Polyester film and production method therefor

Abstract

A polyester film containing a polybutylene terephthalate and a polyethylene terephthalate, wherein the absolute value of the difference between the degree of lateral orientation (Δnv) and the degree of longitudinal orientation (Δnp) of the polyester film is 57 or less.

Description

Polyester film and its manufacturing method

[0001] The present invention relates to a shaping polyester film, a method for producing the same, and a laminate and an exterior material comprising the shaping polyester film.

[0002] Conventionally, polyester films, particularly polyethylene terephthalate films and polyethylene naphthalate films, have excellent mechanical properties, heat resistance, and chemical resistance, and have been used in a variety of applications, such as packaging, electronic components, electrical insulation, metal lamination, optical applications such as display components for foldable displays, bendable displays, and rollable displays, touch panels, anti-reflection applications, and glass shatter prevention applications (see, for example, Patent Documents 1 and 2).

[0003] In these applications, polyester films are often molded and processed rather than used as is, and many of these products place emphasis on mechanical properties. Therefore, polyester films alone cannot meet the required quality requirements, and many products are composited with other layers (e.g., resin layers, metal layers, etc.). For example, in lithium-ion batteries, exterior packaging materials are used as packaging materials to seal electrodes, electrolytes, etc. Traditionally, metal exterior packaging materials have been widely used. Meanwhile, with the recent advances in performance of electric vehicles, hybrid electric vehicles, personal computers, cameras, mobile phones, etc., lithium-ion batteries are required to have a variety of shapes, as well as to be thinner and lighter. However, traditionally widely used metal exterior packaging materials have had problems with being unable to keep up with the increasing variety of shapes and also have limitations on how much they can be made lighter.

[0004] For this reason, in recent years, bags (pouches) made of a laminate in which a plastic film is laminated on a metal foil such as aluminum foil have come into use as packaging materials that can be easily processed into a variety of shapes and can be made thinner and lighter. In pouch-shaped packaging materials, a recess is generally formed by cold-forming the laminate, and electricity storage device elements such as electrodes and electrolyte are placed in the space formed by the recess, and the laminates are heat-sealed to each other to obtain a lithium-ion battery or the like in which the electricity storage device elements are housed inside the packaging material.

[0005] Polyamide is widely used as the substrate of the laminate because of its excellent formability. In addition to formability, moisture resistance, sealing properties, and chemical resistance may be required, and a structure consisting of a polyester film / polyamide film / metal foil / sealant film in this order from the outer layer is also being considered.

[0006] In addition, in recent years, electronic devices have been required to be smaller and thinner, and sharper corners of pouches have been considered to efficiently install lithium-ion batteries together with printed circuit boards and other components. Furthermore, in order to further increase the capacity and energy density of batteries per volume by increasing the content volume per pouch, there has been increasing emphasis in recent years on the development of pouch films with excellent formability that allow for the formation of deeper recesses during cold forming, such as deep drawing. For example, Patent Documents 3 and 4 discuss the use of polyester films primarily composed of polybutylene terephthalate, which has good formability, as battery exterior materials. Patent Documents 5 to 7 also discuss the use of polyester films containing polybutylene terephthalate as battery exterior materials.

[0007] JP 2006-341546 A JP 2004-122767 A JP 2012-077292 A WO 2014 / 017457 A JP 2017-177412 A JP 2022-056851 A WO 2017 / 057773

[0008] However, the polyester films described in Patent Documents 1 and 2 are not intended for use as battery exterior packaging. The polybutylene terephthalate disclosed in Patent Document 5 is a polyester-based elastomer, and various mechanical properties may be insufficient. When attempting to manufacture exterior packaging materials using laminates containing polyester films primarily composed of polybutylene terephthalate as disclosed in Patent Documents 3, 4, and 6, the laminates exhibit excellent elongation and good moldability, but suffer from poor puncture resistance. Specifically, there is a large variation in the amount of displacement upon puncture and in the puncture strength, making them unsuitable for use as exterior packaging materials. Furthermore, even Patent Document 7 fails to adequately prevent such variation, and there is a risk that mechanical strength, such as puncture resistance, may not be sufficiently improved.

[0009] Therefore, an object of the present invention is to provide a polyester film for forming that has good puncture resistance.

[0010] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by incorporating polyethylene terephthalate and polybutylene terephthalate into a moldable polyester film and setting the absolute value of the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) of the polyester film to a certain value or less, and have completed the following invention. That is, the present invention provides the following [1] to

[22] .

[0011] [1] A polyester film for extrusion molding containing polybutylene terephthalate and polyethylene terephthalate, wherein the absolute value of the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) of the polyester film is 57 or less. [2] The polyester film for extrusion molding according to [1] above, wherein the ratio of the polyethylene terephthalate content to the polybutylene terephthalate content (PET / PBT) is 55 / 45 or more and 95 / 5 or less by mass. [3] The polyester film for extrusion molding according to [1] or [2] above, wherein the puncture strength is 480 N / mm or more. [4] The polyester film for extrusion molding according to any of [1] to [3] above, wherein the displacement upon puncture is 4 mm or more. [5] The polyester film for extrusion molding according to any of [1] to [4] above, wherein the puncture strength variation is 0.036 or less. [6] The polyester film for extrusion molding according to any one of [1] to [5] above, having a variation in displacement upon piercing of 0.034 or less. [7] The polyester film for extrusion molding according to any one of [1] to [6] above, having a planar orientation degree of 100 or more and 200 or less. [8] The polyester film for extrusion molding according to any one of [1] to [7] above, having a heat shrinkage rate of less than 5% in one of the longitudinal and transverse directions of the polyester film and a heat shrinkage rate of more than 2% in the other direction after heat treatment at 160°C for 15 minutes. [9] The polyester film for extrusion molding according to any one of [1] to [8] above, having a haze of 10% or less.

[10] The polyester film for extrusion molding according to any one of [1] to [9] above, having a melting point (1st run) of 230°C or more.

[11] The polyester film for extrusion molding according to any one of [1] to

[10] above, wherein the intrinsic viscosity of the polyester film is 0.6 dL / g or more and 0.85 dL / g or less.

[12] The polyester film for extrusion molding according to any one of [1] to

[11] above, wherein the polyester film is a biaxially stretched polyester film.

[13] A method for producing the polyester film for extrusion molding according to any one of [1] to

[12] above, comprising a step of stretching in the longitudinal direction and the transverse direction, wherein the preheating temperature during transverse stretching is 60°C to 90°C and the stretching temperature is 70°C to 120°C.

[14] A method for producing the polyester film for extrusion molding according to

[13] above, wherein the heat setting temperature after the stretching is 190°C or higher.

[15] A method for producing the polyester film for extrusion molding according to

[13] or

[14] above, wherein the ratio of the transverse stretching ratio to the longitudinal stretching ratio (TD / MD) is 1 to 1.7.

[16] A method for producing the polyester film for extrusion molding according to any one of

[13] to

[15] above, wherein the stretching temperature during longitudinal stretching is 50°C to 85°C.

[17] A method for producing the polyester film for extrusion molding according to any one of

[13] to

[16] above, wherein the longitudinal stretching ratio is 2.5 to 4.5.

[18] A method for producing the polyester film for extrusion molding according to any one of

[13] to

[17] above, wherein the transverse stretching ratio is 3.5 times or more and 5.5 times or less.

[19] A laminate comprising the polyester film for extrusion molding according to any one of [1] to

[12] above, and at least one of a resin layer and a metal layer.

[20] An exterior packaging material comprising the polyester film according to any one of [1] to

[12] above or the laminate according to

[19] above.

[21] The exterior packaging material according to

[20] above, which is an exterior packaging material for a battery.

[22] A battery comprising the exterior packaging material according to

[20] or

[21] above.

[0012] According to the present invention, a polyester film for extrusion molding having good puncture resistance can be provided.

[0013] [Polyester film for molding] The polyester film for molding of the present invention (hereinafter sometimes simply referred to as "the polyester film") contains polybutylene terephthalate (hereinafter sometimes referred to as "PBT") and polyethylene terephthalate (hereinafter sometimes referred to as "PET").

[0014] (PBT) The PBT constituting the present polyester film is a polycondensate of a dicarboxylic acid component and a diol component, and is a polyester containing terephthalic acid as the dicarboxylic acid component and 1,4-butanediol as the diol component. The PBT is primarily composed of terephthalic acid and 1,4-butanediol, and preferably contains 50 mol% or more of terephthalic acid as the dicarboxylic acid component and 50 mol% or more of 1,4-butanediol (BDO) as the diol component. The proportion of terephthalic acid in the dicarboxylic acid component of the PBT is more preferably 70 mol% or more, even more preferably 90 mol% or more, and most preferably 100 mol%. Furthermore, the proportion of 1,4-butanediol in the diol component of the PBT is more preferably 70 mol% or more, even more preferably 90 mol% or more, and most preferably 100 mol%. Therefore, homo-PBT is most preferred as the PBT. By increasing the proportion of terephthalic acid and BDO in PBT, the crystallinity of the polyester film is increased, and the degree of planar orientation is easily increased.

[0015] In PBT, the dicarboxylic acid other than terephthalic acid is not particularly limited, and examples thereof include aromatic dicarboxylic acids such as phthalic acid, isophthalic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-benzophenonedicarboxylic acid, 4,4'-diphenoxyethanedicarboxylic acid, 4,4'-diphenylsulfonedicarboxylic acid, and 2,6-naphthalenedicarboxylic acid, alicyclic dicarboxylic acids such as 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid, and aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. These dicarboxylic acid components other than terephthalic acid may be used alone or in combination of two or more.

[0016] In the present invention, the diol component other than BDO is not particularly limited, and examples thereof include aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 1,8-octanediol, and dibutylene glycol, alicyclic diols such as 1,2-cyclohexanediol, 1,4-cyclohexanediol, 1,1-cyclohexanedimethylol, and 1,4-cyclohexanedimethylol, and aromatic diols such as xylylene glycol, 4,4'-dihydroxybiphenyl, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone. These diol components other than BDO may also be used alone or in combination of two or more.

[0017] Furthermore, in PBT, one or more of the following can be used as copolymerization components: hydroxycarboxylic acids such as lactic acid, glycolic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, 6-hydroxy-2-naphthalenecarboxylic acid, and p-β-hydroxyethoxybenzoic acid; monofunctional components such as alkoxycarboxylic acids, stearyl alcohol, benzyl alcohol, stearic acid, benzoic acid, t-butylbenzoic acid, and benzoylbenzoic acid; and trifunctional or higher polyfunctional components such as tricarballylic acid, trimellitic acid, trimesic acid, pyromellitic acid, gallic acid, trimethylolethane, trimethylolpropane, glycerol, and pentaerythritol.

[0018] The intrinsic viscosity of PBT is not particularly limited, but is preferably 0.7 dL / g to 1.4 dL / g, more preferably 0.73 dL / g to 1.37 dL / g, even more preferably 0.77 dL / g to 1.33 dL / g, and even more preferably 0.8 dL / g to 1.3 dL / g. Setting the intrinsic viscosity of PBT within the above range facilitates improving the heat resistance, productivity, and film-forming properties of the polyester film. The intrinsic viscosity of PBT can be appropriately set within the above range and may be, for example, but not limited to, 0.8 dL / g to 1.2 dL / g, 0.8 dL / g to 1.1 dL / g, or 0.8 dL / g to 1 dL / g.

[0019] In addition, when two or more types of PBT or the like having different intrinsic viscosities are used, the intrinsic viscosity refers to the intrinsic viscosity of the mixed polyester. The intrinsic viscosity can be measured by a conventional method, for example, using an Uperohde viscometer at 30°C using a phenol:tetrachloroethane=1:1 solvent.

[0020] (PET) The PET constituting the present film is a polycondensate of a dicarboxylic acid component and a diol component, and is a polyester containing terephthalic acid as the dicarboxylic acid component and ethylene glycol as the diol component. PET is primarily composed of terephthalic acid and ethylene glycol, and preferably contains 50 mol% or more of terephthalic acid as the dicarboxylic acid component and 50 mol% or more of ethylene glycol as the diol component. The proportion of terephthalic acid in the dicarboxylic acid component of PET is more preferably 70 mol% or more, even more preferably 90 mol% or more, and most preferably 100 mol%. Furthermore, the proportion of ethylene glycol in the diol component is more preferably 70 mol% or more, even more preferably 90 mol% or more, and most preferably 100 mol%. Therefore, homo-PET is most preferred. By increasing the proportions of terephthalic acid and ethylene glycol in PET, the crystallinity of the present polyester film is increased, making it easier to increase the degree of planar orientation, etc. In addition, it becomes easier to impart a certain level of mechanical strength, and it becomes easier to increase the puncture resistance, Martens hardness, indentation hardness, elastic deformation power, and the like.

[0021] In PET, there are no particular limitations on the dicarboxylic acid other than terephthalic acid, and examples of dicarboxylic acids other than terephthalic acid are as listed above for PBT. In PET, the dicarboxylic acid other than terephthalic acid may be used alone or in combination of two or more. Furthermore, there are no particular limitations on the diol component other than ethylene glycol, and specific examples include 1,4-butanediol and the diols other than ethylene glycol listed above for PBT. Furthermore, in PET, one or more of hydroxycarboxylic acids, monofunctional components, trifunctional or higher polyfunctional components, etc. may be used as copolymer components, and specific examples of these are as listed above for PBT.

[0022] The intrinsic viscosity of PET is not particularly limited, but is preferably 0.5 dL / g to 1 dL / g, more preferably 0.53 dL / g to 0.96 dL / g, even more preferably 0.57 dL / g to 0.93 dL / g, and even more preferably 0.6 dL / g to 0.9 dL / g. Setting the intrinsic viscosity of PET within the above range facilitates improving the heat resistance, productivity, and film-forming properties of the polyester film. The intrinsic viscosity of PET can be appropriately set within the above range and may be, for example, but not limited to, 0.6 dL / g to 0.8 dL / g or 0.6 dL / g to 0.75 dL / g.

[0023] (Mass Ratio of PBT to PET) In the present polyester film, the ratio of the content of polyethylene terephthalate to polybutylene terephthalate (PET / PBT) is preferably 55 / 45 or more and 95 / 5 or less by mass. By setting the mass ratio to 55 / 45 or more, the mechanical strength of the present polyester film is increased, and puncture resistance is further improved. Furthermore, heat resistance is also easily improved. Furthermore, by setting the mass ratio to 95 / 5 or less, elongation is easily ensured, and the amount of displacement upon puncture, as described below, is also easily increased. The mass ratio is more preferably 60 / 40 or more, even more preferably 63 / 37 or more, even more preferably 65 / 35 or more, particularly preferably 72 / 28 or more, and more preferably 90 / 10 or less, even more preferably 85 / 15 or less, and even more preferably 82 / 18 or less.

