Polyester Film and Its Uses

Abstract

To provide a polyester film for protecting organic EL modules in foldable displays occurring no cracks in the folded part capable of providing a folding type display, which is excellent in mass productivity and there is no risk of disturbing the image displayed at the folded part after repeatedly folding.SOLUTION: The polyester film for protecting organic EL modules for foldable displays satisfies the following conditions: (1) the refractive index in the bending direction is 1.590 to 1.620; (2) the refractive index in the direction of the folding part is 1.670 to 1.700; (3) the refractive index in the thickness direction is 1.520 or less; and (4) the density is 1.380 g / cm3 or more (here, the bending direction means the direction orthogonal to the folding part when the polyester film is folded.)SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a polyester film for protecting an organic EL module of a foldable display, a foldable display, and a portable terminal device, and to a foldable display and a portable terminal device in which images are less likely to be disturbed due to film deformation even when repeatedly folded, and to a polyester film for protecting an organic EL module of the foldable display.

Background Art

[0002] The thin film of portable terminal devices has been made lighter, and portable terminal devices represented by smartphones have been widely spread. While various functions are required for portable terminal devices, convenience is also required. Therefore, the widespread portable terminal devices should be able to be operated with one hand for simple operations, and further, since it is premised on being stored in a pocket of clothes or the like, it is necessary to have a small screen size of about 6 inches.

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

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

[0005] Therefore, in recent years, portable terminals incorporating flexible displays and foldable displays have been proposed. With this method, it can be conveniently carried as a portable terminal device equipped with a large-screen display without the image being interrupted.

[0006] Here, for displays and mobile 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 portion corresponding to a certain folding part is repeatedly bent, the film at that portion is deformed over time, and there are problems such as distorting the image displayed on the display. In addition to the surface protection film, various parts of the foldable display use films, such as a polarizing plate, a retardation plate, a touch panel substrate, a substrate of a display cell such as an organic EL (electroluminescence), and a protective member on the back surface. Durability against repeated folding is also required for these films.

[0007] Therefore, a method of partially changing the film thickness has been proposed (see Patent Document 2), but 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, as the refractive index in the bending direction is decreased, the pencil hardness during hard coat application decreases, and there is a problem of deterioration of the surface protection function of the display. Also, as the refractive index in one direction is decreased, the deformation during folding is improved, but the uniaxial orientation in the folding direction increases, and there are problems such as cracks or breakage occurring in the folding part.

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 with excellent mass productivity and no possibility of causing image distortion at the folded portion after repeated folding, and a mobile terminal device equipped with such a foldable display. Therefore, it is intended to provide a polyester film for protecting an organic EL module of a foldable display, in which no folding marks or cracks occur in the folding portion.

Means for Solving the Problems

[0011] That is, the present invention has the following configuration. 1. A polyester film for protecting an organic EL module of a foldable display that satisfies the following conditions. (1) The refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding portion is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less (4) The density is 1.380 g / cm 3 or more (Here, the bending direction refers to the direction orthogonal to the folding portion when folding the polyester film.) 2. The polyester film for protecting an organic EL module of the foldable display according to the first item above, wherein the elastic modulus in the bending direction is 2.7 GPa or less and the elastic modulus in the direction of the folding portion is 4.5 GPa or more. 3. The polyester film for protecting an organic EL module of the foldable display according to the first or second item above, wherein the total light transmittance is 85% or more, the haze is 3% or less, and the maximum thermal shrinkage rate is 6% or less. 4. The polyester film for protecting an organic EL module of the foldable display according to any one of the first to third items above, which has an easy-adhesion layer on at least one surface. 5. The polyester film for protecting an organic EL module of the foldable display according to any one of the first to fourth aspects, having a hard coat layer with a thickness of 1 to 50 μm on at least one surface. 6. The foldable display including the polyester film for protecting an organic EL module of the foldable display according to any one of the first to fifth aspects as a protective film of the organic EL module, wherein a protective film of a single continuous organic EL module is disposed through a folding portion of the foldable display. display. 7. A portable terminal device having the foldable display according to the sixth aspect.

Advantages of the Invention

[0012] The foldable display using the polyester film for protecting an organic EL module of the foldable display of the present invention maintains mass productivity, and the polyester film does not crack at the folding portion, does not deform after repeated folding, 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 beautiful images, is rich in functionality, and is excellent in convenience such as portability.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0014] (Display) As used in the present invention, the term "display" generally refers to a display device. Examples of display types include LCD, organic EL display, inorganic EL display, LED, FED, etc. Among them, LCD, organic EL, and inorganic EL with 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. Although it can be said that the smaller the bending radius, the better, the smaller the bending radius, the easier it is to leave a folding mark. The bending radius is preferably 0.1 mm or more, but may be 0.5 mm or more, or 1 mm or more. Even if the bending radius is 1 mm, practical thinning can be achieved when carried. The bending radius when folded refers to the measurement at the position of reference numeral 11 in the schematic diagram of FIG. 1 and means the radius inside the folded part when folded. Note that the surface protection film described later may be located on the outside or inside of the folded foldable display. In addition, the foldable display may be three-fold, four-fold, or even a rollable type called a rollable type, and all of these are within the scope of the foldable display as used in the present invention.

[0016] The polyester film for a foldable display 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 typical configuration of a foldable display and the parts where the polyester film of the present invention can be used will be described. In the following, 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) The essential configuration of a foldable organic EL display is an organic EL module, and further, a circularly polarized plate, a touch panel module, a surface protection film, a back protection film, etc. may be provided as required. (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 the base material film is required to have high water vapor and oxygen barrier properties, 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] (Protection film for organic EL module) It is also preferable that a protection film is provided on the non-viewing side of the organic EL module. An organic EL module is generally formed on a glass substrate. In the case of a flexible display such as a foldable display, the glass substrate is peeled off, and a film is provided as a substitute protection layer. The polyester film of the present invention can be used as this protection film for the organic EL module. A hard coat may be applied to the protection film to prevent damage.

