Polyester film for coating metal plate, method for producing same, and resin-coated metal plate

Abstract

The present invention provides a polyester film for coating a metal plate, the polyester film having both laminability and can-forming processability, a method for producing the same, and a resin-coated metal plate using the film. This polyester film for coating a metal plate according to the present invention is a polyester film including a polyester resin in a proportion of 70 mass% or more, characterized in that: in both the longitudinal and transverse directions, the film thickness unevenness, calculated from the average, maximum, and minimum values when the film thickness is measured at 200 points at 1 mm intervals, is 0.0 to 15%; and the ratio of the Young's modulus EMD in the longitudinal direction to the Young's modulus ETD in the transverse direction (EMD / ETD) is 1.1 to 10, and the EMD is 5.5 to 19  GPa.

Description

Polyester film for metal plate coating, method for manufacturing the same, and resin-coated metal plate 【0001】 This invention relates to a polyester film for coating metal plates, a method for manufacturing the same, and a resin-coated metal plate. 【0002】 Traditionally, the inner and outer surfaces of metal containers made of tin-free steel (hereinafter referred to as TFS) or aluminum have been widely coated with paint for corrosion protection. Various thermosetting resins, such as epoxy resins and phenolic resins, have been used for such coatings. However, coating methods using thermosetting resins have problems: the drying time of the paint is long, which not only reduces productivity but also consumes a great deal of energy and discharges a large amount of solvent. 【0003】 Therefore, in order to solve these problems, a method has been proposed in which a thermoplastic resin is laminated onto a metal plate. One method for laminating a thermoplastic resin onto a metal plate is to heat a metal plate that has undergone various surface treatments such as plating, and then heat-press a thermoplastic resin film onto the metal plate to laminate it. 【0004】 However, conventional thermoplastic resin films have problems such as thermal shrinkage when exposed to high-temperature atmospheres during heat-pressing, resulting in the inclusion of air bubbles and wrinkles between the film and the metal plate, meaning they do not have sufficient lamination properties. Furthermore, applying high tension to the film to suppress thermal shrinkage causes the film to stretch, which exacerbates the inclusion of air bubbles and wrinkles. Thermal shrinkage of thermoplastic resin films reduces the width of the product, resulting in decreased productivity. In addition, the inclusion of defects such as air bubbles and wrinkles can lead to problems with can manufacturing, for example, when attempting to mold the film into a can that requires high processability, as these defects can cause the film to break or abrade, resulting in poor can manufacturing processability. 【0005】To prevent thermal shrinkage of thermoplastic film, one might consider lowering the temperature of the metal plate during heat bonding. However, lowering the temperature of the metal plate during heat bonding can lead to problems such as poor adhesion between the metal plate and the thermoplastic film, or, for example, delamination of the film when attempting to form a can requiring high processability, resulting in poor can manufacturing processability. 【0006】 Therefore, various technologies have been proposed to resolve problems caused by the poor laminating properties of thermoplastic resin films and to improve the adhesion between metal plates and thermoplastic resin films. 【0007】 For example, Patent Document 1 proposes a polyester composite film having a structure in which three layers, A, B, and C, are laminated. Layer A is a layer made of a resin with a high crystallization temperature that is responsible for heat resistance, layer B is a layer made of a resin with a high melting point that is responsible for impact resistance and deformation resistance, and layer C is a layer made of a resin with a low glass transition temperature that is responsible for adhesion to metal plates. 【0008】 Furthermore, Patent Documents 2 to 4 propose lamination methods that suppress the entrapment of air bubbles. Specifically, Patent Document 2 controls the diameter of the laminating roll and the thickness of the rubber lining. Patent Document 3 describes blowing gas onto the moving metal strip near the laminating roll. Patent Document 4 describes reducing the pressure in the space upstream of the compression roll when performing continuous lamination. 【0009】 Furthermore, Patent Document 5 proposes a method for optimizing the Young's modulus and thermal shrinkage rate by controlling the manufacturing conditions of the film, thereby suppressing bubbles and wrinkles. 【0010】 Generally, metal containers are broadly classified into two types: two-piece cans, which consist of a can body and a lid that are integrated with the bottom, and three-piece cans, which consist of a can body, a top lid, and a bottom lid. Two-piece cans have a beautiful appearance because they do not have welded joints on the can body, but they generally require high workability. 【0011】On the other hand, in recent years, from the perspective of reducing material costs and conserving resources, the thickness of resin-coated metal sheets used in metal containers has been decreasing. When manufacturing cans of the same shape using these thinned resin-coated metal sheets, the degree of processing increases, which can lead to fracture or abrasion, particularly in the resin coating layer located on the inner and outer surfaces of the metal container. 【0012】 Therefore, in order to manufacture a highly processed two-piece can body, a material design is required that prevents the rupture or abrasion of the resin coating layer. 【0013】 Patent Document 6 proposes a technique for suppressing the rupture or abrasion of the resin coating layer when manufacturing a highly processed two-piece can body, which involves adding an organic lubricating component to the resin coating layer. 【0014】 Furthermore, Patent Documents 7 and 8 propose a method for suppressing thickness variations in resin films, which can worsen the processability of can manufacturing, by controlling the film formation conditions of the resin film. 【0015】 JP 7-290666 JP 6-8335 JP 7-214724 JP 7-186353 International Publication No. 2020 / 090552 JP 2017-30210 JP 2002-292734 JP 2009-149109 【0016】 However, when attempting to use resin-coated metal sheets for manufacturing under high lamination temperature conditions or for two-piece cans requiring high processability, the reality is that resin-coated metal sheets that combine high-quality lamination and can-making processability have yet to be realized. 【0017】 - Lamination properties: For example, in order to suppress the inclusion of defects such as air bubbles and wrinkles when heat-press laminating a thermoplastic resin film onto a metal plate, it is necessary to suppress the thermal shrinkage of the film or to improve the stiffness of the film itself, as mentioned above. 【0018】Although Patent Document 1 improves heat resistance by using a three-layer polyester composite film, the heat resistance assumed in that document is heat resistance during can manufacturing and heat treatment after can manufacturing. Therefore, it is not possible to sufficiently suppress thermal shrinkage at high temperatures during lamination. 【0019】 Similarly, the methods described in Patent Documents 2 to 4 cannot essentially suppress the thermal shrinkage of the film and are ineffective under high lamination temperature conditions. 【0020】 Furthermore, while the method described in Patent Document 5 is effective in suppressing thermal shrinkage of the film and improving its stiffness, it cannot control the unevenness of the film thickness, which is another cause of air bubbles and wrinkles under high lamination temperature conditions, and therefore its effectiveness is limited. 【0021】 - Can manufacturing processability: In order to suppress rupture and abrasion of the resin coating layer when manufacturing the can body of a highly processed two-piece can, it is necessary to improve the can manufacturing processability. To this end, methods such as adding lubricants to the resin coating layer to ensure the sliding properties of the resin coating layer surface, reducing the crystallinity of the resin coating layer to ensure elongation, and suppressing thickness variations in the resin coating layer so that localized stress is not applied during molding are employed. 【0022】 For example, the method described in Patent Document 6 attempts to achieve both processability and ink adhesion by adding a lubricant to the resin coating layer and controlling the water contact angle of the resin coating layer. However, when heat-pressed under high lamination temperature conditions, the inclusion of air bubbles and wrinkles is unavoidable, and it is not possible to completely eliminate the processability of can manufacturing. 【0023】 Furthermore, the method described in Patent Document 7 attempts to suppress variations in film thickness and thermal shrinkage by controlling the film stretching conditions. However, because it is a biaxially oriented film stretched in both the longitudinal and width directions, deterioration of lamination properties is observed, particularly due to high thermal shrinkage in the width direction, and it is not possible to completely eliminate the issue of can-making processability. 【0024】Similarly, the method described in Patent Document 8 attempts to suppress thickness variations in the film by controlling the extrusion conditions of the polyester resin. However, because it is an unstretched film, the effect of suppressing thickness variations is limited, and it is not possible to completely eliminate the processability required for can manufacturing. 【0025】 This invention has been made in view of the above circumstances. Its purpose is to provide a polyester film for coating metal sheets that combines lamination properties and can-making processability, a method for manufacturing the same, and a resin-coated metal sheet using the film. 【0026】 The inventors of this invention conducted diligent research to solve the above problems and obtained the following findings. 【0027】 When coating a metal plate with a thermoplastic resin film using the thermocompression lamination method, it is important to use polyester resin as the main component of the film and to control the balance of the Young's modulus in the longitudinal and width directions within a certain range in order to suppress the inclusion of air bubbles and wrinkles due to the thermal shrinkage of the film. Furthermore, it is important to increase the Young's modulus in the longitudinal direction so that the film does not deform even when high tension is applied. 【0028】 Furthermore, in order to suppress air bubbles and wrinkles that may still be present even after thermal shrinkage is suppressed, it is important to suppress thickness variations in the longitudinal and width directions of the film. 【0029】 Therefore, the inventors have found that by uniaxially stretching a polyester film in the longitudinal direction under optimal conditions, the balance of Young's modulus in the longitudinal and width directions, the Young's modulus in the longitudinal direction, and the film thickness variations in the longitudinal direction can be controlled. 【0030】 While it has been conventionally known that film thickness variations in the width direction can be suppressed by adjusting the slit width of the T-die or by stretching the film in the width direction, in the case of unstretched films or longitudinally uniaxially oriented films, the spacing of thickness variations is narrow, making suppression difficult by T-die adjustment alone. However, the inventors have found that film thickness variations in the width direction can be suppressed without stretching the film in the width direction by controlling the shear rate of the molten resin inside the T-die and the ambient temperature of the molten resin discharged from the T-die. 【0031】By suppressing the inclusion of air bubbles, wrinkles, and thickness unevenness caused by the heat shrinkage of the film in this way, it has been found that both the laminating property and the can manufacturing processability can be achieved. 【0032】 Further, for example, when used as the can body of a two-piece can capable of printing on the outer surface side of the can, it is necessary to contain an inorganic pigment in order to increase the whiteness and conceal the color of the metal plate. Since the whiteness unevenness can be suppressed by suppressing the heat shrinkage and thickness unevenness of the film, there is also an effect of enhancing the design property of the can appearance. 【0033】 The present invention has been completed based on the above findings, and the gist thereof is as follows. 【0034】 [1] A polyester film containing a polyester resin in a proportion of 70% by mass or more, wherein the film thickness unevenness obtained from the average value, maximum value, and minimum value measured at 200 points at intervals of 1 mm in each of the longitudinal direction and the width direction is 0.0 to 15%, and the Young's modulus E １６０ＴＤ , １６０ＴＤ , １６０ＭＤ in the longitudinal direction and the Young's modulus E ＴＤ in the width direction have a ratio (E ＭＤ / E ＴＤ ) of 1.1 to 10, and the E ＭＤ is 5.5 to 19 GPa, a polyester film for coating a metal plate. 【0035】 [2] The polyester film for coating a metal plate according to [1], wherein the E ＴＤ is 1.0 to 5.0 GPa. 【0036】 [3] The polyester film for coating a metal plate according to [1] or [2], wherein the 160 ° C heat shrinkage rate S １６０ＭＤ in the longitudinal direction is 0.0 to 20%. 【0037】 [4] The polyester film for coating a metal plate according to any one of [1] to [3], wherein the 160 ° C heat shrinkage rate S １６０ＴＤ in the width direction is 0.0 to 10%. 【0038】 [5] The sum (S １６０ＭＤ ) of the S １６０ＴＤ and the S １６０ＭＤ + S １６０ＴＤA polyester film for coating metal plates according to [3] or [4], wherein the content of [3] is 0.0 to 30%. 【0039】 [6] A polyester film for coating metal plates according to any one of [1] to [5], wherein the melting point is 220 to 255°C. 【0040】 [7] A polyester film for coating metal plates according to any one of [1] to [6], containing 5.0 to 25% by mass of an inorganic pigment. 【0041】 [8] A polyester film for coating metal plates according to any one of [1] to [7], wherein the film thickness is 5.0 to 35 μm. 【0042】 [9] A resin-coated metal plate having at least one side of the polyester film for metal plate coating described in any one of [1] to [8] above. 【0043】

