Painted metal plate and its manufacturing method

Abstract

To provide a method for manufacturing a coated metal plate having a coated film containing a fluororesin which is difficult to be changed in workability over time, and is difficult to generate a pressure mark in a coated film during winding and storage.SOLUTION: A method for manufacturing a coated metal plate includes the steps of: coating a fluororesin-based coating material containing a fluororesin onto a metal plate, and forming a coated film; heating the coated film to 200°C or higher, and curing the coated film; cooling the cured coated film; and re-heating the cooled coated film to 55°C or higher and 140°C or lower.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a coated metal sheet and a method for producing the same. [Background technology]

[0002] Coated metal sheets generally have excellent durability, weather resistance, and design properties, and are suitable for use, for example, as exterior building materials. Among coated metal sheets for exterior building materials, coated metal sheets having a coating film containing a fluororesin are suitable for coated metal sheets that require long-term durability.

[0003] However, coatings containing fluororesins (for example, coatings containing polyvinylidene fluoride) can be highly crystalline and have poor processability. Therefore, it has been proposed to incorporate not only fluororesins but also thermoplastic acrylic resins into coatings (Patent Documents 1 to 3). It has also been proposed to suppress the crystallization of polyvinylidene fluoride by rapidly cooling the coating after curing it (Patent Document 4), thereby improving the processability of coated metal sheets.

[0004] On the other hand, coating films containing polyvinylidene fluoride with improved processability have very low hardness immediately after film formation. Therefore, there is a problem in that when the coated metal sheet is wound on a roll or the like, the coating film is prone to develop indentations (hereinafter also referred to as "pressure marks"). To prevent such pressure marks, it has been proposed to form a soft polyester coating film with a glass transition temperature of 10 to 20°C on the back surface of the coated metal sheet (Patent Document 5). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 45-9662 [Patent Document 2] Special Publication No. 3-11266 [Patent Document 3] Japanese Patent Application Publication No. 2-174977 [Patent Document 4] Japanese Patent Application Publication No. 114846 / 1983 [Patent Document 5] Japanese Patent Application Laid-Open No. 2008-87242 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the hardness of a coating film in which the crystallinity of polyvinylidene fluoride is adjusted as in Patent Documents 1 to 4 gradually increases over several months after production. Therefore, even if the pressure marks caused by winding can be suppressed by the method of Patent Document 5, there is a problem that the processability differs greatly between immediately after production and after a certain time has passed since production, making it difficult to handle.

[0007] Therefore, the present invention provides a coated metal sheet having a coating film containing a fluororesin, which is less likely to change in processability over time and is less likely to produce pressure marks or the like in the coating film, and a method for producing the same. [Means for solving the problem]

[0008] The present invention provides the following method for producing a coated metal sheet. [1] A method for producing a coated metal plate, comprising the steps of applying a fluororesin-based paint containing a fluororesin onto a metal plate to form a coating film, heating the coating film to 200°C or higher to harden it, cooling the hardened coating film, and reheating the cooled coating film to a temperature of 55°C or higher and 140°C or lower.

[0009] [2] The method for producing a coated metal sheet according to [1], wherein the step of reheating the coating film is a step of subjecting the coating film to flame treatment and / or high-frequency induction heating. [3] The method for producing a coated metal sheet according to [1] or [2], wherein the metal sheet is a metal strip, and further comprises a step of winding the coated metal sheet onto a roll after reheating the coating film.

[0010] [4] The method for producing a coated metal sheet according to any one of [1] to [3], wherein the step of cooling the cured coating film is a step of water-cooling the coating film. [5] The method for producing a coated metal sheet according to any one of [1] to [4], wherein in the step of cooling the cured coating film, the cooling rate of the coating film is 600 to 20° C. / second.

[0011] The present invention provides the following coated metal sheet. [6] A metal plate and a coating film containing a fluororesin disposed on the metal plate, wherein the infrared spectrum of the coating film has a wavelength of 840 cm -1 The absorbance of the β-type crystals at a wavelength of 794 cm -1 The ratio of absorbance derived from α-type crystals in the vicinity is 0.5 or less, and the wavelength determined by the two-point base method is 789 cm -1 The absorbance of α, β, and γ crystals at a wavelength of 840 cm -1 A coated metal sheet having an absorbance ratio of 0.4 or more derived from beta crystals in the vicinity. [Effects of the Invention]

[0012] According to the present invention, there are provided a method for producing a coated metal sheet having a coating film containing a fluororesin, which coating film is less likely to change in processability over time and is less likely to produce pressure marks on the coating film during winding, storage, etc., and a method for producing the same. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the temperature of the coating film in the reheating step and the X-ray diffraction pattern of the coating film after the reheating step. [Figure 2] Figure 2A is an SEM photograph of a coating film that has not been subjected to a reheating process, Figure 2B is an SEM photograph of a coating film that has been reheated to 130°C, and Figure 2C is an SEM photograph of a coating film that has been reheated to 150°C. [Figure 3] FIG. 3 is a graph showing the relationship between the coating temperature in the reheating step and the composite elastic modulus of the coating after the reheating step. DETAILED DESCRIPTION OF THE INVENTION

[0014] 1. Manufacturing method of painted metal sheets The manufacturing method of the present invention produces a coated metal sheet having a coating film containing a fluororesin. As mentioned above, when a coating film containing a fluororesin is produced on a metal sheet using conventional methods, the hardness is low immediately after production, and pressure marks are easily formed when the sheet is wound or stacked. On the other hand, the hardness of a coating film containing a fluororesin increases over time. Therefore, the ease of processing differs between when the sheet is processed immediately after production and when it is processed several months after production. Therefore, it has been necessary to adjust the processing conditions in some cases.

[0015] Generally, fluororesin becomes amorphous when cooled after coating and hardening, but over time, the fluororesin crystallizes. As a result, the hardness of the coating film is very low immediately after production, making it prone to pressure marks. However, as the fluororesin crystallizes, pressure marks become less likely to occur, but the processability of the coating film decreases.

[0016] In contrast, the manufacturing method of the present invention involves the steps of applying a fluororesin-based paint containing a fluororesin onto a metal plate to form a coating film (coating film forming step), heating the coating film to 200°C or higher to harden it (hardening step), cooling the hardened coating film (cooling step), and reheating the cooled coating film to a temperature of 55°C or higher and 140°C or lower (reheating step).

[0017] Through intensive research by the present inventors, it has been found that there are at least three types of fluororesin crystals: α-type crystals, β-type crystals, and γ-type crystals. Increasing the amount of β-type crystals reduces the likelihood of pressure marks and changes in the workability of the coating film. When a coating film containing fluororesin contains a large amount of β-type crystals, pressure marks are less likely to occur, even when the coating film is wound up immediately after production. Furthermore, in a coating film containing a large amount of β-type crystals, α-type crystals and γ-type crystals are less likely to form over time, stabilizing the performance of the coating film. In other words, the workability of the coated metal sheet remains good and does not change over time, making it extremely easy to handle. Furthermore, since γ-type crystals are obtained by heat-treating α-type crystals at high temperatures, they are thought to rarely appear in the manufacturing method of the present invention.

[0018] Furthermore, the inventors' investigations have revealed that by applying a fluororesin coating material, curing it, and then sufficiently cooling the coating film and then performing a reheating process, the growth of β-type crystals of the fluororesin is selectively promoted. In other words, simply performing the cooling and reheating processes can stabilize the performance of the coating film and suppress the occurrence of pressure marks. Therefore, there is also the advantage that large-scale equipment and complicated processing are not required.

[0019] The correlation between the temperature of the reheating process and the crystalline structure of the fluororesin is supported by the following verification results. A fluororesin-based paint made from a 70 / 30 mixture of polyvinylidene fluoride and an acrylic resin with a weight-average molecular weight of 100,000 and an acid value of 3 mg / g was applied to a metal plate, cured at 200°C or higher, and cooled to 30°C. The paint was then reheated so that the maximum temperature of the paint film reached was 80°C, 130°C, or 150°C. X-ray diffraction was then performed on the reheated paint film. X-ray diffraction patterns at each reheating temperature are shown in Figure 1. For comparison, the X-ray diffraction pattern of a paint film that was not reheated is also shown in Figure 1.