[0019] FIG. 3 is a schematic cross-sectional view of an example of an organic EL module 3 provided with a polyester film for protecting an organic EL module of the foldable display of the present invention. However, the present invention is not limited thereto. The organic EL module 3 has a thin film transistor (TFT) 33 and an organic EL element 34 in this order on a back substrate (such as a polyimide substrate or a polyester substrate) 32, and the organic EL element 34 is sealed with a sealing adhesive layer 35. In order to protect the back substrate 32 of this organic EL panel, the polyester film of the present invention can be used as a protective film 30 via an adhesive 31. Although not shown, the protective film 30 may have a hard coat layer on at least the surface opposite to the adhesive 31.

[0020] (Touch panel module) It is preferable that the portable terminal device has a touch panel. When an organic EL display is used, it is preferable that a touch panel module is disposed above the organic EL display or between the organic EL module and the circular polarizing 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 the transparent base material of the touch panel, it is preferable to provide a hard coat layer or a refractive index adjusting layer on the polyester film.

[0021] (Circular polarizing plate) The circular polarizing plate suppresses the reduction in image quality caused by external light being reflected by members inside the display. The circular polarizing plate has a linear polarizing plate and a retardation plate. The linear polarizing plate has a protective film on at least the viewing side surface of the polarizer. It may also have a protective film on the surface opposite to the viewing side of the polarizer, 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. A deviation of up to 10 degrees, preferably up to 5 degrees, from this parallel or orthogonal is allowed.

[0022] (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 on the outermost surface of the display, or something called an after that can be self - adhered, peeled off, and replaced by the user. 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 preferably a film having a hard coat layer 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.

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

[0024] Hereinafter, the polyester film for protecting the organic EL module of the foldable display in the present invention may be simply referred to as the polyester film of the present invention or a polyester film.

[0025] The polyester film of the present invention may be a single-layer film composed of one or more polyester resins. When two or more polyesters are used, it may be a multilayer structure film or a super-multilayer laminated film with a repeating structure.

[0026] Examples of the polyester resin used for the polyester film include polyester films made of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers mainly composed of the constituent components of these resins. Among them, a stretched polyethylene terephthalate film is particularly preferable in terms of mechanical properties, heat resistance, transparency, price, etc.

[0027] When using a polyester copolymer for a 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 with an average molecular weight of 150 to 20,000. The mass ratio of the copolymer components of the preferred copolymer is less than 20% by mass. When it is less than 20% by mass, it is preferable as the film strength, transparency, and heat resistance are maintained.

[0028] 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 0.50 to 1.0 dl / g. When the intrinsic viscosity is 0.50 dl / g or more, the impact resistance of the obtained film is improved, and it is preferable that the 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 preferable that the film production can be stably operated.

[0029] The thickness of the polyester film is preferably 10 to 80 μm, and more preferably 25 to 75 μm. When the thickness is 10 μm or more, the effect of improving the pencil hardness and the impact resistance improvement effect are observed. When the thickness is 80 μm or less, it is advantageous for weight reduction, and in addition, it is excellent in flexibility, processability, and handleability.

[0030] The surface of the polyester film of the present invention may be smooth or may have irregularities. Since it is for use in protecting an organic EL module, it is preferably highly transparent, and a decrease in optical properties due to irregularities is not preferable. As haze, 3% or less is preferable, 2% or less is more preferable, and 1% or less is most preferable. If the haze is 3% or less, the visibility of an image can be improved. Although the lower limit of the haze is preferably as small as possible, from the viewpoint of stable production, 0.1% or more is preferable, and 0.3% or more may be sufficient.

[0031] For the purpose of reducing haze as described above, it is preferable that the irregularities on the film surface are not too large. However, from the viewpoint of handleability, in order to impart a certain degree of slipperiness, as a method of forming irregularities, particles can be blended in the surface polyester resin layer, or a coating layer containing particles can be formed by coating during film formation.

[0032] As a method of blending particles in the polyester resin layer, a known method can be adopted. For example, it can be added at any stage of manufacturing 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 be advanced. Further, 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 a method of blending dried particles and a polyester raw material using a kneading extruder.

[0033] Among these, a method is preferable in which aggregated inorganic particles are homogeneously dispersed in a monomer solution that is part of the polyester raw material, and then the filtered product is added to the remainder of the polyester raw material before, during, or after the esterification reaction. According to this method, since the monomer solution has a low viscosity, homogeneous dispersion of the particles and highly accurate filtration of the slurry can be easily performed. In addition, 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 viewpoint, in particular, it is preferable to add to the remainder of the raw material in a low-temperature state before the esterification reaction.

[0034] Further, 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 extrusion of the pellets and the pellets not containing particles (masterbatch method).

[0035] The polyester film may also contain various additives within a range that maintains a preferable range of total light transmittance. Examples of the additives include an antistatic agent, a UV absorber, and a stabilizer.

[0036] The total light transmittance of the polyester film is preferably 85% or more, 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.

[0037] The maximum heat shrinkage rate of the polyester film after heat treatment at 150°C for 30 minutes is preferably 6% or less, more preferably 5% or less. If the heat shrinkage rate is 6% or less, planar defects such as curl and undulation after attaching the organic EL module 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 if it is -1% or more, there is no risk of causing planar defects, which is preferable.

