Biaxially stretched film

Abstract

A biaxially stretched film, characterized in that it contains a polyethylene naphthalate copolymer (A), wherein the average value of the hysteresis loss rate of the biaxially stretched film in a stretching cycle test up to a tensile strain of 5% in each of the MD and TD directions is 47.0% or less. A biaxially stretched film having excellent folding resistance and heat resistance can be provided.

Description

Technical Field

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

[0002] Polyester has excellent properties such as heat resistance, weather resistance, mechanical strength, transparency, chemical resistance, and gas barrier properties, and is also easily available at a low price, so it has high versatility and is currently widely used as a resin for beverage / food containers, packaging materials, molded products, films, etc. The main component of polyester resin is polyethylene terephthalate (hereinafter sometimes referred to as "PET"), which has excellent mechanical properties, electrical properties, chemical resistance, etc. and is widely used, but it has difficulties in heat resistance, folding resistance, etc.

[0003] Also, polybutylene naphthalate (PBN) obtained by polymerizing naphthalene dicarboxylic acid and 1,4-butanediol is known as a product using naphthalene dicarboxylic acid as an acid component. However, the glass transition temperature is as low as about 75°C and the melting point is as low as about 240°C, so there is a problem in heat resistance.

[0004] In addition, since the crystallization rate is too fast, it is not suitable for film formation by extrusion molding.

[0005] On the other hand, as the demand for flexible displays has increased in recent years, there has been a strong demand for films having high heat resistance, excellent recovery properties, and excellent resistance to repeated bending.

[0006] For example, Patent Document 1 studies a film based on a cyclic olefin resin film that is resistant to repeated bending.

[0007] In addition, as a film having excellent heat resistance and bending resistance, a polyimide film has been proposed (Patent Document 2).

[0008] Patent Document 3 proposes a polyethylene naphthalate resin in which the crystallinity is improved without impairing the heat resistance by blending an ethylene oxide adduct of a bisphenol compound or a derivative thereof into all alcohol components.

[0009] Patent Document 4 proposes a modified polyester resin in which heat resistance, impact strength, etc. are improved by blending 2,6-naphthalene dicarboxylic acid as an acid component and blending a bisphenol compound and 1,4-cyclohexanedimethanol as a diol component.

[0010] Furthermore, Patent Document 5 discloses a polyethylene naphthalate resin having a high glass transition temperature by blending N,N-bis-(2-hydroxyethyl)-4-4′-bisphthalimide in all alcohol components.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Publication No. 2014-104687

[0014] Patent Document 2: International Publication No. 2016 / 060213

[0015] Patent Document 3: Japanese Patent Application Laid-Open No. 8-48759

[0016] Patent Document 4: Japanese Patent Application Publication No. 2004-107559

[0017] Patent Document 5: Japanese Patent Application No. 2015-504105 Summary of the invention

[0018] Problem that the invention aims to solve

[0019] However, the film disclosed in Patent Document 1 has a low level of resistance to repeated folding and does not meet market demands.

[0020] In addition, since cyclic olefin resins are poor in coating properties and adhesiveness, it is considered difficult to laminate them with other members as a member for a flexible display.

[0021] Furthermore, although the polyimide film described in Patent Document 2 has bending resistance, its production process is a molding method by coating using a solvent, so productivity is poor and costs are high.

[0022] The polyethylene naphthalate resin described in Patent Document 3 increases the crystallization rate by blending an ethylene oxide adduct of a bisphenol compound or its derivative into all alcohol components, thereby improving the trouble of drying the raw materials and improving the moldability. However, if the crystallization rate is too fast, the film forming properties and stretchability of the film formed by extrusion molding deteriorate, and therefore, it is not suitable for stretched films.

[0023] The modified polyester resin described in Patent Document 4 exhibits high heat resistance, but on the contrary, the temperature during molding cannot be set high, and concerns about thermal decomposition become apparent, which may reduce moldability and properties of the material after molding.

[0024] The polyethylene naphthalate resin described in Patent Document 5 has extremely excellent heat resistance, but since a diol component having a rigid bisphthalimide skeleton is blended, there is a possibility that the folding resistance may be deteriorated.

[0025] An object of the present invention is to provide a biaxially stretched film having excellent folding resistance and heat resistance while solving the above-mentioned problems.

[0026] Solutions for solving problems

[0027] The present inventors have conducted intensive studies to achieve the above-mentioned objects and have completed the present invention. The present invention has the following aspects.

[0028] [1] A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), wherein the average value of the hysteresis loss rate of the biaxially stretched film in a stretching cycle test up to a tensile strain of 5% in each of MD and TD directions is 47.0% or less.

[0029] [2] A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), wherein the biaxially stretched film has an average residual strain of 0.900% or less when subjected to a stretching cycle test up to a tensile strain of 5% in each of MD and TD directions.

[0030] [3] The biaxially stretched film according to [1] or [2], wherein the polyethylene naphthalate copolymer (A) comprises 2,6-naphthalene dicarboxylic acid units as the dicarboxylic acid component (a-1), and comprises bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units and comprises ethylene glycol units as the diol component (a-2).