[0020] Figure 1 confirms that the closer the maximum temperature reached by the coating film during reheating is to 130°C, the more likely it is that beta crystals of the fluororesin will become dominant. The data also confirms that when the coating film temperature during reheating reaches 150°C (over 140°C), the beta crystals disappear and alpha crystals appear. The reason beta crystals disappear above 140°C is thought to be because the coating film softens at temperatures above 140°C, causing the beta crystals to transition to the more stable alpha crystals.

[0021] 2A to 2C show SEM (scanning electron microscope) photographs of a coating film that was not reheated (unreheated), a coating film that was reheated to 130°C, and a coating film that was reheated to 150°C. As shown in FIGS. 2A and 2B, no clear crystalline structure was observed in the coating film that was not reheated and in the coating film that reached a maximum temperature of 130°C in the reheating process. In contrast, a lamellar structure due to α-type crystals was observed in the coating film that reached a maximum temperature of 150°C in the reheating process.

[0022] In a coating film with a lamellar structure, as shown in Figure 2C, there is a mixture of high-strength and low-strength areas due to differences in the structure. Therefore, although the occurrence of pressure marks is suppressed by the high-strength areas, when processing a coating film in which alpha-type crystals of fluororesin are dominant, a local load is applied to the boundary between the high-strength and low-strength areas, which can cause breakage.

[0023] In contrast, a coating film in which β-type crystals of fluororesin are dominant, as shown in Figure 2B, has a uniform structure, making it less likely for strength variations to occur within the coating film. Therefore, a coating film in which β-type crystals are dominant is less likely to be subjected to localized loads during processing, making it less likely to break. Furthermore, because the strength of the coating film is uniform, it can be said that pressure marks are also less likely to occur.

[0024] In the unreheated coating film shown in Figure 2A, although the lamellar structure shown in Figure 2C is not formed, it can be said that the amount of β-type crystals is not sufficiently large based on the results of the X-ray diffraction pattern described above, etc. Therefore, it is difficult to obtain sufficient strength with such a coating film.

[0025] Figure 3 shows a graph illustrating the relationship between the maximum temperature reached during the reheating process and the composite modulus of the coating film. As shown in Figure 3, when the temperature during the reheating process is set to 55°C or higher, the composite modulus of the coating film increases. This is thought to be because heating to 55°C or higher during the reheating process selectively grows β-type crystals of the fluororesin. Meanwhile, the composite modulus of the coating film reached its maximum around 130°C and dropped significantly at 150°C. It is thought that the amount of α-type crystals began to become dominant above 130°C, and at 150°C, α-type crystals became overwhelmingly dominant. The composite modulus in Figure 3 was measured using a nanoindenter under the following conditions. The composite modulus was calculated as the average of five measurements taken on the same coating film.

[0026] Composite Elasticity Measuring Device HYSITRON TI Premier Multi Scale Measurement conditions) Indenter: Berkovich Load control measurement conditions: Maximum load 300μN Loading and unloading time 5 seconds Maximum load holding time 5 seconds

[0027] Each step in the method for producing a coated metal sheet of the present invention will be described below, but other steps may also be included as long as they do not impair the object and effect of the present invention.

[0028] (Coating film formation process) The coating film forming step is a step of applying a fluororesin-based paint containing a fluororesin to a metal plate to form a coating film. The method for applying the fluororesin-based paint is not particularly limited, and known methods can be used, including roll coating, curtain flow coating, spray coating, and dip coating. The amount of fluororesin-based paint applied is appropriately adjusted depending on the desired thickness of the fluororesin layer. For example, when the coated metal plate is used as an exterior building material, it is preferable to adjust the amount of application so that the thickness of the coating film after curing (after the reheating step) is 3 to 30 μm. If the thickness of the coating film formed in the coating film forming step is too thin, the durability and hiding power of the resulting coating film may be insufficient. On the other hand, if the thickness of the coating film formed in the coating film forming step is too thick, popping may occur in the curing step described below.

[0029] The fluororesin-based paint should contain at least a fluororesin and have fluidity that allows it to be applied to a desired position on a metal plate. In addition to the fluororesin, the fluororesin-based paint may also contain a curing agent, a (meth)acrylic resin, inorganic particles, organic particles, coloring pigments, wax, an organic solvent, a hydrophilizing agent, etc. In this specification, the fluororesin refers to a resin that contains fluorine atoms in its structure. The fluororesin is preferably a polyvinylidene fluoride-based fluororesin, but the fluororesin is not limited to this. The fluororesin-based paint may contain only one type of fluororesin, or may contain two or more types of fluororesin.

[0030] The polyvinylidene fluoride resin may be polyvinylidene fluoride (PVDF), which is a homopolymer of 1,1-difluoroethylene (hereinafter also referred to as "vinylidene fluoride"), or may be a copolymer of 1,1-difluoroethylene and another monomer. However, the polyvinylidene fluoride resin preferably contains 50 mol % or more, and more preferably 60 mol % or more, of a monomer derived from vinylidene fluoride relative to the total amount of monomers constituting the polyvinylidene fluoride resin.

[0031] Examples of monomers copolymerizable with vinylidene fluoride include fluoroolefins, vinyl ethers, vinyl esters, etc. The polyvinylidene fluoride resin may contain only one type of structure derived from a monomer other than vinylidene fluoride, or may contain two or more types.

[0032] When the fluororesin is a polyvinylidene fluoride resin, its weight-average molecular weight is preferably 100,000 or more, more preferably 200,000 or more, and even more preferably 400,000 or more. On the other hand, the weight-average molecular weight is preferably 1,300,000 or less, more preferably 1,000,000 or less. When the weight-average molecular weight of the polyvinylidene fluoride resin is within this range, the compatibility of the polyvinylidene fluoride resin with other components in the fluororesin paint is improved, and a coating film with high strength can be obtained. The weight-average molecular weight is a value measured by gel permeation chromatography (styrene equivalent value).

[0033] The amount of fluororesin (polyvinylidene fluoride resin) is preferably 30 to 100 parts by mass, more preferably 40 to 90 parts by mass, and even more preferably 50 to 80 parts by mass, relative to 100 parts by mass of the total resin solids content of the fluororesin-based paint. The resin solids content of the fluororesin-based paint refers to components excluding volatile components such as solvents, pigments, UV absorbers, light stabilizers, wax, and matting agents such as resin beads, mica, and silica. When the amount of fluororesin in the fluororesin-based paint is within this range, the resulting coating film is likely to have good long-term durability.

[0034] The fluororesin-based paint may contain a curing agent together with the fluororesin. When a curing agent is contained together with the fluororesin, a crosslinked structure is easily formed, and the resulting coating film tends to be tougher. Examples of curing agents include isocyanate-based curing agents, aminoplast-based curing agents, polybasic acid-based curing agents, polyamine-based curing agents, etc. The fluororesin-based paint may contain only one type of curing agent, or may contain two or more types.

[0035] Examples of isocyanate curing agents include polyisocyanate compounds and their blocked products, modified polyisocyanate compounds, and polymers of polyisocyanate compounds. Polyisocyanate compounds are compounds having two or more isocyanate groups. Examples of polyisocyanate compounds include aliphatic polyisocyanate compounds such as ethylene diisocyanate, propylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, hexamethylene triisocyanate, and lysine diisocyanate; alicyclic polyisocyanate compounds such as isophorone diisocyanate, dicyclohexylmethane diisocyanate, and diisocyanate methylcyclohexane; and aromatic isocyanate compounds such as m-xylene diisocyanate and p-xylene diisocyanate. Examples of modified polyisocyanate compounds include urethane modified compounds, urea modified compounds, isocyanurate modified compounds, biuret modified compounds, allophanate modified compounds, carbodiimide modified compounds, and the like.