[10] A step of melting and kneading a raw material resin containing polyester resin in a proportion of 70% by mass or more in an extruder to obtain a molten resin, wherein the molten resin temperature in the T-die is 30°C or higher to 50°C or lower than the melting point of the polyester resin, and the molten resin shear rate in the T-die is 60 to 1000 sec. －１ A method for manufacturing a polyester film for metal plate coating according to any one of [1] to [8], comprising the steps of: discharging a sheet from a T-die under the conditions described above, cooling and solidifying it on a casting drum under the conditions that the ambient temperature between the T-die and the casting drum is 30 to 50°C to obtain an unstretched film; stretching the unstretched film in the longitudinal direction under the conditions that the stretching ratio is 3.5 to 7.0 times, the stretching temperature is 5°C or more above the glass transition temperature of the polyester resin and 50°C or less above the glass transition temperature, and the time for which the temperature of the unstretched film is 1.8 to 15 seconds; heat-fixing the stretched film while relaxing it in the stretching direction for a heat-fixing time of 0.2 to 5 seconds; and cooling the heat-fixed film to room temperature. 【0044】The polyester film for metal plate coating according to the present invention provides a resin-coated metal plate that combines lamination properties and can-making processability. Specifically, according to the present invention, in the thermocompression lamination method, the inclusion of air bubbles and wrinkles due to thermal shrinkage of the film can be suppressed, especially at high temperatures. Furthermore, according to the present invention, in the manufacture of two-piece can bodies where a high degree of processing is required, the rupture and abrasion of the resin coating layer during can-making can be suppressed. In addition, the polyester film for metal plate coating according to the present invention can suppress color unevenness when colored, and can provide a high level of design appeal to the product. 【0045】 The present invention will now be described in detail. In the embodiments of the present invention, A (numerical value) to B (numerical value) means A or greater and B or less. 【0046】 [Polyester film for metal plate coating] The polyester film for metal plate coating in one embodiment of the present invention is a polyester film containing polyester resin in a proportion of 70% by mass or more per 100% by mass of the film. The proportion of polyester resin is preferably 75% by mass or more, and more preferably 80% by mass or more. 【0047】 (Polyester resin) The polyester resin is preferably a resin obtained by polymerization of monomers mainly composed of aromatic dicarboxylic acids or aliphatic dicarboxylic acids and diols, or a mixture thereof. 【0048】 Examples of the aforementioned aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenyletherdicarboxylic acid, diphenylsulfondicarboxylic acid, diphenoxyethanedicarboxylic acid, and 5-sodium sulfisophthalic acid. 【0049】 Examples of the aliphatic dicarboxylic acids include oxalic acid, succinic acid, adipic acid, suberic acid, sebacic acid, dimer acid, maleic acid, fumaric acid, dodecanedionic acid, cyclohexanedicarboxylic acid and their ester derivatives. 【0050】These dicarboxylic acids may be used individually, or two or more may be used in combination. Furthermore, oxycarboxylic acids such as p-oxybenzoic acid may be copolymerized. 【0051】 Examples of the aforementioned diol components include ethylene glycol, propanediol, butanediol, pentanediol, hexanediol, neopentyl glycol, cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, 2,2-bis(4-hydroxyethoxyphenyl)propane, isosorbide (1,4:3,6-dianhydroglucitol, 1,4:3,6-dianhydro-D-sorbitol), spiroglycol, bisphenol A, and bisphenol S. Among these, it is preferable to use at least one of ethylene glycol and butanediol. Only one of these diol components may be used, or two or more may be used in combination. 【0052】 As the polyester resin constituting the polyester film for metal plate coating of the present invention, it is preferable to use at least one selected from the group consisting of polyethylene terephthalate and its copolymers, and polybutylene terephthalate and its copolymers, from the viewpoint of cost, ease of molding as a film, and adhesion to the metal plate. Mixtures thereof can also be preferably used. 【0053】The polyester resin constituting the polyester film for metal plate coating of the present invention may be copolymerized with polyfunctional compounds such as trimellitic acid, trimesic acid, and trimethylolpropane, as long as they do not hinder the effects of the present invention. Furthermore, resin components other than polyester may be added for the purpose of imparting functionality. Other resin components besides the polyester mentioned above include chain-like polyolefins such as polyethylene, polypropylene, poly(4-methylpentene-1), and polyacetal; alicyclic polyolefins which are ring-opening metathesis polymerization, addition polymerization, and addition copolymers with other olefins of norbornene; biodegradable polymers such as polylactic acid and polybutyl succinate; polyamides such as nylon 6, nylon 11, nylon 12, and nylon 66; aramids; polymethyl methacrylate; polyvinyl chloride; polyvinylidene chloride; polyvinyl alcohol; polyvinyl butyral; ethylene vinyl acetate copolymer; polyacetal; polyglycolic acid; polystyrene; styrene copolymer polymethyl methacrylate; polycarbonate; polyethersulfone; polyetheretherketone; modified polyphenylene ether; polyphenylene sulfide; polyetherimide; polyimide; polyarylate; tetrafluoroethylene resin; trifluoroethylene resin; trifluoroethylene chloride resin; tetrafluoroethylene-hexafluoropropylene copolymer; and polyvinylidene fluoride. These may be copolymers or mixtures. 【0054】 The polyester film for metal plate coating of the present invention has a film thickness variation of 0.0 to 15% in both the longitudinal and width directions, determined from the average, maximum, and minimum values ​​obtained by measuring the film thickness at 200 points at 1 mm intervals. By setting the film thickness variation to 0.0 to 15% in both directions, lamination and can-making processes are improved. It is more preferable that the film thickness variation be 0.0 to 10% in both the longitudinal and width directions, even more preferable that be 0.0 to 7.0%, and most preferable that be 0.0 to 5.0%. The lower the film thickness variation, the better the lamination and can-making processes. 【0055】Furthermore, there are inevitably areas of film that are thicker and thinner. The degree of this variation in thickness is indicated by the film thickness variation. The film thickness variation is calculated using the following formula, which is obtained by measuring the film thickness at 200 points at 1 mm intervals and using the average, maximum, and minimum values: Film Thickness Variation (%) = ((Maximum Value - Minimum Value) / Average Value) × 100 【0056】 Here, the "longitudinal direction" of the film is defined as the direction in which the film travels during its formation. For example, in the case of a film manufactured by winding it into a roll, the winding direction of the roll corresponds to the longitudinal direction. The "width direction" of the film is defined as the direction perpendicular to the longitudinal direction. 【0057】 To ensure that the film thickness variation in the longitudinal direction is within the above range, the polyester film should be uniaxially stretched in the longitudinal direction under the conditions described later. To ensure that the film thickness variation in the width direction is within the above range, the polyester film should be melt-extruded under the conditions described later. 【0058】 The polyester film for metal plate coating of the present invention has a Young's modulus E in the longitudinal direction. ＭＤ and Young's modulus E in the width direction ＴＤ The ratio (E ＭＤ / E ＴＤ ) is between 1.1 and 10. E ＭＤ / E ＴＤ By setting this to 1.1 or higher, deformation and shrinkage of the film at high temperatures during heat sealing can be suppressed, improving lamination performance. ＭＤ / E ＴＤ It is preferably 1.2 or higher, more preferably 3.6 or higher, and most preferably 5.1 or higher. On the other hand, E ＭＤ / E ＴＤ By setting this value to 10 or less, film breakage and abrasion can be suppressed when molding resin-coated metal sheets into two-piece cans, improving can manufacturing processability. ＭＤ / E ＴＤ It is preferably 9.0 or less, more preferably 8.0 or less, and most preferably 7.0 or less. ＭＤ / E ＴＤThe value is preferably 1.2 to 9.0, more preferably 3.6 to 8.0, and most preferably 5.1 to 7.0. 【0059】 E ＭＤ / E ＴＤ To keep the above range within limits, the polyester film can be uniaxially stretched in the longitudinal direction under the conditions described later, and then heat-fixed. 【0060】 The polyester film for metal plate coating of the present invention is E ＭＤ The pressure ranges from 5.5 to 19 GPa. ＭＤ By setting the pressure to 5.5 GPa or higher, high tension can be applied to the film at high temperatures during heat bonding, improving lamination performance. ＭＤ It is preferable that it be 7.0 GPa or higher, more preferably 8.5 GPa or higher, and most preferably 10 GPa or higher. On the other hand, E ＭＤ By keeping the pressure below 19 GPa, film breakage and abrasion can be suppressed when molding resin-coated metal sheets into two-piece cans, improving can manufacturing processability. ＭＤ The pressure is preferably 18 GPa or less, more preferably 16 GPa or less, and most preferably 15 GPa or less. ＭＤ The pressure is preferably 7.0 to 18 GPa, more preferably 8.5 to 16 GPa, and most preferably 10 to 15 GPa. 【0061】 E ＭＤ To keep the above range within limits, the polyester film can be uniaxially stretched in the longitudinal direction under the conditions described later, and then heat-fixed. 【0062】 The polyester film for metal plate coating of the present invention is E ＴＤ It is preferable that the pressure is between 1.0 and 5.0 GPa. ＴＤ By setting the pressure to 1.0 GPa or higher, the stiffness of the film is improved, which suppresses the inclusion of wrinkles during heat-press lamination and improves lamination performance. ＴＤ It is more preferably 1.2 GPa or higher, even more preferably 1.4 GPa or higher, and most preferably 1.6 GPa or higher. On the other hand, E ＴＤBy setting the pressure to 5.0 GPa or less, deformation and shrinkage of the film at high temperatures during heat bonding can be suppressed, improving lamination performance. ＴＤ It is more preferably 4.5 GPa or less, even more preferably 4.0 GPa or less, and most preferably 3.5 GPa or less. ＴＤ It is more preferably 1.2 to 4.5 GPa, even more preferably 1.4 to 4.0 GPa, and most preferably 1.6 to 3.5 GPa. 【0063】 E ＴＤ To keep the above range within limits, the polyester film can be uniaxially stretched in the longitudinal direction under the conditions described later, and then heat-fixed. 