Copolymer powder, method for producing said powder, dope, method for producing said dope, and method for producing optical film

Abstract

The present disclosure provides a method for producing a powder containing a copolymer containing a structural unit derived from α-methylene lactone and a structural unit derived from an alkyl (meth)acrylate, the method being characterized by comprising: a polymerization step in which monomers including α-methylene lactone and an alkyl (meth)acrylate are subjected to suspension polymerization in a solvent in the presence of a polymerization initiator and an emulsifier to obtain a copolymer; a drying step in which the copolymer is dried to obtain a first powder; and a heat treatment step in which the first powder is heated at a temperature higher than that in the drying step to obtain a second powder in which the total content of the α-methylene lactone and the alkyl (meth)acrylate is smaller than that in the first powder.

Description

Copolymer powder and method for producing the same, dope and method for producing the same, and method for producing optical film

[0001] The present disclosure relates to a copolymer powder and a method for producing the same, a dope and a method for producing the same, and a method for producing an optical film.

[0002] Optical films containing copolymers containing structural units derived from α-methylene lactone are known. For example, Patent Document 1 discloses a method for producing a copolymer containing structural units derived from α-methylene lactone, and the copolymer produced by this production method. Patent Document 1 also discloses an optical film produced by a solution casting method including the steps of casting a dope containing the copolymer and a solvent onto a support and volatilizing the solvent from the cast dope.

[0003] Japanese Patent Application Laid-Open No. 2023-044845

[0004] When producing an optical film by volatilizing a solvent from a dope, if the drying efficiency of the solvent can be improved, the productivity of the optical film can be increased. The drying efficiency of the solvent can be improved by increasing the drying temperature, but increasing the drying temperature may cause the solvent to bump and cause bubble marks on the optical film. In other words, there is a limit to how much the drying efficiency can be improved by adjusting the drying temperature alone.

[0005] To solve these problems, the present disclosure provides a powder containing a copolymer having structural units derived from α-methylene lactone, which powder can improve the drying efficiency of a solvent and increase the productivity of optical films, and a method for producing the same. The present disclosure also provides a dope using such a powder, a method for producing the same, and a method for producing an optical film.

[0006] The present disclosure provides the following: a method for producing a powder as described in [1] to [8]; a method for producing a dope as described in [9]; a method for producing an optical film as described in

[10] ; a powder as described in

[11] to

[14] ; and a dope as described in

[15] . [1] A method for producing a powder containing a copolymer containing structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate, the method comprising: a polymerization step of suspension-polymerizing monomers containing α-methylene lactone and alkyl (meth)acrylate in a solvent in the presence of a polymerization initiator and an emulsifier to obtain a copolymer; a drying step of drying the copolymer to obtain a first powder; and a heat-treatment step of heating the first powder at a temperature higher than that in the drying step to obtain a second powder having a lower total content of α-methylene lactone and alkyl (meth)acrylate than that of the first powder. [2] The method for producing the powder as described in [1], wherein the second powder has a total content of α-methylene lactone and alkyl (meth)acrylate of 0.70% by mass or less. [3] The method for producing a powder according to [1] or [2], wherein the content of the emulsifier in the second powder is 100 ppm by mass or less. [4] The method for producing a powder according to any one of [1] to [3], wherein the emulsifier is an ammonium sulfate salt. [5] The method for producing a powder according to any one of [1] to [4], wherein the content of structural units derived from α-methylene lactone in the copolymer is 10% by mass or more and 40% by mass or less. [6] The method for producing a powder according to any one of [1] to [5], wherein the drying temperature in the drying step is 80°C or more and 105°C or less. [7] The method for producing a powder according to any one of [1] to [6], wherein the glass transition temperature Tg of the copolymer measured by the starting point method is 115°C or more, and the heating temperature in the heat treatment step is (Tg - 10)°C or more and (Tg + 70)°C or less. [8] The method for producing a powder according to any one of [1] to [7], wherein the rate of decrease in the weight average molecular weight of the copolymer in the heat treatment step is 10% or less. [9] A method for producing a dope, comprising: a dope preparation step of mixing the powder obtained by the method for producing a dope according to any one of the above-mentioned [1] to [8] with a solvent to obtain a dope.

[10] A method for producing an optical film, comprising: a casting step of casting the dope obtained by the method for producing a dope according to the above-mentioned [9] on a support to form a casting film; and a volatilization step of volatilizing the solvent from the casting film.

[11] A powder containing a copolymer containing structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate, wherein the total content of α-methylene lactone and alkyl (meth)acrylate is 0.70% by mass or less.

[12] The powder according to

[11] , wherein the content of an emulsifier is 100 ppm by mass or less.

[13] The powder according to

[12] , wherein the emulsifier is an ammonium sulfate salt.

[14] The powder according to any one of

[11] to

[13] , wherein the weight-average molecular weight of the copolymer measured by gel permeation chromatography (GPC) is 200,000 or more and 1,000,000 or less.

[15] A dope containing a copolymer containing structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate, and a solvent, wherein the total content of α-methylene lactone and alkyl (meth)acrylate is 0.70% by mass or less based on the total amount of the copolymer, α-methylene lactone, and alkyl (meth)acrylate.

[0007] According to the present disclosure, there are provided a powder containing a copolymer including a structural unit derived from α-methylene lactone, which can improve the drying efficiency of a dope solvent and increase the productivity of an optical film, and a method for producing the same. Furthermore, according to the present disclosure, there are provided a dope using such a powder, a method for producing the same, and a method for producing an optical film.

[0008] 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 ranges shown as X to Y mean from X to Y.

[0009] <Method for Producing Powder> A method for producing a powder according to one embodiment is a method for producing a powder containing a copolymer including structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate, and includes the following steps: a polymerization step of suspension-polymerizing monomers including α-methylene lactone and alkyl (meth)acrylate in a solvent in the presence of a polymerization initiator and an emulsifier to obtain a copolymer; a drying step of drying the obtained copolymer to obtain a first powder; and a heat-treatment step of heating the first powder at a temperature higher than that in the drying step to obtain a second powder having a lower total content of the α-methylene lactone and the alkyl (meth)acrylate than the first powder.

