Hard coating for foldable displays and its applications

Abstract

This invention provides a hard coating for foldable displays that prevents creases or cracks in the folded portion and effectively suppresses rainbow-like colors (interference spots) caused by minor cracks in the easy-bond resin layer, etc. The hard coating for foldable displays comprises a hard coating having an easy-bond resin layer and a hard coating layer sequentially on at least one side of a polyester film with a thickness of 10-80 μm. The easy-bond resin layer is formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds, and a polyester resin. The polyester film having the easy-bond resin layer and the hard coating layer satisfies the following conditions (1) to (4): (1) Refractive index in the bending direction is 1.590-1.620; (2) Refractive index in the folded portion direction is 1.670-1.700; (3) Refractive index in the thickness direction is 1.520 or less; (4) Density is 1.380 g / cm³ or more (here, the bending direction refers to the direction orthogonal to the folded portion when the polyester film is folded).

Description

Hard Coating Film for Foldable Display and Its Use The present invention relates to a hard coating film for a foldable display, a foldable display, and a portable terminal, and relates to a foldable display and a portable terminal in which images are not easily disturbed due to deformation of the film even when repeatedly folded, and the hard coating film for the foldable display described above. With the progress of weight reduction of the film of portable terminals, portable terminals represented by smartphones have been widely popularized. In portable terminals, various functions are required, but on the other hand, convenience is also required. Therefore, popular portable terminals can be easily operated with one hand, and furthermore, since they are designed to be able to be stored in a clothes pocket or the like, they must be set to a small screen size of about 6 inches. On the other hand, in tablet terminals with a screen size of 7 inches to 10 inches, they have high functionality not only for image content or music, but also for business use, drawing use, reading, etc. However, they cannot be operated with one hand, and their portability is also poor, and there are problems in terms of convenience. In order to achieve these, a method of miniaturizing by joining a plurality of displays (refer to Patent Document 1) has been proposed, but since the border part remains, the image is cut off, and the reduction in visual recognition becomes a problem and it cannot be popularized. Therefore, in recent years, portable terminals equipped with flexible displays and foldable displays have been proposed. In this method, the image is not cut off, and as a portable terminal equipped with a large-screen display, it has good convenience and portability. Here, regarding conventional displays or portable terminals that do not have a folding structure, although the surface of the display can be protected by materials such as glass that do not have flexibility, in a foldable display, when the display becomes a single surface through the folding part, a hard coating film or the like that has flexibility and can protect the surface must be used. However, in a foldable display, since a certain part corresponding to the folding part is repeatedly bent, the film in this part is deformed over time, and there are problems such as deformation of the image displayed on the display. Also, not only the surface protective film, but also in a foldable display, films are used in various parts such as a polarizing plate, a retardation plate, a touch panel substrate, a substrate of a display unit such as an organic EL, and a protective member on the back surface. For these films, durability against repeated folding is also required. As a method of improving durability, a method of partially changing the film thickness has also been proposed (refer to Patent Document 2), but there is a problem of lack of mass productivity. In addition, a method of adjusting the refractive index in the bending direction of a polyester film has been proposed (see Patent Document 3). As the refractive index in the bending direction decreases, the pencil hardness during hard coat application decreases, and there is a problem of degradation of the surface protection function of the display. Further, although the deformation during folding is improved when the refractive index in one direction is decreased, the uniaxial orientation in the folding direction increases, and there is a problem of cracks or breaks occurring in the folded portion. On the other hand, visual recognition or designability is also required for the above-mentioned hard coat film. Therefore, in order to suppress glare or rainbow-like colors (interference fringes) caused by reflected light when viewed from an arbitrary angle, etc., an antireflection layer having a multilayer structure in which a high refractive index layer and a low refractive index layer are laminated on top of the hard coat layer is generally provided. However, recently, 3-wavelength type fluorescent lamps have become mainstream due to the reproducibility of daylight color, and interference fringes caused by reflected light are more easily seen. Furthermore, the requirement to reduce costs by simplifying the antireflection layer is gradually increasing. Therefore, it is required to suppress interference fringes as much as possible with only the hard coat film without an antireflection layer. As described above, as a method of suppressing interference fringes, a method of providing one or two layers of optically adjusted layers with adjusted refractive indices on a polyester film has been proposed, but the durability of the polyester film against repeated folding must be considered. For example, an optically adjusted layer containing an excessive amount of metal fine particles causes interference fringes due to minute cracks starting from the fine particles, and thus has not been fully satisfactory. [Prior Art Documents] [Patent Documents] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-228391 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-155124 [Patent Document 3] International Publication No. 2018 / 150940 [Problems to be Solved by the Invention] The present invention aims to solve the problems of the conventional display members as described above, and to provide a foldable display excellent in mass productivity and without fear of image disturbance in the image displayed in the folded portion after repeated folding, and a portable terminal equipped with such a foldable display, and to provide a hard coat film for a foldable display in which no creases or cracks occur in the folded portion and which can effectively suppress rainbow-like colors (interference fringes) caused by the influence of minute cracks such as an easy-to-adhere resin layer. [Means for Solving the Problems] That is, the present invention includes the following configurations. 1. A hard coating film for a foldable display, which is a hard coating film having an easy-to-adhere resin layer and a hard coating layer in sequence on at least one side of a polyester film with a thickness of 10 to 80 μm, wherein the aforementioned easy-to-adhere resin layer is cured from a composition containing at least one compound selected from titanium compounds and zirconium compounds and a polyester resin, and the polyester film before having the easy-to-adhere resin layer and laminating the hard coating layer satisfies the following conditions (1) to (4). (1) The 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) The density is 1.380 g / cm 3 Herein, the so-called bending direction refers to the direction orthogonal to the folding part when the polyester film is folded). 2. The hard coating film for a foldable display according to item 1 above, wherein the refractive index of the aforementioned easy-to-adhere resin layer is lower than the refractive index in the bending direction and the refractive index in the direction of the folding part of the polyester film before having the easy-to-adhere resin layer and laminating the hard coating layer, and is higher than the refractive index of the aforementioned hard coating layer. 3. The polyester film for a foldable display according to item 1 or 2 above, wherein the refractive index of the aforementioned easy-to-adhere resin layer satisfies the following conditions (5) and (6). (5) The refractive index of the easy-to-adhere resin layer is lower than the refractive index in the bending direction of the polyester film before having the easy-to-adhere resin layer and laminating the hard coating layer, and the difference in refractive index is greater than 0 and 0.07 or less (6) The refractive index of the easy-to-adhere resin layer is lower than the refractive index in the direction of the folding part of the polyester film before having the easy-to-adhere resin layer and laminating the hard coating layer, and the difference in refractive index is 0.080 or more and 0.150 or less 4. The polyester film for a foldable display according to any one of items 1 to 3 above, which is a polyester resin containing a naphthalenedicarboxylic acid component as at least a part of the dicarboxylic acid component among the dicarboxylic acid component and the diol component constituting the polyester resin contained in the aforementioned easy-to-adhere resin layer. 5. The hard coating film for a foldable display according to items 1 to 4 above, wherein the total light transmittance of the polyester film before having the easy-to-adhere resin layer and laminating the hard coating layer is 85% or more, the haze is 3% or less, and the maximum thermal shrinkage rate is 6% or less. 6. The hard coating film for a foldable display according to any one of items 1 to 5 above, wherein the thickness of the aforementioned hard coating layer is 1 to 50 μm. 7. A foldable display, which is a foldable display that configures the hard coating film for a foldable display according to item 6 above with the hard coating layer on the surface as a surface protection film, and is configured with a single hard coating film continuous through the folding part of the foldable display. 8. A portable terminal having the foldable display according to item 7 above. [Effects of the Invention] The foldable display using the hard coating film for a foldable display of the present invention maintains mass productivity on the one hand, and on the other hand, the hard coating film does not crack at the folding part and does not deform after repeated folding. Furthermore, in addition to cracks at the folding part, lifting at the interface between the hard coating layer and the easy-to-adhere resin layer, and lifting at the interface between the easy-to-adhere resin layer and the polyester film, etc., it is also possible to effectively suppress rainbow-like colors (interference fringes) caused by fine cracks, etc., and no image disturbance occurs at the folding part of the display. A portable terminal equipped with a foldable display using the aforementioned hard coating film provides beautiful images, is rich in functionality, and has excellent portability and other conveniences. [Embodiments for Carrying out the Invention] (Display) The display referred to in the present invention generally refers to a display device. As types of displays, there are LCDs, organic EL displays, inorganic EL displays, LEDs, FEDs, etc., but preferably an LCD or an organic EL or inorganic EL having a bendable structure. Particularly preferably, it is an organic EL or inorganic EL that can reduce the layer structure, and more preferably an organic EL with a wide color gamut. (Foldable display) A foldable display is one in which a continuous single display can be folded in half or the like when carried. By folding, the size is halved, and portability can be improved. The bending radius of the foldable display is preferably 5 mm or less, and more preferably 3 mm or less. If the bending radius is 5 mm or less, thinning in the folded state becomes possible. Although it can be said that the smaller the bending radius, the better, the smaller the bending radius, the easier it is to cause creases. The bending radius is preferably 0.1 mm or more, and can also be 0.5 mm or more, or 1 mm or more. Even if the bending radius is 1 mm, sufficient thinning for practical use can be achieved when carried. The so-called bending radius during folding is the place where the symbol 11 in the schematic diagram of FIG. 1 is measured, and it means the radius inside the folding part during folding. In addition, the following surface protection film can be located on the outside of the fold of the foldable display or on the inside. Also, the foldable display can be folded three times or four times, and furthermore, it can also be a winding type called a rollable type, and all of these are within the scope of the foldable display referred to in the present invention. The hard coating film for a foldable display of the present invention can be used in any part as long as it is a component of the foldable display. Hereinafter, taking an organic EL display as an example, the representative structure of the foldable display and the parts where the hard coating film of the present invention can be used will be described. In addition, hereinafter, the hard coating film for a foldable display of the present invention will sometimes be simply referred to as the hard coating film of the present invention. (Foldable organic EL display) As a necessary component of the foldable organic EL display, there is an organic EL module, but a circular polarizing plate, a touch panel module, a surface protection film, a back protection film, etc. can be further provided as required. (Organic EL module) The general structure of the organic EL module includes an electrode / electron transport layer / light emitting layer / hole transport layer / transparent electrode. (Touch panel module) In a portable terminal, it preferably has a touch panel. When using an organic EL display, it is preferable to dispose the touch panel module above the organic EL display or between the organic EL module / circular polarizing plate. The touch panel module has a transparent substrate such as a thin film and a transparent electrode disposed thereon. The hard coating film of the present invention can be used as the transparent substrate. When used as the transparent substrate of the touch panel, it is preferable to provide a refractive index adjustment layer. (Circular polarizing plate) The circular polarizing plate suppresses the reduction in image quality due to the reflection of external light by the components inside the display. The circular polarizing plate has a linear polarizing plate and a retardation plate. The linear polarizing plate has a protective film on at least the visually recognizable side of the polarizer. It is also possible to have a protective film on the side of the polarizer opposite to the visually recognizable side, and the retardation plate can also be directly laminated on the polarizer. The retardation plate is a resin film having retardation such as polycarbonate or cyclic olefin, or a retardation layer composed of a liquid crystal compound provided on the resin film. The hard coating film of the present invention can be used as the protective film of the polarizer. In these cases, when the base film of the hard coating film of the present invention is a polyester film, it is preferable that the slow axis direction of the polyester film is parallel or orthogonal to the absorption axis direction of the polarizer. In addition, a deviation of 10 degrees, preferably within 5 degrees, from the parallel or orthogonal is allowed. (Surface protective film) If an impact is applied to the display from above, there is a risk of disconnection of the circuit of the organic EL module or the touch panel module. Therefore, a surface protective film is provided in most cases. The hard coating film of the present invention can be used as the surface protective film. The surface protective film includes a so-called cover window incorporated into the outermost surface of the display, or a so-called after-sticker that can be attached, peeled off, and exchanged by the user himself / herself. The hard coating film of the present invention can be used in both cases. The hard coating layer is provided on the surface of the foldable display with the hard coating layer as the visually recognizable side. In addition, the hard coating layer can also be provided on both sides. (Back protective film) On the back side of the display, it is preferable to provide a protective film. The hard coating film of the present invention can be used as the protective film on the back side. The hard coating film of the present invention can also be other than the above as long as it is used in the folded part of the components constituting the foldable display. Among these, the hard coating film of the present invention is preferably used for the cover window surface protective film, the after-sticker surface protective film, the base film of the touch panel module, and the back protective film. More preferably, it is used for the cover window surface protective film and the after-sticker surface protective film. Also, as a foldable display, it is not necessary to use the hard coating film of the present invention in all of the above uses. In a foldable display, hard coating films based on polyester