Optical films, polarizing plates, and liquid crystal panels

Abstract

The present invention provides an optical film that reduces the yellowing of the liquid crystal panel when viewed from an oblique angle, while maintaining heat resistance and transparency. [Solution] The optical film comprises a resin composition, and the thickness direction phase difference Rth at a wavelength of 590 nm is between -35.0 nm and -5.0 nm. The resin composition comprises an acrylic resin having a methyl methacrylate unit content of 98% by weight or more and a tripletacticity of 55% or more, and a copolymer containing (meth)acrylonitrile units and aromatic vinyl units, and has a glass transition temperature of 120°C or higher. The copolymer has a (meth)acrylonitrile unit content of 22% by weight or more and 28% by weight or less. The optical film has an internal haze of 0.20% or less.

Description

[Technical Field]

[0001] This invention relates to optical films, polarizing plates, and liquid crystal panels. [Background technology]

[0002] Optical films containing acrylic resin have excellent transparency, color tone, appearance, heat resistance, and processability, and are therefore used, for example, in polarizer protective films (see, for example, Patent Document 1). Here, the polarizer protective film is applied to a liquid crystal panel by laminating it to both sides of a polarizer to form a polarizing plate, and then placing it on both sides of a liquid crystal cell.

[0003] On the other hand, IPS-type liquid crystal panels are preferred for applications such as LCD televisions because they offer a wide viewing angle and excellent color reproduction. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-25333 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, when an optical film containing acrylic resin is applied to an IPS-type liquid crystal panel, the black display on the panel may appear slightly yellowish when viewed from an oblique angle.

[0006] In this case, to reduce the yellowness when viewing the liquid crystal panel from an oblique angle, it is conceivable to use an optical film further containing a copolymer containing aromatic vinyl units, but the transparency of the optical film may not be maintained.

[0007] The present invention aims to provide an optical film that reduces the degree of yellowness when viewed from an oblique angle on a liquid crystal panel, while maintaining heat resistance and transparency. [Means for solving the problem]

[0008] [1] An optical film comprising a resin composition, wherein the thickness direction phase difference Rth at a wavelength of 590 nm is -35.0 nm or more and -5.0 nm or less, the resin composition comprising an acrylic resin having a methyl methacrylate unit content of 98% by weight or more and a triple syndiotacticity of 55% or more, and a copolymer comprising (meth)acrylonitrile units and aromatic vinyl units, the glass transition temperature is 116°C or higher, the copolymer having a (meth)acrylonitrile unit content of 22% by weight or more and 28% by weight or less, and an internal haze of 0.20% or less.

[0009] [2] The optical film according to [1], wherein the resin composition has a content of (meth)acrylonitrile units that is greater than 0% by weight and 4.0% by weight or less, and a content of aromatic vinyl units that is greater than 0% by weight and 10.0% by weight or less.

[0010] [3] The optical film according to [1] or [2], wherein the copolymer has a weight-average molecular weight of 80,000 or more and 200,000 or less, and a polydispersity of 1.6 or more and 2.5 or less.

[0011] [4] The optical film according to any one of [1] to [3], wherein the acrylic resin has a weight-average molecular weight of 80,000 or more and 200,000 or less, and a polydispersity of 1.6 or more and 2.5 or less.

[0012] [5] An optical film according to any one of items [1] to [4], wherein the average value of the in-plane phase difference Re is 4.0 nm or less.

[0013] [6] The optical film according to any one of [1] to [5], wherein the resin composition has a 5% weight loss temperature of 335°C or higher.

[0014] [7] The optical film according to any one of [1] to [6], which does not contain an ultraviolet absorber.

[0015] [8] The optical film according to any one of [1] to [7], which is a biaxially stretched film.

[0016] [9] A polarizing plate comprising the optical film according to any one of [1] to [8].

[0017]

[10] A liquid crystal panel comprising the polarizing plate described in [9].

