Biaxially stretched film

Abstract

To provide a biaxially stretched film that has excellent folding resistance and heat resistance.SOLUTION: A biaxially stretched film contains a polyethylene naphthalate copolymer (A), wherein the average of hysteresis loss ratios after a tensile cycle test up to 5% tensile strain in the MD direction and in the TD direction is 47.0% or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a biaxially stretched film having excellent heat resistance and folding resistance.

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 resins for containers and packaging materials for beverages and foods, molded products, films, etc. The main polyester resin is polyethylene terephthalate (hereinafter sometimes referred to as "PET"), which is excellent in mechanical properties, electrical properties, chemical resistance, etc. and has a wide range of uses, but has drawbacks in heat resistance, folding resistance, etc.

[0003] In addition, as a material using naphthalenedicarboxylic acid as an acid component, polybutylene naphthalate (PBN) obtained by polymerizing naphthalenedicarboxylic acid and 1,4-butanediol is known, but since its glass transition temperature is about 75°C and its melting point is about 240°C, which is low, there is a problem in heat resistance. In addition, since the crystallization rate is too fast, it is not suitable for film formation by extrusion molding.

[0004] On the other hand, in recent years, as the need for flexible displays has been increasing, there has been a strong demand for a film having high heat resistance, excellent resilience, and excellent repeated folding resistance.

[0005] For example, Patent Document 1 examines a film with repeated folding resistance using a cyclic olefin resin film.

[0006] In addition, a polyimide film has been proposed as a film having excellent heat resistance and bending resistance (Patent Document 2).

[0007] In addition, Patent Document 3 proposes a polyethylene naphthalate resin in which an ethylene oxide adduct of a bisphenol compound or its derivative is blended in all alcohol components, improving crystallinity without impairing heat resistance.

[0008] Further, Patent Document 4 proposes a modified polyester resin in which 2,6-naphthalenedicarboxylic acid is used as the acid component and a bisphenol-based compound and 1,4-cyclohexanedimethanol are blended in the glycol component, improving heat resistance, impact strength, etc.

[0009] Furthermore, Patent Document 5 discloses a polyethylene naphthalate resin having a high glass transition temperature by blending N,N-bis-(2-hydroxyethyl)-4,4'-biphthalimide in all alcohol components.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0011] However, the film disclosed in Patent Document 1 has a low level of repeated bending resistance and does not meet the market requirements. In addition, since cyclic olefin resins are poor in coatability and adhesiveness, it is considered difficult to laminate them with other members as members for flexible displays.

[0012] Also, although the polyimide film described in Patent Document 2 has flex resistance, in its manufacturing process, since it is a molding method by coating using a solvent, productivity is poor and cost is also high.

[0013] The polyethylene naphthalate resin described in Patent Document 3 improves the crystallization rate, improves troubles during raw material drying, and improves moldability by blending an ethylene oxide adduct of a bisphenol compound or its derivative in all alcohol components. However, if the crystallization rate is too fast, the film forming property and drawability of the film by extrusion molding deteriorate, so it is not suitable for a drawn film.

[0014] The modified polyester resin described in Patent Document 4 exhibits high heat resistance. On the other hand, the temperature during molding must be set high, concerns about thermal decomposability become apparent, and there is a possibility that the moldability and the properties of the material after molding deteriorate.

[0015] The polyethylene naphthalate resin described in Patent Document 5 has extremely excellent heat resistance. However, since it contains a diol component having a rigid biphthalimide skeleton, the fold resistance may deteriorate.

[0016] The problem to be solved by the present invention is to solve the above problems and provide a biaxially oriented film excellent in fold resistance and heat resistance.

Means for Solving the Problem

[0017] As a result of intensive studies to achieve the above problems, the present inventors have completed the present invention. The present invention has the following aspects. [1] A biaxially oriented film containing a polyethylene naphthalate copolymer (A), and having an average value of the hysteresis loss rate of 47.0% or less when a tensile cycle test is performed up to a 5% tensile strain in each of the MD and TD directions. [2] A biaxially stretched film containing a polyethylene naphthalate copolymer (A), wherein the average value of the residual strain is 0.900% or less when a tensile cycle test is performed up to a 5% tensile strain in each of the MD and TD directions. [3] The biaxially stretched film according to [1] or [2] above, wherein the polyethylene naphthalate copolymer (A) contains 2,6-naphthalenedicarboxylic acid units as the dicarboxylic acid component (a-1), and contains a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units and ethylene glycol units as the diol component (a-2). [4] The biaxially stretched film according to [3] above, wherein the terephthalic acid units as the other dicarboxylic acid component other than (a-1) are less than 2 mol% in all the dicarboxylic acid components. [5] The biaxially stretched film according to [3] or [4] above, wherein the diol units having a biphtalimide skeleton as the other diol component other than (a-2) are less than 1 mol% in all the diol components. [6] A biaxially stretched film containing a polyethylene naphthalate copolymer (A), containing a dicarboxylic acid component (a-1) and a diol component (a-2), the dicarboxylic acid component (a-1) contains at least 2,6-naphthalenedicarboxylic acid units, the diol component (a-2) consists of two components of a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units and ethylene glycol units, and the terephthalic acid units as the other dicarboxylic acid component other than (a-1) are less than 2 mol% in all the dicarboxylic acid components. [7] The biaxially stretched film according to any one of [3] to [6] above, wherein the diol component (a-2) contains 4 mol% or more and 70 mol% or less of a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units, and contains 30 mol% or more and 96 mol% or less of ethylene glycol units. [8] The biaxially stretched film according to any one of [1] to [7] above, wherein the glass transition temperature is 75°C or higher and 150°C or lower. [9] The biaxially stretched film according to any one of [1] to [8] above, having a crystal melting temperature of 220°C or higher and 300°C or lower.

