WO2024122626A1 - Acrylic resin powder and film - Google Patents

Abstract

Provided is an acrylic resin powder, wherein the electrical conductivity at 23°C, 50% RH of an aqueous solution obtained by mixing the acrylic resin powder and ultrapure water in a 1:10 weight ratio, placing the resultant mixture in a pressure-proof container, heating for 20 hours at 100°C, and then filtering, is 300 μS/cm or less and the content of volatile components is less than 1.0 wt%.

Description

Acrylic resin powder and film

The present invention relates to acrylic resin powder, a dope solution, a method for producing a dope solution, a film, and a method for producing a film.

　Traditionally, triacetyl cellulose (TAC) film has been used as a polarizer protection film for LCD displays. However, TAC film is highly hygroscopic, and as screens become larger and more highly defined, issues have become apparent in which LCD panels warp during transportation, causing a deterioration in image quality.

On the other hand, acrylic resin film has been attracting attention as an alternative to TAC film due to its excellent optical properties and low moisture absorption.

Patent Document 1 describes an acrylic resin powder used in the production of films by a solution casting method, which contains an acrylic polymer whose constituent units are 30 to 100% by weight of methyl methacrylate units and 0 to 70% by weight of other monomer units copolymerizable therewith, 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.

International Publication No. 2022/124402

However, acrylic resin powder contains hygroscopic substances used during polymerization (e.g., ferrous sulfate heptahydrate, sodium formaldehyde sulfoxylate, sodium persulfate, sodium hydroxide, disodium ethylenediaminetetraacetate, anhydrous disodium hydrogen phosphate, etc.). For this reason, during film production, after the solvent evaporates from the dope solution cast onto the surface of the support, polar substances such as moisture adsorbed to the hygroscopic substances evaporate and foam, i.e., foam marks may appear on the film. In addition, if the film is stored for a long time in a high-temperature, high-humidity environment, the hygroscopic substances contained in the film will absorb moisture, which may reduce the transparency of the film.

The present invention aims to provide an acrylic resin powder that can suppress foaming marks on films and the loss of transparency in high-temperature, high-humidity environments.

(1) An acrylic resin powder that is prepared by mixing the acrylic resin powder with ultrapure water in a weight ratio of 1:10, placing the mixture in a pressure vessel, heating at 100°C for 20 hours, and filtering the mixture to obtain an aqueous solution that has an electrical conductivity of 300 μS/cm or less at 23°C and 50% RH and has a volatile component content of less than 1.0% by weight.

(2) The acrylic resin powder described in (1) has a glass transition temperature of 110°C or higher.

(3) The acrylic resin powder according to (1) or (2), wherein the acrylic resin contained in the acrylic resin powder has a structural unit containing a heterocycle in the main chain.

(4) The acrylic resin powder described in (3) has a structural unit derived from an N-substituted maleimide.

(5) The acrylic resin powder according to (4), wherein the N-substituted maleimide is N-cyclohexylmaleimide or N-phenylmaleimide.

(6) The acrylic resin powder according to (4) or (5), wherein the aqueous solution has a sulfate ion content of 350 ppm or less.

(7) An acrylic resin powder or granule according to any one of (1) to (6), having a weight average molecular weight of 400,000 or more and 4,000,000 or less.

(8) An acrylic resin powder or granule described in any one of (1) to (7) for use in the production of a film by a solution casting method.

(9) A dope solution in which the acrylic resin powder particles described in any one of (1) to (8) are dissolved in a solvent.

(10) A method for producing a dope solution, comprising dissolving the acrylic resin powder described in any one of (1) to (8) in a solvent to produce the dope solution.

(11) A film produced by casting the dope solution described in (9) onto the surface of a support and then volatilizing the solvent.

(12) A method for producing a film, comprising casting the dope solution described in (9) onto the surface of a support, and then volatilizing the solvent to produce a film.

(13) A film containing an acrylic resin, the acrylic resin having a structural unit containing a heterocycle in the main chain, a glass transition temperature of 110°C or higher, a weight average molecular weight of 400,000 or more and 4,000,000 or less, a change in haze ΔHz before and after storage for 96 hours at 85°C and 95% RH when the film has a thickness of 40 μm being 2.5% or less, and a change in yellowness ΔYI before and after storage for 96 hours at 85°C and 95% RH when the film has a thickness of 40 μm being 3.5 or less.

(14) The film described in (13) has a haze Hz of 2% or less when the thickness is 40 μm, and a yellowness index YI of 2.0 or less when the thickness is 40 μm.

(15) The film according to (13) or (14), in which the acrylic resin has a structural unit derived from an N-substituted maleimide.

(16) The film according to (15), wherein the N-substituted maleimide is N-cyclohexylmaleimide or N-phenylmaleimide.

The present invention provides an acrylic resin powder that can suppress foaming marks on the film and the loss of transparency in high-temperature, high-humidity environments.

The following describes an embodiment of the present invention.

(Acrylic resin powder)
The acrylic resin powder of the present embodiment is used, for example, in the production of a film by a solution casting method, and has a volatile component content of less than 1.0% by weight. In particular, the volatile component content is preferably less than 0.5% by weight. The acrylic resin powder of the present embodiment and ultrapure water are mixed in a weight ratio of 1:10, placed in a pressure-resistant container, heated at 100° C. for 20 hours, and then filtered to obtain an aqueous solution having an electrical conductivity of 300 μS/cm or less at 23° C. and 50% RH. If the electrical conductivity of the aqueous solution at 23° C. and 50% RH is 300 μS/cm or less, the content of the hygroscopic substance in the acrylic resin powder of the present embodiment is reduced, and therefore, foaming marks on the film and a decrease in transparency in a high-temperature and high-humidity environment can be suppressed. From the viewpoint of further suppressing foaming marks on the film and a decrease in transparency in a high-temperature and high-humidity environment, the electrical conductivity of the above aqueous solution at 23°C and 50% RH is preferably less than 300 μS/cm, more preferably 290 μS/cm or less, even more preferably 280 μS/cm or less, and particularly preferably 260 μS/cm or less.

In this specification and claims, acrylic resin means a polymer of a monomer having an acryloyl group and/or a monomer having a methacryloyl group. In this case, the acrylic resin may be either a homopolymer or a copolymer. When the acrylic resin is a copolymer, it may be a copolymer of a monomer not having an acryloyl group or a methacryloyl group.

Also, powder and granules refer to powders with a volume average particle diameter of 0.01 mm or more and less than 0.1 mm, and/or granules with a volume average particle diameter of 0.1 mm or more and less than 10 mm.

From the viewpoint of heat resistance of the film, the glass transition temperature of the acrylic resin powder particles of this embodiment is preferably 110°C or higher, more preferably 114°C or higher, even more preferably 115°C or higher, even more preferably 117°C or higher, even more preferably 119°C or higher, even more preferably 120°C or higher, even more preferably 121°C or higher, even more preferably 122°C or higher, particularly preferably 125°C or higher, and most preferably 126°C or higher.

