Polyester film for folding displays, laminated film, and mobile terminal device

Abstract

The purpose of the present invention is to provide a polyester film for folding displays which achieves a high level of both resistance to folding marks and resistance to bending breakage. The present invention provides a polyester film for folding displays wherein the difference (Ny-Nx) between the refractive index Nx in the bending direction and the refractive index Ny in a direction orthogonal to the bending direction is not less than 0.05, and the limiting viscosity of at least a surface layer is not less than 0.65 dl / g.

Description

Polyester films for foldable displays, laminated films, and mobile terminal devices

[0001] The present invention relates to a polyester film to be applied to the folding portion of a mobile terminal device having a folding display, and preferably to a surface protection film to be attached to the surface of the folding display.

[0002] In a foldable display, a certain portion (the portion corresponding to the folding portion) is repeatedly folded. Therefore, when a conventional polyester film is used as a surface protection film, the portion corresponding to the folding portion deteriorates with frequency of use, and the image displayed on the display may become distorted at that portion. Patent Document 1 describes a polyester film that has been designed to prevent deterioration of the folding portion, with a refractive index in the bending direction of 1.590 to 1.620, a refractive index in the direction perpendicular to the bending direction (the folding portion direction) of 1.670 to 1.700, a refractive index in the thickness direction of 1.520 or less, and a density of 1.380 g / cm ３ The polyester film of Patent Document 1 is resistant to crease marks, and for example, when bent at a bending radius of 1.5 mm and then placed on a horizontal surface, the film rises by 5 mm or less from the horizontal surface. The polyester film of Patent Document 1 also has excellent flex fracture resistance, and no wrinkles (cracks) occur on the film surface even after being bent 200,000 times at a curvature radius of 0.5 mm.

[0003] International Publication No. 2020 / 162119

[0004] Considering the usefulness of a foldable display when mounted on a mobile terminal, a polyester film for a foldable display is required to achieve high levels of resistance to creases and flexural breakage resistance (resistance to breakage at the bend), and preferably, in addition to these, to achieve high levels of impact resistance. Therefore, the inventors aimed to provide a polyester film for a foldable display that combines high levels of resistance to creases and flexural breakage resistance. Specifically, the objective of the invention was to provide a polyester film that does not produce creases even when bent at a small curvature radius of 0.5 mm, while maintaining good flexural breakage resistance even in a flex test at a level of 100,000 cycles, preferably 200,000 cycles, and more preferably 300,000 cycles. Resistance to crease marks and resistance to flexural breakage are contradictory properties, and generally, the more resistant to crease marks, the poorer the resistance to flexural breakage (for example, the smaller the refractive index in the bending direction, the smaller the elongation at break, resulting in poorer resistance to flexural breakage, but conversely, improved resistance to crease marks). Therefore, it is difficult to achieve both at a high level. Furthermore, as the thickness of the film increases, the impact resistance improves, but the resistance to crease marks and resistance to flexural breakage become poorer, so it is even more difficult to achieve both at a high level in a thick film. Note that resistance to crease marks and resistance to flexural breakage are sometimes collectively referred to as flexibility.

[0005] The present invention has the following features that have solved the above problems. [1] A polyester film for a foldable display, in which the difference (Ny-Nx) between the refractive index Nx in the bending direction and the refractive index Ny in the direction perpendicular to the bending direction is 0.05 or more, and at least the surface layer has an intrinsic viscosity of 0.65 dL / g or more. [2] A polyester film for a foldable display, in which the cube of the 0.2% proof stress point strain in at least one of the longitudinal direction and the width direction divided by the square root of the thickness (μm) is 2.1 or more, and at least the surface layer has an intrinsic viscosity of 0.65 dL / g or more. [3] A polyester film for a foldable display, in which the value of the breaking elongation in at least one of the longitudinal direction and the width direction divided by the refractive index in that direction is 13 to 90, and at least the surface layer has an intrinsic viscosity of 0.65 dL / g or more. [4] The polyester film for foldable displays according to [1], wherein the value obtained by dividing the cube of the 0.2% proof stress strain in at least one of the longitudinal direction and the width direction by the square root of the thickness (μm) is 2.1 or more. [5] The polyester film for foldable displays according to [1], wherein the value obtained by dividing the breaking elongation in at least one of the longitudinal direction and the width direction by the refractive index in that direction is 13 to 90. [6] The polyester film for foldable displays according to any one of [1] to [5], wherein the refractive index Nx in the bending direction is 1.570 to 1.630. [7] The polyester film for foldable displays according to any one of [1] to [6], wherein the refractive index Ny in the direction perpendicular to the bending direction is 1.670 to 1.760. [8] The polyester film for foldable displays according to any one of [1] to [7], wherein the refractive index Nz in the thickness direction is 1.500 to 1.530. [9] The polyester film for a foldable display according to any one of [1] to [8], which is a polyethylene terephthalate film.

[10] The polyester film for a foldable display according to any one of [1] to [9], which has a surface layer and a core layer.

[11] The polyester film for foldable displays according to any one of [1] to

[10] , which, after undergoing an unloaded clamshell folding test in which the film is bent 50,000 times at a bending radius of 0.5 mm and a speed of 1 folding / second, exhibits a maximum lift-up height of 5 mm or less when placed horizontally on a flat surface with the inside of the bend facing down.

[12] The polyester film for foldable displays according to any one of [1] to

[11] , which, after being folded at a bending radius of 0.5 mm and maintained at room temperature for 24 hours, exhibits a maximum lift-up height of 7 mm or less when placed horizontally on a flat surface with the inside of the bend facing down.

[13] The polyester film for foldable displays according to any one of [1] to

[12] , which, after undergoing an unloaded clamshell folding test in which the film is bent 100,000 times at a bending radius of 0.4 mm and a speed of 1 folding / second, exhibits no cracks or breaks when the surface on the outside of the bend is observed under a microscope at 700x magnification.

[14] The polyester film for foldable displays according to any one of [1] to

[13] , wherein after a no-load clamshell folding test in which the film is bent 200,000 times at a bending radius of 0.4 mm and a speed of 1 bending / second, neither cracks nor breaks are observed when the surface on the outer side of the bend is observed under a microscope at 700x magnification.

[15] Use of the polyester film according to any one of [1] to

[14] in a foldable display.

[16] A method for obtaining a foldable display using the polyester film according to any one of [1] to

[14] .

[17] A laminate film for foldable displays, comprising the polyester film according to any one of [1] to

[14] and an easy-adhesion layer laminated on at least one surface of the polyester film.

[18] The laminate film for foldable displays according to

[17] , further comprising a hard coat layer on the easy-adhesion layer.

[19] A foldable display comprising the laminate film according to

[17] or

[18] as a surface protective film.

[20] A mobile terminal device comprising the foldable display according to

[19] .

