JPS622640B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reverse coating method, and more particularly to a reverse coating method in which a part of an object to be coated is coated with synthetic resin powder, and the remaining part of the object to be coated is further coated with an electrodeposition coating.

The reverse painting method is a painting method that is effective in saving labor and resources, and is a highly corrosion-resistant painting method, so it is a painting method that has been attracting attention in recent years, particularly in the field of painting automobile bodies.

In the reverse coating method, a part of the object to be coated is generally coated with dry (electrostatic) powder (dry powder), wet powder (slurry), or electrodeposited powder (EPC), and then the coating film is coated. This is a method in which the material is heat-sealed at a temperature above its softening point and at a temperature that does not cause crosslinking and hardening to make it immobile, and then the other parts of the object are coated by regular electrodeposition.

For example, when painting an automobile body, (i) the outer panel of the automobile body is coated in advance with a thick coat of paint containing the above-mentioned powder, and this coating film is heat-fused to make it immobile and especially corrosion-resistant. A high-performance paint film with excellent properties is applied, and (ii) the interior of the vehicle is painted using regular electrodeposition coating.

According to this method, it is possible to easily apply a thick film coating to the outer panel, and since powder etc. can be used, it is possible to reduce the number of times of painting up to the final painting of the automobile body, and it is possible to achieve good quality. A coating film is obtained.
It also has advantages such as good throwing power when electrodeposited on the inner plate.

In such a reverse coating method, after a part of the object to be coated is coated with the above-mentioned powder-containing paint, the coating film on the area coated with the above-mentioned powder-containing paint is made immovable before the next electrodeposition coating. Therefore, a process is usually required to heat-fuse the coating film to the object to be coated at a temperature that is above the softening point of the powder but does not cause crosslinking and hardening. Next, other parts of the object to be coated are usually electrocoated, and after passing through a water washing process, a baking drying process is carried out to bake the entire coating film.

In the reverse coating method described above, after the coating film is formed with the powder-containing paint in the first stage, the portion where the coating film is thermally fused has become hydrophobic at this stage. For this reason, after the subsequent ordinary electrodeposition coating process is completed, the washing water after rinsing adheres in the form of large water droplets or puddles on the coating film formed by the powder-containing coating material. It was found that if the film was subjected to the final baking step as it was, problems would occur in the coating film performance due to the water pooling. In other words, the water puddle area has a concave finish,
It has been found that the appearance of the coating film may be impaired, and that moisture resistance may deteriorate in terms of performance.

The reason for this is that in the dry powder, slurry coating, or EPC coating described above, in the post-painting heat fusion process, the powder, which is a coating film forming component, is heat fused and flows, resulting in a uniform and even coating film. As a result, the coating film adheres to the object being coated and becomes hydrophobic in terms of performance. Water that adheres to this hydrophobic coating film during the next normal electrodeposition and water washing process will interfere with heat conduction in the areas where the water has adhered during the final baking drying process, causing localized areas that prevent uniform and smooth baking of the coating film. Lose. Furthermore, it is thought that the water becomes boiling and obstructs the smoothing of the sticky coating film, which ultimately causes defects in the coating film. This coating film defect caused by water is generally called a water mark, a water drop mark, a water mark, or the like.

As a method for solving the above-mentioned problem of water marks in the reverse coating method, a method can be considered in which a coating film made of a powder-containing paint is cured by baking, and then the coating film is usually subjected to electrodeposition coating. However, even in this case, it is necessary to bake and dry the coating film after the subsequent ordinary electrodeposition coating, and carrying out baking and drying twice is a big disadvantage from the point of view of energy saving. Further, when such intermediate baking drying is performed, a problem arises in that a boundary line is formed between the powder coating film in the first stage and the electrodeposition coating film in the second stage. It was also found that although this method could eliminate water marks, it could not completely solve the problem of moisture resistance caused by water evaporation.

In order to eliminate the drawbacks of the conventional reverse coating method described above, the present inventors applied heat to a part of the object to be coated after forming a coating film using powder coating at a temperature that is above the soft point of the powder and does not harden the coating film. After fusing, the other parts of the object to be coated are painted with ordinary electrocoating paint, washed with water, pre-dried at a temperature below the softening point of the powder, and then baked to remove the water marks. They found that the formation of marks) can be prevented.

Furthermore, according to the method of the present invention, since the amount of water that is normally introduced into the baking drying oven after electrodeposition coating is reduced, it is possible to prevent coating defects due to tar sag that occurs in the oven.

For powder electrodeposition paints that can be used in the method of the present invention, see, for example, Japanese Patent Publication No. 51-40585, for powder paints see Japanese Patent Publication No. 48-29836, and for wet powder paints see Japanese Patent Publication No. 55-50514. I can do it. Further, any known electrodeposition paint can be used as the electrodeposition paint.

Examples are described below. All parts and percentages herein are by weight. Note that the present invention is not limited to these Examples.

Example 1 Preparation of cationic water-dispersed resin: Epicote #1001 (manufactured by Ciel) 488 parts Isopropyl alcohol 250〃 Diethanolamine 105〃 Lactic acid 76.5〃 Ion exchange water 1057〃 Amine-added organic acid neutralization using the above formulation A binder with a solid content of epoxy cationic electrodeposition resin A of 30% was obtained.

