Painted metal sheet and method for manufacturing the same, and ionizing radiation-curing paint

Abstract

The objective is to provide a painted metal sheet that has good processability and a coating that is less prone to pressure marks. [Solution] The painted metal plate that solves the above problem comprises a metal plate and a coating film which is a cured product of an ionizing radiation-curable paint, disposed on the metal plate. The ionizing radiation-curable paint contains an N-substituted (meth)acrylamide monomer, a polyfunctional urethane (meth)acrylate, and a polyfunctional (meth)acrylate monomer. The amount of the N-substituted (meth)acrylamide monomer is 1% by mass or more and 60% by mass or less of the total amount of curable components of the ionizing radiation-curable paint.

Description

[Technical Field]

[0001] This invention relates to painted metal sheets, a method for manufacturing the same, and ionizing radiation-curing paints. [Background technology]

[0002] Painted metal sheets generally offer excellent durability and weather resistance, and are used in a variety of applications, including building materials. The coating on these painted metal sheets requires high workability and aesthetic appeal, depending on the application.

[0003] In general, painted metal sheets are manufactured in a continuous coil painting line while being transported at high speeds of 100 m / min or more. In recent years, there has been a demand for further increases in the speed of the manufacturing line in order to improve productivity. To increase the speed of the manufacturing line, it has been considered to cure the paint applied to the metal sheet by irradiating the paint with an electron beam (for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 08-243484 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Electron beam curing allows for uniform hardening of the paint's interior in a short time. However, curing paint with electron beams tends to make the paint film hard, which can lead to cracking when the film is bent or otherwise processed. On the other hand, using urethane (meth)acrylate or similar materials as a paint binder, which can form a highly flexible paint film, has also been considered. However, in this case, when the painted metal sheet is wound onto a roll or similar device, pressure marks tend to appear on the paint film. In other words, processability and pressure mark resistance are in a trade-off relationship for painted metal sheets, and it has been difficult to achieve both simultaneously.

[0006] The present invention has been made in view of the above problems. The present invention aims to provide a painted metal sheet having good processability and a coating that is less prone to pressure marks, a method for manufacturing the same, and an ionizing radiation-curing paint for obtaining the painted metal sheet. [Means for solving the problem]

[0007] This invention provides the following painted metal sheets. [1] A painted metal plate having a metal plate and a coating film which is a cured product of an ionizing radiation-curable coating disposed on the metal plate, wherein the ionizing radiation-curable coating comprises an N-substituted (meth)acrylamide monomer, a polyfunctional urethane (meth)acrylate, and a polyfunctional (meth)acrylate monomer, and the amount of the N-substituted (meth)acrylamide monomer is 1% by mass or more and 60% by mass or less of the total amount of curable components of the ionizing radiation-curable coating. [2] The painted metal sheet according to [1], wherein the N-substituted (meth)acrylamide monomer is at least one selected from the group consisting of N-acryloylmorpholine, N-hydroxyethylacrylamide, and N,N-dimethylaminopropylacrylamide. [3] The painted metal sheet according to [1] or [2], wherein the amount of the polyfunctional (meth)acrylate monomer is 1% by mass or more and 50% by mass or less with respect to the total amount of curable components of the ionizing radiation-curable paint.

[0008] This application provides a method for manufacturing the following painted metal sheet. [4] A step of applying an ionizing radiation-curable paint containing an N-substituted (meth)acrylamide monomer, a polyfunctional urethane (meth)acrylate, and a polyfunctional (meth)acrylate monomer onto a metal sheet, and a step of irradiating the film of the ionizing radiation-curable paint with ionizing radiation, wherein the amount of the N-substituted (meth)acrylamide monomer in the ionizing radiation-curable paint is 1% by mass or more and 60% by mass or less based on the total amount of the curable components of the ionizing radiation-curable paint. A method for manufacturing a painted metal sheet. [5] The method for manufacturing a painted metal sheet according to [4], wherein the step of irradiating the ionizing radiation is a step of irradiating the film with an electron beam. [6] The method for manufacturing a painted metal sheet according to [4], wherein the step of irradiating the ionizing radiation is a step of irradiating the film with ultraviolet rays having a wavelength of 150 nm or more and 300 nm or less and then irradiating with an electron beam.

[0009] This application provides the following ionizing radiation-curable paint. [7] An ionizing radiation-curable paint for obtaining the paint film of the painted metal sheet according to any one of [1] to [3] above, containing an N-substituted (meth)acrylamide monomer, a polyfunctional urethane (meth)acrylate, and a polyfunctional (meth)acrylate monomer, wherein the amount of the N-substituted (meth)acrylamide monomer is 1% by mass or more and 60% by mass or less based on the total amount of the curable components of the ionizing radiation-curable paint. An ionizing radiation-curable paint. [Advantages of the Invention]

[0010] According to the present invention, there is provided a painted metal sheet having a paint film with good processability and hardly generating pressure marks. [Modes for Carrying Out the Invention]

[0011] The present invention relates to a painted metal plate having a metal plate and a coating film, a method for manufacturing the painted metal plate, and an ionizing radiation-curable paint used for forming the coating film of the painted metal plate. Hereinafter, the ionizing radiation-curable paint will be described first, and then the painted metal plate and its manufacturing method will be described.

[0012] 1. Ionizing radiation-curable paint The ionizing radiation-curable paint (hereinafter, also simply referred to as "paint") of the present invention is a paint that is cured by irradiation with ionizing radiation (electromagnetic waves or charged particle beams). The type of ionizing radiation irradiated when curing the paint is not particularly limited, but from the viewpoint that a large-scale device is not required, ultraviolet rays having a wavelength of 300 nm or less (hereinafter, also referred to as "short-wavelength UV") or electron beams (hereinafter, also referred to as "EB") are preferably used.

[0013] Here, the paint of the present invention contains at least an N-substituted (meth)acrylamide monomer, a polyfunctional urethane (meth)acrylate, and a polyfunctional (meth)acrylate monomer. Further, the amount of the N-substituted (meth)acrylamide monomer in the paint is 1% by mass or more and 60% by mass or less with respect to the total amount of the curable components in the paint. In the present specification, (meth)acryl means methacryl, acryl, and both of them. The same applies to (meth)acrylate, (meth)acryloyl, (meth)acryloyloxy, etc.