[0006] The polyester film for foldable displays of the present invention can achieve both resistance to crease marks and flexural breakage resistance at a higher level. Preferably, the polyester film for foldable displays of the present invention can achieve both resistance to crease marks and flexural breakage resistance at a higher level even when the thickness is increased. Such polyester films are useful for laminate films for foldable displays, foldable displays (also known as foldable displays), mobile terminal devices, and the like. For example, when the polyester film of the present invention is used as a surface protective film for a foldable display, repeated folding is unlikely to cause image distortion due to deformation of the surface protective film located on the surface or breakage at the fold of the surface protective film, thereby improving convenience such as image beauty, functionality, and portability. Furthermore, increasing the thickness of the polyester film can further improve screen protection. When the polyester film of the present invention is used in parts other than the surface protective film, durability against repeated folding can be improved. As a result of the investigation, it was found that in one embodiment of the present invention, if the value obtained by dividing the cube of the 0.2% proof stress point strain in at least one of the longitudinal direction and the width direction by the square root of the thickness (μm) is 2.1 or more, image distortion due to deformation of the surface protection film and breakage at the folded portion of the surface protection film are less likely to occur even if the thickness is increased. Furthermore, as a result of the investigation, it was found that in one embodiment of the present invention, by increasing the intrinsic viscosity, the breaking elongation relative to the refractive index can be made larger than when the intrinsic viscosity is low. Furthermore, it was found that if the value obtained by dividing the breaking elongation in at least one of the longitudinal direction and the width direction by the refractive index in that direction is 13 to 90, deformation upon bending is less likely to occur, and if the intrinsic viscosity of at least the surface layer is 0.65 dl / g or more, breakage upon bending is less likely to occur.

[0007] FIG. 1 is a schematic perspective view illustrating the folding of the polyester film for foldable displays of the present invention.

[0008] 1. Foldable Display In the present invention, the display is not particularly limited as long as it can be used for a foldable display, and examples thereof include a liquid crystal display (LCD), an organic EL display, an inorganic EL display, an LED, and an FED. An LCD, an organic EL display, or an inorganic EL display is preferred. An organic EL display or an inorganic EL display is more preferred in that the layer structure required for the display can be reduced.

[0009] A foldable display encompasses a single continuous display that can be folded in half or otherwise when needed, such as for carrying. By folding, the size of the display when carried can be reduced by half or more, improving the portability of the portable device. The bending radius of a foldable display is preferably 5 mm or less, more preferably 0.1 to 5 mm, even more preferably 0.5 to 3 mm, and particularly preferably 1 to 3 mm. A bending radius of 5 mm or less enables a thin display in the folded state. From the perspective of thinning, a smaller bending radius is preferable. Even a bending radius of 1 mm can achieve a practically sufficient thinness when folded. On the other hand, from the perspective of reducing the occurrence of creases, a larger bending radius is preferable. The surface protective film described below is one application of the polyester film and laminate film for a foldable display, and the surface protective film may be located on the outer surface of the folded display or on the inner surface of the folded display. Furthermore, the foldable display may be a tri-fold type, a quadruple-fold type, or even a rollable type known as a rollable type. All of these are encompassed by the foldable display of the present invention.

[0010] The polyester film for a foldable display and the laminate film for a foldable display of the present invention may be used in any part of a foldable display as long as it is a constituent member of the foldable display. Below, using a foldable organic EL display as an example, a typical configuration of a foldable display and parts of the foldable display in which the polyester film and the laminate film of the present invention can be used will be described.

[0011] 2. Foldable Organic EL Display A foldable organic EL display includes an organic EL module, and may further include a circular polarizer, a touch panel module, a front protective film, a back protective film, and the like, as needed.

[0012] 3. Substrate Film Used in Organic EL Module An organic EL module generally comprises an electrode, an electron transport layer, a light-emitting layer, a hole transport layer, and a transparent electrode. A polyester film or a laminate film thereof can be used as the substrate film for each layer, such as the transparent electrode. The polyester film or laminate film for a foldable display of the present invention may be used as the polyester film.

[0013] 4. Organic EL Module Protective Film A protective film is preferably provided on the visible side of the organic EL module. It is also preferable to provide a protective film on the non-visible side of the organic EL module. The polyester film and laminate film for foldable displays of the present invention may be used as these protective films. Organic EL modules are generally formed on glass substrates. Glass substrates support and protect the organic EL module, but cannot be used in foldable displays because they cannot be bent. In foldable displays, a protective film is used as a protective member in place of glass, and the polyester film and laminate film for foldable displays of the present invention can be used as this protective film. The protective film may be provided with a hard coat layer to prevent scratches.

[0014] 5. Substrate Film Used in Touch Panel Module A mobile terminal device preferably has a touch panel. In the case of an organic EL display, the touch panel module is preferably disposed on top of the organic EL display or between the organic EL module and a circular polarizer. The touch panel module may have a transparent substrate such as a film and a transparent electrode disposed thereon. The polyester film and laminate film for a foldable display of the present invention can be used as this transparent substrate. When the polyester film and laminate film for a foldable display of the present invention are used as a transparent substrate included in a touch panel module, the polyester film and laminate film are preferably provided with a hard coat layer, a refractive index adjustment layer, or the like.

[0015] 6. Protective Films and Resin Films Used in Circularly Polarizing Plates Circularly polarizing plates have the function of suppressing degradation of image quality due to reflection of external light by components inside the display. Circularly polarizing plates are composed of a linear polarizing plate and a retardation plate. The linear polarizing plate may have a protective film on at least the viewing side of the polarizer. The linear polarizing plate may also have a protective film on the side opposite the viewing side of the polarizer, or a retardation plate may be directly laminated on the polarizer. The polyester film and laminate film for foldable displays of the present invention can be used as a polarizer protective film, a resin film for a retardation plate, etc. In these cases, the polyester film and laminate film for foldable displays of the present invention preferably have a slow axis direction parallel or perpendicular to the absorption axis direction of the polarizer. Note that a deviation of up to 10 degrees, preferably 5 degrees, from this parallel or perpendicular orientation is acceptable.

[0016] 7. Surface Protection Film Impact on a display from above can cause disconnection of the circuits in the organic EL module and touch panel module. For this reason, displays are often provided with a surface protection film. Films with high light transmittance and low haze, such as polyimide film, polyester film, polycarbonate film, acrylic film, TAC film, and cycloolefin polymer film, can be used as the surface protection film. Among these, polyester film, which has high impact resistance and can be produced inexpensively, is preferred. The polyester film and laminate film for foldable displays of the present invention are preferably used as this surface protection film. Examples of surface protection films include those that are already installed on the outermost surface of the display at the time of factory shipment (called cover windows) and those that can be attached, peeled, and replaced by the user (called after-films). The polyester film and laminate film for foldable displays of the present invention can be used for either of these. When the polyester film and laminate film for foldable displays of the present invention are used as a surface protection film, the laminate film having a hard coat layer laminated thereon is provided on the surface of the foldable display with the hard coat layer facing the viewing side of the display. The hard coat layer may be provided on both sides of the laminate film. The polyester film and laminate film for a folding display of the present invention are preferably used as a surface protection film for an organic EL module of a folding display.

[0017] 8. Polyester Film for Foldable Displays 8.1 Layer Structure and Materials The polyester film for foldable displays of the present invention may be a single-layer film made of one or more types of polyester resin, or may be a multilayer film when two or more types of polyester are used.

[0018] Examples of polyester resins include polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, and polyester resins (copolymer polyester resins) made of copolymers containing these resin components as main components. Among these, polyethylene terephthalate films are particularly preferred in terms of mechanical properties, heat resistance, transparency, price, etc.

[0019] 8.2 Thickness The thickness of the polyester film for a foldable display is preferably 2 to 500 μm, more preferably 10 to 400 μm, even more preferably 10 to 250 μm, and even more preferably 20 to 100 μm. A film thickness of 2 μm or more is advantageous in that the film has sufficient rigidity, is easy to handle, and can protect foldable display components such as organic EL modules and touch panels. A film thickness of 500 μm or less is advantageous in that the film is easy to transport when transported using multiple rolls, the produced film is easy to handle, and breakage due to folding is suppressed. From the viewpoint of improving screen protection, the thickness of the polyester film for a foldable display is preferably 40 to 150 μm, more preferably 45 to 130 μm, even more preferably 55 to 110 μm, and even more preferably 60 to 105 μm.

