CN115011116A

CN115011116A - Polyimide film

Info

Publication number: CN115011116A
Application number: CN202110984862.2A
Authority: CN
Inventors: 李浩庸; 安炅日; 柳多英; 金东演; 沈在铉
Original assignee: Doosan Corp
Current assignee: Doosan Corp
Priority date: 2021-03-04
Filing date: 2021-08-24
Publication date: 2022-09-06
Also published as: JP2022135876A; TW202235489A; JP7271620B2; KR102493648B1; KR20220125107A; TWI784696B

Abstract

The present invention provides a polyimide film obtained by copolymerization of at least one diamine and at least one acid dianhydride, wherein the yield strength measured according to ASTM D882 is 50-200 MPa, and the modulus of resilience based on the following formula 1 is 0.5-5.0 MPa, where σ is the yield strength and E is the modulus, when the thickness is 30-100 μm. In more detail, a polyimide film capable of being used as an overcoat window of a display due to high Modulus of Resilience (Modulus of Resilience) and yield strength while ensuring excellent restorability and high bending characteristics is provided. Formula 1

。

Description

Polyimide film

Technical Field

The present invention relates to a polyimide film, and more particularly, to a polyimide film that can be used as an overcoat window of a display due to high Modulus of Resilience (Modulus of Resilience) and yield strength while having excellent restorability and high bending characteristics.

Background

An outer Cover Window (Cover Window) of a protective panel is applied to the surface of a Display device such as a Liquid Crystal Display (LCD) or an Organic Light Emitting Display (OLED). Conventionally, tempered glass having excellent flatness, heat resistance, chemical resistance, and barrier properties against moisture or gas, a small coefficient of linear expansion (CTE), and high light transmittance is mainly used as a material for an exterior window.

On the other hand, in recent years, flexible displays such as curved displays and in-folding displays have been developed. In order to be applied to such a flexible display, the exterior window should have flexibility, but the exterior window made of glass is generally heavy, fragile and low in flexibility, and thus is not suitable for the flexible display.

In order to solve the above problem, in recent years, an exterior window made of a plastic material having relatively free moldability has been proposed. The outer covering window made of plastic has the advantages of light weight, difficult breakage and various designs. As plastic materials for exterior windows, polycarbonate, polyethylene terephthalate, polymethyl methacrylate, and the like, which are excellent in transparency, have been mainly used. Although the above materials have an advantage of excellent transparency, they have a glass transition temperature (Tg) of 150 ℃ or less and poor heat resistance, and have low chemical resistance and mechanical strength and limited applications.

In order to solve such a problem, an excessively thick hard coat layer or the like has been introduced into the plastic material, but in this case, cracks occur in the hard coat layer during molding or the bendability is significantly reduced. In particular, in recent years, displays in a flexible state have been used in large numbers, but there is a limitation in improving the restoring force with respect to physical deformation caused by an external force.

Disclosure of Invention

Problems to be solved

The present invention has been made to solve the above problems, and an object of the present invention is to provide a novel polyimide film which has excellent restorability and various physical properties such as high bendability, mechanical strength, and transparency, and which can be used as an exterior window.

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

Means for solving the problems

In order to achieve the above object, the present invention provides a polyimide film obtained by copolymerization of at least one diamine and at least one acid dianhydride, having a Yield Strength (Yield Strength) of 50 to 200MPa and a Modulus of Resilience (Modulus of Resilience) of 0.5 to 5.0MPa as measured according to ASTM D882 at a thickness of 30 to 100 μm.

According to an embodiment of the present invention, the polyimide film is bent at a curvature radius of 1 to 10mm based on the thickness of 30 to 100 μm to have an initial recovery height (H) after 72 hours of bending _I ) A recovery height (H) of 5cm or less after at least 1 hour _F ) Is less than 3 cm.

According to an embodiment of the present invention, in a Stress-Strain Curve (Stress-Strain Curve) of the above-described film measured according to ASTM D882, a Yield Strain (Y/S) defined by a Strain at a critical point (Strain) corresponding to at least 80% of a slope in an interval of tensile strength of 20 to 40MPa may be 2.0% to 5.0%.

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

According to an embodiment of the present invention, the modulus measured according to ASTM D882 may be 3-8 GPa.

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

According to an embodiment of the present invention, the at least one diamine may include at least one selected from the group consisting of a fluorinated aromatic first diamine, a sulfone aromatic second diamine, a hydroxyl aromatic third diamine, an ether aromatic fourth diamine, a non-fluorinated aromatic fifth diamine, and an alicyclic sixth diamine.

According to an embodiment of the present invention, the first diamine to the sixth diamine may be contained in an amount of 10 to 100 mol% based on 100 mol% of the entire diamine.

According to an embodiment of the present invention, the at least one acid dianhydride may include one or more selected from the group consisting of fluorinated aromatic first acid dianhydride, non-fluorinated aromatic second acid dianhydride, sulfone-based aromatic third acid dianhydride, and alicyclic fourth acid dianhydride.