[0014] As described above, when the paint is applied on a metal plate and cured with ionizing radiation to form a coating film, the coating film was likely to become hard. Therefore, when processing the painted metal plate, cracks or the like were likely to occur in the coating film. On the other hand, when the coating film was made flexible, pressure marks were likely to occur when winding up the painted metal plate. In contrast, the coating film obtained from the paint of the present invention has high processability and is less likely to form pressure marks. The reason is not clear, but it is considered as follows.

[0015] The N-substituted (meth)acrylamide monomer contained in the paint of the present invention exhibits excellent curing reactivity to ionizing radiation, and the surface of the paint film hardens easily when irradiated with an electron beam. On the other hand, the cured product of the polyfunctional urethane (meth)acrylate has moderate flexibility. Therefore, although the cured film (paint film) of this paint has a hard surface, its interior has moderate flexibility. Thus, it is considered that both processability and pressure mark resistance are achieved. The following describes each component.

[0016] (N-substituted (meth)acrylamide monomer) In this specification, an N-substituted (meth)acrylamide monomer is a monomer in which at least one of the two hydrogen atoms bonded to the N atom in (meth)acrylamide is substituted with various substituents such as hydrocarbon groups. An N-substituted (meth)acrylamide monomer may also be a monomer in which both of the two hydrogen atoms bonded to the N atom in (meth)acrylamide are substituted with various substituents. An ionizing radiation-curable coating may contain only one type of N-substituted (meth)acrylamide monomer, or it may contain two or more types.

[0017] N-substituted (meth)acrylamide monomers may have two or more (meth)acryloyl groups in their molecule. However, considering the viscosity and storage stability of the paint, it is preferable that N-substituted (meth)acrylamide monomers have only one (meth)acryloyl group in their molecule.

[0018] Here, examples of N-substituted (meth)acrylamide monomers having only one (meth)acryloyl group in the molecule include compounds represented by the following general formula. [ka] In the above general formula, R 1 represents a hydrogen atom or a methyl group. R 2 and R 3is, independently of each other, a hydrogen atom; a saturated or unsaturated alkyl group having 1 to 20 carbon atoms (preferably 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms) such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a dodecyl group, an octadecyl group; a hydroxyalkyl group having 1 to 20 carbon atoms (preferably 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms) such as a hydroxymethyl group, a hydroxyethyl group, a hydroxypropyl group; an alkoxyalkyl group having 2 to 20 carbon atoms (preferably 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms) such as a methoxymethyl group, an ethoxymethyl group, a propoxymethyl group, a butoxymethyl group, a methoxyethyl group, an ethoxyethyl group, a butoxyethyl group, a methoxypropyl group, an ethoxypropyl group, a methoxybutyl group, an ethoxybutyl group; a cycloalkyl group having 6 to 10 carbon atoms such as a cyclohexyl group; an aromatic ring having 6 to 12 carbon atoms such as a phenyl group; a 2-(3,4-dihydroxyphenyl)ethyl group; phenylboronic acid; a 3-(dimethylamino)propyl group; (however, R 2 and R 3 shall not be a hydrogen atom at the same time).

[0019] Also, R 2 and R 3 may be linked to each other to form a cyclic structure having 5 to 20 carbon atoms such as a piperidine ring, a hexamethyleneimine ring, a heptamethyleneimine ring, an octamethyleneimine ring, a decamethyleneimine ring, a morpholine ring. Further, a substituent having 1 to 5 carbon atoms may be bonded to these cyclic structures.

[0020] The N-substituted (meth)acrylamide monomer is preferably a monomer in which either one of R 2 and R 3 is hydrogen, or a monomer in which R 2 and R 3 are linked to form a cyclic structure. According to such a monomer, the viscosity of the paint is likely to be within a desired range, and the storage stability of the paint is likely to be good.

[0021] Furthermore, the molecular weight of the N-substituted (meth)acrylamide monomer is preferably 80 to 300, more preferably 90 to 250, and even more preferably 100 to 200. When the molecular weight of the N-substituted (meth)acrylamide monomer is within this range, the viscosity of the paint tends to fall within the desired range. In addition, when the molecular weight of the N-substituted (meth)acrylamide monomer is within this range, the compatibility between the N-substituted (meth)acrylamide monomer and other components tends to be good.

[0022] Specific examples of preferred N-substituted (meth)acrylamide monomers include N,N-dimethylacrylamide, N,N-diethylacrylamide, N-isopropylacrylamide, N-acryloylmorpholine, N-hydroxyethylacrylamide, and N,N-dimethylaminopropylacrylamide. Among these, N-acryloylmorpholine, N-hydroxyethylacrylamide, and N,N-dimethylaminopropylacrylamide are particularly preferred in terms of availability and curability.

[0023] Here, the amount of N-substituted (meth)acrylamide monomer in the paint should be 1% by mass or more and 60% by mass or less, with 5% by mass or more and 50% by mass or less, relative to the total amount of curable components of the paint. If the amount of N-substituted (meth)acrylamide monomer is 1% by mass or more, as described above, the surface of the paint film obtained from the paint tends to harden. Also, if the amount of N-substituted (meth)acrylamide monomer is 60% by mass or less, the storage stability of the paint tends to be good. In this specification, the curable components of the paint refer to the components contained in the paint that polymerize upon irradiation with ionizing radiation. That is, N-substituted (meth)acrylamide monomer, urethane (meth)acrylate, and polyfunctional (meth)acrylate monomer, as well as other curable components that are included as needed.

[0024] (Polyfunctional urethane (meth)acrylate) A polyfunctional urethane (meth)acrylate is a compound having two or more (meth)acryloyloxy groups and a urethane bond in its molecule. The paint may contain only one type of polyfunctional urethane (meth)acrylate, or it may contain two or more types.

