Polyester film and use thereof

Abstract

To provide a polyester film for a foldable display which is excellent in mass productivity and which is free from occurrence of fold marks or cracks in a foldable portion so as to provide a foldable display which is free from occurrence of disturbance in an image displayed in the foldable portion after bending and a portable terminal device on which such a foldable display is mounted.SOLUTION: A polyester film for a foldable display is configured so that a hold angle in a bending direction of 155° or more, and a maximum heat shrinkage rate at 150°C is 1.5% or less. (Here, the hold angle refers to an angle formed by bending after fixing at a room temperature for 72 hours so that strain of 1.7% is applied to both surfaces of a bent portion. Also, the bending direction refers to a direction orthogonal to the folded portion.)SELECTED DRAWING: Figure 4

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, but this method has not become widespread because it leaves a bezel, which means the image is cut off and visibility is reduced.

[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. 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, for example, Patent Document 1), but this method has the problem of being poorly suited for mass production.

[0008] Additionally, methods have been proposed to adjust the refractive index of polyester film in the bending direction, but as the refractive index in the bending direction is lowered, the pencil hardness when applying a hard coat decreases, resulting in a problem of reduced surface protection for displays. Lowering the refractive index in one direction improves deformation during folding, but increases uniaxial orientation in the folding direction, resulting in problems such as cracks or breakage at the fold. Biaxially oriented films do not break and are highly productive, but they are prone to deformation during folding and have poor resistance to bending. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-155124 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 fold after folding, as well as a polyester film for foldable displays that does not produce creases or cracks at the fold, 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 foldable displays with a hold angle of 155° or more in the bending direction and a maximum heat shrinkage rate of 1.5% or less at 150°C. (Here, the hold angle refers to the angle formed by the fold marks after fixing the material at room temperature for 72 hours so that a strain of 1.7% is applied to each surface of the bent part. The bending direction refers to the direction perpendicular to the fold.) 2. The polyester film for folding displays according to claim 1, which has a total light transmittance of 85% or more and a haze of 3% 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. 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 3, and a hard coat layer having a thickness of 1 to 50 μm on at least one surface of the polyester film for a foldable display. 5. A foldable display in which the hard coat film for a foldable display according to item 4 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. 6. A mobile terminal device having the foldable display described in item 5 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 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 the like. [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. [Figure 3] FIG. 2 is a schematic diagram showing a method for measuring a hold angle in a bending direction in the present invention. [Figure 4] FIG. 2 is a schematic diagram of a bent portion of a sample film for explaining a method for calculating the thickness of a spacer used when measuring a hold angle in the bending direction in 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 portion of a component 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 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.

[0037] 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.

[0038] 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.

[0039] 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 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 through a filter into a sheet, which is cooled and stretched to form a substrate film.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] (Maximum heat shrinkage rate) In the present invention, the maximum heat shrinkage of the polyester film after heat treatment at 150°C for 30 minutes is preferably 1.5% or less, more preferably 1.3% or less, even more preferably 1.0% or less, and particularly preferably 0.5% or less. A maximum heat shrinkage of 1.5% or less can suppress flatness defects such as curling and undulation during hard coating processing. The lower the heat shrinkage, the better, but a value of -1.0% or more is preferred, and a value of 0% or more is even more preferred. A negative heat shrinkage indicates expansion after heating, and even a value less than -1.0% may result in flatness defects. The maximum heat shrinkage can be effectively adjusted by adjusting the stretch ratio or by using offline annealing or aging treatment.

[0045] (About hold angle) In the present invention, the hold angle in the bending direction is preferably 155° or greater, more preferably 158° or greater, or even more preferably 160° or greater. Here, the hold angle refers to the angle formed by the fold marks after fixing at room temperature for 72 hours so that both surfaces of the bent portion are subjected to a strain of 1.7% (calculated as described below). Furthermore, in the present invention, the bending direction refers to the direction perpendicular to the folding portion (21) expected for use in a foldable display, as shown by reference numeral 22 on the polyester film (reference numeral 2) in Figure 2. If the hold angle is 155° or less, deformation of the film may occur when the display is opened after folding and use, potentially adversely affecting the display's functionality, such as reducing the display's visibility. If the hold angle is 155° or greater, deformation is minimal, allowing good visibility to be maintained. The hold angle can be effectively adjusted by controlling the refractive index by adjusting the stretching ratio and stretching temperature. Furthermore, a relaxation process in the stretching direction, offline annealing treatment, or aging treatment may be used to increase the hold angle. The hold angle in the bending direction is best 180°, but may be 175° or less, or may be 170° or less.

