Polyester film and film laminate

Abstract

The present invention provides a polyester film that ensures a certain level of mechanical strength while remaining intact even when heated to high temperatures, and capable of properly forming an insulating film. [Solution] A laminated polyester film having a polyester film and a resin layer (a) on one surface side of the polyester film, wherein the resin layer (a) contains a resin and a filler (Za), and the content of the filler (Za) in the resin layer is 20% by mass or more, and is for use in batteries.

Description

[Technical Field] 【0001】 This invention relates to polyester films and film laminates used for batteries. [Background technology] 【0002】 Conventionally, polyester films possess transparency, dimensional stability, mechanical properties, heat resistance, and chemical resistance, and are used in a variety of applications, including packaging, electronic components, electrical insulation, metal lamination, display components such as flexible displays, touch panels, anti-reflective coatings, and glass shatterproof coatings. In recent years, with the aim of achieving carbon neutrality, the development of batteries for electric vehicles has progressed, and polyester films are also being considered for use as battery casing materials (see, for example, Patent Document 1). Furthermore, flame retardants have sometimes been added to polyester films to improve their flame retardancy (see, for example, Patent Document 2). 【0003】 To improve the performance of electric vehicles, it is necessary to increase the energy density of the battery. On the other hand, batteries with high energy density are prone to thermal runaway, and when thermal runaway occurs, the temperature of the battery cells rises rapidly and can reach over 400°C. Conventionally, various measures have been taken to suppress thermal runaway. For example, Patent Document 3 proposes a thermal runaway prevention sheet that suppresses heat conduction to adjacent cells when one cell experiences thermal runaway. [Prior art documents] [Patent Documents] 【0004】 [Patent Document 1] Japanese Patent Publication No. 2020-140775 [Patent Document 2] Japanese Patent Publication No. 2015-081271 [Patent Document 3] Japanese Patent Publication No. 2018-206605 [Overview of the project] [Problems that the invention aims to solve] 【0005】 Incidentally, if thermal runaway occurs in a battery cell, using a conventional battery cell casing material such as that described in Patent Document 1 may cause the casing material to disappear, exposing the conductive material inside the battery element. When the conductive material inside the battery element is exposed, sparks may occur, leading to a fire or chain explosions between battery cells. On the other hand, while the thermal runaway prevention sheet described in Patent Document 3 can prevent so-called chain explosions, where thermal runaway between battery cells causes a chain reaction to adjacent cells, it does not suggest that it can prevent exposure of conductive materials or the generation of sparks. 【0006】 Furthermore, in order to prevent exposure of conductive materials and the generation of sparks, the inventors have considered the possibility of using polyester film, which is used as an exterior material or protective material, to remain even after high-temperature heating and form an insulating film. Resin films can form a carbonized layer and leave residue when heated at high temperatures by containing flame retardants, but if flame retardants are included in polyester film, the mechanical strength may decrease due to the flame retardants. 【0007】 Therefore, the object of the present invention is to provide a polyester film that can maintain a certain level of mechanical strength, remain even when heated to high temperatures, and properly form an insulating film, making it suitable for use in batteries. [Means for solving the problem] 【0008】 As a result of diligent research, the inventors have found that the above problems can be solved by providing a predetermined structure for the resin layer on one surface side of the polyester film, and have completed the present invention as follows. The present invention is as follows [1] to

[33] . 【0009】 [1] A laminated polyester film having a polyester film and a resin layer (a) on one surface side of the polyester film, The resin layer (a) comprises resin and filler (Za), The content of the filler (Za) in the resin layer (a) is 20% by mass or more, A laminated polyester film for use in a battery. [2] The laminated polyester film according to [1] above, wherein the content of the filler (Za) in the resin layer (a) is 10% by volume or more. [3] The mass per unit area of the resin layer (a) is 10 mg / m 2 or more and 4000 mg / m 2 or less. The laminated polyester film according to [1] or [2] above. [4] The laminated polyester film according to any one of [1] to [3] above, wherein the thickness of the resin layer (a) is less than 2 μm. [5] The laminated polyester film according to any one of [1] to [4] above, wherein the average particle diameter of the filler (Za) is less than 1 μm. [6] The laminated polyester film according to any one of [1] to [5] above, wherein the tensile breaking strength is 175 MPa or more in both the MD and TD directions. [7] The laminated polyester film according to any one of [1] to [6] above, wherein the filler (Za) is an inorganic filler. [8] The laminated polyester film according to any one of [1] to [7] above, wherein the filler (Za) is at least one selected from the group consisting of titanium oxide, silica, zirconium oxide, and carbon black. [9] The laminated polyester film according to any one of [1] to [8] above, wherein the polyester film has a polyester resin (X) and a flame retardant (Y).

[10] The content of phosphorus element (P1) per 100 parts by mass of the polyester resin (X) on one surface of the polyester film is 0.1 part by mass or more, and the content of phosphorus element (P1) is more than the content of phosphorus element (P2) per 100 parts by mass of the polyester resin (X) at the central portion in the thickness direction of the polyester film. The laminated polyester film according to [9] above.

[11] The phosphorus element content rate (pt) in the laminated polyester film is 0.01% by mass or more, The laminated polyester film according to [9] or

[10] above, wherein the phosphorus element content (p1) on one surface of the polyester film is higher than the phosphorus element content (p2) at the center in the thickness direction of the polyester film.

[12] The phosphorus element content (pt) in the laminated polyester film is 0.01% by mass or more, The laminated polyester film according to any one of [9] to

[11] above, having a tensile breaking strength of 175 MPa or more in both MD and TD.

[13] The laminated polyester film according to any one of [1] to

[12] above, wherein the polyester film contains a filler (Z).

[14] The laminated polyester film according to

[13] above, wherein the filler (Z) is an inorganic filler.

[15] The laminated polyester film according to

[13] above, wherein the filler (Z) is at least one selected from the group consisting of titanium oxide, silica, carbon black, calcium carbonate, and aluminum oxide.

[16] The laminated polyester film according to any one of

[13] to

[15] above, wherein the content (z) of the filler (Z) in the polyester film is 20% by mass or less.

[17] The laminated polyester film according to any one of [9] to

[16] above, wherein the phosphorus element content (pt) of the laminated polyester film is 0.05% by mass or more and 1% by mass or less.

[18] The polyester film according to any one of [1] to

[17] above, wherein the total (pt + zt) of the phosphorus element content (pt) and the filler content (zt) in the laminated polyester film is 0.3% by mass or more and 20% by mass or less.

[19] The laminated polyester film according to any one of [1] to

[18] above, wherein the polyester film includes a surface layer (A) and an intermediate layer (B).

[20] The laminated polyester film according to

[19] above, further having a back surface layer (C).

[21] A laminated polyester film according to any of [1] to

[20] above, wherein the weight retention rate measured by thermogravimetric differential thermal analysis (TG-DTA) at 700°C is greater than 1%.

[22] A laminated polyester film according to any of [1] to

[21] above, wherein the weight retention rate measured by thermogravimetric differential thermal analysis (TG-DTA) at 500°C is 15% or more.

[23] A laminated polyester film according to any of [1] to

[22] above, wherein a brass plate with a thickness of 1 mm and the polyester film are laminated together, and after being held in a 700°C atmosphere for 30 minutes, the surface of the brass plate does not conduct electricity as measured by a tester.

[24] The laminated polyester film according to any one of [1] to

[23] above, wherein the resin contained in the resin layer is at least one selected from the group consisting of polyester resin, (meth)acrylic resin, and urethane resin.

[25] A film laminate comprising a laminated polyester film as described in any of [1] to

[24] above, and at least one of a functional layer and a metal layer provided on at least one surface side of the laminated polyester film.

[26] The film laminate according to

[25] above, wherein the functional layer is an adhesive layer.

[27] The film laminate according to

[25] or

[26] above, comprising the metal layer.

[28] The film laminate according to

[27] , wherein the metal constituting the metal layer is at least one of copper and aluminum.

[29] A laminated polyester film according to any of [1] to

[24] above, for use as battery protection or battery housing.

[30] A battery protective material comprising a laminated polyester film as described in any of [1] to

[24] above.

[31] A battery comprising a laminated polyester film as described in any of [1] to

[24] above.

[32] An electronic device equipped with the battery described in

[31] above.

[33] A method of use in which the laminated polyester film described in any of [1] to

