Polyamide-imide film and flexible display panel including the polyamide-imide film

Abstract

This invention relates to a polyamide-imide film, a window covering film, and a display panel including the window covering film. More specifically, it relates to a polyamide-imide film comprising a polyamide-imide structure derived from dianhydride, diamine, and aromatic diacyl chloride, wherein the chlorine content in the film is 5-30 ppm, and the film will be subjected to a yield of 0.55 W / m at 40°C. 2 The process of irradiating with 340nm long-wave ultraviolet (UVA) for 20 hours followed by a 4-hour block was repeated three times, and the change in the yellow index ΔYI, as measured by ASTM E313, was less than 5.

Description

Technical Field

[0001] The present invention relates to a polyamide-imide film, a window covering film, and a display panel including the window covering film. Background Technology

[0002] Thin display devices such as liquid crystal displays (LCDs) or organic light emitting diode displays (OLEDs) are implemented in the form of touch screen panels. They are widely used not only in smartphones and tablet PCs, but also in various wearable devices and other smart devices characterized by portability.

[0003] In this type of display device based on a portable touchscreen panel, a window cover is provided on the display panel to protect the display from scratches or external impacts. In recent years, with the development of flexible foldable display devices that can be folded and unfolded, the glass of this window cover has been replaced by a film made of plastic material.

[0004] Polyimide resins, due to their high aromatic ring density, are colored yellow or brown, resulting in low transmittance in the visible light region and exhibiting a yellowish hue. Therefore, research is underway to explore the use of polyimide resins as optical films, specifically, as improved methods for use as window covering films for display panels.

[0005] However, the yellow index of polyimide-based resins is above 4, which is insufficient to meet the physical properties required by the market, and they exhibit rapid yellowing upon prolonged exposure to ultraviolet light. Furthermore, even with improved transparency, there are issues with reduced heat resistance and mechanical properties.

[0006] [Existing Technical Documents]

[0007] [Patent Literature]

[0008] Korean Patent Publication No. 10-2055476 (December 6, 2019) Summary of the Invention

[0009] Technical problems to be solved

[0010] One aspect of the present invention for solving the above problems is to provide a film with excellent transparency, excellent mechanical and physical properties and heat resistance, and a yellow index as low as 4 or less, which, even after long-term exposure to ultraviolet light, specifically, after accelerated UV weathering test, shows little change in the yellow index, specifically, a change of less than 5.

[0011] Furthermore, one aspect of the present invention aims to provide a membrane that exhibits excellent flexibility and does not undergo permanent deformation and / or damage even with repeated bending, and thus can be used as a window covering film for flexible displays.

[0012] Technical solution

[0013] One embodiment of the present invention for solving the aforementioned technical problem provides a polyamide-imide membrane comprising a polyamide-imide structure derived from dianhydride, diamine, and aromatic diacyl chloride, wherein the chlorine content in the membrane is 5-30 ppm, and the membrane will be subjected to a flow rate of 0.55 W / m at 40°C. 2 The process of irradiating with 340nm long-wave ultraviolet (UVA) for 20 hours followed by a 4-hour block was repeated three times, and the change in yellow index ΔYI measured according to ASTM E313 satisfied the following formula.

[0014] [Formula 1]

[0015] △YI≤5

[0016] (Where, △YI = Yellow index after accelerated ultraviolet (UV) weathering test - Initial yellow index.)

[0017] As one embodiment, the initial yellow index of the polyamide-imide film can be below 4.

[0018] As one embodiment, the initial yellow index of the polyamide-imide film can be below 2, and the ΔYI can be below 4.

[0019] As one implementation, the content of the aromatic diacyl chloride can be 20-80 moles relative to 100 moles of diamine.

[0020] As one embodiment, the polyamide-imide film may have a modulus of 4 GPa or higher according to ASTM D111 and an elongation at break of 8% or higher.

[0021] As one embodiment, the polyamide-imide film has a transmittance of more than 5% as measured at 388 nm according to ASTM E313, a total light transmittance of more than 87% as measured at 400-700 nm, a haze of less than 2.0%, and a b* value of less than 2.0.

[0022] As one embodiment, the diamine may comprise a fluoroaromatic diamine.

[0023] As one implementation, the dianhydride may comprise aromatic dianhydrides and alicyclic dianhydrides.

[0024] As one embodiment, relative to 100 moles of the diamine, it may comprise 10-50 moles of the aromatic dianhydride and 10-60 moles of the alicyclic dianhydride.

[0025] As one embodiment, the alicyclic dianhydride may comprise cyclobutanetetracarboxylic dianhydride (CBDA).

[0026] As one implementation, the molar ratio of the sum of aromatic diacyl chloride and dianhydride to the diamine can be 1:0.9-1.1.

[0027] As one embodiment, the polyamide-imide film may be composed of a block polyamide-imide structure.

[0028] As one embodiment, the polyamide-imide film may comprise units derived from fluorinated aromatic diamines, units derived from aromatic dianhydrides, units derived from alicyclic dianhydrides, and units derived from aromatic diacyl chlorides.

[0029] As one embodiment, the thickness of the polyamide-imide film can be 10-500 μm.

[0030] Another embodiment of the present invention provides a window covering film comprising: a polyamide-imide film according to the first embodiment; and a coating formed on one or both sides of the polyamide-imide film.

[0031] As one implementation, the coating may be selected from any one or more of the following: hard coating, antistatic layer, anti-fingerprint layer, anti-fouling layer, anti-scratch layer, low refractive layer, anti-reflective layer, and shock-absorbing layer.

[0032] Another embodiment of the present invention provides a flexible display panel comprising a polyamide-imide film according to the first embodiment.

[0033] Beneficial effects

[0034] The polyamide-imide film of one embodiment of the present invention has high visible light transmittance, thus exhibiting excellent transparency, and also possesses excellent mechanical and physical properties and heat resistance, thereby providing physical properties suitable for use as a window cover film for flexible display panels.

[0035] In addition, a film can be provided that satisfies the transparency and mechanical-physical properties described above, and has a yellow index as low as 4 or less, with minimal change in the yellow index even after long-term exposure to ultraviolet light, thus exhibiting excellent weather resistance. Detailed Implementation

[0036] The present invention will now be described in more detail. However, the following embodiments are merely for the purpose of illustrating the invention in detail, and the invention is not limited thereto; the invention can be implemented in various forms.

[0037] Furthermore, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of effectively describing specific embodiments only and is not intended to limit the invention.

[0038] In addition, unless otherwise stated, the singular form used in the specification and claims also includes the plural form.

[0039] Furthermore, unless otherwise stated to the contrary, when a part is described as "containing" or "including" a constituent element, it means that other constituent elements may also be included, rather than excluding other constituent elements.

