JP5456496B2 - Method for forming high-saturation multilayer coating film and painted product - Google Patents

Description

  The present invention is a method for forming a multilayer coating film comprising at least an intermediate coating film, a first base coating film, a second base coating film, and a clear coating film, which are sequentially formed on an object to be coated, with high saturation. And it is related with the said method of forming the multilayer coating film which has a deep design property.

  Car body painting is usually carried out by sequentially forming an electrodeposition coating film, an intermediate coating film, and a top coating film on an object to be coated. In the conventional method, after electrodeposition coating is performed by immersing an object to be coated in an electrodeposition paint, the paint is cured by baking at a high temperature to form an electrodeposition paint film. The process of applying an intermediate coating on the top, baking to form an intermediate coating, and further applying the top coating on the intermediate coating and then baking to form the top coating It was the target. However, it is not preferable to perform the baking treatment every time when each coating film is formed as described above from the viewpoint of resource and cost reduction. Therefore, in recent years, wet-on-wet coating has been performed in which the baking process is not performed after the intermediate coating and / or top coating is applied, and the coating of the next process is applied over the uncured coating. It has become.

  For example, metallic paint colors and mica paint colors, which have become the mainstream in recent years, are base paints that include glossy pigments for obtaining glossiness and colored pigments for obtaining chroma as transparent paints, and transparent clear paints. Is used. Usually, as the glitter pigment, an aluminum flake pigment having metallic luster is used in the case of metallic paint color, and mica having interference is used in the case of mica paint color. In the case of a multi-layer coating film of these coating colors, it was obtained after forming the uncured coating film by wet-on-wet coating of the base paint and clear paint on the baked intermediate coating film. The uncured coating is cured by a single baking process.

  However, when wet-on-wet coating of metallic or mica is used, the metallic luster, metallic design and / or flip-flop properties are caused by disturbing the orientation of the glitter pigment contained in the base coating. There are problems such as reduction in gloss, uneven gloss, and insufficient saturation.

  Various methods have been proposed to solve the above problems.

  For example, Patent Document 1 discloses that an uncured first coating film is formed by applying a first paint containing a coloring component and / or a bright material on an object to be coated. Applying a second paint containing a coloring component on top to form an uncured second paint film, and then applying a clear paint on the uncured second paint film to form a clear paint film A method for forming a multi-layer coating film is described. In this document, by specifying the coloring component content in the second paint in the above method, problems such as color unevenness, color fading and deterioration in weather resistance do not occur, and the color has a depth, It is described that it is possible to form a highly design multilayer multilayer coating film having high saturation.

  In Patent Document 2, an aqueous first base glitter paint is applied on an object to be coated to form an uncured first base paint film, and an aqueous second base glitter is formed on the uncured first base paint film. An uncured second base coating film is formed by applying a functional coating, and a clear coating is formed on the uncured second base coating film to form a clear coating, and the uncured first base and first coating A glittering coating film forming method including a step of heat-curing two bases and a clear coating film at once is described. In this document, by specifying the solid content of the paint in the water-based first base glitter paint and the water second base glitter paint in the above method, the brightness unevenness is not obtained in aluminum flake pigments having metallic gloss. It is described that it is possible to obtain a glittering coating film exhibiting a metallic appearance without any interference and exhibiting a very high flip-flop property with a mica pigment having an interference property.

  In Patent Document 3, a glittering material-containing solvent-based basecoat paint (A) is applied on an object to be coated to form an uncured coating film (a), and the glittering material is formed on the uncured coating film (a). A solvent-based base coat paint (B) is applied to form an uncured coating film (b), and a top clear paint is applied to the uncured coating film (b) to form a clear coating film. A coating film forming method including a step of simultaneously baking an uncured coating film (a), a coating film (b) and a clear coating film is described. In this document, by specifying the content of the glitter pigment and the solid content of the paint in the base coat paint and the dry film thickness of the paint film in the above-described method, the paint film has extremely excellent metallic luster and metal design. It is described that can be obtained.

  In Patent Document 4, an aqueous base coat paint (A1) containing a glitter pigment is applied to an object to be coated to form an uncured first base coat film, and the glitter is formed on the uncured first base coat film. A method for forming a glittering multi-layer coating film comprising a step of applying a pigment-containing water-based base coat paint (A2) to form an uncured second base coat film and then heating and curing both coating films simultaneously is described. . In this document, by defining the solid content of the paint in the aqueous base coat paint and the particle size of the inorganic fine particles contained in the bright pigment-containing aqueous base coat paint (A1) in the above-described method, It is described that a coating film having excellent film smoothness can be obtained.

JP 2001-314807 A JP 2004-351389 A JP 2006-150169 A Special table 2009-505807

  A water-based paint that uses water instead of an organic solvent as a diluting solvent in order to reduce the amount of organic solvent released into the environment in the field of automobile body painting due to social demands to reduce environmental impacts in recent years. There are active efforts to utilize this. However, the water-based paint has a slow volatilization rate of water as a diluting solvent, and the volatilization rate is greatly influenced by coating environmental conditions such as temperature and humidity. For this reason, in the case of wet-on-wet coating using a water-based paint, the orientation of the glitter pigment is more likely to be disturbed than in the case of using an organic solvent-type paint, and as a result, the decrease in saturation is more pronounced. It was. Therefore, the present invention improves the orientation of the glitter pigment by using the first aqueous base paint containing the glitter pigment and the second aqueous base paint containing the color pigment, and has high saturation and depth. It aims at providing the formation method of the multilayer coating film which has the designability.

  As a result of various studies on means for solving the above-mentioned problems, the present inventors have determined the concentration of each component of the first and second water-based base paints used when forming the multilayer coating film, and the dry film thickness of the base coating film. The present invention has been completed by finding suitable conditions for the above.

  That is, the gist of the present invention is as follows.

(1) Intermediate coating film formed on the object to be coated, first base coating film formed on the intermediate coating film, second base coating film formed on the first base coating film, A method for forming a multilayer coating film comprising at least a clear coating film formed on a second base coating film, the method comprising:
A first base coating film forming step in which an uncured first base coating film is formed by applying a thermosetting first aqueous base coating material containing a glittering pigment on the intermediate coating film;
A second base coating film forming step in which a thermosetting second water-based base coating material containing a coloring pigment is applied on the uncured first base coating film to form an uncured second base coating film;
A clear coating film forming step in which a thermosetting clear coating is applied on the uncured first base coating film and the second base coating film to form an uncured clear coating film;
The uncured first base coating film, the second base coating film, and the clear coating film are baked to cure each coating film by heating, and the cured first base coating film, second base coating film and A baking process for forming a multilayer coating including a clear coating;
The solids concentration of the paint contained in the first water-based base paint is in the range of 5 to 15% by weight with respect to the total weight of the first water-based base paint, and the paint solids contained in the second water-based base paint The partial concentration is in the range of 15 to 45% by mass with respect to the total mass of the second aqueous base paint, and the thickness of the cured first base coating film in the range of 2 to 8 μm is cured. The method for forming a multilayer coating film, wherein a ratio of a film thickness between the first base coating film and the second base coating film in a dry state is in a range of 1: 1.5 to 1: 6.

