Optical Film

Abstract

To provide an optical film that has excellent transparency and folding resistance.SOLUTION: The optical film includes a polyimide resin. The polyimide resin includes a structure represented by formula (3) and has a weight-average molecular weight of at least 160,000.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an optical film and a polyimide resin used as materials for flexible display devices, and a flexible display device including the optical film. [Background technology]

[0002] Display devices such as liquid crystal displays (LCDs) and organic electroluminescence (EL) displays (OLEDs) are widely used in a variety of applications, including mobile phones and smartwatches. Glass has traditionally been used as the front panel for these devices, but its rigidity and fragility make it difficult to use as the front panel material for flexible display devices. Optical films with high heat resistance, such as those made from polymers such as polyimide resins, are being considered as an alternative to glass. Patent Document 1 discloses a polyimide resin having, as a monomer component, an ester-type tetracarboxylic acid dianhydride formed by esterifying biphenyl-4,4'-diols with two trimellitic acids, and a polyimide film formed from the polyimide resin. Polyimide films formed from such polyimide resins are advantageous in that they have high heat resistance and a low coefficient of linear thermal expansion. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2014 / 046180 Summary of the Invention [Problem to be solved by the invention]

[0004] Front panel films, which are materials for flexible display devices, often play a role in protecting the internal components of the flexible display device, and are therefore often required to be thicker to cushion against external impacts. However, polyimide resins such as those described in Patent Document 1 contain structures derived from rigid ester-type tetracarboxylic dianhydrides, and the resulting polyimide films lack sufficient transparency. In particular, as the thickness increases, the haze increases and the transmittance decreases significantly, making it impossible to ensure the transparency required for flexible display device materials. Furthermore, optical films used as materials for flexible display devices are also required to have excellent folding endurance.

[0005] Therefore, an object of the present invention is to provide an optical film and polyimide resin having excellent transparency and folding resistance, and a flexible display device including the optical film. [Means for solving the problem]

[0006] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by including a structure represented by formula (3) as Y in formula (1) in an optical film containing a polyimide-based resin and adjusting the weight-average molecular weight of the polyimide-based resin, and have thus completed the present invention. That is, the present invention includes the following preferred embodiments.

[0007] [1] An optical film containing a polyimide resin, The polyimide resin has the formula (1): [ka] [In formula (1), X represents a divalent organic group, Y represents a tetravalent organic group; * represents a bond. Y in formula (1) is a structural unit represented by formula (3): [ka] [In formula (3), R 1each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; R 2 ~R 5 each independently represents a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom; m's each independently represent an integer of 0 to 3; n represents an integer of 1 to 4; * represents a bond, except for R 2 ~R 5 In at least one benzene ring having R 2 ~R 5 at least one of which is a monovalent hydrocarbon group which may have a halogen atom. and having a weight average molecular weight of 160,000 or more. [2] The optical film according to [1], wherein X in formula (1) contains at least one of a divalent aromatic group, a divalent alicyclic group, and a divalent aliphatic group. [3] As X in formula (1), formula (4): [ka] [In formula (4), A represents a single bond, —O—, a diphenylmethylene group, a divalent hydrocarbon group which may have a halogen atom, —SO—, —S—, —CO—, —PO—, —PO—, —N(R A1 )- or -Si(R A2 )2-, R A1 and R A2 each independently represents a hydrogen atom or an alkyl group which may have a halogen atom; R 6 each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; s's each independently represent an integer of 0 to 4; * represents a bond. The optical film according to [1] or [2], which contains a structure represented by the following formula: [4] The optical film according to any one of [1] to [3], which has a thickness of 35 μm or more. [5] The optical film according to any one of [1] to [4], which has a total light transmittance of 85% or more. [6] The optical film according to any one of [1] to [5], which has a yellowness index of 3.0 or less. [7] The optical film according to any one of [1] to [6], which has an elastic modulus of 3.5 GPa or more. [8] The optical film according to any one of [1] to [7], which is a film for a front panel of a flexible display device. [9] A flexible display device comprising the optical film according to any one of [1] to [8].

[10] The flexible display device according to [9], further comprising a touch sensor.

[11] The flexible display device according to [9] or

[10] , further comprising a polarizing plate.

[12] Formula (1): [ka] [In formula (1), X represents a divalent organic group, Y represents a tetravalent organic group; * represents a bond. Y in formula (1) is a structural unit represented by formula (3): [ka] [In formula (3), R 1 each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; R 2 ~R 5 each independently represents a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom; m's each independently represent an integer of 0 to 3; n represents an integer of 1 to 4; * represents a bond, except for R 2 ~R 5 In at least one benzene ring having R 2~R 5 at least one of which is a monovalent hydrocarbon group which may have a halogen atom. and having a weight average molecular weight of 160,000 or more. [Effects of the Invention]

[0008] The optical film of the present invention has excellent transparency and folding resistance, and can therefore be suitably used as a material for flexible display devices. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Optical Film] The optical film of the present invention contains a polyimide resin. <Polyimide resin> The polyimide resin contained in the optical film of the present invention is a polyimide resin represented by the formula (1): [ka] [In formula (1), X represents a divalent organic group, Y represents a tetravalent organic group; * represents a bond. It contains a structural unit represented by:

[0010] In formula (1), Y's each independently represent a tetravalent organic group, preferably a tetravalent organic group having 4 to 80 carbon atoms, and more preferably a tetravalent organic group having 4 to 60 carbon atoms and a cyclic structure. Examples of the cyclic structure include an alicyclic, aromatic ring, and heterocyclic structure. The organic group is an organic group in which a hydrogen atom in the organic group may be substituted with a substituent, and the substituent is preferably a halogen atom, a hydrocarbon group (e.g., an alkyl group, an aryl group, etc.) that may have a halogen atom, an alkoxy group, or an aryloxy group that may have a halogen atom, and in such cases, the hydrocarbon group, alkoxy group, or aryloxy group that may have a halogen atom preferably has 1 to 8 carbon atoms. A polyimide-based resin that is one embodiment of the present invention may contain multiple types of Y's, and the multiple types of Y's may be the same or different from one another.

[0011] The polyimide resin of the present invention is a polyimide resin represented by the formula (3): [ka] [In formula (3), R 1 each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; R 2 ~R 5 each independently represents a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom; m's each independently represent an integer of 0 to 3; n represents an integer of 1 to 4, * represents a bond, except for R 2 ~R 5 In at least one benzene ring having R 2 ~R 5 at least one of which is a monovalent hydrocarbon group which may have a halogen atom. and has a weight average molecular weight of 160,000 or more.

[0012] The present inventors have found that in a polyimide resin containing a structure represented by formula (3) as Y in formula (1), as described in Patent Document 1, the transparency of the resulting optical film is insufficient, particularly when the film is thick. Furthermore, they have also found that a reaction solution for polymerizing a polyimide resin having such a structure has high viscosity, making it difficult to proceed with polymerization, and therefore it is difficult to increase the molecular weight of the polyimide resin. The present inventors have succeeded in increasing the weight-average molecular weight of the polyimide resin to 160,000 or more by optimizing the production conditions of the resin, and have unexpectedly found that this leads to a significant improvement in the transparency of optical films containing the polyimide resin, as well as an improvement in the folding endurance of the optical films.

[0013] Therefore, an optical film containing the polyimide resin of the present invention can have excellent transparency and folding resistance. On the other hand, if the weight-average molecular weight of the polyimide resin contained in the optical film is less than 160,000, the optical film tends to have insufficient transparency and folding resistance.

[0014] The weight-average molecular weight (hereinafter sometimes abbreviated as Mw) of the polyimide resin in the present invention is 160,000 or more, preferably 180,000 or more, more preferably 200,000 or more, even more preferably 250,000 or more, even more preferably 300,000 or more, particularly preferably 350,000 or more, and preferably 1,000,000 or less, more preferably 800,000 or less, even more preferably 700,000 or less, particularly preferably 600,000 or less. When the Mw of the polyimide resin is above the above-mentioned lower limit, the transparency, elastic modulus, and folding endurance of the resulting optical film are more likely to be improved. When the Mw is below the above-mentioned upper limit, gelation of the resin varnish is more likely to be suppressed, and the optical properties of the resulting optical film are more likely to be improved. The weight-average molecular weight can be determined, for example, by gel permeation chromatography (hereinafter sometimes abbreviated as GPC) measurement in terms of standard polystyrene, for example, by the method described in the Examples.

[0015] In equation (3), R 1are each independently a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom. Examples of halogen atoms include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Examples of alkyl groups include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, a 2-methylbutyl group, a 3-methylbutyl group, a 2-ethylpropyl group, and an n-hexyl group. Examples of alkoxy groups include a methoxy group, an ethoxy group, a propyloxy group, an isopropyloxy group, an n-butoxy group, an isobutoxy group, a sec-butoxy group, a tert-butoxy group, a pentyloxy group, a hexyloxy group, and a cyclohexyloxy group. Examples of aryl groups include a phenyl group, a tolyl group, a xylyl group, a naphthyl group, and a biphenyl group. Examples of the aryloxy group include a phenoxy group, a naphthyloxy group, and a biphenyloxy group. 1 are each independently preferably a halogen atom, an alkyl group having 1 to 6 carbon atoms which may have a halogen atom, an alkoxy group having 1 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, or an aryloxy group having 6 to 12 carbon atoms. From the viewpoint of easily improving the transparency, elastic modulus, and folding resistance of the optical film, R 1 are preferably, independently of each other, an alkyl group or an alkoxy group which may have a halogen atom, and more preferably an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms which may have a halogen atom.

[0016] In formula (3), m's each independently represent an integer of 0 to 3, preferably an integer of 0 to 2, more preferably 0 or 1, and even more preferably 0, from the viewpoint of easily increasing the transparency, elastic modulus, and folding endurance of the optical film.

[0017] In equation (3), R 2 , R 3 , R 4 and R 5 each independently represents a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom; R2 ~R 5 In at least one benzene ring having R 2 ~R 5 At least one of the groups is a monovalent hydrocarbon group which may have a halogen atom. Examples of the hydrocarbon group include aromatic hydrocarbon groups, alicyclic hydrocarbon groups, and aliphatic hydrocarbon groups. Examples of the aromatic hydrocarbon group include aryl groups such as phenyl, tolyl, xylyl, naphthyl, and biphenyl. Examples of the alicyclic hydrocarbon group include cycloalkyl groups such as cyclopentyl and cyclohexyl. Examples of the aliphatic hydrocarbon group include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, 2-methylbutyl, 3-methylbutyl, 2-ethylpropyl, n-hexyl, n-heptyl, n-octyl, tert-octyl, n-nonyl, and n-decyl. Examples of the halogen atom include those described above. R 2 ~R 5 are each independently preferably a hydrogen atom or an aryl group having 6 to 12 carbon atoms, a cycloalkyl group having 4 to 8 carbon atoms, or an alkyl group having 1 to 6 carbon atoms, each of which may have a halogen atom. From the viewpoint of easily improving the solubility of the resin in a solvent and the transparency, elastic modulus, and folding resistance of the optical film, R 2 ~R 5 are each independently preferably a hydrogen atom or an alkyl group which may have a halogen atom, more preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms which may have a halogen atom, and even more preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms which may have a halogen atom. From the viewpoint of easily improving the solubility of the resin in a solvent and the transparency, elastic modulus, and folding endurance of the optical film, R 2 ~R 5 In at least one benzene ring having R 2 ~R 5 At least two of R are preferably monovalent hydrocarbon groups which may have a halogen atom, 2 ~R 5It is more preferable that at least three of the groups be monovalent hydrocarbon groups which may have a halogen atom.

[0018] In formula (3), n represents an integer of 1 to 4, and from the viewpoint of easily improving the transparency, elastic modulus, and folding endurance of the optical film, n is preferably an integer of 1 to 3, more preferably 2 or 3, and even more preferably 2. The constitutional unit represented by formula (1) may contain one or more structures represented by formula (3) as Y.

[0019] In a preferred embodiment of the present invention, formula (3) is represented by formula (3'): [ka] [In formula (3'), * represents a bond] That is, at least some of the multiple Ys in formula (1) are represented by formula (3'). In this form, the transparency, elastic modulus, and folding endurance of the optical film are likely to be improved.

[0020] In one embodiment of the present invention, the proportion of the structural units represented by formula (1) in which Y is represented by formula (3) is preferably 30 mol% or more, more preferably 50 mol% or more, and even more preferably 70 mol% or more, relative to the total molar amount (100 mol%) of the structural units represented by formula (1). When the proportion of the structural units represented by formula (3) in which Y is represented by formula (3) is at least the above lower limit, the transparency, elastic modulus, and folding endurance of the optical film are likely to be improved. The upper limit of the proportion of the structural units represented by formula (3) is 100 mol% or less. The proportion of the structural units represented by formula (3) in which Y is represented by formula (3) can be, for example, 1 It can be measured using H-NMR, or can be calculated from the ratio of raw materials used.

