Storage method of fluororesin coated steel plate

Abstract

To provide a coated steel plate with an excellent processability over time while a property such as a processability and a weather resistance is favorably maintained.SOLUTION: The present invention provides a storage method where a fluororesin coated steel plate is stored at a temperature lower than a glass formation temperature, the fluororesin coated steel plate being a coated steel plate in which a chemical conversion film, a primer layer, and a top coating layer are formed on a hot-dip Al-Zn based plated steel plate. The top coating layer includes a resin component having a containing mass ratio of polyvinylidene fluoride resin of 50 to 80%, and a coloring pigment.

Description

Detailed Description of the Invention

Technical Field

[0001] The present invention relates to a coated steel sheet having excellent processability over time.

Background Art

[0002] A hot-dip Al-Zn plated steel sheet having a standard composition containing about 55% by mass of aluminum and about 1.6% by mass of silicon in the plating layer and the balance being zinc (hereinafter sometimes referred to as "55% Al-Zn plated steel sheet" or "galvalume steel sheet") exhibits excellent corrosion resistance, and thus may meet the SDGs goals, which are the requirements of the international community. In recent years, the demand has been increasing mainly in the building materials field. In addition, a fluororesin-coated steel sheet obtained by coating a 55% Al-Zn plated steel sheet with a fluororesin is used as a metal siding having a combined shape of a fitting shape with a complex molded pattern imitating stone, wood, or porcelain by an embossing roll, due to the excellent processability and weather resistance of the fluororesin, and is used as a wall material for houses and stores.

[0003] As a fluororesin-coated steel sheet, for example, in Document 1, a coated metal sheet is disclosed in which a topcoat layer containing a specific vinylidene fluoride resin is formed on the uppermost layer of a metal plate having a chemical conversion layer formed on the surface and a primer layer further formed thereon. For example, as described in Document 1, since the vinylidene fluoride resin is a thermoplastic and crystalline resin, excellent processability is imparted to the fluororesin-coated steel sheet by suppressing crystallization and making it amorphous by rapid cooling after baking.

Patent Document 1

[0004] Except for special applications, generally, vinylidene fluoride resin is rarely used alone. The reason is that fluorine-based resins are expensive. In such cases, an acrylic resin that is inexpensive and has good compatibility with the fluorine-based resin is blended and used within a range that does not reduce the weather resistance and processability of the fluorine-based resin. Their weight ratio is Vinylidene fluoride resin : Acrylic resin = 80 wt% : 20 wt% to = 50 wt% : 50 wt% It is. (Refer to Document 2)

Patent Document 2

[0005] That the vinylidene fluoride resin is a crystalline resin can be known, for example, from the observation of sharp peaks in wide-angle X-ray diffraction analysis as disclosed in Patent Document 2. Further, that the fluororesin-coated steel sheet has been amorphized by rapid cooling can be confirmed by performing the same X-ray diffraction analysis or by measuring the melting point during heating by DSC measurement (differential scanning calorimetry).

[0006] When the vinylidene fluoride resin crystallizes, why it causes a decrease in processability has not been clearly elucidated. Currently, the reason is considered to be that the polyvinylidene fluoride molecules forming the fluororesin are thermodynamically stabilized by arranging regularly, the molecular backbone extends and flexibility is lost. The energy of the stabilization is about 30 - 40 (J / g) in terms of the heat of fusion by DSC measurement (differential scanning calorimetry).

[0007] In addition, since the glass transition point due to the intense intramolecular thermal motion of the vinylidene fluoride resin is as low as minus 35°C, there is a problem that crystallization progresses over time and processability is lost when stored in a warehouse or the like under a room temperature atmosphere (problem of processability over time).

[0008] In low-molecular-weight molecules, crystallization can be said to occur when the molecules move, aggregate, and then arrange themselves neatly to become thermodynamically stable. At this time, it is essential for molecular motion to become active for the molecules to move. At the same time, contrary operations such as cooling are required to remove the heat generated by stabilization during crystallization. As a result, crystallization proceeds efficiently under the conditions that are the greatest common divisor of the two factors. Although the same can be said for high-molecular-weight polymers, it is thermodynamically disadvantageous for the center of gravity of a single polymer molecule to move. For this reason, in polymers, crystallization is explained by the thermal motion of segment units that do not involve the movement of the center of gravity of the polymer. (See Reference 3)

Non-Patent Document 3

[0009] For fluororesin-coated steel sheets in which crystallization has progressed, although good processability can be restored by heating above the melting point of the fluororesin and then rapidly cooling, the change in gloss due to the change in surface unevenness caused by reheating and the loss in aesthetics and economy when passing through the production process again are greater.

