JPS59140400A - Reverse coating method by electrodeposition - Google Patents

Abstract

PURPOSE:To obtain a paint film having superior performance, especially a significant anticorrosive effect at the boundary of a paint film formed by cationic electrodeposition while giving satisfactory economical efficiency, hue and finished appearance by specifying the thickness of said paint film formed by cationic electrodeposition when a paint film is formed by a reverse coating method. CONSTITUTION:An electrically conductive substrate is coated with powder paint or other paint by electrostatic coating or other method, and the coated substrate is optionally heated to form a paint film. The uncoated part of the substrate is coated with paint by cationic electrodeposition. In this reverse coating method, the thickness of a paint film formed by cationic electrodeposition is adjusted to >=30mum. A sufficient paint film is formed even at the boundary part between both paint films, and the desired paint film can be obtd.

Description

[Detailed description of the invention] The present invention relates to a reverse coating method.

Conventionally, a substrate with a conductive surface is coated with various paints such as powder paint using various coating methods such as electrostatic coating, heated as necessary to form a coating film, and then the unpainted areas are removed. A reverse painting method is known in which cationic electrodeposition paint is applied. be.

However, in this case, the coating film was not sufficiently formed at the boundary between the coating film and the cationic electrodeposition coating film, and the coating film performance in this area, particularly corrosion resistance, was extremely poor. This is because the electrodeposition coating is not formed due to the dust at the end of the coating film and the insulating properties of the linear thin film portion 1, and the thin film portion remains as it is.

Various attempts have been made to improve the above drawbacks.

Special Publication No. 57-37337 and Special Publication No. 55-4131
Publication No. 7 Ic discloses a method of containing paint (conductive powder).It is also known to apply a wax paint to a thin film part.However, these are economical, have only one hue and finish It is not something that should be determined based on appearance, etc.

The inventors of the present invention have carried out research in order to solve the above drawbacks, and have found that if the cationic electrodeposition coating is applied to a thickness of 30μ or more (conventional production lines were at most 20μ). It has been found that an electrodeposited coating film is formed on the thin film portion, and the above drawbacks are eliminated.

That is, the gist of the present invention is a reverse coating method, which is characterized in that after coating a conductive substrate by various methods, one cationic electrodeposition coating is performed so that the electrodeposition coating film thickness is 30μ or more. do.

Although any paint may be used for the coating of the present invention, powder paints, solvent-based paints, or water-based paints are generally used. The coating method may be electrostatic coating, spray coating, electrodeposition coating, or the like.

Any combination of the above paints and painting methods may be used as long as it is possible.
, powder electrostatic coating is preferred for energy saving reasons.

It is preferable to apply the powder coating to a film thickness of 30 μm or more in order to achieve a uniform film thickness with the cationic electrodeposition coating film.

The coated article may be cured and then subjected to cationic electrodeposition coating, but from the viewpoint of appearance, energy saving, etc., it is preferable to apply cationic electrodeposition coating while it is semi-cured or uncured.

As the cationic electrodeposition paint used in the present invention, common cationic electrodeposition paints such as those based on polyamino resins, acrylic resins, and polybutadiene resins can be used. In particular, amine-modified epoxy resin, amine-modified epoxy resin,
Paints such as urethane crosslinked cationic electrodeposition paints are preferred.

In the cationic electrodeposition method according to the present invention, the resin is coated with an organic acid (
For example, acetic acid, lactic acid) or an inorganic acid is used to form an electrodeposition bath mixed with a solvent, pigment, and additives, and the cathode and anode are immersed in the bath. The method consists of applying a voltage to the conductive thread to deposit a cationic electrodeposition resin on the conductive surface.

The content of the solvent in the electrodeposition bath is preferably 6% by weight or less based on the total weight of the bath from the viewpoints of ensuring good throwing power, low pollution, and saving resources.

The electrodeposition time is usually 1 to 5 minutes, preferably 2 to 3 minutes.

The applied voltage is 100-400 volts, preferably 200-400 volts
It is 300 volts.

After washing the electrodeposition-coated article as described above with water (this step
When baked and cured at a temperature of 50 to 200° C., preferably 160 to 180° C., for 10 to 30 minutes, the formation of a coating film at the boundary between the coating film and the cationic electrodeposited coating film was sufficient.

The feature of the present invention is that the film thickness of the cationic electrodeposition coating film is made to be 30μ or more, but in the conventional reverse coating method, even if the film thickness of the cationic electrodeposition coating film is increased, the coating film is It was not thought that electrodeposition coating would be formed only on the unformed areas, and that a coating film would be formed at the boundary areas. Therefore, the present invention breaks the conventional common sense and achieves excellent effects.

