JP2022113338A - Multi-layered coating film formation method - Google Patents

Abstract

To provide a multi-layered coating film formation method which enables efficient formation of a white-based multi-layered coating film that is excellent in weather resistance, suppresses white color unevenness and luminosity unevenness, and has high lightness.SOLUTION: There is provided a multi-layered coating film formation method that is excellent in production efficiently and is reduced in environmental loads by sequentially coating specific first intermediate coating (P1), second intermediate coating (P2), first aqueous base coating (BC1), second aqueous base coating (BC2) and clear coating (CC) on a cured electrodeposition coating film formed on a steel plate, and thereby forming a first intermediate coating film, a second intermediate coating film, a first base coating film, a second base coating film, and a clear coating film having specific pigment composition, brightness and film thickness.SELECTED DRAWING: None

Description

The present invention relates to a method for forming a multi-layer coating film, and in particular, it is possible to efficiently form a high-brightness white multi-layer coating film that has excellent weather resistance and suppresses white unevenness and brightness unevenness. The present invention relates to a method for forming a multilayer coating film.

It is known to form a multilayer coating film including an electrodeposition coating film, an intermediate coating film, a base coating film and a clear coating film on an object to be coated such as an automobile outer panel. The formation of such a multi-layered coating film in a normal paint color consists of the process of applying an intermediate paint on a cured electrodeposition coating → baking → painting a base paint → preheating → painting a clear paint → baking. It can be formed by a method including each step.

In addition, a white color consisting of an electrodeposition coating film, an intermediate coating film, a white base coating film, a white pearl-like or silver pearl-like glittering base coating film, and a clear coating film is applied to an object to be coated such as an automobile outer panel. It is also known to form a multi-layer coating film (for example, Patent Document 1). In such a white multi-layer coating film, the color tone of the white base coating film and the design properties of the glittering base coating film are combined by allowing light to pass through the clear coating film and the glittering base coating film, resulting in a white pearl coating. It is possible to form a coating film having a high-class appearance with excellent brilliance such as silver pearl tone or silver pearl tone.

By the way, when forming a high-brightness white multi-layer coating film, in order to achieve both high brightness and suppression of weather peeling, the first intermediate coating → baking → high-brightness second intermediate coating → baking →Applying a bright base paint→Preheating→Applying a clear paint→Baking can be used. According to this method, by assigning different functions to the first intermediate coating and the second intermediate coating, it is possible to achieve both high brightness and suppression of weather resistance peeling. Including the process of painting → baking → painting the high-brightness second intermediate paint → baking. Therefore, the baking process of the intermediate coating film is required twice, so there is a problem in terms of production efficiency and energy consumption. Since it is large, there is also a problem in terms of environmental load. In addition, in such a high-brightness white multi-layer coating film, suppression of white unevenness, brightness unevenness, and the like is required in order to provide high designability.

JP-A-8-164358

In view of such problems of the prior art, the present invention is a multi-layer coating that can efficiently form a high-brightness white multi-layer coating film that has excellent weather resistance and suppresses white unevenness and brightness unevenness. An object of the present invention is to provide a coating film forming method.

That is, the present invention provides the following steps (1) to (7):
(1) A step of applying an electrodeposition paint on a steel plate and heating and curing to form a cured electrodeposition coating film;
(2) A first intermediate coating (P1) containing a binder component (A _P1 ), a titanium dioxide pigment (B) and a scale-like aluminum pigment (C) on the cured electrodeposition coating film obtained in step (1). is applied, and the cured film thickness (T _P1 ) is within the range of 15 to 25 μm, and the lightness L ^* value (L ^* _P1 ) at the time of curing is within the range of 80 to 90. forming a
(3) A second intermediate coating (P2) containing a binder component (A _P2 ) and a titanium dioxide pigment (B) is applied wet-on-wet onto the first intermediate coating film obtained in step (2). to form a second intermediate coating film having a cured film thickness (T _P2 ) within the range of 15 to 25 μm and a lightness L ^* value (L ^* _P2 ) at the time of curing within the range of 85 to 93. process,
(4) A first aqueous base paint (BC1) containing a binder component (A _BC1 ) and a titanium dioxide pigment (B) is applied onto the second intermediate coating film obtained in step (3) to form a cured film. forming a first base coating film having a thickness (T _BC1 ) within the range of 5 to 12 μm and a lightness L ^* value (L ^* _BC1 ) upon curing within the range of 87 to 95;
(5) A binder component (A _BC2 ) and a light interference pigment (D) are contained on the first base coating film obtained in step (4), and the content of the light interference pigment (D) is The second aqueous base paint (BC2) in the range of 3 to 8 parts by mass based on 100 parts by mass of the solid content of the binder component (A _BC2 ) is applied wet-on-wet, and the cured film thickness (T _BC2 ) is forming a second base coating film having a thickness within the range of 5 to 15 μm;
(6) a step of applying a clear coat paint (CC) wet-on-wet onto the second base paint film obtained in step (5) to form a clear coat paint film; and (7) steps (4) to A step of simultaneously curing the multilayer coating film formed in (6) by heating the multilayer coating film including the first base coating film, the second base coating film and the clear coat coating film;
A method for forming a multilayer coating film comprising
(a) the L ^* _P2 is higher than the L ^* _P1 , and the difference between the L ^* _P2 and the L ^* _P1 is within the range of 1 to 10;
(b) the L ^* _BC1 is higher than the L ^* _P2 , and the difference between the L ^* _BC1 and the L ^* _P2 is within the range of 1 to 5;
(c) the ratio of T _BC1 to T _BC2 is within the range of T _BC1 /T _BC2 = 1/1 to 1/3;
The present invention relates to a method for forming a multilayer coating film.

According to the present invention, there is provided a method for forming a high-brightness white multi-layer coating film with excellent weather resistance and suppressed white unevenness, brightness unevenness, etc., which is excellent in production efficiency and reduces environmental load. can provide a method.

DETAILED DESCRIPTION OF THE INVENTION Embodiments for carrying out the present invention will be described in detail below.

[Formation of cured electrodeposition coating film]
In the present invention, first, an electrodeposition coating is applied on a steel plate and cured by heating to form a cured electrodeposition coating (step (1)). In the present specification, the electrodeposition paint is applied to the surface of the steel plate, which is the object to be coated, to prevent rust and corrosion of the steel plate, and to improve the impact resistance of the surface of the article on which the multilayer coating film is formed. It is a paint used to enhance the

Examples of steel sheets to be coated include cold-rolled steel sheets, alloyed hot-dip galvanized steel sheets, electro-galvanized steel sheets, electro-zinc-iron two-layer electro-plated steel sheets, organic composite plated steel sheets, Al materials, Mg materials, and the like. can. Moreover, after washing the surface of these metal plates by alkali degreasing or the like, if necessary, the metal plates may be subjected to surface treatment such as phosphate chemical treatment, chromate treatment, or composite oxide treatment.

The electrodeposition paint used in this step is preferably a thermosetting water-based paint commonly used in the field, and either cationic electrodeposition paint or anionic electrodeposition paint can be used. Such an electrodeposition paint is preferably a water-based paint containing a base resin, a curing agent, and an aqueous medium comprising water and/or a hydrophilic organic solvent.

From the viewpoint of rust resistance, it is preferable to use, for example, an epoxy resin, an acrylic resin, a polyester resin, or the like as the base resin. Among them, from the viewpoint of rust resistance, it is preferable to use a resin having an aromatic ring as at least one of the base resins, and it is particularly preferable to use an epoxy resin having an aromatic ring. As the curing agent, it is preferable to use, for example, a blocked polyisocyanate compound, an amino resin, or the like. Examples of hydrophilic organic solvents include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, and ethylene glycol. By applying the electrodeposition paint, a highly rust-resistant coating film can be obtained.

In this step, as means for applying the electrodeposition paint onto the steel plate, an electrodeposition coating method commonly used in the relevant field can be adopted. By this coating method, a highly rust-resistant coating film can be formed over almost the entire surface of an object to be coated that has been preformed.

The electrodeposition coating film formed in this process prevents the occurrence of a mixed layer with the first intermediate coating film formed on the same coating film, and the coating appearance of the resulting multi-layer coating film is improved. In order to improve it, after applying a thermosetting electrodeposition coating, the uncured coating is baked and cured by heating. In this specification, the term "cured electrodeposition coating film" means a coating film obtained by heating and curing an electrodeposition coating film formed on a steel plate.

In general, if the baking treatment is performed at a temperature exceeding 190°C, the coating film becomes too hard and brittle. do not have. Therefore, in this step, the temperature for baking the uncured electrodeposition coating is generally 110 to 190.degree. C., preferably 120 to 180.degree. Also, the baking treatment time is preferably 10 to 60 minutes. By performing the baking treatment under the above conditions, a hardened and dried electrodeposition coating film can be obtained.

The dry film thickness of the cured electrodeposition coating film after baking treatment under the above conditions is usually in the range of 5 to 40 μm, preferably 10 to 30 μm.

By forming the electrodeposition coating film according to the above, the rust resistance of the coated steel sheet can be improved.

[Formation of first intermediate coating film]
A first intermediate coating (P1) is applied onto the cured electrodeposition coating obtained in step (1) to form a first intermediate coating (step (2)). The first intermediate coating (P1) contains a binder component (A _P1 ), titanium dioxide pigment (B) and scale-like aluminum pigment (C). In addition, the first intermediate coating film formed by the first intermediate coating (P1) has a cured film thickness (T _P1 ) within the range of 15 to 25 μm, and the lightness L ^* value (L ^* _P1 ) is a coating within the range of 80-90. In this specification, the lightness L ^* value means the lightness L ^* value in the L ^* a ^* b ^* color system. In addition, the lightness L ^* value (L ^* _P1 ) of the first intermediate coating film when cured is the same as that of the first intermediate coating film in the state in which the first intermediate coating film formed on the cured electrodeposition coating film is cured. It is the lightness obtained by measuring from the surface of the coating film opposite to the side in contact with the cured electrodeposition coating film.

