Method for manufacturing color steel plate

Abstract

To provide a method for manufacturing a color steel plate capable of forming a cured coating film having sufficient performance even at a low baking temperature.SOLUTION: A method for manufacturing a color steel plate includes: a step of supplying a two-liquid type urethane coating material, obtained by mixing a main agent containing a polyol component and a curing agent containing a polyisocyanate component using a two-liquid mixing device, to a roll coater and applying the two-liquid type urethane coating material to a steel plate by using the roll coater to form an uncured coating film; and a step of baking the uncured coating film to form a cured coating film.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a color steel sheet. [Background technology]

[0002] The term "color steel sheet" generally refers to a steel sheet having a coating film formed on its surface, and is sometimes called a "painted steel sheet." Note that the color steel sheet in the present invention also encompasses a steel sheet having a coating film formed on its surface that does not contain either a pigment or a dye.

[0003] In industrial production lines for color steel sheets, color steel sheets are generally produced by applying a paint to a steel sheet using a roll coater to form an uncured coating film, and then baking the uncured coating film in a heating furnace to form a cured coating film. In such industrial production lines, a one-component urethane paint (i.e., a paint containing a polyol component and a blocked polyisocyanate component) as described in Patent Document 1 has traditionally been used to form the cured coating film. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-179998 Summary of the Invention [Problem to be solved by the invention]

[0005] One reason why one-component urethane paints have been used in conventional industrial production lines for color steel sheets is their long pot life. While one-component urethane paints have the advantage of a long pot life, they have the disadvantage of requiring a high baking temperature to form a cured coating film with sufficient performance. In the examples of Patent Document 1, which use one-component urethane paint as the topcoat, the baking is carried out at 232°C for 60 seconds.

[0006] Due to recent increases in fuel costs and efforts toward carbon neutrality, there is a demand for a method for forming a cured coating film with sufficient performance even at a low baking temperature. The present invention has been made in light of these circumstances, and aims to provide a method for producing a color steel sheet that can form a cured coating film with sufficient performance even at a low baking temperature. [Means for solving the problem]

[0007] The present invention that can achieve the above object is as follows. [1] A process of using a two-component mixer to mix a base agent containing a polyol component and a curing agent containing a polyisocyanate component, thereby obtaining a two-component urethane coating material, supplying the resulting coating material to a roll coater, and applying the two-component urethane coating material to a steel plate using the roll coater to form an uncured coating film; and A step of baking the uncured coating film to form a cured coating film. A method for manufacturing a color steel sheet, comprising: [2] The method according to [1], wherein the uncured coating film is baked using a heating furnace, the atmospheric temperature of the heating furnace is 150 to 250°C, and the baking time is 20 to 80 seconds. [Effects of the Invention]

[0008] According to the method for producing a color steel sheet of the present invention, a cured coating film having sufficient performance can be formed even at a low baking temperature. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 shows a schematic cross-sectional view of a roll coater that can be used in one embodiment of the present invention. [Figure 2] FIG. 2 shows a schematic cross-sectional view of a roll coater that can be used in another embodiment of the present invention. [Figure 3] FIG. 3 shows a schematic cross-sectional view of a roll coater that can be used in another embodiment of the present invention. [Figure 4]FIG. 4 shows a schematic cross-sectional view of a roll coater that can be used in another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] One of the features of the present invention is that a cured coating film is formed using a two-component urethane paint. The cured coating film formed may be a top coat, a primer layer, or a back coat. That is, the manufacturing method of the present invention can form any of a top coat, a primer layer, and a back coat.

[0011] In this specification, the term "top coat" refers to a coating film having performance suitable for use in building materials or home appliances. Examples of building materials include roofing materials and wall materials. Examples of home appliances include refrigerators and washing machines. In this specification, the term "primer layer" refers to a coating film that ensures adhesion between the steel sheet and the top coat, or the performance of the color steel sheet (for example, corrosion resistance, workability, etc.). In this specification, the term "reverse coating" refers to a coating film formed on the surface (reverse surface) opposite to the surface (front surface) on which the top coat of a color steel sheet is formed.

[0012] In this specification, "two-component urethane paint" refers to a paint that consists of a base agent containing a polyol component and a curing agent containing a polyisocyanate component, and is used by mixing these before application, while "one-component urethane paint" refers to a paint that contains a polyol component and a blocked polyisocyanate component.

