Biaxially oriented polyester film for metal plate lamination, laminated metal plate and container

Abstract

One purpose of the present invention is to provide a biaxially oriented polyester film for metal plate lamination, the biaxially oriented polyester film making it possible to reduce or prevent the occurrence of excessively large cracks that could be formed in the polyester film during molding, and to suppress or reduce corrosion of the metal plate caused by a retort treatment. Another purpose of the present invention is to provide a laminated metal plate and a container. The biaxially oriented polyester film for laminating a metal plate includes a layer A and a layer B. Layer A comprises a polyester, which contains copolymerized polyethylene terephthalate including an aliphatic dicarboxylic acid component. Layer B comprises an amorphous polyester, which contains copolymerized polyethylene terephthalate including an aliphatic dicarboxylic acid component. The laminated metal plate includes a metal plate and a biaxially oriented polyester film for laminating a metal plate. The container includes a laminate.

Description

Biaxially oriented polyester film for bonding to a metal plate, laminated metal plate, and container

[0001] The present invention relates to a biaxially oriented polyester film for bonding to a metal plate, a laminated metal plate, and a container.

[0002] Metal cans are generally coated to prevent corrosion of the inner and outer surfaces. In recent years, as a method of imparting rust prevention without using organic solvents for the purpose of simplifying processes, improving hygiene, and preventing pollution, coating with a thermoplastic resin film such as a polyester film has been carried out. That is, after laminating a thermoplastic resin film on a metal plate such as tinplate, tin-free steel, or aluminum, food cans, beverage cans, and aerosol cans are formed by subjecting them to severe forming processes such as drawn cans or thin-walled drawn cans. Cans (i.e., food cans, beverage cans, aerosol cans, etc.) used for these applications have come to be manufactured by subjecting them to even more severe processing conditions such as thinning drawing or ironing processes from the perspective of cost reduction. In addition, the contents have become diverse and are also being used for contents with high metal corrosiveness or contents that are sterilized at high temperatures. For this reason, higher adhesion between the metal plate and the thermoplastic resin film has been required.

[0003] Methods for bonding a polyester film to a metal plate include, in addition to the method using an adhesive, for example, a method of imparting thermoadhesive properties to the polyester film and directly bonding it to the metal plate. Furthermore, there is a method of imparting thermoadhesive properties to the polyester film by making the structure of the polyester film a laminated structure of a polyester base material layer and a polyester layer having thermoadhesive properties (hereinafter sometimes referred to as a "thermoadhesive layer").

[0004] Among these, a polyester film having a laminated structure, that is, a polyester film containing a thermoadhesive layer, is excellent in that it is easy to impart thermoadhesive properties, film strength, and dimensional stability (for example, Patent Documents 1 and 2). However, when using a metal plate that is difficult to thermoadhere, such as tinplate or aluminum, sufficient adhesion cannot be obtained.

[0005] On the other hand, Patent Document 3 describes a laminated polyester film that can be heat-bonded at low temperatures. However, while this polyester film can be molded into shallow-drawn cans with a low degree of moldability, when molded into deep-drawn cans with a high degree of moldability or into cans formed by ironing, it has problems such as the occurrence of minute cracks like pinholes or film cracks (specifically, excessively large cracks) at the top of the can.

[0006] Furthermore, Patent Document 4 describes a polyester film that provides sufficient adhesion with low-temperature lamination and exhibits excellent moldability when molded after lamination.

[0007] JP 6-340047 JP 7-101015 JP 5-42643 JP 2015-174382

[0008] The objective is to provide a biaxially oriented polyester film for metal sheet lamination that can reduce or prevent excessively large cracks that may occur in the polyester film during molding, and can also suppress or reduce corrosion of the metal sheet that may occur due to retort processing. The objective is also to provide laminated metal sheets and containers.

[0009] The present invention comprises the following configuration [1]: [1] A biaxially oriented polyester film for laminating metal plates, comprising: an A layer containing polyester; and a B layer containing amorphous polyester, wherein the polyester in the A layer contains copolymerized polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group having 4 to 8 carbon atoms; the amount of the aliphatic dicarboxylic acid component in the copolymerized polyethylene terephthalate in the A layer is 0.2 mol% to 5 mol% of 100 mol% of the dicarboxylic acid component; and the polyester in the B layer contains copolymerized polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group having 4 to 8 carbon atoms.

[0010] The present invention preferably comprises the configurations described in [2] and later below. [2] The biaxially oriented polyester film for laminating metal plates according to [1], wherein the melting point of the polyester in the B layer is 180°C or higher and 215°C or lower. [3] The biaxially oriented polyester film for laminating metal plates according to [1] or [2], wherein the copolymerized polyethylene terephthalate in the B layer further comprises copolymerized components other than the aliphatic dicarboxylic acid component. [4] The biaxially oriented polyester film for laminating metal plates according to any one of [1] to [3], which is a biaxially oriented polyester film for laminating metal plates for molding. [5] The biaxially oriented polyester film for laminating metal plates according to any one of [1] to [4], wherein the polyester in the A layer is the copolymerized polyethylene terephthalate in the A layer. [6] The biaxially oriented polyester film for laminating metal plates according to any one of [1] to [5], wherein the copolymerized polyethylene terephthalate in the A layer is crystalline. [7] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to [6], wherein the aliphatic dicarboxylic acid component of the copolymerized polyethylene terephthalate in layer A is a sebaciic acid component. [8] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to [7], wherein the amount of the aliphatic dicarboxylic acid component is 0.4 mol% or more or 0.5 mol% or more in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in layer A. [9] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to [8], wherein the amount of the aliphatic dicarboxylic acid component is 4.5 mol% or less or 4 mol% or less in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in layer A.