【0064】 The polyester film for metal plate coating of the present invention has a thermal shrinkage rate of S in the longitudinal direction at 160°C. １６０ＭＤ It is preferable that the amount is between 0.0 and 20%. １６０ＭＤ By setting this to 0.0-20%, shrinkage of the film at high temperatures during heat bonding can be suppressed, improving lamination performance. １６０ＭＤ It is more preferably 0.0 to 18%, even more preferably 0.0 to 16%, and most preferably 15% or less. １６０ＭＤ The lower the value, the better the lamination. 【0065】 S １６０ＭＤ To keep the above range within limits, the polyester film can be uniaxially stretched in the longitudinal direction under the conditions described later, and then heat-fixed. 【0066】 The polyester film for metal plate coating of the present invention has a heat shrinkage rate of S in the width direction at 160°C. １６０ＴＤ It is preferable that the amount is 0.0 to 10%. １６０ＴＤ By setting this to 0.0-10%, shrinkage of the film at high temperatures during heat bonding can be suppressed, improving lamination performance. １６０ＴＤ It is more preferably 0.0 to 9.0%, even more preferably 0.0 to 8.0%, and most preferably 0.0 to 7.0%. １６０ＴＤ The lower the value, the better the lamination. 【0067】S １６０ＴＤ To keep the above range within limits, the polyester film can be uniaxially stretched in the longitudinal direction under the conditions described later, and then heat-fixed. 【0068】 The polyester film for metal plate coating of the present invention is S １６０ＭＤ and S １６０ＴＤ The sum of (S １６０ＭＤ +S １６０ＴＤ ) is preferably 0.0 to 30%. １６０ＭＤ +S １６０ＴＤ By setting this to 0.0-30%, shrinkage of the film at high temperatures during heat bonding can be suppressed, improving lamination performance. １６０ＭＤ +S １６０ＴＤ It is more preferably 0.0 to 26%, even more preferably 0.0 to 23%, and most preferably 0.0 to 20%. １６０ＭＤ +S １６０ＴＤ The lower the value, the better the lamination. 【0069】 S １６０ＭＤ +S １６０ＴＤ To keep the above range within limits, the polyester film can be uniaxially stretched in the longitudinal direction under the conditions described later, and then heat-fixed. 【0070】 The polyester film for metal plate coating of the present invention preferably has a melting point of 220 to 255°C. By setting the melting point to 220°C or higher, lamination is improved, and melting of the resin coating layer can be suppressed in environments where the can body is exposed to high temperatures, such as in printing, heat treatment, and retort processing processes, after the resin-coated metal plate has been molded into a two-piece can. The melting point is more preferably 225°C or higher, even more preferably 227°C or higher, and most preferably 229°C or higher. On the other hand, by setting the melting point to 255°C or lower, the adhesion of the resin coating layer of the resin-coated metal plate can be improved, and the thermocompression bonding temperature can be lowered. The melting point is more preferably 250°C or lower, even more preferably 248°C or lower, and most preferably 246°C or lower. The aforementioned melting point is more preferably 225 to 250°C, even more preferably 227 to 248°C, and most preferably 229 to 246°C. 【0071】In addition, when polyester resins are copolymers or mixtures, or resins with multiple crystalline forms, multiple melting peaks may be observed when measured with a differential scanning calorimeter (DSC). Here, "melting point" refers to the peak top temperature of the melting peak with the largest heat of fusion. The melting point of polyester film can be controlled by the type of aromatic dicarboxylic acid or aliphatic dicarboxylic acid and diol that are the main constituent components of the polyester resin, the type and amount of copolymerization components in the case of copolymers, or the amount of mixture in the case of mixtures. 【0072】 (Inorganic Pigments) The polyester film for metal plate coating of the present invention preferably contains 5.0 to 25% by mass of inorganic pigment. By setting the inorganic pigment content to 5.0 to 25% by mass, the design and opacity of the metal plate can be improved. The inorganic pigment content is more preferably 8.0 to 25% by mass, even more preferably 10 to 25% by mass, and most preferably 12 to 25% by mass. Furthermore, by setting the inorganic pigment content to 25% by mass or less, film breakage and abrasion can be suppressed when molding the resin-coated metal plate into a two-piece can, and the can-making processability is improved. 【0073】 The inorganic pigment is not particularly limited and any inorganic pigment can be used. Preferably, the inorganic pigment is one that turns white when added to a polyester film. For example, at least one selected from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, zirconium oxide, magnesium oxide, zinc oxide, barium titanate, lead zirconate titanate, and other oxide ceramics, talc, mica, calcium carbonate, and barium sulfate can be used. Among these, titanium dioxide is preferred in terms of dispersibility in polyester resin and whiteness, and rutile-type titanium dioxide is more preferred. 【0074】The method of adding the inorganic pigment is not particularly limited. For example, the inorganic pigment may be added together with the polyester resin in the extruder for the A-layer raw material during film production, and then mixed and dispersed in the film-making extruder. Alternatively, the polyester resin and inorganic pigment may be mixed in advance using a separate extruder to prepare a masterbatch, and then the masterbatch of inorganic pigment may be added together with the polyester resin in the extruder for the A-layer raw material during film production. It should be noted that the method of adding the inorganic pigment as a masterbatch is preferred because it allows for uniform and well-dispersible addition of the inorganic pigment to the polyester resin. 【0075】 The polyester film for metal plate coating of the present invention may have a laminated structure in which multiple layers with different compositions are stacked. 【0076】 When the polyester film has a laminated structure, the outermost layer (the outermost layer that is exposed and does not come into contact with the metal plate) is defined as layer B, and the layer in contact with the inside of layer B is defined as layer A. In addition, layers other than layer A and layer B may be included as long as they do not hinder the effects of the present invention. The laminated structure may be a two-layer structure of layer A and layer B, a three-layer structure including other layers (for example, layer C), or a number of layers greater than that. 【0077】 For example, in a three-layer structure, if the raw material composition of the outermost B layer and the layer sandwiched between the A layer and the metal plate are the same, it is expressed as a B / A / B three-layer structure. When using two types of layers, A and B, a B / A / B laminated structure is preferred as the three-layer structure. When using three types of layers, A, B, and C, a B / A / C laminated structure is preferred as the three-layer structure, with the C layer placed on the side in contact with the metal plate. In the case of a laminated structure of four or more layers, it is preferable from the viewpoint of film manufacturing and metal plate coating that the outermost surface side of the resin-coated metal plate (the side that is exposed and does not come into contact with the metal plate) and the side that is in direct contact with the metal plate are both B layers, that is, in the case of the film alone, both the front and back surfaces of the film are B layers. 【0078】The method for producing the polyester film having the above-described laminated structure is not particularly limited and can be produced by any known method. For example, co-extrusion using a feed block method or a multi-manifold method, a method of bonding with other films, or a lamination method in which molten resin is directly laminated onto the film can be used. Among these, the feed block method or the co-extrusion method using a multi-manifold method is preferred from the viewpoint of lamination accuracy and productivity. 【0079】 (Inorganic Lubricant) The polyester film for metal plate coating of the present invention may further contain an inorganic lubricant, and in a laminated structure, it is preferable to contain the inorganic lubricant in the B layer. The amount of the inorganic lubricant is not particularly limited, but if it is 100 ppm by mass or more relative to the total components of the B layer, the film can be given slipperiness and wrinkle formation during heat-seal lamination can be suppressed. For this reason, the amount of the inorganic lubricant is preferably 100 ppm by mass or more, more preferably 150 ppm by mass or more, and even more preferably 200 ppm by mass or more, relative to the total components of the B layer. On the other hand, if the amount of the inorganic lubricant is 5000 ppm by mass or less relative to the total components of the B layer, the processability for can making is improved and film abrasion can be suppressed. For this reason, the amount of the inorganic lubricant is preferably 5000 ppm by mass or less, more preferably 3000 ppm by mass or less, and even more preferably 1000 ppm by mass or less, relative to the total components of the B layer. The content of the inorganic lubricant is preferably 100 to 5000 ppm by mass, more preferably 150 to 3000 ppm by mass, and even more preferably 200 to 1000 ppm by mass, relative to the total components of layer B. 【0080】 Any inorganic lubricant that can impart slipperiness when added to a polyester film can be used. Examples of inorganic lubricants include oxide ceramics such as silicon dioxide, aluminum oxide, titanium dioxide, zirconium oxide, magnesium oxide, zinc oxide, barium titanate, and lead zirconate titanate, as well as talc, mica, calcium carbonate, and barium sulfate, and mixtures thereof may also be used. 【0081】The method of adding the inorganic lubricant is not particularly limited and can be done by any method. For example, the inorganic lubricant may be added to the extruder together with the polyester resin during film production and mixed and dispersed in the film-making extruder. Alternatively, the polyester resin and inorganic lubricant may be mixed in advance using a separate extruder to prepare a masterbatch, and then the inorganic lubricant masterbatch may be added to the extruder together with the polyester resin during film production. It should be noted that the method of adding the inorganic lubricant as a masterbatch is preferred because it allows for the uniform and well-dispersible addition of a small amount of inorganic lubricant to the polyester resin. 【0082】 (Wax) The polyester film for metal plate coating of the present invention may further contain wax, and it is preferable that the wax be contained in the B layer in the laminated structure. The amount of wax is not particularly limited, but if it is 0.050% by mass or more relative to the total components of the B layer, the wax will be well dispersed in the polyester resin of the B layer, and as a result, excellent can-making processability will be obtained even at high processing levels. For this reason, the amount of wax is preferably 0.050% by mass or more, more preferably 0.060% by mass or more, and even more preferably 0.07% by mass or more, relative to the total components of the B layer. Similarly, if the amount of wax is 1.0% by mass or less relative to the total components of the B layer, the wax will be well dispersed in the polyester resin of the B layer, and as a result, excellent can-making processability will be obtained even at high processing levels. For this reason, the amount of wax is preferably 1.0% by mass or less, more preferably 0.40% by mass or less, and even more preferably 0.18% by mass or less, relative to the total components of the B layer. The wax content is preferably 0.050 to 1.0% by mass, more preferably 0.060 to 0.40% by mass, and even more preferably 0.070 to 0.18% by mass, relative to the total components of layer B. 