JPH0341225B2 - - Google Patents

Description

[Detailed description of the invention]

Technical Field The present invention relates to a method for forming a laminated coating film, and more specifically, the present invention relates to a method for forming a laminated coating film, and more specifically, after coating and curing an undercoat film on an object to be coated, and polishing the cured undercoat film, the intermediate coat film and top coat film are then applied. The present invention relates to a method for forming a laminated coating film with excellent finished appearance by applying the intermediate coating film and curing the intermediate coating film and the top coating film without polishing. Conventional technology Conventionally, the technology for forming laminated coatings, as typified by automobile coatings, has focused on the pursuit of functions such as corrosion resistance and chipping resistance, suitability for mass production, and economic efficiency. With the spread of electrodeposition paints, chipping resistance has been improved by using specific polyester resins and by incorporating talc and mica with specific particle sizes. Publication No. 58-74349, Japanese Unexamined Patent Publication No. 1983
It has been proposed in Japanese Patent Publication No. 141459 and Japanese Patent Publication No. 52-43657. However, although these proposed technologies are achieving practicality for the coated objects and economical efficiency based on mass production, they are still lacking in terms of providing a high-quality exterior finish with excellent durability, image clarity, and gloss. It was enough. For passenger cars, the standard method is to apply a chemical coating, an electrodeposited undercoat, bake harden, apply a first intermediate coat, and a second intermediate coat, polish after bake harden, and then finish by laminating a metallic base, top clear, or solid coat. It is a paint type. Also, JP-A-58-70866 and JP-A-58-
Publication No. 3678 discloses polishing the electrodeposition coating film and the intermediate coating film, but when polishing is done in the intermediate coating process, the top coating paint is sucked in, resulting in an excellent texture. A glossy coating film could not be obtained. On the other hand, the necessity of a polishing step during the painting process is often due to the unevenness of the paint film as well as the material itself. In other words, dull steel plates, which are often used as steel plates for automobiles, have dullness (unevenness) of 0.5 to 0.8 μm, and this unevenness cannot be eliminated simply by laminating a coating film. Polishing of the paint film is required. OBJECTS OF THE INVENTION Therefore, an object of the present invention is to eliminate the problems of the conventional techniques described above and to provide a method for forming a laminated coating film with excellent finished appearance. Structure of the Invention That is, according to the present invention, a cationic electrodeposition paint is applied to an object to be coated and cured, the cured cationic electrodeposition coating is polished, and then a pigment volume concentration of 10 to 50% is applied.
and two layers of primary and secondary intermediate coats having a pigment volume concentration of 5 to 40% are sequentially laminated, and at least one top coat is further applied, without polishing the intermediate coat and top coat. Provided is a method for forming a laminated coating film characterized by curing. Specific Description of the Structure and Effects of the Invention In view of the current status of the method for forming a laminated coating film as described above, the present inventors have conducted intensive studies to develop a method for forming a laminated coating film with an excellent finished appearance. The present inventors have discovered that the problems of conventional multilayer coatings can be effectively solved by adopting the configuration described above, and have completed the present invention. That is, according to the present invention, first, the object to be coated is subjected to pretreatment such as film conversion treatment as necessary, and then,
An undercoat film is applied and cured, and the cured undercoat film is polished. The paint used for the undercoat film is the cationic electrodeposition paint, which has been the most widely used conventionally.However, the undercoat used in the present invention is not limited to the cationic electrodeposition paint. Polishing may be performed after the reverse step of the mold primer or powder coating/cationic or anionic electrodeposition coating. The coating thickness of the undercoat film is reduced by about 5 μm by polishing as described below, so if the thickness is not maintained at least about 20 μm, the anticorrosion properties may deteriorate or the base layer may be partially polished. Paint film thickness is
The thickness is preferably 30 μm or more, but since it is difficult to apply an extremely thick electrodeposition coating, the maximum thickness is about 50 μm. If you want to apply an undercoat film to a thickness greater than that, it is preferable to use a solvent-based primer or powder paint, but recently, thick-film cationic electrodeposition paints have also been developed, so use these. You can also do that. The undercoat film applied to the object to be coated is cured by baking according to a conventional method. The standard bake-curing conditions when using cationic electrodeposition paints are 165 to 185°C x 20 minutes, but bake-curing conditions that give the abrasive a semi-cured state without entanglement of the uncured paint film, for example, 140-160