[0018]

[11] A resin composition comprising an acrylic resin having a methyl methacrylate unit content of 98% by weight or more and a syndiotacticity of triads of 55% or more, and a copolymer containing (meth)acrylonitrile units and aromatic vinyl units, having a glass transition temperature of 116°C or more, wherein the copolymer has a (meth)acrylonitrile unit content of 22% by weight or more and 28% by weight or less, and the internal haze of a 40-μm-thick film formed by molding the resin composition is 0.20% or less. [Advantages of the Invention]

[0019] According to the present invention, it is possible to provide an optical film that reduces the yellowness when viewed from an oblique direction of a liquid crystal panel and maintains heat resistance and transparency. [Embodiments for Carrying Out the Invention]

[0020] Hereinafter, embodiments of the present invention will be described.

[0021] (Optical Film) The optical film of this embodiment contains a resin composition.

[0022] The thickness-direction phase difference Rth of the optical film of this embodiment at a wavelength of 590 nm is preferably -35.0 nm or more and -5.0 nm or less, preferably -30.0 nm or more and -6.0 nm or less, preferably -20.0 nm or more and -7.0 nm or less, more preferably -15.0 nm or more and -8.0 nm or less, and particularly preferably -14.0 nm or more and -9.0 nm or less. When Rth is -35.0 nm or more and -5.0 nm or less, the yellowness when viewed from an oblique direction on the liquid crystal panel is reduced. In this case, the yellowness of the optical film of this embodiment when viewed from an oblique direction on the liquid crystal panel is preferably 50 or less, more preferably 45 or less, and even more preferably 40 or less.

[0023] The average value of the in-plane phase difference Re of the optical film in this embodiment is preferably 4.0 nm or less, and more preferably 3.8 nm or less. When the average value of Re is 4.0 nm or less, the uniformity of the in-plane phase difference Re is improved. In this case, it is preferable that the standard deviation of the in-plane phase difference Re of the optical film in this embodiment is less than 0.8.

[0024] Note that Re and Rth are expressed in formula Re=(nx-ny)×d Rth = [(nx + ny) / 2 - nz] × d It is calculated by the following formula. Here, nx, ny, and nz are the refractive indices in the X-axis direction, Y-axis direction, and Z-axis direction, respectively, with the MD direction being the X-axis, the TD direction being the Y-axis, and the film thickness direction being the Z-axis. Also, d is the thickness of the film.

[0025] The internal haze of the optical film in this embodiment is 0.20% or less, preferably 0.18% or less, more preferably 0.17% or less, even more preferably 0.16% or less, and even more preferably 0.15% or less. Since the internal haze of the optical film in this embodiment is 0.20% or less, the transparency of the optical film in this embodiment is maintained. For example, the internal haze of the optical film in this embodiment is 0.05% or more.

[0026] The yellowness of the optical film in this embodiment is preferably 0.01 to 5.00, and more preferably 0.1 to 2.0. When the yellowness of the optical film in this embodiment is 5.00 or less, the coloration of the optical film in this embodiment is minimal, and the impact on the color rendering of the display is small.

[0027] The absorbance of the optical film of this embodiment at a wavelength of 380 nm is preferably 0.01 or more and 1.00 or less, and more preferably 0.10 or more and 0.80 or less. If the absorbance of the optical film of this embodiment at a wavelength of 380 nm is 1.00 or less, the optical film of this embodiment is substantially free of ultraviolet absorbers.

[0028] The optical film of this embodiment can be applied, for example, to a polarizer protective film. In this case, the optical film of this embodiment can be bonded to a polarizer to form a polarizing plate. The polarizer is not particularly limited, and known polarizers can be used. The polarizing plate can be combined with a liquid crystal cell to form a liquid crystal panel. In this case, it is preferable to use an IPS type liquid crystal cell with a wide viewing angle. Furthermore, when the optical film of this embodiment is placed on the side facing the liquid crystal cell, it does not need to contain an ultraviolet absorber, or may not contain an ultraviolet absorber at all.

[0029] (Resin composition) The resin composition includes an acrylic resin having a methyl methacrylate unit content of 98% by weight or more and a triplet syndiotacticity of 55% or more.

[0030] In this specification and in the claims, acrylic resin means a polymer of monomers 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. If the acrylic resin is a copolymer, it may be a copolymer of monomers that do not have an acryloyl group or a methacryloyl group.