  (2) The first base coating that has been pre-heated by preheating the uncured first base coating film and the second base coating film between the second base coating film forming step and the clear coating film forming step. The method for forming a multilayer coating film according to (1), further comprising a preheating step of forming a film and a second base coating film.

  (3) The above-mentioned (1), wherein the concentration of the glitter pigment contained in the first aqueous base paint is in the range of 10 to 60 parts by mass with respect to 100 parts by mass of the solid content of the paint contained in the first aqueous base paint. Or the formation method of the multilayer coating film of (2).

  (4) The above-mentioned (1) to (4), wherein the concentration of the color pigment contained in the second aqueous base paint is in the range of 0.1 to 20 parts by mass with respect to 100 parts by mass of the solid content of the paint contained in the second aqueous base paint. The method for forming a multilayer coating film according to any one of (3).

  (5) A coated article comprising a multilayer coating film formed by the method according to any one of (1) to (4).

  According to the present invention, it is possible to provide a method for forming a multilayer coating film that improves the orientation of the glitter pigment and has a design with high chroma and depth.

FIG. 3 is a diagram showing the relationship between the dry film thickness of the first base coating film and the reflection intensity of incident light from the front direction of the coating board in the coating plate including the first base coating film. FIG. 4 is a diagram showing the relationship between the dry film thickness of the first base coating film and the saturation of the coating film surface in the coated plate provided with the multilayer coating film of the present invention. FIG. 3 is a diagram showing the relationship between the solid content concentration of the first water-based base coating material and the saturation of the coating film surface in a coated plate including the multilayer coating film of the present invention. FIG. 4 is a diagram showing the relationship between the solid content concentration of the second aqueous base coating material and the saturation of the coating film surface in a coating plate provided with the multilayer coating film of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail.
1. Method for forming multilayer coating film The method for forming a multilayer coating film of the present invention includes an intermediate coating film formed on an object to be coated, a first base coating film formed on the intermediate coating film, A method for forming a multilayer coating film comprising at least a second base coating film formed on a base coating film and a clear coating film formed on the second base coating film, the method comprising: It is a method including a base coating film forming process, a second base coating film forming process, a clear coating film forming process, and a baking process.

  The method for forming a multilayer coating film of the present invention may further include an electrodeposition coating film forming process and / or an intermediate coating film forming process before performing the first base coating film forming process. In this case, the multilayer coating film of the present invention is formed on the electrodeposition coating film formed on the object to be coated, the intermediate coating film formed on the electrodeposition coating film, and the intermediate coating film. The first base coating film, the second base coating film formed on the first base coating film, and the clear coating film formed on the second base coating film.

  In addition, the method for forming a multilayer coating film of the present invention includes preheating the uncured first base coating film and the second base coating film between the second base coating film forming step and the clear coating film forming step. A preheating step of forming an uncured preheat-treated first base coating film and second base coating film may be further included.

  In the present specification, the “object to be coated” means an object on which a coating film is scheduled to be formed by the method for forming a multilayer coating film of the present invention. The thing provided with the multilayer coating film formed by the formation method of a coating film on the surface of a to-be-coated object. Examples of the material of the object to be coated used in the method for forming a multilayer coating film of the present invention include metals, alloys containing them, and plastics. When the multilayer coating film of the present invention further includes an electrodeposition coating film formed on the object to be coated, the material of the object to be coated is a conductive metal or an alloy containing them, or a conductive treatment in advance. The applied plastic is preferred. As an example, a metal such as iron, copper, aluminum, tin or zinc or an alloy containing them is particularly preferable. Moreover, as a use of the coated product obtained by the method for forming a multilayer coating film of the present invention, for example, an automobile body or a part such as a passenger car, a truck or a motorcycle is preferable, and the shape of the coated product is as described above. Shapes for use in applications are preferred. Therefore, particularly preferred objects to be coated in the method for forming a multilayer coating film of the present invention are plates and molded products for producing automobile bodies and / or parts, which are made of the above-mentioned preferred metals or alloys containing them. is there. By applying the method for forming a multilayer coating film of the present invention to an object to be coated having the above-described material and shape, a coated article having a multilayer coating film having high chroma and deep design is manufactured. It becomes possible.

Hereinafter, each step will be described in detail.
1-1. Electrodeposition coating film formation process This process is intended to form an electrodeposition coating film by coating a thermosetting electrodeposition paint on an object to be coated.

  In this specification, “electrodeposition paint” is applied to the surface of the object to be coated as described above to prevent rust and corrosion of the object to be coated and to enhance the impact resistance of the surface of the object to be coated. It means the paint used for. The electrodeposition paint used in this step is preferably a thermosetting aqueous paint commonly used in the art, and either a cationic electrodeposition paint or an anionic electrodeposition paint can be used. Such an electrodeposition paint is preferably an aqueous paint containing at least a base resin and a curing agent and an aqueous medium composed of water and / or a hydrophilic organic solvent. From the viewpoint of rust resistance, it is preferable to use an epoxy resin, an acrylic resin or a polyester resin as the base resin, and a blocked polyisocyanate compound or an amino resin as the curing agent. Here, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, ethylene glycol, or the like can be used as the hydrophilic organic solvent. By applying the electrodeposition paint having the above-mentioned preferred configuration, an electrodeposition coating film having high rust resistance and impact resistance can be obtained.

  In the present specification, the “electrodeposition coating film” means a coating film formed by coating the above-mentioned electrodeposition coating material on an object to be coated.

  In this step, an electrodeposition coating method commonly used in the art can be adopted as means for coating the electrodeposition paint on the object to be coated. By the above-described coating method, it is possible to form a highly rust-resistant coating film over substantially the entire surface of an object to be coated that has been previously molded.