[24] above is used as an insulating layer. [Effects of the Invention] 【0010】 According to the present invention, it is possible to provide a polyester film that ensures a certain level of mechanical strength, remains intact even when heated to high temperatures, and properly forms an insulating film, making it suitable for use in batteries. [Brief explanation of the drawing] 【0011】 [Figure 1] This is a schematic diagram illustrating the method for evaluating insulating properties. [Modes for carrying out the invention] 【0012】 Next, embodiments of the present invention will be described. However, the present invention is not limited to the embodiments described below. 【0013】 <<Laminated polyester film>> The laminated polyester film of the present invention is used for batteries and comprises a polyester film and a resin layer (a) provided on one surface side of the polyester film. In the following description, the laminated polyester film may be referred to as "this laminated film" and the polyester film as "this film". 【0014】 [Resin layer (a)] The resin layer (a) contains a resin and a filler (Za). By containing the filler (Za), the resin layer (a) remains intact even when heated to high temperatures, allowing for the proper formation of an insulating film. 【0015】 <Filler (Za)> The filler (Za) contained in the resin layer (a) may be either an inorganic filler or an organic filler, but an inorganic filler is preferred from the viewpoint of insulating film formation due to the filler itself remaining when heated at high temperatures. The shape of the filler (Za) may be spherical particles, flakes, plates, fibers, needles, etc., but a spherical shape is preferred from the viewpoint of being easily distributed uniformly in the resin layer (a). 【0016】 Examples of inorganic fillers include titanium dioxide, silica, carbon black, talc, mica, calcium carbonate, magnesium carbonate, barium oxide, lead titanate, potassium titanate, barium titanate, zirconium oxide, magnesium oxide, calcium oxide, aluminum oxide, zinc sulfide, antimony oxide, zinc oxide, boron nitride, aluminum nitride, and barium sulfate. Examples of organic fillers include crosslinked polymers such as crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked styrene-acrylic resin particles, and crosslinked polyester resin particles, as well as organic particles such as calcium oxalate and ion exchange resins. Among the above, titanium dioxide, silica, zirconium oxide, and carbon black are preferred as the filler (Za), with silica being more preferred. The filler (Za) may be used alone or in combination of two or more types. Therefore, a combination of two or more suitable particles such as titanium dioxide, silica, zirconium oxide, and carbon black can also be used. 【0017】 The average particle size of the filler (Za) is preferably 1 nm or more and less than 1 μm, more preferably 2 nm or more and 500 nm or less, even more preferably 4 nm or more and 100 nm or less, even more preferably 6 nm or more and 60 nm or less, even more preferably 8 nm or more and 40 nm or less, and even more preferably 10 nm or more and 30 nm or less. Within this range of average particle size, the generation of coarse protrusions due to particle aggregation and process contamination due to particle detachment can be suppressed. Furthermore, if the particles are in powder form, the average particle size can be determined by using a centrifugal sedimentation particle size distribution analyzer (e.g., Shimadzu Corporation's "SA-CP3" model) to measure the particle size distribution at 50% of the cumulative volume fraction (d50). For particles in films, layers, or resins, the average particle size can be determined by observing 10 or more particles with a scanning electron microscope (SEM), measuring the diameter of each particle, and taking the average value. In the case of non-spherical particles, the average of the longest and shortest diameters can be used as the diameter of each particle. 【0018】 The content of the filler (Za) in the resin layer (a) is 20% by mass or more, based on the total amount of the resin layer (a). If the content of the filler (Za) is less than 20% by mass, it becomes difficult to properly form an insulating film with the resin layer (a) when heated at high temperatures. From the viewpoint of good insulating film formation when heated at high temperatures, the content of the filler (Za) is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, even more preferably 60% by mass or more, and particularly preferably 65% ​​by mass or more, based on mass. Furthermore, from the viewpoint of improving the film-forming properties of the resin layer (a), the content of the filler (Za) in the resin layer (a) is preferably 99% by mass or less, more preferably 95% by mass or less, even more preferably 90% by mass or less, even more preferably 85% by mass or less, and particularly preferably 80% by mass or less, based on mass. 【0019】 The content of the filler (Za) in the resin layer (a) is preferably 10 volume% or more on a volume basis relative to the resin layer (a). When the content of the filler (Za) is 10 volume% or more on a volume basis, it becomes easier for the resin layer (a) to form an insulating film more appropriately when heated at high temperatures. The content of the filler (Za) is more preferably 15 volume% or more, even more preferably 24 volume% or more, even more preferably 32 volume% or more, even more preferably 40 volume% or more, even more preferably 48 volume% or more, and even more preferably 52 volume% or more on a volume basis. Furthermore, from the viewpoint of improving the film-forming properties of the resin layer (a), the content of the filler (Za) in the resin layer (a) is preferably 80 volume% or less, more preferably 76 volume% or less, even more preferably 72 volume% or less, even more preferably 68 volume% or less, and even more preferably 64 volume% or less on a volume basis. 【0020】 Furthermore, the content (z3) of the filler (Za) in the resin layer (a) is, on a mass basis, 0.1% by mass or more relative to the total amount of the laminated film, preferably 0.3% by mass or more, more preferably 0.4% by mass or more, and even more preferably 0.5% by mass or more. Alternatively, it may be 2% by mass or less, 1.75% by mass or less, preferably 1.5% by mass or less, and more preferably 1.25% by mass or less. Setting the content (z3) of the filler (Za) in the resin layer relative to the laminated film to be above the lower limit makes it easier to improve the formation of an insulating film. Also, setting it to be below the upper limit prevents the content of the filler (Za) in the resin layer (a) from becoming unnecessarily high or the resin layer (a) from becoming too thick. 【0021】 <Resin> The resin in resin layer (a) is a binder resin, and the binder resin is defined as a polymer compound having a number-average molecular weight (Mn) of 1000 or more, as measured by gel permeation chromatography (GPC), and possessing film-forming properties, in accordance with the "Flow Scheme for Safety Evaluation of Polymer Compounds" (November 1985, sponsored by the Chemical Substances Council). However, the binder resin excludes those exemplified as crosslinking agents described later. There are no particular restrictions on the binder resin, and conventionally known binder resins such as polyester resin, (meth)acrylic resin, polyurethane resin, polyvinyl resin (polyvinyl alcohol, vinyl chloride vinyl acetate copolymer, etc.), polyalkylene glycol, polyalkyleneimine, methylcellulose, hydroxycellulose, and starches can be used. Among these, at least one selected from polyester resin, (meth)acrylic resin, and polyurethane resin is preferred from the viewpoint of film-forming properties and adhesion to polyester film, and at least one selected from polyester resin and (meth)acrylic resin is more preferred. In the resin layer (a), one type of binder resin may be used alone, or two or more types may be used in combination. For example, it is also preferable to use polyester resin and (meth)acrylic resin in combination. By containing the binder resin, the resin layer (a) can hold and fix the filler (Za). 【0022】 (Polyester resin) Polyester resins include those composed of polycarboxylic acids and polyhydroxy compounds. Examples of polycarboxylic acids include terephthalic acid, isophthalic acid, orthophthalic acid, phthalic acid, 4,4'-diphenyldicarboxylic acid, 2,5-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2-potassium sulfoterephthalic acid, 5-sodium sulfisoisophthalic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, glutaric acid, succinic acid, trimellitic acid, trimesic acid, pyromellitic acid, trimellitic anhydride, phthalic anhydride, p-hydroxybenzoic acid, monopotassium salt of trimellitic acid, and their ester-forming derivatives. Examples of polyvalent hydroxy compounds that can be used include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, p-xylylene glycol, bisphenol A-ethylene glycol adduct, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polytetramethylene oxide glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, sodium dimethylolethylsulfonate, and potassium dimethylolpropionate. From these compounds, one or more can be appropriately selected, and a polyester resin can be synthesized by a conventional polycondensation reaction. 【0023】 Furthermore, as part of the polycarboxylic acid, sulfoisophthalic acids such as 5-sodium sulfisophthalic acid are copolymerized to introduce sulfonic acid groups into the polyester skeleton, which are then neutralized and hydrophilized. The amount copolymerized is usually 1 to 13 mol%, preferably 2 to 10 mol%, and more preferably 3 to 9 mol%, relative to the total polycarboxylic acid. By introducing an appropriate amount of sulfonic acid groups, the hydrophilicity of the resin can be increased, and the water dispersion stability can be improved. 【0024】 ((meth)acrylic resin) (Meth)acrylic resin is a polymer composed of polymerizable monomers, including acrylic and methacrylic monomers. These may be homopolymers, copolymers, or copolymers with polymerizable monomers other than acrylic and methacrylic monomers. (Meth)acrylic polymers are polymers having structural units derived from (meth)acrylic acid or alkyl (meth)acrylate esters. A (meth)acrylic polymer may be at least one polymer selected from (meth)acrylic acid and alkyl (meth)acrylate esters, or it may be a copolymer of at least one selected from these and at least one selected from other monomers, such as styrene or styrene derivatives, monomers containing hydroxyl groups, etc. Furthermore, copolymers of these polymers with other polymers (e.g., polyester, polyurethane, etc.) are also included. Examples include block copolymers and graft copolymers. In other words, the (meth)acrylic resin may be a (meth)acrylic-modified polyester resin or a (meth)acrylic-modified polyurethane resin. Alternatively, polymers (or mixtures of polymers, in some cases) obtained by polymerizing polymerizable monomers in a polyester solution or polyester dispersion are also included. Similarly, polymers (or mixtures of polymers, in some cases) obtained by polymerizing polymerizable monomers in a polyurethane solution or polyurethane dispersion are also included. In the same manner, polymers (or mixtures of polymers, in some cases) obtained by polymerizing polymerizable monomers in other polymer solutions or dispersions are also included, and these are also referred to as (meth)acrylic-modified polyester resins or (meth)acrylic-modified polyurethane resins in this specification. The polyesters and polyurethanes used in (meth)acrylic resins described above can be appropriately selected from the polyesters and polyurethanes exemplified as those used in binder resins, as described later. Furthermore, (meth)acrylic resin may contain hydroxyl groups and amino groups to further improve adhesion with polyester film. 【0025】 The polymerizable monomers mentioned above are not particularly limited, but some representative compounds include, for example, various carboxyl group-containing monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, and citraconic acid, and their salts; various hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, monobutyl hydroxyfumarate, and monobutyl hydroxyitaconate; and methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, and lauryl (meth)acrylate. Examples include various alkyl(meth)acrylic acid esters; various nitrogen-containing monomers such as (meth)acrylamide, diacetone acrylamide, or (meth)acrylonitrile; hydroxyl-containing nitrogen-containing monomers such as N-methylol(meth)acrylamide; various styrene derivatives such as styrene, α-methylstyrene, divinylbenzene, and vinyltoluene; various vinyl esters such as vinyl propionate; various silicon-containing polymerizable monomers such as γ-methacryloxypropyltrimethoxysilane and vinyltrimethoxysilane; phosphorus-containing vinyl monomers; various vinyl halides such as vinyl chloride and vinylidene chloride; and various conjugated dienes such as butadiene. 【0026】 Among the (meth)acrylic resins mentioned above, polymers obtained by polymerizing polymerizable monomers containing acrylic and methacrylic monomers are preferred, and polymers containing alkyl (meth)acrylic acid esters are more preferred. Furthermore, the resin layer composition containing (meth)acrylic resin is preferably diluted with a solvent to form a coating solution, as described later, and it is preferable that the solvent is primarily water (50% by mass or more). In other words, from the viewpoint of facilitating dissolution or dispersion when the coating solution is aqueous, it is preferable that the polymerizable monomer has hydrophilic groups such as hydroxyl groups or carboxyl groups. Therefore, polymers obtained by polymerizing alkyl (meth)acrylic acid esters with polymerizable monomers containing hydrophilic group monomers such as monomers containing hydroxyl groups and monomers containing carboxyl groups are also preferred as acrylic resins. The content of alkyl(meth)acrylic acid esters in the polymerizable monomer is, for example, 50% by mass or more and 99% by mass or less, preferably 70% by mass or more and 98% by mass or less, and more preferably 80% by mass or more and 97% by mass or less. Furthermore, the content of hydrophilic group-containing monomers in the polymerizable monomer is, for example, 1% by mass or more and 25% by mass or less, preferably 2% by mass or more and 20% by mass or less, and more preferably 3% by mass or more and 12% by mass or less. Furthermore, the acrylic resin may also be an emulsion polymer obtained by polymerizing polymerizable monomers in the presence of a surfactant, for example. 【0027】 (Polyurethane resin) Polyurethane resin is a polymer compound having urethane bonds within its molecule, and is preferably water-dispersible or water-soluble. In this invention, it may be used alone or in combination of two or more types. To impart water dispersibility or water solubility, it is common and preferable to introduce hydrophilic groups such as hydroxyl groups, carboxyl groups, sulfonic acid groups, sulfonyl groups, phosphate groups, and ether groups into the polyurethane resin. Among these hydrophilic groups, carboxyl groups or sulfonic acid groups are particularly preferred from the viewpoint of adhesion between the resin layer and the polyester film. For example, the introduction of carboxyl groups can be carried out using carboxyl-containing polyhydric alcohols such as dimethylolpropionic acid or dimethylolbutanoic acid. 【0028】 One method for producing polyurethane resin involves the reaction of a hydroxyl group-containing compound with an isocyanate. Polyols are preferably used as the hydroxyl group-containing compound, including, for example, polyester polyols, polyether polyols, polycarbonate-based polyols, polyolefin polyols, and acrylic polyols. Among these, polyester polyols are preferred due to their excellent adhesion to polyester films. These compounds may be used individually or in combination. 【0029】 Polyester polyols include those obtained from the reaction of polycarboxylic acids or their acid anhydrides with polyhydric alcohols. Examples of polycarboxylic acids include malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, sebacic acid, fumaric acid, maleic acid, terephthalic acid, isophthalic acid, etc. Examples of polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2-methyl-1,3-propanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2-methyl-2,4-pentanediol, 2-methyl-2-propyl-1,3-propanediol, 1 Examples include ,8-octanediol, 2,2,4-trimethyl-1,3-pentanediol, 2-ethyl-1,3-hexanediol, 2,5-dimethyl-2,5-hexanediol, 1,9-nonanediol, 2-methyl-1,8-octanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-butyl-2-hexyl-1,3-propanediol, cyclohexanediol, bishydroxymethylcyclohexane, dimethanolbenzene, bishydroxyethoxybenzene, alkyldialkanolamines, lactonediols, etc. 【0030】 Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, and polyhexamethylene ether glycol. 【0031】 Examples of polycarbonate-based polyols include polycarbonate diols obtained by de-alcoholization reactions of polyhydric alcohols with dimethyl carbonate, diethyl carbonate, diphenyl carbonate, ethylene carbonate, etc., such as poly(1,6-hexylene) carbonate and poly(3-methyl-1,5-pentylene) carbonate. 【0032】 Examples of polyisocyanate compounds used to obtain polyurethane resins include aromatic diisocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylenediphenyl diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, and tolidine diisocyanate; aliphatic diisocyanates having aromatic rings such as α,α,α',α'-tetramethylxylylene diisocyanate; aliphatic diisocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; and alicyclic diisocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, and isopropylidene dicyclohexyl diisocyanate. These may be used individually or in combination of multiple types. 【0033】 A chain extender may be used when synthesizing polyurethane resin. The chain extender is not particularly limited as long as it has two or more active groups that react with isocyanate groups. Generally, chain extenders having two hydroxyl groups or amino groups can be used. Examples of chain extenders having two hydroxyl groups include aliphatic glycols such as ethylene glycol, propylene glycol, and butanediol; aromatic glycols such as xylylene glycol and bishydroxyethoxybenzene; and ester glycols such as neopentyl glycol hydroxypivalate. 【0034】 Examples of chain extenders having two amino groups include aromatic diamines such as tolylenediamine, xylylenediamine, and diphenylmethanediamine; aliphatic diamines such as ethylenediamine, propanediamine, hexanediamine, 2,2-dimethyl-1,3-propanediamine, 2-methyl-1,5-pentanediamine, trimethylhexanediamine, 2-butyl-2-ethyl-1,5-pentanediamine, 1,8-octanediamine, 1,9-nonanediamine, and 1,10-decanediamine; and alicyclic diamines such as 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, dicyclohexylmethanediamine, 1,4-diaminocyclohexane, and 1,3-bisaminomethylcyclohexane. 【0035】 (Crosslinking agent) The resin layer (a) of the present invention may be further crosslinked with a crosslinking agent. There are no particular restrictions on the crosslinking agent, and conventionally known crosslinking agents can be used. Examples include melamine compounds, oxazoline compounds, epoxy compounds, carbodiimide compounds, isocyanate compounds, and silane coupling compounds. In particular, from the viewpoint of increasing the strength of the coating film and improving adhesion with the polyester film, it is preferable to include one or more selected from the group consisting of melamine compounds, isocyanate compounds, and oxazoline compounds, and it is especially preferable to include a melamine compound. Furthermore, the resin layer can be made into a cured resin layer by using a crosslinking agent. 【0036】 (Melamine compound) Melamine compounds are compounds that have a melamine skeleton in their composition. Examples include alkylolated melamine derivatives, compounds obtained by reacting alkylolated melamine derivatives with alcohol to partially or completely etherify them, and mixtures thereof. Examples of alkylolation include methylolation, ethylolation, isopropylroleation, n-butyrolation, and isobutylolation. Among these, methylolation is preferred from the viewpoint of reactivity. Suitable alcohols for etherification include methanol, ethanol, isopropanol, n-butanol, and isobutanol, with methanol being the most preferred among these. Furthermore, the melamine compound may be a monomer, a polymer of two or more commensals, or a mixture thereof. In addition, a compound in which urea or the like is co-condensed with a portion of the melamine can be used, and a catalyst may also be used in this composition to increase the reactivity of the melamine compound. 【0037】 (Oxazoline compounds) Oxazoline compounds are compounds having an oxazoline group in their molecule, and polymers containing an oxazoline group are particularly preferred. These can be produced by polymerization of an addition-polymerizable oxazoline group-containing monomer alone or with other monomers. Examples of addition-polymerizable oxazoline group-containing monomers include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline. One or more of these can be used. Among these, 2-isopropenyl-2-oxazoline is preferred because it is readily available industrially. Other monomers are not limited as long as they are copolymerizable with addition-polymerizable oxazoline group-containing monomers, for example (meth)acrylic acid esters such as alkyl (meth)acrylates (alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, and cyclohexyl groups); unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, styrenesulfonic acid and their salts (sodium salt, potassium salt, ammonium salt, tertiary amine salt, etc.); unsaturated nitriles such as acrylonitrile and methacrylonitrile; (meth)acrylamide, N-alkyl(meth) Examples of unsaturated amides such as acrylamide and N,N-dialkyl(meth)acrylamide (alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, and cyclohexyl groups); vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether and ethyl vinyl ether; α-olefins such as ethylene and propylene; halogen-containing α,β-unsaturated monomers such as vinyl chloride and vinylidene chloride; and α,β-unsaturated aromatic monomers such as styrene and α-methylstyrene. One or more of these monomers can be used. Furthermore, the oxazoline compound may have a polyalkylene oxide chain, such as a polyethylene oxide chain, and other monomers such as (meth)acrylates having a polyalkylene oxide chain may also be used. From the viewpoint of improving the adhesion of the resin layer (a) to the polyester film, the amount of oxazoline groups in the oxazoline compound is preferably in the range of 0.5 to 10 mmol / g, more preferably 1 to 9 mmol / g, and even more preferably 3 to 8 mmol / g. 