KR102493648B1 - Polyimide film having excellent resilence property - Google Patents

Abstract

The present invention relates to at least one diamine; And at least one kind of acid dianhydride as a copolymerized polyimide film, more specifically, due to its high modulus of resiliency and yield strength, it simultaneously secures excellent resilience and high flexural properties and can be used as a cover window for a display. A polyimide film that can be used is provided.

Description

Polyimide film having excellent restoring properties {POLYIMIDE FILM HAVING EXCELLENT RESILENCE PROPERTY}

The present invention relates to a polyimide film, and more particularly, to a polyimide film that can be used as a cover window of a display because of its high modulus of resilience and yield strength, and thus has excellent resilience and high bending properties at the same time. will be.

A cover window for protecting a panel is applied to a surface of a display device such as a liquid crystal display (LCD) or an organic light emitting display (OLED). As a material of a conventional cover window, tempered glass having excellent flatness, heat resistance, chemical resistance, and barrier performance against moisture or gas, a low coefficient of linear expansion (CTE), and high light transmittance has been mainly used.

Meanwhile, flexible displays such as curved displays and in-folding displays are being developed. In order to be applied to such a flexible display, the cover window must also have flexibility, but a conventional glass cover window is generally heavy and easy to break, and is not suitable for a flexible display due to poor flexibility.

In order to solve the above problems, recently, cover windows using plastic materials having relatively free moldability have been proposed. A cover window made of a plastic material has the advantage of being light, hard to break, and capable of implementing various designs. Currently, as a plastic material for a cover window, polycarbonate, polyethylene terephthalate, and polymethyl methacrylate having excellent transparency are mainly used. Although these materials have the advantage of excellent transparency, their glass transition temperature (Tg) is less than 150° C., and their application is limited due to their poor heat resistance and poor chemical resistance and mechanical strength.

In order to compensate for this problem, a thick hard coating layer or the like has been introduced to the above-mentioned plastic material, but in this case, cracks occur in the hard coating layer or flexibility is significantly reduced during molding. In particular, in recent years, a flexible display has been widely used, but there is a limit to improving the restoring force against physical deformation caused by an external force.

The present invention has been made to solve the above-mentioned problems, and a technical task is to provide a novel polyimide film that can be applied as a cover window with excellent restoring force, high flexibility, mechanical strength, transparency, and other physical properties at the same time. .

Other objects and advantages of the present invention can be more clearly described by the following detailed description and claims.

In order to achieve the above technical problem, the present invention is at least one diamine; And a polyimide film copolymerized with at least one acid dianhydride, wherein the yield strength measured according to ASTM D882 is 50 to 200 MPa at a thickness of 30 to 100 μm, and the Modulus of Resilience) provides a polyimide film that is 0.5 to 5.0 MPa.

In one embodiment of the present invention, the polyimide film has an initial restoration height (H _I ) of 5 cm after bending the film for 72 hours at a radius of curvature of 1 to 10 mm based on a film thickness of 30 to 100 μm. or less, and the restoration height (H _F ) after at least 1 hour may be 3 cm or less.

In one embodiment of the present invention, in the stress-strain curve of the film measured according to ASTM D882, the deformation of the critical point corresponding to at least 80% of the slope of the tensile strength range of 20 to 40 MPa Yield Strain (Y/S), defined as strain, may be 2.0% to 5.0%.

In one embodiment of the present invention, when the film is bent with a radius of curvature of 1 to 5 mm, the number of times of bending until breakage may be 100,000 or more.

For one embodiment of the present invention, the modulus measured according to ASTM D882 may be 3 to 8 GPa.

In one embodiment of the present invention, at a thickness of 30 to 100 μm, light transmittance at a wavelength of 550 nm is 85% or more, and yellowness according to the ASTM E313-73 standard may be 10 or less.

In one embodiment of the present invention, the at least one type of diamine is a fluorinated aromatic first diamine, a sulfone-based aromatic second diamine, a hydroxy-based aromatic third diamine, an ether-based aromatic fourth diamine, a non-fluorinated aromatic fifth It may include at least one selected from the group consisting of diamines and alicyclic 6th diamines.

For one embodiment of the present invention, the content of each of the first diamine to the sixth diamine may be 10 to 100 mol% based on 100 mol% of the total diamine.

In one embodiment of the present invention, the at least one acid dianhydride is from the group consisting of a fluorinated aromatic primary acid dianhydride, a non-fluorinated aromatic secondary acid dianhydride, a sulfonic aromatic tertiary acid dianhydride, and an alicyclic quaternary acid dianhydride. One or more selected species may be included.

In one embodiment of the present invention, the content of the first to fourth acid dianhydrides may be 10 to 100 mol% based on 100 mol% of the total acid dianhydride.

In one embodiment of the present invention, the ratio (a/b) of the number of moles of the diamine (a) and the acid dianhydride (b) may be in the range of 0.7 to 1.3.

For example, the polyimide film may be used as a cover window of a display device.