films, polyimide films, polyamide films, polyamide-imide films, polycarbonate films, acrylic films, triacetyl cellulose films, cyclic olefin polymer films, polyphenylene sulfide films, poly-4-methyl-1-pentene films, etc. can be appropriately used according to adaptability. When the base film constituting the hard coating film of the present invention is a polyester film, it may be a single-layer film composed of one or more polyester resins. When two or more polyesters are used, it may be a multilayer film or a super-multilayer laminated film with a repeating structure. Examples of the polyester resin used for the polyester film as the base film of the hard coating film include polyester films composed of polyethylene terephthalate, polybutylene terephthalate, poly(ethylene-2,6-naphthalate), or copolymers having the constituent components of these resins as the main components. Among them, from the viewpoints of mechanical properties, heat resistance, transparency, price, etc., a stretched polyethylene terephthalate film is particularly preferred. When a copolymer of polyester is used in the polyester film as the base film of the hard coating film, examples of the dicarboxylic acid component of the 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. Also, examples of the glycol component 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; alicyclic glycols such as 1,4-cyclohexanedimethanol; and polyethylene glycol having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerized components of the preferred copolymer is less than 20% by mass. When it is less than 20% by mass, the film strength, transparency, and heat resistance can be maintained, which is more suitable. Moreover, in the manufacture of the polyester film as the base film of the hard coating film, the intrinsic viscosity of at least one or more resin particles is preferably in the range of 0.50 to 1.0 dl / g. If the intrinsic viscosity is 0.50 dl / g or more, the impact resistance of the obtained film increases, and it is less likely to cause disconnection of the internal circuit of the display due to external impact, which is more suitable. On the other hand, if the intrinsic viscosity is 1.00 dl / g or less, the filtration pressure of the molten fluid does not increase excessively, and it is easy to stably operate the film manufacture, which is more suitable. The thickness of the polyester film as the base film of the hard coating film is preferably 10 to 80 μm, more preferably 25 to 75 μm. If the thickness is 10 μm or more, the effect of improving the pencil hardness and the impact resistance can be seen. If the thickness is 80 μm or less, it is beneficial for weight reduction, and the flexibility, processability, or disposability is excellent. The surface of the hard coating film of the present invention may be smooth or may have unevenness, but from the viewpoint of the surface covering use for a display, it is not suitable because the optical properties derived from the unevenness are reduced. The haze in the state before laminating the hard coating layer by laminating an easy-adhering resin layer on the polyester film is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visual recognition of the image can be improved. The lower limit of the haze is preferably as small as possible, but from the aspect of stable production, it is preferably 0.1% or more, and may also be 0.3% or more. As described above, for the purpose of reducing haze, the unevenness on the surface of the film should not be too large. From the perspective of processability, in order to impart a certain degree of slidability, as a method of forming unevenness, it can be formed by blending particles in a base polyester film, or by coating a particle-containing coating as an easy-to-adhere resin layer during the film formation of the base polyester film. As a method of blending particles in a base polyester film, well-known methods can be adopted. For example, it can be added at any stage of manufacturing polyester, but it is preferably added at the esterification stage, or at the stage after the transesterification reaction and before the start of the polycondensation reaction, added as a slurry dispersed in ethylene glycol or the like, and the polycondensation reaction can also be carried out. Also, it can be carried out by a method of blending a slurry of particles dispersed in ethylene glycol or water and a polyester raw material using a kneading extruder with an exhaust port, or a method of blending dried particles and a polyester raw material using a kneading extruder. Among them, it is preferably a method of uniformly dispersing aggregated inorganic particles in a monomer liquid that is part of the polyester raw material, and then adding the filtered product to the remaining part of the polyester raw material before, during, or after the esterification reaction. By this method, since the monomer liquid has a low viscosity, the uniform dispersion of particles or the high-precision filtration of the slurry is easy to carry out. At the same time, when added to the remaining part of the raw material, the dispersibility of the particles is good, and new aggregates are not easily generated. From this perspective, it is particularly preferably added to the remaining part of the raw material in a low-temperature state before the esterification reaction. Also, after obtaining polyester containing particles in advance, by a method such as kneading and extruding its granules with granules without particles (masterbatch method), the number of protrusions on the film surface can be further reduced. Also, within the preferred range of maintaining the total light transmittance, the polyester film as the base material can contain various additives. Examples of additives include antistatic agents, UV absorbers, and stabilizers. The total light transmittance of the polyester film in a state where it has an easy-to-adhere resin layer and does not have a hard coat layer is preferably 85% or more, more preferably 87% or more. If the transmittance is 85% or more, visual recognition can be sufficiently ensured. In order to improve the total light transmittance of the following hard coat film, the total light transmittance of the aforementioned polyester film is also preferably 85% or more. The higher the total light transmittance of the aforementioned polyester film, the better it can be said, but from the aspect of stable production, it is preferably 99% or less, and can also be 97% or less. The maximum heat shrinkage rate of the polyester film after heat treatment at 150°C for 30 minutes in a state with an easy-to-adhere resin layer and without a hard coat layer is preferably 6% or less, more preferably 5% or less. If the heat shrinkage rate is 6% or less, planar defects such as curling or undulating during HC processing can be suppressed. The lower the heat shrinkage rate, the better, but it is preferably -1% or more, more preferably 0% or more. The negative coefficient here means expansion after heating. If it is -1% or more, the planar state is good and it is more suitable. In order to impart sufficient pencil hardness to the hard coat film for the foldable display of the present invention, the polyester film in a state with an easy-to-adhere resin layer and without a hard coat layer preferably has the following characteristics. In the past, for the pencil hardness evaluation of the pencil hardness of the hard coat film after laminating the hard coat layer on the base polyester film, since the film deformed in the thickness direction, it was judged that the pencil hardness would decrease. In the present invention, it is preferable that the indentation depth after unloading the test force applied in the thickness direction to the base polyester film in the above state by the following dynamic ultra-micro hardness tester falls within a specific range. In the pencil hardness evaluation of the hard coat film using the above base polyester film, high hardness can be achieved and it is more suitable. The indentation depth in the thickness direction of the base polyester film in the above state after unloading the test force is preferably 1.5 μm or less, more preferably 1.4 μm or less, still more preferably 1.3 μm or less. If the indentation depth after unloading the test force (the final deformation amount after applying the load) is 1.5 μm or less, in the pencil hardness evaluation of the hard coat film after laminating the hard coat layer, the film is not easily deformed in the thickness direction, and the pencil hardness can be improved. If the pencil hardness of the hard coat film can be improved, damage and dents are not easily generated on the surface of the display, and the visual recognition of the display is increased. The lower the indentation depth after unloading the test force, the better, but from the viewpoints of stable production or saturation of the effect, it is preferably 0.3 μm or more, more preferably 0.5 μm or more. In order to reduce the indentation depth after unloading the test force, it is effective to adjust the refractive index in the thickness direction of the polyester film in a state with an easy-to-adhere resin layer and without a hard coat layer to 1.520 or less. As a means of making the refractive index 1.520 or less, as described below, it can be exemplified that within the range where other physical properties, the refractive index in the bending direction or the folding direction can be controlled within a preferable range, increasing the elongation ratio in the bending direction or the folding direction, or setting a low elongation temperature in the bending direction or the folding direction, setting conditions such as a high heat setting temperature, etc. On the non-hard coat surface of the hard coat film of the present invention, a treatment for improving the coating of the adhesive or the adhesion to the hard coat layer can be performed. As a surface treatment method, for example, roughening treatment such as sandblasting treatment, solvent treatment, etc., or oxidation treatment such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone-ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, etc. can be used without particular limitation. Furthermore, it is preferable to improve the adhesion by an adhesion enhancing layer such as an easy adhesion layer. As the easy adhesion layer, acrylic resin, polyester resin, polyurethane resin, polyether resin, etc. can be used without particular limitation, and can be formed by a general coating method, preferably by an in-line coating formulation. The above-mentioned polyester film can be manufactured, for example, through the following steps: homogeneously dispersing inorganic particles in a monomer liquid that is part of the polyester raw material, filtering, adding to the remaining part of the polyester raw material, and performing a polymerization step of polymerizing the polyester; and melt-extruding the polyester through a filter into a sheet shape, cooling it, and then stretching to form a substrate film in a film forming step. Next, regarding the manufacturing method of the polyester film as the substrate, an example of using poly(ethylene terephthalate) (hereinafter sometimes referred to as PET) granules as the raw material of the substrate film will be described in detail, but it is not limited thereto. Also, the number of layers such as single-layer structure and multi-layer structure is not limited. After mixing and drying the PET granules in a specified ratio, they are supplied to a well-known extrusion machine for melt lamination, extruded into a sheet shape from a slit-shaped die, and cooled and solidified on a casting roll to form an unstretched film. In the case of a single layer, one extrusion machine can be used. When manufacturing a multi-layer film, two or more extrusion machines, two or more manifolds or a confluence block (for example, a confluence block having a square confluence part) can be used to laminate a plurality of film layers constituting each outermost layer, extrude a sheet of two or more layers from a nozzle, and cool it on a casting roll to form an unstretched film. In this case, during melt extrusion, at any place where the molten resin is maintained at about 280 °C, in order to remove foreign substances contained in the resin, it is preferable to perform high-precision filtration. The filter medium used for high-precision filtration of the molten resin is not particularly limited, but a filter medium of a sintered stainless steel body is more suitable because of its excellent removal performance of condensates mainly composed of Si, Ti, Sb, Ge, Cu and high-melting-point organic substances. Furthermore, the filtration particle size of the filter medium (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 medium (initial filtration efficiency 95%) exceeds 20 μm, foreign substances larger than 20 μm cannot be sufficiently removed. By using a filter medium with a filtration particle size of 20 μm or less (initial filtration efficiency 95%) for high-precision filtration of the molten resin, although there is a situation where productivity decreases, it is more suitable in terms of obtaining a film with fewer protrusions caused by coarse particles. (Refractive Index in the Bending Direction) In the present invention, the refractive index in at least any one of the longitudinal direction (mechanical flow direction) and the width direction of the polyester film having an easy-to-adhere resin layer and before laminating the hard coat layer is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. Moreover, the refractive index in the bending direction of the polyester film having an easy-to-adhere resin layer and before laminating the hard coat layer is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. Here, the so-called bending direction means, as shown by the symbol 22 on the polyester film (symbol 2) in FIG. 2, the direction orthogonal to the folding portion (symbol 21) assumed in the use of the foldable display. If the refractive index in at least any one of the longitudinal direction and the width direction is 1.590 to 1.620, there is less deformation during repeated folding, and there is no fear of degrading the image quality of the foldable display, which is more suitable. The refractive index is more preferably 1.591 to 1.600. Of course, the direction is preferably the aforementioned bending direction. If it is 1.590 or more, there is no fear of introducing cracks in the folding portion direction after the following bending test, and of course, breakage does not occur, so good visual recognition of the display can be maintained. The refractive index of the polyester film can be effectively adjusted by adjusting the draw ratio and draw temperature. Also, in order to adjust the refractive index, a relaxation step in the drawing direction and multi-stage drawing can be used. When performing multi-stage drawing, it is preferable to increase the draw ratio of the second stage and subsequent stages more than that of the first stage. In addition, the difference between the refractive index of the polyester film having an easy-to-adhere resin layer and before laminating the hard coat layer and the refractive index of the polyester film alone without even the easy-to-adhere resin layer is negligibly small. By controlling the refractive index in at least any one of the longitudinal direction (mechanical flow direction) and the width direction of the polyester film having an easy-to-adhere resin layer and before laminating the hard coat layer within the above range, more preferably by controlling the refractive index in the bending direction within the above range, the fatigue caused by the compressive stress applied to the inside of the fold during folding can be reduced. It is considered that the fatigue caused by the compressive stress mainly occurs in the crystalline part, and the less the crystal in the bending direction, the less likely it is to fatigue. Therefore, it is considered that by reducing the refractive index, the amount of oriented crystals in the bending direction can be reduced, and the compressive fatigue can be suppressed. Also, by reducing the refractive index, the creep phenomenon occurring due to the tensile stress applied to the outside of the fold during folding can be suppressed. It is considered that the fatigue caused by the tensile stress mainly occurs in the amorphous part, and deformation occurs due to the alignment of molecular chains caused by repeated application of stress. It can be speculated that the less the molecular chains arranged side by side in the bending direction, the less the deformation caused by alignment. Also, since less amorphous part can suppress the fatigue caused by stretching, it is preferably the one with a high crystallinity, that is, a high density. In the present invention, the stretching ratio of the unstretched polyester film in at least one of the length direction (mechanical flow direction) and the width direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. Moreover, the stretching direction is preferably the aforementioned bending direction. If the stretching ratio is 1.2 times or more, it is suitable because there is no deformation during subsequent processing such as hard coat application. If the stretching ratio is 2.0 times or less, it is suitable because there is no thickness unevenness of the film. The stretching temperature is preferably 75 to 120 °C, more preferably 75 to 105 °C. In addition, as the heating method