Metal plate for container, method for manufacturing metal plate for container, and method for manufacturing can

Abstract

Provided, inter alia, is a metal plate for a container having a resin coating layer that satisfies the physical properties required of a resin coating layer when being processed as a metal container, and has a satisfactory appearance even when subjected to a molding process and a heat treatment. The metal plate for a container has a resin coating layer provided on at least one surface of the metal plate. The resin coating layer includes a polyester resin and is obtained by laminating a plurality of resin layers. The resin coating layer has a complex viscosity at 280°C of 31 to 100 Pa·s. The plurality of resin layers have a pigment layer containing a white pigment and a lubricating layer containing 0.01 to 1.35 mass% of a lubricating component. The lubricating layer is disposed on the outermost side of the plurality of resin layers. The ratio of the film thickness of pigment layer to the film thickness of the lubricating layer disposed on the outermost side of the plurality of resin layers is 1.0 / 1.0 to 16.0 / 1.0. The lubricating component of the lubricating layer includes a wax containing a polar group.

Description

Metal plate for container, manufacturing method of metal plate for container, and manufacturing method of can

[0001] The present invention relates to a metal sheet for containers having a resin coating layer provided on at least one surface of the metal sheet, a method for manufacturing the metal sheet for containers, and a method for manufacturing cans.

[0002] Metal containers are broadly classified into two-piece cans and three-piece cans. Two-piece cans consist of two components: a cylindrical can body with a bottom and a lid that closes an opening formed at one axial end of the can body. Three-piece cans consist of three components: a cylindrical can body, a top lid that closes an opening formed at one end of the can body, and a bottom lid that closes an opening formed at the other end of the can body. Two-piece cans have an excellent appearance because they do not have welds on the can body.

[0003] Metal containers are made of metal plates such as tin-free steel (TFS) and aluminum, which are coated with paint to improve corrosion resistance and weather resistance.

[0004] The technology of applying a coating to a metal sheet requires a long processing time due to the complicated coating and baking process, and further has the problem of discharging a large amount of solvent. To solve these problems, metal sheets for containers have been developed that have a thermoplastic film on the surface of the metal sheet. The thermoplastic film is also called a resin coating layer. Such metal sheets for containers are widely used industrially, mainly as materials for beverage cans.

[0005] In recent years, metal plates used in metal containers have been thinned in order to reduce material costs and conserve resources. When can bodies are manufactured using thinned metal plates, the degree of processing is high, which may result in breakage or chipping (hereinafter also referred to as breakage, etc.) of the resin coating layer. In particular, two-piece cans tend to be highly processed, which makes them more susceptible to breakage, etc. of the resin coating layer. Therefore, in order to manufacture two-piece cans, a material design that prevents breakage, etc. of the resin coating layer is required.

[0006] Various printing techniques are applied to the resin coating layer to enhance its design. However, if the affinity between the printing ink and the resin coating layer is low, the ink will not adhere well. This can lead to the ink peeling off the resin coating layer, damaging the appearance. Therefore, the resin coating layer needs to be designed with a material that has a high affinity with the printing ink.

[0007] Here, methods such as drawing and DI processing are used to manufacture two-piece can bodies from metal container sheets (see Patent Documents 1, 2, and 3).Furthermore, by adding a white pigment to the resin coating layer after forming the metal container sheet, the design of the can body can be improved by printing or the like (see Patent Documents 4 and 5).

[0008] In the production of two-piece cans, attempts have been made to prevent breakage of the resin coating layer and to improve adhesion between the resin coating layer and printing ink, for example, by adding an acid-modified polyolefin or an oxidized polyolefin to the resin coating layer as a lubricating component (see Patent Documents 6 and 7).

[0009] However, cases have been reported in which metal container plates provided with a resin coating layer have a poor appearance during processing. For example, residual stress is generated in the resin coating layer when the metal container plate is formed. This residual stress is relieved by subsequent heat treatment, causing the resin coating layer to deform unevenly. The uneven deformation of the resin coating layer causes uneven distribution of the pigment. As a result, cases have been reported in which spot-like defects have occurred in the resin coating layer. To prevent the occurrence of such defects, the resin coating layer is formed from a resin material whose difference between the heat of crystallization and the heat of fusion is within a predetermined range (see Patent Document 8).

[0010] The resin coating layer contains a mixture of rigid crystalline and rigid amorphous phases and a flexible mobile amorphous phase. For example, when a metal container sheet undergoes intensive processing, such as forming a two-piece can, the mixture of these two types of layers can cause uneven distortion in the resin coating layer. When the metal container sheet is subjected to heat treatment in this state, the softened resin coating layer deforms unevenly. As a result, wrinkle-like defects have been reported in some cases. To prevent such defects, the resin coating layer contains a predetermined amount of ethylene terephthalate units, a predetermined amount of mobile amorphous matter, and a predetermined range of lubricating components (see Patent Document 9). The defects in the resin coating layer described in Patent Documents 8 and 9 are caused by can processing and heat treatment.

[0011] Japanese Patent Application Laid-Open No. 2-303634 Japanese Patent Application Laid-Open No. 4-91825 Japanese Patent Application Laid-Open No. 2004-148324 Japanese Patent Application Laid-Open No. 8-169098 Japanese Patent Application Laid-Open No. 2004-130536 International Publication No. 2019 / 116706 International Publication No. 2019 / 116707 International Publication No. 2013 / 030972 International Publication No. 2021 / 182256

[0012] As described above, by adding a polyolefin containing a polar group as a lubricating component to the resin coating layer, it is possible to prevent the resin coating layer from breaking when molding a two-piece can, which requires a high degree of processing, and to improve the adhesion between the resin coating layer and printing ink.

[0013] Furthermore, when molding a two-piece can, which requires a high degree of processing, residual stress occurs in the resin coating layer. The residual stress in the resin coating layer can be alleviated by subjecting the metal sheet for containers to a heat treatment near the melting point of the resin coating layer. This heat treatment can prevent the resin coating layer from peeling off from the metal sheet and the ink from peeling off from the resin coating layer, which are caused by the residual stress.