According to one embodiment of the present invention, high resilience and yield strength can be secured through the adoption of certain components constituting the polyimide film and control of their contents, thereby exhibiting excellent resilience against physical deformation. In addition, it is possible to realize high yield strain and high flexural characteristics by adjusting the slope of the tensile elastic region.

In addition, in the present invention, it exhibits high light transmittance, low yellowness, excellent modulus and surface hardness, so that the workability and reliability of the final product can be improved.

Accordingly, the polyimide film of the present invention can be usefully applied as a cover window for display devices, flexible displays, etc. in the field including flat panel display panels, and other IT products, electronic products, home appliances, etc. known in the art. is also applicable.

Effects according to the present invention are not limited by the contents exemplified above, and more diverse effects are included in the present specification.

1 is a stress-strain curve in which yield strength, yield strain, and modulus of resilience are defined in a tensile test using a polyimide film according to the present invention. Curve) is a graph.

Hereinafter, the present invention will be described in detail. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art, and the following examples may be modified in various other forms, and the scope of the present invention is not limited to the following embodiments. It is not limited to examples. Here, like reference numerals designate like structures throughout this specification.

Unless otherwise defined, all terms (including technical and scientific terms) used in this specification may be used in a meaning commonly understood by those of ordinary skill in the art to which the present invention belongs. In addition, terms defined in commonly used dictionaries are not interpreted ideally or excessively unless explicitly specifically defined.

Also, throughout the specification, when a certain part "includes" a certain component, it means that it may further include other components, not excluding other components, unless otherwise stated. In addition, throughout the specification, "above" or "on" means not only the case of being located above or below the target part, but also the case of another part in the middle thereof, and must necessarily specify the direction of gravity It does not mean that it is located above the standard. Also, terms such as "first" and "second" in the present specification do not indicate any order or importance, but are used to distinguish components from each other.

<Polyimide film>

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

Here, the cover window refers to a film disposed on the outermost surface of the flexible display device to protect the display device. The cover window may be a window film alone or a film having a window coating layer formed on a separate substrate made of an optically transparent resin.

On the other hand, the cover window of the display exposed to the outside needs to have processing characteristics such as flexibility and excellent optical characteristics in daily life, as well as a certain degree of stability against physical deformation caused by external loads. In particular, when the deformation due to stress exceeds the yield deformation when the cover window is folded, permanent deformation is generated, and thus recovery is impossible.

In the present invention, among the inherent physical properties of film materials, specific physical properties related to restoring force, such as modulus of resilience and yield strength, are adopted, and these physical properties are conventionally known plastic materials and/or poly We want to improve the stability by controlling it higher than the mid film. Accordingly, when the above-described polyimide film is applied to a cover window of a foldable display device, film resilience before and after static folding is excellent and high reliability can be provided. Such a display device may be applied to any display device known in the art without limitation, and may be in particular an in-folding or out-folding type foldable display.

For one embodiment of the present invention, the polyimide film may include at least one type of diamine; And at least one type of acid dianhydride copolymerized, at a thickness of 30 to 100 μm, the yield strength measured according to ASTM D882 is 50 to 200 MPa, and the Modulus of Resilience (R ) may be 0.5 to 5.0 MPa.

Here, the resilience may be defined by the following [Equation 1].

[Equation 1]

(In the above formula, σ is the yield strength and E is the modulus)

Specifically, resilience (R) is one of the rheological characteristics of a material, and means energy per unit volume that a material can absorb without permanent deformation. For example, when a predetermined material is deformed, the deformation at a small amount of deformation is elastic, but under certain conditions, when the elastic deformation is transferred to the plastic deformation, permanent deformation that is not recovered anymore occurs. That is, resilience is the maximum energy that a unit volume of a material can store within the elastic limit, that is, it represents the elastic recovery force. Physical properties such as resilience (R), yield strength, and yield strain described later to be controlled in the present invention can be known through a stress-strain curve (S-S curve) . This stress-strain curve (S-S curve) is a curve on a graph showing the relationship between the stress applied to the polymer material through the tensile test of the polymer material and the strain of the polymer material in response to the stress , it can be said to be a curve showing the change in the amount of stress required as the strain of the polymer material increases. As shown in FIG. 1 below, in general, tensile strain is displayed on the horizontal axis (eg, x-axis) and stress (tensile strength) is displayed on the vertical axis (eg, y-axis).

In the present invention, the above-described Modulus of Resilience and Yield Strength parameters and their values may be affected by the thickness of the polyimide film. The values of resilience and yield strength may be measured based on a thickness of 30 to 100 μm of the polyimide film, specifically 30 to 90 μm, and more specifically 80 ± 5 μm.

Specifically, the polyimide film according to the present invention may have a yield strength of 50 to 195 MPa and a modulus of resilience (R) of 0.5 to 4.0 MPa, and more specifically, the yield strength (Yield Strength). Strength) is 100 to 190 MPa, and the resilience (Modulus of Resilience, R) may be 2.0 to 3.5 MPa. In the case of the polyimide film of the present invention that satisfies the above-described resilience and yield strength parameters, it can exhibit excellent restoring characteristics by securing high elasticity without permanent deformation.

