JP4116577B2 - Birefringent film, optical film, polarizing plate, and liquid crystal display device - Google Patents

Description

  The present invention relates to a birefringent film, an optical film, a polarizing plate using them, and a liquid crystal display device using them.

  Conventionally, a twisted nematic (TN) mode has been widely used as an operation mode of a liquid crystal cell of a liquid crystal display device. The liquid crystal display device using the TN mode liquid crystal cell has problems such as a slow response speed, a decrease in contrast in the perspective direction, and a color shift in the perspective direction in the black display portion. In comparison, there is also a problem that the display quality is inferior.

  Therefore, an OCB (Optically Compensate bend) mode liquid crystal cell has been proposed for the purpose of improving the response speed and display characteristics in the perspective direction (see, for example, Patent Document 1). Compared with the conventional TN mode liquid crystal cell, the OCB mode liquid crystal cell has a twist angle of the orientation of the liquid crystal carried between the upper and lower cell substrates, and the liquid crystal molecules are substantially in the upper and lower portions of the liquid crystal cell. The difference is that a nematic liquid crystal composition that is oriented in the opposite direction (contrast) (bend orientation) and that does not contain a chiral agent is used. Because of these characteristics, the OCB mode liquid crystal display device has a very wide viewing angle, high contrast, and a very fast response speed (about 1/20 to 1) compared to the conventional TN mode liquid crystal display device. / About 500) can be provided, and the possibility of CRT substitution is expected (see, for example, Patent Documents 2 and 3).

  Here, an OCB mode liquid crystal panel will be described with reference to FIG. FIG. 3A is an exploded perspective view schematically showing the configuration of the OCB mode liquid crystal panel. The liquid crystal panel includes an OCB mode liquid crystal cell 1 including liquid crystal molecules 4 bent-aligned between cell substrates, a biaxial retardation plate 2 for optical compensation, and two polarizing plates 3. The polarizing plate 3 is laminated on one surface of the cell 1, and the polarizing plate 3 is laminated on the other surface via the biaxial retardation plate 2. In the figure, an arrow a indicates the rubbing direction (liquid crystal alignment direction) of the cell substrate of the liquid crystal cell 1, the arrow b indicates the slow axis direction of the biaxial retardation plate 2, and the arrow c indicates a polarizing plate. 3 shows the absorption axis direction. FIG. 3B is a refractive index ellipsoid of the OCB mode liquid crystal cell 1 and represents positive birefringence (nx ′ <ny ′ <nz ′). In the figure, nx ′, ny ′, and nz ′ represent refractive indexes, ny ′ is the in-plane direction of the liquid crystal cell 1 and the refractive index in the alignment direction of the liquid crystal molecules (arrow a), and nx ′ is The refractive index in the in-plane direction of the liquid crystal cell 1 and perpendicular to the ny ′ direction, and nz ′ represents the refractive index in the thickness direction of the liquid crystal cell 1. FIG. 3C is a refractive index ellipsoid of the biaxial retardation plate 2 and represents negative birefringence (nx> ny> nz). In the figure, nx, ny, and nz represent refractive indexes, nx is the refractive index in the in-plane direction of the biaxial retardation plate 2 and in the slow axis direction (arrow b), and ny is 2 The refractive index is in the in-plane direction of the axial retardation plate 2 and perpendicular to the nx direction, and nz represents the refractive index in the thickness direction of the biaxial retardation plate 2.

  In order to obtain a good black display in such an OCB mode liquid crystal display device, the display mode is preferably a normally white (NW) mode in which white is applied when no electric field is applied and black is applied when an electric field is applied. The alignment of the liquid crystal molecules in the black display is parallel to the normal direction of the cell substrate of the liquid crystal cell, that is, the liquid crystal molecules are in a substantially upright state. As shown in FIG. 3B, the anisotropy in the thickness direction is very large (nx <ny <nz). Therefore, the biaxial retardation plate used for optical compensation of the liquid crystal cell has a negative biaxial birefringence (nx> ny> as shown in FIG. 3C so that the birefringence ellipsoid of the liquid crystal panel is spherical. nz) optical anisotropy, that is, the thickness direction retardation represented by “(nx-nz) × thickness” is large and represented by “(nx-nz) / (nx-ny)”. This is because the ellipticity coefficient Z of the refractive index ellipsoid to be formed is required to be large.

However, with the conventional technology, it is difficult to produce a biaxial retardation plate having a large retardation in the thickness direction and a large elliptic coefficient Z. As a typical biaxial retardation plate, for example, a film obtained by biaxially stretching a polycarbonate or a norbornene polymer can be mentioned. These films have a thickness of, for example, about 25 μm to 100 μm and can be obtained. The phase difference range is also narrow. Therefore, for example, in order to obtain sufficient optical characteristics capable of optically compensating the OCB mode liquid crystal display device, it is necessary to laminate a plurality of the films. As a result, the liquid crystal display device is made thicker and heavier. However, there is a problem that the optical axis shift and the light transmittance decrease due to the lamination of the films, and the display characteristics of the liquid crystal display device deteriorate. Further, when the film becomes thick, when a polarizing plate is laminated and used for a liquid crystal display device, the biaxial retardation plate may be deformed due to distortion of the polarizing plate caused by shrinkage. Due to this deformation, a slow axis shift or a phase difference change occurs in the biaxial retardation plate, and for example, there is a problem that light leaks around the display screen of the liquid crystal display device. . It should be noted that such a shift of the slow axis and a change in the retardation value increase as the thickness of the biaxial retardation plate increases.
Japanese Patent Laid-Open No. 7-84254 US Pat. No. 4,583,825 US Pat. No. 5,410,422

  As described above, in the conventional optical film, when optical compensation is performed for a liquid crystal cell, in particular, an OCB mode liquid crystal cell, there is a problem that an appearance defect occurs on the display screen due to thickening or lamination of the film. is there.

  Therefore, the present invention has excellent optical characteristics for optical compensation, and when used in an image display device, for example, a composite image that can maintain excellent display quality without problems in appearance such as light leakage. The object is to provide a refractive film.

In order to achieve the above object, the birefringent film of the present invention has a thickness (d), a thickness direction retardation (Rth) measured with light having a wavelength of 590 nm, and an elliptic coefficient (Z) of a refractive index ellipsoid. All the following conditions (i) to (iii) are satisfied.
0.2 μm ≦ d ≦ 10 μm (i)
300 nm ≦ Rth ≦ 1500 nm (ii)
6 <Z <20 (iii)
In the above conditions, Rth and Z are respectively calculated by the following formulas, where nx, ny and nz represent the refractive indexes in the X-axis, Y-axis and Z-axis directions of the birefringent film, The X-axis direction is an axial direction showing the maximum refractive index in the plane of the birefringent film, the Y-axis direction is an axial direction perpendicular to the X-axis in the plane, and the Z-axis direction is The thickness direction is perpendicular to the X axis and the Y axis.
Rth = (nx−nz) · d
Z = (nx−nz) / (nx−ny)

  If the birefringent film satisfies all of the above conditions (i) to (iii), for example, a liquid crystal cell in which a large thickness direction retardation (Rth) is required for optical compensation, particularly an OCB mode liquid crystal cell. It can be fully compensated and can maintain excellent display quality when used in a liquid crystal display device.

