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

Abstract

Provided is a resin-coated metal sheet having a smooth and attractive outward appearance, the method minimizing any defects (roughness) in external appearance that are produced in a resin coating layer during heat treatment after can processing and minimizing any decrease in the smoothness of the resin-coating-layer surface during coating. This resin-coated metal sheet 1 comprises resin coating layers 3, 4 on at least one surface of a metal sheet 2, the resin coating layers 3, 4 having a polyester resin content of at least 90 mass% with respect to the total resin content, wherein the crystallinity of the resin coating layers is 15% or less, the maximum value SaMAX of the arithmetic average height Sa for the surfaces of the resin coating layers is 0.30 μm or lower, and the difference ΔSa between the maximum value SaMAX and the minimum value SaMIN of the arithmetic average height Sa for the surfaces of the resin coating layers is 0.15 μm or less.

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 thermoplastic resin film is pressed against a metal plate at a low temperature and then heat-treated for a very short time 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 forming. 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] The present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows.

[0012] [1] A resin-coated metal sheet having a resin coating layer containing 90% by mass or more of polyester resin on at least one surface of a metal sheet, wherein the amount of crystals in the resin coating layer is 15% or less, and the maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer ＭＡＸ The maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer is 0.30 μm or less. ＭＡＸ and the minimum value Sa ＭＩＮ A resin-coated metal sheet in which the difference ΔSa between the

[0013] [2] The resin-coated metal plate according to [1], wherein the resin coating layer is provided on both the front and back surfaces of the metal plate, and the difference in melting point of the resin coating layer on both the front and back surfaces is 25°C or less.

[0014] [3] The resin-coated metal sheet according to [1] or [2], wherein the resin coating layer on at least one side contains 0.010% by mass or more and 1.0% by mass or less of wax.

[0015] [4] A two-piece can made using the resin-coated metal sheet according to any one of [1] to [3] above.

[0016] [5] The two-piece can according to [4], wherein the resin coating layer is located on the outer surface side of the two-piece can.

[0017] [6] A method for producing a resin-coated metal plate, comprising: pressing a thermoplastic resin film containing a thermoplastic resin onto at least one surface of a metal plate heated to a temperature of (melting point of the thermoplastic resin film - 40°C) or higher (melting point of the thermoplastic resin film + 5°C) or lower, followed by cooling to obtain a pre-heat-treated resin-coated metal plate; and heating the pre-heat-treated resin-coated metal plate to a heat treatment temperature of (melting point of the thermoplastic resin film + 5°C) or higher (melting point of the thermoplastic resin film + 30°C) in 0.5 seconds or higher and 1.5 seconds or lower, maintaining the heat treatment temperature for 0.5 seconds or higher and 1.5 seconds or lower, and then cooling to obtain a resin-coated metal plate.

[0018] 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.

[0019] FIG. 1 is a diagram showing a cross section of an example of a resin-coated metal plate.

[0020] 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.

[0021] The resin-coated metal sheet according to the present invention is a resin-coated metal sheet having a resin coating layer on at least one surface of a metal sheet, the resin coating layer containing 90% by mass or more of polyester resin based on the total resin, wherein the crystal content of the resin coating layer is 15% or less, and the maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer is ＭＡＸ is 0.30 μm or less, and the maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer ＭＡＸ and the minimum value Sa ＭＩＮ The difference ΔSa between the above is 0.15 μm or less.

[0022] 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.

[0023] Fig. 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 Fig. 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.

[0024] [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.

[0025] 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 ２ The 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] [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.

[0031] Polyester resins are polymers consisting of dicarboxylic acid units and glycol units.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] [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, high post-processing adhesion of the resin coating layer can be achieved. 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.

[0037] 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)

[0038] 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)

[0039] [Surface smoothness of resin coating layer] The maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer ＭＡＸ The maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer is 0.30 μm or less. ＭＡＸ If the maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer is more than 0.30 μm, the surface roughness of the resin coating layer is so large that a colored resin coating layer will have a mottled appearance, and a colorless resin coating layer will have a cloudy appearance. ＭＡＸ The lower limit of the maximum value Sa ＭＡＸ is preferably 0.10 μm or more.

