JPH06218325A - Preparation of coated steel sheet - Google Patents

Abstract

PURPOSE:To obtain a method of preparing a coated steel sheet with superb corrosion inhibition and moldability which meets a guaranty about durability of at least, 30 years by using a specific undercoat and a specific intermediate coat consisting of a solid portion containing a specific vehicle as a main component when forming a coat using a fluororesin paint as a top coating. CONSTITUTION:An undercoat paint is applied to a plated steel sheet, when an intermediate coat paint and a finishing paint are applied over each other, as a method of preparing a coated steel sheet. A coat is formed as a top coating using a fluororesin paint. In addition, when forming a coat of finishing paint using the fluororesin paint, the undercoat paint comprising a vehicle as a main component which contains 50 to 90wt.% of linear polyester resin as a solid with a hydroxyl group of 8 to 25mgKOH/g and 40 to 100 pts.wt. of pigment for 100 pts.wt. of vehicle solid portion, and the intermediate coat paint containing a vehicle composed mainly of (polyvinylidene fluoride) resin and thermally curable acrylic resin are used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a coated steel sheet. More specifically, the present invention relates to a method for producing a coated steel sheet having more excellent durability than conventional coated steel sheets.

[0002]

2. Description of the Related Art A coated steel sheet having a thickness of 0.3 to 0.8 mm, which is generally called a pre-coated steel sheet, is cut and formed into a desired use for a roof or a building. It is currently widely used in parts such as exteriors that require long-term durability, electrical equipment, indoor equipment, vehicle interiors, building interiors, and the like. "Durability 20-year warranty" for coated steel sheets used for parts requiring long-term durability among the above applications
Is now the norm in the industry. However, in the case of coated steel sheet, the cut end surface is always exposed when it is formed and processed. Even in the construction property, the cut end face is the part where blisters and rusts are most likely to occur, and even with painted steel plates guaranteed for 20 years, the processing part is a disclaimer in the industry. In general,
The coated plate used for the coated steel plate is a cold rolled steel plate, a galvanized steel plate, an aluminum / zinc alloy plated steel plate, a stainless steel plate, an aluminum plate, a metal plate such as an aluminum alloy plate, etc., but is guaranteed for 20 years. Mainly used as coated steel sheets are hot-dip galvanized steel sheets with high corrosion resistance, aluminium / zinc alloy plated steel sheets and stainless steel sheets. The coating used is also vinylidene fluoride resin coating with excellent corrosion resistance. The one that is painted and baked is generally used. Known techniques relating to these vinylidene fluoride resin-coated steel sheets include, for example, a coating film using an epoxy resin as a base coating, an acrylic resin as an intermediate coating, and an acrylic modified vinylidene fluoride resin as a top coating as a main component (Japanese Patent Publication No. 63- No. 11950), an epoxy resin as a base coating, an acrylic modified vinylidene fluoride resin as an intermediate coating, and an acrylic modified vinylidene fluoride resin as a main coating as a main component (Japanese Patent Laid-open No. Sho 5 (1999) -58242).
No. 9-179340, Japanese Examined Patent Publication No. 3-11266), a vinylidene fluoride resin-based top coating film having a crystallinity of 18% or less and a vinylidene fluoride resin content of 70% by weight or more (Japanese Patent Laid-Open No. 61-114846). ), And a coating film finished with a vinylidene fluoride resin coating material containing glass fibers (JP-A-61-2336869, JP-A-57-17).
No. 7379), an overcoat paint finish using an undercoat paint of an acrylic modified polyester resin and containing a polyvinylidene fluoride resin and an acrylic resin (JP-A-2-23318).
No. 3), a finish coating film composed of polyvinylidene fluoride and a thermoplastic acrylic resin (Japanese Patent Laid-Open No. 4-11672).
Gazette), using an intermediate coating composition containing polyvinylidene fluoride and a thermoplastic acrylic polymer as main components, and finishing coating composition containing polyvinylidene fluoride (JP-A-2-174).
977) and the like are disclosed. However,
In recent years, it has become difficult to easily repair building exteriors due to the increase in the number of buildings in the coastal area, so that the coated steel sheets for building exterior materials are required to have a high degree of corrosion resistance, and the cut end surface is also guaranteed. Must be considered. Against this background, there has been a demand for a coated steel sheet having higher durability than ever before, that is, a “durability of 30 years or more guaranteed”. Therefore, there is a demand in the art for the development of a coated steel sheet that has a corrosion resistance that is about 2-3 times that of a conventional coated steel sheet, including the cut end surface, the scratched portion, and the bent portion. However, in the above-mentioned coated steel sheet for the 20-year warranty, the following problems (1) to (3) have become apparent, and there is a problem that the above requests cannot be met. (1) The total coating film thickness of the undercoat, middle coat and top coat is 50
Since it is as thin as μm or less, moisture easily penetrates the coating film,
In addition, it does not have sufficient corrosion resistance, such as swelling easily on the cut end surface and the processed part. (2) Conventionally, an epoxy-based paint has been usually applied as the undercoat paint to be combined with the fluorine-based finish paint. Since the undercoating paint has a hard coating film and poor workability, even a thick coating film can be applied only to a coating film thickness of at most about 10 μm. Therefore, the corrosion resistance cannot be exhibited, and the total coating thickness of the coated steel sheet is 50 μm as described in (1).
It is also one of the reasons why it cannot be more than m. If an attempt is made to apply a thicker undercoating film, there are problems such as cracks during baking of the coating film and cracks during molding. Therefore, the thickness of the coating film is about 10 μm. (3) Finishing paints (referring to intermediate paints and topcoat paints) in the prior art are paints mainly containing polyvinylidene fluoride as a color-developing agent component and a thermoplastic acrylic resin, and these also have the following problems. there were. That is, there is no particular problem in obtaining a coating film thickness of less than 40 μm by coating the coating material once, but when coating the coating material twice or more, since the coating film on the lower portion is thermoplastic, the upper coating material is applied. There is a problem that the lower coating film is redissolved by the solvent in the coating material, and an attempt to obtain a coating film thickness of 40 μm or more causes a side effect. Therefore, the total coating thickness of the finish paint is limited to about 40 μm, which is (1)
As described in (1), it is also one of the reasons why the total coating film thickness of the coated steel sheet cannot be 50 μm or more. As for the quality of coated steel sheet that can meet the "durability of 30 years or more", (1) discoloration of color after 2000 hours of accelerated weather resistance test, NBS color difference (ΔE) is 7 or less, (2) Salt spray test 2
It is required to have performance of ASTM: 8F or higher in 500 hours.

