Polyamide-imide-based film, method for producing the same, and cover window and display device comprising the same

Abstract

The embodiments aim to provide a polyamide-imide-based film including a polyamide-imide-based polymer, in which the average value of surface energy (SE) of a first side and a second side of the film is 43 to 70 mN / m, and thus it is excellent in optical properties, mechanical properties, and surface properties; a method of manufacturing the same; and a cover window and a display device including the polyamide-imide-based film.

Description

Technical Field

[0001] The embodiments relate to a polyamide-imide-based film, a method for preparing the same, and a cover window and display device comprising the polyamide-imide-based film. Background Technology

[0002] Polyimide-based resins, such as poly(amide-imide) (PAI), exhibit excellent resistance to abrasion, heat, and chemicals. Therefore, they are used in applications such as primary electrical insulation, coatings, adhesives, extrusion resins, heat-resistant coatings, heat-resistant sheets, heat-resistant adhesives, heat-resistant fibers, and heat-resistant films.

[0003] Polyimide is used in a variety of fields. For example, it is made into powder form and used as a coating for metal or magnetic wires. Depending on the application, it can be mixed with other additives. Furthermore, polyimide is used to coat kitchenware, utilizes its heat and chemical resistance as a membrane for gas separation, and is used in natural gas wells to filter contaminants such as carbon dioxide, hydrogen sulfide, and impurities.

[0004] In recent years, polyimide has been developed into thin films that are inexpensive and possess excellent optical, mechanical, and thermal properties. These polyimide-based films can be used in display materials for organic light-emitting diodes (OLEDs), liquid crystal displays (LCDs), and, if delay properties are achieved, can be used in anti-reflective films, compensation films, and delay films.

[0005] However, conventional polyimide-based films, especially conventional polyamide-imide-based films, suffer from the problem that polyamide-imide polymers inevitably contain fluorine atoms, which may be subject to environmental regulations. Therefore, the demand for developing polyamide-imide-based films prepared from fluorine-free polyamide-imide-based polymers and possessing excellent optical and mechanical properties is steadily increasing. Summary of the Invention

[0006] Technical issues

[0007] The embodiments are intended to provide a polyamide-imide-based thin film with excellent optical and mechanical properties, a process for preparing the polyamide-imide-based thin film, and a cover window and display device comprising the polyamide-imide-based thin film.

[0008] Solution to the problem

[0009] According to one embodiment, a polyamide-imide-based film comprises a polyamide-imide-based polymer, wherein the average surface energy (SE) of the first and second sides of the film is 43 to 70 mN / m, as measured by the following method:

[0010] [Measurement Method]

[0011] A 1 μL water droplet was placed on the film surface using a KRUSS contact angle measuring device, and the contact angle between the droplet and the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the polar surface energy (γ) of the film with respect to water. s p ).

[0012] Using a KRUSS contact angle measuring device, 1 μL of diiodomethane was dropped onto the film surface, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the nonpolar surface energy (γ) of the film to diiodomethane. s d ).

[0013] The surface energy of a thin film is the sum of its polar surface energy for water and its nonpolar surface energy for diiodomethane.

[0014] According to another embodiment, the cover window for the display device comprises a polyamide-imide-based film and a functional layer, wherein the polyamide-imide-based film comprises a polyamide-imide-based polymer, and according to the above measurement method, the average surface energy (SE) of the first and second sides of the film is 43 to 70 mN / m.

[0015] According to another embodiment, the display device includes: a display unit; and a cover window disposed on the display unit, wherein the cover window includes a polyamide-imide-based thin film and a functional layer, the polyamide-imide-based thin film comprising a polyamide-imide-based polymer, and according to the above measurement method, the average surface energy (SE) of the first and second sides of the thin film is 43 to 70 mN / m.

[0016] According to one embodiment, a process for preparing the polyamide-imide-based film includes: polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent to prepare a polyamide-imide-based polymer solution; casting the polymer solution and then drying it to prepare a gel sheet; and heat-treating the gel sheet.

[0017] Beneficial effects of the invention

[0018] According to one embodiment, the polyamide-imide-based film not only has excellent optical and mechanical properties, but also excellent surface properties. Attached Figure Description

[0019] Figure 1This is a schematic exploded view of a display device according to one embodiment.

[0020] Figure 2 This is a schematic perspective view of a display device according to one embodiment.

[0021] Figure 3 This is a schematic cross-sectional view of a display device according to one embodiment.

[0022] Figure 4 This is a schematic flow chart of a process for preparing polyamide-imide-based films according to one embodiment. Detailed Implementation

[0023] The embodiments will now be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement them. However, the embodiments can be implemented in many different ways and are not limited to those described herein.

[0024] Throughout this specification, when it is mentioned that films, windows, panels, layers, etc., are formed "above" or "below" another film, window, panel, layer, etc., it not only means that one element is directly formed above or below another element, but also that there are other elements in between, and that one element is not directly formed above or below another element. Furthermore, the terms "above" or "below" for each element can be found in the accompanying drawings. For descriptive purposes, the sizes of the elements in the drawings may be enlarged and do not represent actual sizes. Moreover, throughout this specification, the same reference numerals refer to the same elements.

[0025] Throughout the instruction manual, when a section is referred to as "containing" an element, it should be understood that other elements may be included, rather than excluded, unless otherwise expressly stated.

[0026] In this specification, unless otherwise stated, singular expressions should be interpreted to encompass the singular or plural as presented in the context.

[0027] Furthermore, unless otherwise stated, all figures and expressions used herein relating to the number of components, reaction conditions, etc., should be understood to be modified by the term “about”.

[0028] This document uses terms such as "first," "second," etc., to describe various components, but does not use these terms to limit the components. The terms are only used to distinguish one component from another.

[0029] Furthermore, as used herein, the term "substitution" means substitution by at least one of the following substituent groups: deuterium, -F, -Cl, -Br, -I, hydroxyl group, cyano group, nitro group, amino group, amidine group, hydrazine group, hydrazone group, ester group, ketone group, carboxyl group, substituted or unsubstituted alkyl group, substituted or unsubstituted alkenyl group, substituted or unsubstituted alkynyl group, substituted or unsubstituted alkoxy group, substituted or unsubstituted alicyclic organic group, substituted or unsubstituted heterocyclic group, substituted or unsubstituted aryl group, and substituted or unsubstituted heteroaryl group. The substituents listed above can be linked together to form a ring.

[0030] Polyamide-imide based film

[0031] One embodiment provides a polyamide-imide-based film having excellent optical properties (such as yellowness index and haze) as well as excellent mechanical and surface properties.

[0032] According to one embodiment, a polyamide-imide-based film comprises a polyamide-imide-based polymer, wherein the average surface energy (SE) of the first and second sides of the film is 43 to 70 mN / m, as measured by the following method:

[0033] [Measurement Method]

[0034] A 1 μL water droplet was placed on the film surface using a KRUSS contact angle measuring device, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the polar surface energy (γ) of the film with respect to water. s p );

[0035] Using a KRUSS contact angle measuring device, 1 μL of diiodomethane was dropped onto the film surface, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the nonpolar surface energy (γ) of the film to diiodomethane. s d );and

[0036] The surface energy (γ) of the thin film s ) is the polar surface energy of water (γ) s p ) and p-diiodomethane (γ) s d The sum of the nonpolar surface energies of ).

[0037] The average surface energy (SE) of the first and second sides of the thin film is the average surface energy (SE1) of the first side and the surface energy (SE2) of the second side of the thin film.

[0038] The surface energy on one side of the thin film can be derived from the Owens-Wendt model using Equations 1 and 2. Specifically, the surface energy on one side of the thin film is the sum of the polar surface energy of that side of the film with respect to water and the nonpolar surface energy with respect to diiodomethane. The polar surface energy of the thin film with respect to water (γ s p ) and the nonpolar surface energy (γ) of the thin film to diiodomethane s d The following equations 1 and 2 can be derived from the Owens-Wendt model.

[0039] [Equation 1]

[0040]

[0041] [Equation 2]

[0042]

[0043] Here, in Equation 1, θ represents the contact angle between one side of the film and the liquid. For diiodomethane, γ L It is 50.8 mN / m, γ L d It is 49.5 mN / m, and γ L p It is 1.3 mN / m. For water, γ L It is 72.8 mN / m, γ L d It is 21.8 mN / m, and γ L p It is 51 mN / m.

[0044] In one embodiment, according to the above measurement method, the average surface energy (SE) of the first and second sides of the polyamide-imide-based film can be 43 mN / m or higher, 44 mN / m or higher, 45 mN / m or higher, 46 mN / m or higher, 47 mN / m or higher, 48 mN / m or higher, 49 mN / m or higher, or 50 mN / m or higher. The average surface energy (SE) of the first and second sides of the polyamide-imide-based film can be 70 mN / m or lower, 67 mN / m or lower, 65 mN / m or lower, 63 mN / m or lower, 60 mN / m or lower, 58 mN / m or lower, 57 mN / m or lower, or 55 mN / m or lower.

[0045] Specifically, according to the above measurement method, the average surface energy (SE) of the first and second sides of the polyamide-imide-based film can be 43 to 68 mN / m, 43 to 65 mN / m, 43 to 63 mN / m, 43 to 60 mN / m, 43 to 58 mN / m, 43 to 57 mN / m, 43 to 55 mN / m, 45 to 70 mN / m, 45 to 67 mN / m, 45 to 65 mN / m, 45 to 63 mN / m, 46 to 60 mN / m, 46 to 58 mN / m, 46 to 57 mN / m, or 46 to 55 mN / m, but is not limited thereto.

