Polyester Film and Its Uses

Abstract

[Problem] To provide a polyester film for foldable displays, the polyester film being free from the occurrence of creases or cracks in a folding part, for the purpose of making it possible to provide a foldable display that has exceptional mass productivity and does not raise concerns regarding distortion of an image displayed on a folding part after repeated folding, and a mobile terminal device equipped with such a foldable display. [Solution] A polyester film for foldable displays, the polyester film having a bending-direction hold angle of 155° or greater and a maximum heat shrinkage rate at 150°C of 1.5% or less. (The hold angle refers to an angle formed after bending after having been fixed at room temperature for 72 hours so as to exert a strain of 1.7% on both surfaces of the bent portion. In addition, the bending direction refers to a direction orthogonal to a folding part.)

Description

Technical Field

[0001] The present invention relates to a polyester film for a foldable display, a hard coat film for a foldable display, a foldable display, and a portable terminal device, and more particularly to a foldable display and a portable terminal device in which images are less likely to be disturbed due to film deformation even when repeatedly folded, and to the polyester film and hard coat film for the foldable display.

Background Art

[0002] The thin film of portable terminal devices has been made lighter, and portable terminal devices typified by smartphones have become widely popular. While various functions are required for portable terminal devices, convenience is also required. Therefore, popular portable terminal devices should be able to be operated with one hand for simple operations, and furthermore, since they are assumed to be stored in a pocket of clothing or the like, it is necessary to have a small screen size of about 6 inches.

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

[0004] To achieve these, a method of making a device compact by connecting a plurality of displays has been proposed. However, since the bezel part remains, the video is interrupted, resulting in a problem of reduced visibility and lack of widespread use.

[0005] Therefore, in recent years, portable terminals incorporating flexible displays and foldable displays have been proposed. With this method, it is possible to carry a portable terminal device equipped with a large screen display conveniently without interruption of images.

[0006] Here, for displays and portable terminal devices that do not have a conventional folding structure, the surface of the display could be protected with a non-flexible material such as glass. However, in a foldable display, when making a single-sided display through the folding part, it is necessary to use a hard coat film or the like that has flexibility and can protect the surface. However, in a foldable display, since the part corresponding to a certain folding part is repeatedly bent, the film at that part is deformed over time, and there are problems such as distorting the image displayed on the display. Also, not only the surface protection film, but also in a foldable display, films are used in various parts such as a polarizing plate, a retardation plate, a touch panel substrate, a substrate of a display cell such as an organic EL, and a protection member on the back surface, and durability against repeated folding is required for these films.

[0007] Therefore, a method of partially changing the film thickness has been proposed (see, for example, Patent Document 1), but there is a problem of poor mass productivity.

[0008] Also, a method of adjusting the refractive index in the bending direction of a polyester film has been proposed. However, as the refractive index in the bending direction is decreased, the pencil hardness during hard coat application decreases, and there is a problem of deterioration of the surface protection function of the display. Also, as the refractive index in one direction is decreased, the deformation during folding improves, but the uniaxial orientation in the folding direction increases, and there are problems such as cracks occurring or breaking in the folding part. In a biaxially stretched film, breakage does not occur and productivity is good, but deformation during folding easily occurs and the resistance to bending is inferior.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] The present invention aims to solve the problems of conventional display members as described above, and provides a foldable display excellent in mass productivity and having no possibility of causing image distortion at the folded portion after folding, and a mobile terminal device equipped with such a foldable display. In order to achieve this, the present invention provides a polyester film for a foldable display in which no folding marks or cracks occur in the folding portion.

Means for Solving the Problems

[0011] That is, the present invention has the following configurations. 1. A polyester film for a foldable display having a hold angle in the bending direction of 155° or more and a maximum thermal shrinkage rate at 150°C of 1.5% or less. (Here, the hold angle refers to the angle formed by the fold after being fixed at room temperature for 72 hours so that a strain of 1.7% is applied to both surfaces of the bent portion. The bending direction refers to the direction orthogonal to the folding portion.) 2. The polyester film for a foldable display according to the first item above, having a total light transmittance of 85% or more and a haze of 3% or less. 3. The polyester film for a foldable display according to the first or second item above, having an easy adhesion layer on at least one side of the polyester film. 4. A hard coat film for a foldable display having a hard coat layer with a thickness of 1 to 50 μm on at least one side of the polyester film for a foldable display according to any one of the first to third items above. 5. A foldable display in which the hard coat film for a foldable display according to the fourth item above is arranged as a surface protection film with the hard coat layer positioned on the surface, and a single continuous hard coat film is arranged through the folding portion of the foldable display. 6. A mobile terminal device having the foldable display according to the fifth item above. [Effect of the Invention]

[0012] The foldable display using the polyester film or hard coat film for the foldable display of the present invention can maintain mass productivity, and the polyester film does not generate cracks in the folded portion, does not cause deformation after folding, and does not cause image disturbance in the folded portion of the display. A portable terminal device equipped with the foldable display using the polyester film or hard coat film as described above provides beautiful images, is rich in functionality, and is excellent in convenience such as portability. [Brief Description of the Drawings]

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

[0014] (Display) The display referred to in the present invention generally refers to a display device. Examples of the types of displays include LCD, organic EL display, inorganic EL display, LED, FED, etc. Among them, an LCD, organic EL, or inorganic EL having a foldable structure is preferable. In particular, organic EL and inorganic EL, which can reduce the layer structure, are particularly preferable, and organic EL having a wide color gamut is even more preferable.