[0003]

Under the circumstances, the present invention manufactures a coated steel sheet having excellent corrosion resistance and forming workability, which is considered to be able to meet the "durability of 30 years or more guarantee". The purpose is to provide a method.

[0004]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies in order to achieve the above-mentioned object, and as a result, a coating thickness of about 30 μm can be obtained by one coating, and the molding processability is excellent. In addition, a total of 6 by using a specific undercoat paint that has corrosion resistance on the cut end face, scratched part, and molding part, and a specific intermediate paint that does not re-dissolve when applying the topcoat paint
The inventors have found that a coating film having a thickness of 0 to 120 μm can be obtained and that the object can be achieved, and have completed the present invention based on this finding. That is, in the present invention, when a primer coating, an intermediate coating and a top coating are overlaid on a plated steel sheet and a coating film is formed using a fluororesin type coating as the top coating, a hydroxyl value of 8 to 25 mgKOH / An undercoat paint containing, as a main component, a color-developing agent containing 50 to 90% by weight of a linear polyester resin (g), and 40 to 100 parts by weight of a pigment based on 100 parts by weight of the solid content of the color-developing agent.
Another object of the present invention is to provide a method for producing a coated steel sheet, which comprises using an intermediate coating material containing a polyvinylidene fluoride resin and a thermosetting acrylic resin as main components of a color developing agent.