[10] The biaxially stretched film according to any one of [1] to [9] above, having a thickness of 1 μm or more and 250 μm or less.

[11] The biaxially stretched film according to any one of [1] to

[10] above, for use in a display.

[12] A foldable display comprising the biaxially stretched film according to any one of [1] to

[11] above.

Effect of the Invention

[0018] According to the present invention, a biaxially stretched film excellent in fold resistance and heat resistance can be proposed.

Brief Description of the Drawings

[0019]

Figure 1

Mode for Carrying Out the Invention

[0020] Hereinafter, the present invention will be described in detail. However, the present invention is not limited to the embodiments described below.

[0021] <Biaxially Stretched Film> In an embodiment of the present invention, the biaxially stretched film of the first aspect contains a polyethylene naphthalate (hereinafter sometimes referred to as "PEN") - based copolymer (A), and the average value of the hysteresis loss rate when a tensile cycle test is performed up to 5% tensile strain in each of the MD and TD directions is 47.0% or less.

[0022] The biaxially stretched film of the second aspect contains a polyethylene naphthalate - based copolymer (A), and the average value of the residual strain when a tensile cycle test is performed up to 5% tensile strain in each of the MD and TD directions is 0.900% or less.

[0023] The biaxially stretched film of the third aspect contains a polyethylene naphthalate copolymer (A), includes a dicarboxylic acid component (a-1) and a diol component (a-2), the dicarboxylic acid component (a-1) contains at least 2,6-naphthalenedicarboxylic acid units, the diol component (a-2) consists of two components of bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units and ethylene glycol units, and in the total dicarboxylic acid component, the terephthalic acid units as other dicarboxylic acid components other than (a-1) are less than 2 mol%. Since this film is a biaxially stretched film, it can be made into a thin film and has excellent folding resistance. Note that MD means the flow direction of the film, and TD means the direction perpendicular to MD. Also, the biaxially stretched films of the first to third aspects described above may be referred to as "this film".

[0024] The present invention has found that a biaxially stretched film containing a PEN copolymer having a hysteresis loss rate or a residual strain of a specific value or less has excellent folding resistance and heat resistance as a film for display, and is particularly suitable for foldable applications, and has been completed. Since this film has a low hysteresis loss rate or residual strain, it is considered to have excellent resilience and exhibit folding resistance. Also, a biaxially stretched film containing a PEN copolymer having a specific composition also has excellent folding resistance and heat resistance as a film for display, and is particularly suitable for foldable applications.

[0025] 1. Physical properties First, the physical properties of this film will be described.

[0026] (1) Hysteresis loss rate When a tensile cycle test is performed on this film up to 5% tensile strain in the MD and TD directions at 23°C, the average value of the hysteresis loss rate is 47.0% or less, more preferably 46.0% or less, still more preferably 45.0% or less, and even more preferably 44.0% or less. The lower limit is not particularly limited, but it is 0.100% or more. When the hysteresis loss rate is 47.0% or less, the restoring force of the film increases and the fold resistance (flex resistance) of the film is maintained within a practical range. Also, by taking the average value of the hysteresis loss rate in the MD and TD directions respectively, it can be used as a characteristic index for the entire film. The hysteresis loss rate can be adjusted according to stretching conditions and the like. The hysteresis loss rate of this film can be measured by the method described in the examples in accordance with JIS K 7312:1996. More specifically, when a stress-strain curve graph as shown in Fig. 1 is obtained by a tensile cycle test, the ratio of the area enclosed by abcef to the area of the whole (abcda) is defined as the hysteresis loss rate.

[0027] Also, the hysteresis loss rate in the MD and TD directions of this film at 23°C is preferably 47.0% or less, more preferably 46.0% or less, still more preferably 45.0% or less, and even more preferably 44.0% or less. It is preferable that the hysteresis loss rate of one of MD and TD is within the above numerical range, and it is more preferable that the hysteresis loss rates of both MD and TD are within the above numerical range.

