Transparent conductive polyester film and application of the same

Abstract

To provide a foldable display which has excellent mass-producibility and has no risk of causing distortion of images displayed at the folded portion and a transparent conductive polyester film for a foldable display which is used for portable terminal equipment and does not cause fold marks or breaks at the folded portion.SOLUTION: There is provided a transparent conductive polyester film for a foldable display which is a transparent conductive polyester film in which a transparent conductive layer is laminated on at least one surface of a polyester film, wherein the polyester film satisfies the following conditions. (1) The refractive index in the bending direction is 1.590 to 1.620, (2) The refractive index in the folding direction 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 / cm3 or more. (Here, the bending direction refers to the direction perpendicular to the folding section when the polyester film is folded.)SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a transparent conductive polyester film for a foldable display, a foldable display, and a mobile terminal device, and more particularly to a foldable display and a mobile terminal device that are less likely to suffer from image distortion due to deformation or tearing of the film even when folded repeatedly, and to a transparent conductive polyester film and a touch panel for the foldable display. [Background technology]

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

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

[0004] To achieve these goals, a method has been proposed of connecting multiple displays to make them more compact, but this method has not become widespread because it leaves a bezel, which means the image is cut off and visibility is reduced.

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

[0006] Image display devices, particularly image display devices equipped with touch panels, which have become increasingly popular in recent years, require optical films with excellent durability and foldability that do not break or crack even when repeatedly folded, for their image display surfaces.

[0007] Furthermore, touch panels often use glass for the display screen, but although glass is highly hard, it breaks when folded, making it impossible to provide foldable functionality. Furthermore, because glass is a material with a high specific gravity, it needs to be made thinner to reduce weight, but thinner glass reduces its strength and makes it more susceptible to breakage. Foldable displays use films in various parts, such as polarizers, retardation films, touch panel substrates, display cell substrates such as organic electroluminescence (EL), and backside protective materials, and these films are also required to be durable enough to withstand repeated folding.

[0008] Furthermore, for example, Patent Document 1 discloses an optical film having flexibility, in which two hard coat layers with different Vickers hardnesses are provided on one surface of a substrate film. However, with such optical films, repeated folding can cause the substrate film to tear or leave folding marks, and the films do not satisfy the bending resistance required in recent years.

[0009] Furthermore, Patent Document 2 proposes using a polyimide film or an aramid film as a resin substrate with flex resistance. However, polyimide film and aramid film are very expensive, which is undesirable because it increases the price of the flexible terminal itself.

[0010] Patent Document 3 proposes laminating two substrate units each having a substrate film layer, a hard coat layer, and a conductive layer in this order so that the substrate film layers directly face each other, and using a cycloolefin resin as the substrate film layer. However, this method could not be applied to foldable displays because the overall thickness was too thick to reduce the weight and the cycloolefin resin substrate itself did not have sufficient bending resistance. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-186210 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-33034 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-90925 Summary of the Invention [Problem to be solved by the invention]

[0012] In view of the above problems, the object of the present invention is to provide a foldable display that is easy to mass-produce and does not cause distortion of the image displayed at the folded portion after repeated folding, and a transparent conductive polyester film for a foldable display that does not cause creases or cuts at the folded portion, so as to be able to provide a foldable display that is easy to mass-produce and does not cause distortion of the image displayed at the folded portion after repeated folding, and a mobile terminal device equipped with such a foldable display. [Means for solving the problem]

[0013] That is, the present invention comprises the following: 1. A transparent conductive polyester film for a folding display, comprising a polyester film and a transparent conductive layer laminated on at least one side thereof, wherein the polyester film satisfies the following conditions: (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) Refractive index in the thickness direction is 1.520 or less (4) Density is 1.380 g / cm 3 End (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.) 2. The transparent conductive polyester film for a foldable display described in 1 above, wherein the transparent conductive layer contains at least one selected from a conductive fibrous filler, a metal oxide, and a conductive polymer. 3. The transparent conductive polyester film for a foldable display according to claim 1 or 2, wherein the polyester film has a total light transmittance of 85% or more, a haze of 3% or less, and a maximum heat shrinkage of 6% or less. 4. The transparent conductive polyester film for a foldable display according to any one of the above items 1 to 3, which has an easy-adhesion layer on at least one surface of the polyester film. 5. The transparent conductive polyester film for a foldable display according to any one of the above items 1 to 4, which has a hard coat layer having a thickness of 1 to 50 μm on at least one surface of the polyester film. 6. A touch panel for a folding display, comprising the transparent conductive polyester film for a folding display according to any one of items 1 to 5 above. 7. A foldable display having a touch panel for the foldable display described in 6 above. 8. A mobile terminal device having the foldable display described in item 7 above. [Effects of the Invention]

[0014] A foldable display using the transparent conductive polyester film for a foldable display of the present invention maintains mass productivity, and the polyester film does not crack or break at the folding portion, does not deform after repeated folding, and does not cause image distortion at the folding portion of the display. Mobile terminal devices equipped with a foldable display using such a polyester film provide beautiful images, are highly functional, and are convenient in terms of portability and other factors. [Brief explanation of the drawings]

[0015] [Figure 1]FIG. 2 is a schematic diagram showing the bending radius when the foldable display of the present invention is folded. [Figure 2] FIG. 2 is a schematic diagram showing the bending direction of a polyester film for a transparent conductive polyester film of a foldable display according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] (display) The term "display" as used herein refers to display devices in general, and examples of the display include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs. LCDs, organic EL displays, and inorganic EL displays that have a bendable structure are preferred. Organic EL displays and inorganic EL displays that can reduce the number of layers are particularly preferred, and organic EL displays that have a wide color gamut are even more preferred.

[0017] (foldable display) A foldable display is a single continuous display that can be folded in half or otherwise when carried. Folding reduces the size by half, improving portability. The bending radius of a foldable display is preferably 5 mm or less, and more preferably 3 mm or less. A bending radius of 5 mm or less allows for a thinner display when folded. A smaller bending radius is preferable, but the smaller the bending radius, the more likely it is that creases will form. A bending radius of 0.1 mm or more is preferable, but it can also be 0.5 mm or more, or even 1 mm or more. Even a bending radius of 1 mm can achieve a practically thin display when carried. The bending radius when folded is measured at the location indicated by the symbol 11 in the schematic diagram of Figure 1 and refers to the radius of the inside of the folded portion when folded. The surface protection film, described below, may be located on either the outside or inside of the folded portion of the foldable display. Furthermore, the foldable display may be tri-folded or quadruple-folded, or may be a rollable type, all of which are considered to fall within the scope of the foldable display of the present invention.

[0018] The transparent conductive polyester film for a folding display of the present invention may be used in any part of a touch panel module of a folding display. Hereinafter, a typical configuration of a folding display and a touch panel module in which the transparent conductive polyester film of the present invention can be used will be described using an organic EL display as an example. A polyester film having a transparent conductive layer is called a transparent conductive polyester film, and a transparent conductive polyester film that is used as a component to incorporate wiring and other components and turn into a sensor that detects contact (approach) by a finger or other object is called a touch panel module. Hereinafter, the transparent conductive polyester film for a folding display of the present invention may be simply referred to as the transparent conductive polyester film of the present invention, the conductive film of the present invention, or the conductive polyester film of the present invention.

[0019] (foldable OLED display) The essential component of a foldable organic EL display is an organic EL module, but a circular polarizer, a touch panel module, a surface protective film, a back protective film, etc. may also be provided as needed.

[0020] (touch panel module) It is preferable that a mobile terminal device has a touch panel. When an organic EL display is used, it is preferably disposed on the viewing side of the organic EL module, and more preferably, a touch panel module is disposed between the organic EL module and the circular polarizer. The touch panel module has a transparent substrate such as a film and a transparent conductive polyester film having a transparent conductive layer disposed thereon. In the present invention, a specific polyester film can be used as the transparent substrate for this transparent conductive polyester film. When used as the transparent substrate for the transparent conductive polyester film, it is preferable to provide the polyester film with a refractive index adjustment layer or a hard coat layer.

[0021] (Transparent conductive layer) The transparent conductive layer in the present invention is not particularly limited as long as it is a layer that is transparent and conductive, and examples thereof include a conductive filler-containing layer, a metal oxide layer, and a conductive polymer-containing layer.

