JPH0713022A - Optical compensator sheet and liquid crystal display element using the same - Google Patents

Abstract

PURPOSE:To obtain an optical compensator sheet excellent in visual angle characteristics which can be produced at low cost with a simple process by forming the sheet in such a manner that refractive index in the thickness direction is made larger than two refractive indices equal to each other in the plane. CONSTITUTION:This optical compensator sheet consists of a first optical anisotropic film in which at least the normal direction of the film shows no double refraction, and a second optical anisotropic film having double refraction in the normal direction of the film. The first optical anisotropic film is a positive uniaxial optical anisotropic material having the optical axis in the normal direction. The refractive index in the film thickness direction is larger than the two refractive indices equal to each other in the plane. When this sheet is to be used for a liquid crystal display element, the display element has at least a liquid crystal cell, two polarizing plates disposed on both sides of the cell, and at least one optical compensator sheet between the liquid crystal cell and two polarizing plates. In the first optical anisotropic film, double refraction is gradually shown in the direction with larger angle from the normal direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical compensation sheet and a liquid crystal display device using the same, and more particularly, an optical compensation sheet useful for improving the viewing angle characteristics of display contrast and display color and a liquid crystal using the same. Regarding display element.

[0002]

2. Description of the Related Art Liquid crystal display devices have many features such as low voltage and low power consumption, which can be directly connected to an IC circuit, various display functions, and weight reduction. It is widely used as a display device for word processors and personal computers. inside that,
A twisted nematic liquid crystal display device (hereinafter STN-LCD) having a twist angle of liquid crystal molecules of 160 ° or more is capable of displaying a larger capacity than a conventional twisted nematic liquid crystal display device (TN-LCD) having a twist angle of 90 °. Because of its excellent high-speed response, it is currently the mainstream of liquid crystal display devices.

However, the STN-LCD has a drawback in that the background of the displayed image is colored blue or yellow (blue mode or yellow mode). Therefore, in black and white display, the contrast and the visibility are lowered, and colorization is extremely made. There was also the problem of difficulty.

As means for solving the above-mentioned problems, Japanese Patent Laid-Open No. 63-
No. 167303, No. 63-167304, No. 63-1
89804, 63-261302, 63-14.
9624, JP-A-1-201607 and 1-201.
608, 1-105217, JP-A-2-2853.
No. 03, No. 2-59702, No. 2-24406, No. 2-146002, No. 2-257103, JP-A-3.
-23404, 3-126012, 3-181.
As described in Japanese Patent Publication No. 905, No. 3-194503, etc., a method using a retardation plate has been proposed, and STN-
It was found that the coloring of the image displayed by the LCD was significantly improved, but the viewing angle characteristics were hardly improved.

Therefore, in order to improve this viewing angle characteristic,
Japanese Patent Application Laid-Open No. 2-385303 discloses a method in which a birefringent film having a refractive index in the thickness direction larger than at least one of main refractive indexes parallel to a surface is prepared by electric field orientation and used as a retardation plate. was suggested. According to this method, the change in contrast depending on the viewing angle is reduced, and the viewing angle characteristics are improved, but the effect is still small, and it is necessary to apply a high voltage to the molten polycarbonate for a long time, and the manufacturing process is also Due to the complexity, it was difficult to reduce costs. In addition, JP-A-2-
In Japanese Patent No. 160204, there is proposed a method in which a rod-shaped polycarbonate obtained by extrusion molding is cut into a plate shape and polished and used as a retardation plate.
With this method, it was extremely difficult to produce a large-area retardation plate at low cost. Furthermore, in EP482626A2, a method of using a film obtained by laminating a heat-shrinkable film on a polycarbonate film, uniaxially stretching, and then peeling the heat-shrinkable film as a retardation plate is proposed. There is a problem that a heat-shrinkable film having at least the same area as or more than twice the area of the retardation plate is required, and it cannot be produced at low cost.