In order for the polyimide film according to the present invention to be used as a cover window for a mobile communication terminal or a tablet PC, in addition to the aforementioned resilience and yield strength properties, excellent optical properties such as excellent flexibility, high transparency and light transmittance, high It is desirable to have mechanical properties at the same time.

In particular, the polyimide film of the present invention is another feature of having high yield strain and high flexural properties at the same time compared to conventionally known plastic materials and/or polyimide films. When such a polyimide film is applied to a cover window of an in-folding type foldable mobile phone, it can exhibit excellent folding endurance and strength through high flexibility and high strength.

For example, the polyimide film corresponds to at least 80% of the slope of the tensile strength of 20 to 40 MPa in the stress-strain curve of the film measured according to ASTM D882. A yield strain (Y/S) defined as a strain at a critical point may be 2.0% to 5.0%.

In the present specification, the yield strain is the stress-strain slope in a specific tensile strength range (eg, 20 to 40 MPa) in the stress-strain curve of the polyimide film measured according to ASTM D882 ( It is defined as the x-intercept value, that is, tensile strain, of a predetermined threshold slope (X ₂ ≥X ₁ × 0.8) having at least 80% of the Stress-Strain slope, X ₁ ) (see FIG. 1 below) . This yield strain is newly defined in the present invention, and since it is a unique physical property due to the specific composition of the polyimide film, it may correspond to a characteristic different from the conventional polyimide film.

Specifically, in a stress-strain curve (S-S curve) obtained by a tensile test measured at 25 ° C, the yield strain (Y / S) of the polyimide film may be 2.0% to 4.8%. The yield strain (Y/S) is preferably from 2.0% to 4.0%, more preferably from 2.0% to 3.8% in view of improving the bending resistance. In the case of a polyimide film that satisfies such a yield strain (Y/S) parameter, flexibility is remarkably excellent, and crack generation due to repeated bending fatigue can be suppressed and excellent flexibility can be exhibited.

In another embodiment, the polyimide film has an initial restoration height (H _I ) of 5 cm or less after bending the film for 72 hours at a radius of curvature of 1 to 10 mm based on a film thickness of 30 to 100 μm, The restoration height after 1 hour (H _F ) may be 3 cm or less. Specifically, the initial restoration height (H _I ) may be 3 cm or less, and the restoration height (H _F ) after 1 hour may be 0.5 cm or less. At this time, the lower limit of the restoration height of the film at the beginning or after 1 hour has elapsed is not particularly limited, and may be, for example, 0 cm or more.

For another specific example, when the polyimide film is bent with a radius of curvature of 1 mm to 5 mm, the number of bends until breakage may be 100,000 or more. In particular, when bending with a bending radius of 1 mm, the number of bendings until breakage may be 100,000 or more, specifically 200,000 or more, and preferably 1,000,000 or more. At this time, the upper limit of the deformation of the number of times of bending until fracture is not particularly limited, and may be, for example, 3,000,000 times or less, and specifically, 2,500,000 times or less.

The polyimide film according to the present invention preferably has excellent mechanical properties of high modulus in consideration of scratch resistance and reliability of a display.

For example, the polyimide film may have a modulus of 3 to 8 GPa, and may be 3.5 to 7 GPa in terms of simultaneously exhibiting mechanical hardness and excellent flexibility. Here, modulus means a value measured according to ASTM D882. When the modulus is smaller than the above-described value, it is difficult to exhibit sufficient hardness, and when the modulus is larger than the above-mentioned value, flexibility may decrease and foldability may deteriorate.

The polyimide film according to the present invention preferably has excellent optical properties such as high transparency and light transmittance at the same time in order to increase the visibility of the display screen.

For example, the polyimide film may have a light transmittance of 85% or more, specifically 89% or more, and more specifically 90% to 99% at a thickness of 30 to 100 μm and a wavelength of 550 nm. In addition, yellowness (Y.I.) according to the ASTM E313-73 standard may be 10 or less, specifically 7 or less, and more specifically 5 or less.

In this specification, the above-described physical properties of the polyimide film are based on a film thickness of 10 to 100 μm, and may be specifically 30 to 80 μm, unless otherwise specified. However, it is not limited to the aforementioned thickness range, and may be appropriately adjusted within a normal thickness range known in the art.

The polyimide film according to the present invention, as long as it satisfies the above-described resilience and yield strength characteristics, is not particularly limited to components constituting the polyimide resin and/or its composition.

For example, the polyimide film may include at least one type of diamine; and at least one acid dianhydride, and may be prepared by imidizing and heat-treating a polyamic acid composition including diamine, acid dianhydride, and, if necessary, a solvent, at a high temperature.

