Polyimide film with excellent weather resistance

Abstract

The present invention relates to a polyimide film obtained by copolymerizing at least one diamine and at least one acid dianhydride, and more particularly to a polyimide film having excellent transparency, excellent weather resistance and excellent visibility due to minimal color change caused by external ultraviolet exposure, and thus capable of being used as an exterior window covering.

Description

Technical Field

[0001] The present invention relates to a polyimide film, and more particularly to a polyimide film having excellent transparency and capable of simultaneously ensuring excellent weather resistance and excellent visibility by preventing color change caused by external ultraviolet exposure. Background Art

[0002] Cover windows are used on the surface of display devices such as liquid crystal display (LCD) and organic light emitting display (OLED). As the material of the cover window, tempered glass with excellent flatness, heat resistance, chemical resistance, water or gas barrier performance, small linear expansion coefficient (CTE), and high light transmittance is mainly used.

[0003] On the other hand, in recent years, flexible displays such as curved displays and foldable displays have been developed. In order to be applied to such flexible displays, the outer cover window should be flexible. However, the outer cover window made of glass material is usually not only bulky and fragile, but also has low flexibility, and is therefore not suitable for flexible displays.

[0004] In order to solve the above-mentioned problems, recently, an outer covering window using a plastic material with relatively free formability has been proposed. The outer covering window using a plastic material has the advantages of being light and not easy to break, and being able to present a variety of designs. As the current plastic materials for outer covering windows, polycarbonate, polyethylene terephthalate, polymethyl methacrylate, etc. with excellent transparency are mainly used. Although such materials have the advantage of excellent transparency, the glass transition temperature (Tg) is below 150°C and the heat resistance is poor, and the chemical resistance and mechanical strength are low and the application is limited. In addition, the outer covering window will be arranged on the outermost side of the flexible display device. When the above-mentioned material is used and continuously exposed to external ultraviolet rays, yellowing will occur and adversely affect the visibility of the display. Summary of the invention

[0005] Technical issues

[0006] The present invention is proposed to solve the above-mentioned problems, and its object is to provide a polyimide film which minimizes the change in a* and b* values ​​in the CIE Lab color space caused by external ultraviolet (UV-B) exposure and can simultaneously achieve high weather resistance and excellent visibility.

[0007] Technological solutions

[0008] In order to achieve the above-mentioned object, the present invention provides a polyimide film, which is copolymerized by at least one diamine and at least one acid dianhydride, and in the UV-B wavelength region of 280 to 315 nm, through a 72-hour exposure test, the change value of a* (Δa*) and the change value of b* (Δb*) in the CIE Lab color space measured by a spectrophotometer simultaneously satisfy the following equations 1 and 2.

[0009] [Formula 1]

[0010] -0.5≤Δa*≤0.5 (where, Δa*=a*1–a*0)

[0011] [Formula 2]

[0012] -1.5≤Δb*≤1.5 (where Δb*=b*1–b*0)

[0013] In the above formulas 1 and 2,

[0014] a*1 and b*1 are the a* and b* values ​​of the CIE Lab color space measured after 72 hours of exposure in the UV-B wavelength region,

[0015] a*0 and b*0 are the a* and b* values ​​of the CIE Lab color space measured before exposure to the UV-B wavelength region, respectively.

[0016] According to a specific example of the present invention, the polyimide film can have a Δa* in the range of -0.4 to 0 and a Δb* in the range of 0 to 1.0 measured by a spectrophotometer in a 72-hour exposure test in the UV-B wavelength region of 280 to 315 nm.

[0017] According to a specific example of the present invention, when the polyimide film has a thickness of 10 to 100 μm, the transmittance at a wavelength of 550 nm may be above 85%, the yellowness based on the ASTM E313-73 standard may be below 5, and the glass transition temperature (Tg) may be 250 to 450° C.

[0018] According to a specific example of the present invention, when the polyimide film has a thickness of 10 to 100 μm, the average coefficient of linear expansion (CTE) measured at 50 to 250° C. may be less than 100 ppm / ° C., and the Young's modulus may be 2 to 8 GPa.

[0019] According to a specific example of the present invention, the at least one diamine may include one or more selected from the group consisting of a fluorinated first diamine, a sulfone second diamine, a hydroxyl third diamine, an ether fourth diamine, an alicyclic fifth diamine and a non-fluorinated sixth diamine.

