Optical transparent film for flexible display device

Abstract

The present invention relates to an optically transparent film for a flexible display device, and more specifically, an optically transparent film for a flexible display device according to one embodiment of the present invention comprises: a first polymer substrate comprising a thermoplastic polyurethane; a second polymer substrate disposed below the first polymer substrate and comprising a polyester copolymer; and a hard coating layer disposed on the first polymer substrate.

Description

Technology Field

[0001] The present invention relates to an optically transparent composite film for a flexible display device, and more specifically, to an optically transparent film for a flexible display device that is optically transparent and satisfies folding characteristics while having excellent wear resistance and durability, and is applicable to a flexible display device. Background Technology

[0002] With the recent entry into the information age, the field of displays that visually represent electrical information signals has advanced rapidly, and in response to this, various display devices with excellent performance characteristics such as thinness, lightness, and low power consumption are being developed. In addition, foldable display devices that utilize flexible materials to achieve flexible characteristics and maintain display performance even when the device is folded are rapidly emerging as next-generation display devices.

[0003] Tempered glass substrates, widely used for display devices, offer excellent transparency but are susceptible to impact and have limitations in terms of weight reduction and thinning. Furthermore, glass substrates present significant process constraints for implementing curved or foldable display devices. Consequently, various flexible substrates have been developed to replace glass. In particular, polymer substrates are optically transparent, lightweight, and flexible, and are easy to process into thin films, offering advantages in weight reduction and thinning. Above all, they possess high impact resistance and flexibility, making them easy to process into various shapes.

[0004] Meanwhile, polymer substrates have lower moisture permeability compared to glass substrates, allowing moisture or oxygen to easily penetrate, which can reduce the lifespan of the display device. To prevent this, various protective layers and functional coating layers are laminated on the top and bottom of the polymer substrate.

[0005] In order for films made of these polymers to serve as cover windows for flexible display devices, they require excellent surface hardness and wear resistance, excellent moisture and oxygen barrier properties, and excellent optical properties, which directly affect the lifespan of the display device. However, since conventional hard coating films actually have very poor moisture and oxygen barrier capabilities, experiments are actively being conducted to achieve the above properties by coating functional coating layers in multiple layers.

[0006] To this end, a transparent protective film composed of a double layer of polymer films is used; however, there have been problems such as delamination or fracture between the layers or reduced durability as the adhesion between the two polymer films decreases over time. The problem to be solved

[0007] The problem that the present invention aims to solve is to provide an optically transparent film for a flexible display device that has high hardness and scratch resistance, while also having excellent wear resistance and durability, so that it can replace a conventional glass substrate for use as a cover window for a flexible display device.

[0008] The problems of the present invention are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below. means of solving the problem

[0009] To solve the problem described above, an optically transparent film for a flexible display device according to one embodiment of the present invention comprises: a first polymer substrate comprising a thermoplastic polyurethane; a second polymer substrate disposed below the first polymer substrate and comprising a polyester copolymer; and a hard coating layer disposed on the first polymer substrate.

[0010] Specific details of other embodiments are included in the detailed description and drawings. Effects of the invention

[0011] The present invention provides the effect of improving durability and scratch resistance by laminating a first polymer substrate and a second polymer substrate.

[0012] The present invention provides the effect of improving interlayer adhesion compared to conventional double-layer films by using a thermoplastic polyurethane film and a polyester copolymer film, and thereby improving durability and lifespan.

[0013] The effects according to the present invention are not limited to those exemplified above, and various other effects are included in this specification. Brief explanation of the drawing

[0014] FIG. 1 is a schematic cross-sectional view of an optically transparent film for a flexible display device according to one embodiment of the present invention. Specific details for implementing the invention

[0015] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

[0016] Meanwhile, the advantages and features of the present invention and the methods for achieving them will become clear by referring to the embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below but may be implemented in various different forms. These embodiments are provided merely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the scope of the claims. Furthermore, the size and thickness of each component shown in the drawings are illustrated for convenience of explanation and the present invention is not necessarily limited to the size and thickness of the illustrated components.

[0017] Where terms such as 'includes,' 'have,' 'consists of,' etc. are used in this specification, other parts may be added unless 'only' is used. Where a component is expressed in the singular, it includes cases where it is in the plural unless specifically stated otherwise.

