Polyester film and use thereof

Abstract

To provide a polyester film for a protective film of a folding type display which is excellent in mass productivity and does not generate cracks in a folding part in order to provide the folding type display which has no possibility of generating disturbance in an image displayed in the folding part after repeated folding.SOLUTION: The polyester film for the protective film of the foldable display satisfies the following conditions: (1) The refraction index in the bending direction is 1.590 to 1.620 (2) The refraction index in the direction of the folded portion is 1.670 to 1.700 (3) The refraction index in the thickness direction is 1.520 or less (4) The concentration is 1. 380g / cm3 or more (Here, the bending direction refers to a direction orthogonal to the folded portion when the polyester film is folded. ) SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

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

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

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

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

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

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

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

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-228391 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-155124 [Patent Document 3] International Publication No. 2018 / 150940 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention seeks to solve the problems associated with conventional display components as described above, and aims to provide a foldable display that is easy to mass-produce and does not pose a risk of image distortion at the foldable portion after repeated folding, as well as a polyester film for foldable displays that does not produce fold marks or cracks at the foldable portion, in order to enable the provision of a mobile terminal device equipped with such a foldable display. [Means for solving the problem]

[0011] That is, the present invention comprises the following: 1. A polyester film for folding displays having a thickness of 10 to 80 μm and satisfying the following conditions: (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) Refractive index in the thickness direction is 1.520 or less (4) Density is 1.380 g / cm 3 End (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.) 2. The polyester film for folding displays 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. 3. The polyester film for a folding display according to claim 1 or 2, which has an easy-adhesion layer on at least one side of the polyester film. 4. The polyester film for folding displays according to any one of the above items 1 to 3, which has an in-plane retardation (Re) of 3,000 to 30,000 nm. 5. A hard coat film for a foldable display, comprising the polyester film for a foldable display according to any one of the above items 1 to 4, and a hard coat layer having a thickness of 1 to 50 μm on at least one surface thereof. 6. A foldable display in which the hard coat film for a foldable display according to item 5 above is arranged as a surface protective film so that the hard coat layer is positioned on the surface, and a single continuous hard coat film is arranged across the folding portion of the foldable display. 7. A mobile terminal device having the foldable display described in 6 above. [Effects of the Invention]

[0012] A foldable display using the polyester film for foldable displays or the hard coat film 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. Mobile terminal devices equipped with a foldable display using such a polyester film or hard coat film provide beautiful images, are highly functional, and are convenient in terms of portability and other factors. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a schematic diagram showing the bending radius when the foldable display of the present invention is folded. [Figure 2] FIG. 1 is a schematic diagram showing the bending direction of a polyester film for a foldable display according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] (display) The term "display" as used herein refers to display devices in general, and examples of the display include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs. LCDs, organic EL displays, and inorganic EL displays that have a bendable structure are preferred. Organic EL displays and inorganic EL displays that can reduce the number of layers are particularly preferred, and organic EL displays that have a wide color gamut are even more preferred.

[0015] (foldable display) A foldable display is a single continuous display that can be folded in half or otherwise when carried. Folding reduces the size by half, improving portability. The bending radius of a foldable display is preferably 5 mm or less, and more preferably 3 mm or less. A bending radius of 5 mm or less allows for a thinner display when folded. A smaller bending radius is preferable, but the smaller the bending radius, the more likely it is that creases will form. A bending radius of 0.1 mm or more is preferable, but it can also be 0.5 mm or more, or even 1 mm or more. Even a bending radius of 1 mm can achieve a practically thin display when carried. The bending radius when folded is measured at the location indicated by the symbol 11 in the schematic diagram of Figure 1 and refers to the radius of the inside of the folded portion when folded. The surface protection film, described below, may be located on either the outside or inside of the folded portion of the foldable display. Furthermore, the foldable display may be tri-folded or quadruple-folded, or may be a rollable type, all of which are considered to fall within the scope of the foldable display of the present invention.

[0016] The polyester film for a folding display of the present invention may be used in any part of a folding display as long as it is a component of the display. Below, a typical configuration of a folding display and the parts in which the polyester film of the present invention can be used are described using an organic EL display as an example. Hereinafter, the polyester film for a folding display of the present invention may be simply referred to as the polyester film of the present invention.

[0017] (foldable OLED display) The essential component of a foldable organic EL display is an organic EL module, but a circular polarizer, a touch panel module, a surface protective film, a back protective film, etc. may also be provided as needed. (organic EL module) An organic EL module generally comprises an electrode / electron transport layer / light-emitting layer / hole transport layer / transparent electrode. The polyester film of the present invention can be used as a substrate on which an electrode is provided and an electron transport layer, a light-emitting layer, and a hole transport layer are further provided. In particular, it can be preferably used as a substrate for a transparent electrode. In this case, since the substrate film is required to have high barrier properties against water vapor and oxygen, it is preferable that the polyester film of the present invention be provided with a barrier layer such as a metal oxide layer. To improve the barrier properties, multiple barrier layers may be provided, or multiple polyester films each provided with a barrier layer may be used.

[0018] (touch panel module) It is preferable that a mobile terminal device has a touch panel. When an organic EL display is used, it is preferable that a touch panel module is disposed above the organic EL display or between the organic EL module and the circular polarizer. The touch panel module has a transparent substrate such as a film and a transparent electrode disposed thereon. The polyester film of the present invention can be used as this transparent substrate. When used as a transparent substrate for a touch panel, it is preferable that a hard coat layer or a refractive index adjustment layer is provided on the polyester film.

[0019] (Circular polarizer) Circular polarizers prevent deterioration of image quality due to reflection of external light by components inside a display. Circular polarizers have a linear polarizer and a retardation plate. A linear polarizer has a protective film on at least the viewing side of the polarizer. A protective film may also be provided on the side of the polarizer opposite the viewing side, or a retardation plate may be directly laminated on the polarizer. The retardation plate may be a resin film having a retardation, such as a polycarbonate or cyclic olefin, or a resin film provided with a retardation layer made of a liquid crystal compound. The polyester film of the present invention can be used as a polarizer protective film or a resin film for a retardation plate. In these cases, the slow axis direction of the polyester film of the present invention is preferably parallel or perpendicular to the absorption axis direction of the polarizer. A deviation of up to 10 degrees, preferably up to 5 degrees, from this parallel or perpendicular orientation is permitted.

[0020] (Surface protection film) When an impact is applied to a display from above, there is a risk of the circuits of an organic EL module or a touch panel module being disconnected, and therefore a surface protective film is often provided. The polyester film of the present invention is used as this surface protective film. Surface protective films include those called cover windows that are incorporated into the outermost surface of a display, and those called after-films that can be attached, peeled off, and replaced by the user. In either case, the polyester film of the present invention is used. When the polyester film of the present invention is used as a surface protective film, it is preferable that a hard coat layer is laminated on at least the front surface side of the polyester film. The hard coat layer is provided on the surface of a folding display with the hard coat layer on the viewing side. The hard coat layer may be provided on both sides.

