polyester film

Abstract

To provide a polyester film that offers low retardation properties and also excels in flex resistance and heat resistance.SOLUTION: A polyester film comprises a main component resin that is a resin composition (X) mainly composed of polyethylene terephthalate (A), with an extensional intrinsic birefringence of 0.0350 or less. The polyester film has a fusion enthalpy of 5-50 J / g.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a polyester film.

Background Art

[0002] Polyester is excellent in properties such as heat resistance, weather resistance, mechanical strength, transparency, chemical resistance, and gas barrier properties, and is also easily available in terms of price. Therefore, it has high versatility and is currently widely used in containers and packaging materials for beverages and foods, molded products, films, etc. The main polyester resin is polyethylene terephthalate, which is excellent in mechanical properties, chemical resistance, etc. and has a wide range of uses, but has drawbacks in terms of flex resistance, etc.

[0003] On the other hand, in recent years, with the miniaturization and weight reduction of electronic devices, etc., there is a tendency to use flexible substrates and flexible printed circuits. Along with this trend, the demand for flexibility in display applications is increasing, and there is a strong demand for a film that is excellent in resilience and repeated fold resistance (flex resistance).

[0004] In addition, for films used as various optical members such as liquid crystal displays, touch panels, OLEDs (Organic Light Emitting Diodes), etc., for example, it is required to be able to suppress the occurrence of interference colors and rainbow unevenness associated with optical interference under polarized light. As a solution to such problems, there is a method of controlling retardation within a specific range, but it is desirable to control the retardation of the film low in terms of reducing the anisotropy of the film and maintaining the physical property balance between the longitudinal direction (MD) and the width direction (TD).

[0005] For example, Patent Document 1 discloses a polyester film for a foldable display that is excellent in mass productivity and in which no crack occurs in the folded portion in order to provide a foldable display in which the image displayed in the folded portion after repeated folding is not disturbed.

[0006] Patent Document 2 discloses a polyester resin having a fluorene skeleton that also possesses a high refractive index, low birefringence, and high toughness, as well as a molded article containing the resin.

[0007] Furthermore, Patent Document 3 discloses a polyarylate resin composition that is excellent in moldability, heat resistance, impact resistance, and transparency, and hardly discolors during molding, and is characterized by containing 10 to 90 parts by mass of polyarylate resin and 90 to 10 parts by mass of polyester, which is made up of at least terephthalic acid and 1,4-cyclohexanedimethanol, wherein the amount of terephthalic acid used is 50 to 100 mol% of the total dicarboxylic acid components of the polyester, and the amount of 1,4-cyclohexanedimethanol used is 50 to 100 mol% of the total diol components of the polyester. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2021-9349 [Patent Document 2] Japanese Patent Publication No. 2016-69643 [Patent Document 3] Japanese Patent Publication No. 2002-302596 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, although the film disclosed in Patent Document 1 has excellent flexibility in the longitudinal direction (MD), its flexibility in the width direction (TD) is insufficient. As a result, the bending characteristics are constrained by the orientation of the film, which can increase the workload in the display manufacturing and processing processes, as it may be necessary to align the orientation of the film each time it is manufactured. Furthermore, due to its high retardation, while interference can be prevented, the anisotropy of the film cannot be reduced, resulting in differences in bending characteristics depending on the bending direction, as described above, which may make it difficult to apply to some displays.

[0010] Patent documents 2 and 3 do not consider controlling the retardation of the film to a low level in order to maintain a balance between the physical properties of the film in the longitudinal direction (MD) and the width direction (TD) while suppressing the occurrence of interference colors and rainbow unevenness due to light interference.

[0011] Furthermore, the films made of resins described in Patent Documents 2 and 3 are highly likely to be amorphous, and may have insufficient heat resistance at high temperatures exceeding the glass transition temperature. In addition, because the film is amorphous, deformation when an external force is applied becomes significant at high temperatures near or above the glass transition temperature, which may result in insufficient bending resistance under high-temperature conditions.

[0012] Furthermore, a challenge is that the resins described in Patent Documents 2 and 3 are expensive, and the films made from these resins are also expensive.

[0013] The problem that this invention aims to solve is to provide a polyester film that solves the above-mentioned problems, has low retardation characteristics, and also exhibits excellent flexibility and heat resistance. [Means for solving the problem]

[0014] The inventors of this invention have diligently studied and conducted research to achieve the above objectives, and as a result, have completed this invention. In one aspect, the present invention is summarized in the following [1] to

[13] . [1] A polyester film comprising a resin composition (X) as the main component resin, having polyethylene terephthalate (A) as the main component and having a stretched intrinsic birefringence of 0.0350 or less, and having a melting enthalpy of 5 to 50 J / g. [2] The polyester film according to [1] above, wherein the glass transition temperature of the resin composition (X) is 75 to 130°C. [3] The polyester film according to [1] or [2] above, wherein the resin composition (X) further contains polyarylate (B). [4] The polyester film according to [3] above, comprising 1 part by mass or more and 60 parts by mass or less of the polyethylene terephthalate (A) per 100 parts by mass of the polyarylate (B). [5] The polyester film according to [3] or [4] above, wherein the resin composition (X) further contains a compatibilizer (C). [6] The polyester film according to [5] above, comprising 1 part by mass or more and 50 parts by mass or less of the compatibilizer (C) per 100 parts by mass of polyethylene terephthalate (A). [7] The polyester film according to any one of [1] to [6] above, wherein the polyethylene terephthalate (A) is homopolyethylene terephthalate. [8] A polyester film according to any one of [1] to [7] above, wherein the in-plane retardation is 500 nm or less. [9] A polyester film according to any one of [1] to [8] above, wherein the average value of the bending angle after high-temperature bending tests in the longitudinal direction (MD) and the width direction (TD) is 140 degrees or less.

[10] A polyester film according to any one of the above [1] to [9], wherein the average value of the hysteresis loss rate when a tensile cycle test is performed in both the longitudinal (MD) and widthwise (TD) directions up to a 5% tensile strain is 50% or less.

[11] A polyester film according to any one of the above [1] to

[10] , wherein the thermal shrinkage rate when heated at 150°C for 30 minutes is -10 to 10% in both the longitudinal direction (MD) and the width direction (TD).

[12] A polyester film according to any one of [1] to

[11] above, which is stretched in at least one direction.

[13] A polyester film according to any one of [1] to

[12] above, which is for a display. [Advantages of the Invention]

[0015] The polyester film of the present invention has low retardation characteristics and is also excellent in flex resistance and heat resistance. Therefore, the polyester film of the present invention can be suitably used for displays, particularly for flexible displays and the like. [Brief Description of the Drawings]

[0016] [Figure 1] It is a diagram showing a method for measuring the bending angle after a high-temperature bending test. [Figure 2] It is a profile of a stress-strain curve. [Modes for Carrying Out the Invention]

[0017] Hereinafter, the present invention will be described in detail. However, the present invention is not limited to the embodiments described below.