  The electrodeposition coating film formed in this step is heated in order to prevent the occurrence of a mixed layer with the subsequently formed intermediate coating film and to improve the coating appearance of the resulting multilayer coating film. After applying a curable electrodeposition coating, it is preferable to bake the uncured coating film and heat cure it. In this specification, “baking treatment” refers to heating a coated product with a thermosetting coating on its surface under high-temperature conditions to volatilize an aqueous medium contained in the coating, and a base resin. And a treatment for polymerizing the curing agent to form a cured dry coating film. In order to polymerize the base resin and the curing agent, it is necessary to perform a baking treatment at a temperature exceeding the curing temperature necessary for the polymerization. However, in general, when the baking treatment is performed at a temperature exceeding 180 ° C., the coating film becomes too hard and brittle, and when the baking treatment is performed at a temperature of less than 110 ° C., polymerization of the above components becomes insufficient. It is not preferable. Therefore, the baking treatment carried out in the method for forming a multilayer coating film of the present invention is 10 to 50 ° C. higher than the curing temperature of the base resin and the curing agent contained in the thermosetting paint used. Temperature is preferred. In this step, the baking temperature of the uncured electrodeposition coating film is preferably 110 to 180 ° C. Moreover, it is preferable that the time of a baking process is 10 to 60 minutes. By performing the baking treatment under the above conditions, it is possible to obtain a cured and dried electrodeposition coating film.

  Moreover, it is preferable that the dry film thickness of the cured electrodeposition coating film after being baked under the above conditions is in the range of 5 to 40 μm.

  In the present specification, the “dry film thickness” is a cured film formed by applying a thermosetting paint to form an uncured coating film and then baking the uncured coating film. It means the thickness of the dried film. The dry film thickness can be measured, for example, according to JIS K 5600-1-7 (1999).

  By forming the electrodeposition coating film having the above-mentioned preferred configuration, it becomes possible to improve the rust resistance and impact resistance of the coated object.

1-2. Intermediate coating film forming step This step is intended to form an intermediate coating film by applying a thermosetting intermediate coating on the electrodeposition coating film.

  In this specification, “intermediate paint” refers to a coating that ensures the smoothness of the surface of the coating film and that has impact resistance and chipping resistance (resistance to damage to the coating film caused by collision of obstacles such as pebbles). It means a paint used to enhance film properties. The thermosetting intermediate coating used in this step is a thermosetting water-based paint commonly used in the industry, and is a water-based composition comprising a base resin and a curing agent and water and / or a hydrophilic organic solvent. An aqueous paint containing at least a medium is preferable. Here, as the base resin and the curing agent, a known compound commonly used in the art may be used. As the base resin, for example, polyethylene, polyester resin, polyurethane resin, acrylic resin, or the like is cured. As the agent, for example, an amino resin, a polyisocyanate compound, a blocked polyisocyanate compound, or the like can be used. As the hydrophilic organic solvent, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, ethylene glycol, or the like can be used. In addition to the above-mentioned components, the intermediate coating used in the method for forming a multilayer coating film of the present invention may optionally contain an ultraviolet absorber, an antifoaming agent, a thickener, a rust inhibitor, a surface conditioner, or You may contain a pigment etc. suitably.

  By applying the intermediate coating material having the above-mentioned preferred configuration, it becomes possible to improve the impact resistance and chipping resistance of the coated object.

  In the present specification, the “intermediate coating film” means a coating film formed by coating the above-mentioned intermediate coating composition on an object to be coated or an electrodeposition coating film.

  As a method for applying the intermediate coating, a normal coating method commonly used in the art can be adopted. Examples of such a coating method include a coating method using a brush or a coating machine. A coating method using a coating machine is preferred. The coating machine is preferably an airless spray coating machine, an air spray coating machine, or a rotary atomizing electrostatic coating machine such as a paint cassette type. Particularly preferred is a rotary atomizing electrostatic coating machine. By using the above-mentioned paint and coating method, it is possible to obtain a good coating appearance without causing undesirable problems such as color unevenness and sagging.

  The intermediate coating film formed in this step may be uncured and contain an aqueous medium, and the baking treatment described above in advance before the first base coating film forming step to be performed subsequently. By carrying out the above, a dry state in which the coating film is cured and the aqueous medium contained in the paint is volatilized may be used. In order to prevent the occurrence of a mixed layer with the first base coating film formed in the first base coating film forming process and improve the orientation of the glitter pigment contained in the first base coating film, A cured intermediate coating film in a dry state is preferred. In such a case, the baking temperature of the uncured intermediate coating film is preferably 110 to 180 ° C. Moreover, it is preferable that the time of a baking process is 10 to 60 minutes. By performing the baking treatment under the above conditions, an intermediate coating film having a high degree of polymerization can be obtained.

  Moreover, it is preferable that the film thickness of the cured intermediate coating film after baking treatment under the above conditions is in the range of 10 to 50 μm.

  By forming the intermediate coating film having the above-mentioned preferred configuration, it is possible to improve the appearance of the coated object and improve the impact resistance and chipping resistance.

1-3. First base coating film formation process This process consists of coating the intermediate coating film with a thermosetting first water-based base paint containing a luster pigment to form an uncured first base coating film. With the goal.

  In the present specification, the “first aqueous base paint” is an aqueous paint containing a luster pigment, which imparts a metallic luster, a metallic design and / or flip-flop property, and is an electrodeposition serving as a base. It means a paint used to conceal the coating film and the intermediate coating film. The thermosetting first water-based paint used in this step is a thermosetting water-based paint commonly used in the industry, comprising a base resin and a curing agent, water and / or a hydrophilic organic solvent. It is preferable that the aqueous paint contains at least an aqueous medium. Here, as the base resin and the curing agent, a known compound commonly used in the art may be used. As the base resin, for example, an acrylic resin, a polyester resin, a polyurethane resin, or the like is used as the curing agent. For example, an amino resin, a polyisocyanate compound, a blocked polyisocyanate compound, or the like can be used. As the aqueous medium, a medium composed of water and / or at least one hydrophilic organic solvent may be used. Examples of the hydrophilic organic solvent include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, ethylene glycol, and the like. Can be used. In addition to the above components, the thermosetting first water-based base paint used in the method for forming a multilayer coating film of the present invention includes a color pigment, an extender pigment, an ultraviolet absorber, an antifoaming agent, an increase agent, if desired. You may contain a viscosity agent, a rust preventive agent, a surface conditioner, etc. suitably.

