Resin-coated metal sheet, two-piece can, and method for producing resin-coated metal sheet

Abstract

Provided is a resin-coated metal sheet that suppresses appearance defects and reductions in smoothness and that exhibits an excellent processability and post-processing adhesion. A resin-coated metal sheet 1 according to the present invention comprises, on at least one side of a metal sheet 2, resin coating layers 3, 4 that contain at least 90 mass% polyester resin with reference to the total resin. The crystal content of the resin coating layers is not more than 15%; the arithmetic mean height Sa of the surface of the resin coating layers is not more than 0.30 µm; the resin coating layers contain 8 mass% to 30 mass% titanium dioxide; and the amount of Ti detected at the interface of a resin coating layer with the metal sheet, according to elemental analysis by X-ray photoelectron spectroscopic measurement, is not more than 2 atom%.

Description

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

[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 a resin-coated metal sheet.

[0002] Conventionally, metal plates such as tin-free steel (TFS) or aluminum used as materials for metal containers have been coated with paint to improve corrosion resistance. However, the production of coated metal plates involves complex painting and baking processes, which results in low productivity, requires a long processing time, and further involves a large environmental load due to the emission of large amounts of solvents and carbon dioxide.

[0003] To solve these problems, resin-coated metal sheets, which are made by laminating a thermoplastic resin film on the surface of a heated metal sheet, have been developed as an alternative to painted metal sheets. These sheets are now widely used industrially, primarily for beverage cans and food cans.

[0004] Metal containers are generally broadly classified into two-piece cans and three-piece cans. A two-piece can is a metal container composed of two parts: a can body and a lid, which are integral with the can bottom. On the other hand, a three-piece can is a metal container composed of three parts: a can body, a top lid, and a bottom lid. Two-piece cans have a beautiful appearance because they have no welds. However, metal sheets used as the raw material for two-piece cans generally require a high degree of processing. Furthermore, with the development of can-making processing technology, the degree of processing has increased, creating a new problem for resin-coated metal sheets for two-piece cans: appearance defects (rough surfaces) can occur in the resin coating layer during heat treatment after can-making.

[0005] For two-piece cans, techniques have been proposed for manufacturing can bodies using resin-coated metal sheets by drawing or DI (Draw and Ironing) methods (Patent Documents 1 and 2). Also, a technique has been proposed for controlling the amount of crystals in the resin coating layer in order to prevent surface roughness from occurring in the resin coating layer during heat treatment after can-making (Patent Document 3).

[0006] JP 04-091825 A JP 2004-148324 A International Publication No. 2013 / 030972

[0007] The techniques described in Patent Documents 1 and 2 are fundamental techniques for manufacturing two-piece cans. However, as the degree of processing increases with the development of can-making processing technology, new problems have arisen, such as surface roughness occurring on the resin coating layer during heat treatment after can-making processing, making it more important to control the performance of resin-coated metal sheets used in manufacturing two-piece cans.

[0008] The technology described in Patent Document 3 can suppress the occurrence of roughness on the resin coating layer during heat treatment after can manufacturing. However, there is room for improvement in that the resin coating layer, whose surface is heated to a high temperature during coating, is pressed by a laminating roll, which may reduce the smoothness of the surface of the resin coating layer.

[0009] The present invention has been made in view of the above circumstances. That is, an object of the present invention is to suppress appearance defects (rough surfaces) that occur in the resin coating layer of a resin-coated metal sheet during heat treatment after can forming, and to suppress a decrease in the smoothness of the surface of the resin coating layer during coating. Another object of the present invention is to thereby provide a resin-coated metal sheet that has a smooth and beautiful appearance, and is excellent in processability and adhesion of the resin coating layer after processing.

[0010] The surface roughness of the resin coating layer occurs when the resin coating layer is pressed by a laminating roll while the surface is heated to a high temperature. After extensive research, the inventors discovered the following: By performing a two-stage process in which a resin coating layer is coated on a metal sheet at a low temperature and then a very short heat treatment at a temperature above the melting point of the resin coating layer, it is possible to suppress appearance defects (skin roughness) that occur in the resin coating layer during heat treatment after can manufacturing. This makes it possible to provide a resin-coated metal sheet with a smooth and beautiful appearance on the surface of the resin coating layer.

[0011] Furthermore, by reducing the crystallinity of the resin coating layer, it is possible to reduce the residual stress introduced during processing. In addition, by reducing the amount of inorganic additives present at the adhesive interface between the metal sheet and the resin coating layer, it is possible to obtain a resin-coated metal sheet that is excellent in processability and adhesiveness of the resin coating layer after processing.

[0012] The gist and configuration of the present invention, which has been completed based on the above findings, is as follows.

[0013] [1] A resin-coated metal sheet provided on at least one side of a metal sheet with a resin coating layer containing polyester resin in an amount of 90 mass% or more relative to the total resin, wherein the amount of crystals in the resin coating layer is 15% or less, 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 mass% to 30 mass% of titanium dioxide, and elemental analysis by X-ray photoelectron spectroscopy reveals that the amount of Ti detected at the interface between the resin coating layer and the metal sheet is 2 atomic % or less.

[0014] [2] The resin-coated metal sheet according to [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.

[0015] [3] The resin-coated metal sheet according to [1] or [2], 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, and the first layer has a thickness of 2 μm or more and contains 2 mass% or less of titanium dioxide.

[0016] [4] The resin coating layer has a multilayer structure further including a third layer located on the second layer and forming a surface of the resin coating layer, and the third layer has a thickness of 2 μm or more and contains 2 mass% or less of titanium dioxide. [3] The resin-coated metal sheet according to [3].

[0017] [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.

[0018] [6] A two-piece can made using the resin-coated metal sheet according to any one of [1] to [5], wherein the resin coating layer is located on the outer surface side.

