resin film

Abstract

An acrylic resin composition for use in producing films by a solution casting method, the composition comprising an acrylic polymer comprising, as constituent units, 30-100 wt% methyl methacrylate units and 0-70 wt% units of a monomer copolymerizable therewith and an ionic emulsifier, the content of the ionic emulsifier being 0.1-10 parts by weight per 100 parts by weight of the acrylic polymer.

Description

[Technical Field]

[0001] This invention relates to an acrylic resin composition used for manufacturing films by solution casting, and to a resin film manufactured by solution casting using the composition. [Background technology]

[0002] In recent years, as screen sizes and resolutions have increased, the use of TAC (triacetylcellulose) in polarizer protective films for liquid crystal displays has revealed a problem: its high moisture permeability and water absorption can cause panels to warp during transport, leading to a decrease in image quality.

[0003] Acrylic resin films have excellent optical properties, low moisture permeability, and low water absorption, making them a promising alternative to TAC films. Patent Document 1 discloses a technique for suppressing whitening of the resulting film and the formation of bubbles in the film by optimizing conditions such as the amount of residual solvent and temperature during the drying process when manufacturing an acrylic resin film by solution casting.

[0004] Furthermore, Patent Document 2 discloses that a film with excellent optical properties, dimensional stability, and adhesiveness can be obtained by using an acrylic polymer obtained by suspension polymerization in the presence of a suspension polymerization dispersant having a specific structure, in a solution casting method. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2014-177089 [Patent Document 2] Special Publication No. 2019-533203 [Overview of the project] [Problems that the invention aims to solve]

[0006] The technology described in Patent Document 1 requires the control of complex manufacturing conditions such as solvent residue and temperature, and the technology described in Patent Document 2 requires the use of a suspension polymerization dispersant with a special structure, leaving room for improvement. In addition to the aforementioned problem of bubbles forming in the film, it was found that depending on the composition of the acrylic resin composition used in the solution casting method, there is a problem of deterioration in the transparency of the dope in the solution casting method.

[0007] In view of the above situation, the present invention aims to provide an acrylic resin composition used for manufacturing films by solution casting, which improves the transparency of the dope containing the composition and suppresses foam marks on the surface of the acrylic resin film manufactured by solution casting. [Means for solving the problem]

[0008] As a result of diligent research, the inventors focused on components other than the main polymer contained in the acrylic resin composition (components referred to as auxiliary raw materials or impurities used in the production of the main polymer), and found that by controlling their type and content, the transparency of the dope containing this acrylic resin composition was improved, and foam marks during film drying were less likely to occur on the surface of the acrylic resin film manufactured by the solution casting method, thus completing the present invention.

[0009] In other words, the present invention relates to an acrylic resin composition for film production by solution casting, comprising an acrylic polymer having 30 to 100% by weight of methyl methacrylate units and 0 to 70% by weight of other monomer units copolymerizable thereto as constituent units, and an ionic emulsifier, wherein the content of the ionic emulsifier is 0.1 to 10 parts by weight per 100 parts by weight of the acrylic polymer. Preferably, the ionic emulsifier is a sulfonate. Preferably, the sulfonate salt comprises at least one selected from the group consisting of lithium salts, sodium salts, and potassium salts. Preferably, the sulfonate includes at least one selected from the group consisting of dialkyl sulfosuccinate, alkanesulfonate, alphaolefin sulfonate, alkylbenzene sulfonate, naphthalene sulfonate-formaldehyde condensate, alkylnaphthalene sulfonate, and N-methyl-N-acyl taurate salt. Preferably, the copolymerizable monomer units include (meth)acrylic acid ester units (excluding methyl methacrylate) having 1 to 20 carbon atoms in the ester moiety, and / or maleimide units. Preferably, the content of the copolymerizable other monomer units is 0.1% to 50% by weight of the total amount of constituent units of the acrylic polymer. The acrylic resin composition may further contain 1 to 50 parts by weight of a graft copolymer having a core-shell structure, per 100 parts by weight of the acrylic polymer. Preferably, the weight-average molecular weight of the acrylic polymer is 500,000 or more. Preferably, the haze of a solution dope containing the acrylic resin composition at a concentration of 5% by weight in a mixed solvent of 95% by weight of methylene chloride and 5% by weight of methanol is 5% or less. The present invention also relates to a resin film formed by molding the acrylic resin composition by a solution casting method. Preferably, the haze of the resin film is 2% or less. Preferably, the resin film is a protective film for lamination onto the surface of another substrate. Preferably, the resin film is a polarizer protective film. Furthermore, the present invention also relates to a polarizer and a polarizing plate formed by laminating the resin film, and also to a display device including the polarizing plate. The present invention also relates to a method for producing the acrylic resin composition, comprising the steps of: carrying out emulsion polymerization or suspension polymerization in the presence of an ionic emulsifier to obtain a mixture containing the acrylic polymer and water; and carrying out a drying operation on the mixture without performing a washing operation. Furthermore, the present invention also relates to a method for producing a resin film, which includes a step of forming a film from a dope containing the acrylic resin composition and a solvent by a solution casting method. Preferably, the solvent contains 1 to 25% by weight of an alcohol. Preferably, the alcohol is ethanol and / or methanol. [[Effect of the Invention]]

[0010] According to the present invention, there is provided an acrylic resin composition used for producing a film by a solution casting method, in which the transparency of a dope containing the composition is improved, and foaming marks on the surface of an acrylic resin film produced by the solution casting method can be suppressed. An acrylic resin film produced by a solution casting method using the acrylic resin composition according to the present invention is unlikely to have foaming marks during film formation drying on the film surface, has excellent appearance, and can be a highly transparent film. Such an acrylic resin film has few optical defects and high light extraction efficiency, and thus can be suitably used as an optical film for a liquid crystal display member, particularly as a polarizer protection film. [[Brief Description of the Drawings]]

[0011] [Figure 1] Microscopic photograph of the surface of a film prepared for foamability evaluation using the resin composition obtained in Example 1 [Figure 2] Microscopic photograph of the surface of a film prepared for foamability evaluation using the resin composition obtained in Comparative Example 1 [[Modes for Carrying Out the Invention]]

[0012] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to these embodiments.

[0013] (Acrylic Resin Composition) The acrylic resin composition of the present invention contains at least an acrylic polymer comprising 30 to 100% by weight of methyl methacrylate units and 0 to 70% by weight of other monomer units copolymerizable thereto, and an ionic emulsifier, wherein the content of the ionic emulsifier is 0.1 to 10 parts by weight per 100 parts by weight of the acrylic polymer. With such a composition, when producing a resin film by solution casting, foam marks due to the drying process are less likely to occur, and a highly transparent film can be obtained.

