Polyester film and use thereof

Abstract

To provide a polyester film for a folding type display that is excellent in mass-productivity and is free from occurence of cracks in the folding section, for providing the holding type display free from a fear of occurence of generation of distorted image displayed in a folded portion after repeated bending.SOLUTION: A polyester film for foldable displays with a thickness of 10 to 125 μm and a high temperature hold angle of 70° or more in a bending direction; here, the high temperature fold angle refers to an angle formed of a folding trace remained after fixing for 18 hours under 85°C heating such that strain of 1.7% is applied on both surfaces of a bent portion respectively. Further, a flexure direction refers to a direction perpendicular to the folded portion.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a polyester film for a foldable display, a hard coat film for a foldable display, a foldable display, and a portable terminal device, and to a foldable display and a portable terminal device in which image distortion due to film deformation is unlikely to occur even when repeatedly folded, and to the polyester film for the foldable display.

Background Art

[0002] The thin film of portable terminal devices has been made lighter, and portable terminal devices typified by smartphones have become widely popular. While various functions are required for portable terminal devices, convenience is also required. Therefore, popular portable terminal devices need to have a small screen size of about 6 inches because simple operations can be performed with one hand and they are assumed to be stored in a pocket of clothes or the like.

[0003] On the other hand, tablet terminals with a screen size of 7 inches to 10 inches are assumed to be used for video content, music, business applications, drawing applications, reading, etc., and have high functionality. However, they cannot be operated with one hand, have poor portability, and have problems with convenience.

[0004] To achieve these, a method of making it compact by connecting a plurality of displays has been proposed (see Patent Document 1). However, in the invention of Patent Document 1, since a bezel portion remains, the video is interrupted, and a decrease in visibility becomes a problem and it has not become widespread.

[0005] On the other hand, in recent years, portable terminals incorporating flexible displays and foldable displays have been proposed. With this method, it is possible to carry a portable terminal device equipped with a large-screen display conveniently without the image being interrupted.

[0006] Here, for displays and portable terminal devices that do not have a conventional folding structure, the surface of the display could be protected with a non-flexible material such as glass. However, in a foldable display, when using a single-sided display through the folding part, it is necessary to use a hard coat film or the like that has flexibility and can protect the surface. However, in a foldable display, since the part corresponding to a certain folding part is repeatedly bent, there are problems such as the film at that part being deformed over time and distorting the image displayed on the display. Also, not only the surface protection film, but also in foldable displays, films are used in various parts such as polarizing plates, retardation plates, touch panel substrates, substrates of display cells such as organic ELs, and protective members on the back surface, and durability against repeated folding is required for these films.

[0007] Therefore, a method of partially changing the film thickness has also been proposed (see Patent Document 2). However, In the invention of Patent Document 2, the manufacturing process becomes complicated to change the film thickness, and there is a problem of poor mass productivity.

[0008] Also, a method of adjusting the refractive index in the bending direction of a polyester film has been proposed (see Patent Document 3). However, there was a concern that films using polyethylene terephthalate could not be used in applications that require higher reliability (high temperature regions).

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] The present invention aims to solve the problems of conventional display members as described above, and provides a foldable display that is excellent in mass productivity and has no risk of causing image distortion in the folded portion after repeated folding, and a portable terminal device equipped with such a foldable display, and a polyester film for a foldable display that enables the provision of the above. Furthermore, the present invention aims to provide a polyester film for a foldable display in which no fold marks occur in the folded portion in a high temperature range.

Means for Solving the Problems

[0011] That is, the present invention has the following configuration. 1. A polyester film having a thickness of 10 μm or more and 125 μm or less, and having a high-temperature hold angle in the bending direction of 70° or more, for a foldable display. (Here, the high-temperature hold angle refers to the angle formed by the fold after being fixed at 85° C. for 18 hours under heating so that a strain of 1.7% is applied to both surfaces of the bent portion. The bending direction refers to the direction orthogonal to the folded portion.) 2. The polyester film for a foldable display according to the first item, having a density of 1.349 g / cm 3 or more. 3. The polyester film for a foldable display according to the first or second item, wherein the polyester is polyethylene naphthalate. 4. The polyester film for a foldable display according to any one of the first to third items, having an easy adhesion layer on at least one surface of the polyester film. 5. A foldable display using the polyester film for a foldable display according to any one of the above 1 to 4, which is a foldable display arranged as a back surface protection film, and a single continuous polyester film is arranged through the folding portion of the foldable display. 6. A portable terminal device having the foldable display according to the above 5.

Effect of the Invention

[0012] The foldable display using the polyester film for a foldable display of the present invention maintains mass productivity, and the polyester film does not deform after repeated folding even in a high temperature region, and does not cause image distortion at the folding portion of the display. A portable terminal device equipped with the foldable display using the polyester film as described above provides a beautiful image, is rich in functionality, and is excellent in convenience such as portability.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0014] (Display) As used in the present invention, the display refers generally to a display device. Examples of the types of displays include LCD, organic EL display, inorganic EL display, LED, FED, etc. For example, LCD, organic EL, and inorganic EL having a foldable structure are preferred. In particular, organic EL and inorganic EL, which can reduce the layer structure, are particularly preferred, and organic EL with a wide color gamut is even more preferred.

[0015] (Foldable display) A foldable display is one in which a single continuous display can be folded, such as in half, when carried. By folding, the size can be halved, improving portability. The bending radius of the foldable display is preferably 5 mm or less, and more preferably 3 mm or less. If the bending radius is 5 mm or less, thinning in the folded state becomes possible. It can be said that the smaller the bending radius, the better. In the present invention, even with such a bending radius, the fold marks can be suppressed. The bending radius is preferably 0.1 mm or more, and may be 0.5 mm or more, or 1 mm or more. Even if the bending radius is 0.1 mm, practically sufficient thinning can be achieved when carried. The bending radius when folded is the value measured at the location indicated by reference numeral 11 in the foldable display 1 in the schematic diagram of FIG. 1, and means the inner radius of the folded portion when folded. Note that the surface protection film described later may be located on the outer side or the inner side of the folded foldable display. In addition, the foldable display may be foldable three times, four times, or may even be a rollable type called rollable, and all of these are within the scope of the foldable display as referred to in the present invention. Moreover, with the polyester film of the present invention, not only bending in the longitudinal direction as shown in FIG. 1 but also bending in the width direction is possible.