【0083】The wax is preferably a modified polyolefin wax with an acid value of 50 to 100 mg KOH / g. If the acid value of the wax is 50 mg KOH / g or higher, the wax disperses well in the polyester resin, resulting in excellent can-making processability. Also, when printing is applied during can-making, adhesion is improved depending on the type of ink. For this reason, the acid value of the wax is preferably 50 mg KOH / g or higher, more preferably 55 mg KOH / g or higher, and even more preferably 60 mg KOH / g or higher. On the other hand, if the acid value of the wax is 100 mg KOH / g or lower, the wax can maintain a good dispersion state without excessive reaction with the polyester resin. As a result, excellent can-making processability is obtained. For this reason, the acid value of the wax is preferably 100 mg KOH / g or lower, more preferably 90 mg KOH / g or lower, and even more preferably 80 mg KOH / g or lower. The acid value of the wax is more preferably 55 to 90 mg KOH / g, and even more preferably 60 to 80 mg KOH / g. 【0084】The modified polyolefin waxes include oxidized polyolefin waxes obtained by introducing functional groups through an oxidation reaction by introducing air into polymers of olefin monomers having 2 to 8 carbon atoms, such as ethylene, propylene, 1-butene, 1-pentene, isobutene, isobutylene, and butadiene, or their thermal decomposition products, while the polymers are in a molten state at 140°C to 180°C; unsaturated carboxylic acids having 3 to 8 carbon atoms, such as acrylic acid, methacrylic acid, vinyl acetate, vinyl propionate, maleic acid, maleic anhydride, itaconic acid, and monomethyl maleic acid; and monovalent or divalent metals such as sodium, potassium, lithium, zinc, magnesium, and calcium, in whole or in part. Examples include acid-modified polyolefin waxes obtained by random copolymerization, block copolymerization, or graft copolymerization of functional group-containing monomers such as metal salts neutralized with cations, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isobutyl acrylate, isobutyl methacrylate, n-butyl acrylate, n-butyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, monomethyl maleate, glycidyl acrylate, glycidyl methacrylate, vinyl acetate, acrylamine, acrylamide, etc., or mixtures thereof. In particular, it is preferable that the modified polyolefin wax is one of acid-modified polyethylene wax, acid-modified polypropylene wax, oxidized polyethylene wax, or oxidized polypropylene wax, or a mixture thereof. The acid modification method is preferably maleic acid modification and / or maleic anhydride modification, and it is preferable that the functional group introduced by oxidation is a carboxyl group, a ketone group, and / or a hydroxyl group. 【0085】The number average molecular weight of the modified polyolefin wax is not particularly limited, but is preferably 500 to 20,000, more preferably 1,000 to 10,000, and even more preferably 1,500 to 5,000. If the number average molecular weight of the polyolefin wax is 500 to 20,000, the polyolefin wax disperses well in the polyester resin of layer B, further improving the processability of the resin-coated metal sheet for can manufacturing. The number average molecular weight of the modified polyolefin wax is preferably 500 or more, more preferably 1,000 or more, and even more preferably 1,500 or more. Furthermore, the number average molecular weight of the modified polyolefin wax is preferably 20,000 or less, more preferably 10,000 or less, and even more preferably 5,000 or less. 【0086】 The melting point of the modified polyolefin wax is not particularly limited, but is preferably 70°C to 180°C. It is more preferably 90°C to 160°C, and particularly preferably 100°C to 140°C. If the melting point of the modified polyolefin wax contained in layer B is 70°C to 180°C, high sliding properties are exhibited due to the heat generated during processing during can manufacturing, resulting in further improvement of can manufacturing processability. The melting point of the modified polyolefin wax is preferably 70°C or higher, more preferably 90°C or higher, and even more preferably 100°C or higher. Also, the melting point of the modified polyolefin wax is 180°C or lower, more preferably 160°C or lower, and even more preferably 140°C or lower. 【0087】 The method of adding the wax is not particularly limited. For example, the wax may be added to the extruder together with the polyester resin during film production, and the mixture may be kneaded and dispersed within the film-making extruder. Alternatively, a masterbatch may be prepared in advance by kneading the polyester resin and the wax in a separate extruder, and then the wax masterbatch may be added to the extruder together with the polyester resin during film production. The method of adding the wax as a masterbatch is preferred because it allows for the uniform and well-dispersible addition of a small amount of wax to the polyester resin. 【0088】(Antioxidant) The polyester film for metal plate coating of the present invention may further optionally contain an antioxidant. Adding an antioxidant improves heat resistance. The content of the antioxidant is not particularly limited, but from the viewpoint of improving heat resistance, it is preferably 0.0001 to 1.0% by mass, and more preferably 0.001 to 1.0% by mass. The antioxidant is not particularly limited and any antioxidant can be used. As the antioxidant, for example, at least one selected from the group consisting of hindered phenols, hydrazines, phosphites, etc., can be used. The content of the antioxidant is preferably 0.0001% by mass or more, more preferably 0.001% by mass or more. Also, the content of the antioxidant is preferably 1.0% by mass or less. 【0089】 The polyester film for metal plate coating of the present invention may contain various additives in addition to the antioxidant described above, as long as they do not impair the effects of the present invention. As additives, for example, at least one selected from the group consisting of crystal nucleating agents, heat stabilizers, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and coloring pigments can be used. 【0090】The polyester film for metal plate coating of the present invention is not particularly limited and can have any thickness (hereinafter referred to as "film thickness"). However, if the film thickness is 5.0 μm or more, the film has stiffness, exhibiting better film transportability, ensuring reliable coating during heat-press bonding with the metal plate, and providing even better corrosion resistance when used as a container. For this reason, the film thickness is preferably 5.0 μm or more, more preferably 8.0 μm or more, even more preferably 10 μm or more, and most preferably 12 μm or more. On the other hand, if the film thickness is 35 μm or less, it is possible to suppress cost increases without impairing the corrosion resistance when the resin-coated metal plate is used as a container. For this reason, the film thickness is preferably 35 μm or less, more preferably 30 μm or less, even more preferably 25 μm or less, and most preferably 20 μm or less. The film thickness is preferably 5.0 to 35 μm, more preferably 8.0 to 30 μm, even more preferably 10 to 25 μm, and most preferably 12 to 20 μm. 【0091】 [Manufacturing Method] Next, a preferred method for manufacturing a polyester film for metal plate coating in one embodiment of the present invention will be described. However, the present invention is not limited to the following disclosure. 【0092】 First, a mixture containing dried polyester resin and additives such as inorganic pigments (also referred to as "raw material resin") is melt-kneaded in an extruder to form a molten resin. The form of the polyester resin is not particularly limited, but may be in the form of pellets, for example. The polyester resin is dried under hot air or vacuum as needed, and then supplied to the extruder together with additives such as inorganic pigments. 【0093】 In the extruder, the raw material resin is heated to a temperature above the melting point of the polyester resin and kneaded to form a molten resin. When extruding the molten resin, it is preferable to equalize the extrusion amount using a gear pump or the like. It is also preferable to remove foreign matter and modified resin by extruding through a filter or the like. 【0094】 When manufacturing a film with a laminated structure, the raw material resin for layer A and the raw material resin for layer B can be molten in separate extruders, and each resin can be supplied to the lamination device through different flow paths. For example, a feed block or a multi-manifold die can be used as the lamination device. 【0095】 Next, the molten resin is extruded from the T-die in a sheet form and cooled and solidified on a casting drum to form an unstretched film. At this time, in order to improve the adhesion between the cooling body such as the casting drum and the molten sheet, it is preferable to use electrodes such as wire, tape, needle, or knife shapes to adhere them by electrostatic force and rapidly cool and solidify them. Methods of blowing air from a slit-shaped, spot-shaped, or surface-shaped device to adhere and rapidly cool and solidify the film, or methods of adhering and rapidly cooling and solidifying the film using a nip roll, or combinations thereof are also preferred. 【0096】 - Molten resin temperature in T-die: (Melting point + 30°C) or higher, (Melting point + 50°C) or lower. If the molten resin temperature in the T-die is less than the melting point of the polyester resin + 30°C, the flow of the molten resin inside the T-die becomes unstable, and in particular, variations in film thickness in the width direction may become large. For this reason, the molten resin temperature should be the melting point of the polyester resin + 30°C or higher, preferably the melting point + 33°C or higher, and more preferably the melting point + 36°C or higher. On the other hand, if the molten resin temperature in the T-die exceeds the melting point of the polyester resin + 50°C, the molten resin deteriorates and its viscosity decreases, and it may no longer be extruded from the T-die in a sheet-like form. For this reason, the molten resin temperature should be the melting point of the polyester resin + 50°C or lower, preferably the melting point + 47°C or lower, and more preferably the melting point + 44°C or lower. The molten resin temperature is preferably the melting point of the polyester resin + 33 to + 47°C, and more preferably the melting point + 36 to + 44°C. 