阅读说明：本技术 聚酰亚胺类膜的制造方法和由此制造的聚酰亚胺类膜 (Method for producing polyimide film and polyimide film produced thereby ) 是由 洪起一 郑鹤基 于 2018-12-04 设计创作，主要内容包括：本发明涉及一种聚酰亚胺类膜的制造方法和由此制造的聚酰亚胺类膜,具体地,涉及一种聚酰亚胺类膜的制造方法和由此制造的聚酰亚胺类膜,其中,所述聚酰亚胺类膜由于其由屈服伸长率表示的挠曲特性优异而有效地用作柔性电子器件的覆盖基板。(The present invention relates to a method for producing a polyimide-based film and a polyimide-based film produced thereby, and in particular, to a method for producing a polyimide-based film which is effectively used as a cover substrate for flexible electronic devices because of its excellent flexing characteristics as represented by yield elongation, and a polyimide-based film produced thereby.)

1. A method of manufacturing a polyimide-based film in a roll-to-roll manner, comprising:

subjecting a polyimide-based film obtained in a gel state, which is formed by casting on a support, to a first heat treatment (S1) while stretching the polyimide-based film in a Machine Direction (MD) at an elongation of not less than 100% and less than 135% and shrinking the polyimide-based film in a Transverse Direction (TD) at a shrinkage rate of more than 75% and not more than 100%, and excluding a case where the elongation in the machine direction is equal to the shrinkage rate in the transverse direction; and

subjecting the polyimide-based film after the first heat treatment to a second heat treatment (S2) while being oriented in the Machine Direction (MD) at not less than 30N/mm²And less than 160N/mm²Further stretching the polyimide-based film,

wherein the maximum heating temperature in the course of the first heat treatment is 250 ℃ to 330 ℃, and the second heat treatment is performed at a temperature within-10 ℃ to +30 ℃ of the maximum heating temperature reached in the course of the first heat treatment for 200 seconds or more.

2. The method according to claim 1, wherein, in the step (S1), the elongation in the longitudinal direction is 105% to 130%.

3. The method according to claim 2, wherein, in the step (S1), the elongation in the longitudinal direction is 115% to 130%.

4. The method of claim 1, wherein, in the step (S1), the shrinkage in the transverse direction is 80% to 100%.

5. The method of claim 4, wherein, in the step (S1), the shrinkage in the transverse direction is 80% to 95%.

6. The method of claim 1, wherein, in the step (S2), the time of the second heat treatment is not less than 200 seconds and less than 1,500 seconds.

7. The method of claim 1, wherein, in the step (S2), the pulling force is 30N/mm²To 150N/mm²。

8. A polyimide-based film produced according to the method of any one of claims 1 to 7.

9. The polyimide-based film according to claim 8, wherein the yield strain of the polyimide-based film in a uniaxial direction is 3% or more.

10. The polyimide-based film according to claim 9, wherein the polyimide-based film has a pencil hardness of 1H or more as measured in accordance with ASTM D3363, and has a yellowness of 5.0 or less as measured in accordance with CM-3700D manufactured by Konica Minolta, Inc and a light transmittance of 85% or more at 550 nm.

Technical Field

This application claims the benefit of priority from korean patent application No. 10-2017-.

The present disclosure relates to a method for manufacturing a polyimide-based film and a polyimide-based film manufactured thereby.

Background

In particular, devices such as foldable or rollable devices have been developed such that flexible displays can be obtained, and at the same time, new types of flexible substrates are needed that can protect the bottom element.

Various high-hardness plastic substrates are considered as candidate materials for flexible display substrates. Among them, a transparent polyimide film capable of achieving high hardness with a small thickness is considered as a main candidate material for a cover substrate. However, the substrate for the display cover is mainly required to maintain high hardness, low moisture permeability, chemical resistance, and light transmittance to protect components included in the display device. However, the conventional transparent plastic substrate has a limitation in ensuring abrasion resistance and low surface hardness, compared to glass. For this reason, many techniques have been developed to improve the surface hardness of the polymer film. However, it is difficult for these techniques to achieve bending characteristics with a radius of curvature of 1mm or less while securing surface hardness.

Meanwhile, in order to improve the surface hardness of the plastic substrate, korean patent laid-open publication No.2010-0041992 provides a high-hardness hard coating film composition including an ultraviolet-curable urethane acrylate oligomer, and international publication No. wo2013-187699 proposes a high-hardness silicone resin composition including an alicyclic epoxy group, a method of preparing the same, and an optical film including the cured product.

