Optical film and flexible display

Abstract

To provide an optical film containing a copolymer comprising a structural unit derived from α-methylene lactone and having excellent resistance to bending.SOLUTION: An optical film 10, 20 is provided. The optical film 10, 20 includes: a base layer 1 containing a copolymer having a structural unit derived from α-methylene lactone and a structural unit derived from an alkyl (meth)acrylate; and a coating layer 2 disposed on one side of the base layer 1 and containing a thermoplastic resin. The total light transmittance of the optical film 10, 20 is 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 having 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 having 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 having 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 having 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 disposing a coating layer containing a thermoplastic resin on one side of a substrate layer containing a copolymer having 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 having a structural unit derived from α-methylene lactone and a structural unit derived from alkyl (meth)acrylate; a coating layer containing a thermoplastic resin arranged on one side of the base 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 resin contains a resin having an aromatic ring in the main chain. [3] The optical film according to [2], wherein the resin having an aromatic ring in the main chain is at least one selected from the group consisting of polyarylate resins, phenoxy resins, and aromatic polycarbonate resins. [4] The optical film according to any one of [1] to [3], wherein the thermoplastic resin contains a resin soluble in at least one of methyl ethyl ketone and tetrahydrofuran. [5] The optical film according to any one of [1] to [4], wherein the thermoplastic resin has a glass transition temperature of 120° C. or higher. [6] The optical film according to any one of [1] to [5], wherein the structural unit derived from α-methylene lactone contains a structural unit derived from α-methylene-γ-butyrolactone. [7] The optical film according to any one of [1] to [6], wherein the structural unit derived from alkyl (meth)acrylate contains a structural unit derived from methyl methacrylate. [8] The optical film according to any one of [1] to [7], wherein the copolymer has a glass transition temperature of 110 to 160°C. [9] The optical film according to any one of [1] to [8], wherein the copolymer has a weight average molecular weight of 200,000 to 1,000,000.

[10] The optical film according to any one of [1] to [9], wherein the thickness of the substrate layer is 20 to 70 μm.

[11] The optical film according to any one of [1] to

[10] , wherein the coating layer has a thickness of 1 to 20 μm.

[12] A flexible display that can be bent or rolled, The optical film according to any one of [1] to

[11] is provided, The optical film is configured 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. [Effects of the Invention]

[0009] According to the present disclosure, there is provided an optical film having excellent bending resistance and containing a copolymer having 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 with reference to the accompanying drawings, although the present disclosure is not limited to the following embodiments.

[0012] In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in an example.

[0013] In the present disclosure, alkyl (meth)acrylates encompass alkyl acrylates and the corresponding alkyl methacrylates.

[0014] <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).

[0015] [Base material layer] The substrate layer 1 contains a copolymer having structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate.

[0016] ·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 may be, for example, a 4- to 8-membered ring, a 5- to 6-membered ring, or a 5-membered ring.

[0017] An example of the five-membered α-methylene lactone is a compound represented by the following formula (1).

[0018] [ka]

[0019] In formula (1), R 1 ~R 4 R 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.

[0020] The structural unit derived from α-methylene lactone is formed, for example, 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).

[0021] [ka]

[0022] In formula (2), R 1 ~R 4 is R in Eq. (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.

[0023] 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.

[0024] The structural unit derived from α-methylene lactone is represented by R in formula (2). 1 ~R 4 It is preferable that the compound contains a structural unit derived from α-methylene-γ-butyrolactone in which all of are hydrogen atoms.

[0025] The content of the structural unit derived from α-methylene lactone in the copolymer is preferably 5 to 50 mass %, more preferably 10 to 40 mass %, which can further improve the heat resistance, transparency, and strength of the resulting optical film.

[0026] (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.

[0027] The structural unit derived from alkyl (meth)acrylate is formed, for example, 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.

[0028] The content of structural units derived from alkyl (meth)acrylate in the copolymer is preferably 50 to 95 mass %, more preferably 60 to 90 mass %, which can further improve the heat resistance, transparency, and strength of the resulting optical film.

