Resin-coated metal sheet and its manufacturing method

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

[Technical Field]

[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. [Background technology]

[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 not only involves a complicated baking process, but also requires a long processing time and generates 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 laminate of a resin layer (I) of a crystalline polyester resin consisting of crystalline polyethylene terephthalate / crystalline polybutylene terephthalate, a resin layer (II) of a polyester resin having a melting point of 180°C or higher 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, and a resin layer (III) of a water-dispersible copolymer polyester resin on the side of the resin layer (II) that will be bonded to the metal plate. The use of this film improves adhesion to the metal plate and impact resistance. Furthermore, because this film has a multilayer structure and contains an olefin-based polymer dispersed in a polyester resin, it has excellent adhesion and processability. However, contact with contents such as food was not taken into consideration, leaving the film with poor content removal ability as a problem.

[0007] 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 a specific ratio, with the heat shrinkage at 130°C for 15 minutes adjusted to fall within a specific range. 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 metals, impact resistance, and flavor and aroma retention. However, the addition of 80 to 40% by mass of the PBT-based polyester (I), which has a melting point in the range of 200 to 223°C, results in a rapid crystallization rate. This results in excessive crystallization of the film during the paint baking heat treatment after printing, leading to embrittlement of the film and 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. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-240985 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-221315 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-166856 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[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 sheet is 1725±5 cm by Raman spectroscopy after retort treatment. -1 C=O peak intensity of 1615±5cm -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 the gist of the present invention is as follows. [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 and 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, which is performed 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±5cm -1 to the C=C peak intensity, I 1725 / I 1615 is between 0.50 and 0.65, The peak intensity ratio I after the resin-coated metal plate was subjected to a retort treatment at 130 ° C. for 90 minutes 1725 / I 1615 is 0.55 or more and 0.75 or less, 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.

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

[0015] [3] A resin-coated metal sheet 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 sheet, 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], 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 and (Tm+40)°C or lower; (ii) the surface temperature of the laminating roll is Tg°C or higher and (Tg+80)°C or lower, where Tg is the glass transition point of the polyester resin; (iii) the contact time between the biaxially stretched film and the laminating roll is 10 milliseconds or more and 30 milliseconds or less; Next, a step of 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; A method for producing a resin-coated metal sheet having the above structure. [Effects of the Invention]

[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. DETAILED DESCRIPTION OF THE INVENTION

[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 , chromium hydroxide layer is 10-30mg / 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 containing polyethylene terephthalate as a main component. "Containing polyethylene terephthalate as a main component" 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 is 90 mol % or more, the resin coating layer is suitable for applications requiring heat resistance.

[0023] Terephthalic acid, 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 high-process applications.

[0024] Furthermore, one or both of other dicarboxylic acid components and glycol components may be copolymerized within a range that does not impair the above-mentioned properties. 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, biaxially stretched films are 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 -1 C=O peak intensity of 1615±5cm -1 to the C=C peak intensity, I 1725 / I 1615 (Hereafter, peak intensity ratio I 1725 / I 1615 It is important that the variance (also called the variance) is between 0.50 and 0.65.

[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 explained below.

