JPH04311902A - 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 mutirefractive film 1 with positive refraction aeolotropy which has uniaxial orientation with an angle (visible angle) of 37-42 deg. showing that the ratio of retardation determined in the condition of inclining a lagging axis, if the multirefractive film has positive refraction aeolotropy, or an advancing axis, if it has negative refraction aeolotropy, as the axis of rotation to retardation determined in a horizontal condition is 1.10 and a multirefractive film 3 with negative refraction aeolotropy which has uniaxial orientation with a visible angle of 37-42 deg. are laminated so that the respective lagging axes can be in the same direction. Because changes in retardation due to inclination are compensated with each other, a total change is made smaller and an excellent phase difference plate which has a visible angle of 42 deg. or more 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 angular dependence of the birefringence of the liquid crystal cell, but also on the angular dependence of the birefringence, or retardation, of the retardation plate. It is known that the smaller the angular change in retardation for a plate, the better. In recent years, JP-A-2-
As shown in Japanese Patent Application Laid-Open No. 191914, Japanese Patent Application Laid-Open No. 2-285303, etc., there is a method of shrinking a film in a direction perpendicular to the stretching axis during uniaxial stretching, or a method of forming a film from a liquid polymer 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 problem] As a result of intensive studies to solve the above problems, we have developed a birefringent film with optically positive refractive index anisotropy and a birefringent film with negative refractive index anisotropy. The inventors discovered that a retardation plate with good viewing angle characteristics can be obtained by stacking and using films with different intensities such that the slow axes of each film are in the same direction, leading to the present invention. be.

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, that is, the viewing angle characteristics are good.

The viewing angle of conventional retardation plates is 37 to 40° when stretched using the longitudinal uniaxial stretching method, 42° even when completely uniaxially oriented, and 20° when stretched using the tenter stretching method. 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. This is a retardation plate obtained by laminating layers in the following manner.

In the present invention, by laminating a birefringent film having optically positive refractive index anisotropy and a birefringent film having negative refractive index anisotropy, the viewing angle can be easily increased to 42° or more. It can be done.

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 preferred.

[0011] 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 an inter-roll stretching method or an inter-roll compression stretching method may be used.

[0012] Furthermore, as the adhesive or pressure-sensitive adhesive 3 for laminating the birefringent films having positive and negative refractive index anisotropy, any known adhesive or pressure-sensitive adhesive 3 may be used as long as it has excellent transparency and durability. can be used. 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 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 the viewing angle characteristics of each. It is possible to arbitrarily set the angle θ of each retardation plate from 1.10 to a value exceeding 60°.

The method for 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. Alternatively, a method such as bonding with a polarizing plate may be used. Either of the retardation plates 1 and 2 may be placed on the liquid crystal cell side, or between the upper polarizing plate and the liquid crystal cell or between the lower polarizing plate and the liquid crystal cell.

[0015]

[Effect of the invention] In the present invention, when a birefringent film with uniaxial orientation and a viewing angle of 37 to 42° is used as the birefringent film with positive and negative refractive index anisotropy. , the viewing angle characteristics can be easily improved to 42° or more. Therefore, in order to improve the viewing angle characteristics of conventional retardation plates, we have created completely uniaxial orientation,
There is no need to control the orientation in the thickness direction, and viewing angle characteristics can be easily improved dramatically compared to the case of each film alone. By using this as an optical compensator, liquid crystal The display characteristics of the display device can be significantly improved.

[0016]

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

[0017] 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 197 mm by longitudinal uniaxial stretching.
Stretched 1.7 times at °C, R1 = 315 nm, θ1.10
A birefringent film having a diameter of 40.0° and a thickness of 80 μm was obtained. In addition, a polystyrene film was stretched 1.5 times at 125°C by a longitudinal uniaxial stretching method, R2 = 278 nm, θ
A birefringent film having a diameter of 1.10=38.5° and a thickness of 162 μm was obtained. When these two films were laminated so that their slow axes were in the same direction, R' = 591 nm, R40/R0
A retardation plate with a very small angle dependence of retardation of =1.10 was obtained.

Example 2 A polycarbonate film was stretched to 198 mm by longitudinal uniaxial stretching.
Stretched 2.0 times at °C, R1 = 510 nm, θ1.10
A birefringent film having a diameter of 41.3° and a thickness of 76 μm was obtained. In addition, a polystyrene film was stretched 2.0 times at 127°C by a longitudinal uniaxial stretching method, R2 = 82 nm, θ1
．． A birefringent film having 10=41.0° and a thickness of 134 μm was obtained. When these two films were laminated so that their slow axes were in the same direction, R' = 588 nm.
, θ1.10=45.0°, a retardation plate with good viewing angle characteristics was obtained.

Comparative Example 1 A transparent polycarbonate film was stretched by longitudinal uniaxial stretching.
A birefringent film was obtained by stretching 1.8 times at 98°C. This film has R=437nm and θ1.10=41.0
It was °.

Comparative Example 2 Polycarbonate film was stretched to 20% by tenter stretching method.
A birefringent film was obtained by stretching 2.5 times at 0°C. This film has R=590nm and θ1.10=31.9°
Met.

[Brief explanation of drawings]

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 (3)

[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. A retardation plate obtained by laminating layers so that [Claim 2] The ratio of retardation (R0) measured from the vertical direction of the film surface to 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 1.10 (θ1
．． 10) is a birefringent film having a positive refractive index anisotropy with a uniaxial orientation of 37° or more and 42° or less, and a retardation (R
0) and the retardation (R) measured from a direction tilted by θ degrees from the vertical axis with the fast axis as the rotation axis (R/
The angle (θ1.10) at which R0 ) is 1.10 is 37
2. The retardation plate according to claim 1, characterized in that a birefringent film having negative refractive index anisotropy and uniaxial orientation of .degree. or more and 42.degree. or less is used. 3. A liquid crystal display device comprising a stack of retardation plates according to claim 1.