Biaxially stretched polyester film for laminating to metal sheet, laminate, and container

Abstract

A purpose of the present invention is to provide a biaxially stretched polyester film for laminating to a metal sheet, which can be prevented or inhibited from peeling from the metal sheet during forming and which can prevent or inhibit the metal sheet from suffering corrosion that can be caused by retorting. Another purpose is to provide a laminate and a container. The biaxially stretched polyester film for laminating to a metal sheet comprises an A layer including poly(ethylene terephthalate) and a B layer including a polyester which is substantially amorphous. T1 (i.e., the melting point of the polyester which is substantially amorphous) is 180-215°C. The difference between T1 and T2 (i.e., the melting sub-peak temperature of a differential scanning calorimetry curve of the biaxially stretched polyester film for laminating to a metal sheet) is 15-40°C. The laminate comprises a metal sheet and the biaxially stretched polyester film for laminating to a metal sheet. The container includes the laminate.

Description

Biaxially oriented polyester film for laminating metal sheets, laminate, and container

[0001] The present invention relates to a biaxially oriented polyester film for laminating metal sheets, a laminate, and a container.

[0002] Metal cans are generally coated with a coating to prevent corrosion on the interior and exterior surfaces. In recent years, coating with a thermoplastic resin film, such as a polyester film, has been used to provide rust prevention without using organic solvents, with the aim of simplifying processes, improving hygiene, and preventing pollution. Specifically, a thermoplastic resin film is laminated onto a metal plate, such as tinplate, tin-free steel, or aluminum, and then the can is molded into food cans, beverage cans, aerosol cans, and other cans, which require rigorous molding processes, such as drawn cans and thin-walled drawn cans. To reduce costs, cans used for these applications (i.e., food cans, beverage cans, aerosol cans, etc.) are increasingly manufactured using thin-walled drawn and ironed processes, which are more stringent than ever before. Additionally, the contents of cans have become more diverse, including those highly corrosive to metals and those requiring sterilization at high temperatures. This has created a need for stronger adhesion between the metal plate and the thermoplastic resin film.

[0003] Methods for bonding a polyester film to a metal plate include, in addition to a method using an adhesive, a method of imparting thermal adhesiveness to a polyester film and then directly bonding it to a metal plate, and a method of imparting thermal adhesiveness to a polyester film by forming the polyester film into a laminate structure of a polyester base layer and a polyester layer having thermal adhesiveness (hereinafter sometimes referred to as a "thermal adhesive layer").

[0004] Among these, polyester films with a laminated structure, i.e., polyester films including a thermal adhesive layer, are excellent in that they can easily provide thermal adhesiveness, film strength, and dimensional stability (see, for example, Patent Documents 1 and 2). However, when metal plates that are difficult to thermally bond, such as tinplate or aluminum, are used, there is a problem that sufficient adhesiveness cannot be obtained.

[0005] On the other hand, Patent Document 3 describes a laminated polyester film that can be thermally bonded at low temperatures. However, this polyester film is a polyester film for laminating three-piece metal cans that is not subjected to forming processes such as drawn cans or thin-walled drawn cans, and when this polyester film is laminated to a metal plate at low temperature to produce a laminated metal plate, there is a problem in that the polyester film peels off during forming.

[0006] Furthermore, Patent Document 4 describes a polyester film that provides sufficient adhesiveness through low-temperature lamination and exhibits excellent molding processability when molded after lamination.

[0007] Japanese Patent Laid-Open No. 6-340047 Japanese Patent Laid-Open No. 7-101015 Japanese Patent Laid-Open No. 2003-80654 Japanese Patent Laid-Open No. 2015-174382

[0008] The present invention aims to provide a biaxially oriented polyester film for laminating to metal sheets, which can prevent or reduce peeling of the biaxially oriented polyester film from the metal sheet during molding and can also prevent or reduce corrosion of the metal sheet that may occur during retort treatment.Another object is to provide a laminate and a container.

[0009] The present invention has the following configuration [1]: [1] A biaxially oriented polyester film for metal sheet bonding, comprising: an A layer containing polyethylene terephthalate; and a B layer containing a substantially amorphous polyester, wherein T1 is 180°C or higher and 215°C or lower, T1 is the melting point of the polyester in the B layer, the difference between T1 and T2 is 15°C or higher and 40°C or lower, and T2 is the melting sub-peak temperature in a differential scanning calorimetry curve of the biaxially oriented polyester film for metal sheet bonding.

[0010] The present invention preferably includes the following configuration [2] or later. [2] The biaxially stretched polyester film for metal sheet bonding according to [1], wherein the plane orientation coefficient of Layer B is greater than 0.05 and not greater than 0.10. [3] The biaxially stretched polyester film for metal sheet bonding according to [1] or [2], wherein the polyester of Layer B contains a polycarboxylic acid component, the polycarboxylic acid component contains a terephthalic acid component, and the content of the terephthalic acid component in 100 mol % of the polycarboxylic acid component is 60 mol % or more, and the polyester of Layer B contains a polyol component, the polyol component contains an ethylene glycol component, and the content of the ethylene glycol component in 100 mol % of the polyol component is 90 mol % or more. [4] The biaxially stretched polyester film for metal sheet bonding according to any of [1] to [3], wherein the polyethylene terephthalate of Layer A is copolymerized polyethylene terephthalate. [5] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to [4], which is a biaxially oriented polyester film for bonding to a metal sheet for forming. [6] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to [5], wherein the polyethylene terephthalate in Layer A is at least one copolymerized polyethylene terephthalate selected from the group consisting of isophthalic acid-copolymerized polyethylene terephthalate, 1,4-cyclohexanedimethanol-copolymerized polyethylene terephthalate, neopentyl glycol-copolymerized polyethylene terephthalate, 2,6-naphthalenedicarboxylic acid-copolymerized polyethylene terephthalate, and diethylene glycol-copolymerized polyethylene terephthalate. [7] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to [6], wherein the polyethylene terephthalate in Layer A is at least one copolymerized polyethylene terephthalate selected from the group consisting of isophthalic acid-copolymerized polyethylene terephthalate, 1,4-cyclohexanedimethanol-copolymerized polyethylene terephthalate, and neopentyl glycol-copolymerized polyethylene terephthalate.[8] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to [7], wherein the polyethylene terephthalate in Layer A is polyethylene terephthalate copolymerized with isophthalic acid. [9] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to [8], wherein the polyethylene terephthalate in Layer A is crystalline polyethylene terephthalate.

