JPH0474184B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for producing a coated metal sheet with excellent weather resistance and workability, which is suitable for uses such as roofing materials, side wall materials, tank inner wall materials, automobile and bicycle parts, and home appliance parts. .

[Conventional technology]

Conventionally, painted metal sheets used for exterior building materials and interior parts are made of cold-rolled steel sheets, galvanized steel sheets, stainless steel sheets, aluminum sheets, aluminum alloy sheets, etc., as the base metal sheet. Generally used is a metal plate coated with a thermosetting resin paint such as a polyester resin paint, a silicon-modified polyester resin paint, or a silicon-modified acrylic resin paint and then baked. These painted metal plates have good weather resistance, but
They have poor workability, and if these coated metal plates are used in areas that require severe forming processes such as tight bending, cracks may occur in the coating film, which may lead to peeling or damage to the coating film. There was a problem in that the corrosion resistance deteriorated significantly even if it did not peel off.

[Problems to be solved by the invention]

The inventors of the present invention conducted intensive research to eliminate the problems with conventional painted metal plates, and as a result, they used a vinylidene fluoride resin paint that has vinylidene fluoride resin as its main component as a paint that has excellent weather resistance and processability. However, since vinylidene fluoride resin is a crystalline resin, vinylidene fluoride resin is applied to the surface of the metal plate, which has a chemical conversion treatment layer on the surface and an undercoat layer on top of it. It was found that when the paint was applied and baked, the crystallization of the resin progressed excessively during the coating process, making it impossible to obtain the desired properties.

[Means to solve the problem]