[0015] (Folding Display) A folding display is a continuous single display that can be folded, such as in half, when carried. By folding, the size can be halved, improving portability. The bending radius of the folding display is preferably 5 mm or less, and more preferably 3 mm or less. If the bending radius is 5 mm or less, thinning in the folded state is possible. It can be said that the smaller the bending radius, the better, but the smaller the bending radius, the easier it is to leave folding marks. The bending radius is preferably 0.1 mm or more, but may be 0.5 mm or more, or may be 1 mm or more. Even if the bending radius is 1 mm, practical thinning can be achieved when carried. The bending radius when folded is measured at the location of reference numeral 11 in the schematic diagram of FIG. 1, and means the radius inside the folded portion when folded. Note that the surface protection film described later may be located on the outside or inside of the folded folding display. Further, the folding display may be tri-folded, quad-folded, or even a rollable type called a roll-up type, and all of these are within the scope of the folding display referred to in the present invention.

[0016] The polyester film for a folding display of the present invention may be used in any part as long as it is a component of the folding display. Hereinafter, taking an organic EL display as an example, the typical configuration of the folding display and the parts where the polyester film of the present invention can be used will be described. Note that 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] (Folding Organic EL Display) The essential configuration of a folding organic EL display is an organic EL module, and further, a circularly polarizing plate, a touch panel module, a surface protection film, a back protection film, etc. may be provided as necessary. (Organic EL Module) A general structure of an organic EL module consists of an electrode / electron transport layer / light emitting layer / hole transport layer / transparent electrode. As a substrate for providing an electrode and further providing an electron transport layer, a light emitting layer, and a hole transport layer, the polyester film of the present invention can be used. In particular, it can be preferably used as a substrate for the transparent electrode. In this case, since a high water vapor and oxygen barrier property is required for the substrate film, it is preferable that a barrier layer such as a metal oxide layer is provided on the polyester film of the present invention. In order to increase the barrier property, a plurality of barrier layers may be provided, or a plurality of polyester films provided with a barrier layer may be used.

[0018] (Touch panel module) It is preferable that a portable terminal device has a touch panel. When an organic EL display is used, it is preferable that a touch panel module is disposed on the upper part of the organic EL display or between the organic EL module / circular polarizing plate. 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 of a touch panel, it is preferable to provide a hard coat layer and a refractive index adjustment layer on the polyester film.

[0019] (Circular polarizing plate) The circular polarizing plate suppresses the reduction in image quality caused by external light being reflected by members inside the display. The circular polarizing plate has a linear polarizing plate and a retardation plate. The linear polarizing plate has a protective film on at least the viewing side surface of the polarizer. A protective film may also be provided on the surface of the polarizer opposite to the viewing side, or the retardation plate may be directly laminated on the polarizer. As the retardation plate, a resin film having retardation such as polycarbonate or cyclic olefin, or a resin film provided with a retardation layer made of a liquid crystal compound is used. The polyester film of the present invention can be used as the protective film for the polarizer or the resin film of the retardation plate. In these cases, it is preferable that the slow axis direction of the polyester film of the present invention is parallel or orthogonal to the absorption axis direction of the polarizer. A deviation of up to 10 degrees, preferably up to 5 degrees, from this parallel or orthogonal is allowed.

[0020] (Surface protective film) When an impact is applied to the display from above, there is a risk that the circuits of the organic EL module or the touch panel module may be disconnected. Therefore, in many cases, a surface protective film is provided. The polyester film of the present invention is used as this surface protective film. The surface protective film may be a cover window incorporated in the outermost surface of the display, or an after that can be self-attached, peeled off, and replaced by the user. In any case, the polyester film of the present invention is used. When the polyester film of the present invention is used as the surface protective film, it is preferable that a hard coat layer is laminated on at least the surface side of the polyester film. The hard coat layer is provided on the surface of the foldable display with the viewing side facing outward. Note that the hard coat layer may be provided on both sides.

[0021] (Back surface protective film) It is also preferable to provide a protective film on the back surface side of the display. The polyester film of the present invention can be used as this protective film on the back surface side.

[0022] The polyester film of the present invention may be other than the above as long as it is used at the folding portion of a component of a foldable display. Among these, the polyester film of the present invention is preferably used as a cover window surface protection film, an after surface protection film, a base film of a touch panel module, and a back surface protection film. More preferably, it is used as a cover window surface protection film and an after surface protection film.

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

[0024] The polyester film of the present invention may be a single-layer film composed of one or more types of polyester resins, or when using two or more types of polyesters, it may be a multilayer structure film or an ultra-multilayer laminated film with a repeating structure.

[0025] Examples of the polyester resin used for the polyester film include polyester films composed of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers having the constituent components of these resins as the main components. Among them, a stretched polyethylene terephthalate film is particularly preferable from the viewpoints of mechanical properties, heat resistance, transparency, price, etc.