[10] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to [9], wherein the polyethylene terephthalate in Layer A has a terephthalic acid component content of 82 mol% or more or 85 mol% or more relative to 100 mol% of polycarboxylic acid components.

[11] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[10] , wherein the polycarboxylic acid component in the polyethylene terephthalate in Layer A is a dicarboxylic acid component.

[12] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[11] , wherein the polyethylene terephthalate in Layer A has an ethylene glycol content of 90 mol% or more or 95 mol% or more relative to 100 mol% of the polyol component.

[13] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[12] , wherein the polyol component of the polyethylene terephthalate in Layer A is a diol component.

[14] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[13] , wherein the polyethylene terephthalate content in Layer A is 85 mass% or more or 90 mass% or more.

[15] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[14] , wherein the polyethylene terephthalate content in Layer A is 95 mass% or more or 98 mass% or more.

[16] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[15] , wherein the polyester in the layer B is copolymerized polyethylene terephthalate.

[17] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[16] , wherein the polyester in the layer B has a terephthalic acid content of 80 mol% or less or 78 mol% or less based on 100 mol% of polycarboxylic acid components.

[18] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[17] , wherein the polycarboxylic acid component of the polyester in the B layer is a dicarboxylic acid component.

[19] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[18] , wherein the content of an ethylene glycol component in 100 mol% of a polyol component in the polyester in the B layer is 90 mol% or more or 95 mol% or more.

[20] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[19] , wherein the polyol component of the polyester in the B layer is a diol component.

[21] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[20] , wherein the content of the polyester in the B layer is 85 mass% or more or 90 mass% or more.

[22] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[21] , wherein the content of the polyester in the B layer is 95 mass% or more or 98 mass% or more.

[23] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[22] , wherein the difference between T1 and T2 is 16°C or more, or 20°C or more.

[24] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[23] , wherein the difference between T1 and T2 is 38°C or less, or 36°C or less.

[25] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[24] , wherein the crystalline melting peak temperature in the differential scanning calorimetry curve of the biaxially oriented polyester film for metal sheet bonding is 215°C or more, or 220°C or more.

[26] The biaxially oriented polyester film for metal sheet bonding according to any one of [1] to

[25] , wherein the crystalline melting peak temperature in the differential scanning calorimetry curve of the biaxially oriented polyester film for metal sheet bonding is 260°C or less, or 255°C or less.

[27] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[26] , wherein the crystalline melting peak temperature in the differential scanning calorimetry curve of the biaxially stretched polyester film for metal sheet bonding is 240°C or less, or 235°C or less.

[28] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[27] , wherein the difference between the crystalline melting peak temperature and the melting sub-peak temperature (i.e., the crystalline melting peak temperature - the melting sub-peak temperature) is 35°C or higher, or 40°C or higher.

[29] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[28] , wherein the difference between the crystalline melting peak temperature and the melting sub-peak temperature is 90°C or lower, or 80°C or lower.

[30] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[29] , wherein the difference between the crystalline melting peak temperature and the melting sub-peak temperature is 70°C or lower, or 60°C or lower.

[31] The biaxially stretched polyester film for metal sheet bonding according to any one of [1] to

[30] , wherein the crystalline melting peak temperature is derived from the polyethylene terephthalate of Layer A.

[32] The biaxially oriented polyester film for metal sheet lamination according to any one of [1] to

[31] , wherein the melting subpeak temperature is derived from the polyethylene terephthalate of Layer A.

[33] A laminate comprising a metal sheet and the biaxially oriented polyester film for metal sheet lamination according to any one of [1] to

[32] laminated to the metal sheet.

[34] The laminate according to

[33] , wherein the metal sheet is a tinplate, a tin-free steel sheet, or an aluminum sheet.

[35] The laminate according to

[33] or

[34] , wherein the metal sheet is a tinplate.

[36] A container comprising the laminate according to any one of

[33] to

[35] .

[0011] According to the present invention, it is possible to provide a biaxially oriented polyester film for laminating to metal sheets, which can suppress or reduce peeling of the biaxially oriented polyester film from the metal sheet during molding and can also suppress or reduce corrosion of the metal sheet that may occur during retort treatment. According to the present invention, it is also possible to provide a laminate and a container.

[0012] 1 is an example of a differential scanning calorimetry curve of a polyester film according to the present embodiment. In this example, the crystalline melting peak temperature (Tp) of the differential scanning calorimetry curve is 255°C, and the melting sub-peak temperature (T2) is 194°C.

[0013] <Introduction> Hereinafter, an embodiment of the present invention will be described in detail.

[0014] The biaxially oriented polyester film for metal sheet bonding of this embodiment (hereinafter may be referred to as "polyester film") comprises: Layer A containing polyethylene terephthalate; and Layer B containing a substantially amorphous polyester (hereinafter may be referred to as "amorphous polyester"), wherein T1 is 180°C or higher and 215°C or lower, T1 is the melting point of the polyester in Layer B, the difference between T1 and T2 is 15°C or higher and 40°C or lower, and T2 is the melting sub-peak temperature in a differential scanning calorimetry curve (hereinafter may be referred to as "DSC curve") of the biaxially oriented polyester film for metal sheet bonding of this embodiment.