Further, JP-A-2-256023, JP-A-3-141303, JP-A-3-14122 and JP-A-3-24.
In Japanese Patent No. 502, there is proposed a method in which one film each having a positive and negative intrinsic birefringence or a laminated film is used as a retardation plate. According to this method, the viewing angle characteristics can be improved by adjusting the birefringence of the two films in accordance with the characteristics of the liquid crystal cell. However, two or more birefringent films prepared separately are used. Things are necessary and that alone adds to the cost.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical compensatory sheet having excellent visual angle characteristics which can be manufactured by a simple process at low cost and uses the optical compensatory sheet. An object of the present invention is to provide a liquid crystal display device having good viewing angle characteristics.

[0008]

Means for Solving the Problems The above problems are solved by a first optically anisotropic film which does not exhibit birefringence at least in the normal direction of the film, and a second optical anisotropic film which exhibits birefringence in the normal direction of the film. In the optical compensatory sheet comprising an anisotropic film, the first optically anisotropic film is a positive uniaxial optically anisotropic body having an optical axis in the normal direction and having a refractive index in the thickness direction of the surface. Of the two optical polarizers having the same refractive index as each other, and when used in a liquid crystal display element, at least a liquid crystal cell and two polarizing plates and a liquid crystal cell arranged on both sides of the liquid crystal cell. This has been achieved by a liquid crystal element characterized by having at least one of the above optical compensation sheet between the polarizing plate and the liquid crystal cell and the polarizing plate.

The details will be described below. The first optically anisotropic film used in the present invention is a positive uniaxial optically anisotropic substance having no birefringence in the normal direction of the film and having an optical axis in the normal direction, The feature is that birefringence develops as the distance from the direction increases. Therefore, the refractive index in the normal direction is Nz, the two in-plane refractive indexes are Nx,
If Ny, the relationship between Nx, Ny and Nz is Nz> Nx
= Ny. This first optically anisotropic film can be prepared by irradiating a polymer compound having a photoisomerizable functional group represented by azobenzene and stilbenzene with light. That is, Katsumi Yoshino, "Electronics
"Photofunctional polymer", Takahiro Seki "Control of molecular alignment by light"
As described in "Polymer", page 844, vol. 41, December issue (1922), etc., the polymer is controlled in orientation by utilizing photoisomerization to control the refractive index. That is, the first optically anisotropic film is a methylene chloride, acetone, a polymer compound containing a photoisomerizable functional group such as azobenzene or stilbenzene in its side chain or main chain.
It can be obtained by dissolving it in an organic solvent such as methanol or methyl ethyl ketone, coating it on a polymer film, and irradiating it with white light while heating it at a glass transition temperature of the polymer substance + 10 ° to 20 °. . Although the solution concentration varies greatly depending on the viscosity of the polymer, it is usually used in the range of 5 to 50%, preferably in the range of 8 to 30%, and the application is possible by using a wire bar, a gieser or the like. . After coating, the solvent is removed by drying, and the cis / trans isomerization of the above-mentioned photoisomerizable functional group is promoted by white light irradiation to increase the refractive index in the thickness direction. The illuminance of white light is not particularly limited, and in order to quickly obtain the target refractive index,
Higher illuminance is desirable.

A liquid crystalline polymer can also be used as the first optically anisotropic film. The liquid crystal polymer used has a refractive index (Nz) in the thickness direction in the liquid crystal state in at least one direction in the plane (such as described in JP-A-5-27235 and JP-A-5-34678). Nx, N
y) Any structure having a larger size and having a liquid crystal transition point or less and being in a glass state can be used. By applying such a liquid crystalline polymer in a molten state to a substrate or a substrate subjected to vertical alignment treatment, heat-treating and cooling
It is possible to obtain the first optically anisotropic film oriented in the relationship of z> Nx = Ny. The liquid crystal polymer, main chain type liquid crystal polymer such as polyester, polyamide, polycarbonate, polyester imide, polyacrylate,
Although a side chain type liquid crystal polymer such as polymethacrylate can be used, polyester is preferable in view of transparency, orientation and glass transition point. Furthermore, the structure of polyester is described in JP-A-5-27.
Nos. 235 and 5-34678 can be cited. As described above, the liquid crystalline polymer is applied on the substrate in a solution state or a molten state. Although the solvent varies depending on the polymer, acetone, methyl ethyl ketone, cyclohexane or the like can be used. The concentration of the solution depends on the viscosity of the polymer, but is usually 5 to 50%, preferably 8 to 30%.
% Range. As a coating method, a spin coating method, a roll coating method, a printing method, a car run cone method, a dipping and pulling method or the like is adopted. After coating, the solvent is removed by drying, and heat treatment is performed at a predetermined temperature for a predetermined time to align the liquid crystal. It is preferable to perform heat treatment at a glass transition temperature according to the characteristics of the polymer, and a range of 50 ° C to 300 ° C, particularly 100 ° C to 250 ° C is preferable. The heat treatment time is preferably 5 seconds to 120 minutes, particularly 10 seconds to 60 minutes, in order to obtain sufficient orientation.
Then, the alignment is fixed by cooling to a temperature not higher than the glass transition point of the polymer liquid crystal.