[0021] The polyimide resin of the present invention further includes a compound represented by the formula (1), where Y is a compound represented by the formula (20), the formula (21), the formula (22), the formula (23), the formula (24), the formula (25), the formula (26), the formula (27), the formula (28), or the formula (29): [ka] It may include a structure represented by:

[0022] In formulas (20) to (29), * represents a bond, and W 1 represents a single bond, -O-, a diphenylmethylene group, a divalent hydrocarbon group which may have a halogen atom, such as -CH2-, -CH2-CH2-, -CH(CH3)-, -C(CH3)2-, -C(CF3)2-, -Ar-, -SO2-, -S-, -CO-, -PO-, -PO2-, -N(R W1 )- or -Si(R W2 )2-, -O-Ar-O-, -Ar-O-Ar-, -Ar-CH2-Ar-, -Ar-C(CH3)2-Ar-, or Ar-SO2-Ar-. Ar represents an arylene group having 6 to 20 carbon atoms which may have a fluorine atom, and a specific example is a phenylene group. R W1 and R W2 are each independently a hydrogen atom or an alkyl group which may have a halogen atom. The hydrogen atoms in the groups represented by formulas (20) to (29) may be groups substituted with a methyl group, a fluoro group, a chloro group, or a trifluoromethyl group; and may be tetravalent chain hydrocarbon groups having 6 or less carbon atoms. The hydrogen atoms on the rings in formulas (20) to (29) may be substituted with an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms. The alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms, and the aryl group having 6 to 12 carbon atoms are each substituted with R in formula (3). 1 Examples of such a substance include those exemplified above.

[0023] Among the groups represented by formulas (20) to (29), from the viewpoint of easily improving the transparency, elastic modulus, and folding endurance of the optical film, groups represented by formula (26), formula (28), or formula (29) are preferred, and groups represented by formula (26) are more preferred. 1From the viewpoint of easily improving the transparency, elastic modulus, and folding endurance of the optical film, preferably represents a single bond, -O-, -CH2-, -CH2-CH2-, -CH(CH3)-, -C(CH3)2-, or C(CF3)2-, more preferably a single bond, -O-, -CH2-, -CH(CH3)-, -C(CH3)2-, or C(CF3)2-, even more preferably a single bond, -C(CH3)2-, or C(CF3)2-, particularly preferably a single bond or C(CF3)2-, and most preferably -C(CF3)2-.

[0024] In a preferred embodiment of the present invention, formula (26) is a compound represented by formula (5): [ka] [In formula (5), B represents a single bond, —O—, a diphenylmethylene group, a divalent hydrocarbon group which may have a halogen atom, —SO—, —S—, —CO—, —COO—, —PO—, —PO—, or —N(R B1 )- or -Si(R B2 )2-, R B1 and R B2 each independently represents a hydrogen atom or an alkyl group which may have a halogen atom; R 7 each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; t's each independently represent an integer of 0 to 3; * represents a bond. When the polyimide resin further contains a structure represented by formula (5) as Y in formula (1), the solubility of the resin in a solvent, and the transparency, elastic modulus, and folding endurance of the optical film are likely to be improved.

[0025] In equation (5), R 7 are each independently a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom. The halogen atom, the alkyl group, the alkoxy group, the aryl group, and the aryloxy group which may have a halogen atom are each R in formula (3).1 From the viewpoint of transparency, elastic modulus and folding endurance of the optical film, R 7 are each independently preferably an alkyl group having 1 to 6 carbon atoms which may have one or more halogen atoms, and more preferably an alkyl group having 1 to 3 carbon atoms which may have one or more halogen atoms.

[0026] In formula (5), t represents an integer of 0 to 3, and preferably represents an integer of 0 to 2, more preferably represents 0 or 1, and even more preferably 0, from the viewpoint of easily increasing the transparency, elastic modulus, and folding endurance of the optical film.

[0027] In formula (5), B's each independently represent a single bond, -O-, a diphenylmethylene group, a divalent hydrocarbon group which may have a halogen atom, -SO2-, -S-, -CO-, -COO-, -PO2-, -PO2-, or -N(R B1 )- or -Si(R B2 )2-, R B1 and R B2 each independently represents a hydrogen atom or an alkyl group which may have a halogen atom.

[0028] The divalent hydrocarbon group which may have a halogen atom includes R 2 ~R 5 Among the monovalent hydrocarbon groups which may have a halogen atom in the formula (5), a divalent group in which one hydrogen atom has been further removed can be mentioned. The divalent hydrocarbon group which may have a halogen atom can form a ring by replacing two of the hydrogen atoms contained in the group, that is, the two hydrogen atoms can be replaced with bonds and the two bonds can be linked to form a ring, and examples of such rings include cycloalkane rings having 3 to 12 carbon atoms. In addition, -N(R B1 )- and -Si(R B2 )2-R B1 and R B2 The alkyl group which may have a halogen atom in the formula (3) is R 1 Examples of the alkyl group which may have a halogen atom include those exemplified above.

[0029] As B in formula (5), from the viewpoint of easily improving the transparency, elastic modulus, and folding resistance of the optical film, a single bond or a divalent hydrocarbon group which may have a halogen atom is preferred, a single bond, -CH2-, -CH2-CH2-, -CH(CH3)-, -C(CH3)2-, or -C(CF3)2- is more preferred, a single bond, -C(CH3)2-, or -C(CF3)2- is even more preferred, a single bond or -C(CF3)2- is even more preferred, and -C(CF3)2- is particularly preferred.

[0030] In a preferred embodiment of the present invention, formula (5) is represented by formula (5'): [ka] [In formula (5'), * represents a bond] That is, at least some of the multiple Ys in formula (1) are represented by formula (5'). In this form, the transparency, elastic modulus, and folding endurance of the optical film are likely to be improved.

[0031] In one embodiment of the present invention, when Y in formula (1) contains a structure represented by formula (5), the proportion of the structural units represented by formula (5) in which Y is the structural unit represented by formula (5) is preferably 5 mol% or more, more preferably 10 mol% or more, even more preferably 15 mol% or more, even more preferably 20 mol% or more, particularly preferably 25 mol% or more, and even more particularly preferably 30 mol%, and is preferably 80 mol% or less, more preferably 70 mol% or less, and even more preferably 60 mol% or less, relative to the total molar amount of the structural units represented by formula (1). When the proportion of the structural units represented by formula (5) in which Y is the structural unit represented by formula (5) is within the above range, the transparency, elastic modulus, and folding endurance of the optical film are likely to be improved. The proportion of the structural units represented by formula (5) in which Y is the structural unit represented by formula (5) can be, for example, 1 It can be measured using H-NMR, or can be calculated from the ratio of raw materials used.

[0032] In one embodiment of the present invention, when Y in formula (1) contains a structure represented by formula (5), the total proportion of the structural unit in which Y is represented by formula (3) and the structural unit in which Y is represented by formula (5) is preferably 50 mol% or more, more preferably 70 mol% or more, even more preferably 90 mol% or more, and preferably 100 mol% or less, relative to the total molar amount of the structural units represented by formula (1). When this total proportion is within the above range, the transparency, elastic modulus, and folding endurance of the optical film are likely to be improved. Note that this total proportion can be, for example, 1 It can be measured using H-NMR, or can be calculated from the ratio of raw materials used.

[0033] In formula (1), X represents a divalent organic group, preferably a divalent organic group having 4 to 40 carbon atoms.

[0034] In order to easily improve the transparency, modulus of elasticity, and folding resistance of the optical film, the polyimide resin in the present invention preferably contains, as X in formula (1), at least one of a divalent aromatic group, a divalent alicyclic group, and a divalent aliphatic group, and more preferably contains a divalent aromatic group. Examples of the divalent aromatic group include, for example, R 2 ~R 5 a divalent aromatic hydrocarbon group in which one hydrogen atom among the hydrogen atoms in the aromatic hydrocarbon group exemplified above is replaced by a bond; and at least one of the divalent aromatic hydrocarbon groups is connected to a linking group, for example, V 1 Examples of the divalent alicyclic group include groups bonded via a linking group such as R 2 ~R 5 a divalent alicyclic hydrocarbon group in which one hydrogen atom among the hydrogen atoms in the alicyclic hydrocarbon group exemplified above is replaced by a bond; at least one of the divalent alicyclic hydrocarbon groups is connected to a linking group, for example, 1 Examples of the divalent aliphatic group include groups bonded via a linking group such as R 2 ~R 5a divalent aliphatic hydrocarbon group in which one hydrogen atom among the hydrogen atoms in the aliphatic hydrocarbon group exemplified above is replaced by a bond; at least one of the divalent aliphatic hydrocarbon groups is connected to a linking group, for example, V 1 Examples of such groups include groups bonded via a linking group such as the above.

[0035] X in formula (1) preferably represents a divalent organic group having 4 to 40 carbon atoms and a cyclic structure such as an alicyclic, aromatic, or heterocyclic structure, more preferably a divalent aromatic group having 4 to 40 carbon atoms or a divalent alicyclic group having 4 to 40 carbon atoms, and even more preferably a divalent aromatic group having 4 to 40 carbon atoms. The organic group may have a hydrogen atom substituted with a hydrocarbon group or a fluorine-substituted hydrocarbon group, and in such a case, the hydrocarbon group or the fluorine-substituted hydrocarbon group preferably has 1 to 8 carbon atoms. In one embodiment of the present invention, the polyimide resin of the present invention may contain multiple types of X, which may be the same or different. Examples of X include groups represented by formula (10), formula (11), formula (12), formula (13), formula (14), formula (15), formula (16), formula (17) and formula (18); groups in which hydrogen atoms in the groups represented by formulas (10) to (18) are substituted with methyl groups, fluoro groups, chloro groups or trifluoromethyl groups; and chain hydrocarbon groups having 6 or less carbon atoms.

[0036] [ka]

[0037] In formulas (10) to (18), * represents a bond, V 1 , V 2 and V 3 are each independently a single bond, -O-, -S-, -CH2-, -CH2-CH2-, -CH(CH3)-, -C(CH3)2-, -C(CF3)2-, -SO2-, -CO-, or -N(Q)-. Here, Q represents a monovalent hydrocarbon group having 1 to 12 carbon atoms which may have a halogen atom. Examples of the monovalent hydrocarbon group having 1 to 12 carbon atoms which may have a halogen atom include R in formula (3). 2~R 5 Examples of the monovalent hydrocarbon group which may have a halogen atom in the formula (I) include those exemplified above. One example is V 1 and V 3 is a single bond, -O- or -S-, and V 2 is -CH2-, -C(CH3)2-, -C(CF3)2- or -SO2-. 1 and V 2 The bonding positions of each ring and V 2 and V 3 The bonding positions of the rings are preferably, independently of one another, meta or para positions, more preferably para positions, relative to each ring. In addition, the hydrogen atoms on the rings in formulas (10) to (18) may be substituted with an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms. The alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms, and the aryl group having 6 to 12 carbon atoms are respectively R in formula (3). 1 Examples of such a substance include those exemplified above.

[0038] In a preferred embodiment of the present invention, the polyimide resin of the present invention further comprises a compound represented by the formula (4): [ka] [In formula (4), A represents a single bond, —O—, a diphenylmethylene group, a divalent hydrocarbon group which may have a halogen atom, —SO—, —S—, —CO—, —PO—, —PO—, —N(R A1 )- or -Si(R A2 )2-, R A1 and R A2 each independently represents a hydrogen atom or an alkyl group which may have a halogen atom; R 6 each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; s's each independently represent an integer of 0 to 4; * represents a bond. The structural unit represented by formula (1) may contain one or more structures represented by formula (4) as X. In this embodiment, the transparency, elastic modulus, and folding endurance of the optical film are easily improved.

[0039] R 6 are each independently a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom. The halogen atom, the alkyl group, the alkoxy group, the aryl group, and the aryloxy group which may have a halogen atom are each R in formula (3). 1 Examples of such a substance include those exemplified above.

[0040] Among these, R 6 are each independently preferably an alkyl group having 1 to 6 carbon atoms or a halogenated alkyl group having 1 to 6 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms, and even more preferably a perfluoroalkyl group, from the viewpoint of easily improving the transparency, elastic modulus, and folding resistance of the optical film. 6 are each independently a methyl group, a chloro group, or a trifluoromethyl group. s's are each independently an integer of 0 to 4, and from the viewpoint of easily improving the transparency, elastic modulus, and folding endurance of the optical film, preferably represent an integer of 1 to 3, more preferably represent 1 or 2, and even more preferably represent 1. In a preferred embodiment of the present invention, in each benzene ring, s is 1, and R 6 is substituted and R 6 is preferably a methyl group, a fluoro group, a chloro group or a trifluoromethyl group.

[0041] In formula (4), the positions of the bonds are preferably meta or para, more preferably para, based on -A-, from the viewpoint of easily increasing the transparency, elastic modulus, and folding endurance of the optical film.