From the viewpoint of film toughness and film formability, the weight average molecular weight of the acrylic resin powder of this embodiment is preferably 400,000 or more and 4,000,000 or less, more preferably 500,000 or more and 3,500,000 or less, even more preferably 500,000 or more and 3,000,000 or less, and particularly preferably 600,000 or more and 3,000,000 or less. The weight average molecular weight of the acrylic resin powder of this embodiment may be 500,000 or more and 2,500,000 or less, or may be 500,000 or more and 2,000,000 or less. The weight average molecular weight of the acrylic resin powder of this embodiment may be 700,000 or more, or may be 800,000 or more.

The acrylic resin powder of this embodiment preferably contains an ionic emulsifier from the viewpoint of suppressing foaming marks on the film.

(acrylic resin)
The acrylic resin constituting the acrylic resin powder of this embodiment has, for example, a content of structural units derived from methyl methacrylate of 30% by weight or more and a content of structural units derived from other monomers capable of copolymerizing with methyl methacrylate (hereinafter referred to as other monomers) of 70% by weight or less.

From the viewpoint of the appearance and weather resistance of the film, the content of structural units derived from methyl methacrylate in the acrylic resin is preferably 50% by weight or more, more preferably 60% by weight or more, even more preferably 70% by weight or more, and particularly preferably 80% by weight or more. Furthermore, from the viewpoint of the optical properties and heat resistance of the film, the content of structural units derived from methyl methacrylate in the acrylic resin 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.

The acrylic resin may contain structural units derived from a polyfunctional monomer having two or more polymerizable functional groups in the molecule.

Other monomers 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-hydroxyethyl methacrylate, 2-hydroxypropyl 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, 2-hydroxypropyl acrylate, etc. Examples of the vinyl are (meth)acrylic acid esters having 1 to 20 carbon atoms in the ester moiety (excluding methyl methacrylate); (meth)acrylamides such as methacrylamide, N-methylol methacrylamide, acrylamide, and N-methylol acrylamide; carboxylic acids and salts thereof such as methacrylic acid and acrylic acid; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl arenes such as styrene, α-methylstyrene, monochlorostyrene, and dichlorostyrene; N-substituted maleimides such as N-phenylmaleimide, N-cyclohexylmaleimide, and N-methylmaleimide; maleic acid, fumaric acid, and esters thereof; 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. Two or more of these may be used in combination. Among these, (meth)acrylic acid esters (excluding methyl methacrylate) having 1 to 20 carbon atoms in the ester moiety, vinylarenes, and N-substituted maleimides are preferred, and (meth)acrylic acid esters (excluding methyl methacrylate) having 1 to 20 carbon atoms in the ester moiety and N-substituted maleimides are particularly preferred.

Here, when an N-substituted maleimide is used as the other monomer, using a persulfate as the polymerization initiator may result in a large change in the yellowness of the film. For this reason, the content of sulfate ions in the aqueous solution used to measure the electrical conductivity at 23°C and 50% RH described above is preferably 350 ppm or less, more preferably 150 ppm or less, and particularly preferably 100 ppm or less. Here, sulfate ions are generated by hydrolysis of persulfate.

Since the acrylic resin powder of this embodiment is used to manufacture a film by a solution casting method, it is preferable that the other monomers include a drying-accelerating comonomer that can increase the evaporation rate of the solvent.

Drying-accelerating comonomers include N-substituted maleimides, methacrylic acid esters 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 esters in which the ester moiety is a saturated hydrocarbon group having 7 to 16 carbon atoms and a condensed ring structure, methacrylic acid esters in which the ester moiety is a linear or branched group containing an ether bond, vinyl arenes, etc., and two or more of these may be used in combination. The use of drying-accelerating comonomers increases the heat resistance of the acrylic resin and also increases the rate at which the solvent evaporates from the dope solution cast onto the surface of the support during film production.

Examples of N-substituted maleimides include N-phenylmaleimide, N-benzylmaleimide, N-cyclohexylmaleimide, and N-methylmaleimide. Among these, N-phenylmaleimide and N-cyclohexylmaleimide are preferred.

Methacrylic acid esters in which the ester moiety is a primary or secondary hydrocarbon group having 2 to 8 carbon atoms, or an aromatic hydrocarbon group, include, for example, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, phenyl methacrylate, and benzyl methacrylate. Among these, ethyl methacrylate, n-butyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, and benzyl methacrylate are preferred.

Examples of methacrylic acid esters in which the ester moiety is a saturated hydrocarbon group having 7 to 16 carbon atoms and a condensed ring structure include dicyclopentanyl methacrylate and isobornyl methacrylate. The saturated hydrocarbon group preferably has 8 to 14 carbon atoms, and more preferably has 9 to 12 carbon atoms. The condensed ring structure is preferably a structure in which two five-membered rings are condensed by three consecutive carbon atoms.

An example of a methacrylic acid ester in which the ester moiety is a linear or branched group containing an ether bond is 2-methoxyethyl methacrylate.

Vinyl arenes include, for example, styrene, α-methylstyrene, monochlorostyrene, dichlorostyrene, etc. Among these, styrene is preferred.

The content of structural units derived from other monomers in the acrylic resin 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. On the other hand, since the optical properties and heat resistance of the film can be adjusted, the content of structural units derived from other monomers in the acrylic resin is preferably 0.1% by weight or more, more preferably 1% by weight or more, even more preferably 3% by weight or more, and particularly preferably 5% by weight or more.

From the viewpoint of the heat resistance of the film, it is preferable that the acrylic resin has a structural unit containing a heterocycle in the main chain. Examples of heterocycles include glutarimide rings, lactone rings, maleic anhydride rings, maleimide rings, glutaric anhydride rings, etc., and two or more of them may be used in combination. Among these, from the viewpoint of the heat resistance and optical properties of the film, glutarimide rings, lactone rings, and maleimide rings are preferred.

(Acrylic resin polymerization method)
The polymerization method of the monomers constituting the acrylic resin is not particularly limited, but from the viewpoints of the degree of freedom in the structural design of the acrylic resin, the ease of polymerization, productivity, etc., emulsion polymerization and suspension polymerization are preferred. Here, when N-substituted maleimide is used as the other monomer, the N-substituted maleimide remaining without polymerization tends to hydrolyze and color the acrylic resin, but from the viewpoint of reducing the amount of remaining N-substituted maleimide, emulsion polymerization is preferred. At this time, a mixture in which the N-substituted maleimide and a monomer other than the N-substituted maleimide constituting the acrylic resin are dissolved and mixed in advance may be supplied to a reactor and reacted. In addition, the N-substituted maleimide may be directly supplied to a reactor containing a monomer other than the N-substituted maleimide constituting the acrylic resin, and dissolved and mixed in the reactor and reacted. Here, a mixture in which the N-substituted maleimide and a monomer other than the N-substituted maleimide constituting the acrylic resin are dissolved and mixed may be supplied directly to a reactor and reacted. However, for example, when the polymerization reactivity of the N-substituted maleimide with a monomer other than the N-substituted maleimide constituting the acrylic resin is low, the N-substituted maleimide tends to remain. Therefore, by supplying the mixture to a reactor and reacting it, and then supplying a monomer other than the N-substituted maleimide constituting the acrylic resin and reacting it, the N-substituted maleimide can be effectively polymerized, and the amount of remaining N-substituted maleimide, which is the cause of coloring of the acrylic resin, can be reduced.

