JPH05317806A - Highly corrosion resistant coated steel sheet favorable for food sanitation - Google Patents

Abstract

PURPOSE:To obtain a coated steel sheet excellent in food sanitation and corrosion resistance in such a use as the inner box of a microwave oven brought into indirect contact with food. CONSTITUTION:A steel sheet plated with Zn or Zn alloy on both sides and subjected to chemical treatment is used as a base material. A film of a coating material contg. 100 pts.wt. resin blended with 5-100 pts.wt. phosphate pigment as a rust preventive pigment and 1-60 pts.wt. wet process silica by total of both <=120 pts.wt. and is formed as an undercoat layer on the guarantee side (front side) of the steel sheet brought into indirect contact with food and a film of a coating material contg. 100 pts.wt. resin blended with 5-120 pts.wt. chromate as a rust preventive pigment is formed as an undercoat layer on the non-guarantee side (rear side). The objective highly corrosion resistant coated steel sheet favorable for food sanitation is provided.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is a double-sided Z processed by chemical conversion treatment.
For the undercoat of the n-plated steel sheet that comes into indirect contact with food (guarantee side), a phosphate-based rust-preventive pigment-containing coating is used, and on the opposite side (non-guaranteed side) of the coating layer The present invention relates to a highly corrosion-resistant coated steel sheet having a coating film containing a chrome-based pigment, which is preferable in food hygiene and has excellent corrosion resistance on the end faces.

[0002]

2. Description of the Related Art Conventionally, chromium-based pigments (for example, strontium chromate, zinc chromate, etc.) have been widely used as anticorrosion pigments in undercoat paints in precoat metal based on Zn-plated steel sheet, mainly from the viewpoint of corrosion resistance. (For example, JP-A-58-177475).
issue). Further, as a means for further improving the corrosion resistance, a similar undercoat coating film layer is provided on the back surface to deal with the problem.

Further, from the viewpoint of food hygiene, a coating film containing zinc phosphate and silica is provided on one side or both sides as in the case of the chromium-based pigment to deal with the problem (Japanese Patent Application No. 4-1480).
No. 1).

[0004]

However, these methods, that is, the method of using a paint containing a chromium-based pigment represented by strontium chromate and zinc chromate as the undercoat of the front and back of the pre-coated metal has sufficient corrosion resistance. However, the elution pigment from the end face, particularly the elution pigment from the primer on the guarantee surface side may wrap around to the guarantee surface, and the chromium-based pigment may be deposited on the guarantee surface side depending on the dry environment.

Therefore, there is a problem in hygiene when such a pre-coated metal is applied to an indirect contact with food such as a microwave oven inner box. Further, the method of providing a coating film containing zinc phosphate and silica on one side or both sides has no problem in safety and hygiene, but the corrosion resistance is slightly inferior. It is an object of the present invention to provide a pre-coated metal that is excellent in both food hygiene and corrosion resistance in applications where it comes into indirect contact with food, such as the microwave oven inner box.

[0006]

DISCLOSURE OF THE INVENTION The present invention is a coated steel sheet having a double-sided zinc-based plated steel sheet subjected to chemical conversion treatment as a base material, which is an undercoat coating on the guarantee surface (surface) side that indirectly contacts food. The film layer contains 5 to 100 parts of a phosphate pigment and wet silica as a rust preventive pigment with respect to 100 parts by weight of the resin.
It is a coating film of a coating composition in which the phosphate pigment and the wet silica are added in a total amount of 120 parts by weight or less in a proportion of -60 parts by weight, and the non-guaranteed surface (back surface) side undercoating film layer is 100 parts by weight of resin. On the other hand, the present invention provides a highly corrosion-resistant coated steel sheet which is a coating film in which a chromate is mixed in an amount of 5 to 120 parts by weight as a rust preventive pigment in terms of food hygiene. This steel sheet has a top coat layer on at least the guarantee surface side.

The present invention will be described in more detail below.

The coated steel sheet of the present invention is a double-sided Zn-plated steel sheet that has been subjected to a chemical conversion treatment as a base material, and a phosphate-based rust-preventive pigment is used as an undercoat on the side that indirectly contacts food when used as a member. A coating film containing a chromium-based rust-preventive pigment-containing coating film on the opposite side of the coating film,
The present invention was made while observing the flow of ventilation when the above-mentioned coated steel sheet was used as a box in a microwave oven.

