Hard coating film for foldable display and use thereof

Abstract

To provide a hard coat film for a folding type display which does not generate a folding mark or a crack in a folding part and is excellent in suppression of an iris-like color (interference spot) affected by a fine crack or the like of an easily adhesive resin layer or the like.SOLUTION: A hard coat film for a folding-type display, comprising an easily adhesive resin layer and a hard coat layer in this order on at least one surface of a polyester film having a thickness of 10 to 80 μ m, wherein the easily adhesive resin layer is obtained by curing a composition containing at least one compound selected from a titanium compound and a zirconium compound, and a polyester resin, and the polyester film having the easily adhesive resin layer and before laminating the hard coat layer satisfies characteristics in a specific range.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hard coat film for a folding display, a folding display, and a mobile terminal device, and more particularly to a folding display and a mobile terminal device that are less likely to suffer image distortion due to film deformation even when folded repeatedly, and the hard coat film for the folding display. [Background technology]

[0002] As mobile devices become thinner and lighter, smartphones and other mobile devices are becoming more and more popular. While mobile devices are required to have a variety of functions, they also need to be convenient. For this reason, the most popular mobile devices must be able to be operated with one hand for simple operations and have a small screen size of around 6 inches, as they are designed to be stored in a pocket or similar.

[0003] On the other hand, tablet devices with screen sizes of 7 to 10 inches are highly functional and are intended for not only video content and music, but also business use, drawing, reading, etc. However, they cannot be operated with one hand, are less portable, and have issues with convenience.

[0004] To achieve these goals, a method has been proposed of connecting multiple displays to make them more compact (see Patent Document 1), but this method has not become widespread because the bezel remains, resulting in a truncated image and reduced visibility.

[0005] In recent years, mobile devices incorporating flexible and foldable displays have been proposed, allowing users to conveniently carry around large-screen mobile devices without image interruption.

[0006] In conventional displays and mobile terminal devices that do not have a folding structure, the display surface can be protected with an inflexible material such as glass. However, in foldable displays, when a full-surface display is used via a folding section, it is necessary to use a flexible hard-coated film or the like that can protect the surface. However, in foldable displays, the area corresponding to the folding section is repeatedly folded, causing problems such as deformation of the film in that area over time and distorting the image displayed on the display. In addition to surface protection films, foldable displays also use films in various parts such as polarizing plates, retardation plates, touch panel substrates, display cell substrates such as organic electroluminescence (EL) displays, and backside protective members, and these films are also required to be durable against repeated folding.

[0007] As a method for improving durability, a method of partially changing the film thickness has been proposed (see Patent Document 2), but this method has the problem of being poorly suited for mass production.

[0008] A method for adjusting the refractive index in the bending direction of a polyester film has also been proposed (see Patent Document 3), but as the refractive index in the bending direction is reduced, the pencil hardness when a hard coat is applied decreases, resulting in a problem of reduced surface protection function for displays. Furthermore, lowering the refractive index in one direction improves deformation when folded, but increases uniaxial orientation in the folding direction, resulting in problems such as cracks or breakage at the folded portion.

[0009] On the other hand, the hard coat film also requires visibility and design. Therefore, in order to suppress glare and iridescent colors (interference fringes) caused by reflected light when viewed from any angle, it is common to provide a multilayer antireflection layer on top of the hard coat layer, in which a high refractive index layer and a low refractive index layer are alternately laminated. However, in recent years, three-wavelength fluorescent lamps have become mainstream in order to reproduce daylight color, making interference fringes caused by reflected light more visible. Furthermore, there is an increasing demand for cost reductions through the simplification of antireflection layers. Therefore, there is a demand for a hard coat film that suppresses interference fringes as much as possible without the addition of an antireflection layer.

[0010] As a method for suppressing interference fringes as described above, a method of providing one or two optical adjustment layers with adjusted refractive indexes on a polyester film has been proposed, but with polyester films, it is necessary to consider durability against repeated bending. For example, an optical adjustment layer containing excessive metal fine particles generates interference fringes due to microcracks originating from the fine particles, so a fully satisfactory optical adjustment layer has not been obtained. [Prior art documents] [Patent documents]

[0011] [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 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention seeks to solve the problems associated with conventional display components such as those described above, and aims to provide a foldable display that is easy to mass-produce and does not pose a risk of image distortion at the foldable portion after repeated folding, as well as a mobile terminal device equipped with such a foldable display. The present invention seeks to provide a hard coat film for a foldable display that does not produce fold marks or cracks at the foldable portion and that can effectively suppress iridescent colors (interference patterns) that are affected by fine cracks in the easy-adhesion resin layer, etc. [Means for solving the problem]

[0013] That is, the present invention comprises the following: 1. A hard coat film for a folding display, comprising an easy-adhesion resin layer and a hard coat layer, in that order, on at least one surface of a polyester film having a thickness of 10 to 80 μm, wherein the easy-adhesion resin layer is formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds, and a polyester resin, and the polyester film having the easy-adhesion resin layer and before laminating the hard coat layer satisfies the following conditions (1) to (4): (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) Refractive index in the thickness direction is 1.520 or less (4) Density is 1.380 g / cm 3 End (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.) 2. The hard coat film for a folding display described in 1 above, wherein the refractive index of the easy-adhesion resin layer is lower than the refractive index in the bending direction and the refractive index in the direction of the fold of the polyester film having the easy-adhesion resin layer before laminating the hard coat layer, and higher than the refractive index of the hard coat layer. 3. The polyester film for a folding display according to the above item 1 or 2, wherein the refractive index of the easy-adhesion resin layer satisfies the following conditions (5) and (6): (5) The refractive index of the easy-adhesion resin layer is lower than the refractive index of the polyester film having the easy-adhesion resin layer and before laminating the hard coat layer in the bending direction, and the difference in refractive index is greater than 0 and not greater than 0.07. (6) The refractive index of the easy-adhesion resin layer is lower than the refractive index of the polyester film having the easy-adhesion resin layer and before laminating the hard coat layer in the direction of the folded portion, and the difference in refractive index is 0.080 or more and 0.150 or less. 4. A polyester film for a folding display according to any one of claims 1 to 3, wherein the polyester resin contained in the easy-adhesion resin layer 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 diol component constituting the polyester resin. 5. A hard coat film for a folding display according to any one of claims 1 to 4 above, wherein the polyester film having the easy-adhesion resin layer and before laminating the hard coat layer has a total light transmittance of 85% or more, a haze of 3% or less, and a maximum heat shrinkage of 6% or less. 6. The hard coat film for a folding display according to any one of the above items 1 to 5, wherein the hard coat layer has a thickness of 1 to 50 μm. 7. A foldable display in which the hard coat film for a foldable display according to 6 above is arranged as a surface protective film so that the hard coat layer is positioned on the surface, and a single continuous hard coat film is arranged across the folding portion of the foldable display. 8. A mobile terminal device having the foldable display described in item 7 above. [Effects of the Invention]

[0014] A foldable display using the hard coat film for a foldable display of the present invention maintains mass productivity, while the hard coat film does not develop cracks at the foldable portion and does not deform after repeated folding. Furthermore, cracks at the foldable portion, lifting at the interface between the hard coat and the easy-adhesion resin layer, lifting at the interface between the easy-adhesion resin layer and the polyester film, and iridescent colors (interference patterns) caused by fine cracks can be effectively suppressed, and image distortion does not occur at the foldable portion of the display. Mobile terminal devices equipped with a foldable display using such a hard coat film provide beautiful images, are highly functional, and are convenient in terms of portability and other factors. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a schematic diagram showing the bending radius when the foldable display of the present invention is folded. [Figure 2] FIG. 2 is a schematic diagram showing the bending direction of a polyester film constituting the hard coat film for a foldable display in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] (display) The term "display" as used herein refers to display devices in general, and examples of the display include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs. LCDs, organic EL displays, and inorganic EL displays that have a bendable structure are preferred. Organic EL displays and inorganic EL displays that can reduce the number of layers are particularly preferred, and organic EL displays that have a wide color gamut are even more preferred.

[0017] (foldable display) A foldable display is a single continuous display that can be folded in half or otherwise when carried. Folding reduces the size by half, improving portability. The bending radius of a foldable display is preferably 5 mm or less, and more preferably 3 mm or less. A bending radius of 5 mm or less allows for a thinner display when folded. A smaller bending radius is preferable, but the smaller the bending radius, the more likely it is that creases will form. A bending radius of 0.1 mm or more is preferable, but it can also be 0.5 mm or more, or even 1 mm or more. Even a bending radius of 1 mm can achieve a practically thin display when carried. The bending radius when folded is measured at the location indicated by the symbol 11 in the schematic diagram of Figure 1 and refers to the radius of the inside of the folded portion when folded. The surface protection film, described below, may be located on either the outside or inside of the folded portion of the foldable display. Furthermore, the foldable display may be tri-folded or quadruple-folded, or may be a rollable type, all of which are considered to fall within the scope of the foldable display of the present invention.

[0018] The hard coat film for a folding display of the present invention may be used in any part of a folding display as long as it is a component of the folding display. Below, a typical configuration of a folding display and the parts in which the hard coat film of the present invention can be used will be described using an organic EL display as an example. Hereinafter, the hard coat film for a folding display of the present invention may be simply referred to as the hard coat film of the present invention.

[0019] (foldable OLED display) The essential component of a foldable organic EL display is an organic EL module, but a circular polarizer, a touch panel module, a surface protective film, a back protective film, etc. may also be provided as needed. (organic EL module) The general structure of an organic EL module is composed of an electrode, an electron transport layer, an emitting layer, a hole transport layer, and a transparent electrode.

[0020] (touch panel module) It is preferable that a mobile terminal device has a touch panel. When an organic EL display is used, it is preferable that a touch panel module is disposed above the organic EL display or between the organic EL module and the circular polarizer. The touch panel module has a transparent substrate such as a film and a transparent electrode disposed thereon. The hard coat film of the present invention can be used as this transparent substrate. When used as the transparent substrate of a touch panel, it is preferable to provide a refractive index adjustment layer.

[0021] (Circular polarizer) Circular polarizers prevent deterioration of image quality due to reflection of external light by components inside a display. A circular polarizer has a linear polarizer and a retardation plate. A linear polarizer has a protective film on at least the viewing side of the polarizer. A protective film may also be provided on the side of the polarizer opposite the viewing side, or a retardation plate may be directly laminated on the polarizer. The retardation plate may be a resin film having a retardation, such as a polycarbonate or cyclic olefin, or a resin film provided with a retardation layer made of a liquid crystal compound. The hard-coated film of the present invention can be used as a polarizer protective film. In these cases, when the base film of the hard-coated film of the present invention is a polyester film, the slow axis direction of the polyester film is preferably parallel or perpendicular to the absorption axis direction of the polarizer. A deviation of up to 10 degrees, preferably up to 5 degrees, from this parallel or perpendicular orientation is permitted.

[0022] (Surface protection film) When an impact is applied to a display from above, there is a risk of the circuits in an organic EL module or a touch panel module being disconnected, so a surface protective film is often provided. The hard coat film of the present invention is used as this surface protective film. Surface protective films include those called cover windows that are incorporated into the outermost surface of the display, and those called after-films that can be attached, peeled off, and replaced by the user. In either case, the hard coat film of the present invention is used. The hard coat layer is provided on the surface of a folding display with the hard coat layer on the viewing side. The hard coat layer may be provided on both sides.

[0023] (Back protection film) It is also preferable that a protective film is provided on the back side of the display, and the hard coat film of the present invention can be used as this protective film on the back side.

[0024] The hard coat film of the present invention may be any other than those described above, as long as it is used in a folded portion of a component of a folding display. Among these, the hard coat film of the present invention is preferably used as a cover window surface protective film, an after-surface protective film, a base film for a touch panel module, or a back surface protective film, and more preferably as a cover window surface protective film or an after-surface protective film.