In order to prevent foaming marks on the film, it is preferable that the acrylic resin is produced by emulsion polymerization of the monomers in the presence of an ionic emulsifier. In this case, the ionic emulsifier may be added all at once or gradually. For example, if the latex obtained by emulsion polymerization of the monomers in the presence of an ionic emulsifier is dried without washing, an acrylic resin powder containing the ionic emulsifier is obtained.

The ionic emulsifier may be any of a cationic emulsifier, an anionic emulsifier, and an amphoteric emulsifier, but is preferably an anionic emulsifier.

Examples of anionic emulsifiers include dialkyl sulfosuccinates, alkanesulfonates, α-olefinsulfonates, alkylbenzenesulfonates, naphthalenesulfonate-formaldehyde condensates, alkylnaphthalenesulfonates, N-methyl-N-acyltaurate salts, etc. Among these, dialkyl sulfosuccinates and alkylbenzenesulfonates are preferred from the viewpoints of suppressing foam marks on the film and improving polymerization stability in emulsion polymerization.

Examples of anionic emulsifiers include lithium salts, sodium salts, potassium salts, calcium salts, and magnesium salts. Among these, lithium salts, sodium salts, and potassium salts are preferred from the viewpoint of foaming marks on the film.

The weight ratio of the ionic emulsifier to the monomer is preferably 0.1% to 10%, more preferably 0.3% to 7%, even more preferably 0.4% to 6%, even more preferably 0.5% to 5%, particularly preferably 0.8% to 3%, and most preferably 1% to 3%. When the weight ratio of the ionic emulsifier to the monomer is 10% or less, foaming marks on the film are suppressed, and when it is 10% or less, the polymerization stability of the emulsion polymerization is improved.

When polymerizing the monomers, a polymerization initiator can be used. Examples of polymerization initiators include persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate; and organic peroxides such as t-butyl hydroperoxide, t-butylperoxyisopropyl carbonate, cumene hydroperoxide, p-menthane hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, bis(3,5,5-trimethylhexanoyl) peroxide, dilauroyl peroxide, and benzoyl peroxide.

When an organic peroxide is used as a polymerization initiator, the organic peroxide may be cleaved by thermal decomposition alone to generate radicals, which are then used to polymerize the monomer. Alternatively, an organic peroxide may be combined with an oxidizing agent such as ferrous sulfate and a reducing agent such as sodium formaldehyde sulfoxylate to generate radicals at low temperatures, and the monomer may be polymerized by redox polymerization (see, for example, Japanese Patent No. 3,960,631).

When polymerizing the monomers, a chain transfer agent may be used to adjust the molecular weight of the acrylic resin. Examples of chain transfer agents include thioglycolic acid esters such as alkyl mercaptans, alkyl sulfides, alkyl disulfides, and 2-ethylhexyl thioglycolate; mercapto acids such as α-methylstyrene dimer and β-mercaptopropionic acid; and aromatic mercaptans such as benzyl mercaptan, thiophenol, thiocresol, and thionaphthol.

(Graft Copolymer)
The acrylic resin constituting the acrylic resin powder of the present embodiment may be a graft copolymer having a core-shell structure, which increases the mechanical strength of the film, such as bending resistance and crack resistance.

A graft copolymer having a core-shell structure can be obtained, for example, by polymerizing a monomer composition in the presence of crosslinked polymer particles (core layer) to form a shell layer. The core layer and shell layer may each be composed of one layer, or two or more layers. There are no particular limitations on the method for producing a graft copolymer having a core-shell structure, but examples include a method in which a monomer composition mainly composed of an acrylic acid ester is polymerized in the presence of a crosslinking agent to form rubber-like polymer particles, and then a monomer composition mainly composed of a methacrylic acid ester is polymerized.

A graft copolymer having a core-shell structure can be obtained, for example, by emulsion polymerization of a monomer composition. From the viewpoint of suppressing foaming marks on the film, it is preferable to emulsion polymerize the monomer composition in the presence of an ionic emulsifier. This suppresses mutual aggregation of the graft copolymer particles having a core-shell structure, resulting in a good dispersion state and improved stability over time of the dope solution.

(Dope solution)
In the dope solution of the present embodiment, the acrylic resin powder of the present embodiment is dissolved in a solvent. The solvent is not particularly limited as long as it is a good solvent for the acrylic resin powder of the present embodiment, and examples of the solvent include chlorine-based organic solvents such as methylene chloride, and non-chlorine-based organic solvents such as methyl acetate, ethyl acetate, acetone, methyl ethyl ketone, and tetrahydrofuran. Among these, methylene chloride is preferred from the viewpoint of the solubility of the acrylic resin powder of the present embodiment.

The solvent preferably contains alcohol. Alcohol is a poor solvent for the acrylic resin powder of this embodiment, and not only does it increase the drying efficiency of the dope solution of this embodiment, but it also increases the adhesion of the film to other substrates such as a polarizer. The alcohol is not particularly limited, but examples include linear or branched aliphatic alcohols having 1 to 4 carbon atoms. Among these, ethanol and methanol are preferred.

The alcohol content in the solvent is preferably 1% by weight or more and 25% by weight or less, more preferably 2% by weight or more and 20% by weight or less, and even more preferably 3% by weight or more and 15% by weight or less.

The dope solution of this embodiment may further contain, as necessary, known additives such as light stabilizers, ultraviolet absorbers, heat stabilizers, antioxidants, matting agents, light diffusing agents, colorants, dyes, pigments, antistatic agents, heat reflecting materials, lubricants, plasticizers, and fillers, styrene resins such as acrylonitrile-styrene resin and styrene-maleic anhydride resin, fluororesins such as polycarbonate resin, polyvinyl acetal resin, cellulose acylate resin, polyvinylidene fluoride, and polyfluorinated alkyl (meth)acrylate resin, silicone resin, polyolefin resin, polyethylene terephthalate resin, and polybutylene terephthalate resin, and other resins, as well as inorganic fine particles having birefringence (see Patent No. 3648201 and Patent No. 4336586), low molecular weight compounds having birefringence and a molecular weight of 5000 or less, preferably 1000 or less (see Patent No. 3696649), etc.

The dope solution of this embodiment is produced by dissolving the acrylic resin powder of this embodiment in a solvent. When dissolving the acrylic resin powder of this embodiment in a solvent, the temperature and pressure can be adjusted as appropriate. After dissolving the acrylic resin powder of this embodiment in a solvent, post-treatment such as filtration and degassing can be performed as necessary.