[10] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to [9], wherein the copolymerized polyethylene terephthalate in layer A contains an isophthalic acid component.

[11] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[10] , wherein the amount of the isophthalic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in the A layer is 5 mol% or more or 7 mol% or more.

[12] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[11] , wherein the amount of the isophthalic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in layer A is 15 mol% or less or 13 mol% or less.

[13] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[12] , wherein the total amount of the aliphatic dicarboxylic acid component, the isophthalic acid component and the terephthalic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in layer A is 95 mol% or more or 98 mol% or more.

[14] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[13] , wherein the content of the ethylene glycol component in 100 mol% of the diol component of the copolymerized polyethylene terephthalate in layer A is 92 mol% or more or 95 mol% or more.

[15] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[14] , wherein the polyester content in the A layer is 85% by mass or 90% by mass or more.

[16] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[15] , wherein the polyester content in the A layer is 95% by mass or 98% by mass or more.

[17] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

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

[18] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[17] , wherein the aliphatic dicarboxylic acid component of the copolymerized polyethylene terephthalate in the B layer is the sebaciate component.

[19] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[18] , wherein the amount of the aliphatic dicarboxylic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in the B layer is 1 mol% or more or 3 mol% or more.

[20] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[19] , wherein the amount of the aliphatic dicarboxylic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in the B layer is 5 mol% or more or 6 mol% or more.

[21] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[20] , wherein the amount of the aliphatic dicarboxylic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in the B layer is 20 mol% or less or 16 mol% or less.

[22] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[21] , wherein the copolymerized polyethylene terephthalate in the B layer contains an isophthalic acid component.

[23] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[22] , wherein the amount of the isophthalic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in the B layer is 10 mol% or more or 12 mol% or more.

[24] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[23] , wherein the amount of the isophthalic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in the B layer is 30 mol% or less or 25 mol% or less.

[25] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[24] , wherein the total amount of the aliphatic dicarboxylic acid component, the isophthalic acid component and the terephthalic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in the B layer is 95 mol% or more or 98 mol% or more.

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

[25] , wherein the content of the ethylene glycol component in 100 mol% of the diol component of the copolymerized polyethylene terephthalate in the B layer is 92 mol% or more or 95 mol% or more.

[27] The biaxially oriented polyester film for laminating metal plates according to any one of [1] to

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

[28] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

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

[29] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[28] , wherein the melting point (T) of the polyester in the B layer is 190°C or higher or 195°C or higher.

[30] A biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[29] , wherein the melting point (T) of the polyester in the B layer is 212°C or lower or 200°C or lower.

[31] A laminated metal plate comprising a metal plate and a biaxially oriented polyester film for laminating metal plates according to any one of [1] to

[30] laminated to the metal plate.

[32] The laminated metal plate according to

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

[33] The laminated metal sheet according to

[31] or

[32] , wherein the metal sheet is a tin plate.

[34] A container comprising the laminated metal sheet according to any one of

[31] to

[33] .

[0011] According to the present invention, it is possible to provide a biaxially oriented polyester film for laminating metal sheets that can reduce or prevent excessively large cracks that may occur in the polyester film during molding, and can also suppress or reduce corrosion of the metal sheet that may occur due to retort processing. According to the present invention, it is also possible to provide laminated metal sheets and containers.

[0012] <Introduction> The embodiments of the present invention will be described in detail below.

[0013] The biaxially oriented polyester film for metal plate lamination of this embodiment (hereinafter sometimes referred to as "polyester film") comprises a polyester A layer and an amorphous polyester B layer, wherein the polyester of the A layer comprises copolymerized polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group having 4 to 8 carbon atoms, and the amount of the aliphatic dicarboxylic acid component is 0.2 mol% to 5 mol% of 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in the A layer, and the polyester of the B layer comprises copolymerized polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group having 4 to 8 carbon atoms.

[0014] Since layer B contains amorphous polyester, the polyester film of this embodiment can suppress or reduce corrosion of the metal plate that may occur due to retort processing. This will be explained. The polyester film of this embodiment can be bonded to a metal plate, that is, laminated. The metal plate to which the polyester film of this embodiment is laminated, i.e., the laminated metal plate (hereinafter sometimes referred to as "laminated body"), can be formed into, for example, a container (a can, for example), or components that make up the container (for example, a can lid, a can body, or a can bottom). If layer B were to contain crystalline polyester instead of amorphous polyester, then crystallization of the crystalline polyester would proceed actively when the container is subjected to retort processing, and the volume of layer B would decrease. As a result, a space would be created between layer B and the metal plate. In contrast, according to this embodiment, since layer B contains amorphous polyester, it is possible to avoid the active crystallization of layer B due to retort processing, and therefore, the reduction in the volume of layer B due to retort processing can be avoided or reduced. Therefore, the generation of a space between layer B and the metal plate due to a decrease in the volume of layer B can be avoided or mitigated. As a result, the intrusion of water (for example, water originating from the contents packed in the container) into the interface between layer B and the metal plate can be suppressed or reduced. Thus, the polyester film of this embodiment can suppress or reduce corrosion of the metal plate that may occur due to retort processing.

