Polyester film and use thereof

Abstract

[PROBLEM] To provide a foldable display in which no distortion occurs in an image displayed in a folded portion after repeated bending, and which is excellent in mass productivity, a polyester film for a foldable display in which no cracks occur in a folding portion is provided. [SOLUTION] A polyester for a foldable display, which is a polyester film having a film thickness of 10 to 125 μm, the polyester film having a high-temperature retention angle in a bending direction of 70° or more; here, the high-temperature retention angle refers to an angle of a bending mark formed after heating and fixing at 85°C for 18 hours in a manner in which 1.7% strain is applied to both surfaces of a bent portion. In addition, the bending direction refers to a direction orthogonal to the folding portion.

Description

TECHNICAL FIELD

[0001] The present application 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 relates to a foldable display and a portable terminal device which are less likely to cause image distortion due to deformation of a film even when repeatedly folded, and the aforementioned polyester film for a foldable display. BACKGROUND

[0002] The film of a portable terminal device is being made lighter, and portable terminal devices typified by smartphones have become widespread. Various functions are required for portable terminal devices, and, on the contrary, convenience is also required. Therefore, in the widespread portable terminal devices, simple operation with one hand and further storage in a pocket of clothes or the like are assumed as a premise, and thus a small screen size of about 6 inches is sometimes required.

[0003] On the other hand, in tablet terminal devices having a screen size of 7 inches to 10 inches, not only for image content and music, but also for business use, drawing use, reading, and the like, high functionality is assumed. However, it cannot be operated with one hand, and mobility is poor, and thus there is a problem in terms of convenience.

[0004] In order to achieve these problems, a method of making a plurality of displays compact by connecting them has been proposed (see Patent Literature 1). However, the invention of Patent Literature 1 has a problem in that visibility is reduced because the image is cut off due to the portion in which the frame remains, and has not become widespread.

[0005] On the other hand, in recent years, portable terminals equipped with a flexible display and a foldable display have been proposed. By this means, the image is not cut off, and it is possible to carry the portable terminal device equipped with a large-screen display with good convenience.

[0006] Here, in the case of a display and a portable terminal device that do not have a folding structure, the surface of the display can be protected by a raw material such as glass that does not have flexibility. However, in the case of a foldable display in which a one-surface display is formed by folding a portion, a hard coat film or the like that has flexibility and can protect the surface must be used.

[0007] However, in a foldable display, the portion that contacts the constant folding portion is repeatedly bent, and thus the film of the portion is deformed over time, and there is a problem in that the image displayed in the display is distorted. In addition, not only the surface protection film but also various portions such as a polarizing plate, a phase difference plate, a touch panel substrate, a substrate of a display unit such as organic EL, and a back protection member can use a film in a foldable display, and durability to repeated folding is also required for these films.

[0008] 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 due to the change in film thickness, and there is a problem of lacking mass productivity.

[0009] In addition, a method of adjusting the refractive index of the bending direction of the polyester film has also been proposed (see Patent Document 3). However, in a film using polyethylene terephthalate, there is a concern that it cannot be used for applications that require more reliability (high temperature region).

[0010] Prior Art Documents

[0011] Patent Documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 2010-228391

[0013] Patent Document 2: Japanese Patent Application Publication No. 2016-155124

[0014] Patent Document 3: International Publication No. 2018 / 150940 SUMMARY

[0015] PROBLEMS TO BE SOLVED BY THE INVENTION

[0016] The present invention aims to solve the problems of the related art display member described above, and provides a polyester film for a foldable display that can provide a foldable display with excellent mass productivity, without concern of distortion in an image displayed in a folded portion after repeated bending, and a portable terminal device equipped with such a foldable display.

[0017] Further, the present invention aims to provide a polyester film for a foldable display that does not produce a fold in a folded portion in a high temperature region.

[0018] SOLUTION TO PROBLEM

[0019] That is, the present invention includes the following configuration.

[0020] 1. A polyester film for a foldable display, which is a polyester film with a thickness of 10 to 125 μm, the polyester film having a high temperature retention angle in the bending direction of 70° or more.

[0021] (Note that the high temperature retention angle refers to the angle of a fold mark formed after heating and fixing at 85°C for 18 hours in a manner that 1.7% strain is applied to both surfaces of the bent portion. In addition, the bending direction refers to the direction orthogonal to the folded portion.)

[0022] 2. The polyester film for a foldable display according to item 1, having a density of 1.349 g / cm 3 and above.

[0023] 3. The polyester film for a foldable display according to claim 1 or 2, wherein the polyester is polyethylene naphthalate.

[0024] 4. The polyester film for a foldable display according to any one of claims 1 to 3, wherein an easy-adhesion layer is provided on at least one side of the polyester film.

[0025] 5. A foldable display, which is a foldable display provided with the polyester film for a foldable display according to any one of claims 1 to 4 as a back surface protective film.

[0026] The polyester film is provided as a continuous single polyester film via a folding portion of the foldable display.

[0027] 6. A portable terminal device provided with the foldable display according to claim 5.

[0028] Effects of the Invention

[0029] The foldable display using the polyester film for a foldable display according to the present application maintains productivity, and the polyester film does not cause deformation even after repeated folding in a high temperature region, and does not cause distortion of an image in a folding portion of the display. The portable terminal device provided with the foldable display using the aforementioned polyester film provides a beautiful image, is rich in functionality, and is excellent in portability and the like. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a schematic view for showing a bending radius when the foldable display of the present application is folded.

[0031] Figure 2 is a schematic view for showing a bending direction of the polyester film for a foldable display in the present application.

[0032] Figure 3 is a schematic view for explaining a measuring method of a holding angle of the bending direction.

[0033] Figure 4 is an enlarged schematic view of a sample film (symbol 4) in a state of being sandwiched between two PTFE plates. DETAILED DESCRIPTION

[0034] (Display)

[0035] The display according to the present application means all display devices, and as a kind of display, there are LCD, organic EL display, inorganic EL display, LED, FED, and the like. For example, it is preferable to have an LCD, organic EL, inorganic EL having a structure capable of being bent. In particular, it is particularly preferable to have an organic EL, inorganic EL capable of reducing the layer constitution, and further preferable to have an organic EL having a wide color gamut.

[0036] (Foldable display)

[0037] For the foldable display, one continuous display can be folded in half or the like when moved. By halving the size by folding, the mobility can be improved. The bending radius of the foldable display is preferably 5 mm or less, and further preferably 3 mm or less. When 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. According to the present application, even with such a bending radius, the crease can be suppressed.

[0038] The bending radius is preferably 0.1 mm or more, can be 0.5 mm or more, and can be 1 mm or more. Even with a bending radius of 0.1 mm, thinning sufficient for use when moved can be achieved.