(film)
The film of the present embodiment is produced by casting the dope solution of the present embodiment on the surface of the support and then volatilizing the solvent. For example, the dope solution of the present embodiment delivered by the liquid delivery pump is first cast from the slit of the pressure 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 casting film, and then the casting film is heated on the support to volatilize the solvent, thereby producing the film of the present embodiment. The support is not particularly limited, but examples thereof include a PET film and a glass plate. The temperature conditions for volatilizing the solvent can be appropriately determined depending on the boiling point of the solvent used. Next, after peeling the film from the support, post-treatment such as drying, heating, and stretching can also be performed as necessary.

The thickness of the film of this embodiment is preferably 5 μm or more and 200 μm or less, more preferably 5 μm or more and 100 μm or less, and particularly preferably 10 μm or more and 80 μm or less. When the thickness of the film of this embodiment is 5 μm or more, the handling property and the function as a protective film are improved, and when the thickness is 200 μm or less, the uniformity of the optical properties and the drying speed are improved.

The film of this embodiment contains an acrylic resin, which has a structural unit containing a heterocycle in the main chain, has a glass transition temperature of 110°C or higher, and has a weight average molecular weight of 400,000 or more and 4,000,000 or less.

When the thickness of the film of this embodiment is 40 μm, the change in haze ΔHz before and after storage for 96 hours under conditions of 85° C. and 95% RH is 2.5% or less, preferably 2% or less, and more preferably 1.5% or less. When ΔHz is 2.5% or less, the film of this embodiment can be suitably applied to optical components that require light transparency.

In this case, when the thickness of the film of this embodiment is 40 μm, the haze Hz is preferably 2% or less, more preferably 1.5% or less, 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 Hz is 2% or less, the film of this embodiment can be suitably applied to optical components that require light transparency.

In addition, when the thickness of the film of this embodiment is 40 μm, the change in yellowness ΔYI before and after storage for 96 hours under conditions of 85° C. and 95% RH is 3.5 or less, preferably 3.0 or less, and more preferably 2.0 or less. When ΔYI is 3.5 or less, the film of this embodiment can be suitably applied to optical components that require light transparency.

In this case, when the thickness of the film of this embodiment is 40 μm, the yellowness index YI is preferably 2.0 or less, more preferably 1.5 or less, more preferably 1.0 or less, even more preferably 0.8 or less, even more preferably 0.65% or less, and particularly preferably 0.5% or less. When YI is 2.0 or less, the film of this embodiment can be suitably applied to optical components that require light transparency.

The film of this embodiment can be suitably used, for example, as an optical film for displays. Examples of optical films for displays include protective films such as polarizer protective films.

When the film of this embodiment is used as a polarizer protective film, it is preferable that the film of this embodiment has small optical isotropy. In particular, it is preferable that the film of this embodiment has small optical isotropy not only in the in-plane directions (length direction, width direction) but also in the thickness direction. Specifically, the absolute value of the in-plane retardation of the film of this embodiment 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 thickness direction retardation of the film of this embodiment 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.

Here, the retardation is an index value calculated based on birefringence, and the in-plane retardation (Re) and the thickness direction retardation (Rth) are expressed by the formula: Re=(nx-ny)×d
Rth = ((nx + ny) / 2 - nz) x d
(In the formula, nx, ny, and nz are the refractive indexes in the respective axial directions, where the stretching direction (the orientation direction of the polymer chain) in the plane of the molded article is the X axis, the direction perpendicular to the X axis is the Y axis, and the thickness direction of the film is the Z axis, d is the thickness of the molded article, and nx-ny is the orientation birefringence.)
In an ideal molded body having perfect optical isotropy, the in-plane retardation (Re) and thickness direction retardation (Rth) are 0. The MD direction of the molded body is taken as the X-axis, but in the case of a stretched molded body, the stretching direction is taken as the X-axis.

The orientation birefringence of the film of this embodiment is preferably -2.6 x 10 ^-4 or more and 2.6 x 10 ^-4 or less, more preferably -1.7 x 10 ^-4 or more and 1.7 x 10 ^-4 or less, even more preferably -1.0 x 10 ^-4 or more and 1.0 x 10 ^-4 or less, particularly preferably -0.5 x 10 ^-4 or more and 0.5 x 10 ^-4 or less, and most preferably -0.2 x 10 ^-4 or more and 0.2 x 10 ^-4 or less. When the orientation birefringence of the film of this embodiment is -2.6 x 10 ^-4 or more and 2.6 x 10 ^-4 or less, the birefringence during the molding process of the film of this embodiment is suppressed, and the optical characteristics are stabilized. Therefore, the film of this embodiment can be used as an optical film used in liquid crystal displays and the like.

The film of this embodiment has a photoelastic constant of preferably -6 x 10 ^-12 Pa ^-1 or more and 6 x 10 ^-12 Pa ^-1 or less, more preferably -4 x 10 ^-12 Pa ^-1 or more and 4 x 10 ^-12 Pa ^-1 or less, even more preferably -2 x 10 ^-12 Pa ^-1 or more and 2 x 10 ^-12 Pa ^-1 or less, even more preferably -1 x 10 ^-12 Pa ^-1 or more and 1 x 10 ^-12 Pa ^-1 or less, particularly preferably -0.5 x 10 ^-12 Pa ^-1 or more and 0.5 x 10 ^-12 Pa ^-1 or less, and most preferably -0.2 x 10 ^-12 Pa ^-1 or more and 0.2 x 10 ^-12 Pa ^-1 or less. When the photoelastic constant of the film of this embodiment is -6×10 ^-12 Pa ^-1 or more and 6×10 ^-12 Pa ^-1 or less, even when the film of this embodiment is subjected to stress and deformed, birefringence is suppressed and optical isotropy is reduced. For example, when the film of this embodiment is used as a polarizer protective film, even if the liquid crystal panel is deformed during transportation due to the influence of moisture in the air or temperature, the optical properties are maintained and image quality is less likely to deteriorate.

The above describes an embodiment of the present invention, but the present invention is not limited to the above embodiment, and the above embodiment may be modified as appropriate within the scope of the spirit of the present invention.

