Film manufacturing method and dope solution

Abstract

To provide a method for producing a film that can reduce visual imperfections in the film and can also suppress deterioration in its strength, hardness and transparency.SOLUTION: The present invention provides a method for producing a film that includes a copolymer with a lactone ring structure. The method includes the step of making a film through the solvent casting of a dope liquid including the copolymer and a first solvent, a second solvent and a third solvent. The first solvent has a boiling point of lower than 60°C, the second solvent has a boiling point of 60-110°C, and the third solvent has a boiling point of higher than 110°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a film and a dope solution. [Background technology]

[0002] Copolymers having a lactone ring structure are excellent in transparency, heat resistance, and optical isotropy, and are expected to be used in optical applications. For example, Patent Document 1 describes that an optically isotropic acrylic resin film containing a thermoplastic copolymer containing a predetermined lactone ring-containing monomer unit and satisfying predetermined physical properties is preferably used as an optical film such as a polarizer protective film. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-41007 Summary of the Invention [Problem to be solved by the invention]

[0004] One example of a film production method is a solution casting method, in which a dope solution containing a solvent, a polymer, and the like is prepared and then film formation is performed. When applying this solution casting method to film production from a copolymer having a lactone ring structure, the inventors discovered that when a solvent typically used in solution casting is used, the resulting film suffers from poor appearance and problems with strength, hardness, transparency, and the like. The problem of poor appearance is thought to be caused by the formation of bubbles on the film surface due to the solvent bumping during solvent removal (drying). Furthermore, the problems with film strength, hardness, transparency, and the like are thought to be caused by the solvent not being sufficiently removed during solvent removal (drying), resulting in a relatively large amount of residual solvent remaining in the film. Furthermore, removing the residual solvent requires an extended drying time and high-temperature drying at 200°C or higher, which significantly reduces productivity and is prone to poor appearance, such as film discoloration.

[0005] The present invention has been made in consideration of the above circumstances, and its main object is to provide a method for producing a film that can suppress the occurrence of poor appearance of the film and the decrease in strength, hardness, and transparency of the film. [Means for solving the problem]

[0006] The present invention provides the following methods for producing a film as described in [1] to [7], and the dope solution as described in [8] and [9]. [1] A method for producing a film containing a copolymer having a lactone ring structure, comprising the step of forming a film by a solution casting method from a dope solution containing the copolymer and a first solvent, a second solvent, and a third solvent, wherein the boiling point of the first solvent is less than 60°C, the boiling point of the second solvent is 60 to 110°C, and the boiling point of the third solvent is higher than 110°C. [2] The manufacturing method according to [1], further comprising a step of stretching the film. [3] The manufacturing method according to [1] or [2], wherein the dope solution contains 5 to 80 mass % of the first solvent based on the total amount of the first solvent, the second solvent, and the third solvent. [4] The method according to any one of [1] to [3], wherein the dope solution contains 3 to 50 mass% of the second solvent based on the total amount of the first solvent, the second solvent, and the third solvent. [5] The method according to any one of [1] to [4], wherein the copolymer further comprises a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms. [6] The method according to any one of [1] to [5], wherein the viscosity of the dope solution is 0.1 to 500,000 cP. [7] The method according to any one of [1] to [6], wherein the thickness of the unstretched film is 60 to 200 μm. [8] A dope solution containing a copolymer having a lactone ring structure and a first solvent, a second solvent, and a third solvent, wherein the boiling point of the first solvent is less than 60°C, the boiling point of the second solvent is 60 to 110°C, and the boiling point of the third solvent is higher than 110°C. [9] The dope solution according to [8], which has a viscosity of 0.1 to 500,000 cP. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a method for producing a film that can suppress the occurrence of poor appearance of the film and the decrease in strength, hardness, and transparency of the film. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments.

[0009] [Copolymer] The copolymer in the film of this embodiment has a lactone ring structure.

[0010] The lactone ring structure is not particularly limited and may be, for example, a 4- to 8-membered ring, but is preferably a 5- or 6-membered ring because of its excellent stability as a ring structure.

[0011] The copolymer is not particularly limited in the constituent unit for forming the lactone ring structure, but it is preferable that the copolymer has a constituent unit derived from α-methylene lactone.

[0012] The constitutional unit derived from α-methylene lactone is formed by polymerization of α-methylene lactone in which a methylene group is bonded to the carbon atom at the α-position. The specific structure of the constitutional unit derived from α-methylene lactone is not particularly limited.

[0013] Specific examples of 5- or 6-membered α-methylene lactones include α-methylene-γ-butyrolactone and α-methylene-δ-valerolactone, which may have a substituent.

[0014] The constitutional unit derived from α-methylene lactone is preferably a constitutional unit having a structure shown in the following formula (1).

[0015] [ka]

[0016] R in Equation (1) 1 ~R 4 are each independently a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms.

[0017] The structural unit having the structure shown in formula (1) can be formed by polymerizing a monomer containing α-methylene-γ-butyrolactone shown in formula (2) below.

