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

Abstract

Provided is a resin-coated metal plate which has a smooth and beautiful appearance by suppressing appearance defects (surface roughness) occurring in a resin coating layer during heat treatment after can-making processing and suppressing a decrease in smoothness of the surface of the resin coating layer during coating. This resin-coated metal plate (1) is provided on at least one surface of a metal plate (2) with resin coating layers (3, 4) containing 90 mass% or more of a polyester resin with respect to the total resin, the crystal content of the resin coating layers being 15% or less, and the maximum value SaMAX of the arithmetic mean height Sa of the surfaces of the resin coating layers being 0.30 [mu] m or less. The difference [Delta] Sa between the maximum value SaMAX and the minimum value SaMIN of the arithmetic mean height Sa of the surface of the resin coating layer is 0.15 [mu] m or less.

Description

Technical Field

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

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

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

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

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

[0006] [Prior art literature]

[0007] [Patent Document]

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

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

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

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

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

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

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

[0015] [Means for solving the problem]

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

[0017] The present invention was made based on the above findings. That is, the gist of the present invention is as follows.

[0018] [1] A resin-coated metal plate comprising a resin coating layer containing 90% by mass or more of a polyester resin relative to the total resin on at least one surface of the metal plate, wherein:

[0019] The crystallization amount of the resin covering layer is less than 15%,

[0020] The maximum value Sa of the arithmetic mean height Sa of the surface of the resin cover layer MAX 0.30μm or less,

[0021] The maximum value Sa of the arithmetic mean height Sa of the surface of the resin cover layer MAX With the minimum value Sa MIN The difference ΔSa is 0.15 μm or less.

[0022] [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.

[0023] The difference in melting points between the resin covering layers on the front and back surfaces is 25° C. or less.

[0024] [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.

[0025] [4] A two-piece can obtained by coating a metal plate with the resin described in any one of [1] to [3] above.

[0026] [5] The two-piece can according to [4] above, wherein the resin coating layer is located on the outer surface side of the two-piece can.

[0027] [6] A method for producing a resin-coated metal plate, wherein a thermoplastic resin film containing a thermoplastic resin is pressed against at least one side of a metal plate heated to a temperature not lower than (the melting point of the thermoplastic resin film - 40°C) and not higher than (the melting point of the thermoplastic resin film + 5°C), and then cooled to obtain a resin-coated metal plate before heat treatment.

[0028] The resin-coated metal plate before heat treatment is heated to a heat treatment temperature of not less than (the melting point of the thermoplastic resin film + 5° C.) and not more than (the melting point of the thermoplastic resin film + 30° C.) over a period of not less than 0.5 seconds and not more than 1.5 seconds, maintained at the heat treatment temperature for not less than 0.5 seconds and not more than 1.5 seconds, and then cooled to obtain a resin-coated metal plate.

[0029] [Effects of the Invention]

[0030] According to the present invention, it is possible to suppress appearance defects (surface roughness) that may occur in the resin coating layer of a resin-coated metal sheet during heat treatment after canning, and to suppress a decrease in the smoothness of the surface of the resin coating layer during coating. Furthermore, it is possible to provide a resin-coated metal sheet having a smooth and beautiful appearance while achieving both workability and adhesion of the resin coating layer after processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a diagram showing a cross section of an example of a resin-coated metal plate. DETAILED DESCRIPTION

[0032] Hereinafter, an embodiment of the method for producing a resin-coated metal sheet of the present invention will be described. It should be noted that the embodiment described below is an example of a specific embodiment of the present invention, and the configuration of the present invention is not limited to the specific example.

[0033] The resin-coated metal sheet of the present invention comprises a resin coating layer containing 90% by mass or more of a polyester resin relative to the total resin on at least one surface of the metal sheet, wherein the crystallization amount 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 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 With the minimum value Sa MIN The difference ΔSa is 0.15 μm or less.

[0034] According to the present invention, it is possible to suppress appearance defects (surface roughness) in the resin coating layer of a resin-coated metal sheet during heat treatment after canning, and to suppress a decrease in surface smoothness of the resin coating layer during coating. Furthermore, it is possible to provide a resin-coated metal sheet having a smooth and beautiful appearance. Furthermore, it is possible to provide a resin-coated metal sheet having reduced residual stress after canning and excellent workability of the resin coating layer and post-processing adhesion.

[0035] exist Figure 1 1 shows an example of a cross section of the resin-coated metal plate 1 according to one embodiment. Figure 1 The resin-coated metal sheet 1 shown has a resin coating layer 3 provided on the front side of the metal sheet 2, and a resin coating layer 4 provided on the back side of the metal sheet 2. It should be noted that the resin coating layer may be provided on only one side of the metal sheet 2. The resin coating layer 3 provided on the front side of the metal sheet 2 and the resin coating layer 4 provided on the back side of the metal sheet 2 are located on the outer surface and inner surface of the two-piece can, respectively, after canning.