That is, dozens of holes are punched in the coated steel sheet used as the inner box member of the microwave oven by punching from the inner side (the side indirectly contacting the food) to the outer side. When punching, the burrs are exposed to the outside, and the undercoat coating on the inside is sagged on the end face of the material. On the other hand, the outer undercoating film does not drip at the end surface of the material.

The inner box of the microwave oven has an air supply port and an air exhaust port, and the flow of ventilation air takes in outside air from the air supply port and exhausts air containing a large amount of moisture from food heated by an electron beam. Exhaust through the mouth. Therefore, the elution of the rust preventive pigment occurs and the corrosion resistance becomes a problem on the exhaust port side.
On the exhaust port side, the direction of air flow and the direction of burrs due to punching coincide with each other, and as described above, air containing a large amount of water mainly elutes the rust preventive pigment in the inner undercoating film. On the other hand, the anticorrosion pigment in the outer undercoat coat elutes slightly while assisting its ability on the inner side, but it does not come into contact with food when the flow of exhaust gas is added, and from the viewpoint of food hygiene and end face corrosion resistance. It shows excellent performance.

Explaining the contents of the present invention in detail, the base material applicable to the present invention is Zn-based plating which can be used for ordinary coated steel plates such as electrolytic zinc, alloyed electrolytic zinc, molten zinc, and alloyed molten zinc. They have been subjected to a chemical conversion treatment with a phosphate such as zinc phosphate.
In applications where corrosion resistance is required, hot-dip zinc or alloyed hot-dip zinc having a large coating weight is preferably used, and particularly in wet environment, it is desirable to use hot-dip galvanized steel sheet.

As the resin for the undercoating material for this base material, there is no problem in adhesion to the chemical conversion surface and the top coating film,
Basically any resin can be used as long as it does not cause a problem in the appearance of the top coating film, such as epoxy, acrylic, urethane,
A polyester resin or the like can be used. This can be used regardless of whether the anticorrosive pigment is chrome-based or non-chrome-based. As for the topcoat paint, basically any resin may be used as long as it satisfies the required performance.

As the rust preventive pigment in the undercoat film on the guarantee side, a phosphate type pigment such as zinc phosphate, calcium phosphate, aluminum phosphate and wet silica are used. As the phosphate-based pigment, zinc phosphate is preferable from the viewpoint of corrosion resistance.

Chromates such as styrontium chromate, barium chromate and zinc chromate are used as rust preventive pigments in the undercoating film on the non-guaranteed side.
Strontium chromate is preferable from the viewpoint of corrosion resistance.

Examples of silica to be combined with a phosphate-based pigment such as zinc phosphate include wet silica, gas phase silica, colloidal silica and the like, and wet silica is also preferable from the viewpoint of durability of corrosion resistance.

The wet silica is manufactured by Nissan Chemical Co., Ltd.
Representative examples of Snowtex-20, -30, -40 include those having a large particle diameter such as Snowtex-20L and -0L, and those having a small particle diameter such as represented by Snowtex-S.

The mixing ratio of the above resin and pigment will be further described. Regarding the undercoat paint on the guarantee side, resin 100 is used.
It is preferable that the phosphate pigment and the wet silica are 5 to 100 parts by weight and 1 to 60 parts by weight, respectively, and the total amount of the phosphate pigment and the wet silica is not more than 120 parts by weight with respect to parts by weight. The reason for this is that if the amount of the phosphate pigment is less than 5 parts by weight, even if a large amount of wet silica is introduced, the corrosion resistance is poor, and if it exceeds 100 parts by weight, the appearance and adhesion of the top coating surface This is because it is unsuitable from the viewpoint of workability. Therefore, the amount of the phosphate pigment is set to 5 to 100 parts by weight. At this time, if the wet silica is less than 1 part by weight, the durability of the corrosion resistance is poor, and if it exceeds 60 parts by weight, the appearance of the top coating surface is poor. Cause. Therefore, wet silica is 1-6
The range was 0 part by weight. Further, in consideration of coatability, it is preferable that the total amount of the phosphate pigment and the wet silica does not exceed 120 parts by weight.

