Polyimide film for cover windows and display device containing the same

Abstract

To provide a polyimide film for cover windows that is colorless and transparent, has no optical unevenness, and also has excellent thermostability and mechanical properties.SOLUTION: A polyimide film contains, as constitutional units derived from a dianhydride, a constitutional unit derived from a compound of the following structure and a constitutional unit derived from 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, and as constitutional units derived from a diamine, a constitutional unit derived from 2,2'-bis(trifluoromethyl)benzidine and a constitutional unit derived from 4,4'-diaminodiphenyl sulfone.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a polyimide film for a cover window and a display device including the same.

Background Art

[0002] A polyimide film (PI) has excellent heat resistance with insolubility and infusibility, and also has excellent oxidation resistance, heat resistance, radiation resistance, low-temperature characteristics, and chemical resistance. Therefore, polyimide films are used in a wide range of technical fields such as heat-resistant advanced materials for automobiles, aircraft, and spacecraft, and electronic materials such as insulating coating agents, insulating films, semiconductors, and electrode protection films for TFT-LCDs. In recent years, 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 must have insulation properties above a certain level. In addition, as portable electronic devices and communication devices become thinner, slimmer, and more flexible, flexibility is required along with mechanical properties such as high hardness and high rigidity. Also, generally, as a coating layer is laminated on a substrate to impart various physical properties, light diffused reflection is induced in the cover window, optical unevenness may occur, and visibility may deteriorate. Therefore, optical physical properties such as high display quality and 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 embodiment provides a polyimide film for cover windows that can meet the performance requirements of advanced cover windows.

[0006] In detail, one embodiment provides a polyimide film for cover windows that has a low thickness-direction phase difference in the visible light region, which is effective in preventing reflections over a wide viewing angle and can significantly reduce unevenness.

[0007] In detail, one embodiment provides a polyimide film for cover windows that is colorless and transparent, has no degradation in optical properties, is free from optical unevenness, has excellent optical properties such as visibility, and is excellent in heat resistance and mechanical properties.

[0008] Another embodiment provides a cover window for a display device that includes the polyimide film. Another embodiment provides a flexible display device including the polyimide film or the cover window. [Means for solving the problem]

[0009] A polyimide film for a cover window according to one embodiment may include structural units derived from dianhydrides and structural units derived from diamines, wherein the structural units derived from dianhydrides include structural units derived from a compound represented by the following chemical formula 1 and structural units derived from a compound represented by the following chemical formula 2, and the structural units derived from diamines include structural units derived from a compound represented by the following chemical formula 3 and structural units derived from a compound represented by the following chemical formula 4, and the polyimide film for a cover window 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.

[0010] [Chemical formula 1] [ka]

[0011] [Chemical Formula 2] [Chemistry]

[0012] [Chemical Formula 3] [Chemistry]

[0013] [Chemical Formula 4] [Chemistry]

[0014] [[ID=‘35]] The polyimide film may satisfy the wavelength dispersibility with the following Formula 1 and the following Formula 2. [Formula 1] 1.06 ≦ Rth(450nm) / Rth(550nm) ≦ 1.10 [Formula 2] 0.95 ≦ Rth(650nm) / Rth(550nm) ≦ 0.98

[0015] [In the above Formula 1 and Formula 2, Rth(450nm) is the absolute value of the thickness direction retardation at a wavelength of 450 nm (unit: nm), Rth(‘550nm) is the absolute value of the thickness direction retardation at a wavelength of 550 nm (unit: nm), Rth(650nm) is the absolute value of the thickness direction retardation at a wavelength of 650 nm (unit: nm).]

[0016] The polyimide film may have a modulus of 4 GPa or more and an elongation at break of 15% or more according to ASTM E111. In the polyimide film, the structural unit derived from the compound represented by the above 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, 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] The polyimide film may have a thickness of 40 to 80 μm, an absolute value of the retardation in the thickness direction (Rth) at a wavelength of 550 nm of 50 to ......................................................

[0019] Also, the laminate according to another embodiment may include the polyimide film formed on one surface of a substrate. Also, the cover window for a display device according to still another embodiment may include the polyimide film and a coating layer formed on the polyimide film.

[0020] The coating layer may be a hard coating layer, an antistatic layer, an anti-fingerprint layer, an antifouling layer, a scratch-resistant layer, a low-refractive-index layer, an antireflection layer, a shock-absorbing layer, or a combination thereof. Also, the flexible display device according to still another embodiment may include the polyimide film.

Advantages of the Invention

[0021] The polyimide film for a cover window according to one embodiment can significantly improve the unevenness phenomenon that causes deterioration in visibility, particularly the rainbow phenomenon due to retardation.

