Laminated polyester film for metal sheet bonding 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 laminated film for metal plate bonding is provided, comprising a biaxially oriented polyester film containing polyester resin A, with a layer containing amorphous polyester resin B on one side, wherein the highest peak temperature Te1(°C) and the next highest peak temperature Te2(°C), measured using a dynamic viscoelasticity measuring device for the laminated film, satisfy 15 ≤ Te1(°C) - Te2(°C) ≤ 90.

Description

Technical Field

[0001] The present invention relates to a laminated polyester film for metal plate bonding and 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, a laminate of a thermoplastic resin film on a metal plate such as tinplate, tin-free steel, aluminum, etc. is used for metal cans such as food cans and beverage cans and their lids. 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 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.

[0005] Among these, in the case of a laminated polyester film having a thermoadhesive layer, it is excellent in that it is easy to have thermoadhesiveness 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 thermoadhere, 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.

[0008] To prevent corrosion of end-of-life (EOE), Patent Document 5 discloses a technique for coating a metal surface with a film via an adhesive. However, even with the use of an adhesive, it may not be able to adequately follow the lamination process and subsequent molding, and in some cases, corrosion may progress because the adhesion cannot be maintained during retort processing. [Prior art documents] [Patent Documents]

[0009] [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 [Patent Document 5] Japanese Patent Publication No. 2008-296440 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] The present invention was devised to solve the problems of the prior art described above, and its purpose is to provide a laminated polyester film for metal sheet bonding and molding that has excellent adhesion and excellent opening properties that do not cause feathering when opening when a can lid is manufactured by heat pressing in the lamination process, and that can prevent corrosion even with contents that are prone to metal corrosion. [Means for solving the problem]

[0011] As a result of diligent research to achieve the above objective, the inventors of the present invention have discovered that by using polyester of a specific composition for the polyester constituting the film, it is possible to manufacture a metal can lid that exhibits excellent adhesion to metal plates and corrosion resistance, while simultaneously having excellent opening properties that prevent feathering when opened, by ensuring that the peak of the film's loss modulus is within a specific temperature range. This has led to the completion of the present invention.

[0012] In other words, the present invention consists of the following configurations (1) to (6). (1) A laminated polyester film for metal plate lamination and molding, comprising a biaxially oriented polyester film containing polyester resin A, with a layer containing amorphous polyester resin B provided on one side, wherein the highest temperature peak temperature Te1 (°C) of the loss modulus of the film, measured using a dynamic viscoelasticity measuring device for the laminated film, and the temperature of the next highest peak Te2 (°C) satisfy the following formula. 15 ≤ Te1(°C) - Te2(°C) ≤ 90 (2) The laminated polyester film for metal plate lamination molding according to (1), wherein the polyester resin B contains 0.5 to 5 mol% of trifunctional or higher polycarboxylic acid components with respect to 100 mol% of the total polycarboxylic acid components constituting the polyester resin B. (3) A laminated polyester film for metal plate bonding and molding according to (1), wherein the polyester resin B consists of a plurality of ester constituent units containing two or more polycarboxylic acid components and two or more polyol components. (4) The laminated polyester film for metal plate laminating and forming processing according to (1), wherein the layer containing polyester resin B is a coating layer. (5) A method for manufacturing a metal plate, comprising a step of thermocompression bonding the biaxially stretched polyester film for metal plate laminating and forming processing according to any one of (1) to (4) to a metal plate. (6) A method for manufacturing a metal container, comprising a step of forming the metal plate obtained by the method for manufacturing a metal plate according to (5). [Advantages of the Invention]

[0013] The laminated polyester film for metal plate laminating and forming processing of the present invention can obtain excellent adhesiveness when thermocompression bonded to a metal plate, prevent deterioration of the opening property due to film peeling during opening, and prevent corrosion even for contents that are likely to cause metal corrosion. In addition, the film can be opened without generating feathering due to a design in which the film tears with an appropriate stress, so it can be suitably used for metal can lid applications. [Brief Description of the Drawings]

[0014] [Figure 1] It is an explanatory diagram of Te1 and Te2 in the present invention. [Embodiments for Carrying Out the Invention]

[0015] Hereinafter, embodiments of the present invention will be described in detail. The laminated polyester film for metal plate laminating and forming processing of the present invention is a laminated film provided with a layer containing an amorphous polyester resin B on one side of a biaxially stretched polyester film containing a polyester resin A, and is characterized in that the highest peak temperature Te1 (°C) of the loss elastic modulus of the laminated film and the next higher peak temperature Te2 (°C) satisfy a specific formula.

