Polyamide-imide-based film, process for preparing the same, and cover window and display device comprising the same

Abstract

This embodiment aims to provide a polyamide-imide film with a TGA weight loss area value (TDA) of 0.01%·min / ℃ or less and excellent mechanical properties, optical properties and post-processing suitability, a method for preparing the film, and a cover window and display device containing the film.

Description

Polyamide-imide film, preparation method thereof, cover window and display device containing the same Field of the Invention This embodiment relates to a polyamide-imide film, a method for preparing the same, and a cover window and a display device comprising the same. Background of the Invention Polyimide resins, such as poly(amide-imide) (PAI), have excellent abrasion resistance, heat resistance, and chemical resistance. Therefore, they are used in applications such as primary electrical insulation, coatings, adhesives, resins for extrusion, heat-resistant coatings, heat-resistant sheets, heat-resistant adhesives, heat-resistant fibers, and heat-resistant films. Polyimides are used in a variety of fields. For example, they are formulated in powder form and applied as coatings to metals or magnet wires. Depending on the application, they are mixed with other additives. Polyimides are also used with fluoropolymers as decorative and anti-corrosion coatings. They also serve to bond fluoropolymers to metal substrates. Furthermore, polyimides are used to coat kitchenware, as membranes for gas separation due to their heat and chemical resistance, and in natural gas wells to filter out pollutants such as carbon dioxide, hydrogen sulfide, and impurities. In recent years, polyimide has been developed into thin films, which are relatively inexpensive and possess excellent optical, mechanical, and thermal characteristics. These polyimide-based films can be used as display materials for organic light-emitting diodes (OLEDs), liquid crystal displays (LCDs), and similar devices. If retardation properties are achieved, they can also be used in antireflection films, compensation films, and retardation films. When a functional layer (such as a hard coat) is laminated on such a polyimide-based film, there is a problem that the functional layer may detach from the film when immersed in a solvent during post-processing, or the mechanical and optical properties of the film may deteriorate. Therefore, there is an increasing demand for the development of films with excellent mechanical and optical properties and suitability for post-processing. Summary of the Invention Technical issues The present embodiment aims to provide a polyamide-imide film having excellent mechanical properties, optical properties and post-processing suitability, a method for preparing the same, and a cover window and a display device comprising the same. Problem Solving Methods According to one embodiment, the polyamide-imide film has a TGA weight loss area (TDA) value of 0.01%·min / ° C. or less. According to another embodiment, a cover window for a display device includes a polyamide-imide film and a functional layer, wherein the polyamide-imide film has a TGA weight loss area value (TDA) of 0.01%·min / ° C. or less. According to another embodiment, a display device includes a display unit; and a cover window disposed on the display unit, wherein the cover window includes a polyamide-imide film and a functional layer, and the polyamide-imide film has a TGA weight loss area value (TDA) of 0.01%·min / °C or less. A method for preparing a polyamide-imide film according to one embodiment includes polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent to prepare a polyamide-imide polymer solution; casting the solution to prepare a gel sheet; and heat-treating the gel sheet. Advantageous Effects of the Invention Since the polyamide-imide film according to one embodiment has a TGA weight loss area value (TDA) adjusted to 0.01%·min / °C or less, it has excellent optical and mechanical properties, as well as post-processing suitability such as adhesion to functional layers and solvent resistance. Once the polyamide-imide film is prepared, it is post-processed, such as laminating a functional layer to impart functions such as anti-fingerprint, anti-static, anti-scattering, and adhesion enhancement, and / or immersed in a solvent. After post-treatment, the functional layer may detach from the film, or the optical and mechanical properties may deteriorate, such as a sharp increase in haze after immersion in solvents. Therefore, post-treatment suitability, such as adhesion to the functional layer and solvent resistance, plays a crucial role in the quality reliability of the final product. Since the polyamide-imide film according to the embodiment has a TGA weight loss area (TDA) value adjusted to a specific range, the quality reliability and product yield of the final product (such as a cover window for a display device or a display device coated with the film) can be improved. Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement the embodiments. However, the embodiments can be implemented in many different ways and are not limited to the ways described herein. Throughout this specification, when referring to a film, window, panel, layer, or the like formed "on" or "under" another film, window, panel, layer, or the like, it is intended that one element is formed directly on or under another element, and also that one element is formed indirectly on or under another element with other elements interposed therebetween. In addition, the terms "on" or "under" with respect to individual elements may refer to the drawings. For descriptive purposes, the sizes of individual elements in the drawings may be exaggerated and do not indicate actual sizes. In addition, the same reference numerals refer to the same elements throughout this specification. Throughout the specification, unless specifically stated otherwise, when a part is referred to as “comprising” an element, it should be understood that other elements may be included rather than excluded. In this specification, unless otherwise specified, a singular expression is interpreted as encompassing a singular or plural number as construed in the context. Moreover, unless otherwise indicated, all numbers and expressions relating to quantities of ingredients, reaction conditions, and so forth used herein may be understood as being modified by the term "about." The terms "first", "second", etc. are used herein to describe various elements, and these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Furthermore, as used herein, the term "substituted" means substitution with at least one substituent selected from the group consisting of deuterium, -F, -Cl, -Br, -I, hydroxyl, cyano, nitro, amino, amidino, hydrazine, hydrazone, ester, keto, carboxyl, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted alkoxy, substituted or unsubstituted alicyclic organic group, substituted or unsubstituted heterocyclic group, substituted or unsubstituted aryl, and substituted or unsubstituted heteroaryl. The substituents listed above may be linked together to form a ring. polyamide - Imide film One embodiment provides a polyamide-imide film having a TGA weight loss area (TDA) value that satisfies a specific range, thereby having not only excellent heat resistance and yellowness index but also excellent post-processing suitability such as solvent resistance and adhesion to functional layers. According to one embodiment, the polyamide-imide film has a TGA weight loss area (TDA) value of 0.01%·min / ° C. or less. Specifically, the TGA weight loss area value (TDA) of the polyamide-imide film can be 0.009%·min / ℃ or less, 0.008%·min / ℃ or less, 0.007%·min / ℃ or less, 0.006%·min / ℃ or less, 0.005%·min / ℃ or less, 0.004%·min / ℃ or less, 0.003%·min / ℃ or less, 0.0022%·min / ℃ or less, or 0.002%·min / ℃ or less, and 0.0001%·min / °C or more, 0.0002%·min / °C or more, 0.0003%·min / °C or more, 0.0004%·min / °C or more, 0.0005%·min / °C or more, 0.0006%·min / °C or more, 0.0007%·min / °C or more, 0.0008%·min / °C or more, 0.0009%·min / °C or more, or 0.001%·min / °C or more. More specifically, the TGA weight loss area value (TDA) of the polyamide-imide film can be 0.0001 to 0.01%·min / °C, 0.0001 to 0.008%·min / °C, 0.0001 to 0.006%·min / °C, 0.0001 to 0.004%·min / °C, 0.0001 to 0.003%·min / °C, 0.0001 to 0.0022%·min / °C, min / ℃、0.0001 to 0.002%·min / ℃、0.0005 to 0.01%·min / ℃、0.0005 to 0.008%·min / ℃、0.0005 to 0.006%·min / ℃、0.0005 to 0.004%·min / ℃、0.0005 to 0.003%·min / ℃、0.0005 to 0.0022%·min / ℃、0.0 0.005 to 0.002%·min / ℃, 0.0008 to 0.01%·min / ℃, 0.0008 to 0.008%·min / ℃, 0.0008 to 0.006%·min / ℃, 0.0008 to 0.004%·min / ℃, 0.0008 to 0.003%·min / ℃, 0.0008 to 0.0022%·min / ℃, 0.0008 to 0.002 %·min / ℃, 0.001 to 0.01%·min / ℃, 0.001 to 0.008%·min / ℃, 0.001 to 0.006%·min / ℃, 0.001 to 0.004%·min / ℃, 0.001 to 0.003%·min / ℃, 0.001 to 0.0022%·min / ℃, or 0.001 to 0.002%·min / ℃, but it is not limited thereto. In TGA analysis, the first weight loss temperature (Tw) refers to the temperature at which the rate of change in weight loss reaches a maximum within the temperature range of 270°C to 330°C. Furthermore, in a graph of the rate of change in weight loss (y-axis) versus temperature (x-axis), when a straight line is drawn connecting the point corresponding to a temperature of (Tw – 30)°C and the point corresponding to a temperature of (Tw + 30)°C, the area between the straight line and the graph, representing the portion at the first weight loss temperature (Tw), is referred to as the TGA weight loss area (TDA). TGA analysis was performed using a TA Thermogravimetric Analyzer Q500 model by measuring a 2 g polyamide-imide film sample and heating it at a rate of 10° C. / min within a range of 25° C. to 700° C. Because the TGA weight loss area (TDA) value of polyamide-imide films is controlled within the above range, they exhibit excellent optical and mechanical properties. Furthermore, when subjected to post-processing, such as lamination with a functional layer, the films exhibit enhanced adhesion to the functional layer, significantly reducing delamination between layers when used in products. Furthermore, they exhibit excellent solvent resistance, maintaining physical properties even after immersion in solvents commonly used in post-processing. Conversely, if the TGA weight loss area (TDA) value of a polyamide-imide film falls outside the above range, adhesion to the functional layer may deteriorate, and optical properties such as yellowness may deteriorate after immersion in a