Biaxially oriented polyester film for metal sheet lamination and molding processes.

Abstract

The present invention provides a biaxially oriented polyester film for metal sheet lamination and molding processes that, when heat-sealed during the lamination process to produce can lids, exhibits excellent adhesion and superior opening properties that prevent feathering during opening, and can prevent corrosion even with contents that are prone to metal corrosion. [Solution] A biaxially oriented polyester film for metal plate lamination forming, comprising at least two layers including a polyester-containing layer A and an amorphous polyester-containing layer B, wherein the amorphous polyester contained in layer B is copolymerized polyethylene terephthalate having two or more copolymerized monomers as copolymerized components, each containing at least one aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms, and the loss modulus peak of the biaxially oriented polyester film is within a specific temperature range.

Description

Technical Field

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

Background Art

[0002] Metal cans are generally painted 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 purposes 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, it is used for metal plates such as tinplate, tin-free steel, and aluminum laminated with a thermoplastic resin film, and for metal cans such as food cans and beverage cans and their lids. Among them, in the case of metal lids, in recent years, there has been a tendency to attach a can lid called an easy-open end (EOE) that can be opened by hand without using an opening tool such as a can opener. For this reason, there is a need for an opening property that allows the film to open the can lid without generating feathering.

[0003] In order to achieve good opening properties, the following two requirements are necessary. (i) The film adheres to the metal plate and does not peel off from the metal plate during opening. (ii) The film is broken and torn with appropriate stress.

[0004] Regarding (i), in addition to the method of using an adhesive to bond a polyester film to a metal plate or the like, for example, a method of directly bonding to an adherend by imparting thermo-adhesiveness to the polyester film, and further, a method of imparting thermo-adhesiveness by forming a laminated structure of a layer having a thermo-adhesive layer and a layer using the polyester film as a base material.

[0005] Among these, in the case of a laminated polyester film having a thermo-adhesive layer, it is excellent in that it is easy to have both thermo-adhesiveness and film strength and dimensional stability (see, for example, Patent Documents 1 and 2). However, for example, when using a metal plate that is difficult to thermo-adhere, such as tinplate or aluminum, sufficient adhesiveness cannot be obtained.

[0006] On the other hand, Patent Document 3 describes a laminated polyester film that can be heat-bonded at low temperatures. However, when this film is used for EOE (End of End of Effect), feathering is likely to occur because the film stretches and is difficult to tear.

[0007] Furthermore, regarding (ii), Patent Document 4 discloses a biaxially stretched polyester resin film for metal can lids in which the surface orientation coefficient is defined within a certain range. This film is intended to have excellent moldability, impact resistance, and opening properties. However, when this film is used for EOE, corrosion may occur depending on the contents. [Prior art documents] [Patent Documents]

[0008] [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. 2001-335650 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The present invention was devised to solve the problems of the prior art described above, and its purpose is to provide a biaxially oriented polyester film for metal sheet lamination that, when heat-sealed in a lamination process to produce a can lid, has excellent adhesion and excellent opening properties that do not cause feathering when opened, and can prevent corrosion even with contents that are prone to metal corrosion. [Means for solving the problem]

[0010] As a result of diligent research to achieve the above objective, the inventors of the present invention have discovered that by using a polyester of a specific composition that does not have crystalline properties as a specific heat-adhesive layer in the polyester constituting the film, it is possible to manufacture a metal can lid that exhibits excellent adhesion to metal plates and corrosion resistance, and at the same time, by designing the film so that the peak of the loss modulus of elasticity is within a specific temperature range, it is possible to manufacture a metal can lid that also has excellent opening properties that do not cause feathering when opened, thus completing the present invention.

[0011] In other words, the present invention consists of the following configurations (1) to (3). (1) A biaxially oriented polyester film comprising at least two layers, an A layer containing polyester and a B layer containing amorphous polyester, wherein the amorphous polyester contained in the B layer is copolymerized polyethylene terephthalate having two or more copolymerized monomers as copolymerized components, each containing at least one aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms, and the biaxially oriented polyester film satisfies the following conditions (i) to (iii). (i) The highest peak temperature Te1 (°C) of the loss modulus of the film measured using a dynamic viscoelasticity measuring device, and the next highest peak temperature Te2 (°C) (ii) 90 ≤ Te1 (°C) ≤ 120 (iii) 25 ≤ Te2 (°C) ≤ 70 (2) A laminated metal sheet obtained by laminating a biaxially oriented polyester film for metal sheet lamination molding described in (1) with a metal sheet. (3) A metal can lid comprising the laminated metal plate described in (2). [Effects of the Invention]

[0012] The biaxially stretched polyester film for metal plate laminating and forming processing of the present invention can obtain excellent adhesiveness when thermocompression-bonded to a metal plate, can prevent deterioration of the opening property due to film peeling during opening, can prevent corrosion even for contents that are likely to cause metal corrosion, and can be opened without generating feathering due to the design that the film tears with an appropriate stress, so it can be suitably used for metal can lid applications.