The present invention will be described in detail below. Examples of the coated plate used in the method of the present invention include cold rolled steel plate, galvanized steel plate, aluminum / zinc alloy plated steel plate, stainless steel plate, aluminum plate, aluminum alloy plate, and other metal plates, but blister and end surface processing The hot dip galvanized steel sheet, 5% aluminum / zinc alloy plated steel sheet, and 55% aluminum / zinc alloy plated steel sheet are preferable in terms of excellent corrosion resistance of the part. The surface of the coated plate can be subjected to a chemical conversion treatment prior to coating in consideration of adhesion to the coating film and the like. The chemical conversion treatment may be that generally used as a coating base treatment. For hot dip galvanized steel sheet, 5% aluminum / zinc alloy plated steel sheet, zinc phosphate treatment, or coating type or reactive type chromate treatment is used. It can be preferably applied, and coating type or reactive type chromate treatment can be preferably applied to a 55% aluminum / zinc alloy plated steel sheet. In the method of the present invention, a coated steel sheet is manufactured by coating the above steel sheet with the undercoat paint, the intermediate paint and the topcoat paint. As the undercoat paint, a paint containing a linear polyester resin (a high molecular polyester resin mainly having a linear structure) as a main component of the color-developing agent is used. Further, this undercoat paint preferably contains an epoxy resin and a melamine resin together with the linear polyester resin. As the linear polyester resin, it is necessary to use one having a hydroxyl value in the range of 8 to 25 mgKOH / g, and a number average molecular weight of 5.0.
Those having a glass transition point of 0 to 25,000 and a glass transition point of 10 to 50 ° C. are preferable. As such a linear polyester resin, for example, commercially available Byron 59C is used.
S, GK13CS [manufactured by Toyobo Co., Ltd., trade name], Beckolite M-6801-30 [manufactured by Dainippon Ink Industry Co., Ltd.,
Product name] etc. In the linear polyester resin, if the number average molecular weight is less than 5,000, the coating film is excessively hardened to lower the processability, and if it exceeds 25,000, a large amount of solvent for dissolving the resin is required, and the coating composition Since the solid content is low, it becomes difficult to thicken the coating film. Further, when the glass transition point is lower than 10 ° C, the coating film hardness is lowered, and when the glass transition point is higher than 50 ° C, the moldability is lowered, so that both are not preferable. Furthermore, the hydroxyl value is 8 mg
If it is less than KOH / g, the cross-link density of the coating film will be insufficient, and the paint applied on it will be easy to open when baking and drying.
If it exceeds H / g, the cross-linking density of the coating film becomes high and the moldability decreases.

The epoxy resin has a number average molecular weight of 400 to 2,700 and an epoxy equivalent of 200 to 2.0.
Those in the range of 00 are preferable. As such an epoxy resin, commercially available products such as Epicoat # 828, #
834, # 1001, # 1004, # 1007 [trade name of Yuka Shell Epoxy Co., Ltd.] and the like. Further, as the melamine resin, a methylated melamine resin or a butylated melamine resin which is compatible with the linear polyester resin is suitable. The linear polyester resin, the epoxy resin and the melamine resin, which constitute the color-developing agent component in the undercoat paint, are used in proportions of 50 to 90% by weight of the linear polyester resin and the epoxy resin and the melamine resin as solid contents. 10 to 50% by weight in total
It should be in the range of. The mixing ratio of the epoxy resin and the melamine resin is 50:50 to 80: 2 by weight.
It is preferably in the range of 0. If the linear polyester resin is less than 50% by weight, the thickness of 30μ can be obtained by applying the undercoat paint once.
It is difficult to obtain a dry film thickness of about m and the moldability is poor, and when it exceeds 90% by weight, the adhesion to the base material is deteriorated, and thus neither of the objects of the present invention can be achieved. Pigments such as rust preventive pigments, coloring pigments and extender pigments are used in the undercoat paint, and solvents and various additives generally used in paints can be added. As the rust preventive pigment, a chromic acid-based rust preventive pigment, that is, strontium chromate, calcium chromate, barium chromate, zinc chromate and the like can be used, but strontium chromate is most suitable. Examples of extender pigments that can be used include calcium carbonate, clay, talc, synthetic silica, natural silica, antimony trioxide, barium sulfate, precipitated barium sulfate, and kaolin. The amount of addition of all pigments is 40 to 1 with respect to 100 parts by weight of the solid content of the above-mentioned color developing agent.
It is selected in the range of 00 parts by weight. If it is less than 40 parts by weight, it is easy to cause armpits and sufficient rust preventive power cannot be obtained.
When it exceeds the weight part, the bending workability is deteriorated and the corrosion resistance of the processed part is deteriorated. As the solvent, toluene, an aromatic hydrocarbon solvent such as xylene, ethyl acetate, butyl acetate,
A cellosolve-based solvent, methyl isobutyl ketone, isophorone, cyclohexanone or the like can be used. in this way,
By using a linear polyester resin as the main component of the color-developing agent of the undercoat paint, it is possible to suppress the occurrence of armpits and achieve a thick coating of about 30 μm with one coating, and in a thick film of about 60 μm with two coatings. It is also possible to provide excellent moldability.

