Polyester film and application thereof

Abstract

To provide a polyester film for a touch panel module substrate of a foldable display which is excellent in mass productivity and causes no cracking at a folded part in order to provide a foldable display with no risk of distortion of an image displayed at the folded part after repeated folding.SOLUTION: There is provided a polyester film for a touch panel module substrate of a foldable display which satisfies the following conditions. (1) The refractive index in the bending direction is 1.590 to 1.620. (2) The refractive index in the direction of a folded part is 1.670 to 1.700. (3) The refractive index in the direction of thickness is 1.520 or less. (4) The density is 1.380 g / cm3 or more (here, the bending direction refers to the direction perpendicular to the folded part when the polyester film is folded.)SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a polyester film for a touch panel module base material of a foldable display, a hard coat film for a touch panel module base material of a foldable display, a foldable display, and a portable terminal device, and 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 the hard coat film for a touch panel module base material of 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 need to have a small screen size of about 6 inches because simple operations can be performed with one hand and they are assumed to be stored in a pocket of clothes or the like.

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

[0004] To achieve these, a method of making a device compact by connecting a plurality of displays has been proposed (see Patent Document 1), but since the bezel part remains, the video is interrupted, and the decrease in visibility becomes a problem and it has not become widespread.

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

[0006] Here, for displays and mobile 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 using 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, causing problems such as distorting the image displayed on the display. In addition to the surface protection film, various parts of the foldable display use films, 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 protective member on the back surface. Durability against repeated folding is also required for these films.

[0007] Therefore, a method of partially changing the film thickness has been proposed (see Patent Document 2), 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 (see Patent Document 3). However, as the refractive index in the bending direction is decreased, the pencil hardness during hard coat application decreases, resulting in a problem of deterioration of the surface protection function of the display. In addition, although the deformation during folding improves as the refractive index in one direction is decreased, there is a problem that the uniaxial orientation in the folding direction increases, causing cracks or breakage in the folding part.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

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 repeated folding, and a portable terminal device equipped with such a foldable display. Therefore, it is intended to provide a polyester film for a touch panel module substrate of 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 configuration. 1. A polyester film for a touch panel module substrate of a foldable display that satisfies the following conditions. (1) The refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding portion is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less (4) The density is 1.380 g / cm 3 or more (Here, the bending direction means the direction orthogonal to the folding portion when the polyester film is folded.) 2. The polyester film for a touch panel module substrate of the foldable display according to the first item above, wherein the elastic modulus in the bending direction is 2.7 GPa or less and the elastic modulus in the direction of the folding portion is 4.5 GPa or more. 3. The polyester film for a touch panel module substrate of the foldable display according to the first or second item above, wherein the total light transmittance is 85% or more, the haze is 3% or less, and the maximum heat shrinkage rate is 2% or less. 4. In the evaluation of the bending resistance after heat treatment at 150°C for 30 minutes (bending radius: 1.5 mm), the polyester film for a touch panel module substrate of the foldable display according to any one of the above 1 to 3, in which cracks or deformations cannot be confirmed. 5. The polyester film for a touch panel module substrate of the foldable display according to any one of the above 1 to 4, which has an easy adhesion layer on at least one surface of the polyester film for a touch panel module substrate of the foldable display. 6. The polyester film for a touch panel module substrate of the foldable display according to any one of the above 1 to 5, which has a hard coat layer with a thickness of 1 to 50 μm on at least one surface of the polyester film for a touch panel module substrate of the foldable display. 7. The foldable display in which the polyester film for a touch panel module substrate of the foldable display according to any one of the above 1 to 6 is included as a substrate film of the touch panel module, and a substrate film of a single touch panel module continuous through the folding part of the foldable display is included. 8. A portable terminal device having the foldable display according to the above 7.

Advantages of the Invention

[0012] The foldable display using the polyester film for a touch panel module substrate of the foldable display of the present invention maintains mass productivity, and the polyester film does not generate cracks in the folding part, does not cause deformation after repeated folding, and does not cause image distortion in the folding part of the display. A portable terminal device equipped with a foldable display using the polyester film as described above for a touch panel module substrate 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

Embodiments for Carrying out the Invention

[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, LCDs, organic ELs, and inorganic ELs having a foldable structure are preferred. In particular, organic ELs and inorganic ELs that can reduce the layer structure are particularly preferred, and organic ELs with a wide color gamut are even more preferred.

[0015] (Foldable Display) A foldable display is one in which a continuous single display can be folded, such as being folded in half, when carried. By folding, the size can be halved and the portability can be improved. The bending radius of the foldable 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 becomes possible. It can be said that the smaller the bending radius, the better. However, 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, practically sufficient thinning can be achieved when carried. The bending radius when folded means measuring the position of reference numeral 11 in the schematic diagram of FIG. 1, and refers to the inner radius of the folded portion when folded. Incidentally, the surface protection film described later may be located on the outside of the folded foldable display or may be located on the inside. In addition, the foldable display may be tri-fold or quad-fold, or may even be a rollable type that can be wound up, and all of these are within the scope of the foldable display as referred to in the present invention.

[0016] The polyester film for a foldable display of the present invention may be used in any part as long as it is a component of a foldable display. Hereinafter, taking an organic EL display as an example, the representative configuration of a foldable display and the parts where the polyester film of the present invention can be used will be described. In the following, the polyester film for a foldable display of the present invention may sometimes be simply referred to as the polyester film of the present invention.

[0017] (Foldable organic EL display) As an essential configuration of a foldable organic EL display, there is an organic EL module. Further, if necessary, a circularly polarized light plate, a touch panel module, a front protection film, a back protection film, etc. are provided.

[0018] (Touch panel module) A portable terminal device preferably has a touch panel. When using an organic EL display, it is preferably arranged on the viewing side of the organic EL module, and further preferably, a touch panel module is arranged between the organic EL module and the circularly polarized light plate. The touch panel module preferably has a transparent conductive film having a transparent base material such as a film and a transparent conductive layer arranged thereon. The polyester film in the present invention can be used as the transparent base material of this transparent conductive film. When used as the transparent base material of the transparent conductive film, it is preferable to provide a refractive index adjustment layer or a hard coat layer on the polyester film.

[0019] (Transparent conductive layer) In the present invention, the conductive layer preferably included in the touch panel module may be a layer that is transparent and has conductivity, and is not particularly limited. Examples thereof include a conductive filler-containing layer, a metal layer, a metal oxide layer, and a conductive polymer-containing layer.

[0020] Here, the term "transparent" means that in the state processed to function as a touch panel, it is sufficient that it appears transparent to the naked eye, and it is not necessarily required that the site itself having conductivity is transparent. For example, if an electrode pattern is provided so that the conductive layer functions as a touch panel, even if the wiring itself is opaque with a metal such as gold, the conductive layer can be regarded as transparent as long as the electrode pattern cannot be seen and the image can be observed when the touch panel is viewed with the naked eye.