[0028] Furthermore, the difference in the hysteresis loss rate between the MD and TD directions of this film at 23°C is preferably 20.0% or less, more preferably 15.0% or less, and still more preferably 10.0% or less. Since the difference in the hysteresis loss rate in the MD and TD directions is within the above numerical range, the bending resistance of the film, and thus the anisotropy of various film properties, is reduced. Therefore, when using the film as a member or during secondary processing, there is no need to select a specific direction, and defects specific to only a particular direction due to anisotropy are less likely to occur, resulting in a film with excellent handleability. In addition, for the invention according to the first aspect of the present invention, it is necessary that the average value of the hysteresis loss rate when a tensile cycle test is performed up to 5% tensile strain in each of the MD and TD directions at 23°C is 47.0% or less, and it is further preferable to satisfy the requirements for residual strain described below.

[0029] (2) Residual strain The average value of the residual strain when a tensile cycle test is performed up to 5% tensile strain in each of the MD and TD directions of this film at 23°C is 0.900% or less, more preferably 0.890% or less, even more preferably 0.880% or less, and still more preferably 0.870% or less. The lower limit is not particularly limited, but it is 0.100% or more. When the residual strain is 0.900% or less, the restoring force of the film increases, and the bending resistance (folding resistance) of the film is maintained within a practical range. Also, by taking the average value of the residual strain in each of the MD and TD directions, it can be used as a characteristic index for the entire film. The residual strain can be adjusted by stretching conditions and the like. The residual strain of this film can be measured by the method described in the examples in accordance with JIS K 7312:1996. More specifically, when a graph of a stress-strain curve as shown in FIG. 1 is obtained by a tensile cycle test, the value of f is defined as the residual strain.

[0030] In addition, the residual strain in the MD and TD directions of this film at 23°C is preferably 0.900% or less, more preferably 0.890% or less, even more preferably 0.880% or less, and still more preferably 0.870% or less. It is preferable that the residual strain of one of MD and TD is within the above numerical range, and it is more preferable that the residual strains of both MD and TD are within the above numerical range.

[0031] Furthermore, the difference in the residual strains in the MD and TD directions of this film at 23°C is preferably 0.900% or less, more preferably 0.500% or less, still more preferably 0.200% or less, and even more preferably 0.100% or less. When the difference in the residual strains in the MD and TD directions is within the above numerical range, the bending resistance of the film, and thus the anisotropy of various film properties, becomes small. Therefore, when using the film as a member or during secondary processing, there is no need to select a specific direction, and since defects specific to only a specific direction due to anisotropy are less likely to occur, the film has excellent handling properties.

[0032] (3) Glass transition temperature The glass transition temperature (Tg) of this film is preferably 75°C or higher and 150°C or lower. More preferably, it is 76°C or higher and 140°C or lower, and still more preferably 77°C or higher and 130°C or lower. If Tg is 75°C or higher, it can be said that this film has excellent heat resistance because it does not deform when used for display applications. On the other hand, if Tg is 150°C or lower, it is also suitable for processability. The glass transition temperature (Tg) of this film is measured using a differential scanning calorimeter (DSC) at a heating rate of 10°C / min in accordance with JIS K7121 (2012).

[0033] (4) Crystal melting temperature The crystal melting temperature (Tm) of this film is preferably 220°C or higher and 300°C or lower. More preferably, it is 221°C or higher and 295°C or lower, even more preferably 222°C or higher and 290°C or lower, and particularly preferably 223°C or higher and 285°C or lower. If the crystal melting temperature Tm of this film is within such a range, this film is excellent in the balance between heat resistance and melt extrusion moldability. Here, the crystal melting temperature Tm is measured for this film using a differential scanning calorimeter (DSC) at a heating rate of 10°C / min in accordance with JIS K7121 (2012).

[0034] (5) Thickness The thickness of this film is preferably 1 μm or more and 250 μm or less, and more preferably 5 μm or more and 200 μm or less. By setting it to 1 μm or more, the film strength is maintained within a practical range. By being 250 μm or less, the fold resistance is likely to be exhibited. The thickness can be adjusted according to the film forming and stretching conditions. Regarding the thickness of this film, it was measured at 5 unspecified locations in the plane using a dial gauge of 1 / 1000 mm, and the average value was taken as the thickness.

[0035] 2. Components Next, the components constituting this film will be described.

[0036] The present invention has been made by finding that a biaxially stretched film containing a polyester resin, which generally exhibits a relatively high yield stress and elastic modulus compared to polyethylene resins and polypropylene resins, is excellent in flexural resistance. Even in the case of a polyester resin, which is a material with a high yield stress and elastic modulus, there has been a problem that when the stress and strain applied by deformation are large, deformation occurs and strain that is not dissipated in the material remains. However, in the present invention, it has been found that if the hysteresis loss rate of the film is below a specific value, the restoring force of the film increases and strain is less likely to occur. Generally, when deformation occurs beyond the elastic deformation region, large strains are generated in the material. However, even within the elastic deformation region, strains may remain in the material, which can contribute to the formation of deformation marks such as creases and wrinkles, and is considered to affect the appearance quality and the material properties themselves. That is, the smaller the hysteresis loss rate, the higher the resilience to deformation and the smaller the strain remaining in the material. Therefore, even when deformation within the elastic deformation region or large strains beyond the elastic deformation region are applied, it is considered that deformation marks are less likely to occur and the material has excellent deformation resistance.