[0031] The glass transition temperature of the resin composition is 116°C or higher, preferably 118°C or higher, more preferably 119°C or higher, still more preferably 120°C or higher, and even more preferably higher than 120°C. Since the glass transition temperature of the resin composition is 116°C or higher, the heat resistance of the optical film of the present embodiment is enhanced. Also, orientation relaxation is less likely to progress in a high-temperature and high-humidity environment, the stability of the retardation is improved, and the dimensional stability is also excellent. The glass transition temperature of the resin composition is, for example, 160°C or lower.

[0032] The internal haze of a 40-μm-thick film formed by molding the resin composition is 0.20% or lower, preferably 0.18% or lower, more preferably 0.17% or lower, still more preferably 0.16% or lower, and even more preferably 0.15% or lower. Since the internal haze of a 40-μm-thick film formed by molding the resin composition is 0.20% or lower, the transparency of the optical film of the present embodiment is maintained. Note that the internal haze of a 40-μm-thick film formed by molding the resin composition is, for example, 0.05% or higher.

[0033] The birefringence development Δnxy of the resin composition is preferably -1.2×10 -3 or higher and -0.1×10 -3 or lower, more preferably -1.0×10 -3 or higher and -0.25×10 -3 or lower, still more preferably -1.0×10 -3 or higher and -0.2×10 -3 or lower, and even more preferably -0.8×10 -3 or higher and -0.12×10 -3 or lower. When Δnxy is -1.2×10 -3 or higher, it becomes easier to develop a desired retardation in the thickness direction when the resin composition is biaxially stretched, and since it is -0.1×10 -3 or lower, the in-plane retardation is likely to be uniform when the resin composition is biaxially stretched.

[0034] In this specification and in the claims, the birefringence Δnxy of a resin composition means the birefringence that occurs when an unstretched film of the resin composition is uniaxially stretched at its free end at a temperature 5°C higher than the glass transition temperature of the resin composition, such that the stretching ratio in the longitudinal direction (longitudinal direction) becomes twice as large.

[0035] Note that Δnxy is expressed by the formula Δnxy=nx-ny=Re / d It is calculated by the following formula. Here, nx and ny are the refractive indices in the X-axis and Y-axis directions, respectively, with the MD direction as the X-axis, the TD direction as the Y-axis, and the film thickness direction as the Z-axis. Re is the in-plane phase difference of the film, and d is the thickness of the film.

[0036] The resin composition further comprises a copolymer containing (meth)acrylonitrile units and aromatic vinyl units. The content of (meth)acrylonitrile units in the copolymer is 22% by weight or more and 28% by weight or less, preferably 23% by weight or more and 27% by weight or less. When the content of (meth)acrylonitrile units in the copolymer is 22% by weight or more and 28% by weight or less, the transparency of the optical film of this embodiment is maintained.

[0037] Aromatic vinyls are not particularly limited, but examples include styrene, α-methylstyrene, methoxystyrene, vinyltoluene, and halostyrene. Among these, styrene is preferred from the viewpoint of reducing the yellowness when the liquid crystal panel is viewed from an oblique angle. Here, (meth)acrylonitrile is acrylonitrile or methacrylonitrile.

[0038] The weight-average molecular weight of the copolymer is preferably between 80,000 and 200,000, and more preferably between 90,000 and 150,000. When the weight-average molecular weight of the copolymer is 80,000 or more, the mechanical properties of the molded article of the resin composition tend to improve, and when it is 200,000 or less, the compatibility with acrylic resin increases.

[0039] The polydispersity of the copolymer (ratio of weight-average molecular weight to number-average molecular weight) is preferably 1.6 or more and 2.5 or less, and more preferably 1.6 or more and 2.2 or less. When the polydispersity of the copolymer is 1.6 or more, the fluidity of the resin composition tends to improve and it tends to become easier to mold, and when it is 2.5 or less, the mechanical properties of the molded article of the resin composition tend to improve, such as impact resistance, toughness, and bending resistance.