[0019] [7] A method for producing a resin-coated metal sheet, comprising: preparing a thermoplastic resin film having a thickness of 2 μm or more and a multilayer structure including a first layer containing 2 mass% or less of titanium dioxide and a second layer in contact with the first layer, the thermoplastic resin film containing 8 mass% or more to 30 mass% or less of titanium dioxide overall and containing 90 mass% or more of polyester resin based on the total resin; contacting the first layer with the thermoplastic resin film and pressing the first layer against at least one side of a metal sheet heated to a temperature of (melting point of the thermoplastic resin film - 40°C) or more to (melting point of the thermoplastic resin film + 5°C) or less; heating the metal sheet to a heat treatment temperature of (melting point of the thermoplastic resin film + 5°C) or more to (melting point of the thermoplastic resin film + 30°C) in 0.5 seconds or more to 1.5 seconds or less, holding the metal sheet at the heat treatment temperature for 0.5 seconds or more to 1.5 seconds or less, and then cooling the metal sheet to obtain a resin-coated metal sheet.

[0020] [8] The thermoplastic resin film has a third layer in contact with the second layer, and the third layer has a thickness of 2 μm or more and contains 2 mass% or less of titanium dioxide. [7] The method for producing a resin-coated metal sheet.

[0021] According to the present invention, it is possible to suppress appearance defects (rough surfaces) that occur in the resin coating layer of a resin-coated metal sheet during heat treatment after can-making, and to suppress a decrease in the smoothness of the surface of the resin coating layer during coating. As a result, it is possible to provide a resin-coated metal sheet that has a smooth and beautiful appearance, and also has good processability and adhesion of the resin coating layer after processing.

[0022] 1 is a diagram showing a cross section of an example of a resin-coated metal sheet, and FIG. 2 is a diagram showing a cross section of an example of a resin-coated metal sheet provided with a resin coating layer of a three-layer structure.

[0023] Hereinafter, an embodiment of the method for producing a resin-coated metal sheet according to the present invention will be described. Note that the embodiment described below is an example of the present invention, and the configuration of the present invention is not limited to this specific example.

[0024] The resin-coated metal sheet of the present invention is a resin-coated metal sheet provided on at least one side of a metal sheet with a resin coating layer containing polyester resin in an amount of 90 mass% or more relative to the total resin, wherein the crystalline amount of the resin coating layer is 15% or less, 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 mass% to 30 mass% of titanium dioxide, and elemental analysis by X-ray photoelectron spectroscopy reveals that the amount of Ti detected at the interface between the resin coating layer and the metal sheet is 2 atomic % or less.

[0025] According to the present invention, it is possible to suppress appearance defects (rough surfaces) that occur in the resin coating layer of a resin-coated metal sheet during heat treatment after can-making, and 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. Furthermore, it is possible to provide a resin-coated metal sheet that reduces residual stress after can-making, and has excellent workability and adhesion of the resin coating layer after processing.

[0026] Figure 1 shows an example of a cross section of a resin-coated metal sheet 1 according to one embodiment. The resin-coated metal sheet 1 shown in Figure 1 has a resin coating layer 3 on the front side of a metal sheet 2 and a resin coating layer 4 on the back side of the metal sheet 2. 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 and inner surfaces of a two-piece can after can forming, respectively. At least one of the resin coating layers 3 and 4 is a resin coating layer containing titanium dioxide.

[0027] [Metal Sheet] First, the metal sheet will be described. As the metal sheet for the resin-coated metal sheet, steel sheets, aluminum sheets, and the like, which are widely used as materials for cans, can be used.

[0028] As the metal plate, tin-free steel (TFS) is particularly suitable from the viewpoint of resin adhesion in a high-temperature, humid environment such as retort sterilization. The TFS is not particularly limited by the amount of the metal chromium layer and the chromium oxide layer attached, but a preferred amount is 50 mg / m ２ More than 200g / m ２The following metal chromium layer and a metal chromium equivalent coating amount of 3 mg / m ２ 30g / m or more ２ It is preferable that the surface has the following chromium oxide layer.

[0029] The type of metal sheet is not particularly limited as long as it can be formed into the desired shape, but steel sheets having the following chemical compositions and manufacturing methods are preferred: (1) A steel sheet obtained by recrystallization annealing using low-carbon steel with a C (carbon) content of more than 0.003% by mass and not more than 0.10% by mass through continuous annealing; (2) A steel sheet obtained by recrystallization annealing using low-carbon steel with a C content of more than 0.003% by mass and not more than 0.10% by mass through continuous annealing and overaging treatment; (3) A steel sheet obtained by recrystallization annealing using low-carbon steel with a C content of more than 0.003% by mass and not more than 0.10% by mass through box annealing; (4) A steel sheet obtained by secondary cold rolling (DR (double reduced) rolling) using low-carbon steel with a C content of more than 0.003% by mass and not more than 0.10% by mass through recrystallization annealing using continuous annealing or box annealing. (5) A steel sheet obtained by using IF (Interstitial Free) steel, which is made by adding elements that fix dissolved C, such as Nb and Ti, to ultra-low carbon steel having a C content of 0.003 mass% or less, and by recrystallization annealing using continuous annealing.

[0030] The mechanical properties of the metal sheet are not particularly limited as long as they can be formed into the desired shape. To maintain workability and sufficient can body strength, it is preferable to use a metal sheet with a yield point (YP) of 220 MPa or more and 580 MPa or less. Furthermore, the Lankford value (r value), which is an index of plastic anisotropy, is preferably 0.8 or more. Furthermore, the absolute value of the in-plane anisotropy Δr of the r value is preferably 0.7 or less.