  The paint solid content concentration contained in the thermosetting first water-based base paint is represented by the ratio of the weight of the paint solid content to the total weight of the first water-based base paint. In the present specification, the “paint solid content” means non-volatile components such as a base resin and a curing agent that are contained in the paint and remain after being dried at 110 ° C. for 1 hour. Therefore, the solid concentration of the paint contained in the thermosetting first water-based base paint was measured over the bottom of the container by weighing the uncured first water-based base paint into a heat-resistant container such as an aluminum foil cup. Then, drying at 110 ° C. for 1 hour, weighing the mass of the paint component remaining after drying, and calculating the ratio of the mass of the paint component remaining after drying to the total mass of the paint before drying can be calculated. it can.

  The conventional multi-layer coating method, in which the thermosetting first base paint, second base paint, and clear paint are sequentially applied in an uncured state (wet-on-wet paint), increases the viscosity after painting. In order to prevent sagging and improve the orientation of the glitter pigment, it has been considered preferable that the solid content concentration of the thermosetting first water-based base coating is in a range higher than about 15% by mass. It was. For example, Patent Document 3 describes that the concentration is preferably 18% by mass or more. On the other hand, in the method for forming a multilayer coating film of the present invention, a multilayer coating film with higher saturation can be obtained by using a thermosetting first aqueous base coating material having a low coating solid content concentration. it can. When the solid content of the paint in the thermosetting first aqueous base paint is low, the content of the aqueous medium is high, so that the uncured coating is accompanied by the volatilization of the aqueous medium in the preheating process or baking process described below. The film contracts significantly in the film thickness direction. As a result, the film thickness of the cured dry film formed in the baking process is reduced. When the uncured coating film shrinks in the film thickness direction, the force that orients paint components such as glitter pigments contained in the uncured coating film acts in the film thickness direction, that is, in the lower coating film surface direction. To do. Thus, the shrinkage force generated by the shrinkage of the uncured coating film accompanying the volatilization of the aqueous medium becomes the driving force for the orientation acting on the glitter pigment contained in the uncured first base coating film. When the solid content concentration of the thermosetting first aqueous base paint exceeds 15% by mass, the shrinkage force is reduced and the orientation of the glitter pigment is lowered, which is not preferable. On the other hand, when the solid content concentration of the thermosetting first water-based base paint is less than 5% by mass, not only the underlying concealability of the resulting first base paint film is lowered but also the paint film This is not preferable because the strength of the resin decreases. Therefore, the concentration of the solid content of the paint contained in the thermosetting first aqueous base paint is preferably in the range of 5 to 15% by mass, and more preferably in the range of 8 to 12% by mass.

  The mass of the solid content of the paint contained in the thermosetting first aqueous base paint can be calculated by the same method as described above.

  By using the thermosetting first water-based base paint containing the solid content of the paint with the above composition and concentration, it is possible to obtain a first base coating film having a highly oriented glitter pigment.

  The glitter pigment contained in the thermosetting first aqueous base paint is a pigment that imparts glitter or light interference to the coating film. For example, aluminum flake pigment, vapor-deposited aluminum flake pigment, metal oxide coating Aluminum flake pigments, colored aluminum flake pigments, mica (mica), titanium oxide-coated mica, iron oxide-coated mica, mica-like iron oxide, titanium oxide-coated silica, titanium oxide-coated alumina, iron oxide-coated silica, or iron oxide-coated alumina Can be mentioned. Aluminum flake pigments or colored aluminum flake pigments are preferred.

  By using the thermosetting first water-based base paint containing the glitter pigment, it is possible to form a multi-layer coating film having high chroma and deep design.

  The glitter pigment contained in the thermosetting first aqueous base paint is preferably in the shape of a scale or a thin plate. When it is said shape, it is preferable that a luster pigment has an average particle diameter of 5-20 micrometers, and it is more preferable to have an average particle diameter of 8-15 micrometers. Further, the glitter pigment preferably has an average thickness of 0.05 to 0.40 μm, and more preferably has an average thickness of 0.05 to 0.30 μm. By using a thermosetting first water-based base paint containing a bright pigment of the above shape, it is possible to improve the light reflection intensity by the bright pigment and improve the saturation of the surface of the paint. Become.

  In this specification, the average particle diameter of the glitter pigment means the median diameter (d50) of the volume-based particle size distribution measured by the laser diffraction scattering method. For example, the microtrack particle size distribution measuring device (MT3300, Nikkiso) Etc.). In addition, in this specification, the average thickness of the glitter pigment is determined by observing a cross section perpendicular to the surface of the formed coating film with an electron microscope, and positioned on a straight line drawn at random on the electron microscope image. It means a value obtained by averaging the thicknesses of the 20 bright pigments.

  The thermosetting first aqueous base paint used in the method for forming a multilayer coating film of the present invention may contain the above-mentioned glitter pigments alone or in combination of two or more. When the concentration of the glitter pigment is less than 10 parts by mass, it is not preferable because the effect of imparting a metallic luster and the like by the glitter pigment cannot be sufficiently exhibited, and the concealing property of the base is also lowered. Further, when the concentration of the glitter pigment exceeds 60 parts by mass, it is not preferable because not only the physical properties of the paint are deteriorated and the workability during coating is impaired, but also the surface smoothness of the coating film is impaired. Therefore, the concentration of the glitter pigment contained in the thermosetting first aqueous base paint is in the range of 10 to 60 parts by mass with respect to 100 parts by mass of the solid content of the paint contained in the first aqueous base paint. It is preferable that the range is 15 to 40 parts by mass.

  The mass of the solid content of the paint contained in the thermosetting first aqueous base paint can be calculated by the above method, and the mass of the glitter pigment is contained in the uncured first aqueous base paint. It can be determined based on the mass of the glitter pigment.

  By using the first aqueous base paint containing the glitter pigment at the above-mentioned concentration, it becomes possible to improve the concealability of the base and the surface smoothness of the coating film.

  In the present specification, the “first base coating film” means a coating film formed by applying the first water-based base coating material on the intermediate coating film.

  As a method of applying the thermosetting first water-based base paint, a usual coating method commonly used in the art can be adopted. Examples of such a coating method include a coating method using a brush or a coating machine. A coating method using a coating machine is preferred. The coating machine is preferably an airless spray coating machine, an air spray coating machine, or a rotary atomizing electrostatic coating machine such as a paint cassette type. Particularly preferred is a rotary atomizing electrostatic coating machine. By using the above-mentioned paint and coating method, it is possible to obtain a good coating appearance without causing undesirable problems such as color unevenness and sagging.

