WO2013039066A1

WO2013039066A1 - Double-layer coating film formation method and double-layer coating film

Info

Publication number: WO2013039066A1
Application number: PCT/JP2012/073196
Authority: WO
Inventors: 小島　圭介; 孝昌青木; 孝則橋本; 小川　剛志
Original assignee: 本田技研工業株式会社
Priority date: 2011-09-13
Filing date: 2012-09-11
Publication date: 2013-03-21
Also published as: CN103796764B; JP5881719B2; BR112014005780A2; CN103796764A; JPWO2013039066A1

Abstract

The present invention relates to a double-layer coating film formation method by which a deterioration or an exfoliation at an interface between a final coating film and an undercoating film may be limited even without a surface coating and a coating performance may be improved regardless of a color. The present invention comprises a first base coating film formation step in which a first base coating film is formed by coating an electrodeposited coating target with a first base coating material, a second base coating film formation step in which a second base coating film is formed by coating the first base coating film with a second base coating material, and a clear coating film formation step in which a clear coating film is formed by coating the second base coating film with a clear coating material. In the first base coating film formation step, the first base coating film is formed by coating the first base coating material such that a pigment concentration of the first base coating film after a bake hardening is from 40 wt% to 60 wt% and a film thickness is at least 20µm. In the second base coating film formation step, the second base coating film is formed by coating the second base coating material such that the film thickness of the second base coating film after the bake hardening is at least 8µm.

Description

Method of forming multilayer coating and multilayer coating

The present invention relates to a method of forming a multilayer coating and a multilayer coating. More particularly, the present invention relates to a method of forming a multilayer coating for an automobile body and a multilayer coating.

Conventionally, in the coating of an automobile body, for example, a middle coat coating process, a baking process, a top coat base coating process, a preheating process, a top coat clear coating process and a baking process on an object to which electrodeposition coating as an undercoat is applied. By sequentially passing through, a multilayer coating film is formed. As described above, since a plurality of processes have to be performed, shortening of the processes is required from the viewpoint of reduction of CO ₂ emissions and cost reduction.

For example, a technique is proposed in which an intermediate coating film, a color base coating film and a clear coating film are sequentially formed on a substrate, and these three layers are simultaneously baked and cured (see Patent Document 1). According to this technique, the baking process of the intermediate coating film can be omitted, and the cost can be reduced.

However, in recent years, the middle coat painting itself has been abolished, and further shortening of the process is required. Therefore, for example, there has been proposed a technique for abolishing the intermediate coating and for containing a fine organic pigment having an average particle diameter of 0.05 μm or less and an inorganic ultraviolet shielding agent having an average particle diameter of 0.10 μm or less in the top coating ( Patent Document 2). According to this technology, it is said that deterioration and peeling at the interface between the top coat and the bottom coat due to light transmission, which has been a problem in the prior art when the middle coat is abolished, can be suppressed.

Japanese Patent Laid-Open No. 2003-47893 JP 2001-38286 A

However, in the technique of Patent Document 2, the shielding effect by the inorganic ultraviolet shielding agent largely varies depending on the hue, and in particular, the shielding effect to light rays of colors having high brightness is low. Therefore, it was easy to transmit the light ray of the color with high brightness through the coating, and it was not possible to sufficiently suppress the deterioration and the peeling at the interface between the top coat and the undercoat.

Further, in the technology of Patent Document 2, the light shielding effect per unit amount of the inorganic ultraviolet shielding agent is not sufficient, and when the content is increased, the content of other components such as a pigment decreases, There was a risk of impairing the curability.

Furthermore, conventionally, a base coating film that expresses a hue has a place where the pigment concentration greatly varies depending on the hue (for example, 40 parts by mass or more is required for high brightness white, and 20 parts by mass or less for low brightness black) In the technique of Patent Document 2, the pigment concentration in the base coating remains the same despite the elimination of the middle coat coating. Therefore, since the pigment concentration in the base coating film is largely different depending on the hue, the stress propagation property to an impact received from the outside, that is, the chipping resistance largely varies depending on the hue. In particular, when the pigment concentration is low, the stress input from the outside can not be dispersed, and problems such as exfoliation of the coating film and exposure of the substrate occur to obtain excellent coating film performance regardless of the hue. I could not.

The present invention has been made in view of the above, and the object thereof is to suppress deterioration and peeling at the interface between the top coat film and the under coat film sufficiently even if the middle coat coating is abolished, regardless of the hue. An object of the present invention is to provide a technology capable of obtaining excellent coating film performance.