during stretching, conventional means such as hot air heating method, roll heating method, infrared heating method, etc. can be adopted. By setting the stretching temperature to 75 to 120 °C, it is possible to prevent significant thickness unevenness caused by the stretching of the above-mentioned stretching ratio. Also, as described above, by stretching at as low a temperature as possible within the range where significant thickness unevenness does not occur, the refractive index in the thickness direction can be reduced. (Regarding the refractive index in the direction of the folding part) The refractive index in the direction orthogonal to the direction in which the refractive index of the polyester film before laminating the hard coat layer with the easy-to-adhere resin layer is 1.590 to 1.620 is preferably 1.670 to 1.700. That is, the refractive index in the direction orthogonal to the bending direction (the direction of the folding part) is preferably 1.670 to 1.700. By setting it to 1.670 to 1.700, deformation during folding in the bending direction can be reduced. By setting it to 1.700 or less, the introduction of cracks or fractures in the direction of the folding part can be suppressed. By setting it to 1.670 or more, the bendability in the bending direction can be improved and the surface hardness can be increased. More preferably, it is 1.680 to 1.695. As a method for adjusting the refractive index in the direction orthogonal to the bending direction, stretching ratio, stretching preheating temperature, stretching temperature, multi-stage stretching, and film relaxation can be mentioned. The stretching ratio is preferably 4.0 to 6.0 times, more preferably 4.4 to 6.0. Also, the stretching preheating temperature in the direction orthogonal to the bending direction is preferably 70 to 110 °C. When performing multi-stage stretching in the direction orthogonal to the bending direction, it is preferable to increase the stretching ratio of the second stage and subsequent stages more than the first stage. Film relaxation can be carried out by 1 to 10% in both the mechanical flow direction (length direction) and the vertical direction (width direction). (Refractive Index in the Thickness Direction) The refractive index in the thickness direction of the polyester film with an easy-to-adhere resin layer and before laminating the hard coat layer 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 lower, a decrease in the hardness of the film surface can be suppressed, and both bendability and surface hardness can be achieved. By setting it to 1.520 or less, the indentation depth after unloading the test force in the thickness direction decreases, and the hardness of the film surface, especially the pencil hardness of the hard coat film after laminating the hard coat layer, can be improved. More preferably, it is 1.515 or less, still more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. The refractive index in the thickness direction should preferably be low, but in terms of stable production, it is preferably 1.3 or more, and further can be 1.4 or more. Particularly preferably, it is 1.410 or more. It can be said that the above range can be achieved by increasing the draw ratio in both the bending direction and the folding direction. However, after controlling the refractive indices in the bending direction and the width direction within a preferable range, in order to control the refractive index in the thickness direction, it is preferable to set the conditions while confirming the balance of the conditions in each step of the film-forming process. The method for controlling the refractive index in the thickness direction within the above range includes the temperature settings of the preheating temperature for stretching in the bending direction, the stretching temperature, the draw ratio, the preheating temperature for stretching in the direction of the folding part, the stretching temperature, multi-stage stretching, high-ratio stretching, or heat setting. The preheating temperature for stretching in the bending direction is preferably 70°C to 110°C. The stretching temperature in the bending direction is preferably 75 to 120°C. The draw 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 draw ratio, while maintaining the bendability in the bending direction, the refractive index in the thickness direction can be effectively reduced. The preheating temperature for stretching in the direction of the folding part is also preferably 75°C to 110°C. The stretching temperature is preferably 75 to 120°C. The draw ratio of the folding part 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 of high-ratio stretching, multi-stage stretching can be used. In this case, it is more appropriate to increase the draw ratio of the second stage more than that of the first stage, and the refractive index can be effectively controlled. Also, a method of stretching again after the crystallization step can be used. Accelerated stretching, where the stretching speed is increased from the initial stage to the latter half of stretching, can also be used. The heat setting temperature is preferably 180 to 240°C. By performing heat setting, the orientation crystallization in the stretching direction progresses, and the refractive index in the thickness direction can be reduced. The reason for the increase in the hardness of the film surface due to the reduction of the refractive index in the thickness direction is not necessarily clear, but it is considered that aromatic groups such as benzene rings in the molecular chain are oriented in the plane direction, and have the effect of suppressing deformation caused by stress applied in the thickness direction. (Regarding the density of the polyester film) The density of the polyester film with an easy-to-adhere resin layer before laminating the hard coating layer is preferably 1.380 g / cm 3 or more. More preferably, it is 1.383 g / cm 3 or more. By setting it to 1.380 g / cm 3 or more, the flexibility is improved, the surface hardness of the film can be increased, and in particular, the pencil hardness of the hard coating film after laminating the hard coating layer can be increased. The higher the density, the better. Although it is also somewhat affected by the presence or absence of particles in the film, it is preferably 1.40 g / cm 3 or less. By setting the heat setting temperature during film formation at 180 - 240 °C, crystallization can proceed, effectively increasing the density. In addition, the difference in density between the polyester film with an easy-to-adhere resin layer before laminating the hard coating layer and the polyester film alone without even an easy-to-adhere resin layer is negligible. The bending direction of the polyester film is preferably corresponding to the length direction (machine flow direction). In this way, when biaxially stretched, it is easy to reduce the refractive index in the bending direction and easy to improve the flexibility. That is, the unstretched polyester sheet is stretched in the length direction at a stretching ratio of 1.2 - 2.0 times, more preferably 1.7 - 2.0 times, to obtain a preferable polyester film. Then, in the width direction, stretching at a stretching ratio of 4.0 - 6.0 times, more preferably 4.4 - 6.0 times, is a preferable aspect. Also, in the present invention, it is a particularly preferred aspect that the polyester film with an easy-to-adhere resin layer before laminating the hard coating layer simultaneously has the following 4 characteristics: (1) The refractive index in the bending direction is 1.590 - 1.620; (2) The refractive index in the direction of the folded part is 1.670 - 1.700; (3) The refractive index in the thickness direction is 1.520 or less; (4) The density is 1.380 g / cm 3 or more. However, even for combinations within the above preferred manufacturing conditions, for example, a combination where the stretching ratio in the bending direction is 1.4 times or less, the stretching ratio in the direction of the folded part is less than 4.4 times, and the heat setting temperature is 220 °C or less, there may be a situation where it is not the best condition combination among each preferred manufacturing condition range, and it may not satisfy the above 4 characteristics simultaneously. In this case, by finely adjusting any one condition or a combination thereof, such as increasing the stretching ratio in the bending direction to 1.7 times or more, or increasing the stretching ratio in the direction of the folded part to 4.4 times or more, or increasing the heat setting temperature to around 230 °C, or reducing the stretching temperature in the bending direction and / or the direction of the folded part, it can be made to satisfy the above 4 characteristics simultaneously. In order to adjust film-forming properties, film strength, thermal dimensional stability, appearance defects, etc., any film-forming method such as stretching, relaxation, heat setting, surface treatment, etc. can be adopted. However, controlling the refractive index and density of the film within the above-mentioned preferred ranges is a particularly preferred aspect in the present invention. By controlling the refractive index and density within the preferred ranges, a polyester film suitable for a foldable display can be provided, which can obtain better bending resistance and surface hardness than conventional films, especially the high pencil hardness of the hard-coated film after laminating a hard coating layer. Specifically, for example, after sufficiently vacuum-drying PET granules, they are supplied to an extruder and melt-extruded into a sheet at about 280 °C, and then cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, in the length direction with a roller heated to 75 to 120 °C to obtain a uniaxially oriented PET film. Furthermore, by grasping the end of the film with a clamp and guiding it to a hot air zone heated to 75 to 120 °C, after drying, it is stretched 4.0 to 6.0 times, more preferably 4.4 to 6.0 times, in the width direction. Then, it can be guided to a heat treatment zone at 180 to 240 °C for heat treatment for 1 to 60 seconds. In this heat treatment step, a relaxation treatment of 0 to 10% can be applied in the width direction or the length direction as required. The intrinsic viscosity of the polyester film is preferably in the range of 0.50 to 1.0 dl / g. If the intrinsic viscosity is 0.50 dl / g or more, the impact resistance increases, and it is less likely to occur the disconnection of the internal circuit of the display caused by external impact, which is more suitable. On the other hand, if the intrinsic viscosity is 1.00 dl / g or less, the filtration pressure of the molten fluid does not increase excessively, and the film manufacturing is stable, which is more suitable. (Easy-to-adhere resin layer) In the present invention, in order to improve the adhesion between the polyester film and the hard coating layer, etc., it is preferable to laminate an easy-to-adhere resin layer on the polyester film. The easy-to-adhere resin layer can be obtained by so-called in-line coating: after applying the coating liquid for forming the easy-to-adhere resin layer on one or both sides of the unstretched or longitudinally uniaxially stretched film, heat treatment and drying are carried out as required, and it is further stretched in at least one direction of the unstretched film. Heat treatment can also be carried out after biaxial stretching. The coating amount of the final easy-to-adhere layer is preferably controlled at 0.005 to 0.20 g / m 2 . If the coating amount is 0.005 g / m 2 or more, adhesion is obtained, which is more suitable. On the other hand, if the coating amount is 0.20 g / m 2 or less, anti-bonding property is obtained, which is more suitable. As the resin contained in the coating liquid used in the laminate as the easy-adhesion layer, for example, polyester resin, polyether polyurethane resin, polyester polyurethane resin, polycarbonate polyurethane resin, acrylic resin, etc. can be used, and there is no particular limitation. However, from the viewpoints of high adhesion to the polyester film and refractive index, it is preferably a polyester resin containing a naphthalenedicarboxylic acid component as at least a part of the dicarboxylic acid component in the dicarboxylic acid component and the diol component constituting the polyester resin. Furthermore, a crosslinked structure can also be formed in the binder resin contained in the easy-adhesion resin layer to improve the adhesion durability of these easy-adhesion resin layers. As the crosslinking agent contained in the coating liquid for forming the easy-adhesion layer, examples include melamine compounds, isocyanate compounds, oxazoline compounds, epoxides, carbodiimide compounds, etc., and self-crosslinking type polyurethane resins, etc. can also be blended. Two or more kinds of crosslinking agents can also be used in combination. In terms of the properties of coating in the above-mentioned wire, it is preferably coated with an aqueous coating liquid, and the aforementioned resin or crosslinking agent is preferably a water-soluble or water-dispersible resin or compound. The polyester resin contained in the easy-adhesion resin layer is preferably a linear polyester composed of a dicarboxylic acid component and a diol component (diol component). As the above-mentioned dicarboxylic acid component, examples include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4-diphenyldicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, adipic acid, sebacic acid, phenylindanedicarboxylic acid, dimer acid, etc. Two or more of these components can be used. Furthermore, unsaturated polyacids such as maleic acid, fumaric acid, itaconic acid, etc. or hydroxycarboxylic acids such as p-hydroxybenzoic acid, p-(β-hydroxyethoxy)benzoic acid, etc. can be used in a smaller proportion together with these components. The proportion of the unsaturated polyacid component or the hydroxycarboxylic acid component is 10 mol% or less, preferably 5 mol% or less. As the dicarboxylic acid component of the above-mentioned polyester resin, by containing a component derived from naphthalenedicarboxylic acid, the refractive index increases, and it becomes easier to control the rainbow-like color under a fluorescent lamp. Also, the wet heat resistance can be improved. Of course, the polymerization step and the copolymerization step of making the polyester contain a naphthalenedicarboxylic acid component, etc. can be the so-called direct polymerization method or the transesterification method, and the dicarboxylic acid component such as the naphthalenedicarboxylic acid component can also be introduced in the form of its ester derivative. As the naphthalenedicarboxylic acid as described above, 2,6-naphthalenedicarboxylic acid is preferred. The proportion of the above naphthalenedicarboxylic acid component in the total dicarboxylic acid component constituting the polyester resin is preferably 20 mol% or more, more preferably 30 mol% or more, still more preferably 50 mol% or more, and even more preferably 60 mol% or more. If it is 20 mol% or more, the effect of increasing the refractive index of the easy-adhesion resin layer is significant and it is more suitable. The proportion of the above naphthalenedicarboxylic acid component in the total dicarboxylic acid component constituting the polyester resin may also be 100 mol%, but for the flexibility of the easy-adhesion resin layer, it is more preferably 95 mol% or less. As long as it is within the range of exerting the effects of the present invention, as the glycol component in the polyester resin, ethylene glycol, 1,3-propanediol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, 1,4-cyclohexanedimethanol, xylene glycol, dimethylolpropionic acid, glycerol, trimethylolpropane, poly(ethyleneoxy) glycol, poly(tetramethyleneoxy) glycol, epoxy alkane adduct of bisphenol A, etc. can also be further used, and two or more of these can be used. Also, when considering the weight of the cross-linking agent in the easy-adhesion resin layer and there is a risk of easily losing flexibility and cracks may occur in the easy-adhesion resin layer after the bending test, it is a preferred form that the polyester resin contains the dicarboxylic acid component represented by the following formula (1) and / or the glycol component represented by the following formula (2). (1) HOOC-(CH 2 )n-COOH (In the formula, n is an integer of 4≤n≤10) (2) HO-(CH 2 )n-OH (In the formula, n is an integer of 4≤n≤10) In this way, by containing the dicarboxylic acid component and / or glycol component having a carbon component with a specific length, flexibility is imparted to the polyester resin. Even after the bending test, it is easy to maintain the coating film, and cracks at the starting point of particle aggregates can be suppressed. Examples of the dicarboxylic acid component of formula (1) include adipic acid, sebacic acid, azelaic acid, etc. Also, examples of the glycol component of formula (2) include 1,4-butanediol, 1,6-hexanediol, etc. The polyester resin can be one that is dissolved or dispersed in water, or a water-soluble organic solvent (for example, an aqueous solution containing less than 50% by mass of alcohol, alkyl cellulose, ketone-based, ether-based), or an organic solvent (for example, toluene, ethyl acetate, etc.). When using the polyester resin as an aqueous coating solution, a water-soluble or water-dispersible polyester resin can be used, but for this