Polyamide-based film, method for preparing the same, and cover window and display device including the same

Abstract

The present invention provides a polyamide-based film, which comprises a polyamide-based polymer and has an RSR of 3.4 m / N to 5.0 m / N expressed by the following Equation 1 based on the thickness of the film of 50 μm. A The film has excellent ring stiffness, static bending characteristics and folding characteristics, while ensuring at least a certain level of mechanical and optical properties. The present invention also discloses a method for preparing the polyamide-based film, as well as a cover window and a display device including the film. <Equation 1>RSR A =(RSR MD +RSR TD ) / 2 In Equation 1, RSR MD is the ring stiffness value measured with the MD direction of the film as the longitudinal direction at room temperature, and RSR TD is the ring stiffness value measured with the TD direction of the film as the longitudinal direction at room temperature.

Description

Technical Field

[0001] This embodiment relates to a polyamide-based film, a method for preparing the same, and a cover window and a display device including the same. Background Art

[0002] Polyamide-based resins, such as poly(amide-imide) (PAI), have excellent abrasion resistance, heat resistance, and chemical resistance. Therefore, they are used in applications such as primary electrical insulation, coatings, adhesives, resins for extrusion, heat-resistant coatings, heat-resistant plates, heat-resistant adhesives, heat-resistant fibers, and heat-resistant films.

[0003] Polyamides are applied in various fields. For example, polyamides are made into powder form and used as coatings for metal or magnetic wires. Depending on the application, they can be mixed with other additives. In addition, polyamides are used as decorative and anti-corrosion paints together with fluoropolymers. It also serves to bond fluoropolymers to metal substrates. In addition, polyamides are used to coat kitchen utensils, used as gas separation membranes utilizing their heat resistance and chemical resistance, and used in natural gas wells to filter contaminants such as carbon dioxide, hydrogen sulfide, and impurities.

[0004] In recent years, polyamides have been developed in the form of films, which are inexpensive and have excellent optical, mechanical, and thermal properties. Such polyamide-based films can be applied to display materials such as organic light-emitting diodes (OLEDs) or liquid crystal displays (LCDs), and if retardation performance is achieved, they can be applied to anti-reflection films, compensation films, and retardation films.

[0005] When such a polyamide-based film is applied to a cover window or a foldable or flexible display device, there is a problem that folding marks remain in the folding point portion due to repeated folding operations.

[0006] Therefore, there has been a continuous need to develop a film that has excellent resilience and excellent mechanical and optical properties while solving the above problems. Summary of the Invention

[0007] Technical Problem

[0008] An object of this embodiment is to provide a polyamide-based film, a method for preparing the same, and a cover window and a display device including the same, the film having excellent ring stiffness, static bending characteristics, and folding characteristics while ensuring at least a certain degree of mechanical and optical properties.

[0009] Solution to the Problem

[0010] One embodiment provides a polyamide-based film including a polyamide-based polymer and having an RSR of 3.4 m / N to 5.0 m / N as represented by the following Equation 1 based on the thickness of the film of 50 μm A value.

[0011] <Equation 1>RSR A =(RSR MD +RSR TD ) / 2

[0012] In Equation 1, RSR MD is the ring stiffness value measured with the MD direction of the film as the longitudinal direction at room temperature, and RSR TD is the ring stiffness value measured with the TD direction of the film as the longitudinal direction at room temperature.

[0013] Another embodiment provides a cover window for a display device, which includes a polyamide-based film and a functional layer. Among them, the polyamide-based film includes a polyamide-based polymer and has an RSR of 3.4 m / N to 5.0 m / N as represented by Equation 1 above based on the thickness of the 50-μm film A value.

[0014] Another embodiment provides a display device, which includes a display unit; and a cover window disposed on the display unit. Among them, the cover window includes a polyamide-based film and a functional layer, and the polyamide-based film includes a polyamide-based polymer and has an RSR of 3.4 m / N to 5.0 m / N as represented by Equation 1 above based on the thickness of the 50-μm film A value.

[0015] One embodiment provides a method for preparing a polyamide-based film, which includes the following steps: polymerizing a diamine compound, a dicarbonyl compound, and an optional dianhydride compound in an organic solvent to prepare a polyamide-based polymer solution; casting the solution and then drying it to prepare a gel sheet; and performing heat treatment on the gel sheet. The step of performing heat treatment on the gel sheet is to fix the lateral ends of the gel sheet with fixing parts and perform heat treatment on it while changing the width of the fixed gel sheet.

[0016] Advantages of the Invention

[0017] Since the polyamide-based film according to this embodiment has an RSR A value within a predetermined range, it exhibits excellent flexibility and mechanical durability. Therefore, the folding marks that may be left due to the repeated folding operation of the film can be minimized, and the resilience can be improved.

[0018] When the polyamide-based film is applied to the cover window of a display device or a foldable or flexible display device, the folding operation can be performed with an appropriate force, and the deformation that may occur in the folding point part during repeated folding can be minimized, thereby improving the stability and quality reliability during long-term use. Description of the Drawings

[0019] Figure 1is a schematic exploded view of a display device according to an embodiment;

[0020] Figure 2 is a schematic perspective view of a display device according to an embodiment;

[0021] Figure 3 is a schematic cross-sectional view of a display device according to an embodiment;

[0022] Figure 4 is a schematic flow chart of a method for preparing a polyamide-based film according to an embodiment;

[0023] Figure 5 schematically shows a method for measuring the ring stiffness of a polyamide-based film according to an embodiment;

[0024] Figure 6 schematically shows a method for measuring the deformation angle of a polyamide-based film according to an embodiment. Detailed Description of the Embodiments

[0025] Best Mode for Carrying Out the Invention

[0026] Hereinafter, these embodiments will be described in detail with reference to the accompanying drawings so that those skilled in the art to which the present invention pertains can easily implement them. However, these embodiments can be implemented in many different ways and are not limited to those described herein.

[0027] Throughout this specification, when it is mentioned that each film, window, panel, layer, etc. is formed "on" or "under" another film, window, panel, layer, etc., it means not only that one element is directly formed on or under another element, but also that one element is indirectly formed on or under another element with other elements intervening therebetween. In addition, the terms "on" or "under" for each element can be referred to the accompanying drawings. For ease of description, the sizes of the respective elements in the drawings may be exaggerated and do not represent actual sizes. In addition, throughout the specification, the same reference numerals denote the same elements.

[0028] Throughout this specification, when a component is referred to as "comprising" an element, it should be understood that other elements may be included, rather than excluding other elements, unless otherwise specifically stated.

[0029] In this specification, singular expressions are interpreted as covering the singular or plural as interpreted in the context, unless otherwise stated.

[0030] In addition, unless otherwise specified, all numbers and expressions related to the amounts of components, reaction conditions, etc. used herein should be understood to be modified by the term "about".

[0031] The terms first, second, etc. are used herein to describe various elements, and these elements should not be limited by these terms. These terms are only used to distinguish one element from another element.

[0032] In addition, the term "substituted" as used herein means substituted by at least one selected from the group consisting of the following substituents: deuterium, -F, -Cl, -Br, -I, hydroxyl, cyano, nitro, amino, amidino, hydrazine, hydrazone, ester, keto, carboxyl, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted alkoxy, substituted or unsubstituted alicyclic organic, substituted or unsubstituted heterocyclic, substituted or unsubstituted aryl, and substituted or unsubstituted heteroaryl. The substituents listed above may be connected to each other to form a ring.

[0033] Polyamide-based film

[0034] One embodiment provides a polyamide-based film having an RSR within a predetermined range. A value, thereby exhibiting excellent flexibility and mechanical durability; therefore, it is possible to minimize folding marks around the folding point portion that may be caused by repeated folding operations of the film and improve the restoring force.

[0035] The polyamide-based film according to one embodiment includes a polyamide-based polymer.

[0036] The polyamide-based film has an RSR of 3.4 m / N to 5.0 m / N as expressed in the following Equation 1 based on a film thickness of 50 μm. A value.

[0037] <Equation 1>RSR A =(RSR MD +RSR TD ) / 2

[0038] In Equation 1, RSR MD It is the ring stiffness value measured at room temperature with the MD direction of the film as the longitudinal direction, RSR TD It is the ring stiffness value measured at room temperature with the TD direction of the film as the longitudinal direction.

[0039] Specifically, based on a film thickness of 50 μm, the RSR of the polyamide-based film is A The value may be 3.4 m / N or more, 3.5 m / N or more, 3.6 m / N or more, 3.7 m / N or more, or 3.8 m / N or more, and may also be 5.0 m / N or less, 4.8 m / N or less, 4.5 m / N or less, or 4.2 m / N or less, RSR A The value is expressed as in Equation 1 above.

[0040] The method for measuring the ring stiffness of the polyamide-based film is as follows Figure 5 as shown. Specifically, referring to Figure 5 (a), the ring stiffness tester (TOYOSEIKI) includes a fixing part (10), a pressing part (20), and a sensor (not shown). Both ends of the polyamide-based film (100) with a width of 15 mm, a length of 120 mm, and a thickness of 50 μm are fixed to the fixing part (10). Subsequently, referring to Figure 5 (b), the pressing part (20) is used to press the polyamide-based film (100) at a pressing speed of 3.3 mm / s until the final separation distance (L) between the pressing part (20) and the fixing part (10) is 20 mm. Then, the ring stiffness of the polyamide-based film (100) is measured with the sensor.