[0036] Examples of aminoplast curing agents include methylol melamines, methylol guanamines, methylol ureas, etc. Examples of methylol melamines include methylol melamines etherified with lower alcohols such as butylated methylol melamine and methylated methylol melamine; epoxy-modified methylol melamine; etc. Examples of methylol ureas include alkylated methylol ureas such as methylated methylol urea and ethylated methylol urea.

[0037] Examples of polybasic acid curing agents include long-chain aliphatic dicarboxylic acids, aromatic polycarboxylic acids, and the like, and may also be acid anhydrides thereof. Examples of polyvalent amine curing agents include ethylenediamine, ethylenetriamine, and the like.

[0038] The fluororesin-based paint preferably contains 0.1 to 100 parts by mass, and more preferably 1 to 50 parts by mass, of the curing agent per 100 parts by mass of the fluororesin. When the amount of curing agent per 100 parts by mass of the fluororesin is 0.1 part by mass or more, the hardness of the coating film tends to be increased. On the other hand, when the amount of curing agent per 100 parts by mass of the fluororesin is 100 parts by mass or less, the processability and impact resistance of the coating film tend to be good.

[0039] From the viewpoint of increasing the fluidity of the fluororesin-based coating material and improving the adhesion between the resulting coating film and the metal plate, the fluororesin-based coating material preferably further contains a (meth)acrylic resin. The (meth)acrylic resin may be thermoplastic or thermosetting. In this specification, (meth)acrylic refers to methacrylic or acrylic, or both.

[0040] Examples of thermoplastic (meth)acrylic resins include polymers containing 70 mol% or more of monomers derived from (meth)acrylic acid alkyl esters relative to the total amount of monomer units constituting the (meth)acrylic resin. Examples of (meth)acrylic acid alkyl esters include polymers having an alkyl group with 3 to 12 carbon atoms, more specifically, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, amyl (meth)acrylate, octyl (meth)acrylate, etc. The thermoplastic (meth)acrylic resin may contain only one type of structure derived from (meth)acrylic acid alkyl ester, or may contain two or more types.

[0041] Furthermore, the thermoplastic (meth)acrylic resin may have a structure derived from a monomer other than the (meth)acrylic acid alkyl ester, and examples thereof include styrene, vinyl toluene, (meth)acrylonitrile, vinyl chloride, and the like.

[0042] The weight-average molecular weight of the thermoplastic (meth)acrylic resin contained in the fluororesin-based coating material is preferably 40,000 to 300,000, more preferably 50,000 to 200,000. The weight-average molecular weight is a value (styrene equivalent) measured by GPC. When the weight-average molecular weight of the thermoplastic (meth)acrylic resin is within this range, the fluidity of the fluororesin-based coating material is increased, and the adhesion between the resulting coating film and the metal sheet is likely to be improved.

[0043] The amount of thermoplastic (meth)acrylic resin in the fluororesin paint is preferably 150 parts by mass or less, more preferably 10 to 50 parts by mass, per 100 parts by mass of the fluororesin. When the fluororesin paint contains the thermoplastic (meth)acrylic resin in this range, the fluidity of the fluororesin paint tends to be good.

[0044] On the other hand, examples of thermosetting (meth)acrylic resins include (meth)acrylic resins having crosslinkable reactive groups such as hydroxyl groups, carboxyl groups, glycidyl groups, active halogens, isocyanate groups, etc. The thermosetting (meth)acrylic resins are used together with curing agents such as alkylated melamine, polyols, polyamines, polyamides, and polyoxiranes.

[0045] The weight-average molecular weight of the thermosetting (meth)acrylic resin contained in the fluororesin-based coating material is preferably 1,000 to 20,000, more preferably 2,000 to 10,000. The weight-average molecular weight of the thermosetting (meth)acrylic resin is a value (styrene equivalent) measured by GPC. When the weight-average molecular weight of the thermosetting (meth)acrylic resin is within this range, the fluidity of the fluororesin-based coating material is increased, and the adhesion between the resulting coating film and the metal sheet is likely to be improved.

[0046] The amount of thermosetting (meth)acrylic resin in the fluororesin-based paint is preferably 150 parts by mass or less, more preferably 10 to 50 parts by mass, per 100 parts by mass of the fluororesin. When the thermosetting (meth)acrylic resin is contained in this range, the fluidity and other properties of the paint tend to be good.

[0047] The fluororesin-based coating material may further contain inorganic particles or organic particles. When the fluororesin-based coating material contains these, the surface roughness of the resulting coating film can be adjusted. The average particle size of the inorganic or organic particles is preferably 4 to 80 μm, more preferably 10 to 60 μm. The average particle size of the inorganic or organic particles is a value measured by the Coulter counter method. The shape of the inorganic or organic particles is not particularly limited, but a roughly spherical shape is preferred from the viewpoint of making it easier to adjust the surface condition of the resulting coating film.

[0048] Examples of inorganic particles include silica, barium sulfate, talc, calcium carbonate, mica, glass beads, glass flakes, etc. Examples of organic particles include resin beads made of acrylic resin or polyacrylonitrile resin. These resin beads may be produced by known methods or may be commercially available. Examples of commercially available acrylic resin beads include "Tuftic AR650S (average particle size 18 μm)," "Tuftic AR650M (average particle size 30 μm)," "Tuftic AR650MX (average particle size 40 μm)," "Tuftic AR650MZ (average particle size 60 μm)," and "Tuftic AR650ML (average particle size 80 μm)," all manufactured by Toyobo Co., Ltd. Examples of commercially available polyacrylonitrile resin beads include "Tuftic A-20 (average particle size 24 μm)," "Tuftic YK-30 (average particle size 33 μm)," "Tuftic YK-50 (average particle size 50 μm)," and "Tuftic YK-80 (average particle size 80 μm)," manufactured by Toyobo Co., Ltd.

[0049] The amount of inorganic particles and / or organic particles contained in the fluororesin-based paint is appropriately selected depending on the desired surface condition of the coating film, etc. Usually, the total amount of inorganic particles and / or organic particles is preferably 1 to 40 parts by mass per 100 parts by mass of the solid content of the fluororesin-based paint.

[0050] Furthermore, the fluororesin-based paint may contain a coloring pigment. The coloring pigment preferably has an average particle size of, for example, 0.2 to 2.0 μm. Examples of such coloring pigments include titanium oxide, iron oxide, yellow iron oxide, phthalocyanine blue, carbon black, cobalt blue, etc. When the fluororesin-based paint contains a coloring pigment, the amount thereof is preferably 20 to 60 parts by mass, more preferably 30 to 55 parts by mass, per 100 parts by mass of the solid content of the fluororesin-based paint.

[0051] Furthermore, the fluororesin-based paint may contain wax. Examples of wax include, but are not limited to, polyolefin-based waxes such as polyethylene and polypropylene; fluorine-based waxes such as polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, and polyvinyl fluoride; paraffin-based wax; and stearic acid-based wax. The amount of wax is appropriately selected depending on the type of wax, but is preferably about 2 to 15 parts by mass per 100 parts by mass of the solid content of the fluororesin-based paint.

[0052] The fluororesin-based paint may contain an organic solvent as needed. The organic solvent is not particularly limited as long as it can sufficiently dissolve or disperse each component contained in the fluororesin-based paint. Examples of organic solvents include hydrocarbon solvents such as toluene, xylene, Solvesso® 100 (trade name, manufactured by ExxonMobil Corp.), Solvesso® 150 (trade name, manufactured by ExxonMobil Corp.), and Solvesso® 200 (trade name, manufactured by ExxonMobil Corp.); ketone-based solvents such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and isophorone; ester-based solvents such as ethyl acetate, butyl acetate, ethylene glycol monoethyl ether acetate, and dimethyl phthalate; alcohol-based solvents such as methanol, isopropyl alcohol, and n-butyl alcohol; and ether-alcohol-based solvents such as ethylene glycol monoethyl ether and diethylene glycol monobutyl ether. The fluororesin-based paint may contain only one of these, or two or more. Among these, isophorone, xylene, ethylbenzene, cyclohexanone, and dimethyl phthalate are preferred from the viewpoint of compatibility with the fluororesin.