[0018] [ka]

[0019] R in Equation (2) 1 ~R 4 are each independently a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms.

[0020] The hydrocarbon group is an aliphatic hydrocarbon group or an aromatic hydrocarbon group. The aliphatic hydrocarbon group is, for example, an alkyl group. The number of carbon atoms in the alkyl group is preferably 1 to 10, more preferably 1 to 8. The alkyl group may be linear or branched, or may be cyclic. Examples of the alkyl group include 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, and a cyclohexyl group.

[0021] The aromatic hydrocarbon group is not particularly limited and may contain, for example, a heterocyclic structure. Examples of the aromatic hydrocarbon group include a phenyl group, a tolyl group, and a benzyl group.

[0022] R 1 ~R 4 are preferably, each independently, a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and more preferably, all are hydrogen atoms.

[0023] The copolymer contains structural units of any other monomer in addition to the structural units for forming the lactone ring structure. Specific examples include structural units derived from monomers such as alkyl (meth)acrylate, benzyl (meth)acrylate, chloromethyl (meth)acrylate, 2-chloroethyl (meth)acrylate, styrene, vinyl toluene, α-methylstyrene, acrylonitrile, methyl vinyl ketone, ethylene, propylene, and vinyl acetate. These may be used alone or in combination of two or more.

[0024] Among the structural units of any other monomers described above, the copolymer preferably contains a structural unit derived from an alkyl (meth)acrylate, and more preferably contains a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms, from the viewpoint of further improving the heat resistance, transparency, etc. of the resulting film.

[0025] The structural unit derived from alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms is formed by polymerization of alkyl (meth)acrylate. Examples of the alkyl group having 1 to 6 carbon atoms in the alkyl (meth)acrylate include 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, and a cyclohexyl group. These may be used alone or in combination of two or more.

[0026] The alkyl group in the alkyl (meth)acrylate preferably has 1 to 3 carbon atoms, more preferably 1 or 2 carbon atoms, and even more preferably 1 carbon atom.

[0027] The content of the structural units for forming a lactone ring structure in the copolymer is preferably 5 to 60% by mass, more preferably 7.5 to 50% by mass, and even more preferably 10 to 45% by mass, from the viewpoint of further improving heat resistance, etc. The content of each structural unit in the copolymer is determined by dissolving the copolymer in a heavy solvent and 1 It can be determined by measuring H-NMR and calculating the area ratio of the peaks corresponding to each structural unit.

[0028] The content of structural units derived from alkyl (meth)acrylate having an alkyl group with 1 to 6 carbon atoms in the copolymer is preferably 95 to 40 mass %, more preferably 92.5 to 45 mass %, and even more preferably 90 to 50 mass %.

[0029] In the copolymer, the content of structural units other than structural units for forming a lactone ring structure and structural units derived from alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less.

[0030] 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, and even more preferably 150,000 to 500,000. In particular, from the viewpoint of improving the strength of the obtained film, the weight-average molecular weight of the copolymer is preferably more than 200,000. The number-average molecular weight (Mn) of the copolymer is, for example, 30,000 to 500,000, and the dispersity (Mw / Mn) is, for example, 5 or less.

[0031] 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, from the viewpoint of facilitating an increase in the drying temperature to promote drying during film production. Furthermore, from the viewpoint of further increasing the strength of the film, Tg is preferably 200° C. or lower, more preferably 190° C. or lower.

[0032] The copolymer's HSP value (Hansen Solubility Parameters) δD is 14 to 22 MPa, which is necessary to ensure that the film is highly hydrophobic and to prevent deterioration of film properties due to water and problems caused by moisture permeation when incorporated into devices. 1 / 2 , δP is 8 to 15 MPa 1 / 2 , δH is 5 to 10 MPa 1 / 2 It is preferable that δD is 16 to 21 MPa. 1 / 2 , δP is 10 to 13 MPa 1 / 2 , δH is 6 to 9 MPa 1 / 2 It is more preferable that δD is 17 to 20 MPa. 1 / 2 , δP is 11 to 13 MPa 1 / 2 , δH is 6 to 9 MPa 1 / 2It is more preferable that the HSP value of the copolymer is 0.05 to 0.05. The HSP value of the copolymer was calculated using HSPiP version: 5.3.08 based on the results of a solubility test in a solvent whose HSP value is known, according to the method described in Hansen Solubility Parameters: A User's Handbook, Second Edition, by Charles M. Hansen. The calculation method was determined using Classic GA.

[0033] The copolymer can be produced by, for example, solution polymerization or polymerization in an aqueous solvent, although the method is not particularly limited. Polymerization in an aqueous solvent includes a polymerization step of polymerizing a compound having a structural unit for forming a lactone ring structure and any other monomer, and is characterized in that all of the monomers are present in the system from the early stage of the polymerization step.

[0034] The aqueous solvent may be water alone or may contain a non-aqueous solvent (particularly a water-soluble organic solvent). Examples of the water-soluble organic solvent 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.

[0035] The proportion of the 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.