[0031] [4] The biaxially stretched film according to [3] above, wherein the content of terephthalic acid units as dicarboxylic acid components other than the above-mentioned (a-1) in all dicarboxylic acid components is less than 2 mol%.

[0032] [5] The biaxially stretched film according to [3] or [4], wherein the diol unit having a biphenylimide skeleton as a diol component other than the above (a-2) is less than 1 mol% in all diol components.

[0033] [6] A biaxially stretched film comprising a polyethylene naphthalate copolymer (A),

[0034] The copolymer (A) comprises a dicarboxylic acid component (a-1) and a diol component (a-2).

[0035] The dicarboxylic acid component (a-1) contains at least 2,6-naphthalene dicarboxylic acid units.

[0036] The diol component (a-2) contains two components, namely, a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol unit and an ethylene glycol unit.

[0037] The content of terephthalic acid units as dicarboxylic acid components other than the dicarboxylic acid component (a-1) in the total dicarboxylic acid components is less than 2 mol%.

[0038] [7] A biaxially stretched film according to any one of [3] to [6] above, wherein the diol component (a-2) contains 4 mol% to 70 mol% of bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units, and contains 30 mol% to 96 mol% of ethylene glycol units.

[0039] [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.

[0040] [9] The biaxially stretched film according to any one of [1] to [8] above, wherein the crystalline melting temperature is 220° C. or higher and 300° C. or lower.

[0041]

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

[0042]

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

[10] above, which is used for a display.

[0043]

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

[11] above.

[0044] Effects of the Invention

[0045] According to the present invention, a biaxially stretched film having excellent folding resistance and heat resistance can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is the distribution diagram of stress-strain curve. DETAILED DESCRIPTION

[0047] Hereinafter, the present invention will be described in detail, but the present invention is not limited to the embodiments described below.

[0048] <Biaxially stretched film>

[0049] In an embodiment of the present invention, the biaxially stretched film of the first scheme contains a polyethylene naphthalate (hereinafter sometimes referred to as "PEN") based copolymer (A), and the average value of the hysteresis loss rate of the biaxially stretched film when subjected to a stretching cycle test up to a tensile strain of 5% in each of the MD and TD directions is 47.0% or less.

[0050] The biaxially stretched film of the second embodiment comprises a polyethylene naphthalate-based copolymer (A), wherein the average value of residual strain of the biaxially stretched film in each of MD and TD directions when subjected to a stretching cycle test up to a tensile strain of 5% is 0.900% or less.

[0051] The biaxially stretched film of the third embodiment comprises a polyethylene naphthalate-based copolymer (A), wherein the copolymer (A) comprises a dicarboxylic acid component (a-1) and a diol component (a-2), wherein the dicarboxylic acid component (a-1) comprises at least a 2,6-naphthalene dicarboxylic acid unit, and the diol component (a-2) comprises two components, namely, a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit and an ethylene glycol unit, and among all the dicarboxylic acid components, the terephthalic acid unit as a dicarboxylic acid component other than the (a-1) is less than 2 mol%.

[0052] Since the present film is a biaxially stretched film, it can be formed into a thin film and has excellent folding resistance.

[0053] It should be noted that MD refers to the flow direction of the film, and TD refers to the direction perpendicular to MD. In addition, the biaxially stretched films of the first to third embodiments described above may be referred to as "this film".

[0054] The present invention has been completed by finding that a biaxially stretched film containing a PEN-based copolymer having a hysteresis loss ratio or residual strain of a specific value or less has excellent folding resistance and heat resistance as a display film and is particularly suitable for foldable applications.

[0055] Since the hysteresis loss rate and residual strain of the present film are low, it is considered that the film has excellent recovery properties and exhibits folding resistance.

[0056] In addition, a biaxially stretched film containing a PEN-based copolymer having a specific composition also has excellent folding resistance and heat resistance as a film for a display, and is particularly suitable for foldable applications.

[0057] 1.Physical properties

[0058] First, the physical properties of the film will be described.

[0059] (1) Hysteresis loss rate

[0060] The average value of the hysteresis loss rate of the film when subjected to a stretching cycle test to a tensile strain of 5% in each of the MD and TD directions at 23°C is 47.0% or less, more preferably 46.0% or less, further preferably 45.0% or less, and further preferably 44.0% or less. The lower limit is not particularly limited, but is 0.100% or more.

[0061] When the hysteresis loss ratio is 47.0% or less, the restoring force of the film becomes large, and the bending resistance (folding resistance) of the film is maintained within a practical range.

[0062] In addition, by making the hysteresis loss ratio an average value in each of the MD and TD directions, it can be regarded as a characteristic index of the entire film. The hysteresis loss ratio can be adjusted by the stretching conditions and the like.

[0063] The hysteresis loss rate of the present film can be measured according to JIS K 7312:1996 by the method described in Examples.

[0064] More specifically, through the tensile cycle test, Figure 1 When the stress-strain curve is plotted as shown, the ratio of the area enclosed by abcef to the area of ​​the whole (abcda) is defined as the hysteresis loss rate.