[0010] The powder obtained by this manufacturing method is subjected to a heat treatment process, so that the amount of residual monomers in the powder is sufficiently reduced. When such powder is dissolved in a solvent to obtain a dope, the drying efficiency of the solvent is improved. Therefore, the productivity of the optical film produced by using such a dope can be improved.

[0011] [Copolymer] The copolymer contains structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate.

[0012] (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 a 4- to 8-membered ring, a 5- or 6-membered ring, or a 5-membered ring.

[0013] An example of the five-membered α-methylene lactone is a compound represented by the following formula (1).

[0014]

[0015] R in formula (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.

[0016] 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.

[0017] 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 shown in the following formula (2), and more preferably contains a structural unit derived from α-methylene-γ-butyrolactone. The structural unit shown in the following formula (2) is formed, for example, by polymerization of a monomer containing a compound shown in formula (1).

[0018]

[0019] R in formula (2) １ ~R ４ is R in formula (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.

[0020] 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.

[0021] The content of structural units derived from α-methylene lactone in the copolymer is preferably 5% by mass or more and 50% by mass or less, and more preferably 10% by mass or more and 40% 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, of the total amount of structural units derived from α-methylene lactone, the content of structural units derived from α-methylene-γ-butyrolactone 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.

[0022] (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.

[0023] 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.

[0024] 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, more preferably 60% by mass or more and 90% by mass or less. This can further improve the heat resistance, transparency, and strength of an optical film obtained from this copolymer. 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.

[0025] (Other Structural Units) The copolymer may, if necessary, contain 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. The content of 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.

[0026] The content of each structural unit in the copolymer was determined by dissolving the copolymer in a heavy solvent and１ H-NMR is measured and the area ratio of the peaks corresponding to each structural unit is calculated.

[0027] 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 measured by gel permeation chromatography (GPC).

[0028] 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 measured by gel permeation chromatography (GPC).

[0029] 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 an optical film obtained from this copolymer can be further improved. The glass transition temperature (Tg) of the copolymer is measured in accordance with the provisions of JIS K 7121.

[0030] [Polymerization Step] In the polymerization step, the above-mentioned monomers containing α-methylene lactone and alkyl (meth)acrylate are suspension polymerized in a solvent in the presence of a polymerization initiator and an emulsifier to obtain a copolymer.

[0031] 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 (water-soluble organic 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.

[0032] Examples of the polymerization initiator 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 the polymerization initiator added may be adjusted as appropriate, but is preferably 0.01 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the monomer.

[0033] 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 powder. The addition of an emulsifier can improve the stability of the polymerization reaction. The content of the emulsifier 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.

[0034] 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 step 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).

[0035] 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.

[0036] When polymerizing the monomers, a chain transfer agent and / or additives may be added as necessary.

[0037] 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 parts by mass or more and 1 part by mass or less relative to 100 parts by mass of the monomer.

[0038] Examples of the additive include a water-insoluble organic solvent such as an alkane, and a radical scavenger. The amount of the additive added may be adjusted as appropriate, but is preferably 0.001 part by mass or more and 1 part by mass or less relative to 100 parts by mass of the monomer.

[0039] 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.

[0040] [Drying Step] In the drying step, the copolymer obtained in the polymerization step is dried to obtain a first powder. That is, 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 powder. Before the drying step, the copolymer obtained in the polymerization step may be subjected to solid-liquid separation. Examples of solid-liquid separation methods include filtration, centrifugation, and a combination thereof. By performing solid-liquid separation, the aqueous solvent can be removed more efficiently in the drying step.

[0041] The drying temperature in the drying step is preferably 80° C. or higher and 105° C. or lower, and more preferably 85° C. or higher and 100° C. or lower, thereby enabling the aqueous solvent to be removed more efficiently.

[0042] The drying time in the drying step is preferably from 1 hour to 24 hours, more preferably from 3 hours to 15 hours, and even more preferably from 5 hours to 12 hours, which allows the aqueous solvent to be removed more efficiently.

[0043] The total content of α-methylene lactone and alkyl (meth)acrylate in the first powder is preferably 1.7% by mass or less, more preferably 1.5% by mass or less, and even more preferably 1.3% by mass or less.

[0044] The content of α-methylene lactone in the first powder is preferably 0.7% by mass or less, and more preferably 0.5% by mass or less.

[0045] The content of alkyl (meth)acrylate in the first powder is preferably 1.0% by mass or less, and more preferably 0.8% by mass or less.

[0046] The content of the emulsifier in the first powder is preferably 200 ppm by mass or less, and more preferably 180 ppm by mass or less.

[0047] [Heat Treatment Step] In the heat treatment step, the first powder obtained in the drying step is heated at a temperature higher than that in the drying step to obtain a second powder having a lower total content of α-methylene lactone and alkyl (meth)acrylate than the first powder. That is, the heat treatment step is a step for obtaining a second powder in which the α-methylene lactone and alkyl (meth)acrylate contents have been reduced from the first powder. In the heat treatment step, it is preferable that the content of emulsifiers and the like can also be reduced. By performing the heat treatment step on the first powder from which the aqueous solvent has been removed in advance by the drying step, the α-methylene lactone and alkyl (meth)acrylate contents can be efficiently reduced.

[0048] 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 improve the bending resistance of an optical film produced using the second powder. Furthermore, the yellowness index (YI) of the optical film can be reduced, and transparency can be sufficiently increased.

[0049] 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.

[0050] In the heat treatment step, if the heating temperature is high, the copolymer may be depolymerized, resulting in a decrease in the weight-average molecular weight of the copolymer. Therefore, the decrease rate of the weight-average molecular weight of the copolymer before and after the treatment step is preferably 0% to 10%, more preferably 0.01% to 5%, and even more preferably 0.1% to 1%. This can further improve the yellowness index (YI) and bending resistance of the optical film obtained from this copolymer.

[0051] The total content of α-methylene lactone and alkyl (meth)acrylate in the second powder is preferably 0.70% by mass or less, more preferably 0.60% by mass or less, 0.50% by mass or less, 0.40% by mass or less, or 0.30% by mass or less, even more preferably 0.20% by mass or less, and particularly preferably 0.10% by mass or less. This further improves the drying efficiency of the dope solvent when producing an optical film from the dope, thereby further increasing the productivity of the optical film. The total content of α-methylene lactone and alkyl (meth)acrylate in the second powder may be 0% by mass or more, or may be 0.01% by mass or more.