[0025] Furthermore, the hard coat film of the present invention does not necessarily need to be used for all of the above-mentioned applications of the foldable display. In the foldable display, a hard coat film having a base material such as a polyester film, a polyimide film, a polyamide film, a polyamideimide film, a polycarbonate film, an acrylic film, a triacetyl cellulose film, a cycloolefin polymer film, a polyphenylene sulfide film, or a polymethylpentene film can be used appropriately according to suitability.

[0026] When the substrate film constituting the hard coat film of the present invention is a polyester film, it may be a single-layer film made of one or more types of polyester resin, or when two or more types of polyester are used, it may be a multilayer structure film or an ultra-multilayer laminate film with a repeating structure.

[0027] Examples of polyester resins used in the polyester film that is the base film of the hard coat film include polyester films made of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers containing these resin components as the main components. Among these, stretched polyethylene terephthalate films are particularly preferred in terms of mechanical properties, heat resistance, transparency, cost, etc.

[0028] When a polyester copolymer is used for the polyester film that serves as the base film of the hard coat 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. 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 glycols having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerization components in the copolymer is preferably less than 20% by mass. A content of less than 20% by mass is preferred because film strength, transparency, and heat resistance are maintained.

[0029] In addition, in the production of polyester films that serve as the base film for hard coat films, the intrinsic viscosity of at least one type of resin pellets is preferably in the range of 0.50 to 1.0 dL / g. When the intrinsic viscosity is 0.50 dL / g or higher, the impact resistance of the resulting film is improved, making it less likely that external impacts will cause breaks in the internal circuits of the display. On the other hand, when the intrinsic viscosity is 1.00 dL / g or lower, the increase in filtration pressure of the molten fluid is prevented from becoming too large, making it easier to operate the film production stably.

[0030] The thickness of the polyester film serving as the base film of the hard coat film is preferably 10 to 80 μm, and more preferably 25 to 75 μm. A thickness of 10 μm or more provides improved pencil hardness and impact resistance, while a thickness of 80 μm or less is advantageous for weight reduction and is excellent in flexibility, processability, handleability, etc.

[0031] The surface of the hard-coated film of the present invention may be smooth or uneven, but since it is used as a surface cover for a display, deterioration of optical properties due to unevenness is undesirable. The haze of the polyester film after laminating an easy-adhesion resin layer and before laminating a hard-coat layer is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. A haze of 3% or less can improve image visibility. The lower the haze limit, the better, but from the standpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0032] As mentioned above, in order to reduce haze, it is better if the unevenness on the film surface is not too large. However, in order to provide a certain degree of slipperiness from the viewpoint of ease of handling, unevenness can be formed by blending particles into the base polyester film or by coating the base polyester film with a coating layer containing particles as an easy-adhesion resin layer during the film formation process.

[0033] The method of incorporating particles into the base polyester film can be a known method. For example, they can be added at any stage of polyester production, but preferably they can be added as a slurry dispersed in ethylene glycol or the like at the stage of esterification, or after the completion of the transesterification reaction and before the start of the polycondensation reaction, to proceed with the polycondensation reaction. Alternatively, they can be added by a method of blending a slurry of particles dispersed in ethylene glycol or water with polyester raw materials using a vented kneading extruder, or a method of blending dried particles with polyester raw materials using a kneading extruder.

[0034] Among these, a method in which aggregate inorganic particles are homogeneously dispersed in a monomer liquid that will become a part of the polyester raw material, and then the filtered product is added to the remainder of the polyester raw material before, during, or after the esterification reaction is preferred. This method facilitates homogeneous dispersion of the particles and high-precision filtration of the slurry, since the monomer liquid has a low viscosity. Furthermore, when the monomer liquid is added to the remainder of the raw material, the particles are well dispersible and new aggregates are unlikely to form. From this perspective, it is particularly preferred to add the monomer liquid to the remainder of the raw material at a low temperature before the esterification reaction.

[0035] Furthermore, the number of protrusions on the film surface can be further reduced by a method (masterbatch method) in which a polyester containing particles is prepared in advance and then the pellets are kneaded and extruded with pellets containing no particles.

[0036] The polyester film substrate may contain various additives, such as antistatic agents, UV absorbers, and stabilizers, as long as the total light transmittance remains within a preferred range.

[0037] The total light transmittance of the polyester film having an easy-adhesion resin layer but no hard coat layer is preferably 85% or more, more preferably 87% or more. A transmittance of 85% or more can ensure sufficient visibility. The total light transmittance of the polyester film is preferably 85% or more in order to increase the total light transmittance of the hard coat film described below. The higher the total light transmittance of the polyester film, the better, but from the viewpoint of stable production, it is preferably 99% or less, and may be 97% or less.

[0038] The maximum heat shrinkage of a polyester film having an easy-adhesion resin layer but no hard coat layer after heat treatment at 150°C for 30 minutes is preferably 6% or less, more preferably 5% or less. A heat shrinkage of 6% or less can suppress flatness defects such as curling and undulation during HC processing. The lower the maximum heat shrinkage, the better, but it is preferably -1% or more, and more preferably 0% or more. Here, a negative value indicates expansion after heating, and a value of -1% or more is preferable because the flatness is good.

[0039] In order to provide the hard-coated film for a folding display of the present invention with sufficient pencil hardness, the polyester film preferably has an easy-adhesion resin layer and, in the absence of the hard-coat layer, has the following properties. When a conventional substrate polyester film is laminated with a hard-coat layer and then subjected to pencil hardness evaluation, it is believed that the pencil hardness is reduced due to deformation of the film in the thickness direction. In the present invention, it is preferable to set the indentation depth of the substrate polyester film in this state after unloading the test force in the thickness direction using a dynamic ultramicrohardness tester described below within a specific range. In pencil hardness evaluation of a hard-coated film using the substrate polyester film, high hardness can be achieved, which is preferable. The indentation depth of the substrate polyester film in this state after unloading the test force in the thickness direction is preferably 1.5 μm or less, more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. If the indentation depth after unloading the test force (the final deformation amount under load) is 1.5 μm or less, the film is less likely to deform in the thickness direction in a pencil hardness evaluation of the hard coat film after laminating a hard coat layer, and the pencil hardness can be increased. Increasing the pencil hardness of the hard coat film reduces the likelihood of scratches and dents on the display surface, improving the visibility of the display. Although it can be said that the lower the indentation depth after unloading the test force, the better, in terms of stable production and saturation of the effect, a depth of 0.3 μm or more is preferable, and even more preferably 0.5 μm or more.

[0040] In order to reduce the indentation depth after the test force is removed, it is effective to adjust the refractive index in the thickness direction of a polyester film having an easy-adhesion resin layer but no hard coat layer to 1.520 or less. As a means for adjusting the refractive index to 1.520 or less, as will be described later, examples of conditions include adjusting the stretch ratio in the bending direction or folding direction to a high value, setting the stretching temperature in the bending direction or folding direction to a low value, or setting the heat setting temperature to a high value, within a range in which other physical properties and the refractive index in the bending direction or folding direction can be controlled within preferred ranges.

[0041] The non-hard-coated surface of the hard-coated film of the present invention may be coated with an adhesive or subjected to a treatment to improve adhesion to the hard-coated layer.

[0042] Examples of surface treatment methods include roughening treatments such as sandblasting and solvent treatment, and oxidation treatments such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone / ultraviolet irradiation treatment, flame treatment, chromic acid treatment, and hot air treatment, and any of these methods can be used without particular limitation.

[0043] It is also preferable to improve adhesion by using an adhesion-improving layer such as an easy-adhesion resin layer. The easy-adhesion resin layer can be made of any resin, such as an acrylic resin, a polyester resin, a polyurethane resin, or a polyether resin, and can be formed by a general coating method, preferably a so-called in-line coating method.

[0044] The polyester film can be produced, for example, through a polymerization step in which inorganic particles are homogeneously dispersed in a monomer liquid that becomes a part of the polyester raw material, the resulting dispersion is filtered, and the resulting dispersion is added to the remainder of the polyester raw material to polymerize the polyester; and a film formation step in which the resulting polyester is melt-extruded into a sheet through a filter, cooled, and stretched to form a substrate film.

[0045] Next, a method for producing a polyester film substrate will be described in detail using an example in which polyethylene terephthalate (hereinafter sometimes referred to as PET) pellets are used as the raw material for the substrate film, but the present invention is not limited to this. Furthermore, the number of layers, such as a single layer or a multilayer structure, is not limited.

[0046] After mixing and drying PET pellets in a predetermined ratio, the mixture is fed into a known melt lamination extruder, extruded through a slit die into a sheet, and cooled and solidified on a casting roll to form an unstretched film. While a single extruder is sufficient for a single-layer film, multilayer films can be produced using two or more extruders and two or more manifolds or merging blocks (e.g., merging blocks with rectangular merging sections) to laminate the multiple film layers that make up the outermost layers, extrude a two or more layer sheet from the die, and cool it on a casting roll to form an unstretched film.

[0047] In this case, it is preferable to perform high-precision filtration to remove foreign matter contained in the resin at any location where the molten resin is maintained at about 280°C during melt extrusion. The filter material used for high-precision filtration of the molten resin is not particularly limited, but a stainless steel sintered filter material is preferred because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, and Cu and high-melting-point organic matter.

[0048] Furthermore, the filtration particle size (initial filtration efficiency 95%) of the filter material is preferably 20 μm or less, particularly preferably 15 μm or less. If the filtration particle size (initial filtration efficiency 95%) of the filter material exceeds 20 μm, foreign matter of 20 μm or more in size cannot be sufficiently removed. Although high-precision filtration of molten resin using a filter material with a filtration particle size (initial filtration efficiency 95%) of 20 μm or less may reduce productivity, it is preferable to obtain a film with fewer protrusions due to coarse particles.

[0049] (Refractive index in bending direction) In the present invention, the refractive index in at least one of the longitudinal direction (machine flow direction) and width direction of the polyester film having an easy-adhesion resin layer and before laminating a hard coat layer is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. The refractive index in the bending direction of the polyester film having an easy-adhesion resin layer and before laminating a hard coat layer is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. Here, the bending direction refers to the direction perpendicular to the folding portion (reference numeral 21) expected in the use of a foldable display, as indicated by reference numeral 22 on the polyester film (reference numeral 2) in FIG. 2. A refractive index of 1.590 to 1.620 in at least one of the longitudinal direction and width direction is preferred because it reduces deformation during repeated folding and does not degrade the image quality of the foldable display. A refractive index of 1.591 to 1.600 is more preferred. Of course, the bending direction is preferably the bending direction. If the refractive index is 1.590 or higher, there is no risk of cracks occurring in the folded portion direction after the bending test described below, and of course no breakage occurs, so the visibility of the display can be maintained at a good level. The refractive index of the polyester film can be effectively adjusted by adjusting the stretching ratio and stretching temperature. In addition, a relaxation process in the stretching direction and multi-stage stretching may be used to adjust the refractive index. When multi-stage stretching is performed, it is preferable to make the stretching ratio in the second stage and thereafter higher than the stretching ratio in the first stage. Note that the difference in refractive index between the polyester film having an easy-adhesion resin layer and before laminating the hard coat layer and the polyester film alone without the easy-adhesion resin layer is negligibly small.

[0050] By controlling the refractive index in at least one of the longitudinal direction (machine flow direction) and width direction of the polyester film having an easy-adhesion resin layer and before laminating the hard coat layer within the above range, and more preferably by controlling the refractive index in the bending direction within the above range, fatigue due to compressive stress applied to the inside of the fold when folded can be reduced. Fatigue due to compressive stress is thought to occur mainly in crystalline parts, and the fewer crystals there are in the bending direction, the less fatigue there is. Therefore, it is thought that lowering the refractive index reduces the amount of oriented crystals in the bending direction, thereby suppressing compressive fatigue.