[0038] The polyester film of the present invention can provide sufficient pencil hardness to the hard coat film after laminating the hard coat layer. It is considered that in the pencil hardness evaluation of the hard coat film of the conventional polyester film, the pencil hardness has decreased because the film is deformed in the thickness direction after laminating the hard coat layer. In the present invention, by setting the indentation depth after unloading the test force in the film thickness direction by the dynamic ultra-micro hardness tester described later within a specific range, high hardness can be achieved in the pencil hardness evaluation of the hard coat film. The indentation depth after unloading the test force is preferably 1.5 μm or less, more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. When the indentation depth after unloading the test force (the final deformation amount under load) is 1.5 μm or less, in the pencil hardness evaluation of the hard coat film after laminating the hard coat layer, the film is less likely to be deformed in the thickness direction and the pencil hardness can be increased. When the pencil hardness of the hard coat film can be increased, scratches and dents are less likely to occur on the display surface, and the visibility of the display is improved. Although it can be said that the lower the indentation depth after unloading the test force, the better, in terms of stable production and saturation of the effect, 0.3 μm or more is preferable, and further, 0.5 μm or more is preferable.

[0039] In order to reduce the indentation depth after unloading the test force, it is effective to adjust the refractive index in the thickness direction to 1.520 or less. As means for making the refractive index 1.520 or less, as will be described later, within the range where other physical properties, the refractive indices in the bending direction and the folding direction can be controlled within a preferable range, adjusting the stretching ratio in the bending direction and the folding direction to be high, setting the stretching temperature in the bending direction and the folding direction to be low, setting the heat setting temperature to be high, etc. can be exemplified as conditions.

[0040] The polyester film for a foldable display of the present invention does not generate creases, cracks, or breaks during folding, and can adjust the neutral plane of the display. Furthermore, it can protect the organic EL panel from external impacts. The neutral plane refers to the plane where the inner side is under compressive stress and the outer side is under tensile stress when folded, but no stress is applied in between. In a foldable display, generally, the neutral plane is designed for the organic EL layer. The neutral plane can be adjusted by the elastic modulus and thickness of each layer. Therefore, the elastic modulus in the bending direction of the polyester film is preferably 2.7 GPa or less, more preferably 2.6 GPa or less, and even more preferably 2.5 GPa or less. By reducing the elastic modulus in the bending direction, it can be said that the bendability is improved, but it is preferably 1.8 GPa or more for adjusting the neutral plane. The elastic modulus in the folding direction is preferably 4.5 GPa or more, more preferably 4.6 GPa or more, and even more preferably 4.7 GPa or more. By increasing the elastic modulus in the folding direction, the flatness of the display surface can be maintained during display production. Also, the organic EL can be protected from external impacts. Although the higher the elastic modulus in the folding direction is, the better, it is preferably 8.0 GPa or less from the viewpoint of film-forming properties.

[0041] On the surface of the polyester film of the present invention, a treatment can be performed to improve the adhesion to resins such as an adhesive layer and a hard coat layer.

[0042] Examples of the method by surface treatment include 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 they can be used without particular limitation.

[0043] Also, the adhesion can 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 it can be formed by a general coating method, preferably a so-called in-line coat formulation.

[0044] The above polyester film can be manufactured through a polymerization process in which inorganic particles are homogeneously dispersed and filtered in a monomer solution that is part of the polyester raw material, and then added to the remaining part of the polyester raw material to perform the polymerization of the polyester, and a film forming process in which the polyester is melt-extruded into a sheet shape through a filter, cooled, and then stretched to form a base film.

[0045] Next, a method for manufacturing 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 a single-layer configuration or a multi-layer configuration is not limited.

[0046] 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 shape 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 used, but when manufacturing a multi-layer 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.

[0047] 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 280°C to remove foreign substances contained in the resin. The filter medium used for high-precision filtration of the molten resin is not particularly limited, but a filter medium made 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.

[0048] Furthermore, the filtration particle size of the filter medium (initial filtration efficiency of 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 of 95%) exceeds 20 μm, foreign matters with a size of 20 μm or more cannot be sufficiently removed. Although the high-precision filtration of the molten resin may reduce productivity by using a filter medium with a filtration particle size of the filter medium (initial filtration efficiency of 95%) of 20 μm or less, it is preferable for obtaining a film with few protrusions caused by coarse particles.

[0049] (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 1.590 to 1.620, more preferably 1.591 to 1.600. And the refractive index in the bending direction of the polyester film is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. Here, the bending direction refers to the 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 1.590 to 1.620, there is little deformation during repeated folding, and it is preferable because there is no risk of degrading the image quality of the foldable display. It is more preferable that the refractive index is 1.591 to 1.600. Of course, that direction is preferably the above-mentioned bending direction. When it is 1.590 or more, there is no risk of cracks in the folding portion direction after the bending test described later, and of course, no breakage occurs, so the visibility of the display can be kept good. The refractive index of the polyester film can be effectively adjusted by adjusting the draw ratio and the draw temperature. Also, 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 to make the draw ratio in the second and subsequent stages higher than the draw ratio in the first stage.

[0050] 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 crystalline part, and the less the crystals in the bending direction, the less likely it is to fatigue. Therefore, by reducing the refractive index, the amount of oriented crystals in the bending direction is reduced, and it is considered that the compression fatigue is suppressed.

[0051] In addition, 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 less the amorphous part, the more the fatigue due to tension can be suppressed, it is preferable that the degree of crystallinity, that is, the density is higher.