[0014] (Acrylic polymer) The acrylic polymer contained in the acrylic resin composition according to this embodiment consists of 30 to 100% by weight of methyl methacrylate units and 0 to 70% by weight of other monomer units copolymerizable therewith.

[0015] From the viewpoint of appearance and weather resistance, the acrylic polymer may contain 30% by weight or more of methyl methacrylate units in the total amount of constituent units of the polymer, but it is preferable that it contains 50% by weight or more, more preferably 60% by weight or more, even more preferably 70% by weight or more, and particularly preferable 80% by weight or more. Furthermore, from the viewpoint of optical properties and heat resistance, the upper limit is preferably 99.9% by weight or less, more preferably 99% by weight or less, even more preferably 97% by weight or less, and particularly preferably 95% by weight or less. In addition, from the viewpoint of processability and appearance, it is preferable that the acrylic polymer does not contain polyfunctional monomer units having two or more polymerizable functional groups in the molecule.

[0016] Other monomer units copolymerizable with methyl methacrylate units include, for example, ethyl methacrylate, propyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, benzyl methacrylate, octyl methacrylate, stearyl methacrylate, glycidyl methacrylate, epoxycyclohexylmethyl methacrylate, dimethylaminoethyl methacrylate, 2-hydroxylethyl methacrylate, 2-hydroxylpropyl methacrylate, dicyclopentanyl methacrylate, 2,2,2-trifluoroethyl methacrylate, 2,2,2-trichloroethyl methacrylate, isobornyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, glycidyl acrylate, epoxycyclohexylmethyl acrylate, 2-hydroxyethyl acrylate, and 2-hydroxypropyl acrylate. Examples include (meth)acrylic acid ester units with 1 to 20 carbon atoms in the ester group, such as droxypropyl (excluding methyl methacrylate); (meth)acrylamide units such as methacrylamide, N-methylolmethacrylamide, acrylamide, and N-methylolacrylamide; carboxylic acids such as methacrylic acid and acrylic acid and their salts; vinyl cyanide units such as acrylonitrile and methacrylonitrile; vinylarene units such as styrene, α-methylstyrene, monochlorostyrene, and dichlorostyrene; maleimide units such as N-phenylmaleimide, N-cyclohexylmaleimide, and N-methylmaleimide; maleic acid, fumaric acid, and their esters; vinyl halides such as vinyl chloride, vinyl bromide, and chloroprene; vinyl esters such as vinyl formate, vinyl acetate, and vinyl propionate; and alkenes such as ethylene, propylene, butylene, butadiene, and isobutylene. Among these, (meth)acrylic acid ester units (excluding methyl methacrylate), vinylarene units, and / or maleimide units having 1 to 20 carbon atoms in the ester moiety are preferred, and (meth)acrylic acid ester units (excluding methyl methacrylate), and / or maleimide units having 1 to 20 carbon atoms in the ester moiety are particularly preferred.These monomers can be used individually or in combination of two or more types.

[0017] The acrylic resin composition according to this embodiment is used to manufacture an acrylic resin film using a solution casting method. Therefore, it is preferable that the composition contains, as other copolymerizable monomer units, drying-promoting comonomers that increase the rate of solvent evaporation as structural units.

[0018] Preferably, the drying-promoting comonomer unit, which has good heat resistance and can increase the rate of solvent evaporation, is at least one selected from the group consisting of maleimide units, methacrylic acid ester units in which the ester moiety is a primary or secondary hydrocarbon group having 2 to 8 carbon atoms, or an aromatic hydrocarbon group, methacrylic acid ester units in which the ester moiety is a saturated hydrocarbon group having 7 to 16 carbon atoms having a condensed ring structure, methacrylic acid ester units in which the ester moiety is a linear or branched group containing an ether linkage, and vinylarene units. By using these drying-promoting comonomer units, it is possible to increase the rate of solvent evaporation from the cast film in the solution casting method while maintaining the excellent heat resistance of the acrylic polymer.

[0019] Examples of the maleimide units include N-phenylmaleimide, N-benzylmaleimide, N-cyclohexylmaleimide, and N-methylmaleimide, with N-phenylmaleimide, N-benzylmaleimide, and N-cyclohexylmaleimide being preferred.

[0020] Examples of methacrylic acid ester units in which the ester moiety is a primary or secondary hydrocarbon group having 2 to 8 carbon atoms, or an aromatic hydrocarbon group, include ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, phenyl methacrylate, and benzyl methacrylate. Of these, ethyl methacrylate, n-butyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, and benzyl methacrylate are preferred.

[0021] Examples of methacrylic acid ester units in which the ester moiety is a saturated hydrocarbon group having 7 to 16 carbon atoms and a fused ring structure include dicyclopentanyl methacrylate and isobornyl methacrylate. The number of carbon atoms in the saturated hydrocarbon group is preferably 8 to 14, and more preferably 9 to 12. The fused ring structure is not particularly limited, but it is preferably a structure in which two five-membered rings are fused by three consecutive carbon atoms.

[0022] Examples of methacrylic acid ester units in which the ester moiety is a linear or branched group containing an ether linkage include 2-methoxyethyl methacrylate.

[0023] Examples of the vinylarene units include styrene, α-methylstyrene, monochlorostyrene, and dichlorostyrene. Of these, styrene is preferred.

[0024] The acrylic polymer is not particularly limited as long as it contains 0 to 70% by weight of the copolymerizable other monomer units out of the total amount of constituent units of the polymer. However, in order to adjust the optical properties and heat resistance of the resulting resin composition, the acrylic polymer preferably contains 0.1% by weight or more of the copolymerizable other monomer units, more preferably 1% by weight or more, even more preferably 3% by weight or more, and particularly preferably 5% by weight or more. Furthermore, the upper limit is preferably 50% by weight or less, more preferably 40% by weight or less, even more preferably 30% by weight or less, and particularly preferably 20% by weight or less.

[0025] The aforementioned acrylic polymers have excellent heat resistance, and therefore, it is preferable that they have a ring structure in the main chain. Examples of ring structures include glutarimide ring structures, lactone ring structures, maleic anhydride-derived structures, maleimide ring structures (including N-substituted maleimide-derived structures), and glutaric acid anhydride ring structures. Also, acrylic resins containing (meth)acrylic acid structural units in the molecule are also examples. Specifically, examples include maleimide acrylic resins (acrylic resins in which an unsubstituted or N-substituted maleimide compound is copolymerized as a copolymer component), glutarimide acrylic resins, lactone ring-containing acrylic resins, acrylic resins or methacrylic resins containing hydroxyl groups and / or carboxyl groups, partially hydrogenated styrene unit-containing acrylic polymers obtained by partially hydrogenating the aromatic ring of a styrene-containing acrylic polymer obtained by polymerizing a styrene monomer and other monomers copolymerizable thereto, and acrylic polymers having cyclic acid anhydride structures such as glutaric acid anhydride structures or maleic acid anhydride-derived structures.