[0024] The polyester film may contain a resin other than the above-mentioned PET and PBT, as long as the effect of the present invention is not impaired. Examples of the resin other than the above-mentioned PET and PBT include polyester resins other than PET and PBT and resins other than polyester resins. Examples of resins other than polyester resins include polystyrene-based resins, polyvinyl chloride-based resins, polyvinylidene chloride-based resins, chlorinated polyethylene-based resins, polycarbonate-based resins, polyamide-based resins, polyacetal-based resins, acrylic-based resins, ethylene-vinyl acetate copolymers, polymethylpentene-based resins, polyvinyl alcohol-based resins, cyclic olefin-based resins, polylactic acid-based resins, polybutylene succinate-based resins, polyacrylonitrile-based resins, polyethylene oxide-based resins, cellulose-based resins, polyimide-based resins, polyurethane-based resins, polyphenylene sulfide-based resins, polyphenylene ether-based resins, polyvinyl acetal-based resins, polybutadiene-based resins, polybutene-based resins, polyamide-imide-based resins, polyamide bismaleimide-based resins, polyetherimide-based resins, polyether ether ketone-based resins, polyether ketone-based resins, polyethersulfone-based resins, polyketone-based resins, polysulfone-based resins, aramid-based resins, and fluorine-based resins.

[0025] In the present polyester film, PBT and PET are preferably the main components, and specifically, the total amount of PBT and PET is preferably 50% by mass or more, preferably 70% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, based on the total amount of the present polyester film. There is no particular upper limit for the total amount of PBT and PET, as long as it is 100% by mass or less, but from the viewpoint of blending additives, etc., it is preferable that the total amount of PBT and PET is less than 100% by mass, based on the total amount of the present polyester film.

[0026] The thickness of the polyester film is not particularly limited as long as it is within a range that allows film formation, but is, for example, in the range of 6 μm to 90 μm, preferably 8 μm to 70 μm, more preferably 9 μm to 50 μm, and even more preferably 10 μm to 40 μm. The polyester film is preferably a biaxially stretched polyester film. By forming the polyester film into a biaxially stretched polyester film, it becomes easier to achieve a certain degree of planar orientation, as described below. In addition, it becomes easier to adjust |Δnv−Δnp| within the desired range.

[0027] The polyester film may have a single layer structure (single layer film) or a multilayer structure (multilayer film) having two or more layers. In the case of a multilayer structure, it may have a surface layer and a core layer. The surface layer is a layer that constitutes one surface of the polyester film, and the core layer is a layer disposed inside the surface layer. The polyester film may have a surface layer that constitutes the other surface of the polyester film in addition to the surface layer and core layer that constitute one surface. In the case of a multilayer structure, the polyester film may have a two-layer structure of surface layer / core layer, a three-layer structure of surface layer / core layer / surface layer, or a layer structure of four or more layers, but a three-layer structure of surface layer / core layer / surface layer is preferred. In a layer structure of four or more layers, it is preferred that the core layer has two or more layers.

[0028] In the case of a multilayer structure, each layer may contain PET and PBT, and the ratio of the PET to PBT content in each layer may be as described above. The ratio of the PET to PBT content in some of the layers may be as described above, or the ratio of the PET to PBT content in all of the layers may be as described above. The ratio of the PET to PBT content in the entire multilayer structure may be as described above. Furthermore, each layer may contain a resin other than PET and PBT as described above, and the total content of PET and PBT in each layer may be as described above based on each layer instead of the total content of the polyester film.

[0029] However, as described below, for example, the core layer may contain PET and PBT, while the surface layer may contain PET but not PBT. Furthermore, in the case of a multilayer structure, the material composition (e.g., resin composition) of one surface layer may be different from the material composition of at least one of the other layers. For example, the material composition of the surface layer may be different from the material composition of the core layer. Specifically, the PET and PBT contents in each layer may be adjusted so that the contents at the film surface and the center in the film thickness direction are as shown in a preferred embodiment described below. Furthermore, for example, it is preferable that the PET content in at least one surface layer is higher than the PBT content in at least one of the other layers, based on the content ratio. In particular, it is preferable that the PET content in at least one surface layer (hereinafter, content (A1)) is higher than the PET content (A2) in the core layer. Note that "different material compositions" means that the material compositions are not the same or are not substantially the same. "Not substantially identical" means that, when comparing the components constituting each material, the difference in the content of each component exceeds ±1% by mass, and / or, if the materials contain components that are different from each other, the total content of the different components is 2% by mass or more relative to the total material.

[0030] The thickness of the surface layer is preferably smaller than that of the core layer. The thickness of each surface layer is preferably 1% to 20% of the total thickness of the polyester film, more preferably 2% to 15%, and even more preferably 4% to 12%. If the thickness of the surface layer is within this range, when the surface layer contains particles as described below, the particles can impart lubricity without impairing transparency, and problems such as particle shedding are less likely to occur. From the same perspective, the thickness of each surface layer is preferably 0.2 μm to 10 μm, more preferably 0.4 μm to 7.5 μm, even more preferably 0.8 μm to 6 μm, and even more preferably 1 μm to 4 μm.

[0031] The core layer is preferably a layer that constitutes the central portion of the polyester film in the thickness direction. The thickness of the core layer is preferably 65% ​​to 98% of the total thickness of the polyester film, more preferably 70% to 96%, and even more preferably 75% to 94%. The thickness of the core layer is preferably 7 μm to 88 μm, more preferably 10 μm to 70 μm, and even more preferably 15 μm to 55 μm. The thickness ratio of each surface layer to the core layer is, for example, 1 / 99 to 40 / 60, preferably 2 / 98 to 35 / 65, preferably 4 / 96 to 30 / 70, and even more preferably 6 / 94 to 25 / 75. Furthermore, when the present polyester film has a three-layer structure of a surface layer, a core layer, and another surface layer, the thickness ratio (surface layer thickness: core layer thickness: surface layer thickness) is preferably 1-20:65-98:1-20, more preferably 2-15:70-96:2-15, and even more preferably 3-12.5:75-94:3-12.5. The thickness of the core layer referred to here means the total thickness of the core layers when the core layer has two or more layers.

[0032] [Content of Each Resin on the Film Surface and Center] A preferred embodiment of the content of each resin on the film surface and center is described below. In a preferred embodiment, the present polyester film has a PET content (hereinafter also referred to as "content (A1)") on at least one surface of the polyester film, which is greater than the PET content (hereinafter also referred to as "content (A2)") in the center of the film thickness direction, based on mass percentage. When the present polyester film contains both PBT and PET and the PET content (A1) on the surface is increased, the puncture strength, Martens hardness, indentation hardness, etc. are increased, resulting in excellent pinhole resistance and resistance to fracture when subjected to external force. When used as a battery exterior material, the battery is less likely to break even if damaged due to an accident, etc. Furthermore, the high Martens hardness and indentation hardness reduce swelling of the battery when heated, preventing contact with the battery casing, for example, outside the battery exterior material. Furthermore, when the present polyester film contains both PBT and PET and the PET content (A1) at the surface is greater than the PET content (A2), the elastic deformation power can also be increased. A high value of the elastic deformation power means that a molded article can easily return to its original shape even if it is deformed. This makes it easier for the shape of the molded article to be maintained during molding, resulting in a molded article with excellent design and shape recovery, and reducing molding defects. Furthermore, even when an external force such as puncture is applied, the original shape is easily restored when the external force is released, thereby more effectively suppressing deformation of the molded article. It is sufficient for the PET content (A1) at one surface to be greater than the PET content (A2) at the center of the film. However, it is preferable for the PET content (A1) at both surfaces to be greater than the PET content (A2).

[0033] In a preferred embodiment, the PET content (A1) on at least one surface of the polyester film is, for example, 70% by mass or more, preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 89% by mass or more, and even more preferably 92% by mass or more. The PET content (A1) may be 100% by mass or less. However, when PBT is contained on one surface, for example, it is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 97% by mass or less. The PET content (A1) on the surface having a higher PET content than the above-mentioned content (A2) may be at least the lower limit and at most the upper limit as described above. However, it is preferred that the PET content (A1) on both surfaces be at least the lower limit and at most the upper limit as described above. In a preferred embodiment, the PET content (A2) in the central portion of the polyester film in the thickness direction may be, for example, 50% by mass or more, but is preferably 55% by mass or more, more preferably 60% by mass or more, even more preferably 63% by mass or more, still more preferably 65% ​​by mass or more, and particularly preferably 72% by mass or more, and is preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less, and still more preferably 82% by mass or less.

[0034] In a preferred embodiment, the PET content (A1) on at least one surface is greater than the PET content (A2) in the central region, but the difference (A1-A2) between the PET content (A1) and the PET content (A2) is preferably within a certain range, specifically, from 3% to 35% by mass. By maintaining the difference in content within a certain range, the mechanical strength of the polyester film is improved, and the puncture resistance, Martens hardness, indentation hardness, and elastic deformation power are likely to be further improved. The difference in content (A1-A2) is more preferably 5% by mass or more, even more preferably 8% by mass or more, even more preferably 10% by mass or more, and more preferably 30% by mass or less, even more preferably 25% by mass or less, and even more preferably 22% by mass or less.

[0035] In a preferred embodiment, the polyester film has a mass ratio (B1 / A1) of the PBT content (B1) to the PET content (A1) on at least one surface of the film. This mass ratio (B1 / A1) is preferably 20 / 80 or less, more preferably 15 / 85 or less, even more preferably 10 / 90 or less, and even more preferably 8 / 92 or less. By maintaining the mass ratio (B1 / A1) at or below the above-mentioned certain value, the Martens hardness, indentation hardness, elastic deformation power, and the like are easily increased. The polyester film does not need to contain PBT on at least one surface; therefore, the mass ratio (B1 / A1) may be 0 / 100 or more. However, the polyester film may contain PBT on at least one surface. In this case, the mass ratio (B1 / A1) is preferably 1 / 99 or more, more preferably 2 / 98 or more, and even more preferably 3 / 97 or more. The content ratio (B1 / A1) may be equal to or less than the upper limit value and equal to or more than the lower limit value on the surface where the content (A1) is greater than the content (A2), but it is preferable that the content ratio (B1 / A1) on both surfaces is equal to or less than the upper limit value and equal to or more than the lower limit value.

[0036] In a preferred embodiment, the polyester film has a mass ratio (B2 / A2) of the PBT content (B2) to the PET content (A2) at the center of the film thickness direction, of 5 / 95 or more and 45 / 55 or less. A mass ratio of 45 / 55 or less enhances the mechanical strength of the polyester film, making it easier to improve pinhole resistance, breaking strength, and heat resistance. Furthermore, a mass ratio of 5 / 95 or more improves elongation and shape recovery. The ratio (B2 / A2) is more preferably 40 / 60 or less, even more preferably 35 / 65 or less, even more preferably 30 / 70 or less, more preferably 10 / 90 or more, even more preferably 15 / 85 or more, and even more preferably 18 / 82 or more.

[0037] In this specification, the PET content (A1) and PBT content (B1) in one surface refer to the PET and PBT contents in the surface layer of a multilayer film. However, in the present polyester film, even within the same layer, as in the monolayer film having a concentration gradient described below, the blending may vary along the thickness direction near the surface, resulting in changes in the PET content and PBT content. When the PET content varies near the surface, for example, the PET content and PBT content in the region from the film surface to 5% of the total thickness may be referred to as the content (A1) and content (B1), respectively. Furthermore, the PET content (A2) and PBT content (B2) in the thickness direction center may refer to the PET content and PBT content, respectively, in the core layer of a multilayer film. However, when the PET content and the PBT content vary in the thickness direction of the core layer or the central part of the film in the thickness direction, such as when the composition of the core layer varies in the thickness direction of a multilayer film or when the composition varies in the thickness direction of a monolayer film, the PET content (A2) and the PBT content (B2) are defined as the PET content and the PBT content, respectively, in a region that is 10% of the total thickness and centered on the central part of the film in the thickness direction.

[0038] When the present film is, for example, a monolayer film, the PET and PBT may have a concentration gradient in the thickness direction. By providing a concentration gradient, the PET contents (A1) and (A2) and the PBT contents (B1) and (B2) may be adjusted to fall within the ranges specified in the above preferred embodiment. Specifically, the PET content may have a concentration gradient that decreases from one surface of the present film toward the center in the thickness direction. It is also preferable that the PET content decreases from one surface of the present film toward the center in the thickness direction, and then increases again toward the other surface. That is, for PET, a region with a high content (first region) and a region with a low content (second region) may be provided from one surface to the other surface, but it is preferable that the region with a high content (first region), a region with a low content (second region), and a region with a high content (first region) are provided in this order. On the other hand, the PBT content may have a concentration gradient that increases from one surface of the present film toward the center in the thickness direction. Furthermore, it is preferable that the PBT content of the present film increases from one surface toward the center and then increases again toward the other surface. That is, with regard to PBT, it is sufficient to have a region with a low content (first region) and a region with a high content (second region) from one surface toward the other, but it is preferable that the region with a low content (first region), the region with a high content (second region), and the region with a low content (first region) are provided in this order. Note that with regard to PBT, the region with a low content is a concept that also includes a region that does not contain PBT.

[0039] (Particles) The polyester film may contain particles. By containing particles, the polyester film can be imparted with properties such as easy slippage, thereby improving the handleability of the film. The particles are not particularly limited, but examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, 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. Of these, silica and aluminum oxide are preferred, and silica is more preferred.

[0040] In consideration of both the transparency and ease of handling of the film, the average particle size of the particles is usually in the range of 0.05 μm to 10 μm, preferably 0.1 μm to 6 μm, more preferably 0.3 μm to 5 μm, and even more preferably 0.6 μm to 4.5 μm. The average particle size of the particles is the particle size at an integrated volume fraction of 50% (d50) in an equivalent sphericity distribution measured using a centrifugal sedimentation particle size distribution analyzer.

