JP7723583B2 - Painted metal plate and its manufacturing method - Google Patents

Description

The present invention relates to a coated metal sheet and a method for manufacturing the same.

Coated metal sheets generally have excellent durability, weather resistance, and design, and are ideal for use as exterior building materials, for example. Among painted metal sheets for exterior building materials, those requiring long-term durability are best suited to coated metal sheets with a coating containing fluororesin.

However, coatings containing fluororesin (for example, coatings containing polyvinylidene fluoride) can be highly crystalline and have poor processability. Therefore, it has been proposed to include not only fluororesin but also thermoplastic acrylic resin in the coating (Patent Documents 1 to 3). It has also been proposed to suppress the crystallization of polyvinylidene fluoride by rapidly cooling the coating after it has hardened, thereby improving the processability of the coated metal sheet (Patent Document 4).

However, 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 developing indentations (hereinafter also referred to as "pressure marks"). To prevent such pressure marks, it has been proposed, for example, 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).

Special Publication No. 45-9662 Special Publication No. 3-11266 Japanese Unexamined Patent Publication No. 2-174977 Japanese Unexamined Patent Publication No. 114846/1986 JP 2008-87242 A

However, as in Patent Documents 1 to 4, coating films in which the crystallinity of polyvinylidene fluoride is adjusted gradually become harder over the course of several months after production. Therefore, even if a method such as that in Patent Document 5 could suppress pressure marks caused by winding, there was an issue of difficulty in handling, as processability differs significantly between immediately after production and after some time has passed since production.

The present invention therefore provides a coated metal sheet having a coating film containing a fluororesin, which is resistant to changes in processability over time and is less likely to develop pressure marks or other defects in the coating film, as well as a method for producing the same.

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.

[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.

[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 the cooling rate of the coating film in the step of cooling the cured coating film is 600 to 20°C/second.

The present invention provides the following coated metal sheet.
[6] A coated metal sheet comprising a metal sheet and a coating film containing a fluororesin disposed on the metal sheet, wherein in the infrared spectrum of the coating film, the ratio of the absorbance attributable to α-type crystals at a wavelength of around 794 cm ^-1 to the absorbance attributable to β-type crystals at a wavelength of around 840 cm ^-1 is 0.5 or less, and the ratio of the absorbance attributable to β-type crystals at a wavelength of around 840 cm-1 to the absorbance attributable to α-, β-, and γ-type crystals at a wavelength of around 789 cm ^{- 1} as determined by a two-point base method is 0.4 or more.

The present invention provides a method for producing a coated metal sheet having a coating film containing a fluororesin, which is resistant to changes in processability over time and is also resistant to pressure marks on the coating film during winding or storage, as well as a method for producing the same.

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. FIG. 2A is an SEM photograph of a coating film that has not been subjected to a reheating process, FIG. 2B is an SEM photograph of a coating film that has been reheated to 130°C, and FIG. 2C is an SEM photograph of a coating film that has been reheated to 150°C. 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.

1. Manufacturing Method of Coated Metal Sheet The manufacturing method of the present invention manufactures a coated metal sheet having a coating film containing a fluororesin. As mentioned above, when a coating film containing a fluororesin is formed on a metal sheet using conventional methods, the hardness is low immediately after formation, 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. As a result, it has been necessary to adjust the processing conditions in some cases.

Generally, fluororesin becomes amorphous when cooled after coating containing it is applied and hardened, but over time, the fluororesin crystallizes. As a result, the hardness of the coating film immediately after production is very low, making it prone to pressure marks and other problems. On the other hand, as the fluororesin crystallizes, pressure marks and other problems become less likely to occur, but the workability of the coating film decreases.

In contrast, the manufacturing method of the present invention involves the steps of applying a fluororesin-based paint containing a fluororesin to a metal plate to form a coating film (coating film formation process), heating the coating film to 200°C or higher to harden it (curing process), cooling the hardened coating film (cooling process), and reheating the cooled coating film to a temperature of 55°C or higher but not exceeding 140°C (reheating process).

The inventors' extensive research has revealed 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. When a coating containing fluororesin contains a large amount of β-type crystals of the fluororesin, pressure marks are less likely to form even when the coating is rolled up immediately after production. Furthermore, in coatings 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. In other words, the workability of the coated metal sheet remains good and unchanged 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 with the manufacturing method of the present invention.

