Polyester film for surface protective film of folding type display and application of the same

Abstract

To provide a folding type display which is excellent in mass productivity and which hardly causes image irregularity appearing in the folding portion after repeated folding of the display, and a portable terminal having such a folding type display mounted thereon, and to provide a polyester film for a surface protective film of the display, and a hard coat film for the surface protective film.SOLUTION: A polyester film is for a surface protective film of a folding type display. The polyester film is cut in a sheet form, has thickness of 10-75 μm, has an intrinsic viscosity of 0.55-0.65 dl / g, and the amount of rise of a cut plane on at least one side of a cut end portion is 35 μm or less. Provided are, a hard coat film using the polyester film for the surface protective film of the folding type display, the folding type display, and a portable terminal device.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

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

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

[0004] To achieve these goals, a method has been proposed of connecting multiple displays to make them more compact (Patent Document 1), but this method has not become widespread because the bezel remains, resulting in a truncated image and reduced visibility.

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

[0006] In conventional displays and mobile terminal devices that do not have a folding structure, the display surface can be protected with an inflexible material such as glass, but in a foldable display, when a full-surface display is formed via a folding portion, it is necessary to use a hard-coated film or the like that is flexible and can protect the surface. However, in a foldable display, the portion that corresponds to the folding portion is repeatedly folded, and the film in that portion deforms over time, causing problems such as distorting the image displayed on the display.

[0007] Therefore, a method of partially changing the film thickness has been proposed (see Patent Document 2), but this method has the problem of being poorly suited for mass production.

[0008] Hard coat films capable of protecting the above-mentioned surfaces are required to avoid cracks or significant creases caused by repeated bending. When a film of a certain thickness is bent, compressive stress is applied to the inner surface of the bent film, and tensile stress is applied to the outer surface of the bent film. When bent to the same bending radius, a thicker film will have a larger amount of deformation due to bending on both the inner and outer surfaces, resulting in larger compressive and tensile stresses, which is likely to be disadvantageous in terms of repeated bending resistance.

[0009] The influence of film thickness on flex resistance is not limited to the average thickness of the film, but also applies to localized locations, such as the edges of the cut surface of the film. In other words, if the thickness of the cut portion increases locally, cracks and deformations are more likely to occur due to repeated bending, causing problems such as distorting the image displayed on the display. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-228391 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-155124 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention seeks to solve the problems associated with conventional display surface protection members as described above, and aims to provide a foldable display that is easy to mass-produce and does not pose a risk of image distortion at the fold after repeated folding, as well as a mobile terminal device equipped with such a foldable display, by providing a polyester film for a surface protection film for a foldable display and a hard coat film for a surface protection film. [Means for solving the problem]

[0012] That is, the present invention comprises the following: 1. A polyester film for use as a surface protection film for folding displays, which is a polyester film cut into sheets, has a thickness of 10 to 75 μm, an intrinsic viscosity of the film of 0.55 to 0.65 dl / g, and has a protrusion of 35 μm or less on the cut surface on at least one side of the cut edge. 2. A polyester film for surface protection films of folding displays, as described in 1 above, in which the amount of swelling at the cut surfaces at both ends cut in the bending direction of the polyester film is 35 μm or less. (Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.) 3. The polyester film for surface protection of a folding display according to the above item 1 or 2, wherein the edge of the polyester film is cut using laser light. 4. A hard coat film for surface protection films of foldable displays, comprising the polyester film for surface protection films of foldable displays described in any one of 1 to 3 above, and a hard coat layer having a thickness of 1 to 50 μm on at least one side thereof. 5. A hard coat film for surface protection of a folding display according to the above item 4, wherein the pencil hardness of the hard coat layer measured under a load of 750 g in accordance with JIS K5600-5-4:1999 is H or more. 6. A foldable display in which the hard coat film for surface protection of a foldable display described in 4 or 5 above is arranged as a surface protection film so that the hard coat layer is positioned on the surface, and the foldable display has a bending radius of 5 mm or less when folded. 7. The foldable display according to claim 6, wherein a single continuous hard coat film is disposed across the fold of the foldable display. 8. A mobile terminal device having the foldable display described in item 6 or 7 above. [Effects of the Invention]

[0013] A foldable display using the polyester film or hard coat film for surface protection of a foldable display of the present invention maintains mass productivity, and the polyester film or hard coat film does not deform after repeated folding, so that image distortion does not occur at the folded portion of the display. Mobile terminal devices equipped with such a foldable display provide beautiful images, are highly functional, and are convenient in terms of portability and other factors. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a schematic diagram showing the measurement points of the bending radius when folded in the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the bending direction of the polyester film for a surface protective film of a folding display according to the present invention. [Figure 3] 5 is a schematic diagram for explaining the amount of swelling of the cut surface of the cut end portion in the present invention. FIG. [Figure 4]FIG. 2 is a schematic diagram illustrating an end portion cut in the bending direction of the polyester film of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] (display) The term "display" as used herein refers to a general display device, and examples of the display include LCD, organic EL display, inorganic EL display, LED, and FED. However, LCD, organic EL, and inorganic EL, which have a bendable structure, are preferred. In particular, organic EL and inorganic EL, which can reduce the layer structure, are particularly preferred, and organic EL, which has a wide color gamut, is even more preferred.

[0016] (foldable display) A foldable display preferably has a structure in which a single continuous display can be folded in half when carried, reducing its size by half and improving portability. At the same time, a thin and lightweight display is also desirable. Therefore, 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 thin display when folded. While a smaller bending radius is preferable, a bending radius of 0.1 mm or more is acceptable, even a bending radius of 0.5 mm or more is acceptable. Even a bending radius of 1 mm or more provides sufficient practicality compared to conventional displays without a folding structure. The bending radius when folded is measured at the location indicated by reference numeral 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 display. In other words, a mobile terminal device with a foldable display may have the foldable display located on either the outside or inside of the folded display.

[0017] (Organic EL) The general structure of an organic EL display consists of an organic EL layer consisting of an electrode / electron transport layer / light-emitting layer / hole transport layer / transparent electrode, a retardation plate to improve image quality, and a polarizing plate.

