JPH04311903A - Phase difference plate and liquid crystal display - Google Patents

Abstract

PURPOSE:To obtain a phase difference plate which has good visible angle property by laminating a multirefractive film with optically positive refraction aeolotropy and a multirefractive film with negative refraction aeolotropy so that the respective lagging axes in the film planes can be in the same direction. CONSTITUTION:A multirefractive film 1 with optically positive refraction aeolotropy and a multirefractive film 3 with negative refraction aeolotropy are laminated so that the respective lagging axes in the film planes can be in the same direction and an angle (visible angle) can be 42 deg. or more, showing that the ratio of retardation determined from the direction perpendicular to the film plane to retardation determined from the direction angled theta to a perpendicular axis with the lagging axis as a rotational axis is 1.10. A film which biaxial orientation with a visible angle of 37 deg. or less is used as either one of films. As a result, changes in retardation arising in the respective films are compensated with each other and a phase difference plate which has productivity and good visible angle property is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel retardation plate used in liquid crystal display devices and the like.

0002

[Prior Art] Retardation plates are disclosed in Japanese Patent Application Laid-open Nos. 189804/1989, 96623/1999, and 118/1999.
As shown in Publication No. 805, etc., it is a polymer film that has optical homogeneity and durability and has uniaxial orientation, and is used as an optical compensator to improve the display quality of liquid crystal display devices. ing. The STN type liquid crystal display device using a retardation plate is a two-layer type STN in which two liquid crystal cells are laminated.
Although they have advantages such as being lighter, thinner, and cheaper than conventional liquid crystal display devices, they also have disadvantages such as poor viewing angle characteristics and inferior black and white quality. Although these shortcomings have been considerably improved by methods such as laminating two retardation plates, the viewing angle characteristics have not yet reached a satisfactory level.

The viewing angle characteristics of a liquid crystal display device depend not only on the angle dependence of the birefringence of the liquid crystal cell, but also on the birefringence of the retardation plate,
That is, it largely depends on the angular dependence of retardation, and it is known that in conventional retardation plates, the smaller the angular change in retardation, the better. In recent years, JP-A-2
As shown in Japanese Patent Application Laid-open No. 191914 and Japanese Patent Application Laid-Open No. 2-285303, there is a method of shrinking a film in a direction perpendicular to the stretching axis during uniaxial stretching, and a method of forming a liquid polymer into a film under the application of an electric field. Studies have been conducted to improve the viewing angle characteristics by reducing the angular change in the retardation of the retardation plate by using various methods such as controlling the birefringence characteristics by stretching the film.
Many of these methods have problems with mass production, and few methods can be expected to significantly improve viewing angle characteristics.

0004

SUMMARY OF THE INVENTION An object of the present invention is to provide a retardation plate which not only has better viewing angle characteristics than ever before, but also is highly suitable for mass production.

[0005]

[Means for Solving the Problems] As a result of intensive studies to solve the above problems, the present inventors have developed a birefringent film having optically positive refractive index anisotropy and a birefringent film having negative refractive index anisotropy. By laminating and using birefringent films having a characteristic so that the in-plane slow axes of each film are in the same direction,
We discovered that a retardation plate with good viewing angle characteristics could be obtained.
This led to the present invention.

In order to evaluate the viewing angle characteristics of a birefringent film, in the case of a birefringent film with positive refractive index anisotropy, the slow axis is In addition, in the case of a birefringent film with negative refractive index anisotropy, the retardation (R) is measured with the fast axis tilted as the rotation axis.
The inclination angle (θ1.
10), and this value is called the viewing angle of the film. The smaller the angular change in retardation, the smaller this θ1.
10 is large, which means that the viewing angle characteristics are good.

The viewing angle of conventional retardation plates is 37 to 42° when stretched by longitudinal uniaxial stretching, 42° even when completely uniaxially oriented, and 20° when stretched by tenter stretching. Only ~37° was obtained.