[0029] (Other structural units) The copolymer may further have other structural units different from the structural units derived from α-methylene lactone and the structural units derived from alkyl (meth)acrylate. Examples of 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 may contain two or more types. Of course, the copolymer may not contain any other structural units. The content of the other structural units in the copolymer may be 0 to 20% by mass, or may be 0 to 10% by mass.

[0030] The content of each structural unit in the copolymer was determined by dissolving the copolymer in a heavy solvent and 1 It can be determined by measuring H-NMR and calculating the area ratio of the peaks corresponding to each structural unit.

[0031] The weight-average molecular weight (Mw) of the copolymer is preferably 200,000 to 1,000,000, more preferably 200,000 to 800,000, and even more preferably 200,000 to 600,000. This further improves the bending resistance of the resulting optical film. The weight-average molecular weight (Mw) of the copolymer is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0032] The number average molecular weight (Mn) of the copolymer is preferably 50,000 to 500,000, more preferably 70,000 to 400,000, and even more preferably 90,000 to 300,000. This further improves the bending resistance of the resulting optical film. The number average molecular weight (Mn) of the copolymer is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0033] The glass transition temperature (Tg) of the copolymer measured by the onset method is preferably 110 to 160°C, more preferably 115 to 155°C, and even more preferably 120 to 150°C. This further improves the heat resistance of the resulting optical film. The glass transition temperature (Tg) of the copolymer can be measured in accordance with the provisions of JIS K 7121.

[0034] 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.

[0035] In suspension polymerization, a copolymer is obtained by polymerizing a monomer containing α-methylene lactone and an alkyl (meth)acrylate in a solvent in the presence of a polymerization initiator and an emulsifier.

[0036] The solvent used in suspension polymerization is an aqueous solvent. The aqueous solvent is preferably water alone, but may contain a non-aqueous solvent (particularly a water-soluble organic solvent) as long as the effects of the present disclosure are not impaired. Examples of 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 5% by mass or less, 2% by mass or less, or 1% by mass or less.

[0037] 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 per 100 parts by mass of the monomer.

[0038] 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 can be adjusted as needed, but is preferably 0.1 to 4 parts by weight, and more preferably 0.2 to 2 parts by weight, per 100 parts by weight of the monomer.

[0039] 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).

[0040] In the 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.

[0041] When polymerizing the monomers, a chain transfer agent, an additive, etc. may be added.

[0042] 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.

[0043] 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.

[0044] The polymerization temperature is preferably 40 to 100° C., more preferably 50 to 95° C., and even more preferably 60 to 90° C. The polymerization time is preferably 0.5 to 20 hours, and more preferably 1 to 10 hours.

[0045] After the polymerization, the copolymer can be recovered by solid-liquid separation, which can be performed by filtration, centrifugation, or a combination thereof.

[0046] The obtained copolymer may be subjected to a drying step. The drying step is a step for converting the obtained copolymer into powder (first powder). The drying temperature is preferably 80 to 105°C, and more preferably 85 to 100°C. The drying time is preferably 1 to 24 hours, more preferably 3 to 15 hours, and even more preferably 5 to 12 hours.

[0047] 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.

[0048] When the glass transition temperature of the copolymer is Tg, the heating temperature in the heat treatment step is preferably from (Tg-10) to (Tg+70)° C., more preferably from (Tg-5) to (Tg+60)° C., and even more preferably from (Tg) to (Tg+50)° C. This allows α-methylene lactone and alkyl (meth)acrylate to be reduced more efficiently.

[0049] The heating time in the heat treatment step is preferably 0.1 to 25 hours, more preferably 1 to 12 hours, and even more preferably 1 to 8 hours, which allows α-methylene lactone and alkyl (meth)acrylate to be reduced more efficiently.

[0050] · 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.

[0051] 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.

[0052] 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.