[0029] The above peak intensity ratio I 1725 / I 1615 is an index of the orientation state of the PET molecular chains. This peak intensity ratio I 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 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 1615 It is also important that the ratio (also called the "ratio") is between 0.55 and 0.75. The conditions for retort treatment are that the atmospheric temperature in the retort sterilization furnace is 130°C and that 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 ratio is less than 0.55, the amorphous content increases, the interaction between molecular chains decreases, and the impact resistance decreases. 1725 / I 1615 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, making it difficult to obtain adhesion and coating properties. 1725 / I 1615 is 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 spectroscopic analyzer LabRAM HR manufactured by Horiba Ltd., with an excitation light source: semiconductor laser (λ = 532 nm), a microscope magnification: ×100, and an aperture: 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 distance of 1725 ± 5 cm -1C=O peak intensity of 1615±5cm -1 to the C=C peak intensity, I 1725 / I 1615 Here, the boundary between the metal plate and the resin coating layer is set as the starting point (0 μm), and a 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 / I 1615 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 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 have a multilayer 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 or fewer layers. An example of a multilayer structure in which each layer has a function is to incorporate wax into the outermost layer to reduce the amount of wax in the first resin coating layer as a whole and effectively control the processability of the wax. It is also possible to control the color tone of the first resin coating layer as a whole while ensuring processability by adding a larger amount of pigment to the intermediate layer in the multilayer structure. In such cases, the 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 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. To achieve both whiteness and processability as a layer, the outermost and lowermost layers preferably contain 0% to 2% titanium dioxide, and the intermediate layer preferably 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 of the container after molding and processing, the first resin coating layer preferably has a multi-layer structure including at least an outermost layer and an underlayer, and the outermost layer that comes into contact with the contents is preferably the wax-containing layer. In this case, the wax compound is preferably contained in an amount of 0.1 to 2.0% by mass relative to the total weight 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 the ease of 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 content to 2.0 mass % or less is that if the content 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 film 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 are 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 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 consisting of an outermost layer (upper layer), an intermediate layer (main layer), and a lowermost layer (lower layer). Preferably, it has three or fewer layers. By having three or fewer layers, film production costs 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 organic pigment as a coloring pigment. Although disazo organic pigments are yellow, they are transparent, allowing for 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 the colored resin). This occurs when the oil-soluble dye dissolves in the resin to be colored, causing it to rise 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, and CI Pigment Yellow 180 is particularly suitable. This pigment is registered with the FDA (Food and Drug Administration) and is therefore safe and capable of achieving excellent color tones. The amount added is preferably 0.1% to 20.0% by mass of the total layer to which the pigment is added. If the amount is less than 0.1% by mass, the color development is poor and the desired color tone is not achieved. Conversely, if the amount is more than 20.0% by mass, the transparency of the resin may be poor, resulting in a color tone lacking in brilliance. The particle size of the pigment is preferably less than 1 μm. If the particle size is 1 μm or more, the transparency of the polyester resin may be lost, which is undesirable. 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 enhance the clarity of the printed surface, resulting in a good appearance. The pigment to be added must be able to exhibit excellent design properties after the container is formed, 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 formed. The amount added is preferably 10.0% to 30.0% by mass of 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 formation 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 producing a resin-coated metal sheet according to one embodiment of the present invention will be described. The method for producing a resin-coated metal sheet includes the steps of laminating a biaxially oriented film onto a metal sheet to obtain a first resin coating layer, and starting cooling within 2 seconds of completing lamination until the surface temperature of the first resin coating layer reaches (Tg-20)°C or higher and Tg°C or lower. First, an example of a method for obtaining a biaxially oriented film to be laminated onto a metal sheet will be described, in which the biaxially oriented film has a multilayer structure. An example of the production 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 may melt excessively, potentially causing 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 sheet, thereby ensuring good adhesion to the metal sheet. On the other hand, if the surface temperature of the laminating roll exceeds (Tg + 80)°C, the biaxially oriented film may melt excessively, potentially causing adhesion of the biaxially oriented film to the laminating roll. Furthermore, it becomes difficult to control the crystalline structure (peak intensity ratio) of the first resin coating layer within the range specified in 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 thermally analyzing the biaxially stretched film or the resin coating layer of the resin-coated metal plate under the following conditions. Measurement equipment: TA Instruments DSC Q100 Measurement temperature range: -50℃~290℃ Heating rate: 10℃ / min Atmosphere: Nitrogen atmosphere, flow rate 50m / min

[0051] Furthermore, adjusting the contact time between the biaxially stretched film and the laminating rolls is also an important factor. The contact time must 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 rolls 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 also allows the crystalline structure of the first resin coating layer to undergo gradual structural changes from the interface with the metal plate to the surface layer, allowing for 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, the resin layer must be promptly cooled (quenched) to fix its crystalline structure. The cooling start time must be limited to within 2 seconds from the completion of lamination, and 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 set to (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 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. [Example]

[0056] Examples of the present invention will be described below. (Metal plate manufacturing method) The steel sheets were cold rolled, annealed, and temper rolled to a thickness of 0.22 mm and a width of 977 mm. After degreasing and pickling, they were chrome-plated to produce chrome-plated steel sheets (TFS). The chrome plating was performed using CrO3, F - , SO4 2- Electroplating is performed in a plating bath containing CrO3, F, after intermediate rinsing. - During the chemical conversion treatment, the electrolysis conditions (current density, amount of electricity, etc.) were adjusted to achieve a chromium metal deposition amount and a chromium hydroxide deposition amount of 120 mg / m2, respectively, in terms of Cr. 2 and 15 mg / m 2 It was decided.

[0057] (Method of manufacturing resin coating film) A polyester resin having the resin composition shown in Table 1-1 was prepared, and the wax in the amount shown in Table 1-1 was added. The resin was dried and melted in the usual manner, co-extruded through a T-die, and then cooled and solidified on a cooling drum to obtain an unstretched film. The unstretched film obtained 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 melting temperature Tm (° C.), and 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 films are shown in Table 1-2.

