Optical film and flexible display

Abstract

To provide an optical film that has excellent bending resistance and contains a copolymer including a structural unit derived from α-methylene lactone.SOLUTION: Optical films 10 and 20 are provided. The optical films 10 and 20 include: a substrate layer 1 containing a copolymer including a structural unit derived from α-methylene lactone and a structural unit derived from alkyl (meth)acrylate; and a coating layer 2 that is disposed on one side of the substrate layer 1 and contains thermoplastic elastomer. The optical films 10 and 20 have total light transmittance of 90% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to optical films and flexible displays. [Background technology]

[0002] Copolymers containing structural units derived from α-methylene lactone are expected to be applied to optical components due to their excellent transparency, etc. For example, Patent Document 1 discloses an optical film containing a copolymer containing structural units derived from α-methylene lactone. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-041007 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been an increasing demand for foldable or rollable flexible displays for mobile applications such as smartphones and tablet terminals. Optical films used in flexible displays are also required to have excellent bending resistance as well as transparency.

[0005] However, conventional optical films containing copolymers containing structural units derived from α-methylene lactone do not have sufficient bending resistance, and there is still room for improvement.

[0006] A primary object of the present disclosure is to provide an optical film having excellent bending resistance and containing a copolymer including structural units derived from α-methylene lactone. [Means for solving the problem]

[0007] The present inventors conducted research to solve the above-mentioned problems and discovered that bending resistance can be improved by placing a coating layer containing a thermoplastic elastomer on one side of a substrate layer containing a copolymer including structural units derived from α-methylene lactone, which led to the completion of the invention of the present disclosure.

[0008] The present disclosure provides the optical films described in [1] to [8] and the flexible display described in [9]. [1] A substrate layer containing a copolymer including a structural unit derived from α-methylene lactone and a structural unit derived from alkyl (meth)acrylate; a coating layer containing a thermoplastic elastomer disposed on one side of the substrate layer; Equipped with An optical film with a total light transmittance of 90% or more. [2] The optical film according to [1], wherein the thermoplastic elastomer comprises at least one selected from the group consisting of an acrylic thermoplastic elastomer, a styrene thermoplastic elastomer, and a urethane thermoplastic elastomer. [3] The optical film according to [1] or [2], wherein the structural unit derived from α-methylene lactone contains a structural unit derived from α-methylene-γ-butyrolactone. [4] The optical film according to any one of [1] to [3], wherein the structural unit derived from alkyl (meth)acrylate contains a structural unit derived from methyl methacrylate. [5] The optical film according to any one of [1] to [4], wherein the copolymer has a glass transition temperature of 115° C. or higher and 180° C. or lower. [6] The optical film according to any one of [1] to [5], wherein the weight average molecular weight of the copolymer is 200,000 or more and 1,000,000 or less. [7] The optical film according to any one of [1] to [6], wherein the thickness of the substrate layer is 20 μm or more and 70 μm or less. [8] The optical film according to any one of [1] to [7], wherein the thickness of the coating layer is 0.5 μm or more and 25 μm or less. [9] A flexible display that can be bent or rolled, The optical film according to any one of [1] to [8] is provided, The optical film is configured so that the radius of curvature of the covering layer becomes larger than the radius of curvature of the base layer when the flexible display is bent or rolled. [Effects of the Invention]

[0009] According to the present disclosure, there is provided an optical film having excellent bending resistance and containing a copolymer including structural units derived from α-methylene lactone, and a flexible display using such an optical film. [Brief explanation of the drawings]

[0010] [Figure 1] 1(a) and 1(b) are schematic cross-sectional views showing one embodiment of an optical film. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described. However, the present disclosure is not limited to the following embodiments. The upper and lower limit values ​​of the numerical ranges specified in the present disclosure may be replaced with any values ​​shown in the examples. Furthermore, the upper and lower limit values ​​individually described may be arbitrarily combined. In the present disclosure, the term "(meth)acrylic acid" means acrylic acid and methacrylic acid. The numerical range shown as X to Y means not less than X and not more than Y.

[0012] <Optical film> An optical film according to one embodiment includes a substrate layer and a coating layer disposed on one side of the substrate layer. FIGS. 1(a) and 1(b) are schematic cross-sectional views illustrating one embodiment of the optical film. The coating layer is not particularly limited as long as it is disposed on one side of the substrate layer. For example, the coating layer 2 may be disposed directly on the substrate layer 1, as in the optical film 10 shown in FIG. 1(a), or may be disposed on the substrate layer 1 via a third layer 3, as in the optical film 20 shown in FIG. 1(b).

[0013] [Base material layer] The substrate layer 1 contains a copolymer containing structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate.

[0014] ·Copolymer (Structural unit derived from α-methylene lactone) α-Methylene lactone is a general term for compounds in which an exomethylene group is bonded to the α-carbon atom of a lactone ring. The number of ring members in the lactone is not particularly limited, but it may be a 4- to 8-membered ring, a 5- to 6-membered ring, or even a 5-membered ring.

[0015] An example of the five-membered α-methylene lactone is a compound represented by the following formula (1).

[0016] [ka]

[0017] R in Equation (1) 1 ~R 4 are each independently a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms. 1 ~R 4 are preferably each independently a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and more preferably all are hydrogen atoms.

[0018] Examples of the compound represented by formula (1) include α-methylene-γ-butyrolactone, α-methylene-β-methyl-γ-butyrolactone, α-methylene-β-ethyl-γ-butyrolactone, α-methylene-γ-methyl-γ-butyrolactone, α-methylene-γ-ethyl-γ-butyrolactone, α-methylene-β,β-dimethyl-γ-butyrolactone, α-methylene-β-methyl-γ-methyl-γ-butyrolactone, and α-methylene-γ,γ-dimethyl-γ-butyrolactone.

[0019] The structural unit derived from α-methylene lactone is formed by polymerization of α-methylene lactone. The copolymer may contain only one type of structural unit derived from α-methylene lactone, or may contain two or more types. The structural unit derived from α-methylene lactone preferably contains a structural unit represented by the following formula (2). The structural unit represented by the following formula (2) is formed, for example, by polymerization of a monomer containing a compound represented by formula (1).

[0020] [ka]

[0021] R in Equation (2) 1 ~R 4 is R in Equation (1). 1 ~R 4 and each independently represents a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms. 1 ~R 4 are preferably each independently a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and more preferably all are hydrogen atoms. 1 ~R 4 When all of are hydrogen atoms, the structural unit shown in formula (2) is a structural unit derived from α-methylene-γ-butyrolactone.