Furthermore, the inventors' research has revealed that by applying and curing a fluororesin-based paint, thoroughly cooling the coating, 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 described above can stabilize the performance of the coating and suppress the occurrence of pressure marks. This also has the advantage of eliminating the need for large-scale equipment or complex processing.

The correlation between the temperature in the reheating process and the crystalline structure of the fluororesin is also 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 reached by the paint film was 80°C, 130°C, or 150°C. X-ray diffraction was then performed on the reheated paint film. The 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.

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 (above 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.

Scanning electron microscope (SEM) 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 are shown in Figures 2A to 2C. As shown in Figures 2A and 2B, no clear crystalline structure was observed in the coating film that was not reheated or in the coating film that reached a maximum temperature of 130°C during the reheating process. In contrast, a lamellar structure resulting from α-type crystals was observed in the coating film that reached a maximum temperature of 150°C during the reheating process.

In coatings 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 in which alpha-type crystals of fluororesin are dominant, localized loads are applied to the boundaries between the high-strength and low-strength areas, which can lead to fractures.

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, coating films in which β-type crystals are dominant are less likely to be subjected to localized loads during processing, making them 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.

In addition, although the unreheated coating film shown in Figure 2A does not have the lamellar structure shown in Figure 2C, the results of the X-ray diffraction pattern described above indicate that the amount of β-type crystals is not sufficiently large. Therefore, it is difficult to obtain sufficient strength with such a coating film.

Figure 3 shows a graph illustrating the relationship between the maximum temperature reached in the reheating process and the composite modulus of the coating film. As shown in Figure 3, when the temperature in 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 in the reheating process selectively grows beta crystals of the fluororesin. Meanwhile, the composite modulus of the coating film reaches its maximum around 130°C and drops significantly at 150°C. It is thought that the amount of alpha crystals begins to become dominant above 130°C, and at 150°C, alpha crystals become overwhelmingly dominant. The composite modulus in Figure 3 was measured using a nanoindenter under the following conditions. The composite modulus was also calculated as the average of five measurements taken on the same coating film.

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

The following describes each step in the method for producing coated metal sheets of the present invention, but other steps may also be included as long as they do not impair the purpose and effects of the present invention.

(Coating film formation process)
The coating film formation process is a process 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 adjusted appropriately 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 process) is 3 to 30 μm. If the thickness of the coating film formed in the coating film formation process 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 formation process is too thick, popping may occur during the curing process described below.

Fluororesin-based paints must contain at least a fluororesin and have the fluidity to be applied to the desired location on a metal plate. In addition to the fluororesin, fluororesin-based paints may also contain curing agents, (meth)acrylic resins, inorganic particles, organic particles, coloring pigments, wax, organic solvents, hydrophilizing agents, etc. In this specification, fluororesin refers to a resin that contains fluorine atoms in its structure. The fluororesin is preferably a polyvinylidene fluoride-based fluororesin, but is not limited to this. Fluororesin-based paints may contain only one type of fluororesin, or two or more types.

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 it 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 vinylidene fluoride-derived monomers relative to the total amount of monomers constituting the polyvinylidene fluoride resin.

Examples of monomers copolymerizable with vinylidene fluoride include fluoroolefins, vinyl ethers, vinyl esters, etc. Polyvinylidene fluoride resins may contain only one type of structure derived from a monomer other than vinylidene fluoride, or may contain two or more types.

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, resulting in a coating film with high strength. The weight-average molecular weight is the value measured by gel permeation chromatography (styrene equivalent value).

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, based on 100 parts by mass of the total resin solids content of the fluororesin paint. The resin solids content of the fluororesin paint refers to the 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 paint is within this range, the resulting coating tends to have good long-term durability.

Fluororesin-based paints may contain a curing agent in addition to the fluororesin. When a curing agent is included with the fluororesin, a crosslinked structure is more likely to form, making the resulting coating stronger. Examples of curing agents include isocyanate-based curing agents, aminoplast-based curing agents, polybasic acid-based curing agents, and polyamine-based curing agents. Fluororesin-based paints may contain only one type of curing agent, or two or more types.