[0051] Furthermore, the creep phenomenon caused by tensile stress applied to the outside of the fold when folded can be suppressed by reducing the refractive index. Fatigue due to tensile stress is thought to occur mainly in the amorphous portion, and repeated stress causes molecular chains to align, resulting in deformation. It can be assumed that the fewer molecular chains aligned in the bending direction, the less deformation due to alignment. Furthermore, since fatigue due to tension can be suppressed with fewer amorphous portions, a higher degree of crystallinity, i.e., a higher density, is preferable.

[0052] In the present invention, the stretching ratio of the unstretched polyester sheet in at least one of the longitudinal direction (machine flow direction) and the transverse direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. The stretching direction is preferably the bending direction. A stretching ratio of 1.2 times or more is preferred because it prevents deformation during post-processing, such as hard coating, while a stretching ratio of 2.0 times or less is preferred because it prevents thickness unevenness in the film. The stretching temperature is preferably 75 to 120°C, more preferably 75 to 105°C. Heating methods during stretching can be conventionally known, such as hot air heating, roll heating, and infrared heating. Setting the stretching temperature to 75 to 120°C can prevent significant thickness unevenness due to stretching at the above stretching ratio. Furthermore, by stretching at a temperature as low as possible within a range that does not cause significant thickness unevenness, the refractive index in the thickness direction can be reduced.

[0053] (Refractive index in the direction of the fold) The refractive index in the direction perpendicular to the direction in which the refractive index of the polyester film having the above-mentioned easy-adhesion resin layer and before laminating the hard coat layer is 1.590 to 1.620 is preferably 1.670 to 1.700. That is, the refractive index in the direction perpendicular to the bending direction (the direction of the folded portion) is preferably 1.670 to 1.700. By adjusting the refractive index to 1.670 to 1.700, deformation when folded in the bending direction can be reduced. By adjusting the refractive index to 1.700 or less, cracks and breaks in the direction of the folded portion can be suppressed. By adjusting the refractive index to 1.670 or more, flexibility in the bending direction and surface hardness can be improved. A refractive index of 1.680 to 1.695 is more preferable. Methods for adjusting the refractive index in the direction perpendicular to the bending direction include the stretching ratio, stretching preheating temperature, stretching temperature, multi-stage stretching, and film relaxation. The stretching ratio is preferably 4.0 to 6.0, more preferably 4.4 to 6.0. The preheating temperature for stretching in the direction perpendicular to the bending direction is preferably 70 to 110°C. When multi-stage stretching is performed in the direction perpendicular to the bending direction, it is preferable to use a higher stretch ratio in the second and subsequent stages than in the first stage. The film may be relaxed by 1 to 10% in either the machine direction (longitudinal direction) or the perpendicular direction (width direction).

[0054] (Refractive index in the thickness direction) The refractive index in the thickness direction of a polyester film having an easy-adhesion resin layer and before laminating a hard coat layer is preferably 1.520 or less. By setting the refractive index to 1.520 or less, even if the refractive index in the bending direction is designed to be low, a decrease in the hardness of the film surface can be suppressed, thereby achieving both flexibility and surface hardness. By setting the refractive index to 1.520 or less, the indentation depth in the thickness direction after unloading the test force can be reduced, and the hardness of the film surface, particularly the pencil hardness of the hard coat film after laminating a hard coat layer, can be improved. The refractive index is more preferably 1.515 or less, even more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. A low refractive index in the thickness direction is preferable, but from the perspective of stable production, it is preferably 1.3 or more, and may even be 1.4 or more. A refractive index of 1.410 or more is particularly preferred. It can be said that the above range can be achieved by increasing the stretching ratio in both the bending direction and the folding direction. However, in order to control the refractive index in the thickness direction after controlling the refractive index in the bending direction and width direction to a preferred range, it is preferable to set the conditions while checking the balance of the conditions for each process in the film-forming process.

[0055] Methods for controlling the refractive index in the thickness direction within the above range include the preheating temperature, stretching temperature, and stretching ratio in the bending direction, and the preheating temperature, stretching temperature, multi-stage stretching, high-ratio stretching, or heat setting temperatures in the folding direction. 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°C to 120°C. The stretching ratio in the bending direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. By lowering the stretching temperature and stretching at a low stretching ratio, the refractive index in the thickness direction can be effectively reduced while maintaining the flexibility in the bending direction. The preheating temperature for stretching in the folding direction is also preferably 75°C to 110°C. The stretching temperature is preferably 75°C to 120°C. The stretching ratio in the folding direction is preferably 4.0 to 6.0 times, 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. Multistage stretching may be used as a method for high-ratio stretching. In this case, it is preferable to set the stretching ratio in the second stage higher than that in the first stage, as this allows for effective control of the refractive index. A method in which stretching is performed again after the crystallization step may also be used. Accelerated stretching, in which the stretching speed is increased from the initial stage to the latter stage of stretching, may also be used. The heat setting temperature is preferably 180 to 240° C. By performing heat setting, oriented crystallization in the stretching direction progresses, and the refractive index in the thickness direction can be reduced. The reason why lowering the refractive index in the thickness direction improves the hardness of the film surface is not entirely clear, but it is thought that aromatic groups such as benzene rings in the molecular chain are oriented in the plane direction, which has the effect of suppressing deformation due to stress applied in the thickness direction.

[0056] (Regarding polyester film density) The density of the polyester film having an easy-adhesion resin layer and before laminating the hard coat layer is 1.380 g / cm 3 It is preferable that the density is 1.383 g / cm or more. 3 More preferably, it is 1.380 g / cm or more. 3By setting the density to 1.40 g / cm or more, it is possible to improve the flexibility and the surface hardness of the film, particularly the pencil hardness of the hard coat film after laminating the hard coat layer. The higher the density, the better. Although it depends somewhat on the presence or absence of particles in the film, it is preferable to set the density to 1.40 g / cm or more. 3 It is preferable that the temperature is 180 to 240°C or less. By setting the heat setting temperature during film formation to 180 to 240°C, crystallization can be promoted and the density can be effectively increased. Note that the density of the polyester film having an easy-adhesion resin layer and before laminating the hard coat layer is negligibly small compared to the density of the polyester film alone having no easy-adhesion resin layer.

[0057] The bending direction of the polyester film is preferably aligned with the longitudinal direction (machine flow direction). This facilitates biaxial stretching to reduce the refractive index in the bending direction and improve flexibility. That is, a polyester film is preferably obtained by stretching an unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.2 to 2.0, more preferably 1.7 to 2.0. Furthermore, a preferred embodiment is to stretch the polyester sheet in the width direction at a stretching ratio of 4.0 to 6.0, more preferably 4.4 to 6.0.

[0058] In the present invention, the polyester film has an easy-adhesion resin layer and is not laminated with a hard coat layer. (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) Refractive index in the thickness direction is 1.520 or less (4) Density is 1.380 g / cm 3 End Although it is particularly preferable to simultaneously achieve the above four properties, even within the above-mentioned preferred manufacturing conditions, if the combination of conditions is not optimal within each preferred manufacturing condition range, such as a stretch ratio in the bending direction of 1.4 or less, a stretch ratio in the folded direction of less than 4.4, and a heat setting temperature of 220° C. or less, it may not necessarily be possible to simultaneously achieve the above four properties. In this case, the above four properties can be simultaneously achieved by fine-tuning any of the conditions or by combining them, such as increasing the stretch ratio in the bending direction to 1.7 or more, increasing the stretch ratio in the folded direction to 4.4 or more, increasing the heat setting temperature to about 230° C., or lowering the stretching temperature in the bending direction and / or the folded direction.

[0059] Although any film-forming method such as stretching, relaxation, heat setting, or surface treatment may be used to adjust film-forming properties, film strength, thermal dimensional stability, and appearance defects, controlling the refractive index and density of the film within the above-mentioned preferred ranges is a particularly preferred embodiment of the present invention. By controlling the refractive index and density within the preferred ranges, a polyester film suitable for foldable displays can be provided that has superior flex resistance and surface hardness compared to conventional films, particularly high pencil hardness of the hard coat film after lamination of a hard coat layer.

[0060] Specifically, for example, PET pellets are thoroughly vacuum-dried, fed into an extruder, melt-extruded into a sheet at approximately 280°C, and cooled to solidify, forming an unstretched PET sheet. The resulting unstretched sheet is stretched 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, in the longitudinal direction using rolls heated to 75 to 120°C to obtain a uniaxially oriented PET film. The film is then gripped at its edges with clips and introduced into a hot air zone heated to 75 to 120°C, where it is dried and then stretched 4.0 to 6.0 times, more preferably 4.4 to 6.0 times, in the transverse direction. Subsequently, the film is introduced into a heat treatment zone at 180 to 240°C, where it is heat-treated for 1 to 60 seconds. During this heat treatment process, if necessary, a relaxation treatment of 0 to 10% may be performed in the transverse or longitudinal direction.

[0061] The intrinsic viscosity of the polyester film is preferably in the range of 0.50 to 1.0 dL / g. When the intrinsic viscosity is 0.50 dL / g or more, the impact resistance is improved and disconnection of the internal circuit of the display due to external impact is less likely to occur, which is preferable. On the other hand, when the intrinsic viscosity is 1.00 dL / g or less, the increase in filtration pressure of the molten fluid is not too large, which is preferable because film production is stable.

[0062] (Easy adhesive resin layer) In the present invention, it is preferable to laminate an easy-adhesion resin layer on the polyester film in order to improve the adhesion between the polyester film and a hard coat layer, etc. The easy-adhesion resin layer can be obtained by so-called in-line coating, in which a coating liquid for forming the easy-adhesion resin layer is applied to one or both sides of an unstretched or uniaxially stretched film in the machine direction, followed by heat treatment and drying as necessary, and further stretching in at least one direction that is not stretched. Heat treatment can also be performed after biaxial stretching. The final coating amount of the easy-adhesion layer is 0.005 to 0.20 g / m 2 It is preferable to control the coating amount to 0.005 g / m 2 On the other hand, when the coating amount is 0.20 g / m or more, adhesiveness can be obtained, which is preferable. 2 If it is less than this, blocking resistance can be obtained, which is preferable.

[0063] Resins contained in the coating solution used to laminate the adhesion layer can be, for example, polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc., and are not particularly limited. However, polyester resins are preferred in terms of high adhesion to polyester films and refractive index. Furthermore, among the dicarboxylic acid components and diol components constituting the polyester resin, polyester resins are preferably copolymerized with a naphthalene dicarboxylic acid component as at least a portion of the dicarboxylic acid component, which can increase the refractive index of the adhesion layer. Furthermore, to improve the adhesion durability of these adhesion layer, a crosslinked structure may be formed in the binder resin contained in the adhesion layer. Examples of crosslinking agents contained in the adhesion layer-forming coating solution include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, carbodiimide compounds, etc., and self-crosslinking polyurethane resins can also be blended. Two or more types of crosslinking agents can also be used in combination. Due to the nature of the inline coating, these are preferably applied using an aqueous coating solution, and the resins and crosslinking agents are preferably water-soluble or water-dispersible resins or compounds.

[0064] The polyester resin contained in the easy-adhesion resin layer is preferably a linear polyester containing a dicarboxylic acid component and a diol component (glycol component) as constituent components.

[0065] Examples of the dicarboxylic acid component 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, and dimer acid. Two or more of these components can be used. Furthermore, small proportions of unsaturated polybasic acids such as maleic acid, fumaric acid, and itaconic acid, and hydroxycarboxylic acids such as p-hydroxybenzoic acid and p-(β-hydroxyethoxy)benzoic acid, can be used together with these components. The proportion of the unsaturated polybasic acid component or hydroxycarboxylic acid component is 10 mol % or less, preferably 5 mol % or less.