Brief Description of the Drawings

[0013] [Figure 1] It is an explanatory diagram of Te1 and Te2 in the present invention.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described in detail. The biaxially stretched polyester film for metal plate laminating and forming processing of the present invention is a biaxially stretched polyester film composed of at least two layers including an A layer containing polyester and a B layer containing amorphous polyester, and the amorphous polyester contained in the B layer is a copolyethylene terephthalate having at least two types of copolymerized monomers including at least one aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms as copolymerization components, and the maximum peak temperature Te1 (°C) of the loss elastic modulus of the polyester film and the peak temperature Te2 (°C) at the next higher temperature satisfy a specific formula.

[0015] (Polyester contained in the A layer) The polyester constituting the A layer is not particularly limited, but preferably examples include homopolyethylene terephthalate, polyethylene terephthalate copolymer, and polyethylene-2,6-naphthalate copolymer. For suppressing feathering during opening, homopolyethylene terephthalate is particularly preferred.

[0016] When the polyester of layer A is copolymerized polyethylene terephthalate, the copolymerization component can be either an acid or an alcohol. Examples of acid components include aliphatic dicarboxylic acids such as adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid, 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. Among these, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol are particularly preferred because they have the effect of improving moldability while maintaining heat resistance. Here, terephthalic acid and ethylene glycol are not used as copolymerization components. When copolymerized polyethylene terephthalate contains other copolymer components as described above, it is preferable that the total amount of copolymer components be less than 5 mol%, when the total amount of all dicarboxylic acid components and all diol components constituting the copolymerized polyethylene terephthalate is 200 mol%. This makes it easier to improve feathering resistance when opening.

[0017] Layer A may contain a blend of other resins in addition to polyethylene terephthalate or its copolymer. 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.

[0018] In addition, it is preferable to contain a lubricant in the polyester. The lubricant can be either inorganic or organic, but inorganic ones are preferred. Examples of inorganic lubricants include silica, alumina, titanium dioxide, calcium carbonate, barium sulfate, etc. Examples of organic lubricants include silicone resin particles, crosslinked polystyrene particles, etc. Particularly, a lubricant 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.

[0019] Other additives such as antioxidants, dispersants, viscosity modifiers, fluorescent brighteners, heat stabilizers, ultraviolet absorbers, antistatic agents, etc. can also be added to the polyester.

[0020] (Amorphous polyester contained in the B layer) The polyester constituting layer B is amorphous polyester, specifically copolymerized polyethylene terephthalate. Here, amorphous means that no melting peak is observed 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. Preferably, at least one of the copolymerization components of the copolymerized polyethylene terephthalate is an aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms, i.e., adipic acid, pimelic acid, suberic acid, azelaic acid, or sebacic acid. The carbon number mentioned above does not include carbon atoms in the carboxyl group. By using these dicarboxylic acids as copolymerization components, the thermal adhesion to the metal plate is improved, which prevents deterioration of opening performance due to film peeling during opening, and at the same time greatly improves corrosion resistance when subjected to impact at low temperatures. The copolymerization amount of aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms is preferably 1 to 20 mol%, and particularly preferably 3 to 15 mol%, when the total dicarboxylic acid component constituting the copolymerized polyethylene terephthalate is 100 mol%.

[0021] To obtain an amorphous resin from copolymerized polyethylene terephthalate, it is preferable to further add at least one monomer other than an aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms 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, 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, with isophthalic acid being particularly preferred. Here, terephthalic acid and ethylene glycol are not used as copolymer 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 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 having alkylene groups with 4 to 8 carbon atoms, be 20 mol% or more, more preferably 22 mol% or more, and particularly preferably 25 mol% or more. For example, when sebacic acid is added at 8 mol% of the total carboxylic acid, if the second copolymer component is isophthalic acid, it is preferable to add 12 mol% or more, more preferably 14 mol% or more, and particularly preferably 17 mol% or more of the total carboxylic acid. Similarly, in the case of neopentyl glycol, which is the diol component, it is preferable to add 12 mol% or more, more preferably 14 mol% or more, and particularly preferably 17 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 copolymerized polyethylene terephthalate are preferably added during the polymerization of polyethylene terephthalate to form a sufficiently random copolymer polymer. There is a method to obtain a polyester of a desired composition by kneading two or more types of polyester, obtained by separately polymerizing copolymerized polyesters, during the melting process. However, it is preferable to use this resin only after confirming that the transesterification reaction has proceeded sufficiently and the resin has become amorphous.