[0014] However, when a resin coating layer containing a polyolefin containing a polar group as a lubricating component is subjected to heat treatment at a temperature close to the melting point of the resin coating layer, many fine pits are formed on the surface of the resin coating layer. These fine pits occur even without processing. Even when the resin coating layers described in Patent Documents 8 and 9 are used, the problem of fine pits being formed on the resin coating layer remains.

[0015] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a metal plate for containers, etc., which has a resin coating layer that satisfies the physical properties required of a resin coating layer when processed into a metal container, and which has a good appearance even when subjected to molding processing and heat treatment.

[0016] In order to solve the above problems, the present invention has the following features: [1] A metal sheet for containers having a resin coating layer provided on at least one surface of the metal sheet, wherein the resin coating layer contains a polyester resin and is formed by laminating multiple resin layers, the resin coating layer having a complex viscosity at 280°C of 31 Pa·s or more and 100 Pa·s or less, the multiple resin layers having a pigment layer containing a white pigment and a lubricating layer containing 0.01 mass % or more and 1.35 mass % or less of a lubricating component, the lubricating layer being disposed on the outermost side of the multiple resin layers, the ratio of the thickness of the pigment layer to the thickness of the outermost lubricating layer of the multiple resin layers being 1.0 / 1.0 or more and 16.0 / 1.0 or less, and the lubricating component of the lubricating layer including a wax containing a polar group. [2] The metal sheet for containers according to [1], wherein the white pigment is titanium oxide, and the pigment layer contains 5 mass % or more and 30 mass % or less of the titanium oxide. [3] The metal sheet for containers according to [1] or [2], wherein the lubricating layer is provided on a surface of the metal sheet in the stacking direction of the plurality of resin layers, the pigment layer is provided on the surface of the lubricating layer, and the lubricating layer is provided on the surface of the pigment layer. [4] The metal sheet for containers according to any of [1] to [3], wherein the polyester resin is a copolymerized polyethylene terephthalate resin containing isophthalic acid as an acid component. [5] The method for manufacturing a metal sheet for containers according to any of [1] to [4], comprising: a hot-melt extrusion step of hot-melt extruding components of the lubricating layer and components of the pigment layer in different extruders; an extrusion molding step of extruding the components of the lubricating layer and the components of the pigment layer through a T-die after laminating them in a feed block; and a lamination step of thermocompression laminating the metal sheet and the resin coating layer together. [6] A method for manufacturing a can, comprising a can forming step of forming the metal sheet for containers according to any one of [1] to [4] into a can, and a heat treatment step of applying heat treatment to the metal sheet for containers at least once at a temperature equal to or higher than the glass transition point of the resin coating layer and equal to or lower than the melting point of the resin coating layer, wherein the heat treatment step is carried out before the can forming step.[7] The method for manufacturing a can according to [6], wherein the heat treatment includes ink baking.

[0017] According to the metal plate for containers of the present invention, the resin coating layer contains a polyester resin and is composed of multiple laminated resin layers. Its complex viscosity at 280°C is 31 Pa·s or more and 100 Pa·s or less. The ratio of the thickness of the pigment layer to the thickness of the outermost lubricating layer is 1.0 / 1.0 or more and 16.0 / 1.0 or less. Furthermore, the multiple resin layers include a pigment layer containing a white pigment and a lubricating layer containing 0.01% by mass to 1.35% by mass of a lubricating component. Therefore, the metal plate for containers of the present invention satisfies the physical properties required of a resin coating layer when processed into a metal container, and maintains a good appearance even when subjected to molding and heat treatment.

[0018] It is a cross-sectional view of the metal plate for containers which is one embodiment of the present invention. It is a flow diagram showing a method for manufacturing the metal plate for containers and a method for manufacturing a can. It is a cross-sectional view of the metal plate for containers according to a modified example.

[0019] The inventors of the present invention conducted extensive research to solve the above-mentioned problems, and as a result, they discovered the following: In order to prevent breakage of the resin coating layer during high-process molding, it is necessary to add a lubricating component to the resin coating layer to reduce friction on the surface of the resin coating layer.

[0020] Furthermore, in order to avoid impeding adhesion between the printing ink and the resin coating layer, ink adhesion can be ensured by selecting a wax containing a polar group as a lubricating component. However, when a metal plate for a container provided with a resin coating layer containing a wax containing a polar group is subjected to heat treatment at a temperature close to the melting point of the resin coating layer, many fine dents are formed in the resin coating layer. It is important to suppress the occurrence of these dents.

[0021] Hereinafter, a metal plate for a container according to one embodiment of the present invention will be described. FIG. 1 shows a cross section of a metal plate for a container according to one embodiment of the present invention. As shown in FIG. 1, a metal plate for a container 100 according to one embodiment of the present invention has a metal plate 10, a resin coating layer 20 provided on a front surface 11 of the metal plate 10, and a resin coating layer 30 provided on a back surface 12 of the metal plate 10. The resin coating layer 20 is located on the outside of the metal container when the metal plate for a container 100 is formed. The resin coating layer 30 is located on the inside of the metal container when the metal plate for a container 100 is formed.

[0022] The metal container is not particularly limited as long as it is obtained by forming the metal plate for containers 100, and examples thereof include food cans, beverage cans, and 18L cans. In particular, the metal container is preferably a two-piece can such as an aerosol can, which is highly processed and subjected to printing. In this embodiment, an example in which a two-piece can is used as the metal container will be described.