[0041] Since the RSR A value of the polyamide-based film is controlled within the above range, a polyamide-based film can be obtained in which, when the film is applied to a flexible display device, the folding marks around the folding point part that may be caused by the repeated folding operation of the film are minimized, and the resilience is improved.

[0042] On the other hand, if the RSR A value of the polyamide-based film exceeds the above range, the flexibility of the film is reduced, and an excessive force is required to fold it, which causes inconvenience during the folding or bending process. If the RSR A value of the polyamide-based film is lower than the above range, the wrinkles after repeated folding may be clearly visible, thereby reducing the quality reliability of the product.

[0043] When measuring the ring stiffness of the prepared specimen with the longitudinal direction along the MD direction of the polyamide-based film at room temperature, and the measured ring stiffness value is RSR MD , the RSR MD value is 3.35 m / N to 5.0 m / N.

[0044] Specifically, the RSR MD value of the polyamide-based film can be 3.35 m / N or greater, 3.4 m / N or greater, 3.5 m / N or greater, or 3.6 m / N or greater, and can also be 5.0 m / N or less, 4.8 m / N or less, 4.5 m / N or less, 4.2 m / N or less, or 4.0 m / N or less.

[0045] When measuring the ring stiffness of the prepared specimen with the longitudinal direction along the TD direction of the polyamide-based film at room temperature, and the measured ring stiffness value is RSR TD , the RSR TD value is 3.3 m / N to 5.0 m / N.

[0046] Specifically, the RSR of the polyamide-based filmTD The value can be 3.3 m / N or greater, 3.4 m / N or greater, 3.5 m / N or greater, or 3.6 m / N or greater, and can also be 5.0 m / N or less, 4.8 m / N or less, 4.5 m / N or less, 4.2 m / N or less, or 4.1 m / N or less.

[0047] If the RSR of the polyamide-based film MD value and the RSR TD value satisfy the above ranges, then in the MD and TD directions of the film, that is, regardless of the direction of the film, it has the desired level of ring stiffness value; therefore, the mass deviation between the MD / TD directions is low, thus ensuring mass reliability and increasing product yield.

[0048] On the other hand, if either the RSR MD value or the RSR TD value of the polyamide-based film is not within the above ranges, then the mass deviation of the film in the film direction will increase, which will deteriorate the quality of the film itself and directly result in a high product defect rate when applied to the final product.

[0049] In addition, if both the RSR MD value and the RSR TD value of the polyamide-based film are not within the above ranges, then the flexibility or mechanical durability of the film may deteriorate significantly.

[0050] Based on a film thickness of 50 μm, the polyamide-based film has an RSL value of 3.4 m / N to 5.0 m / N as represented by Equation 2 below A value.

[0051] <Equation 2>RSL A =(RSL MD +RSL TD ) / 2

[0052] In Equation 2, RSL MD is the ring stiffness value measured with the MD direction of the film as the longitudinal direction within 1 minute after the film is placed at -20 °C for 2 hours, and RSL TD is the ring stiffness value measured with the TD direction of the film as the longitudinal direction within 1 minute after the film is placed at -20 °C for 2 hours.

[0053] Specifically, based on a film thickness of 50 μm, the RSL A value of the polyamide-based film can be 3.4 m / N or greater, 3.5 m / N or greater, 3.6 m / N or greater, 3.7 m / N or greater, or 3.8 m / N or greater, and can also be 5.0 m / N or less, 4.8 m / N or less, 4.5 m / N or less, or 4.2 m / N or less, RSL AThe value is represented as in Equation 2 above.

[0054] Since the RSL of the polyamide-based film A is controlled within the above range, even when placed at an extremely low temperature of -20°C for 2 hours, the required level of ring stiffness can be achieved. Specifically, when the film is applied to a flexible display device, the defect rate of the product can be reduced. In particular, even when the flexible display device is used in a harsh environment, the mechanical properties or appearance quality will not deteriorate.

[0055] After the polyamide-based film is placed at -20°C for 2 hours, the ring stiffness of the prepared specimen with the MD direction of the film as the longitudinal direction is measured within 1 minute, and the measured ring stiffness value is RSL MD When MD the value of RSL is 3.4 m / N to 5.0 m / N.

[0056] Specifically, the RSL of the polyamide-based film MD value can be 3.4 m / N or greater, 3.5 m / N or greater, 3.6 m / N or greater, or 3.8 m / N or greater, and can also be 5.0 m / N or less, 4.8 m / N or less, 4.5 m / N or less, or 4.2 m / N or less.

[0057] After the polyamide-based film is placed at -20°C for 2 hours, the ring stiffness of the prepared specimen with the TD direction of the film as the longitudinal direction is measured within 1 minute, and the measured ring stiffness value is RSL TD When TD the value of RSL is 3.4 m / N to 5.0 m / N.

[0058] Specifically, the RSL of the polyamide-based film TD value can be 3.4 m / N or greater, 3.5 m / N or greater, 3.6 m / N or greater, or 3.8 m / N or greater, and can also be 5.0 m / N or less, 4.8 m / N or less, 4.5 m / N or less, or 4.2 m / N or less.

[0059] If the RSL MD value and the RSL TD value of the polyamide-based film satisfy the above range, then in the MD and TD directions of the film, that is, regardless of which direction of the film, even after exposure to extremely low temperature, it has the required level of ring stiffness value; therefore, when the film is applied to a flexible display device, the quality deviation is small, thus improving the reliability of the product.

[0060] In one embodiment, when a polyamide-based film with a thickness of 50 μm is folded into a curvature radius of 2 mm, then placed at 25°C for 24 hours, and the force applied to the film is released, the inner angle of the film is 140° or greater.

[0061] Specifically, when a polyamide-based film with a thickness of 50 μm is folded to have a radius of curvature of 2 mm, then placed at 25°C for 24 hours, and the force applied to the film is released, the inner angle of the film can be 145° or greater, 150° or greater, 155° or greater, or 160° or greater, but not limited thereto.

[0062] Regarding the static bending characteristics of the polyamide-based film, Figure 6 a method of measuring the deformation angle is shown. Specifically, referring to Figure 6 (a), a polyamide-based film (100) with a width of 20 mm, a length of 150 mm, and a thickness of 50 μm is placed in a glass fixture such that the radius of curvature is 2 mm, and placed in this folded state at 25°C for 24 hours. Referring to Figure 6 (b), after 24 hours, the film is taken out of the glass fixture. When the force applied to the film is released, the inner angle (AG) of the film is measured.

[0063] If the inner angle of the measured polyamide-based film evaluated by the deformation angle of the film is within the above range, good resilience can be ensured; therefore, when it is applied to a foldable display device or a flexible display device, image distortion will not occur. In particular, for example, when it is applied to a foldable display device, it tends to remain in the folded state for a long time before being unfolded. In this case, permanent deformation that may occur in the appearance of the device can be minimized.

[0064] According to one embodiment, the modulus of the polyamide-based film is 5.0 GPa or greater. Specifically, the modulus can be 5.5 GPa or greater, 5.7 GPa or greater, or 6 GPa or greater.

[0065] The transmittance of the polyamide-based film can be 80% or higher. For example, the transmittance can be 85% or higher, 88% or higher, and can be 100% or lower, or 99% or lower.

[0066] The haze of the polyamide-based film is 1% or lower. Specifically, the haze can be 0.8% or lower, 0.7% or lower, 0.6% or lower, or 0.5% or lower, but not limited thereto.

[0067] The yellowness index of the polyamide-based film is 5 or smaller. For example, the yellowness index can be 4.5 or smaller, 4.0 or smaller, or 3.5 or smaller, but not limited thereto.

[0068] In one embodiment, based on a film thickness of 50 μm, the thickness deviation of the polyamide-based film is 3 μm or smaller, or 2 μm or smaller. In addition, the thickness deviation rate can be 5% or lower, 4% or lower, or 3% or lower, but not limited thereto.

[0069] The polyamide-based film may have a compressive strength of 0.4 kgf / μm or greater. Specifically, the compressive strength may be 0.45 kgf / μm or greater, or 0.5 kgf / μm or greater, but is not limited thereto.

[0070] When perforating the polyamide-based film with a 2.5-mm spherical tip at a speed of 10 mm / min in the UTM compression mode, the maximum diameter (mm) of the perforation including cracks is 60 mm or less. Specifically, the maximum diameter (mm) of the perforation may be 55 mm or less, or 50 mm or less, and may also be 1 mm or greater, 3 mm or greater, 5 mm or greater, 10 mm or greater, 15 mm or greater, or 20 mm or greater.

[0071] The surface hardness of the polyamide-based film may be HB or higher. Specifically, the surface hardness may be H or higher, or 2H or higher, but is not limited thereto.

[0072] The tensile strength of the polyamide-based film may be 15 kgf / mm 2 or greater. Specifically, the tensile strength may be 18 kgf / mm 2 or greater, 20 kgf / mm 2 or greater, 21 kgf / mm 2 or greater, or 22 kgf / mm 2 or greater, but is not limited thereto.

