Resin-coated metal plate and method for manufacturing same

Abstract

Provided is a resin-coated metal plate excelling in all of adherence, coverability, impact resistance after a retort treatment, and ease of content removal. The resin-coated metal plate according to the present invention comprises: a metal plate; and a first resin coating layer that is formed on one surface among the front and back surfaces of the metal plate and comprises a biaxially stretched film containing a polyester resin. In addition, the polyester resin contains 90 mol% or more of ethylene terephthalate, and the first resin coating layer has a wax-containing layer that has a peak intensity ratio I1725 / I1615, which is a ratio of a C=O peak intensity of 1725±5 cm-1 to a C=C peak intensity of 1615±5 cm-1, of 0.50-0.65 as determined by Raman spectroscopic analysis, has a peak intensity ratio I1725 / I1615 of 0.55-0.75 after the resin-coated metal plate has been subjected to a retort treatment at 130ºC for 90 minutes, and contains 0.1-2.0 mass% of a wax compound.

Description

Resin-coated metal sheet and its manufacturing method

[0001] The present invention relates to a resin-coated metal sheet used for can bodies and lids of food cans and beverage cans, for example, and a method for producing the same.

[0002] Conventionally, metal sheets such as tin-free steel (TFS) and aluminum, which are materials used for food cans, have been painted to improve corrosion resistance, durability, weather resistance, etc. However, this painting process has problems such as a complicated baking process, a long processing time, and the discharge of a large amount of solvent.

[0003] To solve these problems, resin-coated metal sheets (film-laminated metal sheets) have been developed as an alternative to painted steel sheets. These sheets are made by pressing a thermoplastic resin film onto a heated metal sheet. Resin-coated metal sheets are currently used industrially as materials for food cans, beverage cans, and aerosol cans.

[0004] These materials are required to have not only basic properties such as adhesion and coating ability, but also properties related to impact resistance. Therefore, in order to improve the impact resistance of conventional resin-coated metal sheets having a resin coating layer containing polyester resin, other components have been added or the film thickness has been increased.

[0005] Here, adhesion refers to the property of the resin coating layer not peeling off when processed into a container shape, and coverability refers to the property of not cracking the resin coating layer and not exposing the underlying metal plate when processing the blank into a can body. Also, impact resistance refers to the ability to prevent damage to the resin coating layer, such as denting of the container due to impact during transportation.

[0006] For example, Patent Document 1 discloses a film comprising a resin layer (I) of a crystalline polyester resin consisting of crystalline polyethylene terephthalate / crystalline polybutylene terephthalate, a resin layer (II) in which an olefin-based polymer is dispersed in the form of particles with an average particle size of 3.0 to 5.0 μm in a polyester resin having a melting point of 180°C or higher, and a resin layer (III) of a water-dispersible copolymer polyester resin on the side of the resin layer (II) that adheres to the metal plate. Use of this film improves adhesion to the metal plate and impact resistance. Furthermore, this film has a multilayer structure and, because the olefin-based polymer is dispersed in the polyester resin, has excellent adhesion and processability. However, contact with contents such as food was not taken into consideration, leaving the problem of poor content removal.

[0007] Furthermore, Patent Document 2 proposes the use of a polyester film containing at least two polyesters with different crystallinity, i.e., at least two polyesters including a PBT-based polyester (I) and a PET-based polyester (II), blended in specific ratios, with a heat shrinkage rate adjusted within a specific range at 130°C for 15 minutes. This polyester film can suppress the occurrence of shrinkage wrinkles during drying after application of the adhesive layer. It also exhibits excellent can formability, particularly in draw and ironing processes, and is excellent in thermal lamination with metal, impact resistance, and flavor and aroma retention. However, because 80 to 40% by mass of the PBT-based polyester (I), which has a melting point in the range of 200 to 223°C, is added, the crystallization rate is rapid. As a result, excessive crystallization of the film occurs during the paint baking heat treatment after printing, leading to embrittlement of the film, resulting in insufficient formability after heating (coating ability).

[0008] Patent Document 3 discloses a laminated metal sheet for a two-piece can, comprising a metal sheet, a first polyester resin layer formed on the surface of the metal sheet that will become the outer surface of the container after molding, and a second polyester resin layer formed on the surface of the metal sheet that will become the inner surface of the container after molding. In this laminated metal sheet, the first polyester resin layer contains 30% to 60% by mass of polyethylene terephthalate or copolymerized polyethylene terephthalate having a copolymerization component content of less than 6 mol%, 40% to 70% by mass of polybutylene terephthalate or copolymerized polybutylene terephthalate having a copolymerization component content of less than 5 mol%, and a polyolefin wax in an external proportion of 0.01% to 3.0%. The second polyester resin layer is copolymerized polyethylene terephthalate having a copolymerization component content of less than 22 mol%, and the first and second polyester resin layers have a residual orientation degree of less than 30%. This laminated metal sheet has a polyester resin layer formed on the metal sheet surface that will become the outer surface of the container after molding, which can suppress retort whitening. However, there is a risk that excessive crystallization of the film will progress during retort treatment, leading to embrittlement of the film, and therefore there is a need to improve the impact resistance after retort.

[0009] JP 2010-240985 A JP 2009-221315 A JP 2014-166856 A

[0010] As described above, it is essential that resin-coated metal sheets have excellent adhesion and coating properties, which are required for food can materials. Furthermore, in container applications, a retort process is performed in which the beverage or food sealed in the container is pressurized, heated, and sterilized, and it is also important that the resin-coated metal sheets have excellent impact resistance after the retort process. Furthermore, in container applications, easy removal of contents such as food leads to increased consumer purchasing motivation, so it is also important that the resin-coated metal sheets have excellent content removal properties.

