JP7271620B2 - Polyimide film with excellent recovery properties - Google Patents

Description

TECHNICAL FIELD 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 by simultaneously having excellent restoring force and high bending characteristics due to high modulus of resilience and yield strength. .

A cover window is applied to the surface of a display device such as a liquid crystal display (LCD) or an organic light emitting display (OLED) to protect the panel. ing. Conventionally, tempered glass, which has excellent flatness, heat resistance, chemical resistance, and barrier performance against moisture and gases, has a small coefficient of linear expansion (CTE), and has high light transmittance, has been the main material used for cover windows. was used.

In recent years, flexible displays such as curved displays and in-folding displays have been developed. In order to be applied flexibly in this way, the cover window also needs to be flexible. do not have.

In order to solve the above-mentioned problems, recently, a cover window using a plastic material having relatively good moldability has been proposed. A cover window made of a plastic material has the advantage of being lightweight, hard to crack, and capable of implementing various designs. At present, polycarbonate, polyethylene terephthalate, polymethyl methacrylate, etc., which are excellent in transparency, are mainly used as plastic materials for cover windows. Although such materials have the advantage of having excellent transparency, they have a glass transition temperature (Tg) of 150° C. or less, have poor heat resistance, and have low chemical resistance and mechanical strength, which limits their application. be.

Attempts have been made to introduce a thick hard coat layer or the like into the plastic material in order to compensate for such inconveniences, but in this case, cracks occur in the hard coat layer during molding, or flexibility is significantly reduced. In particular, in recent years, many flexible display devices have been used, but there has been a limit to improving the restoring force against physical deformation due to external force.

The present invention has been devised to solve the above-mentioned problems, and provides a cover window that has excellent restoring force and high flexibility, as well as excellent properties such as mechanical strength and transparency. There is a technical problem in providing a new applicable polyimide film.

Other objects and advantages of the present invention will become more apparent from the detailed description and claims that follow.

In order to achieve the above technical object, the present invention provides a copolymerized polyimide film containing at least one diamine and at least one acid dianhydride, which has a thickness of 30-100 μm and meets ASTM D882 provide a polyimide film having a Yield Strength of 50-200 MPa and a Modulus of Resilience of 0.5-5.0 MPa, measured according to

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

In one embodiment of the present invention, the stress-strain curve of the film measured according to ASTM D882 has a threshold value corresponding to at least 80% of the slope of the section of the tensile strength of 20 to 40 MPa. The yield strain (Y/S) defined as the strain of is 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 before breaking is 100,000 times or more.

In one embodiment of the invention, the modulus measured according to ASTM D882 is 3-8 GPa.

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

In one embodiment of the present invention, the at least one diamine is a fluorinated aromatic primary diamine, a sulfone-based aromatic secondary diamine, a hydroxy-based aromatic tertiary diamine, an ether-based aromatic quaternary diamine. One or more selected from the group consisting of diamines, non-fluorine aromatic fifth diamines, and alicyclic sixth diamines.

In one embodiment of the present invention, the content of each of the first to sixth diamines is 10 to 100 mol % with respect to 100 mol % of all diamines.

In one embodiment of the present invention, the at least one dianhydride is a fluorinated aromatic primary dianhydride, a non-fluorinated aromatic secondary dianhydride, a sulfonic aromatic tertiary and an alicyclic fourth acid dianhydride.

In one embodiment of the present invention, the content of each of the first to fourth acid dianhydrides is 10 to 100 mol% with respect to 100 mol% of the total acid dianhydride.

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

In one embodiment of the present invention, the polyimide film can be used for cover windows of display devices.

According to an embodiment of the present invention, the polyimide film exhibits excellent resilience against physical deformation by securing high resilience and yield strength by selecting predetermined components constituting the polyimide film and adjusting the content thereof. be able to. Also, by adjusting the tensile elastic region slope, high yield strain and high bending properties can be achieved.

In addition, the present invention exhibits high light transmittance, low yellowness, excellent modulus and surface hardness, and can improve the workability and reliability of the final product.