[0040] The content of (meth)acrylonitrile units in the resin composition is preferably greater than 0% by weight and 4.0% by weight or less, more preferably 0.3% by weight or more and 4.0% by weight or less, and even more preferably 0.4% by weight or more and 3.5% by weight or less. When the content of (meth)acrylonitrile units in the resin composition is 0.3% by weight or more and 4.0% by weight or less, the transparency of the optical film of this embodiment is maintained.

[0041] The content of aromatic vinyl units in the resin composition is preferably greater than 0% by weight and 10.0% by weight or less, more preferably 1.0% by weight or more and 10.0% by weight or less, and even more preferably 2.0% by weight or more and 9.0% by weight or less. When the content of aromatic vinyl units in the resin composition is 1.0% by weight or more and 10.0% by weight or less, the yellowness when the liquid crystal panel is viewed from an oblique direction is reduced. It is preferable that the acrylic resin contained in the resin composition does not contain aromatic vinyl units.

[0042] The resin composition may further contain a methyl methacrylate-styrene copolymer.

[0043] The resin composition may further contain additives, provided that they do not impair the objectives of the present invention. The additives are not particularly limited, but examples include antioxidants, heat stabilizers, light stabilizers, ultraviolet absorbers, specific wavelength absorbers or specific wavelength absorbing dyes for blue light filtering, light-resistant stabilizers such as radical scavengers, phase difference adjusters, catalysts, plasticizers, lubricants, antistatic agents, colorants, shrinkage inhibitors, antibacterial and deodorizing agents, fluorescent whitening agents, and compatibilizers. Two or more of these may be used in combination.

[0044] The 5% weight loss temperature of the resin composition is preferably 335°C or higher, more preferably 336°C or higher, and even more preferably 337°C or higher. When the 5% weight loss temperature of the resin composition is 335°C or higher, contamination of the cooling rolls during the production of the raw film is suppressed, and the film-forming properties of the raw film are improved. The 5% weight loss temperature of the resin composition is, for example, 380°C or lower.

[0045] (Acrylic resin) The syndiotacticity of the triple-marked acrylic resin is 55% or higher, preferably 56% or higher, more preferably 57% or higher, and even more preferably 58% or higher. When the syndiotacticity of the triple-marked acrylic resin is 55% or higher, the heat resistance of the resin composition is increased.

[0046] The syndiotacticity of the triple-marked acrylic resin is preferably 70% or less, more preferably 67% or less, even more preferably 65% ​​or less, and even more preferably 60% or less. When the syndiotacticity of the triple-marked acrylic resin is 70% or less, the extrusion processability of the resin composition is improved.

[0047] The syndiotacticity of the triplet configuration of acrylic resin is the proportion of the three structural unit chains (triplets) that are rr. Note that in two structural unit chains (doublets), those with the same stereochemistry are called meso (m), and those with the opposite stereochemistry are called racemo (r).

[0048] The content of methyl methacrylate units in the acrylic resin is preferably 98% by weight or more, more preferably 99% by weight or more, and even more preferably 100% by weight. When the content of methyl methacrylate units in the acrylic resin is 98% by weight or more, the transparency of the optical film of this embodiment is maintained.

[0049] The synthesis method for acrylic resin is not particularly limited, but examples include anionic polymerization and radical polymerization. Among these, radical polymerization is preferred from an environmental standpoint (see, for example, International Publication No. 2023 / 238886). Here, the glass transition temperature and the syndiotacticity expressed in triplicate of the acrylic resin can be controlled by the polymerization temperature of the acrylic resin. For example, lowering the polymerization temperature of the acrylic resin increases the glass transition temperature and syndiotacticity of the acrylic resin. Furthermore, the glass transition temperature of the acrylic resin can also be controlled by the molecular weight of the acrylic resin.

[0050] Monomers other than methyl methacrylate that can be used when synthesizing acrylic resins are not particularly limited, but include, for example, alkyl acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate; aryl acrylates such as phenyl acrylate; cycloalkyl acrylates such as cyclohexyl acrylate and norbornenyl acrylate; alkyl methacrylates other than methyl methacrylate such as ethyl methacrylate, propyl methacrylate, and butyl methacrylate; aryl methacrylates such as phenyl methacrylate; cycloalkyl methacrylates such as cyclohexyl methacrylate and norbornenyl methacrylate; aromatic vinyl compounds such as styrene and α-methylstyrene; acrylamide; methacrylamide; acrylonitrile; and methacrylonitrile.

