Polyester film

Abstract

To provide a polyester film with both dynamic and static bending properties.SOLUTION: A polyester film in which the activation energy Ea obtained from the following calculation method is 450 kJ / mol or more and 800 kJ / mol or less, and the surface orientation coefficient is 0.120 or more and less than 0.164. [Calculation method] The tanδ peak temperature T is calculated from a ratio of the loss modulus E" and the storage elastic modulus E' at each measurement frequency k (0.1 Hz, 0.5 Hz, 1.0 Hz, 2.0 Hz, 5.0 Hz, 10 Hz) obtained from the dynamic viscoelasticity measurement, and a linear approximation is taken from the Arrhenius plot with the horizontal axis as 1 / T and the vertical axis as Ln (k), and Ea is calculated from the slope [-Ea / R], and an average value in both of the longitudinal direction and the width direction is the activated energy Ea. T: tanδ peak temperature [K] k: Measurement frequency [Hz] Ln: Natural logarithm R: Gas constant 8.314[J K-1 mol-1]SELECTED DRAWING: None

Description

[Technical field]

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

[0002] In recent years, image display devices using self-luminous bodies called organic light-emitting diodes (sometimes abbreviated as OLEDs) (hereinafter referred to as "organic electroluminescent display devices") have been put to practical use. Compared to conventional liquid crystal display devices, organic electroluminescent display devices are not only superior in terms of visibility and response speed because they use self-luminous bodies, but also enable display devices to be made thinner and more flexible because they do not require auxiliary lighting devices such as backlights. For this reason, the development of flexible displays that can be folded, rolled up, and repeatedly folded is accelerating, and bending resistance is also required for cover films that prevent scratches on the display surface and OLED support films (sometimes called backplate films) that are intended to protect the OLEDs mounted inside OLED display devices.

[0003] For example, for flexible displays, an antireflection film has been proposed in which an antireflection layer is provided on at least one surface of a flexible transparent resin film containing a polyester film (Patent Document 1). Furthermore, polyesters for organic electroluminescence display devices that focus on specific bending properties have been proposed (Patent Documents 2 and 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-75869 A [Patent Document 2] JP 2018-124367 A [Patent Document 3] International Publication No. 2021 / 182191 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the film used for image display devices described in Patent Document 1 does not take into consideration the bending resistance of the base film, making it difficult to apply it to flexible displays. In addition, bending resistance includes dynamic bending resistance determined by repeated bending tests and static bending resistance, which is measured by leaving the film in a folded state for a certain period of time and then determining the degree of crease thereafter. Patent Documents 2 and 3 focus on specific bending characteristics to achieve static bending resistance, but do not take dynamic bending resistance into consideration, and therefore do not achieve both static bending resistance and dynamic bending resistance at the same time.

[0006] An object of the present invention is to provide a polyester film that satisfies both the dynamic flex resistance and the static flex resistance described above. [Means for solving the problem]

[0007] In order to solve the above problems, the polyester film of the present invention employs the following means. (1) A polyester film having an activation energy Ea of 450 kJ / mol or more and 800 kJ / mol or less, calculated by the following calculation method, and a plane orientation coefficient of 0.120 or more and less than 0.164. [Calculation method] The tan δ peak temperature T is calculated from the ratio of the loss modulus E'' to the storage modulus E' at each measurement frequency k (0.1 Hz, 0.5 Hz, 1.0 Hz, 2.0 Hz, 5.0 Hz, 10 Hz) obtained from the dynamic viscoelasticity measurement. A linear approximation is taken from the Arrhenius plot with 1 / T on the horizontal axis and Ln(k) on the vertical axis, and Ea is calculated from the slope [-Ea / R]. The activation energy Ea is the average value in both the longitudinal and transverse directions. T: tan δ peak temperature [K] k: Measurement frequency [Hz] Ln: Natural logarithm R: Gas constant 8.314 [J·K -1 mol -1 ] (2) The polyester film according to (1), having a network stretch ratio λnet of 1.4 or more. (3) 1386cm by Fourier transform infrared spectroscopy -1 The polyester film according to (1) or (2), wherein the peak intensity is 0.6 or more and 0.75 or less. (4) The density N between entanglement points is 4×10 26 (m -3 ) or more 12×10 26 (m -3 The polyester film according to any one of (1) to (3), which is: (5) The polyester film according to any one of (1) to (4), which is used as a protective film. (6) The polyester film according to any one of (1) to (4), which is used as a protective film for an organic electroluminescence display device. Effect of the Invention

[0008] According to the present invention, by controlling the activation energy Ea and the plane orientation coefficient within a specific range, a polyester film that can achieve both dynamic and static bending resistance can be provided. Such a polyester film can be particularly suitably used as a film for flexible displays such as organic electroluminescence display devices. [Brief description of the drawings]

[0009] [Figure 1] 1 is a graph showing elongation-stress curves of an unstretched sheet and a stretched sheet as a measurement sample for determining a network stretch ratio λnet. [Diagram 2] 1 is an Arrhenius plot of tan δ peak temperature T obtained at each measurement frequency k, with the horizontal axis being 1 / T and the vertical axis being Ln(k) in order to determine activation energy Ea. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The polyester film according to the present invention will be described in detail below together with embodiments.

[0011] The polyester film of the present invention is mainly composed of polyester. Examples of glycols or derivatives thereof that give polyester include aliphatic dihydroxy compounds such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and neopentyl glycol, polyoxyalkylene glycols such as diethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol, alicyclic dihydroxy compounds such as 1,4-cyclohexanedimethanol and spiroglycol, aromatic dihydroxy compounds such as bisphenol A and bisphenol S, and derivatives thereof.

[0012] Examples of dicarboxylic acids or derivatives thereof that provide the polyester used in the present invention include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenoxyethanedicarboxylic acid, and 5-sodiumsulfonedicarboxylic acid, aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, and fumaric acid, alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, and oxycarboxylic acids such as paraoxybenzoic acid, as well as derivatives thereof. Examples of dicarboxylic acid derivatives include esters such as dimethyl terephthalate, diethyl terephthalate, 2-hydroxyethyl methyl terephthalate, dimethyl 2,6-naphthalenedicarboxylate, dimethyl isophthalate, dimethyl adipate, diethyl maleate, and dimethyl dimerate.