[0042] In formula (4), each A independently represents a single bond, -O-, a diphenylmethylene group, a divalent hydrocarbon group which may have a halogen atom, -SO2-, -S-, -CO-, -PO2-, -PO2-, or -N(R A1 )- or -Si(R A2 )2-, R A1 and R A2 each independently represents a hydrogen atom or an alkyl group which may have a halogen atom. The divalent hydrocarbon group which may have a halogen atom is R in formula (3). 2 ~R 5 Among the monovalent hydrocarbon groups which may have a halogen atom in the above formula (4), a divalent group in which one hydrogen atom has been further removed can be mentioned. The divalent hydrocarbon group which may have a halogen atom can form a ring by replacing two of the hydrogen atoms contained in the group, that is, the two hydrogen atoms can be replaced with bonds and the two bonds can be linked to form a ring, and examples of such rings include cycloalkane rings having 3 to 12 carbon atoms. Furthermore, -N(R A1 )- and -Si(R A2 )2-R A1 and R A2 The alkyl group which may have a halogen atom in the formula (3) is R 1 Examples of the alkyl group which may have a halogen atom include those exemplified above.

[0043] In terms of easily improving the transparency, elastic modulus, and folding resistance of the optical film, A in formula (3) is preferably a single bond, -CH2-, -CH2-CH2-, -CH(CH3)-, -C(CH3)2-, or -C(CF3)2-, more preferably a single bond, -C(CH3)2-, or -C(CF3)2-, even more preferably a single bond or -C(CF3)2-, and particularly preferably a single bond.

[0044] In a preferred embodiment of the present invention, in order to easily improve the transparency, elastic modulus, and folding endurance of the optical film, R 6are each independently a halogenated alkyl group having 1 to 6 carbon atoms, s is 1 or 2, and A is a single bond, -C(CH3)2- or -C(CF3)2-.

[0045] In a preferred embodiment of the present invention, formula (4) is represented by formula (4'): [ka] That is, at least a part of the multiple X's in formula (1) are represented by formula (4'). In this form, the transparency, elastic modulus, and folding endurance of the optical film are easily improved. The structural unit represented by formula (1) may contain one or more structures represented by formula (4') as X.

[0046] In one embodiment of the present invention, when X in formula (1) contains a structure represented by formula (4), the proportion of structural units represented by formula (4) in which X is the structural unit is preferably 30 mol% or more, more preferably 50 mol% or more, even more preferably 70 mol% or more, and preferably 100 mol% or less, relative to the total molar amount of structural units represented by formula (1). When the proportion of structural units represented by formula (4) in which X is the structural unit is within the above range, the transparency, elastic modulus, and folding endurance of the optical film are likely to be improved. The proportion of structural units represented by formula (4) in which X is the structural unit is, for example, 1 It can be measured using H-NMR, or can be calculated from the ratio of raw materials used.

[0047] The polyimide resin in the present invention may contain, in addition to the constitutional unit represented by formula (1), a constitutional unit represented by formula (30) and / or a constitutional unit represented by formula (31). [ka]

[0048] In equation (30), Y 1 is a tetravalent organic group, preferably an organic group in which a hydrogen atom may be substituted with a hydrocarbon group or a fluorine-substituted hydrocarbon group. 1Examples of the Y include groups represented by formula (20), formula (21), formula (22), formula (23), formula (24), formula (25), formula (26), formula (27), formula (28) and formula (29), groups in which hydrogen atoms in the groups represented by formulas (20) to (29) are substituted with methyl groups, fluoro groups, chloro groups or trifluoromethyl groups, and tetravalent chain hydrocarbon groups having 6 or less carbon atoms. In one embodiment of the present invention, the polyimide resin contains a plurality of types of Y 1 and multiple types of Y 1 may be the same as or different from each other.

[0049] In equation (31), Y 2 is a trivalent organic group, preferably an organic group in which a hydrogen atom may be substituted with a hydrocarbon group or a fluorine-substituted hydrocarbon group. 2 Examples of the Y include groups in which one of the bonds in the groups represented by the above formulas (20), (21), (22), (23), (24), (25), (26), (27), (28), and (29) has been replaced with a hydrogen atom, and trivalent chain hydrocarbon groups having 6 or less carbon atoms. In one embodiment of the present invention, the polyimide resin contains a plurality of types of Y 2 and multiple types of Y 2 may be the same as or different from each other.

[0050] In formula (30) and formula (31), X 1 and X 2 are each independently a divalent organic group, and preferably an organic group in which a hydrogen atom may be substituted with a hydrocarbon group or a fluorine-substituted hydrocarbon group. 1 and X 2 Examples of such a group include groups represented by the above formulas (10), (11), (12), (13), (14), (15), (16), (17) and (18); groups in which hydrogen atoms in the groups represented by formulas (10) to (18) have been substituted with methyl groups, fluoro groups, chloro groups or trifluoromethyl groups; and chain hydrocarbon groups having 6 or less carbon atoms.

[0051] In one embodiment of the present invention, the polyimide resin comprises a structural unit represented by formula (1), and optionally at least one structural unit selected from the structural units represented by formula (30) and formula (31). From the viewpoint of easily improving the transparency, modulus of elasticity, and folding endurance of the optical film, the proportion of the structural unit represented by formula (1) in the polyimide resin is preferably 80 mol% or more, more preferably 90 mol% or more, and even more preferably 95 mol% or more, based on the total molar amount of all structural units contained in the polyimide resin, for example, the structural unit represented by formula (1), and optionally at least one structural unit selected from the structural unit represented by formula (30) and formula (31). The upper limit of the proportion of the structural unit represented by formula (1) in the polyimide resin is 100 mol% or less. The above proportion can be expressed, for example, as follows: 1 The polyimide resin in the present invention is preferably a polyimide resin, since it can easily improve the transparency, elastic modulus, and folding endurance of the optical film.

[0052] In a preferred embodiment of the present invention, the polyimide resin of the present invention may contain halogen atoms such as fluorine atoms, which can be introduced, for example, by the above-mentioned fluorine-containing substituents. When the polyimide resin contains halogen atoms, the yellowness index (hereinafter sometimes referred to as YI value) of the optical film is easily reduced, and the elastic modulus and folding endurance are easily increased. Furthermore, a high elastic modulus of the optical film is easily suppressed from occurring scratches and wrinkles. Furthermore, a low YI value of the optical film is easily improved in transparency and visibility of the film. The halogen atoms are preferably fluorine atoms. Examples of fluorine-containing substituents preferred for incorporating fluorine atoms into the polyimide resin include a fluoro group and a trifluoromethyl group.

[0053] The content of halogen atoms in the polyimide resin is preferably 1 to 40 mass%, more preferably 5 to 40 mass%, and even more preferably 5 to 30 mass%, based on the mass of the polyimide resin. When the content of halogen atoms is equal to or greater than the above lower limit, the YI value of the optical film is easily reduced and the elastic modulus and folding endurance are easily increased. When the content of halogen atoms is equal to or less than the above upper limit, synthesis is facilitated.

[0054] The imidization ratio of the polyimide resin is preferably 90% or more, more preferably 93% or more, and even more preferably 96% or more. From the viewpoint of easily improving the optical properties of the optical film, it is preferable that the imidization ratio is equal to or greater than the above-mentioned lower limit. Furthermore, the upper limit of the imidization ratio is 100% or less. The imidization ratio indicates the ratio of the molar amount of imide bonds in the polyimide resin to twice the molar amount of structural units derived from tetracarboxylic acid compounds in the polyimide resin. When the polyimide resin contains a tricarboxylic acid compound, the imidization ratio indicates the ratio of the molar amount of imide bonds in the polyimide resin to the sum of twice the molar amount of structural units derived from tetracarboxylic acid compounds in the polyimide resin and the molar amount of structural units derived from tricarboxylic acid compounds. Furthermore, the imidization ratio can be determined by IR, NMR, or the like.

[0055] In one embodiment of the present invention, the polyimide resin contained in the optical film is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, particularly preferably 80% by mass or more, and preferably 100% by mass or less, relative to 100% by mass of the optical film.

[0056] <Method of manufacturing polyimide resin> The method for producing the polyimide resin contained in the optical film of the present invention is not particularly limited. In one embodiment of the present invention, the polyimide resin containing the structural unit represented by formula (1) can be produced by a method including the steps of reacting a diamine compound with a tetracarboxylic acid compound to obtain a polyamic acid, and imidizing the polyamic acid. In addition to the tetracarboxylic acid compound, a tricarboxylic acid compound may also be reacted.

[0057] The tetracarboxylic acid compound used in the production of the polyimide resin is at least a compound represented by the formula (X): [ka] [In formula (X), R 1 ~R 5 , n and m are R in formula (3), 1 ~R 5 , n and m] In addition, it is preferable that the compound represented by R in formula (X) is 1 ~R 5 Suitable forms of n and m are also R 1 ~R 5 , n and m.

[0058] The compound represented by formula (X) may be obtained by a conventional method, for example, by reacting trimellitic anhydride or a derivative thereof with an aromatic diol, or a commercially available product may be used.

[0059] The structural units represented by formula (1) and formula (30) are usually derived from a diamine compound and a tetracarboxylic acid compound, and the structural unit represented by formula (31) is usually derived from a diamine compound and a tricarboxylic acid compound.

[0060] Examples of tetracarboxylic acid compounds used in the synthesis of polyimide resins include aromatic tetracarboxylic acid compounds such as aromatic tetracarboxylic acid dianhydrides, and aliphatic tetracarboxylic acid compounds such as aliphatic tetracarboxylic acid dianhydrides. The tetracarboxylic acid compounds may be used alone or in combination of two or more. The tetracarboxylic acid compounds may be dianhydrides or tetracarboxylic acid compound analogs such as acid chloride compounds.

[0061] Specific examples of the aromatic tetracarboxylic acid dianhydride include non-condensed polycyclic aromatic tetracarboxylic acid dianhydrides, monocyclic aromatic tetracarboxylic acid dianhydrides, and condensed polycyclic aromatic tetracarboxylic acid dianhydrides. Examples of the non-condensed polycyclic aromatic tetracarboxylic dianhydride include an ester of trimellitic anhydride and 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol (hereinafter sometimes referred to as TAHMBP), an ester of trimellitic anhydride and 2,2',3,3'-tetramethyl-4,4'-biphenol, an ester of trimellitic anhydride and 3,3',5,5'-tetramethyl-4,4'-biphenol, 4,4'-oxydiphthalic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter sometimes referred to as BPDA), 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenoxyphenyl)propane dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (hereinafter sometimes referred to as 6FDA), 1,2-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(2,3-di 1,2-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, 4,4'-(p-phenylenedioxy)diphthalic dianhydride, and 4,4'-(m-phenylenedioxy)diphthalic dianhydride.Furthermore, examples of monocyclic aromatic tetracarboxylic acid dianhydrides include 1,2,4,5-benzenetetracarboxylic acid dianhydride (also represented as pyromellitic dianhydride, hereinafter sometimes referred to as PMDA), and examples of condensed polycyclic aromatic tetracarboxylic acid dianhydrides include 2,3,6,7-naphthalenetetracarboxylic acid dianhydride. Among these, preferred are esters of trimellitic anhydride and 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol, esters of trimellitic anhydride and 2,2',3,3'-tetramethyl-4,4'-biphenol, esters of trimellitic anhydride and 3,3',5,5'-tetramethyl-4,4'-biphenol, PMDA, 4,4'-oxydiphthalic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic ... dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenoxyphenyl)propane dianhydride, 6FDA, 1,2-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis( 2,3-dicarboxyphenyl)ethane dianhydride, 1,2-bis(3,4-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, 4,4'-(p-phenylenedioxy)diphthalic dianhydride, and 4,4'-(m-phenylenedioxy)diphthalic dianhydride are preferred, and a mixture of trimellitic anhydride and 2,2',3,3',5,5'-hexamethasone is more preferred. Examples of suitable phthalic anhydrides include esters of trimellitic anhydride and 2,2',3,3'-tetramethyl-4,4'-biphenol, esters of trimellitic anhydride and 3,3',5,5'-tetramethyl-4,4'-biphenol, 4,4'-oxydiphthalic dianhydride, BPDA, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 6FDA, bis(3,4-dicarboxyphenyl)methane dianhydride, and 4,4'-(p-phenylenedioxy)diphthalic dianhydride. These may be used alone or in combination of two or more.

[0062] Examples of aliphatic tetracarboxylic acid dianhydrides include cyclic and acyclic aliphatic tetracarboxylic acid dianhydrides. Cyclic aliphatic tetracarboxylic acid dianhydrides are tetracarboxylic acid dianhydrides having an alicyclic hydrocarbon structure. Specific examples include cycloalkane tetracarboxylic acid dianhydrides such as 1,2,4,5-cyclohexane tetracarboxylic acid dianhydride, 1,2,3,4-cyclobutane tetracarboxylic acid dianhydride, and 1,2,3,4-cyclopentane tetracarboxylic acid dianhydride; bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acid dianhydride; dicyclohexyl-3,3',4,4'-tetracarboxylic acid dianhydride; and positional isomers thereof. These may be used alone or in combination of two or more. Specific examples of acyclic aliphatic tetracarboxylic acid dianhydrides include 1,2,3,4-butane tetracarboxylic acid dianhydride and 1,2,3,4-pentane tetracarboxylic acid dianhydride. These may be used alone or in combination of two or more. Furthermore, a cyclic aliphatic tetracarboxylic acid dianhydride and an acyclic aliphatic tetracarboxylic acid dianhydride may be used in combination.