[0022] The method for forming this transparent conductive layer is not particularly limited, but examples thereof include dry film-forming methods such as sputtering, vacuum deposition, CVD, and ion plating, and wet film-forming methods such as sol-gel and coating, and can be appropriately selected depending on the type of transparent conductive layer. When used as a capacitive touch panel, the conductive layer is often patterned, such as a mesh-like straight line lattice pattern with lines intersecting at almost right angles, a wavy line lattice pattern with the conductive portion between the intersections having at least one curved portion, or a diamond pattern.

[0023] Patterning methods include chemical etching, laser etching, and other methods of patterning after film formation, as well as pattern printing during coating. Methods for printing patterns include gravure printing, letterpress printing, offset printing, screen printing, and inkjet printing, and can be selected according to the properties of the paint and the fineness of the pattern.

[0024] The surface resistivity of the conductive layer is not particularly limited, but is preferably 0.1 Ω / □ or more and 200 Ω / □ or less.

[0025] In the present invention, "transparent" means that the conductive layer is transparent to the naked eye when processed to function as a touch panel, and the conductive layer itself does not necessarily have to be transparent. For example, even if an electrode pattern is provided so that the conductive layer functions as a touch panel, and the wiring itself is made of a metal such as gold and is opaque, the conductive layer can be considered transparent as long as the electrode pattern is not visible when the touch panel is viewed with the naked eye and an image can be observed.

[0026] The conductive filler in the conductive filler-containing layer is preferably a filler or fiber of metals such as gold, silver, copper, aluminum, nickel, titanium, iron, zinc, or tin, or an alloy thereof, a metal oxide filler, a metal-coated synthetic fiber, or a conductive carbon fiber such as carbon nanotubes. Metal, alloy, or metal oxide fillers of various shapes, such as spherical particles, flat particles, flake-like particles, needle-like particles, or fibrous particles, can be used. Among these, flake-like particles, needle-like particles, and fibrous fillers (fibers of metals or their alloys, metal oxide fibers, metal-coated synthetic fibers, and conductive carbon fibers) are preferred in terms of flex resistance, with fibrous fillers being even more preferred.

[0027] The conductive filler-containing layer preferably contains a binder resin. Examples of binder resins include polyester resins, polyurethane resins, polyamide resins, and acrylic resins. Furthermore, these resins are preferably crosslinked. Crosslinking agents may be used in accordance with the respective resins, and examples 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 per 100 parts by mass of the resin components constituting the conductive layer.

[0028] The conductive filler-containing layer can be formed by a coating method. The electrode pattern can be formed by chemical etching or laser etching after coating, or by printing. Examples of printing methods include gravure printing, letterpress printing, offset printing, screen printing, and inkjet printing, and can be selected depending on the properties of the coating material and the fineness of the pattern.

[0029] Examples of metals for the metal layer include gold, silver, copper, aluminum, nickel, titanium, iron, zinc, tin, etc. The metal layer can be formed by a vapor deposition method, a sputtering method, etc., and the electrode pattern is preferably formed by chemical etching or laser etching after the metal layer is formed.

[0030] Examples of metal oxide layers include ZnO, CeO2, Sb2O3, SnO2, indium tin oxide (abbreviated as ITO), In2O3, antimony-doped tin oxide (abbreviated as ATO), aluminum-doped zinc oxide (abbreviated as AZO), etc. The metal oxide layer can be formed by a method such as sputtering, and the electrode pattern is preferably formed by chemical etching or laser etching after the metal oxide layer is formed.

[0031] Examples of the conductive polymer-containing layer and the conductive polymer that can be used include aromatic conjugated poly(paraphenylene), heterocyclic conjugated polypyrrole, polythiophene, aliphatic conjugated polyacetylene, heteroatom-containing conjugated polyaniline, mixed conjugated poly(phenylene vinylene), multi-chain conjugated systems that have multiple conjugated chains in the molecule, and conductive composites that are polymers in which the above-mentioned conjugated polymer chains are grafted or block-copolymerized to a saturated polymer.

[0032] The conductive layer containing a conductive polymer may contain the resin component listed for the conductive filler-containing layer. The amount of the conductive polymer contained in the conductive layer containing a conductive polymer can be directly applied to the amount described for the conductive filler-containing layer. The conductive polymer-containing layer can be provided by a coating method, and the electrode pattern can be provided by the same method as described for the conductive filler-containing layer.

[0033] Among the above transparent conductive layers, a conductive filler-containing layer is more preferred from the viewpoints of transparency in the visible light region, bending resistance, and the like.

[0034] Preferred conductive fillers include fillers or fibers of metals such as gold, silver, aluminum, nickel, titanium, iron, zinc, and tin, or alloys thereof, metal oxide fillers, metal-coated synthetic fibers, and conductive carbon fibers. Metal, alloy, and metal oxide fillers of various shapes, such as spherical particles, flat particles, flake-like particles, and fibrous particles, can be used. Among these, flake-like particles and fibrous fillers (fibers of metals or alloys thereof, metal oxide fibers, metal-coated synthetic fibers, and conductive carbon fibers) are preferred in terms of flex resistance, with fibrous fillers being even more preferred.

[0035] The fiber diameter of the fibrous filler is preferably 200 nm or less, and the fiber length is preferably 1 μm or more. A fiber diameter of 200 nm or less is preferable because the haze value of the produced transparent conductive layer is not high and sufficient light transmission performance is obtained. The lower limit of the fiber diameter of the conductive fibrous filler is preferably 10 nm from the viewpoint of the conductivity of the transparent conductive layer, and the more preferred range of the fiber diameter is 15 to 180 nm. Furthermore, a fiber length of the conductive fibrous filler of 1 μm or more is preferable because a transparent conductive layer with sufficient conductivity can be formed and aggregation can be suppressed, thereby preventing an increase in the haze value and a decrease in light transmission performance. The upper limit of the fiber length is preferably 500 μm, and the more preferred range of the fiber length is 3 to 300 μm, and even more preferably 10 to 30 μm. The fiber diameter and fiber length of the conductive fibrous filler can be determined, for example, using an electron microscope such as a SEM or a TEM.

[0036] Examples of the conductive carbon fiber include vapor grown carbon fiber (VGCF), carbon nanotube (CNT), wire cup, wire wall, etc. These conductive carbon fibers can be used alone or in combination of two or more.

[0037] The CNTs may be single-walled CNTs, double-walled CNTs, or multi-walled CNTs with three or more layers, but those with a diameter of 0.3 to 100 nm and a length of approximately 0.1 to 20 μm are preferably used. In order to increase the transparency of the conductive layer and reduce the surface resistance, single-walled CNTs or double-walled CNTs with a diameter of 10 nm or less and a length of 1 to 10 μm are more preferred. Furthermore, it is preferable that the CNT aggregate contain as few impurities as possible, such as amorphous carbon and catalytic metals.

[0038] As the metal fibers, for example, fibers produced by a wire drawing method or a cutting method, which draws metal thin and long, can be used. One or more types of such metal fibers can be used. Among these metal fibers, metal fibers using silver are preferred because of their excellent conductivity.

[0039] Examples of metal-coated synthetic fibers include acrylic fibers coated with a metal. One or more types of such metal-coated synthetic fibers can be used. Among these metal-coated synthetic fibers, silver-coated synthetic fibers are preferred because of their excellent conductivity.

[0040] The content of the conductive filler in the transparent conductive layer is preferably, for example, 20 to 3,000 parts by mass per 100 parts by mass of the resin component constituting the transparent conductive layer. When the content is 3,000 parts by mass or less, the haze of the transparent conductive polyester film of the present invention is not increased and light transmission performance is sufficient, which is preferable. Furthermore, when the content is 20 parts by mass or more, the amount of binder resin that enters the contact points of the conductive filler is not too large, so that the conductivity of the transparent conductive layer is maintained and the resistance value of the transparent conductive polyester film of the present invention is low, which is preferable. The lower limit of the conductive filler content is more preferably 50 parts by mass, and the upper limit is more preferably 1,000 parts by mass.

[0041] Examples of metal oxide fillers include ZnO, CeO2, Sb2O3, SnO2, indium tin oxide (often abbreviated as ITO), In2O3, Al2O3, antimony-doped tin oxide (abbreviated as ATO), and aluminum-doped zinc oxide (abbreviated as AZO). The average particle size of the metal oxide filler is preferably 0.1 nm to 0.1 μm. By keeping the particle size within this range, a highly transparent conductive layer with almost no haze and good total light transmittance can be obtained.

[0042] The content of the metal oxide filler is preferably 10 to 400 parts by mass relative to 100 parts by mass of the resin component constituting the transparent conductive layer. If the content is less than 10 parts by mass, a transparent conductive layer having sufficient conductive properties may not be formed, whereas if the content exceeds 400 parts by mass, the haze of the transparent conductive polyester film of the present invention may increase or the light transmission performance may become insufficient.