[0026] Among these, glutarimide ring structures and maleimide ring structures are particularly preferred because they can effectively improve the heat resistance of acrylic resin films and offer an excellent balance with optical properties. These can also be used in combination to impart optical properties, high thermal stability, and solvent resistance to acrylic polymers.

[0027] The weight-average molecular weight of the acrylic polymer is not particularly limited, but from the viewpoint of achieving a balance between the toughness of the resulting acrylic resin film and good film-forming properties, it is preferably 400,000 to 4,000,000, more preferably 800,000 to 3,500,000, even more preferably 800,000 to 3,000,000, and particularly preferably 1,000,000 to 3,000,000. The weight-average molecular weight may also be 800,000 to 2,500,000, or 800,000 to 2,000,000. Furthermore, when forming a film by melt extrusion, it is necessary to melt the acrylic polymer to lower its viscosity, which requires the polymer to have a relatively low molecular weight. However, in this embodiment, since the film is formed by solution casting, it is possible to easily form a film even if the polymer has a high molecular weight. From this viewpoint, the weight-average molecular weight of the acrylic polymer may be 500,000 or more. The weight-average molecular weight can be calculated using gel permeation chromatography (GPC) and the standard polystyrene equivalent method.

[0028] The acrylic polymer preferably has excellent heat resistance, and the glass transition temperature can be used as an indicator of heat resistance. The acrylic polymer preferably exhibits a glass transition temperature of 110°C or higher, more preferably 114°C or higher, even more preferably 115°C or higher, even more preferably 119°C or higher, particularly preferably 122°C or higher, and most preferably 125°C or higher.

[0029] (Method for producing acrylic polymers) The method for producing the acrylic polymer according to this embodiment is not particularly limited as long as it can exhibit the effects of the invention. However, from the viewpoint of the degree of freedom in designing the structure of the acrylic polymer, the ease of polymerization, and productivity, it is preferable that it be produced by emulsion polymerization or suspension polymerization.

[0030] When manufacturing acrylic resin films by solution casting, it is more preferable to manufacture them by emulsion polymerization, in which polymerization is carried out in the presence of an ionic emulsifier, from the viewpoint of obtaining a highly transparent film with excellent appearance, as foaming marks during film formation and drying are less likely to occur on the film surface and inside the film. In particular, in acrylic polymers containing a maleimide ring structure in the main chain, maleimide monomers that remain unreacted during the polymerization process tend to hydrolyze and discolor the acrylic polymer. Since the amount of remaining maleimide monomer can be effectively reduced, it is preferable to manufacture the polymer using an emulsion polymerization method.

[0031] The acrylic resin composition according to this embodiment contains an ionic emulsifier. The ionic emulsifier may be an ionic emulsifier that remains in the acrylic polymer after being used in emulsion polymerization during the production of the acrylic polymer.

[0032] When acrylic polymers are recovered from the reaction system after emulsion polymerization is complete, washing with water or organic solvents will wash away the ionic emulsifiers, resulting in recovered acrylic polymers that are substantially free of ionic emulsifiers. Therefore, when producing the acrylic resin composition according to this embodiment, it is preferable to perform only a drying operation on the reaction system after emulsion polymerization is completed, without performing a washing operation. Since the acrylic polymer recovered by drying alone contains an ionic emulsifier, it can constitute the acrylic resin composition according to this embodiment.

[0033] It is desirable from the viewpoint of energy cost and productivity to omit the washing operation. If the washing operation is not performed, the ionic emulsifier used in emulsion polymerization remains in the resulting acrylic polymer, so the total amount of ionic emulsifier used in emulsion polymerization and the amount of ionic emulsifier in the acrylic resin composition will be substantially equal. In emulsion polymerization, the ionic emulsifier may be added all at once or sequentially.

[0034] Despite containing residual emulsifier, the acrylic resin composition according to this embodiment can suppress foaming marks on the film surface because the emulsifier is an ionic emulsifier. On the other hand, if a nonionic emulsifier remains in the resin composition, foaming marks are more likely to occur on the film surface.

[0035] The ionic emulsifier may be a cationic emulsifier, anionic emulsifier, or amphoteric emulsifier. Of these, anionic emulsifiers are preferred. However, nonionic emulsifiers are not included in the category of ionic emulsifiers. The type of ionic emulsifier is not particularly limited as long as it is used to provide an acrylic resin composition that can exhibit the effects of the invention, and known types can be used. Examples include carboxylates, sulfonates, sulfate esters, and phosphate esters, but sulfonates are preferred because they can highly suppress foaming marks during film formation and drying, and also have excellent polymerization stability.

[0036] More specifically, examples include dialkyl sulfosuccinates, alkanesulfonates, alphaolefin sulfonates, alkylbenzene sulfonates, naphthalene sulfonate-formaldehyde condensates, alkylnaphthalene sulfonates, and N-methyl-N-acyl taurate salts. Among these, dialkyl sulfosuccinates or alkylbenzene sulfonates are preferred.

[0037] These sulfonates are not particularly limited as long as they can exhibit the effects of the invention, and may include lithium salts, sodium salts, potassium salts, calcium salts, magnesium salts, etc. Particularly from the viewpoint of effectively suppressing foam marks, it is preferable to include at least one selected from the group consisting of lithium salts, sodium salts, and potassium salts. When the sulfonates exist as salts of these monovalent cations, even if salt remains in the acrylic resin composition, it is thought that the salt dissolves in the alcohol component in the doping solvent and finely disperses the salt at a microscopic level in the solution dope, thus suppressing foaming to a microscopic level.

[0038] According to this embodiment, the amount of ionic emulsifier used during polymerization is less restricted, and the range of polymerization designs can be broadened. Furthermore, not only can the number of washing steps be reduced, but polymer acquisition methods that do not require washing, such as granulation methods such as spray drying, can also be applied, thereby significantly improving the productivity when manufacturing acrylic resins.

[0039] On the other hand, in the case of sulfonates, which are salts formed from polyvalent cations such as calcium ion salts or magnesium salts, they tend to be insoluble in alcohol components. Therefore, for example, it is preferable to reduce the salt content in the acrylic resin composition to some extent by solidifying polymerized latex produced by emulsion polymerization with a coagulant, heat-treating it, and then washing the resulting slurry particles using a known washing method, in order to suppress foaming marks during film drying.

