Polyester film and its applications

Abstract

To provide a polyester film for a touch panel module substrate of a foldable display which is excellent in mass productivity and causes no cracking at a folded part in order to provide a foldable display with no risk of distortion of an image displayed at the folded part after repeated folding.SOLUTION: There is provided a polyester film for a touch panel module substrate of a foldable display which satisfies the following conditions. (1) The refractive index in the bending direction is 1.590 to 1.620. (2) The refractive index in the direction of a folded part is 1.670 to 1.700. (3) The refractive index in the direction of thickness is 1.520 or less. (4) The density is 1.380 g / cm3 or more (here, the bending direction refers to the direction perpendicular to the folded part when the polyester film is folded.)SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polyester film for a touch panel module substrate of a foldable display, a hard coat film for a touch panel module substrate of a foldable display, a foldable display, and a mobile terminal device, and more particularly to a foldable display and a mobile terminal device that are less prone to image distortion due to film deformation even after repeated folding, and to the polyester film and hard coat film for the touch panel module substrate of the foldable display. [Background technology]

[0002] With the advancement of thin-film and lightweight mobile devices, mobile devices such as smartphones have become widely popular. While mobile devices are required to have various functions, convenience is also a key requirement. Therefore, popular mobile devices need to be small, around 6 inches in size, as they are designed to be easily operated with one hand and to be stored in clothing pockets.

[0003] On the other hand, tablet devices with screen sizes ranging from 7 to 10 inches offer high functionality, as they are intended for use not only with video content and music, but also for business, drawing, and reading. However, they cannot be operated with one hand, and their portability is poor, posing challenges to convenience.

[0004] To achieve these goals, a method has been proposed to make the display more compact by connecting multiple displays together (see Patent Document 1). However, because the bezel remains, the image is cut off, resulting in reduced visibility, which has prevented its widespread adoption.

[0005] Therefore, in recent years, mobile devices incorporating flexible displays and foldable displays have been proposed. With this method, images are not interrupted, and the device can be conveniently carried as a mobile terminal equipped with a large screen display.

[0006] In conventional displays and mobile devices without a folding structure, the surface of the display could be protected with a non-flexible material such as glass. However, in folding displays, when the folding part forms a single display surface, it is necessary to use a flexible hard coat film or similar material that can protect the surface. However, in folding displays, certain areas that are folded are repeatedly bent, causing the film in those areas to deform over time, leading to problems such as distortion of the image displayed on the display. In addition to the surface protection film, folding displays use films in various parts, such as polarizing plates, phase difference plates, touch panel substrates, substrates for display cells such as organic EL, and protective materials on the back, and these films also need to be durable against repeated folding.

[0007] Therefore, methods for partially changing the film thickness have been proposed (see Patent Document 2), but these have the problem of being unsuitable for mass production.

[0008] Furthermore, methods for adjusting the refractive index of polyester film in the bending direction have also been proposed (see Patent Document 3), but there was a problem that as the refractive index in the bending direction was lowered, the pencil hardness when hard coat was applied decreased, resulting in a decrease in the surface protection function of the display. Also, while lowering the refractive index in one direction improved deformation when folded, it increased the uniaxial orientation in the folding direction, leading to problems such as cracks occurring or breakage at the folded part. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2010-228391 [Patent Document 2] Japanese Patent Publication No. 2016-155124 [Patent Document 3] International Publication No. 2018 / 150940 [Overview of the project] [Problems that the invention aims to solve]

[0010] The present invention aims to solve the problems of conventional display components as described above, and to provide a foldable display that is easy to mass-produce and does not cause distortion of the image displayed at the folded part after repeated folding, and a mobile terminal device equipped with such a foldable display, by providing a polyester film for the touch panel module substrate of a foldable display that does not cause creases or cracks at the folded part. [Means for solving the problem]

[0011] In other words, the present invention consists of the following configuration. 1. A polyester film for a touch panel module substrate of a foldable display that satisfies the following conditions. (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less. (4) Density is 1.380 g / cm³ 3 That's all. (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.) 2. A polyester film for a touch panel module substrate of a foldable display as described in item 1 above, wherein the modulus of elasticity in the bending direction is 2.7 GPa or less, and the modulus of elasticity in the folding direction is 4.5 GPa or more. 3. A polyester film for a touch panel module substrate of a foldable display according to the first or second above, wherein the total light transmittance is 85% or more, the haze is 3% or less, and the maximum thermal shrinkage rate is 2% or less. 4. A polyester film for a touch panel module substrate of a foldable display described in any of the above 1 to 3, which shows no cracks or deformation in a bending resistance evaluation (bending radius 1.5 mm) after heat treatment at 150°C for 30 minutes. 5. A polyester film for a touch panel module substrate of a foldable display according to any one of the first to fourth claims, having an easy-adhesion layer on at least one side of the polyester film for the touch panel module substrate of the foldable display. 6. A polyester film for a touch panel module substrate of a foldable display according to any one of the first to fifth claims, having a hard coat layer with a thickness of 1 to 50 μm on at least one side of the polyester film for the touch panel module substrate of the foldable display. 7. A foldable display in which a polyester film for a touch panel module substrate of a foldable display described in any of the above 1 to 6 is included as a substrate film for a touch panel module, and the foldable display includes a single continuous substrate film for a touch panel module via the foldable portion of the foldable display. 8. A portable terminal device having a foldable display as described in item 7 above. [Effects of the Invention]

[0012] The foldable display using the polyester film for the touch panel module substrate of the present invention maintains mass producibility while preventing cracks from occurring in the folded portion of the polyester film, preventing deformation after repeated folding, and preventing image distortion at the folded portion of the display. A portable terminal device equipped with a foldable display using such a polyester film as a touch panel module substrate provides beautiful images, is highly functional, and offers excellent portability and other conveniences. [Brief explanation of the drawing]

[0013] [Figure 1] It is a schematic diagram for illustrating the bending radius when the foldable display according to the present invention is folded. [Figure 2] It is a schematic diagram for illustrating the bending direction of the polyester film for touch panel module substrates of the foldable display according to the present invention. MODE FOR CARRYING OUT THE INVENTION

[0014] (Display) The display referred to in the present invention generally refers to all display devices. Examples of types of displays include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs, among which bendable LCDs, organic ELs, and inorganic ELs are preferable. In particular, organic EL and inorganic EL, which can reduce the number of layer structures, are particularly preferable, and organic EL with a wide color gamut is further preferable.

[0015] (Foldable Display) A foldable display is one in which a single continuous display can be folded, such as folded in half, for carrying. By folding the display, the size can be halved and portability can be improved. The bending radius of the foldable display is preferably 5 mm or less, more preferably 3 mm or less. If the bending radius is 5 mm or less, thickness reduction in the folded state can be achieved. It can be said that the smaller the bending radius, the better, but the smaller the bending radius, the more likely folding marks are to form. The bending radius is preferably 0.1 mm or more, but may be 0.5 mm or more, or 1 mm or more. Even if the bending radius is 1 mm, practically sufficient thickness reduction for carrying can be achieved. The bending radius when folded refers to the measurement at the position indicated by reference numeral 11 in the schematic diagram of FIG. 1, and means the inner radius of the folded portion when folded. The surface protection film described later may be positioned on the outer folded side or the inner folded side of the foldable display. Furthermore, the foldable display may be tri-fold, quad-fold, or even rollable, and all of these fall within the scope of the foldable display as defined in this invention.

[0016] The polyester film for foldable displays of the present invention may be used in any part of a foldable display. Below, using an organic EL display as an example, a typical configuration of a foldable display and the parts in which the polyester film of the present invention may be used will be described. Hereinafter, the polyester film for foldable displays of the present invention may simply be referred to as the polyester film of the present invention.

[0017] (Foldable OLED display) The essential component of a foldable organic EL display is the organic EL module, but additional components such as a circular polarizer, touch panel module, surface protective film, and back protective film may be provided as needed.