[0065] In addition, the hysteresis loss rates in the MD and TD directions of the film at 23° C. are preferably 47.0% or less, more preferably 46.0% or less, further preferably 45.0% or less, and further preferably 44.0% or less, respectively. It is preferred that the hysteresis loss rate in one of MD and TD is within the above numerical range, and it is more preferred that the hysteresis loss rates in both MD and TD are within the above numerical range.

[0066] Furthermore, the difference between the hysteresis loss rates in the MD and TD directions at 23° C. of the present film is preferably 20.0% or less, more preferably 15.0% or less, and even more preferably 10.0% or less.

[0067] The difference between the hysteresis loss rates in the MD and TD directions is within the above-mentioned numerical range, so that the bending resistance of the film and the anisotropy of various film properties are reduced. Therefore, when the film is used as a component or during secondary processing, there is no need to select a specific direction, and it is not easy to cause adverse conditions in only a specific direction due to anisotropy. Therefore, the film becomes a film with excellent operability.

[0068] The first aspect of the present invention requires that the average value of the hysteresis loss rate when a stretching cycle test is performed in each of the MD and TD directions at 23°C until the tensile strain is 5% is 47.0% or less, and preferably further satisfies the residual strain characteristics described below.

[0069] (2) Residual strain

[0070] The average value of the residual strain of the film when subjected to a stretching cycle test to a tensile strain of 5% in each of the MD and TD directions at 23°C is 0.900% or less, more preferably 0.890% or less, further preferably 0.880% or less, and further preferably 0.870% or less. The lower limit is not particularly limited, but is 0.100% or more.

[0071] When the residual strain is 0.900% or less, the restoring force of the film becomes large, and the bending resistance (folding resistance) of the film is maintained within the practical range.

[0072] In addition, by making the residual strain an average value in each of the MD and TD directions, it can be regarded as a characteristic index of the entire film. The residual strain can be adjusted by the stretching conditions and the like.

[0073] The residual strain of the present film can be measured according to JIS K 7312:1996 by the method described in Examples.

[0074] More specifically, through the tensile cycle test, Figure 1 When the stress-strain curve is plotted, the value of f is defined as the residual strain.

[0075] The residual strains in the MD and TD directions of the present film at 23° C. are each preferably 0.900% or less, more preferably 0.890% or less, further preferably 0.880% or less, and further preferably 0.870% or less.

[0076] It is preferred that the residual strain in one of MD and TD is within the above numerical range, and it is more preferred that the residual strain in both MD and TD is within the above numerical range.

[0077] Furthermore, the difference between the residual strains in the MD and TD directions of the present film at 23° C. is preferably 0.900% or less, more preferably 0.500% or less, further preferably 0.200% or less, and further preferably 0.100% or less.

[0078] The difference between the residual strains in the MD and TD directions is within the above-mentioned numerical range, so that the bending resistance of the film and the anisotropy of various film properties are reduced. Therefore, when the film is used as a component, there is no need to select a specific direction during secondary processing, and it is not easy to cause adverse conditions in only a specific direction due to anisotropy. Therefore, it becomes a film with excellent operability.

[0079] (3) Glass transition temperature

[0080] The glass transition temperature (Tg) of the present film is preferably 75° C. to 150° C., more preferably 76° C. to 140° C., and further preferably 77° C. to 130° C. When Tg is 75° C. or higher, the present film does not deform when used for display applications, and thus can be said to have excellent heat resistance.

[0081] On the other hand, when Tg is 150° C. or less, processability is also suitable.

[0082] The glass transition temperature (Tg) of the film is measured at a heating rate of 10° C. / min using a differential scanning calorimeter (DSC) in accordance with JIS K7121 (2012).

[0083] (4) Crystal melting temperature

[0084] The crystal melting temperature (Tm) of the present film is preferably 220°C or higher and 300°C or lower. In particular, it is more preferably 221°C or higher and 295°C or lower, further 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 the present film is within the above range, the balance between the heat resistance and the melt extrusion moldability of the present film is excellent. Here, the crystal melting temperature Tm is measured according to JIS K7121 (2012) using a differential scanning calorimeter (DSC) at a heating rate of 10°C / min.

[0085] (5) Thickness

[0086] The thickness of the film is preferably 1 μm or more and 250 μm or less, more preferably 5 μm or more and 200 μm or less. By setting it to be 1 μm or more, the film strength is kept within the practical range. By being 250 μm or less, it becomes easy to reflect the folding resistance. The thickness can be adjusted according to the film making and stretching conditions.

[0087] The thickness of the film was measured with a 1 / 1000 mm micrometer at 5 unspecified locations on the surface and the average value was taken as the thickness.

[0088] 2. Ingredients

[0089] Next, the components constituting the present film will be described.

[0090] The present invention is accomplished by finding that a biaxially stretched film containing a polyester resin having a higher yield stress and elastic modulus than polyethylene resins and polypropylene resins generally has excellent bending resistance. Even in the case of a polyester resin, which is a material having a high yield stress and elastic modulus, deformation occurs if stress and strain applied by deformation are large, and there is a problem that unrelieved strain remains in the material.

[0091] However, the present invention has found that when the hysteresis loss rate of a film is equal to or less than a specific value, the restoring force of the film increases and strain is less likely to occur.