[0040] The amount of ionic emulsifier is preferably 0.1 to 10 parts by weight per 100 parts by weight of acrylic polymer. From the viewpoint of suppressing foam marks during film formation and drying and having an excellent balance with polymerization stability, it is more preferably 0.3 to 7 parts by weight, even more preferably 0.4 to 6 parts by weight, even more preferably 0.5 to 5 parts by weight, particularly preferably 0.8 to 3 parts by weight, and most preferably 1 to 3 parts by weight. If the amount is greater than 10 parts by weight, the effect of suppressing foam marks in the acrylic resin film will be reduced, and the transparency of the acrylic resin film may decrease. In addition, physical properties other than foaming properties, such as the thermal stability of the acrylic resin film, may decrease, or salt may bleed onto the metal rolls during film formation in the solution casting method, potentially contaminating the metal rolls.

[0041] When polymerizing the aforementioned acrylic polymer, known polymerization initiators can be used, but examples include persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate; and organic peroxides such as tertiary butyl hydroperoxide, tertiary butyl peroxyisopropyl carbonate, cumene hydroperoxide, paramentane hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, di8,5,5-trimethylhexanoyl peroxide, dilauroyl peroxide, and benzoyl peroxide.

[0042] These initiators may be used to polymerize by generating radicals through a thermal decomposition mechanism alone, or they may be used as redox initiators that generate radicals at low temperatures by combining them with an oxidizing agent such as ferrous sulfate and a reducing agent such as sodium formaldehyde sulfoxylate, as described in the examples of Japanese Patent Publication No. 3960631. By combining these according to the composition of the acrylic polymer, it is possible to suppress discoloration.

[0043] Furthermore, in order to adjust the molecular weight of the acrylic polymer, known chain transfer agents may be used when polymerizing the acrylic polymer. Examples of chain transfer agents include alkyl mercaptans, alkyl sulfides, alkyl disulfides, thioglycolic acid esters such as 2-ethylhexyl thioglycolate, alpha-methylstyrene dimers, mercapto acids such as β-mercaptopropionic acid, benzyl mercaptans, thiophenols, thiocresol, thionaphthol, and other aromatic mercaptans.

[0044] (graft copolymer) The acrylic resin composition according to this embodiment may further contain a graft copolymer having a core-shell structure. Furthermore, when forming the dope in the solution casting method, the acrylic resin composition and the graft copolymer having a core-shell structure may be added to the solvent, respectively. The graft copolymer having a core-shell structure can impart mechanical strength, such as bending resistance and crack resistance, to the acrylic resin film.

[0045] Graft copolymers having a core-shell structure are also called multi-stage polymers, multilayer polymers, or core-shell polymers. These polymers have a polymer layer (shell layer) obtained by polymerizing a monomer mixture in the presence of crosslinked polymer particles (core layer). The core layer and shell layer may each consist of one layer or two or more layers. Such graft copolymers are not particularly limited, and known ones can be used as appropriate. As an example, a graft copolymer can be obtained by polymerizing a monomer mixture mainly composed of acrylic acid ester with a crosslinking agent to form an acrylic acid ester-based rubbery polymer, and then polymerizing a monomer mixture mainly composed of methacrylic acid ester in the presence of the acrylic acid ester-based rubbery polymer.

[0046] The graft copolymer can be produced by conventional emulsion polymerization using known emulsifiers. However, from the viewpoint of suppressing foam marks during film drying of acrylic resin films, it is preferable to produce it by emulsion polymerization using an ionic emulsifier that is soluble in alcohol. Furthermore, for example, when the graft copolymer is granulated using a coagulant such as calcium chloride or magnesium chloride, the ionic emulsifier will exist as a salt of polyvalent cations. Therefore, from the viewpoint of suppressing foam marks on the resin film, it is preferable to wash the graft copolymer using a known washing method to reduce the salt content in the graft copolymer.

[0047] In the acrylic resin composition, the blending ratio of the acrylic polymer to the graft copolymer having a core-shell structure is preferably 1 to 50 parts by weight, more preferably 5 to 40 parts by weight, and particularly preferably 7 to 30 parts by weight, per 100 parts by weight of the acrylic polymer.

[0048] When the proportion of graft copolymer having a core-shell structure is 1 part by weight or more, the strength improvement effect due to the incorporation of graft copolymer having a core-shell structure can be obtained. Furthermore, when the proportion is 50 parts by weight or less, the acrylic resin film exhibits excellent heat resistance and elastic modulus, and good processability during film formation.

[0049] When dissolving and dispersing the graft copolymer in a solvent used for solution doping, it is preferable to use a solvent that does not cause significant swelling. For example, a graft copolymer with a high crosslink density in the crosslinked polymer of the core layer is thought to suppress solvent penetration into the core layer and reduce swelling of the graft copolymer. As a result, the density of the molecular chains in the shell layer does not decrease, and the steric repulsion effect between particles is maintained, leading to good particle dispersibility.

[0050] The acrylic resin composition according to this embodiment contains an ionic emulsifier, and when it contains graft copolymer particles having a core-shell structure, it suppresses mutual aggregation of the graft copolymer particles and exhibits a good dispersion state. It also contributes to improving the long-term stability of the solution dope (aggregation is less likely to occur even when stored for a long period of time).

[0051] (Other ingredients) When manufacturing an acrylic resin film using the solution casting method, the acrylic resin composition may optionally contain known additives such as light stabilizers, ultraviolet absorbers, heat stabilizers, antioxidants, matting agents, light diffusing agents, colorants, dyes, pigments, antistatic agents, heat reflectors, lubricants, plasticizers, and fillers, or styrene resins such as acrylonitrile styrene resin, methyl methacrylate styrene resin, and styrene-maleic anhydride resin, polycarbonate resin, polyvinyl acetal resin, cellulose acylate resin, polyvinylidene fluoride, and polyalkyl fluoride. Fluorine-based resins such as methacrylate resins, silicone-based resins, polyolefin-based resins, polyethylene terephthalate resins, polybutylene terephthalate resins, and other resins may be used. Furthermore, to adjust the orientation birefringence of the formed film, a solution dope may be prepared by appropriately mixing in birefringent inorganic fine particles described in Japanese Patent Publication No. 3648201 and Japanese Patent Publication No. 4336586, or a birefringent low-molecular-weight compound with a molecular weight of 5000 or less, preferably 1000 or less, as described in Japanese Patent Publication No. 3696649.

[0052] (Solution casting method) The acrylic resin composition according to this embodiment is used to manufacture a resin film using a solution casting method. Specifically, a solution dope is prepared by dissolving the acrylic resin composition in a so-called good solvent in which it dissolves well. Then, the prepared dope is cast onto a support surface, and the solvent is evaporated to manufacture the resin film.

[0053] The type of good solvent is not particularly limited as long as it can dissolve the acrylic resin composition, but examples include chlorinated organic solvents such as methylene chloride, and non-chlorinated organic solvents such as methyl acetate, ethyl acetate, acetone, methyl ethyl ketone, and tetrahydrofuran. Among these, methylene chloride can be given as a preferred example because it can dissolve the acrylic resin composition well.