In order to achieve the above object, according to the present invention, a first base paint film forming step of coating a first base paint on a substrate to which electrodeposition coating has been applied to form a first base paint film; 1) A second base paint film forming step of coating a second base paint on the base paint film to form a second base paint film, and a clear paint film on the second base paint film, by applying a clear paint Providing a clear coating film forming step of forming a multilayer coating film. In the method for forming a multilayer coating film according to the present invention, in the first base coating film forming step, the pigment concentration of the first base coating film after baking and curing becomes 40 to 60 mass% and the film thickness becomes 20 μm or more As described above, the first base paint is applied to form the first base paint film, and in the second base paint film forming step, the film thickness of the second base paint film after baking and curing becomes 8 μm or more. The second base paint is applied to form a second base coating.

In the method of forming a multilayer coating film according to the present invention, a first base coating film forming step of forming a first base coating film on an electrodeposition coating film as a base coating film forming step, instead of eliminating the middle coat coating. And a second base coating formation step of forming a second base coating on the first base coating. Further, the pigment concentration of the first base coating film is set to 40 to 60% by mass, the film thickness is set to 20 μm or more, and the film thickness of the second base coating film is set to 8 μm or more.
According to the present invention, a base coating film which develops a hue is divided into two layers, and a first base coating film having a high pigment concentration is formed in a lower layer of the second base coating film with a predetermined film thickness or more secured. Thus, it is possible to securely block light rays of high brightness and to ensure excellent chipping resistance. As a result, even if the intermediate coating is abolished, deterioration and peeling at the interface with the electrodeposition coating can be sufficiently suppressed, and excellent coating performance can be obtained regardless of the hue.

The method for forming a multilayer coating film according to the present invention further comprises a baking step of baking and curing the first base coating film and the second base coating film simultaneously, and the first base paint and the second base paint. It is preferable to use a combination in which the first base paint has a lower curing temperature than the second base paint.

In the present invention, the first base coating and the second base coating are formed using a combination in which the first base coating has a lower curing temperature than the second base coating, and the first base coating and the second base are formed. The coating is simultaneously baked and cured.
Conventionally, when curing of the second base coating is initiated prior to curing of the first base coating at the time of simultaneous printing, curing has already started along with curing shrinkage of the first base coating A distortion occurs in the second base coating film, causing a problem that the smoothness of the multilayer coating film is impaired. On the other hand, according to the present invention, at the time of simultaneous printing, the curing of the second base coating film is started after the curing of the first base coating film is started. The smoothness of the film can be improved, and the gloss as an appearance characteristic can be improved.

In the method of forming a multilayer coating film according to the present invention, as a combination of the second base paint and the clear paint, the clear paint is a coating film more than the second base paint at the curing temperature of the second base paint. It is preferred to use a combination having a low viscosity.

In this invention, the second base coating film and the clear coating film are formed using a combination in which the clear coating composition has a lower viscosity than the second base coating composition at the curing temperature of the second base coating composition.
According to this invention, the viscosity of the clear coating film is lower than that of the second base coating film at the curing temperature of the second base paint, so that the mixed layer between the second base coating film and the clear coating film can be suppressed. In addition, it is possible to release stress (hereinafter referred to as "curing strain") in the coating film shrinkage direction which occurs at the time of curing shrinkage of the second base coating film, and it is possible to suppress the curing strain from remaining in the coating film. Thus, the gloss as the appearance characteristic can be improved, and simultaneously with the input of external stress such as chipping, the curing strain remaining in the coating film can be released and peeling can be suppressed.

The present invention also provides a multilayer coating film formed by the method for forming a multilayer coating film described above.

According to the multilayer coating film of the present invention, the same effect as the method of forming a multilayer coating film described above can be obtained.

According to the present invention, it is possible to sufficiently suppress the deterioration and peeling at the interface between the top coat film and the under coat film even if the middle coat coating is abolished, and a technology capable of obtaining excellent coat film performance regardless of paint color. Can be provided.

FIG. 6 is a view showing the relationship between the temperature and the viscosity of each paint used in Example 3. FIG. 2 is a view showing the relationship between the temperature and the viscosity of each paint used in Example 1;

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
In the method for forming a multilayer coating film according to the present embodiment, the first base paint, the second base paint, and the clear paint are sequentially applied on the substrate to which the electrodeposition coating has been applied, and then they are simultaneously baked and cured. As a result, the first base coating, the second base coating and the clear coating are formed. Moreover, the multilayer coating film which concerns on this embodiment is a multilayer coating film formed by such a formation method.