  In the method for forming a multilayer coating film of the present invention, each coating film is obtained by simultaneously baking the uncured first base coating film, the second base coating film, and the clear coating film in the baking step described below. Is cured by heating to form a multilayer coating film including the cured first base coating film, second base coating film, and clear coating film. Therefore, no baking treatment is performed in this step. The first base coating film formed in this step is uncured and contains an aqueous medium, and is subsequently subjected to a second base coating film forming step.

  By forming the first base coating film having the above-mentioned preferred configuration, the orientation of the glitter pigment can be improved.

1-4. Second base coating film forming process This step is to apply a thermosetting second water-based base paint containing a color pigment on the uncured first base coating film to form an uncured second base coating film. The purpose is to form.

  In the present specification, the “second aqueous base paint” means an aqueous paint containing a color pigment and used for imparting a desired color. The thermosetting second water-based base paint used in this step is a thermosetting water-based paint commonly used in the industry, comprising a base resin and a curing agent, and water and / or a hydrophilic organic solvent. It is preferable that the aqueous paint contains at least an aqueous medium. Here, as the base resin and the curing agent, a known compound commonly used in the art may be used. As the base resin, for example, an acrylic resin, a polyester resin, a polyurethane resin, or the like is used as the curing agent. For example, an amino resin, a polyisocyanate compound, a blocked polyisocyanate compound, or the like can be used. As the aqueous medium, a medium composed of water and / or at least one hydrophilic organic solvent may be used. Examples of the hydrophilic organic solvent include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, ethylene glycol, and the like. Can be used. In addition to the above components, the thermosetting second aqueous base paint used in the method for forming a multilayer coating film of the present invention may be an extender pigment, an ultraviolet absorber, an antifoaming agent, a thickener, You may contain a rust preventive agent or a surface conditioner suitably.

  The paint solid content concentration contained in the thermosetting second water-based base paint is represented by the ratio of the weight of the paint solid content to the total weight of the second water-based base paint. When the solid content concentration of the paint contained in the thermosetting second aqueous base paint is less than 15% by mass, it is not preferable because the strength of the coating film is lowered. On the other hand, when the solid content concentration of the thermosetting second aqueous base paint exceeds 45% by mass, the viscosity increases and the workability at the time of coating is impaired, and the surface smoothness of the resulting coating film This is not preferable because the transmittance of reflected light due to the bright pigment contained in the first base coating film is lowered and the saturation on the surface of the coated product is lowered. Therefore, the solid content concentration of the paint contained in the thermosetting second aqueous base paint is preferably in the range of 15 to 45% by mass.

  By using a thermosetting second water-based base paint containing paint solids with the above composition and concentration, a second base coating film having high surface smoothness and high transparency of reflected light by the glitter pigment is obtained. Is possible.

  The mass of the solid content of the paint contained in the thermosetting second aqueous base paint can be calculated by the same method as described above.

  The color pigment contained in the thermosetting second aqueous base paint is a pigment that imparts a desired color to the coating film. For example, a white pigment such as titanium oxide, carbon black, acetylene black, lamp black, bone Black pigments such as black, graphite, iron black or aniline black, perylene maroon, red iron oxide, naphthol AS azo red, ansanthrone, anthraquinonyl red, quinacridone red pigment, diketopyrrolopyrrole, watching red or permanent red Red pigments such as yellow iron oxide, titanium yellow, monoazo yellow, condensed azo yellow, azomethine yellow, bismuth vanadate, benzimidazolone, isoindolinone, isoindoline, quinophthalone, benzidine yellow or permanent yellow Orange pigments such as permanent orange, purple pigments such as cobalt purple, quinacridone bio red or dioxazine bio red, blue pigments such as cobalt blue, phthalocyanine blue or selenium blue, and green pigments such as phthalocyanine green. . Perylene maroon, red iron oxide or phthalocyanine blue is preferable.

  The thermosetting second water-based base paint used in the method for forming a multilayer coating film of the present invention can contain the above-mentioned color pigments alone or in combination of two or more. The color pigment concentration contained in the thermosetting second aqueous base paint is preferably in the range of 0.1 to 20 parts by weight with respect to 100 parts by weight of the solid content of the paint contained in the second aqueous base paint. The range of 0.1 to 5 parts by mass is more preferable.

  By using the thermosetting second aqueous base paint containing the above-mentioned color pigment, it becomes possible to form a multilayer coating film having a deep color.

  The mass of paint solids contained in the thermosetting second aqueous base paint can be calculated by the above method, and the mass of the color pigment is the color contained in the uncured second aqueous base paint. It can be determined based on the mass of the pigment.

  In the present specification, the “second base coating film” means a coating film formed by applying the second aqueous base coating material on the first base coating film.

  As a method for applying the thermosetting second water-based base paint, a method similar to the method for applying the first water-based base paint described above can be employed. Particularly preferred is a coating method using a rotary atomizing electrostatic coating machine. By using the above-mentioned paint and coating method, it is possible to obtain a good coating appearance without causing undesirable problems such as color unevenness and sagging.

  In the multi-layer coating film forming method of the present invention, in the baking step described below, each uncoated first base coating film, second base coating film, and clear coating film are simultaneously baked, whereby each coating film is processed. Heat curing is performed to form a multilayer coating film including the cured first base coating film, second base coating film, and clear coating film. Therefore, no baking treatment is performed in this step. The second base coating film formed in this step is uncured and contains an aqueous medium, and is then subjected to a preheating step or a baking step that is subsequently performed.

  By forming the second base coating film having the above-mentioned preferable configuration, it is possible to improve the saturation of the coated object.

1-5. Preheating process The method for forming a multilayer coating film of the present invention may further include a preheating process between the second base coating film forming process and the clear coating film forming process. In this step, the first base coating film and the second base coating film that are uncured and containing an aqueous medium are preheated to volatilize the aqueous medium contained in the coating film in a short time, and are uncured and An object is to form a first base coating film and a second base coating film in a dry state.

  In the present specification, “preheating treatment” is a time during which the base resin and the curing agent contained in the coating film are not polymerized / cured or are not substantially polymerized / cured. And by heating at a temperature condition, the aqueous medium and / or other volatile substances contained in the coating film are reduced to a content that does not substantially affect the quality of the finally obtained multilayer coating film. It means the process to volatilize. Further, in this specification, “dry state” means a state where the aqueous medium contained in the coating film has been volatilized to a content that does not substantially affect the quality of the finally obtained multilayer coating film. To do.

  In the method for forming a multilayer coating film according to the present invention, the thermosetting first base paint and the second base paint are sequentially applied in an uncured state (wet-on-wet paint). Therefore, by performing the preheating treatment before the coating film is cured by the baking treatment, the aqueous medium is volatilized from the first base coating film and the second base coating film in a short time, and the uncured and dried first medium is removed. 1 base coating film and 2nd base coating film (the 1st base coating film and 2nd base coating film which were pre-heat-processed) can be formed.

