JPH0448347B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a triple-coated precoated metal plate that is mainly used as a building material and has particularly excellent weather resistance, workability, and corrosion resistance. Pre-coated metal sheets are metal substrates coated with paint in advance, and are usually mass-produced continuously by painting on cold-rolled steel sheets, galvanized steel sheets, or aluminized steel sheets using a roll coater. Because it is of excellent quality, uniform, suitable for mass applications, and does not require painting after construction, demand for it is increasing as a material for building materials, home appliances, office equipment, etc. The main performance requirements for these pre-coated metal plates include weather resistance, corrosion resistance, adhesion, workability, and scratch resistance, but it is impossible to satisfy all of these quality performances with one type of paint. , is technically very difficult. For this reason, conventionally, paints were divided into two types, with the undercoat responsible for adhesion and corrosion resistance, and the topcoat responsible for weather resistance, workability, scratch resistance, etc., thereby further improving the quality and performance of each paint. It has been attempted to improve the quality and performance of pre-coated steel sheets by applying these paints in a so-called two-coat, two-bake method, in which an undercoat is painted and baked, and then a top coat is painted and baked. However, the top coat of a two-coat product still requires many conflicting performances at the same time. for example,
If the coating film is hardened for the purpose of improving scratch resistance, processability deteriorates. Even with polyester paint, which is commonly used as a top coat for pre-coated metal sheets, if the skeleton resin is selected with the aim of improving weather resistance and scratch resistance, the processability deteriorates and it becomes unable to withstand normal bending processing. Conversely, if the skeleton resin is selected to improve processability, performance such as weather resistance and scratch resistance will deteriorate. For this reason, conventional top coat paints for two-coat products have been improved by mixing various resins to balance performance. However, conventionally, the two-coat top coat must contain coloring pigments, etc. for coloring, and when processed, the addition of pigments reduces the elongation rate of the coating, resulting in cracks in the processed area, causing cracks in the processed areas. This had the disadvantage that more water penetrated and the corrosion resistance of the pre-coated metal sheet was impaired. The inventor of the present invention particularly aimed to prevent corrosion occurring from processed parts by forming a top coat that does not contain pigment and has a higher elongation rate than the intermediate coat on top of the conventional two-coat film. As a result of repeated research, we completed an invention related to a pre-coated metal plate coated with an undercoat film, an intermediate coat film, and a top coat film, in which the top coat film has a higher elongation rate than the intermediate coat film. This is what led to this. In the present invention, first, an undercoat is applied and baked onto a metal substrate. As the metal substrate, a steel plate, a galvanized steel plate, a zinc alloy plated steel plate, a lead plated steel plate, a lead alloy plated steel plate, an aluminum plated steel plate, an aluminum alloy plated steel plate, a stainless steel plate, an aluminum plate, etc. are used. Furthermore, on top of this
It also includes those having a chemical conversion treatment layer of about 0.1 to 5μ. Chemical conversion treatment is performed as a surface treatment for metal plates in order to improve the corrosion resistance of the metal substrate and the adhesion with paint. For example, zinc phosphate treatment,
This is done by iron phosphate treatment or electrolytic chromate treatment. As the undercoat paint, a paint commonly used for pre-coated metal plates can be used. for example,
Paints prepared by blending coloring pigments, anti-corrosion pigments, extender pigments, etc. with a resin solution whose main component is resin such as epoxy resin, oil-free polyester, acrylic resin, or urethane resin are used. It is preferable to use an undercoat paint whose main component is an epoxy resin with good corrosion resistance. After the base coat film is formed, an intermediate coat paint is applied and baked thereon to form an intermediate coat film. As intermediate coating paint, acrylic resin, polyester resin,
Although silicone polyester resins and the like are used, acrylic-modified vinylidene fluoride resins are preferred from the viewpoint of weather resistance. Commercially available acrylic modified vinylidene fluoride resins are available. Generally, a material containing about 70% by weight of vinylidene fluoride resin as a fluorine content is used, and in order to further improve weather resistance, this proportion may be increased. For paints using acrylic-modified vinylidene fluoride resin, if the dry coating film is 7μ, approximately 95% of UV rays will be blocked when UV transmittance is measured in the 300 nm range, which is said to have a high energy level. . And the annual decrease in paint film due to outdoor exposure is
It has a thickness of about 0.2 to 0.3μ, indicating excellent weather resistance. After the intermediate coat is formed, a top coat is applied and baked on top. By selecting the top coat in such a way that the elongation rate of the top coat is higher than that of the intermediate coat, it not only greatly improves workability but also reduces distortion of the paint film during processing. , weather resistance is also improved. Furthermore, we have found that by selecting the top coat to have an elongation rate that can withstand OT bending, corrosion resistance, especially in the processed areas, can be improved more than expected. However, the elongation rate that can withstand OT (Zelty) bending is defined as the elongation rate when bent according to the JIS G3312 (colored galvanized iron plate) bending test method, where the thickness of the base steel plate is T and the inner diameter of the bend is OT. , refers to the elongation rate of a paint film that shows no cracks when the surface of the paint film is observed with a magnifying glass of 10x magnification. In addition, considering weather resistance, workability, and scratch resistance during construction, JIS G3311 (colored galvanized iron plate)