[0020] According to a specific example of the present invention, based on 100 mol % of the total diamine, the content of the first to sixth diamines may be 10 to 100 mol %, respectively.

[0021] According to a specific example of the present invention, the at least one acid dianhydride may include one or more selected from the group consisting of a fluorinated aromatic first acid dianhydride, an alicyclic second acid dianhydride, a non-fluorinated aromatic third acid dianhydride and a sulfone aromatic fourth acid dianhydride.

[0022] According to a specific example of the present invention, based on 100 mol % of the total acid dianhydride, the content of the first acid dianhydride to the fourth acid dianhydride may be 10 to 100 mol % respectively.

[0023] According to a specific example of the present invention, the molar ratio (a / b) of the diamine (a) to the dianhydride (b) may be in the range of 0.7 to 1.3.

[0024] According to a specific example of the present invention, the polyimide film can be used as a cover window of a display device.

[0025] Effects of the Invention

[0026] According to one embodiment of the present invention, by selecting a predetermined component constituting the polyimide film and adjusting its content, the Δa* and Δb* characteristic values ​​of the CIE Lab color space are controlled, thereby minimizing the amount of color coordinate change caused by external ultraviolet exposure, as well as high weather resistance and excellent visibility.

[0027] Furthermore, the present invention exhibits high light transmittance, low yellowness, excellent modulus, and low coefficient of linear expansion (CTE), thereby being able to improve the operability and reliability of the final product.

[0028] Therefore, the polyimide film of the present invention can be effectively used as a cover window of display devices in the art, such as flat panel display panels, flexible displays, etc., and can also be applied to IT products, electronic products, home appliances, etc. known in the art.

[0029] The effects of the present invention are not limited to the above-exemplified contents, and this specification includes more various effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a diagram showing the CIE Lab color space (L*, a*, b*). DETAILED DESCRIPTION

[0031] The present invention is described in detail below. The embodiments of the present invention are provided to more completely describe the present invention to those skilled in the art. The following embodiments may be modified into various other forms, and the scope of the present invention is not limited by the following embodiments. At this time, throughout this specification, the same reference symbols refer to the same structure.

[0032] Unless otherwise defined, all terms (including technical and scientific terms) used in this specification may be used according to the meanings commonly understood by those skilled in the art to which the present invention belongs. In addition, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless otherwise clearly defined.

[0033] In addition, throughout the specification, when it is indicated that a certain part "includes" a certain constituent element, unless there is a special record to the contrary, it means that it further includes other constituent elements, rather than excluding other constituent elements. In addition, throughout the specification, "above" means not only including the situation where it is located above or below the object part, but also including the situation where there are other parts in the middle, and it does not necessarily mean that it is located on the upper side based on the direction of gravity. Moreover, in the specification of this application, the terms "first", "second", etc. are used to distinguish constituent elements from each other, rather than representing any order or importance.

[0034] <Polyimide film>

[0035] The polyimide resin film according to an embodiment of the present invention is a transparent film that can be disposed on a display device, and more specifically, can be used as a cover window of a flexible display.

[0036] Here, the outer cover window refers to a film disposed on the outermost side of the flexible display device to protect the display device. Such an outer cover window may be a single window film, or may be a film in which a window coating is formed on another substrate (Substrate) composed of an optically transparent resin.

[0037] On the other hand, when the outermost cover window disposed on the display device is made of conventional plastic material, although the initial transparency or light transmittance is relatively excellent, it will be damaged (damaged) when continuously exposed to external ultraviolet rays and will inevitably cause yellowing phenomenon, thereby reducing visibility. In particular, as the Δa* and Δb* values, which are color coordinate values ​​in the CIE Lab color space, increase, the color change increases, which has an adverse effect on the visibility of the display.

[0038] In contrast, the polyimide film of the present invention is characterized in that, compared with conventionally known plastic films, in the CIE Lab color space based on an external ultraviolet exposure test, the amount of change in the color coordinate (L*, a*, b*) value, such as the change in a* value (Δa*) and the change in b* value (Δb*), is minimized to below a specific range. As a result, when the polyimide film is applied to a flexible display device that is continuously exposed to external ultraviolet rays in daily life, specifically, the outer cover window of a foldable mobile phone, it can continuously exert excellent weather resistance and visibility effects.