[0015] Because Layer B contains an amorphous polyester, the polyester film of this embodiment can suppress or reduce corrosion of a metal plate that may occur during retort treatment. This will be explained below. The polyester film of this embodiment can be attached to a metal plate, i.e., laminated. A metal plate laminated with the polyester film of this embodiment, i.e., a laminate (hereinafter sometimes referred to as a "laminated metal plate"), can be formed into, for example, a container (e.g., a can) or a component constituting a container (e.g., a can lid, can body, or can bottom). If Layer B contains a crystalline polyester instead of an amorphous polyester, retort treatment of the container actively crystallizes the crystalline polyester, reducing the volume of Layer B. As a result, a space is created between Layer B and the metal plate. In contrast, according to this embodiment, because Layer B contains an amorphous polyester, active crystallization of Layer B during retort treatment can be avoided, and therefore, reduction in the volume of Layer B during retort treatment can be avoided or reduced. This can prevent or mitigate the generation of a space between Layer B and the metal plate due to a decrease in the volume of Layer B. As a result, the intrusion of water (e.g., water from the contents of the container) into the interface between Layer B and the metal plate can be suppressed or reduced. Therefore, the polyester film of this embodiment can suppress or reduce corrosion of the metal plate that can occur during retort treatment.

[0016] Furthermore, since the difference between T1 and T2, i.e., T1-T2, is 15°C or more, the polyester film of this embodiment can further suppress or reduce corrosion of metal plates that may occur during retort treatment. This will be explained below. T2, i.e., the melting subpeak temperature in the differential scanning calorimetry curve of the polyester film of this embodiment, can be regarded as an indicator of the amount of heat the polyester film received during the production process, particularly the amount of heat received after biaxial stretching. This is because the greater the amount of heat received after biaxial stretching (e.g., the amount of heat received by heat setting), the higher the melting subpeak temperature. Since the difference between T1 and T2 is 15°C or more, i.e., T2 is not excessively high, the amount of heat the polyester film of this embodiment received during the production process, particularly the amount of heat received after biaxial stretching, is not excessively large. Therefore, during the production process of the polyester film of this embodiment, the molecular chains of the polyester (e.g., amorphous polyester) in Layer B are prevented from changing from a planar orientation to a completely random orientation. If the polyester molecular chains in Layer B were completely randomly oriented, retorting the container would generate spherulites larger than oriented crystals, resulting in excessively large density variations (i.e., density variations) within Layer B and making Layer B brittle. In contrast, in this embodiment, the polyester molecular chains in Layer B are not completely randomly oriented but are somewhat planar oriented, reducing the generation of spherulites during retort processing and thus reducing density variations within Layer B. This avoids or reduces excessive brittleness of Layer B. As a result, water penetration into the interface between Layer B and the metal plate can be further suppressed or reduced. Therefore, the polyester film of this embodiment can further suppress or reduce corrosion of the metal plate that may occur during retort processing.

[0017] Since the difference between T1 and T2, i.e., T1-T2, is 40°C or less, peeling of the polyester film from the metal plate during molding can be suppressed or reduced. This will be explained below. Since the difference between T1 and T2 is 40°C or less, i.e., T2 is not excessively low, the amount of heat received by the polyester film of this embodiment during the production process, particularly the amount of heat received after biaxial stretching, is not excessively small. Therefore, during the production process of the polyester film of this embodiment, the degree of planar orientation of the molecular chains of the polyester (e.g., amorphous polyester) in Layer B is alleviated to some extent. Therefore, the degree of thermal shrinkage of Layer B during molding of a container or a component constituting the container, i.e., during molding, can be reduced. This makes it possible to avoid or reduce an excessive decrease in the interfacial adhesive strength between Layer B and the metal plate. Therefore, peeling of the polyester film from the metal plate during molding can be suppressed or reduced.

[0018] The structure, physical properties, etc. of the polyester film of this embodiment will be described in detail below.

[0019] <Biaxially oriented polyester film for metal sheet bonding> The polyester film of this embodiment includes a layer A and a layer B. In the polyester film of this embodiment, the layer B may be provided on the layer A. That is, the layer B may be laminated on the layer A.

[0020] The layer A can form one of the two surfaces of the polyester film of this embodiment, and the layer B can form the other surface of the polyester film of this embodiment.

[0021] <Layer A> Layer A contains polyethylene terephthalate. Examples of polyethylene terephthalate include homopolyethylene terephthalate and copolymer polyethylene terephthalate. Homopolyethylene terephthalate may contain a diethylene glycol component that may be by-produced during its production. Among these, copolymer polyethylene terephthalate is preferred because it allows the production of cans or can bodies with a high degree of formability (i.e., drawing ratio).

[0022] The copolymerization component of copolymerized polyethylene terephthalate may be an acid component or an alcohol component. Examples of the acid component include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, and sebacic acid; and alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid. Examples of the alcohol component include aliphatic diols such as 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, and triethylene glycol; and alicyclic diols such as 1,4-cyclohexanedimethanol. These may be used alone or in combination. Among these, isophthalic acid, 2,6-naphthalenedicarboxylic acid, diethylene glycol, 1,4-cyclohexanedimethanol, and neopentyl glycol are preferred, with isophthalic acid and 1,4-cyclohexanedimethanol being particularly preferred.

[0023] The polyethylene terephthalate is preferably crystalline polyethylene terephthalate. Here, crystalline polyethylene terephthalate means polyethylene terephthalate in which a crystalline melting peak is observed in differential scanning calorimetry (i.e., DSC).

[0024] In polyethylene terephthalate, the content of the terephthalic acid component in 100 mol % of the polycarboxylic acid component is preferably 82 mol % or more, and more preferably 85 mol % or more. The polycarboxylic acid component is preferably a dicarboxylic acid component.

[0025] In polyethylene terephthalate, the content of the ethylene glycol component in 100 mol % of the polyol component is more preferably 90 mol % or more, and even more preferably 95 mol % or more. Note that the polyol component is preferably a diol component.

[0026] The content of polyethylene terephthalate in Layer A is preferably 85% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and still more preferably 98% by mass or more. Thus, the polyester constituting Layer A can have polyethylene terephthalate as a main constituent component.

[0027] Layer A may contain other resins in addition to polyethylene terephthalate, such as polyesters such as polybutylene terephthalate, polyethylene-2,6-naphthalate, and poly-1,4-cyclohexanedimethylene terephthalate, polycarbonate, polyethylene, polypropylene, polyamide, polyvinyl chloride, ionomer, and silicone resin.