[0021] (Back protection film) It is also preferable that a protective film is provided on the back side of the display, and the polyester film of the present invention can be used as this protective film on the back side.

[0022] The polyester film of the present invention may be any other film than those described above, as long as it is used in a folded position as a constituent member of a folding display. Among these, the polyester film of the present invention is preferably used as a cover window surface protective film, an after-surface protective film, a base film for a touch panel module, or a back surface protective film, and more preferably as a cover window surface protective film or an after-surface protective film.

[0023] Furthermore, it is not necessary for the polyester film of the present invention to be used in all of the above foldable displays. In addition to the polyester film of the present invention, other films such as polyimide films, polyamide films, polyamideimide films, polyester films other than the polyester film of the present invention, polycarbonate films, acrylic films, triacetyl cellulose films, cycloolefin polymer films, polyphenylene sulfide films, and polymethylpentene films can also be used in foldable displays, depending on their suitability.

[0024] The polyester film of the present invention may be a single-layer film made of one or more types of polyester resin, or when two or more types of polyester are used, it may be a multilayer film or an ultra-multilayer laminate film with a repeating structure.

[0025] Examples of polyester resins used in polyester films include polyester films made of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers containing these resin components as main components. Among these, stretched polyethylene terephthalate films are particularly preferred in terms of mechanical properties, heat resistance, transparency, cost, etc.

[0026] When a polyester copolymer is used for the polyester film, examples of the dicarboxylic acid component of the polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; and polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of the glycol component include fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; aromatic glycols such as p-xylene glycol; alicyclic glycols such as 1,4-cyclohexanedimethanol; and polyethylene glycols having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerization components in the copolymer is preferably less than 20% by mass. A mass ratio of less than 20% by mass is preferred because film strength, transparency, and heat resistance are maintained.

[0027] In addition, in the production of polyester films, the intrinsic viscosity of at least one type of resin pellets is preferably in the range of 0.50 to 1.0 dL / g. When the intrinsic viscosity is 0.50 dL / g or higher, the impact resistance of the resulting film is improved, and disconnection of the internal circuitry of a display due to external impact is less likely to occur, which is preferable. On the other hand, when the intrinsic viscosity is 1.00 dL / g or lower, the increase in filtration pressure of the molten fluid is prevented from becoming too large, which facilitates stable film production operations, which is preferable.

[0028] The thickness of the polyester film is preferably 10 to 80 μm, and more preferably 25 to 75 μm. A thickness of 10 μm or more improves pencil hardness and impact resistance, while a thickness of 80 μm or less is advantageous for weight reduction and is excellent in flexibility, processability, handleability, etc.

[0029] The surface of the polyester film of the present invention may be smooth or uneven, but since it is used as a surface cover for a display, deterioration of optical properties due to unevenness is undesirable. The haze is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. A haze of 3% or less can improve image visibility. The lower limit of the haze is better, but from the viewpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0030] As mentioned above, in order to reduce haze, it is better if the film surface does not have too much unevenness. However, in order to provide a certain degree of slipperiness from the viewpoint of ease of handling, unevenness can be formed by blending particles into the surface polyester resin layer or by coating a coating layer containing particles during film formation.

[0031] The method of blending particles into the polyester resin layer can be a known method. For example, they can be added at any stage of polyester production, but preferably they can be added as a slurry dispersed in ethylene glycol or the like at the stage of esterification, or after the completion of the transesterification reaction and before the start of the polycondensation reaction, to proceed with the polycondensation reaction. Alternatively, they can be added by a method of blending a slurry of particles dispersed in ethylene glycol or water with polyester raw materials using a vented kneading extruder, or a method of blending dried particles with polyester raw materials using a kneading extruder.

[0032] Among these, a method in which aggregate inorganic particles are homogeneously dispersed in a monomer liquid that will become a part of the polyester raw material, and then the filtered product is added to the remainder of the polyester raw material before, during, or after the esterification reaction is preferred. This method facilitates homogeneous dispersion of the particles and high-precision filtration of the slurry, since the monomer liquid has a low viscosity. Furthermore, when the monomer liquid is added to the remainder of the raw material, the particles are well dispersible and new aggregates are unlikely to form. From this perspective, it is particularly preferred to add the monomer liquid to the remainder of the raw material at a low temperature before the esterification reaction.

[0033] Furthermore, the number of protrusions on the film surface can be further reduced by a method (masterbatch method) in which a polyester containing particles is prepared in advance and then the pellets are kneaded and extruded with pellets containing no particles.

[0034] The polyester film may contain various additives, such as antistatic agents, UV absorbers, and stabilizers, as long as the total light transmittance remains within a preferred range.

[0035] The total light transmittance of the polyester film is preferably 85% or more, and more preferably 87% or more. A transmittance of 85% or more ensures sufficient visibility. The higher the total light transmittance of the polyester film, the better, but from the viewpoint of stable production, a total light transmittance of 99% or less is preferable, and 97% or less is also acceptable.

[0036] The maximum heat shrinkage of polyester film after heat treatment at 150°C for 30 minutes is preferably 6% or less, and more preferably 5% or less. A heat shrinkage of 6% or less can suppress flatness defects such as curling and undulation during HC processing. The lower the heat shrinkage, the better, but it is preferably -1% or more, and more preferably 0% or more. A negative value here means expansion after heating, and flatness defects may occur if the value is below -1%.

[0037] The polyester film for foldable displays of the present invention can provide sufficient pencil hardness to the hard-coated film after laminating a hard-coat layer. It is believed that when conventional polyester films are laminated with a hard-coat layer, the pencil hardness is reduced due to deformation of the film in the thickness direction during pencil hardness evaluation. In the present invention, by setting the indentation depth after unloading the test force in the film thickness direction using a dynamic ultra-microhardness tester (described below) within a specific range, high hardness can be achieved in pencil hardness evaluation of the hard-coated film. The indentation depth after unloading the test force in the film thickness direction is preferably 1.5 μm or less, more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. When the indentation depth after unloading the test force (final deformation amount under load) is 1.5 μm or less, the film is less likely to deform in the thickness direction during pencil hardness evaluation of the hard-coated film after laminating a hard-coat layer, allowing for high pencil hardness. Increasing the pencil hardness of the hard coat film reduces the likelihood of scratches and dents on the display surface, improving the visibility of the display. The lower the indentation depth after unloading the test force, the better, but from the perspective of stable production and saturation of the effect, a depth of 0.3 μm or more is preferable, and even more preferably 0.5 μm or more.