[0025] The number of (meth)acryloyloxy groups in the polyfunctional urethane (meth)acrylate is two or more, but preferably between two and ten, and more preferably between two and six. When the number of (meth)acryloyloxy groups in the polyfunctional urethane (meth)acrylate is within this range, the curability of the polyfunctional urethane (meth)acrylate is good.

[0026] Here, the polyfunctional urethane (meth)acrylate may be obtained by reacting a urethane prepolymer having an isocyanate group with a (meth)acrylate monomer having a hydroxyl group, or by reacting a urethane prepolymer having a hydroxyl group with a (meth)acrylate monomer having an isocyanate group, or by reacting a urethane prepolymer having an amino group with a (meth)acrylate monomer having an isocyanate group.

[0027] Urethane prepolymers are obtained, for example, by the reaction of polyisocyanate and polyol. In this process, urethane bonds are formed in the molecular chains of the urethane prepolymer by the reaction of polyisocyanate and polyol.

[0028] The polyols that can be used in the preparation of urethane prepolymers are not particularly limited, as long as they are compounds containing two or more hydroxyl groups. Examples of polyols include polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polyoxytetramethylene glycol (PTMG); polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); polycaprolactone polyols such as poly-ε-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; acrylic polyols; and the like. These can be used individually or in combination of two or more.

[0029] The number-average molecular weight of the polyol is preferably between 300 and 10,000, and more preferably between 500 and 8,000. When the number-average molecular weight of the polyol is 300 or higher, the interior of the coating film tends to become more flexible, and processability tends to improve further. The above number-average molecular weight is a styrene-equivalent value measured by gel permeation chromatography (GPC).

[0030] Furthermore, when preparing urethane prepolymers, low molecular weight polyols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, and other diols; and triols such as glycerin, trimethylolpropane, and hexanetriol may be used. These can be used individually or in combination of two or more.

[0031] On the other hand, polyisocyanates that can be used in the preparation of urethane prepolymers are any compounds containing two or more isocyanate groups. Examples of polyisocyanates include aromatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, a mixture of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate (TDI), 4,4'-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3'-vitrylene-4,4'-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethyl xylylene diisocyanate (TMXDI), and paraphenylenedi diisocyanate (PPDI); and 4,4'-dicyclohexylmethane diisocyanate (H 12 This includes alicyclic or aliphatic polyisocyanates such as MDI, hydrogenated xylylene diisocyanate (H6XDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), norbornene diisocyanate (NBDI), and derivatives of these polyisocyanates. These can be used individually or in combination of two or more. Examples of derivatives of the above polyisocyanates include adduct-modified products obtained by reacting diisocyanate with polyhydric alcohols; isocyanurate-modified products of diisocyanate; burette-modified products; alohanate-modified products, etc. Among these, alicyclic or aliphatic polyisocyanates, or their derivatives, are more preferred from the viewpoint of weather resistance of the resulting coating film.

[0032] Furthermore, when preparing the above-mentioned urethane prepolymer, a polyamine may be reacted together with the polyol and polyisocyanate. The polyamine can be any compound having two or more amino groups. Examples of polyamines include aliphatic polyamines such as ethylenediamine, propylenediamine, butylenediamine, and hexamethylenediamine; alicyclic polyamines such as isophoronediamine and piperazine; and aromatic polyamines such as phenylenediamine, toluenediamine, diethyltoluenediamine, dimethylthiotoluenediamine, xylylenediamine, and 4,4'-diaminodiphenylmethane.

[0033] Examples of (meth)acrylates to be reacted with the above-mentioned urethane prepolymer include hydroxy-containing (meth)acrylates such as hydroxyalkyl (meth)acrylates like 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; and isocyanate-containing (meth)acrylates such as 2-methacryloyloxyethyl isocyanate, 2-acryloyloxyethyl isocyanate, and methacryloyloxyethyl isocyanate ethyl ether.

[0034] The number-average molecular weight of the above-mentioned polyfunctional urethane (meth)acrylate is preferably 900 to 20,000, more preferably 1,000 to 17,000, and even more preferably 1,200 to 16,000. The number-average molecular weight of the polyfunctional urethane (meth)acrylate can be measured in the same way as the number-average molecular weight of the above-mentioned polyol. When the number-average molecular weight of the polyfunctional urethane (meth)acrylate is within this range, the viscosity of the paint tends to fall within the desired range. In addition, the coating film tends to have appropriate processability.

[0035] The viscosity of polyfunctional urethane (meth)acrylate at 25°C, as measured by a conical-plate rotational viscometer, is preferably between 5,000 mPa·s and 3,000,000 mPa·s, and more preferably between 10,000 mPa·s and 2,500,000 mPa·s. When the viscosity of the polyfunctional urethane (meth)acrylate is within the above range, the viscosity of the paint tends to fall within the desired range. Furthermore, from the viewpoint of processability, it is preferable that the glass transition temperature of the single-cured product be 80°C or lower.

[0036] The amount of polyfunctional urethane (meth)acrylate in the paint is preferably 20% to 90% by mass, and more preferably 30% to 70% by mass, relative to the total amount of curable components in the paint. When the amount of polyfunctional urethane (meth)acrylate is 20% by mass or more, the processability of the painted metal sheet tends to improve further. Also, when the amount of polyfunctional urethane (meth)acrylate is 70% by mass or less, the viscosity of the paint tends to fall within the desired range.

[0037] (Polyfunctional (meth)acrylate monomer) A polyfunctional (meth)acrylate monomer is a monomer containing two or more (meth)acryloyloxy groups in its molecule. However, substances equivalent to the above-mentioned polyfunctional urethane (meth)acrylate monomer are not included in the definition of a polyfunctional (meth)acrylate monomer. A paint may contain only one type of polyfunctional (meth)acrylate monomer, or it may contain two or more types.

[0038] The number of (meth)acryloyloxy groups in the polyfunctional (meth)acrylate monomer is preferably 2 to 4, and preferably 2 or 3, from the viewpoint of the hardness of the resulting coating film and suppressing excessive curing shrinkage.