Therefore, the polyimide film of the present invention can be usefully applied to display devices in the field including flat panel displays, cover windows such as flexible displays, and other IT products, electronic products, and home appliances known in the field. It can also be applied to products and the like.

The effects of the present invention are not limited by the above examples, and various other effects are included in this specification.

FIG. 1 shows 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. ) is a graph.

The present invention will be described in detail below. The examples of the present invention are provided so that the present invention will be more complete and complete for those of ordinary skill in the art, and the following examples may be practiced with various modifications. However, the scope of the invention is not limited by these examples. In this specification, the same reference numerals refer to the same structure.

Unless defined otherwise, all terms (including technical and scientific terms) used herein have the meaning commonly understood by one of ordinary skill in the art to which this invention belongs. Also, terms defined in commonly used dictionaries should not be interpreted ideally or unduly unless specifically defined.

In addition, throughout the specification, when a part "includes" another component, it does not mean that the other component is excluded, but that the other component can be further included, unless otherwise specified. means In addition, in the specification as a whole, "above" or "above" means not only the case where the target part is located above or below, but also the case where there is another part in the middle. It does not mean that it is located above the direction of gravity. In this specification, terms such as "first" and "second" are used to distinguish between constituent elements without indicating arbitrary order or importance.

<Polyimide film>
A polyimide resin film according to an embodiment of the present invention is a transparent film provided in a display device, and more specifically, it can be used as a cover window of a flexible display.

Here, the cover window refers to a film that is disposed on the outermost shell of the flexible display device and protects the display device. Such a cover window may be a window film alone or a film in which a window coating layer is formed on another substrate film made of an optically transparent resin.

On the other hand, the cover window of the display that is exposed to the outside needs to have processing characteristics such as flexibility and excellent optical characteristics in daily life. must have resilience. In particular, when the cover window is folded, if the strain due to the stress exceeds the yield strain, a permanent deformation occurs, which cannot be recovered any more and becomes irreversible.

In the present invention, among the inherent physical properties inherent in the film material, specific physical properties that are related to the restoring force, such as modulus of resilience and yield strength, are adopted, and these physical properties are known in the art. We are trying to improve the resilience by controlling the plastic material and / or polyimide film higher than that of other plastic materials. Accordingly, when the polyimide film is applied to a cover window of a foldable display device, it has excellent film restoring force before and after static folding and provides high reliability. be able to. Such display devices can be applied without limitation to display devices known in the art, particularly in-folding and out-folding type foldable displays. can be

In one embodiment of the present invention, the polyimide film comprises at least one diamine and at least one acid dianhydride and is copolymerized, and has a thickness of 30 to 100 μm and a thickness of 30 to 100 μm according to ASTM D882. can be 50 to 200 MPa, and the Modulus of Resilience (R) can be 0.5 to 5.0 MPa. Here, the resilience may be defined by Equation 1 below.

（式中、σは、降伏強度であり、Ｅは、モジュラスである。）

(Where σ is the yield strength and E is the modulus.)

Specifically, resilience (R) is one of the rheological properties of a material, and means energy per unit volume that the material can absorb without permanent deformation. For example, when deformation is applied to a given material, deformation with a small amount of deformation is elastic, but under certain conditions, when elastic deformation is converted to plastic deformation, permanent deformation that cannot be restored occurs. end up That is, resilience is the maximum energy that can be stored per unit volume of a material within its elastic limit, ie, elastic recovery force. Physical properties such as resilience (R) to be controlled in the present invention, yield strength (Yield Strength), and yield strain (Yield Strain) described later are stress-strain curves (Stress-Strain Curve: SS Curve). can be done. Such a stress-strain curve (SS curve) is a stress applied to a polymer material by a tensile test of the polymer material and a strain of the polymer material corresponding to this stress. ), and can be said to be a curve showing the change in the amount of stress required as the deformation of the polymeric material increases. As shown in FIG. 1, generally the horizontal axis (eg, x-axis) is tensile strain and the vertical axis (eg, y-axis) is stress.