[0013] In the polyester composition of the present invention, 80 mol% or more of the glycol units are preferably structural units derived from ethylene glycol, more preferably 85 mol% or more, and even more preferably 90 mol% or more. In addition, 80 mol% or more of the dicarboxylic acid units are preferably structural units derived from terephthalic acid, more preferably 85 mol% or more, and even more preferably 90 mol% or more. From the viewpoint of further improving static bending resistance while maintaining dynamic bending resistance without decreasing rigidity, it is preferable to use a composition in which isophthalic acid is copolymerized in the range of 2 mol% to 6 mol%. More preferably, it is in the range of 2.5 mol% to 4 mol%. Incidentally, when isophthalic acid is copolymerized in the range exceeding 6 mol%, the static bending resistance is excellent, but the rigidity decreases and the dynamic bending resistance may be impaired. In addition, other polyester resins may be mixed instead of a single resin. For example, the Young's modulus can be decreased by mixing polybutylene terephthalate, and the Young's modulus tends to increase by adding polyethylene naphthalate. They can be mixed appropriately according to the required characteristics. In addition, the polyester film of the present invention is preferably a biaxially oriented polyester film.

[0014] The concept of achieving both dynamic bending resistance and static bending resistance in polyester films is shown below. Dynamic bending resistance requires resistance to repeated strain, and the higher the mechanical strength and rigidity of the film, the more advantageous it is. In general, in the concept of polyester films, in order to achieve high rigidity, the film is highly oriented by stretching at a high ratio. On the other hand, static bending resistance is achieved by maintaining the film in a folded state for a certain period of time, so that if the rigidity of the film is high, the film is likely to be creased after being held for a certain period of time due to its high rigidity, resulting in poor static bending resistance. In the present invention, in order to achieve both dynamic bending resistance and static bending resistance, the film orientation is kept below a certain level, static bending resistance is ensured, and the film is highly rigidified by other means than high orientation. In addition, in order to obtain static bending resistance, it is also important to have a dense molecular structure despite being low oriented.

[0015] The polyester film of the present invention has an activation energy Ea of 450 kJ / mol or more and 800 kJ / mol or less. Here, the calculation method of the activation energy Ea is as described in the measurement method (13) Activation Energy Ea in the examples. The activation energy Ea in the present invention is a parameter that indicates the activation state of the amorphous part in the polyester film. A large activation energy Ea indicates that the amorphous part is in a more constrained state, and unlike the amorphous state in which the so-called amorphous molecular mobility is high, the amorphous chain is in a tensioned state and shows high rigidity even in the amorphous state. That is, even if the orientation of the film is low, dynamic bending resistance can be expressed by controlling the activation energy Ea to 450 kJ / mol or more. Therefore, if the activation energy Ea is less than 450 kJ / mol, the dynamic bending resistance is poor. On the other hand, if the activation energy Ea exceeds 800 kJ / mol, the restraint state of the amorphous portion becomes very high, and the dimensional stability due to heat is significantly reduced, so that problems such as shrinkage, deformation, curling, etc. occur during subsequent thermal processing such as providing a hard coat layer, making the film less practical. For this reason, the activation energy Ea must be 450 kJ / mol or more and 800 kJ / mol or less. It is preferably 480 kJ / mol or more and 750 kJ / mol or less, 500 kJ / mol or more and 700 kJ / mol or less, and most preferably 550 kJ / mol or more and 650 kJ / mol or less.

[0016] The polyester film of the present invention has a planar orientation coefficient of 0.120 or more and less than 0.164. As described above, when the film is highly oriented, it has excellent dynamic bending resistance but poor static bending resistance. Therefore, the planar orientation coefficient, which indicates the orientation state of the film, must be less than 0.164. The planar orientation coefficient can be determined by the method described in the measurement method (5) Planar orientation coefficient fn of polyester film in the examples. From the viewpoint of dynamic bending resistance, the planar orientation coefficient is preferably 0.140 or more, preferably 0.155 or more, and most preferably 0.160 or more. The planar orientation coefficient can be adjusted, for example, by the areal stretch ratio of the film, and the higher the areal stretch ratio, the higher the planar orientation coefficient.

[0017] In order to make the polyester film of the present invention have an activation energy Ea of 450 kJ / mol or more and 800 kJ / mol or less and a plane orientation coefficient of 0.12 or more and less than 0.164, it is important to precisely control the tension state of the amorphous part of the film, which can be controlled by the film-forming conditions ((1) stepwise stretching, (2) cooling to 50°C or less after biaxial stretching of the film, (3) final heat treatment temperature), the intrinsic viscosity of the film, and the heat treatment temperature after biaxial stretching. This will be explained in detail below.

[0018] First, as the conditions for biaxial stretching of the film, it is preferable to perform stepwise stretching (for example, stretching in three or more steps) in the stretching direction of each axis in order to form a dense molecular structure. For example, when the longitudinal stretching is performed in three steps on the first axis, and the stretching ratios of the three steps are MD1, MD2, and MD3 in order, it is preferable to set the stretching ratio as follows in order to obtain a dense bulk structure. For convenience, in the present invention, the machine flow direction (MD direction) is the longitudinal direction, and the direction perpendicular to the longitudinal direction is the width direction (TD direction). However, when the longitudinal direction and width direction of the film are unknown, the breaking strength is measured in any one direction (0°) of the film, and in directions of 15°, 30°, 45°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, and 165° from that direction, and the direction with the highest breaking strength is regarded as the width direction, and the direction perpendicular to the width direction is regarded as the longitudinal direction.

[0019] (Example of distribution of stretch ratio for the first axis) MD1: 1.03 times or more and 1.08 times or less. MD2: 1.05 times or more and 1.12 times or less. MD3: 2.0 times or more and 3.3 times or less, and MD3 accounts for 80% or more of the total draw ratio in the first axis (MD1×MD2×MD3).

[0020] Furthermore, when the biaxial stretching in the width direction is performed in three stages, and the stretching ratios in the three stages are designated TD1, TD2, and TD3, respectively, it is preferable to set the stretching ratios as follows in order to obtain a dense bulk structure.

[0021] (Example of biaxial stretching ratio allocation) TD1: 1.3 times or more and 2.0 times or less. TD2: 1.3 times or more and 1.8 times or less. TD3: 1.3 times or more and 1.8 times or less. The total biaxial stretching ratio (TD1 × TD2 × TD3): is 3.5 times or less. The difference in draw ratios among TD1, TD2, and TD3 is 0.3 times or less.