[0016] The polyester film for a foldable display of the present invention may be used in any part as long as it is a component of a foldable display. Hereinafter, taking an organic EL display as an example, the representative configuration of a foldable display and the parts where the polyester film of the present invention can be used will be described. Hereinafter, the polyester film for a foldable display of the present invention may sometimes be simply referred to as the polyester film of the present invention.

[0017] (Foldable organic EL display) As an essential configuration of a foldable organic EL display, there is an organic EL module. Further, if necessary, a circularly polarized light plate, a touch panel module, a surface protection film, a back surface protection film, etc. are provided. (Organic EL module) A general configuration of an organic EL module consists of an electrode / electron transport layer / light emitting layer / hole transport layer / transparent electrode. As a base material for providing an electrode and further providing an electron transport layer, a light emitting layer, and a hole transport layer, the polyester film of the present invention can be used. In particular, it can be preferably used as a base material for a transparent electrode. In this case, since a high water vapor and oxygen barrier property is required for the base film, it is preferable that the polyester film of the present invention is provided with a barrier layer such as a metal oxide layer. In order to increase the barrier property, a plurality of barrier layers may be provided, or a plurality of polyester films provided with a barrier layer may be used.

[0018] (Touch panel module) It is preferable for portable terminal devices to have a touch panel. When using an organic EL display, it is preferable that a touch panel module is disposed on the upper part of the organic EL display or between the organic EL module / circularly polarized light plate. The touch panel module has a transparent base material such as a film and a transparent electrode disposed thereon. The polyester film of the present invention can be used as this transparent base material. When used as a transparent base material for a touch panel, it is preferable to provide a hard coat layer or a refractive index adjustment layer on the polyester film.

[0019] (Circularly Polarizing Plate) The circularly polarizing plate suppresses the reflection of external light by members inside the display and the resulting degradation of image quality. The circularly polarizing plate has a linearly polarizing plate and a retardation plate. The linearly polarizing plate has a protective film on at least the viewing side surface of the polarizer. A protective film may also be provided on the surface of the polarizer opposite to the viewing side, or the retardation plate may be directly laminated on the polarizer. As the retardation plate, a resin film having retardation such as polycarbonate or cyclic olefin, or a resin film provided with a retardation layer made of a liquid crystal compound is used. The polyester film of the present invention can be used as the protective film for the polarizer or the resin film of the retardation plate. In these cases, it is preferable that the slow axis direction of the polyester film of the present invention is parallel or orthogonal to the absorption axis direction of the polarizer. Note that a deviation of up to 10 degrees, preferably up to 5 degrees, from this parallel or orthogonal is allowed.

[0020] (Surface Protective Film) When an impact is applied to the display from above, there is a risk that the circuits of the organic EL module or the touch panel module may be disconnected. Therefore, in many cases, a surface protective film is provided. The polyester film of the present invention is used as this surface protective film. The surface protective film may be something called a cover window incorporated in the outermost surface of the display, or something called an after that can be attached, peeled off, and replaced by the user himself / herself. In any case, the polyester film of the present invention is used. When the polyester film of the present invention is used as the surface protective film, it is preferable that a hard coat layer is laminated on at least the surface side of the polyester film. The hard coat layer is provided on the surface of the foldable display with the viewing side facing outwards. Note that the hard coat layer may be provided on both sides.

[0021] (Back Protective Film) It is also preferable that a protective film is provided on the back side of the display. Specifically, an adhesive layer is provided on the non-viewable side of the organic EL module, and it has a laminated structure. The polyester film of the present invention can be used as this protective film on the back side.

[0022] The polyester film of the present invention can be used for other applications as long as it is used at the folding part of the constituent members of the foldable display. Among these, the polyester film of the present invention is preferably used for a cover window surface protective film, an after surface protective film, a base film of a touch panel module, and a back surface protective film. Further, it is preferably used for a cover window surface protective film and an after surface protective film.

[0023] Also, it is not necessary for the polyester film of the present invention to be used in all of the above for the foldable display. In a foldable display, in addition to the polyester film of the present invention, a polyimide film, a polyamide film, a polyamideimide film, a polyester film other than the polyester film of the present invention, a polycarbonate film, an acrylic film, a triacetyl cellulose film, a cycloolefin polymer film, a polyphenylene sulfide film, a polymethylpentene film, etc. can be appropriately used according to the suitability.

[0024] The polyester film of the present invention may be a single-layer film composed of one or more polyester resins, or when two or more polyesters are used, it may be a multilayer structure film or a super-multilayer laminated film with a repeating structure.

[0025] Examples of the polyester resin used for the polyester film include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, for example, polyethylene-2,6-naphthalate, or a polyester film composed of a copolymer having the constituent components of these resins as the main component. Among them, from the viewpoints of mechanical properties, heat resistance, transparency, etc., a polyethylene naphthalate film, particularly a stretched polyethylene naphthalate film, is preferable. In an embodiment mainly composed of a polyethylene naphthalate resin, when mixing other polyesters, the other polyester resin may be 40% by weight or less, for example 10% by weight or less, may be 5% by weight or less, and preferably less than 5% by weight, based on 100% by weight of the resin in the polyester film. On the other hand, the polyethylene naphthalate resin may be 60% by weight or more, may be 90% by weight or more, may be 95% by weight or more, for example, preferably contained in an amount exceeding 95% by weight, based on 100% by weight of the resin in the polyester film. When the other polyester resin is less than 5% by weight, the crystallinity of the polyester film can be highly maintained, and the high-temperature hold angle can be favorably maintained. In one embodiment, the proportion of polyethylene naphthalate in the raw material ratio of the polyester film is 100% by weight. In the present invention, the polyester film may contain a plurality of types of polyethylene naphthalates having different characteristics. By increasing the proportion of polyethylene naphthalate, the polyester film does not deform after being repeatedly folded even in a high-temperature region, and can suppress image disturbance at the folded portion of the display. Furthermore, a portable terminal device equipped with a foldable display using the polyester film of the present invention provides beautiful images, is rich in functionality, and is excellent in convenience such as portability.

