Polyimide-based film and flexible display panel including the same

Abstract

The present application provides a polyimide-based film, a window cover film, and a display device including the same. More specifically, a polyimide-based film is provided, which has a modulus of 5 GPa or more measured using a universal testing machine (UTM) according to ASTM D882, undergoes plastic deformation at a strain of 4% or more during stretching, and the difference between the machine direction modulus Mmd and the transverse direction modulus Mtd satisfies the following formula 1: [Formula 1] |Mmd – Mtd| ≤ 0.7 GPa.

Description

[0001] Related Application

[0002] This application claims the priority of Korean Patent Application No. 10-2020-0098616, filed with the Korean Intellectual Property Office on August 6, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a polyimide-based film, a window cover film, and a display panel including the same.

[0004] More specifically, the following disclosure relates to a polyimide-based film (which solves the problem of leaving marks on the folded portion when the film is kept in a folded state for a long time and then unfolded), a window cover film, and a display panel including the same. Background Art

[0005] Thin display devices are implemented in the form of touch screen panels and are used in various smart devices, including various wearable devices as well as smartphones and tablet PCs.

[0006] A display based on a touch screen panel is provided with a window cover made of tempered glass on a display panel to protect the display panel from scratches or external impacts.

[0007] However, in recent years, since tempered glass is not suitable for lightweighting and is vulnerable to external impacts, technologies of optical plastic films having strength or scratch resistance comparable to that of tempered glass, as well as flexibility and impact resistance, have been developed.

[0008] As these plastic materials, polyethylene terephthalate (PET), polyethersulfone (PES), polyethylene naphthalate (PEN), polyacrylate (PAR), polycarbonate (PC), polyimide (PI), polyaramide (PA), etc. are used.

[0009] Generally, polymers have viscoelastic properties. The properties of an elastomer (elastic deformation) are exhibited in the fine deformation portion, but when the deformation increases, plastic deformation is exhibited due to the properties of a viscous body. Here, when the elastic properties of a polymer are large, the polymer may have a high modulus and strength but a low elongation rate. However, when the viscosity is large, the polymer exhibits a high elongation rate but a low modulus and strength, and thus has weak mechanical strength.

[0010] Optical films for window cover films applied to foldable displays and flexible displays need to have high mechanical strength to replace glass and should not leave marks even when kept in a deformed state such as folding and bending.

[0011] Therefore, a physical property is required such that no trace is left when a device having a film for a window covering film is kept in a folded state for a long time and then unfolded.

[0012] [Related Technical Literature]

[0013] [Patent Literature]

[0014] (Patent Document 1) Korean Patent Laid-Open Publication No. 10-2017-0028083A (March 13, 2017) Summary of the Invention

[0015] One embodiment of the present invention aims to provide a polyimide-based film for a window covering film applied to a flexible display. When it is fixed to a folding tester (YUASA SYSTEMS CO., LTD.) using an adhesive and folded in a state where the folding radius ( Figure 1 R1 in it) is 3 mm, kept at 25 °C / 50% RH for 240 hours, and then unfolded, the folded area of the polyimide-based film will not re-fold at the folded part, can be restored to the original state with minimal deformation, and the change in optical physical properties is very small.

[0016] Generally, when an optical film made of an organic material is deformed to a plastically deformed part and then kept in a state where a constant stress is applied, even if the stress is removed, the polymer will not return to its original state. This may cause a problem that a device with a window covering film will leave a trace and cannot be restored to the original state when it is kept in a folded state for a long time and then unfolded. Therefore, the physical properties of the window covering film for a flexible display are particularly important.

[0017] Therefore, as a result of research to solve this problem, the present invention found that by adjusting the magnitude of the stress applied to the film and the stress relaxation during the film production process, the magnitude of the modulus in the film and the strain causing plastic deformation are adjusted to achieve the above object.

[0018] More specifically, as an example, the magnitude of the stress applied to the film and the stress relaxation can be adjusted by adjusting the stretching and heat setting steps, and it was found that by the following method: when the modulus is 5.0 GPa or more, plastic deformation occurs at a strain of 4% or more during the stretching process, and the difference between the modulus in the machine direction (MD) and the width direction (TD) is 0.7 GPa or less, the desired effect of the present invention can be obtained, thus completing the present invention.

[0019] In addition, the present invention also found that when the stress at the time of plastic deformation is 1000 kgf / cm 2 or more, the effect of the present invention can be further improved, thus completing the present invention.

[0020] In addition, in the stress-strain curve measured using a universal testing machine (UTM), at the point where plastic deformation occurs, the energy required per unit thickness μm of the polyimide-based film can be 30 J / m 2 / μm or more, more specifically 30 - 100 J / m 2 / μm, more specifically 35 - 60 J / m 2 / μm. Within this range, a film with better resilience even after long-term folding can be provided.

[0021] When a film satisfying the physical properties can be obtained, regardless of the preparation method and means, as an exemplary embodiment, first, the transparent polyimide solution of the present invention is cast on a substrate, and then the film is peeled off from the substrate while retaining 15 - 30 wt% of the residual solvent through the first drying.

[0022] Subsequently, the peeled film is stretched 1.01 - 1.5 times in the MD direction (film traveling direction) at a temperature below 150°C, and then dried again in a drying chamber for secondary drying to dry the solvent to 5 wt% or less, preferably 3 wt% or less, more preferably 0.5 wt% or less. Here, the drying temperature is preferably maintained at 150°C - 300°C. During the secondary drying process, a step of fixing the film using clips or needle-type jigs to suppress shrinkage in the TD direction (direction perpendicular to the film traveling direction) to impart a stretching effect in the TD direction is performed. Then, the film with the stretching effect is heat-treated at a temperature around the glass transition temperature (Tg) ± 30°C for 10 seconds to 10 minutes to obtain a polyimide-based film having the characteristics of the present invention.

[0023] Therefore, the film of the present invention is a transparent polyimide-based film having a high modulus and causing plastic deformation at high strain, and can be used as a window covering film for foldable and flexible devices, which shows a significantly improved recovery to its original state when held in a state of being folded and then unfolded for a long time.

[0024] In addition, the polyimide-based film having adjusted creep characteristics according to the present invention can have a high pencil hardness, dynamic bending characteristics, and durability.