[0073] The elongation rate of the polyamide-based film may be 15% or higher. Specifically, the elongation rate may be 16% or higher, 17% or higher, or 18% or higher, but is not limited thereto.

[0074] When the polyamide-based film with a thickness of 50 μm is folded into a curvature radius of 3 mm, the number of folds before fracture can be 200,000 or more. When the film is folded into a curvature radius of 3 mm and then unfolded, the number of folds is counted as 1.

[0075] Since the number of folds of the polyamide-based film satisfies the above range, it can be advantageously applied to a foldable display device or a flexible display device.

[0076] The surface roughness of the polyamide-based film may be from 0.01 μm to 0.07 μm. Specifically, the surface roughness may be 0.01 μm or greater or 0.02 μm or greater, and may also be 0.07 μm or less, 0.06 μm or less, or 0.05 μm or less, but is not limited thereto.

[0077] Since the surface roughness of the polyamide-based film satisfies the above range, it can be beneficial to achieve the brightness conditions or texture when it is applied to a display device.

[0078] The content of the residual solvent in the polyamide-based membrane can be 1,500 ppm or less. For example, the content of the residual solvent can be 1,200 ppm or less, 1,000 ppm or less, 800 ppm or less, or 500 ppm or less, but is not limited thereto.

[0079] The residual solvent refers to the solvent that remains in the finally produced membrane without volatilization during the membrane production process.

[0080] If the content of the residual solvent in the polyamide-based membrane exceeds the above range, the durability of the membrane may decrease, and it may affect the quality deviation of the membrane. In particular, since it affects the mechanical strength, it may have an adverse effect on the post-treatment of the membrane. Due to the accelerated hygroscopicity of the membrane, the optical properties and heat resistance, not to mention the mechanical properties, may also deteriorate.

[0081] The polyamide-based membrane according to one embodiment includes a polyamide-based polymer, which is prepared by polymerizing a diamine compound, a dicarbonyl compound, and optionally a dianhydride compound.

[0082] For example, the polyamide-based polymer can be prepared by polymerizing a diamine compound and a dicarbonyl compound. It can be prepared by polymerizing a diamine compound, a dicarbonyl compound, and a dianhydride compound.

[0083] The polyamide-based polymer is a polymer including amide repeating units. In addition, the polyamide-based polymer may optionally further include imide repeating units.

[0084] Specifically, the polyamide-based polymer includes amide repeating units derived from the polymerization of a diamine compound and a dicarbonyl compound; and optionally includes imide repeating units derived from the polymerization of a diamine compound and a dianhydride compound.

[0085] The diamine compound is a compound that forms an imide bond with a dianhydride compound and an amide bond with a dicarbonyl compound to form a copolymer.

[0086] The diamine compound is not particularly limited. For example, it can be an aromatic diamine compound containing an aromatic structure. For example, the diamine compound can be a compound represented by the following formula 1.

[0087] [Formula 1]

[0088] H2N-(E) e -NH2

[0089] In formula 1, E can be selected from a substituted or unsubstituted divalent C6-C 30 aliphatic cyclic group, a substituted or unsubstituted divalent C4-C 30 aliphatic heterocyclic group, a substituted or unsubstituted divalent C6-C 30Aryl group, substituted or unsubstituted divalent C4-C 30 Heteroaryl group, substituted or unsubstituted C1-C 30 Alkylene group, substituted or unsubstituted C2-C 30 Alkenylene group, substituted or unsubstituted C2-C 30 Alkynylene group, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, -C(CH3)2- and -C(CF3)2-.

[0090] e is an integer from 1 to 5. When e is 2 or greater, the Es can be the same as or different from each other.

[0091] (E) in Formula 1 e can be selected from the groups represented by Formulae 1-1a to 1-14a below, but is not limited thereto.

[0092]

[0093] Specifically, (E) in Formula 1 e can be selected from the groups represented by Formulae 1-1b to 1-13b below, but is not limited thereto.

[0094]

[0095] More specifically, (E)e in Formula 1 can be a group represented by Formula 1-6b above or a group represented by Formula 1-9b above.

[0096] In one embodiment, the diamine compound can include a compound having a fluorine-containing substituent or a compound having an ether group (-O-).

[0097] The diamine compound can be composed of a compound having a fluorine-containing substituent. In this case, the fluorine-containing substituent can be a fluorinated hydrocarbon group, and specifically can be a trifluoromethyl group. However, it is not limited thereto.

[0098] In some embodiments, the diamine compound can include one diamine compound. That is, the diamine compound can be composed of a single component.

[0099] For example, the diamine compound can include 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) represented by the following formula, but is not limited thereto.

[0100]

[0101] The dianhydride compound has a low birefringence value, so it can contribute to improving optical properties such as the transmittance of a film including a polyamide-based polymer.

[0102] The dianhydride compound is not particularly limited, but it may be, for example, an aromatic dianhydride compound containing an aromatic structure. For example, the aromatic dianhydride compound may be a compound represented by the following formula 2.

[0103] [Formula 2]

[0104]

[0105] In formula 2, G may be selected from substituted or unsubstituted tetravalent C6-C 30 aliphatic ring groups, substituted or unsubstituted tetravalent C4-C 30 aliphatic heterocyclic groups, substituted or unsubstituted tetravalent C6-C 30 aromatic ring groups or substituted or unsubstituted tetravalent C4-C 30 heteroaromatic ring groups, where the aliphatic ring group, aliphatic heterocyclic group, aromatic ring group or heteroaromatic ring group may exist alone, be fused to each other to form a fused ring, or be bonded through a bonding group selected from substituted or unsubstituted C1-C 30 alkylene, substituted or unsubstituted C2-C 30 alkenylene, substituted or unsubstituted C2-C 30 alkynylene, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, -C(CH3)2- and -C(CF3)2-.

[0106] G in the above formula 2 may be selected from the groups represented by the following formulae 2-1a to 2-9a, but is not limited thereto.

[0107]

[0108] For example, G in formula 2 may be the group represented by the above formula 2-2a, the group represented by the above formula 2-8a, or the group represented by the above formula 2-9a.

[0109] In one embodiment, the dianhydride compound may include a compound having a fluorine-containing substituent, a compound having a biphenyl group, or a compound having a keto group.

[0110] The fluorine-containing substituent may be a fluorinated hydrocarbon group, and specifically, it may be a trifluoromethyl group. However, it is not limited thereto.

[0111] In another embodiment, the dianhydride compound may consist of a single component or a mixture of two components.

[0112] For example, the dianhydride compound may include at least one selected from the group consisting of 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), which have the following structures, but are not limited thereto.

[0113]

[0114] Diamine compounds and dianhydride compounds can be polymerized to form polyamic acids.

[0115] Subsequently, the polyamic acid can be converted into a polyimide through a dehydration reaction, and the polyimide includes imide repeating units.

[0116] The polyimide can form repeating units represented by the following formula A.

[0117] [Formula A]

[0118]

[0119] In formula A, E, G, and e are as described above.

[0120] For example, the polyimide can include repeating units represented by the following formula A-1, but is not limited thereto.

[0121] [Formula A-1]

[0122]

[0123] In formula A-1, n is an integer from 1 to 400.

[0124] The dicarbonyl compound is not particularly limited, but it can be, for example, a compound represented by the following formula 3.

[0125] [Formula 3]

[0126]

[0127] In formula 3, J can be selected from substituted or unsubstituted divalent C6-C 30 aliphatic cyclic groups, substituted or unsubstituted divalent C4-C 30 aliphatic heterocyclic groups, substituted or unsubstituted divalent C6-C 30 aromatic ring groups, substituted or unsubstituted divalent C4-C 30 heteroaromatic ring groups, substituted or unsubstituted C1-C 30 alkylene groups, substituted or unsubstituted C2-C 30 alkenylene groups, substituted or unsubstituted C2-C 30 alkynylene groups, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, -C(CH3)2-, and -C(CF3)2-.

[0128] j is selected from integers from 1 to 5. When j is 2 or greater, Js can be the same as or different from each other.

[0129] X is a halogen atom. Specifically, X may be F, Cl, Br, I, etc. More specifically, X may be Cl, but is not limited thereto.

[0130] (J) in Formula 3 above j may be selected from the groups represented by Formulae 3-1a to 3-14a below, but is not limited thereto.

[0131]

[0132] Specifically, (J) in Formula 3 above j may be selected from the groups represented by Formulae 3-1b to 3-8b below, but is not limited thereto.

[0133]

[0134] More specifically, (J) in Formula 3 j may be the group represented by Formula 3-1b, the group represented by Formula 3-2b, the group represented by Formula 3-3b, or the group represented by Formula 3-8b above.

[0135] For example, (J) in Formula 3 above j may be the group represented by Formula 3-1b or the group represented by Formula 3-2b.

[0136] In one embodiment, a single dicarbonyl compound may be used alone, or a mixture of at least two different dicarbonyl compounds may be used as the dicarbonyl compound. If two or more dicarbonyl compounds are used, (J) in Formula 3 above j at least two dicarbonyl compounds selected from the groups represented by Formulae 3-1b to 3-8b above may be used as the dicarbonyl compound.

[0137] In another embodiment, the dicarbonyl compound may be an aromatic dicarbonyl compound containing an aromatic structure.