  Specifically, since the birefringent film of the present invention is thin, for example, even if it is laminated with a polarizing plate and used in an image display device, the shift and position of the slow axis due to the distortion of the polarizing plate as described above. Changes in the phase difference value can be suppressed, and deterioration in display quality such as light leakage around the screen can be prevented. Furthermore, since the birefringent film of the present invention is thin, for example, the image display device can be thinned. Further, since the birefringent film of the present invention has a large thickness direction retardation, it is particularly effective for optical compensation of an OCB mode liquid crystal cell.

  In the birefringent film of the present invention, the thickness (d) is 0.2 μm or more and 10 μm or less (condition (i)). When the thickness is 0.2 μm or more, a phase difference capable of optical compensation can be exhibited. Moreover, if the said thickness is 10 micrometers or less, it can manufacture uniformly, and thickness reduction is further attained. The thickness of the birefringent film of the present invention is preferably 1 μm to 8 μm, and more preferably 2 μm to 6 μm.

  The birefringent film of the present invention has a thickness direction retardation (Rth) measured with light having a wavelength of 590 nm of 300 nm to 1500 nm (condition (ii)). If the thickness direction retardation is 300 nm or more, it is sufficient for optical compensation, and for example, an OCB mode liquid crystal cell can be compensated. The thickness direction retardation is preferably 300 nm to 1000 nm, and more preferably 400 nm to 1000 nm.

  In the birefringent film of the present invention, the ellipticity coefficient Z of the refractive index ellipsoid is more than 6 and less than 20 (condition (iii)). When the elliptic coefficient Z exceeds 6, for example, sufficient contrast is obtained when used in a liquid crystal display device such as an OCB mode. When the elliptic coefficient Z is less than 20, a conventional negative uniaxial film is obtained. Excellent viewing angle characteristics compared to when using. The elliptic coefficient Z is preferably more than 6 and less than 15, more preferably more than 7 and less than 10.

  Because all of the conditions (i) to (iii) are satisfied, the birefringent film of the present invention has the excellent effects of excellent optical compensation and display quality as described above.

In the birefringent film of the present invention, the front phase difference (Re) measured with light having a wavelength of 590 nm is preferably 20 nm or more, more preferably 30 nm to 400 nm, and even more preferably 40 nm to 300 nm. The front phase difference (Re) is calculated by the following formula. In the following formula, nx, ny and d are as described above.
Re = (nx−ny) · d

  Next, the polymer material used for the birefringent film of the present invention will be described.

The polymer material contained in the birefringent film of the present invention is not particularly limited as long as it can form a birefringent film satisfying all of the above conditions (i) to (iii), but the repeating unit consisting of the following general formula (1) The polyimide which has is preferable, More preferably, it is a polyimide which has a repeating unit which consists of following Structural formula (2).

In the general formula (1), A and B are each independently a substituent selected from the group consisting of halogen, alkyl having 1 to 3 carbon atoms, alkyl halide having 1 to 3 carbon atoms, phenyl and substituted phenyl. And the substituents of the substituted phenyl are each independently a substituent selected from the group consisting of hydrogen, halogen, alkyl having 1 to 3 carbon atoms, and alkyl halide having 1 to 3 carbon atoms. m, x, and y each represent the number of substitutions (integer), m is 0 to 3, x and y are 0 or 1, and at least one of x and y is 1. The number of substitutions of 0 indicates that the substituent is hydrogen.

  The polyimide having the repeating unit consisting of the general formula (1) and the structural formula (2) is a novel polyimide synthesized by the present inventors to obtain a birefringent film that satisfies the above conditions (i) to (iii). . Such a polyimide is preferable because it is a polyimide having a higher birefringence in the thickness direction and higher solubility in an organic solvent.

  Polyimide having a repeating unit of the above structural formula (2) is, for example, 2,2′-dichloro-4,4 ′, 5,5′-biphenyltetracarboxylic dianhydride (DCBPDA), 2,2 ′. -Bis (trifluoromethyl) -4,4'-diaminobiphenyl (PFMB or TFMB). Although the synthesis method is not particularly limited, for example, it can be synthesized using a conventionally known method called so-called thermal imidization or chemical imidization. These (1) and (2) have a weight average molecular weight (MW) of, for example, 10,000 to 1,000,000, preferably 20,000 to 500,000.

  The polymer material may be further mixed with another resin having a different structure as long as the orientation is not significantly lowered. Examples of such a mixing resin include general-purpose resins, engineering plastics, thermoplastic resins, and thermosetting resins. When blending these mixing resins into the polymer material, the blending amount is not particularly limited as long as the orientation does not significantly decrease, but usually, for example, 0 to 50% by mass with respect to the polymer material. Preferably, it is 0-30 mass%.

  Examples of the general-purpose resin include polyethylene (PE), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), ABS resin, and AS resin. Examples of the engineering plastic include polyacetate (POM), polycarbonate (PC), polyamide (PA: nylon), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT). Examples of the thermoplastic resin include polyphenylene sulfide (PPS), polyethersulfone (PES), polyketone (PK), polyimide (PI), polycyclohexanedimethanol terephthalate (PCT), polyarylate (PAR), and liquid crystal polymer. (LCP) and the like. When the liquid crystal polymer is used as the mixing resin, particularly when the mixture is compatible with each other and the mixture exhibits liquid crystallinity, a magnetic field, an electric field, an alignment film, stretch alignment, fluid alignment It can also be expected to control the orientation by, for example. When the mixing resin is a material containing a photoreactive functional group that undergoes photoisomerization or photodimerization, three-dimensional molecular orientation can be controlled by light irradiation. Examples of the thermosetting resin include an epoxy resin and a phenol novolac resin.

  Next, the manufacturing method of the birefringent film of this invention is demonstrated. The birefringent film of the present invention includes, for example, a step of applying a solution containing the above-described polymer to a substrate to form a coating film, and a step of aligning the coating film to form a birefringent film. It can manufacture with the manufacturing method containing. In addition, the coating film may be subjected to a drying treatment after being formed on the substrate and before the orientation treatment. Below, an example of the manufacturing method of this invention is demonstrated concretely.

(1: coating process)
First, a polymer material containing polyimide is dissolved in a solvent to prepare a polymer solution, which is applied to a substrate to form a coating film. This coating treatment is performed by an appropriate method such as a spin coating method, a roll coating method, a flow coating method, a print coating method, a dip coating method, a casting film forming method, a bar coating method, or a gravure printing method. be able to. The amount of the polymer material added to the solvent is preferably a viscosity suitable for the workability of coating, and is, for example, 5 to 50 parts by weight, preferably 10 to 100 parts by weight of the solvent. Parts by weight to 40 parts by weight. The thickness of the coating film is preferably 0.4 μm to 200 μm, more preferably 2 μm to 100 μm, when the polyimide concentration of the polymer solution is, for example, 10% by mass.