[0054] For solution doping, it is preferable to add a poor solvent, such as an alcohol, along with a good solvent. As the alcohol, for example, a linear or branched aliphatic alcohol having 1 to 4 carbon atoms can be used. Among these, ethanol and / or methanol are preferred. By adding the aforementioned alcohol, the drying efficiency of the dope is increased, and the evaporating alcohol creates numerous voids in the areas where it was previously present in the film, resulting in a film that is less dense and therefore has excellent adhesion to other substrates such as polarizers. The amount of alcohol added is preferably 1 to 25% by weight, more preferably 2 to 20% by weight, and even more preferably 3 to 15% by weight, based on the total amount of solvent added to the dope.

[0055] Methods for preparing a solution dope include first preparing pellets containing an acrylic resin composition, and optionally other components such as a graft copolymer, and then mixing these pellets with a solvent to dissolve and disperse each component in the solvent to prepare a solution dope; or adding and mixing each component separately in a solvent to prepare a solution dope; or preparing two or more dope preparation solutions and mixing these solutions to prepare a solution dope. Of these, the method of first preparing pellets containing an acrylic resin composition, and optionally other components such as a graft copolymer, and then mixing these pellets with a solvent to dissolve and disperse each component in the solvent to prepare a solution dope is preferred from the viewpoint of uniformly mixing and dispersing the components in the solution dope.

[0056] The resulting solution dope must have few insoluble substances and exhibit excellent transparency at the solution doping stage. This is necessary to obtain a highly transparent resin film that is less prone to foaming marks on the film surface during film formation and drying. The presence or absence of insoluble substances in the alcohol component, which cause foaming marks, can be detected in advance by evaluating the transparency of the solution dope.

[0057] One method for measuring the transparency of a solution-doped acrylic resin composition is to measure the haze of a solution-doped acrylic resin composition prepared by dissolving it in a solvent with a specific good solvent / poor solvent alcohol composition at a specific solid content concentration.

[0058] In this embodiment, it is preferable that the acrylic resin composition, when dissolved at a concentration of 5% by weight in a mixed solvent of 95% by weight of methylene chloride and 5% by weight of methanol, has a haze of 5% or less. When an acrylic resin composition that can produce a solution dope with such low haze is produced by the solution casting method, foam marks are less likely to occur on the film surface during film formation and drying, resulting in a highly transparent film with excellent appearance.

[0059] The dope dissolution process can be carried out by adjusting the temperature and pressure as appropriate. After the dissolution process, the obtained solution dope can be filtered and degassed. Next, the solution dope is supplied to a pressurized die by a liquid transfer pump, and the solution dope is cast from the slit of the pressurized die onto the surface (mirror surface) of a support such as an endless belt or drum made of metal or synthetic resin to form a dope film. The formed dope film is heated on the support to evaporate the solvent and form a film. The temperature conditions for evaporating the solvent can be appropriately determined according to the boiling point of the solvent used. The film thus obtained is peeled off the support. After that, the obtained film may be subjected to drying, heating, stretching, etc., as appropriate.

[0060] (Resin film) The resin film according to this embodiment is formed by solution casting using the solution dope described above. The thickness of the resin film is not particularly limited, but is preferably 5 to 200 μm, and more preferably 5 to 100 μm. When the thickness of the resin film is 200 μm or less, the cooling after molding becomes uniform, which tends to result in uniform optical properties and a faster drying speed. Furthermore, when the thickness of the resin film is 5 μm or more, the resin film becomes easier to handle and tends to have excellent protective film function.

[0061] The aforementioned resin film, when measured at a thickness of 40 μm, preferably has a haze of 2% or less, more preferably 1.5% or less, even more preferably 1% or less, even more preferably 0.8% or less, even more preferably 0.6% or less, and particularly preferably 0.4% or less. When the haze satisfies this range, it has high transparency and can therefore be suitably used in optical components where light transmittance is required.

[0062] Furthermore, the resin film formed by the acrylic resin composition according to this embodiment using a solution casting method can preferably be used as a laminated protective film on the surface of another substrate, more preferably as an optical film, and particularly preferably as a polarizer protective film.

[0063] When used as a polarizer protective film, it is preferable to have low optical isotropy. In particular, it is preferable that the optical isotropy of the resin film is low not only in the in-plane direction (length direction and width direction) but also in the thickness direction.

[0064] More specifically, the absolute value of the in-plane phase difference is preferably 10 nm or less, more preferably 5 nm or less, and particularly preferably 3 nm or less. Furthermore, the absolute value of the phase difference in the thickness direction is preferably 50 nm or less, more preferably 20 nm or less, even more preferably 10 nm or less, and particularly preferably 5 nm or less. A resin film having such a phase difference can be suitably used as a polarizer protective film for a polarizer plate in a liquid crystal display device.

[0065] Here, the phase difference is an index value calculated based on birefringence, and the in-plane phase difference (Re) and the thickness-direction phase difference (Rth) can be calculated using the following formulas. In an ideal molded body that is perfectly optically isotropic in three dimensions, both the in-plane phase difference (Re) and the thickness-direction phase difference (Rth) are zero. Re=(nx-ny)×d Rth = ((nx + ny) / 2 - nz) × d In the above formula, nx, ny, and nz represent the refractive indices in the axial directions, where the stretching direction (the orientation direction of the polymer chains) in the plane is the X-axis, the direction perpendicular to the X-axis is the Y-axis, and the thickness direction of the molded body is the Z-axis. Also, d represents the thickness of the molded body, and nx - ny represents the orientation birefringence. Note that the MD direction of the molded body is taken as the X-axis, and in the case of a stretched molded body, the stretching direction is taken as the X-axis.

[0066] The resin film formed by molding the acrylic resin composition according to this embodiment by the solution casting method has an orientation birefringence of -2.6×10 -4 ~2.6×10 -4 which is preferably, -1.7×10 -4 ~1.7×10 -4 more preferably, -1.0×10 -4 ~1.0×10 -4 even more preferably, -0.5×10 -4 ~0.5×10 -4 particularly preferably, -0.2×10 -4 ~0.2×10 -4 and most preferably. If the orientation birefringence is within the above range, birefringence during molding can be avoided, and stable optical properties can be obtained. Also, it is very suitable as an optical film used in liquid crystal displays and the like.