water-solubilization or water-dispersion, it is preferred to carry out copolymerization with a compound containing a sulfonate group or a compound containing a carboxylate group. The number average molecular weight of the polyester resin is preferably 5,000 to 40,000, more preferably 10,000 to 30,000, and particularly preferably 12,000 to 25,000, from the viewpoints of film strength, ease of water dispersion, etc. The content of the solid component of the polyester resin in the solid component of the easy-to-adhere resin layer is preferably 20% by mass or more and 90% by mass or less, more preferably 30% by mass or more and 80% by mass or less, from the viewpoints of adjusting adhesion and refractive index, etc. In addition, the polyester resin may be a single one or a mixture of two or more. In the case of a mixture of two or more, the total of the polyester resin components is preferably the above composition. (Urethane resin) The urethane resin that can be used in the easy-to-adhere resin layer, as a constituent component, at least includes a polyol component, a polyisocyanate component, and optionally a chain extender. In the above urethane resin, these constituent components are mainly high molecular compounds copolymerized through urethane bonds. Including polycarbonate polyol as a constituent component of the urethane resin can make the coating film have flexibility, so it is one of the preferred forms. In addition, the constituent components of these urethane resins can be specified by nuclear magnetic resonance analysis, etc. (Polyurethane resin having a polycarbonate skeleton) In the polyurethane resin having a polycarbonate skeleton, the glycol component of the constituent components preferably contains an aliphatic polycarbonate polyol having excellent heat resistance and hydrolysis resistance. In the optical use of the present invention, from the viewpoint of preventing yellowing, it is preferable to use an aliphatic polycarbonate polyol. Examples of the aliphatic polycarbonate polyol include aliphatic polycarbonate diol, aliphatic polycarbonate triol, etc., but aliphatic polycarbonate diol is preferably used. Examples of the aliphatic polycarbonate diol as a constituent component of the urethane resin of the present invention include those obtained by reacting one or more of glycols such as ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, 1,8-nonanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, etc. with carbonate esters such as dimethyl carbonate, diphenyl carbonate, ethylene carbonate, phosgene, etc. The polyisocyanates that are constituent components of the urethane resin of the present invention include, for example, aromatic aliphatic diisocyanates such as xylylene diisocyanate, isophorone diisocyanate, 4,4-dicyclohexylmethane diisocyanate, alicyclic diisocyanates such as 1,3-bis(isocyanatomethyl)cyclohexane, aliphatic diisocyanates such as hexamethylene diisocyanate and 2,2,4-trimethylhexamethylene diisocyanate, or polyisocyanates obtained by pre-addition of single or plural of these compounds with trimethylolpropane and the like. The aforementioned polyisocyanates have no yellowing problem and are more suitable for optical applications requiring high transparency. Also, for the aforementioned polyisocyanates, the coating film is not too hard, can relieve the stress caused by shrinkage and swelling of photocurable resins and the like, and maintain adhesion, so it is more suitable. In order to impart water solubility to the urethane resin, a sulfonic acid (salt) group or a carboxylic acid (salt) group can be introduced (copolymerized) into the urethane molecular skeleton. Since the sulfonic acid (salt) group is strongly acidic and it is difficult to maintain moisture resistance due to its hygroscopic property, it is advisable to introduce a weakly acidic carboxylic acid (salt) group. Also, a nonionic group such as a polyoxyalkylene group can be introduced. In order to introduce a carboxylic acid (salt) group into the urethane resin, for example, a polyol compound having a carboxylic acid group such as dimethylolpropionic acid and dimethylolbutyric acid is introduced as a copolymerization component as the polyol component and neutralized with a salt former. Specific examples of the salt former include trialkylamines such as ammonia, trimethylamine, triethylamine, triisopropylamine, tri-n-propylamine, and tri-n-butylamine, N-methyl quinoline, N-ethyl quinoline and other N-alkyl quinolines, N-dialkylalkanolamines such as N-dimethylethanolamine and N-diethylethanolamine. These can be used alone or in combination of two or more. When a polyol compound having a carboxylic acid (salt) group is used as a copolymerization component to impart water solubility, when the total polyol component of the urethane resin is 100 mol%, the compositional molar ratio of the polyol compound having a carboxylic acid (salt) group in the urethane resin is preferably 3 to 60 mol%, more preferably 5 to 40 mol%. When the aforementioned compositional molar ratio is 3 mol% or more, the water dispersibility is good and it is more suitable. Also, when the aforementioned compositional molar ratio is 60 mol% or less, water resistance and moisture and heat resistance can be maintained, so it is more suitable. The glass transition temperature of the urethane resin in the present invention is preferably less than 0 °C, more preferably less than -5 °C. When the glass transition temperature is less than 0 °C, it is more suitable because appropriate flexibility is easily exhibited from the viewpoint of stress relaxation of the coating layer. In order to form a crosslinked structure in the easy-bonding resin layer, the easy-bonding resin layer can also be formed by including a crosslinking agent. By containing a crosslinking agent, the adhesion under high temperature and high humidity can be further improved. As specific crosslinking agents, urea-based, epoxy-based, melamine-based, isocyanate-based, oxazoline-based, carbodiimide-based, etc. can be mentioned. Among these, from the perspective of the stability of the coating solution over time and the effect of improving the adhesion under high temperature and high humidity treatment, melamine-based, isocyanate-based, oxazoline-based, carbodiimide-based crosslinking agents are preferred. Also, in order to promote the crosslinking reaction, a catalyst or the like can be appropriately used as needed. In the case where a crosslinking agent is included in the easy-bonding resin layer, the content of the crosslinking agent is preferably 5% by mass or more and 50% by mass or less in the total solid content of the coating layer. More preferably, it is 10% by mass or more and 40% by mass or less. If it is 10% by mass or more, the strength of the resin in the easy-bonding resin layer can be maintained, and the adhesion under high temperature and high humidity is good. If it is 40% by mass or less, the flexibility of the resin in the coating layer can be maintained, and the adhesion after repeated folding tests at normal temperature, high temperature and high humidity can be maintained, which is more suitable. Preferably, at least one compound selected from titanium compounds and zirconium compounds is contained in the easy-bonding resin layer of the present invention. The rainbow-like color (interference fringes) of the hard coating film is said to occur because the refractive index of the polyester film of the substrate (for example, 1.62 to 1.65) and the refractive index of the hard coating layer composed of an acrylic resin or the like (for example, 1.52) have a large difference. In order to reduce the refractive index difference between the layers to prevent the occurrence of interference fringes, it is very important to control the refractive index of the easy-bonding resin layer so that the refractive index difference between the polyester film and the easy-bonding resin layer and the refractive index difference between the easy-bonding resin layer and the hard coating layer become smaller. When controlling the refractive index of the easy-bonding resin layer composed of the binder resin or particles of the main component, it becomes easy to control by containing the above-mentioned compounds with a high refractive index. As titanium compounds, for example, water-soluble titanium chelates or water-soluble titanates, titanium oxide, titanium chloride, etc. can be mentioned, and among them, titanium dioxide (titania) is preferably used. As zirconium compounds, for example, water-soluble zirconium chelates or water-soluble zirconates, zirconium acetate, zirconium hydroxide, zirconium oxide, etc. can be mentioned, and among them, zirconium dioxide (zirconia) is preferably used. It is also preferred that the above-mentioned compounds with a high refractive index are in the form of particles. The average particle diameter of metal oxide particles such as zirconium dioxide is preferably 5 nm or more and 150 nm or less. More preferably, it is 10 nm or more and 100 nm or less, and even more preferably, it is 30 nm or more and 70 nm or less. By designing the average particle diameter of the metal oxide fine particles within the above range, the haze of the film can be reduced. Further, for the particulate metal compound, in order to obtain the buffering effect during repeated folding, it is desirable to design its particle diameter to be smaller than the film thickness of the easy-to-adhere resin layer, and preferably a particle diameter at which particle aggregation does not easily occur. If particle aggregation is suppressed, it is more suitable because it will not become the starting point of cracks or the like in the easy-to-adhere resin layer during folding. The addition amount of the particulate metal compound is preferably designed to be 0.1 mass% concentration or more and less than 15 mass% concentration with respect to the easy-to-adhere resin layer, in order to achieve the buffering effect during folding of the easy-to-adhere resin layer, and moreover, it will not conversely become the starting point of cracks or the like caused by particle aggregation. Furthermore, it is preferably 0.5 mass% concentration or more and 14 mass% concentration or less, and more preferably 1 mass% concentration or more and 13 mass% concentration or less. If particles are added within this range, aggregated particles are not likely to occur in the easy-to-adhere resin layer, and cracks will not occur in the easy-to-adhere resin layer during repeated folding caused by such aggregated particles. Therefore, it is more suitable that interference fringes are not easily generated after the folding test. The particulate metal compound in the easy-to-adhere resin layer preferably has a lower proportion in the folding part direction than in the bending direction. It is considered that fine cracks occurring during repeated folding are likely to occur in the folding part direction where a load is applied in the thickness direction. Therefore, by reducing the particle frequency in the folding part direction, the occurrence of cracks can be suppressed. It is considered that by reducing the particle frequency, the aggregation ratio of particles or the frequency of crack propagation can be reduced. The proportion of particles in each different direction can be confirmed by observing with a transmission electron microscope (TEM) of the cross section or the like. As a method of changing the proportion of the particulate metal compound in the easy-to-adhere resin layer in different directions, it is preferably formed by an in-line coating method. In the in-line coating method, there is a step of extending in at least one direction after coating a coating liquid containing the easy-to-adhere resin material. Therefore, it is considered that by adjusting the extension ratio, the proportion of particles in the easy-to-adhere resin layer can be changed. In order to prevent aggregation, a dispersant can also be used in combination with the particulate metal compound. As the dispersant used in the present invention, any polymer compound that can maintain the binder resin containing the emulsion, dissolve or disperse the following cross-linking agent, and disperse the metal oxide fine particles is acceptable. Specifically, well-known polymer dispersants such as polyethylene, polyacrylic acid, polycarboxylic acid, and polyurethane can be used. More specifically, as polyethylene polymers, polyvinyl alcohol, polyvinylpyrrolidone, polyvinyl acetate, polyvinyl esters, etc. and copolymers thereof can be used. As polyacrylic acid polymers, polyacrylic acid, sodium polyacrylate, ammonium polyacrylate, etc. and copolymers thereof can be used. As polycarboxylic acid polymers, polycarboxylic acid, sodium polycarboxylate, ammonium polycarboxylate, etc. and copolymers thereof can be used. As polyurethane polymers, polyurethane, etc. and copolymers thereof can be used. Also, copolymers of these or copolymers with sulfonic acid polymers, etc. can be used. Among them, as a dispersant for metal fine particles, an acrylic resin such as a polyacrylic acid polymer is preferred. It is more preferable that the metal oxide particles are particles having a dispersant on a part or all of their surfaces. By containing a dispersant, an effect of suppressing the aggregation of metal oxide particles can be obtained. By making the easy-to-adhere resin layer contain the particles, the transparency of the coating film can be maintained, and stress relaxation during the repeated folding test can be expected. By containing a dispersant, according to the coating film formation process, aggregation of metal oxide particles is less likely to occur in the easy-to-adhere resin layer, and cracks are less likely to occur, which is preferable. The method of surface-treating metal oxide particles with an acrylic resin is not particularly limited. Specifically, methods such as adding a mixture of metal oxide particles and an acrylic resin pre-mixed to a solvent and then dispersing it; and adding metal oxide particles and an acrylic resin in sequence in a solvent and dispersing them can be cited. As a device for performing such dispersion, a dissolver, a high-speed mixer, a homogenizer, a kneader, a ball mill, a roll mill, a sand mixer, a paint shaker, an SC mill, a ring mill, a needle mill, etc. can be used. As the amount of the dispersant added to the metal oxide particles, based on the mass of the metal oxide particles, it is preferably 5% by mass concentration or more and less than 40% by mass concentration. When the addition amount of the dispersant is 5% by mass concentration or more, it is preferable in terms of easily obtaining a good dispersion state of the metal oxide particles in the easy-to-adhere resin layer. When the addition amount of the dispersant is less than 40% by mass concentration, it is preferable in terms of easily utilizing the characteristics of the metal oxide particles to adjust the refractive index of the easy-to-adhere resin layer. Furthermore, an addition amount of 10% by mass concentration or more and 30% by mass concentration or less is more preferable. In the present invention, the refractive index of the easy-to-adhere resin layer is designed within a certain range to satisfy the thin film interference principle, thereby reducing rainbow-like colors (interference fringes). It is considered that by further filling a certain amount of a metal compound for adjusting the refractive index into the easy-to-adhere resin layer, it has an effect of buffering the damage caused to the easy-to-adhere resin layer during repeated folding. The thickness of the easy - adhesion resin layer that can suppress rainbow - like colors (interference fringes) can be adjusted to satisfy the formula 2nd = λb / 4. Here, n represents the refractive index of the easy - adhesion resin layer, d represents the thickness of the easy - adhesion resin layer, and λb represents the bottom wavelength of the reflection spectrum, which can be appropriately set within the range of 450 - 650 nm. In the polyester film with an easy - adhesion resin layer in the present invention, since it is designed such that the refractive indices in the bending direction and the direction of the folding part are different, it is preferable to control the refractive index of the easy - adhesion resin layer after considering each direction. Relative to the refractive index in the bending direction of the polyester film before laminating the hard - coat layer with the easy - adhesion resin layer, it is desirable to control the refractive index of the easy - adhesion resin layer to be lower. As the range of the refractive index difference between the easy - adhesion resin layer and the refractive index in the aforementioned bending direction, it is preferably greater than 0 and 0.070 or less. More preferably, it is 0.005 or more and 0.065 or less. Even more preferably, it is 0.010 or more and 0.060 or less. If the refractive index of the easy - adhesion resin layer is lower than that in the bending direction of the polyester film, the refractive index difference from the laminated hard - coat layer becomes smaller, and it is more suitable for effectively suppressing rainbow - like colors (interference fringes). Also, when the aforementioned refractive index difference is 0.070 or less, the refractive index difference from the polyester film does not become too large, and it is more suitable for effectively suppressing rainbow - like colors (interference fringes). Regarding the bending direction, when the easy - adhesion resin layer