Polyimide film having low reflexibility

Abstract

The present invention relates to a polyimide film copolymerized to include at least one diamine and at least one acid anhydride, wherein at least one of the diamine and the acid anhydride comprises a cycloaliphatic compound, and more specifically, to a polyimide film that can be used as a cover window for a display through excellent visibility by securing low reflectance and high transmittance characteristics.

Description

Technology Field

[0001] The present invention relates to a polyimide film, and more specifically, to a polyimide film that can be used as a cover window for a display by securing low reflectance and high transmittance characteristics and providing excellent visibility. Background Technology

[0002] A cover window is applied to the surface of display devices, such as Liquid Crystal Displays (LCDs) and Organic Light Emitting Displays (OLEDs), to protect the panel. Conventionally, tempered glass has been primarily used as the material for cover windows due to its excellent flatness, heat resistance, chemical resistance, and barrier performance against moisture or gas, as well as its low coefficient of linear expansion (CTE) and high light transmittance.

[0003] Meanwhile, flexible displays such as curved displays and in-folding displays are being developed recently. In order to be applied to these flexible displays, the cover window must also be flexible; however, conventional glass cover windows are generally heavy and brittle, and lack flexibility, making them unsuitable for flexible displays.

[0004] To address the aforementioned problems, cover windows made of plastic materials with relatively free moldability have recently been proposed. Cover windows made of plastic materials have the advantages of being lightweight, resistant to breakage, and capable of realizing various designs. Currently, plastic materials with excellent transparency, such as polycarbonate, polyethylene terephthalate, and polymethyl methacrylate, are mainly used for cover windows. Although these materials have the advantage of excellent transparency, their application is limited because their heat resistance is poor with a glass transition temperature (Tg) of 150°C or lower, and their chemical resistance and mechanical strength are poor.

[0005] In addition, the cover window is placed at the outermost edge of the flexible display device. If the aforementioned materials are applied and continuously exposed to external ultraviolet rays, yellowing occurs, which adversely affects the visibility of the display. In particular, external light continuously encountered in daily life has a significant impact on the visibility of the display device, and one of the factors affecting this visibility is reflectivity. As the aforementioned reflectivity value increases, it can easily obstruct or interfere with the image generation seen from the side of the display device, which can cause the visibility of the display device to deteriorate rapidly. The problem to be solved

[0006] The present invention has been devised to solve the aforementioned problems, and its technical objective is to provide a novel polyimide film that can be applied as a cover window by exhibiting high transparency and excellent mechanical properties while securing improved low reflectivity characteristics.

[0007] Other objects and advantages of the present invention may be more clearly explained by the following detailed description of the invention and claims. means of solving the problem

[0008] To achieve the above technical objectives, the present invention provides a polyimide film copolymerized to include at least one diamine and at least one acid anhydride, wherein at least one of the diamine and the acid anhydride comprises an alicyclic compound, and the polyimide film has a reflectance of 10.0% or less at a wavelength of 550 nm as measured by ASTM E1164.

[0009] For example, in one embodiment of the present invention, the polyimide film may have a difference between transmittance and reflectance of 78.0 to 83.0% at 550 nm.

[0010] For example, in one embodiment of the present invention, the polyimide film has a thickness of 30 to 100 μm, a transmittance of 85% or more at a wavelength of 550 nm, and a yellowness of 5 or less according to ASTM E313-73.

[0011] For example, in one embodiment of the present invention, the polyimide film may have an average coefficient of linear expansion (CTE) of 100 ppm / ℃ or less measured at 50 to 250℃, and a Young's modulus of 2 to 8 GPa.

[0012] For example, in one embodiment of the present invention, the alicyclic compound comprises at least one of an alicyclic diamine and an alicyclic acid anhydride, and may be included in a ratio of 10 to 100 mol% based on the total mol% of the combined diamine and acid anhydride.

[0013] For example, in one embodiment of the present invention, the diamine may include at least one aromatic diamine, or may include an aromatic diamine and a diamine alicyclic.

[0014] For example, in one embodiment of the present invention, the aromatic diamine and the alicyclic diamine may be included in a ratio of 0 to 100 : 100 to 0 mol% based on 100 mol% of the total diamine.

[0015] For example, in one embodiment of the present invention, the aromatic diamine may include one or more selected from the group consisting of a fluorinated first diamine; a sulfonated second diamine, a hydroxyl third diamine, an ether-based fourth diamine, and a non-fluorinated fifth diamine.