[0015] Furthermore, since layer B contains copolymer polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group with 4 to 8 carbon atoms, it is possible to relieve the stress generated in the polyester film when forming a container or a component constituting the container, that is, the stress generated in the polyester film during the molding process. Therefore, it is possible to reduce or prevent excessively large cracks that may occur in the polyester film during the molding process. In particular, it is possible to reduce or prevent excessively large cracks that may occur in the polyester film during molding processes that include strongly squeezing the laminated metal plate.

[0016] Furthermore, since layer A contains copolymerized polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group with 4 to 8 carbon atoms, the stress generated in the polyester film during molding can be further reduced. Therefore, excessively large cracks that may occur in the polyester film during molding can be further reduced or prevented.

[0017] Furthermore, since the amount of aliphatic dicarboxylic acid component in the copolymerized polyethylene terephthalate in layer A is 0.2 mol% or more, the stress generated in the polyester film during molding can be further reduced. Therefore, excessively large cracks that may occur in the polyester film during molding can be further reduced or prevented.

[0018] Furthermore, since the amount of aliphatic dicarboxylic acid component in the copolymerized polyethylene terephthalate in layer A is 5 mol% or less, it is possible to avoid an excessive decrease in the strength of layer A, and therefore, excessive damage that the polyester film may receive from tools (for example, a punch) during molding can be reduced or prevented. As a result, minute cracks that may occur throughout the polyester film during molding can be reduced or prevented.

[0019] The following describes in detail the structure and physical properties of the polyester film of this embodiment.

[0020] <Biaxially Oriented Polyester Film for Metal Plate Lamination> The polyester film of this embodiment includes layer A and layer B. In the polyester film of this embodiment, layer B can be provided on layer A. That is, layer B can be laminated on layer A.

[0021] Layer A can form one of the two surfaces of the polyester film of this embodiment. Layer B can form the other surface of the polyester film of this embodiment.

[0022] <Layer A> Layer A contains polyester. Preferably, the polyester in layer A is crystalline. That is, it is preferable that the polyester in layer A is a crystalline polyester. Here, a crystalline polyester means a polyester in which a crystalline melting peak is observed by differential scanning calorimetry (i.e., DSC).

[0023] The polyester in layer A contains copolymerized polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group with 4 to 8 carbon atoms. That is, the polyester in layer A contains copolymerized polyethylene terephthalate copolymerized with at least an aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms. Because the copolymerized polyethylene terephthalate in layer A contains an aliphatic dicarboxylic acid component having an alkylene group with 4 to 8 carbon atoms, it is possible to relieve the stress generated in the polyester film during molding, and therefore, it is possible to reduce or prevent excessively large cracks that may occur in the polyester film during molding.

[0024] Examples of aliphatic dicarboxylic acids having an alkylene group with 4 to 8 carbon atoms include adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. The copolymerized polyethylene terephthalate preferably contains at least one of adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid as a copolymer component. The carbon atoms mentioned above do not include carbon atoms of carboxyl groups adjacent to the alkylene group. That is, carbon atoms of carboxyl groups adjacent to the alkylene group are not counted as carbon atoms of the alkylene group. Sebacic acid is preferred as the aliphatic dicarboxylic acid.

[0025] In layer A, the amount of aliphatic dicarboxylic acid component is 0.2 mol% or more, preferably 0.4 mol% or more, and more preferably 0.5 mol% or more, out of 100 mol% of the dicarboxylic acid component of copolymerized polyethylene terephthalate. Since it is 0.2 mol% or more, the stress generated in the polyester film during molding can be further reduced, and therefore, excessively large cracks that may occur in the polyester film during molding can be further reduced or prevented. In layer A, the amount of aliphatic dicarboxylic acid component is 5 mol% or less, preferably 4.5 mol% or less, and more preferably 4 mol% or less, out of 100 mol% of the dicarboxylic acid component of copolymerized polyethylene terephthalate. Since it is 5 mol% or less, it is possible to avoid an excessive decrease in the strength of layer A, and as a result, minute cracks that may occur throughout the polyester film during molding can be reduced or prevented.

[0026] Copolymerized polyethylene terephthalate may further contain other copolymer components. Such copolymer components may be acidic or alcoholic components. Examples of acidic components include isophthalic acid, phthalic acid, aromatic dicarboxylic acids such as 2,6-naphthalenedicarboxylic acid, and alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid. Examples of alcoholic components 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 can be used individually or in combination of two or more. Among these, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol are particularly preferred because they enhance moldability while maintaining heat resistance. Here, terephthalic acid and ethylene glycol are not considered copolymer components. When copolymerized polyethylene terephthalate contains other copolymer components as described above, it is preferable that the total amount of copolymer components, including dicarboxylic acid having alkylene chains with 4 to 8 carbon atoms, is 5 mol% to 15 mol% when the total amount of dicarboxylic acid and diol components of copolymerized polyethylene terephthalate is 200 mol%. This may also be 10 mol% to 14 mol%.

[0027] In particular, the copolymerized polyethylene terephthalate of layer A preferably contains an isophthalic acid component. Specifically, the copolymerized polyethylene terephthalate of layer A is preferably polyethylene terephthalate copolymerized with at least isophthalic acid and an aliphatic dicarboxylic acid (specifically, an aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms).

[0028] In layer A, the amount of isophthalic acid component is preferably 5 mol% or more, and more preferably 7 mol% or more, out of 100 mol% of the dicarboxylic acid component of copolymerized polyethylene terephthalate. In layer A, the amount of isophthalic acid component is preferably 15 mol% or less, and more preferably 13 mol% or less, out of 100 mol% of the dicarboxylic acid component of copolymerized polyethylene terephthalate.