It is preferable to use a paint whose coating film has a hardness of at least F on a pencil hardness scale according to the method specified in (1). As such a top coating, a coating that does not contain any pigment and has an acrylic modified vinylidene fluoride resin as its main component is preferable. The coating film elongation rate of the coating material is improved compared to the case where the top coating material contains a pigment. or,
As mentioned above, it also has excellent weather resistance. When an acrylic modified vinylidene fluoride resin is used for the top coat, it is preferable to further add about 0.1 to 5% by weight of an ultraviolet absorber to the top coat, since deterioration of the intermediate coat due to ultraviolet rays can be prevented. As the ultraviolet absorber, any of the commercially available UV absorbers, such as salicylic acid, benzophenone, benzolitoazole, and cyanoacrylate, can be used. In this case, the acrylic-modified vinylidene fluoride resin used for the intermediate coat can reduce the expensive fluorine content compared to the resin used for the top coat, and also allows the use of inexpensive silicone polyester resin for the intermediate coat. . In addition to the main resin component, which is the coating film forming element, the paints used in the present invention include solvents, pigments, curing agents,
It is adjusted by adding the required amount of additives commonly used in paints. Next, the three-coat method of the present invention has the advantage that the top coat film and intermediate coat film can each be thinner than the conventional two-coat top coat film. For example, the dry film thickness of the top coat is 3 to 20μ, preferably 5 to 20μ.
15μ, and the intermediate coating film has a thickness of 15 to 30μ, preferably 18 to 25μ. In both cases, the lower limit is a necessary value from the viewpoint of processability and ultraviolet transmission, and the upper limit is a necessary value from the viewpoint of coating economy. When the film thicknesses of both coating films are determined in this way, it is possible to obtain a precoated metal plate that is rich in color tone and has excellent weather resistance and corrosion resistance in processed parts. The present invention will be explained in more detail with reference to Examples below. Example An epoxy resin paint containing a rust preventive pigment was applied onto a galvanized steel plate treated with zinc phosphate and having a thickness of 0.35 mm, and baked at 180°C for 50 seconds to obtain an undercoat film with a dry film thickness of 5 μm. Then, an intermediate coat film was applied. As intermediate paints, acrylic modified vinylidene fluoride resin paints and silicone polyester resin paints with different elongation rates were used. The former was baked at 240°C for 65 seconds to give a film thickness of 20μ. The latter was baked at 210°C for 65 seconds to give a film thickness of 20μ. In both cases, colored pigments are used as pigments in paints.
% by weight. Then, as a top coat, an acrylic modified vinylidene fluoride resin paint containing 74% vinylidene fluoride resin was baked at 240°C for 65 seconds to give a film thickness as shown in Table 1. As a general rule, the top coating was done with clear, but as an example for comparison, a case where 5% by weight of colored pigment was contained in the coating material was shown (Experiment No. 6). Moreover, the case where an ultraviolet absorber is contained is shown. Also, as an example for comparison, the undercoat is an epoxy resin paint of 5μ, and the intermediate coat is an acrylic modified vinylidene fluoride resin (74% vinylidene fluoride resin content) paint.
A commercially available two-coat product coated with 30μ was used as a test plate (Experiment Nos. 9 and 12). Performance tests were conducted on the above test plates using the test method described later, and the results are shown in Table 1. Note that the fluorine content is a value converted to vinylidene fluoride resin. Furthermore, the film thicknesses of the top coat and intermediate coat are dry film thicknesses.
Also, the percentages in the table are percentages by weight. Moreover, the performance test was conducted as follows. (1) Corrosion resistance: After 3000 hours of salt spray test (JIS Z2371), we observed the occurrence of white rust on the OT bent part. After 3000 hours, the blister width on the cross-cut part (the scratches that reach the substrate with a cutter are marked with an x) and the white rust occurrence on the Erichsen part (extruded 6 mm onto the paint film surface using an Erichsen tester) observed. White rust is judged using a 5-point system, where 5 means no white rust and 1 means significant white rust. (2) Workability: Compliant with impact deformation test and OT bending test (JIS G3312). Judgment is 5 on the 5 point system
1 means no cracks, 1 means severe peeling. The denominator value is determined after the paint film is peeled off with tape. (3) Paint film hardness: Indicated by the highest pencil hardness that does not cause scratches on the paint film using a method based on JIS G3312. (4) Scratch resistance: Visual judgment was made by scratching the coating film with a 10 yen coin. ◎ mark is the best, ○ mark is average.

【table】

【table】

[Table] *marks are examples for comparison.
(5) Weather resistance: Ducycle Weather Meter 2000
Color difference (△E) and gloss retention rate of painted film surface after time
Indicated by (GR).

Claims (1)

[Scope of Claims] 1. The undercoat film is formed from a paint whose main component is an epoxy resin, and the intermediate coat film has a fluorine content of 50 to 75%.
The main component is acrylic-modified vinylidene fluoride resin or silicone polyester resin with a silicon content of 25 to 35%, and is formed by a paint containing color pigments, and the top coat film has a higher paint elongation rate than the intermediate coat film. A pre-coated metal plate made of a paint containing no pigment and containing an acrylic modified vinylidene fluoride resin with a high fluorine content of 70 to 75%. 2. The top coat is formed from a paint whose main component is an acrylic modified vinylidene fluoride resin with a fluorine content of 70 to 75% or more, and which does not contain pigments and has an ultraviolet absorber added from 0.1 wt% to 5 wt%. A precoated metal plate according to claim 1.