[0036] In the polymerization step, it is preferable to have all of the monomers present in the system from the beginning of the polymerization step. As a method for having all of the monomers present in the system from the beginning of the polymerization step, it is sufficient that all of the monomers are charged into the reactor before the polymerization substantially starts, and for example, all of the monomers can be charged into the reactor before the temperature of the reactor is raised to the polymerization temperature.

[0037] When dispersing the monomer in an aqueous solvent, the dispersion may be carried out by stirring with a paddle blade or the like, or may be carried out using an emulsifying / dispersing device such as a high-speed shear turbine type disperser, a high-pressure jet homogenizer, an ultrasonic type emulsifying / dispersing machine, a medium stirring / dispersing machine, or a forced gap passing type disperser.

[0038] When polymerizing the monomers, a polymerization initiator, a chain transfer agent, a dispersant and / or an additive may be added as necessary.

[0039] Examples of the polymerization initiator include organic peroxides such as 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 content of the polymerization initiator is not particularly limited and may be appropriately set depending on the combination of monomers used, reaction conditions, etc., but is preferably 100 to 50,000 ppm by mass, more preferably 500 to 30,000 ppm by mass, and even more preferably 1,000 to 20,000 ppm by mass, based on the total monomers.

[0040] Examples of chain transfer agents include monofunctional thiol compounds such as n-dodecyl mercaptan and β-mercaptopropionic acid; bifunctional thiol compounds such as polysiloxanes modified at both ends with mercapto groups; and side-chain polyfunctional mercapto-modified polysiloxanes in which the side chains are mercapto-modified. The molecular weight of the resulting copolymer can be adjusted by adding or not adding a chain transfer agent, and by adjusting the amount added if added. The content of the chain transfer agent can be appropriately set depending on the combination of monomers used, reaction conditions, etc., and is not particularly limited, but is preferably 10 to 10,000 ppm by mass, more preferably 100 to 3,000 ppm by mass, based on the total monomers.

[0041] Examples of dispersants 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, thorium hydroxide, and metal oxide powders. The addition of a dispersant can improve the stability of the polymerization reaction. The content of the dispersant is not particularly limited, but is preferably 0.1 to 3 mass%, more preferably 0.2 to 1 mass%, based on the total monomers.

[0042] The copolymer of the present embodiment may contain various additives other than the dispersant. Examples of additives include ultraviolet absorbers such as benzophenone compounds, salicylate compounds, benzoate compounds, triazole compounds, and triazine compounds; phenolic, phosphorus, and sulfur-based antioxidants; polymerization inhibitors such as 4-tertiarybutylcatechol (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; retardation adjusters such as retardation increasers, retardation decreasers, and retardation stabilizers; antistatic agents including anionic, cationic, and nonionic surfactants; colorants such as inorganic pigments, organic pigments, and dyes; organic and inorganic fillers; resin modifiers; and organic and inorganic fillers. The content of the additive in the copolymer of the present embodiment may be 0 to 5% by mass, or may be 0 to 3% by mass.

[0043] Examples of the polymerization reaction include suspension polymerization, emulsion polymerization, etc. Among these, suspension polymerization, in which the reaction is carried out by suspending monomers in an aqueous solvent in the presence of a dispersant, is preferred, since it can further improve the transparency of the resulting copolymer.

[0044] After the polymerization step, the copolymer particles can be recovered by solid-liquid separation, which can be performed by filtration, centrifugation, or a combination thereof.

[0045] The resulting copolymer is preferably dried at a temperature of, for example, 60°C or higher and 120°C or lower.

[0046] Alternatively, water may be removed by direct drying without solid-liquid separation. The copolymer can be directly obtained as a powder by drying using various dryers.

[0047] [Film manufacturing method] The film manufacturing method of the present embodiment includes a step of forming a film by a solution casting method from a dope solution containing the copolymer having a lactone ring structure, a first solvent, a second solvent, and a third solvent, and is further characterized in that the boiling point of the first solvent is less than 60°C, the boiling point of the second solvent is 60 to 110°C, and the boiling point of the third solvent is greater than 110°C.

[0048] As the solution casting method, a conventionally known method can be applied. For example, a solution casting method including the steps of preparing a dope solution containing the copolymer having a lactone ring structure and a first solvent, a second solvent, and a third solvent, applying the dope solution to a support, and removing the solvent from the applied dope solution to obtain a film can be applied.

[0049] The first solvent is not particularly limited as long as it has a boiling point of less than 60°C, and may include, for example, acetone, methylene chloride, 1,1-dichloroethane, diethyl ether, ethyl formate, methyl acetate, pentane, etc. Among these, acetone or methylene chloride is preferred. These solvents may be used alone or in combination of two or more. The lower limit of the boiling point of the first solvent is not particularly limited, but may be, for example, 30°C or higher.

[0050] The second solvent is not particularly limited as long as it has a boiling point of 60 to 110°C, and may include, for example, methyl ethyl ketone, methanol, ethanol, isopropanol, n-butanol, 2-butanol, 2-methyl-2-propanol, chloroform, 1,2-dichloroethane, ethyl acetate, methyl propionate, ethyl propionate, acetonitrile, heptane, hexane, cyclohexane, benzene, etc. Among these, it is preferable to include methyl ethyl ketone or ethanol. These solvents may be used alone or in combination of two or more.