According to an embodiment of the present invention, the contents of the first to fourth acid dianhydrides may be each 10 to 100 mol% based on 100 mol% of the total acid dianhydride.

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

According to an embodiment of the present invention, the polyimide film may be used as an exterior window of a display device.

Effects of the invention

According to an embodiment of the present invention, selection of predetermined components constituting a polyimide film and adjustment of the content thereof ensure high modulus of restitution and yield strength, and thus exhibit excellent recovery from physical deformation. Further, adjusting the slope of the tensile elastic region enables high yield strain and high bending characteristics to be achieved.

In addition, in the present invention, high transmittance, low yellowness, excellent modulus and surface hardness are exhibited, so that the operability and reliability of the final product can be improved.

Therefore, the polyimide film of the present invention can be effectively used as an overcoat window for display devices, flexible displays, and the like in the art including flat panel display panels, and can also be applied to IT products, electronic products, home electric appliances, and the like known in the art.

The effects of the present invention are not limited to the above-described examples, and more various effects are included in the present specification.

Drawings

FIG. 1 is a graph showing a Stress-Strain Curve (Stress-Strain Curve) defined by Yield Strength (Yield Strength), Yield Strain (Yield Strain) and Modulus of Resilience (Modulus of Resilience) in a tensile test using a polyimide film of the present invention.

Detailed Description

The present invention will be described in detail below. The embodiments of the present invention are provided to more fully explain to those skilled in the art, and the following embodiments may be modified into various other forms without limiting the scope of the present invention. At this time, the same reference numerals are used throughout the specification to designate the same structures.

Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the same sense as commonly understood by one of ordinary skill in the art to which this invention belongs. In addition, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless expressly defined otherwise.

In addition, throughout the specification, when a part is referred to as "including" a certain component, unless otherwise stated, it means that other components may be further included, but other components are not excluded. In addition, the meaning of "at … …" throughout the specification includes not only the case where the object portion is located above or below but also the case where the object portion is located in the middle of the object portion and is not located above with reference to the direction of gravity. In the present specification, the terms "first", "second", and the like do not denote any order or importance, and are used to distinguish a plurality of 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 provided to a display device, and more particularly, can be used as a Cover Window (Cover Window) of a flexible display.

Here, the cover window is a film disposed at the outermost side of the flexible display device to protect the display device. Such an outer cover window may be a single window film, or may be a film in which a window coating layer is formed on another Substrate (Substrate) film composed of an optically transparent resin.

On the other hand, an exterior window of a display exposed to the outside should have not only processing characteristics such as flexibility and excellent optical characteristics but also a predetermined resilience to physical deformation caused by an external load in daily life. In particular, when the outer covering window is folded, if the deformation caused by stress exceeds the yield strain, permanent deformation occurs without recovery and recovery is not possible.

In the present invention, it is desired to improve the restorability by selecting specific physical properties related to the restorability, such as modulus of Resilience (modulus of Resilience) and Yield Strength (Yield Strength), among the original physical properties of the film material and controlling these physical properties to be higher than those of conventionally known plastic materials and/or polyimide films. Therefore, when the polyimide film is used as an outer cover window of a foldable display device, the film restoring force before and after static folding (static folding) is excellent, and high reliability can be provided. Such a display device can be applied to display devices known in the art without limitation, and particularly, can be an in-folding (in-folding) type foldable display (out-folding) type foldable display.

According to an embodiment of the present invention, the polyimide film comprises at least one diamine and at least one acid dianhydride copolymerized, and has a Yield Strength (Yield Strength) of 50 to 200MPa and a Modulus of Resilience (R) of 0.5 to 5.0MPa as measured according to ASTM D882 at a thickness of 30 to 100. mu.m.

Here, the modulus of resilience may be defined by [ formula 1] below.

[ formula 1]

(in the above formula, σ represents yield strength, and E represents modulus)

In particular, the modulus of restitution (R), as one of the rheological (Rheology) properties of a material, means the energy per unit volume that the material is able to absorb without permanent deformation. For example, when a predetermined material is deformed, although the strain at a small deformation amount is elastic, permanent deformation which cannot be recovered any more occurs once the strain is converted from elastic deformation to plastic deformation under a certain condition. That is, the modulus of resilience is the maximum energy that can be stored in the monomer volume of the material within the elastic limit, i.e., it represents the elastic restoring force. Physical properties such as the modulus of resilience (R), the Yield Strength (Yield Strength), and the Yield Strain (Yield Strain) to be described later can be understood from a Stress-Strain Curve (S-score). Such a stress-strain curve (S-S curve) is a curve on a graph showing a relationship between a stress (stress) applied to a polymer material by a tensile test of the polymer material and a strain (strain) of the polymer material expressed in accordance with the stress, and may be referred to as a curve showing a change in the magnitude of the stress required as the strain of the polymer material increases. As shown in fig. 1 below, generally, the horizontal axis (e.g., x-axis) represents tensile Strain (Stress) and the vertical axis (e.g., y-axis) represents Stress (tensile strength).