Preparation of powder coatings: Epicote #1004 (manufactured by Ciel) 40 parts Adduct B-1065 (manufactured by Hyurus) 30〃 Titanium oxide R-550 (manufactured by Ishihara Sangyo Co., Ltd.) 19〃 Kaolin ASP-200 (manufactured by Hayashi Kasei Co., Ltd.) 10〃 Carbon Black MA-100 (manufactured by Mitsubishi Kasei Corporation) 1〃 Benzoin 0.5〃 A conventional method that involves premixing the isocyanate-cured epoxy thermosetting powder coating with the above formulation, heat kneading, cooling, coarse pulverization, and fine pulverization. Prepared according to.

8.48 tons of the above powder coating, 21.70 tons of the above cationic electrodeposition resin A binder, and ion-exchanged water.
100 tons of powder electrodeposition paint bath liquid with solid content concentration of 15% and pH of 5.3 was prepared using 69.82 tons of liquid.

Preparation of cationic electrodeposition paint: Epicote #1001 (manufactured by Ciel) 1000 parts Tetrahydrophthalic anhydride 304〃 Hydroxyethyl methacrylate 260〃 Cellosolve acetate 919.7〃 Diethylethanolamine 234〃 Tolylene diisocyanate 348〃 Hydroquinone 10〃 Triethylamine 3 Formic acid 49.4 Based on the above formulation, a cationic resin B having a solid content of 70% was prepared by a conventional method.

Cationic resin B 50 parts Titanium oxide 7〃 Carbon black 0.75〃 Strontium chromate 0.3〃 Lead silicate 0.7〃 Ion-exchanged water 87.1〃 Cationicity can be obtained by using the above-mentioned formulation under the conventional process of premix, ball mill dispersion, and letdown. An electrodeposition paint with a solid content of 30% was obtained.

90 tons of the above cationic electrodeposition paint and ion exchange water
180 tons of cationic electrodeposition paint with a solid content concentration of 15% and a pH of 6.0 were prepared using 90 tons of paint.

1800 c.c. displacement with zinc phosphate treatment, coated area
A ^70m2 car body was coated with the above powder electrodeposition paint bath solution at a conveyor speed of 4m and a body pitch of 5.5m.
Cationic powder electrodeposition coating was carried out in a vacuum chamber under the following conditions: electrodeposition temperature: 25° C., distance between electrodes: 60 cm, electrodeposition time: 15 to 30 seconds, and electrodeposition voltage: 500 V.

Subsequently, the sample was washed with water, drained and set for 3 minutes, and then passed through a drain drying oven for about 10 minutes in an atmosphere of 80 to 85°C. It was confirmed that the outer panel was immobile.

Next, the powder was electrodeposited in 180 tons of the above cationic electrodeposition paint bath solution under the following conditions: electrodeposition temperature 25℃, electrode distance 50cm, total electrodeposition time 120 seconds, electrodeposition voltage 250V. The interior of the room, bags, and gaps that were not covered were coated with cationic electrodeposition using an energized tank method, and then washed with water.

Passage time 8 minutes, 1500m ³ /
min, 60mmAq, 50KW circulation fan, 150×
Burner heat capacity of 10 ⁴ Kcal/Hr, 300m ³ /min, 30mm
Aq, 7.5KW exhaust fan, effective length 35m with 3m front and rear air curtain zones, volume ^315m3 ,
It was passed through a drain drying oven with a furnace temperature of 70 to 75°.

After being taken out of the drain drying oven and moved through an open space of approximately 1 m, the powder was introduced into a baking drying oven with an ambient temperature of 185 to 190°C and a residence time of 25 minutes. The electrodeposition coating film and the cationic electrodeposition coating film applied to the inner plate were simultaneously baked and cured. The specifications of the baking drying oven at this time are ^1000m3 /min, 70mm
Aq, 25KW circulation fan, 110×10 ⁴ Kcal/Hr burner heat, 105m ³ /min burner blower,
It was equipped with three sets of 60m ³ /min, 30mmAq, 1.5KW exhaust fans, and had an effective length of 90m and a volume of 810m ³ with air curtain zones of 3m each at the front and rear.

No problems such as "water droplets" or "water marks" were observed on the surface of the baked powder electrodeposition coating.

Comparative Example 1 An automobile was painted using the same painting process as in Example 1, except that the draining and drying oven after regular electrodeposition washing was not installed, and all other conditions were the same as in Example 1. Many water marks were observed on the exterior of the body, and a polishing process was required to obtain a good appearance.

Example 2 The powder coating shown in Example 1 was pulverized to an average particle size of 25 μm, and electrostatically applied to the outer panel of a body to a particle size of 50 μm using a conventional method. The size, shape, conveyor speed, body pitch, etc. of the body were the same as in Example 1. Drying to make the outer panel part immobile was carried out at a body temperature of 170° for 10 minutes. Thereafter, the body was subjected to electrodeposition, washing with water, draining and drying, and final baking drying under the same conditions as in Example 1. The finish on the outer panel was good.

Comparative Example 2 Reverse painting was carried out in the same manner as in Example 2, except that the drain drying oven was not used. Many water marks were observed on the outer panel of the body.

Claims (1)

[Claims] 1 After coating a part of the object to be coated with powder, wet powder, or powder electrodeposition paint, the coating film is heat-fused at a temperature above the softening point of the powder but not crosslinking and hardening, and then the object to be coated is In the reverse coating method, in which other parts of the body are painted with ordinary electrodeposition paint, washed with water, and then baked, a powder coating film is applied at a temperature below the softening point of the powder after washing with water and before baking. A reverse painting method characterized by preliminary drying.