[0022] The hydrocarbon group may be an aliphatic hydrocarbon group or an aromatic hydrocarbon group. Examples of the aliphatic hydrocarbon group include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, a t-butyl group, an n-pentyl group, an n-hexyl group, a cyclopentyl group, and a cyclohexyl group. Examples of the aromatic hydrocarbon group include a phenyl group, a tolyl group, and a benzyl group.

[0023] The content of structural units derived from α-methylene lactone in the copolymer is preferably from 5% to 50% by mass, and more preferably from 10% to 40% by mass. This allows the heat resistance, transparency, and strength of an optical film obtained from this copolymer to be further improved. Furthermore, the content of structural units derived from α-methylene-γ-butyrolactone in the total amount of structural units derived from α-methylene lactone is preferably from 80% to 100% by mass, more preferably from 90% to 100% by mass, even more preferably from 95% to 100% by mass, and particularly preferably 100% by mass.

[0024] (Structural units derived from alkyl (meth)acrylate) Alkyl (meth)acrylate is a general term for esters of (meth)acrylic acid and monohydric alkyl alcohol. Examples of alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, n-pentyl (meth)acrylate, and n-hexyl (meth)acrylate.

[0025] The structural unit derived from alkyl (meth)acrylate is formed by polymerization of alkyl (meth)acrylate. The copolymer may contain only one type of structural unit derived from alkyl (meth)acrylate, or may contain two or more types. The structural unit derived from alkyl (meth)acrylate preferably contains a structural unit derived from alkyl (meth)acrylate having an alkyl group with 1 to 6 carbon atoms, more preferably contains a structural unit derived from alkyl (meth)acrylate having an alkyl group with 1 to 3 carbon atoms, and even more preferably contains a structural unit derived from methyl methacrylate.

[0026] The content of structural units derived from alkyl (meth)acrylate in the copolymer is preferably 50% by mass or more and 95% by mass or less, and more preferably 60% by mass or more and 90% by mass or less. This allows the heat resistance, transparency, and strength of an optical film obtained from this copolymer to be further improved. Furthermore, the content of structural units derived from methyl methacrylate in the total amount of structural units derived from alkyl (meth)acrylate is preferably 80% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less, even more preferably 95% by mass or more and 100% by mass or less, and particularly preferably 100% by mass.

[0027] (Other structural units) The copolymer may further contain other structural units different from the structural units derived from α-methylene lactone and the structural units derived from alkyl (meth)acrylate, as necessary. Examples of the other structural units include structural units derived from monomers such as benzyl (meth)acrylate, chloromethyl (meth)acrylate, 2-chloroethyl (meth)acrylate, styrene, vinyl toluene, α-methylstyrene, acrylonitrile, methyl vinyl ketone, ethylene, propylene, and vinyl acetate. The copolymer may contain only one type of other structural unit, or two or more types. The content of the other structural units in the copolymer may be 0% by mass or more and 20% by mass or less, or 0% by mass or more and 10% by mass or less.

[0028] The content of each structural unit in the copolymer was determined by dissolving the copolymer in a heavy solvent and 1 It is determined by measuring H-NMR and calculating the area ratio of the peaks corresponding to each structural unit.

[0029] The weight-average molecular weight (Mw) of the copolymer is preferably 200,000 or more and 1,000,000 or less, more preferably 220,000 or more and 900,000 or less, and even more preferably 250,000 or more and 700,000 or less. In particular, by making the weight-average molecular weight (Mw) of the copolymer 200,000 or more, preferably 220,000 or more, the bending resistance of the optical film obtained from this copolymer can be further improved. The weight-average molecular weight (Mw) of the copolymer is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0030] The number average molecular weight (Mn) of the copolymer is preferably 80,000 or more and 400,000 or less, more preferably 90,000 or more and 300,000 or less, and even more preferably 100,000 or more and 250,000 or less. In particular, by making the number average molecular weight (Mn) of the copolymer 80,000 or more, preferably 90,000 or more, the bending resistance of the optical film obtained from this copolymer can be further improved. The number average molecular weight (Mn) of the copolymer is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0031] The glass transition temperature (Tg) of the copolymer measured by the onset method is preferably 115°C or higher, more preferably 115°C or higher and 180°C or lower, and even more preferably 120°C or higher and 150°C or lower. In particular, by adjusting the glass transition temperature (Tg) to 115°C or higher, preferably 120°C or higher, the heat resistance of the optical film obtained from this copolymer can be further improved. The glass transition temperature (Tg) of the copolymer is measured in accordance with the specifications of JIS K 7121.

[0032] Copolymer manufacturing method The copolymer can be produced by a known polymerization method such as solution polymerization, bulk polymerization, suspension polymerization, emulsion polymerization, etc. The polymerization method is preferably suspension polymerization, since it can further improve the transparency of the resulting copolymer.

[0033] In suspension polymerization, monomers including α-methylene lactone and alkyl (meth)acrylate are polymerized in a solvent in the presence of a polymerization initiator and an emulsifier to obtain a copolymer.

[0034] The solvent used in suspension polymerization is an aqueous solvent. The aqueous solvent is preferably water alone, but may contain a non-aqueous solvent (water-soluble organic solvent) within an acceptable range. Examples of non-aqueous solvents (water-soluble organic solvents) include alcohol solvents such as methanol, ethanol, propanol, butanol, 2-methylpropyl alcohol, and 2-methyl-2-propanol; ketone solvents such as acetone and methyl ethyl ketone; ester solvents such as ethyl acetate; and ether solvents such as dioxane, diethyl ether, and tetrahydrofuran. The content of the non-aqueous solvent in the aqueous solvent may be 0% by mass or more and 5% by mass or less, 0% by mass or more and 2% by mass or less, or 0% by mass or more and 1% by mass or less.

[0035] Examples of polymerization initiators include organic peroxides such as dilauroyl peroxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, di-t-butyl peroxide, lauroyl peroxide, benzoyl peroxide, t-butylperoxyisopropyl carbonate, t-amylperoxy-2-ethylhexanoate, and t-butylperoxy-2-ethylhexanoate; and azo compounds such as 2,2'-azobis(isobutyronitrile), 1,1'-azobis(cyclohexanecarbonitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and dimethyl-2,2'-azobis(2-methylpropionate). The amount of polymerization initiator added can be adjusted as appropriate, but is preferably 0.01 to 5 parts by mass relative to 100 parts by mass of the monomer.

