JP2012068430A - Retardation film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative retardation film capable of achieving a high glass-transition temperature while retaining an original moldability and optical characteristics of acrylic resin.SOLUTION: This retardation film comprises a thermoplastic resin composition (C) that includes an acrylic polymer (A) having a positive intrinsic birefringence, and a polymer (B) having a negative intrinsic birefringence, wherein a mass ratio (a/b) of a repeating unit (a) represented by a general formula (1) to an aromatic vinyl monomer unit (b) is 0.5-2, the content of the repeating unit (a) with respect to the total sum of the acrylic polymer (A) and the polymer (B) is 25 mass% or more, and the content of the aromatic vinyl monomer unit (b) is 20 mass% or more. This retardation film has retardation Rth in the thickness direction of -30 nm or less.

Description

  The present invention relates to a retardation film made of a thermoplastic resin composition having negative intrinsic birefringence, and a polarizing plate and an image display device including the film.

  A stretched film obtained by uniaxially stretching or biaxially stretching a resin film is widely used in the field of image display. One type is a retardation film that utilizes birefringence based on the orientation of polymer chains generated by stretching, and the retardation film is widely used for color tone compensation and viewing angle compensation in liquid crystal display devices (LCD). . Conventionally, a λ / 4 plate whose optical path length difference (retardation) based on the phase difference caused by birefringence is ¼ of the wavelength is a typical retardation film used in LCDs.

  In recent years, retardation films capable of supporting various optical designs have been demanded due to advances in optical design technology and to improve the appeal of LCDs to consumers. For example, an in-plane switching (IPS) mode, which is a kind of liquid crystal display mode, is characterized in that a wide viewing angle can be realized without using a retardation film. However, due to the optical characteristics of the liquid crystal cell, light leakage occurs when the screen is viewed from an oblique direction, and the contrast of the display image is lowered due to so-called “black floating”. On the other hand, there is a vertical alignment (VA) mode as a liquid crystal display mode competing with the IPS mode. In the VA mode, although a wide viewing angle as in the IPS mode cannot be obtained, a high contrast image display with less light leakage is obtained. realizable. At present, a technique for widening the viewing angle in the VA mode is rapidly progressing, and in order to counter this, suppression of light leakage in the IPS mode by arrangement of a retardation film is required.

  The refractive index in the thickness direction of the IPS mode liquid crystal cell is smaller than the refractive index in the in-plane direction. For this reason, in order to suppress light leakage, a “negative retardation film” having a negative retardation Rth in the thickness direction is required. The retardation Rth is defined as nx as the refractive index of the slow axis in the film plane, ny as the refractive index of the fast axis in the film plane, nz as the refractive index in the thickness direction of the film, and d as the film thickness. Sometimes given by the expression {(nx + ny) / 2−nz} × d. The negative retardation film can be obtained by stretching a resin having negative intrinsic birefringence.

  By the way, polymethyl methacrylate (PMMA) having a good balance of molding processability and surface hardness and excellent optical characteristics such as high light transmittance and low wavelength dependency is widely used as an optical material. Thus, a negative retardation film is obtained by stretching a film made of PMMA (Patent Document 1). However, the glass transition temperature (Tg) of the retardation film made of PMMA is slightly low, about 100 ° C., and is used for applications requiring higher Tg (use for applications requiring higher heat resistance: for example, image display It is difficult to use the device.

  On the other hand, as an acrylic resin having both transparency and heat resistance, a resin containing an acrylic polymer having a ring structure in the main chain has been developed. For example, a polymer having a lactone ring structure in the main chain obtained by subjecting a polymer having a hydroxyl group and an ester group in the molecular chain to a lactone cyclocondensation reaction (for example, see Patent Documents 2 and 3) or a glutarimide ring Polymers containing a structure (for example, see Patent Document 4), polymers containing a glutaric anhydride structure (for example, see Patent Document 5), etc. are being applied to optical film applications such as retardation films. ing.

  These films containing an acrylic polymer having a ring structure in the main chain were able to impart heat resistance while maintaining excellent optical properties such as the original light transmittance and low wavelength dependency of the acrylic resin. However, since the ring structure of the main chain has a positive intrinsic birefringence, application to a negative retardation film is difficult. Therefore, Patent Document 4 discloses a negative retardation film including a polymer containing a glutarimide ring structure in a main chain obtained by copolymerizing a styrene monomer imparting negative intrinsic birefringence. Patent Document 6 discloses a negative retardation film containing an acrylic polymer containing a lactone ring structure in the main chain and a styrene polymer.

Japanese Patent Laid-Open No. 05-066400 JP 2006-096960 A JP 2008-009378 A WO2005 / 054311 Publication JP 2006-241197 A WO2009 / 139353

  However, in order to cancel the positive intrinsic birefringence derived from the ring structure of the main chain and obtain a negative retardation film, a styrene system that imparts a negative intrinsic birefringence to the acrylic polymer having a ring structure in the main chain It is necessary to copolymerize a large amount of monomer, but when a large amount of styrene monomer is copolymerized, in addition to the acrylic polymer having a ring structure in the main chain, the styrene monomer Since the derived structural unit is also brittle, it is not easy to obtain high film strength even if the polymer chain is oriented by stretching.

  In addition, acrylic polymers having a ring structure in the main chain may introduce a ring structure into the main chain in the cyclization reaction after polymerization, but the cyclization reaction was performed after copolymerizing styrene monomers. In some cases, the structural unit derived from the styrenic monomer does not contribute to the cyclization reaction, resulting in insufficient cyclization, and the heat resistance and strength of the film may be reduced. Furthermore, a crosslinking reaction may occur due to an uncyclized reactive group, and the fluidity of the molten resin may change during molding, resulting in a decrease in molding processability, and gelation may occur, resulting in defects in the appearance of the film. May increase.

