JPS6124175B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a pre-coated steel sheet with an organic film formed on only one side, and more specifically, a single-sided pre-coated steel sheet with an organic film that can be baked at a low temperature on only one side so as not to deteriorate the chemical conversion properties of the non-painted surface and the adhesion of the paint film. It concerns steel plates. In recent years, surface-treated steel sheets such as metal-plated steel sheets and painted steel sheets have come to be used for parts that are prone to corrosion in order to cope with the harsher usage environments (for example, driving on salt-sprayed roads) for automotive steel sheets. . Surface-treated steel sheets used for automobiles require welding and forming performance in addition to corrosion resistance, and are usually subjected to chemical conversion treatment, electrodeposition coating, and electrostatic coating after forming and assembly. It is important that the product can be successfully treated in the rust prevention process, and that the exterior surface should be aesthetically pleasing, such as smoothness, gloss, and sharpness. On the other hand, with regard to corrosion, it is known from experience that corrosion occurs most severely in the crevices inside the structure where it is difficult for the paint to spread during painting after molding. For this reason, galvanized steel sheets and organic coated steel sheets have recently come to be used for lower parts of car bodies that are likely to come into contact with corrosive fluids while driving. However, for areas where one side is the outermost surface and aesthetic appearance is required,
Steel plates with surface treatment applied to only one side are often used. This is a treatment to prevent differences in appearance (color tone, gloss, etc.) after painting between the surface-treated surface and the surface using ordinary steel sheets. Further, a steel plate that has been surface-treated on both sides is disadvantageous in terms of welding and formability than a steel plate that has been surface-treated on only one side.
Particularly in the case of organic coated steel sheets, it is not preferable to apply coating to both sides, as this significantly reduces weldability and formability. Therefore, when organic coated steel sheets are applied to automobile parts that are subject to particularly poor corrosive environments, a method is adopted in which only the inner surfaces are coated, and the outer surfaces, where appearance is important, are left unpainted. There is. However, when paint applied to one side is passed through a baking oven, the non-painted side (steel side) becomes oxidized.
This poses a problem as the adhesion of paint films applied during the automobile production process may deteriorate. In order to solve the problem of poor adhesion on non-painted surfaces, the present inventors identified the limit temperature at which the surface of the steel plate is not affected by heating, and developed a paint that can provide sufficient coating performance under baking conditions below that temperature. By separately developing and applying this, we were able to invent a new single-sided coated steel sheet suitable for subsequent chemical conversion treatment and painting processes. That is, the present invention
Hydroxy content 2-5 to 100 parts by weight of linear polyether polyol with hydroxy content 6-9%
Polyol resin A containing 30 to 100 parts by weight of epichlorohydrin-bisphenol A type epoxy resin and both A and B of diphenylmethane diisocyanate prepolymer B having an isocyanate content of 20 to 30% in NCO/OH equivalent ratio. This is a single-sided precoated steel sheet having a coating layer on only one side with a thickness of 3 μm to 50 μm, which is obtained by curing a paint compounded at a ratio of 1/1.2 to 1.5 at 150° C. or less within 5 minutes. The contents will be explained in detail below. The present inventors first conducted the following experiment to determine under what heating conditions the coating adhesion of a conventional cold-rolled steel sheet deteriorates. That is, after heating a cold-rolled steel sheet in an air atmosphere, it was treated with zinc phosphate, and the crystal adhesion state and secondary adhesion of the cationic coating were investigated. It was confirmed that it deteriorates when held for more than 5 minutes.

