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

Abstract

Provided is a metal sheet for containers, etc., having a resin coating layer that satisfies the physical properties required for a resin coating layer when processed into a metal container and maintains a good appearance even after molding and heat treatment. The metal sheet for containers has a resin coating layer provided on at least one surface of the metal sheet. The resin coating layer contains a polyester resin and 0.1 mol % to 3.4 mol % of diethylene glycol as an alcohol component, and is formed by laminating multiple resin layers. The multiple resin layers include a pigment layer containing a white pigment and a lubricating layer containing 0.01 mass % to 0.75 mass % of a lubricating component. The lubricating layer is disposed on the outermost of the multiple resin layers. The lubricating component of the lubricating layer includes a polyolefin containing a polar group.

Description

[Technical Field]

[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. [Background technology]

[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 there are no welds in the can body.

[0003] Metal containers are made of tin-free steel (TFS) and aluminum plates, which are painted 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 cause breakage or chipping (hereinafter also referred to as breakage, etc.) in the resin coating layer. In particular, two-piece cans tend to be highly processed, which makes them more susceptible to breakage, etc. in the resin coating layer. Therefore, in order to manufacture two-piece cans, a material design that prevents breakage, etc. in the resin coating layer is required.

[0006] Various printing techniques are applied to the resin coating layer to enhance its design. If the printing ink has low affinity with the resin coating layer, the ink cannot adhere well to the surface. This can lead to the ink peeling off the resin coating layer and damaging the appearance. Therefore, the resin coating layer needs to be designed with a material that has 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 other processes (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 acid-modified polyolefin or oxidized polyolefin to the resin coating layer as a lubricating component (see Patent Documents 6 and 7).

[0009] However, there have been reported cases where metal container plates provided with a resin coating layer have had poor appearances 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, there have been reported cases where 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 phases, namely, crystalline and rigid amorphous phases, and a flexible phase, namely, mobile amorphous phases. 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 causes 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, cases have been reported in which wrinkle-like defects have occurred in the resin coating layer. To prevent the occurrence of such defects, the ethylene terephthalate unit content of the resin coating layer is set to a predetermined value or more, the mobile amorphous content of the resin coating layer is set to a predetermined value or more, and the lubricating component contained in the resin coating layer is set to a predetermined range (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. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2-303634 [Patent Document 2] Japanese Patent Application Publication No. 4-91825 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-148324 [Patent Document 4] Japanese Patent Application Publication No. 8-169098 [Patent Document 5] Japanese Patent Application Laid-Open No. 2004-130536 [Patent Document 6] International Publication No. 2019 / 116706 [Patent Document 7] International Publication No. 2019 / 116707 [Patent Document 8] International Publication No. 2013 / 030972 [Patent Document 9] International Publication No. 2021 / 182256 Summary of the Invention [Problem to be solved by the invention]

[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 two-piece cans, 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 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. [Means for solving the problem]

[0016] In order to solve the above problems, the present invention has the following features. [1] A metal plate for a container having a resin coating layer provided on at least one surface of the metal plate, the resin coating layer contains a polyester resin and 0.1 mol % or more and 3.4 mol % or less of diethylene glycol as an alcohol component, and is formed by laminating a plurality of resin layers; the plurality of resin layers include a pigment layer containing a white pigment and a lubricating layer containing 0.01% by mass or more and 0.75% by mass or less of a lubricating component, the lubricating layer is disposed on the outermost of the plurality of resin layers, The metal plate for a container, wherein the lubricating component of the lubricating layer includes a polyolefin containing a polar group. [2] the white pigment is titanium oxide, The metal plate for containers according to [1], wherein the pigment layer contains the titanium oxide in an amount of 5% by mass or more and 30% by mass or less. [3] The metal plate for containers according to [1] or [2], wherein the lubricating layer is provided on the surface of the metal plate 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 plate for containers according to any one of [1] to [3], wherein the resin coating layer is a copolymerized polyethylene terephthalate resin containing isophthalic acid as an acid component of the polyester resin and diethylene glycol as an alcohol component. [5] A method for producing a metal sheet for containers according to any one of [1] to [4], a hot-melt extrusion step of hot-melt extruding the components of the lubricating layer and the components of the pigment layer in different extruders; an extrusion molding step in which the components of the lubricating layer and the components of the pigment layer are laminated in a feed block and then extruded through a T-die; a laminating step of thermocompression laminating the metal plate and the resin coating layer together. [6] A can manufacturing method including a can forming step of forming the metal sheet for containers according to any one of [1] to [4] into a can, a heat treatment step of subjecting the metal plate for containers to at least one heat treatment 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, The method for manufacturing a can, wherein the heat treatment step is carried out before the can forming step is carried out. [7] The method for manufacturing a can according to [6], wherein the heat treatment includes ink baking. [Effects of the Invention]