  This invention is made | formed in view of the said present condition, and aims at provision of the negative phase difference film which can implement | achieve a high glass transition temperature, maintaining the original moldability and the outstanding optical characteristic of an acrylic resin.

  The retardation film of the present invention comprises a thermoplastic resin composition (C) that satisfies the following conditions (I) to (IV), and the refractive index of the slow axis in the film plane is nx, and the fast axis in the film plane: Where Nth is the refractive index of the film, nz is the refractive index in the thickness direction of the film, and d is the thickness of the film, the thickness direction is represented by Rth = {(nx + ny) / 2−nz} × d. The retardation film has a retardation Rth of −30 nm or less.

    (I): negative containing an acrylic polymer (A) having a positive intrinsic birefringence containing a repeating unit (a) represented by the following general formula (1) and an aromatic vinyl monomer unit (b) A polymer (B) having an intrinsic birefringence of

    (II): The mass ratio (a / b) of the repeating unit (a) to the aromatic vinyl monomer unit (b) is 0.5-2.

    (III): The content of the repeating unit (a) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 25% by mass or more.

    (IV): The content of the aromatic vinyl monomer unit (b) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 20% by mass or more.

［式中、Ｒ１、Ｒ２、Ｒ３はそれぞれ独立して水素原子または、炭素数１〜２０の有機残基を表す。］
本発明の偏光板は、上記本発明の位相差フィルムを備える。

[Wherein, R 1, R 2 and R 3 each independently represents a hydrogen atom or an organic residue having 1 to 20 carbon atoms. ]
The polarizing plate of the present invention includes the retardation film of the present invention.

  The image display device of the present invention includes the retardation film of the present invention.

  Here, the intrinsic birefringence of a polymer is the refraction of light in a direction parallel to the direction (orientation axis) in which the molecular chains in the layer are aligned, assuming a layer in which the molecular chains of the polymer are uniaxially aligned. The value obtained by subtracting the refractive index of light in the direction perpendicular to the orientation axis from the ratio. The intrinsic birefringence of the resin composition is determined by the balance of intrinsic birefringence of each polymer contained in the resin composition.

  According to the present invention, a thermoplastic comprising an acrylic polymer (A) having a positive intrinsic birefringence and a polymer (B) having a negative intrinsic birefringence containing an aromatic vinyl monomer unit (b). By using the resin composition (C), it is possible to obtain a highly flexible retardation film capable of realizing a high glass transition temperature while maintaining the original moldability and excellent optical properties of the acrylic resin.

  Based on this effect of the retardation film of the present invention, the polarizing plate of the present invention can be suitably used for various applications such as an image display device. Further, the image display device of the present invention is excellent in image display characteristics such as less light leakage when the screen is viewed from an oblique direction.

  Unless otherwise specified in the following description, “%” means “mass%” and “part” means “part by mass”. In addition, “A to B” indicating a range indicates that the range is A or more and B or less.

[Acrylic polymer (A)]
The acrylic polymer (A) containing the repeating unit (a) represented by the following general formula (1) has positive intrinsic birefringence.

［式中、Ｒ１、Ｒ２、Ｒ３はそれぞれ独立して水素原子または、炭素数１〜２０の有機残基を表す。］
上記一般式（１）において、Ｒ１、Ｒ２はそれぞれ独立して、水素原子またはメチル基であることが好ましく、Ｒ３は水素原子、メチル基、エチル基、プロピル基、ブチル基、またはシクロヘキシル基であることが好ましい。

[Wherein, R 1, R 2 and R 3 each independently represents a hydrogen atom or an organic residue having 1 to 20 carbon atoms. ]
In the general formula (1), R1 and R2 are preferably each independently a hydrogen atom or a methyl group, and R3 is a hydrogen atom, a methyl group, an ethyl group, a propyl group, a butyl group, or a cyclohexyl group. It is preferable.

  Usually, an acrylic polymer such as polymethyl methacrylate has a negative intrinsic birefringence, but the acrylic polymer (A) of the present invention has a positive intrinsic property by containing the repeating unit (a). Has birefringence.

  The acrylic polymer (A) is a polymer having a (meth) acrylic acid ester unit as a structural unit, and is not particularly limited as long as the effects of the present invention are not impaired, and a known thermoplastic acrylic polymer is used. I can do it. The content of the (meth) acrylic acid ester unit in the acrylic polymer (A) is preferably 10% by mass or more, more preferably 30% by mass or more, and particularly preferably 50% by mass or more. Further, when the acrylic polymer (A) has a ring structure in the main chain, the total of the proportion of (meth) acrylic acid ester units in the total structural units and the content of the ring structure is 30% by mass or more. More preferably, it is 50 mass% or more, More preferably, it is 70 mass% or more, Most preferably, it is 90% weight or more.

  Preferable specific examples of the (meth) acrylate monomer include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate, (meth ) (Meth) acrylic acid alkyl esters such as t-butyl acrylate, n-hexyl (meth) acrylate, cyclohexyl (meth) acrylate; benzyl (meth) acrylate; chloromethyl (meth) acrylate; (meth) 2-chloroethyl acrylate; (meth) acrylic acid dicyclopentanyloxyethyl; (meth) acrylic acid dicyclopentanyl; (meth) acrylic acid 2-hydroxyethyl; (meth) acrylic acid 3-hydroxypropyl; ) Acrylic acid 2,3,4,5,6-pentahydroxyhexyl and (meth) acrylic acid 2,3 Such as 4,5-tetrahydroxy-pentyl and the like, may contain one kind alone of these structural unit derived from (meth) acrylic acid ester monomer may be present together two or more. Among these, (meth) acrylic acid alkyl ester is preferable, and methyl methacrylate is most preferable in terms of excellent thermal stability and optical characteristics.