[0036] [Metal Sheet]

[0037] First, the metal plate will be described. As the metal plate of the resin-coated metal plate, steel plates and aluminum plates, which are widely used as can materials, can be used.

[0038] As the metal plate, tin-free steel (TFS) is particularly preferred from the viewpoint of resin adhesion in a high-temperature, humid environment such as retort sterilization. The amount of the metal chromium layer and the chromium oxide layer applied to the TFS is not particularly limited, but preferably has an amount of 50 mg / m2 applied to the surface. 2 Above 200g / m 2The following metal chromium layer and the amount of metal chromium on it is 3mg / m 2 Above 30g / m 2 Below the chromium oxide layer.

[0039] The type of metal plate is not particularly limited as long as it can be formed into a target shape, but steel plates having the following component compositions and production methods are preferred.

[0040] (1) A steel sheet obtained by using low-carbon steel having a C (carbon) content of more than 0.003 mass % and 0.10 mass % or less and performing recrystallization annealing by continuous annealing.

[0041] (2) A steel sheet obtained by using a low-carbon steel having a C content of more than 0.003 mass % and 0.10 mass % or less and performing recrystallization annealing and overaging treatment by continuous annealing.

[0042] (3) A steel sheet obtained by using low-carbon steel having a C content exceeding 0.003 mass % and 0.10 mass % or less and performing recrystallization annealing by box annealing.

[0043] (4) A steel sheet obtained by using a low-carbon steel having a C content of more than 0.003 mass % and 0.10 mass % or less, performing recrystallization annealing by continuous annealing or box annealing, and then performing secondary cold rolling (DR (Double Reduced) rolling).

[0044] (5) A steel plate obtained by continuous annealing and recrystallization annealing using IF (Interstitial Free) steel in which elements such as Nb and Ti for fixing dissolved C are added to ultra-low carbon steel having a C content of 0.003 mass % or less.

[0045] The mechanical properties of the metal sheet are not particularly limited as long as it can be formed into the desired shape. To ensure sufficient can strength without compromising workability, a metal sheet with a yield point (YP) of 220 MPa to 580 MPa is preferred. Furthermore, the Lankford value (r-value), an indicator of plastic anisotropy, is preferably 0.8 or higher. Furthermore, the absolute value of the in-plane anisotropy Δr of the r-value is preferably 0.7 or lower.

[0046] The composition of the metal plate is not particularly limited. For example, a steel plate containing component elements such as Si, Mn, P, S, Al and N may also be used. The Si content is preferably 0.001% by mass or more, and preferably 0.1% by mass or less. The Mn content is preferably 0.01% by mass or more, and preferably 0.6% by mass or less. The P content is preferably 0.002% by mass or more, and preferably 0.05% by mass or less. The S content is preferably 0.002% by mass or more, and preferably 0.05% by mass or less. The Al content is preferably 0.005% by mass or more, and preferably 0.100% by mass or less. The N content is preferably 0.0005% by mass or more, and preferably 0.020% by mass or less. In addition, the composition may also contain other components such as Ti, Nb, B, Cu, Ni, Cr, Mo and V. From the perspective of ensuring corrosion resistance, the content of the above-mentioned component elements is preferably 0.02% by mass or less in total.

[0047] The thickness of the metal plate is not particularly limited, and may be, for example, 0.10 mm or more and 0.50 mm or less.

[0048] [Composition of resin cover layer]

[0049] The resin-coated metal sheet comprises a resin coating layer composed primarily of polyester resin on at least one surface of the metal sheet. The proportion of the polyester resin in the resin constituting the resin coating layer is 90% by mass or greater, calculated as solids. If the resin coating layer contains inorganic additives (such as inorganic pigments), the proportion of the polyester resin in the resin, after deducting the weight of these inorganic additives, is 90% by mass or greater.

[0050] Polyester resin is a polymer composed of dicarboxylic acid units and ethylene glycol units.

[0051] As the dicarboxylic acid unit, units derived from aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenoxyethane dicarboxylic acid, 5-sodium sulfonatoisophthalic acid, and phthalic acid; aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, and fumaric acid; alicyclic dicarboxylic acids such as cyclohexane dicarboxylic acid; and hydroxycarboxylic acids such as p-hydroxybenzoic acid can be used.

[0052] The polyester resin preferably contains 90 mol% or more of terephthalic acid units among the dicarboxylic acid units. When the polyester resin contains 90 mol% or more of terephthalic acid units among the dicarboxylic acid units, sufficient heat resistance against frictional heat during continuous canning can be ensured, and more stable moldability and coating properties can be achieved.