[0046] In one embodiment, according to the above measurement method, the surface energy (SE1) of the first side of the polyamide-imide-based film can be 43 to 68 mN / m, 43 to 65 mN / m, 43 to 63 mN / m, 43 to 60 mN / m, 43 to 58 mN / m, 43 to 57 mN / m, 43 to 55 mN / m, 45 to 70 mN / m, 45 to 67 mN / m, 45 to 65 mN / m, 45 to 63 mN / m, 46 to 60 mN / m, 46 to 58 mN / m, 46 to 57 mN / m, or 46 to 55 mN / m.

[0047] 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 phase used in the formation of the polyamide-imide-based film. Specifically, in the process of preparing the film, the air side may refer to the side that is not in contact with the strip phase on which the polyamide-imide-based polymer solution is cast and dried.

[0048] In addition, according to the above measurement method, the surface energy (SE2) of the second side of the polyamide-imide-based film can be 43 to 68 mN / m, 43 to 65 mN / m, 43 to 63 mN / m, 43 to 60 mN / m, 43 to 58 mN / m, 43 to 57 mN / m, 43 to 55 mN / m, 45 to 70 mN / m, 45 to 67 mN / m, 45 to 65 mN / m, 45 to 63 mN / m, 46 to 60 mN / m, 46 to 58 mN / m, 46 to 57 mN / m, or 46 to 55 mN / m.

[0049] In one embodiment, the second side may be the strip side of the film. The strip side refers to the side that is in contact with the support phase used in the formation of the polyamide-imide-based film. Specifically, in the process of preparing the film, the strip side may refer to the side that is in contact with the strip phase on which the polyamide-imide-based polymer solution is cast and dried.

[0050] When the average surface energy (SE) of the first and second sides of a polyamide-imide-based film according to one embodiment, and / or the surface energy (SE1, SE2) of one side of the film, meets the above ranges, the adhesion of the film to other materials is enhanced, and the adhesion efficiency during lamination is improved, thereby reducing delamination. This increases the durability of the film and the reliability of the product, making it suitable for electronic device components such as cover windows, foldable displays, and touchscreens. Furthermore, appropriate surface energy can control the wettability of the film, thereby improving the film's quality uniformity and further enhancing processability, thus improving production efficiency.

[0051] Conversely, if the surface energy of the polyamide-imide film according to one embodiment does not meet the above range, the adhesion between the film and other materials may be insufficient, which may lead to coating or printing defects in some cases, and poor adhesion during lamination may result in deterioration of product quality and durability. In addition, inadequate surface wettability will result in poor film uniformity and may reduce plasticity during processing or post-processing, thus reducing production efficiency.

[0052] In one embodiment, according to the above measurement method, the difference between the surface energy of the first side and the surface energy of the second side of the thin film (|SE1-SE2|) can be 10 mN / m or lower, 9 mN / m or lower, 8 mN / m or lower, 7 mN / m or lower, 6 mN / m or lower, 5 mN / m or lower, 4 mN / m or lower, or 3 mN / m or lower. The difference between the surface energy of the first side and the surface energy of the second side of the thin film (|SE1-SE2|) can be 0 mN / m or higher, 0.5 mN / m or higher, 1 mN / m or higher, 1.5 mN / m or higher, or 2 mN / m or higher.

[0053] Specifically, according to the above measurement method, the difference between the surface energy of the first side and the surface energy of the second side of the thin film (|SE1-SE2|) can be 0 to 10 mN / m, 0.5 to 9 mN / m, 0.5 to 8 mN / m, 0.5 to 7 mN / m, 1 to 6 mN / m, or 1 to 5 mN / m, but is not limited to these.

[0054] When the difference (|SE1-SE2|) between the surface energy of the first side and the surface energy of the second side of the polyamide-imide-based film according to one embodiment meets the above range, uniform surface properties on both sides of the film are guaranteed, thus enhancing adhesion and coating properties, and providing excellent processability in post-processing, making it suitable for manufacturing electronic device components.

[0055] Conversely, if the difference (|SE1-SE2|) between the surface energy of the first side and the surface energy of the second side of the polyamide-imide-based film according to one embodiment does not meet the above range, the surface properties on both sides of the film may differ, which may lead to problems such as poor adhesion or poor coating uniformity, thereby reducing processability in subsequent processing.

[0056] In one embodiment, the polyamide-imide-based polymer may be a fluorine-free polyamide-imide-based polymer.

[0057] Specifically, fluorine atoms may not be detected when analyzing polyamide-imide-based polymers using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). For example, fluorine atoms may not be detected when using a Bruker FlatQUADXFlash150 instrument for SEM-EDS analysis of polyamide-imide-based polymers.

[0058] In one embodiment, the polyamide-imide-based film may be substantially free of fluorine atoms.

[0059] Specifically, polyamide-imide-based films may contain 500 ppm or less of fluorine atoms.

[0060] More specifically, the polyamide-imide-based film may contain 400 ppm or less, 300 ppm or less, 200 ppm or less, 100 ppm or less, or 50 ppm or less of fluorine atoms.

[0061] For example, polyamide-imide-based films may be free of fluorine atoms, but are not limited to this.

[0062] When the polyamide-imide-based film and / or the polyamide-imide-based polymer according to one embodiment are free of fluorine atoms or contain fluorine atoms within the range described above, an environmentally friendly film can be provided by minimizing the amount of harmful substances contained in the film, the film's lifespan can be extended by improving chemical stability, and it can be exempt from fluorine-related environmental regulations (PFAS). In this case, even if the polyamide-imide-based polymer is free of fluorine atoms, the polyamide-imide-based film according to one embodiment still possesses excellent optical and mechanical properties.

[0063] In one embodiment, based on a film thickness of 50 µm, the modulus of the polyamide-imide-based film may be 5 GPa or higher, 5.2 GPa or higher, 5.3 GPa or higher, 5.4 GPa or higher, 5.5 GPa or higher, 5.6 GPa or higher, 5.7 GPa or higher, 5.8 GPa or higher, 5.9 GPa or higher, or 6 GPa or higher, and may be 8 GPa or lower, 7.5 GPa or lower, or 7 GPa or lower.

[0064] Specifically, based on a film thickness of 50 µm, the modulus of the polyamide-imide-based film can be 5 to 8 GPa, 5 to 7 GPa, 5.4 to 8 GPa, 5.4 to 7 GPa, 6 to 8 GPa, or 6 to 7 GPa.

[0065] When a sample is cut to a length of 10 cm or longer in the direction perpendicular to the main contraction direction, and to a length of 10 mm in the main contraction direction, mounted on a fixture at 10 cm intervals, and then stretched at a rate of 10 mm / min at room temperature until fracture to obtain a stress-strain curve, the slope of the load at the initial deformation in the stress-strain curve can be taken as the modulus (GPa). For example, the modulus can be measured using Instron's universal testing machine UTM 5566A, but it is not limited to this.

[0066] When the modulus of the polyamide-imide-based film according to one embodiment meets the above range, the mechanical strength and durability of the polyamide-imide-based film are enhanced, the heat resistance of the film is improved, and the film is suitable for use in electronic device components such as cover windows.

[0067] On the other hand, if the modulus of the polyamide-imide-based film according to one embodiment does not meet the above range, the film may deform due to heat or external force, or its formability may deteriorate during processing.

[0068] In one embodiment, based on a film thickness of 50 µm, the transmittance of the polyamide-imide-based film at a wavelength of 380 nm can be 3% or less. Specifically, based on a film thickness of 50 µm, the transmittance of the polyamide-imide-based film at a wavelength of 380 nm can be 2.5% or less, 2.3% or less, 2% or less, 1.7% or less, 1.5% or less, 1% or less, 0.7% or less, 0.5% or less, 0.4% or less, 0.35% or less, or 0.3% or less.

[0069] For example, transmittance at a wavelength of 380 nm can be measured using a JASCO V-670 UV / Vis / NIR spectrophotometer, but it is not limited to this.

[0070] When the transmittance of the polyamide-imide-based film according to one embodiment meets the above range at a wavelength of 380 nm, the ultraviolet stability increases, thereby enhancing the optical stability of the film, and the film can be suitable for protecting ultraviolet-sensitive electronic device components and displays.

[0071] In one embodiment, the polyamide-imide-based film does not contain a UV shielding agent.

[0072] In one embodiment, when measured in the visible light wavelength range, the total transmittance of the polyamide-imide-based film can be 78% or higher or 80% or higher. For example, the total transmittance can be 82% or higher, 84% or higher, 85% or higher, or 86% or higher, and can be 100% or lower, 99% or lower, 95% or lower, 90% or lower, 89% or lower, or 88% or lower.

[0073] In one embodiment, the transmittance of the polyamide-imide-based film at a wavelength of 550 nm can be 78% or higher or 80% or higher. For example, the transmittance at a wavelength of 550 nm can be 82% or higher, 84% or higher, 85% or higher, or 86% or higher, and can be 100% or lower, 99% or lower, 95% or lower, 90% or lower, 89% or lower, or 88% or lower.

[0074] The haze of the polyamide-imide-based film can be 1% or lower. Specifically, the haze can be 0.8% or lower, 0.7% or lower, 0.6% or lower, 0.5% or lower, 0.4% or lower, 0.3% or lower, or 0.25% or lower, but is not limited thereto. The haze of the film can be a value measured in the visible light wavelength range (400 to 700 nm).