[0138] The dicarbonyl compound may include terephthaloyl chloride (TPC), 1,1'-biphenyl-4,4'-dicarbonyl dichloride (BPDC), isophthaloyl chloride (IPC), or a combination thereof as shown in the following formula, but is not limited thereto.

[0139]

[0140] The diamine compound and the dicarbonyl compound may polymerize to form a repeating unit represented by the following Formula B.

[0141] [Formula B]

[0142]

[0143] In Formula B, the descriptions of E, J, e, and j are as described above.

[0144] For example, a diamine compound and a dicarbonyl compound can be polymerized to form amide repeating units represented by the following Formulas B-1 and B-2.

[0145] Alternatively, a diamine compound and a dicarbonyl compound can be polymerized to form amide repeating units represented by the following Formulas B-2 and B-3.

[0146] [Formula B-1]

[0147]

[0148] In Formula B-1, x is an integer from 1 to 400.

[0149] [Formula B-2]

[0150]

[0151] In Formula B-2, y is an integer from 1 to 400.

[0152] [Formula B-3]

[0153]

[0154] In Formula B-3, y is an integer from 1 to 400.

[0155] According to one embodiment, the polyamide-based polymer may include repeating units represented by the following Formula B; and it may optionally include repeating units represented by the following Formula A:

[0156] [Formula A]

[0157]

[0158] [Formula B]

[0159]

[0160] In Formulas A and B, E and J are each independently selected from substituted or unsubstituted divalent C6-C 30 aliphatic cyclic groups, substituted or unsubstituted divalent C4-C 30 aliphatic heterocyclic groups, substituted or unsubstituted divalent C6-C 30 aromatic ring groups, substituted or unsubstituted divalent C4-C 30 heteroaromatic ring groups, substituted or unsubstituted C1-C 30 alkylene groups, substituted or unsubstituted C2-C 30 alkenylene groups, substituted or unsubstituted C2-C 30An alkynylene group, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, -C(CH3)2-, and -C(CF3)2-

[0161] e and j each independently represent an integer from 1 to 5

[0162] When e is 2 or greater, two or more Es are the same as or different from each other

[0163] When j is 2 or greater, two or more Js are the same as or different from each other

[0164] G is a substituted or unsubstituted tetravalent C6-C 30 aliphatic cyclic group, a substituted or unsubstituted tetravalent C4-C 30 aliphatic heterocyclic group, a substituted or unsubstituted tetravalent C6-C 30 aromatic cyclic group, or a substituted or unsubstituted tetravalent C4-C 30 heteroaromatic cyclic group, where the aliphatic cyclic group, aliphatic heterocyclic group, aromatic cyclic group, or heteroaromatic cyclic group can exist alone, be fused to each other to form a fused ring, or be bonded through a bonding group selected from substituted or unsubstituted C1-C 30 alkylene group, substituted or unsubstituted C2-C 30 alkenylene group, substituted or unsubstituted C2-C 30 alkynylene group, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O)2-, -Si(CH3)2-, -C(CH3)2-, and -C(CF3)2-

[0165] The polyamide group polymer may contain imide group repeating units and amide group repeating units in a molar ratio of 0:100 to 70:30. Specifically, the molar ratio of imide group repeating units to amide group repeating units can be 0:100 to 70:30, 0:100 to 60:40, 0:100 to 50:50, or 0:100 to 45:55, but is not limited thereto.

[0166] If the molar ratio of imide repeating units to amide repeating units of the polyamide group polymer is within the above range, combined with a specific treatment method, the RSR A and RSL A values can be effectively controlled, and the flexibility and mechanical durability of the membrane can be improved.

[0167] In the polyamide-based polymer, the molar ratio of the repeating unit represented by Formula A to the repeating unit represented by Formula B may be from 0:100 to 70:30. Specifically, the molar ratio of the repeating unit represented by Formula A to the repeating unit represented by Formula B may be from 0:100 to 70:30, from 0:100 to 60:40, from 0:100 to 50:50, or from 0:100 to 45:55, but is not limited thereto.

[0168] In addition to the polyamide-based polymer, the polyamide-based film according to one embodiment may further include at least one selected from the group consisting of a filler, a blue pigment, and a UVA absorber.

[0169] The filler may include, for example, oxides, carbonates, or sulfates of metals or metalloids. For example, the filler may include silica, calcium carbonate, barium sulfate, etc., but is not limited thereto.

[0170] The filler may be used in the form of particles. In addition, the surface of the filler is not subjected to a special coating treatment and may be uniformly dispersed throughout the film.

[0171] Since the polyamide-based film contains the filler, a wide viewing angle can be ensured without degrading the optical properties of the film, and not only the roughness and winding property can be improved, but also the improvement effect of scratches caused by sliding during the film preparation process can be improved.

[0172] The filler may have a refractive index of 1.55 to 1.75. Specifically, the refractive index of the filler may be from 1.60 to 1.75, from 1.60 to 1.70, from 1.60 to 1.68, or from 1.62 to 1.65, but is not limited thereto.

[0173] If the refractive index of the filler satisfies the above range, the birefringence values related to nx, ny, and nz can be appropriately adjusted, and the brightness of the film at different angles can be improved.

[0174] On the other hand, if the refractive index of the filler is outside the above range, problems such as the filler being visibly apparent on the film or an increase in haze due to the filler may occur.

[0175] Based on the total weight of the solid content of the polyamide-based polymer, the content of the filler may be from 100 ppm to 15,000 ppm. Specifically, based on the total weight of the solid content of the polyamide-based polymer, the content of the filler may be from 100 ppm to 14,500 ppm, from 100 ppm to 14,200 ppm, from 200 ppm to 14,500 ppm, from 200 ppm to 14,200 ppm, from 250 ppm to 14,100 ppm, or from 300 ppm to 14,000 ppm, but is not limited thereto.

[0176] If the content of the filler is outside the above range, the haze of the film will increase sharply, and the fillers may aggregate with each other on the surface of the film, so that a foreign object feeling can be visually observed, or it may cause problems with the sliding performance, or make the winding property worse during the preparation process.

[0177] The blue pigment may include OP-1300A manufactured by Toyo, but is not limited thereto.

[0178] In some embodiments, the blue pigment may be used in an amount of 50 ppm to 5,000 ppm based on the total weight of the polyamide-based polymer. Preferably, based on the total weight of the polyamide-based polymer, the amount of the blue pigment used may be 100 to 5,000 ppm, 200 to 5,000 ppm, 300 to 5,000 ppm, 400 to 5,000 ppm, 50 to 3,000 ppm, 100 to 3,000 ppm, 200 to 3,000 ppm, 300 to 3,000 ppm, 400 to 3,000 ppm, 50 to 2,000 ppm, 100 to 2,000 ppm, 200 to 2,000 ppm, 300 to 2,000 ppm, 400 to 2,000 ppm, 50 to 1,000 ppm, 100 to 1,000 ppm, 200 to 1,000 ppm, 300 to 1,000 ppm, or 400 to 1,000 ppm, but is not limited thereto.

[0179] The UVA absorber may include an absorber that absorbs electromagnetic waves having a wavelength of 10 nm to 400 nm used in the art. For example, the UVA absorber may include a benzotriazole-based compound, and the benzotriazole-based compound may include an N-phenol benzotriazole-based compound. In some embodiments, the N-phenol benzotriazole-based compound may include N-phenol benzotriazole in which the phenol group is substituted with an alkyl group having 1 to 10 carbon atoms. It may be substituted with two or more alkyl groups, and the alkyl group may be linear, branched, or cyclic.

[0180] In some embodiments, the UVA absorber may be used in an amount of 0.1% to 10% by weight based on the total weight of the polyamide-based polymer. Preferably, relative to the total weight of the polyamide-based polymer, the amount of the UVA absorber used may be 0.1% to 5%, 0.1% to 3%, 0.1% to 2%, 0.5% to 10%, 0.5% to 5%, 0.5% to 3%, 0.5% to 2%, 1% to 10%, 1% to 5%, 1% to 3%, or 1% to 2% by weight, but is not limited thereto.

[0181] The physical properties of the polyamide-based film as described above are based on a thickness of 40 μm to 60 μm. For example, the physical properties of the polyamide-based film are based on a thickness of 50 μm.

[0182] The characteristics of the composition and properties of the polyamide-based film described above can be combined with each other.

[0183] In addition, the RSR of the polyamide-based film described above A value, RSR MD value, RSR TD value, RSL A value, RSL MD value, RSL TD value, the inner angle of the film evaluated according to the deformation angle, modulus, transmittance, haze, and yellowness index can be adjusted by combining the chemical and physical properties of the components constituting the polyamide-based film and the specific conditions of each step in the method for preparing the polyamide-based film described below.

[0184] For example, by combining the composition and content of the components constituting the polyamide-based film, the polymerization conditions and heat treatment conditions during the film preparation process, etc., to achieve the RSR within the desired range A value, RSR MD value, RSR TD value, RSL A value, RSL MD value, RSL TD value, and the inner angle of the film evaluated according to the deformation angle.

[0185] Cover window for display device

[0186] A cover window for a display device according to an embodiment includes a polyamide-based film and a functional layer.

[0187] The polyamide-based film includes a polyamide-based polymer and has an RSR of 3.4 m / N to 5.0 m / N as represented by Equation 1 above based on a thickness of 50 μm A value.