[0020] 8.3 Haze The surface of the polyester film for folding displays may be smooth or uneven, but when used as a surface protection film, deterioration of optical properties due to unevenness is undesirable. The haze is preferably 3.5% or less, more preferably 0.1 to 3.5%, and even more preferably 0.3 to 2%. A haze of 3.5% or less can improve image visibility when used as a surface protection film. The lower limit of the haze is not particularly limited, but a haze of 0.1% or more is preferred from the standpoint of stable production.

[0021] 8.4 Total Light Transmittance The total light transmittance of the polyester film for foldable displays is preferably 85% or more, more preferably 85 to 99%, and even more preferably 87 to 97%. When the total light transmittance is 85% or more, sufficient visibility can be ensured when used as a surface protection film. There is no particular upper limit to the total light transmittance, but from the perspective of stable production, it is preferably 99% or less.

[0022] 8.5 Film Direction (Bending Direction, Direction Perpendicular to the Bending Direction) Figures 1(a) to 1(d) are schematic perspective views illustrating the directions of a polyester film for a foldable display. Figure 1(a) shows polyester film 11a before folding, Figure 1(b) shows polyester film 11b in the process of folding, and Figure 1(d) shows polyester film 11d after folding. As shown in Figure 1(b), polyester film 11b for a foldable display is bent on a virtual plane 13 perpendicular to a specific virtual axis 12 so that polyester film 11b is symmetrical with respect to a virtual line 14 intersecting axis 12. Figure 1(c) is a conceptual diagram showing the relationship between virtual plane 13, polyester film 11b, and virtual line 14. The direction of this virtual line 14 projected onto polyester surface 11x before bending is referred to as bending direction 15, and the direction of axis 12 is referred to as bending axis direction 16. The direction perpendicular to the bending direction 15 means the bending axis direction 16, which is also sometimes referred to as the "folded portion direction." When the polyester film 11 is completely folded, the bending axis 12 becomes the folded portion, and the radius of the bending axis 12 is referred to as the bending radius.

[0023] 8.6 Intrinsic Viscosity The intrinsic viscosity of the polyester film for foldable displays is 0.65 dl / g or more, preferably 0.65 to 1.0 dl / g, more preferably 0.67 to 0.9 dl / g, and even more preferably 0.70 to 0.8 dl / g, at least in the surface layer of the film. Combining the high intrinsic viscosity of the polyester film with the control of the refractive index described below can achieve both resistance to crease marks and resistance to flex fracture at a higher level than before. Therefore, for example, when the polyester film for foldable displays is used as a surface protection film, deterioration in the image quality of the foldable display is suppressed, improving the visibility of the display. The intrinsic viscosity has traditionally been set low, for example, at around 0.6 dl / g. However, increasing the intrinsic viscosity has made it possible to achieve both resistance to crease marks and resistance to crease marks. The upper limit of the intrinsic viscosity is not particularly limited, but is, for example, 1.0 dl / g or less. The intrinsic viscosity may be the value for the entire film.

[0024] 8.7 Refractive Index 8.7.1a Difference between the Refractive Index Nx in the Bending Direction and the Refractive Index Ny in the Bending Axis Direction The difference between the refractive index Nx in the bending direction of the polyester film for foldable displays and the refractive index Ny in the bending axis direction (folded portion direction) (Ny-Nx) is 0.05 or more, preferably 0.05 to 0.2, even more preferably 0.05 to 1.5, particularly preferably 0.08 to 1.3, and most preferably 0.09 to 1.2. By setting Ny-Nx to 0.05 or more and the intrinsic viscosity to 0.65 dl / g or more, it is possible to achieve both resistance to crease marks and resistance to flex fracture at a higher level than before. The higher the lower limit of Ny-Nx, the higher the level of resistance to crease marks and resistance to flex fracture. The upper limit of Ny-Nx is not particularly limited, but is, for example, 0.2 or less. The smaller the upper limit of Ny-Nx is compared to 0.2, the higher the balance between resistance to crease marks and resistance to flex fracture can be achieved.

[0025] 8.7.1b Difference between refractive index Ny in bending axis direction and refractive index Nz in thickness direction The difference between the refractive index Ny in the bending axis direction and the refractive index Nz in the thickness direction of the polyester film for foldable displays (Ny-Nz) is, for example, 0.160 to 0.300, preferably 0.163 to 0.270, and more preferably 0.165 to 0.250. The larger Ny-Nz is, the higher the balance between resistance to crease marks and flex fracture resistance can be achieved.

[0026] 8.7.2 Refractive Index Nx in the Bending Direction The refractive index Nx in the bending direction (the direction in which the film is folded) of a polyester film for foldable displays is preferably 1.570 to 1.630. A refractive index Nx in the bending direction of 1.570 to 1.630 can achieve both high levels of resistance to crease marks and high resistance to flex fracture. This is because reducing the bending index Nx reduces fatigue due to compressive stress on the inside of the fold. Fatigue due to compressive stress is believed to occur primarily in crystalline regions, and the fewer crystals in the bending direction, the less fatigue there is. In other words, lowering the refractive index Nx reduces the amount of oriented crystals in the bending direction, which is thought to suppress compressive fatigue. Furthermore, the refractive index Nx can also control creep caused by tensile stress applied to the outside of the fold during folding. Fatigue due to tensile stress is believed to occur primarily in amorphous regions, where repeated stress causes molecular chain alignment, resulting in deformation. It can be assumed that fewer molecular chains aligned in the bending direction results in less deformation due to alignment. Furthermore, a higher degree of crystallinity is preferred because a smaller amorphous portion can suppress fatigue due to tension. The refractive index Nx is preferably 1.570 to 1.600, more preferably 1.580 to 1.590. The more the refractive index Nx is set within a preferred range, the higher the level of both resistance to crease marks and flex fracture resistance can be achieved.

[0027] 8.7.3 Refractive Index Ny in the Bending Axis Direction (Folded Portion Direction) The refractive index Ny in the bending axis direction (folded portion direction) of the polyester film for foldable displays is preferably 1.670 to 1.760. By setting the refractive index Ny in the bending axis direction to 1.670 to 1.760, both resistance to crease marks and resistance to flexural breakage can be achieved at an even higher level. Furthermore, by setting the refractive index Ny in the bending axis direction to 1.670 or more, the flexibility in the bending direction can be improved, and the surface hardness can be improved. On the other hand, by setting the refractive index Ny to 1.760 or less, the resistance to flexural breakage in the bending axis direction (folded portion direction) can be improved. The refractive index Ny is preferably 1.680 to 1.730, and more preferably 1.685 to 1.710. The more the refractive index Ny is set within the preferred range, the higher the level of both resistance to crease marks and resistance to flexural breakage can be achieved.

