WO2024004773A1

WO2024004773A1 - Film for back face plate and polyester film, and image display apparatus and device with film for back face plate or polyester film mounted therein

Info

Publication number: WO2024004773A1
Application number: PCT/JP2023/022869
Authority: WO
Inventors: 龍太郎鎌田; 研鈴木; 亘合田
Original assignee: 東レ株式会社
Priority date: 2022-06-27
Filing date: 2023-06-21
Publication date: 2024-01-04

Abstract

The present invention addresses the problem of providing a film for a back face plate, the film being used for an image display apparatus capable of acquiring normal image information in a photoelectric conversion element behind an image display face. The spirit of the present invention is to provide a film for a back face plate, the flim being used for an image display apparatus in which a polarization plate, an image display light-emitting element array, a back face plate, and a photoelectric conversion element are mounted in this order from the viewer side, wherein a phase difference value obtained when light having a wavelength of 590 nm enters, along a plane including a fast axis of the film and the normal axis with respect to the film surface, the film at an incident angle of 40° with the normal axis as reference 0° is 0-1000 nm. The spirit of the present invention is also to provide a polyester film preferably used to the film for a back face plate.

Description

Back plate film and polyester film, and image display devices and devices equipped with the back plate film or polyester film

The present invention relates to a back plate film used for light emission for image display, a polyester film, an image display device, and a device having the image display device.

With the development of display terminals, image display devices are required to be small, thin, and lightweight.

Electroluminescent display devices, typified by organic light emitting display devices, are self-luminous display devices, and unlike liquid crystal display devices, do not require a separate light source, can be easily made thin and lightweight, and have excellent hue reproduction, response speed, It also has excellent viewing angles and contrast ratios, and is used in display devices.

On the other hand, as display devices become smaller, it is desired that the display device has a larger screen (wider display area), and the area other than the display area of the display device is also required to be reduced. Display is attracting attention.

On the other hand, smartphones and tablet devices are often equipped with cameras for the purpose of taking photos, keeping records, and sharing them with friends. In such terminals, a camera may be provided not only on the back side but also on the image display surface side.

When a camera is placed on the image display surface side, most cases use a slot or opening method. That is, a hole or slot is provided on the image display surface side to allow light from the outside world to enter the camera lens. During imaging, light rays from the outside pass through such slots or apertures and are incident on a camera located below the image display surface. Since the area where such slots or openings are present is a non-display area, development of an image display device without slots or openings is underway (for example, Patent Document 1).

Furthermore, by reducing the bezel area, which corresponds to the non-display area that is the outer part of the display area, and by developing a foldable display device that allows multiple image display devices to be opened and closed in a foldable manner, it is possible to improve display on large screens. Realization and miniaturization and weight reduction of terminals are being considered. In these developments, thin-film and flexible image display devices that can maintain display performance even when warped have been proposed. However, such image display devices are so thin that they cannot be restored when folded, or the creases are fixed. There are things to do. Therefore, in order to support the flexible substrate and maintain the curvature within an appropriate range during bending, a back plate is placed below the flexible substrate.The back plate is made of stainless steel. Those made of metal, such as, and those made of plastic, such as polymethyl methacrylate, polycarbonate, polyvinyl alcohol, and polyethylene terephthalate, are known (for example, Patent Document 2). In addition, it is known that plastic films can cause color unevenness or rainbow unevenness depending on the combination with specific components (polarizing plates, etc.), and various countermeasures have been taken (for example, Patent Document 3 5).

Special Publication No. 2021-511649 JP 2021-107918 Publication Japanese Patent Application Publication No. 2022-133016 Patent No. 6862654 specification Patent No. 6950731 specification

However, when a specific plastic film is used as a film for the back plate, rainbow-like color unevenness occurs in images captured by a camera installed on the image display surface without slots or holes. It became clear that a problem would arise. The occurrence of such color unevenness not only leads to a decrease in image quality, but also causes problems such as a decrease in the quality of information obtained when other photoelectric conversion elements such as a fingerprint authentication sensor, light sensor, distance sensor, etc. are used instead of a camera. cause. Such color unevenness can be improved by inserting a depolarizing member between the camera and the plastic film as in the technology of Patent Document 3, but it is also possible to solve this problem by using a liquid crystal material or forming a complicated uneven pattern. Because it is necessary to do so, the transmittance decreases, causing problems such as a decrease in the quality of the information obtained.

Furthermore, by controlling the retardation to be low in the film described in Patent Document 4, and by controlling the retardation to be high in the film described in Patent Document 5, color unevenness when used in a liquid crystal display device can be improved. It has been described that it suppresses However, not only is there no knowledge of how to eliminate color unevenness when used as a back plate, but there are also problems with heat resistance and bending resistance. In particular, the film described in Patent Document 4 is brittle, and the film described in Patent Document 5 has strong anisotropy and is difficult to handle due to the high retardation, and the film is thick, so its bending resistance is poor. There are challenges.

Therefore, an object of the present invention is to eliminate the above-mentioned color unevenness without using a depolarizing member. An object of the present invention is to provide an image display device with excellent imaging quality even when using a conversion element. It is also preferable to provide an image display device for use in a flexible image display device.

The gist of the present invention for solving the above problems is as follows.

That is, as follows.
[1] A film for a back plate used in an image display device, in which a polarizing plate, a light emitting element array for image display, a back plate, and a photoelectric conversion element are mounted in this order when viewed from the viewer's side. , the retardation value found by incident a light beam with a wavelength of 590 nm at an incident angle of 40 degrees with the vertical axis as a reference of 0 degrees along a plane including the fast axis of the film and an axis perpendicular to the film surface is 0 nm or more and 1000 nm or less A film for the back plate.
[2] The film for a back plate according to [1], which includes a biaxially oriented polyester film as one of its constituent elements.
[3] The biaxially oriented polyester film is a single layer film or a film in which a plurality of layers are laminated, and the isophthalic acid component is used in an amount of 6 mol% or more of the total dicarboxylic acid components of the polyester resin constituting the layer. The film for a back plate according to [2], which has at least one layer of polyester resin.
[4] The polyester constituting the biaxially oriented polyester film is a group consisting of naphthalene dicarboxylic acid, cyclohexane dicarboxylic acid, fluorene derivatives, paraxylene glycol, bisphenol A, isosorbate, cyclohexanedimethanol, and ester-forming derivatives thereof. The film for a back plate according to [2] above, wherein at least one selected from the following is a polymerized or copolymerized polyester, or a polyester containing polyetherimide.
[5] The film for a back plate according to the above [2], which has a crystallinity of 30% or less and a rigid amorphous content of 20% or more and 50% or less.
[6] A film in which two or more layers are laminated, where the refractive index in the thickness direction of the layer with the highest refractive index in the thickness direction is nA, and the refractive index in the thickness direction of the layer with the smallest refractive index in the thickness direction is nB. The film for a back plate according to [1], which satisfies the following formula (1).
nA-nB ≧ 0.02 (1)
[7] The film for a back plate according to [1], wherein the increase in haze (Δ haze) when heat-treated at 120° C. for 5 hours is 0% or more and 2.0% or less.
[8] The film for a back plate according to [2], wherein the biaxially oriented polyester film has a relaxation time of a mobile amorphous phase of 0.200 ms or less in TD-NMR measurement after heat treatment at 120° C. for 3 minutes. .
[9] In the Fourier transform infrared spectroscopy (FT-IR) spectrum of the thickest layer among the layers constituting the film, the spectral intensity r1 observed at 1388 cm ^-1 and the spectral intensity r1 observed at 1372 cm ^-1 The film for a back plate according to [2], wherein the ratio R (r2/r1) of spectral intensity r2 satisfies the following formula (2).
1.0 < R < 1.8 (2)
[10] The film for a back plate according to [1], which has a bending rigidity of 0.2 mN or more and 20.0 mN or less in both the longitudinal direction and the width direction.
[11] The biaxially oriented polyester film is obtained by dividing the endotherm ΔHg observed in the irreversible curve of temperature modulated DSC (30 to 150°C) by the specific heat capacity difference ΔCp before and after the glass transition point observed in the reversible curve. The film for a back plate according to [2], wherein the amorphous density A calculated by ΔHg/ΔCp is 1.0 or more.
[12] The film for a back plate according to [1], wherein the recovery angle is 140° or more in both the longitudinal direction and the width direction.
[13] The film for a back plate according to [1], which has a breaking elongation of 50% or more in both the longitudinal direction and the width direction.
[14] The film for a back plate according to [1], wherein the number of bending failures evaluated using an MIT bending tester is 1000 times or more in both the longitudinal direction and the width direction.
[15] The difference between the average transmittance of P-waves when p-polarized light is incident at an incident angle of 40° and the average transmittance of S-waves when s-polarized light is incident at an incident angle of 40° is 10% or less. A film for a back plate according to [1],
[16] A polyester film consisting of a dicarboxylic acid component and a diol component, in which the terephthalic acid component of the dicarboxylic acid component is 50 mol% or more and 95 mol% or less, and the ethylene glycol component of the diol component is 50 mol% or more 95 mol% or less, the melting point observed at the lowest temperature is 240°C or less, the density is 1.360 g/cc or less, and it is perpendicular to the fast axis of the film and the axis perpendicular to the film surface. A polyester film having a retardation value of 0 nm or more and 1000 nm or less, which is determined by incident light with a wavelength of 590 nm at an incident angle of 40 degrees with the axis as a reference of 0 degrees.
[17] The polyester film is a single layer film or a film in which a plurality of layers are laminated, and the polyester resin contains an isophthalic acid component of 6 mol% or more of the total dicarboxylic acid components of the polyester resin constituting the layer. The polyester film according to [16], having at least one layer.
[18] The polyester constituting the polyester film is selected from the group consisting of naphthalene dicarboxylic acid, cyclohexane dicarboxylic acid, fluorene derivatives, paraxylene glycol, bisphenol A, isosorbate, cyclohexanedimethanol, and ester-forming derivatives thereof. The polyester film according to [16], wherein at least one is a polymerized or copolymerized polyester or a polyester containing polyetherimide.
[19] The polyester film according to [16], which has a crystallinity of 30% or less and a rigid amorphous content of 20% or more and 50% or less.
[20] The polyester film according to [16], wherein the relaxation time of the mobile amorphous phase in TD-NMR measurement after heat treatment at 120° C. for 3 minutes is 0.200 ms or less.
[21] In the Fourier transform infrared spectroscopy (FT-IR) spectrum of the thickest layer among the layers constituting the film, the spectral intensity r1 observed at 1388 cm ^-1 and the spectral intensity r1 observed at 1372 cm ^-1 The polyester film according to [16], wherein the ratio R (r2/r1) of spectral intensity r2 satisfies the following formula (2).
1.0 < R < 1.8 (2)
[22] The polyester film according to [16], which has a bending rigidity of 0.2 mN or more and 20.0 mN or less in both the longitudinal direction and the width direction.
[23] The heat absorption amount ΔHg observed on the irreversible curve of temperature modulated DSC (30 to 150°C) is divided by the specific heat capacity difference ΔCp before and after the glass transition point observed on the reversible curve, which is calculated as ΔHg/ΔCp. The polyester film according to [16], which has a crystal density A of 1.0 or more.
[24] The polyester film according to [16], which has a breaking elongation of 50% or more in both the longitudinal direction and the width direction.
[25] An image display device equipped with the back plate film according to [1] or the polyester film according to [16].
[26] The image display device according to [25], wherein the light emitting device array for an image display device used in the image display device is an organic electroluminescent device array.
[27] The image display device according to [25], wherein the photoelectric conversion element mounted on the image display device has a light-receiving angular range of 60° or more and 140° or less.
[28] A foldable display equipped with the back plate film according to [1] or the polyester film according to [16],
[29] A device comprising, as one of its components, an image display device on which the back plate film according to [1] or the polyester film according to [16] is mounted.

According to the present invention, by using the film for a back plate according to the present invention, even a photoelectric conversion element such as a camera disposed on the side opposite to the viewer's side of the light emitting element array for image display can have rainbow unevenness. Accordingly, it is possible to provide a flexible image display device that suppresses the occurrence of images and has excellent imaging quality. Furthermore, it is possible to provide a polyester film that can suppress the occurrence of rainbow unevenness when used in optical devices such as image display devices.

A schematic front view of an example of an image display device of the present invention Reference diagram for explanation of R40 measurement test Reference diagram for understanding bending stiffness measurement test Diagram explaining the configuration of the device used for the visibility test Reference diagram for understanding the recovery angle measurement test

An image display device in which the back plate film of the present invention is used includes a polarizing plate, a light emitting element array for image display, , a back plate, and a photoelectric conversion element are mounted in this order. Such an image display device may include components other than these components. For example, it is possible to provide an adhesive layer between the image display light emitting element array and the back plate, or to provide a color filter, lens film, diffusion sheet, antireflection film, etc. as necessary. Furthermore, hereinafter, the side opposite to the viewer's side (the side on which no image is displayed) may be referred to as the back side.

