Polyester film and film laminate

Abstract

To provide a low-cost and versatile polyester film which contributes to mitigating thermal runaway reactions by increasing the ability to interrupt current during a short circuit through melting and carbonization.SOLUTION: A polyester film comprises inorganic particles and / or a flame retardant and a polyester resin. The polyester film comprises, as the flame retardant, a phosphorus-based flame retardant. The combined content of the inorganic particles and the flame retardant (calculated as phosphorus element) in the film is 0.8 mass% or more. The weight residue rate at 500°C as measured by thermogravimetric differential thermal analysis (TG-DTA) is 15.0% or more. The polyester film is capable of forming a carbonized layer.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a polyester film capable of forming a carbonized layer and a film laminate. [Background technology]

[0002] Background Art Polyester films have traditionally been used in a variety of applications, including packaging, electronic components, electrical insulation, metal lamination, display components such as flexible displays, touch panels, anti-reflection, and shatterproofing of glass, due to their transparency, dimensional stability, mechanical properties, heat resistance, and chemical resistance.

[0003] In recent years, electric vehicles have been attracting attention from the perspective of achieving carbon neutrality. To further improve the performance of electric vehicles, it is necessary to increase the energy density of the batteries used, and high energy density is essential to achieve a practical driving distance. However, batteries with high energy density are often prone to thermal runaway, and when thermal runaway occurs, the temperature of the battery cell rises rapidly and can reach over 400°C. In addition, when one cell experiences thermal runaway, there is a high possibility that adjacent cells will also experience thermal runaway in a chain reaction, posing a safety issue. Therefore, from the perspective of ensuring higher safety, there is a need for materials that can suppress thermal runaway even when used in high temperature ranges of around 500°C.

[0004] In addition, various measures have been taken to suppress thermal runaway. For example, Patent Document 1 proposes the installation of a plastic thermal runaway prevention wall between adjacent secondary batteries as a measure to prevent so-called consecutive explosions, in which multiple cells go out of control in succession. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4958409 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the thermal runaway prevention wall in Patent Document 1 has a complex structure in which the thermal runaway prevention wall is part of the heat conductive cylinder, and is therefore not very versatile. On the other hand, silicone-based resins or fluororesins are generally used as resin materials in high temperature ranges around 500°C. However, the surface of these resin sheets has issues such as being highly water-repellent, poor secondary processability (for example, adding functional layers by coating), and difficulty in making them into thin films, which has tended to limit the range of use.

[0007] Therefore, the present invention has been made in consideration of the above-mentioned situation, and an object of the present invention is to provide an inexpensive, versatile polyester film and film laminate that can contribute to suppressing thermal runaway reactions by improving the current interruption function during a short circuit caused by melting or carbonization. [Means for solving the problem]

[0008] As a result of intensive research, the present inventors have found that the above-mentioned problems can be solved by a specific configuration. The present invention has the following aspects [1] to

[15] .

[0009] [1] A polyester film capable of forming a carbonized layer, comprising inorganic particles and / or a flame retardant and a polyester resin, wherein the flame retardant is a phosphorus-based flame retardant, the total content of the inorganic particles and the flame retardant (calculated as phosphorus element amount) in the film is 0.8 mass% or more, and the weight residual rate measured by thermogravimetric differential thermal analysis (TG-DTA) at 500°C is 15.0% or more. [2] The polyester film according to the above [1], wherein the inorganic particles are at least one selected from the group consisting of titanium oxide, carbon black and silica. [3] The polyester film according to [1] or [2] above, wherein the polyester resin is polyethylene terephthalate and / or 2,6-polyethylene naphthalate. [4] The polyester film according to any one of the above [1] to [3], which is a biaxially stretched film. [5] The polyester film according to any one of the above [1] to [4], which, when laminated with a metal plate and the film, is held in an atmosphere of 500°C for 10 minutes, and then, when measured with a tester, shows that the surface of the metal plate is not conductive. [6] The polyester film according to any one of the above [1] to [5], which has a thickness of 25 μm or more and 125 μm or less. [7] A film laminate in which at least one surface of the polyester film according to any one of the above [1] to [6] is laminated to a metal layer directly or via another layer. [8] The film laminate according to [7] above, wherein the other layer is a functional layer. [9] The film laminate according to [8] above, wherein the functional layer is an adhesive layer.

[10] The film laminate according to any one of the above [7] to [9], wherein the metal layer is made of copper or aluminum.

[11] A method for using the polyester film according to any one of the above [1] to [6], wherein the carbonized layer is used as an insulating layer.

[12] A method for using the film laminate according to any one of the above [7] to

[10] , wherein the carbonized layer is used as an insulating layer.

[13] The polyester film according to any one of the above [1] to [6], which is for insulation purposes.

[14] The film laminate according to any one of the above [7] to

[10] , which is for insulation purposes.

[15] An electronic device equipped with a battery comprising, as an insulating layer, the polyester film according to any one of claims 1 to 6 or the film laminate according to any one of claims 7 to 10. Effect of the Invention

[0010] According to the present invention, it is possible to provide an inexpensive and versatile polyester film and film laminate which can contribute to suppressing thermal runaway reactions by improving the current interruption function during a short circuit caused by melting or carbonization. [Brief description of the drawings]

[0011] [Figure 1] 4 is a photograph showing the state of the polyester film produced in Example 2 after the insulation property was evaluated. [Diagram 2] 1 is a photograph showing the state of the polyester film produced in Comparative Example 1 after evaluation of its insulating properties. [Diagram 3] 1 is a photograph showing the state of the polyester film produced in Example 2 before the insulation property was evaluated. [Figure 4] FIG. 2 is a schematic diagram showing a method for evaluating insulation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Next, an example of an embodiment of the present invention will be described, however, the present invention is not limited to the embodiment described below.