[0022] It should be noted that in the translation of the sentence in ID=7, the content after "250nmであり" is incomplete in the original text you provided. I have translated it as far as possible according to the existing content. You can check and supplement the complete information for a more accurate translation.Furthermore, it is possible to achieve colorless and transparent optical properties even in a thickness range that has mechanical strength comparable to that of tempered glass. In addition, by having a low thickness-direction phase difference (Rth) over a wide visible light range, the reflective appearance can be dramatically improved. At the same time, in addition to the high strength characteristics mentioned above, it has excellent bending properties, which prevents cracking and breakage due to bending. Therefore, the polyimide film for cover windows according to one embodiment can be usefully applied to optical applications such as foldable display devices or flexible display devices. [Modes for carrying out the invention]

[0023] Hereinafter, one embodiment will be described in detail 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 may be realized in various other forms and is not limited to the embodiment described herein. Nor is it intended to limit the scope of protection as defined by the claims.

[0024] Furthermore, unless otherwise defined, technical and scientific terms used herein may have the meanings that are ordinarily understood by a person with ordinary skill in the art to which this invention pertains.

[0025] Throughout this specification, when a part “includes” a component, unless otherwise stated, it may include other components rather than excluding them.

[0026] In this specification, unless otherwise defined, when a part such as a layer, film, thin film, region, or plate is said to be "on top of" or "above" another part, this may include not only when it is "directly above" the other part, but also when another part exists between them.

[0027] Hereinafter, unless otherwise defined, “these combinations” may mean a mixture or copolymerization of the components. Hereinafter, unless otherwise defined, "A and / or B" may mean a configuration that includes both A and B simultaneously, or a configuration selected from both A and B.

[0028] Hereinafter, unless otherwise defined, "derived" means that at least one of the functional groups of a compound has been modified, and may specifically include forms in which the reactive and / or detaching groups of a compound have been modified or detached in response to a reaction. Furthermore, if structures derived from different compounds are identical to each other, a structure derived from one compound may also be derived from any one of the other compounds and have the same structure.

[0029] In this specification, unless otherwise defined, “polymer” means a relatively high molecular weight molecule whose structure may include multiple repeats of units derived from low molecular weight molecules. In one embodiment, the 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 another embodiment, the polymer may be a homopolymer (e.g., a copolymer containing one monomer).

[0030] Hereinafter, unless otherwise defined, "polyamic acid" means a polymer containing structural units having an amic acid molecule, and "polyimide" may mean a polymer containing structural units having an imide molecule.

[0031] Hereinafter, unless otherwise defined, the term "unevenness" may be interpreted as encompassing all light-induced distortion phenomena that can occur at a particular angle. Examples of such distortions include blackout phenomena where the screen appears black, hotspot phenomena, and rainbow phenomena with iridescent unevenness in the screen of a display device containing a polyimide film.

[0032] A polyimide film for a cover window according to one embodiment will be described below. Polyimide film is attracting attention as a replacement for the expensive tempered glass traditionally used as cover windows, but polyimide film is susceptible to distortion caused by light. However, since the cover window formed on the outermost surface of a display device is directly visible to the naked eye, it is crucial that the film does not experience distortion caused by light. Therefore, a polyimide film that can fundamentally solve the problem of distortion caused by light is needed.

[0033] A polyimide film for a cover window according to one embodiment includes structural units derived from dianhydrides and structural units derived from diamines. Specifically, the structural units derived from dianhydrides include structural units derived from a compound represented by the following chemical formula 1 and structural units derived from a compound represented by the following chemical formula 2, and the structural units derived from diamines may include structural units derived from a compound represented by the following chemical formula 3 and structural units derived from a compound represented by the following chemical formula 4. Here, the polyimide film for the cover window 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. As a result, even with a thickness of 30 μm or more, it is possible to reduce the optical distortion of a cover window containing the polyimide film for the cover window by having excellent transparency and reducing light distortion. Furthermore, while it can replace tempered glass, it significantly improves the iridescent unevenness that occurs when viewed from various angles, thereby providing superior optical properties compared to conventional polyimide films.

[0034] [Chemical formula 1] [ka]

[0035] [Chemical formula 2] [ka]

[0036] [Chemical formula 3] [ka]

[0037] [Chemical formula 4] [ka]

[0038] The aforementioned thickness-direction phase difference value may be measured at room temperature before heating the film, and this room temperature may be the temperature when the temperature has not been artificially controlled. For example, this room temperature may be 20°C to 40°C, or 20°C to 30°C, or 23°C to 26°C.