An image display device using the film for a back plate of the present invention has an array of light-emitting elements for image display in order to prevent a decrease in the visibility of a displayed image and a decrease in contrast ratio due to light entering the inside of the display device from the outside. A polarizing plate is placed on the front side when viewed from above.

The polarizing plate used in the present invention can be applied to organic electroluminescent display devices, etc. as a circularly polarizing plate used together with an optical film (λ/4 retardation film), so that it can be used in organic electroluminescent devices, etc. at all wavelengths of visible light. It has the effect of blocking reflected light from metal electrodes, etc., and can prevent reflections during observation, as well as improve black expression. A circularly polarizing plate is composed of a polarizer that converts incident light into linearly polarized light and a retardation plate that converts linearly polarized light into circularly polarized light.

As the polarizing plate, any polarizing plate or coated polarizing film used in the field related to image display devices can be appropriately selected and used. Typical polarizing plates include polyvinyl alcohol films dyed with dichroic materials such as iodine, but are not limited to these, and include polarizing plates that are publicly known or that may be developed in the future. can be selected and used as appropriate.

As the retardation plate, a retardation plate used in a field related to image display devices can be appropriately selected and used. As the retardation plate, a plastic film stretched in a specific direction can be used, and examples thereof include polycarbonate, polyester, polysulfone, polystyrene, polymethyl methacrylate, and the like. Although it is possible to form a retardation plate with a single layer of birefringent film, it is possible to laminate multiple birefringent films to reduce the wavelength dependence of the retardation value and function over the entire visible light wavelength range. It may also be formed by

Furthermore, the polarizer and the retardation plate can be bonded together using a transparent acrylic adhesive or adhesive that does not have optical anisotropy.

Note that an antireflection film can be provided on the surface of the polarizing plate from the viewpoint of preventing a decrease in visibility due to reflection of external light on the surface. For example, in addition to directly forming a multilayer film on the surface of a polarizing plate, it is also possible to attach an antireflection film. Further, a fine structure such as a moth-eye structure may be provided, or an appropriate anti-glare treatment may be applied.

The image display light emitting element array, which is one of the constituent elements of the image display apparatus in which the back plate film of the present invention is used, is selected from any image display light emitting element array used in the field related to image display apparatuses. and can be used. Typical light emitting element arrays for image display include those using organic electroluminescent elements, those using inorganic electroluminescent elements, organic light emitting diodes, micro light emitting diodes, etc. However, the present invention is not limited thereto, and any known or future-developed light emitting element array for image display can be appropriately selected and used. Among these, it is preferable to use an organic electroluminescent device because it can achieve high contrast and form a clear image.

An organic electroluminescent element array can be suitably used as the image display light emitting element array. An example of an organic electroluminescent device array includes a substrate, a thin film transistor, an organic electroluminescent device, and a sealing layer, and the organic electroluminescent device includes an anode, a light emitting layer, and a cathode. For example, it may have the following layer structures (i) to (vi). Further, the light-emitting layer described below may consist of a blue light-emitting layer, a green light-emitting layer, and a red light-emitting layer.

Representative examples of the structure of the organic electroluminescent device are shown below.
(i) Anode/Hole injection transport layer/Emissive layer/Electron injection transport layer/Cathode (ii) Anode/Hole injection transport layer/Emissive 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/emissive layer/hole blocking layer/electron transport layer/electron injection layer/cathode.

The substrate is a member for supporting various elements of the light emitting element array for image display, and can be formed of an insulating material. The film for a back plate of the present invention can be suitably used as the main substrate. If this base material is thin and has low rigidity, sag will occur when various elements are arranged. This back plate film supports the image display light emitting element array so that it does not sag, and functions to protect the image display light emitting element array from moisture, heat, impact, and the like. In order to bond the image display light emitting element array and the back plate film, an adhesive layer can be disposed between the image display light emitting element array and the back plate film.

The film for a back plate of the present invention is determined by making a light beam with a wavelength of 590 nm incident at an incident angle of 40° with the vertical axis as a reference of 0° along a plane including the fast axis of the film and an axis perpendicular to the film surface. The retardation value (hereinafter sometimes referred to as "R40") is 0 nm or more and 1000 nm or less. By controlling R40 within the above range, the accuracy of information obtained by the photoelectric conversion element is high, for example, color unevenness in images taken with a camera can be reduced, and recognition accuracy of the sensor is improved. The smaller R40 is, the higher the accuracy of information obtained by the photoelectric conversion element is, and R40 is preferably 800 nm or less, more preferably 500 nm or less. In the present invention, R40 is determined by the method described in (4) Front phase difference Re, R40, which will be described later.

A method for controlling R40 in the range of 0 nm or more and 1000 nm or less is to control the film thickness and the birefringence in the thickness direction of the film. In particular, since an unstretched film of an amorphous resin has a small retardation, it is a commonly used method to use an amorphous resin. The amorphous resin referred to here refers to a resin whose crystal fusion energy (hereinafter also referred to as "ΔHm") obtained from a differential scanning calorimeter (hereinafter also referred to as "DSC") is less than 5 J/g, As examples of typical amorphous resins for optical applications, cyclic olefin resins, polystyrene resins, polycarbonate resins, acrylic resins, triacetylcellulose, and copolymerized polyesters are often used from the viewpoint of optical quality. In particular, from the viewpoint of heat resistance and productivity, cyclic olefin resins such as bicyclo[2,2,1]hept-2-ene (hereinafter referred to as norbornene), tricyclo[4,3,0,12.5]dec- Tricyclic olefins having 10 carbon atoms (hereinafter referred to as tricyclodecene) such as 3-ene, and carbon atoms such as tetracyclo[4,4,0,12.5,17.10]dodec-3-ene, etc. 12 tetracyclic olefins (hereinafter referred to as tetracyclododecene), cyclopentadiene, or 1,3-cyclohexadiene are widely used. Further, it is desirable that the film thickness be made thin as long as the supporting function is not impaired. Moreover, the crystalline resin refers to a resin whose ΔHm is 5 J/g or more.

Furthermore, the film for a back plate of the present invention is preferably a biaxially oriented polyester film from the viewpoint of being compatible with a foldable display described below. If the polyester film is biaxially oriented, the Young's modulus and elongation at break, which will be described later, can be controlled by appropriately setting the stretching conditions. Moreover, by biaxially oriented, the thickness unevenness of the film is reduced, and a high quality film can be obtained. Note that the biaxially oriented film referred to in the present invention refers to the refractive index of one or more layers constituting the film nx, ny, nz (here, the refractive index in the longitudinal direction is nx, the refractive index in the width direction is ny, The refractive index in the thickness direction is nz), and the average value of nx, ny, and nz is smaller than nx and ny and larger than nz. However, in general, biaxial orientation progresses planar orientation and increases R40, so the purpose of the present invention can be achieved by thinning the film or by using the resin composition and film forming process as described below. It is necessary to control the higher-order structure of the film within a range that does not inhibit the

In addition, the polyester film as used in the present invention refers to a film whose main component is polyester as a resin constituting the film. This is the obtained polymer. In addition, in this invention, a structural component shows the minimum unit which can be obtained by hydrolyzing polyester. In addition, the main component refers to the resin with the highest content among the resins constituting the film, and preferably, the resin accounts for 50% by mass when the total mass of the resins constituting the film is 100% by mass. It is more preferably contained in the film in an amount of 70% by mass or more. Furthermore, oxyacids such as hydroxybenzoic acid may be used in part as long as they do not impede the object of the present invention.

The dicarboxylic acid constituents constituting the polyester used in the present invention include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 1 , 8-naphthalenedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 9,9-bis(carboxymethyl)fluorene, 9,9-bis[2-carboxyethyl]fluorene, 9,9 -Bis[1-carboxyethyl]fluorene, 9,9-bis[2-carboxy-1-cyclohexylethyl]fluorene, 9,9-bis[2-carboxy-1-phenylethyl]fluorene, 4,4'-diphenyl Aromatic compounds such as dicarboxylic acid, 4,4'-diphenyl ether dicarboxylic acid, 5-sodium sulfoisophthalic acid, phenyl endane dicarboxylic acid, anthracenedicarboxylic acid, phenanthrene dicarboxylic acid, 9,9'-bis(4-carboxyphenyl)fluorene acid, etc. Aliphatic acids such as dicarboxylic acid, malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, dimer acid, eicosandioic acid, pimelic acid, azelaic acid, methylmalonic acid, ethylmalonic acid, etc. Examples include dicarboxylic acids, alicyclic dicarboxylic acids such as adamantane dicarboxylic acid, norbornene dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, and decalin dicarboxylic acid. Among these, it is preferable to use aromatic dicarboxylic acid as the main dicarboxylic acid constituent. These dicarboxylic acid constituents may be used alone or in combination of two or more. Among them, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, 9,9-bis[2-carboxyethyl]fluorene, 9,9-bis[1-carboxyethyl]fluorene, 9,9-bis[2-carboxy-1-cyclohexyl It is preferable to use at least one selected from the group consisting of ethyl]fluorene and 9,9-bis[2-carboxy-1-phenylethyl]fluorene.

Examples of diol components constituting the polyester used in the present invention include ethylene glycol, 1,3-butanediol, 1,4-butanediol, tetramethylene glycol, 1,6-hexanediol, and 1,2-cyclohexanediol. Methanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, paraxylene glycol, diethylene glycol, triethylene glycol, polyalkylene glycol, isosorbate, 1,2-propanediol, 1,3-propanediol, neopentyl Glycol, 1,5-pentanediol, 1,7-heptanediol, 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene, 9,9-bis[4-(2-hydroxyethoxy)- 3-methylphenyl]fluorene, 9,9-bis[4-(2-hydroxyethoxy)-3,5-dimethylphenyl]fluorene, 2,2-bis(4-hydroxyethoxyphenyl)propane, bisphenol A, spiro Examples include glycol. These diol constituents may be used alone or in combination of two or more. Among them, paraxylene glycol, bisphenol A, isosorbate, cyclohexanedimethanol, 9,9-bis(t-butoxycarbonylmethyl)fluorene, 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene, 9, At least one selected from 9-bis[4-(2-hydroxyethoxy)-3-methylphenyl]fluorene and 9,9-bis[4-(2-hydroxyethoxy)-3,5-dimethylphenyl]fluorene It is preferable to use the following components.

In particular, when the sum of all dicarboxylic acid components and diol components is 100 mol%, the total amount of naphthalene dicarboxylic acid, cyclohexane dicarboxylic acid, the fluorene derivatives exemplified above, paraxylene glycol, bisphenol A, isosorbate, and cyclohexanedimethanol is It is preferable that it accounts for 5 mol% or more, more preferably 7.5 mol% or more. By using such a copolymerized polyester, the amorphousness of the polyester can be improved and the R40 value can be controlled low, and the ratio of the amorphous structure can be increased, so that the agglomerated amorphous components mentioned above can be reduced. This makes it easier to control the value of ΔHg, which indicates the amount of aggregated amorphous, to a high value. From the viewpoint of increasing amorphousness, it is more preferable to use two types of these copolymerization components. Furthermore, the glass transition temperature Tg can be increased, and even under processing temperatures when incorporated into an image display device, an increase in haze can be suppressed, and a film with excellent transparency can be obtained.

Furthermore, polyester may contain resins other than polyester, and examples of such resins include polyetherimide. By including polyetherimide, the proportion of the amorphous structure mentioned above can be increased, and the value of R40 can be controlled to be low, and by increasing the proportion of the amorphous structure, the amount of agglomerated amorphous components can be reduced. can be increased.

The recovery angle of the film for a back plate of the present invention is preferably 140° or more in both the longitudinal direction and the width direction, and more preferably 160° or more. The recovery angle can be evaluated using the measurement method shown in (18) Flexibility, which will be described later. By controlling the recovery angle within this range, even if the image display device is folded and unfolded several times, the back When used as a plate film, the suppression of wrinkles and lifting phenomena is even more remarkable.