[0053] As the ethylene glycol unit, units derived from aliphatic diols such as ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, and neopentyl glycol; alicyclic diols such as cyclohexanedimethanol; aromatic diols such as bisphenol A and bisphenol S; and diethylene glycol can be used.

[0054] In addition, the above-mentioned dicarboxylic acids and diols may be used in combination of two or more types within a range that does not impair heat resistance and processability.

[0055] [Amount of crystallization of the resin cover layer]

[0056] The crystallization amount of the resin covering layer is set to 15% or less. By setting the crystallization amount to 15% or less, the high moldability required for the molding of two-piece cans can be obtained, and the appearance defects (rough surface) generated in the resin covering layer during the heat treatment after the canning process can be suppressed. In addition, by setting the crystallization amount to a low value, the residual stress in the resin covering layer after the canning process is reduced, thereby achieving high post-processing adhesion of the resin covering layer. The crystallization amount of the resin covering layer is preferably 12% or less, and more preferably 10% or less. When the resin covering layer contains inorganic additives (inorganic pigments, etc.), the crystallization amount in the resin material obtained by subtracting the mass of the inorganic additives needs to be 15% or less. The lower limit of the crystallization amount is not particularly limited, and the crystallization amount can be 1% or more. It should be noted that, as shown below, the crystallization amount is calculated based on the inorganic additive content obtained by thermogravimetric measurement and the crystallization heat and fusion heat obtained by differential scanning calorimetry. Here, inorganic additives refer to inorganic pigments and inorganic additives among additives other than inorganic pigments.

[0057] The determination of the content of inorganic additives is carried out as follows. First, the resin-coated metal plate is immersed in a mixed solution of concentrated hydrochloric acid (12 mol / L): distilled water = 1:1 at room temperature to dissolve the metal plate to separate the resin coating layer. The separated resin coating layer is thoroughly washed with distilled water and then vacuum-dried. The thermogravimetric measurement is performed using a thermogravimetric measuring device with the temperature range set from room temperature to 800°C, an air flow rate of 300 mL / min, and a heating rate of 10°C / min. As shown in the following formula (1), the ratio of the weight at 800°C to the weight at room temperature is defined as the inorganic additive content.

[0058] Content of inorganic additives [%] = weight at 800°C [mg] / weight at room temperature [mg] × 100 (1)

[0059] The determination of the amount of crystallization is carried out as follows. In the same manner as the determination of the content of inorganic additives, the metal plate is dissolved from the resin-coated metal plate to separate the resin coating layer, and the resin coating layer is dried. The dried resin coating layer is measured from 0°C to 300°C at a heating rate of 10°C / min using a differential scanning calorimeter (DSCQ100) manufactured by TA Instruments. The heat of crystallization is calculated based on the area of the exothermic peak observed during the period of 100-200°C, and the heat of fusion is calculated based on the area of the endothermic peak measured during the period of 200°C to 280°C. Based on the obtained heat of crystallization and heat of fusion, the amount of crystallization is calculated according to the following formula (2). It should be noted that, with respect to the content of inorganic additives, the value obtained by the above-mentioned method is used.

[0060] Crystallization amount [%] = (heat of fusion [J / g] - heat of crystallization [J / g]) × 100 / (100 - inorganic additive content [%]) / 140.2 [J / g] × 100… (2)

[0061] [Surface smoothness of resin coating]

[0062] The maximum value Sa of the arithmetic mean height Sa of the surface of the resin cover layer MAX The maximum value Sa of the arithmetic mean height Sa on the surface of the resin coating layer is set to 0.30 μm or less. MAX If the surface roughness of the resin coating layer exceeds 0.30 μm, the surface roughness of the resin coating layer is large, resulting in a mottled pattern in the case of a colored resin coating layer and a blurred appearance in the case of a colorless resin coating layer. MAX There is no particular lower limit for Sa, but the maximum value Sa is preferably MAX It is set to be 0.10 μm or more.

[0063] The maximum value Sa of the arithmetic mean height Sa of the surface of the resin cover layer MAX With the minimum value Sa MIN The difference ΔSa is set to be 0.15 μm or less. The maximum value Sa of the arithmetic mean height Sa on the surface of the resin coating layer MAX With the minimum value Sa MIN If the difference ΔSa exceeds 0.15 μm, the appearance of the product will vary depending on the position, and a metal container with a beautiful appearance cannot be stably obtained. MAX With the minimum value Sa MIN The lower limit of the difference ΔSa is not particularly limited, but it is preferable that the ΔSa be 0.05 μm or more.

