Polyester film for surface protection of foldable displays and its applications

Abstract

To provide a foldable display which is well-suited for mass production and which causes little distortion to an image displayed on a folded portion even after repeated folding, and a portable terminal device with the foldable display installed thereon, and to provide a polyester film serving as a surface protective film for the foldable display, and a hard coat film serving as a surface protective film therefor.SOLUTION: A polyester film serving as a surface protective film for a foldable display is a polyester film with thickness of 10 to 75 μm and a deformation at 0.2% proof stress of 2.6 to 5.0% at least in either the lengthwise or widthwise direction. A hard coat film, the foldable display, and a portable terminal device use the polyester film serving as a surface protective film for the foldable display.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] As mobile terminal devices become thinner and lighter, smartphones and other mobile terminal devices are becoming more and more popular. While mobile terminal devices are required to have a variety of functions, they are also required to be convenient. For this reason, the most popular mobile terminal devices must be able to perform simple operations with one hand 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 (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 or 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, but in a foldable display, when a full-surface display is formed via a folding portion, it is necessary to use a hard-coated film or the like that is flexible and can protect the surface. However, in a foldable display, the portion that corresponds to the folding portion is repeatedly folded, and the film in that portion deforms over time, causing problems such as distorting the image displayed on the display.

[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. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-228391 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-155124 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention seeks to solve the problems associated with conventional display surface protection members 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 fold after repeated folding, as well as a mobile terminal device equipped with such a foldable display, by providing a polyester film for a surface protection film for a foldable display and a hard coat film for a surface protection film. [Means for solving the problem]

[0010] That is, the present invention comprises the following: 1. A polyester film for surface protection of folding displays, characterized in that it is a polyester film having a thickness of 10 to 75 μm and has a 0.2% yield point strain in at least one of the longitudinal and transverse directions of 2.6 to 5.0%. 2. The polyester film for surface protection films of folding displays according to claim 1, characterized in that the 0.2% yield point strain in the bending direction is 2.6 to 5.0%. (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.) 3. The polyester film for surface protection of a folding display according to the above item 1 or 2, characterized in that the intrinsic viscosity of the film is 0.60 to 1.0 dl / g. 4. A hard coat film for surface protection films of foldable displays, characterized in that it has a hard coat layer having a thickness of 1 to 50 μm on at least one side of the polyester film for surface protection films of foldable displays described in any one of 1 to 3 above. 5. A hard coat film for surface protection of a folding display, as described in 4 above, characterized in that the pencil hardness of the hard coat layer measured under a load of 750 g in accordance with JIS K5600-5-4:1999 is H or higher. 6. A foldable display in which the hard coat film for surface protection of a foldable display described in 4 or 5 above is arranged as a surface protection film so that the hard coat layer is positioned on the surface, and the foldable display has a bending radius of 5 mm or less when folded. 7. The foldable display according to claim 6, wherein a single continuous hard coat film is disposed across the folding portion of the foldable display. 8. A mobile terminal device having the foldable display described in item 6 or 7 above. [Effects of the Invention]

[0011] A foldable display using the polyester film or hard coat film for surface protection of a foldable display of the present invention maintains mass productivity, and the polyester film or hard coat film does not deform after repeated folding, so that image distortion does not occur at the folded portion of the display. Mobile terminal devices equipped with such a foldable display provide beautiful images, are highly functional, and are convenient in terms of portability and other factors. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic diagram showing the measurement points of the bending radius when folded in the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the bending direction of the polyester film for a surface protective film of a folding display according to the present invention. [Figure 3] FIG. 2 is a schematic diagram for explaining the 0.2% proof stress strain in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] (display) The term "display" as used herein refers to a general display device, and examples of the display include LCD, organic EL display, inorganic EL display, LED, and FED. However, LCD, organic EL, and inorganic EL, which have a bendable structure, are preferred. In particular, organic EL and inorganic EL, which can reduce the layer structure, are particularly preferred, and organic EL, which has a wide color gamut, is even more preferred.