【0097】 - Molten resin shear rate in T-die: 60-1000 sec －１ The molten resin shear rate in the T-die is 60 sec. －１ If the shear rate is less than 60 sec, the flow of molten resin inside the T-die becomes unstable, and in particular, variations in film thickness in the width direction may become larger. Therefore, the molten resin shear rate should be 60 sec.－１ Preferably 100 sec －１ More preferably 300 sec －１ That concludes the explanation. On the other hand, the molten resin shear rate in the T-die is 1000 sec. －１ If it exceeds this, excessive shear will be applied, and stretching the unstretched sheet which is strongly oriented in the longitudinal direction will further increase the orientation in the longitudinal direction, E ＭＤ and E ＴＤ The temperature can become too high. Also, in some cases, film breakage may occur. Therefore, the molten resin shear rate is set to 1000 sec. －１ Preferably 800 sec －１ More specifically, 600 sec －１ The following applies: The molten resin shear rate shall be 100 to 800 sec. －１ Preferably, 300 to 600 seconds －１ This is preferable. 【0098】 The molten resin shear rate in a T-die is expressed by equation (1). Shear rate (sec －１ ) = 6Q / ρWt ２ ...(1) Q: Flow rate (kg / sec) ρ: Specific gravity (kg / cm ３ ) W: Groove width (cm) T: Groove spacing (cm) 【0099】 - Ambient temperature between T-die and casting drum: 30 to 50°C If the ambient temperature between the T-die and casting drum is less than 30°C, cooling unevenness may occur in the section where the sheet-like molten resin discharged from the T-die is air-cooled before being cooled in the casting drum, and in particular, unevenness in film thickness in the width direction may become large. For this reason, the ambient temperature should be 30°C or higher, preferably 32°C or higher, and more preferably 34°C or higher. On the other hand, if the ambient temperature between the T-die and casting drum exceeds 50°C, the contact point when the molten resin adheres to the casting drum becomes unstable, and in particular, unevenness in film thickness in the width direction may become large. For this reason, the ambient temperature should be 50°C or lower, preferably 48°C or lower, and more preferably 46°C or lower. The ambient temperature is preferably 32 to 48°C, and more preferably 34 to 46°C. 【0100】Subsequently, the unstretched film is stretched in the longitudinal direction. Here, "longitudinal direction" is synonymous with "longitudinal direction," and longitudinal stretching refers to stretching to impart a longitudinal molecular orientation to the film. Longitudinal stretching is usually performed using a pair of rolls with different peripheral speeds, and the stretching ratio is controlled by the difference in peripheral speeds. Longitudinal stretching may be performed in one stage, or it may be performed in multiple stages using multiple pairs of rolls. 【0101】 - Stretching ratio: 3.5 to 7.0 times If the stretching ratio in the stretching process is less than 3.5 times, stretching unevenness may occur, resulting in large variations in film thickness in the longitudinal direction, and poor productivity. Therefore, the stretching ratio in the stretching process should be 3.5 times or more, preferably 4.0 times or more, and more preferably 4.5 times or more. On the other hand, if the stretching ratio exceeds 7.0 times, the orientation in the stretching direction may become too pronounced, making it prone to thermal shrinkage, and the Young's modulus E in the direction perpendicular to the stretching direction may be affected. ＴＤ A decrease in this can make the fabric more prone to wrinkles. Therefore, the stretching ratio in the stretching process is set to 7.0 times or less, preferably 6.0 times or less, and more preferably 5.5 times or less. The stretching ratio in the stretching process is preferably 4.0 to 6.0 times, and more preferably 4.5 to 5.5 times. 【0102】 ・Stretching temperature: (Tg + 5°C) to (Tg + 50°C) If the stretching temperature is too low, the film may break during stretching, or the orientation in the stretching direction may become too pronounced, making it prone to thermal shrinkage. For this reason, it is necessary to heat the polyester resin to a temperature above its glass transition temperature (hereinafter referred to as Tg) to soften it before stretching. For this reason, the stretching temperature should be (Tg + 5°C) or higher, preferably (Tg + 10°C) or higher, and more preferably (Tg + 15°C) or higher. On the other hand, if the stretching temperature is too high, the unstretched film may undergo thermal crystallization, causing the film to break during stretching, or conversely, it may become too soft and stick to the roll, resulting in large variations in film thickness in the longitudinal direction. For this reason, the stretching temperature should be (Tg + 50°C) or lower, preferably (Tg + 40°C) or lower, and more preferably (Tg + 35°C) or lower. The stretching temperature is preferably between (Tg + 10°C) and (Tg + 40°C), and more preferably between (Tg + 15°C) and (Tg + 35°C). 【0103】- Time for the unoriented film temperature to reach Tg or higher: 1.8 to 15 seconds Note that the heating conditions to reach the stretching temperature are important in suppressing both insufficient film heating and roll adhesion. Until the unoriented film reaches Tg (first stage of preheating), the temperature of each roll and the contact time with the rolls are not particularly limited as long as the film is heated while in contact with multiple rolls. However, the time for the unoriented film temperature to reach Tg or higher (second stage of preheating) should be 1.8 to 15 seconds. If the time for the unoriented film temperature to reach Tg or higher is too short, heating variations in the unoriented film may occur, resulting in larger film unevenness in the longitudinal direction or insufficient heating before stretching. For this reason, the time for the unoriented film temperature to reach Tg or higher should be 1.8 seconds or more, preferably 3.0 seconds or more, and more preferably 4.0 seconds or more. On the other hand, if the time during which the temperature of the unoriented film is above Tg is too long, the softening of the unoriented film may progress too much, causing film deformation or roll adhesion, which can result in large variations in film thickness in the longitudinal direction. Therefore, the time during which the temperature of the unoriented film is above Tg should be 15 seconds or less, preferably 12 seconds or less, and more preferably 10 seconds or less. The time during which the temperature of the unoriented film is above Tg is preferably 3.0 to 12 seconds, and more preferably 4.0 to 10 seconds. 【0104】 It is preferable to heat-set the longitudinally stretched film afterward. 【0105】 • Heat setting temperature: (Stretching temperature + 10°C) to (Melting point - 50°C) Heat setting is preferably performed by running the film on a roll heated to a high temperature. If the heat setting temperature is below (Stretching temperature + 10°C), the heat setting will be insufficient and E ＭＤ and E ＴＤ If the S decreases, １６０ＭＤ and S １６０ＴＤmay increase. Therefore, the heat setting temperature is preferably (drawing temperature + 10°C) or higher, more preferably (drawing temperature + 20°C) or higher, and even more preferably (drawing temperature + 30°C) or higher. On the other hand, when the heat setting temperature exceeds (melting point - 50°C), the film may soften and the film thickness unevenness in the longitudinal direction may increase. Therefore, the heat setting temperature is preferably (melting point - 50°C) or lower, more preferably (melting point - 60°C) or lower, and even more preferably (melting point - 70°C) or lower. The heat setting temperature is more preferably (drawing temperature + 20°C) to (melting point - 60°C), and even more preferably (drawing temperature + 30°C) to (melting point - 70°C). 【0106】 ・ Heat setting time: 0.2 to 5 seconds When the time during which the film contacts the heating roll at the heat setting temperature, that is, the heat setting time, is less than 0.2 seconds, the heat setting is insufficient and E ＭＤ and E ＴＤ may decrease, or S １６０ＭＤ and S １６０ＴＤ may increase. Therefore, the heat setting time is 0.2 seconds or more, preferably 0.3 seconds or more, and more preferably 0.4 seconds or more. The longer the heat setting time, the greater the heat setting effect, and while there is an effect of reducing S １６０ＭＤ and S １６０ＴＤ , E ＭＤ and E ＴＤ may become too high. Therefore, the heat setting time is 5 seconds or less, preferably 3 seconds or less, and more preferably 2 seconds or less. The heat setting time can be controlled by adjusting the number of heating rolls, the expansion and contraction of the diameter, or the running speed of the film. The heat setting time is preferably 0.3 to 3 seconds, and more preferably 0.4 to 2 seconds. 【0107】 ・ Relaxation rate: 0.5 to 5% Further, the heat setting may be performed while relaxing the film in the stretching direction. By relaxing simultaneously with the heat setting, the residual stress of the film oriented in the longitudinal direction can be further reduced. If the relaxation rate is too low, the effect of reducing the residual stress in the longitudinal direction cannot be obtained, and E ＭＤ or S １６０ＭＤ may become too high, or E ＴＤ or S １６０ＴＤIn some cases, the relaxation rate may become too low. Therefore, the relaxation rate is preferably 0.5% or higher, more preferably 0.8% or higher, and even more preferably 1% or higher. On the other hand, if the relaxation rate is too high, E ＭＤ Ya S １６０ＭＤ If it becomes too low, E ＴＤ Ya S １６０ＴＤ The slack can sometimes become too high. Also, the film may not shrink completely, causing it to sag during transport. Therefore, the slack is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less. The slack is more preferably 0.8 to 4%, and even more preferably 1 to 3%. The heat-set film is then cooled to obtain a polyester film. 【0108】 The polyester film for metal plate coating of the present invention can suppress wrinkles and thermal shrinkage of the film, especially at high temperatures, during the thermocompression lamination method. Furthermore, in conjunction with controlling the film thickness variations and Young's modulus, it can suppress the inclusion of air bubbles and wrinkles, and when used as a can body for two-piece cans requiring a high degree of processing, it can suppress the occurrence of rupture or abrasion of the resin coating layer during can manufacturing. For this reason, it can be suitably used for surface coating of metal plates for containers, and can be suitably used as a resin-coated metal plate by coating at least one side of the metal plate. 【0109】 The present invention will be further described below based on the following examples. However, the present invention is not limited to the following examples. 【0110】 In this example, the properties of the polyester film for metal plate coating were measured and evaluated using the following method. 