  The base material is not particularly limited as long as the base material can be coated with a polymer material. For example, it may be a plastic substrate or a substrate of an inorganic compound such as a glass substrate or a silicon wafer. Examples of the plastic substrate include those produced by a casting method, those produced by forming a melted polymer and then subjecting it to a stretching treatment, and among these, precise coating accuracy is possible. Therefore, a plastic substrate whose mechanical strength is increased by the stretching treatment is preferable.

  When the laminate in which the birefringent film of the present invention is formed on the substrate is used as it is as the optical film of the present invention, the substrate is preferably a transparent film formed from a polymer having excellent transparency, for example. . Further, since the birefringent film of the present invention functions as an optical film that is a composite with a phase difference plate, it becomes possible to design a more precise optical characteristic for wide viewing angle of the liquid crystal display panel. As the substrate, for example, a retardation plate that produces a phase difference when stretched can also be used. Specifically, examples of the base material include base materials that can change the wavelength dispersion of the refractive index, the angle of the slow axis, and the like in each layer. Examples of a material for forming such a substrate include acetate resin such as triacetyl cellulose (TAC), polyester resin, poly (ether sulfone) resin, poly (ether ketone) resin, poly (amidoimide) resin, polysulfone resin, polycarbonate Resins, polyamide resins, polyimide resins, polyolefin resins, acrylic resins, norbornene resins, cellulose resins, polyarylate resins, polystyrene resins, polyvinyl alcohol resins, polyvinyl chloride resins, polyvinylidene chloride resins, polymethyl methacrylate resins, and mixtures thereof Etc. Further, a liquid crystal polymer or the like can be used, and further, for example, a thermoplastic resin having a substituted imide group or an unsubstituted imide group in the side chain as described in JP-A-2001-343529 (WO01 / 37007). Also, a mixture of a thermoplastic resin having a substituted phenyl group or an unsubstituted phenyl group and a nitrile group in the side chain can be used. Specific examples include a resin composition having an alternating copolymer composed of isobutene and N-methylmaleimide and an acrylonitrile / styrene copolymer. These substrates have been subjected to appropriate treatment on the surface for any purpose, for example, to improve adhesion with polyimide or to prevent cracking of the substrate due to solvent penetration. May be.

  The thickness of the substrate is, for example, 12 μm or more and 200 μm or less, preferably 20 μm or more and 150 μm or less, and more preferably 25 μm or more and 100 μm or less. If the thickness is 12 μm or more, sufficiently precise coating accuracy can be obtained, and if it is 200 μm or less, the amount of distortion can be further suppressed and appearance defects can be prevented when mounted on a liquid crystal panel.

  Examples of the commercially available optical film that can be used as the base material described above include, for example, a product name “Fujitac” manufactured by Fuji Photo Film Co., Ltd., a product name “Zeonor” manufactured by Nippon Zeon Co., Ltd., and a product name “Arton” manufactured by JSR Corporation. Or the like. Moreover, you may use an optical film provided with the optical compensation layer already comprised from the inclination alignment layer of a liquid crystal polymer. Examples of such an optical film include a product name “WV film” manufactured by Fuji Photo Film Co., Ltd. and a product name “NH film” manufactured by Nippon Oil Corporation.

  The solvent for the polymer material is not particularly limited as long as it can sufficiently dissolve the polymer material, and can be appropriately determined according to the type of the polymer material. The solvent is preferably a solvent that does not have too strong a dissolving power for the substrate. Specifically, for example, halogenated hydrocarbons such as chloroform, dichloromethane, carbon tetrachloride, dichloroethane, tetrachloroethane, trichloroethylene, tetrachloroethylene, chlorobenzene and orthodichlorobenzene; phenols such as phenol and parachlorophenol; benzene, toluene, Aromatic hydrocarbons such as xylene, methoxybenzene, 1,2-dimethoxybenzene; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, cyclopentanone, 2-pyrrolidone, N-methyl-2-pyrrolidone; Ester solvents such as ethyl acetate and butyl acetate; t-butyl alcohol, glycerin, ethylene glycol, triethylene glycol, ethylene glycol monomethyl ether, diethylene glycol Alcohol solvents such as dimethyl ether, propylene glycol, dipropylene glycol, 2-methyl-2,4-pentanediol; amide solvents such as dimethylformamide and dimethylacetamide; nitrile solvents such as acetonitrile and butyronitrile; diethyl ether And ether solvents such as dibutyl ether and tetrahydrofuran; carbon disulfide, ethyl cellosolve, butyl cellosolve and the like. These solvents may be used alone or in combination of any two or more in order to improve the smoothness of the birefringent film to be coated. Among the solvents, preferable solvents include ketone solvents such as cyclopentanone, cyclohexanone, methyl isobutyl ketone, and methyl ethyl ketone.

  Moreover, you may mix | blend an additive with the said polymer solution as needed. Examples of the additive include additives for an arbitrary purpose such as an ultraviolet ray preventing agent, a deterioration preventing agent, a plasticizer, an antistatic agent, and an additive for improving the adhesion to the substrate described above. Examples of the preventive deterioration agent include an antioxidant, a peroxide decomposer, a radical inhibitor, a metal deactivator, an acid scavenger, and an amine. The blending amount of the additive is preferably within a range that does not impair the optical properties of the birefringent film of the present invention.

(2: Drying process)
Next, the coating film formed on the base material is subjected to a drying treatment, and the coating film is fixed, whereby an optically negative uniaxial property (nx> nz and nx>) is formed on the base material. ny) immobilized membrane (immobilized membrane) is formed. Examples of the drying treatment include natural drying and heat drying (for example, 40 ° C. to 350 ° C.). The heat drying may be a one-step heat treatment, but a drying treatment including two or more heat-drying steps is preferable. This is because the amount of residual solvent can be reduced without impairing the appearance uniformity by performing at least two stages of heat drying treatment. The number of drying processes of two or more stages is not particularly limited, but is, for example, 2 to 30 stages, preferably 2 to 20 stages, and more preferably 2 to 10 stages.

  A specific example of a two-stage drying process including a first drying process (also referred to as a pre-cure process) and a second drying process (a post-cure process) will be described below. The treatment temperature of the pre-curing treatment is, for example, 40 ° C. or more and less than 150 ° C., preferably 40 ° C. to 140 ° C., more preferably 40 ° C. to 120 ° C., and further preferably 40 ° C. to 100 ° C. The treatment time for the pre-curing treatment is, for example, 0.5 minutes to 10 minutes, preferably 0.5 minutes to 8 minutes, and more preferably 0.5 minutes to 5 minutes. And the processing temperature of a post-cure process is 150 degreeC or more and less than 350 degreeC, for example, 150 to 300 degreeC is preferable, More preferably, it is 150 to 200 degreeC. The processing time of the post-curing treatment is, for example, 1 minute to 60 minutes, preferably 1 to 40 minutes, and more preferably 1 to 30 minutes.

