Biaxially oriented polyester film for metal lamination molding processes.

Abstract

To provide a biaxially oriented polyester film for metal lamination molding that exhibits excellent adhesion in the lamination process, high moldability, and resistance to corrosion by contents after the formation of a metal container. [Solution] A biaxially oriented polyester film comprising at least two layers, an A layer made of polyester and a B layer made of amorphous polyester, wherein the polyester resin constituting the B layer contains copolymerized polyethylene terephthalate having two or more copolymerized monomers as copolymerized components, each containing at least one aliphatic dicarboxylic acid having 30 or more carbon atoms, and the polyester constituting the A layer contains copolymerized polyethylene terephthalate containing 0.1 to 1 mol% of aliphatic dicarboxylic acid having 30 or more carbon atoms as a copolymerized component in the total dicarboxylic acid components, for use in metal plate lamination molding.

Description

Technical Field

[0001] The present invention relates to a biaxially stretched polyester film for metal plate bonding forming processing.

Background Art

[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, preventing pollution, etc., 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, aluminum, etc., it is used for food cans, beverage cans, and aerosol cans that are subjected to severe forming processes such as drawn cans and thin-walled drawn cans. Cans used for these applications are being manufactured by subjecting them to further thinning drawing processes and ironing processes with stricter processing conditions from the perspective of cost reduction. In addition, the contents have become diverse, and they are also being used for contents with high metal corrosiveness and contents that are sterilized at high temperatures. For this reason, higher adhesion between the metal and the film has been required.

[0003] In addition to the method using an adhesive, methods for bonding a polyester film to a metal plate or the like include, for example, a method of imparting thermoadhesiveness to the polyester film and directly bonding it to the adherend, and further, a method of imparting thermoadhesiveness by forming a laminated structure of a layer having a thermoadhesive layer with a polyester film as a base material.

[0004] Among these, in the case of a laminated polyester film having a thermoadhesive layer, it is excellent in that it is easy to impart thermoadhesiveness, film strength, and dimensional stability (for example, Patent Documents 1 and 2). However, for example, when using a metal plate that is difficult to thermally adhere, such as tinplate or aluminum, sufficient adhesiveness 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 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 ironed cans produced by ironing, problems such as abrasion of the can wall occur.

[0006] Furthermore, Patent Document 4 describes a polyester film that provides sufficient adhesion even when laminated at low temperatures and exhibits excellent moldability when molded after bonding. However, there is a problem in that when containers made using this film are filled with highly corrosive contents and subjected to impacts such as dropping the can in a low-temperature environment, corrosion is likely to occur starting from the impact point. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 6-340047 [Patent Document 2] Japanese Patent Application Publication No. 7-101015 [Patent Document 3] Japanese Patent Application Publication No. 5-42643 [Patent Document 4] Japanese Patent Publication No. 2015-174382 [Overview of the project] [Problems that the invention aims to solve]

[0008] The present invention has been made in view of the above, and its objective is to provide a biaxially oriented polyester film for metal-laminated sheet bonding that provides excellent adhesion when heat-pressed during the lamination process, allows for the manufacture of highly moldable metal containers without the film on the can wall being scraped, and is resistant to corrosion even when the metal container is filled with contents and subjected to impact at low temperatures. [Means for solving the problem]

[0009] As a result of diligent research to solve the aforementioned problems, the present inventors have found that by using polyester copolymerized with a specific copolymer component in the polyester constituting the film, it is possible to mold cans with a high degree of moldability without the film on the can wall being scraped off. Furthermore, by using a polyester resin of a specific composition that does not have crystalline properties as a specific heat-bonding layer, it is possible to manufacture metal cans that are resistant to corrosion even when subjected to impact at low temperatures. This led to the completion of the present invention.