[0025] Other features and aspects will become apparent from the following detailed description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A schematic diagram showing the folded state of a window covering film according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0027] In the following, the present invention will be described in more detail. However, each of the following exemplary embodiments is only for reference in describing the present invention in detail, and the present invention is not limited thereto and can be implemented in various forms.

[0028] In addition, unless otherwise defined, all technical terms and scientific terms have the same meaning as commonly understood by those skilled in the art to which the present invention pertains. The terms used herein are only for effectively describing a specific example and are not used to limit the present invention.

[0029] In addition, unless otherwise indicated in the context, the singular forms used in the specification and the appended claims may also include the plural forms.

[0030] In the present invention, polyimide is used as a term including polyimide or polyamide-imide.

[0031] The inventors of the present invention have developed a film, as Figure 1 shown, when it is fixed to a folding tester (YUASA SYSTEMS CO., LTD.) using an adhesive, in a state of being folded with a folding radius ([R1 in Figure 1 ) set to 3 mm, maintained at 25 °C / 50% RH for 240 hours, and then unfolded, it can return to its original state without re-folding or causing deformation in the folded part, and the optical and physical properties change less. Deformation means that when the folded part is unfolded on a flat floor, the folded part bends again or is re-folded, or there are optical stains or a blurry haze in the folded part.

[0032] In addition, the film of the present invention has a modulus of 5 GPa or more according to ASTM D882 and undergoes plastic deformation at a strain of 4% or more during the stretching process. When the difference between the modulus in the machine direction Mmd and the modulus in the width direction Mtd satisfies the following formula 1, the film can exhibit better resilience and is therefore preferred. Specifically, in the following formula 1, the difference is preferably 0.7 GPa or less, 0.4 GPa or less, 0.3 GPa or less, and more specifically 0.2 GPa or less. There is no limit to the lower limit, and it can be 0.

[0033] [Formula 1]

[0034] |Mmd – Mtd| ≤ 0.7 Gpa.

[0035] The boundary between elasticity and plasticity is determined by defining the point at which the differential slope in the S-S curve decreases by 75% relative to the stress-strain slope in the initial elastic part (0% - 0.5% strain part) as the plastic part.

[0036] In addition, a polyimide-based film having a total light transmittance of more than 87% measured at 400 - 700 nm according to ASTM D1746, a light transmittance of more than 5% measured at 388 nm according to ASTM D1746, a haze of 2.0% or less, and a yellowness index of 5.0 or less is preferred.

[0037] Within the range that satisfies all physical properties, the film can be applied to a window covering film, which has excellent resilience to recover its original state even after being folded for a long time, and excellent optical and physical properties even after folding, thereby providing a window covering film suitable for flexible displays.

[0038] In addition, in the measurement of the stress-strain curve using a universal testing machine (UTM), at the point where plastic deformation occurs, the energy required for the polyimide-based film per unit thickness μm can be 30 J / m 2 or more, more specifically 30 - 100 J / m 2 . Within this range, a film with excellent resilience even after being folded for a long time can be provided, which is therefore preferred.

[0039] In addition, the stress of the polyimide-based film at the point where plastic deformation occurs can be 1000 kgf / cm 2 or more, more specifically 1000 - 3000 kgf / cm 2 .

[0040] In addition, as a method for manufacturing a polyimide-based film that satisfies all physical properties, although there is no particular limitation on the method in the present invention, as an example of a means to achieve the purpose, the transparent polyimide of the present invention is used, and the drying conditions, stretching conditions, and heat treatment conditions are adjusted, and the content of the solvent in each step is adjusted to change the creep characteristics, thereby obtaining the physical properties of the present invention.

[0041] As an example of a specific means, the polyimide-based resin solution of the present invention is cast, the film is peeled off in a state where the residual solvent is 15 - 30% by weight, and it is finely stretched in the MD direction at a temperature of 150°C or lower, and while performing secondary drying, it is tightly fixed in the TD direction with a clip so as not to shrink but to be stretched, thereby imparting a stretching effect. Subsequently, the film is heat-treated at a glass transition temperature (Tg) ± 30°C close to the glass transition temperature, thereby manufacturing a polyimide film having the resilience characteristics of the present invention.

[0042] Hereinafter, this will be described in more detail as an example.

[0043] <Polyimide-based film>

[0044] In an exemplary embodiment of the present invention, the thickness of the polyimide-based film may be 10 - 500 μm, 20 - 250 μm, or 30 - 100 μm.

[0045] In an exemplary embodiment of the present invention, the polyimide-based film may be a polyimide-based resin, particularly a polyimide-based resin having a polyamide-imide structure.

[0046] Preferably, the polyimide-based film may be a polyamide-imide-based resin containing fluorine atoms and an aliphatic ring structure, and the physical properties of the present invention can be achieved by subjecting the resin to the drying, stretching, and heat treatment conditions of the present invention, so that better mechanical and physical properties and dynamic bending properties can be obtained.

[0047] In an exemplary embodiment of the present invention, the polyamide-imide-based resin containing fluorine atoms and an aliphatic ring structure may include units derived from fluorinated aromatic diamines, units derived from aromatic dianhydrides, and units derived from aromatic diacid dichloride.

[0048] More preferably, in an exemplary embodiment of the present invention, as the polyamide-imide-based resin containing fluorine atoms and an aliphatic ring structure, a quaternary copolymer containing units derived from fluorinated aromatic diamines, units derived from aromatic dianhydrides, units derived from alicyclic dianhydrides, and units derived from aromatic diacid dichloride is preferably used because this quaternary copolymer is more suitable for exhibiting the required physical properties.

[0049] In an exemplary embodiment of the present invention, as an example of the polyamide-imide-based resin containing fluorine atoms and an aliphatic ring structure, the polyamide-imide polymer is preferred because the present invention is better realized. It is prepared by the following method: preparing an amine-terminated polyamide oligomer derived from a first fluorinated aromatic diamine and an aromatic diacid dichloride, and polymerizing the amine-terminated polyamide oligomer with monomers derived from a second fluorinated aromatic diamine, an aromatic dianhydride, and an alicyclic dianhydride to prepare a polyamide-imide polymer.

[0050] The first fluorinated aromatic diamine and the second fluorinated aromatic diamine may be of the same or different types. More specifically, an exemplary embodiment of the polyamide-imide-based resin may include a block composed of an amine-terminated polyamide oligomer derived from a first fluorinated aromatic diamine and an aromatic diacid dichloride and polyimide units at both ends, and based on mass, the content of the block may be 50% or more.