[0092] Generally, deformation beyond the elastic deformation range will cause a large strain in the material. However, even if the deformation is within the elastic deformation range, there may be residual strain in the material, which may become a factor in the residual deformation marks such as creases and wrinkles, resulting in poor appearance and affecting the material properties themselves.

[0093] That is, it is believed that the smaller the hysteresis loss rate, the higher the recovery to deformation and the smaller the residual strain in the material. Therefore, even when deformation within the elastic deformation area or large strain exceeding the elastic deformation area is applied, it is not easy to cause deformation marks and the deformation resistance is excellent.

[0094] <Polyethylene naphthalate copolymer (A)>

[0095] The present film contains a polyethylene naphthalate-based copolymer (A).

[0096] Here, in the present invention, "comprising" the PEN-based copolymer (A) means that the PEN-based copolymer (A) is contained within a range that exhibits the effects of the present invention, and the content of the PEN-based copolymer (A) in the present film is preferably 50 mass % or more, more preferably 60 mass % or more, further preferably 70 mass % or more, further preferably 80 mass % or more, and most preferably 90 mass % or more (including 100 mass %).

[0097] It is generally known that glycol-modified polyethylene terephthalate obtained by introducing a copolymer component into polyethylene terephthalate has improved impact resistance compared with ordinary polyethylene terephthalate.

[0098] On the other hand, there is a concern about a decrease in heat resistance due to a decrease in crystallinity.

[0099] In addition, since the molding temperature needs to be increased by raising the glass transition temperature and the melting point, there is a concern that the resin may be thermally decomposed.

[0100] The present invention has found that a biaxially stretched film containing a PEN copolymer (A) having improved impact resistance, excellent bending resistance, and excellent heat resistance and molding processability can be formed by introducing a copolymer component into polyethylene naphthalate.

[0101] The PEN-based copolymer (A) in the present invention contains a dicarboxylic acid component (a-1) and a diol component (a-2).

[0102] As the dicarboxylic acid component (a-1), 2,6-naphthalenedicarboxylic acid is an essential component, and other copolymerization components may be added thereto as necessary. As other copolymer components, there can be mentioned aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, 2,5-furan dicarboxylic acid, 2,4-furan dicarboxylic acid, 3,4-furan dicarboxylic acid, benzophenone dicarboxylic acid, 4,4'-diphenyl dicarboxylic acid, 3,3'-diphenyl dicarboxylic acid, and 4,4'-diphenyl ether dicarboxylic acid; aliphatic dicarboxylic acids such as cyclohexane dicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and dimer acid; hydroxycarboxylic acids such as p-hydroxybenzoic acid, and the like. Among them, isophthalic acid, 2,5-furan dicarboxylic acid, 2,4-furan dicarboxylic acid, and 3,4-furan dicarboxylic acid are preferred from the viewpoint of moldability.

[0103] These copolymerization components may be used alone or in combination of two or more.

[0104] On the other hand, as the diol component (a-2), ethylene glycol is an essential component, and other copolymerization components thereof as needed include diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer glycol, 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 glycol, and bisphenols are preferred. From the viewpoint of maintaining film strength, 1,4-cyclohexanedimethanol and bisphenols are particularly preferably used.

[0105] Moreover, as a bisphenol, it is preferable to use a bisphenol A-ethylene oxide adduct.

[0106] These copolymerization components may be used alone or in combination of two or more.

[0107] As described above, the PEN copolymer of the present invention is obtained by containing at least one of the dicarboxylic acid component (a-1) and the diol component (a-2) as a copolymer component in PEN obtained from 2,6-naphthalene dicarboxylic acid and ethylene glycol. By containing such a copolymer component, the bending resistance, heat resistance and molding processability of PEN can be further improved.

[0108] The PEN-based copolymer (A) used in the present invention is preferably a polyethylene naphthalate-based copolymer containing 2,6-naphthalene dicarboxylic acid units as the dicarboxylic acid component (a-1), bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units, and ethylene glycol units as the diol component (a-2).

[0109] Furthermore, the diol component (a-2) more preferably contains two components of a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol unit and an ethylene glycol unit.

[0110] The PEN-based copolymer (A) preferably contains 90 mol% or more, more preferably 92 mol% or more, further preferably 94 mol% or more, further preferably 96 mol% or more, particularly preferably 98 mol% or more of 2,6-naphthalenedicarboxylic acid units in the dicarboxylic acid component (a-1), and the entirety (100 mol%) of the dicarboxylic acid component (a-1) may be 2,6-naphthalenedicarboxylic acid.

[0111] When the content of the 2,6-naphthalene dicarboxylic acid unit in the dicarboxylic acid component (a-1) is within the above numerical range, the glass transition temperature and melting point of the polyethylene naphthalate polymer are increased, and the heat resistance of the film is improved.

[0112] For the purpose of improving moldability and heat resistance, the copolymerization component in the dicarboxylic acid component (a-1) may be copolymerized in an amount of less than 10 mol%.

[0113] Among them, from the viewpoint of heat resistance and folding resistance, terephthalic acid as the dicarboxylic acid component of polyethylene terephthalate is preferably less than 2 mol %, more preferably less than 1 mol %, and further preferably 0 mol %.

