Resin-coated metal plate, two-piece can, and method for manufacturing resin-coated metal plate

Abstract

Provided is a resin-coated metal plate having excellent processability and post-processing adhesion, while suppressing appearance defects and reduction in smoothness. This resin-coated metal plate (1) is provided on at least one surface of a metal plate (2) with resin coating layers (3, 4) containing 90 mass% or more of a polyester resin with respect to the total resin, the crystal content of the resin coating layers being 15% or less, and the arithmetic average height (Sa) of the surface of the resin coating layers being 0.30 [mu] m or less. The resin coating layer contains 8-30 mass% of titanium dioxide, and the amount of Ti detected at the interface between the resin coating layer and the metal plate as determined by elemental analysis by X-ray photoelectron spectroscopy is 2 at% or less.

Description

Technical Field

[0001] The present invention relates to a resin-coated metal sheet having a resin coating layer, a two-piece can, and a method for producing the resin-coated metal sheet. Background Art

[0002] Currently, metal sheets such as tin-free steel (TFS) and aluminum used as raw materials for metal containers are coated metal sheets that have been painted to improve corrosion resistance. However, the production of coated metal sheets presents challenges: the coating and baking process is complex and has low productivity, requiring significant processing time, and emitting large amounts of solvents and carbon dioxide, placing a significant environmental burden.

[0003] To solve the above-mentioned problems, resin-coated metal sheets, which are formed by laminating a thermoplastic resin film on the surface of a heated metal sheet, have been developed as an alternative to painted metal sheets. Currently, these sheets are widely used industrially, mainly in beverage cans and food cans.

[0004] Metal containers are generally divided into two-piece cans and three-piece cans. A two-piece can is a metal container consisting of a can body and a lid that are integrated with the can bottom. On the other hand, a three-piece can is a metal container consisting of a can body, a top lid, and a bottom lid. Two-piece cans have a beautiful appearance because they do not have welded parts. On the other hand, the metal sheets used as the raw materials for two-piece cans are generally required to have a high degree of processing. In addition, the high degree of processing associated with the development of canning processing technology has created a new problem: the resin-coated metal sheets for two-piece cans may produce appearance defects (rough surface) on the resin coating during heat treatment after canning.

[0005] Regarding two-piece cans, there are proposed technologies for manufacturing can bodies using resin-coated metal sheets as raw materials through deep drawing or DI (draw and ironing) processes (Patent Documents 1 and 2). Furthermore, there is a proposed technology for controlling the amount of crystallization in the resin coating layer to suppress surface roughness generated during heat treatment after canning (Patent Document 3).

[0006] [Prior art literature]

[0007] [Patent Document]

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 04-091825

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2004-148324

[0010] Patent Document 3: International Publication No. 2013 / 030972 Summary of the Invention

[0011] [Problems to be Solved by the Invention]

[0012] The technologies described in Patent Documents 1 and 2 are fundamental technologies related to the production of two-piece cans. However, the advancement of canning processes, accompanied by higher degrees of processing, has also led to new challenges, such as surface roughness on the resin coating during post-canning heat treatment. This has made it even more important to control the properties of resin-coated metal sheets used in two-piece can production.

[0013] The technology described in Patent Document 3 can suppress surface roughness that occurs in the resin coating during heat treatment after canning. However, the resin coating, whose surface has reached a high temperature during coating, is pressed by the laminating rollers, potentially reducing the smoothness of the resin coating surface. This leaves room for improvement.

[0014] The present invention has been developed in light of the above-mentioned circumstances. Specifically, the present invention aims to suppress appearance defects (surface roughness) that may occur in the resin coating layer of a resin-coated metal sheet during heat treatment after canning, and to prevent a decrease in the smoothness of the resin coating layer during coating. Furthermore, the present invention aims to provide a resin-coated metal sheet having a smooth and beautiful appearance, excellent workability, and excellent adhesion of the resin coating layer after processing.

[0015] [Means for solving the problem]

[0016] The surface roughness of the resin coating is caused by the coating being pressed by the laminating rollers while the coating is still hot. The present inventors conducted extensive research and discovered the following: By implementing a two-step process: applying the resin coating to the metal sheet at a low temperature, followed by a very short heat treatment at a temperature exceeding the melting point of the resin coating, it is possible to suppress the appearance defects (surface roughness) that may occur in the resin coating during the heat treatment after canning. Furthermore, it is possible to provide a resin-coated metal sheet with a smooth and beautiful resin coating surface.

[0017] Furthermore, by reducing the crystallinity of the resin coating, residual stress introduced during processing can be reduced. Furthermore, by reducing the amount of inorganic additives present at the interface between the metal sheet and the resin coating, a resin-coated metal sheet with excellent processability and adhesion of the resin coating after processing can be obtained.

[0018] The present invention was made based on the above findings. That is, the gist of the present invention is as follows.

[0019] [1] A resin-coated metal plate comprising a resin coating layer containing 90% by mass or more of a polyester resin relative to the total resin on at least one surface of the metal plate, wherein:

[0020] The crystallization amount of the resin covering layer is less than 15%,

[0021] The arithmetic mean height Sa of the surface of the resin coating layer is 0.30 μm or less,

[0022] The resin coating layer contains 8% by mass or more and 30% by mass or less of titanium dioxide,

[0023] The amount of Ti detected at the interface between the resin coating layer and the metal plate, as determined by elemental analysis using X-ray photoelectron spectroscopy, was 2 atomic % or less.

[0024] [2] The resin-coated metal sheet according to [1] above, wherein the amount of Ti detected on the surface of the resin coating layer as determined by elemental analysis using X-ray photoelectron spectroscopy is 2 atomic % or less.

[0025] [3] The resin-coated metal sheet according to [1] or [2] above, wherein the resin coating layer has a multilayer structure including a first layer in contact with the metal sheet and a second layer located on the first layer,

[0026] The first layer has a thickness of 2 μm or greater and contains 2 mass % or less of titanium dioxide.

[0027] [4] The resin-coated metal sheet according to [3] above, wherein the resin coating layer has a multilayer structure further including a third layer, the third layer forming a surface of the resin coating layer and being located on the second layer,

[0028] The third layer has a thickness of 2 μm or more and contains 2% by mass or less of titanium dioxide.

[0029] [5] The resin-coated metal sheet according to any one of [1] to [4], wherein the resin coating layer contains 0.010% by mass or more and 1.0% by mass or less of wax.