[0039] The bending radius at the time of folding is Figure 1 The value obtained by measuring the position of the symbol 11 in the foldable display 1 in the schematic view is the radius of the inside of the folded portion at the time of folding. Note that the surface protection film described later can be located on the outside of the fold of the foldable display, or can be located on the inside.

[0040] In addition, the foldable display can be tri-folded, quad-folded, and further can be a so-called rollable roll-up type, all of which fall within the scope of the so-called foldable display of the present application.

[0041] In addition, the polyester film according to the present application can not only be bent in the length direction as shown in Figure 1 , but also can be bent in the width direction.

[0042] The polyester film for foldable displays of the present application can be used in any part as long as it is a constituent member of the foldable display. Hereinafter, the representative constitution of the foldable display and the part in which the polyester film of the present application can be used will be described taking an organic EL display as an example. Note that hereinafter, the polyester film for foldable displays of the present application will sometimes be referred to simply as the polyester film of the present application.

[0043] (Foldable organic EL display)

[0044] As a necessary constitution of the foldable organic EL display, an organic EL assembly is provided, but a circular polarizing plate, a touch panel assembly, a surface protection film, a back protection film, and the like can further be provided as needed.

[0045] (Organic EL assembly)

[0046] The general constitution of an organic EL component is formed of an electrode / electron transport layer / light emitting layer / hole transport layer / transparent electrode. As a substrate on which an electrode, further an electron transport layer, a light emitting layer, a hole transport layer are provided, the polyester film of the present application can be used. It can be particularly preferable to use as a substrate of a transparent electrode. In this case, the substrate film is required to have high barrier properties against water vapor and oxygen, and therefore, it is preferable to provide a barrier layer such as a metal oxide layer on the polyester film of the present application. In order to improve the barrier properties, a plurality of barrier layers can be provided, and a plurality of polyester films provided with a barrier layer can be used.

[0047] (touch panel component)

[0048] It is preferable to provide a touch panel in a portable terminal device. In the case of using an organic EL display, it is preferable to arrange a touch panel component between the upper portion of the organic EL display, or between the organic EL component / circular polarizing plate. The touch panel component has a transparent substrate such as a film and a transparent electrode arranged thereon. The polyester film of the present application can be used as this transparent substrate. In the case of using as a transparent substrate of a touch panel, it is preferable to provide a hard coat layer, a refractive index adjusting layer on the polyester film.

[0049] (circular polarizing plate)

[0050] A circular polarizing plate reflects external light by a member inside a display, and suppresses a decrease in image quality. The circular polarizing plate has a linear polarizing plate and a phase difference plate. The linear polarizing plate has a protective film on at least the viewable side of the polarizing plate. It is also possible to have a protective film on the side opposite to the viewable side of the polarizing plate, and it is also possible to directly laminate the phase difference plate on the polarizing plate. The phase difference plate uses a resin film having a phase difference such as polycarbonate, a cyclic olefin, or a member in which a phase difference layer formed of a liquid crystal compound is provided on a resin film. The polyester film of the present application can be used as the protective film of the polarizing plate, the resin film of the phase difference plate. In these cases, it is preferable that the slow axis direction of the polyester film of the present application is parallel or orthogonal to the light absorption axis direction of the polarizing plate. Note that a deviation of 10 degrees, preferably 5 degrees or less from this parallel or orthogonal is allowed.

[0051] (surface protective film)

[0052] When an impact is applied to a display from the upper portion, there is a concern that the circuit of an organic EL component, a touch panel component will be broken, and therefore, a surface protective film is provided in many cases. The polyester film of the present application is used as this surface protective film. The surface protective film has a cover window which is incorporated into the outermost surface of a display, a back film which is attached by a user himself / herself, can be peeled off, and can be exchanged, but the polyester film of the present application can be used in all of these cases. In the case of using the polyester film of the present application as a surface protective film, it is preferable to laminate a hard coat layer on at least the surface side of the polyester film. The hard coat layer is provided on the viewable side, or on the surface of a foldable display. Note that the hard coat layer can be provided on both surfaces.

[0053] (back surface protective film)

[0054] It is also preferable to provide a protective film on the back surface side of the display. Specifically, a configuration is adopted in which an adhesive layer is provided on the non- visible side of the organic EL assembly and is attached. The polyester film of the present application can be used as the protective film on the back surface side.

[0055] The polyester film of the present application can also be used for purposes other than those described above, as long as it is used in the folding portion of the constituent member of the folding-type display.

[0056] Among these, the polyester film of the present application is preferably used for a cover window surface protective film, a back film surface protective film, a base film of a touch panel assembly, and a back surface protective film. Furthermore, it is preferably used for a cover window surface protective film and a back film surface protective film.

[0057] In addition, the polyester film of the present application need not be used for all of the above, as a folding-type display. In the folding-type display, a polyimide film, a polyamide film, a polyamide-imide film, a polyester film other than the polyester film of the present application, a polycarbonate film, an acrylic film, a triacetyl cellulose film, a cyclic olefin polymer film, a polyphenylene sulfide film, a polymethyl pentene film, or the like can be appropriately used according to suitability in addition to the polyester film of the present application.

[0058] The polyester film of the present application can be a single-layer film constituted by one or more polyester resins, and in the case of using two or more polyester resins, can be a multilayer structure film, or a super-multilayer laminated film of a repeating structure.

[0059] As the polyester resin used in the polyester film, for example, a polyester film formed of polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, for example, polyethylene 2,6-naphthalate, or a copolymer in which these resins are used as the main component can be given. Among these, from the aspects of mechanical properties, heat resistance, transparency, and the like, a polyethylene naphthalate film is preferable, and a stretched polyethylene naphthalate film is particularly preferable.

[0060] In the case of mixing other polyester resins as the main component of the polyethylene naphthalate resin, the other polyester resins can be 40% by weight or less, for example, 10% by weight or less, can be 5% by weight or less, and are preferably less than 5% by weight, with respect to 100% by weight of the resin in the polyester film.

[0061] On the other hand, the polyethylene naphthalate resin can be 60% by weight or more, can be 90% by weight or more, can be 95% by weight or more, and is preferably contained in an amount exceeding 95% by weight, with respect to 100% by weight of the resin in the polyester film.

[0062] Other polyester resins are less than 5% by weight, thereby the crystallinity of the polyester film can be kept higher, and the high-temperature retention angle can be kept well.

[0063] In one embodiment, the proportion of polyethylene naphthalate in the raw material ratio of the polyester film is 100% by weight.

[0064] It should be noted that in the present application, the polyester film can contain a plurality of polyethylene naphthalate with different properties.

[0065] By increasing the proportion of polyethylene naphthalate, the polyester film will not cause deformation after repeated folding in the high temperature region, and the distortion of the image in the folding part of the display can be inhibited. Furthermore, the portable terminal device equipped with the folding display using the polyester film of the present application provides a beautiful image, rich in functionality, and the mobility and other convenience become excellent.