[0040] In this invention, the "film" is obtained by coating the "polyamide-imide resin solution" onto a substrate and then drying and peeling it off. It may be stretched or unstretched.

[0041] The present invention will now be described.

[0042] The inventors of this invention, through extensive research to solve the aforementioned technical problem, surprisingly discovered that when preparing polyamide-imide using a dicarboxylic acid organic compound with a chloryl group, a membrane with a chlorine content in the final membrane ranging from 5 to 30 ppm can be provided. This membrane exhibits a low yellow index, minimal yellowing even after prolonged exposure to ultraviolet light, and excellent mechanical and physical properties as well as transparency. This led to the completion of this invention.

[0043] In this invention, as long as yellowing occurs in small amounts, specifically, as long as the physical property of a yellow index change ΔYI of less than 5 is met, the composition and preparation method are not limited. The change in the yellow index is measured as follows: at 40°C, at a rate of 0.55 W / m 2The process of irradiating 340nm UVA for 20 hours followed by blocking for 4 hours was repeated three times and measured according to ASTM E313. However, taking one method to obtain the above physical properties as an example, it was surprisingly found that when aromatic diacyl chloride is included as a monomer and the chlorine content in the final film is in the range of 5-30ppm, a film can be provided with a low yellow index that meets the requirements of 4 or less, preferably 3 or less, more preferably 1-2, and yellowing occurs less even after long-term exposure to ultraviolet light, and it has excellent physical properties in terms of both mechanical and physical properties and transparency.

[0044] In this invention, it was confirmed that the yellow index was as high as 2 or more in the range of chlorine content exceeding 30 ppm, and that the rate of change of the yellow index increased sharply after exposure to ultraviolet light, and the mechanical and physical properties decreased.

[0045] Furthermore, when the chlorine content is less than 5 ppm or when diacyl chloride is not used as a monomer to make the chlorine content less than 5 ppm, the transparency and mechanical and physical properties are reduced, especially after UV irradiation, the rate of change of the yellow index increases, so it is not preferred.

[0046] Therefore, it was found that using diacyl chloride as a monomer and controlling the chlorine content to 5-30 ppm resulted in the low yellow index and low yellow index change rate of this invention, thus completing this invention.

[0047] When the chlorine content is adjusted by the following methods, namely, in the step of purifying the oligomer after using sufficient diacyl chloride as a monomer, or after purifying the polyamic acid by imidization and then vacuum drying in the drying step to control the chlorine content, or by adjusting the content to the above-mentioned level by various methods, the desired effect of the present invention can be achieved. Therefore, there are no particular limitations on the method of adjusting the chlorine content.

[0048] Preferably, the chlorine content can be easily controlled within the above-mentioned range by preparing an intermediate polymer in the form of a polyamic acid oligomer using a polymeric composition containing a dibasic base as a monomer, and purifying the intermediate polymer, but it is not limited thereto.

[0049] The significance of this invention lies in confirming that when polyamide-imide is prepared by including diacyl chloride as a monomer, the chlorine content is 5-30 ppm, resulting in a low yellow index, minimal change in the yellow index after long-term maintenance under ultraviolet light, and excellent mechanical and physical properties and transparency.

[0050] A random polyamide-imide resin can be prepared by mixing aromatic diacyl chloride, diamine and dianhydride and polymerizing and imidizing, followed by chlorine purification, instead of preparing intermediates first as in this invention. However, in this case, a large amount of energy is required in the purification or vacuum drying steps, so it is not preferred, but this situation is not excluded in this invention.

[0051] Furthermore, compared to the absence of chlorine in existing technologies, when the present invention contains the chlorine content of the present invention, even with the same polymer structure, the present invention exhibits a superior rate of change in the yellow index, and therefore has a superior retention rate of mechanical and physical properties in environments such as long-term exposure to ultraviolet light, and is therefore preferred.

[0052] The present invention will now be described in detail.

[0053] <Polyamide-imide film>

[0054] In one embodiment of the present invention, the polyamide-imide membrane comprises a polyamide-imide structure derived from dianhydride, diamine, and aromatic diacyl chloride, and the chlorine content in the membrane is 5-30 ppm. It will be subjected to a reaction at 40°C with a flow rate of 0.55 W / m 2 The process of irradiating with 340nm UVA for 20 hours followed by a 4-hour blocking period was repeated three times, and the change in yellow index ΔYI, measured according to ASTM E313, satisfied Equation 1 below. Furthermore, a film can be provided in which the change in yellow index is 5 or less, more preferably 4 or less, and even more preferably 3 or less, exhibiting excellent weather resistance and minimal variation in mechanical and physical properties.

[0055] [Formula 1]

[0056] △YI≤5

[0057] (Where, △YI = Yellow index after UV accelerated weathering test - Initial yellow index.)

[0058] Furthermore, according to ASTM E313, the initial yellow index can be below 4, preferably below 3, more preferably 0.1-3, and even more preferably 0.1-2.

[0059] In one embodiment of the present invention, the modulus of the polyamide-imide film according to ASTM D882 may be 3 GPa or more, 4 GPa or more, or 5 GPa or more, more preferably 7 GPa or more, and the elongation at break may be 8% or more, 12% or more, or 15% or more.

[0060] In one embodiment of the present invention, the transmittance of the polyamide-imide film, measured at 388 nm according to ASTM D1746, can be 5% or more or 5-80%, and the total transmittance measured at 400-700 nm can be 87% or more, 88% or more, 89% or more or 90% or more; the haze according to ASTM D1003 can be less than 2.0%, less than 1.5%, or less than 1.0%; and the b* value according to ASTM E313 can be less than 2.0, less than 1.3, or 0.4-1.3.

[0061] In one embodiment of the present invention, the thickness of the polyamide-imide film can be 10-500 μm, 20-250 μm or 30-100 μm.

[0062] In one embodiment of the present invention, the polyamide-imide film may contain fluorine atoms and an aliphatic ring structure, thereby exhibiting excellent mechanical and physical properties and dynamic bending characteristics.

[0063] The polyamide-imide resin described in this invention is not particularly limited, but it is described in the present invention based on Example 1 as follows.

[0064] As one embodiment of the present invention, as shown in Example 1, as an example of a method for preparing a polyamide-imide-based resin containing fluorine atoms and an aliphatic cyclic structure, it can be prepared as follows, but is not limited thereto.

[0065] First, an amine-terminated polyamide oligomer derived from a first fluoroaromatic diamine and an aromatic diacyl chloride is prepared, and the amine-terminated polyamide oligomer, a second fluoroaromatic diamine, an aromatic dianhydride, and an alicyclic dianhydride can be polymerized to prepare a polyamide imide polymer.

[0066] The first fluoroaromatic diamine and the second fluoroaromatic diamine may be of the same or different types.