[0010] Under such circumstances, various techniques have been studied to solve the problem of the processability of fluororesin-coated steel sheets over time. For example, Patent Document 4 discloses a technique in which fine fluororesin particles with a size of 5 μm or less are used as the material constituting the topcoat layer to improve the compatibility with the acrylic resin and suppress the crystallization of the fluororesin. However, although a certain effect of improving processability can be obtained by the method of Patent Document 4, the crystallization of the fluororesin topcoat layer cannot be sufficiently suppressed after 3 months, and cracks occur in the small bending R parts of the processed parts during siding formation. Therefore, a technique with improved processability over time has still been desired.

Patent Document 4

Disclosure of the Invention

Problems to be Solved by the Invention

[0011] In view of such circumstances, an object of the present invention is to provide a painted steel sheet having excellent workability over time while maintaining good previous physical properties such as workability and weather resistance without degrading them.

Means for Solving the Problems

[0012] As a result of examining a fluororesin-coated steel sheet in which a chemical conversion coating film, a primer layer, and a topcoat layer containing a fluororesin are formed on a molten Al-Zn-based plated steel sheet in order to solve the above problems, the present inventors have found that even when the ratio of polyvinylidene fluoride and acrylic resin forming the topcoat layer is a previous ratio that is advantageous both in terms of workability and economy, a storage method without a decrease in workability over time has been found. Previous ratio: polyvinylidene fluoride resin : acrylic resin = 80% by weight : 20% by weight to = 50% by weight : 50% by weight

[0013] The present invention has been made based on the above-described experiments, and the gist thereof is as follows. 1. Crystallization of polyvinylidene fluoride resin near room temperature occurs due to the thermal motion of segments. 2. The thermal motion of the segments stops below the glass transition temperature. 3. Therefore, crystallization can be effectively suppressed by storing the fluororesin-coated steel sheet in an atmosphere below the glass transition temperature (minus 35 °C). 4. Since the fluororesin-coated steel sheet is pre-cooled and stored, it becomes easy to keep the fluororesin-coated steel sheet below the glass transition point with a cooler or the like from the storage warehouse to the delivery destination. 5. As the refrigerant for low-temperature storage of the fluororesin-coated steel sheet, dry ice, liquid nitrogen, liquid oxygen, liquid air, etc. can be appropriately used.

Effects of the Invention

[0014] According to the present invention, it is possible to provide a painted steel sheet having excellent workability over time while maintaining other physical properties such as workability and weather resistance well.

Brief Description of the Drawings

Figure 1

Figure 2

[0015] As for the method for testing the opening intensity of this X-ray, for example, using a device with a copper target material, a nickel filter, a 1-degree divergent slit, and a 0.3-mm receiving slit for diffracted X-rays, a focused X-ray beam with a tube voltage of 40 kV and a tube current of 80 mA was used, and the diffraction intensity was measured by scintillation counting under the measurement conditions of a time constant of 1 second and a scanning speed of the goniometer of 1 degree / minute.

[0016] That is, the fluororesin-coated steel sheet used in the present invention is a coated steel sheet in which a chemical conversion treatment layer, a primer layer thereon, and a top coat layer are formed on a molten Al-Zn-based plated steel sheet. And the top coat layer contains a polyvinylidene fluoride resin and an acrylic resin, and is a fluororesin-coated steel sheet excellent in weather resistance and processability, which contains a resin component in which the content ratio of the polyvinylidene fluoride resin is 50 to 80%, a coloring pigment according to the use, and an inorganic filler or an organic aggregate.

[0017] Hereinafter, each member constituting the fluororesin-coated steel sheet used in the present invention will be briefly described.

[0018] (Molten Al-Zn-based plated steel sheet) The fluororesin-coated steel sheet of the present invention uses a molten Al-Zn plated steel sheet as the steel sheet. The mainstream of the molten Al-Zn plated steel sheet is a molten Al-Zn-Si plated steel sheet written in descending order of the weight% of the contained elements.

[0019] Furthermore, in addition to Al, Zn, and Si described above, the plated steel sheet may also contain optional components within 5% in order to improve the workability of the plating layer. Here, as the optional components, for example, alkaline earth metals such as Ca and Mg, and Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B are added.

[0020] Also, when the plating layer does not contain optional components, products have been commercialized in which the elongation limit of the molten Al-Zn-based plated steel sheet before film formation is improved by about several tens of percent by performing heat treatment (referred to as annealing treatment) at about 200 °C for 24 hours. In addition to this, in addition to the molten Al-Zn-based plated steel sheet in which optional components are added to the plating layer described above, the disclosed method can also be applied to molten Zn (containing a small amount of Al) steel sheets and molten Zn (containing several percent of Al) steel sheets. By using a molten Al-Zn plated steel sheet, high corrosion resistance can be ensured.