[0046] (About offline annealing) In the present invention, in order to increase the hold angle and reduce the maximum thermal shrinkage, the produced film can be wound into a roll and then annealed offline. The annealing temperature is 150°C or higher and 200°C or lower, more preferably 170°C or higher and 190°C or lower. The time for applying the temperature is preferably 3 seconds or higher and 90 seconds or lower, and more preferably 5 seconds or higher and 60 seconds or lower. By setting the temperature and time within the above ranges, the desired annealing can be achieved and a good film that maintains transparency can be obtained.

[0047] (About offline aging processing) In the present invention, in order to increase the hold angle and reduce the maximum thermal shrinkage, the produced film can be wound into a roll and then subjected to offline aging treatment. The aging temperature is preferably 50°C to 70°C, more preferably 55°C to 65°C. The treatment time is preferably 72 hours or more, more preferably 120 hours or more. It is believed that offline aging treatment does not promote crystallization, but rather causes conformational changes in the amorphous portion, resulting in densification. It is believed that fatigue due to tensile stress applied to the outside during folding causes elongation of the amorphous portion, and therefore densification can reduce tensile fatigue. There is no upper limit to the aging time, but since a long aging time can make production difficult, it is preferably one month or less, and may be 480 hours or less, or may be 360 ​​hours or less.

[0048] In the present invention, the stretching ratio of the unstretched polyester sheet is not particularly limited, but is preferably 1.2 to 6.0 times.

[0049] The stretching ratio in at least one of the longitudinal direction (machine flow direction) and the width direction, which is the bending direction, is preferably 1.2 to 2.0, more preferably 1.7 to 2.0. By lowering the stretching ratio in the bending direction, the stress applied to the folded part during folding can be reduced, and compressive and tensile fatigue can be suppressed, thereby increasing the hold angle in the bending direction.

[0050] The stretching ratio in the direction perpendicular to the bending direction is preferably 4.2 times or less, and more preferably 4.0 times or less. By reducing the stretching ratio in the direction perpendicular to the bending direction, a maximum heat shrinkage of 1.5% or less can be achieved.

[0051] When stretching is performed at a ratio of 1.2 to 2.0 in the bending direction, the stretching temperature is preferably 75 to 120°C, and more preferably 75 to 105°C. Heating during stretching can be performed using conventionally known methods such as hot air heating, roll heating, and infrared heating. By setting the stretching temperature at 75 to 120°C, it is possible to prevent significant thickness unevenness due to stretching at the above stretch ratio.

[0052] The preheating temperature for stretching in the direction perpendicular to the bending direction is preferably 70 to 110°C. When multi-stage 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).

[0053] (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 It is more preferable that the density is 1.380 g / cm 3 By 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 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.

[0054] 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.

[0055] (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. 2 If it is less than this, blocking resistance can be obtained, which is preferable.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] (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.

[0060] (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.

[0061] (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.

[0062] (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.

[0063] (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.

[0064] (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 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.

[0065] 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.

[0066] 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]

[0067] 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.

[0068] (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.

[0069] (2) 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).

[0070] (3) Pencil hardness Using the hard coat film as a sample, the pencil hardness 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 hardness of 3H or more was considered acceptable.

[0071] (4) Total light transmittance, haze The measurement was carried out using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0072] (5) Density The density was measured according to the method (density gradient tube method) in accordance with JIS K 7112:1999 (unit: g / cm 3 ).

[0073] (6) 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 of the sample film 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 (%).