[0067] In one embodiment of the present invention, when an oligomer containing an amine-terminated structure formed in the polymer chain by an aromatic diacyl chloride is used as a monomer of a diamine, optical physical properties can be improved, particularly mechanical strength including modulus, and dynamic bending properties can be further improved.

[0068] In one embodiment of the present invention, when the polyamide oligomer block is as described above, the polyamide oligomer containing amine-terminated polyamide, the diamine monomer containing the second fluoroaromatic diamine, and the dianhydride monomer containing the aromatic dianhydride and alicyclic dianhydride of the present invention described above can preferably be used in a molar ratio of 1:0.8-1.1, more preferably in a molar ratio of 1:0.9-1, and even more preferably in a molar ratio of 1:1.

[0069] Furthermore, relative to the total diamine monomers mentioned above, the content of amine-terminated polyamide oligomers is more than 20 mol%, preferably more than 50 mol%, more preferably more than 70 mol%, and more specifically 20-80 mol%, which is more conducive to satisfying the mechanical and physical properties, yellow index, and optical properties of the present invention. However, there is no particular limitation on the content of the amine-terminated polyamide oligomers.

[0070] Furthermore, there are no particular restrictions on the composition ratio of aromatic dianhydride and alicyclic dianhydride, but when considering the transparency, yellow index, mechanical and physical properties of the present invention, it is preferred to use a ratio of 20-80 mol%: 80-20 mol%, but it is not necessarily limited to this.

[0071] In one embodiment of the present invention, the fluoroaromatic diamine component can be used in combination with 2,2'-bis(trifluoromethyl)-benzidine and other known aromatic diamine components, but 2,2'-bis(trifluoromethyl)-benzidine can also be used alone.

[0072] By using the fluorinated aromatic diamine described above as the polyamide-imide-based film, excellent optical properties can be improved based on the mechanical and physical properties required in this invention, and the yellow index can be enhanced. Furthermore, by increasing the tensile modulus of the polyamide-imide-based film, the mechanical strength of the hard coating film can be increased, and the dynamic bending properties can be further improved.

[0073] Aromatic dianhydrides may be used in combination with at least one or more of the following: 4,4'-hexafluoroisopropylidene phthalic anhydride (6FDA), biphenyl tetracarboxylic dianhydride (BPDA), oxyphthalic anhydride (ODPA), sulfonyl phthalic anhydride (SO2DPA), (isopropylidene diphenoxy)bis(phthalic anhydride) (6HDBA), 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic dianhydride (TDA), 1,2,4,5-phenyltetracarboxylic dianhydride (PMDA), benzophenone tetracarboxylic dianhydride (BTDA), bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride (SiDA), and bis(dicarboxyphenoxy)diphenyl sulfide dianhydride (BDSDA), but the present invention is not limited thereto.

[0074] Alicyclic dianhydrides can be selected, for example, from any one or a mixture of two or more of 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), 5-(2,5-dioxotetrahydrofuranyl)-3-methylcyclohexene-1,2-dicarboxylic dianhydride (DOCDA), bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BTA), bicyclooctene-2,3,5,6-tetracarboxylic dianhydride (BODA), 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA), 1,2,4-tricarboxy-3-carboxymethylcyclopentane dianhydride (TMDA), 1,2,3,4-tetracarboxycyclopentane dianhydride (TCDA), and their derivatives.

[0075] In one embodiment of the present invention, when an amide structure is formed within the polymer chain by means of an aromatic diacyl chloride, optical physical properties can be improved, and in particular, mechanical strength, including modulus, can be significantly improved, and dynamic bending properties can be further enhanced.

[0076] Aromatic diacyl chlorides may be selected from, but are not limited to, mixtures of two or more selected from isophthaloyl dichloride (IPC), terephthaloyl dichloride (TPC), 1,1'-biphenyl-4,4'-dicarbonyl dichloride (BPC), 1,4-naphthalenedicarboxylic dichloride (NPC), 2,6-naphthalene dicarboxylic dichloride (NTC), 1,5-naphthalene dicarboxylic dichloride (NEC) and their derivatives.

[0077] The following is an example of a method for preparing polyamide-imide films.

[0078] In one embodiment of the present invention, the polyamide-imide film can be prepared by coating a "polyamide-imide resin solution" containing polyamide-imide resin and solvent onto a substrate, and then drying or drying and stretching. That is, the polyamide-imide film can be prepared by solution casting.

[0079] As an example, a polyamide-imide membrane can be prepared by a method comprising the following steps: reacting a fluoroaromatic diamine with an aromatic diacyl chloride to prepare an oligomer; reacting the prepared oligomer with a fluoroaromatic diamine, an aromatic dianhydride, and an alicyclic dianhydride to prepare a polyamic acid solution; imidizing the polyamic acid solution to prepare a polyamide-imide resin; and coating the polyamide-imide solution to prepare a membrane, wherein the polyamide-imide solution is obtained by dissolving the polyamide-imide resin in an organic solvent.

[0080] In the process of preparing the membrane, the method is not particularly limited as long as the chlorine content of the present invention can be adjusted to 5-30 ppm through coating, drying or stretching steps, stretching and drying steps, etc.

[0081] The following will use the preparation of the block polyamide-imide film based on Example 1 as an example to explain each step in more detail.

[0082] First, the steps of preparing oligomers using diacyl chloride as a monomer may include the following steps: reacting a fluoroaromatic diamine with an aromatic diacyl chloride in a reactor; and purifying and drying the obtained oligomers.

[0083] In this case, a fluoroaromatic diamine is added at a molar ratio of 1.01-2 relative to an aromatic diacyl chloride to prepare an amine-terminated polyamide oligomer monomer. The molecular weight of the oligomer monomer is not particularly limited; for example, a weight-average molecular weight in the range of 1000-3000 g / mol can yield superior physical properties. Furthermore, by suppressing side reactions through oligomer polymerization in the presence of pyridine, a resin with even better physical properties can be prepared.

[0084] As a representative example of the diacyl chloride, aromatic carbonyl halide monomers such as terephthaloyl chloride or isophthaloyl chloride can be used.

[0085] Furthermore, after preparing the oligomer, as a method to reduce the chlorine content, there may be a step of adding it to methanol, water, etc., and washing it several times, or a method of easily reducing the chlorine content by film drying during drying, etc., and there are no particular limitations on this method. In addition, in order to adjust the chlorine content, the oligomer can be prepared and the chlorine content can be reduced by alternating washing with solvent and non-solvent. Furthermore, after washing with water, by vacuum drying at a temperature of 40-60°C or lower for 5 hours or more, specifically 5-10 hours, a resin with less yellowing can be prepared.