[0016] [Polyester Resin A] The polyester resin A that constitutes the biaxially oriented polyester film is not particularly limited as long as it is a polyester resin, but preferably examples thereof include homopolyethylene terephthalate, polyethylene terephthalate copolymer, and polyethylene-2,6-naphthalate copolymer. For suppressing feathering during opening, homopolyethylene terephthalate is particularly preferred.

[0017] The polyester resin A may be a polyester resin copolymerized with a third component as long as it does not affect the feathering property during opening. Such copolymerization components may be either a polycarboxylic acid component or a polyol component. Examples of the polycarboxylic acid component include aliphatic dicarboxylic acids such as adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, fumaric acid, and maleic 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 the alcohol component include aliphatic diols such as 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, and triethylene glycol, and alicyclic diols such as 1,4-cyclohexanedimethanol. Among these, isophthalic acid, 2,6-naphthalenedicarboxylic acid, fumaric acid, maleic acid, 1,4-cyclohexanedimethanol, and neopentyl glycol are preferred because they have the effect of improving the moldability without deteriorating the opening property. When the polyester resin A contains the above copolymerization components, when the total of all polycarboxylic acid components and all polyol components constituting the polyester is 200 mol%, it is preferably less than 10 mol% in total of the copolymerization components. This makes it easier to improve the feathering resistance during opening.

[0018] The layer containing the polyester resin A may contain other resins blended therein. Examples of other resins include polycarbonate, polyethylene, polypropylene, polyamide, polyvinyl chloride, ionomer, and silicone resin.

[0019] Furthermore, it is preferable to include a lubricant in polyester resin A. 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 average particle size is 0.8 μm.

[0020] Polyester resin A may also contain other additives, such as antioxidants, dispersants, viscosity modifiers, fluorescent whitening agents, heat stabilizers, UV absorbers, and antistatic agents.

[0021] <Polyester resin B> Polyester resin B is an amorphous polyester. Here, amorphous means that, according to the method described in JIS-K-7121, no melting peak is observed when a mass of approximately 10 mg is heated from 20°C at a rate of 20°C / min in DSC measurement. Polyesters are condensates of polycarbonates and polyols. Examples of polycarboxylic acid components include aromatic polycarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, 2,6-naphthalenedicarboxylic acid, trimellitic acid, and pyromellitic acid; aliphatic polycarboxylic acids such as adipic acid, azelaic acid, sebacic acid, dimer acid, fumaric acid, and maleic acid; and alicyclic polycarboxylic acids such as 1,4-cyclohexanedicarboxylic acid. Examples of polyol components include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol, glycerol, pentaerythritol, trimethylolethane, and trimethylolpropane; and alicyclic polyols such as 1,4-cyclohexanedimethanol.

[0022] These can be used individually or in combination of two or more, but it is more preferable to use multiple ester constituent units containing two or more polycarboxylic acid components and two or more polyol components in order to reduce the crystallinity of the resin. Among these, terephthalic acid, isophthalic acid, orthophthalic acid, 2,6-naphthalenedicarboxylic acid, fumaric acid, maleic acid, and trimellitic acid are preferred as polycarboxylic acid components, and ethylene glycol, 1,2-propanediol, 1,4-cyclohexanedimethanol, and neopentyl glycol are preferred as polyol components. The polycarboxylic acid and polyol components are preferably designed so that the polyester does not have crystallinity, and the most abundant polycarboxylic acid component is preferably 90 mol% or less, more preferably 85 mol% or less, and particularly preferably 80 mol% or less, based on 100 mol% of the total polycarboxylic acid components. Similarly, the most abundant polyol component is preferably 90 mol% or less, more preferably 85 mol% or less, and particularly preferably 80 mol% or less, based on 100 mol% of the total polyol components.

[0023] Furthermore, the Tg of polyester resin B is preferably 25°C to 75°C, more preferably 30°C to 75°C, and even more preferably 50°C to 70°C. If the Tg of polyester is 25°C or higher, blocking is easily suppressed when winding the laminated film, which is made by laminating polyester resin B onto a biaxially oriented polyester film made of polyester resin A, into a roll shape. On the other hand, if it is 75°C or lower, lamination is easier without impairing adhesion to metal.