The present inventors have found that the amount of change in retardation of a birefringent film having positive refractive index anisotropy is positive when measured with the in-plane slow axis of the film tilted as the rotation axis. On the other hand, a birefringent film with negative refractive index anisotropy has a negative refractive index, and similarly, when measured with the fast axis tilted as an axis, the birefringent film has a positive refractive index anisotropy. We focused on the phenomenon that the change in retardation of birefringent films with negative refractive index anisotropy is negative, while it is positive with birefringent films with negative refractive index anisotropy. When laminated in the same direction, the change in retardation that occurs in each film due to tilting is compensated for by each other, so the change in overall retardation becomes extremely small, resulting in excellent viewing angle characteristics. It has been found that a retardation plate can be obtained. That is,
The present invention optically uses a birefringent film having a positive refractive index anisotropy and a birefringent film having a negative refractive index anisotropy so that the slow axes in each film plane are in the same direction. The ratio of the retardation (R0) measured from the direction perpendicular to the film surface and the retardation (R) measured from the direction tilted by θ degrees from the vertical axis with the slow axis as the rotation axis. (R/R0) is 1.1
This is a retardation plate in which the angle (θ1.10) that becomes 0 is 42° or more.

The birefringent film having positive refractive index anisotropy used in the present invention includes, for example, polycarbonate resin, polyethylene terephthalate, polyester resin such as polyester copolymer, nylon 6, nylon 6, etc.
Examples include films made of polyamide resins such as No. 6, polysulfone, polyethersulfone, polyarylate, and modified products thereof, oriented by stretching or the like. In addition, the birefringent film having negative refractive index anisotropy used in the present invention may be obtained by copolymerizing polymethyl methacrylate or methyl (meth)acrylate as a main component with other ethylene comonomers. Poly(meth)acrylate resins such as methyl meth)acrylate copolymers; polystyrene resins such as styrene copolymers obtained by copolymerizing styrene as a main component with other ethylene comonomers; Examples include films made of modified products of or oriented by stretching or the like. Considering the development of birefringence, strength, heat resistance, etc., birefringent films with positive refractive index anisotropy include polycarbonate, polysulfone, polyethersulfone, and negative refractive index anisotropy. As the birefringent film, polystyrene and polymethyl methacrylate are preferable.

[0010] As a stretching method for orienting a film having positive or negative refractive index anisotropy to obtain a birefringent film used in the present invention, any stretching method that can obtain homogeneity of retardation may be used. Any known method such as a tenter stretching method, an inter-roll stretching method, or an inter-roll compression stretching method may be used, but the tenter stretching method is most preferable from the viewpoint of homogeneity and imparting biaxial orientation.

[0011] In the present invention, a laminated film is used by laminating birefringent films having positive and negative refractive index anisotropy, but as an adhesive or pressure-sensitive adhesive for laminating, Any known material can be used as long as it has excellent transparency and durability. FIG. 1 shows an example of a retardation plate according to the present invention, in which one birefringent film having positive and one negative refractive index anisotropy is used.

The retardation of the retardation plate according to the present invention is determined by the retardation of each of the laminated birefringent films having positive and negative refractive index anisotropy (R1, R2
) is added (R'=R1 +R2). In addition, θ1.10 can be adjusted by adjusting the retardation values (R1, R2) of the laminated birefringent films with positive and negative refractive index anisotropy and their respective viewing angle characteristics. It is possible to arbitrarily set θ from 1.10, which each individual retardation plate has, to a value exceeding 60°.

[0013] At this time, if a film having relatively good viewing angle characteristics with a viewing angle of 37 to 42° is used as the birefringent film having positive and negative refractive index anisotropy, an arbitrary In order to obtain a retardation plate having viewing angle characteristics, it is necessary to laminate films having positive and negative refractive index anisotropy with a considerable difference in retardation. In particular, in order to obtain a viewing angle of 42 to 60 degrees with a retardation plate that is laminated and has a retardation of 500 nm or less, a birefringent film having either positive or negative refractive index anisotropy is used. dation is 40n
It is necessary to use less than m, but generally 5
Since it is very difficult to obtain a birefringent film having a retardation of 0 nm or less with uniformity by a normal stretching method, there is a problem in mass production.