[0026] When a polyester copolymer is used for the polyester 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 glycol having an average molecular weight of 150 to 20,000. The mass ratio of the copolymer components of the preferred copolymer is less than 3% by mass. When it is less than 3% by mass, it is preferable because film strength, transparency, and heat resistance are maintained.

[0027] In addition, in the production of the polyester film, the intrinsic viscosity of at least one type of resin pellet is preferably in the range of 0.40 to 1.0 dl / g. When the intrinsic viscosity is 0.40 dl / g or more, the impact resistance of the obtained film is improved, and it is preferable because disconnection of the internal circuit of the display due to an external impact is less likely to occur. On the other hand, when the intrinsic viscosity is 1.00 dl / g or less, the filtration pressure rise of the molten fluid does not become too large, and it is easy to stably operate the film production, which is preferable. preferable. For example, the intrinsic viscosity of at least one type of resin pellet is 0.40 to 0.8 dl / g, and the intrinsic viscosity may be 0.40 to 0.7 dl / g.

[0028] The thickness of the polyester film is 10 μm or more and 125 μm, and more preferably, for example, 25 μm or more and 100 μm or less. When the thickness is 10 μm or more, an effect of improving the pencil hardness and an effect of improving the impact resistance are observed. When the thickness is 125 μm or less, it is advantageous for weight reduction, and in addition, it is excellent in flexibility, processability, handling property, and the like.

[0029] The surface of the polyester film of the present invention may be smooth or may have irregularities. When used for the surface cover of a display, it preferably has a smooth film surface. The haze is preferably 3% or less, more preferably 2% or less, and particularly preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. The lower limit of the haze is preferably as small as possible, but from the viewpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0030] For the purpose of reducing the haze as described above, the irregularities on the film surface are preferably not too large. However, from the viewpoint of handling, it may have irregularities in order to provide a certain degree of slipperiness. As a method of forming the surface irregularities, it can be formed by a method of blending particles into the surface polyester resin layer, or by coating a coating layer containing particles during film formation.

[0031] As a method of blending particles into the polyester resin layer, a known method can be adopted. For example, it can be added at any stage of producing polyester, but preferably at the stage of esterification or at the stage after the transesterification reaction and before the start of the polycondensation reaction, and added as a slurry dispersed in ethylene glycol or the like, and the polycondensation reaction may proceed. Also, it can be carried out by a method of blending a slurry of particles dispersed in ethylene glycol or water and a polyester raw material using a kneading extruder with a vent, or by a method of blending dried particles and a polyester raw material using a kneading extruder.

[0032] Among these, a method is preferred in which, after homogeneously dispersing aggregated inorganic particles in a monomer liquid that is part of the polyester raw material and then filtering it, the filtered product is added to the remainder of the polyester raw material before, during, or after the esterification reaction. According to this method, since the monomer liquid has a low viscosity, homogeneous dispersion of the particles and highly accurate filtration of the slurry can be easily performed. Moreover, when adding to the remainder of the raw material, the dispersibility of the particles is good and new aggregates are less likely to occur. From such a perspective, it is particularly preferable to add to the remainder of the raw material in the low-temperature state before the esterification reaction.

[0033] In addition, after obtaining a polyester containing particles in advance, the number of protrusions on the film surface can be further reduced by a method such as kneading and extruding the pellets containing the particles and the pellets not containing the particles (masterbatch method).

[0034] Also, the polyester film may contain various additives within a range that maintains a preferable range of the total light transmittance. Examples of the additives include an antistatic agent, a UV absorber, and a stabilizer.

[0035] The total light transmittance of the polyester film is preferably 85% or more, and more preferably 87% or more. If the transmittance is 85% or more, sufficient visibility can be ensured. Although it can be said that the higher the total light transmittance of the polyester film, the better, from the aspect of stable production, it is preferably 99% or less, and may be 97% or less.

[0036] The maximum heat shrinkage rate of the polyester film after heat treatment at 150°C for 30 minutes is preferably 2% or less, more preferably 1.5% or less, for example, 1.2% or less. If the heat shrinkage rate is 2% or less, dimensional changes due to heat generation of the organic EL display itself can be suppressed. Although it can be said that the lower the heat shrinkage rate, the better, it is preferably -1% or more and preferably 0% or more. Here, the minus means expansion after heating, and when it is less than -1%, flatness defects may also occur in some cases.

[0037] On the surface of the polyester film of the present invention, a treatment can be performed to improve the adhesion to a resin for forming a hard coat layer or the like.

[0038] Examples of the method by surface treatment include, for example, roughening treatment such as sandblasting treatment and solvent treatment, and oxidation treatment 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 can be used without particular limitation.

[0039] In addition, the adhesion can also be improved by an adhesion improving layer such as an easy adhesion layer. As the easy adhesion layer, acrylic resin, polyester resin, polyurethane resin, polyether resin, etc. can be used without particular limitation, and can be formed by a general coating method, preferably a so-called in-line coat formulation.

[0040] The above-mentioned polyester film can be produced, for example, through a polymerization step of homogeneously dispersing inorganic particles in a monomer solution that is part of the polyester raw material, filtering the solution, adding it to the remaining part of the polyester raw material, and performing polyester polymerization, and a film forming step of melt-extruding the polyester in a sheet form through a filter, cooling it, and stretching it to form a base film.

[0041] Next, a method for producing a biaxially stretched polyester film 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 of the base film, but it is not limited thereto. Also, the number of layers such as single-layer configuration and multi-layer configuration is not limited. In addition, in an embodiment using a polyethylene naphthalate (PEN) film instead of the PET film, the polyester film according to the present invention can be produced in the same manner.

[0042] After mixing and drying PET pellets at a predetermined ratio, they are supplied to a known extrusion machine for melt lamination, extruded into a sheet form from a slit die, and cooled and solidified on a casting roll to form an unstretched film. In the case of a single layer, one extruder may be sufficient, but when manufacturing a multilayer film, two or more extruders, a manifold or a merging block with two or more layers (for example, a merging block having a square merging portion) are used to laminate a plurality of film layers constituting each outermost layer, extrude two or more sheets from the die, and cool them with a casting roll to form an unstretched film.