In the present invention, the above-mentioned parameters of Modulus of Resilience and Yield Strength (Yield Strength) and their values may be affected by the thickness of the above-mentioned polyimide film. The numerical values of the modulus of resilience and the yield strength may be measured based on the thickness of the polyimide film of 30 to 100. mu.m, specifically 30 to 90 μm, more specifically 80. + -.5. mu.m.

Specifically, the Yield Strength (Yield Strength, R) of the polyimide film of the present invention may be 50 to 195MPa, the Modulus of Resilience (Modulus of Resilience, R) may be 0.5 to 4.0MPa, more specifically, the Yield Strength (Yield Strength, R) may be 100 to 190MPa, and the Modulus of Resilience (Modulus of Resilience, R) may be 2.0 to 3.5 MPa. In the case of the polyimide film of the present invention satisfying the above-mentioned parameters of the modulus of resilience and Yield Strength (Yield Strength), high elasticity can be ensured without causing permanent deformation, and excellent recovery characteristics can be exhibited.

In order to be used as an exterior window of a mobile communication terminal or a tablet PC, etc., the polyimide film of the present invention preferably has excellent optical characteristics such as excellent flexibility, high transparency, and light transmittance, and high mechanical characteristics, together with the above-described characteristics of modulus of elasticity and yield strength.

In particular, the polyimide film of the present invention is characterized by having both high Yield Strain (Yield Strain) and high bending property as compared with conventionally known plastic materials and/or polyimide films. When such a polyimide film is used as an outer cover window of an in-folding (in-folding) type foldable cellular phone, it is possible to exhibit an excellent folding strength effect by high bendability and high strength.

In one embodiment, the polyimide film may have a Yield Strain (Yield Strain, Y/S) defined by a Strain at a critical point (Strain) corresponding to at least 80% of a slope in a region of tensile strength of 20 to 40MPa in a Stress-Strain Curve (Stress-Strain Curve) of the film measured according to ASTM D882, the Yield Strain (Y/S) being 2.0% to 5.0%.

In the present specification, the yield Strain is determined by the Stress-Strain slope (Stress-Strain slope, X) in a specific tensile strength range (for example, 20 to 40MPa) in the Stress-Strain Curve (Stress-Strain Curve) of the polyimide film measured according to ASTM D882 ₁ ) At least 80% or more of a predetermined critical point (threshold slope), X ₂ ≥X ₁ X 0.8), i.e. Tensile strain (see fig. 1 below). Such yield strain is newly defined in the present invention, is an inherent physical property due to a specific composition of the polyimide film, and is a characteristic that is different from the conventional polyimide film.

Specifically, the yield strain (Y/S) of the polyimide film may be 2.0% to 4.8% in a stress-strain curve (S-Scurve) obtained by a tensile test at 25 ℃. From the viewpoint of improving the bending resistance, the yield strain (Y/S) may be preferably 2.0% to 4.0%, more preferably 2.0 to 3.8%. In the case of a polyimide film satisfying such yield strain (Y/S) parameters, the flexibility is sufficiently excellent to suppress the occurrence of cracks in repeated bending fatigue, and excellent bendability is exhibited.

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

In yet another specific example, the number of times of bending until breaking of the polyimide film when the film is bent with a radius of curvature of 1mm to 5mm may be 100,000 or more. In particular, when bending is performed with a bending radius of 1mm, the number of bending times to break may be 10 ten thousand or more, specifically 20 ten thousand or more, and preferably million (1,000,000) or more. In this case, the upper limit of the deformation of the number of bending times to fracture is not particularly limited, and may be, for example, 3,000,000 times or less, specifically 2,500,000 times or less.

The polyimide film of the present invention preferably has excellent mechanical characteristics of high modulus in view of scratch resistance, reliability, etc. of a display.

Specifically, the polyimide film may have a Modulus (Modulus) of 3 to 8GPa, and may have a Modulus of 3.5 to 7GPa in order to exhibit both mechanical hardness and excellent flexibility. Here, modulus (modulius) means a value determined according to ASTM D882. When the modulus is less than the above value, it is difficult to exhibit sufficient hardness, and when the modulus is more than the above value, flexibility may be reduced and folding property may be reduced.

In order to improve the visibility of a display screen, the polyimide film of the present invention preferably has excellent optical characteristics such as high transparency and light transmittance at the same time.

Specifically, the polyimide film has a light transmittance of 85% or more, specifically 89% or more, more specifically 90% to 99% at a wavelength of 550nm, when the thickness is 30 to 100 μm. The yellowness index (Y.I.) according to ASTM E313-73 may be 10 or less, specifically 7 or less, and more specifically 5 or less.