In order to control the recovery angle within the above range, the bending rigidity of the film is preferably 20.0 mN or less, more preferably 10.0 mN or less in both the longitudinal direction and the width direction. Further, by setting the bending rigidity to 0.2 mN or more, it becomes possible to support the light emitting element array for image display with a sufficient margin so as not to sag. From the viewpoint of supportability, the bending rigidity is more preferably 1.0 mN or more. The method of controlling the bending rigidity within the range is not particularly limited, but the most effective method is to control the thickness of the film, and the thickness of the film is preferably 10 μm or more and 70 μm or less, and 10 μm or more and 50 μm or less. is more preferable. In addition, in order to control the bending rigidity to a low level, the main method is to lower the Young's modulus, and the Young's modulus in both the longitudinal direction and the width direction is preferably 1.5 GPa or more and 4.5 GPa or less, and 2. It is preferable to set it to 0 GPa or more and 4.0 GPa or less. As a method of controlling the Young's modulus, it can also be increased by using a resin with a high Young's modulus such as a polyester resin. Further, among polyester resins, if the amorphousness is increased by including a copolymer component, the Young's modulus can be lowered. In addition, the Young's modulus is related to the oriented crystalline state of the film, and can also be controlled by the stretching ratio, stretching temperature, heat treatment temperature, heat shrinkage treatment, etc. For example, if the stretching ratio in one direction is high and the stretching ratio in the other direction is low, the Young's modulus in the high-magnification direction will be higher and the Young's modulus in the low-magnification direction will be lower, so the Young's modulus is set to 4.0 GPa or less. In order to achieve this, the Young's modulus can be controlled within the above range by setting the stretching ratio in both the longitudinal direction and the width direction to 2 times or more and 4.5 times or less. In addition, as the orientation state of the film is relaxed, the Young's modulus tends to decrease, so in order to make the Young's modulus 2.0 GPa or more, the melting point of the layer with the lowest melting point among the resins constituting the film is used as a reference. It is necessary to perform heat treatment at +10°C or lower. For example, when a copolymerized polyester with a melting point of 226°C is the layer with the lowest melting point, the heat treatment temperature is preferably 226°C or more and 236°C or less. Further, a plasticizer or an elastomer may be added to lower the Young's modulus as long as it does not impede the effects of the present invention.

The film for the back plate in the present invention is preferably a polyester film, more preferably a biaxially oriented polyester film, but in that case, temperature-modulated differential scanning calorimetry (temperature-modulated DSC) The specific heat capacity difference ΔCp (unit: J/(g°C)) before and after the glass transition point observed between 30°C and 150°C on the curve, and the endothermic amount observed between 30°C and 150°C on the irreversible curve. It is preferable that ΔHg/ΔCp (hereinafter also referred to as "amorphous density A") divided by ΔHg (unit: J/g) is 1.0 or more.

By controlling the amorphous density A within the above range, the bending resistance can be improved and the recovery angle can be highly controlled. As a preferable range, ΔHg/ΔCp is preferably greater than 1.0 and less than 10.0, and the lower limit is more preferably greater than 2.0, particularly preferably greater than 2.5. , the upper limit is more preferably less than 7.0. The reason why bending resistance can be improved by controlling the amorphous density is as follows.

Since a polyester film is composed of polymer chains, it consists of a crystalline component with relatively high rigidity, a rigid amorphous component, and a movable amorphous component. Due to its heterogeneity, the movable amorphous component includes an agglomerated amorphous component that is relatively rigid due to the aggregation of polymer chains, and a relaxed amorphous component in which the polymer chains are dispersed. The amount of relaxed amorphous components in the polyester film can be determined from the above relationship by subtracting the amount of aggregated amorphous components from the amount of mobile amorphous components. The amount of mobile amorphous components and the amount of aggregated amorphous components can be analyzed by temperature modulation DSC. The specific heat capacity difference ΔCp before and after the glass transition temperature observed between 30° C. and 150° C. in the reversible curve obtained by temperature modulation DSC is proportional to the amount of the mobile amorphous component. In addition, the endothermic amount ΔHg observed between 30°C and 150°C in the irreversible curve is the amount of heat required for the agglomerated amorphous component to relax and transform into a relaxed amorphous component, so the agglomerated amorphous component depends on the amount of Note that the values of ΔCp and ΔHg are obtained as the average value of three measurements.

The method for controlling ΔHg/ΔCp within the above range is, for example, as described in steps (1) and (2) below, and in particular, the temperature at the third stage of the heat treatment step and the relaxation rate in the longitudinal and lateral directions are can be controlled by
Step (1) Step of stretching 3.7 times to 4.5 times in the transverse stretching direction Step (2) Step of relaxing by heat treatment at 210° C. to 245° C. Furthermore, details of each step will be described below.

Step (1): In order to obtain the effect of the present invention regarding the step of stretching 3.7 times or more and 4.5 times or less in the transverse stretching direction, it is important to stretch in three stages, and furthermore, in the first stage It is preferable that the stretching ratios in the second and third stages satisfy the following conditions.
1st stage: 2.0 times or more and 2.5 times or less 2nd stage: ≦1.5 times 3rd stage: ≦1.5 times.

Step (2): Regarding the step of heat treatment relaxation at 220°C or higher and 245°C or lower, in order to obtain more significant effects of the present invention, it is important to perform the heat treatment relaxation in three stages. It is preferable that the heat treatment temperature of the stage and third stage and the relaxation rate in the longitudinal direction and the transverse direction satisfy the following conditions.
1st stage: 150°C or more and 245°C or less 2nd stage: 220°C or more and 245°C or less 3rd stage: 100°C or more and 180°C or less Vertical relaxation rate: 1% or more and 8% or less Lateral relaxation rate: 1% or more 8 %below.

The reason why the bending resistance is improved by increasing the value of ΔHg/ΔCp is considered to be as follows. Since a polyester film is composed of polymer chains, it is composed of a relatively rigid crystalline component, a rigid amorphous component, and a movable amorphous component. Due to its heterogeneity, the movable amorphous component includes an agglomerated amorphous component that is relatively rigid due to the aggregation of polymer chains, and a relaxed amorphous component in which the polymer chains are dispersed. When a polyester film is bent, elongation or compression force is applied to the polymer chain at the bend, and it is thought that the relaxed amorphous component, which has relatively low rigidity, is plastically deformed. Polyester film plastically deforms when bent, so when it is left in a folded state for a certain period of time, the polyester film plastically deforms, causing wrinkles, lifting, cracks, etc. to occur in the polyester film. Further, when repeatedly bent, a base point of breakage occurs in the polyester film due to plastic deformation, leading to breakage. From the above, in order to improve the static bending resistance, it is considered preferable to increase the amount of aggregated amorphous components and decrease the amount of relaxed amorphous components. That is, it is thought that the bending resistance is improved by increasing ΔHg, which indicates the amount of aggregated amorphous components, and decreasing ΔCp, which indicates the amount of mobile amorphous components, that is, by increasing the value of ΔHg/ΔCp. Note that ΔHg and ΔCp are obtained by the method described in the section (12) ΔCp, ΔHg, and rigid amorphous amount Xra of the film.

The image display device in which the back plate film of the present invention is used is preferably flexible, and is preferably a foldable display having a bendable portion with a bending diameter of 1 mm or more and 10 mm or less. A foldable display is a single continuous display that can be folded in half for carrying. The upper limit of the bending diameter is more preferably 8 mm or less, more preferably 6 mm or less, and even more preferably 5 mm or less. If the bending diameter is 10 mm or less, it is possible to reduce the thickness of the folded state. It can be said that the smaller the bending diameter, the better, but the smaller the bending diameter, the more easily creases are 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 thickness for carrying. Further, the foldable display may be folded in three or four, or may be a roll-up type called a rollable display, and all of these fall within the scope of the foldable display in the present invention.

It is preferable that the elongation at break of the film for a back plate of the present invention is 50% or more in both the longitudinal direction and the width direction. By setting the elongation at break to 50% or more, the number of times the flexural breakage is reached is 1000 times or more in both the longitudinal and width directions when evaluated using an MIT bending tester, and it can be repeatedly mounted on a foldable display. It is possible to prevent the film from becoming fatigued and breaking when bent. It is more preferable that the elongation at break is 100% or more from the viewpoint of suppressing breakage when repeatedly bent. Note that the higher the elongation at break, the better. Therefore, there is no particular upper limit, but it is sufficient that it is 300% or less in both the longitudinal direction and the width direction. In order to improve the elongation at break, this can be achieved by imparting entanglement and orientation of molecular chains in the higher-order structure of the film to at least one layer of the layer structure of the film constituting the film for the back plate. An effective method is to include a crystalline polyester resin. Moreover, it is more preferable that ΔHm of the crystalline polyester resin is 25 J/g or more. Increasing the entanglement point density can be achieved by increasing the molecular weight of the crystalline resin, that is, by increasing the intrinsic viscosity (also referred to as "IV") of the film. A preferred IV is 0.60 or more and less than 1.00. However, if it is 1.00 or more, it is necessary to use a resin with high viscosity and melt extrusion is difficult, and if it is less than 0.60, it is difficult to form entanglement points and it is difficult to provide orientation by stretching. becomes difficult. It is preferably 0.65 or more and less than 0.90, most preferably 0.68 or more and less than 0.85. From the viewpoint of the above IV when made into a film, the IV of the resin used is preferably 0.65 or more and less than 1.00, more preferably 0.70 or more and less than 0.90, and 0.80 or more and less than 0.90. Most preferably less than Further, from the viewpoint of imparting orientation, the stretching ratios in both the longitudinal direction and the width direction are preferably 3 times or more and 5 times or less. Note that the higher the number of times the bending breakage is reached, the more preferable it is, and therefore there is no particular upper limit, but it is sufficient if it is 100,000 times or less.

It is preferable that the film for a back plate of the present invention has an increase in haze (Δhaze) of 2.0% or less when heat treated at 120° C. for 5 hours. By controlling the Δ haze within the above range, the image can be maintained without whitening even when the back plate film is subjected to heat during the processing process when manufacturing an image display device using this film for the back plate. High visibility of the display device can be maintained. From the viewpoint of maintaining high visibility, Δ haze is preferably 2.0% or less, more preferably 1.5% or less, even more preferably 1.0% or less, and most preferably 0.5% or less. Note that the lower limit of ΔHaze is zero. Processing here refers to the process of forming a functional layer on the back plate film and adding it to the process before it is commercialized. Examples include a process of applying an adhesive, a process of forming a hard coat layer, a process of vapor depositing a transparent conductive layer, a lamination process of bonding films together, and an annealing process.

As a measure for suppressing Δ haze, one example is to control the thermal crystallization heat amount ΔHc determined from DSC to be low. ΔHc is preferably 15 J/g or less, more preferably 10 J/g or less, and most preferably 7 J/g or less. The increase in haze during heat treatment is mainly due to thermal crystallization in which a mobile amorphous component undergoes a phase transition to a crystalline component due to the application of heat, and thermal crystallization can be achieved by controlling ΔHc low. The proportion of movable amorphous crystals decreases, making it difficult to whiten due to heat treatment, so that Δ haze can be controlled to be small. A method for reducing ΔHc is to use an amorphous resin such as COP, which does not undergo thermal crystallization, to form an alloy state in which thermal crystallization is inhibited. Alternatively, it is necessary to use a crystalline resin and form a higher-order structure of the film by oriented crystallization within a range that does not impair the effects of the present invention. From the viewpoint of controlling ΔHc to the above level, the heat treatment temperature when heat treating the film is preferably a melting point of +7°C or less based on the melting point of the resin that constitutes the layer with the lowest melting point among the layers constituting the film, and the layer with the lowest melting point The melting point is more preferably +5° C. or lower based on the melting point of the resin constituting the layer. As mentioned above, from the viewpoint of reducing the movable amorphous component, it is preferable that the amount of rigid amorphous is 20% or more. The upper limit of the amount of rigid amorphous is 50% or less from the viewpoint of reducing R40 as described later. Note that the amount of rigid amorphous is determined by the method described in the section (11) ΔCp, ΔHg, and amount of rigid amorphous Xra of the film.

In addition, from the viewpoint of suppressing the Δ haze within the above range, it is preferable that the relaxation time of the mobile amorphous phase in TD-NMR (Time domain NMR) measurement after heat treatment at 120°C for 3 minutes is 0.200 ms or less. . In this measurement, the molecular mobility of the mobile amorphous component at 120° C. is evaluated, and the faster the relaxation time, the lower the molecular mobility, and the more difficult it is for thermal crystallization to proceed. In this way, an increase in haze can be suppressed not only by reducing the amount of the mobile amorphous component described above, but also by slowing down the progress of thermal crystallization of the mobile amorphous component. As a method for controlling the relaxation time of the mobile amorphous phase in the TD-NMR measurement within the above range, the IV of the film is preferably set to 0.65 or more, more preferably 0.68 or more. By increasing the IV, the mobility of molecules can be controlled to be low, and the relaxation time of the mobile amorphous amount can be controlled to be low. Although there is no particular restriction on the lower limit of the relaxation time of the mobile amorphous phase, it is practically 0.100 ms or less.