[0037] <Polyethylene naphthalate copolymer (A)> This film contains a polyethylene naphthalate copolymer (A). Here, in the present invention, "containing" the PEN copolymer (A) means containing the PEN copolymer (A) within the scope where the effects of the present invention are achieved. The content of the PEN copolymer (A) in this film is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, even more preferably 80% by mass or more, and most preferably 90% by mass or more (including 100% by mass).

[0038] Generally, glycol-modified polyethylene terephthalate obtained by introducing a copolymer component into polyethylene terephthalate is known to have improved impact resistance compared to ordinary polyethylene terephthalate. On the other hand, there is a concern about a decrease in heat resistance due to a decrease in crystallinity. In addition, since it is necessary to increase the molding temperature by raising the glass transition temperature and melting point, there may be a concern about thermal decomposition of the resin in some cases. The present invention has found a biaxially stretched film containing a PEN copolymer (A) that can be made to have improved impact resistance, and thus excellent bend resistance, and further excellent heat resistance and moldability by introducing a copolymer component into polyethylene naphthalate.

[0039] The PEN-based copolymer (A) in the present invention contains a dicarboxylic acid component (a-1) and a diol component (a-2). As the dicarboxylic acid component (a-1), 2,6-naphthalenedicarboxylic acid is an essential component, and other copolymerization components are added thereto as necessary. Examples of other copolymerization components include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic 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, dimer acid; oxycarboxylic acids such as p-oxybenzoic acid, etc. Among these, isophthalic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, and 3,4-furandicarboxylic acid are preferable from the viewpoint of moldability. These copolymerization components can be used alone or in combination of two or more.

[0040] On the other hand, as the diol component (a-2), ethylene glycol is an essential component, and other copolymerization components include diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, bisphenols (bisphenol compounds such as bisphenol A, bisphenol F or bisphenol S, or their derivatives or ethylene oxide adducts) etc. Among these, 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols are preferable. Particularly from the viewpoint of maintaining film strength, it is preferable to use 1,4-cyclohexanedimethanol and bisphenols. Moreover, as the bisphenols, it is preferable to use a bisphenol A-ethylene oxide adduct. These copolymer components can be used alone or in combination of two or more.

[0041] As described above, the PEN-based copolymer of the present invention contains a copolymer component in at least one of the dicarboxylic acid component (a-1) and the diol component (a-2) with respect to PEN obtained from 2,6-naphthalenedicarboxylic acid and ethylene glycol. By including such a copolymer component, the flexural fatigue resistance, heat resistance, and moldability of PEN can be further improved.

[0042] The PEN-based copolymer (A) used in the present invention preferably contains a 2,6-naphthalenedicarboxylic acid unit as the dicarboxylic acid component (a-1), and a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit, and an ethylene glycol unit as the diol component (a-2), and is a polyethylene naphthalate-based copolymer. Moreover, it is more preferable that the diol component (a-2) consists of two components, a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit, and an ethylene glycol unit.

[0043] The PEN-based copolymer (A) preferably contains 2,6-naphthalenedicarboxylic acid units in the dicarboxylic acid component (a-1) in an amount of 90 mol% or more, more preferably 92 mol% or more, still more preferably 94 mol% or more, even more preferably 96 mol% or more, and particularly preferably 98 mol% or more. All of the dicarboxylic acid component (a-1) (100 mol%) may be 2,6-naphthalenedicarboxylic acid. By setting the content of the 2,6-naphthalenedicarboxylic acid units in the dicarboxylic acid component (a-1) within the above numerical range, the glass transition temperature and melting point of the polyethylene naphthalate-based polymer are improved, and thus the heat resistance of this film is improved. The copolymer component in the dicarboxylic acid component (a-1) may be copolymerized in an amount of less than 10 mol% for the purpose of improving moldability and heat resistance. However, from the viewpoints of heat resistance and fold resistance, terephthalic acid, which is a dicarboxylic acid component of polyethylene terephthalate, is preferably less than 2 mol%, more preferably less than 1 mol%, still more preferably 0 mol%.

[0044] The PEN-based copolymer (A) preferably contains bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol in the diol component (a-2) in an amount of 4 mol% or more and 70 mol% or less, more preferably 4.2 mol% or more and 60 mol% or less, still more preferably 4.4 mol% or more and 50 mol% or less, even more preferably 4.6 mol% or more and 40 mol% or less, and particularly preferably 4.8 mol% or more and 30 mol% or less. By setting the content of bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol in the diol component (a-2) within the above numerical range, the glass transition temperature and melting point of the PEN-based copolymer (A) are improved, and thus the heat resistance of this film is improved. In addition, since the crystallinity can be controlled, the crystallization rate can be slowed down, and the extrusion moldability and stretching processability of the film can be improved. Also, when the content is 70 mol% or less, the melting point does not become too high. Therefore, it is not necessary to set the molding temperature high, and there is no concern of thermal decomposition.