[0036] Examples of emulsifiers include water-soluble polymer dispersion stabilizers such as polyvinyl alcohol (PVA), polyvinylpyrrolidone, cellulose, gelatin, sodium polyacrylate, and sodium polymethacrylate; anionic surfactants such as sodium lauryl sulfate and polyoxyethylene alkylphenyl ether sulfate salts (e.g., polyoxyethylene distyrylphenyl ether ammonium sulfate); cationic surfactants such as alkylamine salts and quaternary ammonium salts; zwitterionic surfactants such as lauryl dimethylamine oxide; nonionic surfactants such as polyoxyethylene alkyl ethers; and inorganic dispersants such as alginates, zein, and casein; barium sulfate, calcium sulfate, barium carbonate, magnesium carbonate, calcium phosphate, talc, clay, diatomaceous earth, bentonite, titanium hydroxide, sodium hydroxide, and metal oxide powders. The addition of an emulsifier can improve the stability of the polymerization reaction. The amount of emulsifier added may be adjusted as appropriate, but is preferably 0.1 to 4 parts by mass, more preferably 0.2 to 2 parts by mass, relative to 100 parts by mass of the monomer.

[0037] The emulsifier is preferably an ammonium sulfate salt. Ammonium sulfate salts decompose by heating, releasing ammonia. Therefore, the emulsifier remaining in the powder can be sufficiently removed by the heat treatment process described below, further improving the drying efficiency of the solvent in the dope and further increasing the productivity of the optical film. An example of an ammonium sulfate salt emulsifier is polyoxyethylene distyryl phenyl ether sulfate ester ammonium (Hitenol (registered trademark) NF-08).

[0038] In suspension polymerization, when dispersing the monomer in the aqueous solvent, the dispersion may be carried out by stirring with a paddle blade or the like, or may be carried out using an apparatus such as a high-speed shear turbine type disperser, a high-pressure jet homogenizer, an ultrasonic emulsifying disperser, a medium stirring disperser, or a forced gap passage type disperser.

[0039] When polymerizing the monomers, a chain transfer agent, an additive, etc. may be added as needed.

[0040] Examples of chain transfer agents include monofunctional thiol compounds such as n-dodecyl mercaptan and β-mercaptopropionic acid; bifunctional thiol compounds such as polysiloxanes modified at both ends with mercapto groups; and side-chain polyfunctional mercapto-modified polysiloxanes in which the side chains are modified with mercapto groups. The amount of chain transfer agent added may be adjusted as appropriate, but is preferably 0.001 to 1 part by mass relative to 100 parts by mass of the monomer.

[0041] Examples of additives include water-insoluble organic solvents such as alkanes and radical scavengers. The amount of additive added may be adjusted as appropriate, but is preferably 0.001 to 1 part by mass relative to 100 parts by mass of the monomer.

[0042] The polymerization temperature is preferably from 40° C. to 100° C., more preferably from 50° C. to 95° C., and even more preferably from 60° C. to 90° C. The polymerization time is preferably from 0.5 hours to 20 hours, and more preferably from 1 hour to 10 hours.

[0043] After the polymerization, the copolymer can be recovered by solid-liquid separation, which can be performed by filtration, centrifugation, or a combination thereof.

[0044] The obtained copolymer may be subjected to a drying step. The drying step is a step for removing the solvent (aqueous solvent) used in the suspension polymerization and converting the copolymer obtained in the polymerization step into a powder (first powder). The drying temperature is preferably 80°C or higher and 105°C or lower, and more preferably 85°C or higher and 100°C or lower. The drying time is preferably 1 hour or higher and 24 hours or lower, more preferably 3 hours or higher and 15 hours or lower, and even more preferably 5 hours or higher and 12 hours or lower. This allows the aqueous solvent to be removed more efficiently.

[0045] The first powder obtained in the drying step may be further subjected to a heat treatment step, in which the first powder obtained in the drying step is heated at a temperature higher than that in the drying step to obtain a powder (second powder) in which the amount of unreacted residual monomers (α-methylene lactone, alkyl (meth)acrylate, etc.) has been reduced from the first powder.

[0046] The heating temperature in the heat treatment step is preferably (Tg-10)°C or higher (Tg+70)°C or lower, more preferably (Tg-5)°C or higher (Tg+60)°C or lower, and even more preferably (Tg)°C or higher (Tg+50)°C or lower, where Tg is the glass transition temperature of the copolymer. This makes it possible to more efficiently reduce α-methylene lactone and alkyl (meth)acrylate, and when an optical film is produced using the second powder, the bending resistance of the optical film can be improved. Furthermore, the yellowness index (YI) of the optical film can be reduced, and transparency can be sufficiently increased.

[0047] The heating time in the heat treatment step is preferably from 0.1 to 25 hours, more preferably from 1 to 12 hours, and even more preferably from 1 to 8 hours, which allows for more efficient reduction of α-methylene lactone and alkyl (meth)acrylate.

[0048] · Manufacturing method of base layer (film) The base layer 1 may be made of a film 1A. The film 1A may be produced by any known method without any particular limitation. For example, the film 1A may be produced by a so-called solution casting method, which includes a casting step of casting a dope containing a copolymer and a solvent onto a support to form a casting film, and a volatilization step of volatilizing the solvent from the casting film.

[0049] The dope can be obtained by, for example, mixing the copolymer with a solvent. The copolymer may be, for example, a first powder obtained by a drying process, or a second powder obtained by a drying process and a heat treatment process.

[0050] Examples of the solvent include chain ketone solvents such as acetone and methyl ethyl ketone; cyclic ketone solvents such as cyclohexanone (anone) and cyclopentanone; alkyl chloride solvents such as methylene chloride, chloroform, 1,2-dichloroethane, and 1,1-dichloroethane; cyclic ester solvents such as γ-butyrolactone (GBL), γ-valerolactone, and δ-valerolactone; amide solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone (NMP), and N,N'-dimethylimidazolidinone (DMI); sulfoxide solvents such as dimethyl sulfoxide; aromatic solvents such as toluene, xylene, and benzene; and alcohol solvents such as methanol, ethanol, isopropanol, n-butanol, 2-butanol, methyl cellosolve, ethyl cellosolve, and butyl cellosolve. The solvent is preferably a mixture of methylene chloride and ethanol in a volume ratio of 9:1 to 7:3, methyl ethyl ketone, or N,N-dimethylacetamide.

[0051] The content of the copolymer in the dope is preferably 5% by mass to 60% by mass, more preferably 10% by mass to 50% by mass, and even more preferably 15% by mass to 40% by mass, which can increase the productivity of the film 1A.