As described above, techniques for improving the hardness of a substrate material for a display cover have been actively proposed, but these techniques have a limitation in improving the bending characteristics of a film. Furthermore, manufacturing techniques that can significantly improve the bending properties of the film while ensuring low water permeability and abrasion resistance required for plastic substrates used in a wider range of applications other than covering the substrate are still imperfect.

Disclosure of Invention

Technical problem

Accordingly, an object of the present disclosure is to provide a method for manufacturing a polyimide-based film having significantly improved bending characteristics while maintaining basic physical properties of the polyimide-based film, and a polyimide-based film manufactured thereby. Further, another object of the present disclosure is to provide a polyimide-based film having improved bending characteristics while ensuring surface hardness and optical properties.

Technical scheme

According to an aspect of the present disclosure, in order to solve the technical problem, there is provided a method of manufacturing a polyimide-based film in a roll-to-roll manner, including: subjecting a polyimide-based film obtained in a gel state, which is formed by casting on a support, to a first heat treatment (S1) while stretching the polyimide-based film in a Machine Direction (MD) at an elongation of not less than 100% and less than 135% and shrinking the polyimide-based film in a Transverse Direction (TD) at a shrinkage of more than 75% and not more than 100% (excluding a case where the elongation in the machine direction is equal to the shrinkage in the transverse direction); and subjecting the polyimide-based film after the first heat treatment to a second heat treatment (S2) while further in the Machine Direction (MD) at not less than 30N/mm²And less than 160N/mm²The polyimide-based film is stretched at a maximum heating temperature of 250 ℃ to 330 ℃ in the first heat treatment, and the second heat treatment is performed at a temperature within-10 ℃ to +30 ℃ of the maximum heating temperature reached in the first heat treatment for 200 seconds or more.

In step (S1), the elongation in the longitudinal direction may be more preferably 105% to 130%, or 115% to 130%.

In the step (S1), the shrinkage rate in the transverse direction may be more preferably 80% to 100%, or 80% to 95%.

In the step (S2), the time of the second heat treatment may be more preferably not less than 200 seconds and less than 1,500 seconds.

In the step (S2), the pulling force may be more preferably 30N/mm²To 150N/mm²。

According to another aspect of the present disclosure, there is provided a polyimide-based film manufactured according to the above-described method.

In this case, the polyimide-based film has a yield strain in the uniaxial direction of 3% or more, a pencil hardness of 1H or more as measured in accordance with ASTM D3363, a yellowness of 5.0 or less as measured in accordance with CM-3700D manufactured by Konica Minolta, Inc, and a light transmittance at 550nm of 85% or more.

Advantageous effects

The present disclosure provides a polyimide-based film having excellent bending characteristics, and thus can be used as a substrate for various applications of flexible electronic devices having crimpability or foldability.

In addition, the polyimide-based film of the present disclosure has bending characteristics and ensures optical properties such as excellent surface hardness and scratch resistance. In particular, the polyimide-based film is highly useful as a cover substrate for flexible electronic devices.

Detailed description of the preferred embodiments

In one aspect, the present disclosure relates to a method of manufacturing a polyimide-based film in a roll-to-roll manner, including: subjecting a polyimide-based film obtained in a gel state, which is formed by casting on a support, to a first heat treatment (S1) while stretching the polyimide-based film in a Machine Direction (MD) at an elongation of not less than 100% and less than 135% and shrinking the polyimide-based film in a Transverse Direction (TD) at a shrinkage of more than 75% and not more than 100% (excluding a case where the elongation in the machine direction is equal to the shrinkage in the transverse direction); and subjecting the polyimide-based film after the first heat treatment to a second heat treatment (S2) while further in the Machine Direction (MD)Not less than 30N/mm²And less than 160N/mm²The polyimide-based film is stretched at a maximum heating temperature of 250 ℃ to 330 ℃ in the first heat treatment, and the second heat treatment is performed at a temperature within-10 ℃ to +30 ℃ of the maximum heating temperature reached in the first heat treatment for 200 seconds or more.

An object of the present disclosure is to provide a polyimide-based film having physical properties suitable for application to a cover substrate of a foldable or rollable device having a radius of curvature of 1mm (1R) or less. The polyimide-based film should withstand very strong physical stresses in order to enable the film to be applied to such devices. In other words, the polyimide-based film should have a bending property capable of returning the film to its original state even when deformation is applied thereto several tens of thousands times or more.