[0051] In an exemplary embodiment of the present invention, when an amine-terminated oligomer having an amide structure in a polymer chain formed from an aromatic diacid dichloride is used as a monomer of a diamine, not only can the optical and physical properties be further improved, but in particular, the mechanical strength including modulus can be further improved, and the dynamic bending performance can also be further enhanced.

[0052] In an exemplary embodiment of the present invention, when a polyamide oligomer block is included as described above, the molar ratio between the diamine monomer containing an amine-terminated polyamide oligomer and a second fluoro-based aromatic diamine and the dianhydride monomer containing an aromatic dianhydride and an alicyclic dianhydride is preferably 1:(0.9 - 1.1), specifically 1:1.

[0053] Furthermore, the content of the amine-terminated polyamide oligomer relative to all diamine monomers is not particularly limited, but in order to meet the mechanical properties, yellowness index, and optical properties of the present invention, it preferably includes 30 mol% or more, specifically 50 mol% or more, and more specifically 70 mol% or more of the amine-terminated polyamide oligomer.

[0054] In addition, there is no particular limitation on the composition ratio of the aromatic dianhydride to the alicyclic dianhydride, but considering the transparency, yellowness index, and mechanical and physical properties of the present invention, the composition ratio of the aromatic dianhydride to the alicyclic dianhydride is preferably in the ratio of (30 - 80 mol%):(70 - 20 mol%), but the present invention is not necessarily limited thereto.

[0055] In addition, another example of the polyamide-imide resin containing a fluorine atom and an aliphatic cyclic structure in the present invention may be a polyamide-imide resin obtained by mixing, polymerizing, and imidizing a fluoro-based aromatic diamine, an aromatic dianhydride, an alicyclic dianhydride, and an aromatic diacid dichloride.

[0056] This resin has a random copolymer structure and may include 40 mol or more, specifically 50 - 80 mol of aromatic diacid dichloride, 10 - 50 mol of aromatic dianhydride, and 10 - 60 mol of alicyclic dianhydride relative to 100 mol of diamine, and can be prepared by polymerizing at a molar ratio of the sum of diacid dichloride and dianhydride to the diamine monomer of 1:(0.9 - 1.1), specifically 1:1, but the present invention is not necessarily limited thereto.

[0057] Due to different surface energies, the random polyamide-imide resin and the block polyamide-imide resin of the present invention are slightly different in optical properties such as transparency, mechanical and physical properties, and solvent sensitivity, but they may also fall within the scope of the present invention.

[0058] In an exemplary embodiment of the present invention, as the fluoroaromatic diamine component, a mixture of 2,2'-bis(trifluoromethyl)-benzidine and another known aromatic diamine component can be used, or 2,2'-bis(trifluoromethyl)-benzidine can be used alone. By using such a fluoroaromatic diamine, based on the mechanical and physical properties required by the present invention, excellent optical properties can be further improved as a polyamide-imide-based film, and the yellowness index can be further improved. In addition, the tensile modulus of the polyamide-imide-based film can be improved to further improve the mechanical strength and further improve the dynamic bending performance.

[0059] As the aromatic dianhydride, at least one or more of 4,4'-hexafluoroisopropylidene diphthalic anhydride (6FDA), biphenyltetracarboxylic dianhydride (BPDA), oxydiphthalic anhydride (ODPA), sulfonyldiphthalic anhydride (SO2DPA), (isopropylidenediphenoxy)bis(phthalic anhydride) (6HDBA), 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic dianhydride (TDA), 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), benzophenonetetracarboxylic dianhydride (BTDA), bis(carboxyphenyl)dimethylsilane dianhydride (SiDA), and bis(dicarboxyphenoxy)diphenyl sulfide dianhydride (BDSDA) can be used, but the present invention is not limited thereto.

[0060] Examples of the alicyclic dianhydride include one or a mixture of two or more selected from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), 5-(2,5-dioxotetrahydrofuryl)-3-methylcyclohexene-1,2-dicarboxylic dianhydride (DOCDA), bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BTA), bicyclooctene-2,3,5,6-tetracarboxylic dianhydride (BODA), 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA), 1,2,4-tricarboxy-3-methylcarboxycyclopentane dianhydride (TMDA), 1,2,3,4-tetracarboxycyclopentane dianhydride (TCDA), and derivatives thereof.

[0061] In an exemplary embodiment of the present invention, when forming an amide structure in a polymer chain through an aromatic diacyl chloride, not only can the optical physical properties be further improved, but also the mechanical strength (especially including modulus) can be further improved.

[0062] As the aromatic diacyl chloride, one or a mixture of two or more selected from isophthaloyl chloride (IPC), terephthaloyl chloride (TPC), [1,1'-biphenyl]-4,4'-dicarbonyl chloride (BPC), 1,4-naphthalenedicarbonyl chloride (NPC), 2,6-naphthalenedicarbonyl chloride (NTC), 1,5-naphthalenedicarbonyl chloride (NEC), and their derivatives can be used, but the present invention is not limited thereto.

[0063] Hereinafter, taking the case of manufacturing a block polyamide-imide film as an example, each process will be described in more detail.

[0064] The step of preparing the oligomer may include reacting a fluoroaromatic diamine with an aromatic diacyl chloride, and purifying and drying the obtained oligomer.

[0065] In this case, relative to the aromatic diacyl chloride, the fluoroaromatic diamine can be introduced in a molar ratio of 1.01 - 2 to prepare an amine-terminated polyamide oligomer. The molecular weight of the oligomer is not particularly limited, but for example, when the weight average molecular weight is in the range of 1000 - 3000 g / mol, better physical properties can be obtained. Here, side reactions can be suppressed by polymerizing the oligomer in the presence of pyridine to prepare a resin with better physical properties.

[0066] In addition, it is preferable to use an aromatic carbonyl halide monomer such as terephthaloyl chloride or isophthaloyl chloride instead of terephthalate or terephthalic acid itself for introducing the amide structure because the chlorine element has an impact on the physical properties of the film.

[0067] Next, the step of preparing the polyamic acid can be carried out by a solution polymerization reaction, in which the oligomer thus prepared reacts with a fluoroaromatic diamine, an aromatic dianhydride, and an alicyclic dianhydride in an organic solvent. Here, the organic solvent used for the polymerization reaction can be, for example, one or more polar solvents selected from dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethyl cellosolve, methyl cellosolve, acetone, ethyl acetate, and m-cresol.