[0043] The resin component of the transparent conductive layer is not particularly limited, and may be any of conventionally known materials. Examples of suitable crosslinking agents include polyester resins, polyurethane resins, polyamide resins, and acrylic resins. These resins are preferably crosslinked. Examples of suitable crosslinking agents include isocyanate compounds, epoxy resins, melamine compounds, oxazolines, carbodiimides, and compounds having two or more double bonds.

[0044] The conductive layer containing the conductive filler can be provided by applying (printing) a conductive paint containing the conductive filler and a resin component onto a polyester film of a transparent substrate, and then drying the applied paint.

[0045] When the transparent conductive layer is a conductive polymer-containing layer, examples of the conductive polymer that can be used include highly molecular weight conductive agents such as aromatic conjugated poly(paraphenylene), heterocyclic conjugated polypyrrole, polythiophene, aliphatic conjugated polyacetylene, heteroatom-containing conjugated polyaniline, mixed conjugated poly(phenylene vinylene), multi-chain conjugated systems that have multiple conjugated chains in the molecule, and conductive composites that are polymers in which the above-mentioned conjugated polymer chains are grafted or block-copolymerized to a saturated polymer.

[0046] The transparent conductive layer containing a conductive polymer may contain the above-mentioned resin component. The transparent conductive layer containing a conductive polymer can be provided by coating (printing) a conductive paint containing a conductive polymer and, if necessary, a resin component on a polyester film as a transparent substrate, and then drying the paint.

[0047] The conductive coating material may contain, in addition to the conductive filler or conductive polymer and resin component, additives and solvents generally used in coating materials, such as a crosslinking catalyst, a dispersant, a dispersion stabilizer, a thickener, a leveling agent, etc. Furthermore, if the coating material is a radiation-curable type, it may further contain a polymerization initiator, a polymerization inhibitor, etc. Furthermore, a mixture of multiple types of conductive fillers may be used, and both a conductive polymer and a conductive filler may be used.

[0048] The transparent conductive layer may contain refractive index adjusting particles. Examples of the refractive index adjusting particles include high refractive index particles and low refractive index particles. The high refractive index particles are not particularly limited, and examples include high refractive index resins containing aromatic rings, sulfur atoms, or bromine atoms in resin materials such as aromatic polyimide resins, epoxy resins, (meth)acrylic resins (acrylate and methacrylate compounds), polyester resins, and urethane resins, as well as their precursors, and other high refractive index materials, as well as the above-mentioned metal oxide filler particles and metal alkoxide particles. The low refractive index particles are not particularly limited, and examples include low refractive index resins containing fluorine atoms in resin materials such as epoxy resins, (meth)acrylic resins, polyester resins, and urethane resins, as well as their precursors, as well as magnesium fluoride particles and hollow or porous particles (organic or inorganic).

[0049] When the transparent conductive layer is a metal oxide layer, the metal oxides exemplified above as the metal oxide filler are preferably used as the metal oxide, and these are preferably formed by a dry film formation method.

[0050] As the transparent substrate film of the transparent conductive polyester film of the present invention, a polyester film having specific properties is used. The transparent substrate film of the transparent conductive polyester film of the present invention may be simply referred to as a transparent substrate film or a polyester film.

[0051] The polyester film may be a single-layer film made of one or more types of polyester resin, or when two or more types of polyester are used, it may be a multilayer structure film or an ultra-multilayer laminate film with a repeating structure.

[0052] Examples of polyester resins used in polyester films include polyester films made of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers containing these resin components as main components. Among these, stretched polyethylene terephthalate films are particularly preferred in terms of mechanical properties, heat resistance, transparency, cost, etc.

[0053] When a polyester copolymer is used for the polyester film, examples of the dicarboxylic acid component of the polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; and polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of the glycol component include fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; aromatic glycols such as p-xylene glycol; alicyclic glycols such as 1,4-cyclohexanedimethanol; and polyethylene glycols having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerization components in the copolymer is preferably less than 20% by mass. A mass ratio of less than 20% by mass is preferred because film strength, transparency, and heat resistance are maintained.

[0054] In addition, in the production of polyester films, the intrinsic viscosity of at least one type of resin pellets is preferably in the range of 0.50 to 1.0 dL / g. When the intrinsic viscosity is 0.50 dL / g or higher, the impact resistance of the resulting film is improved, and disconnection of the internal circuitry of a display due to external impact is less likely to occur, which is preferable. On the other hand, when the intrinsic viscosity is 1.00 dL / g or lower, the increase in filtration pressure of the molten fluid is prevented from becoming too large, which facilitates stable film production operations, which is preferable.

[0055] 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. A thickness of 10 μm or more provides satisfactory impact resistance and flex resistance, while a thickness of 300 μm or less is advantageous for weight reduction and also provides excellent flexibility, processability, handleability, etc.

[0056] The surface of the polyester film of the present invention may be smooth or uneven, but since it is used for display touch panel applications, deterioration of optical properties due to unevenness is undesirable. The haze is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. A haze of 3% or less can improve image visibility. The lower limit of the haze is the better, but from the viewpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0057] As described above, in order to reduce haze, it is preferable that the unevenness of the film surface is not too large. However, in order to provide a certain degree of slipperiness from the viewpoints of adhesion to the transparent conductive layer and ease of handling, unevenness can be formed by blending particles into the surface polyester resin layer or by coating a coating layer containing particles during film formation.

[0058] The method of blending particles into the polyester resin layer can be a known method. For example, they can be added at any stage of polyester production, but preferably they can be added as a slurry dispersed in ethylene glycol or the like at the stage of esterification, or after the completion of the transesterification reaction and before the start of the polycondensation reaction, to proceed with the polycondensation reaction. Alternatively, they can be added by a method of blending a slurry of particles dispersed in ethylene glycol or water with polyester raw materials using a vented kneading extruder, or a method of blending dried particles with polyester raw materials using a kneading extruder.

[0059] Among these, a method in which aggregate inorganic particles are homogeneously dispersed in a monomer liquid that will become a part of the polyester raw material, and then the filtered product is added to the remainder of the polyester raw material before, during, or after the esterification reaction is preferred. This method facilitates homogeneous dispersion of the particles and high-precision filtration of the slurry, since the monomer liquid has a low viscosity. Furthermore, when the monomer liquid is added to the remainder of the raw material, the particles are well dispersible and new aggregates are unlikely to form. From this perspective, it is particularly preferred to add the monomer liquid to the remainder of the raw material at a low temperature before the esterification reaction.

[0060] Furthermore, the number of protrusions on the film surface can be further reduced by a method (masterbatch method) in which a polyester containing particles is prepared in advance and then the pellets are kneaded and extruded with pellets containing no particles.

[0061] The polyester film may contain various additives, such as antistatic agents, UV absorbers, and stabilizers, as long as the total light transmittance remains within a preferred range.

[0062] The total light transmittance of the polyester film is preferably 85% or more, and more preferably 87% or more. A transmittance of 85% or more ensures sufficient visibility. The higher the total light transmittance of the polyester film, the better, but from the viewpoint of stable production, a total light transmittance of 99% or less is preferable, and 97% or less is also acceptable.

[0063] The maximum heat shrinkage of the polyester film after heat treatment at 150°C for 30 minutes is preferably 6% or less, and more preferably 5% or less. A heat shrinkage of 6% or less can suppress flatness defects such as curling and undulation during lamination processing of the hard coat layer or application processing of the transparent conductive layer. The lower the heat shrinkage, the better, but a value of -1% or more is preferable, and 0% or more is more preferable. A negative value here indicates expansion after heating, and a value below -1% may also result in flatness defects.

[0064] The polyester film used in the transparent conductive polyester film of the foldable display of the present invention can provide sufficient pencil hardness to the hard-coated film after laminating a hard-coat layer. It is believed that when conventional polyester films are laminated with a hard-coat layer, the pencil hardness of the hard-coated film is reduced due to deformation in the thickness direction. In the present invention, by setting the indentation depth in the film thickness direction after unloading the test force using a dynamic ultra-microhardness tester (described below) within a specific range, high hardness can be achieved in the pencil hardness evaluation of the hard-coated film. The indentation depth in the film thickness direction after unloading the test force is preferably 1.5 μm or less, more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. When the indentation depth after unloading the test force (final deformation amount under load) is 1.5 μm or less, the film is less likely to deform in the thickness direction during pencil hardness evaluation of the hard-coated film after laminating a hard-coat layer, resulting in high pencil hardness. Increasing the pencil hardness of the hard coat film reduces the likelihood of scratches and dents on the display surface, improving the visibility of the display. The lower the indentation depth after unloading the test force, the better, but from the perspective of stable production and saturation of the effect, a depth of 0.3 μm or more is preferable, and even more preferably 0.5 μm or more.