[0011] In view of the above circumstances, an object of the present invention is to provide a resin-coated metal sheet that is excellent in all of adhesion, coating properties, impact resistance after retort treatment, and ease of content removal.

[0012] The present inventors have conducted extensive research into means for solving the above problems. As a result, they have found that a resin-coated metal sheet obtained by coating a metal sheet with a biaxially stretched film made of polyester resin has excellent adhesion, coating properties, and impact resistance after retort treatment under the following conditions: That is, the polyester resin contains 90 mol % or more of ethylene terephthalate, and the surface roughness of the resin-coated metal sheet is 1725±5 cm by Raman spectroscopy after retort treatment. －１ C═O peak intensity of 1615±5 cm －１ Peak intensity ratio I to the C=C peak intensity １７２５ / I １６１５ The present inventors have also found that by providing a layer containing a predetermined amount of a wax compound, it is possible to obtain a resin-coated metal sheet that is excellent in terms of ease of content removal.

[0013] The present invention has been made based on the above findings, and its gist is as follows: [1] A resin-coated metal sheet having a metal sheet and a first resin coating layer made of a biaxially stretched film containing a polyester resin formed on one of the front and back surfaces of the metal sheet, wherein the polyester resin contains 90 mol % or more of ethylene terephthalate, and the first resin coating layer has a Raman polarization of 1725±5 cm by Raman spectroscopy, the measurement direction being set so that the laser polarization plane is perpendicular to the thickness direction of the first resin coating layer. －１ C═O peak intensity of 1615±5 cm －１ The peak intensity ratio I is the ratio of the C═C peak intensity to the C═C peak intensity １７２５ / I １６１５ is 0.50 or more and 0.65 or less, and the peak intensity ratio I after the resin-coated metal plate is subjected to a retort treatment at 130°C for 90 minutes １７２５ / I １６１５ A resin-coated metal sheet having a wax-containing layer containing a wax compound in an amount of 0.1 mass % to 2.0 mass %.

[0014] [2] The resin-coated metal sheet according to [1], wherein the first resin coating layer has a multilayer structure including at least an outermost layer and an underlayer, and the outermost layer is the wax-containing layer.

[0015] [3] A resin-coated metal plate according to [1] or [2], which has a second resin coating layer containing a polyester resin formed on the other of the front and back surfaces of the metal plate, the second resin coating layer having a multilayer structure including at least an outermost layer and an underlying layer, and in which the layers of the second resin coating layer other than the outermost layer contain either titanium dioxide or a disazo-based organic pigment or both as coloring pigments.

[0016] [4] The resin-coated metal sheet according to any one of [1] to [3], wherein the wax compound is carnauba wax.

[0017] [5] A method for producing a resin-coated metal sheet according to any one of [1] to [4], comprising: a step of laminating the biaxially stretched film onto the metal sheet under the following conditions (i) to (iii) to obtain the first resin coating layer; and a step of: (i) the surface temperature of the metal sheet at the start of lamination is Tm°C or higher and (Tm + 40)°C or lower, where Tm is the melting point of the polyester resin; (ii) the surface temperature of a laminating roll is Tg°C or higher and (Tg + 80)°C or lower, where Tg is the glass transition point of the polyester resin; and (iii) the contact time between the biaxially stretched film and the laminating roll is 10 milliseconds or higher and 30 milliseconds or lower; and then, starting cooling within 2 seconds from the completion of the lamination, and continuing cooling until the surface temperature of the first resin coating layer is (Tg - 20)°C or higher and Tg°C or lower.

[0018] According to the present invention, it is possible to provide a resin-coated metal sheet that is excellent in all of adhesion, coating properties, impact resistance after retort treatment, and ease of content removal.

[0019] A resin-coated metal sheet according to one embodiment of the present invention will be described in detail below. First, the metal sheet used in the embodiment of the present invention will be described.

[0020] As the metal sheet, aluminum sheet or mild steel sheet, which are widely used as materials for cans, can be used. In particular, a surface-treated steel sheet (hereinafter referred to as TFS) having a two-layer coating formed thereon, consisting of a lower layer of metallic chromium and an upper layer of chromium hydroxide, is optimal. The coating mass of the TFS is not particularly limited, but from the viewpoint of post-processing adhesion and corrosion resistance, the metallic chromium layer should be 70 to 200 mg / m2 in terms of Cr. ２ , the chromium hydroxide layer is 10 to 30 mg / m ２ The thickness of the metal plate is preferably 0.10 to 0.40 mm.

[0021] One of the front and back surfaces of the metal plate has a first resin coating layer made of a biaxially stretched film containing a polyester resin. The first resin coating layer made of this biaxially stretched film will now be described.

[0022] The first resin coating layer contains a polyester resin mainly composed of polyethylene terephthalate. "Mainly composed of polyethylene terephthalate" means that 90 mol% or more of the polyester structural units are ethylene terephthalate. The content of ethylene terephthalate in the polyester structural units is more preferably 92 mol% or more, and most preferably 100 mol%. When the ethylene terephthalate unit content is 90 mol% or more, the resin coating layer is suitable for applications requiring heat resistance.

[0023] Terephthalic acid, as an acid component of polyethylene terephthalate, is essential for ensuring properties such as mechanical strength, heat resistance, and corrosion resistance. Furthermore, copolymerization of terephthalic acid with isophthalic acid improves processability. That is, copolymerization of 2 to 10 mol % of an isophthalic acid component with respect to the terephthalic acid component improves deep drawability, making it suitable for applications requiring high processing efficiency.