[0023] The metal plate 10 is not particularly limited, but a steel plate such as tinplate or tin-free steel can be used. For tinplate, a plating amount of 0.5 g / m ２ Above, 15g / m ２ It is preferable to use the following:

[0024] The tin-free steel preferably has a metal chromium layer and a chromium oxide layer formed on the surface of the steel sheet. The metal chromium layer has a coating weight of 50 mg / m2 relative to the steel sheet. ２ More than 200g / m ２ The chromium oxide layer preferably has a coating weight of 3 mg / m2 or less relative to the steel sheet in terms of metallic chromium layer. ２ Above, 30g / m ２ It is preferable that:

[0025] The type of steel sheet is not particularly limited as long as it can be formed into the desired can shape. The steel sheet preferably has the following composition and is produced by the following production method.

[0026] (1) A steel sheet that has been recrystallized annealed by continuous annealing using low-carbon steel with a C (carbon) content of approximately 0.010% by mass or more and 0.10% by mass or less. (2) A steel sheet that has been recrystallized annealed by continuous annealing and overaging treated using low-carbon steel with a C content of approximately 0.010% by mass or more and 0.10% by mass or less. (3) A steel sheet that has been recrystallized annealed by box annealing using low-carbon steel with a C content of approximately 0.010% by mass or more and 0.10% by mass or less. (4) A steel sheet that has been recrystallized annealed by continuous annealing or box annealing using low-carbon steel with a C content of approximately 0.010% by mass or more and 0.10% by mass or less, and then subjected to secondary cold rolling (DR: Double Reduced) rolling. (5) An IF (Interstitial Free) steel made by adding elements such as Nb and Ti, which have a higher reactivity with C than iron, to an ultra-low carbon steel with a C content of approximately 0.003 mass% or less, and then recrystallizing and annealing the steel by continuous annealing.

[0027] The mechanical properties of the steel sheet are not particularly limited as long as they can be formed into a desired shape. The mechanical properties of the steel sheet preferably include a yield point (YP) in the range of about 220 MPa to 580 MPa. Having such a yield point (YP) of the steel sheet allows the workability to be maintained without impairing the strength of the can body.

[0028] The Lankford value (r value), which is an index of plastic anisotropy, is preferably 0.8 or more, and the absolute value of the in-plane anisotropy Δr of the Lankford value (r value) is preferably 0.7 or less.

[0029] The steel sheet may contain any of the following components to achieve the above-described properties: Si, Mn, P, S, Al, and N. The Si content of the steel sheet is preferably 0.001% by mass or more and 0.1% by mass or less. The Mn content of the steel sheet is preferably 0.01% by mass or more and 0.6% by mass or less. The P content of the steel sheet is preferably 0.002% by mass or more and 0.05% by mass or less. The S content of the steel sheet is preferably 0.002% by mass or more and 0.05% by mass or less. The Al content of the steel sheet is preferably 0.005% by mass or more and 0.100% by mass or less. The N content of the steel sheet is preferably 0.0005% by mass or more and 0.020% by mass or less. The steel sheet may also contain other components such as Ti, Nb, B, Cu, Ni, Cr, Mo, and V. From the viewpoint of ensuring corrosion resistance and the like, the steel sheet preferably contains these components in a total amount of 0.02 mass % or less.

[0030] The resin coating layer 20 is not particularly limited, but may be formed in the form of a film, for example. The resin coating layer 20 is formed by laminating a first resin layer 21, a second resin layer 22, and a third resin layer 23 in this order from the metal plate 10 side. That is, the first resin layer 21 is provided on the surface 11 of the metal plate 10. The second resin layer 22 is provided on the surface of the first resin layer 21. The third resin layer 23 is provided on the surface of the second resin layer 22. In this embodiment, the third resin layer 23 is the outermost layer of the multiple resin layers. Hereinafter, the third resin layer 23 will also be referred to as the outermost layer.

[0031] Each of the first resin layer 21, the second resin layer 22, and the third resin layer 23 contains a polyester resin. In other words, the resin coating layer 20 contains a polyester resin. Among the components constituting the resin coating layer 20 or the first resin layer 21, the second resin layer 22, and the third resin layer 23, polyester resin is contained in the largest amount. The melting point of the polyester resin is preferably 234°C or higher and 254°C or lower, and more preferably 244°C or higher and 254°C or lower. A melting point of 234°C or higher of the polyester resin is preferable from the viewpoint of molding processability, since softening due to heat treatment can be suppressed. Furthermore, a melting point of 254°C or lower of the polyester resin is preferable from the viewpoint of molding processability, since it has moderate crystallinity. The melting point of the resin coating layer 20 can be, for example, the melting point of the polyester resin.

[0032] Various dicarboxylic acid components and glycol components can be used as raw materials for the polyester resin, and the polyester resin may be obtained by copolymerizing a plurality of dicarboxylic acid components and glycol components within a range that does not impair heat resistance or processability.

[0033] Examples of the dicarboxylic acid component of the polyester resin include 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.

[0034] Examples of the glycol component of the polyester resin include 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.

[0035] From the viewpoint of economic efficiency, the polyester resin preferably contains terephthalic acid and ethylene glycol as its main components. Furthermore, the polyester resin is more preferably polyethylene terephthalate copolymerized with isophthalic acid. By using polyethylene terephthalate copolymerized with isophthalic acid as the polyester resin, flexibility can be imparted to the resin coating layer 20, and breakage of the resin coating layer 20 can be suppressed during high-process can-making processes.

[0036] The resins of the first resin layer 21, the second resin layer 22, and the third resin layer 23 may contain additives such as fluorescent whitening agents, antioxidants, heat stabilizers, ultraviolet absorbers, antistatic agents, etc. Adding a fluorescent whitening agent to any of the first resin layer 21, the second resin layer 22, and the third resin layer 23 can improve the whiteness of the resin coating layer 20.

[0037] The resin coating layer 20 has a pigment layer containing a white pigment. The position of the pigment layer is not particularly limited, but in this embodiment, the second resin layer 22 serves as the pigment layer.

[0038] The pigment layer preferably contains 5% by mass or more and 30% by mass or less of a white pigment, and more preferably 10% by mass or more and 25% by mass or less. By containing 5% by mass or more of a white pigment, the pigment layer can be imparted with sufficient hiding power and can impart a desirable whiteness after processing. Furthermore, by containing 30% by mass or less of a white pigment, the pigment layer can be imparted with appropriate stretchability and can improve processability.