In the present specification, unless otherwise stated, the physical properties of the polyimide film may be 30 to 80 μm based on the film thickness of 10 to 100. mu.m. However, the thickness is not limited to the above range, and can be adjusted as appropriate within a range of a usual thickness known in the art.

The polyimide film of the present invention is not particularly limited in terms of the components constituting the polyimide resin and/or the composition thereof, as long as the film satisfies the above-described characteristics of the modulus of elasticity and yield strength.

For example, the polyimide film is obtained by copolymerizing at least one diamine and at least one acid dianhydride, and specifically, can be produced by imidizing and heat-treating a polyamic acid composition containing a diamine, an acid dianhydride, and, if necessary, a solvent at a high temperature.

Generally, a Polyimide (PI) resin is a highly heat-resistant resin produced by polymerizing an aromatic acid dianhydride and an aromatic diamine or an aromatic diisocyanate in a solution to produce a polyamic acid derivative, and then subjecting the polyamic acid derivative to ring-closing dehydration at a high temperature to imidize the polyamic acid derivative. Such a polyimide resin is a polymer containing an imide (imide) ring, and is excellent in chemical stability, heat resistance, chemical resistance, abrasion resistance, and electrical characteristics based on the imide ring. The polyimide resin may be in the form of a random copolymer (randomcopolymer) or a block copolymer (block copolymer).

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

In particular, in the present invention, when mechanical properties such as high Modulus of resilience and yield strength, high bendability, Modulus (Modulus), etc. of the polyimide film are considered; when optical properties such as High Transmittance (High Transmittance), low Y.I, and low Haze (Haze) are required, at least one aromatic diamine may be used or the aromatic diamine and the alicyclic diamine may be used in combination.

Specific examples of the aromatic diamine include fluorine-based diamines, Sulfone-based diamines, Hydroxyl-based diamines, Ether-based diamines, non-fluorine-based diamines, and the like, each having a fluorinated substituent. Therefore, in the present invention, as the diamine compound, a fluorinated aromatic primary diamine, a sulfone aromatic secondary diamine, a hydroxyl aromatic tertiary diamine, an ether aromatic quaternary diamine, and a non-fluorinated aromatic tertiary diamine, each of which has a fluorine substituent introduced therein, may be used alone or in combination of two or more thereof as appropriate.

As non-limiting examples of the diamine monomer (a) that can be used, Oxydianiline (ODA), 2' -bis (trifluoromethyl) -4,4' -Diaminobiphenyl (2,2' -TFDB), 2' -bis (trifluoromethyl) -4,3' -Diaminobiphenyl (2,2' -bis (trifluoromethylphenyl) -4,3' -Diaminobiphenyl), 2' -bis (trifluoromethyl) -5,5' -Diaminobiphenyl (2,2' -bis (trifluoromethylphenyl) -5,5' -Diaminobiphenyl), 2' -bis (trifluoromethyl) -4,4' -Diaminobiphenyl ether (2,2' -bis (trifluoromethylphenyl) -4,4' -diaminodipheny-lether, 6-FODA), bisaminohydroxyphenylhexafluoropropane (DBOH), bis (bromodiphenyl oxide), bis (trifluoromethyl) and bis (trifluoromethyl) ether can be used, Bisaminophenoxyphenylhexafluoropropane (4BDAF), bisaminophenoxyphenylpropane (6HMDA), bisaminophenoxydiphenylsulfone (DBSDA), bis (4-aminophenyl) sulfone (4,4 '-DDS), bis (3-aminophenyl) sulfone (3,3' -DDS), sulfonyldiphthalic anhydride (SO2DPA), 4 '-oxydianiline (4,4' -ODA), or a mixture of one or more of these.

When high transparency, high glass transition temperature and low yellowness of the polyimide film are considered, the fluorinated first diamine may use 2,2' -Bis (trifluoromethyl) -4,4' -diaminobiphenyl (2,2' -TFDB), 1,4-Bis (4-amino-2-trifluoromethylphenoxy) benzene (1,4-Bis (4-amino-2-trifluoromethylphenoxy) benzene, 6-FAPB), 2-Bis (3-amino-4-methylphenyl) -hexafluoropropane (2,2-Bis (3-amino-4-methylphenyl) -hexafluoro-pane, Bis-AT-AF) capable of inducing linear type of high molecular weight. In addition, bis (4-aminophenyl) sulfone (4,4 '-DDS) or 3,3' -DDS may be used as the sulfone-based second diamine. In addition, 2-Bis (3-amino-4-hydroxyphenyl) -hexafluoropropane (2,2-Bis (3-amino-4-hydroxyphenyl) -hexafluoro propane, Bis-AP-AF) can be used as the hydroxyl tertiary diamine. Further, 2'-bis (trifluoromethyl) -4,4' -diaminophenyl ether (6-FODA) or Oxydianiline (ODA) can be used as the ether-based fourth diamine. Further, as the non-fluorine-based fifth diamine, m-tolidine (m-tolidine) or p-phenylenediamine (p-PDA) can be used. The first to fifth diamine components may be used singly or in combination of two or more.