[0013] <<Polyester film>> The polyester film of the present invention (hereinafter also referred to as "the film") has a weight residual rate (%) of 15.0% or more at 500° C. as measured by thermogravimetric differential thermal analysis (TG-DTA). When the weight residual rate is 15.0% or more, the polyester film can maintain a film state even if the polyester film itself is carbonized, and a carbonized layer is formed. That is, the polyester film of the present invention is a polyester film capable of forming a carbonized layer. The physical properties of the polyester film of the present invention will be described in detail later. The weight remaining rate can be measured by the method described in the Examples.

[0014] The present film is not particularly limited as long as it satisfies the above-mentioned requirements, and the present film may have a single-layer structure or a laminated (multi-layer) structure. When the present film has a laminated structure, the present film may have a two-layer structure, a three-layer structure, or may have four or more layers as long as it does not deviate from the gist of the present invention. The number of layers to be laminated is not particularly limited, but is preferably 10 layers or less. If the number is 10 layers or less, the thickness of each layer is sufficient, so that the lamination property during film formation is sufficient, flow marks and the like are unlikely to occur, and the quality of the film is sufficiently maintained. When the present film has a laminated structure of two or more layers, three layers of two kinds or three layers of three kinds are preferable, and three layers of two kinds are more preferable.

[0015] The present film may be a non-stretched film (sheet) or a stretched film. Of these, a stretched film stretched in a uniaxial or biaxial direction is preferable. Of these, a biaxially stretched film is more preferable in terms of excellent balance of mechanical properties and flatness.

[0016] The present film contains polyester as a main component resin, and when the present film has a laminated structure, it is preferable that the main component resin of each layer is polyester. The term "main component resin" refers to the resin that is most abundant among the resins constituting each layer, and is, for example, a 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.

[0017] <Polyester> The polyester used as the raw material of the present film is not particularly limited, and may be a homopolyester or a copolymer polyester. Specific examples include polyesters obtained by polycondensation of a dicarboxylic acid component and a diol component.

[0018] Examples of the dicarboxylic acid component 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-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic 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.

[0019] Examples of the diol component include ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, and bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or derivatives thereof, or ethylene oxide adducts thereof).

[0020] Representative examples of polyesters include polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, and polybutylene naphthalate.

[0021] Examples of the copolymer polyester include a copolymer polyester containing a third component as a copolymerization component other than the compound that is the main component of the dicarboxylic acid component constituting the polyester and the compound that is the main component of the diol component.

[0022] In particular, from the viewpoint of easily adjusting the weight residual ratio to a desired value, it is preferable that the present film contains polyethylene terephthalate (hereinafter also referred to as "PET") or 2,6-polyethylene naphthalate (hereinafter also referred to as "PEN") as the polyester. PET and PEN can be used either alone or in combination. The content of the polyester in the present film is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more (including 100% by mass) of the resin constituting the present film. When the present film has a laminated structure, it is preferable that the content of polyester in each layer satisfies the above range.

[0023] Specifically, the polyester contains a dicarboxylic acid component (a-1) and a diol component (a-2). More specifically, it is preferable that the dicarboxylic acid component (a-1) contains terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and the diol component (a-2) contains ethylene glycol.

[0024] The polyester preferably contains 80 mol % or more, more preferably 90 mol % or more, and even more preferably all (100 mol %) of the dicarboxylic acid component (a-1) is terephthalic acid and / or 2,6-naphthalenedicarboxylic acid. By making the content of terephthalic acid and / or 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid component (a-1) 80 mol % or more, the above weight residual rate can be maintained high.

[0025] The polyester 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 particularly preferably 90 mol% or more. On the other hand, the upper limit of 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 particularly preferably 95 mol% or less. By setting the content of ethylene glycol in the diol component (a-2) within this range, the above weight residual rate can be maintained high.

[0026] In addition, when polyester is produced (polycondensed) using ethylene glycol as one of the raw materials, diethylene glycol is usually 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 type of polycondensation, but is about 5 mol% or less of ethylene glycol. In the present invention, 5 mol% or less of diethylene glycol is considered to be by-product diethylene glycol, and the by-product diethylene glycol is also included in ethylene glycol and is distinguished from a copolymerization component. On the other hand, depending on the content of diethylene glycol, more specifically, when diethylene glycol is contained in excess of 5 mol%, diethylene glycol is treated as a copolymerization component rather than as a by-product diethylene glycol.

[0027] The polyester may contain a copolymerization component other than terephthalic acid and / or 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid component (a-1). The content of the copolymerization component is preferably 20 mol % or less, more preferably 10 mol % or less, and further preferably all of the dicarboxylic acid component (a-1) is terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and the other copolymerization components are 0 mol %. The polyester preferably contains 49 mol% or less of the copolymerization component 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, the lower limit of the copolymerization component 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. By setting the content of the copolymerization component in the dicarboxylic acid component (a-1) and / or the diol component (a-2) within this range, the above weight residual rate can be maintained high.

[0028] The copolymerization components added to the dicarboxylic acid component (a-1) are dicarboxylic acids other than terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, such as aromatic dicarboxylic acids such as isophthalic acid, orthophthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenone dicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenylether 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-furandicarboxylic acid, 2,4-furandicarboxylic acid, and 3,4-furandicarboxylic acid are preferred. These copolymerization components can be used alone or in combination of two or more.

[0029] Examples of copolymerization components added to the diol component (a-2) include diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or derivatives thereof, or ethylene oxide adducts thereof). From the viewpoint of maintaining film strength, bisphenols are more preferred, and as the bisphenols, it is preferable to use bisphenol A-ethylene oxide adducts. These copolymerization components can be used alone or in combination of two or more.