The film for a back plate of the present invention is a laminated film in which two or more layers are laminated, and the refractive index in the thickness direction of the layer with the largest refractive index in the thickness direction (such layer is referred to as "layer A") is nA, When the refractive index in the thickness direction of the layer with the smallest refractive index (such layer is referred to as "layer B") is nB, it is preferable that the following formula (1) is satisfied.
nA−nB ≧ 0.02 (1).

By controlling the refractive index of each layer of the film so as to satisfy formula (1), the mechanical strength of the film can be increased by having layer B with a small refractive index in the thickness direction, and the elongation at break can be controlled within the above range. can do. The higher nA−nB is, the more preferable it is, but practically it is 0.15 or less. Further, by having the layer A having a large refractive index in the thickness direction, the retardation in the thickness direction can be controlled to be low, and R40 can be controlled within the above-mentioned preferable range. By controlling the refractive index of each layer in this manner, it is possible to achieve both optical properties and mechanical properties. In addition, from the viewpoint of controlling R40 low, it is preferable that the sum of the thicknesses of the layers having the same refractive index as layer B is as small as possible; It is preferable that the total sum is larger. From the above, it is preferable that the total thickness of the layers having the same refractive index as layer B is 4 μm or more and 10 μm or less.

As a method of controlling to satisfy formula (1), when the melting point of the resin constituting layer A is TmA, and the melting point of the resin constituting layer B is TmB, TmA>TmB, TmB - 15°C or more TmA This can be achieved by heat treatment at a heat treatment temperature of -5°C or lower. Note that the higher the heat treatment temperature is, the more the orientation relaxation progresses and the R40 tends to decrease, but if the heat treatment is performed at a heat treatment temperature higher than TmA - 5°C, the film will melt and it will be difficult to form a film. may not be possible. The method of controlling the melting point of the constituent resins is not limited, but a method of controlling by adding a copolymer component is preferably used. For example, by copolymerizing isophthalic acid with a dicarboxylic acid component, the melting point can be lowered compared to the case where no copolymerization is performed, and the difference in melting point between the resin constituting layer A and the resin constituting layer B can be reduced. You can attach it.

From the viewpoint of easily ensuring a melting point difference, it is preferable to have at least one layer of a polyester resin in which an isophthalic acid component is used in an amount of 6 mol % or more of the total dicarboxylic acid components of the polyester resin constituting the layer.

In addition, the polyester film used in the present invention is made of a polyester resin in which the terephthalic acid component is 50 mol% or more and 95 mol% or less among the dicarboxylic acid components, and the ethylene glycol component is 50 mol% or more and 95 mol% or less among the diol components. It is preferable to use it in at least one of the layers constituting the film. Further, it is more preferable that the melting point on the lowest temperature side measured by DSC is 240° C. or lower from the viewpoint of promoting orientation relaxation. The melting point at the lowest temperature measured by DSC is more preferably 230°C or lower. Although there is no particular restriction on the lower limit, it is preferably 200°C or higher.

In addition, as a method of reducing the refractive index in the thickness direction, a method of increasing the amorphousness of the resin is generally used. Examples include using a polyester resin containing any one of bisphenol A, isosorbate, cyclohexanedicarboxylic acid, and cyclohexanedimethanol.

The density of the polyester film is preferably 1.360 g/cc or less from the viewpoint of controlling R40 to a small value. There is no particular lower limit, but it is preferably 1.34 g/cc or more. Examples of methods for reducing the density to 1.360 g/cc or less include forming a film as exemplified above and employing a heat treatment process.

In the Fourier transform infrared spectroscopy (FT-IR) spectrum of the thickest layer among the layers constituting the film, the spectral intensity r1 observed at 1388 cm ^-1 and the spectral intensity r2 observed at 1372 cm ^-1 It is preferable that the ratio R (r2/r1) satisfies the following formula (2).
1.0 < R < 1.8 (2).

It is known that the methylene group moiety of polyethylene terephthalate has two types of conformations: a gauche structure and a trans structure. Of these two types, the trans structure is more advantageous than the gauche structure in terms of regular arrangement of molecular chains. From this, the gauche-type structure reflects a part where the molecular chains of polyethylene terephthalate are not regularly arranged (hereinafter also referred to as the "amorphous part"), and the trans-type structure reflects a structure in which the molecular chains of polyethylene terephthalate are regularly arranged. are doing. In a structure in which molecular chains are regularly arranged, there is a part that forms a crystalline structure (hereinafter also referred to as "crystalline part") and an amorphous part (hereinafter referred to as "orientated part") in which molecular chains are arranged in an intermediate position between the above-mentioned amorphous part and crystalline part. These can be observed by FT-IR.In other words, r1 reflects the trans-type structure originating from the crystalline part, and r2 reflects the gauche-type structure originating from the amorphous part. Reflects the structure. Regarding the crystal structure, since it has molecular orientation, a phase difference occurs and the value of R40 tends to increase. On the other hand, for the amorphous part, the molecules are random compared to the crystal structure. Therefore, the higher the ratio of the amorphous portion, the lower the R40 tends to be.As shown in equation (2), by increasing R larger than 1.0, R40 can be controlled low. On the other hand, by setting R to less than 1.8, Δ haze can be suppressed.For the same reason, the crystallinity of the film is preferably 30% or less. As a method of controlling the value of R so as to satisfy formula (2) and making the degree of crystallinity 30% or less, the heat treatment temperature is set to -15°C or higher, with the melting point of the thickest layer being the standard. One example of this is a method in which heat treatment is performed at a temperature below the melting point of +20° C. based on the melting point of the thick layer.

Next, a preferred embodiment of manufacturing the film for a back plate of the present invention will be described below. However, the present invention is not construed as being limited to such examples.

The most suitable combination of resins in the present invention is that resin a constituting layer A and resin b constituting layer B each contain a terephthalic acid component or a naphthalene dicarboxylic acid component among the acid components. In a preferable form, it contains 50 mol% or more and 95 mol% or less, and 50 mol% or more and 95 mol% of the ethylene glycol component among the diol components. In particular, it is preferable to include a terephthalic acid component as an acid component from the viewpoint of controlling R40 to 1000 nm or less. Furthermore, it is preferable to use either cyclohexanedicarboxylic acid, a fluorene derivative, paraxylene glycol, bisphenol A, isosorbate, or cyclohexanedimethanol as a monomer for both resin a and resin b, and it is preferable to use any two of them. More preferred. By using it as resin a, it is particularly preferable in terms of controlling R40 to 1000 nm or less. By using it as resin b, the amorphous density A can be controlled to be low. Furthermore, by using it as both resin a and resin b, it is excellent in thermal dimensional stability and heat resistance reliability tests. Further, from the viewpoint of lowering the melting point of resin b compared to resin a, it is preferable to use 6 mol % or more of isophthalic acid as an acid component.

The resins a and b described above are each supplied to a vented twin-screw extruder and melt-extruded. At this time, it is preferable to control the inside of the extruder under a flowing nitrogen atmosphere, with the oxygen concentration being 0.7% by volume or less, and the resin temperature being controlled at 240°C to 320°C. Then, through filters and gear pumps, foreign matter is removed and the extrusion amount is equalized, and in a three-layer merging block with a rectangular laminated section, the resin A layer is placed on the surface layer on both sides, and the resin B layer is placed on the inner layer. They are laminated in this manner and discharged in a sheet form from a T-die onto a cooling drum. At that time, the electrostatic application method uses electrodes to which high voltage is applied to bring the cooling drum and resin into close contact with static electricity, the casting method creates a water film between the casting drum and the extruded polymer sheet, and the casting drum temperature is controlled to increase the temperature of the polyester resin. A sheet-shaped polymer is brought into close contact with a casting drum by a method of making the extruded polymer adhere to a temperature lower than the glass transition point, or by a method of combining two or more of these methods, and is cooled and solidified to obtain an unstretched film. Among these casting methods, when polyester is used, a method of applying electrostatic charge is preferably used from the viewpoint of productivity and flatness.

The film for a back plate of the present invention is preferably stretched in at least one direction from the viewpoints of heat resistance, dimensional stability, mechanical strength, flatness, and thickness unevenness. In the case of biaxial orientation, the unstretched film is stretched in the longitudinal direction and then stretched in the width direction, or by a sequential biaxial stretching method in which the unstretched film is stretched in the width direction and then stretched in the longitudinal direction. It can be obtained by stretching by a simultaneous biaxial stretching method in which the longitudinal direction and the width direction are stretched almost simultaneously. Further, after biaxial orientation, the film may be further stretched in the longitudinal direction or the width direction.

In the present invention, after stretching in the longitudinal direction, it is preferable to perform at least the following steps (1) and (2) in order.
Step (1) Step of stretching 3.7 times to 4.5 times in the transverse stretching direction Step (2) Step of relaxing by heat treatment at 210° C. to 245° C. Furthermore, details of each step will be described below.

The stretching speed in step (1) is preferably 500%/min or more and 100,000%/min or less. In particular, from the viewpoint of reducing R40, it is more preferable to slow the stretching speed, and the second-axis stretching speed is preferably 10,000%/min or less. Further, the preheating temperature is preferably at least -20°C, the glass transition temperature of the resin, and the glass transition temperature of the resin, +20°C, and the stretching temperature is preferably at least the glass transition temperature of the resin, and at most 60°C, the glass transition temperature of the resin.

Furthermore, at least one side can be subjected to corona treatment or coated with an easily adhesive layer. A method for providing a coating layer in-line during the film manufacturing process is to uniformly apply a coating layer composition dispersed in water onto a film that has been uniaxially stretched using a metaling wire bar, gravure roll, etc. However, a method of drying the coating material while stretching is preferred, and in this case, the thickness of the easily adhesive layer is preferably 10 nm or more and 1000 nm or less. Additionally, 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. good. The resin preferably used for the easily adhesive layer is preferably at least one resin selected from acrylic resins, polyester resins, and urethane resins from the viewpoint of adhesiveness and handleability. Furthermore, off-annealing under conditions of 90 to 200° C. is also preferably used.

Regarding the laminated structure, a laminated polyester film can be obtained by coextruding using two or more extruders as described above, or a single layer film can be obtained. Examples of the laminated structure include an A/B laminated structure in which layer A and layer B are laminated one by one, a multilayer laminated structure in which multiple layers of layer A and layer B are laminated alternately, and a laminated structure in which three or more types of layers are laminated. , can be set as necessary. In the case of a laminated polyester having 101 or more layers, a static mixer or a feed block is applied, and layer A consisting of the polyester composition of the present invention and layer B consisting of the other polyester composition are merged so that they are alternately laminated, It is preferable that both surface layer portions become layer B.

Photoelectric conversion elements used in image display devices using the back plate film of the present invention include cameras that acquire light information in the near-ultraviolet to visible light and near-infrared wavelengths of about 300 nm to 1500 nm, and fingerprint Examples include imaging devices such as authentication sensors, light sensors, and distance sensors. A plurality of photoelectric conversion elements may be arranged in one image display device.

In the image display device of the present invention, light transmitted through the display panel is provided to the photoelectric conversion element disposed at the bottom of the display panel, but the form in which light is transmitted through the display does not impair the performance of the present invention. Any method is fine. For example, the method described in Japanese Patent Publication No. 2021-529411 may be mentioned, but the method is not limited thereto.

FIG. 1 is a schematic diagram illustrating an example of an image display device according to the present invention. As shown in FIG. It includes a light emitting element array for image display having a screen area 102 where the image sensor is placed on the back side thereof, and a screen area 103 where no image sensor is placed on the back surface thereof, and a bezel portion 105 where no light emitting element array is placed. In a portion corresponding to the screen area 102 in which the image sensor 104 is arranged on the back side of the image display surface, the light emitting element array for image display has a transparent material that uses transparent materials for the cathode, the anode, and the light emitting element substrate. It has a light emitting element for image display with high transparency, and a part corresponding to the other screen area 103 may have a light emitting element for image display with low transparency, or a light emitting element for image display with high transparency may be included. It may have a light emitting element.

Since this screen area 102 has high transparency, the image sensor 104 can collect off-screen information that has passed through the screen area 102 and has entered, and this image display device can realize full-screen display. . In this example, an example in which the image sensor 104 is arranged is explained, but in the area corresponding to the screen area 102, there may be a camera with a different viewing angle than the image sensor 104, a fingerprint authentication sensor, a light sensor, a distance sensor, etc. Other photoelectric conversion elements may be arranged.