[0018] [[Polyester Film]] The polyester film of the present invention (hereinafter, also referred to as "the present film") contains a resin composition (X) having polyethylene terephthalate (A) as a main component and an intrinsic birefringence of stretching of 0.0350 or less as a main resin, and has a melting enthalpy of 5 to 50 J / g.

[0019] This film may have a single-layer structure or a laminated (multilayer) structure. If this film has a laminated structure, it may have a two-layer structure, a three-layer structure, or more, and may have four or more layers, as long as it does not depart from the gist of the present invention. The number of layers to be laminated is not particularly limited, but it is preferably 10 layers or less. If there are 10 layers or less, the thickness of each layer will be sufficient, resulting in sufficient lamination during film formation, making it less likely for flow marks to occur, and ensuring that the quality of the film is well maintained. In particular, from the viewpoint of reducing the manufacturing cost of this film, a single-layer structure or a laminated structure of two to three layers is preferred.

[0020] Furthermore, this film may be an unoriented film (sheet) or an oriented film. In particular, an oriented film stretched in one or two axes is preferred. Among these, a biaxially oriented film is more preferred in terms of its excellent balance of mechanical properties and flatness, its ability to be made into a thin film, and its ease of controlling retardation to a low level.

[0021] The present film preferably has resin composition (X) as its main component resin. Furthermore, if the present film has a laminated structure, it is preferable that the main component resin of each layer is resin composition (X). The term "main component resin" refers to the resin that makes up the largest proportion of each layer, for example, the resin that accounts for 50% by mass or more, preferably 70% by mass or more, and more preferably 80% by mass or more (including 100% by mass) of the resins that make up each layer.

[0022] <Resin composition (X)> The resin composition (X) is characterized by having polyethylene terephthalate (A) as its main component and having a stretched intrinsic birefringence of 0.0350 or less.

[0023] The stretched intrinsic birefringence of the resin composition (X) is 0.0350 or less. The above-mentioned intrinsic birefringence due to stretching refers to the slope of birefringence with respect to the stretch ratio when stretching is performed under conditions where the orientation of the polymer chain is hardly relaxed due to stretching. Therefore, it can be used as an indicator to evaluate the effect of stretching on birefringence. In the present invention, when it is desirable to control the retardation of the film to a low level, it is preferable to use a resin with a low intrinsic birefringence, and more preferably one that is less affected by stretching conditions. Therefore, if the stretched intrinsic birefringence exceeds 0.0350, the birefringence with respect to stretching becomes high, making it difficult to control the retardation to a low level. From the viewpoint of obtaining low retardation characteristics, the stretched intrinsic birefringence is 0.0350 or less, preferably 0.0330 or less, more preferably 0.0310 or less, and even more preferably 0.0290 or less. Furthermore, the smaller the stretched intrinsic birefringence, the better, and although there is no particular lower limit, it is about 0.0010. For example, in general crystalline polyester resins, as described later, resins with a glass transition temperature of 75 to 130°C suitable for the present invention typically have a stretched intrinsic birefringence of about 0.0380 to 0.0800. The stretched intrinsic birefringence is the value calculated using the method described in the example.

[0024] Furthermore, the glass transition temperature of the resin composition (X) is preferably 75 to 130°C. If the glass transition temperature is 75°C or higher, the film will have good flexibility and heat resistance under high-temperature conditions. On the other hand, if the glass transition temperature is 130°C or lower, the stretch moldability will not decrease, and it can be applied to stretched films without any problems. From the viewpoint of achieving both flexibility, heat resistance, and stretchability under high-temperature conditions, the glass transition temperature is more preferably 78 to 120°C, even more preferably 80 to 110°C, particularly preferably 83 to 100°C, and especially preferably 85 to 90°C.

[0025] (Polyethylene terephthalate (A)) The resin composition (X) is characterized by having polyethylene terephthalate (A) as its main component. By using polyethylene terephthalate (A) as the main component, the cost of the resin composition (X), and consequently the cost of the film, can be reduced. Furthermore, "main component" means that it is included as the resin with the highest content proportion among the resins constituting each layer. For example, it is included as a resin that accounts for 50% by mass or more, preferably 70% by mass or more, and more preferably 80% by mass or more (including 100% by mass) among the resins constituting each layer.

[0026] The polyethylene terephthalate (A) is a polyester that contains terephthalic acid as a dicarboxylic acid component (a-1) and ethylene glycol as a diol component (a-2), with terephthalic acid and ethylene glycol as the main components, respectively. In other words, it is a polyester that contains 50 mol% or more of terephthalic acid as a dicarboxylic acid component (a-1) and 50 mol% or more of ethylene glycol as a diol component (a-2). In particular, the polyethylene terephthalate (A) used in the present invention preferably contains 80 mol% or more of terephthalic acid as the dicarboxylic acid component (a-1), more preferably 90 mol% or more, and even more preferably 100 mol%. Furthermore, the polyethylene terephthalate (A) preferably contains 80 mol% or more of ethylene glycol as the diol component (a-2), more preferably 90 mol% or more, and even more preferably 100 mol%. Among these, the most preferred is that the polyethylene terephthalate (A) is a polyester (homopolyethylene terephthalate) containing 100 mol% of terephthalic acid as the dicarboxylic acid component (a-1) and 100 mol% of ethylene glycol as the diol component (a-2).

[0027] The polyethylene terephthalate (A) may be copolymerized with a dicarboxylic acid component other than terephthalic acid as the dicarboxylic acid component (a-1). Specifically, examples include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,5-frandicarboxylic acid, 2,4-frandicarboxylic acid, 3,4-frandicarboxylic acid, benzophenone dicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenyl ether dicarboxylic acid; aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and dimer acid; and oxycarboxylic acids such as p-oxybenzoic acid. These dicarboxylic acid components can be used individually or in combination of two or more. Furthermore, the content of dicarboxylic acid components other than terephthalic acid is preferably 20 mol% or less, and more preferably 10 mol% or less, of the total dicarboxylic acid (a-1) components including terephthalic acid.

[0028] The polyethylene terephthalate (A) may be copolymerized with a diol component other than ethylene glycol as the diol component (a-2). Specifically, examples include diethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or their derivatives, or ethylene oxide adducts thereof). These diol components can be used individually or in combination of two or more. Furthermore, the content of diol components other than ethylene glycol is preferably 20 mol% or less, and more preferably 10 mol% or less, of the total diol components (a-2) including ethylene glycol.