[0021] (Conversion coating) The fluororesin-coated steel sheet used in the present invention forms a conversion coating on the plating layer of the molten plated steel sheet. By forming a conversion coating on the plated steel sheet, the corrosion resistance of the fluororesin-coated steel sheet and the adhesion between the plating layer and the primer layer can be further enhanced.

[0022] The type and formation conditions of the conversion coating are not particularly limited and can be appropriately selected according to the required performance. For example, it is formed by applying a chromate treatment solution or a chromium-free conversion coating solution such as vanadium.

[0023] (Primer layer) The fluororesin-coated steel sheet used in the present invention forms a primer layer between the chemical conversion film and the top coat layer. By applying the primer, the adhesiveness between the top layer and the chemical conversion film can be enhanced, and the corrosion resistance and rust prevention property can also be further improved. As the primer, a general-purpose type suitable for the top coat layer and the chemical conversion film can be used, or a type according to the application may be used.

[0024] (Top coat layer) The fluorine-coated steel sheet used in the present invention has a top coat layer in addition to the above-mentioned hot-dip Al-Zn alloy coated steel sheet, chemical conversion treatment film and primer layer. The top coat layer may be an off-the-shelf product of a paint manufacturer such as Nippon Paint or Kansai Paint. It may also be a paint to which a heat-reflective pigment is added according to the purpose to impart a heat insulation function for countermeasures against global warming.

[0025] Regarding the resin component contained in the aforementioned top coat, for polyvinylidene fluoride resin, it is preferable to use a resin having a weight average molecular weight of 150,000 to 700,000 and a melting point of 150 to 180°C. For example, "Kynar 500 (weight average molecular weight: 350,000, melting point: 160 - 165°C)" manufactured by Arkema, "Kynar 500 (weight average molecular weight: 650,000, melting point 160 - 165°C)" manufactured by Nippon Penwalt Co., Ltd., etc. can be exemplified.

[0026] As the acrylic resin to be mixed with the polyvinylidene fluoride resin, a resin having a weight average molecular weight of 50,000 to 250,000 is preferable. Also, an acrylic resin modified by copolymerization between different acrylic monomers can be appropriately used.

Examples

[0027] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples at all.

[0028] 〈Examples 1 - 9 of the present invention, Comparative Examples 1 - 9〉 (Preparation of test materials 1) The plate thickness is 0.35 mm, and the plating adhesion amount is 300 g / m on both sides2 , a hot-dip galvanized steel sheet having a composition of Zn-55% Al-1.6% Si and subjected to skin pass treatment on the plating layer was prepared. After degreasing the hot-dip galvanized steel sheet, the prepared vanadium-based chromate-free chemical conversion treatment liquid was adhered by dipping, squeezed with a roll coater, and then dried at a reaching temperature of 90 ° C of the steel sheet and a baking time of 10 seconds, and the chromate-free chemical conversion treatment amount was an adhesion amount of 0.2 g / m 2 It was adjusted to be. Thereafter, a primer layer paint based on an epoxy-modified urethane resin and using vanadium phosphate as an anti-rust pigment was applied onto the above chemical conversion treatment with a bar coater, baked at a reaching temperature of 210 ° C of the steel sheet and a baking time of 25 seconds, and a primer layer was formed so that the resulting film thickness became 5 μm. Then, on the steel sheet on which the primer layer was formed, based on a resin component in which 30% by weight of PVdF resin and 15% by weight of acrylic resin were 45% by mass, 40% by weight of an iron-chromium composite oxide (``Black411'' manufactured by Shepherd Color Co., Ltd.) as a black pigment was added to the resin, A top coat layer paint was prepared by adding 5% by mass of titanium oxide and 3% by mass of acrylic beads (average particle size 20 μm) and isophorone as a solvent to a desired viscosity.

[0029] 〈Examples 1 to 3 and Comparative Examples 1 to 3 of the present invention〉 The fluororesin-coated steel sheet obtained by adjusting the above test specimens was stored in the following atmosphere. Test specimen 1-1: Stored for 75 hours in an accelerating aging atmosphere (50 ° C) Test specimen 1-2: Stored for 9 months in a room temperature atmosphere (20 ° C) Test specimen 1-3: Stored for 9 months in an atmosphere in a household freezer (-18 ° C) Test specimen 1-4: Stored for 9 months in an atmosphere in a tuna freezer (-50 ° C) Test specimen 1-5: Stored for 9 months in a dry ice atmosphere (-79 ° C) Test specimen 1-6: Stored for 9 months in a liquid nitrogen atmosphere (-196 ° C)

[0030] (Adjustment of test specimens 2) The ratio of the fluororesin and the acrylic resin used in the above Preparation 1 of the test specimen was based on 40% by mass of the fluororesin and 5% by mass of the acrylic resin, and the other components were the same as those in Preparation 1 of the test specimen, and a paint for a top coat layer was prepared.