[0016] For example, in one embodiment of the present invention, the acid anhydride may include at least one aromatic acid anhydride, or may include an aromatic acid anhydride and a cycloaliphatic acid anhydride.

[0017] For example, in one embodiment of the present invention, the aromatic acid anhydride and the alicyclic acid anhydride may be included in a ratio of 10 to 100 : 0 to 90 mol% based on 100 mol% of the total acid anhydride.

[0018] For example, in one embodiment of the present invention, the aromatic acid anhydride may include one or more selected from the group consisting of a fluorinated aromatic first acid anhydride, a non-fluorinated aromatic second acid anhydride, and a sulfonated aromatic third acid anhydride.

[0019] For example, in one embodiment of the present invention, the ratio of the number of moles (a / b) of the diamine (a) and the acid anhydride (b) may be in the range of 0.7 to 1.3.

[0020] For example, in one embodiment of the present invention, the polyimide film can be used as a cover window of a display device. Effects of the invention

[0021] According to one embodiment of the present invention, low reflectance characteristics can be secured by adopting a specific component constituting a polyimide film and controlling its content, while simultaneously continuously exhibiting optical characteristics such as high light transmittance and low yellowness.

[0022] In addition, the present invention exhibits an excellent modulus and a low coefficient of linear expansion (CTE), thereby improving the workability and reliability of the final product.

[0023] Accordingly, the polyimide film of the present invention can be usefully applied as a cover window for display devices in the field of this field, such as flat panel displays and flexible displays, and can also be applied to other IT products, electronic products, home appliances, etc. known in the field.

[0024] The effects according to the present invention are not limited to those exemplified above, and a wider variety of effects are included in this specification. Specific details for implementing the invention

[0025] The present invention will be described in detail below. Embodiments of the present invention are provided to more completely explain the invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the present invention is not limited to the following embodiments. Throughout this specification, the same reference numerals refer to the same structures.

[0026] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) may be used in a meaning commonly understood by those skilled in the art to which the present invention pertains. Additionally, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless explicitly and specifically defined otherwise.

[0027] Furthermore, throughout the specification, when a part is described as "comprising" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components. Also, throughout the specification, the terms "above" or "on" mean not only cases where a part is located above or below the subject part but also cases where another part is located in between, and do not necessarily mean that it is located above based on the direction of gravity. Furthermore, in this specification, terms such as "first," "second," etc., are used to distinguish components from one another, rather than indicating an arbitrary order or importance.

[0028] Polyimide Film

[0029] A polyimide resin film according to one embodiment of the present invention can be used as a transparent film that can be provided in a display device, and more specifically as a cover window of a flexible display.

[0030] Here, the cover window refers to a film placed at the outermost edge of a flexible display device to protect the display device. This cover window may be a window film alone, or a film in which an adhesive layer and a window coating layer are formed on a separate substrate film made of an optically transparent resin.

[0031] Meanwhile, the cover window of a display exposed to the outside must not only possess excellent optical properties such as flexibility and transmittance in daily life, but also support low reflectivity in consideration of the display's visibility. However, conventional plastic cover windows have focused only on improving light transmittance and have not recognized low reflectivity, and there have been limitations in lowering reflectivity below a certain range.

[0032] In contrast, the polyimide film according to the present invention is characterized by having a lower reflectance compared to conventionally known plastic films, as well as controlling the reflectance value to be below a predetermined range. Accordingly, the polyimide film can be applied without limitation to display devices known in the art, and in particular, when applied as a cover window for in-folding or out-folding type foldable mobile phones, it can continuously exhibit excellent visibility and high optical properties through its low reflectance characteristics.

[0033] In one embodiment of the present invention, the polyimide film comprises at least one diamine; and at least one acid anhydride, wherein at least one of the diamine and the acid anhydride is copolymerized with an alicyclic compound, and the reflectance at a wavelength of 550 nm may be 10.0% or less.

[0034] Here, 'reflectance' refers to specific reflectance characteristics of the polyimide film in question measured according to a prescribed evaluation standard (ASTM E1164 standard). The aforementioned reflectance and corresponding values ​​may differ from those measured by other evaluation standards.

[0035] For one specific example, the polyimide film according to the present invention, at a thickness of 30 to 100 μm, may have a reflectance of 10.0% or less at a wavelength of 550 nm as measured by ASTM E1164, specifically 9.8% or less. In particular, it is preferable that the reflectance be 9.5% or less in order to improve the optical properties and visibility of the film.