[0029] Normally, when polyester is produced (polycondensed) using ethylene glycol as one of the raw materials, diethylene glycol is produced as a by-product from ethylene glycol. In this specification, this diethylene glycol is referred to as by-product diethylene glycol. The amount of diethylene glycol produced as a by-product from ethylene glycol varies depending on the type of polycondensation, but it is approximately 5 mol% or less of the ethylene glycol. In this invention, diethylene glycol of 5 mol% or less is considered by-product diethylene glycol, and this by-product diethylene glycol is also included in ethylene glycol and distinguished from copolymer components. On the other hand, depending on the content of diethylene glycol, more specifically, if the content of diethylene glycol exceeds 5 mol%, the diethylene glycol is treated as a copolymer component rather than as by-product diethylene glycol.

[0030] (Polyarylate (B)) Preferably, the resin composition (X) further contains polyarylate (B). Generally, the heat resistance of a resin composition can be improved by increasing its glass transition temperature. Therefore, the resin composition (X) according to the present invention, by mixing polyethylene terephthalate (A) with polyarylate (B), yields a resin composition with a higher glass transition temperature than polyethylene terephthalate (A) alone, and exhibits superior heat resistance. Furthermore, obtaining a resin composition with a higher glass transition temperature than polyethylene terephthalate (A) alone results in superior flexibility under high-temperature conditions.

[0031] Polyarylate (B) is a polycondensate of an aromatic dicarboxylic acid component and a divalent phenol component. The dicarboxylic acid component (b-1) constituting the polyarylate (B) is not particularly limited as long as it is a divalent aromatic carboxylic acid, but a mixture of terephthalic acid and isophthalic acid is preferred. The ratio (mol%) of terephthalic acid to isophthalic acid is preferably terephthalic acid / isophthalic acid = 99 / 1 to 1 / 99, more preferably 90 / 10 to 10 / 90, even more preferably 80 / 20 to 20 / 80, particularly preferably 70 / 30 to 30 / 70, and especially preferably 60 / 40 to 40 / 60. By having the ratio of terephthalic acid to isophthalic acid as the dicarboxylic acid component (b-1) within the above range, polyarylate (B) exhibits excellent heat resistance and extrusion moldability.

[0032] Polyarylate (B) may also be copolymerized with a dicarboxylic acid component other than terephthalic acid and isophthalic acid as the dicarboxylic acid component (b-1). Specifically, aromatic dicarboxylic acids such as phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, benzophenone dicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenyletherdicarboxylic acid are preferred. Furthermore, in order to avoid impairing the heat resistance of polyarylate (B), it is preferable that the copolymerization ratio of dicarboxylic acid components other than terephthalic acid and isophthalic acid be 10 mol% or less.

[0033] The divalent phenol component (b-2) constituting the polyarylate (B) is not particularly limited as long as it is a divalent phenol, but it is preferable that it contains either bisphenol A (2,2-bis(4-hydroxyphenyl)propane), bisphenol TMC (1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane), or both bisphenol A and bisphenol TMC.

[0034] Generally, the inclusion of bisphenol A results in polyarylate (B) with excellent extrudeability (fluidity). On the other hand, the inclusion of bisphenol TMC improves the glass transition temperature, resulting in a polyarylate (B) with excellent heat resistance. When a balance between extrusion moldability and heat resistance is desired, it is preferable to use both bisphenol A and bisphenol TMC. In this case, the ratio (mol%) of bisphenol A to bisphenol TMC is preferably bisphenol A / bisphenol TMC = 99 / 1 to 1 / 99, more preferably 90 / 10 to 10 / 90, even more preferably 80 / 20 to 20 / 80, particularly preferably 70 / 30 to 30 / 70, and especially preferably 60 / 40 to 40 / 60. By adjusting the ratio of bisphenol A to bisphenol TMC within a certain range, a polyarylate (B) with an excellent balance of heat resistance and melt moldability is obtained.

[0035] Polyarylate (B) may also be copolymerized with bisphenol A and bisphenol TMC as the divalent phenol component (b-2). Specifically, bisphenol AP (1,1-bis(4-hydroxyphenyl)-1-phenylethane), bisphenol AF (2,2-bis(4-hydroxyphenyl)hexafluoropropane), bisphenol B (2,2-bis(4-hydroxyphenyl)butane), bisphenol BP (bis(4-hydroxyphenyl)diphenylmethane), bisphenol C (2,2-bis(3-methyl-4-hydroxyphenyl)propane), bisphenol E (1,1-bis(4-hydroxyphenyl)ethane), bisphenol F (bis(4-hydroxyphenyl)methane), bis Examples include phenol G (2,2-bis(4-hydroxy-3-isopropylphenyl)propane), bisphenol M (1,3-bis(2-(4-hydroxyphenyl)-2-propyl)benzene), bisphenol S (bis(4-hydroxyphenyl)sulfone), bisphenol P (1,4-bis(2-(4-hydroxyphenyl)-2-propyl)benzene), bisphenol PH (5,5'-(1-methylethylidene)-bis[1,1'-(bisphenyl)-2-ol]propane), and bisphenol Z (1,1-bis(4-hydroxyphenyl)cyclohexane). To avoid impairing the heat resistance of polyarylate (B), the copolymerization ratio of the above compound is preferably 10 mol% or less.

[0036] To enhance the compatibility of polyarylate (B) with polyethylene terephthalate (A), it is preferable to select a mixture of terephthalic acid and isophthalic acid as the dicarboxylic acid component (b-1), and either bisphenol A, bisphenol TMC, or a mixture of bisphenol A and bisphenol TMC as the divalent phenol component (b-2).

[0037] If the resin composition (X) contains polyarylate (B), it is preferable that it contains polyarylate (B) in a ratio of 1 part by mass or more and 60 parts by mass or less per 100 parts by mass of polyethylene terephthalate (A). If the content of polyarylate (B) in the resin composition (X) is 1 part by mass or more, the glass transition temperature of the resin composition (X) increases, improving its heat resistance and flexibility under high-temperature conditions. On the other hand, if the content is 60 parts by mass or less, the crystallinity of the resin composition (X) is maintained, making it easier to achieve a desired melting enthalpy for the resulting film. Furthermore, if the content is within this range, it is easier to control the stretched intrinsic birefringence to a desired value, thereby maintaining the low retardation characteristics of the film. From the above viewpoint, the content ratio of polyarylate (B) is more preferably 10 parts by mass or more and 55 parts by mass or less per 100 parts by mass of polyethylene terephthalate (A), and even more preferably 20 parts by mass or more and 50 parts by mass or less.