[0024] Furthermore, one or both of other dicarboxylic acid components and glycol components may be copolymerized to the extent that the above-mentioned properties are not impaired. Examples of dicarboxylic acid components include aromatic dicarboxylic acids such as diphenylcarboxylic 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, fumaric acid, and cyclohexanedicarboxylic acid; and oxycarboxylic acids such as p-oxybenzoic acid. Examples of other glycol components include aliphatic glycols such as propanediol, butanediol, pentanediol, hexanediol, and neopentyl glycol; alicyclic glycols such as cyclohexanedimethanol; aromatic glycols such as bisphenol A and bisphenol S; diethylene glycol; and polyethylene glycol. Two or more of these dicarboxylic acid components and glycol components may be used in combination. Furthermore, as long as the effects of the present invention are not impaired, polyfunctional compounds such as trimellitic acid, trimesic acid, and trimethylolpropane may also be copolymerized.

[0025] Furthermore, the resin material used as the raw material for the first resin coating layer is not limited by its manufacturing method. For example, the resin material can be formed by esterifying terephthalic acid, ethylene glycol, and a copolymerization component, followed by polycondensation of the resulting reaction product to form a copolymerized polyester. Alternatively, the resin material can be formed by transesterifying dimethyl terephthalate, ethylene glycol, and a copolymerization component, followed by polycondensation of the resulting reaction product to form a copolymerized polyester. In the production of the copolymerized polyester, additives such as fluorescent brighteners, antioxidants, heat stabilizers, UV absorbers, and antistatic agents may be added as needed. Adding a fluorescent brightener is effective for improving whiteness.

[0026] The first resin coating layer is a resin coating layer made of a biaxially stretched film. The biaxially stretched film is used because it can improve can-forming properties by isotropically orienting polyester molecules in the film plane. In addition, the biaxially stretched film is advantageous in terms of film productivity and film thickness uniformity.

[0027] The first resin coating layer has a Raman spectroscopic analysis of 1725±5 cm by setting the measurement direction so that the laser polarization plane is perpendicular to the thickness direction of the first resin coating layer. －１ C═O peak intensity of 1615±5 cm －１ The peak intensity ratio I is the ratio of the C═C peak intensity to the C═C peak intensity １７２５ / I １６１５ (Hereinafter, the peak intensity ratio I １７２５ / I １６１５ It is important that the σ is 0.50 or more and 0.65 or less.

[0028] That is, the peak intensity ratio I of the first resin coating layer １７２５ / I １６１５ By making the ratio of 0.50 to 0.65, it is possible to ensure the adhesion, coating properties, and impact resistance after retort treatment, which are the objects of the present invention. The reasons for this will be described below.

[0029] The above peak intensity ratio I １７２５ / I １６１５ is an index of the orientation state of the PET molecular chains. １７２５ / I １６１５ If the peak intensity ratio I is less than 0.50, the amorphous content increases and the interaction between molecular chains decreases, resulting in a deterioration in impact resistance. １７２５ / I １６１５ is 0.50 or more, and preferably 0.53 or more. １７２５ / I １６１５ If the peak intensity ratio I exceeds 0.65, the amorphous content decreases, the flexibility of the first resin coating layer decreases, and cracks may occur, resulting in a deterioration in the coating properties (ability to prevent exposure of the metal plate) after processing. １７２５ / I １６１５ is set to 0.65 or less, and preferably 0.63 or less.

[0030] Furthermore, after the resin-coated metal plate was subjected to a retort treatment at 130°C for 90 minutes, the first resin coating layer had the above-mentioned peak intensity ratio I １７２５ / I １６１５ (Hereinafter, the peak intensity ratio after retort treatment I １７２５ / I １６１５It is also important that the ratio (ratio) of the atmospheric temperature in the retort sterilization furnace is 130°C and the temperature is maintained at 130°C for 90 minutes.

[0031] That is, the first resin coating layer is heated and crystallized by the retort treatment. Therefore, the peak intensity ratio I １７２５ / I １６１５ The peak intensity ratio after retort treatment I １７２５ / I １６１５ If the peak intensity ratio after retort treatment I is smaller than 0.55, the amorphous content increases, the interaction between molecular chains decreases, and the impact resistance decreases. １７２５ / I １６１５ is 0.55 or more, and preferably 0.58 or more. On the other hand, the peak intensity ratio after retort treatment I １７２５ / I １６１５ If the peak intensity ratio after retort treatment I exceeds 0.75, the amorphous content decreases and the film becomes brittle, so that adhesion and coating properties cannot be obtained. １７２５ / I １６１５ is set to 0.75 or less, and preferably 0.72 or less.

[0032] Peak intensity ratio I １７２５ / I １６１５ and peak intensity ratio after retort treatment I １７２５ / I １６１５ can be measured by the following method. Measurements are carried out using a Raman spectrometer LabRAM HR manufactured by Horiba Ltd., with a semiconductor laser (λ=532 nm) as the excitation light source, a microscope magnification of ×100, and an aperture of 25 μmφ. The measurement direction is the direction in which the laser polarization plane is perpendicular to the thickness direction of the resin coating layer, and the measurement is carried out at a wavelength of 1725±5 cm －１ C═O peak intensity of 1615±5 cm －１ The peak intensity ratio I is the ratio of the C═C peak intensity to the C═C peak intensity １７２５ / I １６１５ Here, the boundary between the metal plate and the resin coating layer is set as the starting point (0 μm), and line analysis of the resin coating layer is performed every 1 μm along the thickness direction of the resin coating layer from the starting point. The peak intensity ratio I obtained by each measurement １７２５ / I１６１５ The smallest value of the peak intensity ratio I １７２５ / I １６１５ The largest value among the peak intensity ratios is the peak intensity ratio I １７２５ / I １６１５ In addition, after the resin-coated metal plate is subjected to a retort treatment at 130°C for 90 minutes, the peak intensity ratio after retort treatment I １７２５ / I １６１５ The peak intensity ratio I before and after the retort treatment is calculated. １７２５ / I １６１５ The upper and lower limits of the test shall be such that the minimum value of the measurement result is equal to or greater than the lower limit, and the maximum value of the measurement result is equal to or less than the upper limit.