[0014] (foldable display) A foldable display preferably has a structure in which a single continuous display can be folded in half for portability, reducing its size by half. At the same time, it is desirable for the display to be thin and lightweight. Therefore, the bending radius of the 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 folded display. While a smaller bending radius is preferable, a bending radius of 0.1 mm or more is acceptable, even a bending radius of 0.5 mm or more is acceptable. Even a bending radius of 1 mm or more provides sufficient practicality compared to conventional displays without a folding structure. 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 foldable display.

[0015] (Organic EL) The general structure of an organic EL display consists of an organic EL layer consisting of an electrode / electron transport layer / light-emitting layer / hole transport layer / transparent electrode, a retardation plate to improve image quality, and a polarizing plate.

[0016] (Mobile terminal device with touch panel) When an organic EL display is used in a mobile terminal device with a touch panel, a touch panel module is placed on top of the organic EL display or between the organic EL layer and the retardation film. In this case, if an impact is applied from above, the circuits of the organic EL and the touch panel may be disconnected, so a surface protection film is required. It is preferable that the film placed on the front surface of the display as a surface protection film has a hard coat layer laminated on at least the front surface side of the display.

[0017] (Surface protection film for foldable displays) As the surface protection film, any film having high light transmittance and low haze, such as a polyimide film, a polyester film, a polycarbonate film, an acrylic film, a triacetyl cellulose film, or a cycloolefin polymer film, can be used. Among these, polyimide films and polyester films having high impact resistance and sufficient pencil hardness are preferred, and polyester films that can be produced inexpensively are particularly preferred.

[0018] In the present invention, the polyester film 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 structure film or an ultra-multilayer laminate film with a repeating structure.

[0019] Examples of polyester resins 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.

[0020] 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 preferred copolymer is 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.

[0021] 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 0.60 to 1.0 dL / g. An intrinsic viscosity of 0.60 dL / g or higher improves the impact resistance of the resulting film, making it less likely for the internal circuit to break due to external impact. This also contributes to minimizing deformation when repeatedly bent, which is also preferable. On the other hand, an intrinsic viscosity of 1.00 dL / g or lower is preferable because it prevents excessive increases in the filtration pressure of the molten fluid, facilitating stable film production.

[0022] Regardless of whether the film has a single-layer or multilayer structure, the intrinsic viscosity of the film is preferably 0.60 dL / g or more. It is more preferably 0.62 dL / g or more. It is even more preferably 0.68 dL / g or more. If it is 0.60 dL / g or more, fatigue resistance can be imparted and sufficient flex resistance can be obtained. On the other hand, a film having an intrinsic viscosity of 1.00 dL / g or less is preferable because it can be produced with good operability.

[0023] The thickness of the polyester film is preferably 10 to 75 μm, and more preferably 25 to 75 μm. A thickness of 10 μm or more improves pencil hardness, while a thickness of 75 μm or less is advantageous for weight reduction and is excellent in flexibility, processability, handleability, etc.

[0024] 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 preferably as small as possible, but it can be 0.1% or more, or even 0.3% or more.

[0025] As mentioned above, in order to reduce haze, it is better for the film surface to have small irregularities. However, in order to provide a certain degree of slipperiness from the viewpoint of ease of handling, irregularities can be formed by blending a filler into the surface polyester resin layer or by coating a filler-containing coating layer during film formation.

[0026] The method of incorporating particles into the base film 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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 it may be 99% or less, or even 97% or less.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] Next, a method for producing a biaxially stretched polyester film will be described in detail using an example in which polyethylene terephthalate (hereinafter referred to as PET) pellets are used as the raw material for the base film, but the method is not limited to this. In addition, the number of layers may be limited to a single layer or a multilayer structure. It is not something that can be done.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Furthermore, the 0.2% proof stress strain in at least one of the longitudinal direction (machine flow direction) and the transverse direction of the polyester film is preferably 2.6 to 5.0%, and more preferably 3.0 to 5.0%. The 0.2% proof stress strain is a value used as a substitute for the yield point, regardless of whether a yield point appears or not in the stress-strain curve, and can be used as an index of the elastic region. Figure 3 shows a schematic diagram of a stress-strain curve for determining the 0.2% proof stress strain. Figure 3 shows an example of a case where a yield point does not appear. The 0.2% proof stress strain is determined by a conventional tensile test. On the stress-strain curve, a line parallel to OA, where Hooke's law holds, is drawn from point Q on the stress-strain curve. The point where it intersects with the stress-strain curve is designated as the proof stress point P. A parallel line is then dropped from this point on the vertical axis. The strain value at point H where it intersects with the strain axis is the 0.2% proof stress point strain. The 0.2% proof stress point strain can be determined from the stress-strain curve as described above. However, if the stress-strain curve is obtained using a tensile testing machine (Shimadzu AUTOGRAPH AG-X Plus 1kN), the 0.2% proof stress point strain can then be calculated directly using Shimadzu's AUTOGRAPH X software by setting the strain to 0.2%.