[0060] [Table 1-1]

[0061] [Table 1-2]

[0062] (Method of manufacturing 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 the 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 the 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 rolls were internally water-cooled, and the temperature during film bonding was controlled by circulating cooling water inside the rolls. 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 sheets) The properties of the resin-coated metal sheets obtained as described above were evaluated by the following methods. In addition, the peak intensity ratio I 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 chromium-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] [Coating] Cans were fabricated from the resin-coated metal sheet using the following method, and the coating properties of the first resin coating layer were evaluated. After applying wax to both sides of the resin-coated metal sheet, a 165 mm diameter disk was punched out, and a shallow-drawn can was obtained at a drawing ratio of 1.50. This shallow-drawn can was then redrawn at 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 placed in contact with this 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 between 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. Specifically, if the current value is less than 0.1 mA, the metal plate is sufficiently covered by 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 by 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] [Coverability evaluation criteria] ◎: Current value: Less than 0.01mA ○: Current value: 0.01mA or more and less than 0.1mA △: Current value: 0.1mA or more and less than 1mA ×: Current value: 1mA or more

[0070] [Impact resistance after retort processing (impact resistance)] Wax was applied to both sides of the resin-coated metal sheet, and then a disk with a diameter of 165 mm was punched out from the sheet at a drawing ratio of 1.50 to form a shallow-drawn can. The shallow-drawn can was then redrawn at 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 dented area 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 dented area on the inner surface of the can. A platinum electrode was placed in contact with the sponge to serve as the cathode, and the upper end of the can body (the exposed metal sheet area) served as the anode. A voltage of 6.2 V was then applied between the platinum electrode and the can body, and the current value was measured after 4 seconds. This evaluation based on the current value is the same as the evaluation criteria for coating property described above, but the smaller the current value, the less likely a surface defect will occur even if the can is dented by a foreign object colliding with it during transportation, etc., and the better the impact resistance can be said to be. Specifically, if the current value is less than 0.1 mA, surface defects are less likely to occur even if the can 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] [Impact resistance evaluation criteria] ◎: Current value: Less than 0.01mA ○: Current value: 0.01mA or more and less than 0.1mA △: Current value: 0.1mA or more and less than 1mA ×: Current value: 1mA or more

[0072] [Content removal] 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, the cup was filled with cured luncheon meat (protein content in the solids: 60% by mass), and the lid was tightened. The cup was then subjected to retort sterilization (130°C, 90 minutes). The lid was then removed, the cup was turned upside down, and the contents were removed. The ease of removing the contents was evaluated by observing the amount of contents 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 residue can be seen. ○: When the cup is turned upside down, the contents remain inside the cup, but when the cup is shaken up and down (such as by shaking the cup with your hand), the contents can be removed. When the inside of the cup is observed with the naked eye after removal, almost no residue can be seen. ×: It is difficult to remove the contents by simply vibrating the cup up and down (such as by shaking the cup with your hand). After the speed of the up and down vibration is increased significantly or the contents are forcibly removed using an implement such as a spoon, the contents are clearly visible when the inside of the cup is examined with the naked eye.

[0074] [Table 2]

[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. [Industrial Applicability]

[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, which is performed 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 is 0.50 or more and 0.65 or less, The peak intensity ratio I after the resin-coated metal plate 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 underlying layer, and the outermost layer is the wax-containing layer.

3. 2. The resin-coated metal plate according to claim 1, further comprising 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 underlayer, and 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.

4. 3. The resin-coated metal plate according to claim 2, further comprising 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 underlayer, and 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.

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 surface temperature of the laminating roll is Tg°C or higher (Tg+80)°C or lower, where Tg (°C) is the glass transition point of the polyester resin; (iii) the contact time between the biaxially stretched film and the laminating roll is 10 milliseconds or more and 30 milliseconds or less; Next, a step of starting cooling within 2 seconds from the completion of the lamination and continuing cooling until the surface temperature of the first resin coating layer reaches a cooling stop temperature of (Tg-20)°C or higher and Tg°C or lower; A method for producing a resin-coated metal sheet having the above structure.

7. The method for producing a 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 surface temperature of the laminating roll is Tg°C or higher (Tg+80)°C or lower, where Tg (°C) is the glass transition point of the polyester resin; (iii) the contact time between the biaxially stretched film and the laminating roll is 10 milliseconds or more and 30 milliseconds or less; Next, a step of starting cooling within 2 seconds from the completion of the lamination and continuing cooling until the surface temperature of the first resin coating layer reaches a cooling stop temperature of (Tg-20)°C or higher and Tg°C or lower; A method for producing a resin-coated metal sheet having the above structure.

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