[0039] As described above, the polyimide film for cover windows contains structural units derived from the compounds represented by chemical formulas 1 to 4, thereby further improving the light distortion phenomenon compared to polyimide films containing polyimide polymers with rigid structures. For example, in a polyimide film for cover windows according to one embodiment, the structural units derived from the dianhydride do not have to include rigid structural units, and do not have to include structural units derived from a dianhydride in which two anhydride groups are condensed into one 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 do not have to include structural units derived from pyromellitic dianhydride (PMDA), structural units derived from cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA), or a combination thereof.

[0040] As a result, the polyimide film for cover windows according to one embodiment can achieve low thickness-direction phase difference while remaining transparent even at a thickness of 30 μm or more, further improving visibility. Therefore, eye fatigue can be further reduced when using a cover window containing the polyimide film for cover windows. Furthermore, since it can have excellent optical properties as described above even with a thickness of 30 μm or more, mechanical strength such as modulus can be further improved, and dynamic bending characteristics are further improved, making it even more suitable for application to cover windows of flexible display devices that repeatedly fold and unfold.

[0041] As an example, the polyimide film for the cover window may satisfy the following formula 1. By satisfying formula 1, the polyimide film for the cover window can have positive dispersion characteristics and maintain a nearly constant thickness-direction phase difference value even when moving to the long-wavelength region. As a result, the polyimide film for the cover window can be given an even better viewing angle and visibility, and by minimizing wavelength dispersion, the reflective appearance can be further improved.

[0042] [Formula 1] 1.06≦Rth(450nm) / Rth(550nm)≦1.10 [In the above formula 1, Rth(450nm) is the absolute value (in nm) of the phase difference in the thickness direction at a wavelength of 450 nm. Rth(550nm) is the absolute value (in nm) of the phase difference in the thickness direction at a wavelength of 550 nm. For example, in formula 1, Rth(450nm) / Rth(550nm) may be 1.095 or less, for example, 1.09 or less.

[0043] As an example, the polyimide film for the cover window may satisfy the following formula 2. By satisfying formula 2, the polyimide film for the cover window can exhibit even more stable positive wavelength dispersion when moving to the long wavelength region. As a result, visibility can be further improved by satisfying formula 2 for the polyimide film for the cover window.

[0044] [Formula 2] 0.95≦Rth(650nm) / Rth(550nm)≦0.98 [In the above formula 2, Rth(550nm) is the absolute value (in nm) of the phase difference in the thickness direction at a wavelength of 550 nm. Rth(650nm) is the absolute value (in nm) of the phase difference in the thickness direction at a wavelength of 650 nm. For example, in formula 2, Rth(650nm) / Rth(550nm) may be 0.955 or greater, for example, 0.96 or greater.

[0045] More specifically, the polyimide film for the cover window may satisfy both formulas 1 and 2 simultaneously, thereby further improving the positive wavelength dispersion.

[0046] As an example, the structural units derived from the compound represented by chemical formula 1 may be included in an amount of 70 to 95 mol%, based on 100 mol% of the structural units derived from the dianhydride. Here, the structural units derived from the dianhydride may specifically be structural units derived from the compound represented by chemical formula 1 and structural units derived from the compound represented by chemical formula 2, and their total moles may be defined as 100 mol%. As described above, by including structural units derived from the dianhydride, even when the thickness of the polyimide film for the cover window is 30 μm or more, it is possible to provide not only transparency but also a low thickness-direction phase difference, as well as 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.

[0047] Specifically, the structural units derived from the compound represented by chemical formula 1 may be present in an amount of 70 to 90 mol%, more specifically 70 to 85 mol%, based on 100 mol% of the structural units derived from the dianhydride. By including the structural units derived from the compound represented by chemical formula 1 within the above range, the mechanical and optical properties can be further improved simultaneously.

[0048] As an example, the structural units derived from the compound represented by chemical formula 3 may be included in an amount of 70 to 95 mol%, based on 100 mol% of the structural units derived from the diamine. Here, the structural units derived from the diamine may specifically be structural units derived from the compound represented by chemical formula 3 and structural units derived from the compound represented by chemical formula 4, and their total moles may be defined as 100 mol%. As described above, by including structural units derived from the diamine, even when the thickness of the polyimide film for the cover window is 30 μm or more, visibility can be further improved due to more stable positive wavelength dispersion, and mechanical properties such as modulus and elongation at break can be further improved. This makes it possible to achieve optical and mechanical properties equivalent to or better than those of tempered glass.

[0049] Specifically, 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%, based on 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 further improved.

[0050] In one embodiment, 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 may be 400 nm or less, 350 nm or less, or 50 to 300 nm.

[0051] Furthermore, in one embodiment of the polyimide film for cover windows, when the thickness is 40 to 80 μm, the absolute value of the phase difference in the thickness direction (Rth) at a wavelength of 550 nm may be 50 to 250 nm, or 80 to 240 nm, or 90 to 240 nm, or 90 to 220 nm.