[0039] According to a specific example of the present invention, the above-mentioned polyimide film is formed by copolymerizing at least one diamine and at least one acid dianhydride. In the UV-B wavelength region of 280 to 315 nm, through a 72-hour exposure test, the change value of a* (Δa*) and the change value of b* (Δb*) in the CIE Lab color space measured by a spectrophotometer can simultaneously satisfy the following equations 1 and 2.

[0040] [Formula 1]

[0041] -0.5≤Δa*≤0.5 (where, Δa*=a*1–a*0)

[0042] [Formula 2]

[0043] -1.5≤Δb*≤1.5 (where Δb*=b*1–b*0)

[0044] In the above formulas 1 and 2,

[0045] a*1 and b*1 are the a* and b* values ​​of the CIE Lab color space measured after 72 hours of exposure in the UV-B wavelength region,

[0046] a*0 and b*0 are the a* and b* values ​​of the CIE Lab color space measured before exposure to the UV-B wavelength region, respectively.

[0047] In the present invention, the L*a*b* color system means the color values ​​specified by the CIE Lab color space. Figure 1 As shown in the L*a*b* color system, L* represents lightness, a*b* represents hue and saturation. Specifically, a*b* represents the direction of color, a* represents the red direction, -a* represents the green direction, b* represents the yellow direction, and -b* represents the blue direction (see below). Figure 1 ).

[0048] According to a specific example, the polyimide film of the present invention can have a Δa* of -0.5 or more and a Δb* of 1.5 or less measured by a spectrophotometer in a UV-B wavelength region of 280 to 315 nm through a 72-hour exposure test. In particular, from the viewpoint of improving the optical properties and weather resistance of the film, Δa* is preferably in the range of -0.4 to 0, and Δb* is preferably in the range of 0 to 1.0.

[0049] Here, the change values ​​of a* and b* (Δa*, Δb*) may be affected by the thickness of the corresponding film. The numerical value of such a color coordinate change amount (Δa*, Δb*) can be measured based on the thickness of the corresponding polyimide film of 10 to 100 μm, specifically 30 to 80 μm, and more specifically 80 ± 5 μm. In addition, in the present invention, the change values ​​of a* and b* (e.g., Δa*, Δb*) mean the values ​​measured based on the predetermined thickness of the corresponding polyimide film, but are not limited to this. The situation represented by the ratio of the change values ​​of a* and b* (e.g., Δa*, Δb*) based on the change in the thickness of the polyimide film also belongs to the scope of the present invention.

[0050] In the case of the polyimide film of the present invention having the above-mentioned color coordinate change amount (Δa*, Δb*) parameters and corresponding values, excellent optical properties and weather resistance can be continuously maintained even in a harsh environment due to minimized color change.

[0051] The polyimide film of the present invention not only needs to adjust the above-mentioned color coordinate changes, but also needs to have excellent optical properties such as high transparency and light transmittance in order to improve the visibility of the display screen.

[0052] According to another specific example of the present invention, when the polyimide film has a thickness of 10 to 100 μm, the light transmittance at a wavelength of 550 nm is 85% or more, specifically 89% or more, and more specifically 90% to 99%. In addition, the yellowness (YI) based on the ASTM E313-73 standard can be 5 or less, specifically 4.5 or less.

[0053] According to another specific example of the present invention, the polyimide film may have a Young's modulus of 2 to 8 GPa, and may be 3 to 6 GPa from the perspective of simultaneously showing mechanical hardness and excellent flexibility. Here, Young's modulus refers to a value measured according to ISO 527-3 standard. When the modulus is less than the above value, it is difficult to exert sufficient hardness, and when it is greater than the above value, the flexibility is reduced and the foldability may be reduced.

[0054] According to another specific example of the present invention, when the thickness of the polyimide film is 10 to 100 μm, the average linear expansion coefficient (CTE) measured at 50 to 250°C can be less than 100 ppm / °C, specifically less than 70 ppm / °C, and more specifically 30 to 65 ppm / °C.

[0055] In addition, the glass transition temperature (Tg) of the polyimide film may be 250°C or higher, and specifically may be 250 to 450°C.

[0056] In this specification, the above properties of the polyimide film are based on a corresponding film thickness of 10 to 100 μm, specifically 30 to 80 μm, unless otherwise specified. However, the thickness is not limited to the above range and can be appropriately adjusted within the usual thickness range known in the art.