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

[0027] In addition, in the production of the polyester film, the intrinsic viscosity of at least one type of resin pellet is preferably in the range of 0.50 to 1.0 dl / g. When the intrinsic viscosity is 0.50 dl / g or more, the impact resistance of the obtained film is improved, and it is preferable that disconnection of the internal circuit of the display due to an external impact hardly occurs. On the other hand, when the intrinsic viscosity is 1.00 dl / g or less, the increase in the filtration pressure of the molten fluid does not become too large, and it is preferable that the film production can be stably operated.

[0028] The thickness of the polyester film is preferably 10 to 80 μm, and more preferably 25 to 75 μm. When the thickness is 10 μm or more, an effect of improving the pencil hardness and an effect of improving the impact resistance are observed. When the thickness is 80 μm or less, it is advantageous for weight reduction, and in addition, it is excellent in flexibility, processability, handling properties, and the like.

[0029] The surface of the polyester film of the present invention may be smooth or may have irregularities. However, since it is used for the surface cover of a display, a decrease in optical properties due to irregularities is not preferable. As haze, 3% or less is preferable, 2% or less is more preferable, and 1% or less is most preferable. If the haze is 3% or less, the visibility of the image can be improved. Although the lower limit of haze is preferably as small as possible, from the viewpoint of stable production, 0.1% or more is preferable, and 0.3% or more may be sufficient.

[0030] For the purpose of reducing haze as described above, it is preferable that the irregularities on the film surface are not too large. However, from the viewpoint of handling properties, in order to provide a certain degree of slipperiness, as a method of forming irregularities, particles can be blended into the surface polyester resin layer, or a coating layer containing particles can be formed by coating during film formation.

[0031] As a method of blending particles into the polyester resin layer, a known method can be adopted. For example, it can be added at any stage of manufacturing polyester, but preferably at the stage of esterification or after the transesterification reaction and before the start of the polycondensation reaction, and added as a slurry dispersed in ethylene glycol or the like, and the polycondensation reaction may be advanced. Further, it can be carried out by a method of blending a slurry of particles dispersed in ethylene glycol or water and a polyester raw material using a kneading extruder with a vent, or a method of blending dried particles and a polyester raw material using a kneading extruder.

[0032] Among them, a method of homogeneously dispersing aggregated inorganic particles in a monomer liquid that is part of the polyester raw material, filtering it, and adding it to the remainder of the polyester raw material before, during, or after the esterification reaction is preferable. According to this method, since the monomer liquid has a low viscosity, homogeneous dispersion of particles and high-precision filtration of the slurry can be easily performed. In addition, when adding to the remainder of the raw material, the dispersibility of the particles is good and new aggregates are less likely to occur. From such a viewpoint, it is particularly preferable to add it to the remainder of the raw material in a low-temperature state before the esterification reaction.

[0033] Further, after obtaining a polyester containing particles in advance, the number of protrusions on the film surface can be further reduced by a method such as kneading and extrusion of the pellets containing the particles and the pellets not containing the particles (masterbatch method).

[0034] The polyester film may also contain various additives within a range that maintains a preferable range of total light transmittance. Examples of the additives include an antistatic agent, a UV absorber, and a stabilizer.

[0035] The total light transmittance of the polyester film is preferably 85% or more, more preferably 87% or more. If the transmittance is 85% or more, sufficient visibility can be ensured. Although it can be said that the higher the total light transmittance of the polyester film, the better, from the aspect of stable production, 99% or less is preferable, and 97% or less may be acceptable.

[0036] A treatment for improving the adhesion to a resin for forming a hard coat layer or the like can be performed on the surface of the polyester film of the present invention.

[0037] Examples of the surface treatment method include unevenness treatment such as sandblast treatment and solvent treatment, and oxidation treatment such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone-ultraviolet irradiation treatment, flame treatment, chromic acid treatment, and hot air treatment, and can be used without particular limitation.

[0038] Also, the adhesion can be improved by an adhesion improvement layer such as an easy adhesion layer. As the easy adhesion layer, an acrylic resin, a polyester resin, a polyurethane resin, a polyether resin, etc. can be used without particular limitation, and can be formed by a general coating method, preferably a so-called in-line coat formulation.

[0039] The above polyester film can be manufactured through a polymerization process in which inorganic particles are homogeneously dispersed and filtered in a monomer solution that is part of the polyester raw material, and then added to the remaining portion of the polyester raw material to perform the polymerization of the polyester, and a film forming process in which the polyester is melt-extruded into a sheet shape through a filter, cooled, and then stretched to form a base film.

[0040] Next, a method for manufacturing a biaxially stretched polyester film will be described in detail using an example in which polyethylene terephthalate (hereinafter sometimes referred to as PET) pellets are used as the raw material for the base film, but it is not limited thereto. Also, the number of layers such as single-layer configuration and multi-layer configuration is not limited.

[0041] After mixing and drying PET pellets at a predetermined ratio, they are supplied to a known extrusion machine for melt lamination, extruded into a sheet shape from a slit die, and cooled and solidified on a casting roll to form an unstretched film. In the case of a single layer, one extruder may be used, but when manufacturing a multi-layer film, two or more extruders, a manifold or a confluence block with two or more layers (for example, a confluence block having a square confluence part) are used to laminate a plurality of film layers constituting each outermost layer, extrude two or more sheets from the die, and cool them with a casting roll to form an unstretched film.