[0022] The melting point (T) of copolymerized polyethylene terephthalate is preferably between 180°C and 215°C. Since the melting point of amorphous polyester cannot be measured by DSC, it is measured according to JIS-K-0064:1992 "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, as described in 3.1 of the same standard, by the "visual method". A T of 215°C or lower is preferable because it is easier to obtain sufficient adhesive strength regardless of the type of metal to be laminated. On the other hand, a T of 180°C or higher is preferable because it improves heat resistance and prevents the film from being destroyed by heat generated during the can manufacturing process. Note that layer B may contain other resins other than copolymerized polyethylene terephthalate, and the other resin is preferably an amorphous resin.

[0023] It is preferable to include a lubricant in the copolymerized polyethylene terephthalate. 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. Since layer B is composed of amorphous resin, it is difficult to form protrusions during biaxial stretching, so it is preferable to add a larger amount of lubricant. For example, when using silica, it is preferable to add silica with an average particle size of 2 μm or more in the range of 0.01% to 0.3% by mass.

[0024] Polyester can also be treated with other additives, such as antioxidants, dispersants, viscosity modifiers, fluorescent whitening agents, heat stabilizers, UV absorbers, and antistatic agents.

[0025] <Film properties> The polyester film of the present invention is used in a biaxially stretched and heat-fixed state. In this case, the polyester film of the present invention preferably satisfies the following conditions: (i) the highest peak temperature Te1 (°C) of the loss modulus of the film measured using a dynamic viscoelasticity measuring device, and the next highest peak temperature Te2 (°C), as shown in the example in Figure 1. (ii) 90 ≤ Te1 (°C) ≤ 120 (iii) 25 ≤ Te2 (°C) ≤ 70

[0026] Te1 is preferably 90°C to 120°C, more preferably 93°C to 117°C, and particularly preferably 95°C to 115°C. When Te1 is 90°C or higher, the film is moderately easy to tear, which is preferable in terms of opening properties. On the other hand, when Te1 is 120°C or lower, the molecular orientation and crystallinity of the film do not become too high, and the moldability is good even in lid applications where the degree of moldability is low, which is also preferable. The value of Te1 can be adjusted by adjusting the copolymerization components and copolymerization amount, and by the film formation conditions, particularly by the biaxial stretching ratio or stretching temperature.

[0027] Te2 is preferably 25°C to 70°C, more preferably 30°C to 68°C, and particularly preferably 34°C to 65°C. A Te2 of 25°C or higher is preferable because it prevents the B layer from becoming too soft and reduces feathering during opening. On the other hand, a Te2 of 70°C or lower is preferable because it provides good thermal adhesion to the metal plate during the lamination process, prevents the film from peeling off during opening, and provides good opening properties. The value of Te2 can be adjusted by the copolymerization components and copolymerization amount.

[0028] For example, as a copolymer monomer component other than the aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms contained in the amorphous copolymer polyethylene terephthalate contained in layer B, isophthalic acid has a moderate effect of lowering Te2 and is particularly preferred in the present invention. On the other hand, for example, 1,4-cyclohexanedimethanol has a smaller effect of lowering Te2 than isophthalic acid, while polyethylene glycol 4000 (PEG-4000) has a larger effect of lowering Te2 than isophthalic acid. Te2 can be appropriately adjusted by adjusting the copolymerization amount depending on the type of copolymer monomer used.

[0029] Furthermore, the difference between Te1 and Te2 (Te1-Te2) is preferably 30°C or more and less than 80°C, more preferably 35°C or more and less than 75°C, and particularly preferably 40°C or more and less than 70°C. Satisfying these conditions makes it easier to achieve both thermal adhesion with the metal sheet during the lamination process and ease of opening during the opening process.

[0030] Here, Te1 and Te2 are measured using a dynamic viscoelasticity measuring device at a frequency of 10 Hz and a dynamic displacement of ±25 × 10⁻⁶. ―4 The measurement is determined in cm at a heating rate of 2°C / min. However, the film should be stored in a desiccator overnight before measurement.

[0031] <Film thickness> The thickness of the biaxially oriented polyester film of the present invention can be changed as needed, but a range of 6 to 75 μm is preferred, with a range of 10 to 75 μm being particularly preferred, and a range of 15 to 50 μm being especially preferred. A thickness of 6 μm or more makes tearing and other damage less likely during molding, while a thickness of 75 μm or less is more economical.