[0039] Titanium oxide or the like can be used as the white pigment. The titanium oxide is not particularly limited, but for example, rutile-type titanium oxide with a purity of 90% by mass or more can be used. Using rutile-type titanium oxide with a purity of 90% by mass or more can improve the dispersibility of titanium oxide when mixed with a resin material, thereby making it possible to achieve uniform quality for the resin coating layer 20.

[0040] The resin coating layer 20 has a lubricating layer containing a lubricating component. The position of the lubricating layer is not particularly limited, but in this embodiment, the first resin layer 21 and the third resin layer serve as the lubricating layer.

[0041] The lubricating component may be a wax containing a polar group. By using a wax containing a polar group as the lubricating component, it is possible to ensure the affinity between the polyester resin and the lubricating component and the processability of the metal plate for containers 100.

[0042] Examples of waxes containing a polar group include oxidized polyolefins such as polyethylene oxide, polyolefins such as acid-modified polyolefins such as ethylene acrylic acid copolymers and ethylene maleic anhydride copolymers, and ester waxes such as montanic acid, its ethylene glycol esters, and glycerol esters.

[0043] The affinity between the polyester resin and the ink (hereinafter simply referred to as ink) printed on the resin coating layer 20 changes depending on the acid value of the lubricating component. Specifically, the affinity between the polyester resin and the ink increases as the acid value of the lubricating component increases. As the affinity between the polyester resin and the ink increases, the dispersion size of the lubricating component on the surface of the resin coating layer 20 tends to decrease. In order to obtain a high affinity between the polyester resin and the ink, it is advisable to control the acid value of the lubricating component.

[0044] The acid value of the lubricating component is preferably 1.0 mgKOH / g or more and 100 mgKOH / g or less, more preferably 5.0 mgKOH / g or more and 80 mgKOH / g or less, and more preferably 10.0 mgKOH / g or more and 80 mgKOH / g or less. By making the acid value of the lubricating component 1.0 mgKOH / g or more, good affinity with the resin coating layer 20 and ink can be obtained. As a result, adhesion of the ink to the resin coating layer 20 can be improved.

[0045] Furthermore, by setting the acid value of the lubricating component to 100 mgKOH / g or less, it is possible to prevent excessive dispersion of the lubricating component in the resin coating layer 20. As a result, it is possible to ensure a friction coefficient required when the metal plate for containers 100 is subjected to can manufacturing, for example.

[0046] The lubricating layer preferably contains 0.01% by mass or more and 1.35% by mass or less of the lubricating component, preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.07% by mass or more. By containing 0.01% by mass or more of the lubricating component, the lubricating layer can ensure sufficient sliding properties during can manufacturing and can prevent scratches caused by scraping of the resin coating layer 20.

[0047] The lubricating layer preferably contains 0.50 mass % or less, more preferably 0.25 mass % or less, of the lubricating component. When the lubricating layer contains 1.35 mass % or less of the lubricating component, the formation of fine irregularities in the resin coating layer 20 can be suppressed.

[0048] Thus, the lubricating layer contains a lubricating component, which ensures the surface's sliding properties during severe processing, such as the manufacture of two-piece cans. It is believed that an excessive content of the lubricating component in the lubricating layer promotes softening of the resin coating layer 20 when the temperature of the resin coating layer 20 approaches its melting point. It is also believed that an excessive content of the lubricating component in the lubricating layer inhibits thermal crystallization of the resin coating layer 20.

[0049] By using the first resin layer 21 and the third resin layer 23 as lubricating layers and the second resin layer 22 as a pigment layer, the resin coating layer 20 can be easily formed using, for example, extrusion molding, thereby improving the productivity of the resin coating layer 20. Furthermore, since the first resin layer disposed on the surface of the metal plate 10 is a lubricating layer, the adhesion between the resin coating layer 20 and the metal plate 10 can be improved. Furthermore, since the lubricating layer contains a lubricating component, good moldability into metal containers such as cans can be obtained.

[0050] The ratio of the film thickness of the second resin layer 22 to the film thickness of the third resin layer 23 ([film thickness of pigment layer] / [film thickness of outermost lubricating layer]) is preferably 1.0 / 1.0 or more and 16.0 / 1.0 or less, more preferably 2.0 / 1.0 or more and 14.0 / 1.0 or less, and even more preferably 3.0 / 1.0 or more and 8.0 / 1.0 or less.

[0051] By setting the film thickness ratio to 1.0 / 1.0 or more, it is possible to prevent the loss of the color of the white pigment in the pigment layer. Furthermore, by setting the film thickness ratio to 16.0 / 1.0 or less, it is possible to prevent the formation of fine irregularities. In other words, the pigment layer contains a pigment with high surface energy. As the distance from the pigment layer to the air interface increases, the force required to indent the outermost third resin layer 23 also increases. As a result, it is believed that the number of indentations in the third resin layer 23 decreases.

[0052] The above film thickness ratio ([film thickness of pigment layer] / [film thickness of outermost lubricating layer]) can be adjusted by changing the extrusion rate (resin weight per unit time), which is the amount of resin extruded by the extruder for the lubricating layer and the extruder for the pigment layer. For example, when the total extrusion rate is constant, an increase in the extrusion rate of the extruder for the pigment layer increases the film thickness ratio ([film thickness of pigment layer] / [film thickness of outermost lubricating layer]).

[0053] The thickness of the resin coating layer 20, i.e., the total thickness of the first resin layer 21, the second resin layer 22, and the third resin layer 23, is preferably 10 μm or more and less than 30 μm, and more preferably 15 μm or more and 25 μm or less. By setting the thickness of the resin coating layer 20 in this manner, the functions of the lubricating layer and the pigment layer can be properly exhibited, and manufacturing costs can be kept reasonable.