[0018] (Mobile terminal device with touch panel) When an organic EL display is used in a mobile terminal device with a touch panel, a touch panel module is placed on top of the organic EL display or between the organic EL layer and the retardation film. In this case, if an impact is applied from above, the circuits of the organic EL and the touch panel may be disconnected, so a surface protection film is required. It is preferable that the film placed on the front surface of the display as a surface protection film has a hard coat layer laminated on at least the front surface side of the display.

[0019] (Surface protection film for foldable displays) As the surface protection film, any film having high light transmittance and low haze, such as a polyimide film, a polyester film, a polycarbonate film, an acrylic film, a triacetyl cellulose film, or a cycloolefin polymer film, can be used. Among these, polyimide films and polyester films having high impact resistance and sufficient pencil hardness are preferred, and polyester films that can be produced inexpensively are particularly preferred.

[0020] In the present invention, the polyester film may be a single-layer film made of one or more types of polyester resin, or when two or more types of polyester are used, it may be a multilayer structure film or an ultra-multilayer laminate film with a repeating structure.

[0021] Examples of polyester resins include polyester films made of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers containing these resin components as main components. Among these, stretched polyethylene terephthalate films are particularly preferred in terms of mechanical properties, heat resistance, transparency, cost, etc.

[0022] When a polyester copolymer is used for the polyester film, examples of the dicarboxylic acid component of the polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; and polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of the glycol component include fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; aromatic glycols such as p-xylene glycol; alicyclic glycols such as 1,4-cyclohexanedimethanol; and polyethylene glycols having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerization components in the preferred copolymer is less than 20% by mass. A mass ratio of less than 20% by mass is preferred because film strength, transparency, and heat resistance are maintained.

[0023] 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 0.55 to 0.75 dL / g. An intrinsic viscosity of 0.55 dL / g or higher improves the impact resistance of the resulting film, making it less likely for the internal circuit to break due to external impact. This also contributes to minimizing deformation when repeatedly bent, which is also preferable. On the other hand, an intrinsic viscosity of 0.75 dL / g or lower is preferable because it prevents excessive increases in the filtration pressure of the molten fluid, facilitating stable film production.

[0024] Regardless of whether the film has a single-layer or multilayer structure, the intrinsic viscosity of the film is preferably 0.55 dL / g or more. It is more preferably 0.58 dL / g or more. A limiting viscosity of 0.55 dL / g or more can impart fatigue resistance and provide sufficient flex resistance. On the other hand, a film having an intrinsic viscosity of 0.65 dL / g or less is preferable because it can be produced with good operability. Furthermore, when the film is fused with laser light, the amount of swelling at the cut surface can be reduced, which is preferable.

[0025] The thickness of the polyester film is preferably 10 to 75 μm, and more preferably 25 to 75 μm. A thickness of 10 μm or more improves pencil hardness, while a thickness of 75 μm or less is advantageous for weight reduction and is excellent in flexibility, processability, handleability, etc.

[0026] The surface of the polyester film of the present invention may be smooth or may have some irregularities. However, since the film is used as a surface cover for a display, deterioration of optical properties due to irregularities is undesirable. The haze is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. A haze of 3% or less can improve image visibility. The lower limit of the haze is preferably as small as possible, but it may be 0.1% or more, or even 0.3% or more.

[0027] As mentioned above, in order to reduce haze, it is better for the film surface to have small irregularities. However, in order to provide a certain degree of slipperiness from the viewpoint of ease of handling, irregularities can be formed by blending a filler into the surface polyester resin layer or by coating a filler-containing coating layer during film formation.

[0028] The method of incorporating particles into the base film can be a known method. For example, they can be added at any stage of polyester production, but preferably they can be added as a slurry dispersed in ethylene glycol or the like at the stage of esterification, or after the completion of the transesterification reaction and before the start of the polycondensation reaction, to proceed with the polycondensation reaction. Alternatively, they can be added by a method of blending a slurry of particles dispersed in ethylene glycol or water with polyester raw materials using a vented kneading extruder, or a method of blending dried particles with polyester raw materials using a kneading extruder.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] The total light transmittance of the polyester film is preferably 85% or more, and more preferably 87% or more. A transmittance of 85% or more ensures sufficient visibility. The higher the total light transmittance of the polyester film, the better, but it may be 99% or less, or even 97% or less.

[0033] 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.

[0034] 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.

[0035] Furthermore, adhesion can be improved by an adhesion-improving layer such as an easy-adhesion layer. The easy-adhesion layer can be formed on the surface of a polyester film by applying a coating liquid to at least one side of the polyester film at any stage in the polyester film production process. For example, the easy-adhesion layer may be formed on one side of the polyester film after a uniaxially oriented PET film is obtained. The solids concentration of the resin composition in the coating liquid is preferably 2 to 35% by mass, and particularly preferably 4 to 15% by mass.

[0036] The adhesive layer can be made of any resin, including acrylic resins, polyester resins, polyurethane resins, and polyether resins, without any particular limitations. These adhesive layers may also be crosslinked to improve their adhesion durability. Specific examples of crosslinking agents include urea-based, epoxy-based, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based crosslinkers. Lubricant particles can also be added to impart slipperiness to the surface of the adhesive layer. The particles may be inorganic or organic, and include, but are not limited to, inorganic particles such as silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, zirconium oxide, and titanium dioxide, as well as organic particles such as acrylic or methacrylic, vinyl chloride, vinyl acetate, melamine, polycarbonate, urea, epoxy, urethane, phenol, diallyl phthalate, and polyester. Silica is particularly preferred for imparting adequate slipperiness to the coating layer.

[0037] The adhesive layer can be formed on the surface of a polyester film by applying a coating solution to at least one side of the polyester film at any stage during the polyester film manufacturing process. For example, after obtaining a uniaxially oriented PET film, the adhesive layer can be formed by any known coating method. Examples include reverse roll coating, gravure coating, kiss coating, reverse kiss coating, die coating, roll brushing, spray coating, air knife coating, wire bar coating, pipe doctor coating, impregnation coating, and curtain coating. In the present invention, the thickness of the finally obtained adhesive layer is preferably 0.03 to 0.20 g / m². A thickness of less than 0.03 g / m² reduces adhesion, while a thickness greater than 0.20 g / m² reduces anti-blocking properties and slippage, which is undesirable.