[0052] The dope may contain other polymers different from the copolymer, if necessary. Examples of other polymers include olefin polymers such as polyethylene, polypropylene, ethylene-propylene copolymer, and poly(4-methyl-1-pentene); halogen-containing polymers such as vinyl chloride and chlorinated vinyl resin; acrylic polymers such as polymethyl methacrylate; styrene polymers such as polystyrene, styrene-methyl methacrylate copolymer, styrene-acrylonitrile copolymer, and acrylonitrile-butadiene-styrene block copolymer; polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyamides such as nylon 6, nylon 66, and nylon 610; polyacetal; polycarbonate; polyphenylene oxide; polyphenylene sulfide; polyether ether ketone; polysulfone; polyethersulfone; polyoxybenzylene; polyamide imide; elastic organic particles such as polybutadiene rubber and acrylic rubber; and rubbery polymers such as ABS resin and ASA resin blended with polybutadiene rubber or acrylic rubber. The dope may contain only one kind of other polymer, or may contain two or more kinds of other polymers, and the content of the other polymer in the dope may be adjusted appropriately according to the composition of the film 1A to be obtained.

[0053] The dope may contain additives as needed. Examples of additives include antioxidants; stabilizers such as light stabilizers, weather stabilizers, and heat stabilizers; reinforcing materials such as glass fibers and carbon fibers; ultraviolet absorbers; near-infrared absorbers; flame retardants; antistatic agents; colorants such as inorganic pigments, organic pigments, and dyes; organic or inorganic antiblocking agents; resin modifiers; plasticizers; lubricants; fluidizing agents; and compatibilizers. The dope may contain only one type of additive, or two or more types. The content of the additives in the dope may be adjusted appropriately depending on the composition of the film 1A to be obtained.

[0054] Examples of the support include a stainless steel endless belt; a rotating metal drum; a metal sheet such as aluminum or copper foil; and a plastic film such as a polyimide film or a polyester film (polyethylene terephthalate film).

[0055] When the dope is cast, an applicator such as a die coater, a doctor blade coater, a roll coater, a comma coater, or a lip coater may be used.

[0056] The method for volatilizing the solvent from the casting film may be, for example, a method of heating the casting film within a temperature range in which no foaming marks are formed on the film 1A.

[0057] The heating temperature for volatilizing the solvent from the casting film is preferably 20° C. or higher and 200° C. or lower, more preferably 20° C. or higher and 150° C. or lower, and further preferably 20° C. or higher and 100° C. or lower, thereby improving the drying efficiency of the solvent while suppressing the generation of foam marks.

[0058] The heating time for volatilizing the solvent from the casting film is preferably 5 minutes to 120 minutes, more preferably 10 minutes to 80 minutes, which can improve the efficiency of drying the solvent.

[0059] It is preferable that the film 1A is stretched. This allows the film 1A to have sufficient strength. The film 1A may be stretched at any time after the optical film is peeled from the support. For example, the film 1A may be stretched after being dried again and before being rolled into a film. Alternatively, the film may be stretched after being rolled into a film roll and then pulled out from the film roll.

[0060] Examples of the stretching method for the film 1A include uniaxial stretching such as free width uniaxial stretching and fixed width uniaxial stretching; and biaxial stretching such as sequential biaxial stretching and simultaneous biaxial stretching.

[0061] The heating temperature for stretching film 1A is near the glass transition temperature (Tg) of the copolymer described above, and more specifically, is preferably from (Tg-30)°C to (Tg+100)°C, more preferably from (Tg-20)°C to (Tg+50)°C, and even more preferably from (Tg-10)°C to (Tg+30)°C.

[0062] The stretching speed in stretching the film 1A is preferably 5% / min or more and 500% / min or less.

[0063] The stretching ratio of the film 1A in the longitudinal and transverse directions is preferably in the range of 1.05 to 10 times.

[0064] From the viewpoint of use as a flexible display, the thickness of the film 1A (base layer 1) is preferably 20 μm or more and 70 μm or less, and more preferably 30 μm or more and 50 μm or less.

[0065] The copolymer content in film 1A is preferably 50% by mass to 100% by mass, more preferably 70% by mass to 100% by mass, and even more preferably 90% by mass to 100% by mass, which allows film 1A to have sufficient heat resistance, transparency, and strength.

[0066] Film 1A may contain the above-mentioned other polymers as needed. Film 1A may contain only one type of other polymer, or may contain two or more types. The content of the other polymers in Film 1A may be 0% by mass or more and 50% by mass or less, 0% by mass or more and 30% by mass or less, or 0% by mass or more and 10% by mass or less.

[0067] Film 1A may contain the above-mentioned additives as needed. Film 1A may contain only one type of additive, or may contain two or more types of additives. The content of the additives in Film 1A may be 0% by mass or more and 5% by mass or less, 0% by mass or more and 2% by mass or less, or 0% by mass or more and 0.5% by mass or less.

[0068] The film 1A becomes the base layer 1 by providing a covering layer 2, which will be described later.

[0069] [Coating layer] The coating layer 2 contains a thermoplastic elastomer. The coating layer 2 is disposed on one side of the base layer 1. For example, the coating layer 2 is disposed directly on the base layer 1, as in the optical film 10 shown in FIG. 1(a).

[0070] When the base layer 1 is bent, a strong tensile stress is applied to the outer side of the base layer 1 (the side with a larger radius of curvature). In particular, if the surface of the base layer 1 has minute depressions, such as foaming marks, the depressions tend to be the starting point for fracture. By disposing the coating layer 2 on one side of the base layer 1 (the outer side of the base layer 1), fracture of the base layer 1 can be suppressed when the base layer 1 is bent so that the radius of curvature of the coating layer 2 is larger than the radius of curvature of the base layer 1 (the base layer 1 faces inward), improving the bending resistance of the optical film 10. This is thought to be because the frictional force at the interface between the base layer 1 and the coating layer 2 weakens the tensile stress acting on the outer side of the base layer 1. On the other hand, since the coating layer 2 contains a thermoplastic elastomer and has rubber elasticity as described below, it is thought to have strong resistance to tensile stress acting on the outer side and is less likely to fracture.