[0036] In the case of the polyimide film of the present invention having the aforementioned reflectance parameters and corresponding values, the reflectance due to external light is minimized even under harsh environments, so excellent visibility of the display screen can be continuously maintained.

[0037] In order for the polyimide film according to the present invention to be used as a cover window for a mobile communication terminal or a tablet PC, it is desirable to simultaneously possess excellent optical properties, such as high transparency and light transmittance, and mechanical properties, in addition to the aforementioned low reflectivity characteristics.

[0038] In another embodiment of the present invention, the polyimide film may have a thickness of 30 to 100 μm and a light transmittance of 85% or more at a wavelength of 550 nm, specifically 89% or more, more specifically 90% to 99%. In addition, the yellowness (YI) according to ASTM E313-73 may be 5 or less, specifically 4 or less, more specifically 3.5 or less.

[0039] In another embodiment of the present invention, the polyimide film may have a difference between transmittance and reflectance of 78.0 to 83.0% at 550 nm, specifically 79 to 82.0%, and more specifically 80 to 82.0%.

[0040] In another embodiment of the present invention, the polyimide film may have a Young's modulus of 2 to 8 GPa, specifically 3 to 6 GPa. In terms of simultaneously exhibiting mechanical hardness and excellent flexibility, it may be 4 to 6 GPa. Here, Young's modulus refers to a value measured according to ISO 527-3. If the modulus is smaller than the aforementioned value, it is difficult to exhibit sufficient hardness, and if it is larger than the aforementioned value, flexibility decreases, which may lead to reduced foldability.

[0041] In another embodiment of the present invention, the polyimide film may have an average coefficient of linear expansion (CTE) measured at 50 to 250°C at a thickness of 30 to 100 μm, which is 100 ppm / °C or less, specifically 70 ppm / °C or less, and more specifically 30 to 65 ppm / °C.

[0042] The polyimide film according to the present invention is not particularly limited to the components constituting the polyimide resin and / or its composition, etc., as long as it satisfies the low reflectance characteristics measured according to a predetermined evaluation standard and the corresponding values.

[0043] For a specific example, the polyimide film comprises at least one diamine and at least one acid anhydride, wherein at least one of the diamine and the acid anhydride is copolymerized with an alicyclic compound.

[0044] In the present invention, the alicyclic compound may include at least one of a conventional alicyclic diamine and an alicyclic acid anhydride known in the art. Such alicyclic diamines and alicyclic acid anhydrides are not particularly limited as long as they include at least one alicyclic ring in their molecular structure, and the specific components may be the diamine and acid anhydride components described below. The alicyclic compound may be included in a ratio of 10 to 100 mol% based on the total mol% of the combined diamine and acid anhydride, for example, the total mol% of 200 mol% of the combined diamine and acid anhydride, and specifically, the ratio may be 50 to 90 mol%.

[0045] The diamine (a) component constituting the polyimide film of the present invention is not limited to compounds having an intramolecular diamine structure, and any ordinary diamine compound known in the art may be used without limitation. For example, aromatic, alicyclic, or aliphatic compounds having a diamine structure, or combinations thereof, may be used.

[0046] In particular, in the present invention, considering optical properties such as low reflectance, high light transmittance, and low YI of the polyimide film, at least one aromatic diamine may be used, or said aromatic diamine and a diamine alicyclic may be mixed.

[0047] Specific examples of the aforementioned aromatic diamines include diamines having fluorinated substituents, such as fluorinated, sulfone, hydroxyl, ether, and non-fluorinated diamines, which may be used individually or appropriately combined to form one or more types. Accordingly, in the present invention, as a diamine compound, a fluorinated aromatic primary diamine, a sulfone-based aromatic secondary diamine, a hydroxyl-based aromatic tertiary diamine, an ether-based aromatic quaternary diamine, and a non-fluorinated aromatic quintide diamine may be used individually or in a mixed form of two or more types.