[0063] Among the above tetracarboxylic dianhydrides, from the viewpoint of easily increasing the transparency, modulus of elasticity and folding resistance of the optical film, esters of trimellitic anhydride and 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol, esters of trimellitic anhydride and 2,2',3,3'-tetramethyl-4,4'-biphenol, esters of trimellitic anhydride and 3,3',5,5'-tetramethyl-4,4'-biphenol, PMDA, 4,4'-oxydiphthalic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-2,2-bis(2,2-bis(3,4-dicarboxyphenyl)-2,2-di ... Preferred are 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol, 2,2',3,3'-tetramethyl-4,4'-biphenol, 2,2',3,3'-tetramethyl-4,4'-biphenol, 2,3',5,5'-tetramethyl-4,4'-biphenol, 6FDA, and mixtures thereof. More preferred are 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol, 2,2',3,3'-tetramethyl-4,4'-biphenol, 2,2',3,3'-tetramethyl-4,4'-biphenol, and 2,3',5,5'-tetramethyl-4,4'-biphenol, and 2,3',5,5'-tetramethyl-4,4'-biphenol.

[0064] Examples of diamine compounds used in the synthesis of polyimide resins include aliphatic diamines, aromatic diamines, and mixtures thereof. In this embodiment, "aromatic diamine" refers to a diamine in which an amino group is directly bonded to an aromatic ring, and may contain an aliphatic group or other substituents as part of its structure. This aromatic ring may be a single ring or a condensed ring, and examples include a benzene ring, a naphthalene ring, an anthracene ring, and a fluorene ring, but are not limited to these. Among these, a benzene ring is preferred. Furthermore, "aliphatic diamine" refers to a diamine in which an amino group is directly bonded to an aliphatic group, and may contain an aromatic ring or other substituents as part of its structure.

[0065] Examples of aliphatic diamines include acyclic aliphatic diamines such as hexamethylenediamine, and cyclic aliphatic diamines such as 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, norbornanediamine, and 4,4'-diaminodicyclohexylmethane, which can be used alone or in combination of two or more.

[0066] Examples of aromatic diamines include aromatic diamines having one aromatic ring, such as p-phenylenediamine, m-phenylenediamine, 2,4-toluenediamine, m-xylylenediamine, p-xylylenediamine, 1,5-diaminonaphthalene, and 2,6-diaminonaphthalene; 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, bis[4-(4-aminophenoxy)phenyl]sulfone, and bis[4-(3-aminophenoxy)phenyl]sulfone. Examples of aromatic diamines include aromatic diamines having two or more aromatic rings, such as 2,2-bis[4-(4-aminophenoxy)phenyl]sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 2,2'-dimethylbenzidine, 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl (hereinafter sometimes referred to as TFMB), 4,4'-(hexafluoropropylidene)dianiline (hereinafter sometimes referred to as 6FDAM), 4,4'-bis(4-aminophenoxy)biphenyl, 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-methylphenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene, and 9,9-bis(4-amino-3-fluorophenyl)fluorene. These can be used alone or in combination of two or more.

[0067] The aromatic diamine is preferably 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 1,4-bis(4-aminophenoxy)benzene, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, or 2,2-bis[4-(3-aminophenoxy)phenyl]propane. 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 1,4-bis(4-aminophenoxy)benzene, bis[4-(4-aminophenoxy)phenyl]sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2'-dimethylbenzidine, TFMB, 6FDAM, and 4,4'-bis(4-aminophenoxy)biphenyl are more preferred. These can be used alone or in combination of two or more.

[0068] Among the above diamine compounds, from the viewpoint of easily improving the transparency, elastic modulus, and folding resistance of the optical film, it is more preferable to use one or more selected from the group consisting of 2,2'-dimethylbenzidine, TFMB, 4,4'-bis(4-aminophenoxy)biphenyl, 6FDAM, and 4,4'-diaminodiphenyl ether, and it is even more preferable to use TFMB and / or 6FDAM.

[0069] The polyimide resin may be a resin obtained by further reacting, in addition to the tetracarboxylic acid compound used in the resin synthesis, other tetracarboxylic acids and tricarboxylic acids, and anhydrides and derivatives thereof, within the range that does not impair various physical properties of the optical film.

[0070] Other tetracarboxylic acids include water adducts of the anhydrides of the above tetracarboxylic acid compounds.

[0071] Examples of tricarboxylic acid compounds include aromatic tricarboxylic acids, aliphatic tricarboxylic acids, and their analogous acid chloride compounds and acid anhydrides, and two or more of these may be used in combination. Specific examples include 1,2,4-benzenetricarboxylic acid anhydride; 2,3,6-naphthalenetricarboxylic acid-2,3-anhydride; and compounds in which phthalic anhydride and benzoic acid are linked via a single bond, -O-, -CH2-, -C(CH3)2-, -C(CF3)2-, -SO2-, or a phenylene group.

[0072] In the production of the polyimide resin, the amounts of the diamine compound and the tetracarboxylic acid compound used can be appropriately selected depending on the desired ratio of each structural unit of the resin. In a preferred embodiment of the present invention, the amount of the diamine compound used is preferably 0.94 mol or more, more preferably 0.96 mol or more, even more preferably 0.98 mol or more, particularly preferably 0.99 mol or more, and is preferably 1.20 mol or less, more preferably 1.10 mol or less, even more preferably 1.05 mol or less, particularly preferably 1.02 mol or less, relative to 1 mol of the tetracarboxylic acid compound. When the amount of the diamine compound used relative to the tetracarboxylic acid compound is within the above range, it is easy to adjust the weight-average molecular weight of the polyimide resin to 160,000 or more.

[0073] The reaction temperature between the diamine compound and the tetracarboxylic acid compound is not particularly limited and may be, for example, 5 to 200°C, and the reaction time is also not particularly limited and may be, for example, about 30 minutes to 72 hours. In a preferred embodiment of the present invention, from the viewpoint of easily adjusting the weight-average molecular weight of the polyimide resin to 160,000 or more, the reaction temperature is preferably 5 to 50°C, more preferably 5 to 40°C, and even more preferably 5 to 25°C, and the reaction time is preferably 3 to 24 hours, more preferably 5 to 20 hours.

[0074] The reaction between the diamine compound and the tetracarboxylic acid compound is preferably carried out in a solvent. The solvent is not particularly limited as long as it does not affect the reaction, and examples thereof include alcoholic solvents such as water, methanol, ethanol, ethylene glycol, isopropyl alcohol, propylene glycol, ethylene glycol methyl ether, ethylene glycol butyl ether, 1-methoxy-2-propanol, 2-butoxyethanol, and propylene glycol monomethyl ether; ester solvents such as ethyl acetate, butyl acetate, ethylene glycol methyl ether acetate, γ-butyrolactone, γ-valerolactone, propylene glycol methyl ether acetate, and ethyl lactate; acetone, methyl ethyl ketone, cyclopentanone, cyclohexanone, 2-heptanone, and methylisobutyl ether; Examples of suitable solvents include ketone solvents such as ethanol ketone; aliphatic hydrocarbon solvents such as pentane, hexane, and heptane; alicyclic hydrocarbon solvents such as ethylcyclohexane; aromatic hydrocarbon solvents such as toluene and xylene; nitrile solvents such as acetonitrile; ether solvents such as tetrahydrofuran and dimethoxyethane; chlorine-containing solvents such as chloroform and chlorobenzene; amide solvents such as N,N-dimethylacetamide (hereinafter sometimes referred to as DMAc) and N,N-dimethylformamide (hereinafter sometimes referred to as DMF); sulfur-containing solvents such as dimethyl sulfone, dimethyl sulfoxide, and sulfolane; carbonate solvents such as ethylene carbonate and propylene carbonate; and combinations thereof. Among these, amide solvents are preferred from the viewpoint of solubility. In a preferred embodiment of the present invention, from the viewpoint of easily adjusting the weight average molecular weight of the polyimide resin to 160,000 or more, it is preferable that the solvent used in the reaction is a solvent that has been strictly dehydrated to a water content of 700 ppm or less.

[0075] The reaction of the diamine compound with the tetracarboxylic acid compound may be carried out, as necessary, in an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere, or under reduced pressure. From the viewpoint of easily adjusting the weight-average molecular weight of the polyimide resin to 160,000 or more, it is preferable to carry out the reaction under the inert atmosphere in a strictly controlled dehydrated solvent while stirring.

[0076] Examples of the imidization catalyst used in the imidization step include aliphatic amines such as tripropylamine, dibutylpropylamine, and ethyldibutylamine; alicyclic amines (monocyclic) such as N-ethylpiperidine, N-propylpiperidine, N-butylpyrrolidine, N-butylpiperidine, and N-propylhexahydroazepine; azabicyclo[2.2.1]heptane, azabicyclo[3.2.1]octane, azabicyclo[2.2.2]octane, and azabicyclo[2.2.1]heptane. [3.2.2] Examples of suitable imidation catalysts include alicyclic (polycyclic) amines such as nonane; and aromatic amines such as pyridine, 2-methylpyridine (2-picoline), 3-methylpyridine (3-picoline), 4-methylpyridine (4-picoline), 2-ethylpyridine, 3-ethylpyridine, 4-ethylpyridine, 2,4-dimethylpyridine, 2,4,6-trimethylpyridine, 3,4-cyclopentenopyridine, 5,6,7,8-tetrahydroisoquinoline, and isoquinoline. Furthermore, in order to facilitate the imidation reaction, it is preferable to use an acid anhydride together with the imidation catalyst. Examples of suitable acid anhydrides include conventional acid anhydrides used in imidation reactions. Specific examples include aliphatic acid anhydrides such as acetic anhydride, propionic anhydride, and butyric anhydride, and aromatic acid anhydrides such as phthalic acid.

[0077] In a preferred embodiment of the present invention, the imidization step is preferably carried out stepwise, with the temperature elevated to an optimal reaction temperature. By carrying out the imidization stepwise, decomposition of the resin is suppressed, making it easier to adjust the weight-average molecular weight of the resulting polyimide resin to 160,000 or more. The reaction temperature elevated in the stepwise imidization step is preferably 40 to 85°C, more preferably 45 to 80°C. When the reaction temperature is within this range, the imidization reaction tends to proceed sufficiently, and the molecular weight tends to be less likely to decrease during the imidization of the amic acid in the imidization step. The reaction time is preferably 30 minutes to 10 hours, more preferably 30 minutes to 5 hours. When the reaction time is within this range, the resin decomposition does not occur, resulting in a sufficient increase in molecular weight. Furthermore, the imidization rate tends to be low, which tends to prevent the polymer from becoming low in molecular weight in subsequent steps. In this way, by controlling the imidization step in addition to the synthesis conditions described above, a resin with a high weight-average molecular weight can be obtained.

[0078] The polyimide resin may be isolated by separation and purification using a conventional method, for example, a separation means such as filtration, concentration, extraction, crystallization, recrystallization, column chromatography, or a combination of these. In a preferred embodiment, the resin can be isolated by adding a large amount of alcohol such as methanol to a reaction solution containing the resin to precipitate the resin, followed by concentration, filtration, drying, etc.

[0079] <Additives> The optical film of the present invention may contain at least one filler in addition to the polyimide resin. Examples of fillers include organic particles and inorganic particles, with inorganic particles being preferred. Examples of inorganic particles include metal oxide particles such as silica, zirconia, alumina, titania, zinc oxide, germanium oxide, indium oxide, tin oxide, indium tin oxide (ITO), antimony oxide, and cerium oxide, and metal fluoride particles such as magnesium fluoride and sodium fluoride. Among these, silica particles, zirconia particles, and alumina particles are preferred, with silica particles being more preferred, from the viewpoint of easily achieving both the elastic modulus and transparency of the optical film. These fillers can be used alone or in combination of two or more.

[0080] The average primary particle diameter of the filler, preferably silica particles, is usually 1 nm or more, preferably 5 nm or more, more preferably 10 nm or more, even more preferably 15 nm or more, and particularly preferably 20 nm or more; and preferably 100 nm or less, more preferably 90 nm or less, even more preferably 80 nm or less, even more preferably 70 nm or less, particularly preferably 60 nm or less, even particularly preferably 50 nm or less, and most preferably 40 nm or less. When the average primary particle diameter of the filler, preferably silica particles, is within the above range, it is easy to achieve both the elastic modulus and transparency of the optical film. Furthermore, it is easy to suppress aggregation of the filler, preferably silica particles, and improve the transparency of the resulting optical film. The average primary particle diameter of the filler can be measured by the BET method. It is also possible to measure the average primary particle diameter by image analysis using a transmission electron microscope or a scanning electron microscope.

[0081] When the optical film of the present invention contains a filler, preferably silica particles, the filler content is usually 0.1% by mass or more, preferably 1% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, even more preferably 20% by mass or more, particularly preferably 30% by mass or more, and preferably 60% by mass or less, based on the mass of the optical film. When the filler content is equal to or greater than the above-mentioned lower limit, the resulting optical film is likely to have both high elastic modulus and high transparency. When the filler content is equal to or less than the above-mentioned upper limit, the optical properties of the optical film are likely to be improved.