[0066] In the case of using a copolymer of polyester in the polyester film, as the dicarboxylic acid component of the polyester, for example, aliphatic dicarboxylic acids such as adipic acid, sebacic acid, etc.; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalene dicarboxylic acid, etc.; and polyfunctional carboxylic acids such as trimellitic acid, pyromellitic acid, etc. can be mentioned. In addition, as the diol component, for example, aliphatic diols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, neopentyl glycol, etc.; aromatic diols such as terephthalyl alcohol; alicyclic diols such as 1,4-cyclohexane dimethanol; and polyethylene glycol having an average molecular weight of 150 to 20,000 can be mentioned. The mass ratio of the copolymer component of the preferred copolymer is less than 3% by mass. In the case of less than 3% by mass, the film strength, transparency, and heat resistance can be maintained, and it is preferred.

[0067] In addition, in the production of the polyester film, the inherent viscosity of at least one or more resin pellets is preferably in the range of 0.40 to 1.0 dl / g. When the inherent viscosity is 0.40 dl / g or more, the impact resistance of the obtained film is improved, and the display internal circuit is not easily broken by external impact, which is preferred. On the other hand, when the inherent viscosity is 1.00 dl / g or less, the filter pressure of the melt fluid does not become excessively large, and the film production can be easily and stably operated, which is preferred.

[0068] For example, the inherent viscosity of at least one or more resin pellets is 0.40 to 0.8 dl / g, and the inherent viscosity can also be 0.40 to 0.7 dl / g.

[0069] The thickness of the polyester film is preferably 10 μm or more and 125 μm or less, and more preferably 25 μm or more and 100 μm or less. If the thickness is 10 μm or more, the pencil hardness improvement effect and the impact resistance improvement effect are seen, and if the thickness is 125 μm or less, the lightness is improved, and in addition, the flexibility, the processability, the handleability, and the like are excellent.

[0070] The surface of the polyester film of the present application can be smooth or can have unevenness. In the case of a surface covering use for a display, a film surface having smoothness is preferred.

[0071] 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 an image can be improved. The lower limit of the haze is preferably 0.1% or more, and can be 0.3% or more, from the viewpoint of stable production.

[0072] As described above, in order to reduce the haze, the unevenness of the film surface is preferably not too large, but from the viewpoint of handleability, in order to impart a certain degree of slipperiness, the unevenness can be present.

[0073] As a method of forming the surface unevenness, a method of compounding particles in the polyester resin layer of the surface layer, and a method of forming by coating a coating layer containing particles in the middle of film production can be used.

[0074] As a method of compounding particles in the polyester resin layer, a publicly known method can be used. For example, it can be added at any stage of the production of polyester, and preferably, it can be added in the form of a slurry of particles dispersed in ethylene glycol or the like at the esterification stage, or at a stage after the completion of the transesterification reaction and before the start of the polycondensation reaction, and the polycondensation reaction is promoted. Alternatively, a method of using a mixing extruder with a vent to blend a slurry of particles dispersed in ethylene glycol or water or the like with polyester raw materials, or a method of using a mixing extruder to blend dry particles with polyester raw materials can be used.

[0075] Among them, a method of adding a product obtained by homogeneously dispersing aggregate inorganic particles in a monomer solution that is a part of the polyester raw materials, and filtering the product before the esterification reaction, in the middle of the esterification reaction, or in the remaining part of the polyester raw materials after the esterification reaction is preferred. According to this method, since the monomer solution has a low viscosity, homogeneous dispersion of the particles and high-precision filtration of the slurry can be easily performed, and when added to the remaining part of the raw materials, the dispersibility of the particles is good, and new aggregates are not easily generated. From the above viewpoints, it is particularly preferred to add to the remaining part of the raw materials in a low-temperature state before the esterification reaction.

[0076] In addition, by melt-kneading the pellets containing the particles and the pellets not containing the particles after obtaining the polyester containing the particles in advance (master batch method), the number of projections on the surface of the film can be further reduced.

[0077] In addition, the polyester film can contain various additives within a range where the total light transmittance is maintained within a preferable range. As the additives, for example, an antistatic agent, a UV absorber, a stabilizer can be given.

[0078] The total light transmittance of the polyester film is preferably 85% or more, further preferably 87% or more. If the total light transmittance is 85% or more, visibility can be sufficiently ensured. The higher the total light transmittance of the polyester film is, the better, but from the viewpoint of stable production, it is preferably 99% or less, and can be 97% or less.

[0079] The maximum heat shrinkage of the polyester film after heat treatment at 150°C for 30 minutes is preferably 2% or less, further preferably 1.5% or less, for example, 1.2% or less.

[0080] If the heat shrinkage is 2% or less, dimensional change due to heat release from the organic EL display itself can be suppressed. It can be said that the lower the heat shrinkage is, the better, but it is preferably -1% or more, and preferably 0% or more. The negative value here means that expansion occurs after heating, and in the case of less than -1%, sometimes, planar defects occur.

[0081] The surface of the polyester film of the present application can be subjected to a treatment for improving adhesion to a resin such as a hard coat layer.

[0082] As the method based on surface treatment, for example, sandblasting treatment, concave-convex treatment based on solvent treatment or the like, corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone / ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, and the like can be used without particular limitation.

[0083] In addition, by an adhesion-improving layer such as an easy-adhesion layer, adhesion can be improved. As the easy-adhesion layer, an acrylic resin, a polyester resin, a polyurethane resin, a polyether resin, and the like can be used without particular limitation, and can be formed by a general coating method, preferably a so-called in-line coating method.

[0084] The polyester film described above can be manufactured, for example, by a polymerization step of homogeneously dispersing inorganic particles in a monomer liquid that is part of a polyester raw material, filtering, and adding to the balance of the polyester raw material to perform polymerization of the polyester; and a film formation step of melt-extruding the polyester into a sheet shape with the aid of a filter, cooling, and stretching to form a base film.

[0085] Next, regarding the manufacturing method of the biaxially stretched polyester film, an example in which pellets of polyethylene terephthalate (hereinafter, sometimes referred to as PET) are used as a raw material of a base film will be described in detail, but the present application is not limited to these. Also, the number of layers is not limited to a single layer, a multilayer, and the like.

[0086] Note that, in the case of using a polyethylene naphthalate (PEN) film instead of the PET film, the polyester film of the present application can also be manufactured in the same manner.