[0053] The content of the copolymer in the dope is preferably 5 to 60% by mass, more preferably 10 to 50% by mass, and even more preferably 15 to 40% by mass. By keeping the content in this range, the productivity of the film 1A can be improved.

[0054] The dope may further contain other polymers different from the copolymer. 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. Of course, the dope does not necessarily contain any other polymers. The content of the other polymers in the dope can be appropriately adjusted according to the composition of the film 1A to be obtained.

[0055] The dope may further contain additives. Examples of additives include antioxidants; stabilizers such as light stabilizers, weather stabilizers, and heat stabilizers; reinforcing materials such as glass fiber and carbon fiber; 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. Of course, the dope may not contain any additives. The content of the additives in the dope can be appropriately adjusted according to the composition of the film 1A to be obtained.

[0056] Examples of the support include a stainless steel endless belt; a rotating metal drum; a metal sheet such as aluminum foil or copper foil; and a plastic film such as a polyimide film or a polyester film (polyethylene terephthalate film).

[0057] Examples of methods for applying the dope include methods using a die coater, doctor blade coater, roll coater, comma coater, and lip coater.

[0058] 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.

[0059] The heating temperature when volatilizing the solvent from the casting film is preferably 20 to 200° C., more preferably 20 to 100° C., and further preferably 20 to 50° C. This can improve the drying efficiency of the solvent while suppressing the generation of foam marks.

[0060] The heating time for volatilizing the solvent from the casting film is preferably 5 to 120 minutes, more preferably 10 to 80 minutes, which can improve the efficiency of drying the solvent.

[0061] The film 1A can be stretched to form a stretched film, which allows the film 1A to have sufficient strength.

[0062] 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.

[0063] The heating temperature for stretching film 1A is near the glass transition temperature (Tg) of the copolymer, and more specifically, is preferably (Tg-30) to (Tg+100)°C, more preferably (Tg-20) to (Tg+50)°C, and even more preferably (Tg-10) to (Tg+30)°C.

[0064] The stretching speed in stretching the film 1A is preferably 5 to 500% / min.

[0065] The stretching ratio in stretching the film 1A is preferably in the range of 1.05 to 10 times in both the longitudinal and transverse directions.

[0066] From the viewpoint of use as a flexible display, the thickness of the film 1A (substrate layer 1) is preferably 20 to 70 μm, and more preferably 30 to 50 μm.

[0067] The copolymer content in film 1A is preferably 50 to 100% by mass, more preferably 70 to 100% by mass, and even more preferably 90 to 100% by mass, which allows film 1A to have sufficient heat resistance, transparency, and strength.

[0068] Film 1A may further contain the other polymers described above. Film 1A may contain only one type of other polymer, or may contain two or more types. Of course, film 1A does not necessarily have to contain other polymers. The content of other polymers in film 1A may be 0 to 50% by mass, 0 to 30% by mass, or 0 to 10% by mass.

[0069] Film 1A may further contain the above-mentioned additives. Film 1A may contain only one type of additive, or may contain two or more types of additives. Of course, film 1A does not have to contain any additives. The content of the additive in film 1A may be 0 to 5 mass %, 0 to 2 mass %, or 0 to 0.5 mass %.

[0070] The film 1A becomes the base material layer 1 by providing a covering layer 2, which will be described later.

[0071] [Coating layer] The coating layer 2 contains a thermoplastic resin. 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).

[0072] When the base layer 1 is bent, a strong tensile stress is applied to the outside of the base layer 1. In particular, if there are minute depressions such as foaming marks on the surface of the base layer 1, the depressions tend to be the starting point for fracture. In contrast, by disposing the coating layer 2 on one side of the base layer 1 (the outside 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 is on the inside), and the bending resistance of the optical film 10 can be improved. 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 outside of the base layer 1. On the other hand, the coating layer 2 contains a thermoplastic resin, and is therefore thought to have high resistance to tensile stress acting on the outside, making it less likely to fracture.