[0038] The polyester film can be produced, for example, through a polymerization step in which inorganic particles are homogeneously dispersed in a monomer liquid that forms 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 polyester is melt-extruded through a filter into a sheet, which is cooled and stretched to form a substrate film. In order to provide the film with strength, dimensional stability, and heat resistance, it is preferable to perform stretching in biaxial directions to obtain a biaxially oriented polyester film.

[0039] Next, a method for producing a biaxially oriented polyester film will be described in detail using an example in which polyethylene terephthalate (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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] From the viewpoint of the mechanical properties of the film, it is preferable that the stretching ratio in the direction perpendicular to the bending direction (the direction of the fold) be larger than that in the bending direction, and an example of the stretching ratio in the direction perpendicular to the bending direction is 2.5 to 5.0. A stretching ratio of 2.5 or more can ensure stable productivity, while a stretching ratio of 5.0 or less is preferable because it can provide good bending resistance. Here, the bending direction refers to the direction perpendicular to the fold (reference numeral 21) expected in the use as a surface protection film for a foldable display, as shown by reference numeral 22 on the polyester film (reference numeral 2) in Figure 2. The bending direction is not limited to either the longitudinal or transverse direction of the film.

[0044] 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 1.0 to 3.4 times, with 1.4 to 2.0 times being more preferable from the viewpoint of flex resistance. The stretching direction is preferably the bending direction. A stretching ratio of 1.4 times or more is preferred because deformation during hard coating is prevented, and a stretching ratio of 2.0 times or less is preferred because good flex resistance is obtained. A stretching ratio of 3.0 to 3.4 times is more preferable from the viewpoint of stable productivity. 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.

[0045] Specifically, for example, PET pellets are thoroughly vacuum-dried, fed into an extruder, melt-extruded into a sheet at approximately 280°C, and cooled to solidify, forming an unstretched PET sheet. The resulting unstretched sheet is stretched 1.0 to 3.4 times in the longitudinal direction using rolls heated to 80 to 130°C to obtain a uniaxially oriented PET film. The film is then gripped at its edges with clips and introduced into a hot air zone heated to 80 to 180°C, where it is dried and then stretched 2.5 to 5.0 times in the transverse direction. The film is then introduced into a heat treatment zone at 180 to 250°C and heat-treated for 1 to 60 seconds to complete the crystal orientation. During this heat treatment process, a relaxation treatment of 1 to 12% may be performed in the transverse or longitudinal direction, if necessary.

[0046] (Hard coat layer) The polyester film positioned on the surface of a foldable display to protect the display preferably has a hard coat layer on its surface. The hard coat layer is preferably positioned on the display surface side of the polyester film when used in the display. The resin forming the hard coat layer may be any resin that transmits visible light, but one with high light transmittance is preferred. Examples of materials that can be used include acrylic resins, polycarbonate resins, vinyl chloride resins, polyester resins, urethane resins, epoxy resins, melamine resins, siloxane resins, organic-inorganic hybrid resins, and active energy ray-curable resins.

[0047] Resins that are cured by active energy rays such as ultraviolet rays and electron beams include compounds having a (meth)acrylate functional group, such as polyester (meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, and silicone (meth)acrylate, as well as compounds having a functional group with an unsaturated double bond, such as an allyl group or a vinyl group. Furthermore, polyfunctional monomers may be used in combination to increase the hardness of the hard coat layer. Examples of polyfunctional monomers include trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and neopentyl glycol di(meth)acrylate. The above materials may be used alone or in combination.

[0048] When the active energy ray is ultraviolet light, it is preferable to add a photopolymerization initiator. The photopolymerization initiator may be a radical polymerization system, a cationic polymerization system, or a mixed system of cationic polymerization and radical polymerization, but a radical polymerization system is particularly preferable because of its high reaction rate and excellent productivity. Examples of ultraviolet radical polymerization initiators include alkylphenones, benzoins, benzophenones, phosphine oxides, ketals, anthraquinones, thioxanthones, azo compounds, peroxides, 2,3-dialkyldione compounds, disulfide compounds, fluoroamine compounds, aromatic sulfonium compounds, titanocenes, and phenyl oxyacetates, which may be used alone or in combination of two or more. Further specific examples include carbonyl compounds such as acetophenone, 2,2-diethoxyacetophenone, p-dimethylacetophenone, p-dimethylaminopropiophenone, benzophenone, 2-chlorobenzophenone, 4,4'-dichlorobenzophenone, 4,4'-bisdiethylaminobenzophenone, Michler's ketone, benzil, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, methylbenzoyl formate, p-isopropyl-α-hydroxyisobutylphenone, α-hydroxyisobutylphenone, 2,2-dimethoxy-2-phenylacetophenone, and 1-hydroxycyclohexyl phenyl ketone; sulfur compounds such as tetramethylthiuram monosulfide, tetramethylthiuram disulfide, thioxanthone, 2-chlorothioxanthone, and 2-methylthioxanthone; and peroxide compounds such as benzoyl peroxide and di-t-butyl peroxide. The amount of photopolymerization initiator added can be in the range of 0.1 parts by mass or more, more preferably 1 part by mass or more, and 30 parts by mass or less, more preferably 20 parts by mass or less, relative to 100 parts by mass of the active energy ray-curable resin. An amount of 0.1 parts by mass or more is preferred because the hardness of the hard coat layer can be increased. Furthermore, an amount of 30 parts by mass or less is preferred because there is no risk of yellowing of the hard coat layer and the hard coat layer is sufficiently cured.

[0049] Furthermore, various additives may be contained within a range that does not impair the performance of the hard coat, such as a polymerization inhibitor, a crosslinking agent, an antistatic agent, an adhesion improver, an antioxidant, a leveling agent, a coupling agent, an antifoaming agent, a filler, a solvent, an antiglare agent, an antireflection agent, an inorganic filler, or an organic filler.