[0053] The fluororesin-based paint may further contain a hydrophilizing agent. When the fluororesin-based paint contains a hydrophilizing agent, the hydrophilizing agent orients on the surface of the coating film, imparting resistance to rain streaks to the coating film. Examples of hydrophilizing agents include alkoxysilanes such as tetramethoxysilane and tetraethoxysilane, and silicone resins. Among these, silicone resins are preferred because they are easily orientated on the surface of the coating film and easily increase the hydrophilicity of the coating film surface.

[0054] In this specification, silicone resin refers to a compound obtained by partial hydrolysis and condensation of alkoxysilane, which is a polymer that is mainly composed of a three-dimensional crosslinked structure but does not reach the stage of gelation and is soluble in organic solvents. The three-dimensional crosslinked structure contained in the silicone resin is not particularly limited, and may be, for example, a cage structure, a ladder structure, or a random structure. In this specification, tetraalkoxysilane and condensates (organosilicates) obtained by hydrolysis and condensation of only tetraalkoxysilane are not included in the silicone resin.

[0055] The structure of the silicone resin is not particularly limited, and can be a homopolymer of trialkoxysilane, or a copolymer of trialkoxysilane with tetraalkoxysilane and / or dialkoxysilane, etc. However, the amount of Si atoms derived from trialkoxysilane relative to the total molar amount of Si atoms contained in the silicone resin is preferably 50 to 100 mol%, more preferably 60 to 100 mol%. When the proportion of Si atoms derived from trialkoxysilane is 50 mol% or more, the silicone resin tends to be uniformly concentrated on the coating film surface. As a result, the coating film has very good resistance to rain streaks. The amount of Si atoms derived from trialkoxysilane is 29 It can be identified by Si-NMR analysis.

[0056] The amount (molar number) of silanol groups in the silicone resin is preferably 5 to 50 mol% relative to the total molar amount of Si atoms, more preferably 15 to 40 mol%. If the amount of silanol groups exceeds 50 mol% relative to the total molar amount of Si atoms, the reactivity of the silicone resin becomes too high and it becomes more likely to polymerize in the fluororesin-based paint. On the other hand, if the amount of silanol groups is less than 5 mol% relative to the total molar amount of Si atoms, the silicone resin will not easily interact with other components in the fluororesin-based paint, and it will be more likely to detach from the paint film, which may result in insufficient rain streak resistance.

[0057] Here, the weight-average molecular weight of the silicone resin is preferably 700 to 50,000, more preferably 1,000 to 10,000. If the weight-average molecular weight of the silicone resin is less than 700, the silicone resin will be more likely to evaporate during the curing process described below, contaminating the heating device and reducing the amount of silicone resin in the coating film. On the other hand, if the weight-average molecular weight exceeds 50,000, the viscosity of the fluororesin-based coating material will increase, making it difficult to form a coating film. The weight-average molecular weight of the silicone resin is the polystyrene equivalent measured by gel permeation chromatography (GPC).

[0058] The amount of the hydrophilizing agent is preferably 1 to 10 parts by mass, more preferably 2 to 7 parts by mass, per 100 parts by mass of the solid content of the fluororesin-based coating material.

[0059] The fluororesin-based paint can be prepared by mixing the fluororesin and other components and stirring or dispersing the mixture.

[0060] On the other hand, the metal sheet to which the fluororesin-based paint is applied in the coating film formation step can be selected from known metal sheets as long as the effects of the present invention can be obtained. Examples of the metal sheet include cold-rolled steel sheet, zinc-plated steel sheet, Zn-Al alloy-plated steel sheet, Zn-Al-Mg alloy-plated steel sheet, aluminum-plated steel sheet, stainless steel sheet (including austenitic, martensitic, ferritic, and ferritic-martensite dual-phase), aluminum sheet, aluminum alloy sheet, and copper sheet.

[0061] The metal sheet is preferably a plated steel sheet or a stainless steel sheet from the viewpoints of corrosion resistance and weight reduction, and more preferably a plated steel sheet from the viewpoint of cost-effectiveness. Furthermore, the metal sheet is preferably a hot-dip 55% Al-Zn alloy-plated steel sheet, a Zn-Al-Mg alloy-plated steel sheet, or an aluminum-plated steel sheet from the viewpoint of further improving corrosion resistance. Among these, a zinc-plated steel sheet is preferred, and a magnesium-containing zinc-plated steel sheet, such as a Zn-Al-Mg alloy-plated steel sheet, is more preferred. The metal sheet may have a chemical conversion coating, a primer coating, or the like formed on its surface, provided that the effects of the present invention are not impaired. Furthermore, the metal sheet may be subjected to a roughening process, such as embossing or drawing, provided that the effects of the present invention are not impaired.

[0062] The thickness of the metal sheet can be appropriately determined based on the intended use of the coated metal sheet, etc. For example, when the intended use of the coated metal sheet is an exterior building material, the thickness of the metal sheet can be 0.2 to 1.6 mm.

[0063] The metal sheet may be in the form of a sheet, but is preferably in the form of a band, i.e., a metal band, from the viewpoint of the effect of being able to suppress pressure marks. When the metal sheet is a metal band, the coating film forming step may be carried out while unwinding the metal sheet wound around a roll.

[0064] (hardening process) In the curing step, the coating film of the fluororesin paint formed in the coating film formation step is heated to 200°C or higher to cure and baked onto the metal plate. The method for heating the coating film is not particularly limited, and known heating devices can be used. From the viewpoint of production efficiency of the coated metal plate, it is preferable to carry out the curing step on the same line as the coating film formation step.

[0065] The heating temperature should be 200° C. or higher, but from the viewpoint of obtaining a uniform coating film, 240 to 300° C. is more preferable. The heating time is appropriately selected depending on the thickness of the coating film, etc., but from the viewpoint of obtaining a uniform coating film, 3 to 90 seconds is preferable, 10 to 70 seconds is more preferable, and 20 to 60 seconds is even more preferable.

[0066] In order to cure the coating film uniformly in a shorter time, air may be blown onto the plate surface at a velocity of 0.9 m / s or more simultaneously with the heating.

[0067] (cooling process) In the cooling step, the coating film after the curing step is cooled. If the coating film temperature after the cooling step is 50°C or higher and the reheating step is performed in this state, α-type crystals are likely to grow during the reheating step. Furthermore, if the coating film is stored for a long period of time at a temperature of 50°C or higher after the cooling step, α-type crystals will also grow, and processability will likely decrease. Therefore, in the cooling step, it is preferable to cool the coating film so that its temperature is less than 50°C, and it is also preferable to keep the coating film at a temperature less than 50°C after the cooling step and until the reheating step (during storage). The lower the coating film temperature after the cooling step, the smaller the ratio of the α-type crystal amount to the β-type crystal amount of the fluororesin in the coating film, i.e., the ratio (α-type crystal amount / β-type crystal amount). Furthermore, the smaller the (α-type crystal amount / β-type crystal amount) after the cooling step, the more likely β-type crystals will grow during the reheating step described below. Furthermore, the above ratio of the α-type crystal amount / β-type crystal amount is maintained after the cooling step and until the reheating step, i.e., during coating film storage. Therefore, the temperature of the coating film from the cooling step to the reheating step is preferably less than 50°C, and more preferably less than 30°C. At this time, the coating film may be cooled as necessary. The cooling step is preferably carried out on the same line as the coating film formation step and curing step.

[0068] The method for cooling the coating film in the cooling step is not particularly limited, and may be, for example, air cooling, water cooling, natural cooling, contact with a cooling member, or a combination of these, but water cooling is preferred from the viewpoint of cooling efficiency. By shortening the cooling time, the production line can be shortened. The cooling rate of the coating film in the cooling step is preferably 600 to 20°C / sec, and more preferably 400 to 50°C / sec. The temperature drop rate of the coating film from the start to the end of the cooling step may be constant, or may be changed intermittently or continuously.