[0038] In order to reduce the indentation depth after the test force is removed, it is effective to adjust the refractive index in the thickness direction to 1.520 or less. As a means for adjusting the refractive index to 1.520 or less, as will be described later, examples of conditions that can be used include adjusting the stretch ratio in the bending direction or folding direction to a high value, setting the stretching temperature in the bending direction or folding direction to a low value, or setting the heat setting temperature to a high value, within a range that allows other physical properties and the refractive index in the bending direction or folding direction to be controlled within preferred ranges.

[0039] The surface of the polyester film of the present invention may be subjected to a treatment to improve adhesion to a resin forming a hard coat layer or the like.

[0040] Examples of surface treatment methods include roughening treatments such as sandblasting and solvent treatment, and oxidation treatments such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone / ultraviolet irradiation treatment, flame treatment, chromic acid treatment, and hot air treatment, and any of these methods can be used without particular limitation.

[0041] Furthermore, adhesion can be improved by an adhesion-improving layer such as an easy-adhesion layer. The easy-adhesion layer can be made of any resin, such as an acrylic resin, a polyester resin, a polyurethane resin, or a polyether resin, and can be formed by a general coating method, preferably a so-called in-line coating method.

[0042] The polyester film can be produced, for example, through a polymerization step in which inorganic particles are homogeneously dispersed in a monomer liquid that becomes a part of the polyester raw material, the resulting dispersion is filtered, and the resulting dispersion is added to the remainder of the polyester raw material to polymerize the polyester; and a film formation step in which the resulting polyester is melt-extruded into a sheet through a filter, cooled, and stretched to form a substrate film.

[0043] Next, a method for producing a biaxially stretched polyester film will be described in detail using an example in which polyethylene terephthalate (hereinafter sometimes referred to as PET) pellets are used as the raw material for the base film, but the method is not limited to this. Furthermore, the number of layers, such as a single layer or a multilayer structure, is not limited.

[0044] After mixing and drying PET pellets in a predetermined ratio, the mixture is fed into a known melt lamination extruder, extruded through a slit die into a sheet, and cooled and solidified on a casting roll to form an unstretched film. While a single extruder is sufficient for a single-layer film, multilayer films can be produced using two or more extruders and two or more manifolds or merging blocks (e.g., merging blocks with rectangular merging sections) to laminate the multiple film layers that make up the outermost layers, extrude a two or more layer sheet from the die, and cool it on a casting roll to form an unstretched film.

[0045] In this case, it is preferable to perform high-precision filtration to remove foreign matter contained in the resin at any location where the molten resin is maintained at about 280°C during melt extrusion. The filter material used for high-precision filtration of the molten resin is not particularly limited, but a stainless steel sintered filter material is preferred because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, and Cu and high-melting-point organic matter.

[0046] Furthermore, the filtration particle size (initial filtration efficiency 95%) of the filter material is preferably 20 μm or less, particularly preferably 15 μm or less. If the filtration particle size (initial filtration efficiency 95%) of the filter material exceeds 20 μm, foreign matter of 20 μm or more in size cannot be sufficiently removed. Although high-precision filtration of molten resin using a filter material with a filtration particle size (initial filtration efficiency 95%) of 20 μm or less may reduce productivity, it is preferable to obtain a film with fewer protrusions due to coarse particles.

[0047] (Refractive index in bending direction) In the present invention, the refractive index of the polyester film in at least one of the longitudinal direction (machine flow direction) and width direction is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. The refractive index of the polyester film in the bending direction is preferably 1.590 to 1.620, even more preferably 1.591 to 1.600. Here, the bending direction refers to the direction perpendicular to the folding portion (reference numeral 21) expected in the use of a folding display, as indicated by reference numeral 22 on the polyester film (reference numeral 2) in FIG. 2. A refractive index of 1.590 to 1.620 in at least one of the longitudinal direction and width direction is preferably 1.590 to 1.620, as this reduces deformation during repeated folding and thus does not risk degrading the image quality of the folding display. A refractive index of 1.591 to 1.600 is more preferable. Of course, this direction is preferably the bending direction. If the refractive index is 1.590 or more, there is no risk of cracks occurring in the folded direction after the bending test described below, and of course no breakage occurs, so the visibility of the display can be maintained at a good level. The refractive index of the polyester film can be effectively adjusted by adjusting the stretching ratio and stretching temperature. A relaxation step in the stretching direction and multi-stage stretching may also be used to adjust the refractive index. When multi-stage stretching is performed, it is preferable to set the stretching ratios in the second and subsequent stages higher than the stretching ratio in the first stage.

[0048] By controlling the refractive index in at least one of the longitudinal direction (machine flow direction) and width direction of the polyester film within the above range, and more preferably by controlling the refractive index in the bending direction within the above range, fatigue due to compressive stress applied to the inside of the fold when folded can be reduced. Fatigue due to compressive stress is thought to occur mainly in crystalline parts, and the fewer crystals there are in the bending direction, the less fatigue there is. Therefore, it is thought that lowering the refractive index reduces the amount of oriented crystals in the bending direction, thereby suppressing compressive fatigue.

[0049] Furthermore, the creep phenomenon caused by tensile stress applied to the outside of the fold when folded can be suppressed by reducing the refractive index. Fatigue due to tensile stress is thought to occur mainly in the amorphous portion, and repeated stress causes molecular chains to align, resulting in deformation. It can be assumed that the fewer molecular chains aligned in the bending direction, the less deformation due to alignment. Furthermore, since fatigue due to tension can be suppressed with fewer amorphous portions, a higher degree of crystallinity, i.e., a higher density, is preferable.

[0050] In the present invention, the stretching ratio of the unstretched polyester sheet in at least one of the longitudinal direction (machine flow direction) and the transverse direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. The stretching direction is preferably the bending direction. A stretching ratio of 1.2 times or more is preferred because it prevents deformation during post-processing, such as hard coating, while a stretching ratio of 2.0 times or less is preferred because it prevents thickness unevenness in the film. The stretching temperature is preferably 75 to 120°C, more preferably 75 to 105°C. Heating methods during stretching can be conventionally known, such as hot air heating, roll heating, and infrared heating. Setting the stretching temperature to 75 to 120°C can prevent significant thickness unevenness due to stretching at the above stretching ratio. Furthermore, by stretching at a temperature as low as possible within a range that does not cause significant thickness unevenness, the refractive index in the thickness direction can be reduced.