[0087] After the pellets of PET are mixed in a predetermined ratio and dried, they are supplied to a publicly known melt laminating extruder, extruded from a slit-like die into a sheet shape, and cooled and solidified on a casting roll to form an unstretched film. In the case of a single layer, one extruder can be used, and in the case of a multilayer film, two or more extruders, two or more manifold blocks, or a combination block (for example, a combination block having a square-shaped combination portion) can be used to stack a plurality of film layers constituting the outermost layers, extrude two or more sheets from a nozzle, and cool them on a casting roll to form an unstretched film.

[0088] In the above case, during melt extrusion, high-precision filtration is preferably performed at an arbitrary position at which the molten resin is kept at about 300°C or higher to remove foreign matter contained in the resin. The filter material used in the high-precision filtration of the molten resin is not particularly limited, and a filter material of a stainless steel sintered body is preferable because it is excellent in the removal performance of aggregates having Si, Ti, Sb, Ge, and Cu as main components and high-melting-point organic matter.

[0089] Further, the filtration particle size (initial filtration efficiency 95%) of the filter material is preferably 20 μm or less, and particularly preferably 15 μm or less. When the filtration particle size (initial filtration efficiency 95%) of the filter material exceeds 20 μm, foreign matter having a size of 20 μm or more cannot be sufficiently removed. Although the high-precision filtration of the molten resin using a filter material having a filtration particle size (initial filtration efficiency 95%) of 20 μm or less sometimes reduces the productivity, it is preferable in terms of obtaining a film having fewer protrusions caused by coarse particles.

[0090] (Refractive index with respect to the bending direction)

[0091] In the present application, the refractive index in at least either of the length direction (mechanical flow direction) and the width direction of the polyester film is preferably 1.610 or more and 1.750 or less, for example, 1.610 or more and 1.710 or less, and further preferably 1.630 or more and 1.680 or less.

[0092] In one approach, by making the refractive index of the polyester film along its length 1.610 or higher, the crystallinity can be effectively improved, thus improving the high-temperature holding angle. If it is below 1.750, the stress during bending can be reduced, both of which can improve the holding angle at room temperature and the high-temperature holding angle.

[0093] Conversely, if the refractive index of the polyester film in the width direction is within the aforementioned range, it is desirable that the refractive index of the polyester film in the length direction be higher than that in the width direction.

[0094] The refractive index of the polyester film in the bending direction is preferably 1.610 or higher and 1.750 or lower, for example, 1.610 or higher and 1.710 or lower, more preferably 1.630 or higher and 1.680 or lower.

[0095] Here, as Figure 2 As shown by symbol 22 on the polyester film (symbol 2), the bending direction refers to the direction orthogonal to the fold (symbol 21) envisioned in the application of the foldable display.

[0096] If the refractive index in at least one of the length and width directions is above 1.610 and below 1.750, the deformation during repeated folding is minimal, and there is no concern about reducing the image quality of the foldable display, making it a preferred option.

[0097] The refractive index of the polyester film in the bending direction is more preferably 1.630 to 1.680. Of course, this direction is preferably the aforementioned bending direction. If it is 1.610 or higher, the crystallinity can be effectively improved, and the high-temperature holding angle can be improved. If it is 1.750 or lower, the stress during bending can be reduced, and both can improve the holding angle at room temperature and the high-temperature holding angle.

[0098] The refractive index of polyester film can be effectively adjusted by regulating the stretching ratio and stretching temperature. Furthermore, to adjust the refractive index, a relaxation process in the stretching direction or multi-stage stretching can be used. When performing multi-stage stretching, it is preferable to further increase the stretching ratio of the second stage and subsequent stages compared to the stretching ratio of the first stage.

[0099] By controlling the refractive index of the polyester film in at least one of its longitudinal (mechanical flow) and transverse directions within the aforementioned range, and more preferably within the aforementioned range in the bending direction, fatigue caused by compressive stress applied to the inner side of the fold during folding can be reduced. It is believed that fatigue caused by compressive stress mainly occurs in the crystalline regions, and the fewer crystals in the bending direction, the less prone to fatigue. Therefore, it is considered that by making the refractive index in the bending direction less than the refractive index in the direction perpendicular to the bending direction, the amount of oriented crystals in the bending direction can be reduced, thereby suppressing compressive fatigue.

[0100] In addition, the creep phenomenon due to the tensile stress applied to the outer side of the fold at the time of folding can be suppressed by the reduction in the refractive index. It is considered that the fatigue due to the tensile stress mainly occurs in the amorphous portion, and the molecular chain merging due to the repeatedly applied stress occurs, and deformation occurs. It can be inferred that the less the molecular chains aligned in the bending direction, the less the deformation due to the merging. In addition, when the amorphous portion is small, the fatigue due to the stretching can be suppressed, and thus, the degree of crystallization, that is, the density is preferably high.

[0101] In the present application, for the unstretched polyester sheet, the stretching ratio in at least either of the length direction (mechanical flow direction) and the width direction is preferably 1.0 times or more and 3.4 times or less, and further preferably 1.4 times or more and 2.3 times or less. Furthermore, the stretching direction is preferably the aforementioned bending direction. If the stretching ratio is 3.4 times or less, the thickness unevenness of the film does not occur, and thus, it is preferable. As the stretching temperature, 120°C or higher and 150°C or lower, and further preferably 125°C or higher and 145°C or lower are preferable. Note that the heating method at the time of stretching can employ the conventionally known means such as the hot air heating method, the roll heating method, the infrared heating method, and the like. By making the stretching temperature 125°C or higher and 145°C or lower, the large thickness unevenness due to the stretching at the aforementioned stretching ratio can be prevented.

[0102] (Refractive index with respect to the direction of the folded portion)

[0103] The refractive index of the polyester film in the direction orthogonal to the direction in which the refractive index is 1.610 or more and 1.750 or less is preferably 1.750 to 1.870. That is, the refractive index of 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 being 1.750 or more and 1.870 or less, the deformation at the time of folding in the bending direction can be reduced. By being 1.870 or less, the case where the crack enters the direction of the folded portion can be suppressed, and further, the breakage can be suppressed. In addition, the breakage in the winding process after the stretching can be suppressed. By being 1.750 or more, the density can be increased, and the high-temperature retention angle can be improved.

[0104] For example, in the case where the length direction of the polyester film is the bending direction, the width direction of the polyester film corresponds to the direction orthogonal to the bending direction (the direction of the folded portion).

[0105] The refractive index of the direction orthogonal to the bending direction is more preferably 1.770 to 1.830.

[0106] In addition, in the case where the refractive index of the bending direction is compared with the refractive index of the direction orthogonal to the bending direction (the direction of the folded portion), it is desirable that the refractive index of the bending direction be low.

[0107] According to this method, deformation at the time of folding in the bending direction can be reduced. In addition, the occurrence of cracks in the direction of the fold can be suppressed, and further, breakage can be suppressed. Furthermore, breakage in the coiling process after stretching can be suppressed. In addition, the density can be increased, and the high-temperature retention angle can be improved.