[0018] In interpreting the components, they are interpreted to include a margin of error even in the absence of a separate explicit description.

[0019] In the case of describing a positional relationship, for example, when the positional relationship between two parts is described using expressions such as 'on,' 'upper,' 'lower,' or 'next to,' one or more other parts may be located between the two parts unless 'immediately' or 'directly' is used.

[0020] Although terms such as "first," "second," etc., are used to describe various components, these components are not limited by these terms. These terms are used merely to distinguish one component from another. Accordingly, the first component mentioned below may be the second component within the technical scope of the present invention.

[0021] The features of each of the various embodiments of the present invention may be combined or combined with one another, either partially or wholly, and as will be fully understood by those skilled in the art, various technical interlocking and operation are possible, and each embodiment may be implemented independently of one another or together in an interlocking relationship.

[0022] FIG. 1 is a schematic cross-sectional view of an optically transparent film (100) for a flexible display device according to one embodiment of the present invention.

[0023] Referring to FIG. 1, an optically transparent film (100) for a flexible display device according to one embodiment of the present invention comprises a first polymer substrate (110), a second polymer substrate (120), and a hard coating layer (130).

[0024] The first polymer substrate (110) is a substrate for forming various layers that constitute an optically transparent film (100) for a flexible display device. The first polymer substrate (110) protects the display device from external impacts, etc.

[0025] The first polymer substrate (110) comprises thermoplastic polyurethane (TPU). Thermoplastic polyurethane has high transparency and excellent flexibility and impact resistance.

[0026] Thermoplastic polyurethanes can be derived from polyfunctional polyols with two or more functions and polyfunctional isocyanates with two or more functions, and may additionally include chain extenders. The polyols may include one or more of aromatic polyols, aliphatic polyols, and alicyclic polyols. Preferably, the polyurethane may be formed from one or more of aliphatic polyols and alicyclic polyols. In this case, yellowing of the transparent film may occur less. The polyols may include, but are not limited to, polyester diols, polycarbonate diols, polyolefin diols, polyether diols, polythioether diols, polysiloxane diols, polyacetal diols, and polyesteramide diols. The polyfunctional isocyanates may include any aliphatic, alicyclic, or aromatic isocyanates. The chain extenders may include diols, for example, aliphatic diols, amino alcohols, diamines, hydrazines, hydrazides, or mixtures thereof. The Shore hardness of the thermoplastic polyurethane can be obtained by varying the block level amount of the polyfunctional isocyanate and / or chain extender in the thermoplastic polyurethane or by controlling the polymerization time. When manufacturing the thermoplastic polyurethane, tin compounds, such as tin salts of carboxylic acids, amines, such as dimethylcyclohexylamine or triethylenediamine, may be further included as catalysts to promote the formation of bonds in the urethane. When manufacturing the thermoplastic polyurethane, other conventional components, such as surfactants, flame retardants, fillers, pigments, etc., may also be further included.

[0027] Meanwhile, the first polymer substrate (110) may be formed solely of thermoplastic polyurethane, or may be formed by further including an additive that prevents yellowing, such as a stabilizer, in the thermoplastic polyurethane.

[0028] The thickness of the first polymer substrate (110) may be 50 µm to 400 µm or 100 µm to 200 µm. Within this range, the thickness of the optically transparent film (100) for a flexible display device can be made slim, and the flexibility and folding characteristics are excellent.

[0029] The elastic modulus of the first polymer substrate (110) may be 10 MPa to 1000 MPa or 30 MPa to 50 MPa. When the elastic modulus of the first polymer substrate (110) satisfies the above range, the folding ability and scratch resistance of the optically transparent film may be excellent.

[0030] The refractive index of the first polymer substrate (110) may be 1.40 to 1.65 or 1.45 to 1.60. Within the above range, the refractive index is appropriate so that the optical properties of the protective film may be good and screen visibility may be improved.

[0031] The light transmittance of the first polymer substrate (110) at a wavelength of 550 nm may be 85% to 100% or 90% to 99%. Within the above range, it may be used in a display device.