[0064] The maximum value Sa of the arithmetic mean height Sa of the surface of the resin cover layerMAX and minimum value Sa MIN The surface roughness was measured using a 3D shape measuring machine. Using a Keyence one-touch 3D shape measuring machine, the shape was measured at a magnification of 160 times on a 1.9mm x 1.4mm field of view. The arithmetic mean height Sa was calculated using the surface roughness analysis. Measurements were performed at five randomly selected locations on the same surface of the resin-coated metal plate, and the maximum value was set as the maximum value Sa of the arithmetic mean height Sa of the resin-coated metal plate. MAX , the minimum value is set to the minimum value Sa of the arithmetic mean height Sa of the resin-covered metal plate MIN The obtained Sa MAX with Sa MIN The difference is set as ΔSa.

[0065] Because it is highly relevant to the product's appearance, the present invention uses the arithmetic mean height (Sa) as an indicator of surface smoothness. There are two types of arithmetic mean height: Ra, which represents the arithmetic mean height of a line, and Sa, which represents the arithmetic mean height of a surface. Using Sa as an indicator of surface smoothness eliminates the discrepancies in measurement results caused by the direction of measurement, as occurs with Ra, and allows evaluation of the entire surface.

[0066] It should be noted that the crystallinity and surface smoothness of such a resin covering layer are achieved, for example, by a two-stage treatment described in the manufacturing method described later. The two-stage treatment means: after covering the metal plate with a resin covering layer at a low temperature, a heat treatment is performed at a temperature exceeding the melting point for an extremely short time.

[0067] In order to improve the sliding properties during processing, wax may also be added to the resin covering layer. Wax may be added in an amount of 0.010% by mass or more, or in an amount of 1.0% or less by mass. In particular, it is preferred to add wax in an amount of 0.010% by mass or more but not more than 1.0% by mass to the resin covering layer located on the outer surface side of the two-piece can after molding. By adding 0.010% by mass or more of wax to the resin covering layer, the friction coefficient of the surface of the resin covering layer during processing can be reduced, thereby suppressing the wear of the resin covering layer. On the other hand, if the amount of wax added is 1.0% by mass or less, the film formation of the resin covering layer is easier, so it is preferred that the amount of wax added is 1.0% by mass or less.

[0068] As the wax, at least one selected from polyolefin waxes such as polyethylene and polypropylene and modified products thereof, natural waxes such as palm wax, polyamide wax, and polyester wax, or a mixture thereof can be used.

[0069] Furthermore, as needed, additives such as antioxidants, heat stabilizers, lubricants, crystal nucleating agents, ultraviolet absorbers, and antistatic agents may be added to the resin coating, provided that they do not impair the effects of the present invention. Furthermore, coloring pigments may be added to the resin coating to enhance the aesthetics of the inner and outer surfaces of the metal container.

[0070] A white pigment may also be added to the resin covering layer. By adding a white pigment to the resin covering layer, the base metal can be shielded, the clarity of the printing can be increased, and a particularly good appearance can be obtained. As a white pigment, for example, titanium dioxide, aluminum oxide, calcium carbonate, barium sulfate, or a mixture thereof can be added. Titanium dioxide is preferably added as a white pigment because it has strong tinting power and can ensure a good appearance even after the can is formed. 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, its addition amount is preferably set to 8% by mass or more of the resin covering layer, and is also preferably set to 30% by mass or less.

[0071] When forming resin coatings on both sides of a metal plate, the difference in melting point between the two resin coatings is preferably 25°C or less for manufacturing reasons, in order to achieve low crystallinity and a good appearance for both resin coatings. Details will be described later in the description of the manufacturing method.

[0072] The thickness of the resin coating layer is not particularly limited, and may be, for example, 6 μm or more, or 50 μm or less.

[0073] Furthermore, the resin covering layer is not limited to a single-layer structure, and may be a multi-layer structure within the scope of satisfying the requirements specified in the present invention.

[0074] By using the above-mentioned resin-coated metal sheet, a two-piece can having a smooth and beautiful appearance can be manufactured. The two-piece can can be manufactured according to conventional methods. In this two-piece can, the resin coating layer is preferably located on the outer surface side of the two-piece can.

[0075] Next, an example of a method for manufacturing a resin-coated metal plate will be described.

[0076] The method for manufacturing the resin-coated metal sheet of the present invention is as follows:

[0077] A thermoplastic resin film containing a thermoplastic resin is press-bonded to at least one surface of a metal plate heated to a temperature not lower than (the melting point of the thermoplastic resin film - 40°C) and not higher than (the melting point of the thermoplastic resin film + 5°C), followed by cooling to obtain a pre-heat-treated resin-coated metal plate.

[0078] The resin-coated metal plate before heat treatment is heated to a heat treatment temperature of not less than (the melting point of the thermoplastic resin film + 5°C) and not more than (the melting point of the thermoplastic resin film + 30°C) over a period of not less than 0.5 seconds and not more than 1.5 seconds, maintained at the heat treatment temperature for not less than 0.5 seconds and not more than 1.5 seconds, and then cooled to obtain a resin-coated metal plate.

