Stretched film and method for manufacturing the same

Abstract

To provide a method for manufacturing a stretched film containing a copolymer having a constituent unit derived from α-methylene lactone by employing a solution film forming method, and to provide a method for manufacturing a stretched film having sufficient strength and hardness.SOLUTION: A method for manufacturing a stretched film containing a copolymer having a composition unit derived from α-methylene lactone, comprises the steps of: obtaining an unstretched film by a solution film forming method by using a dope containing the copolymer and a solvent; and stretching the unstretched film at an areal ratio of 1.8 to 10 times, wherein a residual solvent content in the unstretched film is 0.05 to 5.0 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to stretched films and methods for manufacturing the same. [Background technology]

[0002] Copolymers having structural units derived from α-methylene lactone exhibit excellent transparency and other properties, making them promising for application in optical components. For example, Patent Document 1 discloses an optically isotropic acrylic resin film containing a copolymer having structural units derived from α-methylene lactone. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2009-41007 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] While methods for manufacturing films, such as melt deposition and solution deposition, are known, films containing copolymers having α-methylene lactone-derived structural units, as disclosed in Patent Document 1, were often manufactured by melt deposition. However, copolymers having α-methylene lactone-derived structural units have high viscosity in the molten state, which meant there was room for improvement in the efficiency of film manufacturing. Therefore, manufacturing by solution deposition was considered preferable.

[0005] Incidentally, films require sufficient strength and hardness, and generally, this requirement is met by stretching unstretched films obtained by melt-forming or solution-forming methods. However, there was room for improvement in terms of strength and hardness for stretched films obtained by stretching unstretched films obtained by solution-forming methods.

[0006] The main object of this disclosure is to provide a method for producing a stretched film containing a copolymer having structural units derived from α-methylene lactone, using a solution film formation method, and to provide a method for producing a stretched film having sufficient strength and hardness. [Means for solving the problem]

[0007] This disclosure includes methods for manufacturing stretched films as described in [1] to [7] below, and stretched films as described in [8] to [9]. [1] A method for producing a stretched film comprising a copolymer having structural units derived from α-methylene lactone, comprising the steps of: obtaining an unstretched film by a solution film formation method using a dope comprising the copolymer and a solvent; and stretching the unstretched film to a surface magnification of 1.8 to 10 times, wherein the amount of residual solvent in the unstretched film is 0.05 to 5.0% by mass. [2] The method for producing a stretched film according to [1], wherein the amount of residual solvent in the unstretched film is 0.1 to 5.0% by mass. [3] The method for producing a stretched film according to [1] or [2], wherein the thickness of the unstretched film is 60 to 200 μm. [4] The method for producing a stretched film according to any one of [1] to [3], wherein the copolymer further comprises a constituent unit derived from alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms. [5] The method for producing a stretched film according to any one of [1] to [4], wherein the solvent comprises methylene chloride. [6] The method for producing a stretched film according to any one of [1] to [5], wherein the viscosity of the dope is 0.1 to 50.0 Pa·s. [7] The method for producing a stretched film according to any one of [1] to [6], wherein the solid content concentration of the dope is 5 to 40% by mass. [8] A stretched film comprising a copolymer having structural units derived from α-methylene lactone, wherein the stretched film contains 10 to 20,000 ppm by mass of methylene chloride. [9] The stretched film according to [8], wherein the absolute value of the phase difference in the thickness direction is 30 nm or less. [Effects of the Invention]

[0008] According to this disclosure, a method for producing a stretched film containing a copolymer having structural units derived from α-methylene lactone, employing a solution film formation method, can be provided, which provides a method for producing a stretched film having sufficient strength and hardness. Furthermore, according to the present invention, a stretched film containing a copolymer having structural units derived from α-methylene lactone can be provided, which has sufficient strength and hardness. [Modes for carrying out the invention]

[0009] Embodiments of the present disclosure are described below. However, the present disclosure is not limited to the embodiments described below. In the following description, "(meth)acrylic" is a term that encompasses both acrylic and methacrylic. Resin and polymer are synonymous terms. A numerical range indicated as "X~Y" means X or more and Y or less. For example, "1.8~10 times" means 1.8 times or more and 10 times or less.

[0010] A method for producing a stretched film according to one embodiment of the present disclosure is a method for producing a stretched film comprising a copolymer having structural units derived from α-methylene lactone, comprising the steps of: obtaining an unstretched film by a solution film formation method using a dope comprising a copolymer having structural units derived from α-methylene lactone and a solvent; and stretching the unstretched film to a surface magnification of 1.8 to 10 times, wherein the amount of residual solvent in the unstretched film is 0.05 to 5.0% by mass.

[0011] [Copolymer having structural units derived from α-methylene lactone] The constituent units derived from α-methylene lactone have a structure derived from an α-methylene lactone monomer in which a methylene group is bonded to the carbon atom at the α-position of the lactone ring. Typically, the constituent units derived from α-methylene lactone may also have structures derived from monomers such as α-methylene-γ-butyrolactone and α-methylene-δ-valerolactone.

[0012] From the viewpoint of further improving the mechanical properties of the obtained stretched film, the lactone ring is preferably a 5-membered ring (γ-lactone) or a 6-membered ring (δ-lactone).

[0013] The structural unit derived from α-methylene lactone preferably has a structure represented by the following formula (1).

[0014] [Chemical formula]

[0015] R in formula (1) 1 to R 4 are each independently a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms.

[0016] The hydrocarbon group is an aliphatic hydrocarbon group or an aromatic hydrocarbon group. The aliphatic hydrocarbon group may be, for example, an alkyl group. The alkyl group may be linear, branched or cyclic. Specifically, the alkyl group may be a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, a t-butyl group, an n-pentyl group, an n-hexyl group, a cyclopentyl group, a cyclohexyl group, or the like. Specifically, the aromatic hydrocarbon group may be a phenyl group, a tolyl group, a benzyl group, or the like.

[0017] R 1 to R 4 are preferably each independently a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, more preferably a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, and even more preferably all are hydrogen atoms.

[0018] The structural unit having the structure represented by formula (1) can be formed by polymerization of a compound represented by the following formula (2).

[0019] [Chemical formula]

[0020] R in equation (2) 1 ~R 4 R in equation (1) 1 ~R 4 It is identical to [the other one]. Therefore, a detailed explanation will be omitted.

[0021] The copolymer may have only one structural unit derived from α-methylene lactone, or it may have two or more, but it is preferable that it has at least one structural unit derived from α-methylene-γ-butyrolactone.