is bent inward, compressive stress is applied, and when it is bent outward, tensile stress is applied. Therefore, compared with resin components with a large amount of crystalline structure, it is preferably designed with resin components having a large amount of amorphous structure. More preferably, a metal compound is appropriately added to compensate for the refractive index. Relative to the refractive index in the direction of the folding part of the polyester film before laminating the hard - coat layer with the easy - adhesion resin layer, it is also desirable to control the refractive index of the easy - adhesion resin layer to be lower. As the range of the refractive index difference between the easy - adhesion resin layer and the refractive index in the aforementioned folding part direction, it is preferably 0.080 or more and 0.150 or less. More preferably, it is 0.085 or more and 0.14 or less. Even more preferably, it is 0.090 or more and 0.13 or less. If the refractive index is lower than that in the folding part direction of the polyester film, the refractive index difference from the laminated hard - coat layer becomes smaller, and it is more suitable for effectively suppressing rainbow - like colors (interference fringes). Also, when the aforementioned refractive index difference is 0.150 or less, the refractive index difference from the polyester film does not become too large, and it is more suitable for effectively suppressing rainbow - like colors (interference fringes). In the easy adhesion layer, in order to impart lubricity, it is preferable to add particles. The average particle diameter of the fine particles is preferably 2 μm or less. If the average particle diameter of the particles exceeds 2 μm, the particles tend to fall off from the easy adhesion layer. Examples of the particles contained in the easy adhesion layer include inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silicon dioxide, aluminum oxide, talc, kaolin, clay, calcium phosphate, mica, lithium bentonite, zirconium dioxide, tungsten oxide, lithium fluoride, calcium fluoride, etc., or organic polymer particles such as styrene-based, acrylic-based, melamine-based, phenylguanidine-based, polysiloxane-based particles, etc. These can be added to the easy adhesion layer alone, or two or more kinds can be added in combination. In the coating layer, in order to impart appropriate slidability, it is particularly preferable to use silicon dioxide particles having an average particle diameter of 200 nm or more and 700 nm or less. Further, as the addition amount of the particles for imparting lubricity, it is preferably less than 1% by mass with respect to the easy adhesion resin layer. If it is less than 1% by mass, there are a small number of particles in the easy adhesion resin layer that are larger than the thickness of the easy adhesion resin layer, so cracks are not easily propagated during folding, which is preferable. A state of 0.5% by weight or less is a more preferable aspect. As a method for coating the coating liquid, well-known methods can be used in the same manner as the above-mentioned coating layer. For example, reverse roll coating method, gravure coating method, kiss coating method, roller brush coating method, spraying method, air knife coating method, wire bar coating method, pipedoctor method, etc. can be mentioned, and these methods can be used alone or in combination. (Hard coating layer) When the polyester film of the present invention is located on the surface of a foldable display and used as a surface protective film for protecting the display, it is preferably provided with a hard coating layer on at least one of its surfaces. The hard coating layer is preferably located on the display surface side of the polyester film and is used in the display. As the resin for forming the hard coating layer, silicone-based, inorganic hybrid-based, acrylic-based, urethane acrylate-based, polyester acrylate-based, epoxy-based, etc. can be used without particular limitation. Further, two or more kinds of materials can be mixed and used, and particles such as inorganic fillers or organic fillers can also be added. As the resin for forming the hard coating, examples thereof include compounds having (meth)acrylate-based functional groups such as polyester (meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, and polysiloxane (meth)acrylate, or compounds containing functional groups having unsaturated double bonds such as allyl or vinyl. Further, in order to improve the hardness of the hard coating, a polyfunctional monomer may be used in combination. As the polyfunctional monomer, for example, trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, neopentyl glycol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and neopentyl glycol di(meth)acrylate can be exemplified. The above materials can be used alone or in combination of two or more materials. When the active energy ray for hardening the hard coating is ultraviolet light, it is preferably to add a photopolymerization initiator. The photopolymerization initiator can also be a radical polymerization system, a cationic polymerization system, or a mixed system of cationic polymerization and radical polymerization. However, due to the fast reaction rate and excellent productivity, a radical polymerization system is particularly preferred. As examples of the ultraviolet radical polymerization initiator, alkylbenzophenones, benzoins, benzophenones, phosphine oxides, ketals, anthraquinones, thioxanthones, azo compounds, peroxides, 2,3-dialkyldione compounds, disulfides, fluoroamine compounds, or aromatic sulfonium salts, titanocenes, phenyl glycollates can be mentioned, and they can be used alone or in combination of two or more. As more specific examples, acetophenone, 2,2-diethoxyacetophenone, p-dimethylacetophenone, p-dimethylaminopropiophenone, benzophenone, 2-chlorobenzophenone, 4,4'-dichlorobenzophenone, 4,4'-bisdiethylaminobenzophenone, Michler's ketone, benzil, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, methyl benzoyl formate, p-isopropyl-α-hydroxy isobutylphenone, α-hydroxy isobutylphenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone and other carbonyl compounds, tetramethylthiuram monosulfide, tetramethylthiuram disulfide, thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone and other sulfides, benzoyl peroxide, di(tert-butyl) peroxide and other peroxides can be mentioned. The addition amount of the photopolymerization initiator is preferably in the range of not less than 0.1 part by mass, more preferably not less than 1 part by mass, and not more than 30 parts by mass, more preferably not more than 20 parts by mass, based on 100 parts by mass of the active energy ray-curable resin. If the addition amount is not less than 0.1 part by mass, the hardness of the hard coating can be improved, which is more suitable. Further, if the addition amount is not more than 30 parts by mass, there is no risk of yellowing of the hard coating, and the hardening of the hard coating is also sufficient, which is more suitable. Furthermore, various additives can be included within the range that does not hinder the performance of the hard coating. Examples of various additives include polymerization inhibitors, crosslinking agents, antistatic agents, adhesion improvers, antioxidants, leveling agents, coupling agents, defoaming agents, fillers, solvents, antiglare agents, antireflection agents, inorganic fillers, or organic fillers, etc. From the viewpoint of suppressing the rainbow-like colors (interference fringes) of the hard coating film, the refractive index of the hard coating is preferably less than that of the easy-to-adhere resin layer. (Film thickness of the hard coating) The film thickness of the hard coating is preferably 1 to 50 μm. If it is 1 μm or more, it can be sufficiently hardened. In order to improve the pencil hardness, it is more preferably 5 μm or more. Also, by making the thickness 50 μm or less, curling caused by the hardening shrinkage of the hard coating can be suppressed, and the processability of the thin film can be improved. (Coating method) As the coating method of the hard coating, a Meyer bar, a gravure coater, a die coater, a knife coater, etc. can be used without particular limitation, and can be appropriately selected according to the viscosity and film thickness. (Hardening conditions) As the hardening method of the hard coating, energy rays such as ultraviolet rays and electron beams, or hardening methods caused by heat, etc. can be used. In order to reduce damage to the thin film, a hardening method by ultraviolet rays or electron beams, etc. is preferably used. (Pencil hardness) The pencil hardness of the hard coating is preferably 3H or more, and more preferably 4H or more. If the pencil hardness is 3H or more, it is not easily damaged and does not reduce visual recognition. Generally, a higher pencil hardness of the hard coating is preferred, but it is okay to be 9H or less, 8H or less, and 6H or less can also be used without problems in practical applications. (Properties of the hard coating) The hard coating in the present invention can be used for the purpose of improving the pencil hardness of the surface as described above and protecting the display, and a high transmittance is preferred. The transmittance of the hard coating film is preferably 85% or more, more preferably 87% or more, and even more preferably 88% or more. If the transmittance is 85% or more, sufficient visual recognition can be obtained. Generally, the higher the total light transmittance of the hard coating film, the better, but from the aspect of stable production, it is preferably 99% or less, and can also be 97% or less. Also, generally, the lower the haze of the hard coating film, the better, and it is preferably 3% or less. The haze of the hard coating film is more preferably 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visual recognition of the image can be improved. Generally, the lower the haze, the better, but from the aspect of stable production, it is preferably 0.1% or more, and can also be 0.3% or more. For the hard coating, other functions can also be further added. For example, a hard coating with added functions such as an antiglare layer, an antiglare and antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer having a certain pencil hardness as described above can also be preferably applied to the present invention. In addition, when using the base film as the substrate of the touch panel module, a hard coat layer may also be provided. When using, for example, an ITO layer as the transparent electrode layer of the touch panel module, in order to make the electrode pattern less visible, it is preferable to provide a refractive index adjustment layer between the base film and the transparent electrode layer. In this case, the hard coat layer itself may also serve as the refractive index adjustment layer, and furthermore, a refractive index adjustment layer may be laminated separately. [Examples] Next, the present invention will be described using examples and comparative examples. First, the evaluation methods of the characteristic values implemented by the polyester film of the present invention are shown below. (1) Intrinsic viscosity After pulverizing and drying the film or polyester resin, it is dissolved in a mixed solvent of phenol / tetrachloroethane = 60 / 40 (mass ratio). After subjecting this solution to centrifugal separation treatment to remove inorganic particles, an Ubbelohde viscometer is used to measure the flow-down time of a solution with a concentration of 0.4 (g / dl) at 30°C and the flow-down time of only the solvent. From the time ratio of these, using the formula of Huggins and assuming the Huggins constant is 0.38, the intrinsic viscosity is calculated. (2) Density The density is measured according to the method of JIS K7112:1999 (density gradient tube method). (Unit: g / cm 3 ). (3) Average particle diameter of particles in the polyester film or the easy-to-adhere resin layer The following method is used: The particles on the cross-section of the film are observed with a scanning electron microscope, 50 particles are observed, and their average value is taken as the average particle diameter. The particle diameter of non-spherical irregularly shaped particles can be calculated as the equivalent circular diameter. The equivalent circular diameter is the value obtained by dividing the area of the observed particle by pi (π), calculating the square root, and doubling it. (The unit varies depending on the average particle diameter, but mainly nm is used). (4) Refractive index of the polyester film before laminating the hard coat layer with the easy-to-adhere resin layer According to JIS K7142:2008 "Method for Measuring the Refractive Index of Plastics (Method A)", using an Abbe refractometer (NAR-4T manufactured by ATAGO Co., Ltd., measurement wavelength 589 nm), the refractive index in the length direction, the refractive index in the width direction, and the refractive index in the thickness direction of the polyester film before laminating the hard coat layer with the easy-to-adhere resin layer are obtained. (5) Refractive index of the easy-bonding resin layer The refractive index of the easy-bonding resin layer can be calculated by fitting the reflection spectrum measured using a spectrometer (product name: "UV-3150", manufactured by Shimadzu Corporation) and the reflection spectrum calculated from the optical model of a thin film using Fresnel coefficients. On the side opposite to the surface of the PET substrate where the easy-bonding resin layer is formed (the back surface; when the easy-bonding resin layer is formed on both sides, it is the surface of the easy-bonding resin layer on the side where the refractive index is not measured), a black plastic tape with a width larger than the measurement spot area (for example, product name: "Yamato vinyl tape NO200-38-21", manufactured by YAMATO CO., LTD., 38 mm width) is attached to prevent back reflection and the measurement is then carried out. (6) Flex resistance of the polyester film sample (bending radius: 1.5 mm) A polyester film sample with a size of 20 mm in the width direction × 110 mm in the flow direction is prepared. Using an unloaded U-shaped stretch tester (DLDMLH-FS manufactured by YUASA SYSTEM Machine Co., Ltd.), it is set at a bending radius of 1.5 mm and bent 200,000 times at a speed of 1 time per second. At this time, the sample has its two end portions at positions 10 mm on the long side fixed, and the bent portion is 20 mm × 90 mm. Here, FIG. 1 is a schematic diagram for showing the bending radius when folding a foldable display. Considering the case where the polyester film is disposed on the inner surface in its folded state, the position of symbol 11 in FIG. 1 is set at 1.5 mm and a model bending test is carried out. After the bending treatment, the sample is placed flat with the inner side of the bend facing downwards and visually observed. ○: No cracks or deformations are visible on the sample. ×: There are cracks or creases on the sample, and when placed horizontally, the maximum height of the bulge is 5 mm or more. (7) Flexibility resistance of the polyester film sample (bending radius 0.5 mm) In the same manner as the above bending test, it is set at a bending radius of 0.5 mm and bent 200,000 times at a speed of 1 time / second. Here, Fig. 1 is a schematic diagram for showing the bending radius when folding the foldable display. When considering arranging the polyester film on the inner surface in its folded state, the position of symbol 11 in Fig. 1 is set at 0.5 mm, and a model bending test is conducted. Observe the outer film surface of the bent part at 700 times magnification with a digital microscope (RH8800 manufactured by HIROX Co., Ltd.) to check for wrinkles (cracks). Different from the visual inspection of the flexibility resistance with the above bending radius of 1.5 mm, in this test where the bending radius is reduced to 0.5 mm, a hard coating or other components are laminated or attached, and an attempt is made to evaluate in a state close to the actual use state of the foldable display. Different from the visual observation with the aforementioned bending radius of 1.5 mm, this is a test for detecting fine defects that are difficult to detect visually, such as easy breakage or easy introduction of cracks. ○: There are no defects on the outer film surface of the bend. ×: Breakage or wrinkles (cracks) can be seen on the outer film surface of the bend. (8) Penetration depth after unloading the test force Cut the sample to be about 2 cm square, and fix the opposite side of the measurement surface on a micro cover glass 18×18 mm (manufactured by Matsunami Glass Ind., Ltd.) with an adhesive (CEMEDINE (registered trademark) Highsuper 30). After attachment and fixation, leave it at room temperature for more than 12 hours, and then use a dynamic ultra-micro hardness tester "DUH-211" (manufactured by Shimadzu Corporation) to measure the penetration depth (μm) after unloading the test force under the following conditions. 