In the present invention, the parameters and numerical values of the modulus of resilience and yield strength are affected by the thickness of the polyimide film. The numerical values of such resilience and yield strength are measured based on the thickness of the polyimide film of 30 to 100 μm, specifically 30 to 90 μm, more specifically 80±5 μm. can.

Specifically, the polyimide film according to the present invention may have a yield strength of 50 to 195 MPa and a resilience (R) of 0.5 to 4.0 MPa. ) can be 100 to 190 MPa, and resilience (R) can be 2.0 to 3.5 MPa. The polyimide film of the present invention, which satisfies the parameters of resilience and yield strength, does not undergo permanent deformation, ensures high elasticity, and exhibits excellent recovery characteristics.

In order for the polyimide film according to the present invention to be used for cover windows of mobile communication terminals, tablet PCs, etc., it is necessary to have excellent optical properties such as excellent flexibility, high transparency, and light transmittance in addition to the resilience and yield strength characteristics. It is preferred to have properties and high mechanical properties at the same time.

In particular, the polyimide film of the present invention is characterized in that it has both high yield strength and high flexural properties compared to conventionally known plastic materials and/or polyimide films. When such a polyimide film is applied to the cover window of an in-folding type foldable mobile phone, it has high bending characteristics and high strength, so that it has excellent folding endurance effect. can play.

As one specific example, the polyimide film has a stress-strain curve of the film measured according to ASTM D882, with a tensile strength of 20 to 40 MPa section slope of at least 80%. The yield stress (Y/S), defined as the corresponding threshold strain, can be between 2.0% and 5.0%.

In this specification, the yield strain is the stress in the polyimide film measured in accordance with ASTM D882 - the stress in a specific tensile strength section (for example, 20 to 40 MPa) in the strain curve (Stress-Strain Curve). - the x-intercept value of a predetermined threshold (threshold slope, X ₂ ≥ X ₁ × 0.8) having at least 80 or more for the strain slope (Stress-Strain slope, X ₁ ), i.e. Tensile strain (see Figure 1). Such a yield strain is newly defined in the present invention, and is an inherent physical property due to the specific composition of the polyimide film, so it can be said that it is a feature that distinguishes it from conventional polyimide films. .

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 is 2.0% to 4.8%. can be. The yield strain (Y/S) is preferably 2.0% to 4.0%, more preferably 2.0% to 3.8%, from the viewpoint of improving bending resistance. A polyimide film that satisfies such a yield strain (Y/S) parameter is remarkably excellent in flexibility, so cracks due to repeated bending fatigue are suppressed, and excellent flexibility can be exhibited.

As another specific example, the polyimide film has a curvature radius of 1 to 10 mm based on a film thickness of 30 to 100 μm, and the initial restoration height (H _I ) after bending the film for 72 hours is It may be 5 cm or less, and the restored height (H _F ) after 1 hour 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 restored height of the film at the beginning or after 1 hour is not particularly limited, and may be, for example, 0 cm or more.

As another specific example, the polyimide film can be bent 100,000 times or more before breaking when the film is bent with a radius of curvature of 1 mm to 5 mm. In particular, when it is bent with a radius of curvature of 1 mm, the number of times of bending until breakage is 100,000 times or more, specifically 200,000 times or more, preferably 1 million (1,000,000) times or more. At this time, the upper limit of variation in the number of bends until breakage is not particularly limited, and may be, for example, 3,000,000 times or less, specifically 2,500,000 times or less.

Considering the scratch resistance and reliability of the display, the polyimide film according to the present invention preferably has high modulus and excellent mechanical properties.

As a specific example, the polyimide film has a modulus of 3 to 8 GPa, which can be 3.5 to 7 GPa in terms of exhibiting mechanical hardness and excellent flexibility at the same time. Here, modulus means a value measured according to ASTM D882. If the modulus is smaller than the above numerical value, it is difficult to exhibit sufficient hardness, and if it is larger than the above numerical value, flexibility and foldability are reduced.

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 improve the visibility of the display screen.

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

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

As long as the polyimide film according to the present invention satisfies the resilience and yield strength characteristics, there are no particular restrictions on the components and/or the composition of the polyimide resin.