[0032] The second polymer substrate (120) is placed below the first polymer substrate (110). The second polymer substrate (120) protects the display device from external impacts, etc. together with the first polymer substrate (110). The second polymer substrate (120) can improve the wear resistance and durability of the optical transparent film (100) for the flexible display device.

[0033] The second polymer substrate (120) comprises a polyester copolymer. The polyester copolymer constituting the second polymer substrate (120) is a copolymer prepared by copolymerizing a dicarboxylic acid component, a diol component, and a trifunctional compound.

[0034] The dicarboxylic acid component refers to a major monomer that constitutes the polyester copolymer together with the diol component. In particular, the dicarboxylic acid includes terephthalic acid, and the physical properties of the polyester copolymer according to the present invention, such as heat resistance, chemical resistance, and weather resistance, can be improved by the terephthalic acid.

[0035] The dicarboxylic acid component may additionally include an aromatic dicarboxylic acid component, an aliphatic dicarboxylic acid component, or a mixture thereof, in addition to terephthalic acid. In this case, the dicarboxylic acid component other than terephthalic acid may be included in an amount of 1 to 30 weight% relative to the total weight of the total dicarboxylic acid component. The aromatic dicarboxylic acid component may be an aromatic dicarboxylic acid having 8 to 20 carbon atoms, preferably 8 to 14 carbon atoms, or a mixture thereof. Examples of the aromatic dicarboxylic acid include isophthalic acid, naphthalene dicarboxylic acids such as 2,6-naphthalene dicarboxylic acid, diphenyl dicarboxylic acid, 4,4'-stilbene dicarboxylic acid, 2,5-furandicarboxylic acid, 2,5-thiophene dicarboxylic acid, etc., but specific examples of the aromatic dicarboxylic acid are not limited thereto. The aliphatic dicarboxylic acid component may be an aliphatic dicarboxylic acid component having 4 to 20 carbon atoms, preferably 4 to 12 carbon atoms, or a mixture thereof. Examples of aliphatic dicarboxylic acids include cyclohexanedicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and 1,3-cyclohexanedicarboxylic acid, linear, branched, or cyclic aliphatic dicarboxylic acid components such as phthalic acid, sebacic acid, succinic acid, isodecylsuccinic acid, maleic acid, fumaric acid, adipic acid, glutaric acid, and azelaic acid, but specific examples of the above aliphatic dicarboxylic acids are not limited thereto.

[0036] The diol component forms a polyester copolymer together with the dicarboxylic acid component. In this case, the diol component includes cyclohexanedimethanol, ethylene glycol, and isosorbide.

[0037] The above cyclohexanedimethanol (e.g., 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, or 1,4-cyclohexanedimethanol) is a component that contributes to improving the transparency and impact strength of the polyester copolymer being manufactured. Preferably, the cyclohexanedimethanol residue may be included in an amount of 30 to 70 parts by weight relative to 100 parts by weight of the total diol component residue.

[0038] Ethylene glycol is a component that contributes to improving the transparency and impact strength of a polyester copolymer prepared together with cyclohexanedimethanol. Preferably, the ethylene glycol residue may be included in an amount of 5 to 25 parts by weight per 100 parts by weight of the total diol component residue.

[0039] Isosorbide is used to improve the processability of the polyester copolymer being manufactured. Although the transparency and impact strength of the polyester copolymer are improved by the diol components of the aforementioned cyclohexanedimethanol and ethylene glycol, for processability, shear fluidity characteristics must be improved and the crystallization rate delayed, but it is difficult to achieve these effects with cyclohexanedimethanol and ethylene glycol alone. Accordingly, when isosorbide is included as a diol component, the processability of the polyester copolymer being manufactured is improved by improving shear fluidity characteristics and delaying the crystallization rate while maintaining transparency and impact strength. Preferably, the isosorbide residue may be included in an amount of 0.1 to 5 parts by weight relative to 100 parts by weight of the total diol component residue.

[0040] The trifunctional compound is a component used in the manufacture of polyester copolymers in addition to the aforementioned dicarboxylic acid and diol components, and is added to further improve processability, particularly shear fluidization properties.