Therefore, the present inventors investigated various conditions to prevent excessive crystallization of the resin when applying and baking vinylidene fluoride resin paint, and as a result, the baking temperature was set within a specific range. , and by performing cooling after baking under specific conditions regarding the cooling start temperature, cooling rate, and temperature reached at that cooling rate, the degree of crystallization of the vinylidene fluoride resin paint can be reduced to 18% or less. The present invention was completed by determining that the desired characteristics could be obtained while suppressing the amount of heat. That is, the present invention applies a chemical conversion treatment to the surface of a metal plate, then applies and bakes an undercoat paint, and then further coats the surface with a vinylidene fluoride resin paint containing 70% by weight or more of vinylidene fluoride resin in a non-volatile content. 230 to 270 after application when applying and baking.
Weather resistance and processing characterized by baking at a temperature of 160°C and rapidly cooling at a cooling rate gradient of 160°C/sec or more to a temperature of 70°C or less while the paint temperature is higher than the recrystallization temperature of the resin. The present invention provides a method for manufacturing a coated metal plate with excellent properties. EMBODIMENT OF THE INVENTION Hereinafter, the manufacturing method of the coated metal plate with excellent weather resistance and workability according to the present invention will be explained in detail with reference to the drawings. Figure 1 is a cross-sectional view showing the structure of a coated metal plate with excellent weather resistance and workability manufactured by the method of the present invention, and Figure 2 is a cross-sectional view showing the structure of a coated metal plate with excellent weather resistance and workability. A graph showing an example of recrystallization characteristics, and FIG. 3 is a diagram showing a method of calculating the degree of crystallinity by X-ray diffraction. In the present invention, the metal plate 1 which is the base material is as follows:
Cold-rolled steel sheets, depending on the purpose of the painted metal sheets being manufactured.
Galvanized steel sheets, stainless steel sheets, aluminum sheets, aluminum alloy sheets, etc. are used. The chemical conversion treatment applied to the surface of the metal plate 1 includes an undercoat coating layer 3 on which the resulting chemical conversion treatment layer 2 is formed.
Phosphate treatment or chromate treatment similar to the chemical conversion treatment applied to ordinary coated metal plates is not particularly limited as long as it has good adhesion to the metal plate and improves the corrosion resistance of the metal plate 1. is preferred. As an undercoat paint to be applied and baked on the chemical conversion treatment layer 2, the obtained undercoat film layer 3 has good adhesion to the lower chemical conversion treatment layer 2 and the vinylidene fluoride resin top coat layer 4 above it. Epoxy resin paints are preferred. Apply on top of the undercoat film layer 3.
As the vinylidene fluoride resin top coat material to be baked, use one in which the amount of vinylidene fluoride resin is 70% by weight or more in terms of non-volatile content (the same applies to the resulting vinylidene fluoride resin top coat layer 4). . The reason for this is that if the amount of vinylidene fluoride resin is less than 70% by weight, the excellent weather resistance and processability properties of vinylidene fluoride resin will be impaired by additives such as resin. In the method of the present invention, first, a chemical conversion treatment is performed on the surface of a metal plate 1 to form a chemical conversion treatment layer 2;
After coating and baking an undercoat paint to form an undercoat film layer 3 with a thickness of preferably 4 to 6 μm, a non-volatile amount of vinylidene fluoride resin of 70% by weight is applied on the undercoat film layer 3. % or more of vinylidene fluoride resin paint is applied to obtain a film thickness of preferably 18 to 22 μm, then baked at a temperature of 230 to 270°C, and cooled while the paint temperature is higher than the recrystallization temperature of the resin. It is sufficient to rapidly cool the sample at a rate gradient of 160°C/sec or more until the temperature reaches 70°C or less. This is because the baking temperature of vinylidene fluoride resin paint is
If the temperature is lower than 230°C, the vinylidene fluoride resin will not be melted sufficiently, resulting in pinholes, which will reduce weather resistance and workability.If the baking temperature exceeds 270°C, This is because the resin deteriorates and the strength of the coating film decreases. In addition, after applying and baking vinylidene fluoride resin paint, the cooling rate gradient is increased to 70°C at a rate of 160°C/sec or more while the paint temperature is higher than the recrystallization temperature of the resin.
The reason for rapid cooling to the temperature below is that the vinylidene fluoride resin will not recrystallize if the cooling rate gradient of the resin is less than 160°C/second or if the resin quenching start point is lower than the recrystallization temperature of the resin. This is because the process progresses significantly and spherulites are formed, resulting in a decrease in elongation properties and impact strength, making it impossible to obtain a top coat layer with excellent weather resistance and processability, which are the objectives of the present invention. The reason why the resin temperature is rapidly cooled to below 70℃ is that if the temperature at the end of rapid cooling is 70℃ or higher, molecular movement of crystal defects in the vinylidene fluoride resin paint is possible, so crystallization occurs from molecular rearrangement. As a result, the degree of crystallinity increases and the elongation properties and impact strength of the vinylidene fluoride resin paint decrease, resulting in a top coat layer having excellent weather resistance and processability, which is the objective of the present invention. This is because it will not be possible to obtain The crystallinity of the vinylidene fluoride resin top coat layer 4 obtained in this manner is suppressed to a low level of 18% or less. As shown in the examples below, when the degree of crystallinity exceeds 18%, not only the weather resistance as a top coat layer but also the processability such as elongation properties and impact strength are rapidly reduced. The coated metal plate obtained by the method of the present invention has layers on the surface of the metal plate 1 in the order from the metal plate 1 side: a chemical conversion layer 2, an undercoat layer 3, and a vinylidene fluoride resin-based top coat layer 4. However, the difference from conventional coated metal sheets is that the top layer, vinylidene fluoride resin top coat layer 4, contains vinylidene fluoride resin in an amount of 70% by weight or more. The degree of crystallinity is kept to a low level of 18% or less. As described above, in the method of the present invention, it is important to be able to suppress the crystallinity of the vinylidene fluoride resin top coat, which is the top coat layer, to a low level of 18% or less. To measure this degree of crystallinity, for example, as shown in Figure 3, an X-ray diffraction intensity test is performed on the vinylidene fluoride resin topcoat layer, and the diffraction area Xc of the crystalline part and the diffraction area Xa of the amorphous part are calculated. It can be calculated using the following formula: [Xc/(Xa+Xc)]×100(%). This X-ray diffraction intensity test method uses, for example, a cupper as a target, a concentrated X-ray beam with a tube voltage of 40 Kv and a tube current of 80 mA, a nickel filter, and a divergent slit. A 0.3 mm one was used as a receiving slit to receive the diffracted X-rays.
Further, as a measurement method, the diffraction intensity may be measured using a scintillation counter under measurement conditions where the time constant is 1 second and the scanning speed of the goniometer is 1 degree/minute.

