railway vehicles, offering excellent drying properties, paint handling workability (pot life, ease of manual stirring, etc.), storage properties, weather resistance, and gloss, by combining a colloidal dispersion type acrylic resin (A1) and a water-soluble acrylic resin (A2). As shown in Patent Document 3, when painting large objects such as railway vehicles, excellent paint handling workability, such as pot life, is required.

[0006] Patent Document 4 describes a two-component mixed paint composition in which, by adding a specific amount of oxime compound to a specific hydroxyl group-containing acrylic resin, the rate of viscosity increase after mixing with a polyisocyanate compound can be gently adjusted, and the appearance changes little even after time has passed since the two components were mixed. This results in a two-component mixed paint composition that has a sufficient pot life and little change in the finished appearance. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2015-058535 [Patent Document 2] Japanese Patent Publication No. 2017-101126 [Patent Document 3] Japanese Patent Publication No. 2021-130812 [Patent Document 4] Japanese Patent Publication No. 2022-151864 [Overview of the project] [Problems that the invention aims to solve]

[0008] Thus, when painting large objects such as railway vehicles and heavy vehicles, a sufficient pot life is required. Furthermore, there is a need for paint designs that are less affected by seasonal factors, such as those that have sufficient pot life and drying properties in both the summer season when paint films dry easily and those that dry slowly in the winter season when paint films dry slowly. However, since achieving this is difficult, it has been common practice to adjust part of the formulation depending on the season, such as using different thinners and hardeners for summer and winter. In addition, paint films on large objects such as railway vehicles and heavy vehicles often require paint film performance such as impact resistance and water resistance, so that the paint film does not crack easily when subjected to external impacts. However, if the paint film has poor film formation, these paint film performances may not be achieved, or if the paint film becomes brittle, its impact resistance may be inferior.

[0009] Therefore, as a result of diligent research by the present inventors, the aforementioned problems were solved by providing an aqueous coating composition containing an aqueous main component and a water-dispersible curing agent, particularly by including multiple water-dispersible polyisocyanates with different isocyanate group content as the water-dispersible curing agent, and by optimizing the equivalent ratio between the isocyanate groups contained in these water-dispersible polyisocyanates and the hydroxyl groups of the water-dispersible hydroxyl group-containing acrylic resin contained in the aqueous main component.

[0010] Therefore, an object of the present invention is to provide an aqueous paint composition that has a sufficient pot life regardless of seasonal factors, and also exhibits excellent paintability and coating performance. Another object of the present invention is to provide a multi-layer coating and a method for forming the same that is suitable for painting railway vehicles and the like, using such an aqueous paint composition. [Means for solving the problem]

[0011] In other words, the gist of this invention is as follows: (1) An aqueous paint composition comprising an aqueous main component (I) and a water-dispersible curing agent (II), The aqueous main component (I) comprises a water-dispersible hydroxyl group-containing acrylic resin (a), The water-dispersible curing agent (II) comprises a water-dispersible polyisocyanate (b), and the water-dispersible polyisocyanate (b) comprises a polyisocyanate (b-1) with an isocyanate group content of 8-14% and a polyisocyanate (b-2) with an isocyanate group content of 16-20%. The blending mass ratio ((b-1) / (b-2)) of the polyisocyanate (b-1) and the polyisocyanate (b-2) is 9.5 / 0.5 to 1.5 / 8.5. An aqueous paint composition characterized in that the equivalent ratio (isocyanate group / hydroxyl group) of isocyanate groups contained in the water-dispersible polyisocyanate (b) and hydroxyl groups contained in the water-dispersible hydroxyl group-containing acrylic resin (a) is 0.5 to 3.0. (2) The aqueous paint composition contains a film-forming aid (c), The aqueous paint composition according to (1), characterized in that the film-forming aid (c) comprises a film-forming aid (c-1) having a boiling point of 80°C or more and less than 200°C, and a film-forming aid (c-2) having a boiling point of 200°C or more and less than 290°C. (3) A method for forming a multilayer coating film comprising at least an undercoat film and one or more coating films on the undercoat film, The process includes (X) applying a primer paint composition to the object to be coated to form a primer film, The process includes (Y) a step of applying an aqueous paint composition onto the undercoat to form one or more coating films, A method for forming a multilayer coating film, characterized in that the aqueous coating composition is the aqueous coating composition described in (1) or (2) above. (4) The method for forming a multilayer coating film according to (3), wherein the step (Y) of forming the coating film includes a step of forming at least a first coating film and a second coating film, and the aqueous coating composition described in (1) or (2) above is used for forming at least one or all of the first coating film and the second coating film. (5) The primer paint composition is (X-1) Water-based paint composition containing epoxy resin, (X-2) Putty composition containing unsaturated polyester resin, and (X-3) Urethane-curing water-based paint composition which is at least one selected from the group consisting of, and forming one or two or more undercoat films using the undercoat coating composition, the method for forming a multilayer coating film according to (3). (6) The method for forming a multilayer coating film according to (5), wherein the aqueous coating composition containing the (X-1) epoxy resin is an aqueous coating composition containing at least an epoxy resin, an acrylic resin, and a pigment. (7) The method for forming a multilayer coating film according to (3), wherein the undercoat coating composition contains a silane coupling agent. (8) The method for forming a multilayer coating film according to (3), wherein the step (Y) of forming the coating film includes a step (Y') of applying a clear coating material to form a clear coating film. (9) A multilayer coating film comprising at least an undercoat film and one or two or more overcoat films on the undercoat film, wherein Any or all of the one or two or more overcoat films are formed from the aqueous coating composition according to (1) or (2), the multilayer coating film.

Effects of the Invention

[0012] According to the present invention, it is possible to provide an aqueous coating composition that has a sufficient pot life regardless of seasonal factors, and is also excellent in coating workability and coating film performance, as well as a multilayer coating film using the same and a method for forming the same.

Modes for Carrying Out the Invention

[0013] Hereinafter, the embodiments of the present invention will be described in detail, but the scope of the present invention is not limited only to the following embodiments, and various modifications and combinations can be made to the following embodiments within the scope not changing the gist of the present invention.

[0014] 1. Aqueous Coating Composition The aqueous coating composition of the present invention is a two-component type comprising an aqueous main agent (I) and a water-dispersible curing agent (II). The aqueous main agent (I) contains a water-dispersible hydroxyl group-containing acrylic resin (a). Further, the water-dispersible curing agent (II) contains a water-dispersible polyisocyanate (b). Herein, the terms "aqueous" and "water-dispersible" are not limited as long as they have the same meanings as those commonly used in the art. For example, "aqueous" refers to being soluble in water or an aqueous medium (described later), having a hydrophilic group in the molecule, or using water or an aqueous medium as a medium. In addition, "water-dispersible" may be partially soluble in water or an aqueous medium, and refers to the property of being dispersed without separation when added to water or an aqueous medium using an emulsifier or a hydrophilic group.

[0015] The aqueous coating composition of the present invention is a two-component curable coating composition, which is used by mixing an aqueous main agent (I) and a water-dispersible curing agent (II) immediately before use. In the aqueous coating composition of the present invention, the equivalent ratio [isocyanate group / hydroxyl group (NCO / OH)] of the isocyanate groups contained in the water-dispersible polyisocyanate (b) to the hydroxyl groups contained in the water-dispersible hydroxyl group-containing acrylic resin (a) is 0.5 to 3.0. The preferable range of the equivalent ratio is 0.7 to 2.5, and the more preferable range is 1.0 to 2.0. When the equivalent ratio is less than 0.5, sufficient coating film strength may not be obtained. In addition, sufficient water resistance and salt spray resistance may not be obtained. On the other hand, when the equivalent ratio exceeds 3.0, sufficient coating workability may not be obtained, and in particular, the coating workability at a temperature assumed for summer (around 40°C) may be poor.

[0016] <Aqueous main agent (I)> [Water-dispersible hydroxyl group-containing acrylic resin (a)] The aqueous main agent (I) in the aqueous coating composition of the present invention contains a water-dispersible hydroxyl group-containing acrylic resin (a) (hereinafter sometimes simply referred to as "acrylic resin (a)" or the like). The acrylic resin (a) is not particularly limited as long as it has one or more, preferably two or more hydroxyl groups. The amount of hydroxyl groups is represented by a hydroxyl value, and the hydroxyl value of the acrylic resin (a) is preferably 5 to 200 mgKOH / g, more preferably 50 to 150 mgKOH / g. When the hydroxyl value is within the above range, the strength of the obtained coating film becomes sufficiently high, and the water resistance is also likely to be improved. The hydroxyl value can be measured in accordance with JIS K 0070.

[0017] Furthermore, the acid value of acrylic resin (a) is preferably 1 to 40 mg KOH / g, and more preferably 5 to 30 mg KOH / g. When the acid value is within the above range, the water dispersibility of acrylic resin (a) is increased, and the water resistance of the resulting coating film is also easily improved. The hydroxyl value can be measured in accordance with JIS K 0070.

[0018] Furthermore, the number-average molecular weight of the acrylic resin (a) is not particularly limited and may be, for example, 1,000 to 100,000, 2,000 to 50,000, or 3,000 to 10,000. When the number-average molecular weight is within the above range, an excessive increase in the viscosity of the aqueous coating composition is suppressed, improving the appearance of the resulting coating film and making it easier to achieve sufficiently high strength. The number-average molecular weight is a styrene homopolymer equivalent value measured using gel permeation chromatography.

[0019] As the acrylic resin (a), acrylic polyols are preferred because they tend to produce good physical properties such as appearance, weather resistance, and chemical resistance of the resulting coating film.

[0020] The acrylic resin (a) is not limited, but for example, it may be in the form of an emulsion dispersed in an aqueous solvent. Examples of aqueous media include various types of water such as pure water, deionized water, tap water, and industrial water. The particle size of the acrylic resin (a) is not particularly limited. In terms of ease of viscosity control and the appearance of the resulting coating film, the average particle size is preferably 0.005 μm or more and 1.0 μm or less, and more preferably 0.001 μm or more and 0.5 μm or less. The average particle size is the 50% diameter (median diameter, D50) in the volume-based particle size distribution measured by dynamic light scattering.