[0051] The third solvent is not particularly limited as long as it has a boiling point above 110°C. For example, it may include N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone (NMP), N,N'-dimethylimidazolidinone (DMI), cyclohexanone (anone), cyclopentanone, γ-butyrolactone (GBL), γ-valerolactone, δ-valerolactone, dimethyl sulfoxide, toluene, xylene, methyl cellosolve, ethyl cellosolve, butyl cellosolve, etc. Among these, it is preferable to include an amide solvent such as N,N-dimethylformamide. These solvents may be used alone or in combination of two or more. The upper limit of the boiling point of the third solvent is not particularly limited, but may be, for example, 210°C or lower.

[0052] The dope preferably contains 5 to 80% by mass of the first solvent, more preferably 15 to 70% by mass, and even more preferably 25 to 60% by mass, based on the total amount of the first, second, and third solvents. When the amount of the first solvent is within the above range, the film can be dried efficiently, improving productivity. Furthermore, the amount of solvent remaining in the film can be easily reduced, preventing deterioration of film properties such as strength, hardness, and transparency. Furthermore, when the amount of the first solvent is 80% by mass or less based on the total amount of the first, second, and third solvents, the progress of drying immediately after coating can be delayed, preventing whitening, surface roughness, and other problems.

[0053] The dope preferably contains 3 to 50% by mass, more preferably 5 to 40% by mass, and even more preferably 7 to 30% by mass of the second solvent, based on the total amount of the first, second, and third solvents. By keeping the amount of the second solvent within the above range, the drying efficiency of the film can be improved and the occurrence of whitening and surface roughness caused by excessive drying immediately after coating can be suppressed.

[0054] Furthermore, the dope preferably contains 5 to 40% by mass, more preferably 7 to 35% by mass, and even more preferably 10 to 30% by mass of the third solvent, based on the total amount of the first, second, and third solvents. When the amount of the third solvent is 5% by mass or more based on the total amount of the first, second, and third solvents, the film surface is less likely to skin, and bumping during drying is less likely to occur. Furthermore, when the amount of the third solvent is 40% by mass or less based on the total amount of the first, second, and third solvents, the amount of residual solvent in the film is easily reduced, and film properties such as strength, hardness, and transparency are less likely to be impaired.

[0055] From the viewpoint of efficient film production, the viscosity of the dope is preferably 0.1 to 500,000 cP, more preferably 0.2 to 250,000 cP, and even more preferably 0.3 to 150,000 cP.

[0056] From the viewpoint of efficient film production, the solid content of the dope solution is preferably 5 to 40 mass %, more preferably 8 to 35 mass %, and even more preferably 10 to 30 mass %, based on the total amount of the dope solution.

[0057] The dope solution may contain other polymers, other additives, etc. in addition to the copolymer having a lactone ring structure described above, depending on the desired film properties.

[0058] 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; cellulose derivatives such as triacetyl cellulose, diacetyl cellulose, and cellulose propionate; 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 or acrylic rubber. The content of the other polymers is preferably 0 to 50% by mass, more preferably 0 to 40% by mass, even more preferably 0 to 30% by mass, particularly preferably 0 to 20% by mass, and most preferably 0 to 10% by mass, based on the total amount of the stretched film (resin composition).

[0059] Other additives include, for example, hindered phenol, phosphorus, and sulfur-based antioxidants; stabilizers such as light stabilizers, weather stabilizers, and heat stabilizers; reinforcing materials such as glass fiber and carbon fiber; ultraviolet absorbers such as phenyl salicylate, (2,2'-hydroxy-5-methylphenyl)benzotriazole, and 2-hydroxybenzophenone; near-infrared absorbers; flame retardants such as tris(dibromopropyl)phosphate, triallyl phosphate, and antimony oxide; antistatic agents such as anionic, cationic, and nonionic surfactants; colorants such as inorganic pigments, organic pigments, and dyes; organic fillers; inorganic fillers such as silicon oxide, zirconium oxide, and titanium oxide; resin modifiers; organic or inorganic fillers; plasticizers; lubricants; antistatic agents; flame retardants; fluidizing agents; and compatibilizers. The film may contain one or more other additives. The content of other additives is preferably 0 to 5 mass %, more preferably 0 to 4 mass %, and even more preferably 0 to 3 mass %, based on the total amount of the stretched film.