  In the case of a drying process consisting of three stages, for example, the first drying process is 50 ° C. to 90 ° C., the second drying process is 100 ° C. to 130 ° C., and the third drying process is 140 ° C. to 350 ° C. The condition may be less than or equal to the condition.

  Although the residual solvent amount of the birefringent film finally obtained is not particularly limited, it is preferably 3% by mass or less, and more preferably 1% by mass or less. The smaller the amount of residual solvent, the better the dimensional stability and the birefringent film in which the change in retardation over time is suppressed. The amount of residual solvent of the birefringent film is measured after, for example, “mass of birefringent film including residual solvent” in which only the birefringent film is placed in an aluminum cup or the like, and after heating the aluminum cup or the like at 200 ° C. for 2 hours. It can be determined from the difference from “mass of birefringent film after heating”.

(3: Orientation process)
Next, the birefringent film of the present invention is formed by subjecting the immobilization film formed on the substrate by the drying step to an orientation treatment that imparts an in-plane refractive index anisotropy. Anisotropy is imparted to the refractive index in the in-plane direction to the immobilization film that is optically negative uniaxial (nx> nz and nx> ny), thereby making the birefringent film optically two-dimensional. Axiality (nx>ny> nz) can be obtained. The method for the alignment treatment is not particularly limited, and examples thereof include a stretching treatment and a shrinking treatment that can easily control the retardation value. These treatments may be performed simultaneously with the drying treatment.

  Examples of the stretching treatment include, for example, free-end longitudinal stretching that extends uniaxially in the longitudinal direction, fixed-end lateral stretching that fixes the longitudinal direction and stretches uniaxially in the width direction, and sequential or simultaneous stretching that extends in both the longitudinal direction and the width direction. Biaxial stretching and the like can be mentioned. These stretching treatments are preferably performed by stretching a base material that can be stretched to the base material. According to this method, the base material is uniformly stretched by the tension applied to the base material, and the double-immobilized film can be indirectly uniformly stretched along with the stretching. This method can be applied to a continuous production process, and is preferable in terms of increasing the mass productivity of the product. In addition, you may extend | stretch the said fixed film | membrane with the said base material.

  As the shrinkage treatment, for example, an anisotropic dimensional change of a plastic base material is used, or a base material having a positive shrinkage performance is used to indirectly shrink the immobilized film on the base material. There are methods. At this time, it is preferable to control the shrinkage rate using a stretching machine or the like. As the control method, for example, a method of temporarily opening the clip of the stretching machine and relaxing in the transfer direction of the base material, a method of gradually narrowing the interval between the clips of the stretching machine, etc. . Another example is a method of fixing a fixed film in a drying stage containing a residual solvent to a metal frame and drying by heating (see, for example, 1994 Electronic Information Communication Society Spring Conference Proceedings C-338).

  The ellipticity coefficient Z (= (nx-nz) / (nx-ny)) of the birefringence ellipsoid of the birefringent film of the present invention is controlled by the thickness direction retardation Rth (= (nx-nz) · d) and This can be done by controlling the front phase difference Re (= (nx−ny) · d). The Rth and Re can be controlled, for example, by adjusting the draw ratio, shrink ratio, stretch ratio, shrink ratio, polymer material used, molecular weight, coating film thickness, etc. in the alignment treatment, and the elliptic coefficient Z can be controlled within the range of the condition (iii) while satisfying the conditions (i) and (ii).

  A specific example in which the elliptic coefficient Z is controlled within the range of the condition (iii) based on, for example, the relationship between the draw ratio and Rth and Re will be described with reference to FIG. FIG. 2A is a graph showing an example of the relationship between the draw ratio of the coating film and the retardation value (Rth, Re), and FIG. 2B shows the relationship between the draw ratio and the value of the elliptic coefficient Z. It is a graph which shows an example. Specifically, a polyimide (weight average molecular weight 82,500) solution composed of the repeating unit represented by the structural formula (2) is applied onto a TAC substrate, and a 7.0 μm thick polyimide coating is applied. As a result of measuring the phase difference (Rth, Re) of the birefringent film in the case where a birefringent film was formed by stretching the substrate together with the coating film at a stretch ratio of 0 to 15%. FIG. 2A is a graph showing an example, and FIG. 2B shows the value of the elliptic coefficient Z obtained from these values. As shown in FIG. 2A, the draw ratio and the thickness direction retardation value Rth, the draw ratio and the in-plane direction retardation value Re are linearly related, and the relation between the draw ratio and the value of the elliptic coefficient Z is, for example, It can be represented by a graph such as 2B. Therefore, the ellipticity coefficient Z can be controlled by adjusting the draw ratio based on these graphs.

  As described above, the birefringent film of the present invention that satisfies all of the conditions (i) to (iii) can be produced. The birefringent film of the present invention may be peeled off from the base material and used as a birefringent film alone, or may be used as an optical film of the present invention comprising a laminate with the base material. It is good also as an optical film of this invention which consists of a laminated body which laminated | stacked the birefringent film of invention on another base material etc. through the adhesive agent or the adhesive. However, since the birefringent film of the present invention and the optical film of the present invention are optical members, they are preferably as colorless and transparent as possible. Specifically, in the birefringent film of the present invention and the optical film of the present invention, the light transmittance in the wavelength region of 400 nm to 800 nm is preferably 80% or more, more preferably 90% or more. . The light transmittance can be measured using, for example, a spectrophotometer.

  Next, the use of the birefringent film of the present invention will be described.

  The birefringent film of the present invention can be used alone as a film for optically compensating a liquid crystal cell, particularly an OCB mode liquid crystal cell. Further, the birefringent film of the present invention can be used as a λ / 4 plate or a λ / 2 plate by appropriately setting the front phase difference Re (Re590) at a wavelength of 590 nm. When the birefringent film of the present invention is used as a λ / 4 plate, the Re590 is preferably set to 120 nm to 160 nm, for example. When the birefringent film of the present invention is used as a λ / 2 plate, the Re590 is preferably set to 240 nm to 320 nm, for example.

  The birefringent film of the present invention can be combined with other optical members as necessary to form the optical film of the present invention used for various optical applications. The optical film of this invention should just contain the birefringent film of this invention, and it does not restrict | limit especially as said optical member, For example, a polarizer, a polarizing plate, a phase difference plate etc. can be used.

  The polarizing plate of the present invention is a polarizing plate including the birefringent film of the present invention and a polarizer, and the structure and configuration are not limited as long as the birefringent film and the polarizer of the present invention are included. The method for laminating the polarizing plate of the present invention is not particularly limited, and can be carried out by a conventionally known method. Generally, an adhesive or pressure sensitive adhesive described later can be used.

  The polarizing plate is usually composed of a polarizer and a transparent protective film for the polarizer. In the polarizing plate of the present invention, for example, the birefringent film of the present invention or the optical film of the present invention is the transparent protective film. Can be used as a film.