[0082] The optical film of the present invention may further contain an ultraviolet absorber. The ultraviolet absorber can be appropriately selected from those commonly used as ultraviolet absorbers in the field of resin materials. The ultraviolet absorber may include a compound that absorbs light with a wavelength of 400 nm or less. Examples of the ultraviolet absorber include at least one compound selected from the group consisting of benzophenone-based compounds, salicylate-based compounds, benzotriazole-based compounds, and triazine-based compounds. The ultraviolet absorbers can be used alone or in combination. By including an ultraviolet absorber in the optical film, resin deterioration can be suppressed, thereby improving visibility when the optical film is applied to an image display device, etc. In this specification, the term "based compound" refers to a derivative of the compound to which the term "based compound" is attached. For example, the term "benzophenone-based compound" refers to a compound having a benzophenone as a parent skeleton and a substituent bonded to the benzophenone.

[0083] When the optical film contains an ultraviolet absorber, the content of the ultraviolet absorber is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, and is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less, relative to the mass of the optical film. The suitable content varies depending on the ultraviolet absorber used, but adjusting the content of the ultraviolet absorber so that the light transmittance at 400 nm is about 20 to 60% increases the light resistance of the optical film and also makes it easy to increase its transparency.

[0084] The optical film of the present invention may further contain additives other than the filler and the UV absorber. Examples of such additives include antioxidants, release agents, stabilizers, bluing agents, flame retardants, pH adjusters, silica dispersants, lubricants, thickeners, and leveling agents. When other additives are contained, the content thereof may be preferably 0.001 to 20% by mass, more preferably 0.01 to 15% by mass, and even more preferably 0.1 to 10% by mass, based on the mass of the optical film.

[0085] <Optical film> The optical film of the present invention has a structure represented by formula (3) as Y in formula (1) and has a weight-average molecular weight adjusted to 160,000 or more, thereby achieving both excellent transparency and excellent folding endurance. Furthermore, the optical film of the present invention also has an excellent elastic modulus. Therefore, the optical film of the present invention is suitable for use as a material for flexible display devices, etc. In this specification, transparency can be evaluated by total light transmittance and haze, and an increase or improvement in transparency means an increase in total light transmittance and a decrease in haze.

[0086] In a preferred embodiment of the present invention, the total light transmittance of the optical film of the present invention, preferably at a thickness of 50 μm, is preferably 85% or more, more preferably 88% or more, even more preferably 89% or more, particularly preferably 90% or more, and most preferably 91% or more. When the total light transmittance is equal to or greater than the above-mentioned lower limit, the transparency of the optical film can be improved, and when used, for example, as the front panel of a display device, high visibility can be achieved. The upper limit of the total light transmittance is usually 100% or less. The total light transmittance can be measured using a haze computer in accordance with JIS K 7105:1981, for example, by the method described in the Examples. In this specification, the total light transmittance can also be the total light transmittance within the thickness range of the optical film of the present invention.

[0087] In a preferred embodiment of the present invention, the haze of the optical film of the present invention, preferably at a thickness of 50 μm, is preferably 2.0% or less, more preferably 1.5% or less, even more preferably 1.0% or less, even more preferably 0.8% or less, particularly preferably 0.5% or less, and most preferably 0.3% or less. When the haze of the optical film is below the above upper limit, the transparency of the optical film can be improved, and when used, for example, as the front panel of a display device, high visibility can be achieved. The lower limit of the haze of the optical film is usually 0% or more. Note that the haze can be measured using a haze computer or the like in accordance with JIS K 7136:2000, for example, by the method described in the Examples. In this specification, the haze can also be the haze within the thickness range of the optical film of the present invention.

[0088] In a preferred embodiment of the present invention, the YI value of the optical film of the present invention is preferably 3.0 or less, more preferably 2.8 or less, even more preferably 2.5 or less, and is usually -5 or more, preferably -2 or more. When the YI value of the optical film is equal to or less than the above upper limit, the transparency of the optical film can be improved, and when used as a front panel of a display device, high visibility can be achieved. The YI value can be calculated based on the formula YI = 100 × (1.2769X - 1.0592Z) / Y by measuring the transmittance for light of 300 to 800 nm using an ultraviolet-visible-near-infrared spectrophotometer to determine the tristimulus values (X, Y, Z). For example, it can be calculated by the method described in the Examples.

[0089] In a preferred embodiment of the present invention, the optical film of the present invention is excellent in transparency, folding resistance, and elastic modulus. The elastic modulus of the optical film of the present invention is preferably 3.5 GPa or more, more preferably 4.0 GPa or more, and even more preferably 4.5 GPa or more. When the elastic modulus is equal to or greater than the above-mentioned lower limit, deformation of the optical film is easily suppressed and durability is easily improved. The upper limit of the elastic modulus is not particularly limited, but is usually 15 GPa or less. The elastic modulus can be measured using a tensile tester. More specifically, the elastic modulus can be determined by measuring a stress-strain curve (SS curve) using a tensile tester under conditions of a chuck distance of 50 mm and a pulling rate of 10 mm / min, and determining the slope of the curve; for example, it can be measured by the method described in the Examples. The elastic modulus is a value at 25°C.

[0090] In a preferred embodiment of the present invention, the optical film of the present invention has excellent folding resistance. The number of folding times of the optical film of the present invention in the MIT folding fatigue test according to ASTM standard D2176-16 is preferably 350,000 or more, more preferably 400,000 or more, and even more preferably 450,000 or more. When the number of folding times is equal to or greater than the lower limit, the occurrence of cracks, breakage, and the like can be effectively suppressed even when repeatedly folded. The MIT folding fatigue test can be measured using an MIT folding fatigue tester, for example, by the method described in the Examples.

[0091] The total light transmittance and haze change depending on the thickness of the optical film, and the greater the thickness, the lower the total light transmittance and the higher the haze. That is, it is difficult to produce an optical film with a high total light transmittance and low haze when the film is thick. On the other hand, the optical film of the present invention has a high level of transparency, and therefore can exhibit high total light transmittance and low haze even when relatively thick. Therefore, the optical film of the present invention can have a thickness of preferably 35 μm or more, more preferably 40 μm or more, and even more preferably 45 μm or more. Furthermore, the upper limit of the thickness of the optical film of the present invention is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 60 μm or less. The thickness of the optical film can be measured with a film thickness meter or the like, for example, by the method described in the Examples.

[0092] The use of the optical film of the present invention is not particularly limited, and it may be used for various purposes. As described above, the optical film of the present invention may be a single layer or a laminate, and the optical film of the present invention may be used as it is, or may be used as a laminate with other films. When the optical film is a laminate, all layers laminated on one or both sides of the optical film are collectively referred to as the optical film.

[0093] When the optical film of the present invention is a laminate, it is preferable that the optical film has one or more functional layers on at least one surface thereof. Examples of the functional layer include a hard coat layer, a primer layer, a gas barrier layer, an ultraviolet absorbing layer, an adhesive layer, a hue adjusting layer, and a refractive index adjusting layer. The functional layers can be used alone or in combination of two or more.

[0094] The thickness of the hard coat layer is not particularly limited and may be, for example, 2 to 100 μm. When the thickness of the hard coat layer is within the above range, impact resistance can be increased, folding resistance is less likely to decrease, and curling due to curing shrinkage tends to be less likely to occur. The hard coat layer can be formed by curing a hard coat composition containing a reactive material that can form a crosslinked structure by irradiation with active energy rays or by applying thermal energy, and active energy ray irradiation is preferred. Active energy rays are defined as energy rays that can decompose a compound that generates active species to generate active species, and examples of such rays include visible light, ultraviolet light, infrared light, X-rays, α-rays, β-rays, γ-rays, and electron beams, with ultraviolet light being preferred. The hard coat composition contains a polymer of at least one type of radically polymerizable compound and cationic polymerizable compound.

[0095] The radical polymerizable compound is a compound having a radical polymerizable group. The radical polymerizable group of the radical polymerizable compound may be any functional group capable of undergoing a radical polymerization reaction, such as a group containing a carbon-carbon unsaturated double bond, and specific examples thereof include a vinyl group and a (meth)acryloyl group. When the radical polymerizable compound has two or more radical polymerizable groups, these radical polymerizable groups may be the same or different. The number of radical polymerizable groups in one molecule of the radical polymerizable compound is preferably two or more, in order to improve the hardness of the hard coat layer. From the viewpoint of high reactivity, the radical polymerizable compound is preferably a compound having a (meth)acryloyl group, and specifically includes compounds called polyfunctional acrylate monomers having 2 to 6 (meth)acryloyl groups in one molecule, and oligomers called epoxy (meth)acrylates, urethane (meth)acrylates, and polyester (meth)acrylates, which have several (meth)acryloyl groups in the molecule and have molecular weights of several hundred to several thousand, and is preferably one or more selected from epoxy (meth)acrylates, urethane (meth)acrylates, and polyester (meth)acrylates.

[0096] The cationically polymerizable compound is a compound having a cationically polymerizable group such as an epoxy group, an oxetanyl group, a vinyloxy group, etc. The number of cationically polymerizable groups that the cationically polymerizable compound has in one molecule is preferably 2 or more, more preferably 3 or more, from the viewpoint of improving the hardness of the hard coat layer. Among the cationically polymerizable compounds, compounds having at least one of an epoxy group and an oxetanyl group as the cationically polymerizable group are preferred. Cyclic ether groups such as an epoxy group and an oxetanyl group are preferred because they cause minimal shrinkage during the polymerization reaction. Among cyclic ether groups, compounds having an epoxy group are readily available in a variety of structures, do not adversely affect the durability of the resulting hard coat layer, and are advantageous in that their compatibility with radically polymerizable compounds is easily controlled. Among cyclic ether groups, an oxetanyl group is more likely to achieve a high degree of polymerization than an epoxy group, is less toxic, and accelerates the network formation rate obtained from the cationically polymerizable compound in the resulting hard coat layer, forming an independent network without leaving unreacted monomers in the film even in regions where the radically polymerizable compound is present. Examples of the cationically polymerizable compound having an epoxy group include alicyclic epoxy resins obtained by epoxidizing polyglycidyl ethers of polyhydric alcohols having an alicyclic ring or cyclohexene ring- or cyclopentene ring-containing compounds with a suitable oxidizing agent such as hydrogen peroxide or peracid; aliphatic epoxy resins such as polyglycidyl ethers of aliphatic polyhydric alcohols or their alkylene oxide adducts, polyglycidyl esters of aliphatic long-chain polybasic acids, and glycidyl (meth)acrylate homopolymers and copolymers; and glycidyl ether-type epoxy resins derived from bisphenols, such as bisphenol A, bisphenol F, and hydrogenated bisphenol A, or derivatives thereof, such as alkylene oxide adducts or caprolactone adducts, and novolac epoxy resins.

[0097] The hard coat composition may further contain a polymerization initiator. Examples of the polymerization initiator include a radical polymerization initiator, a cationic polymerization initiator, and a radical and cationic polymerization initiator, and these are appropriately selected and used. These polymerization initiators are decomposed by at least one of active energy ray irradiation and heating to generate radicals or cations, thereby promoting radical polymerization and cationic polymerization. The radical polymerization initiator may be any as long as it is capable of releasing a substance that initiates radical polymerization upon at least one of irradiation with active energy rays and heating. Examples of thermal radical polymerization initiators include organic peroxides such as hydrogen peroxide and perbenzoic acid, and azo compounds such as azobisbutyronitrile. Active energy ray radical polymerization initiators include Type 1 radical polymerization initiators, which generate radicals through molecular decomposition, and Type 2 radical polymerization initiators, which generate radicals through a hydrogen abstraction reaction in the presence of a tertiary amine, and they are used alone or in combination. The cationic polymerization initiator may be any as long as it is capable of releasing a substance that initiates cationic polymerization upon at least one of active energy ray irradiation and heating. Examples of cationic polymerization initiators that can be used include aromatic iodonium salts, aromatic sulfonium salts, and cyclopentadienyl iron (II) complexes. These can initiate cationic polymerization upon either active energy ray irradiation or heating, or both, depending on their structural differences.

[0098] The polymerization initiator can be contained in an amount of preferably 0.1 to 10% by mass relative to 100% by mass of the entire hard coat composition. When the content of the polymerization initiator is within the above range, curing can be sufficiently promoted, the mechanical properties and adhesive strength of the finally obtained coating film can be kept within good ranges, and poor adhesive strength, cracking, and curling due to cure shrinkage tend to be less likely to occur.

[0099] The hard coat composition may further include at least one selected from the group consisting of a solvent and an additive. The solvent can dissolve or disperse the polymerizable compound and the polymerization initiator, and any solvent known in the art as a solvent for hard coat compositions can be used as long as it does not impair the effects of the present invention. The additives may further include inorganic particles, leveling agents, stabilizers, surfactants, antistatic agents, lubricants, antifouling agents, and the like.

[0100] The ultraviolet absorbing layer is a layer that has the function of absorbing ultraviolet rays, and is composed of a main material selected from, for example, an ultraviolet-curable transparent resin, an electron beam-curable transparent resin, and a thermosetting transparent resin, and an ultraviolet absorber dispersed in this main material.