[0018] (Touch panel module) It is preferable that the mobile terminal device has a touch panel. When an organic EL display is used, it is preferable that the touch panel module is positioned on the viewing side of the organic EL module, and further, it is preferable that the touch panel module is positioned between the organic EL module and the circular polarizing plate. It is preferable that the touch panel module has a transparent conductive film having a transparent substrate such as a film and a transparent conductive layer disposed thereon. The polyester film in this invention can be used as the transparent substrate of this transparent conductive film. When used as the transparent substrate of the transparent conductive film, it is preferable that the polyester film be provided with a refractive index adjustment layer and a hard coat layer.

[0019] (Transparent conductive layer) In the present invention, the conductive layer preferably included in the touch panel module may be any transparent and conductive layer, and is not particularly limited, but examples include a conductive filler-containing layer, a metal layer, a metal oxide layer, and a conductive polymer-containing layer.

[0020] It should be noted that "transparent" in this context means that the material must be transparent to the naked eye in a state where it has been processed to function as a touch panel; the conductive parts themselves do not necessarily need to be transparent. For example, even if the conductive layer has an electrode pattern to function as a touch panel, and the wiring itself is opaque made of a metal such as gold, the conductive layer can be considered transparent if the electrode pattern is not visible to the naked eye and an image can be observed.

[0021] Preferred conductive fillers for the conductive filler-containing layer include metals such as gold, silver, copper, aluminum, nickel, titanium, iron, zinc, and tin, as well as alloys thereof, metal oxide fillers, metal-coated synthetic fibers, and conductive carbon fibers such as carbon nanotubes. Various shapes of metals, alloys, and metal oxides can be used as fillers, including spherical particles, flattened particles, flake-shaped particles, needle-shaped particles, and fibrous particles. Among these, flake-shaped particles, needle-shaped particles, and fibrous fillers (fibers of metals and their alloys, metal oxide fibers, metal-coated synthetic fibers, and conductive carbon fibers) are preferred in terms of flexibility, and fibrous fillers are even more preferred.

[0022] It is preferable that a binder resin is used in the conductive filler-containing layer. Examples of binder resins include polyester resin, polyurethane resin, polyamide resin, and acrylic resin. Furthermore, it is preferable that these resins are crosslinked. The crosslinking agent can be used in accordance with each resin, and examples include isocyanate compounds, epoxy resins, melamine compounds, oxazolines, carbodiimides, and compounds having two or more double bonds. The conductive filler content is preferably 10 to 400 parts by mass per 100 parts by mass of the resin components constituting the conductive layer.

[0023] The conductive filler-containing layer can be formed by a coating method. Electrode patterns can be created after coating by chemical etching or laser etching, or by printing. Examples of such methods include gravure printing, letterpress printing, offset printing, screen printing, and inkjet printing, which can be selected according to the characteristics of the coating and the fineness of the pattern.

[0024] Examples of metals used in the metal layer include gold, silver, copper, aluminum, nickel, titanium, iron, zinc, and tin. The metal layer can be formed by vapor deposition, sputtering, etc., and the electrode pattern is preferably processed by chemical etching or laser etching after the metal layer has been formed.

[0025] Examples of metal oxide layers include ZnO, CeO2, Sb2O3, SnO2, indium tin oxide (ITO), In2O3, antimond-doped tin oxide (ATO), and aluminum-doped zinc oxide (AZO). The metal oxide layer can be formed by sputtering or other methods, and the electrode pattern is preferably processed by chemical etching or laser etching after the metal oxide layer has been formed.

[0026] High molecular weight conductive agents such as conductive polymers can be used as conductive polymers in the conductive polymer-containing layer, for example, aromatic conjugated poly(paraphenylene), heterocyclic conjugated polypyrrole and polythiophene, aliphatic conjugated polyacetylene, heteroatom-containing conjugated polyaniline, mixed conjugated poly(phenylenevinylene), double-chain conjugated systems which have multiple conjugated chains in the molecule, and conductive composites which are polymers in which the aforementioned conjugated polymer chains are grafted or block-coupled onto a saturated polymer.

[0027] The conductive layer containing the conductive polymer may also contain the resin components listed for the conductive filler-containing layer. The amount of conductive polymer in the conductive layer containing the conductive polymer can be the same as that described for the conductive filler-containing layer. The conductive polymer-containing layer can be provided by a coating method, and the electrode pattern can be provided in the same manner as described for the conductive filler-containing layer.

[0028] Furthermore, when producing a conductive layer, the conductive film may be heat-treated to stabilize it (by adding various additives and strengthening the layer). In this case, the heat treatment temperature is preferably 30°C or higher, more preferably 50°C or higher, even more preferably 70°C or higher, particularly preferably 100°C or higher, preferably 200°C or lower, more preferably 150°C or lower, and even more preferably 120°C or lower. A heating temperature above the lower limit is preferable because it promotes the stabilization of the conductive film. A heating temperature below the upper limit is preferable because it does not limit the usable substrate to one with high heat resistance. In this case, the substrate requires high flexibility, meaning that it does not exhibit surface cracks, deformation, or breakage when bent, even after heat treatment. The polyester film for touch panel module substrates of the present invention can maintain high flexibility without losing or worsening its flexibility after heat treatment.

[0029] The polyester film of the present invention is preferably used as a base film for touch panel modules.

[0030] A polyester film with specific properties is used as the base film for the touch panel module of the present invention. The base film for the touch panel module of the present invention may be simply referred to as a base film or polyester film.

[0031] The polyester film of the present invention may be a single-layer film made of one or more types of polyester resins, or, when two or more types of polyester are used, it may be a multilayer film or a super-multilayer laminated film with a repeating structure.

[0032] Examples of polyester resins used in polyester films include polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or polyester films made from copolymers mainly composed of these resin components. Among these, stretched polyethylene terephthalate film is particularly preferred in terms of mechanical properties, heat resistance, transparency, and cost.

[0033] When polyester copolymers are used in polyester films, examples of dicarboxylic acid components of polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; and polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of glycol components include fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; aromatic glycols such as p-xylene glycol; and 1,4-cyclohexanediol. Examples include alicyclic glycols such as tanol; and polyethylene glycols with an average molecular weight of 150 to 20,000. The preferred copolymer has a copolymerization ratio of less than 20% by mass. When it is less than 20% by mass, film strength, transparency, and heat resistance are maintained, which is preferable.

[0034] Furthermore, in the production of polyester film, the intrinsic viscosity of at least one type of resin pellet is preferably in the range of 0.50 to 1.0 dl / g. An intrinsic viscosity of 0.50 dl / g or higher is preferable because it improves the impact resistance of the resulting film, making it less likely for the internal circuitry of the display to break due to external impact. On the other hand, an intrinsic viscosity of 1.00 dl / g or lower is preferable because it prevents the filtration pressure of the molten fluid from rising too much, making it easier to operate the film production stably.

[0035] The thickness of the polyester film is preferably 10 to 300 μm, more preferably 10 to 80 μm, and even more preferably 25 to 75 μm. A thickness of 10 μm or more improves bending resistance, pencil hardness, and impact resistance, while a thickness of 300 μm or less is advantageous for weight reduction and offers superior flexibility, processability, and handling.

[0036] The surface of the polyester film of the present invention may be smooth or uneven. However, since it is used for touch panel applications in displays, a decrease in optical properties due to unevenness is undesirable. The haze is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. The lower limit of the haze is better, but from the standpoint of stable production, it is preferably 0.1% or more, and may also be 0.3% or more.

[0037] As mentioned above, for the purpose of reducing haze, it is preferable to have less surface irregularity on the film. However, in order to provide a certain degree of slipperiness from the standpoint of handling, irregularities can be formed by compounding particles into the surface polyester resin layer or by coating with a particle-containing coating layer during the film-forming process.

[0038] Known methods can be used to incorporate particles into the polyester resin layer. For example, they can be added at any stage of polyester production, but preferably as a slurry dispersed in ethylene glycol or the like, at the esterification stage, or after the transesterification reaction is complete but before the polycondensation reaction begins, to allow the polycondensation reaction to proceed. Alternatively, this can be done by using a vented kneading extruder to blend a slurry of particles dispersed in ethylene glycol or water with the polyester raw material, or by using a kneading extruder to blend dried particles with the polyester raw material.