[0052] Furthermore, the polyimide film for the cover window according to one embodiment may have a yellowness of 3.0 or less, or 2.7 or less, or 1 to 2.7, when its thickness is 30 to 150 μm.

[0053] In one embodiment, the polyimide film for a cover window may have a yellowness of 1.0 to 2.7 or 1.5 to 2.5 when its thickness is 40 to 80 μm.

[0054] Specifically, a polyimide film for a cover window according to one embodiment may simultaneously satisfy the aforementioned thickness-direction phase difference (Rth) and yellowness at a wavelength of 550 nm when the thickness is 30 to 150 μm. Furthermore, the polyimide film for a cover window may simultaneously satisfy the aforementioned thickness-direction phase difference (Rth) and yellowness at a wavelength of 550 nm when the thickness is 40 to 80 μm.

[0055] Furthermore, a polyimide film for a cover window according to one embodiment may 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 may satisfy these mechanical properties along with the thickness direction phase difference (Rth) and yellowness at a wavelength of 550 nm as described above.

[0056] In one embodiment, the polyimide film for cover windows may specifically have a modulus of 4.1 GPa or higher, or 4.1 to 6 GPa, in accordance with 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%, and specifically, the above-mentioned modulus and elongation at break may be satisfied simultaneously. This provides sufficient mechanical properties and durability for application to cover windows.

[0057] A polyimide film for a cover window according to one embodiment can prevent image distortion caused by light and provide improved visibility by satisfying all of the above-mentioned thickness-direction phase difference, yellowness, modulus, and elongation at break ranges. Furthermore, it can exhibit more uniform mechanical properties (such as modulus) and optical properties (such as thickness-direction phase difference) throughout the central and edge portions of the film, further reducing film loss. In addition, because the polyimide film for a cover window 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.

[0058] A cover window containing a polyimide film for a cover window according to one embodiment can have even better visibility, prevent the occurrence of fold marks and microcracks, and provide the flexible display device with even better durability and a longer lifespan.

[0059] A polyimide film for a cover window according to one embodiment may be manufactured from a polyimide resin containing structural units derived from the diamines and dianhydrides exemplified above. Specifically, the polyimide resin may have, but is not limited to, 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.

[0060] The following describes a method for manufacturing a polyimide film for cover windows according to one embodiment. A polyimide film for a cover window according to one embodiment may be manufactured by a method comprising: i) reacting the compounds represented by chemical formulas 1 and 2 with the compounds represented by chemical formulas 3 and 4 in an organic solvent to produce a polyamic acid and / or polyimide solution; and ii) applying the polyamic acid and / or polyimide solution obtained in step 1 onto a substrate and heating to cure it. Here, the compounds represented by chemical formulas 1 and 2 may be dianhydrides, and the compounds represented by chemical formulas 3 and 4 may be diamines.

[0061] Specifically, in a method for producing a polyimide film for cover windows according to one embodiment, the polyamic acid and / or polyimide solution may contain structural units derived from the compound represented by chemical formula 1 in an amount of 70 to 95 mol%, based on 100 mol% of structural units derived from the dianhydride. In this case, the remaining mol%, or 5 to 30 mol%, may consist of structural units derived from the compound represented by chemical formula 2.

[0062] Specifically, in a method for producing a polyimide film for cover windows according to one embodiment, the polyamic acid and / or polyimide solution may contain structural units derived from the compound represented by chemical formula 3 in an amount of 70 to 95 mol%, based on 100 mol% of structural units derived from the diamine. In this case, the remaining mol%, or 5 to 30 mol%, may consist of structural units derived from the compound represented by chemical formula 4.

[0063] Furthermore, the polyamic acid and / or polyimide solution satisfying such mol% may have a solid content of 10 to 40% by weight based on the total weight. Here, the solid content may be the polyamic acid and / or polyimide, and the remainder may be an organic solvent.

[0064] In a method for producing a polyimide film for cover windows according to one embodiment, the solid content of the polyamic acid and / or polyimide solution may be 10 to 30% by weight, or 10 to 20% by weight. More specifically, according to one embodiment, even when the solid content of the polyamic acid and / or polyimide solution is 10 to 15% by weight, it has a low viscosity, thus providing 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, making it difficult to improve these properties simultaneously. However, according to one embodiment, it is significant that these properties can be improved simultaneously even at thicknesses of 30 μm or more.

[0065] In a method for producing a polyimide film for a cover window according to one embodiment, step i) may be carried out under a polar solvent, specifically an amide solvent.