[0043] In this case, during melt extrusion, it is preferable to perform high-precision filtration at an arbitrary location where the molten resin is maintained at about 300 °C in order to remove foreign substances contained in the resin. The filter medium used for the high-precision filtration of the molten resin is not particularly limited, but a filter medium of a sintered stainless steel body is preferable because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, Cu and high-melting-point organic substances.

[0044] Furthermore, the filtration particle size of the filter medium (initial filtration efficiency 95%) is preferably 20 μm or less, particularly preferably 15 μm or less. When the filtration particle size of the filter medium (initial filtration efficiency 95%) exceeds 20 μm, foreign substances with a size of 20 μm or more cannot be sufficiently removed. Although the productivity may decrease by performing high-precision filtration of the molten resin using a filter medium with a filtration particle size of the filter medium (initial filtration efficiency 95%) of 20 μm or less, it is preferable for obtaining a film with fewer protrusions due to coarse particles.

[0045] (Regarding the refractive index in the bending direction) In the present invention, the refractive index in at least one of the longitudinal direction (machine flow direction) and the width direction of the polyester film is preferably 1.610 or more and 1.750 or less. For example, it is 1.610 or more and 1.710 or less, and more preferably 1.630 or more and 1.680 or less. In one aspect, when the refractive index in the longitudinal direction of the polyester film is 1.610 or more, the crystallinity can be efficiently improved, and the high-temperature holding angle can be improved. When it is 1.750 or less, the stress during bending can be reduced, and both the holding angle at room temperature and the high-temperature holding angle can be improved. Conversely, in a case where the refractive index in the width direction of the polyester film is within the above range, it is desirable that the refractive index in the longitudinal direction of the polyester film is higher than the refractive index in the width direction of the polyester film. The refractive index in the bending direction of the polyester film is preferably 1.610 or more and 1.750 or less. For example, it is 1.610 or more and 1.710 or less, and more preferably 1.630 or more and 1.680 or less. Here, the bending direction refers to a direction orthogonal to the folding portion (reference numeral 21) assumed in the use of the foldable display, as shown by reference numeral 22 on the polyester film (reference numeral 2) in FIG. 2. When the refractive index in at least one of the longitudinal direction and the width direction is 1.610 or more and 1.750 or less, there is little deformation when repeatedly folded, and it is preferable because there is no risk of degrading the image quality of the foldable display. The refractive index in the bending direction of the polyester film is more preferably 1.630 to 1.680. Of course, the direction is preferably the above-mentioned bending direction. When it is 1.610 or more, the crystallinity can be efficiently improved, and the high-temperature holding angle can be improved. When it is 1.750 or less, the stress during bending can be reduced, and both the holding angle at room temperature and the high-temperature holding angle can be improved. The refractive index of the polyester film can be effectively adjusted by adjusting the draw ratio and the drawing temperature. Further, a relaxation step in the drawing direction or multi-stage drawing may be used for adjusting the refractive index. When performing multi-stage drawing, it is preferable that the draw ratio in the second and subsequent stages is higher than the draw ratio in the first stage.

[0046] Controlling the refractive index in at least one of the longitudinal direction (machine flow direction) and the width direction of the polyester film within the above range, more preferably, controlling the refractive index in the bending direction within the above range can reduce the fatigue caused by the compressive stress applied to the inside during folding. It is considered that the fatigue caused by the compressive stress mainly occurs in the crystal part, and the less the crystals in the bending direction, the less likely it is to fatigue. Therefore, it is considered that when the refractive index in the bending direction becomes smaller than the refractive index in the direction perpendicular to the bending direction, the amount of oriented crystals in the bending direction is reduced and the compression fatigue is suppressed.

[0047] Also, the creep phenomenon caused by the tensile stress applied to the outside during folding can be suppressed by reducing the refractive index. It is considered that the fatigue caused by the tensile stress mainly occurs in the amorphous part, and the alignment of molecular chains occurs due to the repeatedly applied stress, resulting in deformation. It can be inferred that the less the molecular chains arranged in the bending direction, the less the deformation due to alignment. Also, since the fatigue due to tension can be suppressed with less amorphous part, it is preferable that the degree of crystallinity, that is, the density is higher.

[0048] In the present invention, it is preferable that the draw ratio in at least one of the longitudinal direction (machine flow direction) and the width direction of the unstretched polyester sheet is 1.0 times or more and 3.4 times or less, and more preferably 1.4 times or more and 2.3 times or less. And it is preferable that the drawing direction is the above-mentioned bending direction. When the draw ratio is 3.4 times or less, it is preferable because film thickness unevenness does not occur. As the drawing temperature, 120°C or more and 150°C or less is preferable, and 125°C or more and 145°C or less is more preferable. In addition, as the heating method during stretching, conventionally known means such as a hot air heating method, a roll heating method, and an infrared heating method can be adopted. By setting the drawing temperature to 125°C or more and 145°C or less, significant thickness unevenness due to drawing at the above draw ratio can be prevented.

[0049] (Regarding the refractive index in the direction of the folded part) The refractive index in the direction orthogonal to the direction in which the refractive index of the above polyester film is 1.610 or more and 1.750 or less is preferably 1.750 to 1.870. That is, the refractive index in the direction orthogonal to the bending direction (the direction of the folded portion) is preferably 1.750 or more and 1.870 or less. By making it 1.750 or more and 1.870 or less, the deformation when folded in the bending direction can be reduced. By making it 1.870 or less, the occurrence of cracks in the direction of the folded portion can be suppressed, and further breakage can be suppressed. In addition, breakage in the winding process after stretching can be suppressed. By making it 1.750 or more, the density can be increased and the high-temperature holding angle can be improved. For example, when the longitudinal direction of the polyester film is the bending direction, the direction orthogonal to the bending direction (the direction of the folded portion) corresponds to the width direction of the polyester film, which is the direction orthogonal to the bending direction (the direction of the folded portion). The refractive index in the direction orthogonal to the bending direction is more preferably 1.770 to 1.830. Also, when comparing the refractive index in the bending direction with the refractive index in the direction orthogonal to the bending direction (the direction of the folded portion), it is desirable that the refractive index in the bending direction is low. With this aspect, the deformation when folded in the bending direction can be reduced. Also, the occurrence of cracks in the direction of the folded portion can be suppressed, and further breakage can be suppressed. Moreover, breakage in the winding process after stretching can be suppressed. In addition, the density can be increased and the high-temperature holding angle can be improved. Examples of the method for adjusting the refractive index in the direction orthogonal to the bending direction include draw ratio, pre-drawing temperature, drawing temperature, multi-stage drawing, and film relaxation. The draw ratio is preferably 3.3 to 5.0 times, more preferably 3.5 to 4.5 times. Also, the pre-drawing temperature in the direction orthogonal to the bending direction is preferably 125 to 145 °C. When performing multi-stage drawing in the direction orthogonal to the bending direction, it is preferable to make the draw ratio in the second and subsequent stages higher than that in the first stage. The film relaxation may be carried out by 0 to 10% in either the machine flow direction (longitudinal direction) or the perpendicular direction (width direction).