In the diamine monomer (a) of the present invention, the content of the fluorinated aromatic primary diamine, the sulfone aromatic secondary diamine, the hydroxyl aromatic tertiary diamine, the ether aromatic quaternary diamine, the non-fluorine aromatic fifth diamine, and the like is not particularly limited, and may be 0 to 100 mol%, specifically 10 to 90 mol%, more specifically 20 to 80 mol%, based on 100 mol% of the total diamine. Wherein at least one of the first diamine to the fifth diamine is contained in an amount satisfying 100 mol% of the entire diamine.

According to an embodiment of the present invention, as the diamine monomer (a), a fluorinated first diamine and an ether-based fourth diamine may be used in combination. In this case, the ratio of these is not particularly limited, and may be, for example, 50 to 90:10 to 50 mol%.

According to another embodiment of the present invention, as the diamine monomer (a), at least one fluorinated first diamine may be used in combination. In this case, the ratio of the components may be 50 to 90:10 to 50 mol%, and is not particularly limited.

The alicyclic diamine is not particularly limited as long as at least one alicyclic ring is contained in the molecular structure. As non-limiting examples of the alicyclic diamine that can be used, 2-bis (3-amino-4-hydroxycyclohexyl) hexafluoropropane [2,2-bis (3-amino-4-hydroxycyclohexyl) hexafluoro-propane ], 3' -dimethyl-4, 4' -diaminodicyclohexylmethane (MACM), 4' -methylenedicyclohexylamine (PACM), 1, 3-bis (aminomethyl) cyclohexane (1,3-BAC), 1,4-bis (aminomethyl) cyclohexane (1,4-BAC), cis-1, 2-cyclohexanedimethanamine, trans-1, 2-cyclohexanedimethanamine, 1,4-Cyclohexyldiamine (CHDA), bis (4-aminocyclohexyl) ether, a mixture thereof, or the like.

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

The acid dianhydride (b) monomer constituting the polyimide film of the present invention may use any conventional compound known in the art having an acid dianhydride structure in the molecule, without limitation. For example, an aromatic, alicyclic, or aliphatic compound having an acid dianhydride (dianhydide) structure, or a combination thereof may be used, and specifically, at least one alicyclic acid dianhydride may be used, or at least one aromatic acid dianhydride may be used, or the aromatic acid dianhydride and the alicyclic acid dianhydride may be used in combination.

Specific examples of the aromatic acid dianhydride include fluorinated aromatic first acid dianhydride, non-fluorinated aromatic second acid dianhydride, and sulfone aromatic third acid dianhydride. Therefore, in the present invention, as the acid dianhydride compound, a fluorinated aromatic first acid dianhydride, a non-fluorinated aromatic second acid dianhydride, and a sulfone aromatic third acid dianhydride, to which a fluorine substituent is introduced, may be used alone or in a form of a mixture of two or more thereof in an appropriate combination.

The fluorinated first acid dianhydride monomer is not particularly limited as long as it is an aromatic acid dianhydride to which a fluorine substituent is introduced. As non-limiting examples of fluorinated first acid dianhydrides that can be used, there are 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (2,2-bis (3, 4-dicarboxxyphenyl) hexafluoro propane dianhydride, 6-FDA), 4- (trifluoromethyl) pyromellitic dianhydride (4- (trifluoromethyl) chiral dianhydrides, 4-TFMDA), and the like. They may be used alone or in combination of two or more. In the fluorinated acid dianhydride, the characteristics of 6-FDA restricted Charge Transfer Complexes (CTC) between and within the molecular chain are very significant, and thus, it is a compound very suitable for transparentization.

The non-fluorinated second acid dianhydride monomer is not particularly limited as long as it is a non-fluorinated aromatic acid dianhydride into which a fluorine substituent is not introduced. Non-limiting examples of the non-fluorinated second acid Dianhydride monomer that can be used include Pyromellitic Dianhydride (PMDA), 3',4,4' -Biphenyl tetracarboxylic Dianhydride (3,3',4,4' -biphenyltetracarboxylic acid Dianhydride, BPDA), Benzophenone Tetracarboxylic Dianhydride (BTDA), Oxydiphthalic Dianhydride (ODPA), 4,4- (4,4-Isopropylidenediphenoxy) Bis (phthalic anhydride) (4,4- (4,4-Isopropylidenediphenoxy) Bis (phthalic anhydride), BPADA), Bis (carboxyphenyl) dimethylsilane Dianhydride (Bis (3,4-dicarboxyphenyl) dimethylsilane Dianhydride, SiDA), and the like. They may be used alone or in combination of two or more thereof.