[0071] Thermoplastic elastomers The term "thermoplastic elastomer" refers to, for example, a polymer composed of a soft segment and a hard segment having rubber elasticity. The thermoplastic elastomer has a glass transition temperature of, for example, -10°C or lower, derived from the soft segment, and a glass transition temperature of, for example, 90°C or higher, derived from the hard segment. Due to this structure, the thermoplastic elastomer typically has rubber elasticity at 25°C and softens at high temperatures similar to thermoplastic resins. Examples of thermoplastic elastomers include acrylic thermoplastic elastomers, styrene thermoplastic elastomers, urethane thermoplastic elastomers, polyolefin thermoplastic elastomers, polyester thermoplastic elastomers, polyamide thermoplastic elastomers, and silicone thermoplastic elastomers. Among these, from the viewpoint of further improving the bending resistance of the optical film 10, the thermoplastic elastomer preferably includes at least one selected from the group consisting of acrylic thermoplastic elastomers, styrene thermoplastic elastomers, and urethane thermoplastic elastomers, and more preferably includes at least one selected from the group consisting of acrylic thermoplastic elastomers and styrene thermoplastic elastomers.

[0072] The glass transition temperature of the thermoplastic elastomer is measured in accordance with the provisions of JIS K 7121. The glass transition temperature is measured by the starting point method.

[0073] Examples of acrylic thermoplastic elastomers include ethylene-acrylic acid ester copolymer elastomers, ethylene-methacrylic acid ester copolymer elastomers, and block copolymers composed of polymers of methacrylic acid esters (e.g., polymethyl methacrylate) and polymers of acrylic acid esters (e.g., butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate).

[0074] Styrenic thermoplastic elastomers are elastomers that contain at least styrene-derived structural units. Examples of styrenic thermoplastic elastomers include styrene-ethylene-butadiene-styrene block copolymer (SEBS), styrene-isoprene-styrene block copolymer (SIS), styrene-butadiene-styrene block copolymer (SBS), and styrene-ethylene-propylene-styrene block copolymer (SEPS).

[0075] The urethane-based thermoplastic elastomer is an elastomer having at least a urethane structure, and examples of the urethane-based thermoplastic elastomer include elastomers obtained by reacting a polymeric diol, an organic diisocyanate, and a chain extender.

[0076] Coating layer manufacturing method The coating layer 2 can be produced, for example, by applying a dope containing a thermoplastic elastomer to the film 1A to form a coating film, and then drying the coating film.

[0077] The dope used to produce the coating layer 2 can be obtained, for example, by mixing a thermoplastic elastomer with a solvent. The solvent may be the same as the solvents exemplified in the method for producing the film 1A described above, but is preferably methyl ethyl ketone. When the solvent is methyl ethyl ketone, damage or deterioration of the film 1A due to the solvent is significantly reduced, thereby further improving the bending resistance of the optical film 10.

[0078] The content of the thermoplastic elastomer in the dope is preferably 5% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 55% by mass or less, and further preferably 20% by mass or more and 50% by mass or less.

[0079] The dope may contain other polymers and additives different from the thermoplastic elastomer, as needed, which may be the same as those exemplified in the method for producing the film 1A.

[0080] The method of applying the dope and the method of volatilizing the solvent from the casting film may be the same as the method of applying the dope and the method of volatilizing the solvent from the casting film exemplified in the above-mentioned method of producing the film 1A.

[0081] From the viewpoint of use as a flexible display, the thickness of the covering layer 2 is preferably 0.5 μm or more and 25 μm or less, more preferably 1 μm or more and 20 μm or less, and even more preferably 2 μm or more and 15 μm or less.

[0082] The content of the thermoplastic elastomer in the coating layer 2 is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

[0083] The coating layer 2 may contain the other polymers described above as necessary. The coating layer 2 may contain only one type of other polymer, or may contain two or more types. The content of the other polymer in the coating layer 2 may be 0% by mass or more and 50% by mass or less, 0% by mass or more and 30% by mass or less, or 0% by mass or more and 10% by mass or less.

[0084] The coating layer 2 may contain the above-mentioned additives as needed. The coating layer 2 may contain only one type of additive, or may contain two or more types of additives. The content of the additive in the coating layer 2 may be 0% by mass or more and 5% by mass or less, 0% by mass or more and 2% by mass or less, or 0% by mass or more and 0.5% by mass or less.

[0085] As a modification, the cover layer 2 may be disposed on the substrate layer 1 via a third layer 3, as in the optical film 20 shown in FIG. 1(b).

[0086] [Third Layer] Examples of the third layer 3 include a layer (adhesion-improving layer) for improving the adhesion between the base layer 1 and the coating layer 2, and a layer (refractive index control layer) for controlling optical properties. The third layer 3 may have a single-layer structure or a multi-layer structure. The thickness of the third layer 3 may be, for example, 0.1 μm or more and 20 μm or less.

[0087] The optical film 20 can be produced, for example, by applying the dope used to produce the coating layer 2 onto the third layer 3 integrated with the film 1A to form a coating film, and then drying the coating film.

[0088] From the viewpoint of use as a flexible display, the thickness of the optical films 10, 20 is preferably 20.5 μm or more and 95 μm or less, more preferably 21 μm or more and 90 μm or less, and even more preferably 30 μm or more and 65 μm or less.

[0089] From the viewpoint of hardness, the indentation modulus of the optical films 10 and 20 is preferably 5 GPa or more and 8 GPa or less, and more preferably 5 GPa or more and 7 GPa or less. The indentation modulus of the optical film is measured in accordance with the standard of ISO-14577-1.

[0090] The total light transmittance of the optical films 10 and 20 is 90% or more, and from the viewpoint of transparency, preferably 92% or more. The total light transmittance of the optical film is measured in accordance with the provisions of JIS K 7361.

[0091] From the viewpoint of transparency, the haze of the optical films 10 and 20 is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.5% or less. The haze of the optical films 10 and 20 may be, for example, 0.01% or more, or 0.1% or more. The haze of the optical film is measured in accordance with the specifications of JIS K 7136.

[0092] From the viewpoint of colorability, the yellowness index (YI) of the optical films 10 and 20 is preferably 1.0 or less, more preferably 0.8 or less, and even more preferably 0.5 or less. The yellowness index (YI) of the optical films 10 and 20 may be, for example, 0.01 or more, or 0.1 or more. The yellowness index (YI) of the optical film is measured in accordance with the provisions of JIS K 7373.

[0093] The optical films 10 and 20 are preferably bent 200,000 times or more in a foldable test. At this time, the number of flexures is more preferably 250,000 times or more, and even more preferably 300,000 times or more. This allows for an optical film with sufficient strength to be obtained. The number of flexures in the foldable test may be 5 million times or less, 3 million times or less, or 2 million times or less. The number of flexures in the foldable test may be, for example, 200,000 times or more and 5 million times or less, 250,000 times or more and 5 million times or less, 300,000 times or more and 5 million times or less, 200,000 times or more and 3 million times or less, 250,000 times or more and 3 million times or less, 250,000 times or more and 2 million times or less, 300,000 times or more and 5 million times or less, 300,000 times or more and 3 million times or less, or 300,000 times or more and 2 million times or less.