[0057] The polyimide film of the present invention has no particular limitation on the components and / or composition of the polyimide resin as long as it satisfies the color coordinate change (Δa*, Δb*) parameters and corresponding values ​​of the CIE Lab color space based on the external ultraviolet exposure test.

[0058] According to one example, the polyimide film is copolymerized with at least one diamine and at least one acid dianhydride. Specifically, the polyimide film can be produced by imidizing and heat treating a polyamic acid composition containing the diamine, acid dianhydride, and optionally a solvent at a high temperature.

[0059] Generally speaking, polyimide (PI) resin refers to a high heat-resistant resin manufactured by carrying out solution polymerization of aromatic acid dianhydride and aromatic diamine or aromatic diisocyanate to manufacture polyamic acid derivatives, and then carrying out imidization by ring-closing dehydration at high temperature. Such polyimide resin is a high molecular weight substance containing imide ring, and on the basis of the chemical stability of imide ring, heat resistance, chemical resistance, wear resistance and electrical properties are also excellent. The above-mentioned polyimide resin can be a random copolymer or a block copolymer.

[0060] The diamine (a) component constituting the polyimide film of the present invention is not limited as long as it is a compound having a diamine structure in the molecule, and any common diamine compound known in the art can be used without limitation, for example, aromatic, alicyclic, aliphatic compounds having a diamine structure, or combinations thereof.

[0061] In particular, in the present invention, when considering the optical properties such as minimization of the color coordinate change (Δa*, Δb*) of the CIE Lab color space measured by the external ultraviolet exposure test of the polyimide film, high transmittance, low YI, low haze, etc., thermal properties such as high glass transition temperature (High Tg), low linear expansion coefficient (Low CTE), and mechanical properties such as modulus, etc., fluorine-based, sulfone-based, hydroxyl-based, ether-based, alicyclic, non-fluorine-based, etc. diamines having fluorinated substituents can be used alone, or more than one can be used in appropriate combination. Therefore, in the present invention, as diamine compounds, a fluorinated aromatic first diamine, a sulfone-based second diamine, a hydroxyl-based third diamine, an ether-based fourth diamine, an alicyclic fifth diamine, and a non-fluorine-based sixth diamine introduced with a fluorinated substituent can be used alone, or in the form of a mixture of two or more thereof.

[0062] As non-limiting examples of the diamine monomer (a) that can be used, diaminodiphenyl ether (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'-diaminophenyl ether (2,2'-Bis(trifluoromethyl)-4,4'-diaminodiphenyl) ether, 6-FODA), bisaminohydroxyphenylhexafluoropropane (DBOH), bisaminophenoxyphenylhexafluoropropane (4BDAF), bisaminophenoxyphenylpropane (6HMDA), bisaminophenoxydiphenyl sulfone (DBSDA), bis(4-aminophenyl) sulfone (4,4'-DDS), bis(3-aminophenyl) sulfone (3,3'-DDS), sulfonyldiphthalic anhydride (SO2DPA), 4,4'-diaminodiphenyl ether (4,4'-ODA), bis(carboxyphenyl)dimethylsilane, or a mixture of one or more thereof.

[0063] When considering the high transparency, high glass transition temperature and low yellowness of the polyimide film, the fluorinated first diamine can use 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (2,2'-TFDB) and 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (1,4-Bis(4-amino-2-trifluoromethylphenoxy)benzene, 6-FAPB) which can induce linear polymerization. In addition, the sulfone-based second diamine can use bis(4-aminophenyl)sulfone (4,4'-DDS) or 3,3'-DDS. In addition, the hydroxyl-based third diamine can use 2,2-bis(3-amino-4-methylphenyl)-hexafluoropropane (2,2-Bis(3-amino-4-methylphenyl)-hexafluoropropane, BIS-AT-AF) and 2,2-bis(3-amino-4-hydroxycyclohexyl)hexafluoropropane. In addition, the ether-based fourth diamine can use 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6-FODA) or diaminodiphenyl ether (ODA). In addition, as the alicyclic fifth diamine, 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane (MACM), 4,4'-methylenebiscyclohexylamine (PACM), 1,3-bis(aminomethyl)cyclohexane (1,3-BAC), 1,4-bis(aminomethyl)cyclohexane (1,4-BAC), cis-1,2-cyclohexanedimethylamine, trans-1,2-cyclohexanedimethylamine, bis(4-aminocyclohexyl)ether (H-ODA), N-(4-aminocyclohexyl)-1,4-cyclohexanediamine can be used. In addition, as the non-fluorine-based sixth diamine, m-tolidine or p-phenylenediamine (p-PDA) can be used.