[0033] The first resin coating layer may also have a multi-layer structure, with each layer having its own function. For example, it may have a two-layer structure consisting of an outermost layer (upper layer) and a lower layer, or a structure consisting of at least three layers: an outermost layer (upper layer), an intermediate layer (main layer), and a lowermost layer (lower layer). Preferably, it has three layers or less. An example of a multi-layer structure in which each layer has a function is to incorporate wax into the outermost layer to reduce the amount of wax in the entire first resin coating layer and effectively control the processability using the wax. Furthermore, by adding a larger amount of pigment to the intermediate layer in the multi-layer structure, it is possible to control the color tone of the entire first resin coating layer while ensuring processability, etc. In such cases, the film thickness of the outermost layer and the lowermost layer is preferably 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more. On the other hand, the film thickness of the outermost layer and the lowermost layer is preferably 5.0 μm or less, more preferably 4.0 μm or less, and even more preferably 3.0 μm or less. The thickness of the intermediate layer is preferably 6 μm or more, more preferably 8 μm or more, and even more preferably 10 μm or more. On the other hand, the thickness of the intermediate layer is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. In order to achieve both whiteness and processability as a layer, it is preferable that the outermost layer and the lowermost layer contain 0% to 2% titanium dioxide, and the intermediate layer contains 10% to 30% titanium dioxide.

[0034] The first resin coating layer has a wax-containing layer containing 0.1 to 2.0% by mass of a wax compound. When the first resin coating layer is a single layer, the wax compound content of the first resin coating layer is 0.1 to 2.0% by mass. In particular, when one surface is to be the inner surface after container molding, for example, it is preferable that the first resin coating layer has a multi-layer structure including at least an outermost layer and an underlayer, and that the outermost layer that comes into contact with the contents is the wax-containing layer. In this case, it is preferable that the wax compound is contained in an amount of 0.1 to 2.0% by mass relative to the total amount of the outermost layer. The purpose of adding the wax compound is to reduce the surface free energy of the first resin coating layer that faces the inner surface of the container. The addition of the wax compound makes it difficult for the contents to adhere to the first resin coating layer, improving content removal. The reason for limiting the amount of wax compound to 0.10% by mass or more is that if the wax compound is added less than 0.10% by mass, the above-mentioned effect is poor and content removal is poor. The reason for limiting the amount to 2.0 mass % or less is that if the amount exceeds 2.0 mass %, there is a high possibility that the adhesion between the multiple layers of the first resin coating layer will deteriorate, and the formation of the first resin coating layer itself will become difficult, resulting in a deterioration in productivity.

[0035] As the wax compound to be added, organic or inorganic lubricants can be used, but carnauba wax, a natural wax derived from plants, is particularly suitable. Polyester resins containing carnauba wax can be produced by blending a predetermined amount of wax with polyester and then using a conventional production method. The wax content of the resin coating layer is the same as the amount of wax added to the polyester resin.

[0036] Furthermore, the intrinsic viscosity (IV) of the first resin coating layer is preferably 0.50 dl / g or more, more preferably 0.52 dl / g or more, and even more preferably 0.55 dl / g or more. On the other hand, the intrinsic viscosity (IV) of the first resin coating layer is preferably 0.90 dl / g or less, more preferably 0.80 dl / g or less, and even more preferably 0.75 dl / g or less. If the intrinsic viscosity of the first resin coating layer is 0.50 dl / g or more, the molecular weight of the first resin coating layer is high, ensuring sufficient mechanical strength. On the other hand, if the intrinsic viscosity of the first resin coating layer is 0.90 dl / g or less, excellent film-forming properties can be obtained. The intrinsic viscosity (IV) of the first resin coating layer can be adjusted by controlling the polymerization conditions (e.g., amount of polymerization catalyst, polymerization temperature, polymerization time, etc.) or by solid-state polymerization in an inert atmosphere such as nitrogen or under vacuum after melt polymerization.

[0037] The metal plate preferably has a second resin coating layer containing a polyester resin on either the front or back surface. In particular, the second resin coating layer on the other surface preferably has a multilayer structure including at least an outermost layer and an underlying layer. Furthermore, the layers of the second resin coating layer other than the outermost layer preferably contain either or both of titanium dioxide and a disazo-based organic pigment as color pigments.

[0038] The number of layers in the second resin coating layer is not particularly limited, but may be a two-layer structure consisting of an outermost layer (upper layer) and a lower layer, or a structure consisting of at least three layers consisting of an outermost layer (upper layer), an intermediate layer (main layer), and a lowermost layer (lower layer). Preferably, the number is three or less. By having three or less layers, the film production cost can be reduced.

[0039] In the second resin coating layer, the thickness of the outermost layer and the bottom layer is preferably 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more. On the other hand, the thickness of the outermost layer and the bottom layer is preferably 5.0 μm or less, more preferably 4.0 μm or less, and even more preferably 3.0 μm or less. Furthermore, the thickness of the intermediate layer is preferably 6 μm or more, more preferably 8 μm or more, and even more preferably 10 μm or more. On the other hand, the thickness of the intermediate layer is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less.