[0039] In particular, a method is preferred in which aggregated inorganic particles are homogeneously dispersed in a monomer liquid that will become part of the polyester raw material, and then the filtered solution is added to the remaining polyester raw material before, during, or after the esterification reaction. With this method, since the monomer liquid has low viscosity, homogeneous dispersion of particles and high-precision filtration of the slurry can be easily performed, and when added to the remaining raw material, the particles dispersibility is good and new aggregates are less likely to be formed. From this viewpoint, it is especially preferable to add it to the remaining raw material in a low-temperature state before the esterification reaction.

[0040] Furthermore, by obtaining a polyester containing particles beforehand and then kneading and extruding the resulting pellets with pellets that do not contain particles (masterbatch method), the number of protrusions on the film surface can be further reduced.

[0041] Furthermore, the polyester film may contain various additives, within a range that maintains a desirable range of total light transmittance. Examples of additives include antistatic agents, UV absorbers, and stabilizers.

[0042] The total light transmittance of the polyester film is preferably 85% or higher, and more preferably 87% or higher. A transmittance of 85% or higher is sufficient to ensure adequate visibility. While a higher total light transmittance of the polyester film is desirable, from the standpoint of stable production, it is preferable to have a transmittance of 99% or lower, and may also be 97% or lower.

[0043] The maximum thermal shrinkage rate of the polyester film after heat treatment at 150°C for 30 minutes is preferably 0-2%, more preferably 0-1.5%, and even more preferably 0-1.0%. A maximum thermal shrinkage rate of 2% or less can suppress flatness defects such as curling and waviness after attachment to a touch panel module or after hard coat application. When the maximum shrinkage rate is 0% or more, curling due to thermal dimensional changes between the polyester film layer and the functional layer is less likely to occur after various functional layers such as hard coats are applied to the polyester film by post-processing, resulting in a good yield when set on a touch panel, which is preferable.

[0044] The polyester film for foldable displays of the present invention can provide sufficient pencil hardness to the hard coat film after lamination. In conventional polyester films, it is believed that the pencil hardness of the hard coat film decreased after lamination due to deformation in the thickness direction. In the present invention, by setting the indentation depth after unloading the test force in the film thickness direction using the dynamic ultra-micro hardness tester described later to a specific range, high hardness can be achieved in the pencil hardness evaluation of the hard coat film. The indentation depth after removing the test force in the film thickness direction is preferably 1.5 μm or less, more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. When the indentation depth after removing the test force (the final amount of deformation under load) is 1.5 μm or less, the film is less likely to deform in the thickness direction, and the pencil hardness can be increased in the evaluation of the pencil hardness of the hard coat film after lamination of the hard coat layer. When the pencil hardness of the hard coat film can be increased, scratches and dents are less likely to occur on the display surface, and the visibility of the display is improved. It can be said that the lower the indentation depth after removing the test force, the better, but in terms of stable production and saturation of the effect, it is preferably 0.3 μm or more, and even more preferably 0.5 μm or more.

[0045] To reduce the indentation depth after the test load is removed, it is effective to adjust the refractive index in the thickness direction to 1.520 or less. As a means of reducing the refractive index to 1.520 or less, as will be described later, examples of conditions include adjusting the stretching ratio in the bending and folding directions to a high level, setting the stretching temperature in the bending and folding directions to a low level, and setting the heat-fixing temperature to a high level, while keeping other physical properties, such as the refractive index in the bending and folding directions, within a range where they can be controlled to a desirable range.

[0046] The polyester film for foldable displays of the present invention prevents creases, cracks, and breakages when folded, and allows for adjustment of the neutral surface of the display. The neutral surface is the surface where, when folded, compressive stress is applied to the inside and tensile stress to the outside, but no stress is applied in between. In foldable displays, the neutral surface is generally designed in the organic EL layer. The neutral surface can be adjusted by the elastic modulus and thickness of each layer. Therefore, the elastic modulus in the bending direction of the polyester film is preferably 2.7 GPa or less, more preferably 2.6 GPa or less, and even more preferably 2.5 GPa or less. It can be said that flexibility is improved by reducing the elastic modulus in the bending direction, but 1.8 GPa or more is preferable for adjustment of the neutral surface. The elastic modulus in the folding direction is preferably 4.5 GPa or more, more preferably 4.6 GPa or more, and even more preferably 4.7 GPa or more. By increasing the elastic modulus in the folding direction, the flatness of the display surface can be maintained during display manufacturing. In addition, the touch panel module can be protected from external impacts. A higher modulus of elasticity in the folding direction is preferable, but from the viewpoint of film formation, 8.0 GPa or less is preferable.

[0047] The surface of the polyester film of the present invention can be treated to improve adhesion with resins that form transparent conductive layers, adhesive layers, hard coat layers, etc.

[0048] Surface treatment methods include, for example, sandblasting, solvent treatment to create uneven surfaces, and oxidation treatments such as corona discharge, electron beam irradiation, plasma treatment, ozone / ultraviolet irradiation, flame treatment, chromic acid treatment, and hot air treatment, and can be used without any particular limitations.

[0049] Furthermore, adhesion can be improved by using an adhesion-enhancing layer, such as an easy-adhesion layer. The easy-adhesion layer can be made of any material, including acrylic resin, polyester resin, polyurethane resin, and polyether resin, and can be formed using a general coating method, preferably a so-called in-line coating formulation.

[0050] The polyester film described above can be manufactured, for example, through a polymerization step in which inorganic particles are homogeneously dispersed in a monomer liquid that will be part of the polyester raw material, filtered, and then added to the remainder of the polyester raw material to polymerize the polyester; and a film forming step in which the polyester is melt-extruded into a sheet through a filter, cooled, and then stretched to form a base film.

[0051] Next, we will explain in detail the manufacturing method of polyester film, using polyethylene terephthalate (PET) pellets as the raw material for the base film, but this is not the only method. Furthermore, the number of layers, such as single-layer or multi-layer construction, is not limited.

[0052] PET pellets are mixed in a predetermined ratio, dried, and then supplied to a known molten lamination extruder. The pellets are extruded through a slit-shaped die into a sheet, and then cooled and solidified on a casting roll to form an unstretched film. For single-layer films, one extruder is sufficient. However, when manufacturing multi-layer films, two or more extruders, two or more manifolds or confluence blocks (for example, confluence blocks with a rectangular confluence section) are used to laminate multiple film layers constituting each outermost layer. Two or more sheets are then extruded from a die and cooled on a casting roll to form an unstretched film.

[0053] In this case, during melt extrusion, it is preferable to perform high-precision filtration to remove foreign matter contained in the resin at any location where the molten resin is maintained at approximately 280°C. The filter material used for high-precision filtration of the molten resin is not particularly limited, but a stainless steel sintered body filter material is preferred because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, and Cu, as well as high-melting-point organic matter.

[0054] Furthermore, the filtration particle size of the filter media (initial filtration efficiency 95%) is preferably 20 μm or less, and particularly preferably 15 μm or less. If the filtration particle size of the filter media (initial filtration efficiency 95%) exceeds 20 μm, foreign matter larger than 20 μm cannot be sufficiently removed. Although high-precision filtration of molten resin using a filter media with a filtration particle size (initial filtration efficiency 95%) of 20 μm or less may reduce productivity, it is preferable for obtaining a film with fewer protrusions caused by coarse particles.