The L ^* a ^* b ^* color system is a color system standardized by the International Commission on Illumination (CIE) in 1976 and adopted by JIS Z 8784-1 in Japan ^. and chromaticity indicating saturation are represented by a ^* and b ^* . a ^* indicates the red direction (-a ^* indicates the green direction) and b ^* indicates the yellow direction (-b ^* indicates the blue direction). L ^* , a ^* and b ^* in this specification are measured using a multi-angle spectrophotometer CM512m3 (trade name, manufactured by Konica Minolta, Inc.) with irradiation light at 25 degrees to the axis perpendicular to the coating film surface. , is defined as a numerical value calculated from the spectral reflectance received at 90 degrees with respect to the coating film surface.

As the binder component (A _P1 ) used in the first intermediate coating (P1), a coating film-forming resin composition commonly used in intermediate coatings can be used. Examples of such resin compositions include base resins such as acrylic resins, polyester resins, alkyd resins and urethane resins having crosslinkable functional groups such as hydroxyl groups, melamine resins, urea resins, polyisocyanate compounds (blocked ), which are dissolved or dispersed in a solvent such as an organic solvent and/or water.

The titanium dioxide pigment (B) is a white pigment and can impart a white color to the formed coating film. The crystal form of the titanium dioxide pigment (B) may be either the rutile type or the anatase type, but the rutile type is preferred from the viewpoint of excellent hiding properties and weather resistance of the formed coating film. The titanium dioxide pigment (B) may be obtained by coating the surface of titanium dioxide with inorganic oxides such as aluminum oxide, zirconium oxide and silicon dioxide; organic compounds such as amines and alcohols.

The blending amount of the titanium dioxide pigment (B) is such that the lightness L ^* value (L ^* _P1 ) at the time of curing of the first intermediate coating film formed using the first intermediate coating material (P1) is in the range of 80 to 90. Generally, the titanium dioxide pigment (B) is in the range of 50 to 150 parts by mass based on 100 parts by mass of the solid content of the binder component (A _P1 ). It is preferably in the range of 80 to 140 parts by mass, more preferably in the range of 100 to 130 parts by mass.

The scale-like aluminum pigment (C) is generally produced by pulverizing and grinding aluminum in a ball mill or attritor mill in the presence of a grinding medium liquid using a grinding aid. D50) is about 1 to 50 μm, especially about 5 to 25 μm, and the thickness is 0.01 μm to 10 μm, especially 0.1 μm to 5 μm. It is used from the point of finish of the coating film. The average particle size means the median size of the volume-based particle size distribution measured by the laser diffraction scattering method using a Microtrac particle size distribution analyzer MT3300 (trade name, manufactured by Nikkiso Co., Ltd.). The thickness is defined as the average value of 100 or more measured values obtained by observing the cross section of the coating film containing the scale-like aluminum pigment (C) with a microscope and measuring the thickness using image processing software. and Grinding aids include higher fatty acids such as oleic acid, stearic acid, isostearic acid, lauric acid, palmitic acid and myristic acid, as well as aliphatic amines, aliphatic amides and aliphatic alcohols. Aliphatic hydrocarbons such as mineral spirits are used as grinding media.

The scale-like aluminum pigment (C) can be broadly classified into a leafing type and a non-leafing type depending on the type of grinding aid. The leafing type, when blended in the coating composition, arranges (leafing) on the surface of the coating film obtained by painting, gives a finish with a strong metallic feeling, has a heat reflection effect, and exhibits rust prevention power. Therefore, it is often used for various building materials such as tanks, ducts, pipes, and rooftop roofing. In the water-based coating composition of the present invention, it is preferable to use a non-leafing type scale-like aluminum pigment from the viewpoint of the metallic feeling and finish of the coating film obtained by coating.

As non-leafing type scaly aluminum pigments, those whose surfaces are not particularly treated can be used, but those whose surfaces are coated with resin, those whose surfaces are treated with silica, phosphoric acid, molybdic acid, and silane coupling agents. can be used after the surface has been treated with One of the various surface treatments described above can be used, but a plurality of kinds of treatments may be used. Colored aluminum pigments such as those obtained by coating the surface of the scale-like aluminum pigment with a coloring pigment and then coating the surface with a resin, or those obtained by coating the surface of the scale-like aluminum pigment with a metal oxide such as iron oxide may be used. .

In the water-based coating composition of the present invention, the amount of the scaly aluminum pigment (C) blended is the lightness L ^* value ( L ^* _P1 ) is adjusted appropriately so that it falls within the range of 80 to 90, but from the viewpoint of the metallic feeling of the coating film obtained by coating and the finished appearance, generally the binder component (A _P1 ) is a solid Based on 100 parts by mass per minute, the scale-like aluminum pigment (C) is preferably in the range of 0.01 to 7 parts by mass, more preferably in the range of 0.1 to 6 parts by mass, still more preferably It is in the range of 1 to 4 parts by mass.

The first intermediate coating material (P1) may optionally contain a solvent such as water or an organic solvent, a pigment dispersant, a curing catalyst, an antifoaming agent, an antioxidant, an ultraviolet absorber, a light stabilizer, and a thickening agent. agents, various additives such as surface conditioners, coloring pigments and luster pigments other than titanium dioxide pigments (B) and scaly aluminum pigments (C), extender pigments such as barium sulfate, barium carbonate, calcium carbonate, talc, and silica etc. can be appropriately blended.

As the coloring pigment other than the titanium dioxide pigment (B) used in the first intermediate coating (P1), from the viewpoint of the weather resistance of the white multi-layer coating film to be formed, at least one of them is a carbon black pigment. is preferably used. When the first intermediate coating (P1) contains the carbon black pigment, the content of the carbon black pigment is 100 parts by mass of the total solid content of the binder component (A _P1 ) in the first intermediate coating (P1). Based on, it is preferably in the range of 0.005 to 2 parts by mass, more preferably in the range of 0.01 to 1 part by mass, more preferably in the range of 0.01 to 0.5 parts by mass is.

The first intermediate coating (P1) may be a water-based coating or an organic solvent-based coating. Here, the water-based paint is a term that is contrasted with organic solvent-based paint, and generally, binder components, pigments, etc. are dispersed and/or dissolved in water or a medium containing water as the main component (aqueous medium). means paint. When the first intermediate coating (P1) is a water-based coating, the water content in the first intermediate coating (P1) is preferably about 20-80% by mass, more preferably about 30-60% by mass.

The first intermediate coating (P1) can be prepared by mixing and dispersing the above components. The paint solid content of the first intermediate paint (P1) is preferably adjusted in the range of 30 to 60% by mass, more preferably 40 to 50% by mass.

The first intermediate coating material (P1) is added with water, an organic solvent, or the like to adjust the viscosity to be suitable for coating, and then applied as necessary by a known method such as rotary atomization coating, air spray, or airless spray. and can be painted.

The thickness of the first intermediate coating film formed by the first intermediate coating material (P1) is determined from the viewpoint of suppressing weather resistance peeling of the resulting multi-layer coating film and suppressing sagging of the coating film. T _P1 ) is in the range of 15 to 25 μm, more preferably in the range of 17 to 24 μm, and even more preferably in the range of 19 to 23 μm. In the present invention, by using a paint containing a titanium dioxide pigment (B) and a scaly aluminum pigment (C) as the first intermediate paint (P1), even a relatively thin film has little light transmission and weather resistance. It can suppress peeling. For this reason, in the present invention, since the film thickness of the first intermediate coating film can be set thin, the second intermediate coating film is formed by wet-on-wet on the uncured first intermediate coating film (that is, the second 2 omitting the curing step of the first intermediate coating film before forming the intermediate coating film), and thereafter, a multilayer coating film containing an uncured first intermediate coating film and an uncured second intermediate coating film By heating and curing at once, a high-brightness intermediate coating film can be formed. Thus, the first intermediate coating (P1) is used to form the first intermediate coating, and then the second intermediate coating (P2) is applied wet-on-wet onto the first intermediate coating. By forming the second intermediate coating film and then heating and curing the uncured first intermediate coating film and the uncured second intermediate coating film, a high-brightness intermediate coating can be obtained in a single heating and curing process. Since a coating film can be formed, it is possible to improve production efficiency and reduce environmental load.

In addition, the first intermediate coating film formed by the first intermediate coating (P1) has a lightness L ^* value (L ^* _P1 ) at the time of curing within the range of 80 to 90, and has a second intermediate coating composition described later. The L ^* _P2 ^{is higher than the L*} _P1 ^, _and the L ^* The difference between _P2 and L ^* _P1 is set to be within the range of 1-10. By setting the L ^* _P1 in this way, together with the second intermediate coating film, the first base coating film, and the second base coating film, which will be described later, high brightness and suppression of white unevenness, brightness unevenness, etc. can be achieved. A multi-layered coating film can be formed. The cured lightness L ^* value (L ^* _P1 ) of the first intermediate coating film is preferably in the range of 80-89, more preferably in the range of 81-86. The difference between L ^* _P2 and L ^* _P1 is preferably within the range of 1 to 10, more preferably within the range of 3 to 7, from the viewpoint of suppressing white unevenness.