[0021] As the conductive filler in the conductive filler-containing layer, fillers and fibers of metals such as gold, silver, copper, aluminum, nickel, titanium, iron, zinc, tin, and alloys thereof, metal oxide fillers, metal-coated synthetic fibers, and conductive carbon fibers such as carbon nanotubes are preferable. As fillers of metals, alloys, and metal oxides, those having various shapes such as spherical particles, flat particles, flake-like particles, needle-like particles, and fibrous particles can be used. Among these, in terms of flexural resistance, flake-like particles, needle-like particles, and fibrous fillers (fibers of metals and their alloys, fibers of metal oxides, metal-coated synthetic fibers, conductive carbon fibers) are preferable, and fibrous fillers are more preferable.

[0022] It is preferable that a binder resin is used in the conductive filler-containing layer. Examples of the binder resin include polyester resin, polyurethane resin, polyamide resin, and acrylic resin. Further, it is preferable that these resins are crosslinked. The crosslinking agent may be used according to each resin, and examples thereof include isocyanate compounds, epoxy resins, melamine compounds, oxazolines, carbodiimides, and compounds having two or more double bonds. The content of the conductive filler is preferably 10 to 400 parts by mass with respect to 100 parts by mass of the resin component constituting the conductive layer.

[0023] The conductive filler-containing layer can be provided by a coating method. The electrode pattern can be formed by a method of processing by chemical etching or laser etching after coating, or by a method of providing by printing. Examples of printing methods include gravure printing, letterpress printing, offset printing, screen printing, inkjet printing, etc., and can be selected according to the properties of the paint and the fineness of the pattern.

[0024] Examples of the metals for the metal layer include metals such as gold, silver, copper, aluminum, nickel, titanium, iron, zinc, and tin. The metal layer can be provided by a vapor deposition method, a sputtering method, etc., and it is preferable to process the electrode pattern by chemical etching or laser etching after providing the metal layer.

[0025] Examples of the metal oxide layer include ZnO, CeO2, Sb2O3, SnO2, indium tin oxide (abbreviation: ITO), In2O3, antimony-doped tin oxide (abbreviation: ATO), aluminum-doped zinc oxide (abbreviation: AZO), etc. The metal oxide layer can be provided by a sputtering method, etc., and it is preferable to process the electrode pattern by chemical etching or laser etching after providing the metal oxide layer.

[0026] For the conductive polymer-containing layer, as the conductive polymer, for example, aromatic conjugated poly(p-phenylene), heterocyclic conjugated polypyrrole, polythiophene, aliphatic conjugated polyacetylene, heteroatom-containing conjugated polyaniline, mixed conjugated poly(phenylene vinylene), a double-chain conjugated system which is a conjugated system having a plurality of conjugated chains in the molecule, and a conductive composite which is a polymer obtained by grafting or block copolymerizing the aforementioned conjugated polymer chain with a saturated polymer, etc., can be used as a high-molecular-weight conductive agent.

[0027] The conductive layer containing the conductive polymer may contain the resin component mentioned in the conductive filler-containing layer. The content of the conductive polymer in the conductive layer containing the conductive polymer can be directly applied to the amount described in the conductive filler-containing layer. The conductive polymer-containing layer can be provided by a coating method, and the electrode pattern can be provided in the same manner as described for the conductive filler-containing layer.

[0028] Also, when producing the conductive layer, heat treatment may be performed to stabilize the conductive film (each additive and layer strengthening). In this case, the heat treatment temperature is preferably 30 °C or higher, more preferably 50 °C or higher, still more preferably 70 °C or higher, particularly preferably 100 °C or higher, preferably 200 °C or lower, more preferably 150 °C or lower, still more preferably 120 °C or lower. When the heating temperature is above the above lower limit, the stabilization of the conductive film is promoted, which is preferable. When the heating temperature is below the above upper limit, the base materials that can be used are not limited to those with high heat resistance, which is preferable. In this case, the base material is required to have high flex resistance such that there are no surface cracks, deformations, or cracks during bending even after heat treatment. The polyester film for the touch panel module base material of the present invention can maintain high flex resistance without losing or deteriorating the flex resistance after heat treatment.

[0029] The polyester film of the present invention is preferably used as the base film of the touch panel module.

[0030] As the base film for the touch panel module of the present invention, a polyester film having specific properties is used. Note that the base film for the touch panel module of the present invention may be simply referred to as the base film or the polyester film.

[0031] 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 a super-multilayer laminated film with a repeating structure.

[0032] Examples of the polyester resin used for the polyester film include, for example, polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or a polyester film composed of a copolymer having the constituent components of these resins as the main component. Among them, a stretched polyethylene terephthalate film is particularly preferred from the viewpoints of mechanical properties, heat resistance, transparency, price, etc.

[0033] When using a copolymer of polyester 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 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 because the film strength, transparency, and heat resistance are maintained. 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 the 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 filtration pressure rise of the molten fluid does not become too large, and it is preferable that the film production can be stably operated easily.

[0034] Also, 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 the 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 filtration pressure rise of the molten fluid does not become too large, and it is preferable that the film production can be stably operated easily.

[0035] The thickness of the polyester film is preferably 10 to 300 μm, more preferably 10 to 80 μm, and even more preferably 25 to 75 μm. When the thickness is 10 μm or more, the effects of improving flex resistance, pencil hardness, and impact resistance can be seen. When the thickness is 300 μm or less, it is advantageous for weight reduction, and it is also excellent in flexibility, processability, handling properties, etc.

[0036] The surface of the polyester film of the present invention may be smooth or may have irregularities. However, since it is used for the touch panel application of the display, a decrease in optical properties due to irregularities is not preferable. The haze is preferably 3% or less, 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. The lower limit of the haze is preferably as small as possible, but from the viewpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0037] For the purpose of reducing the haze as described above, the irregularities on the film surface are preferably not too large. However, from the viewpoint of handling properties, in order to give 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.

[0038] As a method of blending particles into the polyester resin layer, known methods can be adopted. For example, it can be added at any stage of producing polyester, but preferably at the esterification stage, or at the stage 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 can be advanced. Also, it can be carried out by using a vented kneading extruder to blend a slurry of particles dispersed in ethylene glycol or water and a polyester raw material, or by using a kneading extruder to blend dried particles and a polyester raw material.

[0039] Among them, a method is preferred in which, after uniformly dispersing aggregated inorganic particles in a monomer solution that is part of the polyester raw material and then filtering it, the filtered product is added to the remainder of the polyester raw material before, during, or after the esterification reaction. According to this method, since the monomer solution has a low viscosity, uniform dispersion of the particles and highly accurate filtration of the slurry can be easily performed. Also, when adding to the remainder of the raw material, the dispersibility of the particles is good and new aggregates are less likely to form. From such a perspective, in particular, it is preferable to add to the remainder of the raw material in the low-temperature state before the esterification reaction.

[0040] In addition, 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 extruding the pellets containing the particles and the pellets not containing the particles (masterbatch method).