[0040] A 0.2% proof stress strain of 2.6% or more is preferable because deformation due to strain generated during folding is unlikely to occur. A 0.2% proof stress strain of 5.0% or less is preferable because good impact resistance is obtained. The 0.2% proof stress strain in the bending direction of the polyester film is preferably 2.6 to 5.0%, and more preferably 3.0 to 5.0%. Here, the bending direction refers to the direction perpendicular to the folding portion (reference numeral 21) expected in the application as a surface protection film for a foldable display, as shown by reference numeral 22 on the polyester film (reference numeral 2) in Figure 2. The bending direction is not limited to either the longitudinal direction or the width direction of the film.

[0041] The 0.2% proof stress strain can be adjusted to fall within the above range by adjusting the stretch ratio, stretching temperature, heat setting temperature, polyester raw material, and the like.

[0042] The 0.2% proof stress strain of a polyester film can be effectively controlled by adjusting the stretching ratio. The stretching ratio of an unstretched polyester sheet in at least one of the longitudinal direction (machine flow direction) and the transverse direction is preferably 1.0 to 3.4 times, more preferably 1.0 to 3.0 times. By setting the stretching ratio to 1.0 to 3.4 times, the 0.2% proof stress strain can be adjusted to a high value within the above range, resulting in less deformation when repeatedly folded. The stretching direction is preferably the bending direction. The stretching temperature is preferably 80 to 130°C, more preferably 90 to 130°C. Heating during stretching can be performed using conventional methods such as hot air heating, roll heating, and infrared heating. Setting the stretching temperature to 80 to 130°C can prevent thickness unevenness due to stretching at the above stretching ratio.

[0043] From the viewpoint of the mechanical properties of the film, it is preferable that the stretching ratio in the direction perpendicular to the bending direction when folded (the direction of the folded part) is larger than that in the bending direction, and the stretching ratio in the direction perpendicular to the bending direction can be, for example, 2.5 to 5.0. A stretching ratio of 2.5 or more can ensure stable productivity, and a stretching ratio of 5.0 or less can ensure good impact resistance.

[0044] The crystallinity of the polyester film is preferably 35 to 54%, more preferably 43 to 54%. Increasing the crystallinity can improve the 0.2% proof stress strain and impact resistance. In order to effectively increase the crystallinity, it is preferable to perform a heat treatment at 180 to 250°C after biaxial stretching. If the temperature is 180°C or higher, the crystallinity can be effectively increased, and if the temperature is 250°C or lower, there is no risk of the polyester film surface melting and the appearance can be maintained in a good condition, which is preferable.

[0045] 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 1.0 to 3.4 times in the longitudinal direction using rolls heated to 80 to 130°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 80 to 180°C, where it is dried and then stretched 2.5 to 5.0 times in the transverse direction. The film is then introduced into a heat treatment zone at 180 to 250°C and heat-treated for 1 to 60 seconds to complete the crystal orientation. During this heat treatment process, a relaxation treatment of 1 to 12% may be performed in the transverse or longitudinal direction, if necessary.

[0046] (Hard coat layer) The polyester film placed on the surface of a foldable display to protect the display preferably has a hard coat layer on its surface. The hard coat layer is preferably placed on the display surface side of the polyester film and used in the display. The resin that forms 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 also be mixed and used, and particles such as inorganic filler or organic filler can also be added.