[0074] (7) Hold angle The strength of the break mark that is left when the bent part is fixed so that a strain of 1.7% is applied to both surfaces of the bent part is evaluated. Figure 3 is a schematic diagram illustrating the method for measuring the hold angle in the bending direction. A sample film (reference number 3) was cut to a width of 10 mm and a machine direction of 50 mm. Two PTFE plates (reference number 31) were stacked on top of each other, and in the case of a 50 μm sample film, a 3 mm-thick PTFE plate (reference number 32) was sandwiched between them as a spacer to create a gap. Double-sided tape was attached to both ends of the sample, and the bent film was sandwiched between the 3 mm gap between the PTFE plates and secured with double-sided tape. After storing the film in an environment of 20°C and 65% RH for 72 hours, the angle (reference number 33) between the fold marks on the film was measured 5 minutes after removal from between the two PTFE plates (reference number 32). This angle was taken as the hold angle. To keep the strain constant, the thickness of the PTFE plate used as a spacer is changed depending on the thickness of the film. Figure 4 shows an enlarged schematic diagram of the sample film (reference number 4) sandwiched between two PTFE plates (reference number 32). The neutral plane, where neither compressive nor tensile stress is applied, is defined as the center in the thickness direction, and the difference between the neutral plane and both surfaces is defined as the strain. In other words, the strain applied to both surfaces can be expressed by the following equation. Strain (1.7%) = (|Semicircumference of outermost or inner surface - Semicircumference of midplane| / Semicircumference of midplane) x 100 Here, the semicircle can be calculated by the following formula, where t (mm) is the thickness of the sample film and d (mm) is the bending diameter (diameter of the outermost surface), i.e., the thickness of the spacer used. Semicircumference of the outermost surface = d × π / 2 Semicircumference of the midplane = (dt) × π / 2 The innermost semicircle = (d-2t) × π / 2 From the above, when the strain is set to 1.7%, the thickness is t (mm), the bending diameter (i.e., the thickness of the spacer used) is d (mm), and the thickness of the PTFE plate used for the spacer is determined using the following formula. Table 1 shows spacer thicknesses for typical film thicknesses. Spacer thickness d (mm) = film thickness (mm) x 60 For the sample film with a thickness of 50 μm, the diameter of the outermost circle (reference numeral 41) is 3 mm, which is the same as the thickness d of the spacer. The diameter of the innermost circle (reference numeral 43) is 2.9 mm, and the diameter of the neutral plane (reference numeral 42) is 2.95 mm.

[0075] [Table 1]

[0076] (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.

[0077] (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).

[0078] (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.

[0079] (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.

[0080] (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.

[0081] (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)

[0082] (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.

[0083] Example 1 Polyethylene terephthalate pellets (a) were fed into an extruder and melted at 285°C. This 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, 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 2 The film was then introduced into a tenter, preheated to 105°C, stretched transversely at 4.4 times its original size at 95°C, heat-set at 220°C for 5 seconds with the width held constant, and then relaxed 4% in the width direction at 180°C to obtain a polyethylene terephthalate film roll with a thickness of 50 μm. This was then subjected to offline annealing at 180°C for 30 seconds to obtain a polyester film. The evaluation results are shown in Table 2.

[0084] Example 2 A film roll obtained in the same manner as in Example 1 was not subjected to offline annealing treatment, but instead was subjected to aging treatment at 60° C. for 1 week to obtain a polyester film.

[0085] Example 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 2.7 times and the stretching ratio in the transverse direction was changed to 4.0 times.

[0086] Example 4 A polyester film was obtained in the same manner as in Example 2, except that the stretching ratio in the longitudinal direction was changed to 2.7 times and the stretching ratio in the transverse direction was changed to 4.0 times.

[0087] Example 5 A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was 3.4 times, the stretching ratio in the transverse direction was 4.0 times, the heat setting temperature was 230°C, and the annealing time was 10 seconds.

[0088] Example 6 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, the stretching ratio in the transverse direction was changed to 4.0 times, and the heat setting temperature was changed to 230°C.

