Method for producing multilayer coating film

Abstract

To provide a method for producing a multilayer coating film which uses a water-based coating composition as a primer coating composition, and can achieve a multilayer coating film having smooth appearance even when being wet-on-wet coated.SOLUTION: A method for producing a multilayer coating film includes a primer coating film formation step of applying a primer coating composition onto a coated object, and forming a primer coating film, a top coating film formation step of applying a top coating composition on the primer coating film by wet-on-wet coating, and forming a top coating film, and a drying step of simultaneously drying the primer coating film and the top coating film, and forming a multilayer coating film, wherein the primer coating composition is a water-based coating composition containing a water-based main agent (I) and a water-based curing agent (II), the water-based main agent (I) contains a water dispersion of an epoxy resin (A), the water-based curing agent (II) contains a polyamine compound (B), at least one of the water-based main agent (I) and the water-based curing agent (II) contains a particle material (D), and a content of the particle material (D) is more than 120 pts.mass and 380 pts.mass or less with respect to 100 pts.mass of a resin content contained in the water-based coating composition.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] In recent years, the awareness of reducing environmental load has increased, and there is a demand for replacing with environmentally friendly products. In the field of paints, for example, there is a demand for reducing the amount of organic solvent used, and this demand can be met by using aqueous paint compositions using water as a solvent. In the field of paints used for industrial machinery and construction machinery, there is also a demand for switching to aqueous paint compositions.

[0003] Industrial machinery and construction machinery are generally large and can withstand heavy loads, and are characterized by having thicker constituent substrates (steel plates) than automobile bodies, etc. Therefore, when such industrial machinery or construction machinery is used as a coating substrate, there is a problem that the heat capacity of the substrate is large and heat is not sufficiently transferred to the substrate in a heating furnace. Therefore, when coating such substrates, a room temperature film-forming coating composition is selected, which does not require a high-temperature heating process and can form a coating film at room temperature.

[0004] Industrial machinery, construction machinery, and the like are often used in physically harsh environments, and therefore the coating film that protects the surface is generally required to have excellent weather resistance in addition to excellent corrosion resistance. As a method for forming a coating film that satisfies both the performance of corrosion resistance and weather resistance, a method of forming a multi-layer coating film using a coating composition with excellent corrosion resistance as an undercoat coating composition and then using a topcoat coating composition with excellent weather resistance can be mentioned.

[0005] For example, Patent Document 1 describes a method for forming a thick-film type corrosion-resistant coating film, which is characterized by undercoating a weak-solvent type high-solid modified epoxy resin paint containing a binder resin component consisting of a modified epoxy resin having an epoxy equivalent of 400 to 2,000 g / eq based on the total mass of the binder resin components, an amine resin, and a reactive diluent, and then overcoating a weak-solvent type high-solid polyurethane resin paint containing a binder resin component consisting of a polyol resin having a hydroxyl value of 10 to 100 mgKOH / g and a polyisocyanate compound.

[0006] It is described that this method can provide the same corrosion prevention effect as that achieved by conventional multi-layer coating of each layer with a single undercoat and topcoat. However, this method requires drying the undercoat at room temperature for 24 hours after the undercoat is applied, and then the topcoat is applied, which requires a long time for the process of forming the multi-layer coating, resulting in poor efficiency of the coating process (also known as coating workability).

[0007] In recent years, from the viewpoint of shortening the coating process, a coating method called wet-on-wet (also called 2 coat 1 bake) has been attracting attention. This coating method is a coating method that can shorten the coating process by applying an undercoat coating composition, applying a topcoat coating composition without drying the undercoat coating composition, and then drying the two types of coating films simultaneously.

[0008] Regarding wet-on-wet painting, Patent Document 2 discusses a method for forming a multilayer coating film, which includes applying an undercoat paint composition containing an epoxy resin, an alicyclic hydrocarbon resin, and a polyisocyanate compound onto an object to be coated to form an uncured undercoat coating film, and then applying a topcoat paint base paint composition containing an acrylic resin and a polyisocyanate compound onto the uncured undercoat coating film. Patent Document 3 studies the adjustment of the pigment content and the extender pigment content in the primer coating and topcoat coating for a laminate coating having a primer coating containing a reaction product of an epoxy resin and a pigment, and a topcoat coating formed on the primer coating and containing a reaction product of an acrylic resin, a blocked isocyanate compound, and a pigment. Patent Document 4 describes the use of a composition containing an acrylic resin, an epoxy resin, an isocyanate compound, and a surface conditioner as an undercoat paint composition, and a composition containing an acrylic resin, an isocyanate compound, and a surface conditioner as a topcoat paint composition, with the difference in surface tension between the undercoat paint composition and the topcoat paint composition being 2 to 8 mN / m. Patent Document 5 describes the use of a composition containing an epoxy resin, an anti-rust pigment, a coloring pigment, and an extender pigment as a primer coating composition, and a composition containing an acrylic resin and an active methylene-blocked polyisocyanate compound as a topcoat coating composition, with the ratio of the acrylic resin to the active methylene-blocked polyisocyanate compound (acrylic resin / active methylene-blocked polyisocyanate) being 60 / 40 to 80 / 20. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2010-188239 A [Patent Document 2] JP 2020-192516 A [Patent Document 3] Patent Publication No. 2021-160120 [Patent Document 4] International Publication No. 2013 / 024784 [Patent Document 5] JP 2018-008205 A Summary of the Invention [Problem to be solved by the invention]

[0010] Here, when the undercoat paint composition and the topcoat paint composition are applied wet-on-wet, there is a problem that the paint composition is mixed between the two undried paint film layers (also called mixed layers), and the appearance of the multi-layer paint film obtained after drying is easily deteriorated, and this problem is particularly noticeable when an aqueous paint composition is used as the undercoat paint composition. In particular, when the substrate is an industrial machine or a construction machine, the paint film thickness is large compared to an automobile body, and as described above, a room temperature paint film forming type paint composition is selected, so that all of the conventionally known paint compositions are solvent-based paint compositions using a solvent as a dispersion medium, and it cannot be said that the case of wet-on-wet painting using an aqueous paint composition has not been fully studied. In particular, in wet-on-wet painting, since two layers of paint film are dried at the same time, the appearance of the paint film is easily deteriorated during drying and / or curing.

[0011] The present disclosure aims to provide a method for producing a multilayer coating film that can realize a multilayer coating film having a smooth appearance even when an aqueous coating composition is used as an undercoat coating composition and wet-on-wet coating is performed. [Means for solving the problem]

[0012] The present disclosure includes the following. [1] A step of forming an undercoat coating film by applying an undercoat coating composition onto a substrate; A topcoat coating film forming step of applying a topcoat coating composition wet-on-wet on the undercoat coating film to form a topcoat coating film; and A method for producing a multi-layer coating film, comprising: a drying step of simultaneously drying the undercoat coating film and the topcoat coating film to form a multi-layer coating film, The undercoat coating composition comprises: An aqueous coating composition comprising an aqueous base agent (I) and an aqueous curing agent (II), The aqueous base agent (I) contains an aqueous dispersion of an epoxy resin (A), The aqueous curing agent (II) contains a polyamine compound (B), At least one of the aqueous base agent (I) and the aqueous curing agent (II) comprises a particulate material (D); A method for producing a multi-layer coating film, wherein the content of the particulate material (D) is more than 120 parts by mass and not more than 380 parts by mass per 100 parts by mass of resin solids contained in the aqueous coating composition. [2] The method according to [1], wherein the polyamine compound (B) comprises at least one selected from the group consisting of an aliphatic polyamine, an alicyclic polyamine, an aromatic polyamine, a polyoxyalkylene group-containing polyamine, a polyoxyalkylene group-containing aromatic polyamine, and a polyamidoamine compound. [3] The method according to [1] or [2], wherein the polyamine compound (B) comprises at least one selected from the group consisting of alicyclic polyamines, aromatic polyamines, polyoxyalkylene group-containing aromatic polyamines, and polyamidoamines. [4] The method for producing a multilayer coating film according to any one of [1] to [3], wherein the epoxy equivalent of the epoxy resin (A) is 100 g / eq or more and 5,000 g / eq or less. [5] The method for producing a multilayer coating film according to any one of [1] to [4], wherein the active hydrogen equivalent of the polyamine compound (B) is 10 g / eq or more and 1,000 g / eq or less. [6] The method for producing a multilayer coating film according to any one of [1] to [5], wherein the ratio of the active hydrogen equivalent of the polyamine compound (B) to the epoxy equivalent contained in the epoxy resin (A) (active hydrogen equivalent / epoxy equivalent) is 0.3 or more and 2.0 or less. [7] The method for producing a multi-layer coating film according to any one of [1] to [6], wherein the internal stress of the undercoat coating film formed from the undercoat coating composition is 0.09 MPa or more and 0.50 MPa or less. [8] The topcoat coating composition is a coating composition comprising a base agent (III) and a curing agent (IV), The base agent (III) contains a film-forming resin, The film-forming resin has a hydroxyl group, The method for producing a multi-layer coating film according to any one of [1] to [7], wherein the curing agent (IV) contains a polyisocyanate compound. Effect of the Invention

[0013] According to the method for producing a multilayer coating film of the present disclosure, it is possible to realize a multilayer coating film having a smooth appearance even when an aqueous coating composition is used as an undercoat coating composition and wet-on-wet coating is performed. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing a method for measuring internal stress. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The method for producing a multilayer coating film according to the present disclosure includes: A step of forming an undercoat coating film by applying an undercoat coating composition onto a substrate to form an undercoat coating film; A topcoat coating film forming step of applying a topcoat coating composition wet-on-wet on the undercoat coating film to form a topcoat coating film; and A method for producing a multi-layer coating film, comprising: a drying step of simultaneously drying the undercoat coating film and the topcoat coating film to form a multi-layer coating film, The undercoat coating composition comprises: An aqueous coating composition comprising an aqueous base agent (I) and an aqueous curing agent (II), The aqueous base agent (I) contains an aqueous dispersion of an epoxy resin (A), The aqueous curing agent (II) contains a polyamine compound (B), At least one of the aqueous base agent (I) and the aqueous curing agent (II) contains a particulate material (D), and the content of the particulate material (D) is more than 120 parts by mass and not more than 380 parts by mass per 100 parts by mass of resin solids contained in the aqueous coating composition.

[0016] According to the method for producing a multilayer coating film of the present disclosure, it is possible to realize a multilayer coating film having a smooth appearance even when an aqueous coating composition is used as an undercoat coating composition and wet-on-wet coating is performed.

[0017] Although the present disclosure is not bound by a specific theory, according to the study of the present inventors, the reason why the manufacturing method of the multilayer coating film of the present disclosure can realize a multilayer coating film having a smooth appearance is considered to be as follows. The present inventors have studied the formation process of the multilayer coating film in the case of wet-on-wet painting, and have focused on the state before drying after applying the topcoat paint composition. At this stage, both the undercoat coating film and the topcoat coating film are in a wet state, and it is considered that the interfaces of the uncured undercoat coating film and the topcoat coating film are mixed to a certain extent. The undercoat coating film contains an aqueous base agent and an aqueous curing agent, and it is considered that internal stress occurs during drying due to curing shrinkage. As described above, at the stage before drying, both the undercoat coating film and the topcoat coating film are in a wet state, and the interfaces are mixed, so it is considered that the internal stress generated when the undercoat coating film dries to become a coating film is also transmitted to the topcoat coating film. Therefore, as the undercoat paint film cures and shrinks, the topcoat paint film also shrinks, which is thought to be manifested as a deterioration in the appearance of the multi-layer paint film.

[0018] The present inventors envisioned suppressing the cure shrinkage of the undercoat coating film in order to improve the appearance of the multi-layer coating film, and investigated the use of a certain amount of particulate material in the undercoat paint composition. As a result, they found that, although the cure proceeds, the cure shrinkage is suppressed, and further, the miscibility of the undercoat coating film and the topcoat coating film is appropriately maintained, so that a multi-layer coating film with good appearance can be obtained without impairing the properties of wet-on-wet painting.

[0019] In the present disclosure, the film formed after application of the coating composition and before drying or curing is also referred to as the coating film, and the film formed after drying or curing is also referred to as the coating film.

[0020] (Undercoat coating process) In the undercoat coating film forming step, the undercoat coating composition is applied onto an object to be coated, thereby forming an undercoat coating film.

[0021] The method of applying the undercoat paint composition is not particularly limited, and examples thereof include commonly used application methods such as immersion, brush, roller, roll coater, air spray, airless spray, curtain flow coater, roller curtain coater, and die coater. In the spray application, a two-liquid mixing gun may be used as necessary. These can be appropriately selected depending on the object to be coated.

[0022] The undercoat paint composition can be applied so that the dry film thickness of the undercoat paint film (hereinafter, "dry film of the paint film" is also called "coating film" and "dry film thickness of the paint film" is also called "film thickness") is 10 to 100 μm, preferably 15 to 70 μm. In one embodiment, the undercoat paint composition can be applied so that the film thickness of the undercoat paint film is 30 to 70 μm, further 40 to 60 μm. By using the undercoat paint composition described later, even if a thick film is applied, a smooth and defect-free multilayer paint film can be obtained. In addition, by making the undercoat paint film 10 μm or more, it becomes easy to sufficiently protect the substrate, and by making it 100 μm or less, it becomes easy to suppress the occurrence of defects such as popping in the multilayer paint film.