[0075] Light transmittance and haze can be measured using a Nippon Denshoku Kogyo haze meter NDH-5000W according to JISK 7105 standard.

[0076] The yellowness index of polyamide-imide-based films can be 5 or lower. For example, the yellowness index can be 4.8 or lower, 4.5 or lower, 4.3 or lower, 4.2 or lower, 4.1 or lower, or 4.0 or lower, but is not limited to these.

[0077] The yellowness index can be measured using a spectrophotometer (UltraScan PRO, Hunter Associates Laboratory) according to ASTM-E313 under d65 and 10° conditions.

[0078] In one embodiment, based on a film thickness of 50 µm, the thickness deviation of the polyamide-imide-based film can be 3 µm or less, or 2 µm or less. Additionally, the thickness deviation rate can be 5% or less, 4% or less, or 3% or less, but is not limited thereto.

[0079] In one embodiment, based on a film thickness of 50 µm, the polyamide-imide-based film may have a total transmittance of 80% or higher, a haze of 1% or lower, and a yellowness index of 5 or lower, but is not limited thereto.

[0080] Specifically, based on a film thickness of 50 µm, the polyamide-imide-based film may have a modulus of 6 GPa or higher, a total transmittance of 85% or higher measured in the visible light wavelength range, a haze of 0.5% or lower, and a yellowness index of 4.5 or lower, but is not limited thereto.

[0081] The compressive strength of the polyamide-imide-based film can be 0.4 kgf / µm or higher. Specifically, the compressive strength can be 0.45 kgf / µm or higher, or 0.46 kgf / µm or higher, but is not limited thereto.

[0082] When perforating the polyamide-imide-based film using a 2.5-mm spherical tip in UTM compression mode at a speed of 10 mm / min, the maximum diameter (mm) of the perforation, including cracks, is 60 mm or less. Specifically, the maximum diameter of the perforation can 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.

[0083] The pencil hardness of the polyamide-imide-based film surface can be HB or higher. Specifically, the pencil hardness can be H or higher, or 2H or higher, but is not limited to these.

[0084] The tensile strength of polyamide-imide based films can reach 15 kgf / mm. 2 Or even higher. Specifically, the tensile strength can be 18 kgf / mm. 2 Or higher, 20 kgf / mm 2 Or higher, 21 kgf / mm 2 Or higher, or 22 kgf / mm 2 Or higher, but not limited to this.

[0085] The elongation of the polyamide-imide-based film can be 15% or higher. Specifically, the elongation can be 16% or higher, 17% or higher, or 18% or higher, but is not limited to these.

[0086] When a polyamide-imide-based film with a thickness of 50 µm is folded to a radius of curvature of 3 mm, the number of folds before breakage can be 200,000 or more.

[0087] When the film is folded to a radius of curvature of 3 mm and then unfolded, the fold count is one.

[0088] With the number of folds of polyamide-imide-based films meeting the above range, they can be advantageously applied to foldable or flexible display devices. Specifically, polyamide-imide films can be used in foldable phones, but are not limited to this.

[0089] The surface roughness of polyamide-imide-based films can be from 0.01 µm to 0.07 µm. Specifically, the surface roughness can be from 0.01 µm to 0.06 µm, but is not limited thereto.

[0090] If the surface roughness of a polyamide-imide-based film meets the above range, it may be beneficial to achieve more suitable brightness conditions or texture for its application in display devices.

[0091] The residual solvent content in the polyamide-imide-based film can be 2,500 ppm or lower. Specifically, the residual solvent content can be 2,200 ppm or lower, 2,000 ppm or lower, 1,500 ppm or lower, 1,200 ppm or lower, 1,000 ppm or lower, 800 ppm or lower, 500 ppm or lower, or 300 ppm or lower, but is not limited thereto.

[0092] Residual solvent refers to the solvent that did not evaporate during film production and remains in the final manufactured film.

[0093] If the residual solvent content in the polyamide-imide-based film exceeds the above range, the film's durability may deteriorate, potentially affecting its quality. In particular, it may adversely impact post-processing due to its effect on mechanical strength. Optical properties may worsen due to accelerated moisture absorption, while mechanical properties will be significantly degraded.

[0094] According to one embodiment, a polyamide-imide-based film comprises a polyamide-imide-based polymer prepared by polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound.

[0095] Polyamide-imide-based polymers are polymers that contain repeating units based on imide and repeating units based on amide.

[0096] Specifically, the polyamide-imide-based polymer comprises imide-based repeating units derived from the polymerization of diamine compounds and dianhydride compounds, and amide-based repeating units derived from the polymerization of diamine compounds and dicarbonyl compounds.

[0097] In one embodiment, the polyamide-imide-based polymer may be a polymer of a diamine compound, a dianhydride compound, or a dicarbonyl compound.

[0098] Diamine compounds are compounds that form copolymers by forming imide bonds with dianhydride compounds and amide bonds with dicarbonyl compounds.

[0099] There are no particular limitations on the diamine compound, but it can be an aromatic diamine compound containing an aromatic structure. For example, a diamine compound can be a compound represented by Formula 1 below.

[0100] [Formula 1]

[0101]

[0102] In Equation 1, E can be a self-substituted or unsubstituted divalent C6-C. 30 Alicyclic groups, substituted or unsubstituted divalent C4-C 30 Heterocyclic groups, substituted or unsubstituted divalent C6-C 30 Aromatic ring groups, substituted or unsubstituted divalent C4-C 30 heteroaromatic ring groups, substituted or unsubstituted C1-C 30 alkylene groups, substituted or unsubstituted C2-C 30 alkenyl groups, substituted or unsubstituted C2-C 30 The following groups are used: alkynyl groups, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, and -C(CH3)2-.

[0103] e is an integer from 1 to 5. When e is 2 or greater, two or more E's can be the same or different from each other.

[0104] In Equation 1, (E) e The groups may be selected from, but are not limited to, those represented by formulas 1-1a to 1-14a below.

[0105]

[0106] Specifically, in Equation 1, (E) e The groups may be selected from, but are not limited to, those represented by formulas 1-1b to 1-13b below.

[0107]

[0108] Specifically, in equation 1 above, (E) e It can be represented by equation 1-7b above, but is not limited to this.

[0109] In one embodiment, the diamine compound may comprise compounds without fluorinated substituents. Specifically, the diamine compound may consist of compounds without fluorinated substituents.

[0110] In another embodiment, the diamine compound may comprise a fluorine-free compound. Specifically, the diamine compound may consist of a fluorine-free compound.

[0111] In one embodiment, the diamine compound may comprise a single diamine compound. That is, the diamine compound may consist of a single component.

[0112] For example, diamine compounds may include, but are not limited to, 2,2'-dimethylbenzidine (m-Tolidine) represented by the following formula.

[0113]

[0114] In one embodiment, the diamine compound may consist of 2,2'-dimethylbenzidine (m-Tolidine), but is not limited thereto.

[0115] Dihydride compounds have low birefringence values, which can help improve the optical properties, such as light transmittance, of films containing polyamide-imide-based polymers.

[0116] There are no particular restrictions on the dianhydride compound, but it can be an aromatic dianhydride compound containing an aromatic structure. For example, an aromatic dianhydride compound can be a compound represented by formula 2 below.

[0117] [Equation 2]

[0118]

[0119] In Equation 2, G is selected from substituted or unsubstituted tetravalent C4-C. 30 Alicyclic groups, substituted or unsubstituted tetravalent C4-C 30 Heterocyclic groups, substituted or unsubstituted tetravalent C6-C 30 Aromatic ring groups, or substituted or unsubstituted tetravalent C4-C 30 A heterocyclic ring group, wherein the alicyclic group, heterocyclic group, aromatic ring group, or heterocyclic ring group exists alone, fused together to form a fused ring, or is formed by means of a C1-C group selected from substituted or unsubstituted C1-C2 groups. 30 alkylene groups, substituted or unsubstituted C2-C 30 alkenyl groups, substituted or unsubstituted C2-C 30The bonding groups of the acetylinyl group, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, and -C(CH3)2- are bonded together.

[0120] In Equation 2 above, G can be obtained by substituting or not substituting tetravalent C4-C. 30 Alicyclic group bonding.

[0121] Specifically, G in Formula 2 above can be selected from groups represented by Formulas 2-1a to 2-9a below, but is not limited thereto.

[0122]

[0123] For example, G in Equation 2 can be represented by Equation 2-2a or Equation 2-8a above.

[0124] In one embodiment, the dianhydride compound may comprise compounds without fluorinated substituents. Specifically, the dianhydride compound may consist of compounds without fluorinated substituents.

[0125] In another embodiment, the dianhydride compound may comprise a fluorine-free compound. The dianhydride compound may consist of a fluorine-free compound.

[0126] In another embodiment, the dianhydride compound may consist of a single component or a mixture of two components.

[0127] For example, the dianhydride compound may contain at least one selected from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) having the following structures, but is not limited thereto.

[0128]

[0129]

[0130] Specifically, the dianhydride compound may include 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA).

[0131] In one embodiment, the dianhydride compound may consist of 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), but is not limited thereto.

[0132] Diamine compounds and dianhydride compounds can polymerize to form polyamic acid.