[0094] In the foldability test, the optical films 10 and 20 are bent into a U-shape. The folded state is defined as a state in which the radius of curvature at the bend is 1 mm and the distance between the edges is 2 mm. In the folded state, the portions other than the bend are approximately parallel. Changing from a flat, unfolded state to a folded state is counted as one bending operation. The bending operation is repeated at a rate of 30 times per minute in an environment of 25°C or below, and the number of bendings until breakage occurs is counted. If the optical film is too large, the test can be performed using a test piece cut to an appropriate size (e.g., 15 mm x 80 mm).

[0095] The optical films 10 and 20 of the present embodiment can be suitably used, for example, in flat panel displays and foldable displays, and in particular, can be suitably used as films (e.g., cover windows) for foldable displays that require resistance to bending.

[0096] <Flexible display> The flexible display of one embodiment is a flexible display that can be bent or rolled. A flexible display that can be bent (folded) is also called a foldable display. A flexible display that can be rolled (wound) is also called a rollable display.

[0097] The flexible display includes the above-described optical films 10 and 20. In this case, the optical films 10 and 20 are provided so that the radius of curvature of the cover layer becomes larger than the radius of curvature of the base layer when the flexible display is bent or rolled. [Example]

[0098] The present disclosure will be described in more detail below with reference to examples and comparative examples. However, the present disclosure is not limited to these examples. Various physical properties were measured and evaluated as follows.

[0099] [Weight average molecular weight (Mw) and number average molecular weight (Mn) of copolymer] The weight average molecular weight (Mw) and number average molecular weight (Mn) of the copolymer were measured in terms of polystyrene using gel permeation chromatography (GPC) using the following measuring device and conditions: Device name: Tosoh GPC system HLC-8220 Measurement column configuration: Guard column: Tosoh Corporation, TSKgel guard column SuperHZ-L) Separation column: Tosoh TSKgel SuperHZM-M, two columns connected in series Reference column configuration: Reference column: Tosoh TSKgel SuperH-RC Developing solvent: chloroform (Wako Pure Chemical Industries, special grade) Developing solvent flow rate: 0.6 mL / min Standard sample: TSK standard polystyrene (Tosoh Corporation, PS-oligomer kit) Column temperature: 40℃

[0100] [Glass transition temperature (Tg) of copolymer] The glass transition temperature of the copolymer was measured in accordance with the provisions of JIS K 7121. Specifically, using a differential scanning calorimeter (Rigaku Thermo plus EVO DSC-8230) in a nitrogen gas atmosphere, approximately 10 mg of a sample was heated from room temperature to 200°C (heating rate 20°C / min), and the temperature was measured by the starting point method from the DSC curve obtained. α-Alumina was used as a reference.

[0101] [Optical film thickness] The thickness of the optical film was measured using a Digimatic micrometer (manufactured by Mitutoyo, ID-C112).

[0102] [Optical film bending resistance (foldable test)] The optical film was cut into a size of 15 mm x 80 mm to prepare a test specimen, which was then fixed with tape to a tension-free folding clamshell type (Yuasa System Co., Ltd., DMLHP-CS). The folding position was set at halfway along the long side of the test specimen. The folded state was defined as a state in which the radius of curvature at the folding position was 1 mm and the distance between the edges was 2 mm. In the folded state, all parts except the folding position were approximately parallel. Bending from a flat, unfolded state to a bent state was counted as one bending operation, and the bending operation was repeated at a rate of 30 times per minute in an environment of 25°C. An optical film that did not break after 250,000 bending operations was rated "A+." A bending operation of 200,000 or more but less than 250,000 times before breaking was rated "A," a bending operation of 100,000 or more but less than 200,000 times was rated "B," and a bending operation of less than 100,000 times was rated "C."

[0103] [Indentation elastic modulus of optical film (E IT )] Indentation modulus of optical film (E IT ) was measured in accordance with the provisions of ISO-14577-1. Specifically, an ultra-microhardness tester (Fisherscope HM-2000, manufactured by Fischer Instruments) was used to measure the substrate layer side with the coating layer of the optical film fixed to a glass substrate. The measurement conditions were a square pyramidal Vickers indenter (facing angle a = 136°), a maximum test load of 10 mN, an application time of 20 seconds when the load was applied, a creep time of 5 seconds, an application time of 20 seconds when the load was reduced, and room temperature (25°C). The average value was obtained by measuring three times.

[0104] [Total light transmittance of optical film] The total light transmittance of the optical film was measured in accordance with the provisions of JIS K 7361. Specifically, the measurement was performed using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH-1001DP).

[0105] [Optical film haze] The haze of the optical film was measured in accordance with the standard of JIS K 7136. Specifically, the measurement was performed using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH-1001DP).

[0106] [Optical film yellowness index (YI)] The yellowness index (YI) of the optical film was measured in accordance with the standard of JIS K 7373. Specifically, the optical film was used as a test piece and the measurement was carried out using a spectral colorimeter / haze meter (manufactured by Nippon Denshoku Co., Ltd., COH7700).

[0107] Example 1 <Production of optical film> [Preparing raw materials] Methyl methacrylate was obtained from Sumitomo Chemical Co., Ltd. α-methylene-γ-butyrolactone was obtained from Fujifilm Wako Pure Chemical Industries, Ltd. Dilauroyl peroxide (Perloyl L) and n-dodecyl mercaptan were obtained from NOF Corporation. Polyoxyethylene distyryl phenyl ether sulfate ester ammonium (Hitenol (registered trademark) NF-08) was obtained from Daiichi Kogyo Seiyaku Co., Ltd.

[0108] In the following explanation, the names of compounds are abbreviated as follows: MMA: methyl methacrylate MBL: α-methylene-γ-butyrolactone LPO: Dilauroyl peroxide (Perloyl® L) NF-08: Polyoxyethylene distyryl phenyl ether sulfate ammonium ester (Hitenol (registered trademark) NF-08) n-DM: n-dodecyl mercaptan

[0109] [Preparation of thermoplastic elastomer] The acrylic thermoplastic elastomer, Kuralyte (registered trademark) LA-4285, was obtained from Kuraray Co., Ltd. The styrene-based thermoplastic elastomer (SEBS), Tuftec (registered trademark) H1052, was obtained from Asahi Kasei Corporation. The urethane-based thermoplastic elastomer, Elastollan (registered trademark) ET685, was obtained from BASF.