[0023] Examples of the substrate include metal substrates such as iron, zinc, tin, copper, titanium, tinplate, etc. These metal substrates may be plated with zinc, copper, chromium, etc., or may be surface-treated using a surface treatment agent such as chromate, zinc phosphate, or zirconium salt.

[0024] The method for producing a multilayer coating film according to the present disclosure can be suitably used for coating objects having a large heat capacity, such as metal substrates, which are difficult to sufficiently transfer heat to in a heating furnace. Specific examples of such coating objects include construction machinery (e.g., bulldozers, scrapers, hydraulic excavators, excavators, transport machines (trucks, trailers, etc.), cranes and loading machines, foundation construction machines (diesel hammers, hydraulic hammers, etc.), tunnel construction machines (boring machines, etc.), road rollers, etc.); industrial machinery such as low-voltage and heavy-voltage equipment, agricultural machinery, steel furniture, machine tools, and large vehicles, which are called general industrial use; and other coating objects having a large heat capacity and difficult to heat up even when heated. The method for producing a multilayer coating film according to the present disclosure can be suitably used for coating construction machinery or industrial machinery, which are coating objects having a large heat capacity and difficult to heat up even when heated.

[0025] (Topcoat paint film formation process) In the topcoat coating film forming step, a topcoat coating composition is applied wet-on-wet on the undercoat coating film to form a topcoat coating film, resulting in a state in which a wet topcoat coating film is formed on the wet undercoat coating film.

[0026] Typically, wet-on-wet includes a coating method in which an undried topcoat coating film is formed on an undried undercoat coating film, and the undried undercoat coating film and the undried topcoat coating film are simultaneously dried to form a multi-layer coating film. Wet-on-wet coating can shorten the coating process, and since there is no need to dry the undercoat coating film, it is energy efficient. In addition, the manufacturing method of the multi-layer coating film of the present disclosure uses the undercoat coating composition described below, so that even if a water-based coating composition is used as the undercoat coating composition and wet-on-wet coating is performed, a multi-layer coating film with a smooth surface can be manufactured.

[0027] In one embodiment, the fact that the films coated with the undercoat paint composition, the topcoat paint composition, and the intermediate coat paint composition described below are not dry means that they are not “dry to the touch” as specified in JIS K 5600-1-1. For example, this can be confirmed by lightly touching the center of the coating surface with a fingertip and checking whether the fingertip becomes dirty.

[0028] The interval between the formation of the undercoat coating film and the application of the topcoat coating composition (hereinafter also referred to as the "coating interval" or "interval") is more than 0 minutes, may be 1 to 60 minutes, may be 1 to 30 minutes, or may be 1 to 15 minutes, from the viewpoint of work efficiency. By using the multilayer coating film forming method of the present disclosure, it is possible to obtain a multilayer coating film with excellent coating film appearance even if the topcoat coating composition is applied when the undercoat coating film is almost wet. The temperature during the interval may be, for example, 0°C or higher and lower than 40°C, or 5°C or higher and 35°C or lower.

[0029] In addition, after forming the undercoat coating film, and before applying the topcoat coating composition, the undercoat coating film can be temporarily dried for about 1 to 10 minutes at a temperature higher than normal room temperature (for example, 40 to 100°C, more preferably 40 to 80°C), and the topcoat coating can be applied when the undercoat coating film is in a semi-dried state.

[0030] The method of applying the topcoat paint composition is not particularly limited, and examples thereof include commonly used application methods such as immersion, brush, roller, roll coater, air spray, airless spray, curtain flow coater, roller curtain coater, and die coater. In the spray application, a two-liquid mixing gun may be used as necessary. These can be appropriately selected depending on the object to be coated.

[0031] The topcoat paint composition can be applied so that the dry film thickness of the topcoat paint film (hereinafter also referred to as "topcoat paint film thickness") is 10 to 100 μm, preferably 20 to 80 μm. In one embodiment, the topcoat paint composition can be applied so that the film thickness of the topcoat paint film is 30 to 70 μm, further 40 to 60 μm. By using the topcoat paint composition and undercoat paint composition described later, a smooth and defect-free multilayer paint film can be obtained even when applied to a thick film. In addition, by making the thickness of the topcoat paint film 10 μm or more, it becomes easy to improve the hiding power of the undercoat paint film, and by making it 100 μm or less, it becomes easy to suppress the occurrence of defects such as popping in the multilayer paint film.

[0032] (drying process) In the drying step, the undercoat coating film and the topcoat coating film are dried simultaneously, thereby forming a multi-layer coating film.

[0033] The drying temperature may be preferably 5 to 100°C, and further 15 to 80°C. In one embodiment, the drying temperature may be 5 to 35°C, and the drying time may be 1 to 10 days. In another embodiment, the drying temperature may be, for example, 50 to 100°C, and further 60 to 80°C, and the drying time in this case may be 15 to 60 minutes. In yet another embodiment, the drying may be performed at 5 to 35°C for 15 to 60 minutes (room temperature drying), and then at 50 to 100°C (preferably 60 to 80°C) for 15 to 60 minutes (forced drying).

[0034] (Middle coat paint film formation process) The manufacturing method of the multilayer coating film of the present disclosure may further include a step of applying an intermediate coating composition wet-on-wet on an undried undercoat coating film to form an undried intermediate coating film, or may further include a step of applying an intermediate coating composition wet-on-wet on the undried intermediate coating film to form an undried intermediate coating film.

[0035] The interval between the formation of the undercoat coating film and the application of the intermediate coating composition, and the interval between the formation of the intermediate coating film and the application of the intermediate coating composition, can be appropriately set to the conditions described above as the interval between the formation of the undercoat coating film and the application of the topcoat coating composition.

[0036] The intermediate coating composition can be applied by any of the methods described above for applying the topcoat coating composition. The intermediate coating composition may be applied so that the dry thickness of the intermediate coating film (hereinafter also referred to as the "intermediate coating film thickness") falls within the above-mentioned range for the dry thickness of the topcoat coating film.

[0037] The intermediate coating composition is not particularly limited and may be either an aqueous coating composition or a solvent-based coating composition. Any coating composition known to those skilled in the art as an intermediate coating composition containing a film-forming resin, a curing agent, an organic and / or inorganic coloring pigment and / or an extender pigment, etc. may be used as appropriate.

[0038] When the manufacturing method of a multi-layer coating film includes a step of forming an intermediate coating film, the undercoat coating film is read as the intermediate coating film in the topcoat coating film forming step. Also, when the manufacturing method of a multi-layer coating film includes a step of forming an intermediate coating film, the wet-on-wet method includes the following: A coating method in which an undried intermediate coating film and a top coating film are formed on an undried undercoat coating film, and the undried undercoat coating film, the undried intermediate coating film, and the undried top coating film are all dried simultaneously to form a multi-layer coating film; A coating method in which an undried intermediate coating film is formed on an undried undercoat coating film, the undried undercoat coating film and the undried intermediate coating film are simultaneously dried to form a preliminary multi-layer coating film, and then a topcoat coating film is formed on the preliminary multi-layer coating film, and the topcoat coating film is dried to form a multi-layer coating film; Also included is a coating method in which an undried undercoat coating film is dried to form an undercoat coating film, and then an undried intermediate coating film and an undried topcoat coating film are formed on the undercoat coating film, and the undried intermediate coating film and the undried topcoat coating film are simultaneously dried to form a multi-layer coating film.

[0039] (Undercoat paint composition) The undercoat paint composition is a two-component curing type aqueous paint composition containing an aqueous base agent (I) and an aqueous hardener (II). The aqueous base agent (I) and the aqueous hardener (II) are stored separately and mixed immediately before painting, and the mixture is used for painting.

[0040] The aqueous coating composition uses water as a dispersion medium, and the drying rate of the dispersion medium is slower than that of a solvent-based coating composition using a solvent as a dispersion medium. In particular, in wet-on-wet painting, when the undercoat coating composition is an aqueous coating composition, there is a significant tendency for mixing (mixed layers) to occur between the undried undercoat coating film and the undried topcoat coating film compared to when a solvent-based coating composition is used. However, in the manufacturing method of the multilayer coating film of the present disclosure, a specific undercoat coating composition is used, so that such mixing layers can be suppressed and a multilayer coating film with a smooth surface can be formed.

[0041] The aqueous base agent (I) contains an aqueous dispersion of an epoxy resin (A), and the aqueous curing agent (II) contains a polyamine compound (B). At least one of the aqueous base agent (I) and the aqueous curing agent (II) may contain a particulate material (D), and at least one of the aqueous base agent (I) and the aqueous curing agent (II) may further contain an organic solvent (C) and / or a viscosity modifier (E) as necessary.

[0042] (A) Epoxy resin water dispersion The epoxy resin water dispersion (A) contained in the aqueous base agent (I) is a component in which an epoxy resin is dispersed in water.

[0043] The epoxy resin preferably has at least two epoxy groups per molecule on average. The epoxy equivalent of the epoxy resin may be 100 g / eq or more and 5,000 g / eq or less, 100 g / eq or more and 3,000 g / eq or less, or 150 g / eq or more and 1,000 g / eq or less. The epoxy equivalent being within such a range has the advantage of ensuring the water dispersion stability of the epoxy resin, the coating workability of the resulting coating composition, the film-forming property of the resulting multi-layer coating film, and good coating film appearance. In addition, the epoxy equivalent in the present disclosure refers to the solid epoxy equivalent and can be measured by a method in accordance with JIS K 7236.

[0044] The epoxy resin may be saturated or unsaturated, may be any of aliphatic, alicyclic, aromatic and heterocyclic, may have a hydroxyl group, and may be a modified product modified with an aliphatic polyol compound.

[0045] The epoxy resin is preferably a polyglycidyl ether type epoxy resin having a skeleton based on polyhydric phenols, hydrogenated products of polyhydric phenols, polyhydric alcohols and / or novolac phenol. The skeleton preferably includes a skeleton based on polyhydric alcohols and / or phenols, and more preferably includes a skeleton based on dihydric alcohols.

[0046] Examples of the polyhydric phenols include resorcinol, hydroquinone, 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), a mixture of isomers of dihydroxydiphenylmethane (bisphenol F), tetrabromobisphenol A, 4,4'-dihydroxydiphenylcyclohexane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,4'-dihydroxybiphenyl, 4,4'-dihydroxybenzophenone, 1,1-bis(4-hydroxyphenyl)ethane, 2,2-bis[4-(2'-hydroxypropoxy)phenyl]propane, 1,1-bis(4-hydroxyphenyl)isobutane, 2,2-bis(4-hydroxy-3-tert-butylphenyl)propane, bis(2-hydroxynaphthyl)methane, 1,5-dihydroxynaphthalene, tris(4-hydroxyphenyl)methane, bis(4-hydroxyphenyl)ether, bis(4-hydroxyphenyl)sulfone, and halogenated versions of the above compounds. The hydrogenation products of the polyhydric phenols include any of the hydrogenation products of the above compounds.

[0047] The polyhydric alcohols are not particularly limited, and examples thereof include ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol (n=4 to 35), 1,2-propylene glycol, polypropylene glycol (n=2 to 15), 1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2,6-hexanetriol, glycerol, neopentyl glycol, trimethylolethane, and trimethylolpropane. Among the above compounds, polypropylene glycol (n=8 to 10) is particularly preferred.

[0048] As the epoxy resin, for example, polyglycidyl ester obtained by reacting polycarboxylic acid with epichlorohydrin or its derivative can be used. The polycarboxylic acid is not particularly limited, and examples thereof include aliphatic, alicyclic or aromatic polycarboxylic acids such as oxalic acid, succinic acid, adipic acid, glutaric acid, phthalic acid, terephthalic acid, hexahydrophthalic acid, 2,6-naphthalenedicarboxylic acid and dimerized linoleic acid. Among them, diglycidyl adipate, diglycidyl phthalate and diglycidyl hexahydrophthalate are preferred.

[0049] Among the above epoxy resins, bisphenol type epoxy resins such as bisphenol A type epoxy resins and bisphenol F type epoxy resins are preferred.

[0050] The epoxy resin may be a polyol-modified epoxy resin obtained by reacting an aliphatic polyol compound with an epoxy resin as required. The polyol-modified epoxy resin can be prepared by condensation reaction of the epoxy resin and the aliphatic polyol compound. In the condensation reaction, the mass ratio of the epoxy resin to the aliphatic polyol compound (mass of the epoxy resin: mass of the aliphatic polyol compound) is preferably within the range of 95:5 to 5:95. The polyol-modified epoxy resin has an advantage of having good water dispersion performance. In the present disclosure, an aliphatic polyol compound means a polyol compound that does not contain an aromatic hydrocarbon group in the molecule.

[0051] The aliphatic polyol compound is not particularly limited, and examples thereof include aliphatic polyether polyols, aliphatic polyester polyols, aliphatic polycarbonate polyols, and aliphatic polyurethane polyols. Among them, polyether polyols are preferred, and polyalkylene glycols are more preferred. The polyalkylene glycol is preferably a polyalkylene glycol having an alkylene group having 4 or less carbon atoms, and examples thereof include polyethylene glycol, polypropylene glycol, polybutylene glycol, and block copolymers of ethylene oxide and propylene oxide. In addition, mixtures or copolymers of the polyalkylene glycols can be used. In addition, the polyalkylene glycols may be partially end-blocked with monohydric alcohols or the like. The polyalkylene glycols may have a partially branched structure, but are more preferably linear polyalkylene glycols.