[0052] The total content of α-methylene lactone and alkyl (meth)acrylate in the second powder may be 0% by mass or more and 0.70% by mass or less, 0% by mass or more and 0.60% by mass or less, 0% by mass or more and 0.50% by mass or less, 0% by mass or more and 0.40% by mass or less, 0% by mass or more and 0.30% by mass or less, 0% by mass or more and 0.20% by mass or less, 0% by mass or more and 0.10% by mass or less, 0.01% by mass or more and 0.70% by mass or less, 0.01% by mass or more and 0.60% by mass or less, 0.01% by mass or more and 0.50% by mass or less, 0.01% by mass or more and 0.40% by mass or less, 0.01% by mass or more and 0.30% by mass or less, 0.01% by mass or more and 0.20% by mass or less, or 0.01% by mass or more and 0.10% by mass or less.

[0053] The content of α-methylene lactone in the second powder is preferably 0.35% by mass or less, more preferably 0.30% by mass or less, even more preferably 0.20% by mass or less, particularly preferably 0.10% by mass or less, and most preferably 0.06% by mass or less. This further improves the drying efficiency of the dope solvent when producing an optical film from the dope, thereby further increasing the productivity of the optical film. The content of α-methylene lactone in the second powder may be 0% by mass or more, or may be 0.005% by mass or more.

[0054] The content of α-methylene lactone in the second powder may be 0% by mass or more and 0.35% by mass or less, 0% by mass or more and 0.30% by mass or less, 0% by mass or more and 0.20% by mass or less, 0% by mass or more and 0.10% by mass or less, 0% by mass or more and 0.06% by mass or less, 0.005% by mass or more and 0.35% by mass or less, 0.005% by mass or more and 0.30% by mass or less, 0.005% by mass or more and 0.20% by mass or less, 0.005% by mass or more and 0.10% by mass or less, or 0.005% by mass or more and 0.06% by mass or less.

[0055] The content of alkyl (meth)acrylate in the second powder is preferably 0.35% by mass or less, more preferably 0.30% by mass or less, even more preferably 0.20% by mass or less, particularly preferably 0.10% by mass or less, and most preferably 0.03% by mass or less. This further improves the drying efficiency of the dope solvent when producing an optical film from the dope, thereby further increasing the productivity of the optical film. The content of alkyl (meth)acrylate in the second powder may be 0% by mass or more, or may be 0.005% by mass or more.

[0056] The content of alkyl (meth)acrylate in the second powder may be 0% by mass or more and 0.35% by mass or less, 0% by mass or more and 0.30% by mass or less, 0% by mass or more and 0.20% by mass or less, 0% by mass or more and 0.10% by mass or less, 0% by mass or more and 0.03% by mass or less, 0.005% by mass or more and 0.35% by mass or less, 0.005% by mass or more and 0.30% by mass or less, 0.005% by mass or more and 0.20% by mass or less, 0.005% by mass or more and 0.10% by mass or less, or 0.005% by mass or more and 0.03% by mass or less.

[0057] The content of the emulsifier in the second powder is preferably 100 ppm by mass or less, more preferably 70 ppm by mass or less, and even more preferably 20 ppm by mass or less. This can further improve the drying efficiency of the dope solvent when producing an optical film from the dope, thereby further increasing the productivity of the optical film. The content of the emulsifier in the second powder may be 0 ppm by mass or more.

[0058] The content of the emulsifier in the second powder may be 0 ppm by mass or more and 100 ppm by mass or less, 0 ppm by mass or more and 70 ppm by mass or less, or 0 ppm by mass or more and 20 ppm by mass or less.

[0059] <Powder> The powder according to one embodiment is a powder containing a copolymer including structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate, wherein the total content of α-methylene lactone and alkyl (meth)acrylate is 0.70 mass% or less. This improves the drying efficiency of the dope solvent when producing an optical film from the dope, thereby increasing the productivity of the optical film.

[0060] The total content of α-methylene lactone and alkyl (meth)acrylate in the powder is preferably 0.60% by mass or less, 0.50% by mass or less, 0.40% by mass or less, or 0.30% by mass or less, more preferably 0.20% by mass or less, and even more preferably 0.10% by mass or less. This further improves the drying efficiency of the dope solvent when producing an optical film from the dope, thereby further increasing the productivity of the optical film. The total content of α-methylene lactone and alkyl (meth)acrylate in the powder may be 0% by mass or more, or may be 0.01% by mass or more.

[0061] The total content of α-methylene lactone and alkyl (meth)acrylate in the powder may be 0% by mass or more and 0.70% by mass or less, 0% by mass or more and 0.60% by mass or less, 0% by mass or more and 0.50% by mass or less, 0% by mass or more and 0.40% by mass or less, 0% by mass or more and 0.30% by mass or less, 0% by mass or more and 0.20% by mass or less, 0% by mass or more and 0.10% by mass or less, 0.01% by mass or more and 0.70% by mass or less, 0.01% by mass or more and 0.60% by mass or less, 0.01% by mass or more and 0.50% by mass or less, 0.01% by mass or more and 0.40% by mass or less, 0.01% by mass or more and 0.30% by mass or less, 0.01% by mass or more and 0.20% by mass or less, or 0.01% by mass or more and 0.10% by mass or less.

[0062] The content of α-methylene lactone in the powder is preferably 0.35% by mass or less, more preferably 0.30% by mass or less, even more preferably 0.20% by mass or less, particularly preferably 0.10% by mass or less, and most preferably 0.06% by mass or less. This further improves the drying efficiency of the dope solvent when producing an optical film from the dope, thereby further increasing the productivity of the optical film. The content of α-methylene lactone in the powder may be 0% by mass or more, or may be 0.005% by mass or more.

[0063] The content of α-methylene lactone in the powder may be 0% by mass or more and 0.35% by mass or less, 0% by mass or more and 0.30% by mass or less, 0% by mass or more and 0.20% by mass or less, 0% by mass or more and 0.10% by mass or less, 0% by mass or more and 0.06% by mass or less, 0.005% by mass or more and 0.35% by mass or less, 0.005% by mass or more and 0.30% by mass or less, 0.005% by mass or more and 0.20% by mass or less, 0.005% by mass or more and 0.10% by mass or less, or 0.005% by mass or more and 0.06% by mass or less.