【0111】(1) Film thickness and film thickness variation were measured using a Fujiwork Corporation FT-A100 continuous thickness measuring machine (load 0.14 N). Using the polyester film of the present invention as a sample, sample A was cut to be 210 mm in the longitudinal direction and 50 mm in the width direction, and sample B was cut to be 50 mm in the longitudinal direction and 210 mm in the width direction. With a measurement pitch of 1 mm, 200 measurement points, a feed speed of 1.0 m / min, and a measurement range of ±100, measurements were taken five times each for sample A and sample B, and the average value was taken as the film thickness. Measurements were taken for film A, and the film thickness variation was calculated from the obtained average value, maximum value, and minimum value. The average value of the film thickness variation obtained by taking this measurement five times was taken as the film thickness variation in the longitudinal direction. Measurements were taken in the same way for film B, and the film thickness variation in the width direction was determined. 【0112】 (2) Young's modulus The Young's modulus was measured using a universal material testing machine (AG-X-5) manufactured by Shimadzu Corporation at a temperature of 23°C. The sample used for measurement was cut from the film so that the length in the measurement direction (longitudinal or widthwise direction) was 130 mm and the length in the direction perpendicular to the measurement direction was 10 mm. The cut sample was stretched to an original length of 50 mm at a tensile speed of 300 mm / min and the Young's modulus was measured. The above measurement was repeated five times in both the longitudinal and widthwise directions. The average value of the five measurements in the longitudinal direction was given as the longitudinal Young's modulus E ＭＤ The average of five measurements in the width direction is Young's modulus E in the width direction. ＴＤ That's what I decided. 【0113】 (3) Thermal shrinkage rate was measured using a thermomechanical analyzer (TMA7100C) manufactured by Hitachi High-Tech Science Corporation. The sample was cut so that the distance between the chucks in the measurement direction (longitudinal or widthwise) was 10 mm and the distance in the direction perpendicular to the measurement direction was 4 mm, and fixed between the chucks. The measurement mode was set to tensile mode (measurement load 29.4 mN), and the temperature was raised from 25°C to 200°C at a rate of 5°C / min under a nitrogen atmosphere with a flow rate of 100 mL / min. The TMA value (displacement) at 160°C was determined from the chart obtained from the measurement. In addition, the displacement was defined as positive for shrinkage and negative for expansion or elongation. The distance between the chucks before measurement was L ０ Let L be the displacement amount at 160°C.１ In this case, the thermal shrinkage rate after heat treatment at 160°C was determined by the value calculated using the formula shown below. Measurements were taken five times each in the longitudinal and width directions, and the average of the five measurements in the longitudinal direction was used as the thermal shrinkage rate S at 160°C in the longitudinal direction. １６０ＭＤ The average of five measurements in the width direction is the 160°C heat shrinkage coefficient S in the width direction. １６０ＴＤ The thermal shrinkage rate (%) = (L １ / L ０ ) × 100 【0114】 (4) Melting point The melting point was measured using a differential scanning calorimeter (DSCQ100) manufactured by TA Instruments Corporation, with the film of the present invention as the sample. First, it was cooled to -50°C in a nitrogen atmosphere, and then heated to 290°C at a rate of 20°C / min (1st Run). After heating to 290°C in the 1st Run, it was held for 5 minutes and then rapidly cooled with liquid nitrogen. After that, it was heated again from -50°C to 290°C at a rate of 20°C / min (2nd Run). The melting point was determined from the chart obtained from the 2nd Run measurement. The same measurement was performed three times for each sample, and the average value was taken as the respective melting point. 【0115】 (5) Inorganic pigment content: According to JIS K7250, initial mass W ０ The film was placed in a platinum crucible, first thoroughly burned with a gas burner, and then treated in an electric furnace at 750-800°C for 1 hour to completely reduce it to ash. The mass of the resulting ash was W. １ The inorganic pigment content was measured and calculated using the following formula. Each measurement was performed three times, and the average value was taken as the inorganic pigment content (i.e., the proportion of inorganic pigment in the total film components). Note that the film of this invention may contain other additives such as inorganic lubricants, but since the content of these additives is extremely small compared to the inorganic pigment, the influence of other additives is ignored in the measurement of the inorganic pigment content. Inorganic pigment content (%) = (W １ / W ０ ) × 100 【0116】 (6) Production of resin-coated metal sheets by pressure lamination. TFS (metal Cr layer: 120 mg / m²) was made using a 0.22 mm thick T3CA as the base metal sheet. ２ Cr oxide layer: 10 mg / m² in terms of metallic Cr ２Using a thermocompression lamination method (film lamination method), at least one side of a metal plate was coated with a polyester film for metal plate coating manufactured in the following examples and comparative examples. The specific lamination conditions were as follows: the metal plate temperature immediately before lamination was set to the melting point of the film + 10°C, the lamination roll temperature was 85°C, the lamination time was 15 msec, and the surface pressure was 6.1 kgf / cm². ２ Subsequently, by water cooling one second after the heat-pressing process, a resin-coated metal plate was obtained in which a resin coating layer was applied to both sides of the metal plate. 【0117】 (7) Lamination Properties The percentage of air bubbles and the number of wrinkles in the resin-coated metal sheets obtained in item 6 above were checked, and the heat-press lamination properties were evaluated according to the following criteria. A (Excellent): Percentage of air bubbles less than 1%, and no wrinkles. B (Good): Percentage of air bubbles 1% or more and less than 5%, or one wrinkle. No practical problems. C (Acceptable): Percentage of air bubbles 1% or more and less than 5%, and one wrinkle. No practical problems. D (Unacceptable): Percentage of air bubbles 5% or more, or two or more wrinkles. 【0118】(8) Can-making processability After applying paraffin wax to the resin-coated metal sheet obtained in (6) above, a blank with a diameter of 123 mm was punched out, and the film was drawn into a cup with an inner diameter of 71 mm and a height of 36 mm using a cupping press so that the film was on the outside. Next, the obtained cup was loaded into a DI molding machine and redrawn at a punch speed of 200 mm / second and a stroke of 560 mm. Subsequently, three stages of ironing were performed to reduce the total reduction rate by 51% (reduction rates for each stage: 23%, 25%, and 25%), and a can with an inner diameter of 52 mm and a can height of 90 mm was formed. The formed can was placed in a hot air drying oven and heat-treated so that the can body temperature reached the melting point of the resin coating - 5° after 1 minute. After that, the can was removed from the hot air drying oven and cooled to room temperature. The resin coating layer on the outer surface of the manufactured can was visually observed, and the can-making processability was evaluated according to the following criteria. A (Excellent): No ears are present that are more than 1 mm higher than the average can height at the can opening, and no film abrasion or breakage is observed. B (Good): Ears are present that are more than 1 mm but less than 2 mm higher than the average can height at the can opening, or slight film abrasion occurs at a height of 2 mm or less from the average can height at the can opening. No practical problems. C (Acceptable): No ears are present that are more than 2 mm higher than the average can height at the can opening, and noticeable film abrasion occurs at a height of 2 mm or less from the average can height at the can opening. No practical problems. D (Unacceptable): Ears are present that are more than 2 mm higher than the average can height at the can opening, or film abrasion or breakage occurs more than 2 mm from the can flange. 【0119】(Example 1) As raw material resins for layer A of the film, polyethylene terephthalate, a polyester resin with a melting point of 249°C and an intrinsic viscosity of 0.65 dL / g, and masterbatch pellets containing rutile-type titanium dioxide as an inorganic pigment in the polyethylene terephthalate were prepared. Each was thoroughly dried under high temperature in a vacuum to prevent moisture from being absorbed. Then, the polyester resin and the pellets were blended so that the inorganic pigment content was 15% by mass to obtain resin composition A. Furthermore, as raw material resins for layer B, polyethylene terephthalate, a polyester resin with a melting point of 249°C and an intrinsic viscosity of 0.80 dL / g, a masterbatch containing silicon dioxide as an inorganic lubricant in the polyethylene terephthalate, and masterbatch pellets containing acid-modified polyethylene wax with an acid value of 60 mgKOH / g as a wax in the polyethylene terephthalate were prepared. Each was thoroughly dried under high temperature in a vacuum to prevent moisture from being absorbed. Subsequently, the polyester resin and the pellets were blended to obtain resin composition B, with an inorganic lubricant content of 800 ppm by mass and wax content of 0.15% by mass. Resin compositions A and B were fed into two different single-screw extruders and melt-kneaded at 278°C. Then, after removing foreign matter through a 25 μm cut sintered filter, the layers were merged in a feed block designed with a layering ratio of 1 (layer B):8 (layer A):1 (layer B) to form three layers in the thickness direction. The temperature of the laminated molten resin was 285°C and the shear rate was 300 sec. －１ The material was extruded from a T-die that was controlled to achieve a specific temperature. After passing through a section where the ambient temperature between the T-die and the casting drum was controlled to 35°C, it was cooled and solidified on a casting drum with a surface temperature controlled to 30°C to obtain an unstretched film. 【0120】Next, the unstretched film was preheated (first stage) using a heated ceramic roll until the film temperature reached 78°C, the glass transition temperature Tg of the polyester resin. Subsequently, the unstretched film was preheated (second stage) until it reached 93°C, so that the contact time with the heated roll, i.e., the temperature at which the unstretched film reaches or exceeds Tg, was 4 seconds. Then, the unstretched film was stretched 4.5 times in the longitudinal direction. After that, it was heat-set at 123°C for 0.4 seconds using a heated mirror-finish HCr plated roll. During this time, a 1% relaxation in the longitudinal direction was applied by utilizing the speed difference between two consecutive mirror-finish HCr plated rolls. Finally, the film was cooled to room temperature, and the film with the edges removed was wound on a winding machine to obtain a polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0121】 (Example 2) The same procedure as in Example 1 was followed, except that the molten resin temperature in the T-die was set to 279°C, to obtain a polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0122】 (Example 3) The molten resin shear rate in the T-die was set to 60 sec. －１ Except for the modifications mentioned above, the same procedure as in Example 1 was followed to obtain a polyester film for coating metal plates with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0123】 (Example 4) The molten resin shear rate in the T-die was set to 1000 sec. －１ Except for the modifications mentioned above, the same procedure as in Example 1 was followed to obtain a polyester film for coating metal plates with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0124】 (Example 5) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249 °C was obtained by fabricating in the same manner as in Example 1, except that the ambient temperature between the T-die and the casting drum was set to 30 °C. The physical properties of the obtained film are shown in Table 1. 