[0065] In order to reduce the indentation depth after the test force is removed, it is effective to adjust the refractive index in the thickness direction to 1.520 or less. As a means for adjusting the refractive index to 1.520 or less, as will be described later, examples of conditions that can be used include adjusting the stretch ratio in the bending direction or folding direction to a high value, setting the stretching temperature in the bending direction or folding direction to a low value, or setting the heat setting temperature to a high value, within a range that allows other physical properties and the refractive index in the bending direction or folding direction to be controlled within preferred ranges.

[0066] The surface of the polyester film of the present invention may be subjected to a treatment to improve adhesion to a resin forming a hard coat layer or the like or to a transparent conductive layer.

[0067] Examples of surface treatment methods include roughening treatments such as sandblasting and solvent treatment, and oxidation treatments such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone / ultraviolet irradiation treatment, flame treatment, chromic acid treatment, and hot air treatment, and any of these methods can be used without particular limitation.

[0068] Furthermore, adhesion can be improved by an adhesion-improving layer such as an easy-adhesion layer. The easy-adhesion layer can be made of any resin, such as an acrylic resin, a polyester resin, a polyurethane resin, or a polyether resin, and can be formed by a general coating method, preferably a so-called in-line coating method.

[0069] The polyester film of the present invention may also be provided with a hard coat layer. As a transparent conductive layer for a touch panel module, the hard coat layer may be provided between the substrate film and the transparent electrode layer as a refractive index adjustment layer to make the electrode pattern less visible. In this case, the hard coat layer itself may also serve as the refractive index adjustment layer, or a separate layer for refractive index adjustment may be laminated. Furthermore, a hard coat layer may be provided to improve the flex resistance and impact resistance of the transparent conductive layer itself, thereby adjusting the elastic modulus of the entire film.

[0070] The polyester film can be produced, for example, through a polymerization step in which inorganic particles are homogeneously dispersed in a monomer liquid that becomes a part of the polyester raw material, the resulting dispersion is filtered, and the resulting dispersion is added to the remainder of the polyester raw material to polymerize the polyester; and a film formation step in which the resulting polyester is melt-extruded into a sheet through a filter, cooled, and stretched to form a substrate film.

[0071] Next, a method for producing a polyester film will be described in detail using an example in which polyethylene terephthalate (hereinafter sometimes referred to as PET) pellets are used as the raw material for the base film, but the method is not limited to this. Furthermore, the number of layers, such as a single layer or a multilayer structure, is not limited.

[0072] After mixing and drying PET pellets in a predetermined ratio, the mixture is fed into a known melt lamination extruder, extruded through a slit die into a sheet, and cooled and solidified on a casting roll to form an unstretched film. While a single extruder is sufficient for a single-layer film, multilayer films can be produced using two or more extruders and two or more manifolds or merging blocks (e.g., merging blocks with rectangular merging sections) to laminate the multiple film layers that make up the outermost layers, extrude a two or more layer sheet from the die, and cool it on a casting roll to form an unstretched film.

[0073] In this case, it is preferable to perform high-precision filtration to remove foreign matter contained in the resin at any location where the molten resin is maintained at about 280°C during melt extrusion. The filter material used for high-precision filtration of the molten resin is not particularly limited, but a stainless steel sintered filter material is preferred because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, and Cu and high-melting-point organic matter.

[0074] Furthermore, the filtration particle size (initial filtration efficiency 95%) of the filter material is preferably 20 μm or less, particularly preferably 15 μm or less. If the filtration particle size (initial filtration efficiency 95%) of the filter material exceeds 20 μm, foreign matter of 20 μm or more in size cannot be sufficiently removed. Although high-precision filtration of molten resin using a filter material with a filtration particle size (initial filtration efficiency 95%) of 20 μm or less may reduce productivity, it is preferable to obtain a film with fewer protrusions due to coarse particles.

[0075] (Refractive index in bending direction) In the present invention, the refractive index of the polyester film in at least one of the longitudinal direction (machine flow direction) and width direction is preferably 1.590 to 1.620, more preferably 1.591 to 1.600. The refractive index of the polyester film in the bending direction is preferably 1.590 to 1.620, even more preferably 1.591 to 1.600. Here, the bending direction refers to the direction perpendicular to the folding portion (reference numeral 21) expected in the use of a folding display, as indicated by reference numeral 22 on the polyester film (reference numeral 2) in FIG. 2. A refractive index of 1.590 to 1.620 in at least one of the longitudinal direction and width direction is preferably 1.590 to 1.620, as this reduces deformation during repeated folding and thus does not risk degrading the image quality of the folding display. A refractive index of 1.591 to 1.600 is more preferable. Of course, this direction is preferably the bending direction. If the refractive index is 1.590 or more, there is no risk of cracks occurring in the folded direction after the bending test described below, and of course no breakage occurs, so the visibility of the display can be maintained at a good level. The refractive index of the polyester film can be effectively adjusted by adjusting the stretching ratio and stretching temperature. A relaxation step in the stretching direction and multi-stage stretching may also be used to adjust the refractive index. When multi-stage stretching is performed, it is preferable to set the stretching ratios in the second and subsequent stages higher than the stretching ratio in the first stage.

[0076] By controlling the refractive index in at least one of the longitudinal direction (machine flow direction) and width direction of the polyester film within the above range, and more preferably by controlling the refractive index in the bending direction within the above range, fatigue due to compressive stress applied to the inside of the fold when folded can be reduced. Fatigue due to compressive stress is thought to occur mainly in crystalline parts, and the fewer crystals there are in the bending direction, the less fatigue there is. Therefore, it is thought that lowering the refractive index reduces the amount of oriented crystals in the bending direction, thereby suppressing compressive fatigue.

[0077] Furthermore, the creep phenomenon caused by tensile stress applied to the outside of the fold when folded can be suppressed by reducing the refractive index. Fatigue due to tensile stress is thought to occur mainly in the amorphous portion, and repeated stress causes molecular chains to align, resulting in deformation. It can be assumed that the fewer molecular chains aligned in the bending direction, the less deformation due to alignment. Furthermore, since fatigue due to tension can be suppressed with fewer amorphous portions, a higher degree of crystallinity, i.e., a higher density, is preferable.

[0078] In the present invention, the stretching ratio of the unstretched polyester sheet in at least one of the longitudinal direction (machine flow direction) and the transverse direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. The stretching direction is preferably the bending direction. A stretching ratio of 1.2 times or more is preferred because it prevents deformation during post-processing, such as hard coating, while a stretching ratio of 2.0 times or less is preferred because it prevents thickness unevenness in the film. The stretching temperature is preferably 75 to 120°C, more preferably 75 to 105°C. Heating methods during stretching can be conventionally known, such as hot air heating, roll heating, and infrared heating. Setting the stretching temperature to 75 to 120°C can prevent significant thickness unevenness due to stretching at the above stretching ratio. Furthermore, by stretching at a temperature as low as possible within a range that does not cause significant thickness unevenness, the refractive index in the thickness direction can be reduced.

[0079] (Refractive index in the direction of the fold) The refractive index of the polyester film in a direction perpendicular to the direction in which the refractive index is 1.590 to 1.620 is preferably 1.670 to 1.700. That is, the refractive index in the direction perpendicular to the bending direction (the direction of the folded portion) is preferably 1.670 to 1.700. By adjusting the refractive index to 1.670 to 1.700, deformation when folded in the bending direction can be reduced. By adjusting the refractive index to 1.700 or less, cracks and breaks in the direction of the folded portion can be suppressed. By adjusting the refractive index to 1.670 or more, flexibility in the bending direction can be improved, and impact resistance and surface hardness can be improved. A refractive index of 1.680 to 1.695 is more preferable. Methods for adjusting the refractive index in the direction perpendicular to the bending direction include the stretching ratio, stretching preheating temperature, stretching temperature, multi-stage stretching, and film relaxation. The stretching ratio is preferably 4.0 to 6.0, more preferably 4.4 to 6.0. The preheating temperature for stretching in the direction perpendicular to the bending direction is preferably 70 to 110°C. When multi-stage stretching is performed in the direction perpendicular to the bending direction, it is preferable to use a higher stretch ratio in the second and subsequent stages than in the first stage. The film may be relaxed by 1 to 10% in either the machine direction (longitudinal direction) or the perpendicular direction (width direction).