[0041] The above trifunctional compound may be a benzene tricarboxylic acid or an anhydride thereof. Specifically, the trifunctional compound is a benzene-1,2,3-tricarboxylic acid, a benzene-1,2,3-tricarboxylic acid anhydride, a benzene-1,2,4-tricarboxylic acid, or a benzene-1,2,4-tricarboxylic acid anhydride. Preferably, the residue of the trifunctional compound is included in an amount of 0.01 to 1 part by weight based on 100 parts by weight of the polyester copolymer. If the content exceeds 1 part by weight, there is a disadvantage that the transparency of the polyester copolymer produced is reduced, and if the content is less than 0.01 part by weight, there is a disadvantage that the improvement in processability is negligible.

[0042] As described above, a polyester copolymer is a copolymer prepared by copolymerizing a dicarboxylic acid component, a diol component, and a trifunctional compound. Accordingly, the polyester copolymer comprises a residue of the dicarboxylic acid component including terephthalic acid; a residue of the diol component including cyclohexanedimethanol, ethylene glycol, and isosorbide; and a residue of the trifunctional compound. In this specification, "residue" refers to a certain portion or unit derived from said specific compound that is included in the product of the chemical reaction when said specific compound participates in the chemical reaction. Specifically, each of the "residue" of the dicarboxylic acid component or the "residue" of the diol component refers to a portion derived from the dicarboxylic acid component or a portion derived from the diol component in the polyester copolymer formed by the esterification reaction or the condensation reaction. In addition, the 'residue' of the trifunctional compound refers to a portion derived from the trifunctional compound in the ester structure formed by the esterification reaction of the functional group with the diol component.

[0043] The thickness of the second polymer substrate (120) may be 10 μm to 100 μm. Within this range, the adhesion between the second polymer substrate (120) and the first polymer substrate (110) is improved, and the flexibility and folding characteristics are excellent.

[0044] Flexural modulus of the second polymer substrate (120) 17,000~20,000 kgf / cm 2 It may be possible. If the flexural modulus of the second polymer substrate (120) satisfies the above range, the folding ability and scratch resistance of the optically transparent film may be excellent.

[0045] The glass transition temperature of the second polymer substrate (120) may be 90°C to 120°C.

[0046] The refractive index of the second polymer substrate (120) may be 1.40 to 1.65, specifically 1.45 to 1.60. Within this range, the optical properties of the transparent optical film may be good, and screen visibility may be improved.

[0047] The light transmittance of the second polymer substrate (120) at a wavelength of 550 nm may be 85% to 100% or 90% to 99%. Within the above range, it may be used in a display device.

[0048] The second polymer substrate (120) is made of a polyester copolymer. Since the polyester copolymer is an amorphous polymer, it can have excellent adhesion to thermoplastic polyurethane, which is also an amorphous polymer, and as a result, durability and wear resistance are improved.

[0049] The second polymer substrate (120) is laminated onto one side of the first polymer substrate (110) without an adhesive layer. For example, the second polymer substrate (120) can be laminated to the first polymer substrate (110) through a pressurizing process using a hot press. However, the second polymer substrate (120) can be placed on the first polymer substrate (110) by various methods other than the pressurizing process, such as co-extrusion or thermal lamination.

[0050] A hard coating layer (130) is disposed on the first polymer substrate (110). The hard coating layer (130) can protect an optically transparent film for a display device and various components mounted on the underside of the optically transparent film for a display device. The hard coating layer (130) has excellent surface hardness, so it can maintain high scratch resistance and wear resistance of the optically transparent film (100) for a flexible display device.

[0051] The hard coating layer (130) may include an acrylate-based resin, a urethane-based resin, or a silicone-based resin. For example, the silicone-based resin may be one or more selected from organic siloxane-based resins and silsesquioxanes. In order to ensure high surface hardness, the hard coating layer (130) may include an organic siloxane-based resin. For example, the organic siloxane-based resin may be one or more selected from acrylic-modified siloxane resins and epoxy-modified siloxane resins. For example, the acrylic-modified siloxane resin may be a resin in which acrylic groups are bonded to both ends of the siloxane resin. For example, the acrylic-modified siloxane resin may be represented by the following chemical formula 1, but is not limited thereto.