[0017] According to the metal sheet for containers of the present invention, the resin coating layer contains a polyester resin. The polyester resin contains 0.1 mol % to 3.4 mol % of diethylene glycol as an alcohol component. The resin coating layer also has a lubricating layer containing 0.01 mass % to 0.75 mass % of a lubricating component. As a result, the metal sheet for containers of the present invention satisfies the physical properties required of the resin coating layer when processed into a metal container, and maintains a good appearance even after molding and heat treatment. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view of a metal plate for a container according to one embodiment of the present invention. [Figure 2] 1 is a flow chart showing a method for manufacturing a metal plate for a container and a method for manufacturing a can. [Figure 3] FIG. 10 is a cross-sectional view of a metal plate for a container according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[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 polyolefin containing a polar group as the lubricating component. However, when a metal plate for a container provided with a resin coating layer containing a polyolefin containing a polar group is subjected to heat treatment at a temperature near 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 containers according to one embodiment of the present invention. As shown in Fig. 1, a metal plate for containers 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 containers 100 is formed. The resin coating layer 30 is located on the inside of the metal container when the metal plate for containers 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 printed. 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 may be a steel plate such as tinplate or tin-free steel. 2 More than 15g / m 2 It is preferable to use the following:

[0024] 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 More than 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 is made of low-carbon steel having a C (carbon) content of about 0.010 mass % or more and 0.10 mass % or less and that is recrystallized and annealed by continuous annealing. (2) A steel sheet that is made of low-carbon steel having a C content of about 0.010% by mass or more and 0.10% by mass or less, and that has been subjected to recrystallization annealing and overaging treatment by continuous annealing. (3) A steel sheet that has been recrystallized by box annealing using low-carbon steel with a C content of about 0.010% by mass or more and 0.10% by mass or less. (4) Made from low-carbon steel with a C content of approximately 0.010% by mass or more and 0.10% by mass or less, it is subjected to recrystallization annealing by continuous annealing or box annealing, and then secondary cold rolling (DR: Double Reduced). (5) IF (Interstitial Free) steel is made by adding elements such as Nb and Ti, which have a higher reactivity with C than iron, to ultra-low carbon steel with a C content of approximately 0.003 mass% or less, and then recrystallizing and annealing it using continuous annealing.

[0027] The mechanical properties of the steel sheet are not particularly limited as long as they can be formed into the desired shape. The mechanical properties of the steel sheet preferably include a yield point (YP) in the range of approximately 220 MPa to 580 MPa. Having such a yield point (YP) of the steel sheet ensures that the workability is not impaired and sufficient can body strength is maintained.

[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 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, for example, in the form of a film. 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.

[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 more preferable from the viewpoint of molding processability because softening due to heat treatment can be suppressed. Furthermore, a melting point of 254°C or lower of the polyester resin is more preferable from the viewpoint of molding processability because it has appropriate 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 alcohol components can be used as raw materials for the polyester resin, and the polyester resin may be obtained by copolymerizing multiple dicarboxylic acid components and alcohol components within a range that does not impair heat resistance or processability.