[0188] Details regarding the polyamide-based film are as described above.

[0189] The cover window for a display device can be advantageously applied to a display device.

[0190] Display device

[0191] A display device according to an embodiment includes a display unit; and a cover window disposed on the display unit, wherein the cover window includes a polyamide-based film and a functional layer.

[0192] The polyamide-based film includes a polyamide-based polymer and has an RSR of 3.4 m / N to 5.0 m / N as represented by Equation 1 above based on a thickness of 50 μm A value.

[0193] Details of the polyamide-based film and the cover window are as described above.

[0194] Figure 1 is a schematic exploded view of a display device according to an embodiment, Figure 2 is a schematic perspective view of a display device according to an embodiment. Figure 3 is a schematic cross-sectional view of a display device according to an embodiment.

[0195] Specifically, Figures 1 to 3 shows a display device, which includes a display unit (400) and a cover window (300) disposed on the display unit (400), wherein the cover window includes a polyamide-based film (100) having a first surface (101) and a second surface (102) and a functional layer (200), and an adhesive layer (500) is inserted between the display unit (400) and the cover window (300).

[0196] The display unit (400) is for displaying an image, and it may have a flexible characteristic.

[0197] The display unit (400) may be a display panel for displaying an image. For example, it may be a liquid crystal display panel or an organic electroluminescent display panel. The organic electroluminescent display panel may include a front polarizing plate and an organic EL panel.

[0198] The front polarizing plate may be disposed on the front side of the organic EL panel. Specifically, the front polarizing plate may be attached to the side of the organic EL panel where the image is displayed.

[0199] The organic EL panel may display an image by self-luminescence of pixel units. The organic EL panel may include an organic EL substrate and a driving substrate. The organic EL substrate may include a plurality of organic electroluminescent units, and each organic electroluminescent unit corresponds to one pixel. Specifically, it may include a cathode, an electron transport layer, a light-emitting layer, a hole transport layer, and an anode. The driving substrate is operably coupled to the organic EL substrate. That is, the driving substrate may be coupled to the organic EL substrate so as to apply a driving signal such as a driving current, such that the driving substrate can drive the organic EL substrate by applying a current to each organic electroluminescent unit.

[0200] In addition, an adhesive layer (500) may be inserted between the display unit (400) and the cover window (300). The adhesive layer may be an optically transparent adhesive layer, but there is no particular limitation.

[0201] The cover window (300) may be disposed on the display unit (400). The cover window is located at an outer position of the display device according to an embodiment, thereby protecting the display unit.

[0202] The covering window (300) may include a polyamide-based film and a functional layer. The functional layer may be at least one selected from the group consisting of a hard coat layer, an antireflection layer, an antifouling layer, and an anti-glare layer. The functional layer may be coated on at least one side of the polyamide-based film.

[0203] According to an embodiment, the polyamide-based film can be applied to the exterior of a display device in the form of a film without changing the display driving method, the color filters within the panel, or the lamination structure, thereby providing a display device having a uniform thickness, low haze, high transmittance, and high transparency. Since neither significant process changes nor cost increases are required, it is beneficial for reducing production costs.

[0204] According to an embodiment, the polyamide-based film can have excellent optical properties in terms of high transmittance, low haze, and low yellowness index, and can have excellent mechanical properties such as modulus and flexibility, and when exposed to ultraviolet light, it can inhibit changes (deterioration) in its optical and mechanical properties.

[0205] Specifically, in the polyamide-based film with an RSR A value within the above range, excellent flexibility and mechanical durability can be achieved, and deformation that may remain around the folding point portion due to repeated folding operations of the film can be minimized, and the resilience can be improved. In addition, since it has similar ring stiffness values in all directions and has the required ring stiffness value at extremely low temperatures, its quality is uniform, and when it is applied to the final product, its quality reliability is enhanced, and its mechanical properties or appearance quality will not deteriorate even when used in a harsh environment.

[0206] Method for preparing polyamide-based film

[0207] An embodiment provides a method for preparing a polyamide-based film.

[0208] The method for preparing a polyamide-based film according to an embodiment includes the following steps: polymerizing a diamine compound, a dicarbonyl compound, and optionally a dianhydride compound in an organic solvent to prepare a polyamide-based polymer solution (S100); casting the solution and then drying it to prepare a gel sheet (S200); and performing a heat treatment on the gel sheet (S300) (see Figure 4 ).

[0209] The method for preparing a polyamide-based film according to some embodiments may further include adjusting the viscosity of the polyamide-based polymer solution (S110), aging the polyamide-based polymer solution (S120), and / or degassing the polyamide-based polymer solution (S130).

[0210] A polyamide-based membrane is a membrane mainly composed of a polyamide-based polymer. The polyamide-based polymer is a resin including imide repeating units and amide repeating units in a specified molar ratio as structural units.

[0211] In the method for preparing a polyamide-based membrane, a polymer solution of the polyamide-based polymer can be prepared by simultaneously or sequentially mixing a diamine compound, a dicarbonyl compound, and an optional dianhydride compound in an organic solvent in a reactor and reacting the mixture (S100).

[0212] In one embodiment, the polymer solution can be prepared by simultaneously mixing a diamine compound and a dicarbonyl compound in an organic solvent and reacting them.

[0213] Specifically, the step of preparing the polymer solution may include mixing a diamine compound and a dicarbonyl compound in a solvent and reacting them to prepare a polyamide solution.

[0214] In another embodiment, a polymer solution can be prepared by simultaneously mixing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent and reacting them.

[0215] Specifically, the step of preparing the polymer solution may include first mixing a diamine compound and a dianhydride compound in a solvent and reacting them to produce a polyamic acid (PAA) solution; and second, mixing the polyamic acid (PAA) solution with a dicarbonyl compound and reacting them to form amide bonds and imide bonds. The polyamic acid solution is a solution including polyamic acid.

[0216] Alternatively, the step of preparing the polymer solution may include first mixing a diamine compound and a dianhydride compound in a solvent and reacting them to prepare a polyamic acid solution; dehydrating the polyamic acid solution to obtain a polyimide (PI) solution; and second, mixing the polyimide (PI) solution with a dicarbonyl compound and reacting them to further form amide bonds. The polyimide solution is a solution including a polymer having imide repeating units.

[0217] In one embodiment, the step of preparing the polymer solution may include first mixing a diamine compound and a dicarbonyl compound in a solvent and reacting them to produce a polyamide (PA) solution; and second, mixing the polyamide (PA) solution with a dianhydride compound and reacting them to further form imide bonds. The polyamide solution is a solution including a polymer having amide repeating units.

[0218] The polymer solution thus prepared can be a solution including a polymer containing at least one selected from the group consisting of polyamic acid (PAA) repeating units, polyamide (PA) repeating units, and polyimide (PI) repeating units.

[0219] Alternatively, the polymer contained in the polymer solution contains polymerized amide repeating units derived from a diamine compound and a dicarbonyl compound, and may optionally contain polymerized imide repeating units derived from a diamine compound and a dianhydride compound.

[0220] Details of the diamine compound, dianhydride compound, and dicarbonyl compound are as described above.

[0221] By weight, the solid content contained in the polymer solution may be 10% to 30%. Alternatively, by weight, the solid content contained in the polymer solution may be 15% to 25% or 15% to 20%, but is not limited thereto.

[0222] If the solid content contained in the polymer solution is within the above range, a polyamide-based film can be effectively prepared in the extrusion and casting steps.

[0223] In another embodiment, the step of preparing the polymer solution may further include introducing a catalyst.

[0224] Here, the catalyst may include at least one selected from the group consisting of β-methylpyridine, acetic anhydride, isoquinoline (IQ), and pyridyl compounds, but is not limited thereto.

[0225] Based on 1 mole of the polyamide-based polymer, the catalyst may be added in an amount of 0.01 to 0.5 molar equivalents, 0.01 to 0.4 molar equivalents, 0.01 to 0.3 molar equivalents, 0.01 to 0.2 molar equivalents, or 0.01 to 0.1 molar equivalents, but is not limited thereto.

[0226] Further adding the catalyst can accelerate the reaction rate and enhance the chemical bonding force between repeating unit structures or within repeating unit structures.

[0227] In one embodiment, the step of preparing the polymer solution may further include adjusting the viscosity of the polymer solution (S110). At room temperature, the viscosity of the polymer solution may be 80,000 cps to 500,000 cps, 100,000 cps to 500,000 cps, 150,000 cps to 500,000 cps, 150,000 cps to 450,000 cps, 200,000 cps to 450,000 cps, 200,000 cps to 400,000 cps, 200,000 cps to 350,000 cps, or 200,000 cps to 300,000 cps. In this case, the film-forming ability of the polyamide-based film can be enhanced, thereby enhancing the thickness uniformity.

[0228] Specifically, the steps of preparing the polymer solution may include simultaneously or sequentially mixing and reacting a diamine compound, a dicarbonyl compound, and optionally a dianhydride compound in an organic solvent to prepare a first polymer solution; and further adding a dicarbonyl compound to prepare a second polymer solution having a target viscosity.

[0229] In the steps of preparing the first polymer solution and the second polymer solution, the polymer solutions have different viscosities from each other. For example, the viscosity of the second polymer solution is higher than that of the first polymer solution.