[0029] The copolymerized polyethylene terephthalate in layer A is preferably crystalline. That is, the copolymerized polyethylene terephthalate in layer A is preferably crystalline copolymerized polyethylene terephthalate. Here, crystalline polyethylene terephthalate means polyethylene terephthalate in which a crystalline melting peak is observed by differential scanning calorimetry (i.e., DSC).

[0030] In layer A, the total amount of aliphatic dicarboxylic acid components, isophthalic acid components, and terephthalic acid components in 100 mol% of the dicarboxylic acid component of copolymerized polyethylene terephthalate is preferably 95 mol% or more, and more preferably 98 mol% or more. This total amount may also be 100 mol%.

[0031] In layer A, the content of ethylene glycol component in 100 mol% of the diol component of copolymerized polyethylene terephthalate is preferably 92 mol% or more, and more preferably 95 mol% or more.

[0032] The copolymerized polyethylene terephthalate content 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 even more preferably 98% by mass or more.

[0033] Layer A may contain other resins in addition to copolymerized polyethylene terephthalate. Examples of other resins include polyesters such as polybutylene terephthalate, polyethylene-2,6-naphthalate, and poly-1,4-cyclohexanedimethylene terephthalate, as well as polycarbonates, polyethylene, polypropylene, polyamides, polyvinyl chloride, ionomers, and silicone resins.

[0034] The A layer preferably contains a lubricant. The lubricant can 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, a preferred lubricant in terms of pinhole resistance is a monodisperse lubricant with a particle size ratio (long diameter / short diameter) 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 preferable to add it in the range of 0.01% to 0.3% by mass if the average particle size is 1.5 μm, or in the range of 0.05% to 0.5% by mass if the silica has an average particle size of 0.8 μm.

[0035] Layer A may contain other additives. For example, antioxidants, dispersants, viscosity modifiers, fluorescent whitening agents, heat stabilizers, UV absorbers, and antistatic agents can be added to layer A.

[0036] <Layer B> Layer B contains amorphous polyester. As mentioned above, since layer B contains amorphous polyester, corrosion of the metal plate that may occur due to retort processing can be suppressed or reduced. Here, "amorphous polyester" means polyester in which no crystal melting peak is observed by differential scanning calorimetry (i.e., DSC). Specifically, "amorphous polyester" means polyester in which no crystal melting peak is observed by differential scanning calorimetry (i.e., DSC) of a sample of approximately 10 mg in mass, heated from 20°C at a rate of 20°C / min, according to the method described in JIS-K-7121-1987.

[0037] The melting point (T) of the amorphous polyester in layer B is preferably 180°C or higher and 215°C or lower. Here, since the melting point of amorphous polyester cannot be measured by DSC, it is measured according to JIS-K0064:1992 "Method for Measuring the Melting Point and Melting Range of Chemical Products". Here, the melting point is defined as the temperature at which the sample melts in the capillary and no solid is observed, according to the "visual method" described in 3.1 of the same standard. If T 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 suppressed or reduced. On the other hand, if T is 180°C or higher, heat resistance is improved, and damage to the polyester film that may occur due to heat generation during molding can be prevented or reduced. The melting point (T) of amorphous polyester may be, for example, 190°C or higher, or 195°C or higher. The melting point (T) of amorphous polyester may be, for example, 212°C or lower, or 200°C or lower.

[0038] The amorphous polyester in layer B contains copolymerized polyethylene terephthalate. That is, the amorphous polyester in layer B contains amorphous copolymerized polyethylene terephthalate. The amorphous copolymerized polyethylene terephthalate in layer B contains an aliphatic dicarboxylic acid component having an alkylene group with 4 to 8 carbon atoms. That is, the amorphous copolymerized polyethylene terephthalate in layer B is polyethylene terephthalate copolymerized with at least an aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms. Because the amorphous copolymerized polyethylene terephthalate in layer B contains an aliphatic dicarboxylic acid component having an alkylene group with 4 to 8 carbon atoms, it is possible to relieve the stress generated in the polyester film during molding, and therefore, it is possible to reduce or prevent excessively large cracks that may occur in the polyester film during molding.

[0039] The explanation of the aliphatic dicarboxylic acid component of amorphous copolymer polyethylene terephthalate in layer B (specifically, the aliphatic dicarboxylic acid component having an alkylene group with 4 to 8 carbon atoms) is omitted because it overlaps with the explanation of the aliphatic dicarboxylic acid component of copolymer polyethylene terephthalate in layer A. The explanation of the aliphatic dicarboxylic acid component for layer A can also be used as an explanation of the aliphatic dicarboxylic acid component for layer B.

[0040] In the amorphous copolymer polyethylene terephthalate in layer B, the amount of aliphatic dicarboxylic acid component is preferably 1 mol% or more, more preferably 3 mol% or more, even more preferably 5 mol% or more, and even more preferably 6 mol% or more, out of 100 mol% of the dicarboxylic acid component. In the copolymer polyethylene terephthalate in layer B, the amount of aliphatic dicarboxylic acid component is preferably 20 mol% or less, more preferably 16 mol% or less, and even more preferably 15 mol% or less, out of 100 mol% of the dicarboxylic acid component.