  The polarizer is not particularly limited, and for example, by dying dichroic substances such as iodine and dichroic dyes on various films by using a conventionally known method, dyeing, crosslinking, stretching, and drying. The prepared one can be used. Among these, a film that transmits linearly polarized light when natural light is incident is preferable, and a film that is excellent in light transmittance and degree of polarization is preferable. Examples of the various films that adsorb the dichroic substance include highly hydrophilic films such as polyvinyl alcohol (PVA) film, partially formalized PVA film, ethylene / vinyl acetate copolymer partially saponified film, and cellulose film. In addition to these, for example, polyene oriented films such as PVA dehydrated products and polyvinyl chloride dehydrochlorinated products can be used. Among these, PVA film is preferable. The thickness of the polarizing film is usually in the range of 1 μm to 80 μm, but is not limited thereto.

  The transparent protective film is not particularly limited, and in addition to the birefringent film and the optical film of the present invention, a conventionally known transparent film can be used, but it has excellent transparency, mechanical strength, thermal stability, moisture barrier properties, etc. Is preferred. For example, the transmittance of the transparent protective film is preferably 80% or more, more preferably 90% or more in a wavelength region of 400 nm to 800 nm. Although there may or may not be a retardation of the transparent protective film, those which do not hinder the optical design of the polarizing plate of the present invention are preferred. As a material for such a transparent protective film, for example, the above-mentioned base material that can be used as an optical film in combination with the birefringent film of the present invention can be used. The thickness of the transparent protective film is not particularly limited, and can be appropriately determined according to, for example, a phase difference or a protective strength, but is usually 500 μm or less, preferably 5 μm to 300 μm, more preferably 5 μm to 150 μm. It is.

  The surface of the transparent protective film may be subjected to conventionally known surface treatment. Examples of the surface treatment include hard coat treatment, antireflection treatment, treatment for preventing sticking and diffusion, antiglare, and the like.

  The hard coat treatment is for the purpose of preventing scratches on the surface of the polarizing plate, for example, a treatment for forming a cured film having excellent hardness and slipperiness composed of a curable resin on the surface of the transparent protective layer. is there. As the curable resin, for example, a silicone-based, urethane-based, acrylic-based, or epoxy-based ultraviolet curable resin can be used, and the treatment can be performed by a conventionally known method. The purpose of preventing sticking is to prevent adhesion between adjacent layers. The antireflection treatment is intended to prevent reflection of external light on the surface of the polarizing plate, and can be performed by forming a conventionally known antireflection layer or the like. The anti-glare treatment is intended to prevent visual interference of the light transmitted through the polarizing plate due to reflection of external light on the surface of the polarizing plate. For example, the anti-glare treatment is finely applied to the surface of the transparent protective layer by a conventionally known method. This can be done by forming an uneven structure. Examples of a method for forming such a concavo-convex structure include a roughening method by sandblasting or embossing, a method of forming the transparent protective layer by blending transparent fine particles in the transparent resin as described above, and the like. It is done. Examples of the transparent fine particles include silica, alumina, titania, zirconia, tin oxide, indium oxide, cadmium oxide, antimony oxide, and the like. In addition, conductive inorganic fine particles, crosslinked or uncrosslinked Organic fine particles composed of polymer particles and the like can also be used. The average particle size of the transparent fine particles is not particularly limited, but is, for example, in the range of 0.5 μm to 20 μm. The blending ratio of the transparent fine particles is not particularly limited, but is generally preferably in the range of 2 to 70 parts by mass and more preferably in the range of 5 to 50 parts by mass per 100 parts by mass of the transparent resin as described above. The antiglare layer containing the transparent fine particles can be used as, for example, the transparent protective layer itself, or may be formed as a coating layer on the surface of the transparent protective layer. Furthermore, the anti-glare layer may also serve as a diffusion layer (visual compensation function or the like) for diffusing the light transmitted through the polarizing plate to expand the viewing angle.

  Examples of the retardation plate used in combination with the birefringent film of the present invention in the optical film of the present invention include a polymer film and a film in which a liquid crystal compound is immobilized. The configuration is not particularly limited except that the birefringent film of the present invention and the retardation plate are laminated directly or indirectly. The method for laminating the birefringent film of the present invention and the retardation plate is not particularly limited, and can be performed by a conventionally known method. In general, the adhesives and pressure-sensitive adhesives described later can be used.

  The polymer film preferably has a light transmittance of 80% or more, and is preferably one in which birefringence hardly occurs due to external force. Examples of such a polymer film include cellulose polymers, norbornene polymers, polymethyl methacrylate, and the like. As the cellulose polymer, cellulose ester is preferable. Examples of the norbornene polymer include trade name “ZEONOR” manufactured by Nippon Zeon Co., Ltd., trade name “ARTON” manufactured by JSR Corporation, and the like. The surface of the polymer film may be subjected to surface treatment such as glow discharge treatment, corona discharge treatment, and ultraviolet irradiation treatment. In addition, the polymer film may be appropriately subjected to uniaxial stretching, biaxial stretching, and Z-axis alignment treatment in accordance with the characteristics of the liquid crystal display device to be used.

  Examples of the liquid crystal material of the film on which the liquid crystal compound is fixed include discotic liquid crystal, nematic liquid crystal, cholesteric liquid crystal, smectic liquid crystal, polymerizable liquid crystal, and lyotropic liquid crystal. Specific examples of the liquid crystal material include Schiff liquid crystal, azoxy liquid crystal, alkylcyanobiphenyl liquid crystal, alkylcyanoterphenyl liquid crystal, alkylcyanoquaterphenyl liquid crystal, cyanophenylcyclohexane liquid crystal, and cyanophenyl ester liquid crystal. Benzoic acid phenyl ester liquid crystal, phenyl pyrimidine liquid crystal, phenyl dioxane liquid crystal and the like. However, the liquid crystal material is not limited thereto, and a compound that exhibits a liquid crystal phase in a certain temperature range can be used. The dielectric anisotropy and refractive index anisotropy of the liquid crystal may be negative or positive. The liquid crystal may be composed of a single liquid crystal material or a mixture of two or more liquid crystal materials. The mixture may exhibit a liquid crystal phase in a certain temperature range, or may exhibit a liquid crystal phase in a temperature range in which at least one compound alone forms the mixture.

  The optical film of the present invention is an optical member used in combination with the birefringent film of the present invention, and examples of the optical member other than the polarizing plate and the retardation plate include a diffusion control film and a brightness enhancement film. . Examples of the diffusion control film include a film using diffusion, scattering, and refraction for controlling the viewing angle, and a film using diffusion, scattering, and refraction for controlling the resolution-related glitter, scattered light, and the like. . Examples of the brightness enhancement film include a film using selective reflection of cholesteric liquid crystal and a λ / 4 plate, a film using anisotropic scattering by the polarization direction, a film using a wire grid polarizer, and the like. Nitto Denko's trade name “PCF350”, Merck's trade name “Transmax”, 3M's trade name “D-BEF”, and the like.

  The optical film and the polarizing plate including the birefringent film of the present invention as described above may be obtained by further laminating one or more optical members, for example. An optical member in which two or more optical layers are laminated in this manner can be formed by a method of sequentially laminating separately, for example, in the manufacturing process of a liquid crystal display device or the like. There are advantages such as excellent quality stability and assembly workability, and improvement in manufacturing efficiency of liquid crystal display devices and the like. In addition, various adhesion | attachment means, such as an adhesive, can be used for lamination | stacking, for example.