【0038】 (Epoxy compound) Epoxy compounds are compounds that have an epoxy group in their molecule. Examples include condensates of hydroxyl or amino groups such as epichlorohydrin, ethylene glycol, polyethylene glycol, glycerin, polyglycerin, and bisphenol A, as well as polyepoxy compounds, diepoxy compounds, monoepoxy compounds, and glycidylamine compounds. Examples of polyepoxy compounds include sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, triglycidyl tris(2-hydroxyethyl) isocyanate, glycerol polyglycidyl ether, and trimethylolpropane polyglycidyl ether. Examples of diepoxy compounds include neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, resorcinol diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and polytetramethylene glycol diglycidyl ether. Examples of monoepoxy compounds include allyl glycidyl ether, 2-ethylhexyl glycidyl ether, and phenyl glycidyl ether, while examples of glycidylamine compounds include N,N,N',N'-tetraglycidyl-m-xylylenediamine and 1,3-bis(N,N-diglycidylamino)cyclohexane. From the viewpoint of improving the adhesion of the resin layer to the polyester film, polyether-based epoxy compounds are preferred. Furthermore, regarding the amount of epoxy groups, polyfunctional polyepoxy compounds with three or more functions are preferred over those with two functions. 【0039】 (Carbodiimide compounds) A carbodiimide compound is a compound having a carbodiimide structure, specifically a compound having one or more carbodiimide structures in its molecule. However, polycarbodiimide compounds having two or more carbodiimide structures in their molecule are more preferable for better adhesion between the resin layer and the polyester film. Carbodiimide compounds can be synthesized using conventionally known techniques, and generally involve the condensation reaction of diisocyanate compounds. The diisocyanate compounds are not particularly limited and can be either aromatic or aliphatic. Specifically, examples include tolylene diisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexyl diisocyanate, and dicyclohexylmethane 4,4'-diisocyanate. Furthermore, to the extent that it does not impair the spirit of the present invention, surfactants may be added, or hydrophilic monomers such as polyalkylene oxides, quaternary ammonium salts of dialkylamino alcohols, and hydroxyalkyl sulfonates may be added to improve the water solubility and water dispersibility of the polycarbodiimide compound. 【0040】 (Isocyanate compounds) Isocyanate compounds are compounds having an isocyanate derivative structure, such as isocyanates or blocked isocyanates, which have a structure in which the isocyanate groups of an isocyanate precursor compound are protected with a blocking agent. Examples of isocyanates include aliphatic isocyanate compounds, alicyclic isocyanate compounds, and aromatic isocyanate compounds. These isocyanate compounds are more preferably compounds having multiple isocyanate groups, i.e., polyisocyanate compounds, from the viewpoint of enabling a higher degree of reaction and improving the durability of the resin layer (a). Furthermore, when using an aqueous coating solution, blocked isocyanates are more preferable. 【0041】 Examples of aliphatic polyisocyanate compounds include polyisocyanate compounds derived from aliphatic diisocyanates such as tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, 2,2,4-trimethyl-1,6-diisocyanatohexane, and lysine diisocyanate, as well as lysine triisocyanate, 4-isocyanatomethyl-1,8-octamethylene diisocyanate, bis(2-isocyanatoethyl)2-isocyanatoglutarate, or compounds derived from these isocyanate compounds. Among these, hexamethylene diisocyanate is preferred due to its ease of industrial availability. 【0042】 Examples of alicyclic polyisocyanate compounds include isophorone diisocyanate, (1,3-bis(isocyanatomethyl)-cyclohexane, 4,4'-dicyclohexylmethane diisocyanate, norbornene diisocyanate, hydrogenated xylylene diisocyanate, or compounds derived from these isocyanate compounds. Among these, isophorone diisocyanate is preferred due to its weather resistance and ease of industrial availability. Examples of aromatic polyisocyanate compounds include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, naphthalene diisocyanate, xylylene diisocyanate, or compounds derived from these isocyanate compounds. 【0043】 Among these polyisocyanate compounds, aliphatic polyisocyanate compounds and alicyclic polyisocyanate compounds are preferred because they have excellent weather resistance. Furthermore, among aliphatic polyisocyanate compounds, aliphatic polyisocyanate compounds derived from aliphatic diisocyanates are preferred. Among these, hexamethylene diisocyanate is particularly preferred. These isocyanate compounds may be used individually or in combination of two or more. 【0044】 Blocked polyisocyanate compounds can be synthesized by reacting the isocyanate group of a polyisocyanate compound with a blocking agent. Examples of blocking agents include activated methylene, oxime, pyrazole, alcohol, alkylphenol, phenol, mercaptan, acid amide, acid imide, imidazole, urea, amine, imine, and bisulfite blocking agents. Among these, activated methylene blocking agents are particularly preferred from the viewpoint of improving the adhesion between the resin layer and the polyester film. Furthermore, two or more of these blocking agents may be used in combination. 【0045】 (Silane coupling compounds) Silane coupling compounds are organosilicon compounds that contain both an organic functional group and a hydrolysis group such as an alkoxy group within a single molecule. For example, epoxy group-containing compounds such as 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; vinyl group-containing compounds such as vinyltrimethoxysilane and vinyltriethoxysilane; styryl group-containing compounds such as p-styryltrimethoxysilane and p-styryltriethoxysilane; (meth)acrylic group-containing compounds such as 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, and 3-(meth)acryloxypropylmethyldiethoxysilane; 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and N-2-(aminoethyl)-3 Examples include amino group-containing compounds such as -aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldiethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, and N-phenyl-3-aminopropyltriethoxysilane; isocyanurate group-containing compounds such as tris(trimethoxysilylpropyl)isocyanurate and tris(triethoxysilylpropyl)isocyanurate; and mercapto group-containing compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane, and 3-mercaptopropylmethyldiethoxysilane. 【0046】 The resin content in the resin layer (a) is preferably 1% by mass or more and 75% by mass or less, more preferably 3% by mass or more and 60% by mass or less, even more preferably 5% by mass or more and 40% by mass or less, even more preferably 6% by mass or more and 25% by mass or less, and particularly preferably 7% by mass or more and 15% by mass or less. Having a resin content above a certain level improves film-forming properties and allows for the formation of a film containing the filler (Za). Furthermore, the adhesion to the polyester film helps to suppress the peeling of the coating film. Furthermore, the amount of crosslinking agent in the resin layer (a) is preferably 1 / 9 or more and 9 or less by mass ratio (crosslinking agent / resin) to the resin contained in the resin layer (a), more preferably 1 / 5 or more and 5 or less, even more preferably 1 / 4 or more and 4 or less, even more preferably 1 / 3 or more and 3 or less, and particularly preferably 1 / 2 or more and 2 or less. By keeping the amount of crosslinking agent within the above range, it becomes easier to improve the coating strength of the resin layer (a) and its adhesion to the polyester film. The content or blending amount of each component in the resin layer (a) is the same as the content ratio of the non-volatile components in the resin layer composition. 【0047】 (Other ingredients) The resin layer (a), that is, the resin layer composition, may contain additives other than the above-mentioned components, such as antistatic agents, crosslinking catalysts, defoaming agents, coating properties improvers, surfactants, thickeners, organic lubricants, ultraviolet absorbers, antioxidants, foaming agents, dyes, and pigments, to the extent that it does not impair the spirit of the present invention. Among these, for example, it is preferable to use an antistatic agent. Examples of antistatic agents include polyol compounds or polyether compounds such as polyalkylene oxide, glycerin, polyglycerin, glycerin, or alkylene oxide adducts to polyglycerin. Examples of alkylene oxide in polyalkylene oxide or alkylene oxide adducts include ethylene oxide or propylene oxide. 【0048】 The resin layer (a) is preferably provided on at least one surface of the present film. From the viewpoint of facilitating the formation of an insulating film, the resin layer (a) is preferably provided at least on a surface having a phosphorus element content (P1) or phosphorus element content ratio (p1) higher than the phosphorus element content (P2) or phosphorus element content ratio (p2) in the central portion described later. However, it is also preferable that the resin layer (a) is provided on both surfaces of the present film. 【0049】 The resin layer (a) has a thickness such that the mass per unit area is 10 mg / m 2 or more and 4000 mg / m 2 or less. By having the mass per unit area within the above range, it becomes easier to appropriately retain the insulating film when heated at a high temperature without making the resin layer (a) thicker than necessary. The mass per unit area of the resin layer (a) is 15 mg / m 2 or more and 3000 mg / m 2 or less is more preferable, 20 mg / m 2 or more and 2500 mg / m 2 or less is even more preferable, 30 mg / m 2 or more and 2000 mg / m 2 or less is even more preferable, 40 mg / m 2 or more and 1500 mg / m 2 or less is even more preferable, 50 mg / m 2 or more and 1000 mg / m 2 or less is even more preferable, 60 mg / m 2 or more and 800 mg / m 2 or less is even more preferable. Note that the mass per unit area of the resin layer (a) can be obtained from the coating amount of the non-volatile component when forming the resin layer composition. In the case of performing drying and stretching, it is the mass per unit area of the resin layer (a) after drying and stretching. Also, when the resin layer (a) is provided on both surfaces of the present film, the mass per unit area of the resin layer (a) is the mass per unit area of the resin layer (a) provided on each surface of the present film. 【0050】 The resin layer (a) preferably has a thickness of 0.005 μm or more and less than 2 μm. Having a thickness within the above range makes it easier to properly retain the insulating film when heated to high temperatures without making the resin layer (a) unnecessarily thick. The thickness of the resin layer (a) is more preferably 0.01 μm or more and 1.2 μm or less, even more preferably 0.03 μm or more and 1.2 μm or less, even more preferably 0.05 μm or more and 1 μm or less, even more preferably 0.07 μm or more and 0.8 μm or less, and particularly preferably 0.09 μm or more and 0.6 μm or less. The thickness of the resin layer (a) can be determined by performing SEM observation on a cross-section of the laminated film in the thickness direction, for example, and taking the average of 10 measurement points. 【0051】 The resin layer (a) may consist of a resin layer composition containing a resin and a filler (Za), as well as components other than the resin and filler (Za), such as a crosslinking agent, which may be added as needed. The resin layer composition may be cured, for example, with a crosslinking agent, so that the resin layer (a) becomes a cured resin layer. The resin layer composition may be diluted with a solvent to form a coating solution. That is, the resin layer composition may be applied as a liquid coating solution to, for example, a polyester film, and dried and cured as necessary to form a resin layer (a). The components constituting the resin layer composition (resin, crosslinking agent, filler (Za), etc.) may be dissolved in the solvent or dispersed in the solvent. When used as a coating solution, the concentration of the total nonvolatile components of the resin layer composition in the coating solution is preferably 0.1 to 50% by mass. 【0052】 There are no particular restrictions on the solvent, and either water or an organic solvent can be used. From the viewpoint of environmental protection, it is preferable to use an aqueous coating solution with water as the main solvent (50% by mass or more of the total solvent). The water content is preferably 60% by mass or more, more preferably 70% by mass or more. The aqueous coating solution may contain a small amount of organic solvent. The specific amount of organic solvent should be less than or equal to the amount of water by mass, for example, 50% by mass or less, preferably 40% by mass or less, and more preferably 30% by mass or less of the solvent. Examples of organic solvents used in combination with water include alcohols such as ethanol, isopropanol, ethylene glycol, and glycerin; ethers such as ethyl cellosolve, t-butyl cellosolve, propylene glycol monomethyl ether, and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone; esters such as ethyl acetate; and amines such as dimethylethanolamine. These can be used individually or in combination. By appropriately selecting and including these organic solvents in the aqueous coating solution as needed, the stability and coating properties of the coating solution can be improved. 【0053】 Furthermore, when using only organic solvents as the solvent, examples of such organic solvents include aromatic hydrocarbons such as toluene; aliphatic hydrocarbons such as hexane, heptane, and isooctane; esters such as ethyl acetate and butyl acetate; ketones such as ethyl methyl ketone and isobutyl methyl ketone; alcohols such as ethanol and 2-propanol; and ethers such as diisopropyl ether and dibutyl ether. These may be used individually or in combination, taking into consideration their solubility, coating properties, boiling point, etc. 【0054】 It can be inferred that the resin layer (a) contains unreacted substances, reacted compounds, or mixtures thereof of each component (resin, crosslinking agent, other components, etc.) that constitute this composition. Furthermore, the individual components in the resin layer (a) can be analyzed by methods such as TOF-SIMS, ESCA, and X-ray fluorescence. 【0055】 <Polyester film> The polyester film (this film) may be a single-layer film having a single-layer structure or a multi-layer film having a multi-layer structure, but a multi-layer film is preferred. In the case of a multi-layer film, it is preferable to have an intermediate layer (B) and a surface layer (A) provided on one side of the intermediate layer (B). The surface layer (A) is a layer that constitutes one side of this film, and the intermediate layer (B) is a layer that is located inside the surface layer (A). In addition to the intermediate layer (B) and the surface layer (A), it is preferable that this film has a back layer (C). The back layer (C) is a surface provided on the side of the intermediate layer (B) opposite to the side on which the surface layer (A) is provided, and is a layer that constitutes the other side of this film. In the case of a multilayer film, this film may have a two-layer structure of surface layer (A) / intermediate layer (B), but a three-layer structure of surface layer (A) / intermediate layer (B) / backside layer (C) is preferred. Furthermore, it is not limited to a two-layer or three-layer structure, and may have a four-layer or more layer structure with two or more intermediate layers (B). In addition, there is no particular limit to the number of layers to be laminated, but it is preferred to be 10 layers or less. With 10 layers or less, the thickness of each layer is sufficient, so the lamination during film formation is sufficient, flow marks and the like are less likely to occur, and the quality of the film is well maintained. 【0056】 Furthermore, this film may be an unstretched film or a stretched film. In particular, a stretched film stretched in one or two axes is preferred. Among these, a biaxially stretched film is more preferred in terms of superior balance of mechanical properties and flatness. 【0057】 This film has polyester resin (X) as its main component resin. Furthermore, if this film is a multilayer film, it is preferable that the main component resin of each layer is polyester resin (X). The term "main component resin" refers to the resin that has the highest content proportion among the resins constituting each layer, for example, the resin that accounts for 50% by mass or more, preferably 70% by mass or more, and more preferably 80% by mass or more (including 100% by mass) of the resins constituting each layer. 【0058】 <Polyester resin (X)> The polyester resin (X) is not particularly limited and may be a homopolyester resin or a copolymerized polyester resin. Specifically, examples include polyester resins obtained by polycondensation of a dicarboxylic acid component and a diol component. 【0059】 Examples of the above-mentioned dicarboxylic acid components include aromatic dicarboxylic acids such as terephthalic acid, 2,6-naphthalenedicarboxylic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-frandicarboxylic acid, 2,4-frandicarboxylic acid, 3,4-frandicarboxylic acid, benzophenone dicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenyletherdicarboxylic acid; and aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and dimer acid. 【0060】 Examples of the above-mentioned diol components include ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimergol, and bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or their derivatives, or ethylene oxide adducts thereof). 【0061】 Examples of copolymerized polyester resins include copolymerized polyester resin (X) that contains a third component other than the compound that is the main component of the dicarboxylic acid component and the compound that is the main component of the diol component as copolymerized components. 【0062】 The polyester resin (X) contains a dicarboxylic acid component (a-1) and a diol component (a-2) as described above, but it is preferable that the dicarboxylic acid component (a-1) contains at least one of terephthalic acid and 2,6-naphthalenedicarboxylic acid, and the diol component (a-2) contains ethylene glycol. 【0063】 In particular, the polyester resin (X) preferably contains 80 mol% or more of at least one selected from terephthalic acid and 2,6-naphthalenedicarboxylic acid in its dicarboxylic acid component (a-1), more preferably 90 mol% or more, and even more preferably all (100 mol%) of the dicarboxylic acid component (a-1) is at least one selected from terephthalic acid and 2,6-naphthalenedicarboxylic acid. Furthermore, it is preferable that the at least one selected from terephthalic acid and 2,6-naphthalenedicarboxylic acid is terephthalic acid. By setting the content of at least one selected from terephthalic acid and 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid component (a-1) to 80 mol% or more, it becomes easier to increase the crystal melting temperature of the film, for example, and to maintain appropriate insulation properties under normal usage conditions. 【0064】 The polyester resin (X) preferably contains 51 mol% or more of ethylene glycol in the diol component (a-2), more preferably 60 mol% or more, even more preferably 70 mol% or more, particularly preferably 80 mol% or more, and especially preferably 90 mol% or more. On the other hand, the ethylene glycol in the diol component (a-2) is preferably 100 mol% or less. Furthermore, if the polyester resin (X) is a copolymer polyester resin, the ethylene glycol in the diol component (a-2) is preferably 99 mol% or less, more preferably 98 mol% or less, even more preferably 97 mol% or less, particularly preferably 96 mol% or less, and especially preferably 95 mol% or less. By setting the ethylene glycol content in the diol component (a-2) within this range, the decrease in crystal melting temperature and decrease in crystallinity due to the transesterification reaction become less likely, allowing for the maintenance of appropriate crystallinity and good mechanical strength, and making it easier to suppress the decrease in mechanical strength, especially under high-temperature conditions. 【0065】 Normally, when polyester resin (X) is manufactured (polycondensed) using ethylene glycol as one of the raw materials, diethylene glycol is produced as a by-product from ethylene glycol. In this specification, this diethylene glycol is referred to as by-product diethylene glycol. The amount of diethylene glycol produced as a by-product from ethylene glycol varies depending on the polycondensation method, but it is approximately 5 mol% or less of the ethylene glycol. In the present invention, diethylene glycol of 5 mol% or less is considered by-product diethylene glycol, and this by-product diethylene glycol is also included in ethylene glycol and distinguished from copolymer components. On the other hand, depending on the content of diethylene glycol, more specifically, if the content of diethylene glycol exceeds 5 mol%, the diethylene glycol is treated as a copolymer component rather than as by-product diethylene glycol. 【0066】 The polyester resin (X) may contain copolymer components other than terephthalic acid and 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid component (a-1). The content of these copolymer components is preferably 20 mol% or less, more preferably 10 mol% or less. Furthermore, it is even more preferable that all of the dicarboxylic acid component (a-1) is one selected from terephthalic acid and 2,6-naphthalenedicarboxylic acid, and the other copolymer components are 0 mol%. Furthermore, in the case of a copolymer polyester resin, the polyester resin (X) preferably contains 49 mol% or less of copolymer components in the diol component (a-2), more preferably 40 mol% or less, even more preferably 30 mol% or less, particularly preferably 20 mol% or less, and especially preferably 10 mol% or less. On the other hand, in the case of a copolymer polyester resin, the lower limit of the copolymer components in the diol component (a-2) is preferably 1 mol% or more, more preferably 2 mol% or more, even more preferably 3 mol% or more, particularly preferably 4 mol% or more, and especially preferably 5 mol% or more. 【0067】 The copolymerization component to be added to the dicarboxylic acid component (a-1) is a dicarboxylic acid other than terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples include aromatic dicarboxylic acids such as isophthalic acid, orthophthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-frandicarboxylic acid, 2,4-frandicarboxylic acid, 3,4-frandicarboxylic acid, benzophenone dicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenyl ether dicarboxylic acid; and aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and dimer acid. From the viewpoint of moldability, isophthalic acid, 2,5-frandicarboxylic acid, 2,4-frandicarboxylic acid, and 3,4-frandicarboxylic acid are preferred. These copolymerization components can be used individually or in combination of two or more. 【0068】 Furthermore, examples of copolymer components to be added to the diol component (a-2) include diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimergol, bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or their derivatives, or their ethylene oxide adducts). From the viewpoint of maintaining film strength, bisphenols are more preferred, and among the bisphenols, bisphenol A-ethylene oxide adducts are preferred. These copolymer components can be used individually or in combination of two or more. 【0069】 Furthermore, if this film is a multilayer film, the types and contents of each component constituting the polyester resin (X) contained in any of the layers, preferably in each layer, may be the same as described above, and the resins constituting each layer and the polyester resin (X) in each layer may be the same or different from each other. 