[0052] The hard coating layer (130) may further include inorganic particles. When further including inorganic particles, the surface hardness is further improved, which can enhance the wear resistance and scratch resistance of the optically transparent film (100) for the flexible display device and provide excellent impact resistance. Additionally, the inorganic particles can minimize deformation at high temperatures by lowering the coefficient of thermal expansion without reducing the flexibility of the hard coating layer (130) and can improve barrier properties. For example, the inorganic particles may be one or more of silicon oxide (SiO2), zirconium oxide (ZrO2), aluminum oxide (Al2O3), tantalum oxide (Ta2O5), niobium oxide (Nb2O5, NbO2), and glass beads. When further including these inorganic particles, it is preferable to use nano-sized inorganic particles so that optical properties are not degraded. For example, the average particle size of the inorganic particles may be 10 nm to 50 nm, and within this range, the optical properties can be maintained at a high level while improving the surface hardness and moisture resistance of the hard coating layer (130). If the average particle size of the inorganic particles is too large, the flatness of the hard coating layer (130) may decrease and the optical properties may be degraded. In order to secure higher optical properties and moisture resistance, silicon oxide (SiO2) with a particle size of 10 nm to 50 nm may be used as the inorganic particles, but is not limited thereto.

[0053] A hard coating layer (130) can be formed on a first polymer substrate (110) by various coating methods. For example, a composition for forming a hard coating layer (130) can be coated on a polymer substrate (110) by any one method selected from dip coating, bar coating, slot die coating, microgravure coating, etc., and then the hard coating layer (130) can be formed by UV curing and / or heat curing.

[0054] The thickness of the hard coating layer (130) may be 3 µm to 30 µm or 5 µm to 20 µm. If the thickness of the hard coating layer (130) is less than 3 µm, the surface hardness may not be sufficient, and thus the scratch resistance and wear resistance may be reduced. If the thickness of the hard coating layer (130) is too thick, the flexibility may decrease and stress may increase, causing breakage during folding. Additionally, as the thickness of the hard coating layer (130) increases, the yellowing index (YI) increases, which may degrade the optical properties of the optical transparent film (100) for the flexible display device.

[0055] The pencil hardness of the hard coating layer (130) may be H or higher, or 4H to 8H. Within this range, excellent scratch resistance and wear resistance can be achieved while satisfying folding characteristics. For example, the pencil hardness can be measured under a load of 7.5N according to ISO 15174 standards.

[0056] The yellowing degree of the hard coating layer (130) may be 2 or less, and within this range, the optical transmittance of the optical transparent film (100) for the flexible display device is high, so the optical properties are excellent. For example, the yellowing degree can be measured using a spectrophotometer.

[0057] The effects of the present invention described above will be explained in more detail below through examples and comparative examples. However, the following examples are for illustrative purposes only and do not limit the scope of the present invention.

[0058] [Example 1]

[0059] A TPU film as the first polymer substrate and a polyester copolymer film as the second polymer substrate are prepared having thicknesses as described in Table 1 below. After placing the polyester copolymer film on top of the TPU film, a polyimide film is placed on the polyester copolymer film and a release film is placed on the TPU film surface, and a specimen is prepared by applying pressure of 2 kgf / cm2 at a temperature of 250°C for 10 minutes using a hot press.

[0060] [Experimental Example 1]

[0061] To evaluate durability, the interlayer adhesion between the first polymer substrate and the second polymer substrate of the specimens according to the example and comparative example was evaluated. Interlayer adhesion was determined by making a 100x100 grid incision in the second polymer substrate and then peeling it off with a constant force in the vertical direction using adhesive tape, based on the number of remaining grids.

[0062] [Experimental Example 2]

[0063] To evaluate the physical properties of the optical film, tensile elongation and modulus were measured using a UTM.

[0064] Example 1 Example 2 Example 3 Comparative Example 1 Film composition (thickness) TPU 62㎛ 149㎛ 218㎛ 75㎛ Polyester 40㎛ 45㎛ 39㎛ - PET - - 50㎛ interlayer adhesion 100 / 100 100 / 100 100 / 100 70 / 100 Tensile elongation 6.3% 7.7% 8.1% 2.9% Young's modulus (Kgf / cm²) 2 ) 4088 3519 1935 9340

[0065] In Table 2 above, Examples 1 to 3, in which a polyester film and a TPU film are laminated without an adhesive layer, were compared with Comparative Example 1, in which a conventional PET film and a TPU film are laminated, and it was confirmed that the interlayer adhesion was improved.