[0030] [6] A two-piece can formed by coating a metal plate with the resin described in any one of [1] to [5] above, wherein the resin coating layer is located on the outer surface side.

[0031] [7] A method for manufacturing a resin-coated metal plate, comprising preparing a thermoplastic resin film having a multilayer structure and containing 8% by mass or more and 30% by mass or less of titanium dioxide as a whole and 90% by mass or more of a polyester resin relative to the total resin, wherein the multilayer structure comprises: a first layer having a thickness of 2 μm or more and containing 2% by mass or less of titanium dioxide; and a second layer in contact with the first layer.

[0032] The thermoplastic resin film is pressed against at least one surface of a metal plate heated to a temperature not lower than (the melting point of the thermoplastic resin film - 40°C) and not higher than (the melting point of the thermoplastic resin film + 5°C) so that the first layer contacts the metal plate.

[0033] The metal plate is heated to a heat treatment temperature of (melting point of the thermoplastic resin film + 5°C) or higher and (melting point of the thermoplastic resin film + 30°C) or lower over a period of 0.5 to 1.5 seconds, maintained at the heat treatment temperature for 0.5 to 1.5 seconds, and then cooled to obtain a resin-coated metal plate.

[0034] [8] The method for manufacturing a resin-coated metal plate according to any one of the above [7], wherein the thermoplastic resin film has a third layer in contact with the second layer, the third layer has a thickness of 2 μm or more and contains 2% by mass or less of titanium dioxide.

[0035] [Effects of the Invention]

[0036] According to the present invention, it is possible to suppress appearance defects (surface roughness) that may occur in the resin coating layer of a resin-coated metal sheet during heat treatment after canning, and to suppress a decrease in the smoothness of the surface of the resin coating layer during coating. Furthermore, it is possible to provide a resin-coated metal sheet having a smooth and beautiful appearance while achieving both workability and adhesion of the resin coating layer after processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a diagram showing a cross section of an example of a resin-coated metal plate.

[0038] Figure 2 This is a diagram showing a cross section of an example of a resin-coated metal plate having a three-layered resin coating. DETAILED DESCRIPTION

[0039] Hereinafter, an embodiment of the method for producing a resin-coated metal sheet of the present invention will be described. It should be noted that the embodiment described below is an example of a specific embodiment of the present invention, and the configuration of the present invention is not limited to the specific example.

[0040] The resin-coated metal sheet of the present invention comprises, on at least one side of the metal sheet, a resin coating layer containing 90% by mass or more of a polyester resin relative to the total resin, and is characterized in that the crystallization content of the resin coating layer is 15% or less, the arithmetic mean surface height Sa of the resin coating layer is 0.30 μm or less, the resin coating layer contains titanium dioxide in an amount of 8% by mass or more and 30% by mass or less, and the amount of Ti detected at the interface between the resin coating layer and the metal sheet, as determined by elemental analysis using X-ray photoelectron spectroscopy, is 2 atomic % or less.

[0041] According to the present invention, it is possible to suppress appearance defects (surface roughness) in the resin coating layer of a resin-coated metal sheet during heat treatment after canning, and to suppress a decrease in surface smoothness of the resin coating layer during coating. Furthermore, it is possible to provide a resin-coated metal sheet having a smooth and beautiful appearance. Furthermore, it is possible to provide a resin-coated metal sheet having reduced residual stress after canning and excellent workability of the resin coating layer and post-processing adhesion.

[0042] exist Figure 1 1 shows an example of a cross section of the resin-coated metal plate 1 according to one embodiment. Figure 1 The resin-coated metal sheet 1 shown has a resin coating layer 3 provided on the front side of the metal sheet 2, and a resin coating layer 4 provided on the back side of the metal sheet 2. It should be noted that the resin coating layer may be provided on only one side of the metal sheet 2. The resin coating layer 3 provided on the front side of the metal sheet 2 and the resin coating layer 4 provided on the back side of the metal sheet 2 are located on the outer surface and inner surface of the two-piece can, respectively, after canning.

[0043] [Metal Sheet]

[0044] First, the metal plate will be described. As the metal plate of the resin-coated metal plate, steel plates and aluminum plates, which are widely used as can materials, can be used.

[0045] As the metal plate, tin-free steel (TFS) is particularly preferred from the viewpoint of resin adhesion in a high-temperature, humid environment such as retort sterilization. The amount of the metal chromium layer and the chromium oxide layer applied to the TFS is not particularly limited, but preferably has an amount of 50 mg / m2 applied to the surface. 2 Above 200g / m 2 The following metal chromium layer and the amount of metal chromium on it is 3mg / m 2 Above 30g / m 2 Below the chromium oxide layer.

[0046] The type of metal plate is not particularly limited as long as it can be formed into a target shape, but steel plates having the following component compositions and production methods are preferred.

[0047] (1) A steel sheet obtained by using low-carbon steel having a C (carbon) content of more than 0.003 mass % and 0.10 mass % or less and performing recrystallization annealing by continuous annealing.

[0048] (2) A steel sheet obtained by using a low-carbon steel having a C content of more than 0.003 mass % and 0.10 mass % or less and performing recrystallization annealing and overaging treatment by continuous annealing.

[0049] (3) A steel sheet obtained by using low-carbon steel having a C content exceeding 0.003 mass % and 0.10 mass % or less and performing recrystallization annealing by box annealing.

[0050] (4) A steel sheet obtained by using a low-carbon steel having a C content of more than 0.003 mass % and 0.10 mass % or less, performing recrystallization annealing by continuous annealing or box annealing, and then performing secondary cold rolling (DR (Double Reduced) rolling).

[0051] (5) A steel plate obtained by continuous annealing and recrystallization annealing using IF (Interstitial Free) steel in which elements such as Nb and Ti for fixing dissolved C are added to ultra-low carbon steel having a C content of 0.003 mass % or less.

[0052] The mechanical properties of the metal sheet are not particularly limited as long as it can be formed into the desired shape. To ensure sufficient can strength without compromising workability, a metal sheet with a yield point (YP) of 220 MPa to 580 MPa is preferred. Furthermore, the Lankford value (r-value), an indicator of plastic anisotropy, is preferably 0.8 or higher. Furthermore, the absolute value of the in-plane anisotropy Δr of the r-value is preferably 0.7 or lower.