[0086] When using scavengers such as amines or salt solutions in this invention, the chlorine content can be reduced, but it cannot be adjusted to the desired 5-30 ppm as in this invention. Even if it can be adjusted to the desired 5-30 ppm as in this invention, according to the inventors' research, the additional compound cannot be completely removed, and further side reactions are caused during the polyimide process. Therefore, the yellow index change rate cannot be maintained at the level of this invention, and it cannot be used in window covering films.

[0087] The organic solvent used in the polymerization reaction in this invention can be any one or more polar solvents selected from dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethyl cellosolve, methyl cellosolve, acetone, ethyl acetate, m-cresol, etc.

[0088] More specifically, the present invention prepares an intermediate in the form of an oligomer containing amide units by reacting a fluoroaromatic diamine with an aromatic diacyl chloride, and then prepares a polyamic acid solution by reacting the oligomer with a fluoroaromatic diamine, an aromatic dianhydride and an alicyclic dianhydride, thereby preparing a polyamide imide resin with a uniform distribution of amide intermediates.

[0089] In one embodiment of the present invention, the weight-average molecular weight of the polyamide-imide resin is not particularly limited, but the weight-average molecular weight of the polyamide-imide resin can be 200,000 g / mol or more, preferably 300,000 g / mol or more, and more preferably 300,000-400,000 g / mol. Within the above range, it has superior mechanical and physical properties and can provide excellent dynamic bending characteristics that do not produce cracks even when subjected to repeated bending, and is therefore preferred, but not necessarily limited thereto.

[0090] As described above, the amide intermediate is uniformly distributed throughout the film, thus providing a film with excellent mechanical and physical properties and excellent optical properties over the entire area, further improving the coatability and coating uniformity of the coating composition used in post-coating processes such as hard coatings, and ultimately further improving the optical and physical properties of the window cover film, thus exhibiting excellent optical properties such as not producing optical patterns such as rainbow patterns and mura.

[0091] In this invention, the imidization step can be carried out by chemical imidization, more preferably by using pyridine and acetic anhydride to chemically imidize the polyamic acid solution. Regarding the reaction temperature, it is preferably below 100°C, below 150°C, and specifically, at a low temperature of 50-150°C, imidization can be carried out using an imidization catalyst and a dehydrating agent, but is not limited thereto. Furthermore, the dehydrating agent can be any one or more selected from acetic anhydride, phthalic anhydride, and maleic anhydride, and is not necessarily limited thereto.

[0092] In addition, polyamide-imide resins can be prepared by mixing additives such as flame retardants, tackifiers, inorganic particles, antioxidants, UV stabilizers, and plasticizers into polyamic acid solutions.

[0093] Furthermore, after imidization, the resin can be purified using a solvent to obtain a solid, and this solid can be dissolved in a solvent to obtain a polyamide-imide solution. The solvent may contain, for example, N,N-dimethylacetamide (DMAc), but is not limited to this.

[0094] The process of preparing a film using a polyamide-imide solution can be performed by coating the polyamide-imide solution onto a substrate and then drying it in a drying step that is divided into drying zones. Furthermore, stretching can be performed before or after drying, and a heat treatment step can be performed after the drying or stretching step, if needed. The substrate can be, for example, glass, stainless steel, or film, but is not limited to these. Coating can be performed using a die coater, air knife coating, reverse roller coating, spraying, blade coating, casting, gravure coating, spin coating, etc.

[0095] In this invention, the chlorine content of the polyamide-imide film can be further adjusted during steps such as stretching, drying, and heat treatment, and is not limited to any one method.

[0096] Window covering film

[0097] Another embodiment of the present invention provides a window covering film comprising the above-described polyamide-imide film and a coating formed on the polyamide-imide film.

[0098] When a hard coating is laminated onto a polyamide-imide film with a specific range of solubility, a window covering film with a haze variation of less than 1.5% and significantly improved visibility can be provided.

[0099] In one embodiment of the present invention, the window covering film can meet all of the following physical properties: a transmittance of more than 3% measured at 388 nm according to ASTM D1746 and a total light transmittance of more than 87%, 88%, or 89% measured at 400-700 nm; a haze of less than 1.5%, 1.2%, or 1.0% according to ASTM D1003; a yellow index of less than 1.5, 1.0, or specifically 0.1-1.5 according to ASTM E313; and a b* value of less than 2.0, 1.5, or 1.2.

[0100] In one embodiment of the invention, the coating is a layer that imparts functionality to a window covering film and can be used for various purposes.

[0101] In specific examples, the coating may include one or more layers selected from hard coating, repair layer, impact diffusion layer, self-cleaning layer, anti-fingerprint layer, anti-scratch layer, low refractive layer and impact absorption layer, but is not limited thereto.

[0102] Even with the various coatings described above formed on the polyamide-imide film, a window cover film can be provided that exhibits excellent display quality and high optical properties, particularly significantly reducing rainbow effects.

[0103] In one embodiment of the invention, the coating can be formed on one or both sides of the polyamide-imide film. For example, the coating can be disposed on the top of the polyamide-imide film, and can be disposed on the top and bottom of the polyamide-imide film respectively. The coating can protect the polyamide-imide film, which has excellent optical and mechanical properties, from external physical or chemical damage.

[0104] In one embodiment of the present invention, the solid content of the formed coating can be 0.01-200 g / m² relative to the total area of ​​the polyamide-imide film. 2 Preferably, the solid content of the formed coating can be 20-200 g / m² relative to the total area of ​​the polyamide-imide film. 2 By providing the above-mentioned weight, functionality can be maintained, and surprisingly, no rainbow effect occurs, thus achieving excellent visibility.

[0105] In one embodiment of the present invention, the coating is specifically formed by coating a polyamide-imide film in the form of a composition comprising a coating solvent for forming the coating. The coating solvent is not particularly limited, but is preferably a polar solvent. For example, the polar solvent may be any one or more solvents selected from ether-based solvents, ketone-based solvents, alcohol-based solvents, amide-based solvents, sulfoxide-based solvents, and aromatic hydrocarbon-based solvents. Specifically, the polar solvent may be any one or more solvents selected from dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, ethyl acetate, propylene glycol methyl ether, m-cresol, methanol, ethanol, isopropanol, butanol, 2-methoxyethanol, methyl cellosolve, ethyl cellosolve, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, methyl phenyl ketone, diethyl ketone, dipropyl ketone, cyclohexanone, hexane, heptane, octane, benzene, toluene, and xylene.

[0106] In one embodiment of the present invention, as a method for forming the coating by applying the composition for forming the coating onto the polyamide-imide film, for example, one or more methods selected from spin coating, dip coating, spray coating, die coating, bar coating, roller coating, meniscus coating, flexographic printing, screen printing, bead coating, air knife coating, reverse roller coating, doctor blade coating, cast coating, and gravure coating can be used, but are not limited thereto.