[0101] The adhesive layer is a layer having adhesive properties and functions to adhere the optical film to other members. A commonly known material can be used to form the adhesive layer. For example, a thermosetting resin composition or a photocurable resin composition can be used. In this case, the thermosetting resin composition or the photocurable resin composition can be polymerized and cured by subsequently supplying energy.

[0102] The adhesive layer may be a layer called a pressure-sensitive adhesive (PSA), which is attached to an object by pressing. The pressure-sensitive adhesive may be a pressure-sensitive adhesive that is "a substance that is adhesive at room temperature and adheres to an adherend with light pressure" (JIS K 6800), or it may be a capsule-type adhesive that is "an adhesive in which specific components are encapsulated in a protective coating (microcapsules) and that maintains stability until the coating is destroyed by appropriate means (pressure, heat, etc.)" (JIS K 6800).

[0103] The hue-adjusting layer is a layer having a hue-adjusting function and capable of adjusting the optical film to a desired hue. The hue-adjusting layer is, for example, a layer containing a resin and a colorant. Examples of the colorant include inorganic pigments such as titanium oxide, zinc oxide, red iron oxide, titanium oxide-based calcined pigments, ultramarine, cobalt aluminate, and carbon black; organic pigments such as azo compounds, quinacridone compounds, anthraquinone compounds, perylene compounds, isoindolinone compounds, phthalocyanine compounds, quinophthalone compounds, threne compounds, and diketopyrrolopyrrole compounds; extender pigments such as barium sulfate and calcium carbonate; and dyes such as basic dyes, acid dyes, and mordant dyes.

[0104] The refractive index adjusting layer is a layer that adjusts the refractive index and has a refractive index different from that of a single-layer optical film, for example, and can impart a predetermined refractive index to the optical film. The refractive index adjusting layer may be, for example, a resin layer containing an appropriately selected resin and, optionally, a pigment, or may be a thin metal film. Examples of pigments that adjust the refractive index include silicon oxide, aluminum oxide, antimony oxide, tin oxide, titanium oxide, zirconium oxide, and tantalum oxide. The average primary particle diameter of the pigment may be 0.1 μm or less. By adjusting the average primary particle diameter of the pigment to 0.1 μm or less, diffuse reflection of light passing through the refractive index adjusting layer can be prevented, thereby preventing a decrease in transparency. Examples of metals used in the refractive index adjusting layer include metal oxides or metal nitrides such as titanium oxide, tantalum oxide, zirconium oxide, zinc oxide, tin oxide, silicon oxide, indium oxide, titanium oxynitride, titanium nitride, silicon oxynitride, and silicon nitride.

[0105] In one embodiment of the present invention, the optical film may have a protective film on at least one side, i.e., one or both sides. For example, when the optical film has a functional layer on one side, the protective film may be laminated on the optical film side or the functional layer side, or may be laminated on both the optical film side and the functional layer side. When the optical film has functional layers on both sides, the protective film may be laminated on one of the functional layer sides or on both functional layer sides. The protective film is a film for temporarily protecting the surface of the optical film or functional layer, and is not particularly limited as long as it is a peelable film that can protect the surface of the optical film or functional layer. Examples of protective films include polyester-based resin films such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyolefin-based resin films such as polyethylene and polypropylene films; and acrylic-based resin films. Preferably, the protective film is selected from the group consisting of polyolefin-based resin films, polyethylene terephthalate-based resin films, and acrylic-based resin films. When the optical film has two protective films, the protective films may be the same or different.

[0106] The thickness of the protective film is not particularly limited, but is usually 10 to 120 μm, preferably 15 to 110 μm, and more preferably 20 to 100 μm. When the optical film has two protective films, the thicknesses of the respective protective films may be the same or different.

[0107] [Method for producing optical film] The optical film of the present invention can be produced, for example, by the following process, although it is not particularly limited thereto: (a) a step of preparing a liquid containing the polyimide resin (sometimes referred to as a resin varnish) (varnish preparation step); (b) a step of applying a resin varnish to a substrate to form a coating film (application step); and (c) A process of drying the applied liquid (coating film) to form an optical film (optical film forming process). It can be produced by a method comprising:

[0108] In the varnish preparation step, the polyimide resin is dissolved in a solvent, and the additives are added as needed, followed by stirring and mixing to prepare a resin varnish.

[0109] The solvent used to prepare the resin varnish is not particularly limited as long as it can dissolve the resin. Examples of such solvents include amide-based solvents such as DMAc and DMF; lactone-based solvents such as γ-butyrolactone (hereinafter sometimes referred to as GBL) and γ-valerolactone; sulfur-containing solvents such as dimethyl sulfone, dimethyl sulfoxide, and sulfolane; carbonate-based solvents such as ethylene carbonate and propylene carbonate; and combinations thereof. Among these, amide-based solvents and lactone-based solvents are preferred. These solvents can be used alone or in combination. The resin varnish may also contain water, alcohol-based solvents, ketone-based solvents, acyclic ester-based solvents, ether-based solvents, and the like. The solids concentration of the varnish is preferably 1 to 25% by mass, more preferably 5 to 15% by mass. In this specification, the solids content of the varnish refers to the total amount of the components of the varnish excluding the solvent.

[0110] In the coating step, the varnish is applied to the substrate to form a coating film by a known coating method, such as roll coating methods such as wire bar coating, reverse coating, and gravure coating, die coating, comma coating, lip coating, spin coating, screen coating, fountain coating, dipping, spraying, and drip molding.

[0111] In the optical film formation process, the coating film is dried and peeled off from the substrate to form an optical film. A drying process for drying the optical film may be performed after peeling. The coating film can be dried typically at a temperature of 50 to 350°C, preferably 50 to 230°C. In a preferred embodiment of the present invention, stepwise drying is preferred. Varnishes containing high-molecular-weight resins tend to have high viscosity, which generally makes it difficult to obtain a uniform film, and may result in a film with excellent transparency. Therefore, stepwise drying allows the varnish containing the high-molecular-weight resin to be dried uniformly, thereby improving transparency. If necessary, the coating film may be dried under inert atmosphere conditions. Furthermore, drying the optical film under vacuum conditions may result in the generation and retention of tiny bubbles in the film, which can reduce transparency. Therefore, drying under atmospheric pressure is preferred.

[0112] Examples of the substrate include PET film, PEN film, other polyimide resin or polyamide resin films, etc. Among these, PET film, PEN film, etc. are preferred from the viewpoint of excellent heat resistance, and PET film is more preferred from the viewpoint of adhesion to the optical film and cost.

[0113] The optical film of the present invention can be suitably used as a front panel (sometimes referred to as a window film) for a display device, particularly a flexible display device, particularly a front panel for a rollable display or a foldable display. That is, the optical film of the present invention is preferably a film for a front panel of a flexible display device. The front panel has the function of protecting the display elements of the flexible display device. A flexible display device is a display device that is used while undergoing operations such as repeated bending and rolling of the image display device. The front panel of a flexible display device that is used while undergoing such repeated bending operations is required to have high folding endurance. The front panel is also required to have high visibility. Compared to films for substrates of image display devices used inside the image display device, the front panel of an image display device, particularly a film for a front panel of a flexible display device, is required to have high visibility as well as high folding endurance. For example, the film of the present invention preferably has the total light transmittance, haze and / or YI value described above, from the viewpoint of easily improving visibility when used as the front panel of a flexible display device, and preferably satisfies the number of times it can be folded in the folding fatigue test described above, from the viewpoint of easily improving folding resistance when used as the front panel of a flexible display device.

[0114] Examples of display devices include televisions, smartphones, mobile phones, car navigation systems, tablet PCs, portable game consoles, electronic paper, indicators, bulletin boards, watches, and wearable devices such as smartwatches. Examples of flexible displays include display devices with flexible properties, such as televisions, smartphones, mobile phones, and smartwatches. Examples of flexible display devices include all image display devices with flexible properties, such as the rollable displays and foldable displays described above. A rollable display is an image display device in which the image display portion, including the front panel, is wound up in a roll and is used in a state where the image display portion is pulled out to form a flat or curved surface. This image display device requires operations such as rewinding into a roll each time it is used. A foldable display is an image display device in which the image display portion, including the front panel, is folded and is used in a state where the image display portion is unfolded to form a flat or curved surface. This image display device requires operations such as folding each time it is used. Such image display devices that require repeated operations such as rewinding and folding are referred to as flexible display devices.

[0115] [Flexible display device] The present invention encompasses a flexible display device comprising the optical film of the present invention. The optical film of the present invention is preferably used as a front panel in a flexible display device, and the front panel is sometimes referred to as a window film. The flexible display device comprises a laminate for a flexible display device and an organic EL display panel, with the laminate for a flexible display device being disposed on the viewing side of the organic EL display panel and configured to be foldable. The laminate for a flexible display device may further contain a polarizing plate and a touch sensor. While these may be stacked in any order, the order from the viewing side is preferably window film, polarizing plate, touch sensor, or window film, touch sensor, polarizing plate. The presence of a polarizing plate on the viewing side of the touch sensor is preferable because it makes the touch sensor pattern less visible, improving the visibility of the displayed image. Each component can be laminated using an adhesive, pressure-sensitive adhesive, or the like. Furthermore, a light-shielding pattern may be formed on at least one surface of any of the window film, polarizing plate, and touch sensor layers.

[0116] [Polarizing plate] As described above, the flexible display device of the present invention preferably further includes a polarizing plate, particularly a circular polarizing plate. A circular polarizing plate is a functional layer that transmits only right- or left-circularly polarized light by laminating a λ / 4 retardation plate on a linear polarizing plate. For example, a circular polarizing plate is used to convert external light into right-circularly polarized light, block the external light that is reflected by an organic EL panel and becomes left-circularly polarized, and transmit only the light-emitting component of the organic EL panel, thereby suppressing the influence of reflected light and making images easier to view. To achieve circular polarization, the absorption axis of the linear polarizing plate and the slow axis of the λ / 4 retardation plate theoretically need to be at an angle of 45°, but in practice, the angle is 45±10°. The linear polarizing plate and the λ / 4 retardation plate do not necessarily need to be laminated adjacent to each other; the relationship between the absorption axis and the slow axis may be within the aforementioned range. While achieving complete circular polarization across all wavelengths is preferable, this is not necessarily required in practice. Therefore, the circular polarizing plate of the present invention also encompasses elliptical polarizing plates. It is also preferable to further laminate a λ / 4 phase difference film on the viewing side of the linear polarizer to make the emitted light circularly polarized, thereby improving visibility when wearing polarized sunglasses.

[0117] A linear polarizing plate is a functional layer that transmits light vibrating in the direction of its transmission axis but blocks polarized light vibrating perpendicularly to the direction of the transmission axis. The linear polarizing plate may be composed of a linear polarizer alone or a linear polarizer with a protective film attached to at least one surface of the linear polarizer. The thickness of the linear polarizing plate may be 200 μm or less, and preferably 0.5 to 100 μm. When the thickness of the linear polarizing plate is within the above range, the flexibility of the linear polarizing plate tends not to decrease.

[0118] The linear polarizer may be a film-type polarizer produced by dyeing and stretching a polyvinyl alcohol (hereinafter sometimes abbreviated as PVA) film. A dichroic dye such as iodine is adsorbed onto the PVA film oriented by stretching, or the dichroic dye is oriented by stretching the PVA film while adsorbed thereto, thereby exhibiting polarization performance. The production of the film-type polarizer may also include other steps such as swelling, crosslinking with boric acid, washing with an aqueous solution, and drying. The stretching and dyeing steps may be performed on the PVA film alone, or may be performed on the PVA film laminated with another film such as polyethylene terephthalate. The thickness of the PVA film used is preferably 10 to 100 μm, and the stretching ratio is preferably 2 to 10. Another example of the polarizer is a liquid crystal coating-type polarizer formed by coating a liquid crystal polarizing composition. The liquid crystal polarizing composition can contain a liquid crystal compound and a dichroic dye compound. The liquid crystal compound may have the property of exhibiting a liquid crystal state, and a highly oriented state such as a smectic phase is particularly preferred because it can exhibit high polarization performance. Furthermore, the liquid crystal compound preferably has a polymerizable functional group. The dichroic dye compound is a dye that exhibits dichroism when aligned together with the liquid crystal compound, and may have a polymerizable functional group, or the dichroic dye itself may have liquid crystal properties. Any of the compounds contained in the liquid crystal polarizing composition has a polymerizable functional group, and the liquid crystal polarizing composition may further contain an initiator, a solvent, a dispersant, a leveling agent, a stabilizer, a surfactant, a crosslinking agent, a silane coupling agent, etc. The liquid crystal polarizing layer is produced by applying a liquid crystal polarizing composition onto an alignment film to form a liquid crystal polarizing layer. The liquid crystal polarizing layer can be formed thinner than a film-type polarizer, and its thickness is preferably 0.5 to 10 μm, more preferably 1 to 5 μm.