In general, polymers have viscoelastic behavior that can exhibit elastic or viscous behavior depending on thermal and physical conditions. A behavior that enables the film to recover to its original state is referred to as "elastic behavior", and a main object of the present disclosure is to provide a polyimide-based film that exhibits elastic behavior when deformed with a radius of curvature of 1mm or less.

Hereinafter, the present disclosure will be described in more detail.

First, in general, when a polyimide-based film obtained in a gel state is naturally dried, a solvent is evaporated, and the film inevitably shrinks during imidization. Therefore, in order to control such shrinkage of the film, the film is usually fixed to a specific holder such as a tenter. The method according to the present disclosure includes: under these control conditions of the film, the polyimide-based film obtained in a gel state is subjected to a first heat treatment (S1) while stretching the polyimide-based film in the Machine Direction (MD) at an elongation of not less than 100% and less than 135%, and shrinking the polyimide-based film in the Transverse Direction (TD) at a shrinkage of more than 75% and not more than 100% (excluding the case where the elongation in the machine direction is equal to the shrinkage in the transverse direction).

At this time, in step (S1), the elongation in the longitudinal direction is 105% or more, or 105% to 130%, or 115% to 130%, and the shrinkage in the transverse direction is 100% or less, or 80% to 100%, or 80% to 95%, because it is advantageous to ensure better bending characteristics.

However, when the elongation in the longitudinal direction is less than 100%, there is a problem that a desired yield strain cannot be obtained due to insufficient orientation of the polymer, and when the elongation in the longitudinal direction is 130% or more, there is a problem that the film is broken due to low breaking strain caused by the properties of polyimide. As used herein, the term "100% elongation" refers to the elongation required to prevent shrinkage. As described above, when the polyimide-based film is not fixed, the film inevitably shrinks during the process of removing the solvent. Thus, it is obvious to the person skilled in the art that 100% elongation cannot avoid any change in the film.

In addition, when the shrinkage rate is 75% or less, there is a problem in that the film sags and contacts the floor due to excessive shrinkage, and finally cracks. When the shrinkage rate exceeds 99%, the desired yield strain cannot be obtained because the uniaxial orientation in the longitudinal direction is relatively insufficient.

Accordingly, the present disclosure controls the shrinkage of the film in the manufacture of polyimide-based films to provide maximum orientation so long as the film does not break, thereby resulting in desirable physical properties, particularly yield strain. Since the polyimide film cannot be stretched to a high level due to the properties of polyimide, uniaxial stretching is used in the present disclosure to stretch the polyimide film as much as possible. It may be difficult to obtain a desired orientation by biaxial stretching.

In addition, since a foldable or rollable device generally requires a bending property in one direction, it may be sufficient to apply the device when the film has a yield strain of 3% or more in the Machine Direction (MD) or Transverse Direction (TD). However, in the present disclosure, elongation and yield strain are on a longitudinal basis, as uniaxial stretching in the longitudinal direction will generally be more advantageous in terms of process efficiency. The directions of elongation and contraction can be interchanged when there are no process related problems. In this case, elongation in the transverse direction and contraction in the longitudinal direction may be performed under the conditions described above, and the yield strain may be measured in the elongation direction.

In the present disclosure, the first heat treatment may be performed after heating the temperature to 250 ℃ to 330 ℃, or after reaching the maximum heating temperature. In terms of process conditions, it is preferred to raise the temperature from 60 ℃ to 120 ℃ until reaching 250 ℃ to 330 ℃. In this case, the first heat treatment may be performed for a time of 1 minute to 8 hours, but is not necessarily limited thereto.

Generally, when forming a polyimide-based film, a polyamic acid (polyimide or polyimide-amide precursor) solution or a polyimide-based resin is cast onto a support, and then heated at 80 ℃ to 200 ℃, preferably at 100 ℃ to 180 ℃, to activate a dehydrating agent and/or an imidization catalyst, and the partially cured and dried film in a gel state is peeled off from the support. In order to prevent deformation of the film, the initial temperature before heating in the first heat treatment of the present disclosure is preferably 60 ℃ to 120 ℃, similar to the temperature of imidization.

However, when the minimum heating temperature is less than 250 ℃, the corresponding imidization cannot be sufficiently achieved, and when the minimum heating temperature exceeds 330 ℃, the excessively high Yellowness (YI) of the polyimide may limit its use as a display material. Therefore, the minimum heating temperature is preferably adjusted within the above range.