[0054] By having the resin coating layer 20 have the multiple resin layers 21, 22, and 23, the metal plate for containers 100 can be obtained at low cost and with good processability as described above. If the resin coating layer 20 is composed of only a pigment layer, the resin coating layer 20 cannot be given appropriate processability, and there is a risk of breakage or the like.

[0055] The resin coating layer 30 is not particularly limited, and may be formed, for example, in the form of a film. The resin coating layer 30 is formed by laminating a first resin layer 31, a second resin layer 32, and a third resin layer 33 in this order from the metal plate 10 side. That is, the first resin layer 31 is provided on the rear surface 12 of the metal plate 10. The second resin layer 32 is provided on the surface of the first resin layer 31. The third resin layer 33 is provided on the surface of the second resin layer 32. The resin coating layer 30 can be formed in the same manner as the resin coating layer 20, and therefore a description thereof will be omitted.

[0056] The resin coating layer 20 and the resin coating layer 30 have a complex viscosity at 280° C. of 31 Pa·s or more and 100 Pa·s or less. The complex viscosity at 280° C. of the resin coating layer 20 and the resin coating layer 30 is preferably 35 Pa·s or more and 95 Pa·s or less, and more preferably 40 Pa·s or more and 90 Pa·s or less.

[0057] The complex viscosity at 280° C. can be determined, for example, by measuring the melt viscoelasticity of a freeze-pulverized resin coating layer using a melt viscoelasticity measuring device.

[0058] Furthermore, the occurrence of minute dents on the surface can be suppressed by making the complex viscosity of the resin coating layer 20 and the resin coating layer 30 at 280°C 31 Pa·s or more. It is believed that the resin flow, which is the initial stage that causes minute dents on the surface, is suppressed by making the complex viscosity of the resin coating layer 20 and the resin coating layer 30 at 280°C 31 Pa·s or more.

[0059] The complex viscosity at 280° C. is 100 Pa·s or less. If the complex viscosity at 280° C. is more than 100 Pa·s, the processability of the resin coating layer deteriorates, resulting in poor can-forming processability.

[0060] The complex viscosity at 280°C can be adjusted by at least one of the above-mentioned film thickness ratio ([film thickness of pigment layer] / [film thickness of outermost lubricating layer]) and the lubricating component content of the lubricating layer. The complex viscosity tends to increase as the above-mentioned film thickness ratio ([film thickness of pigment layer] / [film thickness of outermost lubricating layer]) increases. In other words, the complex viscosity tends to decrease as the above-mentioned film thickness ratio ([film thickness of pigment layer] / [film thickness of outermost lubricating layer]) decreases. Furthermore, the complex viscosity tends to decrease when the lubricating component content of the lubricating layer is high. In other words, the complex viscosity tends to increase when the lubricating component content of the lubricating layer is low.

[0061] A method for manufacturing the metal plate for containers 100 described above and a method for manufacturing a can using the metal plate for containers 100 will now be described. Fig. 2 shows a manufacturing flow of the metal plate for containers 100 and a manufacturing flow of a can using the metal plate for containers 100. As shown in Fig. 2, first, a hot melt extrusion step (step S01) is performed in which components of the lubricating layer and components of the pigment layer are hot melt extruded using different extruders.

[0062] Next, an extrusion molding step (step S02) is carried out in which the components of the lubricating layer and the components of the pigment layer are laminated in a feed block and then extruded from a T-die.

[0063] Thereafter, a stretching step (step S03) is preferably carried out in which the extrusion-molded resin composition is uniaxially stretched at a stretching ratio of 3 times or more and 6 times or less.

[0064] The stretching step of step S03 is performed optionally. The stretching ratio of the uniaxial stretching performed in the stretching step of step S03 is preferably 3 times or more, and preferably 6 times or less. Furthermore, the stretching ratio is more preferably 4 times or more and 5 times or less. By setting the stretching ratio of the uniaxial stretching to 3 times or more, thickness unevenness of the resin coating layer can be reduced. Furthermore, by setting the stretching ratio of the uniaxial stretching to 6 times or less, the physical properties required for the resin coating layer can be satisfied, and therefore a good appearance can be obtained without the occurrence of breakage, etc.

[0065] In other words, since the resin coating layer is obtained by uniaxial stretching and the lubricating layer contains a lubricating component at the specified content described above, even if the metal plate for containers 100 is subjected to heat treatment, the occurrence of fine dents in the resin coating layer 20 can be suppressed.

[0066] A laminating step (step S04) is performed in which the metal plate and the resin coating film formed in the stretching step of step S03 are thermocompression laminated together. The laminating step of step S04 is performed after the metal plate 10 is heated to a temperature equal to or higher than the melting point of the resin coating. By performing the extrusion step of step S01 through the laminating step of step S04, a metal plate for a container 100 is manufactured.

[0067] Next, the metal plate for the container manufactured by the laminating step of step S04 is subjected to a heat treatment step in which heat treatment is performed at least once at a temperature above the glass transition point of the resin coating layer and below the melting point of the resin coating layer (step S05).

[0068] Examples of the heat treatment performed in the heat treatment step of step S05 include baking ink onto the resin coating layer 20 and baking a top coat onto the resin coating layer 20. By performing the heat treatment step in these ways, the can manufacturing process can be performed without any additional steps.

[0069] Furthermore, after the heat treatment step of step S05, even if a heat treatment is subsequently performed at a temperature exceeding the melting point of the resin coating layer, the formation of fine dents in the resin coating layer can be significantly suppressed. This phenomenon is thought to occur because when the heat treatment temperature is above the glass transition point and below the melting point of the polyester resin, thermal crystallization of the resin coating layer is promoted, thereby suppressing resin flow that would result in fine dents when the resin coating layer is subsequently heated above the melting point.