[0031] The composition of the metal sheet is not particularly limited, but a steel sheet containing, for example, Si, Mn, P, S, Al, and N may be used. The Si content is preferably 0.001% by mass or more and 0.1% by mass or less. The Mn content is preferably 0.01% by mass or more and 0.6% by mass or less. The P content is preferably 0.002% by mass or more and 0.05% by mass or less. The S content is preferably 0.002% by mass or more and 0.05% by mass or less. The Al content is preferably 0.005% by mass or more and 0.100% by mass or less. The N content is preferably 0.0005% by mass or more and 0.020% by mass or less. The composition may further contain other elements such as Ti, Nb, B, Cu, Ni, Cr, Mo, and V. From the viewpoint of ensuring corrosion resistance and the like, the total content of these component elements is preferably 0.02 mass % or less.

[0032] The thickness of the metal plate is not particularly limited, but may be, for example, 0.10 mm or more and 0.50 mm or less.

[0033] [Component Composition of Resin Coating Layer] The resin-coated metal sheet has a resin coating layer mainly composed of polyester resin on at least one side of the metal sheet. The resin coating layer has a polyester resin content of 90% by mass or more in terms of solid content. If the resin coating layer contains inorganic additives (such as inorganic pigments), the polyester resin content of the resin, minus the weight of the inorganic additives, is 90% by mass or more.

[0034] The polyester resin is a polymer composed of dicarboxylic acid units and glycol units.

[0035] As the dicarboxylic acid unit, units derived from aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenoxyethanedicarboxylic acid, 5-sodiumsulfoisophthalic 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 cyclohexanedicarboxylic acid; and oxycarboxylic acids such as p-oxybenzoic acid can be used.

[0036] The polyester resin preferably contains 90 mol % or more of terephthalic acid units among the dicarboxylic acid units. When the terephthalic acid units are 90 mol % or more of the dicarboxylic acid units contained in the polyester resin, sufficient heat resistance against frictional heat during continuous can forming can be ensured, and more stable moldability and coatability can be obtained.

[0037] As the glycol unit, units derived from aliphatic glycols such as ethylene glycol, propanediol, butanediol, pentanediol, hexanediol, and neopentyl glycol; alicyclic glycols such as cyclohexanedimethanol; aromatic glycols such as bisphenol A and bisphenol S; and diethylene glycol can be used.

[0038] The above-mentioned dicarboxylic acids and glycols may be used in combination in such a manner that the heat resistance and processability are not impaired.

[0039] In at least one of the resin coating layers, the titanium dioxide content is 8% by mass or more and 30% by mass or less. If the titanium dioxide content is less than 8% by mass, the underlying metal cannot be sufficiently concealed, while if it exceeds 30% by mass, the processability of the resin coating layer is impaired. That is, when the titanium dioxide content is within the above-specified range, the underlying metal can be concealed while increasing the clarity of the print, achieving a good white appearance, and the processability of the resin coating layer is not impaired. Other examples of white pigments besides titanium dioxide include alumina, calcium carbonate, and barium sulfate. However, titanium dioxide has strong coloring power and can ensure good appearance even after can formation. Rutile-type titanium dioxide with a purity of 90% by mass or more is particularly preferred because it has better dispersibility when mixed with resin materials. The titanium dioxide content of the resin coating layer is preferably 10% by mass or more. Furthermore, the titanium dioxide content of the resin coating layer is preferably 22% by mass or less. When the resin coating layer has a multi-layer structure as described below, the content of titanium dioxide in the entire resin coating layer made up of multiple layers is set to 8% by mass or more and 30% by mass or less.

[0040] [Crystalline Amount of Resin Coating Layer] The crystallization amount of the resin coating layer is set to 15% or less. Setting the crystallization amount to 15% or less enables the high moldability required for forming two-piece cans and suppresses appearance defects (rough surfaces) that occur in the resin coating layer during heat treatment after can-making. Furthermore, by setting the crystallization amount to a low value and reducing the residual stress in the resin coating layer after can-making, excellent adhesion can be achieved even after processing. The crystallization amount of the resin coating layer is preferably 12% or less, more preferably 10% or less. If the resin coating layer contains an inorganic additive (e.g., an inorganic pigment), the crystallization amount in the resin material, minus the mass of the inorganic additive, must be 15% or less. The lower limit of the crystallization amount is not particularly limited, but the crystallization amount can be 1% or more. The crystallization amount is calculated based on the inorganic additive content obtained by thermogravimetry and the heat of crystallization and heat of fusion obtained by differential scanning calorimetry, as shown below. Here, inorganic additives refer to inorganic pigments and additives other than inorganic pigments.

[0041] The inorganic additive content is measured as follows. First, a resin-coated metal sheet is immersed in a 1:1 mixed solution of concentrated hydrochloric acid (12 mol / L):distilled water at room temperature to dissolve the metal sheet and isolate the resin coating layer. The isolated resin coating layer is thoroughly washed with distilled water and then vacuum-dried. Thermogravimetric measurement is then performed using a thermogravimetric analyzer at a temperature range of room temperature to 800°C, with an air flow rate of 300 mL / min and a heating rate of 10°C / min. As shown in the following formula (1), the inorganic additive content is the ratio of the weight at 800°C to the weight at room temperature. Inorganic additive content [%] = weight at 800°C [mg] / weight at room temperature [mg] × 100... (1)

[0042] The amount of crystallinity is measured as follows. In the same manner as in measuring the inorganic additive content, the metal plate is dissolved from the resin-coated metal plate to isolate the resin coating layer, which is then 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 from the area of ​​the exothermic peak observed between 100 and 200°C, and the heat of fusion is calculated from the area of ​​the endothermic peak measured between 200°C and 280°C. The amount of crystallinity is calculated from the obtained heat of crystallization and heat of fusion according to the following formula (2). The content of the inorganic additive is determined by the above-mentioned method. Amount of crystallinity [%] = (heat of fusion [J / g] - heat of crystallization [J / g]) x 100 / (100 - inorganic additive content [%]) / 140.2 [J / g] x 100 (2)

[0043] [Surface smoothness of resin coating layer] The arithmetic mean height Sa of the resin coating layer surface is 0.30 μm or less. If the arithmetic mean height Sa of the resin coating layer surface exceeds 0.30 μm, the roughness of the resin coating layer surface will be large, resulting in a mottled appearance in the case of a colored resin coating layer and a cloudy appearance in the case of a colorless resin coating layer. Although there is no particular limitation on the lower limit of the arithmetic mean height Sa of the resin coating layer surface, it is preferable that the arithmetic mean height Sa be 0.10 μm or more.