An uncured multilayer coating film is obtained by forming an uncured second intermediate coating film on an uncured first intermediate coating film in a wet-on-wet manner. may be heat-cured before coating of the first water-based base paint (BC1), or may be subjected to the step (4) and subsequent steps in an uncured state. When subjected to step (4) and after as uncured, the first base coating formed in steps (4) to (6) in step (7) described later, the second base coating, and the clear coat coating Since it can be heat-cured together with the film, it is more advantageous in terms of energy saving. In addition, when the multilayer coating film including the first intermediate coating film and the second intermediate coating film is heat-cured before coating the first water-based base coating (BC1), the surface of the cured second intermediate coating film is The smoothness of the coating film can be further enhanced by polishing with means such as hydropolishing. As heating means for heat curing, for example, hot air heating, infrared heating, high-frequency heating, or the like can be used. The heating temperature is preferably 80 to 180°C, more preferably 100 to 160°C. The heating time is preferably 10 to 60 minutes, more preferably 15 to 40 minutes. If necessary, directly or indirectly heat at a temperature of about 50 to about 110 ° C., preferably about 60 to about 90 ° C. for about 1 to 60 minutes by preheating, air blowing, etc. before the heat curing. may be performed.

[Formation of second intermediate coating film]
A second intermediate coating (P2) is applied onto the first intermediate coating obtained in step (2) to form a second intermediate coating (step (3)). The second intermediate coating (P2) contains a binder component (A _P2 ) and a titanium dioxide pigment (B). In addition, the second intermediate coating film formed by the second intermediate coating (P2) has a cured film thickness (T _P2 ) within the range of 15 to 25 μm, and the lightness L ^* value (L ^* _P2 ) is a coating within the range of 85-93. Here, the lightness L ^* value (L ^* _P2 ) at the time of curing of the second intermediate coating film is obtained when both the first intermediate coating film and the second intermediate coating film formed in a laminated state are cured. It is the lightness obtained by measuring from the surface of the second intermediate coating film opposite to the side in contact with the first intermediate coating film in the wet state.

The binder component used in the second intermediate coating (P2) can be appropriately selected from the same base resins and cross-linking agents as those used in the first intermediate coating (P1).

Also, the titanium dioxide pigment (B) blended in the second intermediate coating (P2) can be the one used in the first intermediate coating. The blending amount of the titanium dioxide pigment (B) in the second intermediate coating (P2) is the lightness L ^* value (L ^* _P1 ) is adjusted to be in the range of 85 to 93, but generally the titanium dioxide pigment (B) is 50 to 150 mass parts based on 100 mass parts of the solid content of the binder component (A _P2 ). parts, more preferably 80 to 140 parts by mass, and even more preferably 100 to 130 parts by mass.

The second intermediate coating (P2) may optionally contain a solvent such as water or an organic solvent, a pigment dispersant, a curing catalyst, an antifoaming agent, an antioxidant, an ultraviolet absorber, a light stabilizer, and a thickening agent. various additives such as agents, surface conditioners, coloring pigments other than the titanium dioxide pigment (B), luster pigments, extender pigments such as barium sulfate, barium carbonate, calcium carbonate, talc, silica, and the like. .

As the coloring pigment other than the titanium dioxide pigment (B) used in the second intermediate coating (P2), from the viewpoint of the weather resistance of the white multi-layer coating film to be formed, at least one of them is a carbon black pigment. is preferably used. When the second intermediate coating (P2) contains the carbon black pigment, the content of the carbon black pigment is 100 parts by mass of the total solid content of the binder component (A _P1 ) in the second intermediate coating (P2). Based on, it is preferably in the range of 0.0050 to 2 parts by mass, more preferably in the range of 0.01 to 1 part by mass, more preferably in the range of 0.01 to 0.5 parts by mass is.

The second intermediate coating (P2) may be a water-based coating or an organic solvent-based coating, but is preferably a water-based coating from the viewpoint of VOC reduction. When the second intermediate coating (P2) is a water-based coating, the water content in the second intermediate coating (P2) is preferably about 20-80% by mass, more preferably about 30-60% by mass.

The second intermediate coating (P2) can be prepared by mixing and dispersing the above components. The paint solid content of the second intermediate paint (P2) is preferably in the range of 30 to 60% by mass, more preferably in the range of 40 to 50% by mass.

The second intermediate coating (P2) is added with water, an organic solvent, or the like to adjust the viscosity to be suitable for coating, and then applied as necessary by a known method such as rotary atomization coating, air spray, or airless spray. and can be painted. The film thickness of the second intermediate coating film formed by the second intermediate coating material (P2) is determined from the viewpoint of suppressing white unevenness in the obtained multi-layer coating film and suppressing sagging of the coating film. T _P2 ) in the range of 15 to 25 μm, preferably in the range of 17 to 24 μm, more preferably in the range of 19 to 23 μm.

The second intermediate coating film formed by the second intermediate coating (P2) has a lightness L ^* value (L ^* _P2 ) in the range of 85 to 93 when cured. Further, as described above, in relation to the first intermediate coating film formed by the first intermediate coating material (P1), the L ^* _P2 is higher than the lightness L ^* _P1 of the first intermediate coating film when cured. , and the difference between L ^* _P2 and L ^* _P1 is set to be within the range of 1-10. Furthermore, in the second intermediate coating film, in relation to the first base coating film formed by the first water-based base coating material (BC1) described later, the lightness L ^* _BC1 at the time of curing of the first base coating film is the above It is set so that it is higher than L ^* _P2 and the difference between L ^* _BC1 and L ^* _P2 is within the range of 1-5. By setting L ^* _P2 in this way, together with the first intermediate coating film, the first base coating film, and the second base coating film, which are the coating films above and below, high brightness, white unevenness and brightness can be achieved. It is possible to form a multi-layer coating film in which unevenness and the like are suppressed. The cured lightness L ^* value (L ^* _P2 ) of the second intermediate coating film is preferably in the range of 86-93, more preferably in the range of 89-92. In addition, the difference between L ^* _BC1 and L ^* _P2 is preferably within the range of 1 to 10, more preferably within the range of 3 to 7, from the viewpoint of suppressing white unevenness in the obtained multi-layer coating film. be.

The second intermediate coating film may be subjected to the formation of the first base coating film in the next step (4) after curing, or may be used as the first base coating film in the next step (4) without being cured. It may be used for film formation. As described above, the uncured multilayer coating film formed by wet-on-wet formation of the first intermediate coating film and the second intermediate coating film is cured by heating before coating the first water-based base coating (BC1). Alternatively, it may be further subjected to step (4) and thereafter in an uncured state. When subjected to the step (4) and thereafter in an uncured state, as already described, in the step (7) described later, the first base coating film and the second base coating formed in steps (4) to (6) It can be heat-cured together with the film and clear coat coating.

[Formation of first base coating film]
In step (4), the first aqueous base coating (BC1), which is a water-based coating, is applied onto the second intermediate coating film obtained in step (3) to give a cured film thickness (T _BC1 ) of 5 to 12 μm. and having a lightness L ^* value (L ^* _BC1 ) when cured within the range of 87-95. Here, the lightness L ^* value (L ^* _BC1 ) at the time of curing of the first base coating film is obtained by curing the first intermediate coating film, the second intermediate coating film, and the first base coating film formed in a laminated state. It is the lightness obtained by measuring from the surface of the first base coating opposite to the side in contact with the second intermediate coating in the coated state. Also, the first water-based base paint (BC1) contains a binder component (A _BC1 ) and a titanium dioxide pigment (B).

As the binder component (A _BC1 ) used in the first aqueous base paint (BC1), a resin composition containing a film-forming resin commonly used in paints can be used. As such a resin composition, a thermosetting resin composition can be suitably used. Specifically, for example, acrylic resins, polyester resins, alkyd resins, and urethane resins having a crosslinkable functional group such as a hydroxyl group. and a cross-linking agent such as a melamine resin, a urea resin, a polyisocyanate compound (including a block body) and the like can be used in combination. These resin compositions can be used by dissolving or dispersing them in a solvent such as an organic solvent and/or water. The ratio of the base resin and the cross-linking agent in the resin composition is not particularly limited. Preferably, it can be used within the range of 30 to 60% by mass.

As the titanium dioxide pigment (B) used in the first aqueous base paint (BC1), those used in the first intermediate paint (P1) and second intermediate paint (P2) can be used. The blending amount of the titanium dioxide pigment (B) is such that the lightness L ^* value (L ^* _BC1 ) at the time of curing of the first base coating film formed using the first aqueous base coating material (BC1) is within the range of 87 to 95. Generally, the titanium dioxide pigment (B) is preferably in the range of 50 to 100 parts by mass based on 100 parts by mass of the solid content of the binder component (A _BC1 ), It is more preferably in the range of 60 to 90 parts by mass, still more preferably in the range of 65 to 85 parts by mass.

The first aqueous base paint (BC1) may further contain various additives such as a pigment dispersant, a curing catalyst, an antifoaming agent, an antioxidant, an ultraviolet absorber, a light stabilizer, a thickener, and a surface control agent, if necessary. Additives, coloring pigments other than the titanium dioxide pigment (B), luster pigments, extender pigments such as barium sulfate, barium carbonate, calcium carbonate, talc, silica, and the like can be appropriately blended.

The paint solid content of the first aqueous base paint (BC1) is suitably in the range of 15 to 70% by mass, preferably in the range of 20 to 60% by mass, more preferably in the range of 30 to 50% by mass. is within.

The first aqueous base coating material (BC1) can be applied using a known coating method such as electrostatic coating, air spray, or airless spray.