[0039] The type of polyfunctional (meth)acrylate monomer is not particularly limited as long as it is compatible with the above-mentioned N-substituted (meth)acrylamide monomers and polyfunctional urethane (meth)acrylates. Specific examples of polyfunctional (meth)acrylate monomers include difunctional (meth)acrylate monomers such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, hydroxypivalate neopentyl glycol di(meth)acrylate, dicyclopentanyl di(meth)acrylate, caprolactone-modified dicyclopentenyl di(meth)acrylate, ethylene oxide-modified phosphate di(meth)acrylate, allylated cyclohexyl di(meth)acrylate, and isocyanurate di(meth)acrylate; trimethylolpropane tri(meth) This includes trifunctional (meth)acrylate monomers such as acrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, propionic acid-modified dipentaerythritol tri(meth)acrylate, pentaerythritol tri(meth)acrylate, propylene oxide-modified trimethylolpropane tri(meth)acrylate, and tris(acryloxyethyl) isocyanurate; and tetrafunctional or more (meth)acrylate monomers such as propionic acid-modified dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ethylene oxide-modified dipentaerythritol hexa(meth)acrylate, and caprolactone-modified dipentaerythritol hexa(meth)acrylate. Among these, trimethylolpropane tri(meth)acrylate and ethylene oxide-modified trimethylolpropane tri(meth)acrylate are particularly preferred from the viewpoint of paint viscosity.

[0040] The molecular weight of the polyfunctional (meth)acrylic is preferably between 100 and 1000, as this allows the viscosity of the paint to easily fall within the desired range and also ensures compatibility with other components.

[0041] The amount of polyfunctional (meth)acrylate monomer in the paint is preferably 1% to 50% by mass, and more preferably 5% to 40% by mass, relative to the total amount of curable components in the paint. When the amount of polyfunctional (meth)acrylate monomer is within this range, not only is it easier for the viscosity of the paint to fall within the desired range, but the processability and hardness of the painted metal sheet tend to be further improved.

[0042] (Other curing components) The paint may further contain curable components other than N-substituted (meth)acrylamide monomers, polyfunctional urethane (meth)acrylates, and polyfunctional (meth)acrylate monomers (other curable components). Examples of other curable components include monofunctional (meth)acrylate monomers. When the paint contains monofunctional (meth)acrylate monomers, the viscosity of the paint is more easily brought within the desired range.

[0043] Examples of monofunctional (meth)acrylate monomers include ethyl carbitol acrylate, methoxyethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, β-(meth)acryloyloxyethyl hydrogen phthalate, β-(meth)acryloyloxyethyl hydrogen succinate, nonylphenoxyethyl (meth)acrylate, 3-chloro-2-hydroxypropyl (meth)acrylate, phenoxyethyl (meth)acrylate, and phenoxypolyethylene glycol. Polyethylene glycol (meth)acrylate, butoxy polyethylene glycol (meth)acrylate, alkyl (meth)acrylate, cyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxyethyl phosphate Acrylic acid, 3-acryloyloxyglycerin mono(meth)acrylate, 2-hydroxybutyl(meth)acrylate, 2-hydroxy-1-(meth)acryloxy-3-(meth)acryloxypropane, polypropylene glycol mono(meth)acrylate, polyethylene glycol mono(meth)acrylate, poly-ε-caprolactone mono(meth)acrylate, dialkylaminoethyl(meth)acrylate, glycidyl(meth)acrylate, mono[2-(meth)acryloyloxyethyl]acidphosphine This includes phosphate, trifluoroethyl (meth)acrylate, 2,2,3,3-tetrafluoropropyl (meth)acrylate, 2,2,3,4,4,4-hexafluorobutyl (meth)acrylate, perfluorooctylethyl (meth)acrylate, dicyclopentenyloxyalkyl (meth)acrylate, dicyclopentenyl (meth)acrylate, tricyclodecanyl (meth)acrylate, tricyclodecanyloxyethyl (meth)acrylate, and isobornyloxyethyl (meth)acrylate, etc. These can be used individually or in combination of two or more.

[0044] The amount of monofunctional (meth)acrylate monomer in the paint is preferably 1% by mass or more and 60% by mass or less, and more preferably 5% by mass or more and 50% by mass or less, relative to the total amount of curable components in the paint. When the amount of monofunctional (meth)acrylate monomer is within this range, the viscosity of the paint tends to fall within the desired range.

[0045] (Other ingredients) Paints may contain components other than the curing component. Examples of other components include inorganic particles, organic particles, and coloring pigments.

[0046] When a paint contains inorganic or organic particles, it is possible to create irregularities on the surface of the coating film obtained from the paint, thereby reducing the gloss of the coating film. The average particle size of such inorganic or organic particles is preferably 4 to 80 μm, and more preferably 10 to 60 μm. The average particle size of 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 substantially spherical shape is preferred from the viewpoint of easily adjusting the surface condition of the resulting coating film. 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. However, as will be explained in the method for manufacturing painted metal plates described later, by including the above-mentioned N-substituted (meth)acrylamide monomer in the paint, it is possible to form the desired irregularities on the surface of the coating film even without including inorganic or organic particles. Therefore, it is preferable that the paint of the present invention contains a small amount of such inorganic or organic particles, or does not contain them at all. In particular, from the standpoint of adjusting the viscosity of the paint, the total amount of inorganic and organic particles is preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably none at all, relative to the total amount of paint.

[0047] On the other hand, examples of coloring pigments include titanium dioxide, iron oxide, yellow iron oxide, phthalocyanine blue, carbon black, and cobalt blue. The average particle size of the coloring pigment, as measured by the Coulter counter method, is preferably about 0.2 to 2.0 μm. The amount of coloring pigment is preferably 0.1% to 60% by mass, and more preferably 10% to 50% by mass, relative to the total amount of paint.

[0048] Furthermore, the paint may contain known additives such as dispersants as needed. In addition, the paint may contain a photopolymerization initiator. However, the paint of the present invention is cured by irradiation with ionizing radiation. Therefore, it is not necessary to include a photopolymerization initiator such as a radical polymerization initiator, and in particular when the painted metal sheet is used as an exterior building material, it is preferable from the viewpoint of weather resistance that the paint film does not contain a photopolymerization initiator. Accordingly, the amount of photopolymerization initiator relative to the total amount of paint is preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably none.