[0051] The weight-average molecular weight of the acrylic resin is preferably between 80,000 and 200,000, and more preferably between 90,000 and 150,000. When the weight-average molecular weight of the acrylic resin is 80,000 or more, the mechanical properties of the molded article of the resin composition tend to improve, and when it is 200,000 or less, the moldability of the resin composition tends to improve.

[0052] The polydispersity of the acrylic resin (ratio of weight-average molecular weight to number-average molecular weight) is preferably 1.6 or more and 2.5 or less, and more preferably 1.6 or more and 2.2 or less. When the polydispersity of the acrylic resin is 1.6 or more, the fluidity of the resin composition tends to improve, making it easier to mold, and when it is 2.5 or less, the mechanical properties of the molded article of the resin composition tend to improve, such as impact resistance, toughness, and bending resistance.

[0053] The number-average molecular weight and weight-average molecular weight of acrylic resins are values ​​measured by gel permeation chromatography (GPC) on a standard polystyrene basis. Furthermore, the number-average molecular weight and weight-average molecular weight of acrylic resins can be controlled by the type and amount of polymerization initiators and chain transfer agents used in the synthesis of the acrylic resin.

[0054] (Method of manufacturing optical film) The optical film of this embodiment can be manufactured using known methods. An example of a method for manufacturing the optical film of this embodiment is described below.

[0055] First, using an extruder equipped with a die at the outlet, the acrylic resin and copolymer are kneaded together, and then the strand is extruded from the die. Next, the strand is cooled in a water bath, and then pelletized using a pelletizer to obtain a resin composition. Next, using an extruder equipped with a T-die at the outlet, the resin composition is melted, and then the sheet is extruded from the T-die and cooled with a cooling roll to obtain a raw film. Next, the raw film is biaxially stretched to obtain the optical film of this embodiment. At this time, the biaxial stretching may be simultaneous biaxial stretching or sequential biaxial stretching.

[0056] The temperature at which the raw film is biaxially stretched is preferably (Tg+5)°C to (Tg+20)°C, more preferably (Tg+6)°C to (Tg+18)°C, and even more preferably (Tg+7)°C to (Tg+15)°C, where Tg is the glass transition temperature of the resin composition. The surface ratio at which the raw film is biaxially stretched is not particularly limited, but for example, it is between 2 and 10 times. The stretching speed at which the raw film is biaxially stretched is not particularly limited, but for example, it is between 1.1 times / min and 100 times / min. When the raw film is sequentially biaxially stretched, the stretching speed of the first stage and the stretching speed of the second stage may be the same or different. In sequential biaxial stretching, the first stage stretching is usually in the longitudinal direction (MD direction), and the second stage stretching is in the width direction (TD direction).

[0057] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and the above embodiments may be modified as appropriate within the scope of the spirit of the present invention. [Examples]

[0058] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0059] (polymerization rate) The polymerization conversion rate [%] was determined by the gravimetric method, based on the ratio of the solid content weight of the polymerization solution to the weight of the added monomer. The solid content weight of the polymerization solution was determined by drying it in an oven set to 150°C for 30 minutes.

[0060] (Syndiotacticity rr with triple display) Using a 400MHz nuclear magnetic resonance spectrometer AVANCEIII (manufactured by Bruker), acrylic resin was subjected to a deuterated chloroform solution under the conditions of 22°C and 16 cumulative cycles. 1 The 1H-NMR spectrum was measured. Next, the area (X) of the region from 0.60 to 0.95 ppm and the area (Y) of the region from 0.60 to 1.25 ppm were measured, with tetramethylsilane (TMS) set to 0 ppm, and then the formula was applied. (X / Y) × 100 The syndiotacticity rr in triple representation was calculated based on this.