[0041] Particles may be incorporated throughout the polyester film, but are preferably incorporated on at least one surface of the polyester film. Therefore, in the multilayer structure described above, particles may be incorporated into at least one surface layer. However, when surface layers are present on both surfaces of the film, it is more preferable to incorporate particles into both surface layers. The particle content is not particularly limited, but is typically less than 5% by mass, preferably 0.0003% to 3% by mass, more preferably 0.001% to 2.5% by mass, and even more preferably 0.01% to 1% by mass. When particles are not incorporated or when the particle content is low, the film exhibits excellent transparency. On the other hand, when particles are incorporated within the above range, the film exhibits improved slipperiness and improved handleability. Even when particles are incorporated, the film's transparency can be sufficiently ensured as long as the particle content is less than 5% by mass. The particle content should be within the above range at least in the region from the surface of the film to a depth of 1 μm in the thickness direction. When the film has a multilayer structure, the particle content in the surface layer should be within the above range. When the film has a single-layer structure, the particle content relative to the entire film should be within the above range. When the film has a multilayer structure, the particle content relative to the entire film is preferably 0.0001% by mass or more and 2% by mass or less, more preferably 0.0005% by mass or more and 1.5% by mass or less, even more preferably 0.001% by mass or more and 1% by mass or less, and even more preferably 0.005% by mass or more and 0.7% by mass or less. When particles are present on the surface of the polyester film, the amount of particles present is not particularly limited, but is preferably about 3 mg / m ２ More preferably, 4 mg / m ２ 15mg / m or more ２ More preferably, 5 mg / m ２ 14mg / m or more ２ The particles present on the surface of the polyester film may be particles exuded from the resin constituting the polyester film, or particles coated on the surface of the polyester film.

[0042] In addition to the above-mentioned particles, the polyester film may contain conventionally known antioxidants, ultraviolet absorbers, antistatic agents, heat stabilizers, lubricants, colorants such as dyes and pigments, etc., as needed.

[0043] One or both surfaces of the present polyester film may be appropriately surface-treated. The surface treatment may be a known modification treatment performed on the surface of a resin film, such as a corona treatment or a plasma treatment. The functional layer described below may be formed on the surface of the present polyester film that has been surface-treated, or on the surface of the present polyester film that has not been surface-treated. However, the surface treatment is preferably performed on the surface on which the functional layer is not to be formed. For example, it is also preferable that the present polyester film has a functional layer on one surface and a corona-treated surface on the other surface on which no functional layer is to be formed.

[0044] <Functional Layer> The present polyester film may have a functional layer such as an easy-adhesion layer, a release layer, an antistatic layer, a coating layer such as an antiblocking layer, a hard coat layer, an inorganic vapor deposition layer, or a printed layer on at least one surface, as long as the effects of the present invention are not impaired. Among these, a coating layer is preferred, and an easy-adhesion layer is more preferred. The easy-adhesion layer is a layer provided to adhere other layers or films to the polyester film, and is not particularly limited, and may be formed from a resin such as a polyurethane resin, a vinyl resin, a polyamide resin, a polyester resin, an acrylic resin, or a polyvinyl acetal resin. The resin in the easy-adhesion layer may be appropriately blended with additives such as various crosslinking agents and particles. When the present polyester film has a functional layer such as functional layer X, the various physical properties of the polyester film described below can be determined by measuring the present polyester film having the functional layer.

[0045] The functional layer has a mass per unit area of ​​1 mg / m ２ 1000mg / m or more ２When the mass per unit area is within the above range, the surface free energy and / or the water droplet contact angle can be easily adjusted to desired values ​​when the functional layer is made into a functional layer X described later without making the functional layer thicker than necessary. The mass per unit area of ​​the functional layer is 5 mg / m or less. ２ 500mg / m or more ２ More preferably, 10 mg / m or less ２ 300mg / m or more ２ More preferably, 15 mg / m ２ 150mg / m or more ２ The following is even more preferable. The mass per unit area of ​​the functional layer can be determined from the amount of nonvolatile components applied when forming the resin composition for forming the functional layer, and when drying and stretching are performed, it is the mass per unit area of ​​the functional layer after drying and stretching. Furthermore, when functional layers are provided on both sides of the present film, the mass per unit area of ​​the functional layer is the mass per unit area of ​​the functional layer provided on each surface of the present film.

[0046] The thickness of the functional layer is, for example, 0.001 μm to 1 μm, preferably 0.005 μm to 0.5 μm, 0.01 μm to 0.4 μm, preferably 0.01 μm to 0.3 μm, and more preferably 0.01 μm to 0.2 μm. In order to improve the slip property, the above-mentioned particles or the like may be contained as necessary.

[0047] (Functional Layer X) In one embodiment of the present invention, the functional layer is preferably a functional layer having a surface free energy of 49 mN / m or less, or a water droplet contact angle of 63° or more. For example, when forming an exterior material such as a pouch, the polyester film may have insufficient releasability from a mold such as a die during extrusion molding, resulting in deformation of the molded product (battery exterior material) obtained upon demolding, and thickness unevenness may also occur. When the surface free energy of the functional layer is below a certain value or the water droplet contact angle is above a certain value, the coefficient of friction (dynamic friction coefficient and static friction coefficient) is low. Therefore, when the polyester film is placed in a mold such as a die, the film slides appropriately, making it easier to follow the mold, and uniform pressure is easily applied during extrusion molding, making it easier for the entire film to stretch uniformly. As a result, the molded product is less likely to deform, and it is easier to obtain a molded product with little thickness variation.

[0048] A functional layer having the above-described surface free energy and / or water droplet contact angle is generally also called a release layer. Hereinafter, the functional layer having the above-described surface free energy and / or water droplet contact angle will be referred to as functional layer X. From the viewpoint of reliably reducing the coefficient of friction, it is more preferable that the surface free energy of the functional layer surface of functional layer X is 49 mN / m or less and the water droplet contact angle is 63° or more.

[0049] From the viewpoint of reducing the coefficient of friction, the surface free energy of the functional layer X surface is more preferably 40 mN / m or less, even more preferably 30 mN / m or less, and even more preferably 27 mN / m or less. The surface free energy of the functional layer X surface is not particularly limited in terms of its lower limit, but from the viewpoint of ease of obtaining it by adjusting the composition of the functional layer X, it is, for example, 5 mN / m or more, preferably 10 mN / m or more, more preferably 15 mN / m or more. From the viewpoint of reducing the coefficient of friction, the water droplet contact angle of the functional layer X surface is more preferably 75° or more, even more preferably 90° or more, and even more preferably 100° or more. The surface free energy of the functional layer X surface is not particularly limited in terms of its upper limit, and is, for example, 140° or less. However, from the viewpoint of ease of obtaining it by adjusting the composition of the functional layer X, it is preferably 130° or less, more preferably 120° or less.

[0050] In the present invention, the water droplet contact angle and surface free energy are measured as follows. [Method for measuring water droplet contact angle] The contact angle is measured using a contact angle meter when 1 μL of pure water is dropped onto the surface of the functional layer of a film that has been conditioned for 24 hours or more in an environment of 23° C. and 50% RH. The contact angle measured 60 seconds after the drop onto the film is used. As the contact angle meter, for example, a contact angle meter (DMo-501 model) manufactured by Kyowa Interface Science Co., Ltd. may be used.

[0051] [Method for Measuring Surface Free Energy] Using a contact angle meter, the contact angle when 1 μL of pure water and methylene iodide is dropped onto the functional layer surface of a film that has been conditioned for 24 hours or more in an environment of 23°C and 50% RH is measured. The contact angle is measured 60 seconds after each liquid is dropped onto the film. For the contact angle measurement, a contact angle meter (DMo-501 model) manufactured by Kyowa Interface Science Co., Ltd. may be used, for example. Using the obtained contact angle and the surface tension component values ​​of each liquid shown in Table 1 below, the surface free energy of the functional layer surface of the film is calculated using the Owens-Wendt-Rable-Kaelble (OWRK) theoretical formula.

[0052]

[0053] Surface free energy is composed of the sum of intermolecular force components. Intermolecular forces are classified into dispersion force, orientation force, induction force, and hydrogen bond force, and each of these constitutes surface free energy as a dispersion component (Dispersion), polar component (Polar), induction component (Induction), and hydrogen bond component (Hydrogen). Of these components, the induction component is very weak and can be ignored, and the hydrogen bond component can be lumped together with the polar component.

[0054] In this specification, the surface free energy γ SV Each component (variance component γ SV d and polar component γ SV p ) is a value determined by the following measurement and calculation methods. LV1 , γ LV1 d and γ LV1 p The contact angle (θ1) between the first liquid and the surface to be measured is known, and the following γ LV2 , γ LV2 d and γ LV2 p The contact angle (θ2) between the second liquid, whose value is known, and the surface to be measured is measured. Next, these values ​​are substituted into the following equations (I-1) and (I-2), and the surface free energy γ of the functional layer surface to be measured is calculated from the simultaneous equations (I-1) and (I-2) below. SV The variance component of γ SV d and the polar component γ SV p In both cases, the unit is mN / m.

[0055] (γ SV d ・γ LV1 d ) 1 / 2 + (γ SV p ・γ LV1 p ) 1 / 2 = γ LV1 (1+cosθ1) / 2...(I-1) (γ SV d ・γ LV2d ) 1 / 2 + (γ SV p ・γ LV2 p ) 1 / 2 = γ LV2 (1+cosθ2) / 2...(I-2)

[0056] gamma SV d : Surface free energy γ of the surface to be measured SV Variance component of γ SV p : Surface free energy γ of the surface to be measured SV The polar component of γ LV1 : surface tension of the first liquid γ LV2 : surface tension of the second liquid θ1: contact angle of the first liquid θ2: contact angle of the second liquid γ LV1 d : dispersion component of the surface tension of the first liquid γ LV1 p : polar component of the surface tension of the first liquid γ LV2 d : dispersion component of the surface tension of the second liquid γ LV2 p : polar component of the surface tension of the second liquid

[0057] The above formulas (I-1) and (I-2) are derived from the following Owens-Wendt-Rable-Kaelble (OWRK) theoretical formula and the following Young's formula: OWRK theoretical formula: γ SL = γ SV +γ LV -2 (γ SV d ・γ LV d ) 1 / 2 -2 (γ SV p ・γ LV p ) 1 / 2 Young's formula: γ SV = γ SL +γ LV cosθ (where γ SL is the interfacial tension between the surface of the object to be measured and the liquid.)

[0058] There are no particular limitations on the method for adjusting the surface free energy and water droplet contact angle of the surface of the functional layer X within the above ranges. For example, it is effective to adjust them by adjusting the composition and thickness of the functional layer X. In adjusting the composition of the functional layer X, it is effective to appropriately adjust the types and content ratios of commonly used release agents, crosslinking agents, binder resins, etc. Selecting the type of release agent and the type of crosslinking agent and adjusting their content ratios is particularly effective. Furthermore, the presence of hydrophilic groups that form weak bonds with water molecules, such as hydroxyl groups, carboxyl groups, amide groups, and thiol groups, tends to increase the surface free energy or decrease the water droplet contact angle. Therefore, appropriately adjusting the content ratio of such hydrophilic groups also makes it easier to adjust the surface free energy or water droplet contact angle within the desired range. Other methods include a method of adjusting by physically changing the surface microstructure, a method of adjusting by introducing functional groups onto the surface through physicochemical treatment such as heat, light, electromagnetic treatment, oxidation, reduction, etc., and a method of adjusting by chemical treatment using a surface treatment agent such as a surfactant, a coupling agent, etc.

[0059] When a functional layer X is provided, the functional layer X may be provided on one surface or both surfaces of the polyester film. However, from the viewpoint of more appropriately exhibiting the effects of providing the functional layer X, it is preferable that the functional layer X be provided on at least one surface where the content (A1) is higher than the content (A2). As described above, the surface of the polyester film having the functional layer X has a low coefficient of friction when the surface free energy is a certain value or less and / or the water droplet contact angle is a certain value or more. The dynamic friction coefficient of the surface of the polyester film having the functional layer X is, for example, 0.3 or less, preferably 0.25 or less, more preferably 0.2 or less, and even more preferably 0.18 or less. The lower limit of the dynamic friction coefficient is not particularly limited, but may be 0.05, 0.08, or 0.1 from the viewpoint of preventing the polyester film from slipping too much when being placed in a mold or the like, thereby reducing the film's handleability. The static friction coefficient of the surface of the polyester film having the functional layer X is, for example, 0.32 or less, preferably 0.28 or less, more preferably 0.25 or less, and even more preferably 0.23 or less. The lower limit of the static friction coefficient is not particularly limited, but may be 0.05, 0.1, or 0.12 from the viewpoint of preventing the polyester film from slipping too much when placing it in a mold or the like, thereby reducing the handleability of the film.

[0060] The static friction coefficient can be measured, for example, using a translational friction tester (MCS-300) manufactured by Yokohama Systems Research Institute under the following measurement conditions. A test piece measuring 15 x 160 mm is cut from the polyester film, and the static friction coefficient between one side of the test piece and the other side is measured. Specifically, one side of the test piece is held in contact with the other side for 15 seconds before the start of the test, and then measurement is performed in the machine direction (MD) under the following conditions. The sample is humidified for at least 6 hours before measurement. The static friction coefficient can then be measured under the following measurement conditions. The dynamic friction coefficient can also be measured in the same way as the static friction coefficient. - Sliding piece: total mass 104 g (contact area is a square with one side measuring 12 mm) - Test speed: 20 mm / min - Temperature: 23°C ± 2°C - Relative humidity: 50% ± 10% - Test value: n = 12, average of 10 points after dividing the maximum and minimum values

[0061] The functional layer X may contain a component used as a so-called release agent. Specific examples include long-chain alkyl group-containing compounds, waxes, fluorine compounds, silicone compounds, etc. These release agents may be used alone or in combination.

[0062] Among the above-mentioned release agents, compounds containing a long-chain alkyl group are preferred from the viewpoint of making it difficult for the release agent to transfer to a mold such as a die during molding, while easily adjusting the surface free energy and / or water droplet contact angle to the above-mentioned specified values. Furthermore, it is preferable to use a compound having a long-chain alkyl group as the main component in the functional layer X. When the release agent contains a long-chain alkyl group, it has good compatibility with components other than the release agent in the functional layer, and can exhibit good release properties even with a small amount. Here, the main component refers to the component with the highest content among the release agents. Furthermore, in the functional layer X, in addition to the long-chain alkyl group-containing compound, wax, a fluorine compound, a silicone compound, etc. may also be used in combination as a release agent.