In general, polyimide (PI) resin refers to a highly heat-resistant resin prepared by solution polymerization of aromatic acid dianhydride and aromatic diamine or aromatic diisocyanate to prepare a polyamic acid derivative, followed by ring-closure dehydration at high temperature to imidize. Such a polyimide resin is a polymer material containing an imide ring, and has excellent heat resistance, chemical resistance, wear resistance and electrical properties based on the chemical stability of the imide ring. The polyimide resin may be in the form of a random copolymer or a block copolymer.

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

In particular, in the present invention, mechanical properties such as high resilience, yield strength, high flexibility, and modulus of the polyimide film; Considering optical properties such as high transmittance, low Y.I, and low haze, at least one type of aromatic diamine may be used, or the aromatic diamine and alicyclic diamine may be mixed. .

Specific examples of the aforementioned aromatic diamine include fluorine-based, sulfone-based, hydroxyl-based, ether-based, and non-fluorine-based diamines having a fluorinated substituent. Accordingly, in the present invention, as the diamine compound, a fluorinated aromatic first diamine having a fluorine substituent introduced therein, a sulfone-based aromatic second diamine, a hydroxy-based aromatic tertiary diamine, an ether-based aromatic fourth diamine, and a fluorine-based aromatic fifth diamine are prepared. Each may be used alone or in a mixed form of two or more by appropriately combining them.

Non-limiting examples of usable diamine monomers (a) include oxydianiline (ODA), 2,2'-bis (trifluoromethyl) -4,4'-diaminobiphenyl (2,2'-TFDB ), 2,2'-bis (trifluoromethyl) -4,3'-diaminobiphenyl (2,2'-Bis (trifluoromethyl) -4,3'-Diaminobiphenyl), 2,2'-bis ( Trifluoromethyl) -5,5'-diaminobiphenyl (2,2'-Bis (trifluoromethyl) -5,5'-Diaminobiphenyl), 2,2'-bis (trifluoromethyl) -4,4 '-diaminophenyl ether (2,2'-Bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, 6-FODA), bis aminohydroxy phenyl hexafluoropropane (DBOH), bis amino phenoxy phenyl hexafluoro Lopropane (4BDAF), Bis Aminophenoxy Phenylpropane (6HMDA), Bis Aminophenoxy Diphenylsulfone (DBSDA), Bis(4-Aminophenyl)sulfone (4,4'-DDS), Bis(3-Aminophenyl ) Sulfone (3,3'-DDS), sulfonyldiphthalic anhydride (SO2DPA), 4,4'-oxydianiline (4,4'-ODA), or one or a mixture of two or more thereof forms, etc. are applicable.

Considering the high transparency, high glass transition temperature, and low yellowness of the polyimide film, fluorinated primary diamine is 2,2'-bis(trifluoromethyl)-4,4, which can lead to linear polymerization. '-Diaminobiphenyl (2,2'-TFDB) and 1,4-Bis(4-amino-2-trifluoromethylphenoxy)benzene (6-FAPB) can be used. In addition, bis(4-aminophenyl)sulfone (4,4'-DDS) or 3,3'-DDS may be used as the sulfone-based secondary diamine. In addition, the hydroxy-based tertiary diamine is 2,2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane (2,2-bis (3-amino-4-hydroxyphenyl) -hexafluoropropane, BIS-AP -AF) can be used. In addition, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6-FODA) or oxydianiline (ODA) may be used as the ether-based quaternary diamine. In addition, as a non-fluorine-based fifth diamine, 2,2-bis (3-amino-4-methylphenyl) -hexafluoropropane (2,2-Bis (3-amino-4-methylphenyl) -hexafluoropropane, BIS-AT-AF ), m-tolidine, or p-phenylenediamine (p-PDA) may be used.

In the diamine monomer (a) of the present invention, the content of the fluorinated aromatic primary diamine, sulfone-based aromatic secondary diamine, hydroxy-based aromatic tertiary diamine, ether-based aromatic quaternary diamine, and non-fluorine-based aromatic tertiary diamine is particularly Although not limited, each may be 0 to 100 mol% based on 100 mol% of the total diamine, specifically 10 to 90 mol%, and more specifically 20 to 80 mol%. However, the content of at least one of the first to fifth diamines is included to satisfy 100 mol% of the total diamines.

For one specific example of the present invention, as the diamine monomer (a), the first fluorinated diamine and the fourth ether diamine may be mixed. At this time, their use ratio is not particularly limited, and may be, for example, 50 to 90: 10 to 50 mol% ratio.

For another specific example of the present invention, at least one type of fluorinated primary diamine may be mixed as the diamine monomer (a). At this time, their use ratio may be 50 to 90: 10 to 50 mol% ratio, but is not particularly limited thereto.

Alicyclic diamine is not particularly limited as long as it contains at least one alicyclic ring in its molecular structure. Non-limiting examples of usable alicyclic diamines include 2,2-bis(3-amino-4-hydroxy cyclohexyl)hexafluoropropane [2,2-bis(3-amino-4-hydroxy cyclohexyl) hexafluoropropane], 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane (MACM), 4,4'-methylenebicyclohexylamine (PACM), 1,3-bis(aminomethyl)cyclohexane (1,3-BAC), 1,4-bis(aminomethyl)cyclohexane (1,4-BAC), cis-1,2-cyclohexanedimethaneamine, trans-1,2-cyclohexanedimethaneamine , 1,4-cyclohexyldiamine (CHDA), bis(4-aminocyclohexyl) ether, or mixtures thereof.