On the other hand, in the present invention, as a birefringent film having positive and/or negative refractive index anisotropy, a film having a biaxial orientation with a viewing angle of 20 to 37° and a viewing angle characteristic not so good is used. By using , it is possible to freely adjust the viewing angle characteristics of the laminate over a wide range even if the difference in retardation of each film is not large. A retardation plate having a viewing angle can be easily obtained. From this, when using a birefringent film produced by the tenter stretching method, which can ensure homogeneity of film orientation but unavoidable in-plane biaxial orientation, optical homogeneity and good arbitrary This technology is extremely effective because it can produce a retardation plate with excellent viewing angle characteristics.

The method of attaching the retardation plate of the present invention to a liquid crystal display device is not particularly limited. For example, an adhesive or the like is applied to one or both sides of the retardation plate shown in FIG. / and bonding with a polarizing plate may be used.

[0016]

【Effect of the invention】

As the birefringent film having positive and negative refractive index anisotropy used in the present invention, good viewing angle characteristics can be obtained simply by laminating two types of birefringent films having similar viewing angle characteristics. Therefore, it is not necessary for each film to have particularly good viewing angle characteristics. Therefore, in order to improve the viewing angle characteristics of conventional retardation plates, there is no need to give them complete uniaxial orientation or to control the orientation in the thickness direction. Even when using a birefringent film with in-plane biaxial orientation, which was said to have poor viewing angle characteristics, the viewing angle can be easily improved to over 42° compared to the case of each film alone. The viewing angle characteristics can be dramatically improved, and by using this as an optical compensator, the display characteristics of a liquid crystal display device can be significantly improved.

[0018]

EXAMPLES The present invention will be explained in detail with reference to Examples below, but the present invention is not limited thereto.

Note that the retardation (R1, R2, R') of each film and laminate was measured using the aforementioned polarizing microscope. Regarding the viewing angle, follow the method described above, and set the slow axis in the case of a birefringent film with positive refractive index anisotropy, and the progressive axis in the case of a birefringent film with negative refractive index anisotropy. The value of the ratio (R
/R0) becomes 1.10, the inclination angle (θ1.10)
However, as an exception, for films whose R/R0 is less than 1.10 in the range of tilt angles from 0 to 60 degrees, the ratio value (R40/R0) when the tilt angle is 40 degrees was used. .

Example 1 A polycarbonate film was stretched to 20% by tenter stretching method.
Stretched 1.7 times at 0°C, R1 = 222 nm, θ1.1
A birefringent film having a temperature of 0=26.5° and a thickness of 72 μm was obtained. In addition, a polystyrene film was stretched 2.5 times at 125°C by a tenter stretching method, and R2 = 220 nm.
A birefringent film having θ1.10=27.5° and a thickness of 174 μm was obtained. When these two films were laminated so that their slow axes were in the same direction, R' = 44
A retardation plate having a thickness of 3 nm and R40/R0 = 1.02 with extremely small angle dependence of retardation was obtained.

Example 2 A polycarbonate film was stretched to 20% by tenter stretching method.
Stretched 2.1 times at 0°C, R1 = 392 nm, θ1.1
A birefringent film having a temperature of 0=25.0° and a thickness of 75 μm was obtained. In addition, a polystyrene film was stretched 2.0 times at 125°C by a tenter stretching method, and R2 = 198 nm.
A birefringent film having θ1.10=27.3° and a thickness of 196 μm was obtained. When these two films were laminated so that their slow axes were in the same direction, R' = 59
A retardation plate having a thickness of 2 nm and θ1.10=45.2° was obtained.

Example 3 Polysulfone film was stretched to 210 mm by tenter stretching method.
Stretched 1.5 times at °C, R1 = 379 nm, θ1.10
A birefringent film having a thickness of 115 μm and a thickness of 115 μm was obtained. In addition, a polymethyl methacrylate sheet was stretched 2.0 times at 130°C by a tenter stretching method, and R2 = 1.
48nm, θ1.10=25.0°, thickness 1142μm
A birefringent film was obtained. When these two films were laminated so that their slow axes were in the same direction,
A retardation plate with R'=527 nm and θ1.10=50.0° was obtained.