[0030] In addition, when the present film has a multilayer structure, the type and content of each component constituting the polyester contained in any layer, preferably each layer, may be the same as above, and the resin constituting each layer and the polyester in each layer may be the same as or different from each other.

[0031] <Polycondensation catalyst> The polycondensation catalyst used in polycondensing the polyester is not particularly limited, and any conventionally known compound can be used. Examples of the polycondensation catalyst include titanium compounds, germanium compounds, antimony compounds, manganese compounds, aluminum compounds, magnesium compounds, and calcium compounds.

[0032] The present film contains inorganic particles and / or a flame retardant. By containing inorganic particles and / or a flame retardant, a weight residual rate of 15.0% or more can be achieved. The inorganic particles and the flame retardant may be used alone or in combination. The inorganic particles and the flame retardant will be described in detail below.

[0033] <Inorganic particles> The film contains inorganic particles from the viewpoint of improving insulation. Examples of inorganic particles include silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, titanium oxide, and carbon black. Among these inorganic particles, silica, aluminum oxide, titanium oxide, and carbon black are preferred from the viewpoint of improving insulation, and silica, titanium oxide, and carbon black are particularly preferred. The inorganic particles can be used alone or in combination of two or more kinds. Thus, the preferred particles of silica, titanium oxide, and carbon black can be used in combination.

[0034] In addition to the inorganic particles, conventional particles may be added to impart slipperiness to the polyester film, prevent scratches during each process, and improve handleability. These particles are not particularly limited as long as they are particles that can impart slipperiness, and specific examples include the above-mentioned inorganic particles, as well as crosslinked polymers such as crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked styrene-acrylic resin particles, and crosslinked polyester particles, and organic particles such as calcium oxalate and ion exchange resins. Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the polyester production process can also be used.

[0035] (Particle shape) The shape of the particles used in the present film is not particularly limited, and any of spherical, block, rod-like, flat, etc. may be used. There is also no particular limitation on the hardness, specific gravity, color, etc. Two or more kinds of these particles may be used in combination as necessary. The particles referred to here include both inorganic particles for improving the insulating properties of the present film and particles for improving the lubricity of the present film.

[0036] (average particle size) The average particle size of the particles used in the present film is usually 0.05 μm to 5.0 μm, preferably 0.10 μm to 4.5 μm, more preferably 0.20 μm to 4.5 μm, and particularly preferably 0.40 μm to 4.5 μm. By using particles in the above range, the desired insulating properties of the present film can be ensured. In addition, when the particles are powder, the average particle size of the particles can be the particle size (d50) at an accumulated volume fraction of 50% in the equivalent sphericity distribution measured using a centrifugal sedimentation type particle size distribution measuring device (e.g., Shimadzu Corporation's "SA-CP3 type"). The average particle size of the particles in the film, layer, or resin can be determined by observing 10 or more particles with a scanning electron microscope (SEM) to measure the diameters of the particles and calculating the average value. In this case, in the case of non-spherical particles, the average value of the longest diameter and the shortest diameter can be measured as the diameter of each particle.

[0037] When particles are incorporated into the present film, it is preferable to, for example, provide a surface layer and an intermediate layer and incorporate particles into the surface layer. In addition, when the film has a three-type, three-layer structure with different front and back designs, it is also possible to incorporate particles into at least one of the surface layers. The content of the particles depends on the average particle size, but is preferably 0.5% by mass or more in the layer containing the particles. More preferably, it is 0.7% by mass or more, and particularly preferably 0.9% by mass or more. On the other hand, the upper limit is 1.5% by mass or less, more preferably 1.3% by mass or less, and particularly preferably 1.0% by mass or less, in terms of film handling. If it is within the above range, the present film can exhibit good insulation properties even when used in a high-temperature atmosphere (for example, 500°C).

[0038] The method of adding particles to the present film is not particularly limited, and any conventionally known method can be used. For example, the particles can be added at any stage of the polyester production, but it is preferable to add the particles after the esterification or transesterification reaction is completed.

[0039] <Flame retardants> In the present invention, the flame retardant includes a phosphorus-based flame retardant. As the phosphorus-based flame retardant, an organic phosphorus-based flame retardant compound is preferably used. The structure of the organic phosphorus-based flame retardant compound is not particularly limited, but examples thereof include carboxymethylphenyl phosphate, (2-carboxyethyl)phenyl phosphate, (2-carboxyethyl)toluyl phosphate, (2-carboxyethyl)2,5-dimethylphenyl phosphate, (2-carboxyethyl)cyclohexyl phosphate, (carboxypropyl)phenyl phosphate, (4-carboxyphenyl)phenyl phosphate, (3-carboxyphenyl)phenyl phosphate, (2-carboxyethyl)methyl phosphate, (2-carboxyethyl)ethyl phosphate, triphenyl phosphate, tributyl phosphate, t-butyldiphenyl phosphate, tris(2-ethylhexyl)phosphate, bisphenol A bis(diphenyl phosphate)-1,3-phenylbis(diphenyl phosphate), phosphonitrilic acid diphenyl ester, or a compound shown in the following formula (1).

[0040] [ka]

[0041] In the above formula (1), A is a divalent or trivalent organic residue, and preferred examples thereof include lower 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 divalent groups such as 1,3-xylylene and 1,4-xylylene.

[0042] Specific examples of the trivalent organic residue include the following.

[0043] [ka]

[0044] In the above formula (1), Q is a hydrocarbon group having 1 to 18 carbon atoms, and examples of such groups include an alkyl group, a cycloalkyl group, an aryl group, an alkoxy group, and an aryloxy group. Z is an ester-forming functional group, specific examples of which include a carboxy group, an alkyl ester of a carboxy group having 1 to 6 carbon atoms, a cycloalkyl ester, an aryl ester, a hydroxy group, and a hydroxylalkoxycarbonyl group having 2 to 7 carbon atoms.