[0045] The PEN-based copolymer (A) preferably contains ethylene glycol in the diol component (a-2) in an amount of 30 mol% or more and 96 mol% or less, more preferably 40 mol% or more and 95.8 mol% or less, still more preferably 50 mol% or more and 95.6 mol% or less, even more preferably 60 mol% or more and 95.4 mol% or less, and particularly preferably 70 mol% or more and 95.2 mol% or less. By setting the content of ethylene glycol in the diol component (a-2) within the above numerical range, the crystallinity of the PEN-based copolymer (A) is maintained, and thus the heat resistance of this film is improved. In addition, for the purpose of improving moldability and heat resistance, the PEN copolymer (A) may copolymerize less than 10 mol% of a bisphenol A-ethylene oxide adduct or a diol component other than 1,4-cyclohexanedimethanol and ethylene glycol. However, a diol component having a rigid phthalimide skeleton deteriorates the folding resistance, so it is preferably less than 1 mol%, more preferably less than 0.5 mol%, and even more preferably 0 mol%. From the viewpoint of folding resistance, specific examples of the diol component other than the bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol and ethylene glycol include 1,2-propanediol, 1,3-propanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol, polyalkylene glycol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, hydroquinone, bisphenol, spiroglycol, 2,2,4,4-tetramethylcyclobutane-1,3-diol, isosorbide, etc. Among these, from the viewpoint of moldability, diethylene glycol, 1,3-propanediol, and 1,3-cyclohexanedimethanol are preferred.

[0046] The crystal melting heat quantity ΔHm of the PEN copolymer (A) is preferably 15 J / g or more and 60 J / g or less, and more preferably 20 J / g or more and 50 J / g or less. If ΔHm (A) is within such a range, the PEN copolymer (A) has appropriate crystallinity excellent in heat resistance, heat and humidity resistance, melt moldability, and stretching processability. The crystal melting heat quantity ΔHm (A) of the PEN copolymer (A) can be measured at a heating rate of 10 °C / min using a differential scanning calorimeter (DSC) in accordance with JIS K7121 (2012).

[0047] The crystal melting temperature Tm(A) of the PEN copolymer (A) is preferably 220°C or higher and 300°C or lower, more preferably 225°C or higher and 290°C or lower, still more preferably 230°C or higher and 280°C or lower, and particularly preferably 235°C or higher and 270°C or lower. If the crystal melting temperature Tm(A) of the PEN copolymer (A) is within such a range, the PEN copolymer (A) is excellent in the balance between heat resistance and melt moldability. The crystal melting temperature Tm(A) of the PEN copolymer (A) can be measured at a heating rate of 10°C / min using a differential scanning calorimeter (DSC) in accordance with JIS K7121 (2012).

[0048] The glass transition temperature Tg(A) of the PEN copolymer (A) is preferably 75°C or higher and 150°C or lower, more preferably 77°C or higher and 145°C or lower, and still more preferably 80°C or higher or 140°C or lower. If the glass transition temperature Tg(A) of the PEN copolymer (A) is within such a range, it is excellent in the balance between heat resistance and melt moldability.

[0049] In the present invention, within a range not impairing the effects of the present invention, it is allowable for this film to contain resins other than the PEN copolymer (A). Examples of other resins include polystyrene resins, polyvinyl chloride resins, polyvinylidene chloride resins, chlorinated polyethylene resins, polyester resins, polycarbonate resins, polyamide resins, polyacetal resins, acrylic resins, ethylene-vinyl acetate copolymers, polymethylpentene resins, polyvinyl alcohol resins, cyclic olefin resins, polylactic acid resins, polybutylene succinate resins, polyacrylonitrile resins, polyethylene oxide resins, cellulose resins, polyimide resins, polyurethane resins, polyphenylene sulfide resins, polyphenylene ether resins, polyvinyl acetal resins, polybutadiene resins, polybutene resins, polyamideimide resins, polyamide bismaleimide resins, polyetherimide resins, polyetheretherketone resins, polyetherketone resins, polyethersulfone resins, polyketone resins, polysulfone resins, aramid resins, and fluorine-based resins, etc.

[0050] In addition, in the present invention, in addition to the components described above, within a range that does not significantly inhibit the effects of the present invention, this film can appropriately contain additives that are generally blended. Examples of the additives include recycled resins generated from trimming losses such as ears, added for the purpose of improving and adjusting the moldability, productivity, and various physical properties of the porous film, inorganic particles such as silica, talc, kaolin, calcium carbonate, pigments such as titanium oxide, carbon black, flame retardants, weather resistance stabilizers, heat resistance stabilizers, antistatic agents, melt viscosity improvers, crosslinking agents, lubricants, nucleating agents, plasticizers, anti-aging agents, antioxidants, light stabilizers, ultraviolet absorbers, neutralizing agents, antifogging agents, antiblocking agents, slip agents, and colorants, etc.

