Composition for forming polyimide film for cover window, manufacturing method and use thereof

Abstract

To provide a composition for forming a polyimide film for a cover window capable of satisfying advanced performance required for a cover window, and a manufacturing method and use thereof.SOLUTION: There is provided a composition for forming a polyimide film for a cover window, comprising: a polyamic acid or polyimide comprising a structural unit derived from a dianhydride and a structural unit derived from a diamine; and a mixed solvent of an amide-based solvent and a hydrocarbon-based solvent, wherein the hydrocarbon-based solvent is included in an amount of 10 to 40 wt.% based on a total weight of the mixed solvent.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a polyimide film forming composition for a cover window, a method for producing the same, and uses thereof.

Background Art

[0002] Polyimide film (PI) has excellent heat resistance with insolubility and infusibility, and has excellent oxidation resistance, heat resistance, radiation resistance, low temperature characteristics, and chemical resistance. Therefore, polyimide film is used in a wide range of technical fields such as heat-resistant advanced materials for automobiles, aircraft, spacecraft, etc. and electronic materials such as insulating coating agents, insulating films, semiconductors, and electrode protection films for TFT-LCDs. Recently, it has also attracted attention as a material for replacing expensive strengthened glass used as a cover window for portable electronic devices and communication devices.

[0003] Cover windows for portable electronic devices and communication devices are for protecting electronic components such as printed wiring boards and lead frames of semiconductor integrated circuits, and need to have insulation properties above a predetermined level. In addition, with the thinning, slimming, and flexibility of portable electronic devices and communication devices, flexibility is required together with mechanical physical properties such as high hardness and high rigidity. Also, generally, in order to impart various physical properties, a coating layer is laminated on a substrate, which may cause irregular reflection of light in the cover window, resulting in optical contamination and a decrease in visibility. Therefore, high display quality and optical physical properties such as no occurrence of Mura phenomenon are also required.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] One example provides a polyimide film-forming composition for cover windows and a method for producing the same, which can meet the performance requirements of advanced cover windows.

[0006] In detail, one example provides a polyimide film forming composition for cover windows and a method for manufacturing the same, which can achieve improved yellowness, have an anti-reflective effect over a wide viewing angle, and significantly reduce unevenness in the appearance of the film.

[0007] Another example is the provision of a polyimide film for cover windows used in optical applications of display devices, which maintains its colorless and transparent optical properties without degradation, is free from optical contamination, has excellent optical properties such as visibility, and possesses excellent heat resistance and mechanical properties.

[0008] Another embodiment provides a multilayer structure including the polyimide film.

[0009] Another embodiment provides a cover window for a display device that includes the polyimide film.

[0010] Another embodiment provides a flexible display device including the polyimide film or the cover window. [Means for solving the problem]

[0011] A polyimide film-forming composition for cover windows according to one embodiment may comprise a polyamine acid or polyimide containing structural units derived from dianhydrides and structural units derived from diamines, and a mixed solvent of an amide solvent and a hydrocarbon solvent, wherein the hydrocarbon solvent is present in an amount of 10 to 40% by weight relative to the total weight of the mixed solvent. In a specific polyimide film-forming composition for cover windows according to one embodiment, the structural units derived from dianhydrides may include structural units derived from the compound represented by the following chemical formula 1 and the compound represented by the following chemical formula 2, and the structural units derived from diamines may include structural units derived from the compound represented by the following chemical formula 3 and the compound represented by the following chemical formula 4. [Chemical formula 1] [ka] [Chemical formula 2] [ka] [Chemical formula 3] [ka] [Chemical formula 4] [ka]

[0012] The aforementioned amide solvent may include dimethylpropionamide.

[0013] The hydrocarbon solvent may be a cyclic hydrocarbon solvent.

[0014] The cyclic hydrocarbon solvent may include toluene, benzene, cyclohexane, or a combination thereof.

[0015] The polyimide film-forming composition for a cover window according to an embodiment may contain 10 to 40% by weight of solid content based on the total weight of the polyimide film-forming composition for a cover window.

[0016] In the polyimide film-forming composition for a cover window, the structural unit derived from the compound represented by Chemical Formula 1 may be contained in an amount of 70 to 95 mol% based on 100 mol% of the structural unit derived from the dianhydride.

[0017] In the polyimide film-forming composition for a cover window, the structural unit derived from the compound represented by Chemical Formula 3 may be contained in an amount of 70 to 95 mol% based on 100 mol% of the structural unit derived from the diamine.

[0018] Also, a method for manufacturing a polyimide film for a cover window according to another embodiment includes: i) reacting a compound represented by the following Chemical Formula 1, a compound represented by the following Chemical Formula 2, a compound represented by the following Chemical Formula 3, and a compound represented by the following Chemical Formula 4 in an amide-based solvent to produce a polyamic acid solution; ii) adding a hydrocarbon-based solvent to the polyamic acid solution to adjust the crystallinity of the polyamic acid; iii) coating the polyimide film-forming composition for a cover window obtained in step ii) on a substrate and curing it, where the hydrocarbon-based solvent in step ii) may be added in an amount of 10 to 40% by weight based on the total weight of the amide-based solvent and the hydrocarbon-based solvent. [Chemical Formula 1] [Chemical Structure] [[ID=|25]] [Chemical Formula 2] [Chemical Structure]<| [Chemical Formula 3] [Chemical Structure] [Chemical formula 4] [ka]

[0019] The curing step of step iii) above may be carried out by heating at 80 to 300°C.

[0020] The procedure may further include a step of leaving the product at room temperature after the application in step iii) above.

[0021] Furthermore, polyimide films for cover windows in other embodiments may be manufactured using the polyimide film-forming composition for cover windows.

[0022] The polyimide film may have a thickness of 30 to 150 μm and a yellowness index (YI) of 3.5 or less according to ASTM E313.

[0023] The polyimide film may have a thickness of 40 to 80 μm and a yellowness index (YI) of 1.0 to 2.7 according to ASTM E313.

[0024] Furthermore, other examples of multilayer structures may include the polyimide film formed on one surface of the substrate.

[0025] The multilayer structure may further include a coating layer formed on the polyimide film.

[0026] The coating layer may be a hard coating layer, an antistatic layer, an anti-fingerprint layer, an anti-fouling layer, an anti-scratch layer, a low refractive index layer, an anti-reflective layer, an impact-absorbing layer, or a combination thereof.

[0027] Furthermore, cover windows for display devices according to other embodiments may include the polyimide film.