[0038] The polyarylate (B) used in this invention may be mixed with polycarbonate to improve extrusion moldability. Since polyarylate (B) and polycarbonate are compatible, mixing polycarbonate with polyarylate (B) can lower the glass transition temperature of polyarylate (B) while maintaining transparency and mechanical properties, thereby improving extrusion moldability. In such cases, the polycarbonate content will be converted to the polyarylate (B) content.

[0039] (Compatibilizer (C)) If the resin composition (X) contains polyethylene terephthalate (A) and polyarylate (B), it is preferable to further contain a compatibilizer (C) because polyethylene terephthalate (A) and polyarylate (B) are immiscible. The resin composition containing the compatibilizer (C) can improve the transparency of the film's appearance.

[0040] The compatibilizer (C) is not particularly limited as long as it can improve the compatibility between polyethylene terephthalate (A) and polyarylate (B), and examples include those that can improve compatibility by transesterification (accelerated). A specific example is a compatibilizer (C) containing polyethylene terephthalate, polyarylate, and a transesterification catalyst. Although polyethylene terephthalate and polyarylate are inherently immiscible, the inclusion of a transesterification catalyst allows them to be made compatible through a transesterification reaction. By incorporating such a compatibilizer (C) into the resin composition (X), the compatibility between polyethylene terephthalate (A) and polyarylate (B) can be improved, resulting in a polyester film with good appearance and transparency.

[0041] The specific and preferred embodiments of polyethylene terephthalate contained in the compatibilizer (C) are the same as those of polyethylene terephthalate (A) described above, and all of these can be used. Furthermore, the specific and preferred embodiments of the polyarylate contained in the compatibilizer (C) are the same as those of the polyarylate (B) described above, and all of these can be used.

[0042] From the viewpoint of compatibility and moldability, the ratio (mass%) of polyethylene terephthalate to polyarylate in the compatibilizer (C) is preferably polyethylene terephthalate / polyarylate = 90 / 10 to 10 / 90, more preferably 80 / 20 to 20 / 80, even more preferably 70 / 30 to 30 / 70, particularly preferably 65 / 35 to 35 / 65, especially preferably 65 / 35 to 40 / 60, and most preferably 65 / 35 to 50 / 50.

[0043] The transesterification catalyst can be one of the conventionally known ones, such as aliphatic carboxylic acid metal salts, more specifically, sodium salts and potassium salts of acetic acid and propionic acid. The amount of the transesterification catalyst added is preferably 0.01 to 0.5 parts by mass, more preferably 0.02 to 0.3 parts by mass, per 100 parts by mass of the total amount of polyethylene terephthalate and polyarylate. The transesterification catalyst can be added between before and immediately after the start of transesterification, and is not particularly limited.

[0044] Furthermore, the compatibilizer (C) may contain an organophosphorus compound to suppress discoloration caused by heat during molding. Examples of the organophosphorus compound include phosphate compounds and / or phosphite compounds, and more specifically, those described in Japanese Patent Publication No. 2002-302596. The amount of the organophosphorus compound added is preferably 0.01 to 0.5 parts by mass, more preferably 0.02 to 0.3 parts by mass, per 100 parts by mass of the total amount of polyethylene terephthalate and polyarylate.

[0045] If the resin composition (X) contains a compatibilizer (C), it is preferable that the compatibilizer (C) is included in an amount of 1 part by mass or more and 50 parts by mass or less per 100 parts by mass of polyethylene terephthalate (A). If the content ratio of the compatibilizer (C) in the resin composition (X) is within this range, the compatibility of polyethylene terephthalate (A) and polyarylate (B) in the resin composition (X) can be improved without impairing the low retardation properties, flexibility, and heat resistance of the film, and the transparency of the film's appearance can be improved. From the above viewpoint, the content ratio of the compatibilizer (C) is more preferably 5 parts by mass or more and 48 parts by mass or less per 100 parts by mass of polyethylene terephthalate (A), even more preferably 6 parts by mass or more and 45 parts by mass or less, and particularly preferably 8 parts by mass or more and 40 parts by mass or less.

[0046] (Other resins) The resin composition (X) may contain other resins other than polyethylene terephthalate (A), polyarylate (B), and compatibilizer (C), as long as the effects of the present invention are not impaired. Other resins include polyesters other than those listed above, polystyrene, polyvinyl chloride, polyvinylidene chloride, chlorinated polyethylene, polyamide, polyacetal, acrylic, ethylene vinyl acetate copolymer, polymethylpentene, polyvinyl alcohol, cyclic olefin, polylactic acid, polybutylene succinate, polyacrylonitrile, polyethylene oxide, cellulose, polyimide, polyurethane, polyphenylene sulfide, polyphenylene ether, polyvinyl acetal, polybutadiene, polybutene, polyamideimide, polyamidebismaleimide, polyetherimide, polyetheretherketone, polyethersulfone, polyketone, polysulfone, etc.

[0047] <particle> This film may contain particles primarily for the purpose of providing slipperiness and preventing scratches during each process. The type of particles to be added is not particularly limited as long as they can provide slipperiness. Specific 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; and organic particles such as acrylic resin, styrene resin, urea resin, phenolic resin, epoxy resin, and benzoguanamine resin. Furthermore, precipitated particles obtained by precipitating and finely dispersing a portion of metal compounds such as catalysts during the polymer manufacturing process, such as polyester, can also be used.

[0048] <Other> Furthermore, this film may contain commonly used additives as appropriate. Examples of such additives include recycled resins generated from trimming losses such as edges, pigments such as titanium dioxide and carbon black, flame retardants, weather stabilizers, heat stabilizers, antistatic agents, melt viscosity modifiers, crosslinking agents, lubricants, nucleating agents, plasticizers, anti-aging agents, antioxidants, light stabilizers, ultraviolet absorbers, neutralizing agents, anti-fogging agents, anti-blocking agents, slip agents, and colorants, which are added for the purpose of improving and adjusting moldability, productivity, and various physical properties.

[0049] <<Method for manufacturing polyester film>> Next, we will explain in detail the manufacturing method of this film. The following description is one example of a method for manufacturing a biaxially oriented film as the film in question, and the film is not limited to films manufactured by this method.

[0050] One example of an embodiment of the present invention is a method for manufacturing the film, in which a resin composition (X) is formed into a film and then biaxially stretched.