[0066] By incorporating a component derived from naphthalenedicarboxylic acid as the dicarboxylic acid component of the polyester resin, the refractive index increases, making it easier to control the iridescent color under fluorescent light. It also makes it possible to improve moist heat resistance. Of course, the polymerization and copolymerization processes for incorporating a naphthalenedicarboxylic acid component or the like into a polyester may be a so-called direct polymerization method or an ester exchange method, and the dicarboxylic acid component such as a naphthalenedicarboxylic acid component may be introduced in the form of its ester derivative.

[0067] As the naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid is preferred. The proportion of the naphthalenedicarboxylic acid component in all dicarboxylic acid components constituting the polyester resin is preferably 20 mol% or more, more preferably 30 mol% or more, even more preferably 50 mol% or more, and even more preferably 60 mol% or more. When it is 20 mol% or more, the effect of increasing the refractive index of the easy-adhesion resin layer is remarkable, which is preferable. The proportion of the naphthalenedicarboxylic acid component in all dicarboxylic acid components constituting the polyester resin may be 100 mol%, but in order to ensure the flexibility of the easy-adhesion resin layer, it is more preferable that it is 95 mol% or less.

[0068] As long as the effects of the present invention are achieved, the glycol component in the polyester resin may further include ethylene glycol, 1,3-propane glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, 1,4-cyclohexanedimethanol, xylene glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, poly(ethyleneoxy)glycol, poly(tetramethyleneoxy)glycol, and alkylene oxide adducts of bisphenol A, and two or more of these may be used.

[0069] Furthermore, when the weight of the crosslinking agent in the easy-adhesion resin layer is taken into consideration, flexibility is likely to be lost, and there is a risk that cracks may occur in the easy-adhesion resin layer after a bending test. In such a case, it is preferable to include a dicarboxylic acid component represented by the following formula (1) and / or a diol component represented by the following formula (2) in the polyester resin. (1) HOOC-(CH)-COOH (wherein n is an integer of 4≦n≦10) (2) HO-(CH)-OH (wherein n is an integer of 4≦n≦10)

[0070] In this way, by containing a dicarboxylic acid component and / or a diol component having carbon components of a specific length, flexibility is imparted to the polyester resin, making it easier to maintain the coating film even after a bending test, and making it possible to suppress cracks originating from particle aggregates. Examples of the dicarboxylic acid component of formula (1) include adipic acid, sebacic acid, azelaic acid, etc. Examples of the diol component of formula (2) include 1,4-butanediol, 1,6-hexanediol, etc.

[0071] The polyester resin can be dissolved or dispersed in water, a water-soluble organic solvent (e.g., an aqueous solution containing less than 50% by mass of alcohol, alkyl cellosolve, ketone, or ether), or an organic solvent (e.g., toluene, ethyl acetate, etc.).

[0072] When a polyester resin is used in a water-based coating liquid, a water-soluble or water-dispersible polyester resin is used. To make the polyester resin water-soluble or water-dispersible, it is preferable to copolymerize a compound containing a sulfonate group or a compound containing a carboxylate group.

[0073] The number average molecular weight of the polyester resin is preferably 5,000 to 40,000 from the viewpoints of coating film strength, ease of water dispersion, etc., more preferably 10,000 to 30,000, and particularly preferably 12,000 to 25,000.

[0074] The solid content of the polyester resin in the solid content of the easy-adhesion resin layer is preferably 20% by mass or more and 90% by mass or less from the viewpoints of adhesion and refractive index adjustment, and more preferably 30% by mass or more and 80% by mass or less. The polyester resin may be a single type or a blend of two or more types. In the case of a blend of two or more types, the total of the polyester resin components preferably has the above composition.

[0075] (urethane resin) The urethane resin that can be used for the easy-adhesion resin layer contains at least a polyol component and a polyisocyanate component as constituent components, and further contains a chain extender as needed. The above-mentioned urethane resin is a polymer compound in which these constituent components are copolymerized mainly through urethane bonds. Inclusion of polycarbonate polyol as a constituent component of the urethane resin is one of the preferred embodiments because it can impart flexibility to the coating film. The constituent components of these urethane resins can be identified by nuclear magnetic resonance analysis or the like.

[0076] (Polyurethane resin with polycarbonate skeleton) The diol component, which is a constituent of the polyurethane resin having a polycarbonate skeleton, preferably contains an aliphatic polycarbonate polyol, which has excellent heat resistance and hydrolysis resistance. In the optical applications of the present invention, it is preferable to use an aliphatic polycarbonate polyol in order to prevent yellowing.

[0077] Examples of the aliphatic polycarbonate polyol include aliphatic polycarbonate diols and aliphatic polycarbonate triols, and preferably aliphatic polycarbonate diols can be used. Examples of the aliphatic polycarbonate diols that are components of the urethane resin of the present invention include aliphatic polycarbonate diols obtained by reacting one or more diols 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, and 1,4-cyclohexanedimethanol with carbonates such as dimethyl carbonate, diphenyl carbonate, ethylene carbonate, and phosgene.

[0078] Examples of polyisocyanates that are components of the urethane resin of the present invention include aromatic aliphatic diisocyanates such as xylylene diisocyanate; alicyclic diisocyanates such as isophorone diisocyanate, 4,4-dicyclohexylmethane diisocyanate, and 1,3-bis(isocyanatomethyl)cyclohexane; aliphatic diisocyanates such as hexamethylene diisocyanate and 2,2,4-trimethylhexamethylene diisocyanate; and polyisocyanates prepared by pre-adding one or more of these compounds with trimethylolpropane or the like. These polyisocyanates are preferred for optical applications that require high transparency and are free from yellowing problems. Furthermore, these polyisocyanates are preferred because they do not make coating films too hard, can alleviate stress caused by shrinkage and swelling of photocurable resins, and maintain adhesion.

[0079] To make urethane resins water-soluble, sulfonic acid (salt) groups or carboxylic acid (salt) groups can be introduced (copolymerized) into the urethane molecular structure. Sulfonic acid (salt) groups are strongly acidic, and their moisture absorption properties can make it difficult to maintain moisture resistance. Therefore, it is preferable to introduce weakly acidic carboxylic acid (salt) groups. Nonionic groups such as polyoxyalkylene groups can also be introduced.

[0080] To introduce carboxylic acid (salt) groups into a urethane resin, for example, a polyol compound having a carboxylic acid group, such as dimethylolpropionic acid or dimethylolbutanoic acid, is introduced as a copolymerization component and neutralized with a salt-forming agent. Specific examples of salt-forming agents include ammonia, trialkylamines such as trimethylamine, triethylamine, triisopropylamine, tri-n-propylamine, and tri-n-butylamine, N-alkylmorpholines such as N-methylmorpholine and N-ethylmorpholine, and N-dialkylalkanolamines such as N-dimethylethanolamine and N-diethylethanolamine. These can be used alone or in combination of two or more.

[0081] When a polyol compound having a carboxylic acid (salt) group is used as a copolymerization component to impart water solubility, the molar ratio of the polyol compound having a carboxylic acid (salt) group in the urethane resin is preferably 3 to 60 mol%, and more preferably 5 to 40 mol%, when the total polyol components of the urethane resin are taken as 100 mol%. A molar ratio of 3 mol% or more is preferred because water dispersibility is good. Furthermore, a molar ratio of 60 mol% or less is preferred because water resistance and moist heat resistance are maintained.

[0082] 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 preferred because it tends to provide suitable flexibility in terms of stress relaxation of the coating layer.

[0083] In order to form a crosslinked structure in the adhesive resin layer, the adhesive resin layer may be formed by containing a crosslinking agent. By incorporating a crosslinking agent, it is possible to further improve adhesion under high temperature and high humidity conditions. Specific examples of crosslinking agents include urea-based, epoxy-based, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based crosslinking agents. Among these, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based crosslinking agents are preferred in terms of the stability of the coating liquid over time and the effect of improving adhesion under high temperature and high humidity treatment. Furthermore, a catalyst or the like can be used as needed to promote the crosslinking reaction.

[0084] When the crosslinking agent is contained in the easy-adhesion resin layer, the content of the crosslinking agent is preferably 5% by mass or more and 50% by mass or less of the total solid components of the coating layer, and more preferably 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-adhesion resin layer is maintained and adhesion is good under high temperature and high humidity conditions, and if it is 40% by mass or less, the flexibility of the resin in the coating layer is maintained and adhesion is maintained after repeated folding tests at room temperature and under high temperature and high humidity conditions, which is preferable.

[0085] The adhesive resin layer of the present invention preferably contains at least one compound selected from a titanium compound and a zirconium compound. It is believed that the iridescent color (interference fringes) of a hard coat film occurs due to the large difference in refractive index between the polyester film substrate (e.g., 1.62 to 1.65) and the hard coat layer (e.g., 1.52) made of an acrylic resin or the like. To minimize the difference in refractive index between the laminated layers and prevent the occurrence of interference fringes, it is important to control the refractive index of the adhesive resin layer so as to minimize the difference in refractive index between the polyester film and the adhesive resin layer, and between the adhesive resin layer and the hard coat layer. The refractive index of the adhesive resin layer, which is primarily composed of binder resin and particles, can be easily controlled by incorporating the above-mentioned high-refractive-index compounds. Examples of titanium compounds include water-soluble titanium chelate compounds, water-soluble titanium acylate compounds, titanium oxide, and titanium chloride, with titanium dioxide (titania) being preferred. Examples of zirconium compounds include water-soluble zirconium chelate compounds, water-soluble zirconium acylate compounds, zirconium acetate, zirconium hydroxide, and zirconium oxide, and among these, zirconium dioxide (zirconia) is preferably used. The compound with a high refractive index is also preferably in particulate form.

[0086] The average particle size of the metal oxide particles such as zirconium dioxide is preferably 5 nm to 150 nm, more preferably 10 nm to 100 nm, and even more preferably 30 nm to 70 nm.

[0087] By designing the average particle size of the metal oxide particles to fall within the above range, film haze can be reduced. Furthermore, the particle size of the particulate metal compound is desirably designed to be smaller than the film thickness of the easy-adhesion resin layer in order to obtain a cushioning effect during repeated folding, and a particle size that is less likely to cause particle aggregation is preferred. Suppressing particle aggregation is preferable because it prevents cracks and the like from forming in the easy-adhesion resin layer during folding.

[0088] The amount of particulate metal compound added is preferably designed to be 0.1% by mass or more and less than 15% by mass relative to the adhesive resin layer to ensure a cushioning effect when the adhesive resin layer is folded and, conversely, to prevent cracks due to particle aggregation from occurring. A concentration of 0.5% by mass or more and 14% by mass or less is more preferable, and a concentration of 1% by mass or more and 13% by mass or less is even more preferable. Adding particles within this range makes it difficult for aggregated particles to form in the adhesive resin layer, and prevents cracks from occurring in the adhesive resin layer during repeated folding due to the aggregated particles, which is preferable because interference patterns are less likely to occur after folding tests.

[0089] It is preferable that the proportion of particulate metal compounds in the easy-adhesion resin layer be lower in the direction of the folded section than in the bending direction. Since fine cracks that occur during repeated folding are more likely to occur in the direction of the folded section where load is applied in the thickness direction, it is thought that crack occurrence can be suppressed by reducing the particle frequency in the direction of the folded section. It is thought that reducing the particle frequency can reduce the rate of particle aggregation and the frequency of crack propagation. The proportion of particles in each direction can be confirmed by observing the cross section with a transmission electron microscope (TEM), etc.