[0010] In other words, the present invention consists of the following configuration. [1] A biaxially oriented polyester film comprising at least two layers: a layer A made of polyester and a layer B made of amorphous polyester, wherein the polyester resin constituting the layer B contains copolymerized polyethylene terephthalate having two or more copolymerized monomers as copolymerized components, each containing at least one aliphatic dicarboxylic acid having 30 or more carbon atoms, and the polyester constituting the layer A contains copolymerized polyethylene terephthalate containing 0.1 to 1 mol% of aliphatic dicarboxylic acid having 30 or more carbon atoms as copolymerized components, for use in metal sheet lamination molding. [2] The polyester film according to [1], wherein the melting point of the polyester constituting the B layer is 180°C or higher and 215°C or lower. [3] A laminated metal sheet obtained by laminating a biaxially oriented polyester film for metal sheet lamination molding described in [1] or [2] with a metal sheet. A metal container comprising the laminated metal plate described in [4] [3]. [Effects of the Invention]

[0011] The biaxially oriented polyester film for metal sheet lamination molding of the present invention exhibits excellent adhesion when heat-pressed onto a metal sheet, and when molded after lamination, it shows excellent formability even at high degrees of moldability. Furthermore, it is resistant to corrosion even when a metal container is formed, filled with contents, and subjected to impact at low temperatures, making it suitable for use in metal container applications. [Modes for carrying out the invention]

[0012] Hereinafter, the present invention will be described in detail.

[0013] <Layer A> (Polyester for Layer A) The polyester constituting Layer A in the present invention includes a copolymerized polyethylene terephthalate having 0.1 to 1 mol% of an aliphatic dicarboxylic acid having 30 or more carbon atoms as a copolymerization component in all dicarboxylic acid components.

[0014] As the copolymerization component of the copolymerized polyethylene terephthalate, a dimer acid obtained by dimerizing an aliphatic dicarboxylic acid having 30 or more carbon atoms, particularly an unsaturated fatty acid having 15 to 25 carbon atoms, and its hydrogenated product are preferably used. Among these, a dimer acid obtained by dimerizing an unsaturated fatty acid having 18 carbon atoms (such as oleic acid or linoleic acid) is preferably used. Here, the carbon number includes the carbon number of the carboxyl group. Considering the heat resistance during melt processing in film production, the upper limit of the carbon number of the aliphatic dicarboxylic acid is preferably 5≤0. The dimer acid contains, as impurities, a trimer that has reacted excessively and an unsaturated aliphatic carboxylic acid that is an unreacted substance. Since these impurities cause gelation of the resin, it is preferable that they be as few as possible. In addition, it is preferable to convert the unreacted unsaturated aliphatic carboxylic acid into a saturated aliphatic carboxylic acid by hydrogenation before use. Compared with using a raw material containing the unreacted unsaturated aliphatic carboxylic acid as it is, it is easier to suppress gelation due to the progress of a branching reaction caused by the unsaturated bond during polymerization, and the color tone of the obtained copolymerized polyethylene terephthalate can be improved. When the total amount of the dicarboxylic acid component is 100 mol%, the copolymerization amount is 0.1 mol% or more, so that when laminating the film to a metal plate and manufacturing a can with a thin side wall at a high draw ratio, the friction with the tool can be reduced, and thus film chipping can be suppressed. On the other hand, if it is 1 mol% or less, problems such as pinholes generated by heat during molding can also be prevented.

[0015] It should be noted that there is an error in the original text where "5≤0" is incorrect. It should be corrected according to the correct content. The above translation is based on the corrected text understanding.Copolymerized polyethylene terephthalate may contain other copolymerization components. These copolymerization components may be acidic or alcoholic. Examples of acidic components include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, terephthalic acid, and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids with fewer than 30 carbon atoms such as adipic acid, azelaic acid, and sebacic 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. 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 copolymerization components. When copolymerized polyethylene terephthalate contains other copolymer components as described above, it is preferable that the total amount of copolymer components, including aliphatic dicarboxylic acids with 30 or more carbon atoms, is 5 to 15 mol%, when the total amount of all dicarboxylic acid components and all diol components constituting copolymerized polyethylene terephthalate is 200 mol%.

[0016] Polyester may contain other resins in addition to polyethylene terephthalate or its copolymers. 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.