[0048] Non-limiting examples of available diamine monomers (a) include oxydianiline (ODA), 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (2,2'-TFDB), 2,2'-bis(trifluoromethyl)-4,3'-diaminobiphenyl (2,2'-Bis(trifluoromethyl)-4,3'-Diaminobiphenyl), 2,2'-bis(trifluoromethyl)-5,5'-diaminobiphenyl (2,2'-Bis(trifluoromethyl)-5,5'-Diaminobiphenyl), 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether (6-FODA), bisaminohydroxyphenyl hexafluoropropane (DBOH), and bisaminophenoxyphenyl Hexafluoropropane (4BDAF), bis-aminophenoxyphenylpropane (6HMDA), bis-aminophenoxydiphenylsulfone (DBSDA), bis(4-aminophenyl)sulfone (4,4'-DDS), bis(3-aminophenyl)sulfone (3,3'-DDS), sulfonyldiphthalic anhydride (SO2DPA), 4,4'-oxydianiline (4,4'-ODA), or a mixture of one or more of these may be applied.

[0049] Considering the high transparency, high glass transition temperature, and low yellowness of the polyimide film, fluorinated primary diamines such as 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (2,2'-TFDB) and 1,4-Bis(4-amino-2-trifluoromethylphenoxy)benzene (6-FAPB), which can induce linear polymerization, may be used. Additionally, sulfonated secondary diamines such as bis(4-aminophenyl)sulfone (4,4'-DDS) or 3,3'-DDS may be used. Additionally, hydroxyl-based tertiary diamines such as 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane, BIS-AP-AF) may be used. In addition, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6-FODA) or oxydianiline (ODA) can be used as the ether-based quaternary diamine. In addition, 2,2-bis(3-amino-4-methylphenyl)-hexafluoropropane (BIS-AT-AF), m-tolidine, or p-phenylenediamine (p-PDA) can be used as the non-fluorinated quinary diamine.

[0050] In the diamine monomer (a) of the present invention, the content of the fluorinated aromatic first diamine, sulfonated aromatic second diamine, hydroxyl aromatic third diamine, etheric aromatic fourth diamine, non-fluorinated aromatic fifth diamine, etc., is not particularly limited, but may be 0 to 100 mol% based on 100 mol% of the total diamine, specifically 10 to 90 mol%, more specifically 20 to 80 mol%. However, the content of at least one of the first diamine to fifth diamine is included to satisfy 100 mol% of the total diamine.

[0051] Non-limiting examples of alicyclic diamines compatible with the aforementioned aromatic diamines include 2,2-bis(3-amino-4-hydroxycyclohexyl)hexafluoropropane, 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane (MACM), 4,4'-methylenebicyclohexylamine (PACM), 1,3-bis(aminomethyl)cyclohexane (1,3-BAC), 1,4-bis(aminomethyl)cyclohexane (1,4-BAC), cis-1,2-cyclohexanedimethaneamine, trans-1,2-cyclohexanedimethaneamine, 1,4-cyclohexyldiamine (CHDA), bis(4-aminocyclohexyl) ethers, or their Mixtures, etc. can be used.

[0052] In the diamine monomer (a) of the present invention, the mixing ratio of the aromatic diamine and the alicyclic diamine may be 0 to 100 : 100 to 0 mol% based on 100 mol% of the total diamine, specifically 0 : 100 to 90 : 10 mol%, and more specifically 0 : 100 to 50 : 50 mol%.

[0053] The acid anhydride (b) monomer constituting the polyimide film of the present invention may be any ordinary compound known in the art having an acid anhydride structure within the molecule without limitation. For example, aromatic, alicyclic, or aliphatic compounds having a dianhydride structure, or combinations thereof, may be used, and specifically, at least one aromatic acid anhydride may be used, or said aromatic acid anhydride and alicyclic acid anhydride may be mixed.

[0054] One specific example of the aforementioned aromatic acid anhydride is that a fluorinated aromatic first acid anhydride, a non-fluorinated aromatic second acid anhydride, and a sulfonated aromatic third acid anhydride may each be used individually or in a combination form in which at least two or more of these are mixed.

[0055] The above-mentioned fluorinated first acid dianhydride monomer is not particularly limited as long as it is an aromatic acid dianhydride with a fluorine substituent introduced. Non-limiting examples of usable fluorinated first acid dianhydrides include 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6-FDA) and 4-(trifluoromethyl)pyromellitic dianhydride (4-TFPMDA). These may be used alone or in a mixture of two or more. Among the fluorinated acid dianhydrides, 6-FDA is a suitable compound for transparency because it has a very high property of limiting the formation of change transfer complexes (CTCs) between and within molecular chains.