[0069] (Reheating process) In the reheating step, the coating film after the cooling step is reheated to a temperature of 55°C or higher and 140°C or lower. The reheating step is a step in which the temperature of the coating film is increased from the temperature at the end of the cooling step. By heating the coating film to a temperature of 55°C or higher and 140°C or lower, energy is imparted to the fluororesin in the coating film, and the amount of β-type crystals of the fluororesin increases, as described above. From the viewpoint of production efficiency of coated metal sheets, it is preferable that the reheating step be performed on the same line as the coating film formation step, curing step, and cooling step. In this specification, the coating film temperature in the reheating step refers to the maximum temperature reached by the coating film during the reheating step.

[0070] The method for reheating the coating film is not particularly limited, and examples include flame treatment, infrared heating, high-frequency induction heating, etc. However, among these, flame treatment and high-frequency induction heating are particularly preferred. In infrared heating, the infrared absorption rate varies depending on the color of the coated metal sheet, so the irradiation amount must be controlled depending on the color of the coated metal sheet. For example, when a coated metal sheet with a white coating film and a coated metal sheet with a black coating film are irradiated with the same amount of infrared light from a halogen lamp, the white coated metal sheet reaches 70°C, while the black coated metal sheet reaches 120°C. In contrast, flame treatment is not affected by the color of the coated metal sheet and allows for easy temperature control. Another advantage of flame treatment is that it does not require large-scale equipment or complex processing.

[0071] On the other hand, the energy efficiency of high-frequency induction heating is over 70%, compared to around 30% for flame treatment and infrared heating. Therefore, high-frequency induction heating is preferable from the viewpoint of energy efficiency. Furthermore, high-frequency induction heating is particularly effective when the metal plate is a steel plate, which is a magnetic material. When high-frequency induction heating a steel plate (metal plate), which is a magnetic material, is performed, rapid heating is possible and temperature controllability and responsiveness are also superior compared to high-frequency induction heating of non-magnetic materials such as copper, aluminum, and austenitic stainless steel.

[0072] When a coated metal sheet having a thickness of 0.2 mm or more and 1.6 mm or less is heated by high-frequency induction heating, it is preferable to heat it at a frequency of 50 kHz or more. This frequency allows the coated metal sheet to be heated efficiently. In this case, to reduce temperature unevenness in the sheet width direction, particularly overheating of the coil width direction edge portion, the frequency of the high-frequency induction heating is preferably 100 kHz or more and 400 kHz or less, and more preferably 150 kHz or more and 400 kHz or less. Frequencies exceeding 400 kHz are suitable for coated metal sheets having a thickness of less than 0.2 mm, but may be unsuitable for coated metal sheets having a thickness of 0.2 mm or more due to the current penetration depth being too shallow.

[0073] Here, the temperature of the coating film in the reheating step may be 55°C or higher and 140°C or lower, more preferably 70 to 130°C, and even more preferably 80 to 120°C. By maintaining the coating film temperature at 55°C or higher, the β-crystallization of the fluororesin in the coating film is more likely to be promoted, as described above, and the processability of the coating film is less likely to change over time. Meanwhile, the general recrystallization temperature of fluororesin is approximately 160°C. Therefore, by maintaining the heating temperature at 140°C or lower, excessive α-crystallization of the fluororesin can be suppressed, and a decrease in the processability of the coating film can be suppressed. Furthermore, by maintaining the coating film temperature at 140°C or lower in the reheating step, the coated metal sheet can be wound onto a roll without a separate cooling step after the reheating step.

[0074] In particular, when the coating film is heated by flame treatment, if the coating film temperature is set to 65°C or higher, the water vapor generated by the combustion of the combustible gas used in the flame treatment is less likely to condense, and the coating film temperature can be raised uniformly. Furthermore, by setting the coating film temperature to 130°C or lower, excessive α-crystallization of the fluororesin can be more reliably suppressed.

[0075] The temperature of the coating film during the reheating process may be measured directly by measuring the temperature of the coating film surface using a contact or non-contact sensor. However, when reheating is performed by flame treatment, the presence of a flame on the coating film side makes it difficult to measure the temperature of the coating film directly. Therefore, in this case, the temperature of the metal plate on the side opposite the coating film may be measured as the temperature of the coating film during the reheating process (flame treatment). Because metal plates have a high heat transfer rate, the temperature of the surface of the metal plate facing the coating film and the temperature of the opposite surface are usually the same. Therefore, the temperature of the metal plate on the side opposite the coating film can be treated as the temperature of the coating film. The temperature of the metal plate may be measured using a contact temperature sensor, but it is preferable to measure it using a non-contact temperature sensor because it is possible to measure the temperature without damaging the metal plate.

[0076] The heating time of the coating film during the reheating step is appropriately selected depending on the heating method, but is usually preferably 5 minutes or less, more preferably 2 minutes or less, and particularly preferably 10 seconds or less. The coating film after the reheating step is preferably allowed to cool at a cooling rate of 5 to 60°C / min. In this case, β-type crystals grow during cooling. On the other hand, if the coating film is rapidly cooled by water cooling or the like after the reheating step, the heating time during the reheating step must be 0.5 to 5 minutes to ensure time for the β-type crystals to grow. Heating for this period of time allows the β-type crystals of the fluororesin in the coating film to selectively grow, as described above. The heating time refers to the time the coating film is held at its maximum temperature. If the heating time is 10 seconds or less, coated metal sheets can be produced particularly efficiently.

[0077] The reheating method using flame treatment will be described in detail below. When the reheating step is performed using flame treatment, a flame is emitted from a flame treatment device while the coated metal sheet after the cooling step is moved in a certain direction.

[0078] The flame treatment device can be an apparatus including a gas supply unit for supplying combustible gas, a burner head for burning the combustible gas supplied from the gas supply unit, and a support member for supporting these. The burner head usually has a substantially rectangular prism-shaped housing connected to the gas supply unit and a flame port located on the bottom surface of the housing, and burns the combustible gas supplied from the gas supply unit in the flame port.

[0079] The internal structure of the burner head housing can be the same as that of a general flame treatment burner, and may include, for example, a flow path for flowing the combustible gas supplied from the gas supply unit to the flame nozzle. The width of the flame nozzle in the direction perpendicular to the transport direction may be equal to or greater than the width of the coating film to be flame treated, and can be, for example, about 50 to 150 cm. On the other hand, the width of the flame nozzle in the transport direction of the coating film can be appropriately set depending on the discharge stability of the combustible gas, and can be, for example, about 1 to 8 mm. The burner head may also have an auxiliary flame nozzle parallel to the flame nozzle. When the burner head includes an auxiliary flame nozzle, the flame can travel more linearly, ensuring reliable flame treatment of the coating film.

[0080] The gas supply unit is a gas flow path connected to the burner head on one side and to the gas mixing unit on the other side. The gas mixing unit is connected to a combustion gas supply source such as a combustion gas cylinder and a combustion supporting gas supply source such as an air cylinder, an oxygen cylinder, compressor air, or air from a blower, and is a component for premixing the combustion gas and the combustion supporting gas. Note that it is preferable that the concentration of oxygen in the combustible gas (mixture of combustion gas and combustion supporting gas) supplied from the gas mixing unit to the gas supply unit is constant, and it is preferable that the gas mixing unit is equipped with an oxygen supplier for supplying oxygen to the gas supply unit as needed.

[0081] Examples of the combustion gas include hydrogen, liquefied petroleum gas (LPG), liquefied natural gas (LNG), acetylene gas, propane gas, and butane. Among these, LPG or LNG is preferred, and LPG is particularly preferred, from the viewpoint of ease of forming a desired flame. On the other hand, examples of the combustion supporting gas include air or oxygen, and air is preferred from the viewpoint of ease of handling.