[0052] 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 1.2 to 2.0 times, and more preferably 1.7 to 2.0 times. And the drawing direction is preferably the above-mentioned bending direction. When the draw ratio is 1.2 times or more, it is preferable because there is no deformation in post-processing such as hard coat coating. When the draw ratio is 2.0 times or less, it is preferable because film thickness unevenness does not occur. As the drawing temperature, 75 to 120 ° C is preferable, and 75 to 105 ° C is more preferable. As for the heating method during drawing, conventionally known means such as hot air heating method, roll heating method, and infrared heating method can be adopted. By setting the drawing temperature to 75 to 120 ° C, large thickness unevenness due to drawing at the above draw ratio can be prevented. Also, by drawing at a temperature as low as possible within the range where large thickness unevenness does not occur as described above, the refractive index in the thickness direction can be reduced.

[0053] (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 1.590 to 1.620 is preferably 1.670 to 1.700. That is, the refractive index in the direction orthogonal to the bending direction (the direction of the folded portion) is preferably 1.670 to 1.700. By setting it to 1.670 to 1.700, deformation when folded in the bending direction can be reduced. By setting it to 1.700 or less, it is possible to suppress cracks or breakage in the direction of the folded portion. By setting it to 1.670 or more, the bendability in the bending direction and the surface hardness can be improved. More preferably, it is 1.680 to 1.695. Examples of methods 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 4.0 to 6.0 times, more preferably 4.4 to 6.0. Also, the pre-drawing temperature in the direction orthogonal to the bending direction is preferably 70 to 110°C. When performing multi-stage drawing in the direction orthogonal to the bending direction, it is preferable to increase the draw ratio in the second and subsequent stages compared to the first stage. Film relaxation may be performed by 1 to 10% in either the machine flow direction (longitudinal direction) or the perpendicular direction (width direction).

[0054] (Regarding the refractive index in the thickness direction) The refractive index in the thickness direction is preferably 1.520 or less. By setting it to 1.520 or less, even if the refractive index in the bending direction is designed to be low, a decrease in the hardness of the film surface can be suppressed, and both flexibility and surface hardness can be achieved. By setting it to 1.520 or less, the indentation depth after unloading the test force in the thickness direction is reduced, and the hardness of the film surface, particularly the pencil hardness of the hard coat film after laminating the hard coat layer, can be improved. More preferably, it is 1.515 or less, still more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. Although the refractive index in the thickness direction is preferably low, from the viewpoint 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. It can be said that the above range can be achieved by increasing the draw ratio in both the bending direction and the folding direction. However, in order to control the refractive index in the thickness direction after controlling the refractive indices in the bending direction and the width direction to the preferable ranges, it is preferable to set the conditions while checking the balance of each process condition in the film forming process.

[0055] The method of controlling the refractive index in the thickness direction has stretching preheating temperature, stretching temperature, stretching ratio in the bending direction, stretching preheating temperature, stretching temperature, multi-stage stretching, high-ratio stretching, or heat setting temperature in the folding part direction. The stretching preheating temperature in the bending direction is preferably 70°C to 110°C. The stretching temperature in the bending direction is preferably 75 to 120°C. The stretching ratio in the bending direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. By lowering the stretching temperature and stretching at a low stretching ratio, the refractive index in the thickness direction can be effectively reduced while maintaining the bendability in the bending direction. The stretching preheating temperature in the folding part direction is also preferably 75°C to 110°C. The stretching temperature is preferably 75 to 120°C. The stretching ratio of the folding part is preferably 4.0 to 6.0 times, more preferably 4.4 to 6.0 times. The refractive index in the thickness direction can be effectively reduced while maintaining or reducing the refractive index in the bending direction. As a method of high-ratio stretching, multi-stage stretching may be used. In that case, it is preferable to increase the stretching ratio of the second stage compared to the stretching ratio of the first stage to effectively control the refractive index. Also, a method of stretching again after the crystallization step may be used. Accelerated stretching that increases the stretching speed from the beginning to the latter half of stretching may be used. The heat setting temperature is preferably 180 to 240°C. By performing heat setting, orientation crystallization in the stretching direction proceeds, and the refractive index in the thickness direction can be reduced. The reason why the hardness of the film surface is improved by reducing the refractive index in the thickness direction is not necessarily clear, but it is considered that aromatics such as benzene rings in the molecular chain are oriented in the plane direction, and there is an effect of suppressing deformation due to stress applied in the thickness direction.

[0056] (Regarding the density of the polyester film) The density of the polyester film is preferably 1.380 g / cm 3 or more. More preferably 1.383 g / cm 3 or more. 1.380 g / cm 3By doing the above, the flexibility can be improved, and the surface hardness of the film, especially the pencil hardness of the hard coat film after laminating the hard coat layer, can be improved. The higher the density, the better, although it is somewhat affected by the presence or absence of particles in the film, etc., and it is preferably 1.40 g / cm 3 or less. By setting the heat setting temperature during film formation to 180 to 240 °C, crystallization can proceed and the density can be effectively increased.

[0057] 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 at the second biaxial stretching stage and easy to improve the flexibility. That is, it is preferable to obtain a polyester film by stretching the unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. And in the width direction, it can be said that it is a preferable embodiment to stretch at a stretching ratio of 4.0 to 6.0 times, more preferably 4.4 to 6.0 times.

[0058] Also, in the present invention, for the polyester film (1) The refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folded part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less (4) The density is 1.380 g / cm 3 or more It can be said that it is a particularly preferable embodiment to simultaneously have these four characteristics. However, even in a combination within the range of the above-mentioned preferable production conditions, for example, a combination in which the stretching ratio in the bending direction is 1.4 times or less, the stretching ratio in the direction of the folded part is less than 4.4 times, and the heat setting temperature is 220 °C or less, each preferable production condition In the case of a combination of conditions that cannot be said to be optimal within the claimed range, it may happen that it is not always possible to obtain a product that simultaneously satisfies the above four characteristics. In this case, by slightly adjusting any one of the conditions or a combination thereof, such as increasing the draw ratio in the bending direction to 1.7 times or more, increasing the draw ratio in the direction of the folded portion to 4.4 times or more, increasing the heat setting temperature to about 230 °C, or lowering the drawing temperature in the bending direction and / or the direction of the folded portion, it is possible to simultaneously satisfy the above four characteristics. This can be achieved.