[0110] In the following description, the names of thermoplastic elastomers will be abbreviated as follows: LA-4285: Acrylic thermoplastic elastomer H1052: Styrenic thermoplastic elastomer (SEBS) ET685: Urethane-based thermoplastic elastomer

[0111] [Synthesis of copolymer and preparation of copolymer powder A] A reactor equipped with a stirrer, temperature sensor, cooling tube, and nitrogen inlet tube was prepared. 75 parts by mass of deionized water containing 0.25 parts by mass of NF-08 was charged into a separate vessel. A previously prepared mixture of 37.5 parts by mass of MMA, 12.5 parts by mass of MBL, 0.5 parts by mass of LPO, and 0.05 parts by mass of n-DM was added. The mixture in the vessel was then stirred at 3000 rpm using a disperser (Primix Corporation; Homomixer MARK II model 2.5). 125 parts by mass of deionized water was then added and transferred to the reactor. The reactor was heated to 65°C while continuing to stir and supply nitrogen gas. The polymerization initiation was considered to occur when the internal temperature reached 65°C. After the liquid temperature reached its peak due to self-heating, the reaction liquid was heated to 75°C and stirred. Two hours after the start of polymerization, the reaction liquid was further heated to 90°C and stirred for 4 hours. The polymerization reaction was then completed. The reaction mixture was then cooled, and the copolymer was collected by filtration and dried at 90°C for 10 hours using a hot air dryer, followed by a heat treatment at 150°C for 1 hour to obtain copolymer powder A. The copolymer had a weight-average molecular weight (Mw) of 225,000, a number-average molecular weight (Mn) of 111,000, and a glass transition temperature (Tg) of 126°C.

[0112] [Preparation of Dope A] The obtained copolymer powder A was dissolved in a mixed solvent of methylene chloride and ethanol in a volume ratio of 9:1 so that the solid content was 28 mass %, and then filtered through a filter with a filtration accuracy of 10 μm to prepare dope A.

[0113] [Preparation of film A3 to serve as base layer A] Dope A was cast onto a stainless steel plate using a Baker applicator to form a cast film. The cast film was heated on a hot plate at 25°C for 10 minutes to obtain film A1. After peeling film A1 from the stainless steel plate, the top, bottom, left, and right sides were fixed and heated in an oven at 140°C for 60 minutes to obtain film A2 with a thickness of 100 μm. The obtained film A2 was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD) and transverse direction (TD) at a temperature of Tg+14°C and a speed of 240 mm / min using a sequential biaxial stretching machine (Toyo Seiki Seisaku-sho, X-6S) to a stretch ratio of 1.6 times in each direction. This resulted in film A3 with a thickness of 40 μm.

[0114] [Optical film fabrication] Film A3 was used as base layer A. A dope obtained by dissolving 2 parts by mass of LA-4285 in 8 parts by mass of methyl ethyl ketone was applied to film A3 so as to have a thickness of 5 μm after drying, and the coating was dried in a hot air dryer at 70° C. for 30 minutes to obtain an optical film of Example 1 having a coating layer on one side of base layer A. The properties of the optical film of Example 1 are shown in Table 1.

[0115] Example 2 An optical film of Example 2 was obtained in the same manner as in Example 1, except that coating was performed so that the coating layer had a thickness of 10 μm after drying. The properties of the optical film of Example 2 are shown in Table 1.

[0116] Example 3 An optical film of Example 3 was obtained in the same manner as in Example 1, except that coating was performed so that the coating layer had a dried thickness of 15 μm. The properties of the optical film of Example 3 are shown in Table 1.

[0117] Example 4 An optical film of Example 4 was obtained in the same manner as in Example 1, except that LA-4285 was changed to H1052. The properties of the optical film of Example 4 are shown in Table 1.

[0118] Example 5 An optical film of Example 5 was obtained in the same manner as in Example 1, except that LA-4285 was changed to ET685. The properties of the optical film of Example 5 are shown in Table 1.

[0119] Example 6 An optical film of Example 6 was obtained in the same manner as in Example 1, except that coating was performed so that the coating layer had a thickness of 1 μm after drying. The properties of the optical film of Example 6 are shown in Table 1.

[0120] (Comparative Example 1) Film A3 having a thickness of 40 μm without providing a cover layer was used as the optical film of Comparative Example 1. The properties of the optical film of Comparative Example 1 are shown in Table 1.

[0121] Example 7 [Synthesis of copolymer and preparation of copolymer powder B] A reactor equipped with a stirrer, temperature sensor, cooling tube, and nitrogen inlet tube was prepared. 75 parts by mass of deionized water containing 0.25 parts by mass of NF-08 was charged into a separate vessel. A previously prepared mixture of 27.5 parts by mass of MMA, 22.5 parts by mass of MBL, 0.5 parts by mass of LPO, and 0.013 parts by mass of n-DM was added. The mixture in the vessel was then stirred at 3000 rpm using a disperser (Primix Corporation; Homomixer MARK II model 2.5). 125 parts by mass of deionized water was then added and transferred to the reactor. The reactor was heated to 65°C while continuing to stir and supply nitrogen gas. The polymerization initiation was considered to occur when the internal temperature reached 65°C. After the liquid temperature reached its peak due to self-heating, the reaction liquid was heated to 75°C and stirred. Two hours after the start of polymerization, the reaction liquid was further heated to 90°C and stirred for 4 hours. The polymerization reaction was then completed. The reaction mixture was then cooled, and the copolymer was collected by filtration and further dried at 90°C for 10 hours using a hot air dryer, followed by a heat treatment at 150°C for 1 hour to obtain copolymer powder B. The copolymer had a weight-average molecular weight (Mw) of 475,000, a number-average molecular weight (Mn) of 220,000, and a glass transition temperature (Tg) of 141°C.

[0122] [Preparation of Dope B] The obtained copolymer powder B was dissolved in a mixed solvent of methylene chloride and ethanol in a volume ratio of 9:1 so that the solid content was 23 mass %, and then filtered through a filter with a filtration accuracy of 10 μm to prepare dope B.