[0040] For example, if the other surface is to be the outer surface after molding of the container, the second resin coating layer may be required to be gold or white to enhance the design after molding or during printing. To impart a brilliant gold color, it is preferable to contain a disazo-based organic pigment as a coloring pigment. Although the disazo-based organic pigment exhibits a yellow color, it is transparent, allowing the gloss of the metal plate to be utilized to efficiently obtain a brilliant gold color.

[0041] Generally, the use of oil-soluble dyes is known as a method for achieving highly transparent coloring. However, one of the drawbacks of these methods is migration (elution of coloring materials from colored resins). This is a phenomenon known as bleeding, whereby oil-soluble dyes dissolved in the resin to be colored float to the surface of the resin coating layer when subjected to a thermal history such as retort sterilization. However, disazo organic pigments generally exist in the resin as particles, which significantly reduces migration.

[0042] As the yellow pigment, a disazo-based organic pigment is preferably used, with CI Pigment Yellow 180 being particularly suitable. This pigment is FDA (Food and Drug Administration) registered, offering excellent safety and the ability to achieve excellent color tones. The amount added is preferably 0.1% by mass or more and 20.0% by mass or less based on the total weight of the layer to which the pigment is added. Less than 0.1% by mass results in poor color development and an undesirable color tone. Conversely, more than 20.0% by mass may result in poor resin transparency and a color tone lacking in brilliance. The pigment particle size is preferably less than 1 μm. Particle sizes of 1 μm or more are undesirable because they may result in a loss of transparency in the polyester resin. Furthermore, higher fatty acid metal salts such as magnesium stearate can be used as dispersants. Using a dispersant allows for a more uniform and transparent color tone to be achieved. The pigment content in the resin coating layer is the same as the amount of pigment added to the polyester resin.

[0043] In addition to yellow pigments, adding a white pigment can conceal the metallic luster of the base and make the printed surface more vivid, resulting in a good appearance. The pigment to be added must be able to exhibit excellent design properties after the container is molded, and from this perspective, titanium dioxide is preferred. Its strong coloring power and excellent ductility make it suitable for ensuring good design properties even after the container is molded. The amount added is preferably 10.0% by mass or more and 30.0% by mass or less, based on the total amount of the layer to which the pigment is added. If the amount is less than 10.0% by mass, the color development is poor and a desirable color tone cannot be obtained. Conversely, if the amount exceeds 30.0% by mass, the flexibility of the resin coating layer is impaired, and it may not be able to follow the container molding process.

[0044] When the resin-coated metal sheet is used as a container, it may be molded so that one of the surfaces is the inner surface of the container and the other surface is the outer surface of the container before use.

[0045] [Manufacturing Method] Next, a method for manufacturing a resin-coated metal sheet according to one embodiment of the present invention will be described. The method for manufacturing a resin-coated metal sheet includes the steps of laminating a biaxially stretched film onto a metal sheet to obtain a first resin coating layer, and initiating cooling within 2 seconds of the completion of lamination until the surface temperature of the first resin coating layer reaches (Tg-20)°C or higher and Tg°C or lower. First, regarding the method for obtaining a biaxially stretched film to be laminated onto a metal sheet, an example in which the biaxially stretched film has a multilayer structure will be described. An example of the manufacturing method will be shown below.

[0046] Each polyester resin, which is the material for each layer of the multilayer structure, is dried as necessary and then fed into a known melt lamination extruder. The polyester resins are co-extruded into a sheet form through a slit die installed in the extruder, and then brought into close contact with a casting drum by electrostatic application or other methods, followed by cooling and solidification to obtain an unstretched sheet. This unstretched sheet is stretched in the longitudinal and width directions of the film to obtain a biaxially stretched film. The stretching ratio can be set as desired depending on the desired film orientation, strength, elastic modulus, etc. From the perspective of film quality, the stretching method is preferably a tenter system, and either a sequential biaxial stretching method in which the film is stretched in the longitudinal direction and then in the width direction, or a simultaneous biaxial stretching method in which the film is stretched in the longitudinal and width directions almost simultaneously, is preferred.

[0047] Next, a method for producing a resin-coated metal sheet by coating a metal sheet with a biaxially stretched film will be described. In the present invention, a method (hereinafter referred to as lamination) can be used in which the metal sheet is heated to a temperature exceeding the melting point of the biaxially stretched film and a film is brought into contact with one or both sides of the metal sheet using a pressure roller (hereinafter referred to as lamination roller) to heat-seal the film.

[0048] The lamination conditions are appropriately set so as to obtain the resin coating layer specified in the present invention. First, the melting point of the polyester resin is Tm (°C), and the surface temperature of the metal plate at the start of lamination must be Tm°C or higher. Specifically, the surface temperature of the metal plate must be controlled to Tm°C or higher and (Tm + 40)°C or lower. By maintaining the surface temperature of the metal plate at Tm°C or higher, the biaxially oriented film melts and wets the surface of the metal plate, thereby ensuring good adhesion to the metal plate. On the other hand, if the surface temperature of the metal plate exceeds (Tm + 40)°C, the biaxially oriented film will melt excessively, which may cause the biaxially oriented film to adhere to the lamination roll. In addition, it becomes difficult to control the crystal structure (peak intensity ratio) of the first resin coating layer within the range specified in the present invention. The surface temperature of the metal plate is preferably (Tm + 25)°C or lower, more preferably (Tm + 15)°C or lower. When the biaxially stretched film is a multilayer film, the melting point of the entire biaxially stretched film is defined as Tm (°C).