Examples of the present invention will be described below, but the present invention is not limited to these examples. The abbreviations respectively mean the following substances. The number of parts of each substance is based on weight and represents the pure content (solid content).
Monomers BA: butyl acrylate ALMA: allyl methacrylate MMA: methyl methacrylate n-BMA: n-butyl methacrylate PhMI: N-phenylmaleimide ChMI: N-cyclohexylmaleimide 2-EHMA: 2-ethylhexyl methacrylate Anionic emulsifier SDSS: sodium dioctyl sulfosuccinate Polymerization initiator t-BHP: t-butyl hydroperoxide FeSO ₄ : ferrous sulfate heptahydrate SFS: sodium formaldehyde sulfoxylate NPS: sodium persulfate LPO: lauroyl peroxide Chain transfer agent 2-EHTG: 2-ethylhexyl thioglycolate Suspension agent HPMC: hydroxypropyl methylcellulose; Metolose 60SH50 (Shin-Etsu Chemical Co., Ltd.)
Others EDTA: Disodium ethylenediaminetetraacetate DSHP: Disodium hydrogen phosphate anhydrous

Example 1
In an 8L glass reactor equipped with a paddle stirrer, deionized water (110 parts), sodium hydroxide (0.004 parts), and SDSS (0.6 parts) were charged, and the mixture was stirred at a rotation speed of 175 rpm, and the temperature was raised to 60° C. while replacing the inside of the reactor with nitrogen. Next, a mixture of BA (8.5 parts) and ALMA (0.043 parts) was added to the reactor, and then t-BHP (0.006 parts), EDTA (0.0055 parts), FeSO ₄ (0.0015 parts), and SFS (0.005 parts) were added to the reactor in sequence, and BA and ALMA were emulsion-polymerized to obtain a latex containing crosslinked polymer particles (cores). Here, after the addition of the mixture was completed, BA and ALMA were emulsion-polymerized for 30 minutes. At this time, the polymerization conversion rate was 99.5%, and the volume average particle size was 850 Å. Next, a mixture of MMA (83.5 parts), n-BMA (1 part), PhMI (7 parts), and 2-EHTG (0.037 parts) was continuously added to the reactor over 70 minutes, and MMA, n-BMA, and PhMI were emulsion-polymerized to form a shell layer. At this time, SDSS (0.3 parts) was continuously added to the reactor in conjunction with the addition of the mixture. In addition, the rotation speed was adjusted to 200 rpm at the start of the addition of the mixture, 235 rpm 10 minutes after the start of the addition, 260 rpm 20 minutes after the start of the addition, 300 rpm 35 minutes after the start of the addition, and 325 rpm 50 minutes after the start of the addition. Next, SFS (0.0052 parts), SDSS (0.2 parts), and t-BHP (0.005 parts) were added to the reactor in sequence, and the temperature was raised to 90° C., and the mixture was reacted for 120 minutes to complete the polymerization, and a latex containing a graft copolymer having a core-shell structure was obtained. At this time, the polymerization conversion rate was 99.9%, and the volume average particle diameter was 1690 Å. Next, the latex was evaporated to dryness in a hot air oven at 50° C. for 24 hours to obtain a white acrylic resin powder having a volume average particle diameter of 120 μm. The volatile component content of the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 791,000.

Example 2
A white acrylic resin powder having a volume average particle size of 120 μm was obtained in the same manner as in Example 1, except that PhMI was replaced with ChMI. The volatile component content of the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 869,000.

The polymerization conversion rate of the latex containing crosslinked polymer particles (core) was 97.0%, and the volume average particle diameter was 700 Å. The polymerization conversion rate of the latex containing the graft copolymer with a core-shell structure was 99.9%, and the volume average particle diameter was 1475 Å.

Example 3
In an 8L glass reactor equipped with a paddle stirrer, deionized water (110 parts), sodium hydroxide (0.004 parts), and SDSS (0.3 parts) were charged, and the mixture was stirred at a rotation speed of 175 rpm, and the temperature was raised to 60° C. while replacing the inside of the reactor with nitrogen. Next, a mixture of BA (8.5 parts) and ALMA (0.043 parts) was added to the reactor, and then t-BHP (0.009 parts), EDTA (0.0055 parts), FeSO ₄ (0.0015 parts), and SFS (0.0076 parts) were added to the reactor in sequence, and BA and ALMA were emulsion-polymerized to obtain a latex containing crosslinked polymer particles (cores). Here, after the addition of the mixture was completed, BA and ALMA were emulsion-polymerized for 30 minutes. At this time, the polymerization conversion rate was 99.1%, and the volume average particle size was 750 Å. Next, a mixture of MMA (80.6 parts), n-BMA (1 part), PhMI (5 parts), and 2-EHTG (0.029 parts) was continuously added to the reactor over 135 minutes, and MMA, n-BMA, and PhMI were emulsion-polymerized to form a shell layer. At this time, SDSS (0.6 parts) was continuously added to the reactor in conjunction with the addition of the mixture. In addition, the rotation speed was adjusted to 200 rpm at the start of the addition of the mixture, 235 rpm 10 minutes after the start of the addition, 260 rpm 20 minutes after, 300 rpm 35 minutes after, and 325 rpm 50 minutes after. Next, 15 minutes after the end of the addition of the mixture, MMA (5 parts) was continuously added to the reactor over 5 minutes, and reacted for 30 minutes to react PhMI in the shell layer. Next, SFS (0.0026 parts) and t-BHP (0.005 parts) were added to the reactor in this order, and the temperature was raised to 80°C. The reaction was continued for 120 minutes to complete the polymerization, and a latex containing a graft copolymer having a core-shell structure was obtained. At this time, the polymerization conversion rate was 100.0%, and the volume average particle diameter was 1610 Å. Next, the latex was evaporated to dryness in a hot air oven at 50°C for 24 hours, and a white acrylic resin powder having a volume average particle diameter of 120 μm was obtained. The content of volatile components in the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 770,000.

Example 4
In an 8L glass reactor equipped with a paddle stirrer, deionized water (110 parts), sodium hydroxide (0.004 parts), and SDSS (0.15 parts) were charged, and the mixture was stirred at a rotation speed of 175 rpm, and the temperature was raised to 60° C. while the inside of the reactor was replaced with nitrogen. Next, t-BHP (0.024 parts), EDTA (0.0055 parts), FeSO ₄ (0.0015 parts), and SFS (0.02 parts) were added to the reactor in sequence, and then a mixture of BA (8.5 parts) and ALMA (0.043 parts) was continuously added to the reactor over 20 minutes, and BA and ALMA were emulsion-polymerized to obtain a latex containing crosslinked polymer particles (cores). Here, after the addition of the mixture was completed, BA and ALMA were emulsion-polymerized for 30 minutes. At this time, the polymerization conversion rate was 99.9%, and the volume average particle size was 775 Å. Next, a mixture of MMA (83.5 parts), n-BMA (1 part), PhMI (7 parts), and 2-EHTG (0.02 parts) was continuously added to the reactor over 70 minutes, and MMA, n-BMA, and PhMI were emulsion-polymerized to form a shell layer. At this time, SDSS (0.75 parts) was continuously added to the reactor in conjunction with the addition of the mixture. In addition, the rotation speed was adjusted to 200 rpm at the start of the addition of the mixture, 235 rpm 10 minutes after the start of the addition, 260 rpm 20 minutes after the start of the addition, 300 rpm 35 minutes after the start of the addition, and 325 rpm 50 minutes after the start of the addition. Next, SFS (0.01 parts), SDSS (0.2 parts), and t-BHP (0.01 parts) were added to the reactor in sequence, and the temperature was raised to 90° C., and the mixture was reacted for 120 minutes to complete the polymerization, and a latex containing a graft copolymer having a core-shell structure was obtained. At this time, the polymerization conversion rate was 100.0%, and the volume average particle diameter was 1670 Å. Next, the latex was evaporated to dryness in a hot air oven at 50° C. for 24 hours to obtain a white acrylic resin powder having a volume average particle diameter of 120 μm. The volatile component content of the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 753,000.