In the method of the present invention, an intermediate coating composition containing a polyvinylidene fluoride resin and a thermosetting acrylic resin as main components of the color developing agent is used. Further, it is preferable that this intermediate coating material contains a thermoplastic acrylic resin and a melamine resin together with the polyvinylidene fluoride resin and the thermosetting acrylic resin. The polyvinylidene fluoride resin is preferably one having a weight average molecular weight of 200,000 or more and a melting point in the range of 150 to 180 ° C. As such a resin, for example, commercially available kiner 5 is used.
00 [manufactured by Atochem Co., Ltd., trade name, weight average molecular weight 35
2,000, melting point 160-165 ° C.] and the like. The thermosetting acrylic resin can react with a melamine resin to form a crosslinked structure, and such an acrylic resin has, for example, a hydroxyl group in a side chain,
Those having a carboxyl group in the side chain and those having a hydroxyl group and a carboxyl group in the side chain can be used. Also,
These can also be mixed and used. For the acrylic resin as described above, a usual method for producing an acrylic resin for paint can be applied as it is, and for example, it can be easily produced by a solution polymerization method, a bulk polymerization method or the like by mixing a polymerization initiator with a monomer. be able to. The thermosetting acrylic resin having a hydroxyl group preferably has a hydroxyl value in the range of 5 to 20 mgKOH / g. Examples of the acrylic monomer having a hydroxyl group for introducing the hydroxyl group into the acrylic resin as described above include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, and 3-hydroxypropyl (meth). ) Acrylate, 2
-Hydroxybutyl (meth) acrylate, 3-hydroxybutyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, glycidyl (meth) acrylate, caprolactone-modified (meth) acrylate and the like, and also Praxel FM monomer or Praxel FA. Monomers [manufactured by Daicel Chemical Industries, Ltd.] and the like can also be used.

The thermosetting acrylic resin having a carboxyl group preferably has an acid value in the range of 5 to 20 mgKOH / g. As the acrylic monomer having a carboxyl group for introducing such a carboxyl group into the acrylic resin, for example, acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, and substituted derivatives thereof can be used. . Other copolymerizable vinyl-based monomers used in combination with the above-mentioned acrylic monomer containing a hydroxyl group or a carboxyl group include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate. Butyl (meth) acrylate,
2-Ethylhexyl (meth) acrylate, lauryl (meth) acrylate, cyclohexyl (meth) acrylate, styrene, pt-butylstyrene, vinyltoluene, vinyl acetate, acyl group-modified (meth) acrylate and the like can be used. As the thermoplastic acrylic resin, for example, commercially available products such as Paraloid A-11 and Paraloid B-44 [Rohm and Haas Japan Co., Ltd.,
Brand name], Dinal BR-80, Dinal BR
-83, Dialnal BR-85, Dialard BR-
87 [trade name, manufactured by Mitsubishi Rayon Co., Ltd.]. Further, as the melamine resin, the same ones as exemplified in the explanation of the undercoat paint can be mentioned. Regarding the blending ratio of each resin component that constitutes the color-developing agent in the intermediate coating composition, it is desirable that the amount of polyvinylidene fluoride be 40 to 80% by weight as a solid content. In addition, the blending ratio of the mixture of the thermosetting acrylic resin and the thermoplastic acrylic resin and the melamine resin is preferably in the range of 80:20 to 95: 5 by weight.
Further, the mixing ratio of the thermosetting acrylic resin and the thermoplastic acrylic resin is preferably in the range of 30:70 to 70:30 by weight.