[0013] In this specification, "polyisocyanate" without the restriction of "blocked" means a polyisocyanate in which the isocyanate group (also called "isocyanato group") is not blocked with a blocking agent, while "blocked polyisocyanate" means a polyisocyanate in which the isocyanate group is blocked with a blocking agent.

[0014] Two-component urethane paints, consisting of a base agent containing a polyol component and a curing agent containing a polyisocyanate component, are more reactive than one-component urethane paints containing a polyol component and a blocked polyisocyanate component, and can form coatings with sufficient performance even at low baking temperatures. However, two-component urethane paints have a short pot life after mixing the base agent and curing agent, so they have not traditionally been used in industrial production lines for color steel sheets. In this regard, the present invention uses a two-component mixing device to mix the base agent and curing agent, and then supplies the two-component urethane paint to a roll coater immediately after mixing. This allows the two-component urethane paint to be applied to steel sheets immediately after mixing, eliminating the pot life issue.

[0015] In this specification, the term "two-component mixing device" refers to a device that can automatically measure and mix component A (e.g., the base component of a two-component urethane paint) and component B (e.g., the curing agent of a two-component urethane paint). A commercially available two-component mixing device can be used. Examples of commercially available two-component mixing devices include the XP50 Plural Component Sprayer (registered trademark) XP-h manufactured by Graco Inc.

[0016] In the present invention, the roll coater is preferably a device having multiple rolls. A roll coater that can be used in one embodiment of the present invention will be described below using Figure 1. Note that Figure 1 is a schematic cross-sectional view, and the ratio of the size between each roll and the ratio of the size of each roll to the thickness of the steel sheet do not match the ratios of an actual device.

[0017] The arrows in the metering roll 1, pickup roll 2, application roll 3, and backup roll 4 in FIG. 1 indicate the direction of rotation of each roll, and the arrow next to the steel plate 6 indicates the direction of movement of the steel plate 6.

[0018] In one embodiment of the present invention, as shown in Figure 1, a two-component urethane paint (i.e., a mixture of a base agent containing a polyol component and a curing agent containing a polyisocyanate component, obtained using a two-component mixing device) is supplied from a two-component urethane paint supply port 5 between a metering roll 1 and a pickup roll 2. In the roll coater shown in Figure 1, a backup roll 4 is a roll for supporting a steel plate 6, an application roll 3 is a roll for applying the two-component urethane paint on said roll to the steel plate 6, a pickup roll 2 is a roll for transferring the two-component urethane paint on said roll to the application roll 3, and a metering roll 1 is a roll for adjusting the appearance or thickness of the coating film.

[0019] In Figure 1, the rolls are in contact with each other. However, Figure 1 is a schematic cross-sectional view, and those skilled in the art will readily understand that in an actual roll coater, a certain amount of space is provided between, for example, the metering roll 1 and the pickup roll 2 in order to adjust the coating thickness. Furthermore, those skilled in the art will be able to appropriately set the space between the rolls and the rotation speed of each roll depending on the required coating thickness or appearance.

[0020] In one embodiment of the present invention, a two-component urethane paint can be applied to a steel sheet using a roll coater having a roll and a supply port for the two-component urethane paint as shown in Figure 1. More specifically, in one embodiment of the present invention, The machine has a metering roll 1, a pickup roll 2, an application roll 3, a backup roll 4, and a supply port 5 for two-component urethane paint. The metering roll 1 and the pickup roll 2 are adjacent to each other, the pickup roll 2 and the application roll 3 are adjacent to each other, and the application roll 3 and the backup roll 4 are adjacent to each other, A supply port 5 for two-component urethane paint is provided between the metering roll 1 and the pickup roll 2; and A roll coater is used in which the steel sheet moves between an application roll 3 and a backup roll 4. Two-component urethane paint is supplied from a supply port 5 of the two-component urethane paint between the metering roll 1 and the pickup roll 2, The two-component urethane paint on the pickup roll 2 is transferred to the application roll 3, and The two-component urethane paint on the application roll 3 can be applied to the steel plate 6 .