[0052] The aliphatic polyol compound may be a mixture of the polyether polyol (particularly, polyalkylene glycol) and other aliphatic polyols. Examples of the other aliphatic polyols include aliphatic polyester polyols, aliphatic polycarbonate polyols, aliphatic polyamide polyols, and aliphatic polyurethane polyols, and aliphatic polyester polyols are particularly preferred. The content of polyether polyol in the aliphatic polyol compound is preferably 70% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less.

[0053] Examples of the other aliphatic polyols include aliphatic polyester polyols of aliphatic dicarboxylic acids and aliphatic diols, dicarboxylic acids having 3 to 40 carbon atoms, diols having 2 to 20 carbon atoms, primary diamines having 2 to 40 carbon atoms, polyalkylene polyamine compounds, amino alcohols, and the like.

[0054] The condensation is preferably carried out at a ratio z(OH):z(EP) of the epoxy equivalent of the epoxy resin to the hydroxyl equivalent of the aliphatic polyol compound is 1:3.6 to 1:10. The z(OH):z(EP) is more preferably within the range of 1:4 to 1:9, and even more preferably within the range of 1:4.5 to 1:8. By carrying out the reaction within the above range, there is an advantage that good water dispersibility can be obtained. The epoxy equivalent of the modified epoxy resin (polyol-modified epoxy resin) is preferably 100 g / eq or more and 10,000 g / eq or less, more preferably 150 g / eq or more and 5,000 g / eq or less, and even more preferably 200 g / eq or more and 2,000 g / eq or less.

[0055] The epoxy resin contained in the epoxy resin water dispersion (A) preferably contains a polyol-modified bisphenol type epoxy resin, such as a polyol-modified bisphenol A type epoxy resin or a polyol-modified bisphenol F type epoxy resin. These polyol-modified bisphenol type epoxy resins have suitable water dispersibility and have the advantage that the physical properties of the resulting multilayer coating film are good. In another embodiment, for example, the polyol-modified epoxy resin (preferably containing a polyol-modified bisphenol type epoxy resin) and a bisphenol type epoxy resin (a bisphenol type epoxy resin without polyol modification) may be used in combination.

[0056] The number average molecular weight of the epoxy resin may be preferably 200 to 20,000, more preferably 300 to 10,000. By keeping it within such a range, there are advantages in that the aqueous dispersion stability of the epoxy resin, the coating workability of the resulting coating composition, and the film-formability of the resulting multilayer coating film can be ensured. In the present disclosure, the number average molecular weight is a value calculated in terms of polystyrene by gel permeation chromatography (GPC).

[0057] The epoxy resin aqueous dispersion (A) can be prepared by carrying out a synthesis reaction of an epoxy resin (polyol-modified epoxy resin) without a solvent or in the presence of a suitable organic solvent, dropping and mixing the resulting mixture into water, and removing excess solvent as necessary to obtain an aqueous dispersion. When dispersing the epoxy resin in water, a dispersant such as a surfactant may be used as necessary.

[0058] The epoxy resin aqueous dispersion (A) may be a commercially available product, such as BECKOPOX series (manufactured by Allnex Japan Co., Ltd.), jER series (manufactured by Mitsubishi Chemical Corporation), and ADEKA RESIN EM series (manufactured by ADEKA Corporation).

[0059] The epoxy resin water dispersions may be used alone or in combination of two or more kinds.

[0060] The aqueous base agent (I) may contain other resin components as necessary in addition to the epoxy resin water dispersion (A). Examples of other resin components include polyurethane resin water dispersion, polyester resin water dispersion, and acrylic resin water dispersion. When the aqueous base agent (I) further contains other resin components, the preferred resin component is polyurethane resin water dispersion from the viewpoint of compatibility with the epoxy resin water dispersion (A). When the aqueous base agent (I) further contains other resin components such as polyurethane resin water dispersion, the preferred amount of the other resin components is such that the performance of the aqueous coating composition and the performance of the multilayer coating film obtained are not impaired. When the aqueous base agent (I) further contains a polyurethane resin water dispersion in addition to the epoxy resin water dispersion (A), the content of the polyurethane resin water dispersion is preferably 0.5 to 20 parts by mass in terms of resin solid content per 100 parts by mass of the resin solid content of the epoxy resin water dispersion (A).

[0061] In the present disclosure, the "resin solids content" of the undercoat paint composition means the total amount of solids content of resin components that can be contained in the aqueous base agent (I) and the aqueous curing agent (II), and specifically means the total amount of solids content of resins that can be contained in film-forming resins such as polyurethane resin water dispersions, polyester water dispersions, and acrylic resin water dispersions that are added as desired, in addition to the solids content of the epoxy resin (A), and the solids content of the polyamine compound (B) described below.

[0062] The amount of resin solids in the undercoat coating composition may be preferably 10 parts by mass or more and 80 parts by mass or less, more preferably 15 parts by mass or more and 70 parts by mass or less, per 100 parts by mass of the total solids contained in the undercoat coating composition. In this disclosure, solids content refers to the residue after drying at 105° C. for 1 hour.

[0063] (B) Polyamine Compound The polyamine compound (B) may be any compound having two or more amino groups in one molecule.

[0064] Examples of the polyamine compound (B) include aliphatic polyamines, alicyclic polyamines, aromatic polyamines, polyoxyalkylene group-containing polyamines, polyoxyalkylene group-containing aromatic polyamines, and polyamidoamine compounds.

[0065] Examples of the aliphatic amine include alkylene polyamines, polyalkylene polyamines, and other aliphatic amines. Examples of alkylene polyamines include HN-(R 1 -NH n -H(wherein, R 1 represents a divalent hydrocarbon group having 1 to 12 carbon atoms which may be substituted with one or more hydrocarbon groups having 1 to 10 carbon atoms, and n represents an integer of 1 to 5. More specific examples include methylenediamine, ethylenediamine, 1,2-diaminopropane, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,3-diaminopentane, neopentanediamine, 1,6-diaminohexane (hexamethylenediamine), 2-methyl-1,5-pentanediamine, 1,7-diaminoheptane, methylhexamethylenediamine, 1,8-diaminooctane, 1,9-diaminononane, and 1,10-diaminodecane.

[0066] Examples of polyalkylene polyamines include diethylenetriamine, bis(3-aminopropylamine), triethylenetetramine, N,N-bis(3-aminopropyl)ethylenediamine, tetraethylenepentamine, pentaethylenehexamine, and hexamethylenetetramine.

[0067] Other aliphatic amines include, for example, tetra(aminomethyl)methane, tetrakis(2-aminoethylaminomethyl)methane, 1,3-bis(2'-aminoethylamino)propane, triethylene-bis(trimethylene)hexamine, bis(3-aminoethyl)amine, and bishexamethylenetriamine.

[0068] Examples of the alicyclic polyamines include 1,4-cyclohexanediamine, 4,4'-methylenebiscyclohexylamine, 4,4'-isopropylidenebiscyclohexylamine, norbornadiamine, bis(aminomethyl)cyclohexane, diaminodicyclohexylmethane, isophoronediamine, menthenediamine (MDA), 1,4-bis(3-aminopropyl)piperazine, and the like.

[0069] Examples of aromatic polyamines include bis(aminoalkyl)benzenes, bis(aminoalkyl)naphthalenes, aromatic polyamine compounds having two or more primary amino groups bonded to a benzene ring, and other aromatic polyamine compounds. The aromatic polyamines are not particularly limited, but more specifically, bis(cyanoethyl)diethylenetriamine, o-xylylenediamine, m-xylylenediamine (MXDA), p-xylylenediamine, phenylenediamine, naphthylenediamine, diaminodiphenylmethane, diaminodiethylphenylmethane, 2,2-bis(4-aminophenyl)propane, 4,4'-diaminodiphenylether, 4,4'-diaminobenzophenone, 4,4 Examples of the diaminodiphenyl ether include 2,2'-diaminodiphenyl sulfone, 2,2'-dimethyl-4,4'-diaminodiphenylmethane, 2,4'-diaminobiphenyl, 2,3'-dimethyl-4,4'-diaminobiphenyl, 3,3'-dimethoxy-4,4'-diaminobiphenyl, bis(aminomethyl)naphthalene, and bis(aminoethyl)naphthalene.

[0070] The polyoxyalkylene group-containing polyamine is a polyamine compound having a polyoxyalkylene chain, which does not fall under the category of the "polyoxyalkylene group-containing aromatic polyamine" shown below, i.e., does not have an aromatic group. Examples of the polyoxyalkylene chain contained in the polyoxyalkylene group-containing polyamine include a polyoxyethylene chain, a polyoxypropylene chain, a poly(oxyethylene-oxypropylene) chain, and a poly(oxytetramethylene) chain.

[0071] Examples of polyoxyalkylene group-containing polyamines include polyoxyalkylene diamines such as polyoxyethylene diamine, polyoxypropylene diamine, poly(oxyethylene-oxypropylene) diamine, etc. These are compounds in which a polyoxyalkylene group is introduced into an aliphatic polyamine, and can also be called polyoxyalkylene group-containing aliphatic polyamines.

[0072] Other examples of polyoxyalkylene group-containing polyamines include polyamines in which two or more hydroxyl groups of a compound obtained by reacting a polyol such as trimethylolpropane or pentaerythritol with an alkylene oxide such as ethylene oxide and / or propylene oxide are converted to amino groups.

[0073] The molecular weight of the polyoxyalkylene group-containing polyamine is preferably 100 to 5,000, more preferably 120 to 3,000, and even more preferably 120 to 500. The molecular weight within the above range has the advantage of improving the appearance of the resulting coating film. When the molecular formula of the polyamine compound is known, the molecular weight can be calculated according to the molecular formula. When the number of repeating oxyalkylene units in the polyoxyalkylene chain is not a natural number, the molecular weight may be a number average molecular weight.

[0074] The polyoxyalkylene group-containing polyamine may be a commercially available product. Examples of the commercially available product include polyoxyalkylene group-containing aliphatic polyamines such as JEFFAMINE M-600, JEFFAMINE M-1000, JEFFAMINE D-230, JEFFAMINE D-2000, JEFFAMINE EDR-148, JEFFAMINE T-403, and JEFFAMINE T-3000 (all manufactured by HUNTSMAN Advanced Materials).

[0075] The polyoxyalkylene group-containing aromatic polyamine is a polyamine compound having a polyoxyalkylene chain and an aromatic group. Specific examples of the polyoxyalkylene chain are the same as those mentioned above.

[0076] Examples of polyoxyalkylene group-containing aromatic polyamines include polyamines in which an amino group-containing aromatic compound is introduced into a polyol such as a diol, trimethylolpropane, or pentaerythritol, or an alkylene oxide such as ethylene oxide, propylene oxide, and / or tetramethylene oxide.

[0077] The polyoxyalkylene group-containing aromatic polyamine may be a commercially available product, such as the Elasmer series (manufactured by Kumiai Chemical Cooperative).

[0078] The polyamidoamine compound used in the production method of the present disclosure is not particularly limited as long as it has a polyamide structure in the molecule and at least two active hydrogens. In this specification, the active hydrogen refers to the hydrogen bonded to the nitrogen atom of the amino group in the polyamidoamine compound and polyamine compound.

[0079] The polyamidoamine compound used in the polyamidoamine compound can be produced by a general method, for example, by a condensation reaction between a polyamine compound and a polycarboxylic acid compound. In this case, the amount of active hydrogen in the resulting polyamidoamine compound can be adjusted by adjusting the ratio of the polyamine compound and the polycarboxylic acid compound used in the reaction.

[0080] The polyamine compound used in the production of the polyamidoamine compound is not particularly limited as long as it has at least two amino groups in the molecule, and at least one selected from the group consisting of aliphatic chain polyamines, aliphatic cyclic polyamines, and aromatic polyamines can be used. As the aliphatic chain polyamine, polyalkylene polyamines such as diethylenetriamine, triethylenetetramine, and tetraethylenepentamine can also be suitably used.

[0081] The polycarboxylic acid compound used in the production of the polyamidoamine compound is not particularly limited as long as it is a compound having at least two carboxy groups in the molecule, but is preferably a dicarboxylic acid such as an aliphatic dicarboxylic acid or a dimer acid.

[0082] In the production of the polyamidoamine compound, in addition to the polyamine compound and the polycarboxylic acid compound, an aminocarboxylic acid compound, a polyol compound, a lactam compound, etc. may be appropriately reacted to form a modified polyamidoamine compound.

[0083] The polyamidoamine compound may contain water or an aqueous solvent in addition to the polyamidoamine compound. Examples of the aqueous solvent include protic polar solvents such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methoxyethanol, 2-ethoxyethanol, 2-propoxyethanol, 2-butoxyethanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, and 1-propoxy-2-propanol, and aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, and N-methylpyrrolidone, and these can be used alone or in combination of two or more. Among the above, it is preferable that the polyamidoamine compound contains water. The solids concentration of the polyamidoamine compound in the polyamine compound (B) is preferably 20% by mass or more and 90% by mass or less.

[0084] As the polyamidoamine compound, commercially available polyamidoamine compounds can be used. Examples of the polyamidoamine compound include Aradur 3986, Aradur 38-1 (manufactured by Huntsman), EPIKURE Curing Agent 8535-W-50, and EPIKURE Curing Agent 8530-W-75 (manufactured by HEXION), and these can be used alone or in combination of two or more.