[0021] The solid content concentration of the acrylic resin (a) is preferably, for example, 15% by mass or more and 95% by mass or less of the total solid content contained in the aqueous main component (I). When the solid content concentration is within the above range, the strength of the resulting coating film tends to increase. The solid content concentration may be 20% by mass or more and 25% by mass or more of the total solid content contained in the aqueous main component (I). Alternatively, the solid content concentration may be 85% by mass or less and 80% by mass or less of the total solid content contained in the aqueous main component (I).

[0022] Acrylic resin (a) can be obtained, for example, by emulsion polymerization of one or more raw material monomers. Emulsion polymerization is carried out by methods commonly used by those skilled in the art. Specifically, an emulsifier is mixed with an aqueous medium containing an organic solvent such as alcohol as needed, and the raw material monomers and polymerization initiators are added dropwise while the resulting mixture is heated and stirred. Alternatively, an emulsion mixture, in which the raw material monomers, emulsifier, and water are pre-emulsified, may be added dropwise to the aqueous medium. It is also possible to obtain acrylic resin by conventionally known methods such as phase conversion, forced emulsification, suspension polymerization, dispersion polymerization, and miniemulsion polymerization, in which monomers are polymerized by methods such as solution polymerization in the presence of a solvent or organic solvent, and then dispersed in water as needed.

[0023] Such acrylic resins (a) can be obtained, for example, by polymerizing a mixture of raw material monomers containing a hydroxyl group-containing ethylenically unsaturated monomer, an alkyl (meth)acrylate monomer, an acid group-containing ethylenically unsaturated monomer, and a styrene-based monomer. Furthermore, acrylic resins (a) may also include modified acrylic resins such as acrylic silicone resins, alkyd-modified acrylic resins, polyester-modified acrylic resins, and epoxy-modified acrylic resins, and may be copolymerized with polymerization components other than acrylic monomers.

[0024] Specific product names for the water-dispersible hydroxyl group-containing acrylic resin (a) in the present invention include, for example, the MACRYNAL series (manufactured by allnex), such as MACRYNAL SM 6825w / 37WA, MACRYNAL SM 6810w / 42WA, MACRYNAL VSM6299 / 42WA; the Bihydrol series (manufactured by Sumika Covestro Urethane), such as Bihydrol A145, Bihydrol A2470, Bihydrol A2542, Bihydrol A2469, Bihydrol A2646, XP2470; the Barnock series (manufactured by DIC), such as Barnock WD-551; and the NeoCryl series (manufactured by DSM), such as NeoCryl XK-555.

[0025] [Other ingredients] The aqueous main component (I) may, as necessary, contain other hydroxyl group-containing resins or other resins that do not contain hydroxyl groups. Examples of other hydroxyl group-containing resins include polyester polyols, polyether polyols, polyurethane polyols, and polycarbonate polyols. The aqueous main component (I) may also, as necessary, contain various known additives. Examples of additives include pigments, dispersants, defoamers, silane coupling agents, UV absorbers, hindered amine light stabilizers, antioxidants, viscosity modifiers, surface modifiers, film-forming aids, rust inhibitors, decontamination agents, anti-algal agents, antifungal agents, preservatives, and thickeners. In particular, the following pigments may be used.

[0026] [Pigments] For example, when forming a colored coating film using the aqueous coating composition of the present invention, a coloring pigment can be used. The coloring pigment can be any pigment commonly used in coatings, but examples include inorganic pigments and organic pigments. Examples of inorganic pigments include titanium dioxide, red iron oxide, yellow iron oxide, and carbon black, while examples of organic pigments include phthalocyanine blue, phthalocyanine green, naphthol red, quinacridone red, benzimidazolon yellow, Hansa yellow, benzimidazolon orange, and dioxazine violet. The composition may also include heat-shielding pigments. These pigments may be used individually or in combination of two or more.

[0027] Furthermore, pigments other than coloring pigments can be used. For example, rust-preventive pigments, extender pigments, and gloss pigments commonly used in paints can be used. Examples of extender pigments include white or colorless pigments. Because they have a low refractive index, they have little effect on opacity when mixed with binders, and are used as fillers to reduce costs in paints, coatings, and cosmetics, and to adjust color strength, gloss, strength, and feel. Known materials can be used as extender pigments, such as precipitated barium sulfate, silica, cristobalite, calcium carbonate, alumina, alum, white clay, magnesium hydroxide, and magnesium oxide. Extender pigments may also be in the form of thin, flat pigments like foil (flak-like pigments), with specific examples including glass flakes, talc, mica, and kaolin clay.

[0028] Furthermore, known materials can be used as rust-preventive pigments, including zinc powder, zinc oxide, barium metaborate, calcium silicate, aluminum phosphate, condensed aluminum phosphate, aluminum tripolyphosphate, zinc phosphate, zinc phosphite, potassium phosphite, calcium phosphite, aluminum phosphite, calcium zinc phosphate, aluminum zinc phosphate, zinc phosphomolybdate, aluminum phosphomolybdate, magnesium phosphate, vanadic acid / phosphate mixed pigments, and the like.

[0029] Furthermore, known materials can be used as luminous pigments. Specific examples include flaky metal pigments made from metals such as zinc, nickel, chromium, tin, copper, silver, platinum, gold, and aluminum, which are thin, flat, foil-like structures, and pearl pigments made by surface-treating talc or mica with metal oxides such as titanium dioxide.

[0030] When using pigments, the pigment volume concentration [PVC (vol %)], which is the proportion of pigment in the paint solids, is not limited, but can usually be 10-50%. For example, in the multi-layer coating described later, the PVC used as an intermediate coat is preferably 20-50%, and more preferably 30-40%. For example, in the multi-layer coating described later, the PVC used as a topcoat is preferably 10-40%, and more preferably 10-20%.

[0031] [Film-forming aid (c)] In the aqueous coating composition of the present invention, it is preferable to include a film-forming aid (c). The film-forming aid is a temporary plasticizer that acts as a plasticizer only during film formation, lowers the minimum film formation temperature, and is a component that volatilizes after film formation is complete. Including it improves film formation and allows for adjustment of paintability. As such a film-forming aid, it is preferable to use a relatively volatile film-forming aid (c-1) with a boiling point of 80°C or higher and less than 200°C, for reasons such as ensuring paintability during painting and volatilizing after film formation so that it does not remain in the coating. Furthermore, it is preferable to use a relatively slow-volatility film-forming aid (c-2) with a boiling point of 200°C or higher and less than 290°C, for reasons such as maintaining a fluid state for a long time while ensuring paintability from the start to the end of painting, even in environments where the coating dries easily in the summer, and softening the water-dispersible resin to improve film formation.

[0032] In other words, in the present invention, it is preferable to use a readily volatile film-forming aid (c-1) and / or a slow-volatile film-forming aid (c-2), with a boiling point of 200°C as the standard. Here, in order to achieve the aforementioned paintability by preventing the volatilization of film-forming aid (c-1) from being too rapid, it is preferable to use one with a boiling point of 80°C or higher. On the other hand, with respect to film-forming aid (c-2), it is possible that the film-forming aid may not completely escape from the paint film and may remain in the paint film as a plasticizer, which could lead to a decrease in paint film performance such as water resistance. Therefore, it is preferable to use one with a boiling point of less than 290°C.

[0033] In particular, using a combination of film-forming aids with different boiling points makes the mixture less susceptible to seasonal factors and facilitates the achievement of excellent coating workability, thus representing a more preferable embodiment. In the aqueous coating composition of the present invention, it is more preferable to include both a film-forming aid (c-1) with a boiling point of 80°C or higher and less than 200°C, and a film-forming aid (c-2) with a boiling point of 200°C or higher and less than 290°C. In this specification, unless otherwise specified, the boiling point refers to the boiling point at 1 atmosphere.

[0034] Examples of film-forming aids (c-1) with a boiling point of 80°C or higher and less than 200°C include propylene glycol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol dimethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monoisopropyl ether, ethylene glycol mono-n-butyl ether (ethylene glycol monobutyl ether), ethylene glycol mono-iso-butyl ether, ethylene glycol mono-tert-butyl ether, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, and diethylene glycol dimethyl ether. The boiling point of the film-forming aid (c-1) is more preferably 110°C or higher and less than 190°C.

[0035] Furthermore, examples of film-forming aids (c-2) with a boiling point of 200°C or higher and less than 290°C include diethylene glycol monopropyl ether, diethylene glycol monoisopropyl ether, diethylene glycol mono-n-butyl ether (diethylene glycol monobutyl ether), diethylene glycol monoiso-butyl ether, diethylene glycol mono-tert-butyl ether, diethylene glycol dibutyl ether, diethylene glycol monomethyl ether acetate, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, ethylene glycol mono-2-ethylhexyl ether, diethylene glycol mono-2-ethylhexyl ether, triethylene glycol monomethyl ether, triethylene glycol monomethyl ether, triethylene glycol monobutyl ether, and dipropylene glycol monobutyl ether. The boiling point of the film-forming aid (c-2) is more preferably 210°C or higher and less than 260°C.

[0036] The mass ratio of the film-forming aid (c-1) to the film-forming aid (c-2) is preferably 9:1 to 1:9, more preferably 8:2 to 2:8, even more preferably 7:3 to 3:7, and particularly preferably 6:4 to 4:6. With such a ratio, it is easier to achieve excellent paint workability without impairing the initial drying properties, film-forming properties, or physical properties of the coating film.

[0037] In the aqueous coating composition of the present invention, the content of film-forming aid (c) is preferably 1.0 to 7.0% by mass, and more preferably 1.5 to 6.0% by mass, as either or both of film-forming aid (c-1) and film-forming aid (c-2). In particular, when film-forming aid (c-1) and film-forming aid (c-2) are used in combination, the content is preferably 3.0 to 7.0% by mass, and more preferably 3.0 to 5.0% by mass. With such a content of film-forming aid (c), it is easier to achieve excellent coating workability without impairing the initial drying properties, film-forming properties, or physical properties of the coating film.