Meanwhile, in order to solve the residual thermal history and residual stress in the film and obtain the maximum orientation effect while preventing the film from being cracked due to excessive stress, the method according to the present disclosure includes performing the second heat treatment (S2) while further performing the second heat treatment in the Machine Direction (MD) at not less than 30N/mm²And less than 160N/mm²The tensile force of (3) stretches the polyimide-based film after the first heat treatment. In this case, the second heat treatment is performed at a temperature within-10 ℃ to +30 ℃ of the maximum heating temperature reached during the first heat treatment for 200 seconds or more. In order to prevent the increase in yellowness while obtaining excellent yield strain, the second heat treatment is more preferably performed for a period of 200 seconds to 1,500 seconds, or 200 seconds to 1,000 seconds.

In the present disclosure, the tensile force applied during the second heat treatment may be understood as a force that the film receives per unit area in the longitudinal direction.

In the case where the temperature during the second heat treatment is lower than-10 ℃ which is the highest heating temperature during the first heat treatment, when an acid anhydride is added as a monomer, crosslinking of the acid anhydride substituted on the terminal group does not occur, whereby heat setting cannot be sufficiently achieved, and it is difficult to obtain a high yield strain due to insufficient orientation even when an acid anhydride is not used. Meanwhile, in the case where the temperature during the second heat treatment exceeds +30 ℃ of the highest heating temperature during the first heat treatment, the polyimide is limited to be used as a display material due to excessively high Yellowness (YI) of the polyimide. That is, when the time in the second heat treatment falls within the above range, there is an effect of obtaining sufficient orientation and preventing an increase in yellowness due to excessive heat treatment.

Meanwhile, in the present disclosure, the polyimide-based film is obtained by reacting (i) a diamine and a dianhydride; (ii) diamines, dianhydrides and aromatic dicarbonyl compounds; (iii) diamines, dianhydrides and anhydrides; or (iv) any combination of diamine, dianhydride, anhydride, and aromatic dicarbonyl compound, and then imidized, wherein the equivalent ratio of all remaining monomers to diamine is 1: 1. Among these combinations, the acid anhydride is preferably added at the time of blocking the polyimide molecular chain, but the present disclosure is not necessarily limited thereto.

Dianhydrides that may be used in the present disclosure include, but are not limited to, those selected from the group consisting of 2, 2-bis (3, 4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), 4- (2, 5-dioxotetrahydrofuran-3-yl) -1,2,3, 4-tetrahydronaphthalene-1, 2-dicarboxylic dianhydride (TDA), pyromellitic dianhydride (1,2,4, 5-benzenetetracarboxylic dianhydride (PMDA)), Benzophenone Tetracarboxylic Dianhydride (BTDA), biphenyl tetracarboxylic dianhydride (BPDA), biscarboxyphenyldimethylsilane dianhydride (SiDA), Oxydiphthalic Dianhydride (ODPA), biscarboxyphenoxydiphenyl sulfide dianhydride (BDSDA), sulfonyl diphthalic anhydride (SO)₂DPA), cyclobutanetetracarboxylic dianhydride (CBDA), isopropylidenedioxybisphthalic anhydride (6HDBA), and the like.

In addition, diamines that may be used in the present disclosure include, but are not limited to, one or a combination of two or more selected from diaminodiphenyl ether (ODA), p-phenylenediamine (pDA), m-phenylenediamine (mPDA), p-methylenedianiline (pMDA), m-methylenedianiline (mda), bistrifluoromethylbenzidine (TFDB), cyclohexanediamine (13CHD, 14CHD), Diaminohydroxyphenylhexafluoropropane (DBOH), diaminophenoxybenzene (133APB, 134APB, 144APB), diaminophenylhexafluoropropane (33-6F, 44-6F), diaminophenylsulfone (4DDS, 3DDS), diaminophenoxyphenylhexafluoropropane (4BDAF), diaminophenoxyphenylpropane (6HMDA), diaminophenoxydiphenylsulfone (DBSDA), and the like.

In addition, acid anhydrides that may be used in the present disclosure include, but are not limited to, one or a combination of two or more selected from the group consisting of bicycloheptene anhydride (nadic anhydride), 4- (9-anthrylethynyl) phthalic anhydride, 1-adamantanecarbonyl chloride, 1, 3-adamantanedicarbonyl dichloride, 5-norbornene-2-carbonyl chloride, 5-norbornene-2, 3-dicarbonyl chloride, cyclopentanecarbonyl chloride, and the like.