[0082] The mixture ratio of combustion gas and combustion supporting gas in the combustible gas supplied to the burner head via the gas supply unit can be appropriately set depending on the types of combustion gas and combustion supporting gas. For example, when the combustion gas is LPG and the combustion supporting gas is air, the volume of air is preferably 24 to 27 parts by volume, more preferably 25 to 26 parts by volume, and even more preferably 25 to 25.5 parts by volume, per 1 part by volume of LPG. Furthermore, when the combustion gas is LNG and the combustion supporting gas is air, the volume of air is preferably 9.5 to 11 parts by volume, more preferably 9.8 to 10.5 parts by volume, and even more preferably 10 to 10.2 parts by volume, per 1 part by volume of LNG.

[0083] The distance between the end of the burner head facing the coating film and the coating film is selected appropriately depending on the desired amount of flame treatment, but can usually be about 10 to 120 mm, preferably 25 to 100 mm, and more preferably 30 to 90 mm. If the distance between the burner head and the coating film is too close, the coating film may come into contact with the burner head due to warping of the metal plate, etc. On the other hand, if the distance between the burner head and the coating film is too far, a large amount of energy is required for flame treatment. During flame treatment, the flame may be emitted from the flame treatment burner perpendicular to the coating film surface, or it may be emitted from the flame treatment burner at a certain angle to the coating film surface.

[0084] Furthermore, the conveying speed of the coating film (coated metal sheet) during the reheating process is appropriately selected. If the reheating process is performed on the same line as the above-mentioned coating film forming process, curing process, cooling process, etc., the conveying speed is adjusted to match the conveying speed of the coated metal sheet (or metal sheet) during these processes. The conveying speed of the coating film (metal sheet) is usually preferably 10 to 150 m / min, more preferably 20 to 120 m / min, and even more preferably 30 to 100 m / min. By moving the coating film at a speed of 10 m / min or more, the flame treatment can be performed efficiently and the temperature of the coating film can be raised to the desired range. On the other hand, if the moving speed of the coating film is too fast, air currents are likely to be generated by the movement of the coating film, and the temperature of the coating film may not be raised sufficiently.

[0085] The amount of flame treatment is selected appropriately depending on the temperature of the coating film after the cooling step, but is usually 30 to 1000 kJ / m 2 is preferred, and 100 to 600 kJ / m 2 is more preferable. In this specification, the term "flame treatment amount" refers to the amount of heat per unit area of the coated metal sheet calculated based on the supply amount of combustion gas such as LP gas. The flame treatment amount can be adjusted by the distance between the burner head of the flame treatment burner and the coating surface, the conveying speed of the coating, etc. A flame treatment amount of 30 kJ / m 2 If the flame treatment amount is less than 1000 kJ / m, it may be difficult to raise the temperature of the coating film to the above range, or uneven treatment may occur. 2 If the temperature exceeds this limit, the temperature of the coating film may become too high, or the coating film may oxidize and turn yellow.

[0086] Furthermore, if the temperature of the coating film (coated metal sheet) after the cooling step is low, a preheating treatment may be performed before flame treatment, in which the coating film surface is heated to 40°C or higher. However, it is preferable to perform this preheating treatment only for a short period of time before flame treatment, i.e., within a range that does not affect the crystallinity of the fluororesin. When a flame is applied to a coating film formed on the surface of a metal sheet with high thermal conductivity (e.g., a metal sheet with thermal conductivity of 10 W / mK or higher), the water vapor generated by the combustion of the combustible gas is cooled and turns into water (liquid), which temporarily accumulates on the surface of the coating film. This water then absorbs the energy during flame treatment and turns into water vapor, which can hinder flame treatment. In contrast, by preheating the coating film surface (metal sheet), the generation of water during flame treatment can be suppressed and the temperature can be sufficiently raised to the desired temperature.

[0087] By using high-frequency induction heating as a method of preheating the coating and combining it with flame treatment, it is possible to impart pressure mark resistance to the coating, and by flame treatment, to make the coating hydrophilic and easy to adhere, all while saving energy.

[0088] The means for preheating the coating film is not particularly limited and is selected appropriately depending on the shape of the coated metal sheet. For example, if the coated metal sheet is strip-shaped, a known heating device may be placed between the cooling device that performs the cooling process and the frame processing device. On the other hand, if the coated metal sheet is in sheet form, a heating device generally called a drying oven may be used. For example, a batch-type drying oven (also called a "safe oven") may be used, and specific examples include a low-temperature incubator manufactured by Isuzu Motors (Model Mini Catalina MRLV-11), an automatic discharge dryer manufactured by Tojo Thermal Engineering Co., Ltd. (Model ATO-101), and a simple explosion-proof dryer manufactured by Tojo Thermal Engineering Co., Ltd. (Model TNAT-1000).

[0089] (winding process) If the coated metal sheet is in a strip shape, a winding step may be further carried out after the reheating step, in which the coated metal sheet is wound up on a roll or the like. The winding step may be carried out on the same line as the above-mentioned reheating step, etc. In the present invention, since the temperature of the coating film after the reheating step is 140°C or less, the sheet can be wound up without a separate cooling step. However, if necessary, a separate cooling step (second cooling step) may be carried out. Note that when the second cooling step is carried out, the cooling rate is preferably slower than the cooling rate in the above-mentioned cooling step, from the viewpoint of not rapidly cooling (amorphizing) the fluororesin in the coating film resin; for example, air cooling or natural cooling is preferred.

[0090] (Cushion layer forming process) In the method for producing a coated metal sheet of the present invention, a cushion layer forming step may be carried out in which a cushion layer is formed on the surface of the metal sheet opposite to the surface on which the coating film is formed. The cushion layer forming step may be carried out before the coating film forming step, or after the coating film forming step, curing step, cooling step, etc., or may be carried out simultaneously with the coating film forming step, curing step, etc. It is preferable that the cushion layer forming step be carried out at a timing that does not affect the crystallinity of the fluororesin.

[0091] When a cushion layer is provided on the surface of the metal sheet opposite to the surface on which the coating film is formed, the fluororesin-containing coating film and the cushion layer are laminated when the coated metal sheet is rolled up or stacked and stored. In this case, the cushion layer relieves stress on the fluororesin-containing coating film, making it less likely that pressure marks will occur in the fluororesin-containing coating film. The cushion layer can be produced, for example, by applying and curing a polyester resin-based paint.

[0092] The polyester resin paint may be a paint containing, for example, a polyester resin, a solvent, a crosslinking agent, and other components as required.

[0093] Examples of polyester resins include polycondensates of dibasic acids (linear monomers) such as terephthalic acid and dihydric alcohols (linear monomers) such as 1,6-hexanediol. The weight-average molecular weight of the polyester resin is preferably 6,000 to 10,000. When the weight-average molecular weight of the polyester resin is within this range, the flexibility of the resulting cushion layer is likely to be increased, and pressure marks are less likely to occur in the coating film.

[0094] Examples of solvents include xylene, isobutyl alcohol, n-butyl alcohol, propylene glycol monomethyl ether, cyclohexane, DBE (dibasic acid ester), ketones, aromatic hydrocarbons, etc. Examples of crosslinking agents include melamine resin. Examples of other ingredients include color pigments (inorganic pigments and organic pigments), fillers, surface conditioners, curing catalysts, etc.

[0095] Furthermore, the method for applying the polyester resin-based paint is not particularly limited, and examples thereof include spraying, bar coater, roller curtain coater, curtain flow coater, roll coater, etc. Furthermore, the method for curing the coating film of the polyester resin-based paint is not particularly limited, and examples thereof include drying and curing in a hot air drying oven and drying and curing in an induction heating device. The thickness of the cushion layer is preferably 5 to 15 μm. Furthermore, the glass transition temperature of the cushion layer obtained by this process is preferably 10 to 20°C. When the glass transition temperature of the cushion layer is within this range, it is easy to relieve stress applied to the coated metal sheet. The glass transition temperature of the cushion layer can be adjusted by, for example, the type of polyester resin in the polyester resin-based paint.