《Measurement conditions》 Test mode: Load-unload test Indenter used: Triangular pyramid indenter with a ridge angle of 115 degrees Indenter elastic modulus: 1.140×10 6 N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50 mN Loading speed: 4.44 mN / sec Loading hold time: 2 sec Unloading hold time: 0 sec (9) Total light transmittance, haze Use a haze meter (NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd.) to measure the polyester film laminated with an easy-adhesion resin layer as a sample. (10) Maximum shrinkage rate Cut a sample film of a polyester film laminated with an easy-to-adhere resin layer into a size of 10 mm in length × 250 mm in width, align the long side with the direction to be measured, attach marks at intervals of 200 mm, and measure the interval A between the marks under a constant tension of 5 g. Next, place the sample film in an oven in a gas environment at 150 °C for 30 minutes without load, then take it out of the oven and cool it to room temperature. Then, find the interval B between the marks under a constant tension of 5 g, and calculate the shrinkage rate (%) by the following formula. In addition, the above shrinkage rate is measured at three equally divided positions in the width direction of the sample film, and the average value of the three points is taken as the shrinkage rate (%). Shrinkage rate (%) = [(A - B) × 100] / A For both the bending direction and the folding direction, cut the sample film in different ways for the length and width of the sample film respectively, conduct the measurement, and take the data in the direction with the larger measured value as the maximum shrinkage rate (%). (11) Refractive index of the hard coat layer The refractive index of the hard coat layer can be calculated by fitting the reflection spectrum measured using a spectrometer (product name "UV-3150", manufactured by Shimadzu Corporation) and the reflection spectrum calculated from the optical model of a multilayer film using Fresnel coefficients. The reflectance of the hard coat layer is measured by coating the hard coat layer composition on a polyethylene terephthalate (PET) substrate with a thickness of 50 μm without applying the easy-to-adhere resin layer, hardening it to form a hard coat layer with a thickness of 1 - 10 μm, and attaching a black plastic tape (e.g., product name "Yamato vinyl tape NO200-38-21", manufactured by YAMATO CO., LTD., 38 mm width) on the opposite side (back side) of the surface of the PET substrate where the hard coat layer is formed to prevent back reflection. (12) Improvement of interference fringes of the hard coating film (rainbow-like color) Cut the hard coating film into an area of 50 mm (in the film width direction) × 110 mm (in the film length direction) to make a sample film. Attach a black glossy tape (Nitto Denko vinyl tape No21; black) to the opposite side of the obtained sample film from the hard coat layer. Make the hard coat layer side of the sample film the top surface, use a 3-wavelength type daylight white (National Palook, F.L 15EX-N 15W) as the light source, and observe it from an oblique upper position where strong reflection can be visually seen (the distance from the light source is 40 - 60 cm, and the angle relative to the film surface is 15 - 45°). The results of visual observation are classified according to the following criteria. In addition, the observation is carried out by 5 people proficient in this evaluation, and the most frequent grade is taken as the evaluation grade. When the number of two grades is the same, the center of the three grades is adopted. For example, when there are 2 people for ◎ and ○ each and 1 person for △, ○ is adopted; when there is 1 person for ◎, 2 people for ○ and △ each, ○ is adopted; when there are 2 people for ◎ and △ each and 1 person for ○, ○ is adopted. ◎: No rainbow-like color is seen from all angles. ○: A slight rainbow-like color is seen from a certain angle. △: A slight rainbow-like color is seen. ×: A clear rainbow-like color is seen. (13) Flexural test of the hard coating film Using the hard coating film, a sample film is made by cutting it into a size of 50 mm in the width direction × 110 mm in the flow direction. Using the obtained sample film, in the same way as the above-mentioned bending test method, it is folded with the easy-adhesion resin layer and the hard coating layer on the inside, set at a bending radius of 3.0 mm, and a bending test is carried out 200,000 times at a speed of 1 time / second. With respect to the bent part of the sample film after the test, the bent inside of the sample is placed downward and placed flat, and visual observation is carried out. ○: No cracks or deformations are seen on the sample. ×: Cracks or creases are seen on the sample. (14) Observation of interference fringes (rainbow-like colors) after the flexural test of the hard coating film With respect to the bent part of the sample film after the flexural test of the above hard coating film, observation is carried out in the same way as the improvement of interference fringes. Specifically, on the opposite side of the hard coating layer on the obtained sample film, a black glossy tape (Nitto Denko vinyl tape No21; black) is attached. With the hard coating layer side of the sample film as the top surface, a 3-wavelength type daylight white (National Palook, F.L 15EX-N 15W) is used as the light source, and observation is carried out at a position relationship where strong reflection can be seen visually from the upper oblique direction (distance from the light source 40 - 60 cm, angle 15 - 45° with respect to the film surface). The results of visual observation are classified according to the following criteria. In addition, the observation is carried out by 5 people proficient in this evaluation, and the most frequent grade is taken as the evaluation grade. When the number of two grades is the same, the center of the three grades is adopted. For example, when there are 2 people for ◎ and ○ each and 1 person for △, ○ is adopted; when there is 1 person for ◎, 2 people for ○ and △ each, ○ is adopted; when there are 2 people for ◎ and △ each and 1 person for ○, ○ is adopted. ◎: No rainbow-like color is seen from all angles. ○: A slight rainbow-like color is seen from a certain angle. △: A slight rainbow-like color is seen. ×: A clear rainbow-like color is seen. This test is implemented for the purpose of detecting interference fringes caused by minute adhesion defects such as interface peeling or cracks between the hard coating layer, the easy-adhesion resin layer, and the polyester film layer by evaluating the interference fringes in this test. (15) Pencil hardness: The pencil hardness of the hard coating film is used as a sample and measured according to JIS K 5600-5-4:1999 under a load of 750 g and a speed of 1.0 mm / s. In the present invention, 3H or above is regarded as qualified. (Preparation of polyethylene terephthalate particles (R1)) As an esterification reaction device, a continuous esterification reaction device composed of a three-stage completely mixed tank having a stirring device, a fractionator, a raw material inlet, and a product outlet is used. Set TPA to 2 tons / hr. Relative to 1 mole of TPA, set EG to 2 moles, and set antimony trioxide to an amount such that the Sb atom becomes 160 ppm relative to the produced PET. Continuously supply such a slurry to the first esterification reaction vessel of the esterification reaction device, and react it at 255 °C under normal pressure with an average residence time of 4 hours. Next, continuously take out the reaction product in the above first esterification reaction vessel out of the system and supply it to the second esterification reaction vessel. In the second esterification reaction vessel, relative to the produced polymer (produced PET), supply 8% by mass of the EG distilled from the first esterification reaction vessel, and further add an EG solution containing magnesium acetate in an amount such that the Mg atom becomes 65 ppm relative to the produced PET, and an EG solution containing TMPA in an amount such that the P atom becomes 20 ppm relative to the produced PET, and react it at 260 °C under normal pressure with an average residence time of 1.5 hours. Subsequently, continuously take out the reaction product in the above second esterification reaction vessel out of the system and supply it to the third esterification reaction vessel, and further add an EG solution containing TMPA in an amount such that the P atom becomes 20 ppm relative to the produced PET, and react it at 260 °C under normal pressure with an average residence time of 0.5 hour. Continuously supply the esterification reaction product produced in the above third esterification reaction vessel to a three-stage continuous polycondensation reaction device for polycondensation, and further filter it with a filter medium of a stainless steel sintered body (90% of particles with a nominal filtration accuracy of 5 μm will be filtered out) to obtain polyethylene terephthalate particles (R1) with an intrinsic viscosity of 0.62 dl / g. (Preparation of polyethylene terephthalate particles (R2)) Regarding the manufacturing steps of polyethylene terephthalate particles (R1), except for adjusting the residence time of the third esterification reaction, adjust the intrinsic viscosity to 0.580 dl / g in the same manner to obtain polyethylene terephthalate particles (R2). (Preparation of polyethylene terephthalate particles (R3)) Using a rotary vacuum polymerization device, perform solid-phase polymerization on polyethylene terephthalate particles (R1) at a reduced pressure of 0.5 mmHg and at 220 °C for a varying time to produce polyethylene terephthalate particles (R3) with an intrinsic viscosity of 0.75 dl / g. (Polymerization of Copolyester Resin) The polymerization of the copolyester resins (a1) to (a3) for forming the easy-adhering resin layer is carried out as follows. In a stainless-steel autoclave equipped with a stirrer, a thermometer, and a partial reflux condenser, 410.3 parts by mass of dimethyl 2,6-naphthalenedicarboxylate, 46.3 parts of sebacic acid, 42.5 parts by mass of sodium 5-sulfoisophthalate dimethyl ester, 175.6 parts by mass of ethylene glycol, 29.2 parts by mass of diethylene glycol, 204.2 parts by mass of 1,6-hexanediol, and 0.5 parts by mass of tetra-n-butyl titanate are placed, and an ester exchange reaction is carried out at a temperature from 160 °C to 220 °C for 4 hours. Then, the temperature is raised to 255 °C, the reaction system is slowly depressurized, and then the reaction is carried out under a reduced pressure of 30 Pa for 1 hour and 30 minutes to obtain the copolyester resin (a1). The obtained copolyester resin is light yellow and transparent. The composition of the copolyester resin (a1) is shown in Table 1. Furthermore, by changing the raw materials and carrying out the operation in the same manner, copolyester resins (a2) and (a3) having the compositions described in Table 1 are obtained. The results of the composition and weight-average molecular weight measured by 1H-NMR for these copolyester resins are shown in Table 1. Table 1 (Preparation of Aqueous Dispersion of Copolyester Resin) In a reactor equipped with a stirrer, a thermometer, and a reflux device, 25 parts by mass of the copolyester resin (a1) and 15 parts by mass of ethylene glycol tertiary butyl ether are added, and the mixture is heated and stirred at 110 °C to dissolve the resin. After the resin is completely dissolved, 60 parts by mass of water is slowly added to the above polyester solution while stirring. After the addition, the liquid is cooled to room temperature while stirring to prepare an aqueous dispersion (Aw-1) of a milky white copolyester having a solid content of 25% by mass. Similarly, by using the copolyester resins (a2) to (a3) instead of the copolyester resin (a1), aqueous dispersions are prepared as aqueous dispersions (Aw-2) to (Aw-3). (Polymerization of urethane resin) In a four-necked flask equipped with a stirrer, a Dimroth cooler, a nitrogen inlet tube, a silica gel drying tube, and a thermometer, 72.96 parts by mass of 1,3-bis(isocyanatomethyl)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, 85.00 parts by mass of acetonitrile as a solvent, and 5.00 parts by mass of N-methylpyrrolidone were charged. Under a nitrogen atmosphere, it was stirred at 75 °C for 3 hours to confirm that the reaction solution reached the specified amine equivalent. Then, after cooling this reaction solution to 40 °C, 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer solution. Subsequently, in a reaction vessel equipped with a homogenizer capable of high-speed stirring, 450 g of water was added and adjusted to 25 °C. While stirring and mixing at 2000 min -1 it was added with an isocyanate group-terminated prepolymer for water dispersion. Then, under reduced pressure, by removing a part of acetonitrile and water, a water-soluble polyurethane resin (B-1) with a solid content of 35% by mass was prepared. (Preparation of self-crosslinking type polyurethane-based resin aqueous solution) 100 parts by mass of a polyester diol (OHV: 2000 eq / ton) having a composition containing adipic acid, 1,6-hexanediol, and neopentyl glycol (molar ratio: 4 / 2 / 3) and 41.4 parts by mass of xylylene diisocyanate were mixed. Under a nitrogen stream, it was reacted at 80 to 90 °C for 1 hour and then cooled to 60 °C. 70 parts by mass of tetrahydrofuran was added for dissolution to obtain a urethane prepolymer solution (NCO / OH ratio: 2.2, free isocyanate group: 3.30% by mass). Then, the aforementioned urethane prepolymer solution was adjusted to 40 °C, and then 45.5 parts by mass of a 20% by mass aqueous solution of sodium bisulfite was added and stirred vigorously, and it was reacted at 40 to 50 °C for 30 minutes. After confirming the disappearance of the free isocyanate group content (in terms of solid content), it was diluted with emulsified water to obtain a self-crosslinking type polyurethane-based resin aqueous solution (B-2) with a solid content of 20% by mass and terminated with sodium bisulfite and containing an isocyanate group. (Polymerization of blocked isocyanate compound) In a flask equipped with a stirrer, a thermometer, and a reflux condenser, 100 parts by mass of a polyisocyanate compound having a trimeric isocyanate structure based on hexamethylene diisocyanate (DURANATETPA manufactured by Asahi Kasei Chemicals), 55 parts by mass of propylene glycol monomethyl ether acetate, and 30 parts by mass of polyethylene glycol monomethyl ether (average molecular weight 750) were placed, and maintained at 70 °C for 4 hours under a nitrogen atmosphere. Thereafter, the temperature of the reaction solution was lowered to 50 °C, and 47 parts by mass of methyl ethyl ketoxime was dropped in. The infrared spectrum of the reaction solution was measured to confirm the disappearance of the absorption of the isocyanate group, and a blocked polyisocyanate liquid having a solid content of 75% by mass was obtained. (Preparation of water-dispersible blocked isocyanate) Water was added to the blocked polyisocyanate liquid obtained above to obtain a blocked polyisocyanate aqueous dispersion (C-1) having a solid content of 40% by mass. (Polymerization of water-soluble carbodiimide compound) In a flask equipped with a thermometer, a nitrogen inlet tube, a reflux condenser, a dropping funnel, and a stirrer, 200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidization catalyst were put in, 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 slowly added thereto at 50 °C to obtain a yellow transparent water-soluble carbodiimide compound (C-2) having a solid content of 40% by mass. (Melamine-based crosslinking agent) As the melamine-based crosslinking agent, BECKAMINE (registered trademark) M-3 (solid content concentration 60%) manufactured by DIC Corporation (melamine-based crosslinking agent (C-3)) was used. ( (Polymerization of oxazoline-based crosslinking agent) In a flask equipped with a thermometer, a nitrogen inlet tube, a reflux condenser, a dropping funnel, and a stirrer, a mixture of 58 parts by mass of ion-exchanged water and 58 parts by mass of isopropyl alcohol as an aqueous medium, and 4 parts by mass of a polymerization initiator (2,2'-azobis(2-methylamidinopropane) dihydrochloride) were put in. On the other hand, 16 parts by mass of 2-isopropenyl-2- as a polymerizable unsaturated monomer having an oxazoline group was put in the dropping funnel. A mixture of 32 parts by mass of oxazoline, 32 parts by mass of methoxypolyethylene glycol acrylate (average number of moles of ethylene glycol added: 9 moles, manufactured by Shin-Nakamura Chemical Co., Ltd.), and 32 parts by mass of methyl methacrylate was added dropwise over 1 hour at 70 °C under a nitrogen atmosphere. After the addition was completed, the reaction solution was stirred for 9 hours and then cooled to obtain a water-soluble resin having a solid content concentration of 40% by mass and having an oxazoline group. (Zirconia particles) In a 3-liter glass container, 2283.6 g of pure water and 403.4 g of oxalic acid dihydrate were added and heated to 40 °C to prepare a 10.72% by mass oxalic acid aqueous solution. While stirring this aqueous solution, 495.8 g of zirconium oxicarbonate powder (ZrOCO 3 , manufactured by AMR International Corp., converted to ZrO 2 contains 39.76% by mass) was added. After mixing for 30 minutes, it was heated at 90 °C for 30 minutes. Then, 1747.2 g of a 25.0% by mass aqueous solution of tetramethylammonium hydroxide (manufactured by Tama Chemical Industry Co., Ltd.) was slowly added over 1 hour. At this point, the mixture was in a slurry state and contained 4.0% by mass in terms of ZrO 2 conversion. The slurry was transferred to a stainless steel autoclave container and subjected to hydrothermal treatment at 145 °C for 5 hours. The product after this hydrothermal treatment was completely sol-gelated without any un-gelled