[0068] Next, the step of imidizing to prepare the polyamide-imide resin can be carried out by chemical imidization. More preferably, the polyamic acid solution is chemically imidized using pyridine and acetic anhydride. Subsequently, imidization can be carried out using an imidization catalyst and a dehydrating agent at a low temperature of 150°C or lower, preferably 100°C or lower, and more specifically 50°C to 150°C.

[0069] By chemical imidization, compared with the case of imidization reaction by high-temperature heating, uniform mechanical and physical properties can be imparted to the entire film.

[0070] As the imidization catalyst, any one or more than two selected from pyridine, isoquinoline, and β-quinoline can be used. Additionally, as the dehydrating agent, any one or more than two selected from acetic anhydride, phthalic anhydride, maleic anhydride, etc. can be used, but the present invention is not limited thereto.

[0071] Furthermore, additives (such as flame retardants, adhesion improvers, inorganic particles, antioxidants, UV inhibitors, and plasticizers) can be mixed with the polyamic acid solution to prepare a polyamide-imide resin.

[0072] Furthermore, after imidization, the resin is purified using a solvent to obtain a solid content, which can be dissolved in a solvent to obtain a polyamide-imide solution. The solvent may include, for example, N,N-dimethylacetamide (DMAc), etc., but is not limited thereto.

[0073] In the present invention, the weight-average molecular weight of the polyamide-imide resin is not particularly limited, but can be 200,000 g / mol or more, preferably 300,000 g / mol or more, and more preferably 300,000 - 400,000 g / mol. Within this range, a film with high modulus, excellent resilience even after long-term bending, excellent mechanical strength, and less curling can be provided, and thus it is preferred.

[0074] <Method for manufacturing a film>

[0075] Hereinafter, a method for manufacturing a polyimide-based film having the characteristics of the present invention will be described.

[0076] In an exemplary embodiment of the present invention, the transparent polyimide of the present invention is cast on a substrate using a solution, and the film is peeled off from the substrate while retaining 15 - 30 wt% of the residual solvent therein.

[0077] Subsequently, the peeled film is stretched 1.01 - 1.5 times in the MD direction (film traveling direction) at a temperature of 150°C or lower, the film is fixed with clips using a pin tenter, and then secondary drying is performed while preventing shrinkage to impart an additional stretching effect. During the second drying, the film is dried to a solvent content of 5 wt% or less, preferably 3 wt% or less, and more preferably 0.5 wt% or less.

[0078] During the first stretching, stretching can be performed in more than two stretching sections, and the temperature is increased not in the first stretching section but in a subsequent section, thereby increasing the stretch ratio. Additionally, the stretching temperature during the first stretching can be 150 °C or lower.

[0079] In an exemplary embodiment, stretching can be performed in two stretching sections; stretched to 105 - 109% at 90 - 120 °C in the first stretching section and stretched to 110 - 115% at 120 - 150 °C in the second stretching section.

[0080] In an exemplary embodiment, stretching can be performed in three stretching sections; stretched to 101 - 104% at 70 - 90 °C in the first stretching section, stretched to 105 - 109% at 90 - 120 °C in the second stretching section, and stretched to 110 - 115% at 120 - 150 °C in the third stretching section. Additionally, the temperature gradually increases from the first stretching section to the third stretching section, preferably increasing the stretch ratio.

[0081] Furthermore, during the second drying, the step of fixing the film using clips or needle-shaped jigs is performed to suppress shrinkage without additional stretching in the TD direction (the direction perpendicular to the film traveling direction), thereby imparting a stretching effect in the TD direction. More specifically, it is preferable to maintain the drying temperature at 200 °C to 300 °C, and during the drying process at 300 °C to 350 °C, it is preferably dried in an N2 atmosphere with an oxygen concentration of 1% or less.

[0082] Subsequently, the film is heat-treated at a temperature around the glass transition temperature (Tg) ± 30 °C for 10 seconds to 10 minutes, thereby manufacturing a polyimide film having the properties of the present invention.

[0083] By using a polyimide resin and adopting the manufacturing method of the present invention, a film as desired by the present invention can be obtained, which has a modulus of 5.0 GPa or more, undergoes plastic deformation at a strain of 4% or more during stretching, and has a modulus difference between MD and TD of 0.7 GPa or less.

[0084] Furthermore, by using a polyimide resin and adopting this manufacturing method, a film having the following properties can be obtained: having a modulus of 5.0 GPa or more, undergoing plastic deformation at a strain of 4% or more during stretching, a modulus difference between MD and TD of 0.7 GPa or less, and the stress at the time of plastic deformation is 1000 kgf / cm 2 or more, preferably 1500 kgf / cm 2 or more, more preferably 2000 kgf / cm2 Thus, the resilience required for the present invention can be obtained.

[0085] More preferably, by using a polyimide resin and adopting this manufacturing method, a film having the following properties can be obtained: a modulus of 5.0 GPa or more, plastic deformation occurring at a strain of 4% or more during stretching, a modulus difference between MD and TD of 0.7 GPa or less, and a stress at the time of plastic deformation of 1000 kgf / cm 2 or more, preferably 1500 kgf / cm 2 or more, more preferably 2000 kgf / cm 2 or more. In the stress-strain curve measured using a universal testing machine (UTM), at the point where plastic deformation occurs, the energy required per unit thickness μm is 30 J / m 2 or more, preferably 30 - 100 J / m 2 Thus, the resilience required for the present invention can be obtained.

[0086] More specifically, a film having excellent resilience can be obtained. As Figure 1 shown, when it is fixed to a folding tester (YUASA SYSTEMS CO., LTD.) using an adhesive and folded at a folding radius of 3 mm ( Figure 1 R1 in

[0087] in the 25°C / 50% RH environment and maintained in the folded state for 240 hours, and then unfolded, it can return to its original state without any re-folding or deformation at the folded part.

[0088] Therefore, the film of the present invention has very good performance as a window covering film for foldable devices and flexible devices, showing a significantly improved recovery state when kept in the folded state for a long time and then unfolded.

[0089] In the present invention, the polyimide solution for forming a film can be prepared by preparing a polyamide-imide, purifying the polyamide-imide, and dissolving the polyamide-imide in a solvent such as N,N-dimethylacetamide (DMAc).