The sulfone-based third acid dianhydride monomer is not particularly limited as long as it is an acid dianhydride to which a sulfone group is introduced, and examples thereof include 3,3',4,4' -diphenylsulfone tetracarboxylic dianhydride (3,3',4,4' -diphenylsulfonnetetracarboxylic dianhydride, DSDA).

In the acid dianhydride monomer (b) of the present invention, the content of the fluorinated aromatic first acid dianhydride, the non-fluorinated aromatic second acid dianhydride, the sulfone aromatic third acid dianhydride, and the like is not particularly limited. For example, the content of each of these acid dianhydrides may be in the range of 0 to 100 mol%, specifically 10 to 90 mol%, more specifically 20 to 80 mol%, based on 100 mol% of the whole acid dianhydride. Wherein at least one of the first to third acid dianhydrides is contained in such a content that the whole acid dianhydride is 100 mol%.

The alicyclic (alicylic) acid dianhydride is not particularly limited as long as it has an alicyclic ring instead of an aromatic ring in the compound and has an acid dianhydride structure. As non-limiting examples of the alicyclic acid dianhydride which can be used, cyclobutanetetracarboxylic dianhydride (CBDA), 1,2,3, 4-cyclopentanetetracarboxylic 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' -bicyclohexane-3, 3',4,4' -tetracarboxylic dianhydride (H-BPDA), 1,2,4, 5-cyclohexanetetracarboxylic dianhydride (H-PMDA), cyclobutenedi-spironorbornane (cyclopropanobione-spironorbomane, cPDA), bicyclo [2.2.2] octane-2,3,5,6-tetracarboxylic acid 2,3:5,6-dianhydride (Bicyclo [2.2.2] octane-2,3,5,6-tetracarboxylic acid 2,3:5, 6-dianhydide (7CI,8CI)), or a mixture of one or more thereof.

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

According to an embodiment of the present invention, as the acid dianhydride (b), a fluorinated aromatic first acid dianhydride and an alicyclic acid dianhydride or a fluorinated aromatic first acid dianhydride and a non-fluorinated aromatic second acid dianhydride may be used in combination. In this case, the ratio of these is not particularly limited, and may be, for example, 10 to 90:90 to 10 mol%, specifically 30 to 80:70 to 20 mol%.

According to another embodiment of the present invention, as the acid dianhydride (b), an alicyclic acid dianhydride and a non-fluorinated aromatic second acid dianhydride may be used in combination. In this case, the ratio of the components may be 30 to 80:20 to 70 mol%, but is not particularly limited thereto.

As another preferred example of the present invention, at least one non-fluorinated aromatic second acid dianhydride may be used in combination as the acid dianhydride (b). In this case, the ratio of the components may be 50 to 80:20 to 50 mol%, 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 0.8 to 1.2, and more preferably 0.9 to 1.1.

The polyamic acid composition of the present invention may use, without limitation, organic solvents known in the art as a solvent for the solution polymerization of the above-mentioned monomers. As examples of the usable solvent, one or more polar solvents selected from the group consisting of m-cresol, N-methyl-2-pyrrolidone (NMP), Dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), acetone, diethyl acetate, and dimethyl phthalate (DMP) may be used. Further, a low boiling point solution such as Tetrahydrofuran (THF), 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 in order to obtain a suitable molecular weight and viscosity of the polyamic acid composition (polyamic acid solution), the content may be preferably 50 to 95% by weight, more preferably 70 to 90% by weight, based on the weight of the whole polyamic acid composition.

The polyamic acid composition can be produced by charging at least one acid dianhydride and at least one diamine into an organic solvent and then reacting them, and for example, the equivalent ratio of the diamine (a) and the acid dianhydride (b) can be adjusted to approximately 1:1 in order to improve the physical properties of the polyimide. The composition of the polyamic acid composition is not particularly limited, and for example, the composition may include 2.5 to 25.0 wt% of acid dianhydride, 2.5 to 25.0 wt% of diamine, and the balance of organic solvent satisfying 100 wt% of the composition, based on 100 wt% of the total polyamic acid composition. Wherein the content of the organic solvent may be 70 to 90% by weight. The polyamic acid composition may be contained in a range of 30 to 70 wt% of acid dianhydride and 30 to 70 wt% of diamine based on 100 wt% of the solid content, and is not particularly limited.

The polyamic acid composition as constituted above may have a viscosity ranging from about 1,000 to 200,000cps, preferably from about 5,000 to 50,000 cps. When the viscosity of the polyamic acid composition is in the above range, the thickness can be easily adjusted when the polyamic acid composition is applied, and the applied surface can be uniformly formed.

The polyamic acid composition may contain, as necessary, at least one additive such as a plasticizer, an antioxidant, a flame retardant, a dispersant, a viscosity modifier, a leveling agent, and the like in a small amount within a range not significantly impairing the object and effect of the present invention.