[0050] (Hard Coat Film Manufacturing Method) To form a hard coat layer, a coating solution prepared by dispersing or dissolving the above-mentioned compound in a predetermined amount of solvent is applied to a polyester film. Examples of organic solvents include aromatic hydrocarbons such as benzene, toluene, and xylene; aliphatic hydrocarbons such as hexane, heptane, octane, nonane, and decane; ketones such as methyl ethyl ketone, diethyl ketone, and diisopropyl ketone; and alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, tridecyl alcohol, cyclohexyl alcohol, and 2-methylcyclohexyl alcohol. It is preferable to select an organic solvent with a boiling point in the range of 60 to 180°C. Using an organic solvent with a boiling point of 60°C or higher can suppress changes in the solids concentration of the coating solution during coating, thereby stabilizing the coating thickness. Setting the temperature to 180°C or lower can suppress deterioration of the flatness of the plastic substrate film due to heat wrinkles that occur during drying.

[0051] The coating method can be any of a variety of methods, including a Mayer bar, gravure coater, die coater, knife coater, blade coater, roll coater, and curtain coater, without any particular limitations. Methods for applying the coating solution to the polyester film and drying it include known methods such as hot air drying and infrared heaters, but hot air drying is preferred because of its fast drying rate. Such rapid drying allows the formation of a hard coat layer while keeping the volatile components substantially uniformly dispersed within the hard coat layer, which is suitable for highly suppressing curling.

[0052] The hard coat layer can be cured using active energy rays such as ultraviolet light or electron beams, or by heat. However, curing methods using ultraviolet light or electron beams are preferred to reduce damage to the film. UV irradiation is typically performed from the coating layer side, but may also be performed from the polyester film side to enhance adhesion to the polyester film. UV light is typically emitted from a high-pressure mercury lamp, fusion H lamp, xenon lamp, or the like, which emits ultraviolet light with a spectral distribution in the wavelength range of 300 to 400 nm. The exposure dose has a lower limit of 50 mJ / m² or more, more preferably 100 mJ / m² or more, and an upper limit of 1000 mJ / m² or less, more preferably 800 mJ / m² or less. An exposure dose of 50 mJ / m² or more is preferred because it increases the hardness of the hard coat layer. On the other hand, an exposure dose of 1000 mJ / m² or less prevents the running speed from becoming too slow, which is advantageous in terms of productivity.

[0053] The thickness of the hard coat layer after curing is preferably 1 to 50 μm. If the thickness is greater than 1 μm, sufficient curing occurs and good pencil hardness is obtained. Furthermore, by keeping the thickness at 50 μm or less, curling due to cure shrinkage of the hard coat can be suppressed, improving the handleability of the film. A thickness of 3 to 45 μm is more preferable, and 5 to 40 μm is even more preferable.

[0054] (Pencil hardness) The pencil hardness of the hard coat layer is preferably B or higher, more preferably H or higher, and particularly preferably 2H or higher. A pencil hardness of B or higher prevents scratches and does not reduce visibility. Generally, a high pencil hardness of the hard coat layer is preferable, but a hard coat layer having a pencil hardness of 10H or lower, 8H or lower, or even 6H or lower can be used without any practical problems.

[0055] (Cutting the film) Methods for cutting a long polyester film or hard-coated film into a desired sheet shape include, for example, a mechanical cutting method using a knife and a laser cutting method using a laser. Among these, the laser cutting method is preferred because it is less likely to produce cutting debris. When a polyester film is irradiated with laser light, the areas of each layer contained in the film that are irradiated with the laser light are heated by the energy of the laser light, causing thermal melting or abrasion. Therefore, the film is cut in the areas irradiated with the laser light.

[0056] The laser oscillator may be a CO2 laser, excimer laser (ArF, KrF, XeCl, XeF), YAG laser, YVO4 laser, YLF laser, fiber laser, or the like. The laser oscillator may be a continuous wave laser oscillator or a pulsed laser oscillator. Of the above, a CO2 laser oscillator is preferred because it has high energy efficiency and a long wavelength, allowing processing by applying heat to materials, making it suitable for processing transparent materials such as polyester film.

[0057] An assist gas can be sprayed onto the irradiation point simultaneously with the laser irradiation. By spraying the assist gas, it is possible to prevent adhesion of molten material generated during cutting to the polyester film or hard-coated film. It is also possible to prevent damage to the polyester film or hard-coated film due to heat. Such an assist gas can be any gas that is inert to the laser light, such as dry air, nitrogen, or argon.

[0058] The laser beam output is preferably 1 W or more but less than 30 W, and more preferably 3 W or more but less than 25 W. An output less than 1 W slows the cutting speed, reducing productivity and possibly even preventing cutting altogether. An output greater than 30 W is undesirable because it widens the cut edge, increases the amount of melted polyester resin due to excessive heating, and causes the molten resin to build up on the cut edge, resulting in a localized increase in the thickness of the cut edge. It is also undesirable because it can cause thermal damage outside the cut edge and increase the production of decomposition products. High-power lasers can be addressed by reducing the output. Pulse lasers can reduce the average output by controlling the pulse width (ns) and pulse frequency (Hz). The cutting speed is preferably 3 m / min or more, more preferably 5 m / min or more, and 10 m / min or more is preferable from a productivity perspective. To reduce thermal damage while simultaneously increasing the cutting speed, repeated laser irradiation with reduced laser output is recommended.

[0059] When a polyester film or hard-coated film is cut with a laser beam, an affected area due to the laser treatment is usually formed around the cut surface. Here, the affected area due to the laser treatment refers to a portion where the resin constituting the film is deformed by the heat generated during cutting with the laser beam, and includes both a thinner and thicker cut surface. In the case of a hard-coated film, the polyester resin constituting the polyester film substrate is prone to deformation. The thickness of the cut surface may increase due to the accumulation of heat-molten resin on the cut surface, causing the edges to bulge. The amount of bulge at the cut edge is preferably 35 μm or less on at least one side of the desired cut shape (usually a square or rectangle) (if a circle or ellipse is cut, the periphery corresponds to one side), more preferably 20 μm or less, and even more preferably 15 μm or less. By reducing the amount of protrusion at the cut edge to 35 μm or less, not only can dimensional changes and wrinkles be suppressed, but also deformation stress concentration at the protrusion at the edge can be suppressed when the film is repeatedly folded. This prevents deterioration of bending deformation, cracking, and breakage of the film. Furthermore, because the film is less likely to deform after repeated folding, a hard-coated film with a small amount of protrusion at the cut edge is preferable because it does not cause image distortion at the folded portion of a display using the hard-coated film as a surface protection film. Ideally, the amount of protrusion at the cut edge is 0 μm, but in practice, the lower limit can be 1 μm or more, or even 2 μm or more. Here, the amount of protrusion at the cut edge refers to the thickness (37) of the hard-coated film (3) in Figure 3, calculated by subtracting the thickness (35) of the hard-coated film unaffected by deformation due to laser light from the thickness (36) of the thickest part of the hard-coated film.