[0064] In the diamine monomer (a) of the present invention, the content of the above-mentioned fluorinated first diamine, sulfone second diamine, hydroxyl third diamine, ether fourth diamine, alicyclic fifth diamine, non-fluorine sixth diamine, etc. is not particularly limited, and can be 10 to 100 mol%, specifically 20 to 90 mol%, and more specifically 20 to 80 mol%, based on 100 mol% of the total diamine.

[0065] According to a preferred example of the present invention, as the diamine monomer (a), the fluorinated first diamine and the ether-based fourth diamine may be mixed and used. In this case, the ratio of the fluorinated first diamine and the ether-based fourth diamine is not particularly limited, and may be, for example, 50 to 90:10 to 50 mol %.

[0066] According to another preferred example of the present invention, at least one fluorinated first diamine may be mixed as the diamine monomer (a), and the ratio of the fluorinated first diamine may be 50 to 90:10 to 50 mol %, but is not particularly limited thereto.

[0067] The acid dianhydride (b) monomer constituting the polyimide film of the present invention can use a common compound known in the art having an acid dianhydride structure in the molecule without limitation. For example, an aromatic, alicyclic, or aliphatic compound having an acid dianhydride structure or a combination thereof can be used. Specifically, a fluorinated aromatic first acid dianhydride, an alicyclic second acid dianhydride, a non-fluorinated aromatic third acid dianhydride, and a sulfone aromatic fourth acid dianhydride can be used alone, or in a combination of at least two or more thereof.

[0068] The above-mentioned fluorinated first acid dianhydride monomer is not particularly limited as long as it is an aromatic acid dianhydride introduced with a fluorine substituent. As non-limiting examples of usable fluorinated first acid dianhydrides, there are 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydrid, 6-FDA), 4-(trifluoromethyl)pyromellitic dianhydride (4-TFPMDA), etc. They can be used alone or in combination of two or more. Among the fluorinated acid dianhydrides, 6-FDA has a very strong property of limiting the formation of charge transfer complexes (CTC: Change transfer complex) between molecular chains and within molecular chains, so it is a very suitable compound for transparency.

[0069] In addition, the alicyclic second acid dianhydride is not particularly limited as long as it is a compound having an alicyclic ring rather than an aromatic ring and having an acid dianhydride structure. Non-limiting examples of usable alicyclic second acid dianhydrides include cyclobutanetetracarboxylic dianhydride (CBDA), 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA), bicyclo[2,2,2]-7-octene-2,3,5,6-tetracarboxylic dianhydride (BCDA), (2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic dianhydride (TDA), 1,1'-bicyclohexane-3,3',4,4'-tetracarboxylic dianhydride. (H-BPDA), 1,2,4,5-cyclohexane-tetracarboxylic dianhydride (H-PMDA), bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic 2,3:5,6-dianhydride (7CI,8CI) or a mixture of more than one thereof.

[0070] In addition, the non-fluorinated third acid dianhydride monomer is not particularly limited as long as it is a non-fluorinated aromatic acid dianhydride without a fluorine substituent. As non-limiting examples of usable non-fluorinated third acid dianhydride monomers, there are pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA), benzophenone tetracarboxylic dianhydride (BTDA), oxydiphthalic dianhydride (ODPA), 4,4-(4,4-isopropylidene diphenoxy)bis(phthalic anhydride) (4,4-(4,4-Isopropylidenediphenoxy)bis(phthalic anhydride), BPADA, etc. They can be used alone or in combination of two or more thereof.

[0071] The sulfone-based quaternary acid dianhydride monomer is not particularly limited as long as it is an acid dianhydride into which a sulfone group is introduced, and an example thereof is 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride (3,3',4,4'-DIPHENYLSULFONE TETRACARBOXYLICDIANHYDRIDE, DSDA).