[0051] In the present invention, in addition to the above-described additives, a coating layer can be provided on the film as long as the effects of the present invention are not significantly inhibited. Examples of the functions of the coating layer include hard coatability, antistatic property, releasability, easy adhesiveness, printability, UV cut property, infrared ray blocking property, gas barrier property, and the like. The coating layer may be provided by in-line coating that treats the film surface during the stretching process, may employ off-line coating that is applied outside the system on the once-produced film, or both may be used in combination.

[0052] <Method for manufacturing the present film> The method for manufacturing the biaxially stretched film of the present invention will be described. However, the following description is an example of the method for manufacturing the present film, and the present film is not limited to the film manufactured by such a manufacturing method.

[0053] The method for manufacturing the present film according to an example of an embodiment of the present invention is a manufacturing method in which a resin composition containing the PEN-based copolymer (A) is formed into a film shape and biaxially stretched.

[0054] The method for kneading the PEN-based copolymer (A), other resins, and additives to obtain a resin composition is not particularly limited. However, in order to obtain the resin composition as simply as possible, it is preferably manufactured by melt-kneading using an extruder. In order to uniformly mix the raw materials constituting the resin composition, it is preferable to perform melt-kneading using a co-rotating twin-screw extruder. The kneading temperature should be equal to or higher than the glass transition temperature of all polymers used, and for crystalline resins, it should be equal to or higher than the crystal melting temperature of the polymer. With respect to the glass transition temperature and crystal melting temperature of the polymers used, the higher the kneading temperature, the easier it is for some transesterification reactions of the polymers to occur and the easier it is to improve compatibility. However, if the kneading temperature is too high, resin decomposition will occur, which is not preferable. Therefore, the kneading temperature is preferably 255°C or higher and 340°C or lower, more preferably 260°C or higher and 330°C or lower, even more preferably 270°C or higher and 320°C or lower, and particularly preferably 280°C or higher and 310°C or lower. Within this range of kneading temperature, compatibility and melt moldability can be improved without causing polymer decomposition.

[0055] The obtained resin composition can be formed by general molding methods, such as extrusion molding, injection molding, blow molding, vacuum molding, pressure air molding, press molding, etc., to produce a biaxially stretched film. In each molding method, the apparatus and processing conditions are not particularly limited. This film is preferably produced, for example, by the following method.

[0056] From the resin composition obtained by mixing, a film that is substantially amorphous and not oriented (hereinafter sometimes referred to as an "unstretched film") is produced by an extrusion method. The production of this unstretched film can adopt, for example, an extrusion method in which the above raw materials are melted by an extruder, extruded from a flat die or an annular die, and then rapidly cooled to form a flat or annular unstretched film. At this time, in some cases, a laminated structure using a plurality of extruders may be adopted.

[0057] Next, the above unstretched film is stretched in at least one direction, usually 1.1 to 5.0 times, preferably 1.1 to 5.0 times in both the longitudinal direction (MD) and the transverse direction (TD) perpendicular to it, from the viewpoints of stretching effect, film strength, etc.

[0058] As the biaxial stretching method, any of the conventionally known stretching methods such as tenter-type sequential biaxial stretching, tenter-type simultaneous biaxial stretching, and tubular-type simultaneous biaxial stretching can be adopted. For example, in the case of the tenter-type sequential biaxial stretching method, an 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, and stretched 1.1 to 5.0 times in the longitudinal direction by a roll-type longitudinal stretching machine, and then stretched 1.1 to 5.0 times in the transverse direction within the temperature range of Tg to Tg + 50 °C by a tenter-type transverse stretching machine. In addition, in the case of the tenter-type simultaneous biaxial stretching or tubular-type simultaneous biaxial stretching method, for example, it can be produced by simultaneously stretching 1.1 to 5.0 times in each axial direction in the longitudinal and transverse directions within the temperature range of Tg to Tg + 50 °C.

[0059] The biaxially stretched film stretched by the above method is subsequently heat-set. By heat-setting, dimensional stability at room temperature can be imparted. In this case, the treatment temperature is preferably selected in the range of the crystal melting temperature Tm - 1 to Tm - 80 °C of the resin composition. If the heat-setting temperature is within the above range, heat-setting is sufficiently performed, the stress during stretching is relaxed, excellent heat resistance and mechanical properties are obtained, and an excellent film without troubles such as breakage or whitening of the film surface can be obtained.

[0060] In the present invention, in order to relax the stress of crystallization shrinkage due to heat-setting, it is preferable to perform relaxation in the width direction in the range of 0 to 15%, preferably 3 to 10% during heat-setting. Since relaxation is sufficiently performed and the film is uniformly relaxed in the width direction, the shrinkage rate in the width direction becomes uniform, and a film excellent in room temperature dimensional stability can be obtained. In addition, since relaxation following the shrinkage of the film is performed, there is no sag or flutter of the film in the tenter, and there is no breakage of the film.

