JPH0131950B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of coating an object to be coated, and more particularly to a method of coating a cationic electrodeposition coating film with a specific topcoat film to prevent yellowing of the coating film. Recently, in order to improve the corrosion resistance of automobile bodies, etc., cationic electrodeposition paints with excellent corrosion resistance have been increasingly used. It has become possible to obtain sufficient corrosion protection with just two coats. However, if the top coat is applied directly onto the cationic electrodeposited coating and baked, there is a problem that the topcoat will yellow, and for this reason, an intermediate coating is applied between the cationic electrodeposited coating and the topcoat. Countermeasures are being taken such as applying paint and providing an intermediate coat. The present inventors have carried out extensive studies to improve the drawbacks of the conventional coating method using cationic electrodeposition coatings, and as a result, we have applied a non-yellowing blocking isocyanate 0.01 to 30% as a top coat on top of the cationic electrodeposition coating. The present inventors have discovered that the above-mentioned problem of yellowing of the coating film can be effectively solved by coating with a powder coating material containing % by weight, and the present invention has been completed. That is, in the present invention, the object to be coated is coated with a cationic electrodeposited coating film that is cured with blocking isocyanate as an undercoat, and then finished with a powder coating containing 0.01 to 30% by weight of a non-yellowing blocking isocyanate as a topcoat. This coating method consists of: According to the present invention, the object to be coated is further coated with a cationic electrodeposition coating film cured by blocking isocyanate as an undercoat, and then a solvent-based, non-aqueous dispersion type,
Or, a coating method in which a water-based metallic base paint is applied, a clear powder paint is applied thereon without baking, and the coating is cured by baking at the same time.
A coating method is provided which comprises incorporating 0.01 to 30% by weight of a non-yellowing blocking isocyanate into at least one of the metallic base paint or the clear powder paint. As described above, according to the present invention, yellowing of the paint film, which has been a problem in the past, can be avoided by applying a specific powder paint on top of the cationic electrodeposition paint film, which has conventionally caused yellowing of top coat paints. It can be solved effectively. Moreover, not only non-yellowing blocking isocyanates but also powder coatings and solvent-based coatings containing blocking isocyanates are known to yellow when baked on iron plates or anionic electrodeposition coatings. By applying paints with such drawbacks onto cationic electrodeposition coatings, it is possible not only to solve the yellowing caused by cationic electrodeposition coatings, but also to improve the effectiveness of powder coatings and solvent-based coatings containing blocking isocyanates. It is surprising and unexpected that the above-mentioned drawbacks can also be overcome. As described above, in the past, a specific intermediate coating was used to overcome yellowing of the paint film, but according to the present invention, the use of such a specific intermediate coating is not necessary and the cationic electrolyte is used. It has the advantage that a top coat, such as a powder, solvent-based, water-based, or non-aqueous dispersion (NAD) top coat, can be applied directly onto the base coat of the deposited film. Furthermore, when performing a so-called metallic coating, as described above, in order to apply a metallic coating on top of a cationic electrodeposition coating in the conventional manner, a metallic base paint is first applied, without baking.
By applying a clear top coat over it, yellowing of the paint film can be prevented. In the latter case, the metallic base paint or top clear paint may be a conventionally known paint, such as powder, solvent-based, water-based, or non-aqueous dispersion (NAD) paint. As mentioned above, when a metallic coating is applied, a blocking isocyanate may be added to at least one of the metallic base paint and the top clear paint. Although the mechanism by which yellowing of paint films can be effectively improved by the method of the present invention is not necessarily clear, the yellowing of top coats on cationic electrodeposition paints is due to water dissociated isocyanates in the cationic electrodeposition paints. The reaction with and subsequent oxidation produces brown amine compounds and nitro compounds, which are transferred to the top coat, causing the top coat to yellow. Additionally, when topcoat paints containing blocking isocyanate are baked, trace amounts of amine compounds and
Yellowing due to formation of nitro compounds. However, by applying a top coat containing blocking isocyanate on the cationic electrodeposited film,
Even if the amine compounds and nitro compounds generated by baking the cationic electrodeposition paint migrate into the top coat,
It is thought that yellowing does not occur because no brown compound remains after reacting with the blocking isocyanate in the top coat, and the amine compound and nitro compound work to prevent the blocking isocyanate from reacting with water and from oxidation. In the present invention, the cationic electrodeposition paint that is cured by blocking isocyanate used as an undercoat paint is an aminoepoxy resin prepared by adding an amine to an epoxy resin, or an aminoepoxy isocyanate resin prepared by adding a polyisocyanate to the epoxy resin. This is a mixture of neutralized resin and blocked polyisocyanate dispersed in water. In the method of the present invention, on the cationic electrodeposition coating film,
Powder coatings applied as top coats are resin-based powders such as polyester melamine resin, acrylic melamine resin, N-alkoxymethyl acrylamide resin, glycidyl acrylic resin (carboxylic acid curing), carboxylic acid acrylic resin (epoxy compound curing), etc. Body paints are preferred. It is preferable that a hydroxyl group be introduced into the resin of these powder coatings in advance, or that a hydroxyl group be present as a result of a reaction between the resin and a curing agent. According to the present invention, these common powder coating compositions contain blocked isocyanate in an amount of 0.01 to 30% by weight, preferably 0.5 to 20% by weight, as described above. In the method of the present invention, on the cationic electrodeposition coating film,
A solvent-based paint that is applied as a metallic base paint.
Examples of non-water dispersible or water-based metallic base paints include resin-based metallic base paints such as acrylic melamine resin, N-alkoxymethyl acrylamide resin, glycidyl acrylic resin (carboxylic acid curing), and carboxylic acid acrylic resin (epoxy compound curing). is suitable. In the resin of these metallic base paints, it is preferable that hydroxyl groups are introduced into the resin in advance, or that hydroxyl groups are present that are generated by the reaction between the resin and a curing agent. Any non-yellowing blocked isocyanate can be used as the non-yellowing blocked isocyanate, and specific examples include isophorone diisocyanate, hexamethylene diisocyanate,
At least one non-yellowing isocyanate such as hydrogenated tolylene diisocyanate and hydrogenated xylylene diisocyanate, or a polyisocyanate compound obtained by reacting with a condensation thereof or a polyhydric alcohol such as ethylene glycol, diethylene glycol, trimethylolpropane, or an amine. Phenols, ε-caprolatatum, oximes,
Examples include blocks made with alcohol, etc. If the content of blocking isocyanate in the powder coating is less than 0.01% by weight, the yellowing of the paint film cannot be improved; on the other hand, if the content of blocking isocyanate in the powder coating exceeds 30% by weight, is not preferable because it deteriorates the physical performance of the coating film. In addition to the above-mentioned components, the above powder coating composition for top coating also contains additives commonly used for powder coatings, such as leveling agents, anti-cissing agents, and antistatic agents. , additives such as anti-blocking agents, etc. can be added. The method of coating the object to be coated with the cationic electrodeposition paint according to the method of the present invention and the method of coating the above-mentioned powder paint on the cationic electrodeposition coating film can be any coating method commonly used in the past. It can be carried out by Examples of the present invention will be described below, but it goes without saying that the scope of the present invention is not limited to these Examples. Example 1 The composition shown in Table 1 below was dry mixed in a super mixer, and then mixed in an extruder (Switzerland).
The mixture was melt-kneaded at 100°C using a Co-kneader PR-46 manufactured by Busu Co., Ltd. This kneaded material was air-cooled, then pulverized in a hammer mill, and sifted through a 150-mesh wire mesh to produce acrylic powder coatings A and B.