[0107] Preferably, in one embodiment of the present invention, the coating may be a hard coating. The hard coating may comprise any one or more elements selected from organic and inorganic substances. For example, the organic substance may contain carbon, and may contain any one or more non-metallic elements selected from hydrogen, oxygen, and nitrogen, centered around carbon. The inorganic substance refers to substances other than organic substances, and may contain any one or more metallic elements selected from alkaline earth metals, alkali metals, transition metals, post-transition metals, and metalloids. As an example, the inorganic substance may exceptionally include carbon dioxide, carbon monoxide, diamond, carbonates, etc.

[0108] In one embodiment of the present invention, the hard coating layer can be a single organic layer or an inorganic layer, or a mixed layer of organic and inorganic materials, and is not particularly limited, but preferably contains 10-90% by weight of organic material and 10-90% by weight of inorganic material, more preferably contains 40-80% by weight of organic material and 20-60% by weight of inorganic material. Even when forming a hard coating layer containing organic and inorganic materials as described above, it exhibits excellent bonding with the polyamide-imide film and does not cause light distortion, particularly showing excellent improvement in the iridescent effect.

[0109] In one embodiment of the present invention, the hard coating is not particularly limited. For example, the hard coating may be a layer comprising one or more polymers selected from acrylic polymers, silicone polymers, epoxy polymers and urethane polymers.

[0110] Specifically, when the hard coating is formed on the polyamide-imide film, it prevents a decrease in optical properties, and to improve surface hardness, the hard coating can be formed from a composition comprising an epoxysilane resin for forming the coating. Specifically, the epoxysilane resin can be a siloxane resin containing epoxy groups. The epoxy groups can be cyclic epoxy groups, aliphatic epoxy groups, aromatic epoxy groups, or mixtures thereof. The siloxane resin can be a polymer compound in which silicon atoms form covalent bonds with oxygen atoms.

[0111] Preferably, for example, the epoxysiloxane resin can be a silsesquioxane resin. Specifically, the epoxysiloxane resin can be a compound in which the silicon atoms of the silsesquioxane compound are directly substituted with epoxy groups, or where the substituents replacing the silicon atoms are substituted with epoxy groups. As a non-limiting example, the epoxysiloxane resin can be a silsesquioxane resin substituted with 2-(3,4-epoxycyclohexyl) or 3-epoxypropoxy (Glycidoxy).

[0112] The epoxysiloxane resin can be prepared in the presence of water by hydrolysis and condensation reactions between an epoxy-containing alkoxysilane and different types of alkoxysilanes. Furthermore, the epoxysiloxane resin can be formed by the polymerization of silane compounds containing epoxycyclohexyl groups.

[0113] For example, the alkoxysilane compound having an epoxy group may be selected from one or more of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, and 3-epoxypropoxypropyltrimethoxysilane.

[0114] According to one embodiment of the present invention, the weight-average molecular weight of the epoxy siloxane resin can be 1000-20000 g / mol, but is not limited to this. Having a weight-average molecular weight within the above range provides an appropriate viscosity, thereby improving the flowability, coatability, curing reactivity, etc., of the composition used to form the coating, and also increasing the surface hardness of the hard coating.

[0115] In one embodiment of the invention, the epoxy silicone resin may be contained in the composition, with a weight ratio of 20-65% by weight, preferably 20-60% by weight, relative to the total weight of the composition used to form the coating. Containing the epoxy silicone resin within the above range can increase the surface hardness of the hard coating and induce uniform curing, thus preventing physical defects such as cracks caused by partial over-curing.

[0116] In one embodiment of the present invention, the composition for forming the coating may further comprise a crosslinking agent and an initiator.

[0117] Specifically, the crosslinking agent is not particularly limited as long as it can form crosslinking bonds with the epoxy siloxane resin, thereby curing the composition used to form the coating and increasing the hardness of the hard coating. However, for example, the crosslinking agent can be selected from (3,4-epoxycyclohexyl)methyl 3',4'-epoxycyclohexane carboxylate, cyclohexane-1,2-dicarboxylic acid diglycidyl ester, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-m-dioxane, bis(3,4-epoxycyclohexylmethyl) adipic acid ester, bis(3,4-epoxy-6-methylcyclohexyl) adipic acid ester, 3,4-epoxy-6-methylcyclohexylmethyl-3',4'-epoxy-6'-methylcyclohexane carboxylate, 1,4-cyclohexanedimethyl The compound may be selected from (3,4-epoxycyclohexane carboxylate), ethylene bis(3,4-epoxycyclohexane carboxylate), 3,4-epoxycyclohexylmethyl (meth)acrylate, bis(3,4-epoxycyclohexylmethyl)adipate, 4-vinylcyclohexene dioxide, vinylcyclohexene monoxide, 1,4-cyclohexanediethanol diglycidyl ether, and 2,2'-[(1-methylethylene)bis(4,1-cyclohexyloxymethylene)]bisepoxyethylene, preferably a compound comprising two 3,4-epoxycyclohexyl groups, and is selected from (3,4-epoxycyclohexyl)methyl 3',4'-epoxycyclohexane carboxylate and bis(3,4-epoxycyclohexylmethyl)adipate.

[0118] In one embodiment of the invention, the content of the crosslinking agent is not particularly limited; for example, 5-150 parts by weight of the crosslinking agent may be included relative to 100 parts by weight of epoxysiloxane resin. Furthermore, according to one embodiment of the invention, 3-30% by weight of the crosslinking agent may be included relative to the total weight of the composition used to form the coating, preferably 5-20% by weight. When the content of the crosslinking agent is within the above range, the coatability and curing reactivity of the composition used to form the coating can be improved.

[0119] In one embodiment of the present invention, the initiator may be a photoinitiator or a thermal initiator. Preferably, the initiator may be a photoinitiator, for example, the photoinitiator may comprise a cationic photoinitiator. The cationic photoinitiator can initiate the polymerization of the epoxysiloxane resin and the epoxy monomer.

[0120] Specifically, the cationic photoinitiator may be selected from one or more of onium salts and organometallic salts, but is not limited thereto. For example, the cationic photoinitiator may be selected from one or more of diaryliodoonium salts, triarylsulfonium salts, aryl diazonium salts, and iron-aromatic complexes, but is not limited thereto.

[0121] In one embodiment of the present invention, the content of the photoinitiator is not particularly limited; for example, 1-15 parts by weight of the photoinitiator may be included relative to 100 parts by weight of epoxysiloxane resin. Furthermore, according to one embodiment of the present invention, 0.1-10% by weight of the photoinitiator may be included relative to the total weight of the composition used to form the coating, preferably 0.3-5% by weight. When the content of the photoinitiator is within the above range, the hard coating exhibits excellent curing efficiency, and after curing, it prevents the degradation of physical properties caused by residual components.