It may be heated at ℃ for about 20 minutes. Since the undercoat film formed in this manner reflects the dull grain, it is necessary to use an abrasive material such as sandpaper to smooth out the corners of the dull grain. This polishing is for polishing the undercoat film, so
The sharpening marks are hidden by the intermediate coat and top coat and are not noticeable, so they can be sanded using coarse sandpaper of about 280 to 600 grit. The undercoat film may be polished by either wet or dry sanding, but wet sanding is preferable to obtain a highly smooth polished surface, and dry sanding is preferable for surfaces with few irregularities due to the ease of work. Good too. In this way, the polished surface absorbs the resin of the paint applied on top because the upper resin layer, which has less suction, has been scraped off, but according to the present invention, the intermediate coat and top coat can be applied. Therefore, the texture and gloss of the finished coating will not be impaired.
In addition, the polished surface has a greatly increased surface area and is hairy, so if the intermediate coat is polished, the solvent and resin will be absorbed by capillary action when the top coat is applied. It is thought that the above-mentioned problems occur because the preferable micro-Brownian motion in the wet film is inhibited and the resin content that should be arranged on the surface layer of the top coat is consumed. However, according to the present invention, the top coat is applied without polishing the intermediate coat, so it seems that this problem can be effectively solved. In the method for forming a laminated coating film according to the present invention, the intermediate coating film is removed by removing the sandpaper marks,
It is preferable that the functions be shared between a primary intermediate coating film, which performs an auxiliary function to enhance corrosion resistance, and a secondary intermediate coating film, which functions to enhance the finish of the top coating film. The primary purpose of the primary intermediate coating is to erase the sandpaper marks caused by polishing and to perform an auxiliary function to increase anti-corrosion properties. things are appropriate,
Particularly preferred is 15-30%. 10% PVC
If it is less than 50%, the ability to hide the scratches caused by the sandpaper grains will not be sufficient, and if it exceeds 50%, a smooth coating film will not be obtained, and more paint will be absorbed, making abnormalities such as rough skin more likely to occur. Pigments used in primary intermediate coating paints include:
Relatively coarse particle size, more than 90% of primary particles are 0.5~
Talc, precipitated barium sulfate, aluminum tripolyphosphate, calcium carbonate, mica, phosphomolybdates, zinc phosphate, zinc white, aluminum silicate, zinc dust, and flakes with a maximum diameter of 30 μm. Aluminum flakes and the like are preferred. By blending appropriate amounts of these pigments, it is effective in hiding scratches on the base and preventing edge rust. It is not clear why these pigments with coarse particle sizes have good scratch-hiding ability, but they do not penetrate into the ground surface, etc. (pigment particles do not get into the gaps of the ground surface, etc.) and exhibit a sealing effect. This is thought to be because Therefore, while coarse particles such as talc and zinc dust are highly effective, they make it difficult to achieve flatness in the secondary intermediate coating, so these points should be taken into consideration when selecting the type and amount of pigment used. It is preferable to select The second intermediate coat is usually applied wet-on-wet (hereinafter referred to as W/W) on top of the first intermediate coat to form a coating with less suction to improve the finish of the top coat. I found that it was preferable. For this purpose, the second intermediate coating film is
A suitable PVC content is 5 to 40%; if the PVC content in the secondary intermediate coat is less than 5%, it will easily reflect the skin condition of the primary intermediate coat, and if it exceeds 40%, the top coat will paint is more likely to be sucked in. When applying the intermediate coating film once, the expected performance described above is achieved in one layer, so the limit range of PVC and film thickness is further limited to PVC: 10 to 30%, film thickness: 35 to 50 μm. It is preferable that In conventional laminated coating film formation technology, the amount of pigment in the intermediate coating film is usually limited to provide clarity to the topcoat film, and the maximum amount of PVC is about 30%. However, according to a preferred embodiment of the present invention, after forming the undercoating electrodeposition coating film, the electrocoating paint is polished, and the functions of the intermediate coating film can be shared between the primary intermediate coating and the second intermediate coating. The limitations can be eliminated and the above objectives can be achieved. In this case, if the primary intermediate coating film and the secondary intermediate coating film are mixed together, the appearance improvement effect will be reduced, so in some cases, the coating of the primary intermediate coating film and the secondary intermediate coating film may be mixed. It may be necessary to perform preheating or flash-off in between. Preheat or flash off is 40-80