In the present invention, aromatic diamine and alicyclic diamine may be mixed as the diamine monomer (a), and the mixing ratio thereof may be appropriately adjusted in consideration of physical properties of the polyimide film. For example, the mixing ratio of aromatic diamine and alicyclic diamine may be 0 to 100: 100 to 0 mol% based on 100 mol% of the total diamine, and specifically 10 to 90: 90 to 10 mol%.

As the acid dianhydride (b) monomer constituting the polyimide film of the present invention, conventional compounds known in the art having an acid dianhydride structure in a molecule may be used without limitation. For example, an aromatic, alicyclic, or aliphatic compound having a dianhydride structure, or a combination thereof may be used. Specifically, at least one type of aromatic acid dianhydride may be used, or the aromatic acid dianhydride and alicyclic compound It is possible to use a mixture of tribasic acid dianhydrides.

Specific examples of the aforementioned aromatic acid dianhydride include fluorinated aromatic primary acid dianhydride, non-fluorinated aromatic secondary acid dianhydride, and sulfone-based aromatic tertiary acid dianhydride. Accordingly, in the present invention, as the acid dianhydride compound, a fluorinated aromatic primary acid dianhydride, a non-fluorinated aromatic secondary acid dianhydride, and a sulfone-based aromatic tertiary acid dianhydride having a fluorine substituent introduced therein are used alone or in combination thereof to form two It can be used in a mixed form of more than one species.

The first fluorinated acid dianhydride monomer is not particularly limited as long as it is an aromatic acid dianhydride having a fluorine substituent introduced therein. Non-limiting examples of usable fluorinated primary acid dianhydrides include 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride, 6-FDA), and 4-(trifluoromethyl)pyromellitic dianhydride (4-(trifluoromethyl)pyromellitic dianhydride, 4-TFPMDA). These may be used alone or in combination of two or more. Among the fluorinated acid dianhydrides, 6-FDA has a very high property of limiting the formation of charge transfer complexes (CTC: Change Transfer Complex) between molecular chains and within molecular chains, so it is suitable for transparent.

In addition, the non-fluorinated secondary acid dianhydride monomer is not particularly limited as long as it is a non-fluorinated aromatic acid dianhydride in which no fluorine substituent is introduced. Non-limiting examples of usable non-fluorinated tertiary acid dianhydride monomers include pyromellitic dianhydride (PMDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (3 ,3′,4,4′-Biphenyl tetracarboxylic acid dianhydride (BPDA), benzophenone tetracarboxylic dianhydride (BTDA), oxydiphthalic dianhydride (ODPA), 4,4-(4,4-Isopropylidenediphenoxy )bis(phthalic anhydride) (BPADA), bis(carboxyphenyl)dimethylsilane dianhydride (Bis(3,4dicarboxyphenyl)dimethylsilanedianhydride, SiDA), and the like. These may be used alone or in combination of two or more thereof.

In addition, the sulfone-based third acid dianhydride monomer is not particularly limited as long as it is an acid dianhydride into which a sulfone group is introduced, and for example, 3,3',4,4'-diphenylsulfotetracarboxylic dianhydride (3,3', 4,4'-diphenylsulfonetetracarboxylic dianhydride, DSDA).

In the acid dianhydride monomer (b) of the present invention, the contents of the fluorinated aromatic primary acid dianhydride, non-fluorinated aromatic secondary acid dianhydride, and sulfonic aromatic tertiary acid dianhydride are not particularly limited. As an example, these may be included in the range of 0 to 100 mol%, specifically 10 to 90 mol%, and more specifically 20 to 80 mol% based on 100 mol% of the total acid dianhydride. However, the content of at least one of the first acid dianhydride to the third acid dianhydride is included to satisfy 100 mol% of the total acid dianhydride.

The alicyclic acid dianhydride is not particularly limited as long as it has an alicyclic ring other than an aromatic ring in the compound and has an acid dianhydride structure. Non-limiting examples of usable alicyclic acid dianhydrides include cyclobutane tetracarboxylic dianhydride (CBDA), 1,2,3,4-cyclopentane tetracarboxylic dianhydride (CPDA), bicyclo [2,2,2]-7-octene-2,3,5,6-tetracarboxylic dianhydride (BCDA), 4-(2,5-dioxotetrahydrofuran-3-yl)-1 ,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic anhydride (TDA), 1,1'-bicyclohexane-3,3',4,4'-tetracarboxylic diane hydride (H-BPDA), 1,2,4,5-cyclohexane-tetracarboxylic dianhydride (H-PMDA), cyclopentanone bis-spironorbornane (cpODA), Bicyclo [2.2.2] octane-2,3,5,6-tetracarboxylic 2,3:5,6-dianhydride (7CI,8CI) or a mixture of one or more thereof.