[0073] The thermoplastic resin refers to, for example, a polymer that has the property of softening at high temperatures, and may be, for example, a polymer that has a glass transition temperature only in the range of 90°C or higher. The glass transition temperature of the thermoplastic resin is preferably 120 to 300°C, more preferably 150 to 280°C, and even more preferably 180 to 260°C. The glass transition temperature of the thermoplastic resin can be measured in accordance with the provisions of JIS K 7121. The glass transition temperature is measured by the starting point method.

[0074] The thermoplastic resin preferably contains a resin having an aromatic ring in the main chain. By containing a resin having an aromatic ring in the main chain of the thermoplastic resin, the bending resistance of the resulting optical film 10 tends to be further improved. Examples of aromatic rings include aromatic carbon rings such as a benzene ring, a naphthalene ring, an anthracene ring, and a phenanthrene ring; and aromatic heterocycles such as a furan ring, a pyrrole ring, a thiophene ring, a pyrazole ring, an oxazole ring, a thiazole ring, a pyridine ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, and a triazine ring. Of these, the aromatic ring is preferably a benzene ring.

[0075] Examples of resins having an aromatic ring in the main chain include polyarylate resins, phenoxy resins, aromatic polycarbonate resins, etc. Among these, from the viewpoint of further improving the bending resistance of the optical film 10, the resin having an aromatic ring in the main chain is preferably at least one selected from the group consisting of polyarylate resins, phenoxy resins, and aromatic polycarbonate resins, more preferably at least one selected from the group consisting of polyarylate resins and phenoxy resins, and even more preferably polyarylate resins.

[0076] Examples of polyarylate resins include resins obtained by reacting dihydric phenols with dibasic acids (such as phthalic acid). The polyarylate resin may be an amorphous polyarylate resin. In the present disclosure, amorphous means one that does not have a melting point (Tm) (an endothermic peak associated with melting in DSC (differential scanning calorimetry)). The polyarylate resin may be a polyarylate resin having terminal OH groups, or a polyarylate resin with blocked ends. Specific examples of polyarylate resins include the Unifiner (registered trademark) series (Unitika Ltd.).

[0077] Examples of phenoxy resins include resins (polyhydroxypolyethers) obtained by reacting bisphenols with epichlorohydrin. Phenoxy resins are also called high-molecular-weight epoxy resins, and can be referred to as epoxy resins having a weight-average molecular weight of 10,000 to 100,000. The phenoxy resin may be an amorphous phenoxy resin. The phenoxy resin may be a phenoxy resin having terminal epoxy groups or a phenoxy resin with blocked terminals. Specific examples of phenoxy resins include PKHH, PKHB, PKHC, PKHA, PKHJ, and PKFE (Tomoe Kogyo Co., Ltd.).

[0078] Examples of aromatic polycarbonate resins include resins obtained by reacting bisphenols with aromatic carbonates. The aromatic polycarbonate resin may be an amorphous aromatic polycarbonate resin. Specific examples of aromatic polycarbonate resins include the Iupizeta (registered trademark) series (Mitsubishi Gas Chemical Company, Inc.).

[0079] The thermoplastic resin preferably contains a resin soluble in at least one of methyl ethyl ketone and tetrahydrofuran. When the thermoplastic resin contains a resin soluble in at least one of methyl ethyl ketone and tetrahydrofuran, the resulting optical film 10 tends to have further improved bending resistance. In the present disclosure, "soluble in methyl ethyl ketone" means that when 10% by mass or more of the thermoplastic resin is dissolved in methyl ethyl ketone at 25°C, the haze of the resulting solution is 15% or less. Similarly, "soluble in tetrahydrofuran" means that when 10% by mass or more of the thermoplastic resin is dissolved in tetrahydrofuran at 25°C, the haze of the resulting solution is 15% or less. The haze of the solution can be determined in accordance with JIS K 7136. Specifically, it can be measured using a haze meter (NDH-1001DP, manufactured by Nippon Denshoku Industries Co., Ltd.) in a quartz cell with an optical path length of 10 mm.