For example, the polyimide film contains at least one diamine and at least one acid dianhydride and is copolymerized. It is produced by imidizing and heat-treating a polyamic acid composition containing at a high temperature.

In general, polyimide (PI) resin is prepared by solution polymerization of aromatic acid dianhydride and aromatic diamine or aromatic diisocyanate to produce a polyamic acid derivative, followed by ring closure dehydration at high temperature and imidization. It refers to a high heat-resistant resin that is Such a polyimide resin is a polymer material containing imide rings, and since the imide rings are chemically stable, it has excellent heat resistance, chemical resistance, and electrical properties. 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 particularly limited as long as it has a diamine structure in the molecule, and diamine compounds known in the art can be used without limitation. Examples thereof include aromatic, alicyclic, and aliphatic compounds having a diamine structure, and combinations thereof.

In particular, in the present invention, high resilience and mechanical properties such as yield strength, high flexibility and modulus, high transmittance and low Y.E.M. I. In consideration of optical properties such as low haze, at least one aromatic diamine can be used, or the aromatic diamine and the alicyclic diamine can be mixed.

Specific examples of the aromatic diamine include fluorine-based, sulfone-based, hydroxyl-based, ether-based, and non-fluorine-based diamines having a fluorinated substituent. Therefore, in the present invention, as the diamine compound, a fluorinated aromatic first diamine into which a fluorine substituent is introduced, a sulfone-based aromatic second diamine, a hydroxy-based aromatic third diamine, an ether-based aromatic fourth diamine, diamine and the non-fluorinated aromatic fifth diamine can be used alone, or can be used in a form of a mixture of two or more of them in appropriate combination.

Usable diamine monomers (a) include, for example, 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), bisaminohydroxyphenylhexafluoropropane (DBOH), bisaminophenoxyphenylhexafluoropropane (4BDAF), bisaminophenoxyphenylpropane (6HMDA) , bisaminophenoxydiphenylsulfone (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 a mixed form of one or more of these, but not limited thereto.

Considering the high transparency, high glass transition temperature, and low yellowness of polyimide films, 2,2'-bis(trifluoro methyl)-4,4'-diaminobiphenyl (2,2'-TFDB), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6-FAPB) can be used. Bis(4-aminophenyl)sulfone (4,4'-DDS) or 3,3'-DDS can be used as the sulfone-based second diamine. Further, as the hydroxy-based third diamine, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane: BIS-AP -AF) can be used. Further, as the ether type fourth diamine, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6-FODA) or oxydianiline (ODA) can be used. . In addition, as the 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-phenyldiamine (p-PDA) can be used.

In the diamine monomer (a) of the present invention, the fluorinated aromatic first diamine, the sulfone-based aromatic second diamine, the hydroxy-based aromatic third diamine, the ether-based aromatic fourth diamine, the non- The content of the fluorine-based aromatic fifth diamine is not particularly limited, but can be 0 to 100 mol%, specifically 10 to 90 mol%, based on 100 mol% of the total diamine. More specifically, it is 20 to 80 mol %. However, the content of any one of the first to fifth diamines is included so as to satisfy 100 mol % of all diamines.

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

In another embodiment of the present invention, at least one fluorinated first diamine can be used as the diamine monomer (a). At this time, the usage ratio of these may be 50-90:10-50 mol %, but is not limited thereto.

The alicyclic diamine is not particularly limited as long as it contains at least one alicyclic ring in its molecule. Usable alicyclic diamines include, for example, 2,2-bis(3-amino-4-hydroxycyclohexyl)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-cyclohexanedimethanamine, trans-1,2-cyclohexanedimethanamine, 1,4-cyclohexyldiamine (1,4-cyclohexyldiamine :CHDA), bis(4-aminocyclohexyl) ether, or mixtures thereof, but are not limited thereto.