  The adhesive or pressure-sensitive adhesive used for the birefringent film, optical film and polarizing plate of the present invention is not particularly limited, and examples thereof include acrylic, vinyl alcohol, silicone, polyester, polyurethane, and polyether. A polymer pressure-sensitive adhesive or a rubber-based pressure-sensitive adhesive can be used. Moreover, it is good also as an adhesive agent etc. which contain microparticles | fine-particles in these materials and show light diffusibility. Among these, as the adhesive or pressure-sensitive adhesive, for example, a material excellent in hygroscopicity and heat resistance is preferable. With such a property, for example, when used in a liquid crystal display device, it can prevent foaming and peeling due to moisture absorption, deterioration of optical characteristics due to a difference in thermal expansion, warpage of the liquid crystal cell, etc., and high quality and durability. The display device is also excellent. As the adhesive, an adhesive composed of a water-soluble crosslinking agent of vinyl alcohol polymer such as glutaraldehyde, melamine or oxalic acid can be used. In addition, when the birefringent film or optical film of the present invention is bonded to a polarizer, and the polarizer is a PVA-based film, for example, from the viewpoint of stability of the bonding treatment, a PVA-based adhesive Is preferred. Although the thickness in particular of the said adhesive agent is not restrict | limited, For example, they are 1 nm-500 nm, Preferably they are 10 nm-300 nm, More preferably, they are 20 nm-100 nm. These adhesives and pressure-sensitive adhesives may be applied, for example, to the surface of the optical film of the present invention or other optical members, or a layer such as a tape or sheet composed of the adhesives or pressure-sensitive adhesives is It may be arranged on the surface. In addition, when apply | coating the said adhesive agent or an adhesive, you may mix | blend other additives and catalysts, such as an acid, with the aqueous solution of the said adhesive agent or an adhesive as needed.

  The birefringent film, the optical film and the polarizing plate of the present invention preferably have a pressure-sensitive adhesive layer or an adhesive layer, for example, since it can be easily laminated to other members such as a liquid crystal cell. These can be arranged on one side or both sides of the birefringent film or the like. From the viewpoint of prevention of foaming and peeling due to moisture absorption, deterioration of optical characteristics due to thermal expansion difference and the like, prevention of warpage of liquid crystal cells, and formation of a liquid crystal display device with high quality and excellent durability, for example, It is preferable that the pressure-sensitive adhesive layer has a low moisture absorption rate and excellent heat resistance. Moreover, the adhesion layer etc. which contain microparticles | fine-particles and show light diffusibility may be sufficient. Formation of the pressure-sensitive adhesive layer on the surfaces of the birefringent film, the optical film and the polarizing plate of the present invention can be achieved, for example, by applying a solution or a melt of various pressure-sensitive adhesive materials by a developing method such as casting or coating. A method of directly adding to a predetermined surface such as a film to form a layer, a method of forming an adhesive layer on a separator to be described later, and transferring it to a predetermined surface of the birefringent film, etc. Can be done by. Such a layer may be formed on any surface of the birefringent film, optical film and polarizing plate of the present invention, for example, formed on the exposed surface of the birefringent film in the polarizing plate of the present invention. Also good.

  As described above, when the surface of the pressure-sensitive adhesive layer or the like provided on the birefringent film, the optical film and the polarizing plate of the present invention is exposed, a separator is used for the purpose of preventing contamination until the pressure-sensitive adhesive layer is put to practical use. It is preferable to cover the surface. This separator is formed on a suitable film such as the above-mentioned transparent protective film by, for example, a method of providing one or more release coats with a release agent such as silicone, long chain alkyl, fluorine, molybdenum sulfide, etc. it can. For example, the pressure-sensitive adhesive layer may be a single layer or a laminate. As the laminate, for example, a laminate in which different compositions and different types of single layers are combined can be used. Moreover, when arrange | positioning on both surfaces, such as the said birefringent film, the same adhesive layer may respectively be sufficient, and a different composition and a different kind of adhesive layer may be sufficient, for example. The thickness of the pressure-sensitive adhesive layer can be appropriately determined according to, for example, the configuration of the birefringent film and the like, and is generally 1 μm to 500 μm.

  As the pressure-sensitive adhesive that forms the pressure-sensitive adhesive layer, for example, one that is excellent in optical transparency and exhibits appropriate wettability, cohesiveness, and adhesive pressure-sensitive adhesive properties is preferable. Specific examples include pressure-sensitive adhesives prepared by appropriately using polymers such as acrylic polymers, silicone polymers, polyesters, polyurethanes, polyethers, and synthetic rubbers as base polymers. Control of the adhesive property of the pressure-sensitive adhesive layer is, for example, the degree of cross-linking depending on the composition and molecular weight of the base polymer forming the pressure-sensitive adhesive layer, the crosslinking method, the content ratio of the crosslinkable functional group, the blending ratio of the crosslinking agent, It can be suitably carried out by a conventionally known method such as adjusting the molecular weight.

  Polymer materials, base materials, transparent protective films, polarizers, retardation plates, adhesives, pressure-sensitive adhesives and the like for forming the birefringent film, optical film and polarizing plate of the present invention as described above are, for example, salicylic acid ester compounds. In addition, an ultraviolet absorbing ability may be imparted by appropriately treating with an ultraviolet absorber such as a benzophenone compound, a benzotriazole compound, a cyanoacrylate compound, or a nickel complex compound.

  Next, in the liquid crystal panel of the present invention, at least one of the birefringent film, the optical film, and the polarizing plate of the present invention is arranged on one side or both sides of the liquid crystal cell. The liquid crystal cell generally has a structure in which liquid crystal is injected into the gap between the opposing liquid crystal cell substrates, and the liquid crystal cell substrate is not particularly limited, and for example, a glass substrate or a plastic substrate can be used. The material for the plastic substrate is not particularly limited, and conventionally known materials can be used. Moreover, when providing a polarizing plate and an optical member on both surfaces of a liquid crystal cell, they may be the same kind and may differ. The operation mode of the liquid crystal cell can be arbitrarily selected, and examples thereof include TN mode, VA mode, and OCB mode liquid crystal cells. Among these, the birefringent film, the optical film, and the polarizing plate of the present invention are particularly excellent in optical compensation of the liquid crystal cell of the OCB mode. Therefore, the OCB mode is particularly preferable as the liquid crystal mode of the liquid crystal panel of the present invention. preferable.

  The liquid crystal display device of the present invention is not particularly limited except that the liquid crystal panel of the present invention is used, and a liquid crystal display device of a reflection type, a semi-transmission type, a transmission / reflection type, or the like can be used. Furthermore, when forming the liquid crystal display device, for example, appropriate components such as a prism array sheet, a lens array sheet, a light diffusing plate, and a backlight can be arranged in one or more layers at appropriate positions. As described above, the OCB mode is particularly preferable as the liquid crystal mode of the liquid crystal display device of the present invention. When the OCB mode liquid crystal display device of the present invention is formed using the birefringent film, optical film or polarizing plate of the present invention, an optical compensation layer containing a discotic compound or a hybrid-oriented rod-shaped liquid crystal compound is further included. It is preferable.