[0060] The dope solution can be prepared by dissolving or dispersing the copolymer having a lactone ring structure, and any other polymers or additives, in a solvent (a mixture of a first solvent, a second solvent, and a third solvent). For example, the copolymer having a lactone ring structure, and any other polymers or additives may be added to a solvent and mixed by appropriate shearing and / or stirring. The order in which the copolymer having a lactone ring structure, and any other polymers or additives are added to the solvent is not particularly limited. The copolymer having a lactone ring structure, and any other polymers or additives may be added all at once or sequentially. Alternatively, the copolymer having a lactone ring structure, and any other polymers or additives may be premixed, preferably heated and melted, and then melt-kneaded by applying appropriate shearing force to prepare a resin composition (e.g., a pellet-shaped or powder-shaped resin composition), which is then mixed with a solvent to prepare the dope solution. In these mixing steps, the temperature and pressure can be appropriately adjusted. The raw materials used for preparing the dope solution may be filtered before use if they are liquid (including the case where the raw materials are dissolved in a solvent and then added), and after the above-mentioned mixing step, the obtained dope solution may be filtered and / or degassed. As a filtration method, known filters such as disc filters and pleated filters can be used, and rough filtering using a wire mesh or the like may be performed before filtration, or the pores may be passed through in order from larger to smaller. The filtration accuracy is preferably 0.1 to 20 μm, more preferably 1 to 15 μm, and even more preferably 2 to 10 μm. As a defoaming method, known methods such as vacuum defoaming and ultrasonic defoaming can be used. When defoaming under reduced pressure, it is preferable to appropriately adjust the degree of vacuum so that the dope placed in the dissolution tank does not develop skin on the surface. It is also preferable to keep the dissolution tank filled with the vapor of the solvent used.

[0061] The support is not particularly limited, and any conventionally known support used in solution casting can be used. Examples of the support include a metal support such as a stainless steel endless belt or a rotating metal drum, and a film (e.g., a plastic film such as a polyimide film or a polyester film (biaxially oriented polyethylene terephthalate film)). The method for applying the dope is not particularly limited, and any conventionally known method can be used. Examples of the method for applying the dope include coating using a die coater, doctor blade coater, roll coater, comma coater, lip coater, etc.

[0062] The method for removing the solvent from the dope solution to produce an unstretched film is not particularly limited, and examples include heating the dope solution to volatilize the solvent. The drying temperature can be appropriately set depending on the solvent used. However, in the film production method according to this embodiment, a combination of a first solvent, a second solvent, and a third solvent is used, so the amount of residual solvent can be sufficiently reduced even under high temperatures such as 200°C or higher. Therefore, in the drying process of the film according to this embodiment, the temperature may be gradually increased, for example, within a range of 40 to 180°C depending on the amount of residual solvent, in order to suppress foaming due to bumping of the solvent. Furthermore, drying in dry air is preferred in order to suppress condensation on the film surface. Furthermore, since blowing hot air or the like directly onto the coating solution before the coating solution surface solidifies can deteriorate the surface properties, drying using radiant heat from a heater or the like from the support side is preferred. The film may be peeled off from the support after drying, or it may be dried to a level where it is self-supporting, and then peeled off from the support and further dried. When the film is further dried, it is preferable to use a method in which the film is dried while holding both ends in the width direction with a tenter, and in some cases adjusting the expansion or contraction according to the sagging of the film or shrinkage due to drying, or a method in which the film is dried by passing it alternately through multiple rolls arranged above and below in an oven (vertical pass method).Whether to use the tenter method or the vertical pass method can be selected appropriately depending on the amount of solvent remaining in the film, and both methods may be used, or they may be used multiple times.

[0063] The thickness of the unstretched film is preferably 60 to 200 μm, more preferably 70 to 180 μm, and even more preferably 80 to 160 μm, from the viewpoint of stretching at a sufficient areal ratio in the subsequent stretching step.

[0064] The unstretched film contains the solvent (residual solvent) used in preparing the dope solution. The amount of residual solvent in the unstretched film of this embodiment is preferably 0.12 to 3 mass%, more preferably 0.15 to 2 mass%, even more preferably 0.17 to 1.8 mass%, and particularly preferably 0.2 to 1.6 mass%, from the viewpoints of exhibiting the strength and hardness properties of the film and suppressing expansion and contraction of the film after stretching.

[0065] In the film manufacturing method according to this embodiment, even if the thickness (dry thickness) of the unstretched film is 100 μm or more, the amount of residual solvent in the unstretched film can be reduced to the above range without the surface being deteriorated by bumping or the like.

[0066] The unstretched film obtained by the solution casting method can be stretched to obtain a film (stretched film) having excellent strength, hardness, retardation, etc.

[0067] As a method for stretching the film, a conventionally known stretching method can be applied, and examples thereof 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. The method for stretching the film is preferably biaxial stretching, since it improves the bending resistance in any two orthogonal directions in the film plane.

[0068] The stretching temperature when stretching the film is preferably around the glass transition temperature of the copolymer, more specifically, preferably from (glass transition temperature - 30)°C to (glass transition temperature + 100)°C, more preferably from (glass transition temperature - 20)°C to (glass transition temperature + 50)°C, and even more preferably from (glass transition temperature - 10)°C to (glass transition temperature + 30)°C.

[0069] The areal stretching ratio when stretching the film of this embodiment is 1.8 to 10 times, and from the viewpoint of improving the strength and hardness of the film after stretching, it is preferably 2 to 7 times, more preferably 2.2 to 6 times, even more preferably 2.5 to 5 times, and particularly preferably 2.6 to 4.5 times.