[0038] In the present invention, the film-forming aid (c) is preferably blended to improve the film-forming properties of the main resin component and to achieve the aforementioned effects; therefore, the film-forming aid (c) used in the present invention is defined as the one contained in the aqueous main component (I). In other words, in the present invention, although the organic solvent blended into the curing agent contributes to lowering the viscosity of the curing agent and improving its miscibility with the main component, it is not treated as the film-forming aid (c).

[0039] <Water-dispersible curing agent (II)> [Water-dispersible polyisocyanate (b)] The water-dispersible curing agent (II) in the aqueous coating composition of the present invention comprises a water-dispersible polyisocyanate (b) (hereinafter, it may be simply referred to as "polyisocyanate (b)"). The water-dispersible polyisocyanate (b) is a polyisocyanate compound that disperses without separation when added to the aforementioned aqueous medium such as water.

[0040] When a water-dispersible curing agent (II) containing a water-dispersible polyisocyanate (b) is dispersed in water, it is presumed that a micelle structure is formed as the hydrophobic structural units, which are isocyanate units, aggregate inside the particles, and the hydrophilic structural units face outwards from the dispersed particles. Due to this action, the water-dispersible curing agent (II) containing the water-dispersible polyisocyanate (b) can exist stably in an aqueous medium, such as water. In other words, since the isocyanate groups, which normally have high reactivity with water, can exist in a hydrophobic state inside the micelles, it is presumed that the decarboxylation reaction in the aqueous medium is suppressed, and storage stability can be improved.

[0041] In the aqueous coating composition of the present invention, a combination of polyisocyanates with different isocyanate (NCO) group content is used as the water-dispersible polyisocyanate (b). By using a combination of polyisocyanates with different NCO group content, pot life, paintability, and coating film performance can be ensured. The polyisocyanates with different NCO group content are a polyisocyanate (b-1) with an NCO content of 8-14% and a polyisocyanate (b-2) with an NCO content of 16-20%. In this specification, the NCO group content is expressed as the amount of NCO groups contained in the polyisocyanate as a mass fraction, and can be measured, for example, in accordance with JIS K 1603-1 (2007).

[0042] Polyisocyanates (b-1) with an NCO content of 8-14% are long-pot-life type isocyanates, offering the advantage of a longer pot life. However, using them as the primary coating can make the paint film difficult to dry. Therefore, if the amount used is high, it can lead to problems such as poor coating performance, sagging when applied to vertical surfaces, and poor drying of the coating film in winter.

[0043] Furthermore, polyisocyanate (b-2) with an NCO content of 16-20% is a common isocyanate that has been used conventionally and is a curing agent with excellent reactivity with hydroxyl group-containing resins. On the other hand, it has a short pot life and the curing reaction proceeds quickly, which presented challenges when painting large objects such as railway vehicles and structures.

[0044] Therefore, in the present invention, by using polyisocyanates (b-1) and (b-2) in combination, and by using them in a manner that satisfies the aforementioned predetermined equivalent ratio, both sufficient pot life and drying properties are achieved in both the summer season when the coating film dries easily and the winter season when the coating film dries slowly, resulting in excellent paint workability that is less affected by seasonal factors, and at the same time, excellent coating film performance such as impact resistance and water resistance.

[0045] Furthermore, when using polyisocyanates with an NCO group content of less than 8%, the curing properties tend to be low. On the other hand, polyisocyanates with an NCO content of more than 20%, as actually used in Patent Document 3, are commercially available, but when the NCO content is too high, the crosslinking density of the coating film increases, and although it is excellent in terms of water resistance, the paint tends to become brittle, and therefore its impact resistance tends to be poor.

[0046] To obtain polyisocyanates with different NCO content, there are no limitations, and methods include obtaining them as commercial products or adjusting the NCO group content. Known methods can be used to adjust the NCO group content, but one example is to appropriately adjust the modification rate in modified polyisocyanates, such as those obtained through hydrophilization, as illustrated later. For example, in the case of modification by the addition reaction of a hydrophilic compound such as a polyalkylene glycol alkyl ether, one method is to adjust the mixing ratio of the hydrophilic compound to the polyisocyanate. Another example is to further mix unmodified polyisocyanate with a mixture of modified polyisocyanate obtained by the addition reaction of a hydrophilic compound and unmodified polyisocyanate, with the aim of adjusting the content of structures derived from isocyanate groups.

[0047] To fully obtain the aforementioned effects, the blending mass ratio (b-1) / (b-2) of polyisocyanate (b-1) to polyisocyanate (b-2) must be 9.5 / 0.5 to 1.5 / 8.5. The preferred range for (b-1) / (b-2) is 9.5 / 0.5 to 3.5 / 6.5, a more preferred range is 9.5 / 0.5 to 4.5 / 5.5, an even more preferred range is 9.5 / 0.5 to 5.5 / 4.5, and a particularly preferred range is 9.5 / 0.5 to 7.5 / 2.5.

[0048] The solid content concentration of the water-dispersible polyisocyanate (b) is preferably, for example, 15% by mass or more and 100% by mass or less of the total solid content contained in the water-dispersible curing agent (II). When the solid content concentration is within the above range, the strength of the resulting coating film tends to increase. The solid content concentration of the water-dispersible polyisocyanate (b) may be 20% by mass or more and 25% by mass or more of the total solid content contained in the water-dispersible curing agent (II). Alternatively, the solid content concentration of the water-dispersible polyisocyanate (b) may be 90% by mass or less and 85% by mass or less of the total solid content contained in the aqueous main component (I).

[0049] The water-dispersible polyisocyanate (b) is not limited in type or chemical structure, as long as it contains (b-1) and (b-2) having the NCO group content described above and is water-dispersible. As such a water-dispersible polyisocyanate (b), it is preferable to use a hydrophilic (hydrophilized) polyisocyanate, for example, obtained by introducing hydrophilic groups into the polyisocyanate. A hydrophilic (hydrophilized) polyisocyanate can be prepared by known methods. For example, it can be obtained by a method including the steps of preparing a precursor polyisocyanate and introducing hydrophilic groups.

[0050] The precursor polyisocyanate (hereinafter simply referred to as "precursor," etc.) is derived from aliphatic and / or alicyclic diisocyanates. Aliphatic and / or alicyclic diisocyanates are compounds that do not contain a benzene ring in their structure. Examples of aliphatic diisocyanate monomers include those having 4 to 30 carbon atoms, and preferred alicyclic diisocyanate monomers have 8 to 30 carbon atoms. Examples include tetramethylene-1,4-diisocyanate, pentamethylene-1,5-diisocyanate, hexamethylene diisocyanate, 2,2,4-trimethyl-hexamethylene-1,6-diisocyanate, lysine diisocyanate, isophorone diisocyanate, 1,3-bis(isocyanatemethyl)cyclohexane, and 4,4'-dicyclohexylmethane diisocyanate. Among these, hexamethylene diisocyanate (hereinafter referred to as "HDI") and isophorone diisocyanate (hereinafter referred to as "IPDI") are preferred due to their weather resistance and ease of industrial availability, with HDI being more preferred. They may be used alone or in combination.

[0051] The precursor can have various types of bonds. Specifically, these include isocyanurate groups, uretdione groups, urethane groups, allophanate groups, biuret groups, iminooxadiazinedione groups, etc., and two or more types may be included. Among these, it is preferable to have an isocyanurate group represented by the following formula (1). Having an isocyanurate group makes it easier to obtain a coating film with excellent weather resistance and gloss.

[0052] [ka]

[0053] Furthermore, the precursor may contain a uretdione dimer consisting of two diisocyanate monomer molecules. The uretdione group consists of two isocyanate groups and is represented by the following formula (2).

[0054] [ka]

[0055] Furthermore, the precursor may have a urethane group and / or an allophanate group. A urethane group is formed from a hydroxyl group and an isocyanate group and is represented by formula (3) below, and an allophanate group is formed from a hydroxyl group and two isocyanate groups and is represented by formula (4) below.

[0056] [ka]

[0057] [ka]

[0058] The bonds and chemical structures that may be contained in such precursors, including those other than those mentioned above, can be formed using known methods.

[0059] The hydrophilic group of the water-dispersible polyisocyanate (b) is not particularly limited and can be anionic or nonionic. Examples of anionic groups include carboxylic acid groups, sulfonic acid groups, and phosphate groups. Examples of nonionic groups include polyethylene glycol type and polyhydric alcohol type. Preferred hydrophilic groups are nonionic groups, carboxylic acid groups, and sulfonic acid groups.

[0060] The following describes how to introduce hydrophilic groups. To introduce the hydrophilic groups exemplified above into the precursor, compounds having both hydrophilic groups and active hydrogen groups are used. Active hydrogen groups are functional groups that react with isocyanate groups, such as hydroxyl groups, mercapto groups, and carboxylic acid groups. Compounds having both anionic groups and active hydrogen groups include oxyacids, which have both hydroxyl and carboxylic acid groups, such as glycolic acid, lactic acid, tartaric acid, citric acid, oxybutyric acid, oxyvaleric acid, hydroxypivalic acid, dimethylolacetic acid, dimethylolpropionic acid, dimethylolbutanoic acid, and polycaprolactone, which has a carboxylic acid group. Compounds having both hydroxyl and sulfonic acid groups include, for example, isethionic acid. Compounds having both a mercapto group and a carboxylic acid group as active hydrogen groups include mercaptoacetic acid, which is a mercaptocarboxylic acid. A compound having both a nonionic group and an active hydrogen group is polyethylene oxide.

[0061] Polyethylene oxide can be obtained, for example, by adding ethylene oxide to a monoalcohol such as methanol, ethanol, or butanol, and may also contain propylene oxide.