[0041] In layer B, the amorphous copolymerized polyethylene terephthalate is preferably copolymerized with at least one monomer other than an aliphatic dicarboxylic acid. In other words, the copolymerized polyethylene terephthalate in layer B is preferably further containing copolymerized components other than the aliphatic dicarboxylic acid component. This copolymerized component may be an acid component or an alcohol component. Examples of acid components include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid, and alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid. Examples of alcohol components 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 can be used individually or in combination of two or more. Among these, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol are preferred, and isophthalic acid is particularly preferred. Here, terephthalic acid and ethylene glycol are not considered copolymerized components. The amount of copolymerization of copolymer components other than aliphatic dicarboxylic acids having alkylene groups with 4 to 8 carbon atoms, which is necessary for copolymerized polyethylene terephthalate to become amorphous, varies depending on the type of copolymer component. When the total of the dicarboxylic acid and diol components of copolymerized polyethylene terephthalate is 200 mol%, the total amount of copolymer components, including aliphatic dicarboxylic acids having alkylene groups with 4 to 8 carbon atoms, is preferably 20 mol% or more, more preferably 22 mol% or more, and particularly preferably 25 mol% or more. For example, when 8 mol% of sebacic acid is added to 100 mol% of the dicarboxylic acid component, if the second copolymer component is isophthalic acid, it is preferable to add isophthalic acid at a rate of preferably 12 mol% or more, more preferably 14 mol% or more, and particularly preferably 17 mol% or more, of the 100 mol% of the dicarboxylic acid component. If the second copolymer component is neopentyl glycol, it is preferable to add neopentyl glycol in an amount of preferably 12 mol% or more, more preferably 14 mol% or more, and particularly preferably 17 mol% or more, of 100 mol% of the diol component.In order to avoid the melting point of the copolyethylene terephthalate from becoming too low, the total of the copolymer components is preferably 35 mol% or less in 200 mol% of the total of the dicarboxylic acid component and the diol component. The copolymer components of the copolyethylene terephthalate are preferably added during the polymerization of the copolyethylene terephthalate and form a sufficiently random copolymer. There is a method of obtaining a polyester with a desired composition by melt-kneading two or more kinds of polyesters polymerized separately, but it is preferable to confirm that the transesterification reaction has sufficiently proceeded and the resin has become amorphous before using this resin.

[0042] Among them, the amorphous copolyethylene terephthalate in the B layer preferably contains an isophthalic acid component. That is, as the amorphous copolyethylene terephthalate in the B layer, at least polyethylene terephthalate copolymerized with isophthalic acid and an aliphatic dicarboxylic acid (specifically, an aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms) is preferable.

[0043] In 100 mol% of the dicarboxylic acid component of the amorphous copolyethylene terephthalate in the B layer, the amount of the isophthalic acid component is preferably 10 mol% or more, more preferably 12 mol% or more, and even more preferably 15 mol% or more. In 100 mol% of the dicarboxylic acid component of the amorphous copolyethylene terephthalate in the B layer, the amount of the isophthalic acid component is preferably 35 mol% or less, more preferably 30 mol% or less, and even more preferably 25 mol% or less.

[0044] In 100 mol% of the dicarboxylic acid component of the amorphous copolyethylene terephthalate in the B layer, the total amount of the aliphatic dicarboxylic acid component, the isophthalic acid component and the terephthalic acid component is preferably 95 mol% or more, more preferably 98 mol% or more. This total amount may be 100 mol%.

[0045] In 100 mol% of the diol component of the amorphous copolyethylene terephthalate in the B layer, the content of the ethylene glycol component is preferably 92 mol% or more, and even more preferably 95 mol% or more.

[0046] The melting point (T) of the amorphous copolyethylene terephthalate in the B layer is preferably 180 °C or higher and 215 °C or lower. Here, since the melting point of the amorphous copolyethylene terephthalate cannot be measured by DSC, it is measured in accordance with JIS-K0064:1992 "Method for Measuring Melting Point and Melting Range of Chemical Products". Here, according to the "visual method" described in 3.1 of this standard, the temperature when the sample melts in the capillary and no solid is observed is taken as the melting point. When T 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 suppressed or reduced. On the other hand, when T is 180 °C or higher, the heat resistance is improved, and destruction of the polyester film that may occur due to heat generation during molding can be prevented or reduced.

[0047] The B layer may contain other resins in addition to the amorphous polyester. The other resin is preferably an amorphous resin.

[0048] The content of the amorphous polyester in the B layer is preferably 85% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more, and still more preferably 98% by mass or more.

[0049] The B layer preferably contains a lubricant. The lubricant may be inorganic or organic, but inorganic is preferred. Examples of inorganic lubricants include silica, alumina, titanium dioxide, calcium carbonate, barium sulfate, etc., and examples of organic lubricants include silicone resin particles, crosslinked polystyrene particles, etc. Since the B layer is composed of an amorphous resin, it is difficult to form protrusions by biaxially stretching and film forming, so it is preferable to add a relatively large amount of a relatively large lubricant. For example, when using silica, it is preferable to add silica with an average particle size of 2 μm or more in the range of 0.01% by mass to 0.3% by mass.

[0050] The B layer may contain other additives. For example, antioxidants, dispersants, viscosity modifiers, fluorescent brighteners, heat stabilizers, ultraviolet absorbers, antistatic agents, etc. can be added to the B layer.

[0051] <Film Thickness> The thickness of the biaxially oriented polyester film in this embodiment can be changed as needed, but a range of 6 μm to 75 μm is preferable, and among these, a range of 10 μm to 75 μm, and particularly 15 μm to 50 μm, is preferable. A thickness of 6 μm or more makes it less likely to tear during molding, while a thickness of 75 μm or less is economical.