[0051] (Refractive index in the direction of the fold) The refractive index of the polyester film in a direction perpendicular to the bending direction (the direction of the folded portion) is preferably 1.670 to 1.700. That is, the refractive index in the direction perpendicular to the bending direction (the direction of the folded portion) is preferably 1.670 to 1.700. By adjusting the refractive index to 1.670 to 1.700, deformation when folded in the bending direction can be reduced. By adjusting the refractive index to 1.700 or less, cracks and breaks in the direction of the folded portion can be suppressed. By adjusting the refractive index to 1.670 or more, flexibility in the bending direction and surface hardness can be improved. A refractive index of 1.680 to 1.695 is more preferable. Methods for adjusting the refractive index in the direction perpendicular to the bending direction include the stretching ratio, stretching preheating temperature, stretching temperature, multi-stage stretching, and film relaxation. The stretching ratio is preferably 4.0 to 6.0 times, more preferably 4.4 to 6.0 times. Furthermore, the stretching preheating temperature in the direction perpendicular to the bending direction is preferably 70 to 110°C. When multistage stretching is performed in the direction perpendicular to the bending direction, it is preferable to use a higher stretch ratio in the second and subsequent stages than in the first stage. The film may be relaxed by 1 to 10% in either the machine direction (longitudinal direction) or the perpendicular direction (width direction).

[0052] (Refractive index in the thickness direction) The refractive index in the thickness direction is preferably 1.520 or less. By setting the refractive index to 1.520 or less, a decrease in the hardness of the film surface can be suppressed even when the refractive index in the bending direction is designed to be low, thereby achieving both flexibility and surface hardness. Setting the refractive index to 1.520 or less reduces the indentation depth in the thickness direction after unloading the test force, thereby improving the hardness of the film surface, particularly the pencil hardness of the hard-coated film after lamination of a hard-coat layer. It is more preferably 1.515 or less, even more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. A low refractive index in the thickness direction is preferable, but from the perspective of stable production, it is preferably 1.3 or more, and even 1.4 or more. It is particularly preferably 1.410 or more. It can be said that the above range can be achieved by increasing the stretch ratio in both the bending direction and the folding direction. However, in order to control the refractive index in the thickness direction while controlling the refractive index in the bending direction and width direction within the preferred range, it is preferable to set the conditions while checking the balance of the conditions for each process in the film formation process.

[0053] Methods for controlling the refractive index in the thickness direction within the above range include the preheating temperature, stretching temperature, and stretching ratio in the bending direction, and the preheating temperature, stretching temperature, multi-stage stretching, high-ratio stretching, or heat setting temperatures in the folding direction. The preheating temperature for stretching in the bending direction is preferably 70°C to 110°C. The stretching temperature in the bending direction is preferably 75°C to 120°C. The stretching ratio in the bending direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. By lowering the stretching temperature and stretching at a low stretching ratio, the refractive index in the thickness direction can be effectively reduced while maintaining the flexibility in the bending direction. The preheating temperature for stretching in the folding direction is also preferably 75°C to 110°C. The stretching temperature is preferably 75°C to 120°C. The stretching ratio in the folding direction is preferably 4.0 to 6.0 times, more preferably 4.4 to 6.0 times. The refractive index in the thickness direction can be effectively reduced while maintaining or reducing the refractive index in the bending direction. Multistage stretching may be used as a method for high-ratio stretching. In this case, it is preferable to set the stretching ratio in the second stage higher than that in the first stage, as this allows for effective control of the refractive index. A method in which stretching is performed again after the crystallization step may also be used. Accelerated stretching, in which the stretching speed is increased from the initial stage to the latter stage of stretching, may also be used. The heat setting temperature is preferably 180 to 240° C. By performing heat setting, oriented crystallization in the stretching direction progresses, and the refractive index in the thickness direction can be reduced. The reason why lowering the refractive index in the thickness direction improves the hardness of the film surface is not entirely clear, but it is thought that aromatic groups such as benzene rings in the molecular chain are oriented in the plane direction, which has the effect of suppressing deformation due to stress applied in the thickness direction.

[0054] (Regarding polyester film density) The density of polyester film is 1.380 g / cm 3 It is preferable that the density is 1.383 g / cm or more. 3 More preferably, it is 1.380 g / cm or more. 3By setting the density to 1.40 g / cm or more, it is possible to improve the flexibility and the surface hardness of the film, particularly the pencil hardness of the hard coat film after laminating the hard coat layer. The higher the density, the better. Although it depends somewhat on the presence or absence of particles in the film, it is preferable to set the density to 1.40 g / cm or more. 3 It is preferable that the density is 1.395 g / cm or less, and more preferably 1.395 g / cm 3 The following is more preferable: By setting the heat setting temperature during film formation to 180 to 240°C, crystallization can be promoted and the density can be effectively increased. 3 If the temperature is lower than this, the crystallization will be insufficient, which will lead to deformation due to bending, and the thermal shrinkage will be high, which will result in poor processability when applying a hard coat.

[0055] The bending direction of the polyester film is preferably aligned with the longitudinal direction (machine flow direction). This facilitates biaxial stretching to reduce the refractive index in the bending direction and improve flexibility. That is, a polyester film is preferably obtained by stretching an unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.2 to 2.0, more preferably 1.7 to 2.0. Furthermore, a preferred embodiment is to stretch the polyester sheet in the width direction at a stretching ratio of 4.0 to 6.0, more preferably 4.4 to 6.0.

[0056] In the present invention, the polyester film (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) Refractive index in the thickness direction is 1.520 or less (4) Density is 1.380 g / cm 3 End Although it is particularly preferable to simultaneously achieve the above four properties, even within the above-mentioned preferred manufacturing conditions, if the combination of conditions is not optimal within each preferred manufacturing condition range, such as a stretch ratio in the bending direction of 1.4 or less, a stretch ratio in the folded direction of less than 4.4, and a heat setting temperature of 220° C. or less, it may not necessarily be possible to simultaneously achieve the above four properties. In this case, the above four properties can be simultaneously achieved by fine-tuning any of the conditions or by combining them, such as increasing the stretch ratio in the bending direction to 1.7 or more, increasing the stretch ratio in the folded direction to 4.4 or more, increasing the heat setting temperature to about 230° C., or lowering the stretching temperature in the bending direction and / or the folded direction.

[0057] Although any film-forming method such as stretching, relaxation, heat setting, or surface treatment may be used to adjust film-forming properties, film strength, thermal dimensional stability, and appearance defects, controlling the refractive index and density of the film within the above-mentioned preferred ranges is a particularly preferred embodiment of the present invention. By controlling the refractive index and density within the preferred ranges, a polyester film suitable for foldable displays can be provided that has superior flex resistance and surface hardness compared to conventional films, particularly high pencil hardness of the hard coat film after lamination of a hard coat layer.

[0058] Specifically, for example, PET pellets are thoroughly vacuum-dried, fed into an extruder, melt-extruded into a sheet at approximately 280°C, and cooled to solidify, forming an unstretched PET sheet. The resulting unstretched sheet is stretched 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, in the longitudinal direction using rolls heated to 75 to 120°C to obtain a uniaxially oriented PET film. The film is then gripped at its edges with clips and introduced into a hot air zone heated to 75 to 120°C, where it is dried and then stretched 4.0 to 6.0 times, more preferably 4.4 to 6.0 times, in the transverse direction. Subsequently, the film is introduced into a heat treatment zone at 180 to 240°C, where it is heat-treated for 1 to 60 seconds. During this heat treatment process, if necessary, a relaxation treatment of 0 to 10% may be performed in the transverse or longitudinal direction.