[0133] Subsequently, polyamic acid can be converted into polyimide through a dehydration reaction, and the polyimide contains repeating imide units.

[0134] Polyimide may contain repeating units represented by the following formula A.

[0135] [Formula A]

[0136]

[0137] E, G, and e in equation A above are as described above.

[0138] For example, polyimide may contain repeating units represented by the following formula A-1, but is not limited thereto.

[0139] [Formula A-1]

[0140]

[0141] In equation A-1, n is an integer from 1 to 400.

[0142] There are no particular restrictions on dicarbonyl compounds, but they can be compounds represented by formula 3 below.

[0143] [Formula 3]

[0144]

[0145] In Equation 3, J can be a self-substituted or unsubstituted divalent C6-C. 30 Alicyclic groups, substituted or unsubstituted divalent C4-C 30 Heterocyclic groups, substituted or unsubstituted divalent C6-C 30 Aromatic ring groups, substituted or unsubstituted divalent C4-C 30 heteroaromatic ring groups, substituted or unsubstituted C1-C 30 alkylene groups, substituted or unsubstituted C2-C 30 alkenyl groups, substituted or unsubstituted C2-C 30 The following groups are used: alkynyl groups, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, and -C(CH3)2-.

[0146] j is selected from an integer from 1 to 5. When j is 2 or greater, two or more Js can be the same or different from each other.

[0147] X can be a halogen atom. Specifically, X can be Cl, Br, I, etc. More specifically, X can be Cl, but is not limited to this.

[0148] In equation 3 above, (J) j The groups may be selected from, but are not limited to, those represented by formulas 3-1a to 3-14a below.

[0149]

[0150] Specifically, in equation 3 above, (J) jThe groups may be selected from, but are not limited to, those represented by formulas 3-1b to 3-8b below.

[0151]

[0152] More specifically, (J) in Equation 3 j It can be represented by Equation 3-1b, Equation 3-2b, Equation 3-3b or Equation 3-8b.

[0153] For example, in equation 3 above, (J) j It can be represented by equation 3-1b or equation 3-2b above.

[0154] In one embodiment, a single dicarbonyl compound may be used, 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, at least two dicarbonyl compounds may be used as the dicarbonyl compound, wherein (J) in Formula 3 above... j The groups are selected from those represented by formulas 3-1b to 3-8b above.

[0155] In another embodiment, the dicarbonyl compound may be an aromatic dicarbonyl compound containing an aromatic structure.

[0156] In one embodiment, the dicarbonyl compound may comprise a fluorine-free compound. Specifically, the dicarbonyl compound may consist of a fluorine-free compound.

[0157] Dicarbonyl compounds may include, but are not limited to, terephthaloyl chloride (TPC), 1,1'-biphenyl-4,4'-dicarboxyyl chloride (BPDC), isophthaloyl chloride (IPC), or combinations thereof represented by the following formula.

[0158]

[0159]

[0160]

[0161] In one embodiment, the dicarbonyl compound may consist of terephthaloyl chloride (TPC) and isophthaloyl chloride (IPC), but is not limited thereto.

[0162] Diamine compounds and dicarbonyl compounds can be polymerized to form repeating units represented by the following formula B.

[0163] [Formula B]

[0164]

[0165] E, J, e, and j in equation B are as described above.

[0166] For example, diamine compounds and dicarbonyl compounds can be polymerized to form amide repeating units represented by the following formulas B-1, B-2 and B-3.

[0167] Alternatively, the diamine compound and the dicarbonyl compound can be polymerized to form an amide repeating unit represented by the following formulas B-2 and B-3.

[0168] [Formula B-1]

[0169]

[0170] In equation B-1, x is an integer from 1 to 400.

[0171] [Formula B-2]

[0172]

[0173] In equation B-2, y is an integer from 1 to 400.

[0174] [Formula B-3]

[0175]

[0176] In equation B-3, y is an integer from 1 to 400.

[0177] In one embodiment, the polyamide-imide-based polymer is a polymer of a diamine compound, a dianhydride compound, and a dicarbonyl compound. The diamine compound may be represented by Formula 1 above, the dianhydride compound may be represented by Formula 2 above, and the dicarbonyl compound may be represented by Formula 3 above.

[0178] According to one embodiment, the polyamide-imide-based polymer may contain repeating units represented by formula A and repeating units represented by formula B.

[0179] [Formula A]

[0180]

[0181] [Formula B]

[0182]

[0183] In formulas A and B, E and J are each independently selected from substituted or unsubstituted divalent C6-C. 30 Alicyclic groups, substituted or unsubstituted divalent C4-C 30 Heterocyclic groups, substituted or unsubstituted divalent C6-C 30 Aromatic ring groups, substituted or unsubstituted divalent C4-C 30 heteroaromatic ring groups, substituted or unsubstituted C1-C 30 alkylene groups, substituted or unsubstituted C2-C30 alkenyl groups, substituted or unsubstituted C2-C 30 The following groups are used: alkynyl groups, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, and -C(CH3)2-.

[0184] e and j are each independently selected from integers from 1 to 5.

[0185] When e is 2 or greater, then two or more E's are either the same or different from each other.

[0186] When j is 2 or greater, then two or more J's are either the same or different from each other, and

[0187] G can be selected from substituted or unsubstituted tetravalent C4-C. 30 Alicyclic groups, substituted or unsubstituted tetravalent C4-C 30 Heterocyclic groups, substituted or unsubstituted tetravalent C6-C 30 Aromatic ring groups, or substituted or unsubstituted tetravalent C4-C 30 A heterocyclic ring group, wherein the alicyclic group, heterocyclic group, aromatic ring group or heterocyclic ring group may exist alone, may be fused together to form a fused ring, or may be formed by means of a C1-C group selected from substituted or unsubstituted C1-C2 groups. 30 alkylene groups, substituted or unsubstituted C2-C 30 alkenyl groups, substituted or unsubstituted C2-C 30 The bonding groups of the acetylinyl group, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, and -C(CH3)2- are bonded together.

[0188] In one embodiment, the diamine compound, the dianhydride compound, and the dicarbonyl compound may each be free of fluorine atoms.

[0189] In one embodiment, the diamine compound comprises 2,2'-dimethylbenzidine (m-Tolidine), and the dianhydride compound may comprise, but is not limited to, 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA).

[0190] Polyamide-imide-based polymers can contain repeating units based on imide and repeating units based on amide in a molar ratio of 2:98 to 70:30. Specifically, the molar ratio of repeating units based on imide to repeating units based on amide can be 2:98 to 60:40, 2:98 to 55:45, 2:98 to 50:50, 2:98 to 45:55, 2:98 to 40:60, 2:98 to 35:65, 2:98 to 30:70, 3:97 to 70:30, 3:97 to 60:40, 3:97 to 55:45, 3:97 to 50:50, 3:97 to 45:55, 3:97 to 40:60, 3:97 to 30:70, 5:95 to 70:30, 5:95 to 60:40, 5:95 to 55:45, 5:95 to 50:50, 5:95 to 40:60, 5:95 to 30:70, or 10:90 to 40:60, but not limited to these.

[0191] When the molar ratio of imide-based repeating units to amide-based repeating units is within the above range, the quality reliability of the film can be enhanced in conjunction with typical processing methods, and excellent optical properties, mechanical properties, and UV shielding can be achieved.

[0192] In polyamide-imide-based polymers, the molar ratio of repeating units represented by Formula A to repeating units represented by Formula B can be from 2:98 to 70:30. Specifically, the molar ratio of repeating units represented by Formula A to repeating units represented by Formula B can be 2:98 to 60:40, 2:98 to 55:45, 2:98 to 50:50, 2:98 to 45:55, 2:98 to 40:60, 2:98 to 35:65, 2:98 to 30:70, 3:97 to 70:30, 3:97 to 60:40, 3:97... The times are 55:45, 3:97 to 50:50, 3:97 to 45:55, 3:97 to 40:60, 3:97 to 30:70, 5:95 to 70:30, 5:95 to 60:40, 5:95 to 55:45, 5:95 to 50:50, 5:95 to 40:60, 5:95 to 30:70, or 10:90 to 40:60, but not limited to these.

[0193] In one embodiment, the polyamide-imide-based polymer may comprise one or more types of amide-based repeating units. Specifically, the polyamide-imide-based polymer may comprise two or more types of amide-based repeating units.

[0194] In one embodiment, the polyamide-imide-based polymer may comprise a first amide-based repeating unit and a second amide-based repeating unit. The first amide-based repeating unit may be formed by a reaction between a first dicarbonyl compound and a diamine compound, and the second amide-based repeating unit may be formed by a reaction between a second dicarbonyl compound and a diamine compound.

[0195] In another embodiment, the polyamide-imide-based polymer may comprise a first amide-based repeating unit derived from a first dicarbonyl compound and a second amide-based repeating unit derived from a second dicarbonyl compound. Specifically, the first amide-based repeating unit may be derived from the first dicarbonyl compound, and the second amide-based repeating unit may be derived from the second dicarbonyl compound.

[0196] The first dicarbonyl compound and the second dicarbonyl compound can be different compounds from each other.

[0197] The first dicarbonyl compound and the second dicarbonyl compound may each contain two carbonyl groups. The angle between the two carbonyl groups contained in the first dicarbonyl compound may be greater than the angle between the two carbonyl groups contained in the second dicarbonyl compound.

[0198] In some embodiments, the first dicarbonyl compound and the second dicarbonyl compound may be structural isomers of each other.