The formation method of the multilayer coating film which concerns on this embodiment has a 1st base coating film formation process, a 2nd base coating film formation process, and a clear coating film formation process.
In the method of forming a multilayer coating film according to the present embodiment, a drying step of drying the coating film is included between the second base coating film forming step and the clear coating film forming step. Later, it has the baking process which bakes and hardens the 1st base coat, the 2nd base coat, and the clear coat simultaneously.

In the first base coating film forming step, the first base coating is formed on the substrate to which the electrodeposition coating has been applied by coating the first base coating.
As a substrate to which electrodeposition coating has been applied, an automobile body to which cationic electrodeposition coating has been applied is preferably applied.

As the first base paint, a water-soluble paint or a water-dispersible paint such as an emulsion containing a resin component and a pigment component can be used.
As the resin component, one containing a polyester resin, an acrylic resin, a urethane resin or the like as a main component is used.
As the pigment component, general-purpose color pigments and extender pigments are used.
Examples of color pigments include white pigments such as titanium oxide, black pigments such as carbon black, yellow pigments such as ocher, red pigments such as red iron oxide and anthraquinone, blue pigments such as phthalocyanine blue, green pigments such as phthalocyanine green and the like Can be mentioned.
Examples of the extender pigment include inorganic pigments such as barium sulfate, calcium carbonate, kaolin and silicate (talc).
The first base paint may contain an organic solvent and various additives as needed.

In the first base coating film forming step according to the present embodiment, the first base coating material in which the pigment concentration of the first base coating film after baking and curing is 40 to 60% by mass is used.
By setting the lower limit value of the pigment concentration of the first base coating film after baking and curing to 40% by mass, it is possible to reliably block light beams of high brightness and sufficiently degrade and peel off the interface with the electrodeposition coating film. While being able to control, excellent chipping resistance can be secured. Further, by setting the upper limit value to 60% by mass, the dispersibility of the pigment can be stabilized and the color unevenness can be suppressed, and the film breakage of the coating film can be suppressed, so an excellent finish appearance and coating film performance can be obtained. .

Further, in the first base coating film forming step according to the present embodiment, the first base coating film is coated by applying the first base paint so that the film thickness of the first base coating film after baking and curing becomes 20 μm or more. Form.
By setting the lower limit value of the film thickness of the first base coating film after baking and curing to 20 μm, it is possible to reliably block light rays of colors with high brightness and sufficiently suppress deterioration and peeling at the interface with the electrodeposition coating film. . The upper limit of the film thickness is preferably 40 μm from the viewpoint of avoiding the occurrence of sagging after coating and the reduction of the finished appearance.

Further, in the first base coating film forming step according to the present embodiment, a first base paint whose curing temperature is lower than the curing temperature of the second base paint described later is used. That is, when simultaneously baking and curing the first base coating film and the second base coating film, the first base coating film is set to be cured prior to the second base coating film.
Here, "hardening temperature" means the temperature at which the drop in viscosity stops and increases while the temperature is rising. Specifically, it is measured by a dynamic viscoelasticity measuring device.

In the second base coating film forming step, the second base coating film is formed on the first base coating film to form a second base coating film. In the present embodiment, the second base paint is applied wet on wet on the first base coating film.

As the second base paint, as in the first base paint, it is possible to use a water-soluble paint or a water-dispersible paint such as an emulsion, which contains a resin component and a pigment component.
As the resin component, one containing a polyester resin, an acrylic resin, a urethane resin or the like as a main component is used as in the first base coating film. However, the resin does not have to be the same as the resin of the first base coating film.
As the pigment component, various color pigments and extender pigments described above are used as in the first base coating film. In addition, the second base coating film plays a role of color development, and a glitter pigment can be used in addition to the above-mentioned color pigment.
As the bright pigment, those generally blended in automobile outer plate paints such as aluminum flakes, mica, mica flakes, glass flakes and the like can be mentioned.
The second base paint may contain an organic solvent and various additives as necessary.