  The acrylic polymer (A) may contain structural units other than those derived from the (meth) acrylic acid ester monomer described above, and a monomer mixture containing monomers other than the (meth) acrylic acid ester monomer. It is obtained by polymerizing. Examples of monomers other than (meth) acrylic acid ester monomers include, for example, acrylonitrile, methyl vinyl ketone, ethylene, propylene, 4-methyl-1-pentene, vinyl acetate, methallyl alcohol, allyl alcohol, and 2-hydroxy. Allyl alcohol such as methyl-1-butene, (meth) acrylic acid such as acrylic acid, methacrylic acid and crotonic acid, 2- (hydroxymethyl) methyl acrylate, 2- (hydroxymethyl) ethyl acrylate 2- ( (Hydroxyalkyl) acrylic acid ester, 2- (hydroxyalkyl) acrylic acid such as 2- (hydroxyethyl) acrylic acid, N-vinylpyrrolidone, N-vinylcarbazole and the like, and only one of these monomers is used. Or two or more of them may be used in combination.

  In the acrylic polymer (A), the content of the styrene monomer unit is preferably less than 5% by mass, more preferably less than 3% by mass, still more preferably less than 1% by mass, and particularly preferably 0.1% by mass. %. When the styrene monomer unit is contained in an amount of 5% by mass or more, it is difficult to obtain high film strength even after stretching. In addition, when the cyclization reaction is carried out after copolymerization at a styrene monomer content of 5% by mass or more, the cyclization becomes insufficient and the heat resistance and strength of the film may be reduced. is there. Furthermore, since a crosslinking reaction or the like occurs due to the uncyclized reactive group, the molding processability may be lowered, or the appearance defect of the film may be increased. The styrene monomer is not particularly limited as long as the effect of the present invention is not impaired as long as it is an aromatic vinyl monomer. For example, styrene, vinyl toluene, α-methyl styrene, α-hydroxymethyl styrene, α -Hydroxyethylstyrene, chlorostyrene, etc. are mentioned.

  In the acrylic polymer (A), the content of the repeating unit (a) is not particularly limited, and is preferably changed depending on, for example, the structure of R3.

  Generally, the content of the repeating unit (a) in the acrylic polymer (A) is preferably 20% by mass or more, more preferably 20% by mass to 95% by mass, and 30% by mass. It is more preferable to set it to -90 mass%, and it is especially preferable to set it as 40 mass%-80 mass%.

  If the content of the repeating unit (a) is within the above range, the heat resistance and transparency of the resulting acrylic polymer (A) will be reduced, the moldability, and the mechanical properties when processed into a film. The strength does not decrease.

  On the other hand, if the content of the repeating unit (a) is less than the above range, the resulting acrylic polymer (A) tends to have insufficient heat resistance or its transparency may be impaired. On the other hand, if the amount is larger than the above range, the heat resistance and melt viscosity are unnecessarily increased, the molding processability is deteriorated, the mechanical strength during film processing becomes extremely brittle, or the transparency is impaired. Tend.

  The acrylic polymer (A) is obtained by polymerizing a monomer mixture containing a (meth) acrylic acid ester monomer, and a known production method can be applied as the production method. Methods for introducing the repeating unit (a) represented by the general formula (1) are described in US Pat. No. 3,284,425, US Pat. No. 4,246,374, JP-A-2-153904, WO2005 / 108438, and the like. Similar to the glutarimide resin, a resin obtained by using methyl methacrylate as a main raw material as a resin having an imidizable unit is used, and the resin having the imidizable unit is imidized using ammonia or a substituted amine. Can be obtained.

  The glass transition temperature of the acrylic polymer (A) is preferably 110 ° C. or higher. More preferably, it is 115 degreeC or more, More preferably, it is 120 degreeC or more, Most preferably, it is 125 degreeC or more. The upper limit of the glass transition temperature is not particularly limited, but is preferably 200 ° C. or less from the viewpoint of moldability. Here, the glass transition temperature is a temperature at which a polymer molecule starts micro-Brownian motion, and there are various measurement methods. In the present invention, a differential scanning calorimeter (DSC) conforms to JIS-K7121. This is defined as the temperature obtained by the starting point method.

  The weight average molecular weight of the acrylic polymer (A) is preferably 10,000 to 300,000, more preferably 30,000 to 300,000, still more preferably 50,000 to 250,000, and particularly preferably 80. , 000-200,000.

[Polymer (B)]
The polymer (B) of the present invention is not particularly limited as long as it contains an aromatic vinyl monomer unit (b) and has negative intrinsic birefringence, and is derived from the aromatic vinyl monomer unit (b). Known polymers containing units can be used. The aromatic vinyl monomer is not particularly limited. For example, styrene, vinyl toluene, α-methyl styrene, α-hydroxymethyl styrene, α-hydroxyethyl styrene, chlorostyrene, N-vinyl carbazole, vinyl naphthalene, vinyl anthracene. Etc. The content of the aromatic vinyl monomer in the polymer (B) is preferably 10% by mass or more, more preferably 30% by mass or more, and particularly preferably 50% by mass or more.

  The polymer (B) may contain a structural unit derived from a (meth) acrylic acid ester monomer. As the (meth) acrylic acid ester monomer, the above (meth) acrylic acid ester monomer can be used.

  Although the specific kind of polymer (B) is not specifically limited, For example, polystyrene, styrene-methyl methacrylate copolymer, acrylonitrile-styrene copolymer, acrylonitrile-styrene-maleimide copolymer styrene-butadiene It may be a block copolymer. A polymer containing a structural unit derived from a vinyl cyanide monomer such as acrylonitrile or methacrylonitrile is preferred because of its excellent compatibility with the acrylic polymer (A), and a structural unit derived from acrylonitrile is preferred. More preferred are polymers containing acrylonitrile-styrene copolymers such as acrylonitrile-styrene copolymers and acrylonitrile-styrene-maleimide copolymers.