[0070] The thickness of the stretched film obtained by the above-mentioned film production method is preferably 10 to 60 μm, more preferably 15 to 55 μm, and even more preferably 20 to 50 μm.

[0071] The film of the present embodiment can be applied to various applications, for example, and can be suitably applied to optical applications. Specific examples of applications include light guide members, film applications, lenses (optical lenses, etc.), covers, and foam applications (for example, cushioning materials, heat-insulating materials, vibration-damping materials, soundproofing materials, sealing materials, packing materials, etc.). Furthermore, since the film of the present embodiment has excellent strength and hardness, it can be suitably used for flexible displays, and can be particularly suitably used as the outermost cover window. It can also be used as a protective film for each layer of a flexible display. Specifically, it can be used by laminating it with another film such as thin-film glass or transparent polyimide. It can also be used as an anti-reflection layer, a protective film for a touch panel, a base portion, etc. [Example]

[0072] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, unless otherwise specified, "parts" means "parts by mass," "%" means "% by mass," and "ppm" means "ppm by mass." Various physical properties were measured and evaluated as follows.

[0073] [Weight average molecular weight (Mw) and number average molecular weight (Mn) of copolymer] The weight-average molecular weight and number-average molecular weight of the copolymer were determined in terms of polystyrene using gel permeation chromatography (GPC) under the following conditions: System: Tosoh GPC system HLC-8220 Measurement column configuration: Guard column (Tosoh, TSKguardcolumn SuperHZ-L) Two separation columns (Tosoh TSKgel SuperHZM-M) connected in series Reference column configuration: Reference column (Tosoh, TSKgel SuperH-RC) Developing solvent: chloroform (Wako Pure Chemical Industries, special grade) Developing solvent flow rate: 0.6 mL / min Standard sample: TSK standard polystyrene (Tosoh, PS-oligomer kit) Column temperature: 40℃

[0074] [Glass transition temperature (Tg) of copolymer] The glass transition temperature of the copolymer was determined in accordance with the provisions of JIS K 7121. Specifically, using a differential scanning calorimeter (Rigaku Thermo plus EVO DSC-8230) in a nitrogen gas atmosphere, approximately 10 mg of a sample was heated from room temperature to 200°C (heating rate 20°C / min), and the temperature was evaluated by the starting point method from the DSC curve obtained. α-Alumina was used as a reference.

[0075] [Dope viscosity] The viscosity of the dope solution was measured at 25° C. using a BHII type viscometer (manufactured by Toki Sangyo Co., Ltd.).

[0076] [Unstretched film thickness (dry film thickness)] The film thickness was measured using a Digimatic Micrometer (Mitutoyo).

[0077] [Amount of solvent remaining in unstretched film] The amount of residual solvent in the unstretched film was measured using a thermogravimetric and differential thermal analyzer (Rigaku Thermoplus EVO TG-DTA-8120). Specifically, approximately 10 mg of sample was heated from room temperature to 300°C (heating rate: 10°C / min) under a nitrogen gas atmosphere, and the weight loss (%) upon reaching 230°C was recorded as the amount of residual solvent. α-Alumina was used as a reference.

[0078] [Appearance of unstretched film] The appearance of the unstretched film was evaluated as follows: if there were 0 to 2 bubbles in each 10 cm piece of film, it was rated as ◯; if there were more than 2 but 5 bubbles, it was rated as △; if there were more than 5 bubbles, it was rated as ×.

[0079] [Strength test of stretched film] The stretched film was cut into a size of 15 mm x 80 mm to form a test specimen, which was then fixed with tape to a tension-free folding clamshell type (Yuasa System Equipment, DMLHP-CS). The test specimen was also folded at half of its long side, with the distance between both ends of the long side of the folded specimen being 5 mm and the radius of curvature of the folded portion of the test specimen being 2.5 mm. The specimen was then folded 100,000 times at a rate of 30 times per minute in an environment of 25 °C, with one bending being defined as a change from a flat, open state to a folded state. If the stretched film in the folded portion after the test was not broken, it was judged as ○, and if it was broken, it was judged as ×.

[0080] [Tensile test of stretched film (measuring elastic modulus)] The stretched film was cut into 120 mm x 10 mm specimens and subjected to tensile testing in accordance with JIS K7127 using an autograph (Shimadzu Corporation: AG-X) at 25°C and 50% relative humidity. The conditions were a tensile speed of 3 mm / min, a chuck distance of 100 mm, and a gauge interval of 80 mm measured with a displacement meter. The test was conducted three times at 25°C, and the average value was used as the measured value. Displacement was measured using a non-contact extensometer (Shimadzu Corporation: TRViewX). The modulus of elasticity was evaluated as the slope between strains of 0.2% and 0.5%. A modulus of elasticity exceeding 4 GPa was marked with a ○, and a modulus of elasticity below 4 GPa was marked with an ×.