[0059] In order to adjust film-forming properties, film strength, thermal dimensional stability, appearance defects, etc., any film-forming method such as stretching, relaxation, heat setting, surface treatment, etc. may be employed, but it can be said that in the present invention, it is a particularly preferred embodiment to control the refractive index and density of the film within the above preferred ranges. By controlling the refractive index and density within the preferred ranges, it is possible to provide a polyester film suitable for a foldable display, which has better bend resistance and surface hardness than conventional films, and in particular, high pencil hardness of a hard coat film after laminating a hard coat layer.

[0060] Specifically, for example, after sufficiently vacuum-drying PET pellets, they are supplied to an extruder and melt-extruded into a sheet shape at about 280 °C, and then cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, in the longitudinal direction with a roll heated to 75 to 120 °C to obtain a uniaxially oriented PET film. Further, the ends of the film are gripped with clips and guided into a hot air zone heated to 75 to 120 °C. After drying, it is stretched 4.0 to 6.0 times, more preferably 4.4 to 6.0 times, in the width direction. Subsequently, it is guided into a heat treatment zone at 180 to 240 °C and heat treatment can be performed for 1 to 60 seconds. During this heat treatment process, a relaxation treatment of 0 to 10% may be performed in the width direction or the longitudinal direction as necessary.

[0061] The intrinsic viscosity of the polyester film preferably ranges from 0.50 to 1.0 dl / g. When the intrinsic viscosity is 0.50 dl / g or more, the impact resistance is improved, and it is preferable that disconnection of the internal circuit of the display due to external impact hardly occurs. 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 film production is stable, which is preferable.

[0062] (Easy adhesion layer) In the present invention, in order to improve the adhesiveness between the polyester film and the hard coat layer etc., it is also preferable to laminate an easy adhesion layer on the polyester film. The easy adhesion layer can be obtained by applying a coating liquid for forming the easy adhesion layer on 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 be 0.005 to 0.20 g / m 2 It is preferable to manage it to. When the coating amount is 0.005 g / m 2 or more, adhesiveness is obtained, which is preferable. On the other hand, when the coating amount is 0.20 g / m 2 or less, blocking resistance is obtained, which is preferable.

[0063] Examples of the resin to be contained in the coating liquid used for laminating the easy adhesion layer include polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc., and they 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 them 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 resins and crosslinking agents are preferably water-soluble or water-dispersible resins and compounds.

[0064] It is preferable to add particles to the easy - adhesion layer in order to impart lubricity. The average particle size of the fine particles is preferably 2 μm or less. If the average particle size of the particles exceeds 2 μm, the particles are likely to fall off from the easy - adhesion layer. Examples of the particles to be included 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 - based particles such as styrene - based, acrylic - based, melamine - based, benzoguanamine - based, silicone - based particles, etc. These may be added to the easy - adhesion layer alone, or two or more of them may be combined and added.

[0065] Also, as a method of 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.

[0066] (Hard - coat layer) When using the polyester film or laminated film of the present invention as a protective film for organic EL, it is preferable to have 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 - based, siloxane - based, inorganic hybrid - based, urethane acrylate - based, polyester acrylate - based, epoxy - based, etc. can be used without particular limitation. Also, two or more types of materials can be mixed and used, and particles such as inorganic fillers and organic fillers can be added.

[0067] (Film thickness of the hard - coat layer) The film thickness of the hard coat layer is preferably 1 to 50 μm. When 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, curling due to the curing shrinkage of the hard coat can be suppressed, and the handleability of the film can be improved.

[0068] (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.

[0069] (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.

[0070] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or more, and more preferably 4H or more. If the pencil hardness is 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.

[0071] (Properties of the hard coat layer) The hard coat layer in the present invention can be used for the purpose of enhancing the pencil hardness of the surface as described above to protect the display, and preferably has a high transmittance. The 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 perspective 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 perspective of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0072] Other functions may be further added to the hard coat layer. For example, a hard coat layer with added functionality such as an antiglare layer having a certain pencil hardness as described above, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer is also preferably applied in the present invention.

[0073] 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 the refractive index adjustment layer, or a separate refractive index adjustment layer may be laminated.

Examples

[0074] 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.

[0075] (1) Limiting viscosity After pulverizing and drying the film or polyester resin, 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 a solution with a concentration of 0.4 (g / dl) at 30°C and the flow-down time of the solvent alone were measured. From the ratio of those times, assuming that the Huggins constant is 0.38 and using Huggins' equation, the intrinsic viscosity was calculated.

[0076] (2) Flexural resistance of the polyester film sample (bending radius 1.5 mm) Prepare a polyester film sample with a size of 20 mm in the width direction × 110 mm in the flow direction. Using a non-load U-shaped expansion and contraction testing machine (manufactured by Yuasa System Co., Ltd., DLDMLH-FS), set the bending radius to 1.5 mm and bend it 200,000 times at a speed of 1 time / second. At that time, the sample was fixed at positions 10 mm from both ends on the long side, and the bending part was 20 mm × 90 mm. Here, Fig. 1 is a schematic diagram for showing the bending radius when folding a foldable display. Considering the case where the polyester film is arranged on the inner surface of the folded state, the bending test was conducted modelly with the position of reference numeral 11 in Fig. 1 set to 1.5 mm. After the bending treatment, place the sample flat with the inner side of the bend facing down and conduct visual observation. ○: No cracks or deformations can be confirmed in the sample. ×: There are cracks or creases in the sample, and when placed horizontally, the maximum height of the lift is 5 mm or more.