In the present invention, an aromatic diamine and an alicyclic diamine can be mixed as the diamine monomer (a), and the mixing ratio of these can be appropriately adjusted in consideration of the physical properties of the polyimide film. can be done. For example, the mixing ratio of the aromatic diamine and the alicyclic diamine is 0 to 100:100 to 0 mol% with respect to 100 mol% of the total diamine, specifically 10 to 90:90 to 10 mol. % percentage.

As the acid dianhydride (b) monomer constituting the polyimide film of the present invention, compounds known in the art having an acid dianhydride structure in the molecule can be used without limitation. For example, an aromatic, alicyclic, or aliphatic compound having a dianhydride structure, or a combination thereof can be used, and specifically, at least one aromatic dianhydride is used. Alternatively, the aromatic dianhydride and the alicyclic dianhydride can be mixed.

Specific examples of the aromatic acid dianhydride include a fluorinated aromatic first acid dianhydride, a non-fluorinated aromatic second acid dianhydride, a sulfonic aromatic third acid dianhydride, and the like. is mentioned. Therefore, in the present invention, as the acid dianhydride compound, a fluorinated aromatic first acid dianhydride having a fluorine substituent introduced, a non-fluorinated aromatic second acid dianhydride, a sulfone-based aromatic second acid dianhydride, The acid dianhydrides of 3 can be used individually, or can be used in a mixed form of two or more by appropriately combining them.

The fluorinated first acid dianhydride monomer is not particularly limited as long as it is an aromatic acid dianhydride into which a fluorine substituent has been introduced. Usable fluorinated first acid dianhydrides include, for example, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanoic dianhydride (2,2-bis(3,4-dicarboxyphenyl ) hexafluoropropane dianhydride: 6-FDA), 4-(trifluoromethyl)pyromellitic dianhydride (4-(trifluoromethyl)pyromellitic dianhydride: 4-TFPMDA) and the like, but are not limited thereto. These can be used alone or in combination of two or more. Among the fluorinated acid dianhydrides, 6-FDA is a compound suitable for transparency because of its very high property of limiting the formation of charge transfer complexes (CTC) between and within molecular chains. is.

Also, the non-fluorinated second acid dianhydride monomer is not particularly limited as long as it is a non-fluorinated aromatic acid dianhydride to which no fluorine substituent has been introduced. Usable non-fluorinated third dianhydride monomers include, for example, pyromellitic dianhydride (PMDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride. substance (3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride: BPDA), benzophenonetetracarboxylic acid dianhydride (BTDA), oxydiphthalic dianhydride (ODPA), 4,4-(4,4-isopropyl Reddendiphenoxy)bis(phthalic anhydride) (BPADA), bis(3,4dicarboxyphenyl)dimethylsilane dianhydride (SiDA), and the like, but are not limited thereto. These can be used singly or in combination of two or more thereof.

Further, the sulfone-based third acid dianhydride monomer is not particularly limited as long as it is an acid dianhydride in which a sulfone group is introduced. For example, 3,3′,4,4′-diphenylsulfone Tetracarboxylic dianhydride (3,3′,4,4′-diphenylsulfonetetracarboxylic dinhydride: DSDA) can be mentioned.

In the acid dianhydride monomer (b) of the present invention, the fluorinated aromatic first acid dianhydride, the non-fluorinated aromatic second acid dianhydride, the sulfone-based aromatic third acid dianhydride, The content of anhydride is not particularly limited. For example, each of these may be contained in the range of 0 to 100 mol%, specifically 10 to 90 mol%, more specifically 20 to 80 mol%, relative to 100 mol% of the total acid dianhydride. in mol %. However, the content of at least one of the first acid dianhydride to the third acid dianhydride is contained so as to satisfy 100 mol % of the total acid dianhydride.

The alicyclic acid dianhydride is not particularly limited as long as it is a compound having an alicyclic ring instead of an aromatic ring and an acid dianhydride structure. Usable alicyclic acid dianhydrides include, for example, 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 dianhydride (TDA), 1,1'-bicyclohexane-3,3',4,4'-tetracarboxylic dianhydride (H-BPDA), 1,2 , 4,5-cyclohexane-tetracarboxylic dianhydride (H-PMDA), cyclopentanone bis-spironorbornane (cpODA), bicyclo[2.2.2]octane-2,3,5 ,6-tetracarboxy 2,3:5,6-acid dianhydrides (7CI, 8CI), or mixtures of one or more thereof, and the like, but are not limited thereto.