The second optically anisotropic film used in the present invention is an optically anisotropic body which exhibits birefringence in the normal direction. The second optically anisotropic film is obtained by, for example, forming a polymer having a positive intrinsic birefringence into a plastic film by a melt extrusion method, a melt casting method, a calender method, or the like, and then uniaxially stretching it. . In this case, the polymer chains are oriented in the stretched direction (X direction), and in the width direction (Y direction) and the thickness direction (Z direction) perpendicular to the stretched direction, Y of the polymer chains is accompanied by orientation of the polymer chains in the X direction. And Z
Orientation in the stretching direction is reduced, resulting in a refractive index N in the stretching direction.
A film in which x is larger than the refractive indexes Ny and Nz in the direction perpendicular to the film is obtained. The relationship of refractive index is Nx> Ny ≧ N
z.

The polymer used for the second optically anisotropic film is preferably a polymer having a positive intrinsic birefringence, and is preferably polycarbonate, polyarylate, polyethylene terephthalate, polyamide, polyimide or the like, and is particularly a polycarbonate polymer or polyarylate. Polymer,
It is preferable to use a polymer having a large intrinsic birefringence value, such as a polyester polymer, which can easily form a planar homogeneous film by solution casting. The above-mentioned polymer may be not only a homopolymer but also a derivative or blend of copolymers thereof. The polymer film preferably has a light transmittance of 70% or more and is substantially transparent and achromatic, and more preferably 90% and has a light transmittance of transparent and achromatic.
The above polymer is solution cast and uniaxially stretched to obtain Nz> N.
A polymer film with y = Nz is obtained.

By combining the first optically anisotropic film and the second optically anisotropic film, STN-LC can be obtained.
The ideal refractive index relationship as a compensation sheet for D, Nx>
Nz> Ny can be obtained, and good viewing angle characteristics can be obtained by incorporating it as an optical compensation sheet into a liquid crystal display device. As a method of combining these two films, a polymer compound having a functional group capable of photoisomerization is coated on the second optically anisotropic film and photoisomerized to obtain the first optical anisotropy. A method of making a film on a second optically anisotropic film, or TAC, Arton,
A polymer compound having a functional group capable of photoisomerization is coated on a film such as Zeonex, which hardly exhibits birefringence, and photoisomerized to form a first optically anisotropic film. Examples include a method using the second optically anisotropic film prepared. In the latter method, T
Create a first optically anisotropic film on AC and attach it to P
From the viewpoint of cost, it is preferable to use it as a protective film for a polarizing film composed of VA / iodine.

As described above, the optical compensation sheet manufactured according to the present invention can prevent the coloring and the contrast reduction due to the change of the viewing angle, which is peculiar to the liquid crystal display, and provide the liquid crystal display with the improved viewing angle characteristics at a low cost. Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited thereto.

[0015]

【Example】

Comparative Example 1 Preparation of Optical Compensation Sheet (1) 17% by weight of polycarbonate having a weight average molecular weight of 100,000
The methylene chloride solution was cast on a stainless steel band, dried until the residual volatile content became 3%, peeled off, and further dried to reduce the residual volatile content to 1% or less, and then transferred at 158 ° C in the conveying direction. It was uniaxially stretched to prepare an optical compensation sheet (1).