[0022] In the present invention, it is preferable to have a heat treatment step after a step of cooling the film to 50°C or less after biaxial stretching. In conventional polyester film manufacturing methods, after sequential biaxial stretching and simultaneous biaxial stretching are performed in terms of productivity and thermal efficiency, heat treatment is performed in a heat treatment step without cooling to 50°C or less. However, in order to form a dense film bulk structure and increase molecular chain entanglement, it is preferable to cool the film to 50°C or less once after biaxial stretching. It is presumed that the film bulk structure becomes dense by stabilizing the crystal structure formed by biaxial stretching at 50°C or less and then performing heat treatment, which makes it easier to control the activation energy Ea within the range of the present invention. It is more preferable to have a heat treatment step after a step of cooling to 35°C or less after biaxial stretching.

[0023] In the heat treatment step after the biaxial stretching in the present invention, the activation energy Ea is preferably controlled to be less than 210°C in order to control the activation energy Ea within the range of the present invention. On the other hand, in terms of practicality such as post-processing and taking dimensional stability into consideration, the heat treatment temperature is preferably 160°C or higher. The heat treatment temperature is preferably 180°C or higher and lower than 200°C.

[0024] From the above viewpoint, it is preferable that the minute endothermic peak temperature Tmeta obtained from DSC (differential scanning calorimetry), which shows the thermal history of the film, is present at 150° C. to 200° C. There is a slight gap between the heat treatment temperature setting and the Tmeta of the film depending on the film formation conditions, and Tmeta tends to be at least the same temperature or lower as the heat treatment temperature.

[0025] In the present invention, the density N between entanglement points tends to increase when the intrinsic viscosity of the film is increased. By controlling this together with other requirements, the number of times of bending resistance to breakage described below can be set within a suitable range. The intrinsic viscosity of the film is preferably 0.66 dl / g or more and 1.15 dl / g or less. Note that, when the intrinsic viscosity is increased, the filtration pressure tends to increase during the extrusion process for forming the film, which may reduce mass productivity. The intrinsic viscosity of the film can be adjusted by the intrinsic viscosity of the resin used. There is no particular upper limit, but the filtration pressure increases during the extrusion process for forming the film, making it necessary to reduce the discharge amount, which may reduce mass productivity. The intrinsic viscosity of the film can be adjusted by the intrinsic viscosity of the resin used.

[0026] The polyester film of the present invention preferably has a network stretch ratio λnet of 1.4 or more. Here, the network stretch ratio λnet can be measured by the method described in the measurement method (8) Network stretch ratio λnet in the examples. The network stretch ratio λnet corresponds to the absolute stretch ratio of the network structure, assuming that the viscoelastic properties of the polymer material are entangled (network) structure formed by the entanglement points of molecular chains as pseudo crosslinking points, as described in, for example, Sen-i-gakkaishi (Sen-i-gakkaishi Journal) (Vol. 65, No. 4 (2009)) Influence of nozzle diameter on the mechanical properties of melt-spun polyethylene terephthalate fiber (by Masato Masuda). In the present invention, it is conceived that a more rigid network structure is formed by this value being large, and it has been possible to achieve a large number of bending breakages. The network stretch ratio λnet is preferably 1.5 or more. One example of a means for achieving a network stretch ratio λnet of 1.4 or more is to use a resin having a weight average molecular weight of 20,000 or more, to simultaneously satisfy the following: an areal stretch ratio of 11 times or more and less than 12.25 times, and a heat treatment temperature of less than 200°C.

[0027] The polyester film of the present invention has an inter-entanglement density N of 4×10 26 (m -3 ) or more 12×10 26 (m -3 ) or less. Here, the density N between entanglement points can be calculated as described in the measurement method (9) density N between entanglement points in the examples. The density N between entanglement points indicates the entanglement density of molecular chains based on the theoretical formula of rubber elasticity, and is a dense molecular structure. By setting the density N between entanglement points in the above range, not only can the activation energy Ea be easily controlled within the preferred range of the present invention, but also the number of bending times until breakage can be increased. In order to increase the density N between entanglement points, it is preferable to use a resin having a weight average molecular weight of 20,000 or more, set the areal stretch ratio to 11 times or more and less than 12.25 times, and set the heat treatment temperature to less than 200°C.

[0028] The polyester film of the present invention preferably has a weight average molecular weight Mw of 20,000 or more and 50,000 or less, more preferably 20,000 or more and 35,000 or less, and most preferably 20,000 or more and 27,500 or less. As described above, by having the weight average molecular weight within this range, the network stretch ratio λnet and the density N between entanglement points can be easily controlled within the range of the present application due to increased entanglement of molecular chains, as compared with a film obtained under the same film-forming conditions. There is no particular limitation on the method for making the weight average molecular weight Mw 20,000 or more and 50,000 or less. For example, the higher the intrinsic viscosity of the polyester used as a raw material, the higher the weight average molecular weight Mw tends to be, and if the melt extrusion temperature is made higher than the melting point of the resin, the weight average molecular weight Mw tends to be lower, and if the residence time from being fed into the extruder until it is bled from the die is extended, the weight average molecular weight Mw tends to be lower, so that a preferable weight average molecular weight Mw can be obtained by controlling each condition. A suitable method is to use a polyester resin having an intrinsic viscosity of 0.70 or more, control the extrusion temperature to the melting point of the resin + 40°C or less, and set the residence time from being fed into the extruder until bleeding from the die to 10 minutes or less. From the viewpoint of increasing the weight average molecular weight Mw, it is more preferable to use a resin having an intrinsic viscosity of 0.80 or more.

[0029] The polyester film of the present invention has a 1386 cm -1 In the case of a polyester film, the peak intensity is preferably 0.6 or more and 0.75 or less. -1 The peak intensity of this peak is a parameter that relatively represents the amount of crystal transformation resulting from the folding structure of the molecular chain, and the greater the peak intensity, the more the crystallization has progressed. In the present invention, the crystallization provides excellent dimensional stability, and the 1386 cm peak intensity measured by Fourier transform infrared spectroscopy -1 It is preferable that the peak intensity of 1386 cm is 0.6 or more. On the other hand, if the crystallinity progresses too much, the rigidity of the film increases and the dynamic bending resistance deteriorates. For this reason, in the present invention, from the viewpoint of dimensional stability and rigidity, -1The measurement method is as described in the Examples. -1 In order to make the peak intensity of 0.6 or more and 0.75 or less, it is preferable to set the plane orientation coefficient to 0.155 or more and the heat treatment temperature after biaxial stretching to 160°C or more and less than 200°C.