【0070】 Typical polyester resins (X) include polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, and polybutylene naphthalate. Among these, the film preferably contains polyethylene terephthalate (hereinafter also referred to as "PET") or 2,6-polyethylene naphthalate (hereinafter also referred to as "PEN") as the polyester resin (X), with PET being more preferred, from the viewpoint of easily maintaining a high crystal melting temperature and appropriate insulation under normal usage conditions. Polyester resin (X) may be used alone or in combination of two or more types. 【0071】 There are no particular restrictions on the polycondensation catalyst used when polycondensing polyester resin (X), and conventionally known compounds can be used, such as titanium compounds, germanium compounds, antimony compounds, manganese compounds, aluminum compounds, magnesium compounds, and calcium compounds. 【0072】 The intrinsic viscosity of the polyester resin (X) is not particularly limited, but from the viewpoint of film-forming properties and productivity, it is preferably 0.4 dL / g or more and 1 dL / g or less, more preferably 0.45 dL / g or more and 0.9 dL / g or less, even more preferably 0.5 dL / g or more and 0.8 dL / g or less, and even more preferably 0.55 dL / g or more and 0.75 dL / g or less. Furthermore, the intrinsic viscosity refers to the intrinsic viscosity of the mixed polyester when two or more polyesters with different intrinsic viscosities are used. The intrinsic viscosity can be measured according to a conventional method. For example, 1 g of polyester from which incompatible components have been removed is accurately weighed, 100 mL of a phenol / tetrachloroethane mixed solvent (50 / 50 by mass ratio) is added to dissolve it, and the viscosity can be measured at 30°C using a viscosity measuring device. 【0073】 To suppress the precipitation of oligomer components, the film may be manufactured using polyester with a low oligomer content as the raw material. Various known methods can be used to manufacture polyester with a low oligomer content, such as a method of solid-phase polymerization after polyester production. Alternatively, the film may be made into a multilayer film, and the surface layer (A) and back layer (C) of the film may be made of polyester raw material with a low oligomer content to suppress the precipitation of oligomer components. Furthermore, the polyester may be obtained by esterification or transesterification, followed by melt polycondensation under reduced pressure at a higher reaction temperature. 【0074】 <Flame retardant (Y)> Preferably, this film contains a flame retardant (Y) in addition to the polyester resin (X) described above. The inclusion of the flame retardant (Y) in this laminated film allows for the formation of a more appropriate insulating film when heated to high temperatures. Examples of flame retardants (Y) include phosphorus-based flame retardants containing phosphorus atoms. The phosphorus-based flame retardant preferably contains one or more compounds selected from phosphorus compounds having an aromatic ring and compounds having a P=O bond, and more preferably contains compounds having both an aromatic ring and a P=O bond. 【0075】 Compounds having a P=O bond include aromatic phosphate esters having the structure of an ester reaction product of an aromatic compound having a phenolic hydroxyl group and phosphoric acid. The compound having a P=O bond may be a phosphate monoester, a phosphate diester, or a phosphate triester, as long as the effects of the present invention are obtained. Examples of aromatic compounds having a phenolic hydroxyl group include phenol and naphthol. Aromatic compounds having a phenolic hydroxyl group may further have substituents other than the phenolic hydroxyl group. Examples of such substituents include lower alkyl groups such as methyl groups. The substituents in a compound having a P=O bond may be singular or plural, and if there are multiple substituents, they may be identical or different. Compounds having a P=O bond can be obtained, for example, by reacting phosphorus oxychloride with a phenol that may have substituents, and are also available commercially. 【0076】 Examples of compounds having a P=O bond include aromatic condensed phosphate esters represented by the following formula (1), which have a structure in which two phosphate esters are condensed, and aromatic phosphate esters represented by the following formula (2). [ka] 【0077】 In formulas (1) and (2), Ar1 is preferably a divalent group represented by formula (3) or formula (4) below, and Ar2 is preferably a monovalent group represented by formula (5) below. In formula (5), R1 independently represents a hydrogen atom or an alkyl group, and if R1 is an alkyl group, it is preferably an alkyl group having 1 to 4 carbon atoms, more preferably an alkyl group having 1 to 2 carbon atoms, and even more preferably a methyl group. 【0078】 [ka] 【0079】 The Ar2 in the aromatic condensed phosphate ester represented by formula (1) and the aromatic phosphate ester represented by formula (2) may be the same or different from each other, as long as the effects of the present invention can be obtained. 【0080】 The phenylene group represented by formula (3) may be any of o-phenylene, m-phenylene, and p-phenylene groups within the range in which the effects of the present invention can be obtained, and may all be the same or different in the aromatic condensed phosphate ester. The position in which the phenylene group of the divalent group represented by formula (4) bonds with the phosphorus atom of the phosphate portion may be ortho, meta, or para relative to the central hydrocarbon group, within the range in which the effects of the present invention can be obtained. The position in which the phenylene group represented by formula (5) bonds with the oxygen atom of the phosphate portion may be any of positions 2, 4, and 5, when the position of R1 is defined as positions 1 and 3, within the range in which the effects of the present invention can be obtained. 【0081】 The aforementioned aromatic condensed phosphate esters can be obtained, for example, by reacting phosphorus oxychloride with a divalent phenol compound and phenol, methylphenol, or dimethylphenol, and are also available commercially. 【0082】 Preferred specific examples of aromatic condensed phosphate esters represented by formula (1) include, for example, the compounds represented by formulas (1-1) to (1-3) below, and preferred specific examples of aromatic phosphate esters represented by formula (2) include, for example, the compounds represented by formulas (2-1) to (2-3) below. 【0083】 [ka] 【0084】 [ka] 【0085】 Examples of commercially available aromatic condensed phosphate esters and aromatic phosphate esters include TPP: triphenyl phosphate, TXP: trixylenyl phosphate, CDP: cresylphenyl phosphate, TCP: tricresyl phosphate, PX-110: cresyl di2,6-xylenyl phosphate, CR-733S: resorcinol bis(diphenyl phosphate), CR-741: bisphenol A bisdiphenyl phosphate, PX200: 1,3-phenylene-tesla-kis(2,6-dimethylphenyl) phosphate, PX201: 1,4-phenylene-tetra-kis(2,6-dimethylphenyl) phosphate, and PX202: 4,4'-biphenylene-tetra-kis(2,6-dimethylphenyl) phosphate, all manufactured by Daihachi Chemical Industry Co., Ltd. 【0086】 In particular, from the viewpoint of minimizing the impact on the basic properties of the film, it is preferable that the aromatic phosphate ester includes the aromatic condensed phosphate ester (compound A) represented by the above formula (1-1). Furthermore, during the condensation reaction of the aromatic condensed phosphate ester (compound A) represented by formula (1-1), an aromatic phosphate ester (compound B) represented by formula (2-1) may be produced. In that case, the mass ratio (compound A:compound B) of the aromatic condensed phosphate ester (compound A) to the aromatic phosphate ester (compound B) may be approximately 80:20 to 99:1, 90:10 to 99:1, or 95:5 to 99:1. In addition to the aromatic condensed phosphate esters represented by formula (1) and the aromatic phosphate esters represented by formula (2), other examples of flame retardants (Y) include condensed phosphites and phosphazene compounds. 【0087】 As the condensed phosphite ester, the compound represented by the following formula (6) is preferred. [ka] In formula (6), R2 independently represents a group having an aromatic ring, R3 independently represents an organic group, and X represents a divalent organic group. 【0088】 In formula (6), the group having an aromatic ring as R2 is, for example, an aryl group, or an alkyl or cycloalkyl group substituted with an aryl group. The aryl group, or the alkyl or cycloalkyl group substituted with an aryl group, may have substituents. Examples of substituents include alkyl groups, alkoxy groups, alkylthio groups, aryl groups, aryloxy groups, arylthio groups, halogen atoms, and aryl halides. Alternatively, the group may be a combination of these substituents, or a combination of these substituents bonded by an oxygen atom, sulfur atom, nitrogen atom, etc. 【0089】 In formula (6), the organic group as R3 can be, for example, an alkyl group, a cycloalkyl group, or an aryl group. The alkyl group, cycloalkyl group, and aryl group may have substituents. Examples of substituents include alkyl groups, alkoxy groups, alkylthio groups, aryl groups, aryloxy groups, arylthio groups, halogen atoms, and aryl halides. Alternatively, the group may be a combination of these substituents, or a combination of these substituents bonded by an oxygen atom, a sulfur atom, a nitrogen atom, etc. 【0090】 In formula (6), the divalent organic group X refers to a group with two or more valents that can be formed by removing one hydrogen atom from the organic group R3 mentioned above. Examples include alkylene groups, phenylene groups, substituted phenylene groups, and polynuclear phenylene groups derived from bisphenols. In formula (6), one or two R3 atoms and X may together form a ring structure. Examples of such ring structures include cycloalkyl structures and aryl structures. Alternatively, the ring structure may be a spiro-ring structure in which the two R3 atoms and X atoms are each bonded via an oxygen atom, containing a phosphorus atom. A commercially available example of condensed phosphite ester is "FireGuard (registered trademark) FCX-210" manufactured by Teijin Corporation. 【0091】 Phosphazene compounds are organic compounds having a -P=N- bond in their molecule, and preferably include cyclic phosphazene compounds represented by the following formula (7), linear phosphazene compounds represented by the following formula (8), and at least one compound selected from the group consisting of at least one phosphazene compound selected from the group consisting of the following formulas (7) and (8) and crosslinked phosphazene compounds obtained by crosslinking with a crosslinking group. 【0092】 [ka] In formula (7), a is an integer between 3 and 25, and R4 independently represents an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryloxy group, an amino group, a hydroxyl group, an aryl group, or an alkylaryl group, preferably at least one of which is an aryl group or an alkylaryl group. 【0093】 [ka] In formula (8), b is an integer between 3 and 10000, and R5 independently represents an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryloxy group, an amino group, a hydroxyl group, an aryl group, or an alkylaryl group, preferably at least one of which is an aryl group. R6 represents at least one selected from -N=P(OR5)3 groups and -N=P(O)OR5 groups. 【0094】 In formulas (7) and (8), examples of alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, t-butyl, pentyl, hexyl, octyl, decyl, and dodecyl groups. C1-C6 alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, t-butyl, pentyl, and hexyl groups are preferred, and C1-C4 alkyl groups such as methyl, ethyl, and propyl groups are particularly preferred. In formulas (7) and (8), examples of cycloalkyl groups include cycloalkyl groups having 5 to 14 carbon atoms, such as cyclopentyl groups and cyclohexyl groups, with cycloalkyl groups having 5 to 8 carbon atoms being preferred. In formulas (7) and (8) above, the alkenyl group can be, for example, a vinyl group or an allyl group, which is an alkenyl group having 2 to 8 carbon atoms. The cycloalkenyl group can be, for example, a cyclopentyl group or a cyclohexyl group, which is a cycloalkenyl group having 5 to 12 carbon atoms. 【0095】 In formulas (7) and (8), examples of alkynyl groups include alkynyl groups having 2 to 8 carbon atoms, such as ethynyl groups and propynyl groups, and alkynyl groups having aryl groups as substituents, such as ethynylbenzene groups. In formulas (7) and (8), examples of aryl groups include aryl groups having 6 to 20 carbon atoms, such as phenyl, tolyl, xylyl, trimethylphenyl, and naphthyl groups. Among these, aryl groups having 6 to 10 carbon atoms are preferred, and phenyl groups are particularly preferred. In formulas (7) and (8), examples of alkylaryl groups include aralkyl groups having 6 to 20 carbon atoms, such as benzyl, phenethyl, and phenylpropyl groups. Among these, aralkyl groups having 7 to 10 carbon atoms are preferred, and benzyl groups are particularly preferred. 【0096】 Among these, it is preferable that R4 in formula (7) and R5 in formula (8) are aryl groups and arylalkyl groups, respectively. By using such aromatic phosphazene compounds, the thermal stability of compositions containing polyester resin can be effectively enhanced. From this viewpoint, it is more preferable that R4 and R5 are aryl groups, and particularly preferable that they are phenyl groups. 【0097】 Examples of cyclic and / or linear phosphazene compounds represented by formulas (7) and (8) include: phenoxyphosphazene; (poly)tolyloxyphosphazenes such as o-tolyloxyphosphazene, m-tolyloxyphosphazene, and p-tolyloxyphosphazene; (poly)xylyloxyphosphazenes such as o,m-xylyloxyphosphazene, o,p-xylyloxyphosphazene, and m,p-xylyloxyphosphazene; and o,m,p-trimethylphenyloxyphosphazene. Examples include (poly)phenoxytolyloxyphosphazenes such as phenoxy-o-tolyloxyphosphazene, phenoxy-m-tolyloxyphosphazene, and phenoxy-p-tolyloxyphosphazene; (poly)phenoxyxyloxyphosphazenes such as phenoxy-o,m-xylyloxyphosphazene, phenoxy-o,p-xylyloxyphosphazene, and phenoxy-m,p-xylyloxyphosphazene; and phenoxy-o,m,p-trimethylphenyloxyphosphazene. Among these, cyclic and / or linear phenoxyphosphazenes are preferred. 【0098】 Among the cyclic phosphazene compounds represented by formula (7), cyclic phenoxyphosphazene in which R4 is a phenyl group is particularly preferred. Examples of such cyclic phenoxyphosphazene compounds include phenoxycyclotriphosphazene, octaphenoxycyclotetraphosphazene, and decaffenoxycyclopentaphosphazene, which are obtained by reacting ammonium chloride and phosphorus pentachloride at a temperature of 120 to 130°C to obtain a mixture of cyclic and linear chlorophosphazene, from which cyclic chlorophosphazene such as hexachlorocyclotriphosphazene, octachlorocyclotetraphosphazene, and decachlorocyclopentaphosphazene are isolated and then substituted with a phenoxy group. Furthermore, the cyclic phenoxyphosphazene compound is preferably a compound in which a in formula (7) is an integer from 3 to 8, and may be a mixture of compounds with different values ​​of a. In formula (7), the average of a is preferably 3 to 5, and more preferably 3 to 4. In particular, a mixture of compounds in which a=3 accounts for 50% by mass or more, a=4 accounts for 10 to 40% by mass, and a=5 or higher accounts for a total of 30% by mass or less is preferred. 【0099】 Among the linear phosphazene compounds represented by formula (8), linear phenoxyphosphazene compounds in which R5 is a phenyl group are particularly preferred. Examples of such linear phenoxyphosphazene compounds include compounds obtained by oxidative polymerization of hexachlorocyclotriphosphazene obtained by the above method at a temperature of 220 to 250°C, and substituting the resulting linear dichlorophosphazene with a degree of polymerization of 3 to 10000 with a phenoxy group. In the linear phenoxyphosphazene compound, b in formula (8) is preferably 3 to 1000, more preferably 3 to 100, and even more preferably 3 to 25. 【0100】 Examples of crosslinked phosphazene compounds include compounds having a crosslinked structure of a 4,4'-diphenylene group, such as compounds having a crosslinked structure of a 4,4'-sulfonyldiphenylene (i.e., a bisphenol S residue), compounds having a crosslinked structure of a 2,2-(4,4'-diphenylene)isopropylidene group, compounds having a crosslinked structure of a 4,4'-oxydiphenylene group, and compounds having a crosslinked structure of a 4,4'-thiodiphenylene group. 【0101】 Furthermore, as the crosslinked phosphazene compound, a crosslinked phenoxyphosphazene compound obtained by crosslinking a cyclic phenoxyphosphazene compound in which R4 is a phenyl group in formula (7) with the crosslinking group, or a crosslinked phenoxyphosphazene compound obtained by crosslinking a linear phenoxyphosphazene compound in which R5 is a phenyl group in formula (8) with the crosslinking group is preferred from the viewpoint of flame retardancy, and a crosslinked phenoxyphosphazene compound obtained by crosslinking a cyclic phenoxyphosphazene compound with the crosslinking group is more preferred. Furthermore, the phenylene group content in the crosslinked phenoxyphosphazene compound is typically 50 to 99.9%, preferably 70 to 90%, based on the total number of phenyl and phenylene groups in the cyclic phosphazene compound represented by formula (7) and / or the linear phenoxyphosphazene compound represented by formula (8). It is also particularly preferable that the crosslinked phenoxyphosphazene compound does not have free hydroxyl groups within its molecule. 【0102】 In this film, it is preferable, from the viewpoint of flame retardancy and mechanical properties of the resin composition containing the polyester resin, that the phosphazene compound is at least one selected from the group consisting of a cyclic phenoxyphosphazene compound represented by formula (7) and a crosslinked phenoxyphosphazene compound obtained by crosslinking a cyclic phenoxyphosphazene compound represented by formula (7) with a crosslinking group. A commercially available phosphazene compound is, for example, FP-110 manufactured by Fushimi Pharmaceutical Co., Ltd. 【0103】 Examples of flame retardants (Y) include compounds represented by the following formula (9). [ka] In formula (9), n is an integer, A is preferably a divalent organic group, Q1 and Q2 are preferably divalent aromatic groups, and Z is preferably a group having an ester-forming functional group. In equation (9), n is preferably an integer between 2 and 40, more preferably an integer between 10 and 30, and even more preferably an integer between 15 and 25. 【0104】 In formula (9), A is, for example, having 1 to 18 carbon atoms, preferably 1 to 6 carbon atoms, and more preferably 1 to 4 carbon atoms. Examples of divalent organic groups represented by A include alkylene groups such as methylene, ethylene, 1,2-propylene, and 1,3-propylene; arylene groups such as 1,3-phenylene and 1,4-phenylene; and substituted arylene groups such as 1,3-xylylene and 1,4-xylylene. In formula (9), the divalent aromatic groups represented by Q1 and Q2 include, for example, the divalent groups represented by formulas (3) and (4). Q1 and Q2 may be the same or different, as long as the effects of the present invention are obtained. 【0105】 In formula (9), examples of groups having an ester-forming functional group represented by Z include groups derived from hydroxycarboxylic acids having 2 to 7 carbon atoms, and groups derived from monoesters of dicarboxylic acids having 2 to 7 carbon atoms and diols having 2 to 7 carbon atoms. When Z is a group derived from a monoester of dicarboxylic acids having 2 to 7 carbon atoms and diols having 2 to 7 carbon atoms, A may be bonded to the dicarboxylic acid-derived site or to the diol-derived site. The compounds represented by formula (9) are preferably those in which a group having an ester-forming functional group represented by Z forms a polyester structure, that is, a structure in which the structural unit represented by formula (9) is repeated multiple times. Examples of such compounds include compounds having a structural unit represented by the following formula (9-1). 【0106】 [ka] In formula (9-1) above, n is an integer between 2 and 40, preferably between 10 and 30, and more preferably between 15 and 25. 【0107】 Using a compound having the structural unit of formula (9-1) is preferable in terms of suppressing blackening during molding, resulting in a superior film appearance, and minimizing the generation of decomposition products during heating. The reason why a compound having the structural unit of formula (9-1) exhibits these excellent effects is not clear, but the inventors surmise that it is related to the reduced influence of hydrolysis reactions. That is, if a compound has such a structural unit, even if hydrolysis occurs, the decomposition products (e.g., phenolic decomposition products) are immobilized within the structure and are therefore less likely to volatilize. In addition, it is thought that recyclability is possible through re-dehydration reactions, making it stable. As a result, it is presumed that the film will have a superior appearance with less blackening, and will also be less prone to odor problems due to sublimation or volatilization of decomposition products. 【0108】 Compounds represented by formula (9) include deethylene glycol polycondensates of 2-(9,10-dihydro-9-oxa-10-oxide-10-phosphaphenanthrene-10-yl)methylsuccinate bis-(2-hydroxyethyl), and for example, compounds represented by the following formula (9-2) are more preferred. [ka] In equation (9-2), n is the same as in equation (9-1). 【0109】 The weight-average molecular weight of the compound represented by formula (9) is, for example, 1170 or more, preferably 2290 or more, and more preferably 3410 or more. When the weight-average molecular weight is 1170 or more, there is no volatilization of the organophosphorus compound and no inhibition of crystallization of the polyester resin (X) during film formation, and furthermore, bleed-out of the compound is suppressed, making it easier to maintain the mechanical strength of the film. The weight-average molecular weight of the compound represented by formula (9) is not particularly limited, but may be, for example, 20000 or less, or 10000 or less. Lowering the molecular weight makes it easier to improve dispersibility within the polyester resin (X). The weight-average molecular weight can be measured using gel permeation chromatography (GPC) with polymethyl methacrylate as the standard substance. 【0110】 Furthermore, examples of flame retardants (Y) include phosphinic acid-based flame retardants such as phosphinates and diphosphinates. Examples of phosphinates or diphosphinates include compounds represented by formula (10) and compounds represented by formula (11). 【0111】 [ka] 【0112】 In formula (10), R 1 and R 2 Each of these independently represents a linear or branched alkyl group with 1 to 6 carbon atoms, or an aryl group with 6 to 10 carbon atoms. M represents a calcium ion, aluminum ion, magnesium ion, or zinc ion. m is a natural number representing the valency of M. In equation (10), R 1 and R 2 Each of these independently represents a linear or branched alkyl group having 1 to 6 carbon atoms, or an aryl group having 6 to 10 carbon atoms, preferably a methyl group, an ethyl group, a propyl group, or a phenyl group. M represents a calcium ion, an aluminum ion, a magnesium ion, or a zinc ion. m is a natural number representing the valency of M, preferably 2 or 3. 