[0070] The heat treatment temperature in step S05 is preferably at least the melting point of the resin coating layer minus 100°C and less than the melting point of the resin coating layer. The heat treatment temperature is preferably at least the melting point of the resin coating layer minus 50°C and less than the melting point of the resin coating layer, and more preferably at least the melting point of the resin coating layer minus 25°C and less than the melting point of the resin coating layer. By setting the heat treatment temperature to at least the melting point of the resin coating layer 20 minus 100°C and less than the melting point of the resin coating layer, it is possible to prevent fine depressions from being formed on the surface of the resin coating layer.

[0071] If the heat treatment temperature exceeds the melting point of the resin coating layer, the formation of fine recesses in the resin coating layer tends to be promoted. Heat treatment at a temperature exceeding the melting point of the resin coating layer causes softening due to crystalline melting of the polyester resin, which is thought to promote the formation of fine recesses in the resin coating layer. The heat treatment step of step S05 may be performed multiple times.

[0072] A can forming step is performed in which the container metal plate is formed into a can (step S06). In other words, the heat treatment step of step S05 is performed before the can forming step of step S06.

[0073] Thereafter, a heat treatment step is performed in which heat treatment is performed at least once at a temperature equal to or higher than the glass transition point of the resin coating layer and equal to or lower than the melting point of the resin coating layer (step S07). The heat treatment step of step S07 can be performed in the same manner as the heat treatment step of step S05. Note that the number of times each step from the heat treatment step of step S05 to the heat treatment step of step S07 is performed can be changed appropriately depending on the type of can, etc. For example, after the heat treatment step of step S07 is performed, the can forming step of step S06 may be performed again.

[0074] The metal plate for containers 100 has a resin coating layer with a lubricating layer containing a polyolefin containing polar groups as a lubricating component. The lubricating component of the lubricating layer generates numerous fine pits on the surface of the resin coating layer when the temperature of the resin coating layer reaches near its melting point. These fine pits are more pronounced when the resin coating layer is subjected to heat treatment above its melting point. Furthermore, the number of these fine pits increases as the content of the lubricating component in the lubricating layer increases.

[0075] The complex viscosity of the resin coating layer at 280°C is within the above-mentioned predetermined range, and the ratio of the thickness of the pigment layer to the thickness of the outermost lubricating layer is within the above-mentioned predetermined range. Furthermore, the content of the lubricating component in the lubricating layer is within a certain range as described above. This makes it possible to suppress the formation of fine dents on the surface of the resin coating layer. Specifically, when the metal plate for containers 100 was heat-treated below the melting point of the resin coating layer, the number of fine dents formed on the surface of the resin coating layer was less than 20 per square centimeter. In other words, it was confirmed that as long as the content of the lubricating component in the lubricating layer is within a certain range, heat treatment below the melting point of the resin coating layer can be performed without affecting the appearance.

[0076] In this embodiment, an example has been described in which the resin coating layer 20 is provided on the front surface 11 of the metal plate 10 and the resin coating layer 30 is provided on the back surface 12. However, it is sufficient that the metal plate 100 for a container has the resin coating layer 20 provided on at least one of the front surface 11 and the back surface 12 of the metal plate 10. When the metal plate 100 for a container is formed in this manner, it is preferable to form the metal container so that the surface of the metal plate 10 on which the resin coating layer 20 is provided is located outside the metal container.

[0077] (Modification) In the above embodiment, an example has been described in which the resin coating layer 20 is composed of a first resin layer, a second resin layer, and a third resin layer. Also, in the resin coating layer 20, the first resin layer and the third resin layer are lubricating layers, and the second resin layer is a pigment layer.

[0078] The lubricating layer may be disposed on the outermost layer in the stacking direction of the resin layers of the resin coating layer. More preferably, the lubricating layer may be disposed only on the outermost layer. By disposing the lubricating layer in this manner, manufacturing costs can be reduced.

[0079] Fig. 3 shows a cross section of a metal plate for a container 200 according to a modified example. As shown in Fig. 3, the metal plate for a container 200 has a resin coating layer 40 provided on the front surface 11 of the metal plate 10 and a resin coating layer 50 provided on the back surface 12 of the metal plate 10. The resin coating layer 40 is located on the outside of the metal container when the metal plate for a container 200 is formed. The resin coating layer 50 is located on the inside of the metal container when the metal plate for a container 100 is formed.

[0080] The resin coating layer 40 is formed by laminating a first resin layer 41 and a second resin layer 42 in this order from the metal plate 10 side. That is, the first resin layer 41 is provided on the surface 11 of the metal plate 10. The second resin layer 42 is provided on the surface of the first resin layer 41.

[0081] In this embodiment, a pigment layer is disposed on the first resin layer 41. A lubricating layer is disposed on the second resin layer 42. When the resin coating layer 40 is thus provided, the ratio of the thickness of the first resin layer 41 to the thickness of the second resin layer 42 ([thickness of pigment layer] / [thickness of outermost lubricating layer]) is preferably 1.0 / 1.0 or more and 16.0 / 1.0 or less, more preferably 2.0 / 1.0 or more and 14.0 / 1.0 or less, and more preferably 3.0 / 1.0 or more and 8.0 / 1.0 or less. By ensuring that the thickness ratio is 1.0 / 1.0 or more, it is possible to prevent the loss of the color due to the white pigment of the pigment layer. Furthermore, by ensuring that the thickness ratio is 16.0 / 1.0 or less, it is possible to prevent the formation of fine irregularities. In other words, the pigment layer contains a pigment with high surface energy. As the distance from the pigment layer to the air interface increases, a larger force is required to dent the outermost second resin layer 42. As a result, it is believed that the number of dents in the second resin layer 42 decreases.

[0082] The thickness of the resin coating layer 40, i.e., the total thickness of the first resin layer 41 and the second resin layer 42, is preferably 10 μm or more and less than 30 μm, and more preferably 15 μm or more and 25 μm or less. By setting the thickness of the resin coating layer 20 in this manner, the functions of the lubricating layer and the pigment layer can be properly exhibited, and manufacturing costs can be kept reasonable.