[0028] Furthermore, flexible display devices according to other embodiments may include a cover window for the display device. [Effects of the Invention]

[0029] According to one example, inhibiting the interaction between polyamine acid and the mixed solvent can significantly reduce the intermolecular packing density during curing. This makes it possible to provide a polyimide film for cover windows that maintains its colorless and transparent properties while simultaneously achieving excellent optical and mechanical properties. Furthermore, it is flexible and has excellent bending characteristics, making it suitable for use as a cover window in flexible displays.

[0030] According to one example, by efficiently controlling the intermolecular interactions that are a drawback of polyimide films, it is possible to achieve not only excellent adhesion, but also superior mechanical properties such as modulus and elongation at break, as well as optical properties. When used as a cover window for display panels, it is possible to effectively suppress the reduction in yellowness, which is a visibility problem, as well as mura phenomena, especially rainbow mura caused by phase differences, thereby increasing the reliability of the display panel containing it. [Modes for carrying out the invention]

[0031] The following description details one embodiment of the present invention so that it can be easily implemented by a person with ordinary skill in the art to which the present invention pertains. However, the present invention can be realized in various different forms and is not limited to the embodiment described herein. Nor does it limit the scope of protection as defined by the claims.

[0032] Furthermore, unless otherwise defined, the technical and scientific terms used herein have the meanings that are ordinarily understood by a person with ordinary skill in the art to which this invention belongs, and descriptions of known functions and configurations that could obscure the gist of this invention in the following description are omitted.

[0033] Throughout this specification, the statement that a part "includes" a certain component may mean, unless otherwise stated, that it does not exclude other components, but rather that it may include other components.

[0034] Hereinafter, unless otherwise specified, when a part such as a layer, film, thin film, region, or plate is said to be "on top of" or "on" another part, this includes not only cases where it is "directly on top of" the other part, but also cases where there is yet another part in between.

[0035] Hereafter, unless otherwise defined in this specification, “these combinations” may mean a mixture or copolymerization of the constituents.

[0036] Hereafter, unless otherwise defined in this specification, "A and / or B" may mean a mode that includes both A and B simultaneously, or a mode selected from A and B.

[0037] Hereafter, unless otherwise defined herein, “polymer” means a relatively high molecular weight molecule whose structure may include multiple repeats of units derived from low molecular weight molecules. In one mode, a polymer may be an alternating copolymer, a block copolymer, a random copolymer, a branched copolymer, a crosslinked copolymer, or a copolymer containing all of these (e.g., a copolymer containing more than one monomer). In other modes, a polymer may be a homopolymer (e.g., a copolymer containing one monomer).

[0038] Hereinafter, unless otherwise defined in this specification, "polyamine acid" may mean a polymer containing structural units having an amine acid moiety, and "polyimide" may mean a polymer containing structural units having an imide moiety.

[0039] Hereinafter, unless otherwise specifically defined in this specification, a polyimide film may be a film containing polyimide, and more specifically, it may be a high heat-resistant film produced by solution polymerization of a dianhydride compound and a diamine compound or diisocyanate compound to produce a polyamine acid, followed by ring-closing dehydration at high temperature to imide it.

[0040] Hereinafter, unless otherwise defined, the term "unevenness" can be interpreted to encompass all light-induced distortion phenomena that may occur at a particular angle. For example, in a display device containing a polyimide film, such light-induced distortions may include blackout phenomena where the screen appears black, hot spot phenomena, or rainbow phenomena with iridescent stains.

[0041] Traditionally, many attempts have been made to improve the optical and mechanical properties of polyimide films while imparting functionality by combining or modifying monomers of various structures. However, since mechanical and optical properties have a trade-off relationship, such attempts have only yielded the very common result of improved mechanical properties but reduced functionality or degraded optical properties. Therefore, a new approach is needed that can simultaneously impart excellent mechanical, functional, and optical properties.

[0042] The composition for forming a polyimide film according to one embodiment (hereinafter also referred to as the polyimide film forming composition) cannot be used as a polymerization solvent for polyamine acid (hereinafter also referred to as the polyimide precursor) and / or polyimide. By applying a nonpolar solvent that has no affinity for polyimide, it is possible to provide a polyimide film with simultaneously improved optical properties, functionality, and mechanical properties. Specifically, the polyimide film forming composition according to one embodiment can provide a polyimide film that has adhesion at or above the level of existing optical adhesive films, improved yellowness, and significantly reduced distortion due to light. As a result, the polyimide film produced by the polyimide forming composition according to one embodiment can be applied to new substrate materials or cover window materials applicable to foldable or flexible display devices, and the polyimide film has excellent visibility, minimizing eye strain for the user.

[0043] A polyimide film-forming composition according to one embodiment may include a polyamine acid and / or polyimide; a polar solvent; and a nonpolar solvent. The polar solvent may be a hydrophilic solvent and may have affinity for polyamine acid and / or polyimide, for example, an amide solvent. The nonpolar solvent may have little affinity for polyamine acid and / or polyimide, for example, a hydrocarbon solvent.

[0044] While not bound by any particular theory, using a mixed solvent of amide and hydrocarbon solvents can effectively inhibit intermolecular interactions between polymers and / or interactions between polymers and the solvent, significantly reducing the intermolecular packing density during curing and simultaneously improving the desired optical and mechanical properties.

[0045] As a result, the polyimide film-forming composition according to this embodiment can exhibit intermolecular behavior and interactions that differ from those of a simple mixed solution in the polymerization step of polyamine acid. For example, if the hydrocarbon solvent is included in the step of polymerizing polyamine acid, it may act as a factor that hinders polymerization, making it impossible to obtain high molecular weight polyamine acid. On the other hand, in the polyimide film-forming composition according to this embodiment, after obtaining a sufficient high molecular weight of polyamine acid and / or polyimide, mixing the polyamine acid and / or polyimide with the hydrocarbon solvent can act as a catalyst that weakens the intermolecular interactions between polymers and / or the strong interactions between polymers and the solvent, and thereafter, the desired optical properties can be obtained during curing.

[0046] A polyimide film-forming composition for cover windows according to one embodiment may comprise a polyamine acid and / or polyimide containing structural units derived from dianhydrides and structural units derived from diamines, and a mixed solvent of an amide solvent and a hydrocarbon solvent, wherein the hydrocarbon solvent may be present in an amount of 10 to 40% by weight relative to the total weight of the mixed solvent. The structural units derived from dianhydrides may comprise structural units derived from compounds represented by the following chemical formula 1 and the following chemical formula 2, and the structural units derived from diamines may comprise structural units derived from compounds represented by the following chemical formula 3 and the following chemical formula 4. This makes it possible to provide a polyimide film for cover windows with improved optical properties by inhibiting packing density and creating an amorphous structure.