【Example】

Examples of the method of the present invention will be described below. Austenitic stainless steel SUS304 and ferritic stainless steel (19Cr/0.5Cu/
A pre-degreased steel plate with a 2D surface finish and a plate thickness of 0.3 mm was treated with a zinc phosphate treatment agent (manufactured by Nippon Paint Co., Ltd., product name: 0.5Nb; manufactured by Nisshin Steel Co., Ltd., product name: NSS442M3). Pre-painting treatment with Granogin #9100)
Epoxy resin paint (manufactured by Nippon Paint Co., Ltd., product name: UNIFLON K Kai) as an undercoat on top of that.
The film was coated using a roll coating method to a dry film thickness of 5 μm, and the maximum plate temperature reached: 200℃ in a conveyor oven.
Baking was performed for 40 seconds to form an undercoat film layer on each steel plate. Then, as a top coat, a vinylidene fluoride resin paint (manufactured by Nippon Paint Co., Ltd., trade name: UNIFLON K) with recrystallization properties shown in Figure 2 is applied as a top coat on top of this coating layer to a dry coating thickness. It was coated to a thickness of 20 μm using a roll coating method, baked again in a conveyor oven, and then rapidly cooled with a large amount of water to form a top coat layer. The combinations of baking conditions and cooling conditions are shown in Table 1. On the other hand, as comparative examples (1' to 4'), the same top coat as above was applied to the same SUS304 stainless steel plate on which the undercoat layer had been formed until the dry film thickness was reached.
Coating was performed to a thickness of 20 μm using a roll coating method, and a top coat layer was formed using the combinations of baking conditions and cooling conditions shown in Table 1. In addition, as a comparative example (5'), an epoxy resin-based paint (manufactured by Nippon Paint Co., Ltd., product name: DIF P) was applied to the same SUS304 stainless steel plate as above that had been subjected to chemical conversion treatment as a paint for forming an undercoat layer. 155), and silicone-modified polyester resin paint (manufactured by Nippon Paint Co., Ltd.,
Using DIF S-30 (trade name), an undercoat film layer and a topcoat film layer were formed, respectively, according to the baking conditions shown in Table 1. Thereafter, a performance test of the obtained coated steel plate was conducted in the following manner. (1) Workability test (a) Bending test Based on JIS G3320 "Painted stainless steel plate",
After bending 180° with zero (0t: close bending), one (1t), and two (2t) scissor plates with the same thickness as the test plate, observe the corner part with a microscope (x40). and evaluated based on the following criteria. ◎ mark: No abnormality ○ mark: Cracks of 10% or less occur ● mark: Cracks of more than 10% and less than 30% occur △ marks: Cracks of more than 30% and less than 70% occur × marks: More than 70% Cracks occur (b) Impact test A Gardner impact test (1.82Kg x 1m condition) was carried out in accordance with ASTM D3281 "Method for testing the processability of organic coatings using an impact tester", and the corner part with a bending radius of 2mmR was examined using a microscope. (×40) and evaluated using the same criteria as above. (2) Adhesion test Samples under normal conditions and after immersion in boiling water for 2 hours were tested in accordance with JIS G3320 "Painted stainless steel plates".
The inner spacing of the bending was determined using zero (0t: close bending), two (2t), and four (4t) scissor plates of the same thickness as the test plate.
After bending by 180°, cellophane tape was applied to the corner, forcibly peeled off, and evaluated according to the following criteria. ◎ mark: No abnormality at all 〇 mark: Very slightly peeled paint film ● mark: Slightly peeled paint △ mark: Significantly peeled paint × mark: Significantly peeled paint film (3) Weather resistance test: 5000
A time irradiation test was conducted. The 60 degree specular gloss of the specimen after the test was measured using a gloss meter manufactured by Murakami Color Research Institute Co., Ltd., and the gloss retention rate relative to the initial gloss was determined and evaluated. Tables 2, 3, and 4 show the results of the performance evaluation test.