[0067] The resin film formed by molding the acrylic resin composition according to this embodiment by the solution casting method has a photoelastic constant of -6×10 -12 ~6×10 -12 Pa -1 which is preferably, -4×10 -12 ~4×10 -12 Pa -1 more preferably, -2×10 -12 ~2×10 -12 Pa -1 even more preferably, -1×10 -12 ~1×10 -12 Pa -1 even further preferably, -0.5×10 -12 ~0.5×10 -12 Pa -1It is particularly preferable that -0.2 × 10 -12 ~0.2 × 10 -12 Pa -1 It is most preferable that this be the case.

[0068] Here, photoelastic birefringence is the birefringence caused by the elastic deformation (strain) of the polymer in a molded body when stress is applied to the molded body. In practice, the degree of photoelastic birefringence of a material can be evaluated by determining the photoelastic constant specific to that polymer. First, stress is applied to the polymer material, and the birefringence when elastic strain occurs is measured. The proportionality constant between the obtained birefringence and the stress is the photoelastic constant. By comparing these photoelastic constants, the birefringence of the polymer when stress is applied can be evaluated. If the photoelastic constant is within the above range, birefringence will not occur even if the molded body is stressed and deformed, thus a molded body with low optical isotropy can be obtained. For example, in polarizer protective film applications, even if the panel is deformed during transportation due to the effects of humidity and temperature in the air, stable optical properties are maintained, thus minimizing quality risks such as image quality degradation. [Examples]

[0069] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The test and evaluation methods for each physical property described in the examples and comparative examples are as follows.

[0070] (1) Weight average molecular weight (Mw) The weight-average molecular weight (Mw) of the acrylic polymer was calculated using a standard polystyrene equivalent method with gel permeation chromatography (GPC). A polystyrene crosslinked gel-packed GPC column (model: Shodex GPC K-806M, manufactured by Showa Denko K.K.) was used, and chloroform was used as the GPC solvent. The sample solution was prepared by dissolving 5 mg of the resin powder of the acrylic resin composition in 2 ml of chloroform, and the column temperature was set to 40°C.

[0071] (2) Volume-average particle size of polymerized latex Using a Microtrac UPA150 (manufactured by Nikkiso Co., Ltd.), the volume-average particle size of polymerized latex of acrylic polymers was determined based on the principle of dynamic light scattering.

[0072] (3) Volume-average particle diameter of bead-shaped particles Using a Microtrac MT3300EXII (manufactured by Nikkiso Co., Ltd.), the volume-average particle diameter of bead-shaped particles was determined based on the principle of laser diffraction scattering.

[0073] (4) Glass transition temperature (Tg) The glass transition temperature (Tg) of acrylic polymers was measured using a differential scanning calorimeter (DSC, model: Q1000, manufactured by TA instruments). The sample was placed under a nitrogen stream and heated to 200°C at a heating rate of 10°C / min, then rapidly cooled to 40°C, and heated again to 200°C at a heating rate of 10°C / min. For the glass transition observed during the second heating cycle, the average of the extrapolation glass transition start temperature and the extrapolation glass transition end temperature was calculated, and this value was defined as the glass transition temperature (Tg).

[0074] (5) Methanol solubility test of surfactants 15 mg of surfactant used in the production of acrylic polymers was weighed (liquid surfactants were evaporated to dryness to obtain a dry powder), added to 10 ml of methanol, and its solubility in methanol was visually confirmed. The solubility indicators were as follows: 〇(Soluble) △(It dissolves, but it takes time) × (Insoluble) "〇 (Soluble)" means that when the dry surfactant powder is added to methanol and shaken, it dissolves quickly in methanol. On the other hand, "△ (Dissolves, but takes time)" means that after adding the dry surfactant powder to methanol, no change is observed for a while, but if shaking is continued for a while, it gradually dissolves and eventually dissolves. "× (Insoluble)" means that it does not dissolve even if shaking is continued.

[0075] (6) Haze measurement of solution-doped samples A mixed solvent consisting of methylene chloride and methanol in a weight ratio of 95:5 was prepared. To this mixed solvent, powder of an acrylic resin composition was added to achieve a solid content concentration of 5% by weight. The mixture was then stirred and mixed using a stirrer tip to prepare a solution dope. After degassing the obtained solution dope, the haze was measured using a haze meter (HAZE Meter NDH4000, manufactured by Nippon Denshoku Industries Ltd.) after setting the mixed solvent (methylene chloride:methanol = 95:5 by weight) as a standard sample and zeroing the meter.

[0076] (7) Evaluation of foaming marks on acrylic resin films A mixed solvent consisting of methylene chloride and methanol in a weight ratio of 80:20 was prepared. An acrylic resin composition was added to this solvent to a solid content concentration of 10% by weight, and then stirred and mixed with a stirrer tip to prepare a solution dope. Next, the solution dope was deposited on a glass plate to a thickness of 1.1 mm using a bar coater by solution casting and held for 10 minutes. The resulting film was then quickly cut to a size of 5.5 cm × 5.5 cm, held in a 6 cm × 6 cm metal frame, and dried in a drying oven at 190°C for 10 minutes.

[0077] After removing the film from the drying oven, the surface of the film that had been held in place by the metal frame was observed using an optical microscope. Films with a high degree of foaming showed whitening (foaming) in areas directly exposed to hot air in the drying oven, suggesting harsh drying conditions. On the other hand, the film held in place by the metal frame showed relatively suppressed foaming even under harsh drying conditions, making it possible to accurately and rapidly detect the occurrence of foam marks.

[0078] The condition of the foam marks on the film surface, observed with an optical microscope, was visually evaluated on a 5-point scale from 1 (poor) to 5 (good) based on the following indicators. 1 (Foam marks are visible across the entire surface) 2 (Although not the entire surface, foaming marks are observed, and there are many of them.) 3 (Although some foaming marks are visible on the surface, they are few in number.) 4 (Slight foaming marks are visible on the surface, but the surface is generally clean.) 5 (The surface is very clean with no foaming marks.)

[0079] (8) Haze measurement of acrylic resin film A solvent consisting of methylene chloride and methanol in a weight ratio of 80:20 was prepared. An acrylic resin composition was added to this solvent to a solid content concentration of 10% by weight, and then stirred and mixed with a stirrer tip to prepare a solution dope. Next, the solution dope was deposited on a glass plate to a thickness of 1.1 mm using a bar coater by solution casting and held for 10 minutes. The film was peeled from the glass plate and its thickness was measured, and the average film thickness was 40 μm. The haze of the obtained film was measured using a haze meter (HZ-V3, manufactured by Suga Test Instruments Co., Ltd.) according to the method described in JIS K7105.