【0125】(Example 6) The same procedure as in Example 1 was followed, except that the ambient temperature between the T-die and the casting drum was set to 50°C, to obtain a polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0126】 (Example 7) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the contact time of the heating roll with the unstretched film at Tg or above was set to 1.8 seconds. The physical properties of the obtained film are shown in Table 1. 【0127】 (Example 8) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the contact time of the heating roll with the unstretched film at Tg or above was set to 15 seconds. The physical properties of the obtained film are shown in Table 1. 【0128】 (Example 9) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the heat setting time was 0.2 seconds. The physical properties of the obtained film are shown in Table 1. 【0129】 (Example 10) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the heat setting time was 5 seconds. The physical properties of the obtained film are shown in Table 1. 【0130】 (Example 11) As raw material resins for the film, polyethylene terephthalate, a polyester resin with a melting point of 243°C and an intrinsic viscosity of 0.64 dL / g, and masterbatch pellets containing silicon dioxide as an inorganic lubricant were prepared. Both were thoroughly dried under high temperature vacuum to remove moisture. Then, the polyester resin and the pellets were blended to obtain a resin composition with an inorganic lubricant content of 800 ppm by mass. The resin composition was fed into a single-screw extruder and melt-kneaded at 275°C. Next, after removing foreign matter through a 25 μm cut sintering filter, the molten resin temperature was 285°C and the shear rate was 300 sec. －１The material was extruded from a T-die controlled to achieve a specific temperature, passed through a section between the T-die and the casting drum where the ambient temperature was controlled to 35°C, and then cooled and solidified on a casting drum with a surface temperature controlled to 30°C to obtain an unstretched film. The process from stretching onward was carried out in the same manner as in Example 1 to obtain a polyester film for metal plate coating with a thickness of 20 μm and a melting point of 243°C. The physical properties of the obtained film are shown in Table 1. 【0131】 (Example 12) A polyester film for metal plate coating was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio was 3.5 times, the stretching temperature was 119°C, the heat setting temperature was 199°C, the heat setting time was 5 seconds, and the longitudinal relaxation ratio was 5%. The properties of the obtained film are shown in Table 1. 【0132】 (Example 13) A polyester film for metal plate coating was prepared in the same manner as in Example 1, except that the longitudinal stretching ratio was 6.5 times, the contact time with the heating roll at Tg or above the unstretched film was 2.5 seconds, and the heat setting temperature was 103°C. A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained. The physical properties of the obtained film are shown in Table 1. 【0133】 (Example 14) The molten resin temperature in the T-die was set to 293°C, and the molten resin shear rate in the T-die was set to 600 sec. －１ Except for setting the ambient temperature between the T-die and the casting drum to 40°C, the same procedure as in Example 1 was followed to obtain a polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0134】 (Example 15) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was 5.5 times. The physical properties of the obtained film are shown in Table 1. 【0135】 (Example 16) A polyester film for metal plate coating was prepared in the same manner as in Example 1, except that the melting point of the polyester resin raw material for layer A of the film was set to 220°C, the melting point of the polyester resin raw material for layer B was set to 220°C, the melting resin temperature in the T-die was set to 256°C, the longitudinal stretching temperature was set to 92°C, and the heat setting temperature was set to 122°C. A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 220°C was obtained. The physical properties of the obtained film are shown in Table 1. 【0136】 (Example 17) A polyester film for coating metal plates was prepared in the same manner as in Example 1, except that the inorganic pigment content was 25% by weight, and a thickness of 20 μm and a melting point of 249°C was obtained. The physical properties of the obtained film are shown in Table 1. 【0137】 (Example 18) The same procedure as in Example 1 was followed, except that the amount of resin discharged from the T-die was adjusted to make the film thinner, to obtain a polyester film for metal plate coating with a thickness of 5.0 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0138】 (Example 19) The same procedure as in Example 1 was followed, except that the amount of resin discharged from the T-die was adjusted to increase the film thickness, to obtain a polyester film for metal plate coating with a thickness of 35 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0139】 (Example 20) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the longitudinal relaxation rate was set to 6%. The physical properties of the obtained film are shown in Table 1. 【0140】 (Example 21) A polyester film for metal plate coating was prepared in the same manner as in Example 1, except that the melting point of the polyester resin raw material for layer A was set to 218°C, the melting point of the polyester resin raw material for layer B was set to 218°C, the melting resin temperature in the T-die was set to 254°C, the longitudinal stretching temperature was set to 92°C, and the heat setting temperature was set to 122°C. A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 218°C was obtained. The physical properties of the obtained film are shown in Table 1. 【0141】 (Example 22) A polyester film for coating metal plates was obtained in the same manner as in Example 1, except that the inorganic pigment content was 26% by weight. The film had a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0142】(Comparative Example 1) The unstretched film from Example 1 was preheated to 100°C using a heated ceramic roll, and then stretched 3.5 times in the longitudinal direction of the film. After that, the film was introduced into a tenter stretcher with its ends held by clips, and stretched 4.5 times in the width direction at 110°C. Then, while heat-setting at 130°C, it was relaxed by 1% in the width direction. Finally, the film was cooled to room temperature, the ends were removed, and the film was wound up on a winding machine to obtain a polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0143】 (Comparative Example 2) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249 °C was obtained by preparing the film in the same manner as in Example 1, except that the molten resin temperature in the T-die was set to 275 °C. The physical properties of the obtained film are shown in Table 1. 【0144】 (Comparative Example 3) The molten resin shear rate in the T-die was set to 50 sec. －１ Except for the modifications mentioned above, the same procedure as in Example 1 was followed to obtain a polyester film for coating metal plates with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0145】 (Comparative Example 4) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249 °C was obtained by the same procedure as in Example 1, except that the ambient temperature between the T-die and the casting drum was set to 25 °C. The physical properties of the obtained film are shown in Table 1. 【0146】 (Comparative Example 5) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the contact time of the heating roll with the unstretched film at Tg or above was set to 1.6 seconds. The physical properties of the obtained film are shown in Table 1. 【0147】 (Comparative Example 6) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the heat setting time was 0.1 seconds. The physical properties of the obtained film are shown in Table 1. 【0148】 (Comparative Example 7) The molten resin shear rate in the T-die was set to 1100 sec. －１Except for the modifications mentioned above, the same procedure as in Example 1 was followed to obtain a polyester film for coating metal plates with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0149】 (Comparative Example 8) The molten resin shear rate in the T-die was set to 60 sec. －１ Except for setting the ambient temperature between the T-die and the casting drum to 51°C, the same procedure as in Example 1 was followed to obtain a polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C. The physical properties of the obtained film are shown in Table 1. 【0150】 (Comparative Example 9) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was 3.4 times. The physical properties of the obtained film are shown in Table 1. 【0151】 (Comparative Example 10) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the contact time of the heating roll with the unstretched film at Tg or above was 16 seconds. The physical properties of the obtained film are shown in Table 1. 【0152】 (Comparative Example 11) A polyester film for metal plate coating with a thickness of 20 μm and a melting point of 249°C was obtained by preparing the film in the same manner as in Example 1, except that the heat setting time was 6 seconds. The physical properties of the obtained film are shown in Table 1. 【0153】 【0154】 According to the present invention, in the thermocompression lamination method, wrinkles and thermal shrinkage of the film can be suppressed, especially at high temperatures. Furthermore, in conjunction with controlling the film thickness variations and Young's modulus, it is possible to not only suppress the inclusion of air bubbles and wrinkles, but also to produce a polyester film with excellent can-making processability. Due to the aforementioned properties, the polyester film can be suitably used as a coating film for metal plates, particularly metal plates for containers. A resin-coated metal plate equipped with the polyester film for metal plate coating of the present invention can be suitably used as a material for manufacturing containers, particularly the can body of a two-piece can that requires a high degree of processability.