[0108] As a method of adjusting the refractive index in the direction orthogonal to the bending direction, the stretching ratio, the stretching preheating temperature, the stretching temperature, multi-stage stretching, and film relaxation can be given. The stretching ratio is preferably 3.3 to 5.0 times, and more preferably 3.5 to 4.5 times. In addition, the stretching preheating temperature in the direction orthogonal to the bending direction is preferably 125 to 145°C. In the case of multi-stage stretching in the direction orthogonal to the bending direction, the stretching ratio is preferably further increased in the second stage and the stages thereafter, as compared with the first stage. The film relaxation can be performed by 0 to 10% in either of the mechanical flow direction (length direction) or the perpendicular direction (width direction).

[0109] (Refractive index with respect to the direction of thickness)

[0110] The refractive index in the thickness direction is preferably 1.520 or less. More preferably, it is 1.515 or less, further preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. The refractive index in the thickness direction is preferably low, but from the viewpoint of stable production, it is preferably 1.3 or more, and further can be 1.4 or more. Particularly preferably, it is 1.410 or more.

[0111] (Density of the polyester film)

[0112] The density of the polyester film is preferably 1.349 g / cm 3 or more, and more preferably 1.350 g / cm 3 or more. By being 1.350 g / cm 3 or more, the high-temperature retention angle can be improved. The higher the density, the more preferable it is, and can be somewhat affected depending on the presence or absence of particles in the film, and is preferably 1.40 g / cm 3 or more, and further more preferably 1.395 g / cm 3 or more.

[0113] By making the density of the polyester film 1.349 g / cm 3 or more, the crystallization of the polyester film of the present application can be sufficiently performed, and the deformation at 85°C can be suppressed. In addition, the case where the heat shrinkage rate becomes high can be suppressed, and the dimensional change due to the heat release of the device can be suppressed.

[0114] By setting the heat setting temperature at the time of film production to 210 to 270°C, the crystallization is performed, and the density can be effectively increased in the above range.

[0115] The bending direction of the polyester film is preferably in correspondence with the length direction (mechanical flow direction). Thus, biaxial stretching easily reduces the refractive index in the bending direction, and easily improves the bendability. That is, when the unstretched polyester sheet is stretched in the length direction at a stretching ratio of 1.0 to 2.3, more preferably at a stretching ratio of 1.4 to 2.1, a preferable polyester film can be obtained. Furthermore, it can be said that stretching at a stretching ratio of 3.3 to 5.0, more preferably at a stretching ratio of 3.5 to 4.5, in the width direction is a preferable method.

[0116] The high-temperature retention angle of the bending direction of the polyester film of the present application is 70° or more. Here, the high-temperature retention angle refers to the angle of the bend mark formed after heating and fixing at 85°C for 18 hours in a manner such that 1.7% strain is applied to both surfaces of the bent portion. In addition, the bending direction refers to the direction orthogonal to the folded portion.

[0117] The high-temperature retention angle of the bending direction is 71° or more, for example, 72° or more. The higher, the better, and most preferably 180°, but the high-temperature retention angle of the bending direction can be 180° or less, and even if it is, for example, 170° or less, it has sufficient functionality.

[0118] By making the high-temperature retention angle of the bending direction within the above range, deformation at 85°C can be suppressed. In addition, the case where the heat shrinkage rate becomes high can be suppressed, and the dimensional change caused by heat release of the device can be suppressed. Thus, according to the present application, deformation does not occur even after repeated folding in a high-temperature region, and the distortion of the image in the folded portion of the display can be suppressed. Furthermore, a portable terminal device equipped with a folding display using the polyester film provides a beautiful image, is highly functional, and is excellent in portability and the like.

[0119] Note that the measurement method of the high-temperature retention angle of the bending direction exemplified in the examples.

[0120] (Easy-adhesion layer)

[0121] In the present application, in order to improve the adhesion of the polyester film to a 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 application. The easy-adhesion layer can be obtained by applying a coating liquid for forming an easy-adhesion layer to one side or both sides of an unstretched or uniaxially stretched film, drying by heat treatment as necessary, and further stretching in at least one direction that has not been stretched. The heat treatment can also be performed after biaxial stretching. The coating amount of the final easy-adhesion layer is preferably managed to be 0.005 to 0.20 g / m 2 . If the coating amount is 0.005 g / m 2 or more, adhesion is obtained and is preferable. On the other hand, if the coating amount is 0.20 g / m 2 or less, blocking resistance is obtained and is preferable.

[0122] As the resin contained in the coating liquid used in the lamination of the easy-adhesion layer, for example, a polyester-based resin, a polyether polyurethane-based resin, a polyester polyurethane resin, a polycarbonate polyurethane resin, an acrylic resin, or the like can be used without particular limitation. As the cross-linking agent contained in the coating liquid for forming the easy-adhesion layer, a melamine compound, an isocyanate compound, an oxazoline compound, an epoxy compound, a carbodiimide compound, or the like can be cited. Two or more kinds can also be mixed and used, respectively. They are preferably coated using a water-based coating liquid in terms of the properties of the aforementioned on-line coating, and the aforementioned resin and cross-linking agent are preferably water-soluble or water-dispersible resins and compounds.

[0123] In order to impart easy slip properties to the easy-adhesion layer, it is preferable to add particles. The average particle diameter of the fine particles is preferably 2 μm or less. If the average particle diameter of the particles exceeds 2 μm, the particles become easily detached from the easy-adhesion layer. As the particles contained in the easy-adhesion layer, for example, 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, and organic polymer-based particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles can be cited. They can be added to the easy-adhesion layer alone or in combination of two or more kinds.

[0124] In addition, as the method of coating the coating liquid, the same known methods as those for the coating layer described above can be used. For example, a reverse roll / coating method, a gravure / coating method, a lip / coating method, a roll brush method, a spray method, an air-knife coating method, a wire bar coating method, a pipe doctor method, and the like can be cited, and these methods can be performed alone or in combination.

[0125] (Hard Coat Layer)

[0126] When the polyester film of the present application is used as a surface protection film for protecting the surface of a display in a folding-type display, it is preferable to have a hard coat layer on at least one surface thereof. The hard coat layer is preferably on the display surface side of the polyester film and is used in the display. As the resin for forming the hard coat layer, an acrylic, a silicone-based, an inorganic mixture-based, an urethane acrylate-based, a polyester acrylate-based, an epoxy-based, or the like can be used without particular limitation. In addition, two or more kinds of materials can be mixed and used, and particles such as inorganic fillers, organic fillers, and the like can be added.

[0127] (Thickness of Hard Coat Layer)

[0128] As the thickness of the hard coat layer, 1 to 50 μm is preferable. If it is 1 μm or more, it is sufficiently cured, and the pencil hardness becomes high and is preferable. Furthermore, by making the thickness 50 μm or less, curling due to the curing shrinkage of the hard coat can be suppressed, and the handleability of the film can be improved.