[0077] (3) Flexural resistance of the polyester film sample (bending radius 0.5 mm) In the same manner as the above bending test, the bending radius was set to 0.5 mm and bent 200,000 times at a speed of 1 time per second. Here, FIG. 1 is a schematic diagram for showing the bending radius when the foldable display is folded. Considering the case where a polyester film is disposed on the inner surface of the folded state, the bending test was performed modelly with the position of reference numeral 11 in FIG. 1 set to 0.5 mm. The film surface on the outer side of the bent portion was observed 700 times with a digital microscope (RH8800 manufactured by HIROX Co., Ltd.), and the presence or absence of wrinkles (cracks) was observed. Separately from the above-mentioned bend resistance visual test with a bending radius of 1.5 mm, by performing this test with the bending radius reduced to 0.5 mm, it is intended to evaluate in a state close to the actual use state of the foldable display on which a hard coat layer or other members are laminated or adhered. This is a test for detecting defects that are fine and difficult to detect visually, such as being easily broken or easily cracked, separately from the visual observation with the bending radius of 1.5 mm. ○: There are no defects on the film surface on the outer side of the bend. ×: Broken or wrinkles (cracks) can be confirmed on the film surface on the outer side of the bend.

[0078] (4) Refractive index In accordance with JIS K 7142:2014 "Method for Measuring Refractive Index of Plastics (Method A)", using an Abbe refractometer (NAR-4T manufactured by Atago Co., Ltd., measurement wavelength 589 nm), the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction were determined.

[0079] (5) Pencil hardness Using the pencil hardness of the hard coat film as a sample, in accordance with JIS K 5600-5-4:1999, it was measured at a load of 750 g and a speed of 1.0 mm / s. In the present invention, 3H or more was regarded as passing.

[0080] (6) Total light transmittance, haze Measured using a haze meter (NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd.).

[0081] (7) Density The density was measured according to the method (density gradient tube method) conforming to JIS K 7112:1999. (Unit: g / cm 3 ).

[0082] (8) Penetration depth after unloading the test force The sample was cut into a square of about 2 cm, and the opposite side of the measurement surface was fixed with an adhesive (Cemedine (registered trademark) High Super 30) on a micro cover glass of 18×18 mm (manufactured by Matsunami Glass Ind., Ltd.). After sticking and fixing, it was left at room temperature for 12 hours or more, and then the penetration depth (μm) after unloading the test force was measured under the following conditions using a dynamic ultra-micro hardness tester "DUH-211" (manufactured by Shimadzu Corporation). ≪Measurement conditions≫ Test mode: Loading-unloading test Indenter used: Triangular pyramid indenter with an included angle of 115 degrees between edges Indenter elastic modulus: 1.140×10 6 N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50 mN Loading rate: 4.44 mN / sec Loading holding time: 2 sec Unloading holding time: 0 sec

[0083] (9) Maximum thermal shrinkage rate The sample film was cut into 10 mm in length and 250 mm in width, marked at 200 mm intervals in the direction where the long side was to be measured, 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. Then, the interval B between the marks was determined under a constant tension of 5 g, and the thermal shrinkage rate (%) was determined by the following formula. The above thermal 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 thermal shrinkage rate (%). Thermal shrinkage rate (%) = [(A - B) × 100] / A For both 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 thermal shrinkage rate (%).

[0084] (10) Elastic modulus (Young's modulus (unit: GPa)) The elastic modulus in the bending direction and the folding direction of the polyester film was measured at 23°C in accordance with JIS K7127.

[0085] (Preparation of polyethylene terephthalate pellet (a)) As an esterification reactor, a continuous esterification reactor consisting of a three-stage completely mixed tank having a stirrer, a partial condenser, a raw material inlet, and a product outlet was used. The amount of TPA was 2 tons / hr, the amount of EG was 2 moles per mole of TPA, the amount of antimony trioxide was such that the amount of Sb atoms was 160 ppm with respect to the produced PET. These slurries were continuously fed into the first esterification reactor of the esterification reactor and reacted at 255°C with an average residence time of 4 hours under normal pressure. Next, the reaction product in the first esterification reactor was continuously taken out of the system and fed into the second esterification reactor. The EG distilled off from the first esterification reactor was supplied to the second esterification reactor in an amount of 8% by mass with respect to the produced polymer (produced PET). Further, an EG solution containing magnesium acetate in an amount such that the amount of Mg atoms was 65 ppm with respect to the produced PET and an EG solution containing TMPA in an amount such that the amount of P atoms was 20 ppm with respect to the produced PET were added, and 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 fed into the third esterification reactor. Further, an EG solution containing TMPA in an amount such that the amount of P atoms was 20 ppm with respect to the produced PET was added, and 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 fed into a three-stage continuous polycondensation reactor for polycondensation, and further filtered with a filter medium of a stainless steel sintered body (nominal filtration accuracy: 90% cut for 5 μm particles) to obtain polyethylene terephthalate pellet (a) having an intrinsic viscosity of 0.62 dl / g.

[0086] (Preparation of polyethylene terephthalate pellet (b)) Regarding the manufacturing process of polyethylene terephthalate pellets (a), the limiting viscosity was adjusted to 0.580 dl / g in the same manner except for adjusting the residence time of the third esterification reaction, and polyethylene terephthalate pellets (b) were obtained.

[0087] (Preparation of polyethylene terephthalate pellets (c)) Polyethylene terephthalate pellets (a) were subjected to solid-phase polymerization at 220 °C for varying times under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus to prepare polyethylene terephthalate pellets (c) with a limiting viscosity of 0.75 dl / g.