Claims

1. A polyester film containing polyester resin in a proportion of 70% by mass or more, wherein the film thickness variation, determined from the average, maximum, and minimum values ​​obtained by measuring the film thickness at 200 points at 1 mm intervals in both the longitudinal and width directions, is 0.0 to 15%, and the Young's modulus in the longitudinal direction is E ＭＤ and Young's modulus E in the width direction ＴＤ The ratio (E ＭＤ / E ＴＤ ) is 1.1 to 10, and the E ＭＤ A polyester film for coating metal plates, with a thermal pressure of 5.5 to 19 GPa.

2. The above E ＴＤ The polyester film for coating metal plates according to claim 1, wherein the pressure is 1.0 to 5.0 GPa.

3. Longitudinal heat shrinkage coefficient S at 160°C １６０ＭＤ A polyester film for coating metal plates according to claim 1 or 2, wherein the content is 0.0 to 20%.

4. Heat shrinkage coefficient S in the width direction at 160°C １６０ＴＤ A polyester film for coating metal plates according to any one of claims 1 to 3, wherein the content is 0.0 to 10%.

5. The S １６０ＭＤ and the S １６０ＴＤ sum of (S １６０ＭＤ + S １６０ＴＤ ) is 0.0 to 30%, the polyester film for metal plate coating according to claim 3 or 4.

6. A polyester film for coating metal plates according to any one of claims 1 to 5, wherein the melting point is 220 to 255°C.

7. A polyester film for coating metal plates according to any one of claims 1 to 6, comprising 5.0 to 25% by mass of an inorganic pigment.

8. A polyester film for coating metal plates according to any one of claims 1 to 7, wherein the film thickness is 5.0 to 35 μm.

9. A resin-coated metal plate having at least one side a polyester film for metal plate coating as described in any one of claims 1 to 8.

10. A process of melting and kneading a raw material resin containing polyester resin in a proportion of 70% by mass or more in an extruder to obtain a molten resin, wherein the molten resin temperature in the T-die is 30°C or higher to 50°C or lower than the melting point of the polyester resin, and the molten resin shear rate in the T-die is 60 to 1000 sec. －１ A method for manufacturing a polyester film for metal plate coating according to any one of claims 1 to 8, comprising the steps of: discharging a sheet from a T-die under certain conditions, cooling and solidifying it on a casting drum under conditions where the ambient temperature between the T-die and the casting drum is 30 to 50°C to obtain an unstretched film; stretching the unstretched film in the longitudinal direction under conditions where the stretching ratio is 3.5 to 7.0 times, the stretching temperature is 5°C or more above the glass transition temperature of the polyester resin and 50°C or less above the glass transition temperature, and the time for which the temperature of the unstretched film is 1.8 to 15 seconds; heat-fixing the stretched film while relaxing it in the stretching direction for a heat-fixing time of 0.2 to 5 seconds; and cooling the heat-fixed film to room temperature.

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