[0085] The polyamine compound (B) may be used alone or in combination of two or more kinds. In one embodiment, the polyamine compound (B) preferably includes at least one selected from the group consisting of aliphatic polyamines, alicyclic polyamines, aromatic polyamines, polyoxyalkylene group-containing polyamines, polyoxyalkylene group-containing aromatic polyamines, and polyamidoamine compounds, and more preferably includes at least one selected from the group consisting of alicyclic polyamines, aromatic polyamines, polyoxyalkylene group-containing aromatic polyamines, and polyamidoamine compounds. The alicyclic polyamines, aromatic polyamines, and polyoxyalkylene group-containing aromatic polyamines have a cyclic structure in the molecule, and the polyamidoamine compounds have a bulky structure. Therefore, due to the steric hindrance of these polyamine compounds, the curing shrinkage of the undercoat coating film is suppressed, the internal stress is reduced, and the appearance of the resulting multilayer coating film is improved.

[0086] The aqueous curing agent (II) may contain a polyamine compound (B), and in one embodiment, may be an aqueous dispersion of the polyamine compound (B). The aqueous dispersion of the polyamine compound (B) can be prepared by dispersing the polyamine compound (B) in an aqueous solvent. Examples of the aqueous solvent include water (ion-exchanged water, pure water, clean water, industrial water, etc.) and mixtures of water and water-miscible organic solvents. Examples of the water-miscible organic solvent include those that are not reactive with the epoxy resin (A) and the polyamine compound (B), such as alcohols such as isopropanol; glycol ethers, etc.

[0087] As a method for dispersing the polyamine compound (B) in an aqueous solvent, when the polyamine compound (B) has a hydrophilic group, the polyamine compound (B) can be dispersed by adding the polyamine compound (B) to water and stirring. In addition, in dispersing the polyamine compound (B), a surfactant, a dispersing resin, etc. may be used in combination, if necessary.

[0088] One embodiment of the dispersion of the polyamine compound (B) is to prepare a polyamine compound aqueous dispersion by mixing the polyamine compound (B) and a surfactant in an aqueous solvent.

[0089] The surfactant preferably includes at least one of an anionic surfactant and a nonionic surfactant. The anionic surfactant is preferably at least one selected from the group consisting of a phosphate ester surfactant, a carboxylic acid surfactant, a sulfonic acid surfactant, and a sulfate ester surfactant. The nonionic surfactant is preferably at least one selected from the group consisting of a polyoxyalkylene glycol fatty acid ester, a polyalkylene glycol fatty acid ester, and a polyoxyalkylene alkyl ether.

[0090] Phosphate ester surfactants, which are a type of anionic surfactant, are surfactants that have a phosphate group as an anionic group. Examples of phosphate ester surfactants, such as surfactants having a phosphate group, include the following: polyoxyalkylene alkyl ether phosphate esters, polyoxyalkylene alkyl phenyl ether phosphate esters and the like; and salts thereof, such as ammonium salts, lithium salts, sodium salts, potassium salts and the like.

[0091] The phosphate ester and its salt may be a commercially available product, such as Disparlon PW-36, Disparlon AQ-330 (Kusumoto Chemical Industries, Ltd.), DISPERBYK-103, DISPERBYK-111, or DISPERBYK-145 (BYK-Chemie Japan).

[0092] Carboxylic acid surfactants, which are a type of anionic surfactant, are surfactants that have a carboxylic acid group as an anionic group. Examples of carboxylic acid surfactants include saturated fatty acids such as propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, 2-ethylcaproic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, stearic acid, isostearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid, melissic acid, and 12-hydroxystearic acid; monounsaturated fatty acids such as crotonic acid, undecylenic acid, myristoleic acid, palmitoleic acid, sapienic acid, oleic acid, elaidic acid, vaccenic acid, gadoleic acid, erucic acid, and nervonic acid; diunsaturated fatty acids such as linoleic acid, eicosaginic acid, and docosadienoic acid; linolenic acid, pinoleic acid, eleostearic acid, mead acid, eicosaginic acid, and linolenic acid; triunsaturated fatty acids such as satrienoic acid; tetraunsaturated fatty acids such as stearidonic acid, arachidonic acid, eicosatetraenoic acid, and adrenic acid; pentaunsaturated fatty acids such as poseopentaenoic acid, eicosapentaenoic acid, osponic acid, sardine acid, and tetracosapentaenoic acid; hexaunsaturated fatty acids such as docosahexaenoic acid and herring acid; vegetable oil derivatives and mixed fatty acids such as castor oil fatty acid, coconut oil fatty acid, linseed oil fatty acid, bran fatty acid, rice oil fatty acid, soybean fatty acid, safflower fatty acid, tall oil fatty acid, and dehydrated castor oil fatty acid; dicarboxylic acids such as sebacic acid, adipic acid, and dimer acid; aromatic carboxylic acids such as benzoic acid, salicylic acid, and cinnamic acid; and salts thereof, for example, ammonium salts, lithium salts, sodium salts, and potassium salts.

[0093] As the carboxylic acid type surfactant, commercially available products may be used, and are available from, for example, Kishida Chemical Co., Ltd., Tokyo Chemical Industry Co., Ltd., Nippon Fine Chemical Co., Ltd., and the like.

[0094] Sulfonic acid surfactants, which are a type of anionic surfactant, are surfactants that have sulfonic acid groups as anionic groups.Surfactants that have sulfonic acid groups include alkylbenzenesulfonic acid, alkyldiphenyletherdisulfonic acid, perfluoroalkyloxybenzenesulfonic acid, alkylsulfonic acid, alkylnaphthalenesulfonic acid, sulfosuccinic acid, N-acylsulfonic acid, polyoxyethylene alkylphenylether sulfate, alkylsulfuric acid, alkylethersulfuric acid, alkylamidesulfuric acid, etc.; and their salts, such as ammonium salt, lithium salt, sodium salt, potassium salt, etc. The surfactant having sulfonic acid group may be a commercially available product. Examples of commercially available products include Pelex SS-H, Neopelex G-25 (Kao Corporation), Lipolan PB-800 (Lion Corporation), Teika Power L128 (Teika Corporation), New Coal 565SNC, New Coal 707SF (Nippon Nyukazai Co., Ltd.), and Aqualon KH-10 (Dai-ichi Kogyo Seiyaku Co., Ltd.). Sulfate type surfactants, which are a type of anionic surfactants, are surfactants that have a sulfate ester group as an anionic group. Examples of sulfate type surfactants include fatty acid sulfate ester salts, alkyl sulfate salts, alkyl ether sulfate salts, and amide ether sulfate salts. Commercially available products may be used as the surfactants that have the sulfate ester group. The nonionic surfactant may be, for example, at least one selected from the group consisting of polyoxyalkylene glycol fatty acid esters, polyalkylene glycol fatty acid esters, and polyoxyalkylene alkyl ethers. Commercially available products may also be used as the nonionic surfactant. Examples of commercially available products include the Genapol series, Genagen series (Clariant Japan), Noigen series (Dai-ichi Kogyo Seiyaku), and Newcol N700 series (Nippon Nyukazai). The surfactants may be used alone or in combination of two or more kinds. For example, only an anionic surfactant may be used, only a nonionic surfactant may be used, or an anionic surfactant and a nonionic surfactant may be used in combination. The temperature and dispersion conditions in the dispersion preparation step can be appropriately selected within the ranges commonly used by those skilled in the art.

[0095] When a surfactant is used, the content of the surfactant in the aqueous dispersion of the polyamine compound (B) is preferably 0.01 to 20 parts by mass relative to 100 parts by mass of the polyamine compound (B). The content of the surfactant within the above range has the advantage that the aqueous dispersion of the polyamine compound (B) obtained has good water dispersibility and the water resistance of the multilayer coating film obtained is good.

[0096] The content of the polyamine compound (B) contained in the aqueous dispersion of the polyamine compound (B) can be appropriately selected depending on the structure and molecular weight of the polyamine compound (B), and is, for example, preferably 30% by mass or more and 90% by mass or less, more preferably 40% by mass or more and 80% by mass or less. By being within the above range, there is an advantage that the dispersion stability of the mixture obtained by mixing the aqueous main agent (I) and the aqueous curing agent (II) is improved.

[0097] In the aqueous dispersion of the polyamine compound (B), the average particle size of the aqueous dispersion (dispersed polyamine compound (B)) is preferably 100 to 1,000 nm, more preferably 100 to 300 nm. The average particle size of the aqueous dispersion within the above range has the advantage of improving the dispersion stability of the mixture of the aqueous base agent (I) and the aqueous curing agent (II). In addition, the epoxy resin aqueous dispersion (A) and the polyamine compound (B) have better reactivity, and the appearance of the resulting multilayer coating film is improved. In the present disclosure, the average particle size of the aqueous dispersion means the average particle size determined by dynamic light scattering, and specifically, can be measured using an electrophoretic light scattering photometer ELSZ series (manufactured by Otsuka Electronics Co., Ltd.) or the like.

[0098] The active hydrogen equivalent of the polyamine compound is preferably 10 g / eq or more and 1,000 g / eq or less, more preferably 20 g / eq or more and 900 g / eq or less, and even more preferably 35 g / eq or more and 800 g / eq or less. In the present disclosure, the active hydrogen equivalent of a polyamine compound refers to the solid content active hydrogen equivalent, and can be measured by a method in accordance with JIS K 7237:1995.

[0099] The ratio of the active hydrogen equivalent of the polyamine compound (B) to the epoxy equivalent of the epoxy resin (A) (active hydrogen equivalent / epoxy equivalent) is preferably 0.3 to 2.0, more preferably 0.4 to 1.5, and even more preferably 0.5 to 1.0. By being within the above range, better reactivity of the epoxy resin water dispersion (A) and the polyamine compound (B) is ensured, and the multilayer coating film obtained from such an undercoat paint composition has the advantage of having good adhesion to the substrate.

[0100] When two or more polyamine compounds are used in combination, the active hydrogen equivalent may be calculated by the following formula: That is, when the active hydrogen equivalent of the polyamine compound obtained by mixing M parts by mass (parts by mass of solid content) of a polyamine compound (B1) having an active hydrogen equivalent of H1 and N parts by mass (parts by mass of solid content) of a polyamine compound (B2) having an active hydrogen equivalent of H2 is Z, Z is calculated according to the following formula. Z=[(M+N)×H1×H2] / (M×H2+N×H1)

[0101] (C) Organic solvent As the organic solvent (C), an organic solvent used in the paint industry may be used. Without being bound by a particular theory, the organic solvent (C) has a polarity and boiling point different from those of water, and therefore the drying property of the undercoat coating film can be controlled by using the organic solvent (C). The organic solvent (C) can also act as a diluent. In one embodiment, the organic solvent (C) can be used to accelerate the drying of the undercoat coating film.

[0102] The organic solvent (C) is not particularly limited, and examples thereof include aromatic hydrocarbon solvents such as xylene and toluene; ether solvents such as dipropylene glycol dimethyl ether, ethylene glycol monobutyl ether, 2-methoxypropanol (propylene glycol monomethyl ether), diethylene glycol monobutyl ether, butyl diglycol, 2-butoxypropanol, methyl ether acetate, propylene glycol monomethyl ether acetate, tetrahydrofuran, and dioxane; alcohol solvents such as ethanol, methanol, propanol, isopropyl alcohol, 2-butanol, and t-butyl alcohol; glycol solvents such as ethylene glycol and propylene glycol; and amide solvents such as N-methylpyrrolidone.

[0103] The organic solvent (C) preferably contains an organic solvent (C1) having a relative evaporation rate of 0.5 to 6, where n-butyl acetate is taken as 1. The relative evaporation rate of the organic solvent (C1) is preferably 1 to 5, more preferably 1 to 2. When the relative evaporation rate is within the above range, there is an advantage that the drying property of the obtained coating composition is good, and the appearance of the obtained multi-layer coating film is good. As the organic solvent having a relative evaporation rate of 0.5 to 6, 2-methoxypropanol (propylene glycol monomethyl ether), ethanol, and isopropyl alcohol are preferred. In the present disclosure, the evaporation rate of the organic solvent (C) is based on a value measured in accordance with the test method specified in ASTM D3539-87 (2004) of the American Society for Testing and Materials, and is expressed as a converted value when the evaporation rate of n-butyl acetate is set to 1.

[0104] When the organic solvent (C) is used, the content of the organic solvent (C) in the undercoat coating composition is preferably 0.1 to 25 mass %, more preferably 0.5 to 20 mass %, and even more preferably 1 to 18 mass %. The content of the organic solvent (C) in the undercoat paint composition is the total amount (parts by mass) of the organic solvent (C) contained in the aqueous base agent (I) and the aqueous curing agent (II) divided by the total amount (parts by mass) of the undercoat paint composition (the total amount of the aqueous base agent (I) and the aqueous curing agent (II)).

[0105] When the organic solvent (C1) is used, the content of the organic solvent (C1) in the organic solvent (C) is preferably 100% by mass in one embodiment, and is preferably 10 to 80% by mass, and more preferably 30 to 75% by mass in another embodiment.

[0106] When the organic solvent (C) is used, the organic solvent (C) may be contained in either the aqueous base agent (I) or the aqueous curing agent (II), and is preferably contained in both the aqueous base agent (I) and the aqueous curing agent (II).