[0033] Examples of dicarboxylic acid components of polyester resins include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenoxyethanedicarboxylic acid, 5-sodium sulfoisophthalic 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. In this embodiment, the resin coating layer 20 is primarily composed of polyester resin. The acid component contained in the resin coating layer 20 can be considered to be contained in the polyester resin. Hereinafter, the acid component content of the resin coating layer 20 refers to the acid component content of the polyester resin.

[0034] The polyester resin contains 0.1 mol % or more and 3.4 mol % or less of diethylene glycol as an alcohol component. The polyester resin preferably contains 0.5 mol % or more and 3.3 mol % or less of diethylene glycol, and more preferably 1.5 mol % or more and 3.1 mol % or less of diethylene glycol. In this embodiment, as described above, the resin coating layer 20 is mainly composed of polyester resin. The alcohol component contained in the resin coating layer 20 can be considered to be contained in the polyester resin. Hereinafter, the alcohol component content of the resin coating layer 20 will be referred to as the alcohol component content of the polyester resin.

[0035] By including 0.1 mol % or more of diethylene glycol in the polyester resin, the formation of fine dents in the resin coating layer 20 can be suppressed. Diethylene glycol is believed to enhance the molecular mobility of the polyester resin. Enhanced molecular mobility of the polyester resin is believed to promote thermal crystallization when the polyester resin is heated. As a result, resin flow of the polyester resin is believed to be suppressed. Therefore, the formation of fine dents on the surface of the resin coating layer 20 can be suppressed.

[0036] The polyester resin may contain an alcohol component other than diethylene glycol, such as an aliphatic glycol (e.g., ethylene glycol, propanediol, butanediol, pentanediol, hexanediol, neopentyl glycol), an alicyclic glycol (e.g., cyclohexanedimethanol), or an aromatic glycol (e.g., bisphenol A, bisphenol S).

[0037] From the viewpoint of economic efficiency, it is preferable to use terephthalic acid and ethylene glycol as the polyester resin. Furthermore, it is more preferable to use polyethylene terephthalate copolymerized with isophthalic acid as the polyester resin. 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 processing.

[0038] 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.

[0039] 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.

[0040] The pigment layer preferably contains 5% to 30% by mass of white pigment, and more preferably 10% to 25% by mass. By containing 5% or more by mass of white pigment, the pigment layer can be imparted with sufficient hiding power and a desirable whiteness after processing. Furthermore, by containing 30% or less by mass of white pigment, the pigment layer can be imparted with appropriate stretchability and can improve processability.

[0041] 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.

[0042] 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.

[0043] The lubricating component may be a polyolefin containing a polar group. By using a polyolefin containing a polar group as the lubricating component, the affinity between the polyester resin and the lubricating component and the processability of the metal plate for containers 100 can be ensured.

[0044] Examples of polyolefins containing polar groups include oxidized polyolefins such as polyethylene oxide, and acid-modified polyolefins such as ethylene acrylic acid copolymers and ethylene maleic anhydride copolymers.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] The lubricating layer preferably contains 0.01% by mass or more and 0.75% by mass or less of the lubricating component, preferably 0.02% by mass or more, and more preferably 0.03% 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.

[0049] The lubricating layer preferably contains 0.50% by mass or less, and more preferably 0.25% by mass or less, of the lubricating component. When the lubricating layer contains 0.75% by mass or less of the lubricating component, the formation of fine irregularities in the resin coating layer 20 can be suppressed.

[0050] The lubricating layer preferably contains 0.02% by mass or more and 0.50% by mass or less of a lubricating component, and more preferably 0.03% by mass or more and 0.25% by mass or less.

[0051] Thus, the lubricating layer contains a lubricating component, which ensures the surface's sliding properties during severe processing, such as the production of two-piece cans. It is believed that an excessive amount 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 amount of the lubricating component in the lubricating layer inhibits thermal crystallization of the resin coating layer 20.

[0052] 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 achieved.