[0066] The amide solvent may mean a compound containing an amide moiety. The amide solvent may be aromatic or aliphatic, for example, it may be aliphatic. Also, for example, the amide solvent may be a cyclic compound or a chain compound, and specifically may have 2 to 15 carbon atoms, for example, 3 to 10 carbon atoms.

[0067] The amide solvent may contain an N,N-dialkylamide molecule, where the dialkyl groups may exist independently, condense with each other to form a ring, or at least one of the dialkyl groups may condense with other substituents in the molecule to form a ring. For example, at least one of the dialkyl groups may condense with an alkyl group linked to the carbonyl carbon of the amide molecule to form a ring. Here, the ring may be a 4- to 7-membered ring, for example, a 5- to 7-membered ring, or 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.

[0068] More specifically, the amide solvent is not limited as long as it is commonly used in polyamic acid and / or polyimide polymerization, but may include, for example, dimethylpropionamide, diethylpropionamide, dimethylacetylamide, diethylacetamide, dimethylformamide, methylpyrrolidone, ethylpyrrolidone, octylpyrrolidone, or a combination thereof, and more specifically, may include dimethylpropionamide.

[0069] As described above, the polyamic acid and / or polyimide solution may have a solid content of 10 to 40% by weight based on the total weight. This allows for a further reduction in the crystallinity of the polyamic acid and / or polyimide solution, enabling the realization of a low thickness-direction phase difference. Specifically, it is possible to achieve a low thickness-direction phase difference of 30 μm or more, which can satisfy mechanical properties comparable to those of tempered glass.

[0070] In a method for manufacturing a polyimide film for a cover window according to one embodiment, step ii) may be performed by thermosetting. Hereinafter, in addition to thermosetting, it may be replaced by a variety of known methods such as chemical curing, infrared curing, batch curing, or continuous curing, or by a different curing method.

[0071] The aforementioned heat curing may be carried out at 80-300°C, 100-280°C, or 150-250°C. The aforementioned heat curing may 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 heat curing may be carried out under two or more temperature conditions selected from these. Furthermore, the heat curing may be carried out in a separate vacuum oven or an oven filled with inert gas, but is not necessarily limited to these.

[0072] 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.

[0073] Furthermore, in a method for manufacturing a polyimide film for a cover window according to one embodiment, the coating for forming the polyimide film may be any coating commonly used in the art, without limitation. 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.

[0074] The substrate may be any substrate commonly used in the 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.

[0075] The following describes the applications of a polyimide film for cover windows according to one embodiment. The first embodiment may be a laminate containing a polyimide film for a cover window according to one embodiment. Here, the laminate may include two or more coating layers of polyimide films containing monomers of different compositions from the polyimide film for a cover window of the present invention.

[0076] Furthermore, the second embodiment 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. Furthermore, the third embodiment may be a flexible display device including a polyimide film for a cover window according to one embodiment.

[0077] In one embodiment, the polyimide film for a cover window has a thickness of 30 to 150 μm, an absolute value of the thickness direction phase difference (Rth) at a wavelength of 550 nm is 500 nm or less, and a yellowness (YI) in accordance with ASTM E313 may be 3.0 or less. Specifically, when the polyimide film for a cover window has a thickness of 30 to 150 μm, the absolute value of the thickness direction phase difference (Rth) at a wavelength of 550 nm may be 400 nm or less, or 350 nm or less, or 50 to 300 nm. Furthermore, when the polyimide film for a cover window has a thickness of 30 to 150 μm, the yellowness may be 3.0 or less, or 2.7 or less, or 1 to 2.7. More specifically, the polyimide film for a cover window may simultaneously satisfy the optical properties such as the thickness direction phase difference (Rth) and yellowness at a wavelength of 550 nm described above.

[0078] For example, when the thickness of the polyimide film for the cover window is 40 to 80 μm, the absolute value of the phase difference in the thickness direction (Rth) at a wavelength of 550 nm may be 50 to 250 nm, or 80 to 240 nm, or 90 to 220 nm.

[0079] For example, when the thickness of the polyimide film for the cover window is 40 to 80 μm, the yellowness may be 1.0 to 2.7, or 1.5 to 2.5.

[0080] As an example, the polyimide film for the cover window may have a thickness of 40 to 80 μm and simultaneously satisfy the above-mentioned thickness-direction phase difference and yellowness at a wavelength of 550 nm.

[0081] Furthermore, the polyimide film for cover windows according to one embodiment may satisfy (a) a modulus of 4 GPa or more in accordance with ASTM E111 and (b) an elongation at break of 15% or more when the thickness is 30 to 150 μm, and most specifically, it may satisfy these mechanical properties in addition to the optical properties described above. Moreover, the polyimide film for cover windows may also satisfy the above mechanical properties in the same way when the thickness is 40 to 80 μm.