[0096] (effect) As described above, the manufacturing method of the present invention involves the above-mentioned coating film formation step, curing step, cooling step, and reheating step. In the manufacturing method of the present invention, after forming a coating film containing a fluororesin, the reheating step promotes the growth of β-type crystals of the fluororesin in the coating film to an extent that the workability of the coated metal sheet can be maintained. Promoting β-type crystallization of the fluororesin reduces the likelihood of pressure marks occurring even when the coating film is wound up immediately after production. Furthermore, since α-type crystallization of the fluororesin in the coating film is less likely to progress over time, the performance of the coating film is stable. In other words, the workability of the coated metal sheet remains good and does not change over time, making it extremely easy to handle.

[0097] Furthermore, in the present invention, the reheating step can be performed by flame treatment and / or high-frequency induction heating, and no special equipment or complicated processing is required. Therefore, according to the production method of the present invention, a high-quality coated metal sheet can be easily produced.

[0098] 2. Painted metal sheets The coated metal sheet produced by the above method has a metal sheet and a coating film containing a fluororesin disposed on the metal sheet. -1 The absorbance of the β-type crystals at a wavelength of 794 cm -1 The ratio of absorbance derived from α-crystals in the vicinity of the wavelength (absorbance derived from α-crystals / absorbance derived from β-crystals) is 0.5 or less. This ratio is more preferably 0.3 or less. The smaller this ratio is, the fewer α-crystals and more β-crystals there are in the fluororesin coating film. Therefore, the lower limit is not particularly limited and may be 0. In addition, the wavelength determined by the two-point base method is 789 cm -1 The wavelength of 840cm for absorbance from α, β, and γ crystals in the vicinity -1The ratio of absorbance derived from β-type crystals near the α-type crystals (absorbance derived from β-type crystals / absorbance derived from α-, β-, and γ-type crystals) is 0.4 or more, and the metal sheet has sufficient β-type crystals. Therefore, as described above, it is possible to obtain a coated metal sheet in which processability is unlikely to change over time and pressure marks are unlikely to occur in the coating film during winding, storage, etc. In this specification, the absorbance derived from α-, β-, and γ-type crystals refers to the absorbance derived from all types of fluororesin, including α-, β-, and γ-type crystals.

[0099] The infrared spectrum of the coating film can be obtained using an infrared spectrophotometer such as Nicolet / iN10MX manufactured by Thermo Scientific under the following conditions. Measurement method: Microscope-ATR method (using germanium prism) Measurement area: 4000cm -1 ~600cm -1 Resolution: 4cm -1 Number of times accumulated: 128 Aperture: 100μm x 100μm [Example]

[0100] 1. First Example [Preparing the metal plate] Plate thickness: 0.27 mm, A4 size (210 mm x 297 mm), plating weight per side: 75 g / m 2 A hot-dip Zn-55%Al alloy-plated steel sheet was prepared, and both sides of the sheet were degreased with alkali. Then, a paint-type chromate treatment solution (NRC300NS manufactured by Nippon Paint Co., Ltd.) was applied to the alloy-plated steel sheet to a Cr deposition amount of 50 mg / m 2 Furthermore, an epoxy resin primer paint (800P manufactured by Nippon Fine Coatings Co., Ltd.) was applied to the surface using a roll coater so that the cured film thickness was 5 μm. The plated steel sheet was then baked so that its maximum temperature reached 215°C, thereby obtaining a plated steel sheet (hereinafter simply referred to as "plated steel sheet") having a primer coating film formed thereon.

[0101] [Preparation of fluororesin-based paint] A fluororesin-based paint was prepared by mixing polyvinylidene fluoride resin (Penwalt Japan, product name: Kynar 500, weight-average molecular weight 650,000, melting point 160-165°C) with thermoplastic acrylic resin (polymer of methyl (meth)acrylate, weight-average molecular weight 10,0000). The blending ratio of polyvinylidene fluoride resin to thermoplastic acrylic resin was 70 / 30 (by mass). Additionally, 50 parts by weight of titanium oxide, a small amount of coloring pigment, and a small amount of matting agent (silica) were added to 100 parts by weight of the solids content of the fluororesin-based paint (excluding components that volatilize when applied to a coating). This resulted in a matte fluororesin-based paint with a gloss of 60° and a gloss of 12.

[0102] [Coating film formation process, curing process, and cushion layer formation process] The above-mentioned fluororesin paint was applied to the above-mentioned plated steel sheet (metal sheet) using a roll coater so that the cured film thickness was 20 μm, and baked for 60 seconds at a maximum sheet temperature of 260°C and a sheet surface air speed of 0.9 m / s. On the other hand, a polyester paint with a glass transition temperature of 20°C ("NSC833-20" manufactured by Nippon Paint Industrial Coatings Co., Ltd., 60°C gloss 30) was used on the back surface, adjusted so that the film thickness after curing was 5 μm, and cured simultaneously with the fluororesin paint to form a cushion layer.

[0103] [Cooling process and reheating process] After the coating was cured (baked), it was cooled to 40°C with water at a rate of 150°C / sec, and then cooled to 20-40°C. The cooled coating was then subjected to flame treatment. The burner used for flame treatment was an F-3000 manufactured by Flynn Burner (USA). The combustible gas used was a mixed gas (LP gas:clean dry air (volume ratio) = 1:25) made by mixing LP gas (combustion gas) and clean dry air in a gas mixer. The flow rate of each gas was adjusted to 1 cm from the burner's flame nozzle. 2The flow rates were adjusted so that the LP gas (combustion gas) was 1.67 L / min and the clean dry air was 41.7 L / min. The length of the burner head's flame nozzle in the direction of film transport was 4 mm. Meanwhile, the length of the burner head's flame nozzle in the direction perpendicular to the film transport direction was 450 mm. Furthermore, the distance between the burner head's flame nozzle and the film surface was 30 mm depending on the desired flame treatment amount. Furthermore, by adjusting the film transport speed, the flame treatment amount could be increased to 100 to 600 kJ / m. 2 The temperature of the coating film during the flame treatment (reheating step) was adjusted to 55°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, and 150°C. The temperature of the metal plate was measured from the coating film side containing the fluororesin with a non-contact thermometer, and this was taken as the temperature of the coating film surface.

[0104] [evaluation] The coated metal sheets prepared by the above method were subjected to the following evaluations of processing stability, pencil coating hardness, and pressure mark acceleration tests. As a comparative example, the same tests were also carried out on coated metal sheets that had not been subjected to flame treatment (reheating step).

[0105] (Processing stability evaluation) The resulting coated metal sheets were stored in a constant temperature and humidity chamber at 23°C and 50% relative humidity for six months. The T-bend workability of each test piece was then tested in the high-temperature, high-humidity chamber every month, starting immediately after production (month 0). Specifically, a metal plate of the same thickness as the test piece was sandwiched inside the test piece and bent 180° (one sheet for 1T, two sheets for 2T, and six sheets for 6T). The test pieces were positioned with the coating facing outward. The bent portion of the bent test piece was then observed and evaluated using a 10x magnifying glass. The T-bend workability limit at which no cracks were observed in the coating was determined to be the workability level. The results are shown in Table 1 below. A better result is one in which workability did not decrease compared to no reheating (flame treatment) and there was no significant change in workability over six months.

[0106] [Table 1]

[0107] (Pencil coating hardness evaluation) The resulting coated metal sheets were subjected to pencil hardness tests every month, starting immediately after production (0 months). Pencil hardness evaluation was performed in accordance with JIS K5600 General Testing Methods for Paints. The hardness of the coating was measured by scratching the surface of the coating with a pencil lead, and expressed as the hardness of the pencil that did not scratch the coating. The results are shown in Table 2. A pencil hardness equal to or higher than the pencil hardness measured after 6 months without reheating (flame treatment) is considered a better result.