The film thickness of the first base coating film formed by the first water-based base coating material (BC1) is in the range of 5 to 12 μm, preferably in the range of 6 to 11 μm as the cured film thickness (T _BC1 ), More preferably, it is within the range of 7 to 10 μm. In addition, the cured film thickness (T _BC1 ) of the first base coating film is in relation to the cured film thickness of the second base coating film formed by the second water-based base coating material described later, and the above T _BC1 and the above T _BC2 is set so that the ratio of T _BC1 /T _BC2 is within the range of 1/1 to 1/3. The ratio of T _BC1 to T _BC2 (T _BC1 /T _BC2 ) is preferably in the range of 1/1 to 1/2.5, more preferably in the range of 1/1 to 1/2.

The first base coating film formed by the first aqueous base coating material (BC1) has a lightness L ^* value (L ^* _BC1 ) in the range of 87-95 when cured. The L ^* _BC1 is preferably in the range of 88-94, more preferably in the range of 90-93. Further, as described above, the L ^* _BC1 is _higher than the L ^* _P2 in relation to the lightness L ^* _P2 of the second intermediate coating film, and the L ^* _BC1 and ^the L ^* The _P2 difference is set to be within the range of 1-5.

By setting the film thickness and brightness of the first base coating film formed by the first aqueous base coating material (BC1) as described above, the first intermediate coating film and the second intermediate coating film, which are the coating films above and below it, Together with the coating film and the second base coating film, it is possible to form a multi-layer coating film with high brightness and suppressed white unevenness and brightness unevenness. In particular, in the present invention, a first base coating film having a higher brightness than the second intermediate coating film having a high brightness is formed, and in the step (5) described later, on the uncured first base coating film By applying a second aqueous base paint containing a relatively small amount of an optical interference pigment (D) in a wet-on-wet manner to form a second base paint film having a thickness greater than that of the first base paint film, It is possible to effectively suppress uneven brightness caused by the thickness of the second light interference base coating film.

[Formation of second base coating film]
In step (5), a second water-based base coating (BC2), which is a water-based coating, is applied onto the uncured first base coating film obtained in step (4) so that the cured film thickness (T _BC2 ) is 5. A second basecoat is formed which is in the range of ˜15 μm. Here, the second aqueous base paint (BC2) contains a binder component (A _BC2 ) and a light interference pigment (D), and the content of the light interference pigment (D) is the binder component (A _BC2 ) is in the range of 3 to 8 parts by mass based on 100 parts by mass of the solid content.

The binder component (A _BC2 ) used in the second aqueous base paint (BC2) is appropriately selected from the base resins and crosslinking agents listed in the description of the binder components used in the first aqueous base paint (BC1). can do.

The light interference pigment (D) is applied on the surface of a scaly substrate such as mica, artificial mica, glass, silica, iron oxide, aluminum oxide, and various metals, and has a refractive index different from that of the substrate such as titanium dioxide and iron oxide. It is a luster pigment coated with a metal oxide. Specifically, metal oxide-coated mica pigments, metal oxide-coated alumina flake pigments, metal oxide-coated glass flake pigments, metal oxide-coated silica flake pigments, and the like shown below can be used.

A metal oxide-coated mica pigment is a pigment in which natural mica or artificial mica is used as a base material and the surface of the base material is coated with a metal oxide. Natural mica is a scaly base material obtained by pulverizing ore mica (mica), and artificial mica is industrial raw materials such as SiO ₂ , MgO, Al ₂ O ₃ , K ₂ SiF ₆ and Na ₂ SiF ₆ . It is synthesized by heating, melting at a high temperature of about 1500° C., cooling and crystallizing, and has less impurities and uniform size and thickness compared to natural mica. Specifically, fluorine phlogopite (KMg ₃ AlSi ₃ O ₁₀ F ₂ ), potassium tetrasilisic mica (KMg ₂₅ AlSi ₄ O ₁₀ F ₂ ), sodium tetrasilisic mica (NaMg ₂₅ AlSi ₄ O ₁₀ F ₂ ), Na Teniolite (NaMg ₂ LiSi ₄ O ₁₀ F ₂ ), LiNa teniolite (LiMg ₂ LiSi ₄ O ₁₀ F ₂ ) and the like are known. Examples of metal oxides used for coating include titanium oxide and iron oxide. Depending on the coating thickness, interference color can be expressed.

A commercially available product can be used as the metal oxide-coated mica pigment. Commercially available products of the metal oxide-coated mica pigment include, for example, "TWINCLE PEARL" series manufactured by Nihon Koken Kogyo Co., Ltd., "Lumina" series and "Magna Pearl" series manufactured by BASF, and "IRIODIN" manufactured by MERCK. series and the like.

A metal oxide-coated alumina flake pigment is a pigment in which alumina flakes are used as a base material and the surface of the base material is coated with a metal oxide. Alumina flake means scaly (flaky) aluminum oxide. It does not have to be a single component of aluminum oxide, and may contain oxides of other metals. Examples of metal oxides used for coating include titanium oxide and iron oxide. Depending on the coating thickness, interference color can be expressed.
Commercially available products can be used as the metal oxide-coated alumina flake pigment. Commercially available products of the metal oxide-coated alumina flake pigment include, for example, "Xirallic" series manufactured by MERCK.

The metal oxide-coated glass flake pigment is obtained by coating a scaly glass substrate with a metal oxide, and since the surface of the substrate is smooth, strong light reflection occurs to express a grainy feel. Metal oxides used for coating are not particularly limited, but titanium oxide and iron oxide are known.
A commercially available product can be used as the metal oxide-coated glass flake pigment. Commercially available products of the metal oxide-coated glass flake pigment include, for example, the "Metashine" series manufactured by Nippon Sheet Glass Co., Ltd., and the like.

The metal oxide-coated silica flake pigment is obtained by coating flaky silica, which is a substrate having a smooth surface and a uniform thickness, with a metal oxide having a different refractive index from that of the substrate.
A commercially available product can be used as the metal oxide-coated silica flake pigment. Commercially available products of the metal oxide-coated silica flake pigment include, for example, the "Colorstream" series manufactured by MERCK.

The optical interference pigment (D) may be subjected to a surface treatment for improving dispersibility, water resistance, chemical resistance, weather resistance, and the like.

Regarding the size of the light interference pigment (D), it is preferable to use one having an average particle size within the range of 5 to 50 μm from the viewpoint of the finish of the applied coating film and the expression of interference color, and more Preferably, the average particle size is within the range of 7 to 35 μm. It is preferable to use a thickness within the range of 0.05 to 7.0 μm. The average particle size as used herein means the median size of the volume-based particle size distribution measured by the laser diffraction scattering method using a Microtrac particle size distribution analyzer MT3300 (trade name, manufactured by Nikkiso Co., Ltd.). The thickness is defined as the average value of 100 or more measured values obtained by observing the cross section of the coating film containing the light interference pigment (C) with a microscope and measuring the thickness using image processing software. and

In addition, in the second water-based base paint (BC2), the content of the light interference pigment (D) is adjusted to the binder component (A _BC2 ) from the viewpoint of suppressing uneven brightness and feeling of brightness of the obtained multi-layer coating film. It is in the range of 3 to 8 parts by mass, preferably in the range of 4 to 7 parts by mass, based on 100 parts by mass of the solid content.

The second water-based base paint (BC2) further contains thickeners, curing catalysts, ultraviolet absorbers, light stabilizers, antifoaming agents, plasticizers, surface conditioners, anti-settling agents, etc. for various paints, if necessary. Additives, coloring pigments other than the light interference pigment (D), luster pigments, extender pigments such as barium sulfate, barium carbonate, calcium carbonate, talc, silica, and the like can be appropriately blended.

The second aqueous base coating material (BC2) can be applied using known coating methods such as electrostatic coating, air spray, and airless spray.

The paint solids content of the second aqueous base paint (BC2) is suitably in the range of 10 to 50% by mass, preferably in the range of 15 to 40% by mass, more preferably in the range of 20 to 30% by mass. be.
Further, the film thickness of the second base coating film formed by the third colored water-based paint (BC2) is within the range of 5 to 15 μm, preferably within the range of 6 to 14 μm, as the cured film thickness (T _BC2 ). More preferably, it is within the range of 7 to 13 μm. Further, as described above, the T _BC2 is adjusted to be within the range of T _BC1 /T _BC2 = 1/1 to 1/3 in relation to the cured film thickness T _BC1 of the first base coating film. be.

By setting the content of the light interference pigment (D) in the second aqueous base paint (BC2) and the cured film thickness of the second base coating film in this way, the first intermediate coating film, the second intermediate coating film Together with the film and the first base coating film, it is possible to form a multi-layer coating film with high brightness and suppressed brightness unevenness. In particular, as described above, a second water-based base coating containing a relatively small amount of the light interference pigment (D) is applied wet-on-wet onto the uncured first base coating to obtain the first base coating. Since the second base coating film having a larger film thickness is formed, it is possible to effectively suppress the uneven brightness caused by the film thickness of the light coherent second base coating film.

[Formation of clear coat film]
In the present invention, a clear coat paint (CC) is applied wet-on-wet onto the uncured second base paint film formed in step (5) to form a clear coat paint film (step (6 )).

As the clear coat paint (CC), for example, those known per se that are commonly used in the painting of automobile bodies can be used. Specifically, for example, hydroxyl group, carboxyl group, epoxy group, silanol group, etc. Base resins such as acrylic resins, polyester resins, alkyd resins, urethane resins, epoxy resins, fluorine resins, melamine resins, urea resins, polyisocyanate compounds that may be blocked, and carboxyl group-containing Organic solvent-based thermosetting coatings, water-based thermosetting coatings, thermosetting powder coatings, etc., containing a cross-linking agent such as a compound or resin, an epoxy group-containing compound or resin as a vehicle component. Among them, an organic solvent-based thermosetting paint containing a carboxyl group-containing resin and an epoxy group-containing resin, or a thermosetting paint containing a hydroxyl group-containing acrylic resin and an optionally blocked polyisocyanate compound are preferable. The clear coat paint may be a one-component paint or a two-component paint such as a two-component urethane resin paint.