  In addition, when the liquid crystal display device of the present invention includes a light source, for example, since the energy of light can be used effectively, the light source is preferably a flat light source that emits polarized light, for example. In the liquid crystal display device of the present invention, for example, a diffusion plate, an antiglare layer, an antireflection film, a protective layer and a protective plate are further disposed on the viewing-side optical film (polarizing plate), or a liquid crystal cell in a liquid crystal panel. A compensation retardation plate or the like may be appropriately disposed between the polarizing plate and the polarizing plate.

  The birefringent film, the optical film, and the polarizing plate of the present invention are not limited to the liquid crystal display device as described above. For example, a self-luminous image display such as an organic electroluminescence (EL) display, PDP, FED, etc. It can also be used for equipment. In addition, the structure of these apparatuses is not restrict | limited at all except using the birefringent film of this invention instead of the conventional birefringent film, an optical film, a polarizing plate, etc. In addition, when used in a self-luminous flat display, for example, circular polarization can be obtained by setting the in-plane retardation value Δnd of the birefringent film of the present invention to λ / 4. Available.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. In addition, each analysis method used by the Example and the comparative example is as follows.

Example (Determination of chemical structural formula)
The structural formula of the chemical substance was determined using 400 MHz ¹ H-NMR LA400 (manufactured by JEOL Ltd.). The measurement was performed by dissolving a 50 mg sample in 0.6 ml of heavy dimethyl sulfoxide (heavy DMSO).

(Molecular weight measurement)
The molecular weight of the chemical substance was prepared using a HLC-8120GPC (manufactured by Tosoh Corporation) after preparing a sample in a 0.1% N, N-dimethylformamide (DMF) solution and filtering with a 0.45 μm membrane filter. It was measured. Polyethylene oxide was used as a standard substance.

(Measurement of phase difference and light transmittance)
The retardation and light transmittance of the birefringent films obtained in each Example and Comparative Example were obtained by transferring the obtained birefringent film to a glass plate via an acrylic adhesive, and the glass plate and the birefringent film. As a laminated body, an automatic birefringence meter KOBRA-21ADH (manufactured by Oji Scientific Instruments Co., Ltd.) was used to measure with a measuring light having a wavelength of 590 nm.

(Measurement of film thickness)
The thickness of the birefringent film was measured using an instantaneous multi-measurement system MCPD-2000 (manufactured by Otsuka Electronics Co., Ltd.).

  2,2′-dichloro-4,4 ′, 5,5′-biphenyltetracarboxylic dianhydride (DCBPDA) and 2,2′-bis (trifluoromethyl) -4,4′-diaminobiphenyl (PFMB) Or TFMB), a polyimide composed of repeating units represented by the following structural formula (2) (weight average molecular weight: Mw = 82,500) is synthesized, and the polyimide is dissolved in methyl isobutyl ketone (MIBK). A 14% by mass polyimide solution was prepared. Next, this polyimide solution is applied on a triacetyl cellulose (TAC) film (trade name: UZ-TAC, manufactured by Fuji Photo Film Co., Ltd.) having a thickness of about 80 μm, dried at 100 ° C. for 5 minutes, and then the TAC film. A polyimide film was formed thereon. This polyimide film was heated to 150 ° C. together with the TAC film, and stretched at a magnification of 2% by free end longitudinal stretching to form a birefringent film having a thickness of 5.7 μm on the stretched TAC film. In addition, extending | stretching 2% of magnification means extending | stretching to 1.02 times the original length in the extending | stretching direction. The optical properties of this birefringent film were Re = 24.0 nm, Rth = 398.1 nm, and Z = 16.6.

  A birefringent film having a thickness of 5.7 μm was formed in the same manner as in Example 1 except that the draw ratio was 2.5%. The optical properties of this birefringent film were Re = 33.5 nm, Rth = 407.6 nm, and Z = 12.2.

  A birefringent film having a thickness of 6.4 μm was formed in the same manner as in Example 1 except that the draw ratio was set to 3.0%. The optical properties of this birefringent film were Re = 35.9 nm, Rth = 418.6 nm, and Z = 11.7.

  A birefringent film having a thickness of 6.2 μm was formed in the same manner as in Example 1 except that the draw ratio was set to 3.5%. The optical properties of this birefringent film were Re = 43.1 nm, Rth = 413.7 nm, and Z = 9.6.

  A birefringent film having a thickness of 5.8 μm was formed in the same manner as in Example 1 except that the draw ratio was 4.0%. The optical properties of this birefringent film were Re = 53.7 m, Rth = 358.2 nm, and Z = 7.3.

  A birefringent film having a thickness of 5.8 μm was formed in the same manner as in Example 1 except that the draw ratio was set to 5.0%. The optical properties of this birefringent film were Re = 58.6 nm, Rth = 380.9 nm, and Z = 6.5.

(Comparative Example 1)
A birefringent film was formed in the same manner as in Example 1 except that the thickness of the film was changed to 14.3 μm. The optical properties of this birefringent film were Re = 54.8 nm, Rth = 741.3 nm, and Z = 13.5.

(Comparative Example 2)
A polyimide (Mw = 51,800) composed of a repeating unit represented by the following structural formula (3) is synthesized from bisphenol A tetracarboxylic dianhydride (Bis-DA) and PFMB. A 14% by mass polyimide solution was prepared by dissolving in cyclopentanone (CPN). Next, a birefringent film having a thickness of 13.9 μm was formed in the same manner as in Example 1 except that this polyimide solution was used. The optical properties of this birefringent film were Re = 51.2 nm, Rth = 153.5 nm, and Z = 3.0.

(Comparative Example 3)
Consists of repeating units represented by the following structural formula (4) from 4,4′-bis (2,3,4,5,6-pentafluorobenzoyl) diphenyl ether (BPDE) and bisphenol A (BA). Polyetherketone (Mw = 421,000) was synthesized, and this polyetherketone was dissolved in methyl isobutyl ketone (MIBK) to prepare a 14% by weight polyetherketone solution. A birefringent film having a thickness of 16.2 μm was formed in the same manner as in Example 1 except that this polyetherketone solution was used. The optical properties of this birefringent film were Re = 32.8 nm, Rth = 20.1 nm, and Z = 6.7.

(Comparative Example 4)
Polycarbonate (manufactured by Nitto Denko Corporation) was stretched by simultaneous biaxial stretching of 30% length and 20% width to form a birefringent film having a thickness of 60 μm. The optical properties of this birefringent film were Re = 10.88 nm, Rth = 610.2 nm, and Z = 6.1.

  Table 1 below shows optical characteristics of the birefringent films obtained in Examples 1 to 6 and Comparative Examples 1 to 4.