[0080] (Refractive index in the thickness direction) The refractive index in the thickness direction is preferably 1.520 or less. By setting the refractive index to 1.520 or less, even if the refractive index in the bending direction is designed to be low, the film's impact resistance and surface hardness can be suppressed, and all of the flexibility, impact resistance, and surface hardness can be achieved. By setting the refractive index to 1.520 or less, the indentation depth in the thickness direction after unloading the test force is reduced, and the hardness of the film surface, particularly cracking and cutting of the transparent conductive polyester film after lamination of the transparent conductive layer, can be prevented. The refractive index is more preferably 1.515 or less, even more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. A low refractive index in the thickness direction is preferable, but from the perspective of stable production, it is preferably 1.3 or more, and may even be 1.4 or more. It is particularly preferably 1.410 or more. It can be said that the above range can be achieved by increasing the stretching 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 index in the bending direction and width direction to a preferred range, it is preferable to set the conditions while checking the balance of the conditions for each process in the film-forming process.

[0081] Methods for controlling the refractive index in the thickness direction within the above range include the preheating temperature, stretching temperature, and stretching ratio in the bending direction, and the preheating temperature, stretching temperature, multi-stage stretching, high-ratio stretching, or heat setting temperatures in the folding direction. The preheating temperature for stretching in the bending direction is preferably 70°C to 110°C. The stretching temperature in the bending direction is preferably 75°C to 120°C. The stretching ratio in the bending direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. By lowering the stretching temperature and stretching at a low stretching ratio, the refractive index in the thickness direction can be effectively reduced while maintaining the flexibility in the bending direction. The preheating temperature for stretching in the folding direction is also preferably 75°C to 110°C. The stretching temperature is preferably 75°C to 120°C. The stretching ratio in the folding direction is preferably 4.0 to 6.0 times, more preferably 4.4 to 6.0 times. The refractive index in the thickness direction can be effectively reduced while maintaining or reducing the refractive index in the bending direction. Multistage stretching may be used as a method for high-ratio stretching. In this case, it is preferable to set the stretching ratio in the second stage higher than that in the first stage, as this allows for effective control of the refractive index. A method in which stretching is performed again after the crystallization step may also be used. Accelerated stretching, in which the stretching speed is increased from the initial stage to the latter stage of stretching, may also be used. The heat setting temperature is preferably 180 to 240° C. By performing heat setting, oriented crystallization in the stretching direction progresses, and the refractive index in the thickness direction can be reduced. The reason why lowering the refractive index in the thickness direction improves the impact resistance and surface hardness of the film is not entirely clear, but it is thought that aromatic groups such as benzene rings in the molecular chain are oriented in the plane direction, which has the effect of suppressing deformation due to stress applied in the thickness direction.

[0082] (Regarding polyester film density) The density of polyester film is 1.380 g / cm 3 It is preferable that the density is 1.383 g / cm or more. 3 More preferably, it is 1.380 g / cm or more. 3By setting the density to 1.40 g / cm or more, it is possible to improve the flexibility and impact resistance of the film, and in particular to prevent cracking and tearing of the transparent conductive polyester film after laminating the conductive film. The higher the density, the better, and although it is somewhat affected by the presence or absence of particles in the film, it is 1.40 g / cm 3 By setting the heat setting temperature during film formation to 180 to 240°C, crystallization can be promoted and the density can be effectively increased.

[0083] The bending direction of the polyester film is preferably aligned with the longitudinal direction (machine flow direction). This facilitates biaxial stretching to reduce the refractive index in the bending direction and improve flexibility. That is, a polyester film is preferably obtained by stretching an unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.2 to 2.0, more preferably 1.7 to 2.0. Furthermore, a preferred embodiment is to stretch the polyester sheet in the width direction at a stretching ratio of 4.0 to 6.0, more preferably 4.4 to 6.0.

[0084] In the present invention, the polyester film (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folding part is 1.670 to 1.700 (3) Refractive index in the thickness direction is 1.520 or less (4) Density is 1.380 g / cm 3 End Although it is particularly preferable to simultaneously achieve the above four properties, even within the above-mentioned preferred manufacturing conditions, if the combination of conditions is not optimal within each preferred manufacturing condition range, such as a stretch ratio in the bending direction of 1.4 or less, a stretch ratio in the folded direction of less than 4.4, and a heat setting temperature of 220° C. or less, it may not necessarily be possible to simultaneously achieve the above four properties. In this case, the above four properties can be simultaneously achieved by fine-tuning any of the conditions or by combining them, such as increasing the stretch ratio in the bending direction to 1.7 or more, increasing the stretch ratio in the folded direction to 4.4 or more, increasing the heat setting temperature to about 230° C., or lowering the stretching temperature in the bending direction and / or the folded direction.

[0085] Although any film-forming method such as stretching, relaxation, heat setting, or surface treatment may be used to adjust film-forming properties, film strength, thermal dimensional stability, and appearance defects, controlling the refractive index and density of the film within the above-mentioned preferred ranges is a particularly preferred embodiment of the present invention. By controlling the refractive index and density within the preferred ranges, a polyester film suitable for foldable displays can be provided that has superior flex resistance and surface hardness compared to conventional films, particularly high pencil hardness of the hard coat film after lamination of a hard coat layer.

[0086] Specifically, for example, PET pellets are thoroughly vacuum-dried, fed into an extruder, melt-extruded into a sheet at approximately 280°C, and cooled to solidify, forming an unstretched PET sheet. The resulting unstretched sheet is stretched 1.2 to 2.0 times, more preferably 1.7 to 2.0 times, in the longitudinal direction using rolls heated to 75 to 120°C to obtain a uniaxially oriented PET film. The film is then gripped at its edges with clips and introduced into a hot air zone heated to 75 to 120°C, where it is dried and then stretched 4.0 to 6.0 times, more preferably 4.4 to 6.0 times, in the transverse direction. Subsequently, the film is introduced into a heat treatment zone at 180 to 240°C, where it is heat-treated for 1 to 60 seconds. During this heat treatment process, if necessary, a relaxation treatment of 0 to 10% may be performed in the transverse or longitudinal direction.

[0087] The intrinsic viscosity of the polyester film is preferably in the range of 0.50 to 1.0 dL / g. When the intrinsic viscosity is 0.50 dL / g or more, the impact resistance is improved and disconnection of the internal circuit of the display due to external impact is less likely to occur, which is preferable. On the other hand, when the intrinsic viscosity is 1.00 dL / g or less, the increase in filtration pressure of the molten fluid is not too large, which is preferable because film production is stable.

[0088] (Easy adhesion layer) In the present invention, it is also preferable to laminate an easy-adhesion layer on the polyester film in order to improve the adhesion between the polyester film and a transparent conductive layer or a hard coat layer, etc. The easy-adhesion layer can be obtained by applying a coating liquid for forming the easy-adhesion layer to one or both sides of an unstretched or uniaxially stretched film in the machine direction, followed by heat treatment and drying as necessary, and then stretching the film in at least one direction that has not been stretched. Heat treatment can also be performed after biaxial stretching. The final coating amount of the easy-adhesion layer is 0.005 to 0.20 g / m 2 It is preferable to control the coating amount to 0.005 g / m 2 On the other hand, when the coating amount is 0.20 g / m or more, adhesiveness can be obtained, which is preferable. 2 If it is less than this, blocking resistance can be obtained, which is preferable.

[0089] Resins contained in the coating liquid used to laminate the easy-adhesion layer include, without particular limitation, polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. Examples of crosslinking agents contained in the coating liquid for forming the easy-adhesion layer include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, carbodiimide compounds, etc. Two or more of each can also be mixed and used. Due to the nature of in-line coating, these are preferably applied using an aqueous coating liquid, and the resins and crosslinking agents are preferably water-soluble or water-dispersible resins or compounds.

[0090] It is preferable to add particles to the adhesion layer to impart slipperiness. The average particle size of the fine particles is preferably 2 μm or less. If the average particle size of the particles exceeds 2 μm, the particles tend to fall off from the adhesion layer. Examples of particles to be contained in the adhesion layer include inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, and organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the adhesion layer alone or in combination of two or more.

[0091] The coating solution can be applied by any known method, similar to that used for the coating layer, such as reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, wire bar coating, and pipe doctor coating, which can be used alone or in combination.