[0081] [Haze of stretched film] The haze was determined in accordance with the provisions of JIS K 7136. Specifically, the haze of the stretched film was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH-1001DP).

[0082] Methyl methacrylate (MMA) and α-methylene-γ-butyrolactone (ML) were obtained from Tokyo Chemical Industry Co., Ltd. Perloyl L (dilauroyl peroxide, LPO) was obtained from NOF Corporation, and polyoxyethylene distyrylphenyl ether sulfate ammonium salt (trade name "Hitenol (registered trademark) NF-08") was obtained from Daiichi Kogyo Seiyaku Co., Ltd.

[0083] <Production of Copolymer 1> A reactor equipped with a stirrer, temperature sensor, cooling tube, and nitrogen inlet tube was prepared. 75 parts of deionized water containing 1 part of Hitenol (registered trademark) NF-08 dissolved as a dispersant was charged into the vessel. A previously prepared mixture of 37.5 parts of MMA as monomers, 12.5 parts of ML, and 0.25 parts of LPO as a polymerization initiator was charged, and the mixture was stirred at 3000 rpm for 15 minutes using a TK homomixer MARK II model 2.5 (manufactured by Primix Corporation) to form a uniform suspension. 125 parts of deionized water was added to the suspension, which was then transferred to a reactor. Nitrogen gas was blown in while stirring, and the reaction solution (suspension) was heated to 65 ° C. The reaction started when the internal temperature reached 65 ° C. The reactor was maintained at 65 ° C. The liquid temperature reached a peak temperature due to self-heating, and then maintained at 75 ° C. Two hours after the start of the reaction, the reaction solution (suspension) was heated to 90 ° C. and stirred for four hours to complete the polymerization reaction. The reaction solution (suspension) was then cooled, and the copolymer was collected by filtration. It was then dried using a hot air dryer to obtain Copolymer 1 (powder). The molecular weight (Mw) of the copolymer was 283,000, Mn was 129,000, and Tg was 127 ° C.

[0084] <Production of Copolymer 2> Copolymer 2 (powder) was obtained in the same manner as in the production of Copolymer 1, except that the amount of MMA was 30 parts and the amount of ML was 20 parts. The molecular weight Mw of the copolymer was 263,000, Mn was 117,000, and Tg was 136°C.

[0085] Example 1 42 parts of acetone, 7 parts of methyl ethyl ketone (MEK), and 21 parts of N,N-dimethylformamide (DMF) were charged into a mixing tank, and 30 parts of the copolymer 1 was added and dissolved under stirring, and then the solution was filtered through a 10 μm filter and further degassed under reduced pressure to obtain a membrane-forming dope solution 1. The viscosity of the membrane-forming dope solution 1 was 10,000 cP.

[0086] The dope solution 1 was cast onto a support (Kapton film) using a coater to a dry thickness of 160 μm. The temperature was raised to 40-180°C over 1 hour and maintained at this temperature for another 1 hour to evaporate the solvent, yielding an unstretched film. The residual solvent content in the resulting unstretched film was 0.9% by mass. The resulting unstretched film was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD) and then the transverse direction (TD) at a stretching rate of 300% / min at a temperature of Tg + 18°C, using a sequential biaxial stretching machine (Toyo Seiki Seisaku-sho, X6-S) to a 4.0x areal stretching ratio. The film was then cooled to obtain a stretched film. The evaluation results of the physical properties of the resulting film are shown in Table 1.

[0087] Example 2 48 parts of acetone, 8 parts of MEK, and 24 parts of DMF were charged into a mixing tank, and 20 parts of the copolymer 1 was added and dissolved under stirring, and then the mixture was filtered through a 10 μm filter and degassed under reduced pressure to obtain a membrane-forming dope solution 2. The viscosity of the membrane-forming dope solution 2 was 2000 cP.

[0088] Dope solution 2 was cast onto a support (Kapton film) using a coater to a dry film thickness of 100 μm. The temperature was raised to 40-180°C over 1 hour and maintained at this temperature for another 1 hour to evaporate the solvent, yielding an unstretched film. The residual solvent content in the resulting unstretched film was 0.9% by mass. The resulting unstretched film was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD) and then the transverse direction (TD) at a stretching rate of 300% / min at a temperature of Tg + 18°C, using a sequential biaxial stretching machine (Toyo Seiki Seisaku-sho, X6-S) to a 4.0x areal stretching ratio. The film was then cooled to obtain a stretched film. The evaluation results of the physical properties of the resulting film are shown in Table 1.

[0089] Example 3 42 parts of acetone, 7 parts of MEK, and 21 parts of DMF were charged into a mixing tank, and 30 parts of the copolymer 2 was added and dissolved under stirring, and then the mixture was filtered through a 10 μm filter and degassed under reduced pressure to obtain a membrane-forming dope solution 3. The viscosity of the membrane-forming dope solution 3 was 13,000 cP.