[0053] The composition of the metal plate is not particularly limited. For example, a steel plate containing component elements such as Si, Mn, P, S, Al and N may also be used. The Si content is preferably 0.001% by mass or more, and preferably 0.1% by mass or less. The Mn content is preferably 0.01% by mass or more, and preferably 0.6% by mass or less. The P content is preferably 0.002% by mass or more, and preferably 0.05% by mass or less. The S content is preferably 0.002% by mass or more, and preferably 0.05% by mass or less. The Al content is preferably 0.005% by mass or more, and preferably 0.100% by mass or less. The N content is preferably 0.0005% by mass or more, and preferably 0.020% by mass or less. In addition, the composition may also contain other components such as Ti, Nb, B, Cu, Ni, Cr, Mo and V. From the perspective of ensuring corrosion resistance, the content of the above-mentioned component elements is preferably 0.02% by mass or less in total.

[0054] The thickness of the metal plate is not particularly limited, and may be, for example, 0.10 mm or more and 0.50 mm or less.

[0055] [Composition of resin cover layer]

[0056] The resin-coated metal sheet comprises a resin coating layer composed primarily of polyester resin on at least one surface of the metal sheet. The proportion of the polyester resin in the resin constituting the resin coating layer is 90% by mass or greater, calculated as solids. If the resin coating layer contains inorganic additives (such as inorganic pigments), the proportion of the polyester resin in the resin, after deducting the weight of these inorganic additives, is 90% by mass or greater.

[0057] Polyester resin is a polymer composed of dicarboxylic acid units and ethylene glycol units.

[0058] As the dicarboxylic acid unit, units derived from aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenoxyethane dicarboxylic acid, 5-sodium sulfonatoisophthalic acid, and phthalic acid; aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, and fumaric acid; alicyclic dicarboxylic acids such as cyclohexane dicarboxylic acid; and hydroxycarboxylic acids such as p-hydroxybenzoic acid can be used.

[0059] The polyester resin preferably contains 90 mol% or more of terephthalic acid units among the dicarboxylic acid units. When the polyester resin contains 90 mol% or more of terephthalic acid units among the dicarboxylic acid units, sufficient heat resistance against frictional heat during continuous canning can be ensured, and more stable moldability and coating properties can be achieved.

[0060] As the ethylene glycol unit, units derived from aliphatic diols such as ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, and neopentyl glycol; alicyclic diols such as cyclohexanedimethanol; aromatic diols such as bisphenol A and bisphenol S; and diethylene glycol can be used.

[0061] In addition, the above-mentioned dicarboxylic acids and diols may be used in combination of two or more types within a range that does not impair heat resistance and processability.

[0062] In at least one of the resin coating layers, the titanium dioxide content is 8% to 30% by mass. If the titanium dioxide content is less than 8% by mass, the base metal cannot be adequately shielded. Furthermore, if it exceeds 30% by mass, the processability of the resin coating is impaired. That is, if the titanium dioxide content is within the above-specified range, the base metal can be shielded, the clarity of the print can be increased, and a white, good appearance can be achieved without compromising the processability of the resin coating. Examples of white pigments other than titanium dioxide include aluminum oxide, calcium carbonate, and barium sulfate. However, titanium dioxide has a strong tinting power and ensures a good appearance even after the can is molded. In particular, rutile-type titanium dioxide with a purity of 90% or more by mass is preferred because it has superior dispersibility when mixed with the resin material. The titanium dioxide content of the resin coating layer is preferably 10% by mass or greater. Furthermore, the titanium dioxide content of the resin coating layer is preferably 22% by mass or less. As described later, when the resin covering layer has a multi-layer structure, the content of titanium dioxide relative to the entire multi-layered resin covering layer is 8% by mass or more and 30% by mass or less.

[0063] [Amount of crystallization of the resin cover layer]

[0064] The amount of crystallization in the resin covering layer is set to 15% or less. By setting the amount of crystallization to 15% or less, the high moldability required for the molding of two-piece cans can be obtained, and the appearance defects (rough surface) generated in the resin covering layer during the heat treatment after the canning process can be suppressed. In addition, by setting the amount of crystallization to a low value, the residual stress in the resin covering layer after the canning process is reduced, thereby also obtaining excellent adhesion after processing. The amount of crystallization in the resin covering layer is preferably 12% or less, more preferably 10% or less. When the resin covering layer contains inorganic additives (inorganic pigments, etc.), the amount of crystallization in the resin material obtained by subtracting the mass of the inorganic additives needs to be 15% or less. The lower limit of the amount of crystallization is not particularly limited, and the amount of crystallization can be 1% or more. It should be noted that, as shown below, the amount of crystallization is calculated based on the content of the inorganic additives obtained by thermogravimetric measurement and the heat of crystallization and heat of fusion obtained by differential scanning calorimetry. Here, inorganic additives refer to inorganic pigments and inorganic additives among additives other than inorganic pigments.

[0065] The determination of the content of inorganic additives is carried out as follows. First, the resin-coated metal plate is immersed in a mixed solution of concentrated hydrochloric acid (12 mol / L): distilled water = 1:1 at room temperature to dissolve the metal plate to separate the resin coating layer. The separated resin coating layer is thoroughly washed with distilled water and then vacuum-dried. The thermogravimetric measurement is performed using a thermogravimetric measuring device with the temperature range set from room temperature to 800°C, an air flow rate of 300 mL / min, and a heating rate of 10°C / min. As shown in the following formula (1), the ratio of the weight at 800°C to the weight at room temperature is defined as the inorganic additive content.

[0066] Content of inorganic additives [%] = weight at 800°C [mg] / weight at room temperature [mg] × 100 (1)

[0067] The determination of the amount of crystallization is carried out as follows. In the same manner as the determination of the content of inorganic additives, the metal plate is dissolved from the resin-coated metal plate to separate the resin coating layer, and the resin coating layer is dried. The dried resin coating layer is measured from 0°C to 300°C at a heating rate of 10°C / min using a differential scanning calorimeter (DSCQ100) manufactured by TA Instruments. The heat of crystallization is calculated based on the area of the exothermic peak observed during the period of 100-200°C, and the heat of fusion is calculated based on the area of the endothermic peak measured during the period of 200°C to 280°C. Based on the obtained heat of crystallization and heat of fusion, the amount of crystallization is calculated according to the following formula (2). It should be noted that, with respect to the content of inorganic additives, the value obtained by the above-mentioned method is used.