[0045] As mentioned above, the structure of the organic phosphorus-based flame retardant compound used in the present invention is not limited, but it is preferable to use a glycol-free polycondensate of 2-(9,10-dihydro-9-oxa-10-oxide-10-phosphaphenanthren-10-yl)methyl succinate bis-(2-hydroxyethyl) (the following formula (2)). This organic phosphorus-based compound contains a phosphorus atom in the molecule, and the lower limit of the average molecular weight measured by GPC is 1170, preferably 2290 or more, and more preferably 3410 or more. When the average molecular weight is 1170 or more, the organic phosphorus-based compound does not volatilize during film formation and the crystallization of the polyester resin is not inhibited, and further, the bleed-out of the organic phosphorus-based compound is suppressed, so that the mechanical strength of the film is maintained. In addition, the upper limit of the average molecular weight of the organic phosphorus-based compound is not particularly specified, but it is considered that if the molecular weight is excessively increased, the dispersibility in the polyester resin is deteriorated.

[0046] [ka]

[0047] When producing a flame-retardant polyester, the method for adding the organic phosphorus-based flame-retardant compound to a polyester production system is not particularly limited. For example, when a polyester is produced by a so-called transesterification method between a dicarboxylic acid diester and a diol, the organic phosphorus-based flame retardant compound may be added during the transesterification reaction, or may be added before the polycondensation reaction after the transesterification reaction or at a relatively early stage of the polycondensation. Also, when a polyester is produced by an esterification method between a dicarboxylic acid and a diol, the organic phosphorus-based flame retardant compound may be added at any esterification stage.

[0048] The content of the organic phosphorus-based flame-retardant compound in the polyester film can be estimated by converting it into the amount of phosphorus element, and is preferably 0.2 mass% to 3.5 mass%, more preferably 0.5 mass% to 3.0 mass%, and particularly preferably 1.0 mass% to 3.0 mass%. By satisfying the above range, good flame retardancy can be imparted.

[0049] <Total content of inorganic particles and flame retardants> The total content of inorganic particles and flame retardant (in terms of elemental phosphorus) in the polyester film (hereinafter, sometimes simply referred to as "total content") is 0.8% by mass or more. If the total content is less than 0.8% by mass, it is difficult to achieve a weight residual rate of 15.0% or more. From the above viewpoints, the total content is preferably 1.0% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2.0% by mass or more. Furthermore, there is no particular upper limit to the total content, but from the viewpoint of fully exerting the effects of the resin components such as polyester resin, the total content is preferably 20 mass% or less, and more preferably 16 mass% or less.

[0050] <Other> In order to suppress the amount of precipitation of oligomer components, the film may be produced using a polyester having a low content of oligomer components as the raw material. As a method for producing a polyester having a low content of oligomer components, various known methods can be used, such as a method of performing solid phase polymerization after the production of a polyester. The amount of precipitation of oligomer components may be suppressed by forming the present film into a three or more layer structure and forming the surface layer of the present film from a polyester raw material having a low content of oligomer components. Furthermore, the polyester may be obtained by carrying out an esterification or transesterification reaction, followed by melt polycondensation at a higher reaction temperature under reduced pressure.

[0051] In addition to the above-mentioned particles, conventionally known ultraviolet absorbents, antioxidants, antistatic agents, heat stabilizers, lubricants, dyes, pigments, etc. may be added to the present film as required.

[0052] The present film may contain resins other than polyester as long as the effects of the present invention are not impaired. Examples of 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, polyamideimide resins, polyamide bismaleimide resins, polyetherimide resins, polyetheretherketone resins, polyetherketone resins, polyethersulfone resins, polyketone resins, polysulfone resins, aramid resins, and fluorine-based resins.

[0053] The thickness of the present film is preferably from 25 μm to 125 μm, more preferably from 38 μm to 125 μm, further preferably from 38 μm to 100 μm, and particularly preferably from 50 μm to 100 μm. By setting the thickness within the above range, the film itself can be used as an insulating layer after carbonization. The thickness of this film was measured at five random points on the surface with a 1 / 1000 mm dial gauge, and the average was used as the thickness.

[0054] <Production method of polyester film> Next, a production example of the present film will be specifically described, but the production example is not limited to the following production example. For example, when producing a biaxially stretched film, a method is preferred in which the dried polyester pellets described above are extruded as a molten sheet from a die using a melt extrusion device such as an extruder, and then cooled and solidified with a cooling roll such as a rotating cooling drum to obtain an unstretched sheet. Here, the cooling is preferably performed to a temperature below the glass transition point of the polymer, for example, to obtain an unoriented sheet (unstretched sheet) in a substantially amorphous state. In addition, in order to improve the flatness of the sheet, it is preferable to increase the adhesion between the sheet and the cooling roll, and an electrostatic application adhesion method and / or a liquid application adhesion method are preferably used.

[0055] Next, the obtained unstretched sheet is stretched in biaxial directions. In this case, the unstretched sheet is first stretched in one direction by a roll or tenter type stretching machine. The stretching temperature is usually 70 to 120°C, preferably 80 to 110°C, and the stretching ratio is usually 2.5 to 7.0 times, preferably 3.0 to 6.0 times.

[0056] Then, the film is stretched in a direction perpendicular to the first-stage stretching direction. In this case, the stretching temperature is usually 70 to 170° C., and the stretch ratio is usually 3.0 to 7.0 times, preferably 3.5 to 6.0 times.