[0060] FIG. 4 is a schematic diagram showing the state of a folded hard-coated film (reference numeral 4), in which the hard-coat layer may be located on the inner surface of the folded film, the outer surface, or both surfaces. In a sheet-like film cut into a desired shape using laser light, the cut edge with a cut surface protrusion of 35 μm or less preferably corresponds to the end cut in the bending direction of the folding (reference numeral 41 in FIG. 4). Although not shown in the figure as reference numeral 41, ends cut in the bending direction of the folding usually have a protrusion on the cut surface. Furthermore, reference numeral 41 represents one end, but it is particularly preferable that the protrusion on both ends of the cut surface, including the opposite end (not shown), is 35 μm or less. Here, the bending direction refers to the direction perpendicular to the fold when the hard-coated film is folded (reference numeral 42 in FIG. 4).

[0061] When cutting a polyester film or a hard-coated film, the film is usually irradiated with laser light while being supported on the support surface of a support having a support surface. The support may be a rigid member such as a plate-like support substrate, or a flexible member such as a film-like support film.

[0062] When cutting a polyester film or hard-coated film with a laser beam, the laser beam is typically irradiated onto the film so that the laser beam scans the film surface along a desired line. Here, the desired line may be an actually drawn line, but typically an imaginary line that is not actually drawn is set. This causes the point at which the laser beam strikes the film to move along the desired line, allowing the film to be cut into the desired shape. To scan the film surface with the laser beam, the laser beam irradiator may be moved, the film may be moved, or both the laser beam and the film may be moved. The speed at which the point at which the laser beam strikes the film moves across the film surface can be set arbitrarily depending on conditions such as the laser beam output and film thickness.

[0063] The surface of the polyester film or hard-coated film to be irradiated with laser light may be either the polyester film base surface or the hard-coated surface. Alternatively, a cover film may be laminated on the surface to be irradiated with laser light and / or the surface opposite to the surface to be irradiated with laser light, and the polyester film or hard-coated film and the cover film may be cut together using the laser light. The cover film is removed after cutting with the laser light. By laminating a cover film and irradiating it with laser light, it is possible to prevent molten material generated during cutting from directly adhering to the polyester film or hard-coated film. It is also possible to prevent damage to the polyester film or hard-coated film due to heat. Furthermore, by appropriately combining the laser intensity and other cutting conditions, it is possible to suppress the formation of raised portions of heat-molten resin on the cut edge, and this method is therefore preferably used.

[0064] The substrate of the cover film is not particularly limited, and examples thereof include resin films such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polycarbonate (PC), polypropylene (PP), polybutadiene, polymethylpentene, polyurethane, polyvinyl chloride (PVC), polystyrene (PS), fluororesin, polylactic acid, and cellulose-based films. The thickness of the cover film is 1 to 50 μm. An adhesive layer may also be provided on one side of the cover film. Examples of materials for the adhesive layer include acrylic, polyester, urethane, rubber, silicone, and vinyl adhesives, and may be active energy ray-curable adhesives. [Example]

[0065] Next, the effects of the present invention will be described using examples and comparative examples. First, the evaluation methods of the characteristic values ​​used in the present invention will be described below.

[0066] (1) Amount of swelling at the cut end The edge of the cut film was cut perpendicularly with a cutter blade, and the surface of the edge was homogenized using a microtome. This cut edge was observed at 600x magnification using a digital microscope RH-2000 (Hirox Corporation). The film thickness and the thickness of the thickest part of the film edge were measured, and the difference between the two was defined as the amount of swelling at the cut edge. In the case of a hard-coated film, for example, reference numeral 37 in Figure 3 indicates the amount of swelling at the cut cross section of the edge. Measurements were taken at five arbitrary positions on one side of the film edge, and the average value was used as the amount of swelling at the cut cross section at the film edge. When measurements were taken at both ends cut in the bending direction of the film, the average values ​​of the swelling at the cut section on both sides are listed in Table 1.

[0067] (2) Film thickness Three 5 cm square samples were cut out from three randomly selected locations on the film. Measurements were taken at five points on each sample (15 points in total) using an electric micrometer (Fine Leaf, Millitron 1245D), and the average value was taken as the film thickness.

[0068] (3) Hard Coat Layer Thickness Sections were cut from three randomly selected locations on the hard-coated film. One edge surface per section was homogenized using a microtome. The edge surfaces were observed at 600x magnification using a RH-2000 digital microscope (Hirox Corporation). The thickness of the hard-coat layer was measured at five points on each edge (15 points in total), and the average value was used as the hard-coat layer thickness.

[0069] (4) Intrinsic viscosity After crushing and drying the film or polyester resin, it was dissolved in a 60 / 40 (mass ratio) phenol / tetrachloroethane mixed solvent. This solution was centrifuged to remove inorganic particles, and then an Ubbelohde viscometer was used to measure the flow time of a 0.4 (g / dL) solution at 30°C and the flow time of the solvent alone. The intrinsic viscosity was calculated from the ratio of these times using the Huggins equation, assuming a Huggins constant of 0.38. For laminated films, the intrinsic viscosity of each individual layer was evaluated by scraping off the corresponding polyester layer of the film according to the laminate thickness.

[0070] (5) Flexibility A polyester film was cut into a 200 mm (bending direction) x 50 mm (folding direction) sample for measurement. Two 5 mm thick glass plates were placed with spacers of each thickness at the edges to create a space between them, and the film was sandwiched between them for 10 seconds. Immediately after that, the film was exposed to fluorescent light, and the folds were observed. The distance at which no fold marks were found was recorded. ○: The interval without folding marks was less than 6.5 mm. △: The gap where no folding marks were left was 6.5 mm or more and less than 7.0 mm ×: The interval where no folding marks were left was 7.0 mm or more.