[0024] To achieve a preferred Tg range, for example, when the total polycarboxylic acid component is 100 mol%, it is preferable to include 50-80 mol% terephthalic acid, 0-30 mol% isophthalic acid, and 0-30 mol% 2,6-naphthalenedicarboxylic acid, and when the total polyol component is 100 mol%, it is preferable to include 20-90 mol% ethylene glycol, 0-80 mol% 1,2-propanediol, and 0-40 mol% 1,4-cyclohexanedimethanol.

[0025] Polyester resin B plays a role in improving opening properties and corrosion resistance through its high adhesion to metal plates. When a polycarboxylic acid component with three or more functions, such as trimellitic acid, is included in polyester resin B, the polyester acquires a branched structure, improving cohesiveness and thus enhancing adhesion. Adhesion to metal plates is easily improved if the content of trifunctional or more polycarboxylic acid components is 0.5 mol% or more of the total 100 mol% polycarboxylic acid components. Furthermore, if it is 5 mol% or less, the cohesiveness is not so high that it impairs opening properties. Additionally, including dicarboxylic acid components with unsaturated bonds, such as fumaric acid and maleic acid, in polyester resin B can also increase the cohesiveness of polyester resin B, thus enhancing adhesion. It is preferable that the content of unsaturated dicarboxylic acids is 5 mol% or more of the total 100 mol% polycarboxylic acid components constituting polyester resin B, as this improves adhesion to metal plates. Side reactions are easily suppressed if the content of unsaturated dicarboxylic acids is 20 mol% or less. In this case, the acid value of polyester resin B is preferably 100 eq / ton or higher, more preferably 200 eq / ton or higher, and particularly preferably 300 eq / ton or higher. Having an acid value above the above lower limit ensures sufficient adhesion of polyester resin B to the metal substrate, and thus improves corrosion resistance.

[0026] It is preferable that polyester resin B is a sufficiently random copolymer polymer, with all its constituent monomer components added during polymerization. There is a method to obtain a polyester of a desired composition by kneading two or more different polyesters, which have been polymerized separately, during melting. In this case, it is preferable that the transesterification reaction proceeds sufficiently during melting so that the resin exhibits amorphous properties, that is, it is preferable that no melting point peak is observed in the DSC measurement described above.

[0027] The reduced viscosity of polyester resin B is preferably 0.2 to 0.6 dl / g, and more preferably 0.3 to 0.5 dl / g. If the reduced viscosity is 0.2 dl / g or higher, sufficient cohesive force is easily achieved, resulting in good adhesion. If it is 0.6 dl / g or lower, it is less likely to stretch too much and hinder opening.

[0028] It is preferable that polyester resin B further contains a catalyst. By including a catalyst, the branching structure formation of polyester resin B can be promoted, and the adhesion to metal can be further improved. Examples of catalysts include acid catalysts such as sulfuric acid, p-toluenesulfonic acid, dodecylbenzenesulfonic acid, naphthalenesulfonic acid, dinonylnaphthalenesulfonic acid, dinonylnaphthalenedisulfonic acid, camphor sulfonic acid, and phosphoric acid, as well as amine blocks (partially neutralized by adding an amine) of these, organotin compounds such as dibutyltin dilaurylate, titanium compounds such as titanium tetrabutoxide, zinc compounds such as zinc acetate, hafnium compounds such as hafnium chloride-THF complex, and rare earth compounds such as scandium triflate. One or more of these can be used in combination. Among these, dodecylbenzenesulfonic acid and its neutralized product are preferred in terms of compatibility with polyester resin B and hygiene.

[0029] The preferred blending ratio of the catalyst to the polyester resin B is (polyester resin B) / (catalyst) = 100 / 0.01 to 100 / 1 (parts by mass), more preferably 100 / 0.05 to 100 / 0.8 (parts by mass), and most preferably 100 / 0.1 to 100 / 0.5 (parts by mass). Being within this range enhances the adhesion of the polyester resin B to the metal, and more effectively prevents feathering when the lid is opened after being processed into a metal lid. Side reactions such as gelation are more likely to occur when the amount of catalyst added is large, but it is preferable to add the catalyst after the production of the polyester resin B because this suppresses side reactions.

[0030] Polyester resin B may contain other additives, such as antioxidants, dispersants, viscosity modifiers, fluorescent whitening agents, heat stabilizers, UV absorbers, and antistatic agents. However, it is important that the amount of these additives is kept to a minimum and does not affect the flavor.