[0028] Layer A preferably contains a lubricant. The lubricant may be inorganic or organic, but inorganic lubricants are preferred. Examples of inorganic lubricants include silica, alumina, titanium dioxide, calcium carbonate, and barium sulfate, while examples of organic lubricants include silicone resin particles and cross-linked polystyrene particles. In particular, lubricants preferred in terms of pinhole resistance are monodisperse lubricants with a particle size ratio (major axis / minor axis) of 1.0 to 1.2. Examples of such lubricants include spherical silica, spherical silicone resin particles, and spherical cross-linked polystyrene particles. When silica is used as a lubricant, it is preferably added in a range of 0.01% to 0.3% by weight for silica with an average particle size of 1.5 μm, and 0.05% to 0.5% by weight for silica with an average particle size of 0.8 μm.

[0029] Layer A may contain other additives, such as antioxidants, dispersants, viscosity modifiers, fluorescent brighteners, heat stabilizers, ultraviolet absorbers, and antistatic agents.

[0030] <Layer B> Layer B contains a polyester that is substantially amorphous, i.e., an amorphous polyester. As described above, because Layer B contains an amorphous polyester, corrosion of the metal plate that may occur during retort treatment can be suppressed or reduced. Here, "amorphous polyester" refers to a polyester for which no crystalline melting peak is observed in differential scanning calorimetry (i.e., DSC). Specifically, "amorphous polyester" refers to a polyester for which no crystalline melting peak is observed in differential scanning calorimetry (i.e., DSC) in which a sample with a mass of approximately 10 mg is heated from 20°C at a rate of 20°C / min according to the method described in JIS-K-7121-1987.

[0031] The amorphous polyester contains a polycarboxylic acid component and a polyol component. That is, the amorphous polyester contains a polycarboxylic acid unit and a polyol unit. Examples of the polycarboxylic acid component include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, terephthalic acid, and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, and sebacic acid; and alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid. Examples of the polyol component include aliphatic diols such as ethylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, and triethylene glycol; and alicyclic diols such as 1,4-cyclohexanedimethanol. The polycarboxylic acid component is preferably a dicarboxylic acid component. That is, the amorphous polyester preferably contains a dicarboxylic acid unit. The polyol component is preferably a diol component. That is, the amorphous polyester preferably contains a diol unit.

[0032] As the amorphous polyester, copolymerized polyethylene terephthalate, specifically copolymerized polyethylene terephthalate that is substantially amorphous (hereinafter, sometimes referred to as "amorphous copolymerized polyethylene terephthalate") is preferred.

[0033] In the amorphous copolymerized polyethylene terephthalate, the content of the terephthalic acid component in 100 mol% of the polycarboxylic acid component is preferably 60 mol% or more, more preferably 65 mol% or more. If the content is 60 mol% or more, peeling of the polyester film, which may occur when a container obtained by molding the laminated metal sheet is used at high temperatures, can be suppressed or reduced. In the amorphous copolymerized polyethylene terephthalate, the content of the terephthalic acid component in 100 mol% of the polycarboxylic acid component may be, for example, 80 mol% or less, or may be 78 mol% or less.

[0034] In the amorphous copolymerized polyethylene terephthalate, the content of the ethylene glycol component in 100 mol % of the polyol component is preferably 60 mol % or more, more preferably 90 mol % or more, and even more preferably 95 mol % or more.

[0035] The copolymerization component of the amorphous copolymerized polyethylene terephthalate may be an acid component or an alcohol component. Examples of the acid component include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, terephthalic acid, and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, and sebacic acid; and alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid. Examples of the alcohol component include aliphatic diols such as 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, and triethylene glycol; and alicyclic diols such as 1,4-cyclohexanedimethanol. These may be used alone or in combination. Among these, isophthalic acid, 2,6-naphthalenedicarboxylic acid, diethylene glycol, 1,4-cyclohexanedimethanol, and neopentyl glycol are preferred, with isophthalic acid and 1,4-cyclohexanedimethanol being particularly preferred. The amount of copolymerization component required for polyethylene terephthalate to become amorphous varies depending on the copolymerization component. When the copolymerization component is isophthalic acid, it is preferable that the isophthalic acid be copolymerized at 22 mol% or more of the polycarboxylic acid component. That is, in amorphous copolymerized polyethylene terephthalate, the content of the isophthalic acid component per 100 mol% of the polycarboxylic acid component is preferably 22 mol% or more. The copolymerization component of copolymerized polyethylene terephthalate is preferably added during the polymerization of polyethylene terephthalate to form a sufficiently random copolymerized polymer. There is a method of obtaining a polyester of the desired composition by kneading two or more polyesters polymerized separately during melting, but it is preferable to confirm that the transesterification reaction has progressed sufficiently to make the resin amorphous before using it.

[0036] The melting point (T1) of the amorphous polyester is 180°C or higher and 215°C or lower. Because the melting point of amorphous polyester cannot be measured by DSC, it is measured in accordance with JIS-K0064:1992, "Method for Determining Melting Point and Melting Range of Chemical Products." Here, the melting point is the temperature at which the sample melts in a capillary tube and no longer appears solid, as determined by the "visual inspection method" described in 3.1 of the standard. When T1 is 215°C or lower, sufficient adhesive strength is easily obtained regardless of the type of metal plate. Therefore, peeling of the polyester film from the metal plate during molding can be further suppressed or reduced. On the other hand, when T1 is 180°C or higher, heat resistance is improved, and peeling of the polyester film, which can occur when a container obtained by molding a laminated metal plate is used at high temperatures, can be suppressed or reduced.

[0037] The content of the amorphous polyester in Layer B is preferably 85% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 98% by mass or more.

[0038] Layer B preferably contains a lubricant. The lubricant may be inorganic or organic, but inorganic lubricants are preferred. Examples of inorganic lubricants include silica, alumina, titanium dioxide, calcium carbonate, and barium sulfate, while examples of organic lubricants include silicone resin particles and cross-linked polystyrene particles. Layer B is composed of amorphous polyester, which makes it difficult to form protrusions during biaxial stretching, so it is preferable to add a larger amount of lubricant. For example, when using silica, it is preferable to add silica with an average particle size of 2 μm or more in a range of 0.01% to 0.3% by mass.