[0122] In one embodiment of the present invention, the composition for forming the coating may further comprise one or more additives selected from fillers, slip agents, light stabilizers, thermal polymerization inhibitors, leveling agents, lubricants, antifouling agents, thickeners, surfactants, defoamers, antistatic agents, dispersants, initiators, coupling agents, antioxidants, ultraviolet (UV) stabilizers, and colorants, but is not limited thereto.

[0123] More specifically, the hard coating may also contain inorganic particles to impart hardness.

[0124] The inorganic particles are preferably silicon dioxide, more preferably surface-treated silicon dioxide, but are not limited thereto. In this case, the surface treatment may include functional groups that can react with the aforementioned crosslinking agent.

[0125] According to one embodiment, the average diameter of the inorganic particles can be 1-500 nm, preferably 10-300 nm, but is not limited thereto.

[0126] When forming the hard coating as described above on existing polyamide-imide films, the rainbow effect cannot be overcome due to light distortion. However, with the polyamide-imide film of the present invention, even when forming the hard coating as described above, the rainbow effect hardly occurs, and excellent visibility can be achieved.

[0127] In one embodiment of the invention, the window covering film may further include a substrate layer. The substrate layer may be formed on the other side of the uncoated polyamide-imide film.

[0128] In one embodiment of the present invention, the polyamide-imide film can be laminated on a substrate layer after being formed into a film, and can be laminated after coating with a polyamic acid resin composition as a precursor of the polyamide-imide film, but there is no particular limitation as long as the above-mentioned laminated structure can be formed.

[0129] In one embodiment of the present invention, the substrate layer is not particularly limited to any commonly used window covering film substrate film. For example, the substrate layer may contain one or more selected from ester-based polymers, carbonate-based polymers, styrene-based polymers, and acrylic-based polymers. In specific examples, the substrate layer may contain one or more selected from polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polybutylene naphthalate, polycarbonate, polystyrene, and polymethyl methacrylate, but is not limited thereto.

[0130] In one embodiment of the present invention, the substrate layer may be a single layer or a multilayer consisting of two or more layers. Specifically, the substrate layer may be a composite layer consisting of optical adhesive layers at the interfaces of two or more substrate films.

[0131] In one embodiment of the present invention, the thickness of the substrate layer can be 50-300 μm, preferably 100-300 μm, and more preferably 150-250 μm. By having the thickness described above, mechanical and physical properties are satisfied, and when laminating polyamide-imide films, light distortion can be significantly reduced.

[0132] In one embodiment of the present invention, in a specific example, the optical adhesive layer may comprise one or more selected from optically clear adhesive (OCA), optically clear resin (OCR), and pressure-sensitive adhesive (PSA), but is not limited thereto.

[0133] In one embodiment of the present invention, a second optical adhesive layer may be included at the interface between the substrate layer of the window covering film and the polyamide-imide film.

[0134] Specifically, the second optical adhesive layer formed at the interface between the substrate layer and the polyamide-imide film can be made of the same or different material as the optical adhesive layer in the substrate layer. For example, the second optical adhesive layer can be formed with a thickness of 20-120 μm, preferably 20-80 μm. When formed with a thickness within the above range, the window cover film as a whole can achieve excellent optical properties and improve optical distortion.

[0135] In one embodiment of the present invention, the window covering film has high surface hardness and excellent flexibility, and is lighter and more durable than tempered glass, thus making it an excellent outermost window substrate for a flexible display panel.

[0136] Another embodiment of the present invention provides a display device comprising a display panel and the aforementioned window cover film formed on the display panel.

[0137] In one embodiment of the present invention, the display device is not particularly limited to any field requiring excellent optical characteristics, and a display panel suitable for the display device can be selected and provided. Preferably, the window cover film can be applied to a flexible display device. In specific examples, it can be applied to any one or more image display devices selected from liquid crystal display devices, electroluminescent display devices, plasma display devices, field emission display devices, etc., but is not limited thereto.

[0138] The display device including the window cover film of the present invention exhibits excellent display quality and significantly reduces light-induced distortion, especially significantly improving the rainbow effect that produces rainbow patterns, and has excellent visibility, thus minimizing eye fatigue for the user.

[0139] The present invention will now be described in more detail based on embodiments and comparative examples. However, the embodiments and comparative examples described below are merely examples for illustrating the present invention in more detail, and the present invention is not limited to the embodiments and comparative examples described below.

[0140] The physical properties are measured as follows.

[0141] 1) Pencil hardness

[0142] For the membrane, according to JISK5400, using a load of 750g, a 20mm line was drawn at a speed of 120mm / s according to the hardness of a pencil (Mitsubishi Corporation), and this was repeated more than 5 times. The pencil hardness was measured based on the condition that more than 2 scratches were produced.

[0143] 2) Modulus / Elongation at Break

[0144] According to ASTM D882, a polyamide-imide film with a length of 50 mm and a width of 10 mm was measured using an Instron UTM 3365 at 25°C and under tensile conditions of 50 mm / min.

[0145] Measure the film thickness and input the value into the instrument. Modulus is expressed in GPa, and elongation at break is expressed as a percentage.

[0146] 3) Light transmittance

[0147] According to ASTM D1746, for a 50 μm thick film, the total transmittance was measured across the entire wavelength range of 400-700 nm using a spectrophotometer (Nippon Denshoku, COH-400), and the single-wavelength transmittance was measured at 388 nm using a UV / Vis (Shimadzu, UV3600) spectrophotometer. The units are percentages.

[0148] 4) Haze

[0149] Measurements were performed according to ASTM D1003 standard, using a 50 μm thick film as a reference, and measured using a spectrophotometer (Nippon Denshoku Kogyo Co., Ltd., COH-400). Units are percentages.

[0150] 5) Weight-average molecular weight (Mw) and polydispersity index (PDI)

[0151] The weight-average molecular weight and polydispersity index of the prepared membrane were measured as follows.

[0152] First, the membrane sample was dissolved in DMAc eluent containing 0.05 M LiBr and used as the sample. Measurements were performed using a GPC system (Waters GPC system, Waters 1515 isocrantic HPLC pump, Waters 2414 refractive index detector), with a GPC column connecting an Olexis column, a Polypore column, and a mixed D column. DMAc solution was used as the solvent, and polymethyl methacrylate (PMMAASTD) was used as the standard. Analysis was conducted at 35°C and a flow rate of 1 mL / min.

[0153] 6) Yellow Index (YI) and b* value

[0154] According to ASTM E313 standard, measurements were performed using a colorimeter (HunterLab, ColorQuest XE) with a film thickness of 50 μm as a reference.