As the undercoat paint on the non-guaranteed surface side, it is preferable to add 5 to 120 parts by weight of a chromate such as strontium chromate to 100 parts by weight of the above resin. If the amount of the chromate is less than 5 parts by weight, the durability of the corrosion resistance is poor, and if it exceeds 120 parts by weight, the coatability and the processing adhesion are poor, so the above range is preferable.

The term "parts by weight" in the present invention means parts by weight in terms of solid content at the time of coating. That is, in the resin mixed with the solvent, the parts by weight of the resin before mixing are shown, and in the silica sol mixed with the solvent, the parts by weight of silica solids are shown.

Further, the above two kinds of undercoat paints may have a film thickness of 2 μm or more and 10 μm or less on both the guaranteed surface side and the non-guaranteed surface side. This is because if the film thickness is 2 μm or less, the corrosion resistance is poor, and if it is 10 μm or more, the workability is poor. The top coating film may have a thickness of 10 μm or more and 20 μm or less.
This is because if the film thickness is 10 μm or less, the hiding property is poor, and if it is 20 μm or more, the processability is poor.

When the appearance on the non-guaranteed surface side is not required, the non-guaranteed surface side may be a coating layer consisting of a resin and chromate as a rust preventive pigment.

[0022]

The present invention will be specifically described with reference to the following examples.

(Example 1) Alloyed hot-dip galvanized steel sheet (GA) (double-sided plating, plating adhesion amount 45 per side)
g / m ² ), after being treated with zinc phosphate (adhesion amount 0.8 g / m ² ), 80 parts by weight of zinc phosphate is added to 100 parts by weight of polyester resin as a guarantee surface primer (undercoat).
A resin coating containing a rust preventive pigment containing 30 parts by weight of wet silica (n-butanol silica sol, NBA-ST, manufactured by Nissan Chemical Industries, Ltd.) as a solid content was applied to a dry film thickness of 5 μm. A resin coating prepared by mixing 30 parts by weight of strontium chromate as an anticorrosive pigment with 100 parts by weight of a polyester resin as a non-guaranteed surface primer (undercoating paint) was applied to a dry film thickness of 5 μm. Then, the maximum plate temperature reached is 200 ± 10 ℃,
After baking under the condition of baking time of 45 seconds, a polyester resin paint as a top coat is applied on both the guaranteed surface and the non-guaranteed surface so that the dry film thickness is 18 μm, and then the maximum attainable plate temperature is 230 ± 10 ° C. Baking was performed under the condition of time of 60 seconds. Table 2 shows the results of evaluating the performance of the coated steel sheet obtained as described above.

(Examples 2 to 4) The galvanized steel sheet in Example 1 was hot-dip galvanized steel sheet (GI) (double-sided plating, coating weight 60 g / m ² per side), electrogalvanized steel sheet (EG) (double-sided plating, Plating coverage 30g / per side
m ² ), an electric Zn-Ni plated steel sheet (Zn-Ni) (double-sided plating, coating amount 30 g / m ² per side) except that the coated steel sheet was prepared in the same manner as in Example 1 and its performance was evaluated. evaluated. The results are shown in Table 2.

(Example 5) A galvannealed steel sheet (both sides plated, coating amount 45 g / m ^{2 on} one side) was treated with Zn phosphate, and a guarantee surface primer (undercoating paint) was applied.
The coated steel sheet was prepared in the same manner as in Example 1 except that the resin was an epoxy resin and the resin of the non-guaranteed surface primer (undercoat paint) was replaced with an epoxy resin, and its performance was evaluated. The results are shown in Table 2.

(Embodiment 6) The performance was evaluated in the same constitution as in Embodiment 1 except that the non-guaranteed top coat (top coating) was omitted. The results are shown in Table 2.

(Example 7) Example 1 was repeated except that Ca phosphate was used instead of Zn phosphate which was the rust preventive pigment on the guarantee side.
A coated steel sheet was prepared in the same manner as in 1. and its performance was evaluated.
The results are shown in Table 2.

(Example 8) A coated steel sheet was prepared in the same manner as in Example 1 except that barium chromate was used instead of strontium chromate, which is the rust preventive pigment on the non-guaranteed surface side, and its performance was evaluated. The results are shown in Table 2.