Examples of isocyanate curing agents include polyisocyanate compounds and their blocked products, modified polyisocyanate compounds, and polymers of polyisocyanate compounds. Polyisocyanate compounds are compounds containing 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, and carbodiimide-modified compounds.

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.

Examples of polybasic acid curing agents include long-chain aliphatic dicarboxylic acids, aromatic polycarboxylic acids, etc., and may also be acid anhydrides of these. Examples of polyamine curing agents include ethylenediamine, ethylenetriamine, etc.

Fluororesin-based paints preferably contain 0.1 to 100 parts by mass of the above-mentioned curing agent per 100 parts by mass of fluororesin, and more preferably 1 to 50 parts by mass. If the amount of curing agent per 100 parts by mass of fluororesin is 0.1 parts by mass or more, the hardness of the coating film is likely to be increased. On the other hand, if the amount of curing agent per 100 parts by mass of fluororesin is 100 parts by mass or less, the processability and impact resistance of the coating film are likely to be good.

From the perspective of increasing the fluidity of the fluororesin-based paint and improving the adhesion between the resulting coating film and the metal sheet, it is preferable that the fluororesin-based paint further contains a (meth)acrylic resin. The (meth)acrylic resin may be thermoplastic or thermosetting. In this specification, (meth)acrylic refers to either methacrylic or acrylic, or both.

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. Thermoplastic (meth)acrylic resins may contain only one type of structure derived from (meth)acrylic acid alkyl esters, or may contain two or more types.

Thermoplastic (meth)acrylic resins may also have structures derived from monomers other than (meth)acrylic acid alkyl esters, examples of which include styrene, vinyl toluene, (meth)acrylonitrile, vinyl chloride, etc.

The weight-average molecular weight of the thermoplastic (meth)acrylic resin contained in the fluororesin-based paint is preferably 40,000 to 300,000, and more preferably 50,000 to 200,000. This 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 paint is increased, and the adhesion between the resulting coating film and the metal sheet is likely to be improved.

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

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, and isocyanate groups. These thermosetting (meth)acrylic resins are used in conjunction with curing agents such as alkylated melamine, polyols, polyamines, polyamides, and polyoxiranes.

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

The amount of thermosetting (meth)acrylic resin in a fluororesin-based paint is preferably 150 parts by mass or less, and more preferably 10 to 50 parts by mass, per 100 parts by mass of fluororesin. Including a thermosetting (meth)acrylic resin within this range tends to improve the fluidity, etc., of the paint.

The fluororesin-based paint may further contain inorganic particles or organic particles. When the fluororesin-based paint 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 using the Coulter counter method. There are no particular restrictions on the shape of the inorganic or organic particles, but a roughly spherical shape is preferred from the perspective of making it easier to adjust the surface condition of the resulting coating film.

Examples of inorganic particles include silica, barium sulfate, talc, calcium carbonate, mica, glass beads, and glass flakes. Examples of organic particles include resin beads made of acrylic resin or polyacrylonitrile resin. These resin beads may be manufactured by known methods or may be commercially available. Examples of commercially available acrylic resin beads include Toyobo's "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)." 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)," all manufactured by Toyobo Co., Ltd.

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

Furthermore, the fluororesin-based paint may contain a coloring pigment. The preferred average particle size of the coloring pigment is, 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, and cobalt blue. When the fluororesin-based paint contains a coloring pigment, the amount thereof is preferably 20 to 60 parts by mass, and more preferably 30 to 55 parts by mass, per 100 parts by mass of the solids content of the fluororesin-based paint.

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 selected appropriately depending on the type of wax, but is preferably approximately 2 to 15 parts by mass per 100 parts by mass of the solid content of the fluororesin-based paint.

Fluororesin-based paints may contain organic solvents as needed. There are no particular limitations on the organic solvent, so long as it is capable of sufficiently dissolving or dispersing the various components of the fluororesin-based paint. Examples of organic solvents include hydrocarbon solvents such as toluene, xylene, Solvesso® 100 (trade name, manufactured by ExxonMobil Corporation), Solvesso® 150 (trade name, manufactured by ExxonMobil Corporation), and Solvesso® 200 (trade name, manufactured by ExxonMobil Corporation); ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and isophorone; ester solvents such as ethyl acetate, butyl acetate, ethylene glycol monoethyl ether acetate, and dimethyl phthalate; alcohol solvents such as methanol, isopropyl alcohol, and n-butyl alcohol; and ether alcohol solvents such as ethylene glycol monoethyl ether and diethylene glycol monobutyl ether. Fluororesin-based paints may contain only one of these solvents, or two or more. Of these, isophorone, xylene, ethylbenzene, cyclohexanone, and dimethyl phthalate are preferred from the standpoint of compatibility with fluororesin.

