JP2024112788A - Polyester film for flexible device and image display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a polyester film for a flexible device having good static bending resistance and dynamic bending resistance; and an image display device having the film mounted thereon.

SOLUTION: A polyester film for a flexible device has ΔCp-ΔHg (relaxed amorphousness A) of 0.0 or less which is obtained by subtracting an endothermic energy amount ΔHg (unit: J/g) observed between 30-130°C on an irreversibility curve of temperature-modulated differential scanning calorimetry from a specific heat difference ΔCp (unit: J/(g°C)) at a glass transformation temperature observed between 30-130°C on a reversibility curve of the calorimetry. An image display device has the film mounted thereon.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a polyester film for flexible devices and an image display device.

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

As for folding films, films with controlled refractive index in the bending direction, folding area, and thickness direction are being considered (Patent Document 1).

Furthermore, films that focus on specific bending properties are being considered (Patent Document 2).

JP 2021-9349 A International Publication No. 2021/182191

In Patent Document 1, the polyester film is designed with bending resistance in mind, but only dynamic bending resistance when repeatedly opened and closed is considered, and the fact that the film is left folded and unused for most of the time when not in use is not taken into consideration. In other words, the design does not take static bending resistance into consideration, resulting in poor static bending resistance. In addition, there are dynamic bending resistance measured 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 measuring the degree of creases that remain. In Patent Document 2, although attention is focused on specific bending characteristics to achieve static bending resistance, dynamic bending resistance is not taken into consideration, and the film does not achieve both static bending resistance and dynamic bending resistance.

The object of the present invention is to provide a polyester film for flexible devices that has both the dynamic and static bending resistance described above, and an image display device equipped with the film.

In order to solve the above problems, the polyester film for flexible devices of the present invention and the image display device incorporating the film employ the following means.
(1) A polyester film for flexible devices, in which ΔCp-ΔHg (relaxed amorphousness A), calculated by subtracting the endothermic heat amount ΔHg (unit: J/g) observed between 30°C and 130°C on an irreversible curve of temperature modulated differential scanning calorimetry from the specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature observed between 30°C and 130°C on a reversible curve of the said measurement, is 0.0 or less.
The specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature observed between 30°C and 130°C in the reversible curve of temperature-modulated differential scanning calorimetry is obtained by calculating the glass transition temperature from the point where a straight line equidistant in the vertical axis direction from the straight line extended from each baseline intersects with the curve of the stepwise change part of the glass transition in the differential scanning calorimetry chart obtained from the reversible curve of temperature-modulated differential scanning calorimetry and then calculating the glass transition temperature from the point where the curve of the stepwise change part of the glass transition intersects with the straight line extended from each baseline at the glass transition temperature. The endothermic amount ΔHg (unit: J/g) observed between 30°C and 130°C in the irreversible curve of the measurement is calculated from the endothermic amount ΔHg (unit: J/g) from the peak area of the endothermic peak observed in the irreversible curve.
(2) A polyester film for flexible devices, in which the specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature observed in a reversible curve of temperature modulated differential scanning calorimetry between 30°C and 130°C and the endothermic heat amount ΔHg (unit: J/g) observed in an irreversible curve of the said measurement between 30°C and 130°C are greater than 1.0, and the ΔHg/ΔCp is calculated by dividing the endothermic heat amount ΔHg (unit: J/g) by the specific heat difference ΔCp (unit: J/(g°C)).
The specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature observed between 30°C and 130°C in the reversible curve of temperature-modulated differential scanning calorimetry is obtained by calculating the glass transition temperature from the point where a straight line equidistant in the vertical axis direction from the straight line extended from each baseline intersects with the curve of the stepwise change part of the glass transition in the differential scanning calorimetry chart obtained from the reversible curve of temperature-modulated differential scanning calorimetry and then calculating the glass transition temperature from the point where the curve of the stepwise change part of the glass transition intersects with the straight line extended from each baseline at the glass transition temperature. The endothermic amount ΔHg (unit: J/g) observed between 30°C and 130°C in the irreversible curve of the measurement is calculated from the endothermic amount ΔHg (unit: J/g) from the peak area of the endothermic peak observed in the irreversible curve.
(3) The polyester film for flexible devices according to (1) or (2), wherein the heat absorption ΔHg is 0.1 J/g or more.
(4) The polyester film for flexible devices according to any one of (1) to (3), wherein the endothermic peak observed in an irreversible curve of temperature modulated differential scanning calorimetry between 30°C and 130°C has an endothermic onset temperature Thg of 70°C or higher.
(5) The polyester film for flexible devices according to any one of (1) to (4), having a bending stiffness in at least one direction measured by a loop stiffness tester of 1 mN or more and 100 mN or less.
(6) The polyester film for flexible devices according to any one of (1) to (5), wherein F30m is the stress at 5% elongation in at least one direction, and F30m is the stress after maintaining the 5% elongation state for 30 minutes, and F30m/F0m (stress relaxation degree) obtained by dividing F30m by F0m is 0.4 or more and 1.0 or less.
(7) The polyester film for flexible devices according to any one of (1) to (6), which has a maximum shrinkage stress of 50 kPa or less observed at 40° C. to 110° C. in at least one direction.
(8) The polyester film for flexible devices according to any one of (1) to (7), which contains a polymer containing one or more of a component derived from naphthalenedicarboxylic acid, a component derived from a diol having a fluorene group, and a component derived from isosorbide in its main chain skeleton, and/or a polymer containing an imide group in its main chain skeleton.
(9) The polyester film for flexible devices according to any one of (1) to (8), having an intrinsic viscosity of 0.60 dL/g or more.
(10) The polyester film for flexible devices according to any one of (1) to (9), having an antimony atom content of 50 ppm or more and 130 ppm or less.
(11) The polyester film for flexible devices according to any one of (1) to (10), wherein the amount of carboxy terminal groups is 40 eq/t or less.
(12) The polyester film for flexible devices according to any one of (1) to (11), wherein the angle between a main orientation axis in the plane of the polyester film and the longitudinal direction of the polyester film is 1° or more and 89° or less.
(13) The polyester film for flexible devices according to any one of (1) to (12), which is used for protecting an image display device.
(14) The polyester film for flexible devices according to (13), which is used as a support film for protecting a light-emitting element of an organic electroluminescence image display device.
(15) An image display device equipped with the polyester film for flexible devices according to any one of (1) to (12).
(16) An image display device equipped with the polyester film for flexible devices described in any one of (1) to (12), the image display device being an organic electroluminescence image display device, the image display device having at least one bent portion, and an angle between a bending direction, which is a direction perpendicular to the bent portion, and a main orientation axis in the plane of the polyester film, is 1° or more.

According to the present invention, a polyester film having good static flex resistance and dynamic flex resistance can be provided as a polyester film for flexible devices. Furthermore, an image display device equipped with the film has excellent static flex resistance and dynamic flex resistance.

1 is a graph showing an example of the temperature modulated differential scanning calorimetry characteristic of a polyester film.

Below, the polyester film for flexible devices according to the present invention will be described in detail along with an embodiment.

In one embodiment of the polyester film for flexible devices of the present invention, the specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature observed in the reversible curve of temperature modulated differential scanning calorimetry (TMDSC) between 30°C and 130°C must be less than or equal to 0.0, subtracting the endothermic heat ΔHg (unit: J/g) observed in the irreversible curve between 30°C and 130°C. By controlling the value within this range, a polyester film suitable for flexible devices with good static flex resistance and dynamic flex resistance can be obtained.

Since polyester films are composed of polymer chains, they consist of crystalline components with relatively high rigidity, rigid amorphous components, and mobile amorphous components. The mobile amorphous components are classified into aggregated amorphous components with relatively high rigidity due to their heterogeneity, and relaxed amorphous components with dispersed polymer chains. The amount of relaxed amorphous components in polyester films can be calculated by subtracting the amount of aggregated amorphous components from the amount of mobile amorphous components according to the above relationship. The amount of mobile amorphous components and the amount of aggregated amorphous components can be analyzed by TMDSC. The specific heat difference ΔCp at the glass transition temperature observed between 30°C and 130°C in the reversible curve obtained by TMDSC is proportional to the amount of mobile amorphous components. In addition, the endothermic amount ΔHg observed between 30°C and 130°C in the irreversible curve is the amount of heat required for the aggregated amorphous components to relax and transition to the relaxed amorphous components, and therefore depends on the amount of aggregated amorphous components. The values of ΔCp and ΔHg are calculated as the average of three measurements, and the analysis method using TMDSC is described in the Examples. The relaxed amorphousness A, which indicates the relative amount of relaxed amorphous components, can be expressed as ΔCp-ΔHg, which is calculated by subtracting ΔHg from ΔCp.

As a result of intensive research, the inventors found that a polyester film having a relaxed amorphous index A of 0.0 or less has good static bending resistance and dynamic bending resistance, and arrived at the present invention. In the polyester film for flexible devices of the present invention, the mechanism by which the static bending resistance and dynamic bending resistance are improved by setting the relaxed amorphous index A within the above range is considered to be as follows. That is, when the polyester film is bent, the polymer chain is subjected to elongation and compression forces at the bent portion, and it is considered that the relaxed amorphous component, which has a relatively low rigidity, is plastically deformed. When the polyester film is plastically deformed by bending, it is plastically deformed when left standing in a folded state for a certain period of time, and wrinkles, lifting, cracks, etc. are generated in the polyester film. In addition, when repeatedly bent, the base point of the polyester film breaks due to plastic deformation, leading to breakage. From the above, it is considered that it is important to have a small amount of relaxed amorphous components in order to improve the static bending resistance and dynamic bending resistance. A small amount of relaxed amorphous components is synonymous with a small relaxed amorphous index A, which indicates the relative amount of relaxed amorphous components, and it has been found that when the relaxed amorphous index A is 0.0 or less, the amount of aggregated amorphous components is relatively large compared to the amount of operational amorphous components, the amount of relaxed amorphous components is relatively small, and the static bending resistance and dynamic bending resistance are good.