Example 1 Preparation of Optical Compensation Sheet (2) By Klaus Andre et al.
Chemie Rapid Communication, 10, 10 g of a polymer having an azobenzene side chain represented by the compound (1) synthesized according to the method described in pp. 478 (1989), and 100 g of methanol: methylene chloride = 9: 1 It was dissolved in a solvent and applied on the above-mentioned uniaxially stretched polycarbonate film by a wire bar so that a dry coating film had a thickness of 8.5 μm. After that, dry and 47 ℃
The plate was placed on a hot plate of No. 1 and irradiated with white light for about 1 hour to prepare an optical compensation sheet (2). At this time, the illuminance was about 15,000 lux.

[0017]

[Chemical 1]

Example 2 Preparation of Optical Compensation Sheet (3) The same as in the preparation of the optical compensation sheet (2) except that cellulose triacetate (TAC) having a thickness of 80 μm was used instead of the uniaxially stretched polycarbonate film used. Then, an optical compensation sheet (3) was prepared.

Example 3 Preparation of Optical Compensation Sheet (4) An optical compensation sheet (4) was prepared according to Example 1 of JP-A-5-27235.

Evaluation Table 1 shows the configurations of the optical compensation sheets obtained in Comparative Example 1 and Examples 1 to 3, and the birefringence values in the normal direction and the triaxial directions of the respective films. As is clear from Table 1,
Each of the first optically anisotropic films does not show birefringence from the normal direction of the film and exhibits positive uniaxiality having an optical axis in the normal direction, and the refractive index in the thickness direction has two refractive indices in the plane. Greater than equal refractive index. The second optically anisotropic film exhibits birefringence in the normal direction of the film.

[0021]

[Table 1]

The structure of the polarizing plate and the optical compensation sheets (2) to (4) obtained in Examples 1 to 3 are shown in FIG. 1, and the structure of the STN liquid crystal display device is shown in FIG. The polarizing plate and the optical compensation sheet of FIG. 1 are incorporated in the liquid crystal display element of FIG. 2 as shown in Table 2, and the contrast ratio of the obtained liquid crystal display element in the driven state and the non-driven state is 10: 1 or more. The viewing angles are shown in Table 3.

[0023]

[Table 2]

[0024]

[Table 3]

As is apparent from Table 3, the optical compensatory sheets (2) to (4) comprising the first optically anisotropic film and the second optically anisotropic film of the present invention are STN liquid crystal display devices. Significantly improves the viewing angle characteristics, particularly the contrast. In addition, the change in color depending on the viewing angle was small.

[Brief description of drawings]

1A shows a cross section of a polarizing plate A which is a normal polarizing plate, and FIG. 1B shows an optical compensation sheet since a cellulose triacetate film is used as a part of the first optical compensation film. The cross section of the polarizing plate B from which the cellulose triacetate on the side in contact with (3) is removed is shown. (C) shows the cross section of the optical compensation sheet (2), (d) shows the optical compensation sheet (3), and (e) shows the cross section of the optical compensation sheet (4).

FIG. 2 shows a configuration of a liquid crystal display element.

[Explanation of symbols]

 1: Cellulose triacetate film 2: Polarizing film composed of PVA / iodine 3: Azobenzene polymer film 4: Polycarbonate film 5: Liquid crystalline polymer 6: Glass 7: Polarizing plate 8: Optical compensation sheet 9: STN liquid crystal cell 10: Optical Compensation sheet 11: Polarizing plate

Claims (2)

[Claims] 1. At least a first optically anisotropic film that does not exhibit birefringence in the normal direction of the film, and a second optically anisotropic film that exhibits birefringence in the normal direction of the film. In the optical compensation sheet, the first optically anisotropic film is a positive uniaxial optically anisotropic body having an optical axis in the normal direction, and has two refractive indices in the thickness direction that are equal to each other in the plane. An optical compensation sheet characterized by being larger than the ratio. 2. A liquid crystal cell and at least two polarizing plates arranged on both sides of the liquid crystal cell, and at least one optical compensation sheet according to claim 1 between the liquid crystal cell and the polarizing plate. Characteristic liquid crystal display element.