[0047] [Chemical formula 1] [ka]

[0048] [Chemical formula 2] [ka] [Chemical formula 3] [ka]

[0049] [Chemical formula 4] [ka]

[0050] As described above, the polyimide film-forming composition for cover windows according to one embodiment can simultaneously improve excellent optical and mechanical properties by using a mixed solvent of an amide solvent and a hydrocarbon solvent, specifically by obtaining a sufficient molecular weight of polyamine acid and / or polyimide, and then further adjusting the crystallinity of the polyamine acid and / or polyimide in the hydrocarbon solvent solution. Here, by using the amide solvent and the hydrocarbon solvent in sequence, the interaction of the polyamine acid and / or polyimide solvent, which are polyimide precursors, can be adjusted within a more appropriate range. Here, this adjustment may mean inhibition.

[0051] The amide solvent refers to a compound containing an amide moiety. The amide solvent can be aromatic or aliphatic, for example, it can be aliphatic. Furthermore, for example, the amide solvent can be a cyclic compound or a linear compound, and specifically, it can have 2 to 15 carbon atoms, for example, 3 to 10 carbon atoms.

[0052] The amide solvent may contain an N,N-dialkylamide moiety, where the dialkyl groups may exist independently, fuse with each other to form a ring, or at least one of the dialkyl groups may fuse with other substituents within the molecule to form a ring, for example, at least one of the dialkyl groups may fuse with an alkyl group linked to the carbonyl carbon of the amide moiety to form a ring. Here, the ring may be a 4- to 7-membered ring, for example, a 5- to 7-membered ring, for example, a 5- or 6-membered ring. The alkyl group may be, for example, a C1- to C10 alkyl group, for example, a C1- to C8 alkyl group, for example, methyl or ethyl.

[0053] Specifically, the amide solvent is not limited as long as it is commonly used in polyamine acid polymerization, but may include, for example, dimethylpropionamide, diethylpropionamide, dimethylacetylamide, diethylacetamide, dimethylformamide, methylpyrrolidone, ethylpyrrolidone, octylpyrrolidone, or combinations thereof, and more specifically, may include dimethylpropionamide.

[0054] As described above, the hydrocarbon solvent can be a nonpolar solvent.

[0055] The hydrocarbon solvent may be a compound composed of carbon and hydrogen. For example, the hydrocarbon solvent may be aromatic or aliphatic, and may be a cyclic or linear compound, but specifically, it may be a cyclic compound. Here, if the hydrocarbon solvent is a cyclic compound, it may contain monocyclic or polycyclic rings, and the polycyclic ring may be a fused or unfused ring, but specifically, it may be a monocyclic ring.

[0056] The hydrocarbon solvent may have 3 to 15 carbon atoms, for example, 6 to 15 carbon atoms, or for example, 6 to 12 carbon atoms.

[0057] The hydrocarbon solvent may be a substituted or unsubstituted C3-C15 cycloalkane, a substituted or unsubstituted C6-C15 aromatic compound, or a combination thereof. Here, the cycloalkane may be cyclobutane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, or a combination thereof, and the aromatic compound may be benzene, naphthalene, or a combination thereof.

[0058] The hydrocarbon solvent may be a cycloalkane substituted with or unsubstituted with at least one C1-C5 alkyl group, an aromatic compound substituted with or unsubstituted with at least one C1-C5 alkyl group, or a combination thereof, where the cycloalkane and aromatic compound are as described above, respectively.

[0059] The aforementioned C1-C5 alkyl group can be, for example, a C1-C3 alkyl group, or, for example, a C1 or C2 alkyl group, and more specifically, a methyl group, but is not limited thereto.

[0060] Furthermore, the hydrocarbon solvent may optionally contain oxygen. For example, if the hydrocarbon solvent contains oxygen, it may contain ketone groups or hydroxyl groups, such as cyclopentanone, cresol, or a combination thereof.

[0061] Specifically, the hydrocarbon solvent may be, but is not limited to, benzene, toluene, cyclohexane, cyclopentanone, cresol, or a combination thereof.

[0062] More specifically, a polyimide film-forming composition for cover windows according to one example may include a mixed solvent comprising an amide solvent containing dimethylpropionamide and a hydrocarbon solvent selected from toluene, benzene, and cyclohexane.

[0063] A polyimide film-forming composition for cover windows according to one example comprises a polyamine acid and / or polyimide containing structural units derived from the diamine and dianhydride as exemplified above.

[0064] The solids content of the polyimide film-forming composition for cover windows according to one embodiment may be in the range of 10 to 40% by weight, 10 to 35% by weight, or 10 to 20% by weight relative to the total weight of the polyimide film-forming composition. Here, the solids may be the polyamine acid and / or polyimide.

[0065] A polyimide film-forming composition for cover windows according to one embodiment may contain 10 to 40% by weight of the hydrocarbon solvent. Here, the weight percentage is based on the total weight of the solvent, and the total weight of the solvent used as the basis means the sum of the total weights of the amide solvent and the hydrocarbon solvent.

[0066] Furthermore, when the above-mentioned conditions for the mixed solvent are met, improved yellowness and haze can be achieved, and simultaneously, the adhesion to substrates such as glass can be significantly improved. Specifically, the mixed solvent can contain the amide solvent and the hydrocarbon solvent in a weight ratio of 90:10 to 60:40.

[0067] The polyamine acid and / or polyimide may have a weight-average molecular weight (Mw) of 10,000 to 80,000 g / mol, or 10,000 to 70,000 g / mol, or 10,000 to 60,000 g / mol.

[0068] One example of a polyimide film-forming composition for cover windows is BPDA (3,3',4,4'-biphenyltetracarboxylic dianehydride), BTDA (3,3',4,4'-benzophenonetetracarboxylic dianehydride), ODPA (4,4'-oxydiphthalic anhydride), BPADA (4,4'-(4,4'-isopropylbiphenoxy)biphthalic anhydride), DSDA (3,3',4,4'-diphenylsulfonetetracarboxylic It may further include structural units derived from one or more dianhydrides selected from dianehydrides, TMHQ (p-phenylene bis-trimellitic monoester anhydride), ESDA (2,2'-bis(4-hydroxyphenyl)propanedibenzoate-3,3',4,4'-tetracarboxylic dianehydride), NTDA (naphthalene tetracarboxylic dianehydride), and TMEG (ethylene glycol bis(anhydrotrimellitate)).