[0063] (Compound Having a Long-Chain Alkyl Group) A compound having a long-chain alkyl group refers to a compound having a linear or branched alkyl group having 4 or more carbon atoms. The number of carbon atoms in the alkyl group is preferably 9 or more, more preferably 12 or more, even more preferably 15 or more, and particularly preferably 18 or more. By increasing the number of carbon atoms in the alkyl group as described above, it is possible to impart appropriate releasability to the functional layer X, reduce the surface free energy of the functional layer X, and / or increase the water droplet contact angle. There is no particular restriction on the upper limit of the number of carbon atoms in the alkyl group, and it is usually about 30, but preferably 25. Having the number of carbon atoms in the alkyl group be 25 or less is preferred from the viewpoint of solubility in a solvent when preparing a coating liquid as described below. Examples of linear or branched alkyl groups having 4 or more carbon atoms include n-butyl, isobutyl, sec-butyl, t-butyl, pentyl, neopentyl, isoamyl, hexyl, heptyl, 2-ethylhexyl, octyl, isooctyl, nonyl, isononyl, decyl, isodecyl, undecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, stearyl, and various octadecyl groups such as isostearyl, and behenyl. Examples of compounds having a long-chain alkyl group include various long-chain alkyl group-containing polymeric compounds, long-chain alkyl group-containing amine compounds, long-chain alkyl group-containing ether compounds, and long-chain alkyl group-containing quaternary ammonium salts. From the viewpoint of exhibiting good mold releasability, long-chain alkyl group-containing polymeric compounds are preferred.

[0064] The long-chain alkyl group-containing polymer compound is preferably a polymer compound having a long-chain alkyl group on the side chain, and methods for producing it include: (1) a method of polymerizing a monomer having a long-chain alkyl group, or copolymerizing a monomer having a long-chain alkyl group with a monomer copolymerizable with said monomer, (2) a method of reacting a polymer having a reactive group with a compound having a long-chain alkyl group that is reactive with said reactive group, etc. Compounds having a long-chain alkyl group can be preferably produced by the above method (2).

[0065] In the above method (1), the monomer having a long-chain alkyl group is preferably a (meth)acrylic monomer, for example, a (meth)acrylic acid ester having an alkyl group having 4 to 30 carbon atoms. More specific examples include (meth)acrylic acid esters such as isobutyl (meth)acrylate, t-butyl (meth)acrylate, n-amyl (meth)acrylate, isoamyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-heptyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, and isostearyl (meth)acrylate, as well as various octadecyl (meth)acrylates such as behenyl (meth)acrylate. In this case, as above, the alkyl group preferably has 9 or more carbon atoms, more preferably 12 or more, even more preferably 15 or more, and particularly preferably 18 or more. The long-chain alkyl group-containing polymer compound obtained by the above method (1) is preferably a (meth)acrylic acid ester (co)polymer, and the content of the structural unit derived from the monomer having the long-chain alkyl group is preferably in the range of 10% by mass to 100% by mass, more preferably in the range of 20% by mass to 80% by mass, and even more preferably in the range of 30% by mass to 60% by mass.

[0066] On the other hand, the monomer copolymerizable with the monomer having a long-chain alkyl group is not particularly limited, but (meth)acrylic monomers, vinyl group-containing monomers, etc. are preferred. Examples of (meth)acrylic monomers include hydroxyl group-containing monomers such as hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 6-hydroxyhexyl (meth)acrylate; carboxy group-containing monomers such as (meth)acrylic acid, itaconic acid, carboxyethyl acrylate, mono(2-acryloyloxyethyl) succinate, ω-carboxy-dicarolactone monoacrylate, and monohydroxyethyl phthalate; and alkyl (meth)acrylates having less than 4 carbon atoms other than the compounds having the long-chain alkyl group, such as methyl (meth)acrylate, ethyl (meth)acrylate, and n-propyl (meth)acrylate. Examples of vinyl group-containing monomers include styrene, vinyl acetate, vinyl propionate, and divinylbenzene. Among these, the copolymerizable monomer preferably includes a monomer containing a functional group such as a hydroxyl group-containing monomer or a carboxy group-containing monomer, from the viewpoint of contributing to the reaction with the crosslinking agent described later, and more preferably includes a carboxy group-containing monomer such as (meth)acrylic acid. In this specification, (meth)acrylic means acrylic or methacrylic.

[0067] In the above method (2), examples of the reactive group of the polymer having a reactive group include a hydroxyl group, an amino group, a carboxy group, and an acid anhydride. Specific examples of the polymer having a reactive group include polyvinyl alcohol, polyethyleneimine, polyethyleneamine, a polyester resin containing a reactive group, and a poly(meth)acrylic resin containing a reactive group. Among these, from the viewpoints of releasability and ease of handling, polyvinyl alcohol and a poly(meth)acrylic resin containing a reactive group are preferred, and polyvinyl alcohol is more preferred.

[0068] In order to exhibit good water solubility of the long-chain alkyl group-containing compound, the reactive group may be neutralized. To neutralize the reactive group, a stabilizer may be used. For example, basic stabilizers include inorganic basic compounds such as calcium hydroxide, magnesium hydroxide, lithium hydroxide, potassium hydroxide, and sodium hydroxide, and amine compounds such as ammonia, trimethylamine, triethylamine, diethylamine, and dimethylaminoethanol. Among these, it is preferable to use inorganic basic compounds, and specifically, calcium hydroxide, magnesium hydroxide, and lithium hydroxide are preferred.

[0069] Examples of compounds having a long-chain alkyl group capable of reacting with the above-mentioned reactive group include long-chain alkyl group-containing isocyanates such as various octadecyl isocyanates such as octyl isocyanate, decyl isocyanate, lauryl isocyanate, isostearyl isocyanate, and stearyl isocyanate, and behenyl isocyanate; long-chain alkyl group-containing acid chlorides such as hexanoyl chloride, octanoyl chloride, decanoyl chloride, lauroyl chloride, octadecanoyl chloride, and behenoyl chloride; long-chain alkyl group-containing amines; long-chain alkyl group-containing alcohols, etc. Among these, from the viewpoints of mold releasability and ease of handling, long-chain alkyl group-containing isocyanates are preferred, and stearyl isocyanate is particularly preferred.

[0070] The number-average molecular weight (Mn) of the long-chain alkyl group-containing polymer compound obtained by the above methods (1) and (2) is preferably 1,000 to 100,000, and more preferably 2,000 to 80,000. When these number-average molecular weights are equal to or greater than the lower limit, the low-molecular-weight components in the functional layer X can be reduced, allowing the long-chain alkyl group moiety to be efficiently localized on the surface of the functional layer, thereby achieving sufficient releasability. On the other hand, when the number-average molecular weight is equal to or less than the upper limit, the functional layer resin composition is easily dissolved in the solvent contained in the coating liquid, which is a preferred form of the functional layer resin composition, and is easily applied to a polyester film. The melting point of the long-chain alkyl group-containing compound is preferably 0°C to 100°C, more preferably 10°C to 100°C, and even more preferably 20°C to 90°C.

[0071] (Wax) Examples of waxes include natural waxes, synthetic waxes, and waxes blended with these. Natural waxes include vegetable waxes, animal waxes, mineral waxes, and petroleum waxes. Vegetable waxes include candelilla wax, carnauba wax, rice wax, Japan wax, and jojoba oil. Animal waxes include beeswax, lanolin, and whale wax. Mineral waxes include montan wax, ozokerite, and ceresin. Petroleum waxes include paraffin wax, microcrystalline wax, and petrolatum.

[0072] Synthetic waxes include synthetic hydrocarbons, modified waxes, hydrogenated waxes, fatty acids, fatty acid amides, amines, imides, ester waxes, and ketones. Synthetic hydrocarbons include Fischer-Tropsch wax (Sazol wax), polyethylene wax, oxidized polyethylene wax, and oxidized polypropylene wax. Low molecular weight polymers (number average molecular weight 500 to 20,000) are also included, specifically polypropylene, ethylene-acrylic acid copolymer, polyethylene glycol, polypropylene glycol, and block or graft bonded polyethylene glycol and polypropylene glycol. Modified waxes include montan wax derivatives, paraffin wax derivatives, and microcrystalline wax derivatives. The term "derivative" as used herein refers to compounds obtained by any of the following processes: refining, oxidation, esterification, and saponification, or a combination thereof. Hydrogenated waxes include hydrogenated castor oil and hydrogenated castor oil derivatives.

[0073] Among these waxes, synthetic waxes are preferred from the viewpoint of excellent release performance and easy availability, synthetic hydrocarbons are more preferred, and oxidized polyethylene wax and oxidized polypropylene wax are even more preferred. Note that, from the viewpoint of efficiently distributing the long-chain alkyl group moiety on the surface of the functional layer X by reducing the low-molecular-weight components in the functional layer X, the number-average molecular weight (Mn) of the wax is preferably in the range of 1,000 to 100,000, and the weight-average molecular weight (Mw) is preferably in the range of 2,000 to 80,000.

[0074] The melting point or softening point of the wax is preferably 80° C. or higher, more preferably 110° C. or higher, taking into consideration durability against heat treatment during use, and is preferably 200° C. or lower, more preferably 170° C. or lower, and even more preferably 150° C. or lower, from the viewpoint of controlling release performance after heat treatment. The melting point or softening point of the wax can be measured by a differential scanning calorimeter (DSC).

[0075] (Fluorine compound) As the fluorine compound, any polymer compound containing fluorine atoms in the molecule can be used, for example, perfluoroalkyl group-containing polymer compounds, polymers of olefin compounds containing fluorine atoms, etc. are mentioned.From the viewpoint of being able to exhibit mold releasability with a small content, perfluoroalkyl group-containing polymer compounds are preferred.As the monomer for forming perfluoroalkyl group-containing polymer compounds, perfluoroalkyl group-containing (meth)acrylates, perfluoroalkyl group-containing vinyl ethers, etc. are preferred.

[0076] Examples of perfluoroalkyl group-containing (meth)acrylates include perfluoroalkyl (meth)acrylate, perfluoroalkylmethyl (meth)acrylate, 2-perfluoroalkylethyl (meth)acrylate, 3-perfluoroalkylpropyl (meth)acrylate, 3-perfluoroalkyl-1-methylpropyl (meth)acrylate, and 3-perfluoroalkyl-2-propenyl (meth)acrylate. Examples of perfluoroalkyl group-containing vinyl ethers include perfluoroalkylmethyl vinyl ether, 2-perfluoroalkylethyl vinyl ether, 3-perfluoropropyl vinyl ether, 3-perfluoroalkyl-1-methylpropyl vinyl ether, and 3-perfluoroalkyl-2-propenyl vinyl ether. These may be polymerized alone or in combination of two or more. From the viewpoint of exhibiting releasability with a small content, the perfluoroalkyl group preferably has 3 to 11 carbon atoms. In addition, from the viewpoint of efficiently distributing the long-chain alkyl group portion on the surface of the functional layer X by reducing the low-molecular-weight components in the functional layer X, the number-average molecular weight (Mn) of the fluorine compound (polymer compound) is preferably in the range of 1,000 or more and 100,000 or less, and the weight-average molecular weight (Mw) is preferably in the range of 2,000 or more and 80,000 or less.

[0077] (Silicone Compound) A silicone compound is a compound having a siloxane bond (—Si—O—) in the molecule, and examples thereof include silicone emulsion, acrylic-grafted silicone, silicone-grafted acrylic, amino-modified silicone, perfluoroalkyl-modified silicone, and alkyl-modified silicone. Curable silicone resins are preferred in terms of releasability, heat resistance, and the like. Examples of curable silicone resins include addition types, condensation types, ultraviolet-curable types, and electron beam-curable types, and any curable type can be used. From the perspective of efficiently localizing the long-chain alkyl group moiety on the surface of the functional layer X by reducing the low-molecular-weight components in the functional layer X, the number-average molecular weight (Mn) of the silicone compound is preferably in the range of 100 to 100,000, and the weight-average molecular weight (Mw) is preferably in the range of 200 to 80,000.

[0078] (Crosslinking Agent) The functional layer X, i.e., the resin composition for the functional layer described below, preferably further contains a crosslinking agent. Examples of crosslinking agents include melamine compounds, oxazoline compounds, epoxy compounds, isocyanate compounds, carbodiimide compounds, and silane coupling agents. Among these crosslinking agents, melamine compounds are preferred because they have high crosslink density and elastic modulus while providing good releasability. Therefore, the functional layer X, i.e., the resin composition for the functional layer described below, preferably contains a long-chain alkyl group-containing compound and a crosslinking agent, and more preferably contains a long-chain alkyl group-containing compound and a melamine compound.

[0079] The crosslinking agent reacts with a release agent such as a long-chain alkyl group-containing compound to improve the performance of the release layer. Therefore, it is preferable that the crosslinking agent reacts with the long-chain alkyl group-containing compound, which is the release agent, in the functional layer X. By using a crosslinking agent, the release agent such as the long-chain alkyl group-containing compound can be effectively localized on the surface of the functional layer X, which makes it easier to lower the surface free energy and increase the water droplet contact angle. However, it is believed that the crosslinking agent exists in the functional layer X in the form of both an unreacted substance and a reacted compound.

[0080] The melamine compound is a compound having a melamine skeleton within the compound. Examples of suitable melamine compounds include alkylolated melamine derivatives, compounds obtained by reacting alkylolated melamine derivatives with alcohols to partially or completely etherify them, and mixtures thereof. Examples of suitable alcohols used for etherification include methanol, ethanol, isopropyl alcohol, n-butanol, and isobutanol. Furthermore, the melamine compound may be a monomer or a dimer or higher polymer, or a mixture thereof. Furthermore, melamine compounds partially co-condensed with urea or the like may also be used, and a catalyst may be used to increase the reactivity of the melamine compound.

[0081] Examples of alkylolation include methylolation, ethylolation, isopropylolation, n-butylolation, and isobutyrolation. Among these, methylolation is preferred from the viewpoint of reactivity. From the viewpoint of improving coating film strength and improving adhesion between the functional layer X and the polyester film, the melamine compound is preferably a partially etherified alkylolated melamine derivative, and more preferably an alkylol etherified with methanol. Therefore, a more preferred form is a partially etherified melamine having a methylol group and a methoxy group. The etherified alkylol group is preferably 0.5 to 5 equivalents, more preferably 0.7 to 3 equivalents, and even more preferably 1.2 to 2.8 equivalents, relative to the unetherified alkylol group. Furthermore, the melamine compound may be either a monomer or a dimer or higher polymer, or a mixture thereof. Furthermore, melamine partially co-condensed with urea or the like may also be used, and a catalyst can be used to increase the reactivity of the melamine compound.

[0082] The oxazoline compound is a compound having an oxazoline group in the molecule, and a polymer containing an oxazoline group is particularly preferred. The polymer containing an oxazoline group can be synthesized by polymerizing an oxazoline group-containing monomer alone or with other monomers. Examples of the oxazoline group-containing monomer 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.

[0083] The epoxy compound is a compound having an epoxy group in the molecule, and examples thereof include condensation products of epichlorohydrin with a hydroxyl group or an amino group of ethylene glycol, polyethylene glycol, glycerin, polyglycerin, bisphenol A, etc., polyepoxy compounds, diepoxy compounds, monoepoxy compounds, glycidylamine compounds, etc.