[0028] 8.7.4 Refractive Index Nz in the Thickness Direction The refractive index Nz in the thickness direction of the polyester film for foldable displays is preferably 1.530 or less. By setting the refractive index Nz to 1.530 or less, even if the refractive index in the bending direction is low, a decrease in the hardness of the film surface can be suppressed, achieving both good flexibility (resistance to creases, resistance to flex fracture) and high surface hardness. By maintaining a high surface hardness of the film, the pencil hardness of the hard coat layer after laminating it on the surface can be improved. While the reason why lowering the refractive index Nz in the thickness direction improves the hardness of the film surface is not entirely clear, it is thought that this is due to the in-plane orientation of aromatic groups such as benzene rings in the molecular chain, which has the effect of suppressing deformation due to stress applied in the thickness direction. The refractive index Nz is more preferably 1.500 to 1.530, even more preferably 1.503 to 1.528, and particularly preferably 1.505 to 1.525. The lower limit of the refractive index Nz is not particularly limited, but from the viewpoint of stable production of the film, it is preferably 1.3 or more.

[0029] 8.8 0.2% Yield Strain The 0.2% yield strain of the polyester film for foldable displays is, for example, 1.0 to 5.0%, preferably 1.5 to 4.0%, and more preferably 2.0 to 3.0%. When the 0.2% yield strain of the polyester film is 1.0 to 5.0% (particularly 2.0 to 3.0%) and the intrinsic viscosity is high (specifically, 0.65 dl / g or more), it is possible to achieve both flex fracture resistance and resistance to crease marks at an even higher level, and in particular, even in a film that is thicker than conventional films and has excellent impact resistance, it is possible to achieve both flex fracture resistance and resistance to crease marks at an even higher level.

[0030] 8.9 (0.2% proof stress point strain^3) / (√thickness) The polyester film for foldable displays preferably has a value obtained by dividing the cube of the 0.2% proof stress point strain in at least one of the longitudinal and width directions by the square root of the thickness (μm) ((0.2% proof stress point strain^3) / (√thickness)) of 2.1 or more, more preferably 2.10 to 6.0, even more preferably 2.11 to 4.0, and most preferably 2.12 to 3.5. If the value of (0.2% proof stress point strain^3) / (√thickness) is within the above range, it is possible to achieve both flex fracture resistance and resistance to crease marks at an even higher level.

[0031] 8.10 Breaking Elongation The breaking elongation of the polyester film for foldable displays is preferably 11% or more, more preferably 20% or more, and is preferably 11 to 160%, more preferably 13 to 140%, even more preferably 16 to 130%, and still more preferably 20 to 120%. A breaking elongation of the film of 11% or more (particularly 20% or more) is advantageous in that the film is less likely to break due to continuous bending and in that the film is less likely to break during processing in subsequent steps.

[0032] 8.11 Breaking Elongation / Refractive Index The polyester film for foldable displays preferably has a breaking elongation in at least one of the longitudinal and width directions divided by the refractive index in that direction (breaking elongation / refractive index) of 13 or more and 90 or less. Within this numerical range, deformation upon bending is unlikely to occur, and if the intrinsic viscosity of at least the surface layer is 0.65 dl / g or more, breaking upon bending is unlikely to occur. It has been found that the lower limit of the breaking elongation in at least one of the longitudinal and width directions divided by the refractive index in that direction is more preferably 15, and particularly preferably 17. It has been found that the upper limit is more preferably 80, and particularly preferably 70. These upper and lower limits can be combined as appropriate; for example, a value of 15 or more and 80 or less is more preferable, and a value of 17 or more and 70 or less is particularly preferable.

[0033] 8.12 Density The density of the polyester film for foldable displays corresponds to the crystallinity of the polyester film. The crystallinity indicates the proportion of crystalline portions in the total crystalline and amorphous portions that make up the polyester film. The density of the polyester film for foldable displays is 1.380 g / cm ３ It is preferable that the density is 1.380 g / cm or more. ３ If the hardness is less than 1.380 g / cm, crystallization is insufficient, deformation due to bending is likely to occur, and the thermal shrinkage rate increases, resulting in poor processability when applying a hard coat. ３ By setting the density to 1.380 to 1.40 g / cm, it is possible to achieve both good flexibility (resistance to creases, resistance to flex fracture) and high surface hardness. By maintaining a high surface hardness of the film, it is possible to improve the pencil hardness of the hard coat layer after laminating the hard coat layer on the surface. ３ is preferred, and 1.383 to 1.395 g / cm ３ The upper limit of the density is appropriately set depending on the presence or absence of particles in the film.

[0034] 8.13 Heat Shrinkage Ratio When the polyester film for a foldable display is heated at 150°C for 30 minutes, the heat shrinkage ratio is preferably 20% or less in both the bending direction and the bending axis direction (folding direction). A heat shrinkage ratio of 20% or less suppresses poor appearance due to deformation of the film under high heat during post-processing, and also suppresses flatness defects such as curling and undulation due to processing involving heating when mounting on a foldable display. A low heat shrinkage ratio is preferable, but from a manufacturing standpoint, a lower limit of 0.01% is preferred. Specifically, the heat shrinkage ratio range is, for example, 0.01 to 20%, preferably 0.01 to 10%, and more preferably 0.01 to 7%.

[0035] 8.14 Resistance to Crease Marking The resistance to crease marks of a polyester film for foldable displays can be evaluated by the strength of the crease when the film is folded and unfolded. Specifically, one aspect of the evaluation of resistance to crease marks involves performing an unloaded clamshell folding test in which the film is bent 50,000 times at a bending radius of 0.5 mm and a speed of 1 bending per second. The film is then placed horizontally on a flat surface with the inside of the bend facing down, and the maximum height of lift from the horizontal surface is measured. When this maximum height is, for example, 5 mm or less, preferably 4 mm or less, and more preferably 3 mm or less, the film can be evaluated as being resistant to crease marks (Crease Marking Evaluation 1). Another aspect of the evaluation of resistance to crease marks involves folding the film with a bending radius of 0.5 mm, holding it at room temperature for 24 hours, and then placing it horizontally on a flat surface with the inside of the bend facing down, and measuring the maximum height of lift from the horizontal surface. When this maximum height is, for example, 7 mm or less, preferably 5 mm or less, the film can be evaluated as being resistant to crease marks (Crease Marking Evaluation 2). In the present invention, if the crease mark evaluation 1 and / or the crease mark evaluation 2 are satisfied, the polyester film is judged to be resistant to crease marks.

[0036] 8.15 Bending Breakage Resistance The bending breakage resistance of a polyester film for a foldable display can be evaluated by determining whether cracks or breaks occur on the surface of the folded portion when the film is folded and unfolded. Specifically, an unloaded clamshell folding test is performed in which the film is bent multiple times at a bending radius of 0.4 mm and a speed of 1 bending / second. The surface on the outer side of the bend is then observed under a microscope at 700x magnification to determine whether cracks or breaks occur. If neither cracks nor breaks are observed, the bending breakage resistance can be evaluated as good. The number of bending cycles at which neither cracks nor breaks are observed is, for example, 100,000, preferably 200,000, more preferably 250,000, and even more preferably 300,000.

[0037] 8.16 Surface Unevenness The surface of the polyester film for a foldable display may be smooth or may have unevenness, but when unevenness is formed, it is preferable to form the unevenness within a range that allows the total light transmittance to be maintained, taking into consideration the use as a surface protective film. Forming unevenness improves the slipperiness of the film.

[0038] Examples of methods for forming the irregularities include a method of blending a filler (particles) into a polyester resin, and a method of coating a coat layer containing fillers (particles) during film formation.

[0039] The particles include inorganic particles, organic particles, etc. Examples of inorganic particles include silica, calcium carbonate, magnesium carbonate, and titanium oxide.