[0030] The thickness of the polyester film of the present invention is preferably 5 μm or more and 100 μm or less. By making the film thickness 5 μm or more, more preferably 9 μm or more, and even more preferably 11 μm or more, the film has excellent handling properties, scratch resistance, and flatness when coated with a curable resin. In addition, by making the film thickness 100 μm or less, more preferably 75 μm or less, and even more preferably 38 μm or less, the film has excellent static bending resistance.

[0031] A preferred embodiment of the polyester film of the present invention is a laminate sheet having a layer containing a curable resin on at least one side of the polyester film. By forming the polyester film into such a laminate sheet, it is possible to enhance the effect of preventing scratches caused by impact from the curable resin layer side, and therefore the polyester film can be suitably used for organic electroluminescence display devices.

[0032] Here, the curable resin refers to a resin that forms a crosslinked structure and hardens by irradiating heat or light. The curable resin is preferably a thermosetting resin or an ultraviolet curable resin, and specifically includes, for example, organic silicone-based, polyol-based, melamine-based, epoxy-based, multifunctional acrylate-based, urethane-based, isocyanate-based, organic-inorganic hybrid-based, which is a composite material of an organic material and an inorganic material, and silsesquioxane-based resins having a curable functional group. More preferably, it is an epoxy-based, multifunctional acrylate-based, organic-inorganic hybrid-based, or silsesquioxane-based resin. Even more preferably, it is a multifunctional acrylate-based, organic-inorganic hybrid-based, or silsesquioxane-based resin.

[0033] As the polyfunctional acrylate-based and silsesquioxane-based resins used as the curable resin, polyfunctional acrylate monomers, oligomers, urethane acrylate oligomers, alkoxysilanes, alkoxysilane hydrolysates, alkoxysilane oligomers, and the like are preferred.

[0034] Examples of the polyfunctional acrylate monomer include polyfunctional acrylates having two or more (meth)acryloyloxy groups in one molecule and modified polymers thereof, and specific examples thereof include pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol triacrylate hexanemethylene diisocyanate urethane monomer, etc. These monomers can be used alone or in combination of two or more.

[0035] In the present invention, the layer containing the curable resin preferably contains one or more types of particles. The particles may be either inorganic particles or organic particles, but inorganic particles are preferred for improving surface hardness.

[0036] Examples of inorganic particles include oxides, silicides, nitrides, borides, chlorides, carbonates, etc. of metals or semimetals. Specifically, at least one selected from the group consisting of silica (SiO2), aluminum oxide (Al2O3), zinc oxide (ZnO), zirconium oxide (ZrO2), titanium oxide (TiO2), antimony oxide (Sb2O3), and indium tin oxide (In2O3+SnO2) is preferred.

[0037] In addition, when particles are introduced for the purpose of improving the surface hardness, the particle size is preferably 1 nm or more and 300 nm or less. In order to achieve a higher level of both surface hardness and bending resistance, the particle size is more preferably 50 nm or more and 200 nm or less, and even more preferably 100 nm or more and 150 nm or less.

[0038] The particle size referred to here means the number average particle size, and refers to the particle size observed in the cross section of the film. If the shape is not a perfect circle, the value converted to a perfect circle with the same area is used as the particle size. Here, the number average particle size Dn can be calculated by the following steps (1) to (4). (1) First, the cross section of the film is cut using a microtome in the thickness direction without crushing it, so that the cut is parallel to the longitudinal direction of the film. (2) A scanning electron microscope is used to obtain a magnified image of the cross section of the cut sample. 2 Photograph the above areas. (3) Next, for each particle observed in the cross section of the image, the cross-sectional area S is determined, and the particle size d is calculated using the following formula. d=2×(S / π) 1 / 2 (4) Using the obtained particle diameter d and the number n of resin particles, Dn is calculated according to the following formula. Dn = Σd / n where Σd is the sum of the particle diameters in the observation area, and n is the total number of particles in the observation area. (5) The above steps (1) to (4) are carried out at five different locations, and the average value is taken as the number average particle size of the particles.

[0039] The content ratio of the curable resin to the particles (particles / resin) is preferably 20 / 80 to 80 / 20 by mass. By making the content ratio 20 / 80 or more, more preferably 30 / 70 or more, and even more preferably 40 / 60 or more, the surface hardness can be effectively obtained. In addition, by making the content ratio 80 / 20 or less, more preferably 70 / 30 or less, and even more preferably 60 / 40 or less, the decrease in bending resistance can be suppressed.

[0040] Next, a specific example of a method for producing the polyester film of the present invention will be described. In this example, polyethylene terephthalate is used as the polyester resin constituting the film, but the present invention is not limited to this example.

[0041] First, polyethylene terephthalate resin with an intrinsic viscosity of 0.85gl / d is dried and pre-crystallized as a resin used for the film, and then fed to a single-screw extruder for melt extrusion. At this time, it is preferable to control the resin temperature to 265 to 290°C. Next, the resin is passed through a filter or a gear pump to remove foreign matter and to equalize the extrusion amount, and is discharged from a T-die into a sheet on a cooling drum. At this time, the sheet-shaped polymer is adhered to the casting drum by using an electrostatic application method in which a high-voltage electrode is used to make the resin adhere to the cooling drum with static electricity, a casting method in which a water film is provided between the casting drum and the extruded polymer sheet, a method in which the casting drum temperature is set to the glass transition point of the polyester resin to (glass transition point -20°C) to make the extruded polymer adhere, or a method in which a combination of these methods is used, and then cooled and solidified to obtain an unstretched film. Among these casting methods, when polyester is used, the electrostatic application method is preferably used from the viewpoints of productivity and flatness.

[0042] The unstretched film obtained in the casting process can be stretched in the longitudinal direction and then in the width direction, or in the width direction and then in the longitudinal direction by a sequential biaxial stretching method, or in the simultaneous biaxial stretching method, in which the film is stretched in the longitudinal direction and the width direction almost simultaneously. In the biaxial stretching method, for example, it is important to stretch the film in multiple stages in the stretching direction of each axis in order to form a dense molecular structure. For example, when the first axis is stretched in the longitudinal direction in three stages, when the stretching ratios of the three stages are MD1, MD2, and MD3 in order, it is preferable to set the ratios as follows in order to obtain a dense bulk structure.