[0080] Coating layer manufacturing method The coating layer 2 can be produced, for example, by applying a dope containing a thermoplastic resin to the film 1A to form a coating film, and then drying the coating film.

[0081] The dope used to produce the coating layer 2 can be obtained, for example, by mixing a thermoplastic resin with a solvent. The solvent may be the same as the solvents exemplified in the production method of the film 1A described above, but is preferably methyl ethyl ketone or tetrahydrofuran. When the solvent is methyl ethyl ketone or tetrahydrofuran, 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.

[0082] The content of the thermoplastic resin in the dope is preferably 5 to 60% by mass, more preferably 10 to 55% by mass, and further preferably 20 to 50% by mass.

[0083] The dope may further contain other polymers, additives, etc. different from the thermoplastic resin. The other polymers and additives may be the same as those exemplified in the method for producing the film 1A.

[0084] 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 production method of the above-mentioned film 1A.

[0085] From the viewpoint of use as a flexible display, the thickness of the covering layer 2 is preferably 1 to 20 μm, and more preferably 2 to 15 μm.

[0086] The content of the thermoplastic resin in the coating layer 2 is preferably 50 to 100% by mass, more preferably 70 to 100% by mass, and even more preferably 90 to 100% by mass.

[0087] The coating layer 2 may further contain the other polymers described above. The coating layer 2 may contain only one type of other polymer, or may contain two or more types of other polymers. Of course, the coating layer 2 does not necessarily need to contain other polymers. The content of the other polymer in the coating layer 2 may be 0 to 50% by mass, 0 to 30% by mass, or 0 to 10% by mass.

[0088] The coating layer 2 may further contain the above-mentioned additives. The coating layer 2 may contain only one type of additive, or may contain two or more types of additives. Of course, the coating layer 2 does not necessarily need to contain any additives. The content of the additive in the coating layer 2 may be 0 to 5 mass %, 0 to 2 mass %, or 0 to 0.5 mass %.

[0089] 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).

[0090] [Third Layer] Examples of the third layer 3 include a layer for improving the adhesion between the base layer 1 and the coating layer 2 (adhesion improving layer), and a layer for controlling optical properties (refractive index control layer). 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 to 20 μm.

[0091] 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.

[0092] From the viewpoint of use as a flexible display, the thickness of the optical films 10 and 20 is preferably 21 to 90 μm, and more preferably 35 to 65 μm.

[0093] The optical films 10 and 20 have a total light transmittance of 90% or more. The total light transmittance is a measure of transparency, and an optical film having a total light transmittance of 90% or more can have sufficient transparency. The total light transmittance of the optical film can be measured in accordance with the provisions of JIS K 7361.

[0094] From the viewpoint of high 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 film can be measured in accordance with the standard of JIS K 7136.

[0095] From the viewpoint of low colorability, the yellowness index (YI) of the optical films 10 and 20 is preferably 1.0 or less, and more preferably 0.8 or less. The yellowness index (YI) of the optical film can be measured in accordance with the standard of JIS K 7373.

[0096] In a foldability test in which the optical films 10 and 20 are repeatedly folded into a U-shape and then returned to their original shape under the following conditions, it is preferable that the optical films 10 and 20 do not break at the folded portion even when folded more than 150,000 times, more preferably do not break at the folded portion even when folded more than 200,000 times, and even more preferably do not break at the folded portion even when folded more than 250,000 times.

[0097] (conditions) Optical films 10 and 20 were cut into 15 mm × 80 mm pieces to prepare test pieces, which were folded at halfway along their long sides so that the radius of curvature of the coating layer was larger than that of the base layer (the base layer was on the inside), and the folded state was set so that the distance between both ends of the long sides of the folded test piece was 2 mm and the radius of curvature of the folded part of the test piece was 1 mm. At 25°C, bending from a flat, unfolded state to a folded state was counted as one bending, and the bending was repeated at a rate of 30 bendings per minute.