[0022] The copolymer has structural units derived from monomers copolymerizable with α-methylene lactone, in addition to structural units derived from α-methylene lactone monomers. Specifically, such structural units may have structures derived from monomers such as alkyl (meth)acrylate, benzyl (meth)acrylate, chloromethyl (meth)acrylate, 2-chloroethyl (meth)acrylate, styrene, vinyltoluene, α-methylstyrene, acrylonitrile, methyl vinyl ketone, ethylene, propylene, and vinyl acetate. The copolymer may have only one structural unit derived from monomers copolymerizable with α-methylene lactone monomers, or it may have two or more.

[0023] From the viewpoint of further improving the heat resistance and transparency of the resulting stretched film, the copolymer preferably has structural units derived from alkyl (meth)acrylate. The structural units derived from alkyl (meth)acrylate have a structure derived from alkyl (meth)acrylate monomers. Specifically, the structural units derived from alkyl (meth)acrylate may have a structure derived from monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl acrylate, cyclopentyl (meth)acrylate, and cyclohexyl (meth)acrylate.

[0024] In particular, the copolymer preferably has a structural unit derived from an alkyl (meth)acrylate having an alkyl group with 1 to 6 carbon atoms, more preferably has a structural unit derived from an alkyl (meth)acrylate having an alkyl group with 1 to 3 carbon atoms, and still more preferably has a structural unit derived from methyl (meth)acrylate. The copolymer may have only one type, or may have two or more types, of the structural unit derived from an alkyl (meth)acrylate, but preferably has at least a structural unit derived from methyl (meth)acrylate.

[0025] The content of each structural unit in the copolymer is not particularly limited, but is preferably as follows. From the viewpoint of further improving the mechanical strength of the obtained stretched film, the content of the structural unit derived from α-methylene lactone is preferably 5 to 60% by mass, more preferably 7.5 to 50% by mass, and still more preferably 10 to 45% by mass. The content of the structural unit derived from an alkyl (meth)acrylate having an alkyl group with 1 to 6 carbon atoms is preferably 40 to 95% by mass, more preferably 50 to 92.5% by mass, and still more preferably 55 to 90% by mass. The content of any structural unit different from the structural unit derived from α-methylene lactone and the structural unit derived from an alkyl (meth)acrylate having an alkyl group with 1 to 6 carbon atoms may be 0 to 30% by mass, may be 0 to 25% by mass, or may be 0 to 20% by mass. The content of each structural unit in the copolymer is obtained by dissolving the copolymer in a deuterated solvent, 1 measuring 1H-NMR, and calculating the area ratio of peaks corresponding to each structural unit.

[0026] The weight average molecular weight (Mw) of the copolymer is preferably 50,000 to 1,500,000, more preferably 100,000 to 1,000,000, still more preferably 150,000 to 800,000, and particularly preferably 200,000 to 600,000. The number average molecular weight (Mn) of the copolymer may be, for example, 30,000 to 500,000. The dispersity (Mw / Mn) may be, for example, 1.5 or more and 5 or less.

[0027] From the viewpoint of further improving the heat resistance and dimensional stability of the resulting stretched film, the glass transition temperature (Tg) of the copolymer is preferably 110°C or higher, more preferably 115°C or higher, even more preferably 120°C or higher, and particularly preferably 125°C or higher. Furthermore, from the viewpoint of further improving the mechanical properties of the resulting stretched film, the glass transition temperature (Tg) of the copolymer is preferably 200°C or lower, and more preferably 190°C or lower.

[0028] The HSP (Hansen Solubility Parameters) value of the copolymer is considered to be 14-22 MPa when δD is set to further improve the water resistance of the resulting stretched film. 1 / 2 δP is 8-15 MPa 1 / 2 δH is 5-10 MPa 1 / 2 Preferably, δD is 16-21 MPa 1 / 2 δP is 10-13 MPa 1 / 2 δH is 6-9 MPa 1 / 2 It is more preferable that δD is 17-20 MPa 1 / 2 δP is 11-13 MPa 1 / 2 δH is 6-9 MPa 1 / 2 It is even more preferable that this is the case. The HSP value of the copolymer is determined by a method in accordance with the description in HansenSolubility Parameters: A User's Handbook, Second Edition, Charles M. Hansen. The HSP value can be calculated using HSPiP version: 5.3.08 based on the results of a solubility test in a known solvent. The calculation method is determined using Classic GA.

[0029] Copolymers can be produced, for example, by suspension polymerization. Suspension polymerization involves a step of dispersing monomers and other additives for obtaining the copolymer in an aqueous solvent, and a polymerization step of polymerizing the monomers.

[0030] The aqueous solvent may be water alone or may contain a water-soluble organic solvent. Specifically, the water-soluble organic solvent may be alcohol solvents such as methanol, ethanol, propanol, butanol, 2-methylpropyl alcohol, 2-methyl-2-propanol; ketone solvents such as acetone, methyl ethyl ketone; ester solvents such as ethyl acetate; ether solvents such as dioxane, diethyl ether, tetrahydrofuran; and so on.

[0031] The content of the water-soluble organic solvent in the aqueous solvent is preferably 5% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less.

[0032] It is preferable that the entire amount of monomers needed to obtain the copolymer is introduced into the reactor before polymerization has substantially begun. For example, it is preferable to introduce the entire amount of monomers into the reactor before raising the reactor temperature to the polymerization temperature.

[0033] When dispersing monomers in an aqueous solvent, dispersion may be carried out by stirring with paddle blades or similar devices, or by using emulsification and dispersion equipment such as a high-speed shear turbine disperser, a high-pressure jet homogenizer, an ultrasonic emulsifier / disperser, a media-stirring disperser, or a forced-gap-pass disperser.

[0034] When polymerizing monomers, additives such as polymerization initiators, chain transfer agents, and dispersants may be added as needed.

[0035] The polymerization initiator may be an organic peroxide such as cumene hydroperoxide, diisopropylbenzene hydroperoxide, di-t-butyl peroxide, lauroyl peroxide, benzoyl peroxide, t-butyl peroxyisopropyl carbonate, t-amyl peroxy-2-ethylhexanoate, or t-butyl peroxy-2-ethylhexanoate; or an azo compound such as 2,2'-azobis(isobutyronitrile), 1,1'-azobis(cyclohexanecarbonile), 2,2'-azobis(2,4-dimethylvaleronitrile), or dimethyl-2,2'-azobis(2-methylpropionate). The amount of polymerization initiator added may be set as appropriate, and may be 0.01 to 5 parts by mass per 100 parts by mass of monomer.