[0090] As a method for changing the directional proportion of the particulate metal compound in the easy-adhesion resin layer, formation by an in-line coating method is preferred. In the in-line coating method, a coating liquid containing an easy-adhesion resin material is applied, and then stretched in at least one direction. Therefore, it is thought that the proportion of the particles in the easy-adhesion resin layer can be changed by adjusting the stretching ratio.

[0091] A dispersant may be used in combination with the particulate metal compound to prevent aggregation. The dispersant used in the present invention may be a polymer compound that can maintain the binder resin in the emulsion, dissolve or disperse the crosslinking agent described below, and disperse metal oxide fine particles.

[0092] Specifically, known polymer dispersants such as polyvinyl, polyacrylic acid, polycarboxylic acid, polyurethane, etc. can be used. More specifically, as polyvinyl polymers, polyvinyl alcohol, polyvinylpyrrolidone, polyvinyl acetate, polyvinyl ester, 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; and copolymers of these or copolymers with sulfonic acid polymers can also be used. Among these, acrylic resins such as polyacrylic acid polymers are preferred as dispersants for metal fine particles.

[0093] It is more preferable that the metal oxide particles have a dispersant on part or all of their surface. The inclusion of a dispersant has the effect of suppressing aggregation of the metal oxide particles. The inclusion of such particles in the easy-adhesion resin layer is expected to maintain the transparency of the coating film while alleviating stress during repeated folding tests. The inclusion of a dispersant is preferable because it makes it difficult for aggregation of the metal oxide particles to occur in the easy-adhesion resin layer and makes it difficult for cracks to occur, depending on the coating film formation process.

[0094] The method for surface treating metal oxide particles with an acrylic resin is not particularly limited, but specific examples include a method in which a mixture of metal oxide particles and an acrylic resin is mixed in advance, added to a solvent, and then dispersed; and a method in which metal oxide particles and an acrylic resin are added in that order to a solvent, and then dispersed.

[0095] Apparatuses that can be used to carry out the dispersion include a dissolver, a high-speed mixer, a homomixer, a meader, a ball mill, a roll mill, a sand mill, a paint shaker, an SC mill, an annular mill, and a pin mill.

[0096] The amount of dispersant added to the metal oxide particles is preferably 5% by mass or more and less than 40% by mass, based on the mass of the metal oxide particles. Addition of 5% by mass or more of the dispersant is preferable because it allows for a good dispersion of the metal oxide particles in the adhesive resin layer. Addition of less than 40% by mass of the dispersant is preferable because it makes it easier to adjust the refractive index of the adhesive resin layer, taking advantage of the characteristics of the metal oxide particles. An addition amount of 10% by mass or more and 30% by mass or less is even more preferable.

[0097] In the present invention, the refractive index of the adhesive resin layer is designed to be within a certain range, and the thin film interference principle is satisfied, thereby reducing the iridescent color (interference pattern). Furthermore, by filling the adhesive resin layer with a certain amount of the metal compound used to adjust the refractive index, it is thought that this has the effect of buffering damage to the adhesive resin layer during repeated folding.

[0098] The thickness of the adhesive resin layer capable of suppressing iridescent colors (interference fringes) can be adjusted to satisfy the formula 2nd=λb / 4, where n is the refractive index of the adhesive resin layer, d is the thickness of the adhesive resin layer, and λb is the bottom wavelength of the reflection spectrum, which can be set appropriately in the range of 450 to 650 nm.

[0099] The polyester film having an easy-adhesion resin layer in the present invention is designed to have different refractive indices in the bending direction and the direction of the folded portion, so it is preferable to control the refractive index of the easy-adhesion resin layer taking each direction into consideration.

[0100] It is desirable to control the refractive index of the easy-adhesion resin layer lower than the refractive index in the bending direction of the polyester film having the easy-adhesion resin layer and before laminating the hard coat layer. The difference in refractive index between the easy-adhesion resin layer and the bending direction is preferably greater than 0 and less than 0.070. It is more preferably 0.005 or more and less than 0.065. It is even more preferably 0.010 or more and less than 0.060. When the refractive index of the easy-adhesion resin layer is lower than that of the polyester film in the bending direction, the difference in refractive index with the laminated hard coat layer is reduced, thereby effectively suppressing iridescent coloring (interference fringes), which is preferable. Furthermore, when the refractive index difference is 0.070 or less, the difference in refractive index with the polyester film is not too large, thereby effectively suppressing iridescent coloring (interference fringes). Because the easy-adhesion resin layer is subjected to compressive stress when bent inward and tensile stress when bent outward in the bending direction, it is preferable to design the resin component with a higher amorphous structure than that with a higher crystalline structure, and it is also preferable to add an appropriate amount of a metal compound to compensate for the refractive index.

[0101] It is desirable to control the refractive index of the easy-adhesion resin layer low relative to the refractive index in the direction of the folds of the polyester film having the easy-adhesion resin layer and before laminating the hard coat layer. The difference in refractive index between the easy-adhesion resin layer and the direction of the folds is preferably in the range of 0.080 to 0.150. It is more preferably 0.085 to 0.14. It is even more preferably 0.090 to 0.13. If the refractive index is lower than the direction of the folds of the polyester film, the difference in refractive index with the laminated hard coat layer becomes small, which is preferable because iridescent coloring (interference fringes) is effectively suppressed. Furthermore, if the refractive index difference is 0.150 or less, the difference in refractive index with the polyester film is not too large, which is preferable because iridescent coloring (interference fringes) can be effectively suppressed.

[0102] It is preferable to add particles to the adhesion layer to impart slipperiness. The average particle size of the fine particles is preferably 2 μm or less. If the average particle size of the particles exceeds 2 μm, the particles tend to fall off from the adhesion layer. Examples of particles to be contained in the adhesion layer include inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, as well as organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the adhesion layer alone, or two or more types may be added in combination. Silica particles with an average particle size of 200 nm to 700 nm are particularly preferred to impart appropriate slipperiness to the coating layer.

[0103] The amount of particles added to impart slipperiness is preferably less than 1% by weight relative to the adhesive resin layer. If the amount is less than 1% by weight, the adhesive resin layer will contain a small number of particles larger than the thickness of the adhesive resin layer, which is preferable because cracks are less likely to propagate when folded. A more preferable embodiment is 0.5% by weight or less.

[0104] The coating solution can be applied by any known method, similar to that used for the coating layer, such as reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, wire bar coating, and pipe doctor coating, which can be used alone or in combination.

[0105] (Hard coat layer) When the polyester film of the present invention is used as a surface protection film for protecting a folding display by being placed on the surface of the display, it is preferable that the film has a hard coat layer on at least one surface. The hard coat layer is preferably placed on the display surface side of the polyester film when used in the display. The resin for forming the hard coat layer can be any resin, including siloxane-based, inorganic hybrid-based, acrylic-based, urethane acrylate-based, polyester acrylate-based, and epoxy-based resins, without any particular limitation. Two or more materials can also be mixed and used, or particles such as inorganic fillers or organic fillers can be added.

[0106] Resins for forming the hard coat layer include compounds having a (meth)acrylate functional group, such as polyester (meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, and silicone (meth)acrylate, as well as compounds having a functional group with an unsaturated double bond, such as an allyl group or a vinyl group. Furthermore, a polyfunctional monomer may be used in combination to increase the hardness of the hard coat layer. Examples of polyfunctional monomers include trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and neopentyl glycol di(meth)acrylate. The above materials may be used alone or in combination.

[0107] When the active energy ray for curing the hard coat layer is ultraviolet light, it is preferable to add a photopolymerization initiator. The photopolymerization initiator may be a radical polymerization system, a cationic polymerization system, or a mixed system of cationic polymerization and radical polymerization. However, radical polymerization systems are particularly preferred because of their high reaction rate and excellent productivity. Examples of ultraviolet radical polymerization initiators include alkylphenones, benzoins, benzophenones, phosphine oxides, ketals, anthraquinones, thioxanthones, azo compounds, peroxides, 2,3-dialkyldione compounds, disulfide compounds, fluoroamine compounds, aromatic sulfonium compounds, titanocenes, and phenyl oxyacetates, which may be used alone or in combination of two or more. Further specific examples include carbonyl compounds such as 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, methylbenzoyl formate, p-isopropyl-α-hydroxyisobutylphenone, α-hydroxyisobutylphenone, 2,2-dimethoxy-2-phenylacetophenone, and 1-hydroxycyclohexyl phenyl ketone; sulfur compounds such as tetramethylthiuram monosulfide, tetramethylthiuram disulfide, thioxanthone, 2-chlorothioxanthone, and 2-methylthioxanthone; and peroxide compounds such as benzoyl peroxide and di-t-butyl peroxide. The amount of photopolymerization initiator added can be in the range of 0.1 parts by mass or more, more preferably 1 part by mass or more, and 30 parts by mass or less, more preferably 20 parts by mass or less, relative to 100 parts by mass of the active energy ray-curable resin. An amount of 0.1 parts by mass or more is preferred because the hardness of the hard coat layer can be increased. Furthermore, an amount of 30 parts by mass or less is preferred because there is no risk of yellowing of the hard coat layer and the hard coat layer is sufficiently cured.

[0108] Furthermore, various additives may be contained within a range that does not impair the performance of the hard coat, such as a polymerization inhibitor, a crosslinking agent, an antistatic agent, an adhesion improver, an antioxidant, a leveling agent, a coupling agent, an antifoaming agent, a filler, a solvent, an antiglare agent, an antireflection agent, an inorganic filler, or an organic filler.

[0109] The refractive index of the hard coat layer is preferably smaller than that of the easy-adhesion resin layer from the viewpoint of suppressing iridescent color (interference fringes) of the hard coat film.

[0110] (Thickness of hard coat layer) The thickness of the hard coat layer is preferably 1 to 50 μm. If it is 1 μm or more, it can be cured sufficiently. To increase the pencil hardness, it is more preferably 5 μm or more. Furthermore, by making the thickness 50 μm or less, curling due to cure shrinkage of the hard coat can be suppressed, and the handling properties of the film can be improved.

[0111] (Application method) The hard coat layer can be applied by any method, including a Mayer bar, gravure coater, die coater, knife coater, etc., without any particular limitation, and can be appropriately selected depending on the viscosity and film thickness.

[0112] (Curing conditions) The hard coat layer can be cured by energy rays such as ultraviolet rays and electron beams, or by heat, and curing methods using ultraviolet rays or electron beams are preferred in order to reduce damage to the film.

[0113] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or more, more preferably 4H or more. A pencil hardness of 3H or more prevents scratches and does not reduce visibility. Generally, a high pencil hardness of the hard coat layer is preferable, but a pencil hardness of 9H or less, 8H or less, or even 6H or less can be used without any practical problems.

[0114] (Hard Coat Layer Characteristics) The hard coat layer of the present invention can be used for the purpose of protecting a display by increasing the pencil hardness of the surface as described above, and preferably has a high transmittance. The transmittance of the hard coat film is preferably 85% or more, more preferably 87% or more, and even more preferably 88% or more. A transmittance of 85% or more provides sufficient visibility. The total light transmittance of the hard coat film is generally preferably higher, but from the standpoint of stable production, it is preferably 99% or less, and may be 97% or less. Furthermore, the haze of the hard coat film is generally preferably low, and preferably 3% or less. The haze of the hard coat film is more preferably 2% or less, and most preferably 1% or less. A haze of 3% or less can improve the visibility of images. The lower the haze, the better, but from the standpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0115] The hard coat layer may further have other functions added thereto. For example, a hard coat layer having the above-mentioned functions, such as an antiglare layer having a certain pencil hardness, an antiglare antireflection layer, an antireflection layer, a low reflection layer, or an antistatic layer, is also preferably used in the present invention.