[0090] That is to say, a film can be prepared by the following method: coating a polyamide-imide solution (also referred to as a polyimide solution) onto a substrate, followed by drying, stretching, and heat treatment steps, and there is no particular limitation on the substrate on which the solution is cast. For example, glass, stainless steel, or other substrate films can be used, but it is not limited thereto. The coating of the polyamide-imide of the present invention onto the substrate can be carried out by a die coating method, an air knife coating method, a reverse roll coating method, a spraying method, a doctor blade coating method, a casting coating method, an intaglio coating method, a spin coating method, etc., but the commonly used solution casting method can be used without limitation.

[0091] The solvent is not particularly limited as long as it can dissolve the polyimide resin or the polyamide-imide resin; however, for example, it can be any one or a mixture of two or more selected from dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, diethyl acetate, m-cresol, etc., but it is not limited thereto.

[0092] Another exemplary embodiment of the present invention provides a window covering film, which includes: the above-mentioned polyimide base film; and a coating formed on the polyimide base film.

[0093] When the coating is laminated on a polyimide base film having a certain range of surface hardness change rate, a window covering film with significantly improved visibility can be provided.

[0094] According to the exemplary embodiment of the present invention, the coating is used to endow the window covering film with functions and can be applied differently according to the purpose.

[0095] Specifically, for example, the coating can include, but is not limited to, a hard coating, an antistatic layer, a repair layer, a shock spread layer, a self-cleaning layer, an anti-fingerprint layer, an anti-scratch layer, a low refractive index layer, a shock absorption layer, etc.

[0096] Even in the case of forming various coatings on the polyimide base film, a window covering film with excellent display quality, high optical performance, and significantly reduced rainbow phenomenon can be provided, and a window covering film with better resilience or resilience of this category than the base film can be provided.

[0097] In an exemplary embodiment of the present invention, specifically, the coating can be formed on one or both surfaces of the polyimide-based film. For example, the coating can be provided on the upper surface of the polyimide-based film, or on each of the upper and lower surfaces of the polyimide-based film. The coating can protect the polyimide-based film having excellent optical and mechanical properties from external physical or chemical damage.

[0098] In an exemplary embodiment of the present invention, based on the total area of the polyimide-based film, the coating can have a solid content of 0.01 - 200 g / m 2 . Specifically, based on the total area of the polyimide-based film, the solid content can be 20 - 200 g / m 2 . By providing the above basis weight, surprisingly, the film can maintain the function of achieving better visibility without causing a rainbow phenomenon.

[0099] In an exemplary embodiment of the present invention, specifically, the coating can be formed by coating the polyimide-based film with a composition for forming the coating in a state containing a coating solvent.

[0100] The coating solvent is not particularly limited, but preferably, it can be a polar solvent. For example, the polar solvent can be any one or more solvents selected from ether solvents, ketone solvents, alcohol solvents, amide solvents, sulfoxide solvents, and aromatic hydrocarbon solvents, etc. Specifically, the polar solvent can be any one or more solvents selected from dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, ethyl acetate, propylene glycol methyl ether, m-cresol, methanol, ethanol, isopropanol, butanol, 2-methoxyethanol, methyl cellosolve, ethyl cellosolve, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, methyl phenyl ketone, diethyl ketone, dipropyl ketone, cyclohexanone, hexane, heptane, octane, benzene, toluene, and xylene, etc.

[0101] In an exemplary embodiment of the present invention, as a method for forming the coating by coating the composition for forming the coating on the polyimide-based film, one or more methods selected from spin coating, dipping, spraying, die coating, bar coating, roll coating, meniscus coating method, flexographic printing, screen printing, bead coating, air knife coating, reverse roll coating, knife coating, casting coating, and gravure coating, etc. can be used, but are not limited thereto.

[0102] Preferably, in an exemplary embodiment of the present invention, the coating can be a hard coating. The hard coating can include any one or more selected from organic materials, inorganic materials, etc.

[0103] For example, organic materials include carbon and may mainly include carbon and any one or more of non-metallic elements selected from, for example, hydrogen, oxygen, and nitrogen.

[0104] Inorganic materials refer to materials other than organic materials and may include any one or more of metallic elements selected from alkaline earth metals, alkali metals, transition metals, post-transition metals, metalloids, etc. For example, as objects of exception, inorganic materials may include carbon dioxide, carbon monoxide, diamond, carbonate, etc.

[0105] In an exemplary embodiment of the present invention, the hard coat may be a single layer of an organic material layer or an inorganic material layer, or a mixed layer of an organic material and an inorganic material, although it is not particularly limited. Preferably, it may include 10-90 wt% of an organic material and 10-90 wt% of an inorganic material. Preferably, the hard coat may include 40-80 wt% of an organic material and 20-60 wt% of an inorganic material. Even in the case of forming a hard coat containing an organic material and an inorganic material as described above, the bonding to the polyimide-based film is excellent, and no optical distortion occurs, especially the effect of improving the rainbow phenomenon is better.

[0106] According to an exemplary embodiment of the present invention, although not particularly limited, the hard coat may be a layer including one or more polymers selected from, for example, acrylic-based polymers, silicon-based polymers, epoxy-based polymers, urethane-based polymers, etc.

[0107] Specifically, the hard coat prevents deterioration of optical properties when formed on a polyimide-based film and may be a layer formed from a composition for forming a coating, which includes an epoxy silane resin for improving surface hardness. Specifically, the epoxy silane resin may be a siloxane resin containing an epoxy group. The epoxy group may be a cyclic epoxy group, an aliphatic epoxy group, an aromatic epoxy group, or a mixture thereof. The siloxane resin may be a polymer compound in which silicon atoms and oxygen atoms form covalent bonds.

[0108] Preferably, for example, the epoxy siloxane resin may be a silsesquioxane resin. Specifically, the epoxy siloxane resin may be a compound in which the silicon atom of a silsesquioxane compound is directly substituted by an epoxy group or a substituent on the silicon atom is substituted by an epoxy group. As a non-limiting example, the epoxy siloxane resin may be a silsesquioxane resin substituted by 2-(3,4-epoxycyclohexyl) or 3-glycidoxy.

[0109] The epoxy siloxane resin may be prepared from an alkoxysilane having an epoxy group alone or by hydrolysis and condensation reaction of an alkoxysilane having an epoxy group and another alkoxysilane in the presence of water. In addition, the epoxy silane resin may be formed by polymerizing a silane compound containing an epoxy cyclohexyl.