[0049] Furthermore, in order to control the crystalline structure (peak intensity ratio) of the resin layer to an appropriate state, the surface temperature of the laminating roll must also be adjusted. Specifically, the glass transition point of the polyester resin is Tg (°C), and the surface temperature of the laminating roll must be controlled to be Tg°C or higher and (Tg + 80)°C or lower. By controlling the surface temperature of the laminating roll to Tg°C or higher, the biaxially oriented film melts and wets the surface of the metal plate, thereby ensuring good adhesion to the metal plate. On the other hand, if the surface temperature of the laminating roll exceeds (Tg + 80)°C, the biaxially oriented film may melt excessively, resulting in the risk of adhesion of the biaxially oriented film to the laminating roll. In addition, it becomes difficult to control the crystalline structure (peak intensity ratio) of the first resin coating layer within the specified range of the present invention. The surface temperature of the laminating roll is preferably (Tg + 40)°C or lower, more preferably (Tg + 20)°C or lower. When the biaxially stretched film is a multilayer film, the glass transition temperature of the entire biaxially stretched film is defined as Tg (° C.).

[0050] The glass transition point Tg (°C) and the melting point Tm (°C) can be measured by thermal analysis of the biaxially stretched film or the resin coating layer of the resin-coated metal plate under the following conditions: Measuring device: DSC Q100 manufactured by TA Instruments Co., Ltd. Measurement temperature range: -50°C to 290°C Heating rate: 10°C / min Atmosphere: Nitrogen atmosphere, flow rate 50m / min

[0051] Furthermore, adjusting the contact time between the biaxially stretched film and the laminating roll is also an important factor. The contact time needs to be controlled to 10 milliseconds or more and 30 milliseconds or less. By adjusting the contact time between the biaxially stretched film and the laminating roll within the above range, the crystalline structure of the resin specified in the present invention can be realized.

[0052] It is preferable to heat the biaxially stretched film before laminating the resin layer to the metal plate. By heating the biaxially stretched film and softening it in advance, the temperature distribution within the cross section of the biaxially stretched film during lamination can be made more uniform. This allows the crystalline structure of the first resin coating layer to undergo a gradual structural change from the interface with the metal plate to the surface layer, thereby achieving more uniform performance. Specifically, it is preferable to adjust the temperature of the biaxially stretched film before lamination to Tg ° C or higher and (Tg + 30) ° C or lower.

[0053] After the lamination is completed, it is necessary to rapidly cool (quench) the resin layer to fix its crystalline structure. The cooling start time must be limited to within 2 seconds from the completion of lamination, preferably within 1 second. Meanwhile, the cooling start time is generally 0.5 seconds or more. Cooling is performed until the surface temperature of the first resin coating layer reaches (Tg-20)°C or higher and Tg°C or lower. That is, the cooling stop temperature is (Tg-20)°C or higher and Tg°C or lower. Specifically, the resin-coated metal sheet immediately after lamination is subjected to water cooling, and the water temperature is controlled to be (Tg-20)°C or higher and Tg°C or lower, until the surface temperature of the first resin coating layer reaches that temperature range. Here, water cooling refers to cooling the resin-coated metal sheet by contacting it with water, and examples include mist cooling, spray cooling, and immersion in a water tank.

[0054] The second resin coating layer can be obtained by the same method as the first resin coating layer described above, thereby producing a resin-coated metal sheet having the first resin coating layer formed on one side and the second resin coating layer formed on the other side.

[0055] For steps and conditions not described in this specification, conventional methods can be used.

[0056] Examples of the present invention will be described below. (Method for manufacturing metal sheet) A steel sheet having a thickness of 0.22 mm and a width of 977 mm, which had been subjected to cold rolling, annealing, and temper rolling, was degreased, pickled, and then chrome-plated to manufacture a chrome-plated steel sheet (TFS). The chrome plating was carried out using CrO ３ , F － , S.O.４ ２－ After intermediate rinsing, electroplating is performed in a plating bath containing CrO ３ , F － During the chemical conversion treatment, the electrolysis conditions (current density, amount of electricity, etc.) were adjusted to achieve a metal chromium deposition amount and a chromium hydroxide deposition amount of 120 mg / m2 in terms of Cr. ２ and 15 mg / m ２ It was decided.

[0057] (Method for manufacturing resin coating film) A polyester resin having the resin composition shown in Table 1-1 was prepared, and the amount of wax shown in Table 1-1 was added. The resin was dried and melted according to a conventional method, co-extruded through a T-die, and then cooled and solidified on a cooling drum to obtain an unstretched film. The obtained unstretched film was biaxially stretched and heat-set to obtain a biaxially stretched film. Note that no wax was added to the resin for the inner surface of the container in Comparative Example 3.

[0058] The obtained biaxially stretched film was subjected to thermal analysis by the method described above to determine the glass transition temperature Tg (°C) and the melting temperature Tm (°C). The results are shown in Table 1-1.

[0059] A polyester resin having the resin composition shown in Table 1-2 was prepared, and the color pigment was added in the amount shown in Table 1-2. The biaxially stretched film was obtained by the method described above. The melting points of the obtained biaxially stretched film are shown in Table 1-2.

[0060]

[0061]

[0062] (Method for producing resin-coated metal sheet) Using a thermocompression laminating device, the biaxially stretched film was laminated onto the chromium-plated steel sheet according to the conditions shown in Table 2, followed by cooling. That is, the biaxially stretched film for the inner surface of a container was laminated onto one of the front and back surfaces of the chromium-plated steel sheet to form a first resin coating layer, and the biaxially stretched film for the outer surface of a container was laminated onto the other of the front and back surfaces of the chromium-plated steel sheet to form a second resin coating layer.