[0072] In the acid dianhydride monomer (b) of the present invention, the content of the above-mentioned fluorinated aromatic first acid dianhydride, alicyclic second acid dianhydride, non-fluorinated aromatic third acid dianhydride, sulfone aromatic fourth acid dianhydride, etc. is not particularly limited. For example, their contents can be in the range of 10 to 100 mol%, specifically 10 to 90 mol%, and more specifically 20 to 80 mol%, based on 100 mol% of the overall acid dianhydride.

[0073] According to a preferred example of the present invention, as the acid dianhydride (b), a fluorinated first acid dianhydride, an alicyclic second acid dianhydride and a non-fluorinated third acid dianhydride can be mixed. At this time, their use ratio is not particularly limited, for example, it can be 5 to 20: 50 to 90: 5 to 15 mol%.

[0074] According to another preferred example of the present invention, as the acid dianhydride (b), alicyclic second acid dianhydride and non-fluorinated third acid dianhydride may be used in combination, and the ratio of these two may be 30-80:20-70 mol %, but is not particularly limited thereto.

[0075] According to another preferred example of the present invention, at least one non-fluorinated third acid dianhydride may be mixed as the acid dianhydride (b). In this case, the ratio of the non-fluorinated third acid dianhydride may be 50-80:20-50 mol %, but is not particularly limited thereto.

[0076] In the polyamic acid composition constituting the polyimide film of the present invention, the molar ratio (a / b) of the diamine component (a) to the dianhydride component (b) may be 0.7 to 1.3, preferably 0.8 to 1.2, and more preferably 0.9 to 1.1.

[0077] The polyamic acid composition of the present invention can use organic solvents known in the art as the solvent for the solution polymerization of the above-mentioned monomer without restriction. As the example of usable solvent, one or more polar solvents selected from meta-cresol, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), acetone, diethyl acetate and dimethyl phthalate (DMP) can be used. In addition, low boiling point solvents such as tetrahydrofuran (THF), chloroform or solvents such as gamma-butyrolactone can be used. At this time, the content of the solvent (polymerization first solvent) is not particularly limited, in order to obtain the molecular weight and viscosity of a suitable polyamic acid composition (polyamic acid solution), based on the overall polyamic acid composition weight, the content can preferably be 50 to 95% by weight, and more preferably can be 70 to 90% by weight.

[0078] The above-mentioned polyamic acid composition can react and manufacture after at least one acid dianhydride and at least one diamine are put into an organic solvent, for example, in order to improve the physical property of polyimide, diamine (a) and acid dianhydride (b) can be adjusted with an equivalent ratio of approximately 1:1. The composition of such a polyamic acid composition is not particularly limited, for example, based on 100% by weight of the overall weight of the polyamic acid composition, 2.5 to 25.0% by weight of acid dianhydride, 2.5 to 25.0% by weight of diamine and an organic solvent satisfying the surplus of 100% by weight of the composition can be included. Here, the content of the organic solvent can be 70 to 90% by weight. In addition, based on 100% by weight of the corresponding solid component, the above-mentioned polyamic acid composition can include 30 to 70% by weight of acid dianhydride and 30 to 70% by weight of diamine, but is not particularly limited thereto.

[0079] The polyamic acid composition thus constituted 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 is within the above range, the thickness can be easily adjusted when the polyamic acid composition is applied, and a coating surface can be uniformly formed.

[0080] If necessary, the polyamic acid composition may contain at least one additive such as a plasticizer, an antioxidant, a flame retardant, a dispersant, a viscosity modifier, a leveling agent, etc. in a small amount within the range not impairing the object and effect of the present invention.

[0081] The polyimide resin film of the present invention can be manufactured according to the usual methods known in the art. For example, it can be manufactured by coating (casting) the above-mentioned polyamic acid composition on a substrate, such as a glass substrate, and then inducing an imide ring-closure reaction (imidization) for 0.5 to 8 hours while slowly raising the temperature in the range of 30 to 350°C.

[0082] At this time, the coating method can use the usual method known in the art without limitation, for example, at least one method selected from the group consisting of spin coating, dip coating, solvent casting, slot die coating and spray coating can be carried out. The polyamic acid composition can be coated at least once to make the thickness of the above-mentioned colorless and transparent polyimide resin layer become hundreds of nm to tens of μm.