[0107] The content of the organic solvent (C) in the aqueous main agent (I) is preferably from 0.1 to 20% by mass, more preferably from 0.5 to 15% by mass, and further preferably from 1.5 to 10% by mass. The content of the organic solvent (C) in the aqueous curing agent (II) is preferably 0.1% by mass or more and 50% by mass or less, more preferably 1% by mass or more and 40% by mass or less, and further preferably 5% by mass or more and 35% by mass or less. By having the content of the organic solvent (C) within the above range, there is an advantage that the drying properties of the resulting coating composition are good, and the appearance of the resulting multi-layer coating film is good.

[0108] The organic solvent (C) may be contained in either the aqueous base agent (I) or the aqueous curing agent (II). In one embodiment, the organic solvent (C) may be contained only in the aqueous base agent (I), in another embodiment, the organic solvent (C) may be contained only in the aqueous curing agent (II), and in yet another embodiment, the organic solvent (C) may be contained in both the aqueous base agent (I) and the aqueous curing agent (II).

[0109] (D) Particle material The particulate material (D) means a particulate material that does not substantially change in volume in the aqueous coating composition and the multi-layer coating film, and the material may be either an inorganic material or an organic material, and is preferably an inorganic material. By including the particulate material (D), a multi-layer coating film with good appearance can be obtained.

[0110] The particulate material (D) preferably comprises one or more selected from the group consisting of an anti-rust pigment (D1), an extender pigment (D2), a coloring pigment (D3) and a filler (D4).

[0111] Examples of the rust-preventive pigment (D1) include iron phosphate, aluminum phosphate, calcium phosphate, aluminum tripolyphosphate, aluminum phosphomolybdate, and zinc aluminum phosphomolybdate.

[0112] The content of the anti-rust pigment (D1) is preferably 0 parts by mass or more and 250 parts by mass or less, relative to 100 parts by mass of the resin solid content of the undercoat paint composition, and in one embodiment, more preferably 5 parts by mass or more and 200 parts by mass or less, even more preferably 10 parts by mass or more and 150 parts by mass or less, and even more preferably 20 parts by mass or more and 100 parts by mass or less, and in another embodiment, it is more preferably 0 parts by mass.

[0113] Examples of the extender pigment (D2) include kaolin, talc, aluminum silicate, calcium silicate, calcium carbonate, mica, and clay.

[0114] The content of the extender pigment (D2) is preferably 0 parts by mass or more and 200 parts by mass or less, relative to 100 parts by mass of the resin solids content of the undercoat paint composition, and in one embodiment, more preferably 1 part by mass or more and 150 parts by mass or less, and even more preferably 5 parts by mass or more and 100 parts by mass or less, and in another embodiment, more preferably 0 part by mass.

[0115] Examples of the color pigment (D3) include organic color pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, isoindoline pigments, diketopyrrolopyrrole pigments, and metal complex pigments; and inorganic color pigments such as yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide.

[0116] The content of the color pigment (D3) is preferably 10 parts by mass or more and 200 parts by mass or less, relative to 100 parts by mass of the resin solid content of the undercoat paint composition, and in one embodiment, more preferably 15 parts by mass or more and 170 parts by mass or less, even more preferably 15 parts by mass or more and 150 parts by mass or less, and even more preferably 15 parts by mass or more and 110 parts by mass or less, and in another embodiment, more preferably 0 part by mass.

[0117] (D4) Filler The filler (D4) may include an inorganic filler.

[0118] The inorganic filler includes inorganic fillers other than the above-mentioned extender pigments, such as glass, silica, colloidal silica, barium sulfate, alumina, bentonite, etc. The inorganic fillers may be used alone or in combination of two or more kinds.

[0119] In one embodiment, the filler (D4) may be present in an amount of preferably 0% by mass or more and 150% by mass or less, more preferably 0% by mass or more and 70% by mass or less, and even more preferably 0% by mass or more and 50% by mass or less, based on a total of 100 parts by mass of the particulate material (D); in another embodiment, the filler (D4) may be present in an amount of preferably 10% by mass or more and 150% by mass or less, more preferably 15% by mass or more and 120% by mass or less, and even more preferably 20% by mass or more and 110% by mass or less, based on a total of 100 parts by mass of the particulate material (D).

[0120] The total content of the antirust pigment (D1), the extender pigment (D2), the coloring pigment (D3) and the filler (D4) may be preferably from 90% by mass to 100% by mass, more preferably from 95% by mass to 100% by mass, based on 100 parts by mass of the particulate material (D).

[0121] The particulate material (D) may contain other particulate materials (D5) in addition to the rust-preventive pigment (D1), the extender pigment (D2), the coloring pigment (D3), and the filler (D4).

[0122] The particulate material (D) may be used alone or in combination of two or more kinds.

[0123] The content of the particulate material (D) is preferably more than 120 parts by mass and not more than 380 parts by mass, more preferably 125 parts by mass or more and not more than 350 parts by mass, even more preferably 130 parts by mass or more and not more than 300 parts by mass, and even more preferably 130 parts by mass or more and not more than 250 parts by mass, relative to 100 parts by mass of the total resin solid content of the undercoat paint composition. When the content of the particulate material (D) is within the above range, a multilayer coating film with good appearance can be obtained.

[0124] The true density of the particulate material (D) is preferably 0.8 g / cm 3 More than 12g / cm 3 Less than 1 g / cm, more preferably 1 g / cm 3 More than 10g / cm 3 More preferably, 1.1 g / cm3 or more and 9 g / cm3 or more. 3 The density of the particulate material (D) is within the above range, and thus a multi-layer coating film having a good appearance can be obtained. In the present disclosure, the true density of the particulate material means a density calculated from the volume of the particulate material itself (the volume of the container excluding the gaps when the particulate material is filled in a container of a certain volume). The true density of the particulate material (D) can be measured by a method conforming to JIS K 0061 using a Gay-Lussac type pycnometer.

[0125] The particulate material (D) has an average particle size of preferably 50 nm or more and 30 μm or less, more preferably 80 nm or more and 20 μm or less, and even more preferably 100 nm or more and 10 μm or less. In the present disclosure, the average particle size of the particulate material (D) refers to the average particle size (D50) determined by a conventional measuring device using a laser scattering method, a diffraction method, or the like, and can be measured using, for example, a laser diffraction type particle size distribution measuring device SALD-2300 (manufactured by Shimadzu Corporation).

[0126] The particulate material (D) may be contained in either the aqueous base agent (I) or the aqueous curing agent (II), and is preferably contained in the aqueous base agent (I).

[0127] The internal stress of the undercoat coating film formed from the undercoat coating composition is preferably 0.09 MPa or more and 0.50 MPa or less, more preferably 0.17 MPa or more and 0.42 MPa or less, and even more preferably 0.27 MPa or more and 0.35 MPa or less.

[0128] In the present disclosure, the internal stress of the undercoat coating film formed from the undercoat coating composition can be measured by the "Inoue and Obata method" (Sato Kozo; Polymer Processing, 42(11), 557(1993)). Specifically, first, a PET film strip having a thickness of 100 μm is coated with the undercoat paint composition so that the thickness of the dried coating film is h1, and a PET film having a coating film is obtained. Next, knife edges are arranged at intervals of 70 mm, and the PET film having the coating film is placed on the knife edges. Then, the temperature is raised to 60° C., and the coating film is cured by holding at 60° C. for 60 minutes to obtain a coating film. Then, the film is cooled to 20° C. over 5 minutes, and the deflection amount (δ) of the PET film at 20° C. at that time is measured. The internal stress is calculated from each measured value according to the following formula. S = 1 / 6h1(h1+h2) × E2h2 3 / (1-ν2 2 )×1 / ρ [In the formula, S: Internal stress h1: thickness of coating h2: Thickness of PET film E2: Elastic modulus of PET film ν2: Poisson's ratio of PET film ρ: radius of curvature [ρ=L 2 / 8δ (where L is the distance between the knife edges (70 mm), and δ is the amount of deflection)

[0129] <Preparation of Undercoat Paint Composition> The aqueous base agent (I) and the aqueous curing agent (II) of the undercoat paint composition can be prepared by mixing the various components by a method known to those skilled in the art. The paint composition can be prepared by a method commonly used by those skilled in the art. For example, a kneading and mixing means using a kneader or roll, or a dispersing and mixing means using a sand grind mill or disperser, or other methods commonly used by those skilled in the art can be used.

[0130] The undercoat paint composition may contain other components in addition to the above components depending on the purpose and use. Examples of other components include organic solvents, resin components, dispersants, curing catalysts, viscosity modifiers, film-forming assistants, and additives commonly used in paint compositions (e.g., ultraviolet absorbers, light stabilizers, antioxidants, defoamers, surface conditioners, pinhole inhibitors, rust inhibitors, etc.). These components can be added to the base agent and / or curing agent in a manner that does not impair the various physical properties of the paint composition of the present disclosure and / or the resulting multilayer coating film.

[0131] Regarding the timing of mixing the water-based main agent (I) and the water-based hardener (II) in the undercoat paint composition, the water-based main agent (I) and the water-based hardener (II) may be mixed before use and applied by a normal painting method. Alternatively, the water-based main agent (I) and the water-based hardener (II) may be mixed by a two-liquid mixing gun, and the liquids may be delivered to the gun and mixed at the tip of the gun.

[0132] (Topcoat paint composition) The topcoat paint composition is not particularly limited, but it is preferable to use a two-liquid curing paint composition containing a base agent (III) and a curing agent (VI). The base agent (III) and the curing agent (VI) are stored separately and mixed immediately before painting, and the mixture is used for painting.

[0133] In one embodiment, the base material (III) contains a film-forming resin having a hydroxyl group, and the curing agent (VI) contains a polyisocyanate compound. The hydroxyl group of the film-forming resin reacts with the isocyanate group of the polyisocyanate compound to form a urethane bond, thereby curing the topcoat coating film.

[0134] The topcoat paint composition may be an aqueous paint composition using water as a dispersion medium, or a solvent-based paint composition using a solvent as a dispersion medium.

[0135] (Water-based topcoat paint composition) The aqueous topcoat paint composition is preferably a two-liquid curing type paint composition containing an aqueous main agent (IIIa) as the main agent (III) and an aqueous curing agent (VIa) as the curing agent (VI). The aqueous main agent (IIIa) preferably contains a film-forming resin having a hydroxyl group, and the aqueous curing agent (VIa) preferably contains a polyisocyanate compound.

[0136] In one embodiment, the aqueous base agent (IIIa) contains an acrylic resin water dispersion (Fa) as a coating film-forming resin, and the aqueous curing agent (VIa) contains a water-dispersible polyisocyanate (Ga) as a polyisocyanate compound.

[0137] Acrylic resin water dispersion (Fa) The acrylic resin contained in the acrylic resin water dispersion (Fa) is a polymer of a monomer mixture containing an ethylenically unsaturated monomer, and has a hydroxyl group. The main component (IIIa) contains the acrylic resin water dispersion (Fa), so that the multilayer coating film can be given good coating film performance such as adhesion and water resistance. In addition, a multilayer coating film with a smooth surface shape can be formed, for example, a multilayer coating film with excellent surface smoothness can be formed.

[0138] The ethylenically unsaturated monomers include hydroxyl group-containing monomers, carboxyl group-containing monomers and other monomers.

[0139] Examples of the hydroxyl group-containing monomer include (meth)acrylic acid hydroxyalkyl esters such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2,3-dihydroxybutyl (meth)acrylate; (meth)acrylic acid polyalkylene glycol monoesters such as polyethylene glycol mono(meth)acrylate; and the (meth)acrylic acid hydroxyalkyl esters and the (meth)acrylic acid polyalkylene glycol monoesters modified with ε-caprolactone (also referred to as "ε-caprolactone modified (meth)acrylates"). Specific examples of the ε-caprolactone modified (meth)acrylates include PLACCEL FA-1, PLACCEL FA-2, PLACCEL FA-3, PLACCEL FA-4, PLACCEL FA-5, PLACCEL FM-1, PLACCEL FM-2, PLACCEL FM-3, PLACCEL FM-4, and PLACCEL FM-5 manufactured by Daicel Chemical Industries, Ltd. In this specification, (meth)acrylic acid means acrylic acid and methacrylic acid.

[0140] Examples of the carboxyl group-containing monomer include monocarboxylic acids such as (meth)acrylic acid, 2-ethylpropenoic acid, and crotonic acid; dicarboxylic acids such as maleic acid, fumaric acid, and itaconic acid; and dicarboxylic acid monoesters such as ethyl maleate, butyl maleate, ethyl itaconate, and butyl itaconate. Preferred carboxyl group-containing ethylenically unsaturated monomers are acrylic acid, methacrylic acid, and the like.

[0141] Examples of other monomers used in the acrylic resin water dispersion (Fa) include styrene-based monomers such as styrene, α-methylstyrene, and vinyltoluene; (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-, i- and t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and lauryl (meth)acrylate; and amides such as (meth)acrylamide. The ethylenically unsaturated monomers may be used alone or in combination of two or more kinds.

[0142] The hydroxyl value of the acrylic resin water dispersion (Fa) is preferably 40 to 200 mgKOH / g, more preferably 50 to 200 mgKOH / g. When the hydroxyl value is 40 mgKOH / g or more, the reactivity with the water-dispersible polyisocyanate (Ga) is good, and the physical strength of the resulting multi-layer coating film is easily maintained. When the hydroxyl value is 200 mgKOH / g or less, the water resistance of the resulting multi-layer coating film is easily maintained.