The fluororesin paint may further contain a hydrophilizing agent. When the fluororesin paint contains a hydrophilizing agent, the hydrophilizing agent orients itself on the surface of the paint film, imparting rain streak resistance to the paint film. Examples of hydrophilizing agents include alkoxysilanes such as tetramethoxysilane and tetraethoxysilane, and silicone resins. Of these, silicone resins are preferred because they are easily orientated on the surface of the paint film and further increase the hydrophilicity of the paint film surface.

In this specification, silicone resin refers to a compound formed by partial hydrolysis and condensation of alkoxysilane, which is a polymer that is primarily composed of a three-dimensional crosslinked structure but does not reach the stage of gelation and is soluble in organic solvents. There are no particular restrictions on the three-dimensional crosslinked structure contained in silicone resin, and it may be, for example, cage-shaped, ladder-shaped, or random-shaped. In this specification, tetraalkoxysilane and condensates (organosilicates) formed by hydrolysis and condensation of only tetraalkoxysilane are not included in the term silicone resin.

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. However, the amount of Si atoms derived from the 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 the 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 excellent resistance to rain streaks. The amount of Si atoms derived from the trialkoxysilane can be determined by ^29Si -NMR analysis.

The amount (number of moles) of silanol groups in the silicone resin is preferably 5 to 50 mol%, more preferably 15 to 40 mol%, based on the total molar amount of Si atoms. If the amount of silanol groups exceeds 50 mol% based on the total molar amount of Si atoms, the reactivity of the silicone resin will be too high, making it more likely to polymerize in the fluororesin-based paint. On the other hand, if the amount of silanol groups is less than 5 mol% based on the total molar amount of Si atoms, the silicone resin will have difficulty interacting with other components in the fluororesin-based paint, making it more likely to detach from the paint film, and sufficient rain stain resistance may not be achieved.

Here, the weight-average molecular weight of the silicone resin is preferably 700 to 50,000, and 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 paint 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).

The amount of hydrophilizing agent is preferably 1 to 10 parts by mass, and more preferably 2 to 7 parts by mass, per 100 parts by mass of the solids content of the fluororesin-based paint.

Fluororesin-based paints can be prepared by mixing fluororesin and other ingredients and stirring or dispersing them.

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

From the viewpoints of corrosion resistance and weight reduction, plated steel sheet or stainless steel sheet is preferred as the metal sheet, and plated steel sheet is even more preferred from the viewpoint of cost-effectiveness. Furthermore, from the viewpoint of further enhancing corrosion resistance, 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. Of these, zinc plated steel sheet is preferred, and zinc plated steel sheet containing magnesium, such as a Zn-Al-Mg alloy plated steel sheet, is more preferred. The metal sheet may have a chemical conversion coating or a primer coating formed on its surface, as long as it does not impair the effects of the present invention. Furthermore, the metal sheet may have a textured finish, such as embossing or drawing, as long as it does not impair the effects of the present invention.

The thickness of the metal plate can be determined appropriately based on the intended use of the painted metal plate. For example, if the intended use of the painted metal plate is as an exterior building material, the thickness of the metal plate can be 0.2 to 1.6 mm or less.

The metal plate may be in the form of a sheet, but a strip, i.e., a metal strip, is preferred from the viewpoint of the effect of suppressing pressure marks. If the metal plate is a metal strip, the coating film formation process may be carried out while unwinding the metal plate wound around a roll.

(Curing process)
In the curing step, the coating film of the fluororesin-based paint formed in the coating film forming 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 a known heating device or the like can be used. From the viewpoint of production efficiency of the coated metal plate, it is preferable that the curing step and the coating film forming step are performed on the same line.