[0057] Then, the film is heat-treated under tension or relaxation of 30% or less at a temperature of 180 to 270° C. to obtain a biaxially stretched film. This heat treatment is also called a heat setting step. The heat treatment may be performed in two or more steps with different temperatures. After the heat treatment, the film may be cooled in a cooling zone. The cooling temperature is preferably higher than the glass transition temperature (Tg) of the polyester constituting the film, and more specifically, is preferably in the range of 100 to 160° C. This cooling may be performed in two or more steps at different temperatures. In the above stretching, a method of stretching in one direction in two or more stages can be adopted. In that case, it is preferable to perform the stretching so that the final stretch ratios in both directions are each within the above range.

[0058] The present film can also be produced by a simultaneous biaxial stretching method, which involves simultaneously stretching and orienting the unstretched sheet in the machine direction (longitudinal direction) and width direction (transverse direction) under a temperature controlled condition usually at 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, the film is subsequently heat-treated under tension or under relaxation of 30% or less at a temperature of 170 to 250° C. to obtain a stretched and oriented film. Regarding the simultaneous biaxial stretching device employing the above-mentioned stretching method, a conventionally known stretching method such as a screw method, a pantograph method, or a linear drive method can be employed.

[0059] The polyester film may be provided with a resin layer in order to impart various functions such as easy adhesion, antistatic properties, and antiblocking properties.

[0060] When the resin layer is provided by in-line coating, it is preferable to prepare the coating solution by preparing each component of the coating solution as an aqueous solution or water dispersion, and coating the coating solution adjusted to a solid content concentration of about 0.1 to 50 mass% on the polyester film. In addition, the coating solution may contain a small amount of organic solvent for the purpose of improving dispersibility in water, improving film-forming properties, etc., within a range that does not impair the gist of the present invention. Only one type of organic solvent may be used, or two or more types may be used as appropriate.

[0061] The thickness of the resin layer provided on the polyester film is usually in the range of 0.003 to 1 μm, preferably 0.005 to 0.5 μm, and more preferably 0.01 to 0.2 μm. By satisfying the above range, the coating appearance is good and the desired functions can be exhibited.

[0062] The resin layer can be formed by any of the conventional 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, calendar coating, and extrusion coating.

[0063] The drying and curing conditions when forming a resin layer on a polyester film are not particularly limited. For example, when forming a resin layer by offline coating, the heat treatment is usually performed at 80 to 200°C for 3 to 40 seconds, preferably at 100 to 180°C for 3 to 40 seconds. On the other hand, when the coating layer is formed by in-line coating, it is usually preferable to carry out heat treatment at 70 to 270° C. for 3 to 200 seconds as a guideline.

[0064] Regardless of whether off-line coating or in-line coating is used, heat treatment and irradiation with active energy rays such as ultraviolet light may be used in combination, if necessary. The polyester film may be previously subjected to a surface treatment such as a corona treatment or a plasma treatment.

[0065] <Physical properties of polyester film> (Weight remaining rate) The weight residual rate of this film measured by thermogravimetric differential thermal analysis (TG-DTA) is 15.0% or more. By satisfying this range, even if the film itself is carbonized, a carbonized layer is formed and the film state can be maintained. In other words, the carbonized layer acts as an insulating layer, improving the current interruption function in the event of a short circuit caused by melting or carbonization. From this viewpoint, the weight residual rate is preferably 20% or more, more preferably 25% or more, and even more preferably 28% or more.

[0066] The weight residual rate can be adjusted by changing the type of polyester constituting the present film, the content of the particles, and the like. The weight remaining rate can be measured by the method described in the Examples.

[0067] (Insulating) It is preferable that the present film, in a laminated state with a metal plate, is held for 10 minutes in an atmosphere at 500° C., and then the surface of the metal plate is not electrically conductive when measured with a tester. The insulating property can be evaluated by the method described in the Examples.

[0068] <<Film laminate>> The film laminate of the present invention is obtained by laminating at least one surface of the present film to a metal layer directly or via another layer. As for the other layers, functional layers are preferred, and among them, an adhesive layer is particularly preferred. The film laminate of the present invention can also be used in the form of an adhesive tape having an adhesive layer on a polyester film.

[0069] <Metal layer> The metal forming the metal layer is not particularly limited as long as it is a metal having electrical conductivity, and examples thereof include aluminum, nickel, gold, silver, copper, cadmium, titanium, etc. Among them, from the viewpoint of versatility, the metal layer is preferably made of copper or aluminum. Here, "made of" means that the metal layer contains copper or aluminum as a main component. The metal layer may contain an element other than a metal having electrical conductivity.

[0070] The metal layer may be in the form of a foil, or may be provided by any of vapor deposition, plating, and sputtering. More specifically, a conventionally known method such as a vacuum vapor deposition method, an electroplating method, and a sputtering method can be used.

[0071] <Functional layer> Examples of the functional layer in the film laminate of the present invention include an easy-adhesion layer, an antistatic layer, and an adhesive layer. Among these, an adhesive layer is preferred since it can be used in the form of an adhesive tape having an adhesive layer on a film.

[0072] <Adhesive layer> The adhesive layer is obtained by curing an adhesive composition. The adhesive composition may be an acrylic adhesive composition containing an acrylic resin as a main component resin, a rubber-based adhesive composition containing rubber as a main component, a urethane-based adhesive composition containing a urethane resin as a main component, or a silicone-based adhesive composition containing a silicone resin as a main component. Among these, acrylic pressure-sensitive adhesive compositions containing an acrylic resin as a main component resin are preferred, because they allow adjustment of a good balance between adhesive strength and peel strength and are inexpensive.

[0073] The above-mentioned "main component resin" means a resin having the highest mass ratio among the resins constituting the adhesive composition. For example, it means a component that occupies 50 mass% or more, preferably 60 mass% or more, more preferably 70 mass% or more of the total amount of resins constituting the adhesive composition. The upper limit is 100 mass%, but is usually 99.99 mass%.