[0047] (film thickness) The thickness of the hard coat layer is preferably 1 to 50 μm, more preferably 1 to 40 μm. If the thickness is greater than 1 μm, sufficient curing occurs and good pencil hardness is obtained. 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. It is more preferably 2 to 25 μm, particularly preferably 2 to 20 μm, and most preferably 3 to 15 μm.

[0048] (Application method) The hard coat layer can be applied by any method, including a Mayer bar, a gravure coater, a die coater, and a knife coater, without any particular limitation, and can be appropriately selected depending on the viscosity and film thickness.

[0049] (Curing conditions) The hard coat layer can be cured by energy rays such as ultraviolet rays and electron beams, or by heat, but curing methods using ultraviolet rays or electron beams are preferred in order to reduce damage to the film.

[0050] (Pencil hardness) The pencil hardness of the hard coat layer is preferably B or higher, more preferably H or higher, and particularly preferably 2H or higher. A pencil hardness of B or higher prevents scratches and does not reduce visibility. Generally, a high pencil hardness of the hard coat layer is preferable, but a hard coat layer having a pencil hardness of 10H or lower, 8H or lower, or even 6H or lower can be used without any practical problems.

[0051] (Type of hard coat layer) The hard coat layer in the present invention may have other functions added thereto, as long as it can be used for the purpose of increasing the pencil hardness of the surface and protecting the display as described above. For example, hard coat layers having the above-mentioned certain pencil hardness and added functions such as an antiglare layer, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer are also preferably used in the present invention. [Example]

[0052] Next, the effects of the present invention will be described using examples and comparative examples. First, the evaluation methods of the characteristic values ​​used in the present invention will be described below.

[0053] (1) 0.2% yield point strain The film was cut into strips measuring 140 mm in the measurement direction and 10 mm wide to obtain specimens. A tensile test was performed at a tensile speed of 100 mm / min using a tensile testing machine (Shimadzu AUTOGRAPH AG-X Plus 1kN). The stress-strain curve was then obtained using Shimadzu TRAPEZIUM X Autograph software, with the strain set to 0.2%. The 0.2% yield point strain was calculated.

[0054] (2) Intrinsic viscosity After crushing and drying the film or polyester resin, it was dissolved in a 60 / 40 (mass ratio) phenol / tetrachloroethane mixed solvent. This solution was centrifuged to remove inorganic particles, and then 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. For laminated films, the intrinsic viscosity of each individual layer was evaluated by scraping off the corresponding polyester layer of the film according to the laminate thickness.

[0055] (3) Crystallinity The density of the film sample was measured at three points. The average value was taken as the degree of crystallinity. The resin component was a mixture of completely amorphous and completely crystalline components, and its density is as described below. Based on the assumption that the density of the sample is the sum of the masses of the components that make up the sample divided by the sum of the volumes of each component, the crystallinity (weight ratio) of each resin was estimated. The density of the sample was measured according to a method (density gradient tube method) in accordance with JISK-7112-1980. The density of each component alone was calculated using the following values ​​(unit: g / cm3): Polyethylene terephthalate resin: completely amorphous 1.34, completely crystalline 1.46 Polyethylene naphthalate resin: completely amorphous 1.32, completely crystalline 1.41

[0056] (4) Flexibility A polyester film was cut into a 200 mm (bending direction) x 50 mm (transverse direction) measurement sample. Two 5 mm thick glass plates were placed with spacers of each thickness at the edges to create a space between them, and the film was sandwiched between them for 10 seconds. Immediately after that, the film was exposed to fluorescent light, and the folded area was observed. The gap at which no fold marks were found was recorded. ○: The interval without folding marks was less than 6.5 mm. △: The gap where no folding marks were left was 6.5 mm or more and less than 7.0 mm ×: The interval where no folding marks were left was 7.0 mm or more.

[0057] (5) Repeated bending resistance A sample measuring 50 mm in the width direction and 100 mm in the machine direction was prepared. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the sample was bent 50,000 times at a rate of 1 bend per second with a bending radius of 3 mm. The sample was fixed at 10 mm from both ends of the long side, and the bending area was 50 mm x 80 mm. After the bending process, the sample was placed on a flat surface with the inside of the bend facing down and visually inspected. ○: No deformation of the sample or, even if there is deformation, the maximum lift when placed horizontally Less than 3mm in height. △: The sample is deformed and when placed horizontally, the maximum lift is 3mm to 5mm less than. ×: The sample has creases or the maximum height of the sample when placed horizontally is 5 mm or more.