[0021] The supply port for supplying the two-component urethane paint to the roll of the roll coater (for example, supply port 5 for the two-component urethane paint in FIG. 1) preferably has multiple slits. By using such a supply port with multiple slits, the two-component urethane paint can be supplied, for example, in the form of a curtain, so that the paint can be supplied uniformly to the roll.

[0022] In the production method of the present invention, a roll coater equipped with a doctor blade may be used to form a coating film with a cleaner appearance. For example, a doctor blade may be installed next to the metering roll 1 in the roll coater shown in Figure 1.

[0023] A roll coater equipped with a doctor blade that can be used in one embodiment of the present invention will be described below with reference to Fig. 2. In Fig. 2, the same components as in Fig. 1 are designated by the same reference numerals. The arrows in Fig. 2 are the same as those in Fig. 1. Fig. 2 is a schematic cross-sectional view, similar to Fig. 1, and the size ratio between the rolls and the like do not match the ratio of an actual device.

[0024] In the roll coater shown in Figure 2, a doctor blade 7 is adjacent to the metering roll 1. This doctor blade 7 smooths the appearance of the two-component urethane paint (i.e., the uncured coating film) supplied onto the metering roll 1. Also, as shown in Figure 2, two-component urethane paint can be supplied between the doctor blade 7 and the metering roll 1 from a supply port 5 for the two-component urethane paint. More specifically, in one embodiment of the present invention, The apparatus has a metering roll 1, a pickup roll 2, an application roll 3, a backup roll 4, a supply port 5 for two-component urethane paint, and a doctor blade 7. The doctor blade 7 and the metering roll 1 are adjacent to each other, the metering roll 1 and the pickup roll 2 are adjacent to each other, the pickup roll 2 and the application roll 3 are adjacent to each other, and the application roll 3 and the backup roll 4 are adjacent to each other, A supply port 5 for two-component urethane paint is provided between the doctor blade 7 and the metering roll 1; and A roll coater is used in which the steel sheet moves between an application roll 3 and a backup roll 4. Two-component urethane paint is supplied from a supply port 5 of the two-component urethane paint between the doctor blade 7 and the metering roll 1, The two-component urethane paint on the pickup roll 2 is transferred to the application roll 3, and The two-component urethane paint on the application roll 3 can be applied to the steel plate 6 .

[0025] In addition, shielding plates may be attached to the sides of the doctor blade 7 and the metering roll 1 to prevent the two-component urethane paint supplied to the doctor blade 7 and the metering roll 1 from overflowing from the sides of these.

[0026] In the manufacturing method of the present invention, a roll coater without a metering roll may be used. Hereinafter, a roll coater without a metering roll used in one embodiment of the present invention will be described with reference to Figures 3 and 4. In Figures 3 and 4, the same reference numerals as in Figures 1 and 2 are used for the same components. The arrows in Figures 3 and 4 are the same as those in Figure 1. Similar to Figure 1, Figures 3 and 4 are schematic cross-sectional views, and the size ratios between the rolls and the like do not match the ratios of the actual device.

[0027] In one embodiment of the present invention, a roll coater having a coating pan 8 can be used, as shown in Figure 3. More specifically, in one embodiment of the present invention, a roll coater having a pickup roll 2, an application roll 3, a backup roll 4, a supply port 5 for a two-component urethane coating material, and a coating pan 8 is used. The pickup roll 2 and the application roll 3 are adjacent to each other, and the application roll 3 and the backup roll 4 are adjacent to each other. A supply port 5 for two-component urethane paint is provided so as to supply two-component urethane paint to the paint pan 8; and A roll coater is used in which the steel sheet moves between an application roll 3 and a backup roll 4. Two-component urethane paint is supplied from a supply port 5 to a paint pan 8, The two-component urethane paint supplied to the paint pan 8 is picked up onto the pickup roll 2, The two-component urethane paint on the pickup roll 2 is transferred to the application roll 3, and The two-component urethane paint on the application roll 3 can be applied to the steel plate 6 .

[0028] In the production method of the present invention, a roll coater equipped with a doctor blade may be used to form a coating film with a cleaner appearance. For example, a doctor blade may be installed next to the pickup roll 2 in the roll coater shown in Figure 3.