[0090] Dope solution 3 was cast onto a support (Kapton film) using a coater to a dry film thickness of 160 μm. The temperature was raised to 40-180°C over 1 hour and maintained at this temperature for another 1 hour to evaporate the solvent, yielding an unstretched film. The residual solvent content in the resulting unstretched film was 0.8% by mass. The resulting unstretched film was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD) and then the transverse direction (TD) at a stretching rate of 300% / min at a temperature of Tg + 18°C, using a sequential biaxial stretching machine (Toyo Seiki Seisaku-sho, X6-S) to a 4.0x areal stretching ratio. The stretched film was then cooled to obtain a stretched film. The evaluation results of the physical properties of the resulting film are shown in Table 1.

[0091] Example 4 42 parts of methylene chloride, 7 parts of ethanol (EtOH), and 21 parts of DMF were charged into a mixing tank, and 30 parts of the copolymer 1 was added and dissolved under stirring, and then the mixture was filtered through a 10 μm filter and degassed under reduced pressure to obtain a membrane-forming dope solution 4. The viscosity of the membrane-forming dope solution 4 was 10,000 cP.

[0092] Dope solution 4 was cast onto a support (Kapton film) using a coater to a dry film thickness of 160 μm. The temperature was raised to 40-180°C over 1 hour and maintained at this temperature for another 1 hour to evaporate the solvent, yielding an unstretched film. The residual solvent content in the resulting unstretched film was 0.9% by mass. The resulting unstretched film was cut into a size of 96 mm x 96 mm and sequentially biaxially stretched in the machine direction (MD) and then the transverse direction (TD) at a stretching rate of 300% / min at a temperature of Tg + 18°C. The stretched film was then cooled to obtain a stretched film. The evaluation results of the physical properties of the resulting film are shown in Table 1.

[0093] (Comparative Example 1) 56 parts of acetone and 14 parts of MEK were charged into a mixing tank, and 30 parts of the copolymer 1 was added and dissolved under stirring, and then the mixture was filtered through a 10 μm filter and degassed under reduced pressure to obtain a membrane-forming dope solution 5. The viscosity of the membrane-forming dope solution 5 was 9000 cP.

[0094] Dope solution 5 was cast onto a support (Kapton film) using a coater to a dry film thickness of 160 μm, and the temperature was raised to 40 to 180°C over 1 hour, and then maintained at this temperature for another 1 hour to evaporate the solvent, yielding an unstretched film. Bumping occurred during the solvent evaporation process. The residual solvent content in the unstretched film was 0.9% by mass. The film had many bubbles and could not be stretched.

[0095] (Comparative Example 2) 70 parts of DMF was added to a mixing tank, and 30 parts of the copolymer 1 was added and dissolved under stirring, and then the mixture was filtered through a 10 μm filter and degassed under reduced pressure to obtain a membrane-forming dope solution 6. The viscosity of the membrane-forming dope solution 6 was 11,000 cP.

[0096] Dope solution 6 was cast onto a support (Kapton film) using a coater to a dry film thickness of 160 μm. The temperature was raised to 80–180°C over 1 hour and maintained at that temperature for an additional 1 hour to evaporate the solvent, yielding an unstretched film. The residual solvent content in the resulting unstretched film was 5% by mass. The resulting unstretched film was cut into a size of 96 mm × 96 mm and sequentially biaxially stretched in the machine direction (MD) and transverse direction (TD) at a temperature of Tg + 18°C ​​at a stretching rate of 300% / min in the machine direction (MD) and transverse direction (TD) to an areal stretching ratio of 4.0. The film was then cooled to obtain a stretched film. The evaluation results of the physical properties of the resulting film are shown in Table 2.

[0097] [Table 1]

[0098] [Table 2]

Claims

1. A method for producing a film containing a copolymer having a lactone ring structure, comprising: a step of forming a film from a dope solution containing the copolymer, a first solvent, a second solvent, and a third solvent by a solution casting method, the boiling point of the first solvent is less than 60°C; The boiling point of the second solvent is 60 to 110°C, The method for producing a film, wherein the boiling point of the third solvent is greater than 110°C.

2. The method of claim 1 further comprising the step of stretching the film.

3. 3. The method according to claim 1, wherein the dope solution contains the first solvent in an amount of 5 to 80 mass % based on the total amount of the first solvent, the second solvent, and the third solvent.

4. 4. The method according to claim 1, wherein the dope solution contains the second solvent in an amount of 3 to 50 mass% based on a total amount of the first solvent, the second solvent, and the third solvent.

5. The method according to any one of claims 1 to 4, wherein the copolymer further comprises a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms.

6. The method according to any one of claims 1 to 5, wherein the viscosity of the dope solution is 0.1 to 500,000 cP.

7. The method according to any one of claims 1 to 6, wherein the unstretched film has a thickness of 60 to 200 µm.

8. A dope solution comprising a copolymer having a lactone ring structure, a first solvent, a second solvent, and a third solvent, the boiling point of the first solvent is less than 60°C; The boiling point of the second solvent is 60 to 110°C, The dope solution, wherein the boiling point of the third solvent is greater than 110°C.

9. 9. The dope according to claim 8, wherein the viscosity is 0.1 to 500,000 cP.

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