[0042] In this case, during melt extrusion, it is preferable to perform high-precision filtration at an arbitrary location where the molten resin is maintained at about 280°C to remove foreign substances contained in the resin. The filter medium used for the high-precision filtration of the molten resin is not particularly limited, but a filter medium of a stainless steel sintered body is preferable because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, Cu and high-melting-point organic substances.

[0043] Furthermore, the filtration particle size of the filter medium (initial filtration efficiency of 95%) is preferably 20 μm or less, particularly preferably 15 μm or less. When the filtration particle size of the filter medium (initial filtration efficiency of 95%) exceeds 20 μm, foreign matters with a size of 20 μm or more cannot be sufficiently removed. Although the productivity may decrease when performing high-precision filtration of the molten resin using a filter medium with a filtration particle size of 20 μm or less (initial filtration efficiency of 95%) for the filter medium, it is preferable for obtaining a film with few protrusions caused by coarse particles.

[0044] (Regarding the maximum heat shrinkage rate) In the present invention, the maximum heat shrinkage rate after heat treatment at 150 °C for 30 minutes of the polyester film is preferably 1.5% or less, more preferably 1.3% or less, still more preferably 1.0% or less, and particularly preferably 0.5% or less. When the maximum heat shrinkage rate is 1.5% or less, flatness defects such as curl and undulation during hard coat processing can be suppressed. Although it can be said that the lower the heat shrinkage rate, the better, it is preferably -1.0% or more, and more preferably 0% or more. A negative heat shrinkage rate means expansion after heating, and flatness defects may occur even when it is less than -1.0%. The maximum heat shrinkage rate can be effectively adjusted by adjusting the draw ratio or using offline annealing treatment or aging treatment.

[0045] (Regarding the hold angle) In the present invention, the hold angle in the bending direction is preferably 155° or more, more preferably 158° or more, and even more preferably 160° or more. Here, the hold angle refers to the angle formed by the fold after fixing at room temperature for 72 hours so that a strain of 1.7% is applied to both surfaces of the bent portion by the calculation described below. In the present invention, the bending direction refers to the direction orthogonal to the folding portion (reference numeral 21) assumed in the use of the foldable display, as shown by reference numeral 22 on the polyester film (reference numeral 2) in FIG. 2. If the hold angle is 155° or less, after folding and then opening the display, deformation of the film may occur, which may adversely affect the function of the display, such as reducing the visibility of the display. If the hold angle is 155° or more, deformation is less and good visibility can be maintained. The hold angle can be effectively adjusted by controlling the refractive index by adjusting the draw ratio and draw temperature. Further, a relaxation process in the drawing direction, an offline annealing process, and an aging process may be used to increase the hold angle. The hold angle in the bending direction is most preferably 180°, but may be 175° or less, or may be 170° or less.

[0046] (Regarding offline annealing treatment) In the present invention, in order to increase the hold angle and also to reduce the maximum heat shrinkage rate, after once winding the produced film into a roll shape, an annealing treatment can also be performed offline. The temperature of the annealing treatment is 150°C or more and 200°C or less, more preferably 170°C or more and 190°C or less. The time for applying the temperature is preferably 3 seconds or more and 90 seconds or less, and more preferably 5 seconds or more and 60 seconds or less. By setting within the above temperature range and time range, the target annealing treatment can be achieved and a good film with transparency maintained can be obtained.

[0047] (Regarding offline aging treatment) In the present invention, in order to increase the holding angle and reduce the maximum heat shrinkage rate, after the film manufactured for this purpose is once wound into a roll, an aging treatment can also be performed offline. The temperature for the aging treatment is preferably 50°C or higher and 70°C or lower, more preferably 55°C or higher and 65°C or lower. The treatment time is preferably 72 hours or longer, and more preferably 120 hours or longer. In the offline aging treatment, crystallization does not progress, and conformational changes occur in the amorphous part, which is considered to result in densification. In the fatigue due to the tensile stress applied to the outside during folding, it is considered that elongation occurs in the amorphous part, so it is considered that the tensile fatigue can be reduced by densification. Although there is no upper limit for the aging time, if it is too long, production may be difficult, so it is preferably within one month, may be 480 hours or less, or may be 360 hours or less.

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

[0049] The draw 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 times, and more preferably 1.7 to 2.0 times. By reducing the draw ratio in the bending direction, the stress applied to the folded part during folding can be reduced, and the holding angle in the bending direction can be increased by suppressing compression and tensile fatigue.

[0050] The draw 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 draw ratio in the direction perpendicular to the bending direction, a maximum heat shrinkage rate of 1.5% or less can be achieved.

[0051] When stretching 1.2 to 2.0 times in the bending direction, the stretching temperature is preferably 75 to 120°C, more preferably 75 to 105°C. In addition, as the heating method during stretching, conventionally known means such as hot air heating method, roll heating method, infrared heating method, etc. can be adopted. By setting the stretching temperature to 75 to 120°C, significant thickness unevenness due to stretching at the above stretching ratio can be prevented.

[0052] The preheating temperature for stretching in the direction perpendicular to the bending direction is preferably 70 to 110°C. When performing multi-stage stretching in the direction perpendicular to the bending direction, it is preferable to increase the stretching ratio in the second and subsequent stages compared to the first stage. Film relaxation may be carried out by 1 to 10% in either the machine flow direction (longitudinal direction) or the perpendicular direction (width direction).