[0114] The PEN-based copolymer (A) preferably contains 4 mol% to 70 mol% in the diol component (a-2), more preferably 4.2 mol% to 60 mol%, further preferably 4.4 mol% to 50 mol%, further preferably 4.6 mol% to 40 mol%, particularly preferably 4.8 mol% to 30 mol% of bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol.

[0115] When the content of bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol in the diol component (a-2) is within the above numerical range, the glass transition temperature and melting point of the PEN copolymer (A) are increased, and the heat resistance of the film is improved.

[0116] 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.

[0117] If the content is 70 mol% or less, the melting point will not be too high, so there is no need to set the molding temperature too high and there is no worry about thermal decomposition.

[0118] The PEN-based copolymer (A) preferably contains 30 mol% to 96 mol% in the diol component (a-2), more preferably 40 mol% to 95.8 mol%, further preferably 50 mol% to 95.6 mol%, further preferably 60 mol% to 95.4 mol%, particularly preferably 70 mol% to 95.2 mol% of ethylene glycol.

[0119] When the content of ethylene glycol in the diol component (a-2) is within the above numerical range, the crystallinity of the PEN-based copolymer (A) is maintained, and the heat resistance of the film is improved.

[0120] In order to improve moldability and heat resistance, the PEN-based copolymer (A) may contain less than 10 mol% of a diol component other than bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol and ethylene glycol.

[0121] However, the diol component having a rigid bisphthalimide skeleton deteriorates the folding resistance, and therefore the content is preferably less than 1 mol %, more preferably less than 0.5 mol %, and further preferably 0 mol %.

[0122] From the viewpoint of folding resistance, specific examples of the diol component other than bisphenol A-ethylene oxide adduct, 1,4-cyclohexanedimethanol and ethylene glycol include 1,2-propylene glycol, 1,3-propylene glycol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol, polyalkylene glycol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, hydroquinone, bisphenol, spirodiol, 2,2,4,4,-tetramethylcyclobutane-1,3-diol, and isosorbide. Among them, from the viewpoint of moldability, diethylene glycol, 1,3-propylene glycol, and 1,3-cyclohexanedimethanol are preferred.

[0123] The crystal melting heat ΔHm of the PEN copolymer (A) is preferably 15 J / g or more and 60 J / g or less, more preferably 20 J / g or more and 50 J / g or less. When ΔHm(A) is within the above range, the PEN copolymer (A) is excellent in heat resistance, moisture and heat resistance, melt moldability and stretching processability, and has moderate crystallinity.

[0124] The crystal fusion heat ΔHm(A) of the PEN-based 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).

[0125] The crystalline melting temperature Tm(A) of the PEN copolymer (A) is preferably 220° C. to 300° C., more preferably 225° C. to 290° C., further preferably 230° C. to 280° C., particularly preferably 235° C. to 270° C. When the crystalline melting temperature Tm(A) of the PEN copolymer (A) is within the above range, the PEN copolymer (A) has an excellent balance between heat resistance and melt moldability.

[0126] The crystalline melting temperature Tm(A) of the PEN-based 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).

[0127] The glass transition temperature Tg(A) of the PEN copolymer (A) is preferably 75° C. to 150° C., more preferably 77° C. to 145° C., further preferably 80° C. or more or 140° C. When the glass transition temperature Tg(A) of the PEN copolymer (A) is within the above range, the balance between heat resistance and melt moldability is excellent.

[0128] In the present invention, the film may contain other resins than the PEN-based copolymer (A) within a range not impairing the effects of the present invention.

[0129] 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, aromatic polyamide resins, and fluorine resins.

[0130] In addition, in the present invention, in addition to the aforementioned components, the film may suitably contain additives that are usually compounded within the range that does not significantly impair the effects of the present invention. As the aforementioned additives, there may be cited additives such as recycled resins, inorganic particles such as silicon dioxide, talc, kaolin, calcium carbonate, titanium oxide, carbon black, flame retardants, weatherability stabilizers, heat stabilizers, antistatic agents, melt viscosity improvers, crosslinking agents, lubricants, nucleating agents, plasticizers, antioxidants, antioxidants, light stabilizers, ultraviolet absorbers, neutralizers, antifogging agents, antiblocking agents, slip agents, and colorants that are added for the purpose of improving / adjusting the molding processability, productivity, and various physical properties of the porous film, and the trimming losses of the edges, etc.

[0131] In addition, in the present invention, in addition to the aforementioned additives, a coating layer may be provided on the film within a range that does not significantly impair the effects of the present invention. As the functions of the aforementioned coating layer, hard coating, antistatic properties, peeling properties, easy adhesion, printability, UV shielding properties, infrared shielding properties, gas barrier properties, etc. may be cited. For the formation of the coating layer, the coating layer may be provided by online coating that treats the film surface during the stretching stroke, or by offline coating that is applied outside the system on a temporarily manufactured film, or both may be used.

[0132] <Method for producing the present film>

[0133] The method for producing the biaxially stretched film of the present invention will be described. The following description is an example of a method for producing the present film, and the present film is not limited to a film produced by the above-mentioned production method.

[0134] A method for producing the film according to an example of an embodiment of the present invention is a method for producing a film by molding a resin composition containing the PEN-based copolymer (A) into a film and then biaxially stretching the film.