[0031] <Film properties> As shown in Figure 1, in the laminated polyester film of the present invention, when the highest temperature peak temperature of the loss modulus of the film measured using a dynamic viscoelasticity measuring device is Te1 (°C), and the next highest peak temperature is Te2 (°C), it is preferable that the difference between Te1 and Te2 (i.e., Te1 (°C) - Te2 (°C)) is 15 or more and 90 or less. The value of Te1 (°C) - Te2 (°C) is more preferably 20 or more and 85 or less, and particularly preferably 25 or more and 80 or less. If Te1 (°C) - Te2 (°C) is 15 or more, the thermal adhesion to the metal plate in the lamination process is good, and feathering is easily suppressed when the metal lid is opened. On the other hand, if it is 90 or less, feathering that occurs due to excessive stretching of the polyester resin B is easily suppressed.

[0032] In the present invention, Te1 is preferably 90°C to 120°C, and more preferably 95°C to 115°C. If Te1 is 90°C or higher, feathering is less likely to occur when opening the metal lid. On the other hand, if Te1 is higher than 120°C, the molecular orientation and crystallinity of the film become too high, which tends to significantly reduce moldability, even for lid applications where the degree of moldability is low. The value of Te1 depends on the copolymerization components and copolymerization amount, but can be adjusted by the film formation conditions, particularly by the biaxial stretching ratio or stretching temperature and the heat setting temperature. Depending on whether polyester resin A is homopolyethylene terephthalate or copolymerized polyethylene terephthalate, and the type and amount of copolymerized monomer components, if the stretching ratio is approximately 3.3 to 4.2 times in both the longitudinal and transverse directions, the stretching temperature is approximately 90 to 105°C, and the heat setting temperature is set to 180 to 220°C, it becomes easier to adjust Te1 to the aforementioned preferred range, and as a result, it becomes easier to appropriately adjust the Te1-Te2 value. Preferably, the Te1-Te2 value is adjusted by adjusting Te1 using the method described above, while adjusting the Te2 value with the copolymerized components and amount of copolymerization, and it is good to set the Tg of polyester resin B to the aforementioned preferred range.

[0033] 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 It can be determined using a temperature of cm and a heating rate of 2°C / min.

[0034] <Film Manufacturing Method> The method for manufacturing the laminated polyester film for metal plate lamination molding of the present invention described above is not particularly limited and can be manufactured by conventionally known methods. For example, first, the polyester resin A is thoroughly dried and then melt-extruded using an extruder at a crystal melting peak temperature (Tp) in DSC + 30 to 70°C to produce an unstretched laminated sheet.

[0035] Next, the unstretched laminated sheet is 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, and the longitudinal stretching ratio to be in the range of 3.0 to 4.2 times and the transverse stretching ratio to be in the range of 3.1 to 4.5 times. The heat fixing temperature is preferably determined according to the Tp of polyester resin A, and it is good to adjust it in the range of Tp-40°C to Tp+15°C. The thickness of the biaxially oriented polyester film can be changed as needed, but the range of 6 to 75 μm is preferable, and among these, the range of 10 to 75 μm, and especially 15 to 50 μm, is preferable. If the thickness is 6 μm or more, tearing etc. is less likely to occur during molding, while if it is 75 μm or less, it is economical.

[0036] The method of laminating a layer containing polyester resin B onto a biaxially oriented polyester film containing polyester resin A manufactured as described above is not particularly limited, but a preferred method is to prepare a coating solution by dissolving or dispersing polyester resin B in water or an organic solvent beforehand, and then coating the biaxially oriented polyester film using a coater.

[0037] Examples of organic solvents used to paint polyester resin B include toluene, xylene, ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, isophorone, methyl cellosolve, butyl cellosolve, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, ethylene glycol monoacetate, methanol, ethanol, butanol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and Solvesso. One or more of these are selected and used, taking into consideration solubility, evaporation rate, etc.

[0038] Polyester resin B can also be dispersed in an aqueous medium and used as a polyester resin aqueous dispersion. Methods for forming an aqueous dispersion include (a) dissolving polyester resin B in a water-soluble organic solvent in which polyester resin B dissolves, and then sequentially adding a basic compound and water as needed to disperse it; and (b) adding polyester resin B, water, a water-soluble organic solvent in which polyester resin B dissolves, and a basic compound as needed, and then heating and dispersing it. Furthermore, if it is desired to reduce the amount of organic solvent or to completely remove it to form an aqueous dispersion, it is also possible to disperse it using an organic solvent having a boiling point of 100°C or less, and then remove the solvent by heating or under reduced pressure. In the case of polyester resin B of the present invention, the former method (a) is preferred from the viewpoint of film-forming properties.