[0055] (Regarding the refractive index in the bending direction) In the present invention, the refractive index of the polyester film in at least one of the longitudinal direction (machine flow direction) and the width direction is preferably 1.590 to 1.620, and more preferably 1.591 to 1.600. Furthermore, the refractive index of the polyester film in the bending direction is preferably 1.590 to 1.620, and more preferably 1.591 to 1.600. Here, the bending direction refers to the direction perpendicular to the folding portion (reference numeral 21) assumed in the application of a foldable display, as shown by reference numeral 22 on the polyester film (reference numeral 2) in Figure 2. A refractive index of 1.590 to 1.620 in at least one of the longitudinal direction and the width direction is preferable because it reduces deformation when repeatedly folded and does not risk degrading the image quality of the foldable display. A refractive index of 1.591 to 1.600 is more preferable. Of course, that direction is preferably the bending direction mentioned above. A refractive index of 1.590 or higher prevents cracking in the folded direction after the bending test described later, and of course, prevents breakage, thus maintaining good visibility of the display. The refractive index of the polyester film can be effectively adjusted by adjusting the stretching ratio and stretching temperature. In addition, a relaxation process in the stretching direction and multi-stage stretching may be used to adjust the refractive index. When performing multi-stage stretching, it is preferable to make the stretching ratio of the second and subsequent stages higher than that of the first stage.

[0056] By controlling the refractive index of the polyester film in at least one of the longitudinal direction (machine flow direction) and the width direction within the above range, and more preferably by controlling the refractive index in the bending direction within the above range, fatigue due to compressive stress applied to the inside of the fold during folding can be reduced. Fatigue due to compressive stress is thought to occur mainly in the crystalline parts, and the fewer crystals there are in the bending direction, the less fatigued the film is. Therefore, it is thought that by lowering the refractive index, the amount of oriented crystals in the bending direction is reduced, thereby suppressing compressive fatigue.

[0057] Furthermore, creep caused by tensile stress on the outside of the fold during folding can be suppressed by reducing the refractive index. Fatigue due to tensile stress is thought to occur mainly in the amorphous region, where repeated stress causes alignment of molecular chains and deformation. It can be inferred that the fewer molecular chains aligned in the bending direction, the less deformation due to alignment. Also, since fatigue due to tensile stress can be suppressed by having fewer amorphous regions, a higher degree of crystallinity, i.e., a higher density, is preferable.

[0058] In the present invention, it is preferable to stretch the unstretched polyester sheet to a ratio of 1.2 to 2.0 times in at least one direction, either the longitudinal direction (machine flow direction) or the width direction, and more preferably 1.7 to 2.0 times. Furthermore, it is preferable that the stretching direction is the bending direction as described above. A stretching ratio of 1.2 times or more is preferable because there is no deformation during post-processing such as hard coat coating, and a stretching ratio of 2.0 times or less is preferable because there is no thickness unevenness in the film. The stretching temperature is preferably 75 to 120°C, and more preferably 75 to 105°C. The heating method during stretching is conventionally known, such as hot air heating, roll heating, and infrared heating. The following methods can be employed. By setting the stretching temperature to 75-120°C, large thickness variations caused by stretching at the above stretching ratio can be prevented. Furthermore, by stretching at the lowest possible temperature within the range that does not produce large thickness variations as described above, the refractive index in the thickness direction can be reduced.

[0059] (Regarding the refractive index in the direction of the folding part) The refractive index of the polyester film described above, in the direction perpendicular to the direction in which the refractive index is 1.590 to 1.620, is preferably 1.670 to 1.700. That is, the refractive index in the direction perpendicular to the bending direction (the direction of the folded part) is preferably 1.670 to 1.700. Setting it to 1.670 to 1.700 reduces deformation when folded in the bending direction. Setting it to 1.700 or less suppresses cracking or breakage in the direction of the folded part. Setting it to 1.670 or more improves flexibility in the bending direction and improves surface hardness. 1.680 to 1.695 is more preferable. Methods for adjusting the refractive index in the direction perpendicular to the bending direction include the stretching ratio, stretching preheating temperature, stretching temperature, multi-stage stretching, and film relaxation. The stretching ratio is preferably 4.0 to 6.0 times, and more preferably 4.4 to 6.0 times. In addition, the stretching preheating temperature in the direction perpendicular to the bending direction is preferably 70 to 110°C. When performing multi-stage stretching in a direction perpendicular to the bending direction, it is preferable to increase the stretching ratio of the second and subsequent stages compared to the first stage. Film relaxation may be performed by 1-10% in both the machine flow direction (longitudinal direction) and the perpendicular direction (width direction).

[0060] (Regarding the refractive index in the direction of thickness) The refractive index in the thickness direction is preferably 1.520 or less. By setting it to 1.520 or less, even if the refractive index in the bending direction is designed to be low, the decrease in hardness of the film surface can be suppressed, and both flexibility and surface hardness can be achieved. Setting it to 1.520 or less reduces the indentation depth after the test force is removed in the thickness direction, and the hardness of the film surface, especially the pencil hardness of the hard coat film after the hard coat layer is laminated, can be improved. More preferably it is 1.515 or less, even more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. A low refractive index in the thickness direction is preferable, but for stable production, 1.3 or more is preferable, and it may even be 1.4 or more. Particularly preferably it is 1.410 or more. The above range can be achieved by increasing the stretching ratio in both the bending direction and the folding direction, but in order to control the refractive index in the thickness direction while controlling the refractive index in the bending direction and width direction to a preferred range, it is preferable to set the conditions while checking the balance of the conditions of each process in the film manufacturing process.

[0061] Methods for controlling the refractive index in the thickness direction to the above range include setting the stretching preheating temperature, stretching temperature, and stretching ratio in the bending direction, the stretching preheating temperature and stretching temperature in the folding direction, multi-stage stretching, high-magnification stretching, or heat-fixing temperature. The stretching preheating temperature in the bending direction is preferably 70°C to 110°C. The stretching temperature in the bending direction is preferably 75°C to 120°C. The stretching ratio in the bending direction is preferably 1.2 to 2.0 times, and more preferably 1.7 to 2.0 times. By lowering the stretching temperature and stretching at a low stretching ratio, the refractive index in the thickness direction can be effectively reduced while maintaining the flexibility in the bending direction. The stretching preheating temperature in the folding direction is also preferably 75°C to 110°C. The stretching temperature is preferably 75°C to 120°C. The stretching ratio in the folding section is preferably 4.0 to 6.0 times, and more preferably 4.4 to 6.0 times. The refractive index in the thickness direction can be effectively reduced while maintaining or reducing the refractive index in the bending direction. As a method for high-magnification stretching, multi-stage stretching may be used. In this case, it is preferable to make the stretching magnification of the second stage higher than that of the first stage in order to effectively control the refractive index. Alternatively, a method of stretching again after the crystallization process may be used. Accelerated stretching, in which the stretching speed is increased from the beginning to the end of the stretching process, may also be used. The preferred heat-fixing temperature is 180-240°C. Heat-fixing promotes oriented crystallization in the stretching direction, which can lower the refractive index in the thickness direction. The reason why lowering the refractive index in the thickness direction improves the hardness of the film surface is not entirely clear, but it is thought that aromatic compounds such as benzene rings within the molecular chain are oriented in the planar direction, which suppresses deformation caused by stress in the thickness direction.

[0062] (Regarding the density of polyester film) The density of the polyester film is 1.380 g / cm³. 3 Preferably, it is 1.383 g / cm³. 3 It is more preferable that the above is true. 1.380 g / cm³ 3By doing so, flexibility can be improved, and the surface hardness of the film, particularly the pencil hardness of the hard coat film after the hard coat layer has been laminated, can be improved. A higher density is preferable, although this is somewhat affected by the presence or absence of particles in the film, but 1.40 g / cm³ is preferable. 3 The following is preferable: By setting the heat-fixing temperature during film formation to 180-240°C, crystallization can be promoted and the density can be effectively increased.

[0063] It is preferable that the bending direction of the polyester film corresponds to the longitudinal direction (machine flow direction). This makes it easier to lower the refractive index in the bending direction at the biaxial stretching stage and improve flexibility. In other words, it is preferable to stretch the unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, to obtain a polyester film. Furthermore, it is preferable to stretch it in the width direction at a stretching ratio of 4.0 to 6.0 times, more preferably 4.4 to 6.0 times.