[Table] Furthermore, even with steel plates heated at temperatures exceeding 150℃,
It has also been found that the secondary adhesion of the paint film can be restored by polishing with sandpaper, zinc phosphate treatment, and cationic electrodeposition painting. This is considered to be because the oxide film formed by heating deteriorated the chemical conversion treatment, resulting in deterioration of the secondary adhesion of the coating film. On the other hand, 150
It was found that chemical conversion treatment properties did not deteriorate even when heated for more than 5 minutes at temperatures below .degree. C., and the secondary adhesion of the coating film was also good. In other words, if you apply a paint that can provide sufficient film performance under baking conditions of 150°C or less for 5 minutes on one side and bake harden it, the chemical conversion properties of the non-painted surface (steel surface) on the other side will be improved. Satisfactory coating film adhesion can be obtained without deterioration. As such a paint, the present inventors used polyol resin A, which is a blend of polyether polyol and epichlorohydrin-bisphenol A type epoxy resin, which were separately developed as described above, and diphenylmethane diisocyanate-based prepolymer B, A and B. There are paints that are a mixture of both, and pre-coated steel sheets having a coating film made by curing such paints are extremely practical as pre-coated steel sheets for automobiles, especially for exterior panels. In addition, when used in some home appliances whose inner surfaces are prone to corrosion, such as refrigerators and washing machines, the outer surface exhibits paintability and appearance exactly the same as ordinary steel sheets, and has excellent weldability and
It has significantly better formability than double-sided coated steel sheets, and has many advantages. The reason why the ratio of polyether polyol and epichlorohydrin-bisphenol A type epoxy resin in polyol resin A of the low-temperature baking paint used in the present invention is 100 parts by weight/30 to 100 parts by weight is that the epoxy resin is 30 parts by weight. If the amount is less than 100 parts by weight, the corrosion resistance of the coating film will be insufficient, whereas if it exceeds 100 parts by weight, the elongation of the coating film will be poor and workability will be reduced. In addition, the reason why the hydroxyl content in the polyether polyol is set to 6 to 9% is because if it is less than 6%, the low temperature curability will decrease and the coating will become too soft.
On the other hand, if it exceeds 9%, the coating film becomes hard and brittle, resulting in poor workability. In addition, the hydroxy content in the epichlorohydrin-bisphenol A epoxy resin is set at 2 to 5%.If it is less than 2%, the low-temperature curing properties will decrease, and if it exceeds 5%, the coating will elongate. This is because it becomes defective and reduces workability. On the other hand, the isocyanate content of diphenylmethane diisocyanate prepolymer B is 20 to 30%.
The reason for this is that if it is less than 20%, low temperature curing properties will decrease.
On the other hand, if it exceeds 30%, the elongation of the coating film will decrease and the processability will decrease. Furthermore, the ratio of A and B is 1/1.2 in terms of NCO/OH equivalent ratio.
The reason for setting it to 1.5 is that if it is less than 1.2, the adhesion of the coating film will decrease, and if it exceeds 1.5, the corrosion resistance of the coating film will be insufficient. Further, the thickness of the coating film formed on one side must be 3μ or more from the viewpoint of corrosion resistance, and if it is less than that, the effect will be small. On the other hand, as the coating thickness increases, workability decreases.
If it exceeds 50μ, cracks will occur in the bent portion, which is not preferable. As the steel plate used in the present invention, in addition to a cold-rolled steel plate, a single-sided plated steel plate whose non-painted surface is a steel surface can be used. In addition, the paint is fine as long as it satisfies the baking hardening conditions of 150°C or less and within 5 minutes, but most of the paints currently used for pre-coated steel sheets are high-temperature baking type to enable high-speed coil coating. Many of them are, for example, 170℃~250℃
Most temperatures are around ℃. In the present invention, the paint used can be baked at 150°C for 5 minutes to obtain sufficient corrosion resistance and film adhesion, and the coating method can be roll coating, spray coating, or curtain flow coating. No problem. In this case, if the baking temperature is further increased, the adhesion of the precoat film, especially the secondary adhesion, will be improved. However, baking at temperatures above 150°C will reduce the chemical conversion properties of the non-painted surface as mentioned above, and will reduce the adhesion of the coating when applied subsequently, so the baking temperature of the pre-coated film must be kept below 150°C. It won't happen. In addition, in order to improve weldability and improve the chemical conversion treatment properties of the painted surface, metal powders such as zinc, aluminum, iron, and stainless steel are added to the coating film at a rate of 60% to 60%.
It is possible to add about 90% by weight, and it does not impair the corrosion resistance of the painted surface or the adhesion of the paint film. The effects of the present invention will be concretely illustrated in Examples below. Example 1 15 coats of paint having the composition shown in Table 2 were applied to one side of a cold-rolled steel plate.
μm coating, 130℃, 140℃, 150℃, 160℃, 170
The coating was cured by baking at ℃ for 3 minutes, and the primary adhesion and secondary adhesion of the coating film were examined. In addition, the unpainted surface was treated with zinc phosphate and cationic epoxy electrodeposition coating, and the primary adhesion and secondary adhesion were examined, and the cold rolled steel sheet was coated with zinc dust-added epoxy paint (baked at 180°C for 1 minute). The adhesion of the cationic electrodeposition coating was compared with that of the iron surface. The results are shown in Table 3.

【table】

【table】

[Table] As a result, it was found that both the primary adhesion and secondary adhesion of the pre-coated film of the steel sheet of the present invention were good when the baking temperature was 160°C or lower. On the other hand, the primary and secondary adhesion of the cationic electrodeposition coating on the non-painted surface (iron surface) is that of the primary and secondary adhesion of the cationic electrodeposition coating on the iron surface of the steel plate coated with the conventional high temperature baking paint (270℃).
Compared to secondary adhesion, it was found to be superior under baking conditions of 150°C or lower. Therefore, in order to ensure the performance of both the pre-coated surface and the steel surface,
The temperature must be below 150℃. Example 2 A coating composition shown in Table 2 in which zinc powder was added to the cured coating film at a concentration of 80% by weight was applied to one side of a cold-rolled steel plate and baked at 150°C for 3 minutes. A notch similar to that in Example 1 was made in a sample treated with dip-type zinc phosphate and coated with 20 μm of epoxy cationic electrodeposition paint on both sides, and the state of cheap peeling after two weeks of salt water spraying and the maximum corrosion depth of the notch. Table 4 shows the results of comparing the performance with a conventional product (one side coated with epoxy paint containing zinc dust).

【table】

[Table] The results show that the painted surface of the steel plate of the present invention exhibits almost the same corrosion resistance and adhesion as the conventional product, and the coating adhesion of the non-painted surface is superior to that of the conventional product. Example 3 The coating film thickness after curing of the paint shown in Table 2 is 3 to 60μ.
Apply it to a 0.8mm cold-rolled steel plate at 150℃,
Table 5 shows the results of evaluating corrosion resistance and workability after baking for 3 minutes.

【table】

【table】

Claims (1)

[Claims] 1. Polyol resin composition A in which 30 to 100 parts by weight of epichlorohydrin-bisphenol A type epoxy resin having a hydroxyl content of 2 to 5% is blended to 100 parts by weight of a linear polyether polyol having a hydroxyl content of 6 to 9%. Isocyanate content 20~
The NCO/OH equivalent ratio of both A and B of 30% diphenylmethane diisocyanate prepolymer B is 1/1.
Paint mixed with a ratio of 1.2 to 1.5 at 150℃ or less,
A coating layer that hardens within 5 minutes is applied to one side only.
A single-sided pre-coated steel sheet characterized by having a thickness of μ~50μ.