[0098] The optical films 10 and 20 of the present embodiment can be suitably used, for example, as a cover window (cover film) and also as a cover window (cover film) for a flexible display that requires resistance to bending.

[0099] <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.

[0100] 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]

[0101] 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.

[0102] <Various physical properties> [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℃

[0103] [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.

[0104] [Optical film thickness] The thickness of the optical film was measured using a Digimatic micrometer (manufactured by Mitutoyo, ID-C112).

[0105] [Optical film bending resistance (foldable test)] The optical film was cut into a size of 15 mm x 80 mm to form a test specimen, which was then fixed with tape to a tension-free folding clamshell type (Yuasa System Equipment, DMLHP-CS). The test specimen was folded at halfway along its long side so that the radius of curvature of the coating layer was larger than that of the base layer (the base layer was on the inside). The folded state was set so that the distance between both ends of the long side of the test specimen in the folded state was 2 mm, and the radius of curvature of the folded portion of the test specimen was 1 mm. The test specimen was then subjected to bending at a rate of 30 times per minute at 25°C, with one bending counting from a flat, unfolded state to a folded state, for a maximum of 250,000 times. The optical film was rated as "S" if it did not break after 250,000 flexions, "A" if it broke after 200,000 or more flexions but less than 250,000 flexions, "B" if it broke after 150,000 or more flexions but less than 200,000 flexions, "C" if it broke after 100,000 or more flexions but less than 150,000 flexions, and "D" if it broke after less than 100,000 flexions.

[0106] [Indentation elastic modulus of optical film (E IT )] Indentation modulus (E IT The elastic modulus was measured using an ultra-microhardness tester (Fisher Instruments, Fischerscope HM-2000) in accordance with ISO-14577-1. The evaluation was carried out with the optical film coating layer 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 calculated by measuring three times.

[0107] [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).

[0108] [Optical film haze] The haze of the optical film was determined in accordance with the provisions of JIS K 7136. Specifically, the measurement was performed using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH-1001DP).

[0109] [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).

[0110] 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.

[0111] 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

[0112] [Preparation of thermoplastic resin] The polyarylate resin, Unifiner (registered trademark) M-2040, was obtained from Unitika Ltd. The resin had a glass transition temperature of 220°C (value announced by the manufacturer) and was soluble in tetrahydrofuran at 25°C. The phenoxy resin, PKHH, was obtained from Tomoe Engineering Co., Ltd. The resin had a glass transition temperature of 92°C (value announced by the manufacturer) and was soluble in methyl ethyl ketone at 25°C. The aromatic polycarbonate resin, Iupizeta (registered trademark) FPC-2136, was obtained from Mitsubishi Gas Chemical Company, Inc. The resin had a glass transition temperature of 131°C (value announced by the manufacturer) and was soluble in methyl ethyl ketone at 25°C.

[0113] In the following description, the names of thermoplastic resins will be abbreviated as follows: M-2040: Polyarylate resin PKHH: Phenoxy resin FPC-2136: Aromatic polycarbonate resin

[0114] [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.

[0115] [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.

[0116] [Preparation of film A3 to serve as base layer A] Dope A was cast onto a stainless steel plate using a coater to form a cast film. The cast film was heated on a hot plate at 25°C for 10 minutes to obtain a 100 μm-thick film A1. After peeling the 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 a film A2. 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 the transverse direction (TD) at a stretch ratio of 1.6 times 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). This resulted in a 40 μm-thick film A3.

[0117] [Optical film fabrication] Film A3 was used as base layer A. A dope obtained by dissolving 2 parts by mass of M-2040 in 8 parts by mass of tetrahydrofuran was applied to film A3 so as to have a thickness of 2 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.

[0118] Example 2 Except for coating so that the thickness after drying was 10 μm, an optical film of Example 2 was obtained in the same manner as in Example 1. The properties of the optical film of Example 2 are shown in Table 1.