[0052] The ratio of the thickness of layer B to the thickness of layer A (i.e., XB / XA: where XA is the thickness of layer A and XB is the thickness of layer B) is preferably in the range of 5 / 1 to 1 / 10 from the viewpoint of film-forming properties, thermal adhesion and corrosion prevention. In other words, the thickness of layer B is preferably 0.1 times or more and 5 times or less 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.

[0053] <Method for Manufacturing Polyester Film> The method for manufacturing the polyester film of this embodiment is not particularly limited. First, an unstretched laminated sheet is prepared by a conventionally known film-forming method, and then it is stretched in two directions. For example, after thoroughly drying the polyester prepared for layer A, it is melted in an extruder at a temperature above the crystal melting peak temperature (Tp) in DSC, usually in the range of Tp+30°C to Tp+70°C. At the same time, after thoroughly drying the amorphous polyester prepared for layer B, it is supplied into the extruder at a temperature above the melting point (T) of amorphous polyester, usually in the range of T+30 to T+70°C. It is preferable to dry the polyester for layer B under reduced pressure at a temperature below the glass transition temperature (Tg). This is because it is possible to avoid the polyester for layer B fusing together. If an extruder with a vent is used, drying may not be necessary.

[0054] Next, it is preferable to produce a laminated, unstretched laminated sheet by a method of laminating both molten resins inside the die, for example, by a simultaneous lamination extrusion method using a multi-manifold die. With such a simultaneous lamination extrusion method, the molten resin forming one layer and the molten resin forming the other layer are laminated inside the die and can be extruded from the die in a sheet shape while maintaining the laminated structure.

[0055] Next, the unstretched laminated sheet can be sequentially or simultaneously biaxially stretched and heat-set. When forming a film by sequential biaxial stretching, the unstretched laminated sheet is heated by roll heating, infrared heating, etc., and first stretched in the longitudinal direction, and then stretched transversely in a stent. At this time, it is preferable to set the stretching temperature to a temperature 20°C to 50°C higher than the Tg of the polyester of layer A, and the longitudinal stretching ratio to be in the range of 2.5 to 3.6 times and the transverse stretching ratio to be in the range of 2.6 to 3.9 times. The heat-set temperature is preferably determined according to the melting point (T) of the amorphous polyester of layer B, and it is good to adjust it in the range of T - 40°C to T + 15°C.

[0056] The metal sheet to which the polyester film of this embodiment is laminated is preferably a metal sheet for can manufacturing. The metal sheet for can manufacturing is preferably a metal sheet such as tinplate, tin-free steel, or aluminum. The lamination temperature is preferably one that balances the adhesion of the polyester film of this embodiment with the moldability into a container. Using the metal sheet laminated with the polyester film of this embodiment (i.e., the laminated metal sheet), a container can be formed using known molding methods. 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 for the can body and lid material of a three-piece can, which is formed by rolling and joining flat sheets.

[0057] The polyester film of this embodiment may be used on the inner surface of a metal container to protect the metal from its 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, can be, for example, T or higher and T+50°C or lower. Furthermore, if the lamination temperature is near the melting point (T) of the amorphous polyester of layer B, specifically T or higher and T+20°C or lower, whitening spots during retort processing can be suppressed.

[0058] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. Each characteristic value was measured by the following method.

[0059] (1) Amount of polyester １The copolymer components of the polyester and their respective amounts were identified using 1H-NMR measurements.

[0060] (2) As a sample for determining the amorphous nature of polyester, approximately 10 mg of polyester (specifically, polyester for forming layer B) was sealed in an aluminum pan for measurement and mounted on 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 crystal melting peak (endothermic peak) was observed in the curve obtained by this differential scanning calorimeter measurement, i.e., the DSC curve, the sample was determined to be amorphous. On the other hand, if a crystal melting peak was observed, the sample was determined to be crystalline.

[0061] (3) Melting point of polyester The melting point of the polyester required to form layer B was determined using a melting point measuring device (MP-21 from Yamato Scientific Co., Ltd.) in accordance with the visual method in JIS-K0064:1992. Specifically, the melting point was determined using the following procedure: A sample of finely crushed polyester required to form layer B was packed into a 3 mm capillary tube. The heating liquid in the heating container was heated until its temperature was approximately 10°C lower than the expected melting point of the sample. The immersion line of the thermometer with an immersion line was aligned with the liquid surface of the heating liquid. At this time, the mercury bulb of the thermometer with an immersion line was positioned approximately in the center of the heating container in the radial direction, and the tip of the thermometer with an immersion line was positioned approximately 20 mm from the bottom of the heating container. With the sample-filled portion of the 3 mm capillary tube in close contact with the mercury bulb of the thermometer with an immersion line, the sample was heated so that the temperature of the heating liquid rose by approximately 1°C per minute. The melting point was measured using an immersion-line thermometer at the temperature at which the sample melted in a 3 mm capillary tube and no solid material was detected. Hereafter, this melting point may be referred to as the "B-layer polyester melting point."

[0062] (4) Heat Adhesion (Lamination Evaluation) A biaxially oriented polyester film was placed on one side of a tinplate with a thickness of 0.25 mm, with layer B facing the tinplate side, and the biaxially oriented polyester film was laminated to the tinplate by heat fusion at 220°C. That is, with layer B facing the tinplate, the biaxially oriented polyester film was laminated to the tinplate at 220°C. The adhesive strength and appearance of the biaxially oriented polyester film on the laminated tinplate obtained in this way were evaluated according to the following procedure. A: Heat adhesion is possible, and the lamination is clean. B: Heat adhesion is possible, but there is a large shrinkage in the width direction of the biaxially oriented polyester film, i.e., width shrinkage. C: Heat adhesion is not possible (i.e., the biaxially oriented polyester film peels off from the tinplate immediately after lamination).