In the present invention, an aromatic dianhydride and an alicyclic dianhydride can be mixed as the acid dianhydride monomer (b). It can be appropriately adjusted in consideration of physical properties. For example, the mixing ratio of the aromatic dianhydride and the alicyclic dianhydride can be 0 to 100:100 to 0 mol% with respect to 100 mol% of the total acid dianhydride. Specifically, the ratio is 10-90:90-10 mol %.

In one embodiment of the present invention, the acid dianhydride (b) is a fluorinated aromatic first acid dianhydride and an alicyclic acid dianhydride, or a fluorinated aromatic second acid dianhydride. A mixture of anhydrides and non-fluorinated aromatic secondary dianhydrides can be used. At this time, the use ratio of these is not particularly limited, and may be, for example, a ratio of 10-90:90-10 mol%, specifically a ratio of 30-80:70:20 mol%.

In another specific example of the present invention, an alicyclic acid dianhydride and a non-fluorinated aromatic second acid dianhydride can be used in combination as the acid dianhydride (b). At this time, the usage ratio of these may be 30-80:20-70 mol %, but is not limited thereto.

As another preferred example of the present invention, at least one non-fluorinated aromatic second acid dianhydride can be used in combination as the acid dianhydride (b). At this time, the usage ratio of these may be 50-80:20-50 mol %, but is not 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) and the number of moles of the acid dianhydride component (b) is 0.7 to It can be 1.3, preferably in the range from 0.8 to 1.2, more preferably from 0.9 to 1.1.

In the polyamic acid composition of the present invention, any organic solvent known in the art can be used without limitation as a solvent for solution polymerization reaction of the monomers. Solvents that can be used include, for example, m-cresol, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), acetone, diethylacetate, and dimethylphthalate. (DMP) can be used. Alternatively, low boiling solutions such as tetrahydrofuran (THF), chloroform, or solvents such as γ-butyrolactone can be used. At this time, the content of the solvent (first solvent for polymerization) is not particularly limited, but in order to obtain an appropriate molecular weight and viscosity of the polyamic acid composition (polyamic acid solution) 50 to 95% by weight, more preferably 70 to 90% by weight.

The polyamic acid composition can be prepared by adding at least one acid dianhydride and at least one diamine to an organic solvent and then reacting them. Diamine (a) and dianhydride (b) can be adjusted so that the equivalence ratio is approximately 1:1. The composition of such a polyamic acid composition is not particularly limited. ˜25.0% by weight, and the balance of the composition being 100% by weight of the organic solvent. Here, the content of the organic solvent can be 70-90% by weight. The polyamic acid composition contains 30 to 70% by weight of acid dianhydride and 30 to 70% by weight of diamine with respect to 100% by weight of the solid content, but is not limited thereto.

Polyamic acid compositions constructed as described above may have viscosities ranging from about 1,000 to 200,000 cps, preferably from about 5,000 to 50,000 cps. When the viscosity of the polyamic acid composition is within the above range, the thickness can be easily controlled and a uniform coating surface can be obtained when the polyamic acid composition is coated.

If necessary, the polyamic acid composition contains at least one of a plasticizer, an antioxidant, a flame retardant, a dispersant, a viscosity modifier, a leveling agent, etc., as long as the objects and effects of the present invention are not significantly impaired. can be added in small amounts.

The polyimide resin film according to the present invention can be produced according to a conventional method well known in the art, for example, after coating (casting) the polyamic acid composition on a substrate such as a glass substrate , while gradually increasing the temperature in the range of 30 to 350° C. for 0.5 to 8 hours to induce an imide ring closure reaction (imidazation).