[0079] When manufacturing the resin-coated metal sheet of the present invention, a thermoplastic resin film serving as the resin coating layer is first produced. The thermoplastic resin film comprises a thermoplastic resin. As previously described in the description of the resin coating layer, a thermoplastic resin containing 90% or more of a polyester resin relative to the total resin content can be used. Additives, etc., are also as previously described in the description of the resin coating layer.

[0080] The manufacture method of thermoplastic resin film is not particularly limited. In one example, it can be manufactured as follows. First, the thermoplastic resin raw material is heated as needed and dried under vacuum and then put into an extruder. In the extruder, the thermoplastic resin is heated and melted. The heated and melted thermoplastic resin is extruded through a filter or the like. Foreign matter and modified resin can be removed by the filter. The extruded resin is molded into a sheet with a T-die and discharged, and extruded onto a cooling body such as a casting drum. By cooling and solidifying the extruded sheet, an unstretched thermoplastic resin film can be obtained.

[0081] The thermoplastic resin film forming the resin coating layer is preferably formed by stretching the unstretched film to form a stretched film from the perspective of reducing the surface roughness of the resin coating layer. The method for obtaining the stretched film is not particularly limited, and can be used, for example, a method of obtaining a uniaxially stretched film by stretching in the longitudinal or width direction of a film forming machine; a method of obtaining a sequentially biaxially stretched film by stretching in the longitudinal or width direction followed by stretching in the other direction; and a method of obtaining a simultaneously biaxially stretched film by stretching in both the longitudinal and width directions. When obtaining a sequentially biaxially stretched film, it is preferred to stretch the unstretched film in the longitudinal direction before stretching in the width direction from the perspectives of uniform quality and space saving of the equipment.

[0082] Next, the method for producing the resin-coated metal sheet of the present invention using the aforementioned thermoplastic resin film will be described. By coating a metal sheet with a resin film at a low temperature and then subjecting it to a very short heat treatment at a temperature exceeding its melting point, a resin-coated metal sheet can be produced that achieves both low crystallinity and surface smoothness in the resin coating layer.

[0083] The thermoplastic resin film is heated to a temperature not lower than the melting start temperature and is pressed against a metal plate using laminating rollers (thermocompression lamination). This thermocompression lamination method is excellent in reducing production costs and enabling energy-saving production.

[0084] One method for reducing the crystallinity of the resin coating is to melt the resin coating by heating the metal plate to a high temperature before applying the resin coating to the metal plate. However, this method makes it difficult to achieve a smooth surface. Therefore, the present invention manufactures a resin-coated metal plate by performing a two-step process: coating the metal plate with a thermoplastic resin film at a low temperature, followed by a very short heat treatment at a temperature exceeding the melting point.

[0085] When the thermoplastic resin film is pressed against the metal plate, it is necessary to control the pressing conditions and ensure a smooth surface. During lamination, the time (thermocompression bonding time) during which the resin film is pressed against the metal plate by the laminating roller is preferably set to more than 10 msec., and is also preferably set to less than 40 msec. By setting the thermocompression bonding time to more than 10 msec., it is possible to more appropriately ensure that the thermoplastic resin film melts and wets and expands on the metal plate surface, making it possible to make the adhesion more suitable. In addition, if the thermocompression bonding time is less than 40 msec., it is possible to more appropriately prevent the laminating roller side of the thermoplastic resin film from softening, making it possible to make the smoothness of the resin covering layer surface more suitable. The thermocompression bonding time is more preferably more than 15 msec. The thermocompression bonding time is more preferably less than 30 msec.

[0086] In order to ensure the smoothness of the surface of the resin covering layer, it is necessary to control the temperature of the metal plate during crimping and suppress the softening of the surface of the resin covering layer during crimping. The temperature of the metal plate during crimping is set to be above (melting point of the thermoplastic resin film - 40°C) and below (melting point of the thermoplastic resin film + 5°C). If the temperature of the metal plate during crimping is lower than (melting point of the thermoplastic resin film - 40°C), there is a case where the metal plate side of the thermoplastic resin film is not fully melted, thereby reducing the adhesion between the metal plate and the resin covering layer. On the other hand, when the temperature of the metal plate during crimping exceeds (melting point of the thermoplastic resin film + 5°C), the surface of the thermoplastic resin film on the laminating roller side softens and the smoothness of the surface of the resin covering layer is impaired, which is not preferred. It should be noted that the temperature of the metal plate is based on the surface temperature of the metal plate. In addition, the melting point of the thermoplastic resin film is the same value as the melting point of the resin covering layer formed by crimping the thermoplastic resin film to the metal plate.