[0063] The laminating roll was internally water-cooled, and the temperature during film bonding was controlled by circulating cooling water inside the roll. The temperature of the biaxially stretched film before lamination was set to Tg°C or higher and (Tg + 30)°C or lower, where Tg (°C) is the glass transition point of the biaxially stretched film, to ensure a uniform temperature distribution within the cross section of the biaxially stretched film. The film was then water-cooled in a metal strip cooling device to produce a resin-coated metal sheet.

[0064] (Evaluation of Resin-Coated Metal Sheet) The properties of the resin-coated metal sheet obtained above were evaluated by the following method. In addition, the peak intensity ratio I of the first resin coating layer before and after the retort treatment was measured by the above-mentioned method. １７２５ / I １６１５ The evaluation results are shown in Table 2.

[0065] [Adhesion] A test piece measuring 120 mm in the longitudinal direction and 30 mm in the width direction was cut from the resin-coated metal sheet, and the first resin coating layer was partially peeled from the long edge of the cut test piece on the inner surface of the can (one side). The peeled first resin coating layer was opened in the opposite direction (angle: 180°) from the chrome-plated steel sheet from which the first resin coating layer was peeled, and a peel test was performed using a tensile tester at a tensile speed of 30 mm / min to evaluate the adhesion strength per 15 mm width. Note that the evaluation target was the film on the inner surface of the can. If this adhesion strength was 5 N / 15 mm or more, adhesion between the film and the metal sheet was maintained even under severe processing such as drawing. On the other hand, if the adhesion strength was less than 5 N / 15 mm, the first resin coating layer may peel under more severe processing conditions such as drawing and ironing.

[0066] [Adhesion evaluation criteria] ◎: 8N / 15mm or more 〇: 5N / 15mm or more and less than 8N / 15mm △: 2N / 15mm or more and less than 5N / 15mm ×: Less than 2N / 15mm

[0067] [Coatability] Cans were fabricated from resin-coated metal sheets using the following method, and the coverability of the first resin coating layer was evaluated. After applying wax to both sides of the resin-coated metal sheet, a 165 mm diameter disk was punched out, resulting in a shallow-drawn can with a drawing ratio of 1.50. These shallow-drawn cans were then redrawn with drawing ratios of 1.80 and 2.00. The first resin coating layer on the upper end of the inner surface of the resulting can was partially scraped off to expose the surface of the metal sheet (chrome-plated steel sheet). A sponge with a diameter of approximately 8 mm containing an electrolyte (1% by mass NaCl aqueous solution) was then placed 10 mm below the can opening in both the longitudinal direction (MD) and transverse direction (TD) of the metal sheet. A platinum electrode was contacted with the sponge to serve as the cathode, and the upper end of the can body (the exposed metal sheet portion) served as the anode. A voltage of 6.2 V was then applied to the platinum electrode and the can body, and the current value was measured after 4 seconds.

[0068] The smaller this current value, the less exposed the metal plate is, and the better the coating performance of the first resin coating layer can be. Specifically, if the current value is less than 0.1 mA, the metal plate is sufficiently covered with the first resin coating layer, so corrosion can be suppressed even when the can is used as a can for general foods such as salmon and seasonings. On the other hand, if the current value is 0.1 mA or more, the metal plate is not sufficiently covered with the first resin coating layer, so there is a concern that corrosion cannot be suppressed even when the can is used as a can for general foods such as salmon and seasonings.

[0069] [Criteria for evaluation of coverage] ◎: Current value: less than 0.01 mA ○: Current value: 0.01 mA or more and less than 0.1 mA △: Current value: 0.1 mA or more and less than 1 mA ×: Current value: 1 mA or more

[0070] [Impact Resistance After Retort Treatment (Impact Resistance)] Wax was applied to both sides of a resin-coated metal sheet, and then a 165 mm diameter disk was punched out. A shallow-drawn can was then formed using a drawing ratio of 1.50. The shallow-drawn can was then redrawn using drawing ratios of 1.80 and 2.00. The drawn can was then retorted at 130°C for 90 minutes. A dent test (1 / 4 inch indenter, 1000 g weight, drop height 100 mm) was then performed on the inner surface of the center of the can body to evaluate the impact resistance of the dent on the inner surface of the can. Specifically, a portion of the first resin coating layer on the inner surface of the upper end of the can was scraped off to expose the metal sheet surface. A sponge with a diameter of approximately 8 mm containing an electrolyte (1% by mass NaCl aqueous solution) was then placed in the dent on the inner surface of the can. A platinum electrode was contacted with the sponge to serve as the cathode, and the upper end of the can body (the exposed metal sheet portion) served as the anode. Next, a voltage of 6.2 V was applied to the platinum electrode and the can body, and the current value after 4 seconds was measured. This evaluation based on the current value was the same as the evaluation criteria for coating property described above, but the smaller the current value, the less likely a surface defect would occur even if the can body was dented due to a collision between the can and a foreign object during transportation, and it can be said that the impact resistance is excellent. Specifically, if the current value is less than 0.1 mA, surface defects are less likely to occur even if the can body is hit by a foreign object. On the other hand, if the current value is 0.1 mA or more, there is a concern that surface defects will occur due to collision with a foreign object, which may become the starting point for corrosion.