[0068] Crystallization amount [%] = (heat of fusion [J / g] - heat of crystallization [J / g]) × 100 / (100 - inorganic additive content [%]) / 140.2 [J / g] × 100… (2)

[0069] [Surface smoothness of resin coating]

[0070] The arithmetic mean surface height Sa of the resin covering layer is set to 0.30 μm or less. If the arithmetic mean surface height Sa of the resin covering layer exceeds 0.30 μm, the surface roughness of the resin covering layer is large, resulting in a mottled pattern in the case of a colored resin covering layer and a hazy appearance in the case of a colorless resin covering layer. The lower limit of the arithmetic mean surface height Sa of the resin covering layer is not particularly limited, but it is preferably set to 0.10 μm or greater.

[0071] The arithmetic mean height (Sa) of the resin coating layer's surface was measured using a 3D profilometer. A KEYENCE One-Touch 3D profilometer was used to measure the surface roughness of a 1.9 mm x 1.4 mm field of view at 160x magnification. The arithmetic mean height (Sa) was calculated using surface roughness analysis. Measurements were taken at five randomly selected locations on the same surface of the resin-coated metal plate, and the maximum value was defined as the arithmetic mean height (Sa) of the resin-coated metal plate.

[0072] Because it is highly relevant to the product's appearance, the present invention uses the arithmetic mean height (Sa) as an indicator of surface smoothness. There are two types of arithmetic mean height: Ra, which represents the arithmetic mean height of a line, and Sa, which represents the arithmetic mean height of a surface. Using Sa as an indicator of surface smoothness eliminates the discrepancies in measurement results caused by the direction of measurement, as occurs with Ra, and allows evaluation of the entire surface.

[0073] It should be noted that the crystallinity and surface smoothness of such a resin covering layer are achieved, for example, by a two-stage treatment described in the manufacturing method described later. The two-stage treatment means: after covering the metal plate with a resin covering layer at a low temperature, a heat treatment is performed at a temperature exceeding the melting point for an extremely short time.

[0074] [Layer structure of resin cover layer]

[0075] The Ti detection amount at the interface of the resin coating layer and the metal plate obtained by elemental analysis by X-ray photoelectron spectroscopy is set to less than 2 atomic %. When the Ti detection amount at the interface of the resin coating layer and the metal plate exceeds 2 atomic %, the titanium dioxide present at the interface of the resin coating layer and the metal plate hinders the close contact of the metal plate and the resin coating layer, and the close contact of the resin coating layer after the canning process is insufficient. Preferably, the Ti detection amount at the interface of the resin coating layer and the metal plate obtained by elemental analysis by X-ray photoelectron spectroscopy is less than 1 atomic %. The lower limit of the Ti detection amount at the interface of the resin coating layer and the metal plate obtained by elemental analysis by X-ray photoelectron spectroscopy is not particularly limited and may also be 0 atomic %. The Ti detection amount at the interface of the resin coating layer and the metal plate obtained by elemental analysis by X-ray photoelectron spectroscopy can be measured by a conventionally known method as described in the embodiments described later. The measurement is preferably performed at a plurality of randomly selected positions within the same surface of the resin-coated metal plate, and the average value of the measurement result is set to the Ti detection amount. Taking into account the variations within the surface, the number of the randomly selected multiple locations is preferably 5 or more.

[0076] In order to reduce the amount of titanium dioxide present at the interface between the resin coating layer and the metal plate, there is a method of forming a multilayer structure having a layer containing no or very little titanium dioxide on the metal plate side of the resin coating layer. Figure 2 It represents a resin-coated metal plate having a multilayer structure in which a first layer 3c, a second layer 3b, and a third layer 3a are sequentially stacked on a metal plate 2. It should be noted that, in the present invention, the third layer 3a is an arbitrary layer. By reducing the titanium dioxide content of the first layer 3c located at the interface between the resin coating layer and the metal plate, the amount of titanium dioxide present at the interface between the resin coating layer and the metal plate can be reduced. The thickness of the first layer 3c is preferably not less than 2 μm for the reason of ensuring sufficient adhesion to the metal plate, and is preferably not more than 5 μm for the reason of ensuring a good appearance of the film. In addition, the titanium dioxide content of the first layer 3c is preferably not more than 2% by mass.

[0077] Furthermore, the amount of Ti detected on the surface of the resin coating layer, as determined by elemental analysis using X-ray photoelectron spectroscopy, is preferably 2 atomic % or less. A Ti content of 2 atomic % or less can suppress wear of the resin coating layer even during rigorous canning processes. More preferably, the amount of Ti detected on the surface of the resin coating layer, as determined by elemental analysis using X-ray photoelectron spectroscopy, is 1 atomic % or less. The lower limit of the amount of Ti detected on the surface of the resin coating layer, as determined by elemental analysis using X-ray photoelectron spectroscopy, is not particularly limited and may be 0 atomic %.

[0078] The amount of Ti detected on the surface of the resin coating layer, as determined by elemental analysis using X-ray photoelectron spectroscopy, was determined using the following method. At room temperature, the resin-coated metal plate was immersed in a mixture of concentrated hydrochloric acid (12 mol / L) and distilled water (1:1), dissolving the metal plate to separate the resin coating layer. The separated resin coating layer was then thoroughly rinsed with distilled water and vacuum-dried. X-ray photoelectron spectroscopy was performed on the interface between the dried resin coating layer and the metal plate using an X-ray photoelectron spectrometer (SSX-100 manufactured by SSI). The X-ray source was monochromatized Al Kα radiation, and the measurement was performed under the conditions of a measurement area of 600 μmφ, a cumulative number of 6 times, and a photoelectron escape angle of 35°. The elements were quantified using the resulting wide-scan spectrum, and the elemental ratio of Ti in the detected elements was calculated. Measurements were performed at five randomly selected locations on each sample, and the average value was used as the amount of Ti detected on the surface of the resin coating layer, as determined by elemental analysis using X-ray photoelectron spectroscopy. It should be noted that the measurement positions were separated by 5 mm or more from each other. The Ti element ratio at the surface of the resin-coated metal plate was also obtained by measuring the surface of the resin-coated metal plate under the same conditions.

[0079] In order to reduce the amount of titanium dioxide present on the surface of the resin coating layer, it is preferable to form a multilayer structure having a layer containing no titanium dioxide or a very low titanium dioxide content on the surface of the resin coating layer. Figure 2 The titanium dioxide content of the third layer 3a shown in FIG. 3 is preferably 2 μm or more, and preferably 5 μm or less. In addition, the titanium dioxide content of the third layer 3a is preferably 2% by mass or less. In addition to the interface between the resin coating layer and the metal plate, in order to reduce the amount of titanium dioxide on the surface of the resin coating layer, it is preferred to have a layer with a low titanium dioxide content on both sides of the resin coating layer ( Figure 2 The at least three-layer structure is composed of a third layer 3a and a first layer 3c).

[0080] To improve the sliding properties during processing, wax may be added to at least one side of the resin coating in an amount of 0.010% by mass or less. Alternatively, wax may be added in an amount of 1.0% by mass or less. In particular, it is preferred to add wax in an amount of 0.010% by mass or less and 1.0% by mass to the resin coating located on the outer surface of the metal container after molding. By adding 0.010% by mass or more of wax to the resin coating, the coefficient of friction on the surface of the resin coating during processing can be reduced, thereby suppressing wear of the resin coating. On the other hand, if the amount of wax added is 1.0% by mass or less, film formation of the resin coating is facilitated, so the amount of wax added is preferably 1.0% by mass or less.

[0081] As described above, when the resin covering layer is a multilayer structure, only the wax added near the surface of the resin covering layer contributes to the slipperiness of the surface of the resin covering layer. Therefore, the wax may be added only to the layer located on the surface. Alternatively, when a multilayer resin covering layer is provided on both sides of a metal plate, the layers on both sides of the resin covering layer may be made of the same raw material, and wax may also be added to the layer on the metal plate side of the resin covering layer. By adding wax only to the surface layer of the resin covering layer, the amount of wax used in the entire resin covering layer can be reduced, thereby suppressing the cost of the resin. It should be noted that if the amount of wax in the surface layer and the adjacent layer is significantly different, the difference in the physical properties of the resin increases, potentially reducing the adhesion between the layers. Therefore, adding a trace amount of wax of less than 0.10% by mass to the adjacent layers is effective.

[0082] As the wax, at least one selected from polyolefin waxes such as polyethylene and polypropylene and modified products thereof, natural waxes such as palm wax, polyamide wax, and polyester wax, or a mixture thereof can be used.

[0083] Furthermore, as needed, additives such as antioxidants, heat stabilizers, lubricants, crystal nucleating agents, ultraviolet absorbers, and antistatic agents may be added to the resin coating, provided that they do not impair the effects of the present invention. Furthermore, coloring pigments may be added to the resin coating to enhance the aesthetics of the inner and outer surfaces of the metal container.

[0084] The thickness of the resin cover layer is not particularly limited, and may be, for example, 6 mm or more, or 50 mm or less.

[0085] By using the above-mentioned resin-coated metal sheet, a two-piece can having a smooth and beautiful appearance can be manufactured. The two-piece can can be manufactured according to conventional methods. In this two-piece can, the resin coating layer is preferably located on the outer surface side of the two-piece can.

[0086] Next, an example of a method for manufacturing a resin-coated metal plate will be described.

[0087] The method for manufacturing the resin-coated metal sheet of the present invention is as follows:

[0088] A thermoplastic resin film having a multilayer structure and containing titanium dioxide in an amount of not less than 8% by mass and not more than 30% by mass, and a polyester resin in an amount of not less than 90% by mass relative to the total resin, is prepared. The multilayer structure comprises a first layer having a thickness of not less than 2 μm and containing titanium dioxide in an amount of not more than 2% by mass, and a second layer in contact with the first layer.

[0089] The thermoplastic resin film is pressed against at least one surface of a metal plate heated to a temperature not lower than (the melting point of the thermoplastic resin film - 40°C) and not higher than (the melting point of the thermoplastic resin film + 5°C) so that the first layer contacts the metal plate.

[0090] The metal plate is heated to a heat treatment temperature of not less than (the melting point of the thermoplastic resin film + 5°C) and not more than (the melting point of the thermoplastic resin film + 30°C) over a period of not less than 0.5 seconds and not more than 1.5 seconds, maintained at the heat treatment temperature for not less than 0.5 seconds and not more than 1.5 seconds, and then cooled to obtain a resin-coated metal plate.

[0091] When manufacturing the resin-coated metal sheet of the present invention, a thermoplastic resin film serving as the resin coating layer is first produced. The thermoplastic resin film comprises a thermoplastic resin and titanium dioxide. As previously described in the description of the resin coating layer, the thermoplastic resin can contain at least 90% by mass of a polyester resin relative to the total resin. Additives and the like are also described previously in the description of the resin coating layer. It should be noted that the amount of titanium dioxide added during production determines the titanium dioxide content in the resin coating layer.

[0092] The thermoplastic resin film comprises a first layer having a thickness of 2 μm or greater and containing 2% by mass or less of titanium dioxide, and a second layer in contact with the first layer. Furthermore, the thermoplastic resin film as a whole contains 8% to 30% by mass of titanium dioxide and contains 90% by mass or more of a polyester resin relative to the total resin content. Furthermore, the thermoplastic resin film preferably has a three-layer structure including a third layer in contact with the second layer. The third layer preferably has a thickness of 2 μm or greater and contains 2% by mass or less of titanium dioxide.

[0093] It should be noted that the thickness of the resin coating as a whole and each resin layer was measured using the following method. The resin-coated metal plate was cut into a size of approximately 20 mm x 15 mm and embedded in the resin for cross-section grinding. The observed surface of the resin-coated metal plate was then cross-sectioned using a cross-section polisher (CP). A cross-sectional photograph of each resin layer of the resin-coated metal plate was then taken at a magnification of 1000 to 3000 times using an FE-SEM. The thickness of the resin coating as a whole and each resin layer was determined by measuring the obtained cross-sectional photographs.

[0094] The method for manufacturing the thermoplastic resin film is not particularly limited. In one example, it can be manufactured as follows. First, the thermoplastic resin raw materials and titanium dioxide constituting each layer are heated as needed and dried under vacuum and then independently put into an extruder. The thermoplastic resin is heated and melted in the extruder. The heated and melted thermoplastic resin is flowed into different flow paths through filters, etc. Foreign matter and modified resins can be removed by the filters. Each thermoplastic resin is fed into a laminating device through different flow paths. As the laminating device, a feed block and a multi-manifold die can be used. In the laminating device, each thermoplastic resin is formed into a sheet with a T-die and discharged, and extruded onto a cooling body such as a casting drum. By cooling and solidifying the extruded sheet, an unstretched thermoplastic resin film with a multilayer structure can be obtained.