(Comparative Example 1) Alloyed hot-dip galvanized steel sheet (double-sided plating, coating weight 45 g / m ^{2 on} one side) was treated with Zn phosphate, and a guarantee surface primer (undercoating)
A coated steel sheet was prepared in the same manner as in Example 1 except that the resin was a polyester system, the amount of Zn phosphate as an anticorrosive pigment was 3 parts by weight, and the amount of wet silica was 40 parts by weight in terms of solid content. The performance was evaluated. The results are shown in Table 2.

(Comparative Example 2) Same as Example 1 except that the amount of Zn phosphate as a rust preventive pigment in the primer was 110 parts by weight and the amount of wet silica was 10 parts by weight in terms of solid content in the primer. A coated steel sheet was prepared and its performance was evaluated. The results are shown in Table 2.

Comparative Example 3 Guaranteed Surface A coated steel sheet was prepared in the same manner as in Example 1 except that silica was not added in the primer and the amount of Zn phosphate was changed to 90 parts by weight, and its performance was evaluated. .. The results are shown in Table 2.

Comparative Example 4 Guaranteed Surface A coated steel sheet was prepared in the same manner as in Example 1 except that the amount of silica in the primer was 70 parts by weight and the amount of Zn phosphate was 40 parts by weight. evaluated. The results are shown in Table 2.

(Comparative Example 5) Non-guaranteed surface A coated steel sheet was prepared in the same manner as in Example 1 except that the amount of strontium chromate in the primer was changed to 3 parts by weight, and its performance was evaluated. The results are shown in Table 2.

(Comparative Example 6) Non-guaranteed surface A coated steel sheet was prepared in the same manner as in Example 1 except that the amount of strontium chromate in the primer was 130 parts by weight, and its performance was evaluated. The results are shown in Table 2.

(Comparative Example 7) The same coating as in Example 1 is shown in the table,
The performance was evaluated by reversing the performance. The results are shown in Table 2.
Shown in.

(Comparative Example 8) The same coating as in Example 1 is shown in the table,
The coating was carried out by replacing the inside with the other, and a coated steel sheet containing no wet silica was prepared and its performance was evaluated. The results are shown in Table 2.

The criteria for determining corrosion resistance, paintability, appearance of coated surface, adhesion, and hygiene are shown below. Corrosion resistance-end surface red rust area (%) after SST (salt spray test) 1000h, average blistering length (mm) Coatability ○ ─ Usual reverse coat coating is possible × ─ Usual reverse coat coating is not possible Appearance ○ ─ No visible unevenness × ─ Visually noticeable unevenness Cohesiveness ○ ─ Separating tape tape after bending 0T There is no peeling between the primer and the top coat by X.-Peeling between the primer and the top coat by the above method. Hygienic property.-No Cr-based pigment is contained in the coating film.-Cr-based pigment is not contained in the coating film. Including Workability ○ ─ No cracks were generated after 0T bending processing × ─ Cracks were generated after 0T bending processing Concealment ○ ─ Undercoat color could not be discerned visually (guarantee side) × ─ Undercoating visually Can distinguish the film color

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[0043]

[0044]

EFFECT OF THE INVENTION The coated steel sheet of the present invention has the same corrosion resistance as in the case of the front and back chromate treatment within the scope of the present invention and is excellent in hygiene. Further, since the steel sheet of the present invention is a non-chromium type coating film on the guarantee side, it is excellent in hygiene and is widely used in the field of indirect contact with food such as microwave oven.

Claims (2)

[Claims] 1. A coated steel sheet comprising a chemical conversion treated double-sided zinc-based plated steel sheet as a base material, wherein an undercoat coating layer on the guarantee surface (surface) side that indirectly contacts food is 100 parts by weight of resin. On the other hand, as a rust preventive pigment, a phosphate pigment and a wet silica are 5 to 100 and 1 to 6 respectively.
This is a coating film of a coating material containing 0 parts by weight and a total of 120 parts by weight or less of a phosphate-based pigment and wet silica. The non-guaranteed surface (back surface) side undercoating film layer is 100 parts by weight of resin. On the other hand, a highly corrosion-resistant coated steel sheet which is a coating film containing 5 to 120 parts by weight of chromate as an anticorrosive pigment, which is preferable in food hygiene. 2. A highly corrosion-resistant coated steel sheet, which is preferable in food hygiene, according to claim 1, which has an overcoat layer on at least the undercoat layer on the guarantee side.