【0113】 [ka] 【0114】 In formula (11), R 4 and R 5 Each of these independently represents a linear or branched alkyl group with 1 to 6 carbon atoms, or an aryl group with 6 to 10 carbon atoms. 3represents a linear or branched alkylene group with 1 to 10 carbon atoms, an arylene group with 6 to 10 carbon atoms, an alkylarylene group with 7 to 10 carbon atoms, or an arylalkylene group with 7 to 10 carbon atoms. M represents a calcium ion, an aluminum ion, a magnesium ion, or a zinc ion. n is a natural number representing the valence of M. n, a, and b are natural numbers that satisfy the relationship 2 × b = n × a. 【0115】 In equation (11), R 4 and R 5 Each of these independently represents a linear or branched alkyl group having 1 to 6 carbon atoms, or an aryl group having 6 to 10 carbon atoms, preferably a methyl group, an ethyl group, a propyl group, or a phenyl group. 3 represents a linear or branched alkylene group having 1 to 10 carbon atoms, an arylene group having 6 to 10 carbon atoms, an alkylarylene group having 7 to 10 carbon atoms, or an arylalkylene group having 7 to 10 carbon atoms, preferably a methylene group, an ethylene group, a propylene group, or a phenylene group. M represents a calcium ion, an aluminum ion, a magnesium ion, or a zinc ion. n is a natural number representing the valence of M. n, a, and b are natural numbers that satisfy the relationship 2 × b = n × a. n is preferably 2 or 3. b is preferably 1, 2, or 3, and more preferably 1 or 3. a is preferably 1 or 2. 【0116】 Phosphinates or diphosphinates specifically include those produced in an aqueous medium using phosphinic acid and metal carbonates, metal hydroxides, or metal oxides. Phosphinates or diphosphinates are basically monomeric compounds, but depending on the reaction conditions, they may also become polymeric phosphinates with a degree of condensation of 1 to 3 under certain environmental conditions. 【0117】 Examples of phosphinic acids or diphosphinic acids include dimethylphosphinic acid, ethylmethylphosphinic acid, diethylphosphinic acid, methyl-n-propylphosphinic acid, methanedi(methylphosphinic acid), benzene-1,4-di(methylphosphinic acid), methylphenylphosphinic acid, and diphenylphosphinic acid. 【0118】 Examples of phosphinates include calcium dimethylphosphinate, magnesium dimethylphosphinate, aluminum dimethylphosphinate, zinc dimethylphosphinate, calcium ethylmethylphosphinate, magnesium ethylmethylphosphinate, aluminum ethylmethylphosphinate, zinc ethylmethylphosphinate, calcium diethylphosphinate, magnesium diethylphosphinate, aluminum diethylphosphinate, zinc diethylphosphinate, calcium methyl-n-propylphosphinate, magnesium methyl-n-propylphosphinate, aluminum methyl-n-propylphosphinate, zinc methyl-n-propylphosphinate, calcium methylphenylphosphinate, magnesium methylphenylphosphinate, aluminum methylphenylphosphinate, zinc methylphenylphosphinate, calcium diphenylphosphinate, magnesium diphenylphosphinate, aluminum diphenylphosphinate, and zinc diphenylphosphinate. 【0119】 Examples of diphosphinates include calcium methanedi(methylphosphinate), magnesium methanedi(methylphosphinate), aluminum methanedi(methylphosphinate), zinc methanedi(methylphosphinate), calcium benzene-1,4-di(methylphosphinate), magnesium benzene-1,4-di(methylphosphinate), aluminum benzene-1,4-di(methylphosphinate), and zinc benzene-1,4-di(methylphosphinate). 【0120】 Among these phosphinates or diphosphinates, aluminum ethylmethylphosphinate, aluminum diethylphosphinate, and zinc diethylphosphinate are preferred. Commercially available phosphinates or diphosphinates include Clariant's EXOLIT OP1230 (aluminum phosphinate) and OP1400. 【0121】 The flame retardant (Y) is preferably a compound having the structural unit of formula (9), more preferably a compound having the structural unit represented by formula (9-1), and even more preferably a compound represented by formula (9-2). By using a compound having the structural unit of formula (9), it is possible to suppress the sublimation and volatilization of the flame retardant and its thermal decomposition products during melt mixing and film formation, while ensuring a certain level of film strength, and to facilitate the proper formation of an insulating film when heated at high temperatures. 【0122】 In preparing this film, the flame retardant (Y) and polyester resin (X) may be pre-mixed and then melt-kneaded, or the flame retardant (Y) may be added to the melted polyester resin (X) and then kneaded. Alternatively, a resin compound such as a masterbatch may be prepared using either of these methods and then melt-kneaded with the polyester resin (X). Furthermore, the flame retardant (Y) may be added to the polyester resin (X) production system during the production of the polyester resin (X). 【0123】 <Phosphorus content> In this film, the phosphorus element content (hereinafter simply referred to as "phosphorus element content (P1)") per 100 parts by mass of polyester resin (X) on one surface of the film is preferably 0.1 parts by mass or more, and the phosphorus element content (P1) is greater than the phosphorus element content (hereinafter simply referred to as "phosphorus element content (P2)") per 100 parts by mass of polyester resin (X) in the central part in the thickness direction of the film. In this film, the phosphorus element is typically derived from the flame retardant (X). By meeting the above requirements, this film contains a large amount of phosphorus element, i.e., the flame retardant (Y), on its laminated surface. Therefore, even when heated to high temperatures, the flame retardant (Y) forms a carbonized layer on the surface of the laminated film, leaving a residue and allowing for the proper formation of an insulating film. As a result, insulation can be ensured when heated to high temperatures, and for example, when used for battery protection, it can prevent the conductive material inside the cell from being exposed or sparks from occurring. Furthermore, because the phosphorus element content (P2) in the central part of the film in the thickness direction is relatively low, it is possible to prevent the demilitarization caused by the reaction between the flame retardant (Y) and the polyester resin (X) in the central part of the film, making it easier to improve the mechanical strength of the laminated film, especially the tensile breaking strength. In this film, from the viewpoint of forming an insulating film more appropriately, if the phosphorus element content (P1) on one surface of the film is higher than the phosphorus element content (P2) in the central part of the film in the thickness direction, it is preferable that the phosphorus element content (P1) on both surfaces of the film is greater than the phosphorus element content (P2) in the central part. 【0124】 The phosphorus content (P1) on the surface of this film, which has a higher phosphorus content (P1) than the phosphorus content (P2) in the central part, is preferably 0.1 parts by mass or more per 100 parts by mass of polyester resin (X), as described above. Having a phosphorus content (P1) of 0.1 parts by mass or more on the surface allows for the proper formation of an insulating film when heated to high temperatures. The phosphorus content (P1) is preferably 0.2 parts by mass or more, more preferably 0.3 parts by mass or more, and even more preferably 0.5 parts by mass or more. Furthermore, the phosphorus content (P1) on the surface having a higher phosphorus content (P1) than the phosphorus content (P2) in the central part may be, for example, 5 parts by mass or less, preferably 4 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 2 parts by mass or less, from the viewpoint of increasing mechanical strength, especially tensile fracture strength. 【0125】 Furthermore, in this film, the phosphorus element content (P2) in the central part of the film is, for example, 0.5 parts by mass or less, preferably 0.3 parts by mass or less, more preferably 0.1 parts by mass or less, and even more preferably 0.05 parts by mass or less, per 100 parts by mass of polyester resin (X). A lower phosphorus element content (P2) in the central part makes it easier to improve the mechanical strength, such as the tensile breaking strength, of this laminated film. The lower the phosphorus element content (P2) in the central part, the better, and its lower limit is 0 parts by mass. 【0126】 From the viewpoint of properly forming an insulating film while further improving mechanical strength such as tensile fracture strength, the difference between the phosphorus element content (P1) on a surface having a higher phosphorus element content (P1) than the phosphorus element content (P2) in the central part and the phosphorus element content (P2) is preferably 0.1 parts by mass or more and 5 parts by mass or less, more preferably 0.2 parts by mass or more and 4 parts by mass or less, even more preferably 0.3 parts by mass or more and 3 parts by mass or less, and even more preferably 0.5 parts by mass or more and 2 parts by mass or less. 【0127】 Furthermore, the phosphorus element content (hereinafter also referred to as phosphorus element content (P)) per 100 parts by mass of polyester resin (X) in the entire film is preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and even more preferably 0.15 parts by mass or more. If the phosphorus element content (P) in the entire film is above a certain amount, it becomes easier to form an insulating film. Also, from the viewpoint of improving mechanical strength such as tensile breaking strength, the phosphorus element content (P) is preferably 2 parts by mass or less, more preferably 1 part by mass or less, more preferably 0.75 parts by mass or less, and even more preferably 0.5 parts by mass or less. 【0128】 <Phosphorus content> Preferably, the phosphorus element content (pt) in the laminated film is 0.01% by mass or more, and the phosphorus element content (p1) on one surface of the film is higher than the phosphorus element content (p2) in the central part of the film in the thickness direction. By meeting the above requirements, this laminated film contains a large amount of phosphorus element, i.e., the flame retardant (Y), on its surface. Therefore, even when heated to high temperatures, the flame retardant (Y) forms a carbonized layer on the film surface, leaving a residue that allows for the proper formation of an insulating film. As a result, the insulating properties can be increased when heated to high temperatures, and for example, when used for battery protection, it can prevent the conductive material inside the cell from being exposed or sparks from occurring. Furthermore, because the phosphorus element content (p2) is low in the central part of the film, the flame retardant (Y) prevents the polyester resin (X) from becoming low-molecular-weight, thereby improving the mechanical strength, especially the tensile breaking strength. 【0129】 The phosphorus element content (pt) in the entire laminated film is preferably, for example, 0.01% by mass or more, as described above, but from the viewpoint of making it easier to form an insulating film, it is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.15% by mass or more. Furthermore, from the viewpoint of improving mechanical strength such as tensile breaking strength, the phosphorus element content (pt) in the entire laminated film is preferably 2% by mass or less, preferably 1% by mass or less, more preferably 0.75% by mass or less, and even more preferably 0.5% by mass or less. 【0130】 For example, this film has a phosphorus content (hereinafter also referred to as phosphorus content (p)) of 0.01% by mass or more, and the phosphorus content on one surface of the film (hereinafter also referred to as phosphorus content (p1)) is higher than the phosphorus content in the center of the film in the thickness direction (hereinafter also referred to as phosphorus content (p2)). By meeting the above requirements, this film contains a large amount of phosphorus element, i.e., the flame retardant (Y), on its surface. Therefore, even when heated to high temperatures, the flame retardant (Y) forms a carbonized layer on the film surface, leaving a residue that allows for the proper formation of an insulating film. As a result, insulation can be ensured when heated to high temperatures, and for example, when used for battery protection, it can prevent the conductive material inside the cell from being exposed or sparks from occurring. Furthermore, because the phosphorus element content (p2) is low in the central part of the film, the flame retardant (Y) prevents the polyester resin (X) from becoming low-molecular-weight, thereby improving the mechanical strength, especially the tensile breaking strength. 【0131】 The phosphorus element content (p) in the entire film is preferably, for example, 0.01% by mass or more, as described above, but from the viewpoint of making it easier to form an insulating film, it is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.15% by mass or more. Furthermore, from the viewpoint of improving mechanical strength such as tensile breaking strength, the phosphorus element content (p) in the entire film is preferably 2% by mass or less, preferably 1% by mass or less, more preferably 0.75% by mass or less, and even more preferably 0.5% by mass or less. If the phosphorus content (p1) on one surface of the film is higher than the phosphorus content (p2) in the central part of the film in the thickness direction, it is preferable that the phosphorus content (p1) on both surfaces of the film is higher than the phosphorus content (p2) in the central part, from the viewpoint of forming an insulating film more appropriately. 【0132】 Furthermore, from the viewpoint of forming an insulating film more appropriately and further improving mechanical strength such as tensile breaking strength, the difference between the phosphorus element content (p1) on the surface having a higher phosphorus element content (p1) than the phosphorus element content (p2) in the central part and the phosphorus element content (p2) is preferably 0.1 mass% or more and 5 mass% or less, more preferably 0.2 mass% or more and 4 mass% or less, even more preferably 0.3 mass% or more and 3 mass% or less, and even more preferably 0.5 mass% or more and 2 mass% or less. 【0133】 In this film, the phosphorus content (p1) on the surface, which has a phosphorus content (p1) higher than the phosphorus content (p2) in the central part, is preferably 0.1% by mass or more and 5% by mass or less, more preferably 0.2% by mass or more and 4% by mass or less, even more preferably 0.3% by mass or more and 3% by mass or less, and even more preferably 0.5% by mass or more and 2% by mass or less. Setting the phosphorus content (p1) on the surface to a certain value or higher as described above makes it easier to form a carbide film more appropriately. In addition, setting the phosphorus content (p1) to a certain value or lower as described above makes it easier to improve mechanical strength such as tensile breaking strength. 【0134】 In this film, the phosphorus content (p2) in the central part of the film thickness direction is, for example, 0.5% by mass or less, preferably 0.3% by mass or less, more preferably 0.1% by mass or less, and even more preferably 0.05% by mass or less. A lower phosphorus content (p2) in the central part makes it easier to improve mechanical strength, such as tensile breaking strength. The lower the phosphorus content (p2) in the central part, the better, and its lower limit is 0% by mass. 【0135】 Note that the phosphorus element content (P1) and phosphorus element content rate (p1) on the surface refer to the phosphorus element content and content rate in the surface layer (A) or back layer (C) in a multilayer film. However, in this film, as with the single-layer film with a concentration gradient described later, the composition may change along the thickness direction near the surface, even within the same layer, causing changes in the phosphorus element content and phosphorus element content. In cases where the phosphorus element content or phosphorus element content changes near the surface in this way, it is preferable to define the phosphorus element content and phosphorus element content rate in the region from the film surface to 5% of the total thickness as the phosphorus element content rate (P1) and phosphorus element content rate (p1), respectively. 【0136】 Furthermore, the phosphorus element content (P2) and phosphorus element content rate (p2) in the central part of the thickness direction described above refer to the phosphorus element content and content rate in the intermediate layer (B) in multilayer films. However, in cases where the phosphorus element content or phosphorus element content changes in the thickness direction, such as when the composition of the intermediate layer (B) changes along the thickness direction in a multilayer film, or when the composition changes along the thickness direction in a single-layer film, the phosphorus element content or phosphorus element content in the intermediate layer (B) or in the central part of the film in the thickness direction shall be defined as the phosphorus element content (P2) and phosphorus element content rate (p2), respectively, in a region of 10% of the total thickness centered on the center of the film in the thickness direction. Furthermore, the phosphorus element content (P) and phosphorus element content rate (p) refer to the phosphorus element content and phosphorus element content rate contained in the film based on the film as a whole. The content rates (z1), (z2), and (z) of the filler (Z) described later are defined in the same way as the phosphorus element content rates (p1), (p2), and (p), respectively. Furthermore, the phosphorus element content (pt) is the phosphorus element content contained in the entire laminated film, based on the entire laminated film. Similarly, the filler content (zt), described later, is the content of all fillers contained in the laminated film (i.e., the sum of filler (Z) and filler (Za)) in the entire laminated film. 【0137】 The phosphorus content and phosphorus content of each element can be determined by known methods and are not particularly limited, but for example, they can be determined using ICP analysis, XPS analysis, or SEM-EDX analysis of the film thickness cross-section. Alternatively, if the blending amount and phosphorus content of each raw material are known, they may be calculated from the blending amount and phosphorus content of each raw material. 【0138】 (Content of flame retardant (X)) The content of the flame retardant (X) in this film should be adjusted so that the phosphorus element content and phosphorus element content ratio fall within the above-mentioned range. For example, in a multilayer film, the flame retardant (X) should be included in the surface layer (A), the back layer (C), or both, so that the phosphorus element content (P1) and phosphorus element content ratio (p1) on the surface are as specified above, and the content of the flame retardant (X) in the surface layer (A), the back layer (C), or both should be adjusted as appropriate. Specifically, the flame retardant content in the surface layer (A) or the back layer (C) is, for example, 1 to 60 parts by mass, preferably 3 to 50 parts by mass, more preferably 5 to 40 parts by mass, and even more preferably 8 to 30 parts by mass, per 100 parts by mass of polyester resin (X). On the other hand, the amount of flame retardant (Y) in the intermediate layer (C) may be, for example, 10 parts by mass or less per 100 parts by mass of polyester resin (X), preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 1 part by mass or less. Furthermore, the intermediate layer (C) does not have to contain a flame retardant, and therefore, the amount of flame retardant in the intermediate layer (C) may be 0 parts by mass or more. Furthermore, the flame retardant content in the entire film is, for example, 0.1 parts by mass to 12 parts by mass, preferably 0.5 parts by mass to 10 parts by mass, more preferably 1 part by mass to 8 parts by mass, and even more preferably 1.5 parts by mass to 5 parts by mass, per 100 parts by mass of polyester resin (X). 【0139】 If the film is a single-layer film, for example, the concentration gradient of the flame retardant (Y) may be adjusted so that either or both of the phosphorus element content and phosphorus element content fall within the specified range. Specifically, the concentration (i.e., content) of the flame retardant (Y) should be gradient such that it decreases from one surface towards the center of the film. Preferably, the concentration of the flame retardant (Y) decreases from one surface towards the center of the film, and then increases again towards the other surface. In other words, it is sufficient to have a region of high concentration (first region) and a region of low concentration (second region) moving from one surface to the other, but it is preferable that they be arranged in the order of a region of high concentration (first region), a region of low concentration (second region), and a region of high concentration (first region). 【0140】 <Filling material (Z)> Preferably, this film further contains a filler (Z). By containing a filler (Z), the film is more likely to retain residue even when heated to high temperatures, and the insulating properties of the insulating film formed by the residue are also more easily improved. The filler (Z) may be either an inorganic or organic filler, but an inorganic filler is preferred from the viewpoint of forming an insulating film due to the filler itself remaining after high-temperature heating. The shape of the filler (Z) may be spherical particles, flakes, plates, fibers, needles, etc., but spherical or plate-shaped particles are preferred from the viewpoint of preventing the film from becoming brittle and its mechanical strength from decreasing due to the formation of voids caused by the filler (Z), and from making it easier to prevent film breakage during film formation. 【0141】 Specific examples of filler (Z) and organic filler are listed under filler (Za), but from the viewpoint of improving insulation, titanium dioxide, silica, carbon black, calcium carbonate, and aluminum oxide are preferred, with titanium dioxide being more preferred among them. Filler (Z) may be used alone or in combination of two or more types. Therefore, two or more of the suitable particles, silica, titanium dioxide, and carbon black, can also be used in combination. The average particle size of the filler (Z) is typically 0.05 μm to 10 μm, preferably 0.1 μm to 7 μm, more preferably 0.15 μm to 5 μm, even more preferably 0.2 μm to 4 μm, and even more preferably 0.25 μm to 3 μm. Using the above range improves particle dispersibility, making it easier to retain the particles as a layer (film), which in turn makes it easier to impart the desired insulating properties to the film and also improves film formation. 【0142】 The method for adding the filler (Z) to the film is not particularly limited, and conventionally known methods can be employed. For example, it can be added at any stage in the production of the polyester resin (X), but it is preferable to add it after the esterification or transesterification reaction is completed. Alternatively, a masterbatch of the filler (Z) can be prepared in advance, blended with other raw materials, and fed into an extruder to produce the film. 