[0082] A polyimide film for a cover window according to one embodiment may specifically have a modulus of 4 GPa or higher, or 4.1 GPa or higher, or 4.1 to 6 GPa, in accordance with ASTM E111. Furthermore, the polyimide film for the cover window 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%. Having such physical properties provides mechanical properties and durability that are even more suitable for application in cover windows.

[0083] The first, second, or third embodiment of one embodiment may include a polyimide film for a cover window that significantly reduces light-induced distortion phenomena by achieving low thickness-direction phase difference and yellowness over a wide visible light range, while also satisfying mechanical properties such as modulus and elongation at break. A functional coating layer may also be included as needed.

[0084] The functional coating layer may be formed on at least one other surface of the polyimide film or substrate for the cover window according to one embodiment. Non-limiting examples include hard coating layers, antistatic layers, anti-fingerprint layers, anti-fouling layers, anti-scratch layers, low refractive index layers, anti-reflective layers, and shock-absorbing layers, and it goes without saying that at least one or more functional coating layers selected from these may be included. In this case, the thickness of the functional coating layer may be 1 to 500 μm, 2 to 450 μm, or 2 to 200 μm, but is not limited thereto.

[0085] As described above, the polyimide film for cover windows according to one embodiment has excellent optical and mechanical properties, allowing it to exhibit sufficient phase difference at various angles, including in cover windows of display devices. This makes it applicable to a wide range of industrial fields where a wide viewing angle is required.

[0086] For example, the display device is not particularly limited as long as it is required in a field where excellent optical properties are needed, 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 displays, electroluminescent displays, plasma displays, and field emission displays.

[0087] Furthermore, the display device including the polyimide film for cover window according to the above-described embodiment not only exhibits superior display quality, but also significantly reduces light-induced distortion, particularly improving the rainbow effect where iridescent discoloration occurs, thus minimizing eye strain for the user due to its excellent visibility. In particular, as the screen size of the display device increases, the screen is often viewed from the side, but when the polyimide film for cover window according to the above embodiment is applied to the display device, it exhibits excellent visibility even when viewed from the side, making it useful for large display devices.

[0088] The following description will illustrate one embodiment in detail, but the present invention is not limited to the following embodiment. In the experiment described below, the physical properties were measured as follows.

[0089] <Phase difference (Rth)> Measurements were taken using AxoScan (OPMF, Axometrics Inc.). The phase difference in the thickness direction (Rth) was measured for wavelengths from 450 nm to 650 nm, and the absolute values ​​of the phase difference in the thickness direction at 450 nm, 550 nm, and 650 nm are shown. The unit is nm.

[0090] <Yellowness (YI)> Measurements were taken using a spectrophotometer (Nippon Denshoku Co., Ltd., COH-5500) in accordance with the ASTM E313 standard.

[0091] <Modus and elongation at break> In accordance with ASTM E111, test specimens with a thickness of 50 μm, a length of 50 mm, and a width of 10 mm were measured using an Instron UTM 3365 at 25°C under conditions of being pulled at 50 mm / min. The unit of modulus is GPa, and the unit of elongation at break is %.

[0092] [Example 1] Manufacturing of polyimide film for cover windows (TFMB (0.9) / DDS (0.1) / BPAF (0.7) / 6FDA (0.3), unit: molar ratio) After filling a stirrer with a nitrogen gas flow with 406.4g of N,N-dimethylpropionamide (DMPA), 26.9g of 2,2'-bis(trifluoromethyl)benzidine (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 dianhydride (BPAF) and 12.45 g of 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) were added at 25°C and dissolved and reacted with stirring for 24 hours. Subsequently, DMPA solvent was further added to achieve a solid content of 14% by weight to produce polyimide film-forming composition 1 for cover windows.

[0093] The obtained polyimide film-forming composition 1 for cover windows was applied to one surface of a glass substrate (1.0T) using a #20 Mayer bar, cured by heating at 80°C for 15 minutes under a nitrogen atmosphere, and then at 350°C for 15 minutes, and then peeled off from the glass substrate to obtain the polyimide film for cover windows of Example 1 with a thickness of 50 μm.

[0094] [Example 2] Manufacturing of polyimide film for cover windows (TFMB (0.85) / DDS (0.15) / BPAF (0.7) / 6FDA (0.3), unit: molar ratio) After filling a stirrer with a nitrogen gas flow with 404.5g of N,N-dimethylpropionamide (DMPA), 25.4g of 2,2'-bis(trifluoromethyl)benzidine (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 dianhydride (BPAF) and 12.45 g of 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) were added at 25°C and dissolved and reacted with stirring for 24 hours. Subsequently, DMPA solvent was further added to achieve a solid content of 14% by weight to produce polyimide film-forming composition 2 for cover windows.