[0069] Furthermore, one example of a polyimide film-forming composition for cover windows is PDA (p-phenylenediamine), m-PDA (m-phenylenediamine), 4,4'-ODA (4,4'-oxydianiline), 3,4'-ODA (3,4'-oxydianiline), BAPP (2,2-bis(4-[4-aminophenoxy]-phenyl)propane), TPE-Q (1,4-bis(4-aminophenoxy)benzene), TPE-R (1,3-bis(4-aminophenoxy)benzene), BAPB (4,4'-bis(4-aminophenoxy)biphenyl), BAPS (2,2-bis(4-[4-aminophenoxy]phenyl)sulfone), m-BAPS (2,2-bis(4-[3-aminophenoxy]phenyl)sulfone), HAB (3,3'-dihydroxy-4, The structure may further include structural units derived from one or more diamines selected from, for example, 4'-diaminobiphenyl), TB(3,3'-dimethylbenzidine), m-TB(2,2'-dimethylbenzidine), 6FAPB(1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene), 6FODA(2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether), APB(1,3-bis(3-aminophenoxy)benzene), 1,4-ND(1,4-naphthalenediamine), 1,5-ND(1,5-naphthalenediamine), DABA(4,4'-diaminobenzanilide), 6-amino-2-(4-aminophenyl)benzoxazole and 5-amino-2-(4-aminophenyl)benzoxazole.

[0070] Furthermore, the aromatic diamine may further include a combination of fluorinated aromatic diamines. Here, a specific example of the fluorinated aromatic diamine is TFMB (2,2'-bistrifluoromethylbenzidine) further comprising 6FAPB (1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene), 6FODA (2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether), or a combination thereof. This makes it possible to provide a film with higher total light transmittance and lower haze.

[0071] A polyimide film-forming composition according to one embodiment can provide a polyamine acid and / or polyimide further comprising structural units derived from the diamine and dianhydride mentioned above.

[0072] Normally, a high solids content of 10% by weight or more (relative to the total weight of the composition) is required to stably coat a thin film. However, in the case of polyimide, viscosity tends to increase as the solids concentration increases. However, if the viscosity is high and the flow of polymers is poor during the manufacturing process of the thin film through coating, it becomes difficult to remove air bubbles, resulting in uneven coating.

[0073] On the other hand, the polyimide film-forming composition according to one embodiment can significantly reduce the viscosity of the composition even when it contains a high solid content by using a mixed solvent of an amide-based solvent and a hydrocarbon-based solvent. This effectively prevents defects that occur during the coating process and enables the realization of improved optical properties. Furthermore, as mentioned above, when an amide-based solvent is used alone, the high viscosity makes it difficult to increase the concentration of solid content, reducing process efficiency. However, the polyimide film-forming composition for cover windows according to one embodiment has low viscosity, thus preventing defects that occur during the coating process, and its high solid content makes it commercially advantageous.

[0074] Furthermore, a cured film formed by curing the polyimide film-forming composition for cover windows to a thickness of 30 to 150 μm, i.e., a polyimide film for cover windows, can exhibit improved light distortion phenomena, not to mention yellowness, compared to a polyimide film containing a polyimide polymer with a rigid structure. For example, in a polyimide film for cover windows according to one embodiment, the structural units derived from the dianhydride may not include rigid structural units, and may not include structural units derived from a dianhydride in which two anhydride groups are fused into a single ring. The ring may be a monoring or a fused ring, and may be an aromatic ring, an aliphatic ring, or a combination thereof. Specifically, the structural units derived from the dianhydride may not include structural units derived from pyromellitic dianhydride (PMDA), structural units derived from cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA), or a combination thereof.

[0075] As a result, the polyimide film for cover windows according to this embodiment can be transparent even with a thickness of 30 μm or more, and can achieve a low phase difference in the thickness direction, thereby improving visibility, and when using a cover window containing the said polyimide film for cover windows, eye strain can be further reduced. Furthermore, even with a thickness of 30 μm or more, it can have the excellent optical properties described above, so mechanical strength such as modulus can be further improved, and dynamic bending characteristics can be further improved, making it even more suitable for application as a cover window for flexible display devices that are repeatedly folded and unfolded.

[0076] In a polyimide film-forming composition for cover windows according to one embodiment, the structural units derived from the compound represented by chemical formula 1 may be present in an amount of 70 to 95 mol% relative to 100 mol% of the structural units derived from the dianhydride. Here, the structural units derived from the dianhydride may specifically be all mol% of the structural units derived from the compound represented by chemical formula 1 and the compound represented by chemical formula 2. As described above, by including the structural units derived from the compound represented by chemical formula 1, even when the thickness of the polyimide film for cover windows is 30 μm or more, it is possible to not only provide greater transparency and a lower thickness-direction phase difference, but also to have better mechanical properties such as modulus and elongation at break. This makes it possible to achieve optical and mechanical properties equivalent to or better than those of tempered glass.

[0077] By including structural units derived from the compound represented by chemical formula 1 within the above-mentioned range, both mechanical and optical properties can be further improved simultaneously. For example, the structural units derived from the compound represented by chemical formula 1 may be present in an amount of 70-90 mol%, or 70-85 mol%, relative to 100 mol%, of the structural units derived from the dianhydride.

[0078] Furthermore, in a polyimide film forming composition for cover windows according to one embodiment, the structural units derived from the compound represented by chemical formula 3 may be present in an amount of 70 to 95 mol% relative to 100 mol% of the structural units derived from the diamine. Here, the structural units derived from the diamine may specifically be all mol% of the structural units derived from the compound represented by chemical formula 3 and the compound represented by chemical formula 4. As described above, by including the structural units derived from the compound represented by chemical formula 3, even when the thickness of the polyimide film for cover windows is 30 μm or more, visibility can be further improved due to more stable positive wavelength dispersion, and it is possible to have even better mechanical properties such as modulus and elongation at break. This makes it possible to achieve optical and mechanical properties equivalent to or better than those of tempered glass.

[0079] For example, the structural units derived from the compound represented by chemical formula 3 may be present in an amount of 70 to 90 mol%, more specifically 75 to 90 mol%, relative to 100 mol% of the structural units derived from the diamine. By including the structural units derived from the compound represented by chemical formula 3 within the above range, the optical properties can be improved.

[0080] The following describes an example of an application of polyimide film for cover windows.

[0081] A first embodiment of the present invention may be a multilayer structure comprising a polyimide film for cover windows of the present invention. Here, the multilayer structure may include two or more coating layers of polyimide films containing monomers of different compositions from the polyimide film for cover windows of the present invention.