[0036] The chain transfer agent may specifically be a monofunctional thiol compound such as n-dodecyl mercaptan or β-mercaptopropionic acid; a difunctional thiol compound such as a polysiloxane with mercapto-modified ends; or a polyfunctional polysiloxane with mercapto-modified side chains. The amount of chain transfer agent added can be set as appropriate, and may be 0.001 to 1 part by mass per 100 parts by mass of monomer.

[0037] The dispersant may specifically be a water-soluble polymer-based dispersion stabilizer such as polyvinyl alcohol (PVA), polyvinylpyrrolidone, cellulose, gelatin, sodium polyacrylate, or sodium polymethacrylate; anionic surfactants such as sodium lauryl sulfate or polyoxyethylene alkylphenyl ether sulfate (e.g., polyoxyethylene distyrylphenyl ether sulfate ammonium); cationic surfactants such as alkylamine salts or quaternary ammonium salts; amphoteric surfactants such as lauryldimethylamine oxide; nonionic surfactants such as polyoxyethylene alkyl ether; other inorganic dispersants such as alginates, zein, casein; barium sulfate, calcium sulfate, barium carbonate, magnesium carbonate, calcium phosphate, talc, clay, diatomaceous earth, bentonite, titanium hydroxide, thorium hydroxide, or metal oxide powders. The amount of dispersant added can be set as appropriate, and may be 0.1 to 3 parts by mass per 100 parts by mass of monomer.

[0038] Furthermore, various additives other than dispersants may be included in the resulting copolymer. Examples of additives other than dispersants include ultraviolet absorbers; antiblocking particles; antioxidants; polymerization inhibitors such as 4-tert-butylcatechol (TBC), hydroquinone, and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4H-TEMPO); stabilizers such as light stabilizers, weather stabilizers, and heat stabilizers; reinforcing materials such as glass fibers and carbon fibers; near-infrared absorbers; flame retardants such as tris(dibromopropyl) phosphate, triallyl phosphate, and antimony oxide; phase difference adjusters such as phase difference enhancers, phase difference reducers, and phase difference stabilizers; antistatic agents including anionic, cationic, and nonionic surfactants; colorants such as inorganic pigments, organic pigments, and dyes; resin modifiers; and the like.

[0039] The ultraviolet absorber may be, for example, a triazole compound, a triazine compound, a benzophenone compound, a salicylate compound, or a benzoate compound. The ultraviolet absorber preferably contains at least one selected from triazole compounds and triazine compounds, and more preferably contains at least one selected from triazole compounds.

[0040] The triazole compounds may specifically include 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazole-2-yl)phenol], 2-(3,5-di-tert-butyl-2-hydroxyphenyl)-5-chlorobenzotriazole, 2-(2H-benzotriazole-2-yl)-p-cresol, 2-(2H-benzotriazole-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, 2-benzotriazole-2-yl-4,6-di-tert-butylphenol, 2-[5-chloro(2H)-benzotriazole-2-yl]-4-methyl-6-t-butylphenol, 2-(2H-benzotriazole-2-yl)-4,6-di-t-butylphenol, and 2-(2H-benzotriazole-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol.

[0041] The triazine compounds may specifically include 2-mono(hydroxyphenyl)-1,3,5-triazine compounds, 2,4-bis(hydroxyphenyl)-1,3,5-triazine compounds, and 2,4,6-tris(hydroxyphenyl)-1,3,5-triazine compounds.

[0042] Benzophenone compounds may specifically include 2,4-dihydroxybenzophenone, 4-n-octyloxy-2-hydroxybenzophenone, and 2,2'-dihydroxy-4,4'-dimethoxybenzophenone. Salicylate compounds may specifically include pt-butylphenyl salicylate. Benzoate compounds may specifically include 2,4-di-t-butylphenyl-3',5'-di-t-butyl-4'-hydroxybenzoate.

[0043] The UV absorber may be a commercially available product, specifically, "Chinubin® 928 (manufactured by BASF Japan)", "ADEKA® LA-32 (manufactured by ADEKA Corporation)", "ADEKA® LA-31 (manufactured by ADEKA Corporation)", "ADEKA® LA-29 (manufactured by ADEKA Corporation)", "ADEKA® LA-24 (manufactured by ADEKA Corporation)", "Eversorb® BL4 (manufactured by Everlight Chemical Co., Ltd.)", etc. Preferably, the UV absorber includes at least one selected from "ADEKA® LA-31 (manufactured by ADEKA Corporation)" and "ADEKA® LA-29 (manufactured by ADEKA Corporation)".

[0044] The antiblocking particles may be inorganic particles or organic particles.

[0045] The inorganic particles may specifically consist of silica, titania, alumina, zirconia, calcium carbonate, talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, calcium phosphate, and the like. It is preferable that the inorganic particles include silica particles. The silica particles may have a hydrophilic or hydrophobic surface treatment. The average primary particle diameter of the inorganic particles may be, for example, 1 to 500 nm.

[0046] The organic particles may specifically consist of silicone resin, fluororesin, (meth)acrylic crosslinked particles, and the like. It is preferable that the organic particles include (meth)acrylic crosslinked particles. The (meth)acrylic crosslinked particles may be copolymerized with styrene and may have a core-shell structure. In the case of (meth)acrylic crosslinked particles having a core-shell structure, the core portion may be composed of (meth)acrylic resin and the shell portion of styrene resin. The average primary particle diameter of the organic particles may be, for example, 10 nm to 10 μm.

[0047] The antioxidant may be, for example, a hindered phenol antioxidant, a hindered amine antioxidant, a phosphorus antioxidant, a sulfur antioxidant, a benzotriazole antioxidant, a benzophenone antioxidant, a hydroxylamine antioxidant, a salicylic acid ester antioxidant, or a triazine antioxidant. Preferably, the antioxidant contains at least one selected from hindered phenol antioxidants, hindered amine antioxidants, phosphorus antioxidants, and sulfur antioxidants, and more preferably at least one selected from hindered phenol antioxidants, hindered amine antioxidants, and phosphorus antioxidants.

[0048] Hindered phenol antioxidants may include, specifically, 2,4-bis[(laurylthio)methyl]-o-cresol, 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl), 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl), and others.