[0080] The following examples will be described in detail, but unless otherwise specified, "parts" and "%" refer to "parts by weight" and "weight percent," respectively. The abbreviations represent the following substances. MMA: Methyl methacrylate BMA: n-butyl methacrylate 2-EHMA: 2-ethylhexyl methacrylate PhMI: N-phenylmaleimide DSS: Sodium dioctyl sulfosuccinate DBS: Sodium dodecylbenzenesulfonate NPS: Sodium Persulfate NDS: Sodium pyrosulfite SFS: Sodium sulfoxylate / formaldehyde ED: Ethylenediaminetetraacetate disodium FeSO4: Ferrous sulfate heptahydrate 2-EHTG: 2-ethylhexyl thioglycolate LPO: Lauroyl peroxide t-BHP: t-butyl hydroperoxide PSF: Semi-hardened bovine potassium fatty acid HPMC: Hydroxypropylmethylcellulose

[0081] (Example 1: Production of acrylic polymer A) In an 8-liter glass reactor equipped with a paddle-type stirrer, 143 parts deionized water, 0.01 parts sodium hydroxide, and 0.005 parts DSS were charged. The mixture was then stirred at 175 rpm, and the temperature was raised to 80°C while purging the reactor with nitrogen. After reaching 80°C, 0.03 parts NPS and 0.001 parts NDS were added. Subsequently, a monomer mixture consisting of 90 parts MMA, 10 parts BMA, and 0.015 parts 2-EHTG was continuously added to the reactor over 80 minutes to allow the reaction to proceed. Furthermore, 15 minutes after the addition of the monomer mixture, 0.495 parts DSS was added dropwise, following the addition of the monomer mixture, and continuously added to the reactor. The stirring speed was increased to 200 rpm at 50 minutes and to 240 rpm at 70 minutes from the start of monomer mixture addition. After the addition of the monomer mixture was complete, the reaction was continued for 60 minutes to complete the polymerization and obtain a polymerized latex. The polymerization conversion rate was 99.5%, and the average particle size was 4500 Å. Next, the obtained polymerized latex was evaporated to dryness in a drying oven at 75°C for 12 hours to obtain a white powdery resin composition containing acrylic polymer A. The weight-average molecular weight of acrylic polymer A was 1,000,000, and the methanol solubility of the DSS used in polymerization was ○ (soluble). The resin composition containing acrylic polymer A contains 0.5 parts by weight of DSS per 100 parts by weight of acrylic polymer A.

[0082] Furthermore, the haze of the solution dope using the white powder of the acrylic polymer A-containing resin composition was 0.7%, and the visual evaluation of the foaming properties of the acrylic resin film produced by solution casting using the white powder of the acrylic polymer A-containing resin composition was 5 points, with a haze of 0.22%. The results are shown in Table 1. In addition, Figure 1 shows a micrograph of the film surface taken during the foaming property evaluation.

[0083] (Example 2: Production of acrylic polymer B) Polymerization was carried out in the same manner as in Example 1, except that the amount of DSS continuously added to the reactor was changed to 4.995 parts, to obtain a polymerized latex. The polymerization conversion rate was 99.7%, and the average particle size was 4300 Å. Using the obtained polymerized latex, a white powdery acrylic polymer B-containing resin composition was obtained in the same manner as in Example 1. The weight-average molecular weight of acrylic polymer B was 1.1 million. The acrylic polymer B-containing resin composition contained 5.0 parts by weight of DSS per 100 parts by weight of acrylic polymer B. The methanol solubility test of the surfactant, the haze of the solution dope, the foaming properties of the film, and the haze of the film were evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0084] (Example 3: Production of acrylic polymer C) Polymerization was carried out in the same manner as in Example 2, except that DSS was replaced with DBS, to obtain a polymerized latex. Using the obtained polymerized latex, a white powdery acrylic polymer C-containing resin composition was obtained in the same manner as in Example 2. The weight-average molecular weight of acrylic polymer C was 900,000. The acrylic polymer C-containing resin composition contained 5.0 parts by weight of DBS per 100 parts by weight of acrylic polymer C. The methanol solubility test of the surfactant, the haze of the solution dope, the foaming properties of the film, and the haze of the film were evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0085] (Example 4: Production of acrylic polymer D) In an 8-liter glass reactor equipped with a paddle-type stirrer, 143 parts deionized water, 0.01 parts sodium hydroxide, and 0.15 parts DSS were charged. The mixture was then stirred at 175 rpm, and the temperature was raised to 85°C while purging the reactor with nitrogen. After reaching 85°C, 0.022 parts NPS and 0.0005 parts SFS were added. Subsequently, a monomer mixture consisting of 85 parts MMA, 5 parts 2-EHMA, and 10 parts PhMI was continuously added to the reactor over 80 minutes to allow the reaction to proceed. Fifteen minutes after the addition of the monomer mixture, 0.55 parts DSS was added dropwise, following the addition of the monomer mixture, and continuously added to the reactor. The stirring speed was increased to 200 rpm at 55 minutes and to 240 rpm at 70 minutes from the start of monomer mixture addition. After the addition of the monomer mixture was complete, a mixed aqueous solution of ED: 0.0055 parts, FeSO4: 0.0015 parts, SFS: 0.03 parts, DSS: 0.3 parts, and t-BHP: 0.03 parts were added to the reactor in order. The reaction was then continued for 60 minutes to complete the polymerization and obtain polymerized latex. The polymerization conversion rate was 99.9%, and the average particle size was 2000 Å. Next, the obtained polymerized latex was evaporated to dryness in a drying oven at 75°C for 12 hours to obtain a white powdery acrylic polymer D-containing resin composition. The weight-average molecular weight of acrylic polymer D was 1.75 million. The acrylic polymer D-containing resin composition contains 1.0 part by weight of DSS per 100 parts by weight of acrylic polymer D. Methanol solubility tests of the surfactant, haze of the solution dope, foaming properties of the film, and haze of the film were evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0086] (Example 5: Production of acrylic polymer E) Polymerization was carried out in the same manner as in Example 2, except that DSS was replaced with PSF, to obtain a polymerized latex. Using the obtained polymerized latex, a white powdery acrylic polymer E-containing resin composition was obtained in the same manner as in Example 2. The weight-average molecular weight of acrylic polymer E was 1,000,000. The acrylic polymer E-containing resin composition contained 5.0 parts by weight of PSF per 100 parts by weight of acrylic polymer E. Methanol solubility tests of the surfactant, haze of the solution dope, foaming properties of the film, and haze of the film were evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0087] (Comparative Example 1: Production of Acrylic Polymer F) 170 parts of deionized water and 0.1 parts of anhydrous disodium hydrogen phosphate were charged into an 8-liter glass reactor equipped with a paddle-type stirrer. The mixture was then stirred at 300 rpm, and the temperature was raised to 40°C while purging the reactor with nitrogen. After charging 0.3 parts of LPO into the reactor, a monomer mixture consisting of 90 parts of MMA, 10 parts of BMA, and 0.02 parts of 2-EHTG was continuously added to the reactor over 30 minutes. 30 minutes after the completion of the monomer mixture addition, 0.4 parts of HPMC (Metholose 60SH50: manufactured by Shin-Etsu Chemical Co., Ltd.) were continuously charged into the reactor over 30 minutes. After 30 minutes, the temperature inside the reactor was raised, and the reaction was started when the internal temperature reached 65°C. 100 minutes after the start of the reaction, the internal temperature of the reactor reached a maximum of 85°C, and thereafter the temperature gradually decreased. Subsequently, the internal temperature of the reactor was raised to 95°C and maintained for 60 minutes to complete the polymerization. The volume-average particle size of the obtained bead-like particles was 50 μm. The suspension slurry containing the bead-like particles was evaporated to dryness in a drying oven at 50°C for 24 hours to obtain an acrylic polymer F-containing resin composition. The weight-average molecular weight of acrylic polymer F was 1,000,000. The acrylic polymer F-containing resin composition contains 0.4 parts by weight of HPMC per 100 parts by weight of acrylic polymer F. Note that HPMC is a nonionic surfactant and is not an ionic emulsifier. The methanol solubility test of the surfactant, the haze of the solution dope, the foaming properties of the film, and the haze of the film were evaluated in the same manner as in Example 1. The results are shown in Table 1. In addition, a micrograph of the film surface taken during the foaming property evaluation is shown in Figure 2.