The polyester film for flexible devices of the present invention preferably has a relaxed amorphism A of 0.0 or less. The relaxed amorphism A is more preferably -0.1 or less, even more preferably -0.2 or less, and particularly preferably -0.3 or less. The smaller the relaxed amorphism A, the better the static flex resistance and dynamic flex resistance, so there is no particular lower limit, and from the viewpoints of mass productivity and economic efficiency, the relaxed amorphism A is preferably -2.0 or more, more preferably -1.0 or more. From the same viewpoint as above, ΔHg/ΔCp is preferably more than 1.0 and less than 10.0, and the lower limit is more preferably greater than 1.5, even more preferably greater than 2.0, and particularly preferably greater than 2.5. It is more preferable that ΔHg/ΔCp is less than 5.0.

In the polyester film for flexible devices of the present invention, in order to control the relaxed amorphous index A and ΔHg/ΔCp within the above ranges, it is important to obtain the film by sequentially carrying out at least the following steps (1) to (3). If the film is obtained by any other method, it may not be possible to control the relaxed amorphous index A and ΔHg/ΔCp within the above ranges.
Step (1) Stretching 2.9 to 3.3 times in the longitudinal direction. Step (2) Stretching 3.7 to 4.5 times in the transverse direction. Step (3) Heat treatment relaxation at 220° C. to 245° C. Further details of each step are described below.

Step (1): In the step of stretching the film in the longitudinal stretching direction by 2.9 to 3.3 times, in order to obtain a more remarkable effect of the present invention, it is important to stretch the film in three stages, and further, it is important that the stretching ratios in the first, second and third stages satisfy the following conditions.
1st stage: ≦1.3 times, 2nd stage: ≦1.5 times, 3rd stage: 2.0 times or more and 2.3 times or less. Step (2): Regarding the step of stretching 3.7 times or more and 4.5 times or less in the transverse stretching direction, in order to obtain the effects of the present invention, it is important to stretch in three stages, and further it is important that the stretching ratios in the 1st, 2nd and 3rd stages satisfy the following conditions.
First stage: 2.0 to 2.5 times, second stage: ≦1.5 times, third stage: ≦1.5 times. Step (3): Regarding the step of performing heat treatment relaxation at 220° C. or more and 245° C. or less, in order to obtain a more remarkable effect of the present invention, it is important to perform the heat treatment relaxation in three stages, and further it is important that the heat treatment temperatures in the first, second and third stages and the relaxation rates in the longitudinal and transverse directions satisfy the following conditions.
1st stage: 150°C or higher and 245°C or lower 2nd stage: 220°C or higher and 245°C or lower 3rd stage: 100°C or higher and 180°C or lower Longitudinal relaxation rate: 1% or higher and 8% or lower Transverse relaxation rate: 1% or higher and 8% or lower In order to obtain the effects of the present invention, it is important that the stretch ratio in the longitudinal direction and the width direction (longitudinal/width stretch ratio) is 0.70 or higher and 0.81 or lower, and it is also important that the areal stretch ratio, which is the product of the longitudinal and width direction stretch ratios, is 10 times or higher and 14 times or lower.

In the above control process, ΔCp can be controlled particularly by the temperature of the heat treatment process, and ΔCp tends to increase as the heat treatment temperatures of the first to third stages are higher. In addition, ΔHg can be controlled particularly by the temperature of the third stage of the heat treatment process and the relaxation rates in the longitudinal and transverse directions, and can be controlled by keeping it within the range of the above process (3).

In the polyester film for flexible devices of the present invention, ΔHg, which depends on the amount of aggregated amorphous components, is preferably 0.1 J/g or more. The lower limit of the preferred range is 0.1 J/g or more, more preferably 0.2 J/g or more, even more preferably 0.3 J/g or more, and particularly preferably 0.4 J/g or more. The higher the amount of aggregated amorphous components, the better the static bending resistance and dynamic bending resistance become, so there is no particular upper limit, but from the viewpoint of economy and mass production, 2.0 J/g or less is preferable, and 1.0 J/g or less is more preferable. The method of controlling ΔHg within the above range is as described in the steps (1) to (3), and in particular, it can be controlled by the temperature in the third stage of the heat treatment step and the relaxation rates in the longitudinal and transverse directions. Furthermore, it is also preferable to set the treatment time of the third stage of the heat treatment step to 3 seconds or more and 60 seconds or less.

In the polyester film for flexible devices of the present invention, the endothermic onset temperature Thg of the endothermic peak observed between 30°C and 130°C in the irreversible curve of temperature modulated differential scanning calorimetry is preferably 70°C or higher. The Thg corresponds to the onset temperature of relaxation of the aggregated amorphous components. Thg is preferably 70°C or higher, more preferably 73°C or higher, even more preferably 75°C or higher, and particularly preferably 80°C or higher. The higher Thg, the better the static bending resistance at high temperatures, so that when the flexible device is exposed to high temperatures, the occurrence of wrinkles and lifting phenomena can be suppressed during folding and unfolding. There is no particular upper limit, but from the viewpoint of economy and mass production, Thg is preferably 130°C or lower, more preferably 120°C or lower. The method of controlling the endothermic onset temperature Thg within the above range can be controlled by setting the glass transition temperature of the resin used in the polyester film to preferably 80°C or higher, more preferably 85°C or higher, and even more preferably 90°C. In addition, as a method for controlling the endothermic onset temperature Thg within the above range, when a plurality of resins are used for the polyester film, it is preferable to include at least a resin having a glass transition temperature of 90° C. or higher, more preferably a resin having a glass transition temperature of 100° C. or higher, and even more preferably a resin having a glass transition temperature of 110° C. or higher. When a plurality of resins are used for the polyester film, it is preferable to use a crystalline resin having a glass transition temperature of 90° C. or higher. When the polyester film is laminated in two or more layers, it is preferable to use a resin having a glass transition temperature of 85° C. or higher in at least one layer. For example, in the case of a three-layer laminate, it is preferable to use a resin having a glass transition temperature of 85° C. or higher in at least one surface layer, and it is preferable to use a resin having a glass transition temperature of 85° C. or higher in all layers.

The polyester film for flexible devices of the present invention preferably has a bending stiffness in at least one direction of 1 mN or more and 100 mN or less. The lower limit of the preferred range is 1 mN or more, more preferably 10 mN or more, and even more preferably 20 mN or more. The upper limit is preferably 100 mN or less, more preferably 70 mN or less, and even more preferably 65 mN or less. The bending stiffness in the above range is preferable because it has good static bending resistance and dynamic bending resistance, does not cause wrinkles or lifting, and has the characteristic of not being restricted in the folding and unfolding operations of the flexible device. When the polyester film for flexible devices of the present invention is applied as a protective film for the surface of an organic electroluminescence image display device, it is particularly preferably 10 mN or more and 50 mN or less. When it is applied as a support film mounted inside an organic electroluminescence image display device, it is particularly preferably 35 mN or more and 65 mN or less. The bending stiffness is determined by a loop stiffness test method, and the detailed measurement conditions are as described in the examples. The method for setting the bending stiffness in at least one direction to the above range is not particularly limited, but examples include a method of adjusting the film thickness or refractive index. Specifically, from the viewpoint of achieving both ease of handling and economic efficiency, the film thickness is preferably 10 μm or more and 80 μm or less, and more preferably 15 μm or more and 60 μm or less. In addition, since it is proportional to the refractive index, it is preferable that the refractive index (measured at a wavelength of 589 nm) in at least one of the longitudinal direction and the width direction is 1.625 or more and less than 1.670. By setting the refractive index in at least one of the longitudinal direction and the width direction to 1.625 or more and less than 1.67, the dynamic bending resistance is improved. The method for controlling the refractive index to the above range is as described in the steps (1) to (3) above, and can be controlled in particular by the longitudinal stretching ratio and the transverse stretching ratio. In addition, from the same viewpoint as above, it is preferable that the bending stiffness in at least one of the longitudinal direction and the width direction satisfies the above numerical range.

The polyester film for flexible devices of the present invention preferably has a birefringence Δn (measured at a wavelength of 589 nm), which is the difference in refractive index between the longitudinal direction and the width direction, of 0.02 or more and 0.05 or less. Birefringence Δn is the difference in refractive index between the longitudinal direction and the width direction, and is the so-called anisotropy of the film. By setting Δn within this range, the static bending resistance and dynamic bending resistance are improved. The birefringence Δn can be controlled to 0.02 or more and 0.05 or less by manufacturing as described in steps (1) to (3) above, and can be controlled in particular by the stretch ratio between the longitudinal direction and the width direction (longitudinal/width stretch ratio).

In the polyester film for flexible devices of the present invention, when the stress at 5% elongation in at least one direction is F0m and the stress after 30 minutes of 5% elongation is F30m, it is preferable that F30m/F0m (stress relaxation degree) obtained by dividing F30m by F0m is 0.4 or more and 1.0 or less. F30m/F0m is an index showing the stress relaxation degree when the polyester film is held for 30 minutes at the F-5 value of the film (elongation 5% stress. Equivalent to F0m). The larger this value, the less likely the film is to undergo plastic deformation, and the better the static bending resistance and dynamic bending resistance can be. Therefore, from the viewpoint of static bending resistance and dynamic bending resistance, it is preferable that it is 0.4 or more, and 1.0 is ideal. From the viewpoint of static bending resistance, F30m/F0m is more preferably 0.5 or more, even more preferably 0.6 or more, and particularly preferably 0.7 or more. Note that F30m and F0m are measured by the method described in the examples. The above F30m/F0m can be controlled by manufacturing as described in steps (1) to (3) above, and can be controlled in particular by the temperature in the third stage of the heat treatment process, the relaxation rates in the longitudinal and transverse directions, and the treatment time. From the same viewpoint as above, it is also preferable that the stress relaxation degree in at least one of the longitudinal and transverse directions satisfies the above numerical range.