[0123] [Preparation of film B3 to serve as base layer B] Dope B was cast onto a stainless steel plate using a Baker applicator to form a cast film. The cast film was heated on a hot plate at 25°C for 10 minutes to obtain film B1. Film B1 was peeled off from the stainless steel plate, and then the top, bottom, left, and right sides were fixed and heated in an oven at 150°C for 60 minutes to obtain film B2 with a thickness of 88 μm. Film B2 was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD) and transverse direction (TD) at a temperature of Tg + 14°C of the copolymer at a speed of 240 mm / min using a sequential biaxial stretching machine (Toyo Seiki Seisaku-sho, X-6S) to a stretch ratio of 1.6 times in each direction. This resulted in film B3 with a thickness of 35 μm.

[0124] An optical film of Example 7 having a coating layer on one side of the base layer B was obtained in the same manner as in Example 1, except that film B3 was used as the base layer B and coating was performed so that the coating layer had a dried thickness of 10 μm. The properties of the optical film of Example 7 are shown in Table 1.

[0125] (Comparative Example 2) Film B3 having a thickness of 35 μm without providing a cover layer was used as the optical film of Comparative Example 2. The properties of the optical film of Comparative Example 2 are shown in Table 1.

[0126] Example 8 [Synthesis of copolymer and preparation of copolymer powder C] A reactor equipped with a stirrer, temperature sensor, cooling tube, and nitrogen inlet tube was prepared. 75 parts by mass of deionized water containing 0.25 parts by mass of NF-08 was charged into a separate vessel. A previously prepared mixture of 37.5 parts by mass of MMA, 12.5 parts by mass of MBL, 0.5 parts by mass of LPO, and 0.005 parts by mass of n-DM was added. The mixture in the vessel was then stirred at 3000 rpm using a disperser (Primix Corporation; Homomixer MARK II model 2.5). 125 parts by mass of deionized water was then added and transferred to the reactor. The reactor was heated to 65°C while continuing to stir and supply nitrogen gas. The polymerization initiation was considered to occur when the internal temperature reached 65°C. After the liquid temperature reached its peak due to self-heating, the reaction liquid was heated to 75°C and stirred. Two hours after the start of polymerization, the reaction liquid was further heated to 90°C and stirred for 4 hours. The polymerization reaction was then completed. The reaction mixture was then cooled, and the copolymer was collected by filtration and dried at 90°C for 10 hours using a hot air dryer, followed by a heat treatment at 150°C for 1 hour to obtain copolymer powder A. The copolymer had a weight-average molecular weight (Mw) of 62,2000, a number-average molecular weight (Mn) of 245,000, and a glass transition temperature (Tg) of 126°C.

[0127] [Preparation of Dope C] The obtained copolymer powder C was dissolved in N,N-dimethylacetamide so that the solid content was 19% by mass, and then filtered through a filter with a filtration accuracy of 10 μm to prepare dope C.

[0128] [Preparation of film C3 to serve as base layer C] Using a Baker applicator, dope C was cast onto a PET film to form a cast film, which was then heated at 130°C for 30 minutes to obtain film C1. After peeling film C1 from the PET film, the top, bottom, left, and right sides were fixed and heated in an oven at 200°C for 60 minutes to obtain film C2 with a thickness of 125 μm. The resulting film C2 was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD) and transverse direction (TD) at a temperature of Tg + 14°C of the copolymer at a speed of 240 mm / min using a sequential biaxial stretching machine (Toyo Seiki Seisaku-sho, X-6S) to a stretch ratio of 1.6 times in each direction, thereby obtaining film C3 with a thickness of 50 μm.

[0129] [Optical film fabrication] Film C3 was used as the base layer C. A dope obtained by dissolving 2 parts by mass of LA-4285 in 8 parts by mass of methyl ethyl ketone was applied to film C3 so that the thickness after drying would be 5 μm, and the coating was dried in a hot air dryer at 70° C. for 30 minutes to obtain an optical film of Example 8 having a coating layer on one side of the base layer C. The properties of the optical film of Example 8 are shown in Table 1.

[0130] (Comparative Example 3) Film C3 having a thickness of 50 μm without providing a cover layer was used as the optical film of Comparative Example 3. The properties of the optical film of Comparative Example 3 are shown in Table 1.

[0131] [Table 1]

[0132] As shown in Table 1, the optical films of Examples 1 to 6, in which a coating layer was disposed on one side of the substrate layer, exhibited better results in the foldability test than the optical film of Comparative Example 1, in which a coating layer was not disposed on one side of the substrate layer. Furthermore, the optical film of Example 7, in which a coating layer was disposed on one side of the substrate layer, exhibited better results in the foldability test than the optical film of Comparative Example 2, in which a coating layer was not disposed on one side of the substrate layer. Furthermore, the optical film of Example 8, in which a coating layer was disposed on one side of the substrate layer, exhibited better results in the foldability test than the optical film of Comparative Example 3, in which a coating layer was not disposed on one side of the substrate layer. These results confirmed that the optical film containing the copolymer containing a structural unit derived from α-methylene lactone according to the present disclosure has excellent bending resistance. [Explanation of symbols]

[0133] 1...substrate layer, 1A...film, 2...covering layer, 3...third layer, 10, 20...optical film.

Claims

1. a substrate layer containing a copolymer including a structural unit derived from α-methylene lactone and a structural unit derived from alkyl (meth)acrylate; a coating layer containing a thermoplastic elastomer disposed on one side of the substrate layer; Equipped with An optical film having a total light transmittance of 90% or more.

2. 2. The optical film according to claim 1, wherein the thermoplastic elastomer comprises at least one selected from the group consisting of an acrylic thermoplastic elastomer, a styrene thermoplastic elastomer, and a urethane thermoplastic elastomer.

3. 3. The optical film according to claim 1, wherein the structural unit derived from α-methylene lactone comprises a structural unit derived from α-methylene-γ-butyrolactone.

4. 3. The optical film according to claim 1, wherein the structural unit derived from alkyl (meth)acrylate contains a structural unit derived from methyl methacrylate.

5. 3. The optical film according to claim 1, wherein the copolymer has a glass transition temperature of 115°C or higher and 180°C or lower.

6. 3. The optical film according to claim 1, wherein the weight average molecular weight of the copolymer is 200,000 or more and 1,000,000 or less.

7. 3. The optical film according to claim 1, wherein the thickness of the substrate layer is 20 μm or more and 70 μm or less.

8. 3. The optical film according to claim 1, wherein the coating layer has a thickness of 0.5 μm or more and 25 μm or less.

9. A flexible display that can be bent or rolled, The optical film according to claim 1 or 2 is provided, The optical film is configured so that the radius of curvature of the covering layer becomes larger than the radius of curvature of the base layer when the flexible display is bent or rolled.

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