In the second base coating formation step of the present embodiment, the second base coating is applied to form a second base coating so that the film thickness of the second base coating after baking and curing becomes 8 μm or more.
The corrosion resistance after chipping can be improved by setting the lower limit value of the film thickness of the second base coating film after baking and curing to 8 μm. The upper limit of the film thickness is preferably 25 μm from the viewpoint of avoiding the occurrence of sagging after coating and the reduction in the finished appearance.

In the present embodiment, a drying step of drying the coating is provided between the second base coating forming step and the clear coating forming step. This drying step removes the solvent components in the first base coating and the second base coating.

In the clear coating film forming step, a clear coating film is formed by applying a clear paint on the second base coating film.
As the clear paint, a general purpose water-based paint for automobile exterior can be used. Examples of the curing mechanism include acrylic-melamine curing, carboxylic acid-glycidyl curing, hydroxyl group- (block) isocyanate curing and the like.
The clear paint can be used either as a one-part or two-part mixed paint. The one-component paint is preferable from the viewpoint of ease of handling in a car coating line because it is inexpensive and there is no restriction such as pot life.
In addition, the clear paint may contain an organic solvent and various additives as needed.

In the clear film formation step of the present embodiment, a clear paint having a viscosity lower than that of the second base paint is used at the curing temperature of the second base paint. That is, when the second base coating film and the clear coating film are simultaneously baked and cured, the viscosity of the clear paint is set to be lower than the viscosity of the second base paint.

In this embodiment, after the clear coating film forming step, there is provided a baking step in which the first base coating film, the second base coating film and the clear coating film are simultaneously baked and cured. The baking conditions are appropriately set according to the types of the first base paint, the second base paint and the clear paint.

According to the present embodiment, the following effects are achieved.
The formation method of the multilayer coating film which concerns on this embodiment forms a 1st base coating film which forms a 1st base coating film on an electrodeposition coating film as a base coating film formation process instead of abolishing middle coat coating. A step of forming a second base coating film on the first base coating film is provided. Further, the pigment concentration of the first base coating film is set to 40 to 60% by mass, the film thickness is set to 20 μm or more, and the film thickness of the second base coating film is set to 8 μm or more.
According to the present embodiment, the base coating film that develops the hue is divided into two layers, and the first base coating film having a high pigment concentration is formed in the lower layer of the second base coating film with a predetermined film thickness or more secured. As a result, it is possible to reliably block light rays of high brightness and to ensure excellent chipping resistance. As a result, even if the intermediate coating is abolished, deterioration and peeling at the interface with the electrodeposition coating can be sufficiently suppressed, and excellent coating performance can be obtained regardless of the hue.

In this embodiment, the first base coating and the second base coating are formed using a combination in which the first base coating has a lower curing temperature than the second base coating, and the first base coating and the second base coating are formed. The base coating is simultaneously baked and cured.
Conventionally, when curing of the second base coating is initiated prior to curing of the first base coating at the time of simultaneous printing, curing has already started along with curing shrinkage of the first base coating A distortion occurs in the second base coating film, causing a problem that the smoothness of the multilayer coating film is impaired. On the other hand, according to the present embodiment, at the time of simultaneous printing, the curing of the second base coating film is started after the curing of the first base coating film is started. The smoothness of the coating film can be improved, and the gloss as an appearance characteristic can be improved.

In the present embodiment, the second base coating film and the clear coating film are formed using a combination in which the clear coating composition has a lower viscosity than the second base coating composition at the curing temperature of the second base coating composition.
According to the present embodiment, the viscosity of the clear coating is lower than that of the second base coating at the curing temperature of the second base coating, so that the mixed layer between the second base coating and the clear coating can be suppressed. . In addition, it is possible to release the stress (curing strain) in the coating film shrinkage direction which occurs at the time of curing shrinkage of the second base coating film, and it is possible to suppress the curing strain from remaining in the coating film. Thus, the gloss as the appearance characteristic can be improved, and simultaneously with the input of external stress such as chipping, the curing strain remaining in the coating film can be released and peeling can be suppressed.

Further, according to the present embodiment, by providing a multilayer coating film formed by the above-described method for forming a multilayer coating film, the same effect as the above-described method for forming a multilayer coating film can be obtained.

The present invention is not limited to the above-described embodiment, and modifications, improvements, and the like as long as the object of the present invention can be achieved are included in the present invention.
In the above embodiment, the first base coating, the second base coating and the clear coating are simultaneously baked and cured, but the present invention is not limited thereto.

Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

Example 1
[Preparation of first base paint]
For a water-based paint for automobiles ("Aquarex" (registered trademark) manufactured by Nippon Paint Co., Ltd.), titanium oxide ("CR-90" (registered trademark) manufactured by Ishihara Sangyo Co., Ltd.) as a pigment component in advance 0.56 parts by mass of carbon black ("FW 200" (registered trademark) manufactured by Evonik Tegsa Co., Ltd.) and 17.05 parts by mass of talc (manufactured by Nippon Talc Co., Ltd. "SG-95" (registered trademark) By blending, the brightness of the coating film after bake curing was adjusted to 10 to 20. Next, a urethane emulsion resin (17.92 parts by mass of “U-Coat UX-310” manufactured by Sanyo Chemical Industries, Ltd., and 9.76 parts by mass of a polyester resin (“HMP 27” manufactured by Mitsui Chemicals, Inc.) are added and sufficiently stirred As a result, the first base paint of Example 1 was obtained in which the pigment concentration in the coating film after baking and curing was adjusted to 40 parts by mass.

[Preparation of second base paint]
A urethane emulsion resin ("U-coat UX-310" manufactured by Sanyo Kasei Co., Ltd.) against a water-based automotive paint ("Aquarex NH 731 P" manufactured by Nippon Paint Co., Ltd.) registered in black pearl in advance. After adding .36 parts by mass, the second base paint of Example 1 was obtained by sufficiently stirring.

[Formation of multilayer coating film]
The first base paint prepared in the present example was spray-coated on a steel plate previously subjected to cationic electrodeposition coating so that the film thickness after baking and curing was 20 μm. Next, the second base paint prepared in the present example was spray-coated by wet on wet so that the film thickness of the coating after baking and curing was 8 μm. Then, the film is allowed to stand for 5 minutes in a drying oven at 80 ° C. for drying, and then allowed to cool to room temperature, and then the clear paint (“Macflow O100” manufactured by Nippon Paint Co., Ltd.) is baked and the film thickness after curing is It spray-coated so that it might be set to 30 micrometers. Subsequently, the multilayer coating film of Example 1 was obtained by standing for 30 minutes in a drying oven at 140 ° C. and simultaneously baking and curing the first base coating film, the second base coating film, and the clear coating film. .

Example 2
[Formation of multilayer coating film]
The multilayer coating of Example 2 is carried out by performing the same operation as in Example 1 except that the first base paint prepared in Example 1 is spray-coated so that the film thickness after baking and curing becomes 23 μm. I got a membrane.

Comparative Example 1
[Formation of multilayer coating film]
The multilayer coating of Comparative Example 1 is carried out by carrying out the same operation as in Example 1 except that the first base paint prepared in Example 1 is spray-coated so that the film thickness after baking and curing becomes 15 μm. I got a membrane.

Comparative Example 2
[Preparation of first base paint]
To the first base paint prepared in Example 1, a water-based paint for automobiles not containing a pigment ("Aquarex" (registered trademark) manufactured by Nippon Paint Co., Ltd.) is blended, and a urethane emulsion resin (SANYO Kasei Co., Ltd.) 22.23 parts by mass of “U-Coat UX-310” manufactured by Co., Ltd. and 12.1 parts by mass of a polyester resin (“HMP 27” manufactured by Mitsui Chemicals, Inc.) were sufficiently stirred. Thereby, the 1st base paint of comparative example 2 adjusted so that the pigment concentration in the coating film after bake-hardening might be 35 mass parts was obtained.

[Formation of multilayer coating film]
A multilayer paint of Comparative Example 2 is carried out by carrying out the same operation as in Example 1 except that the first base paint prepared in this Comparative Example is used instead of the first base paint prepared in Example 1. I got a membrane.

Comparative Example 3
[Formation of multilayer coating film]
The multilayer coating film of Comparative Example 3 is carried out by performing the same operation as in Example 1 except that the second base paint prepared in Example 1 is spray-coated so that the film thickness after baking and curing becomes 5 μm. I got

<Evaluation>
The chipping resistance of the multilayer coatings obtained in Examples 1 and 2 and Comparative Examples 1 to 3 was evaluated. Specifically, the evaluation was performed according to the following procedure.
First, the multilayer coating films of each of the Examples and Comparative Examples, which had been cooled to -20.degree. C. in advance, were set at positions where the shot materials hit at an angle of 90.degree. With a gravelometer tester according to JASO M104. As a shot material, 50 g of crushed stone No. 7 for road was used, and shot with an air pressure of ² kgf / cm ² . After completion of the test, the multilayer coating was taken out and the broken coating and the portion of the coating floating were removed. Thereafter, using a loupe or the like, the amount of peeling of the coating film and the amount of exposure of the steel plate were visually evaluated.