[0230] In the steps of preparing the first polymer solution and the second polymer solution, the stirring speeds may be different from each other. For example, the stirring speed when preparing the first polymer solution may be faster than the stirring speed when preparing the second polymer solution.

[0231] In yet another embodiment, the steps of preparing the polymer solution may further include adjusting the pH value of the polymer solution. In this step, the pH value of the polymer solution may be adjusted to 4 to 7, for example 4.5 to 7.

[0232] The pH value of the polymer solution can be adjusted by adding a pH regulator. The pH regulator is not particularly limited and may include, for example, amine compounds such as alkoxyamines, alkylamines, and alkanolamines.

[0233] By adjusting the pH value of the polymer solution to the above range, defects in the film made from the polymer solution can be prevented, and the desired optical and mechanical properties can be achieved in terms of the yellowness index and modulus.

[0234] Based on the total molar amount of the monomers in the polymer solution, the pH regulator can be used in an amount of 0.1% to 10% by mole.

[0235] In one embodiment, the organic solvent may be at least one selected from the group consisting of dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), m-cresol, tetrahydrofuran (THF), and chloroform. The organic solvent used in the polymer solution may be dimethylacetamide (DMAc), but is not limited thereto.

[0236] In another embodiment, at least one selected from the group consisting of a filler, a blue pigment, and a UVA absorber may be added to the polymer solution.

[0237] Details of the types and contents of the filler, the blue pigment, and the UVA absorber are as described above. The filler, the blue pigment, and the UVA absorber may be mixed with the polyamide-based polymer in the polymer solution.

[0238] The polymer solution can be stored at a temperature of -20°C to 20°C, -20°C to 10°C, -20°C to 5°C, -20°C to 0°C, or 0°C to 10°C.

[0239] If stored at the above temperatures, degradation of the polymer solution can be prevented and the water content can be reduced, thereby preventing defects in the resulting film.

[0240] In some embodiments, the polymer solution or the polymer solution whose viscosity has been adjusted can be aged (S120).

[0241] Aging can be carried out by leaving the polymer solution at a temperature of -10°C to 10°C for 24 hours or longer. In this case, for example, the polyamide-based polymer or unreacted materials contained in the polymer solution can complete the reaction or reach chemical equilibrium, whereby the polymer solution can be homogenized. The mechanical properties and optical properties of the resulting polyamide-based film can be substantially uniform over the entire area of the film. Preferably, aging can be carried out at a temperature of -5°C to 10°C, -5°C to 5°C, or -3°C to 5°C, but is not limited thereto.

[0242] In one embodiment, the method may further include degassing the polyamide-based polymer solution (S130). The degassing step can remove moisture from the polymer solution and reduce impurities, thereby increasing the reaction yield and imparting excellent surface appearance and mechanical properties to the finally produced film.

[0243] Degassing may include vacuum degassing or purging with an inert gas.

[0244] After reducing the internal pressure of the storage tank containing the polymer solution to 0.1 bar to 0.7 bar, vacuum degassing can be carried out for 30 minutes to 3 hours. Performing vacuum degassing under these conditions can reduce the bubbles in the polymer solution. Therefore, surface defects in the resulting film can be prevented, and excellent optical properties such as haze can be achieved.

[0245] In addition, purging can be carried out by purging the tank with an inert gas at an internal pressure of 1 atm to 2 atm. Purging under these conditions can remove moisture from the polymer solution and reduce impurities, thereby increasing the reaction yield and obtaining excellent optical properties such as haze and mechanical properties.

[0246] The inert gas can be at least one selected from the group consisting of nitrogen, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn), but is not limited thereto. Specifically, the inert gas can be nitrogen.

[0247] Vacuum degassing and purging with an inert gas can be carried out in separate steps.

[0248] For example, a vacuum degassing step can be carried out, followed by purging with an inert gas, but not limited thereto.

[0249] Vacuum degassing and / or purging with an inert gas can improve the surface physical properties of the polyamide-based film thus prepared.

[0250] A polymer solution can be cast to prepare a gel sheet (S200).

[0251] For example, the polymer solution can be extruded, coated, and / or dried on a carrier to form a gel sheet.

[0252] In addition, the casting thickness of the polymer solution can be 200 μm to 700 μm. When the polymer solution is cast to a thickness within the above range, the final film made after drying and heat treatment can have a suitable and uniform thickness.

[0253] As described above, the polymer solution can have a viscosity of 150,000 cps to 500,000 cps at room temperature. Since the viscosity meets the above range, the polymer solution can be cast to a uniform thickness without defects, and a polyamide-based film with a substantially uniform thickness can be formed without local / partial thickness change during drying.

[0254] Cast the polymer solution, and then dry it at a temperature of 60 °C to 150 °C, 70 °C to 150 °C, 80 °C to 150 °C, 90 °C to 150 °C, or 90 °C to 120 °C for 5 minutes to 60 minutes or 10 minutes to 40 minutes to prepare a gel sheet. Specifically, the polymer solution is dried at a temperature of 90 °C to 120 °C for 20 minutes to 40 minutes to prepare a gel sheet.

[0255] The solvent of the polymer solution can be partially or completely volatilized during the drying process to prepare a gel sheet.

[0256] The dried gel sheet can be heat-treated to form a polyamide-based film (S300).

[0257] The heat treatment of the gel sheet can be carried out, for example, by a heat treatment device (or a tenter). The heat treatment device can include at least one hot air blower and at least one heater. The heat treatment device can include any one of at least one hot air blower or at least one heater.

[0258] The step of heat-treating the dried gel sheet includes a first heat treatment with hot air supplied by at least one hot air blower; and a second heat treatment with at least one heater.

[0259] The part where the first heat treatment step is carried out is called the first heat treatment part, and the part where the second heat treatment step is carried out is called the second heat treatment part.

[0260] The first heat treatment step and the second heat treatment step can be carried out in sequence. The second heat treatment step can be carried out after the first heat treatment step, or the first heat treatment step can be carried out after the second heat treatment step, but it is not limited thereto. Specifically, the second heat treatment step can be carried out after the first heat treatment step has been carried out.

[0261] The heat treatment of the gel sheet can be carried out by a support that moves continuously in a heat treatment apparatus. Specifically, the gel sheet can be positioned on the support, and the film can move in the longitudinal direction as the support moves in the moving direction.

[0262] The steps for heat-treating the gel sheet include: fixing the transverse ends of the gel sheet (film) with a fixing portion; and changing the width of the gel sheet using the fixing portion. The steps for heat-treating the gel sheet can be carried out by fixing the transverse ends of the gel sheet with a fixing portion and heat-treating it while changing the width of the fixed gel sheet. For example, in a heat treatment apparatus, the transverse ends of the film are fixed with pins, and when the film moves through the support, the width of the gel sheet can be changed as the position of the pins is adjusted.

[0263] When the gel sheet passes through the first heat treatment section and the second heat treatment section, the steps of fixing the transverse ends of the gel sheet with a fixing portion and heat-treating it while changing the width of the fixed gel sheet can be carried out.

[0264] In one embodiment, in the step of changing the width of the gel sheet when the gel sheet passes through the first heat treatment section in the longitudinal direction (moving direction) of the gel sheet, the width of the gel sheet may become narrower.

[0265] In addition, in the step of changing the width of the gel sheet when the gel sheet passes through the second heat treatment section in the longitudinal direction (moving direction) of the gel sheet, the width of the gel sheet may become narrower. Alternatively, in the step of changing the width of the gel sheet, the width of the gel sheet can be repeatedly widened and narrowed.

[0266] The width of the gel sheet at the entrance of the first heat treatment section can be greater than the width of the gel sheet at the exit of the first heat treatment section, and the width of the gel sheet at the entrance of the second heat treatment section can be greater than the width of the gel sheet at the exit of the second heat treatment section.

[0267] In addition, the width of the gel sheet at the entrance of the first heat treatment section can be greater than the width of the gel sheet at the exit of the second heat treatment section, but it is not limited thereto.

[0268] The maximum width of the gel sheet in the first heat treatment section is called Wa, the minimum width of the gel sheet in the first heat treatment section is called Wb, and the minimum width of the gel sheet in the first heat treatment section and the second heat treatment section is called Wc.

[0269] For example, the width of the gel sheet at the entrance of the first heat treatment section can be the maximum width (Wa) of the gel sheet in the first heat treatment section, and the width of the gel sheet at the exit of the first heat treatment section can be the minimum width (Wb) of the gel sheet in the first heat treatment section.

[0270] In addition, the width of the gel sheet at the entrance of the first heat treatment section can be greater than the width of the gel sheet at the exit of the second heat treatment section, and the width of the gel sheet at the exit of the second heat treatment section can be the minimum width (Wc) of the gel sheet in the first and second heat treatment sections.

[0271] As another example, Wb can be greater than or equal to Wc, and Wb can be less than or equal to Wc. Specifically, Wb can be greater than Wc. More specifically, Wa > Wb > Wc, but not limited to this.