[0017] It is also preferable to contain a lubricant in the polyester. The lubricant may be either inorganic or organic, but inorganic ones are 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. In particular, a lubricant that is preferable in terms of pinhole resistance is a monodisperse lubricant 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, spherical crosslinked polystyrene particles, etc. When using silica as the lubricant, it is preferably added in the range of 0.01% to 0.3% by mass for silica with an average particle size of 1.5 μm, and in the range of 0.05% to 0.5% by mass for silica with an average particle size of 0.8 μm or less.

[0018] Other additives such as antioxidants, dispersants, viscosity modifiers, fluorescent brighteners, heat stabilizers, ultraviolet absorbers, antistatic agents, etc. can also be added to the polyester.

[0019] (Polyester for B layer) The polyester constituting layer B in the present invention is amorphous polyester and contains copolymerized polyethylene terephthalate. Here, amorphous means that when approximately 10 mg by mass is heated from 20°C at a rate of 20°C / min in a DSC according to the method described in JIS-K-7121, no melting peak is observed. Of the copolymer components of copolymerized polyethylene terephthalate, at least one is an aliphatic dicarboxylic acid having 30 or more carbon atoms. Dimer acids obtained by dimerizing unsaturated fatty acids having 15 to 25 carbon atoms and their hydrogenated forms are particularly preferred. Among these, dimer acids obtained by dimerizing unsaturated fatty acids having 18 carbon atoms (such as oleic acid and linoleic acid) are particularly preferred. Here, the number of carbon atoms includes the number of carbon atoms constituting the carboxyl group. The upper limit of the number of carbon atoms in the aliphatic dicarboxylic acid is preferably 50 or less, considering the heat resistance during melt processing in film manufacturing. Dimer acids contain impurities such as excessively reacted trimers and unreacted unsaturated aliphatic carboxylic acids. From the viewpoint of suppressing gelation of the resin, it is preferable to have as few of these impurities as possible. Furthermore, it is preferable to convert unreacted unsaturated aliphatic carboxylic acids to saturated aliphatic carboxylic acids by hydrogenation before use. Compared to using raw materials containing unreacted unsaturated aliphatic carboxylic acids as raw materials, this method makes it easier to suppress gelation caused by branching reactions due to the unsaturated bonds during polymerization, and improves the color tone of the resulting copolymerized polyethylene terephthalate. By using these dicarboxylic acids as copolymerization components, it is possible to prevent pinholes and cracks on the top of cans even in can types with high moldability, while also greatly improving corrosion resistance when subjected to impact at low temperatures. The copolymerization amount of these dicarboxylic acids is preferably 1 to 5 mol%, when the total dicarboxylic acid components constituting the copolymerized polyethylene terephthalate are considered to be 100 mol%.

[0020] The copolymerized polyethylene terephthalate is preferably amorphous. To obtain an amorphous resin, at least one monomer other than an aliphatic dicarboxylic acid having 30 or more carbon atoms is added as a copolymer component. This copolymer 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, aliphatic dicarboxylic acids having less than 30 carbon atoms such as adipic acid, azelaic acid, and sebacic 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 copolymer components. The amount of copolymerization of copolymer components other than aliphatic dicarboxylic acids with 30 or more carbon atoms required for polyethylene terephthalate to become amorphous varies depending on the type of copolymer component. However, when the total amount of all dicarboxylic acid components and all diol components constituting copolymerized polyethylene terephthalate is 200 mol%, it is preferable that the total amount of copolymer components, including aliphatic dicarboxylic acids with 30 or more carbon atoms, copolymerizes to 20 mol% or more, more preferably 22 mol% or more, and particularly preferably 25 mol% or more. For example, when adding 4 mol% of a dimer acid with 36 carbon atoms to the total carboxylic acid, if the second copolymer component is isophthalic acid, it is preferable to add 16 mol% or more, more preferably 18 mol% or more, and particularly preferably 21 mol% or more of the total carboxylic acid. Similarly, in the case of neopentyl glycol, which is a diol component, it is preferable to add 16 mol% or more, more preferably 18 mol% or more, and particularly preferably 21 mol% or more of the total diol components. The total amount of the copolymer components is preferably 35 mol% or less in order to prevent the melting point of the copolymerized polyethylene terephthalate from becoming too low.The copolymer components of the copolyethylene terephthalate are preferably added during the polymerization of polyethylene terephthalate to form a sufficiently random copolymer. There is a method of obtaining a polyester with a desired composition by kneading two or more types of polyesters polymerized separately during melting, but it is preferable to confirm that the resin has undergone sufficient transesterification reaction and has become amorphous before use.