Specifically, with regard to coating film peeling, in order from the film coating having a slight peeling, ◎ (slightly peeling), ○ (smallly peeling), Δ (slightly many peeling), × (largely peeling), × × (slarking excess) 5). Also, measure the number of coated film peelings with a diameter or length of 0.5 mm to 1 mm and the number of coated film peelings with a diameter or length of 1 mm or more in a 50 mm × 50 mm square in which scratches after shot are concentrated. did.
The exposure of the steel plate was evaluated in the following five grades in the order from the one with the least exposure: ◎ (less exposure), ((less exposure), ((more exposure slightly), x (more exposure), and x (excessive) . In addition, the multilayer coating film after shot is left to stand in a salt water sprayer for 24 hours in accordance with JIS Z 2371, and in 50 mm x 50 mm squares where scratches are concentrated, the number of rusted areas is measured did.
Also, based on the evaluation results of coating film peeling and the degree of exposure, the chipping resistance is evaluated as ○ with good chipping resistance and as × with poor chipping resistance. did. The results are shown in Table 1.

As shown in Table 1, the film thickness of the first base coating film is 15 μm and is thin and is outside the range (20 μm or more) of the present invention, the pigment concentration in the first base coating film is 35 parts by mass and the present invention is small Comparative Example 2 which is out of the range (40 to 60 parts by mass) and Comparative Example 3 in which the film thickness of the second base coating film is 5 μm and thin and outside the range of the present invention (8 μm or more) And it was judged that it was inferior to chipping resistance, since there were many exposures of a steel plate.
On the other hand, in each of Example 1 and Example 2, the exposure of the coating film peeling and the steel plate was small, and it was determined that the chipping resistance was good.
From these results, the chipping resistance is excellent by setting the pigment concentration of the first base coating film to 40 to 60 mass%, setting the film thickness to 20 μm or more, and setting the film thickness of the second base coating film to 8 μm or more. It was confirmed that sex was obtained.

Example 3
[Preparation of first base paint]
By replacing all the melamine resin components in the first base paint prepared in Example 1 with a completely methylated melamine resin (“Cymel 303” (registered trademark) manufactured by Nippon Cytech Industries, Ltd.), the third of Example 3 was obtained. 1 base paint was prepared. As a result, the curing temperature of the first base paint of this example was lower than the curing temperature of the second base paint (second base paint of Example 1) used in this example.

[Preparation of clear paint]
The clear paint of Example 3 was prepared by adding 1.2 parts by mass of dimethylethanolamine to the clear paint ("Macflow O100" manufactured by Nippon Paint Co., Ltd.) used in Example 1. Thus, at the curing temperature of the second base paint used in this example (the second base paint of Example 1), the viscosity of the clear paint of this example is the same as that of the second base paint used in this example (Example 1) Lower than the viscosity of the second base paint).

[Formation of multilayer coating film]
Using the first base paint prepared in this example instead of the first base paint prepared in Example 1 and using the clear paint prepared in this example instead of the clear paint used in Example 1 The multilayer coating film of Example 3 was obtained by carrying out the same operation as in Example 1.

<Evaluation>
The evaluation of chipping resistance was carried out on the multilayer coating film obtained in Example 3 in accordance with the above-mentioned procedure. Moreover, evaluation of a coating-film external appearance was implemented collectively, making each multilayer coating film of Example 1 and Example 3 contrast.
For the evaluation of the coating appearance, visually, in order from the one with the best gloss of the coating surface, ◎ (slight gloss), 僅か (slight gloss), ((slight gloss), x (slight gloss) Evaluation was made in 5 steps of x) and x (over gloss).
In addition, the arithmetic mean waviness Wa value of the coating film surface was measured using Wave Scan DOI (manufactured by BYK Gardner). The results are shown in Table 2.
Moreover, cross-sectional observation of each multilayer coating film of Example 1 and Example 3 was implemented using the optical microscope.