[0051] The method for kneading the polyethylene terephthalate (A), the polyarylate (B), the compatibilizer (C), the other resins, and additives to obtain a resin composition (X) is not particularly limited, but in order to obtain the resin composition as simply as possible, it is preferable to manufacture it by melt kneading using an extruder. In order to uniformly mix the raw materials constituting the resin composition, it is preferable to melt knead using a coaxial twin-screw extruder. The mixing temperature is preferably above the glass transition temperature of all polymers used, and for crystalline resins, it is preferably above the crystal melting temperature of the polymer. While a higher mixing temperature relative to the glass transition temperature or crystal melting temperature of the polymer is preferable because it facilitates the transesterification reaction of some of the polymers and improves compatibility, excessively high mixing temperatures are undesirable because they cause decomposition of the resin. For this reason, the mixing temperature is preferably 250°C to 330°C, more preferably 255°C to 325°C, even more preferably 260°C to 320°C, and particularly preferably 265°C to 315°C. Within the range of the mixing temperature, compatibility and melt moldability can be improved without causing decomposition of the polymer.

[0052] The obtained resin composition (X) can be molded by general molding methods, such as extrusion molding, injection molding, blow molding, vacuum molding, pressure molding, and press molding, to produce a biaxially oriented film. The apparatus and processing conditions are not particularly limited in each molding method. This film is preferably manufactured by, for example, the following method.

[0053] From the obtained resin composition (X), a substantially amorphous and unoriented film (hereinafter also referred to as "unoriented film") is manufactured by extrusion. This unoriented film can be manufactured, for example, by melting the raw material for the film (resin composition (X), etc.) in an extruder, extruding it through a flat die or annular die, and then rapidly cooling it to produce a flat or annular unoriented film. In this case, a laminated structure using multiple extruders may be used.

[0054] Next, the unstretched film described above is stretched in at least one direction, usually by 1.1 to 6.0 times, and preferably by 1.1 to 6.0 times each in the longitudinal direction (MD) and the width direction (TD) perpendicular thereto, from the viewpoint of stretching effect, film strength, etc. Here, from the viewpoint of controlling the retardation of the film to a low level, when biaxial stretching is performed, it is preferable to stretch the film so that the stretching ratio in the longitudinal direction (MD) and the width direction (TD) are equal.

[0055] As for biaxial stretching methods, any conventionally known stretching method can be used, such as tenter-type sequential biaxial stretching, tenter-type simultaneous biaxial stretching, and tubular-type simultaneous biaxial stretching. For example, in the tenter-type sequential biaxial stretching method, the unstretched film is heated to a temperature range of Tg to Tg+50°C, where Tg is the glass transition temperature of the resin composition (X), stretched 1.1 to 6.0 times in the longitudinal direction using a roll-type longitudinal stretcher, and then stretched 1.1 to 6.0 times in the transverse direction using a tenter-type transverse stretcher within the temperature range of Tg to Tg+50°C. In the case of tenter-type simultaneous biaxial stretching or tubular-type simultaneous biaxial stretching, for example, the film can be manufactured by stretching 1.1 to 6.0 times in each axial direction simultaneously in both the longitudinal and transverse directions within the temperature range of Tg to Tg+50°C.

[0056] The biaxially oriented film stretched by the above method is preferably subsequently heat-set. Heat-setting provides dimensional stability at room temperature. In this case, the processing temperature is preferably selected from the range of the crystal melting temperature Tm-1 to Tm-150°C of the resin composition (X). If the heat-setting temperature is within the above range, sufficient heat-setting is performed, the stress during stretching is relieved, sufficient heat resistance and mechanical properties are obtained, and an excellent film is obtained that is free from problems such as breakage and whitening of the film surface.

[0057] In this invention, in order to alleviate the stress of crystallization shrinkage due to heat setting, it is preferable to perform relaxation in the width direction by 0 to 15%, preferably 1 to 10%, during heat setting. When sufficient relaxation is performed and the film is relaxed uniformly in the width direction, the shrinkage rate in the width direction becomes uniform, and a film with excellent dimensional stability at room temperature is obtained. Furthermore, since the relaxation follows the shrinkage of the film, there is no sagging of the film, flapping in the tenter, and no film breakage.

[0058] <<Physical Properties of Polyester Film>> This film exhibits crystalline properties. This crystalline nature results in excellent heat resistance, particularly in high-temperature regions exceeding the glass transition temperature. Furthermore, the crystalline nature of this film improves its flexibility under high-temperature conditions. More specifically, the enthalpy of melting of this film is 5 to 50 J / g, preferably 10 to 45 J / g, and more preferably 15 to 40 J / g. If the enthalpy of melting is less than 5 J / g, the heat resistance and flexural resistance of the film at high temperatures will be insufficient. On the other hand, if the enthalpy of melting exceeds 50 J / g, the melt-molding properties of the film may decrease.

[0059] The in-plane retardation of this film is preferably 500 nm or less, more preferably 400 nm or less, even more preferably 300 nm or less, and particularly preferably 200 nm or less. If the in-plane retardation is 500 nm or less, the film can be said to have low retardation characteristics, which can suppress the occurrence of interference colors and rainbow unevenness due to light interference, making it suitable for use in displays. Furthermore, the lower the in-plane retardation, the lower the anisotropy of the film, so that the balance of physical properties in the longitudinal direction (MD) and the width direction (TD) of the film can be maintained, and differences in bending characteristics depending on the bending direction are less likely to occur. Therefore, the lower limit of the in-plane retardation may be 0 nm or more, but it may also be 10 nm or more, or 20 nm or more. Furthermore, from a similar viewpoint, the birefringence of this film is preferably 0.01000 or less, more preferably 0.00800 or less, even more preferably 0.00600 or less, particularly preferably 0.00400 or less, and especially preferably 0.00300 or less, and may be 0.00100 or more.

[0060] The bending resistance of this film at high temperatures (90°C) can be evaluated by the average bending angle after a high-temperature bending test. The bending angle in the high-temperature bending test is the bending angle after the film, folded 180 degrees into a 2.0 mm gap and held in that folded state, is left standing in a 90°C oven for 6 hours, then removed to room temperature, the fold is immediately released, and it is left in that state for 24 hours. More specifically, it can be evaluated by the method described in the examples. The smaller the bending angle, the greater the restorative force that causes the film to return to its original state after bending, meaning that the bending resistance is considered good. The bending angle is evaluated using the average value, which refers to the average value of the bending angle in the longitudinal direction (MD) and the bending angle in the width direction (TD). Therefore, it is preferable that the average value of the bending angle after high-temperature bending tests in both the longitudinal (MD) and widthwise (TD) directions be 140 degrees or less, more preferably 139 degrees or less, and even more preferably 138 degrees or less. If the average value of the bending angle is 140 degrees or less, the restorative force of the film is increased, and it can be said that the film has excellent bending resistance under high-temperature conditions. Furthermore, the lower limit of the bending angle should be as small as possible, ideally 0 degrees or greater.