[0021] The melting point (T) of the copolyethylene terephthalate is preferably 180°C or higher and 215°C or lower. Since the melting point of an amorphous polyester cannot be measured by DSC, it is measured in accordance with JIS-K-0064:1992 "Method for Measuring the Melting Point and Melting Range of Chemical Products". Here, according to the "visual method" described in 3.1 of the same standard, the temperature at which the sample melts in the capillary and no solid is observed is taken as the melting point. When T is 215°C or lower, it is preferable because sufficient adhesive strength is easily obtained regardless of the type of metal to be laminated. On the other hand, when T is 180°C or higher, the heat resistance is improved, and the film is not damaged by the heat generation during the can manufacturing process, which is preferable. Note that the B layer may contain other resins other than copolyethylene terephthalate, and the other resins are preferably amorphous resins.

[0022] It is preferable to contain a lubricant in the copolyethylene terephthalate. The lubricant can be either 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, cross-linked polystyrene particles, etc. Since the B layer is composed of an amorphous resin, it is difficult to form protrusions by biaxially stretching the film, so it is preferable to add a relatively large amount of a larger lubricant. For example, when using silica, it is preferable to add silica with an average particle size of 2μm or more in the range of 0.01 mass% to 0.3 mass%.

[0023] Other additives such as antioxidants, dispersants, viscosity regulators, fluorescent brighteners, heat stabilizers, ultraviolet absorbers, antistatic agents, etc. can also be added to the polyester.

[0024] <Film thickness> The thickness of the biaxially oriented polyester film of the present invention can be changed as needed, but is preferably in the range of 6 to 75 μm, and more preferably in the range of 10 to 75 μm, and particularly preferably in the range of 15 to 50 μm. 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.

[0025] Furthermore, the thickness ratio of layer A to layer B (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, adhesion, and corrosion prevention.

[0026] <Film Manufacturing Method> The method for manufacturing the biaxially oriented polyester film for metal sheet lamination molding described above is not particularly limited, and first an unstretched laminated sheet is prepared by a conventionally known film-forming method, and then The material can then be stretched in two directions. For example, after thoroughly drying the polyester prepared for layer A, it is melted in the 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. Simultaneously, after thoroughly drying the polyester prepared for layer B, it is supplied in the extruder at a temperature above the melting point (T) of the 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) to avoid the risk of fusion. Drying is not necessary when using an extruder with a vent.

[0027] Next, it is preferable to produce a laminated, unstretched 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. In 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 it is preferable that they are formed into a sheet shape from the die while maintaining the laminated form.

[0028] Next, the unstretched laminated sheet is manufactured by sequentially or simultaneously biaxially stretching and then heat-fixing it. This can be done. When forming a film by sequential biaxial stretching, the unstretched laminated sheet is roll-heated, and infrared radiation is used. The material is first heated and stretched lengthwise, and then stretched transversely using a stent. It is preferable to set the temperature 20 to 50°C higher than the Tg of the polyester in layer A, with a longitudinal stretch ratio of 2.5 to 3.6 times and a transverse stretch ratio of 2.6 to 3.9 times. The heat-fixing temperature is preferably determined according to the melting point (T) of the polyester in layer B, and is best adjusted within the range of T-40°C to T+15°C.

[0029] The biaxially oriented polyester film for metal sheet lamination processing of the present invention is laminated to a metal sheet, In particular, tinplate, tin-free steel, and aluminum sheets are preferred as metal sheets for can manufacturing. The lamination temperature should be one that balances the adhesion of the film with the ability to form a metal container. Metal containers can be formed from the laminated metal sheet using known molding methods. Specifically, the can body of a container with an integrated body and bottom (2-piece can) can be formed by drawing, ironing, or a combination thereof. This laminated metal sheet can also be used for the can body of a 3-piece can, which is made by rolling and joining flat sheets, as well as for the lid material.