[0155] 7) Accelerated UV weathering resistance test

[0156] It will operate at 0.55 W / m at 40℃. 2 The process of irradiating with 340nm UVA for 20 hours followed by a 4-hour block was repeated three times, and the yellow index was measured according to ASTM E313. That is, the following process was repeated a total of three times at 40°C with a concentration of 0.55W / m 2 Irradiate with 340nm UVA for 20 hours, then block the UVA at 40°C for 4 hours, and repeat the irradiation and blocking process.

[0157] The rate of change of the yellow index is calculated using the following formula.

[0158] △YI = Yellow Index after UV Accelerated Weathering Test - Initial Yellow Index before UV Accelerated Weathering Test

[0159] 8) Methods for measuring chlorine content

[0160] Record the weight of each 20 mL glass vial after adding 0.05 g ± 0.02 g of sample. Add 5 mL ± 1 mL of dimethylformamide (DMF) solution and record the weight. Seal the vial with a paraffin film. Place the vial in a shaker and shake for at least 30 minutes to completely dissolve the sample. Record the weight of each individual 20 mL glass vial. Add 10 mL ± 2 mL of distilled water and record the weight again. Slowly add 5 mL ± 1 mL of the DMF solution containing the sample and vortex mix. Record the weight again. Place the vial in a shaker and shake for at least 3 hours. Separate the precipitate and water-soluble layer using a centrifuge at 1000 rpm. Filter the water-soluble layer sample using a water-soluble 0.2 μm PTFE syringe filter and analyze the resulting aqueous solution by ion chromatography. Using standard solutions of chlorine at each concentration, a calibration curve is obtained by using the concentration-area ratio. The sample is then analyzed, and the concentration is calculated using its area.

[0161] [Example 1]

[0162] Preparation of Polyamide-Imide Films

[0163] In a reactor, terephthaloyl chloride (TPC) and 2,2'-bis(trifluoromethyl)-benzidine (TFMB) were added to a mixed solution of dichloromethane and pyridine, and the mixture was stirred at 25°C for 2 hours under a nitrogen atmosphere. At this point, the molar ratio of TPC:TFMB was set to 300:400, and the solid content was adjusted to 10% by weight. The reactants were then precipitated in excess methanol, filtered, and washed three times with water to obtain a solid. This solid was then vacuum-dried at 50°C for 6 hours to obtain an oligomer with a molecular weight (Formula Weight, FW) of 1670 g / mol.

[0164] N,N-dimethylacetamide (DMAc) as a solvent, 100 moles of the oligomer, and 28.6 moles of 2,2'-bis(trifluoromethyl)-benzidine (TFMB) were added to the reactor and stirred thoroughly. After confirming that the solid raw materials were completely dissolved, fumed silica (with a surface area of ​​95 m²) was added relative to the solids. 2 (g, <1μm) was added to DMAc at a concentration of 1000 ppm, dispersed ultrasonically, and then added. 64.3 mol of cyclobutanetetracarboxylic dianhydride (CBDA) and 64.3 mol of 4,4'-hexafluoroisopropylidene phthalic anhydride (6FDA) were added sequentially and stirred thoroughly, then polymerized at 40 °C for 10 hours. At this point, the solid content was 20% by weight. Next, relative to the total dianhydride content, 2.5 mol of pyridine and acetic anhydride were added sequentially to the solution, and the mixture was stirred at 60 °C for 12 hours.

[0165] After polymerization, the polymerization solution was precipitated in excess methanol, then filtered to obtain a solid. This solid was then vacuum-dried at 50°C for 8 hours to obtain polyamide-imide powder. The powder was then diluted and dissolved in DMAc to a concentration of 20% by weight to prepare a polyamide-imide-based resin solution.

[0166] The polyamide-imide resin solution was coated onto a support (glass) using a coater, then dried at 80°C for 30 minutes, then at 100°C for 1 hour, and finally cooled at room temperature to prepare a film. Subsequently, a stepwise heat treatment was performed at 100-200°C and 250-300°C at a heating rate of 20°C / min for 2 hours.

[0167] The prepared polyamide-imide film had a thickness of 48 μm, a chlorine content of 33 ppm, a total light transmittance of 90.1%, a haze of 0.33%, a yellow index (YI) of 1.7, a b* value of 1.1, a modulus of 7.3 GPa, an elongation at break of 24.1%, a weight-average molecular weight of 325,000 g / mol, a polydispersity index (PDI) of 2.2, and a pencil hardness of HB / 750g. The yellow index after accelerated UV weathering testing was 4.7, confirming an increase of 3 compared to the initial yellow index.

[0168] [Example 2]

[0169] In Example 1, the membrane was prepared by the same method as in Example 1, except that TPC:TFMB was used at a molar ratio of 200:400.

[0170] The prepared polyamide-imide film had a thickness of 52 μm, a chlorine content of 14 ppm, a total light transmittance of 90.4%, a haze of 0.28%, a yellow index (YI) of 1.5, a b* value of 1.0, a modulus of 8.5 GPa, an elongation at break of 20.5%, a weight-average molecular weight of 310,000 g / mol, a polydispersity index (PDI) of 2.6, and a pencil hardness of HB / 750g. The yellow index after accelerated UV weathering testing was 4.3, confirming an increase of 2.8 compared to the initial yellow index.

[0171] [Example 3]

[0172] In Example 1, the membrane was prepared by the same method as in Example 1, except that TPC:TFMB was used at a molar ratio of 120:400.

[0173] The prepared polyamide-imide film exhibits the following physical properties: a thickness of 50 μm, a chlorine content of 25 ppm, a total light transmittance of 90.3%, a haze of 0.31%, a yellow index (YI) of 1.8, a b* value of 1.2, a modulus of 7.9 GPa, an elongation at break of 19.5%, a weight-average molecular weight of 300,000 g / mol, a polydispersity index (PDI) of 2.1, and a pencil hardness of HB / 750 g. The yellow index after accelerated UV weathering testing was 4.8, confirming an increase of 3 compared to the initial yellow index.

[0174] [Example 4]

[0175] In Example 1, the membrane was prepared by the same method as in Example 1, except that 100 moles of oligomer and 28.6 moles of 2,2'-bis(trifluoromethyl)-benzidine (TFMB) were used in the preparation of the polyamide imide, and 128.6 moles of 4,4'-hexafluoroisopropylidene phthalic anhydride (6FDA) were used.

[0176] The prepared polyamide-imide film had a thickness of 42 μm, a chlorine content of 6 ppm, a total light transmittance of 90.5%, a haze of 0.26%, a yellow index (YI) of 1.7, a b* value of 1.2, a modulus of 7.9 GPa, an elongation at break of 24.5%, a weight-average molecular weight of 340,000 g / mol, a polydispersity index (PDI) of 2.1, and a pencil hardness of HB / 750g. The yellow index after accelerated UV weathering testing was 4.6, confirming an increase of 2.9 compared to the initial yellow index.