[0059] The intrinsic viscosity of the polyester film is preferably in the range of 0.50 to 1.0 dL / g. When the intrinsic viscosity is 0.50 dL / g or more, the impact resistance is improved and disconnection of the internal circuit of the display due to external impact is less likely to occur, which is preferable. On the other hand, when the intrinsic viscosity is 1.00 dL / g or less, the increase in filtration pressure of the molten fluid is not too large, which is preferable because film production is stable.

[0060] When a birefringent polymer film is placed on the viewing side of a display module and viewed obliquely, coloration or rainbow-like color spots (iridescence) may occur. This occurs because a portion of the light emitted from the display module is reflected at the interface, generating polarized light. As this polarized light passes through the polyester film, it is converted into linear circular or elliptically polarized light, perpendicular to the original polarization, by the in-plane retardation (Re), which is the product of the birefringence and thickness of the polymer film. Some of this polarized light is then removed by reflection at the film-air interface, etc. This coloration and rainbow spots can be prevented by increasing the in-plane retardation (Re) of the polyester film to 3000 nm or more. The pattern of the conversion into linear circular or elliptically polarized light depends on the wavelength and in-plane retardation (Re). However, by increasing the in-plane retardation (Re), this pattern can be repeated at short wavelength intervals. As a result, the spectral envelope shape of the transmitted light can be made to resemble the emission spectrum of the light source, making it indistinguishable to the human eye.

[0061] A preferable lower limit of the in-plane retardation (Re) is 4500 nm, and a more preferable lower limit is 6000 nm.

[0062] On the other hand, from the viewpoint of the visibility (suppression of rainbow spots), the upper limit of the in-plane retardation (Re) is preferably 30,000 nm. The upper limit of the in-plane retardation (Re) is more preferably 20,000 nm, even more preferably 15,000 nm, particularly preferably 12,000 nm, and most preferably 10,000 nm.

[0063] The in-plane retardation (Re) of the polyester film of the present invention can be determined by measuring the refractive index in two axial directions and the thickness, or by using a commercially available automatic birefringence measuring device such as KOBRA-21ADH (Oji Scientific Instruments Co., Ltd.). The slow axis direction can be determined by using a molecular orientation meter (for example, MOA-6004 molecular orientation meter manufactured by Oji Scientific Instruments Co., Ltd.).

[0064] Since the OLED light source used in foldable displays has a wide emission spectrum, by setting the in-plane retardation (Re) of the polyester film within the above range, it is possible to approximate the envelope shape of the transmitted light spectrum to the emission spectrum of the light source with only a relatively simple configuration.

[0065] When a polarizing plate is present on the viewing side of the display module, the polyester film of the present invention is preferably disposed on the viewing side of the polarizing plate, which is preferable because it not only improves visibility but also provides sufficient anti-reflection function.

[0066] There are no particular limitations on the angle between the absorption axis of the polarizing plate and the slow axis of the polymer film, but from the perspective of ease of cutting the polarizing plate and polymer film, the angle between the absorption axis of the polarizing plate and the slow axis of the polymer film is preferably 0 degrees or 90 degrees. Furthermore, from the perspective of easily reducing coloration and iridescence even when the in-plane retardation (Re) is relatively low, the angle between the absorption axis of the polarizing plate and the slow axis of the polymer film is preferably 0 degrees. In this case, it is preferable that the angle be within a range of ±20 degrees from 0 degrees or 90 degrees, and even ±10 degrees. On the other hand, from the perspective of preventing blackout when viewing a screen through polarized sunglasses, the angle is preferably greater than 20 degrees and less than 70 degrees, and more preferably 30 degrees to 60 degrees.

[0067] (Easy adhesion layer) In the present invention, it is also preferable to laminate an easy-adhesion layer on the polyester film in order to improve the adhesion between the polyester film and a hard coat layer or the like. The easy-adhesion layer can be obtained by applying a coating liquid for forming the easy-adhesion layer to one or both sides of an unstretched or uniaxially stretched film in the machine direction, followed by heat treatment and drying as necessary, and then stretching the film in at least one direction that is not stretched. Heat treatment can also be performed after biaxial stretching. The final coating amount of the easy-adhesion layer is 0.005 to 0.20 g / m 2 It is preferable to control the coating amount to 0.005 g / m 2 On the other hand, when the coating amount is 0.20 g / m or more, adhesiveness can be obtained, which is preferable. 2If it is less than this, blocking resistance can be obtained, which is preferable.

[0068] Resins contained in the coating liquid used to laminate the easy-adhesion layer include, without particular limitation, polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. Examples of crosslinking agents 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 each can also be mixed and used. Due to the nature of in-line coating, these are preferably applied using an aqueous coating liquid, and the resins and crosslinking agents are preferably water-soluble or water-dispersible resins or compounds.

[0069] It is preferable to add particles to the adhesion layer to impart slipperiness. The average particle size of the fine particles is preferably 2 μm or less. If the average particle size of the particles exceeds 2 μm, the particles tend to fall off from the adhesion layer. Examples of particles to be contained in the adhesion layer include inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, and organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the adhesion layer alone or in combination of two or more.

[0070] The coating solution can be applied by any known method, similar to that used for the coating layer, such as reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, wire bar coating, and pipe doctor coating, which can be used alone or in combination.

[0071] (Hard coat layer) When the polyester film of the present invention is used as a surface protection film for protecting a folding display by being placed on the surface of the display, it is preferable that the film has a hard coat layer on at least one surface. The hard coat layer is preferably placed on the display surface side of the polyester film when used in the display. The resin for forming the hard coat layer can be any resin, including acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, and epoxy, without any particular limitation. Two or more materials can be mixed and used, or particles such as inorganic filler or organic filler can be added.

[0072] (Thickness of hard coat layer) The thickness of the hard coat layer is preferably 1 to 50 μm. A thickness of 1 μm or more is preferable because it cures sufficiently and the pencil hardness is high. Furthermore, by keeping the thickness at 50 μm or less, curling due to cure shrinkage of the hard coat can be suppressed, improving the handleability of the film.

[0073] (Application method) The hard coat layer can be applied by any method, including a Mayer bar, gravure coater, die coater, knife coater, etc., without any particular limitation, and can be appropriately selected depending on the viscosity and film thickness.

[0074] (Curing conditions) The hard coat layer can be cured by energy rays such as ultraviolet rays and electron beams, or by heat, and curing methods using ultraviolet rays or electron beams are preferred in order to reduce damage to the film.