[0041] Also, the polyester film may contain various additives within a range that maintains a preferable range of the total light transmittance. Examples of the additives include an antistatic agent, a UV absorber, and a stabilizer.

[0042] 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, it is preferably 99% or less, and may be 97% or less.

[0043] The maximum heat shrinkage rate of the polyester film after heat treatment at 150°C for 30 minutes is preferably 0 to 2% or less, more preferably 0 to 1.5% or less, and even more preferably 0 to 1.0% or less. If the maximum heat shrinkage rate is 2% or less, flatness defects such as curl and undulation after being attached to the touch panel module or after hard coat coating can be suppressed. When the maximum shrinkage rate is 0% or more, it is less likely to generate curl due to the thermal dimensional change between the polyester film layer and the functional layer after applying various functional layers by post-processing on the polyester film such as hard coat, and the yield when setting on the touch panel is good and preferable.

[0044] The polyester film for a foldable display of the present invention can impart sufficient pencil hardness to the hard coat film after laminating the hard coat layer. It is considered that in the pencil hardness evaluation of the hard coat film of a conventional polyester film, the pencil hardness has decreased because the film is deformed in the thickness direction after laminating the hard coat layer. In the present invention, by setting the indentation depth after unloading the test force in the film thickness direction by the dynamic ultra-micro hardness tester described below within a specific range, high hardness can be achieved in the pencil hardness evaluation of the hard coat film. It is preferably 1.5 μm or less, more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. When the indentation depth after unloading the test force (the final deformation amount under load) is 1.5 μm or less, in the pencil hardness evaluation of the hard coat film after laminating the hard coat layer, it is difficult for the film to be deformed in the thickness direction, and the pencil hardness can be increased. If the pencil hardness of the hard coat film can be increased, it becomes difficult for scratches and dents to occur on the display surface, and the visibility of the display is improved. Although it can be said that the lower the indentation depth after unloading the test force, the better, in terms of stable production and saturation of the effect, 0.3 μm or more is preferable, and further, 0.5 μm or more is preferable.

[0045] In order to reduce the indentation depth after unloading the test force, it is effective to adjust the refractive index in the thickness direction to 1.520 or less. As means for making the refractive index 1.520 or less, as will be described later, within the range where other physical properties, the refractive index in the bending direction and the folding direction can be controlled within a preferable range, examples of condition settings include increasing the draw ratio in the bending direction and the folding direction, setting the draw temperature in the bending direction and the folding direction low, and setting the heat setting temperature high.

[0046] The polyester film for a foldable display of the present invention does not generate creases, cracks, or breaks during folding and can adjust the neutral plane of the display. The neutral plane refers to the plane where compressive stress is applied on the inner side and tensile stress is applied on the outer side when folded, but no stress is applied in between. In a foldable display, generally, the neutral plane is designed for the organic EL layer. The neutral plane can be adjusted according to the elastic modulus and thickness of each layer. Therefore, the elastic modulus in the bending direction of the polyester film is preferably 2.7 GPa or less, more preferably 2.6 GPa or less, and even more preferably 2.5 GPa or less. By reducing the elastic modulus in the bending direction, it can be said that the bendability is improved, but for adjusting the neutral plane, it is preferably 1.8 GPa or more. The elastic modulus in the folding direction is preferably 4.5 GPa or more, more preferably 4.6 GPa or more, and even more preferably 4.7 GPa or more. By increasing the elastic modulus in the folding direction, the flatness of the display surface can be maintained during display production. Also, the touch panel module can be protected from external impacts. Although the higher the elastic modulus in the folding direction is, the better, from the viewpoint of film-forming properties, it is preferably 8.0 GPa or less.

[0047] On the surface of the polyester film of the present invention, a treatment can be performed to improve the adhesion to a resin for forming a transparent conductive layer, an adhesive layer, a hard coat layer, etc.

[0048] Examples of the method by surface treatment include roughening treatment such as sandblasting treatment, solvent treatment, etc., and oxidation treatment such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone-ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, etc., and it can be used without particular limitation.

[0049] Also, the adhesion can be improved by an adhesion-improving 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 it can be formed by a general coating method, preferably a so-called in-line coating formulation.

[0050] The above-mentioned 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 part of the polyester raw material for polyester polymerization, and a film forming process in which the polyester is melt-extruded into a sheet shape through a filter, cooled, and stretched to form a base film.

[0051] Next, regarding the method for manufacturing a polyester film, an example in which polyethylene terephthalate (hereinafter sometimes referred to as PET) pellets are used as the raw material for the base film will be described in detail, but it is not limited thereto. Also, the number of layers such as a single-layer structure or a multi-layer structure is not limited.

[0052] 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 merging block with two or more layers (for example, a merging block having a square merging 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.

[0053] 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 high-precision filtration of the molten resin is not particularly limited, but a filter medium made of a sintered stainless steel 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.

[0054] 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 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%) may reduce productivity, it is preferable for obtaining a film with fewer protrusions caused by coarse particles.

[0055] (Regarding the refractive index in the bending direction) In the present invention, the refractive index in at least one of the longitudinal direction (machine flow direction) and the width direction of the polyester film is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. And the refractive index in the bending direction of the polyester film is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. Here, the bending direction refers to the direction orthogonal to the folding portion (reference numeral 21) assumed in the application of the foldable display as shown by reference numeral 22 on the polyester film (reference numeral 2) in FIG. 2. When the refractive index in at least one of the longitudinal direction and the width direction is 1.590 to 1.620, there is little deformation during repeated folding, and there is no possibility of degrading the image quality of the foldable display, which is preferable. It is more preferable that the refractive index is 1.591 to 1.600. Of course, it is preferable that the direction is the above-mentioned bending direction. When it is 1.590 or more, there is no possibility of cracks occurring in the folding portion direction after the bending test described below, and of course, no breakage occurs, so the visibility of the display can be kept good. The refractive index of the polyester film can be effectively adjusted by adjusting the draw ratio and the draw temperature. Also, a relaxation step in the drawing direction or multi-stage drawing may be used for adjusting the refractive index. When performing multi-stage drawing, it is preferable to make the draw ratio in the second and subsequent stages higher than the draw ratio in the first stage.

[0056] Controlling the refractive index in at least one of the longitudinal direction (machine flow direction) and the width direction of the polyester film within the above range, more preferably, controlling the refractive index in the bending direction within the above range can reduce the fatigue caused by the compressive stress applied to the inside during folding. It is considered that the fatigue caused by the compressive stress mainly occurs in the crystal part, and the less the crystals in the bending direction, the less likely it is to fatigue. Therefore, by reducing the refractive index, the amount of oriented crystals in the bending direction is reduced, and it is considered that the compression fatigue is suppressed.

[0057] Also, the creep phenomenon caused by the tensile stress applied to the outside during folding can be suppressed by reducing the refractive index. It is considered that the fatigue caused by the tensile stress mainly occurs in the amorphous part, and the alignment of molecular chains occurs due to the repeatedly applied stress, resulting in deformation. It can be inferred that the less the molecular chains arranged in the bending direction, the less the deformation due to alignment. Also, since the less the amorphous part, the more the fatigue due to tension can be suppressed, it is preferable that the degree of crystallinity, that is, the density is higher.