[0064] Furthermore, in the present invention, the polyester film (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less. (4) Density is 1.380 g / cm³ 3 That's all. It is particularly preferable to simultaneously possess the four characteristics described above. However, even within the range of preferred manufacturing conditions described above, if the combination of conditions is not optimal within each preferred manufacturing condition range, such as a draw ratio of 1.4 times or less in the bending direction, a draw ratio of less than 4.4 times in the folding direction, and a heat-fixing temperature of 220°C or less, it may not be possible to obtain a product that satisfies all four characteristics simultaneously. In such cases, the four characteristics can be simultaneously satisfied by fine-tuning any of the conditions or a combination thereof, such as increasing the draw ratio in the bending direction to 1.7 times or more, increasing the draw ratio in the folding direction to 4.4 times or more, increasing the heat-fixing temperature to around 230°C, or lowering the draw temperature in the bending direction and / or the folding direction.

[0065] To adjust film-forming properties, film strength, thermal dimensional stability, and appearance defects, any film-forming method such as stretching, relaxation, heat fixing, or surface treatment may be used. However, in this invention, controlling the refractive index and density of the film within the above-mentioned preferred range is a particularly preferred embodiment. By controlling the refractive index and density within the preferred range, it is possible to provide a polyester film suitable for foldable displays that exhibits superior flexural resistance and surface hardness compared to conventional films, and in particular, high pencil hardness of the hard-coated film after lamination of the hard-coat layer.

[0066] Specifically, for example, after sufficiently vacuum-drying PET pellets, the pellets are fed into an extruder, melt-extruded into a sheet at approximately 280°C, cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched in the longitudinal direction 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, by rolls heated to 75 to 120°C, to obtain a uniaxially oriented PET film. Further, the end edges of the film are held by clips, guided into a hot air zone heated to 75 to 120°C, and after drying, stretched in the width direction 4.0 to 6.0 times, more preferably 4.4 to 6.0 times. Subsequently, the film can be guided to a heat treatment zone at 180 to 240°C and subjected to heat treatment for 1 to 60 seconds. During this heat treatment step, if necessary, a relaxation treatment of 0 to 10% may be performed in the width direction or the longitudinal direction.

[0067] The intrinsic viscosity of the polyester film is preferably in the range of 0.50 to 1.0 dl / g. When the intrinsic viscosity is 0.50 dl / g or more, the impact resistance is improved, and disconnection of internal circuits of a display due to external impact is less likely to occur, which is preferable. On the other hand, when the intrinsic viscosity is 1.00 dl / g or less, an excessive increase in filtration pressure of the molten fluid is avoided, and film production is stabilized, which is preferable.

[0068] (Easy-Adhesion Layer) In the present invention, in order to improve the adhesion between the polyester film and a transparent conductive layer or a hard coat layer, it is also preferable to laminate an easy-adhesion layer on the polyester film. The easy-adhesion layer can be obtained by applying a coating liquid for forming the easy-adhesion layer onto one or both sides of an unstretched or longitudinally uniaxially stretched film, followed by heat drying as necessary, and further stretching in at least one unstretched direction. Heat treatment can also be performed after biaxial stretching. The coating amount of the final easy-adhesion layer is 0.005 to 0.20 g / m 2 , and it is preferably controlled within this range. When the coating amount is 0.005 g / m 2 or more, sufficient adhesion can be obtained, which is preferable. On the other hand, when the coating amount is 0.20 g / m 2 or less, sufficient blocking resistance can be obtained, which is preferable.

[0069] The resin to be included in the coating solution used for laminating the easy-adhesion layer can be any resin without particular limitation, such as polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, or acrylic resins. Examples of crosslinking agents to be included in the coating solution for forming the easy-adhesion layer include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, and carbodiimide compounds. Two or more of each can also be used in mixture form. Due to the nature of inline coatings, these are preferably applied with a water-based coating solution, and the resins and crosslinking agents are preferably water-soluble or water-dispersible resins or compounds.

[0070] It is preferable to add particles to the easy-adhesion layer to provide slipperiness. The average particle size of the fine particles is preferably 2 μm or less. If the average particle size exceeds 2 μm, the particles tend to fall off the easy-adhesion layer. Examples of particles to be included in the easy-adhesion layer include inorganic particles such as titanium dioxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, as well as organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the easy-adhesion layer individually, or two or more may be added in combination.

[0071] Furthermore, known methods similar to those used for the coating layer can be used for applying the coating solution. Examples include the reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, and pipe doctor method, and these methods can be used individually or in combination.

[0072] (Hard coat layer) It is preferable that the polyester film has a hard coat layer on at least one surface for purposes such as adjusting the refractive index, improving surface hardness, improving flexibility, cracking and tearing resistance, and blocking the adverse effects of oligomers precipitated from the polyester on the transparent conductive layer. The hard coat layer is preferably positioned on top of the polyester film or on top of the easy-adhesion layer. The resin used to form the hard coat layer is not particularly limited and can be acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, epoxy, etc. Furthermore, two or more materials can be mixed and used, or particles such as inorganic fillers or organic fillers can be added.

[0073] (Film thickness of the hard coat layer) The thickness of the hard coat layer is preferably 1 to 50 μm. A thickness of 1 μm or more is preferable as it allows for sufficient curing and increases pencil hardness. Furthermore, by limiting the thickness to 50 μm or less, curling due to hard coat curing shrinkage can be suppressed, improving the handling properties of the film.

[0074] (Application method) The hard coat layer can be applied using various methods, including Meyer bar coating, gravure coating, die coating, and knife coating, without any particular limitations, and can be appropriately selected depending on viscosity and film thickness.

[0075] (Curing conditions) For curing the hard coat layer, methods such as curing with energy rays like ultraviolet light or electron beams, or curing with heat, can be used. However, curing methods using ultraviolet light or electron beams are preferred in order to reduce damage to the film.

[0076] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or higher, and more preferably 4H or higher. A pencil hardness of 3H or higher provides surface protection, preventing scratches and dents from easily occurring, and does not reduce visibility. Generally, a higher pencil hardness for the hard coat layer is preferable, but 9H or lower is acceptable, 8H or lower is acceptable, and even 6H or lower can be used without practical problems.

[0077] (Characteristics of the hard coat layer) The hard coat layer in this invention can be used to protect touch panel modules and displays by increasing the pencil hardness of the surface as described above, and a high transmittance is preferable. The transmittance of the hard coat film is preferably 87% or higher, and more preferably 88% or higher. Sufficient visibility can be obtained with a transmittance of 87% or higher. The total light transmittance of the hard coat film is generally preferable as it is higher, but from the standpoint of stable production, it is preferable to have 99% or less, and may also be 97% or less. In addition, the haze of the hard coat film is generally preferable to be low, and preferably 3% or less. The haze of the hard coat film is more preferable to have 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. The haze is generally preferable as it is lower, but from the standpoint of stable production, it is preferable to have 0.1% or more, and may also be 0.3% or more.

[0078] The hard coat layer may also have other functions added to it. For example, a hard coat layer with added functionality such as an anti-glare layer having a certain pencil hardness, an anti-glare anti-reflective layer, an anti-reflective layer, a low-reflection layer, and an antistatic layer is also preferably applied in the present invention.

[0079] In the polyester film, it is preferable to provide a refractive index adjustment layer between the polyester film and the transparent electrode layer, or between the hard coat layer and the transparent electrode layer, in order to make the electrode pattern of the transparent conductive layer less visible. In this case, the hard coat layer itself may also serve as the refractive index adjustment layer, or a separate refractive index adjustment layer may be laminated. Examples of refractive index adjustment layers include resin layers containing the above-mentioned refractive index adjustment particles, fluorine-containing resin layers, aromatic polyimide resins, epoxy resins, (meth)acrylic resins (acrylate, methacrylate compounds), polyester resins, and urethane resins, as well as resins with high refractive index containing aromatic rings, sulfur atoms, or bromine atoms, and their precursors. These can be provided by coating. In addition, inorganic layers such as ZnO, CeO2, Sb2O3, SnO2, indium tin oxide, In2O3, Al2O3, antimond-doped tin oxide, aluminum-doped zinc oxide, SiO2, and magnesium fluoride are also preferred as refractive index adjustment layers, and these can be provided by a wet film formation method.