In addition, when the polyimide of the present disclosure is a polyamide-imide having an amide structure, an aromatic dicarbonyl compound may be added as a monomer. In this case, the aromatic dicarbonyl compound that can be used includes, but is not limited to, one or a combination of two or more selected from terephthaloyl chloride, terephthalic acid, isophthaloyl chloride, 4' -biphenyldicarbonyl chloride, and the like.

There is no particular limitation on the method of manufacturing the polyimide-based film in a roll-to-roll manner using the above-described monomers, and one example of the manufacturing method according to the present disclosure will be described below.

A monomer selected from the above-mentioned monomers is dissolved in a first solvent, and the resulting solution is polymerized to prepare a polyamic acid solution (precursor of polyimide or polyimide-amide). At this time, although the reaction conditions are not particularly limited, the reaction temperature is preferably-20 ℃ to 80 ℃ and the reaction time is preferably 2 hours to 48 hours. Further, more preferably, the reaction is carried out under an inert atmosphere such as argon or nitrogen.

When an acid anhydride is added as a monomer, the amount of acid anhydride added during the reaction affects the molecular weight. In order to prevent deterioration of inherent physical properties of polyimide, the acid anhydride may be added in an amount of 10 mol% or less, preferably 5 mol% or less, relative to the total mole of dianhydride and acid anhydride. When the acid anhydride is used in a large amount exceeding 10 mol%, the molecular weight decreases, and the optical properties deteriorate, for example, the yellowness increases and the light transmittance decreases. Conversely, increased anhydride content causes crosslinking, which leads to improved thermal properties. However, a large amount of crosslinking also disrupts the arrangement of polymer chains, which causes a decrease in surface hardness.

In addition, in order to obtain a polyimide-based film having a polyamide-imide structure, the molecular weight may be changed depending on the amount of the aromatic dicarbonyl compound added to the monomer. In order to prevent deterioration of the inherent physical properties of the polyimide, the aromatic dicarbonyl compound may be added in an amount of 10 mol% or more and 80 mol% or less, preferably 30 mol% or more and 70 mol% or less, based on the total mol of the dianhydride and the aromatic dicarbonyl compound. When the aromatic dicarbonyl compound is used in a large amount exceeding 80 mol%, optical properties deteriorate, for example, yellowness increases and light transmittance decreases, and gel is formed in the polyamic acid solution, making it difficult to obtain a film during film formation. Further, when the aromatic dicarbonyl compound is used at 10 mol% or less, the optical properties are improved, but the thermal properties are deteriorated, for example, the thermal expansion coefficient is decreased.

The solvent used for polymerization of the monomer is not particularly limited as long as it can dissolve the polyamic acid. As a known reaction solvent, at least one polar solvent selected from the group consisting of m-cresol, N-methyl-2-pyrrolidone (NMP), Dimethylformamide (DMF), dimethylacetamide (DMAc), Dimethylsulfoxide (DMSO), acetone, and ethyl acetate is used. In addition, a low boiling point solution such as Tetrahydrofuran (THF) or chloroform, or a low absorption solvent such as γ -butyrolactone may be used.

The content of the solvent is not particularly limited, but the content of the first solvent is preferably 50 to 95% by weight, more preferably 70 to 90% by weight, relative to the total weight of the polyamic acid solution, in order to obtain an appropriate molecular weight and viscosity of the polyamic acid solution.

As described above, the polyimide-based film may be prepared by polymerizing a polyamic acid solution and imidizing and heat-treating the resultant product at high temperature to form a film. At this time, the produced polyimide-based resin preferably has a glass transition temperature of 200 ℃ to 400 ℃ in view of thermal stability.

Methods of producing polyimide-based films from polyamic acid solutions include thermal imidization, chemical imidization, or a combination of thermal imidization and chemical imidization.

When thermal imidization or a combination of thermal imidization and chemical imidization is employed, the heating condition of the polyamic acid solution may vary depending on the type of polyamic acid solution, the thickness of the polyimide-based film produced, and the like.