[0083] The resin coating layer 50 is formed by laminating a first resin layer 51 and a second resin layer 52 in this order from the metal plate 10 side. That is, the first resin layer 51 is provided on the rear surface 12 of the metal plate 10. The second resin layer 52 is provided on the surface of the first resin layer 51. The resin coating layer 50 can be formed in the same manner as the resin coating layer 40, and therefore a description thereof will be omitted. Even when the resin coating layer 40 is configured in this manner, the same effects as those of the above-described embodiment can be obtained.

[0084] The film thickness of the resin coating layer and the film thickness of each layer of the resin coating layer were measured. The complex viscosity of the resin coating layer at 280°C was also measured. The melting point of the resin coating layer was also measured. The number of dents formed on the surface of the resin coating layer after heat treatment of the metal plate for containers and the static friction coefficient were also measured.

[0085] (Preparation of Metal Plate for Container) A metal plate was prepared using TFS (Tin Free Steel) with a thickness of 0.22 mm, metal Cr layer: 120 mg / m ２ Cr oxide layer: 10 mg / m in terms of metal Cr ２ , temper: T3CA) was used.

[0086] The resin coating layer was made of a polyester resin, which was a main component of the resin coating layer, and was constructed by laminating a first resin layer, a second resin layer, and a third resin layer in this order from the metal plate side. In this example, the first resin layer and the third resin layer were used as lubricating layers, and the second resin layer was used as a pigment layer.

[0087] The resin coating layers of Examples 1 to 14 and Comparative Examples 1 to 3 were prepared by varying the intrinsic viscosity of the polyester resin, the type of lubricating component in the lubricating layer, the content of the lubricating component, etc. The resin coating layers of Examples 1 to 14 and Comparative Examples 1 to 3 were prepared. Each resin coating layer was prepared by adjusting the extrusion rates of the extruder for the lubricating layer and the extruder for the pigment layer so as to achieve the film thickness ratio shown in Table 1. Each layer was laminated and extruded onto a casting drum, and then stretched 4.0 times in the longitudinal direction at 95°C by uniaxial stretching to prepare the resin coating layers of Examples 1 to 14 and Comparative Examples 1 to 3.

[0088] Acid-modified polyethylene (hereinafter, polyethylene will also be referred to as PE) was used as the lubricating component of the lubricating layer in Examples 1 to 10 and Comparative Examples 2 and 3. As the lubricating component of the lubricating layer provided on the resin coating layer, oxidized polyethylene was used in Example 11, acid-modified polypropylene (hereinafter, polypropylene will also be referred to as PP) in Example 12, montanoic acid ethylene glycol ester in Example 13, and montanoic acid glycerol ester in Example 14.

[0089] For the acid-modified PE, maleic anhydride-modified PE was used. For the oxidized PE, PE containing a carboxy group was used. For the acid-modified PP, maleic anhydride-modified PP was used. The acid-modified PE is a polyolefin containing maleic anhydride as a polar group. The oxidized PE is a polyolefin containing a carboxy group as a polar group. The acid-modified PP is a polyolefin containing maleic anhydride as a polar group. The montanic acid ethylene glycol ester and the montanic acid glycerol ester are waxes containing an ester bond as a polar group.

[0090] (1) Measurement of Resin Coating Layer Thickness and Each Layer Thickness The resin coating layers of Examples 1 to 14 and Comparative Examples 1 to 3 were embedded in resin. Next, a cross section was taken in the thickness direction perpendicular to the surface of the resin coating layer using a microtome. The cross section of the resin coating layer was observed with an optical microscope to measure the thickness of the resin coating layer in the direction perpendicular to the surface of the resin coating layer and the thickness of each layer.

[0091] (2) Measurement of Complex Viscosity of Resin Coating Layer The resin coating layer was freeze-pulverized using a vertical reciprocating freeze-pulverizer. The freeze-pulverized resin coating layer was then used to measure the melt viscoelasticity in shear mode using a melt viscoelasticity measuring device. The sample was fused to a 25 mm diameter parallel plate at 280°C, and then cooled to 240°C. The 280°C complex viscosity was then measured from 240°C to 320°C using temperature dispersion measurement at a heating rate of 3°C / min, a frequency of 1 Hz, and in a nitrogen gas atmosphere.

[0092] A resin coating layer was formed on a metal plate by a film lamination method. Specifically, the metal plate was heated to 260°C, which is equal to or higher than the melting point of the resin coating layer. Then, the film-like resin coating layer was thermocompressed onto both sides of the metal plate using a laminating roll at 80°C. After 1.0 second had elapsed since the thermocompression bonding, the metal plate was water-cooled to produce a metal plate for a container in which both sides of the metal plate were coated with a resin coating layer.

[0093] (3) Measurement of Melting Point of Resin Coating Layer Using a differential scanning calorimeter, the resin coating layer peeled from the metal plate for a container was heated from room temperature to 290°C at a heating rate of 10°C / min., and the peak temperature of the endothermic peak between 200°C and 280°C was taken as the melting point of the resin coating layer. The resin coating layer was peeled off by immersing the metal plate for a container in a mixed solution of concentrated hydrochloric acid (12 mol / L):distilled water = 1:1 at room temperature to dissolve the metal plate.

[0094] (4) Heat Treatment Test specimens were prepared by cutting the metal plate for the container into squares measuring 40 mm x 40 mm. The heat treatment was carried out using a hot air circulation dryer. The heat treatment patterns (1) to (5) below were applied to the test specimens of Examples 1 to 14 and Comparative Examples 1 to 3.

[0095] Heat treatment pattern (1): Melting point of resin coating layer -15°C Heat treatment pattern (2): Melting point of resin coating layer Heat treatment pattern (3): Melting point of resin coating layer +10°C Heat treatment pattern (4): Melting point of resin coating layer -15°C → Melting point of resin coating layer -25°C → Melting point of resin coating layer 20 -15°C Heat treatment pattern (5): Melting point of resin coating layer -15°C → Melting point of resin coating layer -25°C → Melting point of resin coating layer +10°C

[0096] Heat treatment patterns (1), (2), and (3) are heat treatments in which a single heat treatment is performed. Heat treatment patterns (4) and (5) are heat treatments in which three heat treatments are performed, assuming ink baking and topcoat baking. The heat treatments are performed at a temperature above the glass transition point of the resin coating layer and below the melting point of the resin coating layer.