[0199] The first dicarbonyl compound and the second dicarbonyl compound can each be an aromatic dicarbonyl compound. Specifically, the first dicarbonyl compound and the second dicarbonyl compound can each have a benzene ring (phenyl group).

[0200] For example, the first dicarbonyl compound and the second dicarbonyl compound can be different aromatic dicarbonyl compounds, but are not limited thereto.

[0201] When the first and second dicarbonyl compounds are aromatic dicarbonyl compounds, they contain a benzene ring. Therefore, they can help improve the mechanical properties, such as pencil hardness and tensile strength, of films containing polyamide-imide-based polymers thus manufactured.

[0202] For example, the angle between the two carbonyl groups contained in the first dicarbonyl compound may be 160 to 180°, and the angle between the two carbonyl groups contained in the second dicarbonyl compound may be 80 to 140°.

[0203] In one embodiment, the first dicarbonyl compound may comprise terephthaloyl chloride (TPC), and the second dicarbonyl compound may comprise isophthaloyl chloride (IPC), but is not limited thereto.

[0204] In one embodiment, the first dicarbonyl compound may be terephthaloyl chloride (TPC), and the second dicarbonyl compound may be isophthaloyl chloride (IPC), but is not limited thereto.

[0205] Using TPC as the first dicarbonyl compound and IPC as the second dicarbonyl compound in an appropriate combination may help increase the viscosity of the film during polymerization, thereby allowing for proper film-forming processes. Films containing the polyamide-imide-based polymer thus produced can have high transmittance and modulus, as well as low haze and yellowness index, while also having improved UV shielding and solvent resistance.

[0206] In one embodiment, the polyamide-imide-based polymer comprises an imide-based repeating unit, a first amide-based repeating unit, and a second amide-based repeating unit, wherein when the sum of the imide-based repeating unit, the first amide-based repeating unit, and the second amide-based repeating unit is 100 mol%, the molar ratio of the first amide-based repeating unit may be 70 mol% or lower. Specifically, when the sum of the imide-based repeating unit, the first amide-based repeating unit, and the second amide-based repeating unit is 100 mol%, the molar ratio of the first amide-based repeating unit may be 65 mol% or lower, 60 mol% or lower, 58 mol% or lower, 55 mol% or lower, 50 mol% or lower, or 40 mol% or lower, but is not limited thereto.

[0207] The molar ratio of the first amide-based repeating unit to the second amide-based repeating unit can be from 21:79 to 79:21. Specifically, the molar ratio of the first amide-based repeating unit to the second amide-based repeating unit can be from 25:75 to 79:21, 30:70 to 79:21, 35:65 to 79:21, 40:60 to 79:21, 21:79 to 75:25, 25:75 to 75:25, 30:70 to 75:25, 35:65 to 75:25, or 40:60 to 75:25, but is not limited thereto.

[0208] By setting the molar ratio of the first amide-based repeating unit and the second amide-based repeating unit within the above range, the physical properties of the polyamide-imide-based film can be controlled within the desired range.

[0209] In addition to the polyamide-imide-based polymer, the polyamide-imide-based film according to one embodiment may further comprise at least one selected from fillers, blue pigments, and UVA absorbers.

[0210] Specifically, the filler may contain oxides, carbonates, or sulfates of metals or non-metals. For example, the filler may contain silicon dioxide, calcium carbonate, barium sulfate, etc., but is not limited to these.

[0211] The filler can be used in particulate form. Furthermore, the surface of the filler is not specially coated, and it can be uniformly dispersed throughout the film.

[0212] Because polyamide-imide-based films contain fillers, a wide viewing angle can be ensured without compromising the film's optical properties. Furthermore, they can not only enhance roughness and winderability but also improve the sliding properties and scratch resistance during film preparation.

[0213] The refractive index of the filler can be from 1.55 to 1.75. Specifically, the refractive index of the filler can be from 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.

[0214] When the refractive index of the filler meets 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 various angles can be improved.

[0215] On the other hand, if the refractive index of the filler exceeds the above range, there may be problems such as the filler being visually obvious on the film or the haze increasing due to the filler.

[0216] The filler content can be from 100 ppm to 15,000 ppm based on the total weight of the solids content of the polyamide-imide-based polymer. Specifically, the filler content can be from 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, but is not limited thereto.

[0217] If the filler content exceeds the above range, the haze of the film will increase sharply, and the filler may aggregate on the film surface, which may result in a visually noticeable foreign object sensation, or may cause problems with sliding performance or reduce the rollability in the manufacturing process.

[0218] In one embodiment, the content of blue pigment may be from 50 to 5,000 ppm based on the total weight of the polyamide-imide-based polymer. Preferably, based on the total weight of the polyamide-imide-based polymer, the content of the blue pigment can 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, but not limited to this.

[0219] UVA absorbers may comprise absorbers used in the art that absorb electromagnetic waves with wavelengths from 10 to 400 nm. For example, UVA absorbers may comprise benzotriazole compounds. Benzotriazole compounds may comprise N-phenolobenzotriazole compounds. Specifically, N-phenolobenzotriazole compounds may comprise N-phenolobenzotriazoles in which the phenolic group is substituted with an alkyl group having 1 to 10 carbon atoms. It may be substituted with two or more alkyl groups, which may be straight-chain, branched, or cyclic.

[0220] In some embodiments, the UVA absorber content may be 0.1 to 10% by weight, based on the total weight of the polyamide-imide-based polymer. Preferably, the UVA absorber content may be 0.1 to 5% by weight, 0.1 to 3% by weight, 0.1 to 2% by weight, 0.5 to 10% by weight, 0.5 to 5% by weight, 0.5 to 3% by weight, 0.5 to 2% by weight, 1 to 10% by weight, 1 to 5% by weight, 1 to 3% by weight, or 1 to 2% by weight, but is not limited thereto, relative to the total weight of the polyamide-imide-based polymer.

[0221] The physical properties of the polyamide-imide-based films described above are based on a thickness of 20 µm to 80 µm. For example, the physical properties of the polyamide-imide-based films are based on a thickness of 50 µm.

[0222] The thickness of the polyamide-imide-based film can be from 20 µm to 100 µm. Specifically, the thickness of the polyamide-imide-based film can be from 20 µm to 80 µm, 20 µm to 60 µm, 20 µm to 50 µm, 25 µm to 100 µm, 25 µm to 80 µm, 25 µm to 60 µm, or 25 µm to 50 µm, but is not limited thereto.

[0223] The thickness of a polyamide-imide-based film can be determined by measuring the thickness at five random points and taking the average. Specifically, a Mitutoyo Corporation digital micrometer 547-401 can be used to measure the thickness of the polyamide-imide-based film at five random points, and the average value is taken as the thickness.

[0224] The characteristics of the composition and properties of polyamide-imide-based films described above can be combined with each other.

[0225] In addition, the presence and content of fluorine atoms in polyamide-imide-based films and / or polyamide-imide-based polymers can be controlled not only by the type of monomer used in the polymerization process, but also by the additives used in the polymerization process and the additives used in post-processing.

[0226] Furthermore, the modulus, transmittance, haze, surface energy, etc. of polyamide-imide-based films can be adjusted by combining the chemical and physical properties of the components constituting the polyamide-imide-based film with the specific conditions of each step in the process for preparing the polyamide-imide-based film, as described below.

[0227] For example, various factors such as the composition and content of the components constituting the polyamide-imide-based film, the residual content of the solvent, the polymerization conditions of the polymer, and the heat treatment conditions in the heat treatment and cooling steps of the film preparation process are combined to achieve the physical properties of the film within the desired range.

[0228] Cover window for a display device

[0229] According to one embodiment, a cover window for a display device comprises a polyamide-imide-based film and a functional layer.

[0230] In polyamide-imide based films, the average surface energy (SE) of the first and second sides of the film is 43 to 70 mN / m, measured according to the following method:

[0231] [Measurement Method]

[0232] A 1 μL water droplet was placed on the film surface using a KRUSS contact angle measuring device, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the polar surface energy (γ) of the film with respect to water. s p );

[0233] Using a KRUSS contact angle measuring device, 1 μL of diiodomethane was dropped onto the film surface, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the nonpolar surface energy (γ) of the film to diiodomethane. s d );and

[0234] The surface energy of a thin film is the sum of its polar surface energy for water and its nonpolar surface energy for diiodomethane.

[0235] Details regarding polyamide-imide-based films are as described above.

[0236] Cover windows for display devices can be advantageously applied to display devices. Specifically, cover windows for display devices can be advantageously applied to foldable display devices, but are not limited thereto.

[0237] Display device

[0238] 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-based film and a functional layer.

[0239] In polyamide-imide based films, the average surface energy (SE) of the first and second sides of the film is 43 to 70 mN / m, according to the following measurement method:

[0240] [Measurement Method]

[0241] A 1 μL water droplet was placed on the film surface using a KRUSS contact angle measuring device, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the polar surface energy (γ) of the film with respect to water. s p );

[0242] Using a KRUSS contact angle measuring device, 1 μL of diiodomethane was dropped onto the film surface, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the nonpolar surface energy (γ) of the film to diiodomethane.s d );

[0243] The surface energy of a thin film is the sum of its polar surface energy for water and its nonpolar surface energy for diiodomethane.

[0244] Details regarding the polyamide-imide-based film and cover window are as described above.

[0245] The display device may be a foldable display device, but is not limited to this.