[0039] Layer B may contain other additives, such as antioxidants, dispersants, viscosity modifiers, fluorescent brighteners, heat stabilizers, ultraviolet absorbers, and antistatic agents.

[0040] <Physical Properties, Thickness, etc. of Biaxially Stretched Polyester Film for Metal Sheet Lamination> (Plane Orientation Coefficient) In the polyester film of this embodiment, the plane orientation coefficient (i.e., Ns) of Layer B, specifically, the plane orientation coefficient (Ns) calculated from the refractive index in the machine direction (hereinafter sometimes referred to as the longitudinal direction, film-forming direction, or MD direction) of the polyester film, the refractive index in the transverse direction (hereinafter sometimes referred to as the width direction or TD direction), and the refractive index in the thickness direction (hereinafter sometimes referred to as the thickness direction or z direction perpendicular to the film surface) of the polyester film, is preferably greater than 0.05 and not greater than 0.10. Here, the plane orientation coefficient is expressed as a value obtained by calculating the refractive index of each directional component of the polyester film measured by the Abbe method according to the following formula: Ns = (nMD + nTD) / 2 - nZ (where nMD is the refractive index in the machine direction of the polyester film, nTD is the refractive index in the TD direction of the polyester film, and nZ is the refractive index in the thickness direction perpendicular to the polyester film surface).

[0041] When the planar orientation coefficient (Ns) exceeds 0.05, the molecular chains of the polyester (e.g., amorphous polyester) in layer B are not oriented in a completely random direction, but are oriented to a certain degree in a planar orientation, thereby further reducing the formation of spherulites during retort treatment and thus further preventing or reducing excessive brittleness of layer B. As a result, water penetration into the interface between layer B and the metal plate can be further suppressed or reduced. Therefore, corrosion of the metal plate can be further suppressed or reduced. On the other hand, when the planar orientation coefficient is 0.10 or less, the degree of planar orientation of the molecular chains of the polyester (e.g., amorphous polyester) in layer B is not excessively high, thereby further reducing the degree of thermal shrinkage of layer B when forming a container with the laminated metal plate. Therefore, excessive reduction in the interfacial adhesive strength between layer B and the metal plate can be further prevented or reduced. Therefore, peeling of the polyester film from the metal plate during forming can be further suppressed or reduced.

[0042] As shown in Figure 1, the differential scanning calorimetry curve (i.e., DSC curve) obtained when differential scanning calorimetry is performed on the polyester film of this embodiment as a sample contains at least a crystalline melting peak, i.e., a main melting peak, and a melting sub-peak. The crystalline melting peak is the endothermic peak with the greatest peak height. On the other hand, the melting sub-peak is an endothermic peak at a lower temperature than the crystalline melting peak. The crystalline melting peak can be an endothermic peak derived from the polyethylene terephthalate of Layer A. The melting sub-peak can also be an endothermic peak derived from the polyethylene terephthalate of Layer A. The temperature at the apex of the crystalline melting peak is the crystalline melting peak temperature (Tp). The temperature at the apex of the melting sub-peak is the melting sub-peak temperature (T2).

[0043] The difference between the melting point (T1) of the amorphous polyester in Layer B and the melting sub-peak temperature (T2) of the DSC curve of the polyester film of this embodiment is 15°C or more and 40°C or less. That is, T1-T2 is 15°C or more and 40°C or less. As described above, since T1-T2 is 15°C or more, corrosion of the metal plate that may occur during retort treatment can be further suppressed or reduced. Since T1-T2 is 15°C or more, breakage that may occur during film formation can also be suppressed or reduced. As described above, since T1-T2 is 40°C or less, peeling of the polyester film from the metal plate during molding processing can be suppressed or reduced. T1-T2 may be, for example, 16°C or more, or 20°C or more. T1-T2 may be, for example, 38°C or less, or 36°C or less.

[0044] The crystalline melting peak temperature (Tp) of the DSC curve of the polyester film of this embodiment is preferably 215°C or higher, more preferably 220°C or higher. On the other hand, Tp is preferably 260°C or lower, more preferably 255°C or lower. The crystalline melting peak temperature (Tp) may be 240°C or lower, or may be 235°C or lower. The crystalline melting peak temperature (Tp) of the DSC curve of the polyester film of this embodiment can be appropriately set. For example, when the polyester film of this embodiment is used to form a can body, the crystalline melting peak temperature (Tp) is preferably 240°C or lower, more preferably 235°C or lower. On the other hand, when the polyester film of this embodiment is used to form a can end, the crystalline melting peak temperature (Tp) is preferably 245°C or higher, more preferably 250°C or higher.

[0045] Regarding the DSC curve of the polyester film of this embodiment, the difference between the crystalline melting peak temperature (Tp) and the melting sub-peak temperature (T2), i.e., Tp - T2, can also be regarded as an index of the amount of heat the polyester film received during the production process, particularly the amount of heat received after biaxial stretching. This is because the greater the amount of heat received after biaxial stretching (for example, the amount of heat received by heat setting), the closer T2 is to Tp.

[0046] Tp - T2 is preferably 35°C or higher, more preferably 40°C or higher. At 35°C or higher, corrosion of the metal plate that may occur during retort treatment can be further suppressed or reduced. This will be explained below. When Tp - T2 is 35°C or higher, T2 is not excessively high, and therefore the amount of heat the polyester film of this embodiment receives during the production process, particularly the amount of heat it receives after biaxial stretching, is not excessively large. Therefore, during the production process of the polyester film of this embodiment, the molecular chains of the polyester (e.g., amorphous polyester) in Layer B are prevented from changing from a planar orientation state to a completely random orientation state. In other words, the molecular chains of the polyester in Layer B are not completely randomly oriented, but are somewhat planarly oriented. This reduces the formation of spherulites during retort treatment, thereby preventing or reducing excessive embrittlement of Layer B. As a result, water penetration into the interface between Layer B and the metal plate can be further suppressed or reduced. Therefore, corrosion of the metal plate that may occur during retort treatment can be further suppressed or reduced.