[0039] The coating manufactured using polyester resin B can be applied to a biaxially oriented polyester film using conventionally known equipment such as a roll coater, bar coater, or knife coater by known methods. Of course, it can also be applied using a spray or other methods. The adhesive is preferably applied to a thickness of 0.1 to 20 μm, preferably 0.5 to 15 μm, and more preferably 1 to 10 μm, in terms of solid content. After drying at 90 to 160°C, the film is wound into a roll.

[0040] The laminated 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 a 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.

[0041] The laminated polyester film for metal sheet bonding and molding 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. [Examples]

[0042] 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.

[0043] (1) Polyester content Polyester resin samples were dissolved in deuterated chloroform and subjected to 1H-NMR analysis using a VARIAN 400-MR nuclear magnetic resonance (NMR) spectrometer. The molar ratio was determined from the ratio of the integrated values.

[0044] (2) Measurement of reduced viscosity (unit: dl / g) A 0.1 g sample of polyester resin was dissolved in 25 cc of a phenol / tetrachloroethane mixed solvent (mass ratio 6 / 4) and measured at 30°C.

[0045] (3) Measurement of acid value 0.2 g of polyester resin sample was dissolved in 40 ml of chloroform and titrated with a 0.01 N potassium hydroxide ethanol solution. 6 The equivalent weight per gram (eq / ton) was determined. Phenolphthalein was used as the indicator.

[0046] (4) 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.

[0047] (5) Glass transition temperature (Tg) 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). The temperature was raised from 20°C to 250°C at a rate of 20°C / min, held for 3 minutes, cooled to 25°C at a rate of 20°C / min, and then raised again from 20°C to 250°C at a rate of 20°C / min to measure the glass transition temperature.

[0048] (6) Peak temperature of loss modulus (Te1 and Te2) The dynamic viscoelasticity was determined using a dynamic viscoelasticity analyzer (PerkinElmer DMA8000) with a measurement frequency of 10 Hz, dynamic displacement of ±25 μm, and heating rate of 2 °C / min.

[0049] (7) Thermal adhesiveness A film sample was laminated to one side of a 0.3 mm thick tinplate sheet by heat fusion at 220°C, with the side laminated with polyester resin B facing the metal plate. 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)

[0050] (8) Moldability and opening properties A tinplate laminated with film was punched out to a diameter of 90 mm with the laminated surface facing the inner surface of the lid according to method (7). Sealing compound was applied to the part that would become the inner surface of the curled portion and allowed to dry. Then, a partial opening type score (score remaining thickness 150 μm, score width 20 μm), rivet processing, and opening tab attachment were performed on the outer surface of the lid to create an easy-open lid. The moldability at this time was evaluated according to the following criteria. ○: Successfully molded △: Film lifting of less than 1 mm occurred after molding. ×: Film lifting or peeling of 1 mm or more occurred after molding. Furthermore, an opening test was conducted on this lid in 50°C water 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 1 mm. ×: Feathering occurred during opening, and the film stretched by more than 1 mm.

[0051] (9) Corrosion resistance The lids prepared in (8) were immersed in 3% acetic acid and subjected to retort treatment at 130°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.

[0052] [Synthesis of polyester resins A1-A3] Monomer raw materials were blended to achieve the compositions shown in Table 1. Manganese acetate was added as a transesterification catalyst at a ratio of 1 part by mass per 2000 parts by mass of monomer raw materials. The mixture was charged into a reactor equipped with a stirrer, a rectification column, and a methanol distillation condenser. The mixture was heated while gradually increasing the temperature from 130°C to 240°C, and the methanol produced as a result of the reaction was distilled off the system for 3 hours to allow the transesterification reaction to proceed. Next, 2 parts by mass of trimethyl phosphate was added as a stabilizer per 2000 parts by mass of monomer raw materials, 1 part by mass of bulk silica with an average particle size of 2.3 μm was added as a lubricant per 2000 parts by mass of monomer raw materials, and 0.3 parts by mass of germanium dioxide was added as a polycondensation catalyst per 2000 parts by mass of monomer raw materials. This reaction mixture was transferred to a reactor equipped with a stirrer and a glycol distillation condenser, and the mixture was subjected to a polycondensation reaction while gradually increasing the temperature from 240°C to 285°C and decreasing the pressure from atmospheric pressure to a high vacuum of 1 mmHg to obtain polyester resins A1 to A3. The reduced viscosity of the obtained polyester resin was 0.75 dl / g, and the acid value was 20 eq / ton.