[0116] A hard coat layer may also be provided when the film is used as a substrate film for a touch panel module. When an ITO layer is used as the transparent electrode layer of the touch panel module, for example, a refractive index adjustment layer is preferably provided between the substrate film and the transparent electrode layer to make the electrode pattern less visible. In this case, the hard coat layer itself may also serve as the refractive index adjustment layer, or a separate refractive index adjustment layer may be laminated on top of it. [Example]

[0117] Next, the present invention will be described with reference to Examples and Comparative Examples. First, the methods for evaluating the properties of the polyester film of the present invention will be described below.

[0118] (1) Intrinsic viscosity The film or polyester resin was crushed and dried, then dissolved in a 60 / 40 (mass ratio) phenol / tetrachloroethane mixed solvent. After centrifuging the solution to remove inorganic particles, an Ubbelohde viscometer was used to measure the flow time of a 0.4 (g / dL) solution at 30°C and the flow time of the solvent alone. The intrinsic viscosity was calculated from the ratio of these times using the Huggins equation, assuming a Huggins constant of 0.38.

[0119] (2) Density The density was measured according to the method (density gradient tube method) in accordance with JIS K7112:1999 (unit: g / cm 3 ). (3) Average particle size of particles in the polyester film or easy-adhesion resin layer The particles on the cross section of the film are observed using a scanning electron microscope, and the average of 50 particles is taken as the average particle size. The particle size of irregular particles that are not spherical can be calculated as the equivalent circle diameter. The equivalent circle diameter is calculated by dividing the area of ​​the observed particle by pi (π), calculating the square root, and then multiplying it by two. (The unit used depends on the average particle size, but nm is mainly used.)

[0120] (4) Refractive index of polyester film having an easy-adhesion resin layer and before laminating a hard coat layer In accordance with JIS K7142:2008 "Method for measuring 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 longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction of the polyester film having an easy-adhesion resin layer and before laminating the hard coat layer were determined.

[0121] (5) Refractive index of the easy-adhesion resin layer The refractive index of the adhesive resin layer can be calculated by fitting the measured reflection spectrum with a spectrophotometer (product name "UV-3150" manufactured by Shimadzu Corporation) to the reflection spectrum calculated from an optical model of a thin film using Fresnel coefficients. To prevent backside reflection, black vinyl tape (e.g., product name "Yamato Vinyl Tape No. 200-38-21" manufactured by Yamato Co., Ltd., 38 mm wide) is attached to the surface opposite to the surface on which the adhesive resin layer is formed (the back surface, or the surface of the adhesive resin layer on the side not to be measured) of the PET substrate on which the refractive index is to be measured.

[0122] (6) Bending resistance of polyester film sample (bending radius 1.5 mm) A polyester film sample measuring 20 mm in width and 110 mm in machine direction was prepared. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the sample was bent 200,000 times at a rate of 1 bend / second with a bend radius of 1.5 mm. The sample was fixed at 10 mm positions on both long sides, and the bending area was 20 mm x 90 mm. Figure 1 is a schematic diagram showing the bend radius when a foldable display is folded. Considering the case where a polyester film is disposed on the inner surface of the folded display, the bending test was performed as a model, with the bending radius set to 1.5 mm at the point indicated by reference numeral 11 in Figure 1. After the bending process, the sample was placed on a flat surface with the bent inner side facing down and visually observed. ○: No cracks or deformations were observed in the sample. ×: The sample has cracks or creases, and when placed horizontally, the maximum lift is 5 mm or more.

[0123] (7) Bending resistance of polyester film sample (bending radius 0.5 mm) Using the same method as in the bending test described above, the display was bent 200,000 times at a bending radius of 0.5 mm and a rate of 1 bend per second. Figure 1 is a schematic diagram showing the bending radius when the foldable display is folded. Taking into account the case where a polyester film is disposed on the inner surface of the folded display, the bending test was performed as a model, with the bending radius set to 0.5 mm at the location indicated by reference numeral 11 in Figure 1 . The film surface outside the bent portion was observed at 700x magnification using a digital microscope (HIROX RH8800) to check for the presence or absence of wrinkles (cracks). In addition to the visual observation of bending resistance with a bending radius of 1.5 mm described above, this test was performed with a bending radius reduced to 0.5 mm to evaluate the display under conditions similar to those in actual use, where a hard coat layer or other components are laminated or attached. This test, separate from the visual observation with a bending radius of 1.5 mm, was also performed to detect minute defects that are difficult to detect visually, such as those prone to breakage or cracking. ○: No defects on the film surface on the outer side of the bend. ×: The film broke or wrinkles (cracks) were observed on the film surface on the outer side of the bend.

[0124] (8) Indentation depth after removing the test force The sample was cut into approximately 2 cm squares and fixed on an 18 × 18 mm microcover glass (manufactured by Matsunami Glass Co., Ltd.) with an adhesive (Cemedine (registered trademark) High Super 30) on the surface opposite the measurement surface. After adhesion and fixation, the sample was left at room temperature for 12 hours or more, and then the indentation depth (μm) after removing the test force was measured using a dynamic ultra-microhardness tester "DUH-211" (manufactured by Shimadzu Corporation) under the following conditions. <Measurement conditions> Test mode: Load-unload test Indenter used: 115 degree ridge angle, triangular pyramid indenter Indenter elastic modulus: 1.140 x 10 6 N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50mN Load speed: 4.44mN / sec Load holding time: 2sec Unloading holding time: 0sec

[0125] (9) Total light transmittance, haze The polyester film having the easy-adhesion resin layer laminated thereon was used as a sample, and the haze was measured using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0126] (10) Maximum heat shrinkage rate A sample film of polyester film laminated with an easy-adhesion resin layer was cut to 10 mm length x 250 mm width, and marks were made at 200 mm intervals along the long side in the direction to be measured, and the distance A between the marks was measured under a constant tension of 5 g. Next, the sample film was left in an oven at 150°C for 30 minutes without load, and then removed from the oven and cooled to room temperature. Thereafter, the distance B between the marks was measured under a constant tension of 5 g, and the thermal shrinkage rate (%) was calculated using the following formula. The thermal shrinkage rate was measured at three equal positions in the width direction of the sample film, and the average value of the three points was taken as the thermal shrinkage rate (%). Heat shrinkage rate (%) = [(AB) x 100] / A The sample film is cut so that the length and width of the sample film are different in both the bending direction and the folding direction, and measurements are taken. The data in the direction where the measurement value is larger is the maximum heat shrinkage rate (%).

[0127] (11) Refractive index of hard coat layer The refractive index of the hard coat layer can be calculated by fitting the reflectance spectrum measured using a spectrophotometer (product name "UV-3150" manufactured by Shimadzu Corporation) to the reflectance spectrum calculated from an optical model of a multilayer thin film using Fresnel coefficients. The reflectance of the hard coat layer is measured after applying a hard coat composition to a 50 μm thick polyethylene terephthalate (PET) substrate without an easy-adhesion resin layer, curing it to form a hard coat layer with a thickness of 1 to 10 μm, and then attaching black vinyl tape (e.g., product name "Yamato Vinyl Tape No. 200-38-21" manufactured by Yamato Co., Ltd., 38 mm wide) with a width larger than the measurement spot area to prevent backside reflection on the surface (backside) of the PET substrate opposite to the surface on which the hard coat layer is formed (rear side).

[0128] (12) Improvement of interference patterns of hard coat film (iris-like color) The hard-coated film was cut into a 50 mm (width direction) x 110 mm (length direction) sample film. A black glossy tape (Nitto Denko, vinyl tape No. 21; black) was attached to the side of the resulting sample film opposite the hard-coated layer. The sample film was placed with the hard-coated side facing up and observed with a three-wavelength daylight light source (National Palook, FL 15EX-N 15W) from above at an angle that provided the strongest visual reflection (40-60 cm from the light source, 15-45° angle relative to the film surface).

[0129] The results of visual observation were ranked according to the following criteria. The observations were made by five people familiar with the evaluation, and the most common rank was taken as the evaluation rank. If there were two ranks with the same number of people, the center of the three ranks was adopted. For example, if two people each chose ◎ and ○ and one person chose △, ○ was adopted; if one person chose ◎ and two each chose ○ and △, ○ was adopted; and if two people each chose ◎ and △ and one person chose ○, ○ was adopted. ◎: No iridescent color is observed from any angle ○: Slight iridescent color can be seen at certain angles △: Slight iridescent coloring is observed ×: Clear iridescent color is observed

[0130] (13) Bending test properties of hard-coated films A sample film was prepared by cutting a hard-coated film into a size of 50 mm in the width direction and 110 mm in the machine direction. The resulting sample film was folded in the same manner as in the bending test described above, with the easy-adhesion resin layer and hard-coat layer facing inward, and the bending radius was set to 3.0 mm. A bending test was performed 200,000 times at a rate of 1 bending / second. After the test, the bent portion of the sample film was placed on a flat surface with the inside of the bend facing down, and visual observation was performed. ○: No cracks or deformations were observed in the sample. ×: Cracks or creases are observed in the sample.

[0131] (14) Observation of interference patterns (iris-like color) after bending test of hard-coated film After the hard-coat film bending test described above, the bent portion of the sample film was subjected to the same observation as for the improvement of interference patterns. Specifically, a black glossy tape (Nitto Denko Corporation, vinyl tape No. 21; black) was attached to the side opposite the hard-coat layer of the obtained sample film. The hard-coat side of the sample film was placed on top, and the sample film was observed from above obliquely using a three-wavelength daylight light source (National Palook, FL 15EX-N 15W) at a position where the strongest reflection was observed (40 to 60 cm from the light source, 15 to 45° angle relative to the film surface). The visual observation results were ranked according to the following criteria. The observations were performed by five experts familiar with this evaluation, and the most frequently ranked rank was used as the evaluation rank. If two or more ranks were the same, the center of the three ranks was used. For example, if there are two ◎s and two ○s and one △, we will hire ○; if there is one ◎ and two ○s and two △s, we will hire ○; and if there are two ◎s and two △s and one ○, we will hire ○. ◎: No iridescent color is observed from any angle ○: Slight iridescent color can be seen at certain angles △: Slight iridescent coloring is observed ×: Clear iridescent color is observed

[0132] This test is conducted to evaluate interference patterns and detect interference patterns caused by minute adhesion defects such as interfacial peeling and cracks between the hard coat layer and the easy-adhesion resin layer and polyester film layer.

[0133] (15) Pencil hardness The pencil hardness of the hard coat film sample was measured at a load of 750 g and a speed of 1.0 mm / s in accordance with JIS K 5600-5-4: 1999. In the present invention, a value of 3H or more was considered acceptable.

[0134] (Preparation of polyethylene terephthalate pellets (R1)) A continuous esterification reactor consisting of a three-stage complete mixing vessel equipped with an agitator, a partial condenser, a raw material inlet, and a product outlet was used. TPA was supplied at 2 tons / hr, EG at 2 moles per mole of TPA, and antimony trioxide in an amount sufficient to provide 160 ppm Sb atoms relative to the PET product. These slurries were continuously fed into the first esterification reactor of the esterification reactor and reacted at atmospheric pressure for an average residence time of 4 hours at 255°C. The reaction product in the first esterification reactor was then continuously removed from the system and fed to the second esterification reactor. EG distilled from the first esterification reactor was fed into the second esterification reactor at 8% by mass relative to the polymer (PET product). An EG solution containing magnesium acetate in an amount sufficient to provide 65 ppm Mg atoms relative to the PET product, and an EG solution containing TMPA in an amount sufficient to provide 20 ppm P atoms relative to the PET product, were then added, and the reaction was continued at atmospheric pressure for an average residence time of 1.5 hours at 260°C. The reaction product in the second esterification reactor was then continuously removed from the system and fed to a third esterification reactor, and an EG solution containing TMPA in an amount such that the P atom concentration in the produced PET was 20 ppm was added, followed by a reaction at atmospheric pressure for an average residence time of 0.5 hours at 260°C. The esterification reaction product produced in the third esterification reactor was continuously fed to a three-stage continuous polycondensation reactor for polycondensation, and further filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cutoff of 5 μm particles) to obtain polyethylene terephthalate pellets (R1) with an intrinsic viscosity of 0.62 dL / g.