[0061] The flexural resistance of this film at room temperature (23°C) can be evaluated by the hysteresis loss rate when a tensile cycle test is performed up to 5% tensile strain. More specifically, it can be evaluated by the method described in the examples. The smaller the hysteresis loss rate, the greater the restorative force that causes the film to return to its original state after bending, meaning that the bending resistance is considered good. The hysteresis loss rate is evaluated as an average value, and the average value here refers to the average value of the hysteresis loss rate in the longitudinal direction (MD) and the hysteresis loss rate in the width direction (TD). Therefore, it is preferable that the average value of the hysteresis loss rate when a tensile cycle test is performed up to 5% tensile strain in both the longitudinal (MD) and widthwise (TD) directions is 50% or less, more preferably 49% or less, and even more preferably 48% or less. If the average value of the hysteresis loss rate is 50% or less, the restoring force of the film is increased, and it can be said that the film has excellent bending resistance under room temperature conditions. Furthermore, the lower limit of the hysteresis loss rate is better the smaller it is, and should be 0% or higher.

[0062] The flexibility of this film is preferably such that it meets the aforementioned range in evaluations under both high-temperature and room-temperature conditions. Excellent flexibility not only at room temperature but also at high-temperature conditions makes it suitable for use in high-temperature environments. Examples of use under high-temperature conditions include using the device after leaving it folded in a car for a long time in the summer, or using it when the device has become hot after fast charging. Therefore, flexibility under high-temperature conditions is necessary. Furthermore, for display applications, it is preferable to have excellent flexibility regardless of the usage environment.

[0063] The thermal shrinkage rate of this film when heated at 150°C for 30 minutes is preferably -10 to 10% in both the longitudinal direction (MD) and the width direction (TD), more preferably -5 to 5%, even more preferably -3 to 3%, and particularly preferably -2 to 2%. This thermal shrinkage rate of -10 to 10% gives the film excellent heat resistance, especially dimensional stability at high temperatures exceeding the glass transition temperature, making it practically suitable for use. Note that positive values ​​indicate shrinkage, and negative values ​​indicate elongation.

[0064] The above physical properties of this film can be adjusted by the type and content of resin used, the stretching ratio, the stretching temperature, and the heat-fixing temperature, among other film-forming conditions.

[0065] The thickness of this film is preferably 1 to 250 μm, more preferably 5 to 200 μm, even more preferably 10 to 150 μm, and particularly preferably 20 to 75 μm. A thickness of 1 μm or more ensures that the film strength is within a practical range. A thickness of 250 μm or less makes it suitable for display applications. The thickness of this film can be adjusted by factors such as stretching conditions.

[0066] <<Functional Layer>> A functional layer may be provided on at least one side of this film. Having a functional layer on at least one side of this film provides various functions.

[0067] Examples of the functional layers include a hard coat layer, an antistatic layer, a release layer, an easy-adhesion layer, an infrared shielding layer, an ultraviolet shielding layer, a printed layer, an adhesive layer, and a bleeding prevention layer. The functional layers may be provided as a single layer or as two or more layers laminated together. The method for forming the functional layer is not particularly limited and may be provided by in-line coating, which treats the film surface during the stretching process; or by off-line coating, which is applied to a film that has already been manufactured outside the system; or both may be used in combination.

[0068] <<Application>> This film has low retardation properties and excellent flexibility and heat resistance, making it suitable for use in displays, especially flexible displays. Flexible displays include foldable displays that can be folded, bendable displays that can be folded and bent, rollable displays that can be rolled up, and stretchable displays that can be expanded and contracted. This film is particularly preferred for use in foldable displays. The foldable display described above may be tri-fold or quad-fold.

[0069] Furthermore, the display is suitable for use with mobile phones, smartphones, digital cameras, personal computers, etc. The type of display is not particularly limited, but examples include LCD, organic EL display, inorganic EL display, LED, FED, etc., with flexible LCD, organic EL, and inorganic EL being preferred. Among these, organic EL and inorganic EL, which can reduce the number of layers, are more preferred, and organic EL with a wide color gamut is even more preferred.

[0070] In the present invention, "for display" refers to any component of a display, and examples include a film that protects the surface side of a display device (surface protection film), a base film for a touch sensor, and a film that protects the back side of a display device (back protection film).

[0071] <<Explanation of terms>> In this invention, the term "film" includes "sheets," and the term "sheet" includes "film." In this invention, when "X~Y" (where X and Y are any numbers) is written, unless otherwise specified, it means "X or greater and Y or less," and also includes the meaning of "preferably greater than X" or "preferably less than Y." Furthermore, when "X or greater" (where X is any number) is written, unless otherwise specified, it includes the meaning of "preferably greater than X," and when "Y or less" (where Y is any number) is written, unless otherwise specified, it also includes the meaning of "preferably less than Y." [Examples]

[0072] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited in any way by the following examples.

[0073] <Evaluation Method> (1) Glass transition temperature (Tg) of resin composition (X) For a resin composition (X) that does not contain crystalline components, the glass transition temperature (Tg) was measured using a DSC8000 (manufactured by PerkinElmer Japan) in accordance with JIS K7121 (2012) during a heating process at a heating rate of 10°C / min.

[0074] (2) The stretched intrinsic birefringence of the resin composition (X) Films of each resin composition (X) were drawn with a grid pattern at 1 cm intervals and passed through a transverse stretching machine (tenter) set to an entrance width of 200 mm and a line speed of 3 m / min. Preheating: stretching was performed at a stretching temperature of -5°C for 32 seconds, stretching was performed at a temperature of Tg +10°C of the resin composition (X) for 32 seconds, stretching in the width direction (TD) to a predetermined magnification in the range of 1.1 to 5 times, and heat fixing: heat treatment was performed at the same temperature as the stretching temperature for 32 seconds to obtain uniaxially oriented films. For the obtained uniaxially oriented film, the length of the grid in the width direction (TD) was measured, and the actual stretching ratio was calculated by dividing it by the grid width before stretching. In addition, birefringence was measured using the method described later, and an XY graph (X: actual stretching ratio, Y: birefringence) was created from the actual stretching ratio and birefringence, and the slope of the graph was defined as the intrinsic birefringence of the stretched film.

[0075] (3) In-plane retardation (Re) of this film The obtained film was subjected to in-plane retardation (Re) measurement at room temperature at a measurement wavelength of 586.4 nm using a phase difference measuring device (KOBURA-WR software: KOBRA-RE, manufactured by Oji Instruments Co., Ltd.). For films with an in-plane retardation (Re) of 4000 nm or less, the above measurement values ​​were used. However, for films where the above measurement value exceeded 4000 nm, or where the value clearly exceeded 4000 nm due to the stretching conditions, or where abnormalities were observed in the measurement results, resulting in a value below 4000 nm due to errors in the above measurement, the following measurement values ​​were used. For the obtained films, the in-plane retardation (Re) was measured at a wavelength of 589 nm, with an aperture diameter of 5 mm, at 23°C using a cell gap inspection device (RETS-1100A, manufactured by Otsuka Electronics Co., Ltd.).