[0040] The maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer ＭＡＸ and the minimum value Sa ＭＩＮ The difference ΔSa between the maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer and the maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer is 0.15 μm or less. ＭＡＸ and the minimum value Sa ＭＩＮ If the difference ΔSa between the maximum value Sa and the maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer exceeds 0.15 μm, the appearance of the product will vary depending on the position, and it will be impossible to obtain a metal container with a consistently beautiful appearance. ＭＡＸ and the minimum value Sa ＭＩＮ The lower limit of the difference ΔSa between the above values ​​is not particularly limited, but it is preferable that the difference ΔSa is 0.05 μm or more.

[0041] The maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer ＭＡＸ and the minimum value Sa ＭＩＮ 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 field of view of 1.9 mm × 1.4 mm at a magnification of 160 times, and the arithmetic mean height Sa is calculated by surface roughness analysis. Measurements are performed at five randomly selected points on the same surface of the resin-coated metal sheet, and the maximum value is taken as the maximum value Sa of the arithmetic mean heights Sa of the resin-coated metal sheet. ＭＡＸ , the minimum value of the arithmetic mean height Sa of the resin-coated metal sheet ＭＩＮ The obtained Sa ＭＡＸ and Sa ＭＩＮ The difference between these is defined as ΔSa.

[0042] 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.

[0043] The amount of crystals and surface smoothness of such a 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 with a resin coating layer at a low temperature, and then heat-treated for a very short time at a temperature above the melting point.

[0044] Wax may be added to the resin coating layer to improve sliding properties during processing. Wax may be added to the resin coating layer in an amount of 0.010% by mass or more, or 1.0% by mass or less. It is particularly preferable to add 0.010% by mass or more to 1.0% by mass or less of wax to the resin coating layer located on the outer surface of the two-piece can after molding. Adding 0.010% by mass or more of wax to the resin coating layer reduces the coefficient of friction of the surface of the resin coating layer during processing, thereby suppressing scraping of the resin coating layer. On the other hand, since the resin coating layer is more easily formed when the wax content is 1.0% by mass or less, the wax content is preferably 1.0% by mass or less.

[0045] 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.

[0046] 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, coloring pigments may be added to the resin coating layer to improve the appearance of the inner and outer surfaces of the metal container.

[0047] A white pigment may be added to the resin coating layer. Adding a white pigment to the resin coating layer can conceal the underlying metal and increase the clarity of the printing, resulting in a particularly good appearance. Examples of white pigments that can be added include titanium dioxide, alumina, calcium carbonate, barium sulfate, and mixtures thereof. Titanium dioxide is preferred as the white pigment because it has strong coloring power and can ensure a good appearance even after can formation. Among white pigments, rutile acid-type white pigments with a purity of 90% or more are particularly preferred because they have better dispersibility when mixed with the resin material. When titanium dioxide is added as a white pigment, the amount added is preferably 8% by mass or more and 30% by mass or less of the resin coating layer.

[0048] When resin coating layers are formed on both sides of a metal plate, the difference in melting point between the resin coating layers on both sides is preferably 25° C. or less for manufacturing reasons, in order to achieve low crystallinity in both resin coating layers and obtain a good appearance. Details will be described later in the explanation of the manufacturing method.

[0049] The thickness of the resin coating layer is not particularly limited, but may be, for example, 6 μm or more and 50 μm or less.

[0050] Furthermore, the resin coating layer is not limited to a single-layer structure, but may have a multi-layer structure as long as it satisfies the requirements of the present invention.

[0051] 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.