In the polyester film for flexible devices of the present invention, it is preferable that the maximum shrinkage stress observed at 40°C to 110°C in at least one direction is 50 kPa or less. The shrinkage stress depends on the amount of relaxed amorphous components stretched, and the smaller the shrinkage stress, the less likely the film is to undergo plastic deformation due to elongation or shrinkage when bent, and the better the static bending resistance and dynamic bending resistance can be. From the viewpoint of static bending resistance, the shrinkage stress is more preferably 40 kPa or less, even more preferably 30 kPa, and particularly preferably 20 kPa or less. There is no particular limit to the lower limit, and the film is 0 kPa or more. The maximum shrinkage stress is measured by the method described in the Examples. The shrinkage stress can be controlled by manufacturing the film as described in the steps (1) to (3), and can be controlled by the relaxation rates in the longitudinal and transverse directions in the heat treatment step. In particular, the areal stretch ratio is preferably 14 times or less, more preferably 13 times or less. From the same viewpoint as above, it is preferable that the maximum shrinkage stress observed at 40°C to 110°C in at least one of the longitudinal and transverse directions falls within the above numerical range.

In the polyester film for flexible devices of the present invention, when the intrinsic viscosity of the polyester film is increased, the static bending resistance and dynamic bending resistance are improved, so that the intrinsic viscosity is preferably 0.60 dL/g or more and 1.5 dL/g or less. The lower limit is preferably 0.60 dL/g or more, more preferably 0.62 dL/g or more, and even more preferably 0.65 dL/g or more, and the upper limit is preferably 1.5 dL/g or less, more preferably 1.15 dL/g or less, and even more preferably 0.90 dL/g or less. It is preferable to set the intrinsic viscosity within the above range because it is possible to suppress the possibility that the filtration pressure becomes too high during the extrusion process for forming the polyester film, thereby reducing the mass productivity. The intrinsic viscosity of the polyester film is determined by the method described in the examples. The intrinsic viscosity of the polyester film can be adjusted by the intrinsic viscosity of the resin used.

In the polyester film for flexible devices of the present invention, the content of antimony atoms is preferably 50 ppm or more and 130 ppm or less in terms of improving transparency, static bending resistance, and dynamic bending resistance. The lower limit is preferably 50 ppm or more, more preferably 55 ppm or more, and even more preferably 60 ppm or more, and the upper limit is preferably 120 ppm or less, more preferably 110 ppm or less, and even more preferably 100 ppm or less. By having the antimony atom content in the above range, it is possible to suppress the possibility of reducing transparency due to light scattering and absorption by antimony atoms while maintaining economic efficiency and mass productivity. The antimony atom content is determined by the method described in the examples. The antimony atom content of the polyester film can be controlled by the antimony atom content of the resin used.

In the polyester film for flexible devices of the present invention, the amount of carboxy terminal groups is preferably 40 eq/t or less. The upper limit is 40 eq/t or less, 35 eq/t or less, or 30 eq/t or less. The lower limit is not particularly limited, but from the viewpoint of mass productivity, 10 eq/t or more is preferable, and 15 eq/t or more is more preferable. By having the amount of carboxy terminal groups in the above range, it is possible to suppress the occurrence of defects due to the polymer chain end that may be the starting point of breakage while maintaining economic efficiency and mass productivity, and it is preferable because static bending resistance and dynamic bending resistance can be improved. The amount of carboxy terminal groups is measured by the method described in the examples. The amount of carboxy terminal groups in the polyester film can be controlled by the amount of carboxy terminal groups of the resin used.

From the viewpoint of use in image display devices, the polyester film for flexible devices of the present invention preferably has a total light transmittance of 90% or more, more preferably 92% or more, and even more preferably 93% or more. The total light transmittance can be controlled by a known method, for example, by providing a thin film layer of an easy-to-adhere resin having a lower refractive index than the polyester film. For example, the refractive index of the thin film layer is preferably 1.58 or less, more preferably 1.53 or less. In addition, from the viewpoint of improving the transmittance by light interference, the layer thickness of the easy-to-adhere resin is preferably in the range of 50 to 140 nm. There is a method for controlling the antimony atom content contained in the polyester film to the above range.

In the polyester film for flexible devices of the present invention, the angle between the main orientation axis in the polyester film plane and the longitudinal direction of the polyester film is preferably 1° or more and 89° or less. When the main orientation axis of the polyester film and the longitudinal direction of the film are in the above range, the static bending resistance and dynamic bending resistance can be maintained at high performance in any direction when the polyester film is incorporated into a flexible device, which is preferable in terms of mass productivity and economic efficiency. The method of controlling the angle between the main orientation axis and the longitudinal direction varies depending on the position in the width direction of the polyester film. For example, in the case of a polyester film obtained by a sequential biaxial stretching method, the angle between the main orientation axis and the longitudinal direction at the central position is 0° or 90°, but the angle of the main orientation axis changes continuously from the central position to the outer position, so the above range can be controlled by using a polyester film other than the central position. In addition, in a uniaxially stretched film obtained by stretching uniaxially in either the longitudinal or width direction, the main orientation axis and the longitudinal direction are at an angle of 0° or 90° throughout the entire width direction of the polyester film, so it may be difficult to achieve both static and dynamic bending resistance. The main orientation axis of the polyester film can be measured using a microwave molecular orientation measurement device, such as the microwave molecular orientation meter MOA-7015 manufactured by Oji Scientific Instruments Co., Ltd., or an optical axis measurement device, such as the ellipsometric measurement device KOBRA-WPR manufactured by Oji Scientific Instruments Co., Ltd.

Examples of glycols or derivatives thereof that give the polyesters of the present invention 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, spiroglycol, and isosorbide; and aromatic dihydroxy compounds such as bisphenol A, bisphenol S, and 9,9'-bis[4-(2-hydroxyethoxy)phenyl]fluorene, as well as derivatives thereof.

In the present invention, from the viewpoint of excellent mechanical properties, it is preferable that the glycol component constituting the polyester is mainly ethylene glycol. Here, "mainly" means that the component occupies a proportion of 70% or more on a molar basis. More preferably, it is 80% or more on a molar basis, even more preferably 90% or more, and particularly preferably 95% or more. Note that when ethylene glycol is mainly polymerized, diethylene glycol may be produced as a by-product and be copolymerized.

Examples of dicarboxylic acids or derivatives thereof that give the polyesters 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, 5-sodiumsulfonedicarboxylic acid, and 9,9'-bis(4-carboxyphenyl)fluorene, 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.

In the present invention, from the viewpoint of low cost and excellent productivity, it is preferable that the acid component constituting the polyester is mainly terephthalic acid or 2,6-naphthalenedicarboxylic acid. Here, "mainly" means that the component occupies 70% or more on a molar basis. More preferably, it is 80% or more on a molar basis, even more preferably 90% or more, and particularly preferably 95% or more.

The polyester composition in the present invention is most preferably made of polyethylene terephthalate resin. Polyethylene terephthalate resin is a polyester resin in which the glycol component is ethylene glycol at 70% or more on a molar basis, and the acid component is terephthalic acid at 70% or more on a molar basis. In addition, other polyester resins may be mixed in place of a single resin. For example, mixing polybutylene terephthalate can reduce the Young's modulus, and adding polyethylene naphthalate tends to increase the Young's modulus. They can be mixed as appropriate according to the required characteristics. However, from the viewpoint of productivity, it is preferable that the polyester composition in the present invention is made of a single polyethylene terephthalate resin.

From the viewpoint of static bending resistance at high temperatures, the polyester film of the present invention preferably contains a polymer containing one or more of a component derived from naphthalene dicarboxylic acid, a component derived from a diol having a fluorene group, and a component derived from isosorbide in the main chain skeleton, and/or a polymer containing an imide group in the main chain skeleton. The glass transition temperature of the above-mentioned aggregated amorphous component can be increased while maintaining dynamic bending resistance, and it is possible to increase Thg, which corresponds to the relaxation onset temperature of the aggregated amorphous component, and the glass transition temperature of the above-mentioned aggregated amorphous component can be increased while maintaining high transparency and dynamic bending resistance. From the same viewpoint, the polyester film preferably contains a total of 5 mass% or more of a polymer containing one or more of a component derived from naphthalene dicarboxylic acid, a component derived from a diol having a fluorene group, and a component derived from isosorbide in the main chain skeleton, and/or a polymer containing an imide group in the main chain skeleton.

In terms of static flex resistance at high temperatures, the composition of the polyester in the present invention is preferably such that, when the sum of all dicarboxylic acid components and all diol components is 200% on a molar basis, the total amount of 2,6-naphthalenedicarboxylic acid, 9,9'-bis[4-(2-hydroxyethoxy)phenyl]fluorene, and isosorbide accounts for 10% or more, and more preferably 15% or more. In particular, when the total dicarboxylic acid components are 100% on a molar basis, it is preferable that 2,6-naphthalenedicarboxylic acid is contained at 10% or more, and the content is more preferably 20% or more, even more preferably 30% or more, and particularly preferably 40% or more. By using such a polyester composition, it is possible to increase the glass transition temperature of the above-mentioned aggregated amorphous components while maintaining dynamic flex resistance, and it is possible to increase Thg, which corresponds to the relaxation onset temperature of the aggregated amorphous components. The polyester may contain resins other than polyester, such as polyetherimide. The content of polyetherimide is preferably 5% by mass or more and 30% by mass or less relative to 100% by mass of the polyester film, and more preferably 7% by mass or more and 20% by mass or less. By containing polyetherimide in the above range, the glass transition temperature of the aggregated amorphous component described above can be increased while maintaining high transparency and dynamic bending resistance.