[0044] The arithmetic mean height Sa of the resin coating layer surface is measured by surface roughness analysis using a 3D shape measuring instrument. Using a Keyence one-shot 3D shape measuring instrument, shape measurement is performed over a 1.9 mm × 1.4 mm field of view at 160x magnification, and the arithmetic mean height Sa is calculated by surface roughness analysis. Measurements are taken at five randomly selected locations on the same surface of the resin-coated metal sheet, and the maximum value is taken as the arithmetic mean height Sa of the resin-coated metal sheet.

[0045] In the present invention, the arithmetic mean height Sa is used as an index of surface smoothness because it has a high correlation with the appearance of the product. Examples of arithmetic mean heights include Ra, which is the arithmetic mean height of a line, and Sa, which is the arithmetic mean height of a surface. In the present invention, by using Sa as an index of surface smoothness, the entire surface can be evaluated without the difference in measurement results caused by the measurement direction that occurs when Ra is used.

[0046] The amount of crystals and surface smoothness of the resin coating layer can be achieved, for example, by a two-stage process, as described in the manufacturing method described below, in which a metal plate is coated on the resin coating layer at a low temperature, and then a very short heat treatment is performed at a temperature above the melting point.

[0047] [Layer Structure of Resin Coating Layer] 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. If the amount of Ti detected at the interface between the resin coating layer and the metal sheet exceeds 2 atomic %, the titanium dioxide present at the interface between the resin coating layer and the metal sheet inhibits adhesion between the metal sheet and the resin coating layer, resulting in insufficient adhesion of the resin coating layer after can forming. Preferably, 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 1 atomic % or less. The lower limit of 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 not particularly limited and may be 0 atomic %. 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, can be measured using a conventionally known method, as described in the Examples below. Preferably, the measurement is performed at multiple randomly selected locations on the same surface of the resin-coated metal sheet, and the average of the measurement results is used as the amount of Ti detected. The number of randomly selected locations is preferably five or more, taking into consideration variations within the surface.

[0048] One way to reduce the amount of titanium dioxide present at the interface between the resin coating layer and the metal sheet is to create a multilayer structure in which the metal sheet side of the resin coating layer contains no or very little titanium dioxide. Figure 2 shows a resin-coated metal sheet having a multilayer structure in which a first layer 3c, a second layer 3b, and a third layer 3a are laminated in this order on a metal sheet 2. Note that the third layer 3a is an optional layer in the present invention. By reducing the titanium dioxide content of the first layer 3c located at the interface between the resin coating layer and the metal sheet, the amount of titanium dioxide present at the interface between the resin coating layer and the metal sheet can be reduced. The thickness of the first layer 3c is preferably 2 μm or more to ensure sufficient adhesion to the metal sheet, and preferably 5 μm or less to ensure a good appearance of the film. Furthermore, the titanium dioxide content of the first layer 3c is preferably 2 mass% or less.

[0049] Furthermore, it is preferable that 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. If the amount of Ti detected is 2 atomic % or less, scraping of the resin coating layer can be suppressed even in more severe can manufacturing 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 %.

[0050] The amount of Ti detected on the resin coating layer surface after elemental analysis by X-ray photoelectron spectroscopy is measured using the following method. The resin-coated metal sheet is immersed in a 1:1 mixed solution of concentrated hydrochloric acid (12 mol / L):distilled water at room temperature to dissolve the metal sheet and isolate the resin coating layer. The isolated resin coating layer is then thoroughly washed with distilled water and vacuum dried. X-ray photoelectron spectroscopy is performed on the interface side of the dried resin coating layer with the metal sheet using an X-ray photoelectron spectroscopy analyzer (SSX-100 manufactured by SSI). The X-ray source is monochromated Al Kα radiation, and the measurement is performed under the conditions of a measurement area of ​​600 μmφ, six accumulations, and a photoelectron escape angle of 35°. The elements are quantified from the wide-scan spectrum obtained by the measurement, and the elemental ratio of Ti among the detected elements is calculated. Measurements are performed at five randomly selected locations on each sample, and the average value is used as the amount of Ti detected on the resin coating layer surface after elemental analysis by X-ray photoelectron spectroscopy. The measurement positions are spaced apart by at least 5 mm. The Ti element ratio on the surface of the resin-coated metal sheet can also be obtained by measuring the surface of the resin-coated metal sheet under the same conditions.

[0051] In order to reduce the amount of titanium dioxide present on the surface of the resin coating layer, it is preferable to use a multi-layer structure in which the surface of the resin coating layer has a layer that does not contain titanium dioxide or has a very low titanium dioxide content. For example, the titanium dioxide content of the third layer 3a shown in Figure 2 can be reduced. The thickness of the third layer 3a is preferably 2 µm or more, and preferably 5 µm or less. Furthermore, the titanium dioxide content of the third layer 3a is preferably 2 mass% or less. In order to reduce the amount of titanium dioxide on the surface of the resin coating layer in addition to the interface between the resin coating layer and the metal sheet, it is preferable to use at least a three-layer structure in which layers with a low titanium dioxide content (third layer 3a and first layer 3c in Figure 2) are provided on both sides of the resin coating layer.