The heating temperature should be 200°C or higher, but 240 to 300°C is more preferable from the perspective of obtaining a uniform coating film. The heating time is selected appropriately depending on factors such as the thickness of the coating film, but from the perspective 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.

To further harden the coating uniformly in a short period of time, air may be blown onto the plate surface at a speed of 0.9 m/s or more simultaneously with the heating.

(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 forming step and the curing step.

The method for cooling the coating film in the cooling process is not particularly limited. For example, it may be air-cooled, water-cooled, left to cool, or contacted with a cooling member, or a combination of these. However, water-cooling is preferred from the perspective of cooling efficiency. By shortening the cooling time, the production line can be shortened. The cooling rate of the coating film in the cooling process is preferably 600 to 20°C/sec, and more preferably 400 to 50°C/sec. The temperature-reducing rate of the coating film from the start to the end of the cooling process may be constant, or may be changed intermittently or continuously.

(Reheating process)
In the reheating step, the coating film after the cooling step is reheated to 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 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 is also performed on the same line as the coating film formation step, curing step, and cooling step. Furthermore, 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.

The method for reheating the coating is not particularly limited, and examples include flame treatment, infrared heating, and high-frequency induction heating. However, among these, flame treatment and high-frequency induction heating are particularly preferred. With infrared heating, the infrared absorption rate varies depending on the color of the painted metal sheet, so the amount of irradiation must be controlled depending on the color of the painted metal sheet. For example, if a halogen lamp is used to irradiate the same amount of infrared light onto a painted metal sheet with a white coating and a painted metal sheet with a black coating, the white painted metal sheet will reach 70°C, while the black painted metal sheet will reach 120°C. In contrast, flame treatment is not affected by the color of the painted metal sheet and allows for easy temperature control. Flame treatment also has the advantage of not requiring large-scale equipment or complex processing.

On the other hand, the energy efficiency of high-frequency induction heating is over 70%, compared to around 30% for flame processing and infrared heating. Therefore, high-frequency induction heating is preferable from the perspective of energy efficiency. High-frequency induction heating is also 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, it is possible to heat it more rapidly and it is also superior in terms of temperature controllability and responsiveness compared to high-frequency induction heating of non-magnetic materials such as copper, aluminum, and austenitic stainless steel.

When using high-frequency induction heating to heat a coated metal sheet with a thickness of 0.2 mm or more and 1.6 mm or less, 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 width direction of the sheet, particularly overheating of the edges in the width direction of the coil, 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 above 400 kHz are suitable for coated metal sheets with a thickness of less than 0.2 mm, but may not be suitable for coated metal sheets with a thickness of 0.2 mm or more, as the current penetration depth may be too shallow.

The temperature of the coating film during the reheating process should be 55°C or higher and 140°C or lower, with 70 to 130°C being more preferable, and 80 to 120°C being even more preferable. By maintaining a coating film temperature of 55°C or higher, the beta-crystallization of the fluororesin in the coating film is more likely to be promoted, as described above, and the workability of the coating film is less likely to change over time. On the other hand, the general recrystallization temperature of fluororesin is around 160°C. Therefore, by setting the heating temperature to 140°C or lower, excessive alpha-crystallization of the fluororesin can be suppressed, and a decrease in the workability of the coating film can be suppressed. Furthermore, by maintaining a coating film temperature of 140°C or lower during the reheating process, the coated metal sheet can be wound onto a roll after the reheating process without the need for a separate cooling process.

In particular, when heating the coating film by flame treatment, setting the coating film temperature to 65°C or higher makes it difficult for water vapor generated by the combustion of the combustible gas used in flame treatment to condense, allowing the coating film temperature to be raised uniformly. Furthermore, setting the coating film temperature to 130°C or lower more reliably prevents excessive alpha-crystallization of the fluororesin.

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 using flame processing, 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 processing). 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, as this allows the temperature to be measured without damaging the metal plate.

The heating time for the coating film during the reheating step is selected appropriately depending on the heating method, but is typically preferably 5 minutes or less, more preferably 2 minutes or less, and especially 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 set to 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. A heating time of 10 seconds or less allows for particularly efficient production of coated metal sheets.

The reheating method using flame treatment is explained in detail below. When the reheating process is performed using flame treatment, a flame is emitted from the flame treatment device while the coated metal sheet after the cooling process is moved in a certain direction.