[0272] In one embodiment, the value of Wb / Wa is from 0.955 to 0.990. For example, the value of Wb / Wa can be 0.955 or greater, 0.960 or greater, 0.965 or greater, 0.968 or higher, or 0.969 or greater, and can also be 0.990 or less, 0.985 or less, 0.980 or less, or 0.975 or less, but not limited to this. As another example, it can be from 0.955 to 0.980.

[0273] In addition, the value of Wc / Wa is from 0.950 to 0.990. For example, the value of Wc / Wa may be 0.950 or greater, 0.953 or greater, 0.955 or greater, or 0.957 or greater, and can also be 0.990 or less, 0.985 or less, 0.980 or less, 0.975 or less, 0.970 or less, or 0.965 or less, but not limited to this. As another example, it can be from 0.950 to 0.970.

[0274] Since in the step of heat-treating the gel sheet, the values of Wb / Wa and Wc / Wa satisfy the above ranges, it is easy to control the RSR A value of the prepared polyamide-based film within 3.4 to 5.0.

[0275] In one embodiment, if hot air supplied by at least one hot air blower is used for heat treatment, the heat can be supplied uniformly. If the heat supply is not uniform, a satisfactory surface roughness cannot be obtained, or the surface quality may be uneven, and the surface energy may be too high or too low.

[0276] The heat treatment with hot air can be carried out in the temperature range of 100°C to 250°C for 5 minutes to 100 minutes. Specifically, the gel sheet can be heat-treated with hot air at a heating rate of 1.5°C / minute to 20°C / minute in the temperature range of 100°C to 250°C for 5 minutes to 60 minutes. More specifically, the heat treatment of the gel sheet can be carried out in the temperature range of 140°C to 250°C.

[0277] In this case, the initial temperature of the heat treatment of the gel sheet with hot air can be 100°C or higher. Specifically, the initial temperature of the heat treatment of the gel sheet with hot air can be 100°C to 180°C. In addition, the maximum temperature of the heat treatment with hot air can be 150°C to 250°C.

[0278] The temperature described in the heat treatment with hot air is the temperature in the heat treatment device where the gel sheet is present. It corresponds to the temperature measured by the temperature sensor located in the first heat treatment part of the heat treatment device.

[0279] In one embodiment, the step of heat-treating the gel sheet may include a second heat treatment by at least one heater. Specifically, the heat treatment is carried out by a plurality of heaters.

[0280] The plurality of heaters may include a plurality of heaters spaced apart from each other in the transverse direction (TD direction) of the gel sheet. The plurality of heaters can be mounted on the heater mounting part, and two or more heater mounting parts can be provided along the moving direction (MD direction) of the gel sheet.

[0281] At least one heater may include an IR heater. However, the type of at least one heater is not limited to the above examples and can be variously changed. Specifically, each of the plurality of heaters may include an IR heater.

[0282] The heat treatment of at least one heater can be carried out in the temperature range of 250°C or higher. Specifically, the heat treatment of at least one heater can be carried out in the temperature range of 250°C to 400°C for 1 minute to 30 minutes or 1 minute to 20 minutes.

[0283] The temperature described in the heat treatment with the heater is the temperature in the heat treatment device where the gel sheet is present. It corresponds to the temperature measured by the temperature sensor located in the second heat treatment part of the heat treatment device.

[0284] Subsequently, after the heat treatment step of the gel sheet, the step of cooling the cured film can be carried out while moving the cured film.

[0285] The step of cooling the cured film when it moves may include a first temperature reduction step of reducing the temperature at a rate of 100 °C / min to 1000 °C / min and a second temperature reduction step of reducing the temperature at a rate of 40 °C / min to 400 °C / min.

[0286] In this case, specifically, the second temperature reduction step is carried out after the first temperature reduction step. The temperature reduction rate of the first temperature reduction step may be faster than that of the second temperature reduction step.

[0287] For example, the maximum rate of the first temperature reduction step is faster than the maximum rate of the second temperature reduction step. Or, the minimum rate of the first temperature reduction step is faster than the minimum rate of the second temperature reduction step.

[0288] If the step of cooling the cured film is carried out in such a multi-stage manner, the physical properties of the cured film can be further stabilized, and the optical and mechanical properties of the film obtained in the curing step can be more stably maintained for a longer time.

[0289] In addition, the process of winding the cooled cured film using a winding machine may also be carried out.

[0290] In this case, the ratio of the moving speed of the gel sheet on the belt during drying to the moving speed of the cured film during winding is 1:0.95 to 1:1.40. Specifically, the ratio of the moving speeds may be 1:0.99 to 1:1.20, 1:0.99 to 1:1.10, or 1:1.01 to 1:1.10, but is not limited thereto.

[0291] If the ratio of the moving speeds is outside the above range, the mechanical properties of the cured film may be damaged, and the flexibility and elastic properties may deteriorate.

[0292] In the method for preparing a polyamide-based film, the thickness change (%) according to the following relational expression 1 may be 3% to 30%. Specifically, the thickness change (%) may be 5% to 20%, but is not limited thereto.

[0293] [Relational expression 1] Thickness change (%) = (M1 - M2) / M2 × 100

[0294] In relational expression 1, M1 is the thickness (μm) of the gel sheet, and M2 is the thickness (μm) of the cured film cooled during winding.

[0295] The polyamide-based film prepared by the above preparation method has excellent mechanical properties, and has excellent resilience when bent for a long time and then the bending force is released, and no visible wrinkles are present after a rigorous folding test. In addition, since it can achieve the required level of ring stiffness not only at room temperature but also in an extremely low temperature environment, it can be applied to various uses that require flexibility and mechanical durability. For example, the polyamide-based film can be applied not only to display devices but also to solar cells, semiconductor devices, sensors, etc.

[0296] Details of the polyamide-based film prepared by the above preparation method of the polyamide-based film are as described above.

[0297] Embodiments of implementing the present invention

[0298] Hereinafter, the above will be described in detail with reference to examples. However, these examples are for illustrating the present invention, and the scope of the present invention is not limited thereto.

[0299] [Example 1]

[0300] Under a nitrogen atmosphere, in a reactor with controllable temperature, dimethylacetamide (DMAc) was added as an organic solvent at 10°C. Then, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB), which is an aromatic diamine, was slowly added thereto and dissolved.

[0301] Then, terephthaloyl chloride (TPC), which is a dicarbonyl compound, was added, and then stirred for 1 hour. Isophthaloyl chloride (IPC) was added in an amount of 94% of the total amount introduced in terms of moles, and then stirred for 1 hour, thereby preparing a first polymer solution.

[0302] The viscosity of the first polymer solution thus prepared was measured. If the measured viscosity did not reach the target viscosity, an IPC solution in a DMAc organic solvent with a concentration of 10% by weight was prepared and added to the first polymer solution, and then stirred for 30 minutes. This step was repeated until the viscosity became 250,000 cps, thereby preparing a second polymer solution.

[0303] The polymer solution thus obtained was coated on a substrate, and then dried with hot air at a temperature in the range of 90°C to 120°C for about 30 minutes to prepare a gel sheet.

[0304] Thereafter, as a heat treatment step of the dry gel sheet, a first heat treatment step and a second heat treatment step are sequentially performed in a heat treatment apparatus. Specifically, as the first heat treatment step (in the first heat treatment section), hot air treatment is performed on the gel sheet at a temperature of 140°C to 250°C. In this case, the temperature in the first heat treatment section is the temperature in the heat treatment apparatus where the gel sheet is present. It corresponds to the temperature measured by a temperature sensor located in the first heat treatment section of the heat treatment apparatus.

[0305] Subsequently, as the second heat treatment step (in the second heat treatment section), the gel sheet is heat-treated at a temperature of 250°C to 400°C by a plurality of IR heaters (IR heaters with a maximum temperature of 1,200°C). In this case, the temperature in the second heat treatment section is the temperature in the heat treatment apparatus where the gel sheet is present. It corresponds to the temperature measured by a temperature sensor located in the second heat treatment section of the heat treatment apparatus.

[0306] In this case, in the step of heat-treating the gel sheet, the lateral ends of the gel sheet are fixed with fixing parts, and heat treatment is performed while changing the width of the fixed gel sheet. Specifically, the maximum width of the gel sheet in the first heat treatment section is referred to as Wa, the minimum width of the gel sheet in the first heat treatment section is referred to as Wb, the minimum width of the gel sheet in the first heat treatment section and the second heat treatment section is referred to as Wc, and the lateral ends of the gel sheet are fixed with fixing parts such that Wa (the same as the width of the gel sheet at the entrance of the first heat treatment section) is 1,660 mm. When the gel sheet moves in the longitudinal direction, heat treatment is performed while changing the width of the gel sheet such that Wb / Wa is 0.969 and Wc / Wa is 0.957.

[0307] Thereafter, a first temperature reduction step is performed by reducing the temperature at a rate of about 800°C / minute, and then a second temperature reduction step is performed by reducing the temperature at a rate of about 100°C / minute, thereby obtaining a polyamide-based film with a thickness of 50 μm, which is wound using a winding machine. In this case, the conveying speed of the gel sheet on the belt during drying is 1 m / s. The speed of the winding machine is controlled such that the ratio of the moving speed of the gel sheet on the belt during drying to the moving speed of the film during winding is in the range of 1:1.01 to 1:1.10.

[0308] The specific composition and molar ratio of the polyamide-based polymer are shown in Table 1 below.