[0082] Furthermore, a second embodiment according to one example may be a cover window for a display device comprising a polyimide film for a cover window according to one embodiment and a coating layer formed on the film.

[0083] Furthermore, a third embodiment, according to one example, can be a flexible display device including the polyimide film for the cover window of one embodiment.

[0084] A polyimide film for a cover window according to one embodiment may have a thickness of 30 to 150 μm, an absolute value of the thickness direction phase difference (Rth) at a wavelength of 550 nm of 500 nm or less, and a yellowness (YI) of 3.5 or less according to ASTM E313. The thickness direction phase difference value may be measured at room temperature before heating the film, and this room temperature may be the temperature without artificial temperature control. For example, this room temperature may be 20°C to 40°C, or 20°C to 30°C, or 23°C to 26°C.

[0085] In one example, when the polyimide film for a cover window has a thickness of 30 to 150 μm, the absolute value of the phase difference in the thickness direction (Rth) at a wavelength of 550 nm can be 350 nm or less, or 50 to 300 nm. As an example, when the polyimide film for a cover window has a thickness of 40 to 80 μm, the absolute value of the phase difference in the thickness direction (Rth) at a wavelength of 550 nm can be 50 to 250 nm, 80 to 240 nm, or 90 to 220 nm.

[0086] Furthermore, when the polyimide film for cover windows according to one example has a thickness of 30 to 150 μm, the degree of yellowness can be 3.5 or less, 3.0 or less, 2.7 or less, or 1 to 2.7. As an example, when the polyimide film for cover windows has a thickness of 40 to 80 μm, the degree of yellowness can be 1.0 to 2.7, or 1.5 to 2.5.

[0087] Specifically, a polyimide film for a cover window according to one embodiment can simultaneously satisfy the thickness-direction phase difference (Rth) and yellowness at a wavelength of 550 nm when the thickness is 30 to 150 μm. Furthermore, the same polyimide film for a cover window can simultaneously satisfy the thickness-direction phase difference (Rth) and yellowness at a wavelength of 550 nm when the thickness is 40 to 80 μm.

[0088] Furthermore, a polyimide film for a cover window according to one embodiment can satisfy the following conditions when its thickness is 30 to 150 μm: (a) a modulus of 4 GPa or more according to ASTM E111, and (b) an elongation at break of 15% or more. More specifically, it can satisfy these mechanical properties simultaneously with the thickness direction phase difference (Rth) and yellowness at a wavelength of 550 nm mentioned above.

[0089] A polyimide film for cover windows according to one embodiment may have a modulus of 4 GPa or higher, or 4.1 GPa or higher, or 4.1 to 6 GPa according to ASTM E111. Furthermore, the polyimide film for cover windows may have an elongation at break of 15% or higher, or 16% or higher, or 18% or higher, or 20% or higher, or 25 to 40%, specifically, it may satisfy the above-mentioned modulus and elongation at break simultaneously. This provides sufficient mechanical properties and durability for application to cover windows.

[0090] The first, second, or third embodiment described above includes a polyimide film for cover windows that significantly reduces light-induced distortion and satisfies mechanical properties such as modulus and elongation at break, and may further include a functional coating layer as needed.

[0091] The coating layer is formed on at least one other surface of the polyimide film or substrate for the cover window in one embodiment, and non-limiting examples include a hard coating layer, an antistatic layer, an anti-fingerprint layer, an anti-fouling layer, an anti-scratch layer, a low refractive index layer, an anti-reflective layer, and an impact-absorbing layer, and may comprise at least one or more coating layers. In this case, the thickness of the coating layer may be 1 to 500 μm, 2 to 450 μm, or 2 to 200 μm.

[0092] With such excellent optical and mechanical properties, the polyimide film for cover windows is expected to have a wide range of applications in various industrial fields, including cover windows for display devices, where it can ensure a wide viewing angle by exhibiting sufficiently low phase difference at various angles.

[0093] A polyimide film for cover windows, according to one embodiment, can prevent image distortion caused by light and provide improved visibility by satisfying all of the following requirements: yellowness, thickness-direction phase difference within the above-mentioned range, modulus, and elongation at break. Furthermore, it can exhibit more uniform mechanical properties (such as modulus) and optical properties (such as yellowness and thickness-direction phase difference) throughout the central and edge portions of the film, further reducing film loss. In addition, because the cover window polyimide film is flexible and has excellent bending properties, even if a predetermined deformation occurs repeatedly, the film will not deform and / or be damaged, and can return to its original shape more easily.

[0094] A cover window containing a polyimide film for cover windows, as described in one example, can have better visibility, prevent the occurrence of creases and microcracks, and provide a more durable and longer-lasting flexible display device.

[0095] A polyimide film for a cover window according to one embodiment can be manufactured from a polyimide resin containing structural units derived from the diamine and dianhydride as exemplified above, and specifically, the polyimide resin may, but is not limited to, have a weight-average molecular weight (Mw) of 10,000 to 80,000 g / mol, or 10,000 to 70,000 g / mol, or 10,000 to 60,000 g / mol.

[0096] As described above, the polyimide film for cover windows, based on this embodiment, possesses excellent optical and mechanical properties, enabling it to exhibit sufficiently low phase differences at various angles, including in the cover windows of display devices. This allows it to be applied in various industrial fields where a wide viewing angle is required.

[0097] For example, the display device is not particularly limited as long as it is in a field that requires excellent optical properties, and a suitable display panel can be selected and provided. Specifically, the polyimide film for the cover window can be applied to a flexible display device. Non-limiting examples include, but are not limited to, various image display devices such as liquid crystal display devices, electroluminescent display devices, plasma display devices, and field emission display devices.

[0098] Furthermore, the display device including the cover window polyimide film according to the above-described example not only exhibits high transparency and excellent display quality, but also significantly reduces light-induced distortion, particularly improving the rainbow effect that causes iridescent stains, thus minimizing eye strain for the user through superior visibility. In particular, as the screen size of the display device increases, the screen is often viewed from the side, but when the cover window polyimide film according to the above-described example is applied to the display device, it offers excellent visibility even when viewed from the side, making it useful for large display devices.

[0099] The following describes a method for manufacturing a polyimide film for cover windows, based on one example.