[0092] (Hard coat layer) The polyester film preferably has a hard coat layer on at least one surface to adjust the refractive index or improve flex resistance and crack / cut resistance. The hard coat layer, located between the polyester film and the transparent conductive layer, can also block the adverse effects of oligomers precipitated from the polyester film. The hard coat layer is preferably positioned on the polyester film or on the easy-adhesion layer. Resins that can be used to form the hard coat layer include, without limitation, acrylics, siloxanes, inorganic hybrids, urethane acrylates, polyester acrylates, and epoxy resins. Two or more materials can also be mixed, or particles such as inorganic or organic fillers can be added.

[0093] (Thickness of hard coat layer) The thickness of the hard coat layer is preferably 1 to 50 μm. A thickness of 1 μm or more is preferable because it cures sufficiently and the pencil hardness is high. Furthermore, by keeping the thickness at 50 μm or less, curling due to cure shrinkage of the hard coat can be suppressed, improving the handleability of the film.

[0094] (Application method) The hard coat layer can be applied by any method, including a Mayer bar, gravure coater, die coater, knife coater, etc., without any particular limitation, and can be appropriately selected depending on the viscosity and film thickness.

[0095] (Curing conditions) The hard coat layer can be cured by energy rays such as ultraviolet rays and electron beams, or by heat, and curing methods using ultraviolet rays or electron beams are preferred in order to reduce damage to the film.

[0096] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 3H or more, more preferably 4H or more. A pencil hardness of 3H or more prevents scratches and does not reduce visibility. Generally, a high pencil hardness of the hard coat layer is preferable, but a pencil hardness of 9H or less, 8H or less, or even 6H or less can be used without any practical problems.

[0097] (Hard Coat Layer Characteristics) The hard coat layer of the present invention can be used for purposes such as adjusting the refractive index, improving flex resistance, cracking and cutting resistance, improving surface hardness, and blocking the adverse effects of oligomers precipitated from the polyester on the transparent conductive layer, as described above. When used in a display, a high transmittance is preferable. The transmittance of the hard coat film is preferably 87% or more, and more preferably 88% or more. A transmittance of 87% or more provides sufficient visibility. While a higher total light transmittance of the hard coat film is generally preferable, from the standpoint of stable production, it is preferably 99% or less, and may be 97% or less. Furthermore, the haze of the hard coat film is generally preferably low, and preferably 3% or less. The haze of the hard coat film is more preferably 2% or less, and most preferably 1% or less. A haze of 3% or less can improve image visibility. While a lower haze is generally preferable, from the standpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more.

[0098] The hard coat layer may further have other functions added thereto, such as an antiglare layer, an antiglare and antireflection layer, an antireflection layer, a low-reflection layer, an antistatic layer, or the like, which has the function of adjusting the refractive index to a certain value or improving resistance to bending and cracking / cutting as described above.

[0099] A hard coat layer can be provided on the polyester film. To obscure the electrode pattern as a transparent conductive layer in a touch panel module, a refractive index adjustment layer is preferably provided between the polyester film and the transparent conductive layer or between the hard coat layer and the transparent electrode layer. In this case, the hard coat layer itself may double as the refractive index adjustment layer, or a separate refractive index adjustment layer may be laminated on top of it. Examples of refractive index adjustment layers include resin layers containing the above-mentioned refractive index adjustment particles, fluorine-containing resin layers, and layers of high-refractive-index resins containing aromatic rings, sulfur atoms, or bromine atoms in resin materials such as aromatic polyimide resins, epoxy resins, (meth)acrylic resins (acrylate and methacrylate compounds), polyester resins, and urethane resins, as well as their precursors. These can be applied by coating. Inorganic layers such as ZnO, CeO2, Sb2O3, SnO2, indium tin oxide, In2O3, Al2O3, antimony-doped tin oxide, aluminum-doped zinc oxide, SiO2, and magnesium fluoride are also preferred as refractive index adjustment layers, and these can be applied by wet film formation.

[0100] Preferred laminate structures of the transparent conductive polyester film in the present invention include, for example, polyester film / transparent conductive layer, polyester film / easy adhesion layer / transparent conductive layer, polyester film / hard coat layer / transparent conductive layer, polyester film / easy adhesion layer / hard coat layer / transparent conductive layer, polyester film / refractive index adjusting layer (one layer or multiple layers with different refractive indexes) / transparent conductive layer, polyester film / easy adhesion layer / refractive index adjusting layer (one layer or multiple layers with different refractive indexes) / transparent conductive layer, polyester film / hard coat layer / refractive index adjusting layer (one layer or multiple layers with different refractive indexes) / transparent conductive layer, polyester film / easy adhesion layer / hard coat layer / refractive index adjusting layer (one layer or multiple layers with different refractive indexes) / transparent conductive layer, and the like. These combinations of laminate structures may be present on one side of the polyester film, or may be present on both sides via the polyester film.

[0101] In the touch panel module of the foldable display of the present invention, the transparent conductive polyester film of the present invention is used as the transparent conductive polyester film constituting the touch panel module, but it does not need to be used for all of the transparent conductive polyester 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, polyimide film, polyamide film, polyamideimide film, polyester film other than the polyester film of the present invention, polycarbonate film, acrylic film, triacetyl cellulose film, cycloolefin polymer film, polyphenylene sulfide film, polymethylpentene film, etc. can be used as the transparent substrate film of the transparent conductive polyester film appropriately according to suitability. [Example]

[0102] Next, the present invention will be described with reference to examples and comparative examples. First, the evaluation methods of the characteristic values ​​used in the present invention will be described below.

[0103] (1) Intrinsic viscosity The film or polyester resin was crushed and dried, then dissolved in a 60 / 40 (mass ratio) phenol / tetrachloroethane mixed solvent. After centrifuging the solution to remove inorganic particles, an Ubbelohde viscometer was used to measure the flow time of a 0.4 (g / dL) solution at 30°C and the flow time of the solvent alone. The intrinsic viscosity was calculated from the ratio of these times using the Huggins equation, assuming a Huggins constant of 0.38.

[0104] (2) Bending resistance of polyester film sample (bending radius 1.5 mm) A polyester film sample measuring 20 mm in width and 110 mm in machine direction was prepared. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the sample was bent 200,000 times at a rate of 1 bend / second with a bend radius of 1.5 mm. The sample was fixed at 10 mm positions on both long sides, and the bending area was 20 mm x 90 mm. Figure 1 is a schematic diagram showing the bend radius when a foldable display is folded. Considering the case where a polyester film is disposed on the inner surface of the folded display, the bending test was performed as a model, with the bending radius set to 1.5 mm at the point indicated by reference numeral 11 in Figure 1. After the bending process, the sample was placed on a flat surface with the bent inner side facing down and visually observed. ○: No cracks or deformations were observed in the sample. ×: The sample has cracks or creases, and when placed horizontally, the maximum lift is 5 mm or more.

[0105] (3) Bending resistance of polyester film sample (bending radius 0.5 mm) Using the same method as in the bending test described above, the display was bent 200,000 times at a bending radius of 0.5 mm and a rate of 1 bend per second. Figure 1 is a schematic diagram showing the bending radius when the foldable display is folded. Taking into account the case where a polyester film is disposed on the inner surface of the folded display, the bending test was performed as a model, with the bending radius set to 0.5 mm at the location indicated by reference numeral 11 in Figure 1 . The film surface outside the bent portion was observed at 700x magnification using a digital microscope (HIROX RH8800) to check for the presence or absence of wrinkles (cracks). In addition to the visual observation of bending resistance with a bending radius of 1.5 mm described above, this test was performed with a bending radius reduced to 0.5 mm to evaluate the display under conditions similar to those in actual use, where a hard coat layer or other components are laminated or attached. This test, separate from the visual observation with a bending radius of 1.5 mm, was also performed to detect minute defects that are difficult to detect visually, such as those prone to breakage or cracking. ○: No defects on the film surface on the outer side of the bend. ×: The film broke or wrinkles (cracks) were observed on the film surface on the outer side of the bend.

[0106] (4) Bending resistance of transparent conductive polyester film sample (bending radius 3.0 mm) Using the same method as in the bending test above, the film was bent 200,000 times at a bending radius of 3.0 mm and a speed of 1 bend / second. Tests were conducted with the conductive film facing inward and outward. ○: No cracks or cuts in the conductive film, and no deformation of the transparent conductive polyester film was observed. ×: Cracks, breaks, or peeling from the substrate were observed in the conductive film, or deformation was observed in the transparent conductive polyester film.