[0032] 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 10 / 1 to 1 / 10 from the viewpoint of film-forming properties, adhesion, and rust prevention.

[0033] <Film Manufacturing Method> The method for manufacturing the biaxially oriented polyester film for metal sheet lamination molding according to the present invention, as described above, is not particularly limited. First, an unstretched laminated sheet can be prepared by a conventionally known film-forming method, and then 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 polyester prepared for layer B, it is supplied in an extruder at a temperature above the melting point (T) of polyester, usually in the range of T+30 to T+80°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.

[0034] 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.

[0035] The unstretched laminated sheet can then be manufactured by sequential or simultaneous biaxial stretching and heat-fixing. 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 with a tenter. At this time, it is preferable to set the stretching temperature to 10 to 50°C higher than the Tg of the polyester of layer A, the longitudinal stretching ratio to be in the range of 2.5 to 4.0 times, and the transverse stretching ratio to be in the range of 2.6 to 4.5 times. The heat-fixing temperature is preferably determined according to the melting point (T) of the polyester of layer B, and is best adjusted within the range of T-40°C to T+15°C.

[0036] The biaxially oriented polyester film for metal sheet lamination processing of the present invention is preferably laminated to a metal sheet, particularly a metal sheet for can manufacturing, such as tinplate, tin-free steel, or aluminum. The lamination temperature is preferably one that balances the adhesion of the film with the moldability into a metal container. The metal sheet laminated with the film can be used to form a metal can lid using known molding methods.

[0037] 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, and may be used not only on the can lid but also on the can body. 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]

[0038] 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.

[0039] (1) Polyester content 1. The copolymer components of the polyester and the amounts of each component were identified by 1H-NMR measurement.

[0040] (2) Determination of the amorphous nature of polyester Approximately 10 mg of polyester was sealed in an aluminum pan for measurement and mounted on a differential scanning calorimeter (TA Instruments, DSCQ100). When the temperature was raised from 20°C to 300°C at a rate of 20°C / min, it was confirmed that no crystal melting peak (endothermic peak) was observed.

[0041] (3) Melting point of amorphous polyester In accordance with JIS-K-0064:1992, the finely crushed sample was packed into a 3 mm capillary tube, and the sample was heated using a melting point analyzer so that the temperature of the heated liquid rose by approximately 1°C per minute. When the sample melted in the capillary tube and no solid material was observed, the temperature was read with a thermometer and defined as the melting point.

[0042] (4) Peak temperature of the loss modulus of the film (Te1 and Te2) A dynamic viscoelasticity measuring device (PerkinElmer, DMA8000) was used, with a measurement frequency of 10 Hz and a dynamic displacement of ±25 × 10⁻¹⁰. -4 The loss modulus was determined in cm, and the highest peak temperature Te1 (°C) and the next highest peak temperature Te2 (°C) were shown. However, the film was stored in a desiccator overnight before measurement.

[0043] (5) Thermal adhesiveness 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 of the film bonded to the tinplate sheet was evaluated according to the following criteria. ○: It can be heat-bonded, and the film tears when you try to peel it off. △: Heat bonding is possible, but it can be peeled off without tearing the film. ×: Cannot be heat-bonded (the film peels off the tin plate immediately after bonding)

[0044] (6) Openness A tinplate laminated with film was prepared according to method (5), and a 70mm diameter die was punched out with the laminated surface facing the inner surface of the lid. Sealing compound was applied to the inner surface of the curled portion and allowed to dry. Then, a partial opening type score (score remaining thickness 110μm, score width 20μm), rivet processing, and opening tabs were attached to the outer surface of the lid to create an easy-open lid. An opening test was conducted on this lid and evaluated according to the following criteria. ○: No feathering occurred when opening the mouth. △: Feathering occurred during opening, but the film stretched by less than 2 mm. ×: Feathering occurred during opening, and the film stretched by more than 2 mm.

[0045] (7) Corrosion resistance The lids prepared in (6) were immersed in 3% acetic acid and subjected to retort treatment at 120°C for 2 hours. After treatment, the lids were 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.

[0046] [Example 1] A layer was composed of homopolyethylene terephthalate, and B layer was composed of copolymerized polyethylene terephthalate containing 0.5% by mass of bulk silica with an average particle size of 2.5 μm, copolymerized with 15 mol% isophthalic acid and 6 mol% sebacic acid relative to the total carboxylic acid. These two layers were dried and melted independently, then co-extruded at 280°C through adjacent dies in an A:B layer ratio of 4:1, and rapidly cooled and solidified to obtain an unstretched laminated film. Next, this unstretched film was longitudinally stretched 3.5 times at 90°C, transversely stretched 3.9 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 15 μm. The evaluation results of the obtained film are shown in Table 1.