[0100] In one example, if a cured film formed with a thickness of 30 to 150 μm is produced that simultaneously satisfies the physical properties of having an absolute value of the thickness direction phase difference (Rth) at a wavelength of 550 nm of 500 nm or less and a yellowness (YI) of 3.5 or less according to ASTM E313, the manufacturing method is not limited, and the method described later is merely an example; as long as a film satisfying the aforementioned physical properties is produced, the method is not limited to the method described later.

[0101] Specifically, a method for manufacturing a polyimide film for cover windows according to one embodiment may be manufactured by applying the above-described polyimide film-forming composition for cover windows onto a substrate such as glass, and then heat-curing it, or by drying and heat-curing it. More specifically, the method includes the steps of i) reacting a dianhydride and a diamine in an amide solvent to produce a polyamine acid and / or polyimide solution, ii) adding a hydrocarbon solvent to the polyamine acid and / or polyimide solution to adjust the crystallinity of the polyamine acid and / or polyimide, and iii) applying the polyimide film-forming composition for cover windows obtained in step ii) onto a substrate and curing it, wherein the hydrocarbon solvent in step ii) may be added in an amount of 10 to 40% by weight relative to the total weight of the amide solvent and the hydrocarbon solvent. Here, the dianhydride and diamine are as described above, for example, the dianhydride may include the compound represented by chemical formula 1 and the compound represented by chemical formula 2, and the diamine may include the compound represented by chemical formula 3 and the compound represented by chemical formula 4.

[0102] Specifically, in a method for producing a polyimide film for cover windows according to one embodiment, the polyamine acid and / or polyimide solution may contain 70 to 95 mol% of the compound represented by chemical formula 1 based on 100 mol% of the dianhydride. Furthermore, the compound represented by chemical formula 2 may be present in an amount of 5 to 30 mol% based on 100 mol% of the dianhydride. Additionally, the compound represented by chemical formula 1 may be present in an amount of 70 to 90 mol%, or 70 to 85 mol%, respectively.

[0103] Specifically, in a method for producing a polyimide film for cover windows according to one embodiment, the polyamine acid and / or polyimide solution may contain 70 to 95 mol% of the compound represented by chemical formula 3 with respect to 100 mol% of the diamine. Furthermore, the compound represented by chemical formula 4 may be present in an amount of 5 to 30 mol% with respect to 100 mol% of the diamine. Additionally, the compound represented by chemical formula 3 may be present in an amount of 70 to 90 mol%, more specifically, 75 to 90 mol%.

[0104] Furthermore, the dianhydride and the diamine may be present in a molar ratio of 1:0.9 to 1:1.1, where the dianhydride may include the compound represented by chemical formula 1 and the compound represented by chemical formula 2. For example, the number of moles of the dianhydride may be the sum of the number of moles of the compound represented by chemical formula 2 and the compound represented by chemical formula 2.

[0105] Furthermore, the polyamine acid and / or polyimide solution satisfying such molar percentages may have a solid content of 10 to 40% by weight relative to the total weight. Here, the solid content may be the polyamine acid and / or polyimide, and the remainder may be an organic solvent.

[0106] In one embodiment of a method for manufacturing a polyimide film for cover windows, the solid content of the polyamine acid and / or polyimide solution can be 40% by weight or less, 35% by weight or less, or in the range of 10 to 20% by weight. More specifically, according to one embodiment, even when the solid content of the polyamine acid and / or polyimide solution is 10 to 20% by weight, it can have low viscosity and provide process advantages. Generally, mechanical properties such as the absolute value of the thickness direction phase difference (Rth) and its modulus are in a trade-off relationship with each other, making it difficult to improve these properties simultaneously. However, according to one embodiment, these properties can be improved simultaneously even at thicknesses of 30 μm or more.

[0107] The step of adjusting the crystallinity of the polyamine acid in step ii) above can be carried out by adding the hydrocarbon solvent to the polyamine acid solution, or by adding the hydrocarbon solvent and a solvent different from the hydrocarbon solvent. Here, the other solvent can be an amide solvent, and the amide solvent can be the same as or different from the amide solvent added in step i). By adding the hydrocarbon solvent, the intermolecular interaction between the polyamine acid and / or polyimide and / or the interaction between the polymer and the solvent can be effectively inhibited, and the intermolecular packing density can be significantly reduced during curing. As a result, according to one embodiment, it is possible to provide a polyimide film for cover windows that is 30 μm or thicker, which significantly improves the yellowness and the thickness direction phase difference (Rth) at a wavelength of 550 nm, and at the same time achieves mechanical properties that reach a level similar to tempered glass. In particular, it is possible to provide a polyimide film for cover windows that is 30 μm or thicker, yet exhibits significantly reduced unevenness due to viewing angle.

[0108] The curing step can be carried out by thermal curing. Here, in addition to thermal curing, various known methods such as chemical curing, infrared curing, batch curing, and continuous curing can be used instead, or different curing methods can be used instead.

[0109] The aforementioned thermosetting can be carried out at 80-300°C, 100-280°C, or 150-250°C.

[0110] The aforementioned thermal curing can be carried out at 80-100°C for 1 minute to 2 hours, or at over 100-200°C for 1 minute to 2 hours, or at over 200-300°C for 1 minute to 2 hours, and stepwise thermal curing can also be carried out under two or more temperature conditions selected from these. Furthermore, thermal curing can be carried out in a separate vacuum oven or an oven filled with inert gas, but is not necessarily limited to these.

[0111] Furthermore, a drying step may be performed before the aforementioned heat curing, if necessary. The drying step may be performed at 30-70°C, 35-65°C, or 40-55°C, but is not limited to these temperatures.

[0112] Furthermore, in the method for manufacturing a polyimide film for a cover window according to one embodiment, the coating for forming the polyimide film can be used without limitation as long as it is commonly used in the relevant field. Non-limiting examples include knife coating, dip coating, roll coating, slot die coating, lip die coating, slide coating, and curtain coating, and it goes without saying that the same or different coatings can be applied sequentially one or more times.

[0113] The substrate can be used without limitation as long as it is commonly used in the relevant field, and non-limiting examples include glass; stainless steel; or plastic films such as polyethylene terephthalate, polyethylene naphthalate, polypropylene, polyethylene, cellulose triacetate, cellulose diacetate, alkyl poly(meth)acrylate, poly(meth)acrylate copolymer, polyvinyl chloride, polyvinyl alcohol, polycarbonate, polystyrene, cellophane, polyvinylidene chloride copolymer, polyamide, polyimide, vinyl chloride-vinyl acetate copolymer, polytetrafluoroethylene, and polytrifluoroethylene.