[0029] In one embodiment of the present invention, as shown in Figure 4, a roll coater can be used in which a doctor blade 7 is provided next to the application 2 and a supply port 5 for the two-component urethane paint is provided between the doctor blade 7 and the pickup roll 2. More specifically, in one embodiment of the present invention, The apparatus has a pickup roll 2, an application roll 3, a backup roll 4, a supply port 5 for two-component urethane paint, and a doctor blade 7. The doctor blade 7 and the pickup roll 2 are adjacent to each other, the pickup roll 2 and the application roll 3 are adjacent to each other, and the application roll 3 and the backup roll 4 are adjacent to each other, A supply port 5 for two-component urethane paint is provided between the doctor blade 7 and the pickup roll 2; and A roll coater is used in which the steel sheet moves between an application roll 3 and a backup roll 4. Two-component urethane paint is supplied from a supply port 5 of the two-component urethane paint between the doctor blade 7 and the pickup roll 2, The two-component urethane paint on the pickup roll 2 is transferred to the application roll 3, and The two-component urethane paint on the application roll 3 can be applied to the steel plate 6 .

[0030] In addition, to prevent the two-component urethane paint supplied to the doctor blade 7 and pickup roll 2 from overflowing from the sides thereof, shielding plates may be attached to the sides of the doctor blade 7 and metering roll 1.

[0031] It is preferable to apply a two-component urethane paint to a steel sheet so that the desired thickness of the cured coating film after drying (after baking) is obtained. When the cured coating film formed by the production method of the present invention is a top coat, its thickness after drying (after baking) is preferably 5 to 30 μm, more preferably 10 to 25 μm. When the cured coating film formed by the production method of the present invention is a primer layer, its thickness after drying (after baking) is preferably 2 to 25 μm, more preferably 3 to 15 μm. When the cured coating film formed by the production method of the present invention is a back coat, its thickness after drying (after baking) is preferably 2 to 15 μm, more preferably 3 to 10 μm.

[0032] In industrial production lines for color steel sheets, baking of the uncured coating film is usually carried out using a heating furnace. The atmospheric temperature of the heating furnace is preferably 150 to 250°C, more preferably 160 to 230°C, and even more preferably 170 to 220°C. The baking time of the uncured coating film is preferably 20 to 80 seconds, more preferably 20 to 70 seconds, and even more preferably 30 to 60 seconds; here, "baking time of the uncured coating film" means the time the uncured coating film remains in the heating furnace.

[0033] The line speed of an industrial production line for color steel sheets (ie, the moving speed of the steel sheets) is usually 30 to 150 m / min, and preferably 50 to 120 m / min.

[0034] After forming the top coat, which is a hardened coating film, by baking, the resulting color steel sheet is preferably cooled, more preferably water-cooled. Water-cooling can be performed using, for example, a water shower or mist.

[0035] Next, we will explain the two-component urethane paint that can be used in the present invention. Below, "two-component urethane paint for forming a top coat" may be abbreviated as "top coat paint," "two-component urethane paint for forming a primer layer" as "primer paint," and "two-component urethane paint for forming a back coat" as "back coat paint." Note that below, the explanation of "two-component urethane paint, its base agent or curing agent" without the limitation of "top coat, primer, or back coat" is common to top coat paint, primer paint, and back coat paint.

[0036] The base material of the two-component polyurethane paint contains a polyol component. Only one type of polyol component may be used, or two or more types may be used in combination. Examples of polyol components include polyester polyols, polyether polyols, silicone-modified polyester polyols, and fluorine-based polyols. From the viewpoints of weather resistance and processability, the polyol component contained in the base material of the top coat paint is preferably a polyester polyol. From the viewpoints of adhesion, processability, or corrosion resistance, the polyol component contained in the base material of the primer paint is preferably a polyester polyol or an epoxy polyol. From the viewpoints of adhesion, processability, or corrosion resistance, the polyol component contained in the base material of the back coat paint is preferably a polyester polyol or an epoxy polyol.

[0037] Examples of carboxylic acids for forming polyester polyols include succinic acid, adipic acid, sebacic acid, dimer acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid, etc. To form polyester polyols, only one type of carboxylic acid may be used, or two or more types may be used in combination.