matter. The resulting sol contained 4.0% by mass in terms of ZrO 2 , had a pH of 6.8, and an average particle diameter of 19 nm. Also, the sol was adjusted with pure water to a ZrO 2 concentration of 2.0% by mass, and the measured transmittance was 88%. Observation of the particles by a transmission electron microscope showed that most were agglomerated particles of primary particles about 7 nm in size of ZrO 2 . Using an ultrafiltration device, while slowly adding pure water, 4000 g of the zirconia sol obtained by the above hydrothermal treatment with a ZrO 2 concentration of 4.0% by mass was washed and concentrated to obtain 953 g of a zirconia sol with a ZrO 2 concentration of 13.1% by mass, a pH of 4.9, and a transmittance of 76% at a ZrO 2 concentration of 13.1% by mass. In 300 g of the zirconia sol obtained by the above washing and concentration 2 To a zirconia sol with a concentration of 13.1 mass%, 3.93 g of a 20 mass% aqueous citric acid solution and 11.0 g of a 25 mass% aqueous tetramethylammonium hydroxide solution were added, and then it was concentrated using an ultrafiltration device. As a result, 129 g of ZrO 2 A high-concentration zirconia sol (D-1) with a concentration of 30.5 mass%. The obtained high-concentration zirconia sol had a pH of 9.3 and an average particle size of 19 nm. Also, the zirconia sol had no sediment and was stable for more than 1 month under the condition of 50 °C. (Zirconia aqueous dispersion) The above-obtained zirconia sol was mixed with a polyacrylic acid dispersant (manufactured by Toagosei Co., Ltd.: Aron A-30SL) to prepare a zirconia aqueous dispersion with a solid content concentration of 13 mass%. A zirconia aqueous dispersion D-1 with a ratio of 3 mass% of the dispersant and 10% of zirconia in the solid content concentration was obtained. (Titanium dioxide particles) 12.09 kg of TiO 2 An aqueous titanium tetrachloride solution containing 7.75 mass% of titanium tetrachloride (manufactured by OSAKA Titanium Technologies Co., LTD.) on a conversion basis was mixed with 4.69 kg of ammonia water containing 15 mass% of ammonia (manufactured by Ube Industries, Ltd.) to prepare a white slurry with a pH of 9.5. Then, the slurry was filtered and washed with pure water to obtain a hydrated titanic acid filter cake with a solid content of 10 mass% and a weight of 9.87 kg. Then, 11.28 kg of hydrogen peroxide water containing 35 mass% of hydrogen peroxide (manufactured by Mitsubishi Gas Chemical Co., Ltd.) and 20.00 kg of pure water were added to the filter cake, and it was heated at 80 °C with stirring for 1 hour, and then 57.52 kg of pure water was added to obtain 98.67 kg of TiO 2 An aqueous titanic acid peroxide solution containing 1 mass% of titanic acid peroxide on a conversion basis. The aqueous titanic acid peroxide solution was transparent yellowish-brown and had a pH of 8.5. Next, 4.70 kg of a cation exchange resin (manufactured by Mitsubishi Chemical Corporation) was mixed into 98.67 kg of the above-mentioned aqueous titanic acid peroxide solution, and 12.33 kg of SnO 2 An aqueous potassium stannate solution containing 1 mass% of potassium stannate (manufactured by Showa Chemical Industry Co., Ltd.) on a conversion basis was slowly added thereto with stirring. Then, after separating the cation exchange resin containing potassium ions, etc., it was placed in an autoclave (manufactured by耐压硝子工業股份有限公司, 120 L) and heated at 165 °C for 18 hours. Next, after cooling the obtained mixed aqueous solution to room temperature, it was concentrated using an ultrafiltration membrane device (ACV-3010 manufactured by Asahi Kasei Corporation) to obtain 9.90 kg of a hydrosol (D-2) containing titanium-based fine particles with a solid component content of 10% by mass. The solid component contained in the sol obtained in this manner was measured by the above method, and the result was titanium-based fine particles (primary particles) composed of a composite oxide containing titanium and tin having a rutile crystal structure. Furthermore, the content of the metal components contained in the titanium-based fine particles was measured, and as a result, each metal component was TiO 2 87.2% by mass, SnO 2 11.0% by mass, and K 2 O 1.8% by mass. Also, the pH of the mixed aqueous solution was 10.0. Furthermore, the hydrosol containing the aforementioned titanium-based fine particles was transparent milky white, the average particle diameter of the aforementioned titanium-based fine particles contained in the hydrosol was 35 nm, and furthermore, the distribution frequency of coarse particles having a particle diameter of 100 nm or more was 0%. Furthermore, the refractive index of the obtained titanium-based fine particles was regarded as 2.42. (Titanium Dioxide Dispersion) The titanium-based fine particles obtained above were mixed with a polyacrylic acid dispersant (manufactured by Toagosei Co., Ltd.: AronA-30SL) to prepare a titanium dioxide aqueous dispersion with a solid component concentration of 13% by mass. A titanium dioxide aqueous dispersion D-2 was obtained in which 3% by mass of the solid component concentration was the dispersant and 10% was titanium dioxide. (Zirconia / Titanium Dioxide Mixed Dispersion) The zirconia sol obtained above was mixed with titanium dioxide-based fine particles and a polyacrylic acid dispersant (manufactured by Toagosei Co., Ltd.: AronA-30SL) to prepare a zirconia / titanium dioxide mixed aqueous dispersion with a solid component concentration of 13% by mass. A mixed dispersion D-3 was obtained in which 3% by mass of the solid component concentration was the dispersant, 7.5% was zirconia, and 2.5% was titanium dioxide. (Silica Particles) As the silica particles, colloidal silica with a particle diameter of 40 nm and a solid component concentration of 30% by mass was used as D-4. (Silica Particles) In order to impart slipperiness, silica particles with a particle diameter of 450 nm and a solid component concentration of 40% by mass were used as D-5. (Surfactant) In order to improve the leveling property of the coating film when forming an easy-to-adhere resin layer, a silicone-based surfactant with a solid component concentration of 100% by mass was used as E-1. (Preparation of Coating Liquid for Easy Adhesion Layer Formation) Mix the following coating agents to prepare coating liquid P-1. Water: 47.52 parts by mass Isopropyl alcohol: 25.00 parts by mass Polyester resin (Aw-1): 17.75 parts by mass Water-dispersible blocked isocyanate compound (C-1): 4.76 parts by mass Zirconia / titania mixed water dispersion (D-3): 4.88 parts by mass Silica particles (D-5): 0.06 parts by mass (silica sol with an average particle diameter of 450 nm and a solid content concentration of 40% by mass) Polysiloxane-based surfactant (E-1): 0.03 parts by mass (polysiloxane-based, solid content concentration 100% by mass) Similarly, prepare coating liquids P-2 to P-14 by setting the blending ratios of the respective coating agents as shown in Table 2. Table 2 (Example 1) Supply poly(ethylene terephthalate) granules (R1) to an extruder and melt them at 285°C. Filter the polymer through a filter medium of a stainless steel sintered body (nominal filtration accuracy: 95% of particles with a diameter of 10 μm will be filtered out), extrude it into a sheet from a nozzle, and then use the electrostatic casting method to cool and solidify it by contacting a casting roller with a surface temperature of 30°C to produce an unoriented film. Heat the unoriented film uniformly at 75°C using a heating roller, heat it to 85°C using a non-contact heater, and perform a roller stretching (longitudinal stretching) of 1.4 times. After that, apply the above-mentioned coating liquid for easy adhesion resin layer formation (P-1) to both sides of the obtained uniaxially oriented film by roll coating, and dry it at 80°C for 20 seconds. In addition, adjust it so that the final (after biaxial stretching) coating amount after drying becomes 0.09 g / m 2 . Then, guide it to a tenter, preheat it at 105°C, laterally stretch it to 4.0 times at 95°C, fix the width, apply heat setting at 230°C for 5 seconds, and further relax it by 4% in the width direction at 180°C to obtain a 50-μm-thick poly(ethylene terephthalate) film. Using a Mayer rod, apply a hard coat coating liquid (the following H-1) to one side of the poly(ethylene terephthalate) film having the above-mentioned easy adhesion resin layer so that the film thickness after drying becomes 10 μm. After drying it at 80°C for 1 minute, irradiate ultraviolet rays (cumulative light quantity: 200 mJ / cm 2 ), and obtain a hard coat film. (Coating Liquid for Hard Coating Formation: H-1) 95 parts by mass of a urethane acrylate-based hard coating agent (BEAMSET (registered trademark) 577 manufactured by Arakawa Chemical Industries, Ltd., solid content concentration 100%), 5 parts by mass of a photoinitiator (IRGACURE (registered trademark) 184 manufactured by BASF JAPAN Co., Ltd., solid content concentration 100%), and 0.1 part by mass of a leveling agent (BYK307 manufactured by BYK JAPAN Co., Ltd., solid content concentration 100%) were mixed and diluted with a solvent of toluene / MEK = 1 / 1 to prepare a coating liquid (H-1) for hard coating formation with a solid content concentration of 40%. (Examples 2 - 3) Except for changing the draw ratio in the longitudinal direction as described in Table 3, the operation was carried out in the same manner as in Example 1 to obtain a polyester film and a hard coating film. (Example 4) Except for changing the draw ratio in the width direction to 5.5 times and the heat setting temperature to 190°C, the operation was carried out in the same manner as in Example 1 to obtain a polyester film and a hard coating film. (Examples 5 - 14) Except for changing the coating liquid for easy adhesion resin layer formation to P-2 - P-11 as described in Table 3, the operation was carried out in the same manner as in Example 1 to obtain a polyester film and a hard coating film. (Examples 15, 16) Except for changing to the following H-2 and H-3 as described in Table 3 in place of the hard coating liquid H-1, the operation was carried out in the same manner as in Example 1 to obtain a polyester film and a hard coating film. (Coating Liquid for Hard Coating Formation: H-2) 30 parts by mass of neopentyl glycol tri- and tetraacrylate (ARONIX (registered trademark) M-306 manufactured by Toagosei Co., Ltd., solid content concentration 100%), 65 parts by mass of polyester acrylate (ARONIX (registered trademark) M9050 manufactured by Toagosei Co., Ltd., solid content 100%), 5 parts by mass of a photoinitiator (IRGACURE (registered trademark) 907 manufactured by BASF JAPAN Co., Ltd., solid content concentration 100%), and 0.1 part by mass of a leveling agent (BYK307 manufactured by BYK JAPAN Co., Ltd., solid content concentration 100%) were mixed and diluted with a solvent of toluene / MEK = 1 / 1 to prepare a hard coating liquid (H-2) with a concentration of 40% by mass. (Coating Liquid for Hard Coating Formation: H-3) 0.1 part by mass of a leveling agent (BYK307 manufactured by BYK JAPAN Co., Ltd., concentration 100%) was added to 100 parts by mass of a hard coating material (OPSTAR (registered trademark) Z7503 manufactured by JSR Corporation, concentration 75%) and diluted with methyl ethyl ketone to prepare a hard coating liquid (H-3) with a solid content concentration of 40% by mass. (Comparative Example 1) A polyester film and a hard coating film were obtained in the same manner as in Example 1, except that stretching in the longitudinal direction was not performed, and stretching was performed only in the width direction to obtain a transverse uniaxially stretched film. (Comparative Example 2) A polyester film and a hard coating film were obtained in the same manner as in Example 4, except that stretching in the longitudinal direction was not performed, and stretching was performed only in the width direction to obtain a transverse uniaxially stretched film. (Comparative Examples 3 to 7) A polyester film and a hard coating film were obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 220 °C to obtain the PET granules and thickness described in Table 1. (Comparative Example 8) A polyester film and a hard coating film were obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to 3.4 times. (Comparative Examples 9 to 11) A polyester film and a hard coating film were obtained in the same manner as in Example 1, except that the coating solution was changed to P-12 to P-14 as described in Table 3. The evaluation results are shown in Table 3. Table 3 The "intrinsic viscosity", "density", "refractive index of the PET film", "continuous bending test with a bending radius of 1.5 mm", "continuous bending test with a bending radius of 0.5 mm", "indentation depth after loading", "total light transmittance", "haze", and "maximum heat shrinkage rate" in the "PET film" column are the evaluation results of a film having an easy-to-adhere resin layer and no hard coating on the PET film. As in Examples 1 to 16, when the refractive index of the polyester film having an easy-to-adhere layer is in a fixed range, and a hard coating is provided on the easy-to-adhere resin layer of the polyester film formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds and a polyester resin, the pencil hardness of the hard coating can also be satisfied, and the improvement of the interference fringes after the continuous bending test is also good. In each example, the cross-section was observed with a transmission electron microscope. As a result, the proportion of the particles contained in the easy-to-adhere resin layer in the direction of the folding portion in the width direction was smaller than that in the bending direction in the longitudinal direction. In Comparative Examples 1 to 8 where the film-forming conditions of the polyester film were changed, the refractive index of the polyester film having an easy-to-adhere resin layer was not in the optimal range, so a hard coating film with satisfactory pencil hardness could not be obtained. In Comparative Example 9 where the easy-to-adhere resin layer was changed, neither titanium compounds nor zirconium dioxide compounds were contained, and the refractive index of the easy-to-adhere resin layer was not sufficiently adjusted. Therefore, no improvement in interference fringes was observed before the bending test of the hard coating film. In addition, the dispersibility of the silica added in place of the aforementioned metal compound was poor, and the interference fringes after the bending test also became worse. In Comparative Example 10 where the easy-adhesion resin layer was changed, although improvement in interference fringes before the bending test was observed, the dispersibility of silica added in place of the titanium compound and zirconium dioxide compound was poor, and after the bending test, the interference fringes deteriorated due to cracks in the easy-adhesion resin layer and the like. In Comparative Example 11 where the easy-adhesion resin layer was changed and the polyester resin was not contained, a large amount of titanium / zirconium mixed oxide had to be added for refractive index adjustment. Under its influence, the metal oxides in the coating film also aggregated, and the interference fringes after the bending test deteriorated. The hard coating films of each Example and Comparative Example were adhered to the organic EL module through an adhesive layer with a thickness of 25 μm to fabricate a foldable display of the smartphone type with a bending radius equivalent to that in FIG. 1 of 3 mm and capable of being folded in half at the center of the whole. The hard coating film was disposed on the surface of one display continuous through the folding portion, and the hard coating layer was disposed so as to be on the surface of the display. Those using the hard coating film of each Example satisfied the operation and visual recognition as a smartphone that can be folded in half at the center for carrying. Also, the surface did not dent due to external force. On the other hand, in the foldable display using the hard coating film of each Comparative Example, as the usage frequency increased, image distortion was felt to occur at the folding portion of the display, and interference fringes were generated, resulting in poor visual recognition, which was not preferable. Also, surface dents and damages were confirmed. [Industrial Applicability] In the foldable display using the hard coating film for a foldable display of the present invention, while maintaining mass productivity, the hard coating film located on the surface of the foldable display does not deform after repeated folding, so there is no image disturbance at the folding portion of the display. In particular, a portable terminal or an image display device equipped with a foldable display using the hard coating film of the present invention as a surface protective film provides beautiful images, is functional, and has excellent convenience such as portability. 1: Foldable display 2: Polyester film for surface protective film of foldable display 11: Bending radius 21: Folding portion 22: Bending direction (direction orthogonal to the folding portion) FIG. 1 is a schematic diagram for showing the bending radius when the foldable display in the present invention is folded. FIG. 2 is a schematic diagram for showing the bending direction of the polyester film constituting the hard coating film for a foldable display in the present invention. 1: Foldable display 11: Bending radius