[0062] Hydroxyl groups are preferred as the active hydrogen groups. Anionic groups are preferably neutralized with organic amines or inorganic bases. This neutralization imparts water dispersibility and water solubility to the aqueous polyisocyanate composition.

[0063] Examples of organic amine compounds include linear, branched, primary, secondary, or tertiary amines with 1 to 20 carbon atoms, such as methylamine, ethylamine, propylamine, isopropylamine, butylamine, 2-ethylhexylamine, cyclohexylamine, dimethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, trimethylamine, triethylamine, triisopropylamine, tributylamine, and ethylenediamine; cyclic amines such as morpholine, N-alkylmorpholine, and pyridine; and hydroxyl-containing amines that have a hydroxyl group in their molecule, such as monoisopropanolamine, methylethanolamine, methylisopropanolamine, dimethylethanolamine, diisopropanolamine, diethanolamine, triethanolamine, diethylethanolamine, and triethanolamine. Examples of inorganic bases include ammonia, potassium hydroxide, and sodium hydroxide.

[0064] The reaction to introduce hydrophilic groups into the precursor can be carried out under known conditions and can be adjusted as appropriate. Generally, it can be carried out at -20 to 150°C, but preferably at 30 to 100°C. Lowering the temperature to 150°C can suppress side reactions, while raising it to -20°C or higher can prevent a decrease in the reaction rate. Furthermore, organometallic salts such as tin, zinc, and lead, as well as tertiary amine compounds and alkali metal alkoxides such as sodium may be used as catalysts in the reaction.

[0065] Furthermore, nonionic, anionic, cationic, or amphoteric surfactants can be added depending on the purpose, such as improving water dispersibility.

[0066] Specific trade names for the water-dispersible polyisocyanate (b) in the present invention include, for example, Barnock DNW-5000, Barnock DNW-5500, Barnock DNW-5010, Barnock DNW-5100, Barnock DNW-5200, Barnock DNW-6000, Barnock DNW-6500 from DIC Corporation, Takenate WD-725, Takenate WD-726, Takenate WD-730 from Mitsui Chemicals, Inc., and Duranate WB40-100, Duranate WB40-80D, Duranate WT30-100, Duranate WT20-100, Duranate WE50-100, Duranate WL70-100, Duranate W from Asahi Kasei Chemicals Corporation. Examples include R80-70P, Bahijur 3100, Bahijur XP2451, Bahijur 304, Bahijur 305, Bahijur XP2700, Bahijur XP2547, Bahijur 2487 / 1, Bahijur 2655, Bahijur 401-70, Bahijur XP2759, Desmodule N3300, Desmodule N3600, Desmodule N3900, Desmodule N3400, Desmodule XP2580, Desmodule XP2565 from Sumitomo Bayer Urethane Co., Ltd., and Coronate AQ-200, Coronate AQ-210, Coronate AQ-120, Coronate AQ-105 from Nippon Polyurethane Co., Ltd.

[0067] [Organic solvents] In the aqueous coating composition of the present invention, the water-dispersible curing agent (II) may contain an organic solvent in addition to the water-dispersible polyisocyanate (b) for purposes such as reducing the viscosity of the coating. Specific examples of organic solvents are not limited as long as they are within the scope of the objectives of the present invention. Examples include 1-methylpyrrolidone, methyl ethyl ketone, acetone, methyl isobutyl ketone, propylene glycol monomethyl ether acetate, diethylene glycol dimethyl ether, ethyl acetate, isopropyl acetate, butyl acetate, toluene, xylene, pentane, iso-pentane, hexane, iso-hexane, cyclohexane, solvent naphtha, and mineral spirits. Two or more of these can be used in combination. The organic solvent is preferably one with a water solubility of 5% by mass or more. Using an organic solvent with a water solubility of 5% by mass or more helps to prevent a decrease in the water dispersibility of the water-dispersible curing agent (II). Furthermore, it is preferable that the organic solvent has a boiling point of 100°C or higher. Using an organic solvent with a boiling point of 100°C or higher allows for relatively slow evaporation of the organic solvent during coating film formation, thus minimizing the impact on the surface appearance of the coating film. The amount of organic solvent used is not limited as long as it is within the scope of the present invention, but it is preferably 0 to 60% by mass of the water-dispersible curing agent (II), more preferably 0 to 50% by mass, and even more preferably 0 to 45% by mass.

[0068] [Other hardening agents] Furthermore, the aqueous coating composition of the present invention may contain curing agents other than the water-dispersible polyisocyanate (b) described above. Other curing agents include, for example, melamine compounds and compounds having alkoxysilyl groups. In this case, it is preferable that the curing components other than the water-dispersible polyisocyanate resin be 50% by mass or less in the water-dispersible curing agent (II).

[0069] [Other resins] Other resins that may be included in the aqueous coating composition of the present invention include, for example, silicone resins, polyester resins, fluororesins, rosin resins, petroleum resins, coumarone resins, phenolic resins, urethane resins, melamine resins, urea resins, cellulose resins, xylene resins, alkyd resins, aliphatic hydrocarbon resins, butyral resins, maleic acid resins, fumaric acid resins, vinyl resins, amine resins, ketimine resins, and the like.

[0070] [Other additives] The aqueous coating composition of the present invention may contain, depending on the purpose, other additives such as curing accelerators, matting agents, flexibility agents, surface modifiers, wetting agents, dispersants, emulsifiers, viscosity modifiers, settling inhibitors, anti-skinning agents, anti-sagging agents, defoaming agents, color separation inhibitors, leveling agents, drying agents, plasticizers, rust inhibitors, film-forming aids, antifungal agents, antibacterial agents, insecticides, light stabilizers, UV absorbers, electrostatic lubricants, antistatic agents, and conductivity modifiers as appropriate. Commercially available products can be suitably used for these components.

[0071] 2. Method for forming a multi-layer coating, multi-layer coating The present invention provides a method for forming a multilayer coating film, comprising at least an undercoat film and one or more coating films on the undercoat film, The process includes (X) applying a primer paint composition to the object to be coated to form a primer film, The method is characterized by comprising the step (Y) of applying an aqueous paint composition onto the primer film to form one or more coating films. Here, the aqueous paint composition may be one of the embodiments shown in "1. Aqueous Paint Composition" above.

[0072] Furthermore, the multilayer coating of the present invention comprises at least a primer coating and one or more coatings on the primer coating, wherein any or all of the one or two or more coatings are formed from the aqueous paint composition described above.

[0073] Specific examples of the multilayer coating method and multilayer coating of the present invention include embodiments (I) to (IV) in which a primer coating is formed on a workpiece, and one or more coatings selected from the group consisting of an intermediate coating and a topcoat are formed thereon. That is, the forming method of the present invention includes a step (X) of applying a primer paint composition to a workpiece to form a primer coating, and a step (Y) of applying the aforementioned aqueous paint composition on the primer coating to form one or more coatings. In this case, as will be described later, steps (X) and (Y) may each form multiple coatings. As illustrated in embodiments (I) to (IV) below, step (X) may include a step of forming a putty coating, and step (Y) may include a step of forming one or more coatings, such as having only a topcoat, having an intermediate coating and a topcoat, or having one intermediate coating layer and two topcoat layers. Furthermore, process (Y) may include a process (Y') in which a clear coating film is formed by applying a clear paint, and the coating film labeled Y' shown below is an example of a clear coating film. Furthermore, the method for forming a multilayer coating film and the multilayer coating film of the present invention are not limited to the embodiments (I) to (IV) described above. (I) Primer film (X-1), topcoat film (Y-1) [clear film (Y'-1)] (II) Primer film (X-1), intermediate coat film (Y-1), topcoat film (Y-2) [clear coat film (Y'-2)] (III) Primer film (X-1), putty film (X-2), intermediate coat film (Y-1), topcoat film (Y-2) [clear coat film (Y'-2)] (IV) Primer film (X-1), putty film (X-2), intermediate coat film (Y-1), topcoat film (Y-2), topcoat film (Y-3), [clear coat film (Y'-3)]

[0074] Among the preferred embodiments described above, for example, in applications such as heavy vehicles like railway cars, but where the undercarriage parts such as bogies require protection by paint but do not require decoration with colored paint, one example is the method described in embodiment (I) above, in which a primer film (X-1) is formed on the object to be painted, and then a topcoat film (clear film) is formed mainly to provide stain resistance and weather resistance.

[0075] On the other hand, in applications where decoration and durability by paint are required, such as on the exterior panels, roofs, and skirts of heavy vehicles like railway cars, it is preferable to form at least a first and second coating film as a covering film on the undercoat film, in order to further ensure aesthetic appeal, durability, and weather resistance, and it is preferable to adopt the embodiments of (II) to (IV) above, which form an intermediate coating film and a topcoat film. In particular, in the case of large objects to be coated that require scratch resistance in the operating environment, such as Shinkansen trains, railway cars, and construction machinery vehicles, it is a more preferable embodiment to form a putty coating film in addition to the undercoat film, as in embodiments (III) and (IV) above, for purposes such as improving aesthetics and forming a smooth surface, or to prevent the undercoat film from peeling off the object when subjected to stress such as impact, thereby protecting the object. In particular, in cases where high design quality and durability are required, such as the outer panels of a Shinkansen, it is preferable to apply a primer film, a putty film, an intermediate film, and a topcoat film, and then finish the topcoat film in multiple colors by using a colored film and a clear film, as in the embodiment described in (IV) above. Furthermore, the present invention's method for forming a coating and multilayer coating may include not only embodiments in which the upper coating is directly formed on the lower coating, but also embodiments in which the upper coating is indirectly formed on the lower coating. That is, in the present invention's method for forming a coating and multilayer coating, for example, other known layers or coatings may exist between the undercoat coating and one or more coatings, and between the undercoat coating and the bottommost coating of the coatings, excluding those coatings.

[0076] The following sections will primarily explain the primer film, putty film, intermediate film, and topcoat film.