(Table 1)
Thickness (μm) Re (nm) Rth (nm) Z
Example 1 5.7 24.0 398.1 16.6
Example 2 5.7 33.5 407.6 12.2
Example 3 6.4 35.9 418.6 11.7
Example 4 6.2 43.1 413.7 9.6
Example 5 5.8 53.7 385.2 7.3
Example 6 5.8 58.6 380.9 6.5
Comparative Example 1 14.3 54.8 741.3 13.5
Comparative Example 2 13.9 51.2 153.5 3.0
Comparative Example 3 16.2 32.8 220.1 6.7
Comparative Example 4 60.0 100.8 610.2 6.1

  The birefringent films obtained in Examples 1 to 6 and Comparative Examples 1 to 4 and the polarizing plate (SEG1224Du, manufactured by Nitto Denko Corporation) are combined with the slow axis of the birefringent film and the absorption axis of the polarizing plate. A polarizing plate was prepared by bonding with an adhesive so that and were perpendicular to each other. Using this polarizing plate and a polarizing plate (SEG1224Du, manufactured by Nitto Denko Corporation) using an acrylic adhesive on both sides of a glass plate having a thickness of 1.2 mm so that the absorption axes of the two polarizing plates are perpendicular to each other. The sample was produced by pasting. The absorption axis of the polarizing plate is in the direction of 45 ° with respect to the long side of the polarizing plate.

  The samples prepared using the birefringent films of Examples 1 to 6 and Comparative Examples 1 to 4 were placed in an oven at 80 ° C. for 16 hours, then placed on a backlight, and light leakage was observed. The results are shown in Table 2 below and FIG. FIG. 1 is a photograph of the sample after heat treatment. FIG. 1A shows the result of the birefringent film of Example 1, and FIG. 1B shows the result of the birefringent film of Comparative Example 1. FIG. C shows the result of the birefringent film of Comparative Example 4. The birefringent films of Examples 2 to 6 had the same results as in FIG. 1A and Comparative Examples 2 and 3 as in FIG. 1B. As shown in the following Table 2 and FIG. 1, when the birefringent films of Examples 1 to 6 (5.7 μm to 6.4 μm) having a small thickness were used, the light leakage was very small, but the comparison was larger in thickness. When the birefringent film (13.9 μm to 16.2 μm) of Examples 1 to 3 is used, light leakage is somewhat recognized and the birefringent film (60.0 μm) of Comparative Example 4 having a larger thickness is used. , Light leakage was observed on the entire surface.

(Table 2)
Light leakage evaluation of the birefringent film sample used
Examples 1 to 6 Light leakage was very small. See Figure 1-A.
Comparative Examples 1 to 3 Some light leakage was observed. See Figure 1-B.
Comparative Example 4 Light leakage was observed on the entire surface. See Figure 1-C.

  As described above, the birefringent film of the present invention is useful for optical compensation and thinning of various image display devices, and is particularly useful for OCB mode liquid crystal display devices.

FIG. 1 is a diagram showing the results of a light leakage test of samples produced using the birefringent films of Examples and Comparative Examples. FIG. 2A is a graph showing an example of the relationship between the stretching ratio and retardation of the birefringent film of the present invention, and FIG. 2B shows the relationship between the stretching ratio of the birefringent film of the present invention and the value of the elliptic coefficient Z. It is a graph which shows an example. FIG. 3 is a diagram showing a configuration of an OCB mode liquid crystal panel and a refractive index ellipsoid of the liquid crystal cell and the phase difference plate of the liquid crystal panel.

Explanation of symbols

1. 1. OCB mode liquid crystal cell 2. Biaxial retardation plate Polarizing plate 4. Rod-like liquid crystalline molecules a. Rubbing direction b. Slow axis c. Absorption axis

Claims (12)

A birefringent film used for optical compensation of an OCB (Optically Compensated Bend) mode liquid crystal cell ,
Including a polyimide having a repeating unit of the following general formula (1),
The thickness (d) of the birefringent film, the thickness direction retardation (Rth) measured with light having a wavelength of 590 nm, and the ellipticity coefficient (Z) of the refractive index ellipsoid all satisfy the following conditions (i) to (iii) A birefringent film characterized by satisfying.
0.2 μm ≦ d ≦ 10 μm (i)
300 nm ≦ Rth ≦ 1500 nm (ii)
6 <Z <20 (iii)
In the above conditions, Rth and Z are respectively calculated by the following formulas, where nx, ny and nz represent the refractive indexes in the X-axis, Y-axis and Z-axis directions of the birefringent film, respectively. The X-axis direction is an axial direction showing the maximum refractive index in the plane of the birefringent film, the Y-axis direction is an axial direction perpendicular to the X-axis in the plane, and the Z-axis direction is The thickness direction is perpendicular to the X axis and the Y axis.
Rth = (nx−nz) · d
Z = (nx−nz) / (nx−ny)

In the general formula (1), A and B are each independently a substituent selected from the group consisting of halogen, alkyl having 1 to 3 carbon atoms, halogenated alkyl having 1 to 3 carbon atoms, phenyl, and substituted phenyl. And each of the substituents of the substituted phenyl is independently a substituent selected from the group consisting of hydrogen, halogen, alkyl having 1 to 3 carbon atoms, and alkyl halide having 1 to 3 carbon atoms. m, x and y each represent the number of substitutions (integer), m is 0 to 3, x and y are 0 or 1, and at least one of x and y is 1. The birefringent film according to claim 1, wherein the polyimide is a polyimide having a repeating unit having the following structural formula (2).

The birefringent film is coated with a solution containing a polyimide having a repeating unit of the general formula (1) or the structural formula (2) to form a coated film, and the coated film is subjected to an orientation treatment. The birefringent film according to claim 1, which is a film obtained. The birefringent film according to claim 3, wherein the orientation treatment is a stretching treatment or a shrinking treatment. The birefringent film according to any one of claims 1 to 4, wherein a front direction retardation (Re) measured with light having a wavelength of 590 nm is 20 nm or more, and the Re is calculated by the following formula.
Re = (nx−ny) · d An optical film comprising the birefringent film according to claim 1. The optical film according to claim 6, comprising a retardation plate, wherein the retardation plate and the birefringent film are laminated. A birefringent film according to any one of claims 1 to 5 or an optical film according to claim 6 or 7, and a polarizer, wherein the birefringent film or optical film and the polarizer are laminated. Polarizer. The polarizing plate according to claim 8, wherein the birefringent film according to any one of claims 1 to 5 or the optical film according to claim 6 or 7 is a transparent protective film for a polarizer. 9. A liquid crystal panel comprising an OCB mode liquid crystal cell and two polarizing plates, wherein the polarizing plates are disposed on both surfaces of the liquid crystal cell, respectively, wherein at least one of the two polarizing plates is claim 8 or A liquid crystal panel which is the polarizing plate according to 9. The liquid crystal panel according to claim 10, wherein the liquid crystal cell is a bend alignment liquid crystal cell including a liquid crystal layer having a twist alignment in a central portion of the cell. A liquid crystal display device including a liquid crystal panel, wherein the liquid crystal panel is the liquid crystal panel according to claim 10.

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