[0074] The adhesive composition may contain, other than the main component, a crosslinking agent described below, and, if necessary, a resin (e.g., an acrylic resin, a rubber, a silicone resin, a urethane resin) that constitutes an adhesive component other than the main component resin. In addition, additives such as tackifiers such as rosin, rosin esters, hydrogenated rosin esters, phenolic resins, aromatic modified terpene resins, aliphatic petroleum resins, alicyclic petroleum resins, styrene resins, and xylene resins, silane coupling agents, antistatic agents, colorants, fillers, antioxidants, ultraviolet absorbers, and functional dyes, as well as additives such as compounds that change color or undergo color change when exposed to ultraviolet light or radiation, can be blended. The blending amount of these additives is preferably 10% by mass or less of the entire pressure-sensitive adhesive composition (based on non-volatile components), and more preferably 5% by mass or less. The additives are generally low-molecular components with a molecular weight of less than 10,000, and it is preferable to avoid these low-molecular components as much as possible in terms of excellent durability.

[0075] (Acrylic resin) An example of an acrylic resin suitable as the main component resin of the pressure-sensitive adhesive composition is a (meth)acrylic polymer. The (meth)acrylic polymer is a polymer having a (meth)acrylic acid alkyl ester as a main constituent unit. Examples of (meth)acrylic acid alkyl 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 alone or in combination of two or more. Among these, methyl (meth)acrylate is preferred from the viewpoints of compatibility with other (meth)acrylates constituting the (meth)acrylic polymer and heat resistance of the cured resin layer. The content of the (meth)acrylic acid alkyl ester in the monomers forming the (meth)acrylic polymer is, for example, 50% by mass or more, 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. The (meth)acrylic polymer may have a radically polymerizable double bond.

[0076] (Crosslinking agent) The pressure-sensitive adhesive composition may contain a crosslinking agent depending on the curing method. Examples of the crosslinking agent include an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, an aziridine-based crosslinking agent, a melamine-based crosslinking agent, an aldehyde-based crosslinking agent, an amine-based crosslinking agent, etc. Among them, an isocyanate-based crosslinking agent is preferably used in terms of improving adhesion to the substrate or reactivity with the acrylic resin. The crosslinking agents may be used alone or in combination of two or more kinds.

[0077] The content of the crosslinking agent is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 5 parts by mass, and particularly preferably 0.1 to 3 parts by mass, relative to 100 parts by mass of the main component resin. If the content of the crosslinking agent is within the above range, the cohesive strength is not insufficient and the desired durability can be obtained, while a decrease in flexibility and adhesive strength can be prevented.

[0078] <<How to use>> A preferred method of using the present polyester film or film laminate (hereinafter sometimes referred to as "the present polyester film, etc.") is to use the carbonized layer in the present polyester film, etc. as an insulating layer. This polyester film is capable of forming a carbonized layer, and during normal use, it can be used as a polyester film, etc. At this stage, no carbonized layer exists. On the other hand, when the polyester film etc. is used in, for example, a battery with high energy density, thermal runaway may occur, causing the temperature of the battery cell to rise rapidly and reach 400° C. or more. Even in such a case, i.e., when a short circuit occurs due to melting or carbonization, the film etc. forms a carbonized layer and functions as an insulating film, improving the current interruption function and contributing to suppression of the thermal runaway reaction. In addition, the term "battery" as used in this invention refers to secondary batteries such as lithium ion batteries, lithium ion polymer batteries, nickel-metal hydride batteries, lithium-sulfur batteries, nickel-cadmium batteries, nickel-iron batteries, nickel-zinc batteries, sodium-sulfur batteries, lead-acid batteries, and air batteries.

[0079] <<Applications>> As described above, the polyester film of the present invention is a polyester film capable of forming a carbonized layer, and therefore is suitable as an insulating film. In addition, a film laminate in which the polyester film of the present invention is combined with a metal layer is also useful for insulating purposes. That is, by using a film laminate in which the polyester film of the present invention is combined with a metal layer, it is possible to suppress a thermal runaway reaction caused by a short circuit. In particular, the film laminate is suitable as an insulating layer for batteries such as lithium batteries and alkaline batteries, electric double layer capacitors, and capacitors, which require higher energy density. Incidentally, various electronic devices equipped with a battery having the polyester film or the like of the present invention as an insulating layer are also included within the scope of the present invention. Specific examples of electronic devices include automobiles, electric bicycles, mobile phones, digital cameras, video cameras, electronic notebooks, personal computers, radios, music players, storage medium recorders, game consoles, televisions, printers, and vacuum cleaners.

[0080] <<Explanation of terms>> In the present invention, the term "film" includes the term "sheet", and the term "sheet" includes the term "film". In the present invention, when it is described as "X to Y" (X and Y are any numbers), unless otherwise specified, it includes the meaning of "X or more and Y or less", as well as "preferably larger than X" or "preferably smaller than Y". In addition, when it is stated that the amount is "X or more" (X is any number), it also means that the amount is "preferably greater than X" unless otherwise specified, and when it is stated that the amount is "Y or less" (Y is any number), it also means that the amount is "preferably smaller than Y" unless otherwise specified. EXAMPLES

[0081] The present invention will now be described in further detail with reference to examples. However, the present invention is not limited to the following examples as long as it does not deviate from the gist of the present invention.

[0082] <Evaluation method> (1) Intrinsic viscosity (IV) 1 g of polyester from which components incompatible with the polyester had been removed was precisely weighed out, dissolved in 100 mL of a mixed solvent of phenol / tetrachloroethane = 50 / 50 (mass ratio), and the viscosity was measured at 30 °C using a viscosity measuring device "VMS-022UPC·F10" (manufactured by Rigo Co., Ltd.).