[0061] <Uses of this film> The biaxially stretched film of the present invention is excellent in fold resistance and heat resistance, and also excellent in transparency, so it can be used as a film for displays, a packaging film, or various protective films. Among them, from the viewpoint of fold resistance, it can be suitably used for foldable displays.

Examples

[0062] Examples are shown below, but the present invention is not limited by these.

[0063] (1) Hysteresis loss rate In accordance with JIS K 7312:1996, the average value of the hysteresis loss rate at 23°C was determined by the following method. As the measuring device, a tensile tester (AG-1kNXplus tensile tester manufactured by Shimadzu Corporation) was used. As the test piece, a rectangle with a length of 100 mm and a width of 10 mm in the measuring direction was cut out from this film. Both ends in the length direction of the test piece were chucked with a chuck distance of 50 mm, and after raising the strain to 5% at a crosshead speed of 0.5 mm / min, a stress-strain curve obtained from a single-cycle tensile cycle test of lowering to the initial position at the same speed was obtained. The stress-strain curve has a profile as shown in FIG. 1, and the hysteresis loss rate was calculated from the obtained stress-strain curve using the area A1 (abcda) of the curve obtained in the ascending operation and the area A2 (abcef), which is the difference between the area A1 and the area of the curve obtained in the descending operation, by the following formula 1. The test was measured 3 times, and the average value was obtained. The above tensile cycle test was performed on the MD and TD of the film, respectively, and the average value was obtained. Hysteresis loss rate = (A2 / A1) × 100 (Formula 1)

[0064] (2) Residual strain In accordance with JIS K 7312:1996, the average value of the residual strain at 23°C was determined by the following method. The measuring device used was a tensile testing machine (AG-1kNXplus tensile testing machine manufactured by Shimadzu Corporation). As the test piece, a rectangle with a length of 100 mm and a width of 10 mm in the measuring direction was cut out from this film. Both ends of the test piece in the length direction were chucked with a chuck distance of 50 mm, and after raising the strain to 5% at a crosshead speed of 0.5 mm / min, the strain at the point where the stress disappeared was taken as the residual strain from the stress-strain curve obtained from one cycle of tensile cycle test of lowering it to the initial position at the same speed. The test was measured 3 times, and the average value was obtained. The above tensile cycle test was carried out for the MD and TD of the film respectively, and the average value was obtained.

[0065] (3) Glass transition temperature (Tg) For the obtained film, using Diamond DSC (manufactured by PerkinElmer Japan), in accordance with JIS K7121 (2012), after raising the temperature to the melting temperature once at a heating rate of 10 °C / min, the temperature was then lowered at a heating rate of 10 °C / min, and the glass transition temperature in the heating process at a heating rate of 10 °C / min was measured.

[0066] (4) Crystal melting temperature (Tm) For the obtained film, using Diamond DSC (manufactured by PerkinElmer Japan), in accordance with JIS K7121 (2012), the crystal melting temperature in the heating process at a heating rate of 10 °C / min was measured.

[0067] [PEN-based copolymer (A)] As PEN-based copolymer (A)-1, dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol%, diol component (a-2): ethylene glycol = 90 mol%, bisphenol A-ethylene oxide adduct = 10 mol% were used. The Tg of the PEN-based copolymer (A)-1 was 119 °C. As the PEN copolymer (A)-2, dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol%, diol component (a-2): ethylene glycol = 95 mol%, bisphenol A-ethylene oxide adduct = 5 mol% were used. The Tg of the PEN copolymer (A)-2 was 120°C. As the PEN copolymer (A)-3, dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol%, diol component (a-2): ethylene glycol = 90 mol%, 1,4-cyclohexanedimethanol = 10 mol% were used. The Tg of the PEN copolymer (A)-3 was 119°C. As the PEN copolymer (A)-4, dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol%, diol component (a-2): ethylene glycol = 80 mol%, 1,4-cyclohexanedimethanol = 20 mol% were used. The Tg of the PEN copolymer (A)-4 was 119°C.

[0068] [PET film (B)] As the PET film (B)-1, a biaxially stretched PET film with a thickness of 50 μm was used.

[0069] [PEN film (C)] As the PEN film (C)-1, a PEN film (Teonex Q51) with a thickness of 50 μm was used.

[0070] (Example 1) The pelletized (A)-1 alone was melt-kneaded in a Φ25 mm twin-screw extruder set at 285°C, extruded as a film from within a T-die with a gap of 1.0 mm, taken up by a casting roll at 110°C, cooled and solidified to obtain a film-like material (cast film) with a thickness of approximately 450 μm. Subsequently, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 132 °C. Subsequently, the obtained longitudinally stretched film was passed through a transverse stretching machine (tenter), and stretched 3.1 times in the transverse direction (TD) at a preheating temperature of 120 - 125 °C, a stretching temperature of 130 °C, and a heat setting temperature of 180 °C. Then, while heat setting in the tenter, a 5% relaxation treatment of the film was performed in the width direction (TD). Table 1 shows the measurement results of the obtained film.