[0056] In addition, the non-fluorinated second acid dianhydride monomer is not particularly limited as long as it is a non-fluorinated aromatic acid dianhydride in which a fluorine substituent has not been introduced. Non-limiting examples of usable non-fluorinated third acid dianhydride monomers include pyromellitic dianhydride (PMDA), 3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride (BPDA), benzophenone tetracarboxylic dianhydride (BTDA), oxydiphthalic dianhydride (ODPA), 4,4-(4,4-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), and bis(carboxyphenyl)dimethylsilanedianhydride (SiDA). These can be used individually or in combination of two or more types.

[0057] In addition, sulfone-based tertiary acid anhydride monomers are not specifically limited as long as they are acid anhydrides with introduced sulfone groups, and an example is 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA).

[0058] In the acid anhydride monomer (b) of the present invention, the content of the fluorinated aromatic first acid anhydride, non-fluorinated aromatic second acid anhydride, sulfonated aromatic third acid anhydride, etc., is not particularly limited. For example, these may each be included in a range of 0 to 100 mol% based on 100 mol% of the total acid anhydride, specifically 10 to 90 mol%, and more specifically 20 to 80 mol%. However, the content of at least one of the first to third acid anhydrides is included to satisfy 100 mol% of the total acid anhydride.

[0059] The above-mentioned alicyclic acid anhydride is not particularly limited to any compound having an acid anhydride structure while possessing an alicyclic ring that is not an aromatic ring within the compound. Non-limiting examples of alicyclic acid anhydrides compatible with the aforementioned aromatic acid anhydrides include cyclobutane tetracarboxylic dianhydride (CBDA), 1,2,3,4-cyclopentane tetracarboxylic dianhydride (CPDA), bicyclo[2,2,2]-7-octene-2,3,5,6-tetracarboxylic acid dianhydride (BCDA), 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic anhydride (TDA), 1,1'-bicyclohexane-3,3',4,4'-tetracarboxylic dianhydride (H-BPDA), 1,2,4,5-cyclohexane-tetracarboxylic dianhydride (H-PMDA), and cyclopentanone There are bis-spironorbornane (cyclopentanone bis-spironorbornane, cpODA), bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic2,3:5,6-dianhydride(7Cl,8Cl), or mixtures of one or more of these.

[0060] In the acid anhydride monomer (b) of the present invention, the mixing ratio of the aromatic acid anhydride and the alicyclic acid anhydride may be 10 to 100 : 0 to 90 mol% based on 100 mol% of the total acid anhydride, and specifically may be 30 : 70 to 90 : 10 mol%.

[0061] As a preferred example of the present invention, a sulfonate-based tertiary acid anhydride and a non-fluorinated secondary acid anhydride may be mixed as the acid anhydride (b). In this case, the ratio of their use is not particularly limited and, for example, may be 10 to 40 : 60 to 90 mol% ratio.

[0062] In the polyamic acid composition constituting the polyimide film of the present invention, the ratio (a / b) of the moles of the diamine component (a) and the moles of the acid anhydride component (b) may be 0.7 to 1.3, preferably 0.8 to 1.2, and more preferably in the range of 0.9 to 1.1.

[0063] The polyimide film of the present invention, configured as described above, comprises at least one diamine and at least one acid anhydride, and includes a repeating unit obtained by including at least one alicyclic diamine or alicyclic acid anhydride. When a predetermined alicyclic repeating unit obtained from such alicyclic diamine / acid anhydride is included, it exhibits low reflectance characteristics, thereby ensuring excellent visibility. In particular, it can be confirmed that the reflectance decreases as the content ratio of the cycloaliphatic structure in the main chain of the polyimide containing aromatic diamine / acid anhydride and alicyclic diamine / acid anhydride increases, and accordingly, optical properties including visibility and mechanical properties can be optimized.

[0064] The polyamic acid composition of the present invention may use any organic solvent known in the art without limitation as a solvent for the solution polymerization reaction of the aforementioned monomers. Examples of usable solvents include one or more polar solvents selected from m-cresol, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), acetone, diethyl acetate, and dimethyl phthalate (DMP). In addition, low-boiling point solutions such as tetrahydrofuran (THF) and chloroform, or solvents such as γ-butyrolactone, may be used. At this time, the content of the solvent (first solvent for polymerization) is not particularly limited, but to obtain a suitable molecular weight and viscosity of the polyamic acid composition (polyamic acid solution), it is preferably 50 to 95 weight% based on the total weight of the polyamic acid composition, and more preferably may be included in the range of 70 to 90 weight%.