[0107] (5) Refractive index In accordance with JIS K 7142:2008 "Method for measuring refractive index of plastics (Method A)", the refractive index in the longitudinal direction, width direction, and thickness direction was determined using an Abbe refractometer (manufactured by Atago Co., Ltd., NAR-4T, measurement wavelength 589 nm).

[0108] (6)Pencil hardness The pencil hardness of the hard coat film sample was measured at a load of 750 g and a speed of 1.0 mm / s in accordance with JIS K 5600-5-4: 1999. In the present invention, a value of 3H or more was considered acceptable.

[0109] (7) Total light transmittance, haze The measurement was carried out using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0110] (8) Density The density was measured according to the method (density gradient tube method) in accordance with JIS K 7112:1999 (unit: g / cm 3 ).

[0111] (9) Indentation depth after removing the test force The sample was cut into approximately 2 cm squares and fixed on an 18 × 18 mm microcover glass (manufactured by Matsunami Glass Co., Ltd.) with an adhesive (Cemedine (registered trademark) High Super 30) on the surface opposite the measurement surface. After adhesion and fixation, the sample was left at room temperature for 12 hours or more, and then the indentation depth (μm) after removing the test force was measured using a dynamic ultra-microhardness tester "DUH-211" (manufactured by Shimadzu Corporation) under the following conditions. <Measurement conditions> Test mode: Load-unload test Indenter used: 115 degree ridge angle, triangular pyramid indenter Indenter elastic modulus: 1.140 x 106 N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50mN Load speed: 4.44mN / sec Load holding time: 2sec Unloading holding time: 0sec

[0112] (10) Maximum heat shrinkage rate The sample film was cut to a size of 10 mm length x 250 mm width, and marks were made at 200 mm intervals along the long side in the direction of measurement. The distance A between the marks was measured under a constant tension of 5 g. The sample film was then left in an oven at 150°C for 30 minutes without load, then removed from the oven and cooled to room temperature. The distance B between the marks was then measured under a constant tension of 5 g, and the thermal shrinkage (%) was calculated using the following formula. The thermal shrinkage was measured at three equal positions across the width of the sample film, and the average value of the three points was taken as the thermal shrinkage (%). Heat shrinkage rate (%) = [(AB) x 100] / A The sample film is cut so that the length and width of the sample film are different in both the bending direction and the folding direction, and measurements are taken. The data in the direction where the measurement value is larger is the maximum heat shrinkage rate (%).

[0113] (Preparation of polyethylene terephthalate pellets (a)) A continuous esterification reactor consisting of a three-stage complete mixing vessel equipped with an agitator, a partial condenser, a raw material inlet, and a product outlet was used. TPA was supplied at 2 tons / hr, EG at 2 moles per mole of TPA, and antimony trioxide in an amount sufficient to provide 160 ppm Sb atoms relative to the PET product. These slurries were continuously fed into the first esterification reactor of the esterification reactor and reacted at atmospheric pressure for an average residence time of 4 hours at 255°C. The reaction product in the first esterification reactor was then continuously removed from the system and fed to the second esterification reactor. EG distilled from the first esterification reactor was fed into the second esterification reactor at 8% by mass relative to the polymer (PET product). An EG solution containing magnesium acetate in an amount sufficient to provide 65 ppm Mg atoms relative to the PET product, and an EG solution containing TMPA in an amount sufficient to provide 20 ppm P atoms relative to the PET product, were then added, and the reaction was continued at atmospheric pressure for an average residence time of 1.5 hours at 260°C. Next, the reaction product in the second esterification reactor was continuously removed from the system and fed to a third esterification reactor, and an EG solution containing TMPA in an amount such that the P atom concentration was 20 ppm relative to the produced PET was added, followed by reaction at atmospheric pressure for an average residence time of 0.5 hours at 260°C. The esterification reaction product produced in the third esterification reactor was continuously fed to a three-stage continuous polycondensation reactor to carry out polycondensation, and then filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cutoff of 5 μm particles) to obtain polyethylene terephthalate pellets (a) with an intrinsic viscosity of 0.62 dL / g.

[0114] (Preparation of polyethylene terephthalate pellets (b)) The intrinsic viscosity was adjusted to 0.580 dl / g in the same manner as in the production process of polyethylene terephthalate pellets (a), except that the residence time of the third esterification reaction was adjusted, to obtain polyethylene terephthalate pellets (b).

[0115] (Preparation of polyethylene terephthalate pellets (c)) The polyethylene terephthalate pellets (a) were subjected to solid-state polymerization at 220°C under a reduced pressure of 0.5 mmHg for various times using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (c) with an intrinsic viscosity of 0.75 dl / g.

[0116] (Polymerization of urethane resin) A four-neck flask equipped with a stirrer, Dimroth condenser, nitrogen inlet tube, silica gel drying tube, and thermometer was charged with 72.96 parts by weight of 1,3-bis(isocyanatemethyl)cyclohexane, 12.60 parts by weight of dimethylolpropionic acid, 11.74 parts by weight of neopentyl glycol, 112.70 parts by weight of polycarbonate diol having a number average molecular weight of 2000, and 85.00 parts by weight of acetonitrile as a solvent and 5.00 parts by weight of N-methylpyrrolidone. The mixture was stirred at 75°C for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction solution reached the predetermined amine equivalent. Next, the reaction solution was cooled to 40°C, and 9.03 parts by weight of triethylamine was added to obtain a polyurethane prepolymer D solution. Next, 450 g of water was added to a reaction vessel equipped with a high-speed homodisperser, and the temperature was adjusted to 25°C. While stirring and mixing at 2000 min-1, the isocyanate-terminated prepolymer was added and dispersed in water. Thereafter, acetonitrile and a portion of the water were removed under reduced pressure to prepare a water-soluble polyurethane resin (A) having a solid content of 35% by mass.

[0117] (Polymerization of Water-Soluble Carbodiimide Compounds) 200 parts by weight of isophorone diisocyanate and 4 parts by weight of 3-methyl-1-phenyl-2-phospholene-1-oxide (carbodiimidation catalyst) were placed in a flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer, and stirred at 180°C for 10 hours under a nitrogen atmosphere to obtain isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Next, 111.2 g of the resulting carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100°C for 24 hours. Water was gradually added at 50°C to obtain a yellow, transparent, water-soluble carbodiimide compound (B) with a solids content of 40% by weight.

[0118] (Preparation of coating solution for forming easy-adhesion layer) The following coating materials were mixed to prepare a coating solution. Water 16.97 parts by mass Isopropanol 21.96 parts by mass Polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (Silica sol with an average particle size of 40 nm, solid content concentration of 40% by mass) Surfactant 0.05 parts by mass (Silicone-based, solid content 100% by mass)

[0119] (Preparation of hard coat coating solution a) To 100 parts by mass of a hard coat material (Opstar (registered trademark) Z7503, manufactured by JSR Corporation, concentration 75%), 0.1 parts by mass of a leveling agent (BYK307, manufactured by BYK Japan, concentration 100%) was added, and the mixture was diluted with methyl ethyl ketone to prepare a hard coat coating solution a with a solids concentration of 40% by mass.

[0120] (Preparation of hard coat coating solution b) 95 parts by mass of pentaerythritol triacrylate (A-TMM-3, manufactured by Shin-Nakamura Chemical Co., Ltd., solids concentration 100%), 5 parts by mass of photopolymerization initiator (Irgacure (registered trademark) 907, manufactured by BASF Japan Ltd., solids concentration 100%), and 0.1 parts by mass of leveling agent (BYK307, manufactured by BYK Japan KK, solids concentration 100%) were mixed and diluted with a solvent of toluene / MEK=1 / 1 to prepare hard coat coating solution b with a concentration of 40% by mass.

[0121] (Preparation of coating solution containing conductive fibrous filler and metal nanowires) A mixed solution of 0.6 g of silver nitrate (Wako Pure Chemical Industries, Ltd.), 36 g of a 1.4 wt% polyvinylpyrrolidone (PVP, Wako Pure Chemical Industries, Ltd., average molecular weight 360,000) solution in ethylene glycol (EG, Kishida Chemical Co., Ltd.), 4 g of a 165 ppm iron (III) chloride (Kishida Chemical Co., Ltd.) solution in EG, and 109 g of EG was prepared and designated as reaction solution 1. Reaction solution 1 was heated from room temperature to 130°C using a personal synthesizer (ChemiStation, PPV-CTRL1, Tokyo Rikakikai Co., Ltd.) and reacted for 187 minutes. A filter thimble (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 poured onto the outside of the filter thimble so that it was flush with the reaction solution inside the filter thimble. After one week, the solution inside the filter thimble was recovered and used as a metal nanowire-containing coating solution.