[0089] Example 7 A polyester film was obtained in the same manner as in Example 2, except that the stretching ratio in the longitudinal direction was changed to 3.4 times, the stretching ratio in the transverse direction was changed to 4.0 times, the heat setting temperature was changed to 230°C, and the aging time was changed to 5 days.

[0090] Example 8 A polyester film was obtained in the same manner as in Example 2, except that the stretching ratio in the longitudinal direction was changed to 3.4 times, the stretching ratio in the transverse direction was changed to 4.0 times, and the heat setting temperature was changed to 230°C.

[0091] Example 9 A polyester film was obtained in the same manner as in Example 2, except that the stretching ratio in the longitudinal direction was 3.4 times, the stretching ratio in the transverse direction was 4.0 times, the heat setting temperature was 230°C, and the aging time was 10 days.

[0092] Example 10 A polyester film was obtained in the same manner as in Example 1, except that the thickness was changed to 38 μm and the heat setting temperature was changed to 190°C.

[0093] Example 11 A polyester film was obtained in the same manner as in Example 2, except that the thickness was changed to 38 μm and the heat setting temperature was changed to 190°C.

[0094] Example 12 A polyester film was obtained in the same manner as in Example 1, except that the thickness was changed to 75 μm and the heat setting temperature was changed to 190°C.

[0095] Example 13 A polyester film was obtained in the same manner as in Example 2, except that the thickness was changed to 75 μm and the heat setting temperature was changed to 190°C.

[0096] (Comparative Example 1) The film roll obtained in the same manner as in Example 1 was subjected to neither annealing nor aging treatment to obtain an untreated polyester film.

[0097] (Comparative Example 2) The film roll obtained in the same manner as in Examples 3 and 4 was not subjected to annealing or aging treatment to obtain an untreated polyester film.

[0098] (Comparative Example 3) The film roll obtained in the same manner as in Examples 5 to 9 was not subjected to annealing or aging treatment to obtain an untreated polyester film.

[0099] Hard coat coating solution a was applied to one side of the prepared film using a Mayer bar so that the film thickness after drying would be 5 μm, and after drying at 80°C for 1 minute, it was irradiated with ultraviolet light (integrated light dose 200 mJ / cm 2 ), a hard-coated film was obtained.

[0100] [Table 2A]

[0101] [Table 2B]

[0102] 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. [Industrial Applicability]

[0103] 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]

[0104] 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) 3: Sample film 31: PTFE board 32: Spacer (PTFE plate) 33: Hold angle 4: Sample film 41: Outermost diameter 42: Midplane diameter 43: innermost diameter

Claims

1. The hold angle in the bending direction is 160° or more, The intrinsic viscosity is 0.50 to 1.0 dl / g, the thickness is 38 to 80 μm, and the density is 1.380 g / cm ３ 1.40g / cm or more ３ is as follows: A polyester film for folding organic EL displays with a total light transmittance of 85% or more. (Here, the hold angle refers to the angle formed by the fold marks after fixing the sheet at room temperature for 72 hours so that a strain of 1.7% is applied to each surface of the bent portion. The bending direction refers to the direction perpendicular to the folded portion.)

2. A polyester film for a foldable organic EL display as described in claim 1, having a maximum thermal shrinkage rate of 1.5% or less.

3. The polyester film has an easy-adhesion layer on at least one surface thereof, 2. The polyester film for a folding organic electroluminescent display according to claim 1, wherein the easy-adhesion layer contains any one of a polyester-based resin, a polyether polyurethane-based resin, a polyester polyurethane resin, a polycarbonate polyurethane resin, and an acrylic resin.

4. The polyester film for a folding display according to any one of claims 1 to 3 has a hard coat layer having a thickness of 1 to 50 µm on at least one surface thereof, The hard coat layer is a hard coat film for foldable organic EL displays that contains one of the following resins: acrylic resin, siloxane resin, inorganic hybrid resin, urethane acrylate resin, polyester acrylate resin, or epoxy resin.

5. Equipped with an organic EL module, A foldable organic EL display in which the hard coat film for a foldable display according to claim 4 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.

6. A mobile terminal device comprising the foldable organic EL display according to claim 5.

Images