[0071] (6) Repeated bending resistance A sample measuring 50 mm in the width direction (direction of the folded portion) and 100 mm in the machine direction (bending direction) was prepared. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the sample was bent 50,000 times at a rate of 1 bend per second with a bending radius of 3 mm. The sample was fixed at 10 mm positions on both long sides, and the bending area was 50 mm x 80 mm. After the bending process, the sample was placed on a flat surface with the inside of the bend facing down and visually inspected. ○: The sample is not deformed, or even if it is deformed, the maximum height of the lift when placed horizontally is less than 3 mm. △: The sample is deformed and when placed horizontally, the maximum floating height is 3 mm or more but less than 5 mm. ×: The sample has creases or the maximum lift height is 5 mm or more when placed horizontally.

[0072] (7) Pencil hardness The hard-coated polyester film thus prepared was measured in accordance with JIS K 5600-5-4:1999 at a load of 750 g and a speed of 0.5 mm / s.

[0073] (Preparation of polyethylene terephthalate pellets A) A continuous esterification reactor consisting of a three-stage complete mixing vessel equipped with an agitator, a partial condenser, a raw material inlet, and a product outlet was used as the esterification reactor. TPA was supplied at a rate of 2 ton / hr, EG at 2 moles per mole of TPA, and antimony trioxide in an amount such that the Sb atom concentration in the produced PET was 160 ppm. These slurries were continuously supplied to the first esterification reactor of the esterification reactor and reacted at normal pressure for an average residence time of 4 hours at 255°C. Next, the reaction product in the first esterification reactor was continuously removed from the system and fed to a second esterification reactor, and EG distilled off from the first esterification reactor was fed into the second esterification reactor in an amount of 8% by mass relative to the produced polymer (produced PET). Further, an EG solution containing magnesium acetate in an amount to give 65 ppm of Mg atoms relative to the produced PET and an EG solution containing TMPA in an amount to give 20 ppm of P atoms relative to the produced PET were added, and the reaction was carried out at atmospheric pressure for an average residence time of 1.5 hours at 260°C. Next, the reaction product in the second esterification reactor was continuously removed from the system and fed to a third esterification reactor, and further, an EG solution containing TMPA in an amount to give 20 ppm of P atoms relative to the produced PET was added, and the reaction was carried out at atmospheric pressure for an average residence time of 0.5 hours at 260°C. The esterification reaction product produced in the third esterification reactor was continuously supplied to a three-stage continuous polycondensation reactor to carry out polycondensation, and then filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cut of 5 μm particles) to obtain polyethylene terephthalate pellets A having an intrinsic viscosity of 0.62 dl / g.

[0074] (Preparation of polyethylene terephthalate pellets B) The polyethylene terephthalate pellets A were subjected to solid-state polymerization using a rotary vacuum polymerization apparatus under a reduced pressure of 0.5 mmHg at 220°C for various times to produce polyethylene terephthalate pellets B having an intrinsic viscosity of 0.72 dL / g.

[0075] (Preparation of polyethylene-2,6-naphthalate pellets C) A conventional transesterification reaction was carried out using 100 parts of dimethyl 2,6-naphthalenedicarboxylate, 60 parts of ethylene glycol, and 0.03 parts of manganese acetate tetrahydrate as a transesterification catalyst, followed by the addition of 0.042 parts of triethylphosphonoacetate to essentially terminate the transesterification reaction. Next, 0.024 parts of antimony trioxide was added, and polymerization was continued under high temperature and high vacuum in a conventional manner to obtain polyethylene-2,6-naphthalate pellets C with an intrinsic viscosity of 0.60 dL / g.

[0076] (Preparation of Copolymer Polyester Resin Aqueous Dispersion) A reaction vessel was charged with 95 parts by mass of dimethyl terephthalate, 95 parts by mass of dimethyl isophthalate, 35 parts by mass of ethylene glycol, 145 parts by mass of neopentyl glycol, 0.1 parts by mass of zinc acetate, and 0.1 parts by mass of antimony trioxide, and a transesterification reaction was carried out at 180°C for 3 hours. Next, 6.0 parts by mass of 5-sodium sulfoisophthalic acid was added, and an esterification reaction was carried out at 240°C for 1 hour. After that, a polycondensation reaction was carried out at 250°C under reduced pressure (10 to 0.2 mmHg) for 2 hours, yielding a copolymerized polyester resin with a number average molecular weight of 19,500 and a softening point of 60°C. 300 parts by mass of the obtained copolymerized polyester resin and 140 parts by mass of butyl cellosolve were stirred at 160°C for 3 hours to obtain a viscous molten liquid, and 560 parts by mass of water were gradually added to this molten liquid, and after 1 hour, a uniform, pale white aqueous dispersion of copolymerized polyester resin with a solids concentration of 30% was obtained.

[0077] (Preparation of aqueous polyurethane resin solution) 100 parts by weight of polyester diol (OHV: 2000 eq / ton) composed of adipic acid, 1,6-hexanediol, and neopentyl glycol (molar ratio: 4 / 2 / 3) was mixed with 41.4 parts by weight of xylylene diisocyanate and reacted under a nitrogen stream at 80-90°C for 1 hour. The mixture was then cooled to 60°C and dissolved with 70 parts by weight of tetrahydrofuran to obtain a urethane prepolymer solution (NCO / OH ratio: 2.2, free isocyanate group: 3.30% by weight). The urethane prepolymer solution was then heated to 40°C, and 45.5 parts by weight of a 20% by weight aqueous solution of sodium bisulfite was added. The mixture was reacted for 30 minutes at 40-50°C with vigorous stirring. After confirming the disappearance of free isocyanate groups (based on solids), the mixture was diluted with emulsified water to obtain a self-crosslinking polyurethane resin solution containing isocyanate groups blocked with sodium bisulfite at a solids content of 20% by weight.