[0129] (coating method)

[0130] As the coating method of the hard coat layer, a Meyer bar, a gravure coater, a die coater, a doctor coater, or the like can be used without particular limitation, and can be appropriately selected depending on the viscosity and the film thickness.

[0131] (curing conditions)

[0132] As the curing method of the hard coat layer, a curing method using energy rays such as ultraviolet rays and electron beams, heat, or the like can be used, and a curing method using ultraviolet rays, electron beams, or the like is preferred in order to reduce damage to the film.

[0133] (pencil hardness)

[0134] As the pencil hardness of the hard coat layer, 3H or more, further preferably 4H or more is preferred. If the pencil hardness is 3H or more, scratches are not easily caused, and visibility is not reduced. Generally, the pencil hardness of the hard coat layer is preferably high, but can be 9H or less, or 8H or less, and even if it is 6H or less, it can be used without problems in practice.

[0135] (characteristics of the hard coat layer)

[0136] The hard coat layer in the present application can be used for the purpose of protecting a display by improving the pencil hardness of the above-mentioned surface, and is preferably high in light transmittance. As the total light transmittance of the hard coat film, 87% or more, further preferably 88% or more is preferred. If the light transmittance is 87% or more, sufficient visibility is obtained. The total light transmittance of the hard coat film is generally more preferably high, but from the aspect of stable production, 99% or less, or 97% or less is preferred. In addition, the haze of the hard coat film is generally preferably low, and 3% or less is preferred. 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 an image can be improved. The haze is generally more preferably low, but from the aspect of stable production, 0.1% or more, or 0.3% or more is preferred.

[0137] The hard coat layer can further have other functions added thereto. For example, a hard coat layer having a certain pencil hardness as described above, to which a function such as an antiglare layer, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer is added, can also be preferably applied to the present application.

[0138] Furthermore, in the case of being used as a base film of a touch panel assembly, a hard coat layer can also be provided. When a transparent electrode layer such as an ITO layer is used as the transparent electrode layer of the touch panel assembly, in order not to easily see the electrode pattern, a refractive index adjustment layer is preferably provided between the base film and the transparent electrode layer. In this case, the hard coat layer itself can have the function of the refractive index adjustment layer, or a refractive index adjustment layer can be further layered separately.

[0139] In another aspect, the polyester film for a folding display of the present application can be used for a folding display configured as a back surface protective film. For example, the polyester film for a folding display of the present application can be configured as a single polyester film continuous through the folding portion of a folding display.

[0140] In another aspect, a portable terminal device having a polyester folding display of the present application is provided.

[0141] Examples

[0142] Next, the present application will be described using examples and comparative examples. First, the evaluation method of the characteristic values implemented in the present application is shown below.

[0143] (1) Inherent viscosity

[0144] After the film or the polyester resin is pulverized and dried, it is dissolved in a mixed solvent of phenol / tetrachloroethane = 60 / 40 (mass ratio). After the solution is subjected to centrifugal separation treatment to remove inorganic particles, the flow time of a solution having a concentration of 0.4 (g / dl) at 30°C and the flow time of the solvent alone are measured using an Ubbelohde viscometer, and from the time ratio thereof, the inherent viscosity is calculated using the Huggins equation, assuming the Huggins constant to be 0.38. Both polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are evaluated using the same calculation formula.

[0145] (2) Refractive index

[0146] Using a laser refractometer (MODEL 2010 PRISM COUPLER) manufactured by Metricon, one sample film is clamped with a pressure of 40 scales of a built-in pressure gauge, and measurement is performed under a laser having a wavelength of 633 nm to obtain a spectrum. On the obtained spectrum, the point at which the detector power sharply decreases is read, and the value is taken as the refractive index. In the measurement mode TE, the refractive index in the length direction and the width direction is measured, and in TM, the refractive index in the thickness direction is measured.

[0147] (3) Total light transmittance, haze

[0148] Measurement is performed using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH5000).

[0149] (4) Density

[0150] The density is measured according to the method (density gradient tube method) according to JIS K 7112:1999. (Unit: g / cm 3 ).

[0151] (5) Maximum heat shrinkage

[0152] The sample film was cut into 10 mm in length x 250 mm in width, and marks were made at 200 mm intervals in the direction of the length for which the shrinkage was to be measured. The interval 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 under no load for 30 minutes, and then taken out of the oven and cooled to room temperature. Then, the interval B between the marks was measured under a constant tension of 5 g, and the heat shrinkage (%) was calculated using the following equation. Note that, for the heat shrinkage, the measurement was performed at three points equally divided in the width direction of the sample film, and the average of the three points was taken as the heat shrinkage (%).

[0153] Heat shrinkage (%) = [(A - B) x 100] / A

[0154] For both the bending direction and the folding direction, the sample film was cut and measured in different ways in the length and width directions, respectively, and the data for the direction for which the measured value was larger was taken as the maximum heat shrinkage (%).

[0155] (6) High-temperature retention angle

[0156] The strength of the folding mark formed when the two surfaces of the bent portion were fixed so as to apply a strain of 1.7% to each surface was evaluated.

[0157] Figure 3 An enlarged schematic view of the sample film (symbol 3) sandwiched between two PTFE plates (symbol 31) is shown in FIG. 6. The neutral plane, to which neither the compressive stress nor the tensile stress is applied, was determined as the center in the thickness direction (dotted line in the figure), and the difference between the neutral plane and the two surfaces was taken as the strain. That is, the strain applied to the two surfaces can be expressed by the following equation.

[0158] In order to make the strain constant, the thickness 32 of the PTFE plate used as the spacer was changed depending on the thickness of the film.

[0159] Figure 4 An enlarged schematic view of the sample film (symbol 3) sandwiched between two PTFE plates (symbol 31) is shown in FIG. 6. The neutral plane, to which neither the compressive stress nor the tensile stress is applied, was determined as the center in the thickness direction (dotted line in the figure), and the difference between the neutral plane and the two surfaces was taken as the strain. That is, the strain applied to the two surfaces can be expressed by the following equation.

[0160] Note that, Figure 4In the above, symbol 41 is the diameter of the outermost surface in the sample film, symbol 42 is the diameter of the neutral surface in the sample film, and symbol 43 indicates the diameter of the innermost surface in the sample film.

[0161] In the evaluation of the high-temperature retention angle, the strain (1.7%) can be expressed by the following method.

[0162] Strain (1.7%)

[0163] = (|semicircumference of outermost surface or innermost surface - semicircumference of neutral surface| / semicircumference of neutral surface) x 100

[0164] Here, for the semicircumference, the thickness of the sample film is set to t (mm), the bending diameter (diameter of the outermost surface), i.e., the thickness of the spacer used is set to d (mm), and the following formulas can be used to calculate them, respectively.