[0095] The thermoplastic resin film forming the resin coating layer is preferably formed by stretching the unstretched film to form a stretched film from the perspective of reducing the surface roughness of the resin coating layer. The method for obtaining the stretched film is not particularly limited, and can be used, for example, a method of obtaining a uniaxially stretched film by stretching in the longitudinal or width direction of a film forming machine; a method of obtaining a sequentially biaxially stretched film by stretching in the longitudinal or width direction followed by stretching in the other direction; and a method of obtaining a simultaneously biaxially stretched film by stretching in both the longitudinal and width directions. When obtaining a sequentially biaxially stretched film, it is preferred to stretch the unstretched film in the longitudinal direction before stretching in the width direction from the perspectives of uniform quality and space saving of the equipment.

[0096] Next, the method for producing the resin-coated metal sheet of the present invention using the aforementioned thermoplastic resin film will be described. By coating a metal sheet with a resin film at a low temperature and then subjecting it to a very short heat treatment at a temperature exceeding its melting point, a resin-coated metal sheet can be produced that achieves both low crystallinity and surface smoothness in the resin coating layer.

[0097] The above-mentioned thermoplastic resin film is heated to a temperature above the melting start temperature and is pressed against the metal plate using a laminating roller (thermocompression bonding film lamination method). This thermocompression bonding film lamination method is excellent in suppressing manufacturing costs and being able to produce energy-efficiently. It should be noted that the first layer of the thermoplastic resin film is brought into contact with the metal plate for compression bonding.

[0098] One method for reducing the crystallinity of the resin coating is to melt the resin coating by heating the metal plate to a high temperature before applying the resin coating to the metal plate. However, this method makes it difficult to achieve a smooth surface. Therefore, the present invention manufactures a resin-coated metal plate by performing a two-step process: coating the metal plate with a thermoplastic resin film at a low temperature, followed by a very short heat treatment at a temperature exceeding the melting point.

[0099] When the thermoplastic resin film is pressed against the metal plate, it is necessary to control the pressing conditions and ensure a smooth surface. During lamination, the time (thermocompression bonding time) during which the resin film is pressed against the metal plate by the laminating roller is preferably set to more than 10 msec., and is also preferably set to less than 40 msec. By setting the thermocompression bonding time to more than 10 msec., it is possible to more appropriately ensure that the thermoplastic resin film melts and wets and expands on the metal plate surface, making it possible to make the adhesion more suitable. In addition, if the thermocompression bonding time is less than 40 msec., it is possible to more appropriately prevent the laminating roller side of the thermoplastic resin film from softening, making it possible to make the smoothness of the resin covering layer surface more suitable. The thermocompression bonding time is more preferably more than 15 msec. The thermocompression bonding time is more preferably less than 30 msec.

[0100] In order to ensure the smoothness of the surface of the resin covering layer, it is necessary to control the temperature of the metal plate during crimping and suppress the softening of the surface of the resin covering layer during crimping. The temperature of the metal plate during crimping is set to be above (melting point of the thermoplastic resin film - 40°C) and below (melting point of the thermoplastic resin film + 5°C). If the temperature of the metal plate during crimping is lower than (melting point of the thermoplastic resin film - 40°C), the metal plate side of the thermoplastic resin film may not be fully melted, thereby reducing the adhesion between the metal plate and the resin covering layer. On the other hand, when the temperature of the metal plate during crimping exceeds (melting point of the thermoplastic resin film + 5°C), the surface of the laminating roller side of the thermoplastic resin film softens, and the smoothness of the surface of the resin covering layer is impaired, so it is not preferred. It should be noted that the temperature of the metal plate is based on the surface temperature of the metal plate.

[0101] The temperature of the laminating roll during pressure bonding is not particularly limited, but is preferably 60° C. or higher to ensure the impact resistance of the film after lamination. It is preferably 150° C. or lower to prevent the film from fusing to the laminating roll during lamination.

[0102] After the thermoplastic resin film is pressed against the metal plate, the resin-coated metal plate is cooled. As a cooling method, water cooling using temperature-regulated water or gas cooling using air, nitrogen, etc. is preferred. From the viewpoint of simplifying the equipment, water cooling is more preferred as a cooling method. As a water cooling method, there can be exemplified a method of immersing the resin-coated metal plate in a water tank filled with water and a method of spraying water onto the resin-coated metal plate from a nozzle, etc. The cooling stop temperature is preferably above 5°C. By setting the cooling stop temperature to above 5°C, condensation on the cooled resin-coated metal plate and peripheral equipment can be more appropriately prevented. In addition, the cooling stop temperature is preferably below (glass transition temperature of the thermoplastic resin film - 10°C). If the cooling stop temperature is below (glass transition temperature of the thermoplastic resin film - 10°C), the fluidity of the amorphous structure inside the resin coating layer can be more appropriately suppressed, and the surface roughness caused by contact between the resin-coated metal plate and rollers after cooling can be more appropriately prevented.

[0103] As described above, resin-coated metal sheets laminated at low temperatures have smooth surfaces, but their high crystallinity makes it difficult to suppress cosmetic defects (roughness of the surface) that may develop in the resin coating during heat treatment after canning. Furthermore, large residual stresses are generated in the resin coating during canning, potentially reducing the adhesion between the resin coating and the metal sheet. Therefore, it is necessary to heat-treat the pre-heat-treatment resin-coated metal sheets, which have been press-bonded at low temperatures, for an extremely short period of time at a temperature exceeding the melting point of the resin coating to reduce the crystallinity of the resin coating without impairing the surface smoothness.

[0104] Preferred heat treatment methods include non-contact methods such as infrared (IR) and induction heating (IH), which allow for rapid temperature increases and require passage through a heating furnace. Furthermore, to ensure smoothness of the resin coating surface, it is preferable that the resin-coated metal sheet not come into contact with rollers or other devices during the period from the start of heat treatment until the end of cooling. Preventing the resin-coated metal sheet from coming into contact with rollers or other devices while in a high-temperature state can more effectively prevent surface roughness.

[0105] The heat treatment temperature is set to be above (melting point of the thermoplastic resin film + 5°C) and below (melting point of the thermoplastic resin film + 30°C). If the heat treatment temperature is lower than (melting point of the thermoplastic resin film + 5°C), the resin coating may not fully dissolve, and the desired crystallinity of the resin coating may not be achieved. On the other hand, if the heat treatment temperature exceeds (melting point of the thermoplastic resin film + 30°C), there is a possibility of thermal degradation of the resin coating, which is not preferred. The heat treatment temperature is based on the temperature of the metal plate.