[0077] [Subject to be coated] The substrates to be coated are not particularly limited and include, for example, metal substrates such as steel, zinc, aluminum, copper, tin, and stainless steel; concrete substrates; and plastic substrates such as polyvinyl chloride, polycarbonate, polyolefin, acrylic, and ABS. In particular, for railway vehicle structures, metal substrates assembled by welding or other processes using metal materials such as steel plates, aluminum materials, and stainless steel materials are commonly used. Substrates that have undergone surface treatment to further improve corrosion resistance can also be applied. Examples of such surface treatments include zinc-rich paint, zinc plating, and metal spraying. Furthermore, metal substrates may have undergone pretreatment such as blasting or chemical treatment before painting to remove rust or improve adhesion. The shape of the substrate can vary depending on its application and may include, for example, plate-like, uneven, three-dimensional, or irregular shapes. The substrates to be coated may be used individually or in combination of two or more types.

[0078] [Undercoat film] In the method of forming the present invention, a primer coating film is formed on the object to be coated as a first coating film. That is, the multi-layer coating film of the present invention has at least a primer coating film. The primer coating film may be formed using any of solvent-based paints, water-based paints, or energy-ray curing paints, but as mentioned above, since the embodiment includes one in which a water-based paint is used as the paint applied on top of it, it is more preferable that the paint used to form the primer coating film is also a water-based paint. By providing the primer coating film, the adhesion to the object to be coated, as well as water resistance, impact resistance, corrosion resistance, etc., can be improved.

[0079] The primer paint composition for forming the undercoat film is applied according to the type and condition of the surface to be coated. By providing an undercoat film between the object to be coated and one or more coating films formed by the aqueous paint composition of the present invention, as described later, the adhesion between the surface to be coated and the coating films, as well as the water resistance and impact resistance of the multi-layer coating, can be improved.

[0080] The undercoat film may be a multi-layered film of two or more layers to improve adhesion. As an undercoat film, for example, when the object to be coated is a metal substrate, a coating film can be obtained by applying and drying a coating composition known in the field of paints, called a primer coating and / or intermediate coating.

[0081] Specifically, the components of the primer paint composition that constitutes the undercoat coating film include, for example, paint compositions containing film-forming resins, crosslinking agents, coloring pigments, extender pigments, gloss pigments, rust-preventive pigments, conductive pigments, etc. Furthermore, the primer paint composition may appropriately contain various known additives such as ultraviolet absorbers, light stabilizers, curing catalysts, plasticizers, adhesion promoters, compatibilizers, defoamers, viscosity modifiers, rust inhibitors, preservatives, surface modifiers, and silane coupling agents.

[0082] As for the main types of film-forming resins, it is preferable to use those that can be used in water-based paints, especially for the purposes of the present invention. While not limited, examples include acrylic resins, polyester resins, alkyd resins, urethane resins, polyolefin resins, epoxy resins, phenolic resins, polyamide resins, and modified resins thereof, as well as hydroxyl group-containing resins, alkyl silicates, etc. These can be used individually or in combination of two or more.

[0083] Furthermore, incorporating a silane coupling agent into the primer paint composition is preferable because it provides excellent adhesion to the substrate and the coating film on top of the primer paint film, as well as improving the coating performance as a multi-layer coating. In the case of a two-component primer paint composition, the silane coupling agent may be included in either the main component or the hardener, or in both. Also, if it is necessary to avoid interference with other components such as the film-forming resin used, the silane coupling agent may be prepared separately from the other components such as the film-forming resin used, either as part of the main component or the hardener, or prepared using other formulations. Known silane coupling agents can be used without limitation. Among these, it is preferable to use a silane coupling agent containing an amino group and / or a silane coupling agent containing an epoxy group, and it is more preferable to include a silane coupling agent containing an amino group.

[0084] Examples of silane coupling agents containing an amino group include γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-aminopropyltriisopropoxysilane, γ-aminopropylmethyldimethoxysilane, γ-aminopropylmethyldiethoxysilane, N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane, N-β-(aminoethyl)-γ-aminopropyltriethoxysilane, and N-β-(aminoethyl)-γ-aminopropylmethyldiethoxysilane. Examples include sisilane, N-β-(aminoethyl)-γ-aminopropyltriisopropoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, γ-ureidopropyltrimethoxysilane, γ-anilinopropyltrimethoxysilane, γ-ureidopropyltrimethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, N-vinylbenzyl-γ-aminopropyltriethoxysilane, N-β-(aminoethyl)-8-aminooctyltrimethoxysilane, and γ-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine. Examples of silane coupling agents containing epoxy groups include glycidoxymethyltrimethoxysilane, glycidoxymethyltriethoxysilane, β-glycidoxyethyltrimethoxysilane, β-glycidoxyethyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyldimethylmethoxysilane, γ-glycidoxypropyl(ethyl)dimethoxysilane, β-3,4-epoxycyclohexylethyltrimethoxysilane, β-3,4-epoxycyclohexylethyltriethoxysilane, 8-glycidoxyoctyltrimethoxysilane, 8-glycidoxyoctylmethyldimethoxysilane, and 8-glycidoxyoctylmethyldiethoxysilane.

[0085] Furthermore, crosslinking may be achieved by incorporating a hardening agent, or the material may remain essentially uncrosslinked without incorporating a hardening agent. Alternatively, crosslinking may be achieved by the penetration of a crosslinking agent contained in the upper coating film.

[0086] As the primer paint composition, either an organic solvent-based paint composition or a water-based paint composition may be used, and it may also be solvent-free. However, considering the impact on the environment and human health, it is preferable to use a high-solids content (solids content of 70% or more) organic solvent-based paint, water-based paint, or solvent-free paint.

[0087] In particular, as a primer paint composition, (X-1) Water-based paint composition containing epoxy resin (X-2) Putty composition containing unsaturated polyester resin, (X-3) Urethane-curing water-based paint composition It is preferable that it be at least one selected from the following.

[0088] The composition of the primer paint can preferably be those described above or known compositions. In particular, it is preferable to form the first undercoat film using an aqueous paint composition (X-1) containing epoxy resin, due to its excellent adhesion to metal substrates and to intermediate and topcoat paints. Furthermore, when the undercoat paint composition is an aqueous paint composition, it is preferable to include acrylic resin from the viewpoint of excellent adhesion to the aqueous paint composition of the present invention used to form the intermediate and topcoat films. Acrylic resin is less water-absorbent than epoxy resin and has excellent compatibility with epoxy resin, resulting in excellent secondary adhesion when a multi-layer coating is formed. In addition, as will be described later, it is preferable to include pigment. Epoxy resins, acrylic resins, and pigments are explained below.

[0089] [Epoxy resin] The epoxy equivalent of the epoxy resin is not limited, but is preferably 100 to 1400 g / eq. Two or more epoxy resins with different epoxy equivalents may be used in combination. From the viewpoint of the finish, curability, and corrosion resistance of the formed coating film, the epoxy equivalent is more preferably 300 to 1400 g / eq, even more preferably 500 to 1400 g / eq, and even more preferably 800 to 1400 g / eq.

[0090] The epoxy resin is not limited as long as it can be used in water-based paints. For example, it is a resin having at least two epoxy groups in one molecule, obtained by reacting a polyhydric alcohol or polyhydric phenol with a halohydrin. The epoxy resin may be used alone or in combination of two or more types.

[0091] Furthermore, the epoxy resin may be a modified and / or copolymerized epoxy resin. Examples include a vinyl-modified epoxy resin aqueous solution obtained by copolymerizing a polymerizable unsaturated group-containing modified epoxy resin, which is obtained by reacting a glycidyl group-containing monomer and amines, with a carboxyl group-containing vinyl monomer, and then neutralizing the copolymer with a basic compound and dispersing or dissolving it in water; a vinyl-modified epoxy resin; and an acrylic-modified epoxy resin.

[0092] Epoxy resins are preferably formulated in the form of epoxy resin emulsions or epoxy resin dispersions. An epoxy resin emulsion refers to an emulsion obtained by dispersing epoxy resin in an aqueous medium mainly composed of water, while an epoxy resin dispersion refers to a dispersion obtained by dispersing epoxy resin in an aqueous medium mainly composed of water. Here, the epoxy resin emulsion is not particularly limited, but it is prepared by emulsifying the epoxy resin in an aqueous medium mainly composed of water using a conventional forced emulsification method (a method using an emulsifier and a high-speed stirrer, etc.). Commercially available epoxy resin emulsions and epoxy resin dispersions can be used.

[0093] From the viewpoint of the curability and corrosion resistance of the coating film, the epoxy resin content is preferably 5 to 60% by mass, more preferably 10 to 50% by mass, and even more preferably 15 to 40% by mass, in terms of the nonvolatile content contained in the primer coating composition (water-based coating composition). Furthermore, the epoxy resin is preferably 5 to 90% by mass, more preferably 10 to 80% by mass, even more preferably 15 to 70% by mass, and even more preferably 20 to 60% by mass, relative to the total mass of the epoxy resin and the acrylic resin described later.

[0094] [Acrylic resin] There are no restrictions on the acrylic resin mentioned above, and known acrylic resins used in aqueous paint compositions are included. The acrylic resin is a polymer of acrylic acid esters or methacrylic acid esters (hereinafter, these may be referred to as (meth)acrylic acid esters), and examples include polymers obtained by polymerizing one or more acrylic components selected from acrylic acid, methacrylic acid, and their esters, amides, and nitriles, and further includes polymers obtained by polymerizing an acrylic component with a non-acrylic component such as styrene.

[0095] Furthermore, it is preferable that the acrylic resin is dispersed in the primer paint composition (aqueous paint composition) in the form of an emulsion. For this reason, it is preferable to use an acrylic resin emulsion when preparing this aqueous paint composition. An acrylic resin emulsion refers to an emulsion in which the acrylic resin is stably dispersed in water, and may contain additives such as emulsifiers as needed. Paints in which the resin is dispersed in the form of an emulsion in this way are also called emulsion paints.