[0065] The polyimide according to the present invention may be a high-heat resistant resin prepared by solution polymerizing using the aforementioned diamine, acid anhydride, and, if necessary, a solvent to prepare a polyamic acid composition (polyamic acid derivative), and then performing ring-closing dehydration at a high temperature to produce an imid. The polyamic acid composition described above may be prepared by reacting at least one acid anhydride and at least one diamine in an organic solvent, and for example, to improve the physical properties of the polyimide, the diamine (a) and the acid anhydride (b) may be adjusted to an equivalent ratio of approximately 1:1. The composition of such a polyamic acid composition is not particularly limited and, for example, may be composed of 2.5 to 25.0 weight% of acid anhydride, 2.5 to 25.0 weight% of diamine, and the remainder of an organic solvent satisfying 100 weight% of the composition, based on 100 weight% of the total weight of the polyamic acid composition. Here, the content of the organic solvent may be 70 to 90 weight%. In addition, the polyamic acid composition may contain 30 to 70 weight% of acid anhydride and 30 to 70 weight% of diamine based on 100 weight% of the solid content, but is not particularly limited thereto.

[0066] The polyamic acid composition configured as described above may have a viscosity in the range of about 1,000 to 200,000 cps, preferably about 5,000 to 50,000 cps. When the viscosity of the polyamic acid composition falls within the aforementioned range, it is easy to control the thickness during coating of the polyamic acid composition, and the coating surface can be uniformly exhibited.

[0067] If necessary, the polyamic acid composition may include a small amount of at least one additive, such as a plasticizer, an antioxidant, a flame retardant, a dispersant, a viscosity modifier, or a leveling agent, within a range that does not significantly impair the purpose and effect of the present invention.

[0068] The polyimide resin film according to the present invention can be manufactured according to conventional methods known in the art, for example, by coating (casting) the aforementioned polyamic acid composition onto a substrate, such as a glass substrate, and then inducing an imid ring-closing reaction (imidazation) for 0.5 to 8 hours while gradually increasing the temperature in the range of 30 to 350°C.

[0069] At this time, the coating method may use any conventional method known in the industry without limitation, and may be carried out by at least one method selected from the group consisting of spin coating, dip coating, solvent casting, slot die coating, and spray coating, for example. The polyamic acid composition may be coated at least once so that the thickness of the colorless transparent polyimide resin layer is from several hundred nm to several tens of μm.

[0070] In the method for manufacturing a polyimide film according to the present invention, the imidation method applied in the step of casting the polymerized polyamic acid onto a support to imidize it may be a thermal imidation method, a chemical imidation method, or a combination of a thermal imidation method and a chemical imidation method.

[0071] The thermoimidation method is a method of obtaining a polyimide film by casting a polyamic acid composition (polyamic acid solution) onto a support and heating it for 1 to 10 hours while gradually increasing the temperature in a temperature range of 30 to 400°C.

[0072] In addition, the chemical imidation method is a method of adding a dehydrating agent, represented by acid anhydrides such as acetic anhydride, and an imidation catalyst, represented by amines such as isoquinoline, β-picoline, and pyridine, to a polyamic acid composition. When the thermal imidation method or the thermal imidation method is used in combination with this chemical imidation method, the heating conditions of the polyamic acid composition may vary depending on the type of polyamic acid composition, the thickness of the polyimide film being produced, etc.

[0073] To explain more specifically the case where the above-mentioned thermoimidation method and chemical imidation method are used in combination, a polyimide film can be obtained by adding a dehydrating agent and an imidation catalyst to a polyamic acid composition, casting it onto a support, and then heating it at 80 to 300°C, preferably 150 to 250°C, to activate the dehydrating agent and the imidation catalyst, thereby partially curing and drying it.

[0074] The polyimide resin formed in this manner is a polymer material containing imide rings, and based on the chemical stability of the imide rings, it has excellent heat resistance, chemical resistance, wear resistance, and electrical properties. The polyimide resin may be in the form of a random copolymer or a block copolymer. The thickness of the polyimide film is not particularly limited and can be appropriately adjusted depending on the field of application. For example, it may be in the range of 10 to 150 μm, and preferably 30 to 100 μm.

[0075] The polyimide film of the present invention and its variations produced as described above can be usefully employed in various fields requiring excellent optical properties such as low reflectivity and high transmittance. In particular, it can be applied as a cover window of a display device to prevent surface scratches and provide excellent flexibility and visibility to a flexible display device.