[0088] (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 with a number average molecular weight of 2000, 85.00 parts by mass of acetonitrile as a solvent, and 5.00 parts by mass of N-methylpyrrolidone were charged. 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 cooling this reaction solution 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. Then, under reduced pressure, a part of acetonitrile and water was removed to prepare a water-soluble polyurethane resin (A) with a solid content of 35% by mass.

[0089] (Polymerization of water-soluble carbodiimide compound) Into a flask equipped with a thermometer, a nitrogen gas inlet tube, a reflux condenser, a dropping funnel, and a stirrer, 200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidization catalyst were charged, and the mixture was 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. To this, water was gradually added at 50 °C to obtain a yellow transparent water-soluble carbodiimide compound (B) having a solid content of 40% by mass.

[0090] (Preparation of coating liquid for forming easy adhesion layer) The following coating agents were mixed to prepare a coating liquid. Water 16.97 parts by mass Isopropanol 21.96 parts by mass Polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (Silica sol with an average particle size of 40 nm, solid content concentration 40% by mass) Surfactant 0.05 parts by mass (Silicone-based, solid content concentration 100% by mass)

[0091] (Preparation of hard coat coating liquid a) To 100 parts by mass of a hard coat material (manufactured by JSR Corporation, Opstar (registered trademark) Z7503, concentration 75%), 0.1 part by mass of a leveling agent (manufactured by BYK Chemie Japan, BYK307, concentration 100%) was added, and the mixture was diluted with methyl ethyl ketone to prepare a hard coat coating liquid a having a solid content concentration of 40% by mass.

[0092] (Preparation of hard coat coating liquid b) 95 parts by mass of pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., A-TMM-3, solid content concentration 100%), 5 parts by mass of a photopolymerization initiator (manufactured by BASF Japan Ltd., Irgacure (registered trademark) 907, solid content concentration 100%), and 0.1 part by mass of a leveling agent (manufactured by BYK-Chemie Japan, BYK307, solid content concentration 100%) were mixed and diluted with a solvent of toluene / MEK = 1 / 1 to prepare a hard coat coating solution b with a concentration of 40% by mass.

[0093] (Example 1) Polyethylene terephthalate pellets (a) were supplied to an extruder and melted at 285°C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut-off 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, forming an unstretched film. This unstretched film was uniformly heated to 75°C using a heating roll, heated to 85°C using a non-contact heater, and subjected to roll stretching (longitudinal stretching) at a ratio of 1.4 times. The above-mentioned coating solution for forming an easy-adhesion layer was applied to both sides of the obtained uniaxially stretched film by a roll coating method and then dried at 80°C for 20 seconds. The coating amount after drying at the end (after biaxial stretching) was adjusted to be 0.06 g / m 2 . Then, it was guided 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 50-μm-thick polyethylene terephthalate film. The evaluation results are shown in Table 1.

[0094] (Examples 2 - 3) A polyester film was obtained in the same manner as in Example 1 except that the longitudinal stretching ratio described in Table 1 was changed.

[0095] (Example 4) A polyester film was obtained in the same manner as in Example 1 except that the lateral stretching ratio was changed to 4.4 times and the heat-fixing temperature was changed to 220°C.

[0096] (Examples 5 - 6) A polyester film was obtained in the same manner as in Example 4, except that the draw ratio in the longitudinal direction was changed as described in Table 1.

[0097] (Example 7) A polyester film was obtained in the same manner as in Example 1, except that the draw ratio in the width direction was changed to 5.5 times and the heat setting temperature was changed to 190°C.

[0098] (Examples 8 - 9) A polyester film was obtained in the same manner as in Example 7, except that the draw ratio in the longitudinal direction was changed as described in Table 1.

[0099] (Example 10) In the production process of Example 5, a polyester film was obtained in the same manner as in Example 5, except that after stretching in the longitudinal direction, a relaxation heat treatment was performed at 100°C for 10%.

[0100] (Example 11) In the production process of Example 5, a polyester film was obtained in the same manner as in Example 5, except that after heat setting, the clip was opened at 200°C and relaxation heat treatment was performed in the longitudinal and width directions. The tenter speed and winding roll speed were adjusted so that the relaxation rate in the longitudinal direction was 3%. The relaxation in the width direction was in a free state.

[0101] (Example 12) A polyester film was obtained in the same manner as in Example 1, except that the temperature during longitudinal stretching was changed to 75°C and the heat setting temperature was changed to 220°C.

[0102] (Example 13) A polyester film was obtained in the same manner as in Example 1, except that the temperature during longitudinal stretching was changed to 75°C, the draw ratio was changed to 1.2 times for stretching, and then the draw ratio in the width direction was changed to 5.0 times for stretching.

[0103] (Example 14) In Example 3, the longitudinal stretching was a two-stage stretching, with the stretching ratio in the first stage being 1.2 times and the stretching ratio in the second stage being 1.67 times. A polyester film was obtained in the same manner as in Example 3 except for this. The total longitudinal stretching ratio is about 2.0 times.

[0104] (Example 15) A polyester film was obtained in the same manner as in Example 5 except that the preheating temperature during widthwise stretching was changed to 95°C and the heat setting temperature was changed to 190°C.

[0105] (Example 16) In Example 2, the widthwise stretching was a two-stage stretching, with the stretching ratio in the first stage being 1.5 times and the stretching ratio in the second stage being 4.0 times. A polyester film was obtained in the same manner as in Example 2 except that the heat setting temperature was changed to 190°C. The total widthwise stretching ratio is 6.0 times.

[0106] (Examples 17 - 18) A polyester film was obtained in the same manner as in Example 2 except that the thickness was changed as shown in Table 1.