[0095] The obtained polyimide film-forming composition 2 for cover windows was applied to one surface of a glass substrate (1.0T) using a #20 Mayer 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 from the glass substrate to obtain the polyimide film for cover windows of Example 2 with a thickness of 50 μm.

[0096] [Example 3] Manufacturing of polyimide films for cover windows Polyimide film-forming composition 1 for cover windows of Example 1 was applied to one surface of a glass substrate (1.0T) using a #20 Mayer bar. The film was cured by heating at 80°C for 15 minutes under a nitrogen atmosphere, followed by heating at 350°C for 15 minutes. The film was then peeled off the glass substrate to obtain a polyimide film for cover windows of Example 3 with a thickness of 80 μm.

[0097] [Examples 4 to 9] Manufacturing of polyimide films for cover windows Polyimide films for cover windows of Examples 4 to 9, with a thickness of 50 μm, were obtained in the same manner as in Example 1, except that the molar ratios of TFMB, DDS, BPAF, and 6FDA were changed as shown in Table 1 below.

[0098] [Comparative Example 1] Manufacturing of polyimide film for cover windows (TFMB (0.99) / BPAF (1), unit: molar ratio) After filling a stirrer with a nitrogen gas flow with 370g of DMPA, 20.74g of TFMB was dissolved while maintaining the reactor temperature at 25°C. 30g of BPAF was then added at 25°C and dissolved and reacted while stirring for 24 hours. Subsequently, DMPA solvent was added to achieve a solid content of 12% by weight to produce polyimide film-forming composition A for cover windows.

[0099] The obtained polyimide film-forming composition A for cover windows was applied to one surface of a glass substrate (1.0T) using a #20 Mayer 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 from the glass substrate to obtain a polyimide film for cover windows of Comparative Example 1 with a thickness of 50 μm.

[0100] [Comparative Example 2] Manufacturing of polyimide film for cover windows (TFMB (0.99) / 6 FDA (1), unit: molar ratio) After filling a stirrer with a nitrogen gas flow with 260g of DMPA, 21.4g of TFMB was dissolved while maintaining the reactor temperature at 25°C. 30g of 6FDA was added at room temperature and dissolved for a certain period of time, and the mixture was stirred and reacted for 24 hours. Subsequently, DMPA solvent was further added to achieve a solid content concentration of 12% by weight, thereby producing polyimide film-forming composition B for cover windows.

[0101] The obtained polyimide film-forming composition B for cover windows was applied to one surface of a glass substrate (1.0T) using a #20 Mayer bar, cured by heating at 80°C for 15 minutes under a nitrogen atmosphere, and then at 350°C for 15 minutes, and then peeled off from the glass substrate to obtain a polyimide film for cover windows of Comparative Example 2 with a thickness of 50 μm.

[0102] [Comparative Example 3] Manufacturing of polyimide film for cover windows ((TFMB(1) / PMDA(0.3) / BPAF(0.7), unit: molar ratio) After filling a stirrer with a nitrogen stream with 484 g of N,N-dimethylpropionamide (DMPA), 29.9 g of 2,2-bis(trifluoromethyl)benzidine (TFMB) was dissolved while maintaining the reactor temperature at 25°C. To this, 30 g of 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) and 6.1 g of pyromellitic dianhydride (PMDA) were added at 25°C and dissolved and reacted while stirring for 24 hours. Subsequently, DMPA solvent was further added to achieve a solid content of 12% by weight to produce polyimide film-forming composition C for cover windows.

[0103] The obtained polyimide film-forming composition C for cover windows was applied to one surface of a glass substrate (1.0T) using a #20 Mayer 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 from the glass substrate to obtain a polyimide film for cover windows of Comparative Example 3 with a thickness of 50 μm.

[0104] [Comparative Example 4] Manufacturing of polyimide film for cover windows ((TFMB(1) / PMDA(0.7) / BPAF(0.3), unit: molar ratio) A polyimide film for cover windows of Comparative Example 4 with a thickness of 50 μm was obtained using the same method as in Comparative Example 3, except that the molar ratios of TFMB, PMDA, and BPAF were changed as shown in Table 1 below.

[0105] Evaluation: Optical and mechanical properties The yellowness (YI), phase difference, modulus, and elongation at break of the polyimide films for cover windows in Examples 1 to 9 and Comparative Examples 1 to 4 were measured and are shown in Tables 2 and 3 below.