[0049] Hindered amine antioxidants, specifically bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis(N-methyl-2,2,6,6-tetramethyl-4-piperidyl) sebacate, N,N′-bis(2,2,6,6-tetramethyl-4-piperidyl)-1,6-hexamethylenediamine, 2-methyl-2-(2,2,6,6-tetramethyl-4-piperidyl)amino-N-(2,2,6,6-tetramethyl- It may also be 4-piperidyl)propionamide, tetrakis(2,2,6,6-tetramethyl-4-piperidyl)(1,2,3,4-butanetetracarboxylate, poly[(6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl)((2,2,6,6-tetramethyl-4-piperidyl)imino)hexamethyl((2,2,6,6-tetramethyl-4-piperidyl)imino)], etc.

[0050] Phosphorus-based antioxidants may include tris(isodecyl)phosphite, tris(tridecyl)phosphite, phenylisooctylphosphite, phenylisodecylphosphite, phenyldi(tridecyl)phosphite, diphenylisooctylphosphite, diphenylisodecylphosphite, diphenyltridecylphosphite, triphenylphosphite, tris(nonylphenyl)phosphite, 4,4'isopropylidenediphenolalkylphosphite, trisnonylphenylphosphite, trisdinonylphenylphosphite, and other oligomer-type and polymer-type compounds having a phosphite structure.

[0051] Sulfur-based antioxidants may include 2,2-thio-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,4-bis[(octylthio)methyl]-o-cresol, 2,4-bis[(laurylthio)methyl]-o-cresol, or other oligomeric or polymeric compounds having a thioether structure.

[0052] After the polymerization process, the copolymer particles can be recovered by solid-liquid separation. The solid-liquid separation method may include filtration, centrifugation, or a combination thereof.

[0053] The resulting copolymer may be dried. The drying temperature may be, for example, between 60°C and 120°C.

[0054] Alternatively, drying may be performed without solid-liquid separation. By drying with a dryer, the copolymer can be obtained as a powder.

[0055] Of course, it goes without saying that copolymers having structural units derived from α-methylene lactone can also be produced by other polymerization methods such as emulsion polymerization.

[0056] [solvent] The solvent is not limited in type as long as it can dissolve the polymer having structural units derived from α-methylene lactone. The solvent may be an organic solvent, for example, a linear ketone solvent, a cyclic ketone solvent, an alkyl chloride solvent, a cyclic ester solvent, an amide solvent, a sulfoxide solvent, an aromatic solvent, or an alcohol solvent. Specifically, linear ketone solvents may be acetone, methyl ethyl ketone, etc. Specifically, cyclic ketone solvents may be cyclohexanone (anone), cyclopentanone, etc. Specifically, alkyl chloride solvents may be methylene chloride, chloroform, 1,2-dichloroethane, 1,1-dichloroethane, etc. Specifically, cyclic ester solvents may be γ-butyrolactone (GBL), γ-valerolactone, δ-valerolactone, etc. The amide solvent may specifically be N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone (NMP), N,N'-dimethylimidazolidinone (DMI), etc. The sulfoxide solvent may specifically be dimethyl sulfoxide, etc. The aromatic solvent may specifically be toluene, xylene, benzene, etc. The alcohol solvent may specifically be methanol, ethanol, isopropanol, n-butanol, 2-butanol, methyl cellosolve, ethyl cellosolve, butyl cellosolve, etc. The solvent may contain only one organic solvent or two or more.

[0057] From the viewpoint of further improving the manufacturing efficiency of stretched films, it is preferable that the solvent be highly volatile. For this reason, it is preferable that the solvent contains methylene chloride. In particular, it is preferable that the solvent contains 50% by mass or more of methylene chloride, more preferably 60% by mass or more, and even more preferably 70% by mass or more. Furthermore, it is preferable that the solvent contains methylene chloride and an alcohol solvent, and more preferably methylene chloride and ethanol. The ratio (mass ratio) of the content of methylene chloride to ethanol (alcohol solvent) is preferably 95:5 to 50:50, more preferably 90:10 to 60:40, and even more preferably 90:10 to 70:30.

[0058] [Dope] The dope is obtained by dissolving or dispersing a copolymer having the α-methylene lactone-derived structural units described above in the solvent described above. In addition to the copolymer having the α-methylene lactone-derived structural units, the dope may also contain other polymers or additives.

[0059] Other polymers may include, specifically, 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; polyether sulfone; polyoxybenzyene; polyamide-imide; elastic organic fine particles such as polybutadiene rubber and acrylic rubber; and rubbery polymers such as ABS resin and ASA resin blended with polybutadiene rubber and acrylic rubber. The dope may contain only one of the other polymers, or two or more.

[0060] The additives may specifically include ultraviolet absorbers; antiblocking particles; antioxidants; polymerization inhibitors such as 4-tert-butylcatechol (TBC), hydroquinone, and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4H-TEMPO); stabilizers such as light stabilizers, weather stabilizers, and heat stabilizers; reinforcing materials such as glass fibers and carbon fibers; near-infrared absorbers; flame retardants such as tris(dibromopropyl) phosphate, triallyl phosphate, and antimony oxide; phase difference adjusters such as phase difference enhancers, phase difference reducers, and phase difference stabilizers; antistatic agents including anionic, cationic, and nonionic surfactants; colorants such as inorganic pigments, organic pigments, and dyes; resin modifiers; and the like.

[0061] These additives are similar to those that can be added to a copolymer when producing a copolymer having structural units derived from α-methylene lactone. In other words, these additives may be added as appropriate when producing a copolymer having structural units derived from α-methylene lactone, or when producing a dope, or both.

[0062] The method for dissolving or dispersing copolymers having α-methylene lactone-derived structural units, and other polymers and additives in a solvent may be a conventionally known method. The order of addition to the solvent may be adjusted as appropriate. The temperature and pressure during preparation may be adjusted as appropriate. The raw materials used for preparing the dope may be purified beforehand. The obtained dope may be filtered and / or degassed.

[0063] Filtration may be carried out using known filters such as disc filters and pleated filters. The filtration accuracy is preferably 0.1 to 20 μm, more preferably 1 to 15 μm, and even more preferably 2 to 10 μm. Furthermore, rough filtration with a wire mesh or the like may be performed before filtration.

[0064] Degassing may be carried out using known methods such as vacuum degassing or ultrasonic degassing. When performing vacuum degassing, the degree of vacuum may be adjusted as appropriate to prevent skin formation on the dope surface. Alternatively, the process may be carried out in an atmosphere filled with solvent vapor.

[0065] From the viewpoint of further improving the manufacturing efficiency of stretched films, the solid content concentration of the dope is preferably 5 to 40% by mass, more preferably 8 to 35% by mass, and even more preferably 10 to 30% by mass.