In addition, the above clear coat paint (CC) can contain, if necessary, coloring pigments, luster pigments, dyes, matting agents, etc., to the extent that transparency is not impaired, and extender pigments, ultraviolet absorbers, and the like. , a light stabilizer, an antifoaming agent, a thickener, an antirust agent, a surface control agent, etc.

The clear coat paint (CC) can be applied by a method known per se, such as an airless spray, an air spray, a rotary atomizer, etc. Static electricity may be applied during application.

The clear coat paint (CC) can be applied so that the cured film thickness is generally 10 to 80 μm, preferably 15 to 60 μm, more preferably 20 to 50 μm. In addition, from the viewpoint of preventing the occurrence of coating film defects, after applying the clear coat paint (CC), if necessary, leave an interval of about 1 to 60 minutes at room temperature, or about 40 to about 80 ° C. can be preheated for about 1 to 60 minutes at a temperature of .

[Heat curing of coating film]
In step (7), by heating the multilayer coating film including the first base coating film, the second base coating film, and the clear coat coating film formed in steps (4) to (6), The layer coating is cured at once.
In the step (3), if the first intermediate coating film and the second intermediate coating film are not heat-cured after the second intermediate coating (P1) is applied, in the step (7), the step (2 By heating the first intermediate coating film, the second intermediate coating film, the first base coating film, the second base coating film, and the clear coating film formed in (6), these five coatings A multi-layer coating including a film can be cured at once. In this case, compared to the case where the first intermediate coating film and the second intermediate coating film are heat-cured after coating the second intermediate coating material (P1), one heat-curing process can be omitted, thereby further improving energy saving. can be made
The heating means can be, for example, hot air heating, infrared heating, high-frequency heating, or the like. The heating temperature is preferably 80 to 160°C, more preferably 100 to 140°C. The heating time is preferably 10 to 60 minutes, more preferably 15 to 40 minutes. If necessary, directly or indirectly heat at a temperature of about 50 to about 110 ° C., preferably about 60 to about 90 ° C. for about 1 to 60 minutes by preheating, air blowing, etc. before the heat curing. may be performed.

[Formed multi-layer coating film]
The multilayer coating film formed by the above steps includes a first intermediate coating film formed on the cured electrodeposition coating film, a second intermediate coating film, a first base coating film, a second base coating film, and a laminate structure with 5 layers of clear coat coating. According to the present invention, using a specific first intermediate paint (P1), a second intermediate paint (P2), a first water-based base paint (BC1), and a second water-based base paint (BC2), a specific Since the first intermediate coating film, second intermediate coating film, first base coating film, and second base coating film are formed with composition, brightness, film thickness, etc., it has excellent weather resistance and white unevenness and brightness unevenness. It is possible to form a high-brightness white multi-layer coating film in which , etc. are suppressed under high production efficiency and reduced environmental load.

Hereinafter, the present invention will be described more specifically with reference to Production Examples, Examples and Comparative Examples. However, the present invention is not limited by these. In each example, "parts" and "%" are based on mass unless otherwise specified. Also, the film thickness of the coating film is based on the cured coating film.

Production of hydroxyl-containing polyester resin Production Example 1
664 parts of isophthalic acid, 496 parts of adipic acid, 237 parts of phthalic anhydride, 788 parts of neopentyl glycol, and 341 parts of trimethylolpropane were placed in a reactor equipped with a stirrer, thermometer, reaction product water removal device, and nitrogen gas inlet tube. and heated to 160° C. under a nitrogen gas atmosphere with stirring. After maintaining the temperature at 160° C. for 1 hour, the temperature was raised to 230° C. over 5 hours while removing the condensed water produced and maintained at the same temperature. When the acid value reaches 7 mgKOH/g, the temperature is cooled to 170°C, 490 parts of ε-caprolactone are added, and the temperature is maintained for 1 hour. A hydroxyl group-containing polyester resin (PE-1) was obtained by diluting with a high-boiling aromatic solvent (manufactured by Maruzen Petrochemical Co., Ltd.). The obtained hydroxyl group-containing polyester resin (PE-1) had a hydroxyl value of 100 mgKOH/g, an acid value of 5.7 mgKOH/g, a number average molecular weight of 2030 and a heating residue of 70%.

Production of organic solvent-based first intermediate coating Production Examples 2 to 9
Each raw material having the composition shown in Table 1 (hydroxyl-containing polyester resin (PE-1) produced in Production Example 1, melamine resin (MF-1), pyrazole-blocked polyisocyanate compound (BNCO-1), titanium dioxide pigment (B) , scaly aluminum pigment (C), color pigment and extender) were mixed and stirred, butyl acetate was added, and the Ford Cup No. 4, each of the first intermediate coating compositions (P1-1) to (P1-8) having a viscosity of 30 seconds was obtained. In addition, the compounding amount of each component is the amount of solid content.

The raw materials used in Table 1 are as follows.
Melamine resin (MF-1): weight average molecular weight 1200, imino group-containing methylbutyl mixed etherified melamine.
Blocked polyisocyanate compound (BNCO-1): A compound obtained by fully blocking hexamethylene diisocyanate with 3,5-dimethylpyrazole.
"Ti-Pure R-706": trade name, manufactured by Chemours, titanium dioxide pigment.
“Aluminum paste PV-H2100”: trade name, manufactured by Asahi Kasei Metals, resin-coated aluminum pigment, average particle diameter D50 of 10 μm.
“Aluminum paste PV-H4070”: trade name, manufactured by Asahi Kasei Metals Co., Ltd., resin-coated aluminum pigment, average particle diameter D50: 7 μm.
“Aluminum paste HR-6601”: trade name, manufactured by Asahi Kasei Metals, resin-coated aluminum pigment, average particle diameter D50 of 14 μm.
“Aluminum paste GX-3100”: trade name, manufactured by Asahi Kasei Metals, aluminum pigment. The average particle diameter D50 is 10 µm.
“Carbon MA-100”: trade name, manufactured by Mitsubishi Chemical Corporation, carbon black pigment.
"TAROX LL-XLO": trade name, manufactured by Titan Kogyo Co., Ltd., yellow iron oxide pigment.
"Toda Color 130R": Trade name, manufactured by Toda Kogyo Co., Ltd., red iron oxide pigment.
““MICRO ACE S-3”: trade name, manufactured by Nippon Talc Co., Ltd., talc pigment.

Production Example 10 Production of hydroxyl group-containing polyester resin In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser and water separator, 174 parts of trimethylolpropane, 327 parts of neopentyl glycol, 352 parts of adipic acid and isophthalic acid were added. 109 parts of 1,2-cyclohexanedicarboxylic anhydride and 101 parts of 1,2-cyclohexanedicarboxylic acid anhydride were charged, and the temperature was raised from 160° C. to 230° C. over 3 hours, and the temperature was maintained at 230° C. while distilling off the produced condensation water using a water separator. and reacted until the acid value became 3 mgKOH/g or less. To this reaction product, 59 parts of trimellitic anhydride is added and an addition reaction is carried out at 170° C. for 30 minutes, then cooled to 50° C. or less, and 2-(dimethylamino)ethanol is added in an amount equivalent to the acid groups. After neutralization, deionized water was gradually added to obtain a hydroxyl group-containing polyester resin solution (PE-2) having a solid content concentration of 45% and a pH of 7.2. The resulting hydroxyl-containing polyester resin had an acid value of 35 mgKOH/g, a hydroxyl value of 128 mgKOH/g, and a weight average molecular weight of 13,000.

Production Example 11 Production of hydroxyl group-containing acrylic resin 35 parts of propylene glycol monopropyl ether was charged into a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen gas inlet tube and dropping device, and the temperature was raised to 85°C. 30 parts of methyl methacrylate, 20 parts of 2-ethylhexyl acrylate, 29 parts of n-butyl acrylate, 15 parts of 2-hydroxyethyl acrylate, 6 parts of acrylic acid, 15 parts of propylene glycol monopropyl ether and 2,2'-azobis (2,4 -dimethylvaleronitrile) was added dropwise over 4 hours, and the mixture was aged for 1 hour after completion of the dropwise addition. After that, a mixture of 10 parts of propylene glycol monopropyl ether and 1 part of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise over 1 hour, and after completion of dropping, the mixture was aged for 1 hour. Further, 7.4 parts of diethanolamine and 13 parts of propylene glycol monopropyl ether were added to obtain a hydroxyl group-containing acrylic resin solution (AC-1) having a solid content of 55%. The resulting hydroxyl-containing acrylic resin had an acid value of 47 mgKOH/g and a hydroxyl value of 72 mgKOH/g.