[0114] In a method for manufacturing a polyimide film for cover windows according to one embodiment, the method may further include a resting step in which the polyimide film-forming composition for cover windows is applied to a substrate and then left at room temperature, if necessary. This resting step allows for more stable maintenance of the optical properties of the film surface. Although not bound by any particular theory, in conventional polyimide film-forming compositions for cover windows, if such a resting step is performed before curing, the solvent absorbs moisture from the air, the moisture diffuses into the interior, and collides with the polyamine acid and / or polyimide, causing clouding and aggregation from the surface of the film, resulting in coating non-uniformity. On the other hand, the polyimide film-forming composition for cover windows according to one embodiment has the advantage of ensuring a film with improved optical properties without clouding or aggregation even when left in the air for a long period of time.

[0115] The aforementioned standing step may be carried out at room temperature and / or high humidity conditions. Here, the room temperature may be 40°C or lower, for example, 30°C or lower, for example, 25°C or lower, and more specifically, 15 to 25°C, for example, 20 to 25°C. The high humidity may be a relative humidity of 50% or higher, for example, 60% or higher, for example, 70% or higher, for example, 80% or higher.

[0116] The aforementioned leaving step can be performed for 1 minute to 3 hours, for example, 10 minutes to 2 hours, or for example, 20 minutes to 1 hour.

[0117] In one embodiment of a method for producing a polyimide film for cover windows, the polyimide film for cover windows can also be produced by mixing one or more additives selected from flame retardants, adhesion improvers, inorganic particles, antioxidants, UV inhibitors, and plasticizers into the polyamine acid solution.

[0118] The following description will illustrate a specific example, but the present invention is not limited to the following example.

[0119] In the experiment described below, the physical properties were measured as follows.

[0120] <Yellowness (YI)> Measurements were taken using a spectrophotometer (Nippon Denshoku Co., Ltd., COH-5500) in accordance with the ASTM E313 standard.

[0121] [Example 1] Manufacturing of polyimide film-forming composition for cover windows (TFMB(0.9) / DDS(0.1) / BPAF(0.7) / 6FDA(0.3)) After filling a stirrer with a nitrogen gas flow with 284.5g of dimethylpropionamide (N,N-dimethylpropionamide, DMPA), 26.9g of 2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine (TFMB) and 2.32g of 4,4'-diaminodiphenyl sulfone (DDS) were dissolved while maintaining the reactor temperature at 25°C. To this, 30 g of 9,9-bis(3,4-dicarboxyphenyl)fluorene anhydride (BPAF) and 12.45 g of 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (FDA) were added at 25°C and stirred for 6 hours to dissolve and react. Next, 121.9 g of toluene was added at 25°C and stirred for 18 hours. Then, DMPA and / or toluene were added to produce polyimide film-forming composition 1 for cover windows, so that the solids content was 14% by weight and the toluene content in the composition was 30% by weight relative to the total weight of DMPA and toluene (i.e., DMPA:toluene = 70% by weight:30% by weight).

[0122] Manufacturing of polyimide films for cover windows The polyimide film-forming composition 1 for cover windows obtained above was applied to one surface of a glass substrate (1.0T) using a #20 Meyer bar. After curing by heating at 80°C for 30 minutes and then at 350°C for 15 minutes under a nitrogen atmosphere, it was peeled off using a glass substrate to obtain the polyimide film for cover windows of Example 1 with a thickness of 50 μm.

[0123] [Example 2] Manufacturing of polyimide film-forming composition for cover windows (TFMB(0.85) / DDS(0.15) / BPAF(0.7) / 6FDA(0.3)) After filling a stirrer with a nitrogen stream with 283.2g of dimethylpropionamide (N,N-dimethylpropionamide, DMPA), 25.4g of 2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine (TFMB) and 3.47g of 4,4'-diaminodiphenyl sulfone (DDS) were dissolved while maintaining the reactor temperature at 25°C. To this, 30 g of 9,9-bis(3,4-dicarboxyphenyl)fluorene anhydride (BPAF) and 12.45 g of 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (FDA) were added at 25°C and stirred for 6 hours to dissolve and react. Next, 121.4 g of toluene was added at 25°C and stirred for 18 hours. Then, DMPA and / or toluene were added to produce polyimide film-forming composition 2 for cover windows, such that the solids content was 14% by weight and the toluene content in the composition was 30% by weight relative to the total weight of DMPA and toluene (i.e., DMPA:toluene = 70% by weight:30% by weight).

[0124] Manufacturing of polyimide films for cover windows Using the acquired polyimide-forming composition 2 for cover windows, a polyimide film for cover windows of Example 2 with a thickness of 50 μm was obtained in the same manner as in Example 1.

[0125] [Examples 3 and 4] The polyimide film-forming compositions 3 and 4 for cover windows were produced using the same method as in Example 1, but with the toluene content (T content) adjusted relative to the total weight of DMPA and toluene as shown in Table 1 below.

[0126] Manufacturing of polyimide films for cover windows Using the respective polyimide film-forming compositions for cover windows obtained, polyimide films for cover windows of Examples 3 and 4 with a thickness of 50 μm were obtained in the same manner as in Example 1.

[0127] [Comparative Example 1 and Comparative Example 2] The same method as in Example 1 was used to produce polyimide film-forming compositions A and B for cover windows, respectively. However, the toluene content (T content) relative to the total weight of DMPA and toluene was adjusted as shown in Table 1 below.

[0128] The acquired polyimide film-forming compositions A and B for cover windows were applied to one surface of a glass substrate (1.0T) using a #20 Meyer bar, cured by heating at 80°C for 15 minutes under a nitrogen atmosphere, and then at 350°C for 15 minutes, and peeled off the glass substrate to obtain polyimide films for cover windows of Comparative Examples 1 and 2 with a thickness of 50 μm.

[0129] [Comparative Example 3] Manufacturing of polyimide film for cover windows ((TFMB(1) / PMDA(0.3) / BPAF(0.7)) After filling a stirrer with a nitrogen stream with 484g of dimethylpropionamide (DMPA), 29.9g of 2,2'-bis(trifluoromethyl)-4,4'-biphenyl diamine (TFMB) was dissolved while maintaining the reactor temperature at 25°C. To this, 30g of 9,9-bis(3,4-dicarboxyphenyl)fluorene anhydride (BPAF) and 6.1g of pyromellitic dianhydride (PMDA) were added at 25°C and stirred for 24 hours to dissolve and react. Next, DMPA solvent was added to bring the solid content to 12% by weight to produce polyimide film-forming composition C for cover windows.