[0165] Semicircumference of outermost surface = d x π / 2

[0166] Semicircumference of neutral surface = (d - t) x π / 2

[0167] Semicircumference of innermost surface = (d - 2t) x π / 2

[0168] According to the above, when the strain is set to 1.7%, the thickness of the sample film is set to t (mm), the bending diameter, i.e., the thickness of the spacer used is set to d (mm), and the thickness of the PTFE plate of the spacer (PTFE plate) is determined by the following formula. The spacer thickness with respect to the representative film thickness is, for example, as shown below.

[0169] Spacer thickness d (mm) = Film thickness (mm) x 60

[0170] For example, in the case of the sample film having a thickness of 50 μm described above, the diameter of the outermost surface (symbol 41) is the same as the thickness d of the spacer, which is 3 mm. The diameter of the innermost surface (symbol 43) is 2.9 mm, and the diameter of the neutral surface (symbol 42) is 2.95 mm. Here, in the formula that represents the strain described above, the semicircumference of the outermost surface or the semicircumference of the innermost surface can be appropriately selected.

[0171] (Preparation of polyethylene naphthalate pellets)

[0172] The transesterification reaction was carried out for 120 minutes while slowly raising the temperature from 150°C to 238°C using 0.03 parts of manganese acetate tetrahydrate as a transesterification catalyst, 100 parts of dimethyl 2,6-naphthalene dicarboxylate, and 60 parts of ethylene glycol. In the middle of the reaction, trimethyl phosphate (added as a solution obtained by heating treatment at 135°C for 5 hours under pressurization of 0.11 to 0.16 MPa in ethylene glycol: 0.023 parts in terms of the amount of trimethyl phosphate) was added at the point when the reaction temperature reached 170°C, and 0.024 parts of antimony trioxide was added after the completion of the transesterification reaction. Thereafter, the reaction product was transferred to a polymerization reactor, the temperature was raised to 290°C, and polycondensation was carried out under high vacuum of 27 Pa or less to obtain polyethylene glycol 2,6-naphthalate having an intrinsic viscosity of 0.48 dl / g, which was substantially free of particles.

[0173] (Preparation of polyethylene terephthalate pellets)

[0174] As the esterification reaction apparatus, a continuous esterification reaction apparatus composed of a stirring apparatus, a phase separator, and a three-stage complete mixing tank having a raw material feed inlet and a product discharge outlet was used. TPA was supplied at 2 tons / hour, EG was supplied at 2 moles per 1 mole of TPA, and antimony trioxide was supplied at 160 ppm of Sb atoms per PET produced. These slurries were continuously supplied to the first esterification reactor of the esterification reaction apparatus, and the reaction was carried out at normal pressure with an average residence time of 4 hours at 255°C.

[0175] Next, the reaction product in the above-mentioned first esterification reactor was continuously taken out of the system and supplied to the second esterification reactor. EG distilled and removed from the first esterification reactor was supplied to the second esterification reactor at 8 mass% relative to the polymer produced (PET produced). Further, an EG solution containing magnesium acetate at an amount of 65 ppm of Mg atoms per PET produced and an EG solution containing TMPA at an amount of 20 ppm of P atoms per PET produced were added, and the reaction was carried out at normal pressure with an average residence time of 1.5 hours at 260°C. Next, the reaction product in the above-mentioned second esterification reactor was continuously taken out of the system and supplied to the third esterification reactor. An EG solution containing TMPA at an amount of 20 ppm of P atoms per PET produced was further added, and the reaction was carried out at normal pressure with an average residence time of 0.5 hours at 260°C. The esterification reaction product produced in the above-mentioned third esterification reactor was continuously supplied to a three-stage continuous polycondensation reaction apparatus and subjected to polycondensation. Further, filtration was carried out using a filter material of stainless steel sinter (nominal filtration accuracy: 5 μm particles 90% cutoff) to obtain polyethylene terephthalate pellets (a) having an intrinsic viscosity of 0.58 dl / g.

[0176] (Polymerization of polyurethane resin)

[0177] In a four-necked flask equipped with a stirrer, a condenser, a nitrogen inlet tube, a silica gel drier, and a thermometer, 72.96 parts by mass of 1,3-bis(isocyanatomethyl)cyclohexane, 12.60 parts by mass of dimethylolpropionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol having 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 a solvent were charged, and stirred at 75°C for 3 hours under a nitrogen atmosphere. The reaction solution was cooled to 40°C, and 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer D solution. Subsequently, in a reaction vessel equipped with a homogenizer capable of high-speed stirring, 450 g of water was added, and adjusted to 25°C. While the temperature was maintained at 25°C, 2000 parts by mass of the polyurethane prepolymer D solution was added dropwise over 1 hour. After the completion of the addition, the mixture was stirred for 1 hour to obtain a water-soluble polyurethane resin (A) having a solid content of 35% by mass. -1 While stirring, the isocyanate group-terminated prepolymer was added dropwise to perform water dispersion. Thereafter, under reduced pressure, a part of the acetonitrile and water was removed to prepare a water-soluble polyurethane resin (A) having a solid content of 35% by mass.

[0178] (Polymerization of water-soluble carbodiimide compound)

[0179] In a flask equipped with a thermometer, a nitrogen inlet tube, a reflux condenser, a dropping funnel, and a stirrer, 200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-l-phenyl-2-phospho-l-oxide as a carbodiimidization catalyst were charged, and stirred at 180°C for 10 hours under a nitrogen atmosphere to obtain an isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Subsequently, 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. To this, water was slowly added at 50°C to obtain a yellow transparent water-soluble carbodiimide compound (B) having a solid content of 40% by mass.

[0180] (Preparation of coating solution for easy-adhesion layer formation)

[0181] A coating solution was prepared by mixing the following coating agents.

[0182]

[0183] (Silica sol having an average particle diameter of 40 nm, solid content concentration 40% by mass)

[0184] Surfactant 0.05 parts by mass

[0185] (Silicone-based, solid content concentration 100% by mass)

[0186] (Example 1)

[0187] The polyethylene naphthalate pellets were supplied to an extruder and melted at 310°C. The polymer was filtered with a filter material of stainless steel sinter (nominal filtration accuracy 10 μm particles 95% cutoff), extruded from a nozzle into a sheet shape, and then, using an electrostatic application casting method, brought into contact with a casting drum having a surface temperature of 60°C to perform cooling and solidification, to produce an unstretched film. The above-mentioned easy-adhesion layer-forming coating liquid was applied to both sides of the unstretched film by roll coating, and then dried at 80°C for 20 seconds. Note that the dried coating amount after the final (after biaxial stretching) was adjusted to 0.06 g / m 2 After that, it was introduced into a tenter, preheated at 140°C, stretched in the transverse direction to 4.2 times at 135°C, width-fixed, heat-set at 240°C for 5 seconds, and further relaxed in the width direction at 180°C by 1%, to obtain a polyethylene naphthalate film having a thickness of 50 μm. The evaluation results are shown in Table 1.