[0087] The temperature of the laminating roll during pressure bonding is not particularly limited, but is preferably 60° C. or higher to ensure the impact resistance of the film after lamination. It is preferably 150° C. or lower to prevent the film from fusing to the laminating roll during lamination.

[0088] After the thermoplastic resin film is pressed against the metal plate, the resin-coated metal plate is cooled. As a cooling method, water cooling using temperature-regulated water or gas cooling using air, nitrogen, etc. is preferred. From the viewpoint of simplifying the equipment, water cooling is more preferred as a cooling method. As a water cooling method, there can be exemplified a method of immersing the resin-coated metal plate in a water tank filled with water and a method of spraying water onto the resin-coated metal plate from a nozzle, etc. The cooling stop temperature is preferably above 5°C, and is preferably below (glass transition temperature of the thermoplastic resin film - 10°C). If the cooling stop temperature is below (glass transition temperature of the thermoplastic resin film - 10°C), the fluidity of the amorphous structure inside the resin coating layer can be more appropriately suppressed, and the surface roughness caused by contact between the resin-coated metal plate and rollers after cooling can be more appropriately prevented. In addition, by setting the cooling stop temperature to above 5°C, condensation on the cooled resin-coated metal plate and peripheral equipment can be more appropriately prevented.

[0089] As described above, resin-coated metal sheets laminated at low temperatures have smooth surfaces, but their high crystallinity makes it difficult to suppress cosmetic defects (roughness of the surface) that may develop in the resin coating during heat treatment after canning. Furthermore, large residual stresses are generated in the resin coating during canning, potentially reducing the adhesion between the resin coating and the metal sheet. Therefore, it is necessary to heat-treat the pre-heat-treatment resin-coated metal sheets, which have been press-bonded at low temperatures, for an extremely short period of time at a temperature exceeding the melting point of the resin coating to reduce the crystallinity of the resin coating without impairing the surface smoothness.

[0090] Preferred heat treatment methods include non-contact methods such as infrared (IR) and induction heating (IH), which allow for rapid temperature increases and require passage through a heating furnace. Furthermore, to ensure smoothness of the resin coating surface, it is preferable that the resin-coated metal sheet not come into contact with rollers or other devices during the period from the start of heat treatment until the end of cooling. Preventing the resin-coated metal sheet from coming into contact with rollers or other devices while in a high-temperature state can more effectively prevent surface roughness.

[0091] The heat treatment temperature is set to be above (melting point of the thermoplastic resin film + 5°C) and below (melting point of the thermoplastic resin film + 30°C). If the heat treatment temperature is lower than (melting point of the thermoplastic resin film + 5°C), the resin coating may not fully dissolve, and the desired crystallinity of the resin coating may not be achieved. On the other hand, if the heat treatment temperature exceeds (melting point of the thermoplastic resin film + 30°C), there is a possibility of thermal degradation of the resin coating, which is not preferred. The heat treatment temperature is based on the temperature of the metal plate.

[0092] When forming resin coatings on both sides of a metal plate, it is assumed that the resin coating on at least one side satisfies the above-mentioned heat treatment temperature conditions. It should be noted that when forming resin coatings on both sides of a metal plate, in order to achieve low crystallinity in the resin coatings on both sides and appropriately obtain a good appearance, it is preferred that the resin coatings on both sides meet the above-mentioned heat treatment temperature conditions. Therefore, it is preferred that the difference in melting point between the resin coatings on both sides is 25°C or less. If the difference in melting point between the resin coatings 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.

[0093] During the heat treatment, the temperature is raised so that the above-mentioned heat treatment temperature is reached within 0.5 seconds to 1.5 seconds. If the temperature is raised for less than 0.5 seconds, temperature control becomes difficult, resulting in temperature differences in the width direction and thus causing variations in the physical properties of the resin coating layer.

[0094] After heating, the heat treatment temperature is maintained for 0.5 seconds to 1.5 seconds. If the heat treatment temperature is maintained for less than 0.5 seconds, the thermoplastic resin film may not be sufficiently melted and the desired crystallinity of the resin coating layer may not be achieved.

[0095] If the heating time or the holding time at the heat treatment temperature exceeds 1.5 seconds, the sheet must be transported over a very long distance without contact with rollers, etc., which increases the size of the equipment and may cause problems such as sheet vibration. Therefore, it is preferred that the heating time and the holding time at the heat treatment temperature be 1.5 seconds or less each, and the total time be 3.0 seconds or less.