[0040] In addition, various additives may be contained within the range that does not impair the total light transmittance or haze, such as a compatibilizer, a plasticizer, a weathering agent, an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, a brightener, a colorant, a conductive agent, a crystal nucleating agent, an ultraviolet absorber, a flame retardant, a flame retardant assistant, a pigment, and a dye.

[0041] 8.17 Surface Treatment The surface of the polyester film for a foldable display may be subjected to a treatment to improve adhesion with a resin forming an adhesive layer, a hard coat layer, or the like.

[0042] 8.18 Stretching Method In order to set the refractive indexes Nx, Ny, Nz, Ny-Nx, density, 0.2% proof stress strain, elongation at break, and the like of the polyester film for foldable displays within the above-mentioned ranges, it is necessary to appropriately set the stretching conditions (stretching preheating temperature, stretching temperature, stretching ratio, use of multi-stage stretching, heat setting, relaxation treatment, etc.) after producing an unstretched polyester film.

[0043] 8.18.1 Stretching Ratio and Heat Setting Temperature First, to reduce the refractive index Nx in the bending direction and increase the refractive index Ny in the bending axis direction so that Ny - Nx is equal to or greater than a predetermined value, it is necessary to uniaxially stretch an unstretched polyester film or to biaxially stretch the film with a large difference in stretching ratios, followed by heat setting at an appropriate temperature. That is, the stretching ratio in the smaller stretching ratio (e.g., in the longitudinal direction (machine flow direction) when stretching significantly in the width direction) is preferably 1.0 to 2.0 times. A stretching ratio of 1.0 times means that no stretching is performed. This stretching ratio is more preferably 1.0 to 1.7 times, even more preferably 1.0 to 1.5 times, and most preferably 1.0 to 1.2 times. The stretching ratio in the larger stretching ratio (e.g., the width direction) is preferably 4.0 to 6.0 times, more preferably 4.2 to 6.0 times. The refractive index difference can be achieved by setting the direction in which the stretching ratio is small (for example, the longitudinal direction) as the bending direction of the film and the direction in which the stretching ratio is large (for example, the width direction (orthogonal direction)) as the bending axis direction of the film (folded portion direction). Note that the direction in which the stretching ratio is small may be set as the width direction, and the direction in which the stretching ratio is large may be set as the longitudinal direction, with this width direction as the bending direction and the longitudinal direction as the bending axis direction. The ratio of the stretching ratio in the direction in which the stretching ratio is large (bending axis direction) to the stretching ratio in the direction in which the stretching ratio is small (bending direction) (stretching ratio in the bending axis direction / stretching ratio in the bending direction) is preferably 1.3 or more, more preferably 1.3 to 6.0, even more preferably 1.5 to 6.0, still more preferably 1.8 to 6.0, and most preferably 2.1 to 5.9.

[0044] Setting the stretching ratio in the direction with the smaller stretching ratio (bending direction) to 1.0 or more is advantageous in terms of reducing deformation during post-processing such as hard coating, etc. Furthermore, setting the stretching ratio in the bending direction to 2.0 or less also has the effect of reducing unevenness in the film thickness.

[0045] To adjust Ny-Nx within the above range, it is necessary to perform a heat treatment (heat setting) while maintaining tension on the stretched film, and to set the heat setting temperature to 190°C or higher. The heat setting temperature is preferably 200°C or higher, and more preferably about 205 to 240°C. Heat setting promotes oriented crystallization in the stretching direction, which can also reduce the refractive index in the thickness direction. It can also promote crystallization and effectively increase density. The heat setting time is 0.5 to 20 seconds, preferably 1 to 15 seconds.

[0046] 8.18.2 Stretching Preheating Temperature and Stretching Temperature The stretching preheating temperature and stretching temperature also affect the refractive indexes Nx, Ny, Nz, Ny-Nx, etc. When stretching in the bending direction (e.g., the longitudinal direction), the heating temperature during stretching is, for example, 75 to 120°C, preferably 75 to 105°C. The stretching preheating temperature or stretching temperature in the bending axis direction (folded portion direction, e.g., the width direction) is, for example, 70 to 110°C. Setting the stretching temperature to 75 to 120°C (bending direction) or 70 to 110°C (bending axis direction) is also effective in preventing significant thickness unevenness due to stretching at the above stretch ratios. Furthermore, lowering the stretching temperature while reducing the stretch ratio is effective in lowering the refractive index in the thickness direction.

[0047] 8.19 Resin pellets The resin pellets used as raw materials are composed of the same resin as the film. The intrinsic viscosity of the resin pellets is preferably 0.65 to 1.2 dl / g, more preferably 0.70 to 1.1 dl / g, and even more preferably 0.80 to 1.0 dl / g.

[0048] 9. Laminate film for foldable displays The laminate film for foldable displays is a film in which an easy-adhesion layer is laminated on at least one surface of the polyester film for foldable displays. That is, the polyester film for foldable displays serves as the base layer of the laminate film. Forming the easy-adhesion layer improves the adhesion between the polyester film and other layers.

[0049] 9.1 Easy-Adhesion Layer For the easy-adhesion layer, for example, polyester-based resins, polyurethane-based resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. can be used. In the easy-adhesion layer, the resin is preferably crosslinked with a crosslinking agent. Examples of the crosslinking agent include melamine-based compounds, isocyanate-based compounds, oxazoline-based compounds, epoxy-based compounds, and carbodiimide-based compounds.

[0050] The easy-adhesion layer can be formed by coating the surface of the substrate layer with a solution containing the resin and, if necessary, a crosslinking agent by a general coating method, preferably an in-line coating method.

[0051] In the in-line coating method, it is preferable to coat one or both sides of an unstretched film or a uniaxially stretched film stretched in the machine direction with a coating liquid containing the resin and, if necessary, a crosslinking agent, and then dry the film at 75 to 150°C and further stretch the film in one or two directions to form an easy-adhesion layer. The final coating amount of the easy-adhesion layer is 0.05 to 0.20 g / m ２ The thickness of the easy-adhesion layer is preferably 10 to 1000 nm, more preferably 20 to 500 nm, and even more preferably 50 to 200 nm.

[0052] The adhesive layer preferably contains particles such as titanium oxide, calcium carbonate, silica, and alumina to provide slipperiness.

[0053] 9.2 Hard Coat Layer In the laminate film for a foldable display, it is preferable that a hard coat layer is laminated on the easy-adhesion layer. In particular, when the laminate film is used as a surface protection film, it is preferable that a hard coat layer is laminated. When an easy-adhesion layer is laminated on one side of the polyester film for a foldable display, it is preferable that the hard coat layer is laminated on the easy-adhesion layer. When easy-adhesion layers are laminated on both sides of the polyester film for a foldable display, the hard coat layer may be laminated on only one of the easy-adhesion layers, or may be laminated on both of the easy-adhesion layers.

[0054] 9.2.1 Resin The resin for forming the hard coat layer is not particularly limited and may be a (meth)acrylic acid ester resin, a siloxane resin, an inorganic hybrid resin, an acrylate resin (such as a urethane acrylate resin or a polyester acrylate resin), or an epoxy resin, and may be a mixture of two or more resins.

[0055] 9.2.2 Film Thickness The thickness of the hard coat layer is preferably 1 to 55 μm. A thickness of 1 μm or more provides good pencil hardness. Furthermore, by keeping the thickness at 55 μm or less, curling of the film due to cure shrinkage of the hard coat is suppressed, improving the handling properties of the film.

[0056] 9.2.3 Method for Forming Hard Coat Layer The hard coat layer can be formed by coating (e.g., applying) a liquid containing the resin and, if necessary, a curing agent (e.g., a photopolymerization initiator) on the easy-adhesion layer, drying the liquid, and curing the resin.