[0064] The content of alkyl (meth)acrylate in the powder is preferably 0.35% by mass or less, more preferably 0.30% by mass or less, even more preferably 0.20% by mass or less, particularly preferably 0.10% by mass or less, and most preferably 0.03% by mass or less. This can further improve the drying efficiency of the dope solvent when producing an optical film from the dope, thereby further increasing the productivity of the optical film. The content of alkyl (meth)acrylate in the powder may be 0% by mass or more, or may be 0.005% by mass or more.

[0065] The content of alkyl (meth)acrylate in the powder may be 0% by mass or more and 0.35% by mass or less, 0% by mass or more and 0.30% by mass or less, 0% by mass or more and 0.20% by mass or less, 0% by mass or more and 0.10% by mass or less, 0% by mass or more and 0.03% by mass or less, 0.005% by mass or more and 0.35% by mass or less, 0.005% by mass or more and 0.30% by mass or less, 0.005% by mass or more and 0.20% by mass or less, 0.005% by mass or more and 0.10% by mass or less, or 0.005% by mass or more and 0.03% by mass or less.

[0066] The content of the emulsifier in the powder is preferably 100 mass ppm or less, more preferably 70 mass ppm or less, and even more preferably 20 mass ppm or less. This can further improve the drying efficiency of the dope solvent when producing an optical film from the dope, thereby further increasing the productivity of the optical film. The content of the emulsifier in the powder may be 0 mass ppm or more.

[0067] The content of the emulsifier in the powder may be 0 ppm by mass or more and 100 ppm by mass or less, 0 ppm by mass or more and 70 ppm by mass or less, or 0 ppm by mass or more and 20 ppm by mass or less.

[0068] The powder can be produced based on the embodiment of the powder production method described above. Therefore, the second powder may be the powder in this embodiment. Therefore, the above-mentioned explanation regarding the copolymer and the explanation regarding the α-methylene lactone, alkyl (meth)acrylate, emulsifier, and the like contained in the second powder can be applied directly to this embodiment.

[0069] The powder containing the copolymer of this embodiment can improve the drying efficiency of the solvent in the dope when producing an optical film, thereby increasing the productivity of the optical film.

[0070] <Method for Producing Dope> A method for producing a dope according to an embodiment includes a dope preparation step of mixing the powder obtained by the method for producing a powder described above with a solvent to obtain a dope. The dope can be suitably used for producing an optical film.

[0071] The content of the powder in the dope is preferably 5% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, and even more preferably 15% by mass or more and 40% by mass or less. This can further improve the productivity of the optical film. In this embodiment, the purity of the copolymer in the powder is sufficiently high, so the content of the powder can be directly considered as the content of the copolymer.

[0072] 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.

[0073] <Dope> A dope according to one embodiment contains a copolymer including structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate, and a solvent. The total content of α-methylene lactone and alkyl (meth)acrylate is 0.70% by mass or less based on the total amount of the copolymer, α-methylene lactone, and alkyl (meth)acrylate. When an optical film is produced from such a dope, the drying efficiency of the dope solvent can be improved, thereby increasing the productivity of the optical film. The total content of α-methylene lactone and alkyl (meth)acrylate may be 0% by mass or more or 0.01% by mass or more based on the total amount of the copolymer, α-methylene lactone, and alkyl (meth)acrylate.

[0074] The total content of α-methylene lactone and alkyl (meth)acrylate in the total amount of the copolymer, α-methylene lactone, and alkyl (meth)acrylate is preferably 0.60 mass% or less, 0.50 mass% or less, 0.40 mass% or less, or 0.30 mass% or less, more preferably 0.20 mass% or less, and even more preferably 0.10 mass% or less, to further improve the drying efficiency of the dope solvent and further improve the productivity of the optical film.

[0075] In the total amount of the copolymer, α-methylene lactone, and alkyl (meth)acrylate, the total content of α-methylene lactone and alkyl (meth)acrylate may be 0% by mass or more and 0.70% by mass or less, 0% by mass or more and 0.60% by mass or less, 0% by mass or more and 0.50% by mass or less, 0% by mass or more and 0.40% by mass or less, 0% by mass or more and 0.30% by mass or less, 0% by mass or more and 0.20% by mass or less, 0% by mass or more and 0.10% by mass or less, 0.01% by mass or more and 0.70% by mass or less, 0.01% by mass or more and 0.60% by mass or less, 0.01% by mass or more and 0.50% by mass or less, 0.01% by mass or more and 0.40% by mass or less, 0.01% by mass or more and 0.30% by mass or less, 0.01% by mass or more and 0.20% by mass or less, or 0.01% by mass or more and 0.10% by mass or less.

[0076] The content of the copolymer in the dope is preferably 5% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, and even more preferably 15% by mass or more and 40% by mass or less, thereby further improving the productivity of the optical film.

[0077] (Other Polymers) The dope may contain other polymers different from the copolymers described above, 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. The content of the other polymer in the dope may be appropriately adjusted depending on the properties of the optical film to be obtained.

[0078] (Additives) 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 may contain two or more types. The content of the additives in the dope may be appropriately adjusted according to the properties of the optical film to be obtained.

[0079] <Method for Producing Optical Film> The method for producing an optical film according to an embodiment is a so-called solution casting method, and includes a casting step of casting the above-described dope onto a support to form a casting film, and a volatilizing step of volatilizing the solvent from the casting film. The optical film according to this embodiment can be produced using a dope prepared by mixing a powder containing the above-described copolymer with a solvent.

[0080] 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).

[0081] 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.

[0082] As a method for volatilizing the solvent from the casting film, for example, a method of heating the casting film within a temperature range in which no foaming marks are formed on the optical film can be mentioned.

[0083] The heating temperature when 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. This can further improve the drying efficiency of the solvent while suppressing the generation of bubble marks, thereby further increasing the productivity of the optical film.

[0084] The heating time for volatilizing the solvent from the casting film is preferably 5 minutes or more and 120 minutes or less, more preferably 10 minutes or more and 80 minutes or less, thereby further improving the drying efficiency of the solvent while suppressing the generation of foam marks, and further increasing the productivity of the optical film.