  Whether or not the polymer (B) is compatible with the acrylic polymer (A) can be confirmed by measuring the Tg of the resin composition obtained by mixing the two by the method described later. Generally, if only one Tg of the composition is confirmed, it can be said that the polymer (B) has compatibility with the acrylic polymer (A).

  When the polymer (B) is an acrylonitrile-styrene copolymer, the proportion of styrene units in all the structural units of the copolymer is not particularly limited, but is usually in the range of about 60 to 80% by mass. Good.

  When the polymer (B) is an acrylonitrile-styrene-maleimide copolymer, the proportion of styrene units in all the structural units of the copolymer is not particularly limited, but is usually in the range of about 55 to 80% by mass. That's fine.

  The polymer (B) may contain a rubbery polymer having an aromatic vinyl monomer unit in the graft chain. The rubbery polymer having an aromatic vinyl monomer unit in the graft chain is not particularly limited. For example, a monomer containing an aromatic vinyl monomer is polymerized in the presence of fine particle acrylic rubber or butadiene rubber. Can be manufactured.

  The rubbery polymer having an aromatic vinyl monomer unit in the graft chain is preferably a rubbery polymer having an aromatic vinyl monomer unit containing a structural unit derived from acrylonitrile in the graft chain. When the graft chain contains a structural unit derived from acrylonitrile, compatibility with the acrylic polymer (A) is improved, so that the rubbery polymer is uniformly dispersed in the resin composition, and the resulting retardation film The total light transmittance is improved. Specific examples include ASA resin, ABS resin, and AES resin obtained by grafting acrylonitrile-styrene copolymer to acrylic rubber, butadiene rubber, or ethylene-propylene rubber, which reduces the negative intrinsic birefringence of the polymer (B). ASA resin is particularly preferable because it is not allowed to occur.

  The weight average molecular weight of the polymer (B) is preferably 10,000 to 500,000, more preferably 50,000 to 400,000, and still more preferably 100,000 to 300,000.

  The content of the aromatic vinyl monomer unit (b) in the polymer (B) can be determined, for example, from the area ratio of 1H-NMR spectrum.

[Thermoplastic resin composition (C)]
The thermoplastic resin composition (C) satisfies the following conditions (I) to (IV).

    (I); negative containing an acrylic polymer (A) having positive intrinsic birefringence containing the repeating unit (a) represented by the general formula (1) and an aromatic vinyl monomer unit (b) A polymer (B) having an intrinsic birefringence of

    (II): The mass ratio (a / b) of the repeating unit (a) to the aromatic vinyl monomer unit (b) is 0.5-2.

    (III): The content of the repeating unit (a) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 25% by mass or more.

    (IV): The content of the aromatic vinyl monomer unit (b) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 20% by mass or more.

  The content of the acrylic polymer (A) having positive intrinsic birefringence in the thermoplastic resin composition (C) is preferably 50% by mass or more and 80% by mass or less, and more preferably 60% by mass or more and 80% by mass or less. It is. The content of the polymer (B) having negative intrinsic birefringence is preferably 20% by mass or more and 50% by mass or less, more preferably 25% by mass or more and 40% by mass or less.

  The mass ratio (a / b) of the repeating unit (a) to the aromatic vinyl monomer unit (b) in the thermoplastic resin composition (C) is 0.5 to 2, more preferably 0. .5 to 1.5.

  In the thermoplastic resin composition (C), the content of the repeating unit (a) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 25% by mass or more. Although there is no upper limit in particular, it is 80 mass% or less, 60 mass% or less is preferable, 50 mass% or less is more preferable, and 40 mass% or less is further more preferable. If it is out of the above range, heat resistance and transparency derived from the acrylic polymer (A) may be impaired.

  In the thermoplastic resin composition (C), the content of the aromatic vinyl monomer unit (b) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 20% by mass or more. Although there is no upper limit in particular, it is 75 mass% or less, 60 mass% or less is preferable, 40 mass% or less is more preferable, and 30 mass% or less is more preferable. If it is out of the above range, the retardation characteristics derived from the polymer (B) may be impaired.

  The thermoplastic resin composition (C) preferably has negative intrinsic birefringence. By having negative intrinsic birefringence, when a retardation film is formed, the thickness direction retardation Rth can be easily set to -30 nm or less.

  The weight average molecular weight of the soluble part of the thermoplastic resin composition (C) is preferably 10,000 to 500,000, more preferably 50,000 to 400,000, and even more preferably 80,000 to 300, 000.

  The glass transition temperature of the thermoplastic resin composition (C) is preferably 110 ° C. or higher. More preferably, it is 115 degreeC or more, More preferably, it is 120 degreeC or more, Most preferably, it is 125 degreeC or more. Moreover, although the upper limit of glass transition temperature is not specifically limited, 200 degreeC or less is preferable from a moldability, and 180 degrees C or less is more preferable.

  The thermoplastic resin composition (C) may contain other thermoplastic resins other than the acrylic polymer (A) and the styrene polymer (B). These other thermoplastic resins are not particularly limited in type, but are, for example, olefin polymers such as polyethylene, polypropylene, ethylene-propylene polymer, poly (4-methyl-1-pentene); vinyl chloride, chlorinated vinyl. Halogen-containing polymers such as resins; acrylic polymers such as polymethyl methacrylate; polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyamides such as nylon 6, nylon 66, and nylon 610; polyacetal: polycarbonate; polyphenylene oxide; Polyphenylene sulfide: Polyetheretherketone; Polysulfone; Polyethersulfone: Polyoxypentylene; Polyamideimide; Rubbery polymerization without styrenic polymer in the graft chain ; And the like.