The polyester film for flexible devices of the present invention is preferably a biaxially oriented polyester film for flexible devices. A biaxially oriented polyester film is produced by stretching an unstretched polyester sheet or film in two directions, the longitudinal direction and the width direction, and refers to a film in which the refractive index in the film thickness direction is smaller than the average value of the refractive index in the longitudinal direction and the width direction in the film plane. A biaxially oriented polyester film has sufficient thermal stability, particularly dimensional stability and mechanical strength, and also has good flatness.

The polyester film for flexible devices of the present invention preferably has a layer containing a curable resin on at least one side. For example, it may be as described in paragraphs [0013] to [0017] of JP-A-2018-124367. In addition, the manufacturing method when providing a curable resin layer may be as described in paragraphs [0025] to [0026] of JP-A-2018-124367.

Next, an example of the polyester film for flexible devices of the present invention and a specific method for producing the polyester film for flexible devices will be described. Here, polyethylene terephthalate is used as the resin constituting the film, but the present invention should not be interpreted as being limited to this example.

First, polyethylene terephthalate resin is dried and pre-crystallized as the 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 295°C. Next, foreign matter is removed and the extrusion amount is balanced through a filter or gear pump, and the resin is discharged in a sheet form onto a cooling drum from a T-die. At this time, the sheet-shaped polymer is adhered to the casting drum by using electrostatic application method in which a high-voltage electrode is used to adhere the resin to the cooling drum with static electricity, 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 temperature of the polyester resin to (glass transition temperature -20°C) to adhere the extruded polymer, or a method in which a combination of these methods is used, and the sheet-shaped polymer is 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.

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 (sequential biaxial stretching method), or in the simultaneous biaxial stretching method, in which the film is stretched in the longitudinal and width directions almost simultaneously.

In the present invention, it is important to carry out at least the following steps (1) to (3) in this order to obtain the above-mentioned product.
Step (1) Stretching 2.9 to 3.3 times in the longitudinal direction. Step (2) Stretching 3.7 to 4.5 times in the transverse direction. Step (3) Heat treatment relaxation at 220° C. to 245° C. Each step is described in detail below.

Step (1): In the step of stretching the film in the longitudinal stretching direction by 2.9 to 3.3 times, in order to obtain a more remarkable effect of the present invention, it is important to stretch the film in three stages, and further, it is important that the stretching ratios in the first, second and third stages satisfy the following conditions.
1st stage: ≦1.3 times, 2nd stage: ≦1.5 times, 3rd stage: 2.0 times or more and 2.3 times or less. Step (2): Regarding the step of stretching 3.7 times or more and 4.5 times or less in the transverse stretching direction, in order to obtain the effects of the present invention, it is important to stretch in three stages, and further it is important that the stretching ratios in the 1st, 2nd and 3rd stages satisfy the following conditions.
First stage: 2.0 to 2.5 times, second stage: ≦1.5 times, third stage: ≦1.5 times. Step (3): Regarding the step of performing heat treatment relaxation at 220° C. or more and 245° C. or less, in order to obtain a more remarkable effect of the present invention, it is important to perform the heat treatment relaxation in three stages, and further it is important that the heat treatment temperatures in the first, second and third stages and the relaxation rates in the longitudinal and transverse directions satisfy the following conditions.
First stage: 150°C to 245°C Second stage: 220°C to 245°C Third stage: 100°C to 180°C Longitudinal relaxation rate: 1% to 8% Transverse relaxation rate: 1% to 8% In order to obtain the effects of the present invention, it is important that the stretch ratio in the longitudinal direction and the width direction (longitudinal/width stretch ratio) is 0.70 to 0.81 in terms of static bending resistance and dynamic bending resistance, and preferably 0.70 to 0.76. It is important that the areal stretch ratio, which is the product of the longitudinal and width direction stretch ratios, is 10 times to 14 times in terms of static bending resistance and dynamic bending resistance, and preferably 12 times to 14 times.

In addition, the stretching temperature in the longitudinal stretching step is preferably set to a level that does not cause stretching unevenness. For example, when using a sequential biaxial stretching method 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, and the stretching temperature is preferably not less than the glass transition temperature of the resin +20°C, the preheating temperature in the width direction is preferably not less than the glass transition temperature of the resin +20°C, and the stretching temperature is preferably not less than the glass transition temperature of the resin +10°C and not more than the glass transition temperature +60°C. Stretching may be performed multiple times in each direction.

In order to obtain the effects of the present invention, the heat treatment step after stretching in the width direction can be performed by any conventional method such as in an oven or on a heated roll. It is important that the heat treatment is performed in multiple zones and the temperature is increased and decreased stepwise, and the temperature and relaxation rate are as described above. A method of slightly stretching the film to about 1.01 to 1.20 times in the width direction in the heat treatment step can also be used. The heat treatment time can be any time within a range that does not deteriorate the properties, and is preferably performed for 1 to 60 seconds, and more preferably for 5 to 30 seconds. The third stage of heat treatment should be performed for 3 to 60 seconds.

In addition, from the viewpoint of adhesion to the layer containing a curable resin, it is preferable that the polyester film for flexible devices of the present invention has an easy-adhesion resin layer having a thickness of 10 nm to 500 nm and a surface free energy of 38 mN/m or more laminated on at least one side. The easy-adhesion resin layer can be formed by coating the surface of the polyester film with an easy-adhesion resin (composite melt extrusion method, hot melt coating method, in-line coating method from a solvent other than water, water-soluble and/or water-dispersible resin, etc.), or by surface lamination of a similar composition or a blend thereof. Among them, the in-line coating method in which a coating agent is applied to one side of a polyester film before the orientation crystallization is completed, stretched in at least one direction, and heat-treated to complete the orientation crystallization is preferred from the viewpoint of forming a uniform coating and from an industrial standpoint. In addition, when the easy-adhesion resin layer is provided by coating, the resin for providing the easy-adhesion resin layer 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.

In addition, in order to further improve the adhesion of the easy-adhesion resin layer used in the present invention, various crosslinking agents can be used in combination with the resin. Melamine-based, epoxy-based, and oxazoline-based resins are generally used as crosslinking agent resins. Examples of particles contained in the resin layer of the present invention include inorganic particles and organic particles, but inorganic particles are more preferred because they improve the slipperiness and blocking resistance. Examples of the inorganic particles that can be used include silica, alumina, kaolin, talc, mica, calcium carbonate, and titanium.

The polyester film for flexible devices of the present invention may be subjected to additional heat treatment such as annealing or aging, as long as the characteristics of the present invention are not impaired. By subjecting the polyester film to annealing or aging treatment, the heat shrinkage characteristics of the polyester film can be controlled, and problems due to differences in heat shrinkage characteristics with other components during the heat processing step when the film is incorporated as an image display device component can be suppressed.

The polyester film for flexible devices of the present invention can be mounted on an image display device. The image display device in the present invention generally refers to display devices, and examples of types of image display devices include organic electroluminescence image display devices, inorganic electroluminescence image display devices, micro LED image display devices, mini LED image display devices, and FED image display devices. In view of the effects of the present invention, the film can be particularly suitably used as a protective film for organic electroluminescence image display devices. By applying the film as a protective film for organic electroluminescence image display devices, the film has excellent static bending resistance and dynamic bending resistance as a support film mounted on the surface of the display device or inside the organic electroluminescence image display device without impairing the flexibility of the display device. Other display applications include use as a touch panel substrate and a shatterproof film.

In addition to optical films, it is also a preferred embodiment to use the properties of the present invention as various protective films, and protective films for industrial materials such as packaging.

The image display device using the polyester film for flexible devices of the present invention is preferably a foldable image display device that can be folded with a bending diameter of 1 to 10 mm. A more preferable range of the bending diameter is 8 mm or less as an upper limit, more preferably 6 mm or less, and even more preferably 5 mm or less. If the bending diameter is 10 mm or less, it can be made thin when folded. The smaller the bending diameter, the better, but the smaller the bending diameter, the more likely it is that creases will be formed. The bending diameter is preferably 0.1 mm or more, but may be 1 mm or more. Even if the bending diameter is 1 mm, it is possible to achieve a practically sufficient thinness when carrying it. In addition, the foldable image display device may be folded in three or four, or may be a rollable type, all of which are considered to fall within the scope of the foldable image display device of the present invention.

In the foldable image display device using the polyester film for flexible devices of the present invention, the angle between the bending direction and the main orientation axis in the polyester film plane is preferably 1° or more and 89° or less. By having the main orientation axis of the polyester film and the bending direction of the image display device in the above range, the static bending resistance and dynamic bending resistance of the foldable image display device can be maintained at a high level, and it is preferable in that mass production and economic efficiency can be improved. The angle between the bending direction and the main orientation axis in the polyester film plane is more preferably 20° or more as a lower limit, more preferably 30° or more, even more preferably 40° or more, and particularly preferably 50° or more, and more preferably 80° or less as an upper limit, more preferably 75° or less, and even more preferably 70° or less.

In the image display device, a polarizer is disposed under the cover member to prevent a decrease in visibility and a decrease in contrast ratio caused by light entering the inside of the display device from the outside. The polarizer in the image display device of the present invention is applied to an organic electroluminescent display device or the like as a circular polarizing plate used together with an optical film (lambda/4 phase difference film), and exerts the effect of blocking specular reflection from metal electrodes of the organic electroluminescent device and the like for all wavelengths of visible light, preventing glare during observation and improving the expression of black. The circular polarizing plate is composed of a polarizer that converts linearly polarized light by transmitting it, and a phase difference plate that converts linearly polarized light by transmitting it into circularly polarized light.