As a result of intensive studies, the present inventors found that in the image display device in which the photoelectric conversion element is arranged on the side opposite to the viewer side of the light emitting element array for image display described above, the R40 of the film used as the back plate is 0 nm or more. It has been found that within a range of 1000 nm or less, color unevenness can be significantly suppressed and sensor accuracy can be improved. The mechanism by which the occurrence of color unevenness is suppressed by the above aspect is considered to be as follows.

The image display device of the present invention includes at least a polarizing plate, a light emitting element array for image display, a light emitting element array for image display, and a light emitting element array for image display in the order from the viewer's side (the side where the image is displayed) toward the direction of the light emitting element array for image display. This is an image display device equipped with a back plate of an element array and photoelectric conversion elements. When a film with birefringence is sandwiched between two optical elements (for example, polarizers, hereinafter referred to as "polarizing optical elements") that have different transmittance of polarized light depending on the in-plane direction, color unevenness can be seen. is widely known. On the other hand, in the configuration shown here, in which the polarizing plate, the light emitting element array for the image display device, the back plate, and the photoelectric conversion element are arranged in this order, no polarizing optical element is included other than the polarizing plate, so the above-mentioned color It is considered that no unevenness is observed. However, when external light incident obliquely or circularly polarized light emitted from a circularly polarizing plate enters the back plate, the transmittance of polarized light (P wave) whose vibration direction is parallel to the incident plane, There is a possibility that the transmittance of polarized light (S wave) in which the vibration direction of light is perpendicular to the incident plane is different. In other words, when light is incident on the surface of the back plate from an oblique direction, the back plate may function as a pseudo polarizing optical element. As a result, it is thought that for light incident from an oblique direction, that is, the part corresponding to the edge of the photograph taken by the camera, the optical path for imaging the part will be pseudo-sandwiched between two polarizing optical elements. It will be done. This is thought to be one of the factors causing color unevenness in the camera and a decrease in sensor accuracy. It is thought that color unevenness can be suppressed and sensor accuracy can be improved by having the film used as the back plate within the above-mentioned retardation range. From the above mechanism, even if the front retardation Re is controlled to be low, it is difficult to suppress color unevenness when R40 is not within the range defined by the present invention. Color unevenness can also be suppressed by reducing the difference in transmittance between S waves and P waves, and the average transmittance of P waves when p-polarized light is incident at an incident angle of 40 degrees and The difference ΔT40 from the average transmittance of S waves when s-polarized light is incident is preferably 10% or less, more preferably 7% or less. More preferably, it is 5% or less. Note that the wavelength range for determining ΔT40 is 400 to 800 nm. The method for setting ΔT40 within this range is not limited, but for example, by applying anti-reflection treatment to the film surface, a low refractive index layer with a refractive index lower than that of the base material is created on the surface layer. A method to improve transmittance by suppressing the reflectance at the film interface.By suppressing the reflectance at the film interface by forming fine protrusions of 1/10 or less of the wavelength of light on the film surface, the transmittance can be improved. A method of controlling the surface structure, such as a method of increasing ΔT40 to reduce it, is preferably used. The antireflection treatment here refers to laminating a layer with a different refractive index on the formed film by methods such as off-coating or vacuum deposition. For example, a method of depositing magnesium fluoride on the surface layer is preferable. used. When forming a low refractive index layer on the film surface, the average value of the refractive index in the longitudinal direction and the width direction of the outermost layer of the film (referred to as "bare film" for convenience) before forming the low refractive index layer is n. When n' is the refractive index of the low refractive index layer formed on the surface layer of the bare film, n/n' is preferably 1.08 or more from the viewpoint of controlling ΔT40 within the above range. Further, the refractive index of the low refractive index layer is preferably 1.35 to 1.58, more preferably 1.35 to 1.45, from the viewpoint of increasing transmittance. The thickness of the low refractive index layer is preferably 1 nm to 1 μm from the viewpoint of not imparting any performance other than optical performance, and preferably from 30 nm to 500 nm from the viewpoint of increasing the transmittance through interference of visible light. is more preferably 60 nm or more and 200 nm or less. When the refractive index is 1.50, the thickness is preferably 60 nm or more and 150 nm or less, and most preferably 85 nm or more and 95 nm or less. As the low refractive index layer, the above refractive index range can be achieved by using, for example, an acrylic resin as the easily adhesive resin layer. Furthermore, the refractive index can also be controlled low by using silica fine particles having internal voids (hereinafter also referred to as hollow silica particles) as particles added as a lubricant to the low refractive index layer. Therefore, it is preferably used. The average particle diameter of the particles added as a lubricant is preferably 40 nm or more and 100 nm or less, more preferably 40 nm or more and 60 nm or less. If the average particle size is less than 40 nm, the refractive index will not be lowered sufficiently, and if it is larger than 100 nm, the film may become white or the surface may become uneven. However, in order to control ΔT40 to a low range of less than 2%, it is necessary to precisely control the surface structure, such as by forming a moth-eye structure by imprinting or stacking multiple vapor-deposited layers as an anti-reflection film. Productivity is poor due to necessity. From that point of view, it is preferable that ΔT40 is 2% or more.

The photoelectric conversion element used in the present invention preferably has a diagonal angle of view of 60 to 140 degrees, which is the maximum angle at which external light irradiated to the image display device can be collected and identified. It is more preferably 70° or more, further preferably 75° or more, and since productivity may decrease if the diagonal angle of view becomes too high, the upper limit is preferably 140° or less, and more preferably 130°. The angle is preferably 120° or less, more preferably 120° or less. Due to the above-mentioned mechanism for suppressing the occurrence of color unevenness, a photoelectric conversion element having a particularly large diagonal angle of view can be used in an image display device, and a wider range of image information can be accurately collected and identified. Further, when a camera is used as a photoelectric conversion element, the number of pixels is preferably 7 million pixels or more in order to take a clear image, and more preferably 12 million pixels or more. The upper limit is not limited, but from the viewpoint of productivity, the upper limit is preferably 48 million pixels or less.

(Methods for measuring characteristics and evaluating effects)
The method of measuring the characteristics and the method of evaluating the effect in the present invention are as follows.

(1) Composition of polyester The film was hydrolyzed with alkali, each component was analyzed by gas chromatography or high performance liquid chromatography, and the composition ratio was determined from the peak area of each component. Dicarboxylic acid constituents and other constituents were measured by high performance liquid chromatography. The measurement conditions can be analyzed by a known method, and an example of the measurement conditions is shown below. Note that the measurement was carried out after inorganic particles were separated by filtration.
Equipment: Shimadzu LC-10A
Column: YMC-Pack ODS-A 150 x 4.6mm S-5μm 120A
Column temperature: 40℃
Flow rate: 1.2ml/min
Detector: UV 240nm
The diol components and other components can be quantitatively analyzed by a known method using gas chromatography. An example of measurement conditions is shown below.
Equipment: Shimadzu 9A (manufactured by Shimadzu Corporation)
Column: SUPELCOWAX-10 capillary column 30m
Column temperature: 140°C to 250°C (heating rate 5°C/min)
Flow rate: Nitrogen 25ml/min
Detector: FID.

(2) Intrinsic viscosity (IV)
A measurement sample (polyester resin (raw material) or polyester film) was dissolved in 100 ml of orthochlorophenol (solution concentration C (measurement sample mass/solution volume) = 1.2 g/100 ml), and the viscosity of the solution at 25 ° C. , measured using an Ostwald viscometer. In addition, the viscosity of the solvent is measured in the same manner. Using the obtained solution viscosity and solvent viscosity, [η] is calculated by the following formula (3), and the obtained value is defined as the intrinsic viscosity (IV) (unit: dl/g).
ηsp/C=[η]+K[η] ²・C Formula (3)
(Here, ηsp=(solution viscosity/solvent viscosity)-1, K is Huggins' constant (assumed to be 0.343).)

(3) Film thickness The film thickness is measured at five arbitrary locations on the film using a dial gauge (standard dial gauge 2109S-10 manufactured by Mitutoyo Co., Ltd.) in accordance with JIS K7130 (1992) A-2 method. We measured the The average value was taken as the film thickness.

(4) Front phase difference Re, R40, slow axis direction, fast axis direction of the film A phase difference measuring device (“KOBRA” (registered trademark)-WPR) manufactured by Oji Scientific Instruments Co., Ltd. was used for measurement. A sample of 3.5 cm x 3.5 cm was cut out from the center in the width direction of the film, and the front retardation Re [nm] at a wavelength of 590 nm and the film orientation angle were measured under the following conditions. The direction of the obtained orientation angle was defined as the slow axis direction, and the direction perpendicular to the slow axis direction was defined as the fast axis direction. When the longitudinal direction and the width direction are unknown, it is assumed that the longitudinal direction and the slow axis direction and the width direction and the fast axis direction are the same, respectively.

Measurement method: High phase difference mode Dispersion curve parameters: a=474.44777, b=2.04281×10 ⁷ , c=250
Order range at λ=750nm: 1-5
Further, the procedure of R40 measurement will be explained using FIG. 2. A light beam with a wavelength of 590 nm is generated along a plane including the fast axis 202 of the film of the film sample 201 and the vertical axis 203 with respect to the film surface from an angle where the incident angle 204 is 40 degrees when the vertical axis is the reference 0 degrees. The phase difference value found by making 205 incident is set as R40. This measurement was also carried out using the same measurement method, dispersion curve parameters, and order range as above.

For the measurement, 5 samples were cut out from the center in the width direction of the film, each was measured, and the average value was used.

(5) Young's modulus, elongation at break A strip-shaped sample having a length of 150 mm and a width of 10 mm was cut out from the central part of the film in the longitudinal direction and the width direction of the film. Young's modulus and elongation at break were measured using a tensile tester according to the method specified in JIS K7127 (1999). The measurements were carried out under the following conditions, and the measurements were made for each of 10 samples, and the average value was determined.
Measuring device: Automatic film strength and elongation measuring device “Tensilon” (registered trademark) AMF/RTA-100 manufactured by Orientech Co., Ltd.
Sample size: width 10mm x sample length 50mm
Tensile speed: 300 mm/min Measurement environment: Temperature 23°C, humidity 65% RH.

(6) Bending rigidity The procedure for measuring bending rigidity will be explained using FIG. 3.

A rectangular sample 201 with a length corresponding to the width direction of the film to be measured of 5 mm and a length corresponding to the longitudinal direction of the film to be measured is 100 mm is prepared from the center of the film width direction. Loop Stiffness Tester (registered trademark) manufactured by J.D. Co., Ltd., is placed on a chuck 301 so that the circumference becomes a ring with a circumference of 50 mm. The probe is lowered from above the ring at a displacement speed of approximately 3.5 mm/sec, and the load is measured when the displacement amount 302 reaches 10 mm after contact with the ring, and the obtained value is measured in the longitudinal direction of the film. The bending stiffness (mN) was taken as . Similarly, a rectangular sample with a length corresponding to the longitudinal direction of the film to be measured of 5 mm and a length corresponding to the width direction of the film to be measured is 100 mm was prepared, and the obtained value was measured in the same manner. The bending stiffness (mN) in the width direction was taken as the width direction bending stiffness (mN). The bending stiffness in the longitudinal direction of the film and the bending stiffness in the width direction of the film were each measured five times, and the average value of all these values was taken as the bending stiffness (mN).

(7) Refractive Index A 3.5 cm x 3.5 cm piece was cut out from the center of the film in the width direction and used as a measurement sample. Using sodium D line (wavelength 589 nm) as a light source and methylene iodide as a mounting liquid, the refractive index in the longitudinal direction, width direction, and thickness direction of the film was measured at 25°C using an Abbe refractometer 4T (manufactured by Atago Co., Ltd.). was measured in accordance with JIS K7142 (2014) A method, and the refractive index in the longitudinal direction of the film was determined as nx, the refractive index in the width direction as ny, and the refractive index in the thickness direction as nz. The test piece placed on the film during measurement had a refractive index of 1.74. When evaluating a laminated film, each layer was peeled off and measured. The measurement was performed five times, and the average value was used.

(8) ΔHaze Measurement was performed using a haze measuring device (manufactured by Nippon Denshoku Kogyo, NDH5000). A square sample having a length of 50 mm and a width of 50 mm was cut out from the center of the film in the width direction. Haze measurement was performed according to the method specified in JIS K7136 (2000). The haze values of the cut samples were determined under the following conditions and were defined as (haze 1) and (haze 2), respectively. By substituting these values into equation (4), Δ haze was calculated. The measurement was performed five times, and the average value was used.
(Haze 1): Haze value when a cut sample is measured as it is.
(Haze 2): Haze value of the sample when the cut sample was set at 120° C. and allowed to stand for 5 hours in a gear oven (GPHH-202, manufactured by ESPEC Co., Ltd.).
ΔHaze = (Haze 2) - (Haze 1) Equation (4).