[0066] The content of each component in the solids of the dope is not particularly limited, but is preferably as follows: The content of copolymers having structural units derived from α-methylene lactone is preferably 80 to 99.99% by mass, more preferably 80 to 99.95% by mass, and even more preferably 90 to 99% by mass. The content of other polymers is preferably 0 to 20% by mass, more preferably 0 to 15% by mass, even more preferably 0 to 10% by mass, and particularly preferably 0 to 5% by mass. The content of ultraviolet absorbers is preferably 0.1 to 10% by mass, more preferably 0.3 to 6% by mass, and even more preferably 0.6 to 4% by mass. The content of antiblocking particles is preferably 0.01 to 1% by mass, more preferably 0.02 to 0.7% by mass, and even more preferably 0.03 to 0.5% by mass. The antioxidant content is preferably 0.01 to 1% by mass, more preferably 0.02 to 0.6% by mass, and even more preferably 0.03 to 0.3% by mass. The content of other additives may be 0 to 5% by mass.

[0067] From the viewpoint of further improving the manufacturing efficiency of stretched films, the viscosity of the dope is preferably 0.1 to 50.0 Pa·s, more preferably 0.2 to 40.0 Pa·s, and even more preferably 0.3 to 30.0 Pa·s. The viscosity of the dope can be adjusted by the type of polymer and solvent, the solid content concentration, the presence or absence of additives, etc.

[0068] [Process for obtaining unstretched film] A method for manufacturing a stretched film according to one embodiment of the present disclosure comprises the step of obtaining an unstretched film by a solution film formation method using the dope described above.

[0069] The solution film formation method may be a conventionally known method, for example, a method comprising the steps of coating a support with the dope described above and removing the solvent from the coated dope to obtain an unstretched film.

[0070] The support may be a conventionally known support used in solution film formation methods. Specifically, such a support may be a metal support such as a stainless steel endless belt or a rotating metal drum, a film (for example, a plastic film such as a polyimide film or polyester film (biaxially oriented polyethylene terephthalate film)), or glass.

[0071] The dope coating method may be a conventionally known method used in solution film formation. Specifically, such a coating method may involve using a die coater, doctor blade coater, roll coater, comma coater, lip coater, etc.

[0072] The method for removing the solvent contained in the dope may be a conventionally known method used in solution film formation, but it is preferable to include the following steps. Step 1: A drying process in which the side opposite the support (the air side) is dried until it solidifies. Step 2: A process to dry the dope until it has self-supporting properties. Step 3: Drying the film that has been peeled from the support.

[0073] In step 1, the drying temperature can be set appropriately according to the solvent used, but from the viewpoint of suppressing bumping of the solvent, it is preferable to gradually increase the drying temperature according to the amount of unvolatilized solvent. Also, from the viewpoint of suppressing condensation on the surface of the film, it is preferable to dry under dry air. Furthermore, it is preferable to dry from the support side using radiant heat from a heater or the like.

[0074] In step 2, the film is dried to a degree that allows it to support itself, that is, to a degree that it does not stretch or sag and break due to its own weight or transport tension. In step 2, it is preferable to heat the film from both the air side and the support side. Warm air may also be blown from the air side. After step 2, the film is peeled off the support. At this time, the amount of residual solvent in the film is preferably 30% by mass or less, more preferably 25% by mass or less, 20% by mass or less, and 15% by mass or less, in that order, and preferably more than 3% by mass, from the viewpoint of further improving the transportability of the film in subsequent steps.

[0075] In step 3, the film is further dried. The drying method is preferably at least one of the tenter method or the vertical pass method. The tenter method is a method in which the film is dried while gripping both ends in the width direction with a tenter and adjusting the expansion and contraction as needed according to the sagging of the film and shrinkage due to drying. The vertical pass method is a method in which the film is dried by alternately passing it through a number of rolls arranged vertically inside an oven.

[0076] In this way, an unstretched film is produced. The thickness of the unstretched film is preferably 60 to 200 μm, more preferably 70 to 180 μm, and even more preferably 100 to 160 μm, from the viewpoint of stretching with a sufficient surface ratio in the next stretching step.

[0077] Unstretched films contain solvents that were present in the dope used in their manufacture. Such solvents are called residual solvents. The amount of residual solvent in an unstretched film is 0.05 to 5.0% by mass, but from the viewpoint of further improving the strength, hardness, and dimensional stability of the resulting stretched film, it is preferably 0.1 to 5.0% by mass, more preferably 0.1 to 3.0% by mass, even more preferably 0.15 to 2.0% by mass, and particularly preferably 0.4 to 1.6% by mass. The residual solvent preferably contains methylene chloride.

[0078] [Process for obtaining stretched film] A method for manufacturing a stretched film according to one embodiment of the present disclosure includes the step of stretching the above-described unstretched film to obtain a stretched film.

[0079] The stretching method may be any conventional stretching method, specifically uniaxial stretching (uniaxial stretching of free width, uniaxial stretching of fixed width), or biaxial stretching (sequential biaxial stretching, simultaneous biaxial stretching).

[0080] The stretching temperature is near the glass transition temperature of the copolymer contained in the unstretched film, and specifically, it is preferably (glass transition temperature -30)°C to (glass transition temperature +100)°C, more preferably (glass transition temperature -20)°C to (glass transition temperature +50)°C, and even more preferably (glass transition temperature -10)°C to (glass transition temperature +30)°C.

[0081] The stretching ratio may be any conventionally known ratio, specifically 1.8 to 10 times. From the viewpoint of further improving the mechanical properties of the resulting stretched film, the stretching ratio is preferably 2 to 7 times, more preferably 2.2 to 6 times, even more preferably 2.5 to 5 times, and particularly preferably 2.8 to 4 times. If the stretching method is biaxial stretching, the stretching ratios in the longitudinal and transverse directions are preferably in the range of 1.05 to 10 times, respectively.

[0082] The surface magnification ratio roughly corresponds to the ratio of the area of ​​the main surface of the film before and after stretching. That is, if the stretching method is uniaxial stretching, the surface magnification ratio is the stretching magnification ratio in one direction, and if the stretching method is biaxial stretching, the surface magnification ratio is the product of the stretching magnification ratio in the longitudinal direction and the stretching magnification ratio in the transverse direction.

[0083] The stretching speed may be a conventionally known speed, and may be, for example, 10 to 1000% / min or 30 to 500% / min in each stretching direction.

[0084] In this way, a stretched film is manufactured. The thickness of the stretched film may be, for example, 10 to 60 μm, but is preferably 15 to 55 μm, and more preferably 20 to 50 μm.