The polarizer may be any polarizer or coated polarizing film used in the art. A representative polarizer may be a polyvinyl alcohol film dyed with a dichroic material such as iodine, but is not limited to this. Any known polarizer or polarizer that may be developed in the future may be appropriately selected and used.

Any retardation plate used in the relevant technical field can be appropriately selected and used. A retardation plate made of a plastic film stretched in a specific direction can be used, and examples of the retardation plate include polycarbonate, polyester, polysulfone, polystyrene, and polymethyl methacrylate. The retardation plate can be formed from a single layer of birefringent film, but it may also be formed by laminating multiple birefringent films so that the wavelength dependency of the retardation is reduced and the plate functions over the entire visible light wavelength range.

The polarizer and retardation plate can be attached to each other using a transparent acrylic adhesive or glue that has no optical anisotropy.

The polyester film for flexible devices of the present invention may also be provided with an adhesive layer. The adhesive layer improves adhesion between layers or adhesion to other members (e.g., a polarizing plate). The position of the adhesive layer is not limited. Examples of components of the adhesive layer include an adhesive and an adhesive. Examples of adhesives include acrylic adhesives, rubber adhesives, and silicone adhesives. The acrylic adhesive is an adhesive containing a polymer of a (meth)acrylic monomer, and when the adhesive layer contains an adhesive, the adhesive layer may contain a tackifier. From the viewpoint of adhesive strength and peelability after molding, the adhesive is preferably an acrylic adhesive. Examples of adhesives include urethane resin adhesives, polyester adhesives, acrylic resin adhesives, ethylene vinyl acetate resin adhesives, polyvinyl alcohol adhesives, polyamide adhesives, and silicone adhesives. The thickness of the adhesive layer is not limited. From the viewpoint of adhesive strength and handling properties, the thickness of the adhesive layer is preferably 5 μm or more and 100 μm or less.

The method of forming the adhesive layer is not limited. For example, it may be formed using a protective film containing an adhesive layer. For example, it may be formed using a composition for forming an adhesive layer containing a component for forming an adhesive layer. The adhesive layer can be formed by applying the composition for forming an adhesive layer onto a polyester film and drying the composition for forming an adhesive layer as necessary. Furthermore, if necessary, after applying the adhesive layer, it can be cured at room temperature or higher and 100°C or lower for one day or more to form the adhesive layer. In addition, various additives such as antioxidants, heat stabilizers, ultraviolet absorbers, infrared absorbers, pigments, dyes, organic or inorganic particles, antistatic agents, nucleating agents, etc. may be added to the adhesive layer.

To improve the effect of preventing reflection of external light, an anti-reflection film can be provided on the surface of the circular polarizer. For example, a multilayer film can be directly formed on the surface of the circular polarizer, or an anti-reflection film can be attached. A fine structure such as a moth-eye structure can also be provided, and an appropriate anti-glare treatment can also be applied.

The configuration of an organic electroluminescence element as an example of a display light-emitting element of an image display device is not particularly limited, but includes a light-emitting element substrate, a thin-film transistor, an organic electroluminescence element, and a sealing layer, and the organic electroluminescence element includes an anode, a light-emitting layer, and a cathode. For example, it may have the layer structure of the following (i) to (vi). In addition, the light-emitting layer described below may be composed of a blue light-emitting layer, a green light-emitting layer, and a red light-emitting layer. The light-emitting element substrate may be the same as the film described later.

Representative examples of the configuration of the organic electroluminescent device are shown below.
(i) anode/hole injection transport layer/light emitting layer/electron injection transport layer/cathode (ii) anode/hole injection transport layer/light emitting layer/hole blocking layer/electron injection transport layer/cathode (iii) anode/hole injection transport layer/electron blocking layer/light emitting layer/hole blocking layer/electron injection transport layer/cathode (iv) anode/hole injection layer/hole transport layer/light emitting layer/electron transport layer/electron injection layer/cathode (v) anode/hole injection layer/hole transport layer/light emitting layer/hole blocking layer/electron transport layer/electron injection layer/cathode (vi) anode/hole injection layer/hole transport layer/electron blocking layer/light emitting layer/hole blocking layer/electron transport layer/electron injection layer/cathode The light emitting element substrate is a base material for supporting various elements of a light emitting element for image display, and may be formed of an insulating material. For example, the light emitting element substrate may be a glass substrate or a plastic substrate. For example, the plastic substrate may be selected from, but is not limited to, polyester, polyimide, polyethersulfone, and polycarbonate.

In the image display device of the present invention, a film may be disposed as a support film under the light emitting element for image display as a support member, and it is preferable to use the polyester film for flexible devices of the present invention as the support film. Flexible glass substrates or plastic substrates are thin and have low rigidity, and may sag when various elements are disposed thereon. The film used as the support film supports the light emitting element for image display so that it does not sag, and protects the light emitting element for image display from moisture, heat, impact, etc. In order to bond the light emitting element for image display and the support film, an adhesive layer may be disposed between the light emitting element for image display and the support film. The adhesive layer may be, but is not limited to, a light transparent adhesive or a reduced pressure adhesive.

The present invention will be explained below with reference to examples, but the present invention is not limited to these examples. The various properties were measured by the following methods.

(1) Antimony Atom and Phosphorus Atom Contents A polyester sample pellet was molded into a cylindrical shape using a melt press, and the fluorescent X-ray intensity was measured using a Rigaku Corporation fluorescent X-ray analyzer (model number: 3270). The antimony atom content and phosphorus atom content were calculated on a mass basis from a calibration curve that had been prepared in advance.

(2) Intrinsic Viscosity A measurement sample was dissolved in 10 mL of orthochlorophenol at 100° C. (solution concentration C (measurement sample mass/solution volume)=0.08 g/mL), and the viscosity of the solution at 25° C. was measured using a viscometer (VMR-052UPC-F10, manufactured by Rigo Co., Ltd.). Similarly, the viscosity of the solvent was measured. Using the obtained solution viscosity and solvent viscosity, [η] was calculated according to the following formula (α), and the obtained value was taken as the intrinsic viscosity.
ηsp/C=[η]+K[η] ²・C (α)
(Here, ηsp=(solution viscosity/solvent viscosity)-1, and K is the Huggins constant (assumed to be 0.343).)
When the solution in which the measurement sample was dissolved contained insoluble matters such as inorganic particles, the measurement was carried out using the following methods (i) to (iv).
(i) A measurement sample is dissolved in 10 mL of orthochlorophenol to prepare a solution having a concentration of more than 0.08 g/mL. The mass of the measurement sample provided in orthochlorophenol is defined as the measurement sample mass.
(ii) Next, the solution containing the insoluble matter is filtered, and the mass of the insoluble matter and the volume of the filtrate after filtration are measured.
(iii) Orthochlorophenol is added to the filtrate after filtration so that (mass of measurement sample (g) - mass of insoluble matter (g)) / (volume of filtrate after filtration (mL) + volume of added orthochlorophenol (mL)) becomes 0.08 g/mL.
(For example, when a concentrated solution of 1.00 g of measurement sample mass/10 mL of solution volume is prepared, if the mass of insoluble matter when the solution is filtered is 0.02 g and the volume of the filtrate after filtration is 9.9 mL, an adjustment is made by adding 5.1 mL of orthochlorophenol. ((1.00 g-0.20 g)/(9.9 mL+0.1 mL)=0.08 g/mL))
(iv) Using a viscometer, the viscosity of the solution obtained in (iii) is measured at 25° C., and using the obtained solution viscosity and solvent viscosity, [η] is calculated according to the above formula (α), and the obtained value is regarded as the intrinsic viscosity.

(3) Carboxy end group amount 60 mL of pure water was added to 1000 mL of orthocresol to prepare an orthocresol adjustment solution. 0.5 g of sample was dissolved in 10 mL of the orthocresol adjustment solution at 100°C, cooled to 25°C, and 3 mL of dichloromethane was added to prepare a solution to be used as a measurement sample. The solution was titrated with a 0.02N KOH/methanol solution using a titration device COM-550 (manufactured by Hiranuma Sangyo Co., Ltd.) to measure the amount of COOH end groups (equivalents/ton). If the solution in which the sample was dissolved contained insoluble matter such as inorganic particles, the solution was filtered to measure the mass of the insoluble matter, and the mass of the insoluble matter was subtracted from the sample mass to obtain the measured sample mass.

(4) Film Thickness When measuring the total thickness of the film, a dial gauge (Model No. XX, manufactured by Mitutoyo Corporation) was used to measure the thickness of five randomly selected points from a sample cut into a 5 cm square from the film, and the average value was calculated.

(5) Thickness of the easy-adhesion resin layer The thickness of the easy-adhesion resin layer on the film was measured by observing the cross section using a transmission electron microscope (Hitachi H-7100 type). The thickness of the easy-adhesion resin layer was read from an image taken with a transmission electron microscope at a magnification of 100,000 times. The thickness of the easy-adhesion resin layer was 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.

(6) Total Light Transmittance Measured according to JIS K7361-1 (1997) using a direct-reading haze computer manufactured by Suga Test Instruments Co., Ltd. Each measurement was carried out three times, and the average value was used.

(7) Main Alignment Axis A sample having dimensions of 100 mm x 100 mm was cut out from any point on the film, and a microwave molecular alignment meter MOA-7015 (frequency 4 GHz) manufactured by KS Systems (now Oji Scientific Instruments) was used.