[0052] For the purpose of improving sliding properties during processing, 0.010% by mass or more of wax may be added to the resin coating layer on at least one side, or 1.0% by mass or less of wax may be added. In particular, it is preferable to add 0.010% by mass or more and 1.0% by mass or less of wax to the resin coating layer located on the outer surface side of the metal container after molding. By adding 0.010% by mass or more of wax to the resin coating layer, the friction coefficient of the resin coating layer surface during processing can be reduced, and scraping of the resin coating layer can be suppressed. On the other hand, if the amount of wax added is 1.0% by mass or less, it is easier to form the resin coating layer, so the amount of wax added is preferably 1.0% by mass or less.

[0053] As described above, when the resin coating layer has a multilayer structure, only the wax added near the surface of the resin coating layer contributes to the sliding properties of the surface of the resin coating layer, so wax may be added only to the layer located on the surface. Furthermore, when a multilayer resin coating layer is provided on both sides of a metal plate, wax may also be added to the layer on the metal plate side of the resin coating layer, for example, by using the same material for both layers of the resin coating layer. Adding wax only to the surface layer of the resin coating layer can reduce the amount of wax used in the entire resin coating layer, thereby reducing resin costs. Note that if the wax amount between the surface layer and the adjacent layer differs significantly, the difference in resin properties may increase, potentially reducing adhesion between the layers. Therefore, it is effective to add a small amount of wax (0.10% by mass or less) to adjacent layers.

[0054] The wax may be at least one selected from the group consisting of polyolefin waxes such as polyethylene and polypropylene, and modified products thereof; natural waxes such as carnauba wax; polyamide waxes; and polyester waxes, or a mixture thereof.

[0055] Furthermore, additives such as antioxidants, heat stabilizers, lubricants, crystal nucleating agents, ultraviolet absorbers, antistatic agents, etc. may be added to the resin coating layer as needed, provided that the effects of the present invention are not impaired. Furthermore, in order to improve the appearance of the inner and outer surfaces of the metal container, coloring pigments other than white pigments may be added to the resin coating layer.

[0056] The thickness of the resin coating layer is not particularly limited, but may be, for example, 6 mm or more and 50 mm or less.

[0057] By using the resin-coated metal sheet described above, two-piece cans having a smooth and beautiful appearance can be manufactured. The two-piece cans can be manufactured by conventional methods. In the two-piece can, it is preferable that the resin coating layer is located on the outer surface side of the two-piece can.

[0058] The method for producing a resin-coated metal sheet according to the present invention includes: preparing a thermoplastic resin film having a thickness of 2 μm or more and a multilayer structure including a first layer containing 2% or less by mass of titanium dioxide and a second layer in contact with the first layer, the first layer containing 8% or more by mass of titanium dioxide overall and 90% or more by mass of polyester resin relative to the total resin; contacting the first layer with the thermoplastic resin film and pressing the first layer against at least one side of a metal plate heated to a temperature of (melting point of the thermoplastic resin film - 40°C) or more but not more than (melting point of the thermoplastic resin film + 5°C); heating the metal plate to a heat treatment temperature of (melting point of the thermoplastic resin film + 5°C) or more but not more than (melting point of the thermoplastic resin film + 30°C) in 0.5 to 1.5 seconds, holding the metal plate at the heat treatment temperature for 0.5 to 1.5 seconds, and then cooling the metal plate to obtain a resin-coated metal sheet.

[0059] In manufacturing the resin-coated metal sheet of the present invention, a thermoplastic resin film that will become the resin coating layer is first manufactured. The thermoplastic resin film contains a thermoplastic resin and titanium dioxide. As described above in the description of the resin coating layer, the thermoplastic resin may be a resin containing 90 mass% or more of polyester resin based on the total resin. The additives are also as described above in the description of the resin coating layer. The amount of titanium dioxide added during manufacturing is the titanium dioxide content in the resin coating layer.

[0060] The thermoplastic resin film includes 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 adjacent to the first layer. Furthermore, the thermoplastic resin film contains 8% by mass or more to 30% by mass or less of titanium dioxide as a whole, and contains 90% by mass or more of polyester resin based on the total resin content. The thermoplastic resin film preferably has a three-layer structure including a third layer adjacent to the second layer. The third layer preferably has a thickness of 2 μm or more and contains 2% by mass or less of titanium dioxide.

[0061] The thickness of the entire resin coating layer and each resin layer is measured using the following method. The resin-coated metal plate is cut into a size of approximately 20 mm x 15 mm, embedded in embedding resin, and cross-section polishing is performed. Furthermore, the cross-section of the surface of the resin-coated metal plate to be observed is polished using a cross-section polisher (CP). Then, a cross-sectional photograph of each resin layer of the resin-coated metal plate is taken at a magnification of 1000 to 3000 times using an FE-SEM. The thickness of the entire resin coating layer and each resin layer is obtained by measuring the obtained cross-sectional photograph.

[0062] The method for producing a thermoplastic resin film is not particularly limited, but in one example, it can be produced as follows. First, the thermoplastic resin raw materials and titanium dioxide constituting each layer are dried under heat and vacuum as necessary and then fed into independent extruders, where the thermoplastic resins are heated and melted. The heated and melted thermoplastic resins are then poured into different flow paths via a filter or the like. Foreign matter and modified resins can be removed by the filter. Each thermoplastic resin is fed into a lamination device through a different flow path. A feed block or a multi-manifold die can be used as the lamination device. Within the lamination device, each thermoplastic resin is formed into a sheet using a T-die, discharged, and extruded onto a cooling body such as a casting drum. The extruded sheet is cooled and solidified to obtain an unstretched thermoplastic resin film having a multilayer structure.