[0057] 9.3 Pencil Hardness The pencil hardness of the hard coat layer is preferably 2H or more. With a pencil hardness of 2H or more, the hard coat layer does not easily deform and does not reduce visibility. Generally, a high pencil hardness of the hard coat layer is preferable, but a pencil hardness of 10H or less is acceptable. The pencil hardness of the hard coat layer is, for example, 2H to 10H, preferably 3H to 9H, more preferably 3H to 8H, and even more preferably 3H to 6H.

[0058] 9.4 Types of Hard Coat Layer The hard coat layer may have other functions in addition to the function of having a predetermined hardness (preferably the function of protecting the display by this hardness). For example, a hard coat layer having functions such as antiglare properties, antireflection properties, low reflectivity, and antistatic properties is also preferred.

[0059] This application claims the benefit of priority based on Japanese Patent Application No. 2024-011752 filed on January 30, 2024, Japanese Patent Application No. 2024-148058 filed on August 29, 2024, and Japanese Patent Application No. 2024-189334 filed on October 28, 2024. The entire contents of the specifications of Japanese Patent Application No. 2024-011752 filed on January 30, 2024, Japanese Patent Application No. 2024-148058 filed on August 29, 2024, and Japanese Patent Application No. 2024-189334 filed on October 28, 2024 are incorporated herein by reference.

[0060] The present invention will be explained in more detail below with reference to examples. However, the present invention is not limited to the following examples, and it is of course possible to carry out the invention by making appropriate modifications within the scope of the above and below-described aims, and all such modifications are included in the technical scope of the present invention.

[0061] (Evaluation Method) (1) Intrinsic Viscosity After pulverizing and drying the film or polyester resin, it was dissolved in a mixed solvent of phenol / tetrachloroethane = 60 / 40 (mass ratio). Since inorganic particles such as additives may be attached to or contained in the film or polyester resin, or an easy-adhesion layer containing inorganic particles such as additives may be applied to the surface of the film, the solution was centrifuged to remove the inorganic particles, and then the flow time of the solution with a concentration of 0.4 (g / dl) at 30 ° C. and the flow time of the solvent alone were measured using an Ubbelohde viscometer. From the time ratio, the intrinsic viscosity was calculated using the Huggins formula, assuming that the Huggins constant was 0.38. In the case of a laminated film, the intrinsic viscosity of each layer alone was evaluated by scraping off the corresponding polyester layer of the film according to the laminate thickness.

[0062] (2) Refractive index Measurement was performed in accordance with JIS-K-7142. The refractive index was measured using an Abbe refractometer with NaD line light. Methylene iodide was used as the mounting liquid, and the refractive index in the longitudinal direction (Nx), width direction (Ny), and thickness direction (Nz) were measured. The measurement was performed at the center of the film width direction.

[0063] (3) Total Light Transmittance and Haze Measurements were made using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.). In the following examples and comparative examples, the total light transmittance and haze of the surface protection film (polyester film) were measured and evaluated according to the following criteria: ◯ (Good): Total light transmittance of 85% or more × (Poor): Total light transmittance of less than 85%

[0064] (4-1) Fold Mark Evaluation 1 (Bending Radius 0.5 mm) A polyester film sample measuring 20 mm in the width direction and 110 mm in the machine direction was prepared. A clamshell-type folding test was performed under tension using a small tabletop clamshell-type durability tester (Yuasa System Co., Ltd., DLDMLP-CS). In the test, the bending radius was set to 0.5 mm, and the sample was bent 50,000 times at a rate of 1 time / second. At that time, the sample was fixed at positions 10 mm from both ends of the long side, and the bending portion was 20 mm x 90 mm, with the width direction of the sample being the bending direction of the test and the machine direction of the sample being the bending axis direction of the test (folded portion direction). Here, Fig. 1 is a schematic diagram showing the bending radius in a folding test. Taking into account the case where a polyester film is disposed on the inner surface when the foldable display is folded, the radius of the bending axis 12 in Fig. 1(b) and Fig. 1(d) was set to 0.5 mm to conduct a model folding test. After 50,000 bending cycles, the sample was placed on a flat surface with the inner side of the bend facing downwards, and visually observed and evaluated according to the following criteria: ◎ (Best): No deformation at the foldable portion, or deformation present, and when placed horizontally, the maximum height of the foldable portion rising from the horizontal surface was 3 mm or less; ○ (Good): When placed horizontally, the maximum height of the foldable portion rising from the horizontal surface was more than 3 mm and less than 5 mm; × (Poor): When placed horizontally, the maximum height of the foldable portion rising from the horizontal surface was more than 5 mm;

[0065] (4-2) Fold Mark Evaluation 2 (Bending Radius 0.5 mm) A polyester film sample measuring 20 mm in width and 110 mm in flow direction was prepared, with the width direction of the sample being the bending direction of the test and the flow direction of the sample being the bending axis direction (folded portion direction) of the test. Here, FIG. 1 is a schematic diagram showing the bending radius in the folding test. Taking into account the case where a polyester film is arranged on the inner surface of a folded display, the radius of the bending axis 12 in FIGS. 1(b) and 1(d) was set to 0.5 mm to perform a model folding test. After maintaining the bending state at room temperature for 24 hours, the sample was placed on a flat surface with the inside of the bend facing down, and visually observed and evaluated according to the following criteria. ◎ (Best): No deformation at the foldable part, or deformation occurs and when placed horizontally, the maximum height of the part that comes into contact with the foldable part rises from the horizontal surface by 5 mm or less. ○ (Good): When placed horizontally, the maximum height of the part that comes into contact with the foldable part rises from the horizontal surface by more than 5 mm and 7 mm or less. × (Bad): When placed horizontally, the maximum height of the part that comes into contact with the foldable part rises from the horizontal surface by more than 7 mm.

[0066] (5) Evaluation of flexural fracture resistance (bending radius 0.4 mm) A polyester film sample measuring 20 mm in the width direction and 110 mm in the machine direction was prepared. A clamshell-type folding test was performed under tension using a small tabletop clamshell-type durability tester (Yuasa System Co., Ltd., DLDMLP-CS). In the test, the bending radius was set to 0.4 mm, and the sample was bent 100,000 times at a rate of 1 time / second. The width direction of the sample was the bending direction of the test, and the machine direction of the sample was the bending axis direction (folded portion direction) of the test. The film surface outside the bent portion was observed at 700x magnification using a digital microscope (HIROX, RH8800) to observe the presence or absence of wrinkles (cracks) and breaks. If there were no cracks or breaks on the film surface after 100,000 flexes, a 200,000 flex test was subsequently conducted, and if there were no cracks or breaks on the film surface after 200,000 flexes, a 300,000 flex test was subsequently conducted. ◎ (Best): No cracks or breaks on the film surface after 300,000 flexes ○ (Excellent): No cracks or breaks on the film surface after 200,000 flexes △ (Good): No cracks or breaks on the film surface after 100,000 flexes × (Bad): Cracks or breaks can be confirmed on the film surface after 100,000 flexes

[0067] (6) 0.2% proof stress strain A polyester film was cut into a strip measuring 140 mm in the measurement direction (longitudinal or width direction) and 10 mm in width (direction perpendicular to the measurement direction) to prepare a sample. A tensile test was performed using a tensile tester (Shimadzu Corporation, AUTOGRAPH AG-X Plus 1kN) at a tension rate of 100 mm / min to obtain a stress-strain curve. Then, using Shimadzu Corporation's autograph software TRAPEZIUM X, the strain was set to 0.2%, and the 0.2% proof stress strain was calculated.