[0106] When forming resin coatings on both sides of a metal plate, in order to achieve low crystallinity and a suitably good appearance for both resin coatings, it is preferable that the resin coatings on both sides meet the aforementioned heat treatment temperature conditions. Therefore, it is preferable that the melting point difference between the resin coatings on both sides is 25°C or less. If the melting point difference between the resin coatings on both sides is 25°C or less, it is easier to adjust the heat treatment temperature on both sides to the aforementioned range of (melting point of the thermoplastic resin film + 5°C) or higher and (melting point of the thermoplastic resin film + 30°C) or lower.

[0107] During the heat treatment, the temperature is raised so that the above-mentioned heat treatment temperature is reached within 0.5 seconds to 1.5 seconds. If the temperature is raised for less than 0.5 seconds, temperature control becomes difficult, resulting in temperature differences in the width direction and thus causing variations in the physical properties of the resin coating layer.

[0108] After heating, the heat treatment temperature is maintained for 0.5 seconds to 1.5 seconds. If the heat treatment temperature is maintained for less than 0.5 seconds, the thermoplastic resin film may not be sufficiently melted and the desired crystallinity of the resin coating layer may not be achieved.

[0109] If the heating time or the holding time at the heat treatment temperature exceeds 1.5 seconds, the sheet must be transported over a very long distance without contact with rollers, etc., which increases the size of the equipment and may cause problems such as sheet vibration. Therefore, it is preferred that the heating time and the holding time at the heat treatment temperature be 1.5 seconds or less each, and the total time be 3.0 seconds or less.

[0110] After the heat treatment, the resin-coated metal plate is cooled. As a cooling method, water cooling using temperature-regulated water or gas cooling using air, nitrogen, etc. is preferred. From the viewpoint of simplifying the equipment, water cooling is more preferred as a cooling method. As a water cooling method, there can be exemplified a method of immersing the resin-coated metal plate in a water tank filled with water and a method of spraying water onto the resin-coated metal plate from a nozzle, etc. The cooling stop temperature is preferably above 5°C. By setting the cooling stop temperature to above 5°C, condensation on the cooled resin-coated metal plate and peripheral equipment can be more appropriately prevented. In addition, the cooling stop temperature is preferably below (glass transition temperature of the thermoplastic resin film - 10°C). If the cooling stop temperature is below (glass transition temperature of the thermoplastic resin film - 10°C), the fluidity of the amorphous structure inside the resin coating layer can be more appropriately suppressed, and the surface roughness caused by contact between the resin-coated metal plate and rollers, etc. after cooling can be more appropriately prevented.

[0111] It should be noted that production conditions other than the above-mentioned conditions can be carried out according to conventional methods.

[0112] Example

[0113] As the metal plate, a 0.22 mm thick metal plate with a chromium coating weight of 120 mg / m 2 、Chromium oxide adhesion 10mg / m 2 (converted in terms of metallic chromium) and a chrome-plated steel plate (TFS) with a tempering degree of T3CA. In each case, a resin having the composition described in Table 1, rutile titanium dioxide with a purity of 90% by mass, and wax were prepared. For each layer of each case, the resin, titanium dioxide and wax were put into an extruder for heating and melting. The molten raw material was transported to a laminating device (feed block) through a filter, formed into a sheet with a T-die, and cooled and solidified on a casting drum to produce a thermoplastic resin film. The thermoplastic resin film was stretched in the longitudinal direction and then stretched in the width direction to obtain a biaxially stretched film in sequence.

[0114] In each case, a resin film was coated on a metal plate using a thermocompression lamination method under the conditions listed in Table 2, followed by water cooling. The thermocompression bonding time was 20 msec, with the first layer of the thermoplastic resin film in contact with the metal plate for compression bonding (in the case of No. 24, the titanium dioxide-added layer was also compressed). Subsequently, a heat treatment was performed under the conditions listed in Table 2, followed by water cooling to produce a resin-coated metal plate. Note that no heat treatment was performed on Nos. 26 and 27.

[0115] The resulting resin-coated metal sheets were measured for the thickness of the resin coating layer, the inorganic additive content, the amount of crystallinity, the surface roughness, the interface between the resin coating layer and the metal sheet, and the Ti element ratio on the surface of the resin coating layer using the aforementioned methods. Furthermore, the melting point of the resin coating layer was measured using the following method. The measurement results are shown in Table 1. It should be noted that the proportion of the polyester resin constituting the resin coating layer in each example was 100% by mass, calculated as a solids content of the total resin. Table 1 shows the composition of the polyester resin in each example.

[0116] [Melting point]

[0117] The resin-coated metal plate was immersed in a 1:1 solution of concentrated hydrochloric acid (12 mol / L): distilled water at room temperature to dissolve the metal plate and isolate the resin coating. The separated resin coating was then rinsed thoroughly with distilled water and vacuum-dried. The dried resin coating was measured using a TA Instruments differential scanning calorimeter (DSCQ100) at a heating rate of 10°C / min from 0°C to 300°C. The peak temperature of the endothermic peak measured between 200°C and 280°C was defined as the melting point of the resin coating.

[0118] [Table 1]

[0119]

[0120] In addition, the resin-coated metal sheets of each example were evaluated for workability, surface roughness, appearance (surface smoothness and width direction), and adhesion by the following methods.

[0121] [Processability]

[0122] After coating each resin-coated metal sheet with paraffin wax, a circular blank with a diameter of 180 mm was punched out. This circular blank was deep-drawn using a deep-drawing press, followed by two-stage redrawing and one-stage ironing, resulting in a can with an inner diameter of 52 mm and a height of 163 mm. The surface of the resin coating on the outer surface of the can was visually inspected, and workability was evaluated according to the following criteria.

[0123] Evaluation "⊚": No chipping was observed.

[0124] Evaluation "○": Wear occurs at a height within 5 mm from the can flange portion. No practical problem.

[0125] Evaluation "X": Wear occurred at a height exceeding 5 mm from the can flange portion, which presents a practical problem.