【0143】 (Content of filler (Z)) The content of filler (Z) in this film relative to the entire film (hereinafter also referred to as content (z)) is, for example, 20% by mass or less, preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. Keeping the content (z) below the above upper limit makes it easier to prevent the film from becoming brittle and its mechanical strength from decreasing due to the filler (Z). The content (z) is, for example, 0.002% by mass or more, but from the viewpoint of making it easier to form an insulating film by including the filler (Z), which allows the filler itself to remain after high-temperature heating, it is preferably 0.2% by mass or more, more preferably 0.5% by mass or more, even more preferably 0.8% by mass or more, and even more preferably 1% by mass or more. 【0144】 The filler (Z) may be contained throughout the entire film (i.e., throughout the thickness direction) or in only a portion of the film in the thickness direction. For example, the filler (Z) may be contained on one or both surfaces of the film (i.e., the surface layer (A) in a multilayer film, or the surface layer (A) and the back layer (C)) while not being contained in the central part of the film (i.e., the intermediate layer (B) in a multilayer film), or vice versa. Furthermore, from the viewpoint of facilitating the formation of an insulating film when heated at high temperatures, if the filler (Z) is contained on at least one or both surfaces of the film (i.e., the surface layer (A) in a multilayer film, or the surface layer (A) and the back layer (C)), it is preferable to formulate it in a quantity that does not reduce mechanical strength. 【0145】 The content (z1) of the filler (Z) on one or both surfaces of the film (i.e., the surface layer (A) in the multilayer film, or the surface layer (A) and the back layer (C)) is not particularly limited, but is, for example, 20% by mass or less, preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, and also, for example, 0.01% by mass or more, preferably 0.2% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more. Furthermore, the content (z2) of the filler (Z) in the central part of the film in the thickness direction (i.e., the intermediate layer (B) in a multilayer film) is not particularly limited, and is, for example, 20% by mass or less, preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. Also, the central part of the film in the thickness direction does not have to contain the filler as described above, and therefore the content (z2) may be 0% by mass or more, but is preferably 0.2% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more. 【0146】 The content of the filler (Z) may be uniform in the thickness direction or may vary in the thickness direction. For example, if the film is a single-layer film, the filler (Z) may have a concentration gradient, similar to the flame retardant (X). That is, the concentration (i.e., content) of the filler (Z) may have a concentration gradient such that it decreases from one surface of the film towards the center, or it may decrease from one surface towards the center of the film and then increase again towards the other surface. 【0147】 The sum of the phosphorus element content (pt) and the filler content (zt) in the laminated film (pt+zt) is preferably 0.3% by mass or more and 20% by mass or less. Having the sum (pt+zt) within this range makes it easier to maintain good mechanical strength, such as tensile breaking strength, while also ensuring insulation even after high-temperature heating due to the formed insulating film. The sum of the content (pt+zt) is preferably 0.5% by mass or more and 16% by mass or less, more preferably 1.5% by mass or more and 11% by mass or less, and even more preferably 2% by mass or more and 6% by mass or less. 【0148】 The filler content (zt) in this laminated film is, for example, 0.002% by mass or more and 19% by mass or less, preferably 0.5% by mass or more and 15% by mass or less, more preferably 1% by mass or more and 10% by mass or less, and even more preferably 1.5% by mass or more and 5% by mass or less. By keeping the filler content (zt) within the above range, it is easier to maintain good mechanical strength, such as tensile breaking strength, while ensuring insulation even after high-temperature heating due to the formed insulating film. 【0149】 Furthermore, the sum of the phosphorus element content (p) and the filler content (z) (p+z) in the film is preferably 0.3% by mass or more and 20% by mass or less. By keeping the sum of the content (p+z) within the above range, it is easier to maintain good mechanical strength, such as tensile breaking strength, while ensuring insulation even after high-temperature heating due to the formed insulating film. The sum of the content (p+z) is preferably 0.7% by mass or more and 16% by mass or less, more preferably 1% by mass or more and 11% by mass or less, and even more preferably 1.5% by mass or more and 6% by mass or less. 【0150】 In addition to the flame retardant (Y) and filler (Z) mentioned above, conventionally known ultraviolet absorbers, antioxidants, antistatic agents, heat stabilizers, lubricants, dyes, pigments, etc., may be added to this film as needed. 【0151】 This film may contain other resins besides polyester resin (X) as long as it does not impair the effects of the present invention. Other resins include polystyrene resins, polyvinyl chloride resins, polyvinylidene chloride resins, chlorinated polyethylene resins, polycarbonate resins, polyamide resins, polyacetal resins, acrylic resins, ethylene vinyl acetate copolymers, polymethylpentene resins, polyvinyl alcohol resins, cyclic olefin resins, polylactic acid resins, polybutylene succinate resins, polyacrylonitrile resins, polyethylene oxide resins, cellulose resins, polyimide resins, polyurethane resins, polyphenylene sulfide resins, polyphenylene ether resins, polyvinyl acetal resins, polybutadiene resins, polybutene resins, polyamide-imide resins, polyamide-bismaleimide resins, polyetherimide resins, polyetherether ketone resins, polyethersulfone resins, polyketone resins, polysulfone resins, aramid resins, and fluorine resins. 【0152】 The thickness of this film is, for example, 20 μm to 200 μm, preferably 30 μm to 150 μm, more preferably 35 μm to 120 μm, and even more preferably 40 μm to 100 μm. By keeping the film thickness within the above range, the film itself can be carbonized and then appropriately used as an insulating film. The thickness of this film was determined by measuring the thickness at five unspecified points in the plane using a 1 / 1000 mm dial gauge and taking the average of these measurements. 【0153】 The thickness of the surface layer (A) and the back layer (C) is preferably 2% to 25% of the total thickness of the film, more preferably 3% to 20%, and even more preferably 5% to 15%. Setting the thickness of the surface layer (A) and the back layer (C) within this range makes it easier to form an insulating film when heated at high temperatures without impairing the mechanical strength of the film. Furthermore, from a similar viewpoint, the thickness of the surface layer (A) and the back layer (C) is, for example, 2 μm or more and 30 μm or less, preferably 3 μm or more and 20 μm or less, more preferably 3.5 μm or more and 15 μm or less, and even more preferably 4 μm or more and 12 μm or less. 【0154】 The thickness of the intermediate layer (B) is preferably 60% to 98% of the total thickness of the film, more preferably 70% to 96%, and even more preferably 75% to 92%. When the thickness of the intermediate layer (B) is within this range, the surface layer (A) appropriately forms an insulating film during high-temperature heating, while the intermediate layer (B) easily imparts appropriate mechanical strength to the entire film. From a similar viewpoint, the thickness of the intermediate layer (B) is, for example, 10 μm or more and 180 μm or less, preferably 15 μm or more and 140 μm or less, more preferably 20 μm or more and 100 μm or less, and even more preferably 25 μm or more and 70 μm or less. 【0155】 [Manufacturing method for this laminated film] Next, we will specifically describe examples of the manufacturing of this laminated film, but the manufacturing examples are not limited to those described below. As an example of a manufacturing method for this laminated film, we will describe a manufacturing method when the film is a biaxially oriented film. However, the manufacturing method is not limited to the one described here. When the film is a biaxially oriented polyester film, it is preferable to first manufacture an unstretched sheet and then stretch it in two directions to obtain a biaxially oriented polyester film. 【0156】 The unstretched sheet is preferably obtained by supplying a polyester resin (X) and components such as a flame retardant (Y), filler (Z), and other additives, which are added as needed, to an extruder and mixing them appropriately to obtain a resin composition, extruding the resin composition as a molten sheet from a die using the extruder, and cooling and solidifying it with a cooling roll. In this case, it is preferable to improve the adhesion between the sheet and the cooling roll in order to improve the flatness of the sheet, and electrostatic application adhesion method and / or liquid coating adhesion method are preferably employed. Furthermore, in the extruder, it is preferable to mix each polyester so that it is compatible, and specifically, it is preferable to knead it at a temperature of 260 to 300°C, preferably 265 to 295°C, more preferably 270 to 290°C. 【0157】 Next, the obtained unstretched sheet is stretched in two directions. In this case, the unstretched sheet is first stretched in one direction using a roll or tenter type stretcher. The stretching temperature is usually 70 to 120°C, preferably 80 to 110°C, and the stretching ratio is usually 2.5 to 6.0 times, preferably 3.0 to 5.0 times. Next, the material is stretched in a direction perpendicular to the stretching direction of the first stage. In this case, the stretching temperature is usually 70 to 170°C, and the stretching ratio is usually 3.0 to 7.0 times, preferably 3.5 to 5.5 times. 【0158】 Next, a heat treatment is performed at a temperature of typically 180-250°C, under tension or with a relaxation of up to 30%, to obtain a biaxially oriented film. This heat treatment is also called the heat setting process. The heat treatment may be carried out in two or more stages at different temperatures. Furthermore, cooling may be performed in a cooling zone after the heat treatment. The cooling temperature is preferably higher than the glass transition temperature (Tg) of the polyester resin (X) constituting the film, and more specifically, it is preferably in the range of 100 to 160°C. This cooling may be carried out in two or more stages at different temperatures. In the stretching described above, a method can be adopted in which stretching is performed in two or more stages in one direction. In that case, it is preferable to perform the stretching so that the final stretching ratios in both directions fall within the above ranges. 【0159】 Furthermore, simultaneous biaxial stretching can also be used in the manufacture of this film. Simultaneous biaxial stretching is a method of simultaneously stretching and oriented the aforementioned unstretched sheet in the machine direction (longitudinal direction) and width direction (transverse direction) while the temperature is controlled to typically 70 to 120°C, preferably 80 to 110°C, and the stretching ratio is preferably 4 to 50 times, more preferably 7 to 35 times, and even more preferably 10 to 25 times in terms of area ratio. Then, heat treatment is carried out at a temperature of typically 180-250°C under tension or under relaxation of 30% or less to obtain a stretched and oriented film. For the simultaneous biaxial stretching apparatus employing the above stretching method, conventional known stretching methods such as screw type, pantograph type, and linear drive type can be used. 【0160】 Furthermore, if the film is a multilayer film, the resin compositions constituting each layer can be obtained, each resin composition can be co-extruded as a polyester layer, and then the resulting film can be stretched and heat-set as described above. 【0161】 Furthermore, if the film is a single-layer film, the flame retardant (X) contained in the single-layer film should have a concentration gradient along the thickness direction as described above. To obtain such a single-layer film, the flame retardant (X) in the resin composition should be adjusted to be unevenly distributed by known methods. Specifically, the flame retardant (X) should not be uniformly dispersed in the resin composition, but rather partially settled due to its own weight or compatibility with the resin, for example, before extrusion molding. It is also possible to adjust the conditions during single-layer film production as appropriate. For example, when melt-mixing the resin composition and extruding it into a film, the cooling rate of the molten resin extruded into a film should be slowed. Specifically, the cooling rate should be slowed by performing stepwise cooling with a cooling roll such as a casting roll, or by slowing down the production speed. It is also possible to adjust the concentration gradient by setting the die temperature lower than the extrusion temperature. In addition, in this manufacturing method, the filler (Z) may also have a concentration gradient. 【0162】 The resin layer (a) may be formed by in-line coating or by off-line coating. When forming the resin layer by in-line coating, it is preferable to manufacture it by applying a coating solution, which is an aqueous solution or aqueous dispersion, to the polyester film. Furthermore, within the scope that does not impair the spirit of the present invention, a small amount of organic solvent may be contained in the coating solution for the purpose of improving dispersibility in water, improving film-forming properties, etc. Only one type of organic solvent may be used, or two or more types may be used as appropriate. 【0163】 As a method for forming the resin layer (a), conventionally known coating methods such as gravure coating, reverse roll coating, die coating, air doctor coating, blade coating, rod coating, bar coating, curtain coating, knife coating, transfer roll coating, squeeze coating, impregnation coating, kiss coating, spray coating, calender coating, and extrusion coating can be used. 【0164】 The resin layer (a) can be formed by applying the coating solution of the resin layer composition to the polyester film as described above, and, if necessary, performing treatments such as drying and curing on the applied composition. The drying and curing conditions when forming the resin layer (a) on the polyester film are not particularly limited. For example, when providing the resin layer by offline coating, it is generally preferable to perform heat treatment at 80-200°C for 3-40 seconds, preferably at 100-180°C for 3-40 seconds. On the other hand, when applying a resin layer (coated layer) by inline coating, it is generally best to perform heat treatment at 70-250°C for 3-200 seconds. Furthermore, regardless of whether it is offline coating or in-line coating, heat treatment and active energy ray irradiation such as ultraviolet irradiation may be used in combination as needed. The polyester film may be pre-treated with surface treatments such as corona treatment or plasma treatment. 【0165】 <Physical properties of laminated polyester film> (Weight remaining rate) Preferably, the laminated film has a weight retention rate of 15% or more, as measured by thermogravimetric differential thermal analysis (TG-DTA) at 500°C. A weight retention rate of 15% or more allows the laminated film itself to carbonize due to high-temperature heating, thereby forming a carbonized layer and an insulating film. Therefore, when this laminated film is used, for example, in battery applications, the insulating film prevents the conductive material from being exposed or sparks from occurring even if thermal runaway occurs in the battery. From this viewpoint, the weight retention rate at 500°C is more preferably 16% or more, and more preferably 18% or more. The higher the weight retention rate at 500°C, the better, but it may be, for example, 50% or less, 40% or less, or 30% or less. 【0166】 The laminated film preferably has a weight retention rate of more than 1% at 700°C, as measured by thermogravimetric differential thermal analysis (TG-DTA). When the weight retention rate at 700°C exceeds 1%, the laminated film can remain in a certain amount and form an insulating film even when heated to extremely high temperatures. The weight retention rate at 700°C is preferably 2% or more, more preferably 2.5% or more, and even more preferably 3% or more. The higher the weight retention rate at 700°C, the better, but it may be, for example, 30% or less, or 20% or less. 【0167】 The laminated film preferably has a 5% weight loss temperature of 365°C or higher, more preferably 375°C or higher, even more preferably 380°C or higher, even more preferably 383°C or higher, and particularly preferably 386°C or higher, as measured by thermogravimetric differential thermal analysis (TG-DTA). When the 5% weight loss temperature is above the above lower limit, initial decomposition such as oxidative decomposition at high temperatures is more easily suppressed, and for example, a larger amount of insulating material tends to be present even at high temperatures of around 370°C. This results in good chemical heat resistance and a higher level of insulation. The 5% weight loss temperature may be, for example, 410°C or lower, or 400°C or lower. 【0168】 The weight retention rate at 500°C and 700°C, and the 5% weight loss temperature, can be adjusted by the type of polyester resin (X) constituting this film, the type and amount of flame retardant (X), the type and amount of filler (Z), and the type and amount of filler (Z) contained in the resin layer (a). Furthermore, the weight retention rate at 500°C can be determined by measuring the weight retention rate at 500°C when the temperature is raised from 25°C to 700°C at 10°C / min in an air atmosphere and held at 700°C for 30 minutes. The weight retention rate at 700°C can be determined by measuring the weight retention rate after being held at 700°C for 30 minutes. The 5% weight loss temperature can be determined by measuring the temperature at which the weight has decreased by 5% from the initial weight. 【0169】 (insulating properties) Preferably, when this laminated film is laminated with a 1 mm thick brass plate and held in a 700°C atmosphere for 30 minutes, the surface of the brass plate does not conduct electricity when measured with a tester. Because it does not conduct electricity when heated under these conditions, this laminated film can maintain its insulating properties as an insulating film even after high-temperature heating, and can be suitably used for applications such as insulation and battery protection. The insulating properties can be evaluated in detail by the method described in the examples. 【0170】 (Tensile breaking strength) The tensile breaking strength of this laminated film is preferably 175 MPa or higher in both the longitudinal direction (MD) and the width direction (TD). A tensile breaking strength of 175 MPa or higher indicates good mechanical strength and makes it suitable for use in various fields, such as batteries. More preferably, the tensile breaking strength of this laminated film is 185 MPa or higher in both the longitudinal direction (MD) and the width direction (TD), and even more preferably 195 MPa or higher. From the viewpoint of mechanical strength, a higher tensile breaking strength is better, but the tensile breaking strength in both the longitudinal direction (MD) and the width direction (TD) may be, for example, 400 MPa or less, or 350 MPa or less. 【0171】 The present invention can provide a polyester film that has high tensile strength while having a phosphorus element content (p) in the film or a phosphorus element content (pt) in the laminated film that is above a certain amount. Specifically, it can provide a polyester film in which the phosphorus element content (p) in the film is 0.01% by mass or more, and the tensile strength is 175 MPa or more in both MD and TD. Furthermore, it can provide a laminated polyester film in which the phosphorus element content (pt) in the laminated film is 0.01% by mass or more, and the tensile strength is 175 MPa or more in both MD and TD. The laminated polyester film having the above configuration ensures a certain level of mechanical strength while remaining intact even when heated to high temperatures, allowing for the proper formation of an insulating film. The preferred values ​​for the phosphorus element content (p), phosphorus element content (pt), and tensile breaking strength in the laminated polyester film having the above configuration are as described above. 【0172】 (Tensile elongation at fracture) The tensile elongation at break in the longitudinal direction (MD) of the laminated film is preferably 120% or more, more preferably 140% or more, even more preferably 160% or more, and particularly preferably 170% or more. While a higher tensile elongation at break in the MD is preferable, it may be, for example, 300% or less, or even 250% or less. The tensile elongation at break in the width direction (TD) of the laminated film is preferably 80% or more, more preferably 100% or more, even more preferably 110% or more, and even more preferably 120% or more. While a higher tensile elongation at break in TD is preferable, it may be, for example, 250% or less, or even 200% or less. When the tensile elongation at break is above a certain value, the flexibility is good, making it suitable for use in a variety of fields. Furthermore, the tensile breaking strength and tensile breaking elongation can be measured by the method described in the examples. 【0173】 (Thermal shrinkage rate) The thermal shrinkage rate in the longitudinal direction (MD) of the laminated film when heat-treated at 150°C for 30 minutes is, for example, 5% or less, preferably 4% or less, more preferably 3% or less, and even more preferably 2% or less. The thermal shrinkage rate in the longitudinal direction (MD) of the laminated film when heat-treated at 150°C for 30 minutes is better the smaller it is, but it may be, for example, 0.1% or more, or 0.3% or more. Furthermore, the thermal shrinkage rate in the width direction (TD) of the laminated film when heat-treated at 150°C for 30 minutes is, for example, 4.5% or less, preferably 3.5% or less, more preferably 2.5% or less, and even more preferably 1.5% or less. The thermal shrinkage rate in the longitudinal direction (TD) of the laminated film when heat-treated at 150°C for 30 minutes is better the smaller it is, but may be, for example, 0.1% or more, or 0.2% or more. 