[0083] (2) Average particle size (In the case of powder) The particle size at an accumulated volume fraction of 50% in the equivalent sphericity distribution measured using a centrifugal sedimentation type particle size distribution analyzer (SA-CP3 type) manufactured by Shimadzu Corporation was taken as the average particle size d50. (For particles in films, layers or resins) Ten or more particles were observed with a scanning electron microscope (SEM) to measure the diameters of the particles, and the average value was taken as the average particle size of the particles. In this case, in the case of non-spherical particles, the average value of the longest and shortest diameters was taken as the diameter of each particle.

[0084] (3) Weight Remaining Rate Using a Shimadzu TG-DTA device (model: DTG60), the temperature was raised from 25°C to 500°C at a rate of 10°C / min in a nitrogen atmosphere in accordance with JIS K7121 (2012). Then, the sample was held at 500°C for 10 minutes in a nitrogen atmosphere. From the above measurements, the 5% weight loss temperature (°C) and the weight remaining rate (%) were obtained.

[0085] (4) Insulation The measurement method will be explained with reference to FIG. The sample film 1 and metal plate 2 (made of brass, 50 x 50 mm x 5 mm thick) were placed in that order on a metal block 3 (made of brass, 50 x 50 mm x 20 mm thick) into which a rod heater (Watlow Fire Rod Heater G1N) had been inserted, and the surface of the metal plate 2 was heated from room temperature to 500°C (at a heating rate of 10°C / min) by heating the rod heater, and then held at 500°C for 10 minutes. After that, it was naturally cooled to room temperature, and the resistance between the surface of the metal plate 2 and the metal block 3 was measured using a tester 4 (manufactured by Hioki E.E. Corporation: Model IR4054) under an applied voltage of 125 V, and the insulation (presence or absence of conductivity) was evaluated according to the following criteria. (Judgment criteria) ◎: Particularly good insulation properties. ◯: Good insulation. ×: Poor insulation. (Conduction is clearly observed)

[0086] <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: 0.85 dL / g), dicarboxylic acid component (a-1): terephthalic acid = 100 mol%, diol component (a-2): ethylene glycol = 100 mol% (4) Polyester D: homopolyethylene terephthalate (intrinsic viscosity: 1.10 dL / g), dicarboxylic acid component (a-1): terephthalic acid = 100 mol%, diol component (a-2): ethylene glycol = 100 mol% (5) Polyester E: 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% (6) Polyester F: A masterbatch in which homopolyethylene terephthalate (polyester A) is blended with 0.7% by mass of silica particles having an average particle size of 2.7 μm (intrinsic viscosity: 0.59 dL / g) (7) Polyester G: A masterbatch in which homopolyethylene terephthalate (polyester A) is blended with 1.0% by mass of silica particles having an average particle size of 3.2 μm (intrinsic viscosity: 0.64 dL / g) (8) Polyester H: A masterbatch in which homopolyethylene terephthalate (polyester A) is blended with 3.2% by mass of silica particles having an average particle size of 3.2 μm (intrinsic viscosity: 0.62 dL / g) (9) Polyester I: A masterbatch in which homopolyethylene terephthalate (polyester A) is blended with 3.5% by mass of silica particles having an average particle size of 4.1 μm (intrinsic viscosity: 0.70 dL / g) (10) Polyester J: A masterbatch in which homopolyethylene terephthalate (polyester A) is blended with 15.0% by mass of silica particles having an average particle size of 4.1 μm (intrinsic viscosity: 0.48 dL / g) (11) Polyester K: A masterbatch in which homopolyethylene terephthalate (polyester A) is blended with 35% by mass of a phosphorus-based flame retardant having the following structure (3% by mass in terms of phosphorus element amount) (intrinsic viscosity: 0.48 dL / g)

[0087] [ka]

[0088] In the above formula (3), n≧4. The flame retardant of the above formula (3) was obtained by the production method described in

[0054] to

[0058] of JP2015-81271A.

[0089] (12) Polyester L: A masterbatch of homopolyethylene terephthalate blended with 20% by mass of carbon black (oil furnace black) having an average particle size of 70 nm (intrinsic viscosity: 0.60 dL / g) (13) Polyester M: A masterbatch of homopolyethylene terephthalate containing 50% by mass of titanium oxide particles having an average particle size of 0.3 μm (intrinsic viscosity: 0.49 dL / g) (14) Polyester N: A masterbatch containing homopolyethylene terephthalate and 1.5% by mass of fluorescent whitening agent (4,4'-Bis(2-benzoxazolyl)stilbene) (intrinsic viscosity: 0.64 dL / g)

[0090] Example 1 A raw material blend of 48.8% Polyester E, 2.5% Polyester H, 44.7% Polyester K, and 4.0% Polyester L by mass was fed into a vented extruder and melt-extruded at 290°C. The material was then cooled and solidified on a cooling roll whose surface temperature was set at 40°C using an electrostatic adhesion method to obtain an amorphous film. The unstretched sheet was then stretched 3.0 times in the machine direction (MD) at 85°C using a roll stretching machine. A resin composition having the following composition was then applied to the sheet so that the thickness after drying was 0.02 μm, and the sheet was preheated at 125°C in a tenter, and then stretched 4.0 times in the transverse direction (TD) at 130°C. Finally, the sheet was heat-set at 215°C to obtain a biaxially stretched polyester film having a resin layer and a thickness of 50 μm. The evaluation results are shown in Table 1.