[0080] When a transparent conductive film is made using the polyester film for touch panel modules according to the present invention, preferred laminated structures include, for example, polyester film / transparent conductive layer, polyester film / easy-adhesion layer / transparent conductive layer, polyester film / hard coat layer / transparent conductive layer, polyester film / easy-adhesion layer / hard coat layer / transparent conductive layer, polyester film / refractive index adjustment layer (one layer or multiple layers with different refractive indices) / transparent conductive layer, polyester film / easy-adhesion layer / refractive index adjustment layer (one layer or multiple layers with different refractive indices) / transparent conductive layer, polyester film / hard coat layer / refractive index adjustment layer (one layer or multiple layers with different refractive indices) / transparent conductive layer, and polyester film / easy-adhesion layer / hard coat layer / refractive index adjustment layer (one layer or multiple layers with different refractive indices) / transparent conductive layer. Combinations of these laminated structures may exist on one side of the polyester film or on both sides via the polyester film.

[0081] In the touch panel module of the foldable display of the present invention, the polyester film of the present invention is used as the substrate for the touch panel module, but it is not necessary to use it for all the films constituting the touch panel module. In the touch panel module of the foldable display, in addition to the polyester film of the present invention, polyimide film, polyamide film, polyamide-imide film, polyester film other than the polyester film of the present invention, polycarbonate film, acrylic film, triacetylcellulose film, cycloolefin polymer film, polyphenylene sulfide film, polymethylpentene film, etc., can be used as substrate films for the touch panel module as appropriate and suitable. [Examples]

[0082] Next, the present invention will be described using examples and comparative examples. First, the method for evaluating characteristic values ​​implemented in the present invention is shown below.

[0083] (1) Intrinsic viscosity After crushing and drying the film or polyester resin, it was dissolved in a mixed solvent of phenol / tetrachloroethane = 60 / 40 (mass ratio). After removing inorganic particles from this solution by centrifugation, the flow time of the solution at a concentration of 0.4 (g / dl) and the flow time of the solvent alone were measured using an Ubbelohde viscometer at 30°C. The intrinsic viscosity was calculated from the ratio of these times using Huggins' equation, assuming that Huggins' constant is 0.38.

[0084] (2) Flexural resistance of polyester film samples (flexural radius 1.5 mm) A polyester film sample measuring 20 mm in width and 110 mm in flow direction was prepared. Using a no-load U-shaped stretch tester (Yuasa System Equipment Co., Ltd., DLDMLH-FS), the bending radius was set to 1.5 mm, and the sample was bent 200,000 times at a speed of 1 time / second. At that time, the sample was fixed at a position 10 mm from both ends of the long side, and the bending area was 20 mm x 90 mm. Here, Figure 1 is a schematic diagram to show the bending radius when a foldable display is folded, and considering the case where the polyester film is placed on the inner surface of the folded state, the bending test was performed model by setting the location indicated by reference numeral 11 in Figure 1 to 1.5 mm. After the bending process was completed, the sample was placed on a flat surface with the inside of the bend facing downwards and observed visually. ○: No cracks or deformation were observed in the sample. ×: The sample has cracks or creases, and when placed horizontally, it lifts up to a maximum height. 5mm or larger.

[0085] (3) Flexural resistance of polyester film samples (flexural radius 0.5 mm) Using the same method as the bending test described above, the bending radius was set to 0.5 mm and the display was bent 200,000 times at a speed of 1 time / second. Here, Figure 1 is a schematic diagram to show the bending radius when a foldable display is folded, and considering the case where a polyester film is placed on the inner surface of the folded state, the bending test is performed model by setting the location indicated by reference numeral 11 in Figure 1 to 0.5 mm. The outer film surface of the bent part was observed with a digital microscope (HIROX RH8800) at 700x magnification to check for the presence or absence of wrinkles (cracks). Separate from the above-mentioned visual test of bending resistance with a bending radius of 1.5 mm, this test, with a reduced bending radius of 0.5 mm, is intended to evaluate the foldable display in a state closer to the actual usage condition, where the hard coat layer and other components are laminated or attached. This test is intended to detect minute defects that are difficult to detect by visual inspection, such as defects that make the display prone to breakage or cracking, separate from the visual observation with a bending radius of 1.5 mm. ○: No defects on the outer surface of the film when bent. ×: Wrinkles (cracks) can be seen on the film surface on the outside of the fracture or bend.

[0086] (4) Flexural resistance of heat-treated polyester film samples (flexural radius 1.5 mm) A sample film was cut to 210 mm x 300 mm in the transverse direction, and the distance A between marks was measured under a constant tension of 5 g. Next, the sample film was left unloaded in an oven at 150°C for 30 minutes, then removed from the oven and cooled to room temperature. After that, it was cut to a size of 20 mm in the width direction x 110 mm in the length direction to prepare a polyester film sample. Using a no-load U-shaped stretch tester (Yuasa System Equipment Co., Ltd., DLDMLH-FS), the bending radius was set to 1.5 mm, and it was bent 200,000 times at a speed of 1 time / second. At that time, the sample was fixed at a position 10 mm from both ends of the long side, and the bending area was 20 mm x 90 mm. Here, Figure 1 is a schematic diagram to show the bending radius when a foldable display is folded, and considering the case in which a polyester film is placed on the inner surface of the folded state, the bending test is performed model by setting the location indicated by reference numeral 11 in Figure 1 to 1.5 mm. After the bending process was completed, the sample was placed on a flat surface with the inside of the bend facing downwards and observed visually. ○: No cracks or deformation (strain) were observed in the sample. ×: The sample has cracks or creases, and when placed horizontally, it lifts up by more than 5mm.

[0087] (5) Refractive index In accordance with JIS K 7142:2014 "Method for measuring the refractive index of plastics (Method A)", the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction were determined using an Abbe refractometer (ATAGO Corporation, NAR-4T, measurement wavelength 589 nm).

[0088] (6) Pencil hardness The pencil hardness of the hard coat film was measured using a sample, in accordance with JIS K 5600-5-4:1999, with a load of 750g and a speed of 1.0mm / s. In this invention, a hardness of 3H or higher was considered acceptable.

[0089] (7) Total light transmittance, haze The measurement was performed using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0090] (8) Density The density was measured according to the method compliant with JIS K 7112:1999 (density gradient pipe method). (Unit: g / cm³) 3 ).

[0091] (9) Depth of indentation after unloading of test force The sample was cut into approximately 2 cm squares and fixed to an 18 × 18 mm microcover glass (manufactured by Matsunami Glass Co., Ltd.) with the opposite side of the measurement surface using adhesive (Cemedine® High Super 30). After fixing, it was left at room temperature for more than 12 hours, and then the indentation depth (μm) after unloading the test force was measured using a dynamic ultramicrohardness tester "DUH-211" (manufactured by Shimadzu Corporation) under the following conditions. <Measurement Conditions> Test mode: Load-unload test Indenter used: 115 degree ridge angle, triangular pyramid indenter Indenter modulus: 1.140 × 10⁶ N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50mN Load speed: 4.44mN / sec Load holding time: 2sec Unloading holding time: 0sec

[0092] (10) Maximum thermal contraction A sample film was cut to a width of 10 mm and a length of 250 mm. Marks were made at 200 mm intervals along the longer side, aligned with the desired measurement direction, and the interval A between the marks was measured under a constant tension of 5 g. Next, the sample film was left unloaded in an oven at 150°C for 30 minutes, then removed from the oven and cooled to room temperature. After that, the interval B between the marks was determined under a constant tension of 5 g, and the thermal shrinkage rate (%) was calculated using the following formula. Note that the thermal shrinkage rate was measured at three equal points along the width of the sample film, and the average of the three points was taken as the thermal shrinkage rate (%). Thermal shrinkage rate (%) = [(AB) × 100] / A The sample film was cut separately in both the bending and folding directions so that the warp and weft directions were different, and measurements were taken. The data in the direction with the larger measurement value was taken as the maximum thermal shrinkage rate (%).

[0093] (11) Elastic modulus (Young's modulus (unit: GPa)) The elastic modulus of polyester film in the bending and folding directions was measured at 23°C in accordance with JIS K7127.