[0050] (Regarding the refractive index in the thickness direction) The refractive index in the thickness direction is preferably 1.520 or less. More preferably 1.515 or less, still more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. Although it is preferable that the refractive index in the thickness direction is low, from the aspect of stable production, 1.3 or more is preferable, and even 1.4 or more may be acceptable. Particularly preferably, it is 1.410 or more.

[0051] (Regarding the density of the polyester film) The density of the polyester film is preferably 1.349 g / cm 3 or more. More preferably, it is 1.350 g / cm 3 or more. By setting it to 1.350 g / cm 3 or more, the high-temperature hold angle can be improved. The higher the density, the more preferable it is. Although it is somewhat affected by the presence or absence of particles in the film, etc., it is preferably 1.40 g / cm 3 or less, and more preferably 1.395 g / cm 3 or less. When the density of the polyester film is 1.349 g / cm 3 or more, the crystallization of the polyester film of the present invention can be sufficiently achieved, and the deformation at 85 °C can be suppressed. Also, an increase in the thermal shrinkage rate can be suppressed, and dimensional changes due to heat generation of the device can be suppressed. By setting the heat setting temperature during film formation to 210 to 270 °C, crystallization can proceed, and the density can be effectively increased within the above range.

[0052] The bending direction of the polyester film is preferably made to correspond to the longitudinal direction (machine flow direction). By doing so, it is easy to lower the refractive index in the bending direction and improve the bendability at the second biaxial stretching stage. That is, it is preferable to obtain a polyester film by stretching an unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.0 to 2.3 times, more preferably 1.4 to 2.1 times. And in the width direction, it can be said that it is a preferable embodiment to stretch at a stretching ratio of 3.3 to 5.0 times, more preferably 3.5 to 4.5 times.

[0053] The polyester film of the present invention has a high-temperature holding angle in the bending direction of 70° or more. Here, the high-temperature holding angle refers to the angle formed by the fold after being fixed at 85°C for 18 hours so that a strain of 1.7% is applied to both surfaces of the bent portion. The bending direction refers to the direction orthogonal to the folded portion. The high-temperature holding angle in the bending direction is 71° or more, for example, 72° or more. The higher it is, the better, and 180° is most preferable. However, the high-temperature holding angle in the bending direction may be 180° or less, for example, 170° or less and still has sufficient function. When the high-temperature holding angle in the bending direction is within the above range, deformation at 85°C can be suppressed. Also, an increase in the heat shrinkage rate can be suppressed, and dimensional changes due to heat generation of the device can be suppressed. Therefore, in the present invention, deformation does not occur even after repeated folding in a high-temperature region, and image disturbance at the folded portion of the display can be suppressed. Furthermore, a portable terminal device equipped with a foldable display using the polyester film provides beautiful images, is rich in functionality, and is excellent in convenience such as portability. Note that the method for measuring the high-temperature holding angle in the bending direction is exemplified in the examples.

[0054] (Easy-adhesion layer) In the present invention, in order to improve the adhesiveness between the polyester film and the hard coat layer or the like, it is also preferable to laminate an easy-adhesion layer on at least one side of the polyester film of the present invention. The easy-adhesion layer can be obtained by applying a coating solution for forming the easy-adhesion layer to one or both sides of an unstretched or uniaxially stretched film in the longitudinal direction, then heat-treating and drying as necessary, and further stretching in at least one direction that has not been stretched. Heat treatment can also be performed after biaxial stretching. The final coating amount of the easy-adhesion layer is preferably controlled to 0.005 to 0.20 g / m 2 It is preferable that the adhesiveness is obtained when the coating amount is 0.005 g / m 2 or more. On the other hand, it is preferable that the anti-blocking property is obtained when the coating amount is 0.20 g / m 2 or less.

[0055] As the resin to be contained in the coating liquid used for laminating the easy-adhesion layer, for example, polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. can be used without particular limitation. Examples of the crosslinking agent to be contained in the coating liquid for forming the easy-adhesion layer include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, carbodiimide compounds, etc. Two or more of these can also be mixed and used. Due to the nature of in-line coating, these are preferably coated with an aqueous coating liquid, and the above-mentioned resin and crosslinking agent are preferably water-soluble or water-dispersible resins and compounds.

[0056] It is preferable to add particles to the easy-adhesion layer in order to impart easy-sliding properties. The average particle size of the fine particles is preferably 2 μm or less. When the average particle size of the particles exceeds 2 μm, the particles are likely to fall off from the easy-adhesion layer. Examples of the particles to be contained in the easy-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, calcium fluoride, etc., and organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, silicone-based, etc. These may be added to the easy-adhesion layer alone, or two or more of them can be added in combination.

[0057] Also, as the method for applying the coating liquid, known methods can be used in the same manner as the above-mentioned coating layer. For example, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, etc. can be mentioned, and these methods can be carried out alone or in combination.

[0058] (Hard coat layer) When used as a surface protection film that protects a display by positioning the polyester film of the present invention on the surface of a foldable display, it preferably has a hard coat layer on at least one of its surfaces. The hard coat layer is preferably located on the display surface side on the polyester film and used in the display. As the resin for forming the hard coat layer, acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, epoxy, etc. can be used without particular limitation. Also, two or more materials can be mixed and used, and particles such as inorganic fillers and organic fillers can be added.