[0135] (Preparation of polyethylene terephthalate pellets (R2)) Polyethylene terephthalate pellets (R2) were obtained by adjusting the intrinsic viscosity to 0.580 dl / g in the same manner as in the production process of polyethylene terephthalate pellets (R1), except that the residence time of the third esterification reaction was adjusted.

[0136] (Preparation of polyethylene terephthalate pellets (R3)) The polyethylene terephthalate pellets (R1) were subjected to solid-state polymerization at 220°C under a reduced pressure of 0.5 mmHg for various times using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (R3) with an intrinsic viscosity of 0.75 dl / g.

[0137] (Polymerization of Copolymerized Polyester Resin) The copolymerization of the copolymer polyester resins (a1) to (a3) ​​for forming the easy-adhesion resin layer was carried out as follows. A stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser was charged with 410.3 parts by mass of dimethyl 2,6-naphthalenedicarboxylate, 46.3 parts by mass of sebacic acid, 42.5 parts by mass of dimethyl sodium 5-sulfoisophthalate, 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, and the mixture was subjected to a transesterification reaction at 160°C to 220°C over 4 hours. The temperature was then raised to 255°C, and the reaction system was gradually reduced in pressure. The reaction was continued for 1 hour and 30 minutes under a reduced pressure of 30 Pa to obtain copolymer polyester resin (a1). The resulting copolymer polyester resin was pale yellow and transparent. The composition of copolymer polyester resin (a1) is shown in Table 1.

[0138] In addition, copolymerized polyester resins (a2) and (a3) ​​having the compositions shown in Table 1 were obtained in the same manner by changing the raw materials. The compositions and weight-average molecular weights of these copolymerized polyester resins were measured by 1H-NMR, and the results are shown in Table 1.

[0139] [Table 1]

[0140] (Preparation of aqueous dispersion of copolymer polyester resin) A reactor equipped with a stirrer, thermometer, and reflux device was charged with 25 parts by mass of copolymer polyester resin (a1) and 15 parts by mass of ethylene glycol t-butyl ether, and the mixture was heated to 110°C and stirred to dissolve the resin. After the resin was completely dissolved, 60 parts by mass of water was gradually added to the polyester solution while stirring. After the addition, the solution was cooled to room temperature while stirring to produce a milky white aqueous dispersion of copolymer polyester (Aw-1) with a solids content of 25% by mass. Similarly, aqueous dispersions (Aw-2) to (Aw-3) were prepared using copolymer polyester resins (a2) to (a3) ​​instead of copolymer polyester resin (a1).

[0141] (Polymerization of urethane resin) A four-neck flask equipped with a stirrer, Dimroth condenser, nitrogen inlet tube, silica gel drying tube, and thermometer was charged with 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, and 85.00 parts by mass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone as solvents. The mixture was stirred at 75°C under a nitrogen atmosphere for 3 hours, and it was confirmed that the reaction solution had reached the required amine equivalent. Next, the reaction solution was cooled to 40°C, and 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisperser capable of high-speed stirring, and the mixture was stirred at 25°C for 2000 min. -1 The isocyanate-terminated prepolymer was added and dispersed in water while stirring and mixing at 100°C. Then, acetonitrile and a portion of the water were removed under reduced pressure to prepare a water-soluble polyurethane resin (B-1) with a solid content of 35% by mass.

[0142] (Preparation of self-crosslinking polyurethane resin aqueous solution) 100 parts by weight of polyester diol (OHV: 2000 eq / ton) composed of adipic acid, 1,6-hexanediol, and neopentyl glycol (molar ratio: 4 / 2 / 3) was mixed with 41.4 parts by weight of xylylene diisocyanate and reacted under a nitrogen stream at 80-90°C for 1 hour. After cooling to 60°C, 70 parts by weight of tetrahydrofuran was added and dissolved to obtain a urethane prepolymer solution (NCO / OH ratio: 2.2, free isocyanate group: 3.30% by weight). The urethane prepolymer solution was then heated to 40°C, and 45.5 parts by weight of a 20% by weight aqueous sodium bisulfite solution was added. The mixture was reacted at 40-50°C for 30 minutes with vigorous stirring. After confirming the disappearance of the free isocyanate group content (solid content equivalent), the solution was diluted with emulsified water to obtain a self-crosslinking polyurethane resin aqueous solution (B-2) containing isocyanate groups blocked with sodium bisulfite and having a solid content of 20% by mass.

[0143] (Polymerization of blocked isocyanate compounds) A flask equipped with a stirrer, thermometer, and reflux condenser was charged with 100 parts by mass of a polyisocyanate compound having an isocyanurate structure (Duranate TPA, 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), and the mixture was maintained at 70°C for 4 hours under a nitrogen atmosphere. The temperature of the reaction mixture was then lowered to 50°C, and 47 parts by mass of methyl ethyl ketoxime was added dropwise. The infrared spectrum of the reaction mixture was measured to confirm that the absorption of the isocyanate group had disappeared, yielding a blocked polyisocyanate solution with a solids content of 75% by mass.

[0144] (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 mass %.

[0145] (Polymerization of Water-Soluble Carbodiimide Compounds) 200 parts by weight of isophorone diisocyanate and 4 parts by weight of 3-methyl-1-phenyl-2-phospholene-1-oxide (carbodiimidation catalyst) were placed in a flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer, 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 resulting carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100°C for 24 hours. Water was gradually added at 50°C to obtain a yellow, transparent, water-soluble carbodiimide compound (C-2) with a solids content of 40% by weight.

[0146] (Melamine-based crosslinking agent) As the melamine-based crosslinking agent, Beckamin (registered trademark) M-3 (solid content concentration 60%) manufactured by DIC Corporation was used (melamine-based crosslinking agent (C-3)).

[0147] (Polymerization of oxazoline-based crosslinking agents) A flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer was charged with a mixture of 58 parts by mass of ion-exchanged water and 58 parts by mass of isopropanol as an aqueous medium, and 4 parts by mass of a polymerization initiator (2,2'-azobis(2-amidinopropane) dihydrochloride). The dropping funnel was charged with a mixture of 16 parts by mass of 2-isopropenyl-2-oxazoline as a polymerizable unsaturated monomer having an oxazoline group, 32 parts by mass of methoxypolyethylene glycol acrylate (average number of moles of ethylene glycol added: 9 moles, Shin-Nakamura Chemical Co., Ltd.), and 32 parts by mass of methyl methacrylate. The mixture was added dropwise over 1 hour at 70°C under a nitrogen atmosphere. After the dropwise addition, the reaction solution was stirred for 9 hours and then cooled to obtain a water-soluble resin (C-4) having an oxazoline group with a solids concentration of 40% by mass.

[0148] (zirconia particles) A 3-liter glass container was charged with 2283.6 g of pure water and 403.4 g of oxalic acid dihydrate and heated to 40°C to prepare a 10.72 mass% oxalic acid aqueous solution. While stirring this aqueous solution, 495.8 g of zirconium oxycarbonate powder (ZrOCO3, manufactured by AMR International Corp., containing 39.76 mass% ZrO2) was gradually added and mixed for 30 minutes, followed by heating at 90°C for 30 minutes. Next, 1747.2 g of a 25.0 mass% tetramethylammonium hydroxide aqueous solution (manufactured by Tama Chemicals Co., Ltd.) was gradually added over 1 hour. At this point, the mixture was in a slurry state, and the ZrO2 The resulting slurry contained 4.0% by mass of ZrO2. The slurry was transferred to a stainless steel autoclave and subjected to hydrothermal treatment at 145°C for 5 hours. The product after this hydrothermal treatment was completely sol-formed with no undissolved matter. The resulting sol contained 4.0% by mass of ZrO2, had a pH of 6.8, and an average particle size of 19 nm. The sol was adjusted to a ZrO2 concentration of 2.0% by mass with pure water and measured for transmittance of 88%. Observation of the particles using a transmission electron microscope revealed that most particles were aggregates of primary ZrO2 particles of approximately 7 nm in size. 4000 g of the zirconia sol with a ZrO2 concentration of 4.0% by mass obtained by the hydrothermal treatment was washed and concentrated using an ultrafiltration device while gradually adding pure water. 953 g of zirconia sol with a ZrO2 concentration of 13.1% by mass, a pH of 4.9, and a transmittance of 76% was obtained.

[0149] To 300 g of the zirconia sol with a ZrO2 concentration of 13.1 wt% obtained by the above washing and concentration, 3.93 g of a 20 wt% aqueous citric acid solution and 11.0 g of a 25 wt% aqueous tetramethylammonium hydroxide solution were added, and the mixture was further concentrated using an ultrafiltration device to obtain 129 g of a high-concentration zirconia sol with a ZrO2 concentration of 30.5 wt% (D-1). This high-concentration zirconia sol had a pH of 9.3 and an average particle size of 19 nm. Furthermore, this zirconia sol was free of sediment and stable for more than one month at 50°C.

[0150] (Zirconia water dispersion) The zirconia sol obtained above was mixed with a polyacrylic acid dispersant (Aron A-30SL, manufactured by Toagosei Co., Ltd.) to prepare a zirconia aqueous dispersion with a solid content of 13% by mass. Zirconia aqueous dispersion D-1 was obtained, in which 3% by mass of the solid content was the dispersant and 10% was zirconia.

[0151] (Titania particles) 12.09 kg of a titanium tetrachloride aqueous solution containing 7.75% by mass of titanium tetrachloride (Osaka Titanium Technologies Co., Ltd.) converted to TiO2 was mixed with 4.69 kg of ammonia water containing 15% by mass of ammonia (Ube Industries, Ltd.) to prepare a white slurry with a pH of 9.5. The slurry was then filtered and washed with pure water to obtain 9.87 kg of a hydrous titanic acid cake with a solids content of 10% by mass. Next, 11.28 kg of hydrogen peroxide water containing 35% by mass of hydrogen peroxide (Mitsubishi Gas Chemical Co., Inc.) and 20.00 kg of pure water were added to the cake, followed by heating at 80°C for 1 hour with stirring. An additional 57.52 kg of pure water was added to obtain 98.67 kg of a titanium peroxide aqueous solution containing 1% by mass of titanium peroxide converted to TiO2. This titanium peroxide aqueous solution was transparent and yellow-brown in color and had a pH of 8.5.

[0152] Next, 98.67 kg of the titanic acid peroxide aqueous solution was mixed with 4.70 kg of a cation exchange resin (manufactured by Mitsubishi Chemical Corporation), and 12.33 kg of a potassium stannate aqueous solution containing 1 mass % of potassium stannate (manufactured by Showa Kako Co., Ltd.) converted to SnO was gradually added thereto with stirring. Next, the cation exchange resin that had incorporated potassium ions was separated, and then placed in an autoclave (manufactured by Taiatsu Glass Industry Co., Ltd., 120 L) and heated at 165°C for 18 hours.

[0153] The resulting mixed aqueous solution was then cooled to room temperature and concentrated using an ultrafiltration membrane device (Asahi Kasei Corporation, ACV-3010) to obtain 9.90 kg (D-2) of a water-dispersed sol containing titanium-based microparticles with a solid content of 10% by mass. The solid content in the sol thus obtained was measured using the method described above, and was found to be titanium-based microparticles (primary particles) composed of a composite oxide containing titanium and tin, with a rutile crystal structure. Furthermore, the content of metal components contained in the titanium-based microparticles was measured, and was found to be 87.2% by mass of TiO, 11.0% by mass of SnO, and 1.8% by mass of KO, calculated on the oxide basis for each metal component. The pH of the mixed aqueous solution was 10.0. The water-dispersed sol containing the titanium-based microparticles was transparent and milky white. The average particle diameter of the titanium-based microparticles contained in this water-dispersed sol was 35 nm, and the distribution frequency of coarse particles with a particle diameter of 100 nm or more was 0%. Furthermore, the refractive index of the obtained titanium-based particles was considered to be 2.42.