[0053] In this case, the thickness of the lubricating layer is preferably 0.5 μm or more and 5.0 μm or less, and more preferably 1.0 μm or more and 3.0 μm or less. By making the thickness of the lubricating layer 0.5 μm or more, good processability can be obtained due to the lubricating component. Furthermore, by making the thickness of the lubricating layer 5.0 μm or less, it is possible to prevent the color of the white pigment in the pigment layer from being impaired.

[0054] 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.

[0055] 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 the ease of processing as described above. If the resin coating layer 20 were composed of only a pigment layer, the resin coating layer 20 would not be able to have appropriate processability, which could result in breakage or the like.

[0056] 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 back 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.

[0057] 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 flow chart for manufacturing the metal plate for containers 100 and a can using the metal plate for containers 100. As shown in Fig. 2, first, a hot melt extrusion step (step S01) is carried out in which components of the lubricating layer and components of the pigment layer are hot melt extruded in different extruders.

[0058] 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.

[0059] In other words, by containing the lubricating component in the lubricating layer 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.

[0060] A laminating step (step S03) is performed in which the metal plate and the formed resin coating are thermocompression laminated together. The laminating step of step S03 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 to the laminating step of step S03, a metal plate for containers 100 is manufactured.

[0061] Next, the metal plate for containers manufactured by the laminating step of step S03 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 S04).

[0062] Examples of the heat treatment performed in the heat treatment step of step S04 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.

[0063] Furthermore, after the heat treatment step in step S04, even if a subsequent heat treatment is performed at a temperature above 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.

[0064] The heat treatment temperature in step S04 is preferably at least 100°C below the melting point of the resin coating layer, and more preferably at least 50°C below the melting point of the resin coating layer. The heat treatment temperature is more preferably at least 25°C below the melting point of the resin coating layer. Setting the heat treatment temperature to at least 100°C below the melting point of the resin coating layer but less than the melting point of the resin coating layer can prevent the formation of fine pits on the surface of the resin coating layer. Heat treatment temperatures exceeding the melting point of the resin coating layer tend to promote the formation of fine pits in the resin coating layer. Heat treatment at temperatures exceeding the melting point of the resin coating layer softens the polyester resin due to crystalline melting, which is believed to promote the formation of fine pits in the resin coating layer. The heat treatment step in step S05 may be performed multiple times.

[0065] A can forming step is performed in which the metal plate for container is formed into a can (Step S05). In other words, before the can forming step of Step S05, the heat treatment step of Step S04 is performed.

[0066] 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 S06). The heat treatment step of step S06 can be performed in the same manner as the heat treatment step of step S04. Note that the number of times each step from the heat treatment step of step S04 to the heat treatment step of step S06 is performed can be changed appropriately depending on the type of can, etc. For example, after the heat treatment step of step S06 is performed, the can forming processing step of step S05 may be performed again.

[0067] 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 fine pits increases as the content of the lubricating component in the lubricating layer increases.

[0068] By setting the content of the lubricating component in the lubricating layer within a certain range as described above, it is possible to prevent the formation of fine dents on the surface of the resin coating layer 20. Specifically, when the metal plate for containers 100 was heat-treated at a temperature 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 20 can be performed without affecting the appearance.

[0069] 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.

[0070] In the above-described can production flow, a stretching step may be performed in which the extrusion-molded resin composition is stretched. The stretching method in the stretching step is not particularly limited, and may be uniaxial stretching or biaxial stretching.

[0071] (Variation) In the above-described 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.

[0072] 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.

[0073] Fig. 3 shows a cross section of a metal plate for containers 200 according to a modified example. As shown in Fig. 3, the metal plate for containers 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 containers 200 is formed. The resin coating layer 50 is located on the inside of the metal container when the metal plate for containers 100 is formed.

[0074] 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.