Example 5
In an 8L glass reactor equipped with a paddle stirrer, deionized water (110 parts), sodium hydroxide (0.004 parts), and SDSS (0.2 parts) were charged, and the mixture was stirred at a rotation speed of 175 rpm, and the temperature was raised to 80° C. while the inside of the reactor was replaced with nitrogen. Next, NPS (0.0321 parts) and SFS (0.0005 parts) were added to the reactor in sequence, and then a mixture of BA (8.5 parts) and ALMA (0.043 parts) was added to the reactor continuously over 20 minutes, and BA and ALMA were emulsion-polymerized to obtain a latex containing crosslinked polymer particles (cores). Here, after the addition of the mixture was completed, BA and ALMA were emulsion-polymerized for 30 minutes. At this time, the polymerization conversion rate was 99.2%, and the volume average particle size was 590 Å. Next, a mixture of MMA (83.5 parts), n-BMA (1 part), PhMI (7 parts), and 2-EHTG (0.0235 parts) was continuously added to the reactor over 70 minutes to emulsion-polymerize MMA, n-BMA, and PhMI to form a shell layer. At this time, SDSS (0.7 parts) was continuously added to the reactor in conjunction with the addition of the mixture. The rotation speed was adjusted to 200 rpm at the start of the addition of the mixture, 235 rpm 10 minutes after the start of the addition, 260 rpm 20 minutes after the start of the addition, 300 rpm 35 minutes after the start of the addition, and 325 rpm 50 minutes after the start of the addition. Next, EDTA (0.0055 parts), FeSO ₄ (0.0015 parts), SFS (0.0077 parts), SDSS (0.2 parts), and t-BHP (0.015 parts) were added to the reactor in order, and the temperature was raised to 90° C., and the reaction was continued for 120 minutes to complete the polymerization, and a latex containing a graft copolymer having a core-shell structure was obtained. At this time, the polymerization conversion rate was 100.0%, and the volume average particle diameter was 1310 Å. Next, the latex was evaporated to dryness in a hot air oven at 50° C. for 24 hours, and a white acrylic resin powder having a volume average particle diameter of 120 μm was obtained. The content of volatile components in the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 692,000.

(Comparative Example 1)
In an 8L glass reactor equipped with a paddle stirrer, deionized water (110 parts), sodium hydroxide (0.004 parts), and SDSS (0.2 parts) were charged, and the mixture was stirred at a rotation speed of 175 rpm, and the temperature was raised to 80° C. while the inside of the reactor was replaced with nitrogen. Next, NPS (0.0321 parts) and SFS (0.0005 parts) were added to the reactor in sequence, and then a mixture of BA (8.5 parts) and ALMA (0.043 parts) was added to the reactor continuously over 20 minutes, and BA and ALMA were emulsion-polymerized to obtain a latex containing crosslinked polymer particles (cores). Here, after the addition of the mixture was completed, BA and ALMA were emulsion-polymerized for 30 minutes. At this time, the polymerization conversion rate was 99.5%, and the volume average particle size was 600 Å. Next, a mixture of MMA (83.5 parts), n-BMA (1 part), PhMI (7 parts), and 2-EHTG (0.02 parts) was continuously added to the reactor over 70 minutes to emulsion-polymerize MMA, n-BMA, and PhMI to form a shell layer. At this time, SDSS (0.7 parts) was continuously added to the reactor in conjunction with the addition of the mixture. The rotation speed was adjusted to 200 rpm at the start of the addition of the mixture, 235 rpm 10 minutes after the start of the addition, 260 rpm 20 minutes after the start of the addition, 300 rpm 35 minutes after the start of the addition, and 325 rpm 50 minutes after the start of the addition. Next, EDTA (0.0055 parts), FeSO ₄ (0.0015 parts), SFS (0.0616 parts), SDSS (0.2 parts), and t-BHP (0.06 parts) were added to the reactor in order, and the reaction was carried out for 60 minutes to complete the polymerization, thereby obtaining a latex containing a graft copolymer having a core-shell structure. At this time, the polymerization conversion rate was 99.8%, and the volume average particle diameter was 1310 Å. Next, the latex was evaporated to dryness in a hot air oven at 50° C. for 24 hours, obtaining a white acrylic resin powder having a volume average particle diameter of 120 μm. The content of volatile components in the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 683,000.

(Comparative Example 2)
In Comparative Example 1, a white acrylic resin powder having a volume average particle size of 120 μm was obtained in the same manner as in Comparative Example 1, except that the latex containing the graft copolymer having a core-shell structure was cooled to 50° C., and then EDTA (0.025 parts) and sodium hydroxide (0.04 parts) were added to the reactor in that order. The volatile component content of the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 683,000.

(Comparative Example 3)
A white acrylic resin powder having a volume average particle size of 120 μm was obtained in the same manner as in Comparative Example 1, except that PhMI was replaced with ChMI. The volatile component content of the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 645,000.

The polymerization conversion rate of the latex containing crosslinked polymer particles (core) was 93.5%, and the volume average particle diameter was 500 Å. The polymerization conversion rate of the latex containing the graft copolymer with a core-shell structure was 100.0%, and the volume average particle diameter was 1120 Å.

(Comparative Example 4)
In an 8L glass reactor equipped with a paddle-type stirrer, deionized water (170 parts) and DSHP (0.1 parts) were charged, and the temperature was raised to 40 ° C. while stirring at a rotation speed of 300 rpm and replacing the inside of the reactor with nitrogen. Next, LPO (0.3 parts) was added to the reactor, and then a mixture of MMA (85 parts), 2-EHMA (5 parts), and PhMI (10 parts) was continuously added to the reactor over 30 minutes. Here, after the addition of the mixture was completed, the mixture was stirred for 30 minutes. Next, HPMC (0.375 parts) was continuously added to the reactor over 30 minutes, and then stirred for 30 minutes. Next, the reactor was heated to 65 ° C., and MMA, 2-EHMA and PhMI were suspension polymerized. At this time, the reactor was heated to 65 ° C. and reached 85 ° C. 100 minutes after it was heated, and then the temperature was gradually lowered. Next, the reactor was heated to 95°C, and the temperature was maintained for 60 minutes to complete the polymerization, and a slurry containing bead-like particles was obtained. At this time, the polymerization conversion rate was 99.5%, and the volume average particle diameter was 50 μm. Next, the slurry was evaporated to dryness in a hot air oven at 50°C for 24 hours, and a white acrylic resin powder with a volume average particle diameter of 55 μm was obtained. The content of volatile components in the acrylic resin powder was less than 0.5% by weight. The weight average molecular weight of the acrylic resin powder was 1,900,000.