It can be carried out under heated conditions of 30 seconds to 5 minutes at °C. It should be noted that heating for more than 5 minutes is not preferable because the heat retention equipment becomes longer and therefore the booth itself needs to be longer. Furthermore, if the temperature exceeds 80°C, the surface of the primary intermediate coat will develop too many wrinkles, making pinholes and the like more likely to occur, which is undesirable. Note that a hot air blowing device can be used as the heat insulation equipment, but in consideration of dust prevention, etc., it is preferable to use a radiant method in which hot water is passed through pipes. As the paint for the intermediate coating film, a polyester-melamine resin system that is generally used in the conventional method of forming a laminated coating film can be used, and if necessary, alkyd resin, epoxy resin, etc. can be used in combination. Can be done. For powder intermediate coatings, polyester/epoxy curing type or acrylic/
Acid curing type can be used, and alkyd type
It is also possible to use melamine-type water-based intermediate coats. After the intermediate coat is baked, dust often adheres to it, and if the top coat is applied as is, dust particles will be generated on the top coat surface. In this case, it is preferable to spot-sand only the dusty parts of the intermediate coating surface with sandpaper of about 360 to 800 grit. If the entire surface is polished, as described above, it causes suction of the top coat, which impairs the achievement of the above-mentioned object of the present invention. The greatest effect of the present invention is a significant improvement in the appearance, but another effect is that the number of steps required for sanding can be reduced compared to conventional methods. Conventionally, when sanding the intermediate coat surface, a sandpaper of about 360 to 800 grit was used as an abrasive, but for polishing the undercoat film, a relatively rough sandpaper of about 280 to 600 grit is sufficient. Yes, because wet sanding is no longer an essential condition as in the past, the need to use water-resistant sandpaper for wet sanding is reduced, and the amount used can be significantly reduced.
Additionally, polishing with fine sandpaper required a long time in the past, but this is also a significant improvement. For example, there is a method of applying the top coat twice in a 4-coat, 4-bake method by changing the pigment concentration of the top coat, as is conventionally done on luxury cars such as Mercedes-Benz, but these methods Not only is the cost extremely high in terms of equipment investment and running costs, but also the appearance quality cannot reach the level of that of the present invention. Furthermore, according to the present invention, the important effect of significantly improving the edge rust prevention ability is also achieved, which is achieved by increasing the concentration of the pigment with a large particle size used in the primary intermediate coating. It is expected that the corner coverage will be increased and the edge rust prevention will be improved compared to conventional systems, making it effective for edge rust prevention. As for the topcoat film, for metallic color systems, it is common to apply a metallic base coat and clear coat in a wet-on-wet system, and acrylic/melamine type is the mainstream for both the base and clear, while urethane type (2 It is possible to use liquid) paints, polyester + cellulose acetate butyrate + melamine paints as a metallic base, and acrylic melamine as a top clearer. Most solid colors are one-coat alkyd melamine type, but two-coat acrylic melamine type is the mainstream.
One-coat bake type solid color and two-component urethane type paints can also be used. Examples 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. Examples 11 to 14 A dull steel plate sample with dimensions of 0.8 x 100 x 300 mm is coated with the primer coating listed in Table 1 to a specified thickness (dry thickness), and this is polished under the conditions listed in Table 1. 1 to a predetermined thickness, preheated under predetermined conditions, then apply intermediate coat 2 to a predetermined pressure, then apply the following top coat to a predetermined thickness (dry thickness), bake and harden to form a laminated coating. A film was formed. Next, this coating film was tested and evaluated for its appearance, edge rust, equipment cost, and running cost.
The results were as shown in Table 2.

【table】

【table】

【table】

【table】

[Table] From the following examples, Examples 1 to 7 are examples of the present invention.
has excellent smoothness and gloss, and at the same time has a sense of fleshiness,
While each evaluation item such as edge rust was well balanced, Comparative Examples 8 to 14 did not require polishing or the polishing process was close to the surface layer, so the level of the conventional technology was mainly that the sharpness appeared. It was hot.

Claims (1)

[Claims] 1. A cationic electrodeposition coating is applied to an object to be coated and cured, the cured cationic electrodeposition coating is polished, and then a pigment volume concentration of 10 to 50% and a pigment volume concentration of 5 to 50% are applied.
Two layers of 40% primary and secondary intermediate coats are laminated in sequence, and at least one top coat is applied.
A method for forming a laminated coating film, which comprises curing the intermediate coating film and the top coating film without polishing. 2 The pigment volume concentration of the primary intermediate coating film is 15 to 30%, and the particle size of 90% or more of the primary particles of the pigment is
The method according to claim 1, wherein the thickness is 0.5 to 30 μm.