[0246] Figure 1 This is a schematic exploded view of a display device according to one embodiment. Figure 2 This is a schematic perspective view of a display device according to one embodiment. Figure 3 This is a schematic cross-sectional view of a display device according to one embodiment.

[0247] Specifically, Figures 1 to 3 Each illustration includes a display unit (400) and a cover window (300) disposed on the display unit (400) and an adhesive layer (500) disposed between the display unit (400) and the cover window (300), wherein the cover window includes a polyamide-imide based film (100) having a first side (101) and a second side (102) and a functional layer (200).

[0248] The display unit (400) is used to display images, and it can have flexible characteristics.

[0249] The display unit (400) may be a display panel for displaying images. 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.

[0250] A front polarizer can be positioned on the front side of the organic EL panel. Specifically, the front polarizer can be attached to the side of the organic EL panel where the image is displayed.

[0251] Organic EL panels display images through the self-illumination of pixel units. An organic EL panel may include an organic EL substrate and a driving substrate. The organic EL substrate may contain multiple 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 driving substrate is operatively coupled to the organic EL substrate. That is, the driving substrate may be coupled to the organic EL substrate to apply a driving signal, such as a driving current, thereby driving the organic EL substrate by applying current to each organic electroluminescent unit.

[0252] Additionally, an adhesive layer (500) may be provided between the display unit (400) and the cover window (300). There are no particular restrictions on the adhesive layer (500), as long as it is an optically transparent adhesive layer.

[0253] A cover window (300) may be provided on the display unit (400). The cover window (300) is located on the outside of the display device, thereby protecting the display unit (400).

[0254] The cover window (300) may comprise a polyamide-imide-based film and a functional layer. The functional layer may be at least one selected from hard coatings, antireflective layers, antifouling layers, and antiglare layers. The functional layer may be coated on at least one side of the polyamide-imide-based film.

[0255] According to one embodiment, a polyamide-imide-based film can be applied in thin film form to the exterior of a display device without altering the display driving method, the color filter inside the panel, or the lamination structure, thereby providing a display device with uniform thickness, low haze, high light transmittance, and high transparency. Since no major process changes or increased costs are required, it is advantageous to reduce production costs.

[0256] According to one embodiment, the polyamide-imide-based film exhibits excellent optical properties in terms of high light transmittance, low haze, and low yellowness index, and also possesses excellent mechanical properties such as modulus and flexibility. Due to its excellent surface properties, it can enhance adhesion to other layers and also improve quality uniformity and processability.

[0257] Specifically, the polyamide-imide-based film according to one embodiment can possess excellent optical, mechanical, and surface properties. Therefore, when the polyamide-imide-based film is applied to the cover window of a display device or the display device itself, the quality reliability and yield of the final product can be improved.

[0258] Process for preparing a polyamide-imide based film

[0259] A process for preparing a polyamide-imide-based 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-based polymer solution (S100); casting the polymer solution and then drying it to prepare a gel sheet (S200); and heat-treating the gel sheet (S300) (see...). Figure 4 ).

[0260] According to one embodiment, a process for preparing a polyamide-imide-based film may further include adjusting the viscosity of a polyamide-imide-based polymer solution (S110), allowing the polyamide-imide-based polymer solution to stand (age) (S120), and / or degassing the polyamide-imide-based polymer solution (S130).

[0261] A polyamide-imide-based film is a film in which a polyamide-imide-based polymer is the main component. The polyamide-imide-based polymer is a resin that contains repeating imide-based units and repeating amide-based units as structural units in a predetermined molar ratio.

[0262] In the process for preparing polyamide-imide-based films, a polymer solution for preparing polyamide-imide-based polymers can be prepared by simultaneously or sequentially mixing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent in a reactor and reacting the mixture (S100).

[0263] In one embodiment, a polymer solution can be prepared by simultaneously mixing and reacting a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent.

[0264] In another embodiment, the step of preparing the polymer solution may include: firstly, mixing and reacting a diamine compound and a dianhydride compound in an organic solvent to produce a polyamic acid (PAA) solution; and secondly, mixing and reacting the polyamic acid (PAA) solution with a dicarbonyl compound to form amide and imide bonds. The polyamic acid solution is a solution containing a polymer comprising repeating amic acid units.

[0265] Additionally, the steps for preparing the polymer solution may include: first, mixing and reacting a diamine compound and a dianhydride compound in an organic solvent to produce a polyamic acid solution; subjecting the polyamic acid solution to dehydration to produce a polyimide (PI) solution; and secondly, 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 repeating imide units.

[0266] Furthermore, the steps for preparing the polymer solution may include: firstly, mixing and reacting a diamine compound and a dicarbonyl compound in an organic solvent to produce a polyamide (PA) solution; and secondly, mixing and reacting the polyamide (PA) solution with a dianhydride compound to further form imide bonds. A polyamide solution is a solution containing a polymer having repeating amide units.

[0267] The polymer solution thus prepared can be a solution containing a polymer that contains at least one selected from polyamic acid (PAA) repeating units, polyamide (PA) repeating units, and polyimide (PI) repeating units.

[0268] For example, the polymer contained in the polymer solution may include imide-based repeating units derived from the polymerization of diamine compounds and dianhydride compounds, as well as amide-based repeating units derived from the polymerization of diamine compounds and dicarbonyl compounds.

[0269] Details regarding diamine compounds, dianhydride compounds, and dicarbonyl compounds are as described above.

[0270] The polymer solution may contain 10% to 30% by weight of solids. Specifically, the polymer solution may contain 15% to 25% by weight of solids, but is not limited to this.

[0271] When the solid content in the polymer solution is within the above range, polyamide-imide-based films can be efficiently manufactured in the extrusion and casting steps. Furthermore, the polyamide-imide-based films thus manufactured can possess excellent optical and surface properties.

[0272] In another embodiment, the step of preparing the polymer solution may further include introducing a catalyst.

[0273] Here, the catalyst may contain at least one selected from β-methylpyridine, acetic anhydride, isoquinoline (IQ) and pyridyl compounds, but is not limited thereto.

[0274] Based on 1 mole of polyamic acid, the catalyst can be added in amounts of 0.01 to 0.5 molar equivalents, 0.01 to 0.4 molar equivalents, or 0.01 to 0.3 molar equivalents, but is not limited thereto.

[0275] Adding a catalyst can accelerate the reaction rate and enhance the chemical bonding between or within the repeating unit structures.

[0276] 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-based film can be enhanced, thereby improving the film thickness uniformity.

[0277] Specifically, the steps for 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 a dicarbonyl compound to prepare a second polymer solution having a target viscosity.

[0278] In the steps of preparing the first polymer solution and the second polymer solution, the polymer solutions may have different viscosities from each other. For example, the second polymer solution may have a higher viscosity than the first polymer solution.

[0279] The viscosity of polymer solutions can be measured using TOKI SANGYO's BH-II device at a constant temperature of 25°C, with RPM set to 4 and the number of rotors set to 4.

[0280] In the steps of preparing the first polymer solution and the second polymer solution, the stirring speeds can be different from each other. For example, the stirring speed when preparing the first polymer solution can be faster than the stirring speed when preparing the second polymer solution.

[0281] In another embodiment, the step of preparing the polymer solution may further include adjusting the pH of the polymer solution. In this step, the pH of the polymer solution may be adjusted to 4 to 7, for example, 4.5 to 7.

[0282] The pH of the polymer solution can be adjusted by adding a pH adjuster. There are no particular limitations on the pH adjuster, and it can include amine compounds such as alkoxyamines, alkylamines, and alkanolamines.

[0283] By adjusting the pH of the polymer solution to the above range, defects in the films made from the polymer solution can be prevented, and the desired optical and mechanical properties in terms of yellowness index and modulus can be achieved.

[0284] Based on the total number of moles of monomers in the polymer solution, a pH adjuster can be used in amounts ranging from 0.1 mol% to 10 mol%.

[0285] In one embodiment, the organic solvent may be at least one selected from dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), m-cresol, tetrahydrofuran (THF), and chloroform. Specifically, the organic solvent used in the polymer solution may be dimethylacetamide (DMAc), but is not limited thereto.

[0286] In another embodiment, at least one selected from fillers, blue pigments, and UVA absorbers may be added to the polymer solution.

[0287] Details regarding the types and amounts of fillers, blue pigments, and UVA absorbers are as described above. The fillers, blue pigments, and UVA absorbers can be mixed with the polyamide-imide-based polymer in the polymer solution.

[0288] 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.

[0289] When stored at the above temperatures, the polymer solution is prevented from degrading and the moisture content is reduced, thereby preventing defects in the film produced.

[0290] In one embodiment, the step of preparing the polymer solution may further involve aging the polymer solution or the viscosity-controlled polymer solution (S120).

[0291] Aging treatment can be performed by placing the polymer solution at a temperature of -10 to 10°C for 24 hours or longer. Under these conditions, the polyamide-imide-based polymer or unreacted materials contained in the polymer solution can react or reach chemical equilibrium, thereby homogenizing the polymer solution. The resulting polyamide-imide-based film exhibits substantially uniform mechanical and optical properties throughout its entire surface area. Preferably, the aging treatment can be performed at temperatures of -5 to 10°C, -5 to 5°C, or -3 to 5°C, but is not limited to these temperatures.