The case of producing a polyimide-based film using a combination of thermal imidization and chemical imidization will be described in more detail below. Adding a dehydrating agent and an imidization catalyst to a polyamic acid solution, casting the resulting solution on a support and heating at 80 to 200 ℃, preferably 100 to 180 ℃ to activate the dehydrating agent and the imidization catalyst, peeling the gel-phase polyamic acid film, which is partially cured and dried, from the support, fixing on the support, and performing heat treatment to obtain a polyimide-based film. A needle-type holder or clip may be used to secure the gel-type membrane. The support used herein may be a glass plate, an aluminum foil, a circulating stainless steel belt, a stainless steel drum, or the like.

Meanwhile, in the present disclosure, a polyimide-based film may be manufactured from the obtained polyamic acid solution as follows. That is, the obtained polyamic acid solution is imidized, the imidized solution is added to a second solvent, precipitated, filtered, and dried to obtain a polyimide resin as a solid, the obtained solid type polyimide resin is dissolved again in the first solvent, cast on a support while slowly raising the temperature in the range of 40 ℃ to 400 ℃, as described above, and heated for 1 hour to 8 hours to obtain a polyimide-based film as a gel.

The first solvent may be the same solvent as that used for polymerization of the polyamic acid solution, and the second solvent may have lower polarity than the first solvent, so as to obtain a solid of the polyimide resin, and specific examples thereof may include at least one selected from the group consisting of water, alcohol, ether, and ketone.

In this case, the content of the second solvent is not particularly limited, but is preferably 5 to 20 times the weight of the polyamic acid solution.

As for the conditions for filtering and drying the obtained polyimide resin solid, it is preferable that the temperature is 50 to 120 ℃ and the time is 3 to 24 hours, in view of the boiling point of the second solvent.

In addition, when a polyimide-based film is manufactured, a filler may be added in the preparation of the polyamic acid solution to improve various properties such as flexibility, thermal conductivity, electrical conductivity, and corona resistance. The filler is not particularly limited, and preferred examples thereof include silica, titanium oxide, layered silica, carbon nanotubes, alumina, silicon nitride, boron nitride, calcium hydrogen phosphate, calcium phosphate and mica.

The particle size of the filler may vary depending on the characteristics of the film to be modified and the type of filler added, but is not particularly limited. In general, the average particle diameter is preferably 0.001 μm to 50 μm, more preferably 0.005 μm to 25 μm, and still more preferably 0.01 μm to 10 μm. In this case, the polyimide-based film effectively exhibits a modification effect and excellent surface properties, conductivity and mechanical properties.

In addition, the amount of the filler may vary depending on the characteristics of the film to be modified and the type of the filler added, and is not particularly limited. Generally, the content of the filler is preferably 0.001 to 20 parts by weight, more preferably 0.01 to 10 parts by weight, relative to 100 parts by weight of the polyamic acid solution, in order to provide the properties to be modified without impairing the bonding structure of the polymer resin.

In the present disclosure, by performing the steps (S1) and (S2) on the polyimide-based film obtained as a gel state formed by casting on the support as described above, a polyimide-based film for a foldable and rollable display having excellent bending characteristics can be manufactured.

The thickness of the polyimide-based film obtained by this method is not particularly limited, and is preferably in the range of 10 μm to 250 μm, more preferably in the range of 25 μm to 150 μm.

In another aspect, the present disclosure relates to a polyimide-based film manufactured by the above method.

The polyimide-based film produced may have physical properties such as a yield strain in a uniaxial direction of 3% or more, further, a pencil hardness of 1H or more, preferably 1H to 3H, measured based on ASTM D3363, and a yellowness of 5.0 or less, preferably 4.0 or less, measured based on CM-3700D manufactured by Konica Minolta, Inc, and a light transmittance at 550nm of 85% or more, preferably 88% or more.

When the yield strain is less than 3%, the deformation in the radius of curvature of 1mm or less may be permanent, thus making the film unable to return to its original shape and causing a limitation in practical use as a cover substrate for a foldable or rollable display device. Since the polyimide-based film of the present disclosure can secure a yield strain of 3% or more by uniaxial stretching, it can be restored to its original shape even when deformed several tens of thousands of times or more at a radius of curvature of 1mm or more, and thus is suitable for a cover substrate of a foldable or rollable display device.

In addition, in the present disclosure, the polyimide-based film may secure scratch resistance to protect a base member due to the surface hardness of 1H or more, and may satisfy optical properties such as a yellowness of less than 5 and a light transmittance of 85% or more, thereby being sufficiently used as a cover substrate of a display.