As shown in Table 2, the curing temperature of the first base paint is lower than the curing temperature of the second base paint, and the viscosity of the clear paint at the curing temperature of the second base paint is lower than the viscosity of the second base paint Particularly in Example 3 in which the steel plate was exposed was less than in Example 1, and it was confirmed that the chipping resistance was improved.
Moreover, in the multilayer coating film of Example 1, the Wa on the coating film surface was large and somewhat glossy, while in the multilayer coating film of Example 3, the Wa on the coating film surface was small and gloss It was confirmed that the closing was slight.
In addition, as a result of cross-sectional observation of the multilayer coating films of Example 1 and Example 3, the boundary between the first base coating film and the second base coating film is clearer in Example 3 than in Example 1. That was confirmed.

Here, the behavior of the viscosity at the time of baking and curing of each paint used in Example 1 and Example 3 will be described with reference to FIG. 1 and FIG. FIG. 1 is a view showing the relationship between temperature and viscosity of each paint used in Example 3, and FIG. 2 is a view showing the relationship between temperature and viscosity of each paint used in Example 1.
In FIG. 1 and FIG. 2, the horizontal axis represents temperature (° C.), and the vertical axis represents the viscosity (Pa · s) of each paint. The viscosity of each paint was measured using a dynamic viscoelasticity measuring apparatus ("Rheosol-G5000" manufactured by UBM), using a parallel plate having a diameter of 25 mm and a gap of 1 mm, a frequency of 1.6 Hz, and an angular amplitude of 8.23 deg. It means the complex viscosity measured when the temperature is raised from 25 ° C. to 140 ° C. in 10 minutes.

As indicated by the arrows in FIGS. 1 and 2, it is considered that curing started when the viscosity began to rise, and the temperature is the curing temperature of each paint. As apparent from FIG. 1, in Example 3, the curing temperature of the first base paint is lower than the curing temperature of the second base paint, and the viscosity of the clear paint at the curing temperature of the second base paint is the second base paint. It was confirmed to be lower than the viscosity of
On the other hand, as is clear from FIG. 2, in Example 1, the curing temperature of the first base paint is higher than the curing temperature of the second base paint, and the viscosity of the clear paint at the curing temperature of the second base paint is the second It was confirmed to be higher than the viscosity of the base paint.
From the above results, it is assumed that the curing temperature of the first base paint is lower than the curing temperature of the second base paint, and the viscosity of the clear paint at the curing temperature of the second base paint is lower than the viscosity of the second base paint. According to the method of forming a multilayer coating film of the present invention, it was confirmed that the glossiness can be improved and the chipping resistance can be improved.

Example 4
[Preparation of first base paint]
While replacing all melamine resin components in a water-based paint for automobiles ("Aquarex" (registered trademark) made by Nippon Paint Co., Ltd.) with a completely methylated melamine resin ("Cymel 303" (registered trademark) made by Nippon Cytech Industries, As a pigment component, 13.16 parts by mass of titanium oxide ("CR-90" (registered trademark) manufactured by Ishihara Sangyo Co., Ltd.) and 0. 4 of carbon black ("FW 200" (registered trademark) manufactured by Evonik Tegsa Co., Ltd.). 13 parts by mass and 6.11 parts by mass of talc (Nippon Talc Co., Ltd. “SG-95” (registered trademark)) were mixed to adjust the brightness of the coating after baking and curing to 50 to 60. . Next, a urethane emulsion resin (17.92 parts by mass of “U-Coat UX-310” manufactured by Sanyo Chemical Industries, Ltd., and 9.76 parts by mass of a polyester resin (“HMP 27” manufactured by Mitsui Chemicals, Inc.) are added and sufficiently stirred As a result, the first base paint of Example 4 was obtained, which was adjusted so that the pigment concentration in the coating film after baking and curing was 40 parts by mass.

[Preparation of second base paint]
14. A urethane emulsion resin ("U-Coat UX-310" manufactured by Sanyo Kasei Co., Ltd.) against a water-based automotive paint ("Aquarex NH 737 M" manufactured by Nippon Paint Co., Ltd.) previously adjusted to silver color. After adding 36 parts by mass, the second base paint of Example 4 was obtained by sufficiently stirring.

[Formation of multilayer coating film]
The first base paint and the second base paint prepared in this example and the clear paint prepared in Example 3 are used instead of the first base paint, the second base paint and the clear paint of Example 1 By carrying out the same operation as in Example 1, a multilayer coating film of Example 4 was obtained.