[Table] 0.8mm thick SPC-1 steel plate treated with zinc phosphate,
Anion electrodeposited plate (Nippon Paint power coat on 0.8mm thick SPC-1 steel plate treated with zinc phosphate)
6600 gray 20μ painted and baked at 170℃ for 30 minutes) and cationic electrodeposited plate (20μ Nippon Paint Power Top U-60 Black painted on a 0.8mm thick SPC-1 steel plate treated with zinc phosphate, 170℃
× Baked for 30 minutes) above acrylic powder paint A
and B were coated to a thickness of 80 microns using an electrostatic powder coating machine (Sames Starjet), and baked at 160°C for 30 minutes. The yellowness of this painted plate was measured using a color difference meter (SM color computer manufactured by Suga Test Instruments) according to the Hunter color difference display method according to JIS Z8730, and determined the b value, Δb value,
The ΔE value was measured. The results are shown in Table 2.

[Table] Example 2 The same procedure as in Example 1 was carried out using the acrylic powder paint shown in Table 3 below instead of the acrylic powder paint B in Example 1. The results are shown in Table 4.

【table】

【table】

【table】

[Table] Example 3 Metallic base paint HI and clear powder paint JK having the following compositions were manufactured.

【table】

【table】

[Table] Zinc phosphate treated plates, anionic electrodeposited plates, and cationic electrodeposited plates were coated with 20μ of the specified metallic base paint using a spray gun and set for 10 minutes at room temperature. During painting, the viscosity of the metallic base paint was adjusted to 22 seconds using a #4 food cup using a mixed solvent of xylene/butanol = 8/2. A specified clear powder coating was applied thereon to a thickness of 60 microns using an electrostatic powder coating machine (SAMES Starjet), and baked at 160°C for 30 minutes to obtain a metallic coating film. The b value and Δb value of this coating film were measured in the same manner as in Example 1.
The results are shown in Table 5.

[Table] *Example

Claims (1)

[Scope of Claims] 1 The object to be coated is coated with a cationic electrodeposition coating film cured by blocking isocyanate as an undercoat, and then coated with a powder coating containing 0.01 to 30% by weight of a non-yellowing blocking isocyanate as a topcoat. A coating method characterized by finishing. 2. The method according to claim 1, wherein the powder coating is an acrylic powder coating containing an acrylic resin obtained by copolymerizing a monomer having a glycidyl group, a dicarboxylic acid, and a non-yellowing blocked isocyanate. 3. The object to be coated is coated with a hardening cationic electrodeposition coating using blocking isocyanate as an undercoat, and then a solvent-based, non-aqueous dispersion type, or water-based metallic base paint is applied, and a clear powder is applied on top of it without baking. 1. A coating method in which a paint is applied and cured by baking at the same time, characterized in that at least one of the metallic base paint or the clear powder paint contains 0.01 to 30% by weight of a non-yellowing blocking isocyanate.