Production Example 12 Production of Acrylic Resin Having Hydroxyl and Phosphate Groups 27.5 parts of methoxypropanol and 27.5 parts of isobutanol are placed in a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube and dropping device. Part of the mixed solvent is added, heated to 110 ° C., 25.0 parts of styrene, 27.5 parts of n-butyl methacrylate, "isostearyl acrylate" (trade name, manufactured by Osaka Organic Chemical Industry Co., Ltd., branched higher alkyl acrylate) 20 .0 parts, 7.5 parts of 4-hydroxybutyl acrylate, 15.0 parts of the following phosphoric acid group-containing polymerizable monomer, 12.5 parts of 2-methacryloyloxyethyl acid phosphate, 10.0 parts of isobutanol, t-butyl per 121.5 parts of a mixture of 4.0 parts of oxyoctanoate was added to the mixed solvent over 4 hours, and 1 part of a mixture of 0.5 parts of t-butyl peroxyoctanoate and 20.0 parts of isopropanol was added. Time dripped. Then, the mixture was stirred and aged for 1 hour to obtain an acrylic resin (AC-2) solution having a solid content of 50% and having a hydroxyl group and a phosphoric acid group. The obtained acrylic resin (AC-2) having a hydroxyl group and a phosphoric acid group had an acid value of 83 mgKOH/g, a hydroxyl value of 29 mgKOH/g and a weight average molecular weight of 10,000.
Phosphate group-containing polymerizable monomer: 57.5 parts of monobutyl phosphate and 41.0 parts of isobutanol are placed in a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube and dropping device, and the temperature is adjusted to 90°C. After the temperature was raised to , 42.5 parts of glycidyl methacrylate was added dropwise over 2 hours, and the mixture was further stirred and aged for 1 hour. Then, 59.0 parts of isopropanol was added to obtain a phosphate group-containing polymerizable monomer solution having a solid concentration of 50%. The acid value of the obtained monomer was 285 mgKOH/g.

Production Example 13 Production of titanium dioxide pigment (B) dispersion Into a stirring and mixing vessel, 78 parts of the hydroxyl group-containing polyester resin solution (PE-2) obtained in Production Example 10 (resin solid content: 35 parts), "Ti-Pure R- 706” (trade name, titanium dioxide pigment manufactured by Chemazu Tayka) and 7 parts of deionized water were added, and 2-(dimethylamino)ethanol was added to adjust the pH to 8.0. Next, the obtained mixed solution is placed in a wide-mouthed glass bottle, glass beads having a diameter of about 1.3 mmφ are added as a dispersion media, the bottle is sealed, and dispersed with a paint shaker for 30 minutes to obtain a titanium dioxide pigment (B) dispersion ( B-1) was obtained.

Production of aluminum pigment (C) dispersion Production Example 14
In a stirring and mixing container, "aluminum paste PV-H2100": trade name, manufactured by Asahi Kasei Metals Co., Ltd., resin-coated aluminum pigment, average particle diameter D50 is 10 μm, resin-coated aluminum content is 50%) 6 parts (solid content 3 parts) ), 10 parts of 2-ethyl-1-hexanol, and 2 parts of the acrylic resin (AC-2) solution having a hydroxyl group and a phosphoric acid group obtained in Production Example 12 (1 part of solid content) are uniformly mixed to obtain an aluminum pigment. A dispersion (C-1) was obtained.

Production example 15
4.1 parts of "aluminum paste GX-3100" (trade name, manufactured by Asahi Kasei Metals Co., Ltd., aluminum pigment, average particle diameter D50 of 10 μm, aluminum content of 74%) (solid content: 3 parts), 10 parts of 2-ethyl-1-hexanol and 2 parts of the acrylic resin (AC-2) solution having a hydroxyl group and a phosphoric acid group obtained in Production Example 12 (1 part of solid content) were uniformly mixed to obtain an aluminum pigment dispersion. (C-2) was obtained.

Production Example 16 Production of black pigment dispersion 18 parts of the hydroxyl group-containing acrylic resin solution (AC-1) obtained in Production Example 11 (resin solid content: 10 parts), "Carbon MA-100" (trade name, manufactured by Mitsubishi Chemical Corporation, Carbon black pigment) 10 parts and 60 parts of deionized water are mixed, adjusted to pH 8.2 with 2-(dimethylamino) ethanol, and then dispersed for 30 minutes with a paint shaker to give a black pigment dispersion (PD-1). got

Production Example 17 Production of yellow pigment dispersion 18 parts of the hydroxyl group-containing acrylic resin solution (AC-1) obtained in Production Example 11 (resin solid content 10 parts), "TAROX LL-XLO" (trade name, manufactured by Titan Kogyo Co., Ltd., 10 parts of yellow iron oxide pigment) and 60 parts of deionized water were mixed, adjusted to pH 8.2 with 2-(dimethylamino) ethanol, and then dispersed for 30 minutes in a paint shaker to obtain a black pigment dispersion (PD-2 ).

Production Example 18 Production of red pigment dispersion 18 parts of the hydroxyl group-containing acrylic resin solution (AC-1) obtained in Production Example 11 (10 parts of resin solid content), "Toda Color 130R" (trade name, manufactured by Toda Kogyo Co., Ltd., red oxidation 10 parts of iron pigment) and 60 parts of deionized water were mixed, adjusted to pH 8.2 with 2-(dimethylamino) ethanol, and dispersed for 30 minutes with a paint shaker to obtain a black pigment dispersion (PD-3). Obtained.

Production Example 19 Production of extender pigment dispersion 7.2 parts of the hydroxyl-containing acrylic resin solution (AC-1) obtained in Production Example 11 (4 parts of resin solid content), "MICRO ACE S-3" (trade name, Nippon Talc Co., Ltd., talc pigment) and 11.7 parts of deionized water are mixed, adjusted to pH 8.0 with 2-(dimethylamino) ethanol, and dispersed for 40 minutes with a paint shaker to form a talc pigment dispersion (PD). -4) was obtained.

Production of water-based first intermediate coating Production Example 20
9.1 parts of the hydroxyl group-containing acrylic resin solution (AC-1) obtained in Production Example 11 (resin solid content: 15 parts), "Ukote UX-8100" (trade name, manufactured by Sanyo Chemical Industries, Ltd., urethane emulsion, solid content: 35 %) 42.9 parts (resin solid content 15 parts), "Cymel 325" (trade name, manufactured by Ornex, melamine resin, solid content 80%) 37.5 parts (resin solid content 30 parts), "Baihydur VPLS2310" (trade name, blocked polyisocyanate compound manufactured by Sumika Bayer Urethane Co., Ltd., solid content 38%) 26.3 parts (resin solid content 10 parts), titanium dioxide pigment (B) dispersion (B) obtained in Production Example 13 -1) 210 parts (125 parts of titanium oxide pigment, 35 parts of resin solid content), 18 parts of scaly aluminum pigment (C) dispersion (C-1) obtained in Production Example 14 (3 parts of scaly aluminum pigment, resin Solid content 1 part), black pigment dispersion obtained in Production Example 16 (PD-1) 0.088 parts, yellow pigment dispersion obtained in Production Example 17 (PD-2) 3.6 parts, in Production Example 18 0.18 parts of the obtained red pigment dispersion (PD-3) and 23.9 parts of the extender dispersion (PD-4) obtained in Production Example 19 (5 parts of extender, 4 parts of resin solid content) were uniformly dispersed. Mixed. Next, "Primal ASE-60" (trade name, manufactured by Dow Chemical Co., thickener), 2-(dimethylamino)ethanol and deionized water were added to the resulting mixture, pH was 8.3, and the solid content of the paint was A water-based first intermediate coating material (P1-9) having a viscosity of 1200 mPa·s when measured at 20° C. with a Brookfield viscometer at a rotational speed of 6 rpm was obtained.

Production Examples 21-22
In Production Example 20, water-based first intermediate coatings (P1-10) to (P1-11) were obtained in the same manner as in Production Example 20, except that the composition was changed to that shown in Table 2 below.

Production of organic solvent-based second intermediate coating Production Examples 23-24
Raw materials having compositions shown in Table 3 (hydroxyl-containing polyester resin (PE-1), melamine resin (MF-1), pyrazole-blocked polyisocyanate compound (BNCO-1), titanium dioxide pigment (B) produced in Production Example 1 , scaly aluminum pigment (c), color pigment and extender) were mixed and stirred, butyl acetate was added, and Ford Cup No. 2 was added at 20°C. Second intermediate coating compositions (P2-1) to (P2-2) having a viscosity of 30 seconds according to No. 4 were obtained. In addition, the compounding amount of each component is the amount of solid content.

「ダイピロキサイド ブルー９４１０」：商品名、大日精化工業社製、青色顔料。

"Dipyroxide Blue 9410": trade name, manufactured by Dainichiseika Kogyo Co., Ltd., blue pigment.

Production Example 25 Production of blue pigment dispersion 18 parts of the hydroxyl group-containing acrylic resin solution (AC-1) obtained in Production Example 11 (10 parts of resin solid content), "Dipyroxide Blue 9410" (trade name, Dainichiseika Kogyo 10 parts of blue pigment) and 60 parts of deionized water, adjusted to pH 8.2 with 2-(dimethylamino) ethanol, dispersed for 30 minutes with a paint shaker to form a black pigment dispersion (PD- 5) was obtained.

Production of water-based second intermediate coating Production Example 26
10.9 parts of the hydroxyl group-containing acrylic resin solution (AC-1) obtained in Production Example 11 (resin solid content 6 parts), "Ukote UX-8100" (trade name, manufactured by Sanyo Chemical Industries, Ltd., urethane emulsion, solid content 35 %) 42.9 parts (resin solid content 15 parts), "Cymel 325" (trade name, manufactured by Ornex, melamine resin, solid content 80%) 37.5 parts (resin solid content 30 parts), "Baihydur VPLS2310" (trade name, blocked polyisocyanate compound manufactured by Sumika Bayer Urethane Co., Ltd., solid content 38%) 26.3 parts (resin solid content 10 parts), titanium dioxide pigment (B) dispersion (B) obtained in Production Example 13 -1) 210 parts, black pigment dispersion obtained in Production Example 16 (PD-1) 0.088 parts, blue pigment dispersion obtained in Production Example 25 (PD-5) 2.0 parts and in Production Example 19 23.9 parts of the obtained extender dispersion (PD-4) was uniformly mixed. Next, "Primal ASE-60" (trade name, manufactured by Dow Chemical Co., thickener), 2-(dimethylamino)ethanol and deionized water were added to the resulting mixture, pH was 8.3, and the solid content of the paint was A water-based second intermediate coating (P2-3) having a viscosity of 48% and a viscosity of 1200 mPa·s when measured using a Brookfield viscometer at 20° C. and a rotational speed of 6 rpm was obtained.