[0085] The stretched film contains 10 to 20,000 ppm by mass of residual solvent. Preferably, the residual solvent content in the stretched film is 15 ppm by mass or more. Preferably, the residual solvent content in the stretched film is 10,000 ppm by mass or less, more preferably 6,000 ppm by mass or less, even more preferably 2,000 ppm by mass or less, and particularly preferably 1,000 ppm by mass or less. The residual solvent preferably contains methylene chloride.

[0086] [Stretched film] Another embodiment of the stretched film of this disclosure is a stretched film comprising a copolymer having structural units derived from α-methylene lactone, characterized in that it contains 10 to 20,000 ppm by mass of methylene chloride. The stretched film can be produced, for example, in the stretched film production method described above, by using a solvent that contains methylene chloride.

[0087] Copolymers having structural units derived from α-methylene lactone are similar to the copolymers having structural units derived from α-methylene lactone described above.

[0088] The stretched film may contain other polymers and additives in addition to the copolymer having structural units derived from α-methylene lactone.

[0089] Other polymers are the same as the other polymers described above, i.e., other polymers that the dope may contain. Additives are the same as the additives described above, i.e., additives that the dope may contain, and may include, for example, the UV absorber described above, the antiblocking particles described above, the antioxidant described above, etc.

[0090] The content of each component in the stretched film is not particularly limited, but is preferably as follows: The content of copolymers having structural units derived from α-methylene lactone is preferably 80 to 99.99% by mass, more preferably 80 to 99.95% by mass, and even more preferably 90 to 99% by mass. The content of other polymers is preferably 0 to 20% by mass, more preferably 0 to 15% by mass, even more preferably 0 to 10% by mass, and particularly preferably 0 to 5% by mass. The content of ultraviolet absorbers is preferably 0.1 to 10% by mass, more preferably 0.3 to 6% by mass, and even more preferably 0.6 to 4% by mass. The content of antiblocking particles is preferably 0.01 to 1% by mass, more preferably 0.02 to 0.7% by mass, and even more preferably 0.03 to 0.5% by mass. The antioxidant content is preferably 0.01 to 1% by mass, more preferably 0.02 to 0.6% by mass, and even more preferably 0.03 to 0.3% by mass. The content of other additives may be 0 to 5% by mass.

[0091] The stretched film contains 10 to 20,000 ppm by mass of methylene chloride. Preferably, the methylene chloride content in the stretched film is 15 ppm by mass or more. Preferably, the methylene chloride content in the stretched film is 10,000 ppm by mass or less, more preferably 6,000 ppm by mass or less, even more preferably 2,000 ppm by mass or less, and particularly preferably 1,000 ppm by mass or less.

[0092] The thickness of the stretched film may be, for example, 10 to 60 μm, preferably 15 to 55 μm, and more preferably 20 to 50 μm.

[0093] The stretched film preferably has an in-plane phase difference (Re) of 15 nm or less, more preferably 10 nm or less, and even more preferably 5 nm or less. Furthermore, the absolute value of the thickness-direction phase difference (Rth) of the stretched film is preferably 30 nm or less, more preferably 20 nm or less, even more preferably 17 nm or less, and particularly preferably 15 nm or less. A smaller absolute value of the phase difference increases the optical isotropy of the stretched film, making it even more suitable for use as a component such as a protective film used in displays and the like.

[0094] The stretched film preferably has a haze of 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less. The haze of the stretched film can be adjusted by selecting the type and content of materials, and by changing the content of methylene chloride in the stretched film.

[0095] From the viewpoint of making the stretched film less susceptible to plastic deformation, it is preferable that the elastic modulus of the stretched film be 4 GPa or higher. The elastic modulus of the stretched film may be, for example, 15 GPa or lower.

[0096] For use in devices such as flexible displays, it is preferable that the stretched film does not break at the folded portion even after being repeatedly folded into a U-shape and then unfolded under predetermined conditions during a strength test. The predetermined conditions are those described in the examples.

[0097] The stretched film may be laminated with one or more layers selected as needed from a range of options including a restorative layer, an impact diffusion layer, a self-cleaning layer, an anti-fingerprint layer, an anti-scratch layer, a refractive index adjusting layer, an impact absorption layer, an ultraviolet absorption layer, a hard coat layer, an adhesive layer, and an easy-adhesion layer.

[0098] The stretched film described above can be applied to a variety of uses. Specifically, it is useful as a protective film or cover film for displays and the like. In particular, it is suitable as a component of flexible displays, and can be used as the outermost cover window film, or laminated with other films such as thin-film glass or transparent polyimide to serve as a protective film for each layer. Furthermore, it can be used as an anti-reflective layer, a protective film for touch panels, or for substrates. [Examples]

[0099] The present disclosure will be explained in more detail with reference to examples. However, the present disclosure is not limited by the examples. Below, the evaluation methods for doped and stretched films, and the manufacturing examples of doped and stretched films will be explained in that order.

[0100] [Weight-average molecular weight (Mw) and number-average molecular weight (Mn) of copolymers] The weight-average molecular weight and number-average molecular weight of the copolymer were determined using gel permeation chromatography (GPC) and converted to polystyrene equivalent. The equipment and measurement conditions used are as follows. System: Tosoh GPC system HLC-8220 Measurement column configuration: • Guard column (manufactured by Tosoh, TSKguardcolumn SuperHZ-L) • Separation columns (Tosoh Corporation, TSKgel SuperHZM-M) - 2 columns connected in series Reference side column configuration: • Reference column (Tosoh Corporation, TSKgel SuperH-RC) Developing solvent: Chloroform (manufactured by Fujifilm Wako Pure Chemical Industries, special grade) Flow rate of developing solvent: 0.6 mL / min Standard sample: TSK standard polystyrene (manufactured by Tosoh, PS-oligomer kit) Column temperature: 40℃

[0101] [Glass transition temperature (Tg) of copolymers] The glass transition temperature of the copolymer was determined in accordance with the provisions of JIS K 7121. Specifically, a differential scanning calorimeter (Rigaku Thermo plus EVO DSC-8230) was used to evaluate the temperature from the starting point method obtained by heating approximately 10 mg of the sample from room temperature to 200°C (heating rate 20°C / min) under a nitrogen gas atmosphere using the DSC curve. α-alumina was used as the reference.

[0102] [Dope viscosity] The viscosity of the dope was measured at 25°C using a BHII type viscometer (manufactured by Toki Sangyo Co., Ltd.).

[0103] [Film thickness] The film thickness was determined using a digital micrometer (manufactured by Mitutoyo).