As other components of the intermediate coating composition, a coloring pigment, an extender pigment, a solvent, an additive and the like which are generally used in coating compositions can be blended. As the color pigment, for example,
Titanium oxide, carbon black, iron oxide, lead chromate,
Aluminum powder, copper powder, cobalt blue, cobalt green, etc. can be used. As the additive, for example, a defoaming agent, a pigment dispersant, an anti-sagging agent, an anti-scratch agent or the like can be used. As described above, by using a composition containing a thermosetting acrylic resin and a melamine resin as the color-developing agent component of the intermediate coating composition, the disadvantage of the conventional intermediate coating composition is that "the intermediate coating composition is redissolved when the top coating composition is applied." It is possible to eliminate coating defects, and the total coating thickness of the intermediate coating and top coating is 40-60.
It became possible to increase the thickness to about μm. In the method of the present invention, a fluororesin type paint is used as the top coat paint. As the fluororesin type paint, a fluororesin type top coat paint for a coated steel sheet which has been conventionally used for a part requiring long-term durability such as a roof or a building exterior can be applied as it is. The main component of the color developing agent of this fluororesin type top coating is usually polyvinylidene fluoride and a thermoplastic acrylic resin, and the mixing ratio of both is 70:30 to 85:15 by weight. It is desirable to be in the range. The polyvinylidene fluoride and the thermoplastic acrylic resin may be the same as those exemplified in the description of the intermediate coating material. Topcoat paints include glass fiber, for the purpose of improving coating hardness and other properties.
Inorganic substances such as glass beads, alumina, mica and silica, and organic aggregates such as polytetrafluoroethylene and polymethylmethacrylate may be blended. Further, as other components of the top coating composition, a coloring pigment, a solvent, an additive or the like generally used in the coating composition can be blended. As the additive, for example, an ultraviolet absorber, an antioxidant, a defoaming agent, a pigment dispersant, a leveling agent, an anti-settling agent, an anti-scratch agent, etc. can be used.

Next, an example of the coating method in the present invention will be shown. First, a hot dip galvanized steel sheet, a 5% aluminium / zinc alloy plated steel sheet or a 55% aluminium / zinc alloy plated steel sheet is used as a coated plate, and a zinc phosphate treatment, or a coating type or reactive type chromate treatment is applied to this. Give. As a coating method, the undercoat coating material of the present invention is applied by roller coating so that the dry coating film thickness is about 20 to 30 μm, and is heated and dried at an ultimate plate temperature of 250 ± 10 ° C. Next, the intermediate coating composition of the present invention is applied by roller coating so that the dry coating film thickness is about 20 to 30 μm, and is dried by heating at the ultimate plate temperature of 250 ± 10 ° C. Further, the top coating composition according to the present invention is applied by roller coating so that the dry coating film thickness is about 20 to 30 μm, and is heated and dried at an ultimate plate temperature of 250 ± 10 ° C. As described above, by baking 3 coats and 3 coats of the undercoat paint, the middle coat paint and the top coat paint, a coated steel sheet having a total coating film thickness of 60 to 90 μm can be obtained. Also, as an alternative to the coating method, the undercoat paint is applied twice to obtain higher corrosion resistance, the intermediate paint and the top paint are applied in the same manner as above, and 4 coats and 4 bakes are applied to obtain a total coating film thickness. It is also possible to obtain a coated steel sheet according to the method of the present invention having a thickness of about 80 to 120 μm.