(8) Longitudinal direction, width direction In the present invention, the flow direction during film production is defined as the longitudinal direction, and the direction perpendicular to the longitudinal direction is defined as the width direction. When the longitudinal direction and width direction are unknown, an arbitrary direction is set as the 0° reference, and the refractive index is measured from 0 to 360° in 15° increments from there, and the direction with the lowest refractive index is defined as the longitudinal direction, and the direction perpendicular to that is defined as the width direction. The refractive index is measured using an Abbe refractometer with sodium D line (wavelength 589 nm) as a light source.

(9) Film refractive index and plane orientation coefficient Using sodium D line (wavelength 589 nm) as a light source and methylene iodide as a mounting liquid, the refractive indexes in the film longitudinal direction, width direction and thickness direction (nMD, nTD, nZD, respectively) were measured in accordance with JIS K7142 (2014) A method using an Abbe refractometer 4T (manufactured by Atago Co., Ltd.) at 25 ° C. The refractive index of the test piece was 1.74. The plane orientation coefficient (fn) was calculated from the obtained refractive index using the following formula (1).
fn=(nMD+nTD)/2-nZD...(1)
(10) Composition of Resin Constituting Film The film is dissolved in hexafluoroisopropanol (HFIP), and the contents of each monomer residue and by-product diethylene glycol are quantified using ¹ H-NMR and ¹³ C-NMR.

(11) Glass transition temperature of raw resin Measurement and analysis were performed using a Thermo plus ECO2 series DSC Vesta manufactured by Rigaku Corporation, and a Thermo plus ECO2 system manufactured by the same company for data analysis, in accordance with JIS K7121 (1987). Specifically, the glass transition temperature was determined from the point where a straight line equidistant in the vertical direction from the extended straight line of each baseline intersects with the curve of the stepwise change in the glass transition in the differential scanning calorimetry measurement chart obtained from the DSC curve when 5 mg of the sample was heated from 25°C to 300°C at 20°C/min.

(12) Specific heat difference ΔCp at the glass transition temperature observed in the film between 30° C. and 130° C.
Using a temperature-modulated differential scanning calorimeter manufactured by TA Instruments, 5 mg of the sample was measured under a nitrogen atmosphere from 0°C to 200°C at a heating rate of 2°C/min, a temperature modulation amplitude of ±1°C, and a temperature modulation period of 60 seconds. The specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature was calculated in the reversible curve in the range of 30°C to 130°C obtained by the measurement. Specifically, in the stepwise change part of the glass transition of the differential scanning calorimeter measurement chart according to JIS K7121 (1987) obtained from the reversible curve, the glass transition temperature was obtained from the point where a straight line equidistant in the vertical axis direction from the straight line extended from each baseline and the curve of the stepwise change part of the glass transition intersect, and the glass transition temperature was obtained from the distance between the two straight lines extended from each of the above-mentioned baselines at the glass transition temperature. The value of ΔCp was the average value of three measurements.

(13) The endothermic amount ΔHg and endothermic onset temperature Thg observed between 30°C and 130°C in the irreversible curve of the film
Using a temperature-modulated differential scanning calorimeter manufactured by TA Instruments, 5 mg of the sample was measured under a nitrogen atmosphere from 0 ° C. to 200 ° C. at a heating rate of 2 ° C. / min, a temperature modulation amplitude of ± 1 ° C., and a temperature modulation period of 60 seconds. The endothermic amount ΔHg (unit: J / g) was calculated from the peak area of the endothermic peak observed in the irreversible curve in the range of 30 ° C. to 130 ° C. obtained by the measurement. The endothermic start temperature Thg was the temperature at the intersection of a straight line extending the baseline on the low-temperature side of the endothermic peak to the high-temperature side and a tangent drawn at the point where the slope of the curve on the low-temperature side of the endothermic peak is maximum. The values of ΔHg and Thg were the average values of three measurements.

(14) Elongation 5% stress (F-5 value)
A rectangular sample having a length of 150 mm and a width of 10 mm was measured using an Instron-type tensile tester according to the method specified in JIS Z1702 (1994). The measurement was carried out under the following conditions, and the stress at an elongation of 5% was measured for 10 samples, and the average value was calculated.
Measuring device: Orientec Co., Ltd.'s automatic film strength and elongation measuring device "Tensilon" (registered trademark) AMF/RTA-100
Sample size: width 10 mm x test length 50 mm
Tensile speed: 300 mm/min. Measurement environment: temperature 23° C., humidity 65% RH.

(15) F0m, F30m
F0m was the stress (unit: MPa) at the time when the elongation reached 5% obtained by the method described in (14). F30m was the stress after the state of 5% elongation was maintained for 30 minutes. The stress was set to F30m (unit: MPa).

(16) Bending stiffness Using a loop stiffness tester manufactured by Toyo Seiki Seisakusho, a sample was cut into a length of 100 mm and a width of 5 mm in the measurement direction, and measurements were performed with a loop circumference of 50 mm, a crushing speed of 3.5 mm/sec, and a crushing distance of 5 mm. The load at the point where the displacement amount reached 10 mm from the start of crushing was taken as the bending stiffness (unit: mN). Next, measurements were performed five times in both the longitudinal direction and the transverse direction, and the average value for each measurement direction was taken as the bending stiffness.

(17) Static bending test lifting height Using a U-shaped stretch tester (Yuasa System Co., Ltd. DLDMLH-FS), a film sample cut to a length of 40 mm x width of 25 mm with the bending direction as the length direction was attached to the end of the tilt clamp with 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 bent state was quickly released and taken out of the device, and the bent inside was placed on a flat floor without applying a load. One minute after releasing the bent state, the height of the center bent part lifted from the ground was read. This measurement was repeated five times, and the average value was calculated to be the static bending test lifting height (unit: mm). The direction in which the length direction of the cut-out film sample was parallel was taken as the measurement result in that direction.

The floating height in a static bending test in the longitudinal and width directions was measured. A smaller floating height is superior. A floating height of 5.0 mm in either the longitudinal or width direction in a static bending test will cause wrinkles and other defects when used as a flexible device, resulting in poor static bending resistance.

(18) MIT Flex Fracture Count A film sample was cut into a rectangle of 110 mm (test direction) x 15 mm width, and a flex test was performed on the film sample using an MIT folding endurance tester (Tester No. 702 manufactured by Mize Co., Ltd.) in accordance with JIS P8115 (2001) at a load of 1,000 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 flexes at which the film sample broke was read as the number of flexes reached at break. This measurement was repeated three times in both the longitudinal and transverse directions, and the average value of the number of flexes less than 1000 was rounded down to the nearest 1000.

The number of MIT flexures to break in the longitudinal and transverse directions was measured. A high number of MIT flexures to break is superior. A material with an MIT flexure to break count of less than 30,000 in either the longitudinal or transverse direction will develop cracks or streaks when used as a flexible device, and will have poor dynamic flexure resistance.

(19) Maximum shrinkage stress A film sample was cut into a rectangle of 50 mm (test direction) x 4 mm width, and TMA (thermo-mechanical analysis) properties were measured using thermal analysis systems EXSTAR-6000 and TMA/SS6000 manufactured by Seiko Instruments Inc. under the conditions of a temperature range of 25°C to 200°C, a heating rate of 10°C/min, a hold time of 5 minutes, a length measurement of 20 mm, and a tensile constant length. The load during length measurement was set to 19.6 mN. The maximum shrinkage stress was defined as the maximum value of the shrinkage stress (unit: kPa) in the range of 40°C to 110°C.

(20) High-temperature static bending test lifting height Using a U-shaped stretching environmental tester (CL09-type-FSC90 manufactured by Yuasa System Equipment), an optical laminate sample cut to a length of 40 mm x width of 25 mm with the bending direction as the length direction was attached to the end of a tilt clamp so that the film sample was on the inside of the bend, horizontal, and the stroke direction was in the length direction of the sample, and the optical laminate was fixed in the most bent state with a face-to-face distance of 3.0 mm and the center of the optical laminate was bent, and the sample was quickly placed in a thermo-hygrostat controlled at 65 ° C. and 30% RH and left to stand for 2 hours. After 2 hours, the sample was quickly removed from the thermo-hygrostat, the bent state of the tilt clamp was released, the optical laminate sample was removed, and the bent inside was placed on a flat floor without applying a load. One minute after the bent state was released, the height of the central bent part lifted from the ground was read. This measurement was repeated five times, and the average value of each was calculated to be the high-temperature static bending test lifting height (unit: mm). The direction in which the length direction of the cut optical laminate sample was parallel was taken as the measurement result in that direction.

The lifting height in a high-temperature static bending test in the longitudinal and transverse directions was measured. The product with the smallest lifting height in the high-temperature static bending test is the best. Products with a lifting height of 7.0 mm or more in either the longitudinal or transverse direction will develop wrinkles when used as a flexible device, and will have poor static bending resistance at high temperatures.

The optical laminate includes the film sample of each example, adhesive layer (25 μm), and glass plate (25 μm) in that order.

The adhesive layer was prepared by adding a mixed solution of 80 parts by weight of ethyl acetate, 70 parts by weight of n-butyl acrylate, 20 parts by weight of methyl acrylate, and 1.0 part by weight of acrylic acid to which 200 parts by weight of acetone and a solution of 0.2 parts by weight of a radical polymerization initiator (2,2'-azobisisobutyronitrile) dissolved in 10 parts by weight of acetone were added in total, and the mixture was reacted at 55°C under nitrogen for 12 hours. Finally, ethyl acetate was added to adjust the concentration of the acrylic resin to 20% by weight. The resulting acrylic resin had a weight average molecular weight Mw of 150,000 and Mw/Mn of 5.0. The resulting acrylic resin was mixed with 0.3 parts by weight of a crosslinking agent ("Coronate L" manufactured by Tosoh Corporation) and 0.5 parts by weight of a silane coupling agent ("X-12-981" manufactured by Shin-Etsu Chemical Co., Ltd.), and ethyl acetate was added so that the total solid concentration was 10% by weight to obtain an adhesive composition.