[0031] 〈Examples 4 to 6 and Comparative Examples 4 to 6 of the Present Invention〉 The fluororesin-coated steel sheet obtained by the above adjustment of the test specimen was stored in the following atmosphere and then immediately evaluated for workability. Test Specimen 2-1: Stored for 75 hours in an accelerated aging atmosphere (50°C) Test Specimen 2-2: Stored for 9 months in a room temperature atmosphere (20°C) Test Specimen 2-3: Stored for 9 months in an atmosphere in a household freezer (-18°C) Test Specimen 2-4: Stored for 9 months in an atmosphere in a tuna freezer (-50°C) Test Specimen 2-5: Stored for 9 months in a dry ice atmosphere (-79°C) Test Specimen 2-6: Stored for 9 months in a liquid nitrogen atmosphere (-196°C)

[0033] (Preparation 3 of the Test Specimen) The above-mentioned non-chromate conversion coating applied in Preparation 1 of the test specimen was formed on a galvanized steel sheet having a plate thickness of 0.35 mm and a plating adhesion amount of 300 g / m² on both sides and having a composition of Zn-55%Al-1.6%Si, which was annealed at 200°C for 24 hours. A primer layer was formed on that layer, and further, a fluororesin film was formed to prepare a fluororesin-coated steel sheet. 2

[0032] 〈Examples 7 to 9 and Comparative Examples 7 to 9 of the Present Invention〉 The fluororesin-coated steel sheet obtained by the above adjustment of the test specimen was stored in the following atmosphere and then immediately evaluated for workability. Test Specimen 3-1: Stored for 75 hours in an accelerated aging atmosphere (50°C) Test Specimen 3-2: Stored for 9 months in a room temperature atmosphere (20°C) Test Specimen 3-3: Stored for 9 months in an atmosphere in a household freezer (-18°C) Test Specimen 3-4: Stored in the atmosphere inside a tuna freezer (-50°C) for 9 months Test Specimen 3-5: Stored in a dry ice atmosphere (-79°C) for 9 months Test Specimen 3-6: Stored in a liquid nitrogen atmosphere (-196°C) for 9 months <Evaluation> For each sample of the coated steel sheet obtained as described above, the evaluation was conducted as follows.

[0034] (1) Workability and workability over time Regarding the coated steel sheet of each sample, a bending test was carried out according to JIS G3322. When bending at 180 degrees with a test piece width of 75 mm, the inner spacing was indicated according to the number of plates, and the limit number of plates (T) at which no cracks occurred in the bent surface coating film was defined as crack-free T. The smaller this crack-free T, the better the workability. This bending test was carried out immediately after sample preparation, after holding at 50°C for 72 hours (after accelerated aging over time) corresponding to 3 months of storage in summer, and after storage for 9 months at each atmosphere temperature (20°C, -18°C, -50°C, -79°C, and -196°C). Table 1 shows the results of crack-free T for each of immediately after sample preparation, after accelerated aging over time, and after storage for 9 months at each atmosphere temperature (20°C, -18°C, -50°C, -79°C, and -196°C). ○: Practical for siding applications, etc., 6T or less. ×: Exceeding 6T

[0035] [Table 1]

[0036] From the results in Table 1, it can be seen that each sample of the present invention example is superior in workability over time compared to each sample of the comparative example. [Industrial Applicability]

[0037] According to the present invention, a coated steel sheet excellent in workability over time can be provided at a desired time while maintaining good workability and other physical properties such as weather resistance.

Claims

1. A coated steel sheet having a chemical conversion coating, a primer layer, and a topcoat layer formed on a molten Al-Zn alloy plated steel sheet, wherein the topcoat layer contains a resin component having a content ratio of polyvinylidene fluoride resin of 50 to 80% and an acrylic resin compatible with the vinylidene fluoride resin of 50 to 20%, and contains a coloring pigment. A method for preventing deterioration of the processability over time of a crystalline fluororesin-coated steel sheet, characterized by storing the fluororesin-coated steel sheet below the glass transition temperature.

2. The coated steel sheet according to claim 1, wherein the molten Al-Zn alloy plated steel sheet constituting the fluororesin-coated steel sheet is a plated steel sheet annealed in advance to improve the elongation of the plating layer, and is a crystalline fluororesin-coated steel sheet.

3. The method for preventing deterioration of processability over time according to claim 1, wherein the temperature for storing the fluororesin-coated steel sheet below the glass transition temperature is -35°C or lower.

4. The method for preventing deterioration of processability over time according to claim 3, wherein the cooling means for storing the temperature at -35°C or lower is a refrigerator capable of variably adjusting the temperature, dry ice, liquid nitrogen, or storage in liquid air.

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