  As described above, when the uncured first base coating film and the second base coating film are preheated, the uncured coating film significantly shrinks in the film thickness direction as the aqueous medium volatilizes. Thereby, the first base coating film and the second base coating film that have been preheated shrink to a film thickness substantially equal to the film thickness of the cured dry film formed in the baking step.

  Since the first base coating film before the preheating treatment is uncured and contains an aqueous medium, the glitter pigment in the coating film can be relatively freely oriented. When preheating treatment is performed here, the shrinkage force generated by the shrinkage of the coating film becomes the driving force for the orientation acting on the glitter pigment, and each glitter pigment is parallel or substantially to the painted surface of the object to be coated. Oriented so as to be substantially parallel to each other. When the glitter pigment has a scaly or thin plate shape, when the glitter pigment itself is aligned as described above, each surface (reflection surface) is also parallel or substantially substantially parallel to the coating surface. Oriented to align. The reflection of light on the surface of the painted object is observed as the sum of the reflections of light on the reflecting surface of each glittering pigment contained in the first base coating film. For this reason, when the orientation of the glitter pigment is improved, the reflection intensity of incident light from the surface direction of the coated object is improved.

  In this step, the preheating treatment temperature is preferably 60 to 100 ° C. Moreover, it is preferable that the time of a preheating process is 1 to 10 minutes.

  By performing the preheating treatment under the above conditions, it is possible to improve the orientation of the glitter pigment in the first base coating film, and as a result, it is possible to improve the saturation on the surface of the coated object having a multilayer coating film. Become.

1-6. Clear film formation process This process is intended to form an uncured clear paint film by applying a thermosetting clear paint on the uncured first base paint film and the second base paint film. To do. The uncured first base coating film and the second base coating film may be a coating film that is uncured and contains an aqueous medium and / or other volatile substances after the second base coating film forming step. An uncured and dried coating film after the preheating step may be used.

  In this specification, the “clear paint” means a paint used to smooth the unevenness of the first and second base coating films and to protect the first and second base coating films. The thermosetting clear paint used in this step is a thermosetting aqueous paint commonly used in the industry, and is an aqueous medium comprising a base resin and a curing agent and water and / or a hydrophilic organic solvent. It is preferable that the water-based paint contains at least. Here, as the base resin and the curing agent, a known compound commonly used in the art may be used. Examples of the base resin include a carboxyl group-containing acrylic resin, an epoxy group-containing acrylic resin, and a hydroxyl group-containing compound. As the curing agent, for example, melamine resin, urea resin, polyisocyanate compound, blocked polyisocyanate compound, or the like can be used. As the aqueous medium, a medium composed of water and / or at least one hydrophilic organic solvent may be used. Examples of the hydrophilic organic solvent include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, ethylene glycol, and the like. Can be used. In addition to the above components, the thermosetting clear paint used in the method for forming a multilayer coating film according to the present invention includes, in addition to the above-described color pigments and glitter pigments, extender pigments, ultraviolet rays. You may contain suitably an absorber, an antifoamer, a thickener, a rust preventive agent, or a surface conditioner.

  By applying the thermosetting clear paint having the above preferred configuration, a clear coating film having a dry film thickness sufficient for protecting the first and second base coating films and excellent in surface smoothness is obtained. It becomes possible.

  In the present specification, the “clear coating film” means a coating film formed by applying the above clear coating on the uncured first base coating film and the second base coating film.

  As a method of applying the clear paint, a method similar to the method of applying the first water-based base paint described above can be employed. Particularly preferred is a coating method using a rotary atomizing electrostatic coating machine. By using the above-mentioned paint and coating method, it is possible to obtain a good coating appearance without causing undesirable problems such as color unevenness and sagging.

  In the multi-layer coating film forming method of the present invention, in the baking step described below, each uncoated first base coating film, second base coating film, and clear coating film are simultaneously baked, whereby each coating film is processed. Heat curing is performed to form a multilayer coating film including the cured first base coating film, second base coating film, and clear coating film. Therefore, no baking treatment is performed in this step. The clear coating film formed in this step is subjected to a subsequent baking step in an uncured state and containing an aqueous medium.

  By forming a clear coating film having the above-mentioned preferred configuration, it is possible to obtain a painted appearance excellent in surface smoothness.

1-7. Baking process This process consists of simultaneously baking the uncured first base coating film, second base coating film, and clear coating film described above, and then curing the dried first base coating film and second base coating film. It aims at forming the multilayer coating film containing a coating film and a clear coating film. The uncured first base coating film and the second base coating film may be a coating film that is uncured after the second base coating film forming step and includes an aqueous medium and / or other volatile substances, It may be an uncured and dried coating film after the preheating step.

  In this step, the baking temperature is preferably 110 to 180 ° C. Moreover, it is preferable that the time of a baking process is 10 to 60 minutes.

  When the film thickness of the cured and dried first base coating film formed in this step is less than 2 μm, the amount of the glitter pigment contained per unit area of the coating film is reduced, and the reflection strength is reduced. Since it falls, it is not preferable. Moreover, when the dry film thickness of the said coating film exceeds 8 micrometers, since the orientation of a luster pigment falls, it is unpreferable. Therefore, the film thickness of the cured first base coating film formed in this step is preferably in the range of 2 to 8 μm, and more preferably in the range of 3 to 5 μm.

  When the coated product to which the method for forming a multilayer coating film of the present invention is applied is a plate and a molded product for manufacturing an automobile body and / or a part, it is a cured and dried compound formed on the workpiece. Of the layer coating, the thickness of the first base coating and the second base coating is usually less than 20 μm. Therefore, the ratio of the film thickness of the cured and dried first base coating film and second base coating film formed in this step is preferably in the range of 1: 1.5 to 1: 6, more preferably 1: The second base coating film is preferably formed so as to be in the range of 1.5 to 1: 4. More specifically, when the thickness of the cured second base coating film formed in this step is less than 3 μm, it is not preferable because the strength of the coating film decreases. In addition, when the dry film thickness of the coating film exceeds 17 μm, in addition to an increase in the dry film thickness of the entire multilayer coating film, transmission of reflected light by the glitter pigment contained in the first base coating film is performed. This is not preferable because the rate decreases and the saturation on the surface of the coated object decreases. Therefore, the film thickness of the cured and dried second base coating film formed in this step is preferably in the range of 3 to 17 μm, and more preferably in the range of 5 to 16 μm.