As for the coating method, any conventional method known in the art can be used without limitation, for example, spin coating method, dip coating method, solvent casting method, slot die coating method. It can be carried out by at least one method selected from the group consisting of coating (slot die coating) and spray coating. The polyamic acid composition may be coated at least once so that the colorless and transparent polyimide resin layer has a thickness of several hundred nanometers to several tens of micrometers.

In the method for producing a polyimide film according to the present invention, the imidization method applied to the step of casting the polymerized polyamic acid onto the support for imidization includes a thermal imidization method, a chemical imidization method, or a thermal imidization method. The chemical imidization method and the chemical imidization method can be used in combination.

In the thermal imidization method, a polyamic acid composition (polyamic acid solution) is cast on a support, and the temperature is gradually increased in the temperature range of 3 to 400° C. while heating for 1 to 10 hours to obtain a polyimide film. is.

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

More specifically, when the thermal imidization method and the chemical imidization method are used in combination, a dehydrating agent and an imidization catalyst are added to the polyamic acid composition, and after casting on a support, the temperature is adjusted to 80 to 300°C. After partial curing and drying by heating, preferably at 150-250° C., to activate the dehydrating agent and the imidization catalyst, a polyimide film is obtained.

The thickness of the polyimide film thus formed is not particularly limited and can be appropriately adjusted according to the field of application. For example, it can range from 10 to 150 μm, preferably from 30 to 100 μm.

The polyimide film of the present invention manufactured as described above and variations thereof can be usefully used in various fields requiring high resilience, high bending properties, and excellent optical properties. In particular, by being applied to the cover window of the display device, it is possible to prevent the surface from being scratched and to provide the flexible display device with excellent flexibility and visibility.

In the present invention, the display device refers to a flexible display device or a non-flexible display device for displaying images, and includes not only a flat panel display device (FPD) but also a curved display device, Foldable display devices, flexible display devices, foldable mobile phones, smart phones, mobile communication terminals, tablet PCs, and the like are included. Specifically, the display device includes a liquid crystal display, an electrophoretic display, an organic light emitting display, an inorganic EL display, and a field emission display. Field emission displays, surface-conduction electron-emitter displays, plasma displays, cathode ray displays, electronic paper and the like. A specific example may be a flat panel display such as LCD, PDP, OLED, or the like. The polyimide film of the present invention is not limited to the applications described above, and can be applied to display devices well known in the art, and can also be used as substrates for flexible display devices and protective films.

A specific example of a display device including the 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 the polyimide film according to an embodiment of the present invention. can include Here, each component constituting the display device is not particularly limited, and may include components known in the art.

The present invention will be described in detail below based on specific examples. The examples described below are merely illustrations for helping understanding of the present invention, and the scope of the present invention is not limited to these examples.

[Examples 1 to 4. Production of polyimide film]
A polyamic acid composition was produced using a composition containing a diamine and an acid dianhydride shown in Table 1 below.

The polyamic acid composition was coated on an LCD glass plate using a bar coater, and then subjected to imidazation in a nitrogen atmosphere convection oven at 80° C. for 30 minutes and 150° C. for 30 minutes. ) was performed. As a result, a polyimide film having a film thickness of 80 μm and an imidization rate of 85% or more was obtained. Next, a polyimide film was separated from the glass plate.

[Comparative Examples 1 to 4. Production of polyimide film]
Polyimide films of Comparative Examples 1 to 4 were produced in the same manner as in Examples 1 to 4, except that the compositions shown in Table 1 were used.

[Experimental example. Evaluation of the physical properties]
The physical properties of the polyimide resin films produced in Examples 1-4 and Comparative Examples 1-4 were evaluated by methods described below, and the results are shown in Table 2 below. Each physical property in Table 2 below is based on a thickness of 50 μm.

<Physical property evaluation method>
1) Measurement of light transmittance Measurement was performed using a UV-Vis NIR spectrophotometer (manufactured by Shimadzu, model number: UV-3150) at a wavelength of 550 nm.

2) Measurement of yellowness The yellowness at 500 nm was measured according to ASTM E131-73 standard using a spectrophotometer (manufactured by Konica Minolta, model number: CM-3600A).