[0061] (Weight-average molecular weight Mw and polydispersity Mw / Mn) Using a high-speed GPC instrument HLC-8220GPC (manufactured by Tosoh), the weight-average molecular weight (Mw) and polydispersity (Mw / Mn) of the acrylic resin were calculated using the standard polystyrene equivalent method. Specifically, a sample solution prepared by dissolving 20 mg of acrylic resin in 10 mL of chloroform was used, and the analysis was performed under the following conditions. Detector: RI detector Solvent: Chloroform Guard column: TSKgel guardcolumn SuperHZ-H (manufactured by Tosoh) Analytical columns: TSKgel SuperHZM-H x 2 (manufactured by Tosoh) Measurement temperature: 40℃ Standard material: Standard polystyrene (manufactured by Tosoh Corporation)

[0062] (Glass transition temperature Tg) Using a high-sensitivity differential scanning calorimeter DSC7000X (manufactured by Hitachi High-Tech Science), 10 mg of acrylic resin or resin composition was heated at a heating rate of 10°C / min under a nitrogen atmosphere, and the glass transition temperature Tg was determined by the midpoint method.

[0063] (5% weight loss temperature Td5) Using a differential thermogravimetric analyzer STA7200 (manufactured by Hitachi High-Tech Science), 10 mg of acrylic resin or resin composition was heated from room temperature at a heating rate of 10°C / min under a nitrogen atmosphere, and the 5% weight loss temperature Td5 was determined.

[0064] (Birefringence-producing Δnxy) After cutting a 30 × 100 mm area from the original film, the free end was uniaxially stretched at a temperature 5°C higher than the glass transition temperature of the original film so that the stretching ratio in the longitudinal direction (longitudinal direction) was doubled, thereby obtaining a uniaxially oriented film. Next, the in-plane phase difference at the center of the uniaxially oriented film was measured using a phase difference measuring device KOBRA-WR (manufactured by Oji Keisoku Kiki), and then divided by the thickness of the uniaxially oriented film to determine the birefringence Δnxy.

[0065] (Phase difference in the thickness direction Rth) The phase difference Rth in the thickness direction of an optical film at a wavelength of 590 nm was measured using a phase difference measuring device KOBRA-WR (manufactured by Oji Instruments).

[0066] (Mean and standard deviation of in-plane phase difference Re) After cutting out a 150 x 150 mm area from the center of the optical film, the mean and standard deviation of the in-plane phase difference Re were measured using a 2D birefringence evaluation system WPA-200 (manufactured by Photonic Lattice). At this time, three lines were drawn in both the MD and TD directions, and the mean and standard deviation of the in-plane phase difference Re were calculated using the line analysis function.

[0067] (Internal haze) In accordance with JIS K7136:2000, the internal haze of the optical film was measured using a HZ-V3 haze meter (manufactured by Suga Test Instruments). The internal haze of the optical film was measured twice, with glycerin and then glass sandwiched between both sides, and the average value was calculated. Furthermore, the measured internal haze was converted to a value equivalent to a film thickness of 40 μm.

[0068] (Yellowness YI when viewed from an oblique angle on the LCD panel) A liquid crystal panel simulation was performed using the LCD Master (manufactured by Syntec) liquid crystal simulator. The setup involved arranging a polarizing plate on the light source side, an IPS-type liquid crystal cell with an in-plane phase difference Re of 295 nm, and another polarizing plate on the viewing side. The measurement results of the thickness-direction phase difference Rth were input as the optical properties of the optical film on the side of the polarizing plates facing the liquid crystal cell. Next, the XYZ color values ​​were simulated when the liquid crystal cell was viewed from a polar angle of 70° and an azimuth angle of 40° with the liquid crystal cell set to a dark state. The yellowness YI was then calculated using the following formula in accordance with JIS K7373:2006. YI = 100(1.2985X - 1.1335Z) / Y