[0091] (Resin layer composition) The following compounds O to Q were mixed so that the solid content ratio was O / P / Q=80:10:10 (mass %) to obtain a resin layer composition. (15) Compound O: A water dispersion of a polyester resin copolymerized with the following composition (Acid components) Terephthalic acid / Isophthalic acid / 5-Sodium sulfoisophthalic acid / (Diol components) Ethylene glycol / 1,4-butanediol / Diethylene glycol = 56 / 40 / 4 / / 70 / 20 / 10 (16) Compound P: A water dispersion of a polyurethane resin copolymerized with the following composition A water dispersion obtained by neutralizing a polyurethane resin consisting of isophorone diisocyanate / polyhexamethylene carbonate polyol / polyoxytetramethylene glycol / dimethylolpropanoic acid = 15 / 78 / 5 / 2 (mol%) with triethylamine. (17) Compound Q: Hexamethoxymethylolmelamine

[0092] (Examples 2 to 11, Comparative Example 1) A polyester film was obtained by producing in the same manner as in Example 1, except that the raw material composition, film thickness, and the presence or absence of a resin layer were different from those in Example 1.

[0093] [Table 1]

[0094] As can be seen from the results in Table 1, the polyester films of Examples 1 to 11 had a residual weight rate of 15.0% or more as determined by thermogravimetric differential thermal analysis (TG-DTA), which demonstrated that they had good insulating properties. It has been discovered that the polyester film of the present invention uses a biaxially oriented polyester film containing a specific amount of inorganic particles and / or flame retardant at 500°C, a temperature range where the polyester resin layer would normally melt and vaporize without remaining, so that the film (resin layer) does not completely vaporize and some of it remains as a carbonized layer. Fig. 1 is a photograph showing the state of the polyester film produced in Example 2 after the insulation evaluation, while Fig. 3 is a photograph showing the state before the insulation evaluation. It can be seen that the film was carbonized by the heat treatment for the insulation evaluation, and a part of it remains as a carbonized layer. The present invention has been completed based on a completely new idea that has not been seen before, that is, to carbonize a polyester film and use it as a carbonized layer. Therefore, the melting and carbonization can further improve the current interruption function during a short circuit, which can contribute to suppressing a thermal runaway reaction. Therefore, the polyester film of the present invention can be suitably used in various applications in which insulation properties are particularly required.

[0095] On the other hand, the polyester film of Comparative Example 1 had a residual weight rate of less than 15.0% as measured by thermogravimetric differential thermal analysis (TG-DTA), and the polyester film was completely vaporized in an atmosphere of 500°C, causing the metal plate surface to become conductive and resulting in insufficient insulation. 2 is a photograph showing the state after the insulation evaluation of the polyester film produced in Comparative Example 1. It is clear that the polyester film has insufficient insulation because no carbonized layer is formed. [Industrial Applicability]

[0096] The polyester film of the present invention is excellent in that it can contribute to suppressing thermal runaway reactions by improving the current interruption function during a short circuit due to melting and carbonization, and even during high-temperature heat treatment (e.g., 500°C), the film does not completely vaporize, but melts and carbonizes, and a portion of it remains as a carbonized layer that acts as an insulating layer, so that even when in contact with a metal layer, it does not become conductive and can maintain its insulation. In addition, by using a film laminate in which the polyester film of the present invention is combined with a metal layer, it is possible to suppress a thermal runaway reaction caused by a short circuit, and the film laminate has a high industrial value. In particular, the film laminate is suitable as an insulating layer for batteries such as lithium batteries and alkaline batteries, electric double layer capacitors, and capacitors, which require a higher energy density. In addition, various electronic devices equipped with a battery having, as an insulating layer, a carbonized layer formed from the polyester film of the present invention are also included in the scope of the present invention. Specific examples of electronic devices include automobiles, electric bicycles, mobile phones, digital cameras, video cameras, electronic notebooks, personal computers, radios, music players, storage medium recorders, game consoles, televisions, printers, and vacuum cleaners. [Explanation of symbols]

[0097] 1. Polyester film 2 metal plate 3 Metal Blocks 4. Tester

Claims

1. The polyester film is capable of forming a carbonized layer, and comprises inorganic particles and / or a flame retardant and a polyester resin, and contains a phosphorus-based flame retardant as the flame retardant, the total content of the inorganic particles and the flame retardant (calculated as the amount of phosphorus element) in the film is 0.8 mass% or more, and the weight residual rate measured by thermogravimetric differential thermal analysis (TG-DTA) at 500°C is 15.0% or more.

2. 2. The polyester film according to claim 1, wherein the inorganic particles are at least one selected from the group consisting of titanium oxide, carbon black and silica.

3. 2. The polyester film according to claim 1, wherein the polyester resin is polyethylene terephthalate and / or 2,6-polyethylene naphthalate.

4. 10. The polyester film of claim 1 which is a biaxially oriented film.

5. 2. The polyester film according to claim 1, which, when laminated on a metal plate and the film, is held in an atmosphere at 500° C. for 10 minutes, is not electrically conductive on the surface of the metal plate when measured with a tester.

6. 2. The polyester film according to claim 1, having a thickness of 25 μm or more and 125 μm or less.

7. 10. A film laminate, comprising the polyester film according to claim 1, at least one surface of which is laminated to a metal layer directly or via another layer.

8. The film laminate according to claim 7 , wherein the other layer is a functional layer.

9. The film laminate according to claim 8 , wherein the functional layer is an adhesive layer.

10. 8. The film laminate of claim 7, wherein the metal layer comprises copper or aluminum.

11. A method for using the polyester film according to any one of claims 1 to 6, which comprises using the carbonized layer according to claim 1 as an insulating layer.

12. A method for using the film laminate according to any one of claims 7 to 10, wherein the carbonized layer according to claim 1 is used as an insulating layer.

13. The polyester film according to any one of claims 1 to 6, which is used for insulation.

14. The film laminate according to any one of claims 7 to 10, which is for insulation.

15. 10. An electronic device equipped with a battery comprising the polyester film according to claim 1 or the film laminate according to claim 7 as an insulating layer.

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