  The thermoplastic resin composition (C) may contain other additives. Other additives include, for example, antioxidants, light stabilizers, weathering stabilizers, heat stabilizers and other stabilizers; phase difference increasing agents, phase difference reducing agents, phase difference adjusting agents such as phase difference stabilizers, Reinforcing materials such as glass fibers and carbon fibers; ultraviolet absorbers; near infrared absorbers; flame retardants such as tris (dibromopropyl) phosphate, triallyl phosphate, antimony oxide; anionic, cationic, and nonionic surfactants Anti-static agents; colorants such as inorganic pigments, organic pigments and dyes; organic fillers and inorganic fillers: resin modifiers; plasticizers; lubricants; The content of other additives in the thermoplastic resin composition is preferably less than 7% by mass, more preferably 2% by mass or less, and still more preferably 0.5% by mass or less.

  The thermoplastic resin composition (C) is not particularly limited, but the acrylic polymer (A) and the styrene polymer (B), other thermoplastic resins and additives, etc. are mixed in a conventionally known mixing method. Can be manufactured by mixing. For example, after pre-blending with a mixer such as an omni mixer, a method of extruding and kneading the obtained mixture can be employed. In this case, the kneader used for extrusion kneading is not particularly limited. For example, a conventionally known kneader such as an extruder such as a single screw extruder or a twin screw extruder or a pressure kneader is used. Can do. The molding temperature is preferably 200 to 350 ° C, more preferably 250 to 320 ° C, still more preferably 255 ° C to 300 ° C, and particularly preferably 260 ° C to 300 ° C.

[Phase difference film]
The retardation film of the present invention comprises a thermoplastic resin composition (C) that satisfies the following conditions (I) to (IV), and the refractive index of the slow axis in the film plane is nx, and the fast axis in the film plane: Where Nth is the refractive index of the film, nz is the refractive index in the thickness direction of the film, and d is the thickness of the film, the thickness direction is represented by Rth = {(nx + ny) / 2−nz} × d. The phase difference Rth is −30 nm or less.

    (I): negative containing an acrylic polymer (A) having a positive intrinsic birefringence containing a repeating unit (a) represented by the following general formula (1) and an aromatic vinyl monomer unit (b) A polymer (B) having an intrinsic birefringence of

    (II): The mass ratio (a / b) of the repeating unit (a) to the aromatic vinyl monomer unit (b) is 0.5-2.

    (III): The content of the repeating unit (a) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 25% by mass or more.

    (IV): The content of the aromatic vinyl monomer unit (b) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 20% by mass or more.

  The retardation film of the present invention is usually a uniaxially or biaxially stretchable film formed by stretching. The birefringence amount Δn due to orientation in stretching is governed by the degree of orientation f, and the birefringence when the polymer chain is completely oriented is defined as the intrinsic birefringence Δn0, and is expressed by the relationship of the following formula.

Δn = f × Δn0
When the polymer polymer chain constituting the thermoplastic resin composition is oriented by stretching, the refractive index anisotropy of the monomer repeating unit is reflected in the entire material made of the thermoplastic resin. And phase difference appears.

  The refractive index of the slow axis in the film plane obtained by stretching a resin having negative intrinsic birefringence is nx, the refractive index of the fast axis in the film plane is ny, and the refractive index in the thickness direction of the film is When the thickness of the film is d, the refractive indexes nx, ny, and nz are in the relationship of nz ≧ nx> ny, nz> nx ≧ ny, or nx> nz> ny, and the expression {(nx + ny) / The phase difference Rth in the thickness direction defined by 2-nz} × d is negative.

  In the present application, such a film having a negative retardation Rth in the thickness direction is defined as a “negative retardation film”. In the negative retardation film, the refractive indexes nx, ny, and nz are in a relationship of nz ≧ nx> ny, nz> nx ≧ ny or nx> nz> ny, and nx, ny, and nz are nz = nx> ny. In this relationship, the retardation film of the present invention becomes a negative A plate. When nx, ny, and nz are in a relationship of nz> nx = ny, the retardation film of the present invention is a positive C plate. nx, ny, and nz may be in a relationship of nx> nz> ny and nz> (nx + ny) / 2.

  That is, in the retardation film of the present invention, when the refractive indexes in the slow axis direction and the fast axis direction in the film plane are nx and ny, respectively, and the refractive index in the thickness direction of the film is nz, nx , Ny and nz may satisfy the following formula (i), (ii) or (iii).

nz> nx = ny (i)
nz = nx> ny (ii)
nx>nz> ny and nz> (nx + ny) / 2 (iii)
The glass transition temperature of the retardation film of the present invention is preferably 110 ° C. or higher, more preferably 115 ° C. or higher, and further preferably 120 ° C. or higher. The upper limit of the glass transition temperature is not particularly limited, but preferably 200 ° C. ° C or lower, more preferably 180 ° C or lower.

  The retardation film of the present invention has a thickness direction retardation Rth of −30 nm or less. The range is preferably −30 nm or less and −1000 nm or more, more preferably −50 nm or less and −500 nm or more, and further preferably −70 nm or less and −200 nm or more. The phase difference Rth is defined as nx as the refractive index of the slow axis in the film plane, ny as the refractive index of the fast axis in the film plane, nz as the refractive index in the film thickness direction, and d as the film thickness. Is given by the expression {(nx + ny) / 2−nz} × d. Note that the refractive indexes nx, ny, and nz in this specification are refractive indexes with respect to light having a wavelength of 589 nm.

  In the retardation film of the present invention, the in-plane retardation Re is, for example, in the range of 0 nm to 1000 nm. Preferably, it is the range of 20 nm or less and 500 nm or more, more preferably 50 nm or less and 300 nm or more. The in-plane phase difference Re is given by the formula (nx−ny) × d.