[0143] The acid value of the acrylic resin water dispersion (Fa) is preferably 2 to 150 mgKOH / g, more preferably 2 to 100 mgKOH / g. When the acid value is within the above range, it is easy to maintain the physical strength of the resulting multilayer coating film. In this specification, the acid value and the hydroxyl value are both values ​​calculated as solid contents and are values ​​measured by a method in accordance with JIS K 0070.

[0144] The number average molecular weight of the acrylic resin water dispersion (Fa) is preferably 1,000 to 100,000, more preferably 1,000 to 50,000. When the number average molecular weight is 1,000 or more, it is easy to maintain the physical strength of the resulting multilayer coating film, and when the number average molecular weight is 100,000 or less, it is easy to maintain the smoothness of the resulting multilayer coating film.

[0145] The acrylic resin water dispersion (Fa) can be prepared by polymerizing the monomer mixture in the absence of a solvent or in the presence of a suitable organic solvent, dropping and mixing the resulting polymer into water, and removing excess solvent as necessary.

[0146] A polymerization initiator may be used in the polymerization reaction, and a radical polymerization initiator may be used as the polymerization initiator. Specific examples of the polymerization initiator include organic peroxides such as benzoyl peroxide, t-butyl peroxide, and cumene hydroperoxide; and organic azo compounds such as azobiscyanovaleric acid and azoisobutyronitrile.

[0147] The polymerization temperature may be, for example, 80 to 140°C, and the polymerization time may be, for example, 1 to 8 hours, which can be appropriately adjusted depending on the polymerization temperature and the reaction scale. The polymerization reaction may be carried out, for example, by dropping the monomer mixture and a polymerization initiator used as necessary into a heated polymerization solvent. The polymerization solvent is not particularly limited, but is preferably one having a boiling point of about 60 to 250°C. Examples of polymerization solvents that can be suitably used include non-water-soluble organic solvents such as butyl acetate, xylene, toluene, methyl isobutyl ketone, propylene glycol, dipropylene glycol dimethyl ether, and methyl ether acetate; and water-soluble organic solvents such as tetrahydrofuran, ethanol, methanol, propanol, isopropanol, 2-butanol, t-butyl alcohol, dioxane, methyl ethyl ketone, ethylene glycol, ethylene glycol monobutyl ether, 2-methoxypropanol, 2-butoxypropanol, diethylene glycol monobutyl ether, butyl diglycol, N-methylpyrrolidone, ethylene carbonate, and propylene carbonate.

[0148] A neutralizing agent may be added to the acrylic resin obtained by polymerization to neutralize at least a part of the acid groups contained in the acrylic resin. This process can impart good water dispersibility to the acrylic resin. The neutralizing agent is not particularly limited, and examples thereof include organic amines such as monomethylamine, dimethylamine, trimethylamine, triethylamine, diisopropylamine, monoethanolamine, diethanolamine, and dimethylethanolamine; and inorganic bases such as sodium hydroxide, potassium hydroxide, and lithium hydroxide. These neutralizing agents may be used alone or in combination of two or more.

[0149] The acrylic resin water dispersion (Fa) can be prepared by mixing water with the acrylic resin, which has been neutralized as necessary, or by mixing the acrylic resin in water. In preparing the acrylic resin water dispersion (Fa), if necessary, the excess organic solvent may be removed before the addition of the neutralizing agent or after the dispersion in water.

[0150] The acrylic resin water dispersion (Fa) may be a commercially available one. The commercially available one is not particularly limited, and examples thereof include MACRYNAL series (manufactured by Surface Specialties) such as MACRYNAL VSM6299 / 42WA, BAYHYDROL series (manufactured by Bayer AG) such as BURNOCK WD-551, BURNOCK series (manufactured by DIC), and NeoCryl series (manufactured by DSM) such as NeoCryl XK-555.

[0151] Water-dispersible polyisocyanate (Ga) The water-dispersible polyisocyanate (Ga) is a compound having water dispersibility and having two or more isocyanate groups in one molecule, and is a polyisocyanate compound that can be dispersed without separation when added to an aqueous medium. The water-dispersible polyisocyanate (Ga) may be modified with a hydrophilic compound having a hydrophilic group, if necessary. The hydrophilic group may be an ionic hydrophilic group or a nonionic hydrophilic group.

[0152] Examples of the water-dispersible polyisocyanate (Ga) include aromatic diisocyanates such as tolylene diisocyanate (TDI), 4,4'-diphenylmethane diisocyanate (MDI), xylylene diisocyanate (XDI), and meta-xylylene diisocyanate (MXDI); aliphatic diisocyanates such as trimethylene diisocyanate, tetramethylene diisocyanate, and hexamethylene diisocyanate (HDI); alicyclic polyisocyanates such as cyclohexane diisocyanate, dicyclohexylmethane diisocyanate, and isophorone diisocyanate (IPDI); and polymers such as biuret, isocyanurate, and trimethylolpropane (TMP) adduct of such aromatic diisocyanates, aliphatic diisocyanates, and alicyclic polyisocyanates. These water-dispersible polyisocyanates (Ga) may be used alone or in combination of two or more. In the present disclosure, "alicyclic" means having an alicyclic structure in the molecule.

[0153] The water-dispersible polyisocyanate (Ga) is preferably an aliphatic diisocyanate and / or an alicyclic polyisocyanate, more preferably hexamethylene diisocyanate (HDI) and / or isophorone diisocyanate (IPDI). Aliphatic diisocyanates and alicyclic polyisocyanates have lower reactivity than aromatic polyisocyanates, and can suppress side reactions with aqueous media such as water.

[0154] In the water-dispersible polyisocyanate (Ga), the polyisocyanate group may be modified, and a crosslinked structure by a plurality of isocyanate groups may exist between a plurality of polyisocyanate compounds or within a single polyisocyanate compound. Since the polymeric polyisocyanate compound is trifunctional or more, at least one of the plurality of isocyanate groups may be modified, and at least two isocyanate groups may contribute to the formation of a crosslinked structure.

[0155] In the aqueous topcoat paint composition, the molar ratio (NCO / OH) of the isocyanate group of the water-dispersible polyisocyanate (Ga) to the hydroxyl group of the acrylic resin water dispersion (Fa) is preferably 0.5 to 3.0, more preferably 0.8 to 2.0. The molar ratio (NCO / OH) in this range has the advantage that the curing reactivity of the aqueous topcoat paint composition can be secured in a good range.

[0156] The aqueous base agent (IIIa) and the aqueous curing agent (IVa) contain water as a dispersion medium. As the water, ion-exchanged water, distilled water, etc. may be used.

[0157] In the aqueous topcoat paint composition, the aqueous main agent (IIIa) and / or the aqueous curing agent (IVa) may contain an organic solvent as necessary. Examples of organic solvents include butyl acetate, xylene, toluene, methyl isobutyl ketone, propylene glycol, dipropylene glycol dimethyl ether, methyl ether acetate, tetrahydrofuran, ethanol, methanol, propanol, isopropanol, 2-butanol, t-butyl alcohol, dioxane, methyl ethyl ketone, ethylene glycol, ethylene glycol monobutyl ether, ethylene glycol monobutyl ether acetate (butyl cellosolve acetate), propylene glycol monomethyl ether acetate, 2-methoxypropanol, 2-butoxypropanol, diethylene glycol monobutyl ether, butyl diglycol, N-methylpyrrolidone, ethylene carbonate, and propylene carbonate. These organic solvents may be organic solvents used in the preparation of the acrylic resin water dispersion (Fa), water-dispersible polyisocyanate (Ga), etc., or may be added separately in the preparation of the aqueous paint composition.

[0158] In the aqueous base agent (IIIa) and the aqueous curing agent (IVa), the water content in the total of water and the organic solvent may be, for example, from 50% by mass to 100% by mass, or from 70% by mass to 100% by mass.

[0159] Other Ingredients In addition to the above components, the water-based topcoat paint composition may contain other components such as pigments, resin particles, resin components, dispersants, curing catalysts, viscosity agents, film-forming assistants, and additives commonly used in paint compositions (e.g., ultraviolet absorbers, light stabilizers, antioxidants, defoamers, surface conditioners, pinhole inhibitors, rust inhibitors, etc.) depending on the purpose and use. These components may be contained in either the water-based main agent (IIIa) or the water-based curing agent (IVa).

[0160] (Solvent-based topcoat paint composition) The solvent-based topcoat paint composition is preferably a two-liquid curing paint composition containing a main agent (IIIb) as the main agent (III) and a curing agent (IVb) as the curing agent (IV). The main agent (IVb) preferably contains an acrylic resin (Fb) as a film-forming resin, and the curing agent (IVb) preferably contains a polyisocyanate compound (Gb). The main agent (IIIb) and / or the curing agent (IVb) may contain an extender pigment and a viscosity modifier.

[0161] Acrylic resin (Fb) The acrylic resin (Fb) is a polymer of a monomer mixture containing an ethylenically unsaturated monomer, and has a hydroxyl group. The main component (IIIb) contains the acrylic resin (Fb), which can impart adhesion of the topcoat coating film to the undercoat coating film, and can impart good coating film performance such as water resistance to the multilayer coating film. In addition, a multilayer coating film with a smooth surface shape can be formed, for example, a multilayer coating film with excellent surface smoothness can be formed.

[0162] The ethylenically unsaturated monomers include hydroxyl group-containing monomers and other monomers.

[0163] Examples of the hydroxyl group-containing monomer include the above-mentioned (meth)acrylic acid hydroxyalkyl ester, the above-mentioned (meth)acrylic acid polyalkylene glycol monoester, and the above-mentioned ε-caprolactone-modified (meth)acrylate.

[0164] Examples of other monomers used in the acrylic resin (Fb) include the carboxyl group-containing monomers; the (meth)acrylic acid alkyl esters; alicyclic (meth)acrylic monomers such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, tricyclodecanyl (meth)acrylate, and adamantyl (meth)acrylate; amino group-containing (meth)acrylic acid esters such as aminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, and butylaminoethyl (meth)acrylate; amide group-containing monomers such as (meth)acrylamide, N-methylol (meth)acrylamide, methoxybutyl (meth)acrylamide, and diacetone (meth)acrylamide; amino group-containing (meth)acrylamides such as aminoethyl (meth)acrylamide, dimethylaminomethyl (meth)acrylamide, and methylaminopropyl (meth)acrylamide; cyanide vinyl monomers such as (meth)acrylonitrile and α-chloroacrylonitrile; saturated aliphatic carboxylic acid vinyl ester monomers such as vinyl acetate and vinyl propionate; and the styrene monomers. The ethylenically unsaturated monomers may be used alone or in combination of two or more kinds.

[0165] As the other monomers used in the acrylic resin (Fb), the (meth)acrylic acid alkyl esters and the alicyclic (meth)acrylic monomers are preferred, and acrylic acid, methacrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, cyclohexyl (meth)acrylate, and the like are more preferred.

[0166] The hydroxyl value of the acrylic resin (Fb) is preferably 40 to 200 mgKOH / g, more preferably 50 to 200 mgKOH / g. When the solid content hydroxyl value is within the above range, it can be appropriately reacted with the polyisocyanate compound (Gb) described later, and desired coating film properties can be obtained.

[0167] The acid value of the acrylic resin (Fb) is preferably 2 to 150 mgKOH / g, more preferably 5 to 30 mgKOH / g. When the acid value is within the above range, desired coating film properties can be obtained. The solid content acid value of the acrylic resin (Fb) is more preferably 5 to 30 mgKOH / g.

[0168] The number average molecular weight of the acrylic resin (Fb) is preferably 3,000 to 20,000, more preferably 3,500 to 15,000, and even more preferably 3,500 to 12,000. By making the number average molecular weight of the acrylic resin (Fb) 3,000 or more, the drying property of the coating composition can be improved, and the adhesion of dust due to the stickiness of the coating composition scattered in the coating booth can be prevented, and a good coating environment can be maintained, and the coating film properties of the obtained multilayer coating film can be made good. In addition, by making the number average molecular weight of the acrylic resin (Fb) 20,000 or less, the gloss of the multilayer coating film can be improved.

[0169] The acrylic resin (Fb) can be produced by polymerizing the monomer mixture without a solvent or in the presence of a suitable organic solvent. Examples of the polymerization method include a radical polymerization method, which can be carried out using a radical polymerization initiator, and specifically, may be a bulk polymerization method, a solution polymerization method, a bulk-suspension two-stage polymerization method in which suspension polymerization is carried out after bulk polymerization, and the like. Among these, the solution polymerization method is particularly preferred, and examples thereof include a method in which the monomer mixture is heated with stirring in the presence of a radical polymerization initiator at a temperature of, for example, 80 to 200°C.

[0170] The acrylic resin (Fb) may be a commercially available one. The commercially available one is not particularly limited, and examples thereof include the Acrydic series (manufactured by DIC Corporation) such as Acrydic A-428, the Dianale series (manufactured by Mitsubishi Chemical Corporation) such as Dianale LC-2657, and the Hitaleoid series (manufactured by Showa Denko Materials Co., Ltd.).

[0171] The film-forming resin may contain, in addition to the acrylic resin (Fb), polyester resin, epoxy resin, etc., as necessary.