[0047] [Example 2] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that a copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm as the B layer, and was copolymerized with 15 mol% isophthalic acid and 10 mol% sebaciic acid relative to the total carboxylic acid. The evaluation results of the obtained film are shown in Table 1.

[0048] [Example 3] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that a copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm as the B layer, and was copolymerized with 15 mol% isophthalic acid and 6 mol% adipic acid relative to the total carboxylic acid, with a longitudinal stretching ratio of 4.0 times and a transverse stretching ratio of 4.2 times. The evaluation results of the obtained film are shown in Table 1.

[0049] [Example 4] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that a copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm as the B layer, and was copolymerized with 15 mol% isophthalic acid, 10 mol% adipic acid, and 10 mol% PEG-4000 relative to the total carboxylic acid components. The longitudinal stretching ratio was 3.2 times and the transverse stretching ratio was 3.3 times. The evaluation results of the obtained film are shown in Table 1.

[0050] [Comparative Example 1] Layer A consisted of 3 mol% isophthalic acid copolymerized polyethylene terephthalate, and Layer B consisted of copolymerized polyethylene terephthalate copolymerized with 11 mol% isophthalic acid relative to the total carboxylic acid, containing 0.5 mass% of bulk silica with an average particle size of 2.5 μm. These two layers were dried and melted independently, then co-extruded at 280°C through adjacent dies in a ratio of Layer A:B = 1:9, and rapidly cooled and solidified to obtain an unstretched laminated film. Next, this unstretched film was longitudinally stretched 3.2 times at 90°C, transversely stretched 3.3 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 20 μm. The evaluation results of the obtained film are shown in Table 1.

[0051] [Comparative Example 2] A biaxially oriented polyester film was obtained in the same manner as in Comparative Example 1, except that a single layer was extruded using homopolyethylene terephthalate containing 0.1% by mass of bulk silica with an average particle size of 2.5 μm. The total thickness of the film was 12 μm. The evaluation results of the obtained film are shown in Table 1.

[0052] [Comparative Example 3] A biaxially oriented polyester film was obtained 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, and copolymerized polyethylene terephthalate obtained by copolymerizing 24 mol% isophthalic acid with total carboxylic acid. The evaluation results of the obtained film are shown in Table 1.

[0053] [Comparative Example 4] A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that a copolymerized polyethylene terephthalate was used, which contained 0.5% by mass of bulk silica with an average particle size of 2.5 μm as the B layer, and was copolymerized with 5 mol% adipic acid relative to the total carboxylic acid and 25 mol% 1,4-cyclohexanedimethanol relative to the total diol components. The evaluation results of the obtained film are shown in Table 1.

[0054] [Comparative Example 5] A biaxially oriented polyester film was obtained 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, and copolymerized polyethylene terephthalate with 12 mol% isophthalic acid, 6 mol% adipic acid, and 15 mol% PEG-4000 relative to the total carboxylic acid components. The evaluation results of the obtained film are shown in Table 1.

[0055] [Table 1]

[0056] As can be seen from Table 1, the polyester films of Examples 1 to 4, which satisfy the conditions of the present invention, received good evaluations, while the polyester films of Comparative Examples 1 to 5, which do not satisfy either condition Te1 or Te2, clearly showed inferior evaluations. [Industrial applicability]

[0057] The biaxially oriented polyester film for metal sheet lamination molding of the present invention exhibits excellent adhesion when heat-pressed to a metal sheet, prevents deterioration of opening performance due to film peeling during opening, and prevents corrosion even with contents that are prone to metal corrosion. Furthermore, because the film is designed to tear under appropriate stress, it can be opened without feathering, making it suitable for use as a metal can lid.

Claims

1. A biaxially oriented polyester film comprising at least two layers, an A layer containing polyester and a B layer containing amorphous polyester, wherein the amorphous polyester contained in the B layer is copolymerized polyethylene terephthalate having two or more copolymerized monomers as copolymerized components, each containing at least one aliphatic dicarboxylic acid having an alkylene group with 4 to 8 carbon atoms, and the biaxially oriented polyester film satisfies the following conditions (i) to (iii). (i) The highest peak temperature Te1 (°C) of the loss modulus of the film measured using a dynamic viscoelasticity measuring device, and the next highest peak temperature Te2 (°C) (ii) 90≦Te1 (°C)≦120 (iii) 25≦Te2 (°C)≦70

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

3. A metal can lid comprising the laminated metal plate described in claim 2.

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