[0058] (6) Pencil hardness The hard-coated polyester film thus prepared was measured in accordance with JIS K 5600-5-4:1999 at a load of 750 g and a speed of 0.5 mm / s.

[0059] (Coating solution 1 for forming hard coat layer) 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 hard coat coating solution 1 with a solids concentration of 40% by weight.

[0060] (Coating solution 2 for forming hard coat layer) 95 parts by weight of a urethane acrylate hard coating agent (Beamset (registered trademark) 577, manufactured by Arakawa Chemical Industries, Ltd., solids concentration 100%), 5 parts by weight of a photopolymerization initiator (Irgacure (registered trademark) 184, manufactured by BASF Japan, solids concentration 100%), and 0.1 parts by weight of a leveling agent (BYK307, manufactured by BYK Japan, solids concentration 100%) were mixed and diluted with a solvent of toluene / MEK=1 / 1 to prepare coating solution 2 with a concentration of 40%.

[0061] (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. TPA was supplied at 2 tons / hr, EG at 2 moles per mole of TPA, and antimony trioxide in an amount sufficient to provide 160 ppm Sb atoms relative to the PET product. These slurries were continuously fed into the first esterification reactor of the esterification reactor and reacted at atmospheric pressure for an average residence time of 4 hours at 255°C. The reaction product in the first esterification reactor was then continuously removed from the system and fed to the second esterification reactor. EG distilled from the first esterification reactor was fed into the second esterification reactor at 8% by mass relative to the polymer (PET product). An EG solution containing magnesium acetate in an amount sufficient to provide 65 ppm Mg atoms relative to the PET product, and an EG solution containing TMPA in an amount sufficient to provide 20 ppm P atoms relative to the PET product, were then added, and the reaction was continued 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 an EG solution containing TMPA in an amount such that the P atom content was 20 ppm relative to the produced PET was added, followed by reaction at atmospheric pressure for an average residence time of 0.5 hours at 260°C. The esterification reaction product was continuously fed 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% cut of 5 μm particles) to obtain polyethylene terephthalate pellets A with an intrinsic viscosity of 0.62 dl / g.

[0062] (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 produce polyethylene terephthalate pellets B having an intrinsic viscosity of 0.72 dL / g.

[0063] (Preparation of polyethylene-2,6-naphthalate pellets C) A conventional transesterification reaction was carried out using 100 parts of dimethyl 2,6-naphthalenedicarboxylate and 60 parts of ethylene glycol, using 0.03 parts of manganese acetate tetrahydrate as a transesterification catalyst. Then, 0.042 parts of triethylphosphonoacetate was added to substantially terminate the transesterification reaction. Next, 0.024 parts of antimony trioxide was added, and a conventional polymerization reaction was carried out at high temperature under high vacuum to obtain polyethylene-2,6-naphthalate (PEN) with an intrinsic viscosity of 0.60 dL / g. The pellets were then pre-dried at 150-160°C for 3 hours, followed by solid-state polymerization at 210°C under 13 kPa in a nitrogen gas atmosphere, yielding polyethylene-2,6-naphthalate pellets C with an intrinsic viscosity of 0.72 dL / g.

[0064] Example 1 The polyethylene terephthalate master pellets B were dried under reduced pressure (3 Torr) at 150 ° C for 8 hours, and then the polyethylene terephthalate pellets B 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 cooled and solidified by contact with a casting drum at a surface temperature of 30 ° C using an electrostatic casting method to produce an unstretched film. The unstretched film was uniformly heated to 75 ° C using a heated roll, heated to 100 ° C using a non-contact heater, and roll-stretched 3.4 times (longitudinal stretching). The resulting uniaxially stretched film was introduced into a tenter, heated to 140 ° C, and stretched 4.0 times laterally. The width was fixed and heat-treated at 240 ° C for 5 seconds, and then further relaxed 4% in the width direction at 210 ° C to obtain a 50 μm thick polyethylene terephthalate film.

[0065] Examples 2 to 6 A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio and heat treatment temperature were changed to those shown in Table 1.