Production of water-dispersible hydroxyl-containing acrylic resin (a) aqueous dispersion Production Example 27
A reactor equipped with a thermometer, a thermostat, a stirrer, a reflux condenser, a nitrogen inlet tube, and a dropping device was charged with 128 parts of deionized water, "Adekari Soap SR-1025" (trade name, manufactured by ADEKA, emulsifier, effective 2 parts of component 25%) were charged, stirred and mixed in a nitrogen stream, and the temperature was raised to 80°C.

Next, 1% of the total amount of the following core monomer emulsion and 5.3 parts of a 6% aqueous solution of ammonium persulfate were introduced into the reactor and maintained at 80° C. for 15 minutes. Thereafter, the remainder of the core monomer emulsion was added dropwise over 3 hours into the reaction vessel maintained at the same temperature, and after completion of the dropwise addition, aging was performed for 1 hour. Next, the following monomer emulsion for the shell part was added dropwise over 1 hour, and after aging for 1 hour, 40 parts of a 5% 2-(dimethylamino)ethanol aqueous solution was gradually added to the reaction vessel while cooling to 30°C. The mixture was discharged while being filtered through a 100-mesh nylon cloth to obtain an aqueous dispersion of a water-dispersible hydroxyl-containing acrylic resin (AC-3) having an average particle size of 100 nm and a solid content of 30%. The resulting water-dispersible hydroxyl-containing acrylic resin (AC-3) had an acid value of 33 mgKOH/g and a hydroxyl value of 25 mgKOH/g.

Monomer emulsion for core part: 40 parts of deionized water, 2.8 parts of "ADEKARI SOAP SR-1025", 2.1 parts of methylenebisacrylamide, 2.8 parts of styrene, 16.1 parts of methyl methacrylate, 28 parts of ethyl acrylate and 21 parts of n-butyl acrylate were mixed and stirred to obtain a monomer emulsion for the core part.

Monomer emulsion for shell: 17 parts of deionized water, 1.2 parts of "Adekari Soap SR-1025", 0.03 parts of ammonium persulfate, 3 parts of styrene, 5.1 parts of 2-hydroxyethyl acrylate, 5 parts of methacrylic acid 1 part, 6 parts of methyl methacrylate, 1.8 parts of ethyl acrylate, and 9 parts of n-butyl acrylate were mixed and stirred to obtain a monomer emulsion for the shell.

Production example 28
In a reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, and water separator, 109 parts trimethylolpropane, 141 parts 1,6-hexanediol, 126 parts 1,2-cyclohexanedicarboxylic anhydride, and 120 parts of adipic acid were added, and the temperature was raised from 160° C. to 230° C. over 3 hours, followed by condensation reaction at 230° C. for 4 hours. Next, 38.3 parts of trimellitic anhydride was added to the obtained condensation reaction product in order to introduce a carboxyl group, reacted at 170° C. for 30 minutes, and then diluted with 2-ethyl-1-hexanol. to obtain a hydroxyl group-containing polyester resin (PE-3) solution with a solid content of 70%. The resulting hydroxyl-containing polyester resin (PE-3) had an acid value of 46 mgKOH/g, a hydroxyl value of 150 mgKOH/g and a number average molecular weight of 1,400.

Production of the first water-based base paint (BC1) Production Example 29
100 parts of the water-dispersible hydroxyl-containing acrylic resin (AC-3) aqueous dispersion obtained in Production Example 27 (solid content: 30 parts), 34.5 parts of the hydroxyl-containing acrylic resin solution (AC-1) obtained in Production Example 11 (Solid content 19 parts), "Cymel 325" (trade name, manufactured by Ornex, melamine resin, solid content 80%) 37.5 parts (resin solid content 30 parts) and titanium dioxide pigment obtained in Production Example 13 (B ) 126 parts of dispersion (B-1) (containing 75 parts of titanium oxide pigment and 21 parts of resin solid content) was uniformly mixed, and further, "Primal ASE-60" (trade name, Dow Chemical Co., Ltd., thickened agent), 2-(dimethylamino) ethanol, and deionized water are added to pH 8.0. s water-based first base paint (BC1-1) was obtained.

Production of optical interference pigment dispersion Production Example 30
In a stirring mixing container, 6 parts of "Metashine 1018RS" (trade name, manufactured by Nippon Sheet Glass Co., Ltd., titanium oxide-coated glass flake pigment), 35 parts of 2-ethyl-1-hexanol, and the hydroxyl group and phosphoric acid obtained in Production Example 12 6 parts of a solution of acrylic resin (AC-3) having a group (solid content: 3 parts) were uniformly mixed to obtain a light interference pigment dispersion (D-1).

Production example 31
In a stirring and mixing container, 5 parts of "Xirallic T60-10 SW Crystal Silver" (trade name, manufactured by Merck Ltd., metal oxide-coated alumina flake pigment), 35 parts of 2-ethyl-1-hexanol and obtained in Production Example 12 6 parts of acrylic resin (AC-3) solution (solid content: 3 parts) having a hydroxyl group and a phosphoric acid group were uniformly mixed to obtain a light interference pigment dispersion (D-2).

Preparation of water-based second base paint Production example 32
100 parts of the water-dispersible hydroxyl-containing acrylic resin (AC-3) aqueous dispersion obtained in Production Example 27 (solid content: 30 parts), 36.4 parts of the hydroxyl-containing acrylic resin solution (AC-1) obtained in Production Example 11 (solid content 20 parts), 28.6 parts of the polyester resin solution (PE-3) obtained in Production Example 28 (solid content 20 parts), "Cymel 325" (trade name, manufactured by Allnex, melamine resin, solid content 80 %) 37.5 parts (resin solid content 30 parts) and 47 parts of the optical interference pigment dispersion (D-1) obtained in Production Example 31 were uniformly mixed, and further "Primal ASE-60" (trade name , Dow Chemical Company, polyacrylic acid-based thickener), 2-(dimethylamino) ethanol, and deionized water are added to pH 8.0, the solid content of the paint is 24%, and the temperature is 20 ° C. using a B-type viscometer. A water-based second base paint (BC2-1) having a viscosity of 4000 mPa·s when measured at a rotational speed of 6 rpm was obtained. The content of the optical interference pigment in the water-based second base paint (BC2-1) is 6 based on 100 parts by mass of the solid content of the binder component (A _BC2 ) in the water-based second base paint (2-1). was parts by mass.

Production Examples 33-36
In Production Example 32, Ford Cup No. 2 at pH 8.0 and 20° C. was prepared in the same manner as in Example 32, except that the formulation composition and the solid content of the paint were as shown in Table 4 below. Aqueous second base paints (BC2-2) to (BC2-5) having a viscosity of 40 seconds according to 4 were obtained. Further, the content of the optical interference pigment (D) in each water-based second base paint is determined based on the solid content of 100 parts by mass of the binder component (A _BC2 ) in the second base paint. ) are also shown in Table 4 as parts by mass.

Preparation of coated object for test A thermosetting epoxy resin-based cationic electrodeposition coating composition (trade name: "Electron GT-10" manufactured by Kansai Paint Co., Ltd.) was applied to a zinc phosphate-treated cold-rolled steel sheet to a film thickness of 20 μm. It was then electrodeposition coated and cured by heating at 170° C. for 30 minutes to obtain a coated object for testing.

Example 1
(Preparation of test coated plate)
The first intermediate coating (P1-1) obtained in Production Example 2 was applied to the four test objects to be coated using a rotary atomizing electrostatic coating machine so that the cured film thickness was 20 μm. An uncured first intermediate coat was electrocoated and allowed to stand for 1 minute. One of the four test substrates was taken out and heated at 140° C. for 30 minutes to cure the uncured first intermediate coating film, thereby obtaining a test substrate A to be coated. Next, for the other three test objects to be coated, the second intermediate coating (P2-1) obtained in Production Example 23 was applied onto the uncured first intermediate coating film by rotary atomization type. to form an uncured second intermediate coating film by electrostatic coating so that the cured film thickness is 20 μm using an electrostatic coating machine, leave it for 7 minutes, heat at 140 ° C. for 30 minutes, and remove the above uncured coating. Curing The first and second intermediate coatings were cured. One of them was used as coated plate B for test.

Next, for the other two test objects to be coated, the first water-based base paint (BC1-1) obtained in Production Example 29 was applied on the second intermediate coating film by a rotary atomization type electrostatic An uncured first base coating film was formed by electrostatic coating using a coating machine so as to give a cured film thickness of 8 μm, and allowed to stand for 1 minute. One of the two test objects to be coated is taken out, preheated at 80 ° C. for 3 minutes, and then heated at 140 ° C. for 30 minutes to cure the uncured first base coating film. A plate to be coated C was obtained. For the remaining one test object to be coated, the second aqueous base paint (BC2-1) obtained in Production Example 33 was applied onto the uncured first base paint film by a rotary atomization type electrostatic An uncured second base coating film was formed by electrostatic coating using a coating machine so as to give a cured film thickness of 10 μm, and allowed to stand for 3 minutes. Then, after preheating at 80° C. for 3 minutes, a thermosetting acrylic resin-based organic solvent-based clear coat paint (trade name “Magiclon KINO-1210TW”, Kansai Paint Co., Ltd.) was applied onto the uncured second base coating film. ) was electrostatically coated using a rotary atomization type electrostatic coating machine so that the cured film thickness was 35 μm to form a clear coat coating film. After leaving it for 7 minutes, it was heated at 140° C. for 30 minutes to cure the uncured first base coating film, the uncured second base coating film and the clear coating film, thereby preparing a test coated plate D. .