[0059] (Film thickness of the hard coat layer) The film thickness of the hard coat layer is preferably 1 to 50 μm. If it is 1 μm or more, it is sufficiently cured and the pencil hardness becomes high, which is preferable. Also, by making the thickness 50 μm or less, curl due to the curing shrinkage of the hard coat can be suppressed, and the handleability of the film can be improved.

[0060] (Coating method) As the coating method of the hard coat layer, a Meyer bar, a gravure coater, a die coater, a knife coater, etc. can be used without particular limitation, and can be appropriately selected according to the viscosity and film thickness.

[0061] (Curing conditions) As the curing method of the hard coat layer, energy rays such as ultraviolet rays and electron beams, and curing methods by heat can be used. In order to reduce damage to the film, a curing method by ultraviolet rays or electron beams is preferable.

[0062] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or more, and more preferably 4H or more. If it has a pencil hardness of 3H or more, it is not easily scratched and does not reduce visibility. Generally, the higher the pencil hardness of the hard coat layer, the better, but it may be 9H or less, 8H or less, or 6H or less, and it can be used practically without problems.

[0063] (Properties of the hard coat layer) The hard coat layer in the present invention can be used for the purpose of protecting the display by increasing the pencil hardness of the surface as described above, and preferably has a high transmittance. The total light transmittance of the hard coat film is preferably 87% or more, more preferably 88% or more. If the transmittance is 87% or more, sufficient visibility can be obtained. Generally, the higher the total light transmittance of the hard coat film, the better, but from the viewpoint of stable production, it is preferably 99% or less, and may be 97% or less. Also, 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. If the haze is 3% or less, the visibility of the image can be improved. Generally, the lower the haze, the better, but from the viewpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0064] Other functions may be further added to the hard coat layer. For example, a hard coat layer with added functions such as an antiglare layer, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer having a certain pencil hardness as described above is also preferably applied in the present invention.

[0065] Also, a hard coat layer may be provided even when used as a base film of a touch panel module. When, for example, an ITO layer is used as the transparent electrode layer of the touch panel module, in order to make the electrode pattern less visible, it is preferable to provide a refractive index adjustment layer between the base film and the transparent electrode layer. In this case, the hard coat layer itself may also serve as a refractive index adjustment layer, or a separate refractive index adjustment may be laminated.

[0066] In another aspect, the polyester film for a foldable display of the present invention can be used for a foldable display disposed as a back surface protection film. For example, the polyester film for a foldable display of the present invention can be disposed on a single continuous polyester film through the folding portion of the foldable display.

[0067] In another aspect, a portable terminal device having the polyester foldable display of the present invention is provided.

Examples

[0068] Next, the present invention will be described using examples and comparative examples. First, the evaluation methods of the characteristic values implemented in the present invention are shown below.

[0069] (1) Intrinsic viscosity After the film or polyester resin was pulverized and dried, it was dissolved in a mixed solvent of phenol / tetrachloroethane = 60 / 40 (mass ratio). After subjecting this solution to centrifugation to remove inorganic particles, using an Ubbelohde viscometer, the flow-down time of the solution with a concentration of 0.4 (g / dl) and the flow-down time of only the solvent were measured at 30 °C, and from the ratio of these times, assuming that the Huggins constant is 0.38, the intrinsic viscosity was calculated using Huggins' equation. The same calculation formula was used for evaluation of both polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).

[0070] (2) Refractive index Using a laser refractometer (Model 2010 Prism Coupler) manufactured by Metricon, a single sample film was sandwiched at a pressure of 40 graduations of the built-in pressure gauge, and measurement was performed with a laser beam having a wavelength of 633 nm to obtain a spectrum chart. On the obtained spectrum chart, the point at which the detector output rapidly decreased was read, and this value was taken as the refractive index. The refractive index in the longitudinal direction and the width direction was measured in the TE measurement mode, and the refractive index in the thickness direction was measured in the TM mode.

[0071] (3) Total light transmittance, haze Measurement was performed using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH5000).

[0072] (4) Density The density was measured according to the method (density gradient tube method) conforming to JIS K 7112:1999. (Unit: g / cm 3 )

[0073] (5) Maximum heat shrinkage rate The sample film was cut into 10 mm in length and 250 mm in width. Marks were made at 200 mm intervals along the long side in the direction where measurement was desired, and the interval A between the marks was measured under a constant tension of 5 g. Subsequently, the sample film was left in an oven at 150 °C in an atmosphere without load for 30 minutes, then taken out of the oven and cooled to room temperature. Thereafter, the interval B between the marks was determined under a constant tension of 5 g, and the heat shrinkage rate (%) was determined by the following formula. The heat shrinkage rate was measured at positions equally divided into three in the width direction of the sample film, and the average value of the three points was taken as the heat shrinkage rate (%). Heat shrinkage rate (%) = [(A - B) × 100] / A For the two directions of the bending direction and the folding direction, the sample film was cut so that the length and width were different, and measured separately. The data in the direction with the larger measured value was taken as the maximum heat shrinkage rate (%).

[0074] (6) High-temperature hold angle Evaluate the strength of the fold formed when a strain of 1.7% is applied to both surfaces of the bent portion respectively. Figure 3 is a schematic diagram for explaining the measurement method of the hold angle in the bending direction. The sample film (reference numeral 3) was cut into 10 mm in the width direction and 50 mm in the flow direction. Two PTFE plates (reference numeral 31) were overlapped, and in the case of a 50 μm sample film, a PTFE plate (reference numeral 32) with a thickness of 3 mm was inserted as a spacer to create a gap. Double-sided tape was attached to both ends of the sample, and it was inserted into the 3 mm gap of the PTFE plate in a bent state, and both ends were fixed with double-sided tape. After being placed in an 85 °C dry environment for 18 hours, it was taken out from between the two PTFE plates (reference numeral 31), and the angle (reference numeral 33) formed by the fold mark on the film was measured 5 minutes later. This angle was taken as the high-temperature hold angle. To keep the strain constant, the thickness 32 of the PTFE plate used as a spacer is changed according to the thickness of the film. Figure 4 shows an enlarged schematic view of the sample film (reference numeral 3) sandwiched between two PTFE plates (reference numeral 31 in Figure 3). The neutral plane, which is not subjected to either the above-mentioned compressive stress or tensile stress, is defined as the center in the thickness direction (the dashed line in the figure), and the differences between the neutral plane and both surfaces are taken as strains. That is, the strains applied to both surfaces can be expressed by the following formula. In Figure 4, reference numeral 41 indicates the outermost diameter of the sample film, reference numeral 42 indicates the diameter of the neutral plane in the sample film, and reference numeral 43 indicates the innermost diameter of the sample film.