  The retardation exhibited by the retardation film of the present invention can also be controlled by adjusting the degree of stretching of the thermoplastic resin composition (C) (for example, adjusting the stretching method, stretching temperature, stretching ratio, etc.).

  Although the thickness of the retardation film of this invention is not specifically limited, For example, they are 10 micrometers-500 micrometers, 20 micrometers-300 micrometers are preferable, and 30 micrometers-150 micrometers are especially preferable.

  The retardation film of the present invention preferably has a total light transmittance of 85% or more. More preferably, it is 90% or more, More preferably, it is 91% or more. The total light transmittance is a measure of transparency, and if it is less than 85%, the transparency is lowered and it is not suitable as an optical film. Since the retardation film of the present invention has good compatibility between the acrylic polymer (A) and the polymer (B), a highly transparent retardation film can be obtained. Moreover, when using the rubbery polymer which has a styrene-type polymer for the said graft chain as a polymer (B), if a graft chain contains the structural unit derived from acrylonitrile, compatibility with an acrylic polymer (A) will be sufficient. Therefore, the rubbery polymer is uniformly dispersed in the resin composition, and the total light transmittance of the resulting retardation film is improved.

  By disposing a negative retardation film having a thickness direction retardation Rth and an in-plane retardation Re in the above ranges on an IPS mode LCD, light leakage when the screen is viewed obliquely can be suppressed. In addition, it is possible to realize image display with high contrast and low color shift.

  In the retardation film of the present invention, the values of the retardation Rth and retardation Re, and the relationship between the refractive indices nx, ny and nz can be selected according to the target optical characteristics.

  The retardation film of the present invention may be uniaxially stretchable or biaxially stretchable. It can be selected according to the desired optical characteristics such as phase difference.

  The retardation film of the present invention may have a laminated structure in which two or more layers having the same or different optical properties are laminated.

  The application of the retardation film of the present invention is not particularly limited, and can be used for the same applications as conventional retardation films. More specifically, the retardation film of the present invention can be used as an optical compensation film in an LCD in an IPS mode or an OCB (optically compensated birefringence) mode.

  The retardation film of the present invention can be combined with other optical members (for example, a retardation film) for the purpose of adjusting the retardation and wavelength dispersion.

  The retardation film of the present invention can be formed by a known method. For example, the thermoplastic resin composition (C) may be formed into a film, and the obtained resin film may be uniaxially or biaxially stretched in a predetermined direction.

  The method for forming the thermoplastic resin composition (C) into a film is not particularly limited. When the thermoplastic resin composition (C) is in the form of a solution, it may be cast, for example. When the thermoplastic resin composition (C) is solid, a molding technique such as melt extrusion or press molding may be used.

  The method of uniaxially or biaxially stretching the obtained resin film is not particularly limited, and a known method may be followed. Uniaxial stretching is typically free-end uniaxial stretching in which the change in the width direction of the film is free. Fixed-end uniaxial stretching is also possible in which the change in the width direction of the film is fixed. Biaxial stretching is typically sequential biaxial stretching, but simultaneous biaxial stretching in which longitudinal and transverse stretching are simultaneously performed can also be suitably used. Furthermore, it is also possible to stretch in the oblique direction with respect to stretching in the thickness direction or film roll. The stretching method, stretching temperature, and stretching ratio may be appropriately selected according to the target optical characteristics and mechanical characteristics.

[Polarizer]
The structure of the polarizing plate of the present invention is not particularly limited as long as it includes the retardation film of the present invention. The polarizing plate of the present invention has, for example, a structure in which a polarizer protective film is bonded to one side or both sides of a polarizer. At this time, at least one polarizer protective film may be the retardation film of the present invention, the polarizing plate has a layer other than the polarizer and the polarizer protective film, and the layer is the present invention. The retardation film may be used.

  The polarizer is not particularly limited. For example, a polarizer obtained by dyeing and stretching a polyvinyl alcohol film; a polyene polarizer such as dehydrated polyvinyl alcohol or dehydrochlorinated polyvinyl chloride; a multilayer laminate or a cholesteric liquid crystal Reflective polarizer used: a known polarizer such as a polarizer made of a thin film crystal film. Among these, a polarizer obtained by dyeing and stretching polyvinyl alcohol is preferable.

  A typical example of the structure of the polarizing plate of the present invention is that the polarizer is protected on one or both sides of a polarizer obtained by uniaxially stretching after dying a polyvinyl alcohol with a dichroic substance such as iodine or a dichroic dye. The film has a structure in which the retardation film of the present invention is bonded.

[Image display device]
The structure of the image display device of the present invention is not particularly limited as long as it includes the retardation film of the present invention. The image display device of the present invention is, for example, a liquid crystal display device (LCD), and the image display unit of the LCD device includes the retardation film of the present invention together with members such as a liquid crystal cell, a polarizing plate, and a backlight. The image display device of the present invention typically includes the retardation film of the present invention as an optical compensation film. The retardation film of the present invention may be provided as a polarizer protective film for a polarizing plate.

  The image display mode of the LCD is not particularly limited, but since the retardation film of the present invention is a negative retardation film, the IPS mode or the OCB mode is preferable.

<Glass transition temperature>
The glass transition temperature (Tg) of the polymer was determined according to JIS K7121. Specifically, a differential scanning calorimeter (manufactured by Rigaku Corporation, DSC-8230) is used to raise a temperature of about 10 mg from room temperature to 200 ° C. (temperature increase rate: 20 ° C./min) in a nitrogen gas atmosphere. From the obtained DSC curve, it was evaluated by the starting point method. Α-alumina was used as a reference.

<Weight average molecular weight>
The weight average molecular weight of the polymer was determined in terms of polystyrene using gel permeation chromatography (GPC). The apparatus and measurement conditions used for the measurement are as follows.