[0292] In one embodiment, the step of preparing the polymer solution may further include degassing the polyamide-imide-based polymer solution (S130). The degassing step removes moisture from the polymer solution and reduces impurities, thereby improving the reaction yield and imparting excellent surface appearance and mechanical properties to the finally manufactured film.

[0293] Degassing may include vacuum degassing or purging with an inert gas.

[0294] Vacuum degassing can be performed for 30 minutes to 3 hours after reducing the internal pressure of the tank containing the polymer solution to 0.1 bar to 0.7 bar. Vacuum degassing under these conditions reduces air bubbles in the polymer solution. As a result, surface defects in the thin film produced can be prevented, and excellent optical properties such as haze can be achieved.

[0295] Alternatively, purging can be performed by purging the vessel with an inert gas at an internal pressure of 1 to 2 atm. Purging under these conditions removes moisture from the polymer solution, reduces impurities, thereby increasing reaction yield, and achieves excellent optical properties (such as haze) and mechanical properties.

[0296] The inert gas may be at least one selected from 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.

[0297] Vacuum degassing and purging with inert gas can be performed in separate steps.

[0298] For example, a vacuum degassing step can be performed, followed by a purging step with an inert gas, but this is not the only option.

[0299] Vacuum degassing and / or purging with an inert gas can improve the physical properties of the resulting polyamide-imide-based film surface.

[0300] In the process used to prepare polyamide-imide films, a polymer solution is cast and then dried to prepare gel sheets (S200).

[0301] For example, a polymer solution can be extruded, coated, and / or dried on a support to form a gel sheet.

[0302] The casting thickness of the polymer solution can range from 200 µm to 700 µm. With polymer solutions cast to thicknesses within this range, the final film produced after drying and heat treatment can have appropriate thickness uniformity.

[0303] As mentioned above, the viscosity of the polymer solution at room temperature can be from 100,000 cps to 500,000 cps or from 150,000 cps to 500,000 cps. With viscosity meeting the above range, the polymer solution can be cast into a uniform thickness without defects, and a substantially uniform thickness based on polyamide-imide can be formed during drying without local / partial thickness variations.

[0304] The polymer solution is cast and then dried at temperatures 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 to prepare a gel sheet. Specifically, the polymer solution is dried at a temperature of 90°C to 140°C for 15 to 40 minutes to prepare a gel sheet.

[0305] The solvent in the polymer solution can be partially or completely evaporated during drying to prepare gel sheets.

[0306] In the process for preparing polyamide-imide films, a dried gel sheet can be heat-treated to form a polyamide-imide-based film (S300).

[0307] The heat treatment of gel sheets can be carried out using a heat curing device.

[0308] Specifically, the heat treatment step of the gel sheet may include heat treatment by at least one heater.

[0309] In addition, the heat treatment step of the gel sheet may further include heat treatment with hot air.

[0310] In one embodiment, the step of heat-treating the gel sheet may include heat treatment with hot air; and heat treatment by at least one heater.

[0311] In one implementation, heat can be supplied uniformly during hot air heat treatment. If the heat supply is uneven, a satisfactory surface roughness cannot be obtained, or the surface quality may be uneven, and the surface energy may rise or fall excessively.

[0312] Heat treatment with hot air can be performed at temperatures ranging from 60°C to 500°C for 5 to 200 minutes. Specifically, the gel sheets can be heat-treated at temperatures ranging from 80°C to 300°C at a heating rate of 1.5°C / min to 20°C / min for 10 to 150 minutes. More specifically, the gel sheets can be heat-treated at temperatures ranging from 140°C to 250°C.

[0313] In this case, the initial temperature for heat treatment of the gel sheet using hot air can be 60°C or higher. Specifically, the initial temperature for heat treatment of the gel sheet can be between 80°C and 180°C. Furthermore, the maximum temperature during heat treatment can be between 200°C and 500°C.

[0314] The heat treatment of the gel sheet can be performed in two or more stages. Specifically, the heat treatment of the gel sheet with hot air can be performed sequentially in a first hot air treatment stage and a second hot air treatment stage. The temperature in the second hot air treatment stage can be higher than the temperature in the first hot air treatment stage.

[0315] In one embodiment, the step of heat-treating the gel sheet may include a second-stage heat treatment via at least one heater (more specifically, via multiple heaters).

[0316] Multiple heaters may include multiple heaters spaced apart from each other in the transverse (TD direction) of the gel sheet. Multiple heaters may be mounted on heater mounting components, and two or more heater mounting components may be arranged along the direction of movement of the gel sheet (MD direction).

[0317] At least one heater may include an IR heater. However, the type of at least one heater is not limited to the examples above and can be modified in various ways. Specifically, multiple heaters may each include an IR heater.

[0318] The heat treatment by at least one heater can be carried out at a temperature of 250°C or higher. Specifically, the heat treatment by at least one heater can be carried out at a temperature of 250°C to 400°C for 1 to 30 minutes or 1 to 20 minutes.

[0319] In the heat treatment performed using a heater, the aforementioned temperature is the temperature within the heat treatment apparatus where the gel sheet is located. It corresponds to the temperature measured by a temperature sensor located in the second heat treatment section of the heat treatment apparatus.

[0320] Subsequently, after the heat treatment step of the gel sheet, its cooling step can be carried out simultaneously with the transfer of the cured film.

[0321] The cooling step of transferring the cured film may include a first cooling step at a rate of decreasing the temperature at a rate of 100°C / min to 1,000°C / min and a second cooling step at a rate of decreasing the temperature at a rate of 40°C / min to 400°C / min.

[0322] In this case, specifically, a second cooling step is performed after the first cooling step. The cooling rate of the first cooling step can be faster than the cooling rate of the second cooling step.

[0323] For example, the maximum rate of the first cooling step can be faster than the maximum rate of the second cooling step. Additionally, the minimum rate of the first cooling step can be faster than the minimum rate of the second cooling step.

[0324] If the cooling process is carried out in such a multi-stage manner, the physical properties of the cured film can be further stabilized, and the optical and mechanical properties of the film achieved during the curing process can be maintained more stably for a longer period of time.

[0325] The step of winding a cooled cured film using a winding machine can be performed.

[0326] In this case, the ratio of the speed at which the gel sheet moves on the tape during winding to the speed at which the cured film moves during winding can be from 1:0.95 to 1:1.40. Specifically, the ratio can be from 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.

[0327] If the ratio of the moving speed exceeds the above range, the mechanical properties of the cured film may be impaired, and its flexibility and elasticity may decrease.

[0328] In the process for preparing polyamide-imide based films, the thickness deviation (%) of the film can be from 3% to 30% according to Equation 1 below. Specifically, the thickness deviation (%) of the film can be from 5% to 20%, but is not limited thereto.

[0329] [Equation 1]

[0330] Thickness change (%) = {(M1-M2) / M1} × 100

[0331] In Equation 1, M1 is the thickness of the gel sheet (µm), and M2 is the thickness of the cured film cooled during winding (µm).

[0332] The polyamide-imide-based thin film, prepared using the process described above, exhibits excellent optical, mechanical, and surface properties. Polyamide-imide-based thin films are suitable for a wide range of applications requiring transparency. For example, they can be used not only in display devices but also in solar cells, semiconductor devices, sensors, and more. In particular, since the polyamide-imide-based thin film according to one embodiment does not contain fluorine atoms that may be restricted by environmental regulations, it can be utilized in a wider range of applications.

[0333] Details regarding the preparation of polyamide-imide-based films using the process for preparing polyamide-imide films are as described above.

[0334] Modes used in this invention

[0335] The above description will now be described in detail with reference to embodiments. However, these embodiments are provided to illustrate the present invention, and the scope of the invention is not limited thereto.

[0336] [Example 1]

[0337] 516.0 g of dimethylacetamide (DMAc) was charged into a 1-liter glass reactor equipped with a temperature-controlled double jacket at 20°C under a nitrogen atmosphere. Then, 46.7 g (0.22 mol) of 2,2'-dimethylbenzidine (m-Tolidine), a diamine compound, was slowly added to dissolve it. Subsequently, 10.75 g (0.055 mol) of 1,2,3,4-cyclobutanetetracarboxylic acid dianhydride (CBDA), a dianhydride compound, was slowly added, followed by stirring for 1 hour. Then, 13.4 g (0.066 mol) of terephthaloyl chloride (TPC), a dicarbonyl compound, was added, followed by stirring for 1 hour; and then 20.1 g (0.099 mol) of isophthaloyl chloride (IPC) was added, followed by stirring for 1 hour, thereby preparing a polyamide-imide-based polymer solution. The resulting polymer solution was coated onto a glass plate and then dried with hot air at 80°C for 30 minutes. It was peeled off from the glass plate, fixed onto the pin frame, and processed at temperatures from 80°C to 300°C at a heating rate of 2°C / min to obtain a polyamide-imide-based film with a thickness of 50 µm.

[0338] The specific composition and molar ratio of the polymer based on polyamide-imide are described in the preparation examples in Table 1 below.

[0339] <Examples 2 to 6 and Comparative Examples 1 to 6>

[0340] Except for changes to the composition and molar ratio of the polymer based on polyamide-imide as shown in Table 1 below, each film was prepared in the same manner as in Example 1.

[0341] In Comparative Example 1, after adding terephthaloyl chloride (TPC) and stirring for 1 hour, a white powder precipitate formed in the solution, making subsequent procedures infeasible.

[0342] Furthermore, in Comparative Examples 3 and 5, the viscosity did not increase to the target viscosity for film formation when polymerization was completed, so subsequent film formation processes could not be carried out.