[0094] (Preparation of polyethylene terephthalate pellets (a)) As the esterification reactor, a continuous esterification reactor consisting of a three-stage complete mixing tank equipped with a stirrer, a partial condenser, a raw material inlet, and a product outlet was used. TPA was supplied at 2 tons / hr, EG at 2 moles per mole of TPA, and antimony trioxide was added in an amount that resulted in 160 ppm of Sb atoms relative to the generated PET. This slurry was continuously supplied to the first esterification reactor of the esterification reactor and reacted at atmospheric pressure at 255°C with an average residence time of 4 hours. Next, the reaction product in the first esterification reactor was continuously removed from the system and supplied to the second esterification reactor. EG distilled off from the first esterification reactor was supplied to the second esterification reactor at 8% by mass relative to the generated polymer (generated PET). Furthermore, an EG solution containing magnesium acetate in an amount that resulted in 65 ppm of Mg atoms relative to the generated PET, and an EG solution containing TMPA in an amount that resulted in 20 ppm of P atoms relative to the generated PET were added, and the reaction was carried out at atmospheric pressure at 260°C with an average residence time of 1.5 hours. Next, the reaction product in the second esterification reaction vessel was continuously removed from the system and supplied to the third esterification reaction vessel. Furthermore, an EG solution containing TMPA in an amount such that the P atom content was 20 ppm relative to the generated PET was added, and the reaction was carried out at atmospheric pressure at 260°C with an average residence time of 0.5 hours. The esterification reaction product generated in the third esterification reaction vessel was continuously supplied to a three-stage continuous polycondensation reactor for polycondensation, and then filtered through a stainless steel sintered filter material (nominal filtration accuracy, 90% cut of 5 μm particles) to obtain polyethylene terephthalate pellets (a) with an intrinsic viscosity of 0.62 dl / g.

[0095] (Preparation of polyethylene terephthalate pellets (b)) The manufacturing process for polyethylene terephthalate pellets (a) was carried out in the same manner as above, except for adjusting the residence time of the third esterification reaction, to adjust the intrinsic viscosity to 0.580 dl / g and obtain polyethylene terephthalate pellets (b).

[0096] (Preparation of polyethylene terephthalate pellets (c)) Polyethylene terephthalate pellets (a) were subjected to solid-phase polymerization at 220°C under reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (c) with an intrinsic viscosity of 0.75 dl / g.

[0097] (Polymerization of urethane resin) In a four-necked flask equipped with a stirrer, a Liebig condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer, 72.96 parts by mass of 1,3-bis(isocyanatemethyl)cyclohexane, 12.60 parts by mass of dimethylolpropionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol with a number average molecular weight of 2000, and 85.00 parts by mass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone as solvents were added. The mixture was stirred at 75°C for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction solution reached the predetermined amine equivalent. Next, the reaction solution was cooled to 40°C, and then 9.03 parts by mass of triethylamine was added to obtain polyurethane prepolymer D solution. Next, in a reaction vessel equipped with a homodisperser capable of high-speed stirring, 450 g of water was added, the temperature was adjusted to 25°C, and the isocyanate-terminated prepolymer was added and dispersed in water while stirring at 2000 min-1. Subsequently, a water-soluble polyurethane resin (A) with a solid content of 35% by mass was prepared by removing some of the acetonitrile and water under reduced pressure.

[0098] (Polymerization of water-soluble carbodiimide compounds) In a flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer, 200 parts by mass of isophorone diisocyanate and 4 parts by mass of the carbodiimide catalyst 3-methyl-1-phenyl-2-phosphorene-1-oxide were added and stirred at 180°C for 10 hours under a nitrogen atmosphere to obtain isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Next, 111.2 g of the obtained carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100°C for 24 hours. Water was gradually added at 50°C to obtain a yellow, transparent, water-soluble carbodiimide compound (B) with a solid content of 40% by mass.

[0099] (Preparation of coating solution for easy adhesion layer formation) The following coating agents were mixed to create the coating solution. Water 16.97 parts by mass Isopropanol 21.96 parts by mass Polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (Silica sol with average particle size of 40 nm, solid content concentration of 40% by mass) Surfactant 0.05 parts by mass (Silicone-based, solid content concentration 100% by mass)

[0100] (Preparation of hard coat coating solution a) 100 parts by mass of hard coat material (Opstar® Z7503, manufactured by JSR, concentration 75%) was mixed with 0.1 parts by mass of leveling agent (BYK307, manufactured by BIK Chemie Japan, concentration 100%), and diluted with methyl ethyl ketone to prepare hard coat coating solution a with a solid content of 40% by mass.

[0101] (Preparation of hard coat coating solution b) 95 parts by mass of pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Industry Co., Ltd., A-TMM-3, 100% solids content), 5 parts by mass of photopolymerization initiator (manufactured by BASF Japan, Irgacure® 907, 100% solids content), and 0.1 parts by mass of leveling agent (manufactured by BIC Chemie Japan, BYK307, 100% solids content) were mixed and diluted with a toluene / MEK = 1 / 1 solvent to prepare a 40% by mass hard coat coating solution b.

[0102] (Preparation of coating solution containing conductive fibrous filler and metal nanowire) A mixed solution was prepared using 0.6 g of silver nitrate (manufactured by Wako Pure Chemical Industries, Ltd.), 36 g of an ethylene glycol (EG, manufactured by Kishida Chemical Co., Ltd.) solution containing 1.4 wt% polyvinylpyrrolidone (PVP, manufactured by Wako Pure Chemical Industries, Ltd., average molecular weight 360,000), 4 g of an EG solution containing 165 ppm iron(III) chloride (manufactured by Kishida Chemical Co., Ltd.), and 109 g of EG. This was designated as reaction solution 1. Reaction solution 1 was heated from room temperature to 130°C and reacted for 187 minutes using a personal synthesis apparatus (ChemiStation, PPV-CTRL1, manufactured by Tokyo Rikakikai Co., Ltd.). A cylindrical filter paper (No. 86R, retained particle size 1 μm, 20 mm x 90 mm, manufactured by Advantec Toyo Co., Ltd.) filled with 20 mL of reaction solution 1 was placed in a 300 mL beaker, and isopropyl alcohol (manufactured by Junsei Chemical Co., Ltd.) was added to the outside of the cylindrical filter paper so that it was at the same height as the reaction solution inside the cylindrical filter paper. After one week, the solution inside the cylindrical filter paper was collected and used as a coating solution containing metal nanowires.

[0103] (Example 1) Pellet(a) of polyethylene terephthalate was fed into an extruder and melted at 285°C. This polymer was filtered through a stainless steel sintered filter (nominal filtration accuracy, 95% cut of 10 μm particles), extruded in sheet form through a die, and then cooled and solidified using an electrostatic casting method by contacting it with a casting drum at a surface temperature of 30°C to produce an unstretched film. This unstretched film was uniformly heated to 75°C using a heating roll, and then heated to 85°C with a non-contact heater to perform 1.4 times roll stretching (longitudinal stretching). The above-mentioned easy-adhesion layer forming coating solution was applied to both sides of the obtained uniaxially oriented film using the roll-coating method, and then dried at 80°C for 20 seconds. The coating amount after final drying (after biaxial stretching) was 0.06 g / m². 2 The material was then adjusted to achieve the desired result. After that, it was guided into a tenter and preheated at 105°C, then stretched transversely to 4.0 times its original size at 95°C, the width was fixed, and heat-set at 230°C for 5 seconds, and then relaxed by 4% in the width direction at 180°C to obtain a polyethylene terephthalate film with a thickness of 50 μm.

[0104] (Examples 2-3) A polyester film was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio was changed as shown in Table 1.

[0105] (Example 4) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the width direction was changed to 4.4 times and the heat setting temperature was changed to 220°C.

[0106] (Examples 5 and 6) A polyester film was obtained in the same manner as in Example 4, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1.

[0107] (Example 7) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the width direction was changed to 5.5 times and the heat setting temperature was changed to 190°C.

[0108] (Examples 8-9) A polyester film was obtained in the same manner as in Example 7, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1.