[0075] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or more, more preferably 4H or more. A pencil hardness of 3H or more prevents scratches and does not reduce visibility. Generally, a high pencil hardness of the hard coat layer is preferable, but a pencil hardness of 9H or less, 8H or less, or even 6H or less can be used without any practical problems.

[0076] (Hard Coat Layer Characteristics) The hard coat layer of the present invention can be used for the purpose of protecting a display by increasing the pencil hardness of the surface as described above, and preferably has a high transmittance. The total light transmittance of the hard coat film is preferably 87% or more, and more preferably 88% or more. A transmittance of 87% or more provides sufficient visibility. The total light transmittance of the hard coat film is generally preferably higher, but from the standpoint of stable production, it is preferably 99% or less, and may be 97% or less. Furthermore, the haze of the hard coat film is generally preferably low, and preferably 3% or less. The haze of the hard coat film is more preferably 2% or less, and most preferably 1% or less. A haze of 3% or less can improve the visibility of images. The lower the haze, the better, but from the standpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0077] The hard coat layer may further have other functions added thereto. For example, a hard coat layer having the above-mentioned functions, such as an antiglare layer having a certain pencil hardness, an antiglare antireflection layer, an antireflection layer, a low reflection layer, or an antistatic layer, is also preferably used in the present invention.

[0078] A hard coat layer may also be provided when the film is used as a substrate film for a touch panel module. When an ITO layer is used as the transparent electrode layer of the touch panel module, for example, a refractive index adjustment layer is preferably provided between the substrate film and the transparent electrode layer to make the electrode pattern less visible. In this case, the hard coat layer itself may also serve as a refractive index adjustment layer, or a separate refractive index adjustment layer may be laminated on top of it. [Example]

[0079] Next, the present invention will be described with reference to examples and comparative examples. First, the evaluation methods of the characteristic values ​​used in the present invention will be described below.

[0080] (1) Intrinsic viscosity The film or polyester resin was crushed and dried, then dissolved in a 60 / 40 (mass ratio) phenol / tetrachloroethane mixed solvent. After centrifuging the solution to remove inorganic particles, an Ubbelohde viscometer was used to measure the flow time of a 0.4 (g / dL) solution at 30°C and the flow time of the solvent alone. The intrinsic viscosity was calculated from the ratio of these times using the Huggins equation, assuming a Huggins constant of 0.38.

[0081] (2) Bending resistance of polyester film sample (bending radius 1.5 mm) A polyester film sample measuring 20 mm in width and 110 mm in machine direction was prepared. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the sample was bent 200,000 times at a rate of 1 bend / second with a bend radius of 1.5 mm. The sample was fixed at 10 mm positions on both long sides, and the bending area was 20 mm x 90 mm. Figure 1 is a schematic diagram showing the bend radius when a foldable display is folded. Considering the case where a polyester film is disposed on the inner surface of the folded display, the bending test was performed as a model, with the bending radius set to 1.5 mm at the point indicated by reference numeral 11 in Figure 1. After the bending process, the sample was placed on a flat surface with the bent inner side facing down and visually observed. ○: No cracks or deformations were observed in the sample. ×: The sample has cracks or creases, and when placed horizontally, the maximum lift is 5 mm or more.

[0082] (3) Bending resistance of polyester film sample (bending radius 0.5 mm) Using the same method as in the bending test described above, the display was bent 200,000 times at a bending radius of 0.5 mm and a rate of 1 bend per second. Figure 1 is a schematic diagram showing the bending radius when the foldable display is folded. Taking into account the case where a polyester film is disposed on the inner surface of the folded display, the bending test was performed as a model, with the bending radius set to 0.5 mm at the location indicated by reference numeral 11 in Figure 1 . The film surface outside the bent portion was observed at 700x magnification using a digital microscope (HIROX RH8800) to check for the presence or absence of wrinkles (cracks). In addition to the visual observation of bending resistance with a bending radius of 1.5 mm described above, this test was performed with a bending radius reduced to 0.5 mm to evaluate the display under conditions similar to those in actual use, where a hard coat layer or other components are laminated or attached. This test, separate from the visual observation with a bending radius of 1.5 mm, was also performed to detect minute defects that are difficult to detect visually, such as those prone to breakage or cracking. ○: No defects on the film surface on the outer side of the bend. ×: The film broke or wrinkles (cracks) were observed on the film surface on the outer side of the bend.

[0083] (4) Refractive index In accordance with JIS K 7142:2008 "Method for measuring refractive index of plastics (Method A)", the refractive index in the longitudinal direction, width direction, and thickness direction was determined using an Abbe refractometer (manufactured by Atago Co., Ltd., NAR-4T, measurement wavelength 589 nm).

[0084] (5) Pencil hardness The pencil hardness of the hard coat film sample was measured at a load of 750 g and a speed of 1.0 mm / s in accordance with JIS K 5600-5-4: 1999. In the present invention, a value of 3H or more was considered acceptable.

[0085] (6) Total light transmittance, haze The measurement was carried out using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0086] (7) Density The density was measured according to the method (density gradient tube method) in accordance with JIS K 7112:1999 (unit: g / cm 3 ).

[0087] (8) Indentation depth after removing the test force The sample was cut into approximately 2 cm squares and fixed on an 18 × 18 mm microcover glass (manufactured by Matsunami Glass Co., Ltd.) with an adhesive (Cemedine (registered trademark) High Super 30) on the surface opposite the measurement surface. After adhesion and fixation, the sample was left at room temperature for 12 hours or more, and then the indentation depth (μm) after removing the test force was measured using a dynamic ultra-microhardness tester "DUH-211" (manufactured by Shimadzu Corporation) under the following conditions. <Measurement conditions> Test mode: Load-unload test Indenter used: 115 degree ridge angle, triangular pyramid indenter Indenter elastic modulus: 1.140 x 10 6 N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50mN Load speed: 4.44mN / sec Load holding time: 2sec Unloading holding time: 0sec

[0088] (9) Maximum heat shrinkage rate The sample film was cut to a size of 10 mm lengthwise and 250 mm widthwise, and marks were made at 200 mm intervals along the long side in the direction to be measured. The distance A between the marks was measured under a constant tension of 5 g. The sample film was then left in an oven at 150°C for 30 minutes without load, and then removed from the oven and cooled to room temperature. The distance B between the marks was then measured under a constant tension of 5 g, and the thermal shrinkage (%) was calculated using the following formula. The thermal shrinkage was measured at three equal positions across the width of the sample film, and the average value of the three points was taken as the thermal shrinkage (%). Heat shrinkage rate (%) = [(AB) x 100] / A The sample film is cut so that the length and width are different in both the bending direction and the folding direction, and measurements are taken. The data in the direction where the measurement value is larger is the maximum heat shrinkage rate (%).