[0076] In the present invention, the display device refers to a flexible display device or a non-flexible display device that displays an image, and includes not only a flat panel display device (FPD), but also a curved display device, a foldable display device, a flexible display device, a foldable mobile phone, a smartphone, a mobile communication terminal, or a tablet PC. Specifically, the display device may be a liquid crystal display, an electrophoretic display, an organic light-emitting display, an inorganic light-emitting display, a field emission display, a surface-conduction electron-emitter display, a plasma display, a cathode ray display, electronic paper, etc. As a specific example, it may be a flat panel such as an LCD, PDP, or OLED. The polyimide film of the present invention is not limited to the aforementioned uses and can be applied to conventional display devices known in the art, and can also be utilized as a substrate or protective film for flexible displays.

[0077] One specific example of a display device equipped with the aforementioned polyimide film comprises a display unit, a polarizing plate, a touchscreen panel, a cover window, and a protective film, wherein the cover window may include a polyimide film according to one embodiment of the present invention. Here, each component constituting the display device is not particularly limited and may include conventional components known in the art.

[0078] The present invention will be explained in more detail below through specific embodiments. The following embodiments are merely examples to aid in understanding the present invention and do not limit the scope of the present invention.

[0079] [Examples 1–9. Preparation of Polyimide Films]

[0080] A polyamic acid composition was prepared using a composition containing a diamine and an acid anhydride as listed in Table 1 below.

[0081] After coating the above polyamic acid composition onto an LCD glass plate using a bar coater, drying and imidation were carried out in a convection oven under a nitrogen atmosphere while gradually increasing the temperature stepwise: 80°C for 30 minutes, 150°C for 30 minutes, 200°C for 1 hour, and 300°C for 1 hour. Accordingly, a polyimide film with a thickness of 80 μm and an imidation rate of 85% or more was prepared. Subsequently, the polyimide film was separated from the glass plate and taken.

[0082] Diamine (Moll%) Acid anhydride (molar%) First monomer Second monomer First monomer Second monomer Example 1 TFDB(100) - CBDA(70) BPDA(30) Example 2 TFDB(100) - CBDA(90) BPDA(10) Example 3 TFDB(100) 　 HBPDA(70) DSDA(30) Example 4 TFDB(100) - H-PMDA(70) BPDA(30) Example 5 TFDB(100) 　 CpODA(70) BPDA(30) Example 6 TFDB(50) CHDA(50) BPDA(70) DSDA(30) Example 7 CHDA(100) - BPDA(70) DSDA(30) Example 8 6FODA(100) - CBDA(70) BPADA(30) Example 9 6FODA(100) - BPDA(70) H-PMDA(30) Comparative Example 1 TFDB(100) - BPDA(100) - Comparative Example 2 TFDB(100) - BPDA(70) DSDA(30) Comparative Example 3 TFDB(100) - BPDA(70) BTDA(30) Comparative Example 4 m-Toldine(100) - BPDA(100) - Comparative Example 5 6FODA(100) 　 BPDA(70) DSDA(30)

[0083] [Comparative Examples 1–5. Preparation of Polyimide Films]

[0084] Polyimide films of Comparative Examples 1 to 5 were each prepared by carrying out the same as Examples 1 to 9 above, except that the composition of Table 1 above was used.

[0085] [Experimental Example. Evaluation of Physical Properties]

[0086] The physical properties of the polyimide resin films prepared in Examples 1 to 9 and Comparative Examples 1 to 5 were evaluated in the following manner, and the results are shown in Table 2 below. In this case, each physical property in Table 2 below is based on a thickness of 80 μm.

[0087] Physical Property Evaluation Methods

[0088] 1) Reflectance measurement

[0089] The reflectance according to ASTM E1164 standards was measured using a UV spectrometer (Konica Minolta, model name: CM-3700d) at a wavelength of 550 nm.

[0090] 2) Yellowness measurement

[0091] The yellowness at 550 nm was measured using a UV spectrometer (Konica Minolta, CM-3700d) according to ASTM E313-73 standards.

[0092] 3) Measurement of light transmittance

[0093] Measurements were taken at a wavelength of 550 nm using a UV-Vis NIR Spectrophotometer (Shimadzu, model name: uv-3150).

[0094] 4) Thickness measurement

[0095] Measurements were taken using the IPC-TM-650 2.4.24.4 method with a DMA (TA Instrument, Q800).