solvent, resulting in reduced film quality and reliability. Furthermore, when using the film to manufacture products, the defect rate of the product may increase. The polyamide-imide film may have a first weight loss temperature (Tw) of 285° C. or higher in TGA analysis. Specifically, the first weight loss temperature (Tw) of the polyamide-imide-based film in TGA analysis may be 290°C or higher, 293°C or higher, 295°C or higher, 298°C or higher, or 300°C or higher, and 330°C or lower, 325°C or lower, 320°C or lower, 315°C or lower, 312°C or lower, 310°C or lower, or 308°C or lower. More specifically, the first weight loss temperature (Tw) of the polyamide-imide based film in TGA analysis may be 280°C to 330°C, 280°C to 320°C, 280°C to 315°C, 280°C to 310°C, 290°C to 330°C, 290°C to 320°C, 290°C to 315°C, 290°C to 310°C, 295°C to 330°C, 295°C to 320°C, 295°C to 315°C, or 295°C to 310°C, but is not limited thereto. If the first weight loss temperature (Tw) of the polyamide-imide film satisfies the above range, adhesion to the functional layer and post-processing suitability such as solvent resistance can be improved. In one embodiment, the polyamide-imide based film may have a 1% weight loss temperature (Td1) of 290° C. or higher in TGA analysis. Specifically, the 1% weight loss temperature (Td1) of the polyamide-imide-based film in TGA analysis may be 295°C or higher, 300°C or higher, 305°C or higher, or 310°C or higher, and 360°C or lower, 355°C or lower, 350°C or lower, 345°C or lower, or 340°C or lower. More specifically, the 1% weight loss temperature (Td1) of the polyamide-imide based film in TGA analysis may be 280°C to 360°C, 280°C to 350°C, 280°C to 345°C, 280°C to 340°C, 290°C to 360°C, 290°C to 350°C, 290°C to 345°C, 290°C to 340°C, 300°C to 360°C, 300°C to 350°C, 300°C to 345°C, 300°C to 340°C, 310°C to 360°C, 310°C to 350°C, 310°C to 345°C, or 310°C to 340°C, but is not limited thereto. For example, when a 2-gram polyamide-imide film sample is measured and heated at a rate of 10°C / min within a range of 25°C to 700°C using TA's thermogravimetric analysis Q500 model, the 1% weight loss temperature (Td1) may refer to the temperature at which 1% weight loss occurs. In one embodiment, the polyamide-imide based film may have a 5% weight loss temperature (Td5) of 400° C. or higher in TGA analysis. Specifically, the 5% weight loss temperature (Td5) of the polyamide-imide-based film in TGA analysis may be 405°C or higher, 410°C or higher, 415°C or higher, 420°C or higher, 425°C or higher, or 430°C or higher, and 470°C or lower, 460°C or lower, 455°C or lower, 450°C or lower, 445°C or lower, or 440°C or lower. More specifically, the 5% weight loss temperature (Td5) of the polyamide-imide-based film in TGA analysis may be 400°C to 470°C, 400°C to 460°C, 400°C to 450°C, 400°C to 440°C, 410°C to 470°C, 410°C to 460°C, 410°C to 450°C, 410°C to 440°C, 420°C to 470°C, 420°C to 460°C, 420°C to 450°C, 420°C to 440°C, 430°C to 470°C, 430°C to 460°C, 430°C to 450°C, or 430°C to 440°C, but is not limited thereto. For example, when a 2-gram polyamide-imide film sample is measured and heated at a rate of 10°C / min over a range of 25°C to 700°C using TA's thermogravimetric analysis Q500 model, the 5% weight loss temperature (Td5) may refer to the temperature at which 5% weight loss occurs. If the 1% weight loss temperature (Td1) and the 5% weight loss temperature (Td5) of the polyamide-imide film respectively meet the above ranges, the residual solvent content in the film becomes low, and the optical and mechanical properties of the film can be improved. The polyamide-imide film may have an adhesion to the functional layer of 4B or more, or 5B. The functional layer may be selected from at least one of the group consisting of a hard coating layer, a reflectivity-reducing layer, an antifouling layer, and an antiglare layer, but is not limited thereto. In one embodiment, a primer layer may be interposed between the functional layer and the polyamide-imide film. Specifically, the polyamide-imide film may have an adhesion to the hard coating layer of 4B or more. More specifically, the polyamide-imide film may have an adhesion to the hard coating layer of 5B. For example, adhesion to a hard coat layer can be measured by applying a primer layer composition onto a polyamide-imide film to form a primer layer having a thickness of 0.1 µm, applying a hard coat layer composition onto the primer layer to form a hard coat layer having a thickness of 5 µm, cutting the surface of the hard coat layer into a grid at regular intervals according to ASTM D 3359 (Method B), and attaching and removing a tape (Nitto Tape 50B) thereon to examine the degree of detachment of the grid sheet from the surface. Here, the hard coating layer may include at least one of an organic component, an inorganic component, and an organic-inorganic composite component as a hard coating agent. As an example, the hard coating layer may include an organic resin. Specifically, the organic resin may be a curable resin. Therefore, the hard coating layer may be a curable coating layer. Specifically, the hard coating layer may include at least one selected from the group consisting of a polyurethane acrylate compound, an acrylate compound, and an epoxy acrylate compound. More specifically, the hard coating layer may include a polyurethane acrylate compound and an acrylate compound. When the polyamide-imide film was immersed in MIBK for 5 seconds and dried at 80°C for 3 minutes, the haze value change (ΔHz) was measured. M ) may be 0.5% or less. Specifically, after immersion in MIBK, the haze value change (ΔHz) of the polyamide-imide film M ) may be 0.4% or less, 0.3% or less, 0.2% or less, 0.15% or less, or 0.1% or less, but is not limited thereto. ΔHz M (%) is Hz M – Hz 0 value, where Hz 0 represents the initial haze value of the film (%), and Hz M It represents the haze value (%) measured after the film was immersed in MIBK solvent for 5 seconds and dried at 80° C. for 3 minutes. When the polyamide-imide film was immersed in IPA for 5 seconds and dried at 80°C for 3 minutes, the haze value change (ΔHz) was measured. I ) may be 0.3% or less. Specifically, after immersion in IPA, the haze value change (ΔHz) of the polyamide-imide film I ) may be 0.2% or less, 0.15% or less, 0.1% or less, or 0.08% or less, but is not limited thereto. ΔHz I (%) is Hz I – Hz 0 value, where Hz 0 represents the initial haze value of the film (%), and Hz I It represents the haze value (%) measured after the film was immersed in IPA solvent for 5 seconds and dried at 80° C. for 3 minutes. The change in haze value (ΔHz) of polyamide-imide film after immersion in MIBK for 5 seconds and drying at 80°C for 3 minutes M ) and polyamide-imide film after being immersed in IPA for 5 seconds and dried at 80℃ for 3 minutes (ΔHz) I ) average value (ΔHz AVG ) may be 0.4% or less. Specifically, the average value of the change in haze value of the polyamide-imide film (ΔHz AVG ) may be 0.3% or less, 0.2% or less, 0.15% or less, 0.12% or less, 0.1% or less, 0.09% or less, or 0.08% or less, but is not limited thereto. Average value of fog value change (ΔHz AVG ) can be a measurement used to determine the solvent resistance of a film. In one embodiment, the polyamide-imide-based film may have a surface tension of 40 dyn / cm or more on the first side. Specifically, the surface tension of the first side of the polyamide-imide-based film may be 43 dyn / cm or more, 45 dyn / cm or more, 46 dyn / cm or more, or 47 dyn / cm or more, and 55 dyn / cm or less, 53 dyn / cm or less, 52 dyn / cm or less, 51 dyn / cm or less, or 50 dyn / cm or less. In one embodiment, the first side may be the air side of the film. The air side refers to the side that is not in contact with the support used to form the polyamide-imide film. Specifically, in a process for preparing the film, the air side may refer to the side that is not in contact with the zone on which the polyamide-imide polymer solution is cast and dried. In one embodiment, the polyamide-imide-based film may have a surface tension of the second side of 40 dyn / cm or more. Specifically, the surface tension of the second side of the polyamide-imide-based film may be 43 dyn / cm or more, 45 dyn / cm or more, 46 dyn / cm or more, or 47 dyn / cm or more, and 55 dyn / cm or less, 53 dyn / cm or less, 52 dyn / cm or less, 51 dyn / cm or less, or 50 dyn / cm or less. In one embodiment, the second side may be a belt side of the film. The belt side refers to the side that contacts the support used to form the polyamide-imide film. Specifically, in a film production process, the belt side may refer to the side that contacts the belt on which the polyamide-imide polymer solution is cast and dried. According to one embodiment, the polyamide-imide film has a modulus of 5.0 GPa or more. Specifically, the modulus may be 5.5 GPa or more, 5.7 GPa or more, or 6 GPa or more. The polyamide-imide film may have a transmittance of 80% or more. For example, the transmittance may be 85% or more, 88% or more, or 100% or less, or 99% or less. The polyamide-imide film has a haze value of 1% or less. Specifically, the haze value may be 0.8% or less, 0.7% or less, 0.6% or less, or 0.5% or less, but is not limited thereto. The transmittance and / or haze value of the film may be values ​​measured within the visible light wavelength range (400 to 700 nm). Specifically, the transmittance of the film may be the total light transmittance measured within the visible light wavelength range. The polyamide-imide film has a yellowness index of 5 or less. For example, the yellowness index may be 4.5 or less, 4.0 or less, or 3.5 or less, but is not limited thereto. In one embodiment, the polyamide-imide film has a thickness deviation of 3 μm or less, or 2 μm or less, based on a thickness of 50 μm. Furthermore, the thickness deviation rate may be 5% or less, 4% or less, or 3% or less, but is not limited thereto. In one embodiment, the polyamide-imide based film may have a modulus of 5 GPa or more, a transmittance of 80% or more, a haze value of 1% or less, and a yellowness index of 5 or less based on a film thickness of 50 μm, but is not limited thereto. The polyamide-imide film may have a compressive strength of 0.4 kgf / µm or more. Specifically, the compressive strength may be 0.45 kgf / µm or more, or 0.46 kgf / µm or more, but is not limited thereto. When a polyamide-imide film is perforated using a 2.5-mm spherical tip in a UTM compression mode at a speed of 10 mm / min, the maximum diameter (mm) of the perforation, including a crack, is 60 mm or less. Specifically, the maximum diameter of the