[0343] <Preparation Examples> Polymer Composition

[0344] [Table 1]

[0345]

[0346] <Evaluation Example>

[0347] The following properties of the films prepared in the Examples and Comparative Examples were measured and evaluated. The results are shown in Table 2 below.

[0348] Evaluation Example 1 : Film thickness measurement

[0349] The thickness was measured at five random points using a digital micrometer 547-401 manufactured by Mitutoyo Corporation. The average of these measurements was taken as the thickness.

[0350] Evaluation Example 2: Transmission and haze measurement

[0351] Total transmittance and haze were measured using a Nippon Denshoku Kogyo NDH-5000W haze meter according to JIS K 7105 standard.

[0352] Evaluation Example 3: Yellowness index measurement

[0353] Yellowness index (YI) was measured according to ASTM-E313 standard at d65 and 10° using a spectrophotometer (UltraScan PRO, Hunter Associates Laboratory).

[0354] Evaluation Example 4: Modulus measurement

[0355] The sample was cut at least 10 cm along a direction perpendicular to the main shrinkage direction and 10 mm along the main shrinkage direction. It was then secured in an Instron UTM 5566A universal testing machine using clamps spaced 10 cm apart. The sample was stretched at 10 mm / min at room temperature until fracture, and stress-strain curves were obtained. The slope of the load relative to the initial strain on the stress-strain curve was taken as the modulus (GPa).

[0356] Evaluation Example 5: Measurement of the average of the surface energy (SE) of the first and second side of the film

[0357] Using a KRUSS contact angle measuring device, 1 μL of water was dropped onto the surface of each thin film, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the polar surface energy (γ) of the thin film with respect to water. s p ).

[0358] Using a KRUSS contact angle measuring device, 1 μL of diiodomethane was dropped onto the surface of each thin film, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the nonpolar surface energy (γ) of the film to diiodomethane. s d ).

[0359] The surface energy on one side of the film is the sum of the polar surface energy of water and the nonpolar surface energy of diiodomethane.

[0360] In addition, the average of the surface energy (SE1) of the first side (air side) of the film and the surface energy (SE2) of the second side (band side) of the film is taken as the average of the surface energy (SE) of the first side and the second side of the film.

[0361] [Table 2]

[0362]

[0363] Referring to Table 2, the film according to the embodiment exhibits excellent optical properties such as transmittance, haze, and yellowness index, as well as excellent modulus. Furthermore, the average surface energy of the first and second sides of the film satisfies a certain range, resulting in excellent surface properties.

[0364] [Explanation of reference numerals in the attached diagram]

[0365] 100: Polyamide-imide based films

[0366] 101: First side 102: Second side

[0367] 200: Functional layer; 300: Cover window

[0368] 400: Display unit; 500: Adhesive layer

Claims

1. A polyamide-imide-based thin film comprising a polyamide-imide-based polymer, wherein the average surface energy (SE) of the first and second sides of the film is 43 to 70 mN / m, as measured by the following method: [Measurement Method] A 1 μL water droplet was placed on the film surface using a KRUSS contact angle measuring device, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the polar surface energy (γ) of the film with respect to water. s p ); Using a KRUSS contact angle measuring device, 1 μL of diiodomethane was dropped onto the film surface, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the nonpolar surface energy (γ) of the film to diiodomethane. s d );as well as The surface energy of the film is the sum of the polar surface energy of water and the nonpolar surface energy of diiodomethane.

2. The polyamide-imide based film according to claim 1, wherein, According to the above measurement method, the surface energy (SE1) of the first side of the film is 43 to 60 mN / m, and according to the above measurement method, the surface energy (SE2) of the second side of the film is 43 to 60 mN / m.

3. The polyamide-imide based film according to claim 1, wherein, According to the above measurement method, the difference (|SE1-SE2|) between the surface energy of the first side and the surface energy of the second side of the thin film is 10 mN / m or lower.

4. The polyamide-imide based film according to claim 1, wherein, The polyamide-imide-based polymer is a fluorine-free polyamide-imide-based polymer.

5. The polyamide-imide based film according to claim 1, wherein, The polyamide-imide-based polymer comprises imide-based repeating units and amide-based repeating units in a molar ratio of 2:98 to 40:

60.

6. The polyamide-imide based film according to claim 1, wherein, The polyamide-imide-based polymer is a polymer of diamine compounds, dianhydride compounds, and dicarbonyl compounds. The diamine compound is represented by Formula 1 below. The dianhydride compound is represented by formula 2 below, and The dicarbonyl compound is represented by the following formula 3: <Formula 1> ; <Formula 2> ; <Formula 3> ; In equations 1 to 3 above, E and J are each independently selected from substituted or unsubstituted divalent C6-C. 30 Alicyclic groups, substituted or unsubstituted divalent C4-C 30 Heterocyclic groups, substituted or unsubstituted divalent C6-C 30 Aromatic ring groups, substituted or unsubstituted divalent C4-C 30 heteroaromatic ring groups, substituted or unsubstituted C1-C 30 alkylene groups, substituted or unsubstituted C2-C 30 alkenyl groups, substituted or unsubstituted C2-C 30 The following groups are used: -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, and -C(CH3)2-. e and j are each independently selected from integers from 1 to 5. When e is 2 or greater, then two or more E's are either the same or different from each other. When j is 2 or greater, then two or more J's are either the same or different from each other. G is selected from substituted or unsubstituted tetravalent C4-C. 30 Alicyclic groups, substituted or unsubstituted tetravalent C4-C 30 Heterocyclic groups, substituted or unsubstituted tetravalent C6-C 30 Aromatic ring groups, or substituted or unsubstituted tetravalent C4-C 30 A heterocyclic aromatic ring group, wherein the alicyclic group, the heterocyclic alicyclic group, the aromatic ring group, or the heterocyclic aromatic ring group exists alone, fused together to form a fused ring, or is formed by means of a C1-C group selected from substituted or unsubstituted C1-C2 groups. 30 alkylene groups, substituted or unsubstituted C2-C 30 alkenyl groups, substituted or unsubstituted C2-C 30 The bonding of acetylinyl groups, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, and -C(CH3)2- groups, and X is a halogen atom.

7. The polyamide-imide based film according to claim 6, wherein, The diamine compound includes 2,2'-dimethylbenzidine.

8. The polyamide-imide based film according to claim 6, wherein, The dianhydride compound includes 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA).

9. The polyamide-imide based film according to claim 1, wherein, The polyamide-imide-based polymer comprises an imide-based repeating unit, a first amide-based repeating unit, and a second amide-based repeating unit, and Wherein, when the sum of the imide-based repeating unit, the first amide-based repeating unit, and the second amide-based repeating unit is 100 mol%, the molar ratio of the first amide-based repeating unit is 70 mol% or lower.

10. The polyamide-imide based film of claim 9, wherein, The first amide-based repeating unit is derived from a first dicarbonyl compound, and the second amide-based repeating unit is derived from a second dicarbonyl compound. The angle between the two carbonyl groups in the first dicarbonyl compound is greater than the angle between the two carbonyl groups in the second dicarbonyl compound.

11. The polyamide-imide based film of claim 1, having the following characteristics based on a film thickness of 50 µm: 5 GPa or higher modulus Total transmittance of 80% or higher measured in the visible light wavelength range 1% or less of haze, and A yellowness index of 5 or lower.

12. A cover window for a display device, comprising a polyamide-imide-based film and a functional layer. in, The average surface energy (SE) of the first and second sides of the polyamide-imide-based film is 43 to 70 mN / m, measured using the following method: [Measurement Method] A 1 μL water droplet was placed on the film surface using a KRUSS contact angle measuring device, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the polar surface energy (γ) of the film with respect to water. s p ); Using a KRUSS contact angle measuring device, 1 μL of diiodomethane was dropped onto the film surface, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the nonpolar surface energy (γ) of the film to diiodomethane. s d );as well as The surface energy of the film is the sum of the polar surface energy of water and the nonpolar surface energy of diiodomethane.

13. A display device comprising a display unit and a cover window disposed on the display unit. in, The cover window comprises a polyamide-imide-based film and a functional layer, and The average surface energy (SE) of the first and second sides of the polyamide-imide-based film is 43 to 70 mN / m, measured by the following method: [Measurement Method] A 1 μL water droplet was placed on the film surface using a KRUSS contact angle measuring device, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the polar surface energy (γ) of the film with respect to water. s p ); Using a KRUSS contact angle measuring device, 1 μL of diiodomethane was dropped onto the film surface, and the contact angle with the film surface was measured. The value calculated using the OWRK (Owens, Wendt, Rabel, and Kaelble) equation is called the nonpolar surface energy (γ) of the film to diiodomethane. s d );as well as The surface energy of the film is the sum of the polar surface energy of water and the nonpolar surface energy of diiodomethane.

14. A process for preparing the polyamide-imide-based film of claim 1, comprising: Polymerization of diamine compounds, dianhydride compounds, and dicarbonyl compounds in organic solvents to prepare polyamide-imide-based polymer solutions; The polymer solution is cast and then dried to prepare a gel sheet; as well as The gel sheet is subjected to heat treatment.

15. The process for preparing polyamide-imide-based films according to claim 14, wherein, The step of preparing the polymer solution includes controlling the viscosity of the polymer solution at room temperature to be between 100,000 cps and 500,000 cps.

Images