[0104] [Amount of residual solvent in unstretched film] The amount of residual solvent in the unstretched film was measured using a differential thermal-thermogravimetric analyzer (Rigaku Corporation, Thermo plus EVO TG-DTA-8120). Specifically, approximately 10 mg of the sample was heated from room temperature to 300°C (heating rate 10°C / min) under a nitrogen gas atmosphere, and the weight loss (%) at 250°C was defined as the amount of residual solvent. Alpha-alumina was used as the reference.

[0105] [Strength testing of stretched film] A stretched film was cut to a size of 15 mm x 80 mm to serve as a test specimen, and fixed with tape to a Tension-Free Folding Clamshell-type (Yuasa System Equipment, DMLHP-CS). The test specimen was then folded at the halfway point of its long side, so that the distance between the two ends of the long side of the folded specimen was 5 mm, and the radius of curvature of the folded portion of the specimen was 2.5 mm. Subsequently, under conditions of 25°C, the specimen was folded from a flat, unfolded state to a folded state, with each fold being defined as one fold, and this was repeated 100,000 times at a rate of 30 folds per minute. After the test, if the stretched film in the folded portion was not broken, it was judged as ○; if it was broken, it was judged as ×.

[0106] [Tensile test of stretched film] Stretched film was cut into 120mm x 10mm specimens and used as test pieces. Tensile tests were conducted in accordance with JIS K7127 using an Autograph (Shimadzu Corporation: AG-X) under conditions of 25°C and 50% relative humidity. The conditions were a tensile speed of 3mm / min, a chuck distance of 100mm, and a gauge spacing of 80mm for measurement with the displacement meter. Three tests were performed at 25°C, and the average value was used as the measurement value. Displacement was measured using a non-contact elongation meter (Shimadzu Corporation: TRViewX), and the modulus of elasticity was evaluated as the slope of the strain between 0.2% and 0.5%. A modulus of elasticity exceeding 4 GPa was marked as ○, and a modulus of elasticity below 4 GPa was marked as ×.

[0107] [Phase difference of stretched film] The in-plane phase difference Re and thickness-direction phase difference Rth of a stretched film with respect to light at a wavelength of 589 nm were measured using a fully automatic birefringent (KOBRA-WR, manufactured by Oji Instruments Co., Ltd.) under conditions of an incident angle of 40°. Specifically, the in-plane phase difference Re and thickness-direction phase difference Rth were calculated from the following formulas, with nx being the refractive index in the slow phase axis direction of the stretched film, ny being the refractive index in the fast phase axis direction of the stretched film, nz being the refractive index in the thickness direction of the stretched film, and d being the thickness of the stretched film. In-plane phase difference Re=|nx-ny|×d Thickness-direction phase difference Rth = [(nx + ny) / 2 - nz] × d

[0108] [Haze in stretched film] The haze of the stretched film was measured using a turbidimeter (NDH5000, manufactured by Nippon Denshoku Industries).

[0109] [Amount of methylene chloride in stretched film] The amount of methylene chloride in the stretched film was determined using gas chromatography (Shimadzu Corporation, instrument name: GC-2014). Specifically, the amount of methylene chloride in the stretched film was calculated by dissolving the film whose mass was measured in N,N-dimethylacetamide and then quantifying the amount of methylene chloride using gas chromatography.

[0110] This section describes the materials used in the manufacture of doped and stretched films. Methyl methacrylate (MMA): Manufactured by Tokyo Chemical Industry Co., Ltd. α-Methylene-γ-Butyrolactone (ML): Manufactured by Tokyo Chemical Industry Co., Ltd. Dilauroyl peroxide (LPO) (Trade name: Perloyl® L): Manufactured by NOF Corporation Polyoxyethylene distyrylphenyl ether sulfate ammonium salt (product name (Hytenol (registered trademark) NF-08): manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)

[0111] <Production of Copolymer 1> A reactor equipped with a stirrer, temperature sensor, condenser, and nitrogen inlet tube was prepared. 75 parts by mass of deionized water containing 1 part by mass of the dispersant, Hytenol® NF-08, was charged into the reactor. Furthermore, a mixture of 37.5 parts by mass of MMA, 12.5 parts by mass of ML, 0.25 parts by mass of the polymerization initiator LPO, and 0.05 parts by mass of the chain transfer agent nDM was charged into the reactor. The mixture was then stirred at 3000 rpm for 15 minutes using a TK homomixer MARK II model 2.5 (manufactured by Primix Corporation) to produce a suspension.

[0112] 125 parts by mass of deionized water were added to the suspension and then transferred to another reactor. In the reactor after transfer, the suspension was stirred while blowing in nitrogen gas and heated to 65°C. The reaction started at 65°C, and the temperature of the reaction solution (suspension) was changed for a while due to self-heating. After reaching the peak temperature, the temperature was raised to 75°C and maintained at a constant temperature for 2 hours from the start of the reaction. After that, the reaction solution (suspension) was heated to 90°C and the polymerization reaction was carried out and completed while stirring for 4 hours. After that, the copolymer was recovered by cooling and filtration. Copolymer 1 (powder) was obtained by drying using a hot air dryer. The molecular weight Mw of the copolymer was 283000, Mn was 129000, and Tg was 127°C. The HSP value calculated from the solubility test was δD 17.5 MPa 1 / 2 δP is 12.2 MPa 1 / 2 δH6.7MPa 1 / 2 That was the case.

[0113] <Production of Copolymer 2> Copolymer 2 (powder) was obtained in the same manner as in the production of copolymer 1, except that 30 parts by mass of MMA and 20 parts by mass of ML were used. The molecular weight Mw of the copolymer was 263,000, Mn was 117,000, and Tg was 136°C. The HSP value calculated from the solubility test was δD 19.1 MPa. 1 / 2 δP is 11.3 MPa 1 / 2 δH7.9MPa 1 / 2 That was the case.

[0114] <Production of Copolymer 3> Copolymer 3 (powder) was obtained in the same manner as in the production of copolymer 1, except that 25 parts by mass of MMA and 25 parts by mass of ML were used. The molecular weight Mw of the copolymer was 213,000, Mn was 87,000, and Tg was 146°C.

[0115] <Manufacturing of Dope 1> 70 parts by mass of methylene chloride and 10 parts by mass of ethanol were added to a mixing tank, and 20 parts by mass of copolymer 1 were added and dissolved while stirring. Dope 1 was obtained by degassing under reduced pressure. The viscosity of dope 1 was 2.0 Pa·s.