[0011]

The present invention will be described in more detail by way of examples, which should not be construed as limiting the invention thereto. The performance of the coated steel sheet was evaluated as follows. (1) Workability test The workability of each test piece cut into a width of 5 cm was tested in a room at 20 ° C. 2T in the table means that two sheets of the same coated plate as the test piece to be processed are sandwiched inside (0T is the test piece as it is) and the coating film is placed outside and bent 180 degrees. Observe the bent tip with a magnifying glass 10 times,
The following criteria were evaluated. ⊚: No cracks ◯: Cracks 10% or less (pass) Δ: Cracks more than 10% and less than 50% (fail) ×: Cracks 50% or more (2) Impact deformation test JIS K-5400 (1979) ) The method of 6.13.3B was applied, the weight was 1 kg, the height was 50 cm, and the coated surface of the test piece was tested upward and downward, and evaluated according to the following criteria. ◎: No peeling of cellophane tape ○: 10% or less of cellophane tape peeling area (pass) △: More than 10% and less than 50% of cellophane tape peeling area (fail) ×: 50% or more of cellophane tape peeling area (3) Boiling water resistance test After immersing the test piece in boiling water for 5 hours, the presence or absence of abnormality in the coating film was visually observed and evaluated according to the following criteria. ◯: No abnormalities on the coated surface (pass) Δ: Slight blistering is recognized (fail) x: Clear blistering is recognized (4) Corrosion resistance test Regarding the test piece subjected to 2T processing, JIS Z-2371
According to (salt spray test), test 2500 hours,
The following conditions were evaluated by observing (a) the blister occurrence on the flat surface portion, (b) the blister occurrence situation on the 2T processed portion, and (c) the blister occurrence situation on the cross-cut portion. ○: No abnormalities on the coated surface (pass) △: Slight blisters are recognized (fail) x: Blisters and white rust are recognized Further, (d) The blister generation width from the end face of the cut is measured and is 6 mm or less. Was accepted.

(5) Presence or absence of armpits When the test pieces were prepared, the presence or absence of armpits was observed on the coated surfaces at the time of drying each of the undercoat, intermediate coating and topcoat, and the following criteria were evaluated. ◯: No abnormalities on the coated surface (pass) Δ: Slight armpits are recognized (fail) X: Clear arms are recognized Note that other test is no longer carried out on the test pieces that have been armored and failed. It was (6) Adhesion test First, JIS K-54 was applied to the test piece coating film in a room at 20 ° C.
According to the method of 6.15 (cross-cut test) of No. 00, a cross-cut is made with a cutter knife, and the test piece is extruded by an Erichsen tester by 6.0 mm from the back surface of the part. After that, sufficiently adhere the cellophane adhesive tape on the grid and immediately immediately peel off the cellophane tape in the direction perpendicular to the coating surface, pass the grid with 100 grids remaining without peeling, 9
9 or less was rejected. (7) Accelerated weathering test A test piece subjected to 3T processing was tested for 2000 hours under the following tester and test conditions (commonly known as Ducycle sunshine weatherometer) Tester: WEL-Sunshine-DC [Suga Tester
Co., Ltd.] Test condition: Light source carbon arc Black panel temperature 63 ° C. Observe the coated surface state after the test, color difference ΔE before and after the test,
The gloss retention percentage was measured. The evaluation method is as follows: ◎: There is no abnormality on the coating surface ○: Almost no abnormality (pass) △: Decrease in gloss, white rust, blistering (fail) ×: Clearly abnormal color difference Regarding the gloss retention rate, 7.0 or less and 70% or more were passed.

Preparation Example 1 Preparation of Undercoat Paint According to the formulation shown in Table 1, first, a linear polyester resin and a part of a mixed solvent were mixed, and then a pigment was added and uniformly mixed, and then an attritor was used. Particle size 30μm
Dispersed below. Furthermore, after adding epoxy resin, melamine resin and acid catalyst, appropriately add the remaining mixed solvent,
Viscosity for Ford Cup # 4 120 ± 10 seconds (25 ℃)
And 13 types of undercoat paints U1 to U13 were prepared. Note that U1 to U5 are undercoat paints according to the present invention, and U6 to U13 are undercoat paints for comparative samples.