3) Measurement of thickness The thickness of the film was measured using a thickness gauge (manufactured by Mitutoyo, model number: 547-401).

4) Measurement of Modulus UTM (manufactured by Instron, model number: 5942) was used to measure tensile strength (MPa) and elastic modulus (GPa) according to ASTM D882 standards.

5) Measurement of yield strain (Yield Strain) Using UTM (manufactured by Instron, model number: 5942), stress in the section of 20 to 40 MPa in accordance with ASTM D882 standards - Gradient of strain curve (Stress-Strain slope threshold, X ₁ ) as a reference, the strain Strain corresponding to a threshold (X ₂ ≧X ₁ ×0.8) that matches 80% or more (x value) was measured.

6) Resilience Resilience was calculated by the following formula using the modulus and yield strength. At this time, the yield strength is Yield/Strength of the yield point and was calculated in the same manner as the yield strain.

（式中、σは、降伏強度であり、Ｅは、モジュラスである。）

(Where σ is the yield strength and E is the modulus.)

7) Measurement of bending characteristics (dynamic folding cycle) The bending characteristics are measured using a planar body unloaded U-shaped stretching tester (DLDMLS-FS, manufactured by YUASA), and the folding cycle is performed by adjusting the curvature radius to 1 mm to 5 mm. (Folding cycle) was measured.

8) Measurement of restoring force A prepared sample was fixed in a static folding holder (curvature radius of 1 to 10 nm) and allowed to stand for 72 hours, then the sample was taken out and the height change of the film over time was measured. and confirmed the resilience.

As shown in Table 2, the polyimide film according to the present invention exhibits higher resilience, yield strength, and yield strain characteristics than the comparative examples, and simultaneously exhibits excellent restoring force and high flexibility. I understand. It was also found to have excellent transparency, mechanical properties and coefficient of thermal expansion. Therefore, it was confirmed that the polyimide film of the present invention can be usefully applied to the cover window of the display device.

Claims (8)

A polyimide film produced by imidizing a polyamic acid composition containing a fluorinated aromatic primary diamine and at least two acid dianhydrides,
The fluorinated aromatic primary diamines are 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (2,2′-TFDB) and 1,4-bis(4-amino-2- trifluoromethylphenoxy)benzene (6-FAPB) in an amount of 70 to 100 mol% based on 100 mol% of all diamines;
The at least two acid dianhydrides are
at least two non-fluorinated aromatic secondary dianhydrides, or
a non-fluorinated aromatic second dianhydride and a fluorinated aromatic first dianhydride or an alicyclic quaternary dianhydride ;
The content of the alicyclic fourth acid dianhydride is 40 mol% or less based on 100 mol% of the acid dianhydride,
Yield strength measured according to ASTM D882 at a thickness of 30 to 100 μm is 50 to 200 MPa,
A polyimide film having a resilience of 2.0 to 5.0 MPa according to the following formula 1.

(Where σ is the yield strength and E is the modulus.) 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 after at least 1 hour has passed. 2. The polyimide film of claim 1, wherein the height (H _F ) is 3 cm or less. Defined as the threshold strain corresponding to at least 80% of the slope of the tensile strength interval of 20 to 40 MPa in the stress-strain curve of the film measured in accordance with ASTM D882. The polyimide film according to claim 1, which has a yield strain (Y/S) of 2.0% to 5.0%. 2. The polyimide film according to claim 1, wherein when the film is bent with a radius of curvature of 1 to 5 mm, the number of times of bending is 100,000 or more until it breaks. 2. The polyimide film according to claim 1, which has a modulus of 3 to 8 GPa as measured according to ASTM D882. At a thickness of 30 to 100 μm, the light transmittance at a wavelength of 550 nm is 85% or more,
2. The polyimide film according to claim 1, which has a yellowness index of 10 or less according to the ASTM E313-73 standard. The polyimide film according to claim 1, wherein the molar ratio (a/b) of the diamine (a) and the acid dianhydride (b) is in the range of 0.7 to 1.3. The polyimide film according to claim 1, which is used for a cover window of a display device.

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