[0053] (Regarding the density of the polyester film) The density of the polyester film is preferably 1.380 g / cm 3 or more. It is more preferably 1.383 g / cm 3 . By making it 1.380 g / cm 3 or more, the flexibility can be improved, and the surface hardness of the film, particularly the pencil hardness of the hard coat film after laminating the hard coat layer, can be improved. The higher the density, the better, although it is also somewhat affected by the presence or absence of particles in the film, etc., but it is preferably 1.40 g / cm 3 or less. By setting the heat setting temperature during film formation to 180 to 240°C, crystallization can proceed and the density can be effectively increased.

[0054] The intrinsic viscosity of the polyester film preferably ranges from 0.50 to 1.0 dl / g. When the intrinsic viscosity is 0.50 dl / g or more, the impact resistance is improved, and it is preferable that the disconnection of the internal circuit of the display due to external impact hardly occurs. On the other hand, when the intrinsic viscosity is 1.00 dl / g or less, the filtration pressure rise of the molten fluid does not become too large, and the film production is stable, which is preferable.

[0055] (Easy adhesion layer) In the present invention, in order to improve the adhesion between a polyester film and a hard coat layer or the like, it is also preferable to laminate an easy-adhesion layer on the polyester film. The easy-adhesion layer can be obtained by applying a coating liquid for forming the easy-adhesion layer to one or both sides of an unstretched or uniaxially stretched film in the longitudinal direction, then heat-treating and drying as necessary, and further stretching in at least one direction that has not been stretched. Heat treatment can also be performed after biaxial stretching. The final coating amount of the easy-adhesion layer is preferably controlled to be 0.005 to 0.20 g / m 2 It is preferable to control it to be 0.005 g / m 2 or more, as adhesion can be obtained, which is preferable. On the other hand, if the coating amount is 0.20 g / m 2 or less, blocking resistance can be obtained, which is preferable.

[0056] As the resin to be contained in the coating liquid used for laminating the easy-adhesion layer, for example, polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. can be used without particular limitation. Examples of the cross-linking agent to be contained in the coating liquid for forming the easy-adhesion layer include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, carbodiimide compounds, etc. Two or more of them can also be mixed and used. Due to the nature of in-line coating, these are preferably coated with an aqueous coating liquid, and the above-mentioned resins and cross-linking agents are preferably water-soluble or water-dispersible resins and compounds.

[0057] It is preferable to add particles to the easy-adhering layer in order to impart slipperiness. The average particle size of the fine particles is preferably 2 μm or less. When the average particle size of the particles exceeds 2 μm, the particles are likely to fall off from the easy-adhering layer. Examples of the particles to be included in the easy-adhering layer include inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, calcium fluoride, etc., and organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, silicone-based, etc. These may be added to the easy-adhering layer alone, or two or more of them may be combined and added.

[0058] Also, as a method of applying the coating liquid, a known method can be used in the same manner as the above-described coating layer. For example, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, etc. can be mentioned, and these methods can be carried out alone or in combination.

[0059] (Hard coat layer) When used as a surface protection film for protecting a display by positioning the polyester film of the present invention on the surface of a foldable display, it preferably has a hard coat layer on at least one of its surfaces. The hard coat layer is preferably positioned on the display surface side on the polyester film and used in the display. As the resin for forming the hard coat layer, acrylic-based, siloxane-based, inorganic hybrid-based, urethane acrylate-based, polyester acrylate-based, epoxy-based, etc. can be used without particular limitation. Also, two or more kinds of materials can be mixed and used, and particles such as inorganic fillers and organic fillers can be added.

[0060] (Film thickness of the hard coat layer) The film thickness of the hard coat layer is preferably 1 to 50 μm. When it is 1 μm or more, it is sufficiently cured and the pencil hardness becomes high, which is preferable. Also, by making the thickness 50 μm or less, curl due to the curing shrinkage of the hard coat can be suppressed, and the handleability of the film can be improved.

[0061] (Coating method) As the coating method of the hard coat layer, a Meyer bar, a gravure coater, a die coater, a knife coater, etc. can be used without particular limitation, and can be appropriately selected according to the viscosity and film thickness.

[0062] (Curing conditions) As the curing method of the hard coat layer, energy rays such as ultraviolet rays and electron beams, and curing methods by heat can be used. In order to reduce damage to the film, a curing method by ultraviolet rays or electron beams is preferable.

[0063] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or more, and more preferably 4H or more. If the pencil hardness is 3H or more, it is not easily scratched and does not reduce visibility. Generally, the higher the pencil hardness of the hard coat layer, the better, but it may be 9H or less, 8H or less, or 6H or less, and it can be used practically without problems.

[0064] (Properties of the hard coat layer) The hard coat layer in the present invention can be used for the purpose of protecting the 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, more preferably 88% or more. If the transmittance is 87% or more, sufficient visibility can be obtained. Generally, the higher the total light transmittance of the hard coat film, the better, but from the perspective of stable production, it is preferably 99% or less, and may be 97% or less. Further, the haze of the hard coat film is generally preferably low, and preferably 3% or less. The haze of the hard coat film is more preferably 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. Generally, the lower the haze, the better, but from the perspective of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0065] Other functions may be further added to the hard coat layer. For example, a hard coat layer added with functions such as an antiglare layer having a certain pencil hardness as described above, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer is also preferably applied in the present invention.