[0106] [Table 1]

[0107] [Table 2]

[0108] [Table 3]

[0109] Referring to Tables 2 and 3, it can be confirmed that the polyimide films for cover windows according to Examples 1 to 9 have an absolute value of 500 nm or less of phase difference in the thickness direction at a wavelength of 550 nm, even at a thickness of 50 μm or more, which is sufficient for use as a cover window, and that the yellowness (YI) according to ASTM E313 is 3.5 or less. Therefore, the polyimide films for cover windows according to Examples 1 to 9 have a low phase difference in the thickness direction (Rth) over a wide visible light range, dramatically improving the reflective appearance, while also possessing high strength characteristics and high elongation at break, making them suitable for application as cover windows in foldable display devices or flexible display devices.

[0110] In contrast, the polyimide film for cover windows according to Comparative Example 1 has a low elongation at break of 5% or less, resulting in poor mechanical properties and therefore it is not suitable for use in cover windows. The polyimide films for cover windows of Comparative Examples 2 to 4 have high absolute values ​​of 500 nm or more for the phase difference in the thickness direction at a wavelength of 550 nm, and therefore they are not suitable for use in cover windows. Furthermore, the polyimide films for cover windows of Comparative Examples 3 and 4 not only have very high absolute values ​​of 1800 nm and 2800 nm for the phase difference in the thickness direction at a wavelength of 550 nm, but are also colored films with very high yellowness values ​​of 53 and 80, respectively, resulting in poor visibility and therefore they are not suitable for use in cover windows.

[0111] Although one embodiment has been described above by limited examples, these are provided only for a more general understanding of the present invention, and the present invention is not limited to the above-described examples. A person with ordinary skill in the art to which the present invention belongs can make various modifications and variations from this description.

[0112] Therefore, the concept of the present invention should not be limited to the embodiments described, and any equivalent or equivalent modifications to the claims described below can be said to fall within the scope of the concept of the present invention.

Claims

1. It includes structural units derived from dianhydrides and structural units derived from diamines. The structural units derived from the two anhydrides include structural units derived from the compound represented by the following chemical formula 1, and structural units derived from the compound represented by the following chemical formula 2. The structural unit derived from the diamine is a polyimide film for cover windows, comprising a structural unit derived from a compound represented by the following chemical formula 3, and a structural unit derived from a compound represented by the following chemical formula 4. The structural unit derived from the compound represented by the chemical formula 1 is, It contains 70 to 95 mol% of structural units derived from the aforementioned dianhydride, based on 100 mol%. The structural unit derived from the compound represented by the chemical formula 3 is, It contains 70 to 95 mol% of structural units derived from the aforementioned diamine, based on 100 mol%, and The polyimide film is a cover window polyimide film having a thickness of 30 to 150 μm, an absolute value of the phase difference in the thickness direction at a wavelength of 550 nm of 500 nm or less, and a yellowness (YI) of 3.5 or less according to ASTM E313. [Chemical formula 1] 【Chemistry 1】 [Chemical formula 2] 【Chemistry 2】 [Chemical formula 3] 【Transformation 3】 [Chemical formula 4] 【Chemistry 4】

2. The aforementioned polyimide film is A polyimide film for a cover window according to claim 1, wherein the wavelength dispersibility satisfies the following formulas 1 and 2. [Formula 1] 1.06≦Rth(450nm) / Rth(550nm)≦1.10 [Formula 2] 0.95≦Rth(650nm) / Rth(550nm)≦0.98 In formulas 1 and 2, Rth (450 nm) is the absolute value (in nm) of the phase difference in the thickness direction at a wavelength of 450 nm. Rth (550 nm) is the absolute value (in nm) of the phase difference in the thickness direction at a wavelength of 550 nm. Rth (650 nm) is the absolute value (in nm) of the phase difference in the thickness direction at a wavelength of 650 nm.

3. The aforementioned polyimide film is A polyimide film for cover windows according to claim 1, wherein the modulus in accordance with ASTM E111 is 4 GPa or higher and the elongation at break is 15% or higher.

4. The aforementioned polyimide film is A polyimide film for a cover window according to claim 1, wherein the thickness is 40 to 80 μm, the absolute value of the thickness direction phase difference (Rth) at a wavelength of 550 nm is 50 to 250 nm, the yellowness (YI) according to ASTM E313 is 1.0 to 2.7, the modulus according to ASTM E111 is 4 GPa or more, and the elongation at break is 15% or more.

5. A laminate comprising a polyimide film according to any one of claims 1 to 4 formed on one surface of a substrate.

6. A polyimide film according to any one of claims 1 to 4, A coating layer formed on the polyimide film, A cover window for a display device, including a cover window.

7. The aforementioned coating layer is A cover window for a display device according to claim 6, comprising 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.

8. A flexible display device comprising a polyimide film according to any one of claims 1 to 4.

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