[0069] (Manufacturing of acrylic resin 1) In a 4L glass reactor equipped with an H-type agitator, 150 parts by weight of deionized water, 0.20 parts by weight of tricalcium phosphate, 0.0075 parts by weight of sodium α-olefin sulfonate, and 0.30 parts by weight of sodium chloride were charged as dispersants. Next, under a nitrogen atmosphere, while stirring at 250 rpm, 100 parts by weight of methyl methacrylate (MMA), 0.289 parts by weight of n-octyl mercaptan as a chain transfer agent, and 0.074 parts by weight of 2,2'-azobis(isobutyrate)dimethyl V-601 (manufactured by Fujifilm Wako Pure Chemical Industries) as a polymerization initiator were added to the reactor. Subsequently, the temperature of the liquid in the reactor was raised to 70°C to start polymerization. Two hours after the start of polymerization, 0.10 parts by weight of tricalcium phosphate was added to the polymerization solution. At this time, an exothermic peak associated with the gel effect was observed 4 hours and 20 minutes after the start of polymerization. Seven hours after the start of polymerization, the temperature was increased to 95°C. At the seven-hour mark, the polymerization conversion rate was 93%. Two hours after the temperature reached 95°C, the reactor was cooled to room temperature to terminate polymerization and obtain an acrylic resin dispersion. At this point, the polymerization conversion rate was 99%. Next, the acrylic resin dispersion was washed with 1 N hydrochloric acid in an amount equal to 0.1 times the weight of the charged monomer, and then washed with water to remove the dispersant. The acrylic resin washed with water was then dehydrated and dried to obtain bead-shaped acrylic resin 1. Acrylic resin 1 had an MMA unit content of 100% by weight, an rr of 57%, an Mw of 116,000, and an Mw / Mn ratio of 2.03.

[0070] (Example 1) Using a 15 mm diameter, coaxial twin-screw extruder (L / D=45) equipped with a die at the outlet, 100 parts by weight of acrylic resin 1 and 2 parts by weight of acrylonitrile-styrene copolymer AS-61NT (manufactured by Shin Chemical Trading Co., Ltd.) (hereinafter referred to as AS resin 1) were kneaded. Here, AS resin 1 has an acrylonitrile (AN) unit content of 26% by weight, a styrene (ST) unit content of 74% by weight, an Mw of 160,000, and an Mw / Mn ratio of 2.03. Next, the strand extruded from the die at the outlet of the extruder was cooled using a water bath, and the strand was pelletized using a pelletizer. After that, it was dried at 100°C for 5 hours to obtain a resin composition. The resin composition had a Tg of 120.1°C, a Td5 of 336.8°C, and a Δnxy of -0.54 × 10⁻⁶. -3 That was the case.

[0071] Using a 15mm diameter, co-rotating twin-screw extruder (L / D=45) equipped with a T-die at the outlet, the resin composition was melted, and then the sheet extruded from the T-die was cooled using a cooling roll to obtain a raw film with a width of 160mm and a thickness of 160μm.

[0072] Using a biaxial film stretching machine IMC-1905 (manufactured by Imoto Seisakusho), the raw film was simultaneously biaxially stretched at a temperature 15°C higher than the glass transition temperature of the resin composition, so that the stretching ratio in the longitudinal and transverse directions was doubled, to obtain an optical film measuring 280 mm x 280 mm.

[0073] (Example 2) An optical film was obtained in the same manner as in Example 1, except that the amount of AS resin 1 added was changed to 5 parts by weight. At this time, the resin composition had a Tg of 119.5°C, a 5% weight loss temperature of 338.2°C, and a Δnxy of -0.94 × 10⁻⁶. -3 That was the case.

[0074] (Example 3) An optical film was obtained in the same manner as in Example 1, except that the amount of AS resin 1 added was changed to 7.5 parts by weight. At this time, the resin composition had a Tg of 118.7°C, a 5% weight loss temperature of 337.9°C, and a Δnxy of -1.03 × 10⁻⁶. -3 That was the case.

[0075] (Example 4) An optical film was obtained in the same manner as in Example 1, except that the amount of AS resin 1 added was changed to 10 parts by weight. At this time, the resin composition had a Tg of 118.0°C, a 5% weight loss temperature of 339.6°C, and a Δnxy of -1.10 × 10⁻⁶. -3 That was the case.