[0309] <Examples 2-5 and Comparative Examples 1 and 2>

[0310] As shown in Table 1 below, the films were all prepared in the same manner as in Example 1, except that the composition and molar ratio of the polymer of the gel sheet, Wb / Wa, Wc / Wa, etc. were changed in the heat treatment step.

[0311] In addition, in the case where the dianhydride compound reacts during the preparation of the polymer solution, after dissolving the aromatic diamine in the organic solvent and before adding the dicarbonyl compound, the dianhydride compound is slowly added and stirred.

[0312] <Evaluation Examples>

[0313] The membranes prepared in the Examples and Comparative Examples were respectively measured and evaluated for the following properties. The results are shown in Table 1 below.

[0314] Evaluation Example 1: Measurement of film thickness

[0315] The thickness was measured at 5 random points using a digital micrometer 547-401 manufactured by Mitutoyo Corporation. Their average value was taken as the thickness.

[0316] Evaluation Example 2: Measurement of modulus

[0317] Samples were cut at least 10 cm in a direction perpendicular to the main shrinkage direction of the membrane and 10 cm in the main shrinkage direction. They were fixed by clamps spaced 10 cm apart in a universal testing machine UTM 5566A of Instron. The samples were stretched at a rate of 12.5 mm / min at room temperature until fracture to obtain a stress-strain curve. The slope of the load with respect to the initial strain on the stress-strain curve was taken as the modulus (GPa).

[0318] Evaluation Example 3: Measurement of transmittance and haze

[0319] According to the JIS K 7136 standard, the transmittance and haze were measured using a haze meter NDH-5000W manufactured by Nippon Denshoku Kogyo.

[0320] Evaluation Example 4: Measurement of yellowness index

[0321] The yellowness index (YI) was measured using a spectrophotometer (UltraScan PRO, Hunter Associates Laboratory) under the conditions of d65 and 10° according to the ASTM-E313 standard.

[0322] Evaluation Example 5: Measurement of ring stiffness

[0323] Fix both ends of a polyamide-based film with a width of 15 mm, a length of 120 mm, and a thickness of 50 μm to the fixing part of a ring stiffness tester (Toyoseiki). Use the extrusion part to extrude the polyamide-based film at an extrusion speed of 3.3 mm / s until the final spacing distance (L) between the extrusion part and the fixing part is 20 mm. Then, measure the ring stiffness of the polyamide-based film with a sensor.

[0324] RSR MD is the ring stiffness value measured at room temperature with the MD direction of the film as the longitudinal direction,

[0325] RSR TD is the ring stiffness value measured at room temperature with the TD direction of the film as the longitudinal direction,

[0326] RSL MD is the ring stiffness value measured within 1 minute with the MD direction of the film as the longitudinal direction after the film is placed at -20 °C for 2 hours, and

[0327] RSL TD is the ring stiffness value measured within 1 minute with the TD direction of the film as the longitudinal direction after the film is placed at -20 °C for 2 hours.

[0328] Evaluation Example 6: Evaluation of deformation angle (static bending test)

[0329] Fold each polymer film with a width of 20 mm, a length of 150 mm, and a thickness of 50 μm into a glass turn shape with a radius of curvature of 2 mm, then place it at 25 °C for 24 hours, and release the force applied to the film. Then measure the inner angle of the film ( Figure 6 (AG in (b)) (see Figure 6 ).

[0330] Evaluation Example 7: Evaluation of wrinkle visibility

[0331] Fold the film with a thickness of 50 μm repeatedly to a radius of curvature of 2 mm, and then unfold it 200,000 times (the number of folding and unfolding times is counted as 1 time). After folding 200,000 times, if almost no wrinkles are visible, it is evaluated as ○; if the wrinkles are slightly visible, it is evaluated as Δ; if the wrinkles are clearly visible, it is evaluated as ×.

[0332] [Table 1]

[0333]

[0334]

[0335] Referring to Table 1, it can be seen that for RSR AThe film properties such as the ring stiffness, modulus, transmittance, haze, and yellowness index of the films of the examples with a value adjusted to 3.4 - 5.0 are excellent, and they are also excellent in terms of the restoring force when bent for a long time and then the bending force is released. Even when folded 200,000 times or more, almost no wrinkles can be seen.

[0336] Specifically, regarding the restoring force, after the deformation angle evaluation test, the films of Examples 1 to 5 have an inner angle of 150° or more, while the films of Comparative Examples 1 and 2 have inner angles of 135° and 130° respectively, showing poor static bending resistance. That is, the films of Comparative Examples 1 and 2 are vulnerable to external extrusion. Especially after being folded for a long time, they will be significantly deformed. Therefore, when applied to a display device, they will not show a uniform screen state or will have wrinkles, resulting in the problem of screen distortion.

[0337] Regarding the ring stiffness, the polyamide-based films of Examples 1 to 5 have excellent ring stiffness when placed at an extremely low temperature of -20°C and at room temperature for 2 hours. Compared with the films of the comparative examples, they also show improved results in the evaluation of the static bending resistance and the visibility of wrinkles after folding. Therefore, they can be advantageously applied to foldable displays, flexible displays, rollable displays, etc.

[0338] Reference numerals

[0339] 100: Polyamide-based film

[0340] 101: First surface 102: Second surface

[0341] 200: Functional layer 300: Cover window

[0342] 400: Display unit 500: Adhesive layer

[0343] 10: Fixing part

[0344] 20: Extrusion part

[0345] L: Final interval distance (L) when measuring the ring stiffness

[0346] AG: Inner angle of the film when evaluating the deformation angle

Claims

1. A polyamide-based membrane, which comprises a polyamide-based polymer and has an RSR value of 3.6 m / N to 4.2 m / N as represented by the following Equation 1 based on the thickness of the 50-μm membrane A value <Equation 1>RSR A =(RSR MD +RSR TD ) / 2 In Equation 1, RSR MD is the ring stiffness value measured with the MD direction of the film as the longitudinal direction at room temperature, and RSR TD is the ring stiffness value measured with the TD direction of the film as the longitudinal direction at room temperature. Among them, The polyamide-based polymer includes polymerized amide repeating units derived from a diamine compound and a dicarbonyl compound, and optionally includes polymerized imide repeating units derived from a diamine compound and a dianhydride compound. The dianhydride compound is at least one selected from the group consisting of 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 3,3',4,4'-biphenyltetracarboxylic dianhydride, the dicarbonyl compound is terephthaloyl chloride, 1,1'-biphenyl-4,4'-dicarbonyl dichloride, isophthaloyl chloride, or a combination thereof, and the diamine compound is 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl. The polyamide-based polymer contains imide group repeating units and amide group repeating units in a molar ratio of 0:100 to 70:

30. Based on a film thickness of 50 μm, it has an RSL of 3.7 m / N to 4.2 m / N as represented by Equation 2 below A Value: <Equation 2>RSL A =(RSL MD +RSL TD ) / 2 In Equation 2, RSL MD is the ring stiffness value measured in 1 minute with the MD direction of the film as the longitudinal direction after the film is placed at -20°C for 2 hours. RSL TD is the ring stiffness value measured in 1 minute with the TD direction of the film as the longitudinal direction after the film is placed at -20°C for 2 hours. The polyamide-based film is prepared by the following method: Polymerizing a diamine compound, a dicarbonyl compound, and an optional dianhydride compound in an organic solvent to prepare a polyamide-based polymer solution; Casting the solution and then drying it to prepare a gel sheet; And Performing a heat treatment on the gel sheet, wherein the step of performing the heat treatment on the gel sheet is to fix the lateral ends of the gel sheet with fixing parts and perform the heat treatment while changing the width of the fixed gel sheet. wherein the step of performing the heat treatment on the gel sheet includes a first heat treatment with hot air supplied by at least one hot air blower and a second heat treatment with at least one heater, and The part where the first heat treatment step is performed is called the first heat treatment part, The part where the second heat treatment step is performed is called the second heat treatment part, The maximum width of the gel sheet in the first heat treatment part is called Wa, The minimum width of the gel sheet in the first heat treatment part is called Wb, and The minimum width of the gel sheet in the first heat treatment part and the second heat treatment part is called Wc, The value of Wb / Wa is 0.955 to 0.990, and the value of Wc / Wa is 0.950 to 0.990, wherein the condition of Wa > Wb > Wc is satisfied; wherein when the film with a thickness of 50 μm is folded into a curvature radius of 2 mm, then placed at 25°C for 24 hours, and the force applied to the film is released, the inner angle of the film is 160° or more; wherein the polyamide-based film further includes at least one selected from the group consisting of a blue pigment and a UVA absorber.

2. The polyamide-based membrane according to claim 1, wherein the RSR MD is from 3.35 m / N to 5.0 m / N, and the RSR TD is from 3.3 m / N to 5.0 m / N.

3. The polyamide-based film according to claim 1, having a modulus of 5 GPa or more, a transmittance of 80% or higher, a haze of 1% or lower, and a yellowness index of 5 or lower.

4. A cover window for a display device, comprising the polyamide-based film according to any one of claims 1 to 3 and a functional layer.

5. A display device, comprising a display unit; and a cover window disposed on the display unit, wherein, The cover window includes the polyamide-based film according to any one of claims 1 to 3 and a functional layer.

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