[0084] Examples of the isocyanate compound include tolylene diisocyanate compounds such as 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate; xylylene diisocyanate compounds such as 1,3-xylylene diisocyanate, 1,4-xylylene diisocyanate and tetramethylxylylene diisocyanate; aromatic isocyanate compounds such as 1,5-naphthalene diisocyanate and triphenylmethane triisocyanate; hexamethylene diisocyanate, isophorone diisocyanate, and adducts of these isocyanate compounds with polyol compounds such as trimethylolpropane; and biuret compounds and isocyanurates of these polyisocyanate compounds.

[0085] Examples of the carbodiimide compound include monocarbodiimide compounds such as dicyclohexylcarbodiimide, diisopropylcarbodiimide, dimethylcarbodiimide, diisobutylcarbodiimide, dioctylcarbodiimide, t-butylisopropylcarbodiimide, diphenylcarbodiimide, di-t-butylcarbodiimide, and di-β-naphthylcarbodiimide; and isocyanate-terminated polycarbodiimides obtained by a condensation reaction of diisocyanate accompanied by decarbonation.

[0086] Examples of the silane coupling agent include γ-mercaptopropyltrimethoxysilane, γ-isocyanatepropyltriethoxysilane, γ-isocyanatepropyltrimethoxysilane, and N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propaneamine.

[0087] As described above, the functional layer X preferably contains a long-chain alkyl group-containing compound and a crosslinking agent. In this case, it is preferable that the content of the long-chain alkyl group-containing compound relative to the total non-volatile components of the functional layer X (the resin composition for the functional layer) is 5% by mass or more and 95% by mass or less, and the content of the crosslinking agent is 5% by mass or more and 95% by mass or less. A content of the long-chain alkyl group-containing compound of 5% by mass or more can impart appropriate releasability to the functional layer X, maintain the surface free energy of the functional layer surface at a certain level or less, and easily adjust the water droplet contact angle to a predetermined value or more. A content of 95% by mass or less makes it easier to incorporate appropriate amounts of components other than the long-chain alkyl group-containing compound, such as a crosslinking agent, into the functional layer X. The content of the long-chain alkyl group-containing compound is more preferably 15% by mass or more and 80% by mass or less, even more preferably 30% by mass or more and 70% by mass or less, and even more preferably 40% by mass or more and 65% by mass or less. Furthermore, when the content of the crosslinking agent is 5% by mass or more, the crosslinking agent can appropriately crosslink the functional layer X, improving the performance of the functional layer X and making it easier to adjust the surface free energy and water droplet contact angle of the functional layer surface to desired values. Furthermore, when the content is 95% by mass or less, it becomes easier to incorporate appropriate amounts of components other than the crosslinking agent, such as long-chain alkyl group-containing compounds, into the functional layer X. The content of the crosslinking agent is more preferably 10% by mass or more and 80% by mass or less, even more preferably 20% by mass or more and 70% by mass or less, and even more preferably 30% by mass or more and 50% by mass or less.

[0088] The functional layer X, i.e., the resin composition for the functional layer, may contain components other than the release agent and the crosslinking agent, such as a binder resin. The binder resin facilitates imparting film-forming properties to the functional layer X. The binder resin is not particularly limited, and conventionally known binder resins such as polyester resins, (meth)acrylic resins, polyurethane resins, polyvinyl resins (polyvinyl alcohol, vinyl chloride-vinyl acetate copolymers, etc.), polyalkylene glycols, polyalkyleneimines, methyl cellulose, hydroxycellulose, and starches can be used. Among these, at least one selected from polyester resins, (meth)acrylic resins, and polyurethane resins is preferred from the viewpoint of film-forming properties and adhesion to polyester films. One type of binder resin may be used alone, or two or more types may be used in combination.

[0089] (Additives) In addition to the above components, the functional layer X, i.e., the resin composition for the functional layer, may further contain particles and additives. The particles are added to improve blocking properties and slip properties, and their type, shape, average particle size, and content are the same as those of the particles that may be incorporated into the polyester film described above. Examples of additives include antistatic agents, UV absorbers, antioxidants, antifoaming agents, lubricants, foaming agents, dyes, pigments, etc.

[0090] The functional layer X is formed on at least one side of the polyester film using a functional layer resin composition. The functional layer resin composition preferably contains a release agent such as a compound having a long-chain alkyl group, and further contains a crosslinking agent. The functional layer resin composition may further contain a binder resin, particles, other additives, and a solvent, as necessary. The functional layer resin composition is preferably in the form of a coating liquid containing a solvent. When used as a coating liquid, the solids concentration of the composition is preferably 0.1% by mass or more and 50% by mass or less, more preferably 0.5% by mass or more and 40% by mass or less, and even more preferably 1% by mass or more and 30% by mass or less.

[0091] The functional layer resin composition preferably contains a solvent to form a liquid coating solution, which is then applied to the surface of a polyester film and dried and cured as necessary to form the functional layer X. In the coating solution, the components forming the functional layer X may be dissolved in the solvent or dispersed in the solvent. Furthermore, in the functional layer resin composition, the solvent is a volatile component. The solvent may be either water or an organic solvent, or a mixed solvent of water and an organic solvent. Examples of 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. The organic solvents may be used alone or in combination.

[0092] (Surface Roughness (Ra)) The surface roughness (Ra) of the functional layer X is not particularly limited, but is, for example, 1 nm or more and 100 nm or less, preferably 5 nm or more and 50 nm or less, and more preferably 10 nm or more and 30 nm or less. When the surface roughness (Ra) of the surface of the functional layer X is within the above range, the polyester film has moderately good sliding processability when placed in a mold such as a die, and also has good sliding properties when unwinding the film when laminating it with other films, etc., tending to improve film handleability. The surface roughness (Ra) is the arithmetic mean roughness, which is one of the line roughness parameters (JIS B0601:1994), and represents the average value of the average height difference from the average plane. That is, when a portion of the reference length L is sampled and the average line of this sampled portion is the x-axis and the direction of longitudinal magnification is the y-axis, and the roughness curve is expressed as y = Z(x), it can be calculated from the following formula.

[0093] <Orientation Degree> The present polyester film has an absolute value (|Δnv - Δnp|) of the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) of 57 or less. As described above, the present polyester film contains PBT and PET, and by setting |Δnv - Δnp| to 57 or less, it can have excellent puncture resistance. Specifically, the variation in displacement and puncture strength upon puncture is small, and the elongation in the thickness direction is more uniform, making it more suitable for use as an exterior packaging material. Although the mechanism behind this is unclear, it is presumed that a low |Δnv - Δnp| reduces the difference in crystallinity between the transverse and longitudinal directions, thereby suppressing the anisotropy of the film, thereby enabling the advantages of both PET and PBT to be expressed and resulting in excellent puncture resistance. Specifically, when a puncture force from a needle or the like is applied to a film, the film stretches and deforms in the thickness direction as if it were being wrapped around the needle or the like. It is believed that this degree of stretching of the film (the degree to which the film is wrapped around the needle or the like) tends to be similar in both the longitudinal and transverse directions of the film, thereby reducing the variation in puncture displacement and puncture strength depending on the puncture position. Thus, by using a film with small variations in puncture displacement and puncture strength, a practical product with superior puncture resistance can be obtained. From the viewpoint of puncture resistance, |Δnv - Δnp| is preferably 50 or less, more preferably 45 or less, and even more preferably 40 or less. Furthermore, |Δnv - Δnp| is not particularly limited and may be 0 or greater, but in practice, it is sufficient that it is 5 or greater, preferably 10 or greater, and more preferably 15 or greater.

[0094] The polyester film preferably has a planar orientation degree (ΔP) of 100 or more, more preferably 120 or more, even more preferably 140 or more, and even more preferably 150 or more. When the polyester film has a planar orientation degree of a certain value or more, the crystallinity of the film is enhanced, and the puncture strength is easily increased. The planar orientation degree is not particularly limited, but is preferably 200 or less, more preferably 180 or less, and even more preferably 170 or less.

[0095] Furthermore, the present polyester film typically has a transverse orientation degree (Δnv) greater than a longitudinal orientation degree (Δnp). The transverse orientation degree (Δnv) is not particularly limited, but is, for example, 50 to 130, preferably 60 to 120, more preferably 70 to 110, and even more preferably 75 to 105. The longitudinal orientation degree (Δnp) is also not particularly limited, but is, for example, 20 to 100, preferably 30 to 90, more preferably 40 to 80, and even more preferably 45 to 70.

[0096] The |Δnv-Δnp|, transverse orientation degree (Δnv), longitudinal orientation degree (Δnp), and planar orientation degree (ΔP) of the present polyester film can be set within the above ranges by adjusting the stretching ratio, stretching temperature, preheating temperature and time of the present polyester film, heat setting temperature and time, etc. in the process of producing the present polyester film.

[0097] The planar orientation degree (ΔP), longitudinal orientation degree (Δnp), and transverse orientation degree (Δnv) of the present polyester film can be determined as follows. According to JIS K 7142:2014 5.1 (Method A), the refractive index in the longitudinal direction (nx), the refractive index in the transverse direction (ny), and the refractive index in the thickness direction (nz) of the present polyester film are measured using an Abbe refractometer with sodium D line as a light source. By applying the measurement results to the following equations, the planar orientation degree (also referred to as planar orientation coefficient; ΔP), the longitudinal orientation degree (also referred to as longitudinal plane orientation coefficient; Δnp), and the transverse orientation degree (also referred to as transverse plane orientation coefficient; Δnv) can be calculated. ΔP = ((nx + ny) / 2 - nz) × 1000 Δnp = (nx - (ny + nz) / 2) × 1000 Δnv = (ny - (nx + nz) / 2) × 1000

[0098] The machine direction (MD) of the film refers to the direction in which the film advances during the film production process, i.e., the winding direction of the film roll. The transverse direction (TD) of the film refers to the direction parallel to the film surface and perpendicular to the machine direction, i.e., the direction parallel to the central axis of the roll when the film is rolled.

[0099] <Puncture Strength> The present polyester film preferably has a puncture strength of 480 N / mm or more. A puncture strength of 480 N / mm or more provides high puncture resistance, making it suitable for use as an exterior material, such as an exterior material for a battery. The puncture strength is more preferably 490 N / mm or more, even more preferably 500 N / mm or more, even more preferably 510 N / mm or more, even more preferably 520 N / mm or more, and even more preferably 530 N / mm or more. The puncture strength is not particularly limited, but is, for example, 1000 N / mm or less, preferably 800 N / mm, more preferably 700 N / mm or less, even more preferably 650 N / mm or less, even more preferably 590 N / mm or less, even more preferably 585 N / mm or less, even more preferably 580 N / mm or less, and even more preferably 575 N / mm or less. When the puncture strength is below the above upper limit, the elongation of the present polyester film is easily improved. Furthermore, by setting the puncture strength to be equal to or less than the above upper limit, when the film is made into an actual product such as a battery exterior material by, for example, extrusion molding, the restoring force of the film trying to return to its original shape is kept low, and the shape retention of the actual product tends to be improved. For example, in the case of a battery exterior material, deformation of the exterior material due to post-processing such as heat fusion or heat generation from an electricity storage device element housed inside can be more effectively suppressed.

[0100] <Displacement upon piercing> The present polyester film preferably has a displacement upon piercing of 4 mm or more. When the displacement upon piercing is 4 mm or more, the film has good extensibility and is sufficiently stretched during shaping, making it easier to increase the molding depth, etc. The displacement upon piercing is more preferably 4.2 mm or more, and even more preferably 4.3 mm or more. The displacement upon piercing is not particularly limited, but may be, for example, 8 mm or less, 6 mm or less, or 5 mm or less.

[0101] <Variation in Puncture Strength> The present polyester film preferably has a coefficient of variation in puncture strength of 0.036 or less, more preferably 0.034 or less, even more preferably 0.032 or less, even more preferably 0.03 or less, even more preferably 0.028 or less, even more preferably 0.026 or less, even more preferably 0.024 or less, even more preferably 0.022 or less, and even more preferably 0.02 or less. A smaller coefficient of variation reduces the variation in puncture strength of the present polyester film, resulting in uniform elongation in the thickness direction, making it suitable for use as an exterior packaging material. The lower the coefficient of variation in puncture strength, the better, but in practice, it is preferably 0.001 or more, more preferably 0.005 or more, and even more preferably 0.01 or more. The coefficient of variation in puncture strength indicates the variation in puncture strength and is calculated as the standard deviation / average value. The average value and standard deviation are the average value and standard deviation of the puncture strength measured at 12 points on the polyester film sample. The coefficient of variation of the displacement upon puncture, which will be described later, indicates the variation of the displacement upon puncture and can be calculated in the same manner, except that the measured value is changed to the displacement upon puncture.

[0102] <Variation in displacement upon piercing> The present polyester film preferably has a coefficient of variation in displacement upon piercing of 0.034 or less, more preferably 0.03 or less, even more preferably 0.027 or less, even more preferably 0.025 or less, even more preferably 0.023 or less, even more preferably 0.021 or less, and even more preferably 0.019 or less. The reduced coefficient of variation in displacement upon piercing reduces the variation in displacement, resulting in uniform elongation in the thickness direction, making the polyester film suitable for use as an exterior packaging material. Furthermore, the film tends to stretch uniformly during shaping, resulting in good formability. The lower the coefficient of variation in displacement upon piercing, the better, but in practice, it is preferably 0.001 or more, more preferably 0.005 or more, and even more preferably 0.01 or more.

[0103] <Martens Hardness> The polyester film has a Martens hardness of 132 N / mm２ It is preferable that the strength is 135 N / mm or more. ２ More preferably, it is 138 N / mm or more. ２ If the Martens hardness is at least a certain level, pinhole resistance, breaking strength, etc. tend to be high. The upper limit of the Martens hardness is not particularly limited, but is, for example, 300 N / mm ２ or 200 N / mm ２ may be.

[0104] <Indentation hardness> The polyester film has an indentation hardness of 200 N / mm ２ It is preferable that the strength is 210 N / mm or more. ２ More preferably, it is 215 N / mm or more. ２ If the indentation hardness is at least a certain level, pinhole resistance, breaking strength, etc. tend to be high. The upper limit of the indentation hardness is not particularly limited, but is, for example, 400 N / mm ２ or 300 N / mm ２ may be.

[0105] <Elastic Deformation Power> The elastic deformation power of the present polyester film is preferably 42% or more, more preferably 43% or more, and even more preferably 44% or more. Having an elastic deformation power of a certain level or higher makes it easier to obtain molded articles with excellent design properties and shape recovery. The upper limit of the elastic deformation power of the present polyester film is not particularly limited, but may be, for example, 60%, 57%, 53%, or 50%.