[0116] <Manufacturing of Dope 2> 66 parts by mass of methylene chloride and 9 parts by mass of ethanol were added to a mixing tank, and 25 parts by mass of copolymer 2 were added and dissolved while stirring. Dope 2 was obtained by degassing under reduced pressure. The viscosity of dope 2 was 11.0 Pa·s.

[0117] <Manufacturing of Dope 3> 75 parts by mass of methylene chloride and 15 parts by mass of ethanol were added to a mixing tank, and 10 parts by mass of copolymer 2 were added and dissolved while stirring. Dope 3 was obtained by degassing under reduced pressure. The viscosity of dope 3 was 0.3 Pa·s.

[0118] <Manufacturing of Dope 4> 85 parts by mass of methylene chloride and 11 parts by mass of ethanol were added to a mixing tank, and 4 parts by mass of copolymer 2 were added and dissolved while stirring. Dope 4 was obtained by degassing under reduced pressure. The viscosity of dope 4 was less than 0.1 Pa·s, the lower limit of quantification of the apparatus.

[0119] <Manufacturing of Dope 5> 70 parts by mass of methylene chloride and 10 parts by mass of ethanol were added to a mixing tank, and 20 parts by mass of copolymer 3 were added and dissolved while stirring. Dope 5 was obtained by degassing under reduced pressure. The viscosity of dope 5 was 3.0 Pa·s.

[0120] <Manufacturing of unstretched film N1> Dope 1 was filtered by passing it sequentially through a filter with an average pore size of 10 μm and then a filter with an average pore size of 3 μm. After filtration, dope 1 was continuously cast from a die onto a stainless steel belt to form a film. Then, dope 1 was dried on the stainless steel belt until the side opposite to the stainless steel belt (the air side) solidified. Casting and drying were carried out in a casting chamber filled with dry air to prevent condensation. After peeling the film from the stainless steel belt, it was continuously transported to a tenter via a multi-roll bridge and dried using the tenter method. After the film transported from the tenter had both ends in the width direction, which were held by clips, cut off, it was continuously transported to an oven via a multi-roll bridge and dried using the vertical pass method. The unstretched film N1 obtained in this way was covered with a 30 μm thick polyethylene protective film (product name: Toretec 7332, manufactured by Toray Film Processing Co., Ltd.) and then wound up using a winding device. In this way, an unstretched film N1 with a thickness of 160 μm and a residual solvent content of 0.8 mass% was obtained.

[0121] <Manufacturing of unstretched films N2-N7> Unstretched films N2 to N7 were obtained by varying the dope used, the dope casting rate, the drying temperature in the casting chamber, the drying temperature in the tenter, and the drying temperature in the oven. Table 1 shows the dope used, thickness, and residual solvent amount for unstretched films N1 to N7. The residual solvent amount was adjusted by adjusting the drying temperature in the casting chamber, the drying temperature in the tenter, and the drying temperature in the oven according to the dope casting rate.

[0122] [Table 1]

[0123] <Manufacturing of stretched film B1> An unstretched film N1 was introduced into an oven stretcher and heated to (Tg+16)°C in the oven, and stretched twice in the longitudinal direction to obtain a uniaxially oriented film. Then, the uniaxially oriented film was continuously introduced into a tenter and stretched twice in the transverse direction to obtain a biaxially oriented film B1. In the tenter, the uniaxially oriented film was first heated to (Tg+16)°C in the preheating zone, and then stretched transversely at (Tg+18)°C in the stretching zone. After cutting off both ends in the width direction that were held by clips, a 30 μm thick polyethylene protective film (product name: Toretec 7332, manufactured by Toray Film Processing Co., Ltd.) was attached to the biaxially oriented film B1 obtained in this way. After that, it was wound up using a winding device.

[0124] <Manufacturing of stretched films B2-B6> Similarly, biaxially oriented films B2 to B6 were obtained from each of the unoriented films N2 to N6, with different stretching ratios. Table 2 shows the unoriented films used, as well as the longitudinal stretching ratio, transverse stretching ratio, and surface stretching ratio for stretched films B1 to B6. Table 3 shows the physical properties of the biaxially oriented films B1 to B6 and the unoriented film N7.

[0125] [Table 2]

[0126] [Table 3]

[0127] Biaxially oriented films B1 to B5 obtained from unoriented films N1 to N5 exhibited excellent strength and hardness. Biaxially oriented film B6 obtained from unoriented film N6 lacked sufficient strength and hardness. Unoriented film N7 lacked sufficient strength but exhibited excellent hardness.

[0128] The superior strength of biaxially oriented films B1-B5 is presumed to be due to the stretching process. Biaxially oriented film B6 was expected to have sufficient strength due to the stretching process, but it is presumed to have insufficient strength due to the excessive methylene chloride content. The insufficient strength of unstretched film N7 is presumed to be due to the lack of stretching.

[0129] The superior hardness of biaxially oriented films B1 to B5 is presumed to be due to the optimized methylene chloride content. The insufficient hardness of biaxially oriented film B6 is presumed to be due to the excessive methylene chloride content. The superior hardness of unoriented film N7 is presumed to be due to the optimized methylene chloride content.

Claims

1. A method for producing a stretched film containing a copolymer having structural units derived from α-methylene lactone, A step of obtaining an unstretched film by a solution film formation method using a dope containing the copolymer and a solvent, The process includes stretching the aforementioned unstretched film to a surface magnification of 1.8 to 10 times, A method for producing a stretched film, wherein the amount of residual solvent in the unstretched film is 0.05 to 5.0% by mass.

2. The method for producing a stretched film according to claim 1, wherein the amount of residual solvent in the unstretched film is 0.1 to 5.0% by mass.

3. The method for producing a stretched film according to claim 1 or 2, wherein the thickness of the unstretched film is 60 to 200 μm.

4. The method for producing a stretched film according to claim 1 or 2, wherein the copolymer further comprises a constituent unit derived from alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms.

5. The method for producing a stretched film according to claim 1 or 2, wherein the solvent comprises methylene chloride.

6. The method for producing a stretched film according to claim 1 or 2, wherein the viscosity of the dope is 0.1 to 50.0 Pa·s.

7. The method for producing a stretched film according to claim 1 or 2, wherein the solid content concentration of the dope is 5 to 40% by mass.

8. A stretched film comprising a copolymer having structural units derived from α-methylene lactone, A stretched film containing 10 to 20,000 ppm by mass of methylene chloride.

9. The stretched film according to claim 8, wherein the absolute value of the phase difference in the thickness direction is 30 nm or less.

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