The polyimide resin film of the present invention can be manufactured according to a general method known in the art, for example, by coating (casting) the polyamic acid composition on a substrate (substrate), such as a glass substrate, and then inducing an imide ring-closure reaction (imidization) for 0.5 to 8 hours while slowly raising the temperature in the range of 30 to 350 ℃.

At this time, the coating method may use a general method known in the art without limitation, and for example, may be performed by at least any one method selected from the group consisting of Spin coating (Spin coating), Dip coating (Dip coating), Solvent casting (Solvent casting), Slot die coating (Slot die coating), and spray coating. The polyamic acid composition may be applied at least once so that the thickness of the colorless and transparent polyimide resin layer may be several hundred nm to several tens of μm.

In the method for producing a polyimide film of the present invention, as the imidization method to be applied in the step of casting the polymerized polyamic acid on the support to perform imidization, a thermal imidization method, a chemical imidization method, or a combination of a thermal imidization method and a chemical imidization method can be applied.

The thermal imidization method is a method in which a polyamic acid composition (polyamic acid solution) is cast on a support and heated for 1 to 10 hours while slowly raising the temperature in a temperature range of 30 to 400 ℃.

The chemical imidization method is a method of adding a dehydrating agent represented by an acid anhydride such as acetic anhydride and an imidization catalyst represented by an amine such as isoquinoline, β -picoline, pyridine to a polyamic acid composition. When the thermal imidization method is used in combination with such a chemical imidization method, the heating conditions of the polyamic acid composition may be changed depending on the kind of the polyamic acid composition, the thickness of the polyimide film to be produced, and the like.

More specifically, the case of using both the thermal imidization method and the chemical imidization method as described above is explained, a dehydrating agent and an imidization catalyst may be added to a polyamic acid composition, cast on a support, and then heated at 80 to 300 ℃, preferably 150 to 250 ℃ to activate the dehydrating agent and the imidization catalyst, thereby partially curing and drying the composition to obtain a polyimide film.

The thickness of the polyimide film thus formed is not particularly limited, and may be appropriately adjusted according to the field of application. For example, the particle size may be in the range of 10 to 150. mu.m, and preferably in the range of 30 to 100. mu.m.

The polyimide film of the present invention and the modified examples thereof produced as described above can be effectively used in various fields where high restorability, high bending properties, and excellent optical properties are required. Particularly, it can be used as a Cover Window (Cover Window) of a display device to prevent surface scratch and impart excellent flexibility and visibility to a flexible display device.

In the present invention, the Display Device is a Flexible Display Device or a non-Flexible Display Device for displaying an image, and includes not only a Flat Panel Display Device (FPD), but also a Curved Display Device (Curved Display Device), a Foldable Display Device (Foldable Display Device), a Flexible Display Device (Flexible Display Device), a Foldable mobile phone, a smart phone, a mobile communication terminal, a tablet PC, and the like. Specifically, the Display device may be a Liquid Crystal Display device (Liquid Crystal Display), an Electrophoretic Display device (Electrophoretic Display), an Organic Light Emitting Display device (Organic Light Emitting Display), an Inorganic EL Display device (Inorganic Light Emitting Display), a Field Emission Display device (Field Emission Display), a Surface-conduction Electron-emitter Display device (Surface-emitter Display), a Plasma Display device (Plasma Display), a Cathode Ray tube Display device (cathoderay Display), an electronic paper, or the like. As a specific example, the flat panel display panel can be LCD, PDP, OLED, etc. The polyimide film of the present invention is not limited to the above-described applications, and can be applied to general display devices known in the art, and can also be used as a substrate or a protective film for flexible displays.

A specific example of the display device including the polyimide film may include a display portion, a polarizer, a touch panel, an overcoat window, and a protective film, and the overcoat window may include the polyimide film according to an embodiment of the present invention. The components constituting the display device are not particularly limited, and may include common components known in the art.

The present invention will be described more specifically with reference to specific examples. The following examples are merely illustrative for facilitating the understanding of the present invention, and the scope of the present invention is not limited thereto.

Examples 1 to 4 production of polyimide films

A polyamic acid composition was produced using a composition including a diamine and an acid dianhydride described in table 1 below.

After the polyamic acid composition was applied to a glass for LCD using a Bar Coater (Bar Coater), drying and imide ring-closure reaction (imidization) were performed while gradually raising the temperature in a convection oven in a nitrogen atmosphere in a stepwise manner at 80 ℃ for 30 minutes, 150 ℃ for 30 minutes, 200 ℃ for 1 hour, and 300 ℃ for 1 hour. Thus, a polyimide film having an imidization ratio of 85% or more and a film thickness of 80 μm was produced. After that, the polyimide film was separated and obtained from the glass.

[ Table 1]

Comparative examples 1 to 4 production of polyimide films

Polyimide films of comparative examples 1 to 4 were produced in the same manner as in examples 1 to 4, respectively, except that the compositions shown in table 1 were used.