[0076] (Comparative Example 1) An optical film was obtained in the same manner as in Example 1, except that AS resin 1 was replaced with acrylonitrile-styrene copolymer AS-20 (manufactured by Shin Chemical Trading Co., Ltd.) (hereinafter referred to as AS resin 2). Here, AS resin 2 had an AN unit content of 30% by weight, an ST unit content of 70% by weight, an Mw of 140,000, and an Mw / Mn ratio of 1.92. At this time, the resin composition had a Tg of 107.9°C and 120.6°C, and a 5% weight loss temperature of 338.1°C. On the other hand, the optical film had an internal haze of 0.46 and was cloudy, so it was not possible to measure properties other than the Δnxy of the resin composition and the internal haze of the optical film.

[0077] (Comparative Example 2) An optical film was obtained in the same manner as in Example 3, except that methyl methacrylate resin parapet HR-S (manufactured by Kuraray) (hereinafter referred to as acrylic resin 2) was used instead of acrylic resin 1. Here, acrylic resin 2 has an MMA unit content of 98% by weight or more, an rr of 51%, an Mw of 91,000, and an Mw / Mn of 1.68. At this time, the resin composition had a Tg of 115.4°C, a 5% weight loss temperature of 337.8°C, and a Δnxy of -1.09 × 10⁻⁶. -3 That was the case.

[0078] Table 1 shows the properties and evaluation results of acrylic resin, AS resin, resin composition, and optical film.

[0079] [Table 1]

[0080] Table 1 shows that the optical films of Examples 1 to 4 have reduced YI when viewed from an oblique angle on the liquid crystal panel, while maintaining heat resistance and transparency. In contrast, the optical film of Comparative Example 1 contains AS resin 2 with an AN unit content of 30% by weight, resulting in a high internal haze of 0.46% and failure to maintain transparency. This is presumed to be due to phase separation between acrylic resin 1 and AS resin 2, as two Tg values ​​were observed. Furthermore, the optical film of Comparative Example 2 does not maintain heat resistance because the Tg of the resin composition is 115.4°C.

Claims

1. Includes a resin composition, The phase difference Rth in the thickness direction at a wavelength of 590 nm is between -35.0 nm and -5.0 nm. The resin composition comprises an acrylic resin having a methyl methacrylate unit content of 98% by weight or more and a triplet syndiotacticity of 55% or more, and a copolymer containing (meth)acrylonitrile units and aromatic vinyl units, and having a glass transition temperature of 116°C or higher. The copolymer has a content of 22% by weight or more and 28% by weight or less of the (meth)acrylonitrile units. An optical film with internal haze of 0.20% or less.

2. The optical film according to claim 1, wherein the resin composition has a content of (meth)acrylonitrile units that is greater than 0% by weight and 4.0% by weight or less, and a content of aromatic vinyl units that is greater than 0% by weight and 10.0% by weight or less.

3. The optical film according to claim 1 or 2, wherein the copolymer has a weight-average molecular weight of 80,000 or more and 200,000 or less, and a polydispersity of 1.6 or more and 2.5 or less.

4. The optical film according to claim 1 or 2, wherein the acrylic resin has a weight-average molecular weight of 80,000 or more and 200,000 or less, and a polydispersity of 1.6 or more and 2.5 or less.

5. The optical film according to claim 1 or 2, wherein the average value of the in-plane phase difference Re is 4.0 nm or less.

6. The optical film according to claim 1 or 2, wherein the resin composition has a 5% weight loss temperature of 335°C or higher.

7. An optical film according to claim 1 or 2, which does not contain an ultraviolet absorber.

8. The optical film according to claim 1 or 2, wherein the optical film is a biaxially oriented film.

9. A polarizing plate comprising the optical film according to claim 1 or 2.

10. A liquid crystal panel comprising the polarizing plate described in claim 9.

11. A resin composition comprising an acrylic resin having a methyl methacrylate unit content of 98% by weight or more and a triplet syndiotacticity of 55% or more, and a copolymer containing (meth)acrylonitrile units and aromatic vinyl units, The glass transition temperature is 116°C or higher. The copolymer has a content of 22% by weight or more and 28% by weight or less of the (meth)acrylonitrile units. A resin composition in which a 40 μm thick film formed by molding the resin composition has an internal haze of 0.20% or less.

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