[0066] Also, a hard coat layer may be provided when it is used as a base film of a touch panel module. When, for example, an ITO layer is used as the transparent electrode layer of the touch panel module, in order to make the electrode pattern less visible, it is preferable to provide a refractive index adjustment layer between the base film and the transparent electrode layer. In this case, the hard coat layer itself may also serve as a refractive index adjustment layer, or a separate refractive index adjustment layer may be further laminated.

Examples

[0067] Next, the present invention will be described using examples and comparative examples. First, the evaluation methods of the characteristic values implemented in the present invention are shown below.

[0068] (1) Limiting viscosity After the film or polyester resin was pulverized and dried, it was dissolved in a mixed solvent of phenol / tetrachloroethane = 60 / 40 (mass ratio). After subjecting this solution to centrifugation to remove inorganic particles, using an Ubbelohde viscometer, the flow-down time of a solution with a concentration of 0.4 (g / dl) at 30 °C and the flow-down time of only the solvent were measured. From the ratio of those times, assuming that the Huggins constant is 0.38, the intrinsic viscosity was calculated using the Huggins equation.

[0069] (2) Refractive index In accordance with JIS K 7142:2008 "Method for Measuring Refractive Index of Plastics (Method A)", using an Abbe refractometer (manufactured by Atago Co., Ltd., NAR-4T, measurement wavelength 589 nm), the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction were determined.

[0070] (3) Pencil hardness Using the hard coat film as a sample, in accordance with JIS K 5600-5-4:1999, the pencil hardness was measured at a load of 750 g and a speed of 1.0 mm / s. In the present invention, 3H or higher was regarded as passing.

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

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

[0073] (6) Maximum thermal shrinkage rate The sample film was cut into a size of 10 mm in length and 250 mm in width. Marks were made at 200 mm intervals along the long side in the direction where measurement was desired, and the interval A between the marks was measured under a constant tension of 5 g. Subsequently, the sample film was left in an oven at 150 °C in a non-loaded atmosphere for 30 minutes, then taken out of the oven and cooled to room temperature. Thereafter, the interval B between the marks was determined under a constant tension of 5 g, and the heat shrinkage rate (%) was determined by the following formula. The heat shrinkage rate was measured at positions equally divided into three in the width direction of the sample film, and the average value of the three points was taken as the heat shrinkage rate (%). Heat shrinkage rate (%) = [(A - B) × 100] / A For the bi-direction of the bending direction and the folding direction, the sample film was cut so that the length and width were different, and measured separately. The data in the direction with the larger measured value was taken as the maximum heat shrinkage rate (%).

[0074] (7) Hold angle The strength of the fold formed when a strain of 1.7% was applied to both surfaces of the bent portion was evaluated. Figure 3 is a schematic diagram for explaining the measurement method of the hold angle in the bending direction. The sample film (reference numeral 3) was cut into 10 mm in the width direction and 50 mm in the flow direction. Two PTFE plates (reference numeral 31) were overlapped, and in the case of a 50 μm sample film, a PTFE plate (reference numeral 32) with a thickness of 3 mm was inserted as a spacer to create a gap. Double-sided tape was attached to both ends of the sample, and it was inserted into the 3 mm gap of the PTFE plate in a bent state, and both ends were fixed with double-sided tape. After being placed in an environment of 20 °C and 65% RH for 72 hours, the angle (reference numeral 33) formed by the crease on the film was measured 5 minutes after taking it out from between the two PTFE plates (reference numeral 32). This angle was taken as the hold angle. To keep the strain constant, the thickness of the PTFE plate used as a spacer was changed according to the thickness of the film. Figure 4 shows an enlarged schematic diagram of the sample film (reference numeral 4) sandwiched between two PTFE plates (reference numeral 32). The neutral plane where neither the above-mentioned compressive stress nor tensile stress is applied was defined as the center in the thickness direction, and the difference between the neutral plane and both surfaces was taken as the strain. That is, the strain applied to both surfaces can be expressed by the following formula. Strain (1.7%) =(|Semicircumference of the outermost or inner surface - Semicircumference of the neutral plane| / Semicircumference of the neutral plane) × 100 Here, when the semicircumference is the thickness t (mm) of the sample film and the bending diameter (outermost diameter), that is, the thickness of the spacer used is d (mm), they can be obtained respectively by the following formulas. Semicircumference of the outermost surface = d × π / 2 Semicircumference of the neutral plane = (d - t) × π / 2 Semicircumference of the innermost surface = (d - 2t) × π / 2 From the above, when determining the strain to be 1.7%, with the thickness t (mm) and the bending diameter, that is, the thickness of the spacer used as d (mm), the thickness of the PTFE plate for the spacer used is determined by the following formula. Table 1 shows the spacer thickness for typical film thicknesses. Spacer thickness d (mm) = Film thickness (mm) × 60 Incidentally, in the case of the sample film with the above thickness of 50 μm, the diameter of the outermost circle (symbol 41) is the same as the thickness d of the spacer and is 3 mm. The diameter of the innermost surface (symbol 43) is 2.9 mm, and the diameter of the neutral plane (symbol 42) is 2.95 mm.