<Test Method>
(Polymerization Conversion Rate)
After taking about 2 g of sample (latex or slurry), the weight of the sample was precisely weighed. Next, the sample was dried in a hot air oven at 120°C for 1 hour, and the weight of the dried sample was precisely weighed as the solid content. Next, the ratio of the sample weights before and after drying was calculated as the solid content ratio in the sample. Finally, the formula {(total weight of charged raw materials x solid content ratio - total weight of raw materials other than water and monomers) / weight of charged monomer} x 100
The polymerization conversion was calculated based on the above formula: The polyfunctional monomer and the chain transfer agent were treated as charged monomers.

(Volume average particle size of latex)
The volume average particle size of the latex was measured by dynamic light scattering using Microtrac UPA150 (manufactured by Nikkiso Co., Ltd.).

(Volume average particle size of slurry)
The volume average particle size of the slurry was measured by a laser diffraction scattering method using a Microtrac MT3300EXII (manufactured by Nikkiso Co., Ltd.).

(Volume average particle size of acrylic resin powder)
Acrylic resin powder particles were dispersed at a predetermined concentration in water containing a surfactant, and then the volume average particle size of the acrylic resin powder particles was measured by a laser diffraction scattering method using a Microtrac MT3300EXII (manufactured by Nikkiso Co., Ltd.).

(Content of Volatile Components)
The content of volatile components in the acrylic resin powder particles was measured using a heat-drying moisture meter MX-50 (manufactured by A&D Co., Ltd.) equipped with a straight-tube halogen lamp. Specifically, the content of volatile components in the acrylic resin powder particles was measured using 5 g of acrylic resin powder particles under conditions of a maximum temperature of 130° C. and a measurement accuracy of MID (minimum display 0.01%, 0.05%/min).

(Weight average molecular weight)
The weight average molecular weight of the acrylic resin powder was calculated by the standard polystyrene conversion method using gel permeation chromatography (GPC). At this time, a TSK gel Super HZM-H (manufactured by Tosoh) filled with polystyrene crosslinked gel was used as the GPC column, and tetrahydrofuran (THF) was used as the GPC solvent. In addition, as the sample solution, 20 mg of the acrylic resin powder was dissolved in 10 ml of THF, and then filtered through a membrane filter with a pore size of 0.2 μm. The column temperature of the GPC was set to 40 ° C.

(Electrical Conductivity)
After weighing 8 g of acrylic resin powder and 80 g of ultrapure water in a Teflon (registered trademark) container, the Teflon (registered trademark) container was placed in a pressure-resistant container, and the outer lid was closed with a special tool to seal it. Next, the pressure-resistant container was gently shaken, the acrylic resin powder and ultrapure water were mixed, and the pressure-resistant container was placed in a hot air oven at 100 ° C. and heated for 20 hours. Next, the pressure-resistant container was left until it was completely cooled, and then the outer lid was loosened with a special tool to remove the Teflon (registered trademark) container. Next, the liquid in the Teflon (registered trademark) container was sucked out, and then filtered with a membrane filter with a pore size of 0.2 μm to obtain an aqueous solution (filtrate). Next, the aqueous solution was stored in a thermostatic room at 23 ° C. and 50% RH for 24 hours, and then the electrical conductivity of the aqueous solution was measured using a compact electrical conductivity meter LAQUAtwin EC-33B (manufactured by HORIBA).

If the volume average particle diameter of the acrylic resin powder is greater than 250 μm, the acrylic resin powder is crushed until the volume average particle diameter is 100 μm or more and 250 μm or less, and then the electrical conductivity of the aqueous solution is measured in the same manner as above. The method for measuring the volume average particle diameter of the acrylic resin powder is the same as described above. Examples of devices for crushing the acrylic resin powder include a mortar and a food cutter.

(Sulfate ion content)
The content of sulfate ions in the aqueous solution used for measuring the electrical conductivity was measured using an ion chromatograph (manufactured by Shimadzu Corporation) under the following measurement conditions: Specifically, the peak area derived from sulfate ions in the aqueous solution was measured, and then the content of sulfate ions (SO ₄ ²⁻ ) was calculated from the ratio to the peak area derived from sulfate ions in a standard solution whose sulfate ion content was known in advance.
<Measurement conditions>
Electrical conductivity detector: CDD-10A VP (manufactured by Shimadzu)
Analytical column: Shim-pack IC-SA2 (manufactured by Shimadzu) with an inner diameter of 4 mm and a length of 250 mm
Guard column: Shim-pack with an inner diameter of 4.6 mm and a length of 10 mm, IC-SA2(G) (manufactured by Shimadzu)
Eluent: aqueous solution of 0.6 _{mmol Na2CO3 and} 12 mmol _NaHCO3 Eluent flow rate: 1.0 mL/min

(Film foaming marks)
Acrylic resin powder was added to a mixed solvent of methylene chloride and methanol (weight ratio 8:2) so that the concentration was 10% by weight, and then the mixture was stirred and mixed with a magnetic stirrer to prepare a transparent dope solution. Next, the dope solution was cast on a glass plate as a support with a thickness of 1.1 mm using a bar coater. Next, the glass plate on which the dope solution was cast was dried at room temperature for 10 minutes, and then the semi-dried film was peeled off from the glass plate. Next, the semi-dried film was cut to a size of 5.5 cm x 5.5 cm, and then dried in a drying oven at 175 ° C. for 7 minutes while fixed to a metal frame of 6 cm x 6 cm to obtain a film for evaluating foam marks. Next, the surface of the film for evaluating foam marks was observed using an optical microscope to evaluate the foam marks. The state of the foam marks was sensorily evaluated on a five-level scale from 1 (good) to 5 (bad) based on the following index.
1: No foaming marks on the film surface, very clean surface 2: A few foaming marks are visible on the film surface, but the surface is generally clean 3: Foaming marks are visible on the film surface, but only a few are visible 4: Foaming marks are visible, but not all over the film, and many are visible 5: Foaming marks are visible all over the film

(Film for constant temperature and humidity testing)
The acrylic resin powder was added to a mixed solvent of methylene chloride and methanol (weight ratio 8:2) so that the concentration was 20% by weight, and then the mixture was stirred and mixed with a magnetic stirrer to prepare a transparent dope solution. Next, the dope solution was left to stand for 24 hours, degassed, and then cast onto a transparent glass plate as a support. At this time, the dope solution was applied in a uniform film shape using an applicator, and the clearance was adjusted so that the thickness of the film for constant temperature and humidity testing was 40 μm. Next, the transparent glass plate on which the dope solution was cast was dried at room temperature for 8 minutes, and then the semi-dried film was peeled off from the transparent glass plate. Next, the semi-dried film was fixed to a stainless steel frame, and then dried in a hot air oven at 160 ° C for 15 minutes to obtain a film for constant temperature and humidity testing.

(Constant temperature and humidity test)
After cutting out a 4 cm x 4 cm test piece from a 40 μm thick constant temperature and humidity test film, a constant temperature and humidity tester LHU-123 (manufactured by Espec) was used to carry out a constant temperature and humidity test on the test piece under conditions of 85 ° C. and 95% RH. The constant temperature and humidity test was carried out by measuring the haze (Hz), internal haze (internal Hz) and yellowness (YI) of the test piece before (initial value) and 96 hours after being placed in the constant temperature and humidity tester. In addition, the difference between the value 96 hours after being placed in the constant temperature and humidity tester and the initial value was calculated, and Δ haze (Δ Hz), Δ internal haze (Δ internal Hz) and Δ yellowness (Δ YI) were also obtained.