[0085] The amount of the solvent remaining in the optical film after the solvent is evaporated from the casting film is preferably 0.1% by mass or more and 60% by mass or less, more preferably 0.1% by mass or more and 50% by mass or less, and further preferably 0.1% by mass or more and 45% by mass or less.

[0086] The amount of residual solvent in the optical film is calculated as follows: First, the mass of the optical film is measured in advance, dried in an oven at 200°C for 1 hour, and the mass of the optical film after drying is measured. The difference in mass between the optical film before and after drying is calculated as the absolute content of residual solvent (unit: g). The calculated absolute content of residual solvent (unit: g) is then divided by the mass (unit: g) of the optical film before drying to obtain the amount of residual solvent (mass %) in the optical film.

[0087] After the solvent is evaporated from the cast film, the optical film can be peeled off from the support, and the optical film peeled off from the support is wound into a film roll by a winder.

[0088] The optical film peeled from the support is preferably dried again before being formed into a film roll.

[0089] The drying temperature for the optical film is preferably 100°C or higher and 250°C or lower, and more preferably 120°C or higher and 230°C or lower.

[0090] The drying time of the optical film is preferably from 5 minutes to 120 minutes, more preferably from 10 minutes to 80 minutes.

[0091] The amount of solvent remaining in the optical film after drying is preferably 0.1% by mass or more and 2.5% by mass or less, more preferably 0.1% by mass or more and 1.5% by mass or less, even more preferably 0.1% by mass or more and 1.3% by mass or less, particularly preferably 0.1% by mass or more and 1.0% by mass or less, and particularly preferably 0.1% by mass or more and 0.8% by mass or less. The amount of solvent remaining in the optical film after drying can be calculated in the same manner as the amount of solvent remaining in the optical film described above.

[0092] It is preferable that the optical film is stretched. This allows an optical film having sufficient strength to be obtained. The optical film may be stretched at any time after it has been peeled from the support. For example, it may be stretched after being dried again and before being made into a film roll. Alternatively, after it has been made into a film roll, the film may be pulled out from the film roll and stretched.

[0093] Examples of methods for stretching optical films 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.

[0094] The heating temperature in stretching the optical film is near the glass transition temperature (Tg) of the copolymer, and more specifically, is preferably (Tg-30)°C or higher and (Tg+100)°C or lower, more preferably (Tg-20)°C or higher and (Tg+50)°C or lower, and even more preferably (Tg-10)°C or higher and (Tg+30)°C or lower.

[0095] The stretching speed in stretching the optical film is preferably 5% / min or more and 500% / min or less.

[0096] The stretching ratio in stretching the optical film is preferably 1.05 to 10 times in both the longitudinal and transverse directions.

[0097] The thickness of the optical film before stretching is preferably 40 μm or more and 200 μm or less, more preferably 50 μm or more and 150 μm or less, and even more preferably 60 μm or more and 120 μm or less.

[0098] The thickness of the optical film after stretching is preferably 15 μm or more and 50 μm or less, more preferably 20 μm or more and 45 μm or less, and even more preferably 25 μm or more and 40 μm or less.

[0099] The content of the copolymer in the optical film 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, thereby making it possible to obtain an optical film having sufficient heat resistance, transparency, and strength.

[0100] In addition, this optical film can also be said to be an optical film containing a copolymer including a structural unit derived from α-methylene lactone and a structural unit derived from alkyl (meth)acrylate, and characterized in that the content of residual solvent is 0.1 mass % or more and 2.5 mass % or less.

[0101] The structural units derived from α-methylene lactone preferably include structural units derived from α-methylene-γ-butyrolactone, and the structural units derived from alkyl (meth)acrylate preferably include structural units derived from methyl methacrylate.

[0102] 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.

[0103] 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.

[0104] The residual solvent is preferably at least one selected from the group consisting of methylene chloride, ethanol, methyl ethyl ketone, and dimethylacetamide. The content of the residual solvent is, for example, more preferably 0.1% by mass to 1.5% by mass, even more preferably 0.1% by mass to 1.3% by mass, particularly preferably 0.1% by mass to 1.0% by mass, and particularly preferably 0.1% by mass to 0.8% by mass.

[0105] The optical film may contain the above-mentioned other polymers as needed. The optical film may contain only one type of other polymer, or may contain two or more types. The content of the other polymers in the optical film 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.

[0106] The optical film may contain the above-mentioned additives as necessary. The optical film may contain only one type of additive, or may contain two or more types of additives. The content of the additives in the optical film 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.

[0107] The optical film preferably has a thickness of 15 μm or more and 50 μm or less (e.g., 40 μm) and a yellowness index (YI) of 1.00 or less. In this case, the yellowness index (YI) is more preferably 0.80 or less, and even more preferably 0.70 or less. This allows for an optical film with sufficient transparency to be obtained. The yellowness index (YI) may be 0 or more, or 0.01 or more. The yellowness index (YI) may be, for example, 0 or more and 1.00 or less, 0 or more and 0.80 or less, 0 or more and 0.70 or less, 0.01 or more and 1.00 or less, 0.01 or more and 0.80 or less, or 0.01 or more and 0.70 or less. The yellowness index (YI) is measured in accordance with the provisions of JIS K 7373.

[0108] The optical film preferably has a thickness of 15 μm or more and 50 μm or less (e.g., 40 μm), and is bent 200,000 times or more in a foldable test. In this case, the number of bends is more preferably 250,000 times or more, and even more preferably 300,000 times or more. This allows an optical film with sufficient strength to be obtained. The number of bends 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 bends 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.

[0109] In the foldability test, the optical film is bent into a U-shape. The folded state is defined as a state in which the radius of curvature at the folding position is 1 mm and the distance between the ends is 2 mm. In the folded state, the portions other than the folding position are approximately parallel. The bending operation is repeated at a rate of 30 times per minute in an environment of 25°C or less, and the number of bendings until breakage occurs is measured. 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).

[0110] The optical film 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 a film (e.g., a cover window) for foldable displays that require bending resistance.

[0111] 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.