System: manufactured by Tosoh Column: TSK-GEL SuperHZM-M 6.0 × 150 2 in series Guard column: TSK-GEL SuperHZ-L 4.6 × 35 1 Reference column: TSK-GEL SuperH-RC 6.0 × 150 Two in series Eluent: Chloroform Flow rate 0.6 mL / min Column temperature: 40 ° C
<Refractive index anisotropy>
The in-plane retardation value Re and thickness direction retardation value Rth of the film at a wavelength of 589 nm were measured using RETS-100 manufactured by Otsuka Electronics Co., Ltd.

The in-plane retardation Re and the thickness direction retardation Rth in the retardation film are represented by Re = (nx−ny) × d and Rth = [(nx + ny) / 2−nz] × d, respectively. Here, nx is the refractive index in the slow axis direction in the plane of the retardation film (direction showing the maximum refractive index in the film plane), ny is the fast axis direction in the plane of the retardation film (in the film plane) In the direction perpendicular to nx), nz is the refractive index in the thickness direction of the retardation film, and d is the thickness of the retardation film.
The thickness d of the film was measured using a Digimatic micrometer (manufactured by Mitutoyo).

<Negative birefringence positive / negative>
The positive or negative of the intrinsic birefringence of the resin composition constituting the retardation film was evaluated based on the orientation angle of the film obtained using a fully automatic birefringence meter (manufactured by Oji Scientific Instruments, KOBRA-WR). . Specifically, free end uniaxial stretching is performed in a state where the film made of the resin composition is heated to a temperature 5 to 10 ° C. higher than the glass transition temperature, and the measured orientation angle is near 0 ° with respect to the stretching direction. In this case, the intrinsic birefringence of the polymer constituting the retardation film is positive, and when the measured orientation angle is around 90 °, the intrinsic birefringence of the resin composition constituting the retardation film is negative.

<Quantification of repeating unit (a)>
Using an acrylic polymer (A) resin pellet, an IR spectrum was measured, the absorption strength of an imide carbonyl group near 1670 cm −1, the absorption strength of an ester carbonyl group near 1720 cm −1, and an acid anhydride near 1760 cm −1. The content of the repeating unit (a) was determined from the ratio of the absorption intensity of the carbonyl group of the product.

<Quantification of aromatic vinyl monomer unit (b)>
Using the resin pellet of the polymer (B), 1H-NMR spectrum was measured, and the aromatic from the ratio of the area of hydrogen derived from the aromatic ring on the low magnetic field side and the area of hydrogen derived from the vinyl group on the high magnetic field side. The content of the vinyl monomer unit (b) was determined.

(Production Example 1)
As an acrylic polymer (A), Asahi Kasei has 70 parts by mass of Pleximide 8813 (glass transition temperature 132 ° C., weight average molecular weight 95000) manufactured by Daicel-Evonik, whose repeating unit (a) is 42% by mass, and polymer (B). 30 parts by mass of Stylac AS783 (73% by mass of styrene, 27% by mass of acrylonitrile, glass transition temperature 108 ° C., weight average molecular weight 220,000) are kneaded using a twin screw extruder, and the repeating unit (a) and aromatic A thermoplastic resin composition (C-1) having a mass ratio (a / b) with the vinyl monomer unit (b) of 1.3 was obtained. The obtained thermoplastic resin composition (C-1) had a glass transition temperature of 126 ° C. and a weight average molecular weight of 142,000.

(Production Example 2)
In the same manner as in Production Example 1, 65 parts by mass of Pleximide 8813 manufactured by Daicel Evonik as an acrylic polymer (A) and 35 parts by mass of Astykasei Stylac AS783 as a polymer (B) were used, and the mass ratio (a A thermoplastic resin composition (C-2) having a / b of 1.0 was obtained. The thermoplastic resin composition (C-2) obtained had a glass transition temperature of 123 ° C. and a weight average molecular weight of 152,000.

(Production Example 3)
In the same manner as in Production Example 1, 60 parts by mass of Daicel-Evonik's Pleximide 8813 as an acrylic polymer (A) and 40 parts by mass of Asahi Kasei's Stylac AS783 as a polymer (B) were used. A thermoplastic resin composition (C-3) having a / b of 0.8 was obtained. The thermoplastic resin composition (C-3) obtained had a glass transition temperature of 121 ° C. and a weight average molecular weight of 154,000.

(Production Example 4)
In the same manner as in Production Example 1, 90 parts by mass of Daicel-Evonik's pleximide 8813 was used as the acrylic polymer (A), and 10 parts by mass of Asahi Kasei Stylac AS783 was used as the polymer (B). A thermoplastic resin composition (C-4) having a / b of 4.9 was obtained. The thermoplastic resin composition (C-4) obtained had a glass transition temperature of 130 ° C. and a weight average molecular weight of 115,000.

(Production Example 5)
In the same manner as in Production Example 1, 80 parts by mass of Daicel-Evonik Pleximimide 8813 was used as the acrylic polymer (A), and 20 parts by mass of Asahi Kasei Stylac AS783 was used as the polymer (B). A thermoplastic resin composition (C-5) having a / b of 2.2 was obtained. The thermoplastic resin composition (C-5) obtained had a glass transition temperature of 128 ° C. and a weight average molecular weight of 128,000.

Example 1
The thermoplastic resin composition (C-1) prepared in Production Example 1 was coated from a coat hanger type T die (width 150 mm) at 280 ° C. using a single screw extruder (φ = 20 mm, L / D = 25). The film was melt-extruded and discharged onto a cooling roll having a temperature of 110 ° C. to produce a film having a thickness of 140 μm. Next, using the Instron universal testing machine, the produced film was uniaxially stretched at a stretching temperature of 131 ° C. so that the stretching ratio in the MD direction was 2.0 times, and a retardation film having a thickness of 94 μm. (FC-1) was obtained. Table 1 shows the physical properties of the retardation film (FC-1).