[0119] Example 3 Film A3 was used as base layer A. A dope obtained by dissolving 2 parts by mass of PKHH 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 3 having a coating layer on one side of base layer A. The properties of the optical film of Example 3 are shown in Table 1.

[0120] Example 4 Except for coating so that the thickness after drying was 10 μm, an optical film of Example 4 was obtained in the same manner as in Example 3. The properties of the optical film of Example 4 are shown in Table 1.

[0121] Example 5 Film A3 was used as base layer A. A dope obtained by dissolving 2 parts by mass of FPC-2136 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 3 having a coating layer on one side of base layer A. The properties of the optical film of Example 3 are shown in Table 1.

[0122] (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.

[0123] Example 6 [Synthesis of copolymer and preparation of copolymer powder B] Copolymer powder B containing a copolymer was obtained in the same manner as in Example 1, except that the 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 replaced with a mixture of 37.5 parts by mass of MMA, 12.5 parts by mass of MBL, 0.5 parts by mass of LPO, and 0.013 parts by mass of n-DM. The copolymer had a weight-average molecular weight (Mw) of 489,000, a number-average molecular weight (Mn) of 227,000, and a glass transition temperature (Tg) of 141°C.

[0124] [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.

[0125] [Preparation of film B3 to serve as base layer B] Dope B was cast onto a stainless steel plate using a coater to form a cast film. The cast film was heated on a hot plate at 25°C for 10 minutes to obtain a 110 μm-thick film B1. After peeling film B1 from the stainless steel plate, the film was fixed at the top, bottom, left, and right and heated in an oven at 150°C for 60 minutes to obtain film B2. The obtained 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 stretch ratio of 1.6 times 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). This resulted in a 43 μm-thick film B3.

[0126] Example 6 An optical film of Example 6 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 dry thickness of 5 μm. The properties of the optical film of Example 6 are shown in Table 1.

[0127] (Comparative Example 2) Film B3 having a thickness of 43 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.

[0128] [Table 1]

[0129] As shown in Table 1, the optical films of Examples 1 to 5, in which a coating layer was disposed on one side of the base 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 base layer. Furthermore, the optical film of Example 6, in which a coating layer was disposed on one side of the base 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 base layer. These results confirmed that the optical film containing the copolymer having a structural unit derived from α-methylene lactone according to the present disclosure has excellent bending resistance. [Explanation of symbols]

[0130] 1...substrate layer, 1A...film, 2...covering layer, 3...third layer, 10, 20...optical film.

Claims

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

2. The optical film according to claim 1 , wherein the thermoplastic resin comprises a resin having an aromatic ring in the main chain.

3. 3. The optical film according to claim 2, wherein the resin having an aromatic ring in the main chain is at least one selected from the group consisting of a polyarylate resin, a phenoxy resin, and an aromatic polycarbonate resin.

4. The optical film according to claim 1 , wherein the thermoplastic resin comprises a resin soluble in at least one of methyl ethyl ketone and tetrahydrofuran.

5. The optical film according to claim 1 , wherein the thermoplastic resin has a glass transition temperature of 120° C. or higher.

6. 6. The optical film according to claim 1, wherein the structural units derived from α-methylene lactone include structural units derived from α-methylene-γ-butyrolactone.

7. 6. The optical film according to claim 1, wherein the structural units derived from alkyl (meth)acrylate include structural units derived from methyl methacrylate.

8. 6. The optical film according to claim 1, wherein the copolymer has a glass transition temperature of 110 to 160°C.

9. 6. The optical film according to claim 1, wherein the copolymer has a weight average molecular weight of 200,000 to 1,000,000.

10. 6. The optical film according to claim 1, wherein the thickness of the substrate layer is 20 to 70 μm.

11. 6. The optical film according to claim 1, wherein the coating layer has a thickness of 1 to 20 μm.

12. A flexible display that can be bent or rolled, The optical film according to any one of claims 1 to 5 is provided, The optical film is configured 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.

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