[0135] The method of kneading the PEN copolymer (A) and other resins and additives to obtain a resin composition is not particularly limited. In order to obtain the resin composition as simply as possible, it is preferably produced by melt kneading using an extruder. In order to uniformly mix the raw materials constituting the resin composition, it is preferably melt kneading using a co-directional twin-screw extruder.

[0136] The kneading temperature must be above the glass transition temperature of all the polymers used, and above the crystal melting temperature of the polymer relative to the crystalline resin. When the kneading temperature is as high as possible relative to the glass transition temperature and crystal melting temperature of the polymer used, it is easy to produce a part of the polymer transesterification reaction, and the compatibility is easy to improve. However, if the kneading temperature is higher than required, it will cause the decomposition of the resin, so it is not preferred. Therefore, the kneading temperature is preferably above 255°C and below 340°C, more preferably above 260°C and below 330°C, further preferably above 270°C and below 320°C, and particularly preferably above 280°C and below 310°C. If the kneading temperature is within the above range, the decomposition of the polymer will not occur, and the compatibility and melt moldability can be improved.

[0137] The obtained resin composition can be molded into a biaxially stretched film by a general molding method such as extrusion molding, injection molding, blow molding, vacuum molding, pressure molding, press molding, etc. In each molding method, the apparatus and processing conditions are not particularly limited.

[0138] The present film is preferably produced, for example, by the following method.

[0139] The resin composition obtained by mixing is used to manufacture a substantially amorphous and unoriented film (hereinafter sometimes referred to as "unstretched film") by extrusion. The unstretched film can be manufactured by, for example, the following extrusion method: the above-mentioned raw materials are melted by an extruder, extruded from a flat die or annular die, and then quenched to form a flat or annular unstretched film. At this time, a stacked structure using multiple extruders can be formed according to the situation.

[0140] Next, for the above-mentioned unstretched film, in the flow direction of the film (longitudinal direction, MD) and the direction perpendicular to it (transverse direction, TD), from the aspects of stretching effect and film strength, it is usually stretched by 1.1 to 5.0 times in at least one direction, and preferably stretched in the biaxial directions of the longitudinal and transverse directions by 1.1 to 5.0 times respectively.

[0141] 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 simultaneous biaxial stretching can be used. For example, in the case of the tenter-type sequential biaxial stretching method, the glass transition temperature of the resin composition is set to Tg, the unstretched film is heated to a temperature range of Tg to Tg+50°C, and stretched to 1.1 to 5.0 times in the longitudinal direction using a roll-type longitudinal stretching machine, and then stretched to 1.1 to 5.0 times in the transverse direction in the temperature range of Tg to Tg+50°C using a tenter-type transverse stretching machine, thereby manufacturing.

[0142] In the case of tenter-type simultaneous biaxial stretching or tubular-type simultaneous biaxial stretching, for example, the film can be produced by simultaneously stretching in each axial direction to 1.1 to 5.0 times in the longitudinal and transverse directions within a temperature range of Tg to Tg+50°C.

[0143] Then, the biaxially stretched film obtained by the stretching method described above is heat-set.

[0144] By performing heat setting, dimensional stability at room temperature can be imparted. The treatment temperature at this time is preferably selected from the range of the crystal melting temperature Tm-1 to Tm-80°C of the aforementioned resin composition. If the heat setting temperature is within the above range, heat setting is fully performed, stress during stretching is relaxed, sufficient heat resistance and mechanical properties can be obtained, and an excellent film without problems such as breakage and whitening of the film surface can be obtained.

[0145] In the present invention, in order to relieve the stress of crystal shrinkage caused by heat setting, it is preferred to relax the film in the width direction by 0 to 15%, preferably 3 to 10% during heat setting. The film is fully relaxed and evenly relaxed in the width direction, so that the shrinkage rate in the width direction becomes uniform, and a film with excellent room temperature dimensional stability can be obtained.

[0146] In addition, since relaxation following the shrinkage of the film is performed, there is no film slack, no swaying in the tenter, and no film breakage.

[0147] <Application of this film>

[0148] The biaxially stretched film of the present invention has excellent folding resistance, heat resistance and transparency, and can be used as a display film, a packaging film, and various protective films. In particular, from the viewpoint of folding resistance, it can be preferably used for foldable displays.

[0149] Example

[0150] Although Examples are shown below, the present invention is not limited to these.

[0151] (1) Hysteresis loss rate

[0152] The average value of the hysteresis loss ratio at 23° C. was determined by the following method in accordance with JIS K 7312:1996.

[0153] The measuring device used is a tensile testing machine (tensile testing machine AG-1kNXplus manufactured by Shimadzu Corporation). The test piece used is a rectangle cut out of the film with a length of 100 mm and a width of 10 mm in the measuring direction. The two ends of the test piece in the length direction are clamped with a chuck distance of 50 mm, and the crosshead is raised at a speed of 0.5 mm / min until the strain is 5%, and then lowered to the initial position at the same speed to obtain the stress-strain curve obtained from this one-cycle tensile cycle test. The stress-strain curve is Figure 1 The distribution diagram shown in FIG. 1 shows that the hysteresis loss rate is calculated from the obtained stress-strain curve using the area A1 (abcda) of the curve obtained by the rising action and the area A2 (abcef) which is the difference between the area A1 and the area of ​​the curve obtained by the falling action, using the following formula 1. The test was measured 3 times and the average value was obtained. The above-mentioned stretching cycle test was performed in the MD and TD of the film, respectively, and the average value was obtained.