[0038] Examples of alcohols for forming polyester polyols include ethylene glycol, propylene glycol, diethylene glycol, neopentyl glycol, trimethylolpropane, glycerin, etc. To form polyester polyols, only one type of alcohol may be used, or two or more types may be used in combination.

[0039] As the polyester polyol, polycaprolactone polyol obtained by ring-opening polymerization of ε-caprolactone using a polyhydric alcohol can also be used.

[0040] From the viewpoint of the viscosity of the coating material or the ease of coating, the number average molecular weight of the polyester polyol is preferably 2,500 to 7,000, more preferably 3,000 to 5,000. This number average molecular weight can be measured by GPC (gel permeation chromatography).

[0041] Polyester polyols are often added in the form of a solution to the base material of two-component polyurethane coatings. From the viewpoint of reactivity or water resistance, the hydroxyl value of the polyester polyol solution is preferably 30 to 120 (mgKOH / g), more preferably 50 to 100 (mgKOH / g).

[0042] The main component of a two-component polyurethane paint usually contains a solvent in addition to a polyol component. Examples of the solvent include xylol, aromatic hydrocarbons, cyclohexanone, and isophorone. The solvent may be used alone or in combination of two or more.

[0043] The solid content of the base agent is preferably 30 to 80% by weight, more preferably 40 to 70% by weight, based on the total weight of the base agent, from the viewpoint of the viscosity of the paint or the ease of application.

[0044] The base material of a two-component polyurethane paint may contain other components in addition to the polyol component and the solvent. Examples of other components contained in the base material of a top coat paint include pigments, aggregates, matting agents, defoaming agents, surface conditioners, scratch prevention agents, curing catalysts, and curing inhibitors. Examples of other components contained in the base material of a primer paint include rust prevention pigments, pigments, aggregates, matting agents, defoaming agents, scratch prevention agents, curing catalysts, and curing inhibitors. Examples of other components contained in the base material of a back coat paint include rust prevention pigments, pigments, aggregates, matting agents, defoaming agents, surface conditioners, scratch prevention agents, curing catalysts, and curing inhibitors. Those skilled in the art can appropriately determine the content of other components in the base material depending on the intended use of the component.

[0045] In order to promote the urethanization reaction between the polyol component and the polyisocyanate component, the base material of the two-component polyurethane coating preferably contains a curing catalyst. Curing catalysts for the urethanization reaction are well known and commercially available. An example of a commercially available curing catalyst is "K-KAT5218" manufactured by King Industries, Inc.

[0046] The base material of the two-component polyurethane coating may contain a cure inhibitor to extend the pot life after mixing the base material with the curing agent. Examples of cure inhibitors include acetylacetone.

[0047] The base agent of the two-component polyurethane paint preferably contains a combination of aluminum bis(ethylacetoacetate) monoacetylacetonate (e.g., "K-KAT5218" manufactured by King Industries, Inc.) and acetylacetone in order to simultaneously accelerate the urethanization reaction and extend the pot life after mixing the base agent and curing agent.

[0048] The curing agent for the two-component polyurethane coating contains a polyisocyanate component. The polyisocyanate component may be used alone or in combination of two or more. Examples of polyisocyanate components include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 2,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, xylylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 1,6-hexamethylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, isophorone diisocyanate, and 1,4-cyclohexane diisocyanate. Among these, 1,6-hexamethylene diisocyanate (HMDI) and isophorone diisocyanate (IPDI) are preferred from the viewpoint of weather resistance or heat yellowing resistance.

[0049] As the polyisocyanate component, a modified product of the above-mentioned diisocyanate (e.g., alcohol-modified diisocyanate), a condensate of the above-mentioned diisocyanate (e.g., biuret (dimer), isocyanurate (trimer)), or a modified product of the condensate (e.g., alcohol-modified isocyanurate), or an adduct of the above-mentioned diisocyanate may be used.

[0050] The curing agent for two-component polyurethane paints usually contains a solvent in addition to the polyisocyanate component. Examples of the solvent include methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), ethyl acetate, butyl acetate, xylol, etc. One type of solvent may be used alone, or two or more types may be used in combination.