[0112] [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 equipment and conditions: Device name: GPC system HLC-8220, manufactured by Tosoh Corporation Measurement side column configuration: Guard column: TSKgel guard column SuperHZ-L, manufactured by Tosoh Corporation Separation column: TSKgel SuperHZM-M, two columns connected in series Reference side column configuration: Reference column: TSKgel SuperH-RC, manufactured by Tosoh Corporation Developing solvent: chloroform (special grade, manufactured by Wako Pure Chemical Industries, Ltd.) Developing solvent flow rate: 0.6 mL / min Standard sample: TSK standard polystyrene (PS-oligomer kit, manufactured by Tosoh Corporation) Column temperature: 40°C

[0113] [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 Corporation; Thermo plus EVO DSC-8230), the glass transition temperature was measured by the starting point method from a DSC curve obtained by heating approximately 10 mg of a sample from room temperature to 200°C (heating rate 20°C / min) under a nitrogen gas atmosphere. α-Alumina was used as a reference.

[0114] [Contents of methyl methacrylate and α-methylene-γ-butyrolactone in powders] The contents of methyl methacrylate and α-methylene-γ-butyrolactone in the first powder and the second powder were measured using a liquid chromatograph (HPLC). The measuring device and measuring conditions are as follows.

[0115] Device name: Shimadzu Corporation, Prominence UFLC XR Column: GLScience, InertSustain C8: 5 μm and C18: 5 μm, two columns in series Mobile phase: Eluent acetonitrile / ultrapure water Flow rate: 0.5 ml / min Detector: UV detector 215 nm

[0116] Five solutions containing known concentrations of each component were prepared, and a calibration curve was created from the detected peak areas. 1 g of resin powder was added to 100 g of a 7 / 3 volumetric solution of acetonitrile and 50 mM sodium phosphate buffer, and the mixture was stirred for 4 hours in a mixing rotor. This solution was quantified under the above conditions.

[0117] [Content of emulsifier in powder] The content of emulsifier in the first powder and the second powder was measured using a liquid chromatograph (HPLC). The measuring device and measuring conditions are as follows.

[0118] Device name: Shimadzu Corporation, Prominence UFLC XR Column: Osaka Soda Co., Ltd., two 90104 columns in series Mobile phase: Eluent acetonitrile / 50 mM sodium phosphate buffer solution in a volume ratio of 7 / 3 Flow rate: 0.5 ml / min Detector: UV detector 225 nm

[0119] Five solutions containing known concentrations of each component were prepared, and a calibration curve was created from the detected peak areas. 1 g of resin powder was added to 100 g of a 1 / 3 volumetric solution of methanol and ultrapure water, and the mixture was stirred for 4 hours using a mix rotor. This solution was quantified under the above conditions.

[0120] [Yellowness Index (YI) of Optical Film] The yellowness index (YI) of the optical film was measured in accordance with the provisions of JIS K 7373. Specifically, the 40 μm optical film obtained in the example was used as a test piece, and the yellowness index (YI) was measured using a spectral colorimeter / haze meter (manufactured by Nippon Denshoku Industries Co., Ltd.; COH7700).

[0121] [Bending Resistance of Optical Film (Foldable Test)] An 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 (DMLHP-CS, manufactured by Yuasa System Co., Ltd.). At this time, the halfway position in the long side direction of the test specimen was set as the folding position. 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 ends was 2 mm. In the folded state, the portions other than the folding position were approximately parallel. One folding operation was defined as changing from a flat, open state to a bent state, 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 even after 250,000 bending operations was evaluated as "A+". The number of times until breakage was evaluated as "A" if it was 200,000 or more but less than 250,000 times, "B" if it was 100,000 or more but less than 200,000 times, and "C" if it was less than 100,000 times.

[0122] [Thickness of Optical Film] The thickness of the optical film was measured using a Digimatic micrometer (manufactured by Mitutoyo; ID-C112).

[0123] [Amount of Residual Solvent in Optical Film] The mass of the prepared optical film was measured, and then dried in an oven at 200°C for 1 hour. The absolute content (g) of residual solvent in the film was calculated from the difference between the film mass after drying and the film mass before drying. The calculated absolute content of residual solvent was divided by the mass of the prepared optical film to determine the amount of residual solvent (mass%) in the optical film.

[0124] 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) was obtained from NOF Corporation. Polyoxyethylene distyryl phenyl ether sulfate ester ammonium (Hitenol (registered trademark) NF-08) was obtained from Daiichi Kogyo Seiyaku Co., Ltd.

[0125] In the following explanation, the names of the compounds will be abbreviated as follows: MMA: methyl methacrylate MBL: α-methylene-γ-butyrolactone LPO: dilauroyl peroxide (Perloyl L) NF-08: polyoxyethylene distyrylphenyl ether sulfate ester ammonium (Hitenol (registered trademark) NF-08) n-DM: n-dodecyl mercaptan

[0126] Comparative Example 1 Synthesis of Copolymer and Preparation of Powder 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 container. 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 thereto. The mixture in the container was then stirred at 3,000 rpm using a disperser (Primix Corporation; Homomixer MARK II model 2.5). 125 parts by mass of deionized water was added to the mixture and then transferred to the reactor.

[0127] In the reactor, the reaction solution was heated to 65°C while continuing to stir and supply nitrogen gas. The point when the internal temperature reached 65°C was considered to be the start of polymerization. After the liquid temperature reached the peak temperature due to self-heating, the reaction solution was heated to 75°C and stirred. Two hours after the start of polymerization, the reaction solution was further heated to 90°C and stirred for four hours. In this manner, the polymerization reaction was completed.

[0128] Thereafter, the reaction solution was cooled, and the copolymer was collected by filtration and further subjected to a drying step at 90°C for 10 hours using a hot air dryer, thereby obtaining a first powder X-1 containing the copolymer. 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.

[0129] The contents of MMA, MBL, and NF-08 contained in the first powder X-1 were measured, and the results are shown in Table 1.

[0130] <Preparation of Dope> The obtained first powder X-1 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 a dope.

[0131] <Preparation of Optical Film A-1c> Using a Baker-type applicator, the dope was cast onto a stainless steel plate to form a cast film. The gap of the Baker-type applicator was set to 380 μm. The cast film was heated at 25° C. for 10 minutes using a hot plate to obtain an optical film A-1c. After peeling the optical film A-1c from the stainless steel plate, the amount of remaining solvent in the optical film A-1c was determined. The results are shown in Table 1.