[0075] 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 disposed in this manner, the thickness of the lubricating layer is preferably 0.5 μm or more and 5.0 μm or less, and more preferably 1.0 μm or more and 3.0 μm or less. This allows for good processability due to the lubricating component. Furthermore, by setting the thickness of the lubricating layer to 5.0 μm or less, it is possible to prevent the color of the white pigment in the pigment layer from being impaired.

[0076] 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.

[0077] 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 back 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. [Example]

[0078] The melting point of the resin coating layer of the metal container plate was measured, and the number of dents formed on the surface of the resin coating layer after heat treatment of the metal container plate and the static friction coefficient were measured.

[0079] (Production of metal plates for containers) The metal plate was TFS (Tin-Free Steel) with a thickness of 0.22 mm, and the metal Cr layer was 120 mg / m 2 Cr oxide layer: 10 mg / m in terms of metallic Cr 2 , tempering grade: T3CA) was used.

[0080] The polyester resin of the resin coating layer was an isophthalic acid copolymerized polyethylene terephthalate resin. The resin coating layer was configured so that a first resin layer, a second resin layer, and a third resin layer were laminated 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. The second resin layer was used as a pigment layer.

[0081] The resin coating layers of Examples 1 to 9 and Comparative Examples 1 to 5 were produced by changing the amount of diethylene glycol in the polyester resin, the type of lubricating component in the lubricating layer, the content of the lubricating component, etc. Acid-modified polyethylene (hereinafter, polyethylene will also be referred to as PE) was used as the lubricating component in the lubricating layer in Examples 1 to 6, 8 to 9, and Comparative Examples 2 to 5. Acid-modified polypropylene (hereinafter, polypropylene will also be referred to as PP) was used as the lubricating component in the lubricating layer provided on the resin coating layer in Example 7.

[0082] The acid-modified PE used was maleic anhydride-modified PE, and the acid-modified PP used was maleic anhydride-modified PP.

[0083] The resin coating layers of Examples 1 to 9 and Comparative Examples 1 to 5 were prepared by stretching the resin compositions that would become the resin coating layers. The stretching in Examples 1 to 9 and Comparative Examples 1 to 5 was carried out by sequential stretching at 95°C, with biaxial stretching performed at a longitudinal stretching ratio of 3.0 times and then a transverse stretching ratio of 3.0 times.

[0084] 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 above 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, and the plate was water-cooled 1.5 seconds after thermocompression bonding to produce a metal plate for containers in which both sides of the metal plate were coated with a resin coating layer.

[0085] (1) Measurement of the melting point of the resin coating layer Using a differential scanning calorimeter, the resin coating layer peeled off from the metal container plate was heated from room temperature to 290°C at a heating rate of 10°C / min, and the peak temperature of the endothermic peak, which was 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 container plate in a 1:1 mixed solution of concentrated hydrochloric acid (12 mol / L):distilled water at room temperature and dissolving the metal plate.

[0086] (2) Heat treatment The metal plate for the container was cut into a 40 mm × 40 mm square to prepare a test piece. Heat treatment was performed using a hot air circulation dryer. The test pieces of Examples 1 to 9 and Comparative Examples 1 to 5 were subjected to the following heat treatment patterns (1) to (5).

[0087] Heat treatment pattern (1): Melting point of resin coating layer -15℃ 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

[0088] 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.

[0089] In the heat treatment patterns (1) to (5), the temperature of the center of the test piece was adjusted so that it reached 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.

[0090] (3) 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.

[0091] Rating: ◎: Fewer than 20 dents. Very good appearance. Evaluation: "Good": 20 to less than 100 dents. Good appearance. Evaluation "△": Number of dents: 100 or more but less than 150. Appearance is problematic for a practical product. Rating "x": 150 or more dents. Abnormal appearance.

[0092] (4) Coefficient of static friction If the static friction coefficient of the resin coating layer is large, a large shear stress will be generated in the resin coating layer when subjected to a high degree of 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.

[0093] 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 for processing. The evaluation results are shown in Table 1.