(Haze (Hz))
The haze (Hz) of the test piece was measured using a haze meter HZ-V3 (manufactured by Suga Test Instruments Co., Ltd.) according to the method described in JIS K7105.

(Internal Haze (Internal Hz))
The haze of the test piece was measured in a state where the influence of surface scattering of the test piece was cancelled, and the internal haze of the test piece was measured. Specifically, glycerin having a refractive index close to that of the acrylic resin powder was dropped on the front and back of the test piece, and then the test piece was sandwiched between glass plates so that the interface between the test piece and the glass plate was filled with glycerin. In this state, the haze was measured as described above, and the internal haze (internal Hz) of the test piece was calculated by subtracting the haze of the glass plate only, which was previously measured.

(Yellowness Index (YI))
The yellowness index (YI) of the test piece was measured in transmission mode using a color meter SC-P (manufactured by Suga Test Instruments Co., Ltd.) according to the method described in JIS K7373.

(Film for measuring phase difference)
The acrylic resin powder was added to a mixed solvent of methylene chloride and ethanol (weight ratio 9:1) so that the concentration was 10% by weight, and then the mixture was stirred and mixed with a magnetic stirrer to prepare a transparent dope solution. Next, the dope solution was left to stand for 24 hours, degassed, and then cast onto a PET film as a support. At this time, the dope solution was applied in a uniform film shape using an applicator, and the clearance was adjusted so that the thickness of the dried film was approximately 60 μm. In addition, Cosmoshine A4100 (manufactured by Toyobo) was used as the PET film. Next, the PET film on which the dope solution was cast was dried in a dry atmosphere at 40° C. for 1 hour, and then the semi-dried film was peeled off from the PET film. Next, the semi-dried film was fixed to a stainless steel frame, and then dried in a dry atmosphere at 140° C. for 60 minutes to obtain a dry film.

Using a stretching machine with a hot air oven function, the dry film was preheated for 5 minutes at a temperature condition of +10°C relative to the glass transition temperature of the dry film, and then the dry film was uniaxially stretched with a stretching speed of 100 mm/min and a stretching ratio of 1.4 times, with the width fixed, to obtain a film for phase difference measurement with a thickness of 40 μm.

(Phase difference)
After cutting out a test piece from the center of the film for retardation measurement, the in-plane retardation Re of the test piece was measured using an automatic birefringence meter KOBRA-WR (manufactured by Oji Measurement Co., Ltd.) under conditions of a wavelength of 590 nm and an incident angle of 0°. At this time, the thickness direction retardation Rth of the test piece was also measured under conditions of an incident angle of 40°. The test piece was moved to change the measurement location and measure the retardation three times each, and the average value was calculated.

(Film for measuring glass transition temperature)
The dried film obtained in the same manner as for the film for retardation measurement was fixed on a stainless steel frame and dried in a hot air oven at 175° C. for 10 minutes to obtain a film for glass transition temperature measurement.

(Glass Transition Temperature (Tg))
The Tg of the glass transition temperature measuring film was measured using a differential scanning calorimeter (DSC) Q1000 (manufactured by TA instruments). Specifically, the glass transition temperature measuring film was heated to 200°C at a heating rate of 10°C/min under a nitrogen gas flow, then rapidly cooled to 40°C, and heated again to 200°C at a heating rate of 10°C/min. The average value of the extrapolated glass transition onset temperature and the extrapolated glass transition end temperature was calculated for the glass transition observed during the second heating, and was taken as Tg.

Table 1 shows the evaluation results for the electrical conductivity, Tg, foaming marks on the film, Hz, internal Hz, and phase difference of the acrylic resin powder.

From Table 1, it can be seen that the use of the acrylic resin powders of Examples 1 to 5 suppresses foaming marks on the film and the decrease in transparency in a high-temperature, high-humidity environment. In contrast, the acrylic resin powders of Comparative Examples 1 to 4 have electrical conductivity of 325 to 386 μS/cm, which causes more foaming marks on the film and decreases transparency in a high-temperature, high-humidity environment.

Claims (16)

1. An acrylic resin powder,
   the acrylic resin powder and ultrapure water are mixed in a weight ratio of 1:10, the mixture is placed in a pressure-resistant container, and the mixture is heated at 100° C. for 20 hours. The aqueous solution obtained by filtering the mixture has an electrical conductivity of 300 μS/cm or less at 23° C. and 50% RH;
   An acrylic resin powder having a volatile component content of less than 1.0% by weight.
2. The acrylic resin powder according to claim 1, having a glass transition temperature of 110°C or higher.
3. The acrylic resin powder according to claim 1, wherein the acrylic resin contained in the acrylic resin powder has a structural unit containing a heterocycle in the main chain.
4. The acrylic resin powder according to claim 3, wherein the acrylic resin has structural units derived from an N-substituted maleimide.
5. The acrylic resin powder according to claim 4, wherein the N-substituted maleimide is N-cyclohexylmaleimide or N-phenylmaleimide.
6. The acrylic resin powder and granules according to claim 4, wherein the aqueous solution has a sulfate ion content of 350 ppm or less.
7. The acrylic resin powder according to claim 1, having a weight average molecular weight of 400,000 or more and 4,000,000 or less.
8. The acrylic resin powder and granules according to claim 1, which are used to manufacture films by a solution casting method.
9. A dope solution in which the acrylic resin powder according to any one of claims 1 to 8 is dissolved in a solvent.
10. A method for producing a dope solution, comprising dissolving the acrylic resin powder according to any one of claims 1 to 8 in a solvent to produce the dope solution.
11. A film produced by casting the dope solution according to claim 9 onto the surface of a support and then volatilizing the solvent.
12. A method for producing a film, comprising casting the dope solution according to claim 9 onto the surface of a support, and then volatilizing the solvent to produce a film.
13. Contains acrylic resin,
    The acrylic resin has a structural unit containing a heterocycle in its main chain, a glass transition temperature of 110° C. or higher, and a weight average molecular weight of 400,000 or higher and 4,000,000 or lower,
    When the thickness is 40 μm, the change in haze ΔHz before and after storage for 96 hours under conditions of 85° C. and 95% RH is 2.5% or less;
    A film having a thickness of 40 μm, in which the change in yellowness index ΔYI before and after storage for 96 hours under conditions of 85° C. and 95% RH is 3.5 or less.
14. The haze Hz is 2% or less when the thickness is 40 μm,
    The film according to claim 13, having a yellowness index YI of 2.0 or less when the film has a thickness of 40 μm.
15. The film according to claim 13 or 14, wherein the acrylic resin has structural units derived from an N-substituted maleimide.
16. The film of claim 15, wherein the N-substituted maleimide is N-cyclohexylmaleimide or N-phenylmaleimide.