【table】

【table】

【table】

[Table] In the workability test shown in Table 2, (No. 1 to 6) according to the method of the present invention showed no abnormality in the coating film at the corner part even in the bending test (0t, 1t, 2t) and impact test. Shows good performance. On the other hand, in the comparative examples (No. 1' to 4'), cracks occurred at the corners at 0t and 1t of the bending test, and in the comparative example (No. 5'), cracks also occurred at the 2t section. In the impact test, cracks occurred in the corners of the comparative examples (Nos. 1' to 5') in a range of 10 to 70%. In addition, as an example of actual molding, a simulated test using a 16-roll molding machine was conducted. In the Examples (Nos. 1 to 6) using the method of the present invention, no cracks were observed even in the closely bent parts, but in all of the comparative examples (Nos. 1' to 5'), cracks occurred in the closely bent parts. did. Cracks that occur during the molding process become a factor in the peeling of the coating film due to the intrusion of rainwater in the actual usage environment. In the coating film adhesion test shown in Table 3, the samples (Nos. 1 to 6) according to the method of the present invention were good, with no coating peeling in both 0t, 1t, and 2t under normal conditions and after immersion in boiling water for 2 hours. On the other hand, in Comparative Examples (Nos. 1' to 5'), peeling of the coating was observed after immersion in boiling water for 2 hours, and it is considered that the adhesion tends to deteriorate over time. In the weather resistance test shown in Table 3, the method of the present invention (No. 1 to
The gloss retention rate of 6) was 96 to 100%, and almost the initial gloss was retained even after 5000 hours, indicating good performance. On the other hand, in the comparative example (No. 5′), the gloss retention rate was
It has fallen to 62%. This is because the paint film deteriorates and
This is because yoking occurs.

【Effect of the invention】

As detailed above, in order to keep the crystallinity of vinylidene fluoride resin paints low, the method of the present invention involves setting the baking temperature within a specific range, and controlling the cooling start temperature, cooling rate, and reaching temperature after baking. By performing rapid cooling under specific temperature conditions, the resulting coated metal sheet can be used in harsh forming processes such as automobile parts and home appliance parts without cracking or peeling of the coating, making it more durable than conventional materials. In comparison, the range of applications is extremely wide. Moreover, even when used outdoors, it has extremely good weather resistance and hardly deteriorates over time, so it can be used without requiring maintenance, so it has great industrial value.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing the structure of a coated metal plate with excellent weather resistance and workability manufactured by the method of the present invention, and Figure 2 is a cross-sectional view showing the structure of a coated metal plate with excellent weather resistance and workability. A graph showing an example of recrystallization characteristics, and FIG. 3 is a diagram showing a method of calculating the degree of crystallinity by X-ray diffraction. In the drawings, 1... Metal plate, 2... Chemical conversion treatment layer, 3
...undercoat film layer, 4...vinylidene fluoride resin-based topcoat film layer.

Claims (1)

[Claims] 1 After applying chemical conversion treatment to the surface of the metal plate, and then applying and baking an undercoat paint, further applying and baking a vinylidene fluoride resin-based paint containing 70% by weight or more of vinylidene fluoride resin in non-volatile content. To do this, bake at a temperature of 230 to 270℃ after application, and while the paint temperature is higher than the recrystallization temperature of the resin, rapidly cool the coating at a cooling rate gradient of 160℃/second or more until the temperature reaches 70℃ or less. A method for producing painted metal sheets with excellent weather resistance and workability.