(Example 2)
Using the thermoplastic resin composition (C-2) prepared in Production Example 2, a film having a thickness of 135 μm was prepared in the same manner as in Example 1, and the draw ratio in the MD direction was determined using an Instron universal testing machine. Free end uniaxial stretching was performed at a stretching temperature of 128 ° C. so as to be 2.0 times to obtain a retardation film (FC-2) having a thickness of 97 μm. Table 1 shows the physical properties of the retardation film (FC-2).

(Example 3)
Using the thermoplastic resin composition (C-3) prepared in Production Example 3, a film having a thickness of 132 μm was prepared in the same manner as in Example 1, and the draw ratio in the MD direction was determined using an Instron universal testing machine. Free end uniaxial stretching was performed at a stretching temperature of 126 ° C. so as to be 2.0 times to obtain a retardation film (FC-3) having a thickness of 94 μm. Table 1 shows the physical properties of the retardation film (FC-3).

Example 4
Using the thermoplastic resin composition (C-2) prepared in Production Example 2, a film having a thickness of 140 μm was prepared in the same manner as in Example 1. Next, using the sequential biaxial stretching machine (X-6S manufactured by Toyo Seiki Seisakusho), the created film is stretched 1.5 times in the MD direction and 2.5 times in the TD direction. The film was sequentially biaxially stretched at a stretching temperature of 133 ° C. to obtain a retardation film (DC-2) having a thickness of 37 μm. Table 1 shows the physical properties of the retardation film (DC-2).

(Example 5)
Using the thermoplastic resin composition (C-3) prepared in Production Example 3, a film having a thickness of 142 μm was prepared in the same manner as in Example 1. Next, using the sequential biaxial stretching machine (X-6S manufactured by Toyo Seiki Seisakusho), the created film is stretched 1.5 times in the MD direction and 2.5 times in the TD direction. The film was sequentially biaxially stretched at a stretching temperature of 131 ° C. to obtain a retardation film (DC-3) having a thickness of 38 μm. Table 1 shows the physical properties of the retardation film (DC-3).

(Example 6)
Using the thermoplastic resin composition (C-3) prepared in Production Example 3, a film having a thickness of 142 μm was prepared in the same manner as in Example 1. Next, the prepared film is uniaxially stretched at a fixed end at a stretching temperature of 126 ° C. using a sequential biaxial stretching machine (X-6S manufactured by Toyo Seiki Seisakusho) so that the stretching ratio in the TD direction is 2.0 times. Thus, a retardation film (EC-3) having a thickness of 67 μm was obtained. Table 1 shows the physical properties of the retardation film (EC-3).

(Comparative Example 1)
Using the thermoplastic resin (C-4) produced in Production Example 4, a film having a thickness of 140 μm was produced in the same manner as in Example 1, and using an Instron universal testing machine, the draw ratio in the MD direction was 2. A free end uniaxial stretching was performed at a stretching temperature of 135 ° C. so as to be 0 times to obtain a retardation film (FC-4) having a thickness of 70 μm. Table 1 shows the physical properties of the retardation film (FC-4).

(Comparative Example 2)
Using the thermoplastic resin (C-5) produced in Production Example 5, a film having a thickness of 152 μm was produced in the same manner as in Example 1, and using an Instron universal testing machine, the draw ratio in the MD direction was 2. A free-end uniaxial stretching was performed at a stretching temperature of 135 ° C. so as to be 0 times to obtain a retardation film (FC-5) having a thickness of 98 μm. Table 1 shows the physical properties of the retardation film (FC-5).

(Comparative Example 3)
Using the thermoplastic resin composition (C-5) prepared in Production Example 5, a film having a thickness of 135 μm was prepared in the same manner as in Example 1. Next, using the sequential biaxial stretching machine (X-6S manufactured by Toyo Seiki Seisakusho), the created film is stretched 1.5 times in the MD direction and 2.5 times in the TD direction. The film was sequentially biaxially stretched at a stretching temperature of 138 ° C. to obtain a retardation film (DC-5) having a thickness of 35 μm. Table 1 shows the physical properties of the retardation film (DC-5).

The negative retardation film of the present invention can be widely used for image display devices such as a liquid crystal display device (LCD) as in the case of conventional retardation films. By using this retardation film, display characteristics in the image display device can be improved.

Claims (5)

It consists of a thermoplastic resin composition (C) satisfying the conditions (I) to (IV), the refractive index of the slow axis in the film plane is nx, the refractive index of the fast axis in the film plane is ny, and the film thickness The thickness direction retardation Rth represented by Rth = {(nx + ny) / 2−nz} × d, where nz is the refractive index in the vertical direction and d is the thickness of the film, is −30 nm or less. Phase difference film.
(I): negative containing an acrylic polymer (A) having a positive intrinsic birefringence containing a repeating unit (a) represented by the following general formula (1) and an aromatic vinyl monomer unit (b) A polymer (B) having an intrinsic birefringence of
(II): The mass ratio (a / b) of the repeating unit (a) to the aromatic vinyl monomer unit (b) is 0.5-2.
(III): The content of the repeating unit (a) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 25% by mass or more.
(IV): The content of the aromatic vinyl monomer unit (b) with respect to the sum of the acrylic polymer (A) and the polymer (B) is 20% by mass or more.

[Wherein, R 1, R 2 and R 3 each independently represents a hydrogen atom or an organic residue having 1 to 20 carbon atoms. ]   The retardation film according to claim 1, wherein the polymer (B) having negative intrinsic birefringence is an acrylonitrile-styrene copolymer.   The retardation film according to claim 1, wherein the glass transition temperature is 110 ° C. or higher.   A polarizing plate comprising the retardation film according to claim 1.   An image display apparatus provided with the retardation film of any one of Claim 1 to 3.