[0110] For example, the alkoxysilane compound having an epoxy group may be any one or more selected from 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, etc.

[0111] In an exemplary embodiment of the present invention, the epoxy silicone resin may have a weight-average molecular weight of 1000 - 20000 g / mol, but is not limited thereto. When the weight-average molecular weight of the epoxy silicone resin is within the above range, it has an appropriate viscosity, thereby improving the fluidity, coatability, curing reactivity, etc. of the composition for forming a coating, and improving the surface hardness of the hard coating.

[0112] In an exemplary embodiment of the present invention, relative to the total weight of the composition for forming a coating, 20 - 65% by weight, preferably 20 - 60% by weight of the epoxy silicone resin may be included. When the content of the epoxy silicone resin is within the above range, the surface hardness of the hard coating can be improved, and uniform curing can be obtained to prevent physical defects (such as cracks) caused by partial over-curing.

[0113] In an exemplary embodiment of the present invention, the composition for forming a coating may further include a crosslinking agent and an initiator.

[0114] Specifically, the crosslinking agent is not particularly limited as long as it can form a crosslink with the epoxy silicone resin to cure the composition for forming the coating and improve the hardness of the hard coating. However, the crosslinking agent can be, for example, selected from any one or more of (3,4-epoxycyclohexyl)methyl-3',4'-epoxycyclohexanecarboxylate, digylcidyl 1,2-cyclohexanedicarboxylate, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-m-dioxane, bis(3,4-epoxycyclohexylmethyl)adipate, bis(3,4-epoxy-6-methylcyclohexyl)adipate, 3,4-epoxy-6-methylcyclohexylmethyl-3',4'-epoxy-6'-methylcyclohexanecarboxylate, 1,4-cyclohexanedimethanol bis(3,4-epoxycyclohexanecarboxylate), ethylidene bis(3,4-epoxycyclohexanecarboxylate), 3,4-epoxycyclohexylmethyl (meth)acrylate, bis(3,4-epoxycyclohexylmethyl)adipate, 4-vinylcyclohexene dioxide, vinylcyclohexene monooxide, 1,4-cyclohexanedimethanol diglycidyl ether, 2,2'-((1-methylethylidene)bis(4,1-cyclohexyleneoxymethylene))bis(oxirane), etc. Preferably, the crosslinking agent can be selected from any one or more of (3,4-epoxycyclohexyl)methyl-3',4'-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl)adipate, and analogs (including compounds in which two 3,4-epoxycyclohexyl groups are connected).

[0115] In an exemplary embodiment of the present invention, the content of the crosslinking agent is not particularly limited. For example, relative to 100 parts by weight of the epoxy silicone resin, it can be 5 - 150 parts by weight. In addition, according to the exemplary embodiment of the present invention, relative to the total weight of the composition for forming the coating, the content of the crosslinking agent can be 3 - 30% by weight, preferably 5 - 20% by weight. Within this range, the coatability and curing reactivity of the composition for forming the coating can be further improved.

[0116] In an exemplary embodiment of the present invention, the initiator can be a photoinitiator or a thermal initiator. Preferably, the initiator can be a photoinitiator. For example, the photoinitiator can include a photo cationic initiator. The photo cationic initiator can initiate the polymerization of the epoxy silicone resin and the epoxy monomer.

[0117] Specifically, the photo cationic initiator can be any one or more selected from onium salts, organometallic salts, etc., but is not limited thereto. For example, the photo cationic initiator can be any one or more selected from diaryliodonium salts, triarylsulfonium salts, aryldiazonium salts, iron-arene complexes, etc., but is not limited thereto.

[0118] In an exemplary embodiment of the present invention, the content of the photoinitiator is not particularly limited. For example, relative to 100 parts by weight of the epoxy silicone resin, it can be 1-15 parts by weight. In addition, according to an exemplary embodiment of the present invention, relative to the total weight of the composition for forming the coating, the content of the crosslinking agent can be 0.1-10% by weight, preferably 0.3-5% by weight. When the content of the photoinitiator is within the above range, the curing efficiency of the hard coating is better, and the deterioration of physical properties caused by residual components after curing can be further prevented.

[0119] In an exemplary embodiment of the present invention, the composition for forming the coating may further include, but is not limited to, fillers, slip agents, light stabilizers, thermal polymerization inhibitors, leveling agents, lubricants, antifouling agents, thickeners, surfactants, defoamers, antistatic agents, dispersants, initiators, coupling agents, antioxidants, ultraviolet stabilizers, colorants, etc.

[0120] The hard coating may further include inorganic particles for imparting hardness. The inorganic particles can preferably be silica, more preferably surface-treated silica, but are not limited thereto. Here, the surface treatment may include functional groups capable of reacting with the above crosslinking agent.

[0121] According to an exemplary embodiment, the average particle size of the inorganic particles can be 1-500 nm, preferably 10-300 nm, but is not limited thereto.

[0122] It can be seen that when a hard coating is formed on a conventional polyimide-based film, sufficient resilience is not shown, but the window covering film of the present invention has sufficiently excellent resilience. In addition, the window covering film can have excellent visibility and mechanical physical properties.

[0123] Another exemplary embodiment of the present invention provides a display device, which includes: a display panel and the above window covering film formed on the display panel.

[0124] In an exemplary embodiment of the present invention, the display device is not particularly limited as long as it belongs to a field that requires excellent optical properties and can be provided by selecting a suitable display panel. Preferably, the window cover film can be applied to a flexible display device, and specifically, for example, it can include and be applied to any one or more image displays selected from various image displays (such as liquid crystal displays, electroluminescent displays, plasma displays, and field emission display devices), but is not limited thereto.

[0125] The display device including the window cover film of the present invention has excellent display quality and significantly reduces the distortion caused by light. Therefore, it can significantly improve the rainbow phenomenon of iridescent stains and minimize the eye fatigue of users with excellent visibility.

[0126] Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples. However, the following examples and comparative examples are only examples for describing the present invention in more detail and do not limit the present invention in any way.