(9) FT-IR measurement A 3.5 cm x 3.5 cm piece was cut out from the center of the film in the width direction and used as a measurement sample. Using Frontier FT-IR manufactured by PerkinElmer Co., Ltd., using a UATR IR unit, the medium crystal was diamond/ZnSe (refractive index 2.4, φ1.5 mm), the light source was MIR, and the measurement range was 4000 to 650 cm. ^-1 , the spectral intensity was measured by the attenuated total reflection method (ATR method, one reflection) using MIR TGS as a detector. The resolution of the spectrometer is 4 cm ⁻¹ and the number of spectral integrations is 16. The spectrum was subjected to ATR correction, and the intensity was expressed as the absorbance (arb.unit) at each wavelength. The measurement was performed five times, and the average value was used.

(10) Film thermal crystallization heat amount ΔHc, melting point Tm, crystal melting energy ΔHm, crystallinity Xc
Weigh 5 mg of the sample from the center in the width direction of the film using an electronic balance, place it in an aluminum sample pan, and use a Thermo plus ECO2 series DSC vesta manufactured by Rigaku Co., Ltd. to measure JIS K7121 (1987) and JIS K7122 (1987). ), the measurement was carried out by increasing the temperature from 25°C to 300°C at a rate of 20°C/min. Data analysis was performed using the company's Thermo plus ECO2 system. The thermal crystallization amount ΔHc, melting point Tm, and crystal melting energy ΔHm were determined from the obtained DSC data. In addition, when multiple melting point peaks were observed, they were named Tm1, Tm2, . . . in order from the one with the lowest peak temperature.

The crystallinity of the film was calculated from equation (5). However, it was calculated as ΔHm0=140.1J/g.

Crystallinity Xc (%) = (ΔHm-ΔHc)/ΔHm0×100 Formula (5).

(11) Film ΔCp, ΔHg, rigid amorphous amount Xra
The specific heat difference ΔCp (unit: J/(g°C)) and endothermic amount ΔHg (unit: J/g) of the film were measured under the following measurement conditions.
・Measurement method: Temperature modulation DSC method ・Measurement device: Q1000 manufactured by TA Instruments
・Data processing: “Universal Analysis 2000” manufactured by TA Instruments
・Atmosphere: Nitrogen flow (50mL/min)
・Temperature and calorific value calibration: High purity indium (Tm=156.61℃, ΔHm=28.70g/J)
・Specific heat calibration: Sapphire ・Temperature range: 0 to 150℃
・Heating rate: 2℃/min
・Sample amount: 5mg
- Sample container: aluminum standard container The rigid amorphous amount Xra of the film was calculated using equation (6). However, it was calculated as ΔCp0=0.4052J/g°C.
Rigid amorphous amount Xra (%)=100-(Xc+ΔCp/ΔCp0×100) Formula (6).
These measurements were performed five times, and the average value was used.

(12) Difference in average transmittance between S wave and P wave when incident at an incident angle of 40° ΔT40
It was determined using the following procedures (b1) to (b3). The following measurements were performed five times, and the average value was used.
(b1) A 3.5 cm x 3.5 cm piece was cut out from the center of the film in the width direction and used as a measurement sample. Variable angle transmission attachment device for U-4100 spectrophotometer, attached to the spectrophotometer (U-4100 Spectrophotometer) manufactured by Hitachi High-Technology Co., Ltd. (formerly Hitachi High-Technologies Co., Ltd.), 8 mm (H) x 8 mm (H) on the light source mask. 5 mm (W) and a polarizer manufactured by GranTerra Co., Ltd., polarized light that becomes p-polarized light at an incident angle of 40° was incident on the film surface that was the measurement specimen, and the transmission spectrum in the wavelength range of 400 to 800 nm was measured. At this time, the measurement was performed with the slow axis direction of the film perpendicular to the s-polarized light direction. As for the measurement conditions, the slit was set to 2 nm, the gain was set to 2, the scanning speed was set to 600 nm/min, the sampling pitch was set to 1 nm, and a polarizer (manufactured by Kenneth Co., Ltd., thin S size polarizing film) was placed just before the detector. , transmittance 0.43, polarization rate 0.9999, product code 1-115-0820) was attached to allow only the p-polarized component of the transmitted light to enter the detector. From the obtained transmission spectrum, the average value of the transmittance in the wavelength range of 400 to 800 nm was calculated, and was taken as the amount of p-polarized light component Tpp [%] in the transmitted light.
(b2) The same experiment as in (b1) above was carried out by changing the incident light to become s-polarized light with respect to the incident plane, and using a polarizer just in front of the detector to allow only s-polarized light to enter the detector. The amount of s-polarized light component Tps [%] in the transmitted light was determined.
(b3) ΔT40 [%] was calculated using the following formula (7).
ΔT40[%]=|Tps−Tpp| Formula (7).

(13) Density A square of 5 mm on each side was cut from the center of the film in the width direction, and the density (g/cc) was measured according to the JIS K7112-2 (2023) standard using a density gradient tube using an aqueous sodium bromide solution. . Note that the measurement was performed three times, and the average value was used.

(14) Relaxation time of mobile amorphous phase in TD-NMR measurement after heat treatment at 120° C. for 3 minutes The relaxation time of mobile amorphous phase was determined using the apparatus and conditions shown below. The measurement was performed five times, and the average value was used.
Equipment: Bruker Biospin mq20
Temperature: 120℃
Measurement nuclear frequency: 19.95MHz
90° pulse width: 2.2μsec
Pulse repetition time: 2sec
Pulse mode: Solido Echo method Measurement was performed by cutting the film, packing the cut film to a height of 1 cm into a glass tube with an outer diameter of 10 mm, and measuring the spin-spin relaxation response of ^{the 1} H nucleus under the above conditions. The decay curve was measured. The measurement was started after the film was placed in the device and kept at the set temperature of 120° C. for 3 minutes. The obtained attenuation curve was fitted by the least squares method using a Gaussian function and an exponential function to separate it into three different components. The lifetime of each component is designated as T1, T2, and T3 from the shortest to the lowest. T1 corresponds to the lifetime of the crystalline component, T2 corresponds to the lifetime of the rigid amorphous component, and T3 corresponds to the lifetime of the movable amorphous component.

(15) Camera visibility (color unevenness evaluation)
As a pseudo evaluation device for the configuration of the present invention, FIG. 4 shows a schematic diagram of a visibility test in a model device configuration of an image display device in which a photoelectric conversion element is arranged below the image display surface.

Film 201 cut out to 50 mm x 50 mm was stacked on polarizing plate 401 (manufactured by Kennis Co., Ltd., polarizing film thin S size, transmittance 0.43, polarization rate 0.9999, product code 1-115-0820). It was used as a sample. The sample was supported with the polarizing plate 401 side facing down, and was installed horizontally on a surface light source 405 (manufactured by Tritech Co., Ltd., Treviewer A4-100), and a camera built-in device 402 (Samsung "Galaxy") (Registered Trademark) S10) ultra-wide-angle lens 403 (diagonal angle of view 122°), and the built-in camera device 402 was set up so that the optical axis of the camera lens was perpendicular to the film 201. . At this time, the photograph was taken so that the entire photographing angle of view 404 was within the range illuminated by the surface light source 405. In this way, images taken with the polarizing plate, back plate, and camera (photoelectric conversion element) arranged in this order were observed, and the visibility was judged as follows. Note that the visibility of A, B, and C was determined to be an acceptable level that can be used sufficiently even in an image display device.
A: Almost no interference color is seen throughout.
B: Although some interference colors are seen at the corners of the angle of view, there is no problem in practical use.
C: Interference color is seen throughout, but can be used for practical purposes.
D: Not suitable for practical use because interference colors are clearly seen.

(16) Supportability Two types of samples with a length of 100 mm and a width of 20 mm were prepared, one with the long side in the slow axis direction and the other with the long side in the fast axis direction. Place the sample on a platform horizontal to the ground surface with 50 mm protruding from the sample. The protruding part of the sample will sag, so measure the amount of sag. The amount of slack is defined as the length perpendicular to the ground surface from the top surface of the sample resting on the stage to the hanging tip of the sample.This measurement is repeated five times, and the bending in the slow axis direction is defined as the length perpendicular to the ground surface. The average value of the stiffness and fast axis direction values obtained 10 times in total is calculated. Supportability was evaluated based on the following criteria. Note that supportability was judged to be acceptable for A and B, which can be used in image display devices.
A: The amount of slack was less than 10 mm, indicating good support.
B: The amount of slack was 10 mm or more and less than 20 mm, and the supportability was at a level that caused no practical problems.
C: The amount of slack was 20 mm or more, and the support was insufficient.

(17) Film forming stability A film with a width of 300 mm and a length of 200 m (6 inch, 350 mm long core wound) was prepared, and it was rewound onto a 3 inch, 350 mm long core under the following conditions, and the conveying speed and tension were varied. However, evaluation was performed using the following criteria. Note that A and B were judged to be at a level that could be sufficiently handled during processing.
A: No tearing or chipping occurred even when the film was rewound at a speed of 15 m/min and a conveying tension of 70 N/m.
B: No tears or chips occurred even when the film was rewound at a speed of 10 m/min and a conveyance tension of 70 N/m, but when the speed was set to 15 m/min, tears or chips sometimes occurred.
C: Tears and chips occurred when the film was rewound at a speed of 10 m/min and a conveyance tension of 70 N/m.

(18) Bending resistance Measurement was performed using a U-shaped stretch tester (DLDMLH-FS manufactured by Yuasa System Instruments). To explain with reference to FIG. 5, a film sample 201 cut out to a length of 60 mm x width of 25 mm is fixed to a tilt clamp with a horizontal clamping surface so that the direction parallel to the long side is the bending direction 501. The film is left standing for 24 hours in a state where the center portion 502 of the film is bent with a distance between surfaces of 3 mm. After 24 hours, the film sample is released from the bent state, taken out from the apparatus, and left standing with the bent outside facing down, and the angle 503 formed by the film sample is measured. This measurement was repeated five times for each of the film sample cut out with the longitudinal direction of the film as the long side and the film sample cut out with the width direction as the long side, and the arithmetic average value was calculated and used as the recovery angle. Angle 403 was read with 0° being a completely folded state and 180° being a state in which the film had recovered to its original unfolded state before folding. Among the measurement results in the longitudinal direction and the width direction, the measurement result with the smaller recovery angle was used to evaluate the bending resistance according to the following criteria. Note that the bending resistances A and B are at a level that can be suitably used as a foldable display.
A: The recovery angle was 160° or more, and no wrinkles or lifting phenomenon occurred due to bending when mounted on a foldable display.
B: The recovery angle is 140° or more and less than 160°, and there is no problem in practical use as a foldable display.
C: The recovery angle is less than 140°, and although it is practical as an image display device, it is not suitable for practical use as a foldable display.

(19) Number of times to reach bending break The film sample was cut into a rectangle of 110 mm (test direction) x 15 mm wide, and tested using an MIT bending tester (testing machine No. 702 manufactured by Mize Co., Ltd.) according to JIS P8115 (2001). A bending test was conducted with 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 film sample was broken. The number of bending times at that time was defined as the number of times the bending break was reached. The test was conducted three times each for samples taken with the MD direction as the test direction and samples taken with the TD direction as the test direction, and the arithmetic mean value was determined. In addition, the following evaluation is performed using the MD direction or the TD direction, whichever has a larger number of bending failures, and the repeated bending resistance A and B are at a level that can be suitably used as a foldable display.
A: The number of bending breaks reached was 2000 times or more, indicating very good repeated bending resistance.
B: The number of bending breaks reached is 1000 or more and less than 2000 times, and there is no problem in practical use as a foldable display.
C: The number of bending failures reached is less than 1000 times, and although it is practical as an image display device, it is not suitable for practical use as a foldable display.

(Production of polyester)
The polyester resin used for film formation was prepared as follows. Note that the expression of mol% is based on the total amount of each dicarboxylic acid component and diol component being 100 mol%. Further, the total amount of dicarboxylic acid components and the total amount of diol components are equal in molar ratio.

(Polyester A)
As dicarboxylic acid components, terephthalic acid component is 92 mol%, isophthalic acid component is 8 mol%, as diol components, ethylene glycol component is 80 mol%, 1,4-cyclohexanedimethanol component is 10 mol%, and isosorbate component is 10 mol%. A copolymerized polyester resin (intrinsic viscosity: 0.80).