[0122] Example 1 Polyethylene terephthalate pellets (a) were fed into an extruder and melted at 285°C. This polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet form from a die, and then contacted with a casting drum with a surface temperature of 30°C using an electrostatic casting method, cooled and solidified, to produce an unstretched film. This unstretched film was uniformly heated to 75°C using a heated roll, heated to 85°C using a non-contact heater, and roll-stretched (longitudinal stretching) by 1.4 times. The above-mentioned coating solution for forming an easy-adhesion layer was applied to both sides of the obtained uniaxially stretched film using a roll coating method, and then dried at 80°C for 20 seconds. The final coating amount after drying (after biaxial stretching) was 0.06 g / m 2 The film was then introduced into a tenter, preheated at 105°C, transversely stretched 4.0 times at 95°C, fixed in width, heat-set at 230°C for 5 seconds, and further relaxed in the width direction by 4% at 180°C to obtain a polyethylene terephthalate film with a thickness of 50 μm.

[0123] (Examples 2 and 3) A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed to that shown in Table 1.

[0124] Example 4 A polyester film was obtained in the same manner as in Example 1, except that the draw ratio in the width direction was changed to 4.4 times and the heat setting temperature was changed to 220°C.

[0125] (Examples 5 to 6) A polyester film was obtained in the same manner as in Example 4, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1.

[0126] Example 7 A polyester film was obtained in the same manner as in Example 1, except that the draw ratio in the width direction was changed to 5.5 times and the heat setting temperature was changed to 190°C.

[0127] (Examples 8 to 9) A polyester film was obtained in the same manner as in Example 7, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1.

[0128] Example 10 A polyester film was obtained in the same manner as in Example 5, except that in the production process of Example 5, after the film was stretched in the longitudinal direction, a 10% relaxation heat treatment was carried out at 100°C.

[0129] Example 11 A polyester film was obtained in the same manner as in Example 5, except that in the production process of Example 5, after heat setting, the clips were released at 200°C and a relaxation heat treatment was performed in the longitudinal and width directions. The tenter speed and the take-up roll speed were adjusted so that the relaxation rate in the longitudinal direction was 3%. The relaxation in the width direction was left in a free state.

[0130] 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.

[0131] Example 13 A polyester film was obtained in the same manner as in Example 1, except that the temperature during longitudinal stretching was changed to 75°C, the stretching ratio was changed to 1.2 times, and then the stretching ratio was changed to 5.0 times in the width direction.

[0132] Example 14 A polyester film was obtained in the same manner as in Example 3, except that the longitudinal stretching in Example 3 was a two-stage stretching in which the stretching ratio in the first stage was 1.2 times and the stretching ratio in the second stage was 1.67 times. The total longitudinal stretching ratio was about 2.0 times.

[0133] 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.

[0134] Example 16 A polyester film was obtained in the same manner as in Example 2, except that the widthwise stretching in Example 2 was changed to two-stage stretching, with the first-stage stretching ratio being 1.5 times and the second-stage stretching ratio being 4.0 times, and the heat setting temperature being 190° C. The total widthwise stretching ratio was 6.0 times.

[0135] (Examples 17 to 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.

[0136] Example 19 A polyester film was obtained in the same manner as in Example 1, except that the relaxation heat treatment in the width direction was not carried out in the production process of Example 1.

[0137] Example 20 An unstretched film was prepared in the same manner as in Example 1, and then the unstretched film was preheated at 75°C in a tenter and stretched laterally at 85°C to 1.4 times its original size. The above-mentioned coating solution for forming an easy-adhesion layer was applied to both sides of the obtained uniaxially stretched film by roll coating, and then dried at 80°C for 20 seconds. The final coating amount after drying (after biaxial stretching) was 0.06 g / m 2The film was uniformly heated to 105°C using a heated roll, then heated to 95°C using a non-contact heater, and roll-stretched (longitudinal stretching) at 4.0 times its original size. The film was then heat-set at 230°C for 5 seconds with the width fixed, to obtain a polyethylene terephthalate film with a thickness of 50 μm.

[0138] (Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that the film was not stretched in the longitudinal direction but was stretched only in the width direction, i.e., transverse uniaxial stretching.

[0139] (Comparative Example 2) A polyester film was obtained in the same manner as in Example 7, except that the film was not stretched in the machine direction but was stretched only in the width direction, i.e., transverse uniaxial stretching.

[0140] (Comparative Examples 3 to 7) A polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 220°C, and the PET pellets and thickness shown in Table 1 were used. As described above, Comparative Examples 3 to 7 had a lower heat setting temperature than Example 1, and the combination of conditions for the longitudinal and transverse stretching ratios was not optimal within the preferred range of conditions. As shown in Table 1, the refractive index in the thickness direction increased, the indentation depth after unloading the test force was large, and the pencil hardness after laminating the hard coat layer was smaller than in each Example.

[0141] (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.

[0142] (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.

[0143] (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.

[0144] (Comparative Example 11) A polyester film was obtained in the same manner as in Example 13, except that the longitudinal stretching temperature was changed to 130°C.

[0145] (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.

[0146] Hard coat coating solution a was applied to one side of the prepared film using a Mayer bar so that the film thickness after drying would be 5 μm, and after drying at 80°C for 1 minute, it was irradiated with ultraviolet light (integrated light dose 200 mJ / cm 2 ) to obtain a hard-coated film. The metal nanowire-containing coating solution was then applied to the surface of the hard-coated layer using a Mayer bar so that the film thickness after drying would be 5 μm, and the coating was dried at 80°C for 10 minutes to obtain a transparent conductive polyester film. The evaluation results are shown in Table 1.

[0147] Example 21 A polyethylene terephthalate film having a thickness of 50 μm was obtained in the same manner as in Example 1, and then a hard-coated film was obtained by coating the film with hard-coat coating solution b. Then, a hard-coated film was obtained in the same manner as in Example 1, except that the hard-coat coating solution a was changed to hard-coat coating solution b.

[0148] A touch panel module using this transparent conductive polyester film was incorporated into an organic EL display to create a smartphone-type foldable display that could be folded in half at the center of the entire display, with a radius of 3 mm, corresponding to the bending radius in Figure 1. The displays using the transparent conductive polyester film of each example satisfied the operation and visibility of a smartphone that could be folded in half at the center and carried around. Furthermore, there was no denting of the surface due to external force. On the other hand, the foldable displays using the transparent conductive polyester film of each comparative example appeared to develop image distortion at the foldable portion of the display as usage frequency increased, which was not very desirable. In addition, some displays had dents on the surface.

[0149] [Table 1]

[0150] [Table 2] [Industrial Applicability]

[0151] A foldable display using the transparent conductive polyester film for a foldable display of the present invention maintains mass productivity and does not cause image distortion at the folding portion of the display. In particular, a mobile terminal device or image display device equipped with a foldable display using the transparent conductive polyester film of the present invention as a touch panel module provides beautiful images, is highly functional, and is excellent in convenience such as portability. [Explanation of symbols]

[0152] 1: Foldable display 11: Bending radius 2: Transparent conductive polyester film for foldable displays 21: Folding section 22: Bending direction (direction perpendicular to the folding part)

Claims

1. A transparent conductive polyester film for a folding display, comprising a polyester film and a transparent conductive layer laminated on at least one side thereof, the polyester film satisfying the following conditions: (1) Refractive index in the bending direction is 1.590 to 1.620 (2) The refractive index in the direction of the folded portion is 1.670 to 1.700 (3) A refractive index in the thickness direction of 1.520 or less (4) Density is 1.380 g / cm ３ End (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.)

2. The transparent conductive polyester film for a foldable display according to claim 1, wherein the transparent conductive layer contains at least one selected from a conductive fibrous filler, a metal oxide, and a conductive polymer.

3. 3. The transparent conductive polyester film for a foldable display according to claim 1, wherein the polyester film has a total light transmittance of 85% or more, a haze of 3% or less, and a maximum heat shrinkage of 6% or less.

4. The transparent conductive polyester film for a foldable display according to any one of claims 1 to 3, having an easy-adhesion layer on at least one surface of the polyester film.

5. The transparent conductive polyester film for a foldable display according to any one of claims 1 to 4, wherein the polyester film has a hard coat layer having a thickness of 1 to 50 µm on at least one surface thereof.

6. A touch panel for a foldable display, comprising the transparent conductive polyester film for a foldable display according to any one of claims 1 to 5.

7. A foldable display comprising the touch panel for a foldable display according to claim 6.

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

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