[0188] (Examples 2 to 6)

[0189] After the unstretched film was obtained in the same manner as in Example 1, the unstretched film was uniformly heated to 120°C with a heated roll, heated to 135°C in a non-contact heater, and subjected to roll stretching (longitudinal stretching) at the MD stretch ratio described in Table 1. The stretch ratio in the length direction was changed to that described in Table 1, and otherwise, a polyester film was obtained in the same manner as in Example 1.

[0190] (Example 7)

[0191] After the unstretched film was obtained in the same manner as in Example 1, the unstretched film was uniformly heated to 120°C with a heated roll, heated to 140°C in a non-contact heater, and subjected to roll stretching (longitudinal stretching) at the MD stretch ratio described in Table 1. The stretch ratio in the length direction was changed to that described in Table 1, and otherwise, a polyester film was obtained in the same manner as in Example 1.

[0192] (Comparative Example 1)

[0193] The polyethylene terephthalate pellets were supplied to an extruder and melted at 285°C. The polymer was filtered with a filter material of stainless steel sinter (nominal filtration accuracy 10 μm particles 95% cutoff), extruded from a nozzle into a sheet shape, and then, using an electrostatic application casting method, brought into contact with a casting drum having a surface temperature of 30°C to perform cooling and solidification, to produce an unstretched film. The unstretched film was uniformly heated to 75°C with a heated roll, heated to 85°C with a non-contact heater, and subjected to roll stretching (longitudinal stretching) at 1.4 times.

[0194] On the obtained uniaxially stretched film, the easy-adhesion layer forming coating liquid was applied on both sides by roll coating method, and then dried at 80°C for 20 seconds. Note that the dried coating amount after the final (biaxially stretched) drying was adjusted to 0.06 g / m 2 In this way, the obtained polyester film was attached to the non-visual side of the organic EL assembly with a 25-μm-thick adhesive layer interposed therebetween, to produce a foldable display of a smartphone type which was folded at the center portion and had a radius of 3 mm as the bending radius. The polyester film was disposed on the non-visual side of the continuous one display with the folded portion interposed therebetween, and was attached to the polyimide film having a barrier layer as the organic EL substrate. The foldable display using the polyester film of each example satisfied the operation and visibility as a smartphone which was folded at the center portion and was portable. In addition, there were no problems in the operation and visibility under high temperature.

[0195] (Comparative Example 2)

[0196] As shown in Table 1, the stretching ratio in the length direction was changed to 1.4 times, and otherwise, the polyester film was obtained in the same manner as in Comparative Example 1.

[0197] The obtained polyester film was attached to the non-visual side of the organic EL assembly with a 25-μm-thick adhesive layer interposed therebetween, to produce a foldable display of a smartphone type which was folded at the center portion and had a radius of 3 mm as the bending radius. The polyester film was disposed on the non-visual side of the continuous one display with the folded portion interposed therebetween, and was attached to the polyimide film having a barrier layer as the organic EL substrate. The foldable display using the polyester film of each example satisfied the operation and visibility as a smartphone which was folded at the center portion and was portable. In addition, there were no problems in the operation and visibility under high temperature. Figure 1 On the other hand, the foldable display using the polyester film of each comparative example felt the distortion of the image at the folded portion of the display as the frequency of use under high temperature increased, and was not so preferable. In addition, a dent and a scratch were confirmed on the surface.

[0198] [Table 1A]

[0199]

[0200]

[0201] [Table 1B]

[0202]

[0203] Industrial applicability

[0204] The foldable display using the polyester film for foldable display according to the present application maintains the mass productivity. And for example, does not cause the deformation after repeatedly folding the polyester film on the back side of the foldable display, and thus, does not cause the distortion of the image in the folded portion of the display. In particular, the portable terminal device or the image display device equipped with the foldable display using the polyester film according to the present application as the back side protective film provides a beautiful image, is rich in functionality, is excellent in the portability and the like, and is high in reliability.​

[0205] Reference Signs

[0206] 1: Foldable display

[0207] 11: Bending radius

[0208] 2: Polyester film for surface protection film of foldable display

[0209] 21: Folded portion

[0210] 22: Bending direction (direction orthogonal to folded portion)

[0211] 3: Test film

[0212] 31: PTFE plate

[0213] 32: Spacer

[0214] 33: Holding angle

[0215] 41: Diameter of outermost surface

[0216] 42: Diameter of neutral plane

[0217] 43: Diameter of innermost surface

Claims

1. A folding organic EL display, which is provided with a polyester film having a thickness of 10 to 125 μm, the refractive index in the bending direction of the polyester film is 1.644 or more and 1.750 or less, the refractive index in the direction of the folding portion orthogonal to the bending direction is 1.770 or more and 1.870 or less, the refractive index in the thickness direction is 1.3 or more and 1.520 or less, Density: 1.349 g / cm 3 Density: 1.40 g / cm 3 Below, wherein the polyester is polyethylene naphthalate, the high-temperature retention angle in the bending direction of the polyester film is 70° or more, the polyester film is provided as a continuous single polyester film via the folding portion of the folding organic EL display, Here, the high-temperature retention angle refers to the angle of the bend mark formed after heating and fixing at 85°C for 18 hours in a manner that 1.7% strain is applied to both surfaces of the bending portion, and the bending direction refers to the direction orthogonal to the folding portion.

2. The folded organic EL display according to claim 1, wherein An easy-adhesion layer is provided on at least one surface of the polyester film.

3. The folded organic EL display according to claim 2, wherein, The easy-adhesion layer contains: at least one resin selected from the group consisting of an acrylic resin, a polyester resin, a polyurethane resin, and a polyether resin; and at least one cross-linking agent selected from the group consisting of a melamine compound, an isocyanate compound, an oxazoline compound, an epoxy compound, and a carbodiimide compound.

4. The folded organic EL display according to any one of claims 1 to 3, wherein The total light transmittance of the polyester film is 85% or more and the haze is 3% or less.

5. The folded organic EL display according to any one of claims 1 to 3, wherein A hard coat layer is provided on at least one surface of the polyester film.

6. The folded organic EL display according to claim 5, wherein, The film thickness of the hard coat layer is 1 to 50 μm, The hard coat layer contains at least one resin selected from the group consisting of an acrylic, a silicone, an inorganic mixture, an urethane acrylate, a polyester acrylate, and an epoxy.

7. A portable terminal device having the folding organic EL display according to any one of claims 1 to 6.

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