[0063] The thermoplastic resin film forming the resin coating layer is preferably obtained by stretching the above-mentioned unstretched film to form a stretched film, from the viewpoint of reducing the surface roughness of the resin coating layer. The method for obtaining the stretched film is not particularly limited, but examples include a method of stretching in the longitudinal or width direction of a film forming machine to obtain a uniaxially stretched film; a method of stretching in the longitudinal or width direction and then stretching in the other direction to obtain a sequentially biaxially stretched film; and a method of simultaneously stretching in the longitudinal and width directions to obtain a simultaneous biaxially stretched film. When obtaining a sequentially biaxially stretched film, it is preferable to stretch the unstretched film in the longitudinal direction and then in the width direction, from the viewpoint of uniform quality and space-saving equipment.

[0064] Next, a method for producing a resin-coated metal sheet of the present invention using the above-mentioned thermoplastic resin film will be described. After coating a metal sheet with the resin film at a low temperature, the metal sheet is subjected to a very short heat treatment at a temperature above the melting point, thereby producing a resin-coated metal sheet that has both low crystallinity in the resin coating layer and good surface smoothness.

[0065] The thermoplastic resin film is heated to a temperature above its melting point and pressed onto a metal plate using a laminating roll (thermocompression film lamination method). This thermocompression film lamination method is advantageous in that it reduces production costs and enables energy-saving production. The first layer of the thermoplastic resin film is pressed onto the metal plate in contact with it.

[0066] One method for reducing the crystallinity of a resin coating layer is to melt the resin coating layer by raising the temperature of the metal sheet when coating the resin coating layer on the metal sheet, but this method makes it difficult to obtain a smooth surface. Therefore, in the present invention, a resin-coated metal sheet is produced by a two-stage process in which the metal sheet is coated with a thermoplastic resin film at a low temperature and then heat-treated for a very short time at a temperature above the melting point.

[0067] When a thermoplastic resin film is pressure-bonded to a metal plate, it is necessary to control the pressure-bonding conditions to ensure a smooth surface. During lamination, the time during which the resin film is pressure-bonded to the metal plate by the laminating roll (thermocompression bonding time) is preferably 10 msec or more, and more preferably 40 msec or less. By setting the thermocompression bonding time to 10 msec or more, the time for the thermoplastic resin film to melt and spread on the metal plate surface can be more suitably ensured, and adhesion can be more suitably improved. Furthermore, if the thermocompression bonding time is 40 msec or less, softening of the laminating roll side of the thermoplastic resin film can be more suitably prevented, and the smoothness of the resin coating layer surface can be more suitably improved. The thermocompression bonding time is more preferably 15 msec or more. The thermocompression bonding time is more preferably 30 msec or less.

[0068] In order to ensure the smoothness of the resin coating layer surface, it is necessary to control the temperature of the metal plate during pressure bonding and suppress softening of the resin coating layer surface during pressure bonding. The temperature of the metal plate during pressure bonding should be at least (melting point of the thermoplastic resin film - 40°C) and not more than (melting point of the thermoplastic resin film + 5°C). If the temperature of the metal plate during pressure bonding is less than (melting point of the thermoplastic resin film - 40°C), the metal plate side of the thermoplastic resin film may not melt sufficiently, and the adhesion between the metal plate and the resin coating layer may decrease. On the other hand, if the temperature of the metal plate during pressure bonding exceeds (melting point of the thermoplastic resin film + 5°C), the surface of the thermoplastic resin film on the laminating roll side softens, impairing the smoothness of the resin coating layer surface, which is undesirable. The temperature of the metal plate is based on the surface temperature of the metal plate.

[0069] 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, and is preferably 150° C. or lower to prevent the film from welding to the laminating roll during lamination.

[0070] After the thermoplastic resin film is pressure-bonded to the metal plate, the resin-coated metal plate is cooled. The cooling method is preferably water cooling using temperature-adjusted water or gas cooling using air, nitrogen, or the like. From the viewpoint of simplifying the equipment, water cooling is more preferable. Examples of water cooling methods include immersing the resin-coated metal plate in a water tank containing water and spraying water onto the resin-coated metal plate from a nozzle or the like. The cooling stop temperature is preferably 5°C or higher. By setting the cooling stop temperature to 5°C or higher, condensation on the resin-coated metal plate and surrounding equipment after cooling can be more effectively prevented. Furthermore, the cooling stop temperature is preferably (the glass transition temperature of the thermoplastic resin film - 10°C) or lower. If the cooling stop temperature is (the glass transition temperature of the thermoplastic resin film - 10°C) or lower, the fluidity of the amorphous structure inside the resin coating layer can be more effectively suppressed, and the resin-coated metal plate can be more effectively prevented from coming into contact with a roll or the like after cooling, resulting in roughness on the surface.

[0071] Although the resin-coated metal sheet laminated at low temperatures as described above has a smooth surface, it has a high degree of crystallinity and is unable to suppress appearance defects (rough surfaces) that occur in the resin coating layer during heat treatment after can-making. Furthermore, there is a concern that large residual stresses will occur in the resin coating layer during can-making, which will reduce the adhesion between the resin coating layer and the metal sheet. Therefore, it is necessary to reduce the crystallinity of the resin coating layer without impairing the surface smoothness by subjecting the resin-coated metal sheet, which has been pressure-bonded at low temperatures, to a very short heat treatment at a temperature above the melting point of the resin coating layer.

[0072] As a heat treatment method, a method of passing the resin-coated metal sheet through a heating furnace that can heat the sheet without contact and in a short time, such as infrared rays (IR) or induction heating (IH), is preferred. Furthermore, from the viewpoint of ensuring the smoothness of the surface of the resin coating layer, it is preferable that the resin-coated metal sheet does not come into contact with various rolls, etc., from the start of the heat treatment to the end of cooling. Preventing the resin-coated metal sheet from coming into contact with various rolls, etc., at a high temperature can more effectively prevent the surface from becoming rough.