[0097] In the heat treatment patterns (1) to (5), the temperature at the center of the test piece was adjusted to a predetermined temperature 60 seconds after the start of heating from room temperature. When the test piece reached the predetermined temperature, it was removed from the hot air circulation dryer and allowed to cool naturally to room temperature.

[0098] (5) Number of dents in the resin coating layer after heat treatment The number of dents occurring within a 10 mm x 10 mm area from the center of the resin coating layer after heat treatment was counted. The number of dents was measured by enlarging and observing the image data of the resin coating layer captured by a camera. The resin coating layer was evaluated according to the number of dents. The evaluation criteria are shown below. The evaluation results are also shown in Table 1.

[0099] Evaluation "◎": Number of dents less than 20. Very good appearance. Evaluation "◯": Number of dents 20 to less than 100. Good appearance. Evaluation "△": Number of dents 100 to less than 150. Appearance problematic for practical use. Evaluation "×": Number of dents 150 or more. Abnormal appearance.

[0100] (6) Static Friction Coefficient If the static friction coefficient of the resin coating layer is large, a large shear stress will be generated in the resin coating layer during high-degree can forming, which will cause the resin coating layer to break, etc. Therefore, the static friction coefficient of the resin coating layer was measured to evaluate the breakage, etc. of the resin coating layer.

[0101] The static friction coefficient was measured using a rotary sliding friction and wear tester at a test temperature of 145°C, a load of 44 N, and a sliding speed of 775 mm / sec. A 10 mm diameter cemented carbide ball was used as the indenter. In this example, the coefficient of friction at the maximum static friction force was taken as the static friction coefficient. Note that 145°C, which was used as an index of the static friction coefficient, is just below the temperature range at which the resin crystallizes, and was set as the temperature condition considered to be the most severe temperature condition during processing. The evaluation results are also shown in Table 1.

[0102] Evaluation "Good": Less than 0.15 Evaluation "Poor": 0.15 or more The resin coating layer is scraped off during molding, causing practical problems.

[0103] As shown in Table 1, Examples 1 to 14 had better results in terms of the number of depressions in the resin coating layer after heat treatment than Comparative Examples 2 and 3. All of Examples 1 to 14 had good static friction coefficients.

[0104] Comparative Example 1 is a test example that does not contain a lubricating component. Comparative Example 1 obtained a better result in terms of the number of depressions in the resin coating layer after heat treatment than Comparative Examples 2 and 3. Comparative Example 1 did not satisfy the required standard for the static friction coefficient.

[0105] In Comparative Examples 2 and 3, the number of recesses in the resin coating layer after heat treatment did not meet the required standard. In Comparative Example 2, the lubricating component content of the lubricating layer was greater than 1.35 mass%, and the complex viscosity at 280°C was less than 31 Pa s. In Comparative Example 3, the ratio of the thickness of the pigment layer to the thickness of the outermost lubricating layer was greater than 16.0 / 1.0.

[0106]

[0107] 100, 200 Metal plate for container 10 Metal plate 20 Resin coating layer 21 First resin layer (lubricating layer) 22 Second resin layer (pigment layer) 23 Third resin layer (lubricating layer) 30 Resin coating layer 40 Resin coating layer 41 First resin layer (pigment layer) 42 Second resin layer (lubricating layer) 50 Resin coating layer

Claims

1. A metal plate for containers having a resin coating layer provided on at least one surface of the metal plate, wherein the resin coating layer contains a polyester resin and is formed by laminating a plurality of resin layers, wherein the complex viscosity of the resin coating layer at 280°C is 31 Pa·s or more and 100 Pa·s or less, wherein the plurality of resin layers have a pigment layer containing a white pigment and a lubricating layer containing 0.01 mass % or more and 1.35 mass % or less of a lubricating component, wherein the lubricating layer is arranged on the outermost side of the plurality of resin layers, and wherein the thickness ratio of the pigment layer to the film thickness of the lubricating layer arranged on the outermost side of the plurality of resin layers is 1.0 / 1.0 or more and 16.0 / 1.0 or less, and the lubricating component of the lubricating layer includes wax containing a polar group.

2. The metal plate for containers according to claim 1, wherein the white pigment is titanium oxide, and the pigment layer contains 5% by mass or more and 30% by mass or less of the titanium oxide.

3. A metal plate for containers as described in claim 1 or 2, wherein the lubricating layer is provided on the surface of the metal plate in the stacking direction of the multiple resin layers, the pigment layer is provided on the surface of the lubricating layer, and the lubricating layer is provided on the surface of the pigment layer.

4. The metal sheet for containers according to any one of claims 1 to 3, wherein the polyester resin is a copolymerized polyethylene terephthalate resin containing isophthalic acid as an acid component.

5. A method for manufacturing a metal plate for containers according to any one of claims 1 to 4, comprising: a hot melt extrusion step in which components of the lubricating layer and components of the pigment layer are hot melt extruded in different extruders; an extrusion molding step in which the components of the lubricating layer and components of the pigment layer are laminated in a feed block and then extruded from a T-die; and a lamination step in which the metal plate and the resin coating layer are thermocompression laminated together.

6. A method for manufacturing cans, comprising a can forming step of forming a metal sheet for containers according to any one of claims 1 to 4 into a can, and a heat treatment step of subjecting said metal sheet for containers to at least one heat treatment at a temperature equal to or higher than the glass transition point of said resin coating layer and lower than the melting point of said resin coating layer, said heat treatment step being carried out before said can forming step.

7. The method of manufacturing a can according to claim 6, wherein the heat treatment includes ink baking.

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