Examples 2-18, Comparative Examples 1-3
In Example 1, the types of the first intermediate coating (P1-1), the second intermediate coating (P2-1), the first water-based base coating (BC1-1), and the second water-based base coating (BC2-1) Test coated plates A to D were produced in the same manner as in Example 1, except that the cured film thicknesses were as shown in Tables 5-1 to 5-4 below.

Example 19
The first intermediate coating (P1-9) obtained in Production Example 20 is applied to the four test objects to be coated using a rotary atomizing electrostatic coating machine so that the cured film thickness is 20 μm. An uncured first intermediate coat was electrocoated and allowed to stand for 1 minute. One of the four test objects to be coated is taken out, preheated at 80 ° C. for 3 minutes, and then heated at 140 ° C. for 30 minutes to cure the uncured first intermediate coating film and test. A plate to be coated A was obtained. Next, for the other three test objects to be coated, the second intermediate coating (P2-3) obtained in Production Example 26 was applied onto the uncured first intermediate coating film by rotary atomization type. Then, an uncured second intermediate coating film was formed by electrostatic coating using the electrostatic coating machine of No. 1 so that the cured film thickness would be 20 μm. One of these test objects was preheated at 80 ° C. for 3 minutes, then heated at 140 ° C. for 30 minutes, and the uncured first intermediate coating film and second intermediate coating The film was cured to obtain coated plate B for test. The remaining two test objects to be coated were left for 3 minutes and then preheated at 80° C. for 3 minutes.

Next, for two test objects to be coated, the first water-based base paint (BC1-1) obtained in Production Example 29 was applied on the uncured second intermediate coating film by a rotary atomization type electrostatic An uncured first base coating film was formed by electrostatic coating using a coating machine so as to give a cured film thickness of 8 μm, and allowed to stand for 1 minute. One of the two test objects to be coated is taken out, preheated at 80 ° C. for 3 minutes, then heated at 140 ° C. for 30 minutes, and the uncured first intermediate coating film and second intermediate coating The coating film and the first base coating film were cured to obtain a coated plate C for test. For the remaining one test object to be coated, the second aqueous base paint (BC2-1) obtained in Production Example 33 was applied onto the uncured first base paint film by a rotary atomization type electrostatic An uncured second base coating film was formed by electrostatic coating using a coating machine so as to give a cured film thickness of 10 μm, and allowed to stand for 3 minutes. Then, after preheating at 80° C. for 3 minutes, a thermosetting acrylic resin-based organic solvent-based clear coat paint (trade name “Magiclon KINO-1210TW”, Kansai Paint Co., Ltd.) was applied onto the uncured second base coating film. ) was electrostatically coated using a rotary atomization type electrostatic coating machine so that the cured film thickness was 35 μm to form a clear coat coating film. After standing for 7 minutes, heat at 140° C. for 30 minutes to cure the uncured first intermediate coating film, second intermediate coating film, first base coating film, second base coating film and clear coat coating film. A coated plate D for test was produced by letting it run.

Example 20, Comparative Example 4
In Example 19, the types of the first intermediate coating (P1-9), the second intermediate coating (P2-3), the first water-based base coating (BC1-1), and the second water-based base coating (BC2-1) Test coated plates A to D were produced in the same manner as in Example 19, except that the cured film thickness was as shown in Tables 5-3 and 5-4 below.

Evaluation Test The test coated plates A to D obtained in Examples 1 to 20 and Comparative Examples 1 to 4 were used for evaluation by the following test methods. The evaluation results are shown in Tables 5-1 to 5-4 below.

(Test method)
Lightness L ^* value: lightness L ^* value (L ^* _P1 ) when curing the first intermediate coating (P1), lightness L ^* value (L ^* _P2 ) when curing the second intermediate coating (P2), and The lightness L ^* value (L ^* _BC1 ) at the time of curing of the first water-based base paint (BC1) was determined by measuring the L ^* values of the coated test panels A to C described above. Specifically, using a multi-angle spectrophotometer "CM-512m3" (manufactured by Konica Minolta), light was irradiated from an angle of 25 ° with respect to the axis perpendicular to the coating surface, and out of the reflected light The L ^* value of light in the direction perpendicular to the coating film surface was measured.

Weather resistance: For each test coated plate D, according to JIS K 5600-7-7, using a “super xenon weather meter” (Suga Test Instruments Co., Ltd., weather resistance tester), irradiance: 180 W / m ² (300 to 400 nm), test piece wetting cycle: 18 minutes/2 hours, black panel temperature: 61 to 65°C. Next, when the irradiation time of the lamp reached 1,000 hours, 2,000 hours and 3,000 hours, the multi-layered coating film of the test plate was cut in a grid shape with a cutter so as to reach the substrate, and the size 100 squares of 2 mm x 2 mm were made. Next, an adhesive cellophane tape was adhered to the surface, and the state of the cross-cut coating film remaining after the tape was rapidly peeled off was examined.
◎: 100 cross-cut coatings remain, and no small cracks of the coating film occur at the edges of the cuts of the cutter ○: 100 cross-cut coatings remain, but the coating film is small at the edges of the cuts Fragmentation Δ: 90 to 99 cross-cut coating films remain ×: 89 or less cross-cut coating films remain.

White unevenness: The test coated plate D was observed with the naked eye, and the degree of occurrence of white unevenness was evaluated according to the following criteria.
◎: Almost no white unevenness is observed, and the coating film has an extremely excellent appearance.
○: Although white unevenness is slightly observed, it has an excellent coating film appearance,
△: White unevenness is observed, the coating film appearance is slightly inferior,
x: A large amount of white unevenness is observed, and the appearance of the coating film is poor.

Brightness unevenness: The test coated plate D was observed with the naked eye, and the degree of brightness unevenness was evaluated according to the following criteria.
◎: Almost no brightness unevenness is observed, and the coating film has an extremely excellent appearance.
○: Although uneven brightness is slightly observed, it has an excellent coating film appearance.
△: Brightness unevenness is observed, the coating film appearance is slightly inferior,
x: A large amount of brightness unevenness is observed, and the appearance of the coating film is poor.

Although the embodiments and examples of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments, and various modifications are possible based on the technical idea of the present invention. For example, the configurations, methods, steps, shapes, materials, numerical values, etc., given in the above-described embodiments and examples are merely examples, and different configurations, methods, steps, shapes, materials, numerical values, etc., may be used if necessary. may be used. Also, the configurations, methods, steps, shapes, materials, numerical values, etc. of the above-described embodiments can be combined with each other without departing from the gist of the present invention.

Claims (3)

Steps (1) to (7) below:
(1) A step of applying an electrodeposition paint on a steel plate and heating and curing to form a cured electrodeposition coating film;
(2) A first intermediate coating (P1) containing a binder component (A _P1 ), a titanium dioxide pigment (B) and a scale-like aluminum pigment (C) on the cured electrodeposition coating film obtained in step (1). is applied, and the cured film thickness (T _P1 ) is within the range of 15 to 25 μm, and the lightness L ^* value (L ^* _P1 ) at the time of curing is within the range of 80 to 90. forming a
(3) A second intermediate coating (P2) containing a binder component (A _P2 ) and a titanium dioxide pigment (B) is applied wet-on-wet onto the first intermediate coating film obtained in step (2). to form a second intermediate coating film having a cured film thickness (T _P2 ) within the range of 15 to 25 μm and a lightness L ^* value (L ^* _P2 ) at the time of curing within the range of 85 to 93. process,
(4) A first aqueous base paint (BC1) containing a binder component (A _BC1 ) and a titanium dioxide pigment (B) is applied onto the second intermediate coating film obtained in step (3) to form a cured film. forming a first base coating film having a thickness (T _BC1 ) within the range of 5 to 12 μm and a lightness L ^* value (L ^* _BC1 ) upon curing within the range of 87 to 95;
(5) A binder component (A _BC2 ) and a light interference pigment (D) are contained on the first base coating film obtained in step (4), and the content of the light interference pigment (D) is as described above. The second aqueous base paint (BC2) in the range of 3 to 8 parts by mass based on 100 parts by mass of the solid content of the binder component (A _BC2 ) is applied wet-on-wet, and the cured film thickness (T _BC2 ) is forming a second base coating film having a thickness within the range of 5 to 15 μm;
(6) a step of applying a clear coat paint (CC) wet-on-wet onto the second base paint film obtained in step (5) to form a clear coat paint film; and (7) steps (4) to A step of simultaneously curing the multilayer coating film formed in (6) by heating the multilayer coating film including the first base coating film, the second base coating film and the clear coat coating film;
A method for forming a multilayer coating film comprising
(a) the L ^* _P2 is higher than the L ^* _P1 , and the difference between the L ^* _P2 and the L ^* _P1 is within the range of 1 to 10;
(b) the L ^* _BC1 is higher than the L ^* _P2 , and the difference between the L ^* _BC1 and the L ^* _P2 is within the range of 1 to 5;
(c) the ratio of T _BC1 to T _BC2 is within the range of T _BC1 /T _BC2 = 1/1 to 1/3;
A method for forming a multilayer coating film. Before the step (4), by heating the multilayer coating film including the first intermediate coating film and the second intermediate coating film formed in the steps (2) to (3), the multilayer coating 2. The method of forming a multi-layer coating film according to claim 1, further comprising the step of simultaneously curing the films. In the step (6), the first intermediate coating film, the second intermediate coating film, the first base coating film, the second base coating film, and the clear coat coating formed in the steps (2) to (5) 2. The method of forming a multi-layer coating film according to claim 1, wherein the multi-layer coating film including these five coating films is cured at once by heating the film.