[0083] In the method for producing a polyimide film of the present invention, as the imidization method used in the step of imidizing the polymerized polyamic acid by casting on a support, a thermal imidization method, a chemical imidization method, or a combination of the thermal imidization method and the chemical imidization method can be used.

[0084] The thermal imidization method is a method of obtaining a polyimide film by casting a polyamic acid composition (polyamic acid solution) on a support and heating it for 1 to 10 hours while gradually increasing the temperature in a temperature range of 30 to 400° C.

[0085] In addition, the chemical imidization method is a method in which a dehydrating agent represented by an acid anhydride such as acetic anhydride and an imidization catalyst represented by amines such as isoquinoline, β-picoline, pyridine, etc. are added to the polyamic acid composition. When a thermal imidization method is used in combination with such a chemical imidization method, the heating conditions of the polyamic acid composition can be changed according to the type of the polyamic acid composition, the thickness of the manufactured polyimide film, etc.

[0086] To more specifically describe the case of using the thermal imidization method and the chemical imidization method in combination, a dehydrating agent and an imidization catalyst are added to the polyamic acid composition, which is cast on a support and then heated at 80 to 300° C., preferably 150 to 250° C. to activate the dehydrating agent and the imidization catalyst, thereby partially curing and drying to obtain a polyimide film.

[0087] The thickness of the polyimide film thus formed is not particularly limited and can be appropriately adjusted according to the field of application, for example, in the range of 10 to 150 μm, preferably in the range of 10 to 100 μm.

[0088] The polyimide film of the present invention prepared as above and its modified examples can be effectively used in various fields requiring minimization of the color coordinate change (Δa*, Δb*) based on the external ultraviolet exposure test, excellent optical properties, etc. In particular, it can be used as a cover window of a display device to prevent surface scratches and continuously give the flexible display device excellent weather resistance and visibility.

[0089] In the present invention, the display device refers to a flexible display device or a non-flexible display device for displaying an image, including not only a flat panel display device (FPD: Flat Panel Display Device), but also a curved display device (Curved Display Device), a foldable display device (Foldable Display Device), a flexible display device (Flexible Display Device), a foldable mobile phone, a smart phone, a mobile communication terminal or a tablet computer, etc. Specifically, the above-mentioned display device can be a liquid crystal display device (Liquid Crystal Display), an electrophoretic display device (Electrophoretic Display), an organic light emitting display device (Organic Light Emitting Display), an inorganic EL display device (Inorganic Light Emitting Display), a field emission display device (Field Emission Display), a surface conduction electron emission display device (Surface-conduction Electron-emitter Display), a plasma display device (Plasma Display), a cathode ray tube display device (Cathode Ray Display), an electronic paper, etc. According to a specific example, it can be a flat panel display panel such as LCD, PDP, OLED, etc. The polyimide film of the present invention is not limited to the above-mentioned uses, and can be applied to a common display device known in the art, and can also be used as a substrate or protective film for a flexible display.

[0090] According to a specific example of a display device having the above-mentioned polyimide film, it can include a display unit, a polarizer, a touch panel, an outer cover window and a protective film, and the outer cover window can include the polyimide film of an embodiment of the present invention. Among them, the components constituting the display device are not particularly limited, and can include common components known in the art.

[0091] The present invention will be described in more detail below by way of specific examples. The following examples are merely illustrative to help understanding of the present invention, and the scope of the present invention is not limited thereto.

[0092] [Examples 1 to 9. Polyimide film production]

[0093] A polyamic acid composition was produced using a composition containing the diamine and acid dianhydride described in Table 1 below.

[0094] After the polyamic acid composition is spin-coated on LCD glass, the temperature is gradually raised in a nitrogen atmosphere convection oven in the manner of 80°C for 30 minutes, 150°C for 30 minutes, 200°C for 1 hour, and 300°C for 1 hour, while drying and imide ring closure reaction (Imidazation) are performed. Thus, a polyimide film with an imidization rate of 85% or more and a film thickness of 80 μm is produced. Afterwards, the glass is etched with hydrofluoric acid to obtain a polyimide film.

[0095] [Table 1]

[0096]

[0097] [Comparative Examples 1 to 5. Polyimide Film Production]

[0098] Polyimide films of Comparative Examples 1 to 5 were produced in the same manner as in Examples 1 to 9 except that the compositions described in Table 1 were used.