[0075] In the evaluation of the high-temperature hold angle, the strain (1.7%) can be expressed by the following method. Strain (1.7%) =(|Half circumference of the outermost or innermost surface - Half circumference of the neutral plane| / Half circumference of the neutral plane) × 100 Here, when the half circumference is calculated based on the thickness t (mm) of the sample film and the bending diameter (outermost diameter), that is, the thickness d (mm) of the spacer used, they can be obtained respectively by the following formulas. Half circumference of the outermost surface = d × π / 2 Half circumference of the neutral plane = (d - t) × π / 2 Half circumference of the innermost surface = (d - 2t) × π / 2 From the above, when determining the strain to 1.7%, taking the thickness t (mm) of the sample film and the bending diameter, that is, the thickness d (mm) of the spacer used, the thickness of the spacer (PTFE plate) used is determined by the following formula. The spacer thickness for typical film thicknesses is shown as follows, for example. Spacer thickness d (mm) = Film thickness (mm) × 60 For example, in the case of the above-mentioned sample film with a thickness of 50 μm, the outermost diameter (reference numeral 41) is the same as the thickness d of the spacer and is 3 mm. The innermost diameter (reference numeral 43) is 2.9 mm, and the diameter of the neutral plane (reference numeral 42) is 2.95 mm. Here, in the formula showing the above strain, the half circumferences of the outermost surface and the innermost surface can be appropriately selected.

[0076] (Preparation of polyethylene naphthalate pellets) 100 parts of dimethyl naphthalene-2,6-dicarboxylate and 60 parts of ethylene glycol were used, and 0.03 part of manganese(II) acetate tetrahydrate was used as a transesterification catalyst. The transesterification reaction was carried out for 120 minutes while gradually raising the temperature from 150°C to 238°C. During the reaction, when the reaction temperature reached 170°C, trimethyl phosphate (added as a solution heat-treated at 135°C for 5 hours under a pressure of 0.11 - 0.16 MPa in ethylene glycol: 0.023 part in terms of trimethyl phosphate) was added. After the transesterification reaction was completed, 0.024 part of antimony trioxide was added. Then, the reaction product was transferred to a polymerization reactor, the temperature was raised to 290°C, and a polycondensation reaction was carried out under a high vacuum of 27 Pa or less to obtain polyethylene-2,6-naphthalenedicarboxylate having an intrinsic viscosity of 0.48 dl / g and substantially containing no particles.

[0077] (Preparation of polyethylene terephthalate pellets) As an esterification reaction apparatus, a continuous esterification reaction apparatus consisting of a three-stage completely mixed tank having a stirrer, a partial condenser, a raw material inlet, and a product outlet was used. TPA was set at 2 tons / hr, EG was set at 2 moles per mole of TPA, and antimony trioxide was set at an amount such that the Sb atom was 160 ppm with respect to the produced PET. These slurries were continuously fed into the first esterification reactor of the esterification reaction apparatus and reacted at 255°C with an average residence time of 4 hours at atmospheric pressure. Next, the reaction product in the first esterification reactor was continuously taken out of the system and supplied to the second esterification reactor. EG distilled off from the first esterification reactor was supplied to the polymer to be produced (produced PET) in an amount of 8% by mass. Further, an EG solution containing magnesium acetate in an amount such that the Mg atom content was 65 ppm with respect to the produced PET and an EG solution containing TMPA in an amount such that the P atom content was 20 ppm with respect to the produced PET were added. The reaction was carried out at 260 °C with an average residence time of 1.5 hours under normal pressure. Next, the reaction product in the second esterification reactor was continuously taken out of the system and supplied to the third esterification reactor. Further, an EG solution containing TMPA in an amount such that the P atom content was 20 ppm with respect to the produced PET was added. The reaction was carried out at 260 °C with an average residence time of 0.5 hours under normal pressure. The esterification reaction product produced in the third esterification reactor was continuously supplied to a three-stage continuous polycondensation reactor for polycondensation. Further, it was filtered through a filter medium of a stainless steel sintered body (nominal filtration accuracy: 90% cut-off for 5-μm particles) to obtain polyethylene terephthalate pellets (a) having an intrinsic viscosity of 0.58 dl / g.

[0078] (Polymerization of Urethane Resin) Into a four-necked flask equipped with a stirrer, Dimroth condenser, nitrogen inlet tube, silica gel drying tube, and thermometer, 72.96 parts by mass of 1,3-bis(isocyanatomethyl)cyclohexane, 12.60 parts by mass of dimethylolpropionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol having a number average molecular weight of 2000, 85.00 parts by mass of acetonitrile as a solvent, and 5.00 parts by mass of N-methylpyrrolidone were charged. The mixture was stirred at 75 °C for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction solution reached a predetermined amine equivalent. Next, after the temperature of this reaction solution was lowered to 40 °C, 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer D solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisper capable of high-speed stirring, adjusted to 25 °C, and while stirring and mixing at 2000 min-1, an isocyanate group-terminated prepolymer was added and dispersed in water. Thereafter, under reduced pressure, a part of acetonitrile and water was removed to prepare a water-soluble polyurethane resin (A) having a solid content of 35% by mass.

[0079] (Polymerization of water-soluble carbodiimide compound) 200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidization catalyst were charged into a flask equipped with a thermometer, a nitrogen gas introduction tube, a reflux condenser, a dropping funnel, and a 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 obtained carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100 °C for 24 hours. Water was gradually added thereto at 50 °C to obtain a yellow transparent water-soluble carbodiimide compound (B) having a solid content of 40% by mass.