perforation may be 5 to 60 mm, 10 to 60 mm, 15 to 60 mm, 20 to 60 mm, 25 to 60 mm, or 25 to 58 mm, but is not limited thereto. The polyamide-imide film may have a surface hardness of HB or higher. Specifically, the surface hardness may be H or higher, or 2H or higher, but is not limited thereto. Polyamide-imide film can have 15 kgf / mm 2 More tensile strength. Specifically, the tensile strength can be 18 kgf / mm 2 or more, 20 kgf / mm 2 or more, 21 kgf / mm 2 or more, or 22 kgf / mm 2 or more, but it is not limited thereto. The polyamide-imide film may have an elongation of 15% or more. Specifically, the elongation may be 16% or more, 17% or more, or 18% or more, but is not limited thereto. When a polyamide-imide film with a thickness of 50 µm is folded to have a curvature radius of 3 mm, the number of folds before breaking can be 200,000 or more. When the film was folded to have a curvature radius of 3 mm and then unfolded, the number of folds was counted as 1. Since the number of folding times of the polyamide-imide film satisfies the above range, it can be advantageously applied to a foldable display device or a flexible display device. The polyamide-imide film may have a surface roughness of 0.01 µm to 0.07 µm. Specifically, the surface roughness may be 0.01 µm to 0.07 µm, or 0.01 µm to 0.06 µm, but is not limited thereto. Since the surface roughness of the polyamide-imide film satisfies the above range, it can advantageously achieve a better brightness condition or texture when applied to a display device. The residual solvent content in the polyamide-imide film may be 1,500 ppm or less. For example, the residual solvent content may be 1,200 ppm or less, 1,000 ppm or less, 800 ppm or less, 500 ppm or less, or 300 ppm or less, but is not limited thereto. Residual solvent refers to the solvent that is not volatilized during the film production process and remains in the final film. If the residual solvent content in polyamide-imide films exceeds the above range, film durability may deteriorate and may affect film quality. In particular, this may negatively impact film post-processing due to its impact on mechanical strength. Furthermore, due to the accelerated hygroscopicity of the film, its optical properties, light resistance, and heat resistance may also deteriorate, not to mention mechanical properties. According to one embodiment, a polyamide-imide film includes a polyamide-imide polymer, which is prepared by polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound. Polyamide-imide polymers are polymers containing an imide repeating unit and an amide repeating unit. Specifically, the polyamide-imide polymer includes a monoimide repeating unit derived from the polymerization reaction of a diamine compound and a dianhydride compound, and a monoamide repeating unit derived from the polymerization reaction of a diamine compound and a dicarbonyl compound. The diamine compound is a compound that forms an imide bond with the dianhydride compound and an amide bond with the dicarbonyl compound, thereby forming a copolymer. The diamine compound is not particularly limited, but may be, for example, an aromatic diamine compound containing an aromatic structure. For example, the diamine compound may be a compound represented by the following Formula 1. [Formula 1] In Formula 1, E can be selected from substituted or unsubstituted divalent C 6-C 30 Aliphatic cyclic group, substituted or unsubstituted divalent C 4-C 30 Heteroaliphatic cyclic group, substituted or unsubstituted divalent C 6-C 30 Aromatic cyclic group, substituted or unsubstituted divalent C 4-C 30 Heteroaromatic cyclic group, substituted or unsubstituted C 1-C 30 Alkylene, substituted or unsubstituted C 2-C 30 Alkenyl, substituted or unsubstituted C 2-C 30 Alkyne, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) 2-、-Si(CH 3) 2-、-C(CH 3) 2-, and -C(CF 3) 2-. e is an integer selected from 1 to 5. When e is 2 or more, E's may be the same as or different from each other. (E) in Formula 1 e The group may be selected from the groups represented by the following formulae 1-1a to 1-14a, but is not limited thereto. Specifically, (E) in Formula 1 e The group may be selected from the groups represented by the following formulae 1-1b to 1-13b, but is not limited thereto. More specifically, (E) in Formula 1 e It may be a group represented by the above formula 1-6b or a group represented by the above formula 1-9b. In one embodiment, the diamine compound may include a compound having a fluorine-containing substituent or a compound having an ether group (—O—). The diamine compound may be composed of a compound having a fluorine-containing substituent. In this case, the fluorine-containing substituent may be a fluorinated hydrocarbon group, and specifically a trifluoromethyl group, but is not limited thereto. In some embodiments, the diamine compound may include one type of diamine compound, that is, the diamine compound may be composed of a single component. For example, the diamine compound may include 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFDB) represented by the following formula, but is not limited thereto. The dianhydride compound has a low birefringence value, so that it can help enhance optical properties such as transmittance of a film including a polyamide-imide-based polymer. The dianhydride compound is not particularly limited, but may be, for example, an aromatic dianhydride compound containing an aromatic structure. For example, the aromatic dianhydride compound may be a compound represented by the following Formula 2. [Formula 2] In Formula 2, G can be selected from substituted or unsubstituted tetravalent C 6-C 30 Aliphatic cyclic group, substituted or unsubstituted tetravalent C 4-C 30 Heteroaliphatic cyclic group, substituted or unsubstituted tetravalent C 6-C 30 Aromatic cyclic group, or substituted or unsubstituted tetravalent C 4-C 30 A group of heteroaromatic cyclic groups, wherein the aliphatic cyclic group, the heteroaliphatic cyclic group, the aromatic cyclic group, or the heteroaromatic cyclic group may exist alone, may be fused to each other to form a condensed ring, or may be selected from substituted or unsubstituted C 1-C 30 Alkylene, substituted or unsubstituted C 2-C 30 Alkenyl, substituted or unsubstituted C 2-C 30 Alkyne, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) 2-、-Si(CH 3) 2-、-C(CH 3) 2-, and -C(CF 3) 2-bonding group and bonded. G in the above Formula 2 may be selected from the groups represented by the following Formulas 2-1a to 2-9a, but is not limited thereto. For example, G in Formula 2 may be a group represented by the above Formula 2-2a, a group represented by the above Formula 2-8a, or a group represented by the above Formula 2-9a. In one embodiment, the dianhydride compound may include a compound having a fluorine-containing substituent, a compound having a biphenyl group, or a compound having a ketone group. The fluorine-containing substituent may be a fluorinated hydrocarbon group, and specifically may be a trifluoromethyl group, but is not limited thereto. In another embodiment, the dianhydride compound may be composed of a single component or a mixture of two components. For example, the dianhydride compound may include at least one selected from the group consisting of 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), which has the following structure, but is not limited thereto. The diamine compound and the dianhydride compound can be polymerized to form polyamide. Thereafter, the polyamine can be converted into polyimide via a dehydration reaction, and the polyimide comprises an imide repeating unit. Polyimide can form a repeating unit represented by the following formula A. [Formula A] In formula A, E, G and e are as described above. For example, the polyimide may include a repeating unit represented by the following formula A-1, but is not limited thereto. [Formula A-1] In formula A-1, n is an integer from 1 to 400. The dicarbonyl compound is not particularly limited, but it may be, for example, a compound represented by the following Formula 3. [Formula 3] In Formula 3, J can be selected from substituted or unsubstituted divalent C 6-C 30 Aliphatic cyclic group, substituted or unsubstituted divalent C 4-C 30 Heteroaliphatic cyclic group, substituted or unsubstituted divalent C 6-C 30 Aromatic cyclic group, substituted or unsubstituted divalent C 4-C 30 Heteroaromatic cyclic group, substituted or unsubstituted C 1-C 30 Alkylene, substituted or unsubstituted C 2-C 30 Alkenyl, substituted or unsubstituted C 2-C 30 Alkyne, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) 2-、-Si(CH 3) 2-、-C(CH 3) 2-, and -C(CF 3) 2-. j is an integer selected from 1 to 5. When j is 2 or more, J may be the same as or different from each other. X is a halogen atom. Specifically, X can be F, Cl, Br, I, or the like. More specifically, X can be Cl, but is not limited thereto. (J) in the above formula 3 j The group may be selected from the groups represented by the following formulae 3-1a to 3-14a, but is not limited thereto. Specifically, (J) in the above formula 3 j The group may be selected from the groups represented by the following formulae 3-1b to 3-8b, but is not limited thereto. More specifically, (J) in Formula 3 j It may be a group represented by the above formula 3-1b, a group represented by the above formula 3-2b, a group represented by the above formula 3-3b, or a group represented by the above formula 3-8b. For example, (J) in the above formula 3 j It may be a group represented by the above formula 3-1b or a group represented by the above formula 3-2b. In one embodiment, a single dicarbonyl compound may be used alone, or a mixture of at least two different dicarbonyl compounds may be used as the dicarbonyl compound. If two or more dicarbonyl compounds are used, then at least two dicarbonyl compounds (wherein (J) in the above formula 3) j is selected from the group represented by the above formulae 3-1b to 3-8b) can be used as the dicarbonyl compound. In another embodiment, the dicarbonyl compound may be an aromatic dicarbonyl compound containing an aromatic structure. The dicarbonyl compound may include terephthaloyl chloride (TPC), 1,1′-biphenyl-4,4′-dicarbonyl dichloride (BPDC), isophthaloyl chloride (IPC), as represented by the following formula, or a combination thereof, but is not limited thereto. The diamine compound and the dicarbonyl compound can be polymerized to form a repeating unit represented by the following formula B. [Formula B] In formula B, E, J, e and j are as described above. For example, a diamine compound and a dicarbonyl compound can be polymerized to