[0052] Next, an example of a method for producing a resin-coated metal sheet will be described. The method for producing a resin-coated metal sheet according to the present invention comprises: pressing a thermoplastic resin film containing a thermoplastic resin onto at least one surface of a metal sheet heated to a temperature of (the melting point of the thermoplastic resin film - 40°C) or higher (the melting point of the thermoplastic resin film + 5°C) or lower, followed by cooling to obtain a pre-heat-treatment resin-coated metal sheet; heating the pre-heat-treatment resin-coated metal sheet to a heat treatment temperature of (the melting point of the thermoplastic resin film + 5°C) or higher (the melting point of the thermoplastic resin film + 30°C) in 0.5 to 1.5 seconds, holding the heat treatment temperature for 0.5 to 1.5 seconds, and then cooling to obtain a resin-coated metal sheet.

[0053] 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. 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 and the like are also as described above in the description of the resin coating layer.

[0054] The method for producing a thermoplastic resin film is not particularly limited, but in one example, it can be produced as follows. First, a thermoplastic resin raw material is heated and dried under vacuum as necessary, and then charged into an extruder, where the thermoplastic resin is heated and melted. The heated and melted thermoplastic resin is extruded through a filter or the like. Foreign matter and denatured resin can be removed by the filter. The extruded 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.

[0055] 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 a 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.

[0056] 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.

[0057] The thermoplastic resin film is heated to a temperature equal to or higher than the melting point and is 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.

[0058] 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.

[0059] 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.

[0060] 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. The melting point of the thermoplastic resin film is the same as the melting point of the resin coating layer formed by pressure bonding the thermoplastic resin film to the metal plate.

[0061] 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.

[0062] 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, and 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 suitably suppressed, and the resin-coated metal plate can be more suitably prevented from coming into contact with a roll or the like after cooling, resulting in surface roughness. Furthermore, 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 suitably prevented.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] When forming a resin coating layer on both sides of a metal plate, the resin coating layer on at least one side must satisfy the above-mentioned heat treatment temperature conditions. When forming a resin coating layer on both sides of a metal plate, it is preferable that the resin coating layers on both sides satisfy the above-mentioned heat treatment temperature conditions, since both resin coating layers achieve low crystallinity and a good appearance can be obtained. 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 to 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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 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, and is preferably (glass transition temperature of the thermoplastic resin film - 10°C) or lower. If the cooling stop temperature is (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 suitably suppressed, and the resin-coated metal sheet can be more suitably prevented from coming into contact with a roll or the like after cooling, resulting in surface roughness. Furthermore, 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 suitably prevented.

[0071] The manufacturing conditions other than those mentioned above can be the same as those in the ordinary method.

[0072] 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 Examples 1 to 19, Comparative Examples 1 to 3, and Comparative Examples 6 and 7, a thermoplastic resin film was pressure-bonded to a metal sheet by a thermocompression film lamination method under the conditions shown in Table 1, and then a heat treatment was performed under the conditions shown in Table 1 to produce a resin-coated metal sheet. In Comparative Examples 4 and 5, a thermoplastic resin film was pressure-bonded to a metal sheet by a thermocompression film lamination method under the conditions shown in Table 1 to produce a resin-coated metal sheet.

[0073] The inorganic additive content, crystal content, and surface roughness of the resin coating layer of the obtained resin-coated metal sheets were measured 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 100% by mass of polyester resin in terms of solid content relative to the total resin, and Table 1 shows the composition of the polyester resin in each example.

[0074] [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.

[0075]

[0076] Furthermore, the resin-coated metal sheets of Examples 1 to 19 and Comparative Examples 1 to 7 were evaluated for workability, surface roughness, appearance, and post-working adhesion by the methods described below. Table 2 shows the evaluation results.

[0077] [Processability] Paraffin wax was applied to the resin-coated metal sheets of Examples 1 to 19 and Comparative Examples 1 to 7, and then circular blanks with a diameter of 180 mm were punched out. These circular blanks were subjected to drawing in 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: "○": Chip-off occurred at a height of 5 mm or less from the can flange. No practical problems were encountered. Evaluation: "×": Chip-off occurred at a height of more than 5 mm from the can flange. Problems were encountered in practical use.