[0047] Tp - T2 is preferably 90°C or less, more preferably 80°C or less. At 90°C or less, peeling of the polyester film from the metal plate during molding can be suppressed or reduced. This will be explained below. Since Tp - T2 is 90°C or less, that is, T2 is not excessively low, the amount of heat received by the polyester film of this embodiment during the production process, particularly the amount of heat received after biaxial stretching, is not excessively small. Therefore, during the production process of the polyester film of this embodiment, the degree of planar orientation of the molecular chains of the polyester (e.g., amorphous polyester) in Layer B is alleviated to some extent. Therefore, the degree of thermal shrinkage of Layer B during molding of a container or a component constituting a container, i.e., during molding, can be reduced. Therefore, excessive reduction in the interfacial adhesive strength between Layer B and the metal plate can be avoided or reduced. Therefore, peeling of the polyester film from the metal plate during molding can be suppressed or reduced. Tp - T2 may be 70°C or less, or may be 60°C or less.

[0048] The thickness of the polyester film of this embodiment can be appropriately changed as needed, but is preferably in the range of 6 μm to 75 μm, more preferably 10 μm to 75 μm, and particularly preferably 15 μm to 50 μm. A thickness of 6 μm or more makes it difficult for breakage to occur when forming a container from the laminated metal sheet, while a thickness of 75 μm or less is economical.

[0049] The thickness of layer B relative to the thickness of layer A (i.e., XB / XA: where XA represents the thickness of layer A and XB represents the thickness of layer B) is preferably in the range of 5 / 1 to 1 / 10 from the viewpoints of film-forming property, thermal adhesiveness, and rust prevention. That is, the thickness of layer B is preferably 0.1 to 5 times the thickness of layer A. The thickness of layer B may be 1 time or less, 0.5 times or less, or 0.3 times or less the thickness of layer A.

[0050] <Method for Manufacturing Biaxially Stretched Polyester Film for Metal Sheet Lamination> The method for manufacturing the polyester film of this embodiment is not particularly limited. An unstretched laminated sheet may be first prepared by a conventional film-forming method, and then bidirectionally stretched. For example, the polyester prepared for Layer A is thoroughly dried and then melted in an extruder at a temperature equal to or higher than the crystalline melting peak temperature (Tp) in DSC, typically in the range of Tp + 30°C to Tp + 70°C. At the same time, the amorphous polyester prepared for Layer B is thoroughly dried and then fed into the extruder at a temperature equal to or higher than the melting point (T1) of the amorphous polyester, typically in the range of T1 + 30°C to T1 + 70°C. The polyester for Layer B is preferably dried under reduced pressure at a temperature equal to or lower than the glass transition temperature (Tg). This prevents the polyester for Layer B from fusing. Drying is not necessary when using a vented extruder.

[0051] Subsequently, it is preferable to produce a laminated unstretched laminate sheet by a method of laminating both molten resins inside a die, for example, by a co-lamination extrusion method using a multi-manifold die. By such a co-lamination extrusion method, a melt of a resin forming one layer and a melt of a resin forming another layer are laminated inside the die, and can be extruded from the die into a sheet while maintaining the laminated configuration.

[0052] The unstretched laminate sheet can then be sequentially or simultaneously biaxially stretched and heat-set. When forming a film by sequential biaxial stretching, the unstretched laminate sheet is first stretched in the longitudinal direction by heating with a roll or infrared heating, and then stretched transversely using a stenter. At this time, it is preferable that the stretching temperature is 20°C to 50°C higher than the Tg of the polyester in Layer A, and that the longitudinal stretching ratio is 2.5 to 3.6 times and the transverse stretching ratio is 2.6 to 3.9 times. The heat-setting temperature is preferably determined depending on the melting point (T1) of the amorphous polyester in Layer B.

[0053] The metal sheet to which the polyester film of this embodiment is laminated is preferably a metal sheet for can manufacturing. Examples of suitable metal sheets for can manufacturing include tinplate, tin-free steel, and aluminum. The lamination temperature is preferably a temperature at which both the adhesiveness of the polyester film of this embodiment and its formability into a container are compatible. A metal sheet laminated with the polyester film of this embodiment (i.e., a laminated metal sheet) can be formed into a container by a known forming method. For example, a container with an integrated can body and bottom (a two-piece can) can be formed by drawing, ironing, or a combination thereof. The laminated metal sheet can also be used to produce a three-piece can body, which is made by rolling and joining flat sheets, and a lid material (i.e., a can lid).

[0054] The polyester film of this embodiment may be used on the inner surface of a metal container to protect the metal from the contents, or on the outer surface of the container to protect the metal from the external environment. The temperature at which the polyester film of this embodiment is bonded to the metal plate, i.e., the lamination temperature, may be, for example, T1 or higher and T1 + 30°C or lower. Note that when the lamination temperature is near the melting point (T1) of the amorphous polyester of Layer B, specifically, T1 or higher and T1 + 20°C or lower, whitening spots during retort treatment can be suppressed.

[0055] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. Each property value was measured by the following method.

[0056] (1) Amount of polyester component １The copolymerization components of the polyester and the amount of each component were identified by H-NMR measurement.

[0057] (2) Determining the amorphousness of polyesters Approximately 10 mg of polyester was used as a sample and sealed in an aluminum pan for measurement, and then attached to a differential scanning calorimeter (TA Instruments, DSCQ100), and the temperature was raised from 20°C to 300°C at a rate of 20°C / min. If no crystalline melting peak (endothermic peak) was observed in the curve obtained by this differential scanning calorimetry, i.e., the DSC curve, the sample was determined to be amorphous. On the other hand, if a crystalline melting peak was observed, the sample was determined to be crystalline.