In the present invention, aromatic acid dianhydride and alicyclic acid dianhydride may be mixed as the acid dianhydride monomer (b), and the mixing ratio thereof may be appropriately adjusted in consideration of physical properties of the polyimide film. For example, the mixing ratio of the aromatic acid dianhydride and the alicyclic acid dianhydride may be 0 to 100: 100 to 0 mol% ratio based on 100 mol% of the total acid dianhydride, and specifically 10 to 90: 90 to 10 mol% ratio. there is.

For one specific example of the present invention, as the acid dianhydride (b), a fluorinated aromatic primary acid dianhydride and an alicyclic acid dianhydride; Alternatively, a fluorinated aromatic second acid dianhydride and a non-fluorinated aromatic second acid dianhydride may be mixed. At this time, their use ratio is not particularly limited, and may be, for example, 10 to 90: 90 to 10 mol% ratio, specifically 30 to 80: 70 to 20 mol% ratio.

For another specific example of the present invention, as the acid dianhydride (b), an alicyclic acid dianhydride and a non-fluorinated aromatic second acid dianhydride may be mixed. At this time, their use ratio may be 30 to 80: 20 to 70 mol% ratio, but is not particularly limited thereto.

For another preferable example of the present invention, as the acid dianhydride (b), at least one non-fluorinated aromatic secondary acid dianhydride may be mixed. At this time, their use ratio may be 50 to 80: 20 to 50 mol% ratio, but is not particularly limited thereto.

In the polyamic acid composition constituting the polyimide film of the present invention, the ratio (a/b) of the number of moles of the diamine component (a) to the number of moles of the acid dianhydride component (b) may be 0.7 to 1.3, preferably. It may range from 0.8 to 1.2, more preferably from 0.9 to 1.1.

In the polyamic acid composition of the present invention, an organic solvent known in the art may be used as a solvent for solution polymerization of the above-described monomers without limitation. For example, m-cresol, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), acetone, diethyl At least one polar solvent selected from acetate and dimethyl phthalate (DMP) may be used. In addition, a low boiling point solution such as tetrahydrofuran (THF) or chloroform or a solvent such as γ-butyrolactone may be used. At this time, the content of the solvent (first solvent for polymerization) is not particularly limited, but is preferably 50 to 95% by weight based on the total weight of the polyamic acid composition in order to obtain an appropriate molecular weight and viscosity of the polyamic acid composition (polyamic acid solution) , More preferably, it may be included in the range of 70 to 90% by weight.

The above-described polyamic acid composition may be prepared by adding at least one kind of acid dianhydride and at least one kind of diamine to an organic solvent and then reacting them. For example, to improve the physical properties of polyimide, diamine (a) and acid dianhydride (b) ) can be adjusted to an equivalence ratio of approximately 1: 1. The composition of the polyamic acid composition is not particularly limited, and for example, based on 100% by weight of the total weight of the polyamic acid composition, 2.5 to 25.0% by weight of acid dianhydride, 2.5 to 25.0% by weight of diamine, and the balance satisfying 100% by weight of the composition. It may be configured to include an organic solvent of. Here, the content of the organic solvent may be 70 to 90% by weight. In addition, the polyamic acid composition may include 30 to 70 wt% of acid dianhydride and 30 to 70 wt% of diamine based on 100 wt% of the solid content, but is not particularly limited thereto.

The polyamic acid composition configured as described above may have a viscosity ranging from about 1,000 to 200,000 cps, preferably about 5,000 to 50,000 cps. When the viscosity of the polyamic acid composition falls within the above-mentioned range, the thickness can be easily controlled during coating of the polyamic acid composition, and the coating surface can be exhibited uniformly.

If necessary, the polyamic acid composition may contain a small amount of at least one additive such as a plasticizer, an antioxidant, a flame retardant, a dispersant, a viscosity modifier, a leveling agent, etc., within a range that does not significantly impair the objects and effects of the present invention. can

The polyimide resin film according to the present invention may be prepared according to a conventional method known in the art, and for example, after coating (casting) the above-described polyamic acid composition on a substrate, for example, a glass substrate, 30 to 350 It can be prepared by inducing an imide ring closure reaction (Imidazation) for 0.5 to 8 hours while gradually raising the temperature in the range of ℃.

At this time, the coating method may use a conventional method known in the art without limitation, and as an example, spin coating, dip coating, solvent casting, slot die coating, and It can be made by at least one method selected from the group consisting of spray coating. The polyamic acid composition may be coated at least once or more so that the colorless and transparent polyimide resin layer has a thickness of several hundred nm to several tens of μm.

In the polyimide film manufacturing method according to the present invention, the imidation method applied to the imidization step by casting the polymerized polyamic acid on a support is a thermal imidation method, a chemical imidation method, or a thermal imidation method and a chemical imidation method. It can be applied in combination with the imidation method.