[0073] The heat treatment temperature is (melting point of the thermoplastic resin film + 5°C) or higher and (melting point of the thermoplastic resin film + 30°C) or lower. If the heat treatment temperature is lower than (melting point of the thermoplastic resin film + 5°C), the resin coating layer may not melt sufficiently, and the desired crystallinity of the resin coating layer may not be achieved. On the other hand, if the heat treatment temperature exceeds (melting point of the thermoplastic resin film + 30)°C, the resin coating layer may deteriorate due to heat, which is not preferable. The heat treatment temperature is based on the temperature of the metal plate.

[0074] When forming resin coating layers on both sides of a metal plate, in order to achieve low crystallinity in both resin coating layers and obtain a good appearance, it is preferable that the resin coating layers on both sides satisfy the above-mentioned heat treatment temperature conditions. Therefore, it is preferable that the difference in melting point of the resin coating layers on both sides is 25°C or less. If the difference in melting point of the resin coating layers on both sides is 25°C or less, it is easy to set the heat treatment temperature on both sides in the above-mentioned range of (melting point of thermoplastic resin film + 5°C) or more and (melting point of thermoplastic resin film + 30°C) or less.

[0075] During the heat treatment, the temperature is raised to the above heat treatment temperature in 0.5 to 1.5 seconds. If the temperature is raised in less than 0.5 seconds, it is difficult to control the temperature, and variations in the physical properties of the resin coating layer may occur, such as temperature differences in the width direction.

[0076] After the temperature is increased, the film is maintained at the heat treatment temperature 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 melt sufficiently, and the desired crystallinity of the resin coating layer may not be achieved.

[0077] If the temperature rise time or the time held at the heat treatment temperature exceeds 1.5 seconds, it will be necessary to transport the plate over a very long distance without contacting rolls, etc., which will require huge equipment and may cause problems such as plate vibration. Therefore, it is preferable that the temperature rise time and the time held at the heat treatment temperature are each 1.5 seconds or less, and that the total time is also 3.0 seconds or less.

[0078] After the heat treatment, the resin-coated metal sheet is cooled. The cooling method is preferably water cooling using temperature-adjusted water or gas cooling using air, nitrogen, or the like. From the viewpoint of simplifying the equipment, water cooling is more preferable. Examples of water cooling methods include immersing the resin-coated metal sheet in a water tank containing water and spraying water onto the resin-coated metal sheet from a nozzle or the like. The cooling stop temperature is preferably 5°C or higher. By setting the cooling stop temperature to 5°C or higher, condensation on the resin-coated metal sheet and surrounding equipment after cooling can be more effectively prevented. Furthermore, the cooling stop temperature is preferably (the glass transition temperature of the thermoplastic resin film - 10°C) or lower. If the cooling stop temperature is (the glass transition temperature of the thermoplastic resin film - 10°C) or lower, the fluidity of the amorphous structure inside the resin coating layer can be more effectively suppressed, and the resin-coated metal sheet can be more effectively prevented from coming into contact with a roll or the like after cooling, resulting in roughness on the surface.

[0079] The manufacturing conditions other than those mentioned above can be the same as those in the ordinary method.

[0080] The metal plate had a thickness of 0.22 mm and a metal chromium coating weight of 120 mg / m ２ , chromium oxide deposition amount 10 mg / m ２ A chrome-plated steel sheet (TFS) with a temper of T3CA (equivalent to metallic chromium) was used. In each example, a resin with the composition shown in Table 1, rutile-type titanium dioxide with a purity of 90% by mass, and wax were prepared. For each layer in each example, the resin, titanium dioxide, and wax were fed into an extruder and heated to melt. The molten raw materials were sent to a lamination device (feed block) through a filter, formed into a sheet using a T-die, and cooled and solidified on a cast drum to produce a thermoplastic resin film. The thermoplastic resin film was stretched in the longitudinal direction and then in the width direction to obtain a sequentially biaxially stretched film.

[0081] In each example, a resin film was coated on a metal plate by a thermocompression film lamination method under the conditions shown in Table 2, followed by water cooling. The thermocompression bonding time was 20 msec, and the first layer of the thermoplastic resin film was brought into contact with the metal plate and pressure-bonded (in No. 24, a titanium dioxide-added layer was pressure-bonded). Then, a heat treatment was performed under the conditions shown in Table 2, followed by water cooling to produce a resin-coated metal plate. Note that Nos. 26 and 27 were not subjected to heat treatment.

[0082] The thickness of the resin coating layer, the content of inorganic additives, the amount of crystals, the surface roughness, and the Ti element ratio at the interface between the resin coating layer and the metal sheet and at the surface of the resin coating layer were measured for the obtained resin-coated metal sheets using the methods described above. The melting point of the resin coating layer was also measured using the method described below. The measurement results are shown in Table 1. In each example, the resin constituting the resin coating layer was polyester resin, with the proportion of polyester resin being 100% by mass in terms of solid content relative to the total resin, and Table 1 shows the composition of the polyester resin in each example.

[0083] [Melting Point] A resin-coated metal sheet was immersed in a 1:1 mixed solution of concentrated hydrochloric acid (12 mol / L):distilled water at room temperature to dissolve the metal sheet and isolate the resin coating layer. The isolated resin coating layer was then thoroughly washed with distilled water and vacuum dried. The dried resin coating layer was measured using a differential scanning calorimeter (DSCQ100) manufactured by TA Instruments from 0°C to 300°C at a heating rate of 10°C / min. The peak temperature of the endothermic peak measured between 200°C and 280°C was taken as the melting point of the resin coating layer.

[0084]

[0085] The resin-coated metal sheets of each example were evaluated for processability, surface roughness, appearance (surface smoothness and width direction), and adhesion by the following methods. Table 2 shows the evaluation results.