[0049] (Paint viscosity) The viscosity of the paints mentioned above is selected appropriately depending on the application and application method. It is usually measured using a conical-plate rotational viscometer at 25°C and a shear rate of 30s. -1 The viscosity of the paint is preferably between 0.1 Pa·s and 10 Pa·s, and more preferably between 0.2 Pa·s and 5 Pa·s. When the viscosity of the paint is within this range, it becomes easier to form a coating of the desired thickness on the metal plate.

[0050] (Method for preparing paint) The above-mentioned coatings can be prepared by mixing N-substituted (meth)acrylamide monomers, polyfunctional urethane (meth)acrylates, polyfunctional (meth)acrylate monomers, and other curable components or other components. The method of mixing these is not particularly limited and can be prepared by stirring or dispersion using known methods.

[0051] 2. Painted metal sheet The painted metal sheet of the present invention comprises at least a metal sheet and a coating film of the above-mentioned paint, wherein the coating film is a cured product of the above-mentioned paint. Therefore, when processing the painted metal sheet, cracks and other damage are less likely to occur in the coating film, and pressure marks are less likely to occur even when the painted metal sheet is wound onto a roll.

[0052] The metal sheet used for the painted metal sheet can be selected from known metal sheets within the range that the effects of the present invention can be obtained. Examples of the above 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 duplex types), aluminum sheets, aluminum alloy sheets, and copper sheets.

[0053] From the viewpoint of corrosion resistance and weight reduction, plated steel sheets or stainless steel sheets are preferred for the above metal sheets, and from the viewpoint of cost-effectiveness, plated steel sheets are preferred. Furthermore, from the viewpoint of further improving corrosion resistance, hot-dip 55% Al-Zn alloy plated steel sheets, Zn-Al-Mg alloy plated steel sheets, or aluminum-based plated steel sheets are preferred for the above metal sheets. Of these, zinc-based plated steel sheets are preferred, and zinc-based plated steel sheets containing magnesium, such as Zn-Al-Mg alloy plated steel sheets, are more preferred. The metal sheets may have a chemical conversion coating or an undercoat coating formed on their surface, to the extent that it does not impair the effects of the present invention. Furthermore, the metal sheets may have embossing or deep drawing processes, or other textured processing, to the extent that it does not impair the effects of the present invention.

[0054] The thickness of the above metal sheet is selected based on the intended use of the painted metal sheet. For example, if the painted metal sheet is used as an exterior building material, the thickness of the above metal sheet can be 0.15 mm to 0.5 mm.

[0055] Furthermore, while the metal plate may be in the form of a single sheet, a strip shape, i.e., a metal strip, is preferable from the standpoint of easily enjoying the effect of suppressing pressure marks.

[0056] On the other hand, the coating film can be any cured product of the aforementioned paint. The thickness of the coating film is preferably 1 μm to 50 μm, and more preferably 3 μm to 30 μm. Within this range, it is possible to protect the metal plate with the coating film. Furthermore, within this range, cracks and peeling of the coating film are less likely to occur during processing of the painted metal plate.

[0057] Furthermore, the gloss value of the coating surface is appropriately selected according to its application. As will be explained later in the section on the manufacturing method of painted metal sheets, the surface condition can be adjusted by the manufacturing method of the painted metal sheet (coating film).

[0058] For example, in applications where high gloss is required for the coating film, the 60° gloss value of the coating film can be set to 70 or higher, and even 80 or higher. Furthermore, when high gloss is required for the coating film, the arithmetic mean surface roughness Ra of the coating film, measured using a needle-type surface roughness measuring instrument (evaluation length 4 mm, speed 0.6 mm / s, cutoff value 0.8 mm, λs value 2.5 μm), is preferably 0.20 μm or less, and more preferably 0.10 μm or less. When the arithmetic mean surface roughness Ra of the coating film is 0.10 μm or less, the gloss value of the coating film tends to be higher.

[0059] On the other hand, in applications where low gloss is required for the coating film, the 60° gloss value of the coating film can be set to 10 or less, and even to 5 or less. When low gloss is required for the coating film, the arithmetic mean surface roughness Ra of the coating film measured using a needle-type surface roughness measuring instrument (evaluation length 4 mm, speed 0.6 mm / s, cutoff value 0.8 mm, λs value 2.5 μm) is preferably 0.1 μm or more, and more preferably 0.5 μm or more. If the arithmetic mean surface roughness Ra of the coating film is 1.0 μm or more, the gloss value of the coating film tends to be lower.

[0060] However, the preferred gloss value and arithmetic mean surface roughness Ra of the coating film are not limited to the above values.

[0061] 3. Method for manufacturing painted metal sheets The above-mentioned painted metal sheet can be manufactured by a method that includes at least the steps of applying the above-mentioned paint to a metal sheet and irradiating the paint film with ionizing radiation. In the method for manufacturing a painted metal sheet of the present invention, the gloss of the resulting coating can be adjusted by the type of ionizing radiation and the irradiation method. The cases of forming a high-gloss coating and forming a low-gloss coating will be described separately below.

[0062] (When forming a high-gloss coating) To form a high-gloss coating, the process involves applying the aforementioned paint to a metal plate (paint application step) and irradiating the paint film with an electron beam (electron beam irradiation step). In this method, only an electron beam is used as the ionizing radiation. This method allows the film to be hardened in the electron beam irradiation step without significantly altering the surface shape of the film formed in the paint application step, resulting in a coating with a smooth surface.

[0063] Comparing the curing reactivity of the N-substituted (meth)acrylamide monomer, urethane (meth)acrylate, and polyfunctional (meth)acrylate monomer contained in the above-mentioned paint, the curing reactivity of the N-substituted (meth)acrylamide monomer is the best. Therefore, when the above film is irradiated with an electron beam in the electron beam irradiation step described above, the N-substituted (meth)acrylamide monomer efficiently cures the urethane (meth)acrylate and polyfunctional (meth)acrylate monomer. As a result, it is believed that a coating film with high gloss value can be obtained, with minimal surface irregularities. Furthermore, in the coating film obtained by this method, pressure marks are less likely to occur on the coating film because the N-substituted (meth)acrylamide monomer efficiently cures the entire coating film. In addition, because the film contains cured products of urethane (meth)acrylate and polyfunctional (meth)acrylate monomer with low hardness, the processability of the coating film is improved.

[0064] The method of applying the paint in the above-described paint application process is not particularly limited as long as it is possible to apply the above-described paint to the surface of the metal plate to the desired thickness. Examples of paint application methods include the roll coating method, curtain flow method, spin coating method, air spray method, airless spray method, and immersion-pull method. Among these, the roll coating method is preferred from the viewpoint of efficiency.

[0065] In the electron beam irradiation step, the film formed in the paint coating step is irradiated with an electron beam. The acceleration voltage of the electron beam used for irradiation is not particularly limited as long as it can cure to the depths of the film, and it is preferable to adjust it appropriately according to, for example, the density of the paint or the desired thickness. For example, if the desired thickness of the coating film is about 20 μm, the voltage can be set to about 50 kV to 120 kV. Within this range, the paint can be cured efficiently.

[0066] Furthermore, the electron dose can be appropriately selected according to the desired degree of hardening. For example, it is preferable to adjust the electron dose so that almost no unhardened monomers are detected in the hardened coating film by GC-MS (gas chromatography / mass spectrometry) or IR (infrared spectroscopy) analysis (for example, to less than 100 ppm). The electron dose can be, for example, 40 kGy to 100 kGy.

[0067] (When forming a low-gloss coating) To form a low-gloss coating, the process involves applying the aforementioned paint to a metal plate (paint application step), irradiating the paint film with ultraviolet light with a wavelength of 150 nm to 300 nm (short-wavelength UV irradiation step), and irradiating the film irradiated with ultraviolet light with an electron beam (electron beam irradiation step). In this method, short-wavelength UV and an electron beam are used as ionizing radiation. This method makes it possible to form uniform, fine irregularities on the surface of the coating film, thereby reducing the surface gloss.

[0068] As described above, when comparing the curing reactivity of the N-substituted (meth)acrylamide monomer, urethane (meth)acrylate, and polyfunctional (meth)acrylate monomer contained in the paint, the curing reactivity of the N-substituted (meth)acrylamide monomer is the best. Therefore, in the short-wavelength UV irradiation process, when short-wavelength UV is irradiated, the N-substituted (meth)acrylamide monomer on the surface side of the film promotes the curing reaction and forms a film. However, at this time, irradiation with short-wavelength UV alone does not completely cure the inside of the film. Therefore, shrinkage associated with the curing reaction of the N-substituted (meth)acrylamide monomer is likely to occur, and the desired irregularities are formed on the surface. Then, by performing an electron beam irradiation process after the short-wavelength UV irradiation process, the urethane (meth)acrylate and polyfunctional (meth)acrylate monomer cure inside the film.

[0069] According to this method, the curing reactivity of the N-substituted (meth)acrylamide monomer is excellent, making it easy to create the desired surface texture even with a small amount of integrated light during the short-wavelength UV irradiation process. Therefore, according to this embodiment, it is possible to manufacture a coated metal sheet having a low gloss film at high speed.

[0070] Furthermore, the coating obtained by this method has increased surface hardness due to the N-substituted (meth)acrylamide monomer film. As a result, pressure marks are less likely to occur on the coating. In addition, because the film contains cured products of low-hardness urethane (meth)acrylate and polyfunctional (meth)acrylate monomers, the processability of the coating is improved.

[0071] The paint application process in this method is the same as the paint application process for forming the high-gloss coating described above.

[0072] The short-wavelength UV irradiation step involves irradiating the film formed in the above-mentioned coating step with short-wavelength UV light (light with a wavelength of 150 nm to 300 nm). The wavelength of the short-wavelength UV light is preferably one that can be absorbed by the carbon-carbon double bond of the N-substituted (meth)acrylamide monomer, preferably between 150 nm and 250 nm, and more preferably between 150 nm and 200 nm. There are no particular restrictions on the light source for irradiating with short-wavelength UV light, but using an excimer light source or UV-LED light source with a single wavelength peak is preferable from the viewpoint of easily obtaining a low-gloss coating film.

[0073] Furthermore, the irradiance and irradiation time of the short-wavelength UV in the short-wavelength UV irradiation process are selected as appropriate, but the integrated light amount is 1 mJ / cm 2 More than 1000mJ / cm 2 It is preferable to set it as follows: 5 mJ / cm 2 More than 100mJ / cm 2 It is preferable to set the value as follows: If it is above the lower limit, the desired irregularities will be more easily formed on the surface of the coating. On the other hand, if it is below the upper limit, the coating will not harden excessively, and in this case as well, the desired irregularities will be more easily formed on the surface of the coating.

[0074] Next, the film irradiated with the short-wavelength UV light is irradiated with an electron beam (electron beam irradiation step). The acceleration voltage of the electron beam used for irradiation is not particularly limited as long as it can cure the film to its depths, and it is preferable to adjust it appropriately according to the density of the coating and the desired thickness. For example, if the desired thickness of the coating film is about 20 μm, the voltage can be set to about 50 kV to 120 kV. Within this range, the coating can be cured efficiently.

[0075] Furthermore, the electron dose can be appropriately selected according to the desired degree of hardening. For example, it is preferable to adjust the electron dose so that almost no unhardened monomers are detected in the hardened coating film by GC-MS (gas chromatography / mass spectrometry) or IR (infrared spectroscopy) analysis (for example, to less than 100 ppm). The electron dose can be, for example, 40 kGy to 100 kGy. [Examples]

[0076] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0077] 1. Preparation of the metal plate Plate thickness 0.3mm, plating adhesion amount per side of A4 size 45g / m² 2 A hot-dip Zn-plated steel sheet was used. After alkaline degreasing the plated steel sheet, a coating-type chromate (NRC300NS, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) with a Cr content of 50 mg / m² was applied. 2 The material was applied to achieve the specified adhesion amount. Furthermore, a primer composition (FL641EU primer, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was applied using a roll coater to a dry film thickness of 5 μm. Afterward, it was baked to a maximum plate temperature of 215°C to form a metal plate.