  Moreover, when the film thickness of the cured dry clear coating film formed in this step is less than 15 μm, it is not preferable because the surface smoothness is lowered. Moreover, when the dry film thickness of the said coating film exceeds 60 micrometers, the transmittance | permeability of the reflected light by a luster pigment falls and it is unpreferable from the saturation on the surface of a coating material falling. Therefore, the film thickness of the cured dry clear film formed in this step is preferably in the range of 15 to 60 μm, and more preferably in the range of 25 to 45 μm.

  By performing the baking treatment under the above-described conditions, it becomes possible to produce a coated product having a multilayer coating film having high chroma and deep design properties.

2. Painted product with multi-layer coating film The method for forming a multi-layer coating film of the present invention described above is used to produce various articles to be coated, preferably automobile bodies and / or parts made of metals or alloys containing them. By applying to this plate or molded product, it becomes possible to obtain a coated product having a multilayer coating film having a design with a high saturation and depth as compared with the multilayer coating film according to the prior art.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.
The effect of the present invention was verified by the following procedure in the three-coat type red metallic coating.

[Coating materials]
The paints used are as follows.
Intermediate coating: TP-65-2 (manufactured by Kansai Paint)
First water-based paint (brilliant pigment): MH-8801 (Asahi Kasei Chemicals)
Second aqueous base paint (color pigment): Perylene Maroon 6438 (manufactured by Sun Chemical Co., Ltd.)
Clear paint: MAGICRON KINO-1210 (manufactured by Kansai Paint)

  In the following examples, the paint solid content concentration (% by mass) of each paint represents the ratio of the mass of the paint solid to the total mass of the paint, and the uncured paint is weighed into an aluminum foil cup and spread over the bottom of the container. And then dried at 110 ° C for 1 hour, weighed the mass of the paint component remaining after drying, and calculated the ratio of the mass of the paint component remaining after drying to the total mass of the paint before drying did. The concentration of the bright pigment or colored pigment (parts by mass) represents the ratio of the mass of the bright pigment or colored pigment to 100 parts by weight of the solid paint contained in the paint, and is based on the mass of the solid paint calculated by the above method. It calculated by calculating | requiring the ratio of the mass of a luster pigment or a coloring pigment. Moreover, the film thickness of the cured dry coating film formed by each coating material was determined by measuring the film thickness of each coating film after being heated and cured by baking treatment with an electromagnetic film thickness meter.

  The bright pigment contained in the first aqueous base paint was an aluminum flake pigment containing aluminum pieces having an average particle diameter of 13 to 15 μm and an average thickness of 0.2 to 0.3 μm.

  In addition, all the coating using the said coating material was performed with the rotary atomization type electrostatic coating machine.

[Coating method]
A thermosetting intermediate coating was applied to the steel plate that had been previously electrodeposited so that the cured film thickness was 35 μm and baked at 140 ° C. for 30 minutes. The thermosetting first water-based base coating material was applied to the coated plate on which the intermediate coating film was formed so that the cured and dried film thickness was a predetermined film thickness. A thermosetting second aqueous base paint was applied to the coated plate on which the uncured first base coating film was formed such that the cured and dried film thickness was a predetermined film thickness. Next, the pre-heat treatment was performed on the coated plate on which the uncured first and second base coating films were formed at 80 ° C. for 3 minutes. After that, a thermosetting clear paint is applied to the coated plate on which an uncured and dry base coating film is formed so that the cured dry film thickness is 40 μm, and baked at 140 ° C. for 30 minutes. By performing the treatment, a coated plate on which a multilayer coating film was formed was produced.

[Example 1: Relationship between dry film thickness of first base coating film and reflection intensity]
According to the above-mentioned coating method, a thermosetting first water-based base paint (paint solid concentration: 10% by mass; pigment concentration: 40 parts by mass) so that the cured film thickness is 3, 8 or 12 μm. ) Was painted. Then, without applying the thermosetting second water-based base paint and clear paint to the coated plate on which the uncured first base coating film was formed, it was cured by baking at 140 ° C. for 30 minutes. A coated plate on which only the dried first base coating film was formed was produced. About said coating plate, it measured from the coating plate front direction using the reflection intensity measuring apparatus (ALCOPE LMR-200; the Kansai Paint company make), and evaluated the reflection intensity. The results are shown in FIG.

  As shown in FIG. 1, the reflection intensity of incident light in the front direction of the coated plate on which the first base coating film was formed was improved as the thickness of the cured first base coating film in the dry state was reduced.

  The thermosetting first water-based base paint contains a luster pigment, and the reflection of light on the surface of the coating plate on which the first base coating film is formed is mainly the surface (reflection surface) of each luster pigment. Observed as the sum of the reflections of light. Therefore, in the coating plate on which the first base coating film is formed, by orienting the glitter pigment so that the reflection surface of each glitter pigment is parallel to the coating plate surface, incident light from the front direction of the coating plate can be obtained. The reflection intensity is considered to improve. In this example, the coating solid content concentration and the glitter pigment concentration of the thermosetting first water-based base paint used for forming each first base coating film are all the same. For this reason, the thinner the film thickness of the cured first base coating film, the lower the degree of freedom of orientation of the glitter pigment in the first base coating film, resulting in improved orientation with respect to the coated plate surface. It becomes. In particular, the thermosetting first aqueous base paint used in this example contains aluminum pieces having an average particle diameter of 13 to 15 μm and an average thickness of 0.2 to 0.3 μm as a bright pigment. Thus, when the film thickness of the cured first base coating film is less than 12 μm, it is considered that the orientation of the glitter pigment in the first base coating film is remarkably limited. Although the results are not shown, if the thickness of the cured first base coating film is less than 2 μm, the amount of glitter pigment contained per unit area of the coating film decreases. On the contrary, the reflection intensity decreased.

[Example 2: Relationship between dry film thickness and saturation of first base coating film]
As in Example 1, a thermosetting first water-based base paint (paint solids concentration: 10% by mass; pigment concentration: 40 parts by mass) so that the cured dry film thickness is 3, 8 or 12 μm. ) Was painted. Then, in accordance with the above coating method, a thermosetting second water-based base paint (paint solids concentration: 30% by mass; pigment concentration: 2 parts by mass) and clear paint are applied and cured by baking. A coated plate on which a multilayer coating film having a different thickness of the dried first base coating film was formed was produced. About said coating board, C ^* (15 degrees) in light reception angle 15 degrees was measured using MA-68II (made by X-Rite), and the saturation of the coating-film surface was evaluated. More specifically, measurement light was irradiated from an angle of 45 ° with respect to an axis perpendicular to the measurement target surface, and C ^* was measured for light received at an angle of 15 ° in the direction of the measurement light from the regular reflection angle. The results are shown in FIG.