Claims

1. A hard coating for a foldable display, comprising a polyester film having an easy-adhesive resin layer and a hard coating layer sequentially on at least one side of a polyester film with a thickness of 10 to 80 μm, wherein the easy-adhesive resin layer is formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds and a polyester resin, wherein the polyester resin comprising the dicarboxylic acid component and the diol component of the polyester resin contained in the easy-adhesive resin layer contains at least a naphthalene dicarboxylic acid component as a dicarboxylic acid component, the easy-adhesive resin layer further contains a crosslinking agent, and the polyester film having the easy-adhesive resin layer and the hard coating layer thereon satisfies all of the following conditions (1) to (4): (1) The refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the folding direction is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less (4) The density is 1.380 g / cm3 or more (here, the bending direction refers to the direction orthogonal to the folding portion when the polyester film is folded).

2. The hard coating for a foldable display as claimed in claim 1, wherein the refractive index of the aforementioned easy-adhesive resin layer is lower than the refractive index of the polyester film having the easy-adhesive resin layer and the refractive index in the direction of bending and folding of the laminated hard coating, and is higher than the refractive index of the aforementioned hard coating.

3. The hard coating for a foldable display as claimed in claim 1, wherein the refractive index of the aforementioned easy-adhesive resin layer satisfies the following conditions (5) and (6): (5) The refractive index of the easy-adhesive resin layer is lower than the refractive index of the polyester film having the easy-adhesive resin layer and before the hard coating layer is laminated, and the difference in refractive index is greater than 0 and less than 0.07; (6) The refractive index of the easy-adhesive resin layer is lower than the refractive index of the polyester film having the easy-adhesive resin layer and before the hard coating layer is laminated, and the difference in refractive index is more than 0.080 and less than 0.

150.

4. The hard coating for a foldable display as claimed in claim 1, wherein the polyester film having the easy-adhesive resin layer and the hard coating layer prior to it has a total light transmittance of 85% or more, a haze of 3% or less, and a maximum heat shrinkage rate of 6% or less.

5. A hard coating for a foldable display as claimed in any of claims 1 to 4, wherein the thickness of the aforementioned hard coating is 1 to 50 μm.

6. A foldable display, wherein the foldable display of claim 5 is configured as a surface protective film by means of a hard coating film so that the hard coating layer is located on the surface, wherein the hard coating film is configured to be a single hard coating film continuous through the folding portion of the foldable display.

7. A portable terminal having a foldable display as claimed in claim 6.

8. A hard coating for a foldable display, comprising a polyester film having an easy-adhesive resin layer and a hard coating layer sequentially on at least one side of a polyester film with a thickness of 10-80 μm, wherein the easy-adhesive resin layer is formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds and a polyester resin, wherein the polyester resin comprising the dicarboxylic acid component and the diol component of the polyester resin contained in the easy-adhesive resin layer contains at least a naphthalene dicarboxylic acid component as a dicarboxylic acid component, the hard coating layer having a pencil hardness of 3H or higher, and the polyester film having the easy-adhesive resin layer and the hard coating layer prior to the lamination of the hard coating layer satisfies all of the following conditions (1) to (6): (1) The refractive index in the bending direction is 1.590-1.620; (2) The refractive index in the folding direction is 1.670-1.

700. (3) The refractive index in the thickness direction is 1.520 or less. (4) The density is 1.380 g / cm3 or more. (5) The refractive index of the easy-bonding resin layer is lower than the refractive index in the bending direction of the polyester film before the easy-bonding resin layer and the laminated hard coating, and the difference in refractive index is greater than 0 and less than 0.

07. (6) The refractive index of the easy-bonding resin layer is lower than the refractive index in the folding direction of the polyester film before the easy-bonding resin layer and the laminated hard coating, and the difference in refractive index is more than 0.080 and less than 0.150 (here, the bending direction refers to the direction orthogonal to the folding part when the polyester film is folded).

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