[0053] [Synthesis of polyester resins B1-B5] Monomer raw materials were blended to achieve the compositions shown in Table 1. 0.5 parts by mass of tetra-n-butyl titanate (TBT) (0.03 mol% of the total polycarboxylic acid component) was added as a catalyst to a 3 L four-necked flask, and the transesterification reaction was carried out while gradually increasing the temperature to 240°C over 4 hours. After the reaction, the pressure in the system was gradually reduced, and polymerization was carried out under reduced pressure to 10 mmHg over 1 hour, while simultaneously raising the temperature to 245°C. Further polymerization was carried out under a vacuum of less than 1 mmHg for 50 minutes. This was then cooled to 210°C under a nitrogen atmosphere. Next, a predetermined amount of trimellitic anhydride was added, and stirring was continued for 30 minutes at 200-230°C under a nitrogen atmosphere. This was then removed to obtain polyester resins B1-B5. The reduced viscosity of the obtained polyester resins was 0.30 dl / g, and the acid value was 300 eq / ton.

[0054] [Table 1]

[0055] [Examples 1-7, Comparative Examples 1 and 2] After drying and melting the polyester resin A listed in Table 2, each was extruded from a die at 280°C and rapidly cooled and solidified to obtain an unstretched laminated film. Next, these unstretched films were longitudinally stretched at the temperatures and magnifications shown in Table 2, then transversely stretched at the temperatures and magnifications shown in Table 2, and subsequently heat-set at the temperatures shown in Table 2 to obtain a biaxially oriented polyester film. The thickness of the film was 20 μm. Next, 100 parts by mass (solids) of polyester resin B listed in Table 2, 0.3 parts by mass of dodecylbenzenesulfonic acid as a catalyst, and methyl ethyl ketone as a solvent were dissolved to obtain a coating solution with a solids content of 20% by weight. Next, this coating solution was applied to the aforementioned biaxially oriented polyester film using a roll coater and dried at 120°C to form a coating film with a thickness of 2 μm, thereby creating the laminated films of Examples 1 to 7 and Comparative Examples 1 and 2. The evaluation results of the obtained films are shown in Table 2.

[0056] [Comparative Example 3] A biaxially oriented polyester film was manufactured using polyester resin A as shown in Table 2, in the same manner as in Example 1. Then, the film of Comparative Example 3 was obtained without coating with polyester resin B. The evaluation results of the obtained film are shown in Table 2.

[0057] [Table 2]

[0058] As can be seen from Table 2, the laminated films of Examples 1 to 7 that satisfy the conditions of the present invention obtained good film evaluations, whereas the laminated films of Comparative Examples 1 and 2 that do not satisfy the Te1-Te2 condition, and the film of Comparative Example 3 that does not use polyester resin B, clearly showed inferior film evaluations. [Industrial applicability]

[0059] 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 ability 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 laminated polyester film for metal plate bonding and molding, comprising a biaxially oriented polyester film containing polyester resin A, with a layer containing amorphous polyester resin B provided on one side, characterized in that the highest temperature peak temperature Te1 (°C) and the next highest temperature peak temperature Te2 (°C) of the loss modulus of the film, as measured using a dynamic viscoelasticity measuring device for the laminated film, satisfy the following formula. 15≦Te1 (℃) - Te2 (℃)≦90

2. The laminated polyester film for metal plate lamination molding according to claim 1, wherein the polyester resin B contains 0.5 to 5 mol% of trifunctional or higher polycarboxylic acid components with respect to 100 mol% of the total polycarboxylic acid components constituting the polyester resin B.

3. The laminated polyester film for metal plate lamination molding according to claim 1, wherein the polyester resin B comprises a plurality of ester constituent units containing two or more polycarboxylic acid components and two or more polyol components.

4. The laminated polyester film for metal plate bonding and molding according to claim 1, wherein the layer containing polyester resin B is a coating layer.

5. A method for manufacturing a metal sheet, comprising the step of heat-pressing a biaxially oriented polyester film for metal sheet lamination processing, as described in any one of claims 1 to 4, onto a metal sheet.

6. A method for manufacturing a metal container, comprising the step of shaping a metal plate obtained by the method for manufacturing a metal plate described in claim 5.

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