[0080] (Preparation of coating liquid for forming easy-adhesion layer) The following coating agents were mixed to prepare a coating liquid. 16.97 parts by mass of water 21.96 parts by mass of isopropanol 3.27 parts by mass of polyurethane resin (A) 1.22 parts by mass of water-soluble carbodiimide compound (B) 0.51 parts by mass of particles (Silica sol having an average particle diameter of 40 nm, solid content concentration: 40% by mass) 0.05 parts by mass of surfactant (Silicone-based, solid content concentration: 100% by mass)

[0081] (Example 1) Polyethylene naphthalate pellets were supplied to an extruder and melted at 310 °C. This polymer was filtered through a stainless sintered filter medium (nominal filtration accuracy: 95% cut of 10 μm particles), extruded in a sheet form from a die, and then brought into contact with a casting drum having a surface temperature of 60 °C using an electrostatic application casting method to be cooled and solidified, thereby producing an unstretched film. The above coating liquid for forming an easy-adhesion layer was applied to both sides of the unstretched film by a roll coating method and then dried at 80 °C for 20 seconds. The coating amount after final (after biaxial stretching) drying was 0.06 g / m 2It was adjusted to become. Then, it was led to a tenter, preheated at 140 °C, laterally stretched 4.2 times at 135 °C, width-fixed, heat-fixed at 240 °C for 5 seconds, and further relaxed by 1% in the width direction at 180 °C to obtain a polyethylene naphthalate film with a thickness of 50 μm. The evaluation results are shown in Table 1.

[0082] (Examples 2 to 6) After obtaining an unstretched film in the same manner as in Example 1, the unstretched film was uniformly heated to 120 °C using a heating roll, heated to 135 °C with a non-contact heater, and roll stretching (longitudinal stretching) was performed at the MD magnification described in Table 1. A polyester film was obtained in the same manner as in Example 1 except that the longitudinal stretching magnification described in Table 1 was changed.

[0083] (Example 7) After obtaining an unstretched film in the same manner as in Example 1, the unstretched film was uniformly heated to 120 °C using a heating roll, heated to 140 °C with a non-contact heater, and roll stretching (longitudinal stretching) was performed at the MD magnification described in Table 1. A polyester film was obtained in the same manner as in Example 1 except that the longitudinal stretching magnification described in Table 1 was changed.

[0084] (Comparative Example 1) Polyethylene terephthalate pellets were supplied to an extruder and melted at 285 °C. This polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut for 10 μm particles), extruded in a sheet form from a die, and then brought into contact with a casting drum having a surface temperature of 30 °C using an electrostatic application casting method to be cooled and solidified to form an unstretched film. This unstretched film was uniformly heated to 75 °C using a heating roll, heated to 85 °C with a non-contact heater, and roll stretching (longitudinal stretching) was performed 1.4 times. After the obtained uniaxially stretched film was coated on both sides with the above coating solution for forming an easy-adhesion layer by a roll coating method, it was dried at 80 °C for 20 seconds. Note that the coating amount after final (after biaxial stretching) drying is 0.06 g / m 2It was adjusted to be as follows. Then, it was led to a tenter, preheated at 105°C, laterally stretched 4.0 times at 95°C, width-fixed, heat-fixed at 230°C for 5 seconds, and further relaxed by 4% in the width direction at 180°C to obtain a polyethylene terephthalate film with a thickness of 50 μm.

[0085] (Comparative Example 2) A polyester film was obtained in the same manner as in Comparative Example 1, except that the longitudinal stretching ratio was changed to 1.4 times as shown in Table 1.

[0086] The obtained polyester film was bonded to the non-visible side of the organic EL module via an adhesive layer with a thickness of 25 μm, and a smartphone-type foldable display that could be folded in half at the center with a corresponding bending radius of 3 mm in FIG. 1 was created. The polyester film was arranged on the non-visible side of a single continuous display through the folding part and was bonded to a polyimide film having a barrier layer that was an organic EL substrate. Those using the polyester film of each example satisfied the operation and visibility as a smartphone that could be folded in half at the center and carried around. Also, there were no problems with operation and visibility even at high temperatures. On the other hand, the foldable displays using the polyester films of each comparative example felt that image distortion occurred at the folding part of the display as the frequency of use at high temperatures increased, and they were not very preferable. Also, some had dents and scratches on the surface.

[0087]

Table 1A

[0088]

Table 1B

Industrial Applicability

[0089] The foldable display using the polyester film for the foldable display of the present invention can maintain mass productivity and, for example, does not cause deformation after the polyester film located on the back surface of the foldable display is repeatedly folded, so that image distortion does not occur at the folded portion of the display. In particular, a portable terminal device or an image display device equipped with a foldable display using the polyester film of the present invention as a back surface protection film provides beautiful images, is rich in functionality, excellent in convenience such as portability, and has high reliability.

Explanation of Signs

[0090] 1: Foldable display 11: Bending radius 2: Polyester film for the surface protection film of the foldable display 21: Folded portion 22: Bending direction (direction orthogonal to the folded portion) 3: Sample film 31: PTFE plate 32: Spacer 33: Hold angle 41: Outermost diameter 42: Neutral plane diameter 43: Innermost diameter

Claims

1. The refractive index in the bending direction is 1.644 or more and 1.730 or less, the refractive index in the folding direction is 1.770 or more and 1.854 or less, the refractive index in the thickness direction is 1.486 or more and 1.508 or less, and the density is 1.349 g / cm ３ The polyethylene naphthalate film for folding displays is as above:

2. The polyethylene naphthalate film for folding displays according to claim 1, having a total light transmittance of 85% or more.

3. The polyethylene naphthalate film for folding displays according to claim 1, having an easy-adhesion layer on at least one surface of the polyethylene naphthalate film.

4. A foldable display comprising the polyethylene naphthalate film for a foldable display according to any one of claims 1 to 3 and a polarizer, the polyethylene naphthalate film being disposed on at least a viewing side surface of the polarizer, A foldable display, wherein the polyethylene naphthalate film is disposed as a continuous single polyethylene naphthalate film across a fold of the foldable display.

5. A mobile terminal device comprising the foldable display according to claim 4.

Images