[0078] (Preparation of coating solution for forming easy-adhesion layer) 7.5 parts by weight of the 30% by weight aqueous dispersion of the copolymerized polyester resin, 11.3 parts by weight of the polyurethane resin aqueous solution, 0.3 parts by weight of an organotin catalyst, 39.8 parts by weight of water, and 37.4 parts by weight of isopropyl alcohol were mixed. Furthermore, 0.6 parts by weight of a 10% by weight aqueous solution of a fluorine-based surfactant (polyoxyethylene-2-perfluorohexyl ethyl ether), 2.3 parts by weight of a 20% by weight aqueous dispersion of colloidal silica (average particle size 40 nm), and 0.5 parts by weight of a 3.5% by weight aqueous dispersion of dry-process silica (average particle size 200 nm, average primary particle size 40 nm) were added. The pH of the mixture was then adjusted to 6.2 with a 5% by weight aqueous solution of sodium bicarbonate, and the mixture was microfiltered through a felt-type polypropylene filter with a filtration particle size of 10 μm (initial filtration efficiency: 95%) to prepare a coating solution for forming an easy-adhesion layer.

[0079] (Coating solution 1 for forming hard coat layer) 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 coating solution 1 for forming a hard coat layer with a solids concentration of 40% by mass.

[0080] (Coating solution 2 for forming hard coat layer) 95 parts by mass of a urethane acrylate hard coating agent (Beamset (registered trademark) 577, manufactured by Arakawa Chemical Industries, Ltd., solids concentration 100%), 5 parts by mass of a photopolymerization initiator (Irgacure (registered trademark) 184, manufactured by BASF Japan, solids concentration 100%), and 0.1 parts by mass of a leveling agent (BYK307, manufactured by BYK Japan, solids concentration 100%) were mixed and diluted with a solvent of toluene / MEK=1 / 1 to prepare coating solution 2 for forming a hard coating layer with a solids concentration of 40%.

[0081] Example 1 The polyethylene terephthalate pellets A were dried under reduced pressure (3 Torr) at 150 ° C for 8 hours, then fed to an extruder and melted at 285 ° C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet form from a die, and then cooled and solidified by contacting a casting drum with a surface temperature of 30 ° C using an electrostatic casting method to produce an unstretched film. The unstretched film was uniformly heated to 75 ° C using a heated roll, heated to 100 ° C using a non-contact heater, and roll-stretched 3.0 times (longitudinal stretching) (the longitudinal direction of the film corresponds to the bending direction). Next, the coating liquid for forming the easy-adhesion layer was applied to the casting drum contact side of the uniaxially stretched film by a reverse kiss coating method so that the thickness of the resin solids after drying was 0.3 μ m. The uniaxially stretched film with the coating layer was introduced into a tenter while drying, heated to 140°C, and stretched transversely by 4.0 times. The width was fixed and heat-treated at 240°C for 5 seconds, and then relaxed 4% in the width direction at 210°C to obtain a 50 μm-thick polyester film (the width direction of the film corresponds to the direction of the folded portion). The adhesive surface of the obtained polyester film was coated with the hard coat layer-forming coating solution 1 using a Mayer bar to a dry thickness of 5.0 μm. After drying at 80°C for 1 minute, the film was irradiated with ultraviolet light (high-pressure mercury lamp, cumulative light intensity 200 mJ / cm2) to obtain a hard-coated film. The obtained polyester film and hard-coated film were each cut to the specified size using a laser processing machine (laser light source: carbon dioxide laser, laser wavelength: 10.6 μm) at an output of 9 W and a processing speed of 64 mm / s, so that the longitudinal direction of the film corresponded to the bending direction, to obtain a sheet-like polyester film and a sheet-like hard-coated film. The hard coat film was irradiated with laser light from the hard coat layer side. The bending resistance and repeated bending resistance were also measured under the same laser processing conditions as above, with a measurement sample of a predetermined size cut so that the longitudinal direction of the film corresponded to the bending direction.

[0082] Example 2 Using the polyester film and hard coat film obtained in the same manner as in Example 1, A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1, except that the laser output was set to 18 W. In addition, for measuring the flex resistance and repeated flex resistance, measurement samples of predetermined sizes were cut under the same laser processing conditions as above, and measurements were performed.

[0083] Examples 3 to 5 A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1 above, except that the stretching ratio and thickness were changed to those shown in Table 1. In addition, for measuring the bending resistance and repeated bending resistance, measurement samples of predetermined sizes were cut under the same laser processing conditions as in Example 1 above, and measurements were performed.

[0084] Example 6 A hard-coated film was obtained in the same manner as in Example 1, except that a polyester film obtained in the same manner as in Example 1 was coated with Coating Solution 1 for forming a hard coat layer so that the film thickness after drying would be 10.0 μm, and laser processing was performed in the same manner as in Example 1 to obtain a sheet-like polyester film and a sheet-like hard-coated film. In addition, measurements of bending resistance and repeated bending resistance were also performed by cutting measurement samples of predetermined sizes under the same laser processing conditions as in Example 1 above.

[0085] Example 7 A hard-coated film was obtained in the same manner as in Example 1, except that a polyester film obtained in the same manner as in Example 1 was coated with Coating Solution 2 for forming a hard-coat layer, and laser processing was performed in the same manner as in Example 1 to obtain a sheet-shaped polyester film and a sheet-shaped hard-coated film. In addition, for measuring the bending resistance and repeated bending resistance, measurement samples of predetermined sizes were cut under the same laser processing conditions as in Example 1, and measurements were performed.

[0086] Example 8 Using the polyester film and hard coat film obtained in the same manner as in Example 1, A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1, except that when cutting with a laser processing machine, a cover film (a polyester-based adhesive tape with a thickness of 25 μm) was attached to the end surface opposite to the surface irradiated with laser light, and the cover film was peeled off after cutting. Regarding the attachment of a protective film to the hard-coated film, the protective film was attached to the polyester film surface of the substrate, not to the hard-coat layer. For measuring the flex resistance and repeated flex resistance, measurement samples of predetermined sizes were cut and measured in the same manner as above.