[0109] (Example 10) In the manufacturing process of Example 5, a polyester film was obtained in the same manner as in Example 5, except that after stretching in the longitudinal direction, a 10% relaxation heat treatment was performed at 100°C.

[0110] (Example 11) In the manufacturing process of Example 5, a polyester film was obtained in the same manner as in Example 5, except that after heat setting, the clips were released at 200°C and relaxation heat treatment was performed in the longitudinal and width directions. In the longitudinal direction, the tenter speed and winding roll speed were adjusted so that the relaxation rate was 3%. Relaxation in the width direction was left free.

[0111] (Example 12) A polyester film was obtained in the same manner as in Example 1, except that the temperature during longitudinal stretching was changed to 75°C and the heat-fixing temperature was changed to 220°C.

[0112] (Example 13) A polyester film was obtained in the same manner as in Example 1, except that the temperature during longitudinal stretching was changed to 75°C, the stretching ratio was changed to 1.2 times, and then the stretching ratio in the width direction was changed to 5.0 times.

[0113] (Example 14) A polyester film was obtained in the same manner as in Example 3, except that the longitudinal stretching was performed in two stages, with the first stage stretching ratio being 1.2 times and the second stage stretching ratio being 1.67 times. The total longitudinal stretching ratio was approximately 2.0 times.

[0114] (Example 15) A polyester film was obtained in the same manner as in Example 5, except that the preheating temperature during widthwise stretching was changed to 95°C and the heat setting temperature was changed to 190°C.

[0115] (Example 16) A polyester film was obtained in the same manner as in Example 2, except that the widthwise stretching was performed in two stages, with the first stage stretching ratio set to 1.5 times and the second stage stretching ratio set to 4.0 times, and the heat setting temperature was changed to 190°C. The total widthwise stretching ratio was 6.0 times.

[0116] (Examples 17-18) A polyester film was obtained in the same manner as in Example 2, except that the thickness was changed as shown in Table 1.

[0117] (Example 19) A polyester film was obtained in the same manner as in Example 1, except that the heat relaxation treatment in the width direction was not performed in the manufacturing process of Example 1.

[0118] (Example 20) After preparing an unstretched film in the same manner as in Example 1, the unstretched film was preheated in a tenter at 75°C and then transversely stretched to 1.4 times its original size at 85°C. The above-mentioned easy-adhesion layer forming coating solution was applied to both sides of the resulting uniaxially oriented film by the roll-coating method, and then dried at 80°C for 20 seconds. The coating amount after final drying (after biaxial stretching) was 0.06 g / m². 2The material was adjusted to achieve the following: It was uniformly heated to 105°C using a heating roll, then heated to 95°C with a non-contact heater, and roll-stretched to 4.0 times its original size (longitudinal stretching). The width was fixed and heat-set at 230°C for 5 seconds to obtain a polyethylene terephthalate film with a thickness of 50 μm.

[0119] (Example 21) A polyethylene terephthalate film with a thickness of 50 μm was obtained in the same manner as in Example 1, and then a hard coat film was obtained by applying hard coat coating solution b.

[0120] (Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that it was stretched only in the width direction and not in the longitudinal direction, resulting in uniaxial stretching.

[0121] (Comparative Example 2) A polyester film was obtained in the same manner as in Example 7, except that it was stretched only in the width direction and not in the longitudinal direction, resulting in uniaxial stretching.

[0122] (Comparative Examples 3-7) A polyester film was obtained in the same manner as in Example 1, except that the heat-fixing temperature was changed to 220°C and the PET pellets and thickness were as described in Table 1. As described above, Comparative Examples 3 to 7 had lower heat-fixing temperatures than Example 1, and the combinations of conditions for the elongation ratio in the longitudinal and width directions were not optimal within the preferred condition range. As shown in Table 1, the refractive index in the thickness direction increased, the indentation depth after unloading the test force was greater, and the pencil hardness after lamination of the hard coat layer was lower compared to each example.

[0123] (Comparative Example 8) A polyester film was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio was changed to 2.7 times and the heat-fixing temperature was changed to 220°C.

[0124] (Comparative Example 9) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to 3.4 times.

[0125] (Comparative Example 10) A polyester film was obtained in the same manner as in Example 4, except that the heat-fixing temperature was changed to 100°C.

[0126] (Comparative Example 11) A polyester film was obtained in the same manner as in Example 13, except that the longitudinal stretching temperature was changed to 130°C.

[0127] (Comparative Example 12) A polyester film was obtained in the same manner as in Example 1, except that the preheating temperature in the width direction was changed to 120°C.

[0128] On one side of the prepared film, hard coat coating solution a was applied using a Meyer burr so that the film thickness after drying was 5 μm. After drying at 80°C for 1 minute, ultraviolet light was irradiated (cumulative light intensity 200 mJ / cm²). 2 A hard coat film was obtained. Subsequently, a metal nanowire-containing coating solution was applied to the surface of the fabricated hard coat layer using a Meyer bar so that the film thickness after drying was 5 μm, and after drying at 80°C for 10 minutes, a transparent conductive polyester film was obtained. The evaluation results are shown in Table 1.

[0129] The transparent conductive polyester film prepared as described above was incorporated into a touch panel module to create a smartphone-type foldable display that could be folded in half at the center, corresponding to the bending radius of 3 mm in Figure 1. The touch panel modules using the transparent conductive polyester film from each example satisfied the functionality and visibility requirements of a smartphone that could be folded in half at the center for portability. Furthermore, the surface did not dent under external force. Here, Examples 6-9, 15, and 16 were slightly inferior to the other examples in terms of the amount of curl after heat processing due to the slightly larger maximum heat shrinkage rate of the polyester film, but overall they were satisfactory. On the other hand, the foldable displays using the polyester film or hard coat film in each comparative example seemed to develop image distortion at the folding part of the display as the frequency of use increased, which was not very desirable. In addition, dents and scratches were observed on the surface in some cases.

[0130] [Table 1]

[0131] [Table 2] [Industrial applicability]

[0132] The foldable display using the polyester film for the touch panel module substrate of the present invention maintains mass producibility, and because the polyester film located in the touch panel module of the foldable display does not deform after repeated folding, there is no image distortion at the folded portion of the display. In particular, a portable terminal device or image display device equipped with a foldable display using the polyester film of the present invention as the touch panel module substrate provides beautiful images, is highly functional, and offers excellent portability and other conveniences. [Explanation of Symbols]

[0133] 1: Foldable display 11: Bending radius 2: Polyester fill for touch panel modules of foldable displays 21: Folding section 22: Bending direction (direction perpendicular to the folding part)

Claims

1. A polyester film for a touch panel module substrate of a foldable display that satisfies the following conditions. (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less. (4) Density of 1.380 g / cm³ ３ That's all. (5) Polyester film is a biaxially oriented polyethylene terephthalate film (6) Total light transmittance of 85% or more, haze of 3% or less, and maximum thermal shrinkage of 2% or less (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.)

2. A polyester film for a touch panel module substrate of a foldable display according to claim 1, wherein the modulus of elasticity in the bending direction is 2.7 GPa or less, and the modulus of elasticity in the folding direction is 4.5 GPa or more.

3. The polyester film for a touch panel module substrate of a foldable display according to Claim 1 or 2, wherein the bending resistance evaluation (bending radius of 1.5 mm) is performed in an unloaded U-shaped stretch test in which the film is bent 200,000 times at a bending radius of 1.5 mm and a speed of 1 time / second after heat treatment at 150°C for 30 minutes, and after the bending is completed, the film is placed on a flat surface with the bent side facing down to evaluate whether cracks or deformation occur.

4. The polyester film for a touch panel module substrate of a foldable display according to any one of claims 1 to 3, wherein the polyester film for the touch panel module substrate of the foldable display has an easy-adhesion layer on at least one side.

5. The polyester film for a touch panel module substrate of a foldable display according to any one of claims 1 to 4, wherein the polyester film for the touch panel module substrate of the foldable display has a hard coat layer with a thickness of 1 to 50 μm on at least one side.

6. A foldable display comprising the polyester film according to any one of Claims 1 to 5.

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