[0051] The solid content of the curing agent is preferably 30 to 80% by weight, more preferably 40 to 70% by weight, based on the total weight of the curing agent from the viewpoints of handling and coating workability. The NCO content of the curing agent is preferably 5 to 30% by weight, more preferably 10 to 20% by weight, based on the total weight of the curing agent from the viewpoints of workability and cost efficiency.

[0052] The base material and curing agent of a two-component polyurethane coating are preferably mixed so that the molar ratio of hydroxy groups in the base material to isocyanate groups in the curing agent is within a desired range. The molar ratio of hydroxy groups in the base material to isocyanate groups in the curing agent is preferably 100:70 to 100:130, more preferably 100:80 to 100:120, and most preferably about 100:100.

[0053] Next, the steel sheet that can be used in the present invention will be described. Examples of steel sheets include cold-rolled steel sheets, hot-dip galvanized steel sheets, electrogalvanized steel sheets, aluminum-zinc alloy-plated steel sheets, aluminum-plated steel sheets, stainless steel sheets, copper-plated steel sheets, and tin-plated steel sheets. The thickness of the steel sheet is preferably 0.1 to 2.0 mm, and more preferably 0.25 to 1.6 mm. [Example]

[0054] The present invention will be described in more detail below with reference to the following Reference Examples, but the present invention is not limited to these. Note that "%" and "parts" used below mean "% by weight" and "parts by weight" unless otherwise specified.

[0055] <Comparative Reference Example 1 and Reference Examples 1 to 4> Comparative Reference Example 1 used a one-component urethane paint (HD6000 U White (1PC)), and Reference Examples 1 to 4 used two-component urethane paints (top coat paints) consisting of a base agent (HD6000 U White (2PC)) and a curing agent (HMDI-based curing agent or IPDI-based curing agent). The base agent and curing agent of the two-component urethane paints (top coat paints) were mixed manually, not with a two-component mixing device. The paints were applied to the steel plate using a bar coater, not with a roll coater.

[0056] The steel sheet used was a commercially available galvalume steel sheet (aluminum-zinc alloy plated steel sheet, steel sheet thickness: 0.35 mm, Al content in the plating: 55%, Zn content in the plating: 45%, plating weight on both sides: 150 g / m 2 ) was used.

[0057] A primer layer was formed by applying "Precolor Primer HP32" manufactured by Akzo Nobel Coatings Co., Ltd. to the surface of the steel plate and drying it (thickness of the primer layer after drying: 5 μm). Furthermore, "Precolor No. 7000" manufactured by Akzo Nobel Coatings Co., Ltd. was applied to the rear surface of the steel sheet and dried to form a rear surface coating (thickness of rear surface coating after drying: 8 μm). A one-component urethane paint or two-component urethane paint was applied to the surface of the steel plate (the surface on which the primer layer was formed) and dried as follows to form a top coat.

[0058] The one-component urethane paint and two-component urethane paint (top coat paint) used were both manufactured by Akzo Nobel Coatings Co., Ltd. Aggregates, matting agents, antifoaming agents, scratch prevention agents, curing catalysts, and curing inhibitors were added to these paints. The amounts of components in the one-component urethane paint (HD6000 U White (1PC)) and the main component of the two-component urethane paint (top coat paint) (HD6000 U White (2PC)) used are shown in Table 1. The components of these paints are as follows: Pigment: Titanium dioxide Polyester polyol solution: Polyester polyol solution manufactured by Akzo Nobel Coatings Co., Ltd. (carboxylic acid components constituting the ester: isophthalic acid, phthalic anhydride, and adipic acid; alcohol components constituting the ester: neopentyl glycol, trimethylolpropane, pentaerythritol, and 1,6-hexanediol; number average molecular weight of polyester polyol: 4,000; hydroxyl value of polyester polyol solution: 50 (mgKOH / g); solvent: cyclohexanone, xylol, aromatic hydrocarbons) Blocked polyisocyanate: Asahi Kasei Corporation's "Duranate TPA-B 80 E" (HMDI isocyanurate type) Aggregate: Sekisui Plastics Co., Ltd. “Techpolymer MBX-30” Matte finish: "SYLYSIA 310P" manufactured by Fuji Silysia Chemical Ltd. Antifoaming agent: "Miki Leveling MK Concentrate" manufactured by Kyoeisha Chemical Co., Ltd. Scratch prevention agent: Kitamura Co., Ltd. "PTFE Wax KTL-8N" Diluents (=solvents): cyclohexanone, xylol, aromatic hydrocarbons Curing catalyst: King Industries, Inc. "K-KAT5218" (aluminum bis(ethylacetoacetate) monoacetylacetonate) Curing inhibitor: acetylacetone

[0059] The 1,6-hexamethylene diisocyanate (HMDI) curing agent or isophorone diisocyanate (IPDI) curing agent used in the two-component urethane paint (top coat paint) was both obtained from Akzo Nobel Coatings Co., Ltd. The solids content and NCO content of these curing agents are shown in Table 2.