[0043] (Example of distribution of stretch ratio for the first axis) MD1: 1.03 times or more and 1.08 times or less. MD2: 1.05 times or more and 1.12 times or less. MD3: 2.0 times or more and 3.3 times or less, and MD3 accounts for 80% or more of the total draw ratio in the first axis (MD1×MD2×MD3).

[0044] Furthermore, when the biaxial stretching in the width direction is performed in three stages, and the stretching ratios in the three stages are designated TD1, TD2, and TD3, respectively, it is preferable to set the stretching ratios as follows in order to obtain a dense bulk structure.

[0045] (Example of biaxial stretching ratio allocation) TD1: 1.3 times or more and 2.0 times or less. TD2: 1.3 times or more and 1.8 times or less. TD3: 1.3 times or more and 1.8 times or less. The total biaxial stretching ratio (TD1 × TD2 × TD3): is 3.5 times or less. The difference in draw ratios among TD1, TD2, and TD3 is 0.3 times or less.

[0046] In the present invention, from the viewpoint of controlling the activation energy Ea and the plane orientation coefficient within the range of the present invention and from the viewpoint of suppressing thickness unevenness, it is preferable to stretch the film by 2.4 times or more in each direction. In addition, in order to set the plane orientation coefficient to 0.120 or more and 0.164 or less, it is preferable to set the areal stretch ratio to 8.0 times or more and less than 12.25 times.

[0047] Furthermore, the stretching temperature is preferably set at a level that does not cause uneven stretching. For example, when a sequential biaxial stretching method is adopted in which stretching is performed in the longitudinal direction and then in the width direction, the preheating temperature in the longitudinal direction is preferably not less than the glass transition temperature of the resin -20°C and not more than the glass transition temperature +0°C, and the stretching temperature is preferably not less than the glass transition temperature of the resin and not more than the glass transition temperature +30°C, the preheating temperature in the width direction is preferably not less than the glass transition temperature of the resin -10°C and not more than the glass transition temperature +20°C, and the stretching temperature is not less than the glass transition temperature of the resin and not more than the glass transition temperature +60°C.

[0048] In producing the polyester film of the present invention, in order to form a dense bulk structure in the internal structure of the film and to enhance molecular chain entanglement, it is preferable to cool the film to 50°C or less after biaxial stretching and before heat treatment.

[0049] In producing the polyester film of the present invention, it is preferable to heat treat the film after biaxial stretching or the above-mentioned cooling. The heat treatment can improve dimensional stability. The heat treatment can be performed by any conventionally known method, such as in an oven or on a heated roll. As described above, the heat treatment can be performed on the premise that the minute endothermic temperature peak Tmeta is less than 200°C. In order to make the minute endothermic temperature peak Tmeta less than 200°C, it is preferable to set the heat treatment temperature to 210°C or less. The heat treatment can also be performed by dividing into multiple zones and gradually increasing and decreasing the temperature, or by slightly stretching the film to about 1.01 to 1.2 times in the width direction in the heat treatment process. The heat treatment can also be performed by relaxing the film in the longitudinal direction and / or width direction. The heat treatment time can be any time within a range that does not deteriorate the properties, and is preferably 10 to 60 seconds, more preferably 15 to 30 seconds.

[0050] In addition, when a layer containing a curable resin is laminated on the polyester film of the present invention, it is preferable to perform a corona treatment on the surface from the viewpoint of adhesion with the layer, or to laminate an easily adhesive resin layer having a thickness of 10 nm to 500 nm and a surface free energy of 38 mN / m or more on at least one side. The method for forming the easily adhesive resin layer includes a method for coating an easily adhesive resin on the film surface, such as a composite melt extrusion method, a hot melt coating method, an in-line coating method from a solvent other than water, a water-soluble and / or water-dispersible resin, and the like. Among them, an in-line coating method in which a coating agent is applied to one side of a film before the orientation crystallization is completed, the film is stretched in at least one direction, and heat-treated to complete the orientation crystallization is preferable from the viewpoint of forming a uniform coating and from the viewpoint of industrialization.

[0051] In addition, when the easy-adhesion resin layer is provided by coating, the resin to which the easy-adhesion resin layer is applied is not particularly limited, but for example, acrylic resins, urethane resins, polyester resins, olefin resins, fluorine resins, vinyl resins, chlorine resins, styrene resins, various graft resins, epoxy resins, silicone resins, etc. can be used, and mixtures of these resins can also be used. From the viewpoint of adhesion, it is preferable to use polyester resins, acrylic resins, or urethane resins. When a polyester resin is used as a water-based coating liquid, a water-soluble or water-dispersible polyester resin is used, but for such water-solubilization or water-dispersion, it is preferable to copolymerize a compound containing a sulfonate group or a compound containing a carboxylate group. In addition, when an acrylic resin is used as a water-based coating liquid, it is necessary to make it dissolved or dispersed in water, and a surfactant (for example, polyether compounds, etc., but not limited thereto) may be used as an emulsifier.

[0052] In addition, in the adhesive resin layer used in the present invention, various crosslinking agents can be used in combination with the resin to further improve adhesion. As the crosslinking agent resin, melamine-based, epoxy-based, and oxazoline-based resins are generally used. As the particles contained in the adhesive resin layer used in the present invention, inorganic particles and organic particles can be mentioned, but inorganic particles are more preferable because they improve slipperiness and blocking resistance. As the inorganic particles, silica, alumina, kaolin, talc, mica, calcium carbonate, titanium, etc. can be used.

[0053] The polyester film of the present invention has excellent dynamic flex resistance and static flex resistance, and therefore can be suitably used, for example, as a protective film that requires handling properties. Furthermore, in view of the effects of the present invention, the polyester film can be particularly suitably used as a protective film for an organic electroluminescence display device (OLED). By applying the polyester film as a protective film for an OLED, the film has excellent flex resistance as a support film mounted on the surface of a display device or inside an OLED without impairing the flexibility of the display device. In addition to optical films, the polyester film can also be used as various protective films utilizing the characteristics of the present invention, and as protective films for industrial materials such as packaging applications, as preferred embodiments. EXAMPLES

[0054] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. Various properties were measured by the following methods.

[0055] [Measurement method] (1) Film thickness When measuring the total thickness of the film, a dial gauge (manufactured by Mitutoyo Corporation) was used to measure the thickness at five randomly selected points on a sample cut into a 5 cm square from the film, and the average value was calculated.