[0106] The Martens hardness, indentation hardness, and elastic deformation power may be measured on at least one surface of the polyester film as long as they are equal to or greater than the lower limit values. Specifically, the values ​​measured on the surface of the polyester film in which the PET content (A1) is greater than the PET content (A2) in the central portion of the film are equal to or greater than the lower limit values. However, it is preferable that the values ​​measured on both surfaces are equal to or greater than the lower limit values. The Martens hardness, indentation hardness, and elastic deformation power may be measured by a load-unload test in which an indenter is pressed against the surface of the polyester film at 20 mN using microhardness measurement. Specifically, they may be measured by the method described in the Examples below.

[0107] <Haze> The haze of the polyester film is preferably 10% or less, more preferably 8% or less, even more preferably 5% or less, and particularly preferably 3% or less. By setting the haze to the above lower limit or less, sufficient transparency is obtained. The haze is not particularly limited, and may be, for example, 0.01% or more, 0.1% or more, or 0.7% or more.

[0108] <Heat Shrinkage> After heat treatment at 160°C for 15 minutes, the present polyester film preferably has a heat shrinkage rate of less than 5% in either the longitudinal or transverse direction of the polyester film and a heat shrinkage rate of more than 2% in the other direction. When the heat shrinkage rate in one direction is less than 5%, the present polyester film has excellent heat resistance. Therefore, dimensional deformation is reduced when laminating the polyester film to other films, making it easier to laminate the polyester film to other films. Furthermore, when laminated to other films to form a laminate, shrinkage stress is less likely to remain, preventing shrinkage during secondary processing into battery exterior materials and other processes, improving secondary processability. Furthermore, when used as a battery exterior material, deterioration of the exterior material is more likely to be prevented even if the battery heats up and the exterior material is heated. On the other hand, when the heat shrinkage rate in the other direction is more than 2%, the film shrinks by a certain amount or more during thermal lamination, improving adhesion to metal layers, other films, and the like.

[0109] From the viewpoint of improving heat resistance, the heat shrinkage rate of the one of the layers is preferably 4.5% or less, more preferably 4% or less, even more preferably 3.5% or less, and even more preferably 3% or less. From the viewpoint of heat resistance, the lower the heat shrinkage rate of the one of the layers, the better. However, from the viewpoint of improving adhesion to the metal layer or other films during thermal lamination, it is preferable that the heat shrinkage rate be a certain value or more, for example, 0.1% or more, preferably 0.5% or more, more preferably 0.8% or more, even more preferably 1% or more, and even more preferably 1.3% or more. From the viewpoint of improving adhesion to the metal layer or other films, the heat shrinkage rate of the other of the layers is more preferably 2.3% or more, even more preferably 2.5% or more, even more preferably 3% or more, even more preferably 3.5% or more, and even more preferably 3.7% or more. The heat shrinkage rate of the other of the layers is preferably 9% or less, more preferably 8% or less, even more preferably 7% or less, even more preferably 6% or less, and even more preferably 5.5% or less. In addition to the heat shrinkage rate of one of the directions, if the heat shrinkage rate of the other direction is set to a certain value or less, the heat resistance of the film will be further improved. This makes it easier to laminate onto other films by lamination, and also improves secondary processability. The ratio of the heat shrinkage rate in the longitudinal direction to the heat shrinkage rate in the transverse direction is not particularly limited, but is, for example, 0.4 to 10, preferably 0.6 to 8, more preferably 0.8 to 6, even more preferably 1 to 4, and even more preferably 0.7 to 2. The heat shrinkage rate can be appropriately adjusted by adjusting the stretching conditions and heat setting temperature of the polyester film, which will be described later.

[0110] <Melting point> The melting point (Tm) of the present polyester film in the first run measured by differential scanning calorimetry is not particularly limited, but is preferably 230°C or higher. A melting point (Tm) of the first run of 230°C or higher provides good heat resistance and facilitates low thermal shrinkage. The melting point (Tm) of the present polyester film in the first run is preferably 235°C or higher, more preferably 240°C or higher. Furthermore, from the viewpoint of easily exerting the effects of using PBT, the melting point (Tm) of the present polyester film in the first run is preferably 260°C or lower, more preferably 255°C or lower, even more preferably 252°C or lower, and even more preferably 250°C or lower.

[0111] The melting point (Tm) of the second run of the polyester film measured by differential scanning calorimetry is not particularly limited, but is preferably 223°C or higher. A melting point (Tm) of the second run of the polyester film of 223°C or higher provides good heat resistance and also facilitates low heat shrinkage. The melting point (Tm) of the second run of the polyester film of the present invention is preferably 228°C or higher, more preferably 233°C or higher. Furthermore, from the viewpoint of easily exerting the effects of using PBT, the melting point (Tm) of the second run of the polyester film of the present invention is preferably 255°C or lower, more preferably 250°C or lower, even more preferably 247°C or lower, and even more preferably 243°C or lower.

[0112] Furthermore, the present polyester film preferably exhibits a single endothermic peak during temperature rise as measured by differential scanning calorimetry, and more preferably exhibits a single melting peak in both the first and second runs. Having a single endothermic peak reduces the likelihood of softening at low temperatures. Therefore, when used as a battery exterior material, deterioration of the exterior material is more likely to be prevented even if the battery dissipates heat and the exterior material is heated. Furthermore, when performing secondary processing such as molding, if two endothermic peaks are present, the processing temperature can be adjusted to match either the lower peak or the higher peak. However, adjusting the processing temperature to the lower peak results in molding being performed in a state where the resin at the higher peak is not yet melted, resulting in reduced moldability. Conversely, adjusting the processing temperature to the higher peak can result in dimensional changes due to the resin at the lower peak before the processing temperature is reached, potentially resulting in problems such as wrinkles due to shrinkage. Therefore, forming the present polyester film so that it exhibits a single endothermic peak also has the advantage of more easily improving secondary processability, such as moldability. Furthermore, the film can be stably produced even at a high heat setting temperature using the manufacturing method described below, making it easier to adjust the heat shrinkage rate. The polyester film can be adjusted to have a single endothermic peak by compatibilizing PBT and PET. This single endothermic peak may have a shoulder at its base, excluding cases where two or more endothermic peaks derived from PBT and PET are clearly observed. The melting point (Tm) is measured in accordance with JIS K7121:2012 at a temperature of -70 to 280°C, a heating rate of 10°C / min, and a cooling rate of 600°C / min. The value at the peak top of the endothermic peak observed during the first heating is defined as the melting point of the first run, and the value at the peak top of the endothermic peak observed during the second heating (reheating) is defined as the melting point of the second run.

[0113] <Glass Transition Temperature (Tg)> The glass temperature (Tg) of the present polyester film measured by differential scanning calorimetry is not particularly limited, but is preferably 50°C or higher, more preferably 55°C or higher, and even more preferably 60°C or higher. When the glass transition temperature (Tg) is a certain value or higher, the heat resistance is good and the heat shrinkage rate and the like tend to be low. The glass temperature (Tg) of the present polyester film is not particularly limited, but is preferably 80°C or higher, more preferably 75°C or lower, and even more preferably 70°C or lower. The glass transition temperature (Tg) is measured in accordance with JIS K7121:2012 at a temperature of -70 to 280°C, a heating rate of 10°C / min, and a cooling rate of 600°C / min, and the midpoint glass transition temperature during reheating is used.

[0114] <Intrinsic Viscosity> The intrinsic viscosity of the polyester film is preferably 0.6 dL / g or more and 0.85 dL / g or less. Setting the intrinsic viscosity of the polyester film within the above range facilitates improving the productivity and film-forming properties of the polyester film. The intrinsic viscosity of the polyester film is more preferably 0.62 dL / g or more and 0.8 dL / g or less, even more preferably 0.63 dL / g or more and 0.78 dL / g or less, and even more preferably 0.64 dL / g or more and 0.75 dL / g or less.

[0115] [Method for producing polyester film] Next, the method for producing the present polyester film (hereinafter, sometimes simply referred to as the present production method) will be described using the case where the present polyester film is a biaxially stretched polyester film as an example. When the present polyester film is a biaxially stretched polyester film, it is preferable to first produce an unstretched sheet, and then stretch it in two directions to obtain a biaxially stretched polyester film.

[0116] The unstretched sheet is preferably obtained by feeding the aforementioned PET and PBT, along with other resins, particles, and other additives, as needed, into an extruder, mixing them appropriately, extruding the mixture as a molten sheet from a die using the extruder, and then cooling and solidifying it on a chill roll. In this case, it is preferable to increase the adhesion between the sheet and the chill roll to improve the sheet's flatness, and an electrostatic adhesion method and / or a liquid application adhesion method are preferably used. Furthermore, in the extruder, the PET and PBT are preferably mixed so as to be compatible with each other; specifically, they are kneaded under conditions of 270 to 300°C, preferably 275 to 295°C, and more preferably 280 to 290°C. A compatibilizer may also be added to enhance compatibility. The compatibilizer may be selected from those commonly used for PBT and PET.

[0117] When kneading using an extruder, it is sufficient to knead the PBT and PET sufficiently to ensure their compatibility. Among these, a method using an extruder in which the ratio of screw length L (mm) to screw diameter D (mm) (L / D) is preferably 15 or more, more preferably 20 or more, and preferably 50 or less, more preferably 40 or less, can be used. The use of such an extruder facilitates further improving the compatibility between PBT and PET, and prevents the resin residence time from becoming too long or the resin temperature from becoming too high, thereby easily suppressing film discoloration due to thermal degradation, outgassing during film formation, and the generation of gel-like foreign matter. The screw configuration of the extruder is preferably a structure having a kneading unit, particularly a spiral kneading unit, to improve kneading performance. One or two kneading units are preferred. It is also effective to use a continuous kneader as the kneader, in which a screw rotatably mounted in the cylinder of the extruder is provided with multiple rotating blades, and a fixed blade is further provided in the cylinder and inserted between the multiple rotating blades.

[0118] It is also effective to adjust the ratio Q / N of the discharge rate Q (kg / hr) to the screw rotation speed N (rpm) during melt-kneading. Q / N is preferably 0.1 or more, more preferably 0.2 or more, and is preferably 10 or less, more preferably 6 or less, and even more preferably 4 or less. By setting Q / N within the above range, discoloration of the film and generation of foreign matter caused by excessively high resin temperature or excessively long residence time can be suppressed, and it becomes easy to achieve sufficient compatibility between PBT and PET.

[0119] Furthermore, it is also effective to make the melt viscosities of PBT and PET similar to each other, which can be controlled by the molecular weight and branching structure.

[0120] The temperature of the cooling roll is preferably 10 to 40° C., more preferably 15 to 35° C., and even more preferably 20 to 30° C. In order to improve the flatness of the sheet, it is preferable to increase the adhesion between the sheet and the rotating cooling drum, and as described above, preferably, an electrostatic application adhesion method or a liquid application adhesion method can be appropriately adopted.

[0121] When the polyester film has a multilayer structure, multiple layers may be coextruded to form an unstretched sheet having a multilayer structure. The polyester raw material may be fed to the extruder as pellets, appropriately dried, or the like. Particles and other additives may be incorporated into the pellets.

[0122] When the present polyester film is a monolayer film, the PET and PBT contained in the monolayer film may have a concentration gradient along the thickness direction. To obtain such a monolayer film, the PET and PBT may be unevenly distributed in the resin composition for forming the monolayer film by a known method. Specifically, this can be achieved by appropriately adjusting the conditions during the production of the monolayer film. For example, the temperature during melt-kneading and extruding the resin composition into a film may be set low, or the degree of kneading may be reduced by appropriately adjusting the screw rotation speed, thereby preventing the reaction between PET and PBT from being adequately promoted. The concentration gradient can be adjusted by utilizing the difference in surface free energy between the two. In addition, in this production method, particles may also have a concentration gradient.

[0123] The unstretched sheet obtained as described above is stretched in the longitudinal and transverse directions. The stretching method is not particularly limited, but may be a roll or tenter-type stretching machine. It is preferable to stretch in the longitudinal direction first, followed by the transverse direction, but the reverse is also possible. The stretching temperature during longitudinal stretching is preferably 50°C to 85°C, more preferably 58°C to 75°C, even more preferably 60°C to 74°C, and even more preferably 62°C to 73°C. The longitudinal stretching ratio is preferably 2.5 to 4.5 times, more preferably 2.8 to 4.2 times, even more preferably 3 to 4 times, and even more preferably 3.2 to 3.8 times. It is also preferable to preheat the polyester film before transverse stretching. The preheating temperature is preferably 60°C to 110°C, more preferably 62°C to 100°C, even more preferably 64°C to 90°C, and even more preferably 65°C to 85°C. The stretching temperature during transverse stretching is preferably 70°C or higher and 120°C or lower, more preferably 74°C or higher and 110°C or lower, even more preferably 77°C or higher and 100°C or lower, and even more preferably 80°C or higher and 95°C or lower. The transverse stretching ratio during transverse stretching is, for example, 3.5 times or higher and 6 times or lower, preferably 3.5 times or higher and 5.5 times or lower, more preferably 4 times or higher and 5.5 times or lower, and even more preferably 4.5 times or higher and 5.2 times or lower. These transverse stretching ratios are relatively low despite the use of PBT. The polyester film is generally produced as a raw film having a TD length (film width) of a certain size or larger during production. By keeping the transverse stretching ratio relatively low as described above, it is possible to keep various physical properties within desired ranges and to suppress variations in various physical properties (e.g., degree of orientation, puncture strength, displacement amount during puncture, etc.) between the center and the end portions of the raw film in the TD direction.

[0124] Furthermore, after stretching the polyester film in the longitudinal and transverse directions, it is preferable to subsequently heat-set the film at a heat-setting temperature of 180°C or higher to obtain a biaxially stretched polyester film. The heat-setting is preferably performed at the heat-setting temperature under tension or with a relaxation of 30% or less, preferably 10% or less, and more preferably 7% or less. The heat-setting temperature is preferably 190°C or higher, more preferably over 190°C, even more preferably 195°C or higher, and even more preferably 200°C or higher. The heat-setting temperature is not particularly limited, but is preferably 240°C or lower, more preferably 230°C or lower, even more preferably 225°C or lower, and even more preferably 220°C or lower. The heat-setting time is preferably 3 to 15 seconds, more preferably 4 to 14 seconds, and even more preferably 5 to 13 seconds. After the heat-setting step, the film is cooled in a cooling zone for 0 to 20%, 0.5 to 15%, and preferably 1 to 10%. More preferably, the cooling treatment may be carried out under a relaxation of 1.5 to 7%. The cooling temperature is, for example, preferably about 120 to 160°C, more preferably about 130 to 150°C.