  As shown in FIG. 2, the saturation of the coated plate on which the multilayer coating film was formed was improved as the thickness of the cured first base coating film in the dry state was reduced.

  In this example, the coated plate on which the multilayer coating film was formed differed only in the dried film thickness of the cured first base coating film, and the second base coating in the dried condition in which the base resin and the curing agent were cured. The film thickness and component composition of the film are the same. From the results of Example 1, it is considered that also in the coated plate of this example, the reflection intensity of incident light on the coated plate was improved as the dry film thickness of the first base coating film was reduced. Here, when the results of FIG. 1 and FIG. 2 are compared, both results show a high correlation. Therefore, the difference in saturation on the coated plate surface on which the multilayer coating film is formed is considered to be a result of reflecting the difference in reflection intensity due to the difference in the dry film thickness of the first base coating film.

[Example 3: Relationship between paint solid content concentration and saturation of first base coating film]
Using a thermosetting first aqueous base paint and second aqueous base paint containing paint solids and pigments at a predetermined concentration, a coated plate on which a multilayer coating film was formed according to the above-described coating method was produced. About said coating board, the said C ^* was measured and the saturation of the coating-film surface was evaluated. Concentrations of paint solids and pigments in the thermosetting first water-based base paint and the second water-based base paint, as well as the thickness and thickness of the cured first and second base paint films formed on each paint plate The surface saturation is shown in Table 1, and the relationship between the solid content concentration of the first aqueous base paint and the saturation of the coated plate surface is shown in FIG.

  As shown in FIG. 3, when the paint solid content concentration in the thermosetting first water-based base paint is in the range of 5 to 15% by mass, the resulting saturation of the coated plate surface showed a high value.

  When the paint solid content concentration in the thermosetting first water-based base paint is low, the moisture content becomes high, and thus the contraction force of the coating film due to moisture evaporation during the preheating treatment becomes large. The glitter pigment in the uncured first base coating film is oriented parallel to the coated plate surface using the shrinkage force as a driving force. Therefore, the lower the coating solid content in the thermosetting first water-based base paint, the better the orientation of the glitter pigment in the first base coating film, resulting in the saturation of the coating plate surface. It is thought to improve.

[Example 4: Relationship between paint solid content concentration and saturation of second base coating film]
Using a thermosetting first aqueous base paint and second aqueous base paint containing paint solids and pigments at a predetermined concentration, a coated plate on which a multilayer coating film was formed according to the above-described coating method was produced. About said coating board, the said C ^* was measured and the saturation of the coating-film surface was evaluated. Concentrations of paint solids and pigments in the thermosetting first water-based base paint and the second water-based base paint, as well as the dry film thickness of the hardened first and second base paint films formed on each paint plate and the surface of the paint plate The saturation is shown in Table 2, and the relationship between the paint solid content concentration in the thermosetting second aqueous base paint and the saturation of the coated plate surface is shown in FIG.

  As shown in FIG. 4, when the paint solid content concentration in the thermosetting second aqueous base paint is in the range of 15 to 45% by mass, there is not much influence on the saturation of the resulting coated plate surface, All the coated plates showed high saturation.

  In the conventional technique for forming a three-coat one-bake type multilayer coating film, it has been known that the smoothness of the surface of the second base coating film is reduced due to the absorption of the solvent into the first base coating film (for example, JP 2006-150169). On the other hand, the multi-layer coating film of this example has a smooth coating surface even when the solid content of the thermosetting second aqueous base coating is relatively high at 15 to 45% by mass. High saturation without damaging the design and design. This result is due to the fact that in the multilayer coating film of this example, the solid content concentration in the thermosetting first water-based base coating is low, so that almost no solvent is sucked into the first base coating. Conceivable.

  The method for forming a multilayer coating film of the present invention includes the first and second concentrations of paint solids and pigments in the thermosetting first aqueous base paint and the second aqueous base paint, and the cured dry state formed by the paint. By defining the film thickness of the second base coating film, it is possible to provide a multi-layer coating film having high chroma and deep design without changing the conventional coating process. Thereby, it becomes possible to manufacture a high-quality coated product without increasing the coating cost.

Claims (5)

Intermediate coating film formed on workpiece, first base coating film formed on intermediate coating film, second base coating film formed on first base coating film, second base A method of forming a multilayer coating film comprising at least a clear coating film formed on a coating film, the method comprising:
A first base coating film forming step in which an uncured first base coating film is formed by applying a thermosetting first aqueous base coating material containing a glittering pigment on the intermediate coating film;
A second base coating film forming step in which a thermosetting second water-based base coating material containing a coloring pigment is applied on the uncured first base coating film to form an uncured second base coating film;
A clear coating film forming step in which a thermosetting clear coating is applied on the uncured first base coating film and the second base coating film to form an uncured clear coating film;
The uncured first base coating film, the second base coating film, and the clear coating film are baked to cure each coating film by heating, and the cured first base coating film, second base coating film and A baking process for forming a multilayer coating including a clear coating;
Including
The solids concentration of the paint contained in the first water-based base paint is in the range of 5 to 15% by weight with respect to the total weight of the first water-based base paint,
The paint solid content concentration contained in the second aqueous base paint is in the range of 15 to 45% by mass with respect to the total mass of the second aqueous base paint,
The thickness of the cured first base coating film in a dry state is in the range of 2-8 μm,
The ratio of the thickness of the cured first base coating film to the second base coating film is in the range of 1: 1.5 to 1: 6.
A method for forming the multilayer coating film.   Between the second base coating film forming step and the clear coating film forming step, the uncured first base coating film and the second base coating film are preheated and the uncured first base coating film and the second base coating film are processed. The method for forming a multilayer coating film according to claim 1, further comprising a preheating step of forming a two-base coating film.   The bright pigment concentration contained in the first aqueous base paint is in the range of 10 to 60 parts by mass with respect to 100 parts by mass of the solid content of the paint contained in the first aqueous base paint. Formation method of layer coating film.   The color pigment concentration contained in the second aqueous base paint is in the range of 0.1 to 20 parts by mass with respect to 100 parts by mass of the solid content of the paint contained in the second aqueous base paint. A method for forming a multilayer coating film according to item 1.   A coated article comprising a multilayer coating film formed by the method according to claim 1.

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