[0132] <Preparation of Optical Film B-1c> An optical film was prepared in the same manner as in the preparation of Optical Film A-1c. The obtained optical film was fixed on each of the top, bottom, left, and right sides and heated in an oven at 120°C for 60 minutes to obtain an optical film B-1c with a thickness of 100 μm. The amount of residual solvent in Optical Film B-1c was determined. The obtained optical film B-1c was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD direction) and the transverse direction (TD direction) using a sequential biaxial stretching machine (X-6S, manufactured by Toyo Seiki Seisaku-sho, Ltd.) at a temperature of Tg of the copolymer + 14°C at a speed of 240 mm / min so that the stretching ratio was 1.6 times in each direction, thereby obtaining a stretched film with a thickness of 40 μm. The YI and folding resistance of the obtained stretched film were measured. The results are shown in Table 1.

[0133] Example 1 First powder X-1 was subjected to a heat treatment step in which it was heated at 150°C for 1 hour using a hot air dryer, to prepare second powder Y-1 of Example 1. The copolymer in second powder Y-1 had a weight average molecular weight (Mw) of 223,000, a number average molecular weight (Mn) of 110,000, and a glass transition temperature (Tg) of 126°C. The contents of MMA, MBL, and NF-08 contained in second powder Y-1 were measured.

[0134] A dope and an optical film A-1 were prepared in the same manner as in Comparative Example 1, except that the first powder X-1 was replaced with the second powder Y-1, and the residual solvent amount of the optical film A-1 was determined. An optical film B-1 was also prepared in the same manner as in Comparative Example 1, and the residual solvent amount, YI, and bending resistance were measured. The results are shown in Table 1.

[0135] Example 2 The first powder X-1 was subjected to a heat treatment step in which it was heated at 125°C for 5 hours using a hot air dryer, to prepare the second powder Y-2 of Example 2. The copolymer in the second powder Y-2 had an Mw of 225,000, an Mn of 110,000, and a glass transition temperature (Tg) of 126°C. The contents of MMA, MBL, and NF-08 contained in the second powder Y-2 were measured.

[0136] A dope and an optical film A-2 were prepared in the same manner as in Comparative Example 1, except that the first powder X-1 was replaced with the second powder Y-2, and the residual solvent amount of the optical film A-2 was determined. Furthermore, an optical film B-2 was prepared in the same manner as in Comparative Example 1, and the residual solvent amount, YI, and bending resistance were measured. The results are shown in Table 1.

[0137] Example 3 First powder X-1 was subjected to a heat treatment step in which it was heated at 115°C for 5 hours using a hot air dryer, to prepare second powder Y-3. The copolymer in second powder Y-3 had an Mw of 224,000, an Mn of 109,000, and a glass transition temperature (Tg) of 126°C. The contents of MMA, MBL, and NF-08 contained in second powder Y-3 were measured.

[0138] A dope and an optical film A-3 were prepared in the same manner as in Comparative Example 1, except that the first powder X-1 was replaced with the second powder Y-3, and the residual solvent amount of the optical film A-3 was measured. An optical film B-3 was also prepared in the same manner as in Comparative Example 1, and the residual solvent amount, YI, and bending resistance were measured. The results are shown in Table 1.

[0139] Example 4 A first powder X-2 was prepared in the same manner as in Comparative Example 1, except that the amount of n-DM used was changed to 0.01 parts by mass. The copolymer had a weight-average molecular weight (Mw) of 622,000, a number-average molecular weight (Mn) of 246,000, and a glass transition temperature (Tg) of 126°C. The contents of MMA and MBL contained in the first powder X-2 were measured. As a result, the MMA content and MBL content in the first powder X-2 were found to be 0.81% by mass and 0.39% by mass, respectively.

[0140] The first powder X-2 was subjected to a heat treatment step in which it was heated at 115°C for 5 hours using a hot air dryer, to prepare a second powder Y-4. The copolymer in the second powder Y-4 had a weight average molecular weight (Mw) of 620,000, a number average molecular weight (Mn) of 245,000, and a glass transition temperature (Tg) of 126°C. The contents of MMA, MBL, and NF-08 contained in the second powder Y-4 were measured.

[0141] A dope and an optical film A-4 were prepared in the same manner as in Comparative Example 1, except that the first powder X-1 was changed to the second powder Y-4, the solid content of the dope was set to 22 mass%, and the gap of the Baker-type applicator was set to 460 μm, and the amount of residual solvent in the optical film A-4 was measured. Furthermore, an optical film B-4 was prepared in the same manner as in Comparative Example 1, and the amount of residual solvent, YI, and bending resistance were measured. The results are shown in Table 2.

[0142] Example 5 A first powder X-3 was prepared in the same manner as in Comparative Example 1, except that n-DM was not used. The weight-average molecular weight (Mw) of the copolymer was 864,000, the number-average molecular weight (Mn) was 293,000, and the glass transition temperature (Tg) was 126°C. The contents of MMA and MBL contained in the first powder X-3 were measured. As a result, the MMA content and MBL content in the first powder X-3 were 0.82% by mass and 0.36% by mass, respectively.

[0143] The first powder X-3 was subjected to a heat treatment step in which it was heated at 115°C for 5 hours using a hot air dryer, to prepare a second powder Y-5. The copolymer in the second powder Y-5 had a weight average molecular weight (Mw) of 865,000, a number average molecular weight (Mn) of 292,000, and a glass transition temperature (Tg) of 126°C. The contents of MMA, MBL, and NF-08 contained in the second powder Y-5 were measured.

[0144] A dope and an optical film A-5 were prepared in the same manner as in Comparative Example 1, except that the first powder X-1 was replaced with the second powder Y-5, the solid content of the dope was 20 mass%, and the gap of the Baker-type applicator was 500 μm. The remaining solvent amount of the optical film A-5 was measured. An optical film B-5 was also prepared in the same manner as in Comparative Example 1, and the remaining solvent amount, YI, and bending resistance were measured. The results are shown in Table 2.

[0145] Example 6 A first powder X-4 was prepared in the same manner as in Comparative Example 1, except that the amount of MMA used was changed to 30 parts by mass and the amount of MBL used was changed to 20 parts by mass. The copolymer had a weight-average molecular weight (Mw) of 237,000, a number-average molecular weight (Mn) of 124,000, and a glass transition temperature (Tg) of 138°C. The contents of MMA and MBL contained in the first powder X-4 were measured. As a result, the MMA content and MBL content in the first powder X-4 were found to be 0.74% by mass and 0.46% by mass, respectively.