form amide repeating units represented by the following formulas B-1 and B-2. Alternatively, the diamine compound and the dicarbonyl compound may be polymerized to form amide repeating units represented by the following formulas B-2 and B-3. [Formula B-1] In formula B-1, x is an integer from 1 to 400. [Formula B-2] In formula B-2, y is an integer from 1 to 400. [Formula B-3] In formula B-3, y is an integer from 1 to 400. According to one embodiment, the polyamide-imide polymer may include a repeating unit represented by the following formula A and a repeating unit represented by the following formula B: [Formula A] [Formula B] In formulas A and B, E and J are independently selected from substituted or unsubstituted divalent C 6-C 30 Aliphatic cyclic group, substituted or unsubstituted divalent C 4-C 30 Heteroaliphatic cyclic group, substituted or unsubstituted divalent C 6-C 30 Aromatic cyclic group, substituted or unsubstituted divalent C 4-C 30 Heteroaromatic cyclic group, substituted or unsubstituted C 1-C 30 Alkylene, substituted or unsubstituted C 2-C 30 Alkenyl, substituted or unsubstituted C 2-C 30 Alkyne, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) 2-、-Si(CH 3) 2-、-C(CH 3) 2-, and -C(CF 3) 2-, e and j are independently selected from integers of 1 to 5, when e is 2 or more, the 2 or more E's may be the same or different from each other, when j is 2 or more, the 2 or more J's may be the same or different from each other, G is a substituted or unsubstituted tetravalent C 6-C 30 Aliphatic cyclic group, substituted or unsubstituted tetravalent C 4-C 30 Heteroaliphatic cyclic group, substituted or unsubstituted tetravalent C 6-C 30 Aromatic cyclic group, or substituted or unsubstituted tetravalent C 4-C 30Heteroaromatic cyclic groups, wherein aliphatic cyclic groups, heteroaliphatic cyclic groups, aromatic cyclic groups, or heteroaromatic cyclic groups exist alone, are fused to each other to form a condensed ring, or are selected from substituted or unsubstituted C 1-C 30 Alkylene, substituted or unsubstituted C 2-C 30 Alkenyl, substituted or unsubstituted C 2-C 30 Alkyne, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) 2-、-Si(CH 3) 2-、-C(CH 3) 2-, and -C(CF 3) 2-bonding group and bonded. The polyamide-imide polymer may include imide repeating units and amide repeating units in a molar ratio of 2:98 to 70:30. Specifically, the molar ratio of imide repeating units to amide repeating units may be 2:98 to 60:40, 2:98 to 55:45, 2:98 to 50:50, 5:95 to 70:30, 5:95 to 60:40, 5:95 to 55:45, 5:95 to 50:50, or 10:90 to 40:60, but is not limited thereto. If the molar ratio of imide repeating units to amide repeating units is within the above range, it is possible to effectively control TGA characteristics such as TGA weight loss area (TDA) of the polyamide-imide film and combine it with a characteristic processing method to improve the quality reliability of the film. In the polyamide-imide polymer, the molar ratio of the repeating unit represented by the above formula A to the repeating unit represented by the above formula B may be 2:98 to 70:30. Specifically, the molar ratio of the repeating unit represented by formula A to the repeating unit represented by formula B may be 2:98 to 60:40, 2:98 to 55:45, 2:98 to 50:50, 5:95 to 70:30, 5:95 to 60:40, 5:95 to 55:45, 5:95 to 50:50, or 10:90 to 40:60, but is not limited thereto. In addition to the polyamide-imide polymer, the polyamide-imide film according to one embodiment may further include at least one selected from the group consisting of a filler, a blue pigment, and a UVA absorber. The filler may include, for example, an oxide, carbonate, or sulfate of a metal or metalloid. For example, the filler may include silicon dioxide, calcium carbonate, barium sulfate, or the like, but is not limited thereto. The filler can be used in the form of particles. In addition, the surface of the filler does not need to be specially coated and it can be evenly dispersed throughout the film. Since the polyamide-imide film contains fillers, it is possible to ensure a wide viewing angle without deteriorating the optical properties of the film, and not only improve the roughness and windability, but also improve the effect of scratching caused by sliding during film preparation. The filler may have a refractive index of 1.55 to 1.75. Specifically, the refractive index of the filler may be 1.60 to 1.75, 1.60 to 1.70, 1.60 to 1.68, or 1.62 to 1.65, but is not limited thereto. If the refractive index of the filler satisfies the above range, the birefringence values ​​related to the x-direction refractive index (nx), y-direction refractive index (ny), and z-direction refractive index (nz) can be appropriately adjusted, and the brightness of the film at specific angles can be improved. On the other hand, if the refractive index of the filler is outside the above range, there may be problems such as the filler being visually conspicuous on the film or the haze value being increased due to the filler. The filler content may be 100 ppm to 15,000 ppm, based on the total weight of the solid content of the polyamide-imide polymer. Specifically, the filler content may be 100 ppm to 14,500 ppm, 100 ppm to 14,200 ppm, 200 ppm to 14,500 ppm, 200 ppm to 14,200 ppm, 250 ppm to 14,100 ppm, or 300 ppm to 14,000 ppm, based on the total weight of the solid content of the polyamide-imide polymer, but is not limited thereto. If the filler content is outside the above range, the haze value of the film increases sharply, and the fillers may aggregate with each other on the surface of the film, thereby visually observing a foreign body feeling, or may cause problems with sliding performance or deterioration of windability during the production process. In some embodiments, the blue pigment may be used in an amount of 50 to 5,000 ppm based on the total weight of the polyamide-imide polymer. Preferably, the amount of the blue pigment used may be 100 to 5,000 ppm, 200 to 5,000 ppm, 300 to 5,000 ppm, 400 to 5,000 ppm, 50 to 3,000 ppm, 100 to 3,000 ppm, 200 to 3,000 ppm, 300 to 3,000 ppm, 400 to 3,000 ppm, 50 to 2,000 ppm, 100 to 2,000 ppm, 200 to 2,000 ppm, 300 to 2,000 ppm, 400 to 2,000 ppm, 50 to 1,000 ppm, 100 to 1,000 ppm, 200 to 1,000 ppm, 300 to 1,000 ppm, or 400 to 1,000 ppm. ppm, based on the total weight of the polyamide-imide polymer, but not limited thereto. UVA absorbers may include absorbers that absorb electromagnetic waves with wavelengths of 10 to 400 nm, as used in the art. For example, UVA absorbers may include benzotriazole compounds. Benzotriazole compounds may include N-phenolic benzotriazole compounds. In some embodiments, N-phenolic benzotriazole compounds may include N-phenolic benzotriazoles in which the phenol group is substituted with an alkyl group having 1 to 10 carbon atoms. These may be substituted with two or more alkyl groups, which may be linear, branched, or cyclic. In some embodiments, the UVA absorber may be used in an amount of 0.1 to 10 wt %. Preferably, the UVA absorber may be used in an amount of 0.1 to 5 wt %, 0.1 to 3 wt %, 0.1 to 2 wt %, 0.5 to 10 wt %, 0.5 to 5 wt %, 0.5 to 3 wt %, 0.5 to 2 wt %, 1 to 10 wt %, 1 to 5 wt %, 1 to 3 wt %, or 1 to 2 wt %, based on the total weight of the polyamide-imide polymer, but is not limited thereto. The physical properties of the polyamide-imide film described above are based on a thickness of 40 µm to 80 µm. For example, the physical properties of the polyamide-imide film are based on a thickness of 50 µm. The components and properties of the polyamide-imide film described above can be combined with each other. Furthermore, in the TGA analysis described above, the TGA weight loss area value (TDA), the first weight loss temperature (Tw), the 1% weight loss temperature (Td1), and / or the 5% weight loss temperature (Td5) of the polyamide-imide film can be adjusted by combining the chemical and physical properties of the components constituting the polyamide-imide film as described below and the specific conditions in the individual steps of the process for preparing the polyamide-imide film. For example, the composition and content of the components constituting the polyamide-imide film, the residual solvent amount, the polymerization conditions and heat treatment conditions in the heat treatment and cooling steps during the film preparation process, and the like are all combined to achieve physical properties such as TGA weight loss area value (TDA), first weight loss temperature (Tw), and the like within the desired range. Cover window for display device A cover window for a display device according to one embodiment includes a polyamide-imide film and a functional layer. The polyamide-imide film has a TGA weight loss area value (TDA) of 0.01%·min / °C or less. The details of the polyamide-imide film are as described above. A cover window for a display device can be advantageously applied to a display device. Display device A display device according to one embodiment includes a display unit; and a cover window disposed on the display unit, wherein the cover window includes a polyamide-imide film and a functional layer. The polyamide-imide film has a TGA weight loss area value (TDA) of 0.01%·min / °C or less. The details of the polyamide-imide film and the cover window are as described above. Figure 1 is an exploded schematic diagram of a display device according to an embodiment. Figure 2 is a perspective schematic diagram of a display device according to an embodiment. Figure 3 is a cross-sectional schematic diagram of a display device according to an embodiment. Specifically, Figures 1 to 3 each illustrate a display device, which includes a display unit (400) and a cover window (300) configured on the display unit (400), wherein the cover window includes a polyamide-imide film (100) having a first side (101) and a second side (102) and a functional layer (200), and an adhesive layer (500) is inserted between the display unit (400) and the cover window (300). The display unit (400) is used to display an image and may have a flexible feature. The display unit (400) may be a display panel for displaying an image. For example, it may be a liquid crystal display panel or an organic electroluminescent display panel. The organic electroluminescent display panel may include a front polarizer and an organic EL panel. The front polarizing plate can be arranged on the front side of the organic EL panel. Specifically, the front polarizing plate can be attached to the side where the image is displayed in the organic EL panel. An organic EL panel can display an image through the self-emission of a pixel unit. The organic