[0078] [Surface Roughness] Paraffin wax was applied to the resin-coated metal sheets of Examples 1 to 19 and Comparative Examples 1 to 7, and then circular blanks with a diameter of 180 mm were punched out. These circular blanks were 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 (melting point of the thermoplastic resin film + 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: "Excellent": No appearance defects such as black spots or wrinkles were observed. "Good": 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. "Average": Appearance defects such as black spots or wrinkles occurred at a height of more than 5 mm but not more than 20 mm from the can flange. Problems in practical use. Evaluation "x": Appearance defects such as black spots and wrinkles occurred at a height of more than 20 mm from the can flange. Problems in practical use.

[0079] [Appearance] The surface smoothness and uniformity of appearance in the width direction of both sides of the resin-coated metal sheets of Invention Examples 1 to 19 and Comparative Examples 1 to 7 were visually inspected, and the appearance was evaluated according to the following criteria: Appearance (surface smoothness) Evaluation "Good": No abnormalities in appearance. Evaluation "Poor": Abnormalities in appearance such as mottled or cloudy appearance. Appearance (Width direction) Evaluation "Good": The appearance is uniform in the width direction. Evaluation "Poor": The appearance differs greatly in the width direction.

[0080] [Post-Processing Adhesion] Paraffin wax was applied to the resin-coated metal sheets of Examples 1 to 19 and Comparative Examples 1 to 7, and then circular blanks with a diameter of 180 mm were punched out. These circular blanks were then drawn in 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. Peel test samples (15 mm wide x 120 mm long) were cut from the can bodies after forming, 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 opening, and the peeled resin coating layer was opened in the opposite direction (at an angle of 180 degrees) 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. The adhesion strength per 15 mm width was evaluated according to the following criteria. The surface to be measured for adhesion strength was the inner 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

[0081]

[0082] As shown in Table 2, the resin-coated metal sheet of Example 1 had good processability, surface roughness, and appearance of the resin coating layer that would become the outer surface of the container after molding (◎ or ◯). The resin-coated metal sheets of Examples 2 to 19 were all good in processability, surface roughness, appearance, and post-processing adhesion. On the other hand, the resin-coated metal sheets of Comparative Examples 1 to 7 had insufficient evaluation results (△ or ×) for any of processability, surface roughness, appearance, and post-processing adhesion.

[0083] 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 having a smooth and beautiful appearance.

[0084] REFERENCE SIGNS LIST 1 Resin-coated metal plate 2 Metal plate 3 Resin coating layer 4 Resin coating layer

Claims

1. A resin-coated metal plate having a resin coating layer containing 90% by mass or more of polyester resin on at least one surface of a metal plate, The amount of crystals in the resin coating layer is 15% or less, The maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer ＭＡＸ is 0.30 μm or less, The maximum value Sa of the arithmetic mean height Sa of the surface of the resin coating layer ＭＡＸ and the minimum value Sa ＭＩＮ The resin-coated metal sheet has a difference ΔSa of 0.15 μm or less.

2. The resin coating layer is provided on both the front and back surfaces of the metal plate, 2. The resin-coated metal sheet according to claim 1, wherein the difference in melting point between the resin coating layers on the front and back sides is 25° C. or less.

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

4. A two-piece can made using the resin-coated metal sheet according to claim 1 or 2.

5. The two-piece can according to claim 4, wherein the resin coating layer is located on an outer surface side of the two-piece can.

6. A thermoplastic resin film containing a thermoplastic resin is pressure-bonded to at least one surface of a metal plate heated to a temperature of (the melting point of the thermoplastic resin film - 40 ° C.) or more and (the melting point of the thermoplastic resin film + 5 ° C.) or less, and then cooled to obtain a pre-heat-treated resin-coated metal plate; A method for producing a resin-coated metal sheet, comprising heating the resin-coated metal sheet before heat treatment to a heat treatment temperature of (melting point of the thermoplastic resin film + 5°C) or more (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 heat treatment temperature for 0.5 seconds or more and 1.5 seconds or less, and then cooling the resin-coated metal sheet to obtain a resin-coated metal sheet.