[0096] Acrylic resin emulsions can be prepared, for example, by emulsion polymerization in water using water as a medium. Alternatively, they may be prepared by emulsifying the acrylic resin in water while applying forced shear force using, for example, a high-speed stirrer. Furthermore, acrylic resin emulsions can be prepared by performing a phase conversion in water on an acrylic resin polymerized in an organic solvent medium, and the organic solvent contained in the acrylic resin emulsion may be removed by distillation or other means as needed.

[0097] Modified acrylic resins may be used. Examples of modified acrylic resins include silicone-modified acrylic resins, urethane-modified acrylic resins, and alkyd-modified acrylic resins.

[0098] From the viewpoint of the curability, moisture resistance, conformability, and secondary adhesion of the coating film, the acrylic resin content is preferably 5 to 90% by mass, more preferably 10 to 80% by mass, and even more preferably 15 to 70% by mass, in terms of the non-volatile content contained in the primer coating composition (water-based coating composition). Furthermore, the acrylic resin is preferably 15 to 98% by mass, more preferably 20 to 90% by mass, even more preferably 30 to 85% by mass, and even more preferably 40 to 80% by mass, relative to the total mass of the epoxy resin and acrylic resin mentioned above.

[0099] [Mixed form of epoxy resin and acrylic resin] The primer paint composition (water-based paint composition) preferably contains an epoxy resin, or an epoxy resin and an acrylic resin, as described above. However, when it contains both an epoxy resin and an acrylic resin, these resins may be used as pre-mixed resins, or they may be separately compounded and mixed together when preparing the water-based paint composition. For example, an emulsion blend produced by the emulsion blending method, in which an epoxy resin emulsion and an acrylic resin emulsion are blended in the predetermined mass ratio described above, can be used.

[0100] Furthermore, it is preferable to use epoxy resins and acrylic resins with improved compatibility and dispersibility. For example, epoxy resins and acrylic resins that have been substantially integrated beforehand by dispersion, compatibility, or bonding through one or more methods such as copolymerization, mixing, and modification can be used. For example, epoxy-acrylic composite resin particles in which epoxy resin and acrylic resin are dispersed or compatible and substantially integrated are exemplified. These states only need to have a higher degree of dispersion or compatibility, and the two do not necessarily need to be compatible. For example, the epoxy resin domains may be uniformly dispersed or compatible and coexist in a form where they are evenly distributed on both the central and surface sides of the acrylic resin particles. Such epoxy-acrylic composite resin particles can be manufactured using known methods. For example, a method is exemplified in which monomers constituting the acrylic resin (which become constituent units) are emulsion polymerized in an aqueous medium containing water or a hydrophilic solvent such as water and methanol, in the presence of the epoxy resin.

[0101] [Pigments] Furthermore, the primer paint composition (water-based paint composition) preferably contains a pigment. The pigment is not limited, and those used in the aforementioned water-based main agent (I) can be used. In this case, the pigment volume concentration (PVC, volume%), which is the proportion of the pigment in the solid content of the paint composition, is preferably 6% to 24%. More preferably 6% to 20%, even more preferably 7% to 18%, and even more preferably 10% to 15%.

[0102] The use of pigments and the preference for PVC are for the following reasons: In other words, in water-based paints using water-based epoxy resins, conventional primers with a high pigment content do not have sufficient density in the paint film, making them prone to peeling with acidic or alkaline cleaning agents. However, we hypothesized that increasing the resin content in the paint film could improve the density of the film and thus improve chemical resistance. As a result, we found that the pigment concentration in the paint film correlates with chemical resistance, and that chemical resistance can be improved by setting the pigment concentration below the aforementioned preferred upper limit. Furthermore, we found that while chemical resistance can be ensured if the pigment concentration is too low, corrosion resistance tends to be poor. Therefore, we also confirmed that both chemical resistance and corrosion resistance can be improved by setting the pigment concentration above the aforementioned preferred lower limit.

[0103] Furthermore, it was confirmed that coatings with low pigment concentrations tend to have poor secondary adhesion because the resin easily absorbs water, causing water to accumulate within the coating. In particular, it was confirmed that secondary adhesion could not be ensured when a multi-layer film was formed by combining it with an intermediate coat or topcoat. This is thought to be because epoxy resin easily absorbs moisture, so when the pigment concentration in the coating is lowered and the amount of epoxy resin is increased, moisture tends to accumulate within the coating. When a multi-layer film is formed, moisture in the undercoat layer does not completely escape, and the undercoat film is likely to swell and peel off from the substrate. As mentioned above, it is preferable to add acrylic resin, which has excellent compatibility with epoxy resin. Using the aforementioned preferred PVC results in excellent corrosion resistance and chemical resistance when painted on metal substrates, as well as excellent secondary adhesion when a multi-layer film is formed.

[0104] Furthermore, the preferred range (embodiment) of PVC in such primer paint compositions (aqueous paint compositions) is a preferred range (embodiment) when forming a water-based multi-layer coating on substrates requiring a high level of chemical resistance and corrosion resistance, such as railway vehicles, and does not apply to other substrates or other applications. In other words, for other substrates or other applications, as described above, primer paints containing PVC of a similar degree to conventional primers or primers containing other resins can also be used. That is, primer paints containing PVC of a similar degree to conventional primers or primers containing other resins can be used in appropriate combination with the aqueous paint composition of the present invention used for forming the coating film.

[0105] [Water absorption rate] Regarding the characteristics described above, it is preferable that the primer paint composition (water-based paint composition) has a low water absorption rate when formed as a paint film. The water absorption rate is preferably 15% or less, more preferably 14% or less, and even more preferably 13% or less.

[0106] [Other ingredients] The preferred primer composition (water-based paint composition) may contain other components in addition to the aforementioned pigments, epoxy resins, and acrylic resins. Examples of these other components include, as mentioned above, curing agents, resins other than the epoxy resins and acrylic resins, and other additives.

[0107] [Hardening agent] The primer paint composition (water-based paint composition) preferably contains a curing agent. The curing agent is preferably an amine resin. The amine resin is not particularly limited; for example, amine compounds are used, and from the viewpoint of reactivity with the epoxy resin, polyamine compounds containing two or more amino groups per molecule are preferred. Known polyamine compounds can be used. The amine resin may be used alone or in combination of two or more types.

[0108] The amine resin is preferably formulated in the form of an amine resin emulsion, an amine resin dispersion, and / or an aqueous solution of the amine resin. An amine resin emulsion refers to an emulsion obtained by dispersing the amine resin in an aqueous medium mainly composed of water, and an amine resin dispersion refers to a dispersion obtained by dispersing the amine resin in an aqueous medium mainly composed of water. Commercially available amine resins in the form of emulsions, dispersions, or aqueous solutions can be suitably used.

[0109] From the viewpoint of curability, corrosion resistance, etc., it is preferable that the amount of amine resin is such that the amount of active hydrogen in the amine resin is in the range of 0.5 to 1.2 equivalents per equivalent of epoxy groups in the epoxy resin.

[0110] Furthermore, the aqueous coating composition of the present invention may contain curing agents other than the amine resin. If the acrylic resin contains hydroxyl groups, other curing agents such as blocked isocyanate resins and melamine resins can be used. The resins used are not particularly limited, and known resins can be used.

[0111] Furthermore, as mentioned above, it is preferable that a silane coupling agent be included in the primer paint composition. However, in cases where other components, such as a film-forming resin, are used as the main component, and the silane coupling agent used is reactive with these other components, it is preferable in this embodiment that the silane coupling agent is prepared to be incorporated into the curing agent. In other words, in the case of the aqueous paint composition (X-1) containing the epoxy resin, when a curing agent is used, it is preferable in this embodiment that a silane coupling agent containing an amino group that is reactive with the epoxy resin is prepared to be incorporated into the curing agent separately from the main component.

[0112] In a preferred embodiment, after forming a first primer coating with an epoxy resin-containing aqueous paint composition (X-1), a putty coating is formed on top of it using a putty composition (X-2) containing an unsaturated polyester resin. As mentioned above, having a putty coating improves aesthetics, creates a smooth surface, and protects the coated object by preventing the first primer coating from peeling off when subjected to stress such as impact. The putty coating can be formed on a part or the entire surface of the first primer coating applied to areas where there are significant irregularities due to the joints between coated objects. For example, an embodiment may be formed on the first primer coating by using a trowel, spatula, roller, etc., to smooth the coated surface.

[0113] As the unsaturated polyester resin for the paint that forms such a putty coating, any known resin can be used without limitation. Among the unsaturated polyester resins, for example, an unsaturated polyester resin obtained by reacting (a) one or more unsaturated polybasic acid components, (b) one or more polyhydric alcohol components, and (c) one or more monobasic acid components or their ester components is more preferred. For (a) to (c), any known components can be used without limitation, and for each, only one type may be used, or two or more types may be used in combination.

[0114] The paint containing the unsaturated polyester resin that forms the putty coating film described above usually contains at least one curing agent to cure the unsaturated polyester resin. The curing agent is not particularly limited, but examples include diacyl peroxide, peroxyester, hydroperoxide, dialkyl peroxide, ketone peroxide, peroxyketal, alkyl perester, and parkervonate types. The curing agent may be used alone or in combination of two or more types.

[0115] The paint used to form the putty film may, if necessary, contain solvents such as organic solvents, and various additives such as pigment dispersants, anti-settling agents, curing catalysts, defoamers, antioxidants, and ultraviolet absorbers.

[0116] The application of the paint that forms the putty film is not particularly limited, but can be done using various trowels and spatulas. The drying time after painting is not particularly limited, but is preferably set at a temperature of 5 to 35°C. The drying time for the paint can be 2 to 24 hours at 23°C and 50% relative humidity. The putty film may be sanded after formation as needed. Methods of sanding include manual sanding using sandpaper and using power tools such as disc sanders. Sanding is usually done after hardening and drying. In addition, the putty film is preferably adjusted to a dry film thickness of 3 to 5 mm in order to correct the distortion of the structure. Typically, 3 to 6 coats (with a dry film thickness of 1 mm or less per coat) are applied to obtain a dry film thickness of 3 to 5 mm.