[0066] [Example 2]

[0067] A TPU film is prepared as the first polymer substrate and a polyester copolymer film as the second polymer substrate, having thicknesses as indicated in Table 2 below. After overlapping the polyester copolymer film onto the TPU film, a polyimide film is placed on the polyester copolymer film and a release film is placed on the TPU film surface, and a hot press is applied at 2 kgf / cm² 2A hybrid coating solution (Solbrain Co., Ltd., NCS-500) with a base composition of epoxysiloxane resin was applied to the upper surface of the TPU using a Mayer bar, and then dried for 10 minutes at an atmosphere temperature of 105°C in a convection oven, followed by irradiation with ultraviolet A light intensity of 700 mJ / ㎠ to form a hard coating layer with a dry thickness of 10㎛.

[0068] [Experimental Example 3]

[0069] To evaluate optical properties and scratch resistance, the light transmittance and pencil hardness of specimens according to the example and comparative example were measured. Light transmittance was measured at 550 nm using a transmission haze meter. Pencil hardness was measured under a load of 7.5 N according to ISO 15174 standards.

[0070] Example 4 Example 5 Comparative Example 2 Film structure (thickness) hard coating layer 10㎛ 10㎛ 10㎛ TPU 149㎛ 218㎛ 200㎛ Polyester 45㎛ 39㎛ - Light transmittance (550 nm) 90.7% 90.9% 91.3% Pencil hardness (750 g) H H HB

[0071] As shown in Table 2 above, Examples 4 and 5, in which a polyester film, a TPU film, and a hard coating layer are sequentially laminated, showed improved pencil hardness when compared to Comparative Example 2, in which a TPU film is used alone.

[0072] Although embodiments of the present invention have been described in more detail with reference to the attached drawings, the present invention is not necessarily limited to these embodiments and may be modified in various ways within the scope of the technical spirit of the present invention. Accordingly, the embodiments disclosed in the present invention are intended to explain, not limit, the technical spirit of the present invention, and the scope of the technical spirit of the present invention is not limited by these embodiments. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. The scope of protection of the present invention shall be interpreted by the claims below, and all technical spirits within an equivalent scope shall be interpreted as being included within the scope of rights of the present invention. Explanation of the symbols

[0073] 100: Optically transparent film for flexible display devices 110: First polymer substrate 120: Second polymer substrate 130: Hard coating layer

Claims

Claim 1 An optically transparent film for a display device comprising: a first polymer substrate comprising a thermoplastic polyurethane; a second polymer substrate disposed below the first polymer substrate and comprising a polyester copolymer; and a hard coating layer disposed on the first polymer substrate, wherein the elastic modulus of the first polymer substrate is 10 MPa to 1000 MPa. Claim 2 An optically transparent film for a display device according to claim 1, wherein the thickness of the first polymer substrate is 50㎛ to 400㎛, the thickness of the second polymer substrate is 10㎛ to 100㎛, and the thickness of the hard coating layer is 5㎛ to 20㎛. Claim 3 An optically transparent film for a display device according to claim 1, wherein the polyester copolymer comprises a residue of a dicarboxylic acid component including terephthalic acid; and a residue of a diol component including cyclohexanedimethanol, ethylene glycol, and isosorbide. Claim 4 An optically transparent film for a display device, wherein the polyester copolymer further comprises a residue of a trifunctional compound. Claim 5 An optically transparent film for a display device according to claim 4, wherein the trifunctional compound is a benzenetricarboxylic acid or an anhydride thereof. Claim 6 An optically transparent film for a display device according to claim 3, wherein, based on 100 parts by weight of the total diol component residue, the ethylene glycol residue is included in an amount of 5 to 25 parts by weight, the isosorbide residue is included in an amount of 0.1 to 5 parts by weight, and the cyclohexanedimethanol residue is included in an amount of 30 to 70 parts by weight. Claim 7 delete Claim 8 In claim 1, the flexural modulus of the second polymer substrate is 17,000 to 20,000 kgf / cm 2 Optically transparent film for display devices. Claim 9 An optically transparent film for a display device according to claim 1, wherein the glass transition temperature of the second polymer substrate is 90 to 120°C.

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