[0127] 1) Evaluation of resilience

[0128] As Figure 1 shown, when the film is fixed to a folding tester (YUASA SYSTEMS CO., LTD.) using an adhesive, it is folded at a folding radius ( Figure 1 R1 set to 3 mm) in a 25 °C / 50% RH environment and held in the folded state for 240 hours, and then unfolded to evaluate whether the film can return to its original state without the folded part folding up again or causing deformation. Here, the film is cut into a size of 100 mm × 50 mm for evaluation, and the load during folding is 1 kgf.

[0129] Good: When visually confirmed, there is no visual appearance change in the folded part and the warpage in the bending direction is less than 2 mm.

[0130] Normal: When visually confirming the folded part, there is a visual appearance change (crease) when visually confirmed, and when the film is placed on a flat place, the warpage in the bending direction is 2 mm to 5 mm.

[0131] Poor: Includes the state where the film remains bent more than 5 mm in the bending direction or does not recover.

[0132] 2) Modulus and elongation at break

[0133] According to ASTM D882, the elongation at break was measured using a UTM3365 purchased from Instron under the condition of stretching a polyamide-imide film with a length of 50 mm and a width of 10 mm at a speed of 50 mm / minute at 25 °C. The thickness of the film was measured and this value was input into the instrument. The unit of modulus is GPa and the unit of elongation at break is %.

[0134] 3) Energy per unit thickness (elastic energy)

[0135] According to ASTM D882, when measured using a UTM3365 purchased from Instron under the condition of stretching a polyamide-imide film with a length of 50 mm and a width of 10 mm at a speed of 50 mm / minute at 25 °C, the energy per unit thickness (elastic energy) was determined as (stress × deformation length) / 2 at the yield point (elastic-plastic deformation transition point) in the S-S curve. The unit is J / m 2 / μm.

[0136] 4) Transmittance

[0137] On a film with a thickness of 50 μm, according to the standard of ASTM D1746, the total light transmittance was measured using a spectrophotometer (purchased from NipponDenshoku, COH-400) in the entire wavelength range of 400 - 700 nm, and the single-wavelength transmittance was measured at 388 nm using a UV / Vis (Shimadzu, UV3600). The unit is %.

[0138] 5) Haze

[0139] According to the standard of ASTM D1003, the haze was measured using a spectrophotometer (purchased from Nippon Denshoku, COH-400) on a film with a thickness of 50 μm. The unit is %.

[0140] 6) Yellow index (YI)

[0141] According to the standard of ASTM E313, using a colorimeter (purchased from HunterLab, ColorQuest XE), the yellow index and b* value were measured based on a film with a thickness of 50 μm.

[0142] 7) Weight-average molecular weight (Mw) and polydispersity index (PDI)

[0143] The weight-average molecular weight and polydispersity index of the prepared film were measured as follows.

[0144] First, the film sample was dissolved in a DMAc eluent containing 0.05 M LiBr and used as a sample. Using GPC (Waters GPC system, Waters 1515 isocratic HPLC pump, Waters 2414 refractive index detector), connecting an Olexis column, a polypore column, and a mixed D column as GPC columns, using a DMAc solution as the solvent, and using polymethyl methacrylate (PMMA STD) as the standard for measurement, analysis was carried out at a flow rate of 1 mL / minute at 35 °C.

[0145] 8) Pencil hardness

[0146] For the films prepared in the examples and comparative examples, according to JIS K5400, a 20-mm line was drawn on the film at a rate of 50 mm / second using a load of 750 g, and this operation was repeated more than 5 times. The pencil hardness was determined based on the case where scratching occurred less than 1 time.

[0147] 9) Measurement of residual solvent content

[0148] For the residual solvent content, the value obtained by subtracting the weight at 370 °C from the weight at 150 °C using TGA (Discovery from TA) was determined as the residual solvent content in the film. Here, the measurement conditions were to heat up to 400 °C at a heating rate of 10 °C / minute and measure the weight change in the range of 150 - 370 °C.

[0149] [Preparation Example 1] Preparation of a composition for forming a polyimide-based film

[0150] p-Terephthaloyl chloride (TPC) and 2,2'-bis(trifluoromethyl)-benzidine (TFMB) were added to a mixed solution of dichloromethane and pyridine in a reactor, and stirred at 25 °C for 2 hours under a nitrogen atmosphere. Here, the molar ratio of TPC:TFMB was adjusted to 300:400, and the solid content was adjusted to 10 wt%. After that, the reactant was precipitated in excess methanol, then filtered to obtain a solid component, which was vacuum dried at 50 °C for more than 6 hours to obtain an oligomer. The molecular weight (FW) of the prepared oligomer was 1670 g / mol.

[0151] N,N-Dimethylacetamide (DMAc), 100 moles of the oligomer, and 28.6 moles of 2,2'-bis(trifluoromethyl)-benzidine (TFMB) were added to the reactor and stirred well. After confirming that the solid raw materials were completely dissolved, fumed silica (surface area of 95 m 2 / g, <1 μm) was added to DMAc at a content based on a solid content of 1000 ppm, and after ultrasonic dispersion, it was added to the reactor. Subsequently, 64.3 moles of cyclobutanetetracarboxylic dianhydride (CBDA) and 64.3 moles of 4,4'-hexafluoroisopropylidene diphthalic anhydride (6FDA) were added, and the mixture was stirred well and polymerized at 40 °C for 10 hours. At this time, the solid content was 12%. Subsequently, pyridine and acetic anhydride were added to the solution in an amount 2.5 times that of the total content of the dianhydride, and the mixture was stirred at 60 °C for 12 hours.

[0152] After the polymerization was completed, the polymerization solution was precipitated in excess methanol and filtered to obtain a solid component, which was vacuum dried at 50 °C for more than 6 hours to obtain polyamide-imide powder. The powder was diluted and dissolved in DMAc at 20 wt% to prepare a polyimide-based resin solution. The polyimide thus prepared had a weight-average molecular weight of 320000 g / mol and a polydispersity index (PDI) of 2.22.

[0153] [Example 1]

[0154] The composition for forming a polyimide-based film prepared in Preparation Example 1 was coated on a glass substrate using a coater, dried in a vacuum oven at 80 °C for 30 minutes, dried at 100 °C for 1 hour, and subjected to a first heat treatment stepwise at 250 - 300 °C for 2 hours, and then cooled to room temperature to form a film. The prepared film had a residual solvent content of 17 wt%.