(Polyester B)
A copolymerized polyester resin (proper Viscosity: 0.80).

(Polyester C)
As dicarboxylic acid components, terephthalic acid component is 88 mol%, isophthalic acid component is 12 mol%, as diol components, ethylene glycol component is 80 mol%, 1,4-cyclohexanedimethanol component is 10 mol%, and isosorbate component is 10 mol%. A copolymerized polyester resin (intrinsic viscosity: 0.80).

(Polyester D)
As dicarboxylic acid components, terephthalic acid component is 92 mol%, isophthalic acid component is 8 mol%, as diol components, ethylene glycol component is 73 mol%, 1,4-cyclohexanedimethanol component is 16 mol%, and isosorbate component is 11 mol%. A copolymerized polyester resin (intrinsic viscosity: 0.80).

(Polyester E)
A copolymerized polyester resin (proper Viscosity: 0.80).

(Polyester F)
A copolymerized polyester resin (intrinsic viscosity: 0.80) containing 88 mol% of a terephthalic acid component as a dicarboxylic acid component, 12 mol% of an isophthalic acid component, and 100 mol% of an ethylene glycol component as a diol component.

(Polyester G)
A polyester resin (intrinsic viscosity: 0.80) containing 100 mol% of a terephthalic acid component as a dicarboxylic acid component and 100 mol% of an ethylene glycol component as a diol component.

(Polyester H)
A copolymerized polyester resin (intrinsic viscosity: 0.80) containing 95 mol% of a terephthalic acid component as a dicarboxylic acid component, 5 mol% of an isophthalic acid component, and 100 mol% of an ethylene glycol component as a diol component.

(Polyester I)
As dicarboxylic acid components, terephthalic acid component is 92 mol%, isophthalic acid component is 8 mol%, as diol components, ethylene glycol component is 80 mol%, 1,4-cyclohexanedimethanol component is 10 mol%, and isosorbate component is 10 mol%. A copolymerized polyester resin (intrinsic viscosity: 0.77).

(Polyester J)
A copolymerized polyester resin (proper Viscosity: 0.77).

(Polyester K)
As dicarboxylic acid components, terephthalic acid component is 92 mol%, isophthalic acid component is 8 mol%, as diol components, ethylene glycol component is 80 mol%, 1,4-cyclohexanedimethanol component is 10 mol%, and isosorbate component is 10 mol%. A copolymerized polyester resin (intrinsic viscosity: 0.69).

(Polyester L)
A copolymerized polyester resin (proper Viscosity: 0.69).

(Polyester M)
A copolymerized polyester resin containing 92 mol% of terephthalic acid component as dicarboxylic acid component, 8 mol% of isophthalic acid component, 90 mol% of ethylene glycol component as diol component, and 10 mol% of isosorbate component (intrinsic viscosity: 0.80 ).

(Polyester N)
A copolyester resin (intrinsic viscosity: 0.80) containing 100 mol% of a terephthalic acid component as a dicarboxylic acid component, 90 mol% of an ethylene glycol component as a diol component, and 10 mol% of an isosorbate component.

(Polyester O)
As dicarboxylic acid components, 82 mol% of terephthalic acid component, 8 mol% of isophthalic acid component, 10 mol% of 2,6-naphthalene dicarboxylic acid component, 90 mol% of ethylene glycol component as diol component, and 1,4-cyclohexane dicarboxylic acid component. A copolyester resin containing 10 mol% of methanol component (intrinsic viscosity: 0.80).

(Polyester P)
As dicarboxylic acid components, terephthalic acid component is 90 mol%, 2,6-naphthalene dicarboxylic acid component is 10 mol%, as diol components, ethylene glycol component is 90 mol%, and 1,4-cyclohexanedimethanol component is 10 mol%. Copolymerized polyester resin (intrinsic viscosity: 0.80).

(Polyester/PEI resin A)
A co-rotating vent type twin-screw kneading extruder equipped with three kneading paddle kneading sections heated to a temperature of 280°C (manufactured by Japan Steel Works, screw diameter 30 mm, screw length/screw diameter = 45.5) Pellets of polyester F obtained by the above method and polyetherimide (PEI) “ ^Ultem ” (registered trademark) Polyester/PEI resin A containing 16% by mass of polyetherimide was obtained by melt extrusion. (Intrinsic viscosity: 0.80)
(Polyester/PEI resin B)
A co-rotating vent type twin-screw kneading extruder equipped with three kneading paddle kneading sections heated to a temperature of 280°C (manufactured by Japan Steel Works, screw diameter 30 mm, screw length/screw diameter = 45.5) Pellets of polyester G and polyetherimide (PEI) “ ^Ultem ” (registered trademark) Polyester/PEI resin B containing 16% by mass of etherimide was obtained. (Intrinsic viscosity: 0.80)
(Olefin A)
"ZEONOR" (registered trademark) 1020R manufactured by Nippon Zeon Co., Ltd. (a resin obtained by ring-opening metathesis polymerization of a cyclic olefin and/or its derivative and then hydrogenation) was used.

(Easy adhesive coating layer)
The easily adhesive coating layer to be laminated on the surface of the film was prepared as follows.

<Coating layer A>
Resin solution (a): Acrylic resin solution copolymerized with 64% by mass of methyl methacrylate, 30% by mass of ethyl acrylate, 5% by mass of acrylic acid, and 1% by mass of acrylonitrile.
Crosslinking agent (b): Methylol-based melamine crosslinking agent.
Particles (c): An aqueous dispersion of collodyl silica particles with a particle diameter of 60 nm.
Fluorine surfactant (d): “Megafac” (registered trademark) F-444 manufactured by DIC Corporation
These were mixed at a solid content mass ratio of (a)/(b)/(c)/(d)=30 parts by mass/8 parts by mass/2 parts by mass/0.6 parts by mass. The refractive index after drying is 1.50.

<Coating layer B>
Resin solution (e): Aqueous solution of polyester resin consisting of an acid component of terephthalic acid (88 mol%), 5-sodium sulfoisophthalic acid (12 mol%) as an acid component, and ethylene glycol (100 mol%) as a diol component. 70 parts by mass of a color coating liquid, acid components terephthalic acid (50 mol%), isophthalic acid (49 mol%), 5-sodium sulfoisophthalic acid (1 mol%), diol components ethylene glycol (55 mol%), neo A solution containing 30 parts by mass of an aqueous dispersion of a polyester resin consisting of an acidic content of pentyl glycol (44 mol%), polyethylene glycol (molecular weight: 4000) (1 mol%), and a diol component.
Crosslinking agent (b): Methylol-based melamine crosslinking agent Crosslinking agent (f): Oxazoline group-containing crosslinking agent Particles (g): Aqueous dispersion of collodyl silica particles with a particle size of about 150 nm Particles (h): Particle size of about 300 nm Aqueous dispersion of collodyl silica particles Fluorine surfactant (d): “Megafac” (registered trademark) F-444 manufactured by DIC Corporation
The solid content mass ratio of these is (e) / (b) / (f) / (g) / (h) / (d) = 47 parts by mass / 19 parts by mass / 20 parts by mass / 4.9 parts by mass / 0 They were mixed at a ratio of .7 parts by mass/0.1 parts by mass. The refractive index after drying is 1.57.

Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention should not be construed as being limited to these.

(Surface processed layer)
<Off coat layer>
219 parts by mass of 3,3,3-trifluoropropyltrimethoxysilane (KBM-7103, manufactured by Shin-Etsu Silicone) was hydrolyzed with 89 parts by mass of 0.5N formic acid while stirring at 20°C±5°C. After 60 minutes, 412 parts by mass of isopropyl alcohol was mixed to prepare a treatment liquid (X1).

Similarly, 158 parts by mass of 3,3,3-trifluoropropyltrimethoxysilane (KBM-7103 manufactured by Shin-Etsu Silicone) was hydrolyzed with 41 parts by mass of 1N formic acid while stirring at 30°C±10°C. After 460 minutes, 521 parts by mass of isopropyl alcohol was mixed to prepare a treatment liquid (X2).

Next, a silica slurry consisting of 144 parts by mass of hollow silica particles having an outer shell with an average primary particle diameter of 50 nm and a porosity of 30% ("Surulia" (registered trademark) manufactured by Catalysts & Chemicals Co., Ltd.) and 560 parts by mass of isopropyl alcohol was prepared. (X3) was prepared.

720 parts by mass of treatment liquid (X1), 720 parts by mass of treatment liquid (X2), 700 parts by mass of silica slurry (X3), 360 parts by mass of methanol, 4272 parts by mass of isopropyl alcohol, and 713 parts by mass of polypropylene glycol monoethyl ether were stirred and mixed. Thereafter, 15 parts by mass of acetoacetoxyaluminum was added as a curing catalyst and stirred and mixed again to prepare a paint having a refractive index of 1.45 after drying and curing.

The prepared paint was applied with a microgravure coater, dried at 80°C, and then heat-treated at 130°C to harden the coating layer to form an off-coat layer with a thickness of about 0.1 μm. The refractive index of this off-coat layer is 1.45.

(Example 1)
Polyester A as the resin component of layer A and polyester B as the resin component of layer B were respectively charged into two single-screw extruders and melted at 280°C. Next, after passing through five FSS type leaf disk filters, a three-layer confluence block with a rectangular laminated portion was measured using a gear pump so that the discharge ratio was polyester A/polyester B = 1/4. Then, a layer of polyester A was layered on the inner layer and a layer of polyester B was layered on the surface layer on both sides, and after extruding it into a sheet from a T-die, an electrostatic casting method was prepared and the layer was heated at 25°C. It was wound around a casting drum and cooled and solidified to produce an unstretched film.

This unstretched film was stretched 3.2 times in the longitudinal direction at 95°C, and then cooled once. Next, both sides of this uniaxially stretched film were subjected to corona discharge treatment, the wet tension of the film was set to 55 mN/m, the easy-adhesion coating layer listed in Table 1 was applied, and then the film was stretched in stages in the width direction to obtain a total After stretching 3.8 times, it was heat-set in three stages at a maximum temperature of 228°C, and then subjected to a 2% relaxation treatment in the longitudinal and width directions at 150°C to obtain a film with a thickness of 20 μm. The stacked thickness of the polyester B layer was 4.0 μm. The thickness of the coating layer after drying was 80 nm. The evaluation results are shown in Tables 2, 3 and 4, and were excellent in visibility, film formation stability, bending resistance, number of times reaching bending break, and heat resistance, but the thin film thickness The support was slightly inferior.

(Example 2)
An unstretched film was produced in the same manner as in Example 1. This unstretched film was stretched 3.2 times in the longitudinal direction at 95° C., and then cooled once. Next, both sides of this uniaxially stretched film were subjected to corona discharge treatment, the wet tension of the film was set to 55 mN/m, the easy-adhesion coating layer listed in Table 1 was applied, and then the film was stretched in stages in the width direction to obtain a total After stretching 3.8 times, it was heat-set in three stages at a maximum temperature of 228°C, and then subjected to a 2% relaxation treatment at 150°C in the longitudinal and width directions to obtain a film with a thickness of 40 μm. The laminated thickness of the polyester B layer was 8.0 μm. The thickness of the coating layer after drying was 80 nm. The evaluation results are shown in Tables 2, 3 and 4, and were excellent in visibility, film formation stability, bending resistance, number of times reaching bending breakage, heat resistance, and supportability.

(Example 3)
An off-coat layer was applied to a film produced in the same manner as in Example 2.
The evaluation results are shown in Tables 2, 3 and 4, and in addition to being excellent in visibility, film formation stability, bending resistance, number of times reaching bending break, heat resistance, and supportability, The value was also lower than that of Example 2.

(Examples 4 to 9)
A film having the thickness shown in Table 1 was obtained in the same manner as in Example 1 by setting the composition and film forming conditions as shown in Table 1, and adjusting the coating thickness as appropriate. The thickness of the coating layer after drying was 80 nm in Examples 4, 5, 6, 8, and 9, and 92 nm in Example 7. As the film thickness increased, R40 and bending rigidity increased, and camera visibility and bending resistance tended to deteriorate. When it was eliminated by a multi-stage stretching process or when the relaxation rate in the longitudinal direction was lowered, the amorphous density A decreased, resulting in poor flexibility. When the refractive index of the easy-adhesive formulation of the surface layer increased, ΔT40 increased, resulting in poor camera visibility. Furthermore, when the total thickness of layer B was less than 4.0 μm, the stability of film formation was somewhat poor, and film tearing may occur during film formation.