[0089] (10) In-plane retardation (Re) This parameter is defined as the product (ΔN × d) of the refractive index anisotropy of biaxially perpendicular films (ΔN = |Nx - Ny|) and the film thickness d (nm), and is a measure of optical isotropy and anisotropy. The biaxial refractive index anisotropy (ΔN) was determined using the following method. The orientation axis direction of the film was determined using two polarizing plates, and a 4 cm × 2 cm rectangle was cut out so that the orientation axes were perpendicular to each other to serve as a measurement sample. The biaxially perpendicular refractive indices (Nx, Ny) and the thickness direction refractive index (Nz) of this sample were measured using an Abbe refractometer (Atago Co., Ltd., NAR-4T), and the absolute value of the difference between the biaxial refractive indices (|Nx - Ny|) was taken as the refractive index anisotropy (ΔN). The film thickness d (nm) was measured using an electric micrometer (Fine Rufu Co., Ltd., Millitron 1245D) and converted to nm. The in-plane retardation (Re) was calculated from the product (ΔN × d) of the refractive index anisotropy (ΔN) and the film thickness d (nm). Of the orientation axes, the axis showing the larger refractive index is defined as the slow axis.

[0090] (11) Iridescence observation The polyester film of the present invention was attached to the surface of an OLED tablet (Samsung Galaxy Tab S3 SM-T820 9.7-inch 32GB tablet) using water, with the bending direction aligned with the tablet's length. The OLED tablet was set to display a white screen (R=0, G=0, B=0), and the angle was changed from an oblique direction relative to the normal direction of the display screen, and the display screen was rotated in the in-plane direction while visually observing the screen. The presence or absence of rainbow spots was judged as follows. Color unevenness observed only on the tablet used was excluded from the judgment. ◎: No rainbow spots observed from any direction. ○: Very faint rainbow spots can be observed in some directions. △: Faint rainbow spots can be observed in some directions but over a fairly wide area. ×: Rainbow spots are clearly visible over a wide area, although only in some directions.

[0091] (Preparation of polyethylene terephthalate pellets (a)) A continuous esterification reactor consisting of a three-stage complete mixing vessel equipped with an agitator, a partial condenser, a raw material inlet, and a product outlet was used as the esterification reactor. TPA was supplied at a rate of 2 ton / hr, EG at 2 moles per mole of TPA, and antimony trioxide in an amount such that the Sb atom concentration in the produced PET was 160 ppm. These slurries were continuously supplied to the first esterification reactor of the esterification reactor and reacted at normal pressure for an average residence time of 4 hours at 255°C. 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 into the second esterification reactor in an amount of 8% by mass relative to the produced polymer (produced PET). Further, an EG solution containing magnesium acetate in an amount to give 65 ppm of Mg atoms relative to the produced PET and an EG solution containing TMPA in an amount to give 20 ppm of P atoms relative to 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 TMPA in an amount to give 20 ppm of P atoms relative to 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 then filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cutoff of 5 μm particles) to obtain polyethylene terephthalate pellets (a) with an intrinsic viscosity of 0.62 dl / g.

[0092] (Preparation of polyethylene terephthalate pellets (b)) The intrinsic viscosity was adjusted to 0.580 dl / g in the same manner as in the production process of polyethylene terephthalate pellets (a), except that the residence time of the third esterification reaction was adjusted, to obtain polyethylene terephthalate pellets (b).

[0093] (Preparation of polyethylene terephthalate pellets (c)) The polyethylene terephthalate pellets (a) were subjected to solid-state polymerization at 220°C under a reduced pressure of 0.5 mmHg for various times using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (c) with an intrinsic viscosity of 0.75 dl / g.

[0094] (Polymerization of urethane resin) A four-neck flask equipped with a stirrer, Dimroth condenser, nitrogen inlet tube, silica gel drying tube, and thermometer was charged with 72.96 parts by weight of 1,3-bis(isocyanatemethyl)cyclohexane, 12.60 parts by weight of dimethylolpropionic acid, 11.74 parts by weight of neopentyl glycol, 112.70 parts by weight of polycarbonate diol having a number average molecular weight of 2000, and 85.00 parts by weight of acetonitrile as a solvent and 5.00 parts by weight of N-methylpyrrolidone. The mixture was stirred at 75°C for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction solution reached the predetermined amine equivalent. Next, the reaction solution was cooled to 40°C, and 9.03 parts by weight 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 high-speed homodisperser, and the temperature was adjusted to 25°C. While stirring and mixing at 2000 min-1, the isocyanate-terminated prepolymer was added and dispersed in water. Thereafter, acetonitrile and a portion of the water were removed under reduced pressure to prepare a water-soluble polyurethane resin (A) having a solid content of 35% by mass.

[0095] (Polymerization of Water-Soluble Carbodiimide Compounds) 200 parts by weight of isophorone diisocyanate and 4 parts by weight of 3-methyl-1-phenyl-2-phospholene-1-oxide (carbodiimidation catalyst) were placed in a flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer, and stirred at 180°C for 10 hours under a nitrogen atmosphere to obtain isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Next, 111.2 g of the resulting carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100°C for 24 hours. Water was gradually added at 50°C to obtain a yellow, transparent, water-soluble carbodiimide compound (B) with a solids content of 40% by weight.

[0096] (Preparation of coating solution for forming easy-adhesion layer) The following coating materials were mixed to prepare a coating solution. 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 of 40% by mass) Surfactant 0.05 parts by mass (Silicone-based, solid content 100% by mass)

[0097] (Preparation of hard coat coating solution a) To 100 parts by weight of a hard coat material (Opstar (registered trademark) Z7503, manufactured by JSR Corporation, concentration 75%), 0.1 parts by weight of a leveling agent (BYK307, manufactured by BYK Japan, concentration 100%) was added, and the mixture was diluted with methyl ethyl ketone to prepare a hard coat coating solution a with a solids concentration of 40% by weight.

[0098] Example 1 Pellets of polyethylene terephthalate (a) were fed into an extruder and melted at 285°C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet form from a die, and then contacted with a casting drum with a surface temperature of 30°C using an electrostatic casting method, where it was cooled and solidified to produce an unstretched film. This unstretched film was uniformly heated to 75°C using a heated roll, heated to 85°C using a non-contact heater, and roll-stretched (longitudinal stretching) by 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 using a roll coating method, and then dried at 80°C for 20 seconds. The final coating amount after drying (after biaxial stretching) was 0.06 g / m 2The film was then introduced into a tenter, preheated at 105°C, transversely stretched 4.0 times at 95°C, heat-set at 230°C for 5 seconds with the width fixed, and then relaxed by 4% in the width direction at 180°C to obtain a polyethylene terephthalate film with a thickness of 50 μm. The evaluation results are shown in Table 1.