[0068] (7) Breaking Elongation A polyester film was cut into a strip measuring 140 mm in the measurement direction (longitudinal or width direction) and 10 mm in width (direction perpendicular to the measurement direction) to prepare a sample. A tensile test was performed using a tensile tester (Shimadzu Corporation, AUTOGRAPH AG-X Plus 1 kN) at a tension rate of 100 mm / min to obtain a stress-strain curve. Then, the elongation at break (breaking elongation) (%) was calculated using Shimadzu Corporation's autograph software TRAPEZIUM X.

[0069] (Preparation of Polyethylene Terephthalate (PET) Pellets (a)) A continuous esterification reactor comprising a three-stage complete mixing vessel having a stirrer, a partial condenser, a raw material inlet, and a product outlet was used as the esterification reactor. A slurry comprising terephthalic acid (TPA), ethylene glycol (EG), and antimony trioxide was continuously supplied to a first esterification reactor of the esterification reactor, and reacted at atmospheric pressure for an average residence time of 4 hours at 255° C. The composition of the supplied slurry was adjusted so that the TPA supply rate was 2 ton / hr, the EG was 2 moles per mole of TPA, and the Sb atoms of the antimony trioxide were 160 ppm (by mass) relative to the produced PET. Next, the reaction product in the first esterification reactor was continuously removed from the system and fed to a second esterification reactor, and EG distilled off from the first esterification reactor was fed to the second esterification reactor in an amount of 8 mass% based on the produced polymer (produced PET). Further, an EG solution containing magnesium acetate in an amount such that the Mg atoms would be 65 ppm based on the produced PET, and an EG solution containing TMPA in an amount such that the P atoms would be 20 ppm based on the produced PET, were added, and the reaction was carried out at atmospheric pressure for an average residence time of 1.5 hours at 260° C. Next, the reaction product in the second esterification reactor was continuously removed from the system and fed to a third esterification reactor, and further, an EG solution containing trimethyl phosphate (TMPA) in an amount such that the P atoms would be 20 ppm (based on mass) based on the produced PET, was added, and the reaction was carried out at atmospheric pressure for an average residence time of 0.5 hours at 260° C. The esterification reaction product produced in the third esterification reactor was continuously supplied to a three-stage continuous polycondensation reactor to carry out polycondensation, and further filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cutoff of 5 μm particles) to obtain polyethylene terephthalate pellets (a) having an intrinsic viscosity of 0.62 dl / g.

[0070] (Preparation of Polyethylene Terephthalate Pellets (b)) The polyethylene terephthalate pellets (a) were subjected to solid-state polymerization using a rotary vacuum polymerization apparatus under a reduced pressure of 0.5 mmHg at 220°C for various times to prepare polyethylene terephthalate pellets (b) having an intrinsic viscosity of 0.73 dl / g.

[0071] (Preparation of Polyethylene Terephthalate Pellets (c)) The polyethylene terephthalate pellets (a) were subjected to solid-state polymerization using a rotary vacuum polymerization apparatus under a reduced pressure of 0.5 mmHg at 220°C for various times to prepare polyethylene terephthalate pellets (c) having an intrinsic viscosity of 1.2 dl / g.

[0072] (Preparation of Polyethylene Terephthalate Pellets (d)) The polyethylene terephthalate pellets (a) were subjected to solid-state polymerization using a rotary vacuum polymerization apparatus under a reduced pressure of 0.5 mmHg at 220°C for various times to prepare polyethylene terephthalate pellets (d) having an intrinsic viscosity of 0.80 dl / g.

[0073] (Example 1) The above polyethylene terephthalate master pellets (c) were dried under reduced pressure (3 Torr) at 180 ° C for 8 hours, then fed to an extruder and melted at 285 ° C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut of 10 μm particles), extruded into a sheet form from a die, and then contacted with a casting drum with a surface temperature of 30 ° C. using an electrostatic casting method to cool and solidify, producing an unstretched film. The unstretched film was gripped with a clip and introduced into a hot air zone heated to 95 ° C., where it was stretched 4.3 times in the width direction. Subsequently, it was introduced into a heat treatment zone at 210 ° C., where it was heat-treated (heat-set) for 5 seconds, to obtain a polyethylene terephthalate film No. 1 in Table 1 having a thickness of 40 μm.

[0074] Example 2 A polyethylene terephthalate film No. 2 in Table 1 having a thickness of 40 μm was obtained in the same manner as in Example 1, except that the polyethylene terephthalate master pellets (b) were used as a raw material.

[0075] Example 3 A polyethylene terephthalate film No. 3 in Table 1 having a thickness of 40 μm was obtained in the same manner as in Example 1, except that the stretching ratio in the width direction was 5.8 times.

[0076] (Example 4) The above polyethylene terephthalate master pellets (c) were dried under reduced pressure (3 Torr) at 180°C for 8 hours, then fed to an extruder and melted at 285°C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut of 10 μm particles), extruded into a sheet form from a die, and then contacted with a casting drum at a surface temperature of 30°C using an electrostatic casting method to cool and solidify, producing an unstretched film. The resulting unstretched sheet was stretched 1.7 times in the longitudinal direction with a roll heated to 85°C to obtain a uniaxially oriented PET film. This uniaxially oriented film was gripped with a clip and introduced into a hot air zone heated to 95°C and stretched 4.3 times in the width direction. Subsequently, it was introduced into a heat treatment zone at 210°C and heat-treated (heat-set) for 5 seconds to obtain a polyethylene terephthalate film No. 4 in Table 1 with a thickness of 40 μm.

[0077] Example 5 A polyethylene terephthalate film No. 5 having a thickness of 40 μm in Table 1 was obtained in the same manner as in Example 4, except that the draw ratio in the width direction was 4.5 times and the heat treatment temperature was 230° C.

[0078] (Example 6) The above polyethylene terephthalate master pellets (b) were dried under reduced pressure (3 Torr) at 180 ° C for 8 hours, then fed to an extruder and melted at 285 ° C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet form from a die, and then contacted with a casting drum with a surface temperature of 30 ° C using an electrostatic casting method to cool and solidify, producing an unstretched film. This unstretched film was gripped with a clip and introduced into a hot air zone heated to 95 ° C, where it was stretched 5.8 times in the width direction. Subsequently, it was introduced into a heat treatment zone at 210 ° C, where it was heat-treated (heat-set) for 5 seconds, resulting in a polyethylene terephthalate film No. 6 in Table 2 having a thickness of 50 μm.

[0079] Example 7 A polyethylene terephthalate film No. 7 in Table 2 was obtained in the same manner as in Example 6, except that the thickness was changed to 65 μm.

[0080] (Example 8) The above polyethylene terephthalate master pellets (b) were dried under reduced pressure (3 Torr) at 180°C for 8 hours, then fed to an extruder and melted at 285°C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut of 10 μm particles), extruded into a sheet form from a die, and then contacted with a casting drum at a surface temperature of 30°C using an electrostatic casting method to cool and solidify, producing an unstretched film. The resulting unstretched sheet was stretched 1.7 times in the longitudinal direction with a roll heated to 85°C to obtain a uniaxially oriented PET film. This uniaxially oriented film was gripped with a clip and introduced into a hot air zone heated to 95°C and stretched 4.5 times in the width direction. Subsequently, it was introduced into a heat treatment zone at 210°C and heat-treated (heat-set) for 5 seconds to obtain a polyethylene terephthalate film No. 8 in Table 2 with a thickness of 65 μm.