The obtained coating solution of the adhesive composition was applied to a film sample using an applicator so that the thickness after drying would be 25 μm. The coating layer was dried at 100°C for 1 minute to obtain a film with an adhesive layer. The exposed surface of the adhesive layer was then protected with a release-treated polyethylene terephthalate film (manufactured by Toray Industries, Inc., product name "Therapeel" (registered trademark) MDA, thickness: 38 μm). The film was then aged for 7 days under conditions of a temperature of 65°C and a relative humidity of 50% RH to obtain a laminate of film sample/adhesive layer/protective film.

The protective film was peeled off from the laminate, and the adhesive-attached film sample and a glass plate (25 μm, product name: CG3, manufactured by Corning) were subjected to corona treatment on their respective bonding surfaces, and then the two were bonded together to obtain an optical laminate.

(Production of polyester)
The polyester resin used for film formation was prepared as follows.

(Polyester A)
A slurry consisting of 86 parts by mass of terephthalic acid and 37 parts by mass of ethylene glycol (1.15 times the molar amount relative to terephthalic acid) was gradually added to an esterification reactor containing 105 parts by mass of bishydroxyethyl terephthalate dissolved at 255°C, and the esterification reaction was allowed to proceed. The temperature in the reaction system was controlled to 245 to 255°C, and the esterification reaction was terminated when the reaction rate reached 95%. 105 parts by mass (corresponding to 100 parts by mass of PET) of the esterification reaction product at 255°C thus obtained was transferred to a polymerization apparatus, and an ethylene glycol solution of 0.048 parts by mass of magnesium acetate tetrahydrate and an ethylene glycol slurry of 0.013 parts by mass of diantimony trioxide were added. Next, an ethylene glycol solution of 0.081 parts by mass of phosphoric acid (85% aqueous solution) was added.

Then, the temperature inside the polymerization apparatus was gradually raised to 290°C, while the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill off ethylene glycol. When the melt viscosity reached an intrinsic viscosity of 0.65 dL/g, the reaction was terminated, the reaction system was returned to normal pressure with nitrogen gas, and the ethylene glycol was discharged in strand form from the bottom of the polymerization apparatus into cold water and cut into chips.

The obtained polyester resin was a polyethylene terephthalate resin with 100 mol% terephthalic acid as the dicarboxylic acid component, 98 mol% ethylene glycol as the glycol component, and 2 mol% diethylene glycol as the glycol component, and had an intrinsic viscosity of 0.65 dL/g, a carboxy end group amount of 35 eq/t, an antimony atom content of 110 ppm, and a phosphorus atom content of 15 ppm.

(Polyester B)
A slurry consisting of 81 parts by mass of terephthalic acid, 5 parts by mass of isophthalic acid, and 38 parts by mass of ethylene glycol (1.18 times the molar amount relative to terephthalic acid) was gradually added to an esterification reactor containing 105 parts by mass of bishydroxyethyl terephthalate dissolved at 255°C, and the esterification reaction was allowed to proceed. The temperature in the reaction system was controlled to 245 to 255°C, and the esterification reaction was terminated when the reaction rate reached 95%. 105 parts by mass (corresponding to 100 parts by mass of PET) of the esterification reaction product at 255°C thus obtained was transferred to a polymerization apparatus, and an ethylene glycol solution of 0.048 parts by mass of magnesium acetate tetrahydrate and an ethylene glycol slurry of 0.008 parts by mass of diantimony trioxide were added. Next, an ethylene glycol solution of 0.014 parts by mass of phosphoric acid (85% aqueous solution) was added.

Then, the temperature inside the polymerization apparatus was gradually raised to 290°C, while the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill off ethylene glycol. When the melt viscosity reached an intrinsic viscosity of 0.55 dL/g, the reaction was terminated, the reaction system was returned to normal pressure with nitrogen gas, and the ethylene glycol was discharged in strand form from the bottom of the polymerization apparatus into cold water and cut into chips.

The resulting polyester chips were transferred to a solid-phase polymerization apparatus and subjected to solid-phase polymerization at 220°C and a vacuum of 133 Pa for 6 hours.

The obtained polyester resin was a polyethylene terephthalate resin with a dicarboxylic acid component of 97 mol % terephthalic acid component, 3 mol % isophthalic acid component, and a glycol component of 98 mol % ethylene glycol component and 2 mol % diethylene glycol component, and had an intrinsic viscosity of 0.75 dL/g, a carboxy end group amount of 10 eq/t, an antimony atom content of 65 ppm, and a phosphorus atom content of 25 ppm.

(Polyester C)
A slurry consisting of 86 parts by mass of terephthalic acid and 37 parts by mass of ethylene glycol (1.15 times the molar amount relative to terephthalic acid) was gradually added to an esterification reactor containing 105 parts by mass of bishydroxyethyl terephthalate dissolved at 255°C, and the esterification reaction was allowed to proceed. The temperature in the reaction system was controlled to 245 to 255°C, and the esterification reaction was terminated when the reaction rate reached 95%. 105 parts by mass of the esterification reaction product at 255°C (corresponding to 100 parts by mass of PET) was transferred to a polymerization apparatus, and an ethylene glycol solution of 0.060 parts by mass of magnesium acetate tetrahydrate and an ethylene glycol slurry of 0.019 parts by mass of diantimony trioxide were added. Then, an ethylene glycol solution of 0.013 parts by mass of trimethyl phosphate was added.

Then, the temperature inside the polymerization apparatus was gradually raised to 290°C, while the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill off ethylene glycol. When the melt viscosity reached an intrinsic viscosity of 0.62 dL/g, the reaction was terminated, the reaction system was returned to normal pressure with nitrogen gas, and the ethylene glycol was discharged in strand form from the bottom of the polymerization apparatus into cold water and cut into chips.

The obtained polyester resin was a polyethylene terephthalate resin with 100 mol% terephthalic acid as the dicarboxylic acid component, 98 mol% ethylene glycol as the glycol component, and 2 mol% diethylene glycol as the glycol component, and had an intrinsic viscosity of 0.62 dL/g, a carboxy end group amount of 35 eq/t, an antimony atom content of 160 ppm, and a phosphorus atom content of 20 ppm.

(Polyester D)
100 parts by mass of dimethyl naphthalenedicarboxylate and 48 parts by mass of ethylene glycol were charged into a reaction vessel and dissolved at 180°C, and then an ethylene glycol solution of 0.048 parts by mass of magnesium acetate tetrahydrate and an ethylene glycol slurry of 0.013 parts by mass of diantimony trioxide were added while stirring to start the transesterification reaction. The temperature was raised to 240°C while distilling off methanol over 3.5 hours to complete the transesterification reaction. An ethylene glycol solution of 0.081 parts by mass of phosphoric acid (85% aqueous solution) was added to distill off the excess ethylene glycol.

Then, the temperature inside the polymerization apparatus was gradually raised to 300°C, while the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill off ethylene glycol. When the melt viscosity reached an intrinsic viscosity of 0.65 dL/g, the reaction was terminated, the reaction system was returned to normal pressure with nitrogen gas, and the ethylene glycol was discharged in the form of strands from the bottom of the polymerization apparatus into cold water and cut into chips.

The obtained polyester resin was a polyethylene naphthalate resin with 100 mol% naphthalenedicarboxylic acid as the dicarboxylic acid component, 98 mol% ethylene glycol as the glycol component, and 2 mol% diethylene glycol as the dicarboxylic acid component, and had an intrinsic viscosity of 0.65 dL/g, a carboxy end group amount of 32 eq/t, an antimony atom content of 110 ppm, and a phosphorus atom content of 15 ppm.

(Polyester E)
The polyester B obtained by the above method and pellets of polyetherimide (PEI) "Ultem" (registered trademark) 1010-1000 manufactured by SABIC Innovative Plastics were fed into a vented twin-screw kneading extruder (manufactured by Japan Steel Works, Ltd., screw diameter 30 mm, screw length/screw diameter = 45.5) of the same direction rotation type equipped with three kneading paddle kneading sections heated to a temperature of 320°C, and melt-extruded at a shear rate of 100 sec ^-1 and a residence time of 1 minute, discharged into cold water in the form of strands, and cut into chips.

The polyester resin obtained was composed of 50 parts by mass of polyethylene terephthalate resin, which had a dicarboxylic acid component of 97 mol % terephthalic acid component, 3 mol % isophthalic acid component, and a glycol component of 98 mol % ethylene glycol component and 2 mol % diethylene glycol component, and 50 parts by mass of PEI resin, and had an intrinsic viscosity of 0.70 dL/g, a carboxy terminal group amount of 20 eq/t, an antimony atom content of 32 ppm, and a phosphorus atom content of 12 ppm.

(Preparation of easy-adhesive resin P)
The easy-adhesion resin layer to be laminated on the surface of the film was prepared as follows.

Resin solution (a): A solution obtained by mixing 70 parts by mass of an aqueous coating liquid of a polyester resin consisting of acid components and diol components, namely, terephthalic acid (88 mol%), 5-sodium sulfoisophthalic acid (12 mol%) as acid components, and ethylene glycol (100 mol%) as diol component, with 30 parts by mass of an aqueous dispersion of a polyester resin consisting of acid components and diol components, namely, terephthalic acid (50 mol%), isophthalic acid (49 mol%), 5-sodium sulfoisophthalic acid (1 mol%) as acid components, and ethylene glycol (55 mol%), neopentyl glycol (44 mol%), and polyethylene glycol (molecular weight: 4000) (1 mol%) as diol components.
Crosslinking agent (b): Methylol group-type melamine crosslinking agent Crosslinking agent (c): Oxazoline group-containing crosslinking agent Particles (d): Water dispersion of colloidal silica particles having a particle diameter of 150 nm Particles (e): Water dispersion of colloidal silica particles having a particle diameter of 300 nm Fluorosurfactant (f): Megafac F-444 manufactured by DIC Corporation
These were mixed in a solid content mass ratio of (a)/(b)/(c)/(d)/(e)/(f) = 47 parts by mass/19 parts by mass/20 parts by mass/4.9 parts by mass/0.7 parts by mass/0.1 parts by mass.
The refractive index is 1.57.