[0107] (Example 19) A polyester film was obtained in the same manner as in Example 1 except that relaxation heat treatment in the width direction was not performed in the manufacturing process of Example 1.

[0108] (Example 20) After creating an unstretched film in the same manner as in Example 1, the unstretched film was preheated at 75°C with a tenter and stretched horizontally 1.4 times at 85°C. After applying the above coating liquid for forming an easy-adhesion layer to both sides of the obtained uniaxially stretched film by the roll coating method, it was dried at 80°C for 20 seconds. Note that the coating amount after drying at the end (after biaxial stretching) was adjusted to be 0.06 g / m 2 . It was uniformly heated to 105°C using a heating roll and heated to 95°C with a non-contact heater, and roll stretching (longitudinal stretching) was performed 4.0 times. After fixing the width, heat setting was performed at 230°C for 5 seconds to obtain a 50-μm-thick polyethylene terephthalate film.

[0109] (Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that it was stretched only in the width direction without stretching in the longitudinal direction and was stretched in the horizontal one-axis direction.

[0110] (Comparative Example 2) A polyester film was obtained in the same manner as in Example 7, except that it was stretched only in the width direction without stretching in the longitudinal direction and was stretched in the horizontal one-axis direction.

[0111] (Comparative Examples 3 to 7) A polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 220°C and the PET pellets and thickness were as described in Table 1. Comparative Examples 3 to 7 have a heat setting temperature lower than that of Example 1 as described above, and are combinations of various condition levels that cannot be said to be the best within the preferable condition range of the stretching ratios in the longitudinal and width directions. As described in Table 1, the refractive index in the thickness direction increased, the indentation depth after unloading the test force was large, and the pencil hardness after laminating the hard coat layer became smaller compared to each example.

[0112] (Comparative Example 8) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to 2.7 times and the heat setting temperature was changed to 220°C.

[0113] (Comparative Example 9) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to 3.4 times.

[0114] (Comparative Example 10) A polyester film was obtained in the same manner as in Example 4, except that the heat setting temperature was changed to 100°C.

[0115] (Comparative Example 11) A polyester film was obtained in the same manner as in Example 13, except that the stretching temperature in the longitudinal direction was changed to 130°C.

[0116] (Comparative Example 12) A polyester film was obtained in the same manner as in Example 1, except that the preheating temperature in the width direction was changed to 120°C.

[0117] Using a Mayer bar, a hard coat coating liquid a was applied to one surface of the prepared film such that the film thickness after drying was 5 μm, and after drying at 80°C for 1 minute, ultraviolet rays were irradiated (integrated light quantity 200 mJ / cm 2 ), and a hard coat film was obtained.

[0118] (Example 21) Also, after obtaining a laminated film in the same manner as in Example 1, a hard coat film was obtained by applying a hard coat coating liquid b in the same manner as described above instead of the hard coat coating liquid a.

[0119] The hard coat film was bonded to an organic EL module via a 25-μm-thick adhesive layer to create a smartphone-type foldable display that could be folded in half at the center with a radius corresponding to the bending radius in FIG. 1 of 3 mm. Those using the hard coat film of each example satisfied the operation and visibility as a foldable smartphone that could be folded in half at the center and carried around. There was no defect in the display due to external force. On the other hand, the foldable displays using the hard coat films of each comparative example seemed to develop image distortion at the folding part of the display as the usage frequency increased, and were not very preferable. Also, there were some cases where the organic EL panel was damaged by external force and could not be displayed correctly.

[0120]

Table 1

Industrial Applicability

[0121] The foldable display using the polyester film for protecting the organic EL module of the foldable display of the present invention can maintain mass productivity, and the polyester film used for protecting the organic EL module does not deform after being repeatedly folded, so that image distortion does not occur at the folded portion of the display. In particular, a portable terminal device equipped with a foldable display using the polyester film of the present invention as a protective film for the organic EL module provides beautiful images, is rich in functionality, and is excellent in convenience such as portability.

Explanation of Signs

[0122] 1: Foldable display 11: Bending radius 2: Polyester film for surface protection film of foldable display 21: Folded portion 22: Bending direction (direction orthogonal to the folded portion) 3: Organic EL module 30: Protective film 31: Adhesive 32: Back substrate 33: Thin film transistor 34: Organic EL element 35: Sealing adhesive layer

Claims

1. A polyester film for protecting the organic EL module of a folding organic EL display that satisfies the following conditions. (1) The refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the fold is 1.670 to 1.700 (3) A refractive index in the thickness direction is 1.520 or less. (4) Density is 1.380 g / cm ３ End (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.)

2. 2. The polyester film for protecting an organic EL module of a folding organic EL display according to claim 1, wherein the polyester film has a Young's modulus in the bending direction of 2.7 GPa or less and a Young's modulus in the direction of the folding portion of 4.5 GPa or more.

3. 3. The polyester film for protecting an organic EL module of a folding organic EL display according to claim 1, which has a total light transmittance of 85% or more, a haze of 3% or less, and a maximum heat shrinkage of 6% or less.

4. The polyester film for protecting an organic EL module of a folding organic EL display according to any one of claims 1 to 3, having an easy-adhesion layer on at least one side.

5. The polyester film for protecting an organic EL module of a folding organic EL display according to any one of claims 1 to 4, which has a hard coat layer having a thickness of 1 to 50 µm on at least one surface.

6. A folding organic EL display including the polyester film for protecting an organic EL module of a folding organic EL display according to any one of claims 1 to 5 as a protective film for the organic EL module, A folding organic EL display in which a protective film of a continuous single organic EL module is disposed via a folding portion of the folding organic EL display.

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