JPS6211907B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a metallic coating method, and more particularly to a low-pollution metallic coating method that provides a coating film with excellent chipping resistance and corrosion resistance. Metallic coating is widely used as a coating method for car bodies, etc., and it not only gives the coated surface a metallic appearance, but also provides long-term durability and durability.
In particular, it is required to have chipping resistance (the coating film does not peel off even when a foreign object such as a stone hits the coating film) and that the coated surface does not corrode from the peeled surface. Furthermore, a coating method that saves resources and reduces pollution is desired. From this point of view, a method has been proposed as a metallic coating method for automobile bodies, in which the coating is applied in the order of anionic electrodeposition paint, solvent-based metallic base, and clear powder paint. A film cannot be obtained. For this reason, attempts have been made to apply a solvent-based intermediate coating on top of the anionic electrodeposition coating, but not only the chipping resistance and corrosion resistance are not sufficiently improved, but the process of solvent-based intermediate coating increases by one step. Therefore, this method increases costs and is also problematic from the viewpoint of resource conservation and pollution prevention. From this point of view, the present inventors have carried out intensive research to improve the corrosion resistance and chipping resistance, which were problems in the coating systems of the solution-type metallic base and clear powder coating mentioned above. A cationic electrodeposition paint is painted and baked on the applied metal material, then an epoxy resin-based or polyester resin-based powder paint is painted and baked, and then a solution-based metallic base paint is applied,
It was discovered that a metallic coating with excellent chipping resistance and corrosion resistance can be obtained by painting a clear powder coating thereon without baking and baking it, which led to the creation of the present invention. The coating method of the present invention will be explained in more detail below. In the first step of the method of the present invention, the surface of the metal to be coated is subjected to, for example, derusting or degreasing treatment as necessary, or surface treatment such as film conversion treatment such as zinc phosphate treatment. , apply cationic electrodeposition paint and bake. As the cationic electrodeposition paint, any conventionally known cationic electrodeposition paint can be used. For example, an epoxy resin-based cationic electrodeposition paint can be suitably used as the resin forming the basic resin core. . This epoxy resin-based cationic electrodeposition paint is made by water-solubilizing a polyamine resin having a large number of amino groups in the epoxy resin core using an organic or inorganic acid as a neutralizing agent.For example, Power Top U-30, U-60 (Nippon Paint Co., Ltd.)
It is possible to use commercially available products such as those available from Such a cationic electrodeposition paint can be applied to a surface to be coated by a general method such as electrodeposition coating, and then baked according to a conventional method to form a cationic electrodeposition paint film. The thickness of this coating film is not particularly limited, but it is generally about 15 to 30 microns thick. Next, in the second step of the method of the present invention, an epoxy resin-based or polyester resin-based powder paint is applied as an intermediate coating on the cationic electrodeposition paint film by a general method, for example, by electrostatic spraying. Then, it is baked according to a conventional method to form an intermediate coating film of the powder paint. The epoxy resin powder coating used as the intermediate coating in the method of the present invention includes, for example, a normal solid bisphenol A type epoxy resin mixed with dicyandiamide, promoted dicyandiamide, amines, acid anhydrides, acid dihydrazide, etc. Types that are cured with acid-terminated polyester resins, etc. can be used, and polyester powder coatings have terephthalic acid as the main acid component, which is mixed with ethylene glycol, neopentyl glycol, 1,4-butanediol, 1, A type in which a polyester resin condensed with a polyol such as 6-hexanediol or trimethylolpropane is cured with a blocked isocyanate, a melamine compound, an epoxy compound, etc., and a self-curing type may be used. The thickness of the intermediate coating film is not particularly limited, but generally a thickness of about 10 to 200 μm is appropriate. Film thickness
If it is less than 10μ, the chipping resistance will be insufficient, so it is not very preferable from a practical standpoint. In particular, when painting the body of a car, the lower part of the car body, which is likely to be hit by stones while the car is running, should be coated with a thick film of, for example, 80μ or more, and the roof and upper side parts should be coated with a film of 10 to 60μ, for example.
By applying the coating to a relatively thin film, it is possible to form a coating film with significantly improved chipping resistance. In particular, according to the method of the present invention, since the powder coating is used as an intermediate coating, there is no limit to the film thickness that can be applied in one coat, as is the case with solvent-based paints, and areas that require thick coating in one coat are not limited. A thick film can be applied to areas where a thin film is suitable, and a thin film can be applied to areas where a thin film is suitable.Furthermore, the film thickness can be gradually changed, so the desired coating can be achieved in one coat and bake without creating any steps on the coating surface. membrane can be obtained. In the third step of the method of the present invention, a commonly used solution-type metallic base paint is applied on the intermediate coat film according to a conventional method, and a clear powder paint is applied as a conventional method without baking. The coating film of the solution-type metallic base paint and the coating film of the clear powder paint are baked at the same time.