[0056] (2) Composition of the resin that makes up the film The film was dissolved in hexafluoroisopropanol (HFIP), 1 H-NMR and 13 The contents of each monomer residue and the by-product diethylene glycol were quantified using C-NMR.

[0057] (3) Glass transition temperature and melting point Tm, Tmeta of the resin that constitutes the polyester film In accordance with JIS K7121 (1987), a differential scanning calorimeter (Robot DSC-RDC6220, manufactured by Seiko Instruments Inc.) was used, and data analysis was performed using thermal analysis rheology system software ("Muse" manufactured by SII NanoTechnology Inc.) to measure and analyze the glass transition temperature [°C], melting point Tm [°C], and minute endothermic peak temperature Tmeta [°C].

[0058] Specifically, 5 mg of sample was heated from 25°C to 300°C at 20°C / min. The temperature at the top of the exothermic peak obtained from the DSC curve was taken as Tcc, the temperature at the top of the endothermic peak obtained from the DSC curve was taken as the melting point Tm, and the temperature of the small endothermic peak observed between Tm and Tcc was taken as Tmeta. The glass transition temperature was determined from the point where the curve of the stepwise change part of the glass transition of the differential scanning calorimetry chart intersected with a straight line equidistant in the vertical direction from the straight line extended from each baseline.

[0059] (4) Intrinsic viscosity 0.100 g of a sample was weighed out to within 0.001 g accuracy, and dissolved in 10 mL of o-chlorophenol by heating at 100°C for 30 minutes. The solution was cooled to room temperature, and 8 mL of the solution was placed in an Ostwald viscometer placed in a water bath at 25°C, and the number of seconds it took to pass the marked line was measured (A seconds). In addition, 8 mL of o-chlorophenol alone was used in the same manner as above, and the number of seconds it took to pass the marked line was measured using an Ostwald viscometer placed in a water bath at 25°C (B seconds). The intrinsic viscosity was calculated using the following formula. IV=-1+[1+4×K×{(A / B)-1}] 0.5 / (2×K×C) Here, K is 0.343 and C is the concentration of the sample solution (g / 100 mL).

[0060] (5) Plane orientation coefficient fn Using sodium D line (wavelength 589 nm) as a light source and methylene iodide as a mounting liquid, the refractive indexes of the film in the longitudinal, transverse and thickness directions (nMD, nTD, nZD, respectively) were measured in accordance with JIS K7142 (2014) A method using an Abbe refractometer (NAR-4T manufactured by Atago Co., Ltd.) at 25°C. The test piece used had a refractive index of 1.74. The plane orientation coefficient (fn) of the film was calculated from the obtained refractive index using the following formula. fn=(nMD+nTD) / 2-nZD (formula).

[0061] (6) Weight average molecular weight Mw To prepare the sample solution, 5 mL of sodium trifluoroacetate-added hexafluoroisopropanol was added to 3 mg of sample as a solvent, and the mixture was gently stirred at 40°C for 3 hours. Then, the mixture was filtered using a 0.5 μm filter. Next, the weight average molecular weight was measured under the following conditions by gel permeation chromatography (GPC) using sodium trifluoroacetate-added hexafluoroisopropanol as a solvent and a differential refractive index (RI) detector (Tosoh RI-8020). Column: Shodex HFIP-LG x 1 (φ8.0 mm x 5 cm, Showa Denko) Shodex HFIP-806M x 2 (φ8.0mm x 30cm, made by Showa Denko) Flow rate: 0.5mL / min Column temperature: 40℃ Injection volume: 0.2mL Molecular weight calibration: Monodisperse polymethyl methacrylate (PMMA) (manufactured by Showa Denko) was used as the standard sample.

[0062] (7) Thickness of the curable resin layer and the easily adhesive resin layer The thickness of the curable resin layer on the film was measured by observing the cross section using a transmission electron microscope (TEM). The thickness of the curable resin layer was read from an image taken by TEM at a magnification of 100,000 times. The thicknesses of the curable resin layer and the easy-adhesive resin layer were measured at a total of 10 points, and the average value was used. The observation magnification may be other than 100,000 times as long as the thickness can be measured.

[0063] (8) Network extension ratio λnet First, prepare a reference sample. Since the reference sample needs to be made in an unstretched state, first measure the intrinsic viscosity of the sample to be measured, and select a resin of the same composition with an equivalent intrinsic viscosity (within ±0.3) for the reference sample. In the examples and comparative examples of the present invention, the main raw material, auxiliary raw material (if used), and particle master specified in each raw material composition were mixed in the mixing ratio specified for each raw material composition to prepare the resin for the reference sample.

[0064] The resin for the reference sample was melt-extruded to obtain an unstretched sheet having a thickness of 400 μm, which was used as the reference sample. The unstretched sheet used as the reference sample had a thickness variation of ±5% or less in the five points measured by the measurement in (1) above. For example, in the case of a 400 μm film, the thickness variation was within 380 to 420 μm.

[0065] Next, the elongation and load of the reference sample were measured using a universal testing machine. The measurement method is as described in (10) Elongation-Stress Curve below. Next, the nominal strain, true strain, nominal stress, and true stress at each elongation were calculated from the obtained elongation and load as follows.

[0066] Nominal strain = elongation (mm) / initial length (mm) *Elongation (%) = Elongation (mm) / Initial length (mm) x 100 True strain = Ln(1 + nominal strain) Nominal stress (MPa) = Load (N) / Initial cross-sectional area (mm 2 ) True stress = nominal stress x (1 + nominal strain) A true strain-true stress curve was created with the calculated true strain on the X-axis and the true stress on the Y-axis.

[0067] Next, the elongation-stress curve was obtained for the measurement sample in the same manner as above, and the true strain-true stress curve was calculated. The obtained true strain-true stress curve was shifted so as to overlap with the breaking point of the reference sample, as shown in Figure 1, and the shift amount of the true strain value at the start of elongation was defined as the network elongation ratio λnet, with 0 as the reference.

[0068] For example, if the nominal strain at break of the reference sample is 5.0 and the nominal stress is 40 MPa, then the true strain is Ln(1 + 5.0) = 1.791759 and the true stress is 40 × (1 + 5) = 240. When the true stress of the measurement sample is superimposed so as to be the same value as the true stress at the break point of the reference sample, and the true strain value at the start of elongation is 1.3, the shift amount is calculated to be 1.3.