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

In the chemical imidation method, a dehydrating agent represented by an acid anhydride such as acetic anhydride and an imidation catalyst represented by amines such as isoquinoline, β-picoline, and pyridine are added to the polyamic acid composition. When a thermal imidation method or a thermal imidation method is used in combination with the chemical imidation method, heating conditions of the polyamic acid composition may vary depending on the type of the polyamic acid composition, the thickness of the polyimide film to be produced, and the like.

In more detail, when the thermal imidation method and the chemical imidation method are used together, a dehydrating agent and an imidation catalyst are added to the polyamic acid composition, casted on a support, and then 80 to 300 ° C., preferably 150 to 250 ° C. After partially curing and drying by heating at °C to activate the dehydrating agent and the imidation catalyst, a polyimide film can be obtained.

The thickness of the polyimide film thus formed is not particularly limited and may be appropriately adjusted depending on the applied field. For example, it may be in the range of 10 to 150 μm, preferably in the range of 30 to 100 μm.

The polyimide film and its modifications of the present invention manufactured as described above can be usefully used in various fields requiring high stability, high flexural properties, and excellent optical properties. In particular, it can be applied as a cover window of a display device to prevent surface scratches and provide excellent flexibility and visibility to a flexible display device.

In the present invention, a display device refers to a flexible display device or a non-flexible display device that displays an image, and includes a flat panel display device (FPD) as well as a curved display device, a foldable display device, and a foldable display device. It includes a foldable display device, a flexible display device, a foldable mobile phone, a smart phone, a mobile communication terminal, a tablet PC, and the like. Specifically, the display device includes a liquid crystal display, an electrophoretic display, an organic light emitting display, an inorganic light emitting display, and a field emission display. It may be a field emission display, a surface-conduction electron-emitter display, a plasma display, a cathode ray display, electronic paper, and the like. For example, it may be a flat panel display panel such as LCD, PDP, OLED. Not limited to the above-mentioned uses, the polyimide film of the present invention can be applied to conventional display devices known in the art, and can also be used as a substrate or protective film for other flexible displays.

A specific example of a display device having the aforementioned polyimide film includes a display unit, a polarizing plate, a touch screen panel, a cover window, and a protective film, and the cover window is made of polyimide according to an embodiment of the present invention. may contain films. Here, each component constituting the display device is not particularly limited, and may include conventional components known in the art.

Hereinafter, the present invention will be described in more detail through specific examples. The following examples are merely examples to aid understanding of the present invention, and the scope of the present invention is not limited thereto.

[Examples 1 to 4. Manufacture of polyimide film]

A polyamic acid composition was prepared using a composition including diamine and acid dianhydride described in Table 1 below.

After coating the polyamic acid composition on a glass plate for LCD using a bar coater, the mixture was heated at 80 ° C for 30 minutes, at 150 ° C for 30 minutes, at 200 ° C for 1 hour, at 300 ° C for 1 hour in a nitrogen atmosphere convection oven. While gradually raising the temperature step by step with time, drying and imide ring closure reaction (Imidazation) were performed. Thus, a polyimide film having an imidation rate of 85% or more and a film thickness of 80 μm was prepared. Then, the polyimide film was separated from the glass plate and taken.

Diamine (mol %) Acid dianhydride (mol %) 1st monomer Second monomer 1st monomer Second monomer Example 1 TFDB (100) - BPADA (70) BPDA (30) Example 2 TFDB (100) - BPDA (80) 6FDA (20) Example 3 TFDB (70) 6FODA (30) CBDA (20) BPDA (80) Example 4 TFDB (70) 6FAPB (30) BPADA (60) CBDA (40) Comparative Example 1 p-PDAs (100) - BTDA (70) DSDA (30) Comparative Example 2 m-Tol (100) - OPDA (50) PMDA (50) Comparative Example 3 BIS-AT-AF (70) p-PDAs (30) CpODAs (30) BTDA (70) Comparative Example 4 m-Tol (80) 4,4-DABAN (20) CpODAs (70) OPDA (30)

[Comparative Examples 1 to 4. Manufacture of polyimide film]

Polyimide films of Comparative Examples 1 to 4 were prepared in the same manner as in Examples 1 to 4, except that the compositions described in Table 1 were used.

[Experimental example. Evaluation of physical properties]

The physical properties of the polyimide resin films prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were evaluated in the following manner, and the results are shown in Table 2 below. At this time, each physical property in Table 2 below is based on a thickness of 50 μm.

<Physical property evaluation method>

1) Measurement of light transmittance

It was measured using a UV-Vis NIR Spectrophotometer (Shimadzu, model name: UV-3150) at a wavelength of 550 nm.

2) Yellowness measurement

Yellowness at 550 nm was measured according to the ASTM E313-73 standard using a spectrophotometer (Konica Minolta, CM-3600A).

3) Thickness measurement

The thickness of the film was measured using a thickness meter (Mitutoyo, model name: 547-401).