[0172] The solid content of the acrylic resin (Fb) in the 100% solid content of the coating film-forming resin is preferably 40% by mass or more and 100% by mass or less, more preferably 50% by mass or more and 100% by mass or less, in one embodiment 50% by mass or more and 90% by mass or less, and in another embodiment 90% by mass or more and 100% by mass or less, from the viewpoint of the water resistance of the coating film and the finish quality. By including the acrylic resin in such a range, a multilayer coating film having excellent drying properties can be formed on the substrate by wet-on-wet coating. In the present disclosure, the "solid content of the film-forming resin" means the total amount of the solid content of the acrylic resin, the solid content of the epoxy resin, and the solid content of other resins that may be contained in the film-forming resin.

[0173] Polyisocyanate compound (Gb) The polyisocyanate compound (Gb) refers to a compound having two or more isocyanate groups in one molecule.

[0174] The polyisocyanate compound (Gb) may be an aliphatic diisocyanate, an alicyclic diisocyanate, an aromatic diisocyanate, or a polymer of an aliphatic diisocyanate, an alicyclic diisocyanate, or an aromatic diisocyanate. Such a polyisocyanate compound (Gb) may be a so-called asymmetric type. The number of carbon atoms contained in the polyisocyanate compound (Gb) is preferably 5 to 24, more preferably 6 to 18.

[0175] Examples of the aliphatic diisocyanate include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate (HDI), 2,2,4-trimethylhexane diisocyanate, undecane diisocyanate-(1,11), lysine ester diisocyanate, diethylene glycol diisocyanate, dipropylene glycol diisocyanate, triethylene glycol diisocyanate, and thiodipropyl diisocyanate. Examples of the alicyclic diisocyanate include cyclohexane diisocyanate, isophorone diisocyanate (IPDI), and dicyclohexylmethane diisocyanate. Examples of the aromatic diisocyanate include 1,5-dimethyl-2,4-bis(isocyanatomethyl)benzene, 1,5-trimethyl-2,4-bis(ω-isocyanatoethyl)-benzene, 1,3,5-trimethyl-2,4-bis(isocyanatomethyl)benzene, 1,3,5-triethyl-2,4-bis(isocyanatomethyl)benzene, 2,4- and / or 2,6-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and 1,4-diisocyanatoisopropylbenzene. Examples of the polymer include a biuret, an isocyanurate, and an adduct with trimethylolpropane (TMP). The polyisocyanate compounds (Gb) may be used alone or in combination of two or more kinds.

[0176] The polyisocyanate compound (Gb) preferably contains at least an isocyanurate of the aliphatic diisocyanate, alicyclic diisocyanate, or aromatic diisocyanate, and more preferably contains an isocyanurate of the aliphatic diisocyanate. When the polyisocyanate compound (Gb) contains such an isocyanurate, the content of the isocyanurate in the polyisocyanate compound (Gb) is preferably 60 mass% or more.

[0177] In the solvent-based topcoat paint composition, the molar ratio (NCO / OH) of the isocyanate group of the polyisocyanate compound (Gb) to the hydroxyl group of the coating film-forming resin is preferably within the range of 0.5 to 2.0, more preferably 0.8 to 1.6. By having the molar ratio (NCO / OH) within this range, the solvent-based topcoat paint composition is sufficiently cured, and the desired coating film properties are obtained.

[0178] In one embodiment, the acrylic resin (Fb) may be combined with a body pigment and a viscosity modifier. This combination can further improve the drying properties of the paint and improve the appearance of the coating film.

[0179] Examples of the extender pigment include talc, clay, calcium carbonate, magnesium carbonate, barium sulfate, silicic acid, silicates, aluminum oxide hydrate, calcium sulfate, gypsum, micaceous iron oxide (MIO), glass flake, szolite mica, and clarite mica.

[0180] In one embodiment, the extender pigment is selected from the group consisting of calcium carbonate, barium sulfate, and talc. For example, heavy calcium carbonate, light calcium carbonate, precipitated barium sulfate, and surface-treated talc can be used. Such extender pigments can be used alone or in combination.

[0181] The amount of the extender pigment is preferably from 0 to 100 parts by mass, more preferably from 0 to 50 parts by mass, per 100 parts by mass of the coating film-forming resin.

[0182] By using the extender pigment in combination with the viscosity modifier, the viscosity and viscosity behavior of the topcoat paint composition, for example, viscosity recovery, can be appropriately adjusted, and appropriate leveling and sagging properties can be imparted.

[0183] As the viscosity modifier, any of the compounds described above as the viscosity modifier used in the undercoat paint composition can be used.

[0184] The viscosity adjusting agent is preferably from 0.01 to 20 parts by mass, more preferably from 0.05 to 10 parts by mass, and even more preferably from 0.1 to 5 parts by mass, relative to 100 parts by mass of the coating film-forming resin.

[0185] For example, when the total solid content of the film-forming resin, the curing agent (IVb), the body pigment and the viscosity modifier is taken as 100% by mass, the solid content of the film-forming resin (Fb) is preferably from 20% by mass to 80% by mass, more preferably from 30% by mass to 60% by mass, and in one embodiment from 35% by mass to 55% by mass.

[0186] In the solvent-based topcoat paint composition, the main agent (IIIb) and the curing agent (IVb) contain an organic solvent as a dispersion medium. The organic solvent can include those that are commonly used in solvent-based paints, such as methyl ethyl ketone, cyclohexanone, Solvesso 100 (manufactured by Exxon Chemical), methoxybutyl acetate, cellosolve acetate, butyl cellosolve acetate, methyl acetate, ethyl acetate, butyl acetate, petroleum ether, and petroleum naphtha. In particular, paint design is possible without selecting organic solvents that are subject to legal restrictions on the specialized side depending on the amount used, such as xylene.

[0187] Other ingredients The solvent-based topcoat paint composition may contain various known additives as necessary. As the various additives, additives used in paint compositions can be appropriately used, such as pigments such as coloring pigments and rust-preventive pigments, anti-sagging and anti-settling agents, curing catalysts (organometallic catalysts), color separation inhibitors, dispersants, defoamers and anti-popping agents, thickeners, leveling agents, matting agents, ultraviolet absorbers, antioxidants, plasticizers, film-forming assistants, organic solvents, etc. The blending amounts of these components are appropriately adjusted within a range that does not impair the effects of the present disclosure. EXAMPLES

[0188] The present disclosure will be described more specifically with reference to the following examples, but the present disclosure is not limited thereto.

[0189] <Production Example 1> Production example of undercoat paint composition <Preparation example of pigment dispersion paste for undercoat paint composition> In a dispersion vessel, 38.00 parts by mass of BECKOPOX EP386W / 52WA as an epoxy resin water dispersion (A-1), 21.79 parts by mass of ion-exchanged water, 2.00 parts by mass of IPA as an organic solvent (C-1), 0.45 parts by mass of BYK-011 as an antifoaming agent, 1.90 parts by mass of BYK-2015 as a dispersing agent, 5.70 parts by mass of LF BOUSEI ZP-DL as a pigment as a particulate material (D1-1), 6.80 parts by mass of SSS talc as a particulate material (D2-1), 21.80 parts by mass of LOMON Titanium Dioxide R-996 as a particulate material (D3-1), and 0.50 parts by mass of TAROX synthetic iron oxide LL-XLO as a particulate material (D3-2) were premixed using a disperser. Thereafter, a dispersion treatment was carried out using an SG mill (dispersion medium: glass beads) at 1,500 rpm until the dispersed particle size of the pigment became 20 μm or less, to obtain a pigment dispersion paste 1 for an undercoat coating composition.

[0190] <Preparation example of undercoat paint composition> (Water-based base for undercoat paint composition) 98.94 parts by mass of pigment dispersion paste 1 for undercoat coating composition and 1.06 parts by mass of Surfynol 440 as a surface conditioner were mixed and stirred using a disper to obtain water-based base agent for undercoat coating composition 1. In addition, water-based base agents 2 to 37 for undercoat coating composition were obtained in the same manner, except that the components and amounts shown in Tables 1 to 4 were used instead of the above components.

[0191] (Water-based hardener for undercoat paint composition) 10.84 parts by mass of LUCKAMIDE WN-720Z as the polyamine compound (B1-1), 5.50 parts by mass of IPA as the organic solvent (C1-1), and 0.25 parts by mass of ion-exchanged water were mixed and stirred using a disper to obtain an aqueous curing agent 1. In addition, aqueous curing agents 2 to 6 were obtained in the same manner, except that the components and amounts shown in Tables 1 to 4 were used instead of the above components.

[0192] (Details of materials used in the undercoat paint composition) Epoxy resin water dispersion (A) (A-1) BECKOPOX EP 386w / 52WA (Allnex, bisphenol A type epoxy resin dispersion), epoxy equivalent: 520g / eq, solid content: 52% by mass (A-2) EM-0427WC (Adeka, bisphenol A type epoxy resin water dispersion), epoxy equivalent: 230g / eq, solid content: 50% by mass (A-3) BECKOPOX EP 2307W / 45WAMP (Allnex, bisphenol A type epoxy resin dispersion), epoxy equivalent: 1,980 g / eq, solid content: 45% by mass Polyamine Compound (B) (B1-1) LUCKAMIDE WN-720Z (DIC Corporation, polyamine compound), active hydrogen equivalent: 177 g / eq, solid content: 50% by mass (B1-2) B-5115 (manufactured by Daito Sangyo Co., Ltd., polyamine compound), active hydrogen equivalent: 39 g / eq, solid content: 100% by mass (B1-3) J-1033 (manufactured by Daito Sangyo Co., Ltd., polyamine compound), active hydrogen equivalent: 60 g / eq, solid content: 100% by mass (B-4) Aradur3986 (manufactured by HUNTSMAN Advanced Material, polyamide amine compound aqueous dispersion), active hydrogen equivalent: 415g / eq, solid content concentration: 40% by mass Organic Solvent (C) (C1-1) IPA (Shoei Chemical Co., Ltd., isopropyl alcohol), evaporation rate: 1.5 (C1-2) Ethanol (Fujifilm Wako Pure Chemical Industries, Ltd.), evaporation rate: 1.54 (C1-3) PGM (Dowanol PM glycol ether, Dow Chemical Company, propylene glycol monomethyl ether), evaporation rate: 0.71 Particle material (D) (D1-1) LF Bousei ZP-DL (Kikuchi Color Co., Ltd., zinc phosphate-based anti-rust pigment), true density: 4.0 g / cm 3 (D1-2) CLF-102 (Guangxi Academy of Chemical Technology, aluminum tripolyphosphate-based anti-rust pigment), true density: 2.6 g / cm 3 (D2-1) SSS talc (manufactured by Nippon Talc Co., Ltd., talc), true density: 2.9 g / cm 3 (D2-2) NC clay (Inamori Mining Co., Ltd., kaolin), true density: 2.7 g / cm 3 (D3-1) LOMON Titanium Dioxide R-996 (manufactured by SICHUAN LOMON TITANIUM INDUSTRY, titanium dioxide), true density: 4.1g / cm 3 (D3-2) TAROX synthetic iron oxide LL-XLO (Titanium Industries, yellow iron oxide), true density: 5.24 g / cm 3 (D3-3) Carbon MA-100 (Mitsubishi Chemical, carbon black), true density: 2.25 g / cm 3 (D4-1) CRYSTALITE VX-S2 (manufactured by Ryumorisha, high purity crystalline quartz filler), true density: 1.8g / cm 3 (D4-2) Precipitated barium sulfate PS07 (Guangxi Xiangzhou Lianzhuang Chemical Co., Ltd., precipitated barium sulfate), true density: 4.5 g / cm 3 Others (materials for undercoat paint composition) BYK-011 (defoamer, manufactured by BYK Japan), solid content: 30% by mass BYK-2015 (BYK Japan, dispersant), solid content: 40% by mass Surfynol 440 (Evonik Japan, surface conditioner), solid content: 100% by mass

[0193] <Production Example 2> Preparation of water-based topcoat paint composition <Preparation example of pigment dispersion paste for water-based topcoat paint composition> (Pigment dispersion paste for water-based topcoat paint composition) In a dispersion vessel, 23.4 parts by mass of ion-exchanged water, 0.5 parts by mass of BYK-420 as a viscosity modifier, 5.6 parts by mass of BYK-2015 as a dispersant, 2.0 parts by mass of Surfynol440 as an antifoaming agent, and 70 parts by mass of TI-PURE R-960 as a pigment were premixed using a disperser. Then, using an SG mill (dispersion medium: glass beads), dispersion treatment was performed at 1,500 rpm until the coarse particles of the pigment became 15 μm or less, and a pigment dispersion paste 1 for aqueous topcoat paint composition was obtained.

[0194] <Preparation of Water-Based Topcoat Paint Composition> (Water-based base agent for water-based topcoat paint composition 1) 56.0 parts by mass of Setaqua 6515 as an acrylic resin water dispersion, 5.0 parts by mass of Solvesso 100 as an organic solvent, 2.0 parts by mass of butyl cellosolve, 2.0 parts by mass of ion-exchanged water, and 103.5 parts by mass of water-based topcoat pigment dispersion paste 1 were mixed and stirred using a disper to obtain main agent 1 for water-based topcoat paint composition.

[0195] (Curing agent for water-based topcoat paint composition 1) 8.10 parts by mass of Duranate TPA-100 as a water-dispersible polyisocyanate and 2.20 parts by mass of Bibidur 401-60 were mixed and stirred using a disper to obtain a curing agent 1 for a water-based topcoat coating composition.