Example 4 A polycarbonate film was stretched to 20% by tenter stretching method.
Stretched 1.7 times at 0°C, R1 = 227 nm, θ1.1
A birefringent film having a temperature of 0=25.2° and a thickness of 80 μm was obtained. In addition, a polystyrene film was stretched 1.5 times at 122°C by a longitudinal uniaxial stretching method, R2 = 373 nm,
A birefringent film having θ1.10=39.5° and a thickness of 175 μm was obtained. When these two films were laminated so that their slow axes were in the same direction, R' = 549.
A retardation plate with θ1.10=47.3° was obtained.

Example 5 A polycarbonate film was stretched to 197 mm by longitudinal uniaxial stretching.
Stretched 1.7 times at °C, R1 = 308 nm, θ1.10
A birefringent film having a diameter of 40.2° and a thickness of 67 μm was obtained. In addition, polystyrene film was stretched 2.5 times at 125°C by tenter stretching method, R2 = 242 nm,
A birefringent film having θ1.10=28.0° and a thickness of 153 μm was obtained. When these two films were laminated so that their slow axes were in the same direction, R' = 530n
A retardation plate with m and θ1.10=49.1° was obtained.

Comparative Example 1 A polycarbonate film was stretched to 198 mm by longitudinal uniaxial stretching.
A birefringent film was obtained by stretching 1.8 times at ℃. This film had R=437 nm and θ1.10=41.0°.

Comparative Example 2 Polycarbonate film was stretched to 19% by tenter stretching method.
A birefringent film was obtained by stretching 2.0 times at 5°C. This film has R=590nm and θ1.10=31.9°
Met.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view showing the structure of an example of a retardation plate according to the present invention.

FIG. 2 is a vertical cross-sectional view showing an example of an STN liquid crystal display device using a retardation plate according to the present invention.

[Explanation of symbols]

1. Birefringent polymer film with positive refractive index anisotropy2. Birefringent polymer film having negative refractive index anisotropy3. Adhesive or adhesive4. Retardation plate 5. Polarizing plate 6. STN liquid crystal cell

Claims (5)

[Claims] [Claim 1] A birefringent film having optically positive refractive index anisotropy and a birefringent film having negative refractive index anisotropy are arranged so that the slow axes in each film plane are in the same direction. The retardation (R0) measured from the direction perpendicular to the film surface and the retardation (R) measured from the direction tilted by θ degrees from the vertical axis with the slow axis as the rotation axis. The ratio (R/R0) is 1.
A retardation plate having an angle of 10 (θ1.10) of 42° or more. [Claim 2] The ratio of retardation (R0) measured from a direction perpendicular to the film surface to retardation (R) measured from a direction tilted by θ degrees from the vertical axis with the slow axis as the rotation axis (R/R0). ) is 1.10 (θ1
．． 10) is a birefringent film with positive refractive index anisotropy with biaxial orientation of 37° or less, the retardation (R0) measured from the direction perpendicular to the film surface, and the fast axis as the rotation axis. If the angle (θ1.10) at which the ratio of retardation (R) (R/R0) measured from the direction tilted by θ degrees from the vertical axis is 1.10 is 37° or less, it has a negative biaxial orientation. 2. The retardation plate according to claim 1, wherein a birefringent film having a refractive index anisotropy of . 3. A birefringent film having positive refractive index anisotropy and biaxial orientation with θ1.10 of 37° or less;
2. The retardation plate according to claim 1, characterized in that a birefringent film having a negative refractive index anisotropy with a uniaxial orientation of 1.10 of 37° or more and 42° or less is used. Claim 4: A birefringent film having a positive refractive index anisotropy having a uniaxial orientation at θ1.10 of 37° or more and 42° or less, and a birefringent film having a biaxial orientation at θ1.10 of 37° or less. The retardation plate according to claim 1, characterized in that a birefringent film having negative refractive index anisotropy is used. 5. A liquid crystal display device comprising a layered retardation plate according to claim 1.