EL panel may include an organic EL substrate and a drive substrate. The organic EL substrate may include a plurality of organic electroluminescent units, each corresponding to a pixel. Specifically, it may include a cathode, an electron transport layer, a light-emitting layer, a hole transport layer, and an anode. The drive substrate is operatively coupled to the organic EL substrate. That is, the drive substrate can be coupled to the organic EL substrate to apply a drive signal such as a drive current, so that the drive substrate can drive the organic EL substrate by applying current to each organic electroluminescent unit. In addition, an adhesive layer (500) may be inserted between the display unit (400) and the cover window (300). The adhesive layer may be an optically transparent adhesive layer, but is not particularly limited. The cover window (300) can be disposed on the display unit (400). The cover window is located outside the display device, thereby protecting the display unit. The cover window (300) may include a polyamide-imide film and a functional layer. The functional layer may be at least one selected from the group consisting of a hard coating layer, a reflectivity-reducing layer, an antifouling layer, and an anti-glare layer. The functional layer may be coated on at least one side of the polyamide-imide film. The details of the functional layer and the hard coating layer are as described above. According to one embodiment, the polyamide-imide film can be applied as a thin film to the exterior of a display device without changing the display driving method, the panel's internal color filter, or the laminate structure. This provides a display device with uniform thickness, low haze, high transmittance, and high transparency. This approach reduces production costs without significant process changes or cost increases. According to one embodiment, the polyamide-imide film can have excellent optical properties in terms of high transmittance, low haze value, and low yellowness index, and can have excellent mechanical properties such as modulus and flexibility. When exposed to ultraviolet rays, changes (deterioration) in its optical and mechanical properties can be suppressed. Specifically, polyamide-imide films with a TGA weight loss area (TDA) value within the aforementioned range or less not only exhibit excellent optical and mechanical properties but also exhibit excellent post-processing suitability, such as adhesion to functional layers and solvent resistance. Consequently, even after post-processing, the film's quality or physical properties do not deteriorate. When applied to display cover windows or display devices, polyamide-imide films can improve the quality, reliability, and production yield of the final product. Used to prepare polyamide - Method for forming imide-based thin films One embodiment provides a method for preparing a polyamide-imide film. According to one embodiment, a method for preparing a polyamide-imide film includes polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent to prepare a polyamide-imide polymer solution (S100); casting the solution to prepare a gel sheet (S200); and heat-treating the gel sheet (S300) (see Figure 4). According to some embodiments, the method for preparing a polyamide-imide film may further include adjusting the viscosity of the polyamide-imide polymer solution ( S110 ), aging the polyamide-imide polymer solution ( S120 ), and / or degassing the polyamide-imide polymer solution ( S130 ). A polyamide-imide film is a film whose main component is a polyamide-imide polymer. A polyamide-imide polymer is a resin containing an imide repeating unit and an amide repeating unit at a predetermined molar ratio as a structural unit. In the method for preparing a polyamide-imide film, a polymer solution for preparing a polyamide-imide polymer can be prepared by simultaneously or sequentially mixing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent in a reaction apparatus and reacting the mixture (S100). In one embodiment, the polymer solution may be prepared by simultaneously mixing and reacting a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent. In another embodiment, the step of preparing a polymer solution may include first mixing and reacting a diamine compound and a dianhydride compound in a solvent to produce a polyamide (PAA) solution; and secondly mixing and reacting the PAA solution with a dicarbonyl compound to form an amide bond and an imide bond. The polyamide solution is a solution containing a polymer containing repeating amide units. Alternatively, the steps of preparing the polymer solution may include first mixing and reacting a diamine compound and a dianhydride compound in a solvent to produce a polyamide solution; dehydrating the polyamide solution to produce a polyimide (PI) solution; and then mixing and reacting the polyimide (PI) solution with a dicarbonyl compound to further form amide bonds. The polyimide solution is a solution containing a polymer having imide repeating units. In yet another embodiment, the step of preparing a polymer solution may include first mixing and reacting a diamine compound and a dicarbonyl compound in a solvent to produce a polyamide (PA) solution; and secondly mixing and reacting the polyamide (PA) solution with a dianhydride compound to further form an imide bond. The polyamide solution is a solution containing a polymer having amide repeating units. The polymer solution prepared in this way may be a solution containing a polymer containing at least one selected from the group consisting of a polyamic acid (PAA) repeating unit, a polyamide (PA) repeating unit, and a polyimide (PI) repeating unit. Alternatively, the polymer contained in the polymer solution includes a monoimide repeating unit derived from the polymerization reaction of a diamine compound and a dianhydride compound, and a monoamide repeating unit derived from the polymerization reaction of a diamine compound and a dicarbonyl compound. The details of the diamine compound, the dianhydride compound, and the dicarbonyl compound are as described above. The solid content of the polymer solution may be 10% to 30% by weight, or 15% to 25% by weight, but is not limited thereto. If the solid content of the polymer solution is within the above range, a polyamide-imide film can be efficiently produced during the extrusion and casting steps. Furthermore, the polyamide-imide film thus produced exhibits similar thermal properties along the film direction, resulting in uniform quality, and can have excellent mechanical properties, optical properties, and heat resistance. In another embodiment, the step of preparing the polymer solution may further include introducing a catalyst. Here, the catalyst may include at least one selected from the group consisting of β-picoline, acetic anhydride, isoquinoline (IQ), and pyridine compounds, but is not limited thereto. The amount of the catalyst added may be 0.01 to 0.5 molar equivalents, 0.01 to 0.4 molar equivalents, or 0.01 to 0.3 molar equivalents, based on 1 mole of polyamine, but is not limited thereto. Further addition of a catalyst can accelerate the reaction rate and strengthen the chemical bonding strength between or within the repeating unit structures. In one embodiment, the step of preparing the polymer solution may further include adjusting the viscosity of the polymer solution ( S110 ). At room temperature, the viscosity of the polymer solution may be 80,000 cps to 500,000 cps, 100,000 cps to 500,000 cps, 150,000 cps to 500,000 cps, 150,000 cps to 450,000 cps, 200,000 cps to 450,000 cps, 200,000 cps to 400,000 cps, 200,000 cps to 350,000 cps, or 250,000 cps to 350,000 cps. In this case, the film forming ability of the polyamide-imide film can be improved, thereby improving the thickness uniformity. Specifically, the step of preparing the polymer solution may include simultaneously or sequentially mixing and reacting a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent to prepare a first polymer solution; and further adding the dicarbonyl compound to prepare a second polymer solution having a target viscosity. In the steps of preparing the first polymer solution and the second polymer solution, the viscosities of the polymer solutions are different from each other. For example, the viscosity of the second polymer solution is higher than the viscosity of the first polymer solution. In the steps of preparing the first polymer solution and the second polymer solution, the stirring speeds may be different from each other. For example, the stirring speed in preparing the first polymer solution is faster than the stirring speed in preparing the second polymer solution. In yet another embodiment, the step of preparing the polymer solution may further include adjusting the pH value of the polymer solution. In this step, the pH value of the polymer solution may be adjusted to 4 to 7, for example, 4.5 to 7. The pH of the polymer solution can be adjusted by adding a pH adjuster. The pH adjuster is not particularly limited and may include, for example, amine compounds such as alkoxyamines, alkylamines, and alkanolamines. Since the pH value of the polymer solution is adjusted to the above range, it is possible to prevent defects in a film produced from the polymer solution and to achieve desired optical and mechanical properties in terms of yellowness index and modulus. The pH adjuster may be used in an amount of 0.1 mol% to 10 mol% based on the total molar number of monomers in the polymer solution. In one embodiment, the organic solvent may be selected from at least one member of the group consisting of dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), m-cresol, tetrahydrofuran (THF), and chloroform. The organic solvent used in the polymer solution may be dimethylacetamide (DMAc), but is not limited thereto. In another embodiment, at least one selected from the group consisting of a filler, a blue pigment, and a UVA absorber may be added to the polymer solution. The types and contents of the filler, blue pigment, and UVA absorber are as described above. The filler, blue pigment, and UVA absorber can be mixed with the polyamide-imide polymer in a polymer solution. The polymer solution can be stored at -20°C to 20°C, -20°C to 10°C, -20°C to 5°C, -20°C to 0°C, or 0°C to 10°C. If it is stored at the above temperature, it is possible to prevent the polymer solution from being degraded and to reduce the water content, thereby preventing defects in a film produced therefrom. In some embodiments, the polymer solution or the polymer solution with adjusted viscosity may be aged ( S120 ). Aging can be performed by placing the polymer solution at a temperature of -10 to 10°C for 24 hours or longer. In this case, the polyamide-imide polymer or unreacted materials contained in the polymer solution can, for example, complete the reaction or reach chemical equilibrium, thereby homogenizing the polymer solution. The mechanical and optical properties of the polyamide-imide film formed therefrom can be substantially uniform across the entire film area. Preferably, aging can be performed at a temperature of -5 to 10°C, -5 to 5°C, or -3 to 5°C, but is not limited thereto. In one embodiment, the method may further include degassing the polyamide-imide polymer solution (S130). The degassing step can remove water and reduce impurities in the polymer solution, thereby increasing the reaction yield and giving the final film excellent surface appearance and mechanical properties. Degassing may include vacuum degassing or purging with an inert gas. After reducing the internal pressure of the tank containing the polymer solution to 0.1 to 0.7 bar, vacuum degassing can be performed for 30 minutes to 3 hours. Vacuum degassing under these conditions can reduce bubbles in the polymer solution. As a result, it is possible to prevent surface defects in the resulting film and achieve excellent optical properties, such as haze value. In addition, purging can be performed by purging the tank with an inert gas at an internal pressure of 1 to 2 atm. Purging under these conditions can remove water from the polymer solution, reduce impurities, thereby increasing the reaction yield, and achieving excellent optical properties such as haze value and mechanical properties. The inert gas may be selected from at least one of the group consisting of nitrogen, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn), but is not limited thereto. Specifically, the inert gas may be nitrogen. Vacuum degassing and purging with inert gas can be performed in separate steps. For example, after the step of purging with an inert gas, a vacuum degassing step may be performed, but it is not limited thereto. Vacuum degassing and / or purging with an inert gas can improve the physical properties of the surface of the resulting polyamide-imide film. The polymer solution may be cast to prepare a gel sheet ( S200 ). For example, a polymer solution can be extruded and coated onto a support to form a gel sheet. Furthermore, the polymer solution can be cast to a thickness of 200 µm to 700 µm. Since the polymer solution is cast to a thickness within the above range, the final film produced after heat treatment can have an appropriate and uniform thickness. As described above, the polymer solution can have a viscosity of 150,000 cps to 500,000 cps at room temperature. Since the viscosity falls within this range, the polymer solution can be cast into a uniform thickness without defects, and a polyamide-imide-based film having substantially uniform thickness can be formed without local / partial thickness variations during subsequent heat treatment. The gel sheet may be heat-treated to form a polyamide-imide film ( S300 ). The heat treatment of the gel sheet can be performed, for example, by a heat setting device. The step of heat-treating the gel sheet may include two or more heat-treating steps and two or more cooling steps. For example, the step of heat-treating the gel sheet may include three or more heat-treating steps and three or more cooling steps. In one embodiment, the step of heat-treating the gel sheet may include a first heat-treating step, a second heat-treating step, and a third heat-treating step. Specifically, the step of heat-treating the gel sheet may include a first heat-treating step, a second heat-treating step, a third heat-treating step, a first cooling step, a second cooling step, and a third cooling step. More specifically, the step of heat-treating the gel sheet may include a first heat-treating step (S310), a first cooling step (S320), a second heat-treating step (S330), a third heat-treating step (S340), a second cooling step (S350), and a third cooling step (S360), which are performed in sequence (see FIG. 5 ). In the first heat treatment step, the gel sheet cast from the polymer solution is heat treated at a temperature of 60° C. to 150° C., 70° C. to 150° C., 80° C. to 150° C., or 90° C. to 150° C. for 5 to 60 minutes. Specifically, the gel sheet is heat treated at a temperature of 90° C. to 140° C. for 10 to 30 minutes. During the first heat treatment step of drying the gel sheet, the solvent of the gel sheet may be partially or completely volatilized. In some embodiments, the first heat treatment step may be performed while it is transferred onto a belt, but it is not limited thereto. The first cooling step may be performed after the first heat treatment step. Specifically, in the first cooling step, the gel sheet may be cooled at a temperature of 10° C. to 70° C., 15° C. to 60° C., 15° C. to 50° C., or 15° C. to 40° C. for 5 to 60 minutes. Specifically, the gel sheet is cooled at a temperature of 15° C. to 35° C. for 5 to 30 minutes. A second heat treatment step may be performed after the first cooling step. Specifically, the second heat treatment step can be performed by hot air treatment. In one embodiment, if the heat treatment is performed using hot air, heat can be supplied uniformly. If the heat supply is uneven, a satisfactory surface roughness cannot be achieved, or the surface quality may be uneven, and the surface energy may increase or decrease too much. Heat treatment using hot air can be performed at a temperature range of 60°C to 500°C for 5 to 200 minutes. Specifically, heat treatment of the gel sheet can be performed at a temperature range of 80°C to 350°C at a heating rate of 1.5°C / min to 20°C / min for 5 to 100 minutes. More specifically, heat treatment of the gel sheet can be performed at a temperature range of 140°C to 300°C. In this case, the initial temperature of the heat treatment of the gel sheet by hot air may be 60° C. or higher. Specifically, the initial temperature of the heat treatment of the gel sheet may be 80° C. to 180° C. Furthermore, the maximum temperature of the heat treatment may be 200° C. to 500° C. Furthermore, the heat treatment of the gel sheet can be performed in two or more stages. Specifically, the heat treatment of the gel sheet using hot air can be performed sequentially in a first hot air treatment stage and a second hot air treatment stage. The temperature of the second hot air treatment stage can be higher than the temperature of the first hot air treatment stage. In one embodiment, the step of heat-treating the gel sheet may include a third heat-treating step of passing through at least one heater. Specifically, the heat-treating step may be performed by passing through a plurality of heaters after the second heat-treating step. The plurality of heaters may include a plurality of heaters spaced apart from each other in the transverse direction (TD) of the gel sheet. The plurality of heaters may be mounted on a heater mounting portion, and two or more heater mounting portions may be arranged along the moving direction (MD) of the gel sheet. The at least one heater may include an IR heater. However, the type of the at least one heater is not limited to the above examples and may be modified in various ways. Specifically, the plurality of heaters may each include an IR heater. The heat treatment by the at least one heater may be performed at a temperature range of 250° C. or higher. Specifically, the heat treatment by the at least one heater may be performed at a temperature range of 250° C. to 400° C. for 1 minute to 30 minutes or 1 minute to 20 minutes. In the heat treatment using a heater, the above temperature is the temperature of the heat treatment device where the gel sheet is present, which corresponds to the temperature measured by a temperature sensor located in the third heat treatment section of the heat treatment device. The second cooling step may be performed after the third heat treatment step. Specifically, in the second cooling step, the gel sheet may be cooled at a temperature of 30° C. to 150° C., 30° C. to 120° C., 50° C. to 120° C., or 70° C. to 120° C. for 1 minute to 60 minutes. Specifically, the gel sheet is cooled at a temperature of 70° C. to 100° C. for 2 minutes to 30 minutes. In one embodiment, the second cooling step may further include a first cooling step of reducing the temperature at a rate of 100° C. / min to 1,000° C. / min. The third cooling step may be performed after the second cooling step. In the third cooling step, the gel sheet may be cooled at a temperature of 10° C. to 70° C., 15° C. to 60° C., 15° C. to 50° C., or 15° C. to 40° C. for 1 minute to 60 minutes. Specifically, the gel sheet is cooled at a temperature of 15° C. to 35° C. for 2 minutes to 30 minutes. In one embodiment, the third cooling step may further include a second cooling step of reducing the temperature at a rate of 40° C. / min to 400° C. / min. In this case, specifically, the second cooling step is performed after the first cooling step. The cooling rate of the first cooling step may be faster than the cooling rate of the second cooling step. For example, the maximum rate of the first cooling step is faster than the maximum rate of the second cooling step, or the minimum rate of the first cooling step is faster than the minimum rate of the second cooling step. If the step of cooling the solidified film is performed in this multi-stage manner, it is possible to further stabilize the physical properties of the solidified film and maintain the optical and mechanical properties of the film achieved during the solidification step more stably over a long period of time. Because the gel sheet is heat-treated by ensuring the proper temperature, time, and process sequence for the heat treatment and cooling steps, effective drying and curing are achieved, minimizing residual solvent in the film and enabling the adjustment of physical properties such as TGA weight loss area (TDA), first weight loss temperature (Tw), and 1% weight loss temperature (Td1) within desired ranges. Furthermore, the film produced by this preparation method exhibits excellent optical and mechanical properties, as well as post-processing suitability, such as adhesion to functional layers and solvent resistance. The temperature of the second cooling step and the temperature of the third cooling step may be different from each other. The temperature of the second cooling step may be higher than the temperature of the third cooling step, but is not limited thereto. In one