There are no particular limitations on the thickness of these coatings, but the coating thickness of solution-type metallic base paints is usually about 10 to 40μ, and the coating thickness of clear powder coatings is usually about 20 to 150μ.
It should be around μ. The metallic base paint used in the third step of the method of the present invention is preferably a solution-type paint from the viewpoint of the metallic feel of the resulting coating. Any metallic base paint such as high solids, non-aqueous dispersion type, water soluble, emulsion, slurry etc. can be used, such as melamine acrylic resin system, N-methylolacrylamide curable acrylic resin system, acrylic resin having glycidyl and carboxyl groups. Resin-based materials such as resin-based and blocked isocyanate-curable acrylic resins are preferred. On the other hand, as the clear paint to be applied on the solution-type metallic base paint film, powder paint is preferable from the viewpoint of weather resistance, paint film appearance, and yellowing, but any clear powder paint can be used. but,
From the viewpoint of weather resistance and durability, it is particularly preferable to use an acrylic resin powder coating. As described above, by applying the cationic electrodeposition paint → epoxy resin or polyester resin powder paint → solution type metallic base paint + clear powder paint in this order, durability and corrosion resistance (salt It is possible to apply a low-pollution metallic coating with excellent sprayability, moisture resistance, peelability after exposure, etc.) and chipping resistance. Although the mechanism by which the chipping resistance of the coating film is significantly improved in such a coating system is not necessarily clear,
According to the findings of the present inventors, the following can be achieved by using a polyester resin-based or epoxy resin-based powder paint as an intermediate coat and a cationic electrodeposition paint as an undercoat on a solution-based base/powder clear metallic coating film. It is presumed that the chipping resistance of the coating film is improved for this reason. The factors that determine the chipping resistance of the paint film are:
(i) strength of the top coat, (ii) flexibility of the intermediate coat, and (iii) flexibility and adhesion to the metal material of the undercoat. In other words, first the top coat (especially the clear coat)
The first condition is to maintain sufficient strength (and flexibility) to withstand the impact against stones that can cause chipping, and from this point of view, the solution-based metallic base paint has a rigid coating and the clear powder coating has a slightly flexible coating. A combination of these is suitable as a top coat for chipping-resistant metallic coatings. Next, if the topcoat is subjected to an impact greater than its strength, if the intermediate and base coats cannot absorb the impact energy (i.e., if the paint is rigid), the paint will peel off. However, if the intermediate coat and base coat can absorb the impact energy (ie, if the coating is flexible), peeling of the coating will not occur. Furthermore, the undercoat film is required to have sufficient adhesion to the material, and if the adhesion to the material is not sufficient, the coating will peel off from the material due to impact, resulting in poor chipping properties. Therefore, sufficient flexibility and sufficient adhesion to the material (and corrosion resistance)
If there is a coating film that has both of these properties, it would be fine to use a single layer for the undercoat and intermediate coating, but in reality, it is difficult to obtain a coating film that has both of these properties.According to the research of the present inventors, although it is rigid, it is difficult to obtain a coating film that has both of these properties. A metallic paint with excellent chipping resistance can be obtained by combining a cationic electrodeposited coating film with excellent adhesion and corrosion resistance with an epoxy resin-based or polyester resin-based powder coating film with excellent flexibility. I found it. It is necessary to use an epoxy resin-based or polyester resin-based powder paint as the intermediate coating; other powder paints such as acrylic resin-based paints cannot provide a coating film with excellent chipping resistance. Furthermore, as mentioned above, the thicker the intermediate coating film, the better the chipping resistance, so a thicker film can be applied to areas where chipping resistance is particularly required, such as the lower part of an automobile body. In conventional coating systems, for example, a metallic coating is coated on top of an anionic electrodeposition coating, but the anionic electrodeposition coating is flexible and absorbs the impact energy of chipping. However, the adhesion to the material is insufficient, resulting in peeling of the coating from the material, and corrosion resistance is also insufficient. On the other hand, when a cationic electrodeposited coating is simply used instead of an anionic coating, the adhesion is good, but the coating is rigid and cannot absorb the impact energy of chipping, resulting in cracking and peeling of the coating. This is not desirable because it causes In contrast, in the coating system of the present invention, as described above, a highly flexible epoxy resin or polyester resin powder is used as an intermediate coating on a cationic electrodeposited coating that has excellent adhesion to materials and corrosion resistance. Since the coating film of the body paint is formed to the required thickness, impact energy can be sufficiently absorbed, and a coating film with excellent chipping resistance can be obtained. The present invention will be described in more detail below based on Examples and Comparative Examples, but it goes without saying that the scope of the present invention is not limited to these Examples. Examples and Comparative Examples Preparation of Powder Coatings Blends having the compositions listed below were dry mixed using a super mixer, and then melted and kneaded at a temperature of 100°C in a co-kneader (model PR-46 manufactured by Swissbus). This was ground in a hammer mill and sifted through a 150-mesh wire mesh to produce epoxy powder coatings A and B, polyester powder coatings C to E, and acrylic powder coating F, respectively.