4) Modulus measurement

Tensile strength (MPa) and modulus of elasticity (GPa) were measured according to the ASTM D882 standard using UTM (Instron, model name: 5942).

5) Measurement of Yield Strain

Using UTM (Instron, model name: 5942), _a threshold value (X ₂ Strain (x value) corresponding to ≥ X ₁ × 0.8) was measured.

6) Resilience

Resilience was calculated according to Equation 1 below using the above-described modulus and yield strength. At this time, the yield strength is Yield/Strength at the yield point, and was calculated in the same way as the yield strain described above.

[Equation 1]

(상기 식에서, σ는 항복강도이고, E는 모듈러스임)

(In the above formula, σ is the yield strength and E is the modulus)

7) Measurement of dynamic folding cycle

The bending characteristics were measured using a flat body unloaded U-folding test equipment (DLDMLS-FS, YUASA), and the folding cycle was measured by adjusting the radius of curvature to 1 mm to 5 mm.

8) Measurement of resilience

After fixing the prepared sample in a static folding holder (curvature radius of 1 ~ 10nm) and standing for 72 hours, when the sample was taken out, the height of the film according to the lapse of a predetermined time was measured to confirm the restoring force.

surrender
transform
(Yield Strain,
%) surrender
robbery
(Yield
strength,
MPa) Modulus of
Resilience
(MPa) Resilience (cm) Dynamic Folding
Cycle
(@1R) modulus
(Gpa) permeability
(%T, @ 550 nm) yellowness
(YI) Early
height Height after 1 hour Height after 3 hours Example 1 3.2 172 3.08 < 0.1 0 0 2000K 4.8 89.5 3.4 Example 2 3.1 160 3.05 1.0 0.1 < 0.1 1800K 4.2 89.8 2.5 Example 3 2.8 180 2.89 1.8 0.8 0.2 1400K 5.6 89.1 3.0 Example 4 3.0 168 2.88 0.5 < 0.1 < 0.1 1700K 4.9 90.0 2.7 Comparative Example 1 1.3 80 0.46 6.9 6.1 5.5 60K 6.9 82.1 17.1 Comparative Example 2 1.5 83 0.49 7.3 5.7 4.8 90K 7.1 84.1 14.7 Comparative Example 3 1.5 72 0.42 6.2 5.3 4.3 80K 6.1 84.7 15.4 Comparative Example 4 1.4 66 0.38 6.3 5.5 4.4 80K 5.8 83.4 16.8

As shown in Table 2, it was found that the polyimide film according to the present invention not only exhibits high resilience, yield strength and yield strain characteristics, but also exhibits excellent resilience and high flexibility at the same time, compared to the comparative example. It was also found that it had excellent transparency, mechanical properties and thermal expansion coefficient.

Accordingly, it was confirmed that the polyimide film of the present invention can be usefully applied as a cover window of a display device.

Claims (12)

at least one diamine; And a polyimide film copolymerized with at least one acid dianhydride,
The diamine is composed of fluorinated aromatic primary diamine,
The acid dianhydride includes at least one aromatic acid dianhydride selected from the group consisting of a fluorinated aromatic first acid dianhydride, a non-fluorinated aromatic second acid dianhydride, and a sulfone-based aromatic third acid dianhydride,
However, alicyclic acid dianhydride is not included,
At a thickness of 30 to 100 μm, the yield strength measured according to ASTM D882 is 50 to 200 MPa,
A polyimide film having a resilience of 3.05 to 5.0 MPa according to the following formula 1:
[Equation 1]

(In the above formula, σ is the yield strength and E is the modulus) According to claim 1,
Based on a film thickness of 30 to 100 μm, the initial restoration height (H _I ) after bending the film for 72 hours at a curvature radius of 1 to 10 mm is 5 cm or less, and the restoration height (H _F ) after at least 1 hour has elapsed is 3 cm The polyimide film below. According to claim 1,
In the stress-strain curve of the film measured according to ASTM D882, the yield strain defined as the strain of the critical point corresponding to at least 80% of the slope of the tensile strength range of 20 to 40 MPa ( Yield Strain, Y / S) is 2.0% to 5.0%, polyimide film. According to claim 1,
A polyimide film whose number of bends until fracture is 100,000 or more when the film is bent with a curvature radius of 1 to 5 mm. According to claim 1,
A polyimide film having a modulus of 3 to 8 GPa, measured according to ASTM D882. According to claim 1,
At a thickness of 30 to 100 μm, the transmittance of light at a wavelength of 550 nm is 85% or more,
A polyimide film having a yellowness of 10 or less according to the ASTM E313-73 standard. delete delete delete According to claim 1,
The content of the first acid dianhydride to the third acid dianhydride is 10 to 100 mol%, respectively, based on 100 mol% of the total acid dianhydride. According to claim 1,
A polyimide film having a ratio (a/b) of the number of moles of the diamine (a) and the acid dianhydride (b) in the range of 0.7 to 1.3. According to claim 1,
Polyimide film used as a cover window for display devices.

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