[0125] In this production method, the ratio of the transverse stretching ratio to the longitudinal stretching ratio (TD / MD) is preferably 1 or more and 1.7 or less. By maintaining the ratio of the transverse stretching ratio to the longitudinal stretching ratio within a certain range, preheating as described above, maintaining the stretching temperature during transverse stretching within a certain range, and setting the heat setting temperature after stretching to a certain temperature or higher, the degree of orientation (planar orientation) of the entire polyester film can be increased while keeping the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) below a certain level. The ratio of the transverse stretching ratio to the longitudinal stretching ratio (TD / MD) is more preferably 1.1 or more and 1.7 or less, and even more preferably 1.2 or more and 1.7 or less. Furthermore, the ratio of the transverse stretching ratio to the longitudinal stretching ratio (TD / MD) is more preferably 1.1 or more and 1.6 or less, and even more preferably 1.2 or more and 1.5 or less. Furthermore, in this manufacturing method, by setting the stretching temperature during longitudinal stretching, as well as the transverse stretching ratio and longitudinal stretching ratio, within a certain range as described above, it is possible to further reduce the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) while increasing the degree of planar orientation. In addition, in this manufacturing method, by stretching under the above-described stretching conditions and setting the heat setting temperature within a certain range, it is possible to easily adjust the heat shrinkage rate within a desired range. In particular, it is effective to set the preheating temperature, stretching temperature, and stretching ratio conditions slightly lower than usual, and to set the heat setting temperature slightly higher than usual.

[0126] In the present production method, the product of the transverse stretching ratio and the longitudinal stretching ratio is preferably 14 or more, more preferably 15 or more, even more preferably greater than 15.5, and preferably 25 or less, more preferably 20 or less, and even more preferably 18 or less. In the present production method, the difference between the transverse stretching ratio and the longitudinal stretching ratio is preferably 0.3 or more, more preferably 0.5 or more, even more preferably 0.7 or more, and is, for example, 2.4 or less, preferably 2 or less, more preferably 1.8 or less, and even more preferably 1.6 or less. By adjusting the transverse stretching ratio and the longitudinal stretching ratio within a certain range as described above, the present polyester film can be improved in various physical properties while suppressing variations in various physical properties (e.g., degree of orientation, puncture strength, amount of displacement during puncture, etc.) between the center and the end portions in the TD direction of the raw film. For example, in the longitudinal direction, the stretching ratio is determined from the ratio of the film transport speed before stretching to the film transport speed after stretching. In the transverse direction, it is determined from the ratio of the film width before stretching to the film width after stretching.

[0127] When the polyester film has a functional layer such as a functional layer X or an easy-adhesion layer, the method for forming the functional layer such as the easy-adhesion layer is not particularly limited, but examples include in-line coating and offline coating. In-line coating is a method of coating within the polyester film production process, and more specifically, it is preferable to coat the polyester after melt extrusion and longitudinal stretching, but before transverse stretching. Off-line coating is a method of providing an easy-adhesion layer using a coating device in a separate process after polyester film production.

[0128] <Laminate> The present polyester film is used, for example, in a laminate. The present polyester film is preferably used as a battery exterior material in the form of a laminate. The laminate of the present invention includes the present polyester film. The laminate of the present invention may include, in addition to the polyester film, at least one of a resin layer, a metal layer, and a sealant layer, and preferably at least one of a resin layer and a metal layer. The metal layer is a layer formed of a metal, a metal oxide, or a mixture of a metal and a metal oxide. Among these, the metal layer is preferably formed from a metal, and examples of metals include aluminum, nickel, titanium, or alloys thereof, stainless steel, titanium steel, and the like, and among these, it is more preferably formed from an aluminum alloy. The metal layer may be formed from a metal foil or by metal vapor deposition, but is preferably formed from a metal foil. From the viewpoint of preventing the occurrence of wrinkles and pinholes, the metal layer is more preferably formed from a soft aluminum alloy foil such as annealed aluminum (JIS H4160:1994 A8021H-O, JIS H4160:1994 A8079H-O, JIS H4000:2014 A8021P-O, JIS H4000:2014 A8079P-O).

[0129] The metal layer may be a single layer or a laminate of two or more layers. The thickness of the metal layer is not particularly limited as long as it exhibits a barrier function against water vapor, etc., but from the viewpoint of reducing the thickness of the present pouch, the thickness is preferably about 100 μm or less, more preferably 3 μm to 100 μm, even more preferably 3 μm to 80 μm, still more preferably 6 μm to 60 μm, and even more preferably 10 μm to 40 μm.

[0130] Furthermore, from the viewpoints of stabilizing adhesion and preventing dissolution and corrosion, it is preferable that at least one surface, more preferably both surfaces, of the metal layer be chemically treated. Here, chemical treatment refers to a process for forming an acid-resistant coating on the surface of the metal layer. The metal layer may be provided with an acid-resistant coating on one side, on both sides, or without an acid-resistant coating. The thickness of the acid-resistant coating is not particularly limited, but from the viewpoints of the cohesive strength of the coating and the adhesion strength with the metal layer and the heat-sealable resin layer, it is preferably 1 nm to 1000 nm, more preferably 3 nm to 100 nm, and even more preferably 5 nm to 50 nm. The thickness of the acid-resistant coating can be measured by observation with a transmission electron microscope, or by a combination of observation with a transmission electron microscope and energy dispersive X-ray spectroscopy or electron energy loss spectroscopy.

[0131] The chemical conversion treatment is preferably carried out by applying a solution containing a compound used to form the acid-resistant coating to the surface of the metal layer by bar coating, roll coating, gravure coating, immersion, or the like, and then heating the metal layer so that the temperature thereof becomes, for example, from 70° C. to 200° C. Before the chemical conversion treatment is carried out on the metal layer, the metal layer may be previously subjected to a degreasing treatment by alkali immersion, electrolytic cleaning, acid cleaning, electrolytic acid cleaning, or the like.

[0132] Examples of resins constituting the resin layer include polyester resins, polyamide resins, polyolefin resins, epoxy resins, acrylic resins, fluororesins, polyurethane resins, silicone resins, and phenolic resins, and mixtures of these resins may also be used. Among these, polyester resins and polyamide resins are preferred, with polyamide resins being particularly preferred. The polyamide resin is not particularly limited, but aliphatic polyamides such as polyamide 6, polyamide 66, polyamide 510, polyamide 12, polyamide 46, copolymers of polyamide 6 and polyamide 66; hexamethylenediamine-isophthalic acid-terephthalic acid copolymer polyamides such as polyamide 6I, polyamide 6T, polyamide 6IT, polyamide 6I6T (I represents isophthalic acid, T represents terephthalic acid) containing structural units derived from terephthalic acid and / or isophthalic acid, polyamides containing aromatics such as polymetaxylylene adipamide (MXD6); alicyclic polyamides such as polyaminomethylcyclohexyl adipamide (PACM6); polyamides copolymerized with a lactam component or an isocyanate component such as 4,4'-diphenylmethane-diisocyanate, polyesteramide copolymers and polyetheresteramide copolymers which are copolymers of copolymerized polyamides with polyesters or polyalkylene ether glycols; copolymers thereof, and the like. These may be used alone or in combination of two or more.

[0133] Furthermore, a resin film may be used as the resin layer, and a stretched film is preferred from the viewpoint of mechanical strength, etc. The stretched film may be a uniaxially stretched film or a biaxially stretched film. Therefore, the resin layer is preferably composed of a polyamide film, and more preferably composed of a stretched polyamide film. The stretched polyamide film can be produced by a conventionally known method. For example, examples of stretching methods for forming a biaxially stretched polyamide film include a sequential biaxial stretching method, an inflation method, and a simultaneous biaxial stretching method, and the sequential biaxial stretching method is preferred.

[0134] The resin layer may be a single layer or a laminate of two or more layers. The thickness of the resin 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.

[0135] The thickness ratio of the present polyester film to the metal layer (present polyester film layer:metal layer) is preferably in the range of 1:1 to 1:5, more preferably 1:1.1 to 1:4, and even more preferably 1:1.2 to 1:3.

[0136] The laminate may also include a sealant layer. By having the sealant layer, the laminate can be adhered to another member by heat fusing with a sealant layer provided on another member. Therefore, by having the sealant layer, the laminate can easily form an exterior material by lamination. The laminate of the present invention may be bonded to another laminate of the present invention via a sealant layer, or the laminate of the present invention may be bonded to a member other than the laminate of the present invention via a sealant layer. The sealant layer may be formed from a heat-sealable resin material, and examples of resins constituting the sealant layer include polyolefins, cyclic polyolefins, carboxylic acid-modified polyolefins, and carboxylic acid-modified cyclic polyolefins. Of these, carboxylic acid-modified polyolefins are preferred, and carboxylic acid-modified polypropylene is more preferred.

[0137] Specific examples of polyolefins include polyethylenes such as low-density polyethylene, medium-density polyethylene, high-density polyethylene, and linear low-density polyethylene; polypropylenes such as homopolypropylene, block copolymers of polypropylene (e.g., block copolymers of propylene and ethylene), and random copolymers of polypropylene (e.g., random copolymers of propylene and ethylene); and ethylene-butene-propylene terpolymers. Among these, polyethylene and polypropylene are preferred, and polypropylene is more preferred.

[0138] The cyclic polyolefin is a copolymer of an olefin and a cyclic monomer, and examples of the olefin constituting the cyclic polyolefin include ethylene, propylene, 4-methyl-1-pentene, butadiene, and isoprene. Examples of the cyclic monomer constituting the cyclic polyolefin include cyclic alkenes such as norbornene; specifically, cyclic dienes such as cyclopentadiene, dicyclopentadiene, cyclohexadiene, and norbornadiene. Among these, cyclic alkenes are preferred, and norbornene is more preferred. Furthermore, styrene can also be used as a constituting monomer.

[0139] The carboxylic acid-modified polyolefin is a polymer obtained by modifying the polyolefin by block polymerization or graft polymerization with a carboxylic acid. Examples of the carboxylic acid used for modification include maleic acid, acrylic acid, itaconic acid, crotonic acid, maleic anhydride, and itaconic anhydride.

[0140] The sealant layer has a thickness of, for example, about 10 μm or more and 100 μm or less, preferably about 15 μm or more and 80 μm or less, and more preferably about 20 μm or more and 60 μm or less.

[0141] An adhesive layer may be present between each layer (e.g., between a resin layer and the polyester film, between a resin layer and a metal layer, or between the polyester film and a metal layer), and each layer may be bonded via the adhesive layer. The adhesive layer may be formed of a known adhesive, such as, but not limited to, a polyether adhesive, a polyester adhesive, a polyurethane adhesive, an epoxy adhesive, a phenolic resin adhesive, a polyamide adhesive, a polyolefin adhesive, a polyacrylic adhesive, an amino resin adhesive, a rubber adhesive, or a silicone adhesive. Furthermore, each layer may be bonded by, but not limited to, a known lamination method, such as a dry lamination method.

[0142] The laminate of the present invention preferably has at least a metal layer. The presence of the metal layer improves the strength of the laminate and functions as a barrier layer that prevents the transmission of water, steam, oxygen, light, etc., making the laminate suitable for use as an exterior packaging material, particularly an exterior packaging material for batteries. When the laminate has a metal layer, the metal layer may be provided on one side of the polyester film. The metal layer may also be bonded to the polyester film via an adhesive layer. When the laminate of the present invention has a metal layer, a resin layer may be further provided between the metal layer and the polyester film. In this case, it is preferable that an adhesive layer be further provided between the polyester film and the resin layer, or between the resin layer and the metal layer, and that the layers be bonded by the adhesive layer.

[0143] The present polyester film is preferably disposed as the outermost layer of a laminate. In particular, the present polyester film is preferably used as the outermost layer of a battery exterior material. The outermost layer of a laminate is prone to breakage due to external forces applied during molding or use, but the present polyester film exhibits excellent puncture resistance, as described above. Furthermore, the Martens hardness and indentation hardness can be increased, resulting in excellent pinhole resistance and breaking strength. Therefore, using the present polyester film as the outermost layer of a laminate facilitates appropriate prevention of damage to the laminate during molding and damage to the battery exterior material containing the laminate. From the same perspective, when the present polyester film has a surface where the content (A1) is higher than the content (A2), the surface where the content (A1) is higher than the content (A2) is preferably disposed on the outermost surface of the laminate. Furthermore, when the present polyester film includes a functional layer X, the functional layer X is preferably disposed in a position that contacts the mold, as described above. Therefore, the functional layer X is preferably disposed on the outermost surface of the laminate. That is, it is also preferred that the surface of the present polyester film having the functional layer X and having a content (A1) higher than the content (A2) is disposed on the outermost surface of the laminate.

[0144] Preferred lamination structures of the laminate of the present invention include the polyester film / adhesive layer / metal layer and the polyester film / adhesive layer / resin layer / adhesive layer / metal layer. Furthermore, the laminate of the present invention also preferably has a laminate structure having a sealant layer in addition to the metal layer. Preferred lamination structures when a metal layer or sealant layer is present include the polyester film / adhesive layer / metal layer / sealant layer and the polyester film / adhesive layer / resin layer / adhesive layer / metal layer / sealant layer. The sealant layer may be bonded directly to the metal layer or via an adhesive layer. Furthermore, the resin constituting the resin layer is preferably a polyamide resin. Therefore, the resin layer in each of the above laminate structures is preferably a polyamide resin layer, with a stretched polyamide film being more preferred.

[0145] <Uses> The polyester film is used for molding. The polyester film is usually molded in the form of the above-mentioned laminate, but may also be molded as a single layer. By molding the polyester film, for example, recesses can be formed, and components constituting a battery, such as an electricity storage device element, can be housed inside the recesses. Molding is a method of molding a polyester film or a laminate into a desired shape using a mold. The molding method is not particularly limited, and may be any molding method such as vacuum molding, pressure molding, or press molding. Furthermore, molding is preferably cold molding. Furthermore, molding is preferably deep drawing molding. The polyester film of the present invention has good extensibility, so it can be appropriately molded without causing breakage, even when molded by molding, particularly deep drawing molding.

[0146] The polyester film is preferably used in molded articles. The polyester film is preferably used as an exterior material for housing and protecting a specified component. In particular, the polyester film has excellent puncture resistance and is expected to also have polyester chemical resistance. Therefore, it can be suitably used as a battery exterior material for packaging battery cells in various batteries, such as lithium ion batteries, lithium ion polymer batteries, lead-acid 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. It is particularly suitable for use as an exterior material for lithium batteries, particularly for automotive lithium batteries. When the polyester film is used as an exterior material, the exterior material may have any configuration as long as it contains the polyester film or the laminate, but it is preferable that the exterior material contain the laminate. That is, the present invention also provides a battery exterior material comprising the polyester film or laminate described above, and a battery comprising the battery exterior material.