【table】

【table】

[Table] Paint test 0.8 x 90 x 150 mm SPC treated with zinc phosphate -
1 Apply 20μ of the electrodeposition paint listed in Table 2 to a steel plate,
Baked at 170°C for 30 minutes. Next, the intermediate powder paint prepared above was electrostatically coated to a thickness of 60μ using a powder electrostatic coating machine (Starjet gun manufactured by Sames).
Baked at 170°C for 30 minutes. After sanding this intermediate powder coating film with #400 sandpaper, 25μ of solvent-based metallic base (Note 9) was applied, and after setting for 10 minutes at 20℃, acrylic clear powder was applied on top. Paint (Note 10) was applied electrostatically to a thickness of 60μ and baked at 170℃ for 30 minutes. The test results for this coated plate are shown in Table 2. (Note 9) Acrylic varnish (10 parts by weight of methyl methacrylate, n-butyl methacrylate)
30 parts by weight, 15 parts by weight of ethyl acrylate, 8 parts by weight of 2-ethylhexyl acrylate
parts by weight, 2 parts by weight of acrylic acid, 5 parts by weight of 2-hydroxyethyl methacrylate,
N-n-butoxymethylacrylamide
Acrylic varnish obtained by solution polymerizing a mixture of 10 parts by weight, 20 parts by weight of styrene, 80 parts by weight of xylene, 20 parts by weight of butanol and 1 part by weight of azobisisobutyronitrile) 100 parts by weight, Alpaste 1109MA (Toyo Aluminum) 5
Part by weight, solvent-based paint (Note 10) manufactured from 0.5 parts by weight of Phthalocyanine Blue and 0.01 parts by weight of Mitsubishi Carbon MA-1000 Acrylic resin (22.5 parts by weight of glycidyl methacrylate, methyl methacrylate)
100 parts by weight of an acrylic resin obtained by suspension polymerization of a mixture of 40 parts by weight, butyl methacrylate, 17.5 parts by weight, 20 parts by weight of styrene, 2 parts by weight of azobisisobutyronitrile and 2.5 parts by weight of t-dodecyl mercaptan, 18 parts by weight of dodecanedioic acid and 1 part of Modaflow (manufactured by Monsanto) were melt-kneaded in a conventional manner, crushed,
Clear acrylic powder paint with 150 mesh

[Table] Next, the physical properties of the epoxy powder paint, polyester powder paint, acrylic powder paint, and solvent-type intermediate paint film used in each of the above experiments were confirmed. Third
The results are shown in the table.

【table】

Claims (1)

[Claims] 1. Coating and baking a cationic electrodeposition paint on a metal material or a metal material that has undergone chemical conversion treatment, then coating and baking an epoxy resin-based or polyester resin-based powder coating, and then applying a solution-type metallic base paint, A metallic coating method characterized by painting a clear powder coating on it and baking it without baking.