[0087] Example 9 Polyester films were obtained in the same manner as in Example 1 above, except that polyethylene-2,6-naphthalate pellets C were used and the heat treatment temperature for film formation was changed as shown in Table 1. Laser processing was then carried out in the same manner as in Example 1 to obtain sheet-shaped polyester films and sheet-shaped hard-coated films. In addition, measurements of bending resistance and repeated bending resistance were also carried out by cutting measurement samples of predetermined sizes under the same laser processing conditions as in Example 1 above.

[0088] Example 10 A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1. In addition, for measuring the bending resistance and repeated bending resistance, measurement samples of predetermined sizes were cut under the same laser processing conditions as in Example 1, and measurements were performed.

[0089] Example 11 A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 10, except that the stretching ratio in the width direction was changed to 5.0 times. In addition, for measuring the bending resistance and repeated bending resistance, measurement samples of predetermined sizes were cut under the same laser processing conditions as in Example 1, and measurements were performed.

[0090] (Examples 12 to 14) A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction was changed as shown in Table 1. In addition, for measuring the bending resistance and repeated bending resistance, measurement samples of predetermined sizes were cut under the same laser processing conditions as in Example 1, and measurements were performed.

[0091] Example 15 Using a polyester film and a hard-coated film obtained in the same manner as in Example 1, a sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1, except that the laser output was 5 W, the processing speed was 320 mm / s, and the number of repeated laser irradiations was 3. For measuring the bending resistance and repeated bending resistance, measurement samples of predetermined sizes were cut in the same manner as above, and measurements were performed.

[0092] (Comparative Example 1) Using the polyester film and hard coat film obtained in the same manner as in Example 1, A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1, except that the laser output was set to 30 W. In addition, for measuring the flex resistance and repeated flex resistance, measurement samples of predetermined sizes were cut under the same laser processing conditions as above, and measurements were performed.

[0093] (Comparative Example 2) A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1 above, except that polyethylene terephthalate pellets B listed in Table 1 were used and the laser output was set to 18 W. In addition, for measuring the flex resistance and repeated flex resistance, measurement samples of predetermined sizes were cut under the same processing conditions as in Example 1 above, and measurements were performed.

[0094] (Comparative Example 3) A sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1, except that the thickness was changed to that shown in Table 1. In addition, for measuring the bending resistance and repeated bending resistance, measurement samples of a predetermined size were cut under the same laser processing conditions as in Example 1, and measurements were performed.

[0095] Comparative Example 4 A hard-coated film was obtained in the same manner as in Example 1, except that a polyester film obtained in the same manner as in Example 1 was used and hard-coat layer-forming coating solution 1 was applied so that the film thickness after drying would be 10.0 μm, and a sheet-shaped polyester film and a sheet-shaped hard-coated film were obtained in the same manner as in Example 1, except that the laser output was set to 30 W. In addition, for measuring the bending resistance and repeated bending resistance, measurement samples of predetermined sizes were cut under the same laser processing conditions as above, and measurements were performed.

[0096] These hard coat films were attached to an organic EL module via a 25 μm thick adhesive layer to produce a smartphone-type foldable display that could be folded in half at the center of the entire display with a radius of 3 mm, corresponding to the bending radius in Figure 1. The hard coat film was arranged on the surface of a single continuous display via the folding portion, and the hard coat layer was arranged so as to be located on the surface of the display. The displays using the hard coat films of each example satisfied the operation and visibility of a smartphone that could be folded in half at the center and carried around. On the other hand, the foldable displays using the hard coat films of each comparative example appeared to develop image distortion at the folding portion of the display as usage frequency increased, which was not very desirable.

[0097] [Table 1] [Industrial Applicability]

[0098] According to a foldable display using the polyester film or hard coat film for surface protection of a foldable display of the present invention, the polyester film or hard coat film located on the surface of the foldable display does not deform after repeated folding while maintaining mass productivity, so that image distortion does not occur at the folded portion of the display. Mobile terminal devices equipped with a foldable display using the polyester film or hard coat film of the present invention as a surface protection film provide beautiful images, are highly functional, and are convenient in terms of portability and the like. [Explanation of symbols]

[0099] 1: Foldable display 11: Bending radius 2: Polyester film for surface protection of foldable displays 21: Folding section 22: Bending direction (direction perpendicular to the folding part) 3: Hard coat film for surface protection of foldable displays 31: Hard coat layer 32: Polyester film substrate 33: Cut edge of hard-coated film 34: Edge cut perpendicular to the cut edge of the hard-coated film 35: Thickness of hard coat film 36: Thickness of the thickest part of the cut edge of the hard-coated film 4: Hard coat film for surface protection of foldable displays 41: End cut in the bending direction (with raised part) 42: Bending direction (direction perpendicular to the folding part)

Claims

1. A sheet-like polyester film having a total light transmittance of 85% or more, A polyester film for a folding display, in which the amount of swelling of at least one of both end portions located at both ends in the folding direction when the polyester film is folded is 35 μm or less. (Here, the bending direction of the polyester film refers to the direction perpendicular to the fold when the polyester film is folded.)

2. The haze is 3% or less, 2. The polyester film for a folding display according to claim 1, wherein the amount of protrusion at both ends is 35 μm or less.

3. 2. The polyester film for a foldable display according to claim 1, having a thickness of 10 μm or more.

4. 2. The polyester film for folding displays according to claim 1, having an intrinsic viscosity of 0.55 dl / g or more.

5. 2. The polyester film for a foldable display according to claim 1, wherein the edge of the polyester film is cut in the bending direction of the polyester film using a laser beam.

6. A hard coat film for a folding display, comprising the polyester film for a folding display according to any one of claims 1 to 5, and a hard coat layer having a thickness of 1 to 50 µm on at least one side thereof.

7. 7. The hard coat film for a folding display according to claim 6, wherein the pencil hardness of the hard coat layer measured under a load of 750 g in accordance with JIS K5600-5-4:1999 is H or more.

8. A foldable display in which the hard coat film for a foldable display according to claim 6 is arranged as a surface protective film so that the hard coat layer is positioned on the surface, and the foldable display has a bending radius of 5 mm or less when folded.

9. 9. The foldable display according to claim 8, wherein a single continuous hard coat film is disposed across the fold of the foldable display.

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

Images