[0058] In the present invention, it is preferable that the draw ratio in at least one of the longitudinal direction (machine flow direction) and the width direction of the unstretched polyester sheet is 1.2 to 2.0 times, and more preferably 1.7 to 2.0 times. And it is preferable that the drawing direction is the above-mentioned bending direction. When the draw ratio is 1.2 times or more, it is preferable because there is no deformation in post-processing such as hard coat coating, and when the draw ratio is 2.0 times or less, it is preferable because film thickness unevenness does not occur. As the drawing temperature, 75 to 120 ° C is preferable, and 75 to 105 ° C is more preferable. The heating method during drawing can adopt conventionally known means such as hot air heating method, roll heating method, infrared heating method, etc. By setting the drawing temperature to 75 to 120 ° C, significant thickness unevenness due to drawing at the above draw ratio can be prevented. Also, by drawing at as low a temperature as possible within the range where no significant thickness unevenness occurs as described above, the refractive index in the thickness direction can be reduced.

[0059] (Regarding the refractive index in the direction of the folded part) The refractive index in the direction orthogonal to the direction in which the refractive index of the above polyester film is 1.590 to 1.620 is preferably 1.670 to 1.700. That is, the refractive index in the direction orthogonal to the bending direction (the direction of the folded portion) is preferably 1.670 to 1.700. By setting it to 1.670 to 1.700, the deformation when folded in the bending direction can be reduced. By setting it to 1.700 or less, the occurrence of cracks or breakage in the direction of the folded portion can be suppressed. By setting it to 1.670 or more, the bendability in the bending direction and the surface hardness can be improved. More preferably, it is 1.680 to 1.695. As methods for adjusting the refractive index in the direction orthogonal to the bending direction, there are stretching ratio, stretching preheating temperature, stretching temperature, multi-stage stretching, and film relaxation. The stretching ratio is preferably 4.0 to 6.0 times, and more preferably 4.4 to 6.0. Also, the stretching preheating temperature in the direction orthogonal to the bending direction is preferably 70 to 110°C. When performing multi-stage stretching in the direction orthogonal to the bending direction, it is preferable to increase the stretching ratio in the second and subsequent stages compared to the first stage. The film relaxation may be performed by 1 to 10% in either the machine flow direction (longitudinal direction) or the perpendicular direction (width direction).

[0060] (Regarding the refractive index in the thickness direction) The refractive index in the thickness direction is preferably 1.520 or less. By setting it to 1.520 or less, even if the refractive index in the bending direction is designed to be low, a decrease in the hardness of the film surface can be suppressed, and both flexibility and surface hardness can be achieved. By setting it to 1.520 or less, the indentation depth after unloading the test force in the thickness direction is reduced, and the hardness of the film surface, particularly the pencil hardness of the hard coat film after laminating the hard coat layer, can be improved. More preferably, it is 1.515 or less, still more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. Although it is preferable that the refractive index in the thickness direction is low, from the viewpoint of stable production, 1.3 or more is preferable, and further, it may be 1.4 or more. Particularly preferably, it is 1.410 or more. It can be said that the above range can be achieved by increasing the draw ratio in both the bending direction and the folding direction. However, in order to control the refractive index in the thickness direction after controlling the refractive indices in the bending direction and the width direction within a preferable range, it is preferable to set the conditions while checking the balance of each process condition in the film forming process.

[0061] The method of controlling the refractive index in the thickness direction includes the stretching preheating temperature, stretching temperature, stretching ratio in the bending direction, stretching preheating temperature, stretching temperature, multi-stage stretching, high-ratio stretching, or heat setting temperature in the direction of the folding part. The stretching preheating temperature in the bending direction is preferably 70°C to 110°C. The stretching temperature in the bending direction is preferably 75 to 120°C. The stretching ratio in the bending direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. By lowering the stretching temperature and stretching at a low stretching ratio, the refractive index in the thickness direction can be effectively reduced while maintaining the bendability in the bending direction. The stretching preheating temperature in the direction of the folding part is also preferably 75°C to 110°C. The stretching temperature is preferably 75 to 120°C. The stretching ratio of the folding part is preferably 4.0 to 6.0 times, more preferably 4.4 to 6.0 times. The refractive index in the thickness direction can be effectively reduced while maintaining or reducing the refractive index in the bending direction. As a method of high-ratio stretching, multi-stage stretching may be used. In that case, it is preferable to increase the stretching ratio of the second stage compared to the stretching ratio of the first stage to effectively control the refractive index. Also, a method of stretching again after the crystallization step may be used. Accelerated stretching that increases the stretching speed from the beginning to the latter half of stretching may be used. The heat setting temperature is preferably 180 to 240°C. By performing heat setting, orientation crystallization in the stretching direction proceeds, and the refractive index in the thickness direction can be reduced. The reason why the hardness of the film surface is improved by reducing the refractive index in the thickness direction is not necessarily clear, but it is considered that aromatics such as benzene rings in the molecular chain are oriented in the plane direction, and there is an effect of suppressing deformation due to stress applied in the thickness direction.

[0062] (Regarding the density of the polyester film) The density of the polyester film is preferably 1.380 g / cm 3 or more. More preferably 1.383 g / cm 3 or more. 1.380 g / cm 3By doing the above, the flexibility can be improved, and the surface hardness of the film, especially the pencil hardness of the hard coat film after laminating the hard coat layer, can be improved. The higher the density, the more preferable it is, and it is somewhat affected by the presence or absence of particles in the film, etc., but 1.40 g / cm 3 It is preferably the following. By setting the heat setting temperature during film formation to 180 to 240 °C, crystallization can proceed and the density can be effectively increased.

[0063] The bending direction of the polyester film is preferably made to correspond to the longitudinal direction (machine flow direction). By doing so, it is easy to lower the refractive index in the bending direction at the second biaxial stretching stage and easy to improve the flexibility. That is, it is preferable to obtain a polyester film by stretching the unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. And in the width direction, it can be said that it is a preferable embodiment to stretch at a stretching ratio of 4.0 to 6.0 times, more preferably 4.4 to 6.0 times.

[0064] Also, in the present invention, for the polyester film (1) The refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folded part is 1.670 to 1.700 (3) The refractive index in the thickness direction is 1.520 or less (4) The density is 1.380 g / cm 3 or more Although it can be said that it is a particularly preferred embodiment to simultaneously have the four characteristics described above, even in the combination within the range of the above-described preferred manufacturing conditions, for example, the stretching ratio in the bending direction is 1.4 times or less, the stretching ratio in the direction of the folded portion is less than 4.4 times, and the heat setting temperature is 220°C or less. In the case of a combination of conditions that are not the best within each range of the above-described preferred manufacturing conditions, it may not always be possible to obtain a product that simultaneously satisfies the above four characteristics. In this case, by finely adjusting any one of the conditions or a combination thereof, such as increasing the stretching ratio in the bending direction to 1.7 times or more, increasing the stretching ratio in the direction of the folded portion to 4.4 times or more, increasing the heat setting temperature to about 230°C, or lowering the stretching temperature in the bending direction and / or the direction of the folded portion, it is possible to simultaneously satisfy the above four characteristics.