[0078] 2. Fabrication of painted metal sheets (1) Materials of the paint The following components were used as materials for the paint. The viscosity of each component was measured at 25°C using the viscosity measurement method based on JIS Z 8803, with a cone-plate rotational viscometer.

[0079] • N-substituted (meth)acrylamide monomer ACMO: Acryloylmorpholine (manufactured by KJ Chemicals, viscosity (25℃) 12 mPa·s) HEAA: N-hydroxyethylacrylamide (manufactured by KJ Chemicals, viscosity (25℃) 280 mPa·s) DMAPAA: N,N-dimethylaminopropylacrylamide (manufactured by KJ Chemicals, viscosity (25℃) 70 mPa·s)

[0080] • Urethane (meth)acrylate UN-353 (Bifunctional urethane acrylate, ART RESIN UN-353, manufactured by Negami Kogyo Co., Ltd., viscosity (25℃) 2,000,000 mPa·s) ETERCURE6106 (bifunctional urethane acrylate, manufactured by Choko Materials Industry Co., Ltd., viscosity (25℃) 9000 mPa·s)

[0081] • (meth)acrylate monomer TMPTA: Trimethylolpropane triacrylate (polyfunctional acrylate, Viscoat #295, manufactured by Osaka Organic Chemical Industry Co., Ltd., viscosity (25℃): 70 mPa·s) IXBA: Isobornyl acrylate (monofunctional acrylate, manufactured by Osaka Organic Chemical Industry Co., Ltd., viscosity (25℃) 7.7 mPa·s)

[0082] ·others Silica particles Coloring pigment: A mixture of titanium dioxide (JR603, manufactured by Teika Co., Ltd.) and a dispersant (DISPERBYK-111, manufactured by BIC Chemie Japan Co., Ltd.) (mass ratio: 99 / 1)

[0083] (2) Preparation of paints (Examples 1-16 and Comparative Examples 1-11) Each component was mixed in the mass ratios shown in Tables 1 and 2.

[0084] (3) Preparation of painted metal sheets having a high-gloss coating (Examples 1-8 and Comparative Examples 1-6) The above-mentioned paints were applied to the metal plates using a bar coater so that the cured film thickness was 20 μm. The metal plates coated with these paints were placed in a batch-type electron beam (EB) apparatus (manufactured by Iwasaki Electric Co., Ltd.) and irradiated with an electron beam (110 kV, 100 kGy) under a nitrogen atmosphere (oxygen concentration less than 300 ppm). As a result, painted metal plates with the desired coating film formed on them were obtained.

[0085] (4) Preparation of painted metal sheets having a low-gloss coating (Examples 9-16 and Comparative Examples 7-11) The above-mentioned paints were applied to the metal plates using a bar coater so that the cured film thickness was 20 μm. While the metal plates coated with the paints were being transported on a conveyor belt, an excimer light irradiation unit (manufactured by Ushio Inc.) was used to irradiate them with light of a wavelength of 172 nm (Xe2) at an irradiance of 130 mW / cm². 2 (50% neutral density filter applied), integrated light intensity 5 mJ / cm 2 The metal plate was irradiated in such a manner. The irradiation unit consisted of an irradiation window length of 60 mm, a light source-to-metal plate distance of 300 mm, and a nitrogen atmosphere (oxygen concentration less than 100 ppm). Subsequently, the metal plate was placed in a batch-processing type electron beam apparatus (manufactured by Iwasaki Electric Co., Ltd.) and irradiated with an electron beam (110 kV, 100 kGy) under a nitrogen atmosphere (oxygen concentration less than 300 ppm). As a result, a painted metal plate with the desired coating film formed on it was obtained. Except for the transport speed when irradiated with short-wavelength UV (light with a wavelength of 172 nm), the integrated light dose of short-wavelength UV is 10 mJ / cm², as described above. 2 , 20 mJ / cm 2 50 mJ / cm² 2 , and 100 mJ / cm 2 The painted metal plates were then fabricated after making adjustments to achieve the desired result.

[0086] 3. Evaluation The paint prepared as described above, and the coating film obtained from said paint, were evaluated as follows. The results are shown in Tables 1 and 2.

[0087] (1) Viscosity of the paint The viscosity of the paint was measured using a conical-plate rotational viscometer at a temperature of 25°C and a shear rate of 30 s. -1 Measurements were taken at the specified location. Furthermore, the evaluation was conducted according to the following criteria. A: Less than 10 Pa·s, liquid. B: 10 Pa·s or higher, no liquidity

[0088] (2) Appearance evaluation of low-gloss coating For Examples 9-16 and Comparative Examples 7-11, the appearance of the coating film was visually evaluated when the integrated light intensity of short-wavelength UV was varied. The evaluation was performed according to the following criteria. In the evaluation below, A and B were considered acceptable. A: Good low gloss across the entire surface of the coating. B: Partially low gloss C: High gloss across the entire coating.

[0089] (3) Gloss value The 60° gloss value of each coating was measured using a gloss meter. For Examples 9-16 and Comparative Examples 7-11, the gloss value was confirmed on a painted metal plate with an integrated short-wavelength UV light intensity of 5 mJ / cm.

[0090] (4) Surface roughness The arithmetic mean roughness Ra value was measured on the coating surface using a needle-type surface roughness analyzer with an evaluation length of 4 mm, a speed of 0.6 mm / s, a cutoff value of 0.8 mm, and a λs value of 2.5 μm. For Examples 9-16 and Comparative Examples 7-11, the integrated light intensity of short-wavelength UV was 5 mJ / cm². 2 The surface roughness was checked on a painted metal plate.