[0086] [Processability] Paraffin wax was applied to the resin-coated metal sheet of each example, and then a circular blank with a diameter of 180 mm was punched out. This circular blank was drawn using a cupping press, followed by two-stage re-drawing and one-stage ironing to form cans with an inner diameter of 52 mm and a can height of 163 mm. After forming, the surface of the resin coating layer on the outer surface of the can body was visually observed, and the processability was evaluated according to the following criteria: Evaluation: "A": No chipping was observed. Evaluation: "B": Chip-off occurred at a height of 5 mm or less from the can flange. No practical problems were encountered. Evaluation: "C": Chip-off occurred at a height of more than 5 mm from the can flange. Problems were encountered in practical use.

[0087] [Surface Roughening] Paraffin wax was applied to each resin-coated metal sheet, and a circular blank with a diameter of 180 mm was punched out. This circular blank was then drawn using a cupping press, followed by two stages of redrawing and one stage of ironing to form cans with an inner diameter of 52 mm and a can height of 163 mm. The formed cans were heated in a hot air oven under conditions such that the can body temperature reached (the melting point of the resin coating layer + 5)°C in 2 minutes, and then rapidly cooled with cold air. After cooling, the condition of the resin coating layer on the outer surface of the can body was visually inspected, and the surface roughness was evaluated according to the following criteria: Evaluation "A": No appearance defects such as black spots or wrinkles were observed. Evaluation "B": Appearance defects such as black spots or wrinkles occurred at a height of 5 mm or less from the can flange. No practical problems were observed. Evaluation "C": Appearance defects such as black spots or wrinkles occurred at a height of more than 5 mm but less than 20 mm from the can flange. Problems were observed in practical use. Evaluation "x": Appearance defects such as black spots and wrinkles were observed at a height of more than 20 mm from the can flange, which poses a practical problem.

[0088] [Appearance] The uniformity of the appearance in terms of surface smoothness on both sides of the resin-coated metal sheet of each example was visually confirmed, and the appearance was evaluated according to the following criteria: Evaluation "Good": No abnormalities in appearance Evaluation "Poor": Abnormalities in appearance such as mottled or cloudy appearance were observed.

[0089] [Adhesion] Paraffin wax was applied to each resin-coated metal sheet, and a circular blank with a diameter of 180 mm was punched out. This circular blank was then drawn using a cupping press, followed by two stages of redrawing and one stage of ironing to form a can with an inner diameter of 52 mm and a height of 163 mm. A sample (15 mm wide x 120 mm long) for a peel test was cut from the body of the formed can, with the can height direction being the longitudinal direction (test direction). A portion of the resin coating layer was peeled from the edge of the cut sample facing the can body opening, and the peeled resin coating layer was opened in the opposite direction (at an angle of 180°) from the metal sheet from which the resin coating layer was peeled. A peel test was performed at a tensile speed of 30 mm / min. Adhesion per 15 mm width was evaluated according to the following criteria. The surface evaluated for adhesion was the outer surface of the can. Evaluation "◎": 3.0N / 15mm or more Evaluation "○": 2.0N / 15mm or more, less than 3.0N / 15mm Evaluation "△": 1.0N / 15mm or more, less than 2.0N / 15mm Evaluation "×": Less than 1.0N / 15mm

[0090]

[0091] As shown in Table 2, in the resin-coated metal sheets of the invention, the processability, surface roughness, appearance (surface smoothness and width direction), and adhesion of the resin coating layer that becomes the outer surface of the container after molding were all good (◎ or ◯). On the other hand, in the comparative examples, the evaluation results of any of the processability, surface roughness, appearance, and adhesion after processing were insufficient (△ or ×).

[0092] According to the present invention, it is possible to suppress appearance defects (rough surfaces) that occur in the resin coating layer of a resin-coated metal sheet during heat treatment after can-making, and to suppress a decrease in the smoothness of the surface of the resin coating layer during coating, thereby making it possible to provide a resin-coated metal sheet that has a smooth and beautiful appearance and is excellent in workability and adhesion of the resin coating layer after processing.

[0093] 1 Resin coated metal plate 2 Metal plate 3 Resin coating layer 3a Third layer 3b Second layer 3c First layer 4 Resin coating layer

Claims

1. A resin-coated metal plate having a resin coating layer containing 90% by mass or more of a polyester resin based on the total resin on at least one surface of the metal plate, The amount of crystals in the resin coating layer is 15% or less, 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, A resin-coated metal sheet, wherein an 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.

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

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

5. The resin-coated metal sheet according to claim 1 or 2, wherein the resin coating layer contains 0.010 mass % or more and 1.0 mass % or less of wax.

6. 3. A two-piece can comprising the resin-coated metal sheet according to claim 1 or 2, wherein the resin coating layer is located on the outer surface side.

7. A thermoplastic resin film is provided, which has a thickness of 2 μm or more, has a multilayer structure including a first layer containing 2 mass % or less of titanium dioxide, and a second layer in contact with the first layer, contains 8 mass % or more and 30 mass % or less of titanium dioxide as a whole, and contains 90 mass % or more of polyester resin based on the total resin, The thermoplastic resin film is pressed against at least one surface of a metal plate heated to a temperature between (the melting point of the thermoplastic resin film - 40 ° C) and (the melting point of the thermoplastic resin film + 5 ° C), with the first layer being brought into contact with the metal plate; The method for producing a resin-coated metal plate includes heating the metal plate to a heat treatment temperature of (melting point of the thermoplastic resin film + 5°C) or more and (melting point of the thermoplastic resin film + 30°C) or less in 0.5 seconds or more and 1.5 seconds or less, holding the metal plate at the heat treatment temperature for 0.5 seconds or more and 1.5 seconds or less, and then cooling the metal plate to obtain a resin-coated metal plate.

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