[0119] The alignment film is produced, for example, by applying an alignment film-forming composition to a substrate and imparting alignment properties by rubbing, polarized light irradiation, or the like. The alignment film-forming composition contains an alignment agent and may further contain a solvent, a crosslinking agent, an initiator, a dispersant, a leveling agent, a silane coupling agent, or the like. Examples of the alignment agent include polyvinyl alcohols, polyacrylates, polyamic acids, and polyimides. When using an alignment agent that imparts alignment properties by polarized light irradiation, it is preferable to use an alignment agent containing a cinnamate group. The weight-average molecular weight of the polymer used as the alignment agent is, for example, about 10,000 to 1,000,000. The thickness of the alignment film is preferably 5 to 10,000 nm, and more preferably 10 to 500 nm, in order to fully exert alignment control force. The liquid crystal polarizing layer can be peeled off from the substrate and transferred and laminated, or the substrate can be laminated as it is. It is also preferable that the substrate serves as a transparent substrate for a protective film, a retardation plate, or a window film.

[0120] The protective film may be any transparent polymer film, and the same materials and additives as those used in the transparent substrate of the window film may be used. Alternatively, the protective film may be a coating-type protective film obtained by applying and curing a cationic curing composition such as an epoxy resin or a radical curing composition such as an acrylate. The protective film may optionally contain a plasticizer, an ultraviolet absorber, an infrared absorber, a colorant such as a pigment or dye, a fluorescent brightener, a dispersant, a heat stabilizer, a light stabilizer, an antistatic agent, an antioxidant, a lubricant, a solvent, or the like. The thickness of the protective film is preferably 200 μm or less, more preferably 1 to 100 μm. When the thickness of the protective film is within the above range, the flexibility of the film tends to be less likely to decrease.

[0121] The λ / 4 retardation plate is a film that imparts a λ / 4 phase difference in a direction perpendicular to the propagation direction of incident light, i.e., in the in-plane direction of the film. The λ / 4 retardation plate may be a stretched retardation plate produced by stretching a polymer film such as a cellulose-based film, an olefin-based film, or a polycarbonate-based film. The λ / 4 retardation plate may contain, as necessary, a retardation adjuster, a plasticizer, an ultraviolet absorber, an infrared absorber, a colorant such as a pigment or dye, a fluorescent brightener, a dispersant, a heat stabilizer, a light stabilizer, an antistatic agent, an antioxidant, a lubricant, a solvent, etc. The thickness of the stretched retardation plate is preferably 200 μm or less, more preferably 1 to 100 μm. When the thickness of the stretched retardation plate is in the above range, the flexibility of the stretched retardation plate tends not to decrease. Another example of the λ / 4 retardation plate is a liquid crystal coated retardation plate formed by coating a liquid crystal composition. The liquid crystal composition contains a liquid crystal compound that exhibits a liquid crystal state such as nematic, cholesteric, smectic, etc. The liquid crystal compound has a polymerizable functional group. The liquid crystal composition may further contain an initiator, a solvent, a dispersant, a leveling agent, a stabilizer, a surfactant, a crosslinking agent, a silane coupling agent, and the like. The liquid crystal-coated retardation plate can be produced by coating a liquid crystal composition on a substrate and curing it to form a liquid crystal retardation layer, similar to the liquid crystal polarizing layer. The liquid crystal-coated retardation plate can be formed thinner than a stretched retardation plate. The thickness of the liquid crystal polarizing layer is preferably 0.5 to 10 μm, more preferably 1 to 5 μm. The liquid crystal-coated retardation plate can be peeled off from the substrate and transferred and laminated, or the substrate can be laminated as it is. It is also preferable that the substrate serves as a transparent substrate for a protective film, a retardation plate, or a window film.

[0122] Generally, many materials exhibit greater birefringence at shorter wavelengths and smaller birefringence at longer wavelengths. In this case, since it is not possible to achieve a λ / 4 retardation across the entire visible light range, the in-plane retardation is preferably designed to be 100 to 180 nm, more preferably 130 to 150 nm, so that it is λ / 4 around 560 nm, where luminosity is high. A reverse dispersion λ / 4 retarder using a material with birefringence wavelength dispersion characteristics opposite to those of ordinary retarders is preferred because it provides good visibility. Examples of such materials include stretched retarders described in JP 2007-232873 A and liquid crystal coated retarders described in JP 2010-30979 A. Another known method is to obtain a wideband λ / 4 retardation plate by combining it with a λ / 2 retardation plate (for example, JP-A-10-90521). λ / 2 retardation plates are manufactured using the same materials and methods as λ / 4 retardation plates. The combination of a stretched retardation plate and a liquid crystal-coated retardation plate is arbitrary, but the thickness of both can be reduced by using a liquid crystal-coated retardation plate. A method of laminating a positive C plate on the circular polarizer to improve visibility in oblique directions is known (for example, JP 2014-224837 A). The positive C plate may be a liquid crystal-coated retardation plate or a stretched retardation plate. The retardation in the thickness direction of the retardation plate is preferably −200 to −20 nm, more preferably −140 to −40 nm.

[0123] [Touch sensor] As described above, the flexible display device of the present invention preferably further includes a touch sensor. The touch sensor is used as an input means. Examples of touch sensors include a resistive film type, a surface acoustic wave type, an infrared type, an electromagnetic induction type, and a capacitance type, and a capacitance type is preferred. A capacitive touch sensor is divided into an active area and an inactive area located on the periphery of the active area. The active area corresponds to a display area of a display panel where a screen is displayed and is an area where a user's touch is sensed, while the inactive area corresponds to a display area where a screen is not displayed on a display device. The touch sensor may include a flexible substrate, a sensing pattern formed in the active area of the substrate, and sensing lines formed in the inactive area of the substrate for connecting to an external driving circuit via the sensing pattern and a pad portion. The flexible substrate may be made of the same material as the transparent substrate of the window film.

[0124] The sensing pattern may include a first pattern formed in a first direction and a second pattern formed in a second direction. The first and second patterns are arranged in different directions. The first and second patterns are formed on the same layer and must be electrically connected to sense a touch point. The first pattern has a structure in which a plurality of unit patterns are connected to each other via joints, while the second pattern has a structure in which a plurality of unit patterns are separated from each other in an island form, requiring a separate bridge electrode to electrically connect the second pattern. A well-known transparent electrode may be used as an electrode for connecting the second pattern. Examples of materials for the transparent electrode include indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), indium zinc tin oxide (IZTO), indium gallium zinc oxide (IGZO), cadmium tin oxide (CTO), PEDOT (poly(3,4-ethylenedioxythiophene)), carbon nanotubes (CNT), graphene, and metal wires, with ITO being preferred. These materials may be used alone or in combination. There are no particular limitations on the metal used for the metal wire, and examples include silver, gold, aluminum, copper, iron, nickel, titanium, tellurium, and chromium, which can be used alone or in combination of two or more. The bridge electrode may be formed on an insulating layer on the sensing pattern, or the bridge electrode may be formed on a substrate, and the insulating layer and sensing pattern may be formed thereon. The bridge electrode may be formed of the same material as the sensing pattern, or may be formed of molybdenum, silver, aluminum, copper, palladium, gold, platinum, zinc, tin, titanium, or an alloy of two or more of these. Since the first and second patterns must be electrically insulated, an insulating layer is formed between the sensing pattern and the bridge electrode. The insulating layer can be formed only between the joint of the first pattern and the bridge electrode, or can be formed as a layer covering the entire sensing pattern. In the case where the insulating layer covers the entire sensing pattern, the bridge electrode can connect to the second pattern through a contact hole formed in the insulating layer.

[0125] The touch sensor may further include an optical modulation layer between the substrate and the electrode as a means for appropriately compensating for a difference in transmittance between a patterned region where a sensing pattern is formed and a non-patterned region where a sensing pattern is not formed, specifically, a difference in light transmittance caused by a difference in refractive index between these regions. The optical modulation layer may include an inorganic insulating material or an organic insulating material. The optical modulation layer may be formed by coating a photocurable composition including a photocurable organic binder and a solvent on the substrate. The photocurable composition may further include inorganic particles. The inorganic particles may increase the refractive index of the optical modulation layer. The photocurable organic binder may include, as long as the effects of the present invention are not impaired, a copolymer of various monomers, such as an acrylate monomer, a styrene monomer, a carboxylic acid monomer, etc. The photocurable organic binder may also be a copolymer including different repeating units, such as an epoxy group-containing repeating unit, an acrylate repeating unit, and a carboxylic acid repeating unit. Examples of the inorganic particles include zirconia particles, titania particles, and alumina particles. The photocurable composition may further contain various additives such as a photopolymerization initiator, a polymerizable monomer, and a curing aid.

[0126] [Adhesive layer] The flexible display laminate includes layers such as a window film, a circular polarizer, and a touch sensor, and film components such as a linear polarizer and a λ / 4 retarder. The adhesive may be a commonly used adhesive, such as a water-based adhesive, an organic solvent-based adhesive, a solventless adhesive, a solid adhesive, a solvent-volatile adhesive, a water-based solvent-volatile adhesive, a moisture-curable adhesive, a heat-curable adhesive, an anaerobic-curable adhesive, an active energy ray-curable adhesive, a curing agent-mixed adhesive, a hot-melt adhesive, a pressure-sensitive adhesive, a pressure-sensitive adhesive, or a remoistening adhesive. The thickness of the adhesive layer can be adjusted appropriately depending on the desired adhesive strength, and is preferably 0.01 to 500 μm, more preferably 0.1 to 300 μm. The flexible display laminate includes multiple adhesive layers, each of which may be the same or different in thickness and type.

[0127] The aqueous solvent-evaporating adhesive can be, as a base polymer, a water-soluble polymer such as a polyvinyl alcohol polymer, starch, or an ethylene-vinyl acetate emulsion or a styrene-butadiene emulsion. In addition to the base polymer and water, a crosslinking agent, a silane compound, an ionic compound, a crosslinking catalyst, an antioxidant, a dye, a pigment, an inorganic filler, an organic solvent, or the like may be blended. When using the aqueous solvent-evaporating adhesive for adhesion, adhesiveness can be imparted by injecting the aqueous solvent-evaporating adhesive between the adherend layers, laminating the adherend layers, and then drying. When using the aqueous solvent-evaporating adhesive, the thickness of the adhesive layer is preferably 0.01 to 10 μm, more preferably 0.1 to 1 μm. When using multiple layers of the aqueous solvent-evaporating adhesive, the thickness and type of each layer may be the same or different.

[0128] The active energy ray-curable adhesive can be formed by curing an active energy ray-curable composition containing a reactive material that forms an adhesive layer upon irradiation with active energy rays. The active energy ray-curable composition can contain at least one polymer of a radical polymerizable compound and a cationic polymerizable compound similar to those contained in the hard coat composition. The radical polymerizable compound can be the same compound as the radical polymerizable compound in the hard coat composition. The cationically polymerizable compound can be the same as the cationically polymerizable compound in the hard coat composition. As the cationically polymerizable compound used in the active energy ray-curable composition, an epoxy compound is particularly preferred. It is also preferred to contain a monofunctional compound as a reactive diluent in order to reduce the viscosity of the adhesive composition.

[0129] The active energy ray composition may contain a monofunctional compound to reduce viscosity. Examples of the monofunctional compound include an acrylate monomer having one (meth)acryloyl group per molecule, and a compound having one epoxy group or oxetanyl group per molecule, such as glycidyl (meth)acrylate. The active energy ray composition may further contain a polymerization initiator. Examples of the polymerization initiator include radical polymerization initiators, cationic polymerization initiators, and radical and cationic polymerization initiators, which are appropriately selected and used. These polymerization initiators are decomposed by at least one of active energy ray irradiation and heating to generate radicals or cations, thereby promoting radical polymerization and cationic polymerization. Among the hard coat compositions, an initiator capable of initiating at least either radical polymerization or cationic polymerization by active energy ray irradiation can be used. The active energy ray-curable composition may further contain an ion scavenger, an antioxidant, a chain transfer agent, an adhesion promoter, a thermoplastic resin, a filler, a flow viscosity modifier, a plasticizer, an antifoaming agent, a solvent, an additive, and a solvent. When two adherend layers are bonded using the active energy ray-curable adhesive, the active energy ray-curable composition is applied to one or both of the adherend layers, followed by lamination, and one or both of the adherend layers are irradiated with active energy rays to cure the adhesive. When the active energy ray-curable adhesive is used, the thickness of the adhesive layer is preferably 0.01 to 20 μm, more preferably 0.1 to 10 μm. When the active energy ray-curable adhesive is used to form multiple adhesive layers, the thickness and type of each layer may be the same or different.

[0130] The pressure-sensitive adhesive is classified into acrylic pressure-sensitive adhesives, urethane pressure-sensitive adhesives, rubber pressure-sensitive adhesives, silicone pressure-sensitive adhesives, etc. depending on the base polymer, and any of these can be used. In addition to the base polymer, the pressure-sensitive adhesive may contain a crosslinking agent, a silane compound, an ionic compound, a crosslinking catalyst, an antioxidant, a tackifier, a plasticizer, a dye, a pigment, an inorganic filler, etc. The components constituting the pressure-sensitive adhesive are dissolved or dispersed in a solvent to obtain a pressure-sensitive adhesive composition, which is then applied to a substrate and dried to form a pressure-sensitive adhesive layer. The pressure-sensitive adhesive layer may be formed directly, or a layer formed separately on a substrate may be transferred. It is also preferable to use a release film to cover the adhesive surface before bonding. When the active energy ray-curable adhesive is used, the thickness of the adhesive layer is preferably 0.1 to 500 μm, more preferably 1 to 300 μm. When multiple layers of the pressure-sensitive adhesive are used, the thickness and type of each layer may be the same or different.