[0030] The metal sheet lamination film of the present invention is intended to be used on the inner surface of a metal container to protect the metal from its contents, but it may also be used on the outer surface to protect the metal from the external environment. By laminating the metal at a temperature near the melting point (T) of the B-layer polyester, between T and T+20°C, whitening spots during retort processing can be suppressed. [Examples]

[0031] 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.

[0032] (1) Polyester content 1. The copolymer components of the polyester and the amounts of each component were identified by 1H-NMR measurement.

[0033] (2) Determination of the amorphous nature of polyester Approximately 10 mg of polyester is sealed in an aluminum pan for measurement and mounted on a differential scanning calorimeter (TA Instruments, DSCQ100). When the temperature is raised from 20°C to 300°C at a rate of 20°C / min, it is confirmed that no crystal melting peak (endothermic peak) is observed.

[0034] (3) Melting point of amorphous polyester In accordance with JIS-K-0064:1992, the finely crushed sample is packed into a 3 mm capillary tube, and the sample is heated using a melting point analyzer so that the temperature of the heated liquid rises by approximately 1°C per minute. When the sample melts in the capillary tube and no solid material is observed, the temperature is read with a thermometer and defined as the melting point.

[0035] (4) Evaluation of lamination with metal plate A film sample was laminated to one side of a 0.25 mm thick tinplate sheet by heat fusion at 220°C, with layer B facing the metal plate side. The adhesive strength and appearance of the film bonded to the tinplate sheet were evaluated according to the following procedure. ○: Can be heat-bonded and laminated neatly. △: Heat bonding is possible, but there is significant shrinkage in width. ×: Cannot be heat-bonded (peels off the tin plate immediately after bonding)

[0036] (5) Molding processability Following method (4), a tinplate laminated with film was cut into a 140mm diameter disc, and processed in three stages using a drawing die and punch to create a seamless container with a height of 140mm and a diameter of 50mm (hereinafter referred to as "can"). The processing status of these cans was observed and evaluated according to the following criteria. ○: The film was processed without any abnormalities, and no chipping or pinholes were observed on the top of the can after molding. △: Minor chipping was observed on the top of the can after molding, but no other defects were found. ×: After molding, large chips appeared on the top of the can and pinholes appeared on the can wall.

[0037] (6) Corrosion test after heating The metal container prepared in (5) was filled with 3% acetic acid and subjected to retort treatment at 135°C for 2 hours. After treatment, the can was disassembled and the inside was observed and evaluated according to the following criteria. ○: No discoloration or corrosion is observed at all. △: Some discoloration is visible, but it has not progressed to corrosion. ×: There are areas that are discolored black and the metal is corroded.

[0038] [Example 1] A copolymer polyethylene terephthalate was prepared by copolymerizing 0.1% by mass of bulk silica with an average particle size of 1.5 μm as layer A, with 12 mol% isophthalic acid and 0.4 mol% C36 hydrogenated dimer acid relative to the total dicarboxylic acid components. A copolymer polyethylene terephthalate was prepared by copolymerizing 0.5% by mass of bulk silica with an average particle size of 2.5 μm as layer B, with 22 moles of isophthalic acid and 2 moles of C36 hydrogenated dimer acid relative to the total dicarboxylic acid components. These two copolymers were dried and melted independently, and then co-extruded at 280°C from adjacent dies in a layer A:B ratio of 4:1, followed by rapid cooling and solidification to obtain an unstretched laminated film. Next, this unstretched film was longitudinally stretched 3.2 times at 110°C, transversely stretched 3.5 times at 120°C, and then heat-set at 180°C to obtain a biaxially oriented polyester film. The total thickness of the film was 25 μm. The evaluation results of the obtained film are shown in Table 1.

[0039] [Example 2] A copolymer polyethylene terephthalate was prepared using two copolymers: Layer A, 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 0.2 mol% C36 hydrogenated dimer acid relative to the total dicarboxylic acid components; and Layer B, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized with 24 moles of isophthalic acid and 1 mole of C36 hydrogenated dimer acid relative to the total dicarboxylic acid components. Films were prepared and evaluated in the same manner as in Example 1. The evaluation results of the obtained films are shown in Table 1.