【0174】 The thermal shrinkage rate in the longitudinal direction (MD) of the laminated film when heat-treated at 180°C for 30 minutes is, for example, 7% or less, preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less. The thermal shrinkage rate in the longitudinal direction (MD) of the laminated film when heat-treated at 180°C for 30 minutes is better the smaller it is, but it may be, for example, 0.1% or more, or 0.5% or more. Furthermore, the thermal shrinkage rate in the width direction (TD) of the laminated film when heat-treated at 180°C for 30 minutes is, for example, 5% or less, preferably 4% or less, more preferably 3% or less, and even more preferably 2% or less. The smaller the thermal shrinkage rate in the width direction (TD) of the film when heat-treated at 180°C for 30 minutes, the better, but it may be, for example, 0.1% or more, or 0.5% or more. The low thermal shrinkage rate when heat-treated at 150°C for 30 minutes or 180°C for 30 minutes indicates that this laminated film has excellent heat resistance and can be suitably used in applications that operate in high-temperature environments, such as battery applications. The thermal shrinkage rate can be measured by the method described in the examples. 【0175】 <<Film Laminate>> The film laminate of the present invention (hereinafter sometimes referred to as "this film laminate") comprises this laminated film and another layer provided on at least one surface side of this laminated film. The other layer is preferably a functional layer, and examples of functional layers include an easy-adhesion layer, an antistatic layer, and an adhesive layer. Among these, an adhesive layer is preferred because it can also be used in the form of an adhesive tape with an adhesive layer provided on this laminated film. Furthermore, as described above, this laminated film has a resin layer (a) provided on at least one surface, but if the resin layer (a) is provided on the surface on which the other layer, such as a functional layer, is provided, the other layer, such as a functional layer, is preferably laminated on top of the resin layer (a). Another example of the other layer is a metal layer. 【0176】 <Adhesive layer> The adhesive layer is formed from an adhesive composition. The adhesive layer is typically obtained by curing the adhesive composition. The adhesive composition may be an acrylic adhesive composition mainly composed of an acrylic resin, a rubber adhesive composition mainly composed of rubber, a urethane adhesive composition mainly composed of a urethane resin, or a silicone adhesive composition mainly composed of a silicone resin. Of these, an acrylic adhesive composition mainly composed of an acrylic resin is preferred because it allows for a good balance between adhesive strength and peeling strength and is inexpensive. The thickness of the adhesive layer is not particularly limited, but is, for example, 5 to 200 μm, preferably 10 to 100 μm. 【0177】 The term "main component resin" above refers to the resin with the highest mass percentage among the resins constituting the adhesive composition. For example, it refers to the component that accounts for 50% or more by mass, preferably 60% or more by mass, and more preferably 70% or more by mass, of the total amount of resin constituting the adhesive composition. The upper limit is 100% by mass, but it is usually 99.99% by mass. 【0178】 In addition to the main components described above, the adhesive composition may also contain, if necessary, a crosslinking agent described later, and other resins that constitute adhesive components other than the main resin (for example, acrylic resins, rubber, silicone resins, urethane resins). In addition, conventionally known additives such as tackifiers like rosin, rosin esters, hydrogenated rosin esters, phenolic resins, aromatic-modified terpene resins, aliphatic petroleum resins, alicyclic petroleum resins, styrene resins, and xylene resins, as well as silane coupling agents, antistatic agents, colorants, fillers, antioxidants, UV absorbers, and functional dyes, and compounds that change color or discolor upon exposure to ultraviolet light or radiation can be incorporated. The amount of these additives is preferably 10% by mass or less of the total adhesive composition (based on non-volatile components), and more preferably 5% by mass or less. As additives, they are usually low-molecular-weight components with a molecular weight of less than 10,000, and it is preferable to include as few of these low-molecular-weight components as possible in order to achieve excellent durability. 【0179】 (Acrylic resin) Suitable acrylic resins for use as the main component resin in adhesive compositions include (meth)acrylic polymers. (Meth)acrylic polymers are polymers whose main constituent unit is alkyl methacrylate. Examples of alkyl (meth)acrylate esters include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, t-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, isobornyl (meth)acrylate, 4-t-butylcyclohexyl (meth)acrylate, tricyclodecanyl (meth)acrylate, dicyclopentaenyl (meth)acrylate, and adamantyl (meth)acrylate. These may be used individually or in combination of two or more. Among these, methyl (meth)acrylate is preferred in terms of compatibility with other (meth)acrylates constituting the (meth)acrylic polymer and the heat resistance of the cured resin layer. The content of alkyl (meth)acrylate in the monomers forming the (meth)acrylic polymer is, for example, 50% by mass or more, but is preferably 60 to 99.99% by mass, more preferably 75 to 98.9% by mass, and even more preferably 87 to 97.8% by mass. Furthermore, the (meth)acrylic polymer may have a double bond that allows for radical polymerization. 【0180】 (Crosslinking agent) The adhesive composition may contain a crosslinking agent depending on its curing method. Examples of crosslinking agents include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, aldehyde-based crosslinking agents, and amine-based crosslinking agents. Among these, isocyanate-based crosslinking agents are preferably used in terms of improving adhesion to the substrate or reactivity with acrylic resins. The crosslinking agent may be used alone or in combination of two or more types. 【0181】 The crosslinking agent content is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 5 parts by mass, and more preferably 0.1 to 3 parts by mass, per 100 parts by mass of the main component resin. If the crosslinking agent content is within the above range, the desired durability can be obtained without insufficient cohesive force, while preventing a decrease in flexibility and tackiness. 【0182】 <Metal layer> The metal layer may be laminated directly onto at least one surface of the laminated film, or via another layer. The presence of the metal layer improves the strength of the laminate and makes it easier to function as a barrier layer that prevents the transmission of water, vapor, oxygen, light, etc. Therefore, the laminated film can be suitably used as an exterior material for batteries. The metal forming the metal layer is not particularly limited as long as it is a conductive metal, and examples include aluminum, nickel, gold, silver, copper, cadmium, and titanium. Among these, from the viewpoint of versatility, it is preferable that the metal constituting the metal layer be copper or aluminum. The metal layer may also contain elements other than conductive metals. 【0183】 The metal layer may be in the form of a foil, or it may be formed by vapor deposition, plating, or sputtering. More specifically, conventionally known methods such as vacuum deposition, electroplating, and sputtering can be used. When the metal layer is laminated to the film via other layers, it may be laminated to the film via the adhesive layer described above, or it may be laminated to the film via an adhesive layer other than the adhesive layer. The thickness of the metal layer is not particularly limited, but is preferably 3 μm to 100 μm, more preferably 3 μm to 80 μm, even more preferably 6 μm to 60 μm, and even more preferably 10 μm to 40 μm. 【0184】 <<How to use>> This laminated film or film laminate (hereinafter sometimes referred to as "this film, etc.") is suitable for use as an insulating material. As described above, this film, etc. can form an insulating film when heated at high temperatures, so by covering various components with it as an insulating layer, arranging it in close proximity to them, or arranging it between components, the insulating properties of the component surfaces and the insulating properties between components can be ensured. 【0185】 Furthermore, this film is intended for use in batteries, and is particularly preferred for battery protection. This film is best placed around the battery. By being used in batteries, especially for battery protection, this film can remain as an insulating film even if the battery experiences thermal runaway and is exposed to temperatures above 400°C. This prevents the discharge of conductive material from inside the battery and prevents sparks from occurring. 【0186】 Examples of batteries include lithium-ion batteries, lithium-ion polymer batteries, lead-acid batteries, lithium-sulfur batteries, nickel-metal hydride batteries, nickel-cadmium batteries, nickel-iron batteries, nickel-zinc batteries, silver oxide-zinc batteries, metal-air batteries, and polyvalent cation batteries. Among these, lithium-ion batteries are preferred. Lithium-ion batteries have a high energy density and tend to become hot when they experience thermal runaway. However, even when the battery is heated to a high temperature due to such thermal runaway, the residue of this film remains and forms an insulating film, making it suitable for electrical protection. 【0187】 When used for battery protection, this film, etc., is not particularly limited, but may be used as a battery protective material to cover the outer surface of the battery element. It may also be attached to the battery housing, for example, to the inner surface of the battery housing in which the battery element is housed. The film, etc., may be housed inside the housing by being adhered to the inner surface of the housing via an adhesive layer, for example. Furthermore, when the battery protective material is applied to the outer surface of the battery element, it may be, for example, a film laminate having an adhesive layer, and the film laminate may be bonded to the outer surface of the battery element via the adhesive layer. The battery protective material may also constitute the battery casing. 【0188】 When this film is used as a battery casing material, the casing material may have any configuration as long as it contains this laminated film or this film laminate, but it is preferable that this film laminate is used. When this film laminate is used as an outer casing for batteries, it is preferable that it has a metal layer. Furthermore, when a battery element is packaged inside the film laminate, it is preferable that the main film is placed on the outside, preferably as the outermost layer, and the metal layer is placed on the inside. In addition, it is preferable that the metal layer is bonded to the main film via an adhesive layer or a bonding layer. 【0189】 When this film laminate is used as an exterior material for batteries, it may further have a sealant layer. Having a sealant layer allows the laminate to be bonded to another component by heat-sealing it with a sealant layer on that component. Therefore, having a sealant layer makes it easy to form an exterior material by lamination. While the film laminates may be bonded to each other via the sealant layer, the laminate of the present invention may also be bonded to components other than the laminate of the present invention via the sealant layer. The sealant layer can be sealed by welding together the sealant layers located around the periphery of the battery element during battery assembly, thereby sealing the battery element with an outer casing (film laminate). 【0190】 The sealant layer can be formed from a heat-sealable resin material. Examples of resins constituting the sealant layer include polyethylene, polyolefins such as polypropylene, cyclic polyolefins, carboxylic acid-modified polyolefins, and carboxylic acid-modified cyclic polyolefins. When the sealant layer is used to package a battery element inside, it is preferable to position it inside the metal layer in the film laminate. The sealant layer is, for example, 10 μm to 100 μm thick, preferably 15 μm to 80 μm, or 20 μm to 60 μm thick. 【0191】 Furthermore, if the film laminate is used as an exterior material for batteries, it may also have a resin film layer in addition to the main laminated film. Examples of resins that make up the resin film layer include polyester resin, polyamide resin, polyolefin resin, epoxy resin, acrylic resin, fluororesin, polyurethane resin, silicon resin, and phenolic resin, and mixtures of these resins may be used. Among these, polyester resin and polyamide resin are preferred, and polyamide resin is preferred in particular. The resin film layer may be placed, for example, between the metal layer and the main film, or between the metal layer and the sealant layer. The resin film layer may consist of a single layer or a laminate of two or more layers. The thickness of the resin film layer is, for example, in the range of 5 μm to 80 μm, preferably 8 μm to 60 μm, and more preferably 10 μm to 40 μm. 【0192】 When used as an exterior material, the film laminate may have an adhesive layer. The adhesive layer is not particularly limited, but it may be composed of known adhesives such as polyether adhesives, polyester adhesives, polyurethane adhesives, epoxy adhesives, phenolic resin adhesives, polyamide adhesives, polyolefin adhesives, polyacrylic adhesives, amino resin adhesives, rubber adhesives, and silicone adhesives. Furthermore, the layers may be bonded together by known lamination methods such as dry lamination, although this is not particularly limited. The adhesive layer may be placed between the metal layer and the main film, the main film and the resin film layer, the resin film layer and the metal layer, the metal layer and the sealant layer, or the sealant layer and the resin film layer in order to bond them together. 【0193】 When used as an exterior material, preferred laminated structures for the film laminate include the main laminated film / adhesive layer / metal layer, or the main laminated film / adhesive layer / resin film layer / adhesive layer / metal layer. Furthermore, the laminate of the present invention may also have a laminated structure having a sealant layer in addition to the metal layer. Preferred laminated structures when a metal layer and a sealant layer are present include the main laminated film / adhesive layer / metal layer / sealant layer and the main laminated film / adhesive layer / resin film layer / adhesive layer / metal layer / sealant layer. 【0194】 The present invention also provides a battery comprising the laminated film or the laminated film. In the battery of the present invention, as described above, the film or the like may be used to cover the outer surface of an electronic element, or to attach the film or the like to the inner surface of a housing that houses the electronic element. The present invention also provides various electronic devices equipped with the above battery. Specific examples of electronic devices include automobiles, electric bicycles, mobile phones, digital cameras, video cameras, electronic organizers, personal computers, radios, music players, storage media recorders, game consoles, televisions, printers, vacuum cleaners, and the like. 【0195】 <<Explanation of terms>> In this invention, the term "film" includes "sheets," and the term "sheet" includes "film." In this invention, when "X~Y" (where X and Y are any numbers) is written, unless otherwise specified, it means "X or greater and Y or less," and also includes the meaning of "preferably greater than X" or "preferably less than Y." Furthermore, when "X or greater" (where X is any number) is written, unless otherwise specified, it includes the meaning of "preferably greater than X," and when "Y or less" (where Y is any number) is written, unless otherwise specified, it also includes the meaning of "preferably less than Y." [Examples] 【0196】 The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following embodiments, unless it exceeds the gist of the invention. 【0197】 <Evaluation Method> (1) Intrinsic viscosity (IV) One g of polyester resin (X), from which incompatible components had been removed, was accurately weighed. 100 mL of a mixed solvent of phenol / tetrachloroethane = 50 / 50 (mass ratio) was added to dissolve it, and the viscosity was measured at 30°C using a viscosity measuring device "VMS-022UPC·F10" (manufactured by Rigosha Co., Ltd.). 【0198】 (2) Weight residue rate, 5% weight reduction temperature Using a TG-DTA device manufactured by Hitachi High-Technologies Corporation (model: NEXTA STA200RV), the temperature was raised from 25°C to 700°C at a rate of 10°C / min in an air atmosphere (air flow rate: 200 mL / min). Then, it was held at 700°C for 30 minutes. At this time, the temperature at which the weight decreased by 5% from the initial weight (5% weight reduction temperature) and the weight residue rate (%) at the 500°C point were determined. Also, the weight residue rate (%) after holding at 700°C for 30 minutes was determined as the weight residue rate (%) at 700°C. The weight residue rate (%) was determined by measuring the ratio of the residue weight to the initial weight. 【0199】 (3) Insulation property As shown in Fig. 1(A), a brass plate 11 with a thickness of 1 mm and dimensions of 50 mm × 50 mm was placed on the heater 10, and a test sample 12 with dimensions of 25 mm × 25 mm was placed thereon. The temperature was raised from 25°C to 700°C at a rate of approximately 13°C / min over 50 minutes in the atmosphere, and after reaching 700°C, it was held at 700°C for 30 minutes. Then, the laminate of the brass plate 11 and the test sample 12 was removed from the heater 10 and cooled to room temperature by air cooling. The temperature was confirmed on the upper surface of the brass plate 11 where the test sample 12 was not placed. Next, as shown in Fig. 1(B), an aluminum foil 13 (10 mm × 10 mm, thickness 12 μm) was placed on the test sample 12, and the resistance between the surface of the aluminum foil 13 and the brass plate 11 was measured using a tester 14 (manufactured by Hioki Electric Co., Ltd.: model IR4054) under the condition of applying 125 V, and the insulation property (presence or absence of conduction) was evaluated according to the following criteria. (Judgment criteria) A: The resistance is higher than 250 MΩ and the insulation property is particularly good (no conduction). B: The resistance is higher than 1 kΩ and less than or equal to 250 MΩ, and the insulation property is good (no conduction). C: The resistance is less than 10 Ω, and the insulation property is poor (conduction exists). 【0200】 (4) Tensile breaking strength and tensile elongation at break of this laminated film Sample pieces measuring 15 mm in width and 150 mm in length were taken from the laminated polyester films or polyester films of the examples and comparative examples. The tensile breaking strength and tensile breaking elongation were measured in a room adjusted to a temperature of 23°C and a humidity of 50% RH using a Shimadzu Autograph AGX-V tensile testing machine. Using a tensile testing machine, the sample films were subjected to tensile tests in the longitudinal direction (MD) or width direction (TD) at a speed of 200 mm / min with a chuck distance of 50 mm. The strength at which the sample was cut (broken) (the value obtained by dividing the tensile load value by the cross-sectional area of ​​the test piece) was defined as the tensile breaking strength (MPa). In addition, the elongation at which the sample was cut (broken) (the percentage of the difference between the gauge length at break and the gauge length before the test divided by the gauge length before the test) was defined as the tensile breaking elongation (%). 【0201】 (5) Thermal shrinkage Samples of laminated polyester film or polyester film measuring 15 mm x 150 mm obtained in the examples and comparative examples were heated in an oven at 150°C or 180°C for 15 minutes in a tension-free state. The length of the sample was measured before and after the treatment, and the thermal shrinkage rates in the longitudinal (MD) and transverse (TD) directions of the film were calculated using the following formula. Thermal shrinkage rate (%) = {(L0-L1) / L0} × 100 (In the above formula, L0 is the sample length before heat treatment, and L1 is the sample length after heat treatment.) Measurements were taken at five points each in the vertical (MD) and horizontal (TD) directions of the film, and the average value was calculated for each. 【0202】 (6) Measurement of phosphorus element content and phosphorus element content The phosphorus content of polyester F was measured using ICP-OES (iCAP6500DUO, manufactured by Thermo Fisher Scientific). As a standard solution, SPEX's XSTC-8 (phosphorus content 10 ppm by mass) was used, and dilutions of 500-fold (0.02 ppm by mass), 200-fold (0.05 ppm by mass), 100-fold (0.1 ppm by mass), and 33.3-fold (0.3 ppm by mass) were prepared to create a calibration curve. The sample was weighed into a Kjeldahl flask (approximately 1 g) and decomposed by wet decomposition with sulfuric acid, nitric acid, and hydrogen peroxide to obtain an acid solution. After cooling, the solution was added to a 50 ml volumetric flask and used as the measurement solution. The wavelength used was 177.440 nm. Based on the obtained results and the blending ratio of raw material F in the surface layer, the phosphorus element content (p1) in the surface layer (i.e., the surface) was calculated. Furthermore, the phosphorus element content (P1) was also calculated from the obtained phosphorus element content (p1), the blending ratio of each raw material, and the polyester content ratio in each raw material. In addition, the phosphorus element content (p) and (pt) were calculated based on the phosphorus element content (p1), the ratio of the discharge amount of each layer when manufacturing the polyester film, and the mass per unit area of ​​the resin layer (a). Finally, the phosphorus element content (P) was also calculated from the obtained phosphorus element content (p), the blending ratio of each raw material, and the polyester content ratio in each raw material. 【0203】 <Materials used> (1) Polyester A: Homopolyethylene terephthalate (intrinsic viscosity; 0.58 dL / g), dicarboxylic acid component (a-1): terephthalic acid = 100 mol%, diol component (a-2): ethylene glycol = 100 mol% (2) Polyester B: Homopolyethylene terephthalate (intrinsic viscosity; 0.70 dL / g), dicarboxylic acid component (a-1): terephthalic acid = 100 mol%, diol component (a-2): ethylene glycol = 100 mol% (3) Polyester C: Homopolyethylene terephthalate (intrinsic viscosity; 1.18 dL / g), dicarboxylic acid component (a-1): terephthalic acid = 100 mol%, diol component (a-2): ethylene glycol = 100 mol% (4) Polyester D: A masterbatch containing 0.7% by mass of silica particles with an average particle size of 2.7 μm in homopolyethylene terephthalate (intrinsic viscosity: 0.59 dL / g) (5) Polyester E: A masterbatch containing 1.0% by mass of silica particles with an average particle size of 3.2 μm in homopolyethylene terephthalate (intrinsic viscosity: 0.64 dL / g) (6) Polyester F: A masterbatch (intrinsic viscosity; 0.48 dL / g) containing 35% by mass (2.9% by mass in terms of phosphorus element content) of a phosphorus-based flame retardant with the following structure in homopolyethylene terephthalate. 【0204】 [ka] In equation (9-2) above, n ≥ 4. The flame retardant of formula (9-2) used was obtained by the manufacturing method described in sections