[0065] In order to adjust film formability, film strength, thermal dimensional stability, appearance defects, etc., any film forming method such as stretching, relaxation, heat setting, surface treatment, etc. may be employed, but it can be said that it is a particularly preferred embodiment in the present invention to control the refractive index and density of the film within the above-described preferred ranges. By controlling the refractive index and density within the preferred ranges, it is possible to provide a polyester film suitable for a foldable display that has better bend resistance and surface hardness than conventional films, and in particular, high pencil hardness of the hard coat film after laminating the hard coat layer.

[0066] Specifically, for example, after sufficiently vacuum-drying PET pellets, they are supplied to an extruder, melt-extruded into a sheet shape at about 280°C, and cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, in the longitudinal direction with a roll heated to 75 to 120°C to obtain a uniaxially oriented PET film. Further, the ends of the film are gripped with clips and guided into a hot air zone heated to 75 to 120°C. After drying, it is stretched 4.0 to 6.0 times, more preferably 4.4 to 6.0 times, in the width direction. Subsequently, it is guided into a heat treatment zone at 180 to 240°C, and heat treatment can be performed for 1 to 60 seconds. During this heat treatment process, if necessary, relaxation treatment of 0 to 10% may be performed in the width direction or the longitudinal direction.

[0067] 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 disconnection of the internal circuit of the display due to external impact is less likely to occur. On the other hand, when the intrinsic viscosity is 1.00 dl / g or less, the filtration pressure increase of the molten fluid does not become too large, and film production is stable, which is preferable.

[0068] (Easy adhesion layer) In the present invention, in order to improve the adhesion between the polyester film and the transparent conductive layer or the hard coat layer, etc., 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 solution for forming the easy adhesion layer on 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 0.005 to 0.20 g / m 2 It is preferable. When the coating amount is 0.005 g / m 2 or more, adhesion is obtained, which is preferable. On the other hand, when the coating amount is 0.20 g / m 2 or less, blocking resistance is obtained, which is preferable.

[0069] Examples of the resin to be incorporated into the coating liquid used for forming the easy-adhesion layer include polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc., and they can be used without particular limitation. Examples of the crosslinking agent to be incorporated into 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 be mixed and used. Due to the nature of in-line coating, these are preferably applied with an aqueous coating liquid, and the above-mentioned resin and crosslinking agent are preferably water-soluble or water-dispersible resins and compounds.

[0070] It is preferable to add particles to the easy-adhesion layer to impart easy-sliding properties. 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-adhesion layer. Examples of the particles to be incorporated into the easy-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, calcium fluoride, etc., and organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, silicone-based, etc. These can be added to the easy-adhesion layer alone, or two or more of them can be combined and added.

[0071] Also, as a method for applying the coating liquid, known methods can be used in the same manner as the above-mentioned 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.

[0072] (Hard coat layer) The polyester film preferably has a hard coat layer on at least one of its surfaces for adjusting the refractive index, improving the surface hardness, improving the flex resistance, improving the cracking and chipping resistance, and blocking the adverse effects of oligomers precipitated from the polyester on the transparent conductive layer. The hard coat layer is preferably positioned and used on the polyester film or on the easy adhesion layer. 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 types of materials can be mixed and used, and particles such as inorganic fillers and organic fillers can be added.

[0073] (Film thickness of the hard coat layer) The film thickness of the hard coat layer is preferably 1 to 50 μm. If 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, curling due to the curing shrinkage of the hard coat can be suppressed, and the handleability of the film can be improved.

[0074] (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.

[0075] (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 the damage to the film, curing methods by ultraviolet rays and electron beams are preferable.

[0076] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or more, and more preferably 4H or more. If it has a pencil hardness of 3H or more, the surface is protected and it is not easily scratched or dented, and the visibility is not reduced. 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.

[0077] (Properties of the hard coat layer) The hard coat layer in the present invention can be used for the purpose of enhancing the pencil hardness of the surface as described above and protecting a touch panel module or a display, and preferably has a high transmittance. The transmittance of the hard coat film is preferably 87% or more, and 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 viewpoint 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 viewpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0078] 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.

[0079] For the polyester film, in order to make the electrode pattern of the transparent conductive layer less visible, it is also preferable to provide a refractive index adjustment layer between the polyester film and the transparent electrode layer or between the hard coat layer and the transparent electrode layer. In that case, the hard coat layer itself may also serve as the refractive index adjustment layer, or a separate refractive index adjustment layer may be laminated. Examples of the refractive index adjustment layer include a resin layer containing the above-mentioned refractive index adjustment particles, a fluorine-containing resin layer, an aromatic polyimide resin, an epoxy resin, a (meth)acrylic resin (acrylate, methacrylate compound), a polyester resin, and a urethane resin, etc., with an aromatic ring, a sulfur atom, or a bromine atom incorporated into the resin material to form a resin with a high refractive index and its precursor layers, etc. These can be provided by coating. Further, as the refractive index adjustment layer, an inorganic layer such as ZnO, CeO2, Sb2O3, SnO2, indium tin oxide, In2O3, Al2O3, antimony-doped tin oxide, aluminum-doped zinc oxide, SiO2, magnesium fluoride, etc. is also preferable, and these can be provided by a wet film-forming method.

[0080] When producing a transparent conductive film using the polyester film for a touch panel module in the present invention, its preferable laminated structure is, for example, polyester film / transparent conductive layer, polyester film / adhesive layer / transparent conductive layer, polyester film / hard coat layer / transparent conductive layer, polyester film / adhesive layer / hard coat layer / transparent conductive layer, polyester film / refractive index adjustment layer (one layer or a plurality of layers with different refractive indices) / transparent conductive layer, polyester film / adhesive layer / refractive index adjustment layer (one layer or a plurality of layers with different refractive indices) / transparent conductive layer, polyester film / hard coat layer / refractive index adjustment layer (one layer or a plurality of layers with different refractive indices) / transparent conductive layer, polyester film / adhesive layer / hard coat layer / refractive index adjustment layer (one layer or a plurality of layers with different refractive indices) / transparent conductive layer, etc. Combinations of these laminated structures may exist on one side of the polyester film, or may exist on both sides through the polyester film.

[0081] As the touch panel module of the foldable display of the present invention, the polyester film of the present invention is used as the base material constituting the touch panel module, but it is not necessary to be used for all the films constituting the touch panel module. In the touch panel module of the 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 as the base material film for the touch panel module according to the suitability.

Example

[0082] 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.

[0083] (1) Intrinsic viscosity After pulverizing and drying the film or the polyester resin, 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 the solvent alone 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.