[0081] Example 9 A polyethylene terephthalate film No. 9 in Table 2 was obtained in the same manner as in Example 7, except that the thickness was changed to 100 μm.

[0082] Example 10 A polyethylene terephthalate film No. 10 in Table 2 was obtained in the same manner as in Example 8, except that the thickness was changed to 100 μm.

[0083] Example 11 A polyethylene terephthalate film No. 11 in Table 2 was obtained in the same manner as in Example 9, except that polyethylene terephthalate pellets (d) were used as the raw material.

[0084] Example 12 A polyethylene terephthalate film No. 12 in Table 2 was obtained in the same manner as in Example 8, except that the stretching ratio in the longitudinal direction was 1.9 times.

[0085] Comparative Example 1 A polyethylene terephthalate film having a thickness of 40 μm and No. 13 in Table 1 was obtained in the same manner as in Example 1, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0086] Comparative Example 2 A polyethylene terephthalate film having a thickness of 40 μm and No. 14 in Table 1 was obtained in the same manner as in Example 3, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0087] (Comparative Example 3) The above polyethylene terephthalate master pellets (a) were dried under reduced pressure (3 Torr) at 180 ° C for 8 hours, then fed to an extruder and melted at 285 ° C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet form from a die, and then contacted with a casting drum with a surface temperature of 30 ° C using an electrostatic casting method to cool and solidify, producing an unstretched film. The obtained unstretched sheet was stretched 3.4 times in the longitudinal direction with a roll heated to 85 ° C to obtain a uniaxially oriented PET film. This uniaxially oriented film was gripped with a clip and introduced into a hot air zone heated to 95 ° C and stretched 3.8 times in the width direction. Subsequently, it was introduced into a heat treatment zone at 210 ° C and heat-treated (heat-set) for 5 seconds to obtain a polyethylene terephthalate film No. 15 in Table 1 with a thickness of 40 μm.

[0088] Comparative Example 4 A polyethylene terephthalate film having a thickness of 40 μm and No. 16 in Table 1 was obtained in the same manner as in Example 4, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0089] (Comparative Example 5) The above polyethylene terephthalate master pellets (b) were dried under reduced pressure (3 Torr) at 180 ° C for 8 hours, then fed to an extruder and melted at 285 ° C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet form from a die, and then contacted with a casting drum with a surface temperature of 30 ° C using an electrostatic casting method to cool and solidify, producing an unstretched film. The obtained unstretched sheet was stretched 3.4 times in the longitudinal direction with a roll heated to 85 ° C to obtain a uniaxially oriented PET film. This uniaxially oriented film was gripped with a clip and introduced into a hot air zone heated to 95 ° C and stretched 3.8 times in the width direction. Subsequently, it was introduced into a heat treatment zone at 210 ° C and heat-treated (heat-set) for 5 seconds to obtain a polyethylene terephthalate film No. 17 in Table 2 with a thickness of 65 μm.

[0090] Comparative Example 6 A polyethylene terephthalate film No. 18 in Table 2 was obtained in the same manner as in Comparative Example 5, except that the thickness was changed to 100 μm.

[0091] Comparative Example 7 A polyethylene terephthalate film No. 19 in Table 2 was obtained in the same manner as in Example 7, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0092] Comparative Example 8 A polyethylene terephthalate film No. 20 in Table 2 was obtained in the same manner as in Example 8, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0093] Comparative Example 9 A polyethylene terephthalate film No. 21 in Table 2 was obtained in the same manner as in Comparative Example 5, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0094] Comparative Example 10 A polyethylene terephthalate film No. 22 in Table 2 was obtained in the same manner as in Example 9, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0095] Comparative Example 11 A polyethylene terephthalate film No. 23 in Table 2 was obtained in the same manner as in Example 10, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0096] Comparative Example 12 A polyethylene terephthalate film No. 24 in Table 2 was obtained in the same manner as in Comparative Example 6, except that polyethylene terephthalate pellets (a) were used as the raw material.

[0097]

[0098]

[0099] The 0.2% proof stress strain in Table 2 is the value in the bending direction.

[0100] The polyester film for a folding display is useful as various components constituting a folding display, and is particularly useful as a surface protection film.

[0101] 11, 11a, 11b, 11d Polyester film for folding display 15 Bending direction 16 Direction perpendicular to the bending direction

Claims

1. A polyester film for folding displays, in which the difference (Ny-Nx) between the refractive index Nx in the bending direction and the refractive index Ny in the direction perpendicular to the bending direction is 0.05 or more, and the intrinsic viscosity of at least the surface layer is 0.65 dl / g or more.

2. A polyester film for folding displays according to claim 1, wherein the value obtained by dividing the cube of the 0.2% proof stress point strain in at least one of the longitudinal direction and the width direction by the square root of the thickness (μm) is 2.1 or more.

3. A polyester film for folding displays according to claim 1, wherein the value obtained by dividing the breaking elongation in at least one of the longitudinal direction and the width direction by the refractive index in that direction is 13 or more and 90 or less.

4. The polyester film for folding displays according to any one of claims 1 to 3, wherein the refractive index Nx in the bending direction is 1.570 to 1.

630.

5. The polyester film for folding displays according to any one of claims 1 to 3, wherein the refractive index Ny in the direction perpendicular to the bending direction is 1.670 to 1.

760.

6. The polyester film for folding displays according to any one of claims 1 to 3, which has a refractive index Nz in the thickness direction of 1.500 to 1.

530.

7. The polyester film for a folding display according to any one of claims 1 to 3, which is a polyethylene terephthalate film.

8. The polyester film for a foldable display according to any one of claims 1 to 3, which has a surface layer and a core layer.

9. The polyester film for foldable displays according to any one of claims 1 to 3, which, after undergoing a no-load clamshell folding test in which it is bent 50,000 times at a bending radius of 0.5 mm and a speed of 1 bending / second, exhibits a maximum lift-up height of 5 mm or less when placed horizontally on a flat surface with the inside of the bend facing down.

10. The polyester film for foldable displays according to any one of claims 1 to 3, which, when folded at a bending radius of 0.5 mm, kept at room temperature for 24 hours, and then placed horizontally on a flat surface with the inside of the bend facing downwards, has a maximum lift height of 7 mm or less.

11. The polyester film for foldable displays according to any one of claims 1 to 3, wherein, after a no-load clamshell folding test in which the film is bent 100,000 times at a bending radius of 0.4 mm and a speed of 1 time / second, when the surface on the outer side of the bend is observed under a microscope at a magnification of 700x, neither cracks nor breaks are observed.

12. The polyester film for foldable displays according to any one of claims 1 to 3, wherein, after a no-load clamshell folding test in which the film is bent 200,000 times at a bending radius of 0.4 mm and a speed of 1 time / second, when the surface on the outer side of the bend is observed under a microscope at a magnification of 700x, neither cracks nor breaks are observed.

13. A laminated film for a folding display, comprising the polyester film according to any one of claims 1 to 3 and an easy-adhesion layer laminated on at least one surface of the polyester film.

14. The laminate film for a folding display according to claim 13, further comprising a hard coat layer on the easy-adhesion layer.

15. A foldable display having the laminate film according to claim 13 as a surface protection film.

16. A mobile terminal device having the foldable display according to claim 15.

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