[0131] [Light blocking pattern] The light-shielding pattern can be applied as at least a part of the bezel or housing of the flexible display device. The light-shielding pattern conceals wiring arranged at the periphery of the flexible display device, making it less visible, thereby improving image visibility. The light-shielding pattern may be in a single-layer or multi-layer form. The color of the light-shielding pattern is not particularly limited and may be various colors such as black, white, and metallic colors. The light-shielding pattern may be formed from a pigment for realizing the color and a polymer such as an acrylic resin, an ester resin, an epoxy resin, a polyurethane, or a silicone. These may be used alone or in combination of two or more types. The light-shielding pattern can be formed by various methods such as printing, lithography, and inkjet printing. The thickness of the light-shielding pattern is preferably 1 to 100 μm, more preferably 2 to 50 μm. It is also preferable to impart a shape such as a slope to the thickness direction of the light-shielding pattern.

[0132] [Polyimide resin] The present invention encompasses polyimide-based resins containing a structural unit represented by formula (1), where Y in formula (1) contains a structure represented by formula (3), and where the polyimide-based resin has a weight-average molecular weight of 160,000 or more. Because the polyimide-based resin of the present invention contains a structure represented by formula (3) as Y in formula (1) and has a weight-average molecular weight of 160,000 or more, optical films containing the polyimide-based resin have excellent transparency and folding endurance. Furthermore, optical films containing the polyimide-based resin of the present invention can have an excellent elastic modulus. Therefore, optical films containing the polyimide-based resin of the present invention can be suitably used as materials for flexible display devices, etc. The polyimide-based resin of the present invention is preferably the same as the polyimide-based resin described above in the section <Polyimide-based Resin>. [Example]

[0133] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples. First, measurement and evaluation methods will be described.

[0134] <Total light transmittance> In accordance with JIS K 7105:1981, the total light transmittance (Tt) of the optical films obtained in the examples and comparative examples was measured using a fully automatic direct-reading haze computer HGM-2DP manufactured by Suga Test Instruments Co., Ltd.

[0135] <Haze> In accordance with JIS K 7136:2000, the optical films obtained in the examples and comparative examples were cut into a size of 30 mm × 30 mm, and the haze (%) was measured using a haze computer (manufactured by Suga Test Instruments Co., Ltd., "HGM-2DP").

[0136] <YI value> For the optical films obtained in the examples and comparative examples, the tristimulus values (X, Y, Z) were determined using an ultraviolet-visible near-infrared spectrophotometer (V-670 manufactured by JASCO Corporation), and the YI value was calculated by substituting them into the following calculation formula. YI = 100×(1.2769X - 1.0592Z) / Y

[0137] <Evaluation of folding resistance> In accordance with ASTM standard D2176-16, the number of folding cycles of the optical films in the examples and comparative examples was determined as follows. The optical film was cut into a strip shape of 15 mm × 100 mm using a dumbbell cutter. The cut optical film was set on the main body of an MIT folding fatigue tester ("Model 0530", manufactured by Toyo Seiki Seisakusho Co., Ltd.), and the number of reciprocating folding cycles in the front and back directions until the optical film broke was measured under the conditions of a test speed of 175 cpm, a folding angle of 135°, a load of 0.75 kgf, and a radius R of the folding clamp of 1 mm. This was taken as the number of folding cycles (also referred to as the folding resistance).

[0138] <Elastic modulus> The elastic modulus of the optical films obtained in the examples and comparative examples was measured using "Autograph AG-IS" manufactured by Shimadzu Corporation. A film with a width of 10 mm in both vertical and horizontal directions was prepared, and the stress-strain curve (S-S curve) was measured under the conditions of a chuck distance of 50 mm and a tensile speed of 10 mm / min, and the elastic modulus was calculated from the slope.

[0139] <Measurement of weight average molecular weight> GPC measurement (1) Pretreatment method A DMF eluent (a 10 mmol / L lithium bromide-added solution) was added to the polyimide resin obtained in each of the Examples and Comparative Examples to a concentration of 2 mg / mL, and the mixture was heated at 80°C for 30 minutes with stirring. After cooling, the mixture was filtered through a 0.45 μm membrane filter to prepare a measurement solution. (2) Measurement conditions Column: Tosoh Corporation TSKgel α-2500 ((7) 7.8 mm diameter x 300 mm) x 1, α-M ((13) 7.8 mm diameter x 300 mm) x 2 Eluent: DMF (10 mmol / L lithium bromide added) Flow rate: 1.0mL / min Detector: RI detector Column temperature: 40℃ Injection volume: 100μL Molecular weight standard: Standard polystyrene

[0140] <Optical film thickness> The thickness of the optical films obtained in the examples and comparative examples was measured using an ABS Digimatic Indicator (manufactured by Mitutoyo Corporation, "ID-C112BS").

[0141] Example 1 [Preparation of polyimide resin (1)] Under a nitrogen atmosphere, TFMB and DMAc rigorously dehydrated to a water content of 700 ppm or less were added to a separable flask equipped with a stirring blade so that the TFMB solids content was 7.81% by mass. TFMB was dissolved in the DMAc with stirring at room temperature. TAHMBP was then added to the flask at a concentration of 101.01 mol% relative to TFMB and stirred at room temperature for 16 hours. 4-picoline (60.61 mol% relative to TFMB) and acetic anhydride (707.07 mol% relative to TFMB) were then added. After stirring for 30 minutes, the internal temperature was gradually increased to 50°C over 20 minutes, then to 60°C over 20 minutes, and finally to 70°C over 20 minutes. The mixture was stirred for an additional 3 hours to obtain a reaction solution. The resulting reaction solution was cooled to room temperature and poured into a large amount of methanol in the form of a thread. The precipitate was removed, immersed in methanol for 6 hours, and then washed with methanol. The precipitate was then dried under reduced pressure at 60°C to obtain polyimide resin (1). The weight-average molecular weight was 484,000.

[0142] [Production of Optical Film (1)] DMAc was added to the obtained polyimide resin (1) so that the solid content concentration was 10% by mass, and polyimide varnish (1) was prepared. The obtained polyimide varnish (1) was applied to the smooth surface of a polyester substrate (manufactured by Toyobo Co., Ltd., product name "A4100") using an applicator so that the thickness of the free-standing film was 55 μm, and the film was dried at 50°C for 30 minutes and then at 140°C for 15 minutes to obtain a free-standing film. The free-standing film was fixed to a metal frame and further dried at 200°C for 60 minutes to obtain an optical film (1) with a thickness of 50 μm.

[0143] <Comparative Example 1> [Preparation of polyimide resin (2)] Under a nitrogen atmosphere, TFMB and DMAc rigorously dehydrated to a water content of 700 ppm or less were added to a separable flask equipped with a stirring blade so that the TFMB solids content was 9.43 mass%. The TFMB was dissolved in the DMAc with stirring at room temperature. Next, 6FDA was added to the flask so that the solids content was 101.01 mol% relative to TFMB, and the mixture was stirred at room temperature for 16 hours. Then, 4-picoline (60.61 mol% relative to TFMB) and acetic anhydride (707.07 mol% relative to TFMB) were added. After stirring for 30 minutes, the mixture was heated to 70°C and stirred for an additional 3 hours to obtain a reaction solution. The resulting reaction solution was cooled to room temperature and poured into a large amount of methanol in the form of a thread. The resulting precipitate was removed, immersed in methanol for 6 hours, and then washed with methanol. The precipitate was then dried under reduced pressure at 60°C to obtain polyimide resin (2). The weight-average molecular weight was 255,000.

[0144] [Production of Optical Film (2)] DMAc was added to the obtained polyimide resin (2) so that the solid content concentration was 10% by mass, to prepare polyimide varnish (2). The obtained polyimide varnish (2) was applied to the smooth surface of a polyester substrate (manufactured by Toyobo Co., Ltd., product name "A4100") using an applicator so that the thickness of the free-standing film was 55 μm, and the film was dried at 50°C for 30 minutes and then at 140°C for 15 minutes to obtain a free-standing film. The free-standing film was fixed to a metal frame and further dried at 200°C for 60 minutes to obtain an optical film (2) with a thickness of 50 μm.

[0145] <Comparative Example 2> [Preparation of polyimide resin (3)] Under a nitrogen atmosphere, TFMB and DMAc rigorously dehydrated to a water content of 700 ppm or less were added to a separable flask equipped with a stirring blade so that the TFMB solids content was 11.48 mass%. TFMB was dissolved in DMAc with stirring at room temperature. Next, BPDA was added to the flask so that the TFMB solids content was 101.01 mol%. The mixture was stirred at room temperature, but the viscosity increased over time, and after 16 hours the mixture became solid. Stirring became difficult, and the preparation was abandoned.

[0146] <Comparative Example 3> [Preparation of polyimide resin (4)] Under a nitrogen atmosphere, TFMB and DMAc rigorously dehydrated to a water content of 700 ppm or less were added to a separable flask equipped with a stirring blade so that the TFMB solids content was 8.52 mass%, and the TFMB was dissolved in the DMAc with stirring at room temperature. Next, an ester of trimellitic anhydride and 4,4'-biphenol was added to the flask so that the solids content was 101.01 mol% relative to the TFMB content. Stirring was continued at room temperature, but the viscosity increased over time, and after 16 hours the mixture became solid, causing poor stirring, so the preparation was abandoned.

[0147] The optical films obtained in Example 1 and Comparative Example 1 were subjected to measurements of total light transmittance, haze, YI value, and modulus of elasticity, and folding endurance evaluation, and the results are shown in Table 1. As for Comparative Examples 2 and 3, as described above, the polyimide resin was no longer kept in a solution state during synthesis, and the polyimide resin could not be extracted, and therefore, evaluation as an optical film was not possible.

[0148] [Table 1]

[0149] As shown in Table 1, the optical film of Example 1 was found to have a high number of folding times, a high total light transmittance, and a low haze. Therefore, it was found that the optical film of the present invention is excellent in transparency and folding resistance.

Claims

1. An optical film containing a polyimide resin, The polyimide resin has the formula (1): 【Chemical Formula 1】 [In formula (1), X represents a divalent organic group, Y represents a tetravalent organic group; * represents a bond. Contains a constitutional unit represented by X in formula (1) is represented by formula (4): 【change】 [In formula (4), A represents a single bond, R Ａ１ and R Ａ２ each independently represents a hydrogen atom or an alkyl group which may have a halogen atom; R ６ each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; s's each independently represent an integer of 0 to 4; * represents a bond. It includes a structure represented by As Y in formula (1), formula (3): 【Chemistry 2】 [In formula (3), R １ each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; R ２ ~R ５ each independently represent a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom; each m independently represents an integer of 0 to 3; n represents an integer of 1 to 4; * represents a bond, provided that R ２ ~R ５ In at least one benzene ring having R ２ ~R ５ at least one of which is a monovalent hydrocarbon group which may have a halogen atom] and having a weight average molecular weight of 160,000 or more.

2. The optical film according to claim 1 , wherein X in formula (1) comprises at least one of a divalent aromatic group, a divalent alicyclic group, and a divalent aliphatic group.

3. 3. The optical film according to claim 1, which has a thickness of 35 μm or more.

4. 4. The optical film according to claim 1, wherein the total light transmittance is 85% or more.

5. 5. The optical film according to claim 1, wherein the yellowness index is 3.0 or less.

6. 6. The optical film according to claim 1, wherein the modulus of elasticity is 3.5 GPa or more.

7. 7. The optical film according to claim 1, which is a film for a front panel of a flexible display device.

8. A flexible display device comprising the optical film according to any one of claims 1 to 7.

9. The flexible display of claim 8 , further comprising a touch sensor.

10. The flexible display device according to claim 8 or 9, further comprising a polarizer.

11. Formula (1): 【Chemistry 4】 [In formula (1), X represents a divalent organic group, Y represents a tetravalent organic group; * represents a bond. Contains a constitutional unit represented by X in formula (1) is represented by formula (4): 【change】 [In formula (4), A represents a single bond, R 6 s each independently represent a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; s's each independently represent an integer of 0 to 4; * represents a bond. It includes a structure represented by As Y in formula (1), formula (3): 【Chemistry 5】 [In formula (3), R １ each independently represents a halogen atom, an alkyl group, an alkoxy group, an aryl group, or an aryloxy group which may have a halogen atom; R ２ ~R ５ each independently represents a hydrogen atom or a monovalent hydrocarbon group which may have a halogen atom; m's each independently represent an integer of 0 to 3; n represents an integer of 1 to 4; * represents a bond, provided that R ２ ~R ５ In at least one benzene ring having R ２ ~R ５ at least two of which are monovalent hydrocarbon groups which may have a halogen atom] and having a weight average molecular weight of 160,000 or more.

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