Example 5
[Preparation of second base paint]
A urethane emulsion resin ("U-coat UX-310" manufactured by Sanyo Chemical Industries, Ltd.) is 14.36 against a water-based paint for automobiles ("Aquarex R540P" manufactured by Nippon Paint Co., Ltd.) registered in red in advance. After adding by mass, the second base paint of Example 5 was obtained by sufficiently stirring.

[Formation of multilayer coating film]
Instead of the first base paint, the second base paint and the clear paint of Example 1, the first base paint prepared in Example 4, the second base paint prepared in this example, and the clear paint prepared in Example 3 The multilayer coating film of Example 5 was obtained by carrying out the same operation as Example 1 except using.

Example 6
[Preparation of second base paint]
A urethane emulsion resin ("U-Coat UX-310" manufactured by Sanyo Kasei Co., Ltd.) is 14.36 against a water-based paint for automobiles ("Aquarex B 548 P" (registered trademark) manufactured by Nippon Paint Co., Ltd.) previously adjusted to blue. After adding by mass, the second base paint of Example 6 was obtained by sufficiently stirring.

Example 7
[Preparation of first base paint]
While replacing all melamine resin components in a water-based paint for automobiles ("Aquarex" (registered trademark) made by Nippon Paint Co., Ltd.) with a completely methylated melamine resin ("Cymel 303" (registered trademark) made by Nippon Cytech Industries, By blending 26.19 parts by mass of titanium oxide ("CR-90" (registered trademark) manufactured by Ishihara Sangyo Co., Ltd.) as a pigment component in advance, the brightness of the coating film after baking and curing becomes 80 to 90. It was adjusted. Next, a urethane emulsion resin (17.92 parts by mass of “U-Coat UX-310” manufactured by Sanyo Chemical Industries, Ltd., and 9.76 parts by mass of a polyester resin (“HMP 27” manufactured by Mitsui Chemicals, Inc.) are added and sufficiently stirred As a result, the first base paint of Example 7 was obtained, which was adjusted so that the pigment concentration in the coating film after baking and curing was 47.4 parts by mass.

[Preparation of second base paint]
A urethane emulsion resin ("U-Coat UX-310" manufactured by Sanyo Kasei Co., Ltd.) is 14.36 against a water-based paint for automobiles ("Aquarex NH578" manufactured by Nippon Paint Co., Ltd.) which has been previously adjusted to white. After addition by mass, the second base paint of Example 7 was obtained by sufficiently stirring.

[Formation of multilayer coating film]
Instead of the first base paint, the second base paint and the clear paint of Example 1, the first base paint prepared in this example, the second base paint prepared in this example and the clear paint prepared in Example 3 The multilayer coating film of Example 7 was obtained by carrying out the same operation as Example 1 except using.

<Evaluation>
The chipping resistance of each of the multilayer coatings of Examples 3 to 7 was evaluated according to the above-mentioned evaluation method. The results are shown in Table 3.

As shown in Table 3, the silver multi-layer coating film of Example 4, the red multi-layer coating film of Example 5, the blue multi-layer coating film of Example 6, the white multi-layer coating of Example 7 It was found that all of the films had the same chipping resistance as the black multilayer coating of Example 3. From this result, it was confirmed that excellent coating film performance can be obtained regardless of the hue by the method of forming a multilayer coating film according to the present invention.

Claims

A first base coating film forming step of coating a first base paint on a substrate to which electrodeposition coating has been applied to form a first base coating film;
A second base coating film forming step of coating a second base paint on the first base coating film to form a second base coating film;
A clear coating film forming step of applying a clear coating on the second base coating film to form a clear coating film;
In the first base paint film forming step, the first base paint is applied so that the pigment concentration of the first base paint film after baking and curing becomes 40 to 60 mass% and the film thickness becomes 20 μm or more. 1 form a base coating,
In the second base coating film forming step, the second base coating is applied to form a second base coating film so that the film thickness of the second base coating film after baking and curing becomes 8 μm or more. Of forming a multilayer coating film.
The method further comprises a baking step of baking and curing the first base coating and the second base coating simultaneously.
The multilayer coating film according to claim 1, wherein the combination of the first base coating and the second base coating uses a combination having a lower curing temperature than the second base coating. How it is formed.
The combination of the second base paint and the clear paint, wherein the clear paint has a lower viscosity of the coating film than the second base paint at the curing temperature of the second base paint. Or the formation method of the multilayer coating film as described in 2.
A multilayer coating film formed by the method for forming a multilayer coating film according to any one of claims 1 to 3.