[0154] Hysteresis loss rate = (A2 / A1) × 100 (Formula 1)

[0155] (2) Residual strain

[0156] The average value of the residual strain at 23° C. was determined by the following method in accordance with JIS K 7312:1996.

[0157] The measuring device used is a tensile testing machine (tensile testing machine AG-1kNXplus manufactured by Shimadzu Corporation). The test piece used is a rectangle cut out of the film with a length of 100 mm and a width of 10 mm in the measuring direction. The two ends of the test piece in the length direction are clamped with a chuck spacing of 50 mm, and after rising to a strain of 5% at a crosshead speed of 0.5 mm / min, it is lowered to the initial position at the same speed. According to the stress-strain curve obtained from the one-cycle tensile cycle test, the strain at the point where the stress disappears is taken as the residual strain. The test is measured 3 times, and the average value is calculated. The above-mentioned tensile cycle test is implemented on the MD and TD of the film respectively, and the average value is calculated.

[0158] (3) Glass transition temperature (Tg)

[0159] The obtained film was heated to the melting temperature at a heating rate of 10°C / min using Diamond DSC (manufactured by Parkin Elmer Japan) in accordance with JIS K7121 (2012), then cooled at a heating rate of 10°C / min, and the glass transition temperature was measured during the heating process at a heating rate of 10°C / min.

[0160] (4) Crystal melting temperature (Tm)

[0161] The obtained film was measured for its crystalline melting temperature in a temperature increase process at a heating rate of 10° C. / min using Diamond DSC (manufactured by Parkin Elmer Japan Co., Ltd.) in accordance with JIS K7121 (2012).

[0162] [PEN-based copolymer (A)]

[0163] As the PEN copolymer (A)-1, a dicarboxylic acid component (a-1): 2,6-naphthalene dicarboxylic acid = 100 mol%, a diol component (a-2): ethylene glycol = 90 mol%, and a bisphenol A-ethylene oxide adduct = 10 mol% were used. The Tg of the PEN copolymer (A)-1 was 119°C.

[0164] As the PEN copolymer (A)-2, a dicarboxylic acid component (a-1): 2,6-naphthalene dicarboxylic acid = 100 mol%, a diol component (a-2): ethylene glycol = 95 mol%, and a bisphenol A-ethylene oxide adduct = 5 mol% were used. The Tg of the PEN copolymer (A)-2 was 120°C.

[0165] As the PEN copolymer (A)-3, a dicarboxylic acid component (a-1): 2,6-naphthalene dicarboxylic acid = 100 mol%, a diol component (a-2): ethylene glycol = 90 mol%, 1,4-cyclohexanedimethanol = 10 mol% was used. The Tg of the PEN copolymer (A)-3 was 119°C.

[0166] As the PEN copolymer (A)-4, a dicarboxylic acid component (a-1): 2,6-naphthalene dicarboxylic acid = 100 mol%, a diol component (a-2): ethylene glycol = 80 mol%, and 1,4-cyclohexanedimethanol = 20 mol% were used. The Tg of the PEN copolymer (A)-4 was 119°C.

[0167] [PET film (B)]

[0168] As the PET film (B)-1, a biaxially stretched PET film having a thickness of 50 μm was used.

[0169] [PEN film (C)]

[0170] As the PEN film (C)-1, a PEN film (Theonex Q51) having a thickness of 50 μm was used.

[0171] (Example 1)

[0172] The individual pellets (A)-1 are melt-kneaded in a Φ25mm twin-screw extruder set at 285°C, extruded in the form of a film from a T-die with a gap of 1.0mm, collected with a casting roll at 110°C, and cooled and solidified to obtain a film (cast film) with a thickness of about 450μm.

[0173] Then, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 132°C. Then, 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 to 125°C, a stretching temperature of 130°C, and a heat setting temperature of 180°C, and then the film was relaxed by 5% in the width direction (TD) while being heat-set in the tenter. The results of the measurements on the obtained film are shown in Table 1.

[0174] (Example 2)

[0175] Using the pellets (A)-2 alone, a film (cast film) having a thickness of about 450 μm was obtained in the same manner as in Example 1.

[0176] Then, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at 135°C. Then, 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, and then the film was relaxed by 5% in the width direction (TD) while being heat-set in the tenter. The results of the measurements on the obtained film are shown in Table 1.

[0177] (Example 3)

[0178] The individual pellets (A)-3 are melt-kneaded in a Φ25mm twin-screw extruder set at 285°C, extruded in the form of a film from a T-die with a gap of 1.0mm, collected with a casting roll at 113°C, and cooled and solidified to obtain a film (cast film) with a thickness of about 450μm.

[0179] Then, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at 135°C. Then, 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, and then the film was relaxed by 5% in the width direction (TD) while being heat-set in the tenter. The results of the measurements on the obtained film are shown in Table 1.