[0058] (3) Melting Point of Polyester The melting point of the polyester used to form Layer B was determined using a melting point measurement device (MP-21, manufactured by Yamato Scientific Co., Ltd.) according to the visual inspection method specified in JIS-K0064:1992. Specifically, the melting point was determined using the following procedure. A 3 mm capillary tube was filled with a finely crushed sample of the polyester used to form Layer B. The heating liquid in the heating vessel was heated until the temperature of the heating liquid reached a temperature approximately 10°C lower than the expected melting point of the sample. The immersion line of the immersion line thermometer was aligned with the liquid surface of the heating liquid. At this time, the mercury bulb of the immersion line thermometer was positioned approximately in the center of the heating vessel in the radial direction, and the tip of the immersion line thermometer was positioned approximately 20 mm from the bottom of the heating vessel. With the sample-filled portion of the 3 mm capillary tube in close contact with the mercury bulb of the immersion line thermometer, the sample was heated so that the temperature of the heating liquid increased by approximately 1°C per minute. The temperature at which the sample melted in the 3 mm capillary tube and no longer became solid was read as the melting point using a thermometer with an immersion line. Hereinafter, this melting point may be referred to as the "melting point of the polyester in layer B."

[0059] (4) Planar Orientation Coefficient of Layer B The refractive index of the biaxially stretched polyester film in each direction was measured using an Abbe refractometer (light source: sodium D line 589 nm, mounting liquid: methylene iodide) in accordance with JIS-K7105:1981. From the obtained refractive indices, the planar orientation coefficient Ns was calculated according to the following formula: Ns = (nMD + nTD) / 2 - nZ (where nMD is the refractive index in the MD direction of the biaxially stretched polyester film, nTD is the refractive index in the TD direction of the biaxially stretched polyester film, and nZ is the refractive index in the thickness direction perpendicular to the plane of the biaxially stretched polyester film.)

[0060] (5) Differential Scanning Calorimetry (Measurement of Melting Peak Temperature and Melting Sub-peak Temperature) Approximately 20 mg of biaxially stretched polyester film as a sample was sealed in an aluminum pan for measurement and attached to a differential scanning calorimeter (TA Instruments, DSCQ100), and the temperature was increased at a heating rate of 20 ° C. / min under nitrogen gas. In the DSC curve drawn by this differential scanning calorimetry, the temperature of a clear endothermic peak due to melting, i.e., the apex of the crystalline melting peak, was read as the crystalline melting peak temperature (Tp). Meanwhile, the temperature of an endothermic peak lower than the crystalline melting peak, i.e., the apex of the melting sub-peak, was read as the melting sub-peak temperature. Indium and tin were used as standard materials for calibration.

[0061] (6) Thermal Adhesion (Lamination Evaluation) A biaxially oriented polyester film was placed on one side of a 0.25 mm thick tinplate with Layer B facing the tinplate, and the biaxially oriented polyester film was then laminated to a metal plate by heat fusion at 220°C. That is, the biaxially oriented polyester film was laminated to the tinplate at 220°C with Layer B facing the tinplate. The adhesive strength and appearance of the biaxially oriented polyester film for the laminated tinplate obtained were evaluated as follows: A: Thermal adhesion was possible and the laminate was clean. B: Thermal adhesion was possible, but the shrinkage in the width direction of the biaxially oriented polyester film, i.e., width shrinkage, was large. C: Thermal adhesion was not possible (i.e., the biaxially oriented polyester film peeled off from the tinplate immediately after lamination).

[0062] (7) Formability A disk-shaped sample with a diameter of 180 mm was cut out from the laminated tinplate prepared in (6). This sample (i.e., a disk-shaped laminated tinplate) was deep-drawn in three stages using a drawing die and a punch to prepare a seamless container (hereinafter abbreviated as "can") with a height of 100 mm and a diameter of 100 mm. When peeling of the biaxially oriented polyester film from the can was not observed, the can was judged as A. On the other hand, when peeling of the biaxially oriented polyester film from the can was observed, the can was judged as B.

[0063] (8) Corrosion resistance (corrosion test after heating) The cans prepared in (7) were filled with 3% acetic acid and subjected to retort treatment at 135°C for 2 hours. After the retort treatment, the cans were disassembled and the interior (i.e., the inner surface of the can) was observed and evaluated according to the following criteria: A: No discoloration or corrosion was observed. B: Discoloration was observed but no corrosion was observed. C: Both discoloration and corrosion were observed. Specifically, black discoloration and some corroded areas were observed.

[0064] (9) Overall Evaluation When the biaxially stretched polyester film did not fall under any of the following (i), (ii), (iii), and (iv), it was rated as A. On the other hand, when the biaxially stretched polyester film fell under at least one of (i), (ii), (iii), and (iv), it was rated as B. (i) Thermal adhesiveness was C. (ii) Formability was B. (iii) Corrosion resistance was C. (iv) Sporadic breakage occurred during film formation.

[0065] Example 1: The raw material for forming Layer A (hereinafter referred to as "Layer A raw material") was polyethylene terephthalate copolymerized with 12 mol% isophthalic acid (hereinafter referred to as "IA") containing 0.1% by weight of aggregate silica with an average particle size of 1.5 μm. The raw material for forming Layer B (hereinafter referred to as "Layer B raw material") was polyethylene terephthalate copolymerized with 24 mol of isophthalic acid containing 0.5% by weight of aggregate silica with an average particle size of 2.5 μm. The Layer A raw material and Layer B raw material were independently dried, melted in an extruder, and co-extruded through adjacent dies at 280°C in a Layer A:Layer B ratio of 4:1. The extrusion was then quenched and solidified to obtain an unstretched laminate film. This unstretched laminate film was then stretched 3.2 times longitudinally at 100°C, then 3.5 times transversely at 120°C, and subsequently heat-set at 180°C to obtain a biaxially oriented polyester film. The thickness of the biaxially stretched polyester film was 20 μm. The evaluation results of the obtained biaxially stretched polyester film are shown in Table 1.

[0066] [Example 2] A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that polyethylene terephthalate copolymerized with 30 mol % 1,4-cyclohexanedimethanol (hereinafter sometimes referred to as "CHDM") containing 0.5 mass % of aggregated silica having an average particle size of 2.5 μm was used as the B layer raw material, and that the film was heat-set at 160° C. The evaluation results of the obtained biaxially stretched polyester film are shown in Table 1.