[0054] to

[0058] of Japanese Patent Publication No. 2015-81271. 【0205】 (7) Polyester G: A masterbatch made by blending 50% by mass of titanium dioxide particles with an average particle size of 0.3 μm with homopolyethylene terephthalate (intrinsic viscosity: 0.49 dL / g). (8) Polyester H: A masterbatch containing 0.3% by mass of silica particles with an average particle size of 2.3 μm (intrinsic viscosity: 0.61 dL / g) (9) Polyester I: Homopolyethylene terephthalate (intrinsic viscosity; 0.80 dL / g), dicarboxylic acid component (a-1): terephthalic acid = 100 mol%, diol component (a-2): ethylene glycol = 100 mol% (10) Polyester J: Homopolyethylene terephthalate (intrinsic viscosity; 0.65 dL / g), dicarboxylic acid component (a-1): terephthalic acid = 100 mol%, diol component (a-2): ethylene glycol = 100 mol% 【0206】 (Resin layer composition 1) The following compounds Q to U were mixed in a solid content (non-volatile component) ratio of Q / R / S / T / U = 4.5 / 4.5 / 9.1 / 9.1 / 72.8 (mass%) to obtain resin layer composition 1. (11) Compound Q: Aqueous dispersion of polyester resin copolymerized with the following composition (Acid component) Terephthalic acid / Isophthalic acid / 5-Sodium sulfoisophthalic acid / / (Diol component) Ethylene glycol / 1,4-Butanediol / Diethylene glycol = 56 / 40 / 4 / / 70 / 20 / 10 (12) Compound R: Aqueous dispersion of an acrylic resin polymerized with the following composition Emulsion polymer of ethyl acrylate / n-butyl acrylate / methyl methacrylate / N-methylol acrylamide / acrylic acid = 65 / 21 / 10 / 2 / 2 (mass%) (Emulsifier: Anionic surfactant) (13) Compound S: Hexamethoxymethylol melamine (14) Compound T: Polyethylene oxide adduct to diglycerin structure. Average molecular weight 350 (15) Compound U: Silica particles (spherical) with an average particle size of 0.014 μm 【0207】 (Resin layer composition 2) The above Compound Q, S and the following Compounds V, W were mixed so that the solid content (non-volatile component) ratio was Q / V / S / W = 75 / 10 / 10 / 5 (mass%) to obtain Resin layer composition 2. (16) Compound V: Aqueous dispersion of a polycarbonate polyurethane resin formed from dimethylolpropionic acid unit: isophorone diisocyanate unit: polyhexamethylene carbonate unit: polyoxytetramethylene glycol unit: pentaethylene glycol unit = 2:12:72:5:9 (mol%) (17) Compound W: Silica particles (spherical) with an average particle size of 0.07 μm 【0208】 (Resin layer composition 3) The above Compounds Q~T were mixed so that the solid content (non-volatile component) ratio was Q / R / S / T = 40.9 / 40.9 / 9.1 / 9.1 (mass%) to obtain Resin layer composition 3. 【0209】 (Example 1) As shown in Table 1, the raw materials for the surface layer (surface layer (A) and back layer (C)) were dry-blended with 52.1% by mass of polyester C, 42.9% by mass of polyester F, and 5.0% by mass of polyester G, while the raw materials for the intermediate layer (B) were dry-blended with 5% by mass of polyester G and 95% by mass of polyester J. The mixed raw materials for the surface layer and the intermediate layer (B) were each fed into separate twin-screw extruders, kneaded at 280°C, co-extruded at 280°C, and then cooled and solidified on a cooling roll set to 25°C using an electrostatic application adhesion method to obtain two types of three-layer (surface layer (A) / intermediate layer (B) / backside layer (C)) unstretched films. Next, the obtained unstretched film was stretched 3.2 times in the longitudinal direction (MD) at 86°C using a roll stretcher. Subsequently, a coating solution containing resin layer composition 1 was applied to both sides of the polyester film by in-line coating. Then, it was guided to a tenter stretcher and stretched 4.2 times in the transverse direction (TD) at 115°C. Subsequently, it was heat-set at a heat-setting temperature of 230°C, and both sides of the polyester film with a thickness of 50 μm (surface layer (A): 5 μm, intermediate layer (B): 40 μm, back layer (C): 5 μm) had a mass per unit area of ​​265 mg / m². 2 A laminated polyester film having a resin layer (a) was obtained. The thickness of the resin layer (a) was 0.352 μm and 0.372 μm. Details of the obtained laminated polyester film and evaluation results are shown in Table 2. 【0210】 (Example 2) The procedure was carried out in the same manner as in Example 1, except that the raw materials for the surface layer and intermediate layer (B) were mixed as shown in Table 1. The thickness of the resin layer (a) was 0.411 μm and 0.412 μm. 【0211】 (Comparative Example 1) As shown in Table 1, the raw materials for the surface layer and intermediate layer (B) were mixed, and the stretching conditions, heat-fixing temperature, and thickness of the surface layer and intermediate layer were changed as shown in Table 1. The procedure was carried out in the same manner as in Example 1, except that the coating solution was not applied to both sides of the polyester film. Details of the obtained polyester film and evaluation results are shown in Table 2. 【0212】 (Comparative Example 2) The raw materials, dry-blended in the mass ratios shown in Table 1, were supplied to a vented extruder, kneaded at 285°C, melt-extruded at 285°C, and cooled and solidified on a cooling roll set to 25°C using an electrostatic application adhesion method to obtain a single-layer unstretched film. Next, the obtained unstretched sheet was stretched 3.0 times in the longitudinal direction (MD) at 85°C using a roll stretcher. Subsequently, by in-line coating, a coating solution containing resin layer composition 2 was applied to both sides of the polyester film to a thickness of 0.02 μm after drying and stretching. Next, it was guided to a tenter stretcher and stretched 4.0 times in the transverse direction (TD) at 130°C. Subsequently, it was heat-set at a heat-setting temperature of 218°C, and a single layer of polyester film with a thickness of 50 μm was applied to both sides to achieve a mass of 58 mg / m² per unit area. 2 A laminated polyester film having a resin layer was obtained. Details and evaluation results of the obtained laminated polyester film are shown in Table 2. 【0213】 (Comparative Example 3) As shown in Table 1, the mixed raw materials for the surface layer and intermediate layer (B) were prepared, and the stretching conditions and heat-fixing temperature were changed as shown in Table 1. Furthermore, after longitudinal stretching and before transverse stretching, a coating solution containing resin layer composition 3 was applied to both sides of the polyester film by in-line coating to a thickness of 0.1 μm after drying and stretching, forming a resin layer (mass per unit area of ​​145 mg / m²). 2 The procedure was carried out in the same manner as in Example 1, except that the ) was formed. 【0214】 [Table 1] 【0215】 [Table 2] *In Table 2, the phosphorus element content and flame retardant (Y) content are expressed per 100 parts by mass of polyester resin (X). *In Table 2, the percentage of filler content (z3) in the resin layer (a) relative to the entire film was calculated based on the density of the filler in the resin layer (a), the density of the coating layer (resin layer), the thickness of the coating layer (resin layer), the density of the polyester film, and the thickness of the polyester film. *In Tables 1 and 2, "surface layer" refers to both the surface layer (A) and the back layer (C). 【0216】 As can be seen from the results in Table 2, in Examples 1 and 2, by satisfying the requirements of the present invention, high tensile breaking strength and good mechanical strength were achieved, and even when heated to 500°C or 700°C, a high weight retention rate was maintained, and an insulating film could be formed even when heated to 700°C. In contrast, in Comparative Examples 1 and 3, since there was no resin layer (a) or the resin layer (a) did not contain a filler, the weight retention rate when heated to 500°C or 700°C was lower than that of Examples 1 and 2, and the insulating properties of the insulating film formed when heated to 700°C were insufficient compared to Examples 1 and 2. Furthermore, in Comparative Example 2, because the polyester film contained a flame retardant (Y), the weight retention rate was high when heated to 500°C or 700°C, and the insulating properties of the insulating film formed when heated to 700°C were also good. However, because the amount of filler in the resin layer (a) was small, and the phosphorus element content and phosphorus element content were the same on the surface and in the center of the thickness direction of the polyester film, the tensile breaking strength was low and the mechanical strength could not be sufficiently high.

Claims

[Claim 1] A laminated polyester film having a polyester film and a resin layer (a) on one surface side of the polyester film, The resin layer (a) comprises resin and filler (Za), The content of the filler (Za) in the resin layer (a) is 20% by mass or more. Laminated polyester film for use in batteries. [Claim 2] The laminated polyester film according to claim 1, wherein the content of the filler (Za) in the resin layer (a) is 10% by volume or more. [Claim 3] The mass per unit area of ​​the resin layer (a) is 10 mg / m². ２ 4000mg / m or more ２ The laminated polyester film according to claim 1, which is as follows: [Claim 4] The laminated polyester film according to claim 1, wherein the thickness of the resin layer (a) is less than 2 μm. [Claim 5] The laminated polyester film according to claim 1, wherein the average particle size of the filler (Za) is less than 1 μm. [Claim 6] The laminated polyester film according to claim 1, wherein the tensile breaking strength is 175 MPa or more in both MD and TD. [Claim 7] The laminated polyester film according to claim 1, wherein the filler (Za) is an inorganic filler. [Claim 8] The laminated polyester film according to claim 1, wherein the filler (Za) is at least one selected from the group consisting of titanium oxide, silica, zirconium oxide, and carbon black. [Claim 9] The laminated polyester film according to claim 1, wherein the polyester film comprises a polyester resin (X) and a flame retardant (Y). [Claim 10] The phosphorus element content (P1) on one surface of the polyester film is 0.1 parts by mass or more per 100 parts by mass of polyester resin (X), The laminated polyester film according to claim 9, wherein the phosphorus element content (P1) is greater than the phosphorus element content (P2) per 100 parts by mass of polyester resin (X) in the central part in the thickness direction of the polyester film. [Claim 11] The phosphorus element content (pt) in the laminated polyester film is 0.01% by mass or more. The laminated polyester film according to claim 9, wherein the phosphorus element content (p1) on one surface of the polyester film is higher than the phosphorus element content (p2) in the central part in the thickness direction of the polyester film. [Claim 12] The phosphorus element content (pt) in the laminated polyester film is 0.01% by mass or more. The laminated polyester film according to claim 9, wherein the tensile breaking strength is 175 MPa or more in both MD and TD. [Claim 13] The laminated polyester film according to any one of claims 9 to 12, wherein the polyester film includes a filler (Z). [Claim 14] The laminated polyester film according to claim 13, wherein the filler (Z) is an inorganic filler. [Claim 15] The laminated polyester film according to claim 13, wherein the filler (Z) is at least one selected from the group consisting of titanium dioxide, silica, carbon black, calcium carbonate, and aluminum oxide. [Claim 16] The laminated polyester film according to claim 13, wherein the content (z) of the filler (Z) in the polyester film is 20% by mass or less. [Claim 17] The laminated polyester film according to any one of claims 9 to 12, wherein the phosphorus element content (pt) of the laminated polyester film is 0.05% by mass or more and 1% by mass or less. [Claim 18] The polyester film according to any one of claims 9 to 12, wherein the sum of the phosphorus element content (pt) and the filler content (zt) in the laminated polyester film (pt + zt) is 0.3% by mass or more and 20% by mass or less. [Claim 19] The laminated polyester film according to any one of claims 9 to 12, wherein the polyester film comprises a surface layer (A) and an intermediate layer (B). [Claim 20] The laminated polyester film according to claim 19, further comprising a back surface layer (C). [Claim 21] A laminated polyester film according to any one of claims 1 to 12, wherein the weight retention rate measured by thermogravimetric differential thermal analysis (TG-DTA) at 700°C is greater than 1%. [Claim 22] A laminated polyester film according to any one of claims 1 to 12, wherein the weight retention rate measured by thermogravimetric differential thermal analysis (TG-DTA) at 500°C is 15% or more. [Claim 23] A laminated polyester film according to any one of claims 1 to 12, wherein a brass plate with a thickness of 1 mm and the polyester film are laminated together, and after being held in a 700°C atmosphere for 30 minutes, the surface of the brass plate does not conduct electricity as measured by a tester. [Claim 24] The laminated polyester film according to any one of claims 1 to 12, wherein the resin contained in the resin layer (a) is at least one selected from the group consisting of polyester resin, (meth)acrylic resin, and urethane resin. [Claim 25] A film laminate comprising a laminated polyester film according to any one of claims 1 to 12, and at least one of a functional layer and a metal layer provided on at least one surface side of the laminated polyester film. [Claim 26] The film laminate according to claim 25, wherein the functional layer is an adhesive layer. [Claim 27] The film laminate according to claim 25, comprising the aforementioned metal layer. [Claim 28] The film laminate according to claim 27, wherein the metal constituting the metal layer is at least one of copper and aluminum. [Claim 29] A laminated polyester film according to any one of claims 1 to 12, for use in battery protection or battery housing. [Claim 30] A battery protective material comprising a laminated polyester film according to any one of claims 1 to 12. [Claim 31] A battery comprising a laminated polyester film according to any one of claims 1 to 12. [Claim 32] An electronic device equipped with the battery described in claim 31. [Claim 33] A method of use in which the laminated polyester film according to any one of claims 1 to 12 is used as an insulating layer.

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