[0130] The acquired polyimide film-forming composition C for cover windows was applied to one surface of a glass substrate (1.0T) using a #20 Meyer bar, cured by heating at 80°C for 15 minutes under a nitrogen atmosphere, and then at 350°C for 15 minutes, and peeled off the glass substrate to obtain a polyimide film for cover windows of Comparative Example 3 with a thickness of 50 μm.

[0131] [Comparative Example 4] Manufacturing of polyimide film for cover windows ((TFMB(1) / PMDA(0.7) / BPAF(0.3)) Cover window polyimide film forming composition D was manufactured in the same manner as in Comparative Example 3, except that the molar ratios of TFMB, PMDA, and BPAF were changed as shown in Table 1 below, and a cover window polyimide film with a thickness of 50 μm was obtained.

[0132] [Table 1]

[0133] Rating: Yellowness The yellowness (YI) of the polyimide films for cover windows in Examples 1 to 4 and Comparative Examples 1 to 4 was measured and is shown in Table 2 below.

[0134] [Table 2]

[0135] Referring to Table 2 above, polyimide film-forming compositions 1 to 4 for cover windows according to Examples 1 to 4 can easily form polyimide films for cover windows having a thickness of 50 μm or more, which is sufficient for use as cover windows. On the other hand, polyimide film-forming composition A for cover windows according to Comparative Example 1 had a high initial polymerization solids content, resulting in a solution viscosity that was too high to be controlled, or precipitation occurred, making polymerization of the polymer and production of the polyimide film impossible.

[0136] Furthermore, it can be confirmed that the polyimide films for cover windows produced with polyimide film-forming compositions 1 to 4 according to Examples 1 to 4 have a lower degree of yellowness compared to the polyimide films for cover windows produced with polyimide film-forming compositions B to D according to Comparative Examples 2 to 4.

[0137] In particular, the polyimide films for cover windows in Comparative Example 3 and Comparative Example 4 are colored films with very high yellowness values ​​of 53 and 80, respectively, resulting in poor visibility, and can be seen as being even more unsuitable for application as cover windows.

[0138] As described above, although one specific example has been used to illustrate the invention, this is provided to facilitate a more general understanding of the invention. The invention is not limited to the above-described example, and various modifications and variations can be made from this description by those who have ordinary skill in the art to which the invention pertains.

[0139] Therefore, the concept of the present invention should not be limited to the embodiments described above. Not only the claims described later, but also all modifications that are equivalent or comparable to the claims described herein can be said to fall within the scope of the concept of the present invention.

Claims

1. a polyamic acid or polyimide comprising structural units derived from a dianhydride and structural units derived from a diamine; A polyimide film-forming composition for a cover window, comprising a mixed solvent of an amide solvent and a hydrocarbon solvent, the hydrocarbon solvent being contained in an amount of 10 to 40% by weight based on a total weight of the mixed solvent, The structural units derived from the dianhydride include structural units derived from a compound represented by Chemical Formula 1 below and a compound represented by Chemical Formula 2 below, and the structural units derived from the diamine include structural units derived from a compound represented by Chemical Formula 3 below and a compound represented by Chemical Formula 4 below. [Chemical formula 1] 【Chemistry 1】 [Chemical formula 2] 【Chemistry 2】 [Chemical formula 3] 【Chemistry 3】 [Chemical formula 4] 【Chemistry 4】

2. The amide solvent is 2. The covering window polyimide film-forming composition of claim 1, comprising dimethylpropionamide.

3. The hydrocarbon solvent is 2. The polyimide film-forming composition for cover windows according to claim 1, wherein the solvent is a cyclic hydrocarbon solvent.

4. The cyclic hydrocarbon solvent is 4. The covering window polyimide film-forming composition of claim 3, comprising toluene, benzene, cyclohexane or a combination thereof.

5. 2. The polyimide film-forming composition for cover windows according to claim 1, comprising 10 to 40% by weight of solids based on the total weight of the polyimide film-forming composition for cover windows.

6. The structural unit derived from the compound represented by Formula 1 is 2. The polyimide film-forming composition for covering windows according to claim 1, wherein the structural unit derived from the dianhydride is contained in an amount of 70 mol % to 95 mol % based on 100 mol %.

7. The structural unit derived from the compound represented by Formula 3 is 2. The polyimide film-forming composition for covering windows according to claim 1, wherein the structural unit derived from the diamine is contained in an amount of 70 mol % to 95 mol % based on 100 mol %.

8. i) reacting a compound represented by the following formula 1 and a compound represented by the following formula 2 with a compound represented by the following formula 3 and a compound represented by the following formula 4 in an amide solvent to prepare a polyamic acid solution; ii) adding a hydrocarbon solvent to the polyamic acid solution to adjust the crystallinity of the polyamic acid; iii) applying the cover window polyimide film-forming composition obtained in step ii) onto a substrate and curing the composition; The method for producing a polyimide film for a cover window, wherein the hydrocarbon solvent in step ii) is added in an amount of 10 to 40% by weight based on the total weight of the amide solvent and the hydrocarbon solvent. [Chemical formula 1] 【Chemistry 5】 [Chemical formula 2] 【Chemistry 6】 [Chemical formula 3] 【Chemistry 7】 [Chemical formula 4] 【Chemistry 8】

9. The curing step of the iii) step is The method for producing a polyimide film for a cover window according to claim 8, wherein the method is carried out by heating at 80 to 300°C.

10. After the application of step iii), The method for producing a polyimide film for a cover window according to claim 8, further comprising a step of leaving the film at room temperature.

11. A polyimide film for cover windows produced from the polyimide film-forming composition for cover windows according to any one of claims 1 to 7.

12. 12. The polyimide film for cover windows according to claim 11, which has a thickness of 30 to 150 μm and a yellowness index (YI) according to ASTM E313 of 3.5 or less.

13. 12. The polyimide film for cover windows according to claim 11, which has a thickness of 40 to 80 μm and a yellowness index (YI) according to ASTM E313 of 1.0 to 2.

7.

14. A multilayer structure comprising the polyimide film of claim 11 formed on one side of a substrate.

15. 15. The multi-layer structure of claim 14, further comprising a coating layer formed on said polyimide film.

16. The coating layer is 16. The multilayer structure of claim 15, which is a hard coating layer, an antistatic layer, an anti-fingerprint layer, an anti-soiling layer, an anti-scratch layer, a low refractive index layer, an anti-reflective layer, an impact absorbing layer, or a combination thereof.

17. A cover window for a display device comprising the polyimide film of claim 11.

18. A flexible display device comprising the cover window for a display device according to claim 17.

Images