It is not clear why a sufficient coating film is formed even at the boundary areas in this way, but in the electrodeposition coating process of the present invention, the coating film is generally affected by the Joule heat at the stage where the film becomes thick and the high temperature I in the baking process. Melting occurs, and this melting is thought to be due to the fact that the film thickness of the adhered paint becomes larger at a point where it exceeds about 30 μm.

The present invention will be explained below from Example 1.

Example 1 Half of the surface-treated board prepared in Step 2 of Table 1 was coated with epoxy resin powder paint under the conditions of Steps 3 and 4 of Table 1.
A cationic electrodeposition paint using baking powder and epoxy thread polyamine resin as a coloring agent was prepared in a plastic container, and the test piece was used as a cathode under the conditions of Step 5 in Table 1. After electrodeposition coating was performed using the material as an anode, washing with water and baking were performed (Step 6 in Table 1) to obtain a test piece having an electrodeposition film bordering the powder coating film. The thickness of the electrodeposited film on this test piece I and the covering condition at the boundary were measured at 35 μm.
A photomicrograph of the cross section of the coating when reverse coating was applied to a film thickness of 45μ (this is submitted as Reference Photo 1) and a photo of the surface condition when reverse coating was applied to a film thickness of 45μ (submitted as Reference Photo 2) are shown. A painted cross section is schematically shown in Figure 1.

In the figure, (1) shows the electrodeposited substrate 5, (2) shows powder paint dust, (3) shows the electrodeposition coating film, and (4) shows the top coat. The test results are shown in Table 2.

Comparative Example 1 In the coating process shown in Table 1, a test piece was prepared using the same coating method as in Example 1, using a conventional epoxy polyamino resin cationic electrodeposition paint having a standard film thickness of 20 μm.

The test results for this test piece are shown in Table 2.

Furthermore, as in Example 1, the electrodeposited film thickness and the covering state at the boundary are shown in a microscopic photograph (reference photograph 3) of reverse coating with a film thickness of 20 μm and a photograph of the same surface condition (reference photograph 4). The painted cross section is shown schematically in Figure 2.

Example 2 The coating process shown in Table 3 was carried out using a melamine alkyd resin system and the same cationic electrodeposition paint as in Example 1 as a solvent-based paint. Solvent-based paint was spray-painted on half of the surface-treated board, cured by baking, and electrodeposited on the remaining half.
A test specimen was obtained having an electrodeposited coating bordered by a coating of solvent-based paint similar to that of the test panel.

The test results for this test piece are shown in Table 4.

Comparative Example 2 Coating process shown in Table 3 (same coating as Example 2 except that the same conventional epoxy polyamino resin cationic electrodeposition paint as Comparative Example 1 was used and electrodeposition was performed according to Step 5) A test piece was prepared using the method.

The test results for this test piece are shown in Table 4.

Example 3 Comparative Example 1 was applied to the powder paint coated board obtained in Step 4 of Table 5.
Conventional cationic electrodeposition paint (cellosolve solvent was added to this material, and the test plate was used as a cathode and the carbon plate was used as an anode in step 5. After electrodeposition coating was carried out, washing and baking were carried out (see Table 5) 6) A test panel was obtained having an electrodeposition coating bordering the powder coating coating.

Corrosion resistance is shown in Table 6.

Comparative Example 3 A polyester resin-based anionic electrodeposition paint was electrodeposited on the powder coated board obtained in Step 4 of Table 7 under the conditions of Step 5, with the test board serving as the @ electrode and the carbon plate serving as the cathode. Thereafter, the sample was washed with water and baked (Step 6 in Table 7) to obtain a test plate having an electrodeposited film bordering the powder coating film. The results are shown in Table 8.

Table 7 Table 8 (Comparative example where the film thickness of anionic electrodeposition paint is 35μ)

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a painted cross section when reverse coating is applied to a film thickness of 35 μm, and FIG. 2 is a schematic diagram of a painted cross section when reverse painting is performed to a film thickness of 20 μm. In the figure, (1) shows the electrodeposited base material, (2) shows powder paint dust, (3) shows the electrodeposition coating film, and (4) shows the top coat.

Claims (1)

[Scope of Claims] 1. In a reverse coating method in which a conductive substrate is coated and then cationic electrodeposition is applied, the cationic electrodeposition is applied so that the thickness of the electrodeposition coating is 30μ or more (this is carried out). 2. The method according to item 1, wherein the coating is performed by powder coating or spray coating. 3. The powder coating is electrostatic powder coating, and the coating has a film thickness of 30μ or more. 4. The first step of applying cationic electrodeposition to the coated article while it remains uncured.
The method described in section. 5. The method according to any one of Items 1 to 3, wherein the cationic electrodeposition coating is performed using an electrodeposition bath having a solvent content of 6% or less.