[0060] When a two-component urethane paint (top coat paint) was used, the base agent and curing agent were mixed manually in the amounts shown in Table 3 so that the molar ratio of hydroxy groups in the base agent to isocyanate groups in the curing agent was 100:100.

[0061] A one-component urethane paint or two-component urethane paint (top coat paint) was applied to the steel sheet using a bar coater so that the thickness of the cured coating (top coat) after drying (baking) would be 18 μm, and the cured coating was formed by baking for 55 seconds at the baking temperature shown in Table 3 using an automatic discharge dryer manufactured by Tojo Nessaku Co., Ltd., to produce a color steel sheet. The performance of the obtained color steel sheet was evaluated as follows. The evaluation results are shown in Table 3.

[0062] (1) Curability A rubbing test was carried out using a rubbing tester (solvent: MEK, load: 1 kg), and the curability was evaluated according to the following criteria. (standard) ◯: The primer layer was not exposed after 100 round trips ×: The primer layer was exposed after less than 100 reciprocal strokes

[0063] (2) Processability The resulting color steel sheet was bent in a T shape with the cured coating facing up and pressed with a press, after which a cellophane tape peeling test was carried out on the bent portion and the processability was evaluated according to the following criteria. (standard) ◯: The cured coating film did not peel off at all ×: The cured coating film peeled off.

[0064] (3) Blocking property Two color steel sheets were stacked together so that their front surfaces (i.e., the surfaces on which the top coat was formed) and back surfaces (i.e., the surfaces on which the back coat was formed) were in contact with each other, and the resulting laminate was applied to the cured coating film at 40°C with a pressure of 40 kg / cm. 2 The laminate was pressed using a hot press so that a load of 1000 kJ / cm2 was applied, and the pressurized state was maintained for 24 hours. After that, the pressure was stopped and the laminate was cooled to room temperature, and then the blocking property was evaluated according to the following criteria. (standard) 〇: Two color steel plates peeled off without any problems ×: The two color steel sheets adhered together and were difficult to peel off.

[0065] [Table 1]

[0066] [Table 2]

[0067] [Table 3]

[0068] As is clear from the results shown in Table 3, in Comparative Reference Example 1, which used a one-component urethane paint, the performance (curability, processability, and anti-blocking properties) of the cured coating film was insufficient when the ambient temperature was as low as 215°C. On the other hand, in Reference Examples 1 to 4, which used a two-component urethane paint (top coat paint), it was possible to form a cured coating film with excellent performance even at an ambient temperature as low as 155 to 215°C. [Industrial Applicability]

[0069] According to the method for producing a color steel sheet of the present invention, a cured coating film (top coat, primer layer, or back coat) with sufficient performance can be formed even at a low baking temperature. The color steel sheet obtained by the production method of the present invention is useful in various fields such as home appliances and building materials. [Explanation of symbols]

[0070] 1 Metering Roll 2 Pickup Roll 3 Application Roles 4. Backup Role 5 Supply port for two-component urethane paint 6 steel plate 7 Doctor Blade 8 paint pans

Claims

1. a step of using a two-component mixer to mix a base agent containing a polyol component and a curing agent containing a polyisocyanate component, supplying the resulting two-component urethane coating material to a roll coater, and applying the two-component urethane coating material to a steel plate using the roll coater to form an uncured coating film; and A step of baking the uncured coating film to form a cured coating film. A method for manufacturing a color steel sheet, comprising:

2. 2. The method according to claim 1, wherein the uncured coating film is baked using a heating furnace, the atmospheric temperature of the heating furnace is 150 to 250° C., and the baking time is 20 to 80 seconds.

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