[0147] When the laminate of the present invention is used as an exterior packaging material and a predetermined component such as a battery cell is packaged inside, the polyester film may be disposed on the outside, preferably as the outermost layer, and the metal layer may be disposed on the inside in each of the above-mentioned laminate structures. That is, when the polyester film is used as a battery exterior packaging material, it is preferably used as the outermost layer of the battery exterior packaging material. Because the polyester film has high chemical resistance, disposing it on the outside (preferably as the outermost layer) of the exterior packaging material can adequately protect the components inside the exterior packaging material. Furthermore, when used as, for example, a battery exterior packaging material, delamination caused by leakage of electrolyte can be adequately prevented. Furthermore, when the laminate of the present invention is used as an exterior packaging material, a predetermined component (e.g., a battery cell such as an electricity storage device element) may be sandwiched between a pair of laminates, and the peripheral portions of the laminates may be heat-sealed via a sealant layer by lamination molding or the like, thereby sealing the component such as the battery cell inside the exterior packaging material. Furthermore, the laminate of the present invention may have recesses formed by molding, and components such as battery cells may be housed in the recesses. The laminates having the recesses may then be heat-sealed together via a sealant layer, and the components such as battery cells may then be sealed within the exterior packaging material.

[0148] <<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 stated that "X to Y" (X and Y are any numbers), it means "X or more and Y or less," unless otherwise specified, and also includes the meaning of "preferably larger than X" or "preferably smaller than Y." Furthermore, when it is stated that "X or more" (X is any number), it means "preferably larger than X" unless otherwise specified, and when it is stated that "Y or less" (Y is any number), it also means "preferably smaller than Y" unless otherwise specified.

[0149] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded.

[0150] <Evaluation Methods> (1) Measurement of intrinsic viscosity (dl / g) of polyester and film 1 g of polyester was precisely weighed, dissolved in 100 ml of a mixed solvent of phenol / tetrachloroethane = 50 / 50 (mass ratio), and measured at 30° C. When particles were blended, the particles were removed and 1 g of polyester was precisely weighed to measure the intrinsic viscosity.

[0151] (2) Degree of transverse orientation (Δnv), degree of longitudinal orientation (Δnp), and degree of planar orientation (ΔP) The degree of transverse orientation (Δnv), degree of longitudinal orientation (Δnp), and degree of planar orientation (ΔP) were measured by the method described in the specification.

[0152] (3-1) Puncture Strength (N / mm) and Displacement Amount (mm) When Puncture Strength Was Measured The puncture strength (N) was measured in accordance with JIS Z1707:2019, and the value per unit thickness obtained by dividing the puncture strength (N) by the film thickness was taken as the puncture strength (N / mm). The displacement amount when measuring the puncture strength was taken as the displacement amount when punctured. The average value of 12 points was used for the measurement. Specifically, a 210 mm x 297 mm sample was cut from the center of a film roll obtained by the method described below, with the short side aligned in the horizontal direction of the film. 12 points were randomly selected and measured evenly from the cut sample, and the average value of these measured values ​​was used.

[0153] (3-2) Variation in puncture strength and variation in displacement upon puncture Using the measured values ​​of the puncture strength and displacement upon puncture obtained above at 12 points, the variation in puncture strength and variation in displacement upon puncture were evaluated using the following formula (1) as the coefficient of variation. A smaller value indicates smaller variation. Formula (1) = standard deviation / average value

[0154] (4) Glass transition temperature (Tg), number of endothermic peaks, and melting point (Tm) Using a differential scanning calorimeter (PerkinElmer Japan "Diamond DSC"), the sample was heated from -70 ° C. to 280 ° C. at a heating rate of 10 ° C. / min, held for 10 minutes, then cooled to -70 ° C. at a heating rate of 600 ° C. / min, and then heated again at a heating rate of 10 ° C. / min. The measurement was performed. The specific heat change due to the transition from the glass state to the rubber state during the second heating process was read, and Tg (glass transition temperature) was determined. The midpoint glass transition temperature was used as Tg (glass transition temperature). When the sample was heated and cooled in the same manner using a differential scanning calorimeter, the peak top temperature of each endothermic peak observed during the first heating process (1st run) and the second heating process (2nd run) was taken as the melting point, and the values ​​are shown in Table 2. When two or more peak tops are observed, both values ​​are recorded. Therefore, one value recorded in the melting point column indicates one endothermic peak, and two values ​​recorded in the melting point column indicates two endothermic peaks. When two or more endothermic peaks are observed, the temperature at the top position of the maximum peak is taken as the melting point.

[0155] (5) Heat Shrinkage Rate A 15 mm x 150 mm sample of the polyester film obtained in each of the Examples and Comparative Examples was heat-treated for 15 minutes in an oven maintained at 160°C in an untensioned state, and the lengths of the sample were measured before and after the treatment, and the heat shrinkage rates in the machine direction (MD) and the transverse direction (TD) of the film were calculated using the following formula: Heat Shrinkage Rate (%) = {(L0 - L1) / L0} x 100 (where L0 is the sample length before the heat treatment, and L1 is the sample length after the heat treatment) Measurements were taken at five points in each of the machine direction (MD) and the transverse direction (TD) of the film, and the average value was calculated for each.

[0156] (6) Haze: Measured in accordance with JIS K 7136:2000 using a haze meter NDH-2000 manufactured by Nippon Denshoku Industries Co., Ltd.

[0157] (7) Martens hardness, indentation hardness, and elastic deformation power Approximately 2 to 8 mg of Aron Alpha (registered trademark) (general-purpose, manufactured by Toa Gosei Chemical Industry Co., Ltd.) was dropped onto a glass slide (S1112, manufactured by Matsunami Glass Industry Co., Ltd.). One side (the side opposite to the surface to be measured) of a polyester film (1.5 cm x 1.5 cm) was placed on top of it as an adhesive surface and allowed to harden. The glass slide with the polyester film was fixed to the sample stage of a hardness tester (dynamic ultra-micro hardness tester (DUH-211S, manufactured by Shimadzu Corporation)), and then a load-unload test was performed on the surface of the polyester film, and the elastic deformation power (η it ) was obtained, and the Martens hardness and indentation hardness were also measured. Each value was calculated as the average of 10 measurements, excluding the first one, out of n = 11 measurements. Wtotal = Wplast + Welast (N m) (Wtotal = total deformation work (N m), Wplast = plastic deformation work (N m), Welast = elastic deformation work (N m)) η it = (Welast / Wtotal) x 100 (%) (Measurement conditions) Indenter used: Diamond regular triangular pyramidal indenter (edge ​​angle: 115) Measurement mode: Load-unload test Test force: 20.00 mN Minimum test force: 0.20 mN Load rate: 0.1464 mN / sec Load holding time: 2 sec Unload holding time: 0 sec Measurement atmosphere: 23±2°C, relative humidity 50±5% Number of measurements: 11

[0158] <Materials used> [Polyester raw materials] (PBT) PBT-A: homobutylene terephthalate (intrinsic viscosity = 0.85 dL / g) PBT-B: homobutylene terephthalate (intrinsic viscosity = 1.26 dL / g) (PET) PET-A: homopolyethylene terephthalate (intrinsic viscosity = 0.64 dL / g) PET-B: homopolyethylene terephthalate (intrinsic viscosity = 0.85 dL / g) PET-C: masterbatch in which 0.55 mass% of silica particles having an average particle size of 3.1 μm were blended into homopolyethylene terephthalate (intrinsic viscosity = 0.61 dL / g)

[0159] Example 1 As shown in Table 2, PBT-A, PET-A, and PET-C were dry-blended in a mass ratio of 20:70:10 to form the surface layer raw materials, and PBT-A and PET-A were dry-blended in a mass ratio of 20:80 to form the core layer raw materials. The mixed raw materials for the surface layer and core layer were each fed into separate twin-screw extruders, kneaded at 280°C, and co-extruded at 280°C. The resulting mixture was cooled and solidified on a cooling roll set at 25°C using an electrostatically applied adhesion method, yielding a two-kind, three-layer (surface layer / core layer / surface layer) unstretched film. The resulting unstretched film was then stretched 3.5 times in the machine direction (MD) at 73°C using a roll stretcher. The film was then introduced into a tenter stretcher, preheated in the tenter at 75°C for 6 seconds, and then stretched 4.5 times in the transverse direction (TD) at 85°C. After stretching, the film was subsequently heat-set at 210°C for 8 seconds and cooled at 140°C under 2% relaxation in the transverse direction (TD) to obtain a biaxially oriented polyester film with a thickness of 25 μm (2.5 μm for each surface layer, 20 μm for the core layer). The biaxially oriented polyester film was wound into a film roll. The obtained polyester film was evaluated. The evaluation results are shown in Table 2. The PET / PBT mass ratio shown in Table 2 is the mass ratio of PET and PBT contained in the entire three-layer biaxially oriented polyester film. Specifically, the PET / PBT mass ratio in the entire polyester film was calculated from the blending ratio of PET and PBT in each layer and the thickness ratio of each layer, and the mass ratio was rounded to one decimal place.

[0160] (Examples 2 to 4, 6, 7, Comparative Examples 1 and 2) The same procedures as in Example 1 were carried out, except that the compositions of the surface layer and core layer and the film production conditions were changed as shown in Table 2. For Example 8, the Martens hardness, indentation hardness, and elastic deformation power of the obtained polyester film were measured, and the Martens hardness was 140 N / mm ２ , indentation hardness is 220 N / mm ２ The elastic deformation power was 45%.

[0161] (Example 5) The compositions of the surface layer and core layer and the film production conditions were changed as shown in Table 2, the preheating time after longitudinal stretching was changed to 7 seconds, the heat setting treatment time was changed to 10 seconds, and a coating solution for an easy-adhesion layer having the following formulation was applied to one side of the polyester film by in-line coating at a stage after longitudinal stretching and before transverse stretching, so that the coating amount of the coating solution on one side of the polyester film (after drying and stretching) was 0.05 g / m ２ The same procedure as in Example 1 was carried out, except that an easy-adhesion layer was formed by coating the coating liquid so that the composition satisfies the following formula: (Composition of coating liquid for easy-adhesion layer) - Water dispersion of polyester resin 90% by mass - Monomer composition: (acid component) polyester resin copolymerized with terephthalic acid / isophthalic acid / 5-sodium sulfoisophthalic acid / / (diol component) ethylene glycol / 1,4-butanediol / diethylene glycol = 56 / 40 / 4 / / 70 / 20 / 10 (mol%) - Melamine compound: hexamethoxymethylolmelamine 9% by mass - Silica particles having an average particle size of 0.07 μm 1% by mass

[0162] (Example 8) The same procedure as in Example 1 was carried out, except that the compositions of the surface layer and core layer and the film production conditions were changed as shown in Table 2, the preheating time after longitudinal stretching was changed to 7 seconds, and the heat setting treatment time was changed to 10 seconds.

[0163]

[0164] The polyester films of the above Examples contained both PBT and PET, and the absolute value of the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) was 57 or less, so they had high puncture strength, large displacement upon puncture, and small variations in displacement and strength upon puncture, resulting in excellent puncture resistance. In contrast, the polyester films of Comparative Examples 1 and 2 contained both PBT and PET, but the absolute value of the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) was greater than 57, so they exhibited large variations in displacement or strength upon puncture, and were unable to achieve excellent puncture resistance. Note that the polyester film of Example 8 contained both PBT and PET, and the PET content (A1) at the surface was higher than the PET content (A2) in the central portion (core layer) in the thickness direction, resulting in high puncture strength, Martens hardness, indentation hardness, and elastic deformation power. Therefore, it can be seen that the polyester film of Example 8 has excellent pinhole resistance, is less likely to break when external force is applied, and also has excellent design and shape recovery properties, making it particularly suitable for battery exterior materials.

Claims

1. A polyester film containing polybutylene terephthalate and polyethylene terephthalate, the absolute value of the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) of the polyester film is 57 or less; A polyester film having a melting point (1st run) of 230°C or higher.

2. 2. The polyester film according to claim 1, wherein the ratio of the content of polyethylene terephthalate to the content of polybutylene terephthalate (PET / PBT) is 55 / 45 or more and 95 / 5 or less in mass ratio.

3. 3. The polyester film according to claim 1, which has a puncture strength of 480 N / mm or more.

4. 3. The polyester film according to claim 1, wherein the displacement when pierced is 4 mm or more.

5. 3. The polyester film according to claim 1, wherein the puncture strength variation is 0.036 or less.

6. 3. The polyester film according to claim 1, wherein the variation in displacement upon piercing is 0.034 or less.

7. The polyester film according to claim 1 or 2, having a planar orientation degree of 100 or more and 200 or less.

8. 3. The polyester film according to claim 1, wherein after heat treatment at 160°C for 15 minutes, the polyester film has a heat shrinkage rate of less than 5% in one of the longitudinal and transverse directions and a heat shrinkage rate of more than 2% in the other direction.

9. 3. The polyester film according to claim 1, which has a haze of 10% or less.

10. 3. The polyester film according to claim 1, wherein the intrinsic viscosity of the polyester film is 0.6 dL / g or more and 0.85 dL / g or less.

11. 3. The polyester film according to claim 1, which is a biaxially oriented polyester film.

12. A method for producing a polyester film containing polybutylene terephthalate and polyethylene terephthalate, wherein the absolute value of the difference between the transverse orientation degree (Δnv) and the longitudinal orientation degree (Δnp) of the polyester film is 57 or less, Stretching in the machine direction and the transverse direction, A method for producing a polyester film, wherein the preheating temperature during transverse stretching is 60°C or higher and 90°C or lower, and the stretching temperature is 70°C or higher and 120°C or lower.

13. The method for producing a polyester film according to claim 12, wherein the heat setting temperature after the stretching is 190°C or higher.

14. The method for producing a polyester film according to claim 12 or 13, wherein the ratio of the transverse stretching ratio to the longitudinal stretching ratio (TD / MD) is 1 or more and 1.7 or less.

15. The method for producing a polyester film according to claim 12 or 13, wherein the stretching temperature during longitudinal stretching is 50°C or higher and 85°C or lower.

16. The method for producing a polyester film according to claim 12 or 13, wherein the longitudinal stretching ratio is 2.5 times or more and 4.5 times or less.

17. The method for producing a polyester film according to claim 12 or 13, wherein the transverse stretching ratio is 3.5 times or more and 5.5 times or less.