[0099] (Examples 2 and 3) 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 that shown in Table 1.

[0100] Example 4 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 4.4 times and the heat setting temperature was changed to 220°C.

[0101] (Examples 5 to 6) A polyester film was obtained in the same manner as in Example 4, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1.

[0102] 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.

[0103] (Examples 8 to 9) A polyester film was obtained in the same manner as in Example 7, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1.

[0104] Example 10 A polyester film was obtained in the same manner as in Example 5, except that in the production process of Example 5, after the film was stretched in the longitudinal direction, a 10% relaxation heat treatment was carried out at 100°C.

[0105] Example 11 A polyester film was obtained in the same manner as in Example 5, except that in the production process of Example 5, after heat setting, the clips were released at 200°C and a relaxation heat treatment was performed in the longitudinal and width directions. The tenter speed and the take-up roll speed were adjusted so that the relaxation rate in the longitudinal direction was 3%. The relaxation in the width direction was left in a free state.

[0106] 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.

[0107] 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 stretching ratio was changed to 1.2 times, and then the stretching ratio was changed to 5.0 times in the width direction.

[0108] Example 14 A polyester film was obtained in the same manner as in Example 3, except that the longitudinal stretching in Example 3 was a two-stage stretching in which the stretching ratio in the first stage was 1.2 times and the stretching ratio in the second stage was 1.67 times. The total longitudinal stretching ratio was about 2.0 times.

[0109] 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.

[0110] Example 16 A polyester film was obtained in the same manner as in Example 2, except that the widthwise stretching in Example 2 was changed to two-stage stretching, with the first-stage stretching ratio being 1.5 times and the second-stage stretching ratio being 4.0 times, and the heat setting temperature being 190° C. The total widthwise stretching ratio was 6.0 times.

[0111] (Examples 17 to 18) Polyester films were obtained in the same manner as in Example 2, except for changing the thickness as shown in Table 1. Here, the polyester film obtained in Example 17 did not necessarily give good results in the observation of rainbow spots, but the polyester film of the present invention can be used in any part of a folding display, and for example, the polyester film obtained in Example 17 can be preferably used in applications such as a back surface protective film for a folding display that are not related to rainbow spots.

[0112] Example 19 A polyester film was obtained in the same manner as in Example 1, except that the relaxation heat treatment in the width direction was not carried out in the production process of Example 1.

[0113] Example 20 An unstretched film was prepared in the same manner as in Example 1, and then the unstretched film was preheated at 75°C in a tenter and stretched laterally at 85°C to 1.4 times its original size. The above-mentioned coating solution for forming an easy-adhesion layer was applied to both sides of the obtained uniaxially stretched film by roll coating, and then dried at 80°C for 20 seconds. The final coating amount after drying (after biaxial stretching) was 0.06 g / m 2 The film was uniformly heated to 105°C using a heated roll, then heated to 95°C using a non-contact heater, and roll-stretched (longitudinal stretching) at 4.0 times its original size. The film was then fixed in width and heat-set at 230°C for 5 seconds to obtain a polyethylene terephthalate film with a thickness of 50 μm. The subsequent steps were the same as in Example 1 to obtain a hard-coated film.

[0114] (Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that the film was not stretched in the longitudinal direction but was stretched only in the width direction, i.e., transverse uniaxial stretching.

[0115] (Comparative Example 2) A polyester film was obtained in the same manner as in Example 7, except that the film was not stretched in the machine direction but was stretched only in the width direction, i.e., transverse uniaxial stretching.

[0116] (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 shown in Table 1 were used. As described above, Comparative Examples 3 to 7 had a lower heat setting temperature than Example 1, and the combination of conditions for the longitudinal and transverse stretching ratios was not optimal within the preferred range of conditions. As shown 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 was smaller than in each Example.

[0117] (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.

[0118] (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.

[0119] (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.

[0120] (Comparative Example 11) A polyester film was obtained in the same manner as in Example 13, except that the longitudinal stretching temperature was changed to 130°C.

[0121] (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.

[0122] (Comparative Example 13) A polyester film was obtained in the same manner as in Example 13, except that the width direction stretching ratio was changed to 5.5 times and the heat setting temperature was changed to 230°C.

[0123] Hard coat coating solution a was applied to one side of the prepared film using a Mayer bar so that the film would have a dry thickness of 5 μm, and the film was dried at 80°C for 1 minute, after which it was irradiated with ultraviolet light (integrated light dose 200 mJ / cm2 ), a hard-coated film was obtained.

[0124] The hard coat film was attached to an organic EL module via a 25 μm thick adhesive layer, creating a smartphone-type foldable display that could be folded in half at the center of the entire display with a radius of 3 mm, corresponding to the bending radius in Figure 1. The hard coat film was arranged on the surface of a single continuous display via the folding portion, with the hard coat layer positioned on the surface of the display. The displays using the hard coat films of each example satisfied the operation and visibility of a smartphone that could be folded in half at the center and carried around. Furthermore, the surface was not dented by external force. On the other hand, the foldable displays using the hard coat films of each comparative example appeared to develop image distortion at the folding portion of the display as usage frequency increased, which was not very desirable. Some displays also had dents and scratches on the surface.

[0125] [Table 1] [Industrial Applicability]

[0126] A foldable display using the polyester film for foldable displays or the hard coat film of the present invention maintains mass productivity, and the polyester film or hard coat film located on the surface of the foldable display does not deform after repeated folding, so that image distortion does not occur at the folded portion of the display. In particular, a mobile terminal device or image display device equipped with a foldable display using the polyester film or hard coat film of the present invention as a surface protective film provides beautiful images, is highly functional, and is excellent in convenience, such as portability. [Explanation of symbols]

[0127] 1: Foldable display 11: Bending radius 2: Polyester film for surface protection of foldable displays 21: Folding section 22: Bending direction (direction perpendicular to the folding part)

Claims

1. A polyester film for foldable displays that satisfies the following conditions: (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folded portion is 1.670 to 1.700 (3) A refractive index in the thickness direction of 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 folding displays 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.

3. The polyester film for a folding display according to claim 1, wherein the polyester film has an easy-adhesion layer on at least one surface thereof.

4. The polyester film for folding displays according to claim 1, wherein the in-plane retardation (Re) is 3000 to 30000 nm.

5. A hard coat film for a folding display, comprising the polyester film for a folding display according to any one of claims 1 to 4, and a hard coat layer having a thickness of 1 to 50 µm on at least one side thereof.

6. A foldable display in which the hard coat film for a foldable display according to claim 5 is arranged as a surface protective film so that the hard coat layer is positioned on the surface, and a single continuous hard coat film is arranged across the folding portion of the foldable display.

7. A mobile terminal device comprising the foldable display according to claim 6.

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