[0014]

[Table 1]

[0015]

[Table 2]

Note 1) Byron 59CS [trade name of Toyobo Co., Ltd. Number average molecular weight: 5000-8000, non-volatile content: 50 wt%, hydroxyl value: 12-22 mgKOH / g, glass transition point: 15 ° C. 2) Byron GK13CS [trade name of Toyobo Co., Ltd.] Number average molecular weight: 5000-8000, nonvolatile content: 50 wt%,
Hydroxyl value: 16-25 mg KOH / g, glass transition point: 24
° C] 3) Beckolite M-6801-30 [trade name of Dainippon Ink and Chemicals, Inc.] Number average molecular weight: 15,000, nonvolatile content: 30 wt%, hydroxyl value: 8 to 15 mgKOH / g, glass transition point: 30 ° C.) 4) Byron 29XS [trade name of Toyobo Co., Ltd.] Number average molecular weight: 20,000 to 25,000, non-volatile content: 35 wt%,
Hydroxyl value: 5-8 mgKOH / g, glass transition point: 70 ° C.] 5) Byron 51CS [trade name of Toyobo Co., Ltd.]. Number average molecular weight: 20,000 to 25,000, non-volatile content: 40 wt%,
Hydroxyl value: 5-8 mg KOH / g, glass transition point: 4 ° C.] 6) DYNAPOL LH812 [Dynamit Nobel Co., Ltd. trade name. Number average molecular weight: 3000, non-volatile content: 60 wt%,
Hydroxyl value: 35 mg KOH / g, glass transition point: 30 ° C. 7) Epicoat 1001 [trade name of Yuka Shell Epoxy Co., Ltd. Nonvolatile content: 60 wt%, epoxy equivalent: 450 to 50
0] 8) Epicoat 834 [trade name of Yuka Shell Epoxy Co., Ltd. Nonvolatile content: 50 wt%, epoxy equivalent: 225-28
0] 9) Epicoat 1007 [trade name of Yuka Shell Epoxy Co., Ltd. Nonvolatile content: 50 wt%, epoxy equivalent: 1750-2
700] 10) Sumimar M55 [trade name of Sumitomo Chemical Co., Ltd. Nonvolatile matter: 70 wt% butanol solution] 11) Solvesso # 150 / cyclohexanone = 50 /
50 (wt%) 12) p-toluenesulfonic acid, 20 wt% solution

Preparation Example 2 Preparation of Thermosetting Acrylic Resin Varnish 5 parts by weight of butyl cellosolve, 40 parts by weight of xylene, and 4.7 parts by weight of butyl alcohol were placed in a reaction vessel heated to 135 ° C. and 34 parts by weight of methyl methacrylate was added. , Butyl acrylate 13 parts by weight, β-hydroxyethyl methacrylate 2 parts by weight, acrylic acid 1 part by weight, cumene hydrobell oxide 0.3 part by weight was added dropwise over 3 hours,
Further, stirring was continued for 2 hours at the same temperature to obtain a thermosetting acrylic resin varnish AS1. The solid content was 50% by weight. The resin solid content had a number average molecular weight of 12,000, a hydroxyl value of 18 mgKOH / g and an acid value of 16. Preparation Example 3 Preparation of intermediate coating and top coating In accordance with the formulation shown in Table 2, first, an acrylic resin and a part of isophorone were mixed, and then a pigment was added and uniformly mixed, and then a particle size of 10 μm was obtained using an attritor. It was dispersed below. While further stirring with a disper, a powder of polyvinylidene fluoride was added and dispersed. Finally, after adding the melamine resin and the acid catalyst, the remaining isophorone was appropriately added to adjust the viscosity to 170 ± 1 with a Ford cup # 4.
Adjusted to 0 seconds (25 ° C), 4 kinds of intermediate coating paints S1 to S4
And a top-coat paint T1 were prepared. S1 to S3 are intermediate coating materials according to the method of the present invention, and S4 is an intermediate coating material for a comparative sample.