[0096] After the heat treatment, the resin-coated metal plate is cooled. As a cooling method, water cooling using temperature-regulated water or gas cooling using air, nitrogen, etc. is preferred. From the viewpoint of simplifying the equipment, water cooling is more preferred as a cooling method. As a water cooling method, there can be exemplified a method of immersing the resin-coated metal plate in a water tank filled with water and a method of spraying water onto the resin-coated metal plate from a nozzle, etc. The cooling stop temperature is preferably above 5°C, and is preferably below (glass transition temperature of the thermoplastic resin film - 10°C). If the cooling stop temperature is below (glass transition temperature of the thermoplastic resin film - 10°C), the fluidity of the amorphous structure inside the resin coating layer can be more appropriately suppressed, and the surface roughness caused by contact between the resin-coated metal plate and rollers after cooling can be more appropriately prevented. In addition, by setting the cooling stop temperature to above 5°C, condensation on the cooled resin-coated metal plate and peripheral equipment can be more appropriately prevented.

[0097] It should be noted that production conditions other than the above-mentioned conditions can be carried out according to conventional methods.

[0098] Example

[0099] As the metal plate, a 0.22 mm thick metal plate with a chromium coating weight of 120 mg / m 2 、Chromium oxide adhesion 10mg / m 2 (calculated as metallic chromium) and a chrome-plated steel sheet (TFS) with a temper degree of T3CA. Inventive Examples 1 to 19, Comparative Examples 1 to 3, and Comparative Examples 6 and 7, resin-coated metal sheets were produced by press-bonding a thermoplastic resin film to a metal sheet using a thermocompression film lamination method under the conditions described in Table 1, followed by heat treatment under the conditions described in Table 1. Furthermore, in Comparative Examples 4 and 5, resin-coated metal sheets were produced by press-bonding a thermoplastic resin film to a metal sheet using a thermocompression film lamination method under the conditions described in Table 1.

[0100] The inorganic additive content, crystallinity, and surface roughness of the resin-coated metal sheets were measured using the aforementioned methods. Furthermore, the melting point of the resin-coated layer was measured using the following method. The results are shown in Table 1. It should be noted that the proportion of the polyester resin constituting the resin-coated layer in each example was 100% by mass, calculated as a solids content of the total resin. Table 1 shows the composition of the polyester resin in each example.

[0101] [Melting point]

[0102] The resin-coated metal plate was immersed in a 1:1 solution of concentrated hydrochloric acid (12 mol / L): distilled water at room temperature to dissolve the metal plate and isolate the resin coating. The separated resin coating was then rinsed thoroughly with distilled water and vacuum-dried. The dried resin coating was measured using a TA Instruments differential scanning calorimeter (DSCQ100) at a heating rate of 10°C / min from 0°C to 300°C. The peak temperature of the endothermic peak measured between 200°C and 280°C was defined as the melting point of the resin coating.

[0103] [Table 1]

[0104]

[0105] The resin-coated metal sheets of Inventive Examples 1 to 19 and Comparative Examples 1 to 7 were evaluated for workability, surface roughness, appearance, and post-processing adhesion by the following methods.

[0106] [Processability]

[0107] After coating the resin-coated metal sheets of Inventive Examples 1-19 and Comparative Examples 1-7 with paraffin wax, circular blanks with a diameter of 180 mm were punched out. These circular blanks were processed using a deep drawing press, followed by two-stage redrawing and one-stage ironing, to form cans with an inner diameter of 52 mm and a height of 163 mm. The surface of the resin coating on the outer surface of the formed cans was visually inspected, and workability was evaluated according to the following criteria.

[0108] Evaluation "⊚": No chipping was observed.

[0109] Evaluation "○": Wear occurs at a height within 5 mm from the can flange portion. No practical problem.

[0110] Evaluation "X": Wear occurred at a height exceeding 5 mm from the can flange portion, which presents a practical problem.

[0111] [Rough surface]

[0112] After coating the resin-coated metal plates of Inventive Examples 1 to 19 and Comparative Examples 1 to 7 with paraffin wax, circular plate blanks with a diameter of 180 mm were punched out. The circular plate blanks were subjected to a deep drawing process using a deep drawing press, followed by a two-stage re-drawing process and a one-stage thinning process, thereby forming a tank with an inner diameter of 52 mm and a tank height of 163 mm. The formed tank was heated using a hot air furnace to a condition where the tank body temperature reached (melting point of the thermoplastic resin film + 5°C) within two minutes, and then rapidly cooled using cold air. The state of the resin coating on the outer surface of the tank body after cooling was visually confirmed, and the surface roughness was evaluated according to the following criteria.

[0113] Evaluation "⊚": No defects in appearance such as black spots and wrinkles were observed.

[0114] Evaluation "0": Appearance defects such as black spots and wrinkles occur within 5 mm of the can flange. No practical problems.

[0115] Evaluation "△": Defects in appearance such as black spots and wrinkles occur at a height exceeding 5 mm and within 20 mm from the can flange. This presents a practical problem.

[0116] Evaluation "X": Appearance defects such as black spots and wrinkles occur at a height exceeding 20 mm from the can flange, which presents a practical problem.