[0091] (5) Pressure mark resistance Two test specimens were prepared by cutting a painted metal sheet to a size of 100mm x 100mm. The surface (paint side) of one test specimen and the back (metal sheet side) of the other test specimen were placed on top of each other and pressed at 50 kg / cm². 2 The load was applied for 24 hours. Then, the two test pieces were peeled off, and the surface of the coating film in contact with the metal plate was visually observed. For Examples 9-16 and Comparative Examples 7-11, the integrated light amount of short-wavelength UV was 100 mJ / cm². 2 Test specimens were prepared from painted metal plates. In the evaluation below, A to C were considered passing grades. A: There are no visible pressure marks, and the surface is not in contact with the back of the object. B: There are some pressure marks, but there is no contact with the back of the object. C: There is a pressure mark and it is in close contact with the back surface of the object, but it is removable. D: The pressure marks are significant, and the material is firmly attached to the back surface, making it impossible to remove.

[0092] (6) Evaluation of processability Painted metal sheets were T-bent in a constant temperature and humidity chamber at 23°C and 50% relative humidity. Specifically, ten metal sheets of the same thickness as the painted metal sheet were sandwiched inside the painted metal sheet (on the metal sheet side). Then, the sheet was bent 180° so that the paint film was on the outside. For Examples 9-16 and Comparative Examples 7-11, the integrated light intensity of short-wavelength UV was 100 mJ / cm². 2 The processability was evaluated using painted metal sheets. A: No cracks will occur in the coating. B: Cracks have appeared in the paint film.

[0093] (7) Pencil hardness evaluation In accordance with JIS K5600 (General Test Methods for Paints), the hardness of the paint film was examined by scratching the surface with a pencil lead, and evaluated using the pencil lead concentration that did not scratch the paint film. For Examples 9-16 and Comparative Examples 7-11, the integrated light intensity of short-wavelength UV was 100 mJ / cm². 2 The test specimens were evaluated using the specified method. A score of 2H or higher was considered acceptable.

[0094] [Table 1]

[0095] [Table 2] As shown in Table 1 above, in Examples 1 to 16, coatings were formed using a paint containing N-substituted (meth)acrylamide monomer, polyfunctional urethane (meth)acrylate, and polyfunctional (meth)acrylate monomer, wherein the amount of N-substituted (meth)acrylamide monomer was 1% by mass or more and 60% by mass or less relative to the total amount of curable components in the paint. In all cases, the coatings exhibited excellent pressure mark resistance and good processability. Furthermore, when curing the coating using the above paint by irradiation with electron beams only, coatings with high gloss values ​​were obtained (Examples 1 to 8). On the other hand, when curing by irradiation with short-wavelength UV and electron beams, coatings with low gloss values ​​were obtained (Examples 9 to 16).

[0096] In contrast, as shown in Table 2, Comparative Example 1, in which a coating film was formed using a paint that did not contain urethane (meth)acrylate, exhibited poor processability. Furthermore, even when containing N-substituted (meth)acrylamide monomer, polyfunctional urethane (meth)acrylate, and polyfunctional (meth)acrylate monomer, Comparative Examples 2 and 7, which contained small amounts of N-substituted (meth)acrylamide monomer, showed poor pressure mark resistance. It is thought that the surface did not harden sufficiently. Similarly, when using paint that did not contain N-substituted (meth)acrylamide monomer, pressure mark resistance was also poor (Comparative Examples 3-5 and 8-11). Moreover, in these comparative examples, a low-gloss coating film could not be obtained even when irradiated with short-wavelength UV and electron beams. [Industrial applicability]

[0097] The present invention provides a painted metal sheet that has good processability and a coating that is less prone to pressure marks. This painted metal sheet is extremely useful in the manufacture of various products with high design appeal, including exterior building materials.

Claims

1. A metal plate and A coating film, which is a cured product of an ionizing radiation-curing paint, is placed on the aforementioned metal plate, It is a painted metal sheet having the following characteristics: The ionizing radiation-curable coating comprises an N-substituted (meth)acrylamide monomer, a polyfunctional urethane (meth)acrylate, and a polyfunctional (meth)acrylate monomer. The amount of the N-substituted (meth)acrylamide monomer is 1% by mass or more and 60% by mass or less relative to the total amount of curable components of the ionizing radiation-curable paint. Painted metal sheet.

2. The N-substituted (meth)acrylamide monomer is at least one selected from the group consisting of N-acryloylmorpholine, N-hydroxyethylacrylamide, and N,N-dimethylaminopropylacrylamide. The painted metal plate according to claim 1.

3. The amount of the polyfunctional (meth)acrylate monomer is 1% by mass or more and 50% by mass or less, relative to the total amount of curable components of the ionizing radiation-curable paint. The painted metal plate according to claim 1.

4. A step of applying an ionizing radiation-curable coating containing an N-substituted (meth)acrylamide monomer, a polyfunctional urethane (meth)acrylate, and a polyfunctional (meth)acrylate monomer onto a metal plate, The process involves irradiating the film of the ionizing radiation-curable paint with ionizing radiation, Includes, The amount of the N-substituted (meth)acrylamide monomer in the ionizing radiation-curable paint is 1% by mass or more and 60% by mass or less, relative to the total amount of curable components of the ionizing radiation-curable paint. A method for manufacturing painted metal sheets.

5. The step of irradiating with ionizing radiation is the step of irradiating the film with an electron beam. The method for manufacturing a painted metal sheet according to claim 4.

6. The step of irradiating with ionizing radiation is a step of irradiating the film with ultraviolet light with a wavelength of 150 nm or more and 300 nm or less, and then irradiating it with an electron beam. The method for manufacturing a painted metal sheet according to claim 4.

7. An ionizing radiation-curing paint for obtaining the coating film on a painted metal plate according to any one of claims 1 to 3, The material comprises an N-substituted (meth)acrylamide monomer, a polyfunctional urethane (meth)acrylate, and a polyfunctional (meth)acrylate monomer. The amount of the N-substituted (meth)acrylamide monomer is 1% by mass or more and 60% by mass or less, relative to the total amount of curable components of the ionizing radiation-curable paint. Ionizing radiation-curable paint.

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