[0076] (4) Birefringence of this film (Δn) The obtained film was subjected to birefringence (Δn) measurement at room temperature at a measurement wavelength of 586.4 nm using a phase difference measuring device (KOBURA-WR, software: KOBRA-RE, manufactured by Oji Instruments Co., Ltd.). For films with an in-plane retardation (Re) of 4000 nm or less, the above measurement values ​​were used. However, for films where the above measurement value exceeded 4000 nm, or where the value clearly exceeded 4000 nm due to the stretching conditions, or where abnormalities were observed in the measurement results, resulting in a value below 4000 nm due to errors in the above measurement, the following measurement values ​​were used. The obtained film was subjected to a cell gap inspection device (RETS-1100A, manufactured by Otsuka Electronics Co., Ltd.) to measure its birefringence (Δn) at a wavelength of 589 nm, with an aperture diameter of 5 mm, at 23°C.

[0077] (5) The enthalpy of melting of this film (ΔHm) The obtained film was measured using a DSC8000 (manufactured by PerkinElmer Japan) in accordance with JIS K7121 (2012) to determine the enthalpy of melting (ΔHm) during a heating process at a heating rate of 10°C / min. Furthermore, if an exothermic peak due to crystallization was observed at a temperature lower than the melting peak during the heating process, the crystallization enthalpy (ΔHc) was calculated and subtracted from the melting enthalpy to obtain the melting enthalpy (ΔHm) of the resulting film.

[0078] (6) The flexibility (bending angle) of this film under high temperature (90°C) conditions. A test piece measuring 30 mm in the longitudinal direction (MD) x 3 mm in the width direction (TD) was cut from any point on the obtained film. The film was held in a 2.0 mm gap with the longitudinal direction (MD) folded in half, and left standing in a 90°C oven without humidification for 6 hours. After that, it was removed to room temperature, the fold was immediately released, and the fold angle (MD) was measured after leaving it for 24 hours. The fold angle (MD) was measured at three points on the same film, and the average value was taken as the fold angle (MD). The fold angle (TD) was measured by cutting a test piece measuring 3 mm in the longitudinal direction (MD) x 30 mm in the width direction (TD) from any point on the obtained film, holding the film in a 2.0 mm gap with the width direction (TD) folded in half, and then measuring the fold angle (TD) in the same manner.

[0079] The aforementioned bending angle was measured in more detail as follows: After leaving the test specimen for 24 hours, it was placed on a stand so that the direction of the fold was vertical, and the angle formed by the fold (fold angle) when viewed from directly above was measured as shown in Figure 1. The bending angle was calculated by subtracting the fold angle from 180 degrees. Furthermore, the average value of the measured bending angle in the longitudinal direction (MD) (the bending angle (MD)) and the bending angle in the width direction (TD) (the bending angle (TD)) was calculated and used as the average bending angle.

[0080] (7) The flexibility (hysteresis loss rate) of this film under room temperature (23°C) conditions. In accordance with JIS K 7312:1996, the average value of the hysteresis loss rate at 23°C was determined by the following method. A tensile testing machine (Shimadzu Corporation AG-1kNXplus) was used for measurement. The test specimen was a rectangle cut from the obtained film, with a length of 100 mm and a width of 10 mm in the measurement direction. The test specimen was chucked at both ends in the longitudinal direction with a chuck distance of 50 mm, and the specimen was raised to a strain of 5% at a crosshead speed of 0.5 mm / min, and then lowered to the initial position at the same speed. A stress-strain curve was obtained from one cycle of tensile cycle testing. The stress-strain curve took the profile shown in Figure 2, and the hysteresis loss rate was calculated from the obtained stress-strain curve using the area A1 (abcda) of the curve obtained during the raising motion and the area A2 (abcef), which is the difference between area A1 and the area of ​​the curve obtained during the lowering motion, using the following formula (1). The test was measured three times, and the average value was calculated. The above tensile cycle test was performed in the longitudinal direction (MD) and the width direction (TD) of the film, and the average value was calculated. Hysteresis loss rate = (A2 / A1) × 100 ... Equation (1)

[0081] (8) Thermal shrinkage rate of this film The obtained film was cut into rectangles with a length of 120 mm and a width of 10 mm in the measurement direction, and marked 100 mm from the edge. These test pieces were suspended by clipping the edges and heated at 150°C for 30 minutes. After cooling, the length from the edge of the test piece to the mark was measured to determine the thermal shrinkage rate. Measurements were taken in both the longitudinal (MD) and transverse (TD) directions.

[0082] (9) Thickness of this film The thickness of the obtained film was determined by measuring it at five unspecified points within the surface using a 1 / 1000 mm dial gauge, and the average of these measurements was taken as the thickness.

[0083] <Materials used> [Resin type: Polyester resin (a)] As the polyester resin (a), a resin was used that contained 60 parts by mass of polyethylene terephthalate (A), 20 parts by mass of polyarylate (B), and 20 parts by mass of a compatibilizer (C). The glass transition temperature of the polyester resin (a) was 88°C, the enthalpy of fusion was 29 J / g, and the melting point was 245°C. Furthermore, the details of the blended polyethylene terephthalate (A), polyarylate (B), and compatibilizer (C) are as follows. [Polyethylene terephthalate (A)] Dicarboxylic acid component: Terephthalic acid = 100 mol%, Diol component: Ethylene glycol = 100 mol% [Polyarylate (B)] Dicarboxylic acid components: Terephthalic acid = 50.4 mol%, Isophthalic acid = 49.6 mol%, Diol component: Bisphenol A = 100 mol% [Compatibilizer (C)] <Polyethylene terephthalate component: 60.8 parts by mass> Dicarboxylic acid component: Terephthalic acid = 100 mol%, Diol component: Ethylene glycol = 96.3 mol%, Diethylene glycol = 3.7 mol% <Polyarylate component: 39.2 parts by mass> Dicarboxylic acid components: Terephthalic acid = 50.1 mol%, Isophthalic acid = 49.9 mol%, Diol component: Bisphenol A = 100 mol% <Transesterification catalyst: Sodium acetate = 0.033 parts by mass>

[0084] [Resin type: Polycyclohexylene dimethylene terephthalate (b)] Dicarboxylic acid components: Terephthalic acid = 91.8 mol%, Isophthalic acid = 8.2 mol%, Diol component: 1,4-Cyclohexanedimethanol = 100 mol%

[0085] [Resin type: Polyethylene terephthalate (c)] Dicarboxylic acid component: Terephthalic acid = 100 mol%, Diol component: Ethylene glycol = 100 mol%

[0086] [Resin type: Polyethylene naphthalate (d)] Dicarboxylic acid component: 2,6-naphthalenedicarboxylic acid = 100 mol%, Diol component: ethylene glycol = 100 mol%

[0087] Table 1 shows the glass transition temperatures and stretched intrinsic birefringences of the above resin types (a) to (d).