[0094] Rating: "Good": Less than 0.15. Evaluation "x": 0.15 or more. The resin coating layer is scraped off during molding, causing practical problems.

[0095] As shown in Table 1, in Examples 1 to 9, the number of recesses in the resin coating layer after heat treatment was better than in Comparative Examples 2 to 5. In all of Examples 1 to 9, a good static friction coefficient was obtained.

[0096] 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 to 5. Comparative Example 1 did not satisfy the required standard for the static friction coefficient.

[0097] In Comparative Examples 2 and 3, the number of dents in the resin coating layer after heat treatment did not meet the required standard. In Comparative Examples 2 and 3, the lubricating layer contained more than 0.75% by mass of the lubricating component. Therefore, it was found that when the lubricating layer contained more than 0.75% by mass of the lubricating component, the number of dents in the resin coating layer after heat treatment tended not to meet the required standard.

[0098] Comparative Example 4 differs from Example 4 in that the polyester resin does not contain diethylene glycol as an alcohol component. In Comparative Example 4, the number of depressions in the resin coating layer after heat treatment did not meet the required standard. Therefore, it was found that when the polyester resin does not contain diethylene glycol as an alcohol component, the number of depressions in the resin coating layer after heat treatment tends to not meet the required standard.

[0099] Comparative Example 5 is an example in which the diethylene glycol content of the polyester resin exceeds 3.4 mol %. In Comparative Example 5, the static friction coefficient did not meet the required standard. Therefore, it was found that when the diethylene glycol content of the polyester resin exceeds 3.4 mol %, the static friction coefficient tends to not meet the required standard.

[0100] [Table 1] [Explanation of symbols]

[0101] 100, 200 Metal plate for containers 10 metal plate 20 Resin coating layer 21 1st 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 a container having a resin coating layer provided on at least one surface of the metal plate, the resin coating layer contains a polyester resin and 0.1 mol % or more and 3.4 mol % or less of diethylene glycol as an alcohol component, and is formed by laminating a plurality of resin layers; the plurality of resin layers include a pigment layer containing a white pigment and a lubricating layer containing a lubricating component in an amount of 0.01% by mass or more and 0.75% by mass or less; the lubricating layer is disposed on the outermost of the plurality of resin layers, The metal plate for a container, wherein the lubricating component of the lubricating layer includes a polyolefin containing a polar group.

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

3. 2. The metal plate for containers according to claim 1, wherein, in the stacking direction of the plurality of resin layers, the lubricating layer is provided on the surface of the metal plate, 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. 3. The metal plate for containers according to claim 2, wherein, in the stacking direction of the plurality of resin layers, the lubricating layer is provided on the surface of the metal plate, 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.

5. The metal plate for containers according to any one of claims 1 to 4, wherein the resin coating layer is a copolymerized polyethylene terephthalate resin containing isophthalic acid as an acid component of the polyester resin and diethylene glycol as an alcohol component.

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

7. A method for manufacturing a metal plate for a container according to claim 5, comprising: a hot-melt extrusion step of hot-melt extruding the components of the lubricating layer and the components of the pigment layer in different extruders; an extrusion molding step in which the components of the lubricating layer and the components of the pigment layer are laminated in a feed block and then extruded through a T-die; a laminating step of thermocompression laminating the metal plate and the resin coating layer together.

8. A can manufacturing method including a can forming step of forming the metal plate for containers according to any one of claims 1 to 4 into a can, a heat treatment step of subjecting the metal plate for containers to at least one heat treatment 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, The method for manufacturing a can, wherein the heat treatment step is carried out before the can forming step is carried out.

9. A method for manufacturing a can, comprising a can forming step of forming the metal plate for containers according to claim 5 into a can, a heat treatment step of subjecting the metal plate for containers to at least one heat treatment 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, The method for manufacturing a can, wherein the heat treatment step is carried out before the can forming step is carried out.

10. The method of claim 8, wherein the heat treatment comprises ink baking.

11. The method of claim 9, wherein the heat treatment comprises ink baking.

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