[0117] Furthermore, a urethane-curing aqueous paint composition (X-3) may be used as the primer paint composition. Such a urethane-curing aqueous paint composition may be, for example, a paint containing an aqueous main component and a water-dispersible curing agent as shown in Patent Document 2 and International Publication No. 2010 / 149169, which is also used in the intermediate coat paint described later, or a similar product may be used as appropriate.

[0118] The film thickness of each primer coating can be adjusted as appropriate depending on the condition of the surface to be coated, but generally, the dry film thickness is preferably in the range of 5 to 150 μm, 5 to 100 μm, and especially 7 to 60 μm, except for the primer coating (X-2). The putty composition (X-2) containing unsaturated polyester resin allows for relatively thick film coating, and generally, the dry film thickness can be appropriately set within the range of 500 μm to 2000 μm (0.5 mm to 2 mm), and especially 700 μm to 1500 μm (0.5 mm to 1.2 mm).

[0119] Conventional painting methods can be used to form the primer film, but spray painting, roller painting, or brush painting are preferred in terms of the finish of the primer film. There are no particular restrictions on drying after painting, but it is preferable to use a temperature condition of, for example, 5 to 35°C. The drying time of the paint can be 2 to 24 hours at, for example, 23°C and 50% relative humidity.

[0120] [Intermediate coating film] In the formation method of the present invention, the intermediate coating film is included in one or more coating films formed in step (Y). In particular, in the present invention, it is formed for purposes such as increasing the total film thickness of the multi-layer coating film to improve durability and improving the aesthetic appearance of the top coating film. Furthermore, when a putty coating film is used, it is preferable that the intermediate coating film is a coating film that suppresses absorption into the putty coating film as a paint on top of the putty coating film. Moreover, it is preferable that the coating film has excellent abrasive properties as it plays a role in making sandpaper marks less noticeable when polishing the putty coating film. Furthermore, it is preferable that the adhesion between the coating layers from the undercoat film to the topcoat film is excellent when forming the topcoat film on the intermediate coating film. That is, in the multi-layer coating film of the present invention, an embodiment is given in which the intermediate coating film is one or more coating films on top of the undercoat film. The intermediate coating film may be a single layer as the first coating film (first coating film) in the coating film, or it may be two layers consisting of the first coating film and the second coating film, or it may be formed from two or more layers. More preferably, the intermediate coating film is formed as a single layer as the first coating film (first coating film) in the coating film.

[0121] The paint used to form the intermediate coating film is not limited to water-based paints, but it is preferable that it be formed using the water-based paint composition of the present invention as described above. As described above, the intermediate coating film formed using the water-based paint composition of the present invention has sufficient pot life and drying properties, resulting in good paint workability and excellent coating performance.

[0122] On the other hand, the intermediate coating paint may be a commonly used, well-known type. Such intermediate coating paints include one-component or multi-component paints of two or more components, such as acrylic urethane resin-based paints, acrylic silicone resin-based paints, urethane resin-based paints, and acrylic resin-based paints. However, from the viewpoint of excellent adhesion to the undercoat film and the toughness of the coating film, acrylic urethane resin-based paints are preferred. Acrylic urethane resin-based paints are generally paints that use acrylic polyol as the main component and polyisocyanate resin as the curing agent, and known types can be used as appropriate. While not limited, for example, paints containing an aqueous main component and a water-dispersible curing agent, as shown in Patent Document 2 and International Publication No. 2010 / 149169, can be used as an example, and similar paints can be used as appropriate.

[0123] Here, the intermediate coating film is not limited, but if it contains pigment, it is preferable that it has a higher PVC content than the undercoat film. The PVC content of the intermediate coating film is preferably 20-50%, and more preferably 30-40%.

[0124] Regarding the method for forming the intermediate coating film, conventionally known painting methods can be used, similar to those for the primer coating film described above. However, from the viewpoint of finish quality, spray painting, roller painting, brush painting, etc., are preferred. There are no particular restrictions on drying after painting, but for example, a temperature condition of 5 to 35°C is preferred. The drying time of the paint can be, for example, 2 to 24 hours at 23°C and 50% relative humidity. The film thickness of the intermediate coating film can be appropriately adjusted depending on the condition of the object to be coated and the primer coating film, but from the viewpoint of the above objective, the dry film thickness is preferably 40 to 70 μm, more preferably 40 to 60 μm, and even more preferably 50 to 60 μm.

[0125] [Topcoat film] The formation method and the topcoat in the multilayer coating of the present invention are included in one or more coating films formed in step (Y). In the present invention, they are often used for the purpose of providing stain resistance and weather resistance, or for decorative purposes. As illustrated in the embodiments (I) to (IV) described above, they are coating films formed on top of the previously formed undercoat, or they are coating films formed on top of the intermediate coating. That is, the multilayer coating of the present invention includes embodiments having one or more topcoat films as coating films on top of the undercoat, and embodiments having one or more intermediate coating films on top of the undercoat, with one or more coating films formed thereon. In other words, the topcoat may be a single layer as the first coating film (first coating film) in the coating film, or it may be two layers consisting of a first coating film and a second coating film, or it may be formed of two or more layers. More preferably, the topcoat is formed in one or two layers as a second or higher coating film (second coating film) in a coating film formed on the intermediate coating film.

[0126] The topcoat film is not particularly limited, but may contain pigments or be a clear coating without pigments. As pigments, those used in the aforementioned aqueous coating compositions can be used, but it is especially preferable that it contains coloring pigments and / or luminescence pigments. Depending on the application, inorganic pigments such as titanium dioxide and iron oxide, and organic pigments such as phthalocyanine and azo pigments are preferably used.

[0127] Regarding the topcoat, examples include a one-coat finish using only a base coat containing coloring pigments and / or gloss pigments, a two-coat finish (multicolor finish) using the base coat (first coloring paint) followed by a second coloring paint of a different color from the base coat, or a two-coat finish using the base coat and a clear coat (topcoat) that does not contain pigments. A multicolor finish with two or more coats is also possible, and the clear coat (topcoat) may be applied in two or more coats. Furthermore, the clear coat (topcoat) may be a colored clear coat or a pearl clear coat, and in any case, conventionally known painting methods can be used, and the choice can be made appropriately depending on the desired color and texture.

[0128] Here, the topcoat film is used either as a first coating film as described above, or as a second or higher coating film formed on the intermediate coating film. Regardless of this, the PVC is not limited when it contains pigment. When the PVC in the topcoat film contains pigment, it is preferably 10-40%, and more preferably 10-20%. When the clear coating without pigment is used, it can be adjusted as appropriate according to the opacity and other factors.

[0129] The topcoat film is formed using the aqueous paint composition of the present invention described above. As described above, the topcoat film formed using the aqueous paint composition of the present invention has sufficient pot life and drying properties, resulting in good paintability and excellent coating performance.

[0130] On the other hand, the topcoat film may be formed by a combination of coating with the aqueous paint composition of the present invention as described above and coating with a commonly used, known paint. Examples of topcoat paints used in such a combination of coatings include the general paints used for the intermediate coat paint, paints with a lower PVC content than the intermediate coat paint, or fluororesin paints, acrylic urethane resin paints, acrylic silicone resin paints, and the like.

[0131] Regarding the method for forming the topcoat film, conventionally known painting methods can be used, similar to those for the undercoat and intermediate coat films described above. However, from the viewpoint of finish quality, spray painting, roller painting, brush painting, etc., are preferred. There are no particular restrictions on drying after painting, but for example, a temperature of 5 to 35°C is preferred. The drying time of the paint can be, for example, 2 to 24 hours at 23°C and 50% relative humidity. The film thickness of the topcoat film can be appropriately adjusted depending on the condition of the underlying substrate and the undercoat and / or intermediate coat films, but from the viewpoint of the above objective, the dry film thickness is preferably 40 to 70 μm, more preferably 40 to 60 μm, and even more preferably 50 to 60 μm. [Examples]

[0132] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to these examples and comparative examples. In the examples and comparative examples, "parts" and "%" refer to mass unless otherwise specified.

[0133] 1. Object to be coated (metal base material) For the test, a polished steel plate JIS G 3141 (SPCC-SB) (150mm x 70mm x 0.8mm) obtained by polishing and degreasing using the following method was used as the metal substrate. (Degreasing method) After degreasing with xylene, it was polished with 120-grit sandpaper / disc sander, and then degreased again with xylene.

[0134] 2. Primer paint composition A primer paint composition was prepared by mixing a main component consisting of a water-dispersible resin (epoxy-acrylic composite emulsion resin, epoxy equivalent 1345 g / eq, non-volatile content: 50% by mass), a glycidyl group-containing silane coupling agent, a rust-inhibiting pigment, titanium dioxide, precipitated barium sulfate, kaolin, a dispersant, an antifoaming agent, a viscosity modifier, and water, with a curing agent consisting of a water-dispersible amine resin (water-dispersible modified polyamine emulsion, active hydrogen equivalent: 323 g / eq, non-volatile content: 40% by mass), an amino group-containing silane coupling agent, a rust inhibitor, a film-forming aid, and water, in an equivalent ratio of 1:1 and containing 15% PVC. The mixture was then prepared using a dispersant immediately before use for painting.