embodiment, the temperature of the second cooling step may be 20°C to 100°C, 40°C to 80°C, or 50°C to 80°C higher than the temperature of the third cooling step. Furthermore, a step of winding the cooled solidified film using a winder may be performed. In this case, the ratio of the moving speed of the gel sheet on the belt during drying to the moving speed of the cured film during winding is 1:0.95 to 1:1.40. Specifically, the moving speed ratio may be 1:0.99 to 1:1.20, 1:0.99 to 1:1.10, or 1:1.0 to 1:1.05, but is not limited thereto. If the ratio of the moving speeds is outside the above range, the mechanical properties of the cured film may be impaired, and flexibility and elasticity may be deteriorated. In the method for preparing a polyamide-imide film, the thickness deviation (%) according to the following equation 1 may be 3% to 30%. Specifically, the thickness deviation (%) may be 5% to 20%, but is not limited thereto. [Equation 1] Thickness variation (%) = (M1 – M2) / M2 × 100 In Equation 1, M1 is the thickness of the gel sheet (μm), and M2 is the thickness of the solidified film cooled while being wound (μm). The polyamide-imide film prepared by the above-described preparation method exhibits excellent optical and mechanical properties, as well as excellent post-processing suitability, such as adhesion to functional layers and solvent resistance. The polyamide-imide film can be used in various applications requiring transparency. For example, the polyamide-imide film can be used not only in display devices, but also in solar cells, semiconductor devices, sensors, and the like. The details of the polyamide-imide-based film prepared by the method for preparing a polyamide-imide film are as described above. example Hereinafter, the above contents will be described in detail with reference to examples. However, these examples are provided for the purpose of illustrating the present invention, and the scope of the present invention is not limited thereto. example 1 In a nitrogen atmosphere at 10° C., a temperature-controllable reactor was filled with dimethylacetamide (DMAc) as an organic solvent. Subsequently, 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFMB) as an aromatic diamine was slowly added thereto and dissolved. Thereafter, 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6-FDA) as a dianhydride compound was slowly added thereto, and the mixture was stirred for 2 hours. Subsequently, terephthaloyl chloride (TPC) as a dicarbonyl compound was added, followed by stirring for 1 hour; and isophthaloyl chloride (IPC) was added, followed by stirring for 1 hour, thereby preparing a polymer solution. The obtained polymer solution is applied to a support to form a gel sheet. Thereafter, in the first heat treatment step, the gel sheet was heat-treated at 140°C for 15 minutes to dry the gel sheet. In the first cooling step, the gel sheet was cooled at 35°C for 5 minutes. Thereafter, as a second heat treatment step, the gel sheet was treated with hot air at 275°C for 5 minutes. In the third heat treatment step, the gel sheet was passed through an IR heater while adjusting the temperature so that the temperature measured by the temperature sensor in the heat treatment section was 280°C. Thereafter, in the second cooling step, the gel sheet was cooled at 100°C for 3 minutes, and in the third cooling step, the gel sheet was cooled at 30°C for 3 minutes to obtain a polyamide-imide film with a thickness of 50 μm. The specific compositions and molar ratios of the polyamide-imide polymers are described in the preparation examples in Table 1 below. example 2 and 3 and comparative examples 1 to 3 Individual films were prepared in the same manner as in Example 1, except that the composition and molar ratio of the polymers, the temperatures and times of the individual heat treatment steps and cooling steps in the heat treatment of the gel sheet, and the like were changed as shown in Tables 1 and 2 below. Preparation example: polymer Composition [Table 1] [Table 2] Evaluation Example The following properties of the films prepared in the respective examples and comparative examples were measured and evaluated. The results are shown in Table 3 below. Evaluation Example 1: Measurement of Film Thickness The thickness was measured at 5 random points using a digital micrometer 547-401 manufactured by Mitutoyo Corporation. The average value was used as the thickness. Evaluation Example 2: Measurement of Transmittance and Haze Value According to JIS K 7136, the light transmittance and haze value were measured using a haze meter NDH-5000W manufactured by Nippon Denshoku Kogyo. Evaluation Example 3: Measurement of Yellowness Index According to ASTM-E313, the yellowness index (YI) was measured using a spectrophotometer (UltraScan PRO, Hunter Associates Laboratory) at d65 and 10°. Evaluation Example 4: Modulus Measurement A sample is cut out by cutting at least 10 cm in a direction perpendicular to the main shrinkage direction of the film and 10 cm in the main shrinkage direction. It is fixed in an Instron universal testing machine UTM 5566A by clamps arranged at 10 cm intervals. A stress-strain curve is obtained when it is stretched at a speed of 10 mm / min at room temperature until the sample breaks. The slope of the load relative to the initial strain on the stress-strain curve is used as the modulus (GPa). Evaluation Example 5: Measurement of surface tension According to the German Industrial Standard (DIN 55660), a mobile surface analyzer from Kruss, Germany, was used to measure the surface tension of the air side and the tape side of each film. Evaluation Example 6: TGA Analysis of Film TGA analysis was performed using a TA Thermogravimetric Analyzer Q500 model by measuring a 2 g sample of the polyamide-imide film and heating it at a rate of 10° C. / min within a range of 25° C. to 700° C. The first weight loss temperature (Tw) refers to the temperature at which the rate of change in weight loss reaches its maximum within the temperature range of 270°C to 330°C. In a graph of the rate of change in weight loss (y-axis) versus temperature (x-axis), if a straight line is drawn connecting the point corresponding to (Tw – 30)°C and the point corresponding to (Tw + 30)°C, the area between the straight line and the graph corresponding to the first weight loss temperature (Tw) is taken as the TGA weight loss area (TDA). In addition, the 1% weight loss temperature (Td1) and the 5% weight loss temperature (Td5) refer to the temperature at which 1% weight loss occurs and the temperature at which 5% weight loss occurs, respectively. Evaluation Example 7: Measurement of Adhesion of Hard Coating Polyester resin (PLASCOAT 446, GOO Chemical, Japan) and polyurethane resin (solid content 28 wt%, H-15, DKS Co. Ltd., Japan) were mixed at a weight ratio of 7:3 (solid content weight ratio), wherein water (H A mixed solvent of 2O) and isopropyl alcohol (IPA) was mixed at a weight ratio of 85:15, and then stirred at room temperature for 30 minutes to prepare a primer layer composition. In addition, 54.32 parts by weight of a polyurethane acrylate oligomer (PU2050, Miwon Specialty Chemical), 23.28 parts by weight of a multifunctional acrylate monomer (M300, Miwon Specialty Chemical), 19.4 parts by weight of a silica sol (MA-ST, Nissan Chemical) (in which silica fine particles with an average particle size of 10 to 15 nm are dispersed in 30% by weight of methanol), and 3 parts by weight of a photoinitiator (I-184, BASF) were mixed to prepare a hard coating composition. The primer layer composition was applied onto the polyamide-imide film sample to form a primer layer having a thickness of 0.1 μm, and the hard coating layer composition was applied onto the primer layer to form a hard coating layer having a thickness of 5 μm. The adhesion strength between the film sample and the hard coating was then evaluated using a cross-hatch test. According to ASTM D3359 (Method B), the surface of the hard coating was cut into a grid pattern at regular intervals. Tape (Nitto Tape 50B) was then applied and removed, and the degree of detachment of the grid cells from the surface was evaluated. This was graded from 0B to 5B according to the following scale, with 5B being the best (see Figure 6). - 5B: The cut surface is clean and the grid squares are not detached (0% of the grid area) - 4B: Small pieces of coating are detached at the intersection (less than 5% of the grid area) - 3B: Small pieces of coating are detached along the edges and at the intersection (5-15% of the grid area) - 2B: The coating is detached at the cut edge and a portion of the rectangular area (15-35% of the grid area) - 1B: The coating is flaking significantly along the cut edge and the squares are detached (35-65% of the grid area) - 0B: The coating is flaking and the squares are detached more severely (greater than 65% of the grid area) Evaluation Example 8: Evaluation of Solvent Resistance A film sample (5 cm × 15 cm) was immersed in the solvent for 5 seconds, dried at 80°C for 3 minutes, and the haze value was measured again according to the method of Evaluation Example 2 above. The smaller the change in haze value, the better the solvent resistance. MIBK or IPA was used as the solvent. [Table 3] As shown in Tables 2 and 3, films with a weight loss area (TDA) of 0.01%·min / °C or less exhibit excellent optical properties such as transmittance, haze value, and yellowness index, as well as mechanical properties such as modulus and adhesion to the hard coating. Changes in haze value after immersion in solvents commonly used for post-treatment remain at or below a certain level, indicating excellent solvent resistance. 100: Polyamide-imide film 101: First side 102: Second side 200: Functional layer 300: Cover window 400: Display unit 500: Adhesive layer AA': Cross-section position S100, S200, S300, S310, S320, S330, S340, S350, S360: Steps FIG. 1 is an exploded schematic diagram of a display device according to an embodiment. FIG. 2 is a perspective view of a display device according to an embodiment. FIG3 is a schematic cross-sectional view of a display device according to an embodiment. FIG. 4 is a schematic flow chart of a process for preparing a polyamide-imide film according to one embodiment. FIG. 5 is a schematic flow diagram illustrating a step of heat-treating a gel sheet in a process for preparing a polyamide-imide film according to an embodiment. FIG. 6 shows the evaluation criteria for the cross-cut test according to ASTM D3359 Method B. 100: Polyamide-imide film 102: Second side 200: Functional layer 300: Covering window 400: Display unit 500: Adhesive layer