[0088] (Example 1) Polyester resin (a) was melt-kneaded in a Φ40 mm twin-screw extruder set to 280°C, extruded as a film through a T-die with a 1.0 mm gap, taken up by a cast roll at 92°C, cooled and solidified to obtain a film-like material (cast film) with a thickness of approximately 450 μm. Next, the obtained cast film was passed through a roll longitudinal stretcher and stretched 3.0 times in the longitudinal direction (MD) at a preheating temperature of 95°C and a stretching temperature of 110°C. Subsequently, the obtained longitudinally stretched film was passed through a transverse stretcher (tenter) and stretched 3.1 times in the width direction (TD) at a preheating temperature of 127°C, a stretching temperature of 130°C, and a heat-setting temperature of 180°C. After that, the film was heat-set in the tenter and relaxed by 3.0% in the width direction (TD) to obtain a biaxially oriented film. Table 2 shows the results of measurements performed on the obtained films.

[0089] (Example 2) A biaxially oriented film was obtained using the same method as in Example 1, except that the film formation conditions, such as the stretching conditions and heat-fixing temperature, were changed as shown in Table 2. Table 2 shows the results of measurements performed on the obtained films.

[0090] (Example 3) A biaxially oriented film was obtained using the same method as in Example 1, except that the preheating temperature during longitudinal stretching was changed to 100°C, and the film formation conditions, such as stretching conditions and heat setting temperature, were changed as shown in Table 2. Table 2 shows the results of measurements performed on the obtained films.

[0091] (Comparative Example 1) Using pelletized (b), a biaxially oriented film was obtained by extrusion molding using a twin-screw extruder, sequential biaxial stretching, and heat setting at the stretching ratio and heat setting temperature described in Table 2, using the same method as in Example 1, i.e., a T-die method using a twin-screw extruder. Table 2 shows the results of measurements performed on the obtained films.

[0092] (Comparative Example 2) Using pelletized (c), a biaxially oriented film was obtained by extrusion molding using a twin-screw extruder, sequential biaxial stretching, and heat setting at the stretching ratio and heat setting temperature described in Table 2, using the same method as in Example 1, i.e., a T-die method, sequential biaxial stretching, and heat setting. Table 2 shows the results of measurements performed on the obtained films.

[0093] (Comparative Example 3) Using pelletized (d), a biaxially oriented film was obtained by extrusion molding using a twin-screw extruder, sequential biaxial stretching, and heat setting at the stretching ratio and heat setting temperature described in Table 2, using the same method as in Example 1, i.e., a T-die method using a twin-screw extruder. Table 2 shows the results of measurements performed on the obtained films.

[0094] [Table 1]

[0095] [Table 2]

[0096] As is clear from Examples 1 to 3 above, the polyester film of the present invention contains a resin composition (X) as the main component resin, which is mainly polyethylene terephthalate (A) and has a stretched intrinsic birefringence of 0.0350 or less. This makes it possible to control retardation to a low level, resulting in a small average bending angle after high-temperature bending tests and a small hysteresis loss rate, thus providing excellent bending resistance under high-temperature and room-temperature conditions. Furthermore, by setting the melting enthalpy of the film to 5 to 50 J / g, it is possible to achieve good heat resistance even in high-temperature regions exceeding the glass transition temperature, and to improve bending resistance under high-temperature conditions. [Industrial applicability]

[0097] The polyester film of the present invention has low retardation properties and excellent flexibility and heat resistance, making it suitable for use in displays, particularly flexible displays. Therefore, the embodiments of this disclosure are useful as flexible display components such as foldable displays, bendable displays, rollable displays, and stretchable displays, taking advantage of the benefits of flexible display panels that can be folded, folded and bent, rolled up, and stretched.

Claims

1. The main component resin is a resin composition (X) which has polyethylene terephthalate (A) as its main component, contains polyarylate (B) and a compatibilizer (C), and has a stretched intrinsic birefringence of 0.0350 or less. The enthalpy of melting is 5 to 50 J / g. A stretched polyester film having an in-plane retardation of 500 nm or less.

2. A resin composition (X) comprising polyethylene terephthalate (A) as the main component, containing polyarylate (B) and a compatibilizer (C), and having a stretched intrinsic birefringence of 0.0350 or less, as the main component resin, A stretched polyester film having a melting enthalpy of 15 to 50 J / g.

3. A resin composition (X) comprising polyethylene terephthalate (A) as the main component, containing polyarylate (B) and a compatibilizer (C), and having a stretched intrinsic birefringence of 0.0350 or less, as the main component resin, The enthalpy of melting is 5 to 50 J / g. Stretched polyester film for use in foldable displays.

4. The stretched polyester film according to any one of claims 1 to 3, wherein the glass transition temperature of the resin composition (X) is 75 to 130°C.

5. The stretched polyester film according to any one of claims 1 to 3, comprising 1 part by mass or more and 60 parts by mass or less of the polyarylate (B) per 100 parts by mass of the polyethylene terephthalate (A).

6. The stretched polyester film according to any one of claims 1 to 3, comprising 1 part by mass or more and 50 parts by mass or less of the compatibilizer (C) per 100 parts by mass of polyethylene terephthalate (A).

7. The stretched polyester film according to any one of claims 1 to 3, wherein the polyethylene terephthalate (A) is homopolyethylene terephthalate.

8. The stretched polyester film according to claim 2 or 3, wherein the in-plane retardation is 500 nm or less.

9. A stretched polyester film according to any one of claims 1 to 3, wherein the average value of the bending angle after high-temperature bending tests in the longitudinal direction (MD) and the width direction (TD) is 140 degrees or less.

10. The stretched polyester film according to any one of claims 1 to 3, wherein the average value of the hysteresis loss rate when a tensile cycle test is performed up to a 5% tensile strain in both the longitudinal direction (MD) and the width direction (TD) is 50% or less.

11. A stretched polyester film according to any one of claims 1 to 3, wherein the thermal shrinkage rate when heated at 150°C for 30 minutes is -10 to 10% in both the longitudinal direction (MD) and the width direction (TD).

12. A stretched polyester film according to claim 1 or 2, for use in displays.

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