[0099] [Experimental example. Physical property evaluation]

[0100] The physical properties of the polyimide resin films produced in Examples 1 to 9 and Comparative Examples 1 to 5 were evaluated by the following methods, and the results are shown in the following Table 2. At this time, each physical property in the following Table 2 was evaluated based on a polyimide film having a thickness of 80 μm.

[0101] <Physical Property Evaluation Method>

[0102] 1) Transmittance measurement

[0103] The color was measured at a wavelength of 550 nm using an ultraviolet-visible near-infrared spectrophotometer (UV-Vis NIR Spectrophotometer, Shimadzu, model name: uv-3150).

[0104] 2) Yellowness determination

[0105] The yellowness at 550 nm was measured using a spectrophotometer (Konica Minolta, model name: CM-3700d) in accordance with ASTM E313-73.

[0106] 3) Glass Transition Temperature (Tg)

[0107] The measurement was performed using DMA (TA Instruments, Q800) according to IPC-TM-650 2.4.24.4 method.

[0108] 4) Thickness measurement

[0109] The thickness of the film was measured using a thickness meter (Mitutoyo, model name: 293-140).

[0110] 5) Determination of coefficient of thermal expansion (CTE)

[0111] The measurement was performed using TMA (TA Instruments, Q400) according to IPC-TM-650 2.4.24.5 method.

[0112] 6) Modulus determination

[0113] The tensile strength (MPa) and elastic modulus (GPa) were measured using UTM (Instron, model name: 5942) in accordance with ISO 527-3.

[0114] 6) Color coordinate value determination

[0115] a* and b* were measured using a spectrophotometer (KONICA MINOLTA, model name: CM-3700d) in accordance with ASTM E313-73.

[0116] [Table 2]

[0117]

[0118] As shown in Table 2, the polyimide film of the present invention can ensure excellent weather resistance and excellent visibility by minimizing the color coordinate change (Δa*, Δb*) of the CIE Lab color space caused by external ultraviolet rays. Therefore, it can be confirmed that the polyimide film of the present invention can be effectively used as an outer cover window of a display device.

Claims

1. A polyimide film used as an outer cover window of a flexible display device, The polyimide film is formed by copolymerizing a diamine composed of 2,2-bis(3-amino-4-methylphenyl)-hexafluoropropane (BIS-AT-AF) and an acid dianhydride composed of at least one non-fluorinated aromatic tertiary acid dianhydride. As the non-fluorinated aromatic third acid dianhydride, one or more of pyromellitic dianhydride PMDA, 3,3′,4,4′-biphenyltetracarboxylic dianhydride BPDA, benzophenonetetracarboxylic dianhydride BTDA, oxydiphthalic dianhydride ODPA and 4,4-(4,4-isopropylidene diphenoxy)bis(phthalic anhydride) BPADA are used, The thickness of the polyimide film is 10 to 100 μm, In the UV-B wavelength region of 280 to 315 nm, the change values ​​Δa* and Δb* of a* and b* in the CIE Lab color space measured by a spectrophotometer according to ASTM E31373 in a 72-hour exposure test satisfy the following equations 1 and 2 at the same time, and Young's modulus is 4.1 to 8 GPa, [Formula 1] -0.5≤Δa*≤0.5, where Δa*=a*1–a*0 [Formula 2] -1.5≤Δb*≤1.5, where Δb*=b*1–b*0 In Formula 1 and Formula 2, a*1 and b*1 are the a* and b* values ​​of the CIE Lab color space measured after 72 hours of exposure in the UV-B wavelength region, a*0 and b*0 are the a* and b* values ​​of the CIE Lab color space measured before exposure to the UV-B wavelength region, respectively. 2 . The polyimide film according to claim 1 , wherein the Δa* of the polyimide film is in the range of −0.4 to 0 and the Δb* is in the range of 0 to 1.0 as measured by a spectrophotometer in a UV-B wavelength region of 280 to 315 nm through a 72-hour exposure test. 3 . The polyimide film according to claim 1 , wherein the light transmittance at a wavelength of 550 nm is 85% or more, the yellowness based on ASTM E313-73 is 5 or less, and the glass transition temperature Tg measured by DMA is 250 to 450° C. The 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. 5 . The polyimide film according to claim 1 , wherein a molar ratio (a / b) of the diamine (a) to the dianhydride (b) is in the range of 0.7 to 1.3.

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