Claims

1. A polyamide-imidene film having a TGA weight loss area value (TDA) of 0.01%·min / ℃ or less, wherein when the film is immersed in MIBK solvent for 5 seconds, dried at 80℃ for 3 minutes, and subsequently the haze value is measured, the haze value change rate (ΔHzM) is 0.2% or less, and wherein when the film is immersed in IPA solvent for 5 seconds, dried at 80℃ for 3 minutes, and subsequently the haze value is measured, the haze value change rate (ΔHzI) is 0.1% or less, wherein the first weight loss temperature (Tw) of the polyamide-imidene film in TGA analysis is 298℃ or higher, wherein the TGA weight loss area value (TDA) is measured by the following analytical method: The TGA analysis was performed using the TA Q500 thermogravimetric analysis model. A 2-gram sample of polyamide-imide film was taken and heated at a rate of 10°C / min within a temperature range of 25°C to 700°C. The first weight loss temperature (Tw) was the temperature at which the rate of weight loss change was maximum within the temperature range of 270°C to 330°C. In a graph of the rate of weight loss change (y-axis) versus temperature (x-axis), when a straight line was drawn connecting the point corresponding to the temperature (Tw–30)°C and the point corresponding to the temperature (Tw+30)°C, the area of ​​the portion corresponding to the first weight loss temperature (Tw) within the area between the line and the graph was taken as the TGA weight loss area value (TDA).

2. The polyamide-imide film of claim 1, wherein the 1% weight loss temperature (Td1) of the film is 300°C or higher in TGA analysis, and the 5% weight loss temperature (Td5) of the film is 420°C or higher in TGA analysis.

3. The polyamide-imide film of claim 1, wherein the film has an adhesion of 5B to the hard coating.

4. The polyamide-amide film of claim 1, wherein the film comprises a polyamide-amide polymer, and the polyamide-amide polymer comprises amide repeating units and amide repeating units in a molar ratio of 2:98 to 70:

30.

5. The polyamide-imide film of claim 1, wherein the film has a surface tension of 40 dyn / cm or more on the first side.

6. The polyamide-imide film of claim 1, having a modulus of 5 GPa or more, transmittance of 80% or more, haze value of 1% or less, and yellow index of 5 or less, based on a film thickness of 50 µm.

7. A cover window for a display device, comprising a polyamide-imide film as claimed in claim 1 and a functional layer.

8. A display device comprising a display unit; and a cover window disposed on the display unit, wherein the cover window comprises a polyamide-imide film as claimed in claim 1 and a functional layer.

9. A method for preparing a polyamide-imidene film as claimed in claim 1, comprising: polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent to prepare a polyamide-imidene polymer solution; casting the solution to prepare a gel sheet; and heat-treating the gel sheet, wherein the step of heat-treating the gel sheet comprises a first heat treatment step, a first cooling step, a second heat treatment step, a third heat treatment step, a second cooling step, and a third cooling step, performed sequentially, wherein in the first heat treatment step, the gel sheet is heat-treated at a temperature of 120°C to 140°C for 10 minutes to 30 minutes, and in the first cooling step, the gel sheet is cooled at a temperature of 30°C to 35°C for 5 minutes to 30 minutes.

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