[0154] (Titania water dispersion) The titanium-based microparticles obtained above were mixed with a polyacrylic acid dispersant (Aron A-30SL, manufactured by Toagosei Co., Ltd.) to prepare a titania aqueous dispersion with a solid content of 13% by mass. Titania aqueous dispersion D-2 was obtained, in which 3% by mass of the solid content was the dispersant and 10% was titania.

[0155] (zirconia / titania mixed dispersion) The zirconia sol obtained above was mixed with titania-based particles and a polyacrylic acid dispersant (Aron A-30SL, manufactured by Toagosei Co., Ltd.) to prepare a zirconia / titania mixed aqueous dispersion with a solid content of 13% by mass. Mixed dispersion D-3 was obtained, with 3% by mass of the solid content being dispersant, 7.5% being zirconia, and 2.5% being titania.

[0156] (silica particles) As the silica particles, colloidal silica having a particle diameter of 40 nm and a solid content of 30 mass % was used as D-4.

[0157] (silica particles) To impart slipperiness, silica particles having a particle diameter of 450 nm and a solid content of 40 mass % were used as D-5.

[0158] (surfactant) In order to improve the leveling properties of the coating film when forming the highly adhesive resin layer, a silicone surfactant with a solid content concentration of 100% by mass was used as E-1.

[0159] (Preparation of coating solution for forming easy-adhesion layer) The following coating materials were mixed to prepare coating solution P-1. Water 47.52 parts by mass Isopropanol 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 size of 450 nm, solid content of 40% by mass) Silicone surfactant (E-1) 0.03 parts by mass (Silicone-based, solid content 100% by mass)

[0160] Similarly, coating solutions P-2 to P-14 were prepared by mixing the coating agents in the ratios shown in Table 2.

[0161] [Table 2]

[0162] Example 1 Polyethylene terephthalate pellets (R1) were fed into an extruder and melted at 285 ° C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet form from a die, and then contacted with a casting drum at a surface temperature of 30 ° C using an electrostatic casting method, cooled and solidified, and an unstretched film was produced. This unstretched film was uniformly heated to 75 ° C using a heated roll, heated to 85 ° C using a non-contact heater, and roll-stretched 1.4 times (longitudinal stretching). The above-mentioned coating liquid (P-1) for forming an easy-adhesion resin layer was applied to both sides of the obtained uniaxially stretched film using a roll coating method, and then dried at 80 ° C for 20 seconds. The final coating amount after drying (after biaxial stretching) was 0.09 g / m 2 The film was then introduced into a tenter, preheated at 105°C, transversely stretched 4.0 times at 95°C, fixed in width, heat-set at 230°C for 5 seconds, and further relaxed in the width direction by 4% at 180°C to obtain a polyethylene terephthalate film with a thickness of 50 μm.

[0163] A hard coat coating solution (H-1 below) was applied to one side of the polyethylene terephthalate film having the above-mentioned easy-adhesion resin layer using a Mayer bar so that the film thickness after drying would be 10 μm, and after drying at 80°C for 1 minute, it was irradiated with ultraviolet light (integrated light amount 200 mJ / cm 2 ), a hard-coated film was obtained.

[0164] (Coating liquid for forming hard coat layer: H-1) 95 parts by mass of a urethane acrylate hard coating agent (Beamset (registered trademark) 577, manufactured by Arakawa Chemical Industries, Ltd., solids concentration 100%), 5 parts by mass of a photopolymerization initiator (Irgacure (registered trademark) 184, manufactured by BASF Japan, solids concentration 100%), and 0.1 parts by mass of a leveling agent (BYK307, manufactured by BYK Japan, solids concentration 100%) were mixed and diluted with a solvent of toluene / MEK = 1 / 1 to prepare a hard coating layer forming coating liquid (H-1) with a solids concentration of 40%.

[0165] (Examples 2 and 3) A polyester film and a hard-coated film were obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to the values ​​shown in Table 3.

[0166] Example 4 A polyester film and a hard-coated film were obtained in the same manner as in Example 1, except that the draw ratio in the width direction was changed to 5.5 times and the heat setting temperature was changed to 190°C.

[0167] (Examples 5 to 14) As shown in Table 3, polyester films and hard coat films were obtained in the same manner as in Example 1, except that the coating solutions for forming the easy-adhesion resin layer were changed to P-2 to P-11.

[0168] (Examples 15 and 16) As shown in Table 4, polyester films and hard-coated films were obtained in the same manner as in Example 1, except that hard-coat coating solution H-1 was replaced with H-2 and H-3 below.

[0169] (Coating liquid for forming hard coat layer: H-2) Pentaerythritol tri- and tetraacrylate (manufactured by Toagosei Co., Ltd., Aronix (registered trademark) M-306, solid content concentration 100%) 30 parts by mass, polyester acrylate (manufactured by Toagosei Co., Ltd., Aronix (registered trademark) M9050, solid content 100%) 65 parts by mass, photopolymerization initiator (manufactured by BASF Japan Ltd., Irgacure (registered trademark) 907, solid content concentration 100%) 5 parts by mass, leveling agent (manufactured by BYK Japan KK, BYK307, solid content concentration 100%) 0.1 parts by mass was mixed and diluted with a solvent of toluene / MEK = 1 / 1 to prepare a hard coat coating solution (H-2) with a concentration of 40% by mass.

[0170] (Coating solution for forming hard coat layer: H-3) To 100 parts by mass of a hard coat material (Opstar (registered trademark) Z7503, manufactured by JSR Corporation, concentration 75%), 0.1 parts by mass of a leveling agent (BYK307, manufactured by BYK Japan, concentration 100%) was added, and the mixture was diluted with methyl ethyl ketone to prepare a hard coat coating solution (H-3) with a solids concentration of 40% by mass.

[0171] (Comparative Example 1) A polyester film and a hard-coated film were obtained in the same manner as in Example 1, except that the film was not stretched in the machine direction but was stretched only in the width direction, i.e., transverse uniaxial stretching.

[0172] (Comparative Example 2) A polyester film and a hard-coated film were obtained in the same manner as in Example 4, except that the film was not stretched in the machine direction but was stretched only in the width direction, i.e., transverse uniaxial stretching.

[0173] (Comparative Examples 3 to 7) A polyester film and a hard coat film were obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 220° C. and the PET pellets and thicknesses shown in Table 1 were used.

[0174] (Comparative Example 8) A polyester film and a hard-coated 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.

[0175] (Comparative Examples 9 to 11) As shown in Table 3, polyester films and hard-coated films were obtained in the same manner as in Example 1, except that the coating solutions were changed to P-12 to P-14.

[0176] The evaluation results are shown in Tables 3 and 4.

[0177] [Table 3]

[0178] [Table 4]

[0179] In the cases of Examples 1 to 16, in which the refractive index of the polyester film having an easy-adhesion layer was within a certain range and a hard coat layer was provided on the easy-adhesion 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 coat was also satisfactory, and the improvement in interference patterns after continuous bending tests was also good.

[0180] In each example, when the cross section was observed with a transmission electron microscope, it was found that the particles contained in the easy-adhesion resin layer were present in a smaller proportion in the width direction, i.e., the direction of the fold, than in the longitudinal direction, i.e., the direction of bending.

[0181] In Comparative Examples 1 to 8, in which the film-forming conditions for the polyester film were changed, the refractive index of the polyester film having the easy-adhesion resin layer was not within the optimum range, and therefore the pencil hardness of the resulting hard-coated film was not satisfactory.

[0182] In Comparative Example 9, in which the easy-adhesion resin layer was changed, neither a titanium compound nor a zirconia compound was contained, and the refractive index of the easy-adhesion resin layer was insufficient, so no improvement in interference patterns was observed before the bending test of the hard coat film. Furthermore, the dispersibility of silica used instead of the metal compound was poor, and a deterioration in interference patterns was also observed after the bending test.

[0183] In Comparative Example 10, in which the easy-adhesion resin layer was changed, an improvement in the interference patterns before the bending test was observed, but the dispersibility of the silica in which the titanium compound was used instead of the zirconia compound was poor, and after the bending test, a worsening of the interference patterns was observed due to the occurrence of cracks in the easy-adhesion resin layer.

[0184] In Comparative Example 11, in which the easy-adhesion resin layer was changed, the absence of polyester resin required the addition of a large amount of titanium / zirconia mixed oxide to adjust the refractive index, which resulted in the occurrence of aggregation in the metal oxides in the coating film, leading to worsening of interference patterns after the bending test.

[0185] The hard coat films of each Example and Comparative Example were attached to an organic EL module via a 25 μm thick adhesive layer to create a smartphone-type foldable display that could be folded in half at the center of the entire display, with a radius of 3 mm corresponding to the bending radius in Figure 1. The hard coat film was disposed on the surface of a single continuous display via the folding portion, with the hard coat layer positioned on the surface of the display. The displays using the hard coat films of each Example satisfied the operation and visibility of a smartphone that could be folded in half at the center and carried around. Furthermore, the surface was not dented by external force. On the other hand, the foldable displays using the hard coat films of each Comparative Example were not very desirable, as the frequency of use increased, and image distortion appeared at the folding portion of the display, and interference patterns were observed, resulting in poor visibility. Some displays also had dents and scratches on the surface. [Industrial Applicability]

[0186] A foldable display using the hard coat film for a foldable display of the present invention maintains mass productivity, and the hard coat film located on the surface of the foldable display does not deform after repeated folding, so that image distortion does not occur at the folded portion of the display. In particular, a mobile terminal device or image display device equipped with a foldable display using the hard coat film of the present invention as a surface protective film provides beautiful images, is highly functional, and is excellent in convenience such as portability. [Explanation of symbols]

[0187] 1: Foldable display 11: Bending radius 2: Polyester film for surface protection of foldable displays 21: Folding section 22: Bending direction (direction perpendicular to the folding part)

Claims

1. A hard coat film for a foldable display, comprising a polyester film with a thickness of 10 to 80 μm, having an easy-adhesion resin layer and a hard coat layer sequentially on at least one side, The easy-to-adhere resin layer is formed by curing a composition containing at least one compound selected from titanium compounds and zirconium compounds, and a polyester resin. A hard-coat film for a foldable display having the aforementioned easy-adhesion resin layer and a polyester film before lamination of the hard-coat layer that satisfies the following conditions; The refractive index in the bending direction is 1.590 to 1.

620. The refractive index in the direction of the folding part is 1.670 to 1.

700. The refractive index in the thickness direction is 1.520 or less. Density is 1.380 g / cm³ ３ That's all. (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.) A hard coat film for a foldable display, wherein the polyester resin contained in the easy-adhesion resin layer is a polyester resin in which at least a portion of the dicarboxylic acid component is a naphthalenedicarboxylic acid component.

2. The refractive index of the easy-adhesion resin layer is lower than the refractive index in the bending direction and the refractive index in the folding direction of the polyester film having the easy-adhesion resin layer and before lamination of the hard coat layer. A hard coat film for a foldable display according to claim 1, wherein the refractive index is higher than that of the hard coat layer.

3. A foldable display in which the hard coat film for a foldable display according to Claim 1 is arranged as a surface protective film such that the hard coat layer is positioned on the surface, and a single continuous hard coat film is arranged through the foldable portion of the foldable display.

4. A portable terminal device having a foldable display as described in Claim 3.

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