[0069] (9) Intertwining density The heat shrinkage stress peak value σ of the sample was measured by the method described in (11) below. It was calculated from the various data obtained according to the following formula. N=σ / kT(λnet 2 -λnet -1 )···formula Here, N: Intertwining density (m -3 ) σ: Peak value of thermal shrinkage stress (N / m 2 ) k: Boltzmann constant (1.380649×10 -23 [J / K]) T: Temperature (298.16(K)).

[0070] (10) Elongation-stress curve A rectangular sample 150 mm long and 10 mm wide was measured using an Instron-type tensile tester (Orientec Co., Ltd., automatic film strength and elongation measuring device "Tensilon" (registered trademark) AMF / RTA-100) according to the method specified in JIS Z1702 (1994). The measurement was performed under the following conditions, and 20 samples were measured.

[0071] From the obtained measurement results, 10 points in total were removed from the data: the 5 points with the lowest elongation and the 5 points with the highest elongation. The average values ​​of elongation and stress were calculated based on the remaining 10 points. The elongation at break was taken as the average of the above 10 points. Meanwhile, the stress at break was only used from the stress of the samples whose elongation at break was above the average among the 10 points, and the average value was used as the stress. This is because the stress at the average elongation of the 10 points for samples whose elongation at break did not reach the average elongation was 0, and if these were included, the average value of the stress at break would deviate from the actual situation. Sample size: width 10mm x length 50mm Pulling speed: 300mm / min Measurement environment: temperature 23℃, humidity 65%RH.

[0072] (11) Heat shrinkage stress peak value σ The film was cut into rectangular samples measuring 50 mm in length and 4 mm in width in the longitudinal and transverse directions, and the thermal shrinkage stress peak value σ (N / m 2 ) was measured. Chuck distance: 15mm Load: 19.6mN Heating rate: 5℃ / min Measurement temperature range: 25~220℃.

[0073] (12) 1,386 cm -1 Peak Intensity The FT-IR spectrum of the surface was measured using a Fourier transform infrared spectrometer (Spectrum 100, manufactured by PerkinElmer). Light source: Special ceramics Detector element: DTGS Resolution: 4cm -1 Number of times accumulated: 256 Measurement wave number range: 4,000~680cm -1 Measurement mode: Attenuated total reflectance (ATR) Attachment: Single reflection ATR crystal (material: diamond / ZnSe).

[0074] The vertical axis is the absorbance [A], and the obtained spectrum was subjected to baseline correction and ATR correction. -1 The peak intensity was normalized to 3, and the peak intensity was 1386 cm -1 The peak intensities were read.

[0075] (13) Activation energy Ea Ea was calculated from the tan δ peak temperature T at each measurement frequency k obtained from dynamic viscoelasticity measurement (DMA). First, the tan δ peak temperature T at each measurement frequency k was measured using DMA. A sample was cut out from the center of the film width direction to 7 cm x 1 cm, and placed on a sample holder so that the sample had a measurement length of 2 cm x film width of 1 cm. Using a viscoelasticity measuring device (Seiko Instruments Inc. DMS6100), the storage modulus E' and loss factor E" were measured in tension mode under the conditions of a temperature range of room temperature 20 ° C. to 200 ° C., a displacement of 10 μm, vibration frequencies of 0.1 Hz, 0.5 Hz, 1.0 Hz, 2.0 Hz, 5.0 Hz, and 10 Hz, and a heating rate of 2 ° C. / min. Next, tan δ was calculated from the ratio of the loss modulus E'' to the storage modulus E'. Both the longitudinal direction and the transverse direction of the film were used as the measurement directions. In the obtained temperature change spectrum of tan δ, the temperature at which tan δ had a maximum value was taken as the tan δ peak temperature.

[0076] Next, the tan δ peak temperature T obtained at each measurement frequency k was plotted in an Arrhenius plot with 1 / T on the horizontal axis and Ln(k) on the vertical axis (see Figure 2). A linear approximation was taken from the obtained plot, and Ea was calculated from the slope [-Ea / R]. The average value in both the longitudinal and transverse directions was taken as the activation energy Ea. T: tan δ peak temperature [K] Ln: Natural logarithm R: Gas constant 8.314 [J·K -1 mol -1 ].

[0077] (14) Number of bending failures Using an MIT folding endurance tester (No. 702 manufactured by Mize Co., Ltd.), samples cut to a size of 110 mm in length (measurement direction) and 15 mm in width were subjected to a bending test in accordance with JIS P8115 (2001) under the conditions of a load of 1000 g, a bending angle of 135° left and right (R: +135°, L: -135°), a bending speed of 175 times / min, and a chuck tip R: 0.38 mm, and the number of times the film was bent when it broke was taken as the number of times it broke. The test was performed three times, and the average value was used.

[0078] (15) Dynamic bending resistance A sample cut to a width of 108 mm and a length of 112 mm was attached to the end of a tilt clamp in a U-shaped extension tester (Yuasa System Co., Ltd. DLDMLH-FS) with the tilt clamp in a horizontal position and the stroke direction in the direction of the sample length, and was bent 10,000 times at a test speed of 60 r / min, a test stroke of 60 mm, and a face-to-face distance of 3 mm. After the test, the dynamic bending resistance of the sample was judged as follows based on the reflected light from a fluorescent lamp and its appearance. A and B were deemed to have passed. A: No change in appearance, no distortion of reflected light, and excellent dynamic bending resistance. B: No change in appearance, but distortion of reflected light was observed. No practical problem. C: Bending lines are clearly observed on the exterior, and the dynamic bending resistance is poor.

[0079] (16) Static bending resistance A film sample cut to a length of 60 mm x width of 25 mm was attached to the end of a tilt clamp in a U-shaped stretch tester (Yuasa System Equipment, DLDMLH-FS) with the bending direction as the length direction, the tilt clamp in a horizontal position, and the stroke direction in the length direction of the sample, and the film was left for 24 hours in the most bent state with the center of the film bent at a face-to-face distance of 1.5 mm. After 24 hours, the film was released from the bent state, taken out of the device, and left to stand with the bent outer side facing down, and the bending angle of the film sample was measured. This measurement was performed five times in both the MD and TD directions, and the average value was calculated to be the static bending test lift angle. The angle was read with 0° as the completely folded state and 180° as the state in which the film recovered to its original unfolded state before folding. The static bending resistance was judged from the obtained recovery angle as follows. ◎ was the best, and ◎ to △ were considered to be acceptable. ◎: The bending angle in the stationary state exceeded 160°. ◯: The bending angle in the stationary state was 150° or more and less than 160°. △: The bending angle in the stationary state was 140° or more and less than 150°. ×: The bending angle in the stationary state was less than 130°.