[0146] The first powder X-4 was subjected to a heat treatment step in which it was heated at 115°C for 5 hours using a hot air dryer, to prepare a second powder Y-6. The weight-average molecular weight (Mw) of the copolymer of the polymer in the second powder Y-6 was 236,000, the number-average molecular weight (Mn) was 124,000, and the glass transition temperature (Tg) was 138°C. The contents of MMA, MBL, and NF-08 contained in the second powder Y-6 were measured.

[0147] <Preparation of Dope> The obtained second powder Y-6 was dissolved in dimethylacetamide to a solid content of 25% by mass, and then filtered through a filter with a filtration accuracy of 10 μm to prepare a dope.

[0148] <Preparation of Optical Film A-6> Using a Baker-type applicator, the dope was cast onto a PET film attached to a stainless steel plate to form a cast film. The gap of the Baker-type applicator was set to 800 μm. The cast film was heated at 120° C. for 30 minutes using a hot air dryer to obtain Optical Film A-6. After peeling Optical Film A-6 from the PET film, the amount of residual solvent in Optical Film A-6 was determined. The results are shown in Table 2.

[0149] <Preparation of Optical Film B-6> An optical film was prepared in the same manner as in the preparation of Optical Film A-6. The obtained optical film was fixed on each of the top, bottom, left, and right sides and heated in an oven at 200°C for 60 minutes to obtain Optical Film B-6 with a thickness of 100 μm. The amount of residual solvent in Optical Film B-6 was determined. The obtained Optical Film B-6 was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD direction) and the transverse direction (TD direction) using a sequential biaxial stretching machine (X-6S, manufactured by Toyo Seiki Seisaku-sho, Ltd.) at a temperature of Tg of the copolymer + 14°C at a speed of 240 mm / min so that the stretch ratio was 1.6 times in each direction, thereby obtaining a stretched film with a thickness of 40 μm. The YI and folding resistance were measured. The results are shown in Table 2.

[0150]

[0151]

[0152] Comparing the residual solvent amounts of optical films A and B produced in each Example and Comparative Example in Tables 1 and 2, it was confirmed that when optical films were produced under the same conditions, the amount of residual solvent was lower when the powder of each Example was used. Therefore, it was confirmed that the dope drying efficiency was higher and the productivity of optical films was superior when the powder of each Example obtained through the heat treatment process was used than when the powder of Comparative Example obtained without the heat treatment process was used. In particular, Examples 1 to 5, which used the second powder having a total content of MBL and MMA of 0.70% by mass or less, showed significantly improved drying efficiency. Example 6, in which the dope solvent was changed, also showed good drying efficiency.

[0153] Furthermore, as shown in Tables 1 and 2, a tendency was observed where the weight average molecular weight (Mw) was larger, and the bending resistance was improved. On the other hand, when Examples 1 to 3, which had the same weight average molecular weight (Mw), were compared, Example 1, which had a lower amount of residual solvent, was superior in bending resistance, and a tendency was observed where the bending resistance of the optical film was improved when the amount of residual solvent was lower.

Claims

1. A method for producing a powder containing a copolymer including structural units derived from α-methylene lactone and structural units derived from alkyl (meth)acrylate, comprising: a polymerization step of suspension polymerizing monomers including α-methylene lactone and alkyl (meth)acrylate in a solvent in the presence of a polymerization initiator and an emulsifier to obtain a copolymer; a drying step of drying the copolymer to obtain a first powder; and a heat treatment step of heating the first powder at a temperature higher than that in the drying step to obtain a second powder having a lower total content of α-methylene lactone and alkyl (meth)acrylate than the first powder.

2. A method for producing a powder as described in claim 1, wherein the total content of the α-methylene lactone and the alkyl (meth)acrylate in the second powder is 0.70 mass% or less.

3. A method for producing a powder as described in claim 1 or 2, wherein the content of the emulsifier in the second powder is 100 mass ppm or less.

4. The method for producing a powder according to claim 1 or 2, wherein the emulsifier is an ammonium sulfate salt.

5. The method for producing a powder according to claim 1 or 2, wherein the content of the structural unit derived from α-methylene lactone in the copolymer is 10% by mass or more and 40% by mass or less.

6. A method for producing a powder according to claim 1 or 2, wherein the drying temperature in the drying step is 80°C or higher and 105°C or lower.

7. The method for producing a powder according to claim 1 or 2, wherein the glass transition temperature Tg of the copolymer measured by an initiation method is 115°C or higher, and the heating temperature in the heat treatment step is (Tg-10)°C or higher and (Tg+70)°C or lower.

8. The method for producing a powder according to claim 1 or 2, wherein the rate of decrease in the weight average molecular weight of the copolymer during the heat treatment step is 10% or less.

9. A method for producing a dope, comprising a dope preparation step of mixing the powder obtained by the method according to claim 1 or 2 with a solvent to obtain a dope.

10. A method for producing an optical film, comprising: a casting step of casting the dope obtained by the method according to claim 9 onto a support to form a casting film; and a volatilization step of volatilizing the solvent from the casting film.

11. A powder containing a copolymer including a structural unit derived from α-methylene lactone and a structural unit derived from alkyl (meth)acrylate, the powder being characterized in that the total content of α-methylene lactone and alkyl (meth)acrylate is 0.70 mass% or less.

12. The powder according to claim 11, wherein the emulsifier content is 100 ppm by mass or less.

13. The powder of claim 12, wherein said emulsifier is an ammonium sulfate salt.

14. The powder according to claim 11 or 12, wherein the weight average molecular weight of the copolymer as measured by gel permeation chromatography (GPC) is 200,000 or more and 1,000,000 or less.

15. A dope comprising a copolymer containing a structural unit derived from α-methylene lactone and a structural unit derived from alkyl (meth)acrylate, and a solvent, wherein the total content of the α-methylene lactone and the alkyl (meth)acrylate is 0.70 mass% or less in the total amount of the copolymer, α-methylene lactone, and alkyl (meth)acrylate.

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