[0180] (Example 4)

[0181] Using the pellets (A)-4 alone, a film (cast film) having a thickness of about 450 μm was obtained in the same manner as in Example 3.

[0182] Then, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 137°C. Then, 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 to 125°C, a stretching temperature of 130°C, and a heat setting temperature of 150°C. After that, the film was subjected to a relaxation treatment of 5% in the width direction (TD) while heat setting in the tenter. The results of the measurements on the obtained film are shown in Table 1.

[0183] (Comparative Example 1)

[0184] Table 1 shows the evaluation results of the biaxially stretched PET film (B)-1.

[0185] (Comparative Example 2)

[0186] Table 1 shows the results of evaluation of PEN film (C)-1.

[0187] [Table 1]

[0188] Table 1

[0189]

[0190] The thin films of Examples 1 to 4 had high crystal melting temperatures and glass transition temperatures and were excellent in heat resistance.

[0191] In addition, the hysteresis loss ratio and residual strain values ​​are significantly lower than those of the PET film of Comparative Example 1 and the PEN film of Comparative Example 2. It can be said that the films containing the PEN copolymer of Examples 1 to 4 are excellent not only in heat resistance but also in folding resistance.

Claims

1. A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), wherein the average value of the hysteresis loss rate of the biaxially stretched film in a stretching cycle test in each of the MD and TD directions up to a tensile strain of 5% is 47.0% or less, and the hysteresis loss rate in the TD direction at 23° C. is 47.0% or less, The polyethylene naphthalate copolymer (A) comprises 2,6-naphthalene dicarboxylic acid units as a dicarboxylic acid component (a-1), bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units, and ethylene glycol units as a diol component (a-2), The diol component (a-2) contains 4 mol% to 70 mol% of bisphenol A-ethylene oxide adduct units and 30 mol% to 96 mol% of ethylene glycol units; or contains more than 10 mol% to 70 mol% of 1,4-cyclohexanedimethanol units and 30 mol% to less than 90 mol% of ethylene glycol units, The film is stretched in the flow direction of the film, that is, the MD direction, by 3.0 to 5.0 times, and in the direction perpendicular to the flow direction of the film, that is, the TD direction, by 1.1 to 5.0 times.

2. A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), wherein the biaxially stretched film has an average residual strain of 0.900% or less when subjected to a stretching cycle test up to a tensile strain of 5% in each of the MD and TD directions, and a residual strain in the TD direction at 23° C. of 0.900% or less, The polyethylene naphthalate copolymer (A) comprises 2,6-naphthalene dicarboxylic acid units as a dicarboxylic acid component (a-1), bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units, and ethylene glycol units as a diol component (a-2), The diol component (a-2) contains 4 mol% to 70 mol% of bisphenol A-ethylene oxide adduct units and 30 mol% to 96 mol% of ethylene glycol units; or contains more than 10 mol% to 70 mol% of 1,4-cyclohexanedimethanol units and 30 mol% to less than 90 mol% of ethylene glycol units, The film is stretched in the flow direction of the film, that is, the MD direction, by 3.0 to 5.0 times, and in the direction perpendicular to the flow direction of the film, that is, the TD direction, by 1.1 to 5.0 times.

3. The biaxially stretched film according to claim 1 or 2, wherein The film is stretched in the direction perpendicular to the flow direction of the film, that is, in the TD direction by 3.1 to 5.0 times.

4. The biaxially stretched film according to claim 1 or 2, wherein The film is stretched in the flow direction of the film, that is, the MD direction, by 3.0 to 3.3 times, and in the direction perpendicular to the flow direction of the film, that is, the TD direction, by 3.1 to 3.3 times.

5. The biaxially stretched film according to claim 1, wherein The difference between the hysteresis loss rates in the MD and TD directions at 23° C. is 20.0% or less.

6. The biaxially stretched film according to claim 2, wherein The difference between the residual strains in the MD and TD directions at 23° C. is 0.900% or less.

7. The biaxially stretched film according to claim 1 or 2, wherein The terephthalic acid units as dicarboxylic acid components other than the above-mentioned (a-1) are less than 2 mol% in all dicarboxylic acid components.

8. The biaxially stretched film according to claim 1 or 2, wherein The content of diol units having a biphenylimide skeleton as diol components other than the above (a-2) in all diol components is less than 1 mol%.

9. The biaxially stretched film according to claim 1 or 2, wherein The diol component (a-2) contains two components: a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol unit, and an ethylene glycol unit. The content of terephthalic acid units as dicarboxylic acid components other than the dicarboxylic acid component (a-1) in the total dicarboxylic acid components is less than 2 mol%. 10 . The biaxially stretched film according to claim 1 , which has a glass transition temperature of 75° C. to 150° C. 11 . The biaxially stretched film according to claim 1 , which has a crystal melting temperature of 220° C. or higher and 300° C. or lower. 12 . The biaxially stretched film according to claim 1 , which has a thickness of 1 μm to 250 μm.

13. The biaxially stretched film according to claim 1 or 2, which is used for a display. 14 . A foldable display comprising the biaxially stretched film according to claim 1 .

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