[0126] [Rough surface]

[0127] After applying paraffin wax to the resin-coated metal plate of each example, a circular plate blank with a diameter of 180 mm was punched out. The circular plate blank was subjected to a deep drawing process using a deep drawing press, followed by a two-stage re-drawing process and a one-stage ironing process, thereby forming a tank with an inner diameter of 52 mm and a tank height of 163 mm. The formed tank was heated using a hot air furnace so that the tank body temperature reached (melting point of the resin coating + 5°C) within two minutes, and then rapidly cooled with cold air. The state of the resin coating on the outer surface of the tank after cooling was visually confirmed, and the surface roughness was evaluated according to the following criteria.

[0128] Evaluation "⊚": No defects in appearance such as black spots and wrinkles were observed.

[0129] Evaluation "0": Appearance defects such as black spots and wrinkles occur within 5 mm of the can flange. No practical problems.

[0130] Evaluation "△": Defects in appearance such as black spots and wrinkles occur at a height exceeding 5 mm and within 20 mm from the can flange. This presents a practical problem.

[0131] Evaluation "X": Appearance defects such as black spots and wrinkles occur at a height exceeding 20 mm from the can flange, which presents a practical problem.

[0132] [Appearance]

[0133] The surface smoothness of both surfaces of the resin-coated metal plate of each example was visually checked for uniformity of appearance, and the appearance was evaluated according to the following criteria.

[0134] Evaluation "○": No abnormality in appearance.

[0135] Evaluation "x": There is an abnormality in appearance such as a mottled pattern or a blurred appearance.

[0136] [Adhesion]

[0137] After applying paraffin wax on the resin-coated metal plate of each example, a circular plate blank with a diameter of 180 mm was punched out. The circular plate blank was subjected to a deep drawing process using a deep drawing press, followed by a two-stage re-drawing process and a one-stage thinning process, thereby forming a tank with an inner diameter of 52 mm and a tank height of 163 mm. A sample (width 15 mm × length 120 mm) for the peeling test was cut out from the main body of the formed tank in such a way that the tank height direction became the length direction (test direction). A portion of the resin coating was peeled off from the end of the tank body opening side of the cut sample, and the peeled resin coating was opened in the opposite direction (angle 180 degrees) to the metal plate from which the resin coating was peeled, and a peeling test was performed at a pulling speed of 30 mm / min. The adhesion was evaluated for each 15 mm width according to the benchmark shown below. The surface for adhesion measurement is the inner surface side of the tank.

[0138] Rating "◎": 3.0N / 15mm or more

[0139] Rating "0": 2.0N / 15mm or more, less than 3.0N / 15mm

[0140] Evaluation "△": 1.0N / 15mm or more, less than 2.0N / 15mm

[0141] Evaluation "×": less than 1.0N / 15mm

[0142] [Table 2]

[0143]

Table 2

[0144]

[0145] As shown in Table 2, the resin-coated metal sheets of the inventive examples had good (⊚ or ☐) evaluations for workability, surface roughness, appearance (surface smoothness and width direction), and adhesion of the resin coating layer that would become the outer surface of the container after molding. On the other hand, the comparative examples had unsatisfactory (Δ or ×) evaluations for any of the following: workability, surface roughness, appearance, and adhesion after processing.

[0146] Industrial applicability

[0147] According to the present invention, it is possible to suppress appearance defects (surface roughness) that may occur in the resin coating layer of a resin-coated metal sheet during heat treatment after canning, and it is possible to suppress a decrease in the smoothness of the surface of the resin coating layer during coating. Thus, it is possible to provide a resin-coated metal sheet having a smooth and beautiful appearance.

[0148] Description of Reference Signs

[0149] 1 Resin-coated metal plate

[0150] 2 metal plates

[0151] 3 Resin covering

[0152] 3a Third floor

[0153] 3b Second floor

[0154] 3c First layer

[0155] 4 Resin covering

Claims

1. A resin-coated metal plate comprising a resin coating layer containing 90% by mass or more of a polyester resin relative to the total resin on at least one surface of the metal plate, wherein: The crystallization amount of the resin covering layer is less than 15%, The arithmetic mean height Sa of the surface of the resin coating layer is 0.30 μm or less, The resin coating layer contains 8% by mass or more and 30% by mass or less of titanium dioxide, The amount of Ti detected at the interface between the resin coating layer and the metal plate, as determined by elemental analysis using X-ray photoelectron spectroscopy, was 2 atomic % or less.

2. The resin-coated metal sheet according to claim 1, wherein The amount of Ti detected on the surface of the resin coating layer as determined by elemental analysis using X-ray photoelectron spectroscopy is 2 atomic % or less.

3. The resin-coated metal sheet according to claim 1 or 2, wherein The resin cover layer has a multilayer structure including a first layer in contact with the metal plate and a second layer located on the first layer. The first layer has a thickness of 2 μm or greater and contains 2 mass % or less of titanium dioxide.

4. The resin-coated metal sheet according to claim 3, wherein The resin cover layer has a multi-layer structure further including a third layer, the third layer forming a surface of the resin cover layer and being located on the second layer, The third layer has a thickness of 2 μm or greater and contains 2% by mass or less of titanium dioxide.

5. The resin-coated metal sheet according to any one of claims 1 to 4, wherein The resin covering layer contains 0.010 mass % to 1.0 mass % of wax. 6 . A two-piece can comprising a metal plate coated with the resin according to claim 1 , wherein the resin coating layer is located on the outer surface side.

7. A method for producing a resin-coated metal plate, wherein: A thermoplastic resin film having a multilayer structure and containing titanium dioxide in an amount of not less than 8% by mass and not more than 30% by mass, and a polyester resin in an amount of not less than 90% by mass relative to the total resin, is prepared. The multilayer structure comprises a first layer having a thickness of not less than 2 μm and containing titanium dioxide in an amount of not more than 2% by mass, and a second layer in contact with the first layer. The thermoplastic resin film is pressed against at least one surface of a metal plate heated to a temperature not lower than (the melting point of the thermoplastic resin film - 40°C) and not higher than (the melting point of the thermoplastic resin film + 5°C) so that the first layer contacts the metal plate. The metal plate is heated to a heat treatment temperature of (melting point of the thermoplastic resin film + 5°C) or higher and (melting point of the thermoplastic resin film + 30°C) or lower over a period of 0.5 to 1.5 seconds, maintained at the heat treatment temperature for 0.5 to 1.5 seconds, and then cooled to obtain a resin-coated metal plate.

8. The method for manufacturing a resin-coated metal sheet according to claim 7, wherein: The thermoplastic resin film includes a third layer in contact with the second layer, the third layer having a thickness of 2 μm or greater and containing 2% by mass or less of titanium dioxide.

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