[0063] (5) Formability A 150 mm diameter disc-shaped sample was cut from the laminated tinplate prepared in (4). This sample, i.e., the disc-shaped laminated tinplate, was subjected to deep drawing in three stages using a drawing die and a punch to produce a seamless side container (hereinafter abbreviated as "can") with a height of 100 mm and a diameter of 80 mm. If no cracks were observed in the biaxially oriented polyester film in the can, it was judged as A. If minute cracks were observed in the biaxially oriented polyester film at the top of the can, but no large cracks were observed in the biaxially oriented polyester film, and no minute cracks were observed in the biaxially oriented polyester film anywhere other than the top of the can, it was judged as B. If large cracks were observed in the biaxially oriented polyester film in the can, it was judged as C. If minute cracks were observed in the biaxially oriented polyester film both at the top of the can and elsewhere, it was also judged as C. Furthermore, the extent to which the supply material would be stretched was estimated based on the area of ​​the portion of the sample, i.e., the disc-shaped laminated tin plate, used for the can wall (hereinafter sometimes referred to as the "supply material") (hereinafter sometimes referred to as the "supply material area") and the area of ​​the can wall. Specifically, this was estimated using the following formula: Supply material area / Can wall area, where the supply material area is π(150 / 2). ２ ―π(80 / 2) ２On the other hand, the area of ​​the can wall is π × 80 × 100. Therefore, the ratio of the area of ​​the supplied material to the area of ​​the can wall is 0.50.

[0064] (6) Corrosion test after heating The can prepared in (5) was filled with 3% acetic acid and retorted at 135°C for 2 hours. After retorting, the can was disassembled and the inside (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, there were parts that were discolored black and corroded.

[0065] (7) Overall evaluation: If all of the thermal adhesion, formability, and corrosion resistance are A, the overall evaluation is determined to be A. If at least one of the thermal adhesion, formability, and corrosion resistance is B, and neither thermal adhesion, formability, nor corrosion resistance is C, the overall evaluation is determined to be B. If at least one of the thermal adhesion, formability, and corrosion resistance is C, the overall evaluation is determined to be C.

[0066] [Example 1] As a raw material for forming layer A (hereinafter sometimes referred to as "layer A raw material"), copolymerized polyethylene terephthalate was prepared by copolymerizing 0.1% by mass of bulk silica with an average particle size of 1.5 μm, with 10 mol% isophthalic acid (hereinafter sometimes referred to as "IA") and 2 mol% sebacic acid (hereinafter sometimes referred to as "SA") relative to the total carboxylic acid (specifically, all carboxylic acids used to polymerize copolymerized polyethylene terephthalate). As a raw material for forming layer B (hereinafter sometimes referred to as "layer B raw material"), copolymerized polyethylene terephthalate was prepared by copolymerizing 0.5% by mass of bulk silica with an average particle size of 2.5 μm, with 12 mol% isophthalic acid and 10 mol% sebacic acid relative to the total carboxylic acid (specifically, all carboxylic acids used to polymerize copolymerized polyethylene terephthalate). The raw materials for layer A and layer B were dried independently and then melted in an extruder. They were co-extruded at 280°C from adjacent dies in a ratio of layer A to layer B of 4:1, and then rapidly cooled and solidified to obtain an unstretched laminated film. This unstretched laminated film was longitudinally stretched 3.2 times at 95°C, then transversely stretched 3.5 times at 110°C, and then heat-set at 180°C to obtain a biaxially oriented polyester film. The thickness of the biaxially oriented polyester film was 25 μm. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0067] [Example 2] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the raw materials for layer A, layer B, and the heat-fixing temperature were changed. As the raw material for layer A, copolymerized polyethylene terephthalate was used, which contained 0.1% by mass of bulk silica with an average particle size of 1.5 μm, copolymerized with 12 mol% isophthalic acid and 0.5 mol% sebacic acid relative to the total carboxylic acid. As the raw material for layer B, copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized with 18 mol% isophthalic acid and 6 mol% sebacic acid relative to the total carboxylic acid. The heat-fixing temperature was set to 175°C. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0068] [Example 3] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the raw materials for layer A, layer B, and the heat-fixing temperature were changed. As the raw material for layer A, copolymerized polyethylene terephthalate was used, which contained 0.1% by mass of bulk silica with an average particle size of 1.5 μm, copolymerized with 8 mol% isophthalic acid and 4 mol% sebacic acid relative to the total carboxylic acid. As the raw material for layer B, copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized with 15 mol% isophthalic acid and 15 mol% sebacic acid relative to the total carboxylic acid. The heat-fixing temperature was set to 175°C. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0069] [Example 4] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the raw materials for layer B and the heat-fixing temperature were changed. As the raw material for layer B, copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized with 24 mol% isophthalic acid and 16 mol% sebacic acid relative to the total carboxylic acid. The heat-fixing temperature was set to 160°C. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0070] [Comparative Example 1] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the raw materials for layer B and the heat-fixing temperature were changed. As the raw material for layer B, copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized with 12 mol% isophthalic acid and 5 mol% sebacic acid relative to the total carboxylic acid. The heat-fixing temperature was set to 190°C. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0071] [Comparative Example 2] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the raw materials for layer A and layer B, the longitudinal stretching temperature, and the transverse stretching temperature were changed. As the raw material for layer A, copolymerized polyethylene terephthalate was used, which contained 0.1% by mass of bulk silica with an average particle size of 1.5 μm and copolymerized with 12 mol% isophthalic acid relative to the total carboxylic acid. As the raw material for layer B, copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm and copolymerized with 20 mol% isophthalic acid relative to the total carboxylic acid. The longitudinal stretching temperature was set to 100°C. The transverse stretching temperature was set to 120°C. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1.