[0040] [Example 3] A copolymer polyethylene terephthalate was prepared using two copolymers: Layer A, which contained 0.1% by mass of bulk silica with an average particle size of 1.5 μm, copolymerized with 10 mol% isophthalic acid and 0.8 mol% C36 hydrogenated dimer acid relative to the total dicarboxylic acid components; and Layer B, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized with 20 moles of isophthalic acid and 4 moles of C36 hydrogenated dimer acid relative to the total dicarboxylic acid components. Films were prepared and evaluated in the same manner as in Example 1. The evaluation results of the obtained films are shown in Table 1.

[0041] [Example 4] The procedure was the same as in Example 1, except that a copolymerized polyethylene terephthalate containing 0.5% by mass of bulk silica with an average particle size of 2.5 μm as layer B, copolymerized with 20 mol% isophthalic acid and 8 mol% C44 hydrogenated dimer acid relative to the total dicarboxylic acid components, and the heat setting temperature was set to 160°C. The evaluation results of the obtained film are shown in Table 1.

[0042] [Comparative Example 1] A copolymer polyethylene terephthalate was prepared and evaluated in the same manner as in Example 1, except that the B layer contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized with 15 mol% isophthalic acid and 2 mol% C36 hydrogenated dimer acid relative to the total dicarboxylic acid components, and the heat setting temperature was set to 190°C. The evaluation results of the obtained film are shown in Table 1.

[0043] [Comparative Example 2] A film was prepared and evaluated in the same manner as in Example 1, except that the A layer was copolymerized polyethylene terephthalate containing 0.1% by mass of bulk silica with an average particle size of 1.5 μm and copolymerized with 10 mol% isophthalic acid relative to the total dicarboxylic acid components, and the B layer was copolymerized polyethylene terephthalate containing 0.5% by mass of bulk silica with an average particle size of 2.5 μm and copolymerized with 18 mol% isophthalic acid and 2 mol% C36 hydrogenated dimer acid relative to the total dicarboxylic acid components. The obtained film showed good lamination evaluation with metal plates, but significant abrasion occurred on the top of the can during the moldability evaluation. For this reason, a corrosion test after heating could not be performed.

[0044] [Comparative Example 3] A film was prepared and evaluated in the same manner as in Example 1, except that the A layer contained 0.1% by mass of bulk silica with an average particle size of 1.5 μm, copolymerized polyethylene terephthalate with 10 mol% isophthalic acid and 2 mol% C36 hydrogenated dimer acid relative to the total dicarboxylic acid components, and the B layer contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized polyethylene terephthalate with 22 moles of isophthalic acid and 2 moles of C36 hydrogenated dimer acid relative to the total dicarboxylic acid components. The obtained film showed good lamination evaluation with metal plates, but numerous pinholes occurred in the can wall during the moldability evaluation. For this reason, a corrosion test after heating could not be performed.

[0045] [Table 1] [Industrial applicability]

[0046] The biaxially oriented polyester film for metal sheet lamination and molding according to the present invention exhibits excellent adhesion when heat-pressed onto a metal sheet, and shows excellent formability when molded after lamination. Furthermore, it is resistant to corrosion even when a metal container is formed, filled with contents, and subjected to impact at low temperatures, making it suitable for use in metal container applications.

Claims

1. A biaxially oriented polyester film comprising at least two layers: a polyester layer A and an amorphous polyester layer B, wherein the polyester resin constituting layer B contains copolymerized polyethylene terephthalate having two or more copolymerized monomers as copolymer components, each containing at least one aliphatic dicarboxylic acid having 30 or more carbon atoms, and the polyester constituting layer A contains copolymerized polyethylene terephthalate containing 0.1 to 1 mol% of aliphatic dicarboxylic acid having 30 or more carbon atoms as a copolymer component, for use in metal sheet lamination molding.

2. The polyester film according to claim 1, wherein the melting point of the polyester constituting layer B is 180°C or higher and 215°C or lower.

3. A laminated metal sheet comprising a biaxially oriented polyester film for metal sheet lamination molding according to claim 1 or 2 and a metal sheet.

4. A metal container comprising the laminated metal plate described in claim 3.

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