[0084] (2) Flex resistance of the polyester film sample (bending radius 1.5 mm) Prepare a polyester film sample with a size of 20 mm in the width direction and 110 mm in the flow direction. Using a no-load U-shaped expansion and contraction testing machine (manufactured by Yuasa System Equipment Co., Ltd., DLDMLH-FS), set the bending radius to 1.5 mm and bend it 200,000 times at a speed of 1 time per second. At that time, the sample was fixed at positions 10 mm from both ends on the long side, and the bending part was 20 mm × 90 mm. Here, Fig. 1 is a schematic diagram for showing the bending radius when the foldable display is folded, and considering the case where the polyester film is arranged on the inner surface of the folded state, the bending test was performed modelly with the position of reference numeral 11 in Fig. 1 set to 1.5 mm. After the bending treatment was completed, the sample was placed flat with the inner side of the bend facing down, and visual observation was carried out. ○: No cracks or deformations can be confirmed in the sample. ×: There are cracks or creases in the sample, and when placed horizontally, the maximum height of the lift is 5 mm or more.

[0085] (3) Flexural resistance of the polyester film sample (bending radius 0.5 mm) In the same manner as the above bending test, the bending radius was set to 0.5 mm and bent 200,000 times at a speed of 1 time per second. Here, Fig. 1 is a schematic diagram for showing the bending radius when the foldable display is folded, and considering the case where the polyester film is arranged on the inner surface of the folded state, the bending test was performed modelly with the position of reference numeral 11 in Fig. 1 set to 0.5 mm. The film surface on the outer side of the bent part was observed at 700 times with a digital microscope (RH8800 manufactured by HIROX), and the presence or absence of wrinkles (cracks) was observed. Separate from the above visual test of the flexural resistance with a bending radius of 1.5 mm, by performing this test with the bending radius reduced to 0.5 mm, it is intended to evaluate in a state close to the actual use state of the foldable display with a hard coat layer or other members laminated or adhered. This is a test for detecting defects that are difficult to detect visually, such as defects that are prone to breakage or cracks. ○: There are no defects on the film surface on the outer side of the bend. ×: Cracks (wrinkles) can be confirmed on the broken or outer surface of the film on the bent side.

[0086] (4) Flexural resistance of the polyester film sample after heat treatment (bending radius: 1.5 mm) The sample film was cut into a size of 210 mm × 300 mm in the lateral direction, and the interval A between marks was measured under a constant tension of 5 g. Subsequently, the sample film was left in an oven at 150 °C in an atmosphere without load for 30 minutes, then taken out of the oven and cooled to room temperature. After that, it was cut into a size of 20 mm × 110 mm in the width direction to prepare a polyester film sample. Using a non-load U-shaped stretching tester (manufactured by Yuasa System Co., Ltd., DLDMLH-FS), the bending radius was set to 1.5 mm, and it was bent 200,000 times at a speed of 1 time / second. At that time, the sample was fixed at positions 10 mm from both ends on the long side, and the bending part was 20 mm × 90 mm. Here, Fig. 1 is a schematic diagram for showing the bending radius when the folding type display is folded, and considering the case where the polyester film is arranged on the inner surface of the folded state, the bending test was carried out modelly with the position of reference numeral 11 in Fig. 1 set to 1.5 mm. After the bending treatment was completed, the sample was placed flat with the inner side of the bend facing down, and visual observation was performed. ○: No cracks or deformation (strain) can be confirmed in the sample. ×: There are cracks or creases in the sample, and when placed horizontally, the maximum height of the lift is 5 mm or more.

[0087] (5) Refractive index In accordance with JIS K 7142:2014 "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.

[0088] (6) Pencil hardness Taking the pencil hardness of the hard coat film as a sample, in accordance with JIS K 5600-5-4:1999, it was measured under a load of 750 g and a speed of 1.0 mm / s. In the present invention, 3H or more was considered qualified.

[0089] (7) Total light transmittance, haze It was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH5000).

[0090] (8) Density The density was measured according to the method conforming to JIS K 7112:1999 (density gradient tube method). (Unit: g / cm 3 ).

[0091] (9) Penetration depth after unloading the test force The sample was cut into approximately 2 cm squares, and the opposite surface of the measurement surface was fixed with an adhesive (Cemedine (registered trademark) High Super 30) on a micro cover glass 18×18 mm (manufactured by Matsunami Glass). After sticking and fixing, it was left at room temperature for 12 hours or more, and then the penetration depth (μm) after unloading the test force was measured using a dynamic ultra-micro hardness tester "DUH-211" (manufactured by Shimadzu Corporation) under the following conditions. ≪Measurement conditions≫ Test mode: Load-unload test Indenter used: Vickers indenter with an included angle of 115 degrees between the edges Indenter elastic modulus: 1.140×106 N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50 mN Loading rate: 4.44 mN / sec Loading hold time: 2 sec Unloading hold time: 0 sec

[0092] (10) Maximum thermal shrinkage rate The sample film was cut into 10 mm in the width direction × 250 mm in the longitudinal direction, marked at 200 mm intervals with the long side aligned in the direction to be measured, 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 an atmosphere without load for 30 minutes, then taken out of the oven and cooled to room temperature. Then, the interval B between the marks was determined under a constant tension of 5 g, and the thermal shrinkage rate (%) was determined by the following formula. The above thermal 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 thermal shrinkage rate (%). Heat shrinkage rate (%) = [(A - B) × 100] / A For the two directions of the bending direction and the folding direction, the sample film was cut so that the vertical and horizontal dimensions were different for each direction, and the data in the direction with the larger measured value was taken as the maximum heat shrinkage rate (%).

[0093] (11) Elastic modulus (Young's modulus (unit: GPa)) In accordance with JIS K7127, the elastic modulus in the bending direction and the folding direction of the polyester film was measured at 23°C.

[0094] (Preparation of polyethylene terephthalate pellets (a)) As an esterification reactor, a continuous esterification reactor consisting of a three-stage completely mixed tank equipped with a stirring device, a partial condenser, a raw material inlet, and a product outlet was used. TPA was set at 2 tons / hr, EG was set at 2 moles per 1 mole of TPA, antimony trioxide was set at an amount such that the Sb atoms were 160 ppm with respect to the produced PET, and 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 under normal pressure. Next, the reaction product in the first esterification reactor was continuously taken out of the system and fed into the second esterification reactor. EG distilled off from the first esterification reactor was supplied to the second esterification reactor in an amount of 8% by mass with respect to the produced polymer (produced PET). Furthermore, an EG solution containing magnesium acetate in an amount such that the Mg atoms were 65 ppm with respect to the produced PET and an EG solution containing TMPA in an amount such that the P atoms were 20 ppm with respect to the produced PET were added, and 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 fed into the third esterification reactor. Furthermore, an EG solution containing TMPA in an amount such that the P atoms were 20 ppm with respect to the produced PET was added, and 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 fed into a three-stage continuous polycondensation reactor for polycondensation, and further filtered through a filter medium of a stainless steel sintered body (nominal filtration accuracy: 90% cut-off for 5 μm particles) to obtain polyethylene terephthalate pellets (a) with an intrinsic viscosity of 0.62 dl / g.