[0135] 3. Water-based paint composition (3-1. Main ingredient) The raw materials listed in Table 1 were sequentially placed into a container, and the main components used in Examples 1-8 and Comparative Examples 1-8 were prepared using a known manufacturing method. The raw materials listed in Table 1 are as follows:

[0136] 〔resin〕 1) Resin A (Hydroxyl group-containing acrylic resin aqueous dispersion) (Number average molecular weight: 4600, Acid value: 8.5 mg KOH / g, Hydroxyl group value: 100 mg KOH / g, Solid content concentration: 45% by mass)

[0137] [Pigments] 2) Coloring pigment (titanium dioxide)

[0138] [Film-forming aid] 3) Film-forming aid (c-1)-1 (ethylene glycol monobutyl ether, boiling point: 171°C) 4) Film-forming aid (c-2)-1 (diethylene glycol dibutyl ether, boiling point: 255°C) 5) Film-forming aid (c-2)-2 (diethylene glycol monobutyl ether, boiling point: 230°C)

[0139] [Other ingredients] 6) Dispersant 7) Antifoaming agent 8) Viscosity modifiers 9) Surface modifiers 10)Water

[0140] (3-2. Hardener) The raw materials listed in Table 1 were prepared sequentially, and the curing agents used in Examples 1-8 and Comparative Examples 1-8 were prepared using known manufacturing methods. The raw materials listed in Table 1 are as follows:

[0141] 〔resin〕 11) Water-dispersible polyisocyanate (b-1)-1 (polyisocyanate compound with NCO content of 13.1% by mass, non-volatile content: 70% by mass, propylene glycol monomethyl ether acetate 30% by mass) 12) Water-dispersible polyisocyanate (b-2)-1 (polyisocyanate compound with NCO content of 16.9% by mass, non-volatile content: 80% by mass, diethylene glycol dimethyl ether 20% by mass) 13) Water-dispersible polyisocyanate (b-2)-2 (polyisocyanate compound with NCO content of 18% by mass, non-volatile content: 100% by mass) 14) Water-dispersible polyisocyanate (b-3) (Polyisocyanate compound with NCO content of 21% by mass, non-volatile content: 100% by mass)

[0142] [Organic solvents] 15) Propylene glycol monomethyl ether acetate

[0143] [Table 1]

[0144] 4. Preparation of aqueous paint composition [Examples 1-8, Comparative Examples 1-8] According to the proportions shown in Table 1, the main component and hardener were mixed using a disperser immediately before use for painting to prepare the aqueous paint compositions for Examples 1-8 and Comparative Examples 1-8.

[0145] 5. Preparation of evaluation specimens (multi-layer coating) The aforementioned metal substrate was coated with the primer paint composition by air spray painting, so that the dry film thickness was 50-60 μm, thereby forming a primer film. Next, the prepared aqueous paint composition was applied to the formed primer film by air spray painting, so that the dry film thickness was 50-60 μm. Subsequently, the material was dried at 23°C for 7 days to form a topcoat film (coating film), and evaluation test pieces (multilayer coating films) for Examples 1 to 8 and Comparative Examples 1 to 8 were obtained.

[0146] 6. Evaluation Using the prepared evaluation specimens (multilayer coatings), the following were evaluated: 23°C pot life, 35°C pot life, pencil hardness, gloss, impact resistance, water resistance, salt spray resistance (SST), paintability 1 (40°C, assuming summer conditions), and paintability 2 (5°C, assuming winter conditions). The results are shown in Table 2.

[0147] <23℃ Pot Life> The aqueous paint compositions obtained in the examples and comparative examples were mixed with a main agent, a hardener, and diluent water to a viscosity that took into account workability (such as dripping during painting), and then allowed to stand at 23°C. For the first 7 hours, the viscosity was measured every hour using an NK-2 viscosity cup (manufactured by Anest Iwata Co., Ltd.), and the time it took to reach the maximum viscosity suitable for this painting method was defined as the pot life. ○: Pot life is 4 hours or more △: Pot life is between 2 hours and less than 4 hours ×: Pot life is less than 2 hours

[0148] <35℃ Pot Life> The aqueous paint compositions obtained in the examples and comparative examples were mixed with a main agent, a hardener, and diluent water to a viscosity that took into account workability (such as dripping during painting), and then allowed to stand at 35°C. For the first 7 hours, the viscosity was measured every hour using an NK-2 viscosity cup (manufactured by Anest Iwata Co., Ltd.), and the time it took to reach the maximum viscosity suitable for this painting method was defined as the pot life. ○: Pot life is 4 hours or more △: Pot life is between 2 hours and less than 4 hours ×: Pot life is less than 2 hours

[0149] <Pencil hardness> The hardness of the multilayer coatings obtained in the examples and comparative examples was evaluated using "JIS K5600-5-4:1999 Scratch hardness (pencil method)". ○:F or more △: B or above, but less than F ×: Less than B

[0150] <Glossy> The gloss value (60° gloss value) of the obtained multi-layer coating was measured using a multi-angle gloss meter micro-TRI-gloss (manufactured by BYK) and evaluated based on the following criteria. ○: Gloss value of 80 or higher △: Gloss value between 60 and 80 ×: Gloss value is less than 60

[0151] <Impact Resistance> The hardness of the multilayer coatings obtained in the examples and comparative examples was evaluated according to "JIS K5600-5-3:1999 Resistance to dropping weights". ○: No abnormalities △: Cracks and peeling are observed in some areas. ×: Cracks and peeling are observed.

[0152] <Water resistance> After immersing the test specimens in water at 23°C for 240 hours, the appearance of the multi-layer coating was visually observed. The evaluation criteria were as follows: ○: No paint film abnormalities (blistering) occurred. △: There are blisters between 1mm and 2mm in diameter on part of the coating. ×: Blisters larger than 2mm in diameter are present across the entire surface of the coating.

[0153] <Salt spray resistance (SST)> Cold-rolled steel sheets conforming to JIS G3141 were polished on the surface with #180 grit waterproof sandpaper, degreased and cleaned with a solvent such as xylene, and then air-spray coated to a dry film thickness of 50 ± 10 μm. After drying under the standard conditions described above, the resulting multi-layer coating was cross-cut with a utility knife to prepare test plates. The prepared test plates were placed in a testing machine conforming to JIS K5600-7-1:1999 and sprayed with salt water for 240 hours. ○: There is no rust or blistering in the general area, and blistering is only present in areas less than 3mm from the cut on one side of the cut. △: No rust or blistering in the general area, and blistering only in areas between 3mm and 6mm on one side from the cut incision. ×: Rust and blistering are present in the general area, and blistering is only present in areas between 3mm and 6mm on one side from the cut incision.

[0154] <Paintability 1 and Paintability 2> The aqueous paint compositions obtained in the examples and comparative examples were mixed with a main agent, a hardener, and diluent water, and applied to vertical surfaces under various temperature conditions. Leveling and sagging of the painted surfaces were visually inspected and evaluated. ○: The paint leveling is good, the paint film is uniform, and there are no abnormalities such as drips. △: The paint leveling is slightly poor, the paint film is somewhat uniform, and there are no abnormalities such as drips. ×: The paint leveling is extremely poor, resulting in an uneven and dripping paint film and a poor appearance.

[0155] [Table 2] [Industrial applicability]

[0156] The aqueous coating composition of the present invention is useful in various fields, and is particularly suitable as a coating composition for forming a coating film (intermediate coating film, top coating film) after a primer coating film has been formed on metal products. The method of forming a multi-layer coating film using the aqueous coating composition of the present invention as the coating film (intermediate coating film, top coating film) is particularly useful in fields that use relatively large and thick objects to be coated, such as steel structures, automobiles, heavy machinery, and railway vehicles.

Claims

1. An aqueous coating composition comprising an aqueous main component (I) and a water-dispersible curing agent (II), The aqueous main component (I) comprises a water-dispersible hydroxyl group-containing acrylic resin (a), The water-dispersible curing agent (II) comprises a water-dispersible polyisocyanate (b), and the water-dispersible polyisocyanate (b) comprises a polyisocyanate (b-1) with an isocyanate group content of 8 to 14% and a polyisocyanate (b-2) with an isocyanate group content of 16 to 20%. The blending mass ratio ((b-1) / (b-2)) of the polyisocyanate (b-1) and the polyisocyanate (b-2) is 9.5 / 0.5 to 1.5 / 8.

5. An aqueous coating composition characterized in that the equivalent ratio (isocyanate group / hydroxyl group) of isocyanate groups contained in the water-dispersible polyisocyanate (b) and hydroxyl groups contained in the water-dispersible hydroxyl group-containing acrylic resin (a) is 0.5 to 3.

0.

2. The aqueous paint composition contains a film-forming aid (c), The aqueous coating composition according to claim 1, characterized in that the film-forming aid (c) comprises a film-forming aid (c-1) having a boiling point of 80°C or more and less than 200°C, and a film-forming aid (c-2) having a boiling point of 200°C or more and less than 290°C.

3. A method for forming a multilayer coating film comprising at least an undercoat film and one or more coating films on the undercoat film, The process includes (X) applying a primer paint composition to the object to be coated to form a primer film, The process includes a step (Y) of applying an aqueous paint composition onto the undercoat to form one or more coating films, A method for forming a multilayer coating film, characterized in that the aqueous coating composition is the aqueous coating composition described in claim 1 or 2.

4. The method for forming a multilayer coating film according to claim 3, wherein the step (Y) of forming the coating film includes a step of forming at least a first coating film and a second coating film, and the aqueous coating composition according to claim 1 or 2 is used for forming at least one or all of the first coating film and the second coating film.

5. The aforementioned primer paint composition (X-1) Aqueous paint composition containing epoxy resin, (X-2) Putty composition containing an unsaturated polyester resin, and (X-3) Urethane-curing water-based paint composition A method for forming a multilayer coating film according to claim 3, characterized in that at least one is selected from the group consisting of the above, and one or more undercoat coating films are formed using the undercoat coating composition.

6. The method for forming a multilayer coating film according to claim 5, characterized in that the aqueous coating composition containing the (X-1) epoxy resin is an aqueous coating composition containing at least an epoxy resin, an acrylic resin, and a pigment.

7. The method for forming a multilayer coating film according to claim 3, characterized in that the primer coating composition contains a silane coupling agent.

8. The method for forming a multilayer coating film according to claim 3, characterized in that the step of forming the coating film (Y) includes a step of applying a clear paint to form a clear coating film (Y').

9. A multilayer coating comprising at least an undercoat film and one or more coating films on the undercoat film, A multilayer coating film characterized in that any or all of the one or two or more coating films are formed from the aqueous coating composition described in claim 1 or 2.

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