[0088] [Table 1]

[0089] As shown in Table 1, the acrylic polymer A-E-containing resin compositions of Examples 1-5 have a solution-doped haze of 5% or less, and the acrylic resin films produced by solution casting of these compositions exhibit excellent foaming properties, resulting in films with a beautiful appearance. Furthermore, the haze of the acrylic resin films is 2% or less, indicating that highly transparent films are obtained. Such aesthetically pleasing and highly transparent acrylic resin films can be suitably applied to optical films such as polarizer protective films. On the other hand, the acrylic polymer F-containing resin composition of Comparative Example 1, which does not contain an ionic emulsifier but contains a nonionic surfactant, had a solution-doped haze of over 5%, and the acrylic resin film produced from this composition by solution casting had a low foaming ability, making it impossible to obtain a film with a beautiful appearance.

Claims

1. This product comprises an acrylic polymer having 30 to 99.9% by weight of methyl methacrylate units and 0.1 to 70% by weight of other monomer units copolymerizable thereto, and an ionic emulsifier. The content of the ionic emulsifier is 0.1 to 10 parts by weight per 100 parts by weight of the acrylic polymer. The aforementioned acrylic polymer does not contain polyfunctional monomer units having two or more polymerizable functional groups within the molecule. The weight-average molecular weight of the aforementioned acrylic polymer is 500,000 or more. The glass transition temperature of the acrylic polymer is 110°C or higher. The copolymerizable monomer unit comprises at least one selected from the group consisting of maleimide units, methacrylic acid ester units whose ester moiety is a primary or secondary hydrocarbon group having 2 to 8 carbon atoms, or an aromatic hydrocarbon group, methacrylic acid ester units whose ester moiety is a saturated hydrocarbon group having 7 to 16 carbon atoms having a condensed ring structure, methacrylic acid ester units whose ester moiety is a linear or branched group containing an ether linkage, and vinylarene units. The aforementioned ionic emulsifier is a sulfonate salt. A resin film is formed by a solution casting method from an acrylic resin composition for film production by a solution casting method, wherein the ionic emulsifier comprises at least one selected from the group consisting of lithium salts, sodium salts, and potassium salts.

2. This product comprises an acrylic polymer having 30 to 99.9% by weight of methyl methacrylate units and 0.1 to 70% by weight of other monomer units copolymerizable thereto, and an ionic emulsifier. The content of the ionic emulsifier is 0.1 to 10 parts by weight per 100 parts by weight of the acrylic polymer. The aforementioned acrylic polymer does not contain polyfunctional monomer units having two or more polymerizable functional groups within the molecule. The weight-average molecular weight of the aforementioned acrylic polymer is 500,000 or more. The glass transition temperature of the acrylic polymer is 110°C or higher. The copolymerizable monomer unit comprises at least one selected from the group consisting of maleimide units, methacrylic acid ester units whose ester moiety is a primary or secondary hydrocarbon group having 2 to 8 carbon atoms, or an aromatic hydrocarbon group, methacrylic acid ester units whose ester moiety is a saturated hydrocarbon group having 7 to 16 carbon atoms having a condensed ring structure, methacrylic acid ester units whose ester moiety is a linear or branched group containing an ether linkage, and vinylarene units. The ionic emulsifier is at least one selected from the group consisting of dialkyl sulfosuccinate, alkanesulfonate, alphaolefin sulfonate, alkylbenzene sulfonate, naphthalene sulfonate-formaldehyde condensate, alkylnaphthalene sulfonate, and N-methyl-N-acyl taurate salt. A resin film is formed by a solution casting method from an acrylic resin composition for film production by a solution casting method, wherein the ionic emulsifier comprises at least one selected from the group consisting of lithium salts, sodium salts, and potassium salts.

3. The resin film according to claim 1 or 2, wherein the copolymerizable monomer units include (meth)acrylic acid ester units (excluding methyl methacrylate) having 1 to 20 carbon atoms in the ester moiety, and / or maleimide units.

4. The resin film according to any one of claims 1 to 3, wherein the content of the copolymerizable other monomer units is 0.1% to 50% by weight of the total amount of constituent units of the acrylic polymer.

5. The resin film according to any one of claims 1 to 4, further comprising 1 to 50 parts by weight of a graft copolymer having a core-shell structure with respect to 100 parts by weight of the acrylic polymer.

6. The resin film according to any one of claims 1 to 5, wherein the haze of a solution dope containing the acrylic resin composition at a concentration of 5% by weight in a mixed solvent of 95% by weight of methylene chloride and 5% by weight of methanol is 5% or less.

7. The resin film according to any one of claims 1 to 6, wherein the haze of the resin film is 2% or less.

8. The resin film according to any one of claims 1 to 7, wherein the resin film is a laminated protective film for the surface of another substrate.

9. The resin film according to any one of claims 1 to 8, wherein the resin film is a polarizer protective film.

10. A polarizing plate comprising a polarizer and a resin film as described in claim 9, laminated together.

11. A display device comprising a polarizing plate as described in claim 10.

12. A method for producing a resin film according to any one of claims 1 to 9, A method for producing a resin film, comprising the step of forming a film of a dope containing the acrylic resin composition and a solvent by a solution casting method.

13. The method for producing a resin film according to claim 12, wherein the solvent contains 1 to 25% by weight of alcohol.

14. The method for producing a resin film according to claim 13, wherein the alcohol is ethanol and / or methanol.

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