(Examples 1 to 9, Comparative Examples 1 to 3)
As shown in the table, the resin type was the polyester shown in the table, vacuum dried at 160 ° C for 3 hours, and then put into an extruder, melted at the extruder temperature shown in the table, and discharged from a T-die into a sheet on a cooling drum controlled at the temperature shown in the table. At that time, electrostatic was applied using a wire-shaped electrode with a diameter of 0.1 mm, and the sheet was adhered to the cooling drum to obtain an unstretched sheet. Then, the sheet was quenched with a cooling roll controlled at a temperature of 20 ° C, and then stretched in the longitudinal direction at the stretching temperature and stretching ratio shown in the table, and then cooled once. Next, corona discharge treatment was performed on both sides of this uniaxially stretched film, the wet tension of the film was set to 55 mN / m, and the easy-adhesion resin P was applied to both sides of the film in the width direction at the stretching temperature and stretching ratio shown in the table, and the film was heat-treated and relaxed in the longitudinal direction and / or width direction at the heat treatment temperature shown in the table in a tenter, and the heat treatment was performed for 5 seconds at the first stage, 5 seconds at the second stage, and 10 seconds at the third stage to obtain a film of the thickness shown in the table. The physical properties of the obtained film are shown in Table 1. The thickness of the easy-adhesive resin layer was 110 nm.

For Examples 1 to 9, both static and dynamic bending resistance were excellent, and in particular, for Examples 7 and 8, both static and dynamic bending resistance were the best. In addition, the static bending resistance at high temperatures was also excellent.

On the other hand, for Comparative Examples 1 to 3, the relaxation amorphousness A was outside the range of the present invention, and the static bending resistance was poor, so the results were not suitable for flexible device applications.

(Comparative Examples 4 and 5)
As shown in the table, the resin type was the polyester shown in the table, vacuum dried at 160 ° C for 3 hours, and then put into an extruder, melted at the extruder temperature shown in the table, and discharged into a sheet form from a T-die onto a cooling drum controlled at the temperature shown in the table. At that time, electrostatic was applied using a wire-shaped electrode with a diameter of 0.1 mm, and the sheet was adhered to the cooling drum to obtain an unstretched sheet. Thereafter, the sheet was quenched with a cooling roll controlled at a temperature of 20 ° C, and then stretched in the longitudinal direction at the stretching temperature and stretching ratio shown in the table, and then cooled once. Next, both sides of this uniaxially stretched film were subjected to a corona discharge treatment, the wet tension of the film was set to 55 mN / m, and the easy-adhesion resin P was applied to both sides of the film, and then the film was stretched in the width direction at the stretching temperature and stretching ratio shown in the table, and the film was heat-treated and relaxed in the longitudinal direction and / or width direction at the heat treatment temperature shown in the table in a tenter. The obtained film roll was then subjected to offline annealing at 180°C for 30 seconds, passing through the first stage for 5 seconds, the second stage for 5 seconds, and the third stage for 10 seconds to obtain a film having the thickness shown in the table. The physical properties of the obtained film are as shown in Tables 1 to 5. The thickness of the easy-adhesion resin layer was 110 nm.

For Comparative Example 4, the relaxation amorphousness A was outside the range of the present invention, so the static bending resistance was poor and the result was unsuitable for flexible device applications. For Comparative Example 5, when held for 30 minutes at a longitudinal elongation stress of 5%, the sample broke during the measurement, so F30m and F30m/F0m could not be measured, and both the static bending resistance and dynamic bending resistance were poor, so the result was unsuitable for flexible device applications.

(Examples 10 to 14)
As shown in the table, a polyester resin was used as the resin type, and a film was obtained and evaluated in the same manner as in Example 1. The physical properties are as shown in the table. The thickness of the easy-adhesion resin layer was 110 nm.

Example 10 was excellent in both static and dynamic bending resistance, with the static bending resistance being particularly excellent. It also had excellent results in static bending resistance at high temperatures.

Examples 11 to 14 were excellent in static bending resistance, high-temperature static bending resistance, and dynamic bending resistance, and Example 11 in particular had the best results in static bending resistance, high-temperature static bending resistance, and dynamic bending resistance.

The polyester film for flexible devices of the present invention and the image display device using the film have good static and dynamic bending resistance, and can be particularly suitably used as a surface protection film or a back support film for an image display device that uses organic electroluminescence as a light-emitting element.

Claims (16)

A polyester film for flexible devices, in which ΔCp-ΔHg (relaxed amorphousness A), calculated by subtracting the endothermic heat amount ΔHg (unit: J/g) observed between 30°C and 130°C on an irreversible curve of temperature modulated differential scanning calorimetry from the specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature observed between 30°C and 130°C on a reversible curve of the said measurement, is 0.0 or less.
The specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature observed between 30°C and 130°C in the reversible curve of temperature-modulated differential scanning calorimetry is obtained by calculating the glass transition temperature from the point where a straight line equidistant in the vertical axis direction from the straight line extended from each baseline intersects with the curve of the stepwise change part of the glass transition in the differential scanning calorimetry chart obtained from the reversible curve of temperature-modulated differential scanning calorimetry and then calculating the glass transition temperature from the point where the curve of the stepwise change part of the glass transition intersects with the straight line extended from each baseline at the glass transition temperature. The endothermic amount ΔHg (unit: J/g) observed between 30°C and 130°C in the irreversible curve of the measurement is calculated from the endothermic amount ΔHg (unit: J/g) from the peak area of the endothermic peak observed in the irreversible curve. A polyester film for flexible devices, in which ΔHg/ΔCp, calculated by dividing the endothermic heat amount ΔHg (unit: J/g) at the glass transition temperature observed between 30°C and 130°C in a reversible curve of temperature modulated differential scanning calorimetry and the endothermic heat amount ΔHg (unit: J/g) observed between 30°C and 130°C in an irreversible curve of the measurement, by the specific heat difference ΔCp (unit: J/(g°C)), is 1.0 or more.
The specific heat difference ΔCp (unit: J/(g°C)) at the glass transition temperature observed between 30°C and 130°C in the reversible curve of temperature-modulated differential scanning calorimetry is obtained by calculating the glass transition temperature from the point where a straight line equidistant in the vertical axis direction from the straight line extended from each baseline intersects with the curve of the stepwise change part of the glass transition in the differential scanning calorimetry chart obtained from the reversible curve of temperature-modulated differential scanning calorimetry and then calculating the glass transition temperature from the point where the curve of the stepwise change part of the glass transition intersects with the straight line extended from each baseline at the glass transition temperature. The endothermic amount ΔHg (unit: J/g) observed between 30°C and 130°C in the irreversible curve of the measurement is calculated from the endothermic amount ΔHg (unit: J/g) from the peak area of the endothermic peak observed in the irreversible curve. The polyester film for flexible devices according to claim 1 or 2, wherein the heat absorption amount ΔHg is greater than 0.1 J/g. The polyester film for flexible devices according to claim 1 or 2, wherein the endothermic peak observed at 30°C to 130°C in the irreversible curve of temperature modulated differential scanning calorimetry has an endothermic onset temperature Thg of 70°C or higher. The polyester film for flexible devices according to claim 1 or 2, wherein the bending stiffness in at least one direction measured by a loop stiffness tester is 1 mN or more and 100 mN or less. The polyester film for flexible devices according to claim 1 or 2, wherein F0m is the stress when 5% elongation is reached in at least one direction, and F30m is the stress after maintaining the 5% elongation state for 30 minutes, and F30m/F0m (stress relaxation degree) obtained by dividing F30m by F0m is 0.4 or more and 1.0 or less. The polyester film for flexible devices according to claim 1 or 2, in which the maximum shrinkage stress observed in at least one direction at 40°C to 110°C is 50 kPa or less. The polyester film for flexible devices according to claim 1 or 2, which contains a polymer containing one or more of a component derived from naphthalenedicarboxylic acid, a component derived from a diol having a fluorene group, and a component derived from isosorbide in its main chain skeleton, and/or a polymer containing an imide group in its main chain skeleton. The polyester film for flexible devices according to claim 1 or 2, which has an intrinsic viscosity of 0.60 dL/g or more. The polyester film for flexible devices according to claim 1 or 2, wherein the content of antimony atoms is 50 ppm or more and 130 ppm or less. The polyester film for flexible devices according to claim 1 or 2, in which the amount of carboxyl end groups is 40 eq/t or less. The polyester film for flexible devices according to claim 1 or 2, wherein the angle between the main orientation axis in the plane of the polyester film and the longitudinal direction of the polyester film is 1° or more and 89° or less. The polyester film for flexible devices according to claim 1 or 2, which is used to protect an image display device. The polyester film for flexible devices according to claim 12, which is used as a support film for protecting a light-emitting element of an organic electroluminescence image display device. An image display device equipped with the polyester film for flexible devices according to claim 1 or 2. An image display device equipped with the polyester film for flexible devices according to claim 1 or 2, the image display device being an organic electroluminescence image display device, the image display device having at least one bent portion, and an angle between the bending direction, which is a direction perpendicular to the bent portion, and the main orientation axis in the plane of the polyester film being 1° or more.

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