[0071] (Example 2) Using pelletized (A)-2 monomer, a film-like material (cast film) with a thickness of about 450 μm was obtained in the same manner as in Example 1. Subsequently, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at 135 °C. Subsequently, the obtained longitudinally stretched film was passed through a transverse stretching machine (tenter), and stretched 3.1 times in the transverse direction (TD) at a preheating temperature of 125 - 130 °C, a stretching temperature of 135 °C, and a heat setting temperature of 180 °C. Then, while heat setting in the tenter, a 5% relaxation treatment of the film was performed in the width direction (TD). Table 1 shows the measurement results of the obtained film.

[0072] (Example 3) The pelletized (A)-3 monomer was melt-kneaded with a Φ25 mm twin-screw extruder set at 285 °C, extruded as a film from a T-die with a gap of 1.0 mm, taken up by a casting roll at 113 °C, cooled and solidified to obtain a film-like material (cast film) with a thickness of about 450 μm. Subsequently, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at 135 °C. Subsequently, the obtained longitudinally stretched film was passed through a transverse stretching machine (tenter), and stretched 3.1 times in the transverse direction (TD) at a preheating temperature of 125 - 130 °C, a stretching temperature of 135 °C, and a heat setting temperature of 180 °C. Then, while heat setting in the tenter, a 5% relaxation treatment of the film was performed in the width direction (TD). Table 1 shows the measurement results of the obtained film.

[0073] (Example 4) Using the pellet-shaped (A)-4 monomer alone, a film-like material (cast film) with a thickness of about 450 μm was obtained in the same manner as in Example 3. Subsequently, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 137°C. Subsequently, the obtained longitudinally stretched film was passed through a transverse stretching machine (tenter), and stretched 3.3 times in the transverse direction (TD) at a preheating temperature of 120 - 125°C, a stretching temperature of 130°C, and a heat setting temperature of 150°C. Then, while heat setting in the tenter, a 5% relaxation treatment of the film was performed in the width direction (TD). The results of measurements on the obtained film are shown in Table 1.

[0074] (Comparative Example 1) The results of evaluating the biaxially stretched PET film (B)-1 are shown in Table 1.

[0075] (Comparative Example 2) The results of evaluating the PEN film (C)-1 are shown in Table 1.

[0076]

Table 1

[0077] The films of Examples 1 to 4 have high crystal melting temperatures and glass transition temperatures and are excellent in heat resistance. Also, compared with the PET film of Comparative Example 1 and the PEN film of Comparative Example 2, the values of the hysteresis loss rate and residual strain are clearly lower. It can be said that the PEN-based copolymer-containing films of Examples 1 to 4 are excellent not only in heat resistance but also in fold resistance.

Claims

1. A biaxially stretched film containing a polyethylene naphthalate copolymer (A), wherein the average value of the hysteresis loss rate in a tensile cycle test up to 5% tensile strain in each of the MD and TD directions is 47.0% or less.

2. A biaxially stretched film containing a polyethylene naphthalate copolymer (A), wherein the average value of the residual strain in a tensile cycle test up to 5% tensile strain in each of the MD and TD directions is 0.900% or less.

3. The biaxially stretched film according to claim 1 or 2, wherein the polyethylene naphthalate copolymer (A) contains 2,6-naphthalenedicarboxylic acid units as the dicarboxylic acid component (a-1), and a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units and ethylene glycol units as the diol component (a-2).

4. The biaxially stretched film according to claim 3, wherein the terephthalic acid units as the other dicarboxylic acid component other than the (a-1) in the total dicarboxylic acid component are less than 2 mol%.

5. The biaxially stretched film according to claim 3 or 4, wherein the diol units having a biphtalimide skeleton as the other diol component other than the (a-2) in the total diol component are less than 1 mol%.

6. A biaxially stretched film containing a polyethylene naphthalate copolymer (A), the copolymer (A) contains a dicarboxylic acid component (a-1) and a diol component (a-2), the dicarboxylic acid component (a-1) contains at least 2,6-naphthalenedicarboxylic acid units, the diol component (a-2) consists of two components, a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units and ethylene glycol units, and the terephthalic acid units as the other dicarboxylic acid component other than the (a-1) in the total dicarboxylic acid component are less than 2 mol%.

7. The biaxially stretched film according to any one of claims 3 to 6, wherein the diol component (a-2) contains 4 mol% or more and 70 mol% or less of a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units, and 30 mol% or more and 96 mol% or less of ethylene glycol units.

8. The biaxially stretched film according to any one of claims 1 to 7, wherein the glass transition temperature is 75°C or higher and 150°C or lower.

9. The biaxially stretched film according to any one of claims 1 to 8, having a crystal melting temperature of 220°C or higher and 300°C or lower.

10. The biaxially stretched film according to any one of claims 1 to 9, having a thickness of 1 μm or more and 250 μm or less.

11. The biaxially stretched film according to any one of claims 1 to 10, for use in a display.

12. A foldable display comprising the biaxially stretched film according to any one of claims 1 to 11.

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