[0096] 5) Measurement of Coefficient of Thermal Expansion (CTE)

[0097] Measurements were taken using the IPC-TM-650 2.4.24.5 method with a TMA (TA Instrument, Q400).

[0098] 6) Modulus measurement

[0099] Tensile strength (MPa) and elastic modulus (GPa) were measured according to ISO 527-3 standards using a UTM (Instron, model name: 5942).

[0100] Thickness (㎛) Reflectance (%) Transmittance (%) Difference between transmittance and reflectance (%) YI Modulus (GPa) CTE (ppm) Example 1 80 9.3 90.1 80.8 1.7 5.6 39 Example 2 8.6 90.3 81.7 1.5 5.5 37 Example 3 9.8 89.2 79.4 3.2 4.7 41 Example 4 9.5 89.5 80 3.1 4.9 45 Example 5 9.4 89.9 80.5 3.4 4.6 38 Example 6 10 89.1 79.1 4.9 4.9 45 Example 7 9.6 89.8 80.2 4.7 4.3 41 Example 8 8.9 90.3 81.4 3.7 4.3 61 Example 9 9.7 88.7 79 4.1 3.6 53 Comparative Example 1 80 13.9 87.6 73.7 6.7 5.9 29 Comparative Example 2 12.7 88.3 75.6 8.9 4.4 38 Comparative Example 3 13.6 87.9 74.3 9.7 4.8 33 Comparative Example 4 15.1 82.8 67.7 59.3 7.2 19 Comparative Example 5 11.2 87.9 76.7 8.7 4.3 31

[0101] As shown in Table 2 above, it was confirmed that the polyimide film according to the present invention has improved optical properties in terms of reflectance, transmittance, yellowness, etc.

[0102] Specifically, it was found that Examples 1 to 9 of the present invention, which contain a certain amount or more of cycloaliphatic diamine and / or acid anhydride, have low light reflectance, high transmittance, and excellent yellowness (YI). In contrast, Comparative Examples 1 to 5, which do not contain cycloaliphatic compounds, not only have significantly high reflectance but also exhibit poor characteristics in terms of transmittance and yellowness.

[0103] Accordingly, it was confirmed that the polyimide film of the present invention can be usefully applied as a cover window of a display device, as it possesses low reflectance, high transmittance, and excellent yellowness characteristics.

Claims

Claim 1 At least one type of diamine; A polyimide film copolymerized to include at least one acid anhydride and used as a cover window for a display device, wherein the diamine comprises an aromatic diamine, a diaminoal diamine, or a mixture of an aromatic diamine and a diaminoal diamine, the acid anhydride comprises a mixture of an aromatic acid anhydride and a diaminoal acid anhydride, the mixing ratio of the aromatic acid anhydride and the diaminoal acid anhydride is 10:90 to 70:30 mol% based on 100 mol% of the total acid anhydride, the aromatic acid anhydride comprises one or more selected from the group consisting of a non-fluorinated aromatic second acid anhydride and a sulfone-based aromatic third acid anhydride, the reflectance at a wavelength of 550 nm measured according to ASTM E1164 is 10.0% or less, and the difference between transmittance and reflectance at a wavelength of 550 nm is 78.0 to Polyimide film with 83.0%. Claim 2 delete Claim 3 A polyimide film according to claim 1, having a thickness of 30 to 100 μm, a transmittance of 85% or more at a wavelength of 550 nm, and a yellowness of 5 or less according to ASTM E313-73. Claim 4 A polyimide film according to claim 1, wherein the average coefficient of linear expansion (CTE) measured at 50 to 250°C is 100 ppm / °C or less, and the Young's modulus is 2 to 8 GPa. Claim 5 delete Claim 6 delete Claim 7 A polyimide film according to claim 1, wherein the aromatic diamine and the alicyclic diamine are each included in an amount of 0 to 100 mol% based on 100 mol% of the total diamine. Claim 8 The polyimide film according to claim 1, wherein the aromatic diamine comprises one or more selected from the group consisting of a fluorinated first diamine; a sulfonate second diamine, a hydroxyl third diamine, an ether fourth diamine, and a non-fluorinated fifth diamine. Claim 9 delete Claim 10 delete Claim 11 delete Claim 12 A polyimide film according to claim 1, wherein the ratio of the moles (a / b) of the diamine (a) and the acid anhydride (b) is in the range of 0.7 to 1.

3. Claim 13 delete