[ Experimental example and evaluation of physical Properties ]

The physical properties of the polyimide resin films produced 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. In this case, the physical properties shown in Table 2 below were based on a thickness of 50 μm.

< methods for evaluating physical Properties >

1) Measurement of light transmittance

The measurement was carried out at a wavelength of 550nm using an ultraviolet-visible near infrared Spectrophotometer (UV-Vis NIR Spectrophotometer, Shimadzu, model name: UV-3150).

2) Determination of yellowness

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

3) Thickness measurement

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

4) Modulus determination

Tensile strength (MPa), modulus of elasticity (GPa) was determined using UTM (Instron, model name: 5942) according to ASTM D882.

5) Yield Strain (Yield Strain) determination

The Strain (Strain, X value) corresponding to a critical value (X2 ≧ X1X 0.8) of 80% or more is determined in the range of 20 to 40MPa based on the slope (Stress-Strain slope, X1) of the Stress-Strain curve by UTM (Instein, model name: 5942) according to ASTM D882.

6) Modulus of restitution

Using the above modulus and Yield Strength (Yield Strength), the modulus of resilience was calculated according to the following formula 1. At this time, the yield strength is calculated as the yield strength at the yield point by the same method as the above-described yield strain.

[ formula 1]

(in the above formula, σ represents yield strength, E represents modulus)

7) Determination of bending characteristics (Dynamic Folding cycle)

The bending characteristics were measured using a flat body unloaded U-fold test apparatus (DLDMLS-FS, Yusa (YUASA)), and the number of Folding cycles (Folding cycles) was measured by adjusting the radius of curvature to 1mm to 5 mm.

8) Determination of restoring force

The restoring force is confirmed by measuring the height of a film after 72 hours have elapsed since a sample set in a Static Folding Holder (Static Folding Holder, radius of curvature 1-10 nm) was fixed and a predetermined time has elapsed after the sample was taken out.

[ Table 2]

As shown in table 2, it is understood that the polyimide film of the present invention not only exhibits higher modulus of resilience, yield strength and yield strain characteristics, but also exhibits excellent recovery force and high bendability, as compared to the comparative examples. In addition, it is known that the film has excellent transparency, mechanical properties and thermal expansion coefficient.

Therefore, it was confirmed that the polyimide film of the present invention can be effectively used as an exterior window of a display device.

Claims

1. A polyimide film obtained by copolymerization comprising at least one diamine and at least one acid dianhydride,

a yield strength of 50 to 200MPa as measured according to ASTM D882 when the thickness is 30 to 100 μm,

based on the following formula 1, the elastic modulus is 0.5 to 5.0MPa,

formula 1

In the formula 1, σ represents yield strength, and E represents modulus.

2. The polyimide film according to claim 1, wherein the film has an initial height H of recovery after being bent for 72 hours at a radius of curvature of 1 to 10mm based on a film thickness of 30 to 100 μm _I 5cm or less, and a recovery height H after at least 1 hour _F Is 3cm or less.

3. The polyimide film of claim 1, having a yield strain Y/S defined by the strain at the critical point corresponding to at least 80% of the slope in the interval of tensile strength 20 to 40MPa in a stress-strain curve of the film determined according to ASTM D882 of 2.0% to 5.0%.

4. The polyimide film according to claim 1, wherein the number of times of bending to break when the film is bent with a radius of curvature of 1 to 5mm is 100,000 or more.

5. The polyimide film of claim 1 having a modulus of 3-8 GPa as determined according to ASTM D882.

6. The polyimide film according to claim 1, having a light transmittance at a wavelength of 550nm of 85% or more at a thickness of 30 to 100. mu.m,

the yellowness index of the composition is 10 or less according to ASTM E313-73.

7. The polyimide film according to claim 1, wherein the at least one diamine contains at least one selected from the group consisting of a fluorinated aromatic first diamine, a sulfone aromatic second diamine, a hydroxyl aromatic third diamine, an ether aromatic fourth diamine, a non-fluorine aromatic fifth diamine, and an alicyclic sixth diamine.

8. The polyimide film according to claim 7, wherein the first to sixth diamines are contained in an amount of 10 to 100 mol% based on 100 mol% of the entire diamine.

9. The polyimide film according to claim 1, the at least one acid dianhydride comprises one or more selected from the group consisting of a fluorinated aromatic first acid dianhydride, a non-fluorinated aromatic second acid dianhydride, a sulfone-based aromatic third acid dianhydride, and an alicyclic fourth acid dianhydride.

10. The polyimide film according to claim 9, wherein the content of each of the first to fourth acid dianhydrides is 10 to 100 mol% based on 100 mol% of the whole acid dianhydride.

11. The polyimide film according to claim 1, wherein a ratio a/b of the number of moles of the diamine a to the acid dianhydride b is in the range of 0.7 to 1.3.

12. The polyimide film of claim 1, used as an exterior cover window for a display device.