A flame treatment device can be a device equipped with 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 typically has a roughly rectangular prism-shaped housing connected to the gas supply unit and a flame nozzle located on the bottom surface of the housing, and burns the combustible gas supplied from the gas supply unit in the flame nozzle.

The internal structure of the burner head housing can be similar to that of a typical flame treatment burner, and may include, for example, a flow path for flowing 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, approximately 50 to 150 cm. On the other hand, the width of the flame nozzle in the transport direction of the coating film can be set appropriately depending on the discharge stability of the combustible gas, and can be, for example, approximately 1 to 8 mm. The burner head may also have an auxiliary flame nozzle parallel to the flame nozzle. If the burner head has an auxiliary flame nozzle, the flame will travel more linearly, ensuring reliable flame treatment of the coating film.

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. 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, oxygen cylinder, compressor air, or air from a blower, and is a component for pre-mixing the combustion gas and combustion supporting gas. It is preferable that the oxygen concentration in the combustible gas (mixture of combustion gas and combustion supporting gas) supplied from the gas mixing unit to the gas supply unit be constant, and it is preferable that the gas mixing unit be equipped with an oxygen supplier for supplying oxygen to the gas supply unit as needed.

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 are preferred, with LPG being particularly preferred, from the perspective of ease of forming the desired flame. On the other hand, examples of the combustion support gas include air or oxygen, with air being preferred from the perspective of ease of handling, etc.

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

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 approximately 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 great deal 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.

The conveying speed of the coating film (coated metal sheet) during the reheating process is selected appropriately. If the reheating process is performed on the same line as the above-mentioned coating film formation process, curing process, cooling process, etc., it 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, 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.

The flame treatment amount is selected appropriately depending on the temperature of the coating film after the cooling process, and is typically preferably 30 to 1000 kJ/ ^m² , and more preferably 100 to 600 kJ/ ^m² . In this specification, the term "flame treatment amount" refers to the heat quantity per unit area of the coated metal sheet calculated based on the supply amount of combustion gas, such as LPG. The flame treatment amount can be adjusted by adjusting the distance between the burner head of the flame treatment burner and the coating film surface, the coating film conveyance speed, and other factors. If the flame treatment amount is less than 30 kJ/ ^m² , it may be difficult to raise the coating film temperature within the above range, or uneven treatment may occur. On the other hand, if the flame treatment amount exceeds 1000 kJ/ ^m² , the coating film may become too hot or may oxidize and yellow.

Furthermore, if the temperature of the coating film (coated metal sheet) after the cooling process is low, a preheating treatment can 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 more), the water vapor generated by the combustion of the combustible gas cools 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.

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, while also making the coating hydrophilic and providing easy adhesion through flame treatment, all while achieving greater energy savings.

The means for preheating the coating 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 can be placed between the cooling device that performs the cooling process described above and the frame processing device. On the other hand, if the coated metal sheet is in sheet form, a heating device commonly known as a drying oven can be used. For example, a batch-type drying oven (also called a "safe oven") can be used, and specific examples include the Isuzu Low-Temperature Incubator (Model Mini Catalina MRLV-11), the Tojo Thermal Engineering Co., Ltd. Automatic Discharge Dryer (Model ATO-101), and the Tojo Thermal Engineering Co., Ltd. Simple Explosion-Proof Dryer (Model TNAT-1000).

(Winding process)
When 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 can 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 coated metal 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 the fluororesin in the coating film resin (not causing it to become amorphous), and for example, air cooling or natural cooling is preferred.

(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.

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

Polyester resin-based paints can be paints that contain, for example, polyester resin, solvent, crosslinking agent, etc., and other components as needed.

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.

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

The method for applying the polyester resin-based paint is not particularly limited, and examples include spraying, bar coating, roller curtain coating, curtain flow coating, and roll coating. The method for curing the polyester resin-based paint coating is not particularly limited, and examples include drying and curing in a hot air drying oven or an induction heating device. The thickness of the cushion layer is preferably 5 to 15 μm. The glass transition temperature of the cushion layer obtained by this process is preferably 10 to 20°C. Having a glass transition temperature of the cushion layer within this range makes it easier to alleviate stress on the coated metal sheet. The glass transition temperature of the cushion layer can be adjusted by the type of polyester resin in the polyester resin-based paint.