[0072] [Comparative Example 3] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the raw materials for layer A and layer B, the longitudinal stretching temperature, and the transverse stretching temperature were changed. As the raw material for layer A, copolymerized polyethylene terephthalate was used, which contained 0.1% by mass of bulk silica with an average particle size of 1.5 μm and copolymerized with 6 mol% isophthalic acid and 6 mol% sebacic acid relative to the total carboxylic acid. As the raw material for layer B, copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm and copolymerized with 12 mol% isophthalic acid and 12 mol% sebacic acid relative to the total carboxylic acid. The longitudinal stretching temperature was set to 90°C. The transverse stretching temperature was set to 105°C. The evaluation results of the obtained biaxially oriented polyester film are shown in Table 1. Although the laminate evaluation of this biaxially oriented polyester film with tinplate was A, a corrosion test was not performed because minute cracks occurred in the biaxially oriented polyester film throughout the can wall.

[0073]

[0074] In a can made from a biaxially oriented polyester film using copolymerized polyethylene terephthalate without sebaciic acid as the raw material for layer A and copolymerized polyethylene terephthalate without sebaciic acid as the raw material for layer B, a large crack occurred in the biaxially oriented polyester film at the top (see Comparative Example 2). It is believed that the crack occurred due to the stress generated in the biaxially oriented polyester film during deep drawing.

[0075] In a can made from a biaxially oriented polyester film using copolymerized polyethylene terephthalate copolymerized with 6 mol% sebaciic acid as the raw material for layer A, minute cracks appeared throughout the biaxially oriented polyester film (see Comparative Example 3). This is thought to be because the strength of layer A was excessively weak, and the biaxially oriented polyester film could not withstand the pressure of the punch.

[0076] In a can made from a biaxially oriented polyester film using crystalline copolymer polyethylene terephthalate as the raw material for layer B, both discoloration and corrosion occurred on the inner surface of the can after retort processing (see Comparative Example 1). This is thought to be because the crystallization of the copolymer polyethylene terephthalate in layer B progressed excessively during retort processing, causing an excessive decrease in the volume of layer B, creating a space between layer B and the tinplate, and as a result, 3% acetic acid penetrated into the interface between layer B and the tinplate. In Comparative Example 1, minute cracks occurred in the biaxially oriented polyester film at the top of the can. This crack is thought to have been caused by stress generated in the biaxially oriented polyester film during deep drawing.

[0077] In Example 4, only one minute crack occurred in the biaxially oriented polyester film at the top of the can. In Example 4, the shrinkage of the biaxially oriented polyester film in the width direction was large when the biaxially oriented polyester film was laminated to the tinplate, suggesting that there was significant thickness unevenness in the biaxially oriented polyester film in the laminated tinplate. Furthermore, the low melting point of layer B indicates that the heat resistance of the biaxially oriented polyester film was low. On the other hand, it is known that the thickness of the laminated tinplate increases between the drawing die and the wrinkle-holding plate as the deep drawing process progresses. In light of these factors, it is thought that excessive force from the punch was applied to the part of the laminated tinplate that forms the top of the can, and that the part of the biaxially oriented polyester film that is the thickest, and that its strength decreased due to the heat during deep drawing, resulting in a minute crack in the biaxially oriented polyester film at the top of the can.

[0078] On the other hand, in Examples 1 to 3, both moldability and corrosion resistance were rated A. Heat adhesion was also rated A.

[0079] Since this invention relates to a biaxially oriented polyester film for bonding metal sheets, a laminated metal sheet, and a container, the invention has industrial applicability.

Claims

1. A biaxially oriented polyester film for laminating metal plates, comprising: Layer A containing polyester; and Layer B containing amorphous polyester, wherein the polyester in Layer A contains copolymerized polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group having 4 to 8 carbon atoms; the amount of the aliphatic dicarboxylic acid component in 100 mol% of the dicarboxylic acid component of the copolymerized polyethylene terephthalate in Layer A is 0.2 mol% to 5 mol%; and the polyester in Layer B contains copolymerized polyethylene terephthalate containing an aliphatic dicarboxylic acid component having an alkylene group having 4 to 8 carbon atoms.

2. The biaxially oriented polyester film for laminating metal plates according to claim 1, wherein the melting point of the polyester in the B layer is 180°C or higher and 215°C or lower.

3. The biaxially oriented polyester film for laminating metal plates according to claim 1, wherein the copolymerized polyethylene terephthalate of the B layer further comprises copolymerized components other than the aliphatic dicarboxylic acid component.

4. The biaxially oriented polyester film for laminating to metal plates according to claim 1, which is a biaxially oriented polyester film for laminating to metal plates for molding.

5. A laminated metal plate comprising a metal plate and a biaxially oriented polyester film for laminating metal plates according to any one of claims 1 to 4, laminated to the metal plate.

6. A container comprising the laminated metal plate according to claim 5.

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