[0155] Subsequently, the dried film was separated. Before being fixed to a pin tenter, the base film was stretched to 1.03 times in the MD direction at 80 °C, and then stretched to 1.05 times and 1.08 times at 100 °C and 130 °C in sequence. Subsequently, the film was fixed with clips using a pin tenter, and then dried in a drying section at 260 °C. Here, an additional stretching effect was imparted by preventing shrinkage during the drying process. After drying, the glass transition temperature (Tg) of the film was 320 °C, and then a heat treatment was performed at the same temperature as the glass transition temperature for 5 minutes. The final prepared film had a solvent content of 0.7 wt%, and its physical properties are listed in Table 1.

[0156] The film had a thickness of 48 μm, a light transmittance at 388 nm of 13%, a total light transmittance of 90.5%, a haze of 0.3%, a yellowness index (YI) of 2.7, a b* value of 0.9, a modulus of 6.5 GPa, an elongation at break of 21.2%, and a pencil hardness of HB / 750 g. In addition, the physical properties related to the resilience are shown in Table 1 below.

[0157] [Example 2 and Example 3]

[0158] The film was prepared in the same manner as in Example 1, except that the solvent content, stretching conditions, and heat treatment conditions were changed as shown in Table 1 below.

[0159] The physical properties of the film were measured and are shown in Table 1 below.

[0160] [Comparative Example 1]

[0161] As shown in Table 1 below, the composition for forming a polyimide-based film prepared in Preparation Example 1 was coated on a glass substrate using a coater, dried in a vacuum oven at 80 °C for 30 minutes, dried at 100 °C for 1 hour, and subjected to a first heat treatment stepwise at 250 - 300 °C for 2 hours, and then cooled to room temperature to obtain a film.

[0162] [Comparative Example 2]

[0163] The film was prepared in the same manner as in Example 1, except that the film was loosely fixed on a tenter so that the film shrank by 5% during the drying process of the second drying. The results are shown in Table 1.

[0164] [Comparative Example 3]

[0165] The film was prepared in the same manner as in Example 1, except that the third stretching in the MD direction was carried out at 200 °C. The results are shown in Table 1.

[0166] [Comparative Example 4]

[0167] The film was prepared in the same manner as in Example 1, except that the stretching in the MD direction was carried out successively by fixing at 160 °C. The results are shown in Table 1.

[0168] [Comparative Example 5]

[0169] The film was prepared in the same manner as in Example 1, except that the heat treatment was carried out at the Tg of 230 °C. The results are shown in Table 1.

[0170] [Comparative Example 6]

[0171] The film was prepared in the same manner as in Example 1, except that the heat treatment was not carried out. The results are shown in Table 1.

[0172] [Table 1]

[0173]

[0174] The polyimide-based film according to the present invention does not deform even when kept in a folded state for a long time and then unfolded, and the optical and physical properties do not change.

[0175] Therefore, a polyimide-based film having long-term stability and stable optical physical properties, and a window covering film and a flexible display using the same can be provided.

[0176] In the foregoing, although the present invention has been described through specific elements (specified matter), specific exemplary embodiments, and the drawings, they are provided only to assist in the overall understanding of the present invention. Therefore, the present invention is not limited to the specific elements of the exemplary embodiments. Those skilled in the art to which the present invention pertains can make various modifications and changes based on this specification.

[0177] Therefore, the spirit of the present invention should not be limited to the above-described exemplary embodiments, but all modifications that are the same as or equivalent to the claims will fall within the scope and spirit of the present invention.

Claims

1. A polyimide-based film, wherein the polyimide-based film is prepared as follows: A polyimide-based resin solution containing units derived from a fluoroaromatic diamine, units derived from an aromatic dianhydride, and units derived from an aromatic dichloride is cast on a substrate. The film is peeled off from the substrate in a state where 15 - 30 wt% of the residual solvent is retained by the first drying. The peeled film is stretched to 1.01 - 1.5 times in the film traveling direction at a temperature below 150 °C, and then subjected to secondary drying in a drying chamber to dry the solvent to 5 wt% or less, and the film is heat-treated at a temperature of the glass transition temperature ± 30 °C for 10 seconds to 10 minutes, thereby preparing the polyimide-based film, wherein during the secondary drying process, the film is fixed using clips or needle-type jigs. The polyimide-based film has a modulus of 5 GPa or more measured using a universal testing machine according to ASTM D882, undergoes plastic deformation at a strain of 4% or more during the stretching process, and the difference between the machine direction modulus Mmd and the transverse direction modulus Mtd satisfies the following formula 1: [Formula 1] |Mmd – Mtd| ≤ 0.7 GPa.

2. The polyimide-based film according to claim 1, wherein in the stress-strain curve measured using a universal testing machine, at the point where plastic deformation occurs, the energy required for the polyimide-based film per unit thickness μm is 30 J / m 2 or more.

3. The polyimide-based film according to claim 2, wherein the energy required for the polyimide-based film per unit thickness μm is 30 - 100 J / m 2 .

4. The polyimide-based film according to claim 1, wherein at the point of plastic deformation, the stress of the polyimide-based film is 1000 kgf / cm 2 or more.

5. The polyimide-based film according to claim 1, wherein the polyimide-based film has a total light transmittance of 87% or more measured at 400 - 700 nm according to ASTM D1746, a light transmittance of 5% or more measured at 388 nm according to ASTM D1746, a haze of 2.0% or less, and a yellowness index of 5.0 or less.

6. The polyimide-based film according to claim 1, wherein the elongation at break of the polyimide-based film according to ASTM D882 is 15% or more.

7. The polyimide-based film according to claim 1, wherein the polyimide-based film is formed of a polyamide-imide-based resin.

8. The polyimide-based film according to claim 1, wherein the polyimide-based film further includes units derived from an alicyclic dianhydride.

9. The polyimide-based film according to claim 1, wherein the thickness of the polyimide-based film is 30 - 110 μm.

10. A window covering film, which comprises: The polyimide-based film according to any one of claims 1 - 9; and A coating formed on one surface or both surfaces of the polyimide-based film.

11. The window covering film according to claim 10, wherein the coating is any one or more selected from a hard coating, an antistatic layer, a recovery layer, an impact diffusion layer, a self-cleaning layer, an anti-fingerprint layer, an anti-fouling layer, an anti-scratch layer, a low refractive index layer, an anti-reflection layer, and an impact absorption layer.

12. A flexible display panel, which comprises the polyimide-based film according to any one of claims 1 - 9.

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