(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.

Furthermore, in the present invention, the reheating process can be performed by flame treatment and/or high-frequency induction heating, eliminating the need for special equipment or complex processes. Therefore, the manufacturing method of the present invention makes it possible to easily produce high-quality coated metal sheets.

2. Coated Metal Sheet The coated metal sheet produced by the above-described method comprises a metal sheet and a coating film containing a fluororesin disposed on the metal sheet. Furthermore, in the infrared spectrum of the coating film, the ratio of the absorbance attributable to α-type crystals at a wavelength of around 794 cm ⁻¹ to the absorbance attributable to β-type crystals at a wavelength of around 840 cm ⁻¹ (absorbance attributable to α-type crystals/absorbance attributable to β-type crystals) is 0.5 or less. This ratio is more preferably 0.3 or less. The smaller this ratio, the fewer α-type crystals and more β-type crystals there are in the fluororesin in the coating film; therefore, the lower limit is not particularly limited and may even be 0. Furthermore, the ratio of the absorbance attributable to β-type crystals at a wavelength of around 840 cm ⁻¹ to the absorbance attributable to α-, β-, and γ-type crystals at a wavelength of around 789 cm ⁻¹ determined by the two-point base method (absorbance attributable to β-type crystals/absorbance attributable to α, β, and γ-type crystals) is 0.4 or more, indicating 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.

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 of accumulation: 128 times Aperture: 100 μm x 100 μm

1. First Example [Preparation of Metal Plate]
A hot-dip Zn-55%Al alloy-plated steel sheet with a thickness of 0.27 mm, A4 size (210 mm x 297 mm), and a coating weight of 75 g/ ^m2 per side was prepared, and both sides of the sheet were alkaline degreased. A paint-type chromate treatment solution (NRC300NS manufactured by Nippon Paint Co., Ltd.) was then applied to the alloy-plated steel sheet so that the Cr coating weight was 50 mg/ ^m2 . An epoxy resin-based primer paint (800P manufactured by Nippon Fine Coatings Co., Ltd.) was then applied to the surface with a roll coater to a cured film thickness of 5 μm. The plated steel sheet was then baked to a maximum temperature of 215°C, yielding a plated steel sheet (hereinafter simply referred to as "plated steel sheet") on which a primer coating film was formed.

[Preparation of fluororesin-based paint]
A fluororesin-based paint was prepared by mixing polyvinylidene fluoride resin (manufactured by Nippon Penwalt Co., Ltd., 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 (amount excluding components that volatilize when formed into a paint film) to prepare a matte fluororesin-based paint with a 60-degree gloss of 12.

[Coating film forming process, curing process, and cushion layer forming process]
The above-mentioned fluororesin-based 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 the sheet was 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° gloss 30) was used on the back surface, adjusted to a film thickness after curing of 5 μm, and cured simultaneously with the fluororesin-based paint to form a cushion layer.

[Cooling step and reheating step]
After curing (baking) the coating film, it was water-cooled to 40°C at a temperature drop rate of 150°C/sec, and then cooled to 20-40°C. The cooled coating film was then subjected to flame treatment. The flame treatment burner used 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) prepared by mixing LP gas (combustion gas) and clean dry air in a gas mixer. The flow rates of the respective gases were adjusted so that the LP gas (combustion gas) was 1.67 L/min and the clean dry air was 41.7 L/min per ^cm2 of the burner nozzle. The length of the burner head nozzle in the direction of transport of the coating film was 4 mm. The length of the burner head nozzle in the direction perpendicular to the transport direction was 450 mm. The distance between the burner head flame and the coating surface was set to 30 mm depending on the desired flame treatment volume. The coating transport speed was adjusted to adjust the flame treatment volume to 100 to 600 kJ/ ^m² , and the coating temperature 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, or 150°C. The temperature of the metal plate was measured from the fluororesin-containing coating side of the coated metal plate using a non-contact thermometer, and this was recorded as the coating surface temperature.

[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 a coated metal sheet that had not been subjected to flame treatment (reheating step).

(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.