[0095] (Preparation of polyethylene terephthalate pellets (b)) Regarding the manufacturing process of the 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).

[0096] (Preparation of polyethylene terephthalate pellets (c)) Polyethylene terephthalate pellets (a) were subjected to solid-phase polymerization at 220 °C under a reduced pressure of 0.5 mmHg for varying times using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (c) having an intrinsic viscosity of 0.75 dl / g.

[0097] (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 having 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. The mixture was stirred at 75 °C for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction solution reached a predetermined amine equivalent. Next, after cooling this 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) having a solid content of 35% by mass.

[0098] (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. The mixture was stirred at 180 °C for 10 hours under a nitrogen atmosphere 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) having a solid content of 40% by mass.

[0099] (Preparation of Coating Liquid for Easy Adhesion Layer Formation) The following coating agents were mixed to prepare a coating liquid. 16.97 parts by mass of water 21.96 parts by mass of isopropanol 3.27 parts by mass of polyurethane resin (A) 1.22 parts by mass of water-soluble carbodiimide compound (B) 0.51 parts by mass of particles (Silica sol with an average particle size of 40 nm and a solid content concentration of 40 mass%) 0.05 parts by mass of surfactant (Silicone-based, solid content concentration 100 mass%)

[0100] (Preparation of Hard Coat Coating Liquid a) To 100 parts by mass of a hard coat material (manufactured by JSR Corporation, Opstar (registered trademark) Z7503, concentration 75%), 0.1 parts by mass of a leveling agent (manufactured by BYK Chemie Japan, BYK307, concentration 100%) was added and diluted with methyl ethyl ketone to prepare a hard coat coating liquid a with a solid content concentration of 40 mass%.

[0101] (Preparation of Hard Coat Coating Liquid b) 95 parts by mass of pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., A-TMM-3, solid content concentration 100%), 5 parts by mass of a photopolymerization initiator (manufactured by BASF Japan Ltd., Irgacure (registered trademark) 907, solid content concentration 100%), and 0.1 parts by mass of a leveling agent (manufactured by BYK Chemie Japan, BYK307, solid content concentration 100%) were mixed and diluted with a solvent of toluene / MEK = 1 / 1 to prepare a hard coat coating liquid b with a concentration of 40 mass%.

[0102] (Preparation of Coating Liquid Containing Conductive Fibrous Filler and Metal Nanowires) 0.6 g of silver nitrate (manufactured by Wako Pure Chemical Industries, Ltd.), 36 g of an ethylene glycol (EG, manufactured by Kishida Chemical Co., Ltd.) solution of 1.4 wt% polyvinylpyrrolidone (PVP, manufactured by Wako Pure Chemical Industries, Ltd., average molecular weight 360,000), 4 g of an EG solution of 165 ppm iron(III) chloride (manufactured by Kishida Chemical Co., Ltd.), and 109 g of EG were mixed to prepare Reaction Solution 1. Reaction Solution 1 was heated from room temperature to 130 °C using a personal synthesis apparatus (ChemiStation, PPV-CTRL1, manufactured by Tokyo Rika Kikai Co., Ltd.) and reacted for 187 minutes. A cylindrical filter paper (No. 86R, retention particle size 1 μm, 20 mm × 90 mm, manufactured by Advantec Toyo Co., Ltd.) filled with 20 mL of Reaction Solution 1 was placed in a 300 mL beaker, and isopropyl alcohol (manufactured by Junsei Chemical Co., Ltd.) was added outside the cylindrical filter paper to the same height as the reaction solution inside the cylindrical filter paper. After one week, the solution inside the cylindrical filter paper was recovered to obtain a metal nanowire-containing coating solution.

[0103] (Example 1) Polyethylene terephthalate pellets (a) were supplied to an extruder and melted at 285 °C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut-off for 10 μm particles), extruded into a sheet from a die, and then cooled and solidified by contacting a casting drum with a surface temperature of 30 °C using an electrostatic application casting method to produce 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 uniaxially stretched (longitudinal stretching) by a factor of 1.4. The above-mentioned coating solution for forming an easy-adhesion layer was applied to both sides of the obtained uniaxially stretched film by a roll coating method and then dried at 80 °C for 20 seconds. The coating amount after final (after biaxial stretching) drying was adjusted to be 0.06 g / m 2 . Then, it was led to a tenter, preheated at 105 °C, laterally stretched 4.0 times at 95 °C, width-fixed, heat-fixed at 230 °C for 5 seconds, and further relaxed by 4% in the width direction at 180 °C to obtain a 50 μm thick polyethylene terephthalate film.

[0104] (Examples 2 - 3) A polyester film was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio described in Table 1 was changed.

[0105] (Example 4) A polyester film was obtained in the same manner as in Example 1, except that the transverse stretching ratio was changed to 4.4 times and the heat setting temperature was changed to 220°C.

[0106] (Examples 5, 6) A polyester film was obtained in the same manner as in Example 4, except that the longitudinal stretching ratio was changed as described in Table 1.

[0107] (Example 7) A polyester film was obtained in the same manner as in Example 1, except that the transverse stretching ratio was changed to 5.5 times and the heat setting temperature was changed to 190°C.

[0108] (Examples 8 - 9) A polyester film was obtained in the same manner as in Example 7, except that the longitudinal stretching ratio was changed as described in Table 1.

[0109] (Example 10) In the production process of Example 5, a polyester film was obtained in the same manner as in Example 5, except that after stretching in the longitudinal direction, a relaxation heat treatment was performed at 100°C for 10%.

[0110] (Example 11) In the production process of Example 5, after heat setting, the clips were opened at 200°C, and relaxation heat treatment was performed in the longitudinal and transverse directions. A polyester film was obtained in the same manner as in Example 5. The tenter speed and the winding roll speed were adjusted so that the relaxation rate in the longitudinal direction was 3%. The relaxation in the transverse direction was in a free state.

[0111] (Example 12) A polyester film was obtained in the same manner as in Example 1, except that the temperature during longitudinal stretching was changed to 75°C and the heat setting temperature was changed to 220°C.

[0112] (Example 13) After changing the temperature during longitudinal stretching to 75°C, changing the stretching ratio to 1.2 times and performing stretching, and then changing the stretching ratio in the width direction to 5.0 times and performing stretching, a polyester film was obtained in the same manner as in Example 1 except for this.

[0113] (Example 14) The polyester film was obtained in the same manner as in Example 3 except that the longitudinal stretching in Example 3 was a two-stage stretching, the stretching ratio in the first stage was 1.2 times, and the stretching ratio in the second stage was 1.67 times. The total longitudinal stretching ratio is about 2.0 times.

[0114] (Example 15) A polyester film was obtained in the same manner as in Example 5 except that the preheating temperature during widthwise stretching was changed to 95°C and the heat setting temperature was changed to 190°C.