[0196] <Production Example 3> <Preparation Example of Water-Based Topcoat Paint Composition 2> (Base agent 2 for water-based topcoat paint composition) 227.3 parts by mass of Burnock WD-551 as an acrylic resin water dispersion and 10.0 parts by mass of AQUATIX-8421 as a polyolefin wax water dispersion were mixed and stirred using a disper to obtain a base 2 for a water-based topcoat paint composition.

[0197] (Water-based hardener for topcoat 2) 69.5 parts by mass of Vibidur 304 as a water-dispersible polyisocyanate and 35.3 parts by mass of 2-butoxyethyl acetate as an organic solvent were mixed and stirred using a disperser to obtain curing agent 2 for water-based topcoat paint.

[0198] (Details of materials for preparing water-based topcoat paint composition) Setaqua 6515 (Allnex, polyol acrylic water dispersion), hydroxyl value: 109 mg KOH / g, solids concentration: 45% by mass Burnock WD-551 (DIC Corporation, acrylic dispersion, acid value: 8.5 mg KOH / g, hydroxyl value: 100 mg KOH / g, number average molecular weight: 4,600, solid content: 44% by mass) Duranate TPA-100 (Asahi Kasei Corporation, isocyanurate-type hexamethylene diisocyanate (HDI)), NCO content: 23.1% by mass, solids concentration: 100% by mass Baybijur 401-60 (manufactured by Sumika Bayer Urethane Co., Ltd., hydrophilic modified isophorone diisocyanate), NCO content: 13.3% by mass, solids concentration: 60% by mass Bayvisuel 304 (manufactured by Sumika Bayer Urethane Co., Ltd., nonionic modified polyisocyanate (hydrophilic polyether is added to HDI trimer to generate urethane group, and HDI trimer is further added to introduce allophanate group, so it is an allophanate modified polyisocyanate with nonionic hydrophilic group and allophanate group introduced to isocyanate chain), solid content: 100% by mass AQUATIX-8421 (BYK, polyolefin wax water dispersion, active ingredient concentration: 20% by weight) Solvesso 100 (Shoei Chemical Industry Co., Ltd., organic solvent) -Butyl cellosolve (Sankyo Chemical Co., Ltd., organic solvent) 2-Butoxyethyl acetate (Kanto Chemical, organic solvent) (Details of materials for water-based topcoat paint composition) BYK-420 (viscosity modifier, manufactured by BYK Japan), solid content: 52% by mass BYK-2015 (BYK Japan, dispersant), solid content: 40% by mass Surfynol 440 (Evonik Japan, surface conditioner), solid content: 100% by mass ·TI-PURE R-960 (Titanium oxide, manufactured by DuPont)

[0199] <Preparation of test pieces with multi-layer coating> Example 1 A JIS G 3141 (SPCC-SB) cold-rolled steel plate with a size of 0.8×70×150 mm was degreased with xylene. Next, 100.0 parts by mass of the water-based base agent 1 for undercoat paint composition and 16.60 parts by mass of the water-based hardener 1 for undercoat paint composition obtained in the above-mentioned manufacturing example were mixed using a disperser (undercoat (1)), and the mixture was applied to the steel plate using an air spray so as to have a dry film thickness of 45 μm, forming an undried undercoat paint film. The undercoat (1) was formulated so that the ratio of the active hydrogen equivalent of the polyamine compound in the aqueous curing agent 1 for the undercoat coating composition to the epoxy equivalent of the epoxy resin, which is the film-forming resin in the aqueous main agent 1 for the undercoat coating composition, (active hydrogen equivalent / epoxy equivalent) was 0.8.

[0200] Next, after a 7-minute interval at room temperature (25°C), while the undercoat coating film was still wet, the isocyanate groups of the polyisocyanate compound in the curing agent 1 for the aqueous topcoat coating composition obtained in the above manufacturing example and the hydroxyl groups of the acrylic resin, which is the coating film-forming resin in the main agent 1 for the aqueous topcoat coating composition, were mixed on the surface of the undercoat coating film using a disper so that the molar ratio (NCO / OH) was 1.0 (topcoat (1)), and a wet-on-wet coating was applied using an air spray to a dry film thickness of 45 µm to form a wet topcoat coating film. After leaving it at room temperature (25°C) for 10 minutes, it was dried at 60°C for 60 minutes (forced drying) to obtain a test piece having a multi-layer coating film with a dry film thickness of 90 µm.

[0201] <Examples 2 to 42 and Comparative Examples 1 to 3> Undercoat paint compositions and topcoat paint compositions were produced in the same manner as in Production Example 1, except that the type and / or amount of each component was changed to the amount shown in Tables 1 to 4. In Example 42, the aqueous topcoat paint composition was prepared by mixing the isocyanate groups of the polyisocyanate compound in the curing agent 2 for the aqueous topcoat paint composition obtained in the above Production Example with the hydroxyl groups of the acrylic resin, which is the film-forming resin in the base agent 2 for the aqueous topcoat paint composition, in a disperser so that the molar ratio (NCO / OH) was 1.0 (topcoat (2)). Test pieces having a multi-layer coating film were obtained in the same manner as in Example 1, except that the undercoat coating composition and the topcoat coating composition were used to form coating films under the conditions (combinations of coating compositions) shown in Tables 1 to 4.

[0202] <Reference example> In the Reference Example, instead of applying the undercoat (1) and topcoat (1) wet-on-wet in Example 1, the undercoat (44) was used, which was then forced-dried, and the topcoat (1) was applied on top of it, and then forced-dried again. In other words, this is an example in which a multi-layer coating film was formed in the same manner as in Comparative Example 3 (2 coats, 2 bakes). That is, the undercoat (44) was applied in the same manner as in Example 1, and then after a 7-minute interval at room temperature (25°C), it was dried at 60°C for 60 minutes (forced drying). After leaving it at room temperature (25°C) for 30 minutes, the topcoat (1) was applied in the same manner as in Example 1, and then it was left at room temperature (25°C) for 10 minutes, and then dried at 60°C for 60 minutes (forced drying) to obtain a test piece having a multi-layer coating film with a dry film thickness of 90 μm.

[0203] <Evaluation items> 1) Coating appearance (visual inspection) The appearance of the multi-layer coating films obtained in the Examples and Comparative Examples was visually observed and evaluated according to the following criteria. A score of 3 or more was considered to be acceptable. 5 points: The coating surface is smooth with no roundness (irregularities). When a fluorescent light is reflected on the coating, the image can be clearly seen. 4 points: The coating surface is slightly rounded (uneven), but when a fluorescent light is projected onto the coating, the image can be seen quite clearly. 3 points: The coating surface is rounded (uneven), but when a fluorescent light is projected onto the coating, the image can be seen. 2 points: The coating surface has rounded edges (convex and concave surfaces). When a fluorescent light is projected onto the coating, the image is slightly blurred but still recognizable. 1 point: The coating has many rounded edges (unevenness). When a fluorescent light is projected onto the coating, the image becomes blurred and is almost unrecognizable.

[0204] 2) Coating appearance (20° and 60° gloss values) The gloss values ​​(20° and 60° gloss values) of the multilayer coating films obtained in the Examples and Comparative Examples were measured using a gloss meter, Micro Trigloss (manufactured by BYK), and evaluated according to the following criteria. In all cases, a score of 3 or more was considered to be acceptable. 20° gloss value 5 points: 70 or more 4 points: 65 or more but less than 70 3 points: 60 or more but less than 65 2 points: 55 or more but less than 60 1 point: Less than 55 60° gloss value 5 points: 85 or above 4 points: 80 or more but less than 85 3 points: 75 or more but less than 80 2 points: 70 or more but less than 75 1 point: Less than 70

[0205] 3) Internal stress The internal stress (S) of the undercoat film at 20°C according to the Inoue and Obata method was calculated based on the following formula, by measuring the strain of the undercoat film and the PET film according to the bimetal method shown in Figure 1 (Sato Kozo; Polymer Processing, 42(11), 557(1993)). Specifically, the undercoat paint composition was first applied to a strip-shaped PET film having a thickness of 100 μm so that the dry thickness of the undercoat film was 40 μm (h1), to obtain a PET film having a coating film. Next, knife edges (shown by △ marks in FIG. 1) were placed at intervals of 70 mm, and the PET film having the coating film was placed on the knife edges. Next, the temperature was raised to 60° C. over 3 minutes, and the coating film was cured by holding at 60° C. for 60 minutes to obtain a coating film. Then, the film was cooled to 20° C. over 5 minutes, and the deflection amount (δ) of the PET film at 20° C. at that time was measured, and the internal stress was calculated from each measured value according to the following formula. S = 1 / 6h1(h1+h2) × E2h2 3 / (1-ν2 2 )×1 / ρ During the ceremony, S: Internal stress (Pa) h1: Dry thickness of the coating (40 μm = 40 × 10 -6 m) h2: Thickness of PET film (100 μm = 100 × 10 -6 m) E2: Elastic modulus of PET film (2.45GPa=2.45×10 9 Pa) ν2: Poisson's ratio of PET film (0.4) ρ: radius of curvature [ρ=L 2 / 8δ (wherein, L is the distance between knife edges (70 mm = 70 × 10 -3 m), δ: distortion amount)] It shows.

[0206] 4) Adhesion The multi-layer coating film of the test piece obtained in the examples and comparative examples was cut with a cutter to a depth that reached the steel plate to be coated, with 11 cuts in each direction at intervals of 1 mm, and Cellophane Tape (registered trademark) (manufactured by Nichiban Co., Ltd.) was applied on the cuts and then peeled off, and the number of remaining squares out of 100 squares was counted (cross-cut test). Note that 100 / 100 indicates that the coating film peeled off area was 0%, for example, 90 / 100 indicates that the coating film peeled off area was 10%, and 50 / 100 indicates that the coating film peeled off area was 50%. A score of 95 / 100 was considered to be acceptable. Note that a score of 94 / 100 or less indicates that there is a clear and noticeable problem in practical use.

[0207] [Table 1]

[0208] [Table 2]

[0209] [Table 3]

[0210] [Table 4]

[0211] Examples 1 to 42 are examples of the present invention, in which an aqueous coating composition was used as the undercoat coating composition, and even when wet-on-wet coating was performed, a multilayer coating film having a smooth appearance could be obtained.

[0212] In Comparative Example 1, the content of the particulate material (D) was less than 120 parts by mass, and the appearance of the resulting multilayer coating film was not fully satisfactory. In Comparative Example 2, the content of the particulate material (D) exceeded 380 parts by mass, and the adhesion between the resulting multi-layer coating film and the substrate was not fully satisfactory. Comparative Example 3 is an example that did not contain the particulate material (D), and the appearance of the resulting multilayer coating film was not fully satisfactory. The reference example is an example in which a multi-layer coating film was formed using the same undercoat and topcoat as in Comparative Example 3 (an example not containing particulate material (D)) without wet-on-wet coating and by a normal method (2 coats, 2 bakes). In this case, a multi-layer coating film having a smooth appearance could be obtained even without containing particulate material (D).

Claims

1. A primer coating film forming step of applying a primer coating composition to an object to be coated to form a primer coating film; A topcoat coating film forming step of applying a topcoat coating composition wet-on-wet on the undercoat coating film to form a topcoat coating film; and A method for producing a multi-layer coating film, comprising: a drying step of simultaneously drying the undercoat coating film and the topcoat coating film to form a multi-layer coating film; The undercoat coating composition comprises: An aqueous coating composition comprising an aqueous base agent (I) and an aqueous curing agent (II), The aqueous base agent (I) contains an aqueous dispersion of an epoxy resin (A), The aqueous curing agent (II) contains a polyamine compound (B), At least one of the aqueous base agent (I) and the aqueous curing agent (II) contains a particulate material (D), The content of the particulate material (D) is more than 120 parts by mass and not more than 380 parts by mass per 100 parts by mass of the resin solid content contained in the aqueous coating composition. A method for manufacturing multi-layer coating films.

2. 2. The production method according to claim 1, wherein the polyamine compound (B) comprises at least one selected from the group consisting of aliphatic polyamines, alicyclic polyamines, aromatic polyamines, polyoxyalkylene group-containing polyamines, polyoxyalkylene group-containing aromatic polyamines, and polyamidoamine compounds.

3. 2. The production method according to claim 1, wherein the polyamine compound (B) comprises at least one selected from the group consisting of alicyclic polyamines, aromatic polyamines, polyoxyalkylene group-containing aromatic polyamines, and polyamidoamines.

4. The method for producing a multi-layer coating film according to claim 1, wherein the epoxy equivalent of the epoxy resin (A) is 100 g / eq or more and 5,000 g / eq or less.

5. 2. The method for producing a multi-layer coating film according to claim 1, wherein the active hydrogen equivalent of the polyamine compound (B) is 10 g / eq or more and 1,000 g / eq or less.

6. 2. The method for producing a multilayer coating film according to claim 1, wherein the ratio (active hydrogen equivalent / epoxy equivalent) of the active hydrogen equivalent of the polyamine compound (B) to the epoxy equivalent contained in the epoxy resin (A) is 0.3 or more and 2.0 or less.

7. The method for producing a multilayer coating film according to claim 1, wherein the internal stress of the primer coating film formed from the primer coating composition is 0.09 MPa or more and 0.50 MPa or less.

8. The topcoat paint composition is a paint composition containing a base agent (III) and a curing agent (IV), The main component (III) contains a film-forming resin, the film-forming resin has a hydroxyl group, The method for producing a multi-layer coating film according to any one of claims 1 to 7, wherein the curing agent (IV) contains a polyisocyanate compound.

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