[0075]

Table 1

[0076] (Preparation of polyethylene terephthalate pellets (a)) As an esterification reactor, a continuous esterification reactor consisting of a three-stage completely mixed tank having a stirrer, a partial condenser, a raw material inlet, and a product outlet was used. With TPA at 2 tons / hr, EG at 2 moles per 1 mole of TPA, and antimony trioxide in an amount such that the Sb atoms are 160 ppm with respect to the produced PET, these slurries were continuously fed into the first esterification reactor of the esterification reactor and reacted at 255 °C with an average residence time of 4 hours at normal pressure. Next, the reaction product in the first esterification reactor was continuously taken out of the system and supplied to the second esterification reactor. EG distilled off from the first esterification reactor was supplied to the polymer to be produced (produced PET) in an amount of 8% by mass. Further, an EG solution containing magnesium acetate in an amount such that the Mg atom content was 65 ppm with respect to the produced PET and an EG solution containing TMPA in an amount such that the P atom content was 20 ppm with respect to the produced PET were added. The reaction was carried out at 260°C with an average residence time of 1.5 hours under normal pressure. Next, the reaction product in the second esterification reactor was continuously taken out of the system and supplied to the third esterification reactor. Further, an EG solution containing TMPA in an amount such that the P atom content was 20 ppm with respect to the produced PET was added. The reaction was carried out at 260°C with an average residence time of 0.5 hours under normal pressure. The esterification reaction product produced in the third esterification reactor was continuously supplied to a three-stage continuous polycondensation reactor for polycondensation. Further, it was filtered through a filter medium made of a sintered stainless steel body (nominal filtration accuracy: 90% cut-off for 5-μm particles) to obtain polyethylene terephthalate pellets (a) having an intrinsic viscosity of 0.62 dl / g.

[0077] (Preparation of polyethylene terephthalate pellets (b)) Regarding the production process of polyethylene terephthalate pellets (a), the intrinsic viscosity was adjusted to 0.580 dl / g in the same manner 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)) Polyethylene terephthalate pellets (a) were subjected to solid-phase polymerization at 220°C for varying times under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus to prepare polyethylene terephthalate pellets (c) having an intrinsic viscosity of 0.75 dl / g.

[0079] (Polymerization of urethane resin) Into a four-necked flask equipped with a stirrer, Dimroth condenser, nitrogen inlet tube, silica gel drying tube, and thermometer, 72.96 parts by mass of 1,3-bis(isocyanatomethyl)cyclohexane, 12.60 parts by mass of dimethylolpropionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol with a number average molecular weight of 2000, 85.00 parts by mass of acetonitrile as a solvent, and 5.00 parts by mass of N-methylpyrrolidone were charged. Under a nitrogen atmosphere, the mixture was stirred at 75 °C for 3 hours, and it was confirmed that the reaction solution reached a predetermined amine equivalent. Next, after cooling the reaction solution to 40 °C, 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer D solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisper capable of high-speed stirring, adjusted to 25 °C, and while stirring and mixing at 2000 min-1, an isocyanate group-terminated prepolymer was added and dispersed in water. Thereafter, under reduced pressure, a part of acetonitrile and water was removed to prepare a water-soluble polyurethane resin (A) with a solid content of 35% by mass.

[0080] (Polymerization of water-soluble carbodiimide compound) Into a flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer, 200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidization catalyst were charged. Under a nitrogen atmosphere, the mixture was stirred at 180 °C for 10 hours to obtain an isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Next, 111.2 g of the obtained carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100 °C for 24 hours. Water was gradually added thereto at 50 °C to obtain a yellow transparent water-soluble carbodiimide compound (B) with a solid content of 40% by mass.

[0081] (Preparation of coating liquid for forming an easy-adhesion layer) The following coating agents were mixed to prepare a coating liquid. Water 16.97 parts by mass Isopropanol 21.96 parts by mass Polyurethane resin (A) 3.27 parts by mass 1.22 parts by mass of water-soluble carbodiimide compound (B) 0.51 part by mass of particles (Silica sol with an average particle size of 40 nm, solid content concentration: 40% by mass) 0.05 part by mass of surfactant (Silicone-based, solid content concentration: 100% by mass)

[0082] (Preparation of hard coat coating liquid a) To 100 parts by weight of a hard coat material (manufactured by JSR Corporation, Optstar (registered trademark) Z7503, concentration: 75%), 0.1 part by weight of a leveling agent (manufactured by BYK Chemie Japan, BYK307, concentration: 100%) was added, and it was diluted with methyl ethyl ketone to prepare a hard coat coating liquid a with a solid content concentration of 40% by weight.

[0083] (Example 1) Pellets (a) of polyethylene terephthalate were fed into an extruder and melted at 285°C. This polymer was filtered through a filter medium of a stainless steel sintered body (nominal filtration accuracy: 95% cut of 10-μm particles), extruded into a sheet form from a die, and then brought into contact with a casting drum having a surface temperature of 30°C using an electrostatic application casting method to be cooled and solidified, thereby producing an unstretched film. This unstretched film was uniformly heated to 75°C using a heating roll, heated to 85°C using a non-contact heater, and subjected to roll stretching (longitudinal stretching) at a ratio of 1.4 times. After applying the above-mentioned coating liquid for forming an easy-adhesion layer to both surfaces of the obtained uniaxially stretched film by a roll coating method, it was dried at 80°C for 20 seconds. Note that the coating amount after drying in the final (after biaxial stretching) was adjusted to be 0.06 g / m 2 . Then, it was led to a tenter, preheated at 105°C, laterally stretched 4.4 times at 95°C, width-fixed, heat-fixed at 220°C for 5 seconds, and further relaxed by 4% in the width direction at 180°C to obtain a polyethylene terephthalate film roll with a thickness of 50 μm. An offline annealing treatment was carried out at 180°C for 30 seconds to obtain a polyester film. The evaluation results are shown in Table 2.