[0018]

[Table 3]

Note 13) Paraloid B-44 [trade name of Rohm and Haas Japan Co., Ltd. Nonvolatile matter: 30 wt% isophorone solution] 14) Cymel 303 [trade name of Mitsui Cyanamid Co., Ltd. Nonvolatile matter: 100%] 15) Kiner 500 [trade name of Atochem Co., Ltd. Weight average molecular weight: 350,000, melting point: 160 to 165 ° C.] Examples 1 to 10 and Comparative Examples 1 to 12 Thickness 0.6 mm, using a coating type chromate treated 5% aluminum / zinc alloy plated steel sheet as a coated plate, According to the type of coating, the number of coatings, the coating thickness and the baking temperature shown in Table 3, the undercoat coating, the intermediate coating and the top coating are sequentially applied to the coated plate, and the total of 22 in the examples and the comparative examples.
Various types of test pieces were prepared. The performance of each test piece was evaluated. The results are shown in Table 4.

[0020]

[Table 4]

[0021]

[Table 5]

[0022]

[Table 6]

[0023]

[Table 7]

[0024]

[Table 8]

As is clear from the results shown in Table 4, in Comparative Examples 1 to 12 which deviate from the method of the present invention, the test in which the occurrence of the side effect was remarkable when the paint was baked and dried was not subjected to another test. For Comparative Example test pieces other than these, although the top coating was a fluororesin coating and the accelerated weather resistance test result was generally good, in other test items, it failed in any of them, there is no satisfactory In Examples 1 to 10 according to the method of the present invention, all the test items are passed, and the manufacturing method of the present coated steel sheet which is considered to be able to meet the requirement of durability of 30 years or more is an excellent method. It is clear.

[0026]

The coated steel sheet according to the method of the present invention uses a pre-treated plated steel sheet as a substrate to be coated, and uses a specified undercoating paint and intermediate coating paint in coating, so that the total coating film thickness is 60%. The thickness can be increased to about 120 μm, and at the same time, the corrosion resistance can be significantly improved. As a result, with the conventional coated steel sheet, only a durability of about 20 years was obtained, but according to the method of the present invention, a coated steel sheet that is considered to have a durability of 30 years or more can be obtained. The method is epoch-making. In the future, maintenance will be unnecessary for a long time on the outer walls of skyscrapers, and the effect will be extremely large.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C09D 127/16 PFJ 9166-4J 133/00 PFZ 7921-4J 167/02 PKW 8933-4J (72) Inventor Kajiyama To: 138-6-103 Setagaya-cho, Hodogaya-ku, Yokohama-shi, Kanagawa Prefecture (72) Inventor Yusuke Maeda 1-7 Amigaido, Matsubara-shi, Osaka Prefecture C-810 Asahi Plaza C-810 (72) Inhei Sakamoto, Chiba Prefecture Togane-shi Kitanoyukiya 202-3 (72) Inventor Koji Kobayashi 3-20-2-4-104 Shirahata, Chuo-ku, Chiba-shi, Chiba (72) Inventor Kenji Shirakami 1795-2-B-Tamajima Otoshima, Kurashiki-shi, Okayama 201 (72) Inventor Hajime Kimura 254-2 Kitanoyukiya, Togane City, Chiba Prefecture

Claims (1)

[Claims] 1. When a primer coating, an intermediate coating and a top coating are applied on a plated steel sheet and a coating is formed by using a fluororesin type coating as the top coating, a hydroxyl value of 8 to 25 mgKOH / g as a solid content is obtained. Linear polyester resin 5
An undercoat paint containing 40 to 100 parts by weight of a pigment based on 100 parts by weight of the solid content of the color-developing agent as a main component, and a polyvinylidene fluoride resin and heat. A method for producing a coated steel sheet, which comprises using an intermediate coating composition containing a curable acrylic resin as a main component of a color-developing agent.