[0066] (Examples 7 and 8) A polyester film was obtained in the same manner as in Example 1 above, except that the thickness was changed to that shown in Table 1.

[0067] Example 9 A polyester film was obtained in the same manner as in Example 1 above, except that polyethylene-2,6-naphthalate pellets C were used as shown in Table 1 and the temperature was adjusted.

[0068] Example 10 A polyester film was obtained in the same manner as in Example 1, except that polyethylene terephthalate master pellets A were used instead.

[0069] (Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio was changed to 3.5 times.

[0070] (Comparative Examples 2 and 3) A polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to that shown in Table 1.

[0071] One side of the polyester films obtained in Examples 1 to 10 and Comparative Examples 1 to 3 was coated with hard coat layer-forming coating solution 1 using a Mayer bar so that the film thickness after drying would be 5.0 μm, and the film was dried at 80°C for 1 minute, followed by irradiation with ultraviolet light (high-pressure mercury lamp, integrated light amount 200 mJ / cm2) to obtain a hard coat film. In Example 11, a hard coat film was obtained in the same manner as in Example 1, except that the polyester film obtained in Example 1 was coated so that the film thickness after drying would be 10.0 μm. In Example 12, a hard coat layer-forming coating solution 2 was coated on the polyester film obtained in Example 1, except that the polyester film obtained in Example 1 was coated with hard coat layer-forming coating solution 3. A hard coat film was obtained in the same manner as in Example 1.

[0072] These hard coat films were attached to an organic EL module via a 25 μm thick adhesive layer to create a smartphone-type foldable display that could be folded in half at the center of the entire display with a radius of 3 mm, which corresponds 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. 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.

[0073] [Table 1] [Industrial Applicability]

[0074] According to a foldable display using the polyester film or hard coat film for surface protection of a foldable display of the present invention, the polyester film or hard coat film located on the surface of the foldable display does not deform after repeated folding while maintaining mass productivity, so that image distortion does not occur at the folded portion of the display. Mobile terminal devices equipped with a foldable display using the polyester film or hard coat film of the present invention as a surface protection film provide beautiful images, are highly functional, and are convenient in terms of portability and the like. [Explanation of symbols]

[0075] 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) O: Origin A: The farthest point from O where Hooke's law holds P: A line parallel to the OA line is drawn from Q and intersects with the stress-strain curve Q: Stress 0 MPa, elongation (strain) 0.2% H: A line drawn parallel to the vertical axis from P intersects with the horizontal axis (0.2% yield point strain (%)) σ0.2: Stress value of P (MPa)

Claims

1. The total light transmittance is 85% or more and 99% or less, the crystallinity is 35 to 54%, and the thickness is 10 to 75 μm, A polyester film for displays, characterized in that the 0.2% yield point strain in the machine direction is 2.6 to 5.0%.

2. 2. The polyester film for displays according to claim 1, wherein the machine direction is a bending direction perpendicular to a fold when the polyester film is folded.

3. A polyester film for displays as described in claim 2, characterized in that the intrinsic viscosity is 0.60 to 1.0 dl / g.

4. 2. The polyester film for displays according to claim 1, wherein the haze is 0.1% or more and 3% or less.

5. In a no-load U-shaped stretch test in which the sample was bent 50,000 times at a bending radius of 3 mm and a speed of 1 time per second, after bending, the sample was placed horizontally on a flat surface with the inside of the bend facing downwards.

2. The polyester film for displays according to claim 1, wherein the maximum lift height is less than 5 mm or the film is not deformed.

6. The polyester film for displays according to any one of claims 1 to 5, wherein a hard coat layer having a thickness of 1 to 50 µm is provided on at least one surface of the polyester film for displays, A hard coat film for displays, characterized in that the pencil hardness of the hard coat layer measured under a load of 750 g in accordance with JIS K5600-5-4:1999 is H or more.

7. A folding display in which the hard coat film for displays according to claim 6 is arranged as a surface protective film so that the hard coat layer is positioned on a surface thereof, The bending radius when folded is 5 mm or less, A single continuous hard-coat film runs through the display fold. A foldable display characterized by its

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

Images