[0071] [Evaluation criteria for impact resistance] ◎: Current value: less than 0.01 mA ○: Current value: 0.01 mA or more and less than 0.1 mA △: Current value: 0.1 mA or more and less than 1 mA ×: Current value: 1 mA or more

[0072] [Content Removal Ease] The laminated resin-coated metal sheet was drawn using a drawing machine with a blank diameter of 100 mm and a drawing ratio (before-drawing diameter / after-drawing diameter) of 1.88 to obtain a cup. Next, salted luncheon meat (protein content in solids: 60% by mass) was filled into the cup, and the lid was tightened. The cup was then subjected to retort sterilization (130°C, 90 minutes). The lid was then removed, and the cup was turned upside down to remove the contents. The ease of content removal was evaluated by observing the amount of content remaining inside the cup.

[0073] [Evaluation criteria for ease of removal] ◎: The contents can be removed simply by turning the cup upside down (without shaking by hand), and when the inside of the cup is observed with the naked eye after removal, almost no deposits are visible. ○: The contents remain inside the cup when the cup is turned upside down, but the contents can be removed by vibrating the cup up and down (by shaking the cup by hand, etc.). When the inside of the cup is observed with the naked eye after removal, almost no deposits are visible. ×: The contents are difficult to remove simply by vibrating the cup up and down (by shaking the cup by hand, etc.). When the speed of the up and down vibrations is increased dramatically or the contents are forcibly removed using an implement such as a spoon, the deposits are clearly visible when the inside of the cup is observed with the naked eye.

[0074]

[0075] As shown in Table 2, the resin-coated metal sheet according to the present invention is excellent in adhesion, coating property, impact resistance after retort treatment, and ease of content removal. Therefore, the resin-coated metal sheet according to the present invention is particularly suitable as a material for containers.

[0076] The resin-coated metal sheet for containers of the present invention can be suitably used as a material for containers such as can bodies and lids for food cans and beverage cans.

Claims

1. A resin-coated metal plate having a metal plate and a first resin coating layer formed on one of the front and back surfaces of the metal plate, the first resin coating layer being made of a biaxially stretched film containing a polyester resin, The polyester resin contains 90 mol % or more of ethylene terephthalate, The first resin coating layer is 1725±5 cm by Raman spectroscopy, in which the measurement direction is set so that the laser polarization plane is perpendicular to the thickness direction of the first resin coating layer. －１ C=O peak intensity of 1615±5 cm －１ The peak intensity ratio I is the ratio of the C=C peak intensity to the １７２５ / I １６１５ is equal to or greater than 0.50 and equal to or less than 0.65, The peak intensity ratio I after the resin-coated metal sheet was subjected to a retort treatment at 130 ° C. for 90 minutes １７２５ / I １６１５ is equal to or greater than 0.55 and equal to or less than 0.75, A wax-containing layer containing a wax compound in an amount of 0.1% by mass or more and 2.0% by mass or less. Resin-coated metal sheet.

2. The resin-coated metal sheet according to claim 1 , wherein the first resin coating layer has a multi-layer structure including at least an outermost layer and an underlayer, and the outermost layer is the wax-containing layer.

3. 2. The resin-coated metal sheet according to claim 1, further comprising a second resin coating layer containing a polyester resin formed on the other surface of the metal sheet, the second resin coating layer having a multilayer structure including at least an outermost layer and an underlayer, and the layers of the second resin coating layer other than the outermost layer contain either one or both of titanium dioxide and a disazo-based organic pigment as a coloring pigment.

4. A resin-coated metal plate as described in claim 2, which has a second resin coating layer containing a polyester resin formed on the other of the front and back surfaces of the metal plate, the second resin coating layer having a multilayer structure including at least an outermost layer and a lower layer, and the layers other than the outermost layer of the second resin coating layer contain either one or both of titanium dioxide and a disazo-based organic pigment as coloring pigments.

5. The resin-coated metal sheet according to claim 1 , wherein the wax compound is carnauba wax.

6. A method for producing a resin-coated metal sheet according to any one of claims 1 to 4, laminating the biaxially stretched film onto the metal plate under the following conditions (i) to (iii) to obtain the first resin coating layer; (i) the melting point of the polyester resin is Tm (°C), and the surface temperature of the metal plate at the start of lamination is Tm°C or higher (Tm+40)°C or lower; (ii) the glass transition point of the polyester resin is Tg (°C), and the surface temperature of the laminating roll is Tg°C or higher (Tg+80)°C or lower; (iii) the contact time between the biaxially stretched film and the lamination roll is 10 milliseconds or more and 30 milliseconds or less; Next, within 2 seconds from the completion of the lamination, cooling is started until the surface temperature of the first resin coating layer is equal to or higher than (Tg-20) °C and equal to or lower than Tg °C. A method for producing a resin-coated metal sheet comprising the steps of:

7. A method for producing the resin-coated metal sheet according to claim 5, laminating the biaxially stretched film onto the metal plate under the following conditions (i) to (iii) to obtain the first resin coating layer; (i) the melting point of the polyester resin is Tm (°C), and the surface temperature of the metal plate at the start of lamination is Tm°C or higher (Tm+40)°C or lower; (ii) the glass transition point of the polyester resin is Tg (°C), and the surface temperature of the laminating roll is Tg°C or higher (Tg+80)°C or lower; (iii) the contact time between the biaxially stretched film and the lamination roll is 10 milliseconds or more and 30 milliseconds or less; Next, within 2 seconds from the completion of the lamination, cooling is started until the surface temperature of the first resin coating layer is equal to or higher than (Tg-20) °C and equal to or lower than Tg °C. A method for producing a resin-coated metal sheet comprising the steps of: