JPH0894838A - Elliptic polarizing plate and liquid crystal display device - Google Patents

Abstract

PURPOSE: To obtain an elliptic polarizing plate with reduced stiching processes, causing no stripping, bubbles nor wrinkles even under a high humidity condition by using an optical anisotropic element provided with an optical anisotropic layer containing a disk like compound on a saponified cellulose triacetate. CONSTITUTION: The elliptic polarizing plate is composed of at least a polarizing element and the optical anisotropic element provided with the optical anisotropic layer containing the disk like compound on the saponified cellulose triacetate. The sticking processes at the time of forming the elliptic polarizing plate are reduced by using the optical anisotropic element as a protective film of the polarizing element. Though it is thought that the remarkable difference of shrinkage property by heat or humidity between the stuck optical anisotropic element and the polarizing plate results in stripping defect, wrinkle deffect, bubble defect or the like on the stuck boundary, the defects are prevented by providing the optical anisotropic layer containing the disk like compound on the supporting body and using the cellulose triacetate, which is conventionally used as the protective film of the polarizing plate, as the supporting body for the optical anisotropic element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an elliptically polarizing plate, especially TN.
TECHNICAL FIELD The present invention relates to an elliptical polarizing plate that improves the viewing angle characteristics of a liquid crystal display device using a liquid crystal cell.

[0002]

2. Description of the Related Art Display devices for OA equipment such as Japanese word processors and desktop personal computers are mainly used in CRTs.
Therefore, it is being converted to a liquid crystal display element (hereinafter referred to as LCD) which has great advantages of thin and light weight and low power consumption. Many of the currently widespread LCDs use twisted nematic liquid crystals. The display method using such a liquid crystal can be roughly classified into a birefringence mode and an optical rotation mode.

A display element using a birefringence mode (STN type LCD) has a twist angle of a liquid crystal molecule arrangement of 90 ° or more and has a steep electro-optical characteristic, so that there is no active element (thin film transistor or diode). However, a large-capacity display can be obtained by time-division driving with a simple matrix electrode structure. However, the response speed is slow (hundreds of milliseconds), and gradation display is difficult, and the display performance of liquid crystal display devices (TFT-LCD or MIM-LCD) using active devices has not been exceeded. .

For the TFT-LCD and the MIM-LCD, an optical rotation mode display mode (TN type LCD) in which the alignment state of liquid crystal molecules is twisted by 90 ° is used. This display method is the most effective method compared with other LCDs because it has a high response speed (several tens of milliseconds), can easily obtain a monochrome display, and has a high display contrast. But,
Since the twisted nematic liquid crystal is used, there is a problem in view angle characteristics that the display color and the display contrast are changed depending on the viewing direction on the principle of the display system, and the display performance of the CRT has not been exceeded.

Japanese Unexamined Patent Publication Nos. 4-229828 and 4-2.
As disclosed in Japanese Patent No. 58923, a method has been proposed in which a viewing angle is increased by disposing a retardation film between a pair of polarizing plates and a TN type liquid crystal cell. The retardation film proposed in the above patent publication has a retardation in the direction perpendicular to the liquid crystal cell of almost zero, exerts no optical action from the front, and retards when tilted. Occurs, and the phase difference developed in the liquid crystal cell is compensated. However, even with these methods, the improvement of the viewing angle is insufficient. Specifically, the phenomenon that the displayed image is colored (coloring phenomenon) or black and white are reversed when tilted vertically or horizontally from the screen normal direction. (Reversal phenomenon) is remarkable, and in particular, when it is considered as an in-vehicle device or a substitute for a CRT, the current situation is that it cannot be dealt with at all.

In JP-A-4-366808 and JP-A-4-366809, a viewing angle is improved by using a liquid crystal cell containing a chiral nematic liquid crystal having an inclined optical axis as a retardation film. This is a layer liquid crystal system, which is expensive and very heavy. Further, JP-A-4-113301, JP-A-5-80323 and JP-A-5-157913 use a retardation film in which a polymer chain, an optical axis or an optical elastic axis is inclined with respect to a liquid crystal cell. Although a method has been proposed, there has been a problem that it is difficult to obtain a large-area retardation film at low cost, for example, by using a uniaxial polycarbonate which is sliced obliquely. Although the improvement of the viewing angle of the STN-LCD is mentioned, no concrete effect is shown on the improvement of the viewing angle of the TN-LCD. In addition, JP-A-5-215
In Japanese Laid-Open Patent Publication No. 921, a pair of substrates subjected to alignment treatment are
A plan for optically compensating an LCD with a birefringent plate having a rod-shaped compound sandwiched between which exhibits liquid crystal properties upon curing has been proposed, but this plan is different from the conventionally proposed so-called double-cell type compensator. It is not suitable for mass production because it causes a huge cost increase. Furthermore T
The effect is not shown for improving the omnidirectional viewing angle of the N-type LCD. Further, in JP-A-3-9326 and JP-A-3-291601, by applying a polymer liquid crystal to a film-shaped substrate having an alignment film, L
Although an optical compensator for CD has been described, it is impossible to orient the molecules obliquely by this method, so that the omnidirectional viewing angle of the TN LCD cannot be improved.

Further, in EP0576304A1 and Japanese Patent Laid-Open No. 6-75116 by the present inventors, a viewing angle characteristic is improved by using a retardation plate having an optically negative uniaxial property and an optical axis inclined. How to do is described. According to this method, the viewing angle is significantly improved as compared with the conventional one, but it is still impossible to improve the viewing angle to the extent that a CRT alternative is considered.

Therefore, the present inventors have filed a patent application No. 6-1265.
21. An optical anisotropic element having an optically negative uniaxial property, the optical axis of which is inclined from the normal direction of the film,
Further, the retardation film having the characteristics of an optically anisotropic element having an optically negative uniaxial property and its optical axis being in the normal direction of the film also significantly improves the viewing angle characteristics of the liquid crystal display device having the TN type liquid crystal. I found a thing.

Generally, in order to improve the viewing angle, it is necessary to install a retardation film between the liquid crystal cell and the polarizing plate. As will be described later, the polarizing plate used here has a protective layer such as cellulose triacetate, which has almost no optical anisotropy, on both sides of a polarizing element in which iodine or a dichroic dye is adsorbed on a stretched polyvinyl alcohol. By bonding the film, the heat resistance and moisture resistance are improved.
Conventionally, a retardation film is attached to this polarizing plate to form an elliptically polarizing plate, which is then attached to a liquid crystal cell to create a liquid crystal display device, which requires many attaching steps. If the device is placed under high temperature or high humidity conditions, peeling failure occurs at the interface where the elliptically polarizing plates are bonded, bubble failure occurs inside the elliptically polarizing plate,
Alternatively, wrinkle failure or the like in which wrinkles are generated from the four corners of the elliptical polarizing plate may occur, resulting in a serious problem of significantly lowering the display quality of the liquid crystal display device. Further, in order to simply reduce the bonding process, it is possible to use the retardation film as a protective film of the polarizing plate, but as described later, it is difficult to bond the polarizing element and the retardation film, and the same as above. In some cases, peeling failure, millet failure, or wrinkle failure occurred.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide an elliptically polarizing plate having a small number of bonding steps and free from peeling failure, foaming failure or wrinkling failure even under high humidity conditions. It is another object of the present invention to provide a liquid crystal display device that does not deteriorate in display quality even when placed under high temperature or high humidity conditions.

[0011]

Means for Solving the Problems The object is (1) at least a polarizing element and an optically anisotropic element having an optically anisotropic layer containing a discotic compound on saponified cellulose triacetate. Characteristic elliptically polarizing plate. (2) The elliptically polarizing plate as described in (1), wherein the optically anisotropic layer is optically uniaxial and its optical axis is inclined at 5 ° to 50 ° from the normal direction of the film. (3) A liquid crystal cell in which a TN type liquid crystal having a twist angle of about 90 ° is sandwiched between at least two electrodes, and the elliptically polarizing plate described in (1) or (2). Achieved by a liquid crystal display device.

It is considered as follows that the elliptically polarizing plate of the present invention does not cause a peeling failure, a wrinkle failure, a wrinkle failure, etc. even if it is placed under a high temperature and high humidity condition. The normally used polarizing element is a film made of a hydrophilic polymer such as ethylene-vinyl acetate copolymer partially saponified polymer, partially formalized polyvinyl alcohol and polyvinyl alcohol, and then iodine or dichroic dye is drawn. It is an adsorbed product or a plastic film such as polyvinyl chloride treated to orient the polyene. In order to improve the heat resistance and moisture resistance of this polarizing element, conventionally, birefringence such as cellulose triacetate is small on both sides of the polarizing element, a film having a low hygroscopicity or a low moisture permeability, and pasted as a protective film, It has been used as a polarizing plate.

In the present invention, an optical anisotropic element is used as a protective film for a polarizing element to reduce the number of laminating steps in producing an elliptically polarizing plate, and to prevent heat or humidity of the optical anisotropic element to be laminated. If the shrinkage characteristic is significantly different from that of the polarizing element, it is considered to be a cause of peeling failure, wrinkle failure, foaming failure, etc. at the bonded interface. The optically anisotropic element of the present invention comprises an optical anisotropic layer containing a discotic compound provided on a support, and as this support, cellulose triacetate conventionally used as a protective film for a polarizing plate. It is estimated that these faults will be eliminated by using. However, it is difficult to install this optically anisotropic layer on ordinary cellulose triacetate in terms of adhesion, and as a result of studying improving this adhesion, saponification of cellulose triacetate is effective. The present invention has been accomplished by discovering that it is a target.

The preferred embodiments of the present invention are described in detail below. The saponified cellulose triacetate is one in which the surface thereof is slightly saponified by immersing the raw material cellulose triacetate film in a dilute alkaline aqueous solution such as 2% potassium hydroxide aqueous solution. is there. The reaction conditions at this time are about 1 to 3 at room temperature.
A relatively mild time of 0 minutes, preferably 10 minutes or less is sufficient.

The cellulose triacetate film as a raw material is an optically anisotropic element having an optically negative uniaxial property and its optical axis being in the normal direction of the film, and at the same time, has a light transmittance of 80% or more. , The principal refractive index in the film plane is nx, n
where y is the main refractive index in the thickness direction and nz is the film thickness, the relationship between the triaxial main refractive indexes is nz <ny = nx
And the expression {(nx + ny) / 2−nz} × d
The retardation represented by is preferably 20 nm to 400 nm. However, the values of nx and ny do not have to be exactly equal, and it is sufficient if they are almost equal. Specifically, if | nx-ny | / | nx-nz | ≦ 0.2, there is no practical problem. The front retardation represented by | nx-ny | xd is preferably 50 nm or less, and more preferably 20 nm or less.

The optically anisotropic layer of the present invention, which is optically uniaxial and has its optical axis tilted from 5 ° to 50 ° from the radial direction of the film, contains an obliquely oriented discotic compound. It is preferable that it is toxic. The optically anisotropic layer has a light transmittance of 80% or more, a main refractive index in the film plane of nx ′, ny ′, a thickness direction refractive index of nz ′, and a thickness of d ′. , The relationship between the triaxial principal refractive indices is nz '<n
y ′ = nx ′ is satisfied, and the expression {(nx ′ + ny ′) /
The retardation represented by 2-nz ′} × d ′ is 50.
It is preferably from nm to 400 nm. However, n
The values of x ′ and ny ′ do not have to be exactly equal, but they need to be approximately equal. Specifically, there is no problem within the following range. | Nx'-ny '| / | nx'-nz' | ≦ 0.2 Further, the angle formed by the optical axis and the film normal direction is 1
More preferably, it is 0 ° to 50 °.

The discotic compound of the present invention is, for example, C,
Report of Destrade et al., Mol. Cryst.
71, page 111 (1981), benzene derivatives and B.I. Report of Kohne et al., Ang
ew. Chem. Vol. 96, p. 70 (1984) and cyclohexane derivatives described in J. Am. M. Lehn et al., J. Chem. Commun. , Pp. 1794 (1985), J. Research report by Zhang et al. A
m. Chem. Soc. 116, 2655 (199
4 years), such as azacrown-type and phenylacetylene-type macrocycles, and these are generally used as the mother nucleus of the molecular center, and straight-chain alkyl groups, alkoxy groups, substituted benzoyloxy groups, etc. It has a structure in which a straight chain is radially substituted, exhibits liquid crystallinity, and includes what is generally called discotic liquid crystal. However, the molecule is not limited to the above description as long as the molecule itself has negative uniaxiality and can give a certain orientation.
Further, in the present invention, the term "formed from a discotic compound" does not mean that the final product is the above compound,
For example, those in which the low molecular weight discotic liquid crystal has a group that reacts with heat, light, etc., and consequently polymerizes or crosslinks by reaction with heat, light, etc., and has a high molecular weight and loses liquid crystallinity are also included. I shall.

Next, the discotic compound in the present invention means the reaction of discotic liquid crystals as listed below, and the reaction of discotic liquid crystals which no longer show liquid crystallinity due to reaction with other low molecular weight compounds and polymers. It means all compounds, such as products, in which the molecule itself has optically negative uniaxiality.

[0019]

[Chemical 1]

[0020]

[Chemical 2]

[0021]

[Chemical 3]

[0022]

[Chemical 4]

In the case where the discotic compound in the present invention is a discotic liquid crystal, a layer containing them is oriented so as to have an optically negative uniaxial axis and an optical axis inclined from 5 ° to 50 ° from the normal direction of the film. The following processing is required for this. Specifically, a rubbing-treated organic alignment film or inorganic alignment film is formed on a saponified cellulose triacetate film, a discotic liquid crystal is applied thereon, and then a liquid crystal phase, more preferably a disconematic phase is formed. To raise the temperature. As a result, the liquid crystal is obliquely aligned, and remains in the solid state at room temperature while maintaining the alignment by subsequent cooling. Further, the discotic nematic liquid crystal phase forming temperature is unique to the discotic liquid crystal, but can be arbitrarily adjusted by mixing two or more different types. The discotic nematic liquid crystal phase-solid phase transition temperature of the discotic liquid crystal used in the present invention is preferably 70 ° C. or higher and 300 or more.
C. or less, particularly preferably 70.degree. C. or more and 150.degree. C. or less.

The organic alignment film may be a polyimide film or a polystyrene derivative, and the water-soluble film may be a gelatin film or polyvinyl alcohol. By subjecting all of them to rubbing treatment, the discotic liquid crystal can be oriented obliquely.
Of these, the alkyl-modified polyvinyl alcohol is particularly preferable, and the present inventors have discovered that it has an excellent ability to uniformly align the discotic liquid crystal. It is speculated that this is due to the strong interaction between the alkyl chains on the alignment film surface and the alkyl side chains of the discotic liquid crystal. The above alkyl-modified polyvinyl alcohol has an alkyl group at the terminal as listed below, and has a saponification degree of 80%.
As described above, the polymerization degree is preferably 200 or more. Further, polyvinyl alcohol having an alkyl group in a side chain can also be used effectively. As a commercial item, Kuraray MP103, M
P203, R1130, etc. are available.

A polyimide film, which is widely used as a liquid crystal alignment film for LCDs, is also preferable as the organic alignment film,
This is a polyamic acid (for example, LQ / L manufactured by Hitachi Chemical
X series, SE series manufactured by Nissan Chemical Co., Ltd.) is applied to the surface of the substrate, baked at 100 to 300 ° C. for 0.5 to 1 hour, and then rubbed.

The rubbing treatment is the same method widely used as a liquid crystal alignment treatment process for LCD, and the surface of the alignment film is made of paper, gauze, felt, rubber,
Alternatively, it is a method of obtaining orientation by rubbing in a certain direction using nylon, polyester fiber or the like. In general, rubbing is performed several times using a cloth or the like in which fibers of uniform length and thickness are evenly flocked.

Further, as a vapor deposition substance for the inorganic oblique vapor deposition film, metal oxides such as TiO ₂ , MgF ₂ and ZnO ₂ and fluorides, and metals such as Au and Al can be cited as representative of SiO.
The metal oxide can be used as the oblique vapor deposition material as long as it has a high dielectric constant, and is not limited to the above. Both a fixed substrate type method and a continuous vapor deposition type method on a film can be used to form a vapor deposition film.
For example, when the vapor deposition angle α is about 65 to 88 °, the discotic liquid crystal is uniformly aligned in a direction whose optical axis is substantially perpendicular to the direction of the vapor deposition particle column.

The above-mentioned alignment film has a function of determining the alignment direction of the discotic liquid crystal molecules coated thereon, but since the alignment property of the discotic liquid crystal depends on the alignment film, its combination is optimized. There is a need. Further, the uniformly aligned discotic liquid crystal molecules are aligned at an angle with the normal line of the film, but the tilt angle does not change much depending on the alignment film and often takes a value specific to the discotic liquid crystal molecules. When two or more discotic liquid crystals are mixed or a compound similar to the discotic liquid crystal is mixed, the tilt angle can be adjusted by the mixing ratio. Therefore, a method of selecting or mixing discotic liquid crystals is more effective for controlling the tilt angle of the oblique alignment.

Another method for orienting the discotic liquid crystal obliquely is magnetic field orientation or electric field orientation. In this case, it is necessary to apply a magnetic field or an electric field at a desired angle while heating the substrate coated with the discotic liquid crystal.

In order to maintain the oblique orientation of the discotic compound thus obtained even under high temperature and high humidity, a polymerizable unsaturated group, an epoxy group, a hydroxyl group and an amino group are previously added to the discotic compound. Groups, functional groups such as carboxyl groups, radical polymerization of polymerizable unsaturated groups by heat or photopolymerization initiator, ring-opening polymerization of epoxy groups by photoacid generator, polyvalent isocyanate, polyvalent It is preferable to crosslink the discotic compound itself by a crosslinking reaction with an epoxy compound. At this time, another compound having the same functional group may be contained.

The polarizing element and the optically anisotropic element used as a protective film are firmly bonded with an acrylic, SBR or silicon adhesive or adhesive to obtain the elliptical polarizing plate of the present invention. In the present invention, one protective film of the polarizing element is the above-mentioned optical anisotropic element, and the other one may be the same optical anisotropic element, but products having small birefringence such as Zeonex, ARTON, and Fujitac. It is preferable to use a film commercially available under the name. When the elliptically polarizing plate of the present invention is used in a TN liquid crystal cell, it is necessary that the optical anisotropic element of the protective film of the elliptically polarizing plate faces the TN liquid crystal cell side.

[0032]

EXAMPLES The present invention will be described in detail below based on examples. Example 1 Preparation of cellulose triacetate film A Triacetyl cellulose having a styrene-equivalent weight average molecular weight of 80,000 was dissolved in methylene chloride, cast on a metal band, and peeled off when the volatile content reached 3%. Then, the cellulose triacetate film A having a thickness of about 115 μm was prepared by performing widthwise stretching with a tenter, MD direction stretching, and relaxation of orientation by heat.

Example 2 Preparation of Saponified Cellulose Triacetate Film B The cellulose triacetate film prepared in Example 1 was immersed in a 2% aqueous potassium hydroxide solution at room temperature for 5 minutes and washed in running water. After drying, a saponified cellulose triacetate film B was prepared.

Example 3 Preparation of Optically Anisotropic Element C Alkyl-modified P using B prepared in Example 1 as a support
VA (MP203: trade name, made by Kuraray) 1.2 μm
Of the above-mentioned discotic liquid crystal TE-8 (m =
4) 0.4 g, trimethylolpropane triacrylate 0.06 g, Irgacure 9070.004 g dissolved in 1.2 g of methyl ethyl ketone, a coating solution is applied by a spin coater (rotation speed 150 rpm / 2 minutes),
After drying, heat up from room temperature to 147 ° C in 10 minutes,
After orienting the discotic liquid crystal, UV irradiation was performed for 2 minutes at 147 ° C. using a high pressure mercury lamp, and the mixture was slowly cooled to room temperature to prepare an optical anisotropic element C having a layer D containing a discotic compound.

Comparative Example 1 Preparation of Optically Anisotropic Element E An optical anisotropic element E of Comparative Example was prepared in the same manner except that the A prepared in Example 1 was used instead of the B used in Example 3. did.

Example 4 Using an ellipsometer, the retardation values of the supports A and B, the retardation value of the optically anisotropic layer D, and the angle of inclination of the optical axis with the normal direction of the film were determined. Are summarized in Table 1.

[0037]

[Table 1]

Example 5 Preparation of Elliptical Polarizing Plate Q An optical anisotropic element C prepared in Example 3 was prepared by using an acrylic adhesive on one side of a polarizing element P in which stretched polyvinyl alcohol was made to adsorb iodine. The elliptically polarizing plate Q of the present invention was prepared by laminating the layer containing the discotic compound on the inside and the cellulose triacetate film A prepared in Example 1 on the opposite side. However, in the elliptically polarizing plate Q, the absorption axis of the polarizing element and the optical anisotropic element C
The direction in which the retardation in (1) has the minimum value is orthogonal to the projection direction on the film surface.

Comparative Example 2 Preparation of Elliptical Polarizing Plate R Instead of the optical anisotropic element C used in Example 5, Comparative Example 1
An elliptically polarizing plate R of a comparative example was prepared in exactly the same manner except that the optical anisotropic element E prepared in 1. was used. The projection direction of the take direction on the film surface is set to be orthogonal.

Example 7 Evaluation of Elliptical Polarizing Plates Elliptic polarizing plates Q and R were adhered to a glass plate with an acrylic adhesive and aged at a high temperature and a pressure of 90.
After being placed in a constant temperature bath at ℃ for 500 and 1000 hours, the occurrence of peeling failure, foaming failure or wrinkling failure was observed, and the results are summarized in Table 2.

[0041]

[Table 2]

Example 8 Preparation of Liquid Crystal Display Devices H1 and H2 The polarizing plate of the TFT type liquid crystal color television 6E-C3 manufactured by Sharp Corporation was peeled off, and the elliptical polarizing plate Q of the present invention was used instead.
Alternatively, the elliptically polarizing plate R of the comparative example was attached to both sides of the liquid crystal cell to prepare liquid crystal display devices H1 and H2. Regarding the elliptically polarizing plate Q, the polarizing element is placed outside, and the projection direction on the film surface in which the retardation of the optical anisotropic element C has a minimum value is 180 ° with the rubbing axis of the adjacent liquid crystal cell. Placed in.

Example 9 Evaluation of Liquid Crystal Display Devices H1 and H2 With respect to the liquid crystal display devices H1 and H2 prepared in Example 7,
The viewing angle dependence of the contrast was measured, and the angle at which the contrast ratio (T0 / T5) between white display and black display was 10 was defined as the viewing angle, and the viewing angle in the vertical and horizontal directions was obtained, and the results are summarized in Table 3. It was

[0044]

[Table 3]

[0045]

The elliptically polarizing plate Q of the present invention and the elliptically polarizing plate R of the comparative example both use an optically anisotropic element as a protective film for the polarizing plate. Is less than before. In addition, these elliptically polarizing plates attached to a glass plate
The elliptically polarizing plate of the present invention has good adhesion between the polarizer and the optically anisotropic element and does not peel at the interface when aged in a constant temperature bath at 90 ° C. ,
Peeling occurred on the interface at the edges, and fine wrinkles were also generated. Also, liquid crystal display devices H1 and H2 to which these are attached
Had a wide viewing angle from the viewpoint of contrast, and almost no reversal was observed.

Claims (3)

[Claims] 1. An elliptically polarizing plate comprising at least a polarizing element and an optically anisotropic element having an optically anisotropic layer containing a discotic compound on saponified cellulose triacetate. 2. The elliptically polarized light according to claim 2, wherein the optically anisotropic layer is optically uniaxial and its optical axis is inclined at 5 ° to 50 ° from the normal direction of the film. Board. 3. A liquid crystal cell in which a TN liquid crystal having a twist angle of about 90 ° is sandwiched between at least two electrodes, and the elliptically polarizing plate according to claim 1 or 2. Liquid crystal display device.