[0080] (17) Dimensional stability The film was cut into rectangular samples measuring 150 mm in length and 10 mm in width in both the longitudinal and transverse directions. Marked lines were drawn on the samples at intervals of 100 mm (50 mm from the center to both ends), and the samples were heat-treated by hanging a 3 g weight and placing them in a hot air oven heated to 150°C for 30 minutes. The distance between the marks after heat treatment was measured, and the thermal shrinkage was calculated from the change in the distance between the marks before and after heating using the following formula. Heat shrinkage rate (%) = {(gauge length before heat treatment) - (gauge length after heat treatment)} / (gauge length before heat treatment) x 100 The dimensional stability was evaluated based on the obtained heat shrinkage rate according to the following criteria: ⊚ was the best, and ⊚ to △ were acceptable. ◎: The average value of MD and TD is less than 3%. ○: The average values ​​of MD and TD are 3% or more and less than 5%. △: The average values ​​of MD and TD are 5% or more and less than 7%. ×: The average value of MD and TD is 7% or more.

[0081] [Polyester and Particle Master] The polyester resin and particle master used for film formation were prepared as follows.

[0082] (Polyester A) A polyethylene terephthalate resin having an intrinsic viscosity of 0.65, in which the dicarboxylic acid component is 100 mol % of a terephthalic acid component and the glycol component is 100 mol % of an ethylene glycol component.

[0083] (Polyester B) A polyethylene terephthalate resin having an intrinsic viscosity of 0.75, in which the dicarboxylic acid component is 100 mol % of a terephthalic acid component and the glycol component is 100 mol % of an ethylene glycol component.

[0084] (Polyester C) A polyethylene terephthalate resin having an intrinsic viscosity of 0.88, in which the dicarboxylic acid component is 100 mol % of a terephthalic acid component and the glycol component is 100 mol % of an ethylene glycol component.

[0085] (Polyester D) A polybutylene terephthalate resin having an intrinsic viscosity of 1.2, in which the dicarboxylic acid component is 100 mol % of a terephthalic acid component and the glycol component is 100 mol % of a 1,4-butanediol component.

[0086] (Polyester E) A polyethylene terephthalate resin copolymerized with 3 mol% isophthalic acid, having an intrinsic viscosity of 0.80, and containing 97 mol% terephthalic acid and 3 mol% isophthalic acid as dicarboxylic acid components, and 100 mol% ethylene glycol as glycol components.

[0087] (Particle Master A) Polyester A contains calcium carbonate particles with an average particle size of 1.2 μm at a particle concentration of 1 mass %. The polyethylene terephthalate particle master has an intrinsic viscosity of 0.65.

[0088] [Examples 1 to 19, Comparative Examples 1 to 4] The resin and particle master were mixed in the types and amounts shown in Table 1 and fed into the extruder.

[0089] Next, the mixture was melted at the extruder temperature shown in Table 1 and extruded from a T-die in the form of a sheet onto a cooling drum controlled at 25° C. At that time, static electricity was applied using a wire electrode with a diameter of 0.1 mm, and the mixture was brought into close contact with the cooling drum to obtain an unstretched sheet.

[0090] Next, it was rapidly cooled with a cooling roll whose temperature was controlled at 20° C., and then stretched in the MD direction with each roll at the stretching temperature and stretch ratio shown in Table 2, and then cooled once (intermediate cooling).

[0091] Next, both sides of this uniaxially stretched film were subjected to a corona discharge treatment to adjust the wet tension of the film to 55 mN / m, and the easy-adhesion resin P was applied to both sides of the film.

[0092] Next, the film was stretched in the TD direction in a first oven tenter at the stretching temperature and stretch ratio shown in Table 2, and then intermediately cooled to the temperature shown in Table 2. Next, in the tenter of the second oven, the film was heat-treated at the heat treatment temperature shown in Table 2 and relaxed in the width direction to obtain a film having the thickness shown in Table 1.

[0093] The physical properties of the obtained films are shown in Tables 3 and 4. In Examples 1 to 14, the films were excellent in bending resistance and flatness after coating with the curable resin.

[0094] [Table 1]

[0095] [Table 2]

[0096] [Table 3]

[0097] [Table 4] [Industrial Applicability]

[0098] The polyester film of the present invention is excellent in dynamic flex resistance and static flex resistance, and therefore can be suitably used, for example, as a protective film that requires good handling properties, particularly as a protective film for an organic electroluminescence display device.

Claims

1. A polyester film having an activation energy Ea of 450 KJ / mol or more and 800 KJ / mol or less, calculated by the following calculation method, and a plane orientation coefficient of 0.120 or more and less than 0.

164. [Calculation method] The tan δ peak temperature T was calculated from the ratio of the loss modulus E″ to the storage modulus E′ at each measurement frequency k (0.1 Hz, 0.5 Hz, 1.0 Hz, 2.0 Hz, 5.0 Hz, 10 Hz) obtained from the dynamic viscoelasticity measurement. A linear approximation was performed from an Arrhenius plot with 1 / T on the horizontal axis and Ln(k) on the vertical axis, and Ea was calculated from the slope [−Ea / R]. The average value in both the longitudinal and transverse directions was taken as the activation energy Ea. T: tan δ peak temperature [K] k: Measurement frequency [Hz] Ln: natural logarithm R: Gas constant 8.314 [J·K －１ ・mol －１ ]

2. 2. The polyester film according to claim 1, having a network stretch ratio λnet of 1.4 or more.

3. 1386 cm by Fourier transform infrared spectroscopy －１ 3. The polyester film according to claim 1, wherein the peak intensity is 0.6 or more and 0.75 or less.

4. The density N between entangled points is 4 × 10 ２６ (m －３ ) or more 12 x 10 ２６ (m －３ 3. The polyester film according to claim 1, wherein the viscosity of the polyester film is 100% or less.

5. 3. The polyester film according to claim 1, which is used as a protective film.

6. 3. The polyester film according to claim 1, which is used as a protective film for an organic electroluminescence display device.

Images