(Examples 10 to 17)
A film having the thickness shown in Table 1 was obtained in the same manner as in Example 1, with the composition and film forming conditions as shown in Table 1. Samples in which the heat treatment temperature was lower than the melting point of the inner layer resin and had a relatively low nA-nB of 0.02 had relatively poor camera visibility. On the other hand, when the heat treatment temperature was higher than the melting point of the inner layer resin, the Δ haze value was slightly inferior. Samples with poor elongation at break had slightly inferior results in the number of times they reached bending break. When the surface layer and inner layer of the film did not contain isosorbate and cyclohexanedicarboxylic acid, the amorphousness decreased and the camera visibility and bending resistance were slightly inferior. When it was contained only in the surface layer of the film, the bending resistance was slightly inferior, and when it was contained only in the inner layer of the film, camera visibility was slightly inferior.

(Examples 18-21)
A film having the thickness shown in Table 1 was obtained in the same manner as in Example 1, with the composition and film forming conditions as shown in Table 1. In Example 17, in which the copolymerization amount of the isophthalic acid component of the dicarboxylic acid component in the inner layer was 5 mol %, the melting point of the resin in the inner layer was high and the difference in melting point between the resin in the inner layer and the resin in the surface layer was small, so the orientation was relaxed by heat treatment. It was difficult to progress, resulting in slightly inferior visibility. In Examples 18, 19, and 20, the resin IV was low and the film IV was low, resulting in slightly inferior film forming stability and Δ haze values.

(Example 22)
A film having the thickness shown in Table 1 was obtained in the same manner as in Example 1, with the composition and film forming conditions as shown in Table 1. The samples obtained by thinning the film had low bending rigidity and slightly poor supportability.

(Example 23)
An unstretched film was produced in the same manner as in Example 1. This unstretched film was stretched in the longitudinal direction under the conditions listed in Table 1-2, and then cooled once. Next, both sides of this uniaxially stretched film were subjected to corona discharge treatment, the wet tension of the film was set to 55 mN/m, the easy-adhesion coating layer listed in Table 1 was applied, and then the film was stretched in stages in the width direction to obtain a total After stretching 3.8 times, it was heat-set in three stages at a maximum temperature of 228°C, and then subjected to a 2% relaxation treatment at 150°C in the longitudinal and width directions to obtain a film with a thickness of 40 μm. The total stacked thickness of the layers made of polyester B was 8.0 μm. The thickness of the coating layer after drying was 80 nm. The evaluation results are shown in Tables 2 to 4, and the film was excellent in film formation stability, number of times reaching flexural breakage, heat resistance, and supportability, but the result was that it did not contain the 1,4-cyclohexanedimethanol component. , visibility and bending resistance were slightly inferior.

(Example 24, 25)
An unstretched film was produced in the same manner as in Example 1. This unstretched film was stretched in the longitudinal direction under the conditions listed in Table 1-2, and then cooled once. Next, corona discharge treatment was applied to both sides of this uniaxially stretched film, the wet tension of the film was set to 55 mN/m, and the easy-adhesion coating layer listed in Table 1-2 was applied, and then the film was stretched stepwise in the width direction. After stretching to a total of 3.8 times, it was heat-set in three stages at a maximum temperature of 228°C, and then relaxed by 2% in the longitudinal and width directions at 150°C to obtain a film with a thickness of 40 μm. Ta. The total stacked thickness of the layers made of polyester B was 8.0 μm. The thickness of the coating layer after drying was 80 nm. The evaluation results are shown in Tables 2 to 4, and in Example 24 containing naphthalene dicarboxylic acid as a copolymerization component and Example 25 containing PEI, visibility, film forming stability, and bending resistance were It was excellent in the number of times it reached flexural breakage, heat resistance, and supportability.

(Example 26)
A film having the thickness shown in Table 1 was obtained in the same manner as in Example 1, with the composition and film forming conditions as shown in Table 1. In addition, since the stretching process was not performed, the coating thickness was made thinner than that in Example 1 when applying the easy-adhesive layer, and the thickness of the coated layer after drying was adjusted to 80 nm. Although the film having the olefin resin component had a low R40, the elongation at break was low and the number of times the film reached flexural breakage was slightly inferior.

(Comparative Examples 1 and 2)
A film having the thickness shown in Table 1 was obtained in the same manner as in Example 1, with the composition and film forming conditions as shown in Table 1. As a result of R40 exceeding 1000 nm, visibility was poor in all cases.
(Comparative example 3)
An unstretched film was produced in the same manner as in Example 1. Both sides of this unstretched film were subjected to corona discharge treatment, the wet tension of the film was set to 55 mN/m, and the easy-adhesion coating layer listed in Table 1-2 was coated, and then 3. After biaxially stretching 4 times, it was heat-set at a maximum temperature of 230°C, and then subjected to a relaxation treatment of 2% in the longitudinal and width directions at 150°C to obtain a film with a thickness of 40 μm. The total stacked thickness of the layers made of polyester B was 8.0 μm. Compared to Example 1, since the total stretching ratio after application of the easy-adhesion layer was higher, the coating thickness at the time of application of the easy-adhesion layer was increased. The thickness of the coating layer after drying was 80 nm. As a result of R40 exceeding 1000 nm, visibility was poor in all cases.

When the back plate film of the present invention is used in an image display device, normal image information can be collected and recognized by the photoelectric conversion element installed below the image display surface, and the film has extremely high industrial applicability. expensive. In addition, the back plate film of the present invention can be used as a screen protection film used for general displays, a polarizer protection film used for liquid crystal displays and organic EL displays, a touch panel base film forming a transparent conductive layer, or a film used for projecting holograms. It can also be suitably used as a film.

101: Image display device 102: Screen area where the image sensor is placed on the back side of the image display surface 103: Screen area where the image sensor is not placed on the back side of the image display surface 104: Image sensor 105: Bezel portion 201: Film (back plate)
202: Fast axis 203: Vertical axis 204: Incident angle 205: Light beam 301: Chuck of loop stiffness tester 302: Displacement amount 401: Polarizing plate 402: Built-in camera device 403: Ultra wide-angle lens 404: Shooting angle of view 405: Surface light source 501: Bending direction 502: Center part of the film 503: Angle formed by the film sample

Claims

A film for a back plate used in an image display device, in which a polarizing plate, a light emitting element array for image display, a back plate, and a photoelectric conversion element are mounted in this order when viewed from the viewer's side. The back surface has a retardation value of 0 nm or more and 1000 nm or less, which is determined by making a light beam with a wavelength of 590 nm incident at an incident angle of 40 degrees with the vertical axis as a reference of 0 degrees along a plane including a fast axis and an axis perpendicular to the film surface. Film for plates.
The film for a back plate according to claim 1, comprising a biaxially oriented polyester film as one of its constituent elements.
The biaxially oriented polyester film is a single layer film or a laminated film of multiple layers, and is a polyester resin in which an isophthalic acid component is used in an amount of 6 mol % or more of the total dicarboxylic acid components of the polyester resin constituting the layer. The film for a back plate according to claim 2, comprising at least one layer of.
The polyester constituting the biaxially oriented polyester film is selected from the group consisting of naphthalene dicarboxylic acid, cyclohexane dicarboxylic acid, fluorene derivatives, paraxylene glycol, bisphenol A, isosorbate, cyclohexanedimethanol, and ester-forming derivatives thereof. The film for a back plate according to claim 2, wherein at least one is a polymerized or copolymerized polyester or a polyester containing polyetherimide.
The film for a back plate according to claim 2, which has a crystallinity of 30% or less and a rigid amorphous content of 20% or more and 50% or less.
A film in which two or more layers are laminated, where the refractive index in the thickness direction of the layer with the largest refractive index in the thickness direction is nA, and the refractive index in the thickness direction of the layer with the smallest refractive index in the thickness direction is nB. The film for a back plate according to claim 1, which satisfies formula (1).
nA-nB ≧ 0.02 (1)
The film for a back plate according to claim 1, wherein the increase in haze (Δhaze) when heat-treated at 120° C. for 5 hours is 0% or more and 2.0% or less.
3. The film for a back plate according to claim 2, wherein the biaxially oriented polyester film has a relaxation time of a mobile amorphous phase of 0.200 ms or less as measured by TD-NMR after heat treatment at 120° C. for 3 minutes.
The biaxially oriented polyester film consists of a plurality of layers, and among the layers constituting the film, the thickest layer has a Fourier transform infrared spectroscopy (FT-IR) spectrum observed at 1388 cm -1 . The film for a back plate according to claim 2, wherein the ratio R (r2/r1) of the spectral intensity r1 and the spectral intensity r2 observed at 1372 cm -1 satisfies the following formula (2).
1.0 < R < 1.8 (2)
The film for a back plate according to claim 1, having bending rigidity of 0.2 mN or more and 20.0 mN or less in both the longitudinal direction and the width direction.
The biaxially oriented polyester film has a ratio of ΔHg/ΔCp obtained by dividing the endotherm ΔHg observed in the irreversible curve of temperature modulated DSC (30 to 150°C) by the specific heat capacity difference ΔCp before and after the glass transition point observed in the reversible curve. The film for a back plate according to claim 2, wherein the amorphous density A calculated by the following is 1.0 or more.
The film for a back plate according to claim 1, wherein the recovery angle is 140° or more in both the longitudinal direction and the width direction.
The film for a back plate according to claim 1, having a breaking elongation of 50% or more in both the longitudinal direction and the width direction.
The film for a back plate according to claim 1, wherein the number of times the film reaches bending breakage when evaluated using an MIT bending tester is 1000 times or more in both the longitudinal direction and the width direction.
A claim that the difference between the average transmittance of P-waves when p-polarized light is incident at an incident angle of 40° and the average transmittance of S-waves when s-polarized light is incident at an incident angle of 40° is 10% or less. The film for a back plate according to item 1.
A polyester film consisting of a dicarboxylic acid component and a diol component, wherein the terephthalic acid component of the dicarboxylic acid component is 50 mol% or more and 95 mol% or less, and the ethylene glycol component of the diol component is 50 mol% or more and 95 mol%. below, the melting point observed at the lowest temperature is 240°C or less, the density is 1.360g/cc or less, and the vertical axis is 0 along the plane including the fast axis of the film and the axis perpendicular to the film surface. A polyester film having a retardation value of 0 nm or more and 1000 nm or less, which is determined by incident light with a wavelength of 590 nm at an incident angle of 40 degrees.
The polyester film is a single layer film or a film in which multiple layers are laminated, and the polyester resin layer contains an isophthalic acid component of 6 mol% or more of the total dicarboxylic acid components of the polyester resin constituting the layer. 17. The polyester film according to claim 16, having at least one layer.
The polyester constituting the polyester film is at least one selected from the group consisting of naphthalene dicarboxylic acid, cyclohexane dicarboxylic acid, fluorene derivatives, paraxylene glycol, bisphenol A, isosorbate, cyclohexanedimethanol, and ester-forming derivatives thereof. The polyester film according to claim 16, which is a polymerized or copolymerized polyester or a polyester containing polyetherimide.
The polyester film according to claim 16, which has a crystallinity of 30% or less and a rigid amorphous content of 20% or more and 50% or less.
The polyester film according to claim 16, wherein the relaxation time of the mobile amorphous phase in TD-NMR measurement after heat treatment at 120° C. for 3 minutes is 0.200 ms or less.
In the Fourier transform infrared spectroscopy (FT-IR) spectrum of the thickest layer among the layers constituting the film, the spectral intensity r1 observed at 1388 cm -1 and the spectral intensity r2 observed at 1372 cm -1 The polyester film according to claim 16, wherein the ratio R (r2/r1) satisfies the following formula (2).
1.0 < R < 1.8 (2)
The polyester film according to claim 16, having bending rigidity of 0.2 mN or more and 20.0 mN or less in both the longitudinal direction and the width direction.
Amorphous density calculated as ΔHg/ΔCp, which is the endotherm ΔHg observed on the irreversible curve of temperature modulated DSC (30 to 150°C) divided by the specific heat capacity difference ΔCp before and after the glass transition point observed on the reversible curve. The polyester film according to claim 16, wherein A is 1.0 or more.
The polyester film according to claim 16, wherein the elongation at break is 50% or more in both the longitudinal direction and the width direction.
An image display device equipped with the back plate film according to claim 1 or the polyester film according to claim 16.
26. The image display device according to claim 25, wherein a light emitting device array for an image display device used in the image display device is an organic electroluminescent device array.
The image display device according to claim 25, wherein a light-receiving angular range of a photoelectric conversion element mounted on the image display device is 60° or more and 140° or less.
A foldable display equipped with the back plate film according to claim 1 or the polyester film according to claim 16.
A device comprising, as one of its components, an image display device on which the back plate film according to claim 1 or the polyester film according to claim 16 is mounted.