[0108] [Table 2]

[0109] (Pressure Mark Accelerated Test) Two samples of coated metal plates cut to a size of 10 cm x 10 cm were stacked on top of each other. Then, the stack was pressed in a press at 4.9 MPa (50 kg / cm). 2 ) was applied for 24 hours. Then, the samples were peeled off one by one, and the appearance of the coating film of each sample was visually observed and evaluated according to the following criteria. ○ and △ are within the range where there is no problem in practical use. ○: No pressure mark occurred △: A slight pressure mark occurred ×: Pressure mark occurred

[0110] [Table 3]

[0111] [result] As shown in Table 1 above, when the reheating step (flame treatment) was not performed, the change in foldability between immediately after production and three months after production was significant. In other words, when attempting precise processing, the number of months since production must be taken into consideration, which poses a problem of difficult handling. In contrast, when reheating to 50 to 140°C by flame treatment, the change in foldability over time is reduced. In particular, when reheating to 90 to 140°C, the physical properties immediately after production and six months after production were essentially the same. On the other hand, when reheating to a temperature higher than 140°C (150°C in the comparative example) by flame treatment, the foldability decreased and the pencil hardness became too high, resulting in a decrease in the workability of the coated metal sheet. It is believed that performing the reheating step to keep the coating temperature at 140°C or below caused the fluororesin in the coating to become β-type crystals, making it less likely for α-type crystals to form over time.

[0112] Furthermore, as shown in Table 2 above, when the reheating step (flame treatment) was not performed, the pencil hardness improved over several months, whereas by performing the flame treatment, the pencil hardness was able to be sufficiently high immediately after production. It is believed that by performing the reheating step so that the temperature of the coating film was 140°C or less, the fluororesin in the coating film became β-type crystals, thereby increasing the hardness of the coating film.

[0113] Furthermore, as shown in Table 3, when the reheating process (flame treatment) was not performed, pressure marks appeared immediately after production and one month later. In contrast, flame treatment made it difficult for pressure marks to appear immediately after production. It is believed that by performing the reheating process so that the temperature of the coating film was 140°C or less, the fluororesin in the coating film became β-type crystals, increasing the hardness of the coating film.

[0114] 2. Second Example [Preparation of painted metal plates] Except for not adding titanium oxide, color pigment, or matting agent (silica) when preparing the fluororesin-based paint for forming the coating film, coated metal sheets were produced in the same manner as in Example 1. In this case, as in Example 1, the temperature of the coating film during the flame treatment (reheating step) was adjusted to 55°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, and 150°C.

[0115] [evaluation] The ratios of α-type crystals / β-type crystals and β-type crystals / α, β, and γ-type crystals of the coated metal sheets prepared by the above-mentioned method were determined by infrared spectrophotometric measurement using the following method. As a comparative example, the same test was also carried out on a coated metal sheet that had not been subjected to flame treatment (reheating step).

[0116] (α-type crystal / β-type crystal ratio, and β-type crystal / α, β, γ-type crystal ratio) The infrared spectrophotometric measurement of the coating film was carried out using the following measuring device and method. Measurement equipment: Thermo Scientific Nicolet / iN10MX Measurement conditions Measurement method: Microscope-ATR method (using germanium prism) Measurement area: 4000cm -1 ~600cm -1 Resolution: 4cm -1 Number of times accumulated: 128 Aperture: 100μm x 100μm

[0117] From the infrared spectrum obtained by the above method, the wavelength of 794 cm was determined by the two-point base method. -1 The absorbance of α-type crystals in the vicinity and the wavelength of 840 cm -1 The absorbance derived from the β-crystals in the vicinity of the α-crystals was determined. The α-crystal / β-crystal ratio was then calculated from the ratio (absorbance derived from the α-crystals / absorbance derived from the β-crystals). This ratio is preferably 0.5 or less, and the smaller the value, the more preferable. The calculated α-crystal / β-crystal ratio is shown in Table 4.

[0118] In addition, to confirm the formation of β-type crystals, the wavelength of 789 cm determined by the two-point base method was -1 The wavelength of 840cm for absorbance from α, β, and γ crystals in the vicinity -1 The ratio of absorbance derived from β-type crystals to that derived from α-, β-, and γ-type crystals was calculated. The ratio is preferably 0.4 or higher, and the larger the value, the more preferable. The calculated β-type crystal / α-, β-, and γ-type crystal ratios are shown in Table 5.

[0119] [result] [Table 4]

[0120] As shown in Table 4, when the reheating step (flame treatment) was not performed and when the reheating step (flame treatment) was performed to 55 to 140 °C, the ratio of α-type crystals to β-type crystals in the fluororesin was 0.5 or less, and it is believed that β-type crystals were overwhelmingly dominant. However, when the reheating step (flame treatment) was not performed, the amount of α-type crystals in the fluororesin was very small, but the amount of β-type crystals was also very small. Therefore, as shown in Table 5 below, the ratio of β-type crystals to α, β, and γ-type crystals was small. Furthermore, when the amount of β-type crystals was small, as shown in the first example above, the processability was likely to change over time. On the other hand, when the temperature in the reheating step was 150 °C, the ratio of α-type crystals to β-type crystals was 2.0 or more, and α-type crystals became dominant. The processability of the coating film in which α-type crystals were dominant was reduced to 9T.

[0121] [Table 5]

[0122] As shown in Table 5, when the reheating step (flame treatment) was not performed, the ratio of beta crystals to alpha, beta, and gamma crystals of the fluororesin was less than 0.4, meaning that the proportion of beta crystals in the crystals was low. In contrast, when the reheating step (flame treatment) was performed at 55 to 140°C, the ratio of beta crystals to alpha, beta, and gamma crystals of the fluororesin was 0.4 or higher, and the proportion of beta crystals tended to increase as the reheating temperature increased, reaching a maximum around 120°C. On the other hand, when the reheating temperature was 150°C, the ratio of beta crystals to alpha, beta, and gamma crystals was 0.2 or less, meaning that the beta crystals had converted to alpha crystals. Therefore, when the reheating step (flame treatment) was performed at 55 to 140°C, the proportion of beta crystals in the crystals was high, making it less likely for pressure marks to form on the coating film during winding or storage. [Industrial Applicability]

[0123] The method for producing a coated metal sheet of the present invention makes it possible to obtain a coated metal sheet whose processability is resistant to change over time and in which pressure marks are resistant to appearing in the coating film during winding, storage, etc. Therefore, it is expected that fluororesin-based coated metal sheets will become even more widespread.

Claims

1. A step of applying a fluororesin-based paint containing a fluororesin onto a metal plate to form a coating film; a step of heating the coating film to 200°C or higher and curing it; Cooling the cured coating; a step of reheating the cooled coating film to 55°C or higher and 140°C or lower; Including, In the step of reheating the coating film, the heating time of the coating film is 10 seconds or less. A method for manufacturing painted metal sheets.

2. The step of reheating the coating film is a step of subjecting the coating film to flame treatment and / or high-frequency induction heating. The method for producing the coated metal sheet according to claim 1.

3. the metal plate is a metal strip, The method further comprises a step of winding the coated metal sheet onto a roll after reheating the coating film. The method for producing a coated metal sheet according to claim 1 or 2.

4. The step of cooling the cured coating film is a step of water-cooling the coating film. The method for producing a coated metal sheet according to any one of claims 1 to 3.

5. In the step of cooling the cured coating film, the cooling rate of the coating film is 600 to 20°C / second. The method for producing a coated metal sheet according to any one of claims 1 to 4.

6. A metal plate; a coating film containing a fluororesin disposed on the metal plate; and In the infrared spectrum of the coating film immediately after production, －１ The absorbance of the β-type crystals at a wavelength of 794 cm －１ The ratio of absorbance derived from α-type crystals in the vicinity is 0.5 or less, and the wavelength determined by the two-point base method is 789 cm －１ The absorbance of the α, β, and γ crystals at a wavelength of 840 cm －１ A coated metal sheet in which the ratio of absorbance derived from β-type crystals in the vicinity of the coated metal sheet is 0.4 or more.

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