[0177] [Comparative Example 1]

[0178] Under a nitrogen atmosphere, N,N-dimethylacetamide (DMAc) was packed into a reactor, and 100 moles of 2,2'-bis(trifluoromethyl)-benzidine (TFMB) were added and stirred thoroughly. Then, 40 moles of 4,4'-hexafluoroisopropylidene phthalic anhydride (6FDA) were added and stirred thoroughly until dissolved. Next, 40 moles of cyclobutanetetracarboxylic dianhydride (CBDA) were added and stirred thoroughly until dissolved. Then, 20 moles of terephthaloyl chloride (TPC) were added and stirred for 6 hours to dissolve and react, thereby preparing a polyamic acid resin composition. The monomers were adjusted to achieve a solid content of 6.5% by weight. Pyridine and acetic anhydride, each in 2.5 times the moles of total dianhydride, were added sequentially to the composition, and the mixture was stirred at 60°C for 1 hour. The solution was then precipitated in excess methanol, filtered to obtain a solid, and vacuum dried at 50°C for at least 6 hours to obtain polyamide-imide powder. The powder was diluted and dissolved in DMAc to 20% by weight to prepare the composition.

[0179] The composition was cast onto a glass substrate to form a film, and the film was then separated from the substrate. The prepared polyamide-imide film had a thickness of 42 μm, a chlorine content of 48 ppm, a yellow index (YI) of 3.5, and a yellow index of 9 after accelerated UV weathering testing, showing an increase of 5.5 compared to the initial yellow index. Furthermore, the modulus of the prepared film was confirmed to be 4 GPa, and the elongation at break was 15.5%, lower than that of Example 1. Additionally, the weight-average molecular weight was confirmed to be 260,000 g / mol, the pencil hardness to be HB / 750 g, the total light transmittance to be 87.5%, and the haze to be 1.5%.

[0180] [Comparative Example 2]

[0181] In Example 1, except that the process of mixing the polyamide-imide resin solution with excess water and filtering to obtain the resin, dissolving the obtained resin again in DMAc solution and precipitating it in excess water was performed 5 times, and then dissolving the obtained polyamide-imide in DMAc to prepare the membrane, was carried out by the same method as in Example 1.

[0182] The obtained film has a thickness of 52 μm, a chlorine content of 4 ppm, a total light transmittance of 90.1%, a haze of 0.8%, a yellow index (YI) of 3, a b* value of 1.2, a modulus of 5 GPa, and an elongation at break of 15%. The yellow index after UV accelerated weathering test is 8.3, and it is confirmed that the yellow index has increased by 5 compared with the initial yellow index.

[0183] [Comparative Example 3]

[0184] In Example 1, the same method as in Example 1 was used, except that the polyamic acid oligomer was prepared, precipitated in excess methanol, filtered, and the resulting solid was vacuum dried at 90°C for 48 hours to obtain the oligomer. The obtained film had a thickness of 52 μm, a chlorine content of 68 ppm, a total light transmittance of 89.5%, a haze of 1.1%, a yellow index (YI) of 3, a b* value of 1.2, a modulus of 5.5 GPa, an elongation at break of 16%, a weight-average molecular weight of 260,000 g / mol, and a pencil hardness of HB / 750 g. The yellow index after accelerated UV weathering testing was 11, confirming an increase of 7 compared to the initial yellow index.

[0185] [Table 1]

[0186]

[0187] As described above, the present invention has been illustrated with specific content and limited embodiments, but this is only provided to help to understand the present invention more fully. The present invention is not limited to the above embodiments, and those skilled in the art can make various modifications and variations based on the above description.

[0188] Therefore, the concept of this invention should not be limited to the illustrated embodiments, and all claims of this invention and all contents that are equivalent to or have equivalent variations of the claims are within the scope of the concept of this invention.

Claims

1. A polyamide-imide membrane comprising a polyamide-imide structure derived from dianhydride, diamine, and aromatic diacyl chloride, wherein the chlorine content in the membrane is 5-30 ppm, and the membrane is subjected to a flow rate of 0.55 W / m at 40°C. 2 The process of irradiating with 340nm long-wave ultraviolet (UVA) for 20 hours followed by a 4-hour blocking period was repeated three times. The change in yellow index ΔYI, measured according to ASTM E313, satisfied the following formula: the initial yellow index was below 2, and the modulus according to ASTM D111 was above 7.3 GPa. [Formula 1] △YI≤4 in, △YI = Yellow index after accelerated UV weathering test - Initial yellow index.

2. The polyamide-imide film according to claim 1, wherein, The content of the aromatic diacyl chloride is 20-80 moles relative to 100 moles of diamine.

3. The polyamide-imide film according to claim 1, wherein, The polyamide-imide film has a transmittance of 5% or more at 388 nm as measured according to ASTM E313, a total transmittance of 87% or more as measured at 400-700 nm, a haze of 2.0% or less, and a b* value of 2.0 or less.

4. The polyamide-imide film according to claim 1, wherein, The diamine includes fluoroaromatic diamines.

5. The polyamide-imide film according to claim 1, wherein, The dianhydrides include aromatic dianhydrides and alicyclic dianhydrides.

6. The polyamide-imide film according to claim 5, wherein, Relative to 100 moles of the diamine, it comprises 10-50 moles of the aromatic dianhydride and 10-60 moles of the alicyclic dianhydride.

7. The polyamide-imide film according to claim 5, wherein, The alicyclic dianhydride includes cyclobutanetetracarboxylic dianhydride (CBDA).

8. The polyamide-imide film according to claim 1, wherein, The molar ratio of the sum of aromatic diacyl chloride and dianhydride to the diamine is 1:0.9-1.

1.

9. The polyamide-imide film according to claim 1, wherein, The polyamide-imide film is composed of a block polyamide-imide structure.

10. The polyamide-imide film according to claim 1, wherein, The polyamide-imide film comprises units derived from fluorinated aromatic diamines, units derived from aromatic dianhydrides, units derived from alicyclic dianhydrides, and units derived from aromatic diacyl chlorides.

11. The polyamide-imide film according to claim 1, wherein, The thickness of the polyamide-imide film is 10-500 μm.

12. A window covering film, comprising: The polyamide-imide film according to any one of claims 1 to 11; as well as A coating formed on one or both sides of the polyamide-imide film.

13. The window covering film according to claim 12, wherein, The coating is selected from one or more of the following: hard coating, antistatic layer, anti-fingerprint layer, anti-fouling layer, anti-scratch layer, low refractive layer, anti-reflective layer, and shock-absorbing layer.

14. A flexible display panel comprising the polyamide-imide film according to any one of claims 1 to 11.

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