[0115] (Example 16) The polyester film was obtained in the same manner as in Example 2 except that the widthwise stretching in Example 2 was a two-stage stretching, the stretching ratio in the first stage was 1.5 times, the stretching ratio in the second stage was 4.0 times, and the heat setting temperature was changed to 190°C. The total widthwise stretching ratio is 6.0 times.

[0116] (Examples 17 - 18) A polyester film was obtained in the same manner as in Example 2 except that the thickness was changed as shown in Table 1.

[0117] (Example 19) A polyester film was obtained in the same manner as in Example 1 except that relaxation heat treatment in the width direction was not performed in the manufacturing process of Example 1.

[0118] (Example 20) After creating an unstretched film in the same manner as in Example 1, the unstretched film was preheated at 75°C with a tenter and stretched horizontally 1.4 times at 85°C. After applying the above coating liquid for forming an easy - adhesion layer to both sides of the obtained uniaxially stretched film by the roll coating method, it was dried at 80°C for 20 seconds. Note that the coating amount after final (after biaxial stretching) drying is 0.06 g / m 2It was adjusted to be as follows. It was uniformly heated to 105 °C using a heating roll and heated to 95 °C with a non-contact heater. Roll stretching (longitudinal stretching) was performed 4.0 times. With the width fixed, heat setting was performed at 230 °C for 5 seconds to obtain a polyethylene terephthalate film with a thickness of 50 μm.

[0119] (Example 21) After obtaining a polyethylene terephthalate film with a thickness of 50 μm in the same manner as in Example 1, a hard coat film coated with the hard coat coating liquid b was obtained.

[0120] (Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that stretching was performed only in the width direction without stretching in the longitudinal direction, and it was stretched uniaxially in the horizontal direction.

[0121] (Comparative Example 2) A polyester film was obtained in the same manner as in Example 7, except that stretching was performed only in the width direction without stretching in the longitudinal direction, and it was stretched uniaxially in the horizontal direction.

[0122] (Comparative Examples 3 - 7) The heat setting temperature was changed to 220 °C, and a polyester film was obtained in the same manner as in Example 1, except that the PET pellets and thickness described in Table 1 were used. Comparative Examples 3 - 7 are combinations of each condition level where the heat setting temperature is lower than that of Example 1 as described above, and the stretching ratios in the longitudinal and width directions are not the best within the preferable condition range. As described in Table 1, the refractive index in the thickness direction increased, the indentation depth after unloading the test force was large, and the pencil hardness after laminating the hard coat layer became smaller compared to each example.

[0123] (Comparative Example 8) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to 2.7 times and the heat setting temperature was changed to 220 °C.

[0124] (Comparative Example 9) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to 3.4 times.

[0125] (Comparative Example 10) A polyester film was obtained in the same manner as in Example 4 except that the heat setting temperature was changed to 100°C.

[0126] (Comparative Example 11) A polyester film was obtained in the same manner as in Example 13 except that the stretching temperature in the longitudinal direction was changed to 130°C.

[0127] (Comparative Example 12) A polyester film was obtained in the same manner as in Example 1 except that the preheating temperature in the width direction was changed to 120°C.

[0128] Using a Mayer bar on one surface of the above-prepared film, a hard coat coating solution a was applied so that the film thickness after drying was 5 μm, dried at 80°C for 1 minute, and then irradiated with ultraviolet light (integrated light quantity 200 mJ / cm 2 ), and a hard coat film was obtained. Then, using a Mayer bar on the surface of the prepared hard coat layer, a metal nanowire-containing coating solution was applied so that the film thickness after drying was 5 μm, dried at 80°C for 10 minutes, and then a transparent conductive polyester film was obtained. The evaluation results are shown in Table 1.

[0129] The transparent conductive polyester film produced above was incorporated into a touch panel module to create a smartphone-type foldable display that could be folded in half at the center of the whole with a corresponding bending radius of 3 mm in Fig. 1. Those using a touch panel module including a transparent conductive polyester film made of the polyester film of each example were satisfactory in terms of operation and visibility as a smartphone that could be folded in half at the center and carried around. Also, the surface did not dent due to external force. Here, Examples 6 to 9, 15, and 16 were slightly inferior to other examples in terms of less curl after heat processing because the maximum heat shrinkage rate of the polyester film was slightly large, but were satisfactory as a whole. On the other hand, the foldable displays using the polyester film or hard coat film of each comparative example seemed to cause image distortion at the folded part of the display as the usage frequency increased, and were not very preferable. Also, some had dents and scratches on the surface.

[0130]

Table 1

[0131]

Table 2

Industrial Applicability

[0132] The foldable display using the polyester film for the touch panel module base material of the foldable display of the present invention can maintain mass productivity, and the polyester film located in the touch panel module of the foldable display does not deform after being repeatedly folded, so that image distortion does not occur at the folded part of the display. In particular, a portable terminal device or an image display device equipped with a foldable display using the polyester film of the present invention as a touch panel module base material provides beautiful images, is rich in functionality, and is excellent in convenience such as portability.

Explanation of Signs

[0133] 1: Foldable display 11: Bending radius 2: Polyester film for the touch panel module of the foldable display 21: Folded part 22: Bending direction (direction orthogonal to the folded part)

Claims

1. A polyester film for use as a substrate for touch panel modules in foldable displays that satisfies the following conditions: (1) The refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the fold is 1.670 to 1.700 (3) A refractive index in the thickness direction is 1.520 or less. (4) Density is 1.380 g / cm ３ End (5) The polyester film is a biaxially oriented polyethylene terephthalate film. (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.)

2. 2. The polyester film for use as a substrate for a touch panel module of a folding display according to claim 1, having an elastic modulus of 2.7 GPa or less in a bending direction and an elastic modulus of 4.5 GPa or more in a direction of the folding portion.

3. 3. The polyester film for use as a substrate for a touch panel module of a folding display according to claim 1 or 2, which has a total light transmittance of 85% or more, a haze of 3% or less, and a maximum heat shrinkage of 2% or less.

4. The bending resistance evaluation (bending radius 1.5 mm) is performed by evaluating whether or not cracks or deformation occurs when the polyester film for touch panel module substrate of a foldable display according to any one of claims 1 to 3 is placed on a flat surface with the inside of the bend facing down after a no-load U-shaped stretching test in which the polyester film is heat-treated at 150°C for 30 minutes and then bent 200,000 times at a bending radius of 1.5 mm and a speed of 1 time / second.

5. The polyester film for a touch panel module substrate of a folding display according to any one of claims 1 to 4, wherein the polyester film for a touch panel module substrate of a folding display has an easy-adhesion layer on at least one side thereof.

6. The polyester film for a touch panel module substrate of a folding display according to any one of claims 1 to 5, wherein the polyester film for a touch panel module substrate of a folding display has a hard coat layer having a thickness of 1 to 50 μm on at least one side of the polyester film for a touch panel module substrate of a folding display.

Images