[0084] (Example 2) The film roll obtained in the same manner as in Example 1 was not subjected to offline annealing treatment. Instead, it was subjected to an aging treatment at 60 °C for one 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 draw ratio in the longitudinal direction was changed to 2.7 times and the draw ratio in the width 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 draw ratio in the longitudinal direction was changed to 2.7 times and the draw ratio in the width 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 draw ratio in the longitudinal direction was changed to 3.4 times, the draw ratio in the width direction was changed to 4.0 times, the heat setting temperature was changed to 230 °C, and the annealing time was changed to 10 seconds.

[0088] (Example 6) A polyester film was obtained in the same manner as in Example 1, except that the draw ratio in the longitudinal direction was changed to 3.4 times, the draw ratio in the width 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 draw ratio in the longitudinal direction was changed to 3.4 times, the draw ratio in the width 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 draw ratio in the longitudinal direction was changed to 3.4 times, the draw ratio in the width 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 draw ratio in the longitudinal direction was changed to 3.4 times, the draw ratio in the width direction was changed to 4.0 times, the heat setting temperature was changed to 230 °C, and the aging time was changed to 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) An untreated polyester film was obtained by not performing annealing and aging treatments on the film roll obtained in the same manner as in Example 1.

[0097] (Comparative Example 2) An untreated polyester film was obtained by not performing annealing and aging treatments on the film roll obtained in the same manner as in Examples 3 to 4.

[0098] (Comparative Example 3) An untreated polyester film was obtained by not performing annealing and aging treatments on the film roll obtained in the same manner as in Examples 5 to 9.

[0099] Using a Mayer bar, a hard coat coating liquid a was applied to one surface of the above-prepared film so that the film thickness after drying was 5 μm, dried at 80°C for 1 minute, and then irradiated with ultraviolet rays (integrated light quantity 200 mJ / cm 2 ), and a hard coat film was obtained.

[0100]

Table 2A

[0101]

Table 2B

[0102] The hard coat film was bonded to the organic EL module via an adhesive layer with a thickness of 25 μm to create a smartphone-type foldable display that can be folded in half at the center with a radius corresponding to the bending radius in FIG. 1 of 3 mm. The hard coat film is arranged on the surface of a single continuous display via the folding portion, and the hard coat layer is arranged so as to be located on the surface of the display. The use of the hard coat film of each example satisfied the operation and visibility as a smartphone that can be folded in half at the center and carried around. Also, the surface did not dent due to external force. On the other hand, the foldable display using the hard coat film of each comparative example seemed to cause image distortion at the folding portion of the display as the usage frequency increased, and was not very preferable. Also, some had dents and scratches on the surface.

Industrial Applicability

[0103] The foldable display using the polyester film or hard coat film for a foldable display of the present invention does not cause deformation after the polyester film or hard coat film located on the surface of the foldable display is repeatedly folded while maintaining mass productivity, and thus does not cause image disturbance at the folding portion of the display. In particular, a portable terminal device or an image display device equipped with a foldable display using the polyester film or hard coat film of the present invention as a surface protection film provides beautiful images, is rich in functionality, and is excellent in convenience such as portability.

Explanation of Signs

[0104] 1: Foldable display 11: Bending radius 2: Polyester film for surface protection film of foldable display 21: Folding part 22: Bending direction (direction orthogonal to folding part) 3: Sample film 31: PTFE plate 32: Spacer (PTFE plate) 33: Holding angle 4: Sample film 41: Diameter of outermost surface 42: Diameter of neutral plane 43: Diameter of innermost surface

Claims

1. The holding angle in the bending direction is 160° or more, the maximum heat shrinkage rate at 150 °C is 1.5% or less, 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 or more and 1.40 g / cm ３ or less, and A polyester film for a foldable organic EL display with a total light transmittance of 85% or more and a haze of 3% or less. (Here, the holding angle refers to the angle formed by the fold after being fixed at room temperature for 72 hours so that a strain of 1.7% is applied to both surfaces of the bent portion. The bending direction refers to the direction perpendicular to the folded portion.)

2. The polyester film has an easy - adhesion layer on at least one side, 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. The polyester film for a foldable organic EL display according to Claim 1.

3. At least one side of the polyester film for a foldable display according to Claim 1 or 2 has a hard - coat layer with a thickness of 1 to 50 μm, The hard - coat layer contains any one of an acrylic - based resin, a siloxane - based resin, an inorganic hybrid - based resin, a urethane acrylate - based resin, a polyester acrylate - based resin, and an epoxy - based resin. A hard - coat film for a foldable organic EL display.

4. An organic EL module is provided, A foldable organic EL display in which the hard - coat film for a foldable display according to Claim 3 is arranged as a surface - protection film with the hard - coat layer positioned on the surface, and a single continuous hard - coat film is arranged through the folded portion of the foldable display.

5. A portable terminal device having the foldable organic EL display according to Claim 4.

Images