(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 coating surface was scratched with a pencil lead to determine the hardness of the coating, and the hardness was expressed as the pencil density that did not scratch the coating. The results are shown in Table 2. A pencil hardness equal to or higher than the pencil hardness after 6 months without reheating (flame treatment) is considered to be a better result.

(Pressure Mark Accelerated Test)
Two samples of coated metal sheets cut to a size of 10 cm x 10 cm were stacked on top of each other. A load of 4.9 MPa (50 kg/ ^cm2 ) was then applied to the stack using a press for 24 hours. The samples were then peeled off one by one, and the appearance of the coating film on each sample was visually observed and evaluated according to the following criteria. ○ and △ indicate a range that is acceptable for practical use.
○: No pressure marks occurred △: A slight pressure mark occurred ×: A pressure mark occurred

[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 had to be taken into consideration, making handling difficult. In contrast, when reheating to 50-140°C by flame treatment, the change in foldability over time was reduced. In particular, when reheating to 90-140°C, the physical properties immediately after production and six months later were essentially the same. On the other hand, when reheating to temperatures 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 converted the fluororesin in the coating into β-type crystals, making it less likely for α-type crystals to form over time.

Furthermore, as shown in Table 2 above, when the reheating process (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 process so that the temperature of the coating film was 140°C or below, the fluororesin in the coating film became β-type crystals, thereby increasing the hardness of the coating film.

Furthermore, as shown in Table 3, when the reheating process (flame treatment) was not performed, pressure marks occurred immediately after production and one month later. In contrast, when flame treatment was performed, pressure marks were less likely to occur immediately after production. It is believed that by performing the reheating process so that the temperature of the coating film was 140°C or below, the fluororesin in the coating film became β-type crystals, increasing the hardness of the coating film.

2. Second Example [Preparation of Coated Metal Sheet]
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, too, 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, as in Example 1.

[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 as follows. 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).

(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 device: Nicolet/iN10MX manufactured by Thermo Scientific
Measurement conditions Measurement method: Microscope - ATR method (using germanium prism)
Measurement area: 4000cm ^-1 ~ 600cm ^-1
Resolution: 4cm ^-1
Number of times of accumulation: 128 times Aperture: 100 μm x 100 μm

From the infrared spectrum obtained by the above method, the absorbance attributable to α-type crystals near a wavelength of 794 cm ^-1 and the absorbance attributable to β-type crystals near a wavelength of 840 cm ^-1 were determined by the two-point base method. The α-type crystal/β-type crystal ratio was then calculated from the ratio of these (absorbance attributable to α-type crystals/absorbance attributable to β-type crystals). This ratio is preferably 0.5 or less, and the smaller the value, the more preferable. The calculated results of the α-type crystal/β-type crystal ratio are shown in Table 4.

To confirm the formation of β-type crystals, the ratio of the absorbance attributable to β-type crystals at a wavelength of approximately 840 cm ^-1 to the absorbance attributable to α-, β-, and γ-type crystals at a wavelength of approximately 789 cm ^- 1 determined by the two-point base method was calculated. This ratio is preferably 0.4 or more, 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.

[result]

As shown in Table 4, when the reheating process (flame treatment) was not performed, and when the reheating process (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, indicating that β-type crystals were overwhelmingly dominant. However, when the reheating process (flame treatment) was not performed, although the amount of α-type crystals in the fluororesin was very small, 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 is small. Furthermore, when the amount of β-type crystals is small, as shown in the first example above, processability is likely to change over time. On the other hand, when the temperature in the reheating process was 150°C, the ratio of α-type crystals to β-type crystals was 2.0 or more, indicating that α-type crystals were predominant. Coatings in which α-type crystals were dominant showed a decrease in processability of 9T.

As shown in Table 5, when the reheating process (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 process (flame treatment) was performed to 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 at around 120°C. On the other hand, when the temperature in the reheating process 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 process (flame treatment) was performed to 55 to 140°C, the proportion of beta crystals in the crystals was high, making it less likely that pressure marks would occur in the coating film during winding or storage.

The method for producing coated metal sheets of the present invention produces coated metal sheets whose workability is less likely to change over time and in which pressure marks are less likely to appear in the coating film during winding or storage. Therefore, it is expected that fluororesin-based coated metal sheets will become even more popular.

Claims (6)

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