[0117] [Appearance]

[0118] The surface smoothness of both surfaces and the uniformity of the appearance in the width direction of the resin-coated metal plates of Inventive Examples 1 to 19 and Comparative Examples 1 to 7 were visually checked, and the appearance was evaluated according to the following criteria.

[0119] Appearance (surface smoothness)

[0120] Evaluation "○": No abnormality in appearance.

[0121] Evaluation "x": There is an abnormality in appearance such as a mottled pattern or a blurred appearance.

[0122] Appearance (width direction)

[0123] Evaluation "◯": Appearance is uniform in the width direction.

[0124] Evaluation "x": The appearance greatly differs in the width direction.

[0125] [Adhesion after processing]

[0126] After paraffin wax was applied to the resin-coated metal plates of Inventive Examples 1 to 19 and Comparative Examples 1 to 7, circular plate blanks with a diameter of 180 mm were punched out. The circular plate blanks were subjected to a deep drawing process using a deep drawing press, followed by a two-stage re-drawing process and a one-stage thinning process, thereby forming a tank with an inner diameter of 52 mm and a tank height of 163 mm. A sample (width 15 mm × length 120 mm) for a peeling test was cut out from the main body of the formed tank in such a way that the tank height direction became the length direction (test direction). A portion of the resin coating was peeled off from the end of the cut sample on the side of the tank opening, and the peeled resin coating was opened in the opposite direction (angle 180 degrees) to the metal plate from which the resin coating was peeled, and a peeling test was performed at a pulling speed of 30 mm / min. The adhesion was evaluated for each 15 mm width according to the following criteria. The surface for adhesion measurement is the inner surface side of the tank.

[0127] Rating "◎": 3.0N / 15mm or more

[0128] Rating "0": 2.0N / 15mm or more, less than 3.0N / 15mm

[0129] Evaluation "△": 1.0N / 15mm or more, less than 2.0N / 15mm

[0130] Evaluation "×": less than 1.0N / 15mm

[0131] [Table 2]

[0132]

[0133] As shown in Table 2, the resin-coated metal sheet of Inventive Example 1 had good workability, surface roughness, and appearance (◎ or ◯) for the resin coating layer that would become the outer surface of the container after molding. Furthermore, the resin-coated metal sheets of Inventive Examples 2 to 19 all had good workability, surface roughness, appearance, and post-processing adhesion. On the other hand, the evaluation results for any of Comparative Examples 1 to 7 were inadequate (△ or ×) for workability, surface roughness, appearance, and post-processing adhesion.

[0134] Industrial applicability

[0135] According to the present invention, it is possible to suppress appearance defects (surface roughness) that may occur in the resin coating layer of a resin-coated metal sheet during heat treatment after canning, and it is possible to suppress a decrease in the smoothness of the surface of the resin coating layer during coating. Thus, it is possible to provide a resin-coated metal sheet having a smooth and beautiful appearance.

[0136] Description of Reference Signs

[0137] 1 Resin-coated metal plate

[0138] 2 metal plates

[0139] 3 Resin covering

[0140] 4 Resin covering

Claims

1. A resin-coated metal plate comprising a resin coating layer containing 90% by mass or more of a polyester resin relative to the total resin on at least one surface of the metal plate, wherein: The crystallization amount of the resin covering layer is less than 15%, The maximum value Sa of the arithmetic mean height Sa of the surface of the resin cover layer MAX 0.30μm or less, The maximum value Sa of the arithmetic mean height Sa of the surface of the resin cover layer MAX With the minimum value Sa MIN The difference ΔSa is 0.15 μm or less.

2. The resin-coated metal sheet according to claim 1, wherein The resin covering layer is provided on both the front and back surfaces of the metal plate. The difference in melting points between the resin covering layers on the front and back surfaces 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 side contains 0.010% by mass or more and 1.0% by mass or less of wax.

4. A two-piece can comprising a metal plate coated with the resin according to any one of claims 1 to 3.

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

6. A method for producing a resin-coated metal sheet, wherein: A thermoplastic resin film containing a thermoplastic resin is pressed against at least one surface of a metal plate heated to a temperature not lower than (the melting point of the thermoplastic resin film - 40°C) and not higher than (the melting point of the thermoplastic resin film + 5°C) and then cooled to obtain a pre-heat-treated resin-coated metal plate. The resin-coated metal plate before heat treatment is heated to a heat treatment temperature of not less than (the melting point of the thermoplastic resin film + 5° C.) and not more than (the melting point of the thermoplastic resin film + 30° C.) over a period of not less than 0.5 seconds and not more than 1.5 seconds, maintained at the heat treatment temperature for not less than 0.5 seconds and not more than 1.5 seconds, and then cooled to obtain a resin-coated metal plate.

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