[0067] [Example 3] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that polyethylene terephthalate copolymerized with 30 mol% neopentyl glycol (hereinafter sometimes referred to as "NPG") containing 0.5 mass% of aggregated silica having an average particle size of 2.5 μm was used as the B layer raw material, and that the film was heat-set at 160° C. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0068] Example 4 A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 190° C. The evaluation results of the obtained biaxially stretched polyester film are shown in Table 1.

[0069] Example 5 A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 170° C. The evaluation results of the obtained biaxially stretched polyester film are shown in Table 1.

[0070] Example 6: As the A layer raw material, polyethylene terephthalate copolymerized with 12 mol% isophthalic acid containing 0.1% by mass of aggregate silica with an average particle size of 1.5 μm was prepared. As the B layer raw material, polyethylene terephthalate copolymerized with 20 mol% isophthalic acid containing 0.5% by mass of aggregate silica with an average particle size of 2.5 μm was prepared. The A layer raw material and the B layer raw material were independently dried and then melted in an extruder. Co-extruded at 280°C through adjacent dies at a Layer A:Layer B ratio of 4:1, and then rapidly solidified to obtain an unstretched laminate film. This unstretched laminate film was then stretched 3.5 times longitudinally at 90°C, then 3.9 times transversely at 110°C, and subsequently heat-set at 175°C to obtain a biaxially stretched polyester film. The biaxially stretched polyester film had a thickness of 20 μm. The evaluation results of the obtained biaxially stretched polyester film are shown in Table 1.

[0071] [Comparative Example 1] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that polyethylene terephthalate copolymerized with 18 moles of isophthalic acid containing 0.5 mass% of aggregated silica having an average particle size of 2.5 μm was used as the raw material for layer B. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0072] [Comparative Example 2] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that polyethylene terephthalate copolymerized with 20 moles of isophthalic acid containing 0.5 mass% of aggregated silica having an average particle size of 2.5 μm was used as the B layer raw material, and the heat setting temperature was changed to 200° C. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0073] Comparative Example 3 A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 165° C. The evaluation results of the obtained biaxially stretched polyester film are shown in Table 1.

[0074] [Comparative Example 4] An attempt was made to produce a biaxially oriented polyester film in the same manner as in Example 1, except that the heat setting temperature was changed to 205°C. However, sporadic breakage occurred during film formation, and therefore a biaxially oriented polyester film could not be obtained.

[0075]

[0076] Regarding the DSC curves of the biaxially stretched polyester films in Examples 1 to 6, the crystalline melting peaks are considered to be derived from the polyester in Layer A. This is because the polyester in Layer B used in Examples 1 to 6 does not have a crystalline melting peak in the DSC curves for determining amorphousness, and therefore could not have formed a crystalline melting peak in the DSC curves of the biaxially stretched polyester films.

[0077] Regarding the DSC curves of the biaxially stretched polyester films in Examples 1 to 6, the melting subpeaks are also thought to be derived from the polyester in the A layer.

[0078] In Examples 1 to 6, the biaxially oriented polyester film did not peel off from the can, and corrosion did not occur on the inner surface of the can after retort treatment.

[0079] In Comparative Example 1, both discoloration and corrosion occurred on the inner surface of the can after retort treatment. This is thought to be because the retort treatment caused excessive crystallization of the polyester in Layer B, resulting in an excessive reduction in the volume of Layer B, which in turn created a space between Layer B and the metal plate, allowing 3% acetic acid to penetrate into the interface between Layer B and the metal plate.

[0080] In Comparative Example 2, both discoloration and corrosion occurred on the inner surface of the can after retort treatment. This is thought to be because the amount of heat the biaxially oriented polyester film received during heat setting was excessively large, causing the molecular chains of the polyester in Layer B to change from a planar orientation to a completely random orientation, resulting in the formation of spherulites during retort treatment and the resulting embrittlement of Layer B.

[0081] In Comparative Example 3, the biaxially oriented polyester film peeled off from the can. This is thought to be because the amount of heat the biaxially oriented polyester film received during heat setting was too small, and therefore the planar orientation of the polyester molecular chains in Layer B was not effectively relaxed, resulting in excessive thermal shrinkage of Layer B when the can was formed.

[0082] INDUSTRIAL APPLICABILITY The present invention has industrial applicability because it relates to a biaxially oriented polyester film for metal sheet lamination, a laminate, and a container.

Claims

1. A biaxially oriented polyester film for metal sheet bonding, comprising: an A layer containing polyethylene terephthalate; and a B layer containing a substantially amorphous polyester, wherein T1 is 180°C or higher and 215°C or lower, T1 is the melting point of the polyester in the B layer, the difference between T1 and T2 is 15°C or higher and 40°C or lower, and T2 is the melting sub-peak temperature in a differential scanning calorimetry curve of the biaxially oriented polyester film for metal sheet bonding.

2. The biaxially oriented polyester film for metal sheet bonding according to claim 1, wherein the plane orientation coefficient of said layer B is greater than 0.05 and not greater than 0.

10.

3. The biaxially oriented polyester film for metal sheet bonding according to claim 1, wherein the polyester of Layer B contains a polycarboxylic acid component, the polycarboxylic acid component contains a terephthalic acid component, and the content of the terephthalic acid component in 100 mol% of the polycarboxylic acid component is 60 mol% or more, and the polyester of Layer B contains a polyol component, the polyol component contains an ethylene glycol component, and the content of the ethylene glycol component in 100 mol% of the polyol component is 90 mol% or more.

4. The biaxially oriented polyester film for metal sheet bonding according to claim 1, wherein the polyethylene terephthalate in the layer A is a copolymerized polyethylene terephthalate.

5. The biaxially oriented polyester film for laminating to metal sheets according to claim 1, which is a biaxially oriented polyester film for laminating to metal sheets for forming.

6. A laminate comprising: a metal plate; and the biaxially oriented polyester film for metal plate lamination according to any one of claims 1 to 5 laminated to the metal plate.

7. A container comprising the laminate of claim 6.

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